Carboranes
Carboranes Second Edition by
Russell N. Grimes
Department of Chemistry University of Virginia
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Academic Press is an imprint of Elsevier 84 Theobald’s Road, London WC1X 8RR, UK Radarweg 29, PO Box 211, 1000 AE Amsterdam, The Netherlands 30 Corporate Drive, Suite 400, Burlington, MA 01803, USA 525 B Street, Suite 1900, San Diego, CA 92101-4495, USA The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, UK Copyright # 2011 Elsevier Inc. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher. Permissions may be sought directly from Elsevier’s Science & Technology Rights Department in Oxford, UK: phone: (þ44) 1865 843830, fax: (þ44) 1865 853333, E-mail:
[email protected]. You may also complete your request online via the Elsevier homepage (http://elsevier.com), by selecting “Support & Contact” then “Copyright and Permission” and then “Obtaining Permissions.” Library of Congress Cataloging-in-Publication Data Grimes, Russell N., 1935Carboranes / Russell N. Grimes. – 2nd ed. p. cm. Includes bibliographical references and index. ISBN 978-0-12-374170-7 (hardback) 1. Carboranes. I. Title. QD412.B1G73 2011 547’.05671–dc22 2010049744 British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. 978-0-12-374170-7 For information on all Academic Press Publications visit our Web site at elsevierdirect.com 11 12 10 9
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Printed and bound in United states of America
To Nancy
Preface The appearance of a second edition 40 years after the original work is sufficiently unusual to warrant an explanation by the author. The truth is that for many years after I wrote Carboranes, I was preoccupied with teaching, research, and proposal writing, along with serving as Chemistry Department chair and editing a volume of Inorganic Syntheses, and I gave no thought to writing a sequel. But as time marched on and retirement from active teaching approached, the idea of a new edition gradually took root. As carborane chemistry expanded rapidly and myriad applications appeared in all sorts of areas of modern technology, the need for a new comprehensive treatment of this field was becoming increasingly apparent; moreover, the prospect of retirement offered the time and freedom to make it happen. Some gentle prodding from fellow boron chemists finally led me to take the plunge, and this book is the result. For anyone taking the trouble to contrast the 1970 Carboranes with the present volume, it will be apparent that there really is no comparison. Indeed, the field itself has changed and expanded in ways that few could have anticipated. In 1970, when the published literature in this area was less than a decade old, carboranes or metallacarboranes were rarely mentioned in textbooks, few chemistry graduates were even aware of their existence, and only the most intrepid organic chemists of that era would have imagined a role for carboranes in organic synthesis. And for workers in medicine and allied fields, carboranes were truly off the radar screen. Not any more! The hundreds of citations of carborane-based publications in biology and biomedicine in this volume hardly scratch the surface, so widespread is the interest and activity in this area. Moreover, carboranes have emerged as major players in nonmedical applications such as extraction of metals from radioactive waste, the development of new materials, and other fields outlined in the last four chapters of this book. Which brings me to the two main reasons for this volume, one of which is to provide a useful introduction to the theory and practice of carborane chemistry for those with no background in this area. The other objective is to furnish a reliable source of information for active researchers in fundamental and applied carborane chemistry. To that end, I have endeavored to include the most up-to-date developments, with coverage of the literature extending well into 2010. Augmenting the text and tables in the printed volume, extended tables of information on specific compounds are provided gratis on the website http://www.elsevierdirect.com/companion.jsp?ISBN=9780123741707, which will be periodically updated with new data. A few comments regarding style and content may be appropriate. I have for the most part employed simplified compound names and chemical formulas consistent with general practice in the boron cluster field, and all drawings are my own, which ensures consistency of presentation and efficient use of space. For similar reasons, spectra and Ortep-style structural drawings are not reproduced in this book; as needed, such items can be found in the original journal articles via the listed citations. Although I have made every effort to be as accurate as possible, with information drawn for the most part from primary sources, errors of commission and omission are inevitable in a project of this size. For these, and especially for instances in which I have overlooked or (worse) inadvertently misrepresented anyone’s work, I apologize in advance. I am indebted to many people, including a couple of generations of hard-working student and postdoctoral colleagues; my longtime friend Walter Siebert, who was my gracious host on two different sabbaticals at the University of Heidelberg, Germany, during the second of which I began the long process of organizing this book; Fred Hawthorne, to this day a major driver and shaper of polyhedral borane chemistry, who kindly read several chapters of the manuscript and offered valuable insights; the Colonel, Bill Lipscomb, who long ago lured me into the alternate universe of boron chemistry; friends whose constant inquiries about The Book over the years kept me from complacency; and above all my wife Nancy Hall Grimes, without whose unfailing support and encouragement I could never have seen this project through to completion. Finally, I thank my colleagues in the Department of Chemistry, a great bunch, for allowing me to keep an office years after retirement when I could have simply been put out to pasture.
Charlottesville, Virginia
Russell N. Grimes February 2011
xvii
CHAPTER
Introduction and History
1
Carboranes (“carbaboranes” in the formal nomenclature) are polyhedral boron-carbon molecular clusters that are stabilized by electron-delocalized covalent bonding in the skeletal framework. In contrast to classical organoboranes such as borabenzene (C5H5B), the skeletal carbon atoms in carboranes typically have at least three and as many as five or six neighbors in the cluster, forming stable—in some cases, exceedingly stable—molecular structures. Carboranes have been known for more than half a century, but for much of that time they were of interest primarily to boron chemists, theoreticians interested in their structures and bonding, and small groups of industrial researchers, who recognized their potential for creating extremely heat-stable polymers. In recent years, the picture has changed dramatically. Carborane chemistry is experiencing a major surge in interest across a wide spectrum of technologies, fueled by developing applications in medicine, nanoscale engineering, catalysis, metal recovery from radioactive waste, and a number of other areas, documented in thousands of publications and patents. This resurgence has been ignited by a wide (if belated) recognition of the unique electronic properties, geometry, and extraordinary versatility of carboranes, and a growing recognition that carborane chemistry affords a whole new realm of possibilities that transcends conventional organic or organometallic synthesis. Carboranes had been envisioned by William Lipscomb, Roald Hoffmann, and others from theoretical considerations [1,2] before reports of the synthesis of any such compounds appeared (although it is entirely possible that they may have been present in some CxByHz products generated by experimentalists as far back as Alfred Stock, the great pioneer of boron hydride chemistry [3,4]). Much of the early speculation centered on the icosahedron, a 12-vertex, 20-sided polyhedron. As icosahedral B12 clusters are present in all known forms of elemental boron and in some metal borides, and several boron hydrides have boron frameworks that are fragments of an icosahedron (e.g., B10H14) [5], it was conjectured that an icosahedral B12H12 hydride might exist. However, in a classic 1955 paper, Longuet-Higgins and Roberts calculated that two additional electrons would be required, stabilizing the icosahedron as a B12H122 dianion [6]. Shortly thereafter, B12H122 salts were isolated by Pitochelli and Hawthorne [7] and found to be incredibly robust, withstanding temperatures above 800 C and exhibiting inertness toward most reagents; in fact, B12H122 is the most stable molecule known to science. Its discovery strongly implied the viability of neutral C2B10H12 clusters, isoelectronic analogues of B12H122 in which two BH units are formally replaced by CH groups whose carbon atoms would be six-coordinate (a fairly startling idea at the time). As it happens, the first icosahedral carboranes had actually been prepared in industrial laboratories in the 1950s but were kept under wraps and not reported in the open literature until late 1963. These compounds were obtained in the course of an industrial effort to synthesize stable organic derivatives of boron hydrides under a post-World War II U.S. government program (Projects ZIP and HEF), whose purpose was to develop practical borane-based aircraft and rocket fuels that could exploit the much higher energies generated by combustion of boron hydrides compared to hydrocarbons [8]. Under this program, conducted by the Callery Chemical Company and the Olin-Mathieson Corporation, stockpiles of diborane (B2H6), pentaborane-9 (B5H9), and decaborane-14 (B10H14) were prepared on pilot-plant scales and used to prepare a variety of alkylated derivatives to be tested as fuel additives. These projects Carboranes. DOI: 10.1016/B978-0-12-374170-7.00017-3 © 2011 Elsevier Inc. All rights reserved.
1
2
CHAPTER 1 Introduction and History
were abandoned after a few years, in part owing to difficulties arising from the solid boron oxide and boron nitride combustion products and their effects on jet and rocket engines. Large stocks of boron hydrides remained, and industrial efforts turned to exploring the possibility of converting them to useful materials such as heat-stable polymers. It was during this period that exploratory reactions of alkynes with boranes led to novel organoboranes with properties far different from their boron hydride precursors. The first reports of carboranes in the journal literature described small, sub-icosahedral clusters. In 1962, Onak, Williams, and Weiss obtained nido-RR0 C2B4H6 (R, R0 ¼ H, Me, C3H7) by reacting B5H9 with alkynes in the presence of 2,6-dimethylpyridine [9]. Soon thereafter, Shapiro and co-workers reported the isolation of closo-C2B3H5 and two isomers of closo-C2B4H6 from the reaction of B5H9 and acetylene in an electric discharge [10,11] (the prefix closo designates a deltahedral structure, i.e., a polyhedron having all triangular faces; nido, Greek for nest, indicates a closo polyhedron minus one vertex). Earlier, in late 1957, an extraordinarily stable compound characterized as B10H10C2H2 (later given the trivial name o-carborane, 1,2-C2B10H12), had been isolated at Reaction Motors, Inc. (now the Reaction Motors Division of Thiokol Chemical Corporation) from the reaction of B10H14 derivatives with acetylene [8]. The original work on the icosahedral C2B10H12 carboranes was published in 1963 in a series of papers from the groups at Thiokol [12–16] and Olin-Mathieson [17–22]. Russian workers were also active in this area, reporting the preparation of “barene” (C2B10H12), which they at first incorrectly characterized as an open B10H10 basket with an 2 2HC5 5CH2 2 bridge spanning the open face [23,24]. Subsequently, Russian groups have played a major role in the development of this area of chemistry that continues to the present time. The two remaining isomers, 1,7- and 1,12-C2B10H12 (m- and p-carborane, respectively) were prepared via thermal cage-rearrangement of o-carborane [16,25–30], and the icosahedral structures of all three isomers were established from 11B NMR and X-ray crystallographic data [22,28,31–39]. Investigations of the C2B10H12 systems soon revealed that their cage C2 2H bonds are highly polar (especially in the 1,2 and 1,7 isomers, and to a lesser degree in the 1,12 compound), imparting a positive charge on the CH hydrogens and making them acidic toward Lewis bases. This has allowed the synthesis of a wide variety of C-substituted derivatives of the 1,2 and 1,7 isomers via replacement of the CH hydrogens (substitution at BH vertexes is also possible using other approaches), opening the way to a vast derivative chemistry that today is the foundation of a broad spectrum of applications in industry and medicine. The icosahedral carboranes are only a part of the story, as was discovered early on through work in several academic and industrial research groups. The C2B10H12 isomers are members of a series of closo-carboranes (Figures 1-1 and 1-2) having the general formula C2Bn2Hn, where n ¼ 5-14 (substituted derivatives of supra-icosahedral 13- and 14-vertex cages are recent exciting discoveries). An isoelectronic series of closo-CBn-1Hn anions (n ¼ 6-12) exists, as do the analogous borane BnHn2 anions (n ¼ 6-12). In addition, there is a host of open-cage carboranes whose cage structures are fragments of polyhedra, examples of which are shown in Figure 1-3. These include nido, arachno (Greek for web), and hypho (net) clusters that are formally derived from closo frameworks by removal of one, two, or three vertexes, respectively, as discussed in the following chapter. In all types of carboranes, different isomers are possible in principle by varying the location of the skeletal carbon atoms, and in many cases isomerism has been observed. Beyond the size, shape, and carbon locations, other variations are possible. For example, the number of carbon atoms in the skeletal framework is not limited to one or two; three- and four-carbon carboranes are well known, and even fiveand six-carbon systems have been prepared. The introduction of functional groups at carbon and boron vertexes adds yet another dimension, linking carborane and organic chemistry, and, in some respects, creating a whole new branch of the latter field. Finally, there is an intrinsic property of carboranes whose significance and potential are so far-reaching that it impacts virtually every field of chemistry. This is their extraordinary ability (shared with polyhedral boranes in general) to accommodate metal and nonmetal atoms of nearly every description in the cage framework [40–47]. Excepting only the extremely electronegative and electropositive elements and the Group 18 gases, carboranes incorporating almost every element in the Periodic Table have been prepared and characterized. There are phospha-, thia-, selena-, and azacarboranes; berylla-, oxa-, alumina-, sila-, germana-, stanna-, arsena-, stiba-, galla-, and indacarboranes;
Introduction and History
= BH
= CH 1
1
4
2
3
1
2
3
4
3
5
2
5
2
4
6
1,2-C2B4H6
1,6-C2B4H6
3
1-CB5H7
•
5 1
• • •
5
6
5
2
3
4
8 8
7
5
1
−
5
2 9
8
2
4 6
4
8
10
10
1,2-C2B8H10
1 2 7
6
3
11 10
2-CB10H−11
2
8
7
6
5
4
9
1,6-C2B8H10
1
−
3
3
7
10
1-CB9H−10
9
4
9 11
4 6
7
8
10
2,3-C2B9H11
FIGURE 1-1 Structures and cage numbering for sub-icosahedral closo-carboranes.
1
5
2
8
6
7
7
1
5
3
9
3
4,5-C2B7H9
1
5
3
•
1,7-C2B6H8
2,4-C2B5H7
1
4
2
6
9
6
2
3
7
2,3-C2B5H7
5
2
3
4
7
4
•
6
H
6
4
1
1
5
4
6
5
1,5-C2B3H5
1
2 9
3
8
6 7 10
1,10-C2B8H10
3
4
CHAPTER 1 Introduction and History
= CH
= BH 1 6 5
1
−
6
2
5
3
4
1 6
2
5
3
4
7
7
8
7
7
9
8
12
12
3
10
9
8
2
4 11
10
9
1-CB11H−12
5
11
10
9
6
2 3
4
11
11 10
1
8 12 1,12-C2B10H12
12
1,2-C2B10H12
1,7-C2B10H12
FIGURE 1-2 Structures and cage numbering for icosahedral closo-carboranes. = BH
= CH
=C
1
6 H
1,2-C2B3H7
4
2,3-C2B4H8
2,3,4-C3B3H7 6 H
4
1
H H
H H
5
2 5
6
H
2
1
3
5
H
4 2
4
1
3 2,3,4,5-C4B2H6
6
H
H
7
9
9
H
H
11
H
4
5,6,7-C3B7H13
H
10
11 7
8 6
5 2
4
2
4 3
3
2 1
H
6 5
3
− 9
7
8
8
10
5,6-C2B8H12
6,8-C2B7H13 10
H
5
8
10
3
7
9
H
7
6
8
9
5
H
3 H
4
2-CB5H9
6
5
H
3 H
4
H
6
5
H
3 3 H
2
2
2
2
4
5
1
1
1
1 7-CB10H−13
FIGURE 1-3 Structures and cage numbering for selected open-cage carboranes.
1 7,8-C2B9H13
Introduction and History
H 9
10
11
H
10 9
7
8
−
11
1
−
2
7
8
6
3
6
6 5
5 2
4
5 2
4 3
3 1 7,8-C2B9H−12
1 7,9-C2B9H−12
5
4
11
10 9
7
8 12 2,3,7,8-C4B8H12
FIGURE 1-3—CONT’D
metallacarboranes involving essentially all of the transition and lanthanide elements; and polyheteroatom clusters incorporating several different elements in the same cage framework. In some cases, even metals as electropositive as lithium and sodium can be considered to participate in skeletal covalent binding. Obviously, one is dealing here with systems that simultaneously impact inorganic, organic, main group, and transition-metal chemistry—exciting in their versatility and potential, but daunting in their complexity. This book is an attempt to draw together as much of this vast sphere as can reasonably be achieved in one volume. In many of the topics to be covered, the level of detail presented is intended primarily as an introduction, to be supplemented as needed by more specialized reviews for which references are provided. Augmenting the text and graphics, tables listing individual compounds with references to the original literature are provided in this volume, with more extensive compilations available on the website http://www.elsevierdirect.com/companion.jsp? ISBN=9780123741707.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
Hoffmann, R.; Lipscomb, W. N. J. Chem. Phys. 1962, 36., 3489. Lipscomb, W. N. Proc. Natl. Acad. Sci. USA 1961, 47, 1791. Stock, A. Hydrides of Boron and Silicon; Cornell University Press: Ithaca, New York, 1933. Stock, A.; Kuss, E. Ber 1923, 56, 789. Lipscomb, W. N. Boron Hydrides; Benjamin: New York, 1963. Longuet-Higgins, H. C.; Roberts, M. Proc. R. Soc. Lond. A 1955, 230, 110. Pitochelli, A. R.; Hawthorne, M. F. J. Am. Chem. Soc. 1960, 82, 6909. Hughes, R. L.; Smith, I. C.; Lawless, E. W. In Production of the Boranes and Related Research; Holzmann, R. T., Ed.; Academic Press: New York, 1967; [and references therein]. Onak, T. P.; Willams, R. E.; Weiss, H. G. J. Am. Chem. Soc. 1962, 84, 2830. Shapiro, I.; Good, C. D.; Williams, R. E. J. Am. Chem. Soc. 1962, 84, 3837. Shapiro, I.; Keilin, B.; Williams, R. E.; Good, C. D. J. Am. Chem. Soc. 1963, 85, 3167. Fein, M. M.; Bobinski, J.; Mayes, N.; Schwartz, N.; Cohen, M. S. Inorg. Chem. 1963, 2, 1111. Fein, M. M.; Grafstein, D.; Paustian, J. E.; Bobinski, J.; Lichstein, B. M.; Mayes, N.; et al. Inorg. Chem. 1963, 2, 1115. Grafstein, D.; Bobinski, J.; Dvorak, J.; Smith, H.; Schwartz, N. N.; Cohen, M. S.; et al. Inorg. Chem. 1963, 2, 1120. Grafstein, D.; Bobinski, J.; Dvorak, J.; Paustian, J. E.; Smith, H. F.; Karlan, S.; et al. Inorg. Chem. 1963, 2, 1125. Grafstein, D.; Dvorak, J. Inorg. Chem. 1963, 2, 1128. Alexander, R. P.; Schroeder, H. A. Inorg. Chem. 1963, 2, 1107. Heying, T. L.; Ager, J. W.; Clark, S. L.; Alexander, R. P.; Papetti, S.; Reid, J. A.; et al. Inorg. Chem. 1963, 2, 1097.
6 [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47]
CHAPTER 1 Introduction and History Heying, T. L.; Ager, J. W.; Clark, S. L.; Mangold, D. J.; Goldstein, H. L.; Hillman, M.; et al. Inorg. Chem. 1963, 2, 1089. Papetti, S.; Heying, T. L. Inorg. Chem. 1963, 2, 1105. Schroeder, H. A.; Heying, T. L.; Reiner, J. R. Inorg. Chem. 1963, 2, 1092. Schroeder, H.; Vickers, G. D. Inorg. Chem. 1963, 2, 1317. Zakharkin, L. I.; Stanko, V. I.; Brattsev, V. A.; Chapovskii, Yu. A.; Struchkov, Yu. T. Izv. Akad. Nauk SSSR Ser. Khim. 1963, 2069 [Russian]. Zakharkin, L. I.; Stanko, V. I.; Brattsev, V. A.; Chapovskii, Yu. A.; Okhlobystin, O. Yu. Izv. Akad. Nauk SSSR Ser. Khim. 1963, 2238 [Russian]. Grafstein, D.; Dvorak, J. U.S. Patent 3,226,429. 1965. Papetti, S.; Heying, T. L. J. Am. Chem. Soc. 1964, 86, 2295. Papetti, S.; Obenland, C.; Heying, T. L. Ind. Eng. Chem., Prod. Res. Dev. 1966, 5, 334. Stanko, V. I.; Klimova, A. I. Zh. Obshch. Khim. 1966, 36, 2214 [Russian p. 2219]. Stanko, V. I.; Klimova, A. I. Zh. Obshch. Khim. 1966, 36, 450 [Russian p. 432]. Zakharkin, L. I.; Kalinin, V. N. Zh. Obshch. Khim. 1966, 36, 376 [Russian p. 362]. Andrianov, V. G.; Stanko, V. I.; Struchkov, Yu. T.; Klimova, A. I. Zh. Strukt. Khim. 1967, 8, 636 [Russian p. 707]. Potenza, J. A.; Lipscomb, W. N. Inorg. Chem. 1964, 3, 1673. Potenza, J. A.; Lipscomb, W. N. Inorg. Chem. 1966, 5, 1471. Potenza, J. A.; Lipscomb, W. N. Inorg. Chem. 1966, 5, 1478. Potenza, J. A.; Lipscomb, W. N. Inorg. Chem. 1966, 5, 1483. Potenza, J. A.; Lipscomb, W. N. J. Am. Chem. Soc. 1964, 86, 1874. Potenza, J. A.; Lipscomb, W. N. Proc. Natl. Acad. Sci. USA 1966, 56, 1917. Stanko, V. I.; Struchkov, Yu. T. Zh. Obshch. Khim. 1965, 35, 930 [Russian]. Voet, D.; Lipscomb, W. N. Inorg. Chem. 1964, 3, 1679. Bubnov, Y. N., Ed.; Boron Chemistry at the Beginning of the 21st Century; Russian Academy of Sciences: Moscow, 2003. Driess, M., No¨th, H., Eds.; Molecular Clusters of the Main Group Elements; Wiley-VCH Verlag GmbH. Co. KgaA: Weinheim, Germany, 2004. Grimes, R. N. In Advanced Inorganic Chemistry, 6th ed.; Cotton, F. A., Wilkinson, G., Murillo, C. A., Bochmann, M., Eds.; Wiley-Interscience: New York, 1999; pp 131–174 [Chapter 5]. Grimes, R. N. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E., Eds.; Pergamon Press: Oxford, 1982; pp 459–542 [Chapter 5.5, review]. Grimes, R. N. In Comprehensive Organometallic Chemistry II; Abel, E., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon Press: Oxford, 1995; vol. 1, pp 373–430 [Chapter 9]. Kennedy, J. D. In The Borane-Carborane-Carbocation Continuum; Casanova, J., Ed.; Wiley: New York, 1998; pp 85–116 [Chapter 3]. Mingos, D. M. P.; Wales, D. J. Introduction to Cluster Chemistry; Prentice Hall: Englewood Cliffs, NJ, 1990. Siebert, W., Ed.; Advances in Boron Chemistry; Royal Society of Chemistry: Cambridge, UK, 1997.
CHAPTER
Structure and Bonding
2
2.1 GENERAL PERSPECTIVE The defining feature of carboranes is the presence of one or more carbon atoms in an electron-delocalized boron cluster framework. As the skeletal carbon and boron atoms are involved in delocalized binding, each typically having five or six neighbors including hydrogen or other attached subsitutuents, these molecules are nonclassical and their structures cannot be described in terms of the usual organic bond diagrams, in which a connecting line between two atoms explicitly indicates an electron pair. Instead, in structures such as those in Figures 1-1–1-3, connecting lines show the polyhedral geometry only, and do not in general represent electron pairs. The range of composition in carborane cages extends from boron-rich clusters such as C2B10H12 and CB11H12 to systems having as many as six skeletal carbon atoms, but those with high boron content are dominant. Consequently, in most theoretical treatments of structure and bonding, carboranes are treated as polyhedral boranes in which one or more skeletal boron atoms are replaced by carbon. The basic principles of bonding in boron clusters are well understood after decades of study, and in recent years the advent of density functional theory (DFT) and other powerful computational tools has led to major advances in the correlation of electronic structure with geometry, reactivity, bond strength, NMR shifts and coupling constants, vibrational frequencies, and other properties. Theoretical findings on particular systems are described at appropriate places in this book; here we focus on basic principles that have general applicability.
2.2 NOMENCLATURE AND NUMBERING Under generally accepted rules, carboranes are named as polyhedral boranes in which one or more boron atoms are replaced by carbon and (in metallacarboranes and heteroatom carboranes) by atoms of other elements, analogous to the oxa-aza replacement nomenclature of organic chemistry. As was noted in Chapter 1, the prefix closo indicates a deltahedral cage (a closed polyhedron with all faces triangular), while nido, arachno, and hypho designate polyhedra with one, two, or three missing vertexes, respectively. For example, the pentagonal bipyramidal C2B5H7 cluster is closo-dicarbaheptaborane (7), in which the (7) designates the number of hydrogen atoms in the parent species; similarly, CB5H9 is nido-carbahexaborane (9). The numbering of cluster vertexes under IUPAC rules is based on the highest-order symmetry axis of the Bn parent polyhedron, numbering successive belts or rings in a clockwise manner as shown in Figures 1-1 and 1-2. Carbon and other heteroatoms in the skeleton are assigned numbers corresponding to their location in the periodic table, with atoms of highest atomic number given the highest priority; for example, in closo-2,4-C2B5H7 the equatorial cage carbons occupy vertexes 2 and 4. Replacement of an apex BH unit in this cluster by CoCp (see Section 2.4) affords the metallacarborane closo-1,2,3-CpCo(C2B4H6). There is an important exception to the general rule: in icosahedral Carboranes. DOI: 10.1016/B978-0-12-374170-7.00016-1 © 2011 Elsevier Inc. All rights reserved.
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CHAPTER 2 Structure and Bonding
metallacarboranes in which one or more boron vertexes are replaced by metal atoms, the numbering system of the parent carborane is retained. Accordingly, in icosahedral 3,1,2-CpCo(C2B9H11), derived from 1,2-C2B10H12 by replacement of B(3)-H by CoCp, the cobalt is numbered 3 rather than 1, which violates the “highest atomic number” rule under which the formula would be 1,2,3-CpCo(C2B9H11), and the entire cage would be numbered differently from the original carborane. This generally accepted convention of numbering the carbon and boron vertexes the same in C2B10H12 and its metallacarborane derivatives, while not officially sanctioned under IUPAC rules, greatly simplifies the discussion and treatment of icosahedral metallacarborane chemistry—a huge subfield in its own right. In this book, for purposes of clarity and concise presentation, line formulas rather than formal names are used in general, and are always written with the skeletal C and B atoms in that order and adjacent to each other; substituents attached to carbon and to boron are placed to the left of C and to the right of B, respectively. Thus, the formula 1,10-Et2C2B8H6-3,5-(CHO)2 denotes a derivative of 1,10-C2B8H10, both of whose skeletal carbon atoms bear ethyl groups, while borons at the 3 and 5 positions have attached formyl substituents. In general, the prefix closo is omitted, and formulas lacking a prefix can be assumed to indicate closo structures. Formulas of all open cage (nido, arachno, hypho) compounds are explicitly labeled as such.
2.3 THE LOCALIZED-BOND APPROACH The first broadly successful theory that illuminated boron hydride structures was advanced by W. N. Lipscomb and his coworkers in work that began over a half-century ago [1] and invoked combinations of 2c2e (two-center, two-electron) and 3c2e (three-center, two-electron) localized electron-pair bonds, in which a single electron pair links either two nuclei (the normal situation in organic chemistry) or three nuclei, as in B2 2H2 2B and B2 2B2 2B bonds. The ability of boron to participate in multi-center bonding involving three or more atoms joined by one localized electron pair is fundamentally a consequence of the fairly low electronegativity of boron and the fact that it has four valence orbitals but only three valence electrons (unlike carbon, which has four of each). Three-center or multicenter bonding is characteristic of so-called “electron-deficient” structures, an often misapplied term about which more will be said later in this chapter. Carbon is also capable of participating in multicenter bonding in certain situations, as in carboranes and other electron-delocalized clusters, metal complexes featuring agostic C2 2H M interactions, and nonclassical carbocations such as C6Me62þ. The localized-bond concept has been primarily applied to boron hydrides that have only boron in the framework [1], but it can be extended to carboranes [1–7]. In Figure 2-1, the molecular structure of nido-2,3-C2B4H8 is presented together with one of several possible localized-bond descriptions [8]. Note that there are 10 2c2e bonds (2 C2 2H, 2 C2 2B, 4 B2 2H, 1 C2 2C, and 1 B2 2B) and 4 3c2e bonds (2 B2 2H2 2B and 2 C2 2B2 2B), requiring 14 electron pairs in total; these are supplied by the two carbons, four borons, and eight hydrogens, which afford 8, 12, and 8 electrons for a total of 28, respectively. Note also that each boron and carbon atom has four nearest-neighbor atoms, requiring four atomic orbitals (approximately sp3) for binding to them, so that there is a precise match between the available orbitals and electron pairs in the molecule. The requirements of electron and orbital balance, together with other considerations, form the basis of Lipscomb’s topological model [1] which for any given borane composition predicts a limited range of possible structures. This theory not only provides a basis for understanding known boron hydride shapes, but allows predictions of many others yet to be discovered. When localized-bond descriptions are applied to deltahedral clusters (polyhedra having only three-sided faces) such as icosahedral B12H122, one must deal with a multitude of resonance forms. The introduction of heteroatoms such as carbon, nitrogen, or sulfur to the skeletal framework adds further complications. For example, even in the simple case of nido-C2B4H8 (Figure 2-1), the valence-bond structure shown is just one of several possible bond arrangements that satisfy the electron-orbital balance requirements, and a rigorous description of the bonding via localized electron pairs would require a resonance combination of several such forms. In general, localized-bond treatments have been superseded by molecular orbital methods that are geared to electron-delocalized cluster systems and can handle a very wide range of compositions and structures.
2.4 Structural patterns in boron clusters H
H C
H B H
H
9
C
B H
B H
H B
H
H Cluster geometry CH
BH
Valence-bond structure (one possible arrangement) closed 3-center, 2-electron bond
H
open 3-center, 2-electron bond 2-center, 2-electron bond
FIGURE 2-1 Geometry and electron-pair bonding in nido-2,3-C2B4H8.
2.4 STRUCTURAL PATTERNS IN BORON CLUSTERS Modern quantitative molecular orbital approaches to polyhedral boron compounds have their roots in perceptive insights dating back several decades, which were based on empirical observations of known structures. From the seminal X-ray structural investigations on lower boranes by Lipscomb and coworkers [1] and on B10H14 by Kasper and coworkers [9] early workers had noted that the boron frameworks in B4H10, B5H11, B6H10, and B10H14 could be viewed as fragments of a B12 icosahedron; however, B5H9, a square pyramid, was an exception to this rule. Williams had recognized [10] as far back as the 1960s that the underlying pattern was based not on the icosahedron alone, but on deltahedra in general [11,12]; thus, square-pyramidal B5H9 can be viewed as a fragment of an octahedral cluster. On the basis of this notion, and drawing on Lipscomb’s topological theory just discussed, as well as quantitative molecular orbital studies [4,13,14], Wade proposed a correlation of geometric structure with the available number of skeletal electrons in each system [15,16], arriving at the now classic Wade’s rules (or Wade-Mingos rules) that have proved to have enormous predictive power in cluster chemistry [17] and become standard fare in inorganic textbooks. Important advances followed through the work of these and other contributors, including R. W. Rudolph, D. M. P. Mingos, R. B. King, A. J. Stone, P. v. R. Schleyer, M. L. McKee, E. D. Jemmis, M. Hoffmann, and others. Polyhedral boranes and carboranes are subsets of covalently bound molecular clusters in general, a large and exceedingly diverse domain [18] that includes transition metal clusters, Zintl ions, and polyhedral cages made of phosphorus, sulfur, and other main-group elements alone or in combination with one another. Carbon clusters, including the fullerenes and their offshoots such as carbon nanotubes, fall into this category as well. While there are some fundamental aspects of cluster binding and architecture that link these various types [18,19], there is considerable variation in their electronic character; for example, in contrast to the boron clusters, fullerenes such as C60 and C70 are classical hydrocarbons in which each carbon is linked to its three neighbors solely by 2c2e bonds. Transition metal clusters present yet another picture, with electronic structures that are different from those of bulk metals, and often exhibit structural features that are not normally found in boranes and carboranes—for example, interstitial atoms (atoms enclosed within the cluster [19–21])—and that require still different bonding descriptions. For purposes of this book, we limit our discussion to concepts that are most directly relevant to carboranes and their metal and nonmetal derivatives.
10
CHAPTER 2 Structure and Bonding
The Wade-Mingos rules, more formally known as the polyhedral skeletal electron pair theory (PSEPT), consist of a set of basic principles that are easily applied. The theoretical rationale underlying these rules, and some caveats and limitations on their use, are dealt with below. Here, we simply outline the main ideas. 1. Cluster molecules are constructed from building-block units such as BH, CH, NH, SH, N, S, M(CO)x, or MCp (in which M is a transition metal and Cp is Z5-C5H5), with each member supplying a given number of electrons to the skeletal framework and, in most instances, using three orbitals for bonding to its neighbors in the skeleton. Structural units that have frontier orbitals of the same symmetry and supply the same number of electrons—for example, BH and Fe(CO)3 (see below)—are designated isolobal, a term coined by Hoffmann and associates who developed the isolobal principle from semiempirical molecular orbital calculations [22,23]. The three-orbital contribution follows from the need to maximize binding with neighboring cluster atoms; main-group donor atoms normally cannot supply more than three energetically accessible orbitals with the required directional geometry, and groups connected to the cage via fewer than three orbitals are commonly regarded as exo-polyhedral substituents, not part of the cage skeleton. Although transition metals have more valence orbitals than do main group elements, the same considerations of geometry and orbital overlap still generally favor a three-orbital contribution (as is also found in metal-cyclopentadienyl interactions in ferrocene and other metallocenes). Four-orbital metal contributions have been proposed for some metal-rich metallaboranes [24–26], a suggestion that is disputed [27] and is not supported by theoretical analysis based on tensor harmonic theory [28]. The electrons donated to the cage are those not involved in exo-polyhedral bonding or sequestered in nonbonding orbitals; thus, a BH unit has a total of four valence electrons of which two are used in the BH bond itself, leaving two electrons for skeletal bonding. Hence, BH is said to be a two-electron donor, and by the same reasoning, CH and NH are three- and four-electron donors, respectively. In the case of a single-atom contributor, such as phosphorus, two of its five valence electrons typically occupy an outward-directed nonbonding orbital, making P a three-electron donor; PH, with an additional electron, is a four-electron donor. An underlying assumption is that the binding in the cluster framework is separate and distinct from the bonds located outside the framework, such as B2 2H and C2 2H. To a first approximation, this is generally true, but there are situations where it does not hold, as, for example, in some polyalogenated boron clusters. In the case of transition metal-ligand groups, the counting is slightly more involved but nonetheless straightforward. In the Fe(CO)3 fragment, for example, the iron atom has eight valence electrons and each CO ligand supplies two electrons, for a total of 14 (note that the formal metal oxidation state is not an issue, as our electron-counting is based on neutral atoms). Iron has nine valence orbitals, of which three are employed in bonding to the CO ligands, three are used for cluster binding (see above), and the remaining three orbitals are nonbonding. Of the 14 electrons available, six are employed in the three Fe2 2CO exo-polyhedral bonds and six are stored in the three nonbonding orbitals, leaving two electrons for cluster bonding. Thus, Fe(CO)3 is isolobal with BH, a fact that is experimentally supported in ferraborane clusters derived from parent boron hydrides via replacement of one or more BH units by Fe(CO)3; compare, for example, B5H9 and (CO)3FeB4H8. Table 2-1 lists some commonly encountered cluster building-block fragments and their electron-donating properties. 2. The cluster geometry is determined by the total number of skeletal electron pairs (SEP) donated by all contributing units. For a closo-polyhedron (deltahedron) of n vertexes, SEP ¼ n þ 1, a relation that was originally deduced from
TABLE 2-1 Skeletal Electron Contributions One-electron donors: FeCp, RuCp, OsCp, Mn(CO)3, Re(CO)3 Two-electron donors: BH, AlH, C, Cr(CO)4, Fe(CO)3, Ru(CO)3, Os(CO)3, CoCp, Ni(CO)2 Three-electron donors: CH, N, P, Co(CO)3, NiCp Four-electron donors: NH, PH, O, S, Se, Ni(CO)3, CuCp Five-electron donors: SH, SeH
2.4 Structural patterns in boron clusters
11
empirical evidence but is supported by molecular orbital theory, as follows. In an n-vertex deltahedron composed of main-group atoms in which each structural unit contributes three orbitals to skeletal binding, there will be a total of 3n MOs. Assuming that, for cluster binding, each unit employs a pair of px and py orbitals tangential to the cluster surface, together with an spz orbital directed toward the center of the cluster (Figure 2-2), theory demonstrates that there will be n spx,y bonding MOs in the polyhedral surface, plus a unique bonding MO inside the cluster that is formed by in-phase overlap of the spz hybrid orbitals directed toward the center. The total number of bonding MOs is therefore n þ 1. 3. Removal of any number of structural units from an n-vertex deltahedron leaves the number of skeletal bonding MOs unchanged. Consequently, an n-vertex closo polyhedron, an (n 1)-vertex nido cluster, and an (n 2)-vertex arachno cluster will all have SEP ¼ n þ 1, as shown in Charts 2-1 and 2-2. For example, a 6-vertex closo octahedron, a 5-vertex nido square pyramid, and a 4-vertex arachno square planar or butterfly cluster (Chart 2-1, second row), all have 7 bonding MOs and hence require 7 SEP. Expressed in terms of the number of vertexes n, the SEP counts for closo, nido, and arachno clusters are therefore n þ 1, n þ 2, and n þ 3; hypho cages (rare in carborane chemistry, and not shown in the charts) correspond to closo systems with four vertexes removed and have an SEP requirement of n þ 4. The patterns for 5- to 15-vertex clusters in Charts 2-1 and 2-2 are presented with selected examples that illustrate general trends. In addition to the common geometries depicted, unusual or unique geometries are found in some cases, particularly in larger cluster systems. Note that the PSEPT rules are intended to predict the most stable cluster geometry in each case; structures that depart significantly from the thermodynamically favored arrangement and hence appear to violate the PSEPT guidelines are not uncommon in carboranes formed under low-energy conditions. For example, the nido-R4C4B8H8 clusters (in which R is a hydrocarbon substituent) are 12-vertex, 28-electron systems that can exhibit any of several different cage geometries, depending on the choice of R, solvent, and other variables [35]. 4. The capping principle dictates that the addition of one vertex by capping a triangular face on an existing n-vertex polyhedron leaves the number of skeletal bonding MOs unchanged; as this increases the value of n by one unit, an n-vertex capped-closo deltahedron (also described as hypercloso) will require exactly n SEP. Thus, a 7-vertex capped octahedron has 7 SEP, as shown in the tricobaltaborane Cp3Co3B4H4 [36,37]. This structural type is rare in carborane chemistry, but an example shown in Figure 2-3(A) is a 7-vertex RuCB5 pentagonal bipyramid, capped on one face by an additional boron vertex [38].
px and py orbitals
spz hybrid orbital
Cluster
FIGURE 2-2 Orientation of px, py, and spz orbitals contributed by a main group atom to skeletal bonding.
12
CHAPTER 2 Structure and Bonding SEP
CLOSO N=5
NIDO
ARACHNO
N=4
N=3
6
C2B3H5
Examples:
N=6
C4H4
CB3H7 Ref. 29 [CB3H4(CMe3)3]
C3H−5
C3H6 N=4
N=5
7 C2B4H6 N=7
C2B3H7, B5H9, C5H5+
C4H42−
B4H10 N=5
N=6
8
C2B5H7
C2B4H8, CB5H9 Cp*2Ru2(Me2C2B2H6) Ref. 30
N=8
N=7
C5H−5
CB4H10 Ref. 31
N=6
9 C2B6H8
C2B5H8− Ref. 32, 33
C2B4H10
N=9
N=8
N=7
C2B7H9
C2B6H10, C4B4H8
[C2B5H11]
N = 10
N=9
Ref. 34
10
N=8
11
Examples:
C2B8H10 CB9H−10
C2B7H11 CB8H−11
CHART 2-1 Structural patterns based on 5- to 10-vertex polyhedra.
C2B6H12 CB7H−12
2.4 Structural patterns in boron clusters SEP
CLOSO
NIDO
N = 11
ARACHNO
N = 10
N=9
C2B8H12 CB9H−12
C2B7H13 CB8H−14
N = 11
N = 10
12 C2B9H11 CB10H−11
N = 12
13 C2B10H12 CB11H−12
C2B9H13 C2B9H−12
C2B8H14 2− C2B8H12
2− C2B9H11 − CB10H13
4− C2B8H10 − CB9H14
N = 11
N = 12
N = 13
4− C2B8H10
14
CpCo(C2B10H12) (CH2)3C2B11H11
2−
C2B10H12
4− C2B9H11
Et4C4B8H8
N = 14
N = 13
(CH2)3C2B12H12
(CH2)3C2B11H11
Cp2Fe2(Me4C4B8H8)
(Ph2PCH2)2Ni(Me4C4B8H8)
Me4C4B7H9
N = 12
15
2−
Cp2Co2(C2B10H12)
N = 15
16
(MeC6H4CHMe2)Ru[(CH2)3C2B12H12]
CHART 2-2 Structural patterns based on 11- to 15-vertex polyhedra.
4− 4− (CH2)3C2B10H11 C2B10H12
H6C6B6Et6
13
14
CHAPTER 2 Structure and Bonding PPh3
Ph3P
H
Ru MeO
OMe B
H
O C
Fe
B
B HC
BH B HC
B H
CH
HC
OMe
A
O C
O C
B
CH
FIGURE 2-3 Structures of 2-(PPh3)2HRu[CB6H4-3,6,8-(OMe)3] and (CO)3Fe(Z4-C4H4).
5. The tetrahedron presents a special case. As a 4-vertex deltahedron, it might be expected to have an SEP requirement of 5. However, most stable tetrahedral clusters have, in fact, 6 SEP. This fact can be reconciled with the WadeMingos rules if one views a tetrahedron as a nido cluster derived from a 5-vertex closo cage (trigonal bipyramid) by removing one of the three-coordinate vertexes; seen in this way, the 6 SEP are as expected. There is, however, another consideration: a tetrahedral cluster with 6 SEP has the exact number of electron pairs required to place a pair on each edge, corresponding to a classical localized system having only 2c2e bonds. Thus 6-SEP tetrahedra can be described, at least qualitatively, in both classical and nonclassical terms. 6. Finally, as mentioned earlier, there are clusters in which the usual assumption of a strict separation between the bonding within the skeleton and the bonds to exo-polyhedral ligands breaks down. This is not uncommon in clusters that have multiple halogen substituents, especially in the polyhedral boron halides (BnXy in which X is F, Cl, Br, or I), where the PSEPT rules fail because of p-donations from X into the boron cage [39–41]. In carboranes and metallacarboranes, this situation rarely arises, and the PSEPT paradigm holds up remarkably well, even in highly halogenated derivatives. The presence of transition metal atoms in the cage framework can also, under some circumstances, lead to deviation from the conventional electron-counting and structural patterns. Some metallaborane structures adopt deltahedral cage geometries other than those in Figures 1-1 and 1-2, and have fewer than n þ 1 skeletal electron pairs; these have been labeled isocloso by Greenwood and Kennedy [42], on the basis of the assumption that the metal center in these cases donates four orbitals to cluster bonding rather than the usual three. An alternative view, espoused by Mingos and coworkers [43] among others, holds that the hypercloso label is more appropriate for these systems inasmuch as it avoids arbitrary assumptions about the nature of the metal-ligand interactions. King has provided a theoretical analysis and modified electron-counting rules for such structures on the basis of localized-bond arguments [26,39,44]. A few examples of such nonconforming clusters are found in metallacarborane chemistry (see Chapter 13).
2.5 EXTENSIONS OF THE ELECTRON-COUNTING RULES Theoreticians have developed and modified the Wade-Mingos (PSEPT) rules in various ways. In order to accommodate multi-cluster systems featuring linked or fused polyhedra, in which two or more cluster units share vertexes, edges, or faces, Jemmis and coworkers [45,46] invoke the mno rule: in a system having m polyhedra, n vertexes, and o shared vertexes, the required number of electron pairs will be the sum of m, n, and o. Condensed-cluster structures are common in boron hydrides and in solid-state borides and carbides. In carborane chemistry, they are essentially confined to metalcontaining clusters, where one finds commo-metallacarboranes in which a metal atom occupies a vertex common to two polyhedral cages as in Ni(C2B9H11)2. Edge- or face-fused carborane systems are quite rare, although Co2(Et4C4B8H8)2 is proposed to have a Co2 2Co edge shared between two 14-vertex polyhedra [47]. Applying the mno rule to Ni(C2B9H11)2,
2.6 Electron-counting in classically bonded clusters
15
we have m ¼ 2, n ¼ 23, and o ¼ 1, so that 26 electron pairs are needed; these are supplied by the four CH, 18 BH, and Ni units, which provide 12, 36, and four electrons (Ni, with 6e stored in its nonbonding orbitals, is a 4e donor), respectively. For Co2(Et4C4B8H8)2, m ¼ 2, n ¼ 26, and o ¼ 2, requiring 30 electron pairs, which are furnished by the two Co atoms (4e, assuming a localized 2e Co2 2Co interaction), eight CEt (24e), and 16 BH (32e). Wang and Schleyer [48] have provided a predictive tool for a proposed new class of borane and carborane clusters, namely those generated from hypothetical BnHn prismanes by capping their non-triangular faces with BH or CH units. For prisms having t triangular faces and m non-triangular faces, the skeletal electron requirement is 6m þ 2t. Thus, a trigonal prism requires 6 3 þ 2 2 ¼ 22 electrons for stabilization. For example, while a B6H6 prism has only 12 skeletal electrons, the extra 10 can be provided by capping its three square faces with CH groups (9e) and adding one electron to create a C3B6H9 anion (not yet synthesized) that is predicted to be stable with a sizeable HOMO—LUMO energy gap of 6.6 eV [48]. This approach is quite general and predicts stability for an extensive family of structurally novel “sea urchin” carboranes, so-called because of their C2 2H and B2 2H bonds radiating outward from the cluster. Capped (hypercloso) polyhedra that satisfy the 6m þ 2t rule are electronically distinct from normal closo-boranes and closo-carboranes that meet the PSEPT requirement of n þ 1 electron pairs (2n þ 2 electrons); thus, the known molecule closo-C2B7H9 (Figure 1-1) has tricapped trigonal prismatic geometry like that predicted for C3B6H9, yet is stable with just 20 skeletal electrons versus 22 for the latter species. In general, the capped-prismane clusters are calculated to have only local aromaticity in the faces, and do not exhibit the full three-dimensional aromaticity that is characteristic of the closo cages. However, some of the predicted capped prismanes satisfy both paradigms; a C3v 14-vertex C2B12H14 cluster is one such example [48].
2.6 ELECTRON-COUNTING IN CLASSICALLY BONDED CLUSTERS Rules based on the total number of valence electrons supplied by cage atoms and substituent groups (polyhedral valence electrons, or PVE) are applicable to localized-bond clusters in which each edge can be assigned an electron pair [49]. This class includes hydrocarbon rings, non-deltahedral polyhedra such as cubane (C8H8) and prismane (trigonalprismatic C6H6), and many metal clusters, but by definition excludes carboranes and other polyhedral boranes. A brief discussion of electron-counting in this type of cage system is presented here in order to accentuate its relationship to, and differences from, electron-delocalized boron clusters. There are three basic rules, easily derived [49]: • • •
Rings of n atoms have PVE ¼ 6n (for main group elements) or 16n (for transition elements). Clusters in which all vertexes have exactly three neighbors in the cage (3-connected) have PVE ¼ 5n and 15n for main group and transition elements, respectively. Replacement of a main group atom by a transition metal in a cluster framework increases the PVE by 10e.
These observations follow directly from the assumptions that all atoms have filled valence shells (8e for main group and 18e for transition metals) and that every edge contains an electron pair. As rings of n atoms have n edges, each of which has an electron pair shared by two atoms, the total electron count must be 8n 2n ¼ 6n for main group elements, or 18n 2n ¼ 16n for transition metals. Thus, the ring compounds C3H6 and Fe3(CO)12 have 18 (6n) and 48 (16n) electrons, respectively; the number of electrons in each case equals the total number of cluster valence electrons. For 3-connected main group clusters, there are 3n/2 edges, so that the PVE is 8n 2(3n/2) ¼ 5n; hence a trigonal prism such as prismane (C6H6) has 30 valence electrons. The PVE model is not generally useful for clusters with vertexes of connectivity four and higher, a group that includes most carboranes and metallacarboranes, because the assumption of localized 2c2e bonds no longer holds. For example, a carbon atom that is bonded to four neighbors in the cluster, in addition to an external substituent, clearly must be involved in multi-center bonding. In such cases, the PVE approach is not valid, but the PSEPT rules are appropriate. Consider for illustration (CO)3Fe(Z4-C4H4), Figure 2-3(B). In the PVE scheme, this is seen as a 34-electron cluster (8e from Fe, 2e from each CO ligand, 4e from each cyclobutadienyl carbon, and 1e from each hydrogen). As the
16
CHAPTER 2 Structure and Bonding
structure is square pyramidal, a localized-bond structure having three Fe2 2CO, four Fe2 2C, four C2 2C, and four C2 2H bonds and three nonbonding electron pairs on Fe (needed to satisfy the 18-electron rule) would require 36 electrons; as there are only 34 electrons available, this molecule cannot be described solely in terms of 2c2e bonds. Hence, a delocalized structure is required. Applying the PSEPT rules, one counts two electrons from the Fe(CO)3 unit and three electrons from each CH group for a total of 14 (SEP ¼ 7), which correctly predicts nido (square pyramidal) geometry for this 5-vertex cluster.
2.7 ISOMER STABILITY AND CAGE REARRANGEMENT Electron-counting schemes such as PSEPT offer a way to identify the thermodynamically preferred shape (closo-nidoarachno, etc.) for a given cage system, but cannot predict the relative stability of isomers on the basis of the locations of cage heteroatoms such as carbon. Empirical observations from the earliest investigations of carboranes established that in general, the most stable isomers are those having the greatest separation between carbon atoms in the skeleton. In the icosahedral C2B10H12 isomers (Figure 1-2), for example, the order of stability from highest to lowest is 1,12 > 1,7 > 1,2; similarly, 1,6-C2B4H6 is more stable than 1,2-C2B4H6, as evidenced by the observed 1,2 ! 1,6 cage rearrangement on heating. The rationale usually invoked for this pattern is that the driving force is mutual repulsion between the relatively electropositive (6þ) carbon nuclei in the cage. Thermal isomerization of polyhedral borane and carborane cages has been extensively studied both experimentally and theoretically, and discussions of particular systems will be found in subsequent chapters of this book. In open cage nido- and arachno-carboranes, other factors become relevant, such as the placement of bridging hydrogens on the skeletal framework. Quantitative calculations of isomer energies in recent years [50–53] have led to increasingly accurate predictions, which for the most part are in agreement with qualitative pattern recognition paradigms put forth by Williams and others [50]. The essential ideas are that (1) skeletal carbon atoms prefer to occupy the lowestcoordinate vertexes and (2) among vertexes of equal connectivity, the carbon atoms will occupy nonadjacent locations if possible. However, there is a caveat: placement of bridging hydrogens is paramount, and can supersede rules (1) and (2). As C2 2H2 2B bridges are rare and B2 2H2 2B groups are restricted to the rims of open (four-sided or larger) faces, carbon atoms in some cases are forced to occupy adjacent and/or higher-coordinate vertexes. An example is found in nido-2,3-C2B3H7 (Figure 1-3) in which one of the carbons is situated in the apex because the alternative—placing both carbons in low-coordinate basal vertexes—would require unacceptable C2 2H2 2B bridging. Other caveats are in order. Like the electron-counting rules described above, predictions on carbon and hydrogen placement and the preferred structures of isomers are based on thermodynamic arguments and do not apply to kinetically stabilized cage structures. Particularly in metallacarboranes prepared under low-energy conditions (as many are), it is not uncommon to find isolable, structurally characterized species that are locked into thermodynamically disfavored structures, which “violate” some or all of the rules outlined above. Examples of such situations can be found throughout the carborane literature, and in this volume. Substituents on the polyhedral cage can also influence the cage structure; for example, McKee has shown that in persubstituted B12X12n clusters (in which n ¼ 0–2), the cage stability increases with increasing electronegativity of X, as electron density is withdrawn from a skeletal bonding molecular orbital [54]. Despite their limitations, the predictive rules that are summarized above are powerful tools for dealing with carborane structures and can at least furnish a starting point for understanding and rationalizing carborane structural patterns.
2.8 CLUSTER AROMATICITY From the early days of polyhedral borane chemistry, it was recognized that the cage framework is stabilized by some delocalization of electrons beyond the conventional 2c2e bond as, otherwise, one could not account for the existence of these molecules as isolable compounds. The discovery of the extraordinarily robust B10H102 and B12H122
2.9 “Electron deficiency” in polyhedral boron clusters
17
deltahedral ions and their closo-carborane counterparts inevitably led to comparisons with aromatic hydrocarbons. Icosahedral B12H122 in particular is often described as a three-dimensional inorganic benzene analogue in which the 26 skeletal electrons occupy 13 filled bonding MOs; indeed, it is sometimes labeled “superaromatic.” In contrast to aromatic hydrocarbons, which feature delocalized carbon-carbon p-bonding, the aromaticity in polyhedral boron clusters utilizes s-bonded interactions involving a set of orbitals tangential to the polyhedral surface and another set directed toward the center, as discussed earlier. Aromaticity in polyhedral boranes and carboranes has been widely explored theoretically [1,49,54–64]. and is supported experimentally by their high stability, magnetic properties, and NMR behavior. Both the large nuclear independent chemical shift (NICS) and the magnetic susceptibility (w) values are indicators of aromatic character in these clusters, as is the topological resonance energy (TRE) which is the added stabilization that is achieved by cyclic conjugation [55]. Most of the theoretical work has focused on all-boron clusters such as the BnHn2 ions, but Schleyer and Najafian obtained similar results for the C2Bn2Hn and CBn1Hn closo-carborane series [62]. A significant finding was that the degree of stabilization conferred by three-dimensional aromaticity tends to increase with increasing cluster size though the trends are not linear, and the species that have 6 and 12 vertexes show special stability. Another discovery was that the incorporation of carbon in the framework causes a leveling effect: the variations in stability are smallest in the C2Bn2Hn series and largest in the BnHn2 family [62]. As will be seen, this spherical aromaticity is strongly reflected in the reaction chemistry of carborane derivatives, where substituent effects are transmitted through the three-dimensional cage skeleton in a manner reminiscent of two-dimensional organic arene chemistry. Although most of the theoretical work on aromaticity in boranes and carboranes has centered on closed polyhedral systems, this property extends to open-cage clusters as well, as expected given the electron-delocalization that is characteristic of all these molecules whether closed or open. However, the presence of open faces in nido- and arachnocarboranes generally results in somewhat less electron-delocalization and hence lower aromaticity than one finds in the closo-carboranes. Again, this will be apparent in the discussions of reaction chemistry in subsequent chapters.
2.9 “ELECTRON DEFICIENCY” IN POLYHEDRAL BORON CLUSTERS Deficient adj. 1: lacking in some necessary quality or element; 2: not up to a normal standard or complement; defective. —Webster’s New Collegiate Dictionary
For a century, dating back to Alfred Stock’s monumental work in isolating and characterizing the lower boron hydrides [65], boron cluster compounds have often been labeled “electron deficient” (e.g., as in the first edition of this book [66]). This descriptive term seemed entirely appropriate in the early days of borane research, but today its use is problematic. This is so partly because we now know that the original interpretation of boron hydride reactivity by Stock and other early workers was incorrect (if understandable), and also because it is now clear that for most boron clusters, there is in fact no “deficiency” of electron density relative to a more stable state. On the contrary, icosahedral cages such as B12H122 and C2B10H12, the most thermodynamically stable molecules known, have precisely the right electron population for maximum stability and cannot accommodate additional electrons without undergoing cage-opening to form higher-energy structures; the same is true of closo-carboranes and -metallacarboranes in general. Nevertheless, the “electron-deficient” label persists as a kind of insider-jargon for nonclassical multicenter bonding. A common error one finds in the literature is the attribution of the well-known inductive electron-withdrawing power of the o-carboranyl (1,2-C2B10H12) cage toward its carbon-bound substituents (Chapter 9) to a supposed electron-deficiency of the entire icosahedral cage. Not so—this property actually reflects the local environment at the electrophilic cage carbon atoms; indeed, this same cluster is typically an electron donor toward groups bonded to its boron atoms. To some chemists, including many with long experience in boron hydride research, continued use of the electrondeficiency terminology is a harmless practice—“part of our culture,” it has been said. But to others, its continued use as applied to carboranes and other polyhedral boranes is seriously misleading, sows confusion among non-boron (and
18
CHAPTER 2 Structure and Bonding
even some boron) chemists, and infects textbooks with misinformation. This writer subscribes to the latter view, and urges that the “electron-deficient” label be discarded as a general descriptor for polyhedral boranes and reserved for special situations where it is in line with general usage by chemists, that is, where there is a genuine electron-poor condition. Some history may be useful. The generally high reactivity toward air and water of the first boron hydrides to be isolated led Stock and his coworkers to associate these properties with a perceived shortage of electrons relative to hydrocarbons. Lacking modern tools such as X-ray crystallography or NMR that would have revealed the actual molecular geometries, these pioneers assumed hydrocarbon-like chain or ring structures for the boranes, but this posed a problem as boron has only three valence electrons. For example, Stock [65] proposed a linear chain structure for B4H10 despite its having only 22 valence electrons vs. 26 in the hydrocarbon C4H10. While it was reasonable at the time to attribute the reactivity of the lower boranes to a shortage of electrons, it is clear in retrospect that this was a misperception. Many of the binary boron hydrides are actually quite stable in air, and the polyhedral BnHn2 anions and their carborane analogues are largely unreactive toward oxygen and water, as has already been noted. Even in neutral boranes, the larger members such as B18H22 are similarly robust, as are most closo-carboranes and metallacarboranes. The lower boron hydrides, therefore, are atypical; their high reactivity is largely a consequence of hydrogen-richness associated with the presence of B2 2H2 2B and BH2 groups (the latter feature being found mostly in the smaller boranes) rather than an inadequate supply of electrons. One can conjecture whether the “electron deficient” label would have been as persistent, or even applied at all, had the stable boranes been discovered first! The working definition employed for years—designating as electron-deficient all molecules for which classical valence bond descriptions using only 2c2e bonds cannot be written—implies that classical hydrocarbon-like structures are the norm, with species such as boranes adopting nonclassical electron-delocalized bonding only as a way of adapting to this supposed shortage of electrons. However, this is not the case. Owing to the much lower electronegativity of boron relative to carbon, and other factors such as orbital size, electron-delocalized cluster architectures are energetically favored for polyboranes and carboranes quite apart from the supply of electrons. This is why polyhedral boron clusters in general do not adopt hydrocarbon-like chain or ring structures even when additional electrons are available. The broad-brush labeling of boron clusters as electron-deficient perpetuates needless confusion, as seen in textbook statements such as “boranes are reactive because of their electron deficiency” and the false notion that electrondelocalized boron clusters are invariably electrophilic (they aren’t; as will be seen throughout this book, carborane cages can function as electron donors or electron acceptors, depending on the point of attachment to the cage and other factors). In this book, “electron-deficiency” is used only in reference to situations involving a genuine paucity of electron density, while the terms “electron-delocalized” or “nonclassical” characterize the type of bonding found in carboranes and other polyboron clusters.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Lipscomb, W. N. Boron Hydrides; Benjamin: New York, 1963. Boer, F. P.; Streib, W. E.; Lipscomb, W. N. Inorg. Chem. 1964, 3, 1666. Fox, M. A.; Wade, K. Pure. Appl. Chem. 2003, 75, 1315. Hoffmann, R.; Lipscomb, W. N. Inorg. Chem. 1963, 2, 231. King, R. B. Collect. Czech. Chem. Commun. 2002, 67, 751. Olah, G. A.; Prakash, G. K. S.; Williams, R. E.; Field, L. D.; Wade, K. Hypercarbon Chemistry; Wiley-Interscience: New York, 1987. Onak, T. P.; Williams, R. E.; Weiss, H. G. J. Am. Chem. Soc. 1962, 84, 2830. Marynick, D. S.; Lipscomb, W. N. J. Am. Chem. Soc. 1972, 94, 8699. Kasper, J. S.; Lucht, C. M.; Harker, D. J. Am. Chem. Soc. 1948, 70, 881Acta. Cryst 1950, 3, 436. Williams, R. E. Inorg. Chem. 1971, 10, 210. Williams, R. E. Adv. Inorg. Chem. Radiochem. 1976, 18, 67. Williams, R. E. Chem. Rev. 1992, 92, 177.
2.9 “Electron deficiency” in polyhedral boron clusters [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40]
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Hoffmann, R.; Lipscomb, W. N. J. Chem. Phys. 1962, 36, 2179. Hoffmann, R.; Lipscomb, W. N. J. Chem. Phys. 1962, 36, 3489. Wade, K. Adv. Inorg. Chem. Radiochem. 1976, 18, 1. Wade, K. J. Chem. Soc., Chem. Commun. 1971, 792. Grimes, R. N. Ann. N. Y. Acad. Sci. 1974, 239, 180. Driess, M., No¨th, H., Eds.; Molecular Clusters of the Main Group Elements; Wiley-VCH Verlag GmbH & Co. KgaA: Weinheim, Germany, 2004. Jemmis, E. D.; Jayasree, E. G.; Parameswaran, P. Chem. Soc. Rev. 2006, 35, 157. Charkin, O. P.; Klimenko, N. M.; Moran, D.; Mebel, A. M.; Charkin, D. O.; Schleyer, P. V. R. Inorg. Chem. 2001, 40, 6913. Jemmis, E. D.; Jayasree, E. G. Collect. Czech. Chem. Commun. 2002, 67, 965. Elian, M.; Chen, M. M. L.; Mingos, D. M. P.; Hoffmann, R. Inorg. Chem. 1976, 15, 1148. Hoffmann, R. Angew. Chem. Int. Ed. 1982, 21, 711. Crook, J. E.; Elrington, M.; Greenwood, N. N.; Kennedy, J. D.; Thornton-Pett, M.; Woollins, J. D. J. Chem. Soc., Dalton. Trans. 1985, 2407. Kennedy, J. D. Inorg. Chem. 1986, 25, 111. King, R. B. Inorg. Chem. 2001, 40, 2699. Baker, R. T. Inorg. Chem. 1986, 25, 109. Wales, D. J.; Stone, A. J. Inorg. Chem. 1989, 28, 3120. Neu, A.; Radacki, K.; Paetzold, P. Angew. Chem. Int. Ed. 1999, 38, 1281. Yan, H.; Beatty, A. M.; Fehlner, T. P. J. Organomet. Chem. 2003, 680, 66. Fox, M. A.; Greatrex, R.; Hofmann, M.; Schleyer, P. V. R.; Williams, R. E. Angew. Chem. Int. Ed. Engl. 1997, 36, 1498. Bausch, J. W.; Matoka, D. J.; Carroll, P. J.; Sneddon, L. G. J. Am. Chem. Soc. 1996, 118, 11423. Beck, J. S.; Quintana, W.; Sneddon, L. G. Organometallics 1988, 7, 1015. DeKock, R. L.; Fehlner, T. P.; Housecroft, C. E.; Lubben, T. V.; Wade, K. Inorg. Chem. 1982, 21, 25. Grimes, R. N. Adv. Inorg. Chem. Radiochem. 1983, 26, 55. Miller, V. R.; Weiss, R.; Grimes, R. N. J. Am. Chem. Soc. 1977, 99, 5646. Pipal, J. R.; Grimes, R. N. Inorg. Chem. 1977, 16, 3255. Pisareva, I. V.; Dolgushin, F. M.; Yanovskii, A. I.; Balagurova, E. V.; Petrovskii, P. V.; Chizhevsky, I. T. Inorg. Chem. 2001, 40, 5318. King, R. B. Inorg. Chem. 1999, 38, 5151. (a) LeBreton, P. R.; Urano, S.; Shahbaz, M.; Emery, S. L.; Morrison, J. A. J. Am. Chem. Soc. 1986, 108, 3937. (b) Swanton, D. J.; Ahlrichs, R.; Ha¨ser, M. Chem. Phys. Lett. 1989, 155, 329. (c) Ho¨nle, W.; Grin, Y.; Burkhardt, A.; Wedig, U.; Schultheiss, M.; von Schnering, H. G.; Kellner, R.; Binder, H. J. Solid. State. Chem. 1997, 133, 59. Morrison, J. A. Chem. Rev. 1991, 91, 35. Kennedy, J. D. In The Borane-Carborane-Carbocation Continuum; Casanova, J., Ed.; Wiley: New York, 1998; pp 85–116 Chapter 3. Johnston, R. L.; Mingos, D. M. P.; Sherwood, P. New. J. Chem. 1991, 15, 831. King, R. B. Inorg. Chim. Acta. 2000, 300–302, 537. Jemmis, E. D.; Balakrishnarajan, M. M.; Pancharatna, P. D. J. Am. Chem. Soc. 2001, 123, 4313. Jemmis, E. D.; Jayasree, E. G. Accounts. Chem. Res. 2003, 36, 816. Wang, Z- T.; Sinn, E.; Grimes, R. N. Inorg. Chem. 1985, 24, 826. Wang, Z- X.; Schleyer, P. V. R. J. Am. Chem. Soc. 2003, 125, 10484. Mingos, D. M. P.; Wales, D. J. Introduction to Cluster Chemistry; Prentice Hall: Englewood Cliffs, NJ, 1990. Hofmann, M.; Fox, M. A.; Greatrex, R.; Schleyer, P. V. R.; Williams, R. E. Inorg. Chem. 2001, 40, 1790. Kiani, F. A.; Hofmann, M. Eur. J. Inorg. Chem. 2005, 2545. Mire, L. W.; Wheeler, S. D.; Wagenseller, E.; Marynick, D. S. Inorg. Chem. 1998, 37, 3099. Williams, R. E.; Bausch, J. W. Appl. Organomet. Chem. 2003, 17, 429. McKee, M. L. Inorg. Chem. 2002, 41, 1299. Aihara, J.-I. Inorg. Chem. 2001, 40, 5042. Chen, Z.; King, R. B. Chem. Rev. 2005, 105, 3613. Ionov, S. P.; Kuznetsov, N. T. Russ. J. Coord. Chem. 2001, 27, 605. Ionov, S. P.; Kuznetsov, N. T.; Sevast’yanov, D. V. Russ. J. Coord. Chem. 1999, 25, 689.
20 [59] [60] [61] [62] [63] [64] [65] [66]
CHAPTER 2 Structure and Bonding Jemmis, E. D.; Balakrishnarajan, M. M. J. Am. Chem. Soc. 2001, 123, 4324. Jemmis, E. D.; Schleyer, P. V. R. J. Am. Chem. Soc. 1982, 104, 4781. King, R. B. Chem. Rev. 2001, 101, 1119. Schleyer, P.v.R.; Najafian, K. Inorg. Chem. 1998, 37, 3454. Schleyer, P.v.R.; Najafian, K.; Mebel, A. M. Inorg. Chem. 1998, 37, 6765. Zhao, M.; Gimarc, B. M. Inorg. Chem. 1993, 32, 4700. Stock, A. Hydrides of Boron and Silicon; Cornell University Press: Ithaca, NY, 1933. Grimes, R. N. Carboranes; Academic Press: New York, 1970.
CHAPTER
Synthesis and Reactivity: An Overview
3
3.1. GENERAL PREPARATIVE ROUTES TO CARBORANES The majority of carborane syntheses involve insertion of carbon into a polyborane framework, as boranes furnish the C unit required electron-delocalized matrix. Alkynes are the most commonly employed insertion reagents, with the C ending up as vicinal carbon atoms in a C2Bn carborane skeleton; in contrast, alkenes are too hydrogen-rich to generate carboranes via reactions with boranes, forming alkylboranes instead. Carboranes having other than two cage carbons can be obtained by other means, as in the formation of CBn clusters from cyanoborane precursors and the insertion of alkynes into dicarbon carboranes to give tetracarbon clusters. As was recounted in Chapter 1, the original discovery of carboranes came about through efforts to “tame” the reac tive boron hydrides B2H6, B5H9, and B10H14 by converting them into organic derivatives that would be more easily (and safely) handled and hence more suitable for use as rocket and jet fuels. Treatment of these boranes with alkenes, alkynes, and other reagents generated a variety of alkyl- and alkenylboranes as anticipated. However, some of the reactions with alkynes also gave unexpected, remarkably stable products that were eventually characterized as the first closo carboranes and their alkyl derivatives. Even today, boron hydride-alkyne reactions remain the most important primary route to carboranes; nonetheless, there are now practical reasons to look for other preparative methods, including cost, safety, and availability of borane starting materials. Here, we outline the main synthetic approaches that have been demonstrated to have some general applicability and point out their advantages and drawbacks. Details and literature references, and descriptions of unique or highly specialized syntheses are deferred to later chapters dealing with specific systems.
3.1.1. Borane-alkyne gas phase reactions Under high-energy conditions such as electric discharge or flash reactions, interactions of lower boron hydrides (B2H6, B4H10, B5H9) with acetylene in the vapor phase produce complex mixtures of small carboranes such as C2B3H5, C2B4H6, and C2B5H7, along with methyl and ethyl derivatives of these. As substantial amounts of nonvolatile amor phous CxByHz solids are also obtained, this is, in general, an inefficient procedure for generating characterizeable carbor anes, but for some members, for example, unsubstituted C2B3H5 and C2B4H6, it remains the only known preparative route. Alkyl- and dialkyl acetylenes react similarly to give C-alkylated carborane derivatives. Vapor phase borane-alkyne reactions under milder conditions (e.g., room temperature) have been employed to syn thesize open-cage carboranes such as nido-2,3-C2B4H8 and nido-1,2-C2B3H7. In general, these processes are more effi cient than the high-energy reactions, tending to give mainly characterizeable gas or liquid products with minimal formation of amorphous solids. However, there are inherent practical constraints on gas phase syntheses—most impor tantly safety, limitation to a small scale, and the need for specialized vacuum-line and/or other gas handling apparatus— that tend to restrict their use to special purposes, mostly in research laboratories.
Carboranes. DOI: 10.1016/B978-0-12-374170-7.00015-X © 2011 Elsevier Inc. All rights reserved.
21
22
CHAPTER 3 Synthesis and Reactivity: An Overview
3.1.2. Borane-alkyne reactions in solution Certain carborane syntheses exploit the moderating influence of solvents, (which in some cases promote the reaction) to effect a high yield of product from alkynes and boron hydrides. A very early example was the preparation of nido RR0 C2B4H6 derivatives (R, R0 ¼ H, alkyl, alkenyl, phenyl) from B5H9 and alkynes in 2,6-dimethylpyridine at room tem perature, a method that was later improved and made practical by employing triethylamine as the base (see Chapter 4). A very important, and versatile, example is the reaction of 6,9-L2B10H12 derivatives of decaborane(14) (L ¼ Lewis base) with alkynes in organic solvents, to generate R2C2B10H10 closo-carboranes: CR0 ! RR0 C2 B10 H10 þ 2L þ H2 L2 B10 H12 þ RC
ðR ¼ H; alkylÞ
However, it has recently been found that direct reactions of B10H14 with alkynes in ionic liquid media generate 1,2-RR0 C2B10H10 derivatives efficiently in the absence of a Lewis base. These and other examples of borane-alkyne carborane syntheses are discussed in detail in Chapter 9. Metal acetylides can, in some cases, be used in lieu of alkynes, affording dicarbon or monocarbon carboranes, for example, C2 2CB5 H7 LiC 2CH3 þ B5 H9 ! C2 H52
3.1.3. Carborane-alkyne reactions Introduction of carbon into an existing carborane framework is one of several methods for preparing carboranes having three or more carbon atoms. Alkyne insertions proceed like those conducted with boranes, as in the gas-phase prepara CH, and the formation of tricarbon carboranes via reactions of tion of nido-C4B2H6 from nido-C2B3H7 and HC arachno-C2B7H13 with alkynes (Chapters 4 and 5).
3.1.4. Carbon insertion via cyano- and isocyanoboranes Reactions of boron hydrides with CN or RNC reagents give cyano- or amino-substituted derivatives that can be con verted into monocarbon carboranes, for example, Hþ
2CB10 H12 B10 H14 þ CN ! B10 H13 CN2 ! H3 N-CB9 H11 þ H3 N2 B10 H14 þ RNC ! RH2 N2 2CB10 H12 Other methods have also been found to convert cyanoboranes to carboranes (Chapters 5 and 9), including reactions of alkylsilyl, alkyltin, or alkyliodo reagents with B10H13CN2, all of which involve incorporation of the cyano carbon atom into a borane cage.
3.1.5. Carboranes from organoboranes Certain small carboranes, especially peralkylated derivatives of 1,5-C2B3H5, have been prepared from alkynylboranes via several routes including hydroboration, heating, reactions with alkynes, or dehalogenation of alkylhaloboranes (Chapter 4). Organoboranes are attractive as carborane precursors as an alternative to boron hydride-based syntheses. At present, however, the synthetic utility of organoborane-based routes is limited by the fact that the carborane products are typically peralkylated derivatives that lack reactive cage CH or BH sites and are not easily functionalized (although in some cases they can serve as ligands in metal sandwich complexes). Nonetheless, continuing research is likely to expand the reach of organoborane-based carborane chemistry in significant ways.
3.2. Interconversion reactions
23
3.2. INTERCONVERSION REACTIONS 3.2.1. Polyhedral rearrangement A characteristic and much-studied feature of the chemistry of closo-carboranes is their cage isomerization on heating. This property is mostly restricted to closed polyhedral clusters, as open-cage molecules generally lack low-energy rear rangement pathways and typically undergo decomposition, loss of hydrogen, or other reactions at high temperatures. The most common type of cage isomerization involves the thermally induced migration of skeletal carbon or other heteroa toms away from each other, as in the quantitative conversion of 1,2-C2B10H12 to the 1,7 isomer at ca. 450 C and the partial rearrangement of the 1,7 to the 1,12 system at higher temperatures, described in Chapter 10; other polyhedral carboranes undergo similar isomerizations, for example, 1,2- to 1,6-C2B4H6 and 1,6- to 1,10-C2B8H10, as discussed in later chapters. The essential driving force in these processes is the mutual repulsion of the carbon nuclei, which carry a higher nuclear charge than their boron neighbors, but the rearrangement mechanisms are still only partially understood after years of investigation. More complex skeletal rearrangement patterns are found in cages having metal or other heteroa toms in addition to carbon, and in carborane anions; in the C2B10H122 isomers, for example, the order of stability is 1,2 > 1,7 > 1,12, the reverse of that in the neutral systems (Chapter 11). Polyhedral rearrangement is an important synthetic tool for generating the more thermally stable carborane systems such as 1,12-C2B10H12.
3.2.2. Metal-promoted cage fusion Metal sandwich compounds of the type HnM(R2C2B4H4)2 (in which M is a transition metal, n ¼ 0–2, R ¼ alkyl, Si(CH3)3, or benzyl, and the carborane ligands are pentagonal-pyramidal nido-R2C2B4H42) readily oxidize with loss of the metal and face-to-face-fusion of the ligands to form nido-R4C4B8H8 tetracarbon carboranes. Such reactions are typically quantitative and extremely facile, occurring in cold (as low as 30 C) solution, and the C4B8 cages exhibit an unusual solvent-dependent cage-fluxionality (Chapter 11). Metal-promoted fusion has not been observed with large carborane ligands such as dicarbollide (C2B9H112), but it does occur with sandwich complexes that contain small metal laborane or metallacarborane ligands; thus, a pair of pyramidal 6-vertex CpCo(C2B3) units can be fused to generate 12-vertex Cp2Co2(C4B6) clusters, demonstrating that cage fusion has some range of applicability as a synthetic approach.
3.2.3. Polyhedral expansion and contraction The electron-counting rules discussed in Chapter 2 imply that the addition of two electrons to a closo cluster should open the cage, forming a nido species, with further addition of two electrons producing an arachno cluster; the process should be reversible, with the removal of electrons (oxidation) inducing arachno ! nido ! closo conversions: 2e 2 2e 4 closo-C2 Bn2 Hn ! nido-C2 Bn2 Hn ! arachno-C2 Bn2 Hn The insertion of a metal or other atom into the open face of the n-vertex nido-dianion creates an (n þ 1)-vertex closo polyhedron, thereby expanding the original cage by one unit; similarly, an (n þ 2)-vertex polyhedron can be formed via addition of two atoms to the arachno species. Examples are the polyhedral expansions of a 6-vertex to a 7-vertex cobaltacarborane via boron addition, and of an 8-vertex carborane to a 9-vertex cobaltacarborane via insertion of cobalt: LiC10 H8
PhBCl2
CpCoðEt2 C2 B3 Br3 Þ ! CpCoðEt2 C2 B3 Br3 Þ2 ! CpCoðEt2 C2 B4 PhBr3 Þ Na
CoCl2
C2 B6 H28 ! C2 B6 H2 8 ! CpCoðC2 B6 H8 Þ
24
CHAPTER 3 Synthesis and Reactivity: An Overview
Cluster expansion in some cases can be achieved in a single step, with the inserted metal itself functioning as the cageopening agent: C2 B3 H5 þ FeðCOÞ5 ! ðCOÞ3 FeðC2 B3 H5 Þ The reverse process, contraction of a polyhedron through oxidative cage-closure, is less common, and in practice has been limited mainly to metal-containing systems: OH
H2 O2
CpCoðC2 B9 H11 Þ ! ! CpCoðC2 B8 H10 Þ Polyhedral expansion and contraction in metallacarborane systems are discussed further in Chapter 13. Carborane cages can also be enlarged via incorporation of boron or carbon, as in the addition of alkynes mentioned in the preceding section; another example is the conversion of a dicarbon to a tricarbon carborane via insertion of cyanide: Hþ
NH3
arachno-C2 B7 H 12 þ MeCN ! nido-MeC3 B7 H9 ! nido-MeC3 B7 H10
Insertion of boron into carboranes can be accomplished by reaction with B2H6 or L:BH3 reagents (in which L is a Lewis base); for example, nido-Et2C2B4H6 and Et3NBH3 combine to generate closo-Et2C2B5H5. Cage degradation via removal of skeletal boron or carbon atoms is a common feature of carborane chemistry, but many such reactions are not clean (often occurring as undesirable side reactions) and in those cases are not particularly useful in synthesis. However, some quantitative degradations have been demonstrated, one of the most important being the alcoholic base-promoted conversion of 1,2-C2B10H12 to nido-carborane anions: þ BðORÞ3 þ H2 C2 B10 H12 þ OR þ 2ROH ! nido-C2 B9 H12
Other examples of selective cage degradations include the synthesis of arachno-C2B7H13 from nido-7,9-C2B9H 12 via chromic acid oxidation and the conversion of 1,6-C2B8H10 to arachno-C2B7H 12 by aqueous hydroxide. In these particu lar cases, only boron is removed from the cage, but there are instances of carbon extraction as well, for example, Hþ
K2 CO3
! 7-CB10 H11 -9-Me-10-OH ! arachno-4-CB7 H13 nido-C2 B10 H13 H2 O
3.3. SUBSTITUTION AT CAGE CARBON AND BORON ATOMS An early discovery in the exploration of carborane chemistry was the acidic character of the C2 2H hydrogens in the C2B10H12 isomers, which is a consequence of the polarity induced by the electronegative carbon atoms. As this polarity is highest in the 1,2 isomer (o-carborane), somewhat less in the 1,7 isomer (m-carborane), and zero in the 1,12 isomer (p-carborane), the C2 2H acidity follows the same trend. Reaction with reagents such as n-butyllithium or Grignards in organic solvents generates C-metallated derivatives that, in turn, react with a wide variety of reagents to give C-substituted organo derivatives, for example, ð1ÞCO2
1; 2-C2 B10 H12 ! Li2 C2 B10 H10 ! ðHOOCÞ2 C2 B10 H12 C4 H10
ð2ÞH2 O
By adjusting the stoichiometry, both mono- and di-C-substituted products can be obtained, although in the lithio deriva tives this is complicated by disproportionation of LiC2B10H11 into Li2C2B10H10 and C2B10H12 (see Chapters 9 and 10). Main-group and transition metal substituents can also be attached to the cage carbon atoms, allowing the synthesis of thousands of C-metallated inorganic and organometallic derivatives (Chapters 6-10). The B2 2H bonds in carboranes are of relatively low polarity as a consequence of the low electronegativity of boron, and the BH hydrogens accordingly exhibit virtually no acidic character toward even the strongest proton acceptors.
3.4. Carboranes as substituents and ligands
25
However, they do readily undergo electrophilic or photochemical reactions with halogenating agents to afford a range of B-fluoro, -chloro, -bromo, and iodo derivatives. For years, this was essentially the extent of B-substitution chemistry on carboranes, as general methods for introducing organic functional groups at boron were lacking. However, in more recent times, metal-promoted organosubstitution techniques such as Suzuki coupling have been applied to carborane sys tems with notable success. The ability to place functional groups at designated carbon and boron positions on the cage has opened up a vast range of carborane-based chemistry that has potential (and actual) application in several fields, such as organometallic polymers and networks, metal trapping agents, carborane-based macrocycles, and liquid crystals, as well as boron neutron capture therapy (BNCT) and other biomedical methodologies, as surveyed in Chapters 14-17.
3.4. CARBORANES AS SUBSTITUENTS AND LIGANDS A notable development in the evolution of carborane chemistry has been its adaption to specific purposes in areas that were seemingly unrelated to boron clusters. The commercial availability of certain carboranes such as the C2B10H12 iso mers has made it feasible to exploit the special electronic, steric, and chemical properties of carboranes and metallacar boranes. For example, as the icosahedral C2B10 cage occupies only a slightly greater volume than a benzene ring rotating on a twofold axis, and functions as a strong electron attractor at its carbon vertexes, replacement of phenyl groups by C2B10 or other carboranyl units offers a controlled way of modifying the properties of organic and organometallic com pounds. Thus, carboranes can function both as scaffolds to which one can attach substituents, and as substituents them selves. Such applications in drug design, liquid crystals, nanostructure construction, and crystal engineering are becoming increasingly common. A much older idea is the use of carboranes as face-bound ligands in metal-carborane sandwich complexes (metalla carboranes), first demonstrated by Hawthorne in 1965. It has long been recognized that dicarbollide (C2B9H112) and other carborane ligands are electronically related to cyclopentadieneide (C5R 5 ), and complexes such as Ni(C2B9H11)2 and CpCo(C2B3H7) are analogues of metallocenes. Cyclic planar C2B3 and C3B2 carborane ligands are not only elec tronic but also steric analogues of C5H5, and readily form stable multidecker and polydecker sandwiches via complex ation to metal centers on both faces (in contrast, few isolable C5R5-bridged metal sandwiches are known). While most of the hundreds of known metallacarboranes have been prepared as part of a systematic exploration of carborane chemistry, attention in this area is increasingly being directed toward the designed synthesis of agents for specific applications in catalysis and other fields, as outlined in Chapters 15-17.
CHAPTER
4
Small carboranes: Four- to six-vertex clusters 4.1 OVERVIEW
Readers of this book whose contact with carborane chemistry has been through applications in biomedical materials or other areas may have the impression that the field is limited to the icosahedral C2B10H12 clusters and their derivatives. In fact, it is not uncommon to see the term “carborane” employed narrowly as a synonym for the C2B10 systems. While it is true that most current carborane applications are based on the icosahedral compounds, the non-icosahedral systems are important for at least two reasons. First, their syntheses and properties lend perspective to an understanding of carborane chemistry as a whole. Second, there are major structural and electronic differences in the various carborane cage systems that can be exploited for specific purposes, and that expand the range of potential applications well beyond what can be achieved with C2B10H12 derivatives alone. A matter of definition requires comment here. What exactly is a carborane? While it is not unreasonable to view twodimensional C2BHn (borirenes and boriranes) and similar three-membered ring systems [1–4], as carboranes (even CBHn species have been so labeled [5,6]), these small molecules are more commonly described as classical organoboranes. For our purposes, we define carboranes as carbon-boron clusters of four or more vertexes in which nonclassical (electrondelocalized multicenter) bonding plays a significant role. In all but a few cases, this definition clearly separates the carboranes from conventional organoboranes whose hydrocarbon-like structures can be described entirely in terms of 2c2e bonds. As will be seen, some 4- and 5-vertex cages straddle the borderline between classical and nonclassical systems, but otherwise there is no difficulty in distinguishing carboranes from other compound types.
4.2 4-VERTEX OPEN CLUSTERS 4.2.1 CB3Hx and C2B2Hx Two nonclassical CB3 derivatives, 4-1 and 4-2, have been isolated and characterized, and several such systems have been explored theoretically (Table 4-1) [7–10]. The nido-carbatetraborane 4-1 has been prepared by reaction of the tetraborane anion B4H3(CMe3)4 with iodomethane [10]. Compound 4-2 was generated from an organotriborane precursor at 80 C and stabilized as the dianion, which was characterized crystallographically and shown to have a classical s-bonded framework [7]. In contrast, multinuclear NMR data on neutral 4-2 support its description as an aromatic, delocalized, and hence nonclassical structure [7]. CMe3 H
4-1
H
C
B
B B CMe3
R H
H
H
CMe3
4-2 R
C
B B
R
B CH2R R = 2,3,5,6-C6Me4H
Carboranes. DOI: 10.1016/B978-0-12-374170-7.00014-8 © 2011 Elsevier Inc. All rights reserved.
27
28
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
TABLE 4-1 CB3Hx and C2B2Hx Derivatives Synthesis and Characterization Compound
Information
References
nido-H2CB3H2(CMe3)3 RHCB3(CH2R)R0 2 (R ¼ SiMe3; R0 ¼ 2,3,5,6-C6H2Me4) nonclassical triboracyclobutane RHCB3(CH2R)R0 2 (R ¼ SiMe3; R0 ¼ 2,3,5,6-C6H2Me4) classical
S, H, B S
[10] [7]
S, X, H, C, B
[7]
DFT GIAO ab initio ab initio ab initio ab initio, classical and nonclassical potential energy surface comparison to Si2C2H2
[7] [10] [9] [8] [11,14, 316–318] [319]
B, C (calculated IGLO) B (calculated IGLO)
[11] [12]
Theoretical Studies Molecular and electronic structure calculations CB3H5 nonclassical triboracyclobutane nido-H2CB3H2(CMe3)3 CB3H7 CB3H4 C2B2H4
NMR calculations C2B2H4 1,3-C2B2H4 X, X-ray diffraction; H, 1H NMR; B,
11
B NMR; C,
13
C NMR.
Although no parent C2B2Hx species has been experimentally characterized at this writing, C2B2H4 has attracted attention from theoreticians (Table 4-1). IGLO, GIAO-SCF, and GIAO-MP2 calculations [11,12] predict a stable existence for the nonclassical boracycloproplylidenediborane structure 4-3 that has a “bare” carbon atom, which is in agreement with X-ray diffraction data for the substituted derivatives (Me3Si)2C2B2(CMe3)2 and (Me3Ge)2C2B2(2,3,5,6-C6Ph4H)2 [13]. Schleyer and coworkers found a somewhat different structure 4-4 as the global energy minimum [14]. H H
4-3
H
C
H H
B B
C
H
4-4
C
B
C
H
B H
As the experimental data on 4-2 [7] illustrate, these small 4-vertex boron-carbon cyclic species occupy a border region between classical and nonclassical structures and properties and, in some cases, can fit into either category, depending on the solvent, temperature, or other factors. A similar situation arises in some 5- and 6-vertex systems, as will be seen later.
4.3 5-VERTEX OPEN CLUSTERS 4.3.1 Nido- and arachno-CB4 systems Nido-CB4H8 and arachno-CB4H10 (analogues of B5H9 and B5H11, respectively), whose 1-isomers are depicted as 4-5 and 4-6, have not been isolated although evidence for CB4H10 in mass spectra of product mixtures has been cited [15,16]. The synthesis of a pentaethyl derivative of nido-2-CB4H8 (with the skeletal carbon occupying a basal vertex and having a bridging CHEt group) was reported [17], but later GIAO-NMR calculations [18] indicate that this species is actually a
4.3 5-Vertex open clusters
29
nido-2,4-C2B4H8 derivative. At present, no compound having a confirmed nido-CB4 cage framework has been characterized. However, several arachno-CB4 clusters are known to be prepared, interestingly, by very different routes. Allene and B4H10 react in the gas phase at 70 C to give 1-MeCB4H9 and 1-MeCB4H82-Me (both having the cage structure of 4-6) [19] that can be isolated when the products are quenched in liquid nitrogen. Gas phase reactions of B4H10 with alkynes at the same temperature [20,21] afford 2-methyl and 2-ethyl derivatives of 4-6, as well as the bridged products 4-7, in which R and/or R0 are H, methyl, ethyl, n-propyl, or tert-butyl, as characterized by multinuclear NMR and mass spectroscopy, and supported by ab initio GIAO-NMR analysis [20,21]. A compound having the composition C3B4H12 that was initially claimed to be the first hypho-carborane [22,23] was later shown to be nido-1-MeCB4H7-m-CH2, a derivative of 4-7 with R ¼ Me and R, R0 ¼ H [20]. H
C
C
H H
H
B
B B
H
H
R
H
H H B
H
B H
H
H B
H
4-5
H
H
B
B B
H
C
H
H
B
H
H
R⬘
H
C
H
H
R⬙
4-6
H B
B
4-7
Structurally similar compounds, arachno-1-(MeR2C)CB4H7-m(2,3)-C6H4 (R ¼ Me, CH2CH2) (4-8) with a benzene ring occupying a dihapto-bridging position between two basal borons, have been isolated from reactions of diborapentafulvenes with excess LiBH4 followed by protonation with HBF4 [24].
B Me B
Me Me
H
H
Me
(1) LiBH4 (2) H+
C
H
B
B
H
B
H
H H B H
4-8
H
H
Still another route to arachno-CB4H10 derivatives involves the treatment of alkynylsilanes with a large excess of diethylborane (“hydride bath”), as in the reaction CMeÞ2 þ Et2 BH !! arachno-1-EtCB4 Et4 H3 -mð2; 3Þ-CEtðSiHMe2 Þ Me2 SiðC which is believed to involve initial Si-C bond cleavage followed by hydroboration of the triple bond [25,26]. Finally, the perethyl arachno-CB4 species 4-9 is reported to be an isolable intermediate in the conversion of diethyl(prop-1-ynyl)borane to closo-1,5-Et2C2B3Et3 (4-10), in the presence of tetraethyldiborane(6) (Figure 4-1) [27]. X-ray data have not been reported for the arachno-CB4 compounds 4-7 – 4-9, and the structures shown are based on spectroscopic evidence and theoretical calculations.
4.3.2 Nido-1,2-C2B3H7 Nido-1,2-dicarbapentaborane (Figure 1-3, top left, and Table 4-2), a structural and electronic analogue of B5H9, is an unusual compound. Known only as the parent species for decades after its discovery until the recent isolation of several derivatives (see below), it has some odd properties and its molecular structure is something of a rule-breaker. The compound was first isolated in 1969 as the main product of the gas phase reaction of B4H10 and acetylene at 25-70 C after quenching in liquid nitrogen, and was characterized from multinuclear NMR and mass spectra [21,28]. This synthesis was independently repeated 30 years later [29] in a study that also reexamined the reaction mechanism and concluded, in accord with an earlier,
30
CHAPTER 4 Small carboranes: Four- to six-vertex clusters Me
Et2B
Et4B2H2
Et4B2H2
(Et2B)3C–Et
Et2B–C≡CMe Et2B
Me Et4B2H2
Et
Et
C Et
B
B
C
Et Et4B2H2
Et
B
B Et Et
C Et Et
C
Et B H
B
H B
Et
H
Et2B
4-10
4-9
FIGURE 4-1 Synthesis of 1,5-Et2C2B3Et3 (4-10) via arachno-1-EtCB4Et4H3-m(2,3)-CEt(BEt2) (4-9).
TABLE 4-2 Open-Cage C2B3Hx Derivatives Synthesis and Characterization Compound
Information
References
nido-1,2-C2B3H7
S, S, S, S,
H, H, H, H,
B, B, B, B,
[21,28] [29] [33] [33]
S, S, S, S,
H, H, H, H,
B, C, MS B, C, MS B, C, MS C, B
[33] [33] [33] [43]
Detailed NMR Studies nido-1,2-C2B3H7
B(detailed), H(detailed)
[21]
Other Experimental Studies nido-1,2-C2B3H7
C2H2 insertion
[38]
Theoretical Studies Molecular and electronic structure calculations nido-C2B3H7, nido-C2B3H6, nido -C2B3H52 nido-C2B3H52 cyclo-C2B3H52
Extended Hu¨ckel Geometry and cage isomerization Geometry
[34] [41] [43]
nido-1/2-R-1,2-C2B3H6 R ¼ Me, Et, n-C4H7, CMe3, Me3Si nido-1-Et-2-Me-1,2-C2B3H5 nido-1-Me-2-Et-1,2-C2B3H5 nido-1,2-Et2C2B3H5 cyclo-(SiMe3)2C2B3R(C6Me4H)22 [R ¼ N(SiMe3)2, OMe]
IR, MS C, ED MS C, MS
Continued
4.3 5-Vertex open clusters
31
TABLE 4-2 Open-Cage C2B3Hx Derivatives—Cont’d Synthesis and Characterization Compound
Information
References
nido-1,2-C2B3H7
Geometry (ab initio), IR Geometry (ab initio) Analogies to P*tBuCB3H5, (tBu)2P*3C2þ, etc.
[11] [35] [320]
B, C, H (IGLO) B (IGLO) C (IGLO) B, C (IGLO) H, B, C, spin-spin coupling (DFT)
[29] [12] [11] [36] [37]
Reaction with NH3 Mechanism of formation from B3H7þ C2H2 Mechanism of formation from B4H8þ C2H2
[42] [31]
NMR calculations nido-1,2-C2B3H7
Reactivity calculations nido-1,2-C2B3H7
X, X-ray diffraction; H, 1H NMR; B,
11
B NMR; C,
[32]
13
C NMR; IR, infrared data; MS, mass spectroscopic data; ED, electron diffraction data.
detailed kinetic investigation [30], that an important early step in the formation of C2B3H7 and other carboranes is the loss of H2 from B4H10 to form B4H8, which in turn reacts with C2H2. A possible competing pathway involving loss of BH3 to give B3H7, suggested by theoretical analysis [31,32], is less consistent with the experimental data [29]. Other products generated from B4H10 and C2H2 in the vapor phase include alkyl derivatives of the known 6-vertex nido-carboranes 1-CB5H9, 2,3C2B4H8, 2,3,4-C3B3H7, and 2,3,4,5-C4B2H6 [29]. Quenched gas-phase reactions of higher alkynes with B4H10 at 70 C generate a series of C(1)-, C(2)-, and C(1,2)substituted derivatives of nido-C2B3H7, listed in Table 4-2 [33], which are the first isolable nido-C2B3 carboranes to be reported other than the parent compound. The square pyramidal structure of 1,2-C2B3H7 shown in Figure 1-3, originally assigned from experimental and calculated spectroscopic data [21,28], is supported by later electron-diffraction [29] and molecular orbital studies [11,34,35], as well as IGLO calculations of NMR shifts [11,12,29,36,37]. The occupancy of an apical vertex by one of the carbon atoms violates the general observation that carbons prefer low-coordinate vertexes (see Chapter 1), which in this case is evidently superseded by the need to avoid C2 2H2 2B bridge-bonding. 1,2-C2B3H7 is a colorless liquid, more volatile than B5H9 and stable as a gas at 50 C, but it decomposes at 110 C to give tan solids, H2, and B2H6. The compound is unchanged over extended periods in dilute hydrocarbon solutions, but in concentrated solutions, or as a pure liquid, it polymerizes irreversibly to a structurally undefined white solid with no evolution of H2 or other side products; polymerization is also induced by HCl, ethers, and other compounds [28]. The compound reacts readily with alkynes [38], undergoing a two-carbon insertion to generate nido-2,3,4,5-C4B2H6 (Figure 1-3, second row center), a previously known carborane that is discussed in detail below. Reaction with propyne affords mainly 3-MeC4B2H5 with some 2-MeC4B2H5, and the interaction with 2-butyne forms 2,3-Me2C4B2H6, accompanied by a smaller amount of the 3,4-Me2 isomer (Figure 4-2). These findings, together with labeling experiments using 13CH, reveal that alkyne insertion into 1,2-C2B3H7 occurs mainly at the C2 CD and H13C 2B bonds, and that there DC is some incorporation into the carborane C2 2C bond as well [38].
32
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
H
C R1–C≡C–R2
H H
B
C H
H
B
H
H
B
B
1
B
H
R1,
R2
= H, Me
R1 R2
2
H
B6
C C
C
5
C4
3
H
H
+ R1
C C
B
C
H
C R2
H
FIGURE 4-2 Conversion of nido-C2B3 to nido-C4B2 carboranes via alkyne insertion.
Other chemistry of 1,2-C2B3H7 has been explored [28]. The compound combines with NEt3 to form a stable 1:1 adduct, and exposure to a limited amount of bromine in solution or in the gas phase results in apparent addition to the cage C2 2C bond, 2C bond, but without release of H2 or HBr. At the time, this finding suggested [28] possible double-bond character for the C2 ˚ determined later from electron diffraction data [29]. Excess Br2 this idea seems inconsistent with the C2 2C distance of 1.626 A attacks and destroys the carborane cage. Reaction with Fe(CO)5 under ultraviolet light [28] affords the nido-ferracarborane 1,2,3-(CO)3Fe(C2B3H7), a known [39,40] complex that contains a planar C2B3 ring ligand. Ethylene combines with 1,2-C2B3H7 in the gas phase at 90 C to give tetraethyldiborane, triethylboron, and unidentified products [28]. Treatment with NaH in THF in an attempt at bridge-deprotonation to form the C2B3H6 ion results in decomposition, while exposure to NaH in mineral oil gives no reaction. Molecular orbital calculations on the experimentally unknown C2B3H52 dianion and its hypothesized cage rearrangement show open pyramidal geometry for the various possible isomers [41]. The reaction of 1,2-C2B3H7 with NH3 has been explored computationally and the C2B3H7NH3 adduct is found to be less stable than the uncomplexed carborane [42].
4.3.3 Other open-cage C2B3 clusters Reductive cage-opening of closo-1,5-C2B3H5 derivatives 4-11 with potassium metal in diethyl ether gives the ring compounds 4-12 whose structures have been established by NMR and X-ray crystallography [43]. The cyclic planar products, which show antiaromatic properties including paramagnetic ring currents, are structurally different from the square pyramidal (nido) species that would be anticipated from a 2-electron reduction of closo-C2B3 clusters. While this might be attributed to substituent effects, ab initio calculations on the parent C2B3H52 anion show that the planar ring geometry has lower energy than any pyramidal structure [43].
R
SiMe3
R⬘
C
B
2 K+
2–
C
B
B
R
2K
Me3Si
C
SiMe3
B
4-11
C R⬘ SiMe3
R = 2,3,5,6-C6Me4H R⬘ = N(SiMe3)2, OMe
B R
B
4-12 R
4.3.4 Nido-C3B2 clusters DFT calculations [44] on the nido-C3B2H5 anion, a currently unknown species, predict three stable square pyramidal isomers, of which the most stable is the 2,3,4 system 4-13 that has all three carbons in the base; however, energy differences between them are small so that thermal interconversion may be possible. In the case of the hypothetical
4.4 5-Vertex closo clusters
33
neutral molecule nido-C3B2H6, extended Hu¨ckel calculations [34] favor the 1,2,3 isomer 4-14 with one apex carbon and a B-H-B bridging hydrogen on the pyramidal base. As in the case of nido-C2B3H7, discussed earlier, it appears that placement of a carbon atom in a normally less favored high-coordinate vertex is favored over alternative structures having C2 2H2 2B or C2 2H2 2C bridges. H
H
−
C
B
4-13
H
B
C C
H
H C
C
C
H B
H
H
H
4-14 B H
H
4.4 5-VERTEX CLOSO CLUSTERS 4.4.1 CB4Hx The closo-carborane anion CB4H5 and its protonated form CB4H6 are 5-vertex cages, with 12 skeletal electrons that are isoelectronic with closo-C2B3H5 and have been studied computationally [45–47]; however, they are currently known only as alkyl or silyl derivatives (Table 4-3). Reduction of the organoborane 4-15 with lithium in diethyl ether forms the aromatic closo-carborane anion, 4-16 which in turn can be protonated to afford neutral 4-17 along with, surprisingly, a bridge-protonated derivative of 4-15 (not shown) that lacks the three-dimensional aromaticity of 4-15 itself [48]. All these molecules have been structurally characterized by X-ray crystallography. The protonated species 4-17 is sufficiently stabilized in the crystal to retain aromaticity although DFT calculations indicate that this is not the case with the protonated parent cluster CB4H6 [48]. SiMe3
B
B
R
B
Li 2e
B Me3SiCH2
R
C B
B B B
CH2SiMe3
Cl
4-15
SiMe3
C
C R
−
SiMe3
R = 2,3,5,6-C6Me4H
R
R
B
B B
CH2SiMe3
CH2SiMe3
4-16
H+
H
R
CH2SiMe3
B CH2SiMe3
4-17
4.4.2 1,5-C2B3H5 4.4.2.1 Synthesis Closo-1,5-dicarbapentaborane (Figure 1-1, top left and Table 4-4) is the smallest member of the closed polyhedral C2Bn2Hn series, and was one of the first carboranes to be prepared, having been isolated in low yield from the products of the gas-phase reaction of B5H9 and acetylene in a glow discharge [49]. Similar results have been obtained in B2H6-acetylene electric discharge experiments [50] and in flash reactions of acetylene with B2H6 or B4H10 [51–53]. A more efficient process involves the pyrolysis of B5H9 and acetylene at 490 C in a stream of H2, which affords a mixture of 1,5-C2B3H5, 1,6-C2B4H6, and 2,4-C2B5H7 in 70% overall yield [54]. However, the best available synthetic routes to the parent compound involve dehydrogenation of nido-2,3-C2B4H8, which is itself obtainable in low-energy
34
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
TABLE 4-3 CB4Hx Derivatives Synthesis and Characterization Compound
closo-1-(H3Si)CB4(C6Me4H)2(CH2SiMe3)2 (aromatic) closo-1-(H3Si)CB4(C6Me4H)2(CH2SiMe3)2H (non-aromatic) arachno-1-RCB4H7-m-CR0 R00 (R ¼ Me, Et, H; R0 ¼ H, Me; R00 ¼ H, Me, Et) arachno-1-MeR2C-CB4H7-m-C6H4 (R ¼ Me, (CH2)2) arachno-1-RCB4H7-m(2,5)-CHR0 (R ¼ H, Me, Et, n-C3H7; R0 ¼ H, Me, Et, n-C3H7, CMe3) arachno-1-RCB4H82-Me (R ¼ H, Me, Et, n-C3H7) arachno-1-RCB4H82-Et (R ¼ Me, Et) arachno-1-EtCB4Et4H3-m-CH(SiMe2) (exo, endo isomers) arachno-m-Et(BEt2)C-1-EtCB4Et4H4 arachno-1-EtCB4Et4H3-m-CEBEt2) “nido-2-EtCB4Et4H-m-CHEt” (later calculated to be a nido-C2B4 derivative; see Ref. [18]) arachno-1-MeCB4H83-Me, -2-Me (fluxional) arachno-syn-1-EtCB4Et4H3-m(2, 5)-CEtSiMe3 arachno-syn, anti-1-EtCB4Et4H3-m(2, 5)-CHMe arachno-anti-1-EtCB4Et4H3-m(2, 5)-CHMe arachno-syn-1-EtCB4Et4H3-m(2, 5)-CHBEt “hypho-C3B4H12” (actually arachno-CB4; see Ref. [20]) “hypho-C3B4H12” (actually arachno-CB4; see Ref. [20])
Information
References
S, S, S, S, S,
X, H, B, C X H, B, C, MS H, B, C, IR, MS H, B, C, MS
[48] [48] [20] [24] [33]
S, S, S, S, S, S,
H, H, H, H, H, H,
IR IR IR
[33] [33] [25] [27] [17] [17]
S, S, S, S, S, S, S,
B, MS H, C, Si, B H, C, Si, B H, C, Si, B H, B, C H, B, MS B(2d), H(2d), C, MS
[19] [26] [26] [26] [26] [23] [22]
B, B, B, B, B, B,
C, C, C C, C, C,
MS MS
Theoretical Studies Molecular and electronic structure calculations CB4H5 (all isomers) CB4H5n (n ¼ 0, 1) arachno-/closo-CB4H6 closo-1-(H3Si)CB4(C6Me4H)2(CH2SiMe3)2 (aromatic) closo-1-(H3Si)CB4(C6Me4H)2(CH2SiMe3)2H (non-aromatic) nido-CB4H8 nido-CB4H53 arachno-1-CB4H10 arachno-1-MeCB4H8-Me (isomers) arachno-1-MeCB4H83-Me, -2-Me (fluxional) arachno-1-MeCB4H83-Me
DFT DFT Extended Hu¨ckel
NMR calculations closo-CB4H5
11
ab initio Energies, ionization potential
ab initio, IGLO ab initio, IGLO ab initio, IGLO DFT
nido-2-CB4H6-m(4, 5)-CH2 arachno-1-RCB4H7-m-CR0 R00 (R ¼ Me, Et, H; R0 ¼ H, Me; R00 ¼ H, Me, Et) arachno-1-MeR2C-CB4H7-m-C6H4 R ¼ Me, (CH2)2 S, synthesis; X, X-ray diffraction; H, 1H NMR; B, IR, infrared data; MS, mass spectroscopic data.
11
B NMR; C,
13
C NMR; Si,
29
B-11B, 11B-13C, 13C-13C spin-spin coupling (DFT) GIAO-NMR, B(calc) ab initio, IGLO ab initio, IGLO
Si NMR; 2d, two-dimensional (COSY) NMR;
[47] [46] [45] [48] [48] [34] [8] [19] [19] [19] [321]
[93] [18] [20] [24]
4.4 5-Vertex closo clusters
35
TABLE 4-4 Closo-C2B3H5 Derivativesa Synthesis and Characterization Derivative
Information
References
Parent
S (high yield, from nido-2,3-C2B4H8) S (B5H9þ C2H2 flow system) S (photolysis of nido-2,3-C2B4H8) S (B5H9þ C2H2 electric discharge), H, B, MS, IR, vapor pressure S (B2H6þ C2H2 electric discharge) H, B, MS, IR S (from nido-2,3-C2B4H8), H, B S (B4H10/B2H6þ C2H2 flash reactions) S (from alkylboron hydrides) S, MS S, H, B
[56] [54] [55] [49] [50] [57] [51,52] [123] [133] [53]
S (B4H10/B2H6þ C2H2 flash reactions) S (B2H6þ C2H2 electric discharge), H, B, MS, IR S, H, B, MS, Raman S (parent þ BMe3), H, B, MS S (parent þ BMe3), H, B, MS S (pyrolysis of BMe3), H, IR,MS S (thermolysis of nido-2-MeCB5H3Et5), MS S (hydroboration of alkylalkynylboranes) X H, B, C CPh), H, B, C, s X (Cl, C
[51,52] [50] [60] [67] [67] [64,65] [61] [62] [27,74] [27] [66]
S [from 1,2-B5H7(SiMe3)2], H, B, IR, MS S [from 1,2-B5H7(SiMe3)2], H, B, IR, MS S [from 1,2-B5H7(SiMe3)2], H, B, IR, MS S (parent þ Cl2), H, B, IR, MS S (parent þ BMe3), H, B, MS S [from nido-2,3-(Me2ClSiCH2)C2B4H8], H, B, MS S (parent þ alkyne, metal-catalyzed), H, B, IR,MS S, H, B S (parent þ atomic S), H, B, IR, MS X, H, B, C,MS S (hydroboration of alkylalkynylboranes)
[70] [70] [70] [67] [67] [71] [69] [109] [68] [72] [63]
S (parent þ BMe3), H, B, MS
[109]
S (PtBr2-catalyzed coupling) S (pyrolysis of parent), H, B, IR, C,MS
[113] [110]
Parent, Me, Me2 derivatives (gas phase flash photolysis) 2-Me 1,5-Me22,3,4-Et3 2,3-Me2 2,3,4-Me3 2,3,4-Me3 1,2,3,4,5-Me5 1,2,3,4,5-Et5
1,5-(Me3C)22,3,4-R3 (R ¼ Cl, CPh, NMe2) CCMe3, C Me, C 1-SiMeH2 1-SiH3 1-SiH32-Me 2-Cl 2-CH2SiMe2Cl 5CHCH2Me 2-CH5 5CH2, 2-CH5 “2-CH5 5CHMe” 2-SH 2,3,4-[N(CHMe2)2]3 1,5-(R0 CH2)22,3,4-R3 (R0 ¼ Et, cyclohexylCH2; R ¼ Me, Et, Pr) 2,20 -(C2B3H4)2 and Me derivatives 2,20 -(C2B3H4)2 (1,5-C2B3H4)2 and (C2B3H3) (C2B3H4)2 isomers
Continued
36
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
TABLE 4-4 Closo-C2B3H5 Derivativesa—Cont’d Synthesis and Characterization Derivative
Information
References
(C2B3H4)(1,6-C2B4H5) isomers 2,2-(1,5-C2B3H4)-(1,6-C2B4H5) 2,20 -(C2B3H4)2 2,20 -3,20 -(C2B3H4)2-C2B3H3
S (copyrolysis of parent þ 1,6-C2B4H6) S (cophotolysis of parent), H, B, IR,MS S(pyrolysis of parent), H, B, IR, UV, Raman, MS S(pyrolysis of parent), H, B, IR, UV, MS
[110] [112] [111] [111]
Raman, IR, B (NMR-IR and Raman-IR correlations) B (NMR-IR correlations) H, C (trans-cage coupling) H(coupling) C C, B (11B, 10B; B2 2C coupling) 2B coupling) B (11B, 10B; B2
[103] [103] [322] [94,95] [96,97] [98] [99]
Detailed NMR Studies Parent
1,5-R22,3,4-R0 3 (R,R0 ¼ Me, Et) 2:20 ,3,100 -(C2B3H4)(10 ,50 -C2B3H3) (100 ,500 -C2B3H4) 2,20 -(C2B3H4)(1,6-C2B4H5) Other Experimental Studies Parent
1,5-Et2
B (11B,
B; B2 2B coupling)
10
Raman, IR UV photoelectron spectra He photoelectron spectra MS (negative ion) ED CH bond polarity, IR, C(d and JCH); comparison with other carboranes XPS: binding energies Metal insertion and polyhedral expansion Reaction with atomic S Reactions with Lewis bases Hydrolysis in CH3OH Reaction with B2H6 Catalytic reactions with alkynes Polymerization reaction of B2H6 þ PtBr2 ! C2B6H12 XPS: binding energies
Theoretical Studies Molecular and electronic structure calculations Parent (all isomers) Isomer stabilities, dipole moments (MNDO) Energy indexes, stabilities Isomer stabilities (ab initio, SCF, DFT) Isomer stabilities, charge distribution (CNDO)
[99]
[103,104] [105] [106] [102] [73] [108] [107] [117,118] [68] [120] [114] [115] [69] [103] [116] [107]
[323–325] [326] [92,327–329] [330] Continued
4.4 5-Vertex closo clusters
37
TABLE 4-4 Closo-C2B3H5 Derivativesa—Cont’d Synthesis and Characterization Derivative
1,2 and 1,5 isomers
C2B3H5 classical structures 1,2-, 1,5- and classical C2B3H5 B–Me, B–F isomers Parent and B–Xm derivatives (X ¼ Li, F, Cl NH2) C2B3Hn (n ¼ 3,5) Parent isomers and B-Me, B-F derivatives 2-m/terminal-B2Hx or -BH4 1,5-Et2 3,4,5-Y3 (Y ¼ NH2, Me, H) 1-CH-1,2-C2B3H4 Parent and 1,5-C2B3H3 2,20 -(C2B3H4)2 2,20 -3,20 -(C2B3H4)2-C2B3H3 Isomerization calculations Parent
C2B3H52
Information
References
Decisive evidence for nonclassical bonding Electronic structure (ab initio) Population density (SCF) Energies, geometries (SCF) Cage structure (ab initio, SCF) Cage structure, dipole moment, ionization potential, heat of formation (AM1) Localized MOs Binding energies (CNDO) Comparison with (CO)9Ru3C2H2 C2 2H bond length compared with halomethanes Vibrations; structure Vibrational spectra Population analysis Stabilities, three-dimensional aromaticity (ab initio) BH and CH capping; isomer stability Geometry, optimized (ab initio) Electron density distribution (ab initio) Second-order NLO properties
[86] [75–77,331] [87] [78] [11,332] [333]
Isomerization energies (DFT) Relative energies Cage structure (SCF, DFT, MP2, CCSD, CCSD[T])
[79,334] [107] [335] [336] [104] [92] [337] [47] [338] [80] [81] [328] [82] [83] [84] [90]
Localized MOs, BCB 3-center bond Relative energies
[89] [84]
Borane-carborane cage coupling (ab initio) Binding energies (CNDO) Bond orders, diamagnetic susceptibility (NLMO) Carboranyl carbenes Structural analogy with hydrocarbons; CBC 3-center bond B2 2B bond rotational barrier) B2 2B bond rotational barrier)
[304] [107] [91] [339] [88] [111] [111]
Cage rearrangement Cage rearrangement, diamond-square-diamond (EHMO) Cage rearrangement of B5H52 (tensor surface harmonic theory) Cage rearrangement
[296–298,340,341] [342] [343] [297] Continued
38
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
TABLE 4-4 Closo-C2B3H5 Derivativesa—Cont’d Synthesis and Characterization Derivative NMR calculations Parent
1,2-MeC2B3H33-Me 1-SiMeH2, 1-SiH3, 1-SiH32-Me Reactivity calculations Parent 1,2- C2B3H5
Information
References
B, C, H spin-spin coupling (DFT) 11 B-11B, 11B-13C, 13C-13C spin-spin coupling (DFT) 11 B-1H coupling correlation with structure 11 B shifts (IGLO) 11 B, 13C shifts (IGLO) 11 B, 13C NMR (GIAO-MP2) C (IGLO) Aromatic solvent-induced 1H NMR shifts: correlation with Hþ charges (PRDDO) 11 B NMR shifts (IGLO, GIAO-MP2) Substituent 1H NMR shift effects
[37] [93] [100] [12] [11] [36] [96] [101]
Protonation to form C2B3Hþ 6 Reaction with NH3 Mechanism of formation from B3H7 and C2H2
[119] [42] [31]
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, a 1,5-C2B3 cage system except where otherwise noted
[59] [70]
13
C NMR; IR, infrared data; MS, mass spectroscopic data.
reactions of B5H9 and C2H2 described later in this chapter. This has been accomplished via photolysis [55], thermolysis [56,57], and electric discharge [57]. At present, 1,5-C2B3H5 is the only definitively characterized closo-C2B3 carborane; theoretical analyses of the hypothetical 1,2 and 2,3 isomers and their interconversions, cited in Table 4-4, show them to be considerably less stable than the 1,5 system. Early reports of the isolation of alkyl derivatives of the 1,2 isomer from electric discharge or flash reactions of alkynes and boranes [50,52,58] appear to be incorrect based on calculated 11B NMR shifts, obtained via the GIAO-MP2 method, showing that the experimentally obtained NMR data are not consistent with a closo-1,2-C2B3 cage structure [59]. An attempted repetition [59] of the original experimental work was inconclusive as the conditions employed in the electric discharge experiments, as well as the product separation techniques, differed from those of the original study [50,52]. The nature of the compounds originally assigned as 1,2-C2B3H5 alkyl derivatives on the basis of mass spectra and 11B and 1H NMR data, which suggested three nonequivalent boron atoms in a C2B3 cluster [50], has not been determined. Alkyl derivatives of 1,5-C2B3H5 have been prepared from organoboranes by various methods (see Table 4-4), including thermolysis [60,61] and reactions with alkylboranes or alkynylboranes [62,63]; thus, pyrolysis of BMe3 at 475-520 C gives 1,5-H2C2B3Me3 in low yield, along with other products [64,65]. The synthesis of 1,5-Et2C2B3Et3 (Figure 4-1, 4-10) from diethyl(prop-1-ynyl)borane and B2H2Et4 has been cited earlier. These routes take advantage of the relative accessibility of organoboranes, but the B-peralkylated products are not generally reactive and, hence, not particularly useful as synthons. On the other hand, 1,1,1-tris(trichloroboryl)-3,3-dimethylbutane generates the tri-B-chloro derivative 1,5(Me3C)2C2B3Cl3 on heating, which in turn is converted to other 1,5-(Me3C)2C2B3R3 compounds in which R is Me, CPh, or NMe2 [66]. B-alkyl derivatives have also been prepared via reaction of parent 1,5-C2B3H5 with CCMe3, C C BMe3 in a hot-cold reactor, which gave mono-, di-, and tri-B-methyl products [67].
4.4 5-Vertex closo clusters
39
Other B-substituted 1,5-C2B3H5 derivatives have been synthesized by various means. Treatment of the parent carborane with 1D atomic sulfur affords 1,5-C2B3H42-SH [68], and metal-catalyzed reactions with alkynes produce B-alkenyl 5CHCH2Me compounds [69]. C-silyl derivatives are obtained by flash derivatives, including the 2-CH5 5CH2 and 2-CH5 thermolysis of 1,2-(SiMe3)2B5H7, which yields, among other products, 1,5-(SiH3)C2B3H4, 1,5-(SiH2Me)C2B3H4, and 1,5-(SiH3)C2B3H32-Me [70], the carboranes evidently forming via a carbon insertion mechanism. The 2-CH2SiMe2Cl derivative is among the products obtained on heating nido-2,3-C2B4H64-CH2SiMe2Cl at 690 C [71]. Most of the reported thermolytic syntheses afford low yields of C2B3 carboranes and are inefficient, although in some cases they constitute the only known pathways to particular compounds. An unusual preparation of a 1,5C2B3H5 derivative involves a retro Diels-Alder reaction of the norbornenyl borane 4-18 to form the tris(diisopropylamino) derivative 4-19 [72]: (Me2CH)2N (Me2CH)2N
B
H
H
B
C H
(Me2CH)2N
B
B
N(CHMe2)2
B
B (Me2CH)2N
H H
H H
C N(CHMe2)2
H
4-18
4-19
H
4.4.2.2 Structure and bonding The trigonal pyramidal cluster geometry of 1,5-C2B3H5 is established from electron diffraction data on the parent compound [73] and from X-ray studies of substituted derivatives [27,66,72,74] (Table 4-4). This cage system is unique in the C2Bn2Hn closo-carborane series, in that it can be represented both as a classical tricyclic organoborane lacking any direct B2 2B bonding interactions, or as a nonclassical electron-delocalized cage in common with ˚ ) and electron density maps showing no charge accuother carboranes. The relatively long B2 2B distance (>1.8 A mulation in the B2 2B vectors [74], together with a number of theoretical studies [75–85], appear to favor the classical organoborane model with an implied absence of B2 2B bonding. However, this conclusion is difficult to reconcile with the relative thermal and chemical stabilities of 1,5-C2B3H5 (for example, its ability to survive temperatures up to 150 C and its unreactivity toward water or air at room temperature), which are behaviors far more typical of closo-carboranes than classical organoboranes. Schleyer and coworkers concluded, on the basis of ab initio calculations of energies and magnetic properties [86], that there is, in fact, a significant B2 2B bonding interaction and hence the nonclassical description is valid for 1,5-C2B3H5 and other 1,5-X2B3H3 clusters in which X is N, P, SiH, or BH. 2B2 2C [87,88] or B2 2C2 2B [89] bondOther studies of the 1,5-C2B3H5 system favor delocalized three-center C2 ing. The full story, however, is more complex. It turns out that the cluster bonding is strongly influenced by substituents attached to the boron atoms (more so than in larger carborane systems) [76,90,91]; 1,5-H2C2B3X3 species in which X can donate electron density via p-interaction with boron (e.g, NH2) are essentially classical, with minimal B2 2B interaction, in contrast to derivatives in which X ¼ H or alkyl, which feature electron-delocalized cluster bonding and are nonclassical [91]. This model is supported by X-ray structural data on 1,5-C2B3Et5 that suggest multicenter bonding in the triangular faces [27,74], reflecting indirect rather than direct B2 2B bonding [91]. A separate study using SCF, DFT, MP2, and other methods found that 1,5-C2B3H5 derivatives having F, Cl, or NH2 substituents on two or more boron atoms have classical structures, while the 2,3-Li2 and Li5 derivatives are nonclassical [90]. Further insight into the bonding in 1,5-C2B3H5 has been gained from ab initio self-consistent field and DFT treatment [92] and other studies (Table 4-4) of the parent molecule and its derivatives, together with experimentally determined NMR data and IGLO calculations on 11B, 13C, and/or 1H chemical shifts and spin-spin coupling constants
40
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
[11,12,36,37,93–100], a study of aromatic solvent-induced proton NMR shifts [101], and analyses of negative-ion mass spectra [102], infrared and Raman spectra [92,103,104], photoelectron spectra [105,106], and ESCA spectra [107]. Sig21H coupling nificant bonding involving s orbitals in the center of the cage is suggested by unusually high antipodal 1H2 [93,94], and the uniquely high C2 2H stretching frequency at 3160 cm1 (vs. 2600 cm1 for other closo-carboranes) is attributed to the low (4) coordination number of carbon [92,108]. In summary, 1,5-C2B3H5 and its derivatives straddle the border between classical and nonclassical systems and can fall into either category depending on the nature of attached substituents. Again, this property is unique among closocarboranes, all of the larger members having unequivocally nonclassical cluster frameworks.
4.4.2.3 Physical properties and reactivity
1,5-C2B3H5 is a gas at room temperature which melts at –126.4 C and boils at –3.7 C, and is thermally stable below 150 C [57]. Experimental studies on parent 1,5-C2B3H5 have been hampered by the relative inaccessibility of this compound, but some information is available. Although somewhat more reactive than its higher congeners in the closo-C2Bn2Hn series, the parent carborane is unaffected by O2, H2O, NEt3, CO2, or acetone at room temperature [49]. However, unlike the larger closo-carboranes, liquid 1,5-C2B3H5 slowly polymerizes on standing, forming a dimer and higher molecular weight products [109]. A 1:1 adduct forms with NMe3 at low temperatures, but on melting it converts to an intractable polymer [109]. 2B Pyrolysis of 1,5-C2B3H5 has been investigated by several groups, who found that the main product is the B2 bonded dimer 2,20 -(1,5-C2B3H4)2 [109–111]; by trapping products on a cold finger in the reactor, higher oligomers including the B2 2B bonded trimer 2,20 -3,20 -(1,5-C2B3H4)21,5-C2B3H3 have been isolated and characterized [110,111]. Under conditions more favorable to secondary reactions, other products have been obtained, including dimers and trimers linked by B2 2C (but not C2 2C) bonds as well as the tetracarbon carborane nido-C4B7H11 [110]. 2B linked mixed-cage species 2,20 -1,5Co-pyrolysis of 1,5-C2B3H5 and 1,6-C2B4H6 generates in 47% yield the B2 0 0 C2B3H41 ,6 -C2B4H5 [110], a compound also obtained in small quantity by Hg-sensitized co-photolysis of the two carboranes [112]. The dimer 2,20 -(1,5-C2B3H4)2 has also been produced, in quantitative yield, by PtBr2-catalyzed dehydrogenation and 2B intercage bonds in the B2 2B linkage at 25 C [113]. MNDO calculations indicate a large rotational barrier for the B2 dimer and trimer, suggesting p-interaction between the polyhedra. In the dimer, the most stable conformation has D2d symmetry with the B3 planes in the two cages mutually perpendicular [111]. This finding is consistent with an analysis of boron-boron spin coupling constants in linked-cage carboranes [99], which gave a calculated value of 39% s character 2B linkage in 2,20 -(1,5-C2B3H4)2 and other linked-cage carboranes, suggesting that (sp1.6 hybridization) for the B2 p-p p-bonding is a distinct possibility. Exposure of neat 1,5-C2B3H5 to Cl2 and NO2 leads to uncharacterizable products [109]; however, reaction with Cl2 in solution gives the 2-chloro derivative, and treatment with BMe3 in the gas phase affords a mixture of B-mono- di-, and trimethylcarboranes [67]. The parent carborane reacts vigorously with water and methanol at room temperature to form triborapentane products [114]: 2CH22 2BðORÞ2 2CH22 2BðORÞ2 þ H2 ðR ¼ H or MeÞ C2 B3 H5 þ 5ROH ! ðROÞ2 B2 The compound initially formed with water (R ¼ H) is hydrolyzed, generating BMe(OH)2 and B(OH)3. Comparative studies show that the larger closo-carboranes 1,6-C2B4H6 and 2,4-C2B5H7 are much less reactive toward these reagents [114]. As mentioned earlier, treatment with atomic (1D) sulfur affords the B-mercapto derivative [68]. Reaction of 1,5-C2B3H5 with B2H6 at 300 C in a flow reactor effects boron insertion to give a new carborane, nido-C2B6H10 (see Chapter 5) [115]. The same reactants combine at room temperature in the presence of a PtBr2 catalyst to afford another new carborane, arachno-5,6-C2B6H12, also discussed in Chapter 5 [116]. Cage expansion of 1,5-C2B3H5 via addition of transition metals to form 6- or 7-vertex metallacarborane clusters [117,118] is described in Chapter 13. The reactivity of 1,5-C2B3H5 toward NH3 and protonating agents has been explored theoretically. The B-NH3 complex is calculated to be less stable than the free carborane by 24 kcal/mole, so complexation with ammonia is not
4.5 6-Vertex open clusters
41
expected [42]. MNDO computations [119] suggest that protonation to form C2B3H6þ occurs preferentially at carbon. Reactions of the parent carborane toward Lewis bases in general give unstable products [120].
4.4.3 Closo-C3B2H5þ A theoretical analysis [121] of the unknown closo-C3B2H5þ cation found an energy minimum for trigonal bipyramidal geometry of C2v symmetry having carbon atoms occupying two apexes and one equatorial position. This structure was rationalized on the basis that CH rather than BH groups are preferred as capping units because of greater ring-cap orbital overlap [121]. At present, unsubstituted CxByHzþ carborane cations are very rare and no small-molecule species of this class have been isolated.
4.5 6-VERTEX OPEN CLUSTERS Successive formal replacement of BHbridge units in hexaborane(10) (B6H10) with C atoms (or of B with Cþ) generates the series of 6-vertex nido species depicted in Figure 4-3, all of which are known as parent compounds or substituted derivatives; in some cases, additional isomers are known. The carboranes shown have been prepared via a diverse range of synthetic routes as described below, none actually involving B6H10. The nonclassical C6H62þ dication has not been prepared per se, but has been characterized as the hexamethyl derivative [122].
H
H
B
B H H
B B H
B
H
B
H
H
H
H B
H
C H B
B B
H
H
B H
H H
C C H
B B
H
H
H
B
B
B
H
B
H B
H H
C H
2,3,4-C3B3H7
C C H
B
C B H
H
C
C
H
C C H
H
2,3,4,5-C4B2H6
+
C
H B
H
C H
2,4-C2B4H8
C H H
C
C H
2,3,4,5,6-C5BH6+
2+
H
H H
B
H
2,3-C2B4H8
H H
H B H
H
H
H
2-CB5H9
B6H10
C H C
H
B
H
H
H
H B
H
H
C C H
C
C H
C H
C6H62+
FIGURE 4-3 Isoelectronic and isostructural 6-vertex nido-CnB6nH10n clusters.
4.5.1 Nido-2-CB5H9 4.5.1.1 Synthesis There are currently no high-yield, efficient routes to nido-2-carbahexaborane(9), 2-CB5H9 (the only known isomer). The parent compound is generated together with other carborane products in the flow pyrolysis of 1-MeB5H8 or 1,2-Me2B5H7 [123]; when 1-ethylpentaborane is used as the reactant, nido-2-MeCB5H8 is obtained, together with methyl derivatives of
42
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
other carboranes. Flash thermolysis of 2-[(ClMe2Si)CH2]B5H8 affords 2-CB5H9 and closo-CB5H7 [124]. In a very different procedure, degradation of closo-1,6-C2B6H8 with BH4 salts, followed by treatment with anhydrous HCl, affords 2-CB5H9, along with several of its methyl, dimethyl, and trimethyl derivatives [125]. Alkyl and other derivatives (Table 4-5) have been synthesized in a variety of ways. In the earliest synthesis of the 2-CB5H9 carborane system, the reaction of acetylene with B5H9 in the vapor phase at 215 C afforded MeCB5H8, CB5H8-3-Me, and CB5H8-4-Me [126]. Methyl derivatives have also been isolated from gas-phase interactions of acetylene and other alkynes with tetraborane(10) (B4H10) [127]. More efficiently, flow pyrolysis of alkenylpentaboranes through a heated tube at 355 C generates alkyl derivatives of CB5H9: for example, 2-trans-1-propenyl-B5H8 produces CB5H8-3-Et 5CH-B5H8 produces 2-, 3-, and 4-Me-2-CB5H8 in a combined yield of and CB5H8-4-Et in 42% total yield, while 2-CH25 51% [128,129].
TABLE 4-5 Nido-2-CB5H9 Derivatives Synthesis and Characterization Compound
Information
References
Parent
S, H, B, IR, MS S, H, B(2d), C S S, H, B B C UV-photoelectron spectra S, H, B, IR, MS H, B, MS IR H, B C MS (detailed) MS (calculated monoisotopic) S, H, B, IR, MS S, H, B, MS S S, H, B, MS S, H, B, IR, MS* B H, B C S, H, B, IR, MS S, H, B, IR, MS* S, B, IR, H, C, P S, H, B, IR, MS S, B, IR, MS S, B, MS, IR
[125] [344] [124] [123] [132] [96] [105] [125] [125,126] [125] [134] [96] [133] [345] [125] [126] [123] [126] [128] [135] [134] [96] [125,128] [128] [137] [130] [70] [127]
1-Me 2-Me
3-Me
4-Me B-R (R ¼ Me, Et; 4 isomers) n-R (n ¼ 1, 2, 3, 4) n-Me (n ¼ 3, 4) Me2 (5 isomers) MeEt (4 isomers) m(Et3P)2PtH 2-Et 2-Me2HSi 2-Me-B-Me
Continued
4.5 6-Vertex open clusters
43
TABLE 4-5 Nido-2-CB5H9 Derivatives—Cont’d Synthesis and Characterization Compound
Information
References
2-Me-n-Et (n ¼ 3,4) 1,2,3-Me3 MeCB5H4Et4?, MeCB5H3Et5? MeCB5H3Et5
S, S, S, S,
[128,129] [125] [15] [61]
Other Experimental Studies Parent
H, B, MS H, B, IR, MS MS H, B, IR, MS
Dipole moment, microwave spectrum Pyrolysis ! 1, 7-C2B10H12 (16%) þ 1, 7-C2B6H8 (5%) þ 1, 6-C2B8H10 (trace) þ arachno-1, 3-C2B7H13 (30%) Pyrolysis ! 1, 7-C2B8H8-B, B0 -Me2 (16%)
3-Me
[131] [136] [136]
Theoretical Studies Molecular and electronic structure calculations Parent Extended Hu¨ckel ab initio, energies of isomers Structure, dipole moment, ionization potential DFT: ionization potential, valence structure UV-photoelectron spectra SCF, MNDO CB5H63
[34] [35] [333] [346] [105] [132,323,347] [8]
Isomerization calculations CB5H8-n-R (n ¼ 1, 2, 3, 4)
Isomerization
[135]
B, C, H, DFT spin-spin coupling B IGLO GIAO/NMR C IGLO
[37] [12] [344] [96]
Protonation
[119]
NMR calculations Parent
Parent, MeCB5H8, CB5H8-n-Me (n ¼ 3, 4) Reactivity calculations Parent 1
S, synthesis; X, X-ray diffraction; H, H NMR; B, spectroscopic data; UV, UV-visible data.
11
B NMR; C,
13
C NMR; 2d, two-dimensional (COSY) NMR; IR, infrared data; MS, mass
Other routes to 2-CB5H9 derivatives involve reactions conducted in solution. Lithium methyl acetylide and B5H9 in diglyme combine to give 2-EtCB5H8 in 5% yield [130], and dehalogenation of alkylhaloboranes also has been successfully employed [15,61]. Remarkably, the action of lithium metal on EtBF2 in THF affords a series of lower carboranes (a rare example of carborane synthesis directly from a monoboron substrate) from which nido-2-MeCB5Et5H3 is isolated in ca. 7 mole % yield together with small amounts of nido-2-MeCB5Et4H4 isomers and tetra- and pentaethyl derivatives of closo-2,4-C2B5H7 [61]. Further, 2-(Me2HSi)CB5H8 is a minor product in the flash thermolysis of 1,2-(Me2HSi)2B5H7 [70].
44
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
4.5.1.2 Structure and properties There are no published crystal structure determinations on CB5H9 or any derivative. However, the pentagonal pyramidal cage geometry (Figure 1-3, top center) has been confirmed by a microwave structural analysis [131] and is further supported by NMR and other spectroscopic evidence in conjunction with ab initio calculations (Table 4-5). Detailed 11B, 1 H, and 13C NMR investigations, including measurements and calculations of chemical shifts [12,96] and nuclear spin-spin coupling constants [37,132], allow insight into the nature of the cluster framework bonding in the parent compound. The mass spectrum has been analyzed and compared with those of related compounds [133]. Several theoretical studies, listed in Table 4-5, have explored the bonding in parent CB5H9 and its isoelectronic nidocarborane relatives (Figure 4-3), all concluding, in line with experiment, that species having carbon in the base of the pyramidal cage are thermodynamically favored. Similar studies have appeared on B-methyl derivatives of 2-CB5H9 [134,135]. Exploration of the chemistry of nido-2-CB5H9 is surprisingly sparse for a molecule that has been known for nearly half a century and is accessible by several different routes. Pyrolysis of the parent carborane gives evidence of cage fusion, affording the closo-carboranes 1,7-C2B10H12 (16%), 1,7-C2B6H8 (5%), and 1,6-C2B8H10 (trace) together with arachno4,5-C2B7H13 (30%) [136]; similar treatment of 2-CB5H8-3-Me produces 1,7-C2B8H8-B, B0 -Me2 in 16% yield. Although MNDO calculations [119] predict that protonation of 2-CB5H9 will occur preferentially at carbon, this has not yet been confirmed experimentally. The platinum-bridged complex 4-20 was obtained in 23% yield from Pt2(m-C8H12)(PEt3)4 and 2-CB5H9 at room temperature in diethyl ether solution, and characterized from infrared and multinuclear NMR spectra [137]. Curiously, this compound is the only reported metal complex of CB5H9 or any of its derivatives (contrasting sharply with the hundreds of known metal complexes of the nido-2,3-C2B4 system). H
B H H H
C
B H
B H
B
H
4-20
B H
Et3P
PEt3 Pt H
Some chemistry of the peralkylated derivative 2-MeCB5Et5H3 has been investigated [61]. The compound reacts with 2H2 2B bridging proton) and forming a salt assumed to be NaþBHEt3, releasing H2 (presumably via abstraction of a B2 NaþMeCB5Et5H2, which has not been characterized; acidification of this salt with HCl or DCl regenerates the neutral carborane and its monodeuterated form, respectively. No deuterium exchange occurs on treatment with B2D2Et4 at 120 C. The compound 2-MeCB5Et5H3 resists oxidation, reacting with atmospheric O2 only very slowly (months) and showing little reactivity toward H2O2 in the presence of H2SO4. It is stable on pyrolysis up to 180 C; above that temperature, it decomposes to give alkyl derivatives of small closo-carboranes and other products [61]. Calculations [135] on the four possible monomethyl derivatives of nido-2-CB5H9 indicate that their relative stabilities are in the order 3-Me > 4-Me > 1-Me > 2-Me, and experimentally [135] it has been found that the 3-Me and 4-Me derivatives are in thermal equilibrium at 225 C.
4.5.2 Hypho-CB5H13 Stepwise degradation of arachno-6-(Me3N)CB9H13 in MeOH/KOH solution at 60 C gave a product characterized from NMR, infrared, and mass spectroscopy as hypho-6-(Me3N)CB5H11, a derivative of the as-yet unknown species hypho-6CB5H13 (“hypho” designates a closo cluster with three missing vertexes and n þ 4 skeletal electron pairs). This compound was isolated, remarkably, in 75% yield as a white crystalline solid that is moderately air-stable [138]. Its proposed structure
4.5 6-Vertex open clusters
45
4-21 is represented as a derivative of B5H11 in which one of the bridging hydrogens is replaced by a 2 2CHNMe32 2 bridge that increases the skeletal electron count from eight to nine pairs [138]. No other hypho-CB5 clusters are known at this time. H
B
H
B H H
H H
B H
H
B H B
H H
4-21
C H
NMe3
4.5.3 Nido-2,3-C2B4H8 Nido-2,3-dicarbahexaborane(8) (Figure 1-3, top right) and its derivatives, discovered by Onak, Williams, and their coworkers in 1962 [139,140], are the preparative starting point for a large area of metal sandwich chemistry involving 6- and 7-vertex nido-MC2B3, closo-MC2B4, and closo-M2C2B3 clusters and related systems, hundreds of which have been prepared and characterized (Chapter 13); among metallacarboranes, only the 12-vertex icosahedral complexes are more numerous.
4.5.3.1 Synthesis All preparative routes of any practical significance for nido-2,3-C2B4 carboranes (Table 4-6) involve the interaction of alkynes with boron hydrides, usually B5H9 (for a general discussion see Chapter 3). Alkene-borane reactions, in contrast,
TABLE 4-6 Nido-2,3-C2B4H8 Derivatives Synthesis and Characterization Compound Derivatives with Main Group Element Substituents No substituents on boron Parent
Information
References
S, H, B S, IR S (large scale) S [macro scale, from nido-2,3(SiMe3)2C2B4H6] S (from closo-2,3-C2B5H7) X X (refinement) H, B, IR, MS H (decoupled) H B C MS (calculated monoisotopic) XPS: binding energies
[57] [127] [348] [155] [349] [162] [163] [170] [171] [71] [172] [29] [345] [107] Continued
46
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
TABLE 4-6 Nido-2,3-C2B4H8 Derivatives—Cont’d Synthesis and Characterization Compound
Information
References
2,3-C2B4H7 [Ph3PCH2]þ 2,3-C2B4H7
S, H, B S, B, H, P S, H, B S, H, B, C, MS S, IR S (large scale) CH/ S (from 1-MeB5H8þ MeC 2-pentyne), H, B S, H, B S, H S, H, B, C, MS S, H, B, UV S (large scale) S, H, B, C, MS S, H, B, IR, MS S, H, B S, H, B, IR, MS* S, IR, MS* S, H, B, IR, MS S, H, B, C, Si, MS S, X, H, B, C, Si, IR, MS S, IR S, H, B S, IR S (large scale) X B S, B S, B, H, IR S, B S (improved) S (detailed), IR, MS, VP S, H, B, C, UV, IR, MS S, H, B, C, UV, IR, MS S, B S, H, B, IR, MS IR, MS* S, H, B, C, Si, MS S, H, B, C, Si, MS S, H, B, C, IR, MS
[170,182] [184] [57,140] [33] [127] [145] [143]
2-Me
2-n-C3H7
2-Ph 2-R (R ¼ Et, CMe3) 2-CH2R (R ¼ fluorenyl, indenyl) 5CMe 2-CH25 2-(CH2)nPh (n ¼ 2, 3) 2-SiH3 2-SiMe3 [2,3-(SiMe3)C2B4H5]2Ba(THF)22þ 2,3-D2 2,3-Me2
2,3-Me2C2B4H5 2,3-(n-C3H7)2 2,3-R2 (R ¼ Et, Ph) 2,3-Et2 2,3- (n-C6H13)2 2,3-Ph2 2,3-Ph2C2B4H5 [(CH2)5-nido-2,3-C2B4H7]2 2,3-(SiH3)2 2,3-(SiMe3)2 2-Me-3-SiMe3 2-R-3-SiMe3 (R ¼ Bu, CMe3)
[140] [139] [33] [57] [145] [33] [146] [57] [147] [141] [141] [154] [350] [197] [140] [127] [145] [162] [172] [170] [187] [172] [157] [148,156] [149] [149] [149] [150] [141] [154] [154] [152] Continued
4.5 6-Vertex open clusters
47
TABLE 4-6 Nido-2,3-C2B4H8 Derivatives—Cont’d Synthesis and Characterization Compound þ
(C4H8ONa )2 (SiMe3)2C2B4H5 Li[(H2N)2C2H4]2þ(SiMe3)2C2B4H5 2,3-(PhCH2)2 2,3-(n-C4H9)2 2,3-(i-C5H11)2 2,3-(CH2R)2 (R ¼ fluorenyl, indenyl) [(CH2)5C2B4H7]2 (2,3-Me2C2B4H5)2 2-Et-3-(CH2)n[3-(2-EtC2B4H6)] (n ¼ 4,6) [2-EtC2B4H63-(CH2)6]2Et2C4B8H8 [2-EtC2B4H63-(CH2)6Et2C4B8H8-]2 (L)Liþ [(SiMe3)RC2B4H4-(m-H)2-exo-Li(L)] [L ¼ (Me2N)2C2H4, none; R ¼ SiMe3, Me, H) LNaþ [(SiMe3)RC2B4H5] [L ¼ THF, (Me2N)2C2H4; R ¼ SiMe3, Me, H] (THF)Naþ [(SiMe3)C2B4H4-exo-Li(THF)] (R ¼ SiMe3, Me, H)
Information
References
S, S, S, S, S, S, S, S, S, S, S, S,
[164] [165] [151] [149] [149] [146] [150] [112] [150] [150] [150] [166]
X, H, B, C, IR X, H, B, Li, C, IR H, B, IR, MS H, B, C, IR, UV, MS H, B, IR, UV, MS H, B, IR, MS H, B, IR, MS H, B, IR, MS H, B, C, MS, UV, IR H, B, MS, UV, IR H, B, MS, UV, IR X[SiMe3, (Me2N)2C2H4], H, B, C, Li, IR
S, X [(Me2N)2C2H4, SiMe3; THF, SiMe3], H, B, C, IR S, H, B, C, IR
D-, hydrocarbon-, B-, or Si-containing substituents on boron S, H, B C2B4H7D n-Me (n ¼ 1, 4, 5) S, ring currents n-Me (n ¼ 1, 2, 4, 5) S, H 4-Et S, H, B, IR, MS 5-Et S, H, B, MS S, H, B, IR, MS,UV 4-CH2Ph S, H, B, IR, MS 4-CH2C6H4Me S, H, B, IR, MS,UV 4-(CH2)3Ph n-(cis-2-but-2-enyl) (n ¼ 1, 4, 5) S, H, B, IR, MS S, H, B, MS 4/5-CH2SiMe2Cl S, H, B 5-SiMe2CH2Cl S, B, IR, MS, H n-SiH3 (n ¼ 1, 2, 4) S, B, IR, MS, H n-SiMe3 (n ¼ 2, 4) S, H, B,VP m(4,5)-CH2SiClMe2 S, B, IR, MS,H m,m0 (4,5)-SiH2(2,3-C2B4H7)2 S, B, IR, MS,H m(4,5)-(CH2)4SiCl S, B, IR, MS,H m(4,5)-SiMeH2 S, B, IR, MS, H m(4,5)-SiR3 R ¼ H, Me 2-Me-n-Et (n ¼ 4, 5) S, H, B, C S (1-MeB5H8þ MeCCH), H, B 2,B-Me2 S, H, B, MS 2,4-Me2 Liþ Naþ [(SiMe3)RC2B4H4]2 (R ¼ H, Me, SiMe3)
[166] [166]
[170,182] [144] [71] [188] [142] [188] [188] [188] [69] [71] [71] [189] [189] [71] [190] [190] [190] [189] [29] [143] [142] [167] Continued
48
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
TABLE 4-6 Nido-2,3-C2B4H8 Derivatives—Cont’d Synthesis and Characterization Compound
Information
References
4,m(4,5)-(SiMe3)2 exo-“closo”-Me(SiMe3)C2B4H41-Na(TMEDA)m(4,5)-H2Na(TMEDA)2 4,5,6-D3 2,3,4-Me3
S, B, IR, MS, H S, H, B, C, IR
[190] [168]
S, H*, B, IR, MS S (from 1-MeB5H8þ MeCCH/2-pentyne), H, B S, H, B, MS S (1-MeB5H8þ MeCCH), H, B S, ring currents S, H, B, IR, MS S, H, B, IR, MS S, H, B, IR, MS, UV S, H, MS S, H, B, MS, UV, IR S, X, H, B, C, i S, H, B, C, IR S, H, B, C, IR
[170] [351] [142] [143] [144] [352] [188] [188] [353] [150] [195] [196] [196]
S, H, B, C, IR
[195]
S, H, B, C, IR S, S, H, B, IR, MS H, B, IR, MS H, B, IR, MS S, H, B, C, IR
[195] [352] [191] [191] [354]
F-, Cl-, Br-, or I-containing substituents on boron 2-Me-4-X (X ¼ Cl, Br, I) 2,3-Me24-X (X ¼ Cl, Br, I) 2,3-Me25-Br 4-X (X ¼ Cl, Br) n-Cl (n ¼ 4, 5) 4-I
S, S, S, S, S, S,
H, H, H, H, H, H,
[201] [201] [187] [200] [187] [201,307]
Main-group metal substituents on boron exo-Me(SiMe3)C2B4H54,5-m(H)2Na(TMEDA)2 (TMEDA)Mg-[(SiMe3)C2B4H5]2 m(4,5)-AlMe2 m(4,5)-Al(Ph3P)3 2,3-Et2-m(4,5)-AlH2(NEt3) 2,3-Et2-m(4,5)-Al(NEt3)(3,4-Et2C2B4H4) m(4,5)-GaMe2
S, S, S, S, S, S, S,
X, H, B, C, IR X, H, B, C, IR MS H, B, IR, MS H, B, IR H, B, IR, MS B, IR, MS
2-Et-3,4-Me2 2,3,n-Me3 (n ¼ 1, 4, 5) 2,3-Me25-Ph 2,3-(PhCH2)24-Me 2,3-(PhCH2)24-CH2Ph 1,3,5-(nido-2,3-Et2C2B4H51-)3C6H3 cyclo-[C2B4H6-(CH2)n-C2B4H6-(CH2)5-]- (n ¼ 4, 5, 6) Na(THF)þ 2,3-(SiMe3)2C2B4H45-iBu 2,3-(Me3Si)25-SiMe2CH2Cl 2,3-(Me3Si)25-SiMe2-(1–1,2-CB10H10C-R) (R ¼ Me, Ph) 2,3-(SiMe3)25-R (R ¼ Me, iBu, CH2CH2Cl, CH2CH2Br) 2,3-(SiMe3)24-Me-5-iBu 5,50 -(2,3-Me2C2B4H5)2 2,3-Me2-m(4,5)-SiMe2R (R ¼ Me, Cl) 2,3-Me24-SiMe3 2,3-(Me3Si)25-SiMe2NMeCMe3
B, B, B, B, B, B,
IR, MS, VP IR, MS,VP MS* IR, MS, VP MS* IR, MS, VP
[168] [355] [192] [192] [356] [356] [192] Continued
4.5 6-Vertex open clusters
49
TABLE 4-6 Nido-2,3-C2B4H8 Derivatives—Cont’d Synthesis and Characterization Compound
Information
References
4-GeMe3 m(4,5)-GeR3 (R ¼ H, Me) m,m0 (4,5)-GeMe2-(4-SiMe3-C2B4H6)2 m(4,5)-SnMe3 m(4,5)-PbMe3
S, S, S, S, S,
B, B, B, B, B,
[189] [189] [189] [190] [190]
S, S, S, S, S, S, S, S,
X, H, B, IR, MS MS H, B, IR, MS H, B, IR, MS B, MS X, H, B, IR, MS X H, B, IR, UV, MS
[357] [358] [357] [185,186] [146] [193] [359] [360]
S, S, S, S, S, S, S, S, S, S, S, X
B, H, C, MS H, B, IR H, B, IR, P H, B, IR, P H, B, IR, P H, B, IR B, H, C, P, IR X, H, B, C, P, IR B, MS H, B, IR, MS X, H, B
[361] [362] [194] [194] [194] [192] [137] [137] [192] [352] [363] [364]
Transition Metal s- and m-Complexes 2,3-[CH2PhCr(CO)3]2 2,3-[(CO)3CrPhCH2]2-m-D 2-CH2Ph-3-CH2PhCr(CO)3 m(4,5)-FeCp(CO)2 Fe[2,3-(indenyl-CH2)2C2B4H6]2 5CMe) 2,3-Et2-m(4,5)-CpFe(CO)(PPh3)-(m-MeC5 2,3-Me2C2B4H5-(2,3-Me2C2B4H3)CoCp 2,3-Me24-[40 -nido-1,2,3-CpCo(Me2C2B3H4)] (isomers) 2,3-Et21-C2RCo2(CO)6 (R ¼ Ph, SiMe3) m(4,5)-NiCl(Ph2PCH2)2 m(4,5)-Cu(Ph3P)2 m(4,5)-Cu(Ph2PCH2)2 2,3-Me2-m(4,5)-CuR2 (R ¼ Ph3P, Ph2PCH2) m(4,5)-Rh(PPh3)3 2,3 Me2-m(4,5)-Pt(Et3P)2H m(4,5)-(Et3P)2PtH m(4,5)-HgPh [2,3-Me2C2B4H5-m(4,5)]2Hg [2,3-(Me3Si)MeC2B4H5-m(4,5)]2Hg Nd3[(SiMe3)2C2B4H4]6[(O-CMe3)(THF)Li]3 Detailed NMR Studies Parent
Parent, 2-Me, 2,3-Et2 2-Me 2,3-Me2 2,3-Et2 2,3-Et2 C2B4H7, parent Et2C2B4H5
MS, H IR, MS, H MS, H IR, MS, H IR, MS, H
B (line narrowing) H (B-H spin relaxation) C C (IGLO) B (line narrowing) B (line narrowing) B (solvent shifts) B (2d) B (paramagnetic effects of Sm2þ) B, C, H spin-spin coupling (DFT) B (paramagnetic effects of Sm2þ) B (2d)
[173] [365] [97] [96] [173] [173] [366] [174] [367] [37] [367] [174] Continued
50
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
TABLE 4-6 Nido-2,3-C2B4H8 Derivatives—Cont’d Synthesis and Characterization Compound Other Experimental Studies Parent
C2B4H7, 2-MeC2B4H6 2-Me
2-CH2-fluorenyl 2,3-Me2
2,3-Et2
2,3-R2 [R ¼ Me, Et, Ph, CH2Ph, indenyl, CH2PhCr(CO)3]
Information
References
MS MS (negative ion) UV photoelectron spectra Photolysis VP IR, Raman Reaction with BMe3 Catalytic reactions with alkynes Deuteration Deprotonation, deuteration High yield conversion to closo-carboranes Co-pyrolysis with BMe3 ! closo-2,4H2C2B5Me5 CMe Reaction with MeC Reaction with Fe(CO)3 Reaction with silanes Reaction with GaMe3, InMe3 Adsorption on silica, angle-resolved photoemission Reactions with CoCl2þ Cp IR, Raman MS (negative ion) VP Kinetics of bridge deprotonation IR, Raman H, MS MS (negative ion) VP Photolysis Deuteration Photoemission spectra Inner shell electron energy-loss spectroscopy (SEELS) Conversion to closo-Et2C2B5H5 Adsorption on silica, angle-resolved photoemission Synchrotron radiation-induced deposition of boron carbide films APP Kinetics of bridge deprotonation
[133] [178] [105] [55,112] [201] [176] [204] [69] [197] [170] [56] [205] [208] [40] [141] [206,368] [202] [369,370] [176] [178] [201] [183] [176] [139] [178] [201] [112] [197] [177] [179,180] [203] [202] [371] [183] Continued
4.5 6-Vertex open clusters
51
TABLE 4-6 Nido-2,3-C2B4H8 Derivatives—Cont’d Synthesis and Characterization Compound
Information
References
2-SiMe3 2,3-(SiMe3)2 2-SiMe33-RC2B4H5 (R ¼ H, Me, SiMe3) 2,3-(Me3Si)2
ED Cage expansion reactions Conversion to closo-1,2-RR0 C2B4H6 alkylation Thermal fusion ! nido-(Me3Si)4C4B8H10 oxidative cage closure
[169] [207] [166] [195] [199] [195]
Valence structures Fractional three-center bonds Three-center bonds DFT: ionization potential, valence structure Localized MOs Localized MOs (SCF) Classical versus nonclassical structures (ab initio) MNDO, dipole moment Energies (ab initio) Electron population analysis Isomer energies (ab initio) XPS: binding energies (CNDO) Extended Hu¨ckel Charge distribution Geometry MNDO MNDO B3LYP optimized geometries
[297,347] [372] [373] [346] [374] [375] [181] [323] [304] [376] [35] [107] [34] [377] [41] [177,179,180] [202] [195]
B3LYP optimized geometry ab initio
[195] [35]
H (solvent shifts) B (IGLO) B (GIAO) B (GIAO) B (GIAO) B (IGLO), GIAO B
[175] [12] [143] [143] [143] [294] [355]
2,3-(SiMe3)25-iBu Theoretical Studies Molecular and electronic structure calculations Parent
Parent, C2B4H7 Parent, C2B4H62 C2B4H62 2,3-Et2 2,3-R2 (R ¼ H, Et) 2,3-(SiMe3)25-R (R ¼ Me, CMe3, CH2CH2Cl, CH2CH2Br) 2,3-(SiMe3)24-Me-5-iBu 2,3-C2B4H62 NMR calculations Parent 2-Me 2,B-Me2 2-Et-3,4-Me2 nido-H2C2B4Cl4 (H2NC2H4NH2)Mg(2,3-C2B4H6)2
Continued
52
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
TABLE 4-6 Nido-2,3-C2B4H8 Derivatives—Cont’d Synthesis and Characterization Compound þ
]2 nido-2,3-(R3Si)2C2B4H42
[M R ¼ H, Me) Parent, 2-Me, 2,3-Et2
Reactivity calculations Parent Parent, 2-Et, 2,3-Me2
(M ¼ Li, Na;
Information
References
B (MP2) covalence or strong ion pairing between M and anions C (IGLO)
[378] [96]
Alkyne incorporation Mechanism of formation from B4H8þ C2H2
[230] [32]
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; 2d, two-dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; UV, UV-visible data; ED, electron diffraction; VP, vapor pressure.
yield organoboranes but not carboranes. Gas-phase thermolysis reactions of small alkynes with B4H10, B5H9, or B5H11 generate mixtures of lower nido-carboranes that invariably include 2,3-C2B4H8 or its derivatives [29,57,127,139, 141,142]; alkyne-alkylpentaborane reactions afford B-alkyl C2B4H8 derivatives [143,144]. However, yields are typically low, and owing to the limitations of scale (and safety) imposed by gas phase operations, only millimoles of pure carboranes are usually obtainable by such methods. An alternative approach utilizes Lewis base–promoted reactions at ambient temperature: the first reported nido-dicarbahexaborane compounds, that is, the 2-methyl, 2,3-diethyl, and 2-n-propyl derivatives, were prepared via liquid phase interaction of B5H9 with alkynes at room temperature promoted by 2,6dimethylpyridine, with separation of the products by vapor phase chromatography [57,140]. The reaction is driven by base-extraction of a BH3 unit from B5H9 to form an adduct: CR0 þ L ! 2;3-RR0 C2 B4 H6 þ L:BH3 B5 H9 þ RC R; R0 ¼ H; Me; Ph; CMe5 5CH2 This procedure was later improved by employing triethylamine as the Lewis base at 0 C, allowing separation by vacuum-line fractionation to afford a range of C-substituted and C,C0 -disubstituted derivatives, in moderate to good yields [145–151]; the reaction works well even with bulky alkynes such as bis(indenyl)- and bis(fluorenyl)acetylene [146,149]. Trimethylsilyl-substituted nido-2,3-dicarbahexaboranes, (Me3Si)RC2B4H6 (R ¼ H, alkyl, or SiMe3), useful synthons in CSiMe3 in a stainless steel cylinder without lower carborane research [152,153], are prepared from B5H9 and Me3SiC solvent or base reagent [154]. (Me3Si)2C2B4H6 generates parent 2,3-C2B4H8 on a multigram scale when heated at 160170 C with HCl gas for several days [155]. Alkyne-pentaborane reactions conducted in the absence of solvent are highly exothermic, and explosions have resulted. For the synthesis of the C,C0 -diethyl-, C-phenyl-, and C,C-diphenyl derivatives, this problem is circumvented by conducting the reaction in cold diethyl ether [156] or THF [157] solution, which moderates the rate of the alkyneborane interaction and takes advantage of the stability of solutions of B5H9 in organic media toward air oxidation [157]; the Et2C2B4H6 product is separated by vacuum-line fractionation from the more volatile solvent. In this manner, the diethylcarborane can be routinely and safely obtained on a scale of 100 grams or more [156]. A further issue remains. What had been the main supply of B5H9 in the world—a large reserve of ca. 100,000 kg originally produced at considerable cost for the borane fuels program described in Chapter 1, and maintained for decades by the U.S. Army—has regrettably been destroyed in a shortsighted decision. The loss of this irreplaceable resource complicates the continued study of R2C2B4H6-based chemistry and much other research on boron clusters [158], including medical applications. For laboratory-scale operations at least, this difficulty can be overcome. A useful route,
4.5 6-Vertex open clusters
53
originally demonstrated in 1979 [145], utilizes the conversion [159] of the inexpensive bulk chemical NaBH4 to NaB3H8, from which B5H9 is generated [160] in situ and reacted with alkynes to give the desired carborane products. NaB3H8 has also been prepared from boric acid and converted to B5H9 [161].
4.5.3.2 Structure and properties The pentagonal-pyramidal molecular geometry of the nido-C2B4 framework (Figure 1-3) is established from X-ray diffraction studies on the parent compound [162,163], Me2C2B4H6 [162], several C- and B-substituted derivatives (Table 4-6), and salts of C-SiMe3 substituted mono- and dianions [164–168], and from a gas phase electron diffraction investigation of (Me3Si)C2B4H7 [169]. Electronic structure and bonding in nido-2,3-R2C2B4H6 carboranes (R ¼ H or alkyl) has been examined in detail via 11B, 1H, and 13C NMR [57,170–175], infrared and Raman [176], ESCA [107], photoemission [177], and negative-ion mass spectroscopy [178], inner shell electron energy-loss spectroscopy (SEELS) [179,180], and in numerous theoretical investigations listed in Table 4-6. The compound 2,3-C2B4H8 is an isoelectronic and isostructural analogue of nido-CB5H9 and other 6-vertex CnB6nH10n clusters depicted in Figure 4-3, and can be described in both localized valence-bond and delocalized molecular orbital language, as discussed earlier in Section 2.3. An ab initio investigation of nonclassical and classical descriptions of the bonding in 2,3-C2B4H8 found the classical form to be less stable by 56.5 kcal than the nonclassical (delocalized) model [181]. The chemistry of 2,3-C2B4H8 and its derivatives is, in large part, based on the Brnsted acidity of its B-H-B bridging protons, one of which is easily removed by metal hydrides in ethereal solvents to form the monoanion [170]. Hydride ion does not attack the remaining bridge proton even at 200 C, but it can be removed by alkyllithium reagents to give the carborane dianion (Figure 4-4). Proton removal occurs exclusively at the B2 2H2 2B bridges, as shown by treatment of the monoanion with anhydrous DCl to generate R2C2B4H5(m-D) [170,182]. A kinetic study [183] of the deprotonation of a wide variety of C,C0 -disubstituted derivatives established that the reaction occurs at the metal hydride surface and is pseudo-first order, independent of the amount of metal hydride present; in general, the reaction rate decreases as the steric bulk of R and R0 are increased. However, in the case of 2,3-Ph2C2B4H6, the acidity of the B-H-B protons is greatly enhanced by electron-withdrawal by the phenyl subsitutuents, resulting in very fast deprotonation [183]. The ylide [Ph3PCH2]þ I deprotonates the parent carborane (but not 2,3-Me2C2B4H6), forming a [Ph3PCH2]þ C2B4H7 salt [184]. The mono- and dianions are highly reactive toward metal ions and other electrophiles, readily forming B2 2M2 2B bridged derivatives in which M can be a transition metal or a main-group element such as Al, Si, Ge, or Sn (Table 4-6), as well as face-bonded complexes in which all five C2B3 ring atoms are coordinated to the metal (Chapters 12 and 13). An early example featuring both types is the reaction of C2B4H7 with CpFe(CO)2I to form yellow, air-stable solid C2B4H7-m(4,5)-Fe(CO)2Cp (4-22), which under UV radiation loses CO to form the ferracarborane sandwiches 4-23, a diamagnetic sublimable orange solid, and brown paramagnetic 4-24 [185,186]. This reaction sequence is unusual, as sandwich complexes such as 4-23 and 4-24 are normally prepared directly from carborane anions as described in Chapter 13. Compounds of this type are analogues of metallocenes in which a cyclopentadieneide (C5H5 or Cp)
H
R⬘ C B
C B H
H
B H
NaH H
−H2
R H
R⬘ C
C B
B H
2−
B
B
B R
H
−
H
H
B
LiCMe3 H
−H2
R H
H
FIGURE 4-4 Bridge-deprotonation of nido-2,3-RR0 C2B4H6 carboranes (R, R0 ¼ H, hydrocarbon, or silyl).
R⬘ C B
C B H
B
H
54
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
ring is replaced by C2B4H62 or another six-electron donor carborane ligand. Further discussion on the carboranearomatic hydrocarbon analogy can be found in later chapters. H
H
B H
H
C B
H
C
B H
hυ
C B
H
C
B
H
+
H
H
H
C B
C
B
H
B H
FeII
Fe
FeIII
C O C O
4-22
H
B H
H
B
H
H
B
H
H
B
4-24
4-23
Halogenation of the nido-2,3-Me2C2B4H5 anion has been examined in detail. Reaction with ICl with or without an AlCl3 catalyst yields the 3- and 4-Cl derivatives in an approximate 1:3 ratio, while treatment with Br2 affords the 3- and 4-Br products, with the former predominating by 9:1 [187]; B2 2X2 2B halogen-bridged intermediate species are believed to form initially and rearrange to the observed B2 2Xterminal products. A similar sequence is found in reactions of the 2,32M2 2B bridged C2B4H7 ion with halomethyl reagents [188] and with halides of the Group 14 elements, which afford B2 derivatives as shown in Figure 4-5 [71,189,190]. At 80-175 C, the Si- and Ge-bridged derivatives of parent 2,3-C2B4H8 rearrange irreversibly to their B(4)-substituted (and equivalent B(6)-substituted) isomers; at higher temperatures, 2,3C2B4H74-SiMe3 is converted to B(1)- and C(2)-SiMe3 derivatives and closo-carborane products [189]. Surprisingly, while 2,3-Me2C2B4H5-m-GeMe3 undergoes thermal isomerization to the 4-GeMe3 isomer, its m-SiMe3 counterpart does not [191]. The corresponding SnMe3-, PbMe3-, AlMe-, and GaMe-bridged derivatives of parent 2,3-C2B4H8 do not isomerize, but all except the m-AlMe derivative react with HCl to regenerate C2B4H8 [190,192]. Transition-metal bridged derivatives are also
R⬘ C
B H
B
H
–
H
R⬙CH2X
H
H
R, R⬘ = Et, CH2Ph R⬙ = H, Me, Ph, p-C6H4Me, (CH2)2Ph X = Cl, Br
C H
C B H
B H –
R
− X−
H
X = Br, Cl
H
R⬘ C B
C B H
H
B
CH2R⬙
H
H
CH2R⬙ H
not isolated
B MR⬙3 = SiH3, SiMe3, GeH3, GeMe3, R, R⬘ = H
B MR⬙3X
R
B H
B
H
R⬘
H
C B
– X–
B C B
R⬘
R
H
R
B
B
B R
C H B
H
H
H
H H
H
C B
C H
R⬘ C B
C H
H
R⬙
R⬙
M R⬙
R, R⬘ = H, Me MR⬙3 = SiH3, SiMe3, SiMe2CH2Cl, GeH3, GeMe3, SnMe3, PbMe3
FIGURE 4-5 Bridge-insertion into nido-2,3-RR0 C2B4H5 anions and rearrangement.
H H
B
MR⬙3
H
H H
MR⬙3 = SnMe3, PbMe3 R, R⬘ = H
B H
B
B
B H
C B
C H
B H
B H
MR⬙3
4.5 6-Vertex open clusters −
H
B Me3Si H
SiMe3 C B
C H
C
C B
H
B
H
B
SiMe3
RX Me3Si
H
B
H
B
H
B
B
H
HCl
H
H H
H
H
B SiMe3
C B
C
B
B
H
55
R
1) NaH
H
2) R⬘X
H
R⬘
H
SiMe3 C B
C B H
B
H
H
R
R
FIGURE 4-6 Mono- and dialkylation of the 2,3-(Me3Si)2C2B4H5 anion. R ¼ Me, i-C4H9; R0 ¼ Me; X ¼ Cl, Br.
known (an example is 4-22 shown above) [185,186,192–194]; further discussion of main group element- and transition element-bridged carboranes can be found in Chapters 12 and 13. Alkylation of the 2,3-(Me3Si)2C2B4H5 anion with alkyl halides leads not to the B(4/6)-substituted products as shown in Figure 4-5, instead giving exclusively the B(5)-alkyl derivatives (Figure 4-6), a finding that is attributed to the steric bulk of the trimethylsilyl groups on the cage carbon atoms [195]. Further alkylation is achieved as shown, by treatment with HCl to remove an SiMe3 group followed by deprotonation with NaH and reaction with R0 X [195]. The compound 2,3-(Me3Si)2C2B4H55-SiMe2CH2Cl is similarly prepared from 2,3-(Me3Si)2C2B4H5 and Me2SiCl (CH2Cl); reaction of this product with 1,2-LiRC2B10H10 gives the mixed-cage linked carborane 4-25 [196]. R H
C
B
B
B
B
H
Me3Si
C
4-25
C
B
B
H
C
B = BH C = CH B
B
B
B
Me3Si
B
Si
H
Me
H
Me
B
B B
Linked-carborane rings and chains such as 4-26 and 4-27 can be prepared via reactions of polyalkynes with B5H9, in C unit in the chain is converted to a nido-C2B4 cage [150]. which each C H
H
B
B
H H
4-26
B
H H
C
B
C
C
B
H
(CH2)n
C
B
B
(CH2)n
Et
H
4-27
H
H
H
B
B H
C
C B
H
H
H
Et
(CH2)5
(CH2)5
B
n = 4,5,6
H
H
H
C
B
H
B H
H
C
C
B
B H
B H
H H
H
B
B H
B H
C B H
H
56
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
While the chemistry of the anions is predominant, reactions involving neutral 2,3-C2B4H8 and its derivatives have been explored to some extent. Photolysis of the vapor generates the lower closo-carboranes 1,5-C2B3H5 and 1,2- and 1,6-C2B4H6 in substantial yields [55]. The parent compound undergoes deuterium exchange with B2D6 at 100 C, with all three basal BH (but not B2 2H2 2B) groups participating [170]; in contrast, D2 exchanges with all terminal and bridging hydrogens [197]. The compound 2,3-(Me3Si)C2B4H7 reduces aldehydes and ketones to the alcohols [198]. On heating at 210 C for 3 days, 2,3-(Me3Si)2C2B4H6 undergoes cage fusion with loss of two SiMe3 groups, to form (Me3Si)2C4B8H10 in good yield [199]. Halogenation in the presence of aluminum halide catalysts occurs with Cl2 or Br2 to afford the 2,3-C2B4H74-X product [200], while iodine monochloride and C2B4H8 combine over AlCl3 to give 2,3-C2B4H74-I [201]. Similarly, the 4-chloro, 4-bromo, and 4-iodo derivatives are obtained via reactions of 2,3-MeC2B4H7 or 2,3-Me2C2B4H6 with Cl2, Br2, and ICl, respectively, under electrophilic conditions [201]. In contrast to the electrophilic halogenation of the 2,3-Me2C2B4H7 anion discussed above, the neutral carboranes afford only the asymmetric B(4)or equivalent B(6)-halogenated product, reflecting the higher negative charge on the borons closest to carbon and demonstrating that there are significant differences in charge distribution between the neutral carborane and its anion. Very few studied reactions of 2,3-C2B4H8 or its derivatives directly involve the cage carbon atoms. In the parent carborane, this reflects the relative inertness of the C2 2H bonds, which, unlike those in closo-carboranes, have low polarity and essentially no acidic character. The C2 2Si bond in silyl derivatives can be cleaved: 2,3-(Me3Si)2C2B4H6 reacts with NaHF2 or HCl at elevated temperatures to lose an SiMe3 group, forming 2,3-(Me3Si)C2B4H7 and Me3SiX (X ¼ F or Cl) quantitatively [169]. Interestingly, an angle-resolved photoemission study of the adsorption of 2,3-Et2C2B4H6 on Si(111) surfaces at 90 K revealed that the ethyl groups dissociate from the molecule in the process [202]. Thermally induced insertion of boron and other elements into the open face of 2,3-C2B4 clusters to create 7-vertex closo polyhedra has been demonstrated with several reagents, including Et3NBH3 [203] and BMe3 [204,205], which afford derivatives of closo-2,4-C2B5H7 and closo-1,7-C2B6H8; GaMe3 and InMe3, which form 1,2,3-MeGa(C2B4H6) and 1,2,3-MeIn(C2B4H6), respectively [206]; and Fe(CO)5, which produces 1,2,3-(CO)3Fe(C2B4H6) and nido-1,2,3-Fe (C2B3H7) [185], the latter compound having an open C2B3 face. A remarkable 4-boron insertion occurs upon reaction of 2,3-(Me3Si)2C2B4H6 with B5H9 to generate 10-vertex arachno-6,9-(Me3Si)C2B8H13 in 21% yield [207]. Another noteworthy cage-expansion is the NiCl2-catalyzed insertion of 2-butyne into 2,3-Et2C2B4H6 to form 8-vertex nido-Me2Et2C4B4H4 (Chapter 5) [208]. However, insertion into the bridge-deprotonated C2B4H7 and C2B4H62 anions and their substituted derivatives is a more versatile approach to the synthesis of closo-MC2B4 clusters in which M is a main-group or transition element (Chapters 12 and 13). A wholly unexpected reaction of bis(2,3-dicarbahexaboranyl) metal complexes was discovered in 1974 [209,210] in studies of HnM(2,3-R2C2B4H4)2 sandwiches, in which M is a transition metal (usually Fe or Co) and R is a hydrocarbon group. As described in Chapter 11, such complexes undergo face-to face oxidative fusion of two nido-C2B4 dianionic ligands to form neutral 12-vertex C4B8 carboranes. Remarkably, this reaction occurs under very mild conditions with a wide variety of R groups, to give typically quantitative yields of fused products [211]. Subsequent investigation [212] has shown that nido-1,2,3-(ligand)M(R2C2B3H4) metallacarboranes exhibit similar metal-promoted oxidative fusion to give 12-vertex M2C4B6 cages. The fused carboranes and metallacarborane products, in turn, have given rise to a subfield centered on large four-carbon carboranes, previously unknown. This chemistry and recent developments are discussed further in Chapters 11 and 13.
4.5.4 Nido-2,4-C2B4H8 4.5.4.1 Synthesis Nido-2,4-dicarbahexaborane(8), with nonadjacent carbons in the cage framework, has not been isolated as a neutral parent species but is known in the form of the 2,4-C2B4H7 ion and as a substituted derivative (Table 4-7). Most known preparative routes to nido-2,4-C2B4 clusters are based on rearrangement or degradation of other carboranes as in the cage-opening of closo-1,6-C2B4H6 with fluoride that affords the nido-2,4-C2B4H6-5-F ion [213] and reaction of closo-1,2-(Me3Si)2C2B4H4 with alkali metals to generate the carborane dianion [214] (Figure 4-7). Closo-to-nido
4.5 6-Vertex open clusters
57
TABLE 4-7 Nido-2,4-C2B4H8 Derivatives Synthesis and Characterization Compound
Information
References
S, B, H S (degradation of C2B5H7) S, B, H, F S, B, H S, B S, B, H S, X, B, H, C, IR S, X, B, H, C, Li, IR, MS S, X, IR S, X, H, B, IR, MS S, H, B, C, MS S S, H, B, C S, H, B, C S, H, B, C X, IR
[182] [215] [213] [120] [120] [182] [219] [214] [220] [224] [17] [217,218] [218] [218] [218] [221]
S, H, B, C, Li S, X
[222] [222]
Detailed NMR Studies 2,4-C2B4H7, 2,4-C2B4H6-3-NMe3
C
[96]
Theoretical Studies Molecular and electronic structure calculations 2,4-C2B4H62 2,4-R2C2B4R42 (R ¼ H, Me) 2,4-R2C2B4R4H (R ¼ H, Me) 2,4-R2C2B4R4H2 (R ¼ H, Me)
ab initio DFT DFT DFT
[35] [218] [218] [218]
NMR calculations 2,4-C2B4H7, 2,4-C2B4H63-NMe3 2,4-R2C2B4R4Hnn2 (R ¼ H, Me) m(5,6)R2NBH-2,4-C2B4H6 (R ¼ H, Me) 2,4-R2C2B4R4H (R ¼ H, Me) [1,2,4-(THF)2M]þ [2,3-(SiMe3)2C2B4H5] (M ¼ Na, Li)
C (IGLO) B (DFT) B (IGLO) B (GIAO) GIAO
[96] [218] [216] [18] [222]
2,4-C2B4H7 2,4-C2B4H7, m-R2NBH-2,4-C2B4H6 (R ¼ Me, Et) 2,4-C2B4H65-F 2,4-C2B4H6-n-PMe3þ (n ¼ 3, 5) 2,4-C2B4H6-3-NMe3þ (n ¼ 3, 5) 2,4-C2B4H6-n-NMe3 (n ¼ 3, 5) [2,4-(SiMe3)2C2B4H4]2(Naþ)2(THF)4 2,4-(SiMe3)2C2B4H4 2 LiLþ (L ¼ THF, TMEDA) 2,4-(SiMe3)2-m-5,6-H2-Ln[(SiMe3)2C2B4H4]2 (Ln ¼ Er, Dy) (classical open) H2C2B4[NH(CHMe2)2]4 2,4-Et2C2B4Et4H2 (reported as nido-2-EtCB4Et4H-m-CHEt)
2,4-Et2C2B4Et4H 2,4-Et2C2B4Et42 Naþ3[(2,4-(SiMe3)2C2B4H4]2Ln[m-5,6-nido-2,4-(SiMe3)2C2B4H4] Ln ¼ Dy, Er 1,2,4-(THF)2Mþ [2,3-(SiMe3)2C2B4H5] (M ¼ Na, Li) 1,2,4-(THF)(TMEDA)Naþ [(SiMe3)2C2B4H5]
3
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; F, 19F NMR; Li, 7Li NMR; IR, infrared data; MS, mass spectroscopic data.
cage-opening can also be achieved with other Lewis bases such as amines and phosphines [120,182]; for example, reaction of 1,6-C2B4H6 with triethylamine gives nido-2,4-C2B4H6þ-5-NEt3,which can be converted to Naþ 2,4-C2B4H7 via reaction with NaH [182]. Both 2,4-C2B4H6þ-5-NEt3 and 2,4-C2B4H6þ-5-PEt3 undergo thermal rearrangement to the 3-amino or 3-phosphino substituted isomer, the reaction in the latter case accompanied by extensive decomposition [120,182].
58
CHAPTER 4 Small carboranes: Four- to six-vertex clusters H
C B H
−
H
H
H
B
B
F−
H
B
B
Na+
H
H
H
B
C B
C
H
C
H
B F
H H
B H
Me3Si
H
B
B
, naphthalene
2−
B
2 LiL+ H
Me3Si
B
C H B
L
C
C
Li0
H
SiMe3 L = THF or TMEDA
B
C
B
SiMe3
H
H
FIGURE 4-7 Conversions of closo-1,6-C2B4H6 to nido-2,4-C2B4H6-5-F and of closo-1,2-(Me3Si)2C2B4H4 to nido-2, 4-(Me3Si)2C2B4H42.
Nido-2,4-C2B4H7 can also be obtained from the 7-vertex closo-carborane 2,4-C2B5H7, via treatment with LiNR2 reagents (R ¼ Me, Et, CHMe2) in acetonitrile at room temperature [215]. The conversion is nearly quantitative and proceeds through a B(5,6)-bridged nido-2,4-C2B4H6-m-BHNR2 intermediate 4-28, as determined by IGLO-NMR studies [216]. H
B H
4-28
H
H
B C
C
H
B
B H
B NR2
H
The hexaethyl derivative 2,4-Et2C2B4Et4H2 (4-29), in which all of the terminal hydrogens are replaced by ethyl groups, can be obtained by thermolysis of the carbon-bridged arachno-CB4 carborane 4-12 described earlier (Figure 4-8) Et C
Et B Et Et
Et
C
B
Et B H
H B
B H
Et
> 100 °C
Et
− Et2BH
Et
Et B C
H
C B Et
Et2B
4-12
4-29
FIGURE 4-8 Conversion of arachno-1-EtCB4Et4H3-m(2,3)-CEt(BEt2) (4-12) to 2,4-Et2C2B4Et4H2 (4-29).
B H
Et
4.5 6-Vertex open clusters
59
[17,27,217]. The structure of 4-29 was originally proposed [17] as a novel carbon-bridged nido-CB4 cluster, but was later identified by GIAO-NMR studies as a nido-2,4-C2B4 carborane [18]. This route avoids the problem of B5H9 availability mentioned earlier, and the protective sheath of relatively unreactive alkyl groups lends stability to the molecule; consequently, the chemistry of this derivative is effectively limited to bridge-deprotonation and metal sandwich formation [218].
4.5.4.2 Structure and properties The pentagonal-pyramidal 2,4-C2B4 geometry with non-vicinal cage carbon atoms is supported by X-ray diffraction data on several anionic derivatives [214,219–222] and metal sandwich complexes (Chapters 12 and 13); a detailed analysis of the 13C NMR spectra of 2,4-C2B4H7 and 2,4-C2B4H63-NMe3 [96]; and calculations of electronic and molecular structure and NMR chemical shifts (Table 4-7). As is often the case with open-cage carboranes, separation of the skeletal carbons does not necessarily lead to greater stability as it typically does in closo systems (Section 2.7): the 2,4C2B4H8 isomer is calculated [18] to be 4.6 kcal mol1 higher in energy than the 2,3 isomer. However, for the C2B4H7 anions, this order is reversed, the 2,4 isomer being more stable by 15.4 kcal mol1. These findings underline the importance of hydrogen placement, as the relative instability of neutral 2,4-C2B4H8 can be attributed to its having only one B2 2B edge on the open face that can accommodate a B2 2H2 2B bridge, forcing one of the carbon atoms to adopt a CH2 group mode; in the anion, with only one “extra” hydrogen, this problem disappears and the tendency of carbons to separate from each other reasserts itself. As with 2,3-C2B4H8 and its derivatives, one or both bridging protons in nido-2,4-C2B4 carboranes can be removed by metal hydrides, amines, or other nucleophiles [182,214,215,218]. Treatment of [Na(TMEDA)þ]2[2,4-(Me3Si)2C2B4H42] with PbCl2 in cold benzene forms the closo-plumbacarborane 1,2,4-PbII[(Me3Si)2C2B4H4] [223], while complexation of the 2,4-Et2C2B4Et4H anion [218] with Fe2þ affords the sandwich H2Fe(2,4-Et2C2B4Et4)2 [217]. The air-stable iron compound, unlike its “carbons-adjacent” analogue H2Fe(2,3-Et2C2B4H4)2 mentioned earlier, shows no tendency to undergo oxidative ligand fusion to form C4B8 carboranes. Most of the known chemistry of nido-2,4-C2B4 clusters centers on the synthesis of metal sandwich complexes, as described in Chapters 12 and 13.
4.5.5 Arachno- and hypho-C2B4Hx clusters Like some molecules described earlier in this chapter, several open-cage 6-vertex carbon-boron species have “borderline” structures that can be described as organoboranes and/or as open-cage carboranes (Table 4-8). An example of strongly electrondonating groups exerting major influence on cage structure is found in the species H2C2B4[N(CHMe2)2]4 (4-30), which
TABLE 4-8 Arachno- and Hypho-C2B4Hx Derivatives Synthesis and Characterization Compound
Information
References
hypho-C2B4H12
B, H B, H, MS ED IR, vapor pressure S, X, H, B, IR, MS
[227] [228] [229] [226] [224]
B2 2C bond energies B2 2C bond energies, ab initio, IGLO 1H, 11B,
[230] [230] [229]
(classical open) H2C2B4[NH(ipr)2]4 Theoretical Studies Molecular and electronic structure calculations arachno-C2B4H10 hypho-C2B4H12 1
H, H NMR; B,
11
13
C NMR
B NMR; IR, infrared data; MS, mass spectroscopic data; ED, electron diffraction.
60
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
was prepared in 73% yield via dehalogenation of a tetrakis(dichloroboryl)ethane derivative, H2C2{B[N(CHMe2)]Cl}4, by Na/K in refluxing benzene [224]. Rather than a nido-carborane structure, X-ray diffraction data revealed this molecule to have, at least in the solid state, the classical geometry shown, described as a 1,3-diboretane having a 2,4˚ , indicating a very 2B distance in the diboretane ring is quite long at 1.93 A bridging B2[N(CHMe2)2]2 group. The B2 weak boron-boron interaction [224].
[CHMe2]2N H
N[CHMe2]2
H
B
B H
H
B
C
B H
B
H
H
B
C
B
C
H
N[CHMe2]2
C
H
H
B
H H
4-30
C
H
C B
H
N[CHMe2]2
4-31
B B
B
4-32
The skeletal framework in 4-30 has been calculated [82,225] as a stable intermediate in the rearrangement of closo-1,2C2B4H6 to the 1,6 isomer, as described below. The 2,4-dimethylenetetraborane 4-31, obtained via reaction of B4H10 [tetraborane(10)] with ethylene [226], has been structurally characterized by 11B, 13C, and 1H NMR [226–229] and gas-phase electron diffraction [229] and can be viewed as a 6-vertex hypho-C2B4H12 carborane cluster. Its 20 skeletal electrons (six and eight from the two CH and four BH units, respectively, plus six electrons from the six “extra” hydrogens) constitute 2n þ 8 electrons or n þ 4 pairs, corresponding to a closo 9-vertex polyhedron with three missing vertexes, according to the electron-counting rules laid out in Chapter 2. With a certain amount of imagination, the geometry of 4-31 fits this description as suggested in 4-32, keeping in mind that this is considered a borderline case. A proposed arachno-C2B4H10 molecule has been explored computationally [230] but has not yet been reported as an isolated species.
4.5.6 Nido-2,3,4-C3B3H7 4.5.6.1 Synthesis The first nido-tricarbahexaborane, indeed the first three-carbon carborane of any type, to be isolated and characterized was 2,3,4-MeC3B3H6, obtained in 1966 as one of several carboranes formed in the gas-phase reaction of B4H10 with acetylene at 25-70 C (whose major product when the reaction is quenched is the previously described nido-1,2C2B3H7) [51,127,231]. Two other products of this reaction, originally thought to be 2,3,4-Me2C3B3H5 isomers, were later identified as the nido-2,3-dicarbahexaboranes 2,3-MeC2B4H64-Me and 2,3-C2B4H75-Et [29]. Alkyl derivatives of nido-2,3,4-tricarbahexaborane (Table 4-9) have been obtained from the reaction of 2-methylbut-1-ene-3-yne, CH, with B4H10 at 70 C, which also affords parent 2,3,4-C3B3H7 [232]. 5CMeC H2C5 A different route to 2,3,4-tricarbahexaboranes utilizes the hydroboration of diethyl(1-propynyl)borane, Et2BC-Me, with tetraethyldiborane, Et4B2H2, catalyzed by trialkyltin chorides, to form the carbon-bridged derivatives C 4-33 and 4-34 [233]. Also obtained in this reaction are alkyl derivatives of 2,3,5-C3B3H7, as discussed below. Et B
4-33 R = Me 4-34 R = Et
H
Et
R C B
C
C
B Et
Et
Me
4.5 6-Vertex open clusters
61
TABLE 4-9 Nido-C3B3H7 Derivatives Synthesis and Characterization Compound
Information
References
2,3,4-C3B3H7 2,3,4-MeC3B3H6
S, S, S, S, S, S, S, S, S, S,
H, B, MS B, MS IR, MS* H H, B, C H, B, C H, B H, B IR, MS H, B, MS
[232] [231] [51] [127] [29] [29] [234] [234] [379] [232]
S, S, S, S, S, S, S, S, S, S,
H, B, C H, B H, B, C H, B, C, MS H, B, C, MS H, B, C B(Et), C(Et), H(Et) B H, B, C H, B, C
[233] [236] [236] [236] [236] [240] [240] [233] [241] [241]
2,3,4-C3B3H61-Me 2,3,4-MeC3B3H3 2,3,4-MeC3B3H3-m(5,6)-D 2,3,4-MeC3B3H3D3, 2-2,3,4-MeC3B3H2D4 2,3,4-RC3B3H5-n-R0 (n ¼ 1, 5; R ¼ H, Me; R0 ¼ H, CH2Me) 2,3,4-HMeEtC3B3Et3-m(5,6)-CH2Me (2 isomers) 2,3,5-Et2(CHMe2)C3B3EtMe2H 2,3,5-Et2(CHMe2)C3B3Et3H 2,3,5-Me(CHMe2)2C3B3EtMe2H 2,3,5-Me(CHMe2)2C3B3Et3H 2,3,5-Et2RC3B3Et3H (R ¼ Me, Et) 2,3,5-Et2RC3B3Et3 (R ¼ Me, Et) 2,3,5-HMeEtC3B3Et3H 2,3,5-(spiro-H2CBEt)H2C3B3Et3 2,3,5-(spiro-H2CBEtR)H2C3B3Et3 (R ¼ NC5H4Me, Bu) 2,3,5-(CMe3)Et2C3B3EtMe2 Other Experimental Studies 2,3,4-MeC3B3H5 Theoretical Studies Molecular and electronic structure calculations C3B3H7 C3B3H7 2,3,4-C3B3H7 C3B3H6 isomers C3B3H6þ isomers
NMR calculations 2,3,4-C3B3H7 2,3,5-R3C3B3R3H (R ¼ H, Me) 2,3,4-HMeEtC3B3Et3-m-5,6)-CH2Me (2 isomers)
S, H, B, MS
[237]
kinetics of formation from B4H10þ C2H2
[30]
MNDO extended Hu¨ckel SCF DFT: ionization potential, valence structure ab initio; energies of isomers Hartree-Fock and B3LYP; isomer stabilities
[323] [34] [347] [346] [35] [121]
GIAO/IGLO GIAO/IGLO GIAO/IGLO
11 11
B shifts B shifts
[12] [18] [233] Continued
62
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
TABLE 4-9 Nido-C3B3H7 Derivatives—Cont’d Synthesis and Characterization Compound
Information
References
2,3,5-(spiro-H2CBEt)H2C3B3Et3 2,3,5-(spiro-H2CBEtR)H2C3B3Et3 (R ¼ NC5H4Me, Bu)
GIAO/IGLO GIAO/IGLO
[241] [241]
mechanism of formation from B4H8 þ C2H2
[32]
Reactivity calculations 2,3,4-MeC3B3H6 1
S, synthesis; X, X-ray diffraction; H, H NMR; B, spectroscopic data; UV, UV-visible data.
11
B NMR; C,
13
C NMR; 2d, two-dimensional (COSY) NMR; IR, infrared data; MS, mass
4.5.6.2 Structure and properties Although no X-ray diffraction or electron diffraction studies of this carborane or any derivative have been reported, the pentagonal pyramidal cage structure with three contiguous carbon atoms in the base (Figure 1-3, second row left) is established with reasonable certainty from multinuclear NMR, infrared, and theoretical investigations (Table 4-9). Nido-2,3,4-C3B3H7 and its 2,3,5 isomer, as discussed below, are members of the family of isoelectronic CnB6nH10n clusters shown in Figure 4-3. The lone B-H-B bridging proton in 2,3,4-MeC3B3H6 can be removed by reaction with NaH in THF or diglyme to form the 2,3,4-MeC3B3H5 anion, which on treatment with DCl generates the bridge-deuterated species 2,3,4-MeC3B3H5-m (5,6)-D [234]. The sodium salt of the anion reacts with BrMn(CO)3 without loss of CO to afford NaBr and a red complex presumed to be 2,3,4-MeC3B3H5-m(5,6)-Mn(CO)3, which at 100 C loses CO to form the manganacarborane sandwich complex 4-35, a thermally stable yellow liquid [234]. This compound, the first reported metallacarborane having more than two skeletal carbon atoms, can also be prepared by reaction of neutral 2,3,4-MeC3B3H6 with Mn2(CO)10 [234,235]. H
B H H
H
C B
4-35
C
Mn C O
C
Me
B
C O
H
C O
4.5.7 Nido-2,3,5-C3B3H7 4.5.7.1 Synthesis Parent nido-2,3,5-tricarbahexaborane, in which one cage carbon atom is separated from the other two, has not been prepared, but alkyl derivatives 4-38 have been obtained by hydroboration of 1,3-dihydro-1,3-diborafulvalenes (4-36) or 4,5-diisopropylidine-1,3-diborolanes (4-37) followed by loss of BEt3 (Figure 4-9) [236–239]. CMe) or bis(diethylboryl)alkenes [R(Et2B)C5 5CEt(BEt2)] can be treated Alternatively, 1-propynyldiethylborane (Et2BC with excess tetraethyldiborane(6) to generate the hexaalkyl compounds nido-2,3,5-Et2RC3B3Et3 (R ¼ Me or Et), accompanied by the 2,3,4-C3B3 derivatives 4-33 and 4-34 discussed above [233,240]. Both the 2,3,4 and 2,3,5 isomers are proposed to form via EtBH2 loss from an unisolated C2-bridged arachno-carborane common intermediate, 4-39 [233]. This process appears
4.5 6-Vertex open clusters
63
R Et
B
Et
R⬘
(HBEt2)2
R⬙
⎯ BEt3 R⬘
B R
B
4-36
Et R
Me
Et
C B
C C
R
B
R
H
R⬙ B
Me
R, R⬘ = Me, Et R⬙ = H, CHMe2, CMe3
⎯ BEt3
H
Me
4-38
(HBEt2)2
Me B R
Me
4-37
FIGURE 4-9 Synthesis of nido-2,3,5-tricarbahexaboranes.
similar to the conversion of arachno-1-EtCB4Et4H3-m(2,3)-CEt(BEt2) (4-12) to nido-2,4-Et2C2B4Et4H2 (4-29) (Figure 4-8), the difference being that 4-12, with only one bridging carbon, gives rise to dicarbon carborane products, while 4-39, with a dicarbon bridge, generates tricarbaboranes. Et C
Et B
4-39
H
H
C
Et
B B
H B
Et
H
C Et
Me
A structurally unique 2,3,5-tricarbahexaborane spiro derivative, 4-40 (Figure 4-10), was isolated from the hydroboration of bis(diethylboryl)acetylene with tetraethyldiborane, along with several other remarkable products including C4B4 − Li+
Et
Et
B H H
Et C C
Et
B B
B Et
4-42
C
n-C4H7Li
CH2
H H
B Et
C C
B
n-C4H7
C
C
B
B Et
H
Et
H
NC6H4Me
H C H C
Et B
B
Et Et
N
4-41 Me
FIGURE 4-10 Reactions of the spiro-carborane 4-40 with g-picoline and n-butyllithium.
H H
B
4-40
C
C
B Et
64
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
and C6B6 carboranes (Chapters 5 and 11, respectively) [241]. The structure assigned to 4-40 is based on experimental NMR data and ab initio/GIAO-IGLO calculations, and is further supported by its reaction chemistry. The compound is extremely O2-sensitive, but treatment with g-picoline results in simple base-substitution on the exo-polyhedral boron to give 4-41 (Figure 4-10), while n-butyllithium opens the ring as shown, to afford the salt 4-42 [241].
4.5.7.2 Structure and properties X-ray studies of 2,3,5-tricarbahexaboranes are not available, but the nido-C3B3 cluster geometry is confirmed by multinuclear NMR spectroscopy and high-level computation (Table 4-9), supported indirectly by X-ray diffraction data on metal sandwich compounds bearing nido-C3B3 ligands [242–245]. In the uncomplexed neutral carboranes (e.g., 4-38), the salient feature is an unusual C2 2HB bridging proton, a consequence of the fact that there is no available B2 2B edge on the open face to accommodate a B2 2H2 2B group. The known chemistry of 2,3,5-tricarbahexaboranes is largely based on removal of this very acidic proton with nucleophiles to generate the carborane monoanion [236,240]. Reactions of both neutral and anionic 2,3,5-tricarbahexaboranes with metal reagents form MC3B3 closometallacarborane clusters and sandwich complexes (Chapter 13), as illustrated by the synthesis of red 4-43 (Figure 4-11) and the related dark yellow cobalt complex (CO)2Co[(Me3C)Et2C3B3Et3), prepared from the neutral carborane and Co2(CO)8 [236,237]. Et B B Et Et
Ni(C3H5)2
Me C C
B B
C
− C3H6 CMe3
H
Et Et
B
R C C
B
C
B Ni
−
Et
Et
R
[(C3H5)NiBr]2 CMe3
4-43
Me
− Br−
Et Et
Et C C
B B
C
CMe3
Et
R = Me, Et
FIGURE 4-11 Formation of 1,2,3,5-NiC3B3 metallacarborane clusters.
4.5.8 Nido-2,3,4,5-C4B2H6 4.5.8.1 Synthesis Nido-2,3,4,5-tetracarbahexaborane(6) (Figure 1-3, second row right), one of the most carbon-rich carborane systems known, is intimately related to classical organoboranes and, indeed, is often obtained as a product of organoborane rearrangements. Peralkyl derivatives of C4B2H6 were obtained by Binger as early as 1966 from cis-bis(dialkylboryl)-1,2dialkylethylenes [(R2B)2C ¼ CR0 2, R, R0 ¼ Me, Et, n-C3H7] by pyrolysis at 150-180 C [246,247], or alternatively, by addition of a catalytic quantity of Et2BCl at 40 C [248]. Parent C4B2H6 was first synthesized by Onak and Wong in 1970 by passing 1,2-tetramethylenediborane [m(1,2)-(CH2)4B2H2] through a flow system at 550 C [123,249]; it was also obtained by Miller and Grimes via insertion of acetylene into nido-1,2-C2B3H7, as discussed earlier in this chapter (see Figure 4-2) [38]. Alkyne-B4H10 flash reactions generate both parent and alkylated C4B2 carboranes [53], CH with B4H10 at 70 C affords HMe3C4B2H2, along with C3B3 carboranes, as 5CMeC while the reaction of H2C5 mentioned earlier [232]. Substituted C4B2 carborane derivatives (Table 4-10) have been prepared from organic or organoborane precursors via a remarkable variety of synthetic approaches, as illustrated in Figures 4-12 and 4-13. Clearly, an important driving force in these reactions is the thermodynamic stability of the nido-C4B2 cluster framework—a consequence of the delocalized framework bonding—which tends to remain intact once formed. However, the substituents bound to boron also
4.5 6-Vertex open clusters
65
TABLE 4-10 Nido-2,3,4,5-C4B2H6 Derivatives Synthesis and Characterization Compound
Information
References
Parent
S, H, B, IR, MS S Microwave structure C ED S(gas phase flash photolysis), H, B S, MS S, H, B, C, MS S, H, B, IR S, H, B, MS C S, H, B, MS S, H, B S, H, B S, H, B S, B, IR, MS S, H, B S, H, B, MS S, X(Et), H, B, C, MS B, C S, H, B S, H, B, C, MS S, H, B, C, MS S, X (C6Me3H3),H, C S, H, B, MS S, H, B, C S, H, B, C, MS S, X, H, B, C, MS S, H, B, C, MS S, H, B, C, MS S, H, B, C, MS Reactions with BX3 and LiEt3BH S, H, B, C S, H, B, C, Sn, MS S, H, B, C, Sn, MS S, H, B, C, Sn, MS S, B, C, P, Se S, B, C, P, IR* S, X(Me), H, B, IR, MS S, H, IR*, Raman* S, H, IR*, Raman*
[38,123,249] [28] [260] [96] [261] [53] [38] [254] [38] [232] [96] [38] [255] [255] [255] [248] [256] [262] [313] [380] [258] [259] [259] [315] [237] [257] [250] [269] [269] [259] [259] [262] [262] [263] [263] [263] [265] [265] [267] [246] [246]
Me6 Parent þ Me and Me2 derivatives 13 12 C2 C2B2H6 1-Me-6-N(CHMe2)2 2/3-Me 3-Me 6-Me 2,3/3,4-R2 (R ¼ D, Me) 3,4-Me2C4B2H2Me2 3,4-Et2C4B2H2-1-Me-6-Et 3,4-Et2C4B2H2-1-Et-6-Me Me2Et2C4B2Et2 isomers Et2H2C4B2Ph2 Et2H2C4B2EtBr R4C4B2I2 (R ¼ Et, Me, Ph) R4C4B2R0 2 (R, R0 ¼ alkyl) 5CH2)2C4B2R2 (R ¼ Me, Cl) (Me2HC)2(MeC5 REt2RC4B2X2 (X ¼ Br, I; R ¼ Et, CH2Et, CHMe2, Bu) (CH2Et)Et2(CH2Et)C4B2MeR (R ¼ Et, Me) 2,3-C4H4-4-SiMe3-C4B2R2 (R ¼ CMe3, 2,4,6-C6Me3H3) H(CMe3)Et2C4B2Me2 (2 isomers) (i-pr) 2Me2C4B2RR0 (R, R0 ¼ Me, CHMe2, Ph) 3-(i-C3H5)-4-(Me2CH)-C4B2(Me2CH)2 new synthesis Cp*Co(H4C4)[(SiMe3)HC4B2(CMe3)2] L(H4C4)[(SiMe3)HC4B2(CMe3)2] L ¼ CpRh, (CO)3Fe BuEt2BuC4B2ClX (X ¼ Cl, Et) REt2RC4B2XH (X ¼ Br, H; R ¼ Et, Bu) MeEtRR0 C4B2-1-R00 -6-Et (R, R, R00 ¼ Me, Et) MeEtRR0 C4B2-1-R00 -6-X (R, R, R00 ¼ Me, Et; X ¼ Br, I, H) Me2Et2C4B2-1-Et-6-SnR3 isomers (R ¼ Me, Ph) Me2Et2C4B2-1-Et-6-SnPh2X isomers (X ¼ Cl, Br) Me2Et2C4B2-1-Et-6-SnX3 isomers (X ¼ Cl, Br) Me4C4B2-1-Me-6-L (L ¼ P(O)Ph2, P(S)Ph2, P(Se)Ph2) Me2Et2C4B2-1-Et-6-L (L ¼ Ir, P(BH3)Ph2, P[W(CO)5]Ph2) Me4C4B2R-6-NSFe2(CO)6 (R ¼ Me, Br) Me4C4B2Et2 Me4C4B2-1-Me-6-Et
Continued
66
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
TABLE 4-10 Nido-2,3,4,5-C4B2H6 Derivatives—Cont’d Synthesis and Characterization Compound
Information
References
Me6
S, H, IR*, Raman* S, MS S, B*, IR*, MS* S, X(Ph, tolyl), H, B, C, MS S, H, B, C, P, MS S, X, H, B, C, IR, MS S, H, B, C, MS S, H, B, C, MS S S, H, B, C, Sn, MS S S, X, H, B, C, Si, MS S, X, H, B, C, MS S, X, H, B, C, MS S, H, B, C, MS S, H, B, C, MS CPh), H, B, C, MS S, X(C S, H, B, C, MS S, H, B, C, MS S, H, B, C, MS halogen exchange S, H, B, C, MS S, H, B, C, MS S, X(Ph), H, B, MS
[246] [314] [248] [266] [266] [266] [266] [266] [266] [266] [266] [266] [266] [266] [266] [266] [264] [264] [264] [264] [264] [270] [270] [270]
S, B, MS
[270]
S, X, H, B, C, MS
[270]
S, H, F, IR, MS S, H, F, IR, MS S, H
[253] [268] [251]
Structure, dipole moment, ionization potential SCF Extended Hu¨ckel DFT: ionization potential, valence structure Classical structures
[333] [347] [34] [346] [82]
ab initio; isomer energies
[35]
Et6 CR (R ¼ Ph, CMe3, SiMe3, p-tolyl) Et4C4B2-1-I-6-C Et4C4B2-1-I-6-PPh2 Et4C4B2-1-I-6-FeCp(CO)2 Et4C4B2H-1-I Et4C4B2-1-I-6-Et Et4C4B2-1-Ph-6-Et Et4C4B2-1-I-6-SnMe3 Et4C4B2-1,6-(SnMe3)2 CSiMe3-6-C CPh Et4C4B2-1-C Et4C4B2-1-I-6-C2PhCo2(CO)6 Et4C4B2-1-I-6-[1-(1,2-C2B10H11)] (Et4C4B2-1-I-6-C6H4-)2 (Et4C4B2-1-I-6-)2O CPh (R¼Br, C CPh) Et4C4B2-1-R-6-C Et4C4B2-1-Br-6-C CCMe3 Et4C4B2-1-I-6-F C-p-CH2C6H4Me]2 1,10 -[Et4C4B2-6-C Et4C4B2-1-I-6-C CPh CR (R ¼ Me, SPh, PPh2) Et4C4B2-1-I-6-C Et4C4B2-1-C CR-6-R (R ¼ n-C4H9, PPh2) CpCo[cyclo-C4R2(Et4C4B2-1-I-6-)2] (R ¼ Ph, SPh) cyclobutadiene complexes CSiMe3-6-)2] CpCo[cyclo-C4R2(Et4C4B2-1-C cyclobutadiene complex CpCo[cyclo-C(O)C4Ph2(Et4C4B2-6-n-C4H9-1-)2] Other C4B2 systems “nido”,arac-R4C4B2F2 (planar) (R ¼ H, Me) “nido”,arac-Me4C4B2F2 (planar) [arac-1,4-H4C4B2(C5H4FeCp)2]2 Theoretical Studies Molecular and electronic structure calculations Parent
Isomerization calculations Parent
Continued
4.5 6-Vertex open clusters
67
TABLE 4-10 Nido-2,3,4,5-C4B2H6 Derivatives—Cont’d Synthesis and Characterization Compound
NMR calculations Parent, 6-Me Parent cyclic planar 1,4-H4C4B2F2
Information
References
Isomer stability; formation from dimerization of C2BH3 [borirene])
[2]
IGLO 13C shifts DFT (H, B, C spin-spin coupling) IGLO 11B shifts IGLO/GIAO, B, F
[96] [37] [12] [294]
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; F, 19F NMR; P, 31P NMR; Li, 7Li NMR; Pt, dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; ED, electron diffraction.
195
Me
I B
B Me Me
Me B
C C
Pt NMR; 2d, two-
C8K, MeBBr2
Me
C
C
R
C
BI3, NaK2.8
R
− KI
R
R
C
I B
C C
C
R
C
R = Me, Et, Ph R
Me
A
Me3C CMe3
Me2C Me
Li C
C
C
H
B
C
(Me3C)2B2Cl2
C
B
CMe2
Me3C
B
B
Cl
B
SiMe3
C
C C
C Ph
H
CHMe2
SiMe3
B
B Me
C H
C
Me
H
D
B
R = CMe3, 2,4,6-C6Me3H3
CMe3 Et
C
C
R
Mg
Me Et
B
B
R
C
H2C=C Me
CMe3 C C
R
R Cl
H
C H
H C
Me
B 100 °C
B Et Et
Me C C
B
C H
C CMe3
FIGURE 4-12 Routes to nido-2,3,4,5-C4B2H6 derivatives from A alkynes [313,314], B allenes [250], C diboraalkenes [315], and D diborapentafulvene [237].
68
CHAPTER 4 Small carboranes: Four- to six-vertex clusters R⬘ Me
Me
R⬘
H
B
C C
R
B Me
Me
A
Δ
Br
Br B
B
C
Sn
R
Me
N(CHMe2)2
N(CHMe2)2
B
Me
B
2− MeBBr2
H
−78 °C
H
C C
N(CHMe2)2
B
C
H
C
B
H
CHMe2
Me C R
C
R B
C Me
CHMe2
B
B C
Me2HC
C CHMe2
Me
R C
B
CHMe2
C Me
R B
B C
C
C
C
hν Me2HC
B
D
C C
R = Me, Ph, CHMe2
R
C C
B
R C
C
CHMe2 Me
R = Me, Cl
B
R R⬘ R⬘
C3H7
MeBBr2 BEt2
Et
R = Me, Et
Et
R
C C
B
Et
R⬘ R⬘
R = Me, Et; R⬘ = Et, n-C3H7, CHMe2, n-C4H9
C3H7
Et
B
BX3 X = Cl, Br, I
C
C X
Sn Et2B
E
MeBBr2
H
C
R, R⬘ = Me, Et
Me
2 Li+
BEt2
Et
B
R⬘ Et
C C
B
X C
R⬘
C Et
FIGURE 4-13 Routes to nido-2,3,4,5-C4B2H6 derivatives from heterocyclic compounds.
play a major role [82,248,250–252]. For example, in contrast to nido-2,3,4,5-R4C4B2R2 carboranes (4-44, R ¼ H or alkyl), the isolated form of R4C4B2F2 (prepared from reactions of BF with alkynes)[253] is a planar heterocycle (4-45), stabilized by attachment of electron-rich fluorines to boron [82]. Similar effects on the C4B2 cluster structure have been observed with other nucleophiles such as amino and ferrocenyl [250–252]. The influence can be subtle: addition of an amino group to one boron in nido-2,3,4,5-H4C4B2H2 stabilizes the carborane, but placing amines on both borons favors a classical structure via electron donation to the cage—a very rare example of an apparent electronically driven nonclassical-to-classical transformation [254].
4.5 6-Vertex open clusters
69
F R R
B R R
R B
C C
C
R C
C
C
R = H, Me
R
C
4-44
B C
R
R
B
4-45
F
R
Figure 4-12 illustrates several synthetic pathways to nido-C4B2 carboranes based on unsaturated hydrocarbons. The bicyclic intermediate in the reaction in Figure 4-12B has been isolated and crystallographically characterized [250], affording remarkable insights into an organoborane ! carborane molecular rearrangement. Other synthetic pathways to nido-C4B2 clusters, involving heterocyclic precursors, are shown in Figures 4-13 A [255,256], B [254], C [257], D [258], and E [259]. The contrast between the various preparative approaches to this carborane system and those used for other small carboranes (most of which are based on boron hydrides) is quite striking, and makes C4B2 clusters somewhat more accessible to organic chemists than are the other lower carboranes.
4.5.8.2 Structure and properties The pentagonal-pyramidal geometry of the nido-2,3,4,5-C4B2 cage is supported by a microwave analysis of parent C4B2H6 [260], a gas-phase electron-diffraction study of Me4C4B2Me2 [261], and X-ray crystallographic and multinuclear NMR investigations of several derivatives (Table 4-10), in all of which boron occupies the high-coordinate apex position. Isomers with carbon in the apex, though possible, would be energetically unfavorable (see Chapter 2) and have not been observed. Most of the known chemistry of this carborane system involves highly C-alkylated derivatives, with B-halogenated compounds [e.g., Figure 4-13] playing a major role. Peralkylated nido-tetracarbahexaboranes react with BBr3 or BI3 to afford 2,3,4,5-R4C4B2R-6-X (R ¼ Me, Et; X ¼ Br, I), quantitatively replacing the B(6)-alkyl group with a halogen atom [262,263]. Treatment of the peralkyl derivatives with Liþ Et3BH generates R4C4B2R-6-H, while excess BBr3 attacks the 5C(BBr2)2Et [262]. Palladium-catalyzed reaccage, degrading it to the tetrakis(dibromoboryl)diethylalkene, Et(BBr2)2C5 CPh with arylzinc reagents (generated in situ from aryl bromides) give 2,3,4,5-Et4C4B2-1-Br-6tions of Et4C4B2-1-I-6-C CCMe3 is treated CPh, which has been crystallographically characterized [264]. However, when Et4C4B2-1-I-6-C C CCMe3)2. The reaction of Et4C4B2-1with PhZnCl and Pd(PPh3)4, the main product is a dialkynyl species, Et4C4B2(C C-p-tolyl with Pd(PPh3)4 and a dialkylzinc reagent affords the apically-linked biscarborane 4-46 [264]. I-6-C Et
Et
4-46
Et
C B
Me-p-C6H4C≡C
C C B
C Et
Et
Et
C C B Et
C
C Et B C≡C-p-C6H4Me
Selective halogen exchange can also be accomplished at the basal boron, provided that the heavier halogen is the leaving atom (with the weaker B2 2X bond). Thus, the diiodo compound 2,3,4, 5-Et4C4B2-1,6-I2 on treatment with excess AgF undergoes selective replacement of the basal iodine with fluorine, forming 2,3,4,5-Et4C4B21-I-6-F in 56% yield; no apically fluorinated product is obtained [264]. The B(2)-brominated compound 2,3,4,5-Me2Et2C4B2Et-2-Br reacts with trimethyl- or triphenylstannyl lithium in THF to give B(2)-SnR3 products. The SnPh3 derivative interacts with electrophiles such as TiCl4 (slowly), SnCl4,
70
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
Br2, and BBr3, progressively cleaving the Sn-C bonds to afford Me2Et2C4B2Et-2-SnPhnX3n (X ¼ Cl, Br; n ¼ 0-3) and ultimately giving the SnX3 derivative [263]. Similarly, reactions of B(2)-Br derivatives with LiPPh2 yield the corresponding B(2)-PPh2 phosphanes, which can be oxidiatively converted to B(2)-PXPh2 products in which X is O, S, or Se [265]. Me2Et2C4B2Et-2-PPh2 reacts with MeI, removing the phosphane group to give the B(2)-I derivative; treatment with BH3THF or W(CO)5 generates Me2Et2C4B2Et-2-PPh2Y complexes [Y ¼ BH3 or W(CO)5] [265]. The B,B0 -diiodo species 2,3,4,5-Et4C4B2I2 is generally reactive toward nucleophiles, allowing the synthesis of B(2)-X CR, PPh2, FeCp(CO)2, and Et [266]. In most such reactions, the apical B(1)-I bond is inert, derivatives in which X is C CPh, it can be replaced by a C CSiMe3 group via Pd0-catalyzed Negishi cross-coupling but in 2,3,4,5-Et4C4B21-I-6-C CZnCl, affording 2,3,4,5-Et4C4B2(C CPh)2. with Me3SiC Nucleophilic substitution on C4B2 carboranes that have reactive groups on boron can be used to generate linked clusters such as 4-46, with further examples (all crystallographically characterized) shown in Figure 4-14 [266,267]. In sharp contrast to the isoelectronic and isostructural 2,3-C2B4H62 and 2,3,4-C3B3H6 anions and their derivatives, which readily form face-bonded (Z5-coordinated) sandwich complexes with metal and metalloid electron-acceptors, as described above, no such compounds featuring nido-C4B2 carborane ligands have been prepared (complexes of classical 1,4-diborabenzene C4B2 rings are known, however [251,268]). It is not clear to what extent this reflects intrinsic inertness of C4B2 ligands toward metal ions—which could be a consequence of depletion of electron density in the ring carbons—or whether it is simply a case of limited experimental study in this area having failed to turn up such species. However, the exo-polyhedral C4 chain in C4B2 benzocarborane derivatives (Figure 4-12C) does coordinate to metals, forming Z5-bound complexes, as depicted in Figure 4-15 [269]. Alkynyl-substituted C4B2 carboranes have been used to construct B(1)- and B(6)-bonded (Z4-cyclobutadiene)cobalt complexes, as illustrated in Figure 4-16 [270].
O C
O C
X
B
B R⬘ R⬘
Co
R C C
X
O C
C
R⬘ C
C
B
C
Ph R⬘
C O
C O
Co2(CO)8
C O
C
S Fe (CO)3
Fe(CO)3
N Fe (CO)3
X = Me, Br; R = Me, n-C4H9 R⬘ = Me, Et
B
Y
C
LiCB10H10CH X = Y = I; R = R⬘ = Et
R
ClZnC6H4C6H4ZnCl X = Y = I; R = R⬘ = Et
H
R C C R
B
B B
B C
B B B
X
X
C
B
C
N
(CO)3 Fe
B
R C C
R⬘
X
−
X B
R⬘
R⬘
C R
X = I; Y = C≡CPh; R = R⬘ = Et
R⬘ C
C
Co
R
S
R C C
B B
B B = BH
B
B R⬘ B R⬘
R C C
C
B
C R
FIGURE 4-14 Syntheses of linked clusters from B-functionalized C4B2 carboranes.
R B
C C R
C C
R⬘ R⬘
4.5 6-Vertex open clusters CMe3 B C C
OC
C O
SiMe3 C O CMe3
CMe3
B
B C C
C H
C
Fe Fe(CO)3(C8H14)2
CMe3
B
B
CMe3 C H
C C
Cp*Co(C2H4)2
C
B
CMe3 C H
C
Co
SiMe3
SiMe3 CpRh(C2H4)2
CMe3 B C C
B
CMe3 C H
C
Rh
SiMe3
FIGURE 4-15 Synthesis of benzo-tetracarbahexaborane metal sandwich complexes.
Et Et
I
C C
C B Et Et
Et C C
C
B
CpCo(C2H4)2 C
C
Co
R
C Et
R = CMe3, SiMe3
I B
Et C
Et
C C
B
C Et
Et
FIGURE 4-16 Synthesis of bis(6-tetracarbahexaborane)(Z4-cyclobutadiene)cobalt complexes.
B
C B Et
I
Et
71
72
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
4.5.9 Nido-2,3,4,5,6-C5BH6þ 4.5.9.1 Synthesis The nido-pentacarbahexaborane(6) cation (Figure 4-3), a cluster type that is commonly designated as boranediyl or borylene but also qualifies as a carborane, is unknown in parent form but can be stabilized by coordination of boron to electron-acceptor groups. At present, all the characterized species are C-permethylated, and many have a halogen substituent on boron (Table 4-11). Several derivatives have been prepared from cyclopentadienyl precursors (Figure 4-17) [271–274], although other routes are known. For example, reaction of the nido-(C5Me5)Snþ cation with BI3 exchanges boron for tin and forms nido-(C5Me5)BIþ [275], while treatment of decamethylsilocene [(C5Me5)2Si] with BCl3 or BBr3 affords the nido-2,3,4,5,6-Me5C5B-SiX2Cp*þ cation, with a SiX2Cp* group coordinated to boron [276]. A side product obtained in the reaction with BBr3 is arachno2SiBr2Cp* (4-47), a neutral species whose X-ray diffraction analysis revealed an open structure, as (Br2Cp*Si)Me5C5B2 2BCl3 [271]. shown. Similarly, diboron tetrachloride (B2Cl4) reacts with (C5Me5)2Si or C5Me5SiMe3 to give neutral Me5C5B2 SiBr2Cp* B
4-47
Me Me
C C
C
Me Me
C
C SiBr2Cp*
Me
TABLE 4-11 C5BHxþ Derivatives Synthesis and Characterization Compound
Information
References
nido-2,3,4,5,6-Me5C5B-SiX2Cp*þ (X ¼ Cl, Br) nido-2,3,4,5,6-Me5C5B-Fe(CO)4
X S, S, S, S, S, S, S, S,
[278] [271] [272] [273] [273] [273,275] [274] [276] [381]
Arachno-C5BH8þ derivatives arachno-(Br2Cp*Si)Me5C5B-SiBr2Cp*þ
S, X, H, B, C, Si, MS
[276]
extended Hu¨ckel SCF: aromaticity; bending of CH hydrogens toward B DFT DFT: singlet ground state and lowest triplet excited state; bond distances and angles DFT: singlet ground state and lowest triplet excited state; bond distances and angles
[34] [277] [279] [280]
þ
Nido-C5BH6 derivatives nido-2,3,4,5,6-Me5C5B-Brþ nido-2,3,4,5,6-Me5C5B-BXCl2 (X ¼ SiCl3, Cl) nido-2,3,4,5,6-Me5C5B-C5Me5þ nido-2,3,4,5,6-Me5C5BClþ nido-2,3,4,5,6-Me5C5BBrþ nido-2,3,4,5,6-Me5C5BIþ
Theoretical Studies Molecular and electronic structure calculations nido-H5C5BHþ nido-2,3,4,5,6-R5C5B-1-BCl3 (R ¼ H, Me) nido-2,3,4,5,6-Me5C5B nido-2,3,4,5,6-Me5C5B-Fe(CO)4 X, X-ray diffraction; H, 1H NMR; B,
11
B NMR; C,
13
X(Cl), H, B, C, MS H, B B B B H, B, C, MS, conductivity X(Cl), H, B, C(Br), MS(Br) X, H, B, C, IR
C NMR; P,
31
P NMR; Si,
29
[280]
Si NMR; IR, infrared data; MS, mass spectroscopic data.
4.6 6-Vertex closo clusters I
Me Me
73
+
B Me GeMe3
BI3 −Me3GeI
Me Me
Me
Me C C
C
C
BI4− Me
C
Me
Me X
Me Me
+
B Me BX2
Me Me
BX3
Me
X = Cl, Br, I Me
Me C C
C
C
Me
BX4−
C Me
FIGURE 4-17 Synthesis of nido-Me5C5BXþ salts.
4.5.9.2 Structure and properties The nido-C5B cage geometry is confirmed by X-ray structures and multinuclear NMR data on several species (Table 4-11). In accord with prediction from theory [277], the methyl groups in the Me5C5BBrþ cation are bent out of the C5 plane toward the boron atom [278]. Stabilization of the nido-R5C5B unit by Lewis acids coordinated to boron has been explored in several theoretical studies [34,277,279,280]; DFT calculations show that the carborane functions as a two-electron s donor toward an Fe(CO)4 group [280]. Beyond their synthesis and characterization, the chemistry of nido-R5C5B clusters has been little studied.
4.6 6-VERTEX CLOSO CLUSTERS 4.6.1 1-CB5H7 and 1-CB5H6 4.6.1.1 Synthesis Parent monocarbahexaborane(7) has been obtained in a variety of high-energy reactions involving organoboranes or borane-hydrocarbon mixtures, including electric discharge or flow pyrolysis of 1-MeB5H8 vapor [123,130,281], pyrolysis of alkenylpentaboranes [128], flash thermolysis of 1,2-(Me3Si)2B5H7 [70], and the reaction of B5H9 with carbon vapor [282]. C- and B-methyl derivatives have been similarly prepared by pyrolyzing 1-MeB5H8 [123,128], 1,2(Me3Si)2B5H7 [70], or 1-(Me3Si)B5H7-m-BMe2 [283]. As in other lower carborane syntheses (Chapter 3), complex product mixtures are obtained with low yields of individual products, and efficient routes to monocarbahexaboranes are not yet available.
4.6.1.2 Structure and properties The octahedral CB5 cage geometry (Figure 1-3, top right), was recognized early on [281] from NMR and other spectroscopic data (Table 4-12) and by analogy with the isoelectronic 14-electron clusters C2B4H6 and B6H62. Gas-phase electron diffraction [284] and microwave [285] studies of CB5H7 have confirmed this structure and locate the “extra” hydrogen over a triangular B3 face. Variable-temperature 11B NMR data reveal that this hydrogen atom tautomerizes rapidly around the molecule, presumably using the four equivalent triboron faces [286] (although one study concluded
74
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
TABLE 4-12 Closo-1-CB5H7 Derivatives Synthesis and Characterization Compound
Information
References
Parent
S, H, B, IR, MS S, MS S, H, B, MS H (coupling) B (variable temp) ED Microwave S, B, IR, MS S, H, B, MS S, MS S, H, B, IR, MS
[281] [282] [123,130] [94,95] [286] [284] [285] [283] [123] [128] [70]
6-Me 1-Me 2(4)-Me (mixture)
Theoretical Studies Molecular and electronic structure calculations Parent Structure, dipole moment, ionization potential Isomerization calculations Parent n-Me (n ¼ 1, 2, 4, 6) CB5H6 CB5H6 (all isomers) NMR calculations Parent CB5H6 2-R (R ¼ H, Me) 1-R (R ¼ H, Me) B-Me (3 isomers) Reactivity calculations Parent CB5H7, CB5H6 CB5H6
[333]
Isomer stabilities Isomer stabilities EI [energy indexes]; stabilities ab initio, DFT ab initio
[135] [135] [326] [382] [47]
11
B, IGLO H, B, C spin-spin coupling (DFT) 11 B-11B, 11B-13C, 13C-13C spin-spin couling (DFT) 13 C, IGLO 11 B, GIAO, IGLO 11 B, GIAO, IGLO
[12] [37] [93] [96] [288] [288]
Protonation Bridge H tautomerism Face-capping Fluorination mechanism; formation of CB5H5F
[305] [287,333,383,384] [385] [289]
S ¼ synthesis; X ¼ X-ray diffraction; H ¼ 1H NMR; B ¼ ED ¼ electron diffraction.
B NMR; C ¼
11
C NMR; IR ¼ infrared data; MS ¼ mass spectroscopic data;
13
4.6 6-Vertex closo clusters
75
that the tautomerizing proton is mainly restricted to the equatorial B2 2B edges [287]). The weakly bound bridging proton is easily removed by NaH or Et3N to generate the CB5H6 anion [282,288], which is the smallest known member of the closo-CBn1Hn carborane anion family. As one of the smallest and structurally simplest carboranes, CB5H7 has been a subject of many theoretical investigations (Table 4-12) centered on proton scrambling, NMR shifts, and (in substituted derivatives) isomer stabilities. Theory is well ahead of experiment with respect to this carborane; for example, while the mechanism of fluorination of the anion has been explored computationally [289], there are as yet no reported experimental studies of fluorination (or halogenation of any type) on this system.
4.6.2 1,2- and 1,6-C2B4H6 Two isomers of closo-dicarbahexaborane(6) having an octahedral cage structure are possible, and both are known; 1,2and 1,6-C2B4H6 (Figure 1-3, top row) were among the first carboranes to be discovered in the early work on alkyneborane reactions in the late 1950s, as described in Chapter 3. As with all lower carboranes, there are no truly efficient, high-yield routes to the parent compounds, and the experimental generation and study of the unsubstituted species (Tables 4-13 and 4-14) have been confined to the very few laboratories having access to small boron hydrides such as B5H9. On the other hand, the C2B4H6 isomers are attractive to theoreticians owing to their simple compositions and small-cage structures.
TABLE 4-13 1,6-C2B4H6 Derivatives Synthesis and Characterization Compound
Information
References
Parent
S (B5H9þ C2H2 flow system) S (pyrolysis of alkylboron hydrides), H S (B2H6þ C2H2 electric discharge), MS, IR S (electric discharge of nido-2, 3-C2B4H8) S (photolysis of nido-2,3-C2B4H8) S (high yield, from nido-2,3-C2B4H8) S (gas phase flash photolysis of B4H10þ C2H2) S (B4H10/B2H6þ C2H2 flash reactions) S (electric discharge of B5H9þ C2H2), MS (detailed), H, B, IR H, B, MS S (B4H10/B2H6þ C2H2 flash reactions) S (gas phase flash photolysis) S (B2H6þ C2H2 electric discharge), H, B, IR, MS S (pyrolysis of nido-2,3-C2B4H8), H, B, MS S (pyrolysis of alkylboron hydrides), H S, B, IR, MS X, ED S (pyrolysis of nido-2,3-C2B4H8) H, B, MS S (oxidation of nido-2,4-Et2C2B4HEt4), H, B, C S, H, B, IR, MS S, H, B, IR, MS
[54] [123] [50] [57] [55] [56] [53] [51,52] [290]
1-Me, 2-Me Me, Me2 derivatives 2-Me 1,6-Me2
1,6-(SiMe3)2 1-C3H7 1,2,3,4,5,6-Et6 2-(cis-2-but-2-enyl) 2,3/2, 4-(cis-2-but-2-enyl)2 isomers
[139] [51,52] [53] [50] [139] [123] [306] [295] [139] [217] [69] [69] Continued
76
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
TABLE 4-13 1,6-C2B4H6 Derivatives—Cont’d Synthesis and Characterization Compound
Information
References
2,3,4-(cis-2-but-2-enyl)3 2,3,4, 5-(cis-2-but-2-enyl)4 1,6-Li2 1,6-(SiMe3)2 1-SiMe36-R (R ¼ n-C4H9, CMe3) 2-CH2SiMe2Cl 2-Cl
S, H, B, IR, MS S, H, B, IR, MS S S (rearrangement of 1,2 isomer), ED S, C, I, MS S, H, B, MS S (photolysis), H, IR, VP, MS S (AlCl3), H, B, MS microwave S, H, B, MS S, H, B, C, Cl, IR
[69] [69] [306] [295] [152] [71] [308] [309] [386] [309] [155]
XPS, B, IR, MS XPS, B, IR, MS S, B, MS S MS S, H, B S, B, VP, MS S, H, B, IR, MS S, H, B, MS S, H, B, IR, MS S (improved) S, IR, H, B, MS S (gas phase pyrolysis of C2B4H6)
[310] [310] [306] [306] [312] [307] [68] [116] [112] [110] [113] [387]
H (coupling) B (NMR-IR correlations) C C 11 B, 10B; B2 2B coupling
[94,95] [103] [96,97] [96] [99]
Raman, IR (detailed) Raman, IR MS MS (negative ion) ED UV photoelectron spectra ion cyclotron resonance, Hþ affinity CH bond polarity, i, C(d and JCH; comparison with other carboranes
[388] [103] [133] [102] [73,389] [105] [390] [108]
2,4-Cl2 H2C2B4Cl4 (probable classical structure [294]) 2-X (X ¼ Cl, Br, I) 2,4-X2 (X ¼ Cl, Br, I) 1-Br-6-Me 1-I-6-Me 2-BBr2 2-I 2-SH 2-(10 ,20 -B2H5) 1,5-C2B3H41,6-C2B4H5 isomers 2-(20 -1,5-C2B3H4) (improved) 2:20 -(1,6-C2B4H5)2 (C2B4H2)n nanoparticles Detailed NMR Studies Parent
2-Cl, 2,4-Cl2 2-(20 -1,5-C2B3H4) Other Experimental Studies Parent
Continued
4.6 6-Vertex closo clusters
77
TABLE 4-13 1,6-C2B4H6 Derivatives—Cont’d Synthesis and Characterization Compound
Information
References
2-X (X ¼ Cl, Br, I)
XPS: binding energies Metal insertion and polyhedral expansion Reaction with BMe3 Reaction with Me3N Reactions with Lewis bases Catalytic reactions with alkynes Competitive electrophilic halogenation, alkylation Ignition mechanism Ignition in H2O vapor—kinetics Ignition promoted by isopropyl nitrate: kinetics Pyrolysis, mechanism of clustering High temperature oxidation Cage-opening reaction with F Hydrolysis in CH3OH Reaction with atomic S Hg-sensitized cophotolysis with 1, 5-C2B3H5 Competitive electrophilic halogenation, alkylation
[106,107] [118] [204] [182] [120] [69] [311] [391] [299] [300] [301] [302] [213] [114] [68] [112] [311]
Theoretical Studies Molecular and electronic structure calculations Parent Population analysis Isomer stabilities (SCF, DFT)
CSiMe3, C N, 1,6-R2 (R ¼ C O, N N) C 1,6-(SiMe3)2 (formation from 1,2 isomer)
Fractional 3-center bonds Electronic structure Localized MOs Geometry (ab initio) Cage structure, dipole moment, ionization potential, heat of formation (AM1) Charge distribution (CNDO) Charge distribution (EHMO) Heat of formation C–H bond length compared with halomethanes Energy indexes, isomer stabilities XPS: binding energies (CNDO) Vibrational modes and structure Second-order NLO properties Y Hybridization, bond order, comparison with X derivatives of other clusters; electronic cage-substituent interactions Geometry (ab initio)
[337,376] [78,92,324,325,327– 330,332,382,392] [372] [79,331] [75,79,334,374,393] [11,47,77,81] [333] [330,394] [395] [396] [336] [326] [107] [397] [328] [293]
[295] Continued
78
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
TABLE 4-13 1,6-C2B4H6 Derivatives—Cont’d Synthesis and Characterization Compound
Information
References
classical C2B4H6 2(m, terminal)-BH4 Nitro- and amino-substituted derivatives
geometry ab initio, linked carboranes DFT, first hyperpolarizability (b)
[82] [304] [398]
Cage rearrangement from 1,2-C2B4 clusters Rearrangement mechanism
[82,225,296– 298,340,399,400] [296]
Isomerization energy (DFT)
[83]
B (IGLO) C (IGLO) B, C (IGLO) B, C, H spin-spin coupling (DFT) 11 B-11B, 11B-13C, 13C-13C spin-spin coupling (DFT) aromatic solvent-induced 1H NMR shifts: correlation with Hþ charges (PRDDO) H (solvent shifts) B-H coupling C (IGLO) 11 B shifts B (IGLO, GIAO)
[12] [96] [11] [37] [93] [101]
Alkyne incorporation Protonation to form 1, 6-C2B4H7þ Combustion mechanism
[230] [119,305] [303]
Isomerization calculations Parent
Parent and classical NMR calculations Parent
2-Cl, 2,4-Cl2 2-X (X ¼ H, F, Cl, Br, I, Me “classical” H2C2B4Cl4 Reactivity calculations Parent
[175] [100] [96] [401] [294]
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; 2d, two-dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; UV, UV-visible data; VP, vapor pressure; XPS, X-ray photoelectron spectra; ED, electron diffraction data.
4.6.2.1 Synthesis The parent 1,2 and/or 1,6 isomers have been obtained via three main routes: (1) reactions of small boranes (usually B2H6, B4H10, or B5H9) with alkynes under high-energy conditions such as electric discharge [50,290], flow pyrolysis [54], and flash photolysis [51–53]; (2) from alkyl derivatives of B5H9 [123]; or (3) from nido-2,3-C2B4H8 via electric discharge [57], pyrolysis [56,139], or photolysis [55]. Many of these reactions also yield C- and B-alkyl derivatives of 1,6-C2B4H6 (Table 4-14), but not the 1,2 isomer, as the latter species rearrange to thermodynamically favored 1,6C2B4 clusters under high-energy reaction conditions. For the synthesis of substituted derivatives, several relatively efficient approaches are available. 1,2-(Me3Si)2C2B4H4 has been prepared in 94% yield via the reaction of the 7-vertex stannacarborane closo-1,2,3-Sn[(Me3Si)2C2B4H4] with PtCl2, probably via formation of an intermediate closo-Pt[(Me3Si)2C2B4H4] cluster (not isolated), which decomposes
4.6 6-Vertex closo clusters
79
TABLE 4-14 1,2-C2B4H6 Derivatives Synthesis and Characterization Compound
Information
References
Parent
S (electric discharge of nido-2,3-C2B4H8), H, B, IR S (photolysis of nido-2,3-C2B4H8) S (pyrolysis of alkylboron hydrides), H S (electric discharge of B5H9þ C2H2 mixture), MS (detailed), H, B, IR S (closo-1,2,3-Sn[(Me3Si)2C2B4H4] þ PtCl2), H, B, C, Si, IR S [oxidation of nido-2,3-(Me3Si)RC2B4H42], H, B, C, IR
[57] [55] [123] [290]
S [oxidation of nido-2,3-(Me3Si)RC2B4H42], H, B, C, IR, MS
[152]
S [oxidation of nido-2,3-(Me3Si)2C2B4H3(i-C4H9)2], H, B, C, IR X, ED
[195] [295]
H(coupling) C, IGLO
[94,95] [96]
microwave (cage structure) ED MS MS (negative ion) He photoelectron spectra ED, thermal rearrangement K þ ErCl3 ! (THF)2Kþ Er[(SiMe3)2C2B4H4]2 reductive cage-opening MS (detailed)
[402] [284] [133] [102] [106] [295] [403]
1-SiMe32-R (R ¼ SiMe3, Me, H) 1-SiMe32-R (R ¼ n-C4H9, CMe3) 1,2-(SiMe3)23-i-C4H9 1,2-(SiMe3)2 Detailed NMR Studies Parent
Other Experimental Studies Parent
1,2-(SiMe3)2
1-CR5 5CH2 (R ¼ H, Me)
Theoretical Studies Molecular and electronic structure calculations Parent isomers EI (energy indexes), stabilities classical versus nonclassical C2B4H6 geometry Parent isomer stabilities (DFT) ab initio SCF C–H bond length compared with halomethanes Charge distribution (CNDO) Cage structure, dipole moment, ionization potential, heat of formation (AM1) Dipole moment, ionization potential, heat of formation Electronic structure
[291] [166]
[404]
[326] [82] [329,382] [47,77,327] [78,332,392] [336] [330] [333] [333] [331,337,376,405] Continued
80
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
TABLE 4-14 1,2-C2B4H6 Derivatives—Cont’d Synthesis and Characterization Compound
Information
References
1,2-R2 (R ¼ H, CH3, NH2, OH, F, SiH3, PH3, SH, Cl) 1,2-(R)2 (R ¼ H, CH3, NH2, OH, F, SiH3, PH3, SH, Cl) 1-CH 1,2-R2 (R ¼ H, OH, SH, NH2, PH2, CH3, SiH3) “classical” H2C2B4Cl4 1,2-(SiMe3)2
Geometry, ab initio electron correlation effects Isomer stabilities Localized MOs DFT; unusually long C2 2C bond distances; influence of substituents on C2 2C length DFT; unusually long C2 2C bond distances; influence of substituents on C2 2C length Carbene DFT optimized structure of singlet and triplet states of neutral species and dianions formed by Hþ removal from R groups IGLO, GIAO,B(Cl) ab initio
[11] [324,325,330] [334,373,374,393] [406]
Isomerization calculations Parent
Cage rearrangement
Parent and classical
3d Hu¨ckel; isomers; rearrangement mechanism Isomerization (DFT)
[82,225,296– 298,340,399,400] [407] [83]
B, C, H spin-spin coupling (DFT) B2 211B, 11B2 213C, 13C2 213C spin-spin coupling (DFT) B2 2H coupling Calculated 11B and 13C NMR shifts (IGLO)
[37] [93] [100] [11,12]
Protonation to form 1,2-C2B4H7þ Alkyne incorporation
[305] [230]
NMR calculations Parent
11
Reactivity calculations Parent
[406] [339] [292] [294] [295]
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; IR, infrared data; MS, mass spectroscopic data; UV, UV-visible data; ED, electron diffraction.
immediately to give the bis(trimethylsilyl)carborane and platinum metal [291]. Metal-promoted oxidation of dilithium salts of nido-2,3-(Me3Si)RC2B4H42 anions yields closo-1,2-(Me3Si)RC2B4H4 products (Figure 4-18) [152,166,195], together with smaller amounts of (Me3Si)2R2C4B8H8 oxidative fusion products (Chapter 11). The nido-to-closo conversion of a 6-vertex cluster via removal of two electrons nicely illustrates the prediction from theory (Chapter 2), as does the oxidation of Naþ nido-2,4-Et2C2B4Et4H with I2 to afford 1,6-Et2C2B4Et4 [217]. The introduction of substituents to 1,6-C2B4 clusters, and related chemistry, are described below.
4.6.2.2 Structure and physical properties The octahedral cluster geometry for 1,2- and 1,6-C2B4H6 is well established from microwave [402] and electron diffraction [73,284,389] studies of the parent compounds and 1,6-C2B4H5-2-Cl [386], supported by X-ray crystallographic and electron diffraction analyses of 1,2- and 1,6-(Me3Si)2C2B4H4 [295]. The C2B4 carboranes have been thoroughly
4.6 6-Vertex closo clusters
Me3Si R
C C
SiMe3
H
H
2−
B
2 Li+ B
B
Me3Si
n-C6H12 0 °C
H
B
C
B NiCl2
H
B
C
H
B
C
R
H
sealed tube H
250-255 °C
B
B H
R = Me, n-C4H9, CMe3, SiMe3
H
B
B
H
C
B
H
81
R
H
FIGURE 4-18 Synthesis of closo-1,2-C2B4 clusters via oxidative closure of nido-2,3-C2B4 dianions.
investigated via mass spectrometric, NMR, IR, UV, Raman, X-ray photoelectron, and other studies on numerous derivatives (Tables 4-13 and 4-14). The tables also list extensive theoretical work on these systems, the results of which correlate closely with experimental findings on electronic structure and charge distribution, proton affinity and other reactivity, cage rearrangement, NMR chemical shifts and coupling constants, and vibrational modes. For both isomers, the observation of high antipodal 1H-1H coupling implies significant bonding involving s orbitals in the center of the cages [94]. Of particular interest is a DFT study of 1,2-R2C2B4H6 derivatives in which R is H, OH, NH2, PH3, SH, CH3, or SiH3; it was found that triplet-state dianions obtained by deprotonation of the R groups undergo major structural change, adopting a nido-C2B4 pyramidal geometry that has an apex BH group [292]. The parent C2B4H6 clusters with their five-coordinate carbon atoms are clearly nonclassical molecules, in contrast to 1,5-C2B3H5, which (as discussed earlier) has been variously treated as classical, nonclassical, or fluctuating between the two. However, electronically active substituents can profoundly influence the cage geometry [293]; an example is the species H2C2B4Cl4 [155] for which NMR evidence and theory point to an open classical structure [294]. The thermal rearrangement of 1,2- to 1,6-C2B4H6, which occurs quantitatively at 250 C [57], has been explored computationally by many workers (see Table 4-13) and is driven by mutual repulsion of the negatively charged carbon atoms. The conversion of 1,2- to 1,6-(Me3Si)RC2B4H4 (R ¼ SiMe3, n-C4H9, CMe3) also requires temperatures of at least 250 C [152,295], which is surprising because the presence of bulky groups on the cage carbon atoms would normally be expected to lower the activation energy for skeletal rearrangement, as is found, for example, in the icosahedral carboranes (Section 10.3). As is generally the case with carborane cage isomerizations (particularly the icosahedral systems, discussed in Chapter 10), it is probable that more than one mechanism is operative although a particular pathway may dominate [296]. For the C2B4H6 system, PRDDO (partial retention of diatomic differential overlap) calculations by Lipscomb and coworkers [82] and a later ab initio study by McKee [225] suggest a mechanism involving a classical benzvalene-like intermediate (Figure 4-19), which invokes a modification of the diamond-square-diamond (dsd) sequence originally proposed by C
B
C
B
B
B
B
B B
C
C
B
B C
B
B
B C
FIGURE 4-19 Simplified representation of the calculated rearrangement of 1,2- to 1,6- C2B4H6 (hydrogens omitted).
82
CHAPTER 4 Small carboranes: Four- to six-vertex clusters
Hoffmann and Lipscomb [297,298]. Indirect experimental support for such an intermediate is given by the synthesis and isolation of a stable B4-tetrakis(triisopropylamino) derivative 4-30, mentioned earlier in this chapter.
4.6.2.3 Reactions of closo-1,2-C2B4 clusters Aside from its thermally induced conversion to the 1,6 system, reports of the chemical reactivity of the 1,2 isomer are sparse. Cage-opening of 1,2-(Me3Si)2C2B4H4 via treatment with alkali metals, as described earlier in Section 4.5, followed by reactions with transition metal or lanthanide salts, generate 1-M[2,4-(Me3Si)2C2B4H4]n2 or 1,2,4-LM[(Me3Si)2C2B4H4]n- “carbons-separated” sandwich complexes, in which L is a donor ligand such as TMEDA (Chapter 13). Calculations on a carbene-substituted species, 1,2-(HC)C2B4H5, show that it is an energy minimum, but full incorporation into the cage to form a 7-vertex C3B4 cluster is favored [339].
4.6.2.4 Reactions of closo-1,6-C2B4 clusters: Hydrolysis, methanolysis, and oxidation The parent carborane is significantly more stable and less reactive than 1,5-C2B3H5, but is degraded by water and methanol to form monoboron products such as B(OR)3 and MeB(OR)2 [114]. The processes occurring during pyrolysis and hightemperature oxidation and combustion have been investigated in detail [299–303], driven in part by interest in using volatile boranes and carboranes as fuels (see Chapter 1).
4.6.2.5 Polyhedral expansion Reductive cage-opening of closo-1,6-C2B4 carboranes to afford nido-2,4-C2B4 clusters—the reverse of oxidative closure—can be achieved with electron-donors, such as fluoride, amines, and phosphines, as described earlier in Section 4.5 and Figure 4-7 [120,182]. Thermal insertion of boron into parent 1,6-C2B4H6 is accomplished by heating with BMe3 at 550-600 C to form a dicarbaheptaborane, 2,4-C2B5H5Me2 [204]. Polyhedral cage expansion of parent 1,6C2B4H6 via direct reaction with transition metal reagents to afford 7-vertex 1,2,4-MC2B4 metallacarborane clusters [118] is discussed in Chapter 13.
4.6.2.6 Cage linkage As mentioned earlier in this chapter, co-pyrolysis of 1,6-C2B4H6 with 1,5-C2B3H5 [110], or Hg-sensitized co-photolysis of the same carboranes [112], yields the BB linked dicluster 2,20 -1,5-C2B3H410 ,60 -C2B4H5. Cage linkage can also be achieved via transition metal catalysis, as in the formation of the dimer 2,20 -(1,6-C2B4H5)2 in quantitative yield by 2B linkage at 25 C [113]. The same platinum reagent promotes the reaction PtBr2-assisted dehydrogenation and B2 of B2H6 with 1,6-C2B4H6 to form the C2B4H5-B2H5 linked species 4-48 [116]. This behavior contrasts with that of 1,5-C2B3H5, which on reaction with B2H6 undergoes cage expansion to form the 8-vertex carborane arachnoC2B6H12; the difference has been explained by ab initio calculations that show a lower activation energy for boron incorporation into the cage in C2B3H5 compared to C2B4H6 [304]. H
C H H
4-48 H
B
B C H
4.6.2.7 Introduction of substituents
H
B
B
H
B H H
B H
Protonation of the C2B4H6 cage to form closo-C2B4H7þ has been explored theoretically [119,305] and the calculated proton affinities are in agreement with experiment; for the 1,6-isomer, protonation is favored at the B2 2B edges [119]. Various approaches have been explored for attaching substituents to the polyhedral framework. As in the icosahedral C2B10H12
4.6 6-Vertex closo clusters
83
carboranes, the CH hydrogens in 1,6-C2B4H6 have protonic character and can be removed with n-butyllithium in ether/hexane to afford the C-mono- or C,C0 -dilithio clusters, from which the C,C0 -dimethyl and C-methyl-C0 -bromo derivatives can be prepared [306]. In comparison with 2,4-C2B5H7 and 1,2-C2B10H12 (Chapters 5 and 9, respectively), the reaction with n-butyllithium is slow, requiring 48 h for completion at room temperature. There is evidence that the monolithio compound in solution exists in equilibrium with the dilithio and parent carboranes [306]. 2C4 H9 Li
MeI
C2 B4 H6 ! Li2 C2 B4 H4 ! Me2 C2 B4 H4 C4 H9 Li
Br2
C2 B4 H6 ! LiC2 B4 H5 ! MeBrC2 B4 H4 Substitution at the boron vertexes in 1,6-C2B4 carboranes has been explored in several studies. Halogenation under Friedel-Crafts conditions yields the B-chloro, B-bromo, or B-iodo derivative [306–310], and has also been used to prepare the corresponding 2,4-dihalo compounds [310]. The B-chloro derivative can also be obtained by photolysis of nido-2,3C2B4H74-Cl or via the photolytic reaction of Cl2 with 1,6-C2B4H6 [308]. In Friedel-Crafts chlorination, there is some decomposition of the cage, apparently owing to reaction with HCl [309]. In the B-monochloro compound, the chlorine substituent enhances the reactivity of the opposite (antipodal) boron [B(4)] toward electrophiles [311], which explains, for example, why dichlorination occurs preferentially at the 2,4 boron positions (this effect is notably absent in 2,4C2B5H7, probably for reasons having to do with cluster symmetry). Competitive reactions of 1,6-C2B4H6 and 2,4C2B5H7 with Cl2/AlCl3 show that the larger carborane is the more reactive [311], apparently reflecting the more varied charge distribution in 2,4-C2B5H7 (which has three different types of BH units) compared to 1,6-C2B4H6, which has identical BH vertexes and, hence, no preferred site for initial attack. X-ray photoelectron spectra reveal strong p-interaction between the halogen substituents and the cluster molecular orbitals [310], exerting a strong influence on the reaction chemistry and NMR chemical shifts. Reaction of 1,6-C2B4H6 with BBr3 at 265 C gives the 2-BBr2 derivative [312]. Other approaches to cage substitution on 1,6-C2B4H6 include reaction with atomic sulfur to generate the 2-mercapto derivative [68] and the use of dicobalt catalysts of the type (R2CR0 )Co2(CO)6 (R,R0 ¼ H, Me, or Et) to prepare alkenylcarboranes via carborane-alkyne interactions [69]: ðMeCCMeÞCo2 ðCOÞ2
CR0 ! C2 B4 H5 -2-ðRC5 C2 B4 H6 þ RC 5CHR0 Þ 5CHR0 Þ2 þ other alkenylcarboranes þ C2 B4 H4 -2; 3=2; 4-ðRC5
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4.6 6-Vertex closo clusters [65] [66] [67] [68] [69] [70] [71] [72] [73] [74] [75] [76] [77] [78] [79] [80] [81] [82] [83] [84] [85] [86] [87] [88] [89] [90] [91] [92] [93] [94] [95] [96] [97] [98] [99] [100] [101] [102] [103] [104] [105] [106] [107] [108] [109] [110] [111] [112] [113] [114] [115]
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4.6 6-Vertex closo clusters [269] [270] [271] [272] [273] [274] [275] [276] [277] [278] [279] [280] [281] [282] [283] [284] [285] [286] [287] [288] [289] [290] [291] [292] [293] [294] [295] [296] [297] [298] [299] [300] [301] [302] [303] [304] [305] [306] [307] [308] [309] [310] [311] [312] [313] [314] [315] [316] [317] [318]
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90 [319] [320] [321] [322] [323] [324] [325] [326] [327] [328] [329] [330] [331] [332] [333] [334] [335] [336] [337] [338] [339] [340] [341] [342] [343] [344] [345] [346] [347] [348] [349] [350] [351] [352] [353] [354] [355] [356] [357] [358] [359] [360] [361] [362] [363] [364] [365] [366] [367] [368] [369]
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4.6 6-Vertex closo clusters [370] [371] [372] [373] [374] [375] [376] [377] [378] [379] [380] [381] [382] [383] [384] [385] [386] [387] [388] [389] [390] [391] [392] [393] [394] [395] [396] [397] [398] [399] [400] [401] [402] [403] [404] [405] [406] [407]
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Grimes, R. N.; Beer, D. C.; Sneddon, L. G.; Miller, V. R.; Weiss, R. Inorg. Chem. 1974, 13, 1138. Perkins, F. K.; Onellion, M.; Lee, S. W.; Li, D. Q.; Mazurowski, J.; Dowben, P. A. Appl. Phys. A Solids Surf. 1992, 54, 442. Marynick, D. S.; Lipscomb, W. N. J. Am. Chem. Soc. 1972, 94, 1748. Lipscomb, W. N. Accounts Chem. Res. 1973, 6, 257. Kleier, D. A.; Halgren, T. A.; Hall, J. H., Jr; Lipscomb, W. N. J. Chem. Phys. 1974, 61, 3905. Marynick, D. S.; Lipscomb, W. N. J. Am. Chem. Soc. 1972, 94, 8699. Cruickshank, D. W. J.; Chablo, A.; Eisenstein, M.; Reidy, P. L. Acta. Chem. Scand. A 1988, A42, 530. Maguire, J. A.; Ford, G. P.; Hosmane, N. S. Inorg. Chem. 1988, 27, 3354. Fox, M. A.; Hughes, A. K.; Johnson, A. L.; Paterson, M. A. J. J. Chem. Soc., Dalton. Trans. 2002, 2009. Franz, D. A.; Howard, J. W.; Grimes, R. N. J. Am. Chem. Soc. 1969, 91, 4010. Wrackmeyer, B. Z. Naturforsch. B 1982, 37, 412. Cowley, A. H.; Lomeli, V.; Voigt, A. J. Am. Chem. Soc. 1998, 120, 6401. Minyaev, R. M.; Minkin, V. I.; Gribanova, T. N.; Starikov, A. G. Russ. Chem. Bull. 2004, 53, 1159. Dewar, M. J. S.; McKee, M. L. J. Am. Chem. Soc. 1977, 99, 5231. McKee, M. L. J. Phys. Chem. 1989, 93, 3426. McKee, M. L.; Bu¨hl, M.; Charkin, O. P.; Schleyer, P. v. R. Inorg. Chem. 1993, 32, 4549. McKown, G. L.; Beaudet, R. A. Inorg. Chem. 1971, 10, 1350. Slutsky, V. G.; Tsyganov, S. A.; Severin, E. S.; Polenov, L. A. Propellants Explos Pyrotechnics 2005, 30, 303. Bragin, J.; Urevig, D. S.; Diem, M., Jr J. Raman Spectrosc. 1982, 12, 86. Mastryukov, V. S.; Dorofeeva, O. V.; Vilkov, L. V.; Golubinskii, A. V.; Zhigach, A. F.; Laptev, V.; et al. Zh. Strukt. Khim. 1975, 16, 171 [Russian]. Dixon, D. A. Inorg. Chem. 1980, 19, 593. Kalinin, V. N.; Mukoseev, Yu. K.; Petrunin, A. B.; Severin, E. S.; Slutskii, V. G.; Tereza, A. M.; et al. Fiz Goreniya Vzryva 1989, 25, 16 [Chem. Abstr. 111:80908a] [Russian]. Epstein, I.; Koetzle, T. F.; Stevens, R. M.; Lipscomb, W. N. J. Am. Chem. Soc. 1970, 92, 7019. Epstein, I. R.; Marynick, D. S.; Lipscomb, W. N. J. Am. Chem. Soc. 1973, 95, 1760. Semenov, S. G. Zh. Strukt. Khim. 1981, 22, 164 [Russian]. Mageswaran, R.; Fitzpatrick, N. J. J. Natl. Sci. Counc. Sri Lanka 1987, 15, 47 [Chem. Abstr. 112:7670h]. Gal’chenko, G. L.; Tamm, N.; Brykina, E. P.; Bekker, D. B.; Petrunin, A. B.; Zhigach, A. F. Khim 1985, 59, 2689 [Russian]. Jensen, J. O. Spectrochim Acta A Mol. Biomol. Spectros. 2004, 60, 57. Suponitsky, K. Y.; Timofeeva, T. V. Cent. Eur. J. Chem. 2003, 1, 1. Halgren, T. A.; Pepperberg, I. M.; Lipscomb, W. N. J. Am. Chem. Soc. 1975, 97, 1248. McKee, M. L. J. Am. Chem. Soc. 1988, 110, 5317. Fehlner, T. P.; Czech, P. T.; Fenske, R. F. Inorg. Chem. 1990, 29, 3103. Beaudet, R. A.; Poynter, R. L. J. Chem. Phys. 1970, 53, 1899. Wang, J.; Li, S.; Zheng, C.; Maguire, J. A.; Hosmane, N. S. Organometallics 2002, 21, 5149. Zhigach, A. F.; Petrunin, A. B.; Bochkarev, V. N.; Siryatskaya, V. N. Zh. Obshch. Khim. 1974, 44, 2787 [Russian]. Semenov, S. G. Vestn Leningr Univ Ser 4: Fiz Khim 1987, 97 [Chem. Abstr. 107:161980n] [Russian]. Oliva, J. M.; Allan, N. L.; Schleyer, P. v. R.; Vin˜as, C.; Teixidor, F. J. Am. Chem. Soc. 2005, 127, 13538. Gimarc, B. M.; Zhao, M. Inorg. Chem. 1996, 35, 825.
CHAPTER
5
Intermediate carboranes: Seven- to nine-vertex clusters
5.1 OVERVIEW Reactions of small boron hydrides with alkynes, which furnish a main synthetic route to the small-cage systems (Chapter 4), do not in general afford cages larger than C2B5 in more than trace amounts. Most carboranes in the 7- to 11-vertex range are prepared by degradation of larger clusters, by reductive cage-opening of closo species, or via insertion of boron or carbon into smaller cages (polyhedral expansion). With one major exception—the nido-C2 B9 H12 (dicarbollide) ions and related 11-vertex open-cage ions, whose characterized metal coordination complexes number in the thousands— the sub-icosahedral carboranes [1] have received relatively little attention and remain a fertile field for investigation. Since the electronic structures and reactivities of individual cage systems vary considerably, even among isomers, few generalizations are possible; each carborane is different and must be studied in its own right.
5.2 7-VERTEX OPEN CLUSTERS 5.2.1 Nido-C2B5H8
The only known and characterized open-cage 7-vertex carboranes are the nido-3,4-C2 B5 H8 anion and several C- and B-substituted derivatives (Table 5-1). The parent and C,C0 -diethyl species are prepared via cage-opening of closo-2,3R2C2B5H5 (R ¼ H or Et) by electron donors such as Et3BH, H, or phosphines, as illustrated in Figure 5-1 [2–5]. The nido cage structure (a dodecahedron with one missing vertex) of the C2 B5 H8 anion depicted in Figure 5-1 is supported by NMR and X-ray crystallographic data on 3,4-Et2 C2 B5 H6 and a zwitterion, 3,4-Et2 C2 B5 H6 -exo-65PMe2 [5]. CH2PMeþ, the latter species forming in the reaction of closo-2,3-Et2C2B5H5 with a phosphorus ylide, CH25
H
−
H
B H H
H H
C B
C
B
Li+Et3BH− H
B B
H
H
2,3-C2B5H7
H
C C H
H
B B
B B
H
B
H
H
C2B5H8−
FIGURE 5-1 Synthesis of the nido-3,4-C2 B5 H8 anion from closo-2,3-C2B5H7. Carboranes. DOI: 10.1016/B978-0-12-374170-7.00013-6 © 2011 Elsevier Inc. All rights reserved.
93
94
CHAPTER 5 Intermediate carboranes: Seven- to nine-vertex clusters
TABLE 5-1 Nido-C2B5H9 Derivatives Compound
Information
Synthesis and characterization 3,4-C2 B5 H8 3,4-Et2 C2 B5 H6
References
3,4-C2B5H7-6-TMEDA 3,4-Et2C2B5H6-6-PMe3 3,4-Et2 C2 B5 H5 -6-CH2PMe3
S, B S, X, H, B, IR S, B (2D), H, IR S, B, MS S, B (2D), H, IR S, X, B (2D), H, C, P, IR, MS
[2] [3] [4] [2] [4] [5]
Other experimental studies C2 B5 H7 2
Metal insertion
[6]
Theoretical studies Molecular and electronic structure discussion C2 B5 H8
[7]
NMR calculations 3,4-C2B5H7-6-NH3 3,4-C2 B5 H8 X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, infrared data; MS, mass spectroscopic data.
IGLO, ab initio IGLO, ab initio 13
C NMR; P,
[2] [2]
31
P NMR; 2D, two-dimensional (COSY) NMR; IR,
The chemistry of nido-C2B5 carboranes has been little explored. Treatment of parent C2 B5 H8 with excess tetramethylethylenediamine (TMEDA) forms an adduct, 3,4-C2B5H6-6-TMEDA, which in the presence of H2O loses boron to afford the known 6-vertex nido-carborane 2,3-C2B4H8 (Section 4.5 and Figure 1-3, top right) [2]. The reduction of closo-2,4-C2B5H7 with sodium naphthalenide in THF generates anionic species, presumed to include nido-C2 B5 H7 2 but not isolated, which combine with CoCl2 and NaC5H5 to give closo-cobaltacarboranes (Chapter 13) [6]. The electronic and geometric structures of nido-C2 B5 H8 ions and neutral 3,4-C2B5H7-6-NH3 have been investigated theoretically [2], and the structural and electronic similarity of this carborane to a known 7-vertex metallaborane, nido-2,7-(Ph3P)2(CO)Os(PhMe2P)ClPtB5H8, has been noted [7].
5.3 7-VERTEX CLOSO CLUSTERS 5.3.1 2-CB6H7
At present the known 7-vertex monocarbon carboranes (Table 5-2) are closo-2-CB6 H7 [8], its neutral protonated form 2-CB6H8 [9], and its 4,5-diiodo and C-phenyl derivatives [8,10,11] that are obtained by cage-degradation of nido-1-CB8H12 and arachno-4-PhCB8H13, respectively (Figure 5-2). An alternative route to parent CB6 H7 , also shown in Figure 5-2, involves conversion of nido-1-CB8H12 to the 5-CB8 H13 anion followed by double-deboronation of the latter carborane with phenylacetylene in refluxing THF [9]. Further discussion of 5-CB8 H13 and its associated chemistry appears below in Section 5.6. The pentagonal pyramidal geometry of 2-CB6 H7 with carbon occupying a low-coordinate equatorial vertex in a 16electron 7-vertex closo system, established from X-ray diffraction and NMR studies (Table 5-2), is in accord with the electron-counting and carbon-placement rules outlined in Chapter 2.
5.3 7-Vertex Closo clusters
95
TABLE 5-2 CB6 and CB7 Derivatives Compound
Information
References
Synthesis and characterization closo-2-CB6 H7 and 2-CB6H8 derivatives PPh4 þ CB6H5-4,5-I2 PPh4 þ CB6 H7 Csþ CB6 H7 NEt4 þ PhCB6 H6 CB6H8
S, S, S, S, S,
X, H, B, C H, B, C H, B (2D), C X, H, B, C H, B (2D), C
[8] [8] [9] [10,11] [9]
S, H, B S, B, C S, B (2D) S (from CB8 H12 ) S, H, B S, X, H, B S, X, H, B, C
[134] [139] [113] [8] [134] [134] [10,136]
B (2D), MS S, H, B (2D), C S (from arachno-C2 B7 H12 )
[113] [9] [114]
EI (energy indexes); stabilities Comparison of 7-vertex polyhedra and 7-coordinate metal complexes Ab initio Ab initio, IGLO NMR Ab initio Ab initio
[13] [12] [14] [143] [14] [111]
Ab initio; relative energies
[112]
11
[15] [16] [10,11] [9] [9]
closo-1-CB7 H8 derivatives CB7 H8
CB7H7I CB7 H6 I2 PhCB7 H7 arachno-CB7H13 derivatives 4-CB7H13 CB7H10Me2 Theoretical studies Molecular and electronic structure calculations CB6 H7
CB6 H7 (all isomers) CB7 H8 CB7 H8 (all isomers) nido-CB7 H8 3 , nido-CB7 H9 2 , nido-CB7 H10 , nido-CB7H11 nido-CB7 H8 3 NMR calculations 1-/2-CB6 H7 2-MeCB6 H6 NEt4 þ 2-PhCB6 H6 CB6 H7 , CB6H8 4-CB7H13
B-11B, 11B-13C, 13C-13C spin-spin coupling (DFT) B predictive rules DFT-GIAO B NMR and geometry optimization GIAO/NMR GIAO/NMR
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; 2D, two-dimensional (COSY) NMR; MS, mass spectroscopic data; DFT, density functional theory; GIAO, gauge including atomic orbital.
96
CHAPTER 5 Intermediate carboranes: Seven- to nine-vertex clusters BKBH
C B B
CKCH
B B
N2 + 5% O2
H
B
B
B
B B
Et3N
+
B
B
B B
B B B
B
1-CB7H8−
2-CB6H7−
1-CB8H12
+
B
−
C B B
C
B
B
H
B
−
B B
C
B
H
B
−
B
B
4-CB8H9−
H+ conc. H2SO4
NaBH4, THF
H+
Et3N
PhC≡CH
H2O
B
− 2 PhCH=CHBH2 C B
B B B
B
−
H
H
H
B B B
H+
B B
B B
B
H
H 2O C
B B
B H
C
+ B(OH)3 + H2
B
H
2-CB5H9
H+ B
H
H2O
H
2-CB6H8
H
B
H
H
5-CB8H13-
B
B
B
H
B
C
H
B
+ B(OH)3 + H2
B
4-CB7H13 H
C B B
−
B
H H
B
B B B
H
H
B
Et3N refluxing toluene
B PhCB8H13
B
B B
B
C
+ 1-PhCB7H7−
B 2-PhCB6H6−
FIGURE 5-2 Synthesis of 7-vertex closo-CB6 clusters and associated reactions.
Treatment of 2-CB6 H7 with I2 in the presence of triethylamine as an HI scavenger gives 2-CB6H5-4,5-I2 , which has been isolated as the PPh4 þ salt and crystallographically characterized [8]. The parent anion can be reversibly protonated in nonaqueous media to give 2-CB6H8, but acid hydrolysis of this species or 2-CB6 H7 results in deboronation to form the 6vertex nido-CB5H9 [9], a carborane described earlier in Chapter 4. Other reports of chemical reactivity on the closo-CB6 system have not appeared, but several theoretical investigations have explored its electronic structure and cluster architecture [9,12–14], B2 2B and B2 2C spin-spin coupling [15], and 11B NMR chemical shifts [9,11,14,16] density functional theory (DFT) calculations on spin-spin coupling constants show that 7-vertex CB6 H7 and C2B5H7 clusters exhibit antipodal interactions (though less so than in 5- and 6vertex carboranes), and that the effect is transmitted mainly through direct bonding between the antipodal nuclei, rather than via coupling through two intervening bonds in the cluster [15].
5.3.2 2,3-C2B5H7 Four isomers are possible for a closo-C2B5 pentagonal bipyramid, but only 2,3- and 2,4-C2B5H7 and their derivatives, in which both carbons occupy low-coordinate equatorial vertexes, are known. The 2,4 isomer with nonadjacent carbons (Figure 1-1, second row) is thermodynamically preferred, as expected (Chapter 2), and can be prepared in high-energy reactions of alkynes and small boranes (see below). closo-2,3-C2B5 clusters are rarely obtained in this way, and are
5.3 7-Vertex Closo clusters
97
generally prepared either by boron insertion into nido-C2B4 carboranes or by extraction of boron from nido-C2B6 cages. Parent 2,3-C2B5H7 can be obtained from nido-4,5-C2B6H10 via flow pyrolysis at 350 C in the gas phase or by heating B-substituted 4,5-C2B6H9-7-R derivatives in solution [2]: D
C2 B6 H9 R ! 2; 3-C2 B5 H7 þ RBH2
R ¼ H; n-hexyl; alkenyl
As later work demonstrated [17], the parent carborane is also generated on thermolysis of silyl derivatives of nido-2,3C2B4H8 [18], though at the time the 2,3-C2B5H7 product was incorrectly assigned as closo-C3B5H7. An early report of the synthesis of 2,3-C2B5H7 [19] was subsequently disproved [2]. C,C-Dimethyl and -diethyl derivatives of 2,3-C2B5H7 were first obtained in low yield from alkyne reactions of alkynes with B8H12, a very rare boron hydride [19,20]. A more viable synthesis employs boron insertion into nido-2,3-Et2C2B4H6 with Lewis base-BH3 adducts as illustrated in Figure 5-3, affording 50-60% yields [4,21]. 10B-labeling experiments prove that the inserted boron is supplied by the BH3 reagent. Insertions conducted with the nido-Et2 C2 B4 H4 2 dianion and PhBCl2 or MeBBr2 afford the apex-substituted 2,3Et2C2B5H4-1-R derivatives (R ¼ Ph or Me) [4]; the same dianion also reacts with BBr3 or BI3 to give apex-halogenated H
H
B
B Et
Et H
C B
C B H
H
B
Et
Et
Et3N•BH3
C
C
140 °C
H
H
B
B
H
B B H
H
H
FIGURE 5-3 Conversion of nido-2,3-Et2C2B4H6 to closo-2,3-C2B5H7 via boron insertion.
2,3-Et2C2B5H4-1-X species (X ¼ Br, I) and with various monoboron reagents to afford 2,3-Et2C2B5H4-1-R derivatives 5CEt(BCl2) forms (Table 5-3) [22]. Reaction of nido-Et2 C2 B4 H4 2 with the bis(dichloroboryl) reagent cis-(Cl2B)CEt5 the linked biscarborane 5-1 [22]. Et
Et
Et
C
C
Et
Et B
B
C
C C
C B
5-1
B B H
H
B
H
H
B
B H
H
Et
B H
B H
B-polyhalogenated derivatives of C2B5H7 can be prepared directly from boron halides, bypassing entirely the use of boron hydride or carborane starting materials. One example is the synthesis of nido-2,3,4,5-Et4C4B2I2 from 3-hexyne, BI3, and a Na/K alloy (see Section 4.5 and Figure 4-12A) that also affords the B-periodo compound 2,3-Et2C2B5I5 as a minor product [23]. Copyrolysis of B2X4 (X ¼ Cl, Br) with CCl4 or CBr4 at 340-420 C is reported to give perhalogenated carboranes, C2B5X7, whose cage structures were not determined [24]; however, the high temperatures employed would seem to favor formation of the carbon-separated 2,4 isomer. A few reactions of the 2,3-C2B5 system have been studied. Bromination of 2,3-Et2C2B5H5 with Br2/AlBr3 forms the 5-Br derivative exclusively [4]. As mentioned in Section 5.2, electron donors open the parent 2,3-C2B5H7 cage to give
98
CHAPTER 5 Intermediate carboranes: Seven- to nine-vertex clusters
TABLE 5-3 1,7-, 1,2-, and 2,3-closo-C2B5H7 Derivatives Compound Synthesis and characterization 2,3-C2B5H7 2,3-C2B5H6-1-butenyl 2,3-Me2C2B5H5 2,3-Et2C2B5H5 2,3-Et2C2B5H4-1-Ph 2,3-Et2C2B5H4-1-Me 2,3-Et2C2B5H4-5-Br 2,3-Et2C2B5H4-1-X (X ¼ Br, I) CR (R ¼ Ph, 2,3-Et2C2B5H4-1-C SiMe3, Me, CMe3) (2,3-Et2C2B5H4-1-)2C2Et2 2,3-Et2C2B5H41-C2RCo2(CO)6 (R ¼ Ph, SiMe3) 2,3-Et2C2B5I5 2,3-C2B5Cl7 (cage isomer not established) Other experimental studies 2,3-Et2C2B5H5
Information
References
S, H, B, C, MS S, H, B, C, MS CMe), B,H, MS S (from B5H9 þ MeC S (from nido-Et2C2B4H6), B, H, MS S, H, B (2D), IR, MS S, B, H, IR, MS S, H, B (2D), IR, MS S, B, H, IR, MS S, B, H, C, MS S, B, H, C, Si(SiMe3), MS
[2] [2] [19,20] [21] [4] [4] [4] [4] [22] [22]
S, S, S, S,
[22] [22] [23] [24]
B, H, C, MS B, H, C, MS MS B, MS
Isomerization to 2,4-Et2C2B5H5 Thermolysis; reactions with Br2, H, PMe3 Conversion to nido-Et2 C2 B5 H6
[21] [4] [3]
1,2/1,7/2,3-C2B5H7 1,2/1,7/2,3- and classical C2B5H7
Isomer stabilities Population analysis Charge distribution (CNDO) BH and CH capping; isomer stability MNDO, dipole moment Molecular conformation (MM3 force field) Electronic structure (ab initio) Stabilities, three-dimensional aromaticity (ab initio) Energy indexes; stabilities 11 B-11B, 11B-13C, 13C-13C spin-spin coupling (DFT) Second-order NLO properties Cage rearrangement Isomerization energy (DFT)
[207–210] [211] [212] [213] [214] [215] [216,217] [14] [13] [15] [209] [25–29] [218]
NMR calculations 2,3-C2B5H7 2,3-C2B5H6–1-Me 2,3-Me2C2B5H5
H, B, C (IGLO) H, B, C (IGLO) Rules for predicting
[2] [2] [16]
Theoretical studies Molecular and electronic structure calculations 1,2/1,7/2,31,2/1,7/2,3-C2B5H7 1,2/1,7/2,3-C2B5H7 1,2/1,7-C2B5H7 1,7/2,3-C2B5H7 2,3-C2B5H7 1,7-C2B5H7 1,2/1,7/2,3-C2B5H7
11
B shifts
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; 2D, two-dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; DFT, density functional theory; MNDO, modified neglect of differential overlap.
5.3 7-Vertex Closo clusters
99
species such as nido-3,4-C2 B5 H8 (Figure 5-1); 2,3-Et2C2B5H5 behaves similarly, reacting with hydride ion to give the nido-3,4-Et2 C2 B5 H8 ion, and with PMe3 to yield neutral nido-3,4-C2B5H5-6-PMe3 [4]. Treatment of 2,3-C2B5H7 with aqueous TMEDA extracts an apex BH unit to generate nido-2,3-C2B4H8 [2], effectively reversing the process shown in Figure 5-3 for the C,C-diethyl derivative. The isomerization of unsubstituted 2,3-C2B5H7 to 2,4-C2B5H7 (calculated [25] to be more stable by 24.2 kcal mol1) has not been reported, but the rearrangement of 2,3-Et2C2B5H5 to 2,4-Et2C2B5H5 occurs in high yield at 320 C in the gas phase [4,21]. Computational studies of the 2,3- to 2,4-C2B5 rearrangement mechanism [25–28] indicate that the putative 1,2-C2B5H7 isomer, which has never been seen experimentally in parent or substituted form, is an intermediate in this conversion. This finding supports a cooperative diamond-square-diamond (dsd) mechanism [29] involving one or more intermediates having open square faces (Figure 5-4). B B C
B B
B
C
C
B C
B C
C B
C
B C
B B
B C
B
C
B B
FIGURE 5-4 Calculated rearrangement of 2,3- to 2,4-C2B5 closo clusters via a dsd mechanism involving a non-isolable 1,2-C2B5 intermediate (simplified presentation).
Calculated activation energy barriers for these processes provide a rationale for the fact that the high-energy 1,2 and 1,7 isomers have not been isolated. Other theoretical investigations, cited in Table 5-3, have explored in detail isomer stabilities, electronic structure, three-dimensional (3D) aromaticity, and 1H, 11B, and 13C NMR shifts and coupling constants of the C2B5H7 isomers.
5.3.3 2,4-C2B5H7 Closo-2,4-dicarbahexaborane(7), the most chemically and thermally stable small carborane, is nearly as robust as the well-known C2B10H12 systems and in some respects even more so (as in its resistance to cage-opening by alcoholic base, for example); however, it is far more volatile and requires handling via a vacuum line or Schlenk techniques. The stability of the C2B5 cluster framework and the relatively straightforward synthesis of substituted derivatives (Table 5-4 and discussion below) have stimulated considerable research interest in this system. 2,4-C2B5H7 is the most intensively studied non-icosahedral carborane in the closo-C2Bn2Hn family and has been explored as a structural component in carborane-siloxane polymers, in liquid crystal systems, and in other applications (Chapters 14 and 17).
5.3.3.1 Synthesis Methods for the preparation of parent 2,4-C2B5H7 have been little advanced in the decades since the first edition of this book [30], the majority involving reactions of lower boranes (usually B2H6 or B5H9) with acetylene under high-energy conditions such as pyrolysis [31] electric discharge [32] and flash reactions [33–35], or thermolysis/electric discharge of nido-2,3-C2B4H8 [36–38] (which is itself derived from small boranes and acetylene [Section 4.5]). Pyrolysis of 1,2-B5H7Me2 generates 2,4-C2B5H7 in low yield [39]. The borane-alkyne reactions typically afford mixtures of 5- to 7-vertex closo-carboranes and alkyl derivatives thereof, with 2,4-C2B5H7, as a thermodynamically favored product, often found as a major component (Table 5-4). While it is possible to obtain fairly good overall carborane yields by adjusting the parameters in flow pyrolysis systems [31,36], there are no known methods for selectively generating C2B5H7 or other individual compounds. Alkylated derivatives of 2,4-C2B5H7 are produced from borane reactants in several nonselective high-temperature processes. Reactions of acetylene with ethyldiborane give apparent peralkylated species, based on mass spectroscopic data [40].
100
CHAPTER 5 Intermediate carboranes: Seven- to nine-vertex clusters
TABLE 5-4 closo-2,4-C2B5H7 Derivatives Compound Synthesis and characterization Parent
2,4-D2 2,4-Li2 Methyl and ethyl derivatives 1-Me, 2-Me 2-Me 5-Me n-Me (n ¼ 1, 3, 5) n-Me (n ¼ 1, 2, 3, 5) 5-Me-6-Cl m-Me-n-Cl (m ¼ 1, 3, 5; n ¼ 3, 5, 7) Me, Me2, Et Me2 1,7-Me2 1,n-Me2 (n ¼ 3, 5, 7) 2,4-Me2 2,B-Me2 isomers 5,6-Me2 n,5-Me2 (n ¼ 3, 6) n,5-Me2-6-Cl (n ¼ 1, 3) 1,5,6-Me3 1,B, B0 -Me3 isomers 1,5,6-Me3 3,5,6-Me3 1,5,n-Me3-6-Cl (n ¼ 3, 7) 1,B-Me4 1,3,5,7-Me4-6-Cl B-Men (n ¼ 1, 2, 3, 5) B-Men (n ¼ 2–5) B-Men (n ¼ 1–4) 1,3,5,6,7-Me5 2,4-Me2-1,3,5,6,7-Me5 þ other derivatives
Information
References
S (B5H9 þ C2H2 flow system) H, IR, MS S (B2H6 þ C2H2 electric discharge), H, B, IR, MS S (from nido-2,3-C2B4H8), H, B S (from 1,2-B5H7Me2), H S (B4H10/B2H6 þ C2H2 flash reactions), IR, MS S (borane-alkyne gas-phase flash photolysis), H, B S, IR, MS, VP, refractive index S, IR, MS, VP, refractive index
[31] [72] [32] [36,37] [39] [34,35] [33] [67] [67]
S (B4H10/B2H6 þ C2H2 flash reactions), IR, MS S, B, IR, VP, MS S, H, B S, H, B, IR, MS S (B4H10/B2H6 þ C2H2 flash reactions) S, H, B S, H, B S (borane-alkyne gas-phase flash photolysis), H, B S (BMe3 þ 1,6-C2B4H6) S (B4H10/B2H6 þ C2H2 flash reactions), IR, MS S, H, B S, B, IR, VP, MS S (pyrolysis of nido-2,3-C2B4H8), H, B, MS CH/2-pentyne), H, B S (1-MeB5H8 þ MeC S, H, B S, H, B S, H, B S, C S, H, B S, H, B S, H, B S, H, B S, H, B S, H, B S, B, H, MS S (BMe3 pyrolysis), MS S (BMe3 þ nido-2,3-C2B4H8) S (BMe3 þ nido-2,3-C2B4H8), H, B, IR, MS S, H, B S, MS
[34,35] [66] [91] [32,59] [34,35] [89] [95] [33] [47] [34,35] [59] [66] [38] [41] [91] [59] [95] [55] [59] [91] [59] [95] [59,91] [95] [47] [43] [48] [48] [59,91] [40] Continued
5.3 7-Vertex Closo clusters
101
TABLE 5-4 closo-2,4-C2B5H7 Derivatives—Cont’d Compound
Information
References
2,4-Me2-1,3,5,6,7-Et5
[46] [44,45] [46] [44,45] [35] [87] [4]
n,5-Et2–3-Cl (n ¼ 1, 6) 2,4-Et2–3,5,6,7-Et4-1-R (R ¼ H, Br)
S, H, B, IR S, MS S, H, B, IR S, MS S (B4H10/B2H6 þ C2H2 flash reactions), H, IR S, H, B S (rearrangement of 2,3-Et2C2B5H5), B (2D), H, IR, MS S, H, B S, H, B, C
Alkenyl derivatives n-(cis-2-butenyl) (n ¼ 1, 3, 5) 1,n-(cis-2-butenyl)2 (n ¼ 3, 5, 7) n,5-(cis-2-butenyl)2 (n ¼ 3, 6) (cis-2-butenyl)3 (five isomers) (cis-2-butenyl)4 (three isomers) (cis-2-butenyl)5
S, S, S, S, S, S,
[92] [92] [92] [92] [92] [92]
Other hydrocarbon derivatives n-Ph (n ¼ 3, 5) 2-(CH2)2Me
S, H, B S, H, B, MS
[87] [38]
Haloalkyl derivatives 5-C(CF3)2OH
S, H, B, F,IR, MS
[93]
Silyl derivatives 2-SiMe2R (R ¼ H, Cl, OMe) 2-SiMe2R (R ¼ Cl, OMe) 2-SiMe3 2,4-(SiMeRH)2 (R ¼ H, Me) 2,4-(Si Me2Cp)2 2,4-(SiMe2R)2 (R ¼ H, Br, Cl, Me, OEt, OMe) 2,4-(SiMe2Br)2 (X ¼ H, Me, Cl, Br, OMe, OEt)
H, IR, MS S, IR, MS, refractive index S, B, IR, VP, MS S, IR, MS S, H, MS H, IR, MS S, IR, MS, VP, refractive index
[72] [67] [66] [219] [78] [72] [67]
Amino and phosphino derivatives n-ClNMe3 (n ¼ 3, 5) 5-NMe3 þ 5-NMe3–6-Meþ 2-P(CF3)2 2,4-[P(CF3)2]2 2-[P(CF3)2]-4-Me 5-PMe3
S, S, S, S, S, S, S,
[89] [88,89] [89] [68] [68] [68] [89]
2-Me-1,3,5,6,7-Et5 2,4-Me-B-Et4 (three isomers) 1-Et n,5-Et2 (n ¼ 1, 3, 6) 2,4-Et2
H, H, H, H, H, H,
B, B, B, B, B, B,
IR, IR, IR, IR, IR, IR,
MS MS MS MS MS MS
H, B H, B H, B B, F,IR, MS B, F,IR, MS B, F,IR, MS B
[87] [49]
Continued
102
CHAPTER 5 Intermediate carboranes: Seven- to nine-vertex clusters
TABLE 5-4 closo-2,4-C2B5H7 Derivatives—Cont’d Compound
Information
References
Alkoxy and mercapto derivatives 5-O[C(CF3)2]2OH n-SH (n ¼ 1,3,5)
S, H, B, F,IR, MS S, H, B, IR, MS
[93] [94]
5-X (X ¼ Cl, I) 5-X-6-Br (X ¼ F, Cl) 5-XNMe3 (X ¼ Cl, Br) 5-ClNMe3 1,n-Cl2 (n ¼ 3, 5) 3,5-X2 (X ¼ Cl, Br, I) 5,6-Cl2 5,6-X2 (X ¼ Cl, I) 1,3,5-Cl3 Cl7 (cage isomer not established) n-Br (n ¼ 2, 5) n-X (n ¼ 3, 5; X ¼ Br, I) n-X (n ¼ 1, 3, 5; X ¼ Br, I) 5-BrNMe3 n,5-Br2 (n ¼ 1, 3, 6; X ¼ Br, I) 1,7-I2-5,6-Br2 1,7-I2-3,5,6-Br3 Br7 (cage isomer not established)
S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S S, S, S S, S, S, S, S, S,
[82] [84] [83,85] [82] [83] [82] [82] [81] [89] [81] [80] [87] [85] [85] [84] [86,89] [87] [84] [80] [85] [87] [24] [66] [87] [57] [86] [57] [85] [85] [24]
Linked-cage derivatives C2B5H6-Cp2Co2C2B5H6 isomers C2B5H6-2,3,8,1,6-Cp3Co3C2B5H6 (2,4-C2B5H6)2 isomers O[2,4-(Me2Si)C2B5H6]2 O[2,4-(Me2Si)C2B5H6]2
S, H, B, IR, UV, MS S, B, MS S, H, B, IR, MS H, IR, MS S, IR, MS, refractive index
Halo derivatives n-F (n ¼ 1, 3) n-X (n ¼ 3, 5; X ¼ F, Cl, Br, I) 5-F 3,5-F2 5,6-F2 1,n-F2 (n ¼ 3, 5) 1-Cl n-Cl (n ¼ 3, 5) n-Cl (n ¼ 1, 3, 5)
H, B B H, B H, H, H, H, H, H, H, B B B H, H, B H, B
B, F,IR, MS
B, F,IR, MS B, B, B, B B, B, B
F, IR, MS F, IR, MS IR, MS IR, MS MS
B B B, MS
B, MS B, IR, MS B H, B B B B B, MS
[102] [102] [103] [72] [67] Continued
5.3 7-Vertex Closo clusters
103
TABLE 5-4 closo-2,4-C2B5H7 Derivatives—Cont’d Compound
Information
References
(2,4-C2B5H6)2-n,n0 -S2 (n, n’ ¼ 1, 3, 5) 2,4-cyclo-{Me2Si(C5H4)Fe[m(CO)]2Fe(C5H4) SiMe2}C2B5H5 2,4-cyclo-{Me2Si(C5H4)Fe[m(CO)]2Fe(C5H4) SiMe2}C2 B5 H5 2 2,4-cyclo-{Me2Si(C5H4)Fe2(CO)2Me2(C5H4) SiMe2}C2B5H5 [Me2Si-C2B5H5SiMe2O]x (polymer) {Me2Si-C2B5H5-SiMe2(O)]x[Me2Si-C2BnHnSiMe2(O)]y (n ¼ 8, 10) (polymer)
S, H, B, IR, MS S, H, IR, MS
[94] [78]
S, H, IR
[78]
S, H, IR
[78]
S, TGA, DSC, elasticity S, IR, DSC
[73,74] [73,74]
H (triple resonance) H (coupling) H (B-H spin relaxation) H, B (B-H 2D) H, B H, B (coupling) B (NMR-IR correlations) B (11B, 10B; B-B coupling) B (aromatic solvent-induced shifts) C C H, B (coupling) C C H, B (coupling) H, B (coupling) H, B (coupling) H, B (coupling) H, B (coupling) B B C C B (aromatic solvent-induced shifts) H, B (coupling) H, B (coupling) C B (aromatic solvent-induced shifts) B B
[220] [54,221] [222] [223] [38] [224] [225] [226] [227] [55,228] [55] [224] [55] [55] [224] [224] [224] [224] [224] [96] [96] [55] [55] [227] [224] [224] [55] [227] [96] [83]
Detailed NMR studies Parent
2-Me 5-Me 5,6-Me2 n-Me, n-Cl (n ¼ 1, 3, 5) 5-Me-6-Cl 5,6-R2 (R ¼ Me, Et) 5-Me3N-6-Me 1,5,6-Me3 n-Et (n ¼ 2, 5) n-Et (n ¼ 1, 3, 5) n-Et2 (n ¼ 1,3; 1,5; 1,7) 1,5,6-Me3 1,3,5,6,7-Me5 5-Cl n-Cl (n ¼ 1, 3, 5) B-Cl2 isomers n-Cl2 (n ¼ 1,5; 3,5; 5,6) 5,6-Cl2 n-Cl-m-Et2 (n ¼ 1, 3, 5; m ¼ B, B0 ) 5-Br
Continued
104
CHAPTER 5 Intermediate carboranes: Seven- to nine-vertex clusters
TABLE 5-4 closo-2,4-C2B5H7 Derivatives—Cont’d Compound
Information
References
3-I 3,5-I2 1,5-(2,4-C2B5H6)2 (2,4-C2B5H6)2 isomers
H, B (coupling) H (triple resonance) H (triple resonance) B (coupling) B (11B, 10B; B-B coupling)
[224] [57] [57] [63] [226]
XPS: binding energies Raman, IR ED Microwave spectra UV-photoelectron spectra Ion cyclotron resonance; Hþ affinity Heat of formation MS MS (negative ion) Decapping (boron extraction) Competitive electrophilic halogenation, Catalytic reactions with alkynes Reaction with atomic S Photolysis Reaction with Lewis bases Hydrolysis in CH3OH Microwave spectra MS H, IR Isomerization Isomerization Isomerization kinetics Competitive electrophilic halogenation, MS (detailed) Isomerization Competitive electrophilic halogenation, VP VP Polymerization VP Microwave spectra He photoelectron spectra He photoelectron spectra Competitive electrophilic halogenation, Isomerization
[229,230] [225] [52] [50,51] [231] [232] [233] [234] [235] [97] [79] [92] [94] [103] [90] [98] [51] [234] [58] [59] [95] [58] [79] [236] [95] [79] [237] [238] [75] [239] [53] [64] [64] [79] [95]
Other experimental studies Parent
B,B0 ,B00 -D3 2-Me 3-Me B-Mex (x ¼ 1, 2, 3, 4) 5-Me n-Me (n ¼ 1,3,5) 5CR (R ¼ H, Me) 2-CH25 5,6-Me2 2-SiMe2X (X ¼ H, Cl) 2-SiMe2X (X ¼ Cl, Br) 2,4-(SiClMe2)2 2,4-R2 (R ¼ SiMe3, SiMe2H, SiMeH2, SiH3) 5-F 5-X (X ¼ Cl, Br I) 5,6-X2 (X ¼ Cl, Br, I) 5-X (X ¼ Cl, I) 5-Cl
alkylation
alkylation
alkylation
alkylation
Continued
5.3 7-Vertex Closo clusters
105
TABLE 5-4 closo-2,4-C2B5H7 Derivatives—Cont’d Compound
Information
References
5-XNMe3 (X ¼ Cl, Br) n-X (n ¼ 3, 5; X ¼ F, Cl, Br, I) 3,5-X2 (X ¼ Cl, Br, I) n-Cl (n ¼ 1, 3, 5) 5,6-Cl2 m-Me-n-Cl (m ¼ 3, 5; n ¼ 1, 3, 6) n,5-Me2–6-Cl (n ¼ 1, 3) 1,5,n-Me3–6-Cl (n ¼ 3, 7) n-Br (n ¼ 1, 3, 5; X ¼ Br, I) 5-Br n,5-Br2 (n ¼ 1, 3, 6; X ¼ Br, I) n-Cl (n ¼ 3, 5) 1,n-Cl2 (n ¼ 3, 5, 7) n,5-Cl2 (n ¼ 3, 6) 3-Cl-5,6-Et2 3,5-I2 1,8,5,6-Cp2Co2C2B5H6–2-(20 ,40 -C2B5H6) (five isomers) [SiMe(CH2CH2CF3)OSiMe(CH2CH2CF3)Cl]2
Halogen exchange Halogen exchange Halogen exchange Isomerization Isomerization Isomerization Isomerization Isomerization Isomerization equilibrium Fluorination Isomerization equilibrium Isomerization kinetics Isomerization kinetics Isomerization kinetics Isomerization kinetics Fluorination Chromatography
[84] [84] [84] [80] [80] [95] [95] [95] [57] [83] [57] [56] [56] [56] [96] [83] [240]
Hydrolytic condensation ! poly(carboranyl) siloxanes Thermochemical behavior; glass transition Glass transition temperature—structure correlations
[77]
HO[(SiMe2)(SiMe2-O-)C2B5H5]nH polymer (C2B5H5-C2B10H10)n linear siloxane Copolymers Theoretical studies Molecular and electronic structure calculations Parent (all isomers) Parent
BH and CH capping; isomer stability Cage structure, dipole moment, ionization potential, heat of formation (AM1) Isomer stabilities (SCF, DFT) Ground states (MNDO) Charge density (ab initio) Charge distribution (CNDO) Binding energies (CNDO) Geometry (ab initio) Population analysis Fractional three-center bonds Localized MOs Electronic structure Molecular conformation (MM3 force field) SCF field C2 2H bond length compared with halomethanes Second-order NLO properties
[69,71] [70]
[213] [241] [207–210,242] [214] [227] [212] [229] [14,243] [211] [244,245] [246–248] [249] [215] [250] [251] [209] Continued
106
CHAPTER 5 Intermediate carboranes: Seven- to nine-vertex clusters
TABLE 5-4 closo-2,4-C2B5H7 Derivatives—Cont’d Compound
Information
References
Parent isomers
Energy indexes; stabilities MO comparison with 7-vertex polyhedra and 7-coordinate metal complexes Isomer stabilities; dipole moments Isomer stabilities; dipole moments Charge density (ab initio) Isomer stabilities; dipole moments Isomer stabilities; dipole moments stabilities; heats of formation; Et-cage angles; energies Stabilities; heats of formation; Et-cage angles; energies Stabilities; heats of formation; Et-cage angles; energies NH2 group orientations (DFT) Stabilities; heats of formation; energies
[13] [12]
[65] [61]
Cage rearrangement Isomerization mechanism Isomerization energy (DFT)
[25,27–29] [26] [218]
B, C,H, spin-spin coupling (DFT) B (IGLO), C(IGLO) B (IGLO) H (solvent shifts) Aromatic solvent-induced 1H NMR shifts: correlation with Hþ charges (PRDDO) B-H coupling 11 B-11B, 11B-13C, 13C-13C spin-spin coupling (DFT) 11 B shift prediction rules C (IGLO) C (IGLO) C (IGLO) GIAO
[252] [243] [253] [254] [255]
Protonation (MNDO) Cage coupling (ab initio)
[257] [122]
n-Me (n ¼ 1, 3, 5) n-Me2 isomers 5-Cl n-Cl (n ¼1, 3, 5) n-Cl2 isomers B-X (X ¼ Cl, Br, I, Et, NMe3 þ ) (three isomers each) B, B0 -X2 (X ¼ Br, I, Et) (five isomers each) B-Cl-B0 ,B0 -Et2 (11 isomers) n-NH2 (n ¼ 1, 2, 3, 5) (2,4-C2B5H6)2 (B-B) (six isomers) Isomerization calculations Parent Parent and classical C2B5H7 NMR calculations Parent
2,4-Me2 n-Me, n-Cl (n ¼ 1, 3, 5) B-Men (n ¼ 2, 3, 5) B-Cl2 isomers 2,4-MeC2B5H5-B-Me isomers Reactivity calculations Parent
[60] [60] [227] [60] [60] [61] [61] [61]
[256] [15,95] [16] [55] [55] [55] [41]
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; F, 19F NMR; P, 31P NMR; 2D, two-dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; UV, UV-visible data; VP, vapor pressure; DFT, density functional theory; GIAO, gauge including atomic orbital; MNDO, modified neglect of differential overlap.
5.3 7-Vertex Closo clusters
107
1-MeB5H8 and propyne react at 230 C to give isomers of 2,4-MeC2B5H5-B-Me as well as nido-2,3-C2B4MenH8n derivatives [41]. Pyrolysis of BMe3 at 475-520 C affords 1,5-H2C2B3Me3 and 2,4-H2C2B5Me5; in the presence of H2, a mixture of 2,4-H2C2B5Me5H5n carboranes is produced in overall 60-66% yield [42,43]. In a very different approach that obviates the need for polyborane reagents and heating, organohaloboranes such as EtBF2 or Et2BCl are treated with lithium in cold THF to give Me2C2B5Et5, which is separable from other products by gas chromatography [44–46]. Other methods for generating 2,4-C2B5 clusters are available, including thermal cage rearrangement of the 2,3 isomer [4,21] and high-temperature reactions of diboron tetrahalides with CCl4 or CBr4 to form perhalocarboranes [24] (both mentioned earlier), as well as polyhedral expansion of smaller systems. Examples of the latter approach, discussed in Section 4.5, are the co-pyrolysis of nido-2,3-C2B4H8 with BMe3 at 300 C to yield 2,4-H2C2B5Me5 and mono-, di-, tri-, and tetramethyl derivatives [47,48], and the reaction of closo-1,6-C2B4H6 with trimethylboron at 590 C to afford 2,4C2B5Me2H5 isomers and other products [47]. Boron insertion into the perethyl anion nido-2,4-Et2C2B4Et4H can be achieved with BBr3 at room temperature, generating 2,4-Et2C2B5Et4-1-H and 2,4-Et2C2B5Et4-1-Br in a 70:15 ratio [49].
5.3.3.2 Structure and electronic properties No X-ray structural data are available for any closo-C2B5 carborane, but the geometry of 2,4-C2B5H7 (Figure 1-1, second row) is well defined from microwave [50,51] and gas-phase electron diffraction [52] studies as well as detailed 1H, 11 B, and 13C NMR, infrared and Raman, mass spectroscopic, and other data, cited in Table 5-4. A microwave investigation of the B(5)-fluoro derivative [53] has revealed a slight enlargement of the carborane cage relative to the unsubstituted compound which is attributed to electron donation by the fluorine to the skeletal framework. Microwave spectroscopic identification of B-deuterated derivatives has allowed unequivocal 11B and 1H NMR assignments of the parent carborane spectra [51]. As in the closo-C2B4H6 isomers discussed in Chapter 4, 1H NMR data on 2,4-C2B5H7 shows a high degree of antipodal 1H-1H coupling, implying significant s-s bonding in the center of the cage [54]. The 2,4-C2B5 cluster system has been the subject of extensive computational studies (Table 5-4), many of which can be correlated with experimental data on cage substitution, skeletal rearrangement, and other properties (see especially Ref. [55]). The relative stabilities of methyl-, chloro, bromo, and iodo mono- and disubstituted species, determined from experimental data on isomer equilibria [56–59], follow the trends 3-Me > 1-Me > 5-Me; 3-X > 5-X > 1-X (X ¼ Cl, Br); and 5-I > 3-I > 1-I. For disubstituted derivatives, the orders are 1,3-Me2 > 3,5-Me2 > 1,7-Me2 > 1,5-Me2 > 5,6-Me2; 3,5-Cl2 1,3-Cl2 > 5,6-Cl2 > 1,5-Cl2 > 1,7-Cl2; 3,5-Br2 5,6-Br2 > 1,3-Br2 > 1,5-Br2 > 1,7-Br2; and 5,6-I2 > 3,5-I2 > 1,5-I2 1,3-I2 > 1,7-I2. The variation in these findings with different substituents implies a complex interplay of factors at work, including orbital overlap, electronegativity, polarizability, and back-bonding of halogen electrons to boron p-acceptor orbitals. Substituent-positional-preference additivity effects [56,59] appear to account for the stability orders in the iodo compounds reasonably well [57], and modified neglect of differential overlap (MNDO) semiempirical calculations on halo-, methyl, and ethyl-B-substituted derivatives correlate nicely with the isomer stability data [60,61]. Ab initio Gaussian-86 calculations on the three distinguishable B-chloro derivatives (i.e., 1 [62], 5 [63], and 3) are inconclusive [61]. He and I photoelectron spectra on 2,4-C2B5H6-5-X and 2,4-C2B5H5-5,6-X2, where X is Cl, Br, or I, together with SCF calculations, indicate that p interactions between filled pp orbitals on the halogens and acceptor orbitals on the cage stabilize the B2 2X bonding and influence the bonding at other cage vertexes [64]. Similar p-overlap in B-amino derivatives, involving the nitrogen unpaired electrons and cage molecular orbitals (MOs), has been investigated [65], and the greatest overlap and highest stability were found to occur in the B(3)-NH2 isomer. These findings are all consistent with the idea that B(3), located as it is between the two electronegative carbon centers, is more electrophilic than other boron vertexes and hence is the preferred site of attack by strong electron donors.
5.3.3.3 Substitution at carbon Apart from thermal isomerization of 2,3-C2B5 derivatives (discussed below), the main route to carbon-substituted 2,4C2B5 carboranes exploits the protonic character of the CH hydrogens. As in the icosahedral 1,2-, 1,7-, and 1,122Hdþ bond polarity C2B10H12 systems (Chapters 9 and 10), the CH groups in 2,4-C2B5H7 have some degree of Cd2 and the protons are displaceable by organolithium reagents in organic solvents to give C-monolithio- and C,C0 -dilithio species [66,67]. However, the reaction is very sluggish compared to that of the larger carboranes (particularly 1,2-C2B10H12), and only the white solid dilithio derivative is stable; the monolithio compound polymerizes in ether-hexane [66].
108
CHAPTER 5 Intermediate carboranes: Seven- to nine-vertex clusters H2 C2 B5 H5 þ 2C4 H9 Li ! Li2 C2 B5 H5 þ 2C4 H10
Reaction of the dilithiocarborane with alkyl or silyl halides forms the corresponding C-alkyl or C,C0 -dialkyl or -silyl derivatives, depending on the ratio of reactants; when monosubstituted products are desired, the remaining lithium is removed by treatment with HCl [66,67]. HCl
Li2 C2 B5 H5 þ Mel ! MeLiC2 B5 H5 ! MeHC2 B5 H5 Lil
LiCl
Li2 C2 B5 H5 þ 2MeLi ! Me2 C2 B5 H5 þ 2Lil HCl
Li2 C2 B5 H5 þ Me3 SiCl ! ðMe3 SiÞLiC2 B5 H5 ! ðMe3 SiÞHC2 B5 H5 LiCl
LiCl
Li2 C2 B5 H5 þ 2Me2 RSiCl ! ðMe2 RSiÞ2 C2 B5 H5 þ 2LiCl
ðR ¼ H; ClÞ
0
The C,C -dilithio compound provides an entry for introducing other carbon-bound substituents such as bis(trifluoromethyl), which can be accomplished via reaction with P(CF3)2I [68], or bromine via interaction with Br2 [66]. Treatment of the bis(chlorodimethylsilyl) carborane with methanol affords bis(alkoxydimethyl) derivatives [67], whose infrared, NMR, mass spectra, and thermochemical behavior have been analyzed in detail [69–72]. The bis(methoxydimethyl) compound, on heating with (Me2ClSi)2C2B5H5 over FeCl3, forms the solid wax carboranylsiloxane polymer 5-2 [73,74]. Polymers of this type can also be obtained by alcoholysis of (Me2ClSi)2C2B5H5 catalyzed by HCl generated in situ [75,76]; 2,4-C2B5-based siloxane polymers having CF3CH2CH2 substituents have also been prepared [77]. H
B
5-2
H
Me
Me
H
B
B C
C
Si
O
B
Si
Me
B H
Me
H
n
When other carborane cages such as C2B8H8 and C2B10H10 are introduced into the chain in small amounts, copolymers having elastomeric properties are obtained [73,74,76]. Similar polymers constructed from icosahedral 1,7- and 1,12-C2B10H12 carboranes have found commercial application and are described in Chapter 14. Like its icosahedral C2B10 cousins which form hundreds of s-bonded exo-metallated complexes (Chapters 9 and 15), the 2,4-C2B5 cage can function similarly, although its potential has barely been scratched. The reaction of 2,4-(Me2RSi)2C2B5H5 with ClMgC5H5 forms 2,4-[Me2(C5H4)]2C2B5H5, which on refluxing with Fe(CO)5 in xylene forms the spectroscopically characterized diiron-bridged macrocycle 5-3 [78]. This compound can be reduced to the dianion with Na/Hg, and the latter species treated with alkyl halides to form species of the type 2,4-[R(CO)2Fe(C5H4)Me2Si]2C2B5H5 in which R is Me or Et. H
B H
Me
5-3 Me
H
B
B C
Si
B
Me
C
B
Si
H
HO
C
Fe
Fe C O
Me
5.3 7-Vertex Closo clusters
109
5.3.3.4 Substitution at boron: Halogenation The reaction of 2,4-C2B5H7 with F2, Cl2, Br2, or I2 results in B-halogenation exclusively, the location varying depending on conditions. In the presence of AlCl3 or other Lewis acids, the reaction with Cl2, Br2, or I2 forms the 5-halo or 5,6-dihalo derivatives [56,57,66,79–81]; however, in the absence of a Lewis acid, halogenation (except for bromine) can follow a very different pattern. Elemental fluorine affords the 1-, 3-, and 5-F as well as the 1, 3-, 1,5-, and 5,6-F2 derivatives, most in Cl > Br is as expected based on the decreasing nucleophilic character of these ions. Halogen exchange can also occur with carborane-amine adducts, as in the reaction of chloride ion or CH2Cl2 with C2B5H5-5,6-Br2NEt3 to generate C2B5H5-5,6-Cl2 and C2B5H5-5-Cl-6-Br [86], and in some cases involve both displacement and rearrangement, for example, the conversion of C2B5H6-5-INEt3 to C2B5H6-3-Cl on reaction with Cl below 100 C [84]. A route that generates primarily B(3)-halo and B(3)-phenyl derivatives (which are typically difficult to isolate from other isomers) employs reactions with BX3 at elevated temperatures and is fairly selective, with only minor formation of other carborane products [87]: BX3
2; 4-C2 B5 H7 ! 2; 4-C2 B5 H6 -3-X X ¼ Cl; Br; I; C6 H5 The reaction is most facile with BI3, requiring only 121 C, compared to 160 C for BBr3 and 270 C for BCl3, and 2H2 2B and B2 2X2 2B bonds may involve bridged intermediates in which the attacking BX3 group is linked to B(3) via B2 prior to loss of HBX2. Another approach to halogenation utilizes reactions with ICl and IBr, affording mainly B-iodo derivatives when 2Cl and B2 2Br products when no Lewis acid is used (Table 5-4) [85]. In the presence AlCl3 is present, but forming B2 of a Friedel-Crafts catalyst, the process likely involves electrophilic Iþ, while in the absence of the catalyst, homolytic cleavage of ICl or IBr occurs to afford reactive Cl or Br radicals that attack the carborane [85]. B-halogenated derivatives can be employed to generate other boron-substituted species, as demonstrated in reactions of 2,4-C2B5H6-n-Cl (n ¼ 3 or 5) with trimethylamine or trimethyl phosphine to form 1:1 adducts. The chlorine substituent can be removed as Cl via reaction with BCl3 to afford cationic species [88,89]: 2; 4-C2 B5 H6 -n-ClLMe3 þ BCl3 ! 2; 4-C2 B5 H6 -n-LMe3 þ BCl4
ðL ¼ N or PÞ
In the case of the apex-chlorinated species 2,4-C2B5H6-1-Cl, the reaction with NMe3 is slow; treatment of the resulting adduct with BCl3 gives the rearranged product 2,4-C2B5H6-3-NMe3 þ BCl4 . The behavior of 2,4-C2B5H6Cl isomers toward trimethylamine and trimethyl phosphine is of interest, given the unreactivity of parent 2,4-C2B5H7 toward those reagents (in contrast, NMe3 and PMe3 react with smaller carboranes, forming unstable adducts with 1,5-C2B3H5 and attacking 1,6-C2B4H6 to produce open-cage products [Chapter 4]). However, 2,4-C2B5H7 is reactive toward dimethylamine, which destroys the cage to form a nonvolatile polymer, Me2NHBH3 [90].
5.3.3.5 B-alkylation and -alkenylation Although B-methylated derivatives of 2,4-C2B5H7 are among the products of gas-phase electric discharge [32], thermal [31], and flash reactions [33–35] of small boranes and alkynes, more straightforward syntheses of the 5-methyl,
110
CHAPTER 5 Intermediate carboranes: Seven- to nine-vertex clusters
5,6-dimethyl, 1,5,6-trimethyl, and tetra- and pentamethyl derivatives are achieved via Friedel-Crafts reaction of the parent carborane with CH3Cl catalyzed by AlCl3 [91]. Substitution at boron can also be accomplished with transition-metal catalysts: alkynes react with 2,4-C2B5H7 in the presence of (R2CR0 )Co2(CO)6 (R,R0 ¼ H, Me, or Et) to generate a series of B-alkenyl derivatives (see Table 5-4) [92], analogous to the reactions of C2B3 and C2B4 carboranes described in Chapter 4.
5.3.3.6 Photochemical B-substitution ˚ forms the B(5)-perfluoropropanol and -perfluorCophotolysis of parent 2,4-C2B5H7 with hexafluoroacetone at 3000 A opinacol derivatives in 21% and 28% isolated yields, respectively; In the absence of light, no reaction is observed [93]. hn
2; 4-C2 B5 H7 þ OðCF3 Þ2 ! 2; 4-C2 B5 H6 -5-CðCF3 Þ2 OH þ 2; 4-C2 B5 H6 -5-½CðCF2 Þ2 2 OH Like its smaller closo-carborane homologues 1,5-C2B2H5 and 1,6-C2B4H6 (Chapter 4), 2,4-C2B5H7 reacts with 1D atomic sulfur generated via the gas-phase photolysis of COS to form B-mercapto derivatives; all possible B(n)-SH compounds (n ¼ 1, 3, 5) are obtained [94]. Photolytic decomposition of each of these isomers affords the corresponding B, B0 -disulfide-bridged S2(C2B5H7)2 dimer.
5.3.3.7 Cage rearrangement in substituted derivatives The thermal isomerization of parent 2,3- to 2,4-C2B5H7 was discussed earlier in this chapter. Rearrangements of derivatives— greatly aided by the ability of the C2B5 framework to survive high temperatures intact—provide information on relative stabilities of substituted isomers. In B-monosubstituted and B,B0 -disubstituted carboranes, the bound atoms or groups serve as labels for their attached vertexes, and can affect the course of the rearrangement [56–59,95]. Data on the high-temperature isomerizations of mono- and disubstituted derivatives of 2,4-C2B5H7, which produce nearly all possible isomers [57,80], favor a dsd mechanism while ruling out rotating-triangle and 1-2-substituent-shift processes in these particular systems [56,58,95,96].
5.3.3.8 Cage opening and polyhedral expansion
In contrast to the icosahedral closo-C2B10H12 isomers, which are converted to nido-C2 B9 H12 ions on treatment with alcoholic base, 2,4-C2B5H7 is unreactive under these conditions [97], although, as mentioned earlier, it is completely degraded by dimethylamine. As described in Section 4.5, reaction with lithium amides in acetonitrile at room temperature produces the nido-2,4-C2 B4 H7 ion in moderate to good yield. Comparative reaction rate studies of the hydrolysis and methanolysis of small closo-carboranes show that 2,4-C2B5H7 is less reactive than 1,5-C2B3H5, but more reactive than 1,6-C2B4H6, toward H2O and MeOH [98]; a similar trend is found in the competitive halogenation studies mentioned above and in Section 4.6 [79]. The higher reactivity of 2,4-C2B5H7 toward these reagents compared to 1,6-C2B4H6 can be attributed to the polarity of the former molecule, which affords a range of BH reaction sites, vs. the nonpolarity of the latter. Reduction of closo-2,4-C2B5H7 with sodium naphthalenide in THF generates dark orange anionic species, presumably including nido-C2 B5 H7 2 , which have not been characterized per se but combine with transition-metal cations and NaC5H5 to give closo-metallacarborane products of the type CpM(C2BnHn) (Chapter 13). With CoCl2, mono- and dimetallic closo-cobaltacarboranes having 7-10 vertexes are obtained following workup in air [6]; the same sequence employing FeCl2 as the metal reagent affords only 7-vertex ferracarboranes [99]. Cage opening and metal insertion are also accomplished in direct reactions of 2,4-C2B5H7 with transition-metal reagents such as Fe(CO)5 [99], CpCo(CO)2 [100], (C2H4)Ni(PPh3)2 [100], and Pt(styrene)(PEt3)2 [101], which generate closo-metallacarborane clusters having 7-10 vertexes and as many as three metal atoms in the cage. Thermal interaction of CpCo(CO)2 with mixtures of (C2B5H6)2 dimers (see below) produces multiple Cp2Co2(C2B5H6)2 2C2B5H6 isomers in which, interestingly, the metals occupy only one cage [102]. Metal insertion processes are more fully discussed in Chapter 13.
5.3.3.9 Cage coupling Linked-cage molecules are ubiquitous in icosahedral carborane chemistry and in large and small metallacarboranes. Organometallic-based techniques that have been successfully employed for effecting B2 2B cage linkage in those systems would very likely work with 2,4-C2B5H7 as well, but this approach remains mostly unexplored. The
5.4 8-Vertex open clusters
111
naphthalenide-promoted cobaltacarborane and ferracarborane syntheses described above give rise to minor amounts of 2(C2B5H6) (M ¼ Co or FeH) in which a boron in the metallacarborane linked-cage products of the type CpM(C2B4H5)2 2H deprotonation [6,99]. Much more is connected to a carbon in the C2B5 cluster, indicating a mechanism involving C2 2B linked (C2B5H6)2 dimers in efficiently, mercury-sensitized photolysis of 2,4-C2B5H7 affords all six of the possible B2 a total yield of 87% [103]. Each of these dimers undergoes thermal rearrangement at 400 C to give an equilibrium mixture of all six isomers, whose relative stabilities have been determined and correlated with MNDO-calculated structures [61]. The most favored conformations in these dimers, based on the calculations, suggest that inter-cage p-interactions in the connecting B2 2B bond may play a stabilizing role. B2 2B coupling in 1,50 -(C2B5H6)2 has been directly observed in 11 B NMR spectra [63].
5.3.4 C3B4 clusters Seven-vertex carboranes having more than two skeletal carbon atoms are at present unknown (a claimed [104,105] hyphoC3B4H12 was subsequently identified [106] as nido-1-MeCB4H7-m-CH2; see Section 4.3). However, several theoretical treatments of C3Bn clusters have been published [107–111]. High-level calculations on a proposed closo-2,4,5C3 B4 H7 þ cation [107,108] (isoelectronic with C2B5H7) indicate that it is more stable by 54 kcal mol1 than the 2,3C2B4H6-1-CH2 þ cation, and suggest that it might be obtained via rearrangement of the latter species in analogy to the well-known benzyl-to-tropylium ion rearrangement.
5.4 8-VERTEX OPEN CLUSTERS
5.4.1 Arachno-CB7H13 and CB7H12 Although open-cage CB7 carboranes have been explored theoretically [111,112], the only characterized examples are arachno-4-CB7H13 and its CB7H10Me2 derivative (Table 5-2), both prepared by degradation of larger cages. As shown in Figure 5-5, reduction of 1,2-C2B10H12 (o-carborane) with sodium affords a C2 B10 H13 anion, which on treatment with K2CO3 forms a species proposed to be nido-7-CB10H11-9-Me-10-OH (5-4). Acid hydrolysis of the latter ion or arachno8-CB8 H13 (Figure 5-2) [9] generates arachno-4-CB7H13 (5-5), characterized from its 11B COSY NMR and mass spectra [113]. Arachno-CB7H10Me2 anions, presumed to have the same cage structure as 5-5, have been obtained as an inseparable mixture of the 3- and 7-Me isomers by reaction of arachno-C2 B7 H12 with NaH, evidently converting one of the cage carbon atoms into an exo-polyhedral methyl group. Acidification of the dianions with trifluoroacetic acid reportedly produces the neutral carboranes, although no data have been published [114]. 2Hþ
C2 B7 H12 þ H ! CB7 H10 CH3 2 ! CB7 H12 CH3 Reaction of the dianion mixture with “proton sponge” (1,8-dimethylaminonaphthalene) and PCl3 generates the 11-vertex nido-phosphacarboranes P3CB7H8, P3CB7H7Me, P3CB7H6ClMe, and P2C2B7H9 [114], described in Chapter 12. Me
C
H
B B
C
Na, naphthalene
B
B
B
B
H
B
B
B
B B C2B10H12
FIGURE 5-5 Synthesis of arachno-4-CB7H13.
thf
-
C2B10H13 BKBH CKCH
K2CO3
OH B
B
C H
B
H
B
B
B B
5-4
H
B
B
B
B B
B HCl
B
C
B
5-5
B
112
CHAPTER 5 Intermediate carboranes: Seven- to nine-vertex clusters
5.4.2 Nido-C2B6H10 Eight-vertex carboranes having 10 skeletal electron pairs (SEP) such as C2B6H10 would be expected, based on the skeletal electron-counting rules outlined in Chapter 2, to adopt a nido geometry derived from a 9-vertex closo polyhedron (tricapped trigonal prism) by removal of one vertex (see Chart 2-1, bottom row). Several carborane cages of this composition are known (Table 5-5), but in the few cases where X-ray structural data are available [2,115] they have been found to adopt not the expected nido geometry, but a more open configuration (5-6) that can be described as an arachno system based on a 10-vertex polyhedron minus two vertexes (the same is true of 10-SEP C4B4 clusters, discussed below). Isomers having this cage structure with varying skeletal carbon atom placements have been synthesized via three different approaches: boron addition to a smaller cluster [116,117], degradation of a larger system [2,115], or cage-opening of a closo-C2B6 polyhedron [118] (vertex numbering is as shown in 5-6): 5
7
6
4 3 1
8 2
5-6 300 C
1; 5-C2 B3 H5 þ B2 H6 ! 3; 6-C2 B6 H10
ðstructure proposed from NMR dataÞ
D
HCl
4; 5-C2 B7 H12 þ MeCN ! 4; 5-C2 B6 H9 ! 4; 5-C2 B6 H10 1=3 Et3 B3 N3 H3
ðanion structure determined by X-ray diffractionÞ 1; 7-C2 B6 H8 þ LiH ! 3; 5-C2 B6 H9 ðstructure based on NMR data and ab initio=IGLO calculationsÞ
TABLE 5-5 C2B6Hx Open-Cage Derivativesa Compound Synthesis and characterization nido-C2B6H10 derivatives 3,6-C2B6H10 4,5-C2B6H10 5CH, 2-butenyl, 4,5-C2B6H9-7-R (R ¼ PhCH5 n-C6H13, n-C8H17) 3,5-C2 B6 H9 3,5-C2B6H8NMe3 4,5-C2 B6 H9 Al(nido-4,7-C2 B6 H8 Þ2 arachno-C2B6H12 derivatives 2,4-C2B6H12 4,5-C2 B6 H11 4,5-C2B6H10-7-Me
Information
References
S (from 1,5-C2B3H5 and B2H6), B, H, IR, MS S (improved) S, H, B(2D), MS, C S, B (2D), H, MS S, X(2-butenyl), H, B(2D), MS
[116] [117] [2] [115] [2]
S, S, S, S,
[118] [118] [2,115] [119,120]
B, H B, H X, B (2D), H, C X, H, B, IR, MS
S, B (2D), H, C, IR, MS B (2D), H (2D) S, H, B(2D)
[121] [123] [2] Continued
5.4 8-Vertex open clusters
113
TABLE 5-5 C2B6Hx Open-Cage Derivativesa—Cont’d Compound
Information
References
S, H, B S, B (2D), H, C
[2] [124]
7,8-(Ph2HP)RC2B6H12 (R ¼ H, Me)
S, H, B(2D) C S, X, H, B, C, P, IR, MS
[125,126] [126] [127]
Detailed NMR Studies nido-3,5-C2 B6 H9
C
[55]
Theoretical studies Molecular and electronic structure calculations nido-C2B6H10 isomers nido-C2 B6 H9 isomers nido-C2 B6 H8 2 isomers
Ab initio Ab initio Ab initio
[111] [111] [111,112]
IGLO IGLO B, H (IGLO) C (IGLO) Spin-spin coupling (DFT) B, H (IGLO) H, B (2D), C (IGLO) IGLO IGLO IGLO C (IGLO) Spin-spin coupling (DFT) IGLO, geometry optimization DFT/GIAO
[258] [2,115] [118] [55] [252] [118] [2] [2] [2] [2] [55] [252] [124] [127]
2
4,5-C2B6H97-Me 4,8-C2 B6 H11
Hypho-C2B6H14 derivatives 7,8-C2 B6 H13
NMR calculations nido-3,6-C2B6H10 nido-4,5-C2B6H10 nido-3,5-C2 B6 H9
nido-3,5-C2B6H8NMe3 nido-4,5-C2 B6 H9 nido-4,5-C2B6H9-7/8-Me arachno-4,5-C2B6H10-7-Me arachno-4,5-C2B6H9-7-Me2 arachno-4,8-C2 B6 H11
hypho-7,8-(Ph2HP)RC2B6H12 (R ¼ H, Me)
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; 2D, two-dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; DFT, density functional theory; GIAO, gauge including atomic orbital. a Cage numbering as given in Section 5.4.
The isolation of several different C2B6 cage isomers in these reactions reflects their very different synthetic origins; thus the symmetrical (C2v) 3,6-C2B6H10 isomer 5-7 obtained at high temperature [116] is calculated to be more stable by 22.5 kcal mol1 than 4,5-C2B6H10 [2]. The latter species, however, has more than one possible arrangement for its two B-H-B bridges. Although multinuclear NMR evidence suggests structure 5-8, ab initio/IGLO calculations favor the less symmetrical 5-9, which in solution apparently alternates between the two enantiomers as shown (only timeaveraged 1H and 11B NMR spectra corresponding to mirror symmetry are obtained, even at 90 C) [2].
114
CHAPTER 5 Intermediate carboranes: Seven- to nine-vertex clusters H H
B
H
C
B
C
B
H B
B B
C
C
H
C
B
B
B
C
H
B
B B
B
5-8
5-7
B
B
B
B
H H
C
B
B
C
B
B B
B
B
5-9
BKBH CKCH
The structure of the 4,5-C2 B6 H9 ion (5-10), with the carbons occupying adjacent vertexes, has been established crystallographically [2], and ab initio calculations show that it is slightly less stable (by 4.5 kcal mol1) than the 3,5 isomer [118] (5-11) which has nonadjacent carbon atoms. These structural findings underscore the essential validity of the empirical rules on carbon and bridge hydrogen placement that were proposed decades ago by Williams and others and are summarized in Chapter 2, especially the preference of carbon for low-coordinate, nonadjacent vertexes. However, they also demonstrate, again, that these rules point to thermodynamically preferred structures, and do not exclude other kinetically stabilized isomers such as 5-9, 5-10, and 5-11 which can be isolated under suitable conditions. H
C C
B
5-10
B
B
B
B BKBH CKCH
B B
H
C B
B
B
5-11
C B
B
The chemistry of nido-4,5-C2B6H10 and 4,5-C2 B6 H9 has been explored to some extent [2]. Hydroboration of alkenes and alkynes by the neutral carborane under mild conditions affords, respectively, B(7)-alkyl and -alkenyl derivatives (Table 5-5). Flow pyrolysis of the same compound at 350 C generates closo-2,3-C2B5H7 (see Section 5.3) in 65% yield, along with smaller quantities of closo-2,4-C2B5H7 and closo-1,7-C2B6H8. Thermolysis of solutions of the 5CHPh, cis-2-butenyl, or n-hexyl) also affords closo-2,3-C2B5H7. 4,5-C2B6H9-7-R derivatives (R ¼ trans-CH5 The tendency of neutral 4,5-C2B6H10 to lose boron from the cage is seen also in the 4,5-C2 B6 H9 anion, as in its reactions with the transition-metal reagents CpCo(CO)I2 and (C6Me6)2Ru2Cl4 to give the 8-vertex metallacarboranes 3,1,2CpCo(C2B5H7) and 3,1,2-(C6Me6)Ru(C2B5H7), respectively (Chapter 13) [2].
5.4.3 Nido-C2B6H82
The nido-C2 B6 H8 2 dianion is unknown as a free species, but an aluminum complex, AlIIIð4; 7-C2 B6 H8 Þ2 (5-12, one enantiomer shown; cage numbering changed from the original), has been isolated in 42% yield from the reaction of Naþ arachno6,8-C2 B7 H12 OEt2 with Et2AlCl [119,120]. Each of the carborane ligands can be viewed as donating two electrons via its two carbon atoms to the Al3þ center, which adopts a distorted tetrahedral coordination geometry. 5-12 is one of a family of aluminum-carborane sandwich complexes that are discussed in Chapter 12. B
B B
C B
C B Al C
B
C B
B B
B
−
B
B
5-12 B = BH C = CH
5.4 8-Vertex open clusters
115
5.4.4 Arachno-C2B6H12
A neutral arachno-dicarbaoctaborane(12) and several arachno-C2 B6 H11 isomers have been characterized. The PtBr2catalyzed cage-growth reaction of diborane with closo-1,5-C2B3H5 generates arachno-2,4-C2B6H12 (Figure 5-6), originally numbered 5,6-C2B6H12. Interestingly, as described earlier in Section 4.6, closo-1,6-C2B4H6 and B2H6 under the same conditions react differently, forming the C2B4H5-B2H5 linked species 4-48 [121]; theoretical analysis indicates that 1,5-C2B3H5 has a lower activation energy for cage expansion compared to 1,6-C2B4H6 [122]. The arachno-C2B6 cage structure shown in Figure 5-6, deduced from 1H, 11B, and 13C NMR spectra [121] but not confirmed crystallographically, is atypical in that it is derived, in a geometric sense, via removal of two vertexes from a C3v-type 10-vertex polyhedron rather than from the usual bicapped square antiprism as depicted in Chart 2-2 (Chapter 2). Isomers of arachno-C2 B6 H11 anions and their derivatives have been obtained by a variety of methods. Selective degradation of closo-1,2-C2B8H10 (removal of two adjacent BH units) by hydroxide ion affords arachno-4,5-C2 B6 H11 (Figure 5-7A), whose cage geometry is that of the 4,5-C2B6H10 carborane discussed above [123]; the same anion is H
B
B
H
C
H
H
BH C
HB
PtBr2
B
C
H
H
B
B
B2H6
H
H
H
H
C
B
H
H
H
B H
H
FIGURE 5-6 Synthesis of arachno-2,4-C2B6H12 (proposed structure). C B
−
B
C
OH−
B B
B
A
B 1,2-C2B8H10
F−
H
− H
B C
B
B C
C B B
FIGURE 5-7 Synthesis of arachno-C2 B6 H11 ions.
B
4,5-C2B7H9
B
B = BH C = CH
B C
B
B B
H
B B
B
H
B
B
B
C
4,5-C2B6H11-
B B
H H
C
LiBEt3H
B
B
B
H
B
B
C
B
B
H
C
B
4,8-C2B6H11-
B
116
CHAPTER 5 Intermediate carboranes: Seven- to nine-vertex clusters
obtained, along with nido-4,5-C2 B6 H9 , on treatment of nido-4,5-C2B6H10 with LiBEt3H [2]. Fluoride ion removes one BH from closo-4,5-C2B7H9 to give arachno-4,8-C2 B6 H11 (originally numbered 4,5) [124], as shown in Figure 5-7B. Also, the reaction of nido-4,5-C2 B6 H9 with MeLi in THF solution affords the C-methylated dianion arachno-4,5C2B6H9-7-Me2, which can be protonated to give arachno-4,5-C2B6H10-7-Me [2]. None of these arachno-C2B6 structures has been secured by X-ray crystal data, but they are well supported by detailed high-resolution NMR spectra, and in several cases by ab initio/IGLO calculations that correlate well with spectroscopic data (Table 5-5).
5.4.5 Hypho-C2B6H13
Hypho-class (8-vertex, 12 SEP) C2B6 carboranes are represented by the 7,8-C2 B6 H13 anion, which can be prepared by the degradation of arachno-6,7-C2B7H13 with aqueous NaCN (Figure 5-8) [125,126]. Derivatives of the same hypho cage system with a PPh2H group attached to one cage carbon atom have been obtained [127] via reaction of PPh2Cl with arachno-6,8-RC2 B7 H11 ions (R ¼ H or Me), whose cage carbons occupy nonadjacent vertexes (see arachno-6,8-C2B7H13, Figure 1-3, second row right). The 7,8-hypho-RC2B6H12-7-PPh2H products have been characterized by X-ray diffraction and exhibit short C2 2P bonds, implying a degree of P5 5C double-bonded ylide-like character, which is also supported by DFT/GIAO (gauge including atomic orbital) calculations [127]. As is frequently the case in electron-rich open-cage carborane structures, alternative descriptions are available: in this case, one could view the CH5 5PPh2H unit as an ylide group appended to an arachno-CB6H10 cluster, or the molecule could be described as a nido-B6H9 (hexaborane) cage with 2X2 2B bridging locations on its base. CH5 5PPh2H and CH2 moieties occupying B2 H
H
B H H
H
H
B
B
H
H
C
H
B
H B
H
BH
C
C NaCN H2O
H
B H
B
H H
B
H
C
H HB
H
−
H
H
BH B
H
H
B H
FIGURE 5-8 Synthesis of 7,8-hypho-C2 B6 H13 from arachno-6,7-C2B7H13.
5.4.6 Nido-C3B5H9
Tricarbon 8-vertex carboranes have not been prepared, but nido-C3B5H9 and nido-C3 B5 H8 have been investigated by ab initio theoretical methods [111]. As expected, the most stable isomer is found to be the 4,6,8 species (numbering as in 5-6) with the framework carbon atoms in low-coordinate nonadjacent locations on the open rim; the calculations predict that other low-energy isomers also have all of their carbons on the open face.
5.4.7 Nido-C4B4H8 5.4.7.1 Synthesis Nido-tetracarbaoctaboranes have been prepared via a remarkable variety of routes (Figure 5-9), including alkyne-ferraborane photolysis, metal-promoted alkyne insertion into carboranes, reduction of thiadiborolenes, dehalogenation of diiodoboryl alkenes, hydroboration of dialkylboryl alkynes, vinyliron-carborane thermolysis, and air-oxidation of vinyltitanium-carborane complexes. Not shown in the figure are accompanying products, reaction intermediates, or postulated
O O C C
A
O C
Me
Fe H
B HB H
H
MeC≡CMe H
B
C C Me
B
Me C
hu
B
H
Me H
C
B B
B
H
H
H
H
H
60%
B
Me
H
Et
C B
H
H
Et H
C
B
Et
C B
MeC≡CMe
B
H
B
C
Me
MxLy
H
MxLy = NiCl2, Ru3(CO)1
B
B
B H
H
H
C C Et
H
31% (NiCl2)
C
Et
Me
C
K
B
B
S
C C Et
B
Et C
THF
B
B
Et
Et
Et
B
Me
Me
Me 30%
Et
D
Et
Et
NaK2.8
I2B
B
Et C
THF
Et C C Et
I
C
B
BI2
B
B
I
I
I 29%
E
H
H
Et2BH Et2B
Et
C
BEt2
C C H
B
H C
B B Et
B Et
Et 9%
Fe
H
C
110 °C Ph2P
Et
B B
B
H
Me
C C
B
C
Et
Me
C O
G
Et
Et
F
Et2C2B4H5
H
H
80%
R B
C B C BB Me Ta Me
B
RC≡CR hu
R
H R = Et, Ph B = B, BH
Me
O2
C C
C H H
R
Et C C Et
H
C
C B C BB Ta
B
R C
B B
B H
H
H
R = Me, Et, Ph 30-36%
FIGURE 5-9 Synthesis of C4B4H8 derivatives. (A) Alkyne-ferraborane photolysis [202]. (B) Metal-promoted insertion of alkynes into nidoC2B4 carboranes [203,204]. (C) Potassium reduction of thiadiborolenes [205]. (D) Dehalogenation of bis(diiodoboryl)-3hexene [23]. (E) Hydroboration of bis(diethylboryl)acetylene [128,133]. (F) Thermolysis of a vinyliron-carborane complex [203]. (G) Air-oxidation of vinyltitanium(V)-C2B4 complexes (discussed in Chapter 13) [206].
118
CHAPTER 5 Intermediate carboranes: Seven- to nine-vertex clusters
mechanisms, but in each case the C4B4 cluster is the major carborane species isolated. In the hydroboration of bis (diethylboryl)acetylene [128] (Figure 5-9E), a second isomer of H4C4B4Et4 having a unique spiro-carborane structure (see 4-40 in Figure 4-10) is obtained as a minor product; in contrast to the C4B4 clusters in Figure 5-9, the spiro compound is highly air sensitive and far less stable.
5.4.7.2 Structure and properties Multinuclear NMR and other spectroscopic data (Table 5-6) point to the same cage structure for all of the C4B4 products, although only that of H4C4B4Et4 (Figure 5-9E) has been confirmed by X-ray diffraction analysis [128]. As in the case of its isoelectronic analogue nido-C2B6H10 discussed earlier, the C4B4H8 system has a nido electron count (8 vertexes, 10 SEP) but adopts the more open arachno-type cage geometry 5-13 rather than the nido structure 5-14 (which in fact is unknown in carborane chemistry, although it is represented by the metallathiaborane [130] Cp2Co2SB5H7). The fact that the same arachno-C4B4 cluster is obtained in such a disparate variety of synthetic routes is striking, and implies a strong thermodynamic preference for this structure. High-level calculations on parent C4B4H8 [111,131,132] and H4C4B4Me4 [128] support this conclusion. Why 5-13, despite its nido electron count, is nonetheless favored over 5-14 is not entirely clear, although it has been noted that in calculated structures the 5-14 geometry is adopted when there are one or two extra hydrogens (as in B8 H9 3 and OB7 H7 ), but otherwise the 5-13 architecture is favored; it has been suggested that one or two such hydrogens attract sufficient electron density from the cage to stabilize 5-14, whereas additional hydrogens create crowding that favors 5-13 [111]. C
C B
C
C B
B B
B
C
C
C
C B B
5-13
5-14
arachno
nido
B
TABLE 5-6 C4Bn Derivatives (n ¼ 4, 5, 6, 7) Compound
Information
References
Me4C4B4H4 Me2Et2C4B4H4 Et4C4B4Me4 Ph4C4B4H4 R2Et2C4B4H4 (R ¼ Me, Et, Ph) Et4C4B4I4
S, S, S, S, S S, S, S,
[128] [133] [202] [203,204] [205] [202] [206] [23]
nido-C4B6H10 derivatives 5, 6, 8, 9-C4B6H10 H4C4B6Me6 (carborane structure?) H4C4B6Me6 (classical cage) 1, 2, 3, 9-Me4C4B6Et6
S, H, B (2D), MS S (pyrolysis of BMe3), MS X S, H, B, C, R, MS
Synthesis and characterization nido-C4B4H8 derivatives H4C4B4Et4
X, H, B, C H, B, C, MS H, B, C, MS H, B, MS* MS H, B, C, MS B, MS
[259] [42] [260] [261] Continued
5.4 8-Vertex open clusters
119
TABLE 5-6 C4Bn Derivatives (n ¼ 4, 5, 6, 7)—Cont’d Compound
Information
References
1, 2, 3, 9-Et4C4B6H6 2, 6, 8, 10-Et4C4B6Et6 Me2Et2C4B6Et6 Et4C4B6(n-C4H9)6
S, S, S, S,
[262] [263] [261] [261]
arachno-C4B6H12 derivatives 5,6,8,10-H4C4B6H6-m(6, 9)-HC¼CH Et4 C4 B6 Et6 2
S, X, H, C H, C, IR, Raman
[264] [263]
nido-C4B7H11 derivatives 2, 7, 9, 10-C4B7H11 7, 8, 9, 10-C4B7H11 1, 7, 8, 10-C4B7H11
S, MS, B (2D), H S, MS, B (2D), H S, H, B, C, IR
[259] [259] [62]
arachno-C4B7H13 derivatives Me4C4B7H8Br 7,8,9,11-H4C4B7H6-10-Me-m(7,11)-CH2 Me4C4B7H9 (two isomers)* Me4 C4 B7 H8 * Me4C4B7H9
S, S, S, S, S,
[265] [264] [265] [265] [266]
Other experimental studies nido-2, 6, 8,10-Et4C4B6Et6
Reduction to arachno-Et4 C4 B6 Et6 2
[263]
[111] [132] [131] [128] [128] [132] [267]
nido-C4B7H11 isomers
Ab initio Second moment Hu¨ckel Extended Hu¨ckel Ab initio GIAO-IGLO Second moment Hu¨ckel DFT: isomer stability, energy penalties for specified structural features DFT, stability
NMR calculations nido-2, 6, 8,10-Et4C4B6Et6 7,8,9,11-nido-H4C4B7H6-10-Me-m(7,11)-CH2 nido-C4B7H11 (three isomers) arachno-5,6,8,10-H4C4B6H6-m(6,9)-HC¼CH
Ab initio, IGLO IGLO Ab initio, IGLO (11B shifts) IGLO
[263] [264] [269] [264]
Theoretical studies Molecular and electronic structure calculations nido-C4B4H8
nido-H4C4B4R4 (R ¼ H, Me) nido-H4C4B4Et4 nido-C4B6H10 nido-C4B6H10 (seven isomers)
H, B (2D), C, MS X, B, MS MS MS
X, H, B, IR, MS H, C, t (IGLO) H, B, IR, MS H, B, R B (2D)
[268]
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; 2D, two-dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; DFT, density functional theory; GIAO, gauge including atomic orbital. *Alternatively described as CHMe-bridged arachno-C3B7 cages (see Table 5-10).
120
CHAPTER 5 Intermediate carboranes: Seven- to nine-vertex clusters
The chemistry of C4B4 carboranes has been almost unexplored beyond their synthesis and structure. However, the reaction of H4C4B4Et4 (Figure 5-9E) with Fe3(CO)12 forms the red crystalline ferracarborane nido-6,9(CO)6Fe2(H4C4B4H4) (5-15), a complex obtained in nearly quantitative yield and characterized via X-ray crystallography [133]. O C C C
C C
B B
Fe3(CO)12
OC
B
O C Fe
Fe C C
C C B
B
O C
B B
O C CO
B
5-15 CKCH BKB-Et
5.5 8-VERTEX CLOSO CLUSTERS 5.5.1 CB7H8
5.5.1.1 Synthesis The closo-carbaoctaborane(1) anion (Table 5-2) can be obtained via boron extraction from nido-1-CB8H12 or arachno-4CB8H12-6-L adducts. As shown in Figure 5-2, 1-CB7 H8 is one of several carborane species formed in the treatment of CB8H12 with triethylamine in an N2 atmosphere containing 5% O2; however, if O2 is rigorously excluded, 1-CB7 H8 is the main product, isolated in 75% yield [8]. The same anion can be prepared from arachno-6-CB8H12-8-NMe3 by reaction with NaH in THF at 60 C [113], by treatment with NaBH4 in 1,2-dimethoxyethane (the latter procedure affording an 83% yield of the 1-CB7 H8 salt) [134], or, alternatively, by reaction of arachno-4-CB8H12-6-SMe2 with aqueous NMe4OH [134]. Similarly, 1-PhCB7 H7 is generated from triethylamine and arachno-4-PhCB8H13 or nido-1-PhCB8H11 [10,135,136]. The monocarbon starting materials in all of these procedures are themselves obtained by carbon extraction from dicarbon species such as C2 B10 H13 , which in turn is prepared by reduction of o-carborane (1,2-C2B10H12) as described in Chapter 11. However, in a major advance in monocarbon carborane synthesis, it has been found that commercially available decaborane(14), B10H14, reacts with aqueous formaldehyde in alkaline solution (probably via a B10H13OH2 intermediate) to generate the arachno-CB9 H14 ion, which in turn can be used to prepare other monocarbon species including CB7 H8 (Figure 5-10) [137–141]. The CB9 clusters are discussed further in Chapter 6. B10 H14 þ 2OH þ H2 CO þ H2 O ! 6-CB9 H14 þ BðOHÞ4 þ H2 6-CB9 H14 þ Fe3þ þ 3H2 O ! arachno-4-CB8 H14 þ Fe2þ þ BðOHÞ3 þ 3=2H2 200 C
2Et3 N
4-CB8 H14 ! nido-1-CB8 H12 ! Et3 NHþ 1-CB7 H8 þ Et3 NBH3
5.5.1.2 Structure
The closo-dodecahedral geometry of 1-CB7 H8 with carbon occupying a low-coordinate vertex, analogous to that of 1,7-C2B6H8 (Figure 1-1, second row), is consistent with NMR data, but the observation of only two 11B NMR resonances in a 3:4 ratio (vs. the expected 2:2:1:1:1 pattern) implies that the cage is fluxional in solution even at low temperatures [134], as is its isoelectronic analogue B8 H8 2 [142]. Definitive X-ray structural information is lacking on CB7 H8 owing to disorder in the crystal, but crystallographic studies of the 1-CB7H7-7-I and 1-CB7H6-7,8-I2 iodo derivatives [134] (obtained by reaction of the parent ion with I2) and 1-PhCB7 H7 [136] confirm the dodecahedral cage geometry. Theoretical investigations of the electronic structure of closo-CB7 H8 support its 3D aromaticity [14] and fluxionality in solution [143].
5.5 8-Vertex Closo clusters
B
H H
H H
H
B
B B B
−
B H2CO
B B
H
B B
OH
B
H
B
−
C B
B
B
B
B B
B
liq. NH3
B
B
B
6-CB9H12-
6-CB9H14-
B10H14
C B
Na
B
−
H H
B
Et3N
B
B
B
I2
H
121
220 °C
HCl
220 °C
FeCl3 B = BH
H H
C = CH
H
B
H
B B
C
H
B B
B B
B
B B
B
B
B B
4-CB8H14 I2
B
4-CB8H9−
200 °C
Et3N
−
C
B
−78 °C I2
−
B B B
−78 °C
B Et3N
B
toluene, reflux B
C
H H
B
B
B
B
Et3N
B B
H
B B
C
B
1-CB7H8−
1-CB8H12
FIGURE 5-10 Synthesis of CB9, CB8, and CB7 carboranes from decaborane(14).
5.5.2 1,7-C2B6H8 Several isomers are possible for a closo-dicarbaoctaborane(8) cluster based on an 8-vertex D2d polyhedron (bisdisphenoid), but only 1,7-C2B6H8 (Figure 1-1, second row) and its substituted derivatives (Table 5-7) have been isolated and characterized. While other isomers may conceivably be formed in some circumstances, they evidently rearrange to the thermodynamically favored 1,7 system in which the cage carbon atoms occupy low-coordinate non-vicinal positions as predicted by theory.
5.5.2.1 Synthesis The known synthetic routes to parent 1,7-C2B6H8 involve relatively less accessible starting materials. Pyrolysis of arachno-6,8-C2B7H13 (Figure 1-3, second row) at 216 C in diphenyl ether affords the unsubstituted compound in 25-30% yield, together with comparable amounts of the 10-vertex cluster 1,6-C2B8H10, suggesting possible disproportionation of the C2B7 cage [144–146]. 1,7-C2B6H8 is also obtained as a minor product (5%) of the thermolysis of nido-2-CB5H9 at 251 C, a reaction described earlier in Section 4.5 [147]. The C-monomethyl and monophenyl derivatives 1,7-RC2B6H7
122
CHAPTER 5 Intermediate carboranes: Seven- to nine-vertex clusters
TABLE 5-7 closo-C2B6H8 Derivatives Compound
Information
References
S S, H, IR S (pyrolysis of nido-2-CB5H9) Microwave spectra S, H, IR S (improved) S, H, IR S, B, H S, B, IR, MS X S (improved) MS (calculated monoisotopic) MS S, MS S, B, H, MS
[144] [145] [147] [149] [145] [144] [145] [4] [146,148] [150] [144] [270] [234] [271] [47]
1,7-PhC2B6H7 1,7-(p-FC6H4)C2B6H7
C C (IGLO) C F, electronic properties
[272] [55] [272] [273]
Other experimental studies 1,7-C2B6H8 1,7-C2B6H8, 1,7-MeC2B6H7, 1,7-Me2C2B6H6
Cage-opening reaction with B2H6 Conversion to nido-CB5H9 derivatives
[118] [144] [152]
Synthesis and characterization 1,7-C2B6H8
1,7-MeC2B6H7 1,7-PhC2B6H7 1,7-Et2C2B6H6 1,7-Me2C2B6H6
1,7-Me2C2B6H5-B-OPh 1,7-H2C2B6Me6 Detailed NMR studies 1,7-C2B6H8
Theoretical studies Molecular and electronic structure calculations Geometry (ab initio) C2B6H8 isomers MNDO BH and CH capping; isomer stability Isomer stability Isomer stability Ab initio Electronic structure 1,7-C2B6H8 Relative stability (ab initio) Localized orbitals SCF Relative stability (ab initio) 3,5(?)-C2B6H8
[14] [214] [213] [207] [210] [217,274] [249] [208] [246] [250] [275] Continued
5.5 8-Vertex Closo clusters
123
TABLE 5-7 closo-C2B6H8 Derivatives—Cont’d Compound
Information
References
Isomerization calculations 1,7-C2B6H8 1,2-C2B6H8
Cage rearrangement Cage rearrangement
[26,27,29,143,276] [276]
IGLO (ab initio) C (IGLO) B-H coupling Aromatic solvent-induced 1H NMR shifts: correlation with Hþ charges (PRDDO)
[143] [55] [256] [255]
NMR calculations C2B6H8 isomers 1,7-C2B6H8
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, UV-visible data.
11
B NMR; C,
13
C NMR; F,
19
F NMR; IR, infrared data; MS, mass spectroscopic data; UV,
(R ¼ Me, Ph) are obtained on pyrolysis of the corresponding arachno-6,8-RC2B7H12 derivative at 216 C in organic solvents [144,145]. Similar treatment of arachno-6,8-Me2C2B7H11 yields 1,7-Me2C2B6H6 [144]. Other routes to 1,7-C2B6H8 derivatives have been demonstrated. 1,7-Me2C2B6H6 has been prepared from B6H10 and dimethylacetylene at 200 C [148], and boron insertion of Et3NBH3 into nido-2,3-Et2C2B4H6 at 171 C affords 1,7Et2C2B6H6 together with other carboranes [4]. Interestingly, the interaction of nido-2,3-C2B4H8 with BMe3 at 221 C results in both cage expansion and replacement of all BH hydrogens with methyl groups, forming 1,7-H2C2B6Me6 and 2,4-H2C2B5Me5 [47]. The conversion of 2,3-C2B4 to 1,7-C2B6 carboranes in the latter two cases reflects the usual tendency of framework carbons in neutral C2Bn carboranes to migrate away from each other at high temperatures.
5.5.2.2 Structure and properties The molecular structure of 1,7-C2B6H8 is established from a gas-phase microwave [149] analysis of the parent species and an X-ray crystallographic study of the C,C-dimethyl derivative [150], and the electronic structure of the cage has been probed by detailed NMR and associated theoretical investigations (Table 5-7). However, the observation of only two 11B NMR resonances in a 4:2 pattern, even in high-resolution spectra, is not consistent with a static dodecahedral cage for C2B6H8. As in the case of 1-CB7 H8 discussed above, these data suggest that reversible polyhedral rearrangement is occurring on the NMR time scale [151]. Ab initio/IGLO computations support the dsd process depicted in Figure 5-11 involving an intermediate species with an open square B4 face that is only slightly less stable than the closo structures [143]. A few studies of closo-C2B6 carborane reactivity have been reported. Diborane interacts with 1,7-C2B6H8 to effect boron insertion, generating the higher carboranes 4,5-C2B7H9 and 1,6-C2B8H10 in 30% and 10% yield, respectively B
B B
B
C
B
B B
C
C
B
B
B
B B B
B C
FIGURE 5-11 Calculated dsd fluxional rearrangement of 1,7-C2B6H8 (B5 5BH; C5 5CH).
B C
B
B C
B
124
CHAPTER 5 Intermediate carboranes: Seven- to nine-vertex clusters
[144]. The reaction of NaBH4 with parent 1,7-C2B6H8 followed by treatment of the resulting solid materials with HCl gas yields nido-2-CB5H9 (Section 4.5) and several of its mono- and dimethyl derivatives; the same procedure conducted with 1,7-MeC2B6H7 or 1,7-Me2C2B6H6 affords similar products [152]. This reaction is unusual in that carbon is extracted from the skeletal framework and converted into a methyl group, which can then undergo cleavage and migrate to different locations on the cage surface. A different type of cage opening in 1,7-C2B6H8 involves the attack of electron donating reagents, as in the reaction with NaH to give the nido-C2 B6 H9 described above. Trimethylamine combines with the carborane to form initially a weakly bound adduct that slowly converts into a strongly bound 1,7-C2B6H8NEt3 complex, whose NMR spectrum and associated calculations indicate that it has the same open-cage structure (5-11) as that of the previously discussed C2 B6 H9 ion [118]. NMR data indicate that the adduct in solution is in equilibrium with free NEt3 and C2B6H8. Insertion of transition metals into the 1,7-C2B6H8 cage or anions formed by reduction of the neutral carborane generates polyhedral metallacarboranes [153–155], as described in Chapter 13.
5.6 9-VERTEX OPEN CLUSTERS The inherent complexity of monocarbon carborane chemistry is further confused by changes in cage numbering over time in different publications, even in papers originating from the same group. As is the case throughout this volume, our numbering of these species conforms to the most recent generally accepted usage and may differ from that found in the cited references.
5.6.1 Nido-CB8H12, arachno-CB8H14, and arachno-CB8H13 5.6.1.1 Synthesis The preparation of arachno-4-CB8H14 in two steps from B10H14 and its dehydrogenation to nido-1-CB8H12 are mentioned in the preceding section (see Figure 5-10 and Table 5-8) [137–139,141]. Alternatively, this compound can be obtained by hydrolytic degradation of nido-CB9 H12 [138,156] or by reacting nido-7,9-C2 B10 H13 with Me2S and HCl to form arachno-6-CB9H11-9-SMe2-m-6,9-CH2B(OH) which is hydrolyzed to afford the arachno-4-CB8H14 product [157]. A different isomer, arachno-5-CB8 H13 , is formed on treatment of nido-1-CB8H12 with BH4 in THF solution, as noted in Section 5.3. Protonation of 5-CB8 H13 in nonaqueous media regenerates 1-CB8H12 (Figure 5-2), but acid hydrolysis extracts a boron atom and converts it to the known carborane arachno-4-CB7H13 [9]. The interaction of 5-CB8 H13 with phenylacetylene to generate closo-2-CB6 H7 with loss of two borons was mentioned earlier (Section 5.3). Other arachno-CB8 cluster isomers have been obtained in the form of Lewis base derivatives (Table 5-8). Boron and carbon removal from the nido-C2 B10 H13 ion with electron donors affords arachno-6-CB8H12-8-L compounds (5-16) [113,158]. The isomeric products arachno-7-CB8H12-8-L (5-17) are obtained on treatment of nido-1-CB8H12 with tertiary Lewis bases at room temperature [159]. 4
B
5-16
3
B
H H
9
8
B
B
B 5
4
L H
C B
6
2
1B
B
7
B = BH 2e
B
5-17
H H
8
B
B
C = CH
5
H
B
B 3
L
9
B B
6
2
C
7
1B
H2 O
1; 2-C2 B10 H12 ! C2 B10 H12 2 ! C2 B10 H13 OH
C2 B10 H13 þ H3 Oþ þ 4H2 O þ L ! 6-CB8 H12 -8-L þ CH3 BðOHÞ2 þ BðOHÞ3 þ 2H2 L ¼ NMe3 ; NEt3 ; quinoline
5.6 9-Vertex open clusters
125
TABLE 5-8 CB8 Derivatives Compound Synthesis and characterization closo-CB8 H9 derivatives 4-CB8 H9 PMePh3 þ 4-CB8 H9 4-PhCB8 H8 4-RCB8 H8 4-CB8H8-n-I (n ¼ 3, 5) 4-CB8H7-5, 6-I2 (n ¼ 3, 5) 4-CB8H8-1-quinolyl QuinolineHþ CB8 H9 nido-CB8H12 derivatives 1-CBH12
1-PhCB8H11 arachno-CB8H14 derivatives 4-CB8H14
4-PhCB8H13 4-CB8 H13
4-CB8 H123 5-CB8 H13 6-CB8H12-8-urotropine 6-CB8H12-8-NMe3 6-CB8H12-8-R (R ¼ pyridyl, SMe2, quinoline) 6-CB8H12-exo-5-8 (R ¼ MeCN, MeNC, PPh3) (6-CB8H12)2-exo, exo-5, 5’-urotropine 7-CB8H12-8-L (L ¼ NMe3, NEt3, quinoline) Other experimental studies nido-1-CB8H12 arachno-4-CB8H14
arachno-4-CB8 H13
Information
References
S S, S, S, S, S, S, S, S,
[8] [193] [192] [10,194] [193] [193] [193] [159] [159]
X, H, B X, H, B, C X, H, B, C X (Me, Ph), H, B H, B X, H, B H, B (2D), C, MS H, B (2D), C, MS
S, H, B, MS S, H, B, C B (2D) S, H, B, C
[137,138] [139] [277] [10]
S, B, C S S, H, B, MS B (2D), H B He photoelectron spectra S, H, C, B B (2D), H S, H, B S, X S, H, B, C S, H, B (2D), C S, X, H, B, MS S, H (2D), B (2D), MS S, H, B (2D), MS S, H, B, MS S, H, B(2D), MS S, X(NEt3), H, B(2D), C, MS
[139] [157] [138,156] [160] [278] [279] [10,136] [160] [138] [157] [161] [9] [158] [113,158] [158] [158] [158] [159]
Oxidation ! 4-CB8 H9 Synthesis of CpNi and CpCo metallacarboranes Synthesis of nido-(Ph3P)2PtCB8H12 Reactions with alkynes ! nido-C2B8 derivatives Reactions with alkynes ! closo-CB7 H8 þ CB6 H7
[192] [164] [163] [162] [162] Continued
126
CHAPTER 5 Intermediate carboranes: Seven- to nine-vertex clusters
TABLE 5-8 CB8 Derivatives—Cont’d Compound Theoretical studies Molecular and electronic structure calculations CB8 H9 isomers nido-1-CB8H12 nido-1-PhCB8H11 arachno-4-CB8H14 arachno-4-PhCB8H13 arachno-4-CB8 H13 NMR calculations 4-CB8H14 4-CB8 H123 4-CB8 H13 5-CB8 H13
Information
References
Ab initio EI (energy indexes); stabilities Ab initio; heat of formation, geometry DFT DFT, geometry optimization Ab initio; heat of formation, geometry DFT DFT, geometry optimization DFT
[14] [13] [280] [139] [10] [280] [139] [10] [157,161]
11
[278] [161] [157,161] [9]
B, CNDO B, 13C DFT/GIAO; fluxional 11 B, 13C DFT/GIAO; fluxional GIAO/NMR 11
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; 2D, two-dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; DFT, density functional theory; GIAO, gauge including atomic orbital.
5.6.1.2 Structure and properties The cluster geometry of nido-1-CB8H12 (Figure 5-10) is well established from 1H, 11B, and 13C NMR data and high-level computations (Table 5-8), although no X-ray diffraction studies are available. Crystallographic structure determinations have not been conducted on neutral, unsubstituted arachno-4-CB8H14, but X-ray analyses of the 4CB8 H13 ion [157], 6-CB8H12-exo-8-urotropine [158], and 7-CB8H12-8-NMe3 [159], confirm the arachno-4-CB8 architecture deduced from NMR and UV-photoelectron spectra (Table 5-8). The 4-CB8 H13 ion in the solid state has the arachno-CB8 framework geometry shown in Figure 5-10 for neutral 4-CB8H14, but with an asymmetric arrangement of B-H-B hydrogen bridges [160]; in solution, the ion is fluxional and has time-averaged Cs symmetry [157,161]. A similar situation exists for the arachno-4-CB8 H13 dianion: DFT/GIAO calculations combined with NMR data support a reversible rearrangement between two asymmetric enantiomers [161]. The assigned structure of 5-CB8 H13 (Figure 5-2) is based on multinuclear NMR data and ab initio GIAO/MP2 calculations [9]. As would be expected for open-cage carboranes with bridging hydrogens on their open faces, the CB8H12 and CB8H14 systems are fairly reactive, especially toward nucleophiles and oxidants. 1-CB8H12 undergoes oxidative closure to form closo-4-CB8 H9 (Section 5.5, Figure 5-10), while the same compound is attacked by Lewis bases to open the cage further and form arachno-CB8 derivatives 5-17 as discussed above. These cage-opening and cage-closing processes furnish clear examples of nido ! closo and nido ! arachno interconversions via withdrawal or addition of electrons to the skeletal framework, in accordance with the electron-counting rules outlined in Chapter 2. Alkyne addition to arachno-4-CB8H14 at 116-121 C affords not tricarbon carboranes as might be expected, but rather derivatives of nido-5,6-C2B8H12. These reactions have been shown to involve conversion of the cage CH2 unit in the arachno-carborane substrate into an exo-polyhedral substituent [162]. In contrast, alkynes and arachno-4CB8 H13 ion lead to cage closure forming closo-CB7 H8 and closo-CB6 H7 [162].
5.6 9-Vertex open clusters
127
Deprotonation of arachno-4-CB8H14 using one and two equivalents of base generate, respectively, the corresponding CB8 H13 and CB8 H12 2 anions. Crystallographic and NMR data show that the first proton removed is from a B-H-B bridge, while the second is an endo-hydrogen from the CH2 group [160,161]. Transition metals can be inserted into the arachno-4-CB8H14 framework to generate 10-vertex MCB8 metallacarboranes, as in the syntheses of nido(Ph3P)2PtCB8H12 [163] via reaction with Pt(PPh3)4 and the preparation of several CpM(CB8H9)n (M ¼ Ni, Co; n ¼ 0, 1) and closo-Cp2Ni2(CB7H8) complexes [164] from CB8 H13 and metal reagents (Chapter 13).
5.6.2 Nido-C2B7H11 Nido-dicarbanonaborane(11) is unknown in parent form at this writing. A carborane that was characterized as such by several groups [19,20,165,166], has been employed in metallacarborane synthesis [167–169], and whose preparation was detailed in Inorganic Syntheses [170], was later proven [171,172] to be arachno-6,7-C2B7H13, discussed below. However, the C,C’-dimethyl derivative nido-1,2-Me2C2B7H9 (5-18) has been obtained in the reaction of B8H12 with dimethyl acetylene [19,20] and structurally characterized by X-ray crystallography [173]. As expected for a 9-vertex, 11 SEP nido system, the cage is formally derived from a bicapped square antiprism by removal of an equatorial vertex (Chart 2-2, top center). Me
H H
B
C
B
5-18
B
B B
B
C
Me
B
Although no unsubstituted nido-C2B7H11 species has been isolated, calculations suggest the existence of a nidoC2B7H9 intermediate in the rearrangement of closo-C2B7H9 isomers [174]. Treatment of closo-4,5-C2B7H9 with trimethylamine forms a C2B7H9NMe3 adduct, existing in equilibrium with free amine and C2B7H9, that is proposed from NMR data and computational studies to have an open nido-type structure (see following section) [175].
5.6.3 Arachno-C2B7H13 Three isomers of the arachno-dicarbanonaborane(13) system are known experimentally: 6,8-C2B7H13 (5-19), 6,7C2B7H13 (5-20), and a C2 B7 H12 ion (5-21) that adopts a different cage geometry corresponding to the boron hydride B9H15. 4
B
H H
6
B
C
B B
1
9
4
H
5
B
H
C B
2
B
3
5-19
8
B
H H
6
B
C
B
7
B
1
BKBH CKCH
H
5
9
B B
2
B
3
8
5 C
H 7
C
6
7
B
B
C4 B
1
H
8 H
-
B H
B
2
B9
B
3
5-20
5-21
5.6.3.1 Synthesis
Arachno-6,8-C2B7H13 (5-19) and its C-methyl, C-phenyl, and C,C0 -dimethyl derivatives have been prepared by chromic acid or ferric ion oxidation of closo-2,3-RR0 C2B9H9 [146,176] or nido-7,9-RR0 C2 B9 H10 [177]: RR0 C2 B9 H10 þ 6H2 O ! RR0 C2 B7 H11 þ 2BðOHÞ3 þ 5Hþ þ 6e ðR ¼ H; Me; R0 ¼ H; Me; PhÞ
128
CHAPTER 5 Intermediate carboranes: Seven- to nine-vertex clusters
Parent 6,8-C2B7H13 has also been isolated as a side product in the pyrolysis of nido-2-CB5H9 [147] and in the oxidation of C2 B8 H10 2 salts [178]. The asymmetric isomer 6,7-C2B7H13 (5-20) has an interesting history. As mentioned earlier, a compound that was described for a decade and a half as “nido-C2B7H11” was eventually discovered to be arachno-6,7-C2B7H13 (originally numbered 4,5) [171,172]. The persistent misidentification of this compound arose mainly from its very facile loss of two hydrogen atoms during mass spectroscopic analysis and the fact that high-quality crystals could not be obtained, preventing X-ray diffraction studies [171]. This carborane has been prepared by various routes, including acetylene insertion into B8H12 [19,20], degradative oxidation of nido-7,8-RR0 C2 B9 H10 ion with formaldehyde [166,170], and oxidation of the latter ion with FeCl3 [165]. The 4,5-C2 B7 H12 ion (5-21) is generated by boron insertion into 4,5-C2 B6 H9 (a species described earlier in Section 5.4) with BH3THF [2]. Further studies on this carborane system have not yet appeared.
5.6.3.2 Structure and properties of 6,8-C2B7H13 The cage geometry of the 6,8-isomer (5-19 and Figure 1-3) is established from X-ray structure determinations on 6,8-Me2C2B7H11 and several other derivatives [179–181] combined with multinuclear NMR and X-ray photoelectron data (Table 5-9). Owing to its open-cage architecture and the presence of four “extra” hydrogens, consisting of two B2 2H2 2B bridges and two CH2 groups, this carborane is a highly reactive and versatile synthetic reagent. It is reversibly deprotonated by reaction with NaH, forming the 6,8-C2 B7 H12 ion; deuterium exchange studies show that the CH2 protons are highly acidic and undergo exchange with the B2 2H2 2B bridging hydrogens in basic media [146]. Several lines of evidence, including deuterium exchange [146], reactivity studies [182–185], and DFT/GIAO calculations 2H2 2B [180], establish that deprotonation of neutral 6,8-C2B7H13 takes place at an endo-CH hydrogen rather than a B2 bridge. The arachno-6,8-C2B7 cage is amenable to both expansion and degradation, and this property is well exploited in synthesis. Boron addition and extraction to generate larger and smaller carborane systems have been demonstrated; for example, the apparent disproportionation of arachno-6,8-C2B7H13 on pyrolysis to afford 1,7-C2B6H8 and 1,6-C2B8H10 was mentioned in Section 5.4. Boron insertion is enhanced by addition of B2H6 during the pyrolysis, affording closo-1,6C2B8H10 in good yield [145,186]. Heating Naþ 6,8-RC2 B7 H11 (R ¼ H, Me) at 200 C generates mainly closo-RC2B7H9 [145]. Direct reaction of 6,8-C2B7H13 with metal reagents gives MC2B7 metallacarboranes where M is Co or Rh (Chapter 13) [187]. Carbon insertion into 6,8-C2 B7 H12 can be effected by reaction with metal carbene complexes of the type 5C(OMe)R to produce carbon-bridged species 5-22, which on acidification and workup forms the tricarbon (CO)5M5 carboranes RC3B7H10 (5-23) (Figure 5-12 and Table 5-10) [188]. Transition-metal-promoted interactions of 6,8C2B7H13 with olefins can be employed to introduce alkyl or alkenyl substituents at boron positions, as shown by the reaction with ethylene in the presence of H2PtCl6 or PtBr2 to generate 6,8-C2B7H12-7-Et as the only product [179].
(CO)5M−
− B
H H
C
B B
H
(CO)5M=C(OMe)R
B
H
C
B B
M = Cr, W; R = Me, Ph
B
B
Carbon insertion into 6,8-C2 B7 H12 with metal carbenes.
HCl
H
C B B
5-22 FIGURE 5-12
C
OMe
H
B B
C
R
R C
H
B B
Na+
B = BH C = CH
B
B
B
C
B
C B B
5-23
H
B
5.6 9-Vertex open clusters
129
TABLE 5-9 C2B7Hx Open-Cage Derivatives Compound
Information
References
S, H, B, IR, MS X S, C, B
[19,20] [173] [175]
6,8-C2B7H12Cl 6,8-RC2B7H12 (R ¼ Me, Ph) 6,8-C2B7H12-7-Me 6,8-C2B7H12-7-Et 5CHR [R ¼ (CH2)2Me, Ph, CH5 5CH2, 6,8-C2B7H12-7-CH5 n-C5H11, (CH2)2Ph] 6,8-C2B7H12-3-X (X ¼ Cl, Br, I, HS) 6,8-C2B7H12-5-X (X ¼ Br, I, S, C4H9) 6,8-C2B7H11-3,5-X2 (X ¼ D, Cl, Br, I) 6,8-C2B7H12-endo/exo-6-CH2CH2CN 6,8-C2 B7 H12 6,8-(NCCH¼CH)C2 B7 H11 6,8-C2B7H12-7-R [R ¼ Et, CH¼CH(CH2)2Me, CH¼CHPh] 6,8-C2B7H12-exo-m(4,5,6)-Cr(CO)5-CR(OMe) (R ¼ Me, Ph) 6,8-C2B7H12-endo,endo-7-m-CðCNÞ2 6,7-C2B7H13 (reported as nido- C2B7H11)
S, E, H, B, IR, MS S (pyrolysis of nido-2-CB5H9) S (cage-degradation of nido-7,9-C2 B9 H12 ) S (oxidation of C2 B8 H10 2 ) S, E, H, B, IR, MS X S (oxidation of C2 B8 H10 2 ) S, E, H, B, IR, MS S S, X, C, B (2D), IR, MS S, X[(CH2)2Me, Ph)], C, B (2D), IR, MS
[146,177] [147] [281] [178] [146] [181] [178] [146] [282] [179] [179]
S, H, B, MS S, H, B, MS S, H, B, MS S, X(endo), H, B, C S, H, B, C S, B, H, IR C, B (2D), DFT), IR, MS S, B (2D),H S, X, H, B, C S
6,7-C2B7H12-6-X (X ¼ Cl, Br, I) 6,7-C2B7H12-6,8-I2 6,7-C2 B7 H12 6,7-C2B7H11-exo-6-CNMe3 5,6-C2 B7 H12
S, S, S, S, S, S,
[189] [189] [189] [180] [180] [184] [179] [188] [180] [19,20,165, 166,170] [171,172] [190] [190] [125] [125] [2]
Hypho-C2B7H15 derivatives 4,9-C2B7H13-exo-5-L (L ¼ NMe3, NEt3)
S, B (2D),H, MS
[125]
B (2D), H, C, MS B (2D,T1), H, C B
[172] [171] [278]
Synthesis and characterization nido-C2B7H11 derivatives 1,2-Me2C2B7H9 C2B7H9NR3 (R ¼ H, Me) arachno-C2B7H13 derivatives 6,8-C2B7H13
6,8-Me2C2B7H11
Detailed NMR studies arachno-6,7-C2B7H13
B (2D), H, C, MS H,C,B (2D) H,C,B (2D) B (2D), H B (2D), H, MS X, H, B (2D), C, H
Continued
130
CHAPTER 5 Intermediate carboranes: Seven- to nine-vertex clusters
TABLE 5-9 C2B7Hx Open-Cage Derivatives—Cont’d Compound
Information
References
arachno-6,7-C2B7H13-xDx arachno-5,6-C2 B7 H12
B(2D,T1), H, C H, B(2D), C, H
[171] [2]
Pyrolysis Chromatography He photoelectron spectra Conversion to 1,6-C2B8H10 Metal insertion Conversion to 4,5-C2B7H9 Pyrolysis and reaction with B2H6 Pd/Pt catalyzed olefin addition þ dehydrogenative borylation þ hydroboration, C, B (2D), DFT), IR, MS halogenation reaction with H! arachno-CB7H10Me2
[20] [283] [279] [145,186] [187] [145] [144,145] [179]
Geometry (ab initio) DFT
[280] [179]
arachno-6,8-C2 B7 H12 arachno-6,8-C2B7H12-endo,endo-7-m-CðCNÞ2 arachno-6,8-C2B7H12-endo/exo-6-CH2CH2CN
Geometry (ab initio) Electronic structure Dipole moment (MNDO) MOs Geometry DFT/GIAO DFT/GIAO DFT/GIAO
[280] [249] [214] [284] [285] [180] [180] [180]
Isomerization calculations nido-C2B7H9
Intermediate in isomerization of closo-C2B7H9
[174]
NMR calculations nido-C2B7H9NR3 (R ¼ H, Me) arachno-5,6-C2B7H13 arachno-5,6-C2 B7 H12
C, B (IGLO) IGLO H, B (2D), C (IGLO)
[175] [2] [2]
Reactivity calculations nido-1,2/2,5-C2B7H11 arachno-4,5/4,6-C2B7H13
Ab initio, heat of formation Ab initio, heat of formation
[280] [280]
Other experimental studies nido-1,2-Me2C2B7H9 arachno-6,8-C2B7H13
arachno-6,8-C2 B7 H12 arachno-6,8-RR’C2B7H13 (R, R0 ¼ H, Me) 5CH(CH2)2Me, arachno-6,8-C2B7H12-7-R [R ¼ Et, CH5 CH5 5CHPh] arachno-6,7-C2B7H13 arachno-6,7-C2 B7 H12 Theoretical studies Molecular and electronic structure calculations nido-1,2/2,5-C2B7H11 5CH(CH2)2Me, arachno-6,8-C2B7H12-7-R [R ¼ Et, CH5 CH5 5CHPh] arachno-6,8/6,7-C2B7H13 arachno-C2B7H13
[190] [114]
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; 2D, two-dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; DFT, density functional theory; GIAO, gauge including atomic orbital; MNDO, modified neglect of differential overlap.
5.6 9-Vertex open clusters
131
TABLE 5-10 C3B6 and C3B7 Derivatives Compound
Information
References
Synthesis and characterization Hypho-C3B6H14 derivatives 1-NCCH2-1,2,5-C3 B6 H12
S, X, H, B (2D), C, IR
[184]
S, H, B (2D), MS S, X, IR, MS S, B (2D) S, H, B (2D), IR, MS S, C S, C S (improved) S, H, B (2D), C, IR, MS S, H, B, C, IR, MS
[286] [287] [182] [182] [288] [288] [183] [188] [289]
S, H, B, C S, X, H, B, C S, H, B (2D), C, IR, MS
[290] [290] [185]
S, S, S, S, S, S, S, S, S, S, S, S, S,
[265] [265] [265] [265] [184,291] [292] [184] [184] [184] [184] [185] [286] [180]
nido-C3B7H11 derivatives 5,6,10-Me3C3B7H8 LiðMeCNÞ2 þ nido-5,6,9-MeC3 B7 H9 MeC3 B7 H9 6,7,9-MeC3B7H10 6-Me-5,6,9-C3B7H10 (13C-labeled) 6-Me-5,6,9-C3 B7 H9 (13C-labeled) 6-Me-5,6,9-C3B7H10 6-R-5,6,9-C3B7H10 (R ¼ Me, Ph) 6-R-5,6,9-C3 B7 H9 [R ¼ C6H5, NC(CH2)4, (p-BrC6H4) (Me3SiO)CH, C14H11, H3BNMe2(CH2)2] 5,6,9-C3 B7 H10 5,6,9-C3B7H11 5,6,10-(MeOOCCH2)C3B7H10 arachno-C3B7H13 derivatives 5,6,10-Me3C3B7H9-m(6,7)-CHMe 5,6,10-Me3C3B7H9-m(6,9)-CHMe 5,6,10-Me3C3B7H8-m(6,7)-CHMe 5,6,10-Me3C3B7H8-9-Br-m(6,9)-CHMe 6-NCCH2-5,6,7-C3B7H12 5,6,9-C3B7H13 6-NCCH2-5,6,7-C3 B7 H11 6-NCCHD-5,6,7-C3B7H12 6-MeOC(O)CH2-5,6,7-C3B7H12 6-MeC(O)CH2-5,6,7-C3B7H12 5,6,10-(MeOOCCH2)(C4H9)C3B7H11 5,6,9-[m(6,9)-RCH]RC3B7H11 (R ¼ H, Me) 5,6,9-[m(6,9)-CH2CH2]C3B7H11 Theoretical studies Molecular and electronic structure calculations C3 B 6 H9 C3 B7 H10 þ
H, B, IR, MS H, B, IR, MS H, B, R X, H, B, IR, MS X, H, B (2D), C, IR, MS H, B (2D), MS H, B H, B, C X, H, B, C, IR, MS H, B, C, IR, MS H, B (2D), C, IR, MS H, B (2D), C(R ¼ H), MS X, H, B, C
[191] [108]
arachno-5,6,9-[m(6,9)-CH2CH2]C3B7H11
DFT; electron-counting rule Hartree-Fock and B3LYP; cation isomer stabilities DFT/GIAO
NMR calculations nido-5,6,n-C3B7H11 (n ¼ 8, 10)
IGLO
[185]
[180]
Continued
132
CHAPTER 5 Intermediate carboranes: Seven- to nine-vertex clusters
TABLE 5-10 C3B6 and C3B7 Derivatives—Cont’d Compound
Information
References
nido-C3B7H11 (four isomers)
DFT: isomer stability, energy penalties for specified structural features DFT: isomer stability, energy penalties for specified structural features DFT: structure DFT: structure IGLO GIAO/IGLO IGLO
[267]
nido-C3 B7 H10 (three isomers) nido-5,6,9-C3 B7 H10 nido-5,6,9-C3B7H11 nido-5,6,10-(MeOOCCH2)C3B7H10 arachno-5,6,9-C3B7H13 arachno-5,6,10-(MeOOCCH2)(C4H9)C3B7H11
[267] [290] [290] [185] [293] [185]
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; 2D, two-dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; DFT, density functional theory; GIAO, gauge including atomic orbital.
Corresponding reactions with other alkenes, such as 1-pentene and styrene, result in both hydroboration and dehydrogenative borylation, for example, H2 PtCl6
6; 8-C2 B7 H13 þ H2 C5 5CHðCH2 Þ2 Me ! or PtBr2
6; 8-C2 B7 H12 -7-trans-CH5 5CHðCH2 Þ2 Me þ 6; 8-C2 B7 H12 -7-trans-C5 H11 H2 PtCl6
6; 8-C2 B7 H13 þ H2 C5 5CHPh ! or PtBr2
6; 8-C2 B7 H12 -7-trans-CH5 5CHPh þ 6; 8-C2 B7 H12 -7-CH2 CH2 Ph However, with PdBr2 the main process is dehydrogenative borylation, as in the reaction with ethylene that affords pri5CH2. These reactions proceed somewhat similarly to metal-promoted hydrosilylation and marily 6,8-C2B7H12-7-CH5 hydroboration of alkenes, and probably involve similar mechanisms [179]. Substitution of halogens or SH on 6,8-C2B7H13 can be achieved by AlCl3-catalyzed reactions with CCl4, Br2, I2, or elemental sulfur to give 6,8-C2B7H12-1-X derivatives (see 5-19 for cage numbering) where X is Cl, Br, I, or SH; when X is Cl or Br, disubstituted 6,8-C2B7H11-1,7-X2 products are also obtained [189]. In the absence of AlCl3, iodination with I2 at 100 C affords a mixture of the 1-I, 7-I, and 1,7-I2 derivatives. 6,8-C2B7H13 also reacts with n-butyllithium to give the 6,8-C2B7H12-5-C4H9, and with DCl in the presence of AlCl3 and CS2 to give 6,8-C2B7H11-1,7-D2 [189]. Cyano-substituted derivatives have been prepared from the 6,8-C2 B7 H12 ion by reactions with cyanoolefins; thus reaction with tetracyanoethylene affords the C,B-bridged product 5-24, while reaction with acrylonitrile forms 6,8(endo-NCCH2CH2)C2 B7 H11 which on acidification gives neutral 6,8-(endo-NCCH2CH2)C2B7H12 [180]. On heating, the latter species undergoes a migration of the cyanoethyl group from the endo to an exo position. N H
C
B H B B
5-24 B
− CN
C
C H C B B B
CN C C N
B = BH C = CH
5.6 9-Vertex open clusters
133
5.6.3.3 Structure and properties of 6,7-C2B7H13
Although no X-ray diffraction study has appeared on this molecule or any derivative, its cluster geometry 5-20 is well established from two-dimensional (2D) NMR and other data (Table 5-9), as mentioned earlier. Some chemistry of this system has been explored. 6,7-C2B7H13 can be halogenated by treatment with anhydrous hydrogen halides over an AlX3 catalyst to form 6,7-C2B7H12-8-X derivatives where X is Cl, Br, or I. The evidence suggests that B(8)-H is initially attacked by HþAlX4 to generate H2 and form a B(8)-X bond with release of AlX3, a process termed electrophileinduced nucleophilic substitution; this sequence differs from electrophilic Friedel-Crafts substitution on aromatic hydrocarbons, reflecting the hydridic character of the B-H bond [190]. In the absence of an AlX3 catalyst, the same monohalo species are obtained along with 6,7-C2B7H11-4,8-I2 [190]. Like its 6,8-isomer, arachno-6,7-C2B7H13 is readily deprotonated by nucleophiles to form the 6,7-C2 B7 H12 anion. Reaction of the neutral carborane with methyl isocyanide gives the derivative arachno-6,7-C2B7H12-8-CNMe [125]. However, the interaction of 6,7-C2B7H13 with tertiary amines takes a different direction, opening the cage further and generating products that have been characterized from 2D NMR and other evidence as hypho-C2B7H13-NR3 (5-25), derivatives of the unknown parent species hypho-C2B7H15 [125]. This system, with 2 CH and 7 BH units plus 6 “extra” hydrogens (in the parent molecule), has 26 skeletal electrons (13 SEP) and can be viewed as a 12-vertex icosahedral framework from which three vertexes have been removed, and is therefore of the hypho class in accordance with the structural rules outlined in Chapter 2. NR3
H
B
C H
5-25
B
C
H
B
H
B
B
B
B = BH C = CH R = Me, Et
B
As was described earlier in Section 5.4, the reaction of arachno-6,7-C2B7H13 with aqueous NaCN results in removal of one boron vertex to afford 7,8-hypho-C2 B6 H13 (Figure 5-8) [125,126]. The formation of C3B7 carboranes via addition of alkynes to 6,7-C2B7H13 is discussed in Chapter 6.
5.6.4 Hypho-C3B6H13
Although nido-C3 B6 H9 has been investigated theoretically via DFT calculations [191], the lone example of an experimentally known 9-vertex three-carbon carborane is the hypho-1-NCCH2-1,2,5-C3 B6 H12 ion 5-26 (Table 5-10), prepared from arachno-5,6,7-(NCCH2)C3B7H12 by degradation with aqueous NH4OH and characterized by NMR and X-ray diffraction [184]. As is often the case with carbon-bridged clusters, alternate descriptions are possible depending on whether the bridging carbon is regarded as a part of the cage framework; thus 5-26 can be viewed as a C2B6 carborane with a C(CH2CN) unit bridging its open face. Both approaches are valid from the standpoint of electron-counting theory but, given its synthetic origin from a C3B7 cluster, it is reasonable to classify 5-26 as a 3-carbon carborane system. −
H
B
C
B
5-26
H
B
B B
C
C H B
CH2CN
B = BH C = CH
134
CHAPTER 5 Intermediate carboranes: Seven- to nine-vertex clusters
5.7 9-VERTEX CLOSO CLUSTERS 5.7.1 4-CB8H9
5.7.1.1 Synthesis
The closo-4-CB8 H9 anion has been obtained via several routes, illustrated in Figure 5-10: pyrolysis of nido-6-CB9 H12 or arachno-6-CB9 H14 at 221 C, and oxidation of nido-1-CB8H12 with iodine in the presence of Et3N as a proton scavenger [8,139,192,193]. It is also formed as a minor byproduct, along with closo-4-CB8H9-1-quinolyl, in the reaction of quinoline with nido-1-CB8H12, which generates mainly arachno-7-CB8H12-8-quinolyl (5-17) as described in the previous section [159]. Thermolysis of arachno-6-MeCB9 H13 or nido-6-PhCB9 H11 generates, respectively, the closo-4MeCB8 H8 and closo-4-PhCB8 H8 ions [10,193,194]. closo-4-PhCB8 H8 can be produced in higher yield by oxidation of nido-1-PhCB8 H10 with iodine [10].
5.7.1.2 Structure and properties
The tricapped trigonal prismatic geometry of closo-4-CB8 H9 (5-27) and several substituted derivatives have been determined by X-ray diffraction studies (Table 5-8) [192–194]. −
C B B
5-27
B B
B
B = BH C = CH B
B B
In this system, which is isoelectronic and isostructural with closo-4,5-C2B7H9 (Figure 1-1), the skeletal carbon, as expected, occupies a low-coordinate (capping) vertex. Reactivity studies on this carborane have been limited to an investigation of its behavior toward elemental iodine in CH2Cl2 solution, which affords the 3-I, 5-I, and 5,6-I2 derivatives (5-28-5-30) in yields of 30%, 30%, and 12%, respectively [193]. The pattern of halogenation here, with iodination occurring preferentially at boron sites remote from the skeletal CH group, is similar to that observed in 10- and 12-vertex closo-CBn1Hn anions (n ¼ 9, 11) [135,195–197] and in the 8-vertex system closo-CB7 H8 [134]. The only observed dihalogenated product is 5-30, in which both iodines are bound to low-coordinate boron atoms, strongly implying a directive effect whereby an iodo substituent at B(5) activates B(6) for attack by a second iodine; interestingly, no such effect is observed when the initial iodination is at B(3) [193]. −
4
−
C 7 B
B B
B
8
B B
2
1 6B
9
B B 3
−
C
B5
B
C B B
B B
B B B
I
I
B B
B B B
B
I
I
5-28
5-29
5-30
5.7.2 4,5-C2B7H9 5.7.2.1 Synthesis Closo-4,5-dicarbanonaborane(9), the only known C2B7H9 isomer, and its C-methyl and C-phenyl derivatives were first prepared in 1968 by pyrolyzing arachno-6,8-RC2B7H12 (R ¼ H, Me, Ph) [144–146], but since that time very few additional
5.7 9-Vertex Closo clusters
135
syntheses of this cage system (none on the parent carborane) have been described. The interaction of 2-butyne with B8H12 gives 4,5-Me2C2B7H7 as one of the products [19], and 4,5-Et2C2B7H7 is among the compounds produced by heating nido2,3-Et2C2B4H6 with Et3NBH3 [4]. Copyrolysis of B2Cl4 with CCl4 or C2Cl4 is reported to generate a C2B7Cl9 perchlorinated cluster of unknown structure [24].
5.7.2.2 Structure and properties A microwave analysis of the parent carborane [198] and an X-ray diffraction study of the 4,5-dimethyl derivative [199] confirm the cage geometry as a tricapped trigonal prism with both skeletal carbons in low-coordinate vertexes (Figure 1-1, second row), which is in accord with the general rules of skeletal electron counting and carbon atom placement outlined in Chapter 2. Multinuclear NMR and other spectroscopic data, as well as a number of theoretical investigations (Table 5-11), further support this cage geometry and illuminate its delocalized electronic structure. Five other distinguishable isomers are possible based on the tricapped trigonal prism, but none has been found experimentally, presumably because all such clusters would have at least one CH unit in a less favorable high-coordinate vertex. The conversion of 4,5-C2B7H9 to other isomers has been explored computationally [26,27,29,174,200] and it has been suggested that the cage is fluxional via a double-dsd mechanism involving a cage-opened nido intermediate [174,200], although rearrangement has not yet been observed experimentally. The possibility that pure 4,5-C2B7H9 might contain a significant amount of an open-cage species in equilibrium with the closo structure has been considered, but is discounted on the basis of NMR data and theoretical calculations [175].
TABLE 5-11 Closo-4,5-C2B7H9 Derivatives Compound Synthesis and characterization Parent
4-Me 4-Ph 6-Me 6-Et 6-Br 4,5-Me2
4,5-Et2 4-Me-6-Br 4,5,6-Me3 4,5-Me2-6-Et 4,5-Me2-6-Br
Information
References
S, H, IR S Microwave spectra Dipole moment S, H, IR S, H, IR S, B, H, IR, MS S, B, H, IR, MS S, B, H, IR, MS S, B, H, IR S, B, H, IR, MS X MS MS (calculated monoisotopic) S, B, H S, B, H, IR, MS S, B, H, IR, MS S, B, H, IR, MS S, B, H, IR, MS S, B, H, IR, MS
[145] [144] [198] [198,294] [145] [145] [201] [201] [201] [146] [19,201] [199] [234] [270] [4] [201] [201] [201] [201] [201] Continued
136
CHAPTER 5 Intermediate carboranes: Seven- to nine-vertex clusters
TABLE 5-11 Closo-4,5-C2B7H9 Derivatives—Cont’d Compound
Information
References
1,3,6,7-Br4 4-Me-1,3,6,7-Br4 4,5-Me2-1,3,6,7-Br4 4,5-Me2-B-OPh Cl9
S, S, S, S, S,
[201] [201] [201] [271] [24]
Detailed NMR studies Parent Parent 4-Ph 4-p-C6H4F
C C, B C F, electronic properties
[55,272] [175] [272] [273]
Reaction with amines Reaction with F
[175] [124]
Electronic structure Isomer stability (ab initio) Isomer stabilities Dipole moment (MNDO) Localized orbitals SCF Hþ charge Geometry (ab initio) EI [energy indexes]; stabilities
[249] [207,208,275] [210] [214] [246] [250] [255] [14,217] [13]
Rearrangement mechanism
[26,27,29,174,200]
C (IGLO) C, B (IGLO) B-H coupling
[55] [175] [256]
Other experimental studies Parent Theoretical studies Molecular and electronic structure calculations Parent
Parent isomers Isomerization calculations Parent NMR calculations Parent
B, H, IR, MS B, H, IR, MS B, H, IR, MS MS MS
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; F, 19F NMR; IR, infrared data; MS, mass spectroscopic data; MNDO, modified neglect of differential overlap.
Reactivity studies on 4,5-C2B7H9 and its derivatives are sparse. As noted earlier, the attack of fluoride ion opens the cage, removing a BH unit to form arachno-4,8-C2 B6 H11 as shown in Figure 5-7B [124]. Trimethylamine forms an Me3NC2B7H9 adduct whose structure, based on 11B and 13C NMR spectra and ab initio/GIAO/NMR calculations with NMe3 and NH3, is believed to have the open-cage geometry 5-31 [175]. NMR evidence indicates that 5-31 exists in equilibrium with the uncomplexed carborane.
5.7 9-Vertex Closo clusters
137
NR3 B C C
5-31
B B
B
BKBH CKCH R = H, Me B
B B
Electrophilic substitution of bromine, methyl, and ethyl groups on 4,5-C2B7H9 occurs preferentially at B(6), the unique low-coordinate boron [201], as expected from its high negative charge relative to other boron atoms in the cluster [199]; in the presence of excess bromine, substitution is observed at four boron vertexes. Similar results are obtained on bromination of the C-methyl and C,C0 -dimethyl derivatives [201]. AlCl3
4; 5-C2 B7 H9 þ MeCl ! 4; 5-C2 B7 H8 -6-Me þ HCl AlCl3
4; 5-C2 B7 H9 þ H2 C5 5CH2 ! 4; 5-C2 B7 H8 -6-Et AlBr3
4; 5-C2 B7 H9 þ Br2 ! 4; 5-C2 B7 H8 -8-Br þ HBr 4; 5-C2 B7 H9 þ excess Br2 ! 4; 5-C2 B7 H8 -1;3;6;7-Br4 þ 4HBr
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5.7 9-Vertex Closo clusters [177] [178] [179] [180] [181] [182] [183] [184] [185] [186] [187] [188] [189] [190] [191] [192] [193] [194] [195] [196] [197] [198] [199] [200] [201] [202] [203] [204] [205] [206] [207] [208] [209] [210] [211] [212] [213] [214] [215] [216] [217] [218] [219] [220] [221] [222] [223] [224] [225] [226] [227] [228]
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142
CHAPTER 5 Intermediate carboranes: Seven- to nine-vertex clusters
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5.7 9-Vertex Closo clusters [276] [277] [278] [279] [280] [281] [282] [283] [284] [285] [286] [287] [288] [289] [290] [291] [292] [293] [294]
143
Gimarc, B. M.; Ott, J. J. J. Am. Chem. Soc. 1987, 109, 1388. Hermanek, S.; Fusek, J.; Sˇtı´br, B.; Plesˇek, J.; Jelinek, T. Polyhedron 1986, 5, 1873. Dolansky, J.; Hermanek, S.; Zahradnik, R. Collect. Czech. Chem. Commun. 1981, 46, 2479. Vondrak, T.; Hermanek, S.; Plesˇek, J. Polyhedron 1993, 12, 1301. McKee, M. L. Inorg. Chem. 1994, 33, 6213. Basˇe, K.; Hermanek, S.; Hanousek, F. J. Chem. Soc. Chem. Commun. 1984, 299. Knyazev, S. P.; Brattsev, V. A.; Stanko, V. I. Zh. Obshch. Khim. 1977, 47, 2627 [Russian]. Plzak, Z.; Sˇtı´br, B. J. Chromatogr. 1978, 151, 363. Hall, J. H., Jr; Dixon, D. A.; Kleier, D. A.; Halgren, T. A.; Brown, L. D.; Lipscomb, W. N. J. Am. Chem. Soc. 1975, 97, 4202. Fox, M. A.; Hughes, A. K.; Malget, J. M. J. Chem. Soc. Dalton Trans. 2002, 3505. Sˇtı´br, B.; Jelinek, T.; Janousek, Z.; Hermanek, S.; Drdakova, E.; Plzak, Z.; et al. J. Chem. Soc. Chem. Commun. 1987, 1106. Wasczcak, M. D.; Wang, Y.; Garg, A.; Geiger, W. E.; Kang, S. O.; Carroll, P. J.; et al. J. Am. Chem. Soc. 2001, 123, 2783. Plumb, C. A.; Sneddon, L. G. Organometallics 1992, 11, 1681. Ramachandran, B. M.; Carroll, P. J.; Sneddon, L. G. Inorg. Chem. 2004, 43, 3467. Holub, J.; Bakardjiev, M.; Hnyk, D.; Cisarˇova´, I.; Sˇtı´br, B. Inorg. Chem. 2008, 47, 760. Su, K.; Barnum, B.; Carroll, P. J.; Sneddon, L. G. J. Am. Chem. Soc. 1992, 114, 2730. Sˇtı´br, B.; Holub, J.; Cisarˇova´, I.; Teixidor, F.; Vin˜as, C. Inorg. Chim. Acta. 1996, 245, 129. Hnyk, D.; Holub, J. Collect. Czech. Chem. Commun. 2002, 67, 813. Echeistova, A. I.; Syrkin, Ya. K.; Rys, E. G.; Kalinin, V. N.; Zakharkin, L. I. Zh. Strukt. Khim. 1974, 15, 154 [Russian].
CHAPTER
Ten-vertex carboranes
6
6.1 OVERVIEW Cage systems of the CB9, C2B8, C3B7, and C4B6 classes are assuming an increasingly important role in carborane chemistry. Many open-cage clusters in this range are amenable to insertion of main-group and transition-metal heteroatoms into their open faces to create heteroatom carboranes and metallacarboranes, thousands of which have been synthesized. Moreover, 10-vertex closo-carboranes have found application in areas such as liquid crystals and nonlinear optics and as nearly noncoordinating counterions for extremely strong Lewis acids, and are under investigation as linkers for attaching radioactive halogen isotopes to tumor-targeting proteins and peptides [1]. Most carboranes in this size regime are prepared via controlled degradation of larger clusters, carbon incorporation into boron hydrides, or, less commonly, by cage expansion of lower carboranes.
6.2 10-VERTEX OPEN CLUSTERS
6.2.1 Nido-CB9H12 and arachno-CB9H14 6.2.1.1 Synthesis Several routes to monocarbon 10-vertex nido- and arachno-carborane anions are known, but recent developments have simplified this area considerably and have made these species, which are valuable synthons in their own right, more readily available. As was mentioned in Section 5.5, the arachno-CB9 H14 ion can be prepared by treating B10H14 with aqueous alkaline formaldehyde, a procedure known as the Brellochs reaction [2–4]. Oxidation of this species with I2 in the presence of triethylamine (as a proton scavenger) affords nido-6-CB9 H12 (see Figure 5-10) [2]; conversely, reduction of the latter ion with sodium in liquid ammonia regenerates arachno-CB9 H14 [5,6]. An earlier route to nido-6CB9 H12 , which is less efficient and requires several steps starting with B10H14, involves the reaction of 6-(Me3N) CB9H11 (itself generated by treatment of the cyanodecaborane anion B10H13CN2 with HCl [7,8]. with sodium in liquid ammonia followed by methanol [9,10]. The Brellochs reaction can be employed using higher aldehydes to afford C-substituted arachno-CB9 H14 derivatives (Table 6-1) [4,12–14,16,17]: B10 H14 þ HCðOÞR þ 2OH þ H2 O ! 6-RCB9 H 13 þ BðOHÞ4 þ H2 ½R ¼ H; Me; CðOÞOH B10 H14 þ C6 H4 CHðOÞCðOÞOH þ 2OH þ H2 O ! 6-½HOðOÞC-C6 H4 CB9 H 13 þ BðOHÞ4 þ H2
However, PhCHO and B10H14 react to give nido-PhCB9 H12 rather than the arachno species [12–14]. B-substituted derivatives of arachno-CB9 H14 can be obtained by several routes. Reaction of the nido-7,9-C2 B10 H13 ion with aqueous trimethylamine in the presence of ethanol extracts both carbon and boron from the cage to afford arachno-6-(endo-Me)CB9H12-8-NMe3 (Figure 6-1) [11,20]. Similar reactions of the C2 B10 H13 ion can be employed to prepare arachno-6-CB9H12-L derivatives in which L is pyridine, urotropine, PPh3, SMe2, or other Carboranes. DOI: 10.1016/B978-0-12-374170-7.00012-4 © 2011 Elsevier Inc. All rights reserved.
145
146
CHAPTER 6 Ten-vertex carboranes
Table 6-1 Open-Cage CB9 Derivatives Compound Synthesis and Characterization No substituents on boron Nido-6-CB9 H12
Nido-6-PhCB9 H11
Nido-6-(p-BrC6H4)CB9 H11 Nido-6-RCB9 H11 [R ¼ Ph, p-C6H4Br, p-C6H4NHC(O)Me, 2-SC4H3] Nido-6-(p-HOCH2-C6H4)CB9 H11 Nido-6-[p-HO(O)C-C6H4]CB9 H11 Nido-6-(H3N)CB9H11 Nido-6-(Me3N)CB9H11
5NH)CB9H11 Nido-(Me2C5 Arachno-6-CB9 H14
Arachno-6-MeCB9 H13 Arachno-6-[HO(O)C]CB9 H13 Arachno-6-(p-OMe-C6H4)CB9 H13 C, N-, or P-containing substituents on boron Nido-9-(Me3CNH2)CB9H8-8-CN-conjuncto-B8H10 Nido-6-CB9H10-8-NMe3-9-Me 2B) Nido-6-(NC5H4-CH2)CB9H10 (N2 Arachno-6-PhCB9H12-9-(NC5H4-o/p-NH2) Arachno-6-CB9H12-8-NMe3-9-endo-Me Arachno-6-CB9H13-9-NCMe Arachno-6-CB9H13-9-PPh3 Arachno-6-CB9H12-9-L [L ¼ NH2Ph, NC5H5, NC5H4-pCH2Ph, NC5H4-p-Ph, NC9H7 (quinoline)] O- or S-containing substituents on boron Nido-6-CB9H10-8-OH-9-Me Arachno-6-CB9H13-9-SMe2
Informationa
References
S, H, B S, B, C H, B S, H, B, C S, X, H, B, C S S, H, B, C S, H, B
[9,10] [2] [7,11] [12] [13] [14] [15] [4]
S, S, X S, S, S S, S, S, S, S, S, S, S, S,
H, B, IR, UV H, B H, B X, B, C H, B (2d), C H, B H, B H, B, C, IR H, B
[16,17] [16,17] [16] [7] [9,10] [8] [18] [7] [7] [2] [6] [4,5,12] [12] [16,17] [4]
S, X, H, B(2d) S, H, B, MS B(2d) S, X, H, B, MS S, X, H, B, MS S, B(2d), MS S, H, B S, H, B, MS S, H, B, MS S, X(all), H, B
[19] [11,20] [20] [21] [22] [20] [11] [9,23] [9,23] [24]
X S, H, B, MS S, X, H, B
[25] [9,23] [24]
H, B, C, IR H, B, C, IR H, B B
Continued
6.2 10-Vertex open clusters
147
Table 6-1 Open-Cage CB9 Derivatives—Cont’d Compound
Informationa
References
Arachno-6-CB9H11-9-SMe2-m(6,9)-CH2BOH
S, H, B(2d)
[26]
Thermolysis ! 4-CB8 H9 Thermolysis ! 4-PhCB8 H8 Oxidative cage closure Thermolysis ! closo-4-CB8 H9 Thermolysis ! 4-CB8 H9 Oxidation ! RCB9 H9 Thermolysis ! closo-4-MeCB8 H8
[27] [12] [16] [12] [27] [28] [12]
DFT: isomer stability, energy penalties for specified structural features Ab initio heat of formation, geometry DFT
[29]
Other Experimental Studies Nido-6-CB9 H12 Nido-6-PhCB9 H11 Nido-6-[p-HO(O)C-C6H4]CB9 H11 Arachno-6-CB9 H14 Arachno-6-CB9 H14 Arachno-6-MeCB9 H13 Theoretical Studies Molecular and electronic structure calculations Nido-n-CB9H10q (n ¼ 1, 2, 5, 6; q ¼ 0–3) Nido-6-CB9H13 Arachno-6-CB9 H14 a
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, spectroscopic data; UV, UV-visible data.
11
B NMR; C,
13
C NMR; 2d, two-dimensional (COSY) NMR; IR, infrared data; MS, mass
−
C
H
B
B C
B B B
B B
[30] [2]
B
thf
B B C2B10H12
C
B Na, naphthalene
B = BH, B C = CH, C
B
B
B
C
B B
B B
H
B B
Me NMe3
H3O+
B
H2O, NMe3
B
−B(OH)3 −H2
B
H
C
H H
H
B B
B
B
B
B
7,9-C2B10H−13
6-MeCB9H12-8-NMe3
FIGURE 6-1 Synthesis of arachno-6-(endo-Me)CB9H12-8-NMe3 from nido-C2 B10 H13 .
Lewis base. As was mentioned in Chapter 5, arachno-6-CB9H11-9-SMe2-m-6,9-CH2B(OH) is obtained from the interaction of nido-7,9-C2 B10 H13 with Me2S and HCl [26]. Arachno-6-CB9H12-L compounds can also be prepared from nido-CB9 clusters, for example, by treating the nido-6CB9 H12 anion with anhydrous HCl and SMe2, MeCN, or PPh3 [9,23], or by reacting nido-6-PhCB9 H11 with 2-electron donor Lewis bases in the presence of hydrated ferric chloride to generate arachno-6-PhCB9H12-L products, where L is SMe2, pyridine or pyridine derivatives, or NC9H7 (quinoline) (Figure 6-2) [22,24]. On extended reaction with hydrated FeCl3, the arachno species having pyridine-type ligands undergo oxidative cage closure to generate neutral closo1-PhCB9H8-L compounds, but the location of the L group on the cage differs. When the ligand is o-aminopyridine, the product is 1-PhCB9H8-2-L, but with pyridine or organosubstituted pyridines attachment occurs at B(6), as shown. The 2-substitution observed in the former case is an unusual feature (closo-CB9 clusters are discussed in the following section).
148
CHAPTER 6 Ten-vertex carboranes Ph − H
C
B H
B B B
B
H
B
B H B B B B
FeCl3(OH2)6
B B
B
C
B L
B
H
H
L
Ph
C
Ph B
FeCl3(OH2)6 L = NC5H5, NC5H4-p-CH2Ph, NC5H4-p-Ph, NC9H7
B
L
B
B = BH, B C = CH, C
B B
B
B
B B
6-PhCB9H12-9-L
6-PhCB9H−11
B
B
1-PhCB9H9-6-L Ph L
FeCl3(OH2)6 L = NC5H4-o-NH2
C B B
B B B
B
B
1-PhCB9H9-2-L
B B
FIGURE 6-2 Synthesis of arachno-6-PhCB9H12-9-L and closo-1-PhCB9H8-n-L derivatives (n ¼ 2 or 6) from the nido-6-PhCB9 H11 anion.
Curiously, the reaction of nido-6-PhCB9 H11 with p-aminopyridine generates an arachno product that is unreactive toward FeCl3 and does not form a closo species. X-ray diffraction data on closo-1-PhCB9H8-2-(NH2-o-NC5H5) reveal intramolecular B2 2H H2 2N dihydrogen bonding, whereas closo-1-PhCB9H8-6-(NH2-p-NC5H5) forms an intermolecular network linked by B2 2H H2 2N interactions [22].
6.2.1.2 Structure and properties
The cage geometry of nido-6-CB9 H12 , based on an 11-vertex closo-polyhedron with its unique high-coordinate vertex removed (see Chart 2-2, second row center and Figure 5-10), is well defined from spectroscopic evidence (Table 6-1) and X-ray diffraction analyses on the derivatives nido-6-[p-HO(O)C-C6H4]CB9 H11 [16] and nido-6-CB9H10-8-OH-9Me [25]. A fused-cage species, nido-9-(Me3CNH2)CB9H8-8-CN-conjuncto-B8H10 (6-1), prepared from the reaction of anti-B18H22 with CNCMe3, has the geometry of anti-B18H22 with one boron replaced by carbon and can be described as a nido-CB9 cage edge-fused to an octaborane fragment [19]. CMe3 B B
NH2 H
6-1
B B
C NC
B B
B
B B
H
B
B B B H B H
B = BH C = CH
B B B
The structure of parent arachno-CB9 H14 , a 10-vertex icosahedral fragment (see Chart 2-2, third row right and Figure 5-10), has been crystallographically determined [2], as have several C- and B-substituted derivatives [22,24] listed in Table 6-1. Exploration of the chemistry of the nido-6-CB9 H12 and arachno-CB9 H14 systems has mostly centered on their interconversion via redox processes, introduction of substituents, and cage closure to generate closed polyhedral CB8 and CB9 species, as described earlier; the synthesis of closo-4-CB8 H9 via pyrolysis of nido-6-CB9 H12 or arachno6-CB9 H14 was mentioned in Section 5.7. An unusual example of cage isomerism in monocarbon carborane chemistry is found in the oxidation of arachno-6-CB9 H14 or nido-6-PhCB9 H11 by I2 to give closo-2-RCB9 H9 clusters (R ¼ H
6.2 10-Vertex open clusters
149
or Ph) in which the carbon atom occupies an uncommon higher coordinate equatorial vertex (Figure 6-3) [13,16,28]. The latter species rearrange at high temperatures to the closo-1-RCB9 H9 isomer; other closo-2-RCB9 H9 and 1-RCB9 H9 derivatives can be obtained in this way (Table 6-4 and Section 6.3). The 10-vertex monocarbon nido- and arachno-carboranes have been explored in detailed NMR and ab initio-DFT theoretical studies (Table 6-1). From considerations of “energy penalties” arising from destabilizing structural features in these species, predictions have been ventured on the stable existence of new isomers that have not yet been found experimentally [29]. − H
Ph
C
B H B
B B B
I2
B B
OH−
B
R
−
B B 6-PhCB9H−11
R B = BH, B
C
B
B
C
B
B
88 °C
B
B
C = CH, C H
B
B
B H B B B B
B B B B
C 2-RCB9H−9
I2
B
B
B
H H
B
B
B
B
−
1-RCB9H−9
R = H, Ph
OH−
B
B 6-CB9H−14
FIGURE 6-3 Synthesis and thermal isomerization of closo-2-RCB9 H9 anions.
6.2.2 Nido-C2B8H12 6.2.2.1 Synthesis Three isomers of nido-dicarbadecaborane(12) are known, of which the first to be prepared was the symmetrical species 5,7-C2B8H12, obtained by treatment of arachno-6,8-C2B7H13 with NaH followed by boron insertion with diborane (Figure 6-4) [70]. Reactions of 6,8-C2B7H13 with RR0 NBH3 adducts in heated ionic liquids generate nido-5,7-C2B8H11-6NRR0 products along with closo-1,6-C2B8H10, as discussed below in Section 6.3. 4
B
B
1
H H
H
6
5
B
C
B
C
9
B
2
9
B
H
8
B
3
6,8-C2B7H13
FIGURE 6-4 Synthesis of nido-5,7-C2B8H12 from 6,8-C2B7H13.
7 B
B
4
B = BH C = CH
6
10
B
7
8
C
B B
(1) NaH (2) B2H6
H H
B
C
5
3
B
1
5,7-C2B8H12
B
2
150
CHAPTER 6 Ten-vertex carboranes
Asymmetric nido-5,6-C2B8H12 and its C,C0 -dimethyl derivative were originally obtained in reactions of B8H12 or CR (R ¼ H or Me) [71,72]. Subsequently, syntheses were developed based on more accessible subn-B9H15 with RC strates such as B9H13-4-L (L ¼ OEt2 or SMe2), nido-7,8-R2 C2 B9 H10 , and 7,9-C2 B10 H13 , which are easily prepared from commercially available B10H14 or 1,2-C2B10H12 (Figure 6-5) [73–81]. In the oxidation of nido-7,8-R2 C2 B9 H10 by FeCl3, the byproducts 5,6-C2B8H11-10-X (X ¼ Cl, OH) are obtained along with the parent carborane [73]. As was mentioned in Chapter 5, derivatives of nido-5,6-C2B8H12 are also formed via alkyne incorporation into arachno-4-CB8H14 [82]. L
H
−
B
H
B
H
B B
H
B
H
RC≡CR
H H
B
C B
B
B
B
C
B
B
B
B
B
B
B
B
B
B9H13-4-L L = SMe2, OEt2
B = BH C = CR R = H, alkyl, aryl
C C
B
FeCl3
B
B
B B
B
B
5,6-C2B8H12
7,8-R2C2B9H−10
FeCl3 −
H
C
B
7,9-C2B10H−13
B
B C
B B
B
B
B
B B
FIGURE 6-5 Synthetic routes to nido-5,6-C2B8H12 and C,C0 -substituted derivatives.
A third isomer, nido-6,9-C2 B8 H10 2 , is known only as an ion and as a ligand in metal complexes; neutral, uncomplexed nido-6,9-C2B8 clusters have not been isolated. The nido-5,6-C2 B8 H11 ion, obtained via deprotonation of 5,6C2B8H12 [78, 83], disproportionates to nido-6,9-C2 B8 H10 2 and closo-1,2-C2B8H10 at 120–200 C [84]: 2 2 nido-5; 6-C2 B8 H 11 ! nido-6; 9-C2 B8 H10 þ closo-1; 2-C2 B8 H10 þ H2
Nido-6,9-C2 B8 H10 2 is also obtained on reduction of 5,6-C2B8H12 with sodium metal [84]. Complexes such as nido-6,9-C2B8H10-m(6,9)AlEt(OEt2) (6-2) and Al(nido-6,9-C2B8H10) 2 can be obtained by reaction of nido-5,6-C2 B8 H11 salts with AlEt2ClOEt2 or AlEt2Cl in refluxing toluene [85–87]; nido-6,9-C2B8H10-m(6,9)ML2 complexes 6-3, 6-4, and 6-5 of analogous structure, where M is Pt or Ni (Table 6-2), have been prepared as well [93]. L M
6-2 M = Al L = Et, L⬘ = OEt 6-3 M = Pt L, L⬘ = PPh3
C
6-4 M = Pt L, L⬘ = SEt2 6-5 M = Ni L, L⬘ = cis-diaminocyclohexane
L⬘
B
C B B B B
B B
B = BH C = CH B
6.2 10-Vertex open clusters
151
Table 6-2 Nido-C2B8H12 Derivatives Compound Synthesis and Characterization Nido-5,7-C2B8H12 Nido-5,7-C2B8H11-6-NMe2 dative B5 5N double bond Nido-5,7-C2B8H11-6-NHCMe3 dative B5 5N double bond Nido-5,7-C2B8H11Cl Nido-5,6-C2B8H12
Nido-5,6-C2B8H11-n-Cl (n ¼ 3,4,7) Nido-5,6-C2B8H10-3,4-Cl2 Nido-5,6-C2B8H9-3,4,7-Cl3 Nido-5,6-C2B8H11-7-Br Nido-5,6-C2B8H10-4,7-Br2 Nido-5,6-C2B8H11-7-I Nido-5,6-Me2C2B8H10
Nido-5,6-R2C2B8H9-9-Me (R ¼ H, Me, Et) Nido-5,6-MeC2B8H10-1-Ph Nido-5,6-(5-CpFeC5H4)C2B8H11 Nido-5,6-(6-CpFeC5H4)C2B8H11 Nido-5,6-(cyclo-CH2OCH2)C2B8H10 Nido-5,6-PhC2B8H11 Nido-5,6-PhC2B8H10-9-Me Nido-5,6-Ph2C2B8H9-9-Me Nido-5,6-C2B8H11-10-OH Nido-5,6-C2B8H11-n-SH (n ¼ 3, 4, 10) Nido-5,6-C2B8H11-10-Cl Nido-5,6-C2B8H11-8-[30 -(10 ,20 -C2B10H11)] Nido-5,6-C2 B8 H11 Nido-5,6-C2B8H10-6-R (R ¼ Me, n-C6H13) Nido-6,9-C2 B8 H10 2 Nido-6,9-C2B8H10-m(6,9)AlEt(OEt2) Al(nido-6,9-C2B8H10)2 Nido-(6,9-C2B8H10)-m-Al2
Informationa
References
S, H, B, IR C S, B, MS
[70] [82] [88]
S, B, MS
[88]
S, S, S, S, S, S, S, S, S, S, S, S, S, S, S S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S,
[70] [80, [73] [79] [78] [72] [89] [89] [89] [89] [89] [89] [71, [80, [76] [82] [82] [74] [74] [77] [80, [82] [82] [73] [90] [73] [91] [92] [83] [83] [84] [85, [85] [87]
H, B H, B(2d), MS B, MS H, B(2d) H, B, IR H, B, IR, MS H, B(2d), MS H, B(2d), MS H, B(2d), MS H, B(2d), MS H, B(2d), MS H, B(2d), MS H, B, IR, MS H, B(2d), MS H, B, C H, B, C H, B X, H, B H, B H, B(2d), MS H, B, C H, B, C B, IR, MS H, B, MS B, MS H, B, IR, MS, UV X H, B(2d) H, B(2d) B, IR X, H, B, IR X, H, B, IR, MS X, H, B, IR
81]
72] 81]
81]
86]
Continued
152
CHAPTER 6 Ten-vertex carboranes
Table 6-2 Nido-C2B8H12 Derivatives—Cont’d Compound
Informationa
References
Nido-6,9-C2B8H10-m(6,9)-Pt(PPh3)2
S, S, S, S,
[93] [94] [44] [93]
H, B, IR, UV X H, B, IR, UV H, B, IR, UV
Nido-6,9-C2B8H10-m(6,9)-Pt(Et2S)2 Nido-6,9-C2B8H10-m(6,9)-Ni[cis-1,2-(H2N)2cyclo-C6H10]2 Nido-6,9-C2B8H10-m(6,9)AlEt(C4H8O) Nido-6,9-C2B8H10-m(6,9)-lEt(C4H8O)
S, H, B, IR S, H, B, IR
[85] [86]
Detailed NMR Studies Nido-5,6-C2B8H12 Nido-5,6-C2B8DnH12-n (n ¼ 3,4) Nido-5,6-C2B8H11-4/8-Cl Nido-5,6-C2B8H11-2-Ph Nido-5,6-C2B8H11-7-X (X ¼ Br, I) Nido-5,6-C2 B8 H11 Nido-6,9-RC2 B8 H9 2 (R ¼ H, Me, Ph) Nido-5,6-C2B8H11-n-Cl (n ¼ 3,4,7) Nido-5,6-C2B8H10-3,4-Cl2 Nido-5,6-C2B8H9-3,4,7-Cl3 Nido-5,6-C2B8H11-7-Br Nido-5,6-C2B8H10-4,7-Br2 Nido-5,6-C2B8H11-7-I
B B B B B H, B H(2d), B B(2d) B(2d) B(2d) B(2d) B(2d) B(2d)
[95] [95] [95] [95] [95] [96] [97] [89] [89] [89] [89] [89] [89]
Deprotonation; nonmetal insertion Metal insertion Chromatography Conversion to arac-C2B8H14 Polyhedral contraction with RuCl2(PPh3)2 ! RuC2B8 cluster Metal insertion Enantiomeric conversion, H, B Pyrolysis ! closo-C2B8 isomers Isomerization Reaction with [LMCl2]2, M ¼ Ru, Rh ! MC2B8 Oxidative closure ! closo-C2B8H10 isomers Oxidation to nido-4,6-C2B7H13 and nido-4,6C2B7H12Cl Reaction with HX
[98] [99] [100] [101] [102]
[107]
Dipole moment (MNDO) Heat of formation, geometry (ab initio)
[108] [30]
Other Experimental Studies Nido-5,6-C2B8H12
Nido-5,6-C2 B8 H11 Nido-d,l-5,6-C2B8H12 Nido-5,6-R2C2B8H12 (R ¼ H, Me) Nido-5,6-C2B8H10-6-R (R ¼ Me,C6H13) Nido-5,6/6,9-C2B8H12 Nido-6,9-C2 B8 H10 2
Theoretical Studies Molecular and electronic structure calculations Nido-C2B8H12 Nido-5,6/6,9-C2B8H12
[103] [96] [84, 104] [83] [105] [106] [106]
Continued
6.2 10-Vertex open clusters
153
Table 6-2 Nido-C2B8H12 Derivatives—Cont’d Compound 2
Nido-C2 B8 H10 Nido-C2 B8 H10 2 Nido-C2 B8 H10 2 (9 isomers) Nido-C2 B8 H11 (5 isomers) Nido-C2B8H12 (32 isomers) NMR calculations Nido-5,7-C2B8H11-6-R (R ¼ NMe2, NHCMe3) a
1
X, X-ray diffraction; H, H NMR; B, data; UV, UV-visible data.
11
B NMR; C,
Informationa
References
Second moment Hu¨ckel Less stable isomer DFT: isomer stability, energy penalties for specified structural features DFT: isomer stability, energy penalties for specified structural features DFT: isomer stability, energy penalties for specified structural features
[109] [110] [29]
DFT/GIAO
[88]
[29] [29]
13
C NMR; 2d, two-dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic
6.2.2.2 Structure No X-ray diffraction data are available on unsubstituted nido-C2B8H12 carboranes, but the structures of the 5,6, 5,7, and 6,9 isomers shown above are supported by multinuclear NMR data on the parent compounds and several derivatives (Table 6-2), detailed 11B NMR studies on 5,6-C2B8H12 [95] and 6,9-RC2 B8 H9 2 anions [97], and X-ray crystallographic data on the linked-cage species nido-5,6-C2B8H11-8-[30 -(10 ,20 -C2B10H11)] [92]. X-ray diffraction analyses have confirmed the cage geometry of 6-2 and related aluminum complexes [85] and of 6-3 [94]. Optically active ()-5,6C2B8H12 has been obtained by the action of (þ)-N-methylcamphidine on racemic ()-5,6-C2B8H12 [96]. The structures and properties of neutral C2B8H12 isomers, the C2 B8 H11 2 anion, and the C2 B8 H10 2 dianions have been explored computationally (for literature references see Table 6-2).
6.2.2.3 Deprotonation and heteroatom insertion Nido-5,6-C2B8H12, as expected for an open-cage carborane having B2 2H2 2B bridges, is a highly reactive species and a useful synthon for other carborane systems, undergoing B2 2H2 2B deprotonation, introduction of metal and nonmetal units into the cage, and other processes; carbon insertion to generate tricarbon carboranes is described in Chapter 7. Reaction with NaH in diethyl ether affords the nido-5,6-C2 B8 H11 ion, which can be treated with PCl3 or AsCl3 and zinc dust in refluxing ether to give the heterocarborane species MC2B8H11 (M ¼ P, As) whose proposed structure is 6-6 [98]. Treatment of the arsenacarborane with aqueous TlOH yields 6-7, formulated as TlAsC2B8H11 from NMR and mass spectroscopic data [98]. Tl H
B
6-6 MC2B8H11
M = P, As
C C
B M B
B B
B B B
B
6-7 TlAsC2B8H11
B
C C
As B
B B
B B B
As was noted earlier, reactions of metal reagents with the nido-5,6-C2 B8 H11 ion in some cases result in cage rearrangement to afford nido-6,9-C2 B8 H10 2 derivatives such as 6-2 and 6-5. However, 11-vertex MC2B8 metallacarboranes in which the 5,6-C2B8 cage geometry is retained have been obtained in reactions of neutral 5,6-C2B8H12 or the 5,6-C2 B8 H11 ion with transition-metal salts or organometallic halides [99, 103, 105] (see Chapter 13). A remarkable polyhedral contraction, with loss of four boron atoms, occurs when 5,6-C2B8H12 and RuCl2(PPh3)2 are heated at 95 C in toluene: although metal
154
CHAPTER 6 Ten-vertex carboranes
insertion occurs to give 11-vertex RuC2B8 clusters as minor products, the major species produced is a 7-vertex ruthenacarborane, closo-(H)Cl(PPh3)2Ru(C2B4H6) [102]. This and related findings [102] suggest that 16-electron metal reagents such as RuCl2(PPh3)2 are unusually powerful boron extractors that can play a special role in synthesis.
6.2.2.4 Introduction of substituents Controlled routes to B- and C-substituted derivatives of 5,6-C2B8H12 are not available in general, but the formation of Bchloro and B-hydroxy compounds as side products in the synthesis of 5,6-C2B8H12 via oxidation of nido7,8-R2 C2 B9 H10 was mentioned earlier. Electrophilic chlorination, bromination, and iodination have been examined, with the general finding that reactivity at the boron atoms decreases in the order B(7) > B(4) > B(3) [89]. Reaction of the parent carborane with elemental sulfur over AlCl3 at 200 C affords the thiols 5,6-C2B8H11-n-SH where n is 3, 4, or 10 [90]. As with other open-cage carborane systems, the only generally available pathway to C-substituted derivaCR0 alkynes into boranes where R and/or R0 are alkyl or aryl groups, as described above. tives is via insertion of RC
6.2.2.5 Cage rearrangement and conversion to other cluster systems 5,6-Dicarbaoctaborane(12) and its conjugate base anions are important precursors to several types of carboranes, forming closo-C2B8 species via hydrogen loss or oxidation (Section 6.3) and undergoing reduction to generate arachno-C2B8 carboranes as discussed below; the disproportionation of nido-5,6-C2 B8 H11 to nido-6,9-C2 B8 H10 2 and closo-1,2-C2B8H10 on heating was noted earlier. As expected, deprotonation of neutral nido-5,6-C2B8H11-6-R (R ¼ H, Me, or n-C6H13) forms the corresponding nido-5,6-C2B8H10-6-R ions; however, deprotonation of the 5-substituted analogues 5,6-C2B8H11-5-R (R ¼ Me or n-C6H13) affords only nido-5,6-C2B8H10-6-R ions, which can be re-protonated to generate neutral nido-5,62R bond C2B8H11-6-R (Figure 6-6). The apparent “migration” of the R group from B(5) to B(6) actually involves no B2 cleavage; it has been explained in terms of fluxional interconversion of the 5,6-C2B8 cage enantiomers (Figure 6-7), which in the case of 5,6-C2B8H11-5-R derivatives leads to a quantitative conversion to the thermodynamically favored 6-substituted isomer [83]. H
B H
B B
B
B
H
C
B C
B H −H+
5,6-C2B8H10-5-R−
rearr
5,6-C2B8H10-6-R−
B B
H+ B
B R
B 5,6-C2B8H11-5-R−
C
C B
B
R
B
B 5,6-C2B8H11-6-R
FIGURE 6-6 Conversion of 5,6-C2B8H11-5-R to 5,6-C2B8H11-6-R derivatives via deprotonation, cage rearrangement, and reprotonation.
B B
B
B B
B
B
C
B B B
B
B
C
B C
B
B B
B C
B B
B
B C
B C B
B B
B B B
C
C B B
B
B
FIGURE 6-7 Proposed fluxional cage interconversion of D- and L- enantiomers of 5,6-C2 B8 H11 anion via “double boron swing” mechanism.
6.2 10-Vertex open clusters
155
The addition of hydrogen halides to the nido-6,9-C2 B8 H10 2 anion in benzene proceeds stereospecifically, giving exclusively arachno-6,9-C2B8H13-5-X products (see Figure 6-4 for cage numbering, which is the same for nido- and arachno-C2B8 cages) [107, 111]: 2 þ 3HX ! 6; 9-C2 B8 H13 -5-X þ 2X ðX ¼ Cl; Br; IÞ 6; 9-C2 B8 H10
This reaction can be viewed as an attack of HX at an “unsaturated” C(6)–B(5) bond, adding two electrons to form arachno-6,9-C2B8H11X2, which is subsequently protonated by two additional HX molecules to give the neutral species. Accordingly, when DCl is employed one obtains the trideuterated species arachno-6,9-C2B8H10D3-5-X [111]. Treatment of nido-6,9-C2 B8 H10 2 with 97% HF affords not only arachno-6,9-C2B8H13-5-F but also an oxo-bridged biscarborane, O-5,50 -(arachno-6,9-C2B8H13)2, suggesting the involvement of a hydroxy-substituted intermediate, 6,9C2B8H13-5-OH [111]. The same compound is obtained along with an analogous OSO2O-bridged biscarborane, in the reaction of nido-6,9-C2 B8 H10 2 with concentrated sulfuric acid [112].
6.2.3 Arachno-C2B8H14 6.2.3.1 Synthesis Several structural isomers having an arachno-C2B8 cage framework are known experimentally, but only 6,9-C2B8H14 (6-8) has been isolated in parent form. This compound was first prepared by reduction of nido-5,6-C2B8H12 with sodium amalgam in ethanol at 50 C [101, 113], but an improved synthesis was later developed based on reduction of the same nido-carborane with NaBH4 in ethanolic NaOH [111, 114]. The preparation of B(5)-halo derivatives of 6,9-C2B8H14 from nido-6,9-C2 B8 H10 2 and hydrogen halides was described above. Remarkably, the C-trimethylsilyl compound 6,9-(Me3Si)C2B8H13 was isolated in 21% yield from a cage-expansion reaction of the 7-vertex carborane nido(Me3Si)2C2B4H6 with B5H9 in which four borons are added to the cage [115]. H
H
H
C
H
C
H
B
6-8
B
H
B = BH
B
B B
B
B
B
Derivatives of other arachno-C2B8H14 isomers (Table 6-3) have been obtained by a variety of methods. Nido-5,6C2B8H12 reacts with ammonia or primary amines to generate arachno-5,6-C2B8H12-exo-9-L products, and with tertiary amines to give arachno-5,10-C2B8H12-exo-6-L species (Figure 6-8) [119]; the parent anion 5,10-C2 B8 H13 can be obtained by reacting arachno-5,10-C2B8H12-exo-6-NEt3 with sodium naphthaleneide in THF [119]. H
L B
B
B B B
9
C
H
B
B
B 5,6-C2B8H12-9-L
L = NH3 or NH2C3H7 B = BH C = CH
7
8
B
2
10
C6
B C
B B
L
C B
H
H H
B
5
3
B1 5,6-C2B8H12
FIGURE 6-8 Synthesis of amine derivatives of arachno-5,6- and 5,10-C2B8H14.
B
B4
L = NEt3 or N(n-C4H9)3
B
B C
L
B
B
H
C B
B
B 5,10-C2B8H12-6-L
L
156
CHAPTER 6 Ten-vertex carboranes
Table 6-3 Arachno-C2B8 Derivatives Compound Synthesis and Characterization Arachno-6,9-C2B8H14
Arachno-6,9-C2B8H10D4 Arachno-6,9-C2B8H13-5-Me Arachno-6,9-C2B8H13-1-HS Arachno-6,9-C2B8H13-5-F Arachno-6,9-C2B8H13-n-X (n ¼ 1, 5; X ¼ Cl, Br, I) Arachno-6,9-C2B8H13-5-OEt Arachno-6,9-(Me3Si)C2B8H13 O-5,50 -(arachno-6,9-C2B8H13)2 OSO3-5,50 -(arachno-6,9-C2B8H13)2 Arachno-5,6-C2 B8 H12 2 arachno-5,6-C2B8H12-9-L (L ¼ NH3, NH2C3H7) Arachno-5,10-C2 B8 H13 Arachno-5,10-C2B8H12-6-Me Arachno-5,10-C2B8H13-6-L (L ¼ NEt3, N(n-C4H9)3) Arachno-5,10-PhC2B8H10-m (6,9)-NHR (R ¼ Et, CHMe2, CMe3) Arachno-5,10-C2B8H11-m-(NHCMe3) (2 isomers) Arachno-5,10-C2B8H11-m(6,9)-O (Ph3P)2(Cl)Ru-arachno-7,8-(RS)MeC2B8H11 (R ¼ Me, Et, CHMe2, n-C4H9, CH2Ph) Detailed NMR Studies Arachno-6,9-C2B8H14 Other Experimental Studies Arachno-C2B8H14 Arachno-C2B8H14 Arachno-5,10-C2B8H12-6-L (L ¼ NH3, primary or secondary amino group) Arachno-5,6-C2B8H12-9-L (L ¼ secondary or tertiary amino group) Theoretical Studies Molecular and electronic structure calculations Arachno-6,9-C2B8H14 Arachno-5,10-PhC2B8H10-m (6,9)-NHR (R ¼ Et, CHMe2, CMe3) Arachno-5,10-C2B8H11-m(6,9)-O
Informationa
References
S, H, B, IR, UV, MS ED S (improved) S, H, MS S, H, B, IR, UV, MS S, H, B, MS S, H, B, MS S, H, B, F S, H, B, MS S, H, B, MS S, X, B(2d), H, IR, MS S, H, B, MS X S, H, B, MS S, B S, H, B(2d), MS S, H, B S, B, C, MS S, X (NEt3), B(2d), H, MS S, H, B S, H, B(2d), MS S, H(2d), B(2d), IR, MS S, X(Me), H, B, IR
[101, 113] [116] [114] [111] [101] [117] [90] [111] [107, 111] [111] [115] [112] [118] [112] [84] [119] [119] [117] [119] [120] [121] [122] [123]
H, B
[5]
Halogenation Chromatography Enantiomeric resolution on HPLC, CD Enantiomeric resolution on HPLC, CD
[111, 114] [100] [124]
Ab initio, GIAO-NMR DFT Ab initio,GIAO-NMR
[124]
[116] [120] [122]
a X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; 2d, two-dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; UV, UV-visible data; ED, gas-phase electron diffraction; CD, circular dichroism.
6.2 10-Vertex open clusters
157
Arachno-5,10-C2B8H14 derivatives have been prepared whose boron atoms in the 6 and 9 positions are bridged by a heteroatom that forms a “handle” across the open face of the basket. Thus, arachno-5,10-PhC2B8H10-m(6,9)-NHR (6-9) is obtained by addition of phenylacetylene to hypho-(R0 H2N)B8H11NHR azaboranes [120], while syn and anti isomers of arachno-5,10-C2B8H11-m(6,9)-(NHCMe3) having the same cage architecture as 6-9 are produced by reaction of closo1,2-C2B8H10 with H2NCMe3 [121, 125]; the direct closo to arachno cage expansion in this case is novel. (As discussed in Section 12.5, the 6-9 species can also be regarded as arachno-azacarboranes with the nitrogen as part of the cage.) A structurally analogous oxo-bridged anion, arachno-5,10-C2B8H11-m(6,9)-O (6-10), has been prepared by treating closo-1,2-C2B8H10 with (n-C4H9)4NOH under mild conditions [122]. H
R
B B = BH R = Et, CHMe2, B CMe3
B
B
H
6-9
C B
B
B
Ph
H
B = BH C = CH
B
B
H
B
B C
6-10
B
B C
−
O
N
C B
B
B
A different arachno-C2B8 cage geometry is found in the ruthenium complexes (Ph3P)2(Cl)Ru-arachno-7,8-(RS) MeC2B8H11 6-11 (R ¼ Me, Et, CHMe2, n-C4H9, CH2Ph), whose cage structure has been determined crystallographically for the R ¼ Me species [123]. The C2B8 framework in 6-11 can be described as an icosahedron from which two nonadjacent vertexes have been extracted to create two five-membered open faces, and is found in several 12-vertex dimetallacarboranes (see Chapter 13) [126]. The 6-11-type complexes are obtained in the reaction of nido-7,8-RMeC2 B9 H10 anions with RuCl2(PPh3)2 in which the B(5)-H group is removed [123]; as shown, the ruthenium is coordinated to the carborane via the thio group and two B2 2H Ru agostic interactions. B
H
B Me
H
C
B
B
B
6-11 B = BH, B
B
H
C
B
S
R
B H
Ru
Cl
Ph3P PPh3
6.2.3.2 Structure Crystal structure determinations are available on arachno-5,10-C2B8H13-6-NEt3 [119], arachno-6,9-(Me3Si)C2B8H13 [115], and O-5,50 -(arachno-6,9-C2B8H13)2 [118] as well as 6-11 (R ¼ Me) [123], and a gas-phase electron diffraction study has been conducted on arachno-6,9-C2B8H14 [116]. The cluster geometries of the arachno-C2B8 isomers are also well supported by multinuclear NMR evidence, cited in Table 6-3.
6.2.3.3 Introduction of substituents The electrophilic halogenation of arachno-6,9-C2B8H14 by CCl4, Br2, or I2 over AlCl3 affords the respective 1-halo derivatives (see Figure 6-8 for cage numbering) [111, 114]; the preference for substitution at B(1) is also found in AlCl3catalyzed deuteration by D2O and DCl [101]. Similarly, arachno-6,9-C2B8H13-1-SH can be obtained by reacting the parent carborane with elemental sulfur in the presence of AlCl3 [90].
158
CHAPTER 6 Ten-vertex carboranes
Derivatives of arachno-6,9-C2B8H14 can often be prepared directly in good yield from relatively accessible nido-C2B8 carboranes. As noted previously, the addition of excess hydrogen halide to the nido-6,9-C2 B8 H10 2 anion in benzene proceeds stereospecifically to give arachno-6,9-C2B8H13-5-X products, while oxo- and sulfato-bridged X-5,50 -(arachno-6,9C2B8H13)2 species (X ¼ O, OSO3) are generated on treatment of nido-6,9-C2 B8 H10 2 with concentrated H2SO4.
6.2.3.4 Enantiomeric resolution and circular dichroism
The arachno-5,6-C2B8H13-6(9)-L and arachno-5,10-C2B8H13-9-L0 carboranes (L ¼ NH3 or a primary or secondary amine; L0 ¼ secondary or tertiary amine) are prochiral (see Figure 6-8), and their enantiomeric selectivities toward cyclodextrin chiral stationary phases (CSPs) and circular dichroism (CD) properties have been investigated [124]. In general, acetyl-bcyclodextrin CSPs are highly effective in separating the enantiomers, with the two carborane series exhibiting different orders of selectivities and, based on the observed CD spectra, different enantiodiscrimination mechanisms. Hydrogen bonding between the amino substituents and the polar acetyl groups on the CSPs evidently plays an important role.
6.2.4 Nido-C3B7H11 and arachno-C3B7H13 6.2.4.1 Synthesis Tricarbon carboranes larger than the 6-vertex C3B3 systems described in Chapter 4 were first obtained by controlled degradation of Me4C4B8H8 via reaction with 95% ethanol in air to afford arachno-5,6,10-Me3C3B7H9-m(6,9)-CHMe (6-12) in a process wherein one framework carbon atom assumes a bridging role [127]. Deprotonation of 6-12 with NaH followed by reprotonation with HCl produces a new isomer, arachno-5,6,10-Me3C3B7H9-m(6,9)-CHMe (6-13) as shown in Figure 6-9. As with many carbon-bridged carboranes, these cages can reasonably be classified as either 10-vertex C3B7 or 11vertex C4B7 systems depending on whether one regards the bridging carbon as a part of the cluster framework—an essentially subjective judgment. In this case, 6-12 and 6-13 might well be considered as 11-vertex arachno species that are formally derived from 13-vertex closo-polyhedra by removal of two vertexes (see Chart 2-2, Me4C4B7H9). A different synthetic route to tricarbon carboranes entails the insertion of alkynes into arachno-6,7-C2B7H13 (5-20) as shown in Figure 6-10 [128]. Reaction of 5-20 with acetylene or 2-butyne at 120 C yields the carbon-bridged arachno species 6-15 (R ¼ H or Me) which is isomeric with 6-12; the reaction with 2-butyne also produces the nido-carborane 6-16. The formation of these species has been rationalized as occurring via an initially formed C-alkenyl intermediate 6-14 that rearranges by different pathways to generate the final products [128]. In the proposed rearrangement of 6-14 to 6-16, the low-coordinate cage carbon C(6) is converted into an exo-polyhedral methyl group. Versatile and efficient syntheses of C3B7 carboranes have been developed based on interactions of nitriles or polarized alkynes with arachno-C2 B7 H12 ions, which are excellent nucleophiles. Figure 6-11 shows a proposed sequence Me H
Me H
C 9
B
8
B C10 4B
H
B
7
B1
6-12
Me NaH B = BH
C B
B C
HCl B
B2 Me
Me3C3B7H8-μ-CMe−
C
H
B
6
B C 5
3
Me
C
Me
C B
B
B
Me
6-13
FIGURE 6-9 Interconversion of arachno-5,6,10-Me3C3B7H9-m-CHMe isomers via deprotonation and reprotonation.
Me
6.2 10-Vertex open clusters H 6
C
R
H
5
4
B
B C 7
B
B
H H
8
B
H
RC≡CR
C
H
R = H, Me
B9
1
2
159
C
B B
B C
B3 B = BH C = CH
5-20
H
6-14
B
B
R
C
H
B
B R = Me
R
H
C H
H
C
B B
B C
6-15
B
R
C
H
B
B
B
Me
B
Me
B C
B C
6-16
B
B
C
B
B Me
FIGURE 6-10 Synthesis of arachno-5,6,9-[m(6,9)-RCH]RC3B7H11 (6-15) and nido-5,6,10-Me3C3B7H8 (6-16) by alkyne addition to arachno6,7-C2B7H13 (5-20, drawn from a different perspective from that shown in Chapter 5), via a proposed intermediate 6-14. H
− N H H
B
− C H C B B
B B B
B
MeCN
:N
H H
Me − C H C B B
B B B
B
Me
C
C
H
B
B B B
C H C B B Me
B
B
6,8-C2B7H−12
B = BH C = CH
B B B
Me Me C
C
B
B B
6-18
B
C
−
Me
H
C
H+
B B
C B
HH
C
C
B
B B
B B B
B B
C B
B
B
B
−NH3
N −
C H C B B B
C C B − B B
6-17
FIGURE 6-11 Suggested mechanism of carbon insertion into arachno-6,8-C2 B7 H12 to form nido-MeC3 B7 H9 (6-17) and nido-5,6,9MeC3B7H10 (6-18).
160
CHAPTER 6 Ten-vertex carboranes
[129, 130] for the attack of a nitrile on arachno-6,8-C2 B7 H12 to give the nido-MeC3 B7 H9 ion (6-17) and nido-5,6,9MeC3B7H10 (6-18), the net result of which is the incorporation of the nitrile carbon into the cage with elimination of NH3. Labeling experiments with Me13CN show that the nitrile methyl group remains attached to its carbon atom throughout the process, as shown [130]. The process is highly dependent on reaction conditions, which can be adjusted to generate product yields in excess of 80% [131]. Other nitriles react similarly with 6,8-C2 B7 H12 , affording a range of C(6)-substituted nido-5,6,9-RC3 B7 H9 derivatives (Table 5-10) [132]. The arachno-6,8-C2 B7 H12 ion reacts similarly with polarized alkynes, but in contrast to the nido species obtained with nitriles, the products formed are arachno-C3B7H13 derivatives [133, 134]: H+ 6,8-C2B7H−12 + HC≡CR ⎯→ arachno-6-RCH2-5,6,7-C3B7H12 (6-19) H
B
CH2R
C C C
B
6-19
B B
B
B = BH C = CH R = CN, C(O)OMe, C(O)Me
B
B
The C(O)OMe-substituted product can also be obtained directly from neutral arachno-6,8-C2B7H13 and methyl propynoate in the presence of 0.1 equivalent of “proton sponge” (1,8-bis[dimethylamino]naphthalene), which initiates a cyclic process involving a 6,8-[MeO(O)C2 2CH5 5CH]C2 B7 H11 intermediate that is protonated by reaction with neutral 6,8-C2B7H13 [135]. However, with arachno-6,7-C2B7H13, the same treatment affords nido-6-MeO(O)CCH2-5,6,10C3B7H12 whose cage structure is the same as that found in 6-16. In contrast to arachno-6,8-C2 B7 H12 , treatment of 6,7-C2 B7 H12 with acetonitrile or methyl propynoate results not in carbon insertion, but rather in cage degradation or decomposition [135]. Differences in reactivity between the arachno-6,7- and 6,8-C2B7H13 isomers, and similar differences in their C2 B7 H12 anions, were noted previously in Chapter 5. Other approaches to C3B7 carboranes have been demonstrated. Heating the cyanocarborane arachno-6,8-(endoNCCH2CH2)C2B7H12 (Section 5.6) with its conjugate base anion generates the dimethylene-bridged product arachno5,6,9-[m(6,9)-CH2CH2]C3B7H11 (6-20) [136]. CH2
CH2 H
C B B
6-20 B
B
C B C
B = BH C = CH B
B
The ubiquitous arachno-6,8-C2 B7 H12 anion reacts with Fischer-type metal carbene complexes to form species of type 6-21, which on protonation yield nido-5,6,9-RC3B7H10 carboranes 6-22 (Figure 6-12) [137]. − (CO)5M
B
H
−
H H
B B
B
C H C B B B
6,8-C2B7H−12
(CO)5M=C(OMe)R
B B
B = BH M = Cr, W C = CH R = Me, Ph
R R + C
H
B B
C H C B B B
H+ −30 °C
B B
H
R
C
OMe
C
C B B
B
C C
C B
H
B
B
B
6-21
FIGURE 6-12 Carbon insertion into 6,8-C2 B7 H12 to generate tricarbon carboranes via Fischer carbene complexes.
B
B B
B
6-22
B
6.2 10-Vertex open clusters
161
Arachno-5,6,9-C3B7H13, apparently the only C3B7 cluster to have been isolated in parent form, has been obtained by oxidative extraction of a boron atom from the nido-7,8,10-C3 B8 H11 anion [138].
6.2.4.2 Structure and properties
An X-ray crystal structure study of one nido-C3B7 derivative, Li(MeCN)2þnido-5,6,9-MeC3 B7 H9 , is available [139], and several crystallographic studies of arachno-5,6,7-C3B7 [134] and 5,6,9-C3B7 [136] systems have been reported. The cage architectures described above are well supported by extensive NMR and other spectroscopic data, including two-dimensional (COSY) NMR studies in many cases (Table 5-10). As might be expected, the open-cage C3B7 carboranes are very reactive species that undergo both degradation to smaller cages (e.g., the conversion of arachno-(NCCH2)C3B7H12 to hypho-(NCCH2)C3 B6 H12 (5-27) as described in Chapter 5), and expansion to larger ones via carbon, boron, or heteroatom insertion. Illustrations of cage enlargement to 11-vertex systems can be found in Section 6.3.
6.2.5 Nido-C4B6H10 6.2.5.1 Synthesis Ten-vertex C4B6 clusters having 12 skeletal electron pairs are expected to adopt a nido cage geometry like that of the isoelectronic C2B8H12 and C3B7H11 systems described above. This structure is indeed found in Et4C4B6Et6, prepared by the reduction of closo-1,5-Et2C2B3Et3 with potassium metal in THF followed by oxidation with iodine (Figure 6-13); the nido-1,2,3,9-Et4C4B6Et6 geometry 6-23 was assigned from 1H, 11B, and 13C NMR data despite the rather unexpected feature that three of the cage carbon atoms occupy high-coordinate vertexes [140]. The same treatment, when applied to closo-1,5-Me2C2B3R3 (R ¼ Et or n-propyl), takes a different course, generating classically bonded (nondelocalized) tetraboraadamantane intermediates 6-24; on heating, these compounds are converted to products 6-25 or 6-26 having the same apparent cage structure as 6-23 [141]. In more recent work, it was found that reduction of closo-1,5-Et2C2B3Et3 with potassium or cesium affords a Et4 C4 B6 H6 2 dianion, which upon oxidation with I2, FeCl2, or HgCl2 gives a
C B = BR C = CR⬘
K R = R⬘ = Et
Et2C2B3Et32−
B B
B B
I2
B C
B
C
C
C B
6-23 Et4C4B6Et6
B B
C
C
B
K R = Et, n-C3H7 R⬘ = Me
160 °C
C C
C
B
B
C
B
B
B
B
6-24
B B
B B
B C
C
C
6-25 Me4C4B6Et6 6-26 Me4C4B6(n-C3H7)6
FIGURE 6-13 Synthesis of 10-vertex tetracarbon carboranes from 1,5-C2B3H5 peralkyl derivatives.
162
CHAPTER 6 Ten-vertex carboranes
tetracarbon carborane characterized by X-ray crystallography as nido-2,6,8,10-Et4C4B6R6 (6-27) [142]. At present, this is the only nido-C4B6 system whose cage structure has been crystallographically established.
6-27
C
B
B = B–Et C = C–Et B
6-28
B B
C C B
B = BH C = CH
C
C
C C B
B C B
B
B
B
B
Other preparative routes to C4B6 carboranes have been explored. Addition of acetylene to arachno-6,7-C2B7H13 yields a small quantity of 5,6,8,9-C4B6H10 (6-28) along with C3B7 and C4B7 clusters [143]. The pyrolysis of BMe3 generated a product characterized from its mass spectrum as Me4C4B6Me6, for which a carborane structure has been proposed [144]; however, in the absence of NMR or other spectroscopic data, a classical organoborane structure such as 6-24 for this compound cannot be ruled out.
6.2.5.2 Structure and properties The general nido-tetracarbadecaborane architecture of compounds 6-23 and 6-25–6-28 is supported by multinuclear NMR and other data (Table 5-6), in addition to the X-ray structure determination on 6-27 already mentioned. However, NMR information does not uniquely establish the proposed location of the skeletal carbons in structures 6-25 and 6-26. Ab initio and individual gauge for localized orbital (IGLO) calculations on 6-27 [142] and DFT, Hu¨ckel, and studies on other C4B6H10 isomers are available (Table 5-6). A second-moment Hu¨ckel calculation [109] on nido-C4B6H10 found an optimized structure that is said to match the “experimentally known” geometry, even though no X-ray determination on any parent C4B6H10 isomer is available. DFT computations on this system indicate that the 4,5,7,9 isomer is slightly more stable than the known 5,6,8,9-C4B6H10 (6-28) [29]. Little is known of the reaction chemistry of nido-C4B6H10 derivatives, other than the reduction of nido-2,6,8,0-Et4C4B6Et6 by alkali metals to generate arachno-Et4C4B6Et62, which can be oxidized back to the neutral compound [142].
6.2.6 Arachno-C4B6H12 The reaction of acetylene with closo-4,5-C2B7H9 affords, in low yield, the dicarbon-bridged product 5,6,9,105CH (6-29), a derivative of arachno-C4B6H12 which can also be viewed as a C6B6 carborane H4C4B6H6-m(6,9)-HC5 if one regards the HCCH bridging group as part of the cluster framework [145] (a true C6B6 cage system has been characterized and is described in Chapter 11). The only other reported arachno-C4B6 species is the Et4 C4 B6 H6 2 dianion, which is generated by alkali-metal reduction of nido-2,6,8,0-Et4C4B6Et6 as described earlier [142]. Definitive structural data on the dianion are not yet available. C
C C
B
6-29
C
B = BH C = CH
B
C B
C B
B
B
6.3 10-VERTEX CLOSO CLUSTERS 6.3.1 1- and 2-CB9H10
Closo 10-vertex monocarbon carboranes, once of interest mainly to specialists in boron cluster chemistry, have assumed increasing importance owing to application in several areas. Among these are the construction of linear rods as synthons for nanoscale assemblies, systems exhibiting liquid crystalline behavior, and, especially, the discovery that some
6.3 10-Vertex closo clusters
163
halogen-substituted derivatives of 1-CB9 H10 are among the least coordinating counterions yet found for stabilizing extremely electrophilic species, an area that has seen spectacular discoveries in recent years. Interest in these and other applications, described in Chapter 17, has spurred the development of efficient synthetic routes that have made this class of carboranes reasonably accessible.
6.3.1.1 Synthesis
The 1-CB9 H10 anion was first prepared by reaction of nido-6-(Me3N)CB9H11 with sodium metal, which results in both deamination and dehydrogenation to give the closo product [18]. The anion is also obtained, along with CB11 H12 , by thermal disproportionation of Csþnido-CB10 H13 at 320–330 C [18]: 2CB10 H 13 ! CB9 H10 þ CB11 H12 þ 2H2
An alternative route to parent 1-CB9 H10 involves the deamination and cage closure of nido-6-(Me3N)CB9H11 by piperidine at 70 C to afford the piperidinium salt [32]: ðMe3 NÞCB9 H11 þ C5 H10 NH ! C5 H10 NHþ 2 1-CB9 H10 þ NMe3
The fact that the product is closo-1-CB9 H10 rather than the 2-isomer that would be expected from a simple cage closure supports a proposed “vertex-flip” mechanism that is initiated by movement of B(9) to a higher-coordinate position, as shown in Figure 6-14 [32]. NMe3
C
B
B
B
B B B
NMe3
B
B
B B
B B
B
B B = BH
C
C
NMe3 B
B B
B
B B B
B
B
B B
B
B
FIGURE 6-14 Suggested mechanism for the conversion of nido-6-(Me3N)CB9H11 to closo-1-CB9 H10 (bridge hydrogen atoms not shown).
An important entry to C-substituted derivatives of CB9 H10 is the previously mentioned Brellochs reaction (Section 6.2), in which commercially available B10H14 is treated with aldehydes in alkaline media to form nido-6-RCB9 H11 or arachno-6-RCB9 H13 (depending on the aldehyde) [2–4]. As shown in Figure 6-3, these clusters are readily oxidized by iodine to form closo-2-RCB9 H9 ions that convert to the thermodynamically preferred closo-1-RCB9 H9 isomers on mild heating [4, 13, 16, 17, 28, 31]. This general synthetic approach, combined with organic transformations on the attached functional groups, has been exploited to prepare a variety of carbon-substituted 1- and 2-RCB9 H9 carboranes (Table 6-4), many of which feature C-aryl groups [3, 13, 14]. Examples are the construction of the linear bis(carboranyl) zwitterion 6-30 in Figure 6-15 [37] and a structurally similar p-C6H4[C(O)NH-C6H4-CB9H9]22 dianion [38]. Like its dicarbon C2B8H10 analogues discussed below, CB9 H10 is lithiated at carbon on treatment with n-butyllithium, and displacement of the lithium by electrophilic groups affords products such as 1-(Me3Si)CB9H10 [18]. Other routes to C-substituted CB9 H10 derivatives include the extrusion of CpCo from the 11-vertex cobaltacarborane CpCo [(Me3N)CB9H9] in refluxing acetonitrile to give 1-(Me3N)CB9H9 [8] and, more practically, the conversion of nido-6(H3N)CB9H11 to closo-1-(H3N)CB9H9 via reaction with Et3NBH3 in the presence of NaBH4 followed by heating at 200 C and treatment with methanol and HCl [36]. In the latter synthesis, the mechanism appears to be a complex one involving formation of a nido-6-(H3BNH2)CB9H11 intermediate that loses H2 on heating to generate closo-6(H3BNH2)CB9H9 via a vertex-flip process similar to that shown in Figure 6-14. Alcoholysis of this species to remove
164
CHAPTER 6 Ten-vertex carboranes
B B
B
−
B = BH C = CH
B B C
B
NH2
CHO
+ OHC
HCl
B B
B
1-(H2NC6H4)CB9H9−
B
H N B B
B
H C
B B C
B B B
N H
B
C H
C
B
B B B
B B B
B
6-30
FIGURE 6-15 Synthesis of p-C6H4[CH¼NH-C6H4-CB9H9]2 from closo-1-ðH2 NC6 H4 ÞCB9 H9 .
Table 6-4 Closo-CB9H10 Derivatives Compound Synthesis and Characterization Nontransition-metal derivatives No substituents on boron 1-CB9 H10
1,10 -(1-CB9H9)22 1-MeCB9 H9 1-PhCB9 H9 1-RCB9 H9 [R ¼ C6H4NH2, p-C6H4Br, 2-SC4H3] 1-(Me3Si)CB9H9 1-(Me3N)CB9H9 1-LCB9H10 (L ¼ H3N, Me3N, Me2S, H2 N, Me2HN) 1-(p-N2-C6H4)CB9H9 anilinyl azo imino 1-(p-H2N-C6H4-N2-C6H4)CB9H9 5NH-C6H4)CB9H9 1-(p-Me2C5 5NH-C6H4-1-CB9H9]2 p-C6H4[CH5 p-C6H4[C(O)NH-C6H4-1-CB9H9]22 1-(HOCH2)CB9H9 1-(p-HOOC-C6H4)CB9 H9 1-(p-HOCH2-C6H4)CB9 H9 1-(p-CHO-C6H4)CB9 H9
Informationa
References
S, B, IR S, H, B, C S (improved), X, B S B, E S, H, B, E S, H, B, C S, X, H, B, C S, X(C6H4Br), H, B, C S, H, B S, H, B, MS, IR S, X, H, B(2d), MS S, X, H, B, C S, X, H, B, C S, X, H, B, C S,H, B, C S, X S, X, H, B, C, IR S, X, H, B, C, IR S, H, B, C, IR S, H, B, C, IR
[18] [31] [32] [33, 34] [35] [35] [31] [13, 14] [4] [18] [8] [36] [37] [37] [37] [37] [38] [16] [16] [16] [16] Continued
6.3 10-Vertex closo clusters
165
Table 6-4 Closo-CB9H10 Derivatives—Cont’d Compound
1-(CHO)CB9 H9 1-[p-CH5 5N-NHC6H3(NO2)2C6H4]CB9 H9 dinitrophenylhydrazone 1-[CH5 5N2 2NHC6H3(NO2)2]CB9 H9 dinitrophenylhydrazone 1-(N2)CB9H9 1-RCB9 H9 R ¼ C(O)OH, C(O)Cl, C(O)NH3, NCO, NHC(O)OCMe, NHC(O)Me 5N1-RCB9 H9 R ¼ cyclo-C5H5Nþ (4 isomers), SMe2þ, N5 C6H4OH, N5 5CHMeþ, NHC(O)Me, cyclo-SC5H10þ, N5 5N-C6H4, N5 5N-C6H4OMe 2-CB9 H10 2-PhCB9 H9 2-(p-C6H4Br)CB9 H9 2-[p-HOCH2C6H4]CB9 H9 2-[HO(O)C]CB9 H9 2-[p-HO(O)C-C6H4]CB9 H9 D or hydrocarbon substituents on boron 1-CB9 H5 D5 1-HCB9 Me9 1-PhCB9H4-6,7,8,9-(C6H4Me)4 1-RCB9H8-6-CH2R0 (R ¼ H, Me; R0 ¼ H, Me, Ph) R-C6H4-OC(O)-CB9H9-C6 H13 ionic liquid crystals R ¼ n-C9H19O, C5H11-cyclo-C6H10-OC(O), C5H11-CB10H10-OC(O), C5H11-cycloC6H10, C5H11-CB10H10C C5 H11 tricyclo-C7H13NCB9H 9 C6 H13 polar additives to nematic liquid crystal hosts C5 H11 cyclo-C5H9SCB9H9C6 H13 polar additives to nematic liquid crystal hosts N- or P-containing substituents on boron 1-PhCB9H8-6-NC5H4CH2Ph 1-PhCB9H8-6-L [L ¼ NC5H5, NC5H4-p-CH2Ph, NC5H4-p-Ph, NC9H7 (quinoline)] 1-PhCB9H8-2-(NC5H4-o-NH2) 1-PhCB9H7-2-(NC5H4-o-NH2)-4-Cl 1-CB9H9-6-PMe2Ph 1,10-[O(O)C]CB9H8-10-NH3 precursor to polar liquid crystals 1,10-[HO(O)C]CB9H8-10-NH3 precursor to polar liquid crystals 1,10-[HO(O)C]CB9H8-10-N2 precursors to polar liquid crystals 1,10-[HO(O)C]CB9H8-10-R R ¼ NC5H5, SCHNMe3 1,10-[HO(O)C]CB9H8-10-SC5H10 1,10-[MeO(O)C]CB9H8-10-R R ¼ NH3, N2 1,10-[C6H11(O)C]CB9H8-10-NC5H5 1,10-[C6H11(O)C]CB9H8-10-SC5H10
Informationa
References
S, S, S, S, S,
[16] [16] [16] [39] [39]
H, B, C, IR X, H, B, C, IR X, H, B, C, IR H, B, C, IR H, B, C, IR
S, H, B, C, IR
[39]
S, S, S, S, S, S,
[28] [13, 28] [4] [16] [16, 17] [16, 17]
H, B, C X, H, B, C X, H, B H, B, C, IR X, H, B, C X, H, B, C, IR
S, H, B, IR S, H, B, C, IR, MS S, H, B, C S, H(2d), B(2d), C, MS S, UV, X-ray powder diffraction, DSC, optical microscopy
[40] [41] [42] [43] [44]
S, X, UV
[45]
S, X, UV
[45]
S, X S, X (all), H, B
[38] [24]
S, X, H, B, MS S, X, H, B, MS S, X, H(2d), B(2d), P S, H, B, IR, Kdiss S, H, B, IR, MS S, X, H, B, IR, MS, UV, E S, X(NC5H5), H, B, IR, MS, UV (NC5H5), E S, H, B, IR, MS S, H, B, IR, MS, E(N2) S, H, B S, H, B
[22] [22] [46] [47] [47] [47] [47] [47] [47] [47] [47] Continued
166
CHAPTER 6 Ten-vertex carboranes
Table 6-4 Closo-CB9H10 Derivatives—Cont’d Compound
Informationa
References
O- or S-containing substituents on boron 1-CB9H5F4(OH)
S, H, B, F, IR, MS
[48]
S, S, S, S, S, S, S, S, S, S, S, S, S, S, S,
[31] [49] [49] [49] [48] [48] [40] [40] [40] [40] [35] [35] [15] [42] [42]
F-, Cl-, Br-, or I-containing substituents on boron 1-RCB9 H9 R ¼ Cl, Br, I 1-CB9H8-6,7-F2 1-CB9H7-6,7,8-F3 1-CB9H6-6,7,8,9-F4 1-CB9H5-6,7,8,9,10-F5 1-CB9H5F4(OH) 1-CB9H9-6-X (X ¼ F, Cl, Br, I) 1-CB9H8-6,7-X2 (X ¼ F, Cl) 1-CB9H8-6,8-X2 (X ¼ F, Cl) 1-CB9H8-6,10-Cl2 1-CB9 H5 Cl5 1,10 -(1-CB9H5Cl4)22 1-PhCB9H8-6-Br 1-PhCB9H4-6,7,8,9-(C6H4Me)4-10-I {1-PhCB9H4-6,7,8,9-(C6H4Me)4-10-I}2 Pd2I2[P(C6H4Me)42þ encapsulated cation 1-PhCB9H4-6,7,8,9,10-I5 Csþ 1-HCB9 H4 Br5 1-RCB9 X9 (R ¼ H, NH2; X ¼ Cl, Br, I) Mþ 1-CB9H5-6,7,8,9,10-Br5 (M ¼ Ag, IrCl(CO)(PPh3)2) Si(CHMe2)3þ 1-CB9H5-6,7,8,9,10-Br5 1-CB9H5F4(NHCOMe) 1-[HO(O)C]CB9H8-10-I 1-(H2N)CB9H8-10-I 2-[HO(O)C]CB9H8-n-I (n ¼ 6,10) 1-PhCB9H8-6-I Kþ(CTV)2(CB9H5Br5) (CF3COH)2, Rbþ(CTV)2(CB9H5Br5) (CH3CN); CTV ¼ cyclotriveratrylene chain polymers with a propeller shape Transition-metal s- and m-complexes FeCp(CO)2þ 1-CB9 H10 Co(phenanthroline)2þ[1-CB9 H10 ]2 Co(bipyridine)32þ[1-CB9 H10 ]2 Ni(phenanthroline)2þ[1-CB9 H10 ]2 Ni(bipyridine)32þ [1-CB9 H10 ]2
H, B, C B, F, MS B, F, MS B, F, MS H, B, F, IR, MS H, B, F, IR, MS H, B(2d), C, F, MS H, B, C, F, MS X(F), H, B, C, F, MS H, B, C, F, MS B H, B H, B, C B, C, MS X, B, C, MS
S, B, C, MS X (molecular and extended crystal) S, X(PhCH2, Cl; H, Br), H, B, C, Si, IR, MS S, X[Ag(toluene), IrCl(CO) (PPh3)2Ag(toluene), B, IR] S, H, B, X S, X, H, B, F, IR, MS S (from B10H14, four steps), H, B S, H, B S, H, B S, X, H, B, C S, X, IR
[42] [50] [41]
S, H, B, MS S, IR, MAG, UV, analysis S, IR, MAG, UV, S, IR, MAG, UV, analysis S, IR, MAG, UV,
COND, thermal
[40] [56]
COND COND, thermal
[57] [56]
COND
[57]
[51] [52] [48] [53] [53] [53] [54] [55]
Continued
6.3 10-Vertex closo clusters
167
Table 6-4 Closo-CB9H10 Derivatives—Cont’d Compound
M(en)3 [1-CB9 H10 ]2 (M ¼ Co, Ni) M þ 1-CB9 H10 (M ¼ Ag, Ph3C) Ag(C6H6)2þ 1-CB9H8-6,8-F2 Ag(toluene)þ 1-CB9 H5 Br5 Ag4I4(MeCN)[1-PhCB9H8-6-I]2.2[1-PhCB9H8-6-I]12MeCN metallo-supramolecular starburst tetrahedron Ag(bipyridyl)þ 1-PhCB9 H9 MeCN (1-D polymer chain) Ag(bis-2-pyridylpyrazine)þ 1-PhCB9 H4 I5 MeCN (1-D polymer chain) Ag(bis-2-pyridylpyrazine)þ 1-PhCB9 H9 MeCN0.5 (1-D polymer chain) Ag(bis-2-pyridylpyrazine)þ 1-PhCB9H8Br [ZnCl(Hpz-CMe3)3]þ PhCB9 H9 (Hpz-Cme3 ¼ 5-tert-butylpyrazole) bowl-shaped host cavity for carboranes and metallacarboranes [ZnCl(Hpz-Cme3)3]þ CB9 H5 Br5 (Hpz-Cme3 ¼ 5-tert-butylpyrazole) 2þ
Other Experimental Studies 1-CB9 H10
1-CB9H9OH 2-RCB9H9 (R ¼ Ph, H) M(en)32þ [1-CB9 H10 ]2 (M ¼ Co, Ni) Theoretical Studies Molecular and electronic structure calculations 1- and 2-CB9 H10 1-CB9 H10 1-CB9H10n (n ¼ 0, 1) 1-CB9 H10 1-CB9H9(C7H6þ) C7H6þ ¼ tropylium 1-[HO(O)C]CB9H8-10-I 1-(H2N)CB9H8-10-I 1-RCB9 H9 R ¼ C(O)OH, C(O)Cl, C(O)NH3, NCO, NHC(O)OCMe, NHC(O)Me, cyclo-C5H5Nþ (4 isomers), Sme2þ, N5 5N-C6H4OH, N5 5CHMe(þ), NHC(O)Me, cyclo-SC5H10þ, N5 5N-C6H4-Ome 2-[HO(O)C]CB9H8-n-I (n ¼ 6,10) 1,10-[HO(O)C]CB9H8-10-N2 1,10-[HO(O)C]CB9H8-10-NC5H5 NMR calculations 1-CB9 H10 1-CB9 H10 1-CB9 H10
Informationa
References
S, S, S, S, S,
[58] [51] [40] [51] [54]
IR, MAG, UV X(Ag), H, B X, H, B, MS X, B, IR H, IR
S, X, IR S, X, IR
[15] [15]
S, X, IR
[15]
S, X, IR S, X, H, MS
[15] [59]
S, X, H, MS
[59]
Halogenation Fluorination Thermogravimetric analysis UV, IR rearrangement to 1-RCB9 H9 Thermal decomposition
[40] [49] [60] [18] [28] [58]
Ab initio Optimized geometry, IR active vibrations, charge distribution) Energies, ionization potentials EI (energy indexes); stabilities Ab initio charge-transfer DFT DFT DFT
[61] [62] [63] [64] [65] [53] [53] [39]
DFT DFT DFT
[53] [47] [47]
Antipodal effect B shifts: antipodal effect B
[66] [67] [68] Continued
168
CHAPTER 6 Ten-vertex carboranes
Table 6-4 Closo-CB9H10 Derivatives—Cont’d Compound Reactivity calculations 1-CB9 H10
Informationa
References
Fluorination mechanism; formation of CB9H9F
[69]
a
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; F, 19F NMR; P, 31P NMR; Si, 29Si NMR, 2d, two-dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; UV, UV-visible data; E, electrochemical data; MAG, magnetic susceptibility; COND, electrical conductivity; DSC, differential scanning calorimetry.
the BH3 followed by acidification yields the neutral product, from which other closo-1-LCB9H9 derivatives (Table 6-4) can be prepared by reactions of the NH3 group [36]. Boron-substituted derivatives of closo-CB9 H10 are generally prepared via reactions of existing closo-CB9 carboranes, as described below. However, in a few cases they have been obtained by other routes, as in the previously described reaction of the nido-6-PhCB9 H11 anion with pyridine or pyridine derivatives to generate arachno cages which are oxidized by hydrated FeCl3 to form closo-1-PhCB9H8-2-L or -6-L products depending on the ligand (Section 6.2 and Figure 6-2) [22, 24]. Other examples are found in the thermal degradation of the 11-vertex platinacarborane nido-8,7-(PMe2Ph)2Pt(CB9H11) to give closo-CB9H9-6-PMe2Ph in 85% yield [46], and the elimination of carbon and boron from 1,2-RR0 C2B10H10 icosahedral carboranes to give closo-1-RCB9H8-6-CH2R0 products as shown in Figure 6-16.[43] In this interesting synthetic approach, one of the two C-R units in the icosahedral cage is converted to an exo-polyhedral CH2R group in the final product.
R⬘
−
C R
B
B
C
B
B
C
B B B
B
B
B B
B
B
B
B
B
R = Me, Ph R⬘ = H, Me
B C
B
B B = BH, B
R
3H+
B
B
C
H
−B3+
B
R
R⬘
H
C
K+ NC4H4−
B
B B
−
R⬘
H
B
B
B B
−
R C B
B B
B B
B
B B B
R⬘
FIGURE 6-16 Conversion of 1,2-RR0 C2B10H10 carboranes to closo-1-RCB9H8-6-CH2R0 anions (mechanism proposed in Ref. [43]).
6.3 10-Vertex closo clusters
169
6.3.1.2 Structure and properties
The bicapped square antiprism cage geometry of closo-1- and 2-CB9 H10 (the only possible isomers) is well established by X-ray crystallographic and NMR data on a variety of derivatives, cited in Table 6-4; however, the only published X-ray study of an unsubstituted species is that on Cp*2Ir2Clþ 3 1-CB9 H10 [32]. Several theoretical investigations (Table 6-4) have explored the electronic structures, three-dimensional aromaticity, charge distribution, NMR antipodal effects, ionization potentials, vibrational energies, and other aspects of the CB9 H10 systems. The prediction from simple rules on heteroatom placement (Section 2.7) that the preferred location of carbon is a low-coordinate apical position, is borne out by detailed calculations [61, 62, 64] and by the observed rearrangement of 2-CB9 H10 to the 1-isomer (see Figure 6-3). Controlled-potential electrolysis of 1-CB9 H10 in acetonitrile produces a 1-electron oxidation affording the dimeric B(10)-B0 (10) coupled species (CB9H10)2 in 90% yield [35], analogous to the oxidative coupling of the isoelectronic B10 H10 2 ion to form (B10H9)4 [146].
6.3.1.3 Introduction of substituents: Deuteration and halogenation
Interest in a range of applications for CB9 H10 derivatives, mentioned earlier, has stimulated the development of methods for placing functional groups (especially halogens) on the polyhedral cage, nearly all such studies being confined to the 1-CB9 H10 isomer. Electrophilic halogenation with F, Cl, Br, and I occurs preferentially at B(6) followed by B(8) [40]. However, electrophilic exchange of deuterium for hydrogen takes place equally fast at all positions on the lower equatorial belt [B(6)-B(9)] and the apical boron, B(10), generating 1-CB9 H5 -6; 7; 8; 9; 10-D5 . Introduction of fluorine at B(6) alters the electronic structure of the cage so that subsequent deuteration occurs much faster at B(8) than at B(10), very different from the behavior of unsubstituted 1-CB9 H10 [40]. Fluorination of 1-CB9 H10 can also be accomplished by treatment with liquid anhydrous HF, which generates the 6-F, 6,7-F2, 6,8-F2, 6,7,8-F3, and 6,7,8,9-F4 derivatives [49], and with so-called N-fluoro reagents such as 1-chloromethyl-4fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (F-TEDA), which introduce up to seven fluorines to the cage but are less sterospecific than liquid HF [48, 49]. Chlorination of 1-CB9 H10 with excess Cl2 affords 1-HCB9Cl5H 4 [35], while treatment of the parent carborane or 1-ðH2 NÞCB9 H9 with excess ICl yields the B-perchloro anions 1-HCB9 Cl9 and 1-ðH2 NÞCB9 Cl9 , respectively [41]. Bromination to give 1-HCB9 Br5 H4 can be achieved by direct reaction with Br2 in refluxing glacial acetic acid [51, 52]; crystallographic studies [50, 52] have confirmed that the bromines occupy the B(6)–B(10) vertexes, that is, those not adjacent to carbon. The B-perbromo and B-periodo anions 1-HCB9 Br9 and 1-HCB9 I9 are obtained by reaction of 1-CB9 H10 with Br2 or I2 in triflic acid [41]. Treatment of 1-PhCB9 H9 with ICl in concentrated aqueous H2SO4 affords the 1-PhCB9H4-6,7,8,9,10-I5 anion [42]. An interesting application of 1-CB9H5-6,7,8,9,10-Br5 (6-31) employs this species as a counterion in Kþ and Rbþ coordination polymers incorporating the bowl-shaped cyclotriveratrylene (CTV) host molecule [55]. Other bowl-shaped hosts such as tert-butylpyrazole form similar complexes, for example, [ZnCl(pyrazole-CMe3)3]þ L where L ¼ 1-PhCB9 H9 or 1-CB9 H5 Br5 [59]. MeO
MeO
OMe OMe HB
OMe
MeO
−
H C HB
Br
Br
BH BH
B
B
B B
Br
Br
B
Br
CTV
6-31
While polyhalogenation is relatively easy to accomplish, the introduction of halogens or other substituents at specific locations is more difficult (as is true for polyhedral boranes generally). Derivatives having functional groups located at the antipodal C(1) and B(10) apical vertexes are potentially valuable synthons for constructing linearly connected chains that may be useful in liquid crystal, nanoengineering, and other applications described in Chapter 17.
170
CHAPTER 6 Ten-vertex carboranes − O HO
C
B
B
C B
HO
B B
C
NIS
B
B C
B B B
B
Δ
B
B
B
I
B
B 2-[HO(O)C]CB9H9−
−
B
O
B
6-33
B = BH, B
6-32 HO
−
O C
HO
C B
C
B
B
B B
B
−
O C
+ B
B B B
B
B B B B
B
B
I
B
I
6-34
3:1
6-35
FIGURE 6-17 Synthesis of 1-[HO(O)C]CB9H8-10-I (6-34) from 2-[HO(O)C]CB9H9.
The carboxylic acid 2-[HO(O)C]CB9H9 (6-32) can be obtained from arachno-CB9 species via a sequence described above (see Figure 6-3) [16]. As shown in Figure 6-17, iodination of this compound with N-iodosuccinimide (NIS) affords 2-[HO(O)C]CB9H8-7-I (6-33), which on heating is converted to 1-[HO(O)C]CB9H8-10-I (6-34) together with a lesser amount of 1-[HO(O)C]CB9H8-6-I (6-35); pure 6-34 is isolated from the mixture by recrystallization [53].
6.3.1.4 Introduction of alkyl, amino, and other organic groups
The carboxyl group in 6-34 can be replaced to generate other linearly disubstituted 1-RCB9H8-10-I derivatives where R is C(O)Cl, NCO, or NH2. These compounds, as well as 1,10-1-[HO(O)C]CB9H8-10-N2 and related species [47], are promising candidates for use in liquid crystals as described in Chapter 17. The B-periodo anion 1-HCB9 I9 reacts with CH3MgBr in THF in the presence of (Ph3P)2PdCl2 in a sealed tube at 25-220 C to give the corresponding permethyl derivative, 1-HCB9 Me9 . This species, like the B-perchloro, -perbromo, and -periodo anions, is thermally stable to 240 C and is unreactive toward strong acids and bases [41].
6.3.1.5 Transition-metal complexes There are currently no closo-CB9 clusters known to have transition metals covalently bound to boron, but cationic metal 2B bridging interactions groups form complexes with 1-CB9 H10 and its derivatives, in some cases involving M H2
6.3 10-Vertex closo clusters
171
þ þ (e.g., FeCp(CO)þ 2B bonding, as in 2 1-CB9 H10 , Ag 1-CB9 H10 , and Ag(C6H6)2 1-CB9H8-6,8-F2 ) [40, 51] or M Br2 þ Ag(toluene) 1-CB9 H5 Br5 which forms a zigzag chain polymer in the solid state [51]. Similarly, 1-PhCB9H5-6-I forms a solid-state complex with an Ag4 L4 4þ supramolecular starburst tetrahedron [54]. In other cases, 1-CB9 H10 serves as a counterion for metal cations ligated by protective groups such as bipyridine and o-phenanthroline where M H2 2B bonding is not present [56–58].
6.3.2 C2B8H10 Of the seven possible isomers having bicapped square antiprism geometry, only the three having one or both carbon atoms in low-coordinate apex positions have been isolated and characterized (Figure 1-1 and Tables 6-5–6-8) although others have been investigated computationally. The first to be identified was 1,6-C2B8H10, which was found to rearrange to the 1,10 isomer on heating; the adjacent-carbon 1,2 isomer was synthesized only later. No experimental evidence for any of the four hypothetical systems lacking apex carbons has been reported.
Table 6-5 1,2-C2B8H10 Derivatives Compound Synthesis and Characterization Parent
1,2-Me2
1,2-Ph2 1,2-Li2
Informationa
References
S, H, B, IR, MS S (from nido-C2B8H12) S (oxidation of C2 B8 H10 2 ) MS (calculated monoisotopic) ED S, H, B, IR, MS S, H, B S [1,2-(B5H8)2 þ Me2C2] S, H, B(2d), MS S, H, B
[78, 104] [84] [106] [147] [148] [104] [149] [150] [81] [149]
Isomer stability (ab initio) Charge distribution
[148, 151, 152] [153, 154] [155]
Cage rearrangement
[156, 157]
11
[68]
Theoretical Studies Molecular and electronic structure calculations Parent
Isomerization calculations Parent NMR calculations Parent a
1
11
X, X-ray diffraction; H, H NMR; B, B NMR; C, spectroscopic data; ED, electron diffraction data.
13
B NMR rules
C NMR; F,
19
F NMR; 2d, two-dimensional (COSY) NMR; IR, infrared data; MS, mass
172
CHAPTER 6 Ten-vertex carboranes
Table 6-6 1,6-C2B8H10 Derivatives Compound
Informationa
References
Synthesis and Characterization Parent S S, IR, MS S, H, IR S (improved) MS MS (calculated monoisotopic) dipole moment 1-Me S, H, IR S (improved) S, B, H, IR 1,6-Me2 S (improved) X MS 6-Me S S (improved) 1-Ph S, H, IR S (improved) 1-Li 1-Me-6-I S 1-Ph-6-Br S 1-Ph-6-Me S 1-Ph-6-HgMe S 1-Ph-6-Li S 1-Ph-6-HgMe S B-Me S, IR, MS B-Cl S, B, H, MS S (pyrolysis of nido-CB5H8-3-Me) B,B0 -Me2 S (1,6-PhC2B8H8-6-)2Hg S, MS 1,6-Me2-B-OPh
[84, 104, 106, 158–160] [161] [162] [163] [164] [147] [165] [162] [163] [166] [163] [167] [164] [160] [163] [162] [163] [160] [160] [168] [160] [168] [76, 169] [169] [161] [70] [170] [168, 169] [171]
Detailed NMR Studies Parent 1-Ph 1-m/p-C6H4F
C C F
[172, 173] [173] [174]
Other Experimental Studies Parent
Competitive electrophilic halogenation/alkylation
[175]
Theoretical Studies Molecular and electronic structure calculations Parent Isomer stability Isomer stability (ab initio)
[151] [153, 154] Continued
6.3 10-Vertex closo clusters
173
Table 6-6 1,6-C2B8H10 Derivatives—Cont’d Compound
Informationa
References
Electronic structure MNDO Localized orbitals Hþ charge Charge distribution
[152, 176, 177] [108] [178] [179] [155]
Cage rearrangement
[156, 157]
C (IGLO) B NMR rules B2 2H coupling Electronic properties
[172] [68] [180] [174]
Isomerization calculations NMR calculations Parent
11
1-m/p-C6H4F a
S, synthesis; X, X-ray diffraction; H, 1H NMR; B,
11
B NMR; C,
13
C NMR; F,
19
F NMR; IR, infrared data; MS, mass spectroscopic data.
Table 6-7 1,10-C2B8H10 Compounda Synthesis and Characterization Parent
1,10-D2 1-Me 1-Ph 1-m/p-C6H4F 1-p-C6H4NO2
Informationb
References
S, H, IR S (large scale) S
[162] [181] [84, 104, 106, 160] [182] [183] [147] [184] [185, 186] [173] [165] [187]
S, H, B ED MS (calculated monoisotopic) Raman, IR Raman (variable temp) C dipole moment CH bond polarity (comparison with other carboranes), IR, C(d and JCH), pKa pKa Raman, IR H, IR S (gas-phase flash photolysis), H, B H, IR C F S
[188] [184] [162] [182] [162] [173] [174] [174] Continued
174
CHAPTER 6 Ten-vertex carboranes
Table 6-7 1,10-C2B8H10—Cont’d Compounda
Informationb
References
1-FeCp(CO)2 1-C5H4FeCp 1,10-(CN)2 C)2 2C 1,10-(Me3Si2 1,10-Me2 1,10-(C5H11)2
S, B, H, IR, MS S, B, H S, X, C, B, IR, UV, MS S, X, C, B, IR, UV, MS S, B, H, IR S, B, C, H, IR (nematic crystals; phase transitions), MS S, H, DSC, opto-electrical properties
[189] [74] [190] [190] [104, 160, 166] [191]
S, H, DSC, opto-electrical properties
[192]
S, H, MS, DSC, polarizing microscopy, ferroelectric liquid crystal properties S, H, MS, DSC, polarizing microscopy, ferroelectric liquid crystal properties S, DSC, polarizing microscopy; mesogenic and dielectric properties
[193]
2CB8H8C2 2C6H42 2C6H2F22 2 OEt 1-C5H112 nematic liquid crystals (FF) 2CB8H8C2 2C6H42 2C6H2F3 nematic 1-C5H112 liquid crystals (FF) 1-C5H11-10-(C6H4)2R (R ¼ C8H17, OC8H17) 1-C5H11-10-C6H4-C4N2H2R (R ¼ C8H17, OC8H17) C5H112 2CB8H8C2 2C6H42 2(C6H2-2,3-X2)2 2nR (X ¼ H, F; n ¼ 0, 1; R ¼ C6H13, C8H17). Liquid crystals (FF) 1-(CH5 5CHCl) 1,10-(CH5 5CHCl)2 CR)2 (R ¼ H, SiMe3) 1,10-(C 1,10-Li2 1,10-(COOH)2 1,10-(CCl3)2 1,10-(SiMeCl)2 1,10-[FeCp(CO)2]2 1,10-[Cp(CO)2Fe]C2B8H9 1,10-[Cp(CO)2Fe]2C2B8H8 {1-[Cp(CO)2Fe]C2B8H8}2Hg C] 2C 1,10-[Cp*Ru(Ph2PCH2CH2PPh2)2 C2B8H9 1,10-[Cp*Ru(Ph2PCH2CH2PPh2)C]2C2B8H8 C 1,10 -(1,10-C2B8H9)2 1-R-10-R0 (R, R0 ¼ H, C5H11, C6H13, COOH C6H4Br) 1-Ph-10-Li 1-Ph-10-R [R ¼ C(O)OH, HgBr, Li] 1-Me-10-Ph 1-C(O)OH-10-Ph 1-Me-10-I 1-NH2-10-Ph 1-Ph-10-SiMe3
S, S, S, S S S S, S, S, S, S, S,
H, B, C, IR, MS X, H, B, C, IR, MS H, B, C, IR, MS
H, B, IR B, H, IR, MS H, B, C, Hg, IR, MS, E X, H, B, C, Hg, IR, MS, E H, B, C, Hg, IR, MS, E X, H, B, C, P, IR, UV
[192]
[193] [194]
[195] [195] [195] [160] [160] [160] [196] [189] [197] [197] [197] [198]
S, X, H, B, C, P, IR, UV
[198]
S S, H, C, B(2d), IR, MS
[199] [75]
Raman S S S S S S
[169] [76] [160] [160] [160] [160] [76] Continued
6.3 10-Vertex closo clusters
175
Table 6-7 1,10-C2B8H10—Cont’d Compounda
Informationb
References
1-Ph-10-PbMe3 1Ph-10-HgMe 1-Ph-10-Br 1-Ph-10-R (R ¼ HgMe, SiMe3, PbMe3) 1-Mn(CO)5-10-Me 1-FeCp(CO)2-10-Me 1-FeCp(CO)2-10-FeCp(CO)(PPh3) 1,10-Me2-B-OPh B-Cl8 (1,10-C2B8H9)2 1,10 -(1,10-C2B8H8-OH)2 (1,10-PhC2B8H8-10-)2Hg (1,10-PhC2B8H8-10-)3As (1,10-PhC2B8H8-10-)4Sn (1,10-PhC2B8H8-10-)2SnMe2 (1,10-PhC2B8H8-10-)3P (1,10-PhC2B8H8-10-)2Hg 1,10-(20 ,30 -C2B9H10-40 -)C2B8H9 1,10-(nido-70 ,90 -C2B9H11-100 -)C2B8H8-10R (R ¼ H, Ph) 1,10-C2B8H8-1,10-(10 -SiMe2-2-Me-1,2C2B10H10)2 CC5H11) R-(1,10-C2B8H8)2-R (R ¼ C7H15, C liquid crystal {Me2Si-2,4-C2B5H5-SiMe2i(O)]x [Me2Si-1,10[C2B8H8-SiMe2(O)]y (polymer)
S S S S S, B, H, IR, S, B, H, IR, S, B, H, IR, S, MS S, B, H, IR, Raman, IR S S S S S S S S, B, H, IR, S, B, H, IR
[76] [168] [168] [169] [189] [189] [189] [171] [160] [200] [199] [168, 169] [76] [76] [76, 169] [76, 169] [169] [201] [201]
Other Experimental Studies 1-PhC2B8H9 1-C5H11-10-C(O)O(C6H4)2OC8H17 2CB8H8C2 2CH22 2 electron transport in 2 2CH22 molecular wire between Au electrodes Theoretical Studies Molecular and electronic structure calculations Parent
1,10-(CH2)2C2B8H8þ 1-m/p-C6H4F
MS MS MS MS
MS
S, X, B, IR, MS
[196]
S, X, UV, C, B, H, MS, t[ab initio enthalpies, free energies, phase transitions; ZINDO (UV-vis)] S, IR
[202] [203]
Friedel-Crafts acylation (triflic acid catalysis); Taft s constants. Phase transitions COND
[205] [206]
MNDO Isomer stability (ab initio) Electronic structure Ab initio structural, vibrational, IR Ab initio electron-transfer [ET] properties Ab initio electron-transfer [ET] properties Electronic properties
[108] [151, 153, 154] [152] [207] [208] [208] [174]
[204]
Continued
176
CHAPTER 6 Ten-vertex carboranes
Table 6-7 1,10-C2B8H10—Cont’d Compounda
Informationb
References
1,10-(CN)2 C)2 2C 1,10-(Me3Si2 1-C7H6-10-L (L ¼ C5H4, C5Me4) (1,10-RC2B8H8)2 (R ¼ heptyl, 1-heptynyl)
MNDO/ZINDO MNDO/ZINDO b (first hyperpolarizability); NLO Polarizability anisotropy; liquid crystals.; transition temp; enthalpy Ab initio enthalpies, free energies, phase transitions; ZINDO (UV-vis) Y Hybridization, bond order, comparison with X derivatives of other clusters; electronic cagesubstituent interactions C2 2C bond lengths DFT, electron transport
[190] [190] [209] [210]
C (IGLO)
[172]
CC5H11) R-(1,10-C2B8H8)2-R (R ¼ C7H15, C liquid crystal CSiMe3, C N, C O, N N) 5C 1,10-R2 (R5 CSiMe3)2 1,10-(C 2CB8H8C2 2CH22 2 molecular wire 2 2CH22 between Au electrodes NMR calculations Parent
[202] [211]
[212] [206]
a
Substituents on the carborane cage. “FF” indicates that the full formula of the compound is given. S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; F, 19F NMR; Hg, 200Hg NMR; 2d, two-dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; UV, UV-visible data; NLO, nonlinear optical properties; COND, electrical conductivity.
b
Table 6-8 Other and Unspecified C2B8H10 Isomers Compound Synthesis and Characterization C2B8H9Cl isomers (C2B8H9)2C2B8H8
Information
References
S (oxidation of C2 B8 H10 2 ) S (oxidation of C2 B8 H10 2 ) MS (calculated monoisotopic)
[106] [106] [147]
Theoretical Studies Molecular and electronic structure calculations Charge distribution C2B8H10 (all isomers) Isomer stabilities (ab initio) C2B8H10 isomers Isomer stability C2B8H10 EI [energy indexes]; stabilities C2B8H10 isomers Relative stability (ab initio) C2B8H10 Ab initio C2B8H10 (all isomers)
[155] [154] [213] [64] [153] [61, 152]
Isomerization calculations 2,7-C2B8H10 2,4-C2B8H10 C2B8H10 C2B8H10
[156] [156] [214–216] [157]
Cage Cage Cage Cage
rearrangement rearrangement rearrangement rearrangement mechanism
6.3 10-Vertex closo clusters
177
6.3.2.1 Synthesis of closo-1,6-C2B8 clusters
As was noted in Chapter 5, arachno-6,8-C2B7H13 (Figure 1-3) disproportionates at 215 C to give 1,6-C2B8H10 and 1,7C2B6H8 [158, 162, 166]; similar treatment of 6,8-RC2B7H12 (R ¼ Me or Ph) affords the corresponding C-substituted carboranes. The yield of 1,6-RC2B8H9 (R ¼ H, Me, Ph) is considerably enhanced if B2H6 is added during the pyrolysis [162, 163], and is increased to 74% when 6,8-C2B7H13 and NaC2B7H12 are combined in THF solution in the presence of B2H6 [163]. 1,6-C2B8H10 and its C-substituted derivatives have also been obtained in reactions of B2H6 or B4H10 with alkynes [159, 161], but more practical synthetic routes utilize the thermal isomerization of 1,6-R2C2B8H8 (R ¼ H or Me) [104] and the dehydrogenation of nido-5,6-R2C2B8H10 (R ¼ H or Me) on heating [70, 84, 104]. Reactions of 6,8-C2B7H13 with amine-borane adducts in heated ionic liquid solvents also generate 1,6-C2B8H10. While Me3NBH3 at 120 C gives only 1,6-C2B8H10, adducts of secondary amines react to form both 1,6-C2B8H10 and novel 2N bond has dative double-bond character (Figure 6-18) [88]. In these nido-5,6-C2B8H11-6-NRR0 products in which the B2 species the sp2-hybridized nitrogen atom is proposed to donate electron density into a vacant p orbital on B(6).
R⬘
C B
H H
H
C
B B B
B
H
RR⬘NHBH3
B
R = R⬘ = Me R = RH, R⬘ = CMe3
B
C B B
6,8-C2B7H13
B
B = BH C = CH
B
B
H H
+
B B C
B B
B
R
B
B
B 1,6-C2B8H10
B C C
B
B
N
5,7-C2B7H11-6-NRR⬘
FIGURE 6-18 Formation of closo- and nido-C2B8 clusters from 6,8-C2B7H13 and Me3NBRR0 adducts.
6.3.2.2 Synthesis of closo-1,2-C2B8 clusters
The controlled thermolysis of nido-5,6-C2B8H12 or its C,C0 -disubstituted derivatives at 120–260 C affords the closo products in low to moderate yield [75, 78, 84, 104]. The temperature required for dehydrogenation is considerably lowered by the addition of N-ethylpiperidine-borane [78]. 1,2-Me2C2B8H8 has also been obtained from 2-butyne and 1,20 (B5H8)2 [150], and 1,2-Ph2C2B8H8 is formed as a side product in the synthesis of nido-5,6-Ph2C2B8H10 from Ph2C2 and arachno-B9H13-4-SMe2 [81]. A more efficient approach [106] involves the oxidation of the nido-6,9-C2 B8 H10 2 dianion to a proposed (but not isolated) unstable 2,6-C2B8H10 intermediate, which rearranges, very likely via a diamond-square-diamond (dsd) mechanism [156], to give three isomeric closo-carborane products (Figure 6-19). Heating the isomer mixture at 350 C affords 1,10-C2B8H10 in pure form.
6.3.2.3 Synthesis of closo-1,10-C2B8 clusters In addition to the method of 6,9-C2 B8 H10 2 oxidation just described, other routes to the thermodynamically preferred 1,10 isomer include the high-temperature rearrangement of 1,2- or 1,6-C2B8H10 or their derivatives [75, 104, 160, 162] and the thermolysis of nido-5,6-C2B8H12 (structure shown in Figure 6-5) at 500 C [84]. The latter procedure has been developed into a large-scale (10 g, 86%) synthesis [181]. Flash photolysis of lower boranes with alkynes, mentioned in earlier chapters, also generates 1,10-R2C2B8H8 products along with smaller carboranes [182]. Thermolysis of 6-ferrocenyl-nido-5,6-C2B8H11 at 400 C affords closo-1,10-(CpFeC5H4)C2B8H9 [74].
178
CHAPTER 6 Ten-vertex carboranes B
2− B
C
C B
B B
B
B
B
B
Δ
C B
−2e
B
B B
CuCl2
B
B C
B = BH C = CH
B
2− 6,9-C2B8H10
C B
B
B
B
C
B B
B
B
+
B
B
B
B
B
+
B
B
C
C
B
B B
B
B
B
C
B
B
B
C
1,2-C2B8H10
1,6-C2B8H10
1,10-C2B8H10
FIGURE 6-19 Oxidative cage closure of nido-6,9-C2 B8 H10 2 to closo-C2B8H10 isomers via a proposed 2,6-C2B8H10 intermediate.
6.3.2.4 Structures of C2B8H10 isomers The only definitive structural investigation of the 1,2-C2B8H10 system is a gas-phase electron diffraction study on the parent compound [148], which confirmed the bicapped square antiprism geometry originally assigned from NMR data ˚ ) and an unusu(Figures 1-1 and 6-19), although it is slightly distorted owing to a short C(1)–C(2) bond distance (1.538 A ˚ ally long C(2)–B(3) edge (1.794 A). The 1,6-C2B8H10 cage architecture is established from an X-ray crystallographic analysis of its C,C0 -dimethyl derivative [167]; structural data have not been reported for the parent compound or any other derivative. For 1,10-C2B8H10, a gas-phase electron diffraction analysis of the parent carborane [183] and X-ray crystallographic studies on several C,C0 -disubstituted derivatives [190, 196, 197, 202] (Table 6-7) have been conducted.
6.3.2.5 Electronic properties and cage rearrangement A number of theoretical investigations on the closo-C2B8H10 isomers, cited in Tables 6-5 – 6-8, have elucidated their electronic structures, charge distribution, magnetic behavior, pathways for cluster isomerization, and other properties. As expected from the topological rules outlined in Chapter 2 and the experimentally observed rearrangements noted above, the increasing order of stability is 1,2 < 1,6 < 1,10, reflecting the preference of carbon for low-coordinate, nonvicinal locations in the cage framework. A self-consistent field (SCF) molecular orbital calculation [156] supports a dsd [214] rearrangement mechanism, via a 2,6-C2B8H10 intermediate (see Figure 6-19) that has not been observed experimentally but serves to stabilize the 1,2 isomer at ambient temperatures via an energy barrier.
6.3.2.6 Introduction of substituents on carbon Methods for cage substitution on C2B8H10 isomers generally parallel those employed for the C2B10H12 icosahedral carboranes (Chapters 9 and 10). The CH hydrogens in all three parent isomers are sufficiently protonic in character to undergo lithiation with butyllithium in ethereal solvents, generating the mono- and dilithio derivatives [76, 149, 160, 169]; in the case of the 1,6 isomer, which has nonequivalent cage CH groups, the 1-Li and 6-Li products are obtained in a 2:1 ratio [160]. The C-lithiated
6.3 10-Vertex closo clusters
179
species afford the main entry to alkyl, aryl, carboxyl, silyl, and other C-substituted derivatives (Tables 6-5 – 6-8) via treatment with appropriate reagents. n-C4 Hg Li n-C4 H9 Li C2 B8 H10 ! LiC2 B8 H9 ! Li2 C2 B8 H8 2SiCl2 Me2
Li2 C2 B8 H8 ! ðClMe2 SiÞ2 C2 B8 H8 ð1Þ CO2
Li2 C2 B8 H8 þ! ½HOðOÞC2 C2 B8 H9 ð2Þ H3 O
Me3 SiCCBr
CuBr
C-CB8 H8 C-C CSiMe3 1; 10-Li2 C2 B8 H8 ! Cu2 C2 B8 H9 ! Me3 SiC Exo-polyhedral metal complexes such as 6-36 and 6-37 [168, 189, 197] as well as the dimer 1,10 -(C2B8H9)2 [199], siloxy polymers [203], and the silyl-linked mixed-carborane 6-38 [196] have also been prepared by this route. Table 6-7 lists numerous C-metallated compounds containing main-group or transition metals.
O C
O C C B
Fe
6-36
B B
B
C
B B
O C
O C
6-37
Fe
B
B
B
B B
C B
Me
Me
B
B
B
C
B
B
B
B B
Me C
Me
Si
B
C
B B
B = BH B B B
C O
C
B B
C O
Me
B
B
Fe
B
B
B C B
B
C
B B
C
6-38
C B
Hg
Me Si
B B
B C
B
C O
C O
B B
Fe
B B
B
B
B B
B
B
Figure 6-20 illustrates the application of C-lithiation in the preparation of homo- and heterodisubstituted derivatives [75, 190] that are synthons for construction of linear chains containing C2B8 clusters for use in liquid crystals, polymers, and other applications as described in Chapters 14–17.
6.3.2.7 Introduction of substituents on boron Boron-substituted derivatives of 1,2-C2B8H10 have not been reported, and methods for the controlled addition of functional groups at boron in the 1,6 and 1,10 isomers have yet to be demonstrated. Direct reaction of 1,10-C2B8H10 with Cl2 affords the B-perchlorinated species 1,10-H2C2B8Cl8 [160], and B-alkylated products of 1,6-C2B8H10 are obtained
180
CHAPTER 6 Ten-vertex carboranes Br
Li
H C B B
B
B B
η-C4H9Li
B B
B
C
B = BH
R = C5H11
B
B
C
C
R
C5H11
B
C
B
O B
B C C5H11
C
B
B B C
OH
B B
O C
B
OH B
B B
C5H11
C
B B
B
O C
B
B B
B B
B
C
(1) CO2 (2) HCl
(1) η-C4H9Li (2) CO2 B
(2) I–C6H4–Br
B B
HO
(1) CuI
B
B
R =H
C
B B
B
FIGURE 6-20 Synthesis of C,C0 -disubstituted derivatives of 1,10-C2B8H10 via lithiation at carbon.
in borane-alkyne reactions [161]. Boron-functionalized 1,6-C2B8H10 derivatives have also been generated via degradation of higher carboranes, as in the decomposition of 1,2-C2B10H11Cl to give 1,6-C2B8H9Cl and the pyrolysis of 1,8-Me2C2B9H9OH in the presence of phenol to give 1,6-Me2C2B8H7OPh [171].
6.3.2.8 Cage expansion and degradation
Hydroboration of 1,6-Me2C2B8H8 with B2H6 at 225 C produces 1,7-Me2C2B10H10 in low yield [166], a rare example of icosahedral carborane synthesis via addition of boron to a smaller cage. Hydrolysis of the parent 1,6-C2B8H10 in aqueous basic ethanol, or with piperidine in diethyl ether, affords the 6,8-C2 B7 H12 ion (Chapter 5) and boric acid [160]; in acidic THF, the carborane degrades completely to boric acid. OH
1; 6-C2 B8 H10 ! C2 B7 H 12 þ BðOHÞ3 In contrast, the 1,10-C2B8H10 isomer is much more resistant to cage degradation, a finding that can be explained in terms of charge distribution [160]. In the 1,6 system B(2) and B(3) (see Figure 1-1) are relatively electropositive, as the only B2 2H units adjacent to both cage carbons, and hence are highly susceptible to nucleophilic attack; in the 1,10 isomer, all eight boron atoms are equivalent and therefore present no particular site for reaction with base, in this respect resembling icosahedral 1,12-C2B10H12 (p-carborane, Chapter 10). One assumes that 1,2-C2B8H10 with its highly polar charge distribution [155] will be even more reactive toward electron donors than the 1,6 isomer, but this has not yet been explored experimentally.
6.3 10-Vertex closo clusters
181
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184
CHAPTER 6 Ten-vertex carboranes
[144] Brown, M. P.; Holliday, A. K.; Way, G. M. J. Chem. Soc. Chem. Commun. 1972, 850. [145] Gru¨ner, B.; Jelinek, T.; Plzak, Z.; Kennedy, J. D.; Ormsby, D. L.; Greatrex, R.; et al. Angew. Chem. Int. Ed. Engl. 1999, 38, 1806. [146] Chamberland, B. L.; Muetterties, E. L. Inorg. Chem. 1964, 3, 1450. [147] McLaughlin, E.; Rozett, R. W. J. Phys. Chem. 1972, 76, 1860. [148] Hnyk, D.; Rankin, D. W. H.; Robertson, H. E.; Hofmann, M.; Schleyer, P. v. R.; Bu¨hl, M. Inorg. Chem. 1994, 33, 4781. [149] Wong, E. H.; Nalband, G. T. Inorg. Chim. Acta 1981, 53, L139. [150] Briguglio, J. J.; Carroll, P. J.; Corcoran, E. W.; Sneddon, L. G. Inorg. Chem. 1986, 25, 4618. [151] Jemmis, E. D. J. Am. Chem. Soc. 1982, 104, 7017. [152] Takano, K.; Izuho, M.; Hosoya, H. J. Phys. Chem. 1992, 96, 6962. [153] Ott, J. J.; Gimarc, B. M. J. Comput. Chem. 1986, 7, 673. [154] Williams, R. E.; Bausch, J. W. Appl. Organomet. Chem. 2003, 17, 429. [155] Boer, F. P.; Potenza, J. A.; Lipscomb, W. N. Inorg. Chem. 1966, 5, 1301. [156] Gimarc, B. M.; Ott, J. J. J. Am. Chem. Soc. 1987, 109, 1388. [157] Gimarc, B. M.; Ott, J. J. Main Group Met. Chem. 1989, 12, 77. [158] Dunks, G. B.; Hawthorne, M. F. Inorg. Chem. 1968, 7, 1038. [159] Grimes, R. N.; Bramlett, C. L.; Vance, R. L. Inorg. Chem. 1969, 8, 55. [160] Garrett, P. M.; Smart, J. C.; Hawthorne, M. F. J. Am. Chem. Soc. 1969, 91, 4707. [161] Grimes, R. N.; Bramlett, C. L. J. Am. Chem. Soc. 1967, 89, 2557. [162] Garrett, P. M.; Smart, J. C.; Ditta, G. S.; Hawthorne, M. F. Inorg. Chem. 1969, 8, 1907. [163] Garrett, P. M.; Ditta, G. S.; Hawthorne, M. F. Inorg. Chem. 1970, 9, 1947. [164] Ditter, J. F.; Gerhart, F. J.; Williams, R. E. Advan. Chem. Ser. No. 1968;, 72, 191–210. [165] Echeistova, A. I.; Syrkin, Ya. K.; Rys, E. G.; Kalinin, V. N.; Zakharkin, L. I. Zh. Strukt. Khim. 1974, 15, 154 [in Russian]. [166] Tebbe, F. N.; Garrett, P. M.; Hawthorne, M. F. J. Am. Chem. Soc. 1968, 90, 869. [167] Koetzle, T. F.; Scarbrough, F. E.; Lipscomb, W. N. Inorg. Chem. 1970, 9, 2279. [168] Zakharkin, L. I.; Kalinin, V. N.; Rys, E. G. Zh. Obshch. Khim. 1973, 43, 847 [in Russian]. [169] Zakharkin, L. I.; Kalinin, V. N.; Rys, E. G. Zh. Obshch. Khim. 1972, 42, 477 [in Russian]. [170] Rietz, R. R.; Hawthorne, M. F. Inorg. Chem. 1974, 13, 755. [171] Mercer, G. D.; Scholer, F. R. Inorg. Chem. 1974, 13, 2256. [172] Diaz, M.; Jaballas, J.; Arias, J.; Lee, H.; Onak, T. J. Am. Chem. Soc. 1996, 118, 4405. [173] Zakharkin, L. I.; Kalinin, V. N.; Rys, E. G.; Antonovich, V. A. Izv. Akad. Nauk. SSSR, Ser. Khim. 1980, 1446 [in Russian, p. 2041]. [174] Zakharkin, L. I.; Kalinin, V. N.; Rys, E. G.; Kvasov, B. A. Izv. Akad. Nauk. SSSR, Ser. Khim. 1972, 507 [in Russian]. [175] Nam, W.; Onak, T. Inorg. Chem. 1987, 26, 1581. [176] Guest, M. F.; Hillier, I. H. Mol. Phys. 1973, 26, 435. [177] Koetzle, T. F.; Lipscomb, W. N. Inorg. Chem. 1970, 9, 2743. [178] Dixon, D. A.; Kleier, D. A.; Halgren, T. A.; Hall, J. H., Jr; Lipscomb, W. N. J. Am. Chem. Soc. 1977, 99, 6226. [179] Jarvis, W.; Inman, W.; Powell, B.; Distefano, E. W.; Onak, T. J. Magn. Reson. 1981, 43, 302. [180] Jarvis, W.; Abdou, Z. J.; Onak, T. Polyhedron 1983, 2, 1067. [181] Holub, J.; Jelinek, T.; Janousek, Z. Collect. Czechoslov. Chem. Commun. 2002, 67, 949. [182] Fox, M. A.; Greatrex, R.; Greenwood, N. N.; Kirk, M. Collect. Czechoslov. Chem. Commun. 1999, 64, 806. [183] Atavin, E. G.; Mastryukov, V. S.; Golubinskii, A. V.; Vilkov, L. V. J. Mol. Struct. (Theochem.) 1980, 65, 259. [184] Leites, L. A.; Vinogradova, L. E.; Bukalov, S. S.; Aleksanyan, V. T.; Rys, E. G.; Kalinin, V. N.; et al. Izv Akad. Nauk. SSSR, Ser. Khim. 1975, 1985 [in Russian]. [185] Bukalov, S. S.; Leites, L. A. J. Raman Spectrosc. 1985, 16, 326. [186] Bukalov, S. S.; Leites, L. A. Opt. Spektrosk. 1984, 56, 10. [187] Leites, L. A.; Vinogradova, L. E. J. Organomet. Chem. 1977, 125, 37. [188] Kruglyak, L. I.; Petrov, E. S.; Kalinin, V. N.; Rys, E. G.; Zakharkin, L. I.; Shatenshtein, A. I. Zh. Obshch. Khim. 1972, 42, 2660 [in Russian, p. 2670]. [189] Owen, D. A.; Smart, J. C.; Garrett, P. M.; Hawthorne, M. F. J. Am. Chem. Soc. 1971, 93, 1362. [190] Pakhomov, S.; Kaszynski, P.; Young, V. G. Inorg. Chem. 2000, 39, 2243. [191] Douglass, A. G.; Both, B.; Kaszynski, P. J. Mater. Chem. 1999, 9, 683.
6.3 10-Vertex closo clusters [192] [193] [194] [195] [196] [197] [198] [199] [200] [201] [202] [203] [204] [205] [206] [207] [208] [209] [210] [211] [212] [213] [214] [215] [216]
185
Jasinski, M.; Jankowiak, A.; Januszko, A.; Bremer, M.; Pauluth, D.; Kaszynski, P. Liq. Cryst. 2008, 35, 343. Januszko, A.; Kaszynski, P.; Wand, M. D.; More, K. M.; Pakhomov, S.; O’Neill, M. J. Mater. Chem. 2004, 14, 1544. Januszko, A.; Glab, K. L.; Kaszynski, P.; Patel, K.; Lewis, R. A.; Mehl, G. H.; et al. J. Mater. Chem. 2006, 16, 3183. Fox, M. A.; Baines, T. E.; Albesa-Jove, D.; Howard, J. A. K.; Low, P. J. J. Organomet. Chem. 2006, 691, 3889. Getman, T. D.; Garrett, P. M.; Knobler, C. B.; Hawthorne, M. F.; Thorne, K.; MacKenzie, J. D. Organometallics 1992, 11, 2723. Wedge, T. J.; Herzog, A.; Huertas, R.; Lee, M. W.; Knobler, C. B.; Hawthorne, M. F. Organometallics 2004, 23, 482. Fox, M. A.; Roberts, R. L.; Baines, T. E.; Le Guennic, B.; Halet, J.-F.; Hartl, F.; et al. J. Am. Chem. Soc. 2008, 130, 3566. Zakharkin, L. I.; Kovredov, A. I. Zh. Obshch. Khim. 1974, 44, 1840 [in Russian]. Vinogradova, L. E.; Leites, L. A.; Bukalov, S. S.; Kovredov, A. I.; Zakharkin, L. I. Izv. Akad. Nauk. SSSR, Ser. Khim. 1977, 2337 [in Russian]. Owen, D. A.; Hawthorne, M. F. J. Am. Chem. Soc. 1969, 91, 6002. Kaszynski, P.; Pakhomov, S.; Tesh, K. F.; Young, V. G. Inorg. Chem. 2001, 40, 6622. Kesting, R. E.; Jackson, K. F.; Klusmann, E. B.; Gerhart, F. J. J. Appl. Polym. Sci. 1970, 14, 2525. Endo, Y.; Taoda, Y. Tetrahedron Lett. 1999, 40, 9073. Douglass, A. G.; Czuprynski, K.; Mierzwa, M.; Kaszynski, P. Chem. Mater. 1998, 10, 2399. Aghaie, H.; Gholami, M. R.; Monajjemi, M.; Ganji, M. D. Physica E Low Dimens. Syst. Nanostruct. 2008, 40, 2965. Salam, A.; Deleuze, M. S.; Francois, J. P. Chem. Phys. 2003, 286, 45. Pati, R.; Pineda, A. C.; Pandey, R.; Karna, S. P. Chem. Phys. Lett. 2005, 406, 483. Allis, D. G.; Spencer, J. T. Inorg. Chem. 2001, 40, 3373. Czuprynski, K.; Kaszynski, P. Liq. Cryst. 1999, 26, 775. Kaszynski, P.; Pakhomov, S.; Young, V. G. Collect. Czechoslov. Chem. Commun. 2002, 67, 1061. Herzog, A.; Jalisatgi, S. S.; Knobler, C. B.; Wedge, T. J.; Hawthorne, M. F. Chem. Eur. J. 2005, 11, 7155. Ott, J. J.; Gimarc, B. M. J. Am. Chem. Soc. 1986, 108, 4303. Lipscomb, W. N. Science 1966, 153, 373. Gimarc, B. M.; Ott, J. J. Stud. Phys. Theory Chem. (Graph Theory Topol. Chem.) 1987, 51, 285. Hoffmann, R.; Lipscomb, W. N. Inorg. Chem. 1963, 2, 231.
CHAPTER
7
Eleven-vertex carboranes
7.1 OVERVIEW In the first edition of this book [1] the known chemistry of 11-vertex closo-carboranes was summarized in a few paragraphs, and the development of nido-C2B9 chemistry was still in its infancy. Since then, as the extensive compound listings in Tables 7-1–7-4 attest, huge advances in carborane chemistry generally have generally been paralleled by extensive studies on 11-vertex clusters, including the discovery of many new systems such as mono-, tri-, and tetracarbon carboranes. Much of this work has benefited from the commercial availability of C2B10H12 and B10H14 starting materials, and has been driven, in large part, by the applications of 11-vertex nido-carborane chemistry in medicine, metal extraction from radioactive waste, development of new materials, and other areas discussed in Chapters 14–17.
7.2 11-VERTEX OPEN CLUSTERS 7.2.1 Nido-CB10H13 7.2.1.1 Synthesis In contrast to dicarbon icosahedral-fragment clusters such as C2B9H13 and C2 B9 H11 2 which are typically generated by boron extraction from icosahedral C2B10 carboranes (see following Section), preparative routes for 11-vertex monocarbon nido-carboranes are quite different. The original synthesis protocol entails the incorporation of carbon into decaborane(14), B10H14 or its derivatives and is accomplished in a multistep sequence, starting with treatment with CN or alkyl isocyanides to form C-amino nido-carboranes, in the former case via a B10H13CN2 dianion [2–8]: H3 Oþ
H2 O
Me2 SO4
B10 H14 þ 2CN ! B10 H13 CN2 ! H3 N2 2CB10 H12 ! 7-ðMe3 NÞCB10 H12 HCN
ð1ÞH3 Oþ
B10 H14 þ RCN ! RH2 N2 2CB10 H12 ! 7-ðRMe2 NÞCB10 H12 ð2ÞMe2 SO4 ;OH
R ¼ Me; Et; n-C3 H7 ; i-C4 H9
Deamination of the C-amino derivatives with sodium or sodium hydride yields the parent nido-carborane anion [5–7]: Na=NH3 or NaH
RMe2 N-CB10 H12 ! 7-CB10 H 13 Alternatively, 7-CB10 H13 salts can be obtained in a single step from nido-B10 H12 2 , a highly reactive species that is easily obtained from B10H14, by reaction with dihalomethanes [9]: CH2 X2
B10 H12 2 ! 7-CB10 H 13 ð25%Þ þ B10 H13
Figure 7-1 illustrates these interconversions. Carboranes. DOI: 10.1016/B978-0-12-374170-7.00011-2 © 2011 Elsevier Inc. All rights reserved.
187
188
CHAPTER 7 Eleven-vertex carboranes
Table 7-1 Nido-CB10H13, Nido-CB10H122, and Nido-CB10H113 Derivatives Compound Synthesis and Characterization No substituents on boron 1-CB10 H13 7-CB10 H13
7-(Me3N)CB10H12 7-(H3N)CB10H12 7-(Me3N)CB10 H10 2 7-(Me3CNH2)CB10H12 7-ðMe3 CNHÞCB10 H12 7-[PhC(O)NH]CB10H11 7-PhCB10 H12 7-[(cyclo-C6H11)H2N]CB10H12 7-(Me3CMeHN)CB10H12 7-(Me3N)CB10H12 7-(cyclo-C6H11)Me2N-CB10H12 7-RCB10 H12 (R ¼ BuMeN, cyclo-C6H11HN) 7-RCB10 H11 (R ¼ Me3N, cyclo-C6H11Me2N) 7-RCB10 H12 [R ¼ NHC(O)Me, NMe3 þ , NHMe2 þ , NH3 þ , succinylamino] 7-(succinylamido)CB10 H12 131I biodistribution in mice 7-(H3N)CB10H12 5NH)CB10H12 7-(Me2C5 5NH)CB10H12 7-((CH2)5C5 5N)]CB10H12 7-[PhCH2(PhCH5 7-(HO2CCH2NH2)CB10H12 7-(AcNH2)CB10H12 7-[(CH2)6N4]CB10H12 7-(MeNH2)CB10H12 7-(EtNH2)CB10H12 7-([CHMe2]NH2)CB10H12 7-(BuNH2)CB10H12 7-(Me3N)CB10H12 7-(Et3N)CB10H12 7-([CHMe2]Me2N)CB10H12 7-(BuMe2N)CB10H12
Information
References
S, B, IR S, H, B, IR S X (variable temperature), thermal studies X H(2d), B(2d) B, E IR, Raman S, H, B(2d), MS S, UV, IR S, H, B S, H, B(2d), E S, X, H, B S, X, H, B S, UV, IR S, H, B, C S, X, H, B, C S, H, B, C S, X, H, B, C S, H, B S, X, H, B, C S, H, B, C S, H, B, C S, H, B, MS
[268] [5] [6,8,9] [19] [20] [112] [250] [289] [21] [6] [43] [23] [290] [290] [6] [17] [16] [2] [2] [2] [2] [2] [2] [24]
S, S S S S S S S S, S, S, S, S, S, S, S,
[24] [12] [12] [12] [12] [12] [12] [12] [10] [10] [10] [10] [10] [10] [10] [10]
H, B, MS
H, H, H, H, H, H, H, H,
B, B, B, B, B, B, B, B,
IR, IR, IR, IR, IR, IR, IR, IR,
MS MS MS MS MS MS MS MS
Continued
7.2 11-Vertex open clusters
189
Table 7-1 Nido-CB10H13, Nido-CB10H122, and Nido-CB10H113 Derivatives—Cont’d Compound
Information
References
7-(Me3N)CB10H12 7-(Me3-nHnN)CB10H12 7-(Et3-nHnN)CB10H12 7-(Me2S)CB10H12 7-(H3N)CB10H12 7-(Me3N)CB10H12 7-(H2PrN)CB10H12 7-ðOCNÞCB10 H12 7-[RHNC(O)NH]CB10 H12 (R ¼ H, Me, CMe3, MeCHC(O) OH, MeCHC(O)OEt, C6H4C(O)OH, CB10H12, gramicidin S)
S, S, S, S S S S S, S,
H, B, C, IR H, B, C, IR
[6] [14] [14] [12] [8] [8] [8] [11] [11]
D or C-containing substituents on boron 7-(Me3CN)CB10D5H6-9-Cl 7-(Me3N)CB10H6D5-9-Cl 7-(Me3N)CB10H6D4-6,9-Cl2 7-(Me3N)CB10H6D4-6,9-Cl2 7-(MeD2N)CB10H12-nDn, (EtD2N)CB10H12-nDn 5CHCH2 7-(Me3N)CB10H11-4-Ph-CH25 7-(H3N)CB10H11-8-CH2Ph 7-(Me3N)CB10H11-8-CH2Ph 7-MeCB10H11-(m-9,10)-MeCH 7-PhCB10H11-(m-9,10)-PhCH [7-CB10H11-(m-9,10)-CH)]22 7-PhCB10H10-(Z5-CO) [-(CH2)n-NH(CB10H12)-]n polymers
S, S, S, S, S, S, S, S, S, S, S, S, S,
H, B, MS H, B (2d), MS, IR H, B (2d), MS, IR H, B (2d), MS, IR MS H, B H, B H, B(2d), MS X X X, H, B, IR X H, B, C, IR (diffuse reflectance)
[21] [22] [22] [22] [10] [23] [13] [13] [35] [38] [36] [40] [43]
N- or P-containing substituents on boron 7-CB10H12-NHC(O)Me 7-(NC5H4-CH2)CB10H11 (NB) 7-CB10H12-8-NEt3 7-CB10H12-8-PPh3 7-[(Me3Si)2CH]CB10H11-9-PPh3 7-(Me3N)CB10H10-m-PPh 7-(Me3N)CB10H10-m-PMe 7-(Me3N)CB10H10-m-PEt 7-MeCB10H9-m(9,10)-CMeH-n-PPh3 (n ¼ 5, 6) 7-MeCB10H10-8-OEt-9-CMeHPPh3 7-CB10H12-8-[(Ph2P-Z5-C5H4)Fe(Z5-C5H4PPh2)]
S, S, S, S, S, S, S, S, S, S, S,
B, IR X, H, B, MS H, B X, H, B, IR, MS X, H, B, IR, MS X, H, B, IR H, B, IR H, B, IR X, H, B, P, MS X, H, B, P, MS H, B, C, P, MS, IR
[250] [291] [21] [27] [30] [41] [41] [41] [39] [39] [31]
O- or S-containing substituents on boron 7-CB10H11-9-Me-10-OH 7-CB10H12-8-OH 7-(H3N)CB10H11-8-OHC4H8O2
H, B, IR, UV H, B, IR H, B, IR
S, B(2d) S, B(2d), H, MS S S
[26] [21] [25] [6] Continued
190
CHAPTER 7 Eleven-vertex carboranes
Table 7-1 Nido-CB10H13, Nido-CB10H122, and Nido-CB10H113 Derivatives—Cont’d Compound
Information
References
7-(Me3N)CB10H11-8-C(O)OH 7-(Me3N)CB10H11-8-OH 7-(Me3NCB10H11-OMe 7-(Me3N)CB10H11-4-SH (7-(Me3N)CB10H11)2S2 7-[PhC(O)NH]CB10H11-SMe2 7-(Me3Si)2CH-7-CB10H11-9-SMe2 7-(Me3N)CB10H11-2, 4-C6H3S(NO2)2 5CH 7-(RNH)CB10H10-2-SMe2-11-Me3SiR0 C5 (R ¼ PhCH2, CMe3, Bu; R0 ¼ SiMe3, Bu) 7-CB10H12-8-SMe2 (2)-1-[(Me3C)HN]CB10H10-3-C6H11-5-SMe2
S, S S, S, S, S, S, S S,
[6] [6] [14] [28] [28] [6] [15] [29] [3]
IR H, B, IR H, B(2d), MS H, B(2d), MS UV, IR H, B, MS, IR H, B, MS
S, H, B, IR, MS S, X, H, B, MS, IR
[27] [4]
Cl-, Br-, or I-containing substituents on boron 7-CB10H12Cl 7-CB10 H11 Cl2 7-CB10H12Br 7-CB10 H11 Br2 7-(Me3N)CB10H11Cl 7-(Me3N)CB10H11-n-Cl (n ¼ 7, 9) 7-(Me3N)CB10H6D5-9-Cl 7-(Me3N)CB10H11-n-Cl (n ¼ 4, 9) 7-(Me3CN)CB10H6D5-9-Cl 7-(Me3N)CB10H6D4-6,9-Cl2 7-(Me2PrN)CB10H10Cl2 7-(Me3N)CB10H11Br 7-(Me2PrN)CB10H11Br 7-(Me3N)CB10H11-4-X (X ¼ I, Br) 7-(Me3N)CB10H10-4,6-X2 (X ¼ I, Br) 7-(Me3N)CB10H11-9-I 7-(Me3N)CB10H9-4-I2 7-(Me3N)CB10H8-4,6 I2 2 anti-9-(Me3CNH2)CB10H10-conjuncto-B8H10
S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S, S,
[5] [5] [5] [5] [5] [22] [22] [21] [21] [22] [5] [5] [5] [23] [23] [23] [23] [23] [292]
Detailed NMR Studies 7-CB10 H13 7-(Me3N)CB10H12 7-CB10H11-4,6 Br2 m-PhCH-PhCB10 H11 (1,2; 1,7) m-MeCH-MeCB10 H11 (1,2; 1,7)
B B B C C
[293] [293] [293] [294] [294]
Other Experimental Studies 7-(Me3N)CB10H12 7-(Me2[CHMe2]N)CB10H12
Reactivity Reactivity
[5,29,42] [5]
IR IR B, IR IR IR, MS H, B (2d), MS, IR H, B (2d), MS, IR H, B(2d), MS H, B, MS H, B (2d), MS, IR IR, MS IR IR, MS H, B (2d; X ¼ I), E (X ¼ I) H, B (2d; X ¼ I), E (X ¼ I) H, B H, B(2d) H, B(2d) X, H, B, MS
Continued
7.2 11-Vertex open clusters
191
Table 7-1 Nido-CB10H13, Nido-CB10H122, and Nido-CB10H113 Derivatives—Cont’d Compound
Information
References
Reactions with amines Thermal decomposition Copolymerization of CB10 H12 anion with dibromoalkanes, crosslinking with Co3þ Controlled chemical and electrochemical substitution [Cl, D, OH, CO, C(O)OH]
[11] [13] [43]
Theoretical Studies Molecular and electronic structure calculations 1-CB10H14 CB10H14 isomers CB10 H13 isomers CB10 H12 2 isomers CB10 H13 7-ðMe3 NÞCB10 H11 7-CB10 H13 7-CB10H12-8-OH 7-(H3N)CB10 H11 7-(H2N)CB10 H12 7-(H2N)CB10H11-4-Cl 7-(H3N)CB10H12 7-ðH3 NÞCB10 H13 þ 7-(H3N)CB10H11-n-Cl (n ¼ 4,9) 7-(H3N)CB10H11-n-OH (n ¼ 8,9) 7-(H3N)CB10H11-8-NH3 7-(Me3N)CB10H12 5CHCH2 7-(Me3N)CB10H11-4-Ph-CH25 7-(Me3N)CB10H11-4-X (X ¼ I, Br) 7-(Me3N)CB10H10-4,6-X2 (X ¼ I, Br) 7-(Me3N)CB10H11-9-I 7-(Me3N)CB10 H10 2 7-(Me3N)CB10H9-4-I2 7-(Me3N)CB10H8-4,6-I2 2
Optimized geometry DFT (stability) DFT (stability) DFT (stability) Vibrational modes Optimized geometry DHf, IP, charge distribution, DHf, IP, charge distribution, DHf, IP, charge distribution, DHf, IP, charge distribution, DHf, IP, charge distribution, DHf, IP, charge distribution, DHf, IP, charge distribution, DHf, IP, charge distribution, DHf, IP, charge distribution, DHf, IP, charge distribution, DHf, IP, charge distribution, DHf, charge distribution DHf, charge distribution DHf, charge distribution DHf, charge distribution DHf, charge distribution DHf, charge distribution DHf, charge distribution
[295] [244] [244] [244] [289] [2] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [23] [23] [23] [23] [23] [23] [23]
NMR calculations 7-CB10 H13 7-CB10 H12 2 7-CB10H10-m(9,10)-CHMe
GIAO (11B) GIAO (11B) GIAO (11B)
[296] [296] [296]
Reactivity calculations 7-MeCB10H12 7-CB10 H12 2 7-CB10H10-m(9,10)-CHMe
pKa pKa pKa
[296] [296] [296]
7-ðOCNÞCB10 H12 [BH2 ðNMe3 Þ2 þ ] [7-CB10 H13 ] 2]n polymers [2 2(CH2)n-NH(CB10H12)2 7-CB10 H13 , 7-(Me3N)CB10H12
bond bond bond bond bond bond bond bond bond bond bond
indices indices indices indices indices indices indices indices indices indices indices
[21]
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; 2d, two-dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; UV, UV-visible data; E, electrochemical data; IP, ionization potential.
192
CHAPTER 7 Eleven-vertex carboranes − CN
H
H
H
B
B
H
B
B H B B B B
−
H
H3N+
B
C
H+
B B
B B
B
B
B B B
B
H2RN+
B
RNC
B B
B B10H14
B
B
B
B
B Me3N-CB10H12
H
B
C
H
B B
B B
−
Me2RN+
H
B
C
Me+
H
B B
B B
B
B B B
B
B B
B H2RN-CB10H12 −
B
H3N-CB10H12
B B
B
B B
H
B B
B
− H H
H H
H
B
C
Me+
B
B B10H13CN2−
Me3N+
B B
H
Me2RN-CB10H12
H
L+
B C
B B
H
B B
B B
Na/liq. NH3 or NaH
B
B
L = NMe3, NRMe2
B
− H
L-CB10H12
B B B B
H
B
C
B
B B B
B B CB10H−13
CH2CX2
B B
H
B
B B
B
H
H
B
X = Br, I
B
B 2−
B10H12
FIGURE 7-1 Synthetic routes to CB10 H13 B ¼ BH.
Carbon-substituted 7-RCB10 H12 derivatives can be prepared similarly, e.g., by reacting B10H14 or its derivatives with alkyl isocyanides [3,4,10], or alternatively, by the conversion of 7-H3NCB10H12 to 7-LCB10H12 species (Table 7-1) [8,11–13]. Trimethylsilyl- and trimethyltin chloride interact with B10H13CN2 to yield, following hydrolysis, 7-H3NCB10H12, while B10H13CN2, on treatment with alkyl iodides, affords 7-LCB10H12 products, where L is NMe3 or NHEt2 [14].
H2 O
B10 H13 CN2 þ Me3 MCl ! 7-ðH3 NÞCB10 H12
ðM ¼ Si; SnÞ
H2 O
B10 H13 CN2 þ Mel ! 7-ðMe3 NÞCB10 H12 C4 H8 O
As mentioned earlier, deamination of the C-amino derivatives generates an unsubstituted CB10 H13 ion.
7.2 11-Vertex open clusters
193
Other synthetic routes to nido-LCB10H12 carboranes have been discovered, in some cases serendipitously. The C2 2C 2SiMe3, which was expected to give closo-1,2dimethyl sulfide-promoted reaction of B10H14 with Me3Si2 (Me3Si)2C2B10H10, instead formed nido-7-[(Me3Si)2CH]CB10H11-9-SMe2 as the major product, along with an alkenyl decaborane derivative [15]; the proposed mechanism involves initial complexation of the alkynyl group with decaborane, followed by intramolecular hydroboration and migration of an SiMe3 group. Nido-7-PhCB10 H12 has been prepared by boron insertion into the 10-vertex PhCB9 H11 anion with LBH3 (L ¼ THF or SMe2) [16,17], and also by thermolysis of nido-6-PhCB9 H11 , but in lower yield [16]. The acid hydrolysis of closo-heterocarboranes 1,12-EðHÞCB10 H13 , where E is P or As (Chapter 12), removes the heteroatom from the cage, forming the 1-CB10 H13 isomer 7-1 in which the skeletal carbon occupies a high-coordinate vertex that is distant from the open face [268]. The geometry of this anion is clearly revealed by its simple 11B NMR spectrum, which exhibits just two resonances of equal intensity, consistent with the presence of just two boron environments on the NMR time scale if rapid tautomerism of the B2 2H2 2B protons on the open face is assumed. -
H
B
H
B B
7-1
B B
B
B
B
B
B
B = BH C = CH
C
7.2.1.2 Structure
X-ray crystallographic structure determinations have been conducted on the parent 7-CB10 H13 ion [19,20] and on several C- and B-substituted derivatives that are listed in Table 7-1. These studies, together with NMR and other spectroscopic data (Table 7-1), confirm an icosahedral-fragment geometry with the skeletal carbon and two nonadjacent B2 2H2 2B bridging units located on the 5-membered open face (Figure 1-3, third row).
7.2.1.3 Substitution at boron: General methods A broad range of synthetic approaches has been employed to alter the chemistry at the boron vertices on the parent 7-CB10 H13 and its C-substituted derivatives. These include direct reaction with halogens, phosphines, and sulfur compounds, electrophilic iodination, electrochemistry, and hydrolysis. Still other boron-substituted nido-CB10 species have been prepared via the insertion of a carbon into B10H14 derivatives and by the reduction of 1,2-R2C2B10H10 carboranes. In many cases, the direct introduction of substituents is an ill-defined and uncontrolled process, affording multiple products and/or compounds in which the location of the entering group is not well established. However, controlled substitution has been achieved in some cases, as will be seen later. The observed patterns of reactivity of the nido-7CB10 cage correlate well with the calculated charge distributions and COSY 11B NMR data [21], which show that B (4) and B(6) are electron-rich and are most prone to electrophilic attack, while the boron sites on the open ring are relatively electropositive and are the favored locations for Lewis base addition.
7.2.1.4 Halogenation
7-CB10 H13 or its neutral charge-compensated counterpart, 7-(Me3N)CB10H12, can be chlorinated electrochemically in a concentrated HCl solution to generate the 4-chloro derivative in 67% yield [21]: 7-ðMe3 NÞCB10 H12 þ 2Cl ! 7-ðMe3 NÞCB10 H11 -4-Cl þ HCl þ 2e Friedel-Crafts nucleophilic chlorination of the same substrate, via reaction with HCl over AlCl3, affords primarily 7-(Me3N)CB10H11-9-Cl accompanied by a small amount of 7-(Me3N)CB10H10-6,9-Cl2 [21,22]; similar treatment with DCl leads to both chorination and deuteration of the BH groups, with the addition of up to 5 D atoms [22]. Treatment
194
CHAPTER 7 Eleven-vertex carboranes
of 7-CB10 H13 or (Me2RN)CB10H12 (R ¼ Me or n-C3H7) with N-chlorosuccinimide, with or without an AlX3 catalyst, gives mono- and dichloro derivatives, while bromination of 7-CB10 H13 with Br2 generates both mono- and dibrominated products [5]. In both cases, spectroscopic and other evidence suggests that halogenation occurs preferentially at the B(4,6) locations. Electrophilic iodination of 7-(Me3N)CB10H12 affords 7-(Me3N)CB10H11-4-I and 7-(Me3N)CB10H10-4,6-I2, the structures of which have been established from COSY two-dimensional 11B NMR spectra [23]. Radioiodination of 7-RCB10H12 derivatives, employing Na131I in the presence of N-chlorosuccinimide, has generated labeled carboranes [24] for biodistribution studies in mice in conjunction with BNCT (boron neutron capture therapy) applications as discussed in Chapter 16.
7.2.1.5 Introduction of other functional groups
Treatment of 7-(Me2RN)CB10H12 (R ¼ Me or n-C3H7) with NaH in THF generates 7-CB10 H13 , in good yield, together with minor side products [5], but the reaction is complex and dependent on certain reaction conditions [21]. At low temperatures, 7-ðMe3 NÞCB10 H11 is initially formed, but on reflux and workup the products obtained are closo-2-CB10 H11 and nido-7-CB10H12-8-OH (7-2) [21,25]. −
H
O 8 7
7-2
C
B 11
3
H 9
H
B
4
B B 10
B
B
6
2B
B
B5
B = BH C = CH
B1
A different B-hydroxyl derivative, 7-CB10H11-9-Me-10-OH, is produced during the reaction of the “reactive” isomer of nido-C2 B10 H13 (see below) with K2CO3 in aqueous THF [26]; in this case, one of the cage carbon atoms in C2 B10 H13 is converted to an exo-polyhedral methyl group. This provides an interesting contrast to the more stable C2 B10 H13 isomer in which one cage carbon adopts a bridging role, as described later in this Section. Oxidative addition of triethylamine, promoted by thallium(III), also leads to a substitution at B(8) [21]: 7-ðMe3 NÞCB10 H12 þ Tl3þ þ NEt3 ! 7-ðMe3 NÞCB10 H10 -8-NEt3 þ Tlþ þ 2Hþ : Controlled-potential electrolysis of Csþ 7-CB10 H13 in acetonitrile results in a 2-electron oxidation process that forms CB10H12-NCMe, which, in turn, is hydrolyzed to the amide 7-CB10H12-NHC(O)Me [250]. This behavior contrasts with that of the dicarbon carboranes, such as C2B8H10 and C2B10H12, which do not undergo electrochemical oxidation, presumably because the presence of two carbon atoms in the cage framework increases the electron density and substantially raises the oxidation potential. The unexpected formation of 7-[(Me3Si)2CH]CB10H11-9-SMe2 from B10H14 and bis(trimethylsilyl)acetylene has been mentioned earlier. A different dimethylsulfide-substituted derivative is obtained on treatment of the parent 7-CB10 H13 with SMe2 in concentrated sulfuric acid [27]: þ 7-CB10 H 13 þ SMe2 þ H ! 7-CB10 H12 -8-SMe2 þ H2
This reaction is proposed [27] to involve the initial formation of a CB10H14 intermediate that loses H2 to generate an electrophilic CB10H12 fragment, which, in turn, combines with the SMe2 electron donor. Disulfide- and thiol-substituted derivatives, such as 7-(Me3N)CB10H11-4-R (R ¼ SH, S2), can be obtained via the interaction of 7-(Me3N)CB10H12 with S2Cl2 [28]. Friedel-Crafts substitution on 7-(Me3N)CB10H12 has been employed to prepare 7-(Me3N)CB10H11-2, 4-C6H3S(NO2)2 for biodistribution studies on melanoma in mice (Chapter 16) [29].
7.2 11-Vertex open clusters
195
Phosphine derivatives are accessible by a simple displacement of the sulfide groups, as seen in the reaction of 7[(Me3Si)2CH]CB10H11-9-SMe2 with triphenylphosphine to afford 7-[(Me3Si)2CH]CB10H11-9-PPh3 [30] and in the interaction of 7-CB10H12-8-SMe2 with the same reagent to generate 7-CB10H12-8-PPh3 [27], both in good yield. Similarly, 1,10 -bis(diphenylphosphino)ferrocene and 7-CB10H12-8-SMe2 combine to form 7-CB10H12-8-[(Ph2P-Z5-C5H4)Fe(Z5C5H4PPh2)] (7-3) [31].
P
7-3
H
B C
Fe
B B
H
B B
B B
B B
Ph2P
B = BH C = CH
B
The versatile compound 7-[(Me3Si)2CH]CB10H11-9-SMe2 interacts with CpCo(CO)2 to produce closo-CpCo [(Me3Si)2CH]CB10H11-n-SMe2] metallacarborane sandwich complexes (Chapter 13) [30]. Phosphino-substituted derivatives have also been obtained in reactions with metal phosphines, as described below.
7.2.1.6 Carbon- and phosphorus-bridged nido-CB10 clusters
The two-electron reduction of icosahedral R2C2B10H10 carboranes (R ¼ H, alkyl, aryl) with alkali metals has long been known to induce cage-opening to form nido-R2 C2 B10 H10 2 dianions, an important process that is described in Chapter 11. Protonation of these dianions generates R2 C2 B10 H11 monoanions that exist in two forms, the kinetic (so-called “reactive”) isomer 7-4 and the thermodynamically stable species 7-5 [32], also known as the “unreactive” isomer because, unlike 7-4, it does not form metallacarboranes with metal reagents [33,34]. In isomer 7-5, one of the original cage carbon atoms is protonated and adopts a bridging role on a nido-CB10 framework, as established by several X-ray diffraction studies and by ab initio calculations(Table 7-1) [35–38]. R H
− H
7-4
B B
C
B
B B
−
R R
B
C
B
R
B B
B B
B
B
B
B B
B C
7-5
B
B = BH
C
B B
Carbon-bridged compounds that are structural analogues of 7-5, 7-MeCB10H9-m(9,10)-CMeH-n-PPh3 (n ¼ 5, 6) have been obtained by refluxing salts of 7-5 (R ¼ Me) with PdCl2(PPh3)2 in ethanol, a reaction that also affords a nonbridged derivative, 7-MeCB10H10-8-OEt-9-CMeHPPh3 [39]. A cluster described as a pentuply-carbonyl-bridged nido-PhCB10 cage, 7-PhCB10H10-(Z5-CO) (7-6) has been prepared by the deprotonation of the o-carborane derivative 1,2-(HO)PhC2B10H10 ˚ in 7-6 is consistent with with 1,8-bis(dimethylamino)naphthalene (“proton sponge”) [40] The C2 2O distance of 1.245 (3) A double-bond character and indicates a transfer of the negative charge from the oxygen to the cage, while the long cluster
196
CHAPTER 7 Eleven-vertex carboranes
˚ ) implies weak but significant interaction between the carbonyl and cluster carbon atoms. C2 2C bond length (2.001 (3) A In truth, 7-6 can be equally well described as a C2B10 cage or as a carbonyl-bridged CB10 system. H
O
O
−
C
C Ph
B
C
B B
B B
B B
B
Ph
−H+
C
B B
B B
B = BH
B
B B
B
7-6
B
B B
B
Phosphorus-bridged derivatives 7-(Me3N)CB10H10-m(9,10)-PR (R ¼ Me, Et, Ph), analogous to 7-5, have been obtained by treating 7-(Me3N)CB10H12 with Et3N, followed by RPCl2, and the structure of the PPh species (7-7) has been established by X-ray crystallography [41]. In this system the phosphorus bridges two boron atoms on the open face but has an essentially nonbonding interaction with the other three facial atoms [41]. In effect, the PPh unit functions as a 2-electron donor that replaces two B2 2H2 2B bridging hydrogens.
P
7-7 Me3N
B
H
C
B B
B B
B
B = BH B
B B B
7.2.1.7 Cage expansion reactions
Boron incorporation into 7-CB10 H13 or 7-(Me3N)CB10H12 by treatment with Et3NBH3 yeilds the icosahedral carboranes closo-1-CB11 H11 and closo-1-ðMe2 NÞCB11 H11 respectively [13,42]; that this occurs via a clean boron insertion has been demonstrated by labeling experiments with Et3N10BH3 [13]. However, the reaction of nido-7-(Me3N)CB10H118-CH2Ph with Et3NBH3 generates closo-1-(Me3N)CB11H10-7-CH2Ph as the sole product, indicating that boron insertion in this case is accompanied by cage rearrangement [13].
7.2.1.8 Polymerization The amine derivative 7-(H3N)CB10H12 can be combined with a,o-dihalogenoalkanes to generate high molecular weight alternating zwitterionic copolymers having pendant carborane groups [43]: 2NHþ ðCB10 H 2ðCH2 Þn2 2gx 7-ðH3 NÞCB10 H12 þ BrðCH2 Þn Br ! f2 12 Þ2 Cross-linking of these chains via coordination of the open-faced carborane cages to Co3þ ions leads to a metallacarborane polymer that has been characterized from spectroscopic evidence as 7-8, in which a total of three bridging protons on each pair of carborane faces is presumably retained in order to maintain electroneutrality, though their fate is not specified.
7.2 11-Vertex open clusters
197
(CH2)nHN−(CH2)n B
C
B B
B
B
B
B
Co B
B
B
B
B B
B B
B B
B C
B
7-8 B = BH
B
(CH2)nHN−(CH2)n
7.2.2 Nido-C2B9H13, nido-C2B9H12, and nido-C2B9H112 The base-promoted, controlled extraction of boron (deboronation) from icosahedral C2B10 carboranes to generate 11-vertex nido-C2B9 species [44], discovered by Wiesboeck and Hawthorne in 1964 [45], was one of the most significant findings of the early exploration of carborane chemistry, and decades later it remains central to the synthesis of many metallacarboranes and hydrophilic functionalized carboranes for medical and other applications. The importance of this reaction is that it affords direct access to a broad array of synthetically useful nido-carboranes from commercially available 1,2-C2B10H12 (o-carborane), 1,7-C2B10H12 (m-carborane), and 1,12-C2B10H12 (p-carborane) and their derivatives using standard organic and organometallic procedures and reagents. Remarkably (and fortunately for synthetic purposes), only one boron atom is normally removed in the controlled degradation of these icosahedral systems, affording in most cases nido-C2 B9 H12 or nido-C2 B9 H11 2 anions or their substituted derivatives (Tables 7-2–7-4). Protonation of these ions in some, but not all, cases generates isolable neutral species, i.e., nido-C2B9H13 isomers or derivatives thereof. Although nine isomers of nido-C2B9H13 are theoretically possible based on skeletal carbon locations, only 7,8- and 2,9-C2B9H13 have been well-characterized as parent species. However, as is evident in the tables, many C- and B-substituted derivatives of 7,8- and 7,9-C2B9H13 and their anions are known, as are a few based on the 2,7- and 2,9-C2B9H13 isomers (Table 7-4).
7.2.2.1 Controlled deboronation of C2B10 carboranes Despite the remarkable stability of the icosahedral C2B10H12 clusters toward attack by acids or oxidizing agents (see Chapters 9 and 10), the 1,2- and 1,7-isomers and their C-substituted derivatives are susceptible to attack by nucleophiles. Bases such as alkoxides [44–53], ammonia [54–56], ammonium hydroxide [57], alkylamines [55,58–60], hydrazine [61–65], piperidine [59,66–71], pyrrolidine [72–74], and iminophosphoranes [75,76] remove a BH unit to create an 11-vertex nido-C2B9 monoanion or, in some instances, nido-C2B9-base adducts (see Tables 7-2 and 7-3). Their anions, in turn, can be deprotonated to afford the corresponding dianion or, alternatively, be protonated to form a neutral nido-carborane. C2 B10 H12 þ OH þ 3CH3 OH ! C2 B9 H 12 þ BðOCH3 Þ3 þ H2 þ H2 O Hþ
C2 B9 H13 ! C2 B9 H12 ! C2 B9 H11 2 þ H
The alkoxide method has been optimized by the use of KOH in boiling methanol, which affords the pure potassium salt with 94% yield [48]. However, in some cases the alkoxide treatment is not suitable. For example, it cannot be employed 2P to generate C-phosphino-nido-C2B9 derivatives from C-phosphino o-carboranes owing to cleavage of the Ccarborane2 bond; for such compounds, piperidine in large excess has been used successfully [67]. Fluoride ion can serve as a cage-deboronating agent [77–88], although the choice of the counterion and the solvent are important. The reaction of ethanolic CsF with 1,2-C2B10H12 or its C-substituted derivatives gives KþRR0 C2 B9 H10
198
CHAPTER 7 Eleven-vertex carboranes
Table 7-2 Selected Nido-7,8-C2B9H13, Nido-7,8-C2B9H12, and Nido-7,8-C2B9H112 Derivativesa Compound
Information
References
Single-cage derivatives of nido-7,8-C2B9H13, nido-7,8-C2B9H12, and nido-7,8-C2B9H112 Neutral single-cage derivatives, no substituents on boron S (modified), B, H C2B9H13 S, IR S, H, B, C Me2C2B9H11 S, IR RC2B9H12 (R ¼ Me, Ph) S, X, H, B, C, P, IR [PH(CHMe2)2]MeC2B9H12 S, X, H, B, C [(Me2CH)2IP]PhC2B9H10 S, X (R, R0 ¼ Ph), H, B, C, P, IR [PR2(O)]R0 C2B9H11 (R ¼ Et, CHMe2, Ph; R0 ¼ H, Me, Ph) S (C5H4NMe)PhC2B9H10 5CH)C2B9H11 (R ¼ CHMe2, Me) S, X(CHMe2), H, B, C, IR, MS (C6H3R2-NH5 S, X, H, B, C, IR, MS [(PhCH2)2NCH2CH2]RC2B9H10 (R ¼ H, Me) S, H, B [H3N(CH2)n]C2B9H11 (n ¼ 2,3) S, X, H, B [H3N(CH2)3]C2B9H11N2H4 S, X, H, B (H5C5NCH2)C2B9H11 S, X, H, B, C, IR (Me2HNCH2)C2B9H11 S, X, H, B, C, IR (Me2NCH2)(HMe2NCH2)C2B9H10 S, X, H, B, C, IR (Ph2P)(HMe2NCH2)C2B9H10 S, X(Al), H, B, C, IR m-Me2M-(Me2NCH2)C2B9H10 M ¼ Al, Ga S, X, H, B, C, IR m-Me2M-(Me2NCH2)(Ph2P)C2B9H10 M¼ Al, Ga S, H, B, C, P, MS. (MePh2P)C2B9H10 selective targeting of mitochondria for BNCT [cyclo-N3P3(C5H10N)4MeCH2]C2B9H12 S, X, P, MS cyclotriphosphazene S, X, H, B, IR m(7,8)-[SCH(PPh3)S]C2B9H10 S, X, H, C m(7,8)-(S-CH2CH2-O-CH2CH2-S)C2B9H11 S, IR X2C2B9H11 (X ¼ Cl, Br, I)
[303] [197] [304,305]
Neutral single-cage derivatives, D or hydrocarbon substituents on boron S, B, IR C2B9H12D S, B C2B9H(13-n)Dn S, B, IR C2B9H12-3-Ph S, H, B, C Me2C2B9Me6H5
[141] [143] [141] [92]
Neutral single-cage derivatives, N-containing substituents on boron S, IR, B (RR0 ¼ H, H) RR0 C2B9H9-3-NC (R, R0 ¼ H, Me; R ¼ Me, R0 ¼ PhCH2) S [HO(O)CCH2]C2B9H10-3-NH3 C2B9H12-9-L [L ¼ NEt3, PhNMe2, C5H4N-C(O)OMe, S, B, H, IR HC(O)NMe2, MeC(O)NMe2] S, IR C2B9H11-9-NC5H5-10-X (X ¼ H, Cl)
[117] [45] [47] [45] [67] [297] [67] [105] [103] [298] [129] [129] [125] [299] [300] [300] [300] [300] [301] [302]
[306] [307] [170] [308] Continued
7.2 11-Vertex open clusters
199
Table 7-2 Selected Nido-7,8-C2B9H13, Nido-7,8-C2B9H12, and Nido-7,8-C2B9H112 Derivativesa—Cont’d Compound
Information
References
R2C2B9H10-9-NC5H5 (R ¼ H, Me) C2B9H11-n-NC5H5 (n ¼ 9,10) Me2C2B9H9-n-NC5H5 (n ¼ 9,10) C2B9H12-5-NC5H5 C2B9H11-9-NC5H4Me C2B9H11-3-(NC5H5-3-Br) R2C2B9H9-9-L (R ¼ H, Me; L ¼ NMe3, pyridyl) RC2B9H11-9-NC5H5 (R ¼ Me, Ph, CH2NC5H5) C2B9H10-9-Me-11-NC5H5 C2B9H12-n-NEt3 (n ¼ 9, 10) C2B9H12-L [L ¼ terpyridine, terpyridine-O(CH2)3] Me[CH2-cyclo-NH(CH2)5]C2B9H10 Me2C2B9H10-B-CH2-cyclo-NR(CH2)5 (R ¼ H, Me) BrC2B9H10-10-BH(NC5H5)2 intermediate in deboronation of closo-1,2-BrC2B10H11 by pyridine
S, B, H, IR H S, H Raman X S, B S, X(H, pyridyl), H, B S, IR, UV S, B, H, MS S, B, H, IR S, MS
[175] [176] [176] [309] [136] [143] [133] [308] [181] [173] [310] [311] [311] [93]
S, X, H, B
Neutral single-cage derivatives, P-containing substituents on boron S, X C2B9H12-9-PPh3 S, X C2B9H11-9-PPh2H S, X (PPh3), H, B, P(PPh3) m(7,8)-S(CH2)3-C2B9H10-11-R (R ¼ PPh3, PMePh2) Neutral single-cage derivatives, O-containing substituents on boron X C2B9H12-10-O(CH2)2O(CH2)2SMe2 S, H C2B9H11-10-OC4H8 S, H Me2C2B9H9-10-OC4H8 S, B, H, IR Me2C2B9H10-9-R [R ¼ H, OC4H8] S, X, H, B, C, IR (PhCH2)2C2B9H9-OC4H8 S, B, H, IR C2B9H12-9-C(O)R (R ¼ Me, Ph) S, B, IR, MS C2B9H12-n-R (R ¼ OEt2, OC4H8; n ¼ 9,10) S, H, B, C C2B9H11-10-O(CH2CH2)2O S, H, B, C, MS Na14{B12H12[O(CH2)6-nido-7,8-RC2B9H8]12} (R ¼ H, Me) (closomer) Neutral single-cage derivatives, S-containing substituents on boron S, B(2d), H C2B9H11-9-SMe2 X S, H, B S, B(2d) S, B, H, MS S, X(Br,I), H, B(2d) C2B9H10-9-SMe2-11-X (X ¼ Cl, Br, I) S, H, B(2d) C2B9H10-9-SMe2-6-Br S, X, H, B(2d) C2B9H9-9-SMe2-6,11-Br2 S, X, H, B, C C2B9H11-10-SMe2
[192] [312] [189]
[313] [176] [176] [175] [174] [188] [187,199] [187] [215]
[112] [130] [133] [181] [177] [314] [314] [314] [127] Continued
200
CHAPTER 7 Eleven-vertex carboranes
Table 7-2 Selected Nido-7,8-C2B9H13, Nido-7,8-C2B9H12, and Nido-7,8-C2B9H112 Derivativesa—Cont’d Compound
Information
References
C2B9H11-10-L [L ¼ SEtPh, SEt3, S(CH2)4] C2B9H11-10-L [L ¼ SMe2, MSEt2,(CH2)4S, OR (CH2CH2)2S, OREt2,(CH2)4O, OR(CH2)2O] C2B9H12-9-CH2SR2 [SR2 ¼ SMe2, MSEt2,(CH2)4S, OR(CH2CH2)2S, (PhCH2)2S] C2B9H11-n-SMe2 (n ¼ 5, 7, 9, 10) C2B9H11-n-SMe2 (n ¼ 9, 10) C2B9H10-9-SMe2-n-Me n ¼ 1-6, 10 C2B9H10-9-SMe2-5,6-Br2 C2B9H11-9-S(O)Me2 C2B9H11-5-S(O)Me2 Me2C2B9H9-9-SMe2
S, X[S(CH2)4], H, B, C S, B, H
[127] [181]
S, B(2d), H
[181]
S, B, H, MS S, B, IR, MS S, X(n ¼ 3, 4), H, B(2d), MS S, H, B(2d), MS S, X Raman S, B, H, IR S, H, B S, H, B, C S, B(2d), H(2d) S, X X S, X(SMe2), H, B(2d) S, X, H, B, C
[49] [199] [315] [315] [316,317] [309] [175] [133] [127] [137] [137] [131] [185] [318]
Me2C2B9H9-10-L [L ¼ SEtPh, SMe2, SEt3, S(CH2)4] PhC2B9H10-n-SMe2 (n ¼ 9, 11) PhC2B9H10-11-SMe2 Ph2C2B9H9-9-SMe2 Ph2C2B9H9-10-L (L ¼ SMe2, SMeEt, SEt2) (Me2ClE)C2B9H10-9-SeMe2 E ¼ Si, Ge
Neutral single-cage derivatives, F-, Cl-, Br-, or I-containing substituents on boron S, B, IR C2B9H11-5,6-Br2 X C2B9H10-9-NC5H5-11-I Neutral single-cage derivatives, main group metal substituents on boron S, X, H, B C2B9H12-10-endo-AlEt(PEt3)2 X C2B9H12-m(9,10)-[AlMe2] S, X(H), H, B, C, IR, MS R(Me2NCH2)C2B9H10-9-AlMe2 (N!Al) R ¼ H, Me S, X(Me), H, B, C, IR, MS R[(PhCH2)2N(CH2)2]C2B9H10-9-AlMe2 (N!Al) R ¼ H, Me S, B, H, IR, MS C2B9H12-m(9,10)-AlR2 (R ¼ Me, Et) S, H, B, C, Al Me2C2B9H11-m(9,10)-AlEt2H2 S, B, H, IR, MS C2B9H12-m(9,10)-GaEt2 IR, Raman (C2B10H11)Sn(C2B9H11) Neutral Multi-Cage Derivatives (C2B9H11)2
(Me2C2B9H10)2 m(9,10)-(C5H5N)2Si(C2B9H11)2 Si[(CH2)2SiMe2CH2)3-RC2B9H10]4 (R ¼ Me, Ph) dendrimers
S, X S, S, S, S,
X, H, B(2d) B, H, IR, UV, MS, pKa B, IR, MS X B, H, C, Si, IR,S
[141] [319]
[320] [321] [194] [194] [193] [47] [193] [322]
[202] [203] [199] [227] [198] [323] Continued
7.2 11-Vertex open clusters
201
Table 7-2 Selected Nido-7,8-C2B9H13, Nido-7,8-C2B9H12, and Nido-7,8-C2B9H112 Derivativesa—Cont’d Compound
Information
References
5N-RC2B10H10 (R ¼ H, Me, Ph) RC2B9H11-N5 5)2 (R ¼ H, Me, Et) (RC2B9H11-1-N5 RC2B9H11-N2Me2-1,2-RC2B10H11 (R ¼ H, Me, Ph) RC2B9H11-N2H2-1,2-RC2B10H11 (R ¼ H, Me, Ph) 2,2-bipyridyl[OC(O)(CH2)3C2B9H11]2
S, H, IR, UV, Raman S, H, IR, UV, Raman S, H, IR (var. temp) S, H, IR (var. temp) S(CsF-promoted deboronation of 2,2bipyridine[1,2-OC(O)(CH2)3-C2B10H11]2) S, H, B, C, IR
[324] [324] [325] [325] [88]
S, X
[132]
2,200 -N2C10H6[C(O)O(CH2)3-nido-7,8-RC2B9H10]2 R ¼ H, Me bipyridyl Cationic 7,8-C2B9H12þ Derivatives [(C5H5N)S]2C2 B9 H10 þ CF3 SO3
Anionic 7,8-C2B9H12 and 7,8-C2B9H112 Single-Cage Derivatives No substituents on boron S, B, H(detailed) C2 B9 H12 S(high yield), B(2d), H(2d), thermal isomerization S (from B10H14 in alkaline solution with HCHO) S, B(2d), H S (CsF-promoted deboronation of 1,2RC2B10H11), H, C, B, IR S, H, B, IR S, B, IR B C IR Raman pKa X Kþ C2 B9 H12 X(variable temp. polymorphs), C Csþ C2 B9 H12 X C10 H6 ðNMe2 Þ2 þ C2 B9 H12 MS (electrospray ionization, Fourier transform Kþ RMeC2 B9 H10 R ¼ H, n-C4H9, n-C6H13, n-C8H17, n-C10H21 ion cyclotron resonance) Neutron diffraction S, H, B (solution þ solid state) S, X, H, B, N, XPS [N-methyl-2,200 -bipyridinium]þ C2 B9 H12 S, X, H RNC5 H5 þ C2 B9 H12 R ¼ n-butyl-, n-hexyl-, n-octyl N-alkylpyridinium salts; ionic liquids S, X, H EtMeN2 C3 MeH2 þ C2 B9 H12 1,2-Me2-3Et-imidazolium salts; ionic liquids S, X, H, B(2d) HðMe2 SOÞ2 þ C2 B9 H12
[326]
[327] [112] [113] [124] [83,88] [44,158] [141] [143] [140,328,329] [45] [330] [331,332] [333] [334] [138] [335] [138] [138] [336] [337]
[337] [123] Continued
202
CHAPTER 7 Eleven-vertex carboranes
Table 7-2 Selected Nido-7,8-C2B9H13, Nido-7,8-C2B9H12, and Nido-7,8-C2B9H112 Derivativesa—Cont’d Compound þ
(Me2N)3PNH2 C2 B9 H12 [(Me2N)3PNHBNP(NMe2)3]2O2þ[C2 B9 H12 ]2 C5 H10 NH2 þ C2 B9 H12 C5H10NH (Me2N)2C10 H6 þ C2 B9 H12 [(Me2N)2C10H6 ¼ proton sponge] C2 B9 H11 2 [Mþ]2 7,8-/7,9-/2,9-C2 B9 H11 2 (M ¼ Li, Na, K) Tl2 2þ PhMeC2 B9 H9 2 {CH[C6H4-p-O(CH2)2NMe2H]CEtPh}þ C2 B9 H11 Tamoxifen analogue for BNCT MnIIð1; 10-phenanÞ3 2þ [C2 B9 H12 ]2 paramagnetic salt; ferromagnetic exchange; evidence for 3D aromaticity MnIIð1; 10-phenanÞ3 2þ [C2 B9 H12 ]2 ferromagnetic interactions near T ¼ 20 K [Csþ5(C2B9H12)4Cl]n M[N2(CHMe2)2CNR2]þ 3 (C9H7)C2 B9 H10 M ¼ Zr, Hf NR2 ¼ NMe2, NEt2, N(CH2)4 (C9H7-Me2C)C2 B9 H11 M4(acac)4ðOHÞ11 þ C2 B9 H12 (M ¼ Zr, Hf) MðenÞ3 3þ C2 B9 H12 (X)2 mH2O (M ¼ Cr, Co; X ¼ Cl, Br; m ¼ 0-3) RuClðdppeÞ2 þ C2 B9 H12 Co(NH2Me)5Br2þðC2 B9 H12 Þ2 NH3Me]2[Co(NH2Me)3BrðC2 B9 H11 Þ2 isomers [Au9M4Cl4(PMePh2)8]þ C2 B9 H12 (M ¼ Au, Ag, Cu) [Au11[Au11(PMePh2)10]3þ[C2B9H12]3 (C4H8O)5LnCl2 þ C2 B9 H12 (Ln ¼ Y, Yb) Tlþ Me2 C2 B9 H10 MeC2 B9 H11
RC2 B9 H11 (R ¼ Me, Ph) Me2 C2 B9 H10 RMeC2 B9 H10 (R ¼ H, C7 H6 þ ) MePhC2 B9 H10 PMePh3 þ Et2 C2 B9 H10 (CH2)3C2 B9 H10 (Me2CH2CH2)RC2 B9 H10 (R ¼ H, CH2CH2OMe)
Information
References
S, X S, X
[76] [76]
S(degradation of 1,2-C2B10H12 with piperidine) S, H, B, C
[70]
S, S, S, S,
[158] [119] [338] [339]
B H, B, C IR X, H, B, C, MS
[90]
S, X, IR, Raman, ESR, MAG
[340]
S, MAG (variable T)
[341]
S, X, H, B, IR S, X, H, B, C
[342] [343]
S, H, B, C, IR S, MAG, IR, H, C S, UV, IR, C
[344] [345] [346]
S, X S, UV, Raman, IR S, UV, Raman, IR S, X, H, B, MS S, X, H, B, UV, MS S, X, H, B, C, IR S, B, H S(CsF-promoted deboronation of 1,2-RC2B10H11), H, C, B, IR UV (detailed; photometric detection) S, IR S (detailed) S, H, B, IR S, H, UV, IR UV (detailed; photometric detection) X S (CsF-promoted deboronation of 1,2-(CH2)3C2B10H10), H, C, B, IR S, X, H, B, IR
[347] [348] [348] [349] [350] [351] [352] [88] [353] [45] [46] [44] [354] [353] [355] [88] [356] Continued
7.2 11-Vertex open clusters
203
Table 7-2 Selected Nido-7,8-C2B9H13, Nido-7,8-C2B9H12, and Nido-7,8-C2B9H112 Derivativesa—Cont’d Compound
(HC5 5CH2)C2 B9 H11 (cyclo-CH5 5CH-CH2)C2 B9 H10 5CH)2C2 B9 H10 (CH25 5CHCH2]C2 B9 H11 [Me(CH2)3CH5 MePhC2 B9 H10
NMe4 þ PhC2 B9 H11 PhC2 B9 H11
Ph2 C2 B9 H10 HNEt3 þ /Me3NCH2Phþ Ph2 C2 B9 H10 (two salts) (PhCH2)2C2 B9 H10 Me(cyclo-HOC6H10)C2 B9 H10 [MeO(CH2)2]C2 B9 H11 K(18-crown-6)þ [HO(CH2)2]C2 B9 H11 [MeOC(O)]C2 B9 H11 [R(O)C]C2 B9 H11 (R ¼ H, MeO) Kþ (Me2NCH2)RC2 B9 H10 (R ¼ H, Me) C)PhC2 B9 H10 (PhC C)PhC2 B9 H9 2 (PhC RR0 C2 B9 H10 [R ¼ H, Me; R0 ¼CH2CN, C(O)NH2, CH2Ph, CN] [cyclo-7,11-CH2CH2N(CH2Ph)2]RC2 B9 H9 (R ¼ H, Me) [PhCH2NH2CH2CH2)2]RC2 B9 H10 (R ¼ H, Me) (m/p-FC6H4)C2 B9 H11 (p-FC6H4)C2 B9 H11 (p-FC6H4)2C2 B9 H10 (p-BrC6H4)C2 B9 H11 (XCH2)2C2 B9 H10 (X ¼ Cl, Br) (XCH2)C2 B9 H11
Information
References
S S, B, H, IR S, B, H, IR S, H, B, IR, MS S, H, B, IR OR S X S, H, B S (from 1-C(O)NH2-2-Ph-1,2-C2B10H10 þ EtONa/EtOH) S, H, B, IR, OR UV (conjugation between Ph and cage) UV (detailed; photometric detection) C S, B, IR, C, H X S, H, B, C, IR S, H, B S, H, B, C, IR S, X, H, B, C, IR S, X, H, B, C, IR S, B, H, C, IR S, H, B S, X (R ¼ H) S, H, B X S, IR S
[55] [357] [357] [86] [44,338] [44] [55] [128] [225] [50] [44] [358] [353] [328] [359] [360] [361] [362] [363] [364] [364] [114] [95] [365] [366] [367,368] [366] [369]
S, X(Me), H, B, C, IR, MS
[298]
S, X(Me), H, B, C, IR, MS F, UV F(Taft constants) S, H, B S, B, C, H,F S, H, B, IR S (from 1,2-(XCH2)2C2B10H10 with NH3, amines) S (from1,2-(XCH2)2C2B10H10 with 1,2-(Et2NH) C2B10H11)
[298] [370] [261] [97] [80] [44] [54] [54] Continued
204
CHAPTER 7 Eleven-vertex carboranes
Table 7-2 Selected Nido-7,8-C2B9H13, Nido-7,8-C2B9H12, and Nido-7,8-C2B9H112 Derivativesa—Cont’d Compound
(Me2NCH2)C2 B9 H11 (p-C6H4NH2)C2 B9 H11 Ph(C6H10-200 -NH2)C2 B9 H10 (200 -C5H4N)(RS)C2 B9 H10 R ¼ Et, CHMe2 (NC5H4CH2)C2 B9 H11 (H5C5N-CH2)2C2 B9 H10 OSO2 CF3 þ triflate [HNC4H3-CH2]C2 B9 H11 pyrrole (H2NCH2)C2 B9 H10 3 (H3N) 2C2 B9 H10 (Me2NCH2)RC2 B9 H10 (Me2NCH2)2(H)C2 B9 H10 (NC5H4CH2)C2 B9 H11 [Me2NH(CH2)3]C2B9H11 (o/m/p-O2NC6H4)C2 B9 H11 [C(O)NH2]C2 B9 H11 [C(O)NH2]CH2C2 B9 H11 (CH2OCH2Me)[cycloN3P3(O2C12H8)2]C2 B9 H10 [cyclo-N3P3(O2C12H8)2](mOCH2)2C2 B9 H10 [MeC(O)]C2 B9 H11 PhCH2 NMe3 þ (MeOCH2)C2 B9 H11 (CH2)nN-cyclo-{C(O)C6H4C(O)}]C2 B9 H11 (n ¼ 2, 3) aminoalkyl NHMe2 þ (CH2)nNHC(O)(C6H4-oCO2)C2 B9 H11 (n ¼ 2, 3) aminoalkyl (CH2-4-Me-5-thio-1,2,4-triazol)C2 B9 H11 cyclo-CH2NHC(¼S)NHCH2-C2 B9 H10 cyclo-(4-MeC6H3)(m-S)2C2 B9 H10 NMe4 þ (Ph2P)2C2 B9 H10 [(H3B)R2P]R0 C2 B9 H10 (R ¼ Et, Ph, CHMe2; R0 ¼ Me, Ph) (PR2)R0 C2 B9 H10 (R ¼ Et, CHMe2, Ph; R0 ¼ H, Me, Ph) (R2P)2C2 B9 H10 (R ¼ Et, Ph, CHMe2, OEt) PðCH2 C2 B9 H11 Þ3 3 NMe4 þ (Ph2OP)2C2 B9 H10 (O5 5PPh2)2C2B9H11 (R2P)2C2 B9 H10 (R ¼ Ph, CHMe2) Hþ (POPh2)2C2 B9 H10 (chelated proton)
Information
References
S, H, B, C, IR S, H, B S, H, B, C, IR S, X, H, B, C, IR S, IR, MS, B, H S, X, H, B S, H, E S, H, B, C, IR S, H, IR S, X, H, B, C, IR S, X, H, B S, H, B, C, IR, MS S, H, B, C, IR, MS S, H, B S[1,2-(C(O)NH2)C2B10H10 þ EtONa/EtOH] S [1-CH2C(O)NH2-2-Ph-1,2-C2B10H10 þ EtONa/EtOH] S, X, H, C, P, IR
[371] [144] [372] [373] [374] [125] [375] [376] [377] [299] [378] [379] [379] [55,97] [50] [50]
S, X, H, C, P, IR
[380]
S, X, H, B, C, IR S, X, H, B, IR S, X(n ¼ 2), H, B, C
[114] [381] [129]
S, X (n ¼ 2)
[129]
S, S, S, S, S,
[377] [377] [382] [69] [134]
H, IR H, IR X, H, B, C X, IR, H, B, P X (CHMe2, Me), H, B, C, P
[380]
S, H, B, C, P, IR (pKa)
[67]
S, H, B, P, IR S, H, B, C, P, IR, MS S, H, C, P, B, IR S, X, H, C, IR S, H, P, H, MS, luminescence; emission excitation S, X, H, B, C, P, IR
[69] [383] [384] [385] [386] [384] Continued
7.2 11-Vertex open clusters
205
Table 7-2 Selected Nido-7,8-C2B9H13, Nido-7,8-C2B9H12, and Nido-7,8-C2B9H112 Derivativesa—Cont’d Compound 0
RR C2 B9 H10 [R ¼ C(O)OH, CH2C(O)OH, C(O)OMe, CH2C(O)OMe] [HOC(O)(CH2)2]C2 B9 H11 (glucosyl-CH2)C2 B9 H11 (MeC5 5CH2)(CH2-O-CH2)C2 B9 H10 Me(HOCH2)C2 B9 H10 (MeOCH2)2C2 B9 H10 5CHMe, H) (ROCH2)2C2 B9 H10 (R ¼ CH5 Me[RO(CH2)3]C2 B9 H10 [R ¼ Et, (CH2)2OMe, nC4H9] agent for extraction of Eu, Sr, Cs R[R0 (O)C-C6H4]C2B9H9-9-125I R ¼ CH2-b-Dglucose, R0 ¼ OH, NH(CH2)2NEt2; R ¼ H, R0 ¼ N (CH2)2NEt2 radiolabeling; binding to melanoma cells [cyanocobalamin-C(O)NH(CH2)4NH-C(O)]C2 B9 H11 (vitamin B-12) Me(PhSCH2)C2 B9 H10 (HS)2C2 B9 H10 ðEtSÞ2 C9 B9 H10 ðRSÞMeC2 B9 H10 (R ¼ Me, Et, CHMe2, n-C4H9, CH2Ph) LL0 C2 B9 H10 [L ¼ H, PPh2; L0 ¼ SEt, S(CHMe2), S(n-C4H9), SCH2Ph] ðEtSÞ2 C2 B9 H10 (PhS)(HOCH2)C2 B9 H10 (PhS)[C9H13O2-C(O)OCH2]C2 B9 H10 (MeC4H2S)C2 B9 H10 (S2NC7H4)2C2 B9 H10 (1,2-RCB10H10C)-S-(nido-7,8-RC2B9H10) (R ¼ H, Me) HNMe3 þ ðHSÞ2 C2 B9 H10 NMe4 þ [S(CHMe2)](PPh2)C2 B9 H10 Naþ m(7,8)SCH2(CH2OCH2)3CH2S-C2 B9 H10 [HNMe3 þ ]2 [anti-7,70 ,8,80 (S2)2(C2B9H10)2]2 NMe4 þ {S[(CH2)2O]3CH2S-C2B9H10} ðMeSÞC2 B9 H11 (SC4H3)2C2 B9 H10 thiophene (RMeHCCH2S)C2 B9 H11 (R ¼ Et, 5CMe2) thioethers CH2CH2CH5 (MeC4H2S)C2 B9 H11 (p-C6H4NCS)C2 B9 H11
Information
References
S
[369]
S, S, S S, S, S, S,
H, B, C, IR, MS H, B, C, IR, MS H, B, C, IR B, IR IR B, H, C, IR
[379] [379] [55] [387] [381] [388] [389]
S, H, B, C, MS
[390]
S, B, UV, MS, biological activity
[391]
S S E S, H, B, IR
[311] [207] [392] [393]
S, H, B, IR
[68]
E S, S, S S, S,
[392] [394] [394] [395] [77] [206]
IR, H, C, B IR, H, C, B H, B, COND X(Me), H, B, C, IR, MS
S, X, H, B S, X, H, B, IR S, X, H, C, IR
[102] [68] [196]
S, X, B
[102]
S, X UV (detailed; photometric detection) S, H, C, MS S, C
[208] [353] [396] [397]
S, B, MS S, H, B
[395] [144] Continued
206
CHAPTER 7 Eleven-vertex carboranes
Table 7-2 Selected Nido-7,8-C2B9H13, Nido-7,8-C2B9H12, and Nido-7,8-C2B9H112 Derivativesa—Cont’d Compound
[(CH2)3-S-thiouracil]C2 B9 H11 cyclo-[SCH(OEt)S]-7,8-C2 B9 H10 cyclo-SC(O)CH2S-7,8-C2 B9 H10 cyclo-S[(CH2)2S]4-C2 B9 H10 cyclo-(S-C6H4-S)C2 B9 H10 cyclo-(S-X-S)C2 B9 H10 (X ¼ various organic chains) [O5 5C(mS2)]C2 B9 H10 MeðMeSÞC2 B9 H10 Me2 Ga-RC2 B9 H10 (R ¼ H, Ph) ClPhC2 B9 H10 I2 C2 B9 H10 D or hydrocarbon substituents on boron C2B9H11D C2B9H12-nDn (n ¼ 0, 2-8) C2B9H11D, C2 B9 H8 D4 (RS)R0 C2B9H9D (R, R0 ¼ Ph; Ph, Me) R2C2B9H9-3-Ph (R ¼ H, Me) C2B9H11-3-Et Ph2C2B9H9-3-Et C2B9H11-9-Me, C2B9H10-9,11-Me2 , C2B9H99,10,11-Me3 C2B9H11-n-R (n ¼ 5, 9) (R ¼ Me, Et, C6H13,Ph, CH5 5CH2) C2B9H11-9-CH2Ph 5CHMe) C2B9H11-9-R (R ¼ Me, Et, n-C4H9, CH25 C2B9H11-3-Ph Tl2C2B9H10-9-Ph Me2C2B9H9-9-CH2Ph CH C2B9H11-9-CH2C C2B9H11-m-Me C2B9H10-9,11-R2 (R ¼ Me, Et, CH2Ph) Csþ C2B9H9-9,10,11-Me3 (Ph3P)2Nþ C2 B9 H2 Me8 Si- or Ge-containing substituents on boron C2B9H11-9-R (R ¼ SiPhMe2, SiMe3) O12Si8[(CH2)3-nido-CB9H10CMe]88 siloxanes R ¼ Me, Ph C2 B9 H11 -GePh3 (m-H)
Information
References
S, H, IR S, IR, H S, IR, H, B, C S, C, H, IR S, IR S, H, C, IR S, IR, MS, COND S, H, IR S, B, MS S NQR (127I)
[377] [303] [398] [208] [208] [197] [399] [208] [400] [55] [401]
S, B, IR Raman S, B S, B S, B, H, C S, H, IR, B S, H, IR, B S, X, H, B, IR S, B, H, IR
[141] [330] [122] [143] [402] [94] [94] [153] [158]
S, B
[169]
S S, S, S, S, S, S, S, S, X S,
[167] [157] [143] [403] [166] [161] [165] [160] [169] [126] [92]
B B IR B B, H B, UV, IR B B X, H, B, C
S, H, C,P S, H, B, C, IR, Ms
[191] [404]
S, B, IR
[190] Continued
7.2 11-Vertex open clusters
207
Table 7-2 Selected Nido-7,8-C2B9H13, Nido-7,8-C2B9H12, and Nido-7,8-C2B9H112 Derivativesa—Cont’d Compound N- or P-containing substituents on boron PhC2 B9 H10 -3-NH3 Me2C2B9H8-9-NMe3-12-Br MeC2B9H10-9-NMe3 (2 isomers) C2B9H11-10-O(CH2)4-N3 C2B9H11-10-O(CH2)2-O-(CH2)2-N3 C2B9H11-7-(200 -pyridyl)H (PhC3N3-C5H4N)PhC2 B9 H10 triazinyl (PhC5H3N-C5H4N)PhC2 B9 H10 bipyridyl C2B9H11-9-R (R ¼ PHPh2, PPh3) C2B9H11-3-R [R ¼ PhC(O)NH, NMe2] C2B9H11-9-PMe2Ph C2B9H10-5(6)-Br-9-L (L ¼ 4-picoline, 3-picoline, 4-benzylpyridine, pyridine, 3-bromopyridine) (single-wall carbon nanotube)[N(CH2)4RC2 B9 H10 Naþ ]n (R ¼ Me, Ph) R2C2B9H8-9-L (R ¼ H, Me; L ¼ NMe3, pyridyl) O- or S-containing substituents on boron C2B9H11-9-CO C2B9H11-9-R(R ¼ SCN, SMe) C2B9H11-9-SMe C2B9H10-9-SMe2 Ph2C2B9H9-9-SMe2 R2C2B9H9-9-SCN (R ¼ H, Me) R2C2B9H8-9-SMe2 (R ¼ H, Me) C2B9H11-10-O(CH2)4-O-C6H4-C(O)O C2B9H11-10-O(CH2)2-O-(CH2)2-O-C6H4-C(O)O C2B9H11-10-O(CH2)2O(CH2)2N3 nucleoside conjugate F-, Cl-, Br-, or I-containing substituents on boron C2B9H12-nFn (n ¼ 1-4) stereochemistry of degradation of 1,2-C2B10 derivatives Ph2C2B9H9-3-F C2B9H10-9,11-X2 X ¼ Cl, Br, I Me2C2B9H8-9,11-X2 X ¼ Cl, Br, I C2B9H11-5-Br C2B9H10-5,6-Br2 C2B9H10-9,11-Br2 C2B9H11X (X ¼ Br, I) C2B9H11-9-X K(þ) X ¼ I, SCN
Information
References
S, S, S, S, S, S, S, S, S, S, S, S,
[405] [58] [58] [187] [187] [406] [98] [98] [191] [407] [408] [150]
H, B, IR H, MS H, MS H, B, C, IR H, B, C, IR X, H, B, C, IR, MS X, H, B, C, MS X, H, B, C, MS H, C, P IR(NMe2) X, H, B, P H, B, IR, UV, XPS
S, H, B, C, IR, boron distribution in tissue
[409]
S
[133]
S, H, C, B, IR UV (detailed; photometric detection) S, B, H, circular dichroism X S, B(2d), H(2d) S, H, B(2d), IR S S, H, B, C, IR S, H, B, C, IR S(dipolar addition [chemical ligation), H, B, IR, MS, UV
[410] [353] [179] [411] [137] [180] [133] [187] [187] [412]
S, B, F
[151]
S, H, B, IR S, H, B, MS S, H, B(2d), IR UV (detailed; photometric detection) S, B NQR (79Br,81Br) S (electrochemical), B, IR S(diaphragm electrolysis)
[153] [147] [413] [353] [143] [401] [149] [414] Continued
208
CHAPTER 7 Eleven-vertex carboranes
Table 7-2 Selected Nido-7,8-C2B9H13, Nido-7,8-C2B9H12, and Nido-7,8-C2B9H112 Derivativesa—Cont’d Compound
C2B9H10-9,11-X2 K(þ) X ¼ Br, I HNMe3 þ C2B9H11-9-I RC2B9H10-3-X (R ¼ H, Me, Ph; X ¼ Br, I) C2B9H11-9-I C2B9H11-5/6-I C2B9H11-n-I (n ¼ 5,9) C2B9H10-9,11-I2 Csþ (p-C6H4NCS)C2B9H10-9-I MeC2B9H10I Me2C2B9H9I MeC2B9H9-9,11-I2 PhC2B9H10I Ph2C2B9H9-5-I Ph2C2B9H9-9-I C2B9H10-2,4-I2 HNM3 þ C2B9H10-9,11-I2 Ph2C2B9H8-9,11-I2 H2 C2 B9 I9 2 H2C2B9H-1,2,4,5,6,9,10,11-I8 2 Main group metal-containing substituents on boron trans-Ir(CO)(PPh3)2(MeCN)þ C2B9H11-10SnPh3 (mH)
Information
References
S(diaphragm electrolysis) S, X S(deboronation of 1,2-C2B10H11-3-X), X (H, I), H, B, C S, B S, H, C, B, IR NQR (127I) NQR (127I) S, X, H, B S, H, B, IR S, H, B, IR S, H, B(2d), IR S, B S, X, H, B, IR S, X, H, B, IR S, X, H, B, C, MS S, X S, X, H, B, IR S, H, B, MS S, X, H, B, MS
[414] [135] [139]
S, X, H, B, P, Sn, IR
[190]
Anionic Multi-Cage Derivatives, Nonmetal substituents ðC2 B9 H11 Þ2 2 S, B, H, IR S, B, IR, UV ðC2 B9 H11 Þ2 S, B, H, IR C2B9H11-(100 ,200 -C2B10H11) 5CHCH2-(C2B9H11)2 S, H, B, IR, MS (C2B9H11)-CH2CH5 S, H, B, C, MS, U, fluorescence C6H3-1,3,5-[(p-C6H4)nCH2CB9H10CMe]333Naþ n ¼ 1, 2 S, H, B, C, MS, U, fluorescence C6H3-1,3,5-[(p-C6H4)nC6H3-3,5-(CH2-CB9H10CMe)2]33 3Naþ n ¼ 0, 1 S, B, MS [B12{Me[(CH2)6C(O)O]C2B9H10}12]14 dodecaboranecarborane closomer S, H, B, C, IR, MS (C2B9H10C-CH2OCH2)3C-C(O)OH3 pentaerythritol dendron building-block for BNCT S, H, MS, IR, UV, interaction with DNA, meso-[MeC2B9H10-7-CH2]4porphyrin resonance light scattering 4Naþ tetrabenzoporphyrin[(C6H4)C2B9H12]44 S, H, UV, MS, toxicity, cell uptake S, H, UV, cell accumulation, 4Kþ (porphyrin)photosensitization 5,10,15,20-ðP-C6 H4 -S-C2 B9 H10 Þ4 4 photodynamic therapy (PDT)
[143] [81] [401] [401] [144] [148] [148] [413] [145] [153] [146] [152] [135] [146] [415] [415]
[199,205] [199] [205] [86] [416] [416] [417] [418] [419] [420] [421]
Continued
7.2 11-Vertex open clusters
209
Table 7-2 Selected Nido-7,8-C2B9H13, Nido-7,8-C2B9H12, and Nido-7,8-C2B9H112 Derivativesa—Cont’d Compound
M[porphyrin-CH(OH)](MeC2B9H11) (M ¼ Co, Cu, 2H) Kþ[Me(CH2)15-O-CH2]2CH-O-CH2-C2B9H11) precursor to liposomes for BNCT CoII(C2B9H11)x porphyrinates MeC2B9H10-(CH2)3-O-P(O)(OR)-O(CH2)3C2B9H10-(CH2)2C(O)NH-C6H4-Ph3 porphyrin carboranyl phosphate diester porphyrin(C6H5)(C6H4-p-NCH2-nido-C2 B9 H10 )3 4Kþ porphyrin[CH(CN)2](C6H4-p-nido-7,8C2B9H11)44 chlorin for BNCT and PDT (photodynamic therapy) [RC2B9H9(C6H4)2]2SO2 2 (R ¼ H, Me, Ph; isomers) sulfone derivatives BNCT [RC2B9H9(CH2)2C(O)NH]2C14 H6 O2 2 (R ¼ H, Me, Ph; isomers) anthraquinone derivatives m-(CH2)n-ðRC2 B9 H11 Þ2 (n ¼ 3-5) (C2B9H10)2C6H4[C(O)OMe] O5 5C[NMe-p-C6H4-C2B9H10]22 urea m-TosN(CH2CH2)2-ðC2 B9 H11 Þ2 1,3/1,4-C6H4(C2B9H9-9-CH2)2 SðCH2 C2 B9 H11 Þ2 S2ðRC2 B9 H10 Þ2 2 (R ¼ H, Me, Ph) disulfide-bridged ðS2 C2 B9 H10 Þ2 2 (C2B9H10)2[m-7,8-S(CH2)nS-]22 (n ¼ 1, 2) [S(Me)C2B9H10]2(CH2)n (n ¼ 2, 3) 2,6-[(C(O)OMe)C2B9H10-8-S-CH2-]2C5H3N SðCH2 C2 B9 H10 Þ2 {7,8-m-[S(CH2CH2O)3CH2CH2-S](7,8-C2B9H11)2}2 [MeO(CH2)2C5H4]6Ti6 ðm3 OÞ8 2þ (1,2C2B10H10)(m-S2)2(nido-7,8-C2B9H10)22 C6H4[p-CH2-O-C6H3(CH2-RC2B9H10)2]24
Information
References
S, H, cytotoxicity
[422]
S, H, B, in vitro toxicity
[423]
S, EPR, reaction with O2 S, H, B, UV (fluorescence), MS BNCT
[424] [425]
S, H, UV, MS S, UV, fluorescence emission spectrum
[426] [427]
S, H, B, C, IR
[428]
S, H, B, C, IR
[428]
S, S, S, S, S, S, S, S, S, S, S, S S, S,
[210] [429] [430] [210] [166] [214] [206] [207] [101] [208] [431] [214] [432] [433]
H, B, C, IR X, H, B, C, IR
S, H, B, C, UV, fluorescence emission
Transition metal s- and m-complexes of 7,8-C2B9H12 Ti, Zr S, (PhCH2)2C2B9H9-m-M(NEt2)2(NEt2H) (M ¼ Ti, Zr) S, C2B9H12-9-(m-H)ZrMe(C5Me4Et)2 S, (HNEt2)(Et2NS2Ti-OCH2)C2B9H10 Cr, Mo, W C2 B9 H11 -3-NC-MðCOÞ5 (M ¼ Cr, Mo, W) m(7,8)-(C3H5)(CO)2Mo(SCH2CH2S)C2B9H10 (MeC6H4)CMo(CO)(PPh2C2H4PPh2)2(O)}þMe2 C2 B9 H10
H, IR, C, B, MS H, IR, C, B, MS X, H H, IR, C, B, MS B H, B, C, IR, MS X(Me), H, B, C, IR, MS X, B B, H, IR, MS H, IR H, B, C, IR, MS
[434]
and 7,8-C2B9H112 X(Ti), H, B, C, IR X X, H, B, C, IR
S, IR, C(W) S, X, H, B S, X, H, C
[174] [435] [436]
[306] [437] [438] Continued
210
CHAPTER 7 Eleven-vertex carboranes
Table 7-2 Selected Nido-7,8-C2B9H13, Nido-7,8-C2B9H12, and Nido-7,8-C2B9H112 Derivativesa—Cont’d Compound
Me2 C2 B9 H9 -3-NC-MðCOÞ5 (M ¼ Mo, W) Mo(CO) ðPh2 PC2 H4 PPh2 Þ2 þ (MeC6H4)C Me2 C2 B9 H10 Me2C2B9H10-(m-H)2W(CO)2(C5Me5) Mn MnIIð1; 10-phenanÞ3 2þ (C2 B9 H12 )2 paramagnetic salt; ferromagnetic exchange; evidence for 3D aromaticity MnIIð1; 10-phenanÞ3 2þ (C2 B9 H12 )2 ferromagnetic interactions near T ¼ 20 K Fe, Ru, Os C2 B9 H11 -3-NC-FeðCOÞ5 C2B9H12-9-Fe(CO)2Cp C2B9H12-9-Fe(CO)2(MeCN)Cp C2B9H12-(C5H4)FeCp C2B9H12-9-FeCp(CO)2 Me2C2B9H10-9-Fe(CO)2Cp [(MePh2P-Z5-C5H4)Fe(Z5-C5 H4 PPh2 þ C2B9H11-9-SMe syn-/anti-Fe[Me(C5H4)C2B9H10] 2RuClðdppeÞ2 þ C2 B9 H12 C2B9H12-5,6,10-(m-H)3RuBr(PPh3)2 m(7,8)-[(PPh3)2ClRu(m-SPh)2]C2B9H10 (2 isomers) m(7,8)-L2ClRu[S(CH2)nS]C2B9H10 (n ¼ 1-4; L ¼ PPh3, PMe2Ph, Me2SO, 0.5 phenanthroline) 2H2 2B] m(7,8)-{(PPh3)2ClRu[S(CH2)2S]}C2B9H10 [Ru2 (PPh2)MeC2B9H10-m3-RuCl(PPh3)L (L ¼ PPh3, EtOH) m(7,8)-[(MeC6H4CHMe2)RuCl(SPh)2]-C2B9H10 Ru[(PPh2)MeC2B9H9(m-H)]2 (2 isomers) R2C2B9H7-exo-5,6,10-(m-H)3RuCl (Ph2PCHMeCH2CHMe) (R ¼ H, Me) 3,1,2-CpFe(C2B9H9)-4-SMe2-8-Hg-1000 nido-700 ,800 -C2B9H8-500 ,600 ,1000 (m-H)3RuCl(PPh3)2 NMe4 þ (C2B9H10-n-S(CH2)nS)2RuCl (n ¼ 2, 3) RuXLL0 -(PPh2)RC2B9H10 (X ¼ Cl, H; R ¼ H, Me, Ph; L ¼ PPh3; L0 ¼ PPh3, CO, tetrahydrothiophene, C2H5OH) (terpyr)Ru[(terpyr)(C2B9H10) Ru[terpyr-O(CH2)3-C2B9H10]þ 1,2C2B10H11-1-(CH3)2-O(terpyr)
Information
References
S, IR S, H, C
[306] [438]
S, X, H, B, C, IR
[439]
S, X, IR, Raman, ESR, MAG
[340]
S, MAG (variable T)
[341]
S, S, S, S, S, S, S,
IR B, H, IR, MS B, H, IR, MS H, B X, H, B, IR, Mo¨ssbauer B, H, IR, MS B, H, C,P, IR
[306] [440] [440] [441] [442] [440] [31]
S, S, S, S, S,
H, B X X, H, B, P, IR X, H, B, P, IR X(PPh3; n ¼ 1, 4), H, B, P, IR
[441] [347] [443] [444] [445]
S, S, S, S, S,
X, IR, H, B X, H, B, P, IR X, H, B, IR X, IR, B, P, H X(Me), H, B, P, IR
[446] [447] [448] [449] [450]
S, X (trans isomer), B, P
[451]
S, X(n ¼ 2), H, B, IR S, H, B, P, IR
[445] [447]
S, H, B, IR, C, MS S, H, B, C, MS
[310] [310] Continued
7.2 11-Vertex open clusters
211
Table 7-2 Selected Nido-7,8-C2B9H13, Nido-7,8-C2B9H12, and Nido-7,8-C2B9H112 Derivativesa—Cont’d Compound
(PhS)RC2B9H10-m-S, H, H)-RuClðPPh3 Þ2 (R ¼ Me, Ph) Ru[(terpyr)C2B9H10]2 [RuH(PPh3)2]-7,8(Ph2P)2C2B9H10 [Ru-P, Ru-H-B(11), Ru-H-B(2)] [RuCl(PPh3)2]-7,8(Ph2P)2C2B9H10 [Ru-P, Ru-H-B(11), Ru-H-B(2)] exo, nido-Cl((C6H5)3P)2Ru-(m-H)3-7,8-nidoC2B9H8-9-Hg-(C2B10H11) Cp*MRu(C8H12)(m-S)2(C2B9H10) M ¼ Co, Rh, Ir Cp*MRu(C8H12)(m-S)2(OMe)(C2B9H9) M ¼ Co, Rh, Ir Me2C2B9H10-(m-H)3OsCl(PPh3)2 RR0 C2B9H10-5,6,10-Os(PPh3)2Cl (3 B2 2H2 2Os) [R, R0 ¼ H, Me, PhCH2, Me; RR0 ¼ 1,2(CH2)2C6H4] Co, Rh, Ir CpCo(C5H4)-C2B9H12 Co(NH2Me)5Br2þ[C2B9H12]2 NH3Me]2[Co(NH2Me)3BrðC2 B9 H11 Þ2 isomers (C2B9H12)n[Co(en)3]X2mH2O (n ¼ 1, 2; X ¼ Cl, Br; m ¼ 0-3) Cp*CoRu(C8H12)(m-S)2(C2B9H10) Cp*CoRu(C8H12)(m-S)2(OMe)(C2B9H9) m(7,8)-[(C8H12)Rh(PPh2)2]C2B9H10 m(7,8)-{(PPh3)2Rh[S(CH2)2S]}C2B9H10 C2B9H11-(n-C9H7)Rh(C9H6) (n ¼ 9, 10) [(C8H12)Rh(PPh2)]PhC2B9H10 enantiomer [Fe(C5H4)2(m-PPh2)2Rh(m-PPh2)]PhC2B9H10 enantiomers (PhS)PhC2B9H10-Rh(C8H12) Me2C2B9H7-5,10-(Ph3P)2Rh-(m-H)2-10endo-AuPPh3 R2C2B9H9-(m-H)3-5,6,10-RhCl(PPh3)2 (R ¼ H, Me) Cp*2Rh2ðm-ClÞ3 þ RR0 C2 B9 H10 R, R0 ¼ H, Me, PhCH2 (R2P)R0 C2B9H10-Rh(PPh3)2 (R ¼ Ph, Et, CHMe2; R0 ¼ H, Me, Ph) (SPh)MeC2B9H10-Rh(PPh3)2 2B2 2H] (Ph2P)MeC2B9H10-Rh(C8H12) [2 Rh2 m(7,8)-[(Me2SCH)(PPh2)]C2B9H10-Rh(C8H12)
Information
References
S, H, B,P, IR
[444]
S, H, B, C, IR, MS H, B, P
[310] [452]
H, B, P
[452]
S, X, H, B, P
[453]
S, S, S, S,
[454] [454] [455] [456]
X(Co, Rh, Ir), H, B, IR X(Co), H, B, IR X, H, P X [Me, (CH2)2C6H4], H, B(2d), P
X S, UV, Raman, IR S, UV, Raman, IR S, UV, IR, C
[457] [348] [348] [346]
S, S, S, S, S, S, S,
[454] [454] [458] [459] [460] [461] [461]
X, H, B, IR X, H, B, IR X, H, B, P, IR X, IR X, H, B(2d), IR X, H, B, C, P, IR, OP H, B, C, P, IR
S, X S, X, H, B, P
[402] [462,463]
S, X (R ¼ H), H, B, P(H) S, X(PhCH2, PhCH2), H, B, C, IR
[464] [465]
S, X (R ¼ Ph, R0 ¼ H), H, B, P (var. T), IR
[466]
S, X, H, B, P, IR S, X, H, P, B, IR S, X, H, B, P, IR
[467] [468] [469] Continued
212
CHAPTER 7 Eleven-vertex carboranes
Table 7-2 Selected Nido-7,8-C2B9H13, Nido-7,8-C2B9H12, and Nido-7,8-C2B9H112 Derivativesa—Cont’d Compound
Information
References
m(7,8)-(CH2)3C2B9H10-10-[(CH2)2C(O)O(CH2)3Me]-Rh(PPh3)2 {m-S-(S-(CH2)2-S)C2B9H10}RhCl{m-(S(CH2)2-S)-C2B9H9} [Cp*M-7,8-(SCH2C(O)OMe)(S)C2B9H10]2 (M ¼ Rh, Ir) [cyclo-SCp*IrRh(C8H12)S]C2B9H9-m(Ir,B)-OMe Cp*RhRu(C8H12)(m-S)2(C2B9H10) Cp*RhRu(C8H12)(m-S)2(OMe)(C2B9H9) NMe4 þ {Cp*ClRh[m(7,8)-(SCH2CH2(OCH2CH2)3)S] C2B9H10} NMe4 þ [m(7,8)-(SCH2CH2S)C2B9H11MClCp*] (M ¼ Rh, Ir) RhL4 þ ðPhSÞMeC2 B9 H10 (L ¼ PPh3, PMePh2, PEt3) [(MeOCH2)C2B9H10]-n-(C9H6)Rh(C9H7) (n ¼ 9, 10) (Ph3P)2Rh-u-SC(O)CH2S-7,8-C2 B9 H10 (PPh3)2Rh-RR0 C2B9H9 [R ¼ PPh3, R0 ¼ Me; R ¼ R0 ¼ Me; RR0 ¼ cyclo-(CH2)3]
S, X, P
[470]
S, X, B, IR
[471]
S, S, S, S, S,
[472] [473] [454] [454] [474]
6,10-(PPh3)[P(cyclohexyl)3]Rh(m-CH2C6H4CH2)C2B9H10 4,9-(PPh3)2Rh-MePhC2B9H10 (Ph3P)2Rh(S, BH)-[m-S(CH2)3]C2 B9 H10 (Ph3P)2Rh-(RS)R0 C2 B9 H10 (R ¼ Ph, Et; R0 ¼ Me, Ph) (C8H12)Rh-(PPh2)C2 B9 H11 (C8H12)Rh(RS)R0 C2B9H11 (R, R0 ¼ Ph, Et, Me) PhC2B9H10-(n-C9H6)Rh(C9H7) (n ¼ 9, 10) LL0 Rh-7,8-ðPh2 PÞ2 C2 B9 H10 (L ¼ CO,PPh3, PMe2Ph,PMePh2,P(OEt)3, 1,2-(Ph2P)2C2B10H10, (Ph2P)2C2B9H10,(Ph2P)2C2H4, pyridine, bipyridine, COphen) (Ph3P)2Rh(m-C4H9S)(m-Ph2P)-C2 B9 H10 [L(m-PPh2)2Rh(m-PPh2)]PhC2B9H10 [L ¼ cyclo-CH-OCMe2-O-CH, binap, Fe(C5H4)2, (CH2)4] enantiomers Cp*2Cl2Rh2[cyclo-(4-MeC6H3)(m-S)2 C2 B9 H10 þ ) [cyclo-(4-MeC6H3)(mS)2C2 B9 H10 m(7,8)-[(PPh3)2Ir(m-SMe)2]C2B9H10 C2B9H10-3,9-(m-H)2IrH2[P(p-MeC6H4)3]2 Me2C2B9H10-(m-H)2AuIrH(PPh3)3 (Ph3P)2Ir[m-7,8-S(CH2)nS-(C2B9H10)] (n ¼ 2-4) [cyclo-EIr2Cp*(C8H12)E]C2B9H9-m(Ir,B)OMe (E ¼ S, Se) [cyclo-SRh(C8H12)CoCp*S]C2B9H10
X, H, B(Rh), IR X, H, B, IR H, B, IR H, B, IR X, H, B, IR
S, X, H, B, IR
S, H, B, IR(2d) S, IR, H, B,P S, H(variable temp), P(variable temp), C [(CH2)3] X S, H(variable temp), P X X S, H, B, P S, H, B, IR, P(variable temp) S, H, B, P, IR S, B(variable temp), H(variable temp), C S, H, B, IR(2d) S, H, B, P, IR
[474] [466] [460] [398] [475] [476] [475] [476] [476] [189] [466] [468] [402] [460] [458]
S, H, B, P, IR S, H, B, C, P, IR
[469]
S, X, H, B, C, R(catalytic hydrogenation and cyclopropanation of alkenes)
[382]
S, S, S, S, S,
B, IR
[477] [478] [462] [477] [473]
S, X, H, B, IR
[479]
X, H, X, H, X, H, H, B X, H,
B IR B, P
Continued
7.2 11-Vertex open clusters
213
Table 7-2 Selected Nido-7,8-C2B9H13, Nido-7,8-C2B9H12, and Nido-7,8-C2B9H112 Derivativesa—Cont’d Compound
Information
References
Cp*IrRu(C8H12)(m-S)2(C2B9H10) [cyclo-(Ph2PMCp*S)]C2B9H9-3-OMe M ¼ Ir, Rh
S, H, B, IR S, X(Ir), H, B, P, IR
[454] [480]
S, S, S, S, S, S,
X, H, C, IR H, IR IR IR X, H, C, IR X, H, C, IR
[108] [481] [459] [481] [385] [111]
S, S, S, S,
X, B X, B, IR X, H, B, IR, P X (R ¼ Me), H, B, P, IR
[482] [481] [398] [483]
Ni, Pd, Pt (m-X)2Ni2[(PPh2)2C2B9H10]2 (X ¼ Cl, Br) (S-CH2CH2-S)C2 B9 H10 NiCl2 [S-(CH2)2-S]C2B9H10NiðClÞ2 [S-(CH2CH2O)3]CH2CH2-S-C2B9H10)2Ni2(Cl)2 (THF)Ni[(m-O5 5PPh2)2C2B9H10]2 [cyclo-Ph2PM(Cl)(PPh3)PPh2]C2B9H10 (M ¼ Ni, Pd, Pt) 2P) (SMe)MeC2B9H10-11-PPh2Pd(PPh3)Cl (B2 m(7,8)-[S(CH2)2S]C2B9H10-Pd(PPh3)(Cl) m(7,8)-SS[CH2C(O)OEt]C2B9H10-Pd(PPh3)2 (PPh2)RC2B9H10-11-PPh2Pd(Cl)PPh3 (R ¼ Me, H, Ph) [(C6H4)PdCMeHNMe2(PPh2)]PhC2B9H10 diastereomers {closo-3,1,2-(PMe2Ph)2Pd[(C4H2RS)C2B9H9-8PMe2Ph]}þ 7,8-(C4H2)(RS)C2 B9 H11 (R ¼ H, Me) [S(CH2CH2O)3CH2CH2S-C2B9H10]2Pd Pd2(m-Cl)2-{(CHMe2)2P}2C2B9H10]2 (S-CH2CH2-S)C2B9H10)2Pd Pd[7,8-(Ph2P)2C2B9H10]2 (Ph2P)PdCl(PPh2)]C2B9H10 Pd[(Ph2P)2C2B9H10]2 (S-(CH2CH2S)4-C2B9H10)Pd(Ph3P)(Cl) Cl2Pd2{2,6-[(C(O)OMe)C2B9H10-8-SCH2-]2C5H3N} Cl(L)Pd(m-PR2)2-7,8-C2B9H10 (L ¼ PPh3, PMePh2) Cu, Ag, Au C2B9H10-9,10-(m-H)2Cu(PPh3)2 m(7,8)-{(PPh3)[C(O)Me2]Cu(PPh2)2}C2B9H10 m(7,8)-[(SCH2CH2S)Cu(PPh3)2]C2B9H10 m(7,8)-(SCH2CH2-O-CH2CH2-OCH2CH2S)M(PPh3)C2B9H10 (M ¼ Cu, Ag) L[(Ph2P)M(PPh3)]C2B9H10 (M ¼ Cu, Au; L ¼ SEt, SCH2Ph) Cu[(m-O5 5PPh2)2C2B9H10]2 [Au9M4Cl4(PMePh2)8]þ C2 B9 H12 (M ¼ Au, Ag, Cu) (S-(CH2)2-S)C2 B9 H10 CuCl2 (Ph3P)Cu-7,8-(Ph2P)2C2B9H10
S, X, P, H, B, C, IR, OP
[461]
S, X (R ¼ H), H, P, B, MS
[395]
S, S, S, S, S, S, S, S,
[481] [110] [481] [110] [484] [484] [481] [431]
X, B, IR X, H, B, P, IR IR H, B, IR,P X, H, C, IR X, H, C, IR IR, H IR, H, B, MS
S, H, B, IR,P
[110]
S, S, S, S,
[485] [104] [486] [487]
X, B, P X X, C, IR X (M ¼ Cu), H, B, IR
S, X (M ¼ Cu, Au; L ¼ SCH2Ph), H, B, P, IR
[488]
S, S, S, S,
[385] [349] [459] [104]
X, H, C, IR X, H, B, MS IR B, H, IR
Continued
214
CHAPTER 7 Eleven-vertex carboranes
Table 7-2 Selected Nido-7,8-C2B9H13, Nido-7,8-C2B9H12, and Nido-7,8-C2B9H112 Derivativesa—Cont’d Compound
Information
References
m(7,8)-[(SCH2S)Ag(PPh3)]C2B9H10 NMe4 þ Ag(m-SCH2CH2S-C2B9H10) 2 m(7,8)-[LAg(PPh2)2]C2B9H10 (L ¼ PPh3, PMePh2, phenanthroline, dppsm, dppe) m(7,8)-[(PPh3)Ag(SCH2CH2-O-CH2CH2S)]C2B9H10 {Ag[m(SCH2CH2-O-CH2CH2-OCH2CH2-O-CH2CH2S)-C2B9H10]}n polymer (C2B9H10)2-9,90 -[Ag(SbPh3)2]2 (C2B9H10)2-9,90 -[Ag(AsPh3)2]2 (bipyridine)Agþm(7,8)-SCH2CH2S]C2 B9 H10 2O2 2CH2CH22 2 Agþ [m(7,8)-SCH2CH22 O2 2CH2CH2S]C2 B9 H10 (Ph3P)Agþ Me2 C2 B9 H10 (bipyridine)Agþ Me2 C2 B9 H10 PhCH2 NMe3 þ C2B9H11-10-Au(PPh3) (R2P)2C2B9H10-m-AuL [R ¼ Ph, CHMe2; L ¼ PPh3, PPh2Me, P(C6H4-4-Me)3] (Ph3PAuPPh2)PhC2B9H10 [(BrAu)Ph2P]2C2B9H10 m(7,8)-(Ph3P)Au(PPh2)2C2B9H10 LAu[P(CHMe2)2]2C2B9H10 (L ¼ Ph3P, Cl2) C2B9H10-9-SMe2-m(10,11)-Au(PPh3) Me2C2B9H9-9-PPh3-(m-H)-10-AuPPh3 Me2C2B9H10-(m-H)3Au3(PPh3)3 (PPh2)2(C3H6)Au2[(PPh2)2C2B9H10]2 (Ph3As)2Au4[(m-PPh2)2(C2B9H10)]2 (C6H4OMe)2Au4[(m-PPh2)2(C2B9H10)]2 (R ¼ Ph, p-C6H4Me, p-C6H4OMe) [Au11 [Au11(PMePh2)10]3þ[C2B9H12]3 [1,2-(m-Ph2P)2C2B10H10]Au[(m-Ph2P)2C2B9H10] [1,2-(m-Ph2P)2C2B10H10]Au[(m-Ph2P)2C2B9H10]0.5 CH2Cl20.5H2O (R3)2Au4[(m-PPh2)2(C2B9H10)]2 (R ¼ Ph, p-C6H4Me, p-C6H4OMe) Au4[(Ph2P)2C2B9H10]2 Ph3PAu-7,8-Me2 C2 B9 H9 7,8-[(CHMe2)2PAu(PPh3)P(CHMe2)2]C2 B9 H9 7,8-[Ph2PAu(PPh3)PPh2]C2 B9 H9 (Me2C2B9H9)AuW(CO)2Cp(m-CC6H3Me2)
S, S, S, P, S,
[489] [490] [107]
X, H, B, IR X, H, B, IR X (L ¼ PPh3, phenanthroline, dppe), H, B, IR, C, MS X, H, B, IR
[487]
S, X, H, B, IR
[489]
S, S, S, S,
[491] [491] [489] [489]
X, H, B, IR B, H, IR IR, H, B IR, H, B
S, IR, H, B S, IR, H, B S, X, H, B, IR, P S, X (CHMe2, PPh3), H, P, H, MS, luminescence; emission excitation S, X, H, P, B S, X, P S, X, H, B, P, IR, C S, X(Cl2), H, B, P, IR S, X, H, B, IR S, X, H, B, C, P S, X, H, B, C, P S, X, H, B, P, IR, C S, X, H, B, P, MS S, X(p-C6H4OMe)
[489] [489] [492] [386] [493] [494] [106] [109] [495] [496] [496] [106] [497] [498]
S, X, H, B, UV, MS S, H, B, P, MS, IR
[350] [497]
X
[499]
S, H, P, MS, luminescence; emission, excitation S, X, H, C, IR S, H, P, B, C S, variable-temp. luminescence
[498]
S, variable-temp. luminescence S, X, H, B, C, IR
[501] [502]
[500] [463] [501]
Continued
7.2 11-Vertex open clusters
215
Table 7-2 Selected Nido-7,8-C2B9H13, Nido-7,8-C2B9H12, and Nido-7,8-C2B9H112 Derivativesa—Cont’d Compound
Information
References
(Me2C2B9H10)AuW(CO)2Cp(m-CC6H3Me2) (Me2C2B9H9)2Au2(CO)2MCp(m-CC6H4Me) (M ¼ W, Mo) (Me2C2B9H10)2Au2(CO)2MCp(m-CC6H4Me) (M ¼ Mo, W)
S, H, B, C, IR S, H, B, C, IR
[502] [502]
S, X(W), H, B, C, IR
[502]
S, X, H, C, IR S, X, distribution in EMT-6 tumor-bearing mice S, X, H, B, IR X S, H, B, IR S, X, H, B, IR S, H, B, IR S, X, H, B, IR S, H, B, P, IR S, B (R ¼ H) S, H, B, IR S, H, B, IR S, H, B
[385] [333] [195] [503] [195] [504] [195] [505] [109] [506] [505] [505] [507]
S, H, B, F S, H, B S, X, H, B, P
[507] [507] [453]
S, H, B S, B, P
[508] [508]
S, X, H, B, C, IR S, X, H, B, C, IR
[351] [509]
S, X, H, B, C, MS, IR
[510]
Pyrolysis to 2,3-C2B9H11 Oxidative degradation Reaction with Lewis bases
[226] [228] [440]
Reaction with Lewis bases
[440]
Zn, Hg Zn[(m-O5 5PPh2)2C2B9H10]2 porphyrin derivatives and Zn complexes (prepared for BNCT) C2B9H11-10-Hg(PPh3) Ph2C2B9H9-10-Hg(PPh3) C2B9H11-HgMe [(MeOCH2)2C2B9H9]-10-Hg(PPh3) Hg(C2B9H10-9-NC5H5)2 m(7,8)-(SCH2S)C2B9H10-Hg(PPh3) Hg[(Ph2P)2C2B9H10]2 (7,8-RC2B9H10)2-10-Hg (R ¼ H, Me, CHMe2) (Ph3P)Hg[7,8-(m-S(CH2)2S)C2B9H9] (Ph3P)Hg[7,8-(Me2S)2C2B9H9] 9-(1,2-C2B10H11)-Hg-10-(7-R-7,8-C2B9H10) (R ¼ H, Ph, CHMe2) 9-(1,2-C2B10H11)-Hg-10-(6-F-7,8-C2B9H10) 10-PhHg-7,8-RC2 B9 H10 (R ¼ H, Ph, CHMe2) exo,nido-Cl[(C6H5)3P]2Ru-(m-H)3-C2B9H89-Hg-(1,2-C2B10H11) C2B9H11-10-Hg-9-[3,1,2-CpCo(C2B9H10)] m(H)3-Ru[P(C6H5)3]2Cl-C2B9H8-10-Hg-9[3,1,2-CpCo(C2B9H10)] Lanthanon and yttrium complexes (C4H8O)5LnCl2 þ C2 B9 H12 (Ln ¼ Y, Yb) (C4H8O)5YCl2 þ (Me2NCH2CH2) (MeOCH2CH2)C2 B9 H10 [(C4H8O)3Ln-(PhCH2)2C2B9H9]2 (Ln ¼ Sm, Yb) Other Experimental Studies Reactivity and kinetics C2B9H13 PhC2B9H12 C2B9H12-9-X (X ¼ Fe(CO)2Cp, Fe(CO)2(MeCN)Cp, Fe(CO)(CNC6H11)2Cp Me2C2B9H10-9-Fe(CO)2Cp
Continued
216
CHAPTER 7 Eleven-vertex carboranes
Table 7-2 Selected Nido-7,8-C2B9H13, Nido-7,8-C2B9H12, and Nido-7,8-C2B9H112 Derivativesa—Cont’d Compound C2 B9 H12
C2 B9 H12 and derivatives (I, OH, SH, CHMe2, Cl, Br, mS) 5CH2) RC2B9H12 (R ¼ Me, Ph, CMeH5 C2B9H9-5,6,10-(mH)3-RuCl(PPh3)2 C2B9H11I Kþ RC2 B9 H11 (R ¼ H, Me, CHMe2) Kþ Me2 C2 B9 H10 C2B9H11.py(X) [py (X) ¼ pyridine derivatives] (Me2NCH2)C2 B9 H11 C2 B9 H11 2
C2B9H10-3-R2 (R ¼ Et, Ph) C2B9H11-9-Me, C2B9H10-9,11-Me2 , C2B9H99,10,11-Me3 C2B9H11-n-I (n ¼ 5, 9), C2B9H10-9,11-I2 Me2C2B9H8-3-Ph2 Me2C2B9H9-9-CH2Ph PhC2 B9 H11 PhC2 B9 H10 2 (optically active) R2C2B9H10-L (L ¼ Me2S, Me2SCH2, C6H5N, C6H5NCH2; R ¼ H, Me, Ph) (FC6H4)C2 B9 H10 2 Catalysis (PPh3)2Rh-(SPh)MeC2B9H10 RR0 C2B9H9-10-L (R, R0 ¼ H, Me; L ¼ SEtPh, SMe2, SEt3, S(CH2)4) (R2P)R0 C2B9H11-Rh(PPh3)2 (R ¼ Ph, Et, CHMe2; R0 ¼ H, Me, Ph) (C5Me4Et)2(CH3)Zr-C2B9H12 Cp*2MeZr-C2B9H12 [CH2-O-bicyclo-C7H4O(CHMe2)CH2RuCl2(C3H4N2mes2)]C2 B9 H11 mes ¼ mesitylene
Information
References
Electrochemical bromination, iodination Deuteration Oxidative degradation Oxidation B insertion Chromatographic separation
[149] [122,158] [183] [511] [219,223] [512]
Kinetics of formation Reaction with Br2 131 I exchange pKa pKa Bridging H tautomerism Ni complexation B insertion Methylation mechanism Me2S-BBr3 insertion ! 1,2-C2B10H11-3-Br B insertion with B2(CH2)4 Sn, Ge, Pb insertion Benzylation B insertion Deuteration
[59] [443] [154] [513] [513] [142] [371] [514] [159] [218] [220] [515] [167] [94] [158]
Pd-catalyzed cross-coupling with RMgX ! 5CH2 C2B9H11R (R ¼ Me, Et, C6H13, Ph, CH5 B insertion Protonation, rearrangement Rearrangement PhB insertion, OR Liquid chromatographic separation of enantiomers, OR F(Taft constants)
[169] [94] [161] [225] [224] [516]
Catalysis of 1-alkene hydrogenation Thermal conversion ! closo-C2B9H11
[467] [127]
Catalytic hydrogenation
[466]
Olefin polymerization catalysis Olefin polymerization catalysis Catalyst for ring-opening metathesis
[435] [435] [517]
[261]
Continued
7.2 11-Vertex open clusters
217
Table 7-2 Selected Nido-7,8-C2B9H13, Nido-7,8-C2B9H12, and Nido-7,8-C2B9H112 Derivativesa—Cont’d Compound
Information
References
m-(CH2)3C2B9H11-8-[(CH2)2C(O)O(CH2)3Me]-Rh(PPh3)2 [L(m-PPh2)2Rh(u-PPh2)]PhC2B9H10 [L ¼ cycloCH2 2O2 2CMe22 2O2 2CH, binap, Fe(C5H4)2, (CH2)4] enantiomers (Ph3P)2(H)Rh2 2C2B9H11
Rh-catalyzed hydroboration; metal-promoted alkene insertion into B-H bond Enantioselective hydrogenation catalysis
[470]
[(Ph2P)2C2B9H10]2Ru (B2 2H2 2Ru) (Ph3P)2Rh(mPh2P)(mH)-RC2B9H10 (R ¼ H, Me) (Z4-C8H10)Rh(mPh2P)(mH)2-MeC2B9H9 (Ph3P)2ClRuþ-7,8-R2 C2 B9 H10 (Ph3P)2Rh-m-(CH2)3-7,8-C2B9H10 m-[-(Ph3P)2(H)Rh2 2CB9H10C2 2CMeCH2-]x[CMe{C(O)OMe}CH2]y- methacrylate copolymer Cp*2ClXRh2[cyclo-(4-MeC6H3)(m-S)2C2 B9 H10 ]þ[cyclo-(4-MeC6H3)(m-S)2C2 B9 H10 (X ¼ Cl,H) Other applications C2 B9 H12
C2B9H11-9-SMe C2B9H11-5-Br C2B9H10-thymidine for BNCT (targeting tumor cells) RC2 B9 H10 (R ¼ Me, Ph, SMe) RR0 C2B9R00 nXm (R,R0 ,R00 ¼ H, aryl, aryl; X ¼ H, halogen) 5CMe)C2 B9 H11 isopropenyl (CH25 5CHMe)C2 B9 H11 (CH25 (thymidine)C2 B9 H10 for BNCT (targeting tumor cells) (nido-C2B9)4porphyrins HO(O)C(CH2)2C2B9H10X (X ¼ H,
131
I,
211
At)
Heterogenized on solid support; catalysis of hydrogenation and isomerization of 1-hexene Catalysis of Kharasch addn of CCl4 to olefins Cyclopropanation catalysis Cyclopropanation catalysis Olefin cyclopropanation catalysis Catalysis of hydrosilanolysis of alkenyl acetates Catalysis of isomerization and hydrogenation of olefins Catalytic hydrogenation and cyclopropanation of alkenes
Herbicide; fungicide; insecticide Mechanism of formation of Ni-B alloys from sulfamic acid electrolytes containing the carborane anion Electroless Ni coating Ni-B electroplates with low B content Ni-B alloy electroplates Electrophoresis; ion mobility; chiral separation) Electrophoresis; ion mobility; chiral separation) Phosphoryl transfer assay Electrophoresis; ion mobility; chiral separation) Defoliant-desiccant Synthesis of methyl methacrylate copolymers) Copolymerization with vinyl monomers Phosphoryl transfer assay Toxicity; DNA damage; light activation; sensitizer for BNCT; photodynamic therapy of tumors in vivo studies of radioiodinated and astatinated derivatives for cancer therapy
[461]
[518] [519] [520] [520] [521] [522] [523,524] [382]
[525] [526]
[527] [528] [529] [530] [530] [74] [530] [531] [532] [533] [74] [534]
[535] Continued
218
CHAPTER 7 Eleven-vertex carboranes
Table 7-2 Selected Nido-7,8-C2B9H13, Nido-7,8-C2B9H12, and Nido-7,8-C2B9H112 Derivativesa—Cont’d Compound
RC2B9H10X (R ¼ (CH2)2C(O)NH(CH2)2Me, amino and amido groups; X ¼ H, 131I, 211At) DTPA {m-[m-C6H4(CH2)2]C2B9H11)(C2B9H10X)}2 X ¼ H, 131I, 211At) “Venus flytrap” complexes C2B9H10X2 biotin derivatives (X ¼ I, 125I, 211At) antibody pretargeting exo,nido-(Ph3P)2ClRu-7,8-Me2C2B9H10 Theoretical Studies Molecular and electronic structure calculations C2B9H13 isomers
C2 B9 H11 2 isomers
C2 B9 H11 C7 H6 þ C2 B9 H12 isomers
C2B9H10-9-SMe2 C10 H6 ðNMe2 Þ2 þ C2 B9 H12 [Mþ]2C2 B9 H11 2 (M ¼ Li, Na, K) cyclo-[(CH2) 2-S-(CH2) 2]O-C2 B9 H10 C2 B9 H11 C7 H6 þ tropylium C2B9H11-9-Me (PH3)Au(Ph2P)2C2B9H10 (model) C10 H6 ðNMe2 Þ2 þ C2 B9 H12 (R3)2Au4[(m- PPh2)2(C2B9H10)]2 (R ¼ Ph, p-C6H4Me, p-C6H4OMe)
Information
References
in vivo studies of radioiodinated and astatinated derivatives for cancer therapy in vivo studies of radioiodinated and astatinated derivatives for cancer therapy
[535]
S, H, B
[536]
Controlled synthesis of poly(methyl methacrylate) with amines
[537]
MNDO, dipole moment DFT, stability Geometry Effects of exo-substitution on stability and electronic properties; interactions with biomolecules ab initio Electron density distribution MNDO, dipole moment MS-Xa electronic structure DFT, stability Effects of exo-substitution on stability and electronic properties; interactions with biomolecules ab initio: face H - double minimum geometry optimization ab initio: face H - double minimum geometry optimization DFT, stability Effects of exo-substitution on stability and electronic properties; interactions with biomolecules Electron density distribution GIAO NMR: MP2 covalence or strong ion pairing between M and anions Molecular modeling [CHARMm] ab initio charge-transfer GIAO; NMR þ geometry DFT population analysis Neutron diffraction ab initio
[538] [244] [118] [539]
[535]
[540] [411] [538] [541] [244] [539]
[121] [121] [244] [539]
[411] [138] [119] [542] [543] [118] [386] [138] [498] Continued
7.2 11-Vertex open clusters
219
Table 7-2 Selected Nido-7,8-C2B9H13, Nido-7,8-C2B9H12, and Nido-7,8-C2B9H112 Derivativesa—Cont’d Compound
Information
References
Isomerization calculations RC2 B9 H11 isomers (R ¼ H, Ph)
Cage rearrangement
[544]
GAIO, 11B shifts GAIO, 11B shifts ab initio GIAO C, IGLO GAIO, 11B shifts GAIO, 11B shifts H NMR ab initio
[296] [296] [122] [140] [296] [296] [90]
Calculated 31P shifts; nido clusters show þI [e donor] effect, closo clusters show -I [e acceptor] effect; pKa
[67]
pKa pKa Electrophilic/nucleophilic attack pKa pKa
[296] [296] [545] [296] [296]
NMR calculations C2B9H13 C2 B9 H12
C2 B9 H11 2 C2B9H11-9/10-SMe2 PSHþ C2 B9 H12 [PS ¼ proton sponge ¼ (Me2N)2C10H6] (PR2)R0 C2 B9 H10 (R ¼ Et,CHMe2, Ph; R0 ¼ H, Me, Ph) Reactivity calculations C2B9H13 C2 B9 H12 C2 B9 H12 isomers C2 B9 H11 2 C2B9H11-9/10-SMe2
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; F, 19F NMR; N, 14N NMR; P, 31P NMR; 2d, two-dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; UV, UV-visible data; E, electrochemical data; ESR, electron spin resonance data; MAG, magnetic susceptibility; COND, electrical conductivity; OR, optical rotation; XPS ¼ X-ray photoelectron spectra. a A more extensive listing (TABLE 7-2 Extended) can be found online at http://www.elsevierdirect.com/companion.jsp? ISBN=9780123741707.
Table 7-3 Nido-7,9-C2B9H13, Nido-7,9-C2B9H12, and Nido-7,9-C2B9H112 Derivatives Compound Synthesis and Characterization Neutral 7,9-C2B9H13 Single-Cage Derivatives C2B9H11-NMe3 Me2C2B9H9-10-NEt3 C2B9H12-C(O)R (R ¼ Me, Ph) (p-FC6H4)2C2B9H11 C2B9H11-10-L [L ¼ NMe3, (Me2CH)2NH, C5H5N] (H3N)C2B9H12 5CMe2, HN5 5C(CH2)6, RC2B9H12 [R ¼ HN5 HN5 5C6H3(OMe)2] iminium
Information
References
S, S, S, S, S, S, S,
[255,256] [155] [188] [80] [170] [546] [546]
H, B, B B, H, H, B, H, B, H, B, H, B,
IR IR C, F IR C, IR, UV C, IR, UV
Continued
220
CHAPTER 7 Eleven-vertex carboranes
Table 7-3 Nido-7,9-C2B9H13, Nido-7,9-C2B9H12, and Nido-7,9-C2B9H112 Derivatives—Cont’d Compound
Information
References
C2B9H11-PMe3 (MePh2P)C2B9H10 selective targeting of mitochondria for BNCT C2B9H11-n-SMe2 (n ¼ 8, 10)
S, H, B, IR S, H, B, C, P, MS.
[255,256] [301]
S, H, B, MS
[49]
Neutral 7,9-C2B9H13 Multi-Cage Derivatives RH2C3N2(C2B9H11)2 (d,l, meso) [R ¼ H, MeOC(O)]
S, H, B, C, IR, MS
[212]
7,9-C2B9H12 and 7,9-C2B9H112 Single-Cage Derivatives No substituents on boron S(deboronation of 1,7-C2B10H12 at 100 C) C2 B9 H12 S, H, B, MS S, B, IR S, H, B, IR S, H, B S, B(T1) S, X, H, B, C PSHþ C2 B9 H12 [PS ¼ proton sponge ¼ (Me2N)2C10H6] S, IR [LnðBH4 Þ2 þ ]2 C2 B9 H12 (Ln ¼ La, Nd, Er) S, X [(Me2N)3PN]2BN(H)PðNMe2 Þ3 þ C2 B9 H12 S [hydrazine þ 1,7-(p-XC6H4)C2B10H11 N2 H5 þ (XC6H4)C2 B9 H11 (X ¼ H, Cl, Br)] S, H, B, IR S, H, B S, H, B, IR Me2 C2 B9 H10 S PhC2 B9 H11 S, H, B Ph2 C2 B9 H10 S (degradation of 1,7-[C(O)NH2]2C2B10H10) Mþ [C(O)NHPh]2C2 B9 H10 (M ¼ H, Cs) S, H, cytotoxicity M[porphyrin-CH(OH)](Me2CH-C2B9H11) (M ¼ Co, Cu, 2H) [CH25 5C(Me)C(O)OCH2]2C2 B9 H10 S, IR D or hydrocarbon substituents on boron C2B9H11D C2B9H12-nDn (n ¼ 0, 2-8) C2B9H11-8-Me C2 B9 H10 Me2 C2B9H10-8-Me2 [Mþ]2 C2 B9 H11 2 (M ¼ Li, Na, K) C2B9H11R (R ¼ Me, Bu) C2B9H11-10-Me C2B9H11-n-Me (n ¼ 3, 8, 10) C2B9H10-8,10-Me2 C2B9H10-10,11-Me2
S, S, S, S, S, S, S, S, S, S S
B, IR B H, B H, B B H, B, C H, B, IR B H, B, C
[88] [49] [141] [44] [158] [155] [90] [547] [75] [65] [44] [225] [44] [52] [87] [548] [422] [388]
[141] [122] [158] [158] [159] [119] [115] [157] [118] [160] [160] Continued
7.2 11-Vertex open clusters
221
Table 7-3 Nido-7,9-C2B9H13, Nido-7,9-C2B9H12, and Nido-7,9-C2B9H112 Derivatives—Cont’d Compound
Information
References
C2B9H9-8,10,11-Me3 C2B9H8-m,8,10,11-Me4 C2B9H11-endo-Me CH C2B9H11-8-CH2C C2B9H11-1,6-(C6H4-p-Me)
S, S S, S, S,
[160] [160] [118] [165] [81]
N- or P-containing substituents on boron C2B9H7Me2NH2OH C2B9H11-10-CD3CN C2 B9 H11 -CHðCNÞ2 C2 B9 H11 -10-CHðCNÞ2 {N2[C(O)OH](CH2C6H5)H}2C2B9H11 [HOC(O)(CH2)2]C2 B9 H11 RC2B9H10-PPh3 (R ¼ H, Ph)
S, H, B S, H, B, C S, H, B, IR S, S, B S, H, B, C, IR, MS S(aqueous F), H, B, C, IR, MS S, X(H), H, B, P, IR
[160] [118] [115] [155] [379] [84] [549]
H, B, IR S (cleavage of 1,7-C2B10H11-9-I), H, B S, H, C, B, IR S, IR
[115] [53] [81] [79]
S, H, B, C, F, IR S, IR S S, X, H, B X, H, B S, B (see ref. 256 for better assignments) S, B S, H, B, IR
[82] [388] [52] [87] [87] [255] [155] [256]
X
[550]
F-, Cl-, Br-, or I-containing substituents on boron C2 B9 H10 Br2 Me2C2B9H9-1,6-Br2 RR0 C2B9H9-n-F (n ¼ 3, 10; R, R0 ¼ Me, Ph, 4-FC6H4) C2B9H11-1/6-X(X ¼ F, Cl, Br, I)
S (from 1,7-C2B10H10-9,10-Br2) S, B S, H, B, C, F, IR S, H, C, B,F, IR
[52] [155] [82] [81]
Transition Metal s- and m-complexes (C2B9H12)[M4(acac)4(OH)11]þ (M ¼ Zr, Hf) RC2B9H10-m(10,11)-OsIVHCl(PPh3)2 (R ¼ H, Ph) C2B9H11-m(10,11)-OsIVH2(PPh3)2
S, MAG, IR, H, COND S, X(H), H, B, P, IR S, X(H), H, B, P, IR
[345] [549] [549]
O- or S-containing substituents on boron C2B9H11-C5 H7 O4 C2B9H11-n-OBu (n ¼ 1, 5) C2B9H11-1,6-OH RR0 C2 B9 H10 (R ¼ C6H4OH, C6H4NO2; R ¼ H; R ¼ R0 ¼ C6H4OPh, C6H4NO2, NC5H4, C6H4NH2) RR0 C2 B9 H10 (R, R0 ¼ Me, Ph, 4-FC6H4) (HOCH2)2RC2 B9 H10 C2 B9 H10 (p-XC6H4)C2 B9 H11 (X ¼ Cl, Br) Ph2C2B9H9-n-OEt10 (n ¼ 2, 3) Ph2C2B9H9-10-OH Me2C2B9H9-OR (R ¼ Me, Et, CHMe2) Me2C2B9H9-m(3,4)-O2 C6 H4 Me2C2B9H9-3-OR (R ¼ Me, Et,CHMe2) (from 1,7-Me2C2B10H10) C2B9H11-8-SMe2
H, B H, B, C B, IR H, C, B, IR
Continued
222
CHAPTER 7 Eleven-vertex carboranes
Table 7-3 Nido-7,9-C2B9H13, Nido-7,9-C2B9H12, and Nido-7,9-C2B9H112 Derivatives—Cont’d Compound
Information
References
S, IR S, H, B, IR S, H, B, IR
[79] [115] [115]
oxidative degradation B insertion deuteration C, IGLO B, C, H Raman Raman F3CSO3H ! cage closure to 2,3-C2B9H11 131 I exchange H2SO4 ! cage closure to 2,3-C2B9H10-X B, proton rearrangement HPLC separation degradation with Br2; B insertion OR B, proton rearrangement H2SO4!cage closure to 2,3-RR0 C2B9H9 F3CSO3H ! cage closure to 2,3(FC6H4)2C2B9H8-4-F F (Taft constants) F (Taft constants) F, UV H2SO4 ! cage closure to 2,3-C2B9H10-B-Me [mixture] MeC(O)OH ! nido-2,8-C2B9H12-11-Me
[228] [223] [122] [140] [551] [552] [552] [118] [154] [118] [161] [512] [553] [44] [161] [118] [118]
geometry DFT, stability MNDO, dipole moment MNDO, dipole moment DFT, stability MNDO, dipole moment Geometry DFT, stability covalence or strong ion pairing between M and anions
[118] [244] [538] [538] [244] [538] [540] [244] [119]
C2B9H12 Multi-Cage Derivatives 1,4-[7,9-(p-BrC6H4)C2B9H10]2C6 H4 2 C2B9H11(1,10-C2B8H9) C2B9H11(1,10-C2B8H8Ph) Other Experimental Studies PhC2B9H12 C2 B9 H12
C2B9H11D C2B9H11-10-OEt C2B9H11I C2B9H11-1(6)-X (X ¼ Cl, I) C2B9H11-8-Me RC2 B9 H11 (R ¼ OH, OMe) PhC2 B9 H11 , Ph2 C2 B9 H10 d,l-PhC2 B9 H11 Me2C2B9H9-8-CH2Ph RR0 C2 B9 H10 (R, R0 ¼ H, Me, Ph) (FC6H4)2C2B9H9-n-F (n ¼ 3, 10) (FC6H4)C2 B9 H11 (FC6H4)2C2 B9 H10 2 (m/p-FC6H4)C2 B9 H11 C2B9H11-n-Me (n ¼ 8, 10) C2B9H11-10(11)-endo-Me Theoretical Studies Molecular and electronic structure calculations C2B9H13 C2B9H13 isomers C2 B9 H12 C2 B9 H12 isomers C2 B9 H11 2 C2 B9 H11 2 isomers [Mþ]2 C2 B9 H11 2 (M ¼ Li, Na, K)
[261] [261] [370] [118] [118]
Continued
7.2 11-Vertex open clusters
223
Table 7-3 Nido-7,9-C2B9H13, Nido-7,9-C2B9H12, and Nido-7,9-C2B9H112 Derivatives—Cont’d Compound
Information
References
cage rearrangement cage rearrangement
[544] [544]
PSHþ C2 B9 H12 [PS ¼ proton sponge ¼ (Me2N)2C10H6] C2B9H11-n-Me (n ¼ 1, 2, 3, 6, 8, 10) C2B9H11-endo-Me C2B9H11-10,11-u-Me C2B9H11-11-MeCN C2B9H11-10-CD3CN Me2C2B9H8-3,4-m-(OCH2CH2O) Me2C2B9H8-10,11-m-(OCH2CH2O)
B, C, H, DFT spin-spin coupling GIAO (11B) IGLO H NMR ab initio GIAO; NMR þ geometry GIAO; NMR þ geometry GIAO; NMR þ geometry GIAO; NMR þ geometry GIAO; NMR þ geometry GIAO; NMR þ geometry GIAO; NMR þ geometry
[551] [296] [140] [90] [118] [118] [118] [118] [118] [118] [118]
Reactivity calculations C2 B9 H12 C2 B9 H12 isomers
pKa charge distribution, reactivity predictions
[296] [545]
Isomerization calculations C2 B9 H12 isomers PhC2 B9 H11 isomers NMR calculations C2 B9 H12
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; F, 19F NMR; 2d, two-dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; UV, UV-visible data; MAG, magnetic susceptibility; OR, optical rotation; COND, electrical conductivity.
Table 7-4 Nido-2,7-, 2,8-, and 2,9-C2B9 Derivatives Compound Synthesis and Characterization C2B9H13 derivatives 2,7-C2B9H12-11-Me
2,7-C2B9H10D2-11-Me 2,7-C2B9H11-6,11-Me2 2,7-Me2C2B9H10-3-CH2Ph 2,7-Me2C2B9H10-11-CH2Ph 2,9-C2B9H13
Information
References
S, B X H C IR, Raman IR, Raman S, H, B S S, X, H, B S, B, IR, UV, MS S, H, C, B ED
[118,158,161] [163] [118,158] [118] [164] [164] [158] [162] [161] [91] [90] [120] Continued
224
CHAPTER 7 Eleven-vertex carboranes
Table 7-4 Nido-2,7-, 2,8-, and 2,9-C2B9 Derivatives—Cont’d Compound
Information
References
S S S S, H, B, C S, H, B S, H, B S (degradation of 1,12-C2B10H12), B, H S, X, H, B, C S, H, B, C
[158] [158] [158] [118] [158] [158] [89] [90] [119]
Deprotonation, rearrangement Deprotonation, rearrangement
[161] [161]
Geometry Geometry Geometry Geometry Geometry Geometry Geometry Covalence or strong ion pairing between M and anions
[118] [118] [118] [118] [118] [118] [118,120] [119]
GIAO GIAO H NMR ab initio NMR: MP2
[118] [118] [90] [119]
C2 B9 H12 derivatives 2,7-C2B9H11-11-Et 2,7-C2B9H11-11-Bu 5CHCH2) 2,7-C2B9H11-11-(CH5 2,7-C2B9H11-11-Me 2,7-C2B9H10-6,11-Me2 2,9-C2 B9 H12 PSHþ2,9-C2 B9 H12 [PS ¼ proton sponge ¼ (Me2N)2C10H6] [Mþ]2 2,9-C2 B9 H11 2 (M ¼ Li, Na, K) covalence or strong ion pairing between M and anions Other Experimental Studies 2,7-C2B9H12-11-Me 2,7-Me2C2B9H10-11-CH2Ph Theoretical Studies Molecular and electronic structure calculations 2,7-C2B9H13 2,7-C2B9H12-11-Me 2,8-C2B9H13 2,8-C2B9H12-7-Me 2,8-C2B9H11-7-Me 2,9-C2B9H13 [Mþ]2 2,9-C2 B9 H11 2 (M ¼ Li, Na, K) NMR calculations 2,7-C2B9H12-11-Me (n ¼ 7, 11) 2,7-C2B9H11-n-Me (n ¼ 7, 11) PSHþ 2,9-C2 B9 H12 [PS ¼ proton sponge ¼ (Me2N)2C10H6] [Mþ]2 2,9-C2 B9 H11 2 (M ¼ Li, Na, K)
S ¼ synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; IR, infrared data; MS, mass spectroscopic data; UV, UV-visible data.
salts in high yields (Table 7-1). Under these conditions, 1,7-C2B10H12 is unreactive, but it is degraded to nido7,9-C2 B9 H12 in 66% yield by CsF in diethylene glycol dimethyl ether at 100 C [88]. Base attack occurs regiospecifically at the most electropositive boron vertex, which in 1,2-C2B10H12 is B(3) or equivalently B(6), and in 1,7-C2B10H12 is B(2) or B(3), these being the boron atoms adjacent to two carbons (Figure 7-2). In 1,12-C2B10H12, a nonpolar system, all 10 borons carry identical charge and hence are equally electrophilic. The decreasing polarity in the series 1,2-, 1,7-, and 1,12-C2B10H12 is reflected in progressively lower reactivity toward base attack in the order 1,2 > 1,7 > 1,12. Indeed, deboronation of 1,12-C2B10H12 does not occur with alkoxide ions and
7.2 11-Vertex open clusters
H 10
8
B
7
B 12
B
C 2
B
11
9
B4 C
B
B5
6
C
5
1,2-C2B10H12
12
B
9
2
B 11 B
B
B4 C
B
B
B
6
8
−H+
7
B
B
3
B
11
B
B C
5
B 4B
2
B C
9C
B
6
8
7
B
B2
3
B B1 2−
7,9-C2B9H11 2−
B
11
11
B
−
B 2
10
7,9-C2B9H−12
8
B
2
2−
B1
B3
12
B
2−
9C
1,7-C2B10H12
C
B
3
H
B 10
7
7
B
7,8-C2B9H11
B B 4
B5
6
6
8
7,8-C2B9H−12
5
1
5
B1
10
−[B+]
C
B B 4
2
B C
9B
B1
−
B
B
3
B3 8
−H+
7
B B
B = BH C = CH
B 10
6
8
11
B
B C
9B
B 4B
10
11
B
−[B+]
1
B
7C
2−
−
B3
225
9
B
B C
10
4
−[B+]
9C 5
1
6
B B 10
1,12-C2B10H12
B5
H
B
B 4B
B
6
8
B 3
B B1
2,9-C2B9H−12
10
11
−H+
9C 5
7
C
11
B
B B
B 2
4B
B
6
8
B 3
B
B B 7
C
2
B1 2− 2,9-C2B9H11
FIGURE 7-2 Conversion of icosahedral C2B10H12 carboranes to nido-C2 B9 H12 and nido-C2 B9 H11 2 (dicarbollide) anions via deboronation and deprotonation. The location of the bridging hydrogen atom in 7,8-C2 B9 H12 can vary (see discussion).
requires stronger conditions such as KOH in propanediol at elevated temperature or KOH in the presence of crown ethers [89–91], affording the 2,9-C2 B9 H12 ion (Figure 7-2). Similar degradation of the B-octamethyl-o-carborane 1,2-H2C2B10H2Me8 in KOEt/DME yields the nido7,8-H2 C2 B9 H2 Me8 ion, which can be protonated to H2C2B9H3Me8 [92]. The mechanism of extraction of boron from C2B10H12 cages is only partially understood and is difficult to study in detail, as in most cases the reaction proceeds rapidly toward completion with the formation of nido-C2B9 and monoboron products. However, an attack by the iminophosphorane N(H)P(NMe2)3 on 1,2-C2B10H12 initially forms adduct 7-9 in which the icosahedral cluster has been opened but the base-complexed boron remains attached to the cage, although connected by just three atoms. Further reaction with the base removes this boron entirely and releases the nido-7,8-C2 B9 H12 anion [75]. While not proved in the general case, adducts such as 7-9 are probable intermediates
226
CHAPTER 7 Eleven-vertex carboranes
in some base-promoted deboronations of icosahedral carboranes. A structurally different intermediate, 7-10, has been isolated during the deboronation of 1,2-BrC2B10H11 by pyridine [93], and its more open geometry reflects the presence of two electron-donor pyridines on the base-complexed boron atom. The crystallographically established cage architecture of 7-9 resembles the structure of the nido-C2 B10 H10 2 cluster that is generated by the reduction of 1,2-C2B10H12, as described in Chapter 11, and is formally derived from a 13-vertex closo polyhedron by the removal of a 6-coordinate vertex. L
H
NC5H5
B
C5H5N B
C C
B B
7-9
B
B
B
B
B B
B
H
Br
B B = BH C = CH L = N(H)P(NMe2)3
C C
B B
7-10
B
B
B
B
B B
The same iminophosphorane reagent also deboronates 1,7-C2B10H12, but the reaction is sluggish compared to that of 2H---N hydrogen-bond interthe 1,2 isomer, initially yielding a 1,7-C2B10H12 N(H)P(NMe2)3 adduct that features a C2 action; on heating at 50 C, this adduct is slowly converted to a salt of nido-7,9-C2 B9 H12 [75]. Para-carborane (1,12C2B10H12) is unreactive toward N(H)P(NMe2)3 even in refluxing toluene. In general, base-induced deboronation of C-substituted and C,C0 -disubstituted derivatives of the C2B10H12 isomers leads to the formation of the corresponding nido-C2B9 anions, but in B-substituted carboranes, the results can be different, depending on the location of the attached groups. In some cases, base attack is completely blocked, as in 3,6-dialkyl derivatives of 1,2-C2B10H12 which are unreactive toward nucleophiles [94]. Not surprisingly, the presence of strongly electron-attracting or electron-releasing groups on the cage can markedly affect the reaction rate toward bases, as will be shown. Deboronation of some 1,2-C2B10H12 derivatives can be accomplished without strong bases, depending on the attached functional groups. Carbon-bound electron-withdrawing substituents such as C(O)R (R ¼ H or Me) [95], sulfoxide [96], XC6H4 (X ¼ F or NO2) [97] and triazinyl [98] induce their rapid conversion to the nido-carborane in wet DMSO at room temperature; the 1-alanine derivative self-degrades in water [99]. Remarkably, 1-lactose-o-carborane is degraded to the nido-7,8-ðlactoseÞC2 B9 H10 in neutral aqueous solution [100]. The dianions (C2B10H10)2[m-7,8-S 2]22 (n ¼ 1, 2), in which two carborane cages are linked by two 2 2S(CH2)nS2 2 bridges, are converted to (CH2)nS2 2]22 by treatment with 1,2-CH2Br2 in ethanol [101]. Similarly, 1,2-S2 C2 B10 H10 2 (nido-C2B9H10)2[m-7,8-S(CH2)nS2 forms nido-7,8-ðHSÞ2 C2 B9 H10 2 when refluxed with NaI in ethanol [102]. Conversion of C-imido and C-phosphino 1,2-C2B10H12 carborane derivatives to their C2B9 analogues has been achieved by refluxing them in methanol and ethanol respectively [103,104], and dihydropyridyl o-carboranes are deboronated by alcoholic I2 [105]. Partial degradation of C,C0 -bis(phosphino) C2B10 derivatives is promoted by transition metal reagents in alcoholic media, thereby allowing the preparation of metal complexes of nido-7,8-(R3P)2C2B9H10-type ligands directly from 1,2-C2B10H12 carboranes [104,106–111]. Nido-7,8-C2 B9 H12 can also be prepared by the degradation of the so-called “reactive” isomer of the nidoC2 B10 H13 anion 7-4 described earlier, with strong oxidants such as aqueous H2O2 [112]. Treatment of 7-4 with aqueous FeCl3 and SMe2 yields nido-7,8-C2B9H11-9-SMe2 [112].
7.2.2.2 Direct synthesis of nido-7,8-C2B9H12 from B10H14 or B10H13 The Brellochs reaction, described in Chapter 6 for the preparation of CB10 carboranes via the insertion of aldehydes into the decaborane(14) cage, can also be used to generate nido-7,8-C2 B9 H12 in good yields via the reaction of B10H14 with excess formaldehyde in strongly alkaline media [113]. The obvious advantage of this approach is that it yields the parent
7.2 11-Vertex open clusters
227
nido-carborane anion directly from decaborane in a single step. On the other hand, its application in the preparation of substituted derivatives or other nido-C2B9 cage isomers has not been demonstrated. Two-carbon insertions into B10 H13 with loss of one boron atom have been accomplished via reactions of 3-butyne-2-one or methyl propiolate, forming [RC(O)]C2 B9 H11 (R ¼ Me or MeO) with good yields [114].
7.2.2.3 Synthesis of nido-7,9-C2B9H12 derivatives from closo-C2B9H11 Although the most practical route to nido-7,9-C2B9 cluster synthesis is the base-promoted degradation of 1,7-C2B10H12 as described above, other approaches are also available. Treatment of the 11-vertex closo-carborane 2,3-C2B9H11, whose carbon atoms occupy nonadjacent vertices (Figure 1-1 and Section 7.3), with BH4 ion opens the cage to generate nido-7,9-C2 B9 H12 [115]; the same carborane reacts with alkyllithium reagents [115] or dimethyl sulfide [49] to yield nido-7,9-C2B9H11-9-L (L ¼ Me, n-C4H9), and nido-7,9-C2B9H11-10-SMe2 respectively. Other Lewis bases combine with 2,3-C2B9H11 to form similar nido-7,9-C2B9H11L adducts [116].
7.2.2.4 Synthesis of nido-C2B9H13 isomers
Acidification of nido-7,8-C2 B9 H12 with HCl affords neutral 7,8-C2B9H13 (Figure 7-3A) [45], a procedure that has been modified [117] by using H3PO4 in place of HCl. The 7,9-C2B9H13 isomer has not been isolated, a fact that may reflect the absence of an available location for a second B2 2H2 2B bridge on the open face of 7,9-C2 B9 H12 . Protonation of 7,9 0 RR C2 B9 H10 anions at ambient temperature leads to cage closure, generating closo-RR0 C2B9H9 products (Figure 7-3B) [118]. (Nido-7,9-RC2 B9 H11 , where R is H or Ph, is also converted to closo-RC2B9H10 and H2 via reaction with polyphosphoric acid at 70 C, as found in early work [116]). A third isomer, 2,9-C2B9H13, is obtained on protonation of the 2,9-C2 B9 H12 ion (Figure 7-2) [90,91], and substituted derivatives of a fourth, 2,7-C2B9H13, are obtained upon reaction of 7,8-C2 B9 H11 2 with alkyl halides (see below). As noted earlier, 7,8- and 2,9-C2B9H13 are the only isomers of this nido-carborane system to have been isolated and characterized in parent form.
−
H
B B
B C
C B
B R B
A
H
H
B R
B
H
B
B
B
B
R
B
B
B
B
7,8-RRC2B9H−10
7,8-RRC2B9H11
B C
B = BH
B C
B B
B B
B
B
R R
C
C B B
H+
B B
B
B
B
R
H
R
B C
C
+
7,9-RRC2B9H−10
B
B B B
B
2,3-RRC2B9H9
FIGURE 7-3 Protonation of 7,8-RR0 C2 B9 H10 and 7,9-RR0 C2 B9 H10 . R ¼ R0 ¼ H, Me, Ph.
R
228
CHAPTER 7 Eleven-vertex carboranes
7.2.2.5 Synthesis of nido-C2B9H112 (dicarbollide) anions
Deprotonation of the nido-C2 B9 H12 anions in solution generates the corresponding C2 B9 H11 2 dianions (Figure 7-2), which were given the trivial name dicarbollide (from the Spanish olla, jar) by their discoverer, M. F. Hawthorne, in recognition of their open molecular structures. As they lack bridging hydrogens and present clean, unimpeded open faces for complexation to metals, they are electronically analogous to cyclopentadeneide ions (C5 R5 ) but are considerably more versatile, and are the foundation of the enormous and highly developed field of icosahedral metallacarborane chemistry (Chapters 12 and 13). The dicarbollide ions are usually employed in situ in complexation reactions with metal reagents, and have been little investigated as species in their own right, in contrast to the well-characterized nido-R2 C2 B4 H4 2 dianions that are discussed in Chapter 4. However, detailed NMR studies of the alkali metal salts of the 7,8-, 7,9-, and 2,9-C2 B9 H11 2 dicarbollide ions in various solvents reveal that they exist in solution as closely associated Mþ C2 B9 H11 2 ion pairs in which the metal is located over the open face, in effect completing the icosahedron [119]. The metal-insertion chemistry of these species is covered in Chapters 13 and 14.
7.2.2.6 Structural studies of nido-C2B9 cage systems
X-ray diffraction data are available for salts of the 2,7-, 2,9-, 7,8-, and 7,9-C2 B9 H12 , as well as C2 B9 H11 2 anions, and for substituted derivatives of these anions and neutral C2B9H13 (Tables 7-2–7-4). No crystallographic studies have been reported for any parent C2B9H13 isomer, but the structure of 2,9-C2B9H13 has been determined by gas-phase electron diffraction [120]. In addition, hundreds of face-coordinated metal complexes containing C2 B9 H11 2 ligands have been crystallographically characterized (Chapter 13, Table 13-3). Although the 11-vertex icosahedral-fragment geometry of the nido-C2B9 cages is well supported by X-ray diffraction, multinuclear NMR, and other data, the location of the “extra” (non-terminal) hydrogen atom in 7,8-C2 B9 H12 and its derivatives has proven elusive. Depending on the cation, the solvent, and/or substituents on the cage, this hydrogen can occupy an asymmetrical bridging position, as in 7-11, or a symmetrical endo-terminal location on B(10) (7-12), or may exhibit fluxional behavior, tautomerizing between these positions, as suggested by NMR evidence and ab initio calculations [121,122]. −
− 10
11
B
9B
7-11
5
B 4B
C 8
H
H
6
B 3
B B1
B C 7
B
B C
B C
7-12
B2
B
B B
B = BH C = CH
B
B B
X-ray diffraction and multinuclear NMR investigations [123] of (Me2SO)2Hþ 7,8-C2 B9 H12 support a symmetrical non-bridged 7-12 structure, both in the solid state and in CD2Cl2 solution, as does a COSY (2d) NMR investigation of the Csþ salt in CD2Cl2 solution [124]. Extended Hu¨ckel molecular orbital calculations indicate that the endo-H interacts with all three borons on the open face via a 4-center 2-electron bond [123]. The 7-12 geometry is also found in the solid state in the zwitterionic 7-mono- and 7,8-dipyridyl derivatives of 7,8-C2 B9 H12 [125], in Csþ7,8-C2B9H19-9,10,11-Me3 [126], and in 7,8-C2B9H11-10-S(CH2)4 [127]. In NMe4 þ PhC2 B9 H11 , the extra hydrogen resides approximately above the center of the C2B3 open face and, in fact, is slightly closer to the carbon atoms than to the boron atoms [128]. 2H2 2B structure 7-11 Crystallographic studies of other substituted derivatives of 7,8-C2 B9 H12 show the bridged B2 [76,107,127,129–137], as does a neutron diffraction study of the proton sponge [1,8-bis(dimethylaminonaphthalene)] salt of parent 7,8-C2 B9 H12 [138] (in some cases, the “extra” hydrogen has not been located). The preferred location for the extra hydrogen can be affected by minor electronic perturbations. For example, in 7,8-C2B9H11-9-I, the electronegative Iþ substituent induces a 7-11-type geometry with a B(10)-H-B(11) bridge, while in the diiodo 7,8-C2B9H109,11-I2 anion, the structure is that of 7-12, with an endo hydrogen on the molecular mirror plane [135], as expected given the two symmetrically placed iodines. In HNMe3 þ 7,8-C2B9H11-3-I, which contains an iodo substituent on B(3) [antipodal to B(10)], an intermediate situation is found in which there is an unsymmetrical B2 2H2 2B bridge ˚ from B(10) and 1.37(10) A ˚ from B(9) [139]. between B(9) and B(10), with the hydrogen located 1.17(10) A
7.2 11-Vertex open clusters
229
NMR data on 7,8-C2 B9 H12 and its derivatives in solution have been interpreted in terms of hydrogen tautomerism between equivalent B2 2H2 2B bridging locations on the open face in 7-11 [121,122,138, 140–142], or, alternatively, as a static 7-12-type structure [124]. There is evidence that in derivatives containing a strong electron-accepting substituent, equilibrium favors the tautomer in which the bridging hydrogen is furthest from the substituted boron atom [142]. Clearly, the bridged and non-bridged arrangements are very close in energy, and in solution it may be difficult to distinguish between a rigid, nonfluxional 7-12 geometry and a time-averaged structure of B2 2H2 2B bridged enantiomers. The structures of 7,9- and 2,9-C2 B9 H12 , shown in Figure 7-2, have been established by X-ray diffraction studies on their PSHþ (proton sponge) salts and correspond to those calculated by ab initio molecular orbital methods [90]; the structure of [(Me2N)3PN]2BN(H)PðNMe2 Þ3 þ 7,9-C2 B9 H12 has also been reported [75]. Both carborane anions exhibit mirror symmetry, with the B2 2H2 2B bridge in each case straddling the molecular mirror plane. Crystallographic analyses and extensive NMR data are also available for a number of derivatives of 7,9-, 2,7-, and 2,9-C2 B9 H12 (Tables 7-3 and 7-4).
7.2.2.7 Introduction of substituents: halogenation and deuteration In comparison to the closo-C2B10 systems and many other polyhedral carboranes, the nido-C2B9 species are highly reactive owing to their open-cage structures and the presence of active bridging or endo hydrogens on the 5-membered open face. This fact both facilitates and complicates the placement of functional groups on the cage. C-substituted derivatives of the C2B9H13 and C2 B9 H12 systems are usually obtained by deboronation of the corresponding closo-C2B10H12 derivatives as described above. This approach can also be employed with B-substituted derivatives, provided that the borons to be extracted (those adjacent to both cage carbons in 1,2- and 1,7-C2B10H12) do not have attached groups that block the base attack. Tables 7-2–7-4 list the isolated and characterized derivatives of the 7,8-, 7,9-, 2,7-, 2,8-, and 2,9-nido-C2B9 neutral and anionic carboranes. Deuterium exchange of the anion or neutral 7,8-C2B9H13 with D2O yields B-deuterated products [141,143]; under basic conditions, only the bridging hydrogens are exchanged, but in acidic media some deuteration at the terminal B-H atoms can be observed. The 7,8-C2 B9 H12 anion is easily halogenated by reaction with I2 in aqueous ethanol, affording C2B9H11-9-I and C2B9H10-9,11-I2 via nucleophilic attack of the formal Iþ at the most electron-rich [{B(9), B(11)] locations [135,144,145]. The C,C0 -diphenyl mono- and diiodo analogues are similarly prepared [146]. Reaction of 7,8-C2 B9 H12 with N-halosuccinimides generates the dihalo products C2B9H10-9,11-X2 (X ¼ Cl, Br, I) in good yield [147], and monoiodo and monobromo derivatives can be obtained via electrochemical methods [148,149]. Adducts of the type 7,8-C2B9H10-9-L, where L is pyridine or a pyridine-type base, react with Br2 to produce 7,8C2B9H10-5(6)-Br-9-L products; here the electron-donating substituent alters the charge distribution in the cage such that the attack of electrophilic Brþ occurs at B(5) or its equivalent B(6) [150]. However, Br2 in acid media and in oxidants such as aqueous FeCl2 destroys the cage framework in 7,8-and 7,9-C2 B9 H12 . [177,553] Many other B-halogenated products have been generated by deboronation of B-halo icosahedral carboranes (Tables 7-2 and 7-3) [52,64,81,139,143,145,151–155], including a per-B-iodinated dicarbollide ion, H2 C2 B9 I9 2 . [415] Attack of the deboronating reagent (n-C4H9)4NþBF4 on 1,7-RR0 C2B10H10 (R, R0 ¼ Me, Ph, 4-FC6H4) results in both boron extraction and fluorination, yielding nido-7,9-RR0 C2B9H9-n-F products (n ¼ 3,10) [82]. B-iodo derivatives of 7,8-C2 B9 H12 , obtained upon reaction of 1,2-C2B10H12 with piperidine, are reported [154] to undergo rapid exchange with Na131I although this has been disputed [156]. At the time of writing this chapter, no halogenated derivatives of nido-C2B9 carboranes other than the 7,8 and 7,9 isomers have been reported.
7.2.2.8 Alkylation, protonation, and cage rearrangement Treatment of the 7,8- and 7,9-C2 B9 H11 2 (dicarbollide) dianions with alkyl halides (RX; R ¼ Me, Et, n-Bu, CH2¼CHMe; X ¼ halogen) produces 7,8-C2B9H11-9-R and 7,9-C2B9H11-10-R as isolable products, via electrophilic attack of formal Rþ at a B-B edge on the open face [157–160]. The methylation of 7,9-C2 B9 H11 2 with MeI in THF/ 2C2 2B-bridged intermediate, 7,9-C2B9H9-m(10,11)-Me, NH3 is proposed, from spectroscopic data, to proceed via a B2 that subsequently rearranges to the terminal B(10)-Me product [160]. Further reaction generates di- and tri-B-methylated species, 7,9-C2B9H10-(n,10)-Me2 (n ¼ 8, 11) and 7,9-C2B9H9-8,10,11-Me3 , in all of which the alkyl groups are bonded only to the boron atoms on the open face.
230
CHAPTER 7 Eleven-vertex carboranes
The alkylation of 7,8-C2 B9 H11 2 by RX reagents and the protonation of the resulting products constitute a mechanistically complex system. When the alkylation is conducted in cold ( 7-38 > 7-40. More broadly, analysis of all nine possible isomers shows that the preference of carbon for low-coordinate sites is more important than the minimization of carbon-carbon bonding interactions [242]. By analogy to their isoelectronic and isolobal C3 B8 H11 and C2 B8 H11 2 counterparts, the nido-C4B7H11 cages are formal 6-electron donors and might be expected to face-bond to suitable transition metal acceptor fragments [for example, M(CO)3 where M is Cr, Mo, or W] to form stable Z5-coordinated MC4B7 complexes. However, at present none have been reported.
7.2.5 Arachno-C4B7H13 7.2.5.1 Synthesis Although methylene-bridged C3B7 cages, such as the previously described 6-12, 6-13, and 6-15 (Section 6.2), can be viewed as arachno 11-vertex systems, only one unambiguous example of an 11-vertex arachno-C4B7 carborane, in which all four carbon atoms are fully integrated into the skeletal framework, is currently known. The reaction of 6,7-C2B7H13
246
CHAPTER 7 Eleven-vertex carboranes
with acetylene in diethyl ether at 120 C affords, in addition to small amounts of other products [243,245], a 4% isolated yield of 7,8,9,11-H4C4B7H6-10-Me-m(7, 11)-CH2 (7-41) [246] that is formally an arachno-C4B7H13 derivative. Me C
7-41
CH2
C
B C
C B
B B
B
B B
It is possible to describe 7-41 as a pentacarbon carborane [246] if the sp3-hybridized bridging carbon is viewed as part of the electron-delocalized cluster. However, this seems something of a stretch and unnecessarily blurs the distinction between carboranes and the classical organoboranes. The same issue arises in other species that contain hydrocarbon bridging groups, as in the C3B7 cages mentioned above and their related derivatives (Table 5-7), and in certain metallacarboranes discussed in Chapter 13.
7.3 11-VERTEX CLOSO CLUSTERS 7.3.1 Closo-CB10H11 7.3.1.1 Synthesis
Parent 2-CB10 H11 is obtained as a minor product during the conversion of (Me3N)CB10H12 to 7-CB10 H13 by treatment with sodium or sodium hydride in refluxing THF, as described in Section 7.2 [6,7]. A more efficient route is the oxidation of the THF adduct Na3CB10H11(OC4H8)1.85 with iodine [5]: Na3 CB10 H11 ðOC4 H8 Þ1:85 þ I2 ! Naþ 2-CB10 H 10 þ 2NaI þ 1:85 C4 H8 O C-substituted derivatives of closo-2-CB10 H11 are synthetically accessible by several methods; for example, oxidation of nido-7-PhCB10 H12 with I2 in aqueous KOH produces 2-PhCB10 H10 [16,247]. Similarly, nido-7-(Me3N)CB10H12 can be oxidized to closo-2-(Me3N)CB10H10, and deprotonation of the iodo derivative 7-(Me3N)CB10H11-4-I followed by oxidation leads to the formation of closo-2-(Me3N)CB10H9-3-I [23]. An entirely different approach, oxidative cage closure, has been employed to generate closo-2[(Me3Si)2CH]CB10 H10 (7-42) from nido-7-[(Me3Si)2CH]CB10H11-9-SMe2, a compound obtained on reaction of B10H14 with silylacetylenes (Section 7.2) [15]. − H
(Me3Si)2CH
B C
H
B B
B B
B B
B B B
SMe2
(Me3Si)2CH Na, NaH, LiEt3BH, or Cp2Co C = CH B = BH
B C
B B
B B
B
B
B
7-42
B
B
A similar process is observed in the previously described reaction of nido-7-MeCB10H9-m(9,10)-CMeH (7-5) with PdCl2(PPh3)2, whose main product is a B-PPh3 derivative of the starting compound (Section 7.2). Also isolated from this reaction is closo-2-MeCB10H9-3-CMeHPPh3 (7-43), whose formation from 7-5 involves both cage closure and conversion of the bridging carbon atom into an exo-polyhedral phosphinoethyl substituent [39].
7.3 11-Vertex closo clusters
247
Me H
− Me
B C
H
Me B B
B B
B B
7-5
C PdCl2(PPh3)2 C = CH B = BH
B
B
B C
B B B
B
B
7-43
B
B B
C
PPh3
Me
B
B
7.3.1.2 Structure
Crystallographic studies of the closo-2-CB10 H11 parent ion have not been reported, but experimental and theoretical investigations [248,249] indicate that the cluster is nonrigid and has some non-triangular faces, thereby deviating from the closo-polyhedral geometry expected for an 11-vertex cluster having 12 skeletal electron pairs (Chapter 2). Indeed, the fact that the 11B NMR spectrum exhibits only three signals (instead of the expected seven) is evidence that the cage is fluxional in solution [250]. In this respect, the molecule resembles its isoelectronic closo-borane analogue B11 H11 2 , whose fluxionality is such that its low-temperature 11B NMR spectrum consists of a single peak [251,252]. Several crystal structures of closo-2-CB10 H11 C- and B-substituted derivatives are available (Table 7-6), all of which show the expected deltahedral cage structure shown in Figure 1-1 and are analogous to that of B11 H11 2 [253], with the cage carbon atom occupying one of the two available low-coordinate vertexes. No other isomer of CB10 H11 has been characterized, in accordance with theoretical calculations that show significantly higher energies for other cage geometries [254].
Table 7-6 Closo-2-CB10H11 Derivatives Compound Synthesis and Characterization 2-CB10 H11 (Li, Na, K, Rb, Cs salts) 2-CB10 H11 2-PhCB10 H10 2-(Me3Si)2CH-2-CB10 H10 2-(Me3Si)2CH-2-CB10H9-6-SMe2 2-(Me3Si)2CH-2-CB10H8-4-OH-6-SMe2 2-Me3N-CB10H10 2-Me3N-CB10H9-3-I 2-MeCB10H9-3-CMeHPPh3 M[porphyrin-CH(OH)](CB10H11) (M ¼ Co, Cu, 2H) Detailed NMR Studies 2-CB10 H11
Information
References
S, S IR S, S, S, S, S, S, S, S,
[558] [5,559] [248] [16,247] [15] [15] [15] [23] [23] [39] [422]
B
IR, UV
X, H, B, C X, H, B, IR X, H, B, IR X, H, B, IR H, B H, B X, H, B, P, MS H, cytotoxicity
[250,560] Continued
248
CHAPTER 7 Eleven-vertex carboranes
Table 7-6 Closo-2-CB10H11 Derivatives—Cont’d Compound Other Experimental Studies 2-CB10 H11
2-CB10 H11 (Li, Na, K, Rb, Cs salts) (Me3N)CB10H10(CO) (Me3N)CB10H10 (Me3N)CB10H10 (Me2PrN)CB10H10 MðenÞ3 2þ CB11 H12 ]2 (M ¼ Co, Ni)
References
E IR, H IR, Raman UV Thermogravimetric analysis Solubility, thermolysis IR, UV
[250] [5] [249] [6] [561] [558] [6] [6] [5] [5] [562]
IR, H, B IR, H IR, MAG, UV, thermal decomposition
Theoretical Studies Molecular and electronic structure calculations CB10 H11 CB10 H11 (all isomers) 2-CB10 H11 2-Me3N-CB10H10 2-Me3N-CB10H9-3-I NMR calculations CB10 H11 1
Information
11
Vibrational analysis; electron density distribution; NOT closo ab initio DFT; nonrigidity Heat of formation, charge distribution Heat of formation, charge distribution
[248]
B
[560] 13
S, synthesis; X, X-ray diffraction; H, H NMR; B, B NMR; C, C NMR; P, UV-visible data; E, electrochemical data; MAG, magnetic susceptibility.
[254] [249] [23] [23]
31
P NMR; IR, infrared data; MS, mass spectroscopic data; UV,
7.3.2 Closo-C2B9H11 7.3.2.1 Synthesis The 11-vertex closo-carborane 2,3-C2B9H11, the only known isomer, can be obtained by dehydrogenation of nidoC2B9H13 at 100 C or by the reaction of nido-7,8- or 7,9-C2 B9 H12 with polyphosphoric acid at 70 C; nidoRR0 C2B9H11 carboranes undergo analogous processes to give the corresponding C-substituted derivatives (Table 7-7) [116,225–227]. As mentioned in the preceding section, protonation of 7,9-RR0 C2 B9 H10 anions (R ¼ H, Me, Ph) with H2SO4 at ambient temperature affords closo-2,3-RR0 C2B9H9 products (see Figure 7-3B) [118]. When conducted with nido-7,9-C2B9H11-n-X ions (n ¼ 1 or 6; X ¼ Br or I), only the 2,3-C2B9H10-10-X product is obtained (Figure 7-12), a finding consistent with a single cage-closing mechanism in which a loss of the bridging proton is followed by the formation of bonds between B(8) and B(10) and B(11) [118]. Oxidative closure of 7,9-C2 B9 H12 to form 2,3-C2B9H11 can be accomplished using SnCl2 [255,256], and the carborane has also been obtained in 49% yield by decomposition of bis(dicarbollyl) nickel, Ni(C2B9H11)2 at 300 C [49]. The B-octamethyl species nido-7,8-H2C2B9H3Me8, mentioned earlier, is dehydrogenated on contact with silica gel in pentane solution and produces two isomers of closo-1,8-H2C2B9Me8 [92]. As described in Chapter 12, oxidation of the aluminacarborane 3,1,2-EtAl(Me2C2B9H9) by SnCl4 extracts the metal and forms the 11-vertex cluster 2,3Me2C2B9H9 [47].
7.3 11-Vertex closo clusters − 8
H
B
7
C
11 3
B
4
B
B B
2
6
−
B
9
C 10 B
H+
B B
5
B
B 1B
X
B B = BH C = CH X = Cl, I
7,9-C2B9H11-1-X−
B
H+
B B
H
B
C
C
B
B
C
249
C B
B B
B B
B
B B
X
X 2,3-C2B9H10-10-X
7,9-C2B9H11-6-X−
FIGURE 7-12 Formation of 2,3-C2B9H10-10-X (X ¼ Br, I) via protonation and cage closure of nido-7,9-C2B9H11-1-X and -6-X anions.
Table 7-7 Closo-C2B9H11 Derivatives Compounds Synthesis and Characterization Single-Cage Derivatives No substituents on boron 2,3-C2B9H11
2,3-MeC2B9H10 2,3-Me2C2B9H9
2,3-Me4C2B9H7 2,3-Me2C2B9H9 PPh3 2,3-Me2C2B9H9 NEt3 2,3-Me2C2B9H9OH
Information
References
S, H, B, C S[thermal decomp of Ni(C2B9H11)2] S, B(“correct”), IR S, H, B, IR, R, MS S, B, IR, MS S, B B, R(Lewis bases) IR, Raman ED Dipole moment S, H, B, IR, MS, R S, B S, H, B, C S, H, B, IR, MS, R S, B, IR, MS S, B X S, E S, B, IR, MS S S S
[118] [49] [255] [116] [227] [226] [18] [249] [120] [563] [116] [226] [47] [116] [227] [226] [257] [265] [227] [116] [116] [116] Continued
250
CHAPTER 7 Eleven-vertex carboranes
Table 7-7 Closo-C2B9H11 Derivatives—Cont’d Compounds
Information
References
2,3-Me2C2B9H9 EtNC 2,3-R2C2B9H9 (R ¼ Me, Ph) 2,3-MePhC2B9H9 2,3-PhC2B9H10
S S, S, S, S, S, S S S S S, S,
[116] [118] [118] [18] [226] [116,225] [116] [116] [116] [116] [116] [118]
2,3-PhC2B9H10 PPh3 2,3-PhC2B9H10 NEt3 2,3-PhC2B9H10 OH 2,3-PhC2B9H10 EtNC 2,3-(p-BrC6H4)C2B9H10 2,3-(FC6H4)2C2B9H9 Hydrocarbon substituents on boron 2,3-C2B9H10-n-Me (n ¼ 1, 4, 8) 2,3-H2C2B9Me9 O- or S-containing substituents on boron 2,3-C2B9H7-4,7-(OH)2 2,3-C2B9H7-4,7-(OD)2 2,3-C2B9H6Br-4,7-(OH)2 2,3-C2B9H6Br-4,7-(OD)2 2,3-Me2C2B9H8-4-OMe 2,3-Me2C2B9H8-4-OH 2,3-Me2C2B9H7-4,7-(OH)2 2,3-Me2C2B9H7-4,7-OC6H4O 2,3-Me2C2B9H7-4,7-(OH)2 2,3-Me2C2B9H7-O2C6H4 2,3-Me2C2B9H7-O2C4H8 2,3-Me2C2B9H7-4,7-O-(CHMe)2-O 2,3-Me2C2B9H7-4,7-O-C6H4-O 2,3-Me2C2B9H7-4,7-O-C6H3Cl-O 2,3-Me2C2B9H7-4,7-O-C2H4-O 2,3-Me2C2B9H7-4,7-OC2H4O 2,3-Me2C2B9H7-4,7-(MeCHO)2CH2 2,3-Me2C2B9H6-10-Br-4,7-(OH)2 F-, Cl-, Br-, or I-containing substituents on boron 2,3-(FC6H4)2C2B9H8-4-F 2,3-C2B9H10-10-Cl
H, B, C H, B, C B, R(Lewis bases) B H, B, IR, MS, R
H, B, IR, MS*, R H, B, C
S, H, B, C S, H,B,C
[118] [92]
S, H, B, IR, MS,R S, H, B, IR, MS,R S, H, B, IR, MS,R S, H, B, IR, MS,R S (from 1,7-Me2C2B10H10), H, B, IR S, E S, H, B, MS, IR,R S, E S, E S, E S, H, B, MS S, H, B, MS S, H, B, MS S, H, B, MS S, H, B, MS S, H, B, MS S, E S, E S, H, B, MS X
[263] [263] [263] [263] [256] [265] [266] [265] [265] [265] [263] [263] [264] [264] [264] [264] [265] [265] [263] [258]
S, H, B, C S, H, B, C
[118] [118] Continued
7.3 11-Vertex closo clusters
251
Table 7-7 Closo-C2B9H11 Derivatives—Cont’d Compounds
Information
References
Multi-Cage Derivatives (2,3-C2B9H10)2 O(2,3-Me2C2B9H8)2 (2,3-Me2C2B9H7-O-)2 2,3-C2B9H10-4-(1-1,10-C2B8H9) 2,3-R2C2B9H8-4-(1,2-C2B10H10R0 ) (R ¼ H, Me; R0 ¼ H, Me)
S, S, S, S, S,
[227] [266] [263] [115] [115]
B, IR, MS H, B, C, MS H, B, IR, MS,R H, B, IR, MS H, B, IR, MS
Detailed NMR Studies 2,3-C2B9H11 2,3-C2B9H11 2,3-C2B9H11 2,3-C2B9H11 2,3-Me2C2B9H9 2,3-EtRC2B9H9 R ¼ Me, Et 2,3-PhC2B9H10 2,3-Ph2C2B9H9 2,3-(FC6H4)C2B9H10 2,3-(m/p-FC6H4)C2B9H10 2,3-Me2C2B9H7-4,7-(OH)2 2,3-Me2C2B9H7-4,7-OC2H4O 2,3-Me2C2B9H7-4,7-OC6H4O 2,3-Me2C2B9H6-10-Br-4,7-(OH)2 2,3-Me2C2B9H3D4-4,7-OC6H4O
H(2d), B(2d) H B C B B C B F(electronic properties) F(electronic properties) B, MS B, MS B, MS B B, MS
[564] [565] [566] [140,328] [566] [566] [328] [566] [261] [260] [566] [566] [566] [566] [566]
Other Experimental Studies 2,3-C2B9H11 2,3-Me2C2B9H7-4,7-(OH)2 2,3-R2C2B9H9 (R ¼ H, Me
Reaction with Me2S Condensation with glycols Metal insertion
[49] [264] [262]
ab initio Hþ charge Localized orbitals Isomer stabilities DFT; nonrigidity ab initio Geometry Geometry MNDO, extended Hu¨ckel, inner-shell electron energy loss spectra
[120] [565] [567] [568] [249] [254,569] [118] [118] [570]
Theoretical studies Molecular and electronic structure calculations 2,3-C2B9H11
C2B9H11 isomers 2,3-Me2C2B9H6-10-Br-4,7-(OH)2 2,3-C2B9H10-n-X (X ¼ F, Cl, Me; n¼ 1, 4, 8, 0) 2,8-C2B9H11
Continued
252
CHAPTER 7 Eleven-vertex carboranes
Table 7-7 Closo-C2B9H11 Derivatives—Cont’d Compounds Isomerization calculations 2,9-C2B9H11 C2B9H11 isomers
NMR calculations 2,3-C2B9H11
2,3-R2C2B9H9 (R ¼ Me, Ph) 2,3-MePhC2B9H9 2,3-C2B9H10-10-Cl 2,3-C2B9H10-n-Me (n ¼ 1, 4, 8) 2,3-(FC6H4)2C2B9H8-4-F 2,3-(FC6H4)2C2B9H9 2,3-C2B9H7-4,7-(OH)2
Information
References
Isomerization Cage rearrangement SCF isomerization Cage rearrangement mechanism Cage rearrangement
[571] [544] [259] [572] [573]
B–H coupling 11 B NMR IGLO GIAO; NMR þ GIAO; NMR þ GIAO; NMR þ GIAO; NMR þ GIAO; NMR þ GIAO; NMR þ GIAO; NMR þ 11 B NMR
[574] [560] [140] [118] [118] [118] [118] [118] [118] [118] [560]
geometry geometry geometry geometry geometry geometry geometry
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; F, 19F NMR; 2d, two-dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; UV, UV-visible data; E, electrochemical data; ED, electron diffraction.
7.3.2.2 Structure The C2v polyhedral geometry shown in Figure 1-1, with both carbons occupying low-coordinate vertexes adjacent to the unique B(1) (the only 7-coordinate boron in the C2Bn-2Hn family), was originally deduced from NMR spectra [18,116] and was later confirmed by an X-ray crystallographic analysis of the C,C0 -dimethyl derivative [257]. Detailed 11B, 13C, and 1H NMR studies have also been published (Table 7-7). No X-ray investigation of the parent carborane has been reported, but a gas-phase electron diffraction study of this compound supports the C2v structure [120]. An X-ray diffraction analysis [258] of 2,3-Me2C2B9H6-10-Br-4,7-(OH)2 (7-44) revealed a cluster geometry that is distorted from the “normal” closed polyhedron due to stretching of the B(1)-B(4) and B(1)-B(7) distances and is intermediate between the closo and nido structures. The partial cage-opening is attributed to the donation of electron density from the hydroxyl group lone pairs to the cage skeleton [258], consistent with earlier assumptions that Lewis base-C2B9 adducts have an open cluster geometry [116]. 1
Me
B
3
C
6
B
B
7-44
2
C
5 4B
8B
HO
B7 B11
B9
B 10
OH
Br
Me
7.3 11-Vertex closo clusters
253
7.3.2.3 Cage isomerization The 11B NMR spectrum of 2,3-C2B9H11 in solution exhibits the expected 1:4:2:2 pattern consistent with a non-fluxional cluster framework, in contrast to its monocarbon analogue CB10 H11 described earlier. No evidence for cage rearrangement has been seen experimentally, presumably because any higher-energy isomers that might form would quickly revert to the stable 2,3 system. However, a SCF theoretical investigation indicates that the energy barrier for interconversion is indeed low, and also predicts that the 2,9- and 2,8 isomers may be metastable and may be capable of stable existence [259].
7.3.2.4 Cage opening
As was noted in Section 7.2, conversion of 2,3-C2B9H11 to the nido-7,9-C2 B9 H12 anion can be accomplished via reaction with BH4 [115], while alkyllithium reagents [115] and dimethyl sulfide [49] afford nido-7,9-C2B9H11-9-L (L ¼ Me, n-C4H9) and nido-7,9-C2B9H11-10-SMe2 respectively. Other electron donors such as NEt3, PPh3, and OH reversibly form nido-7,9-RR0 C2B9H9L adducts (R, R0 ¼ H, Me or Ph) [116,260]. In another example, interaction between 2,3C2B9H11 and its C-substituted derivatives, and C-lithiated closo-carboranes forms nido,closo linked-cage anions 7-45 and 7-46 [115]. R
R
R
B C
C B B
B
B
R
B B
Li
B C
+ B
B
B B
B B
B B
B
B Et2O
B
− R
B
C
B
B
C B C
B
B
B
H
B B B
B
B = BH R = H, Me R = H, Me
B B
C
C
B B
R
B
B
B B
7-45
R
H
B C
C B B
B
B
B
C H
B
C
B
+
B
Et2O
B B
B
B B C
B = BH R = H, Ph
−
B
B
B B
Li
B B
H
H
B C
B C
B B
B B
C
B
R
B B
B
B
H
B
B B
7-46
The electronic interaction between the cage and attached m- and p-fluorophenyl groups in 2,3-(FC6H4)C2B9H10 derivatives has been explored, and the carborane unit is found to be electron-accepting via an inductive mechanism [260,261]. Opening of the closo-C2B9 cluster is also observed with electron-rich d10 transition metals: reaction of 2,3Me2C2B9H11 with reagents such as Ni(1,5-C8H12)2, Pd(Me3C-NC)2, and Pt(PMe2Ph)3 in cold toluene yield 12-vertex icosahedral LnM(Me2C2B9H9) metallacarboranes (Chapter 13) [262].
7.3.2.5 Substitution at boron Treatment of 2,3-Me2C2B9H9 with chromic acid or other oxidants, when carried to completion, generates arachnoMe2C2B7H11 (Chapter 5) [116]. When conducted under air-free conditions at 0 C in benzene, this reaction affords the bis(hydroxy) derivative 2,3-Me2C2B9H7-4,7-(OH)2 (7-47) [263]; bromination of this compound with Br2 forms only the previously cited B(10)-Br derivative 7-44, whose structure has been crystallographically established [258]. Pyrolysis of 7-47 above 150 C gives a dimer that is linked by two oxygen atoms that bridge the B(4)-B(40 ) and B(7)-B(70 ) positions [263]. When 7-47 is heated with organic diols, cyclic condensation products (7-48) are obtained [263] via displacement of the OH groups, as shown by experiments with 18O-labeled glycols [264].
254
CHAPTER 7 Eleven-vertex carboranes
1
Me
B
3
C
6
5
B
7-47
4B
2
C
B B7
8B
B11
HO
B 10
B
Me
Me HO RHC
OH
C
C 4B
B9 OH
B
B
CHR 8B
O H
B
B B
R
7-48
O
B C
Me
C H R
One-electron electrolytic reduction of 7-48-type derivatives produces ESR-active radical anions in which the extra electron is associated with the carborane cage, while the addition of a second electron yields an unstable dianion [265]. The monohydroxyl compound 2,3-Me2C2B9H8-4-OH can be prepared by treatment of 2,3-Me2C2B9H9 with sodium periodate and 2M HCl in benzene. However, if conducted with acetic acid in benzene, the product is the 4,7-(OH)2 derivative; extended reaction times afford (Me2C2B9H8)2O, in which the two carborane units are linked by a single B2 2O2 2B bridge, as the only isolated product [266]. Other than the reaction with Br2 mentioned earlier, direct halogenation of closo-2,3-C2B9 carboranes has not been examined. B-halogenated derivatives can be prepared via cage closure of nido-C2B9H11-X anions, as described above.
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7.3 11-Vertex closo clusters [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [67] [68] [69] [70] [71] [72] [73]
255
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CHAPTER 7 Eleven-vertex carboranes
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7.3 11-Vertex closo clusters [395] [396] [397] [398] [399] [400] [401] [402] [403] [404] [405] [406] [407] [408] [409] [410] [411] [412] [413] [414] [415] [416] [417] [418] [419] [420] [421] [422] [423] [424] [425] [426] [427] [428] [429] [430] [431] [432] [433] [434] [435] [436] [437] [438] [439] [440] [441]
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CHAPTER
Icosahedral carboranes: Closo-CB11 clusters
8
8.1 OVERVIEW The prototype boron clusters, the extremely stable B12H122 dianion and its neutral dicarbon counterparts 1,2-, 1,7-, and 1,12-C2B10H12 (o-, m-, and p-carborane respectively), are 26-electron icosahedral systems that are closely related to elemental boron, all of whose allotropic forms incorporate B12 icosahedra. Their isoelectronic analogue CB11 H12 is similarly resistant to cage degradation and offers important synthetic advantages: unlike B12 H12 2 , it is polar and can be selectively functionalized at its carbon vertex or at three distinct boron sites, and in contrast to the electrically neutral C2B10H12 isomers, it is highly soluble in water. In recent years CB11 H12 , and especially its B-polyhalogenated derivatives, have assumed an increasingly important role as weakly coordinating anions that can form stable salts with extremely electrophilic cations such as R3Siþ, protonated benzene, C60 þ , and H3Oþ. In addition, the water-solubility and stability of CB11 H12 derivatives in aqueous media make them potentially useful in boron neutron capture therapy (BNCT). These and other evolving applications of monocarbon carborane chemistry are elaborated in Chapters 16 and 17. Comprehensive reviews of the chemistry of CB11 H12 [1] and related neutral CB11 cluster systems [2] have been published.
8.2 SYNTHESIS AND STRUCTURE 8.2.1 Parent CB11H12 The original synthesis by Knoth [3,4] involved either disproportionation of Csþnido-7-CB10 H13 (prepared from B10H14 as described in Section 7.2) or boron insertion using triethylamine-borane: 300-320 C CB10 H 13 ! CB10 H11 þ CB11 H12 þ 2H2 Et3 NBH3
CB10 H 13 ! CB11 H12 180 C
The latter approach was improved by Czech workers [5], who found that heating 7-(Me3N)CB10H12 with Et3NBH3 at 180-200 C gives (Me2NH)CB11H11, which is methylated to produce (Me3N)CB11H11; reduction of the latter compound with sodium in liquid ammonia affords the CB11 H12 anion. A more recent, and important, synthesis is based on the conversion of B11 H14 (prepared [6] from the inexpensive bulk chemical NaBH4) to CB11 species via carbon insertion [7,8]: CHCl3
B11 H 14 ! CB11 H12
C2 In a different type of carbon insertion, the reaction of HC 2C5H4N with closo-B11 H11 2 yields a 1-(NC5H4CH2)-CB11H10 product in which one carbon is incorporated into the cage and the pyridyl nitrogen is bound to a boron vertex [9]. The icosahedral structure of the CB11 H12 ion (slightly distorted from ideal Ih symmetry), shown in Figure 1-2, is well established from X-ray diffraction, NMR, and other spectroscopic studies on the parent species and on a number Carboranes. DOI: 10.1016/B978-0-12-374170-7.00010-0 © 2011 Elsevier Inc. All rights reserved.
267
268
CHAPTER 8 Icosahedral carboranes: Closo-CB11 clusters
of derivatives (Table 8-1) [1,10]. In some of its salts, B2 2H X hydrogen-bonding interactions are found between the carborane and the associated cation (even forming 3-D networks) [11], while in others such bonding is effectively absent [12]. The CB11 H12 cluster can function as an electron donor, for example, forming a charge-transfer complex with a methyl viologen [13]. The nature and extent of cation-CB11 H12 interactions is significant because of the interest in this carborane and its derivatives as weakly coordinating anions, a topic that is further developed below and in Chapter 17.
Table 8-1 CB11H12 Derivativesa Compound Synthesis and Characterization CB11 H12 derivatives as weakly coordinating anions CB11 anions (weakly coordinating anions) CB11 H12 C-aryl derivatives C7 H6 þ CB11 H12 Non-transition metal derivatives No substituents on boron CB11 H12
1-(H2N)CB11 H12 (n-C8H17)3NHþ CB11 H12 4,4-bipyridineHþ CB11 H12 salts H-bonded networks with N2 2H N and C2 2H N interactions [2,6-(MeO)2C6H3]SnPh2 þ CB11 H12 stabilization by pincer-type triaryltin ligand M[151.2.2]cryptateþCB11 H12 Ni(TMTAA)]3 M ¼ Na, K; Ni(TMTAA) ¼ tetraazacyclotetradecinenickel(II) [N, N0 -dialkylimidazolium]þ CB11 H12 H2C2N3R(HN2)þ CB11 H12 R ¼ H, Me imidazolium, triazolium salts
Information
References
Review (2002) Review of structural and NQR data Review Review of NLO properties of carboranes
[8] [113] [24] [34]
S, H X(variable temperature), thermal studies B C E IR IR (actual spectrum) MAG, COND Raman Raman (actual spectrum) UV Charge-transfer complexes with MVþ PF6 MV ¼ methyl viologen; triplet excited state generated by 308 nm laser excitation S, X, B, C, MS, IR (actual spectrum), Raman (actual spectrum) IR (nN-H as a measure of acid strength) S, X, IR
[5] [143]
[73] [11]
S, H, B, Sn, IR, MS
[150]
S, X
[12]
S, X, H, IR S, X, UV, IR S(aqueous), H, C, MS
[90] [152] [91]
[3,5,32,46,144,145] [144,146] [145] [3,147–149] [144] [147] [148] [144] [147,149] [13]
[144]
Continued
8.2 Synthesis and structure
269
Table 8-1 CB11H12 Derivativesa—Cont’d Compound
Information
References
Ph[2,6-(MeOCH2)2C6H3]Sn(m-CH2)2Sn[2,6(MeOCH2)2C6H3]Ph2þ [CB11 H12 ]2 catalyst for acetylation of alcohols CB11 H12 (2-10B) MeCB11 H11 PhCB11 H11 (p-BrC6H4)CB11 H11 (H3NCH2)C2 B10 H11 þ (MeCH2)CB11 H11 5NHCH2)C2 B10 H11 þ (MeCH2)CB11 H11 (Me2C5 þ Ag (MeCH2)CB11 H11 RCB11 H11 (R ¼ Li, H, Et, Ph3Si, CF3, Ph2P, CH2Ph) RCB11H10-2-R0 R ¼ H, Ph, NH2, NH(CMe2C6H4-pMe); R’ ¼ NH2, Ph, NMe3 þ Mþ(NC)CB11 H11 X ¼ H, F. Cl, Br, I Mþ ¼ Csþ, Et4Nþ
S, X, H, B, C, Sn, MS
[153]
S, B S, X, H, B, C, MS S, X, H, B, C S, H, B, C S, X S, X S, X S, H, B, P S,X(Ph,NH2; NH2,Ph; Ph, NMe3 þ ),H, B, IR, MS, DSC, pKa S, X(Cs,H; Et4N, Cl), H, B, C, IR, MS, Raman H S, H, B H, B, MS S, H, B, IR, R S, X, H, B, IR, thermal analysis X S, B H, B, MS S, H, B, IR, R S, H, B, IR, R X S, H, B, IR, R S, X, B, C, H, IR
[32] [17] [25,26] [50] [154] [154] [154] [14] [47]
[3] [155] [5] [23] [156] [157] [32] [5] [23] [23] [158] [23] [39]
S, S, S, S,
[23] [15] [15] [15]
(Me3Si)CB11 H11 (H3N)CB11H11 (Me3N)CB11H11
(Me3N)CB11H11 (2-10B) (Me2HN)CB11H11 (NH2Me)CB11H11 (Me2N)CB11 H11 (NHMe)CB11 H11 (C5H11-C7H8N)CB11 H11 (C5H11-C7H8N ¼ pentylquinuclidine) (NHMeEt)CB11H11 LCB11H11 (L ¼ H3N, MeH2N, Me2HN, Me3N) (Me2S)CB11H11 LCB11 H11 [L ¼ H2N, MeHN, Me2N, MeO, HO, HO(O)C] LCB11 H11 (L ¼ MeS, HS) RCB11 H11 [R ¼ NHC(O)Me, NMe3 þ , NHMe2 þ , NH3 þ , succinylamino] (succinylamido)CB11H11 131I biodistribution in mice Me3NHþ CB11 H12 [N, N0 -dialkylimidazolium]þ CB11 H12 [N, N0 -dialkylimidazolium] þ RCB11 H11 (R ¼ Me, Et, C3H7, n-C4H9) ionic liquids
H, H, H, H,
B, IR, R B, MS B, MS, IR B
[22]
S, H, B, IR S, H, B, MS
[15] [159]
S, H, B, MS S (improved: 2 steps from NaBH4 þ BF3 OEt2 via nido-B11 H14 ) S, X, H, IR S, X(Me, Et), H, IR
[159] [7] [90] [90] Continued
270
CHAPTER 8 Icosahedral carboranes: Closo-CB11 clusters
Table 8-1 CB11H12 Derivativesa—Cont’d Compound þ
EtMeN2 C3 MeH2 CB11 H12 1,2-Me2-3-Et-imidazolium ionic liquid RNC5H5þ CB11 H12 R ¼ n-butyl-, n-hexyl-, n-octyl N-alkylpyridinium salts ionic liquids RNC5H5þ R0 CB11 H11 R0 ¼ Me, n-C4H9 N-alkyl pyridinium salts ionic liquids C5 H11 -NC5 H5 þ CB11 H12 N-pentylpyridinium ionic liquid reaction medium in catalytic dehalogenation of aromatic halides [2,6-(MeOCH2)C6H3]SbClþ CB11 H12 organoantimony (III) O2 2C2 2O pincer ligand Na[2.2.2]cryptate (CTV)þ CB11 H12 (CTV ¼ cyclotriveratrylene) supramolecular M(CTV)2(H2O)3ðDMFÞ2 þ CB11 H12 M ¼ Na, Cs, Rb; CTV ¼ cyclotriveratrylene K(CTV)(H2O)(OH) ðDMFÞ2 þ CB11 H12 CTV ¼ cyclotriveratrylene X(CH2)nCB11 H11 (X ¼ Cl, n ¼ 3-7; X ¼ Br) XCB11 H11 (X ¼ F, Cl, Br, I) XCB11 H11 (X ¼ Cl, Br, I) (MeCHOHCH2)CB11 H11 (NMe5 5X)CB11 H11 (X ¼ CHOH, CHO, C(OH)Ph, (O)Ph) (p-XC6H4)CB11 H11 (X ¼ H, F, Cl, Br, I, Ph) 5CH, Ph, (HOCR)CB11 H11 (R ¼ H, n-C3H7, MeCH5 C4H4O) D or hydrocarbon substituents on boron CB11H6-7,8,9,10,11,12-D6 CB11H11-2-R (R ¼ Ph, C6H4Me, F, O(CH2)4Cl, NMe3) MeCB11 H11 RCB11H-2,3,4,5,6,7,8,9,10,11-Me10 R ¼ H, Me RCB11 Me11 R ¼ H, Me (Me2N)CB11H10-2-CH2NH(CHMe2)2 (Me2NH)CB11H10-2-CH2Cl MeCB11H10-12-Me CB11H11-12-R (R ¼ Me, Et, n-C4H9, hexyl, Ph) CH HCB11H10-12-C CHÞ2 HCB11H9-7,12-ðC þ R CB11 Me5 X6 (R ¼ Me, CHMe2; X ¼ Cl, Br) very strong alkylating agents Me3 N3 Cl6 þ HCB11 Me5 Br6 phosphazene cation
Information
References
S, H
[160]
S, X(n-C4H9), H
[160]
S, H
[160]
S, H, B, C, IR
[161]
S, X, H
[162]
X
[124]
S, X
[163]
S, X
[163]
S, H, B, C, IR, MS S, B, C, UV, MS S(improved; methyl triflate), X S, H, IR S, H, B, IR, R
[31] [28] [30] [18] [23]
S (insertion of arylhalocarbenes into nido-B11 H14 ), X(F), H, B, C, IR, UV, MS S, H, IR
[27]
S, S, S, S, S, S, S, S, S, S, S, S,
[32] [48] [29] [29] [29] [49] [164] [33] [33] [36] [36] [69]
B(2d) B(2d), H, F(F), X(O[CH2]4Cl) H, B, C, MS, E H, B, C, MS, E H, B, C, MS, E X, B, H X, B(2d), H H, B, C, IR, MS H, B, C, IR, MS X, H, B, C, IR, Raman, DSC X, H, B, C, IR, Raman, DSC X(CHMe2, Br), H, C, IR
S, X, H, P
[18]
[165] Continued
8.2 Synthesis and structure
271
Table 8-1 CB11H12 Derivativesa—Cont’d Compound þ
H[P3C3(CMe3)] HCB11 Me5 X6 X ¼ Cl, Br stable salt of phosphabenzene cation Me2Si(bicyclo2C6 H2 Me3 þ HCB11 Me5 Br6 silylC6H2Me2C6H3)2 stabilized allyl cation (Me2N)CB11H10-7-CH2Ph (Me3N)CB11H9-2-Ph-8-X (X ¼ I, CH2¼CHCH2) PhCB11H10-12-Ph CB11H11-12-C7H6
(MeC6H4-C6H4)CB11H10-12-C6H4Me (C6H12O2)CB11Me10-12-p-C6H4Br HCB11 Me11 LiCB11 Me11 PtMe[P(CHMe2)3]þ 2 HCB11 Me11 þ Rh6(Pcy3)H16 HCB11 Me11 X(CH2)nCB11 Me11 (X ¼ Cl, Br; n ¼ 3,4,7) Me2BzNþ (CH2)nCB11 Me11 5CH)(CH2)n-2CB11 Me11 (CH25 5CH(CH2)n-2]CB11 Me11 Csþ (n ¼ 2-7) [CH25 5CH(CH2)n-2]CB11 Me11 Liþ polymers from [CH25 [MeC(O)O]CB11 Me11 Ph3Cþ HCB11 Me11 Rh(H)2(H2)ðPR3 Þ2 þ HCB11 Me11 0 RCB11R0 11 R ¼ H, Me; R ¼ Me, Et XCB11 Me11 ,XCB11 H5 Me6 (X ¼ Cl, Br, I) Sn(n-C4H9)þ 3 CB11 Me12 TTFþ CB11 Me12 TTF ¼ tetrathiofulvalene CB11 Me12 stable radical
1-(HMe2N)CB11H11 5CH, Me2N(CH2)2] 1-RCB11Me11 [R ¼ Me3N, H2C5 C] 5CH, Me3SiC 1-RCB11Me11 [R ¼ BrHC5 2CH ¼ CH2 2CB11 Me11 2 Me11 B11 C2 C2 2C 2CB11 Me11 2 Me11 B11 C2 • Me11B11C2 2CH5 5CH2 2CB11 Me11 stable biradical • C2 Me11B11C2 2C 2CB11 Me11 stable biradical CB11 Me12 CB11 Me11 0 reactive intermediate in aromatic substitution ArH þ CB11Me11 ! ArCB11 Me11 þ Hþ
Information
References
S, X, H, P
[166]
S, X, H, C
[95]
S, H, B(2d), MS S, B(2d[I]), H* S, X, H, B, C S, H, B, C, IR, MS, NLO [1st hyperpolarizability (b), later corrected (J. Am. Chem. Soc. 2000, 122, 11274)] S, X S, X, H, B, C S, H, B, C, IR, MS S (improved) S, X, H, B, C, MS X S, H, B, C, IR, MS S, H, B, C, IR, MS S, H, B, C, IR, MS S (improved) S, H, B, C, IR S, H, B, C, IR, MS S, X, H, B, MS S, X S, H, B, C, IR, MS S(improved; methyl triflate), X S, X, H, B, Sn, MS, IR, UV X S, X, E, ESR, UV Dopant in electron-delocalized p-doped hexaarylbenzene cation-radical salts S, H, B, C, IR, MS S, H, B, C, IR, MS S, H, B, C, IR, MS S, H, B, C, IR, MS S, H, B, C, IR, MS S, IR, MS S, IR, MS S, X, H, B, E, IR, MS (electrospray)
[32] [43] [40] [33]
[38] [41] [31] [20] [130] [167] [31] [31] [31] [20] [20] [31] [133] [168] [59] [30] [80] [142] [103] [140]
[141] [141] [141] [141] [141] [141] [141] [19] [135] Continued
272
CHAPTER 8 Icosahedral carboranes: Closo-CB11 clusters
Table 8-1 CB11H12 Derivativesa—Cont’d Compound
Information
References
X
[131]
S, X, EXAFS, B, C, Sn, Pb
[128]
S, X, H, C Catalyst for pericyclic rearrangements S, X, H, B, MS S, X, H, B, MS S, X, H, B, MS S, X, C, H, IR
[139] [136] [117] [117] [117] [81]
S, X, H, C
[70]
S, H, B, C CPh, Cl), H, B, C, IR (actual S, X(C spectrum), Raman (actual spectrum), MS S, H, B, C, IR, Raman, MS S, H, B, C, IR, Raman, MS
[134] [35]
S, H, B, C, IR, Raman, MS
[35]
N- or P-containing substituents on boron CB11H11-7-NR3 R ¼ H, Me CB11H11-n-NMe3 (n ¼ 2,12) (NC5H4-CH2)CB11H10 (NB)
S, H, B(2d) MS S, H, B(2d) MS S, X, H, B, MS
[32] [32] [9]
O- or S-containing substituents on boron HCB11 ðOHÞ11 CB11H11-12-OH CB11 HðOHÞ5 Br6 CB11H11-2-OEt CB11H11-12-O(CH2)2Oþ dioxane zwitterion CB11H10-m(1,2)-(CH2)3CHOMe R ¼ OMe, PPh3 MeCB11H10-2-CH(OMe)Me 1,2-Me2N-CB11H10-2-R R ¼ SH, SMe2 CB11H11-12-L (L ¼ Me2S, MeSCH2SMe)
S, S, S, S, S, S, S, S, S,
B, MS H, B, IR, MS X, IR H, B X, H, B, IR, MS X, H, B, C, IR X, H, B, C, IR X, B(2d), H, MS H, B, MS, IR, R
[169] [170] [75] [7] [170] [171] [171] [172] [15]
S, B, F, MS S, X(Ag salt), B, C, F S, B, F, MS
[46] [45] [46]
/M(C6H6)þ 2
þ
CB11 Me12 M ¼ Tl, Cs, Rb, Li(MeC6H5) K, Na Me3Mþ CB11 Me12 (M ¼ Ge, Sn, Pb) Me3Mþ Me interactions o-(phenothiazinyl)2C6 H4 þ CB11 Me12 Liþ CB11 Me12 HCB11Me9-7,12-Cl2 ZrCp*2ðOHÞ2 þ [HCB11Me10-12-Cl]2 {Pt[CHMe2)3P]2(m-Cl)þ}2 HCB11Me10-12-C6H4F HCB11 Me11 (PPh3)2HIr2þ H(arene)þ HCB11 H5 X6 R ¼ H, Me; X ¼ Cl, Br; arene ¼ benzene, toluene, m-xylene, mesitylene, hexamethylbenzene Rþ MeCB11 Me5 X6 Rþ ¼ CMe3 þ CMe3 þ , CMe2Etþ, cyclo-MeC5 H8 þ ; X ¼ Cl, Br isolation of CMe3 þ cation, etc., at room temperature CB11Me11 boronium ylide; naked B(12) vertex CPh, C CSiMe3, F, I, Cl, Br CB11H11-12-R R ¼ C CR R ¼ Ph, SiMe3 PhCB11H10-12-C CPh, X ¼ F, Cl, Br; CB11H10-7-R-12-X R ¼ C CSiMe3, X ¼ F R ¼ C CPh)22 CB11H10-7,12-(C
F-, Cl-, Br-, or I-containing substituents on boron CB11H11-7-F CB11H11-12-F
[35] [35]
Continued
8.2 Synthesis and structure
273
Table 8-1 CB11H12 Derivativesa—Cont’d Compound
CB11H10-n,12-F2 (n ¼ 2,7) CB11H10-7,12-F2 CB11H9-7,9,12-F3 CB11H11-n-C6F 5 (n ¼ 7,12) CB11 ðCF3 Þ12 HCB11 F11 1-(H2N)CB11 F11 1-(H2N)CB11F10-6-H 1-(H2N)CB11F10-6-OH, 1-(H2N)CB11F9-4,6-ðOHÞ2 MeCB11 F11 SiMe3 þ RCB11 F11 R ¼ H, Et CuðCOÞ2 þ EtCB11 F11 CuðCOÞ4 þ (PhCH2)CB11 F11 H4(tetratolylporphyrin)2þ [CB11 H5 X6 ]2 X ¼ Cl, Br, H N–H—p bonding; evidence against “sitting-atop” metalloporphyrin complexes MeCB11 H5 X6 (X ¼ Cl, Br, I) Csþ 1-HCB11 H5 X6 (X ¼ Cl, Br) CB11H11-2-Cl (Me2HN)CB11H10-12-Cl C6 Me6 þ HCB11 H6 Cl6 C6 H7 þ CB11 H6 Cl6 Ph3Cþ CB11H6-7,8,9,10,11,12-Cl6 CB11 H6 Cl6 CB11H7-7,8,9,10,12-Cl5 Mþ CB11 H6 X6 CTV H2O L (X ¼ Cl, Br; L ¼ CF3CH2OH, MeCN; M ¼ Na, K, Rb, Cs; CTV ¼ cyclotriveratrylene) coordination chains (Ph3P)2H2Irþ CB11 H6 Cl6 olefin hydrogenation catalyst Naþ (CTV)(H2O)CB11 H6 Cl6 cyclotriveratrylene host-guest Hþ CB11 H6 Cl6 PhNC12 H2 Br6 þ CB11 H6 Cl6 HC60 þ CB11 H6 Cl6 HC60 þ • CB11 H6 Cl6 CB11H6-7,8,9,10,11,12-X6 (X ¼ Cl, Br) Hþ HCB11 Cl11 strongest isolable acid Csþ CB11 Cl11 improved synthesis via SbCl5
Information
References
S, S, B, C, F S, B, F, MS S, B, C, F S, H, B S, B(2d), F, MS, IR, Raman S, B, F, MS S, H, B, C, MS, F S, X, B(2d, actual spectrum), C, F(2d, actual spectrum), MS, IR (actual spectrum), Raman (actual spectrum) S, B, C, F S, X, B(2d, actual spectrum), F(2d, actual spectrum), MS S, H, B, MS, F S, H, B, C, Si, COND S, X, IR S, X, IR S, X(Cl), H
[45] [46] [45] [14] [60] [46] [51] [144]
S, X, H, B, C, IR, MS S, X (molecular and extended crystal) S, H, B S (electrochemical), MS S, UV S, H, C, IR S X S, H, B, R S, X, IR
[17] [174] [7] [42] [82] [82] [175] [176] [15] [122]
S, X, H, B, P S, X
[118] [123]
S, H, C, UV, IR S, UV, ESR S, H, C, UV, IR S, H, C, UV, IR, ESR S, H, B, R Acid strength S, H, B, C, MS
[84] [84] [84] [84] [15] [71] [57]
[144] [144] [51] [173] [120] [120] [89]
Continued
274
CHAPTER 8 Icosahedral carboranes: Closo-CB11 clusters
Table 8-1 CB11H12 Derivativesa—Cont’d Compound
Information
References 31
Hþ HCB11 Cl11 Cl–H-Cl bridged linear polymer Hþ HCB11 H5 X6 (X ¼ Cl, Br, I) strongest isolable acids (i-C3H7)3Si(o-C6H4Cl2)þ HCB11 Cl11 silylium ion Et3Siþ HCB11 Cl11 silylium ion Et3Si-H-SiEtþ 3 HCB11 Cl11 silylium ion þ Me3Si-H-SiMe3 HCB11 Cl11 silylium ion H3Oþ 3C6H6 HCB11 Cl11 C6H6 hydronium ion-benzene p-complex H3Oþ HCB11 Cl11 H2 O5 þ HCB11 Cl11 R2Clþ HCB11 Cl11 R ¼ Me, Et chloronium ions (n-C8H17)3NHþ HCB11Cl/ 11HCB11 H5 Cl6 þ 5CHCMe3 HCB11 H5 Br6 cyclo-H6C3(SiMe2)2C]5 vinyl cation H3Oþ HCB11 H5 X6 (X ¼ Cl, Br, I) H3Oþ HCB11 Me5 X6 (X ¼ Cl, Br, I) MeCB11 Cl11 RCB11 Cl11 (R ¼ H, Me) [N, N0 -dialkylimidazolium] þ CB11 H6 X6 (X ¼ Cl, Br) H2 O5 þ HCB11 Me5 Br6 Cp2ZrCH(SiMe3)þ 2 HCB11 Me5 Br6 þ B(subphthalocyanine) HCB11 Me5 Br6 (Ph3P)2H2Irþ CB11 H6 Br6 olefin hydrogenation catalyst [Naþ]2 [CB11 H6 Br6 ]2 (DMF)4(H2O)2(CTV) (CTV ¼ cyclotriveratrylene) coordination chains Ph3Cþ CB11H11Br CB11H11-12-X (X ¼ Cl, Br) CB11H11-2-Br C[C13H6(OMe)2SMe]22þ [HCB11 H5 Br6 ]2 hexacoordinate carbon cation H(C3H7)3þ HCB11 H5 X6 R ¼ CHMe2, C6H11, cycloC5H9; X ¼ H, Br Et3Si(SO2)þ HCB11 Me5 Br6 silylium ion RhP(C5H4)2(C5H7){C6H3(CF3)2B[C6H3(CF3)2]4}þ CB11 H6 Br6 kinetic versus thermodynamic factors in anion coordination CB11 HðOHÞ5 Br6 H(arene)þ HCB11 H5 X6 R ¼ H, Me; X ¼ Cl, Br; arene ¼ benzene, toluene, m-xylene, mesitylene, hexamethylbenzene
Effects on Me3PO P NMR and adsorption on solid acid catalysts IR study of HðH2 OÞn þ structure IR (gas and solid), X Acid strength S, X, Si S, X S, X, Si S, X, Si, IR S, X, IR
[78] [72] [71] [92] [92] [92] [92] [76]
S, X, IR S, IR S, X, IR IR (nN-H as a measure of acid strength) X
[101] [77] [96] [73] [178]
S, S, S, X S, S, S, S, S, S,
X, H, IR IR X, H, B X X, H, B, P X, IR
[101] [101] [56] [58] [90] [77] [179] [180] [118] [122]
S, S, S, S,
H, B X, H, B(Br) X, H, B, C, MS X, H, C, IR
[133] [14] [7] [181]
X, IR X(Br), IR X, H, B, C, IR, MS
[177]
S, H, B, P
[182]
S, X, Si S, X, H, F
[92] [183]
S, X, IR S, X, C, H, IR
[75] [81]
Continued
8.2 Synthesis and structure
275
Table 8-1 CB11H12 Derivativesa—Cont’d Compound þ
(Me2CH)3Si CB11H6-7,8,9,10,11-Br6 HCB11 Br5 I6 Csþ CB11 Br11 improved synthesis via SbCl5 HCB11 Br11 (2,4-C6H3Br2)3NCB11 H6 Br6 C76þ CB11 H6 Br6 Ph3Cþ CB11H6-7,8,9,10,11,12-Br6 Naþ CB11H6-7,8,9,10,11,12-Br6 HCB11 X11 (X ¼ Cl, Br, I) HCB11 X11 (X ¼ Cl, I) CB11 Br12 (n-C8H17)3NHþ HCB11 R5 Br6 (R ¼ H, Me) HCB11 Cl5 Br6 HCB11 Cl5 Br6 , HCB11 Br5 I6 , HCB11 Br6 I5 , HCB11 Br11 HCB11 Cl6 Br5 , HCB11 Cl5 Br6 , HCB11 Cl6 I5 , RCB11 Cl11 (R ¼ H, Me) HCB11 Me5 X6 (X ¼ Cl, Br, I) RCB11 H5 Br6 R ¼ SiMe3, Si(CHMe2)3, C3H5, CH2Ph, P(CMe3)2, C6F5, C3F5, C6F11, Merrifield peptide resin phenoxoniumþ CB11 H6 Br6 vitamin E model compound (2,4,6-C6Me3H2)3Siþ HCB11 Me5 Br6 trimesitylsilyl cation; silylium ion isolated MeCB11 X11 (X ¼ Cl, Br, I) MeCB11 HBr10 PhCB11H10-12-I PhCB11H5-7,8,9,10,11-I6 (C5H11-C7H8N)CB11H10-12-I (C5H11-C7H8N ¼ pentylquinuclidine) R3Siþ CB11 H6 Br6 R3 ¼ Et3, (Me2CH)3, (CMe3)3, (CMe3)2Me H9 O4 þ CB11 H6 Br6 Tl(C7H8)þ 2 CB11 H6 Br6 þ (Me3C)3SiðOHÞ2 CB11 H6 Br6 (Me3N)CB11H10-12-I MeCB11H10-12-I HCB11H10-n-I (n ¼ 7,12) HCB11H10-12-I HCB11H9-7,12-X2 (X ¼ Cl, Br, I) HCB11H9-7,12-I2 HCB11H9-7-I-12-X X ¼ F, Cl, Br, OH
Information
References
S, X, B, Si S, X S, X, H, B, C, MS X S, UV, ESR, IR S, UV, ESR, IR, COND S, H, B, IR Luminescence/phosphorescence S, X, H, B, C, IR, MS(Br) IR study of HðH2 OÞn þ structure S, X, H, B, C, IR, MS IR (nN-H as a measure of acid strength) X S, H, B, C, IR, MS (negative ion)
[93] [58] [57] [58] [87] [87] [114] [184] [21] [78] [21] [73] [58] [58]
S, H, B, C, IR, MS (negative ion)
[58]
S, X(Br), Si (in Me2CHSiþ salts) S, H, P[P(CMe3)2], F(C6F5, C3F5, C6F11), MS S, X, C
[61] [185]
S, X, Si
[94]
S, S, S, S, S,
[21] [21] [40] [50] [39]
X, H, B, C, IR, MS(Br) X, H, B, C, IR, MS H, B, C H, B, C B, C, H, IR, X, MS
[186]
S, X, B, Si, IR
[187]
S, X X S, X, H, B S, H, B S, H, B, C, IR, MS S, H, B S (improved) S, B(2d[Cl]), H S, H, B B(2d), C, MS S, H, B, B(2d), C, MS
[102] [112] [188] [43] [33] [15] [33] [14] [15,189] [189] [189] Continued
276
CHAPTER 8 Icosahedral carboranes: Closo-CB11 clusters
Table 8-1 CB11H12 Derivativesa—Cont’d Compound
MeCB11H5-7,8,9,10,11,12-I6 (Ph3P)2H2Irþ CB11 H6 I6 (MePh2P)2H2Irþ CB11 H6 I6 Ir(CO)(Ph3P)2(HCB11H5Cl6) complex of Vaska’s compound Ir(CO)(arene)(PPh3)þ HCB11 H5 Cl6 arene ¼ C6H6, MeC5H5 complex of Vaska’s compound Ir(CO)2(PPh3)2þ HCB11 H5 Br6 complex of Vaska’s compound IrClPh(CO)(PPh3)2þ HCB11 Cl11 complex of Vaska’s compound HCB11 Cl6 Br5 , HCB11 Cl5 Br6 , HCB11Cl6I5, RCB11 Cl11 (R ¼ H, Me) (n-C8H17)3NHþ HCB11 H5 I6 (n-C8H17)3NHþ HCB11 I11 HCB11 I11 HCB11 I11 enclosing HðH2 OÞn þ cations in nanotubes (H3N)CB11I11 (p-FC6H4)2CFþ HCB11 I11 (p-FC6H4)MeCFþ HCB11 I11 HCB11 Br5 I6 , HCB11Br6I 5 , HCB11Cl6I5 (H3N)CB11I6H5 (Me2CH)3Siþ CB11H6-7,8,9,10,11,12-X2 (X ¼ Cl, Br, I) H(solvent)2þ HCB11 H5 X6 (R ¼ H, Me; X ¼ Cl, Br; solvent ¼ organic O-donor: OEt2, THF, benzophenone, nitrobenzene) O–H–O bonding oxonium ions H(benzophenone)2þ HCB11 H5 Cl6 H(PhNO2)2þ/H(OEt2)2þ/H(THF)2þ HCB11 H5 Cl6 Et2Alþ HCB11 H5 X6 X ¼ Cl, Br Transition Metal s- and m-complexes CB11H12-8-(m-H)-FeCp(CO)2 CB11H12-8(m-H)-FeCp(CO)2XAg (X ¼ Cl, Br, I) Fe(TPP)(C8H10)þ CB11 H6 X6 (TPP ¼ tetraphenylporphyrinate; L ¼ C6H6, C7H8; X ¼ Cl, Br) Fe(TPP)þ CB11 H6 Br6 Fe(H2O)(TPP)þ CB11 H6 Cl6 Fe(OH)(TPP)2þ CB11 H6 Cl6 M[porphyrinPh4]CHOH-CB11 H11 Csþ (M ¼ Co, Cu, none)
Information
References
S, S, S, S,
[33] [118] [118] [190]
H, B, C, IR, MS X, H, B, P X, H, B, P H, B, P
S, X, H, B, P
[190]
S, X, H, B, P
[190]
S, X, H, B, P
[190]
S, H, B, C, IR, MS (negative ion)
[58]
IR (nN-H as a measure of acid strength) IR (nN-H as a measure of acid strength) S, H, B, IR, MS S, X S, H, B, IR, MS S, X, IR S, X, IR S, H, B, C, IR, MS(neg.ion) S, H, B, IR, MS S, X, B, Si
[73] [73] [55] [191] [55] [88] [88] [58] [55] [175]
S
[83]
S, X S, X S, X, H, B
[83] [83] [111]
S, S, S, S,
IR, X, H IR, H X, UV, IR X
[192] [192] [152] [193,194]
S, S, S, S,
X, Mo¨ssbauer, MAG, H, IR X, Mo¨ssbauer, MAG, H, IR X, Mo¨ssbauer, MAG, H, IR MS, IR, UV, H (no metal)
[195] [195] [196] [197] Continued
8.2 Synthesis and structure
277
Table 8-1 CB11H12 Derivativesa—Cont’d Compound þ
Fe(Ph4 porphyrinate) CB11 H12 M(phenanthroline)32þ [CB11 H12 ]2 M ¼ Co, Ni [M(2,200 -bipyridine)3þ [CB11 H12 ]2 (M ¼ Co, Ni) M(en)32þ [CB11 H12 ]2 (M ¼ Co, Ni) ClCu-CB11 F11 CuðCOÞ2 þ EtCB11 F11 CuðCOÞ4 þ (PhCH2)CB11 F11 (C8H12)(MeCN)2Irþ MeCB11 F11 less coordinating than BF4 toward Ir Mþ 1-MeCB11F10-12-SiPh3 (M ¼ Cs, Ag) Cp2ZrMeþ (12-m-Me)CB11HMe10 Cp2Zr(Z2-CH2Ph)-CB11H12 (B-H-Zr) (C5H4Me)2Zr(Me)-CB11H12 (B-H-Zr) Cp*2Zr(Me)2CB11H12 (3 B-H-Zr) Cp0 MoðCOÞ3 þ CB11 H12 (Cp0 ¼ Cp*, Cp) CB11H12-12-MoCp(CO)3 MoCpðCOÞ3 þ CB11H11Br [MoCp(CO)3]2(m-I)þ HCB11 Me11 [CpMo(CO)3I Ag(CB11H12]2 RhðCOÞ4 þ EtCB11 F11 (Z4-C8H12)Rh(THF)2þ CB11 H12 (Z4-C8H12)Rh(Z2-CB11H12) (2 B-H-Rh) (diene)L2Rhþ CB11 H12 (Ph3P)2Rh(m-H)2-CB11H10 (norbornadiene)RhðPPhÞ3 þ CB11 H12 = CB11 H6 Br6 5CH2 (n ¼ 7, 12) (Ph3P)2Rhþ CB11H10-n-CH5 þ (norbornadiene)(Ph3P)2Rh CB11H10-n-R (R ¼ CH5 5CH2; n ¼ 7,12) 5CH2 (n ¼ 7,12) Rh-jj (Ph3P)2Rh-CB11H11-n-CH5 (norbornadiene)(Ph3P)2Rhþ CB11H6-2,4,8,10,12-Et5 (Ph3P)2Rhþ RCB11 H11 [R ¼ Me, Si(CHMe2)3] (Ph3P)2Rhþ MeCB11 H8 Et3 (Ph3P)2[Z2-C2H4)3]Rhþ CB11 H6 Br6 (Ph3P)2Rh-CB11H11-n-Et (n ¼ 7,12) Rh-(m-H)2 (Ph3P)2Rh-CB11H7Et5 Rh-(m-H)2 (Ph3P)2Rh(norbornadiene)þ HCB11 H6 Et5 (Ph3P)2Rh-MeCB11H8Et3 (Ph3P)2RhðMeCNÞx þ MeCB11 H8 Et3 (Z4-C6H8)Rh(PPh3)2þ CB11 H6 Br6 intermediate in catalytic. dehydrogenation of cyclohexane to benzene
Information
References
S, X, Mo¨ssbauer, MAG S, IR, MAG, UV, COND, thermal analysis S, MAG, IR, UV, COND S, IR, MAG, UV S, X, B, F S, X, IR S, X, IR
[198] [199] [147] [200] [51] [120] [120] [201]
S, X, H, B, C S, X, H(2d), B(2d), C, IR S, X, H, B, C S, X, H, B S, X, H, B S, X, H, B, IR S, X, H, IR X, H, B, IR S, H, B, IR S, X, H, B, Ag, IR S, X, supplemental experimental. data S, X, B S, X, H, B S, H, B, P S, X, H, B, P S, H, B, P S, H, B, P S, X, H, B, P
[202] [203] [204] [204] [204] [205] [206] [133] [133] [206] [207] [208] [208] [209] [210] [210] [106] [106]
S, S, S, S, S, S, S, S, S, S, S,
[106,107] [106] [106] [106] [106] [107] [107] [107] [107] [107] [211]
H, B X, H, B, P X[Si(CHMe2)3], H, B, P X, H, B, P X, H, B, P H, B H, B X, H, B X, H, B, MS H, B X, H, C, P
Continued
278
CHAPTER 8 Icosahedral carboranes: Closo-CB11 clusters
Table 8-1 CB11H12 Derivativesa—Cont’d Compound þ
þ HCB11H 8 (m-H)3RhH2
R ¼ CHMe2, C6H11, H(C3H7)3 cyclo-C5H9; X ¼ H, Br (R3P)2Rh2(HCB11H11)2 R ¼ CHMe2, C6H11, cyclo-C5H9 5C-CMe3)Rh(HCB11H5Br6) [(CHMe2)P](H2C5 [(C6H11)P]2(L)Rh(HCB11H5Br6) L ¼ C6H9, C5H7 [Cp*Rh(m-2-Cl)]3(m-3-Cl)2þ CB11 H6 Br6 2 [Cp*RhClþ]2 CB11 H12 2 exo-closo-(R3P)2Rh-CB11H12 [R3P ¼ P(OMe)3, P(MeC6H4CHMe2), 0.5Ph2PCH2CH2PPh2] Rh2(PPh3)4H2(m-H)(m-Cl)þCB11 H12 [(Ph3P)(Ph2P-Ph)Rhþ]2 [CB11 H6 Br6 ]2 Rh6{CH(CH3)2]3P}6H122þ [CB11Me)11H]2 Pd[Ph2P(CH2)2PPh2]2þ [CB11H11Cl]2 {Pd[Ph2P(CH2)2PPh2](CB11H12)}þ CB11 H12 þ/ CB11 H12 Ag(MeCN)þ/ 2 Ag(NCCH2CN)2 Ag[NC(CH2)2CN]5[CB11H12]4 {[2,6-(MeO)2C6H3]SnPh2þ}2 Ag(CB11H12)32 stabilization by pincer-type triaryltin ligand Agþ/(R3P)Agþ/(Ph3P)(Et2O)2Agþ/(Ph3P)2Agþ HCB11 Me11 Ag(bpy)þ 1-PhCB11 H5 I6 MeCN bpy ¼ bipyridyl (1-D polymer chain) Ag2 ðbppzÞ3 2þ [1-(BrC6H4) CB11 H11 ]2 MeCN bppz ¼ bis-2-pyridylpyrazine (1-D polymer chain) Agþ CB11 H6 X6 (X ¼ Cl, Br, I) Agþ CB11H6-7,8,9,10,11,12-Br6 Agþ CB11H6-7,8,9,10,11,12-X6 (X ¼ Cl, I) (PPh3)Agþ CB11 H6 Br6 catalyst for hetero Diels-Alder reactions (Ph3P)2Agþ CB11 H6 Br6 (Ph3P)Agþ CB11 H12 [(Ph3P)Agþ CB11H12]22 2CH3 interactions) Ph3PAgþ HCB11 Me11 (Ag2 PhAg(CB11H12)2 (C5H5N)2Agþ HCB11 Br5 I6 {[(MeCN)4Ag-3][Ag(CB11Br5I6)2]}n [CpMo(CO)3I Ag(CB11H12]2 [Ag(IMes)2þ]2 Ag2(closo-CB11H12)42 (IMes ¼ dimesitylimidazol-2-ylidene) CB11H12-Ag-IrCl(CO)(PPh3)2 CB11H6Br6-Ag-IrCl(CO)(PPh3)2 IrCl(CO)(Ph3P)2.Ag(CB11H12)
Information
References
S, X(C6H11), H, B, P
[182]
S, S, S, S, S, S,
X(C6H11, Br), H, B, P X, H, B, P X(C5H7), H, B, P X X X [P(MeC6H4CHMe2)], H, B, P
[182] [182] [182] [212] [212] [209]
S, S, S, S, S, S, S, S,
H, B, P, X, H, B, P X, H, P X X,H,P X, IR X, IR X, H, B, Sn, IR, MS
[210] [210] [213] [214] [214] [110] [110] [150]
S, X(close Ag-Me contacts), H, B(2d), IR S, X, IR
[116]
S, X, IR
[50]
X S, H, B, IR S, B(2d) S, X, H, B, P, IR
[115] [114] [175] [108,125]
S, S, S, S, S, S, X S, S, S,
[108] [108] [125] [108] [129] [109] [215] [215] [205] [216]
B, P, IR X, B, P, IR H, B, P, IR X, B, P, IR X, H, B(2d), IR X X X, H, B, Ag, IR X, H, B
S, IR, B, P S, IR, B, P S, X, IR
[50]
[192] [192] [217] Continued
8.2 Synthesis and structure
279
Table 8-1 CB11H12 Derivativesa—Cont’d Compound þ
C59N Ag(CB11H6Cl6)2 azafullerenes Fe(TPP)(C8H10)þ Ag(CB11H6Br6)2 (TPP ¼ tetraphenylporphyrinate) Fe(TPP)þ CB11 H12 Fe(TPP)þ Ag(CB11H6Br6)2 [CpMo(CO)3IAg(CB11Br6H6]2 [MoCp(CO)3Iþ Ag(CB11H11Br)]2 {[MoCp(CO)3I]3.Agþ2 [HCB11 Me11 ]2 [CpMo(CO)3I Ag(CB11H12]2 CB11H12-Ag-IrCl(CO)(PPh3)2 CB11H6Br6-Ag-IrCl(CO)(PPh3)2 IrCl(CO)(Ph3P)2.Ag(CB11H12) (Ph3P)2IrH2(m-Br)2CB11H6Br4 (Ph3P)2IrH2(Z2-C2H4)nCB11H6Br6 (n ¼ 2,3) (chlorin)M2 2CB11 H11 Csþ M ¼ Pd, Sn(OH)2, Zn lightindependent cytotoxicity/photodynamic tumor therapy CB11H6Br3-(m-Br)3PtMe3 PtLL0 (PP)CB11 H12 (L ¼ THF, L0 ¼ Me, PP ¼ (CMe3)2P (CH2)3P(CMe3)2) Pt(NCMe)2(PP)þ CB11 H12 PP ¼ (CMe3)2P(CH2)xP (CMe3)2 (x ¼ 2,3), CH2C6H4CH2 [L2P(CH2)3PL2]Pt2 2CB11H12]þ CB11 H12 (2 Pt2 2H2 2B) (L ¼ C6H11, CMe3) 2CB11H12]þ CB11 H12 [(CMe3)2P-Z-P(CMe3)2]Pt2 (2 Pt2 2H2 2B) [Z ¼ (CH2)2, CH2C6H4CH2)] (Me3N)CB11(HgOCOCF3)11 CB11H10(HgOCOCF3)2 (Me3N)CB11H10-12-HgOCOCF3 CB11H11-12-HgOC(O)CF3 [1,3-(Me3Si)2C5H3]2Lnþ CB11 H6 Br6 (Ln ¼ Sm, Er) [1,3-(Me3Si)2C5H3]2Ln(THF)2þ CB11 H6 Br6 (Ln ¼ Sm, Er) Other Experimental Studies Reactivity and kinetics CB11 H12
Information
References
S, X, H, B, C, IR, MS, Raman S, X
[85] [193,194]
ESR, Mossbauer, MAG (Fe spin-state mixing and ligand field strength) ESR, Mossbauer, MAG (Fe spin-state mixing and ligand field strength) S, X, H, B, Ag, IR S, X, H, B S, X, H, B, IR S, X, H, B, Ag, IR S, IR, B, P S, IR, B, P S, X, IR S, X, H, B, P S, X(n ¼ 3), H, B, P S, H, B, MS, IR, UV
[218]
S, X, H, B S, H, B, C, P, IR
[221] [105]
S, H, B, C, P, IR
[105]
S, P, C, H, B, IR
[105]
S, X[(CH2)2], H, B, C, P, IR
[105]
S, S, S, S, S, S,
[44] [44] [44] [14] [119] [119]
B, IR, Raman B, IR, Raman B, IR, Raman X, H, B(Br) IR X, IR, H(Sm), C(Sm), B
Electrophilic substitution, deuteration Fluorination Thermolysis Pd-catalyzed cross-coupling Me methacrylate plasticizer-free polymer; cation exchanger; ionsensitive electrode; Ca-sensitive membrane sensors
[218] [205] [133] [133] [206] [192] [192] [217] [219] [219] [220]
[32] [46] [149] [33] [16]
Continued
280
CHAPTER 8 Icosahedral carboranes: Closo-CB11 clusters
Table 8-1 CB11H12 Derivativesa—Cont’d Compound
þ
5CH(CH2)n-2]CB11 Me11 Li [CH25 HCB11 Me12 Liþ (diene)L2Rhþ CB11H12 (Me3N)CB11H11
M(en)32þ [CB11 H12 ]2 (M ¼ Co, Ni) X(CH2)n CB11 Me11 (X ¼ Cl, Br; n ¼ 3,4,7) HCB11 Me11 1-(H2N)CB11 F11 Catalysis Agþ CB11 H12 R3Siþ HCB11HnCl11-n R ¼ Et, n-C6H13; n ¼ 0, 5 RhCl(cod)þ CB11 H12 (L)Rh(PPh3)2þ CB11 H6 Br6 (L ¼ Z4-C6H8, Z4-C7H10) (norbornadiene)RhðPPh3 Þþ CB11 H12 /CB11 H6 Br6 (Ph3P)Agþ CB11 H12 [(Ph3P)Agþ CB11 H12 ]22, (Ph3P) Agþ CB11 H6 Br6 , (Ph3P)2Agþ CB11 H6 Br6 Et2Alþ HCB11 H5 X6 (X ¼ Cl, Br)
HCB11 H5 X6 X ¼ H, Cl, Br HCB11 H6 Y5 X ¼ Br, Cl; Y ¼ H, Cl, Me Other applications CB11 H12 [-(CH2)n-NH(CB11H11)]n polymers HCB11 Br11
Information
References
Polymerization via radical mechanism Catalysis of radical polymerization of isobutylene Reaction with H2 ! exo-closorhodacarboranes Controlled chemical and electrochemical substitution Cl, D, OH, CO, COOH, B(2d), H, MS Thermal decomposition Side-chain C2 2C cleavage Cage decomposition; B2 2C bond activation Extrusion of C to form closo-3(NC)B11 F10 2
[20] [222]
Activation of Co/Ni/Pd catalysts for vinyl addition/polymerization of norbornene Hydrodehalogenation catalysts for C2 2F, C2 2Cl and C2 2Br bonds Catalyst for addition of arylboronic acids to aldehydes (weakly coordinating anion) Intermediate in catalytic dehydrogenation of cyclohexane to benzene Catalytic hydrogenation of alkenes Catalysis of a Diels-Alder reaction
[209] [42]
[200] [31] [117] [52]
[214]
[97] [223]
[211]
[210] [108]
Lewis acid activation of ethylene; catalytic oligomerization of ethylene; catalytic polymerization of cyclohexane oxide Counterion for hydrodefluorination of perfluoroalkyl groups by SiR3 R ¼ Et, n-C6H13 Si-H/C-F metathesis Gas phase acidity; electron binding energy
[111]
Gravimetric determination Copolymerization of CB11 H11 with dibromoalkanes, TLC Ion exchanger in cation-selective polymeric membrane electrode sensors
[225] [155]
[98]
[224]
[226]
Continued
8.2 Synthesis and structure
281
Table 8-1 CB11H12 Derivativesa—Cont’d Compound 0
RCB11H10-12-R R ¼ C7H6, C5H4; R0 ¼ C5H4, C7H6 RCB11H10-12-R0 R ¼ H, C7H6; R0 ¼ C7H6, H H(arene)þ HCB11 H5 X6 R ¼ H, Me; X ¼ Cl, Br; arene ¼ benzene, toluene, m-xylene, mesitylene, hexamethylbenzene Me3NHþ HCB11 X11 X ¼ Cl, Br, I; Me3NHþ HCB11 H5 Br6
HCB11 Me5 X6 X ¼ Cl, Br, I BPþCB11 H12
Ag[CH(C6H5)3]þ MeCB11 F11 super-weak anion (chlorin)M-CB11 H11 Csþ M ¼ Pd, Sn(OH)2, Zn lightindependent cytotoxicity/photodynamic tumor therapy Theoretical Studies Molecular and electronic structure calculations CB11 H12
C2 [H11B11C2 2C 2CB11H11] biradical with singlettriplet near- degeneracy; candidate for molecular magnet C2 2C 2CB11H11]n n ¼ 1, 2 [H11B11C2 RCB11HnMe11-n R ¼ H, Me •
•
(NC)CB11 H11 X ¼ H, F. Cl, Br, I X@CB11 H12 (X ¼ Liþ, Be2) carborane-encapsulated ions CB11 H12 n (n ¼ 0,1) CB11 Me12 (n ¼ 0,1)
Information
References
Hyperpolarizability; NLO Hyperpolarizability; NLO Stabilization of benzenium ion salts; carborane superacids
[227] [227] [81]
Ion-pairing ability in membranes; selectivity; ion-selective electrodes cation exchangers; ionophore-based sensing platforms; natural population analysis charges Cation interactions (least nucleophilic anion) Ionic liquid for binap-mediated asymmetric catalysis of acetophenone by 3,1,2-H(Ph3P)Rh(C2B9H10)-3CH25 5CHCH2CH2-(m-Rh)] X S, H, B, MS, IR, UV
[228]
ab initio SCF EI [energy indexes]; stabilities Antipodal effect Optimized geometry, IR-active vibrations, charge distribution Vibrational modes Influence of charge, spin, substituents, and atom encapsulation on volume of cage DFT, cage substitution DFT, ab initio
[230,231] [232] [233] [234] [235]
DFT, ab initio Potential energy surfaces, oxidation potentials DFT, stability DFT, stability Energies, ionization potentials, vibrational frequencies Energies, ionization potentials
[61] [229]
[121] [220]
[148] [236]
[237,238] [239]
[239] [29] [22] [240] [241] [241] Continued
282
CHAPTER 8 Icosahedral carboranes: Closo-CB11 clusters
Table 8-1 CB11H12 Derivativesa—Cont’d Compound þ
TTF HCB11 Me12 TTF ¼ tetrathiofulvalene CB11Me11 CB11 H11 C7 H6 þ tropylium (C7H6)CB11H10-12-C5 H4 R ¼ H, C5 H4 (C5H4)CB11H10-12-C7H6 CB11H11-12-C7H6 RCB11H10-12-R0 (R ¼ C7H6, C5H4; R0 ¼ C5H4, C7H6) RCB11H10-12-R0 (R ¼ H, C7H6; R0 ¼ C7H6, H) Fe(TPP)(C8H10) [Ag(CB11H6Br6)2] (TPP ¼ tetraphenylporphyrinate) Fe(TPP)(C8H10) [CB11H6X6] L ¼ C6H6, C7H8 (X ¼ Cl, Br) FeIII(TipsiPP)þ CB11 H6 Br6 4-coordinate porphyrin cation Ph3PAgþ HCB11Me11 exo-closo-(R3P)2Rh-CB11H12 R3P ¼ P(OMe)3, P(MeC6H4CHMe2), 0.5 Ph2PCH2CH2PPh2 Me3NHþ HCB11 X11 (X ¼ Cl, Br, I), Me3NHþ HCB11 H5 Br6 5CHCH2), Me3N-CB11H9-2-Ph-8-X (X ¼ I, CH25 (Me3N)CB11H10-12-I Et2Alþ HCB11 H5 X6 (X ¼ Cl, Br) (Ph3P)Agþ HCB11 Me11 C59NþAgþ[CB11 H6 Cl6 ]2 Hþ HCB11 Cl11 CB11Me11 boronium ylide; naked B(12) vertex CPh, Cl CB11H11-12-R R ¼ C NMR calculations CB11 H12 CPh, Cl CB11H11-12-R R ¼ C 1-(H2N)CB11 H11 1-(H2N)CB11 F11 1-(H2N)CB11F10-6-X (X ¼ H, OH) 1-(H2N)CB11F9-4,6-ðOHÞ2 2 [Ag(IMes)þ 2 ]2 Ag2(closo-CB11H12)4 (IMes ¼ dimesitylimidazol-2-ylidene) Agþ CB11H6-7,8,9,10,11,12-X6 (X ¼ Cl, I) (Me2CH)3Siþ CB11H6-7,8,9,10,11,12-X6 (X ¼ Cl, Br, I)
Information
References
Electronic structure DFT (ab initio) charge-transfer b (first hyperpolarizability); NLO b (first hyperpolarizability); NLO b (first hyperpolarizability); NLO MNDO MNDO DFT
[142] [31] [242] [151] [151] [151] [227] [227] [193]
DFT
[193]
S, X, H, UV
[243]
Structure; energy minimization DFT isomers
[129] [209]
Natural population analysis charges
[228]
Heat of formation; charge distribution Heat of formation; charge distribution DFT geometry and electrostatic AlCB11 bonding) DFT geometry UV-vis and transient absorption spectra DFT: bridged linear polymer structure DFT: isomers DFT geometry
[43] [43] [244]
B F GIAO B, F B, F B, F B, F GIAO Ag-B interactions in solution
[144,245] [144] [35] [144] [144] [144] [144] [216]
IGLO IGLO
[175] [175]
[116] [86] [72] [134] [35]
Continued
8.2 Synthesis and structure
283
Table 8-1 CB11H12 Derivativesa—Cont’d Compound Reactivity calculations CB11 H12
Liþ@CB11 H12 CH HCB11H10-12-C CHÞ2 HCB11H9-7,12 ðC CB11FnH12-n (n ¼ 1,6,12) Hþ CB11 H12 Hþ CB11XnH12-n X ¼ Me, F, Cl, Br, I, CN, CF3 n ¼ 1, 6, 11, 12 Hþ CB11H11X X ¼ CF3SO2, NO2, NMe2, CMe3 MeCB11 F11 CB11H11Br HCB11 H11 (X ¼ H, Me, Cl, F) HCB11 H5 X6 (X ¼ Br, Cl, F) HCB11 Me5 Cl6 (Ph3P)2H2Irþ CB11 H6 Cl6 Me3Mþ CB11 Me12 (M ¼ Ge, Sn, Pb)
Information
References
ab initio, electrophilic substitution Br nsted acidity DFT mechanism of formation from B11H14 Photochemical release of endohedral Liþ DFT: structure DFT: structure Br nsted acidity DFT: structure, gas phase Br nsted acidity DFT: structure, gas phase Br nsted acidity DFT: structure, gas phase Br nsted acidity DFT: MOs ab initio, electrophilic substitution Molecular electrostatic potential; acidity Molecular electrostatic potential; acidity Molecular electrostatic potential; acidity Catalytic activity in olefin hydrogenation Me3Mþ Me interactions
[246] [62] [247] [248] [36] [36] [62] [74] [74] [74] [202] [246] [65] [65] [65] [118] [128]
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; F, 19F NMR; P, 31P NMR; Si, 29Si NMR; Sn, 119Sn NMR; Li, 7Li NMR; Pt, 195Pt NMR; 2d, two-dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; UV, UV-visible data; E, electrochemical data; ESR, electron spin resonance data; MAG, magnetic susceptibility; COND, electrical conductivity; OR, optical rotation; NLO, nonlinear optical properties; NQR, nuclear quadrupole resonance; DSC, differential scanning calorimetry. a A more extensive listing can be found online at http://www.elsevierdirect.com/companion.jsp?ISBN=9780123741707.
8.2.2 Substitution at carbon The acidity of the CH proton in CB11 H12 is comparable to that of acetylene, and C-lithiation via treatment with n-butyllithium followed by reaction with electrophiles such as EtBr, Ph3SiCl, CF3I, and Ph2PCl, generates RCB11 H11 products, where R is an alkyl, silyl, phosphino, carboxylic acid, hydroxy, alcohol, alkenyl, aryl, or other group (Table 8-1) [14–18]. The method also works well with most B-substituted derivatives of CB11 H12 [4,17,19–21], and can be employed to prepare B-perhalogenated 1-(NC)CB11 X11 anions (X ¼ F, Cl, Br) [22]. Highyield syntheses of a wide range of RCB11 H5 Br6 derivatives from the C-lithio anion have been described [185]. Manipulation of the C-amino groups in zwitterionic (Me2NRþ)CB11 H11 compounds, where R is H or Me, produces other C-substituted species. For example, (Me2NH)CB11H11 is easily demethylated with I2 in alkaline solution to give 5CHOH)CB11 H11 (MeNH2)CB11H11, which, in turn, can be treated with formaldehyde and I2 to produce (NMe5 [23]. Deamination of (H3N)CB11H11 with nitrous acid leads instead to (HO)CB11 H11 , which can be methylated to produce (MeO)CB11 H11 . If excess SMe2 is present, reaction of (H3N)CB11H11 with nitrous acid forms (Me2S)CB11H11 in good yield [15].
284
CHAPTER 8 Icosahedral carboranes: Closo-CB11 clusters
The versatile (Me2NH)CB11H11, a stable solid that is soluble in polar solvents, can also be methylated, ethylated, and 5NMe]CB11 H11 , respectively [23]. The benzoylated to generate (Me3N)CB11H11, (EtMeNH)CB11H11, and [Ph(OH)C5 compounds (MenNH3-n)CB11H11 (n ¼ 0-2) are weak acids (pKa 6 in 50% ethanol) and readily lose an amino proton to form the corresponding anion. C-aryl derivatives can be prepared via several routes [24]. As described in Section 6.2, the reaction of B10H14 and PhCHO gives nido-6-PhCB9 H11 . Heating this 10-vertex species with Me3NBH3 at 210 C generates a 2:1 mixture of the Et4Nþ salts of PhCB11 H11 and 4-PhCB9 H9 that can be separated on an ion exchange column by conversion to their Csþ salts, followed by removal of the much less soluble Csþ PhCB9 H9 via fractional crystallization from cold water [25]. Alternatively, if Me2SBH3 is employed as the boronating agent in a Cl2C2H4 solution, Et4Nþ PhCB11 H11 is obtained in 92% yield [26]. The stoichiometry of these cage-expansion reactions can be represented as PhCB9 H 11 þ 2LBH3 ! PhCB11 H11 þ 3H2 þ 2L
In a very different approach, treatment of B11 H14 with arylhalocarbenes (p-XC6H4)CCl: (X ¼ H, F, Cl, Br, I, Ph) achieves carbon atom insertion to form (p-XC6H4)CB11 H11 ions [7,27]. The PhCB11 H11 ion has also been obtained via palladium-catalyzed cross-coupling of phenyl triflate with CIZnCB11 H11 [8]. C-halo derivatives, XCB11 H11 , can be prepared by treating the parent CB11 H12 or CuCB11 H11 with Nhalosuccinimides (for X ¼ Cl, Br, and I) and N-fluorobis(benzenesulfonyl) amine (for X ¼ F) [28]; under carefully controlled conditions, the XCB11 H11 products are obtained in high yields. An improved method for the synthesis of the chloro, bromo, and iodo compounds, using methyl triflate in sulfolane in the presence of CaH to extract the triflic acid formed, has been reported [29,30]. This technique also allows the synthesis of XCB11 Me11 anions (X ¼ Cl, Br, I) from HCB11 Me11 salts, allowing easy access to C-alkenyl derivatives that undergo O2-promoted polymerization, as described below [20,31].
8.2.3 Substitution at boron by electrophilic reagents The introduction of exo-polyhedral atoms or groups at specific boron locations in CB11 H12 is controlled by the charge distribution in the cage skeleton, which gives rise to three different vertex types: the “upper belt” borons adjacent to carbon [B(2)-B(6)], the “lower belt” [B(7)-B(11)], and the unique B(12) that occupies a location antipodal to the carbon atom. As the negative charge increases with increasing distance from the CH vertex, B(12)-H is most negative and hence most susceptible to electrophilic attack, followed by the adjacent (lower-belt) BH groups; the upper-belt borons nearest the carbon are much less reactive, but can nevertheless be functionalized under suitable conditions. This pattern has been clearly revealed by MO calculations, 11B NMR chemical shifts, deuteration experiments, and reactivity studies involving halogens and other electrophiles. Thus, electrophilic deuteration of the parent carborane with DCl/D2O, monitored by 11 B NMR, proceeds in the order B(12) > B(7-11), to give CB11H6-7,8,9,10,11,12-D6 as the final product [32]. The treatment of CB11 H12 with Br2 or N-bromosuccinimide (NBS) affords CB11H11-12-Br; with an excess of NBS, CB11H10-7,12-Br2 is also obtained [14]. Chlorination with N-chlorosuccinimide (NCS) in a 1:1 mole ratio yields CB11H11-12-Cl, with the 7,12-dichloro product also forming if excess NCS is present. The parent anion combines with I2 to generate CB11H11-12-I [15,33], which in turn serves as a precursor to 12-alkyl derivatives via palladium-cross-coupling (Figure 8-1A) [33]. The tropylium ylide CB11 H11 -12-C7 H6 þ , a highly polar molecule (11.2 D) of interest in nonlinear optical (NLO) applications [34] (Chapter 17), is obtained directly from CB11 H12 via reaction with C7 H7 þ ion, as shown in Figure 8-1B [33]. Palladium-cross-coupling reactions have also been employed to prepare 7-alkynyl and 7,12-dialkynyl derivatives (Table 8-1) from their corresponding iodo- and diiodo-carboranyl precursors [35,36]. Derivatives of CB11 H12 that are functionalized at the antipodal 1 and 12 positions have received special attention because their linear “carborod” [37] geometry (8-1) invites possible application in NLO materials, liquid crystals, conducting polymers, and other areas. A number of these have been characterized (Table 8-1) and include homosubstituted (L1 ¼ L2) species and heterosubstituted (L1 6¼ L2) systems; the ligands can be aryl, alkyl, alkenyl, alkynyl, silyl, halo,
8.2 Synthesis and structure H C
H 1
C
6
2
5 11
4 10
RMgX/Pd(PPh3)2
3
8
9 12
I
A
285
THF
7
R = Me, Et, n-C4H9, n-C6H13, Ph
CB10H11-12-R− R
CB10H11-12-I−
H
H C
C
+ H2O/toluene
CB10H−12
CB10H−11-12-C7H6+
+ B FIGURE 8-1 Synthesis of CB11H11-12-R derivatives (in Chapters 8 through 17, carborane structures are depicted in the style of Figure 8-1, with unlabeled vertices representing B or BH units unless otherwise indicated.).
amino, metallo, or other groups [33,38–44]. Most such compounds are prepared by the introduction of an L2 moiety at B (12) on an L1CB11 substrate, as in the preparation of the 1,12-diphenyl derivative from Csþ PhCB11 H11 [40]:
8-1 L1
MeCðOÞOH
C
L2
PhMgBr
PhCB11 H 11 þ I2 ! PhCB11 H10 -12-I ! PhCB11 H10 -12-Ph PdCl2 ðPPh3 Þ2
Fluorinated derivatives of CB11 H12 are obtained by treating it with anhydrous HF [45] or N-fluoro reagents [46]. At 23 C with anhydrous liquid HF, CB11H11-12-F is obtained almost quantitatively, while higher temperatures yield CB11H10-n,12-F2 (n ¼ 2,7) and CB11H9-7,9,12-F3, the latter product being formed at 180 C. Electrophilic B-substitution of CB11 H12 with H2NOSO3H proceeds according to an apparently different mechanism, affording CB11H11-7-NMe3 as the main product, together with a minor amount of the expected 12-NMe3 isomer [32]; this somewhat anomalous result has apparently not been further explored.
286
CHAPTER 8 Icosahedral carboranes: Closo-CB11 clusters
8.2.4 Synthesis of B-substituted derivatives via boron insertion As mentioned at the start of this section, parent CB11 H12 can be obtained from smaller boron clusters by incorporation of boron. This approach can be extended to generate CB11 derivatives bearing substituents at different locations, either by employing starting carboranes that are already B-functionalized, or by using RBX2 as the attacking reagent. Exam5CHCH2) to ples of the former method include the conversion of 11-vertex closo-2-(Me3N)CB10H8IX (X ¼ I, CH25 (Me3N)CB11H9-2-Ph-8-X products [43] and the cage-expansion of nido-(BrC6H4)CB9H11 by reaction with Me2SBH3 and MeC6H4MgBr to give (MeC6H4-C6H4)CB11H10-12-C6H4Me [38]. Insertion using RBX2 reagents, where R is Ph, C6H4Me, F, O(CH2)4Cl, NH2, or NMe3, has been employed in the synthesis of CB11H11-2-R products from CB10 H11 3 salts [47,48]. An interesting illustration of this method is the reaction of nido-7-(Me3N)CB10H10 with (Me2CH)2NBCl2 to form 8-2 in 19% isolated yield, in the course of which a methyl 2N bond [49]: group is eliminated from the trimethylamine group and CH2 is inserted into the exo-polyhedral B2 NMe3
NMe2
C
C
HN(CHMe2)2
(Me2CH)2NBCl2 −2 Cl−
8-2
The reaction is proposed to proceed via a sterically induced activation of C2 2H in which H is transferred from a methyl group to the (Me2CH)2N unit, in the process forming an unstable intermediate that disproportionates to an amine and a carbene, generating 8-2 [49]. In this case the carborane cluster is clearly responsible for the unusual reactivity in which a carbene is inserted into a B2 2N bond.
8.2.5 Polyhalogenation at boron Electrophilic halogenation can place up to six Cl, Br, or I atoms on boron atoms in CB11 H12 or its C-substituted derivatives, with substitution usually restricted to the more negatively charged B(12) and “lower belt” B (7)-B(11) vertices (Table 8-1). A straightforward approach involves elemental halogens [15]: MeCðOÞOH
! CB11 H7 -7; 8; 10; 12-Cl CB11 H 12 þ excess Cl2 5 20 C
MeCðOÞOH
CB11 H 12 þ excess Cl2 ! CB11 H6 -7; 8; 9; 10; 11; 12-Cl6 100 C
MeCðOÞOH
CB11 H ! CB11 H6 -7; 8; 9; 10; 11; 12-Br 12 þ excess Br2 6 80 C
MeCðOÞOH
CB11 H ! CB11 H10 -7; 12-I 12 þ excess I2 2 80 C
Further iodination can be achieved with other reagents; for example, the reaction of PhCB11 H11 with iodine monochloride generates the hexaiodo species PhCB11H5-7,8,9,10,11,12-I6 [50]. Perhalogenated species, of considerable interest as weakly coordinating anions and in other applications (Chapter 17), have been synthesized for all four halogens (Table 8-1). The B-perfluorinated anion HCB11 F11 can be prepared by direct fluorination in liquid HF at room temperature [51]: HFliq
Csþ CB11 H ! Csþ HCB11 F 12 þ 11F2 11 þ 11HF 25 C;48 h
8.2 Synthesis and structure
287
The HCB11 F11 ion is stable in 5M aqueous acid and moderately stable in 3 M KOH, slowly converting to HCB11 ðOHÞF10 and HCB11 ðOHÞ2 F9 over many hours. NMR evidence indicates that HCB11 F11 in strong aqueous 2Cl2 2CB11F11 coorbase is partially deprotonated to CB11 F11 2 [51]. This dianion, with its “bare” carbon, forms a Cu2 dination complex that features a linear Cl2 2Cu2 2C array, illustrating the ability of monocarbon carboranes to coordinate to transition metals [51]. The hydrolysis of (NC)CB11 F11 proceeds similarly and is faster in basic than in acidic media, with (NC)CB11F10-6-OH being identified as an intermediate [22]. The 1-(H2N)CB11 F11 anion, on treatment with lithium diisopropylamide, undergoes a concerted extrusion of carbon from the icosahedral framework to form the 11-vertex closo-3-(NC)B11 F10 2 borane dianion in 90% yield [52]. Carbon extraction/polyhedral contraction is also observed in supra-icosahedral carboranes (Chapter 11) but is otherwise unknown in carborane chemistry as a single-step process; however, 1,2-Me2C2B10H10 can be degraded to CB9 or CB10 clusters via multistep reactions as described in Chapters 6 and 7, respectively [53,54]. The B-perchloro, B-perbromo, and B-periodo HCB11 X11 anions, as well as a number of the C-substituted analogues (RCB11 X11 , R ¼ alkyl or aryl), have been well characterized (Table 8-1). Their synthesis generally requires fairly stringent conditions, and several approaches have been explored [21,55,56]; however, a recently reported preparation of HCB11 Cl11 and HCB11 Br11 using SOCl2 or SbCl5 is simple, efficient, and relatively safe [57]. Another approach employs the reaction of RCB11 H11 (R ¼ H or Me) with halogenating agents in sealed tubes at elevated temperature [21]: ICl=CF3 SO3 H
ð1ÞBuLi
200 C
ð2ÞMeI
Br2 =CF3 SO3 H
Br2 =CF3 SO3 H
CB11 H 12 ! HCB11 Cl11 ! MeCB11 Cl11 CB11 H 12 ! HCB11 Br11 ! BrCB11 Br11 200 C
250 C
ð1ÞBuLi
HCB11 Br 11 ! MeCB11 Br11 ð2ÞMeI
ICl=CF3 SO3 H
ICl
CB11 H 12 ! HCB11 I11 ! MeCB11 Cl11 200 C
200 C
MeCB11 H 11
ICl
! MeCB11 I 11 220 C
The sealed-tube reactions are reported to afford the halogenated products in high yield [21], although this has been questioned [29]. It will be noted that iodine monochloride in the presence of triflic acid generates the B-perchlorinated carborane, but in the absence of triflic acid only the B-iodinated species is obtained. This finding is interpreted in terms of electrophilic substitution to give HCB11 I11 , followed by triflic acid-promoted nucleophilic replacement of the I substituents by Cl to form HCB11 Cl11 [21]. MeCB11 H11 is similarly per-B-iodinated by ICl, but when triflic acid is present the main product is the hexachloro-pentaiodo derivative, MeCB11 Cl6 I5 , suggesting that nucleophilic chlorination is partially inhibited by electron donation from the methyl group. The BrCB11 Br11 ion is the only fully halogenated CB11 carborane to be characterized thus far; it is quantitatively converted to HCB11 Br11 on treatment with aqueous AgNO3 [21]. The sealed-tube method affords an efficient route to mixed-halogen species of the type HCB11 X6 Y5 (X, Y ¼ Cl, Br, I), as illustrated by the synthesis of the hexabromo-pentaiodo derivative [58]: I2 =CF3 SO3 H
HCB11 H5 -7; 8; 9; 10; 11; 12-Br 6 ! HCB11 -2; 3; 4; 5; 6-I5 -7; 8; 9; 10; 11; 12-Br6
8.2.6 Polyalkylation at boron In the search for weakly coordinating anions, an alternative to replacing the BH hydrogens with halogens is to replace them with alkyl or hydroxy substituents. B-peralkylated derivatives lack the electron lone pairs that are associated with halogen atoms (possibly leading to even lower basicity), and also provide an outer sheath of hydrocarbon groups that
288
CHAPTER 8 Icosahedral carboranes: Closo-CB11 clusters
promotes their solubility in nonpolar solvents. However, the cumulative effects of the electron donation from the alkyl groups can be expected to alter the properties of the CB11 cluster to some degree. Methylation of all of the boron vertices in CB11 H12 is achieved upon reaction with methyl triflate; similar treatment of ICB11 H11 affords the dodecamethyl anion [8,19]: CF3 SO3 Me
HCB11 H 11 ! HCB11 Me11 CF3 SO3 Me
ICB11 H 11 ! CB11 Me12
In general, this reaction is allowed to proceed to completion, as it is relatively nonselective and is not useful in the preparation of specific partially alkylated derivatives. However, the placement of a bulky substituent such as Si(CHMe2)3 on the carbon vertex can be used to block the entry of methyl groups at nearby boron vertices; treatment with methyl triflate, followed by the removal of the silyl group with water, produces HCB11H5-7,8,9,10,11,12-Me6 in a good yield [29]. Iodine can also be used as a blocking agent, as its presence on a given boron atom tends to prevent methylation at adjacent borons [29]. Alternatively, B-alkylation can be accomplished by heating CB11 H12 salts with alkyl halides in sealed tubes [59]: MeBr
RCB11 H 11 ! RCB11 Me11
R ¼ H; Me
EtBr
HCB11 H 11 ! HCB11 Et11
The methyl group at B(12) in Li(dioxaborole)þCB11 Me11 (8-3) can be replaced by a p-bromophenyl substituent upon treatment with p-Me3SiC6H4Br at 190 C to give 8-4 [41]. Me
Me
Li+ O
−
Me
C
B
8-3
Me Me
O
Me3Si-C6H4-Br Δ
Me Me Me Me Me
Me
Li+ O B
8-4 O
Me
−
Me
C
Br
Me Me
Me Me
Fluorination of CB11 Me12 , in a two-step process that involves a reaction with F2 followed by treatment with K2NiF6 in HF, affords the dodecakis(trfluoromethyl) cluster CB11 ðCF3 Þ12 (8-5) whose molecular structure is nearly ˚ that is defined by the 36 fluorines on its perimeter [60]. While 8-5 is stable in a spherical ball with a radius of 8 A 20% KOH/EtOH, concentrated H2SO4, and anhydrous CF3COOH, and is unreactive toward O2 þ AsF6 in anhydrous HF, the Csþ salt is shock-sensitive and detonates on scraping with a metal implement. The properties of CB11 Me12 are unusual, and in some respects unique, and are further addressed below.
8.2.7 Mixed alkyl-halo derivatives Clusters combining alkyl and halogen substituents—of interest for “fine-tuning” the electronic properties and basicity of the CB11 system (see below)—are obtained from hexahalo clusters via reaction with methyl triflate at elevated temperature [61]: CF3 SO3 Me
HCB11 H5 X 6 ! HCB11 Me5 X6
X ¼ Cl; Br; I
8.2 Synthesis and structure
289
8.2.8 Acid-base properties The CB11 H12 ion, and especially its polyhalo and polyalkyl derivatives, are among the least nucleophilic species known [62–64], and their conjugate acids are extremely strong Br nsted acids—by several measures, the strongest found to date [65,66]. The CB11 cage is highly aromatic (see Chapter 2), but in contrast to benzene, which is stabilized by p-delocalization in the planar ring, the carborane system features 3-dimensional s aromaticity in which the negative charge is delocalized over the entire icosahedral framework. A measure of this stabilization is the enormous HOMOLUMO energy gap, which exceeds by far that of arenes and other p-aromatic systems [67]. As a consequence, the CB11 skeleton is chemically very robust and remains intact under a wide range of conditions (for example, surviving direct fluorination with F2 as mentioned earlier). The already weak coordinating ability [1] of the parent CB11 H12 is further lowered when the hydrogen atoms are replaced with halogens and/or alkyl groups; halogen substituents withdraw the electron density from the cage and render it even less nucleophilic [66,68], while polyalkyl substitution provides a protective hydrocarbon sheath, affording increased solubility in organic solvents [8]. These two approaches offer competing strategies that are aimed at achieving the synthesis of “least coordinating” anions. For some purposes, mixed-ligand HCB11 Me5 X6 species combine the advantages of both types; thus, Rþ HCB11 Me5 X6 ions (R ¼ Me, Et, CHMe2; X ¼ Cl, Br) are powerful alkylating agents that are stronger than alkyl triflates and can convert benzene to protonated toluene [61,69]: þ Meþ HCB11 Me5 Br 6 þ C6 H6 ! CH3 C6 H6 HCB11 Me5 Br6
The same carborane reagents extract H from alkanes at room temperature or below, forming carbocations that in many cases can be isolated as stable salts [70]: þ Meþ HCB11 Me5 Br 6 þ Cn Hnþ2 ! Cn Hnþ1 HCB11 Me5 Br6 þ CH4
Among the elusive species characterized in this way are the t-butyl and t-pentyl cations. As proton donors, partially or fully halogenated derivatives of CB11 H12 are significantly more powerful than conventional superacids such as CF3SO3H or FSO3H, as shown by their ability to protonate mesityl oxide in liquid SO2 via the reaction 5Ca H2 2CðOÞMe Me2 Cb5
H+
Me2 Cþ 2Ca H5 5CðOHÞMe b2
Moreover, the fact that the carborane acids are nonoxidizing, and therefore benign toward proton acceptors, offers an important additional advantage. As measured by the 13C NMR chemical shift between Cb and Ca, the acids Hþ HCB11 Cl11 and Hþ HCB11 H5 X6 (X ¼ Cl, Br, I), for which Dd > 83, both move the above equilibrium entirely to the right and qualify as the strongest Br nsted acids yet found [68,71]. (The values of Dd for FSO3H, CF3SO3H, HN(SO2CF3)2, and H2SO4 are respectively 74, 73, 72, and 64) [71]. An alternative measure of the acidity of CB11 anions, based on the degree of hydrogen bonding with alkylammonium cations as revealed by the shift in the N2 2H vibrational frequency, marks Hþ HCB11 Cl11 (nN-H ¼ 3163 1 cm1 for its trioctylammonium salt) as the strongest known isolable acid [71–73]. DFT calculations on a variety of Hþ CB11 Xn H12n species (see Table 8-1) show a strong dependence of gas-phase acidity on the nature and location of the substituents [74]. The extreme proton-donating capability of these monocarborane acids has been exploited, especially by Reed and coworkers [68], to stabilize many species that had previously escaped definitive structural characterization, in many cases allowing their isolation and structural study as crystalline salts under ambient conditions. The list is striking, and includes HðH2 OÞn þ (hydrated protons) [75–79]; (n-C4H9)3Snþ [80]; protonated arenes such as C6 H7 þ (benzenium), C6 MeH6 þ and C6Me6Hþ [81,82]; protonated organic solvents [83]; HC60 þ (protonated fullerene) [84] and C59Nþ [84]; (azafullerene) [85,86]; C76 þ [87]; fluorinated carbocations [88]; protonated porphyrins;[89] imidazolium salts; [90,91] R3Siþ salts [92,93]; (mes)3Siþ CB11 Br6 Me5 (mes ¼ 2,4,6-trimethylphenyl);[94] silyl-stabilized allyl cations;[95] and CIR2 þ (chloronium ions, R ¼ Me or Et) [96], among others (Table 8-1). The silylated species represents the closest approach yet to a true R3Siþ (silylium) ion. In an important application having potential environmental consequences,
290
CHAPTER 8 Icosahedral carboranes: Closo-CB11 clusters
silylium salts of HCB11 H5 X6 (X ¼ H, Cl, or Br) have been shown to efficiently catalyze the replacement of fluorine atoms by hydrogen in pefluoroalkanes under mild conditions [97,98]. Some comparative observations may help to place this chemistry in perspective. For example, although C60 and other fullerenes had escaped protonation despite years of effort, HC60 þ CB11 H5 Cl6 has been isolated as a stable salt and its structure has been characterized in detail by 13C CPMAS techniques [99,100]. The sharp singlet exhibited by the 13C NMR spectrum of HC60 þ indicates a rapid movement of the proton, rendering all 60 carbon atoms equivalent on the NMR time scale [84]. Hydrated protons have been isolated in benzene solvent as H3Oþ, H5 O2 þ , H7 O3 þ , and H9 O4 þ [76,101,102], the first of which has been shown from crystallographic studies to be stabilized via hydrogen-bonded interactions between the cation and the p-system of benzene [101]. The tris(n-butyl)tin cation, previously unknown as a structurally characterized entity, is generated [80] by oxidation of Sn2(n-C4H9)6 by the stable free radical CB11 Me12 (further discussed below), which itself is formed on electrochemical oxidation of CB11 Me12 [103]. 1=2ðn-C4 H9 Þ3 Sn-Snðn-C4 H9 Þ3 þ CB11 Me12 ! ðn-C4 H9 Þ3 Snþ CB11 Me 12 In contrast to covalently bound R3SnX compounds, which feature short Sn-X bonds, the (n-C4H9)3Snþ ion is only weakly coordinated to methyl groups of the carborane cation.
8.2.9 Metal complexation The low nucleophilicity of CB11 H12 and its derivatives, especially the halogen- and alkyl-substituted species, can also be used to advantage in constructing novel metal coordination polymers. The architecture and properties of such systems (Table 8-1) are strongly influenced by the nature of the counterions, especially their size, shape, and degree of coordination to the metal cations. The stable and versatile CB11 clusters, and other monocarbon carboranes, have attracted attention because their binding to metals can be controlled and engineered via the choice of exo-polyhedral substituents; hence they are of major interest in supramolecular crystal engineering. There is considerable variation in the role played by carborane anions in metal systems: some interact with metal centers through B2 2H M binding [17,104–110]; certain halogenated CB11 H12 derivatives bind via their halogen atoms [17,21,111–115]; silver salts of HCB11 Me11 show strong CH3 Ag interactions [116] (see below); the same 2C bond cleavage in the presence of Cp2ZrMeþ, H2 IrðPPh3 Þ2 þ carborane anion undergoes B-CH3 activation and B2 2þ and Pt[P(CHMe2)3]2 [117]. In other cases, there is only weak interaction and the carborane functions mainly as a spectator ion [118,119]. Some of these findings are counterintuitive, such as the fact that the polybrominated species CB11 H6 Br6 is actually less nucleophilic than CB11 H12 , despite the lone pairs on the bromines and the much higher polarity of the brominated versus the parent carborane, a discovery that has been attributed to steric effects [114]. An example of crystal engineering in this area is the coordination polymer shown in Figure 8-2, which incorporates (4,4-bipyridine)Agþ and PhCB11 H5 I6 units and consists of zig-zag chains that are held together by interactions between the silver ion and adjacent iodine atoms on the carborane and by silver-bipyridine binding [50]. In contrast, the related complex Ag2(C14H10N4)(BrC6H4)CB11 H11 [C14H10N4 ¼ 2,3-bis(2-pyridyl)pyridine] has no direct interactions between the metal centers and the carborane anions [50]. Essentially nonbinding RCB11 F11 clusters serve as counterions that can stabilize the formerly elusive copper carbonyls, as in CuðCOÞ2 þ PhCH2CB11 F11 and CuðCOÞ4 þ EtCB11 F11 , the latter of which is the first structurally characterized copper (I) tetracarbonyl [120]. The salt Ag[CH(C6H5)3]þMeCB11 F11 is the first example of a trigonal-planar Ag(arene)3þ complex [121]. The parent CB11 H12 anion forms host-guest crystal lattices of the type M[2.2.2]cryptateþCB11 H12 Ni(TMTAA)]3, where M is Na or K and Ni(TMTAA) is tetraazacyclotetradecinenickel(II). In these perovskite-like structures, each cryptateencapsulated Naþ or Kþ cation is surrounded by six Ni(TMTAA) units in a cubic arrangement, with the non-interacting carborane ions located at the corners of the cube [12]. Other examples of novel crystal engineering include the construction of MþCB11 H6 X6 CTV H2O L coordination networks (X ¼ Cl, Br; L ¼ CF3CH2OH, MeCN; M ¼ Na, K, Rb, Cs), which feature bowl-shaped cyclotriveratrylene (CTV) host molecules and Group 1 cations [122–124] (analogous CTV-CB9 cluster arrays such as 6-31 are described in Chapter 6, Section 6.3.1.3).
8.2 Synthesis and structure
291
C I
I
I I
I Ag
N
N
N
N
Ag
Ag
I
I
I
I
I
I
I
I
I
I
I
I
C
C
FIGURE 8-2 Structure of [(4,4-C10H8N2)Ag][PhCB11H5I6] (MeCN solvent not shown).
The versatility of CB11 monocarbon carboranes has been exploited in still other ways. The complex (Ph3P)Agþ CB11 H6 Br6 , when present in only 0.1 mol%, catalyzes the hetero-Diels-Alder reaction between N-benzylidene and Danishefsky’s diene and affords quantitative yields of 8-6 at room temperature [108,125]. The same reaction is catalyzed by (Ph3P)Agþ CB11 H12 but more slowly. OMe
Ph
+ SO
Ph
(Ph3P)Ag+CB11H6Br−6
N
N
CH2Cl2 H
Ph
O
8-6 Ph
Also, silylium derivatives such as R3SiþHCB11 Cl11 and R3SiþHCB11 R5 Br6 (R ¼ H, Me) have been shown to catalyze the ring-opening polymerization of the cyclo-N3P3Cl6 chlorophosphazene trimer [126]. Introduction of vinyl groups at selective boron vertices via a rhodium phosphine complex is illustrated in Figure 8-3. Hydrogenation of these vinyl substituents forms the 2,4,8,10,12-pentaethyl derivative 8-7, as shown in Figure 8-4 [106].
292
CHAPTER 8 Icosahedral carboranes: Closo-CB11 clusters
H
−
H
H
−
−
H2C=CH2 H
C
C
C
+
CH2Cl2
H H
H
Rh
Rh+
PPh3
PPh3
PPh3
Ph3P
Ph3P
Ph3P
Rh+
norbornadiene (nbd)+
Rh(PPh3)2(nbd)+
Rh(PPh3)2 −
H −
C
H C
+
FIGURE 8-3 Synthesis of (Ph3P)2Rh(nbd)þCB11H11-n-CH5 5CH2 (n ¼ 12, 7) from (Ph3P)2RhþCB11 H12 via (Ph3P)2RhþCB11H11-n CH5 5CH2 isomers.
R −
Rh(PPh3)2(nbd)+
C 1) C2H4 H H
repeat x 6
2) H2
Rh+ PPh3
Ph3P
FIGURE 8-4 Synthesis of (Ph3P)2Rh(nbd)þCB11H7-2,4,8,10,12-Et5 (8-7).
norbornadiene
R C
−
8.2 Synthesis and structure
293
8.2.10 Special properties of CB11Me12, HCB11Me11, and CB11Me12 Peralkylated derivatives of CB11 H12 , like their B12 Me12 2 counterpart [127], are effectively spherical hydrocarbons whose CB11 core is sterically protected, so that interactions with solvents and attacking species must necessarily take place at the peripheral alkyl groups. As remarked earlier, its salts dissolve even in low-polarity organic solvents, though this property varies with the cation, with Liþ CB11 Me12 having the highest solubility; saturated solutions of the lithium salt in benzene are electrically conductive. The CB11 Me12 ion is stable toward air, strong bases, and in dilute aqueous strong acids, but decomposes on exposure to concentrated H2SO4 or triflic acid [8]. The methyl groups in CB11 Me12 and HCB11 Me11 have substantial CH3 (methide) character according to DFT calculations [128], and readily bind to transition metal [116,129,130] and main group metal [128,131,132] cations, forming the salts listed in Table 8-1. (Some transition metal salts, such as [MoCp(CO)3]2(m-I)þ HCB11 Me11 [133], do not exhibit significant interaction between the metal cation and the carborane methyls). In certain silver-phosphine complexes of HCB11 Me11 , the Ag H3C binding is sufficiently strong to persist even in CD2Cl2 solution [116]. Depending on the metal-containing cation, the metal-methyl group interaction can be quite strong and, as noted earlier, can lead to B2 2CH3 bond cleavage [117]. In the main-group metal compounds such as Me3 Mþ CB11 Me12 , where 2CH3 covalent bonding takes place as CH3 is extracted from the carborane, leaving a M is Ge, Sn or Pb, actual Me3M2 neutral CB11Me11 cluster (8-8) that has a cage boron atom with a vacant bonding orbital in place of a substituent. This “naked boron” species, described as a boronium ylide, is air-sensitive but stable as a solid below 60 C and can be generated via reactions of the CB11 Me12 radical, as discussed below [134]. Me Me
C
C
Me
Me
Me Me
Me
8-8
Me Me
8-9
Me
Me Me
Me Me
Me Me
Me
Me
Me Me
A different CB11Me11 isomer (8-9), which has a vacant orbital on the cage carbon atom and is described as a carbenoid-carbonium ylide, has been postulated as an intermediate during the extraction of the L substituent from L2 2CB11Me11 carboranes (L ¼ BrCH2CH2 or (CF3)2CHO) by electrophiles [2,31]. This species reacts with arenes in the presence of (CF3)2CHOH to generate 1-aryl-CB11Me11 products [135]. The low nucleophilicity of CB11 Me12 can be exploited in organic synthesis, as illustrated by the use of its lithium salt to catalyze pericyclic rearrangements such as the conversion of quadricyclane to norbornadiene (8-10), cubane to cuneane, diademane to triquinacene, and others [136]. The nearly-nonbinding permethylated carborane anion renders the Liþ cation highly active and effective in catalysis.
Li+CB11Me−11 C6D6
8-10
A different Liþ-catalyzed process is the radical polymerization of the alkenylcarborane salts of the type 5CH(CH2)n-2]CB11 Me11 , which occurs in the solid state or in solution, and is initiated by O2, Liþ[CH25
294
CHAPTER 8 Icosahedral carboranes: Closo-CB11 clusters
azoisobutylnitrile, or di-tert-butylperoxide [20,31,137]. These salts are generated from the LiCB11 Me11 anion, which is obtained via reaction of BrCB11 Me11 or ICB11 Me11 with n-butyllithium at 78 C: TosOðCH2 Þn2 CH¼CH2 n-BuLi þ XCB11 Me 11 ! LiCB11 Me11 ! Li CH2 ¼ CHðCH2 Þn2 CB11 Me11 X ¼ Br;I
n ¼ 37
Organic salts having CB11 Me11 as a counterion display unusually high solubility, making them attractive candidates for the development of “molecular wires” that employ polyarene oligomers (Chapter 17) [138]. Studies of intra- and intermolecular electron exchange in the phenothiazine system have been facilitated by the isolation and structural characterization of the cation radical salt o-(phenothiazinyl)2 C6 H4 þ CB11 Me11 [139]. This radical has also been employed as a p-type dopant in electron-delocalized hexaarylbenzene cation-radical salts [140]. Stable, isolable biradicals of the type • C, can be prepared by two-electron electrochemical oxidation Me11B11C2 2R2 2CB11 Me11 , where R is CH5 5CH or C 2R2 2CB11 Me11 2 dianions [141]. of the corresponding Me11B11C2 The permethylated neutral free radical CB11 Me12 , which is obtained via oxidation of CB11 Me12 , as mentioned earlier, is a remarkably stable solid that dissolves easily in nonpolar solvents, sublimes at 150 C, and is even stable in air for short periods [103]. It is, however, highly reactive and can extract electrons from aromatic hydrocarbons, amines, tetrathiofulvalene [142], and organometallics having M2 2M or M2 2C bonds [103,128]. CB11 Me12 can also function as a methyl transfer agent; for example, it reacts in a 2:1 ratio with Si2(CMe3)6 to give two equivalents of MeSi (CMe3)3, and in the process is converted to the boronium ylide CB11Me11 that has been described earlier [2]. The possible application of CB11 Me12 as a building-block for conducting polymers has been suggested [2].
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8.2 Synthesis and structure [222] [223] [224] [225] [226] [227] [228] [229] [230] [231] [232] [233] [234] [235] [236] [237] [238] [239] [240] [241] [242] [243] [244] [245] [246] [247] [248]
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CHAPTER
9
Icosahedral carboranes: 1,2-C2B10H12 9.1 OVERVIEW
The first carborane to be discovered and characterized (though not the first to be reported in the open journals), 1,2C2B10H12 (closo-1,2-dicarbadodecaborane[12] or trivially ortho-carborane) is still, more than a half-century later, the most widely utilized member of its genre and is commercially available. Its known derivatives, a selection of which is listed in Table 9-1, number in the thousands, many of them synthesized to order for a variety of applications (a more extensive compilation, Table 9-1 Extended, is available at the website http://www.elsevierdirect.com/companion.jsp? ISBN=9780123741707). In the long interim since the first edition of this book [1133] appeared, the icosahedral carboranes have achieved a degree of familiarity among organic and bioorganic chemists, and are finding increasing—indeed almost routine—use in organic synthesis as substituent groups and synthons. This particular subfield of carborane chemistry has been extensively reviewed over the years, with special attention directed to biomedical and other applications (Chapters 14–17). This chapter outlines the main routes of ortho-carborane derivative synthesis and reactivity.
9.2 SYNTHESIS OF THE 1,2-C2B10 CAGE 9.2.1 From B10H14 As is noted in Chapter 3, the principal route to 1,2-C2B10H12 and many C-substituted derivatives (9-1) is the reaction of decaborane(12)-Lewis base adducts with alkynes [1,2,129,143,151,1134]. These adducts are obtained by treatment of B10H14 with an electron donor, and can be used in situ for carborane synthesis: R
H H
H H
R
H
2L
H
RC≡CR
C
9-1
−H2 −2L
−H2 6,9-L2B10H12
B10H14
C
L
L
L = MeCN, R3N, R2S
1,2-RRC2B10H10
R, R = H, alkyl, haloalkyl, aryl, alkenyl, alkynyl
In practice, the L2B10H12 complex is not usually isolated, and the reaction is conducted in a single step by treating B10H14 with an alkyne in the presence of the Lewis base. Dialkynes similarly generate 1,10 -bis(o-carboranyl) products (9-2), usually accompanied by 1-alkynyl species (9-3) [327,328]. A more efficient and practical synthesis of 9-2 from C,C0 -dilithio-o-carborane is described in Section 9.4.
Carboranes. DOI: 10.1016/B978-0-12-374170-7.00009-4 © 2011 Elsevier Inc. All rights reserved.
301
302
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12 R
R C H H
H H
RC≡C−C≡CR
R
C C
C
C +
RC≡C
C
MeCN
9-2
9-3 0
In an important advance, it has been demonstrated that 1,2-RR C2B10H10 derivatives can be obtained directly from B10H14 and alkynes in ionic liquid solvents such as 1-butyl-3-methy-imidazolium chloride (bmimCl) at 120 C, affording a wide range of C-organosubstituted derivatives (a sampling of which is included in Table 9-1) in good yield [152]. This method offers significant advantages over the classical synthesis in that it requires no Lewis base, is generally fast ( B(8,10) > B(4,5,7,11) [163], which corresponds to the calculated charge distribution, in which the negative charge on the BH units increases with increasing distance from the cage carbon atoms. (Although borons 8 and 10 are calculated to be slightly more negative than B(9,12) in the ground state, the five highest filled and most polarizable molecular orbitals place a higher negative charge on B(9) and B(12) [713,717].) The fact that full B-methylation is not achieved is explained by theoretical calculations [268] and experimental evidence [268,1174] showing that methyl groups attached to boron in boron clusters are electron-withdrawing (2 2I effect) rather than electron-donating (þI) as is usually assumed in hydrocarbon chemistry; thus, the added methyl units lower the framework negative charge and inhibit further electrophilic B-methylation.
9.5 Substitution at boron H
H
H
EtBr C
C
C
C H
AlCl3
Et
C
H
397
EtBr
H
Et
C
AlCl3 Et
1,2-H2C2B10H9-9-Et
1,2-H2C2B10H8-9,12-Et2 EtBr AlCl3
1,2-H2C2B10H2-4,5,7,11,8,9,10,12-Et8 H
H
C EtBr
C
H
C
Et Et
AlCl3
EtBr
H
C
Et
AlCl3 Et
Et
1,2-H2C2B10H6-8,9,10,12-Et4
Et
Et
1,2-H2C2B10H7-8,9,12-Et3
FIGURE 9-4 Stepwise electrophilic ethylation of o-carborane.
As has been observed in metallacarboranes [1174], the –I effect evidently is restricted to CH3 and is not observed with ethyl or other alkyl groups. Studies of the reaction with ethyl bromide over AlCl3 have shown that it is self-promoting, in that the addition of the initial ethyl groups increases the electron density on the remaining boron atoms and makes them more susceptible to electrophilic attack; this allows the addition of up to eight ethyl groups (Figure 9-4); the analogous reaction with excess MeI in the presence of AlCl3 affords exclusively 1,2-H2C2B10H2-4,5,7,8,9,10,11,12-Me8 [165]. In the EtBr reaction, it is possible to isolate desired species in reasonable yield by quenching the reaction at a particular stage [163,164,172]. Owing to their low negative charge, the borons adjacent to both framework carbon atoms, B(3) and B(6), are unreactive and cannot be alkylated under electrophilic conditions; however, by treating 1,2-C2B10H11-3-Me (obtained by insertion of BMeþ into nido-C2 B9 H11 2 ) with methyl iodide over AlCl3 one obtains the nonamethyl species H2C2B10HMe9 in good yield [167]. A different nonamethyl derivative, HMeC2B10H2Me8, can be obtained by octa-Bmethylation of HMeC2B10H10 with excess MeI [165]. The formation of the 8,9,10,12-tetraethyl compound is enhanced by the addition to the reaction mixture of ethylene [172], which reacts with the HBr released in the alkylation to generate more EtBr; this product in turn attacks the carborane, thereby promoting overall efficiency of the alkylation process. Other alkyl halides also show electrophilic substitution with 1,2-C2B10H12 with varying degrees of success depending on the agent and the solvent system; for example, while many RX agents work in CS2 solution, in nitromethane only isopropyl bromide is reactive [163]. Isopropyl chloride and bromide in the presence of AlCl3 in CS2 at 20 C give the 8- and 9-isopropyl and 8,9-diisopropyl derivatives [176]. With the same catalyst and solvent at 85-90 C, vinyltrichlorosilane affords 1,2-C2B10H10-n(CH2CH2SiCl3)n products in which n is 1 to 3 [804]. Alkylation of 1,2-(Me2CH)C2B10H11 with 4-chlorobutyric acid over AlCl3 in CS2 generates 9- and 12-(CH2)3C(O)OH derivatives, but this reaction does not take place with parent 1,2-C2B10H12 [148]. However, 5-chlorovaleric acid does alkylate 1,2-C2B10H12: [148] AICI3
1; 2-H2 C2 B10 H10 þ ClðCH2 Þ4 CðOÞOH ! H2 C2 B10 H9 -9-ðCH2 Þ4 CðOÞOH With excess 5-chlorovaleric acid, H2C2B10H10-9,12-[(CH2)4C(O)OH]2 is obtained.
398
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
9.5.2.3 Electrophilic halogenation Reactions of Cl2 [197,216,689,1175–1180], Br2 [133,197,216,689,691,713,1176,1177,1179,1180], and I2 [167,168,197,216,689,691,715,722,1176,1177,1179,1180] with o-carborane in the presence of catalysts such as AlCl3, AlBr3, FeCl3, or strong protonic acids [220,712] follow the same order of substitution as in the interactions with alkyl halides, except that only the relatively negative boron atoms at vertexes 9, 12, 8, and 10 are halogenated; as in the alkylation reactions, no substitution takes place at carbon. With all three halogens, the main products are B(9/12)-X and B (9,12)-X2, accompanied in some cases by B(8/10,9,12)-X3 and B(8,9,10,12)-X4 species. In contrast to the reactions of Cl2 and Br2, iodination requires elevated temperatures. The interaction of I2 with o-carborane in refluxing CH2Cl2 over AlCl3 affords 1,2-C2B10H11-9-I in 93% yield [696], avoiding the formation of chlorinated side products that are often found in reactions conducted in CCl4. The choice of catalyst can affect the course of the reaction; iron filings, which are converted to Fe(FeX3), appear less selective than AlX3 reagents, giving some 8(10),12-dihalo derivatives along with the predominant 9,12-dihalo products. Moreover, CCl4 and CH2Cl2 are effective halogenating agents in the presence of AlCl3 (see below), but not with the iron catalyst [689]. The observed reactivity of parent 1,2-C2B10H12 toward halogens differs from that of its C-substituted derivatives in some cases. For example, while o-carborane itself can only be tribrominated under electrophilic conditions, treatment of 1,2-Me2C2B10H10 at room temperature affords 1,2-Me2C2B10H6Br4; the difference is attributed to an increase in negative charge at the BH groups induced by the methyl groups (the order of halogenation at boron sites is the same in the parent, methyl, and dimethyl compounds). This assumption is supported by molecular orbital calculations on 1-methyl and 1,2-dimethyl-o-carborane, which show that the charge distribution is the same as in the parent carborane but with higher negative charges at all BH vertexes [719]. Kinetic studies of the rates of electrophilic halogenation in parent and 1-methyl-o-carborane indicate that electron donation by methyl substituents bound to carbon occurs via an inductive (þI) mechanism, [690] in contrast to the electron-withdrawing (–I effect) for B2 2CH3 groups noted earlier. Further information on patterns of electrophilic halogen substitution has been obtained from reactions of less symmetric species such as 1-methyl-o-carborane, in which B(9) and B(12) are nonequivalent [1178–1181]. Chlorination of this compound affords the 9-chloro, 12-chloro, 9,12-dichloro, 9,12,8(10)-trichloro, and 9,12,8,10-tetrachloro derivatives [1178,1180], whose structures were determined from their NMR [1177,1182] and 35Cl NQR [287] spectra and X-ray crystallographic data on corresponding bromo and iodo derivatives (Table 9-1). Electrophilic halogenation can be conducted with other reagents as well. With halomethanes the same substitution sequence is observed as with elemental halogens [689,1151,1152,1183,1184]; for example, refluxing o-carborane in CCl4 or CHCl3 over AlCl3 gives the 9,12-dichloro derivative in high yield with minor amounts of mono-, di-, and trichloro-o-carboranes [1183]. Iodobenzene and o-carborane react at 300 C to form mainly the 8- and 9-iodo derivatives [1185], while the AlCl3-catalyzed interaction with iodine monochloride furnishes a simple route to 9,12-diiodo-ocarborane [162]; the same compound can be obtained by treatment with I2 and acetic, nitric, and sulfuric acids [736], or with PhI(O2CCF3)2 in CCl4 [727]. An alternative high-yield route to the 9-I derivative entails the reaction of 1,2-C2B10H12 with I2 and IO3 ion in acidic aqueous media at 80-100 C; with excess iodine/iodate the main product is the 9,12-I2 species [715]: þ 5H2 C2 B10 H10 þ 2i2 þ 10 3 þ H ! 5H2 C2 B10 H9 I þ 3H2 O
The analogous bromination conducted with Br2 and BrO3 affords the 9-Br and 9,12-Br2 derivatives [715]. The reaction of 1,2-C2B10H12 with ICl and triflic acid at 120 C forms 1,2-H2C2B10H2-4,5,7,8,9,10,11,12-I8 in 66% yield [738]. Similarly, Br2 and CF3C(O)OH efficiently convert o-carborane to the 9(12)-monobromo compound [712]. Derivatives containing nine or ten iodo substituents on boron—of interest in various medical and other applications (Chapters 16 and 17)—can be prepared in several steps, outlined in Figure 9-5. As shown, these involve nucleophilic cage-opening of o-carborane and bridge-deprotonation to form the 11-vertex nido-7,8-C2 B9 H11 2 dianion (Chapter 7), insertion of BI2þ via reaction with BI3 to give B(3)-iodo-o-carborane, and finally treatment with ICl to iodinate all of the remaining B-H vertexes except B(6)-H, affording a 94% yield of 1,2-H2C2B10HI9 [167]. As shown, a similar sequence involving the B(3,6)-diiodo carborane affords the fully B-iodinated species 1,2-H2C2B10I10 in 73% yield [739].
9.5 Substitution at boron
2− H
H
C
C
BI3
H
I
I
I H
CC
1) base 2) BuLi
H
H H
H
CC
399
CC
BI3
2− I
1,2-H2C2B10H9-3-I
1) base 2) BuLi
H
1,2-H2C2B10H8-3,6-I2 ICl triflic acid
ICl triflic acid
H
I
I
CC H
I
H
CCI
H H C C
I
I
I
I
I I
I
I
I
I
I 1,2-H2C2B10HI9
I
I
I I
1,2-H2C2B10I10
FIGURE 9-5 Synthesis of B-I9 and B-I10 o-carborane derivatives.
Electrophilic halogenation of aryl-substituted o-carborane derivatives is a useful way to directly compare the electronic properties of the carborane and aryl systems. Reactions of PhC2B10H11 with halogens over FeCl3 or iron filings take place preferentially at the carborane cage, and the reaction can be quenched before the phenyl ring is attacked. The effect is attributed to deactivation of the phenyl group via electron withdrawal by the carborane, since experiments involving competitive bromination of benzene and PhC2B10H11 over a 30-min period yield perbromobenzene while the carborane is mostly unaffected [197]; longer reaction times do result in halogenation at boron. Thus, prolonged treatment with Cl2 over AlCl3 forms (C6Cl5)C2B10H7-8,9,10,12-Cl4, while the corresponding reaction with Br2 affords (C6Br3H2) C2B10H7-8,9,10,12-Br4 [197,216,1179]. Halogenation of 1-vinyl-o-carborane proceeds somewhat differently; attack of Cl2 or Br2 takes place first at the vinyl group followed by substitution at boron, while I2 reacts only with the carborane cage and leaves the vinyl unaffected [1186].
9.5.2.4 Thermal iodination A direct, straightforward approach that offers significant synthetic advantages utilizes the reaction of I2 with 1RC2B10H11 derivatives (R ¼ H, Me, or Ph) at 270 C in sealed tubes without solvent. Under these conditions, the corresponding 1-RC2B10H7-8,9,10,12-I4 derivatives are obtained in high yield together with minor amounts of tri- and diiodo species [140,737].
9.5.2.5 Photochemical (radical) halogenation The reaction of elemental chlorine with o-carborane under ultraviolet light initially results in substitution at B(9) and B (12) followed by B(8) and B(10), as in the Friedel-Crafts processes just described, but the selectivity in this case is lower [27,133,197,706,713,1187]; even B(3) and B(6) are ultimately chlorinated, yielding the B-decachloro derivative. By adjustment of reaction times and conditions it is possible to isolate derivatives having four to ten chlorines at boron
400
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
positions; as in the electrophilic reaction, no halogenation occurs at carbon even under forcing conditions (early reports to the contrary [27,1188] were later corrected [197,702]). However, the perchlorinated 1,2-C2B10Cl12 carborane can be obtained via chlorination in strongly alkaline media in which the acidic C-H hydrogens are removed [197,706]. Treatment of 1-phenyl o-carborane under ultraviolet light ultimately yields 1,2-PhHC2B10Cl10; in contrast to electrophilic chlorination, the phenyl ring is completely unaffected [1183,1187]. Photochemical bromination of o-carborane and 1-methyl-o-carborane is more difficult than chlorination [197,1187] and, unlike electrophilic bromination, yields only monobromo species. From the parent compound one obtains H2C2B10H9-9(12)-Br, while 1-methyl-o-carborane affords HMeC2B10H9-9(12)-Br and HMeC2B10H9-9(12)-Br and HMeC2B10H9-8(10)-Br [1176]. Radical bromination of 1-ethynyl-o-carborane in CCl4 forms mainly trans-(1,2-dibromovinyl)-o-carborane accompanied by a lesser amount of the cis isomer; the boron cage is not brominated under these conditions [1189]. Photochemical bromination of 1-phenyl-o-carborane, as with chlorination, does not attack the phenyl ring, and generates mainly the 1-phenyl-12-bromo derivative [197,1187].
9.5.2.6 Fluorination In contrast to the other halogens, F2 attacks o-carborane with apparently little selectivity to generate derivatives containing 1-10 fluorines. With excess F2 in liquid HF at 0 C, H2C2B10F10 is obtained in 30% isolated yield; no fluorine substitution takes place at the CH vertexes [679]. However, treatment of the parent carborane or its C-alkyl derivatives with SbF5 in fluorocarbon solvents results in stepwise electrophilic substitution starting at B(9)-H and giving successively the B(9)-F, B(9,12)-F2, B(8,9,12)-F3, and B(8,9,10,12)-F4 derivatives [675,678]. Again, the cage carbons are unaffected. A suggested mechanism for this reaction [675] involves initial formation of a hexacoordinate antimony complex 9-47 at the electron-rich B(9) [or equivalent B(12)] vertex, which decomposes to give H2C2B10H9-9-F, HF, and SbF3; the overall reaction then entails reduction of Sb(V) to Sb(III). Subsequent complexation at other B-H bonds leads to further fluorine substitution. H H C C
F F
H
Sb
9-47
F
F F
9.5.2.7 Thermal and photochemical organosubstitution at boron A few reactions of o-carborane or its C-substituted derivatives involving simple thermolysis or UV irradiation have been discovered, in some cases serendipitously, to effect addition of functional groups at boron. Heating with esters at 200-275 C affords 9-alkyl products efficiently, with the carborane itself apparently functioning as a Friedel-Crafts catalyst [1190]: 2CðOÞOR or ROðOÞC2 2C C2
1; 2-H2 C2 B10 H10 ! H2 C2 B10 H9 -9-R ½ROðOÞC3 CH
R ¼ Me; Et; n-C3 H7 ; CHMe2
Carbenes generated by irradiation of ethyl diazoacetate in C6F6 solution attack o-carborane at all B-H locations to form all four possible isomeric B-substituted monocarboxylic acids. The C2 2H bonds are not affected [1191]. 1; 2-H2 C2 B10 H10 þ : CHCðOÞOEt ! 1; 2-H2 C2 B10 Hg -n-CH2 CðOÞOEt
n ¼ 3ð6Þ; 4ð5; 7; 11Þ; 8ð10Þ; 9ð12Þ
Similar results are obtained from thermal or photolytic reactions of o-carborane with CX2 carbenes (X ¼ H, F, or Cl) in C6F6 [161]; in line with theory, insertion occurs first at the more negatively charged B-H locations. Although these authors reported that m- and p-carborane fail to undergo such reactions [161], both isomers have since been found to react with CRH carbenes (R ¼ H or COOEt) to form B2 2CH2R insertion products [1192]. The observed B-H substitutions contrast with the quite different behavior of carbenes toward aromatic hydrocarbons, which typically form bicyclic norcaradienes that then open to give cycloheptatrienes. Expansion reactions of the latter
9.5 Substitution at boron
401
type with o-carborane are highly disfavored because they would disrupt the extremely stable, electron-delocalized icosahedral cage geometry [1191]. Other reagents have also been shown to convert B2 2H to B2 2C bonds. The triosmium methylidine cluster [Os3(m-H)3 (CO)9(m3-C)]3(O3B3O3) reacts with o-carborane with BF3 as a Friedel-Crafts catalyst, forming 1,2-H2C2B10H9-9-(m3-C) Os3(m-H)3(CO)9; the corresponding reaction with 1,7-C2B10H10 (m-carborane) is not observed [927]. Free radicals obtained 2O2 2O2 2CMe3 attack 1,2-Ph2C2B10H10 to yield B2 2C linked polymers [1193]. by thermolysis of Me3C2
9.5.2.8 Synthesis of B22OH and B22ONO2 derivatives via o-carborane oxidation
An early discovery in the investigation of carborane chemistry was the action of 100% nitric acid on 1,2-RR0 C2B10H12 (R, R0 ¼ H, Me), which generates the corresponding B-hydroxy and B-nitrato products in high yield [374,381]. Although definitive structural assignments for these compounds have not been published, the fact that single B2 2OH and B2 2ONO2 isomers are obtained suggests that the point of substitution is B(9/12), which is the most negatively charged BH unit and the most susceptible to oxidative hydroxylation [374]. Treatment of o-carborane with KMnO4 in acetic acid affords all four possible 2B2 2OH isomers, which are converted to the acetoxy derivatives under reaction conditions [372,1194]. 1,2-C2B10H112
9.5.2.9 B22Hg bond formation Boron-mercurated o-carborane derivatives—useful as precursors to other B-substituted species [1195], as noted in other sections of this chapter—are readily obtained by electrophilic substitution. Reaction of 1,2-C2B10H12 with an equivalent amount of Hg[COC(O)CF3]2 in CF3C(O)OH at room temperature proceeds exothermically to afford 1,2-C2B10H11-9HgOC(O)CF3 in good yield [1066–1068,1149]; treatment of this compound with NaCl yields 1,2-C2B10H11-9-HgCl. When Hg[COC(O)CF3]2 is present in excess, polysubstitution occurs to give 1,2-C2B10H12–n[HgOC(O)CF3]n (n ¼ 15) [1065,1067]. The 9-HgOC(O)CF3 derivative can also be obtained, together with Hg(1,2-C2B10H11-9-)2, by refluxing 1,2-C2B10H11-9-Tl[OC(O)CF3]2 with mercury metal in DMF [1056]. Mercuration at boron can also be accomplished by treating 1,2-R2C2B10H10 (R ¼ H, Me) with mercuric acetate in the presence of catalytic amounts of acetic acid, forming 1,2-C2B10H11-9-HgOC(O)Me [1065]. When similar reactions are conducted on derivatives that already have a substituent at B(9), mercury addition occurs at the adjacent B(12) vertex to give a product of the type 1,2-C2B10H10-9-R-12-HgOC(O)CF3, for example 9-48, which is easily converted to 9-49 and 9-50 [1073]. H
H
C
H
C
H
H
C
C
H
C
C
HgO
Cl−
CF3C(O)OH
Me
Me
HgOC(O)CF3
Me
9-48 LiAlH4 Et2O
H
H
C
C H
HgCl
9-49
C
C
Hg
Me
Me
H
9-50
402
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
9.5.2.10 B22Al, B22Ga, B22In, and B22Tl bond formation
Exo-polyhedral B2 2Al bonded o-carborane species have not been reported, but the B2 2Hg bonds in o-carboranyl mercury derivatives are readily cleaved by heavier Group 13 halides to afford B-metallated products [761,1149]: ðH2 C2 B10 H9 Þ2 Hg þ MX3 ! ðH2 C2 B10 H9 Þ2 MX M ¼ Ga; In; Tl n ¼ 04 Ga2 2B bonded species can also be obtained from mercury trifluoroacteate derivatives in trifluoroacetic acid [753]: 1; 2-H2 C2 B10 H9 -9-HgCðOÞOCF3 þ Ga½CðOÞOCF3 3 ! 1; 2-H2 C2 B10 H9 -9-Ga½CðOÞOCF3 2 Thallation is achieved via similar routes, as well as by direct treatment of o-carborane [756,759,762,763]: 1; 2-RR0 C2 B10 H9 -9-Hg½CðOÞOCF3 þ Tl½CðOÞOCF3 3 ! 1; 2-RR0 C2 B10 H9 -9-Tl½CðOÞOCF3 R ¼ H; Me; Ph ðH2 C2 B10 H9 Þ2 Hg þ Tl½CðOÞOCF3 3 ! 1; 2-RR0 C2 B10 H9 -9-Tl½CðOÞOCF3 1; 2-C2 B10 H12 þ Tl½CðOÞOCF3 3 ! 1; 2-RR0 C2 B10 H9 -9-Tl½CðOÞOCF3 In practice, o-carborane is mercurated with HgO and CF3C(O)OH and the reaction mixture is treated with Tl[C(O) OCF3]3 in a one-pot procedure, without isolation of the mercury compound. Direct thallation of o-carborane itself in the absence of mercury occurs with thallium(III) trifluoroacteate in boiling trifluoroacetic acid over several hours, but only in low yield. Boron-thallated o-carborane derivatives, like their B2 2Hg counterparts, are important synthons for the preparation of B-substituted species including those of other main group elements [325,1149]. In some cases, they can function as non-isolated intermediates, as in the synthesis of bis(1,2-dimethyl-o-carboran-9-yl) via oxidation of 1,2-Me2C2B10H10 by thallium(III) acetate and palladium(II) acetate [121]: Pd½CðOÞOMe2
1; 2-Me2 C2 B10 H10 þ Tl½CðOÞOMe3 ! ðMe2 C2 B10 H9 -9-Þ2
9.5.2.11 B22Si, B22Ge, B22Sn, and B22Pb bond formation
Isolated and characterized o-carborane derivatives containing direct exo-polyhedral B2 2Si, B2 2Ge, or B2 2Pb bonds have not been reported at this writing (products having B2 2Hg2 2–Ge arrays are, however, formed in reactions of B-mercuroo-carboranes with germanium reagents [1149]). Compounds with B2 2Sn links can be prepared from o-carboranyl mercury derivatives and tin(II) chloride [822]: 270300 C
ð1; 2-H2 C2 B10 H9 -9-Þ2 Hg þ SnCl2 ! ðH2 C2 B10 H9 -9-Þ2 SnCl2 þ Hg 300320 C
R2 C2 B10 H9 -9-HgCl þ SnCl2 ! H2 C2 B10 H9 -9-SnCl3 þ Hg Another source of B2 2Sn derivatives is via oxidative insertion of tin(II) acetylacetonate into the B2 2Hg bond in R2C2B10H9-9-HgR substrates [826,827]: H2 C2 B10 H9 -9-HgR þ SnðCHAc2 Þ2 ! þH2 C2 B10 H9 9-HgSnðCHAc2 Þ2 R0 þ H2 C2 B10 H9 -9-SnHgðCHAc2 Þ2 R0 AC ¼ MeCðOÞO R ¼ H; Me; Cl; R0 ¼ H; Me; 9-H2 C2 B10 H9 The B2 2Sn bonds in o-carboranyl compounds are much more stable toward nucleophiles than are o-carboranyl C2 2Sn links, allowing extensive substitution chemistry to be conducted on the B-stannyl derivatives as is detailed in Section 9.12.
9.5.2.12 B22N, B22P, B22As, and B22Sb bond formation General methods for binding nitrogen directly to o-carboranyl boron atoms are not available, but B(3)-amino species and related B2 2N compounds can be obtained via reactions of carborane dianions as described earlier in this section. The
9.5 Substitution at boron
403
only reported method for the formation of direct B2 2P bonds in o-carborane involves displacement of mercury from (H2C2B10H9)2Hg by trihalophosphines to form H2C2B10H9-9-PX2 products [1149,1196]. Reactions of bis(9-o-carboranyl)mercury with AsCl3 or SbCl3 generate B(9)-substituted compounds [830,832–834]: ð1; 2-C2 B10 H11 -9-Þ2 Hg þ MX3 ! 1; 2-C2 B10 H11 -9-MCl2
M ¼ As; Sb
9.5.2.13 B22S, B22Se, and B22Te bond formation
Sulfhydrylation of o-carborane via reaction with sulfur over AlCl3 at 100-130 C takes place in high yield to give the 9SH and 9,12-(SH)2 derivatives [655,657,659], a procedure that is analogous to the Friedel-Crafts conversion of benzene to thiophenol. Direct attack of S2Cl2 and SCl2 on 1,2-RR0 C2B10H12 (R, R0 ¼ H, Me) by refluxing over AlCl3 in CH2Cl2 yields S2-linked biscarboranyls 9-51, which on reduction with zinc afford B(9)-SH thiols 9-52 [642,656]. Analogous treatment with Se2Cl2 forms the diselenium-linked counterpart of 9-51 [660,841]; the same compound can be prepared by heating o-carborane with selenium metal over AlCl3 with in an O2-free atmosphere [839]. R R
R R = H, Me
C R
C
S S
C C
Zn C
C
2 HS
R
MeC(O)OH HCl
R
9-52
9-51
When excess S2Cl2 is employed, the di- and tetrathiols RR0 C2B10H10-9,12-(SH)2 and RR0 C2B10H8-8,9,10,12-(SH)4 are obtained [656]. Insertion of elemental sulfur into the B2 2Hg bond in bis(9-o-carboranyl)mercury generates a chain-linked product [1149]: 2Hg2 2S2 2B10 H9 C2 H2 ð1; 2-C2 B10 H11 -9-Þ2 Hg þ S8 ! H2 C2 B10 H92 The o-carboranyl B(9)-thiol and -thiocyanate can be generated from the thallium(trifluoroacetate) derivative [755,1149]: ð1ÞS8
1; 2-RR0 C2 B10 H9 -9-Tl½CðOÞOCF3 2 ! RR0 C2 B10 H9 -9-SH þ ðRR0 C2 B10 H9 Þ2 S2 ð2ÞH2 O 0
R; R ¼ H; Me LiAiH4
ðRR0 C2 B10 H9 Þ2 S2 ! RR0 C2 B10 H9 -9-SH hu
1; 2-RR0 C2 B10 H9 -9-Tl½CðOÞOCF3 2 þ KSCN ! RR0 C2 B10 H9 -9-TlðSCNÞ2 ! RR0 C2 B10 H9 -9-SCN Derivatives having B2 2S bonds can also be obtained from B-halogenated species, as described in Section 9.15. Boron-bonded selenium and tellurium derivatives are accessible via several routes. Reactions of o-carborane with Se2Cl2 over AlCl3 in CH2Cl2 form bis(carboranyl) diselenides analogous to 9-51, that can in turn be reduced with Zn/HCl in acetic acid to give 1,2-RR0 C2B10H11-9-SeH [840,841]. Elemental selenium and bis(o-carboranyl)mercury react in a manner similar to that described above for sulfur, affording Se-linked derivatives: [842,849] 2M2 2Se2 2B10 H9 C2 H2 þ Hg M ¼ Se; Hg ð1; 2-C2 B10 H11 -9-Þ2 Hg þ M ! H2 C2 B10 H92 Although bis(m-carboranyl) Te2 2Hg-linked compounds have been prepared (Chapter 10) [842], the corresponding o-carboranyl species are unknown. However, derivatives with B2 2Te bonds can be generated via reaction of the parent o-carborane with tellurium(IV) chloride under electrophilic conditions [851,852]: AiCi3
Na2 S
C2 B10 H12 þ TeCl4 ! C2 B10 H11 -9-TeCl3 ! H2 C2 B10 H112 2Te2 2Te-S2 2B10 H9 C2 H2
404
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
9.5.3 Addition of transition metals at boron Comparatively few well-characterized derivatives of o-carborane having exo-polyhedral transition-metal atoms directly bonded to boron are known, although boron-metallated species are important intermediates in the transition-metal-catalyzed coupling reactions described below. One approach to B-metallated clusters involves treatment of the B(3)-carboxylyl chloride with anionic metal species to give B-carboxymetal products, which on heating lose CO to form a borontransition metal bond [886,887,897,900,901]: ! R2 C2 B10 H9 -n-ðCOÞFeCpðCOÞ2 R2 C2 B10 H9 -n-CðOÞCl þ Naþ CpFeðCOÞ2 THF
D
! R2 C2 B10 H9 -n-FeCpðCOÞ2
n ¼ 3; 9;
R ¼ H; Me
H2 C2 B10 H9 -n-CðOÞCl þ Naþ ReðCOÞ5 ! H2 C2 B10 H9 -n-ðCOÞReðCOÞ5 THF
D
! H2 C2 B10 H9 -3-ReðCOÞ5
n ¼ 3; 9
The B2 2Fe bond in R2C2B10H9-3-(CO)FeCp(CO)2 (R ¼ H, Me) is cleaved by HgCl2 to afford R2C2B10H9-3-HgCl [897]. Direct addition of iridium to o-carborane can be achieved by reaction with IrCl(EPh3)2 (E ¼ P or As), formed in situ from Ir(C8H14)2Cl, to generate 9-53 [551]. C
C PPh3 + [Ir(C8H14)2Cl]2/cyclohexane
C
H
C
L
−C8H14 C = CH
Ir
9-53
L
L = PPh3, AsPh3 Cl
Another reaction type entails the intramolecular oxidative addition of transition metals to carborane B-H bonds to generate species such as 9-54 to 9-56 [551,883,900]. NEt2
H
C
H
C
C
NEt2
C
9-54
MeRe(CO)5 Re(CO)4
100 C Ph R C
Ph
R
N
N
N C
C
N
C
MeRe(CO)5
9-55
Re(CO)4
100 C R = Me, Ph, CH2 = CMe, Me2CH Me PMe2
P
C C H
Me Cl
C
L
[Ir(C8H14)2Cl]2/cyclohexane −C8H14 L = Me2PJC2B10H11
C
Ir
H H
L
9-56
9.5 Substitution at boron
405
Boron-mercurated derivatives offer another entry to transition-metal-substituted species, as in the reaction of 1,2C2B10H11-9-HgCl with Pt(PPh3)3 in benzene to give 1,2-C2B10H11-9-PtCl(PPh)2 [1011]. As was mentioned earlier, o-carboranyl compounds containing B2 2Ln bonds where Ln is a lanthanide series element, for example (RCB10H9CH-9-)3Ln, can be obtained from (RCB10H9CH-9-)3Hg and metal amalgams [854].
9.5.4 Metal-promoted cross-coupling of B-halo-o-carboranes Halogen atoms bonded to boron in 1,2- and 1,7-C2B10H12 and their C-substituted derivatives are notably unreactive toward nucleophilic and electrophilic attack; even iodine, usually the easiest halogen to remove, is essentially inert toward standard organic reagents. The solution to this problem has come through the application to carborane chemistry of a powerful technique of organic synthesis: coupling catalyzed by palladium, nickel, and other transition metals. It was discovered that B(9)-alkyl, aryl, alkenyl, and other derivatives are readily formed in reactions of 1,2-R2C2B10H9-9-I with Grignard reagents in the presence of phosphino-palladium [131,140,162,168,170,171,326,376] or nickel [170] complexes: ðPh3 PÞn MCl4-n or ðPh3 PÞ4 Pd 1; 2-H2 C2 B10 H9 -9-I þ RMgX ! 1; 2-H2 C2 B10 H9 -9-R
M ¼ Pd; Ni n ¼ 2; 4 R ¼ Me; Et; n-C4 H9 ; CHMe2 ; CH2 CHMe2 Ph; CH2 CH ¼ CH2 ; Ph; m=p-C6 H4 Me; m=p-C6 H4 F; CH2 Ph; p-C6 H4 OMe; p-CH2 C6 H4 Me; 2-thienyl; p-C6 H4 OH; CH2 -1-C2 B10 H11 The reaction proceeds more rapidly in ether-benzene solvent at 60-65 C with (Ph3P)4Pd as catalyst, particularly when R is an aryl group [376]. The same reactions take place, albeit more slowly, with corresponding m- and p-carborane B-iodo derivatives to yield B(9)-R and B(2)-R products, respectively [171]. An early report [1197] of Ullman-type linkage of 1,2-C2B10H11-9-I via reaction with copper powder to form the biscarborane (1,2-C2B10H11-9-)2 has not been confirmed in other laboratories. Coupling of FC6H4Br with 9-iodo-o- or m-carborane or 2-iodo-p-carborane in the presence of (Ph3P)2PdCl2 alone, without Grignards, gives 1,2- and 1,7-H2C2B10H9-9-C6H4F and 1,12-H2C2B10H9-2-C6H4F products, respectively [284]. On the other hand, B(9)-ethynyl derivatives of o- and m-carborane have been obtained from the respective CMgBr (R ¼ Ph, SiMe3) without transition-metal catalysts [336]. 9-iodocarboranes and RC The reaction of 1,2-H2C2B10H8-9,12-I2 with excess arylmagnesium bromide in the presence of trans-(Ph3P)2PdCl2 similarly affords 9,12-diaryl derivatives in good yield [240,410]. When this method is applied to the synthesis of 1,2H2C2B10H8-9,12-R2 dialkyl products from 1,2-H2C2B10H8-9,12-I2, much higher yields are obtained when CuI is added 2PdL2(m-R)(m-I)Cu interas a cocatalyst with (Ph3P)2PdCl2; the reaction in this case is proposed to involve RC2B10H102 mediates [162]. The utility of the cross-coupling method has been expanded by employing organozinc reagents in lieu of RMgX, thereby allowing the synthesis of B(9)-substituted and B(9,12)-disubstituted carboxyl, carbonyl, cyano, and other derivatives containing highly active functional groups [173,175]: ðPh3 PÞ4 Pd
1; 2-H2 C2 B10 H9 -9-I þ RZnX ! 1; 2-H2 C2 B10 H9 -9-R THF;Et2 O
CSiMe3 ; X ¼ Cl; Br R ¼ Et; n-C6 H13 ; CHMe2 ; CH2 Ph; CH2 CH ¼ CH2 ; Ph; CH2 SiMe3 ; C p-CH2 C6 H4 CðOÞPh; p-CH2 C6 H4 CðOÞOMe ðPh3 PÞ4 Pd
1; 2-H2 C2 B10 H8 -9; 12-I2 þ RZnCl ! 1; 2-H2 C2 B10 H8 -9; 12-R2 HF
R ¼ Me; Et
Attempts to employ Suzuki coupling, involving palladium-catalyzed reactions of boronic acids with organic halides, when applied to 9-iodo-o-carborane do not give cross-coupled products. With PhB(OH)2 and Pd(PPh3)4 in benzene containing aqueous Na2CO3, only biphenyl, parent o-carborane, and unreacted 1,2-H2C2B10H9-9-I are obtained. With 9-iodo-m-carborane under the same conditions, the desired 1,7-H2C2B10H9-9-Ph is formed in 27% yield along with
406
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
biphenyl and parent m-carborane [1198]. Both reactions proceed very slowly, and the product distributions are quite different from those of organic Suzuki reactions. Palladium-mediated isotopic exchange reactions with 125I employing Hermann’s catalyst [721] {Pd[P(oC6H4Me)2C6H4CH2]}2(m-OCHMeO)2, in toluene at 100 C enable the preparation of 125I-labeled 1,2-H2C2B10H9-n-I (n ¼ 3, 9) and 1,2-RPhC2B10H9-3-I (R ¼ H, Ph) derivatives in high yield for biomedical applications of the sort described in Chapter 16. Earlier studies have demonstrated that exchange of 131I with the 9-I, 9,12-I2, and 8,9,10,12-I4 derivatives is facilitated by Fe(II) salts [1199], while partial exchange takes place in the mono- and diiodo species in boiling THF even without a metal present [1200].
9.6 ALKYL, HALOALKYL, AND ARYL DERIVATIVES Synthetic routes to C-alkyl and C-aryl o-carborane derivatives, described in the previous section, generally involve either CR0 alkynes into the B10H14 cage or reactions of C-metallated 1,2-C2B10H12 derivatives with alkyl incorporation of RC or aryl halides; other, less general, approaches have also been employed, as noted earlier. Alkylation at boron is achieved 2R groups into nido-C2B9 substrates, or through via electrophilic attack of alkyl halides promoted by AlCl3, insertion of B2 metal-promoted coupling reactions (Section 9.5).
9.6.1 Properties of Alkyl and Haloalkyl derivatives Alkyl substituents on cage carbon atoms tend to increase the electron density at other locations via an electron-releasing mechanism, with noticeable (though usually not drastic) effects on cage reactivity. For example, C-alkylation generally promotes electrophilic substitution; as was observed earlier in this chapter, parent o-carborane can only be tribrominated, whereas 1,2-Me2C2B10H10 is readily tetrabrominated. Notably, the order of halogen substitution on the cage is the same in the parent and C-alkyl derivatives. Alkyl substitution at boron has generally similar consequences, but with an important exception: B-CH3 groups (but not other B-alkyl substituents) are known to function as electron attractors via a -I mechanism, lowering the negative charge in the cluster and inhibiting electrophilic substitution (see Section 9.5). While alkyl groups bound to the o-carborane cage are unreactive toward most attacking reagents, the 9-isopropyl derivative is reported to undergo intra- and intermolecular migration, disproportionation, and rearrangement of the propyl group on treatment with HCl or AlCl3, forming parent 1,2-C2B10H12 along with 4- and 8-isopropyl-o-carborane as well as 8- and 9-propyl-o-carborane [1201]. No mechanistic studies on this system have been reported. Replacement of the fluorine by aryl groups in 1-fluoromethyl-o-carborane can be effected via Friedel-Crafts reaction with arenes [178]: AlCl3
2CH2 F þ arene ! HCB10 H10 C2 2CH2 R R ¼ Ph; C6 H4 Me; C6 H3 Me2 HCB10 H10 C2 However, the corresponding reaction with HCB10H10C2 2CH2Cl does not occur.
9.6.2 Properties of Aryl derivatives 9.6.2.1 Electronic structure and reactivity Phenyl- and other aryl-substituted o-carboranes have been intensively studied experimentally and theoretically for decades, a main focus of interest being the electronic interaction between p-delocalized aromatic rings and the sdelocalized carborane cage [281] (see references in Table 9-1). The special attributes of this class of derivatives have attracted interest in certain areas of application such as nonlinear optics (NLO) [221], which are discussed in Chapter 17. To a large extent, C-aryl derivatives exhibit reaction chemistry similar to that of the parent or C-alkylated o-carboranes, for example undergoing electrophilic halogenation or alkylation at boron as described earlier. However, the electron-withdrawing (-I) character of aryl substituents toward the o-carboranyl system is evident in some reactions. In addition, the steric demands of the phenyl and other aryl groups have consequences in certain cases, as in the
9.6 Alkyl, haloalkyl, and aryl derivatives
407
E(CB10H10CPh)3 (9-22) species discussed above, where E is As or Sb but not P; the latter analogue is apparently precluded by the small radius of phosphorus. Not surprisingly, experimental evidence of such electronic and steric effects is strongest in molecules in which the aryl moiety is directly bound to the cluster framework, and is much less evident when the ring and cage are separated by methylene or other groups. Electronic communication between the o-carborane cage and phenyl rings attached to carbon has been extensively investigated in 1-PhC2B10H11 and 1,2-Ph2C2B10H10 and the related derivatives via X-ray diffraction, 1H, 11B, and 13C NMR, NLO properties, UV-visible, and other spectroscopic techniques, as well as via theoretical calculations (Table 9-1). The electron-withdrawing character of the phenyl group in 1-PhC2B10H11 deshields the attached carboranyl carbon atom and its neighboring cage carbon, as revealed in 13C NMR spectra [44]. At the same time, back donation of electrons from an aromatic ring p system into carborane antibonding orbitals increases the cage C2 2C bond distance, as established from crystallographic studies and molecular orbital calculations on C-aryl derivatives [205]. Changes in electron density at the carborane carbons induced by aryl substitution are reflected in reactivity patterns. 2Li exchange occurs faster than For example, when 1-(p-C6H4Br)-o-carborane (9-57) is treated with n-butyllithium, Br2 does deprotonation of the cage CH group [215]. The Liþ ðC6 H4 ÞC2 B10 H11 that is initially formed evidently undergoes rapid proton exchange between its carboranyl CH and phenyl groups to generate Liþ ðC6 H5 ÞC2 B10 H11 , which reacts with C4H9Br formed in solution to yield the final major product 9-58 accompanied by minor amounts of (BuC6H4) C2B10H11 and PhC2B10H11 [215]. Bu
H
9-57
C
n-BuLi THF, 0 C
C C
Br
H2O
−BuBr
C
H
9-58 85%
Although the phenyl ring in 1-PhC2B10H11 appears to rotate freely according to X-ray and gas-phase electron diffraction evidence [202,207], derivatives containing an electron-acceptor substituent in the ortho position on the aryl ring can show intramolecular C2 2H---O hydrogen bonding in solution, as in 1-(2-methoxyphenyl)-o-carborane (9-59) [212]. Me O
H
C
9-59
C
Other types of intramolecular aryl-cage interactions have been noted, as in 1-Ph-2-Br-o-carborane, whose solid-state structure provides evidence for weak Ph---Br binding [708]. Similarly, bonding interactions between phenyl group ortho hydrogens and metal centers in 1-Ph-2-M o-carborane derivatives can give rise to cyclic structures such as the previously cited rhodium complex 9-28. Effects of attached o-carboranyl clusters on the reactivity of aromatic rings are also of interest, especially since derivatives of the C2B10H12 isomers are increasingly employed in a variety of applications and as agents in organic synthesis. In 1-phenyl-o-carborane, the strong 1-carboranyl -I effect considerably inhibits electrophilic substitution on the benzene ring, to the extent that reactions with acyl halides over AlCl3 do not occur even under rigorous conditions [280,447]. However, phenyl rings separated from the cage carbon by a CH2 unit [447], or bonded to B(9) on the o-carborane cage, behave quite differently, undergoing rapid electrophilic acetylation, bromination, mercuration, and nitration [241]. This underlines the fact that the o-carborane moiety is an inductive electron donor toward substituents attached at boron [183,241,284,1166], in contrast to its function as an acceptor toward groups bound to its highly electrophilic carbon vertexes. It is important to recognize that the electronic behavior of a carborane cluster toward substituents is strongly dependent on the point of attachment on the skeletal framework. Statements that indiscriminately
408
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
attribute effects to the “electron-deficient carborane cage”, commonly found in the literature, are at best misleading, as the cage may be either electron donating or electron attracting toward attached groups depending on their location and properties.
9.6.2.2 Metallation of C-benzyl derivatives An interesting illustration of the C-o-carboranyl -I effect is seen in compounds of the type RCB10H10C2 2CH2Ph (R ¼ alkyl or aryl), which exhibit a marked increase in the acidity of the benzyl protons compared to those of toluene (pKa 19.5 vs. ca. 35). These derivatives react with n-butyllithium in THF solution to generate benzyllithium species, which, in turn, serve as useful precursors to a variety of other derivatives using standard preparative methods (Figure 9-6) [147]. n-C4 H9 Li RCB10 H10 C2 2CH2 Ph ! RCB10 H10 C2 2CHLiPh 20 C
R ¼ Me; CHMe2 ; Ph
n-C4 H9 Li 2CB10 H10 C2 2CH2 Ph ! PhLiCH2 2CB10 H10 C2 2CHLiPh PhCH22 20 C
R
R H
C
H
C
Ph
C C
CH2CH=CH2
C
R = Me, CHMe2, Ph
Ph
C CH2OH
R C3H5Br
H
C
C C
Ph Li
Ph2PCl
CO2
R
PhCHO H
C
C
R H
C
C
C
Ph R
C
CH2O
C O
PPh2
H
C
Ph OH
Ph
C C CH
OH Ph
FIGURE 9-6 Synthesis of C-benzyl o-carboranyl derivatives.
9.6.2.3 Reduction to anionic species Early investigations into the action of alkali metals on aryl o-carboranes showed that potassium reacts with 1,2PhRC2B10H10 (R ¼ H or alkyl) in dimethoxyethane or THF to form colored paramagnetic solutions containing PhRC2 B10 H10 radical anions, with no evolution of H2; ESR spectra of these anions exhibit no fine structure, consistent with delocalization of the unpaired electron in the skeletal framework [1202,1203]. Solutions of the PhC2 B10 H11 ion are rapidly decolorized on contact with O2, water, or alcohol [1202,1203]. In the presence of excess potassium, the
9.6 Alkyl, haloalkyl, and aryl derivatives
409
radical anion forms a stable, colorless diamagnetic PhC2 B10 H11 2 dianion, which on acid hydrolysis is converted to a PhC2 B10 H11 monoanion: The same monoanion is obtained on acid hydrolysis of the radical anion [1202,1203]. Hþ
e
PhC2 B10 H11 þ e ! PhC2 B10 H11 ! PhC2 B10 H11 2 ! PhC2 B10 H11 Hþ
PhC2 B10 H11 ! PhC2 B10 H11 The PhRC2 B10 H10 radical anions are of special interest because of their 2n þ 3 skeletal electron count, which is intermediate between the closo (2n þ 2 electron) and nido (2n þ 4 electron) classes (see Chapter 2). A recent study of Ph2 C2 B10 H10 , obtained by reduction of Ph2C2B10H10 with potassium and also by reversible electrolytic reduction, supplemented by DFT molecular orbital calculations, indicates that it has the same essential closo geometry as the neutral species except for a stretched carborane cage C2 2C bond distance [219]. An absence of p-interaction between the ring and the cluster is evident from its UV-visible spectrum; as before, ESR spectroscopy shows a broad signal lacking fine structure, which implies delocalization of the unpaired electron across the cage framework. The structure of the diamagnetic PhRC2 B10 H10 2 dianions has not been established, although one expects an open nido-type geometry consistent with their 2n þ 4 skeletal electrons. Cyclic voltammetry studies on the reduction of carbon-substituted alkyl and aryl o-carborane derivatives at a glass-carbon electrode have yielded interesting results [70]. While the parent compound and its 1-methyl, 1,2-dimethyl, 1-phenyl, and 1,2-diphenyl derivatives undergo reversible reduction to a monoanion, all except 1,2-Ph2C2B10H10 are destroyed on further reduction. Among these species, only the diphenyl compound exhibits a second reversible reduction to form a stable 1,2-Ph2 C2 B10 H10 2 dianion, presumably because of the ability of the two attached phenyl rings to accommodate the added electronic charge through delocalization.
9.6.2.4 Derivatives with C7 and larger aromatic rings Tropenyliumyl-substituted o-carboranes, also known as ousenes, are obtained by a sequence of reactions starting with the treatment of a C-lithio-o-carborane with tropenyl methyl ether to form the C-(7-cyclohepta-1,3,5-trienyl) derivative 9-60 (Figure 9-7), which is thermally rearranged to the 3-cyclohepta-1,3,5-trienyl species 9-61. Reaction of the latter compound with triphenylcarbonium ion affords the C-tropenyliumyl-o-carborane cation 9-62 [155]. H
H
C
C C7H7OMe C
C
Li
9-60 Δ H
H
C
C C
9-61 FIGURE 9-7 Synthesis of 1-(tropenyliumyl)-o-carborane cation.
Ph3C+
C
−Ph3CH
9-62
+
410
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
Studies of the electronic interaction between the aromatic C7 H6 þ ring and the carborane cage in 9-62 via NMR spectroscopy, UV-visible spectra [155,156,1204], and molecular orbital calculations [1109] demonstrate that in the ground and excited states there is a small electron-withdrawing (-I) effect but no þT ring-to-cage p-electron donation. This compares with the ground-state behavior of the isoelectronic tropenyliumyl B12 H12 2 derivative, 2B12 H11 , which is similar except that in this case there is significant þT charge transfer in the HOMO–LUMO C7H62 transition [1109]. The electronic properties of tropenyliumyl and related o-carborane derivatives have attracted interest in their NLO properties, and theoretical investigations show that some compounds in this class exhibit exceptionally high second-order NLO responses [157,338,1110]. The application of materials in this area is discussed in Chapter 17.
9.6.2.5 Derivatives with C5 aromatic rings
Derivatives of o-carborane having direct cage carbon2 2C5 ring connections are known in the form of ferrocenyl and 2COCl and NaFe(CO)2Cp related cyclopentadienyl-metal complexes (Table 9-1), which are prepared from RCB10H10C2 [893] or via reactions of ferrocenyl alkynes with B10H14 [892]. Species such as 1,2-[cyclo-C5H4FeC5H4]C2B10H10 (9-63) and 1,2-[cyclo-C5H4Fe(CO)(m-CO)2Fe(CO)-C5H4]C2B10H10 (9-64) exhibit strong electron attraction by the carborane cluster (more specifically, by the cage carbons in the framework) as reflected, for example, by substantial deshielding of the C5 ring protons in the 1H NMR spectra [893]. In 9-63, the pronounced difference in the 1H NMR chemical shifts of the Ha and Hb ring protons implies that the C5 rings are bent away from a parallel configuration as shown, with a dihedral angle greater than that (23 ) found in 1,1,2,2-tetramethyl-2-ferroceneophane. X-ray crystallographic confirmation of this finding has not been reported.
O
C C
C Fe
C
Fe OC
9-63 C
CO
9-64
Fe CO
9.6.2.6 B-aryl derivatives o-Carborane derivatives with aryl substituents attached to boron have been less investigated than their C-aryl counterparts, but several 3- and 9-phenyl-substituted species have been prepared (Table 9-1). One useful synthetic approach, discussed in Section 9.4, employs palladium-assisted B2 2C coupling; an example is the aryl-dehalogenation of 3halo-o-carboranes to afford 3-aryl products [234]. UV-photoelectron spectra of 3-phenyl-o-carborane show that the p-p interaction of the C6H5 ring with the carborane cage is weaker by 0.20 eV than that in the 1-phenyl isomer [236], consistent with X-ray structural data on that compound that show little evidence of ring-cage communication [234]. The behavior of B(9)-aryl derivatives toward electrophilic substitution on the hydrocarbon ring was discussed earlier. o-Carboranyl metal complexes having direct boron-cyclopentadienyl links can be obtained from B-carboxylic acids, which in turn are prepared by oxidation of B-methyl derivatives (Figure 9-8). In this sequence the B2 2CO2 2Fe bridged complex 9-67 is thermolyzed to eliminate CO, forming a B2 2Fe species 9-68, which undergoes rearrangement on reaction with bromine to afford 9-69 in high yield [901].
9.7 Alkenyl and alkynyl derivatives
H
H
H
H
C
C
C
CrCO3
C
H
H
C
SOCl2
C
C(O)Cl
C(O)OH
CH3
9-65 H
H
C C
9-66
NaCo(CO)2Cp H
H
C
Br Fe
CO C O
H
Br2
9-69
H
C O C
CCl4, 20 °C −HBr
Δ −CO
C C O C
Fe
9-68
411
CO
9-67
O C
Fe CO
FIGURE 9-8 Synthesis of 1,2-C2B10H9-9-(C5H4)Fe(CO)2Br (9-69).
9.7 ALKENYL AND ALKYNYL DERIVATIVES 9.7.1 Properties of Alkenyl derivatives 9.7.1.1 Reactions with halogens Alkenyl groups directly bonded to o-carboranyl carbon atoms are, in general, relatively inert toward cations and other electrophiles, as a consequence of the –I electron-withdrawing effect of the cluster toward C-bonded (but not B-bonded) substituents. This was observed very early in the exploration of carborane chemistry, where species such as 1-isopropenyl-, 1-vinyl-, and 1-allyl-2-methyl-o-carborane were found to be notably unreactive toward bromine [125,126,143]. Later it was shown that 1-vinyl-o-carborane does react very slowly with Br2 in CCl4 solution at 20 C and more rapidly in boiling CCl4, affording (BrCH2BrHC)C2B10H11 [1186]. As was noted in Section 9.5, in the presence of AlCl3 bromination of some boron atoms on the cage also occurs, yielding (BrCH2BrHC)C2B10H7-8,9,10-12-Br4; treatment of this com5CH)C2B10H7Br4. In the latter compound, the bromine pound with zinc in ethanol generates the vinyl derivative (CH25 atoms increase the –I effect considerably, so that addition of Br2 to the attached vinyl group is substantially slower than 5CH)C2B10H11 itself [1186]. in (CH25 Both the vinyl substituent and the cage are attacked by chlorine in CCl4 solution, in the presence or absence of AlCl3; iodine does not add to the double bond, even over AlCl3, but does add electrophilically to the boron cluster [1186]. AlCl3
ðH2 C5 5CHÞC2 B10 H11 þ I2 ! ðH2 C5 5CHÞC2 B10 H10n In 80 C
HCl and HBr add to 1-vinyl-o-carborane in CS2 over AlBr3, affording the respective 1-(a-haloethyl) product as expected from Markovnikov’s rule [1186].
412
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
1-Isopropenyl-o-carborane reacts rapidly with Br2 over CCl4 under ultraviolet light to give (BrH2C2 2CBrMe) C2B10H11, but the carborane cage is not attacked. In contrast to the behavior of most alkenes, Br2 fails to react at all with the 1-isopropenyl derivative in the dark in ethanolic solution [479]. Alkenyl double bonds that are remote from the o-carborane cluster (i.e., separated by one or more insulating groups) 5CHCH2CH22 2C2B10H11 show essentially normal activity toward halogens and other electrophiles; for example, 1-CH25 combines with 1 equivalent of Br2 to afford 1-CH2BrCHBrCH2CH22 2C2B10H11 [125]. Moreover, vinyl and ethynyl substituents attached to o-carboranyl boron vertexes do not exhibit a -I electron-withdrawing effect. For example, in contrast to the behavior of C-alkenyl-o-carboranes described above, B-alkenyl derivatives readily add electrophiles such as HCl in the presence of AlCl3 [1205], while B(3)- or B(9)-vinyl-o-carborane is brominated by Br2 in CCl4 to form the B(3)bromoethyl derivative in 90% yield. Treatment of the latter compound with sodium amide in liquid ammonia affords 78% 3-ethynyl-o-carborane after acid workup [334]. A similar sequence with the B(9)-allyl derivative affords the 9-propargyl product [335]. Br2
NaNH2
CCl4
NH3
Hþ
CH C2 B10 H11 -3-CH5 5CH2 ! C2 B10 H11 -3-CH2 CH2 Br ! ! C2 B10 H11 -3-C Br2
NaNH2
CHCl3
NH3
Hþ
CH C2 B10 H11 -9-CH2 CH5 5CH2 ! C2 B10 H11 -9-CH2 CHBrCH2 Br ! ! C2 B10 H11 -9-CH2 C
9.7.1.2 Reactions with acids Owing to functional group deactivation via the -I effect, 1-vinyl-, 1-isopropenyl-, and 1-allyl-2-methyl-o-carborane are unreactive toward H2O2, peracetic acid, HOCl, or IBr in glacial acetic acid [125]. However, trifluoroacetic acid reacts with both the 1-allyl and 1-isopropenyl derivatives to form stable epoxides [143,364], for example 9-70, which undergoes ring-opening to form the dihydroxypropyl derivative 9-71. A stereoisomer of the latter species, 9-73, is generated when 1-allyl-o-carborane is converted to the hydroxyformal compound 9-72 and hydrolyzed. H
H
C
C
CH2CH=CH2 C
H
C
CF3C(O)OH
O
C
CH2
CH2CHOHCH2OH C
5% H2SO4
9-70
H2O2 HC(O)OH
CH2CH
9-71
H
H
C
C
CH2CHCH2OH OH−
C
CH2CHOHCH2OH C
O C=O H
9-72
9-73
The 1-(g-butenyl) derivative, in contrast to 1-allyl- and 1-isopropenyl-o-carborane, forms a glycol trifluoroacetate on treatment with CF3C(O)OH. The presence of a 1-o-carboranyl group adjacent to an epoxy ring apparently inhibits acidcatalyzed ring opening, but not when the ring is insulated from the carborane as in 1-(g-butenyl)C2B10H11 [125]. If the reaction is conducted under basic conditions, however, a g-butenyl epoxide (9-74) is formed.
9.7 Alkenyl and alkynyl derivatives
413
H
H
C
C
CH2CH2CHKCH2 C
CH2CH2CH C
CF3C(O)OH
CH2 O
Na2CO3, CH2Cl2
9-74
9.7.1.3 Reactions with other electrophiles The effect of inductive electron withdrawal by the 1-o-carboranyl unit is evident in reactions of 1-alkenyl derivatives with the: CCl2 radical. Phenyl(bromodichloromethyl)mercury attacks 1-allyl-, 1-vinyl-, and 1-isopropenyl-o-carborane to give the gem-dichlorocyclopropane (e.g., 9-75); as expected, the fastest reaction occurs with the allyl compound [1206]. H
H
C
C
CHKCH2 C
PhHgCCl2Br
CH
CCl2
C
CH2
9-75 However, the carboranyl -I effect is demonstrated by the fact that all three alkenyl-o-carboranes are unreactive toward the organomercury reagent in competitive reactions with cyclohexene; under these conditions, 72-75% yields of 7,7dichloronorcarane, but no carboranyl products, are obtained. Notwithstanding some deactivation of adjacent C5 5C bonds by the -I effect, metal p-complexes with olefinic o-car2CH5 5CH2 2CH5 5CH2)Fe(CO)3 from 1-methyl-2-(10 ,30 boranes can be prepared, as in the formation of (MeCB10H10C2 butadienyl)-o-carborane and Fe(CO)5 [1207]. Main group metals also add to 1-alkenyl derivatives; for example, reflux2CHR5 5CH2 (R ¼ H, Me) affords HCB10H10C2 2CHRCH2GeCl3 [1208]. ing HGeCl3 with HCB10H10C2
9.7.1.4 Hydrogenation 1-Isopropenyl-o-carborane reacts with hydrogen over Raney nickel at 50 psi to give 1-(Me2CH)C2B10H11 [142,143]; however, hydrogenation of 1,2-diisopropenyl-o-carborane occurs only under high pressures (ca. 1800 psi) [141]. Quantitative hydrogenation of 1-alkenyl-o-carboranes to the corresponding 1-alkyl derivatives takes place in the presence of CO (which is required) at 120-200 atm. and 130-190 C in ethanol containing Co2(CO)8 or Rh4(CO)12 [1209]. Mechanistic studies employing deuteration of 1-isopropenyl-o-carborane show that the reaction proceeds via 2CR5 5CH2[HCo(CO)3] that interaction with the active catalyst HCo(CO)4 to form a p-complex of the type RCB10H10C2 2 CHRCH2[Co(CO)4]; the latter species combines with HCo rearranges in the presence of CO to form RCB10H10C2 2CHRMe product and Co2(CO)8 [1209]. (CO)4 to generate the RCB10H10C2
9.7.1.5 Reactions with oxidants and radicals 1-Alkenyl derivatives of o-carborane behave more or less like normal alkenes toward most oxidizing agents, such as alkaline permanganate in acetone (which oxidizes only the olefinic C5 5C bond) [143]. Manganese dioxide quantitatively 2CH5 5CHCH2OH (R ¼ Me or Ph) to acroleins at room temperature [1210]. Treatment of 1-isoconverts R2 2CB10H10C2 propenyl-o-carboranes with ozone generates aldehydes quantitatively (see Section 9.9). As mentioned earlier, 1-isopropyl-o-carborane readily undergoes radical addition with bromine under UV light. Dinitrogen tetroxide reacts to generate the nitro nitrite and a dinitro product, which on contact with silica gel are converted to a nitro alcohol and a nitro olefin (9-76 and 9-77), respectively [479].
414
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12 H
H
C
CMe=CH2 C
Me
C
C C
N2O4
CH2NO2 silica
R R = NO2, ONO Me
H
C
Me
H
C
C
CH2NO2
C
C C
OH
CHNO2
+
9-76
9-77
Reaction with tetrafluorohydrazine affords the difluoramino compound 9-78, which on treatment with sodium methoxide forms the difluoraminonitrile 9-79 [479]. H
H
C
CMe=CH2 C
C C
CH2NF2 NF2
150 psi
9-78
Me
C
C
N2F4, 150
H
Me
C
C
N
C
NaOMe
NF2
CH2Cl2
9-79
Similar reactions of tetrafluorohydrazine with alkenyl-o-carborane derivatives are reported in the patent literature [1211]. Silanes add readily to the double bonds in alkenyl-o-carboranes, as described in Section 9.10.
9.7.1.6 Metallation Replacement of the carboranyl C2 2H proton (in monoalkenyl-C-substituted derivatives) with lithium, copper, or other metals via methods described in Section 9.4, in most cases can be conducted as usual with no effect on the alkenyl 2CHgroup. Consequently, lithiation of 1-isopropenyl-o-carborane with butyllithium reagents affords Li2 2CB10H10C2 Me5 5CH2 from which C,C0 -disubstituted carboranes can be obtained via normal synthetic procedures as described ear2CB10H10C2 2CHMe5 5CH2 [364]. Similarly, lier in this chapter; for example, treatment with CO2 generates HO(O)C2 2R derivatives (R ¼ vinyl or isopropenyl) react with PPh2Cl to give Li2 2CB10H10C2 2PPh2, and with Li2 2CB10H10C2 2CB10H10C2 2P(Et2N)2 [564]. (Et2N)2PCl to generate Li2 2alkenyl derivatives where R 6¼ H, the alkenyl When no carboranyl C2 2H group is present, as in R2 2CB10H10C2 group itself can be metallated [431,1212–1216]. In the treatment of allyl-o-carboranes with n-butyllithium, the lithiated species 9-80 serve as synthons for a variety of derivatives (e.g., 9-81 and 9-82) via reactions with water, bromine, CO2, HgCl2, PX3, allyl and silyl halides, ethylene oxide, and other reagents. When R0 is alkyl or phenyl, the formation of allyl derivatives is favored, but when R0 ¼ H the allyl species tend to rearrange to trans-propenyl derivatives such as 9-80 and 9-81 under basic reaction conditions [1217]. In the case of the carboxylic acid 9-82, a 1:1 mixture is obtained [1215].
9.7 Alkenyl and alkynyl derivatives R
R
C
CH2CH=CHR
R
C
C
415
C
CH=CHCHRLi C
C4H9Li
CH=CHCH2Br C
Br2 −LiBr
R = Me, Ph R = H, Me, Ph
R = Me, Ph R = H
9-80
9-81
CO2 R = Me R = H
Me
C
O
C C
C
CH=CHCH2C
OH
O
Me
CH
CH=CH2
C
OH
+ 1:1
9-82
9.7.1.7 Coupling and cyclization reactions Ruthenium-catalyzed homo-and cross-metathesis reactions of 1-(CH2CH5 5CHCH2)-1,2-C2B10H11 afford, respectively, 5CHCH2)(C2B10H11)2 and a variety of 1-CH2CH5 5CHR derivatives listed in Table 9-1 [323]. Formation 1,10 -(CH2CH5 of exo-polyhedral rings from suitably functionalized C-alkenyl-o-carboranes has been demonstrated. In the presence of 2SnMe4 or WOCl42 2SnMe4 in toluene at 80 C, 1,2-diallyl-o-carborane is converted to soluble catalysts such as WCl62 1,4-dihydrobenzocarborane 9-83 [306]. H
C
CH2CH=CH2 C
H2C
CH
C
CH2CH=CH2 catalyst
C
CH2
C
80 C
9-83
Benzocarborane derivatives can also be obtained via nickel-mediated cycloaddition of 1,2-dehydro-o-carborane (carboryne) with alkenes [244,305,309]. Vinyl-o-carboranyl acetic acid undergoes cyclization to form 9-84 on treatment with sodamide in liquid ammonia [1218]. HO(O)C
CH2C(O)O−Na+ C
CH2 CH2
C
CH=CH2 C
H
C
NaNH2 liq. NH3
C
9-84
416
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
9.7.1.8 Addition of amides Lithium alkylamides attack the olefinic double bond in 1-alkyl-2-vinyl-o-carboranes, promoted by the strong 1-carboranyl 2CH2CH2NR0 2 (R ¼ Me, n-C3H7; R0 ¼ Me, Et) [1219]. -I effect, to form aminoalkyl products of the type R2 2CB10H10C2
9.7.1.9 Synthesis and properties of o-carboranyl carbenes Carbon atoms react with o-carborane to give 9-carboranylcarbene, whose triplet state abstracts hydrogen from alkenes in contrast to the more commonly observed carbene addition [1220]. Boron-substituted vinyl derivatives (9-85), prepared by insertion of B-vinyl groups into nido-C2B9 carboranes (Chapter 7), can be used as synthons for the preparation of B (3)-diazo compounds, which on photolysis generate carbenes (9-89) as shown in Figure 9-9. The carbenes react with cisand trans-2-butene to form cyclopropanyl products (9-90) as well as B(3)-methyl o-carboranes and other species not shown in the figure. Detailed studies of the mechanism reveal that singlet-state processes dominate but that triplet carbenes are also significantly involved, and in fact are responsible for the generation of the observed B(3)-methyl o-carborane products via double abstraction of hydrogen [114]. 2C(H) [113], generated by photolysis of HCB10H10C2 2CHN2, Similarly, the 1-o-carboranyl carbene HCB10H10C2 attacks olefins in a mostly stereospecific manner to form 1-cyclopropyl-o-carboranes [112].
R
R CMe3
R
C
R R
C
C
C
CHO
C
R
CH=NNHTs
C
1) O2
H2NNHTs
2) SMe2 R, R = H, Me
9-85
9-86
9-87 1) NaH
R H
R
C
2) 120 °C
R2 R3
C
R
R
R4 R5
hν
C
R C
9-90
C
••
CH hν
R C
CHN2
or
R2 = R3 = Me, R4 = R5 = H R2 = R3 = H, R4 = R5 = Me R2 = R5 = Me, R3 = R4 = H
9-89
9-88
FIGURE 9-9 Conversion of B(3)-vinyl derivatives (9-85) to carbenes (9-89) and reactions of carbenes with alkenes to generate B(3)cyclopropyl products (9-90).
9.7.1.10 Polymerization of 1-alkenyl-o-carboranes One of the main driving forces in the early development of carborane chemistry was the goal of synthesizing novel polymeric materials that exploited the remarkable stability and electronic properties of the icosahedral C2B10 and other carborane cluster systems. A number of synthetic approaches were explored (and are still under investigation in some
9.7 Alkenyl and alkynyl derivatives
417
cases), including the use of alkenyl and alkynyl carborane derivatives as substrates that could be polymerized using established techniques from organic chemistry. In general, polymers generated in this way are of the pendant type, labeled Class II in the first edition of this book [1133], where the carborane units are present in side chains attached to the polymer backbone but are excluded from the main chain as in 9-91. Materials of this type have been prepared via g-irradiation of 1-vinyl-o-carborane and related 1-alkenyl-o-carboranes [352,449,1221–1226].
CH
CH2
C C
9-91
H
n
1-Alkenyl-o-carboranes have been polymerized with styrene and methylmethacrylate to afford copolymers, some of which contain ferrocenylcarbonyl units bound to the cage at C(2) [1227,1228]. Carborane polymers of the pendant type have also been prepared by other routes, described in Chapter 14. With some exceptions, these materials have been largely superseded as targets of interest by Class I systems in which the carborane units are incorporated directly into the polymeric chain.
9.7.2 Properties of Alkynyl derivatives 9.7.2.1 Proton donation
C)C2B10H11 is more acidic than the carboranyl CH proton, as shown in IR spectroThe acetylenic proton in 1,2-(HC scopic studies [330] and in the finding that treatment with Grignards, butyllithium, or Cu2Cl2 results exclusively in metallation of the alkynyl group rather than the cage [296,1229]. The metallated complexes, in turn, can be converted C2 to a variety of RC 2C2B10H11 derivatives where R is a halo, alkyl, carboxyl, b-hydroxyalkyl, or other group, for example, MeHgBr
BuLi
C2 C2 C2 HC 2CB10 H10 CH ! LiC 2CB10 H10 CH ! MeHgC 2CB10 H10 CH D
Cu2 Cl2
I2
NH4 OH
C7 H16
C2 C2 C2 2CB10 H10 CH ! IC 2CB10 H10 CH HC 2CB10 H10 CH ! CuC EtMgBr
1ÞCO2
EtOH
2ÞH3 O
C2 C2 C2 HC 2CB10 H10 CH ! BrMgC 2CB10 H10 CH ! HOðOÞC2 2C 2CB10 H10 CH þ
9.7.2.2 Addition to the triple bond
C)C2B10H11 are much as expected for that functional group. NucleReaction patterns of the alkynyl group in 1,2-(HC ophilic and electrophilic bromination both proceed with high stereoselectivity to yield trans-1-(1,2-dibromovinyl)-ocarborane (9-92). Not surprisingly, radical bromination in CCl4 generates a mixture of the trans and cis isomers 9-92 and 9-93 with the former predominant (Figure 9-10); as noted earlier, no bromination takes place on the carborane cage [296,1189]. Nucleophilic addition of thiophenol occurs easily and stereoselectively to give cis-1-(2-phenylthiovinyl)-ocarborane (9-94), and HI (generated from LiI and acetic acid) similarly reacts nucleophilically to afford the cis-1(2-iodophenyl) species 9-95. Electrophilic addition of HCl over AlCl3 forms the 1-(2-chlorovinyl) derivative 9-96 [1189].
418
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12 H
H
H
Br
Br C
C
C
C
C
C
CH
Br C
C
C
Br2
C
Br2, AcOH
H
Br
C
H
PhSH
HCl CS2, AlCl3
HI
H
C
H
H
C
C C C
H
C C
9-94
C
9-92
C Br
9-93
H
C
9-95
H
C C C
I
C
SPh
H
C
H
H
H
Br
+
CCl4
9-92
H
C
Cl
9-96
FIGURE 9-10 Conversion of 1-ethynyl-o-carborane to cis- and trans-vinyl derivatives.
9.7.2.3 Polymerization Cyclotrimerization of 1-(p-ethynylphenyl)-o-carborane (9-97) together with diethynylbenzene over nickel catalysts is reported to generate p-(1-o-carboranyl)phenyl-diethynylbenzene and p-(1-o-carboranyl)-phenyl-diethynylbenzenephenylacetylene copolymers that have high oxidative thermal stability [1230]. Reaction of 9-97 with 2:1 triphenylphosphine in boiling aromatic solvents produces low molecular weight linear polymers in 92% yield; copolymerization of the same compound with p-diethynylbenzene affords higher molecular weight thermosetting polymers [1231]. H
C
9-97
C
C
CH
9.8 CARBOXYLIC ACIDS AND ESTERS Derivatives containing C(O)OH or CH2OH groups have played a major role in the enormous development of icosahedral carborane chemistry in recent years at both basic and applied levels. Because of the remarkable ability of the C2B10 cluster to remain intact under a wide range of conditions, reactions can be efficiently conducted on these functional groups by standard organic chemical procedures, leaving the cage itself unaffected and affording a large array of desired species. Moreover, carboxylic acids are useful as vehicles for studying electronic effects of the carboranyl framework via pKa and other quantitative measurements.
9.8.1 Synthesis of carboxylic acids 9.8.1.1 C-carboxyl derivatives Some of the general methods outlined in Section 9.5 for the preparation of o-carboranyl derivatives are not applicable, or require modifications, when compounds containing C(O)OH or OH functional groups are sought. Alkyne insertion into the decaborane cage, a frequently used route (Chapter 3), is not a useful approach in this case because the borane
9.8 Carboxylic acids and esters
419
framework is itself destroyed by acetylenic acids and alcohols (conversely, esters can be prepared by this method, as discussed below). C-metallated derivatives such as LiC2B10H11, Li2C2B10H10, NaC2B10H11, or o-carboranyl Grignards can be treated with CO2 and subsequently acidified to afford the C-carboxyl products (Table 9-1). However, as is noted in Section 9.4, the synthesis of [HO(O)C]C2B10H11 from LiC2B10H11 is complicated in diethyl ether and in certain other solvents by the tendency to form Li2C2B10H10 via lithium transfer; one can circumvent this problem by employing alternative media such as benzene. In C-metallated species of the type MCB10H10CR, where R is alkyl, alkenyl, or phenyl, metal exchange is blocked by the absence of a carboranyl CH proton and the synthesis of monocarboxylic acid derivatives is straightforward [52,125,129,363,417,439,1157]: ð1ÞCO2
LiCB10 H10 CR ! HOðOÞC2 2CB10 H10 CR þ ð2ÞH3 O
ð1ÞCO2
2CH2 Þ2 O ! ½HOðOÞC2 2CB10 H10 C2 2CH2 2 O ðLiCB10 H10 C2 þ ð2ÞH3 O
Acids having one or more methylene units (n ¼ 1-4) between the C(O)OH group and the cage are readily prepared via the Grignard [428,434,1161]: Mg
ð1ÞCO2
Et2 O
ð2ÞH3 O
!½HOðOÞCðCH2 Þn CB10 H10 CH BrðCH2 Þn CB10 H10 CH ! BrMgðCH2 Þn CB10 H10 CH þ As is discussed in Section 9.4, some Grignard reactions are highly solvent-dependent. In diethyl ether, the reaction of 2CB10H10CH with magnesium followed by carboxylation generates the o-carboranyl acetic acid HO(O)BrCH22 2CB10H10CH (9-14) as expected, but in THF the C-methylated species HO(O)C2 2CB10H10C2 2Me (9-13) is C2 2CH22 2CB10H10CH intermediate to BrMg2 2CB10H10C2 2Me. The chloroobtained owing to rearrangement of the BrMgCH22 2CB10H10CH undergoes a similar isomerization in both ether and THF. This type of intramethyl derivative ClMgCH22 molecular rearrangement is not found in higher haloalkyl homologues such as g-bromopropyl-o-carborane (9-98), which instead forms the cyclic species 9-99 [434,1145]. CH2
H
C
CH2
C C
(CH2)3Br
Mg
C
CH2
THF
9-99
9-98
A useful route to 1-carboxymethyl (acetic acid) derivatives employs the reaction of NaCB10H10CR substrates with sodium in liquid ammonia and subsequent acidification [432]. ð1ÞNH3
R2 2CB10 H10 C2 2Na þ BrCH2 CðOÞONa!R2 2CB10 H10 C2 2CH2 CðOÞOH R ¼ H; Me;
CH25 5CH;
ð2ÞHCl
CH25 5CHMe; CH2 Br; Ph
A different approach to carboxylic acid synthesis is based on hydrolysis of o-carboranyl esters, which as mentioned earlier, can be obtained by reactions of alkynyl esters with decaborane(14) derivatives [125,129,141,151,364,385,396,436, 439,445,1232]: Hþ
C2 2CðOÞOMe ! HCB10 H10 C2 2CðOÞOMe ! HCB10 H10 C2 2CðOÞOH B10 H12 L2 þ HC L ¼ Et2 S; MeCN In practice, the utility of this method is limited by the fact that many o-carboranyl carboxylic acid esters are resistant to acid hydrolysis, while hydrolysis in basic media frequently leads to decarboxylation. For example,
420
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
1,2-[MeO(O)C]2C2B10H10 is inert toward HCl, CF3C(O)OH, and other acids [125]; ethyl (1-o-carboranyl)acetate is 2CH2C(O)OH on treatment with unaffected by 96% sulfuric acid at 100 C, although it is converted to HCB10H10C2 8 N H2SO4 in dioxane [436]. Decarboxylation is commonly observed in esters whose carboxyl group is directly bound to the carboranyl carbon, as 2C(O)OEt and HCB10H10C2 2C(O)OEt, both of which react with KOH in ethanol to afford parin EtO(O)C2 2CB10H10C2 2C(O)-OKþ. On treatment with catalytic NaOEt in ent C2B10H12 along with the monocarboxylate salt HCB10H10C2 absolute ethanol, both esters are completely decarboxylated to give C2B10H12 and Et2CO3, even at 20 C [123,445,1233], possibly via an ionic intermediate [123,1233]: EtO
2CðOÞOEt ! HCB10 H10 C2 2COðOEtÞ ! HCB10 H10 C ! HCB10 H10 CH HCB10 H10 C2 2 EtOH
EtO
ðEtOÞ2 CO
20 C
Ester cleavage is also effected by organolithium reagents, giving C-lithio-o-carboranes that can be converted to carboxylic acids [397]: nC4 H9 Li
1ÞCO2
ðC4 H9 ÞCðOÞOMe
2ÞH3 Oþ
PhCB10 H10 C2 2CðOÞOMe ! PhCB10 H10 C2 2Li ! PhCB10 H10 C2 2CðOÞOH (Reactions of the esters with Grignards take a different course, forming ketones which on acidification form secondary and tertiary alcohols as is discussed in Section 9.9.) As expected, cleavage of o-carboranyl esters is much less evident in derivatives having methylene or other insulating groups between the carboxyl moiety and the cage, strongly indicating that the process is facilitated by inductive electron 2(CH2)nC(O)OEt (n ¼ 1-3) are withdrawal at the carboranyl carbon atom. Accordingly, the ethyl esters RCB10H10C2 unreactive toward sodium ethoxide [396], although alcoholic KOH does effect partial decarboxylation [436], However, steric influence is also important: decarboxylation at an o-carboranyl carbon is enhanced by the presence of a large sub2C(O)O Kþ to PhCB10H10CH stituent at the adjacent carbon atom, as in the quantitative conversion of PhCB10H10C2 and KHCO3 in ethanol at 20 C [123]. Like the esters, o-carboranyl acyl halides can, in general, be synthesized from decaborane complexes and are readily converted to the carboxylic acids [436]. Hþ
C2 2ðCH2 Þ2 CðOÞCl ! HCB10 H10 C2 2ðCH2 Þ2 CðOÞCl ! HCB10 H10 C2 2ðCH2 Þ2 CðOÞOH B10 H12 ðNCMeÞ2 þ HC The reaction of phenylpropiolic chloride takes a different turn, with the product initially formed cyclizing to 2,3-benzo4,5-carborano-cyclopentanone (9-100) [129,404,436]. B10H12(NCMe)2 + HC≡C(CH2)2C(O)Cl O C
H
C
C C
C
(CH2)2C(O)Cl
9-100 C-carboxylic acid derivatives in some cases can be prepared by oxidizing o-carboranyl alcohols; for example, chromium trioxide in acid solution converts mono- and bis(hydroxyalkyl)-o-carborane derivatives to the acids (the latter in low yield) [145,364,370,436]: CrO3
2ðCH2 Þn CH2 OH ! RCB10 H10 C2 2ðCH2 Þn CðOÞOH RCB10 H10 C2 H2 SO4
CrO3
R ¼ H; Ph
HOCH2 ðCH2 Þn2 2CB10 H10 C2 2ðCH2 Þn CH2 OH ! HOðOÞCðCH2 Þn2 2CB10 H10 C2 2ðCH2 Þn CðOÞOH H2 SO4
9.8 Carboxylic acids and esters
421
In practice, this approach has the disadvantage that o-carboranyl alcohols are not generally accessible via decaboranealkyne reactions as discussed earlier; however, one way around this problem is to transesterify an alkynyl alcohol, convert it to an o-carboranyl ester, hydrolyze the ester to the alcohol, and finally oxidize the alcohol to the acid [129,436,493]. B10 H12 ðNCMeÞ
CðCH2 Þn OCðOÞMe 2! CðCH2 Þn CH2 OH þ ðMeCOÞ2 O ! HC HC MeCðOÞOH
H3 O
þ
CrO3
HCB10 H10 C2 2ðCH2 Þn OCðOÞMe ! HCB10 H10 C2 2ðCH2 Þn OH ! HCB10 H10 C2 2ðCH2 Þn1 CðOÞOH n ¼ 1;2;3
H2 SO4
Reactions of lithio-o-carboranes with ethylene oxide furnish a useful route to C-hydroxyalkyl derivatives which may be converted to the acids, as in the synthesis of 9-101 and 9-102 [320,357,363,364]. O
OH
C (CH2)2OH
CH2 C
C (CH2)2OH
C
CH2
Li2C2B10H10 + 2H2C
CrO3
CH2
C
H+
OH
C
O
O
9-101 (CH2)2OH C
(CH2)2OH CH2
C
CH2 O
CH2
(LiCB10H10C−CH2)2O + 2H2C
C K2CrO7
C
H+
O HO
OH O
C
C CH2
O
CH2
C
CH2 C
CH2 O
C C
9-102 Oxidation of some o-carboranyl alcohols (e.g., [HO(CH2)n]C2B10H11 where n ¼ 2 or 3) to the corresponding acids can be conducted with KMnO4 under basic conditions, but with (HOCH2)C2B10H11 and (HOCH2)2C2B10H10 decarboxylation occurs, affording the parent carborane [125,129,436]. The latter reactions evidently involve initial formation of the respective ketones which in turn undergo base cleavage [1234]. KMnO4
HCB10 H10 C2 2ðCH2 Þ2 OH ! HCB10 H10 C2 2CH2 CðOÞOH OH
KMnO4
2CH2 OH ! HCB10 H10 CH RCB10 H10 C2 OH
R ¼ H; HOCH2
Benzoic acid derivatives of o-carborane, R2 2CB10H10C2 2C6H4-m/p-C(O)OH, are readily prepared by chromic acid oxidation of the corresponding C-tolyl compounds [227,438].
422
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
9.8.1.2 B-Carboxyl derivatives o-Carboranes having a carboxyl group directly attached to boron are rare (Table 9-1). The B(3)-C(O)OH derivative can be prepared by ozonization of 3-vinyl-o-carborane [324], or from H2C2B10H11-3-NH2 which is converted to H2C2B10H11-3-NC(O)H2 and then to H2C2B10H11-3-NC. Hydrolysis of the isonitrile to the amide followed by reaction with NaNO2 yields H2C2B10H11-3-C(O)OH. The same sequence starting with 1-methyl-o-carborane affords HMeC2B10H11-3-C(O)OH [442]. The B(9)-C(O)OH derivative has been obtained by oxidation of 9-ethyl-o-carborane with chromium trioxide and concentrated H2SO4 in glacial acetic acid at 60 C [408,444]. Similar treatment of H2C2B10H10-9-CH2CH2SiCl3 or 29-C(O)-OH [373] or H2C2B10H10–n[C(O)OH]n derivaH2C2B10H10–n(CH2CH2SiMe3)n (n ¼ 1, 2) forms H2C2B10H102 tives [441], respectively. Compounds bearing carboxyl groups on alkyl or aryl substituents attached to a boron vertex are also known. Chromic acid oxidation of 3-tolyl-o-carboranes affords B(3)-C6H4-m/p-C(O)OH derivatives (Tables 9-1 and 9-2) [214,227,229]. CrO3
C2 B10 H11 -3-m=p-C6 H4 Me ! C2 B10 H11 -3-m=p-C6 H4 CðOÞOH AcOH;H2 SO4
A different approach involves electrophilic addition of carboxyalkyl units [148]. AlCl3
ðMe2 HCÞC2 B10 H11 þ ClðCH2 Þ3 CðOÞOH !ðMe2 HCÞC2 B10 H9 -9ð12Þ-ðCH2 Þ3 CðOÞOH This method can be employed to synthesize B-p-carboxybenzyl derivatives such as 9-103, employing an excess of carborane to favor monosubstituted products [409]. H
H
C
C C
H
AlCl3
+ XCH2
CH2Cl2
C
9-103
H
HO C
X = Cl, Br
CH2
O
The same procedure can also be conducted with benzyl halides containing other electron-withdrawing functional groups including C(O)OMe, C(O)OPh, and NO2, yielding the corresponding 1,2-C2B10H11-9(12)-CH2-p-C6H4-R products [409].
TABLE 9-2 Ionization Constants of o-, m-, and p-Carboranyl Carboxylic Acids R
R0
R00 0
pKa
Solvent
References
2.48 2.49 2.61 5.5 13.9
H 2O H 2O 50% EtOH in H2O MeOH MeCN
[1235] [417,418] [324] [423] [423]
00
2R o-Carboranyl Acids 1,2-RR C2B10H92 C-substituted acids H C(O)OH H H C(O)OH H H C(O)OH H H C(O)OH H H C(O)OH H
Continued
9.8 Carboxylic acids and esters
423
TABLE 9-2 Ionization Constants of o-, m-, and p-Carboranyl Carboxylic Acids—Cont’d R
R0
R00
pKa
Solvent
References
C(O)OH C(O)OH Me Me Me Me CHMe2 n-C4H9 CH5 5CH2 Ph CH2OMe H H H H H H H H H H H H
C(O)OH C(O)OH C(O)OH C(O)OH C(O)OH C(O)OH C(O)OH C(O)OH C(O)OH C(O)OH C(O)OH CH2C(O)OH CH2C(O)OH CH2C(O)OH CH2C(O)OH (CH2)2C(O)OH SCH2C(O)OH m-C6H4-C(O)OH m-C6H4-C(O)OH m-C6H4-C(O)OH p-C6H4-C(O)OH p-C6H4-C(O)OH p-C6H4-C(O)OH
H H H H H H H H H H H H H H H H H H H H H H H
3.9 8.0 2.74 2.53 5.75 13.5 2.56 3.01 2.72 3.12 2.33 4.06 3.83 8.1 17.2 4.58 3.71 5.84 5.79 6.57 5.86 5.88 6.55
MeOH MeCN 50% EtOH in H 2O MeOH MeCN H 2O 20% EtOH in H 2O H 2O H 2O H 2O 20% EtOH in MeOH MeCN 20% EtOH in 50% EtOH in 75% EtOH in 75% EtOH in 70% dioxane 75% EtOH in 75% EtOH in 70% dioxane
H 2O H 2O H 2O H 2O in H2O H 2O H 2O in H2O
[423] [423] [52] [52] [423] [423] [418] [418] [418] [418] [418] [418] [418] [423] [423] [418] [426] [279] [227] [227] [279] [227] [227]
B-substituted acids H H H H H H H H H H H H H H
3-C(O)OH 9-C(O)OH 9-SCH2C(O)OH 3-m-C6H4C(O)OH 3-m-C6H4C(O)OH 3-m-C6H4C(O)OH 3-p-C6H4C(O)OH
5.38 7.70[a] 5.13 6.25 7.05 6.25 6.26
50% 50% 50% 20% 70% 75% 75%
H 2O H 2O H 2O H 2O in H2O H 2O H 2O
H H H
3-p-C6H4C(O)OH 4-m-C6H4C(O)OH 4-p-C6H4C(O)OH
6.99 6.61 6.64
70% dioxane in H2O 75% EtOH in H2O 75% EtOH in H2O
[324] [1236] [426] [227,443] [227] [214] [214,227, 443] [227] [214] [214]
3.20 3.34
H 2O 50% EtOH in H2O
[1235] [324]
H H H
2R00 m-Carboranyl Acids 1,7-RR0 C2B10H92 C-substituted acids H C(O)OH H H C(O)OH H
EtOH in EtOH in EtOH in EtOH in dioxane EtOH in EtOH in
H 2O
H 2O
H 2O
Continued
424
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
TABLE 9-2 Ionization Constants of o-, m-, and p-Carboranyl Carboxylic Acids—Cont’d R
R0
R00
pKa
Solvent
References
H H C(O)OH C(O)OH Me Me Me Ph Ph H H H H H H
C(O)OH C(O)OH C(O)OH C(O)OH C(O)OH C(O)OH C(O)OH C(O)OH C(O)OH CH2C(O)OH CH2C(O)OH m-C6H4-C(O)OH m-C6H4-C(O)OH p-C6H4-C(O)OH p-C6H4-C(O)OH
H H H H H H H H H H H H H H H
6.6 16.2 6.8 16.7 3.14 7.1 15.8 6.35 15.1 8.8 21.7 6.17 6.96 6.04 6.79
MeOH MeCN MeOH MeCN 50% EtOH in MeOH MeCN MeOH MeCN MeOH MeCN 75% EtOH in 70% dioxane 75% EtOH in 70% dioxane
H2O in H2O H2O in H2O
[423] [423] [423] [423] [52] [423] [423] [423] [423] [423] [423] [227] [227] [227] [227]
2-C(O)OH 4-C(O)OH 9-C(O)OH 9-SCH2C(O)OH 4-m-C6H4-C(O)OH 4-p-C6H4-C(O)OH
5.11 6.28 7.19a 5.00 6.60 6.53
50% 50% 50% 50% 50% 50%
H2O H2O H2O H2O H2O H2O
[341] [341] [406] [426] [1237] [1237]
2R00 p-Carboranyl Acids 1,12-RR0 C2B10H92 C-substituted acids H C(O)OH H H C(O)OH H H C(O)OH H C(O)OH C(O)OH H C(O)OH C(O)OH H Me C(O)OH H Me C(O)OH H H H SCH2C(O)OH
3.64 7.15 16.7 6.9 17.3 7.15 18.2 4.25
50% EtOH in H2O MeOH MeCN MeOH MeCN MeOH MeCN 50% EtOH in H2O
[87,1238] [423] [423] [423] [423] [423] [423] [426]
B-substituted acids H H H H
6.16 4.65
50% EtOH in H2O 50% EtOH in H2O
[406] [426]
B-substituted acids H H H H H H H H H H H H
a
2-C(O)OH 2-SCH2C(O)OH
Earlier value reported as 5.30 [373] is incorrect [1236].
EtOH EtOH EtOH EtOH EtOH EtOH
in in in in in in
H2O
9.8 Carboxylic acids and esters
425
9.8.2 Properties of o-carboranyl carboxylic acids 9.8.2.1 Acid strength Several broad trends are apparent from experimental measurements supported by theoretical calculations. •
• • •
Electron withdrawal via the -I inductive effect of the 1-C2B10H11 cluster produces moderately strong Brnsted acidity in carboxylic acid groups attached to cage carbon (but not boron) atoms, as can be seen in the pKa values listed in Table 9-2. Acid strength is considerably lower in B-carboxylic acids compared to their C-substituted isomers, reflecting the fact that the carborane is an electron donor to C(O)OH groups attached to boron [183,1236]. 2C(O)OH derivatives, acid strength decreases in the order o- > m- > p-carborane For C2 2C(O)OH and C2 2C6H42 owing to the decreasing polarity of the cage framework. As noted earlier, the inductive effect in C-substituted carboxyl derivatives is much reduced if the carboxyl unit and the carborane cage are separated by alkyl chains or other electronic insulators.
These general observations are further illuminated by pKa measurements of o-carboranyl anilinium ions [279] and of o-, m-, and p-carboranyl B-amino derivatives [1236]. In the latter study, it was concluded from calculated induction constants and from known pKa values that the B(9)-o-carboranyl system is a stronger electron donor than B(9)-mcarboranyl, and that B(9)-p-carboranyl is comparatively weak. The choice of solvent can also affect the measured acid strength, as in B- and C-fluorophenyl-substituted species where there is evidence of hydrogen bonding between the cage and proton-acceptor solvents [280]. The acidity of carboranyl acids may also be also modified if electron-attracting or 2C(O)OH derivatives, where R is an electron -withdrawing substituents are present. For example, in R2 2CB10H10C2 donor such as alkyl, acid strength is lowered, whereas for R ¼ phenyl or vinyl it is increased owing to electron transfer from the carboxyl unit via the cage to the R group [418].
9.8.2.2 Decarboxylation As has been noted, inductive electron withdrawal by the o-carboranyl cage toward C-bonded substituents is most evident in species in which the carboxyl unit is either directly bonded to the cage or connected via an aryl or other group capable of transmitting electronic effects. In acids where the linking group is methylene, such effects are usually minor at best. 1,2-bis(carboxymethyl)-o-carborane 1-101, for example, behaves as expected for a dicarboxylic acid, forming a diamide, dimethyl ester, or diacyl chloride on treatment with ammonia, methanol, or PCl5, respectively. Like adipic acid, on heating it loses carbon dioxide and water to form a cyclic ketone 9-104 [364]. O C
OH
CH2
CH2
C
Δ C
CH2 C
OH
C
C
−CO2−H2O
C
O
CH2
O
9-101
9-104
The 1,2-dicarboxylic acid 9-105 exhibits very different properties. It cannot be esterified, and its dimethyl ester (prepared directly from B10H14 as noted earlier) cannot be hydrolyzed to the diacid. Although the diacid forms salts with ammonia, aniline, hydrazine, and diethylamine, these salts cannot be converted to diamides. The diacid reacts with phosphorus pentachloride or thionyl chloride, forming the anhydride 9-106 rather than the acid dichloride 9-107. The latter compound, however, can be obtained by refluxing with phosphorus oxychloride in chlorine [364]:
426
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12 O
O C
C
Cl
C
C
OH C
C
OH
2 PCl5
C
C
Cl
−2 POCl3 −2 HCl
O
O
9-107
9-105
POCl3 Cl2
PCl5 CH2 C
C C
O
CH2
9-106 Like 9-105, the ether diacid 9-108 (prepared from [LiCB10H10C2 2CH2]2O as previously described), has carboxyl groups directly bonded to the o-carboranyl carbons and behaves similarly, failing to react with thionyl chloride or chlorine in phosphorus oxychloride, or to undero esterification in acid. Phosphorus pentachloride converts it to [Cl(O) C2 2C2B10H10]2O, and the compound decarboxylates at elevated temperature to form the lactone 9-109 and the parent bis(carboranyl) ether 9-110 [363]: O
OH
HO
C C
C
CH2
CH2 O
C
O C
265 C
C
9-108 O
CH2
H
H
C
C
O
C
C
C
C
CH2
CH2 O
C
+ 9-109 75%
9-110 14-17%
The lactone, in turn, can be converted into the synthetically useful acid-carbinol HO(O)C2 2CB10H10C2 2 CH2OH by hydrolysis in NaOH followed by acidification [363].
9.9 Alcohols, hydroxy derivatives, and ethers
427
Decarboxylation of the diester Ph2 2CB10H10C2 2CHPhCH[C(O)OEt]2 by HBr in acetic acid generates the cyclic 2CB10H10C2 2[cyclo-CH(C6H4)C(5 5O)CH2] [405]. C2 2C cleavage can ketone 30 -(phenyl-o-carboranyl)indan-1-one, Ph2 2Me with M(acac)n also be induced by metal acetylacetonates, as shown by the treatment of HO(O)C2 2CB10H10C2 (M ¼ Be or Zr) to form HCB10H10C2 2Me, and the similar conversion of HO(O)C2 2CB10H10C2 2C(O)OH to HCB10H10CH [1157]. Thermal degradation of bis(carboxyphenyl)-o-carboranyl polyesters has been shown to lead to extensive crosslinking, in contrast to analogous conventional organic polyesters which decompose under the same conditions [1239].
9.9 ALCOHOLS, HYDROXY DERIVATIVES, AND ETHERS 9.9.1 Synthesis 9.9.1.1 C-substituted alcohols As in the case of the carboxylic acids, the direct synthesis of o-carboranyl alcohols from B10H12L2 complexes and acetylenic carbinols is ruled out by the degradation of the borane cage in the presence of hydroxy-containing reagents. However, a work-around strategy analogous to that described above for preparing carboxylic acids can be employed; that is, esters derived from acetylenic alcohols are reacted with a decaborane derivative to afford o-carboranyl esters 2CB10H10CR0 (which are distinct from the carboxylic acid-based esters of type ROC(O) of the form R(O)CO(CH2)n2 0 2CB10H10C2 2R described in the preceding section). One then obtains the alcohol by hydrolysis (CH2)n2 [125,127,129,364,1240] or transesterification [141], for example, OH
MeðOÞCOCH22 2CB10 H10 CH ! HOCH22 2CB10 H10 CH orLiAlH4
MeOH
MeðOÞCOCH22 2CB10 H10 CR0 ! HOCH22 2CB10 H10 CH þ MeOCðOÞMe HCl
o-Carboranyl alcohols can be conveniently prepared via treatment of C-metallated derivatives with epoxides or cyclic ethers. The C,C0 -dilithiocarborane and ethylene oxide produce 1,2-bis(hydroxyethyl)-o-carborane [125,188,363, 364], while a-epoxides form secondary alcohols [370]: LiJCB10H10CJLi
O
+
CH2
CH2
HOCH2JCB10H10CJCH2OH
O MeJCB10H10CJLi +
CH2
CHR
R = H, Me, CH2KCH, Ph;
MeJCB10H10CJCH2CHROH R = H, Me, CH2Cl, Ph
Similarly, C-lithio-o-carborane and trimethylene oxide generate HO(CH2)3C2B10H11 [190,200]. The reaction with epichlorohydrin at elevated temperature takes a different course, affording a bis(o-carboranyl) alcohol and a carboranyl epoxide [370,465,466]: MeJCB10H10C−Li
O
+ CH2
CHCH2Cl
(MeJCB10H10CJCH2)2CHOH + MeCB10H10CJCH2CH
O CH2
Secondary alcohols can be obtained from C-lithiated o-carboranes [188,353,354,356,362–364,367,368] and Grignards [1147,1161] via reaction with aldehydes followed by acidification with aqueous HCl, as illustrated by the for2Li and benzaldehyde [354]. mation of 9-111 from Me2 2CB10H10C2
428
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12 Me
Me
C
C
H Li O
C +
Δ
PhCH
C C
HCl
OH
9-111 The interaction of a wide variety of aldehydes and ketones with LiC2B10H11 in dilute (0.1 M) THF solutions at 78 C affords good yields of monosubstituted secondary and tertiary alcohols of the type [RR0 (HO)C]C2B10H11 where R and R0 are H, alkyl, or aryl groups (Table 9-1) [368]. C-Sodio-o-carboranes have also been employed, as shown by the near-quantitative formation of (HOCH2)2C2B10H10 from Na2C2B10H10 and formaldehyde in liquid ammonia [353]. In general, however, carbinol yields obtained from monosodium derivatives are low because of equilibria [353]NH of3 the (l) type RJCB10H10CJCH(ONa)R
RJCB10H10CJNa + RC(KO)H
Alternatively, o-carboranyl alcohols can be prepared via interaction of esters with C-metallated carboranes, although the reactions do not always follow easily predictable patterns. Some esters, for example, combine with 1-phenyl-2-lithio-ocarborane to afford secondary alcohols, while others give ketones [394,399]. Ethyl, propyl, and isopropyl esters of aromatic acids tend to follow the first pathway, while methyl esters of aromatic acids and methyl and ethyl esters of aliphatic acids follow the second: D
HCl
2Li þ PhCðOÞOEt ! ! Ph2 2CB10 H10 C2 2CHðOHÞPh þ LiOEt Ph2 2CB10 H10 C2 D
HCl
Ph2 2CB10 H10 C2 2Li þ PhCðOÞOMe ! ! Ph2 2CB10 H10 C2 2CðOÞPh þ LiOMe Reactions of C-metallated o-carboranes with alkyl formates give carboranyl aldehydes and bis(carboranyl)carbinols, the product ratio depending on conditions: when the metallated carborane reactant is in excess, the carbinol predominates, but when a large excess of formate is employed, the aldehyde is the major product [361]. HCl
2M þ HCðOÞOR0 ! R2 2CB10 H10 C2 2CðOÞH þ ðR2 2CB10 H10 CÞ2 CHðOHÞ R2 2CB10 H10 C2 R ¼ CHMe2 ; Ph; R0 ¼ Me; Et; M ¼ Li; MgBr Another strategy that has been employed utilizes the reaction of a C-lithio-o-carborane containing a protective tertbutyldimethylsilyl group on carbon with methyl chloroformate to give a methyl ester, which on reduction with LiAlH4 affords 1-hydroxymethyl-o-carborane [149]. ð1ÞClCðOÞOMe
ðMe3 CÞMe2 Si2 2CB10 H10 C2 2Li ! HCB10 H10 C2 2CH2 OH ð2ÞLiAlH4
One might expect to prepare o-carboranyl tertiary alcohols from reactions of carboranyl ketones with organolithium or Grignard reagents, and in some cases this is indeed observed [348], for example, ð1ÞPhMgBr
Ph2 2CB10 H10 C2 2CH2 Cð¼ OÞPh ! Ph2 2CB10 H10 C2 2CðOHÞPh2 ð2ÞHCl
However, in many instances the carbonyl group is instead reduced to give a secondary alcohol, often accompanied by 2C(5 5O)Ph with exo-polyhedral C2 2C cleavage [348,391,396,397,1241]. An example is the interaction of PhCB10H10C2 2CH(OH)Ph and PhCB10H10CH. In conEtMgBr which results in both reduction and cleavage, yielding PhCB10H10C2 trast, treatment of the same carborane with n-butyllithium effects primarily cleavage with almost no reduction to the alcohol, even at 50 C. A possible explanation of these findings, and the general failure to obtain tertiary alcohols, is the formation of an unstable tertiary alkoxide 9-112 [397]:
9.9 Alcohols, hydroxy derivatives, and ethers
Ph
Ph
OLi
O
C
C
C Ph
C
C
Ph
C
C4H9Li
C4H9
9-112
Ph
Ph
Ph
C
C C
H
H2O
O
C C
429
C
Li
OH
C
CO2 H3O+
In general, o-carboranyl ketones are cleaved by organolithium reagents and are converted by Grignards to secondary or tertiary alcohols [391,1242]. It may be the case, as has been suggested [391], that tertiary alcoholate intermediates such as 9-112 are more stable when formed by Grignards than are those formed from organolithium reagents, whose O2 2Li bonds are more polar than O2 2Mg interactions; the less stable lithium intermediates consequently undergo cleavage rather than alcohol formation. However, reaction conditions including solvent and temperature are important; at elevated temperature, even the Grignard species give mainly cleavage products. Further discussion of these properties appears in Section 9.10. Secondary alcohols are also generated from o-carboranyl aldehydes via reaction with organolithium reagents, with relatively little C2 2C cleavage occurring [355,385,391,1243]; Grignards, on the other hand, form both secondary and primary alcohols with aldehydes, the ratio of these products varying with the choice of Grignard [385,391]. RMgX
2CðOÞH ! HCB10 H10 C2 2CHðOHÞR þ HCB10 H10 C2 2CH2 OH HCB10 H10 C2 R; R0 ¼ Me; Et; Ph The lower nucleophilicity of Grignards versus organolithium reagents probably accounts for the difference in behavior toward aldehydes. As is noted in Section 9.4, C-substituted o-carboranyl alcohols can be prepared in high yield via palladium-catalyzed addition of aldehydes to 1-(tri-n-butyltin)-o-carborane, or alternatively by silyl displacement from C-silyl-o-carboranes with tetrabutylammonium fluoride (TBAF). Secondary alcohols of the type [(HO)RHC]C2B10H11 can be obtained directly from parent o-carborane by TBAF-promoted reactions with aldehydes (Section 9.4) and by treating lithiobenzyl o-carboranes with aldehydes (Section 9.6 and Figure 9-6). A convenient route to a variety of functionalized alcohols is based on the reaction of o-carboranyl aldehydes with a, b-unsaturated esters, ketones, and nitriles in the presence of 1,4diazabicyclo[2.2.2]octane (DABCO) [1244]. H
H
O
C
O
C C
H
DABCO, r.t.
C
+
OMe
O
OH
C
C
C C
C H
CH2
OMe
430
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
Numerous other approaches to C-substituted o-carboranyl alcohols have been reported, including LiAlH4 reduction of carboxylic acids and ketones [189,348,363,436,1241]; the hydroboration of 1-alkenyl-o-carboranes and oxidation of the resulting boronated species to alcohols [366]; the formation of o-carboranyl diols such as 9-71 via acid-catalyzed opening of epoxide rings (mentioned earlier in Section 9.7); the preparation of o-carboranyl polyols via palladiumpromoted hydrogenation of C-poly(alkoxy)carboranes on charcoal [365]; the reduction of 1-benzoyl-o-carboranes to the alcohols with zinc in ethanol [367]; the formation of (20 -HO-cyclo-C6H10)MeC2B10H10 from MeLiC2B10H10 and cyclohexene oxide [369]; and the conversion of 1-bromohexyl-o-carborane to the corresponding 1-(CH2)6OH derivative by wet N-methylpyrrolidine [193]. The reaction of Li2C2B10H10 with ethylene oxide followed by hydrolysis to afford 1,2-(HOCH2)2C2B10H10 is described in Section 9.8.
9.9.1.2 B-substituted alcohols The two main pathways to B-organosubstituted derivatives in general, outlined earlier in this chapter, entail either the insertion of boron into an 11-vertex nido-C2B9 cluster to generate a B(3/6)–R product, or electrophilic addition to the icosahedral C2B10 cage, which generally favors B(9/12) followed by B(8/10) substitution. Boron-functionalized derivatives obtained by either approach are convertible to alcohols by standard organic methods. As an example, B(3/6)-(CH2)nOH alcohols can be generated by treating 1,2-C2B10H11-3/6-(CH2)nBr (prepared from nido-C2 B9 H11 2 and Cl(CH2)nBr at 78 C) with excess potassium acetate to form the acetate ester 9-113, which is then converted to the carbinol 9-114 [177]. H
H
H
C
C C
H
(CH2)3Br
C
KO2CMe
H
C
(CH2)3OC(O)Me
C
LiAlH4
MeC(O)OH, (MeCO)2O
H
(CH2)3OH
THF
9-113
9-114
Alcohols substituted at B(9), B(12), and other boron vertexes (Table 9-1) can be synthesized by reduction of the corresponding B-substituted ketones or carboxylic acids [373,375].
9.9.1.3 C- and B-hydroxy derivatives Carbinols containing OH groups directly bonded to the o-carboranyl framework are of interest as useful precursors to other derivatives, in part because of their acid-base properties: for example, the pKa for (HO)C2B10H11 is 5.33, comparable to that of benzoic acid (4.25). Several methods are available for the synthesis of C(1)-hydroxy derivatives, including the diazotization of (H2N)C2B10H11 to afford (HO)C2B10H11 [340]. The mono-C-hydroxy derivative can also be obtained via oxidation of lithiocarboranes with benzoyl peroxide [342,344], bis(trimethylsilyl) peroxide [321], or O2 [345]: ð1Þ ðPhCOÞ2 O2
2R!HO2 2CB10 H10 CR 80% þ PhCO2 2CB10 H10 CR Li2 2CB10 H10 C2 ð2Þ 30 C
R ¼ H; Me ð1Þ ðMe3 SiÞ2 O2
Li2 2CB10 H10 CH ! HO2 2CB10 H10 CH 80% ð2Þ 30 C
ð3ÞHCl=MeOH
O2
Li2 2CB10 H10 C2 2R ! HO2 2CB10 H10 CR 30-40% þ HCB10 H10 CR 20 C
In an efficient alternative route to (HO)C2B10H11, a carboranyl boron ester is formed and converted in situ to the desired C-hydroxy derivative by hydrolysis. The same sequence using Li2C2B10H10 affords (HO)2C2B10H10 [339]. ð1Þ BðOMeÞ3
2CB10 H10 CH 80% Li2 2CB10 H10 CH ! HO2 ð2Þ 30% H2 O2 ðexcessÞþMeCðOÞOH
9.9 Alcohols, hydroxy derivatives, and ethers
431
B(3/6)2 2OH derivatives are accessible from 3-amino-o-carboranes via reaction with sodium nitrite, while B(9/12)2 2 OH and -ONO derivatives can be prepared by treating 1,2-RR0 C2B10H12 compounds (R, R0 ¼ H, Me) with 100% nitric acid (Section 9.5). As noted earlier, oxidation of parent o-carborane with KMnO4 in acetic acid generates all four of the 2OH isomers, isolated as their acetoxy derivatives. B(3)2 2OH derivatives are also formed in possible 1,2-C2B10H11-B2 the decomposition of diazonium salts of 3-amino-o-carboranes [134,346]. NaNO2 ;H2 SO4
H2 O
RR0 C2 B10 H10 ! RR0 C2 B10 H9 -3-N2 þ ! RR0 C2 B10 H9 -3-OH MeCðOÞOH;5 C
R; R0 ¼ H; Me
The properties of C- and B-hydroxy-o-carboranes have been explored to a very limited extent. The C2 2OH derivatives, as noted above, are strongly acidic (Table 9-3) and unlike phenol are transparent to ultraviolet light to 205 nm [321]. The B2 2OH derivatives, in contrast, are orders of magnitude less acidic than the C2 2OH species (pKa ca. 5.3 vs. 8.2), the difference arising from the fact that the inductive electron-withdrawing (–I) effect in the C2 2OH compounds is due to a local deficiency of electronic charge in the vicinity of the cage carbon nuclei, and is not (as often erroneously stated) a reflection of electron deficiency in the cluster as a whole. Indeed, as was observed in the discussion of B- and C-substituted carboxylic acids, the C2B10 cluster is actually an electron donor toward substituents attached to boron; as a consequence, one finds much lower polarity of O2 2H bonds in B- versus C-substituted functional groups. Deprotonation of 1-hydroxy-2-phenyl-o-carborane with proton sponge forms the surprising product nido-7PhCB10H10-ðZ5 -COÞ 7-6 Chapter 7, Section 7.3), which has an open-cage CB10 structure with a capping CO group over the open face [1245].
TABLE 9-3 Ionization Constants of o-, m-, and p-Carboranyl C-Hydroxy Derivatives R
pKa
Solvent
References
o-Carboranyl Acids 1,2-(HO)RC2B10H10 H 5.25 H 5.33 Me 5.30
50% EtOH in H2O 50% EtOH in H2O 50% EtOH in H2O
[342,344] [321] [342]
m-Carboranyl acids 1,7-(HO)RC2B10H10 H 8.24 H 8.39 Me 8.33
50% EtOH in H2O 50% EtOH in H2O 50% EtOH in H2O
[342,344] [321] [342]
p-Carboranyl acids 1,12-(HO)RC2B10H10 H 9.03
50% EtOH in H2O
[321]
9.9.1.4 C-substituted ethers The classic route to o-carboranyl ethers is via insertion of alkynyl ethers into B10H12L2 complexes [363,377,398,455, 458,459,461,462,467,1246,1247], for example, C2 2B10 H12 ðNCMeÞ2 þ ðHC 2CH2 Þ2 O !ðHCB10 H10 C2 2CH2 Þ2 O þ 4MeCN þ 2H2 However, this method tends to give low yields of [RCB10H10C(CH2)n]2O products when n > 1. For these compounds, an alternative approach utilizing the lithiation of hydroxyalkyl derivatives followed by reaction with an alkyl halide is more efficient [357]: ð1Þ n-C4 H9 Li 2CB10 H10 C2 2ðCH2 Þ2 OH ! ROðCH2 Þ22 2CB10 H10 C2 2ðCH2 Þ2 OR HOðCH2 Þ22 ð2Þ RX
R ¼ Me; PhCH2
432
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
Similarly, HCB10H10C2 2OH combines with NaH in THF followed by EtOCH2Cl to form HCB10H10C2 2OCH2OEt [321]. Another route is based on phase-transfer catalysis involving the interaction of o-carboranyl alcohols with organic halides [456,457]: RBr; 50% aq: NaOH in CH2 Cl2
HCB10 H10 C2 2CH2 OH ! HCB10 H10 C2 2CH2 OR þ
6295%
Et3 N CH2 Ph
R ¼ n-C4 H9 ; PhCH2 CH2 ¼CHCH2 Br; ðnC4 H9 Þ4 Nþ Br
HOCH22 2CB10 H10 C2 2CH2 OH ! 50% aq: NaOH in CH2 Cl2
CH25 5CHCH2 OCH22 2CB10 H10 C2 2CH2 OCH2 CH5 5CH2
91%
Multicage extended ethers such as 9-115, which fluoresce strongly in various solvents, can be constructed from C-lithio-o-carboranes and polyhalogenated organics, in this case LiPhC2B10H10 and [p-(CH2O-3,5-C6H4Br2)]2C6H4 [468].
C C
C C O O
C C
9-115
C C
In addition to the previously described formation of epoxides such as 9-70 and 9-74 from 1-alkenyl derivatives, cyclic o-carboranyl ethers are formed in condensation reactions of alcohols, as illustrated in Figure 9-11. Reactions of 1-lithio-2-bromomethyl-o-carborane with aldehydes and ketones also generate cyclic ethers, very likely via alcoholate intermediates [188,196]. BrCH2JCB10H10CJLi + RC(O)R ⎯→ [BrCH2JCB10H10CJORRLi] ⎯→ O
R = CHKCHMe, R = H R = R = Me R = Ph, R = H RR = J(CH2)5J
CRR
H2C C
C
9.9 Alcohols, hydroxy derivatives, and ethers O
CH2
H2C
CH2 C
O
C
O CH2
H2C C
H2SO4 140 C
OH
R
CH2 C
H
CH2O
HO
H2C
C
C
C RCH(OC4H9)2
O
O
CH2
H2C C
R = H, Me
C
Me
BF3 toluene
CH2
C
H C
C
O H
C
Me
C Me
OH
O
C
C
CH2
C
HO
C
C
O
O
CH2
H2C
CH2
H
+
CH2
H2C
C C
FIGURE 9-11 Formation of cyclic ethers from 1,2-bis(hydroxymethyl)-o-carborane.
The cyclic ether-carbinol 9-116 is similarly prepared in nearly quantitative yield [382]. H
Li
HO
C C
O
C C
Li 1) HC(O)OMe 2) HCl
C H C OMe
9-116
C
433
434
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
A further illustration of the propensity of o-carboranyl alcohols to form cyclic ethers is given by the synthesis of the macrocycle 9-117, which was guided by a molecular modeling study [463]. C C
CH2
CH2
SH CH2 CH2
TsOCH2CH2OCH2CH2OTs Ts = p-MeC6H4SO2 SH
C
CH2
S CH2
CH2
CH2
CH2
CH2 O
C
CH2
S
CH2
9-117
9.9.1.5 B-substituted ethers Few examples have been synthesized, but palladium-catalyzed cross-coupling reactions of 9-iodo- and 9,12-diiodo-ocarboranes with organomagnesium halides, a method described earlier in Section 9.5, give alkoxyaryl derivatives such as 1,2-H2C2B10H9-9-R and 1,2-H2C2B10H8-9,12-R2 where R is p-C6H4OMe [240,376].
9.9.2 Reactions of alcohols and ethers A long-established route to o-carboranyl carboxylic acids involves the oxidation of the corresponding alcohols, described in Section 9.8. This approach is limited in practice by the fact that the alcohols themselves cannot be prepared directly from B10H12L2 complexes as explained earlier, and also by the observation that alcohol oxidation under some conditions leads to ketones or decarboxylation rather than acids (Section 9.8). To a considerable extent the reactions of o-carboranyl alcohols proceed as expected, undergoing conversion to esters via acylation by aliphatic acids, acid anhydrides, and acid chlorides, while reactions with vinyl ethers afford mixed acetals [123,363]. As mentioned above, tertiary o-carboranyl alcohols undergo NaOH-catalyzed cleavage in ethanol owing to the instability of the alcoholates such as 9-112 that form initially. However, treatment of (HOCH2)2C2B10H10 with active metals in inert solvents readily forms dimetallo alcoholates that are stable except for undergoing hydrolysis on contact with water [364]. Oxidation of secondary a- and b-o-carboranyl alcohols with chromic acid in acid media generates ketones [348,370,401,1234], while primary alcohols are converted to the acids, as described in Section 9.8. Other aspects of the chemistry of o-carboranyl alcohols give strong evidence of electron withdrawal by the cage car2C2B10H11 is unreactive toward NaBr in concentrated H2SO4 or bon atoms via the -I effect. For example, while HOCH22 2C2B10H11 48% HBr in sulfuric acid, it combines with thionyl chloride in the presence of pyridine to form ClOCH22 [123]. Polymer synthesis based on o-carboranyl alcohols was a synthetic goal in earlier years, with limited success; for example, bis(2-hydroxyethyl-1-carboranylmethyl) ether reacts with formaldehyde to give a linear polymer [449,1248]: ðHOCH2 CH22 2CB10 H10 C2 2CH2 Þ2 O þ HCðOÞH ! 2 2½2 2CH2 CH22 2CB10 H10 C2 2CH22 2O2 2CH22 2CB10 H10 C2 2CH2 CH22 2O2 2CH22 2O2 2n2 2 2CB10H10C2 2CH2)2O is unreactive toward formaldehyde. BaseIn contrast, the hydroxymethyl derivative (HOCH22 catalyzed acylation of 1,2-(HOCH2)2C2B10H10 affords methacrylate oligomers; with triethylamine, an intermediate involving coordination of Et3N to both OH groups is involved [1249]. Polyesters of moderate chain length can be prepared from o-carboranyl diols and organic diacids [356,449,1250,1251], or by condensation of monomers [1252]. In general, the tendency of o-carboranyl alcohols to form cyclic ethers (see above) and other exo-polyhedral ring structures [125] has limited their usefulness as polymer precursors. Steric crowding of adjacent functional groups on the o-carboranyl cage, as well as deactivation resulting from inductive electron withdrawal at the carbon atoms [449,1253], both render the o-carboranyl system less favorable as synthons in comparison to m- and p-carboranyl derivatives, which consequently have attracted somewhat more attention in polymer development (Chapter 14).
9.10 Aldehydes and ketones
435
Characteristic reactions of o-carboranyl ethers are mainly limited to the ring opening of epoxides mentioned earlier. As a number of examples cited elsewhere in this chapter indicate, the ether linkage survives a wide range of conditions, allowing standard organic chemistry to be conducted at other functional groups.
9.10 ALDEHYDES AND KETONES 9.10.1 Synthesis of aldehydes 9.10.1.1 C-substituted aldehydes Although o-, m-, and p-carboranyl aldehydes were recognized as potentially important building-block compounds early in the development of carborane chemistry, their synthesis presented a challenge to early investigators since the standard approaches that might have been expected to work were unsuccessful. Oxidation of o-carboranyl alcohols with CrO3 or KMnO4 was found to give carboxylic acids and o-carborane, respectively, while reactions of formates with carboranyl C2 2MgBr derivatives, and reduction of o-carboranyl esters, also failed to form aldehydes [385]. Nevertheless, several synthetic routes to ocarboranyl aldehydes were eventually developed, most of them affording relatively low yields or requiring several steps. The older methods include palladium-catalyzed hydrogenation of o-carboranyl acid chlorides in boiling xylene [385,389], ozonization of vinyl carboranes to form C-epoxy derivatives that undergo ring-opening to give aldehydes [112,114,324, 391,1243], reduction of 1-cyano- to 1-formyl-o-carboranes [386], and the reaction of alkynyl aldehyde diacetates with decaborane-base adducts to form o-carboranyl aldehyde diacetates that are hydrolyzed to generate the aldehydes [385]. dimethylaniline
H Oþ
3 CH½OCðOÞMe2 þ B10 H14 ! HCB10 H10 C2 HC 2CH½OCðOÞMe2 ! HCB10 H10 C2 2CðOÞH
As noted earlier, the treatment of C-metallated o-carboranes with alkyl formates produces both carboranyl aldehydes 2M (R ¼ H, Me), with and bis(carboranyl)carbinols. Reactions of C-lithio- or C-sodio-o-carboranes, RCB10H10C2 BrCH2CH(OEt)2 in a variety of media including benzene, THF, diethyl ether, and liquid ammonia, give acetals, which 2CH2C(O)H products [380,390,1254]. Similarly, 1-formyl derivatives RCB10H10C2 2C on hydrolysis form RCB10H10C2 (O)H, R ¼ H, Me, Ph) have been obtained from LiC2B10H11 and PhOCH(OEt)2 [388]. Application of this approach to 2C(O)H from C-lithio-o-carborane and methyl formate affords the desired 1-formyl the synthesis of HCB10H10C2 derivative in 95% yield in a one-pot procedure [382]. However, attempts to prepare the unknown C,C0 -diformyl-ocarborane via analogous treatment of Li2C2B10H10 have given instead the cyclic ether 9-116 as described in Section 9.9. (In contrast, the C,C0 -diformyl derivatives of m- and p-carborane are easily generated by this method from 1,7- and 1,12Li2C2B10H10 respectively [382].) Still other routes to o-carboranyl aldehydes are known, including the oxidation of 1-hydroxymethyl-o-carborane with dimethyl sulfoxide (DMSO), oxalyl chloride, and triethylamine in cold dichloromethane to afford the aldehyde in moderate yield along with parent carborane [384]. ð1Þ ðClCOÞ2 ; DMSO
2CH2 OH ! HCB10 H10 C2 2CðOÞH ð43%Þ þ HCB10 H10 CH HCB10 H10 C2 ð2Þ Et3 N; CH2 Cl2 78 C
9.10.1.2 B-substituted aldehydes o-Carboranyl compounds with a boron-bound aldehyde functionality are extremely rare, but at least two examples are known. Ozonization of 3-vinyl-o-carborane is reported to generate the 3-formyl derivative, although no yield is given [324]. The 9-pentanal 9-118 has been prepared in 85% yield from the 9-pentanoic acid derivative [393]: EtOH
H2 C2 B10 H92 292 2CHMeCH2 CH2 CðOÞOH ! þ H
ðMe3 CÞ2 AIH
H2 C2 B10 H92 292 2CHMeCH2 CH2 CðOÞOEt ! H2 C2 B10 H92 292 2CHMeCH2 CH2 CðOÞH þ H ;70 C
9-118
436
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
9.10.2 Synthesis of ketones 9.10.2.1 C-substituted derivatives As will be apparent throughout this chapter, many kinds of processes generate o-carboranyl ketones. Among the reactions described earlier are the chromic acid oxidation of secondary o-carboranyl alcohols, the interaction of some ocarboranyl esters with C-metallated carboranes, and the decarboxylation of bis(carboxyl) derivatives to form cyclic ketones. Ketones have also been identified as intermediates in the KMnO4 oxidation of certain o-carboranyl alcohols [e.g., (HOCH2)C2B10H11 and (HOCH2)2C2B10H10] under basic conditions, which leads to decarboxylation (Section 9.8). A fairly general synthesis involves reactions of C-lithio-o-carboranes with a wide variety of acyl halides [398,400,1241]: 2Li þ R0 CðOÞCl ! R2 2CB10 H10 C2 2CðOÞR0 R2 2CB10 H10 C2
R; R0 ¼ alkyl; Ph
This approach allows the construction of bis(o-carboranyl) ketones such as 9-119 [400], 9-120 [411], and 9-121 [400].
C Li
C
C
+
C
C
O
O C
C
Cl
Me
Me
Me
Me
C
C C
9-119 Li
O
C
O C
2
Li
C
2
+
C C
C
C
C
Cl
Cl
9-120 C O
O
O 2
C
+ C
Li
2 Cl
C
O
O
C C Cl
C C
C
C
9-121
C
The fact that ketones, rather than tertiary alcohols, are usually obtained in reactions of C-lithio-o-carboranes with acyl halides illustrates the earlier observation (Section 9.9) that lithium alcoholates of tertiary alcohols are unstable, undergoing cleavage of the exo-polyhedral C2 2C bond to generate ketones and lithiocarboranes. A case in point is the reaction of 1-methyl-2-lithio-o-carborane with 1-phenyl-2-(p-chlorobenzoyl)-o-carborane, for which the equilibria shown in Figure 9-12 are postulated [400]. Ketones are also obtained in the reactions of o-carboranyl Grignards with a wide variety of acyl chlorides, which are very similar to the interactions of these reagents with C-lithio-o-carboranes [1161]. One can also treat o-carboranyl acyl chlorides with organic Grignard reagents [400], for example, 2CðOÞCl þ PhMgX ! Ph2 2CB10 H10 C2 2CðOÞPh þ ðPh2 2CB10 H10 CÞ2 C5 5OÞ Ph2 2CB10 H10 C2
9.10 Aldehydes and ketones
437
Cl Cl Me Me
C
C C C
Li
+
Li
C
C
+
C O
O
C −
C
C
9-122
Me
O
C
C C
Li
C C
+
Cl
FIGURE 9-12 Reaction sequence for the interaction of Me2 2CB10H10C2 2Li with Ph2 2CB10H10C2 2C(O)C6H4Cl via the proposed intermediate alcoholate 9-122.
A mechanism similar to that in Figure 9-12 is very likely involved, with a magnesium alcoholate forming and rearran2MgCl, which in turn reacts with Ph2 2CB10H10C2 2C(O)Cl to form MgCl2 and the bis(phenylging to Ph2 2CB10H10C2 carboranyl ketone) [400]. In a related approach, o-carboranyl acyl chloride derivatives are condensed with benzene [172,367,396,400] or other arenes such as paracyclophane [367] in the presence of AlCl3. C 6 H6
R2 2CB10 H10 C2 2ðCH2 Þn CðOÞCl!R2 2CB10 H10 C2 2ðCH2 Þn CðOÞPh AlCl3
R ¼ H; Me; Ph;
CH; CHMe2 CH2
n ¼ 0; 1; 2
C 6 H6
ClðOÞC2 2CB10 H10 C2 2CðOÞCl ! PhðOÞC2 2CB10 H10 C2 2CðOÞPh AlCl3
Acyl chlorides also combine with acetylene over aluminum chloride, forming b-chlorovinylketones [432]: C2 H2 ; AlCl3
R2 2CB10 H10 C2 2CH2 CðOÞCl ! R2 2CB10 H10 C2 2CH2 CðOÞCH5 5CHCl 5 C; 1;2C2 H4 Cl2
R ¼ H; Me; BrCH2 ;
CH25 5CH
Still other routes to ketone derivatives have emerged from the intensely developed organic derivative chemistry of o-carborane. For example, C-metallated o-carboranes readily undergo 1,4-addition to a, b-unsaturated ketones, again presumably via unstable alcoholate intermediates that rearrange to the ketone products [399,402,1161]. Ph2 2CB10 H10 C2 2M þ PhCH5 5CHCðOÞR ! ½Ph2 2CB10 H10 C2 2CHPh2 2CH5 5CðOMÞR ! Ph2 2CB10 H10 C2 2CHPh2 2CH22 2CðOÞR R ¼ Ph; CMe3 M ¼ Li; Na; MgBr
438
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
Methyl esters of aliphatic and aromatic acids, as well as the ethyl esters of aliphatic acids, combine with lithio-and dilithio-o-carboranes to give ketones [394,399]. However, esters of a, b-unsaturated acids attack C-lithio-o-carboranes to form a, b-unsaturated carboranyl ketone intermediates, which react with a second equivalent of lithiocarborane to generate bis(carboranyl) ketones [399,402]: 2R þ PhCH5 5CHCðOÞOMe ! Li2 2CB10 H10 C2 LiCB10 H10 CR
½PhCH5 5CHCðOÞ2 2CB10 H10 C2 2R ! R2 2CB10 H10 C2 2CHPhCH2 CðOÞ2 2CB10 H10 C2 2R R ¼ Me; Ph
Ketones can be synthesized from acid anhydrides and lithiocarboranes, with cyclic anhydrides generating keto acid derivatives such as 9-123 [399]. R R C
C C
Li
+
O
O
C
C
Me
O
O
Me Me R
R
O
C
C
C C
C
C
Li
O C
OH
C
C
O
+
O
C O
9-123
Cyclic ketones are formed via decarboxylation of C,C0 -dicarboxyl-o-carboranes, as mentioned in Section 9.8, and are also generated from b-carboxylic acids and polyphosphoric acid [145] as well as via insertion of phenyl propiolic chloride into decaborane adducts [129,404,436], which afford 9-124 and 9-125, respectively.
PPA, 200 C
C C
CH2
C O C
O
C
C
9-124
CH2
OH
B10H12(NCMe)2 + PhC≡C−C(O)Cl
C
C C
O
9-125
9.10 Aldehydes and ketones
439
A well-known route to aromatic ether-ketones, employing trifluoromethane-sulfonic acid (TFSA)promoted condensation of dicarboxylic acids with ethers, has been adapted to prepare poly(o-carboranyl ether ketone) products such as 9-126 and 9-127 as building blocks for polymer construction (Chapter 14) [398,1255]. H
H
O
O
C
C HO
C
O
C C
C
TFSA, 20 °C
O
9-126 O
O
C
C
OH
O C HO
C
OMe
C
OMe
9-127
TFSA, 20 C
C
C
OH
C
C
O
O
9.10.2.2 B-substituted ketones Methods for introducing functional groups at boron that are described elsewhere in this chapter are, in general, applicable to the synthesis of boron-attached ketone derivatives. B(9)-acetyl-o-carborane (9-128) is efficiently obtained in three steps from parent 1,2-C2B10H12 [408]:
H
H
H
C
C
C
I2, AlCl3
H
C
CH2Cl2 I 1) Me3SiC≡CMgBr, Pd(PPh3)4 2) KOH, MeOH
H
H
H
C
C
H
C
C
HgO, BF3•OEt2
9-128 C
MeOH
Me
C
O
C H
II
The 9-acetyl derivative can also be prepared by acetylation of Hg (9-C2B10H11)2 with MeC(O)Cl over AlCl3 [406,407]. As noted in Section 9.8, boron-substituted species of the type C2B10H11-9-CH2-p-C6H4COR, including ketones where R is Me or Ph, can be generated via electrophilic alkylation of o-carborane with halobenzyl reagents. Treatment of 9-carboxychloro-o-carborane with benzene in the presence of AlCl3 affords the 9-phenylketone derivative [375]: C 6 H6
! C2 B10 H11 -9-CðOÞPh C2 B10 H11 -9-CðOÞCl AlCl3
B-benzyl ketone derivatives can also be synthesized via palladium-catalyzed cross-coupling of 9-iodo- or 9,12diiodo-o-carboranes with Grignards or organozinc reagents to generate products such as C2B10H11-9-p-CH2C6H4C(O) Ph and C2B10H10-9,12-[p-CH2C6H4C(O)Ph]2 (Section 9.5).
440
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
The preparation of C-substituted ketones 9-126 and 9-127 by co-condensation of o-carboranyl carboxylic acids with ethers, described above, can be applied as well to the synthesis of boron-bound o-carboranyl aryl ketones such as 9-129 and 9-130 [229].
H
H
H
C
C
H
C
H H
C
CrO3, H2SO4
C
OH
O
H
C C
H
CrO3, H2SO4
C
PhOMe TFSA
MeC(O)OH
H
C OMe
9-129
C O
H
H
C
C
H
C PhOMe
C OMe
TFSA
MeC(O)OH
C
OH C
O
HO
C
O C
O
9-130 O
MeO
9.10.3 Reactions of o-carboranyl aldehydes and ketones Earlier examples in this chapter that illustrate the rich chemistry of these derivatives include intramolecular cycloadditions of o-carboranyl aldehydes and ketones to form carboracycles (Section 9.4); the synthesis of secondary alcohols from o-carboranyl aldehydes via reactions with organolithium reagents, Grignards, or a,b-unsaturated esters, ketones, and nitriles (Section 9.9); and the interaction of carboranyl ketones with organolithium or Grignard reagents to afford tertiary or secondary alcohols, in some cases accompanied by cleavage of the exo-polyhedral C2 2C bond (Section 9.9). The last of these has been exploited as a synthetic tool for organic chemistry (see below).
9.10.3.1 Cleavage As was noted in Section 9.9, empirical observations of these reactions reveal a number of trends, though detailed understanding of the mechanisms has been elusive [128,396,526]. Both aldehydes and ketones undergo C2 2C cleavage in basic media, for example, in aqueous or alcoholic ammonia [1256], with sodamide or Group 1 metal ethoxides or sodamide in ethanol [348,385,389,399,1241,1257] and (for ketones) in thin-layer chromatography on basic alumina [348,396,400,526]. Even catalytic amounts of sodium ethoxide induce cleavage: EtONa
R2 2CB10 H10 C2 2CðOÞR0 ! R2 2CB10 H10 CH þ RCðOÞOEt EtOH
R ¼ H; Me; CHMe2 ; Ph
R0 ¼ H; Me; Ph
In the presence of strong bases, the non-carborane cleavage product is not an ester, but a salt of a carboxylic acid, which itself can cause cleavage; an example is the reaction of potassium acetate with Me2 2CB10H10C2 2C(O)Ph in aqueous alcohol, which affords Me2 2CB10H10CH, acetic acid, and potassium benzoate [128]. The cleavage of o-carboranyl ketones is analogous to that of unsymmetrical diaryl ketones, with the more electrophilic unit of the ketone forming a hydrocarbon product. In most cases this is the carborane cluster, so that the
9.10 Aldehydes and ketones
441
o-carboranyl cleavage product is a hydrocarbon rather than a carboxylic acid, as in the splitting of the unsymmetrical ketone 9-131 [399,402].
Me
Me
Me O
C C
C
C H
CH2
C
Me O
C EtONa C
C
C
C H
CH2
C
+ H
ONa
C
9-131 Given the fact that exo-polyhedral C2 2C bonds in o-carboranyl aldehydes, ketones, and carbinols are stable to aqueous and Lewis acids but undergo cleavage in basic media, the 1,2-C2B10H11 unit can be utilized as a removable protective group for carbonyl-containing organic compounds. For a wide range of R2 2C(O)2 2R0 species, reaction with LiC2B10H11 forms secondary or tertiary alcohols of the formula RR0 (HO)C2 2C2B10H11 that are stable in acidic media; treatment of these with base removes the carborane via C2 2C cleavage to afford the original organic species. The o-carboranyl unit consequently has an advantage over conventional acetal or ketal protective groups, which are prone to acid hydrolysis [368].
9.10.3.2 Electronic effects Because of the propensity of o-carboranyl ketones to cleave in basic media, the base-catalyzed replacement of a-hydrogen atoms in these compounds is precluded, and reactions of this kind can only be conducted in neutral or acidic solutions. However, this presents another problem: in acid media, the electron-attracting character of the o-carboranyl cage carbons lowers the basicity of the nearby ketone oxygen, making enolization difficult and thereby hindering a-hydrogen replacement. Bromination of o-carboranyl ketones illustrates this effect: in neutral and weakly acidic media, 2C(O)Me is unreactive toward Br2 in boiling CCl4, although bromine addition can be achieved in acetic Me2 2CB10H10C2 acid at 115 C (the carborane cage is unaffected) [395]. Br2
2CðOÞMe ! N:R: Me2 2CB10 H10 C2 CCl4
Br2 ; 115 C
Me2 2CB10 H10 C2 2CðOÞMe ! Me2 2CB10 H10 C2 2CðOÞCH2 Br ! MeCðOÞOH
MeCðOÞOH
Me2 2CB10 H10 C2 2CðOÞCHBr2 Even when an intervening methylene unit is present between the cage and the carboxyl group, bromination is difficult, but again it can be accomplished in hot acetic acid [395]. The lowered reactivity of the a-hydrogens toward bromine is mirrored in their resistance to deuterium exchange in D2SO4. Because of the -I effect at the cage carbon atoms, o-carboranyl ketones are weaker bases than benzophenone and acetophenone. m-Carboranyl ketones, as expected, are more strongly basic than their o-carboranyl counterparts owing to the reduced -I effect in the latter systems (Chapter 10) [391].
9.10.3.3 Steric effects Comparison of the reactivities of o-carboranyl ketones and aldehydes with corresponding alkyl and aryl ketones suggests that the steric bulk of the carborane unit can play a prominent role. This is reflected, for example, in the fact that 2C(O)R species form 2,4-dinitrophenylhydrazones only sluggishly on treatment with 2,4-dinitrophenylMe2 2CB10H10C2 hydrazine, in contrast to the ease of their formation by MeC(O)R ketones [348]. Similarly, a-(o-carboranyl) aldehydes are often unreactive, for example, failing to form bisulfites, whereas b-(o-carboranyl) aldehydes readily undergo this reaction [1256]: NaHSO3
2CH2 CðOÞH ! Me2 2CB10 H10 C2 2CH2 CðOHÞ2 2SO3 Na Me2 2CB10 H10 C2
442
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
9.10.3.4 Other reactions o-Carboranyl aldehydes and ketones are precursors to a variety of derivatives including acetals (9-132), cyanohydrins (9-133), Schiff bases (9-134), and aniline derivatives (9-135), as illustrated by reactions of the C-formyl derivative. The same compound is reduced by silver oxide to the carboxylic acid and the parent carborane [1256]. H
H
C
OEt C
C
H
C
C C
H
OEt
9-132
C
C
C
OH C
C
OH
H
H+ PhNH2
C
C
N
C
N C
H
H
H
H
H
C
H2O
O C
HCN Et2O
C
+
Ag2O
H
HC(OEt)3 NH4NO3, EtOH
O
H
C
LiAlH4
CH2
H
CN
9-133
9-135
9-134
Palladium-catalyzed reactions of o-carboranyl aldehydes with allyl ethyl carbonate can be converted to allyl derivatives for use as 10B carriers for boron neutron capture therapy (BNCT) [392]: ½Pd; phosphine
5CH2 CHOCðOÞOEt ! R2 2CB10 H10 C2 2CH2 CH5 5CH2 R2 2CB10 H10 CH þ H2 C5
R ¼ C6 H4 CðOÞH
Derivatives containing a,b-unsaturated ketone substituents can be obtained from C-acetyl o-carboranes via treatment with aldehydes over boric acid [412]: H3 BO3
2CðOÞMe þ R0 CHO ! R2 2CB10 H10 C2 2CðOÞ2 2CH5 5CHR0 R2 2CB10 H10 C2 R ¼ Me; Ph; CHMe2
R0 ¼ Ph; p-MeOC6 H4 ; p-Me2 NC6 H4 ; 2-furyl; PhCH5 5CH
9.11 SILICON DERIVATIVES Carborane compounds having attached silyl or siloxy functional groups have been important from the earliest days of carborane chemistry, when intensive effort was directed toward the synthesis of commercially viable silicon-linked carboranyl polymers from o-carborane (Chapter 14). Ultimately, the interest shifted toward m- and p-carborane-based polymers because of the strong tendency of C,C-disubstituted o-carboranyl systems to form cyclic monomers. This latter property, however, has been exploited to create a wide range of structurally novel silicon-carborane ring systems as is discussed below. Silicon-containing o-carborane derivatives are also important in other aspects of synthesis, as in the use of silyl units as removable blocking groups on carbon and the fluoride-promoted desilylation/aldehyde addition reac2SiR3 described in Section 9.4. These facets of o-carborane-silicon chemistry lend enormous vertions of HCB10H10C2 satility to this area that can only be outlined in general terms here.
9.11 Silicon derivatives
443
9.11.1 C-silyl derivatives The introduction of silyl groups at carbon is straightforward via reactions of halosilanes or alkylhalosilanes with lithio-ocarboranes [364,575,766,770,771,785,787,789,1258] or Grignards [684,743], or alternatively, in low yield by combination of alkynylsilanes with decaborane base adducts [555,783]. Table 9-1 includes a representative sampling of o-carboranyl silicon derivatives, and some of the commonly encountered architectures are shown in Figure 9-1. HCB10 H10 C2 2Li þ R3 SiCl ! HCB10 H10 C2 2SiR3
R ¼ alkyl; Ph
HCB10 H10 C2 2Li þ R2 SiCl2 ! ðHCB10 H10 CÞ2 SiR2 HCB10 H10 C2 2Li þ R3 SiCH2 Cl ! HCB10 H10 C2 2CH2 SiR3 PhCB10 H10 C2 2Li þ SiCl4 ! ðPhCB10 H10 CÞ2 SiCl2 Li2 2CB10 H10 C2 2Li þ R2 SiCl2 ! ClR2 Si2 2CB10 H10 C2 2SiR2 Cl Li2 2CB10 H10 C2 2Li þ Me2 SiHCl ! Me2 HSi2 2CB10 H10 C2 2SiMe2 H Li2 2CB10 H10 C2 2Li þ SiCl4 ! Cl3 Si2 2CB10 H10 C2 2SiCl3 CH ! HCB10 H10 C-SiR3 B10 H12 ðNCMeÞ2 þ R3 SiC
9.11.1.1 Cyclization reactions
The strong tendency of C,C0 -silyl o-carboranes to form exo-polyhedral rings, mentioned above, was discovered very early in the exploration of carborane polymer synthesis. Typical cyclizations are seen in the syntheses of 9-136 to 9-141 [460]. SiMe2
C
N C
R
SiMe2
RNH2
SiMe2Cl
C
C
H2O
O C
C
SiMe2Cl
R = H, Me, NH2
9-136
SiMe2
SiMe2
9-137 Cl
SiMeCl2
C C
OH Si
Si Li−CB10H10C−Li
C
Me Cl
C
SiMeCl2
Si
C
H2O
C
C
(ClMe2Si)2O
C
C
C Me
C
Me
O
Si Me
Si
C
9-140
C
Me Si C
C
Me2SiCl2
C
C Li Li
Me
C
9-139
Me
Me Si
Si
Me
Me Si
Me OH
Me
Me
9-138
C
C
9-141
Me Me
C C
Si Me
A different entry to exocyclic o-carboranyl derivatives is afforded by the strained C,C0 -disilacyclobutane species 9-142, which is structurally analogous to o-disilanylphenylene. In contrast to the latter compound, which undergoes Si2 2Si cleavage on contact with ethanol, the reaction of 9-142 (R ¼ Et) with ethanol preserves the Si2 2Si bond but cleaves an Si2 2C link to afford the ethoxydisilyl product 9-143 [767].
444
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
C C
Li
Li
Cl(SiR2)2Cl
C
R = Me, Et
C
SiR2
SiMe2
EtOH
C
R = Me
C -
+
OEtH
SiR2
SiMe2
C
EtOH
SiMe2
OEt
SiMe2
R = Me
C H
9-143
9-142
The disilacyclobutane derivatives are highly reactive toward a variety of reagents, in many, but not all, cases generating cyclic products (Figure 9-13) [794]. Unlike o-disilanylphenylene, which readily dimerizes in the presence of Pd(PPh3)4 and spontaneously polymerizes at room temperature, 9-142 exhibits neither behavior; the difference is consistent with a relative lack of electron delocalization in the exocyclic ring in the carborane species [767]. A similar absence of aromatic character in cyclo-1,2-(CH5 5CH2 2CH5 5CH)C2B10H10 (9-34, “benzocarborane”) was mentioned earlier in Section 9.4. Insertion into the Si2 2Si bond of 9-142 is common, in some cases requiring a palladium catalyst as shown in Figure 9-13. The reaction with trans-cinnamaldehyde takes a different course, with the insertion of two carbonyl groups into the C2 2Si links to generate the heptacyclic species 9-149 [794]. Other chemistry of 9-142 has been investigated; for example, treatment with lithium metal in THF affords the exo-trisilyl derivative 9-150 in 46% yield—a reaction in which SiEt2
R
C C O
H C
R = Ph, C(O)OMe, n-C4H9 R = H, Me, C(O)OMe
RC≡CR C
C
O SiEt2
C
R
SiEt2
9-144
C
SiEt2
O
O2
C SiEt2
O C
9-149
SiEt2
9-145
SiEt2 S
O
9-142
H
SiEt2
C
C S Pd(PPh3)4 catalyst
9-146 SiEt2H
SiEt2 C
C
Pd(PPh3)4 catalyst
C O C
C SiEt2
9-148 FIGURE 9-13 Reactions of C,C0 -disilacyclobutane-o-carborane (9-142).
SiEt2
9-147 NO2
SiEt2
9.11 Silicon derivatives
445
the Si2 2Si bond is initially severed and a third silicon is added intermolecularly [768]. Alkynes undergo palladium-catalyzed alkyne addition to 9-150, enlarging the ring to give 9-151, and benzyl cyanide combines with 9-142 over a nickel-disilacarborane catalyst to generate the styrenyl-amino derivative 9-152 [316].
C C
SiMe2
Li
SiEt2
SiEt2
C
C PhC≡CH
SiEt2
THF, 25 C
Pd(PPh3)4 catalyst
C
SiMe2
SiEt2
9-142
C SiEt2
9-150
9-151
CH2C≡CN
C
SiEt2 N
C
SiEt2
CH
9-152
CH
C
SiEt2 Ni(PEt3)2
SiEt2
catalyst
C SiEt2
The versatile chemistry of 9-142 has been extended still further via insertion of platinum to create the exo-metallo5CH2)(PPh3)2 cycle 9-153 (Figure 9-14), which can also obtained by reaction of 1,2-(Me2HSi)2C2B10H10 with Pt(CH25 [315]. Compound 9-153 is analogous to cyclic bis(silyl)benzene complexes such as o-C6H4(SiMe2)2-m-Pt(PPh3)2 [1259– 1261], which are intermediates in the platinum-catalyzed double silylation of alkynes, alkenes, and other unsaturated molecules. Accordingly, 9-153 is a precursor to a range of cyclic organo derivatives via double silylation [315,790], as illustrated in Figure 9-14. Most of these products retain the original C2 2SiMe2 groups, but the formation of 9-157 involves insertion of two CO units into C2 2Si bonds (as in 9-149), with a third oxygen also introduced via the transcinnamaldehyde reagent to create a 9-membered exo-polyhedral ring [315]. Double silylation can also be promoted by nickel-phosphine catalysts, as illustrated in the formation of the oxadisilacyclohexane 9-162 via a proposed mechanism in which the metal initially coordinates to the C5 5O double bond, followed by CO insertion into the ring and finally expulsion of the diphosphino-nickel unit [797].
C C
SiMe2H
SiMe2H
RC(O)H
C
Ni(PEt3)4 catalyst
C
SiEt2 O R
SiEt2
9-162
C
SiMe2 Ni
C
C
O
R C
C SiMe2 H
SiMe2 O
C
R
Ni SiMe2
9-161
9-160 Ni = Ni(PEt3)2
H
C
446
SiMe2
C
C4H9 C H
SiMe2
SiMe2 C
R
C
SiMe2
n-C4H9C≡CH
9-154 C
C
(Ph3P2Pt(CH2=CH2)
RC≡CR
SiMe2 C
SiMe2
9-142
9-155
PPh3
Pt C
O O
C C
R = H, Me, Et, Ph, C(O)OMe R = H, Me, Et, Ph, C(O)OMe
Me PPh3
SiMe2
O
Ph
9-153
N C N
C O N
O
Ph
SiMe2
Ph
NC
R
SiMe2
C
N
C
N
C N
SiMe2
C
SiMe2
Me2Si O
O
CN
C
O
SiMe2
N SiMe2 SiMe2
Me2Si C
9-158
C
C N
9-159
O
9-156
C C
O
O
C SiMe2
Me2Si O
9-157 FIGURE 9-14 Conversion of 1,2-cyclo-[(Ph3P)2Pt(Me2Si)2]C2B10H10 to 1,2-exo-polyhedral carborane derivatives.
C
Me
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
C
9.11 Silicon derivatives
447
Further examples of nickel-catalyzed cyclo-disilacarborane synthesis are seen in the insertion of nitriles to generate the N-silyl enamine 9-163 and the azodisilacyclohexenes 9-164 [316]. C
SiEt2 N
C
SiEt2
H H
C
C
SiEt2
C
SiMe2H
Ni(PEt3)2
N
C SiEt2
Ni(PEt3)2 C
SiEt2
R
9-164
SiEt2
catalyst
SiEt2
C
RC≡N
C
Me
9-163
SiEt2
C
SiMe2H
C
EtC≡N
catalyst
R = CHMe2, Ph, p-C6H4Me, β-naphthyl
Other silicon-containing exo-polyhedral heterocyclic derivatives, such as 9-138 to 9-141 (see above) and 9-165 [792], can be prepared via different routes. Me Si
C
Me
Me
C Li Li
PhPCl2 or MeAsI2
C
Me
Si C
C
C
C
C E
9-165 E = P−Ph, As−Me The diselenium exocyclic compounds 9-166 have been obtained by reaction of Li2þSe2 C2 B10 H10 2 with R2SiCl2, and the 1,2-cyclo-[Se(SiMe2)2Se] derivative has been prepared from the same dianion and 1,2-dichlorotetramethylsilane [740]. Se
C
R Si
9-166 R
C
Se
R = Me, Ph
9.11.1.2 Metal chelate complexes C-silyl o-carboranes are useful synthons for constructing ligands that form metal chelate complexes of specific design, an area that has relevance to silylation [1262] and other industrially important metal-catalyzed processes. Kinetically stabilized complexes of the types 9-167 to 9-169 are easily prepared, and combine the steric bulk and rigidity of the carborane unit with the ease of chelation of the phosphino groups [313,314,584]. SiMe2
PR2
C
C
DMAD
M
110 C
C
C PR2
SiMe2
H
Pt
(Ph3P)2Pt(C2H4)
C PPh2
9-168
PPh3
R = Ph
C
M C
C
9-167
trans
C
PR2
SiMe2
PR2
SiMe2
SiMe2
M = Pt, Pd R = Me, Ph
C
cis
DMAD = dimethylacetylene-dicarboxylate
MLn
C
SiMe2H Pt(C8H12)2 R = Ph
C PR2
SiMe2
H
C
H
Pt
C
C PPh2
9-169
C
PPh3
448
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
Cyclopentadienylsilyl-substituted carboranes have been employed effectively as ligands in stable, isolable organolanthanide complexes [741,778] as illustrated in Figure 9-15. Similar complexes having trimethylsilylindenyl units bridging the carborane and metal center [802,859], some bearing ether or amino functionalities [1263,1264], have been prepared. In constrained-geometry compounds of this type [858,1265], from which catalyst precursors have been developed as outlined in Chapter 15, the steric bulk of the carborane cage plays a role in stabilization by blocking ligand redistribution reactions.
Me SiMe2Cl
C Li C
C
toluene/Et2O
Li
Li
Me Me
Si
n-C4H9Li
Me
Li
C H
−
THF
Li(THF)4+
C
Si
THF
Li
OEt2
C
LnCl3 Ln = Nd, Sm
C
Si
C
Me Me Li THF
Me LnCl3
Si
C
Me Cl
C H
Ln
Cl
9-170
NdCl3
NdCl3
Ln = Sm, Y, Yb
THF Cl
Me Me
2−
Li Si THF THF
C
Ln
Me Me
Cl
C
Si
C
C
C
Me
Nd Nd
Me
Si
H
Me Me
9-171
Cl Ln
C
Si
C
−
Me
Si
H
C
Me
C
9-172
C
C
Si
C
Me Me
9-173
FIGURE 9-15 Synthetic routes to lanthanide metal o-carboranyl- tetramethylcyclopentadienylsilyl chelate complexes.
9.11.1.3 Dendrimers Multi-branched carbosilane frameworks are a favored motif for dendrimer construction because of their stability and versatility. The incorporation of o-carboranyl clusters can be achieved as in the synthesis of 9-174, an 8-cage secondgeneration dendrimer, by the “divergent” approach in which the branches are built outward from a central core using the Karstedt catalyst, platinum divinyltetramethyldisiloxane [294]. Other silicon-based dendritic systems are accessible by this method, and also by the “convergent” approach wherein pre-assembled branches are brought together as in the construction of 9-176 [294,774,793]. Carboranyl dendrimers are more fully discussed in Chapter 14.
9.11 Silicon derivatives
449
R C C
Si +
[Pt]
Si
Si
Karstedt catalyst
4 R = Me, Ph
C
C
R
R
C C R
Si
C Si
C C
C
Si Si
R
Si
Si
R
Si C
C
Si
Si
Si
Si
C
C R
Si
9-174
Si
R C C
C
R
C
R C C
Si
H
R
C
C
C
C
9-175 R = Me, Ph
Si
R
Si
Si
[Pt] Karstedt catalyst
Si Si
R
Si
C C
C C
R
9-176
o-Carboranylsilyl dendrons are also useful as molecular scaffolds for attaching branches containing a variety of organic groups, allowing the construction of multifunctional reagents as shown by the synthesis of 9-178 to 9-180 from the trivinylsilanes 9-177 [786].
450
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12 Cl Si R
R
C
C Si
C
C
Me2SiHCl [Pt] Karstedt catalyst
9-177 R = Me, Ph, Si(CHCH2)3
Si
Si
Cl
Si
LiH4 Et2O
9-178
Cl
Cl
H
Si
Cl
R C C
Si
Si
CH2Cl
C
Si
H
[Pt] Karstedt catalyst R = Ph
Si
9-179
C
H
9-180 Cl
9.11.1.4 Silicon-linked mixed-carborane derivatives The versatility of silylcarborane chemistry has been encountered in earlier chapters where the syntheses of mixed-cluster 2 species such as 1-SiMe2[50 -2,3-nido-(Me3Si)2C2B4H5]-2-R-C2B10H10 (R ¼ Me, Ph) (4-25, Chapter 4) and 1,10-[Me2 2SiMe2]2C2B8H8 (6-38, Chapter 6) are described. A mixed-isomer chain compound, 1,7-[(SiMe22 2O2 2SiMe22 2 CB10H10C2 2O2 2SiMe2 1,2-MeC2B10H10)]2C2B10H10, in which the carbon atoms of a central m-carboranyl cage are linked via SiMe22 chains to two end-group o-carboranyl clusters, has been prepared from a metathesis reaction of 1,7-(ClSiMe2OSiMe2)2 C2B10H10 with 1,2-LiMeC2B10H10 [787].
9.11.1.5 Silyl as a protecting group in synthesis
The disproportionation of C-monolithio-o-carborane to the C,C0 -dilithio species and parent o-carborane, discussed in Section 9.4, is a major problem in the synthesis of C-monosubstituted o-carborane derivatives as it often leads to mixtures of mono- and disubstituted products. An effective strategy for dealing with this issue is to employ C-silyl-C0 -lithio derivatives whose sterically bulky trialkylsilyl groups block the formation of dilithio species and are removable by treatment with fluoride. C-monosubstituted derivatives can be obtained in near-quantitative yield via this procedure [159,776]: ð1Þ n-C4 H9 Li ð2Þ ClSiMe2 CMe3
ð1Þ n-C4 H9 Li ð2Þ RX
C6 H6 =Et2 O reflux
C6 H6 =Et2 Oreflux
2SiMe2 CMe3 ð99%Þ ! HCB10 H10 CH ! HCB10 H10 C2 ðn-C4 H9 Þ4 Nþ F
R2 2CB10 H10 C2 2SiMe2 CMe3 ! R2 2CB10 H10 CH THF; 76 to 0
R ¼ Me; n-C4 H9 ; CðOÞOMe
By employing bifunctional RX reagents such as 1,2-bis(bromomethyl)benzene, one can efficiently synthesize bis(o-carboranyl) products such as 9-181 [776].
9.11 Silicon derivatives
C
451
C
C
C
H
H
9-181 Related to this chemistry is the TBAF-promoted displacement of SiR3 groups by enals and enones to give 2CH(OH)R or cyclic products as described in Section 9.4. HCB10H10C2 The technique of C-silyl protection/deprotection has been applied to targeted carborane synthesis in several important areas, including the construction of “Venus flytrap” reagents discussed in Chapter 14. In a nonmetal application, the construction of multicage carboracycles by the Hawthorne group is an elegant example. Figure 9-16 illustrates the assembly of 9-182 and 9-183 having three and four o-carboranyl units, respectively, connected by trimethylene chains [266]. The tour de force in this work is the similarly prepared 9-184 whose six carboranes and six tetramethylene connectors form a covalently bonded 36-carbon macrocycle [240].
C
C
C
C
C
C
C
C
C
C
C
C
9-184
452
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12 1) n-C4H9Li SiMe2CMe3
R
C
Me3C
C C
C
C6H6/Et2O 2:1
C
C
N(n-C4H9)4F− THF +
C
C
H
1) n-C4H9Li 2) Br(CH2)3Br
H
C
C C
C
THF
C
C
C
C C
SiMe2
C
C (CH ) OTos 2 3
C H
R
CMe3
Me2Si
C (CH2)3OTos
2)
C
9-182 CMe3 Me3C
Me2Si R
R
C C
1) n-C4H9Li 2) ClSiMe2CMe3
H
C6H6/Et2O 2:1
H
R
SiMe2CMe3
C C
R
1) n-C4H9Li 2) Br(CH2)3Br
SiMe2 C
C C
C
C6H6/Et2O 2:1
H
R
R R R
R
R
R
N(n-C4H9)+4F−
R C
THF
C
C
H
C 1)n-C4H9Li 2) Br(CH2)3Br
H
C
C C
C
THF R
R C
C R
C
R
C
R
R R
9-183
R
FIGURE 9-16 Synthesis of three- and four-carborane carboracycles.
9.11.1.6 Alkoxysilanes Derivatives of o-carborane having alkoxysilyl substituents are of interest as precursors to siloxycarborane polymers, although less favored for this purpose than the corresponding m- and p-carboranyl compounds (Chapter 14). Several routes have been developed, including the interaction of o-carboranyl carbinols with alkylchlorosilanes [317,783,1266,1267]: Me2 SiCl2
2CH2 OH ! ðHCB10 H10 C2 2CH2 OÞ2 SiMe2 HCB10 H10 C2
9.11 Silicon derivatives
453
MeSiCl3
HCB10 H10 C2 2CH2 OH ! ðHCB10 H10 C2 2CH2 OÞ3 SiMe Me3 SiCl
HOCH22 2CB10 H10 C2 2CH2 OH ! Me3 SiOCH22 2CB10 H10 C2 2CH2 OSiMe3 Not surprisingly, the reaction of 1,2-bis(hydroxymethyl)-o-carborane with dichlorodimethylsilane affords the cyclic product 9-185, which is converted back to the diol on exposure to acids or bases [783].
HOCH2JCB10H10CJCH2OH
C
Me2SiCl2
⎯→
O
Me
9-185
Si C
O
Me
Reactions of C-lithio-o-carboranes with chloroalkylsiloxanes can also be employed to generate C-siloxy derivatives [788]. With C,C0 -dilithio-o-carborane, one obtains cyclic o-carboranylsiloxanes [1258], the main product obtained depending on the ratio of siloxane reactant to carborane (but not, apparently, on the siloxane chain length) [1268]. Me
Me C
Si
O
Me
(ClMe2Si−O)nSiMe2Cl 2 equivalents
C
Me
n = 2, 3, 4
C
Si C
9-186
Si
Me
Me
O
Si
Li (ClMe2Si−O)nSiMe2Cl 1 equivalent
C O
n = 2, 3, 4
C
Li
Me Me
9-187
Si Me Me
Somewhat contrary to expectation, neither 9-186 nor 9-187 is polymerized by acid or base catalysts. Alkoxysilane derivatives can also be obtained from o-carboranylalkyl Grignards; with monosubstituted compounds, rearrangements of the type discussed earlier in Section 9.4 lead to alkylation of the carboranyl CH group. When no carboranyl CH is present, the rearrangement cannot occur and C-siloxyalkyl products are obtained [783]. MeSiðOEtÞ3
2CH2 MgBr ! Me2 2CB10 H10 C2 2SiMeðOEtÞ2 HCB10 H10 C2 MeSiðOEtÞ3
Me2 2CB10 H10 C2 2CH2 MgBr ! Me2 2CB10 H10 C2 2CH2 SiMeðOEtÞ2 A different approach to alkoxysilanes employs addition of silyl reagents to C-alkenyl-o-carboranes over platinized carbon, a reaction that works best when the C5 5C bond is remote from the carborane cage [783,1267,1269,1270]. For example, methyldiethoxysilane is unreactive toward 1-vinyl-o-carborane but reacts easily with the 1-allyl derivative and even more rapidly with 1-(30 -butenyl)-o-carborane. HSiMeðOEtÞ2
HCB10 H10 C2 2ðCH2 Þn CH5 5CH2 ! Me2 2CB10 H10 C2 2ðCH2 Þnþ2 SiMeðOEtÞ2 ; Pt=C
n ¼ 1; 2 n 6¼ 0:
Trichlorosilane and alkyldichlorosilanes add to the double bond of 1-alkenyl-o-carboranes, the silicon binding to the carbon furthest from the cage [1271]. HSiCl3
HCB10 H10 C2 2CH5 5CH2 ! HCB10 H10 C2 2ðCH2 Þ2 SiCl3
454
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
When conducted in the presence of water, reactions of alkenylcarboranes with alkyldichlorosilanes generate o-carboranylsiloxanes of varying chain length [1269]: H2 O;Et2 O
MeHSiCl2
HCB10 H10 C2 2ðCH2 Þ2 CH5 5CH2 ! HCB10 H10 C2 2ðCH2 Þ42 2SiMeCl2 ! Me3 SiCl
HCB10 H10 C2 2ðCH2 Þ42 2SiMeðOSiMe3 Þ2 Polymers and oligomers derived from siloxy-o-carboranyl building-block units are discussed in Chapter 14.
9.11.2 B-organosilyl derivatives Only a few boron-substituted o-carboranyl silicon derivatives are known, none featuring direct B2 2Si bonds. Silylation of parent 1,2-C2B10H12 with CH2¼CHSiCl3 over AlCl3 at 80-120 C is reported to give 1,2-H2C2B10H10n(CH2CH2SiCl3)n products [804,1272]. The location of the substituents was not established, but the usual pattern of electrophilic attack on the o-carborane cage is B(9,12) > B(8,10), as is discussed in Section 9.5. Palladium-catalyzed cross-coupling reactions of 9-iodo and 9,12-diiodo-o-carborane furnish an alternative route [173,525,806], as in the synthesis of 9-188.
H
H
C C
I
RZnCl Pd(PPh3)4 THF/Et2O
H
H
X
C
SiMe3
C X = CH2, C≡C
R = Me3SiCH2, Me3SiC≡C
9-188
9.12 GERMANIUM, TIN, AND LEAD DERIVATIVES Much of the chemistry of silicon-containing o-carboranes described in the previous section is reflected in analogous germanium and tin reactions, compounds, and structures; however, there are also notable differences as, for example, in the stability of B2 2Sn versus B2 2Si and B2 2Ge bonded derivatives discussed below.
9.12.1 C-germyl and C-stannyl derivatives As was briefly outlined in Section 9.4 and illustrated in Figure 9-1, C,C0 -dilithio-o-carborane is the starting point for the preparation of a variety of C-germyl and C-stannyl species, many of which are listed in Table 9-1. The range of structures accessible by this approach is further displayed in Figure 9-17 [555,575,808,809]. Monolithio-o-carboranes react readily with stannyl and germyl halides to form derivatives having direct metalcarborane links [555,575,808]:
9.12 Germanium, tin, and lead derivatives
Ge
M C N C
C
C
C
C
9-196
Me
Me
9-190
Me2MX2
C C
MMe2X
Me Me
9-191
Me
H2O
Na
MMe2X
C
Ge
Ge
Li2NCMe3
C
Ge O
CMe3
M Me
Me
Me
Me
Me
Me
Me
455
Li
C
Me2GeCl2
GeMe2Cl
9-189
C
Li
GeMe2Cl
9-194 M = Ge, X = Cl M = Sn, X = Br
Me2SnBr2
NH3
Li2PPh Me C
Me
Me
M
C
Sn Sn
P C
C Me Me
Me
C
Me
C
9-193
Me Me
Me Ge NH
Sn
M
9-195
Me
Me
9-192
Ge Me Me
FIGURE 9-17 Synthesis of C2 2Ge and C2 2Sn derivatives from 1,2-Li2C2B10H10.
Ph2 2CB10 H10 C2 2Li þ SnCl4 ! ðPh2 2CB10 H10 CÞ2 SnCl2 þ ðPh2 2CB10 H10 CÞ3 SnCl Ph2 2CB10 H10 C2 2Li þ GeCl4 ! ðPh2 2CB10 H10 CÞ2 GeCl2 In the reactions with SnCl4 and GeCl4, the steric bulk of the carborane cages evidently prevents the formation of 2CH2)4Sn, (RCB10H10C)3GeCl and (RCB10H10C)4Sn derivatives, which are unknown [575,819]. However, (HCB10H10C2 2CH2MgBr with SnCl4 in which methylene groups relieve the crowding, can be obtained from the reaction of HCB10H10C2 [593]. Cyclic bis(o-carboranyl) structures are accessible via C,C0 -dilithiation, as shown in Figure 9-1 and in the synthesis of 9-197 to 9-199 [601,602,792,795].
456
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12 R Li
M
Li
C
C C
C
R2MX2
C
Li
R C
C
R = Me, Et M = Si, Ge, Sn X = Cl, Br
C
9-197 X
Li
C
C
Me2SiCl2,
C
C
C
C
Me2GeCl2, or MeAsI2
C
C
9-198 X = Me2Si, Me2Ge, MeAs Me Me Li
Li
C
C
M
C
C
Me2MCl2 C
C Me
C
M = Ge, Sn
M
C
M
X = Cl, Br
Me
Me
Me
9-199
M = Si, Ge
C-triethylgermylmercury derivatives have been obtained in exchange reactions of mercury complexes [807]: 2HgR0 þ HgðGeEt3 Þ2 ! R2 2CB10 H10 C2 2HgGeEt3 R2 2CB10 H10 C2 R ¼ H; Me; CH2 Cl; R0 ¼ Me; Ph; Cl Stannyl groups can also be introduced at carbon via thermal reactions of stannylamines with o-carborane [813,815] or by treatment with organolanthanide reagents [772]. 120140 C
HCB10 H10 CH þ R3 Sn2 2NEt3 ! HCB10 H10 C2 2SnR3
R ¼ Et; n-C4 H9
7075%
Ph3 MCl
RYbI
Ph2 2CB10 H10 CH ! ½Ph2 2CB10 H10 C2 2YbI ! Ph2 2CB10 H10 C2 2MEt3 20þ20 C
R ¼ Me; Ph M ¼ Si; Sn 9096%
9.12.1.1 Hydrostannylation of alkenylcarboranes Tin-containing derivatives of o-carborane are generated in reactions of C-vinyl- or other C-alkenyl-carboranes with stannanes [818]; yields are enhanced in the presence of H2PtCl6 or azodiisobutyronitrile (ABN) catalysts. R03 SnH
0
2CR5 5CH2 ! HCB10 H10 C2 2CHR2 2CH2 SnR3 HCB10 H10 C2 R ¼ H; Me R0 ¼ Et; n-C4 H9 ; Ph R02 SnH2
2161%
HCB10 H10 C2 2CR5 5CH2 ! ½HCB10 H10 C2 2CHR02 2CH2 3 SnR0 þ HCB10 H10 C2 2CHR2 2CH2 SnHR02 100120 C
R ¼ H; Me R0 ¼ Et; n-C4 H9
9.12 Germanium, tin, and lead derivatives
457
9.12.1.2 C-stannyl and C-germyl metal chelate complexes Well-designed o-carboranyl ligands containing a dimethylamino unit tethered to carbon (9-200) readily combine with organotin reagents to form derivatives such as 9-201 from which the stannyl group can be displaced by mercury as in 9-202 (Figure 9-18). The interaction of 9-200 with dihalo- or trihalostannanes or SnCl4 leads to stable chelate complexes 9-203 and 9-205 in which the tin center is stabilized in a trigonal-bipyramidal pentacoordinate geometry [820]. Treat˚ ) Sn2 ment of 9-203 with sodium metal effects Wurtz-type coupling to form 9-204, whose extremely long (2.80 A 2Sn bond is ascribed to the bulky carborane ligands.
Me
C
N
C
Me
C
R2SnX2 R = Me, Ph X = Cl, Br
9-202 Me2N
C C
C NMe2 C
9-203
X
R
R
C
NMe2 Na Me Me
C
R
Sn
R
Sn Cl
Hg C
9-201
NMe2
NMe2
C
SnMe3
RSnCl3 R = Ph, Cl
9-205
C
HgCl2
C
Li
9-200
NMe2
Me3SnCl
X = Br R = Me
Sn
C Sn Me
9-204
C
Me
C Me2N
FIGURE 9-18 Synthesis and reactions of o-carboranylamino-tin complexes.
Derivatives of o-carborane having an attached pentacyclic (C5 5N2 2CHR*2 2CH2-O) (oxazolinyl) group are chiral, owing to the presence of the stereogenic ring carbon atom. Placement of an SnMe2X (X ¼ Cl, Br) group on the adjacent cage carbon atom leads to formation of a strong intramolecular N!Sn bond, creating a pentacoordinate tin center and enforcing chiral recognition in these species [529]. Compounds of this type may lead to the development of new carborane-based chiral organotin reagents for organic synthesis. Diphenylphosphinomethylene-tin derivatives of the formula 1,2-(Ph2PCH2)(SnMe2X)C2B10H10 (X ¼ Cl, Br), structurally analogous to 9-201, can be prepared from 1,2-(Ph2PCH2)LiC2B10H10 and SnMe2X2 [824]. Also useful are the complexes of type 9-207 (Figure 9-19) in which the phosphino group is directly bonded to the carborane cage [824]. These compounds are capable of generating a variety of transition-metal complex systems, as shown. Cyclic digermanium o-carborane derivatives of the types 9-214 and 9-215 can be prepared from 1,2-bis(dimethylgermyl)o-carborane (9-212) using the nickel catalyst 9-213, as shown in Figure 9-20. The latter complex is generated by reaction of Ni(PEt3)4 with 9-212 or the previously mentioned 1,1,2,2-tetramethyl-1,2-digermacyclobutane (9-190) [812]. Both 9-212 and its bis(dimethylstannyl) analogue 1,2-(Me2HSn)2C2B10H10 readily combine with Pd(PPh3)4 to give palladium complexes (Ph3P)2Pd-1,2-(Me2M)2C2B10H10 (M ¼ Ge, Sn) that are analogues of 9-213 [809].
458
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
C C
PPh2 Me2SnX2 Li
PPh2
C
dba = dibenzylideneacetone
Me
Me
C
Me
Me
C Ph2P
Sn
X
C
Pd2(dba)3
Pd
Pd P
Me
Sn
9-209
Sn
Me Me
Sn
C
Me
C C
PPh2
C
X = Cl, Br
Me
Sn
Na −NaCl
Pd2(dba)3
Me
O C Fe
C
9-208
9-207
9-206
FeCp(CO2
PPh2 O C
C
)−
X = Cl SnMe2X −NaCl
C
X = Cl, Br
Na+
C
X
P
Ph Ph
Ph Ph
P
P
Ph Ph
Ph Ph
C
9-211
C Pd
9-210
C Sn
C
Sn
Me Me
Me
Me
FIGURE 9-19 Synthesis and reactions of o-carboranylphosphino-tin complexes.
Derivatives that incorporate tin along with heavier Group 16 elements in exo-polyhedral rings are easily accessible from E2 C2 B10 H210- (E ¼ Se, Te), as in the synthesis of 9-216 (counterparts of 9-166) from Se2 C2 B10 H2 10 and Cl2SnR2 (R ¼ Me, Ph) followed by insertion of platinum to create 9-217 [844]. PPh3
C
Se R Sn
C
Se
C
Pt(PPh3)2(CH2=CH2)
C
R
9-216
Se
Pt Sn
Se
PPh3 R R
9-217
Addition of Cl2SnMe2 to a solution of Te2 C2 B10 H2 10 affords the bis(o-carboranyl) product 9-218 which has a direct Te2 2Te bond [853]. Te
Te
C
9-218
C C
C Sn Me
Me
9.12 Germanium, tin, and lead derivatives Me
Me
C
Ge
C
1) Me2GeCl2 2) Na+B(CN)H−3
C
H
C
H
C
Ge
Li
Me
9-212
Me
C C
Me
Me
9-213 catalyst C
Ge PEt3 Ni
Me
Me
Me
Ni(PEt3)4
Ni(PEt3)4
+
9-213 catalyst
Ge Me
9-190
RC≡CR
+
Me
Me
Ge
Li C
459
Me
Me
Ge
Ge C
Ge
R
PEt3
Me Me
9-213
C
Ge
C
Ge
Me Me
Me Me
9-214
9-215
R
R, R = Me, Et, Ph, SiMe3, C(O)OMe
FIGURE 9-20 Synthesis and catalytic reactions of (Et3P)2Ni-1,2-(Me2Ge)2C2B10H10 (9-213).
9.12.1.3 Reactivity of o-carboranyl C-Sn bonds The palladium-catalyzed displacement of Sn(n-C4H9)3 groups from 1-(tri-n-butyltin)-o-carborane by aldehydes, forming 1-CH(OH)R alcohol derivatives, was mentioned in Section 9.4. The catalytic cycle is proposed to involve initial oxidative insertion of Pd[0] into the C2 2Sn bond to generate a Pd[II] species into which the aldehyde is inserted, followed by loss of the Pd[0] and tributyltin moieties to generate the 1-CH(OH)R-C2B10H11 product [351]. A long-known property of C-stannyl-o-carboranes is the facile cleavage of the C2 2Sn link by nucleophiles, especially alcoholic bases which remove the stannyl group almost quantitatively [819]. KOH
Ph2 2CB10 H10 C2 2SnR03 ! Ph2 2CB10 H10 CH þ RSnOH EtOH
R ¼ n-C3 H7 ; Ph
KOH
2CB10 H10 CH ðPh2 2CB10 H10 CÞ2 SnCl2 ! Ph2 EtOH
Comparative studies reveal that C2 2Sn cleavage is most rapid in o-carboranyl compounds and decreases in the order o- >> m- > p-carborane [814,1273–1275], reflecting the corresponding decrease in polarity and cage carbon electronegativity in these systems. Studies of analogous C-silyl and C-germyl derivatives show that the reactivity of HCB10H10C2 2MR3 (M ¼ Si, Ge, Sn) isomers toward alkaline solvolysis in MeOH decreases in the order Si > Sn > 2CH2MR3 the sequence is Sn > Ge > Si [782]. These findings underline the influence Ge; however, in HCB10H10C2 of the electron-deficient cage carbon atom, which increases the polarity of M2 2Ccarborane bonds much more than that of the more remote M2 2CH2 interactions, although other factors (e.g., the affinity of M for the attacking alkoxide) are also clearly at work.
460
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
9.12.2 B-stannyl derivatives As we noted in Section 9.5, there are currently no known and characterized o-carboranyl compounds containing direct B2 2Si, B2 2Ge, or B2 2Pb bonds, a fact which may simply reflect research priorities (or serendipity) rather than intrinsic properties. In contrast, o-carboranes featuring B2 2Sn links are well known. Preparative routes include reactions of B-mercurated o-carboranes with SnCl2 or tin(II) acetylacetonate, described in Section 9.5. In the product obtained from tin(II) chloride (9-219) the metal is tetracoordinate [822], whereas Sn(acac)2 generates in 72% yield a hexacoordinate complex (9-220) in which the acac ligands adopt cis geometry as deduced from NMR data [826,827]. At room temperature in benzene, 9-220 slowly loses mercury (rapidly in THF-benzene) to form 9-221 with retention of the cis coordination geometry.
H
H
H
C
C
C
H
C H
C
C H
Hg
C
H
C H
Sn Cl
Cl Sn(CHAc2)2 C6H6, THF 0 C
9-219 CH Me
Me
Me
CH
C Me
O
C
O Sn
O H
O
Hg
C
C
Me
O
Me C
O
CH Sn
O
C
−Hg
H
Me
C CH C
O
Me
C
C C
H
H
C C
C H
C
9-220
H
C
9-221
H
H
o-Carboranyl B2 2Sn derivatives exhibit markedly different properties from their C2 2Sn counterparts, the principal difference being (as noted in Section 9.5) that the B2 2Sn species are stable in the presence of nucleophiles. The much higher stability of B2 2Sn versus C2 2Sn bonds in o-carboranes allows boron-stannylated species to survive a wide range of conditions, facilitating the designed synthesis of a variety of derivatives for use in medical [823,1276] and other applications (Chapters 16 and 17). In other compounds, the tin is bonded via carboxyl groups attached to cage boron atoms, as in the bis (9-carboxylato-o-carboranyl)-di-n-butyltin compound 9-222 which exhibits antitumor activity exceeding that of cis-platin, against several human tumor cell lines [823]. H
C
C H
H
H
C
(n-C4H9)2SnO
H
C
C
O C
OH C O
C4H9 O C
Sn O
O
9-222
C4H9
C
H
9.13 Nitrogen derivatives
461
9.12.3 C-plumbyl derivatives The known chemistry of o-carboranyl exo-polyhedral-substituted lead compounds is limited to the 1-PbMe3 derivative, on which little has been reported other than a 207Pb-1H NMR study [828]. Heterocarboranes of Si, Ge, Sn, and Pb, in which one or more Group 14 atoms are incorporated into the cage framework, are described in Chapter 11.
9.13 NITROGEN DERIVATIVES The interaction of o-carborane with 100% nitric acid to form B-nitrato products (see below) was an early discovery in carborane chemistry and provided direct evidence, together with halogenation studies, that the BH vertexes are negatively charged relative to the cage CH units and are susceptible to electrophilic attack. These findings opened the way to the facile synthesis of a wide range of B-substituted derivatives, greatly broadening the scope of o-carborane chemistry. In contrast, C-nitrato- and C-nitro-o-carboranes have not been prepared (an incorrect early report [1162] notwithstanding) and there are relatively few known species with direct o-carboranyl C2 2N bonds.
9.13.1 Nitrato, nitro, and related compounds 9.13.1.1 C-substituted derivatives The synthesis of C-nitroso-o-carboranes from NOCl and C-lithio-o-carboranes and their reduction to hydroxylamines was noted in Section 9.4. Undiluted nitric acid, or mixed nitric and sulfuric acids, attack C-hydroxymethyl or C-phenyl o-carboranes only at the functional groups, leaving the C2B10 cluster unchanged and affording C-nitratomethyl and C-nitrophenyl derivatives, respectively [127]. In the nitration of C-phenyl-o-carborane with HNO3, the ratio of m- and p-nitrophenyl products varies with reaction conditions, but the p-nitrophenyl species is predominant in most cases [224,472,493]; the C-(o-nitrophenyl) isomer has been observed only in minute quantity [279]. These findings directly reflect the strong inductive -I effect of the o-carboranyl carbon nuclei; in contrast, the nitration of B(3)-phenyl-ocarborane gives quite different results (see below). HNO3
HCB10 H10 C2 2CH2 OH ! HCB10 H10 C2 2CH2 ONO2 HNO3
HOCH22 2CB10 H10 C2 2CH2 OH ! O2 NOCH22 2CB10 H10 C2 2CH2 ONO2 HNO3
HCB10 H10 C2 2Ph ! HCB10 H10 C2 2C6 H4 -p-NO2 þ HCB10 H10 C2 2C6 H4 -m-NO2 Alternatively, nitrophenyl groups can be introduced at carbon via aromatic nucleophilic substitution [474] or in reactions of C-metallated o-carboranes with nitroarenediazonium salts [278] or nitroaryl iodides [471]: NaH;FC6 H4 -o=m=p-NO2 2CB10 H10 C2 2C6 H4 NO2 Ph2 2CB10 H10 CH ! Ph2 IC6 H4 -m=p-NO2 HCB10 H10 C2 2Cu ! HCB10 H10 C2 2C6 H4 NO2 þ or N2 C6 H4 NO2 BF4
Other synthetic routes to o-carboranes having nitro- or nitrito-containing substituents on carbon are available, including the oxidation of C-aminophenyl derivatives [224], reactions of C-lithio-o-carboranes with p-nitrobenzoyl chloride 2CRMe2 2CH2NO2 (R ¼ NO2 or [400], and the treatment of C-alkenylcarboranes with N2O4 to form HCB10H10C2 ONO) as described in Section 9.7. 90% H2 O2
m-H2 NC6 H42 2CB10 H10 CH ! m-O2 NC6 H42 2CB10 H10 CH CF3 CðOÞOH
O2 N-C6 H4 -CðOÞCl R2 2CB10 H10 C2 2LI ! R2 2CB10 H10 C2 2CðOÞC6 H4 NO2
R ¼ Me; Ph
462
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
Nitro esters such as 9-223 can be prepared by the action of nitrocinnamates, nitroacrylates, and nitrobenzylidenemalonates on C-lithio- or C-magnesio-o-carboranes [480]. O
C(O)OEt PhCH=CHC
Li
C
O O
N NO2
−
C
C
M
Ph
Ph
O
C
OEt
O
C
O
C Ph
N
H + , H2 O
+
Ph
9-223
Ph
OEt
Nitronyl nitroxide mono- and biradicals have been generated from mono- and bis(p-formylphenyl)-o-carborane and characterized by ESR, X-ray diffraction, and magnetic studies. The monoradical crystallizes as a head-to-tail dimer with an intradimer ferromagnetic interaction [475], while the diradical 9-224 is weakly antiferromagnetic and is described as a three-dimensional analogue of o-benzoquinodimethane [476,477]. A manganese(II) complex of this molecule is paramagnetic with an S ¼ 3/2 ground state [478].
H
O
O C
N•O
C H
O
•
N +
+
-O
N
N
O-
(1) HON-C2Me4-NOH/MeOH C
C
(2) NaIO2(aq.), CH2Cl2, 0 °C C
C
9-224
9.13.1.2 B-substituted derivatives
Treatment of o-carboranes with 100% nitric acid in CCl4 solution at 20 C forms B2 2ONO2 and B2 2OH products 9-225 and 9-226 in high yield, the nitrato compound being predominant [374,381]. The extremely unstable nitrate explodes on heating, and is reduced by tin and hydrochloric acid to the same B-hydroxy derivative that is produced from HNO3. 2ONO2 isoMono-C-substituted o-carboranes, having lower symmetry, react with HNO3 to generate two MeC2B10H92 mers in addition to the B-hydroxy compound. The formation of two B-nitrato isomers, as well as NMR evidence, is consistent with the assumption that attack occurs at B(9) and B(12), the most negatively charged BH vertexes. Sn/HCl R
R
C R
R C
C R
C
HNO3
R
C +
9 12
C
O
R, R = H, Me
9-225
OH
O N O
9-226
9.13 Nitrogen derivatives
463
Nitration of B-aryl-o-carboranes, like that of the C-aryl isomers, takes place only on the aromatic ring. In the case of B(3)-phenyl-o-carborane, all three B(3)-nitrophenyl isomers are obtained on reaction with mixed nitric and sulfuric acids, with an observed o-/m-/p-nitrophenyl-o-carborane ratio of 3:4:3 [443]. HNO3 þH2 SO4
H2 C2 B10 H9 -3-Ph ! H2 C2 B10 H9 -3-C6 H4 -o=m=p-NO2 The contrast between this finding and the nitration of C-phenyl derivatives noted earlier, which takes place almost entirely at the para and meta positions on the phenyl ring, can be attributed to the absence of strong inductive electron withdrawal toward substituents attached at boron; indeed, boron-to-ring electron donation is seen. As a consequence there is little directive effect on ring nitration. Further discussion of electrophilic attack on o-carboranyl aryl derivatives can be found in Section 9.6.
9.13.2 Amines, azides, and diazonium salts 9.13.2.1 C-substituted derivatives Amino-o-carboranes with direct Ccage2 2N bonds are relatively uncommon, but can be prepared via hydrogenation of phenylazo derivatives over Raney nickel [484], by reduction of o-carboranyl azides [483] or C-nitroso compounds [492], or by conversion of azides to isocyanates, which in turn react with 94% sulfuric acid to give the amines [439,482]. H2 =Ni
2N5 5NPh ! RCB10 H10 C2 2NH2 RCB10 H10 C2
R ¼ Me; Ph
LiAIH4
RCB10 H10 C2 2N5 5N2 2CB10 H10 CR ! RCB10 H10 C2 2NH2 2NH2 2CB10 H10 CR R ¼ H; Me; Ph Al2 Cl3 H3
CB10 H10 C2 2NO ! HCB10 H10 C2 2NH2 H2 SO4
R ¼ H; Me; Ph H2 SO4
RCB10 H10 C2 2CðOÞN3 ! RCB10 H10 C2 2NCO ! RCB10 H10 C2 2NH2 R ¼ H; Me; Ph The C-azido derivatives are prepared via reactions of C-lithio-o-carboranes with tosyl azide (Section 9.4) or by treating acyl halides with sodium azide in aqueous acetone [439,482]: NaN3
RCB10 H10 C2 2CðOÞCl ! RCB10 H10 C2 2CðOÞN3 In contrast to the rarity of directly bonded C2 2NR2 o-carboranes, derivatives in which the amine unit is separated from the cage by methylene, aryl, or other groups are numerous and varied (Table 9-1). Many such compounds are prepared via alkyne insertion into decaborane-base adducts [129,151,195,486, 488,489,491,496] (Sections 3.1 and 9.2), by reduction of o-carboranyl nitriles [182,190,386], amides [123,182,445,495,1233], azides [182,194], and nitrophenyl derivatives [224,493,494,535], or from C-metallated o-carboranes [320,487,490,495,501]. LiAlH4
HCB10 H10 C2 2ðCH2 Þ3 CN ! HCB10 H10 C2 2ðCH2 Þ4 NH2 LiAlH4
HCB10 H10 C2 2CH2 CðOÞNEt2 ! HCB10 H10 C2 2ðCH2 Þ2 NEt2 H2 AlCl
R2 2CB10 H10 C2 2CðOÞNR02 ! HCB10 H10 C2 2CH2 NR02 NaN3
LiAlH4
HCB10 Me8 H2 C2 2ðCH2 Þ3 Br ! HCB10 Me8 H2 C2 2ðCH2 Þ3 N3 ! HCB10 Me8 H2 C2 2ðCH2 Þ3 NH2
464
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
Sn
HCB10 H10 C-m=p-C6 H42 2NO2 ! HCB10 H10 C-m=p-C6 H42 2NH2 HCl
ClCH2 NR02
R2 2CB10 H10 C2 2Li ! R2 2CB10 H10 C2 2CH2 NR02 0 R ¼ H; CHMe2 ; R ¼ H; Me; Et ClðCH2 Þ2 NMe2 ð1Þn-C4 H9 Li HCB10 H10 C2 2Li ! HCB10 H10 C2 2ðCH2 Þ2 NMe2 ! ð2ÞClðCH2 Þ2 NMe2
ClBðNMe2 Þ2
Me2 NðCH2 Þ22 2CB10 H10 C2 2ðCH2 Þ2 NMe2
HCB10 H10 C2 2Li ! HCB10 H10 C2 2BðNMe2 Þ2 The directed synthesis of 9-227, an o-carboranyl alanine derivative of interest in BNCT, employs a carboranyl Grignard [497]. Other o-carboranyl amines have been prepared for this and similar medical applications [413,427, 499,500,1277]. C
H
CF3
+ MgBr
C
C
N R
C
1) Et2O −78 C + 2) H , H2O
C(O)OMe
H
NHR C
C
CF3
C
H
C
CF3C(O)OH
NH2 C
C
CF3
C
OMe
O
OMe
O
9-227
Alternative routes to C-aminoalkyl-o-carboranes include acid hydrolysis of isocyanates [439,482] and amides [503], hydrogenation of phenylazo derivatives [484], reduction of C-nitroso carboranes [492], and nucleophilic amination of 1-ClCH(aryl)-C2B10H11 derivatives to give C-aminobenzyl derivatives [228]. The versatility of these approaches allows some compounds such as the amino acid 9-228 [496] to be prepared in more than one way: C C
H
O C CH2
C
KCN/NH3 H
C
H
NH2 C
C
HCl conc.
H
H
NH2 C
C
C N
O
9-228 H+, H2O −CO2
O Me NHC(O)Me
Et2NPh
B10H14 + HC≡C−C[C(O)OEt2]
toluene
C C
C H
NH C C O
H
C H
Et C O Et
Deprotection of N-alkylated phthalimides by Gabriel’s method, a procedure commonly employed by organic chemists for the synthesis of primary amines, is normally conducted in strongly basic conditions, which are not suitable for o-carborane derivatives because of degradation of the cage to nido-C2B9 species as described in Chapter 8. However, an alternative deprotection approach using di-tert-butyliminodicarboxylate avoids cage degradation and allows the synthesis of o-carboranyl C-amino HCl adducts [195]. CBr4 =PPh3
HNðBOCÞ2
CH2 Cl2 0 C
Bu4 N HSO4
NaOH
R2 2CB10 H10 C2 2ðCH2 Þ3 OH ! R2 2CB10 H10 C2 2ðCH2 Þ3 Br þ! !
9.13 Nitrogen derivatives
465
HCl
R2 2CB10 H10 C2 2ðCH2 Þ3 NðBOCÞ2 ! HCB10 H10 C2 2ðCH2 Þ3 NH3 Cl R¼H
R ¼ H; Me; ðCH2 Þ3 Br BOC ¼ tert-butyloxycarbonyl In another procedure [189], treatment of C-[p-(a-bromotolyl)]-o-carborane with diethyl phthalimidomalonate forms a phthalimido derivative that can be hydrolyzed and decarboxylated to give a carboranyl phenylalinine, HCB10H10C2 2CH2C6H42 2CH[C(O)OEt]NH2, for potential BNCT therapy of melanoma. A somewhat different approach utilizes reactions of phthalimido derivatives such as 9-229 (n ¼ 2, 3) with hydrazine hydrate under mild conditions to give C-alkylamino carboranes 9-230 [489,491,1278]. On further reaction of 9-230 with hydrazine at elevated temperature, deboronation occurs to form nido-7; 8-H3 NðCH2 Þn C2 B9 H11 ions [489]. C C
Li
C
Br(CH2)nN(CO)2C6H4
(CH2)n N
CMe3
C
C
SiMe2
O
C
Me3C−Me2SiO Bu4N+F−
C
(CH2)n NH2
C
N2H4•H2O 20 C
H
9-230
C
(CH2)n
O N
C
H
C
O C
9-229
(An earlier report [491] that hydrazine degrades 9-230 to the nido species at ambient temperature has been disputed [489].) Alternatively, treatment of 9-229 with NaBH4 and isopropanol followed by HCl and acetic acid affords the HCl adduct of 9-230 [491]. The reaction of C-lithio-C0 -phenyl-o-carborane with N-2-(bromomethyl)phthalimide ion takes a different course, forming an o-carborane derivative bearing a tricyclic benzbicyclo-oxa-aza substituent instead of the expected C-phthalimido product; electron withdrawal by the phenyl ring is the presumed driving force in this mechanism [547]. 1,2-Bis(phthalimidomethyl)-o-carborane, a potential precursor to polyamines, can be obtained via direct reaction of C,C0 -dilthio-o-carborane with N-bromomethylphthalimide [528]. BrCH22N½CðOÞ2 C6 H4
2Li ! C6 H4 ½CðOÞ2 NCH22 2CB10 H10 C2 2CH2 N½CðOÞ2 C6 H4 Li2 2CB10 H10 C2 78 C
As expected from the strong inductive electron-attracting character of the cage carbon atoms, o-carboranyl C-amino derivatives are weakly basic, dissolving in concentrated sulfuric acid but precipitating out of solution on dilution [224,439,482,493]. In general, their behavior parallels that of arylamines, especially for derivatives lacking a direct carboranyl C2 2N bond; an example is the facile oxidation of C-aminophenyl- to C-nitrophenyl-o-carboranes mentioned earlier. Similarly, treatment of C-aminophenyl derivatives with acetic anhydride affords N-acetyl derivatives, which in turn are diazotized by nitrosylsulfuric acid in glacial acetic acid [224,493]. Reactions of the diazonium salts with halide ions and with b-naphthol afford C-halophenyl products and azo dyes, respectively [224,493]. X
HCB10 H10 C2 2p-C6 H4 Nþ ! HCB10 H10 C2 2p-C6 H4 X 2
X ¼ Cl; Br; I
b-C10 H7 OH HCB10 H10 C2 2p-C6 H4 Nþ 2N5 5N2 2C10 H6 OH 2 ! HCB10 H10 C-p-C6 H42
466
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
9.13.2.2 B-substituted derivatives
The preparation of 3-amino-o-carboranes by reduction of RR0 C2B10H10 substrates with sodium in liquid ammonia to form the carborane dianion, followed by oxidation to the neutral RR0 C2B10H9-3-NH2 product, was described in Section 9.5. An alternative approach is boron insertion or “reboronation” into nido-C2B9 (dicarbollide) dianions as outlined in Section 7.2, as in the synthesis of H2C2B10H9-3-NR2 (R ¼ Et, Ph) via the reaction of R2NBCl2 with Na2C2B9H11 in cold THF [383,506]. Since the C2B9 dianions are themselves generated via extraction of boron from a C2B10 cluster, reboronation is, in principle, less efficient as a synthetic route although it may be advantageous in specific cases. B(3)-amino-o-carboranes can also be obtained by treatment of B(3)-amido derivatives with sodium hypobromite, which induces a Hofmann rearrangement to the isocyanate, from which the amine is obtained on hydrolysis, and also by reaction of the B(3)-carboxylic acid with HN3 in sulfuric acid, which leads to the amino product via a Schmidt rearrangement [183]. NaOBr
2C2 B10 H9 -3-CðOÞNHBr ! Me2 2C2 B10 H9 -3-CðOÞNH2 ! Me2 HBr
H2 O
Me2 2C2 B10 H9 -3-NCO ! Me2 2C2 B10 H9 -3-NH2 CO2
NaN3
HC2 B10 H9 -3-CðOÞCl ! HC2 B10 H9 -3-CðOÞNH-Nþ ! 2 þ H
D
H2 O
H
CO2
HC2 B10 H9 -3-CðOÞNH ! HC2 B10 H9 -3-NCO ! HC2 B10 H9 -3-NH2 Direct attachment of amine groups at boron locations other than B(3/6) has not been reported, but the B(9)-azido 29-N3 has been synthesized from HC2B10H92 29-IPhþBF4 and ðn-C4 H9 Þ4 Nþ N3 [539]. B(9)compound HC2B10H92 29-I and H2C2B10H82 29,12-I2 with and B(9,12)-aminophenyl derivatives can be prepared by arylation of H2C2B10H92 29-C6H4NMe2 and p-Me2NC6H4MgBr over PdCl2(PPh3)2 catalyst to give, respectively, H2C2B10H92 29,12-(C6H4NMe2)2 [240]. H2C2B10H82 The properties of B(3/6)-amino-o-carboranes are markedly different from those of the C-amino derivatives. Lacking a strong electron-withdrawing effect toward the amino groups, the boron-substituted compounds are fairly strong bases, easily reacting with acids to form salts. Like aliphatic and aromatic amines in general, they readily undergo alkylation 23/6-NH2 precursors are and acylation [347]. The syntheses of B(3/6)-halo and -hydroxy derivatives from RR0 C2B10H92 described in Section 9.5, and the B-amino compounds can also be converted by standard organic methods to alkyl- and arylamines, amides, isocyanates, and other B2 2N compounds [505,507]. COCl2
R2 2C2 B10 H9 -3-NH2 ! R2 2C2 B10 H9 -3-NHCOCl ! R2 2C2 B10 H9 -3-NCO PhCl;HCl
R ¼ H; Me
HCl
LiAlH4
R2 2C2 B10 H9 -3-NH2 ! R2 2C2 B10 H9 -3-NHMe Et2 O
RCHO
LiAlH4
H2 O
Et2 O
R ¼ H; Me
Me2 2C2 B10 H9 -3-NH2 ! Me2 2C2 B10 H9 -3-N5 5CHR ! Me2 2C2 B10 H9 -3-NHCH2 R R ¼ Ph; m=p-C6 H4 NO2 ; p-C6 H4 Br; p-C6 H4 OMe; o-C6 H4 OH; furanyl RLi
Me2 2C2 B10 H9 -3-N5 5CHPh ! Me2 2C2 B10 H9 -3-NHCHðRÞPh R ¼ H; Me þ H
9.13 Nitrogen derivatives
467
N-protected B(3)-amino acid derivatives that are o-carboranyl analogues of g-aminobutanoic acid, such as HO(O) 2C2B10H92 23-NHC(O)Me, are easily converted in acid media to the corresponding water-soluble B(3)-NH3 þ CCH22 salts that are amenable to biomedical applications (Chapter 16) [507]. Diazotization and subsequent halogenation of 3-amino-o-carboranes are described in Section 9.5.
9.13.3 Nitrogen heterocycles Derivatives of o-carborane bearing pyridyl, bipyridyl, picolyl, porphyrin, thymidine, or other N-heterocyclic substituents are useful as ligands in metal complexes owing to the electron donor properties of the nitrogen atoms, and many such compounds have been characterized (Table 9-1). In general, these are readily prepared by methods described in Section 9.5 for the synthesis of C-aryl derivatives, usually involving either reactions of C-metallated o-carboranes or the insertion of alkynes into decaborane-base adducts. As was noted in Section 9.4, the action of pyridyl aldehydes on C-lithio or C-bromomagnesio-o-carboranes affords C-pyridyl alcohols. Reactions of halopyridines with 1,2-CuC2B10H11 readily generate C-pyridyl-o-carboranes. The interaction of 20 -bromopyridine with the cupracarborane generates the bis(20 -pyridyl) derivative 9-231 in 80% yield with no monosubstituted product detected; the introduction of the first pyridyl unit is presumed to promote addition of the second [274,471]. The mono-C-pyridyl product 9-232 can, however, be prepared via alkyne insertion [471].
C C
H
C
n-C4H9Li, CuCl 2-Br-C5H4N
H
N
9-231
C N
C B10H12(SMe2)2 + HC
C
toluene N
C
N
9-232
H
Pyridyl derivatives having close C2 2H---N contacts such as 9-232 and 9-233, a feature not possible in 30 -pyridyl species such as 9-234, exhibit intramolecular hydrogen bonding, which in the case of 9-232 persists even in solution [274].
C
C
N
C
C H
9-232
9-233
C N
N
C H
H
9-234
Bipyridine, terpyridine, and triazine derivatives are easily accessible via aromatic nucleophilic substitution at an o-carboranyl CH vertex, as in the synthesis of 9-235 and 9-237, which in turn are converted to 9-236 and 9-238, respectively, via reaction with 2,5-norbornadiene in inverse electron-accepting Diels-Alder processes [273].
468
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12 N
Ph
Ph
N
(1)
N
N
N O
C
−
C
C
N C
N
C
toluene, reflux
R
(2) Me2NCOCl R = Me, Ph
C
Ph N
N
N
R
9-236
9-235
Li Ph
N
N
N
R
(1)
N O
N
Ph
Ph
N
N −
N
Ph
N
C
N
N
N
N
O
C C
(2) Me2NCOCl R = Me, Ph
Ph
N
R
R
9-237
C
Ph N
C
N
C
C
N
R
R
C
9-238 A related class of heterocycle-substituted carboranes is the pyrroles, typified by 9-239 to 9-241 [318,511], which are of interest as building-blocks for the construction of tumor-specific agents that may be useful in BNCT, electronics, and other applications [513]. C
H
C
PhSCl, CH2Cl2
SPh
reflux
C
H
C
(1) m-CPBA, CH2Cl2
C
(2) DBU, THF
C
H
SO2Ph
Cl m-CPBA = m-chloroperoxybenzoic acid DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene
C
H
CNCH2C(O)OR NaOCMe3, THF, reflux
C
H
(2) LiAlH4, THF C
C
NH
9-240
NH
9-239
CH2
C O HO
HO
C
Me3N+
Me
2 I−
+ C
Li
+NMe 3
(1) THF, 0 C (2) 80 C
N H
Me
Me C
C C
9-241
2 I−
N H
C
9.13 Nitrogen derivatives
469
Pyrroles are also accessible via palladium-catalyzed Suzuki cross-coupling, as in the synthesis of 9-242 [512]. Br C
Me
+ C
NH
(1) Pd(PPh3)4, 5:1 toluene/MeOH K2CO3
I
C
(2) (n-C4H7)4N+F−, THF, 0 C
N
Me
C
Si(CHMe2)3
9-242
In turn, pyrroles such as HNC4H3(CH2)22 2C2B10H11 can be tetramerized by condensation with benzaldehydes to give b-carboranyl porphyrins [298,318]. However, compounds of the latter type are more easily prepared via crosscoupling, which affords products such as 9-243 more efficiently than earlier methods requiring multiple synthetic steps [512]. Ph
C
Me
B(OH)2
N Br
H
+
C
Ph
N Br
Br N Br
H
N Ph
Ph
Pd(PPh3), toluene, K2CO3 Me
Me
C C
Ph
C
Ph C N H
N
N H
N C C Me
C Ph
Ph
9-243
C Me
Closely related o-carboranyl phthalocynanine derivatives such as the zinc complexes 9-244 (obtained as a mixture of regioisomers in a total yield of 83%) [520] are candidates for use as agents in BNCT and photodynamic therapy (PDT). Boron-substituted derivatives of 5,10,15,20-tetraphenylporphyrin having the general formula (porphyrin)-[p-C6H4NHC(O)-9-C2B10H11]4, prepared by reaction of (porphyrin)(p-C6H4-NH2)4 with 1,2-C2B10H11-9-C(O)Cl and subsequent reduction with SnCl2 in HCl, are of interest as cytotoxic agents toward tumor cells [516,519]. The utility of poly(carboranyl) porphyrins and related derivatives as a means of delivering boron to tumor tissues is discussed in Chapter 16.
470
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12 CN
O
C H
CN
C
ZnOAc 220 C C H
H C
C
C
O
O N N
N N
N
Zn N
N N
O
O
C
C C H
H C
9-244
Other N-heterocyclic o-carboranes have been characterized, and many are listed in Table 9-1. Isoxazole and isoxazoC or CH25 5CH line derivatives, for example, can be generated via reactions of PhCNO with RC2B10H11 where R ¼ HC [531]. A notable example that bridges two distinct boron chemistry areas is the tris(o-carboranyl)borazine derivative 9-245, prepared in 64% yield from N-trimethyl-B-trichloroborazine and 1-lithio-2-n-butyl-o-carborane [501].
C
Me C
N B
C
N
N Me
C B
B
Me
C C
9-245
9.13 Nitrogen derivatives
471
9.13.4 Amides and imides Methods for synthesizing C-amido-o-carboranes include treatment of carboranyl acid chlorides with ammonia or amines [123,182,364,396,445,526,532,1147,1279], reactions of B10H12L2 adducts with alkynyl amides [129], and interactions between C-lithio-o-carboranes and aryl isocyanates [398,399], in the last case forming carboxylic acid anilides (Section 9.4). Amides in which the C(O)NR2 group is directly bonded to the o-carboranyl cage are decarboxylated by sodium amide in liquid ammonia [128], probably via an alcoholate intermediate as in the cleavage of esters and ketones discussed earlier in Sections 9.8 and 9.10. NaNH2
R2 2CB10 H10 C2 2CðOÞNR2 !R2 2CB10 H10 C2 2CðO ÞðNH2 ÞNR02 Naþ ! R2 2CB10 H10 C2 2Na þ NH2 CðOÞNR02 NH3
R; R0 ¼ Me; Ph
Unlike o-carboranyl esters and ketones (Section 9.10), the corresponding amides are not decarboxylated by ethoxide ion; this may simply reflect weakening of the inductive electron withdrawal at the o-carboranyl carbon when an amide group is present, so that stronger bases are required to effect cleavage. Not surprisingly, decarboxylation is affected by proximity of the amide functionality to the o-carborane cage [1279]. With LiAlH4, for example, cleavage occurs only in derivatives having an amide group directly bonded to the carborane; otherwise only reduction to the amine is observed [123,445,1233]. B-amido o-carborane derivatives can be synthesized from the corresponding amines, as in the preparation of B(3)2B(3)-NH2 with formic acid [347]; higher yields are reported when a formic formamides via reaction of R2C2B10H10C2 acid-acetic anhydride mixture is employed [502]. Ac2 O;HCðOÞOH
H2 C2 B10 H9 -3-NH2 ! H2 C2 B10 H9 -3-NHCHO C5 H 5 N
9.13.5 Nitriles, isonitriles, and isocyanates o-Carboranyl nitriles and isonitriles are generated in a variety of reactions, some of which have been mentioned earlier, for example, the action of cyanogen chloride on C-lithiocarboranes (Section 9.4) and the conversion of C-alkenyl carboranes to a C-difluoramine 9-78 and then to the difluoraminonitrile 9-79 as described in Section 9.7. Cyanoethylation of 2CB10H10C2 2(CH2)2CN products has been achieved with Triton-B [546], via R2 2CB10H10CH substrates to yield R2 phase-transfer catalysis in benzene [545], and in two-phase CH2Cl2/H2O or (MeO)2C2H4/H2O systems in the presence of PhCH2 NEt3 þ OH catalyst [446]: ½PhCH2 NEtþ OH 3
R2 2CB10 H10 CH þ CH25 5CH2 2CN ! R2 2CB10 H10 C2 2ðCH2 Þ2 CN CH2 Cl2 =H2 O 20 C
R ¼ H; Ph
Reactions of C-lithio- or C-bromomagnesio-o-carboranes with cynanotosylate afford the corresponding nitriles in good yield [543]. TsOCN
Ph2 2CB10 H10 C2 2Li ! Ph2 2CB10 H10 C2 2CN TsOCN
R2 2CB10 H10 C2 2CH2 MgBr ! R2 2CB10 H10 C2 2CH2 CN
R ¼ H; Me; Ph
Alternatively, phenyl cyanate can be employed [544]. PhOCN
2Li ! R2 2CB10 H10 C2 2CN R2 2CB10 H10 C2 PhOCN
R ¼ Me; Ph
R2 2CB10 H10 C2 2CH2 MgBr ! R2 2CB10 H10 C2 2CH2 CN
R ¼ Me; Ph
472
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
A different approach employs dehydration of carboxamides by P2O5 [182] or trimethylsilylpolyphosphates, as in the synthesis of 9-246 [542]. H
H
O
C
(CH2)n
C
O
C
C
P NH2
O
C
N
C
(CH2)n
OSiMe3 n n = 0, 1
9-246
Boron-substituted o-carboranyl isonitriles are accessible from the corresponding B(3)-formamides (prepared as described above) by reaction with phosphonyl chloride; on heating, the isonitriles 9-247 rearrange to the B(3)-nitriles 9-248 [505,548]. H
C
O
H
N C
C
C
H
N
C
H
H
C
POCl2
N
H
C H
C C
250−300 C
H
NC5H5
9-247
9-248
B(9)-cyano-o-carborane can be obtained via treatment of 1,2-H2C2B10H9-9-C(O)OH with SOCl2 [408]. The nitrile 9-246, as well as the analogous B(3)-isocyanomethyl derivative, can be obtained under milder conditions via an alternative route employing the Burgess reagent, methyl N-(triethylammoniumsulfonyl)carbamate [502]. Et3 NSO2 NCðOÞOMe
H2 C2 B10 H9 -3-ðCH2 Þn NHCHO ! H2 C2 B10 H9 -3-ðCH2 Þn NC
n ¼ 0; 1
Several reactions mentioned earlier generate o-carboranyl isocyanates. Other routes include treatment of C-carboxyl chloride derivatives with azides in refluxing solvents [364,413,533] and condensation of o-carboranyl diols with ClCN [534]. Me3 SiN3
2ðCH2 Þn CðOÞCl ! HCB10 H10 C2 2ðCH2 Þn NCO R2 2CB10 H10 C2 toluene
n ¼ 0; 1; R ¼ H; Me; Ph; CH25 5CMe
LiN3
2CðOÞCl ! OCN2 2CB10 H10 C2 2NCO ClðOÞC2 2CB10 H10 C2 benzene ClCN
2CB10 H10 C2 2CH2 OH ! OCN2 2CB10 H10 C2 2NCO HOCH22 acetone
In another approach, amine-hydrochloride salts are reacted with phosgene in the presence of pyridine [537]. Cl2 CO
HCB10 H10 C2 2ðCH2 Þn NHþ ! HCB10 H10 C2 2ðCH2 Þn NCO 3 Cl pyridine
n ¼ 1; 2; 3
9.13 Nitrogen derivatives
473
B(9)-isocyanato derivatives, for example 9-249, can be obtained from acyl chlorides as in the preparation of C-substituted isocyanates cited above [536].
H
H
H
H
C
C
C
H
C SOCl2 C
H
C
C
Me3SIN3
OH
C
O
OCl
N
C
O
9-249
O
The synthesis of B(3)-isocyanates was described earlier in this section.
9.13.6 Carbamates and ureas Carbon- and boron-substituted o-carboranyl isocyanates combine with alcohols to generate carbamates almost quantitatively; however, while the C-isocyanate derivatives react at room temperature, the boron-bound species require refluxing in toluene for 3-10 h [536]. Once again, the difference in reactivity is ascribed to activation of the carbon-bound substituents by inductive electron withdrawal; in the B(9)-isocyanato derivatives, in contrast, the cage is electron-donating. Me3 COH
H2 C2 B10 H9 -9-NCO ! H2 C2 B10 H9 -9-NHCðOÞCMe3 toluene
Reaction of the B(9)-isocyanato-o-carborane with a difluorooxaazaborolidine complex followed by hydrolysis affords the amino acid-carbamate species 9-250, which is lipophilic and is designed to promote tumor uptake for possible application in BNCT [536]. H
H
C H
O
HO CH
C
C
C
H
MeCN
+ N
C
H2N+
O
− B
C O
Δ N
C O
O
O
F
F
H H2O
+ H2N
H
C
CH
O
− B
C H
F
C
F
O N H
C O
O CH
9-250 H2N
C OH
o-Carboranyl ureas are of interest as boron carriers for BNCT and as modules for construction of novel materials derived from extended carborane-based systems. Diphenyl urea derivatives can be prepared in several steps from m- and p-nitrophenyl-o-carboranes, as in the syntheses of 9-251 and 9-252 [550].
474
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12 C
H
C
H2/Pd-C/EtOH C
H
C
O2N
H2N triphosgene Et3N, Cl(CH2)2Cl
C
H
H
H N
H N
C
C
9-251
C
C O
C
+
H
H
C
9-252 C
C O C N H
N H
Similar methods have been employed in the designed synthesis of multi-carboranyl ureas such as 9-253 and 9-254, which can be used in construction of still larger arrays [549,550]. C
H
H
R
C
N
R
C
N
C
C O R = H, Me C H
C C
C
H
9-253
C
C
C
C
N
N
N Me
C
C O
Me
Me
9-254
C O
N Me
C
9.14 Phosphorus, arsenic, antimony, and bismuth derivatives
475
9.14 PHOSPHORUS, ARSENIC, ANTIMONY, AND BISMUTH DERIVATIVES o-Carboranyl compounds with substituents containing phosphorus, or to a much lesser extent heavier Group 15 elements, are easily prepared and widely used as ligands in metal complexes and in construction of extended nonmetallic systems for use in biomedical and other applications. Attachment of phosphino and related groups to the carborane cage allows the carborane chemist to exploit the well-developed methodology of organophosphorus chemistry in the design and synthesis of a wide variety of structures, and this has been done extensively as the listings in Table 9-1 attest.
9.14.1 Phosphino and related derivatives 9.14.1.1 C-substituted phosphines Some of the more common synthetic approaches to compounds featuring direct Ccage2 2E bonds (E ¼ P, As, Sb) have been outlined in Section 9.4. The most versatile method is via reaction of C-lithio (or less commonly, Grignard) derivatives with halophosphines [50,555,565,575,592] or aminohaloarsines [830]: PR0 R00 Cl
R2 2CB10 H10 C2 2Li ! R2 2CB10 H10 C2 2PR0 R00
R ¼ Me; Ph R0 ; R00 ¼ H; Cl; Me; Ph
R-CB10 H10 C-Li PhPCl2 R2 2CB10 H10 C2 2Li ! R2 2CB10 H10 C2 2PPhCl ! ðR2 2CB10 H10 CÞ2 PPh R5 5Me; Ph
ðEt2 NÞ2 AsCl
HCl
2CB10 H10 C2 2Li ! Me2 HC2 2CB10 H10 C2 2AsðNEt2 Þ2 ! Me2 HC2 H2 O
Me2 HC2 2CB10 H10 C2 2AsCl2 ! Me2 HC2 2CB10 H10 C2 2AsO In many of these reactions, the products obtained are controlled by adjusting the reaction stoichiometry. As was mentioned earlier, mono-C-substituted carboranes of the type HCB10H10C2 2PR2 (9-10) are generated efficiently from 2Li in dimethoxyethane solution, which effectively blocks formation of the dilithio derivative. Tris(oHCB10H10C2 carboranyl)phosphines, (R2 2CB10H10C)3P, and their arsenic and antimony counterparts (9-22) can be obtained from the corresponding Group 15 trichlorides as described in Section 9.4. Similar reactions of C,C0 -dimetallo-o-carboranes lead to bis(phosphino) derivatives, from which exo-polyhedral cyclic systems can often be obtained as illustrated in Figure 9-21 [50,555,575]. Arsenic-bridged analogues are also known [1280]: 2Li þ MeAsBr2 ! ðm-MeAsÞ2 ðC2 B10 H10 Þ2 Li2 2CB10 H10 C2 0
The propensity of C,C -bis(phosphino)-o-carboranes to form exo-polyhedral cycles is further demonstrated in structures such as 9-264 and 9-265 [556], and in metal chelate complexes, discussed below. Reduction of a 1:1 mixture of diastereomers of the chiral bis(phosphino) derivative 9-255, followed by hydrolysis, affords the secondary phosphine 9-262 in a 4:1 mixture of the racamic and meso forms, which react with sulfur on heating to give 9-264 [577].
476
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12 Ph Li
C
C
PhPCl2
C
C
Li
P
C
Cl
NH3
Cl
−2 HCl
P NH
C
Ph
9-255
PCl3
P
Ph
P Ph
9-256 Li2C2B10H10
Cl C C
P
P
Ph C
C
C
C
P
P
Cl
Ph
NaN3
9-257
C C
9-259
N3 NH3
P
C NH2 C C
9-258
P
P
C
P
C
C C
N3
C PPh3
NH2 Ph
Ph Ph P N
C
P
C
9-261 C
P
C
N P Ph
Ph Ph
FIGURE 9-21 Examples of cyclic o-carboranyl phosphine syntheses.
9-260
9.14 Phosphorus, arsenic, antimony, and bismuth derivatives Ph
Ph
Ph
P
P
P
C C
Cl
LiAlH4 H2O
Cl
P
C C
Ph
9-255
H
C
S8
H
P
C
Ph
9-262
9-263
P Ph
Ph H H
S
P
C
S8
477
S
−H2S
C
S
9-264
P Ph
S
F Li
C
C
PF2Br
C
C
Li
P
C
9-265 P
C
F
Chiral bis(aminohalophosphine) derivatives, 1,2-[P*X(NR2)]2C2B10H10 (X ¼ Cl, Br; R ¼ Me, Et, CHMe2) which are structurally analogous to 9-255 have been synthesized and found to have unexpectedly high resistance to methanolysis. This behavior has been traced to an unusually strong intramolecular P2 2P interaction, based on DFT calculations and the fact that the corresponding 1,7-disubstituted m-carboranyl derivatives, in which the phosphorus centers are well-separated, react normally with methanol to afford 1,7-[P*(OMe)(NR2)]2C2B10H10 products [558]. Monofunctional o-carboranes undergo standard substitution chemistry to generate a variety of unsymmetrical Cphosphino derivatives, exemplified by 9-266 to 9-270 [559,572,573], that are useful in metal complex synthesis and other applications, as will be seen. Ph
Ph C C
P Ph
PF2Br
F
Et2O
P
C C
PPh2 Et2O
C
NMe2 P
9-267 NMe2 PPh2Cl
C
Ph
C
Li
F
9-266
P
C
ClP(NMe2)2
Li
C
NMe2
C
Ph
NMe2
9-268
P Ph
Ph C C
H
SH
1) KOH 2) EtBr
C
EtOH
C
9-269
H
S
1) n-C4H9Li 2) PPh2Cl
C
EtOH
C
9-270
P Ph S
478
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
o-Carborane derivatives having phosphino groups not directly attached to the cage, examples of which are listed in Table 9-1, are accessible by general synthetic methods described in earlier sections of this chapter. In structures such as 9-271, generated from the o-carboranyl Grignard and XCl3 (X ¼ P or As), the synthesis is aided by relief of steric crowding afforded by the methylene spacers [597]. 3−
H
H
C
C
C C
(n-C4H9)4NF X
C
H
THF, Δ X=P
C
H
H
C C H
P
C
H
H
C C
C X = P, As
H
9-271
9-272
The tris(o-carboranylmethyl)phosphine 9-271 undergoes controlled cage degradation (see below) to the tris(nido2CH2)3P5 5O [597]. carboranyl)phosphine trianion 9-272, and is converted by hot 30% H2O2 to the oxide (HCB10H10C2
9.14.1.2 B-substituted phosphines o-Carborane derivatives with direct B2 2P bonds are uncommon, and even compounds in which phosphorus is linked to boron through intervening groups are comparatively few in number. The preparation of H2C2B10H9-9-PX2 derivatives from (H2C2B10H9)2Hg and trihalophosphines was noted in Section 9.5, as was the synthesis of 1,2-C2B10H11-9-AsCl2 and its antimony analogue from bis(9-o-carboranyl)mercury. Several types of B2 2S2 2P bonded systems are discussed in the context of B(9)-thiophosphites later in this section. Reactions of B(3)-haloalkyl- and B(9,12)-bis(haloalkyl)-o-carboranes with trimethylphosphine generate water-soluble 1; 2-C2 B10 H11 -3-ðCH2 Þn PMe3 þ X (X ¼ Br, I) and 1; 2-C2 B10 H11 -9; 12-½ðCH2 Þn PMe3 2 2þ 2Br salts, respectively [611].
9.14.2 Phosphates, phosphites, and related derivatives 9.14.2.1 C-substituted oxyphosphorus compounds C-metallated o-carboranes combine readily with phosphoryl trichloride and similar reagents to afford o-carboranyl phosphine oxides and phosphonic chlorides, from which phosphinic and phosphonic acids, phosphates, and phosphites can be prepared (Figure 9-22) [557,591]. Other useful synthetic methods include the preparation of phosphinites from o-carboranyl Grignards [588], catalytic phosphorylation of C-hydroxymethyl derivatives [594,606], and the reaction of ethylphosphoryl dichloride with LiC2B10H11 to afford the cyclic diphosphinite 9-280 [599]. Mg; CIPðOEtÞ2
2CH2 Br ! HCB10 H10 C2 2CH22 2PðOEtÞ2 HCB10 H10 C2 pyridine
0
RR PðOÞCl
HCB10 H10 C2 2CH2 OH ! HCB10 H10 C2 2CH2 OPðOÞRR0 þ HCl Mg
R; R0 ¼ Cl; OCH2 CF3 ; OPh
9.14 Phosphorus, arsenic, antimony, and bismuth derivatives
R
C
C
HCl, C6H6
C
R
C
P(NEt2)2
R
C
H2O, O2
O
C
PCl2
479
P
OH
H
(Et2N)2PCl
9-273
R = Me, Ph
C C
ClO
R POCl3
O P
C
C
R = Me
9-274
Li
C
C Me Me
MePOCl2 R = Me
POCl3 R = Ph Ph
O P
C
C
Me C C
C
9-276
R
C
H2O
2) H+
Cl
C
P
R OH P OH
O
9-278
Me HCl, C6H6
C
1) K2CO3/H2O
Cl
9-277
C
R
C
PCl4 R = H, Me, Ph
C
O Ph
9-275 C
C
C
Me Me
C
P
C C
Ph
C
O
C C
Me
Me
CH2Cl
C C
P O
C
C
9-279
Me
FIGURE 9-22 Synthetic routes to o-carboranyl phosphinic acids (9-273), phosphonic acids (9-278), phosphonic chlorides (9-274 and 9-277), and phosphine oxides (9-275, 9-276 and 9-279).
480
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
O C C
H
EtOPCl2
C
C
9-280 C
Li
P
P
C
O
Though less commonly employed, alkyne insertion into the decaborane cage is useful in some cases, for example, in the preparation of esters of pentavalent phosphorus [1281]: HCC2 2CH2 OPðXÞðOEtÞMe
2CH22 2OPðXÞMeðOEtÞ X ¼ O; S B10 H14 ! HCB10 H10 C2 PNHMe2
The functional-group reactivity of o-carboranyl oxyphosphorus derivatives is largely as expected; for example, 2P(O)(OEt)2 is reduced to Me2 2CB10H10CH on treatment with NaOEt in ethanol [585], and phosphiMe2 2CB10H10C2 nites (9-281) react with p-nitroazobenzene to generate the phosphazido species 9-282 [586].
C
R
C
R
p-O2NC6H4N3 C
9-281
P Me
OEt C
OEt
P R = H, Me, CH2=CMe, Ph
Me
N
NO2
9-282
Phosphonites are converted to alkenyl phosphates on treatment with chloral [1282], and can be transalkylated via reaction with methyl sulfate [1283]. Cl3 CCðOÞH
R2 2CB10 H10 C2 2PðOEtÞ2 ! R2 2CB10 H10 C2 2PðOÞðOEtÞOCH5 5CCl2 R ¼ Me; Ph; CH2 C5 5CMe Me2 SO4
R2 2CB10 H10 C2 2PðOEtÞ2 ! R2 2CB10 H10 C2 2PðOÞðOMeÞMe R ¼ H; CH2 C5 5CMe 110 C
9.14.2.2 Polyphosphate derivatives Oligophosphates containing multiple carborane cage units are of interest as boron carriers for BNCT and in other medical applications (Chapter 16). Water-soluble oligomers 9-283 containing up to three carboranylphosphate repeating units have been prepared from the diol [HO(CH2)3]2C2B10H10 with the aid of coupling reagents [607], and the phosphoramidite 9-284 has been used in construction of oligophosphates targeted to specific antibodies via automated DNA synthesis [1284].
9.14 Phosphorus, arsenic, antimony, and bismuth derivatives
O O
P
481
O O
C
C
O
OC6H4-m-Cl
O
P
C
C
OR
OC6H4-m-Cl n
9-283 N(CHMe2)2 C
C
O
O
P O
DMTr NC
9-284
DMTr = 5-O-dimethoxytrit
Efforts to incorporate carborane units- into nucleosides are illustrated by the synthesis of the o-carboranyl bis(adenosine diphosphate) derivative 9-285 [608] and p-carboranyl analogues [1285]. O
O
N
P
O
N N
HN
P
P
O
MeO
MeO
O
O
O
C
C
O OMe
N
P
O OMe
O
N
N N
N
HN
9-285
CH2Ph
CH2Ph
9.14.3 Thiophosphites and thiophosphates 9.14.3.1 C-substituted derivatives The reaction of elemental sulfur with o-carboranyl secondary phosphines to give C-thiophosphoryl and C-dithiophosphinic cyclic anhydride derivatives 9-263 and 9-264 was noted earlier. Similar reactions of sulfur with C-phenylchlorophosphino derivatives yield chlorothiophosphines, and afford an alternative route to 9-264 [552].
RJCB10H10CJPPhCl ClPhPJCB10H10CJPPhCl
S
S
⎯→
⎯→
RJCB10H10CJP(=S)PhCl
ClPhPJCB10H10CJP(=S)PhCl
R = H, Ph S
Ph C
⎯→
S
P S
C
9-264
P Ph
S
482
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
Thiophosphates and thiophosphonates can be obtained in high yields via disulfide cleavage, and in reactions with sulfur and EtOP(S)MeCl [563]. 2Li þ R0 OÞ2 PðSÞSSPðSÞðOR0 Þ2 ! R2 2CB10 H10 C2 2SPðSÞðOR0 Þ2 R2 2CB10 H10 C2 R ¼ H; Me; Ph R0 ¼ Me; Et; CHMe2 1ÞS
R2 2CB10 H10 C2 2Li ! R2 2CB10 H10 C2 2SMe2 2PðSÞOEt 2ÞEtPðSÞCIMe
C-diethylphosphites undergo an Arbuzov rearrangement with C4H9SCl [590]: C4 H9 SCl
Me2 2CB10 H10 C2 2PðOEtÞ2 ! Me2 2CB10 H10 C2 2Pð¼ OÞðOEtÞSC4 H9 10 - 0
9.14.3.2 B-substituted derivatives The B(9)-thiophosphinite (9-286) is accessible via treatment of the thiol with diphenylchlorophosphine, and can be converted to dithio- and selenothiophosphinates 9-287, the thiophosphinate 9-288, and a stable quasi-phosphonium salt 9-289 [612]. H
H
H H
C
C
H Ph2PCl
C
C H
C
X
C
X = S, Se S
SH H
9-286
chloral
S
PPh2 MeI −20 C H
C
S
9-288
X
9-287 H
C
C H
PPh2
C
PPh2
+ S
O
9-289
PPh2 I−
Me
The thiophosphites 1,2-C2B10H11-9-SP(OEt)2 and (1,2-C2B10H11-9-S2 2)2P(OEt) are similarly prepared from 1,2C2B10H11-9-SH and (EtO)2PCl or (EtO)PCl2 in the presence of triethylamine [609,614]. Et3 N
1; 2-C2 B10 H11 -9-SH þ ðEtOÞ2 PCl ! 1; 2-C2 B10 H11 -9-SPðOEtÞ2 Et3 N
1; 2-C2 B10 H11 -9-SH þ ðEtOÞPCl2 ! ð1; 2-C2 B10 H11 -9-S2 2Þ2 PðOEtÞ 2P Thermolysis of the monocarboranyl thiophosphite at 120 C aided by free-radical generators results in homolytic S2 cleavage, generating multiple boron-substituted products [610].
9.14 Phosphorus, arsenic, antimony, and bismuth derivatives
Δ
C2B10H11-9-SP(OEt)2
483
(C2B10H11-9-S)2POEt (29%) + (C2B10H11-9-S)2P(O)OEt (10%)
The bis(o-carboranyl) thiophosphite, in contrast to analogous alkyl and aryl thiophosphites, is air-stable and can be converted to a variety of other species; for example, heating with sulfur affords the trithiophosphate, while methyl iodide in refluxing benzene gives the methyl dithiophosphonate [609]: S
ð1; 2-C2 B10 H11 -9-SÞ2 PðOEtÞ ! ð1; 2-C2 B10 H11 -9-SÞ2 PðSÞðOEtÞ D
MeI
ð1; 2-C2 B10 H11 -9-SÞ2 PðOEtÞ ! ð1; 2-C2 B10 H11 -9-SÞ2 PðOÞMe
9.14.4 Cyclotriphosphazenes Carborane derivatives that incorporate phosphazene moieties are of interest as possible synthons for novel polymeric materials (Chapter 14). Monomeric species such as 9-290 and 9-291 are obtained from C-lithio-o-carboranes and cyclic chlorophosphazines [595,1286], often accompanied by degradation of the carborane cage to nido-C2B9 species. The tris (o-carboranyl) compound 9-292 has been prepared from N3P3Cl6 and 1,2-(HOCH2) 2C2B10H10 [1287], and other cyclic triphosphazenes such as 9-293 are similarly obtained [596,1288].
C C
R
C N
R
P
R
9-290
N
N
C
N
P Me
N
P Cl
9-291
P R
C
R = Cl, OCH2CF3
Cl
P
N
C
R = Me, Ph
P
R
Cl
R
Cl
R
O
N P
P
N
N
O
O
C
O
C
P O
O C
O
C
O
N P
C
C
O
C
O
C
P
N
N P O
9-292
O
9-293
484
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
9.14.5 Metal complexes Phosphino, arsino, and related o-carborane derivatives easily combine with transition metals to give chelate complexes, a large number of which have been characterized (Table 9-1). These compounds form an extensive and highly versatile Group 15 element derivative chemistry, in which o-carboranyl cages replace aryl and other groups in conventional organophosphorus ligands to give new classes of stable metal complexes for use in catalysis and other applications. Asymmetric derivatives, for example, 9-266 to 9-270, readily bind to rhodium, iridium, and other metals to form active catalysts (Chapter 15) [572]. C,C0 -bis(phosphino) derivatives react with metal halides or carbonyls to form species such as 9-294 to 9-300 having direct metal-phosphorus bonds [560,578,895,980]. Silyl-phosphino metal complexes (9-167 to 9-169) were mentioned in Section 9.11.
C
PPh2 Fe(CO)3
C
C
Fe(CO)5 150 C
C
PPh2
PPh2
C
Ni(CO)4
Ni(CO)2
20 C
C PPh2
PPh2
NiCl2•6H2O 20 C
9-294
PPh2
C
9-295
Cl
PPh2
C
EtOAc reflux –NiCl2
Ni C
Cl
PPh2
Ph2P
2+ C
2Cl−
Ni C
PPh2
Ph2P
C
9-297
9-296
PPhH
PPhH C
(NBD)Mo(CO)4
C
C
20 C
C
PPhH
PPh2
Mo(CO)4
9-298
PPhH
NBD = norbornadiene
CuCl THF PPhH C
PPhH THF
Cu C
9-299
PPhH
C
PPh3
PPh3
Cu C
Cl
PPhH
Cl
9-300
Similar complexes are formed from C,C0 -diarsino-o-carboranes [829,1280]. In addition, bis-chelate species of type 2C2B10H11, which is obtained via reaction 9-301 have been prepared from the “one-armed bandit” carborane Me2AsCH22 of BrMgCH2C2B10H11 with Me2AsI [742].
9.14 Phosphorus, arsenic, antimony, and bismuth derivatives
Me2As C
C
M
C
485
9-301
C
M = Pd, Pt
AsMe2
In some circumstances, metal complexation is accompanied by cage deboronation (see below). The versatility of this chemistry is illustrated by the synthesis of unusual 3- and 4-coordinate gold complexes [1024,1032,1039] including the tetranuclear compound 9-307 [1031]. PPh2
C
Au–THF C
C
+
PPh2 Au
C
PPh2
PPh2
S
PPh2
C H
S
PPh2
C
Au H
C
C
PPh2
S
PPh2
S
PPh2
PPh2
9-302
C
+
PPh2
C
(CH2)
9-303 C
[Au(tht)2]ClO4
PPh2 Au
C PPh2
PPh2
C
C
PPh2
PPh2
9-304 phen
C
PPh2 N
Au C
9-305
N PPh2
C C
PPh2
AuCl
C
+ C
PPh2
AuCl
SH
C
PPh2
C
Au
Au C
S
PPh2 SH
Au
C
C
S
PPh2 S
C
S Ph2P
C
9-306
Au C
9-307
Coordination complexes in which the metal is not bound directly to phosphorus can be generated from o-carboranyl derivatives containing phosphate, ferrocenyl, or other substituents. The P-chiral complexes 9-308 and 9-309, isolated in enantiomerically pure form, and the diferrocenyl compound 9-310 are of interest in the development of chiral metal catalysts [603].
486
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
H
C
C
CpFe[C6H3(PCl2)(CHR−NMe}2)] R = H, Me
C
C
H ROH, Et3N P
P R
R
CH
CH
Me2N
Me2N
Fe
Fe
9-308
9-309 Me2N
C
Li
Fe
P C
CpFe[C6H3(PCl2)(CHR−NMe}2)] R=H
C
OR
C
R = Me, Et, CHMe2
Cl
C
Li
H
Cl Cl
C
Li
P
Me2N
9-310
Fe
Another versatile o-carboranyl ligand is 9-311, whose indenyl ring readily binds to lanthanide metals as in 9-312 [605]. Protonation and oxidation of 9-311 affords a pentavalent-phosphorus oxo derivative 9-314 that forms complexes of type 9-315 with titanium-block metals [604]. N(CHMe2)2 P
Li
C
C N(CHMe2)2
C
[Ln]
P
Ln = Yb, Sm
9-311
C
Ln C
-
9-312
C
P N(CHMe2)2
Me3NHCl
NMe2
C C
H
C
H2O2 N(CHMe2)2
C N(CHMe2)2
C
P
P
M(NMe2)4 M = Ti, Zr, Hf
O C
O (Me2CH)2N
9-313
9-314
NMe2
M
H
9-315
P
9.14 Phosphorus, arsenic, antimony, and bismuth derivatives
487
Metal complexes of B-phosphino-o-carboranyl ligands are comparatively rare, but the formation of B2 2Ir2 2P2 2C exocycles (as in 9-56) via oxidative addition to B2 2H bonds was noted in Section 9.5. Chiral B(9)-BINOL-derived phosphites of type 9-316 have been shown to facilitate the rhodium-catalyzed hydrogenation of dimethyl itaconite with up to 99.8% enantioselectivity [598,615,1289]. H
C H
C O O
P O
9-316
9.14.6 Cage-opening reactions Base-promoted degradation of o-carboranyl C2 2P bonded derivatives to nido-C2B9 species (Section 7.2) is often accompanied by C2 2P cleavage [569], but it is possible to select conditions under which deboronation occurs while preserving the C2 2P bond, thereby allowing efficient synthesis of C-phosphino derivatives of nido-7,8-C2 B9 H12 from their o-carborane precursors. This can be accomplished with piperidine by employing a carborane:base ratio of 1:50 in toluene or 1:10 in ethanol [567]. The nature of the substituents on phosphorus is also important, with arylphosphines affording the best results. In phosphites, the phosphorus atom eliminates an ethoxy group and acquires a positive charge, forming a zwitterion stabilized by piperidine via PH2 2N and NH2 2O hydrogen bonds [567]: C5 H10 NH
Me2 2CB10 H10 C2 2PðOEtÞ2 ! 7; 8-½ðOÞHPþ Me2 2C2 B9 H11 HNC5 H10 Treatment of 1,2-(Ph2P)2C2B10H10 with H2O2 at 0 C in THF has been shown to form a stable chelated-proton species, nido-7; 8-Hþ ðOPh2 PÞ2 C2 B9 H10 , that has been fully characterized structurally [1290]. Nido-carboranyl phosphonium salts are generated from arylphosphino o- and m-carboranes by methylation followed by cage degradation with fluoride. The more robust p-carborane analogue is not deboronated [554]. Mel
CsF
1; 2-=1; 7-HCB10 H10 C2 2PPh2 ! HCB10 H10 C2 2PPh2 Meþ I ! 7; 8=7; 9-MePh2 Pþ C2 B9 H11 Controlled deboronation of prochiral carboranes such as 1-Ph-2-PPh2-C2B10H10 affords chiral nido-7; 8-PhðPh2 PÞ C2 B9 H11 anions that form asymmetric metal complexes. Compounds such as 9-317 can be obtained as pure enantiomers and are effective in enantioselective catalysis of the hydrosilylation of acetophenone with diphenylsilane, the hydrogenation of acetamidocinnamic acid, and other processes [570]. H
Me
C
N Pd
Ph2P H
H
C
Ph
C
9-317
488
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
As noted earlier, the attempted synthesis of o-carboranyl metal-phosphino derivatives often leads to cage degradation, forming exo-nido-C2B9 metal complexes. This is most likely when a nucleophile is present that can attack and remove the relatively electropositive B(3,6) borons, as in reactions conducted in methanol or ethanol solution, for example, the generation of the palladium species 9-318 [921,1001]. Cl Ph3P Ph2P Ph2P C C
(Ph3P)2PdCl2
Pd
Ph2P Ph2P
H
C C
9-318
EtOH
Similarly, reactions of (Ph2P)2C2B10H10 with Cu(II), Zn(II), and Ni(II) halides in ethanol in air form bis(phosphoryl) nido-carborane complexes of the formula M[7,8-(OPPh2)2C2B9H10]2 in which the metal center is coordinated to the phosphoyl oxygen atoms [587]. Deboronation is facilitated by the presence of aryl groups on the phosphines, possibly because of withdrawal of electron density from the cage; when aryl groups are absent, cage degradation may not take place even in nucleophilic media, as in the synthesis of the octahedral ruthenium complex trans-Cl2Ru {[(OEt)2P]2C2B10H10}2 in ethanol [921]. Further discussion of exo-nido metal complexes appears in Chapter 15. Cage-opening without loss of boron to form nido-C2B10 derivatives (a topic covered in Chapter 11) is accomplished in o-carboranes bearing bulky C-phosphino groups via reaction with HCl, a process that is reversible on addition of triethylamine [562]. HCl
Et3 N
Et2 P2 2CB10 H10 C2 2PðCMe3 Þ2 ! nido-ðEt2 PÞ½ðMe3 CÞ2 PC2 B10 H10 ! Et2 P2 2CB10 H10 C2 2PðCMe3 Þ2
9.15 SULFUR, SELENIUM, AND TELLURIUM DERIVATIVES The chemistry of o-carboranyl compounds of the Group 16 elements has been extensively developed and, like that of the phosphorus- and arsenic-based derivatives described in the preceding section, is readily adapted to the designed synthesis of specific target systems. Table 9-1 lists a wide variety of sulfur- and selenium-functionalized compounds along with the few known tellurium derivatives.
9.15.1 Thiols, thioethers, disulfides, and related compounds 9.15.1.1 C22S and C22Se derivatives General approaches for the direct addition of sulfur and selenium at the o-carboranyl carbon atoms, outlined briefly in Section 9.4, include reactions of C-metallated carboranes with sulfur, organic disulfides, or selenium metal. Well2Te2 2Te2 2CB10H10CH and several characterized derivatives with direct C2 2Te bonds are limited to HCB10H10C2 molybdenum and rhodium complexes (Table 9-1). As was noted earlier, a starting point for many syntheses is 1,2-(LiS)2C2B10H10, from which C,C0 -dimercapto-ocarborane, 1,2-(HS)2C2B10H10 (9-25), is generated by hydrolysis, while bis(thioethers), 1,2-(RS)2C2B10H10, are obtained via treatment with alkyl or aryl halides (Section 9.4). Some characteristic reactions of 9-25 and its selenium analogue 9-327 are depicted in Figure 9-23. These include deprotonation to afford the dianion 9-322, with subsequent formation of the disulfur-bridged species 9-320 and 9-324 [627], as well as the generation of the thione 9-319 [665], the bis(chlorosulfenyl) derivative 9-321 [635], the S2 2P2 2S and S2 2B2 2S bridged species 9-325 and 9-326 [629,636], and the diboron-linked dianions 9-323 and 9-328 [662]. Like many of the reactions of o-carboranyl phosphorus derivatives
9.15 Sulfur, selenium, and tellurium derivatives
S
C
C C
S
S
S
S
C
S C
C S
C
9-319
9-320 CSCl2
BrCH2CH2Br EtOH, reflux
SH
SCl
C
CSCl2
S− C
C
C SCl
C
SH
liq. NH3
C
S−
C
I2
or KOH
C
9-25
9-321 R3B
9-322 PhPCl2
C C
S B
R
C
S
S P S
R = Et, η-C4H9, Ph
S
S
C C
S
Cl
2− C
B Cl
S
C
9-325 2−
C
C SeH
B2(NMe2)4 C
9-327
S
9-323
SeH
C
S
C
S
9-326
C
S
9-324
B2(NMe2)4
B C
489
Se
Se
Se Se
B C
C
C
B
Se
Se
C
9-328 FIGURE 9-23 Synthesis of C2 2S and C2 2Se o-carboranyl derivatives from 1,2-(HS)2C2B10H10 (9-25) and 1,2-(HSe)2C2B10H10 (9-327) .
outlined in the previous section, these syntheses may be accompanied by partial degradation to nido-C2B9 cages, especially in alcoholic media and/or in the presence of strong bases. The 1,2-(SH)2 and 1,2-(SeH)2 derivatives 9-25 and 9-327 readily form transition metal chelate complexes as described below, and the dithiol has been employed to modify the surfaces of gold microcrystals (Chapter 17) [628,1291]. Other boron-bridged derivatives similar to 9-326, having the formula R2NB(m-S)2C2B10H10, are obtained on reaction of 9-25 with aminoboranes R2NBH2 (R ¼ Me, n-C4H9) [636]. Boron trichloride is unreactive with 9-25 even in hot benzene, but Cl3B:NCMe and 9-25 combine in a 1:1 ratio to form a BCl-bridged product that on exposure to air regenerates 9-25; when 9-25 is present in 2:1 excess, an anion proposed to be B½ðm-SÞ2 C2 B10 H10 2 2 can be isolated as the tetraethylammonium salt [636].
490
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
The 1,2-diselenido-o-carborane dianion combines with phenyldichlorophosphane to generate the cyclic phospholane 9-329, which in turn is oxidized to the sulfide/selenide 9-330 on reaction with sulfur or selenium, respectively. Partial hydrolysis of the phospholane affords the selenophosphonic acid 9-331 [845]. Se−
Li C
C C
Li
Se
Se−
C
C
PPhCl2
Se Ph
P
Et2O
C
C
E
9-329
Ph P
C
E = S, Se
Se
Se
E
Se
9-330
H2O (trace)
Se
C
Ph
9-331
P C H
E
OH
0
In contrast to the tendency of C,C -bis(mercapto)-o-carborane derivatives to form exo-polyhedral C2 2S2 2X2 2S2 2C rings, mono-C-mercapto-o-carboranes often generate sulfur-linked bis(carboranyl) products. Figure 9-24 illustrates typical reaction sequences starting with the mercaptide monoanion 9-332 and the neutral mercaptan 9-336, leading to the thioether 9-337 [625,626] and the disulfur-bridged derivatives 9-333 [639,647], 9-338 [625], and 9-340 [270,271]. Recently, reactions of Et3 NHþ SC2 B10 H11 with Br(CH2)nCN and Br(CH2)nC(O)OEt (n ¼ 1-4) have been employed as a new route to C- S(CH2)nC(O)OH and other monofunctional derivatives of o- and m-carborane [435]. The S2 2S bond in 9-333 is cleaved via reaction with the monolithiocarborane to form the monosulfur-linked biscar2Me. Hydrogen peroxide attacks 9-333, preserving the S2 2S link but deboronating borane 9-339 and LiS2 2CB10H10C2 one o-carborane unit to a nido-C2B9 cage to form 9-341; piperidine in ethanol degrades both cages, generating 9-342 [639]. R C 1) H2O2 R
R C
S C
S C
R
-
H
C C
2) Me4NCl
C
S
S
9-341
C H
-
R CC
S
S
R
-
H
C C
9-333 piperidine EtOH H
-
S
S C C
R
R
-
H
C C
9-342 Structural and NMR studies of the sulfide monoanion 9-332 (R ¼ Ph) are consistent with a resonance hybrid bond 5S and Ph2 2CB10H10C2 2S forms. Although there is description involving contributions from the Ph2 2CB10 H10 C5
9.15 Sulfur, selenium, and tellurium derivatives R
R
R R = Me, Ph C
S− Li+
C
SMe
MeI
C
C
Li C
S8,
C
H+
1) KOH 2) X−R−X
9-336
1) KOH 2)
Me
Me
C
9-337
Cl
C
S C
C
N Br
9-339 Me
C
S
SMe
Me
Me
+
C
S
Li C
C
1) KOH
C
R
C
C
SH
Br
C
Me
Me
2) Me2SO4
THF
9-335
C
9-333
9-332 Me
C
C
S
C6H6
9-334 Me
S C
I2
liq NH3
R
C
C
491
9-338 R = CH2-C6H4-CH2, (CH2)2, (CH2)3
C
SLi C
S N
9-340
C C
S
Me
Cl
C C
Me
FIGURE 9-24 Reactions of mono-C-mercapto-o-carborane derivatives.
evidence of charge transfer from sulfur into the cage, this occurs to a lesser extent than in the analogous oxygen mono2O despite the higher electronegativity of oxygen, a finding that is ascribed to the higher bond anion Ph2 2CB10H10C2 energy achieved by C5 5O versus C5 5S double-bond character [623].
9.15.1.2 C22X22S and C22X22Se derivatives Compounds in which sulfur or selenium are linked to o-carboranyl carbon atoms through bridging groups are accessible by three general routes: displacement of lithium or other metals from C-metallated derivatives, reactions of existing functional groups on carbon, or insertion of alkynyl-sulfur compounds into B10H12L2 cages. Figure 9-25 depicts some representative syntheses of sulfones (9-343) [500], sulfites (9-344) [184], thiophenes (9-345) [379], thioethers (9-346 [630] and 9-348 [648]), mercaptans (9-347) [630], and triflates (9-349) [654]. The meso-dimercaptosuccinate derivative 9-348 crystallizes in a supramolecular assembly of one-dimensional zigzag ribbons held together by intermolecular hydrogen bonding between the carborane CH and ester CO groups [648].
492
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12 R Li C
R
R
R = H, Me, Ph
C
C
C
O
(ClC6H4)2SO2
C
S
C
O
9-343 H
H
C
CH2OH C
C
SOCl2
H
O
C
S C
O
C5H5N
C
O
9-344 Me
Me C C
C
CH2–C6H4Br SC4H3B(OH)2
C
Pd(PPh3)4 Na2CO3 toluene, EtOH
S
9-345
Me
Me C
C
CMe3 C
C
AlCl3
S
B10H12(SEt)2 + Me3C–S–CH2C≡CH
C6H6, 50 C
9-346
9-347 MeO O
H
C B10H12L2
+
HC≡C
S–CH–C(O)OMe
HC≡C
S–CH–C(O)OMe
R = H, CH2OH
R = H, CH2OSO2CF3
C
C C
OH O(SO2CF3)2 C5H5N
FIGURE 9-25 Syntheses of o-carboranyl C2 2X2 2S derivatives.
C
OSO2CF3
9-349
C S
9-348 R
H
CH
S
L = aniline
R
C O
C CH
C
OMe
C
SH
9.15 Sulfur, selenium, and tellurium derivatives
493
The thioether 9-350 forms the sulfoxide 9-351 on treatment with p-nitroperbenzoic acid, but is unreactive toward hydrogen peroxide in formic acid; in contrast, the m- and p-carboranyl counterparts of 9-350 are oxidized to sulfones by H2O2 (Section 10.15) [624].
S
S C
30% H2O2
C
no reaction
90% HC(O)OH reflux
9-350 O
O
S
S
O2N–C6H4–C(O)2OH
C
C
CHCl2, r.t. −2 O2N–C6H4–C(O)OH
9-351 Other derivatives are generated through standard functional-group chemistry; for example, triflates such as 9-349 are easily converted to the corresponding phthalimides, nitriles, and triphenylphosphine derivatives by Pd-catalyzed replacement of the OSO2CF3 group [654]. Reactions of C-hydroxymethyl o-carboranes with sulfonyl chlorides give methyl esters of sulfonic acids [649], ClSO2 Me
R2 2CB10 H10 C2 2CH2 OH ! HCB10 H10 C2 2CH2 O3 SMe C 5 H5 N
while halogen-induced electrophilic ring cleavage of epithiopropane derivatives affords bis(o-carboranylisopropyl)disulfides (9-352) [643]. R
X
R C
C
S
X2
C
S
CH
C
S
X = Cl, Br, I
CH
C
R
X R = Me, Ph, CHMe2
C
9-352
Benzyl tosylate (toluene p-sulfonate) derivatives of o-, m-, and p-carborane having a phenyl group on the second cage carbon atom undergo hydrolysis to give the corresponding alcohols, and the rates of these reactions are directly influenced by the electronic properties of substituents attached to the phenyl group. H2 O
m=p-RC6 H42 2CB10 H10 C2 2CHðPhÞOSO3 ! HCB10 H10 C2 2CHðPhÞOH R ¼ H; Me; CF3 ; OMe; NMe2 In the m- and p-carborane systems, the hydrolysis rates increase linearly with increasing electron donation by the R substituent, but the opposite effect is observed in the o-carboranyl tosylates; that is, the better electron donor R groups cause the rate to decrease, an effect attributed to an interaction between the solvent and the B(3) vertex in the cage [651]. Detailed studies involving a range of substituents reveal that enantiomeric purity of the alcohol products is retained in systems with weaker electron donors but declines rapidly with increasing electron donation by R, indicating a shift to an SN1 mechanism [653]. Some sulfur-containing o-carboranyl derivatives exhibit useful electronic properties that are of interest for potential application in microelectronics and related fields (Chapter 17). For example, thiophenes such as 9-345 and 9-353 can be
494
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
electropolymerized to give conducting polymers whose thermal and electrochemical stability compares favorably with that of conventional thiophene polymers [379,637,638]. S S B10H14
(2-H3C4S)2C2
C C
9-353
toluene
Linkage of sulfur-functionalized o-carboranyl units to the closo-B12 H12 2 dianion forms high-boron-content multicage entities such as 9-354 that are candidate boron carriers for BNCT [640]. H
H
C
C B12H12(SMeCH2C≡CH)2
C
B10H12L2 reflux
S
S
Me
Me
L = MeCN, SEt2
C
9-354
Dianionic water-soluble B12 derivatives of o-, m-, and p-carborane of the type RCB10 H10 C2 2ðCH2 Þ4 O2 2B12 H11 2 (R ¼ H, Me) have been prepared from C-lithio-carboranes and B12 H112 2OðCH2 Þ4 [464]. The octasulfonate 9-355, obtained by treatment of the corresponding octahydroxy compound with 1,3-trimethylene sultone, is the first example of a water-soluble carboracycle [240]. Other examples of carboracycles are given in Section 9.11 (see Figure 9-16 and 9-184). R
R
R
R
C
C C
C
9-355 R = O(CH2)3SO3Li C
C C
C
R
R
R
R
9.15 Sulfur, selenium, and tellurium derivatives
495
9.15.1.3 B-substituted derivatives The addition of sulfur, selenium, or tellurium at boron via electrophilic attack, discussed in Section 9.5, proceeds as expected with substitution at the most negative atoms B(9,12) followed by B(8,10). The B2 2S and B2 2Se bonds in the products are relatively stable and allow extensive reaction chemistry to be conducted on these derivatives. Treatment of 2S bond but cleaves the S2 2C link, forming H2C2B10H9-9-SH and H2C2B10H9-9-SCN with Kþ 2 C8 H8 2 preserves the B2 2S2 2S2 2H9B10C2H2 [641]. The B(9,12)-(EH)2 derivatives (E ¼ S, Se) are readily methylated with MeI to H2C2B10H92 give H2C2B10H8-9,12-(EMe)2 products, while reaction with benzaldehyde or dimethyl ether gives cyclic heteroacetals (9-356) [660]. EH
H
C C
R
C C
Me2O
C
EH
H
E
H
PhC(O)H or
H
E
R
9-356 R = H, R = Ph R = R = Me
The B2 2SH and B2 2SeH compounds also provide an entry to B-thiophosphites and B-selenophosphites through reactions with diethylchlorophosphites [613]: ðEtOÞ2 PCl
H2 C2 B10 H9 -9-EH ! H2 C2 B10 H9 -9-EPðOEtÞ2 Et3 N
E ¼ S; Se
The selenophosphites, in turn, combine with elemental sulfur or selenium to form thioseleno- and diselenophosphites, respectively [837]: S or Se
H2 C2 B10 H9 -9-SePðORÞ2 ! H2 C2 B10 H9 -9-SeðEÞPðORÞ2 E ¼ S; Se R ¼ Et; n-C4 H9 Alkylation of a selenophosphite (R ¼ Et) by reaction with dimethyl sulfate at 20 C gives the methylselenophosphonate [837]: Me2 SO4
H2 C2 B10 H9 -9-SePðOEtÞ2 ! H2 C2 B10 H9 -9-SePðOÞðOEtÞMe B(9,90 )-diselenium-bridged biscarboranes [1150] (Section 9.4), on refluxing with trialkyl phosphites in toluene, undergo Se2 2Se bond cleavage and the formation of selenophosphates [848]: PðORÞ3
2Se2 2Se2 2H9 B10 C2 H2 ! H2 C2 B10 H92 292 2SePðOÞðORÞ2 H2 C2 B10 H92
R ¼ Me; Et
9.15.2 Metal complexes Like the phosphino-o-carboranes discussed in the preceding section, sulfur- and selenium-functionalized derivatives are excellent ligands for coordination to metals, and a structurally diverse family of complexes is known (Table 9-1). A sampling of representative compounds is presented here.
9.15.2.1 Monodentate systems Most sulfur-coordinated o-carborane complexes feature bidentate S2 2M2 2S binding, but a few compounds of the monodentate type are known. The terpyridine-platinum complexes 9-357 have been prepared and demonstrated to be cytotoxic toward human ovarian cancer cells [632,633].
496
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12 H
H
H
C
C
Li C
PhCH2S(CH2)nBr
C
(CH2)nSCH2Ph C
(CH2)nSH C
AlCl3 C6H6 n = 2, 3
Pt(NCMe)(trpy)(OTf)2
H
OTf−
N
C
+ Pt
(CH2)nS
N
9-357
C N
9.15.2.2 Bidentate systems Many such complexes are known, and some have demonstrated synthetic utility. The square planar cobalt complex 9-358, obtained as the cobaltocenium salt, reacts easily with alkynes to afford the C,C0 -o-carboranyl disulfur derivatives 9-359 to 9-361 in good yield [645], furnishing an efficient process for cobalt-mediated disulfuration and hydrosulfuration of alkynes. SLi
C
+
SLi
C
CpCo(CO)I2
−
S
S C
C Co
Co
THF 0 °C
C
C
S
S
9-358
HC≡C−(C5H4)FeCp Fe
HC≡C−C(O)OMe HC≡C−Ph C
H
O C
C
OMe
CH2 C H
S
S
C
C
S C
C
C
C
C
C
S
9-359
C
CH2
S
S H
9-361
C
9-360
C H
H
OMe C
Fe O
9.15 Sulfur, selenium, and tellurium derivatives
497
Bidentate sulfur and selenium complexes of cobalt, iridium, and other metals such as 9-362 can be linked by organic chains as in 9-363 and 9-364 [673]. S N
(CH=CH)n
N
C
S C
S
n = 0, 1
S
C C
Co
N
(CH=CH)n
N
Co M = Co E = S
C
9-363
E M
Cp*
C
N
E
9-362
N N
N O
O N
N
N
N Ir
E M = Ir
E
C
E = S, Se
Ir
E C
E
C
C
9-364
The versatility of this chemistry and its potential for application in nanoelectronics and other materials are illustrated by the tetranuclear porphyrin system 9-365 [670], the trinuclear tris(pyridyl)-1,3,5-triazine and triethynylbenzene-centered complexes 9-366 [670,971] and 9-367 [671], and numerous others cited in Table 9-1. C
S C
C
Rh
S
N
Rh N
N
H
N N H
N N Rh S C
N S C
C
S
S
9-365 Rh
S C
S
C
498
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
C C
E
E M N
9-366 M = Rh, Ir
E = S, Se
N
N N N
N
M E
C
E C
M
E
E
C
C
C C S
S Rh C N
N
N
S
C
C
Rh
Rh
C
S C
S
9-367
S
C
C
Still other structural motifs have been demonstrated, including the construction of transition-metal-chalcogen mixed clusters such as 9-368 [663], 9-370 [932], and similar systems [664,667,916].
9.15 Sulfur, selenium, and tellurium derivatives
O C C
E
ELi
C
Rh2(CO)4Cl2
C
Rh
E = S, Se
C
Rh
S
9-368
Co
C CO C
CO
Co S
E
ELi
499
C O
9-370 CpCo(C2H4)2
C
SLi CpCo(CO)2
C
Co C
C SLi
S
S
C
Me3SiCHN2
Co
−N2
C S
S
9-369
9-371
R–C≡C–R
C
H
SiMe3
S
C
Co C
C
S
R
C
9-372 R
Oxidative addition of platinum(0) to the previously mentioned cyclic phospholane 9-329 appears, based on NMR monitoring, to proceed via attachment of Pt(PPh3)2 to the lone pair on phosphorus, followed by incorporation of Pt into a C2 2Se2 2Pt2 2P2 2Se2 2C ring. This species, in turn, rearranges to the unusual metallophosphaneselenide 9-373, a complex that is stable only below 20 C, whose structure was deduced from NMR spectra [845].
Se
C
P C Se
9-329
PPh3
P
Se Pt(PPh3)2(C2H2)3
unstable intermediates
C C
Pt Se
PPh3
9-373
Another approach to building exo-polyhedral metal-chalcogen clusters is shown by the reaction of 9-374 with tripyr˚) idyltungsten tricarbonyl to afford the bis(carboranyl) heterodinuclear complex 9-375 [847]. The short (2.23-2.24 A ˚ Co2 2Se distances in the 16-electron complex 9-374 are significantly shorter than normal Co2 2Se bonds (ca. 2.35 A) suggesting multiple-bond character in this ring system [847].
500
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
Co C C
Co
SeLi OC
I
I
C
Se
Se
W(CO)3(C5H5N)2
Co C
SeLi
W
CO Se
C
BF3(OEt)2
Se
O C
Se
C
C Se
C
9-375
9-374
Mo-Ru species that are structurally comparable to 9-375 can be prepared from Mo(CO)3(py)3 and the ruthenium-cymene 2C6H4Me)RuE22 2C2B10H10 (E ¼ S, Se) [669]. 9-374 analogues m-(Me3C2 Tellurium-transition-metal complexes of o-carborane are rare (Table 9-1), but the molybdenum tris-(dimethypyrazolylborate) compound 1,2-cyclo-HB[HMe2C3N2]3Mo(O)Te2C2B10H10 and its sulfur and selenium counterparts (9-376) have been obtained by reaction of the 1,2-Te2 C2 B10 H10 2 dianion (Section 9.4) with 1,2-HB[HMe2C3N2]3Mo(O)Cl2 [668].
N ELi
Cl2Mo
C
BH
N
C
3
O
E
9-376
Mo
C
C
ELi
E = S, Se, Te
(N2C3Me2)3BH
E
A related architecture that is of interest in crystal engineering employs two (Z5-C5H4)M(PPh3)-(m-E)2C2B10H10 units (E ¼ S or Se) linked by 3-oxapentamethylene chains (9-377). The flexibility of the chains allows the two metal-containing end units to adopt different conformations in the solid state [940].
O
E
M
M
PPh3
C C
E
E
Ph3P
9-377 M = Co, Rh
C E
C
E = S, Se
Other exo-polyhedral chalcogen-bridged metal complexes are described in Chapter 14, and an important class of bis(dicarbollyl) metallacarboranes in which the two C2B9 units are linked by sulfur or other bridging units is discussed in Chapter 15.
9.16 Halogen derivatives
501
9.16 HALOGEN DERIVATIVES The first investigations of o-carborane chemistry quickly uncovered two characteristic features: the Lewis acidity of the carbon-bound hydrogens, which allows facile introduction of functional groups at the cage carbon atoms, and the reactivity of the BH units toward halogens and other electrophiles. The latter property is dependent on location in the cluster, with the borons furthest from carbon having the highest negative charge. Very extensive studies over several decades have given a detailed picture of the charge distribution in the 1,2-, 1,7-, and 1,12-C2B10 cages as well as synthetic routes to hundreds of halogenated derivatives, of which many are listed in Table 9-1.
9.16.1 Synthesis The introduction of halogens at o-carboranyl CH and BH vertexes is discussed in detail in Sections 9.4 and 9.5, respectively. Substitution at carbon typically proceeds via C-lithio-o-carboranes or Grignards, while halogenation at boron is usually accomplished in one of several ways, including direct halogenation, displacement of amino or other functional groups by halogens, insertion of haloboron units into C2B9 anions (Section 9.2), or insertion of alkynes into B10H14 halo derivatives. The considerable earlier work in this area has been elaborated by more recent studies, which have opened the way to more efficient synthetic routes to some systems, especially those for which applications have been developed. In this section, the main focus is on reactivity patterns in halogenated o-carboranes, with specific advances in synthesis noted as appropriate.
9.16.2 Properties of B-decahalo-o-carboranes With certain exceptions, the boron-polyhalogenated derivatives are quite unreactive and can survive fairly extreme conditions. For example, H2C2B10Cl10 does not undergo cage degradation in hot aqueous NaOH (though it is deprotonated) [197,701] and is unaffected by SbF3Cl2 at 240 C [27]; an attempted nitration of PhHC2B10Cl10 in mixed acid at 80 C gave no reaction, unlike parent o-carborane which nitrates easily under these conditions [283]. The B-tetrachloro and B-hexachloro derivatives are inert toward ammonia and amines at 20 C, and H2C2B10Cl8H2 is unreactive toward Friedel-Crafts reactions in refluxing benzene [27]. However, the chlorines in all of the B-chlorocarboranes are removed on treatment with H2O2 in 50% aqueous KOH [27], and dimethylsulfoxide destroys the H2C2B10Cl10 cage, forming boron oxychlorides [1292]. In contrast to the other boron-perhalogenated o-carboranes, H2C2B10F10 is considerably more reactive, being easily hydrolyzed by water or moist air and readily undergoing nucleophilic displacement of the fluorines [679]. As would be expected, replacement of B2 2H hydrogen atoms by more electronegative halogens increases the polarity of the o-carboranyl C2 2H bonds via an inductive mechanism [51,237], rendering them considerably more acidic [197,700–702,706,1188]. This effect increases with increasing number of halogens, such that the acid strength of H2C2B10Cl10 is comparable to that of typical organic acids. For example, parent o-carborane has a pKa (calculated from potentiometric data) of 23.3 in dimethoxyethane and that of its 9,12-dichloro derivative is 16.3 [90]. Experimentally determined pKa values (measured in 50% ethanol) of H2C2B10Cl10 (6.89), H2C2B8Cl8Br2 (6.95), and HMeC2B10Cl10 (6.90) may be compared with those of p-NO2-o-Me-C6H3C(O)OH (6.83), p-NO2-C6H4C(O)OH (7.68), Me (CH5 5CH)2C(O)OH (7.00), C6H5SH (7.78), and p-ClC6H4SH (7.78) [702]. A similar trend is found in halogenated o-carboranyl mercury derivatives, whose acidities exceed those of their nonhalogenated counterparts [1059]. In all these systems, the acidity of the C2 2H bond is strongly affected by the choice of solvent [210]. In another demonstration of this effect, infrared and Raman studies have shown that hydrogen bonds of the type C2 2H---X where X is O or N are also much stronger in B-decachloro-o-carboranes than in non-halogenated o-carboranes [703,705]. The effect is measurable even in mono- or dihalogenated derivatives [55], and can be seen, for example, in the crystal structure of 3-iodo-o-carborane [720] which features head-to-tail double chains held together by C2 2H---I bonds. 2H---Cl bonds are larger than those for phenol and In HRC2B10Cl10 (R ¼ H, Me, Et), the enthalpies of formation of C2 acetylene [704]. A more detailed picture of the boron-halogen bonding interaction is available from helium I photoelectron
502
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
spectra of o- and m-carborane and their 9-halo derivatives, which suggest that in the B2 2Cl bond there is significant overlap between filled p orbitals on boron and nonbonding orbitals on chlorine [695]. The role of I_H and I_I intermolecular bonding in the crystal packing of B-iodinated o-carborane derivatives has been investigated in detail by Vin˜as and coworkers, and the resulting solid-state networks are discussed in Chapter 17 [730]. The acid-base chemistry of H2C2B10Cl10 has been extensively studied. In aqueous ethanol it behaves as a monoprotic acid toward Et3N, Et2NH, Me2SO, and dimethylformamide, forming 1:1 adducts with each [702]. On the other hand, on reaction with methylmagnesium iodide in ether, both CH protons are removed, generating two equivalents of methane [197]. 2CB10 Cl10 C2 2MgIþ 2CH4 HCB10 Cl10 CH þ 2MgI ! IMg2 The B-decachloro derivative is soluble in aqueous NaOH or KOH, forming alkali-metal salts that are stable toward hydrolysis in aqueous solution [701,706]. These salts react easily with alkyl halides in aqueous alcoholic media to generate C-alkyl-o-carboranes; with methyl iodide, both the mono- and disodium salts give only the C,C0 -dimethyl derivative. MeI
HCB10 Cl10 C2 2Na ! Me2 2CB10 Cl10 C2 2Me þ NaI EtOH
However, the monomethyl compound can be prepared from the C-monoiodo magnesium salt in THF. MeHgI
MeI
HCB10 Cl10 CH ! HCB10 Cl10 C2 2MgI ! HCB10 Cl10 C2 2Me MgI2 THF;-CH4 Ethyl iodide, interestingly, reacts with both mono- and disodium salts to form exclusively the monoethyl derivative. These findings suggest that equilibria involving the mono- and dimetallated species analogous to those found in the non-halogenated species and discussed earlier in Section 9.4 are involved, although these systems have not been studied in detail. EtI
EtI
EtOH
EtOH
HCB10 Cl10 C2 2Na ! HCB10 Cl10 C2 2Et Na2 2CB10 Cl10 C2 2Na Other C-substituted decachloro-o-carboranes are similarly prepared (Table 9-1) [701]. The near-inertness of HCB10Cl10CH has attracted some interest in biomedical applications; for example, its high acidity has been found to decrease the membrane potential of frog muscle fiber in vitro at pH 5, but decreasingly so at higher pH levels [1293].
9.16.3 Properties of partially B-halogenated o-carboranes 9.16.3.1 Effects of halogen substitution o-Carboranes having fewer than 10 chlorines exhibit markedly less C2 2H acidity than the B-decachloro derivatives. From infrared and NMR spectroscopic evidence on mono-, di-, and trichloro compounds, it appears that the effects of chlorine substitution are felt primarily on the boron directly bonded to it and on those occupying antipodal vertexes [22]. Although at least one H2C2B10Cl8H2 isomer forms a stable adduct with Et3N and can be titrated in aqueous ethanol, derivatives having fewer than seven halogen atoms cannot be titrated with strong bases [27]. Derivatives having more than four bromines on the cage have not been characterized, but mono- to tetra-B-bromo-o-carboranes can be metallated, alkylated, and halogenated via methods similar to those employed for parent C2B10H12 and its chloro derivatives [133]. One would expect the largest effects of cage halogenation to be found in fluoro derivatives, and the calculated electron density distribution in 1,2-C2B10H6-8,9,10-12-F4 based on low-temperature X-ray diffraction data shows that electron withdrawal by the fluorines causes a major shift of electron density from the electron-rich C2 2C bond to the B2 2C bonds [1113]. A characteristic reaction of boron-halogen bonds in o-carboranes is their nucleophilic displacement by CuCl, a process that contrasts with the much more common electrophilic attack on the carborane cage, many examples of which
9.16 Halogen derivatives
503
have been cited in this chapter. This treatment effects the replacement of all bromo and iodo substituents by chlorine atoms in the 9-Br, 9-I, 9,12-Br2, and 9,12-I2 derivatives [1294,1295]; C-substituted derivatives undergo the same reaction, affording the chlorinated products cleanly and leaving aryl or alkyl groups unaffected. CuCl
HCB10 H10n Xn CH ! HCB10 H10n Cln CH CuCl
Ph2 2CB10 H9 ICH ! Ph-CB10 H9 CICH
n ¼ 1; 2
n ¼ 1; 2
X ¼ Br; I
In the 8,9,10,12-tetraiodo derivative, however, only three iodine atoms are replaced by chlorines, even in the presence of excess CuCl [1295]. These reactions may involve a four-center transition state of the type Cu X B
Cl
leading to the formation of a B2 2Cl bond and the expulsion of CuX [1295]. Compared to aryl and metallocene halides, o-carboranyl boron-halogen bonds are considerably less reactive toward nucleophilic substitution. However, replacement of halogen atoms by hydrogen can be accomplished by reaction with alkali metals in liquid ammonia, as in the conversion of the 9-iodo- and 9,12-diiodo derivatives to o-carborane [71,72]: Na
H2 C2 B10 H8 -9; 12-I2 ! H2 C2 B10 H10 liq:NH3
The corresponding treatment of 9-chloro- and 9-bromo derivatives affords only low yields of o-carborane, probably due to a competitive formation of the disodium salt Na2C2B10H10. These observations reflect the increase in electron affinity of the carboranyl carbon atoms as electronegative substituents are added to the cage, and follows the trend of decreasing electronegativity in the order Cl > Br > I. Further evidence is seen in the polarographic reduction potentials [71,73] of o-carborane (2.51 V, –E1/2, measured in 0.002 M dimethylformamide with 0.1 M Et4 Nþ ClO4 ) and its 9-Cl (2.34), 9-Br (2.21), 9-I (2.13), 9,12-Cl2 (2.03), 9,12-Br2 (1.90), 9,12-I2 (1.81), 8,9,12-Cl2 (1.71), 8,9,12-Br3 (1.66), and 8,9,12-I3 (1.41) derivatives. As can be seen, the ease of reduction is greater in the iodo versus the bromo and chloro species and is largest in the polyhalogenated derivatives. B-monohalo-o-carboranes, especially the iodo compounds, are valuable agents in the directed synthesis of specific boron-substituted derivatives via metal-promoted cross-coupling (Section 9.5) and other methods. Activation of the B2 2I bond in B(3)-iodo-o-carborane by copper, nickel, and palladium reagents results in dehydrohalogenation to generate o-carborane; as the same result is obtained with sodium naphthaleneide and with magnesium, oxidative addition can be ruled out as a mechanism in those reactions. Reduction of the same compound opens the cage without loss of boron, affording the 12-vertex nido-7,10-C2 B10 H13 anion 9-378 (Chapter 11) in 97% yield [1296]. H
H
C
C
H
C
C
[M]
H
[M] = Cu, Cu/PPh3, [Ni(PPh3)3], [Pd(PPh3)4], Mg, Na/naphthalene
I
Mg, I2, Br2C2H4 THF, reflux H
C
− C
H
9-378 nido-7,10-C2B10H–13
A particularly useful B-halo-o-carborane is the 9-iodo derivative, whose substituent is easily displaced by attacking groups as seen in metal-promoted cross-coupling (Section 9.5) and in the reaction with copper(I) cyanide to give the
504
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
nitrile 9-379 [548]. Alternatively, one can obtain the 9-ICl2 derivative 9-380 by chlorination or generate phenyliodonium salts such as 9-381 via oxidative condensation with benzene [732]. H
C
H
H
H
C
H
C
C
Cl2
C
H
CuCN
C
Δ
CCl4 ICl2
CN
I
9-380
9-379 1) H2SO4 2) HBF4
C6H6 K2S2O8 H
C
H
C
9-381 IPh+BF4−
The latter process can be reversed by nucleophilic displacement of PhIþ with halide anions or phosphines to regenerate H2C2B10H9-9-I, as was noted in Section 9.5.
9.16.3.2 Deboronation B-halo-o-carboranes are also useful in studies of controlled degradation of the icosahedral cage. As the discussion in Section 7.2 notes, the extraction of B(3)2 2H or its equivalent B(6)2 2H unit from o-carborane yields a single optically inactive nido-7,8-C2 B9 H12 anion. If the hydrogen on B(9) is replaced by a substituent as in H2C2B10H9-9-I, B(3) and B(6) are still equivalent, but their removal leads to a pair of enantiomers as shown in Figure 9-26(A) [726]. However, the degradation of 8,9,12-trifluoro-o-carborane generates two distinct geometric isomers that are obtained in different amounts, the product ratio depending on the nature of the deboronating agent (Figure 9-26(B)) [1297]. In this system, the fluorine located on B(8) lowers the electron density on B(3) relative to B(6), making the latter more susceptible to nucleophilic attack and favoring formation of the 5,6,10-F3 isomer over the 1,5,6-F3 species by more than 2:1. Other bases such as pyridine [1298] and NaSPh [1299] also degrade 9-iodo-, 9-bromo, and other o-carboranes to nido species, in the case of pyridine forming nido-7,8-C2 B9 H11 -10-NC5 H5 þ whose pyridinium group is attached at B(10) on the open face. In general, B-halo-o-carboranes are less reactive than halobenzenes toward nucleophilic replacement of the halogen [1299].
9.16.3.3 Halogen migration B-halo-o-carboranes have played a prominent role in studies of the thermal isomerization of icosahedral carboranes (a topic dealt with in detail in the following chapter), in which the halogen substituents serve as labels allowing the movement of their attached borons to be tracked and thereby help to illuminate cage rearrangement mechanisms [1119,1300,1301]. This approach is complicated, in some cases, by exchange of the halogen atom(s) between different boron atoms under the reaction conditions, and also by electronic influence of the halogen substituents, especially fluorine [1300,1302], on the rearrangement mechanism. Halogen migration occurs in 9-chloro-, 9-bromo-, and 9-iodo-o-carboranes at 390-420 C, generating all possible B-monohalo isomers [1301–1303] via apparent hydrogen-iodine intermolecular exchange while
9.16 Halogen derivatives C 6
C
C
C 3
(n-C4H9)4N+ F−•H2O
−
C
C
I 1,2-H2C2B10H9-9-I
I
I 7,8-C2B9H11-5-I−
C C F
3
MeOH KOH
8
−
C
C
C
C
+
F
F F
F
F 1,2-H2C2B10H7-8,9,12-F3
−
F
F
F
B
7,8-C2B9H11-6-I− 1:1
C = CH
6
−
+
THF, 70 C
A
505
7,8-C2B9H9-5,6,10-F3–
7,8-C2B9H9-1,5,6-F3– 1-Cl > 1-Br > 9-Cl > 1-ClCH2 > 1-BrCH2 > 1-I-CH2 > o-carborane. The replacement of halogens in 1-phenyl-2-halo derivatives follows a similar trend. Hot alcoholic base converts the C-bromo and C-iodo compounds to 1-phenyl-o-carborane, but the C-chloro species is deboronated even by weak bases to give nido-C2B9 products [682]. NaOH
Ph2 2CB10 H10 C2 2X ! Ph2 2CB10 H10 CH þ NaOX X ¼ Br; I EtOH
506
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
The Ph2 2CB10H10C2 2X derivatives (X ¼ Br, I) similarly react with KCN and Na2S to give Ph2 2CB10H10CH, without formation of CN or thio products. Again, the 1-phenyl-2-chloro derivative behaves differently, forming a dicarbaundecaborane salt on reaction with Na2S [682]. NaOH
Ph2 2CB10 H10 C2 2Cl þ Na2 S ! Ph2 2CB9 H10 ClCH Naþ þ H2 EtOH
The same general trend is reflected in reactions with sodium in liquid ammonia. While Ph2 2CB10H10C2 2I is converted quantitatively to o-carborane in 20 min in the presence of Me2NCHO, the corresponding bromo and chloro compounds are 2Br (0.6 V, –E1/2, in 0.002 unreactive under the same conditions. The polarographic reduction potentials of Ph2 2CB10H10C2 2CB10H10C2 2I (1.65 V) reflect the same behavior, as the halogen atoms in M Me2NCHO with 0.1 M Et4 Nþ ClO4 ) and Ph2 these compounds are lost easily and reduction occurs very quickly [71]. In another comparison, 1-Br-o-carborane, as well as the 9,12-Br2 and 8,9,12-Br3 species, combine with pyridine to generate the respective 3,6-dipyridyl derivatives, whereas ocarborane itself is unreactive [1306]. Few studies of C,C0 -dihalo-o-carboranes are available, but all three 1,2-X2C2B10H10 compounds (X ¼ Cl, Br, I) are degraded to nido-X2C2B9H11 species by methanol and ethanol at room temperature [685,697]. The corresponding 1,7X2C2B10H10 derivatives, however, are stable under these conditions [1148].
9.16.5 Properties of C-halomethyl-o-carboranes Derivatives in which the halogen is separated from the o-carboranyl carbon by a methylene group react with Grignards to effect halogen replacement, as in the interaction of 1-chloromethyl-o-carborane with RMgX0 [1305]. 2CH2 Cl þ RMgX0 ! HCB10 H10 C2 2CH2 MgX0 HCB10 H10 C2 0 X ¼ Cl; Br; I R ¼ Et; n-C3 H7 ; n-C4 H9 On treatment with magnesium, HCB10H10C2 2CH2Cl rearranges to form MeCB10H10C2 2MgCl, as described in Section 7.2. Thermolysis of C-bromomethyl- or C-chloromethyl-o-carboranes at 200-300 C induces halogen migration to form Δ by halogenation at boron positions [1185], for example, the respective 1-methyl-B-halo derivative, accompanied ClCH2−CB10H10CH
⎯⎯
H3C−CB10H10−nClnCH
n = 1- 5
Similar products are obtained when Me2 2CB10H10C2 2Cl and Me2 2CB10H10C2 2Br are heated together at 250-300 C [1185]. The fact that these migrations take place at relatively low temperatures indicates that rearrangement of the cluster framework (Chapter 10) is not involved, and that the process is intermolecular.
9.17 MERCURY DERIVATIVES As was described in Section 9.4, o-carborane is easily mercurated at carbon by treatment with mercury halides or organomercury reagents. Reactions of 1,2-LiRC2B10H10 derivatives with mercury(II) halides generally afford symmetrical bis (carboranyl) species such as 9-382, but alkylmercury halides produce unsymmetrical products, for example, 9-383. Both mono- and bis(carboranyl) species are characterized by exceptional stability, which is attributed to electron withdrawal by the carboranyl unit; for example, Hg(CB10H10CPh)2 is unreactive with HCl in boiling ethanol [1057], although RHg2Ccage cleavage under these conditions [203,575]. (CB10H10CPh) compounds (R ¼ Me, Ph) undergo Hg2 R
R C C
9-383
Hg
Me
C MeHgX
R
R C
Li
HgX2
R = H, Ph, Me, CH=CH2 X = Cl, Br
C
C C
Hg
C
9-382
In contrast to most organomercury derivatives, unsymmetrical o-carboranyl mercury compounds tend not to disproportionate, a fact that permits the synthesis of stable RHgR0 derivatives [1057] such as (ferrocenyl)mercury(phenylcarboranyl) and others listed in Table 9-1.
9.17 Mercury derivatives
507
Li-CB10 H10 C-Ph þ CpFeðC5 H4 ÞHgCl ! CpFeðC5 H4 Þ-Hg-CB10 H10 C-Ph Symmetrical bis(o-carboranyl)mercury compounds exhibit remarkable thermal stability and are highly resistant to electrophilic attack, as illustrated by the failure of Hg(CB10H10CPh)2 to react with bromine in refluxing CMe2Cl2/PhBr [1057]. However, this compound does react with dimethylmercury to afford MeHg(CB10H10CPh) in 70% yield [124]. The symmetrical derivatives are generally reactive toward nucleophiles, which typically cleave the Hg2 2Ccage bond to generate carborane or lithiocarborane products, for example, 9-384 and 9-385. Pyridine forms adducts, while alcoholic base attacks and destroys the carborane cage [124].
2 C4H9Li
C
R
C
C
C
2 Li
R
R
R
Hg
C
C
LiAlH4
C
H
C
or Li , 20 C
9-385
9-384
R = Ph, Me
The electron-withdrawing character of the carboranyl cage at the carbon atoms is seen in the reactivity of o-carboranylmercury derivatives, and also in the observation that polarographic reduction takes place at considerably less negative potentials than are required for organomercury compounds in general [203,593,1059]. The unusual stability of Hg2 2Ccage bonds may also reflect, in part, steric hindrance by the cages as well as the high-coordinate bonding environment of the carborane carbon atoms, which may preclude their forming transition states like those normally associated with the cleavage of organomercury compounds [203]. (No such transition states are involved in nucleophilic cleavage.) In contrast to alkylmercuric halides, which react with nitrogen-containing electron donors to form symmetrical complexes, o-carboranyl mercuric halides form stable 1:1 adducts [1062]. In a creative application of mercury-o-carboranyl derivative chemistry, Hawthorne and coworkers have prepared “mercuracarborand” macrocycles exemplified by 9-386 and 9-387 [174,1075,1081–1083,1307–1309], which are inorganic counterparts of crown ethers but differ in an important respect. Unlike the negatively charged oxygens in crown ethers, which trap metal cations, the mercury centers in the carborane systems are electrophilic and coordinate to anions such as Cl, Br, and I, as in 9-388; hence they have been labeled anti-crowns. Other electron-donor substrates give rise to different host-guest architectures.
Li
C
C Hg
C
C Hg C C
Hg C
9-386
C Hg(CO2Me)2
C
Li
Hg
HgCl2
C
C
C
C Hg
Hg
C
C C
Hg
9-387
C
Cl−
Hg
C C
C Hg
Cl
−
Hg C
C C
Hg
9-388
C
508
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
References [1] Hawthorne, M. F.; Andrews, T. D.; Garrett, P. M.; Olsen, F. P.; Reintjes, M.; Tebbe, F. N.; Warren, L. F.; Wegner, P. A.; Young, D. C. Inorg Synth. 1967, 10, 91. [2] Tietze, L. F.; Griesbach, U.; Elsner, O. Synlett 2002, 1109. [3] Beall, H. Inorg. Chem. 1972, 11, 637. [4] Hill, W. E.; Johnson, F. A.; Hosmane, N. S. In Boron Chemistry-4; Parry, R. W., Kodama, G., Eds.; Pergamon Press, 1979; pp 33–39. [5] Stanko, V. I.; Brattsev, V. A.; Gol’tyapin, Yu. V.; Khrapov, V. V.; Babushkina, T. A.; Klimova, T. P. Zh. Obshch. Khim. 1974, 44, 335 [Russian]. [6] Brattsev, V. A.; Stanko, V. I. J. Organomet. Chem. 1973, 55, 205. [7] Davidson, M. G.; Hibbert, T. G.; Howard, J. A. K.; Mackinnon, A.; Wade, K. Chem. Commun. 1996, 2285. [8] Fox, M. A.; Hughes, A. K. Coord. Chem. Rev. 2004, 248, 457 [Review]. [9] Hardie, M. J.; Raston, C. L. Cryst. Growth Design. 2001, 1, 53. [10] Hardie, M. J.; Raston, C. L.; Wells, B. Chem. Eur. J. 2000, 6, 3293. [11] Clark, T. E.; Makha, M.; Raston, C. L.; Sobolev, A. N. Dalton Trans. 2006, 5449. [12] Bohn, R. K.; Bohn, M. D. Inorg. Chem. 1971, 10, 350. [13] Turner, A. R.; Robertson, H. E.; Borisenko, K. B.; Rankin, D. W. H.; Fox, M. A. Dalton Trans. 2005, 1310. [14] Hermanek, S.; Gregor, V.; Sˇtı´br, B.; Plesˇek, J.; Janousek, Z.; Antonovich, V. A. Collect. Czech. Chem. Commun. 1976, 41, 1492. [15] Siedle, A. R.; Bodner, G. M.; Garber, A. R.; Beer, D. C.; Todd, L. J. Inorg. Chem. 1974, 13, 2321. [16] Stanko, V. I.; Khrapov, V. V.; Klimova, T. V.; Babushkina, T. A.; Klimova, T. P. Zh. Obshch. Khim. 1978, 48, 368 [Russian]. [17] Tupciauskas, A.; Stanko, V. I.; Ustynyuk, Yu. A.; Khrapov, V. V. Zh. Strukt. Khim. 1972, 13, 823 [Russian]. [18] Reynhardt, E. C.; Watton, A.; Petch, H. E. J. Magn. Reson. 1982, 46, 453. [19] Beckmann, P.; Leffler, A. J. J. Chem. Phys. 1980, 72, 4600. [20] Reynhardt, E. C.; Froneman, S. Mol. Phys. 1991, 74, 61. [21] Babushkina, T. A.; Klimova, T. P.; Anorova, G. A.; Khrapov, V. V.; Stanko, V. I. Zh. Obshch. Khim. 1976, 46, 1064 [Russian]. [22] Stanko, V. I.; Babushkina, T. A.; Klimova, T. P.; Gol’tyapin, Yu. V.; Klimova, A. I.; Vasil’ev, A. M.; Alymov, A. M.; Khrapov, V. V. Zh. Obshch. Khim. 1976, 46, 1071 [Russian]. [23] Beall, H.; Elvin, A. T.; Bushweller, C. H. Inorg. Chem. 1974, 13, 2031. [24] Bushweller, C. H.; Beall, H.; Grace, M.; Dewkett, W. J.; Bilofsky, H. S.; Am, J. Chem. Soc. 1971, 93, 2145. [25] Baughman, R. H. J. Chem. Phys. 1970, 53, 3781. [26] Bukalov, S. S.; Leites, L. A. Izv. Akad. Nauk. SSSR, Ser. Fiz. 1989, 53, 1715 [Russian]. [27] Schroeder, H. A.; Heying, T. L.; Reiner, J. R. Inorg. Chem. 1963, 2, 1092. [28] Schroeder, H.; Vickers, G. D. Inorg. Chem. 1963, 2, 1317. [29] Stanko, V. I.; Khrapov, V. V.; Klimova, A. I.; Shoolery, J. N. Zh. Strukt. Khim. 1970, 11, 584 [Russian; p. 627]. [30] Vickers, G. D.; Agahigian, H.; Pier, E. A.; Schroeder, H. Inorg. Chem. 1966, 5, 693. [31] Burg, A. B.; Reilly, T. J. Inorg. Chem. 1972, 11, 1962. [32] Reynhardt, E. C. J. Magn. Reson. 1986, 69, 337. [33] Leffler, A. J. J. Chem. Phys. 1984, 81, 2574. [34] Lo¨tz, A.; Voitla¨nder, J. J. Chem. Phys. 1986, 85, 3136. [35] Pascal, Y. L.; Convert, O. Mag. Reson. Chem. 1991, 29, 308. [36] Lo¨tz, A.; Voitla¨nder, J. J. Chem. Phys. 1991, 95, 3208. [37] Olliges, J.; Lo¨tz, A.; Kilian, D.; Voitla¨nder, J.; Wesemann, L. J. Chem. Phys. 1995, 103, 9568. [38] Venable, T. L.; Hutton, W. C.; Grimes, R. N. J. Am. Chem. Soc. 1984, 106, 29. [39] Fa¨cke, T.; Berger, S. Mag. Reson. Chem. 1994, 32, 436. [40] Cendrowski-Guillame, S. M.; Spencer, J. T. Main Group Metal. Chem. 1996, 19, 791. [41] Harris, R. K.; Bowles, J.; Stephenson, I. R.; Wong, E. H. Spectrochim. Acta A 1988, 44A, 273. [42] Wrackmeyer, B.; Herna´ndez, Z. G.; Lang, J.; Tok, O. L.; Anorg, Z. Allg. Chem. 2009, 635, 1087. [43] Subbotin, O. A.; Klimova, T. V.; Stanko, V. I.; Ustynyuk, Yu. A. Zh. Obshch. Khim. 1979, 49, 363 [Russian; p. 415]. [44] Zakharkin, L. I.; Kalinin, V. N.; Rys, E. G.; Antonovich, V. A. Izv. Akad. Nauk. SSSR, Ser. Khim. 1980, 1446 [Russian; p. 2041]. [45] Colella, S. M.; Li, J.; Jones, M., Jr Organometallics 1992, 11, 4346.
9.17 Mercury derivatives
509
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510
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
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CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
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9.17 Mercury derivatives [785] [786] [787] [788] [789] [790] [791] [792] [793] [794] [795] [796] [797] [798] [799] [800] [801] [802] [803] [804] [805] [806] [807] [808] [809] [810] [811] [812] [813] [814] [815] [816] [817] [818] [819] [820] [821] [822] [823] [824] [825] [826]
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531
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CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12
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540 [1472] [1473] [1474] [1475] [1476] [1477] [1478] [1479] [1480] [1481] [1482] [1483] [1484] [1485] [1486] [1487] [1488] [1489] [1490] [1491] [1492] [1493] [1494] [1495] [1496] [1497] [1498] [1499] [1500] [1501] [1502] [1503] [1504] [1505] [1506] [1507] [1508]
CHAPTER 9 Icosahedral carboranes: 1,2-C2B10H12 Wang, Y.-M.; Fu, N.-Y.; Chan, S.-H.; Lee, H.-K.; Wong, H. N. C. Tetrahedron 2007, 63, 8586. White, J. M.; Bateman, S. A.; Kelly, D. P.; Martin, R. F. Acta Cryst. 1996, C52, 2785. Woodhouse, S. L.; Ziolkowski, E. J.; Rendina, L. M. Dalton Trans. 2005, 2827. Wu, D.-H.; Cheng, J.; Li, Y.-Z.; Yan, H. Organometallics 2007, 26, 1560. Xiao, X.-Q.; Lin, Y.-J.; Jin, G.-X. Dalton Trans. 2008, 2615. Xie, Z.; Wang, S.; Zhou, Z.-Y.; Mak, T. C. W. Organometallics 1998, 17, 1907. Yamamoto, K.; Endo, Y. Bioorg. Med. Chem. Lett. 2001, 11, 2389. Yang, X.; Knobler, C. B.; Hawthorne, M. F. J. Am. Chem. Soc. 1992, 114, 380. Yang, X.; Knobler, C. B.; Hawthorne, M. F. J. Am. Chem. Soc. 1993, 115, 4904. Yang, X.; Zheng, Z.; Knobler, C. B.; Hawthorne, M. F. J. Am. Chem. Soc. 1993, 115, 193. Yanovskii, A. I.; Struchkov, Yu. T.; Kalinin, V. N.; Usatov, A. V.; Zakharkin, L. I. Koord. Khim. 1982, 8, 240 [Chem. Abstr. 97:39078j] [Russian]. Yinghuai, Z.; Lo Pei Sia, S.; Kooli, F.; Carpenter, K.; Kemp, R. A. J. Organomet. Chem. 2005, 690, 6284. Yinghuai, Z.; Sia, S. L. P.; Carpenter, K.; Kooli, F.; Kemp, R. A. J. Phys. Chem. Solids 2006, 67, 1216. Yuan, Z.; Entwhistle, C. D.; Collings, J. C.; Albesa-Jove, D.; Batsanov, A. S.; Howard, J. A. K.; et al. Chem. Eur. J. 2006, 12, 2758. Zakharkin, L. I.; Grebennikov, A. V.; Ioffe, S. T.; Savina, L. A. Izv. Akad. Nauk. SSSR, Ser. Khim. 1969, 155 [Russian; p. 163]. Zakharkin, L. I.; Kalinin, V. N.; Kobel’kova, N. I. Synth. React. Inorg. Met. Org. Chem. 1976, 6, 65. Zakharkin, L. I.; Kalinin, V. N.; Snyakin, A. P.; Kvasov, B. A. Zh. Obshch. Khim. 1970, 40, 2419 [Russian]. Zakharkin, L. I.; Kazantsev, A. V.; Ermaganbetov, B. T.; Fonshtein, A. P. Izv. Akad. Nauk. SSSR, Ser. Khim. 1975, 710 [Russian]. Zakharkin, L. I.; Kazantsev, A. V.; Meiramov, M. G. Izv. Akad. Nauk. SSSR, Ser. Khim. 1984, 1641 [Russian]. Zakharkin, L. I.; Kazantsev, A. V.; Meiramov, M. G. Koord. Khim. 1985, 11, 1084 [Chem. Abstr. 105:42929b] [Russian]. Zakharkin, L. I.; Kazantsev, A. V.; Meiramov, M. G. Zh. Obshch. Khim. 1984, 54, 1536 [Russian]. Zakharkin, L. I.; Kovredov, A. I.; Kolobova, N. E.; Isakina, T. A. Izv. Akad. Nauk. SSSR, Ser. Khim. 1970, 497 [Russian]. Zakharkin, L. I.; Kovredov, A. I.; Ol’shevskaya, V. A. Izv. Akad. Nauk. SSSR, Ser. Khim. 1986, 1388 [Russian]. Zakharkin, L. I.; Kozlova, V. S.; Babich, S. A. Zh. Obshch. Khim. 1974, 44, 1891 [Russian]. Zakharkin, L. I.; Lebedev, V. N. Izv. Akad. Nauk. SSSR, Ser. Khim. 1972, 734 [Russian]. Zakharkin, L. I.; Lebedev, V. N. Zh. Obshch. Khim. 1972, 42, 558 [Russian]. Zakharkin, L. I.; Lebedev, V. N.; Shustova, T. V.; Balagurova, E. V. Izv. Akad. Nauk. SSSR, Ser. Khim. 1986, 2813 [Russian]. Zakharkin, L. I.; Ol’shevskaya, V. A.; Guseva, V. V. Russ. Chem. Bull. 1996, 45, 235. Zakharkin, L. I.; Orlova, L. V. Izv. Akad. Nauk. SSSR, Ser. Khim. 1972, 209 [Russian]. Zakharkin, L. I.; Pisareva, I. V.; Vasil’eva, N. S. Zh. Obshch. Khim. 1982, 52, 711 [Russian]. Zakharkin, L. I.; Tarabarina, A. P.; Kalinin, V. N.; Yablokova, N. V.; Yablokov, V. A. Dokl. Akad. Nauk. SSSR 1977, 234, 1332 [Russian]. Zakharkin, L. I.; Zhigareva, G. G. Zh. Obshch. Khim. 1967, 37, 2646 [Russian; p. 2781]. Zheng, Z.; Yang, X.; Knobler, C. B.; Hawthorne, M. F. J. Am. Chem. Socf. 1993, 115, 5320. Zhigach, A. F.; Laptev, V. T.; Bochkarev, V. N.; Petrunin, A. B.; Parfenov, B. P.; Polivanov, A. N. Zh. Obshch. Khim. 1973, 43, 867 [Russian]. Zhu, Y.; Maguire, J. A.; Hosmane, N. S. Inorg. Chem. Commun. 2003, 6, 1344. Zinn, A. A.; Knobler, C. B.; Harwell, C. E.; Hawthorne, M. F. Inorg. Chem. 1999, 38, 2227. Zvereva, T. D.; Churkina, L. A.; Ol’dekop, Yu. A. Vestsi. Akad. Navuk. BSSR, Ser. Khim. Navuk. 1984, 121 [Russian].
CHAPTER
Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
10
10.1 OVERVIEW Although 1,2-C2B10H12 (ortho-carborane) is among the most thermally stable covalently bonded molecules known, on heating above 425 C it isomerizes to the even more robust 1,7-C2B10H12 (meta-carborane). At still higher temperatures (>600 C), further rearrangement occurs with some decomposition, generating the only other possible isomer with icosahedral geometry, 1,12-C2B10H12 (para-carborane), which survives temperatures in excess of 700 C. Electrostatic repulsion between the þ6 charged carbon nuclei is the driving force behind these isomerizations. Many of the standard methods described in Chapter 9 for derivatizing o-carborane via introduction of substituents at the cage carbon and boron atoms are also applicable to m- and p-carborane. However, there are notable differences. Generally these are ascribable to the lower polarity (zero in p-carborane) and/or to the weaker inductive (I) electron attraction in the meta and para isomers, which leads to reduced C2 2H acidity and hence lower reactivity toward metallation at carbon in those systems compared to o-carborane. The diminished I effect in m- versus o-carborane is supported by much chemical evidence, to be described, and by measurement of the polarographic reduction potentials of many derivatives demonstrating that the m- and p-carboranyl species are more difficult to reduce. Other differences in properties are also apparent. The formation of intramolecular exo-polyhedral rings, which is a common theme in the chemistry of o-carborane, is greatly inhibited (though not excluded) in m- and p-carborane by the separation of cage carbons in these systems. As a rule, the lower polarity of m- and p-carborane derivatives is reflected in higher volatility and lower melting points than those of their o-carborane counterparts. Finally, the generally superior thermal and chemical stability of the meta and para isomers allows these clusters to survive more rigorous conditions than o-carborane and favors their use in certain applications such as heat-resistant polymers. In general, derivatives of m- and p-carborane are accessible via two approaches, namely thermal rearrangement of the o-carborane analogue—feasible for derivatives that can survive the high temperatures required—or the introduction of functional groups, which as mentioned above is subject to some limitations because of lower carborane reactivity. Although the known scope of m- and p-carborane chemistry is smaller than that of the ortho isomer, it is nonetheless considerable, and hundreds of derivatives have been prepared (Tables 10-1 and 10-2).
10.2 SYNTHESIS AND STRUCTURE 10.2.1 m-Carborane While the 1,7-C2B10 icosahedron has been obtained in diverse ways, including thermolysis of nido-2-CB5H9 (Section 4.5) [9], g-irradiation of o-carborane [6], cage expansion of 1,6-Me2C2B8H8 to 1,7-Me2C2B10H10 via reaction with B2H6 [85], and the oxidation of 1,12-C2 B10 H12 2 , MðC2 B10 H12 Þ2 2 , or CoðC2 B10 H12 Þ2 anions [7,10–12,736,737], the only practical synthesis of this carborane system is via the thermal isomerization of 1,2-C2B10H12 or its derivatives [1– 5,75,76,102,353,370,444]. When conducted in vacuo or in an inert atmosphere, nearly quantitative yields of m-carborane are obtained in 24-48 h; more rapid conversion (min) is achieved, in 98% yield, in a flow reactor at 600 C [2]. In closed systems, the isomerization is accompanied by formation of bis(m-carboranyl), [(1,7-C2B10H11)2] isomers [4]. Carboranes. DOI: 10.1016/B978-0-12-374170-7.00008-2 © 2011 Elsevier Inc. All rights reserved.
541
542
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-1 Selected 1,7-C2B10H12 Derivativesa Compoundb Non-transition metal derivatives Parent
Informationc
References
S (thermal isomerization of 1,2-C2B10H12) S (gamma irradiation of 1,2-C2B10H12) S (oxidation of 1,12-C2 B10 H12 2 in Na/liquid NH3) S {dealkylation and cage isomerization of 1,2[CHMe2]C2B10H11 at 450 C} S (pyrolysis of nido-2-CB5H9) S [oxidation of M(C2B10H12)2 M ¼ Fe, Co, Ni] S [isomer mixture from oxidation of ðC2 B10 H12 Þ2 ] S [oxidation of 1,12-C2 B10 H12 2 ] X, definitive, cocrystallized with [Me2N]3PO X, N(H)P[NMe2]3 1:1 H-bonded adduct X, C2 2H X solid state interactions X ¼ O, N, S, F. CR, Cp, arene Cl, Br, I, C X, H inclusion complexes with p-CMe3-calix[5] arene ED H H (substituent effects) H (C2 2H shift, coupling constants) H (solid), DSC, X-ray powder (phase transitions, molecular motions) B B (spin-decoupled; interpreted) B (substituent effects) B(2d) B (antipodal shielding) JC,B, JC,C, JB,B NMR coupling constants C C (detailed assignments) C, experimental and IGLO-calculated shifts C (d þ JC2 2H, comparison with other carboranes) C (C2 2H coupling; C hybridization) D (rotational motion in solid; spin-spin and spinlattice relaxation times) S, IR (actual spectrum) IR (actual spectrum, C2 2H intensity) IR (actual spectrum) IR (CH frequency vs. other carboranes) IR (substituent effects) IR (80-300 K), band structure, order-disorder transition
[1–5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17–19] [20,21] [22,23] [24] [25,26] [24,27] [28] [21–23] [29] [20] [30] [20] [31,32] [33] [34] [35,36] [37] [38] [39] [40] [34] [22,23] [41,42] Continued
10.2 Synthesis and structure
543
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
9,10-D2 [RR0 C2B10H10] (FF) radical anions (R, R0 ¼ H, Me) 1,10 -(C2B10H11)2 (FF) 2(CB10H10C)22 2CB10H10CH (FF) HCB10H10C2 Alkyl derivatives 1-Me
Informationc
References
IR (variable T) IR (phase transitions) IR Raman, H(T1) (low-T phase transition) Raman (phase transitions) Raman (variable T) Raman MS (detailed study at 100-250 C) E E (pKa) pKa, acidity relative to 1,2- and 1,12-C2B10H12 pKa, metalation equilibrium constants 2H thermodynamic and kinetic acidity pKa, C2 ESCA, binding energies He photoelectron spectra He photoelectron spectra; ionization potentials Inner shell electron energy-loss spectrum (ISEELS) X-ray fluorescence Photoemission spectra, MO binding energies, absorption on metal surfaces Luminescence, emission spectra △Hcombustion, △Hformation Heat of isomerization from 1,2-C2B10H12 Dipole moment Dielectric data, relaxation dynamics, plastic crystals Ionic fragmentation following photon-induced B 1s and C 1s excitation vs. energetics of decomposition B MS (electron resonance capture mass spectra)
[43] [44] [21,45] [46] [44] [43] [40,45,47,48] [49] [50] [51] [34,52] [53,54] [55] [56] [57] [58] [59] [60] [61]
[71] [72]
S, H, B, C, MS S, IR S, H, B, C, MS
[73] [74] [73]
S S [1,2-(XCH2)C2B10H11 þ Na/NH3 followed by oxidation] H (effect of Me group) B
[75–78] [79]
[62] [63–66] [63] [67,68] [69] [70]
[22,80] [22,80] Continued
544
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
1,7-Me2
1,7-cyclo-(CH2)8 1-Me-7-Et 1-Me-7-Ph 1,7-(n-C3H7)2 1-CHMe2 1-C3H7-7-i-C4H9 1-n-C4H9 1-i-C4H9 1,7-(n/i-C4H9)2 1-C5H11 1,7-(C5H11)2 1-C6H13 1,7-(CR2-C5H5)2 R2 ¼ Me2, (CH2)5 1,7-(C6H13)2 1,7-(C10H21)2 1-C7H6-7-C5H4 1-C7 H6 þ -7-Me tropenyliumyl cation 1-g-C7H7-7-R R ¼ H, Me, g-C7H7 tropenyl n-CHMe2 n ¼ 4, 5, 9 n-(n-C4H9) n ¼ 4, 5, 9 H2C2B10Me8H2 (FF) 9,10-Me2 9-CHMe2 9-Et
Informationc
References
C (detailed assignments) C IR IR (actual spectrum) △Hformation E (reduction; comparison with 1,2- and 1,12C2B10H12) pKa, metalation equilibrium constants 2H thermodynamic and kinetic acidity pKa, C2 S B (antipodal shielding) C E S, X, MS S S S S C (detailed assignments) S C (detailed assignments) Heat of evaporation, vapor pressure
[31] [80,81] [22] [39] [81,82] [83]
S Heat of evaporation, vapor pressure S Heat of evaporation, vapor pressure S, H, B S S Hyperpolarizability; NLO S, H, UV, IR S, H, UV, IR S (electrophilic alkylation) S (electrophilic alkylation) S, H, B, C S, H, B, C, MS S (from RMgX) S (Pd cross-coupling) S {Pd[PPh3]4 catalyzed} S (from RMgX)
[54] [55] [20,78,84,85] [20] [20] [50] [86] [84] [77] [78] [31] [87] [31] [88] [87] [88] [87] [88] [89] [87] [87] [90] [91] [91] [92] [92] [93] [94] [95] [96] [97] [95,98] Continued
10.2 Synthesis and structure
545
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References
9,10-Et2 9-n-C3H7 9-n-C4H9
S, H, B, C, MS S S {Pd[PPh3]4 catalyzed} S (from RMgX) S S, MS S, ESR, MS S, MS
[94] [96] [97] [95,98] [96] [99] [99] [99]
S, F S, H, B, C, F, IR, MS S (thermal cage isomerization) E (reduction; comparison with 1,21,12-C2B10H12 derivatives) S S E (reduction; comparison with 1,21,12-C2B10H12 derivatives) E (reduction; comparison with 1,21,12-C2B10H12 derivatives) S (thermal cage isomerization) E (reduction; comparison with 1,21,12-C2B10H12 derivatives) S, H, B, C, MS S, H, B, C, MS S, H, B, C, MS S (thermal cage isomerization) S S S S S
and
[100] [101] [102] [83]
and
[102] [103] [104]
and
[104]
and
[102] [83]
9-C6H13 C60(C2B10H11)þ (FF) C60(C2B10H11)• (FF) [C60(C2B10H11)]2 Haloalkyl derivatives 1-CH2F 5CHCH2R R ¼ n-C6F13, i-C3F7 1-CH2CH5 1-CH2Cl
1,7-(C2H4Cl)2 1-CH2X-7-R R ¼ H, Me; X ¼ Cl, Br 1,7-(CH2Cl)2 1-CH2Cl-7-R R ¼ H, Me, Ph, I, Cl 1-CH2Br
1-(CH2)3X-B-Me8 X ¼ Cl, Br [Br(CH2)3]HC2B10Me8H2 (FF) [Br(CH2)n]2C2B10Me8H2 n ¼ 3, 4 (FF) 1,7-(CH2Br)2 1,7-(n-C4H9-40 -Br)2 1-C2H4Br 1,7-(C2H4X)2 X ¼ Cl, Br 1,7-(C4H8X)2 X ¼ Cl, Br, I 1,7-(C4H8I)2 Aryl derivatives 1-Ph
S X (bond angles of ipso-C on Ph ring; electron donation) X H C C (detailed assignments)
[105] [106] [106] [102] [102] [102] [102] [102] [102]
[107–110] [111] [112] [107] [107] [31,32] Continued
546
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
1-m/p-C6H4Me 1,7-(p-C6H4Me)2 1-Ph-7-OH 1-Ph-7-SH 1,7-Ph2
1-p-BrC6H4 1,7-(p-IC6H4)2C2B10H10 2CB10H8(9,10-R2) cyclo-[2 2CH2C6H4CH22 C2 2]2 R ¼ H, Me xylylene-linked carboracycles (FF) cyclo-[2 2CH2C6H4CH22 2CB10H8(9,10-Me2) C2 2]4 xylylene-linked carboracycle (FF) 1,3-[C2B10H11]2C6H4 (FF) [1,2-C2B10H10-10 ,40 -C6H4-1,7-C2B10H1010 ,40 -C6H4]2 phenylene-linked o/mcarborane system (FF) 2C2B10H10)3 cyclic trimer cyclo-(10 ,30 -C6H42 (FF) 9-Ph
9,10-Ph2 9,10-(C6H4Me)2
Informationc
References
UV, IR IR E pKa, metalation equilibrium constants 2H thermodynamic and kinetic acidity pKa, C2 pKa S S, H, B, C, IR, MS S, H, B, C, IR, MS S [isomerization of 1,2-Ph2C2B10H10] S X X (redetermination) H C IR IR (variable T) Raman (variable T) E S, H, B, IR, UV S, X, H, B, IR, UV S, X, H, B, C, MS
[113] [107] [114] [54] [55] [115] [116] [117] [117] [118] [107] [119] [120] [107] [107,121] [107] [43] [43] [50] [122] [122] [123]
S, H, B, C, MS
[123]
S H, C, B, IR, UV, MS S, X, H, B, C, IR
[107,124] [124] [125]
S
[107,124]
X, H, C, B, IR, UV S {Pd[PPh3]4 catalyzed} S (from RMgX) S (Pd cross-coupling) S [UV photolysis of Hg(C2B10H10)2] X S, X, H, B, C, MS S S, H, B, C, MS
[124] [97] [95,98] [96] [126] [127] [94] [97] [128] Continued
10.2 Synthesis and structure
547
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References
9-C6H4Me 9-CH2Ph
S (from RMgX) S {Pd[PPh3]4 catalyzed} S (from RMgX) S, IR, MS S, IR, MS S E S, H, B, C, MS S S S S, phase transitions, optical microscopy, DSC, X-ray powder diffraction
[95] [97] [95,98] [129] [129] [77] [50] [94] [97] [130] [131] [132]
S, H, B, C, F F(substituent effects) F(inductive effect; electronic properties) S, H, C, F, IR S, H, C, F, IR S, H, C, F, IR S, H, C, F, IR
[133] [134–136] [137] [138] [138] [138] [138]
S S, H, C, IR S S, H, C, IR S, H, C, IR, MS S S S, F E S F (electronic properties) S [UV photolysis of Hg(C2B10H10)2] S (from RMgX) S S (Pd catalyzed) S, H S, H, B, C, MS
[139,140] [107] [109] [107] [107] [141] [141] [142] [114] [143] [144] [126] [95] [145] [146] [147] [128]
PhB(CB10H10CH)2 (FF) 1,7-(BPh2)2 1,7-(CH2Ph)2 9,10-(CH2Ph)2 5C6H6, MePh, p-MeC6H4Me 1-CH2R R5 2OH)2 1,7-(p-C6H42 2C(O)O]22 2C6H42 2O2 2 1,7-{[p-C6H42 CnH2nþ1}2 n ¼ 9-13 liquid crystals; bent-core mesogens Haloaryl derivatives 1-p-C6H4F 1-m/p-C6H4F 1-m/p-C6F4F-7Me 1-C6F5-7-R R ¼ Me, Ph 1-[p-C6F4(n-C4H9)]-7-Ph p-(RCB10H10C)2C6F4 R ¼ Me, Ph (FF) p-[1,7-PhCB10H10C]-C6F4-[1,2-CB10H10CH] (FF) 1-p-C6H4X X ¼ Cl, Br, I 1-C6H4X X ¼ Cl, Br, I 1-p-C6H4Br 1,7-(C6H4X)2 X ¼ Cl, Br, I 1,4-[BrC6H4-CB10H10C]2C6H4 (FF) 1,4-C6F4(MeC2B10H10)2 (FF) 1-C6F4Cl-7-R R ¼ Me, Ph (m/p-C6H4F)HC2B10Cl10 (FF) 1-p-C6H4I n-p-C6H4F n ¼ 3, 4, 9 4-m/p-C6H4F 9-C6F5 9-C6H4F 9-Xþ C6H4F BF4 X ¼ Cl, Br, I halonium salts 9-m/p-C6H4F 9-m/p-C6H4Br 9,10-(C6H4Cl)2
Continued
548
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb Alkenyl derivatives 5CMe n ¼ 3, 7, 9 n-CH5 5CH2, n-CH25 1-CH5 5CHR R ¼ H, Cl, I 1-CH5 5C5 5CH2 allenyl 5CH2)2 methallyl 1,7-(CH2CMe5 1,7-(CMe5 5CH2)2 5CH)C2B10H2Me8 (FF) 1,7-H(Br2C5 1-CF5 5CXY-7-Me X,Y ¼ F, Cl, n-C4H9, Ph, CF3, SC4H9, SPh, NEt3 2-CH5 5CH2 2CH2CH5 5CHCH22 2 HCB10H10C2 CB10H10CH (FF) n-CH5 5CH2 n ¼ 2, 4 9-CH5 5CH2 (vinyl) 5CH2 (allyl) 9-CH2CH5 Alkynyl derivatives CH 1-C 1-C CR R ¼ H, Ph CH n ¼ 1, 2, 9 n-C n-C CH n ¼ 2, 4 CR)2 R ¼ H, Ph 1,7-(C C2 2C 2C60)-7-Me 1-(C6H42 C2 C2 2C 2C 2 1,7-RCB10H2Me8C2 CH] (FF) CB10H2Me8CR0 [R, R0 ¼ H, C C2 1,7-MeCB10H2Me8C2 2C 2 C2 C2 [C 2CB10H2Me8C2 2C 2]2 C2 2 2C 2CB10H2Me8CMe (FF) [2 2(p-C6H4)2 2CB10H10C2 2pC2 C2 2C 2C20H10(OC8H17)22 2 C 2]n C6H42 C20H10 ¼ binaphthyl chiral conjugated polymers 2CB10H10C2 2p[2 2(p-C6H4)2 C2 2C 2C20H10(OC8H17)22 2 C6H42 C2 C 2]2 C20H10 ¼ binaphthyl C-p-C6H4)2C2B10H10 1,7-(PhC C-p-C6H42 2O)2C6H2](C 2 p-[(C8H172 CB10H10CH)2 CH 9-C CH 9-CH2C C2 9-C 2C4H9
Informationc
References
MS (photoionization mass spectra) S S S S S, H, B, C, MS S, H, F
[148] [149] [150] [151] [152] [153] [154]
S (boron insertion into nido-7,9-C2 B9 H11 2 ) pKa S, H, B, C, F, IR, MS
[155,156] [157] [101]
S (gas phase isomerization of 1,2-C2B10H11-3CH5 5CH2) S S {Pd[PPh3]4 catalyzed}
[158] [159] [97]
S S IR, pKa S (gas phase isomerization of 1,2-C2B10H11-3CH ) C S S, H, C, MS, UV, E, NLO (b hyperpolarizability; fluorescence) S, H, B, C, MS
[160] [161] [162] [158,163]
S, H, B, C, MS
[153]
S, H, B, C, IR, photoluminescence, CD
[166]
S, H, B, C, IR, photoluminescence, CD
[166]
S, X, H, B, IR, UV S, H, B, IR, UV
[122] [122]
S S S
[167,168] [169] [96]
[161] [164,165] [153]
Continued
10.2 Synthesis and structure
549
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb Alcohols and C- and B-hydroxy derivatives 1-OH 1-OH-7-Me 1,7-(OH)2 1-OH-7-Ph n-OH n ¼ 1, 2, 4 n-OH n ¼ 2, 4 2-OH 4-OH 5-OH 9-OH
1-CH2OH 1,7-(CH2OH)2 1,7-H(HOCH2)C2B10H2Me8 (FF) C2 C2 2C 2C 2 1,7-RCB10H2Me8C2 CH] CB10H2Me8CR0 [R, R0 ¼ H, CH2OH, C (FF) 1,7-R2 R ¼ CMe2OH, CMe(CF3)OH, C (CF3)2OH, C(CF3)(CF2Cl)OH 1-(CH2)3OH 2-CH2OH 2-(CH2)3OH 2CH2OH]2 (FF) [CB10H10C2 1,7-(p-CH2C6H4OH)2 [HO(CH2)3]2C2B10Me8H2 (FF) 1-C6H4-m/p-OH n-p-C6H4OH n ¼ 1, 2, 9 1,7-(C6H4OH)2 1,7-R2 R ¼ CHPhOH, CHMe OH 1-R-7-Me R ¼ CH2OH, CH(OH)C5H5N, CH2CH(OH)Ph 1-R-7-Me R ¼ CMe2OH, CPh2OH 1-R-7-R0 R ¼ (CH2)2OH, C2(CH2CH2OH)2; R0 ¼ H, CHMe2, CMe5 5CH2 1,7-[CH(OH)Me]2
Informationc
References
S, H, B, C, IR, MS S, pKa S, pKa S, H, B, C, MS S S, H, B, C, IR, MS pKa S (isomerization of 1,2-C2B10H11-3-OH) S (boron insertion into nido-7,9-C2 B9 H11 2 ) pKa pKa pKa S, H, B, C, IR IR [hydrogen bonding with Et2O] pKa S X (CH2OH), △Hcombustion, △Hformation, △Hsublimation S S, H, B, C, MS S, H, B, C, MS
[170–172] [173,174] [173,174] [170] [175] [117] [176] [177] [155] [177,178] [177,178] [178] [179,180] [181] [177,178] [182] [183] [75,182,184] [153] [153]
S
[152]
S, H, B, C, IR S (boron insertion into nido-7,9-C2 B9 H11 2 ) S, H, B, C, MS S S S, H, B, C, MS pKa; s (inductive mechanism) pKa, hydrophobicity, estrogen receptor binding affinity Dipole moments S S
[185] [155] [186] [187] [188,189] [106] [190] [191] [192] [78] [78]
S, H, IR IR
[193] [194]
IR (variable T), Raman (variable T)
[43] Continued
550
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References
1-R-7-R0 R ¼ H, R0 ¼ OH, CH2OH, NH2; R ¼ R0 ¼ OH complexes with b-cyclodextrin 1-(m/p-C6H4OMe)-7-R R ¼ H, OH, (CH2)nOH n ¼ 1, 2, 3 CMe2(O)OR0 -7-R R ¼ H, 1-C(OH)C CHMe2, CMe2Et; R0 ¼ CMe3, CMe2Et, CMe2(CH2)2Me peroxy alcohols 1,7-{CH(OH)-[2,2]paracyclophane}2 n-CH2OH n ¼ 8, 9 9,10-(CH2CH2OH)2 9-CHMe(CH2)2C(O)OH 9,10-(C6H4OH)2 9-p-C6H4OH 9-CH(OH)Ph
Ka (association constants)
[195]
S, H, MS
[196]
S, IR
[197]
S, IR S, pKa S, IR S S, H, B, C, IR, MS S (RMgX), H S
[198] [199] [200] [201] [128] [202] [203]
Alkoxy and aryloxy derivatives 5CH2 2-OCH2CH5
S, H, B, C, MS
[186]
S, H, B, C, MS S(improved), H, C, MS S, IR S, IR (actual spectrum) S S S, H, B, C, MS S S(improved), H, C, MS IR (variable T), Raman (variable T) IR (80-300 K; band structure; order-disorder transitions) S S S S S S S, H, B, C, MS S, IR
[204] [205] [206] [207] [208] [209] [204] [208,210] [205] [43] [41,42]
S IR (variable T), Raman (variable T)
[52] [43]
Aldehydes 1-C(O)H
1-(CH2)nC(O)H n ¼ 0,1 1,7-[C(O)H]2
2-C(O)H 1-C(O)H-7-R R ¼ Me, Ph 1-C(O)H-7-Me 1-CH2C(O)H 1-CH(OEt)2 acetaldehyde diethyl acetal 1-CH2C(O)H-7-Me 1,7-H[C(O)H]C2B10H2Me8 (FF) 9-R R ¼ CH2C(O)H, CHMeCH2CH2C(O)H Ketones 1-C(O)Ph 1,7-R2 R ¼ C(O)Me, C(O)COPh, CH(OH)C(O)Ph
[156] [208] [193] [211] [211] [193,212] [153] [213]
Continued
10.2 Synthesis and structure
551
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References
1-C(O)Me-7-Me
S IR (actual spectrum) S, IR (actual spectrum) S, pKa S S
[214] [215] [215] [215] [78] [78]
S
[78]
S, IR, MS
[129]
S
[216]
S, IR, UV
[217]
S S pKa S S S, B S
[156] [218,219] [199] [203] [97] [220] [200]
S, IR, pKa S from 1-LiC2B10H11 with no dicarboxylic acid formation X, △Hformation pKa; acidity compared to 1,2- and 1,12-[C(O)OH] C2B10H11 pKa, E (half-neutralization potential) Complex with a-cyclodextrin in aqueous solution; Ka (association constant) S, IR (actual spectrum) pKa Transthyretin (TTR) and COX (cyclooxygenase) inhibition assays S pKa, E (half-neutralization potential) IR (variable T), Raman (variable T) Complex with a-cyclodextrin in aqueous solution; Ka (association constant)
[221] [222]
1-C(O)CH2Br-7-Me 1-C(O)Ph-7-Me 1,7-R2 R ¼ C(O)Me, C(O)Ph 1-R-7-Ph R ¼ C(O)Me, C(O)Ph, CHPhCH2C (O)CMe3 PhCB10H10C-CHPhCH2C(O)-CB10H10C-Ph (FF) 1,7-[1,2-RCB10H10CC(O)]2C2B10H10 R ¼ H, Me (FF) 1,7-[C(O)CH5 5CHR]2 R ¼ Ph, p-MeOC6H4, p-FC6H4, 2-furyl, 2-furylvinyl) a,bunsaturated ketones 1-CH[CH2C(O)R0 ]2 R ¼ CMe3, Ph, p-C6H4OMe 1,5-diketones 2-C(O)Me 9-C(O)Me 9-C(O)Ph 9-CH2C6H4C(O)Ph 9-CH2-p-C6H4C(O)Ph 9,10-[CH2C(O)Ph]2 Carboxylic acids 1-C(O)OH
1-14C(O)OH n-C(O)OH n ¼ 1, 2, 4 1-C(O)OH-7-C6H4-m-F 1,7-[C(O)OH]2
[223] [52,224,225] [226] [227] [228] [176] [229] [75,184,230] [226] [43] [227] Continued
552
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References
1-C(O)OH-7-R R ¼ H, Me, Ph 1-(CH2)nC(O)OH n ¼ 0,1 1-CH2C(O)OH
pKa, E (half-neutralization potential) pKa, induction constants S pKa, E (half-neutralization potential) S, H, B, C, IR S, I (actual spectrum) S, pKa S pKa S, I (actual spectrum) S S Dipole moments S, H, C, IR, MS S (boron insertion into nido-7,9-C2 B9 H11 2 ) pKa S (boron insertion into nido-7,9-C2 B9 H11 2 ) pKa pKa S pKa S, IR S, H, IR
[226] [231] [232] [226] [185] [233] [75] [110] [115] [233] [189] [116] [192] [234] [155] [177] [155] [177] [144,235] [219,236] [199,237] [200] [238]
S S, H, IR, UV
[75,239] [240]
S Cl NQR S, reaction kinetics S S
[110] [241] [242] [182] [243]
S, H, B, C, IR, MS S, H, C, MS S, H, C, MS IR
[244] [205] [205] [194]
S S, H, IR, UV
[152] [245]
1-(CH2)2C(O)OH propionic acid 1-C(O)OH-7-R R ¼ H, Me 1-C(O)OH-7-Me 1-C(O)OH-7-Ph 1-m/p-C6H4C(O)OH 1-CH2C(O)OH-7-Me 1,7-[p-CH2C6H4C(O)OH]2 1,7-[p-C6H4C(O)OH]2 1,7-[C6H4C(O)OH]2 1-C(O)OH-7-NHC(O)OCMe3 2-C(O)OH 2-CH2C(O)OH 4-C(O)OH 4-C6H4-m/p-C(O)OH 9-C(O)OH 9,10-[CH2C(O)OH]2 1,7-[C(O)OH]2 N- and P-containing salts Esters and acyl halides 1,7-[C(O)Cl]2 1,7-[C(O)OR]2 R ¼ alkenyl, alkynyl, and peroxy-containing groups 1-C(O)Cl-7-Ph 1-CH2C(O)Cl 1-C(O)Cl-7-C(O)NHPh 1-CH2C(O)OMe-7-R R ¼ H, CH2C(O)OMe 1-CH2(O)OCR R ¼ Et, Me2CH, Ph, 1-1,7-C2B10H11 (FF) 1-(CH2)2C(O)OMe 1-CH2NH-CH[C(O)OMe]CH2CHMe2 1-CH5 5NCH[C(O)OMe]CH2CHMe2 1-R-7-R0 R ¼ CH2C(O)OX, C2(CH2C(O)OX)2; 5CH2; X ¼ H, Cl R0 ¼ H, CHMe2, CMe5 1,7-[CMe2OC(O)Me]2 2C(O)O2 2(CH2)22 21,71,2-HCB10H10C2 CB10H10C2 2(CH2)22 2O2 2C(O)-1,2CB10H10CH esters (FF)
35
Continued
10.2 Synthesis and structure
553
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References
CH-7-R R ¼ H, 1-C(O)OOCMe2C CHMe2 peroxy alkynes C2 2CMe2OOCMe3] 1-[C(O)OOCMe2C peroxy alkyne HCB10H10C2 2C(O)2 2OO2 2CMe22 2 C2 C 2CMe22 2OO2 2C(O)2 2CB10H10CH peroxy ester (FF) 1-OOR-7-R0 R ¼ CMe3, CH2Ph, H; R0 ¼ Me, 5CH2 peroxides CHMe2, CHMe5 1-C(O)OOR R ¼ CMe3, CMe2Et, C(O)Me, CH2Ph peroxides 1-C(O)OOR-7-C(O)OOH R ¼ CMe3, CMe2Et peroxides CMe2(O)OR0 -7-R R ¼ H, 1-C(OH)C CHMe2, CMe2Et; R0 ¼ CMe3, CMe2Et, CMe2(CH2)2Me peroxy alcohols 1-[CH2C6H4-p-OC(O)Me] 1-[(C6H4)2-p-OC(O)Me] 1,7-[C(O)O(CH2)nOOR]2 R ¼ CMe3, CMe2Et; n ¼ 1, 2 peroxy esters 1,7-[CH2(O)OCR]2 R ¼ Et, Me2CH, Ph 5CH2, 1,7-[C(O)OR]2 R ¼ CH2CH5 CH CH2C 2-C(O)OMe 2-(CH2)3OC(O)Me n-OC(O)Men ¼ 2, 3, 4, 9 9,10-[CH2C(O)Cl]2 9-CH2C(O)OR R ¼ H, Et 9-CHMeCH2CH2C(O)OEt 9-C(O)Cl 9-CH2OC(O)Me
S, heat capacity temperature dependence, △Hcombustion S, heat capacity temperature dependence, △Hcombustion S, △Hformation, △Hcombustion
[246]
S
[248]
S
[249]
S
[250]
S, IR
[197]
S S S
[251] [251] [252]
S S
[243] [253]
S (boron insertion into nido-7,9-C2 B9 H11 2 ) S, H, B, C, MS S S S, IR S, H, IR S, pKa S, pKa
[155] [186] [178] [200] [213] [213] [199] [199]
S S S S S, H, C, IR S, H, C, IR S S, H, B, C, IR
[254] [254] [255] [139] [107] [107] [102] [256]
S S
[78] [257]
Ethers, epoxides, and peroxides 5CH2 1-CH2OCH2CH5 5CH2)2 1,7-(CH2OCH2CH5 1-R-7-R0 R, R0 ¼ CH2OC4H9, CH2OCH2Ph 1-p-C6H4OMe 1-C6H4OPh 1,7-(C6H4OPh)2 2CH2)2O (FF) (Br2 2CB10H10C2 2ðCH2 Þ4 O2 2B12 H11 2 R ¼ H, R2 2CB10 H10 C2 Me (FF) water-soluble compounds for BNCT 1-[CH2(cyclo-CHCH2O-)]-7-Me 9-p-C6H4-OR R ¼ H, Me
[246] [247]
Continued
554
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References
9-p-C6H4OMe 9,10-(C6H4OMe)2 9,10-[C6H4-cyclo-COCH2CH2OMe]2 B-(CH2)2Si(OOR)3 R ¼ CMe3,CMe2Et, 2-cyclohexylisopropyl peroxides
S (RMgX), H S, H, B, C, MS S, H, B, C, MS S
[202] [128] [128] [258]
S S S, S S, S S, S,
[259] [259] [107] [139,260] [107] [261] [147] [220]
Nitro and nitroso derivatives and nitrates 1-NO-7-R R ¼ H, Me 1-NO2-7-R R ¼ H, Me 1-p-C6H4NO2 1,7-(C6H4NO2)2 1,7-[C(O)OC6H4-o-NO2]2 9-m/p-C6H4NO2 9-CH2-p-C6H4NO2 Amines and imines 1-NH2
1,7-(NH2)2 1-NH2-7-R R ¼ H, Ph 1-NRR0 -7-R00 R0 ¼ H, Me, Et; R0 ¼ OH, OC (O)Me, Et, Me, C(O)H; R00 ¼ H, Me, Ph 1-CH2NEt2-7-C(O)OH water-soluble amine carboxylic acid 1-CH2NEt2-7-SH 1-CH2CH(NH2)C(O)OH alanine enantiomers 1-p-C6H4NH2 1-m/p-C6H4NH2 1,7-[C(O)O-p-C6H4NH2]2 aminophenyl oxycarbonyl 1,7-[C(O)NH-p-C6H4NH2]2 aminophenyl carbamoyl 1-OC6H3(OR)CH5 5N(CH2)2NC6H3Me R ¼ Me, Et Schiff bases 1-NHC(O)OCMe3-7-C(O)OH 1-N(boc)NH(boc)-7-C(O)OH boc ¼ tertbutyloxycarbonyl 2C2B10H11)4 (FF) (porphyrin)(9-CH22 2C2B10H11)4 (FF) (porphyrin)(9-CH2CH2CMe2 M(porphyrin-CH(O)H)[CHMe2C2B10H10] M ¼ Co, Cu, 2H (FF)
H, C, IR H, C, IR H B
S (hydrogenation of phenylazo-C2B10H11) IR (C2 2H intensity) S, H, C, MS, complex with a-cyclodextrin in aqueous solution; Ka (association constant) S (hydrogenation of phenylazo-C2B10H11) S S
[262] [39] [227]
S, IR
[265]
S, IR S, H, B, C, IR, optical rotation S pKa, s (inductive mechanism) S
[265] [185] [140] [190] [266]
S
[266]
S, H, I
[267]
S, H, C, IR, MS S, H, B, C, MS
[234] [268]
S, H, cytotoxicity S, H, cytotoxicity S, H, cytotoxicity
[269] [269] [269]
[262] [263] [264]
Continued
10.2 Synthesis and structure
555
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References
1-[CH(OH)(20 -tetraphenylporphyrin)]M-9-R R ¼ NCO, C(O)H, NHC(O)OCMe3; M ¼ 2H, Cu, Zn for BNCT and PDT photodynamic therapy 2-NH2 2-NH2-6-R R ¼ H, Me 2-NH2-6-R R ¼ Me, CCl3 2-NPh2 n-NH2 n ¼ 2, 4 1-(CH2)3NH2-B-Me8 1,7-Me2-3-NH2 9-NH2 9,10-[C6H4NMe2]2 5,10,15,20-(p-C6H4NH2)4 porphyrin derivatives [H2N(CH2)3]HC2B10Me8H2 (FF) [H2N(CH2)n]2C2B10Me8H2 n ¼ 3, 4 (FF) [ClNH3(CH2)n]HC2B10Me8H2 n ¼ 3, 4 (FF) (self-assembly into microrods via sonification) [ClNH3(CH2)n]2C2B10Me8H2 n ¼ 3, 4 (FF) 1-(CH2)3OCH[CH2OCH(CH2OH)2]2-7(CH2)2C4H5[C(O)OH]NH2 water-soluble amino acid cascade polyol for BNCT 2C 6H3(OR)2 2OC(O)2 2 C6H4[o/m/p-NHCH22 CB10H10CH]2 R ¼ Me,Et, 1,3-C6H4, 1,4-C6H4 diamines (FF) 1-(20 -C5H4N) pyridyl 1,7-(20 -C5H4N)2 pyridyl cyclo-[(2,6-NC5H3)-1,7-C2B10H10]3 (trimer) (FF) 9-CH2-(n0 -C5H2N-50 -R-2-C5H4N) R ¼ Ph, p-tolyl; n’ ¼ 30 ,40 bipyridyl 9-CH22 2[C5H(CN)Ph(p-tolyl)N] (2 isomers) pyridyl 9-R R ¼ dioxolylthienyl, methylindolyl, 20 /30 -pyridyl 9-pyridinium-nido-C2B9H12 NC5H3-2,6-(CB10H10CH)2 lutidine (FF) 2CH22 2CB10H10Ccyclo-[2 2(2,6-NC5H3)2 CH22 2]2 lutylidine-linked carboracycle (FF) cyclo-[2 22,6-(O)2CH22 2CB10H10C2 2CH22 2]2 lutylidine NC5H32 N-oxide-linked carboracycle (FF) 9-(20 ,50 -diazabicyclo[20 .20 .20 ]oct-2-ene) derivatives
S, H, B, IR, MS, UV
[270]
S (boron insertion into nido-7,9-C2 B9 H11 2 ) S (from 1,2-C2B10H11-3-NH2 2 ) S (from 1,2-C2 B10 H12 2 in liquid NH3) S, IR S (isomerization of 3-NH2-1,2-C2B10H12) S, H, B, C, MS S pKa, s (inductive mechanism) S, H, B, C, MS S
[155,156] [271] [272] [273] [274] [105] [275] [237] [128] [276]
S, H, B, C, MS S, H, B, C, MS S, H, B, C, MS
[106] [106] [106]
S, H, B, C, MS S, H, C, IR
[106] [277]
S, H, IR
[278]
S, H, B, C, IR S, H, B, C, IR S, H, B, C, IR
[279] [279] [279]
S, X(tolyl,n ¼ 30 ,40 ), H, UV(fluorescence)
[280]
S
[280]
S, H, B, MS
[281]
S S, H, B, C, MS S, X, H, B, C, MS
[180] [123] [123]
S, H, B, C, MS
[123]
S, X, H, MS
[282] Continued
556
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References
S S, IR, MS D, IR, DTA
[75,239] [129] [283]
S S, IR H (high resolution; degree of branching)
[284] [284] [285]
S
[286]
S (from carboxylic chlorides), H S, X(Me, CH2Ph), H, B, C, MS
[287] [288]
S, X, H
[289]
S, H, B, C, IR, MS
[244]
S S
[290] [291]
S, H, B, IR, cytotoxicity
[292]
Azides 1-C(O)N3-7-R R ¼ H, Ph 1-(CH2)3N3-B-Me8 [N3(CH2)n]2C2B10Me8H2 n ¼ 3, 4 (FF) [N3(CH2)3]HC2B10Me8H2 (FF) 9-N3
S S, H, B, C, MS S, H, B, C, MS S, H, B, C, MS S
[263] [105] [106] [106] [180]
Nitriles and isonitriles 1-CN-7-R R ¼ Me, Ph 1-CH2CN-7-R R ¼ Me, Ph [NC(CH2)n]2C2B10Me8H2 n ¼ 3, 4 (FF) n-CN n ¼ 2, 4
S S S, H, B, C, MS S (isomerization of 1,2-C2B10H11-3-CN)
[293,294] [294] [106] [177]
Amides and imides 1,7-[C(O)NH2]2 1,7-[C(O)NHR]2 R ¼ Me, Ph, CMe3 2C(O)NH2 2C6H4-p[2 2C(O)2 2CB10H10C2 NH2 2]n polyamide (FF) 1,7-(maleamidocarboxylic acid)2 1,7-(maleamide)2 2C(O)2 2NH2 2C6H3(NH2)2 2 HCB10H10C2 O2 2C6H3(NHR)2 2NHC(O)2 2CB10H10CH R ¼ H, C(O)CB10H10CH (FF) models for linear and branched polyaminoamides 1-[cyclo-C5 5N2 2O2 2CR5 5CH2 2] R ¼ Ph, C(O)OMe isoxazoles, isoxazolines 9-C(O)NMe2 9-NHC(O)R R ¼ Me, CH2Ph, C6H4Me Ureas 2NMe2 2C(O)2 2 cyclo-[2 2C6H42 NMe2 2C6H42 2CB10H10C2 2]n n ¼ 2,3,4 (FF) Hydrazine derivatives 1-N2{[C(O)OH](CH2Ph)H}2 Isocyanates 1,7-(CH2NCO)2 1-(CH2)nNCO-7-R R ¼ H, Me, Ph, CH25 5CMe; n ¼ 0, 1 9-R R ¼ NCO, NHC(O)OR, NC(O)OCH2cyclo-(CHC(O)OBF2NH2), NC(O)OCH2CH (NH2)C(O)OH, NHC(O)R0 ; R0 ¼ NHCH2Ph, cyclo-(NCH2CH22 2O2 2CH2CH2), cyclo-N (CH2)5, NEt2
Continued
10.2 Synthesis and structure
557
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References
9-CN 9-[4-S-10 ,20 -C6H3(CN)2] 2O2 2C6H3(CN)2 9-p-C6H42
S S (nucleophilic substitution of NO2), H, IR S
[180] [295] [257]
Boron derivatives PhB(CB10H10CH)2 (FF) 1,7-(BPh2)2
S, IR, MS S, IR, MS
[129] [129]
S S S S
[296] [296] [297] [297]
ESR
[298]
S, H, B, C, P, MS
[299]
S X S S S S S S S, X, H, B, C, P, IR, MS S, H, C, P S, B, C, P
[300] [301] [300] [300] [300] [300] [300] [300] [302] [303] [304]
S S
[182] [182]
S
[182]
S, H, B, C, P, MS, IR
[305]
S, X, H, B, C, MS, IR S [UV irradiation of Hg(C2B10H11)2 in P(OMe)3], H, IR S
[305] [306]
Phosphorus derivatives 1-P(OH)2 phosphonous acid 1-P(O)(OH)2 phosphonic acid 5CMe, Ph 1-PMe(OEt)-7-R R ¼ H, Me, CH25 1-[PMe(OEt)5 5N-C6H4-p-NO2]-7-R R ¼ H, Me, CH25 5CMe, Ph 1-PMe(OEt)R• R ¼ OCMe3, Me phosphoranyl radicals 1-(MePh2P)þI selective targeting of mitochondria for BNCT 1,7-(PPh2)2 1,7-(PPhCl)2 1,7-(PCl2)2 1,7-(P(OMe)2]2 1,7-(PPhOMe)2 1,7-[P(NMe2)2]2 1,7-[PPh(NMe2)]2 1,7-{P*Br[N(CHMe2)2]}2 chiral 5PSiMe3 1-C(OSiMe3)5 1,7-{CH2OP[(m-O)2binaphthyl]}2 chiral diphosphites for Rh-catalyzed asymmetric hydrogenation of dimethyl itaconite (HCB10H10C2 2CH2O)3P phosphite (FF) 2CH2O)PCl2 (HCB10H10C2 phosphorodichloridite (FF) (HCB10H10C2 2CH2O)2PCl phosphorodichloridate (FF) 1,7-{P(X)[OCH2-cyclo-C5H5(OH)4O]2}2 X ¼ O,S water-soluble glycophosphonates for BNCT 1,7-[P(NMe2)2]2 9-P(O)(OMe)2 phosphonate
[307] Continued
558
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb Sulfur derivatives 1-SH 1-SH-n-X n ¼ 9, 10; X ¼ Cl, Br n-SH n ¼ 1, 4, 9 1,7-R2 R ¼ SH, SPh, S(p-C6H4Me) 1,7-(SMe)2-9,10-X2 X ¼ H, Br 1,7-(SPh)2 1-[S(O)Ph]2-7-Ph 1-SH-7-R R ¼ H, Me 1-SH-7-Ph (RCB10H10CS)2 R ¼ H, Me (FF) 1-SO3H-7-R R ¼ H, Me 1-SX-7-R R ¼ H, Me; X ¼ Cl, Br 1-CMe5 5S 1-CH2SH 1-R R ¼ C(5 5S)SMe, CH2SH 2 1-(terpyridine)PtSCH22 C2 B10 H11 þ OSO2 CF3 (FF) 2Ph]2 sulfoxide (FF) [S(O)2 2CB10H10C2 1,7-[S(O)2Ph]2 sulfone 2CH2O)2SO sulfite (FF) (HCB10H10C2 1,7-(SCl)2 1,7-R2 R ¼ SO2Cl, SNH2, SOEt, SCN, SH, S[cyclo-CH(Cl)CH5 5CH(CH2)4CH2 2] 1,7-(C4H3S)2 thiophene precursor to conducting polymers with high electrochemical and thermal resistance 2C4H3S)2 thiophene (FF) p-C6H4(CB10H10C2 2] n ¼ 6-8 1,7-cyclo-[2 2S(CH2)nS2 (C2B10H10)2{1,7-cyclo-[S(CH2)nS-]}2 n ¼ 6-8 (FF) 1-SCl-7-SOEt 1,7-R0 C2B10H9-9-S2 2CH2CH22 2P(5 5X)RR0 R ¼ Me, Ph, EtO; R0 ¼ EtO, p-NO2C6H4O, Ph, 1,2-C2B10H11; X ¼ O, S esters, thioesters (FF) 1-R-7-CH2O3SMe R ¼ H, Me methyl esters of sulfonic acids 1-CPhH(OSO3C6H4Me)-7-C6H4R R ¼ H, CF3, Me, OMe, NMe2 tosylates 1-C(OTs)C6H4X X ¼ H, CF3, F, OMe OPh, Me2 tosylates
Informationc
References
S, H, B, pKa X-ray photoelectron S, H, B, pKa MS, pKa, dipole moment S S, B (antipodal shielding), C S S, IR S S, H, B, C, IR, MS S S S S, H, B, C S, H, B, C S, X, H, B, C, MS S, X, H, B(2d), C, MS
[308] [309] [308] [176] [310] [20] [311] [311] [312] [117] [312] [312] [312] [313] [313] [314] [314]
S, S, S, S, S,
[311] [311] [182] [315] [315]
IR IR IR MS, IR H, IR
S, E, UV, TGA,
[316]
S, H, C, MS S, X(n ¼ 7, 8), H, B S, X(n ¼ 7), H, B
[317] [86] [86]
S S
[315] [318]
S
[319]
S, rate constants, electronic effects of substituents
[320]
S, solvolysis; mechanism of hydrolysis; Hammett plots; electron-donating effects of substituents
[321] Continued
10.2 Synthesis and structure
559
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References
1-benzyl p-toluenesulfonates tosylates 1-S[cyclo-PO2(C20H12)2] chiral thiophosphite 9,n-(SH)2 n ¼ 5, 10 9-R R ¼ SO2H, SMe, SO2Me, SO2Cl 9-SO2Ph 9-SCH2C(O)OH 9-X X ¼ SH, SMe 9-SH
hydrolysis kinetics S, H, B, P S S S MS, pKa, dipole moment X-ray photoelectron S, H, B S X S (electrophilic sulfuration with S2Cl2), H, B S S, H, B S
[322] [323] [324] [325] [180] [176] [309] [326] [327–329] [330] [326] [331] [326] [331]
S, H, B S S
[326] [325,329,332] [333]
S S
[334] [334]
S, IR (actual spectrum) S
[38] [152]
S S, F S S S S, IR pKa S H, B, F IR S, H, B, F S ESR S (from F2), IR (actual spectrum) S (from F2), IR (actual spectrum) S (from F2)
[335] [100] [336] [336] [156] [273] [157] [179,337–339] [179, 368] [179] [339] [340] [341] [38] [38] [342]
9-SO2Me sulfonyl 9,10-SR2 R ¼ H, Me 9,10-cyclo-[2 2S-CPhH-S2 2] 2] R ¼ H, 9,10-cyclo-[2 2S-CRR0 -S2 Me; R0 ¼ Ph, Me heteroacetals (C2B10H11-9-)2S2 disulfide (FF) (RR0 C2B10H9-9-)2S2 R, R0 ¼ H, Me disulfide (FF) (2 2CB10H10CF2 2S2 2)n polysulfide (FF) 2S2 2)n polydisulfide (FF) (2 2S2 2CB10H10C2 Fluoro derivatives 1,7-F2 1,7-R2 R ¼ CMe(CF3)OH, C(CF3)2OH, C(CF3)(CF2Cl)OH 1-R-9-F R ¼ H, Me 1-CH2F 1-CF5 5CFC(O)OH-7-Me 1-CF5 5CFR-7-Me R ¼ H, F 2-F
9-F
9,10-F2 B-CF(CF3)-C•(CF2CF3)[CF(CF3)2] radical B-F10 F12
Continued
560
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb Chloro derivatives 1-Cl
1-R R ¼ Cl, CH2Cl 1-Cl-7-Me 1-Cl-7-R R ¼ H, Me 1,7-Cl2
1,7-Me2-5,12-Cl2 3,9,10-Cl3
n,9,10-Cl3 n ¼ 3, 4 3, 8, 9, 10-Cl4 n-Cl n ¼ 4, 5, 9 9-Cl
9-ClPhþ BF4 9-ClRþ BF4 R ¼ Ph, C6H4F 10-Cl 1-Me-9-Cl 1-Me-7-R-9-Cl R ¼ H, Me 1-R-9,10-Cl2 R ¼ H, Me 9,10-Cl2
Informationc
References
IR (C2 2H; H-bonding with solvents) H (C2 2H shift, coupling constants) Dipole moment E (reduction; comparison with 1,2- and 1,12-C2B10H12 derivatives) H, B, IR (substituent effects) 35 Cl NQR S (thermal cage isomerization) S H, B, IR (substituent effects) ED IR (actual spectrum) S (thermal isomerization of 1,7-Me29,12-Cl2-1,2-C2B10H12), X H, B, IR (substituent effects) H (C2 2H shift, coupling constants) H S H (C2 2H shift, coupling constants) H S S, H, B, C, F, IR S S, IR (actual spectrum) H IR (C2 2H intensity) E B (comparison with 1,2- and 1,12C2B10H12 derivatives) He photoelectron spectra 35 Cl NQR, polarity of B2 2Cl bond Dipole moment S, H S H (C2 2H shift, coupling constants) S IR (C2 2H intensity) H IR (C2 2H intensity) S
[343] [24] [344] [83] [22,23] [345] [102] [346,347] [22] [348] [39,347] [349] [23] [24] [350] [351] [24] [350] [352] [179] [338,352–357] [358] [350] [39] [114] [359] [360] [359,361,362] [363,364] [365] [145] [24] [354] [39] [366] [39] [353,355,358] Continued
10.2 Synthesis and structure
561
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
1-Me-7-R-9,10-Cl2 R ¼ H, Me 1-R-9,10-Cl2 R ¼ H, Me 9,10-Cl2 9,12-Cl2
1-Me-9,10-Cl2 1,7-Me2-9,10-Cl2 3,9,10-Cl3 4,9,10-Cl3 H2C2B10H7Cl3 (FF) H2C2B10H7Cl3 (2 isomers) (FF) H2C2B10H6Cl4 (FF)
4,8,9,10-Cl4 H2C2B10Cl8Br2 (FF) H2C2B10Cl10 (FF)
(m/p-C6H4F)HC2B10Cl10 (FF) R2C2B10Cl10 R ¼ H, D (FF) R2C2B10Cl10L R ¼ H, D; L ¼ dioxane, Ph3PO, pyridine, DMSO (FF) 1-R-HC2B10Cl10X R ¼ Me, Et; X ¼ Et3N, Me3N, pyridine (FF) C2B10Cl12 (FF)
Informationc
References
B (antipodal shielding), H, C Dipole moment IR (actual spectrum) H 35 Cl NQR H (C2 2H shift, coupling constants) S H B IR (C2 2H; H-bonding with solvents) S S 35 Cl NQR X Dipole moment IR (C2 2H intensity) S S 35 Cl NQR E Dipole moment S S, B S, IR (detailed) S X IR, Raman IR (C2 2H intensity) IR (H-bonding with bases; solvent effects) IR (CH frequency vs. other carboranes) pKa 35 Cl NQR, polarity of B2 2Cl bond Dipole moment S, F IR, Raman (C-H X bonds) IR, Raman (C-H X bonds)
[20] [363,364] [358] [366] [361] [24] [357] [350] [367] [343] [354] [354] [361] [368] [364] [39] [354] [354] [361] [50] [364] [369] [27] [370] [371] [372] [45] [39] [373] [34] [34,374] [361] [364] [142] [375] [375]
IR (C-H N bonds)
[376]
(Thermal isomerization of 1,2-C2B10Cl12)
[377] Continued
562
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb Bromo derivatives 1-Br
1-Br-7-Me 1-Me-7-R-9-Br R ¼ H, Me 1-Me-7-R-9,10-Br2 R ¼ H, Me 1,7-Me2-7-R-9,10-Br2 R ¼ H, Me 1,7-Me2-9,10-Br2 R ¼ H, Me 1-Me-9-Br (C2B10H11-9-)2Brþ BF4 bromonium salt (FF) C2B10H11-9-BrPhþ phenylbromonium salt (FF) 2-(CH2)4Br 9-Br
9-BrPhþ 9-BrRþ BF4 R ¼ Ph, C6H4F 1,7-Br2
B-Br2 9,10-Br2
Informationc
References
S, IR (actual spectrum) E (reduction; comparison with 1,2- and 1,12-C2B10H12 derivatives) H, B, IR (substituent effects) H (C2 2H shift, coupling constants) IR (C2 2H intensity) S H, B, IR (substituent effects) H H S S, B(antipodal shielding), C S S X S
[347] [83]
S, H, B, C, MS E S, H, B, C, IR S H, B (antipodal shielding), C H B (comparison with 1,2- and 1,12-C2B10H12 derivatives) IR (actual spectrum) Dipole moment He photoelectron spectra S, H S Dipole moment S H, B, IR (substituent effects) IR (actual spectrum) S S 211B/10B quadrupolar-induced H (variable T; 1H2 decoupling) H (C2 2H shift, coupling constants) H X
[22] [24] [39] [378] [22] [366] [366] [369] [20] [354] [379] [380] [180] [186] [114] [179] [180,338,354, 356–358,381] [20] [350] [359] [39,358] [363,364] [360] [365,379] [145] [68] [378] [22] [39] [370] [354,357,358,382] [383] [24] [350] [384] Continued
10.2 Synthesis and structure
563
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
1-Me-9,10-Br2 1,7-(SMe)2-9,10-Br2 H2C2B10Br2H8 (FF) H2C2B10Br3H7 (FF)
H2C2B10Br3H7 (2 isomers) (FF) 1-Me-4,9,n-Br3 n ¼ 10, 12 MeHC2B10Br3H7 (FF) H2C2B10Br4H6 (FF)
MeHC2B10Br4H6 (FF) MeHC2B10Br5H5 (FF) H2C2B10Br6H4 (FF)
H2C2B10Cl8Br2 (FF) H2C2B10Br10 (FF) Iodo derivatives 1-I
n-I n ¼ 2, 9 1,7-I2
1-I-7-Me 1-I-7-CH2Cl
Informationc
References
B B (antipodal shielding), H, C IR (actual spectrum) Dipole moment S IR (C2 2H intensity) S, B (antipodal shielding), C S X S IR (actual spectrum) Dipole moment S S, X(n ¼ 12) S S IR (actual spectrum) E S IR (C2 2H intensity) S IR (C2 2H intensity) IR (actual spectrum) S Dipole moment
[367] [20] [39,358] [363,364] [354] [39] [20] [369] [385] [369] [39] [363,364] [354] [386] [351,354] [355] [39] [50] [351] [39] [355] [39] [39] [369] [363,364]
S, IR (actual spectrum) H, B, IR (substituent effects) E (reduction; comparison with 1,2- and 1,12-C2B10H12 derivatives) Dipole moment 75 Br labeling using Pd-catalyzed H exchange for BNCT S S, E H, B, IR (substituent effects) IR (actual spectrum) E (reduction; comparison with 1,2- and 1,12C2B10H12 derivatives) ED H, B, IR (substituent effects) E (reduction; comparison with 1,2- and 1,12C2B10H12 derivatives)
[347] [22] [104] [344] [387] [347,378] [388] [22] [347] [104] [389,390] [22] [104] Continued
564
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References
1-I-7-Ph [RR0 C2B10H9-9-I] • (FF) radical anions (R, R0 ¼ H, Me) (FF) 9-I
S MS (electron resonance capture mass spectra)
[110] [72]
S, H, B, F, IR S, IR (actual spectrum) S
[179] [358] [180,338,354, 356–358,381] [350] [24] [359]
9-125I 1-Me-7-R-9-I R ¼ H, Me 9-IR2 R ¼ Cl, O2CCF3 9-IRþ BF4 R ¼ Ph, C6H4F 9-IPhþ X X ¼ I, BF4 9-IPhþ BF4 9-IPhþ X X ¼ BF4, Cl, I 9-IRþ R ¼ Ph, 4-anisyl, 4-C6H4F, 3-O2NC6H4, 2,4,6-C6H2Me3 aryliodonium salts 9-IRþ R ¼ OCH2Ph, N3,N5 5PPh3, NH2, p-O2SC6H4Me, O2SPh, NCS, SCN, NHMe 9-IRþn R ¼ Ph, p-C6H4OMe, p-C6H4F, m-C6H4NO2, C6H2Me3 diaryliodonium salts 9,10-I2
1-Me-7-R-9,10-I2 R ¼ H, Me
H H (C2 2H shift, coupling constants) B (comparison with 1,2- and 1,12-C2B10H12 derivatives) IR (actual spectrum, C2 2H intensity) E He photoelectron spectra 127 I NQR Dipole moment S (radiolabeling via Pd-catalyzed isotopic exchange) H S S S X 127 I NQR quadrupole coupling S
[39] [50,114] [360] [391] [363,364] [392] [366] [393] [145] [393] [394] [395] [396]
S
[397]
S
[398]
S S (high yield), MS X (two different forms) H (C2 2H shift, coupling constants) H B B (shift re-assignments; BH coupling) C IR (actual spectrum) E Dipole moment H
[357,358,382] [94] [94,394] [24] [94,350] [94,367] [399] [94] [358] [114,400] [363,364] [366] Continued
10.2 Synthesis and structure
565
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References
1-Me-9,10-I2 4,5,6,8,9,10,11,12-I8
S S, H, B. C, IR, MS
[355] [401]
Exo-polyhedral main-group metal and metalloid element derivatives Magnesium and calcium 2C(O)S [2 2O(O)CR2 2NH(O)C2 2CB10H10C2 NHR2 2C(O)O-M2þ 2 2]n M ¼ Mg, Ca; R ¼ CHMe, (CH2)m, m ¼ 2, 6, 7, 9 oligomeric salts (FF) 2C(O)O2 2 M2þ2 2]n S, COND, thermal and electrophysical properties [2 21,7-O(O)C2 2CB10H102 M ¼ Mg, Ca, oligomeric salts (FF) 1-Me-7-MgI Grignards S Aluminum, gallium, and indium (HCB10H10C)nAlH4nM n ¼ 1, 2; M ¼ Li, Na (FF) Thallium Tl½OCðOÞO2 2þ ½Me2 C2 B10 H9 2 2 (FF) 1-TlClC4H9Cl-7-R R ¼ H, Ph 1-R-7-R0 -B-Tl(O2CCF3) R, R0 ¼ H, Me ClTl(CB10H10CR)2 R ¼ H, Ph, CH2Cl (FF) 1,7-Me2-9-Tl(O2CCF3)2 1,7-R2-9-TlSn{CH[MeC(O)O]2}2Br2 R ¼ H, Me TlX(B-C2B10H11)2 X ¼ Cl, Br (FF) B-TlX2 X ¼ Cl, Br 1,7-Me2-B-TlBr 1,7-Me2-B-TlCl 1,7-RR0 -B-Tl(O2CCF3) R, R0 ¼ H, Me, Ph 9-Tl(CO2CF3)2 9-TlRX R ¼ R ¼ H, Me, n-C4H9; X ¼ Cl, Br, I 9-Tl(SCN) 9-Tl(SCN)2 9-Tl(Cl)M(CO)5 M ¼ Mn, Re 9-Tl(Cl)(C5H3R)Mn(CO)3 R ¼ H, CH2NMe2 9-Tl(OCOCF3) 9-Tl(2,20 -bipyridine)[O(O)CCF3]2 Silicon 1-SiMe3
[402]
[403,404] [405]
S
[406]
S S S, kinetics of thallation S S S
[407] [408] [409] [408] [410] [411]
S, S, S, S, S S S S S S, S, S, X
[412] [412] [412] [412] [413] [414] [415] [415] [415] [416] [417] [179] [418]
IR, Raman IR, Raman IR, Raman X, IR, Raman
B, C, IR IR H, B, C, F, IR
S B(substituent effects), H, IR C(H) detailed assignments IR (C2 2H intensity)
[419] [21] [31] [39] Continued
566
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References
1-SiMe3-7-R R ¼ H, SiMe3, SiMe2Cl, SiMe2OH, SiMe2OMe, SiMe2OSiMe2Cl, SiMe2OSiMe2OH 1-SiMe2CMe3-7-R R ¼ H, Me (1,7-C2B10H11)2SiMe2 (FF) O[SiMe2CB10H10CSiMe3]2 (FF) 1-SiMe2OR R ¼ H, Me 1-SiMe2R R ¼ H, Me, OMe, OH, Cl 1-SiMe2R R ¼ H, OMe, OEt, Cl, OSiMe2OC(O)OH, SiClMe2 1,7-[CH2SiMe2OH]2 1,7-[2 2C5 5CH2 2SiMe2OSiMe2]2 dendrimers with polyhedral oligomeric silsesquioxane (POSS) units 1-SiMe)OR-7-R0 R ¼ H, SiMe2OH; R0 ¼ H, SiMe3 1-SiR3-7-R R ¼ H, SiR3 1-OSiR3-7-R R ¼ H, OSiR3 siloxanes 1-cyclo-[(CH2)3SiMe]-7-Me [1-Me-1,7-C2B10H10]2[cyclo-SiMe(CH2)3] (FF) 1-CH2SiMe2OMe-7-R R ¼ H, CH2SiMe2OMe 1-SiMe2R-7-SiMeR0 R00 R ¼ H, OMe, OEt; R0 ¼ Me, (CH2)2CF3; R00 ¼ H, Cl, OMe 1-SiMeRR0 -7-SiMeR00 OSiMeRR0 R, R00 ¼ Me, Ph, (CH2)2CF3; R0 ¼ H, OMe, OEt,OH 1-CH2SiMe2OSiMe3-7-Me 5PSiMe3 1-C(OSiMe3)5 1,7-(SiR2H)2 R ¼ Me, Ph 1,7-R2 R ¼ SiMe2Cl, SiMeCl2, SiPh2Cl, SiPh2Me, SiMe2Ph, SiMe2OH, SiMe(OH)2, SiPh2OH, SiMe2OMe, SiPh2OMe, SiMe2NH2 5CH2}2 1,7-{O[SiMe2]2CH5 1,7-[(CH2)3SiMe2Cl]2 1,7-[(CH2)3SiMe2(OMe)]2 2] 1,7-cyclo-[2 2(CH2)3SiMe2-O-SiMe2(CH2)32 1,7-[(SiMe2-O)n(SiR2R0 )]2 R ¼ Me, Ph; R0 ¼ Me, Ph, p-C6H4Cl, 1,7-MeC2B10H10, 1,2C2B10H10; n ¼ 1-3 (FF) 1,7-[(SiMe2O)2(SiMe3)]2-9-Cl 1,7-[(SiMe2R]2 R ¼ H, OMe, OEt 1,7-[SiMe2OR]2 R ¼ SiMe2Cl, SiMePhCl, Si (CH5 5CH2)MeCl, SiMe2H, SiMe2(OMe), SiHMeOC(O)Me, SiMe2OC(O)Me disiloxanyl
S
[420]
S, H, B, C, IR, MS S S S H, MS H, MS
[421] [422] [420] [420] [423] [423]
X S, IR, TGA
[424] [425]
IR (I inductive effect)
[426]
IR (detailed study) IR (detailed study) S S S
[427] [427] [428] [428] [429,430]
S
[431]
S
[432]
S S, H, C, P S, H, C, Si, IR, S
[433] [303] [434] [435]
S S S S S, H
[436] [437] [437] [437] [438]
S, H S S
[438] [439] [440]
Continued
10.2 Synthesis and structure
567
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References
1,7-R2 R ¼ SiMe2OEt, SiMe2OMe, SiMe2OPh 1,7-[SiR2(OMe)]2 R ¼ Me, Et 1,7-[SiMe2(OH)]2 1,7-[SiMe2OEt]2 2O2 2SiMe22 2R]2 R ¼ CH5 5CH2, 1,7-[SiMe22 CH hydrosilylation via Karstedt catalysis C ! elastomeric network polymers 1,7-[CH2SiMe2Cl]2 1,7-[SiMe2Cl]2 1,7-(SiPh2R)2 R ¼ Cl, Me 1,7-(SiPhMe2)2 5CH2)-1,7-C2B10H11}2 (FF) {(SiPh)2]2C(5 1,7-{[SiMe2O][1,2SiMe2C2B10H11]}2C2B10H10 (FF) 1-{[SiMe2O]2Si(CH2)3}-7-Me [CB10H10C-SiMe3]2 (FF) {[(C2B10H11-1-C[O]O)(n-C4H9)2Si]2O}2 anti-tumor compound (FF) 2]n R ¼ Me, Et, n-C4H7, 2 2[CB10H10C-SiR22 Ph (FF) polymers C2 C2 [2 2C 2C6H42 2C 2SiPhC2 (CH5 5CH2)]x[C 2SiPh(CH2CH2SiMe22 2CB10H10C2 2SiMe2C2 C}2 CH2CH22 2SiPh{C 2C6H42 2C 2)]y (FF) silylene-carborane polymers C2 C2 2 2 2[2 2C 2C 2SiMe22 O2 2SiMe22 2CB10H10C2 2SiMe22 2O2 2 SiMe22 2(CB10H10C2 2SiMe22 2O2 2SiMe2)z2 2]2 2 (z ¼ 1, 3, 8) (FF) diacetylene-disioxane polymers 2O)22 2SiMe22 2CB10H10C]x 2 2{[(SiMe22 C2 C2 2 2(SiMe22 2O)22 2SiMe22 2C 2C 2}n (FF) diacetylene-disioxane polymers 2 2{CB10H10C2 2[(SiMe22 2O)22 2SiMe22 2 CB10H10C]x2 2(SiMe22 2O)22 2 C2 C2 SiMe22 2[C 2C 2(SiMe22 2O)22 2 SiMe2]y}n (FF) diacetylene-disioxane polymers 2 2[Me2Si-CB10H10C(SiMe2O]m2 2]n m ¼ 1-3 (FF) siloxane copolymers 2(SiMe2O)2SiMe22 2]n (FF) siloxy [2 2CB10H10C2 polymers {2 2Me2Si2 2CB10H10C[SiOMe2]m2 2}n m ¼ 3, 5 (FF) siloxy polymers, MW ¼ 16,000-30,000 (soluble waxes and liquids)
S
[422]
S S, reaction mechanisms S DSC, TGA
[441] [441] [442] [443]
S (thermal isomerization of o-carboranyl isomer) S S S S S, H
[444] [445] [446] [446] [447] [438]
S, H S X
[438] [187] [448]
S, IR, molecular weight
[449]
S, C (solid), Si (solid), B (solid)
[450]
S, H, C, IR, DSC, TGA (effect of concentration dilution of cross-linkable diacetylenes on plasticity)
[451]
S, H(solid, var. temp.), C, IR, DSC, TGA (dependence of thermal properties on copolymer sequence) S, H(solid, var. temp.), C, IR, DSC, TGA (dependence of thermal properties on copolymer sequence)
[452]
S, TGA, DTA
[453]
Ultrasound propagation; glass transition
[454]
S
[455]
[452]
Continued
568
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References
2CB10H10C[SiOMe2]22 2 {2 2Me2Si2 [SiOMePh]y(SiOMe2)2 2}n (y ¼ 1, 2, 3) (FF) siloxy polymers C2 C2 2C6H42 2SiMe2]x2 2 [2 2C 2C 2SiMe22 C2 C2 [C 2C 2SiMe22 2O2 2SiMe2 2 2CB10H10C2 2SiMe22 2O2 2SiMe22 2]y (FF) (linear diacetylene polymers; disiloxanes; 5 compounds; x ¼ 0-100, y ¼ 0-100) 2O2 2SiMe22 2CB10H10C2 2 2 2[SiMe22 SiMe22 2O2 2SiMe22 2(C5H4)2 2Fe(C5H4)2 2 SiMe22 2O2 2SiMe22 2CB10H10C2 2 C2 C2 2O2 2SiMe22 2C 2C 2]n siloxylSiMe22 ferrocene polymer (FF) C2 PhC 2[2 2SiMeH2 2 CPh CB10H10C2 2]n2 2SiMeH2 2C phenylacetylene-terminated silane polymer (FF) 5CMe2 B-CH2CH2SiR3 R ¼ Me, Cl, Et,ON5 B-CH2SiCl3 B-CH2CH2SiMe3 B,B0 -(CH2CH2SiMe3]2 B-CH2CH2SiR3 R ¼ Cl, Me B,B0 -(CH2CH2SiR3)2 R ¼ Cl, Me B-(CH2)2Si(OOR)3 R ¼ CMe3,CMe2Et, 2-cyclohexylisopropyl peroxy 9-C(O)OSiMe3 9-CH2SiMe3 CSiMe3 9-C 9-(CH2)2Si[(CH2)2O]3N silatrane
S
[455]
S, H, C, IR, DSC, TGA
[456,457]
S, H, C, IR, DSC, TGA
[458]
S, H, C, Si, IR, gel permeation chromatography, TGA
[459]
IR (detailed study; inductive effect) IR (detailed study; inductive effect) MS (detailed) MS (detailed) S S S
[460] [460] [461] [461] [462] [462] [258]
S, pKa S S X
[199] [463] [97] [464]
S B (substituent effects), H, IR C(H) detailed assignments S S S S
[419] [21] [31] [465] [466] [466] [466]
S, IR, molecular weight
[449]
S, IR S, IR
[467] [467]
S, IR
[467]
Germanium 1-GeMe3
1-HgGeEt3-7-R R ¼ H, Me, CH2Cl, Ph 1,7-(GeR2Cl)2 R ¼ Ph, Cl 2]n polymer (FF) 2 2[GeMe2-CB10H10C2 2SnMe)22 2CB10H10C]n 2 2[GeMe2-CB10H10C2 polymer (FF) 2GeR22 2]n R ¼ Me, Et, 2 2[CB10H10C2 n-C4H7, Ph (FF) 9-HgGe(C6F5)3 HCB10H10C-9-HgGe(C6F5)2Ge(C6F5)2-9CB10H10CH (FF) 9-HgPt[PPh3]2Ge(C6F5)3
Continued
10.2 Synthesis and structure
569
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb Tin 1-SnMe3
1-SnEt3 1-SnR3 R ¼ H, Me 1-SnCl2 SnMe3-CB10H10C-7-Hg-CB10H10CSnMe3 (FF) 1,7-(SnR3)2 R ¼ n-C4H9, Ph 1,7-[SnPh2Cl]2 (R0 CB10H10C)2SnR2 R ¼ Me, Ph; R0 ¼ Me, Ph (FF) 1-R-7-R0 R, R0 ¼ SnEt3, Sn(n-C4H9)3, SnMe3 1-SnR3-7-Ph R ¼ Me, Ph 1-SnEt3-7-R 1,7-[SnMe3]2-10-(CH2)2SiMe3 1-[(CH2)nC(O)O SnMe3 þ ]-7-R n ¼ 0, 1; R ¼ Me, Ph 1,7-[(CH2)nC(O)O SnMe3 þ ]2 n ¼ 0, 1 1,7-(SnR3)2-B,B0 -[CH2CH2SiMe)3]2 R ¼ Me, Et [1-SnMe)3]-7-SnEt3-B,B0 -[CH2CH2SiMe3]2 2C(O)O2 2Sn(OH)2 2]n [2 2OC(O)2 2CB10H10C2 (FF) 2CB10H9BrC2 2]n polymer (FF) 2 2[SnMe22 2[1,7-CB10H10C]2 2SnMe)22 21, 2 2{SnMe22 12-CB10H10C]}n polymer (FF) 2 2[GeMe22 2CB10H10C2 2SnMe)22 2 CB10H10C]n polymer (FF) 2SnR22 2]n R ¼ Me, Et, n-C4H7, 2 2[CB10H10C2 Ph (FF) (B-C2B10H11)2SnMe2 (FF) 2-CH2CH2SnEt3 9-SnCl2 C2B10H11-9-Sn{CH[MeC(O)O]2}2-Hg-Sn{CH [MeC(O)O]2}2-9-C2B10H11 (FF) 9-SnCl2[O,O0 -3,6-[CMe3]2-o-simiquinolate] 9-SnCl3-1,7-R2 R ¼ H, Me SnCl2(9-R2C2B10H9)2 R ¼ H, Me (FF) [CB10H10C-SnMe3]2 (FF)
Informationc
References
S B (substituent effects), H, IR C(H) detailed assignments S H(C2 2H shift), 119Sn (coupling constants) 117 H ( Sn and 119Sn coupling constants, solvation) IR, Raman B (substituent effects), H, IR
[419,468] [21] [31] [469] [24] [470] [471] [21]
S S S
[472] [472] [472]
S, IR S 119 Sn g-resonance spectra; quadrupole splittings; acceptor properties of cage X Mo¨ssbauer, pKa, E (half-neutralization potential)
[473] [474] [475]
Mo¨ssbauer, pKa, E (half-neutralization potential) IR (detailed study; inductive effect)
[226] [460]
IR (detailed study; inductive effect) S, IR, molecular weight, COND, TGA, DTA
[460] [477]
S S
[466] [466]
S
[466]
S, IR, molecular weight
[449]
IR, Raman S S S
[471] [478] [332] [479]
S, H S S S
[480] [481] [481] [187]
[476] [226]
Continued
570
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References
{[(C2B10H11-1-C[O]O)(n-C4H9)2Sn]2O}2 anti-tumor compound (FF) 2NH(O)C-CB10H10C2 2C(O) [2 2O(O)C(CH2)m2 2C(O)O-Sn2þ2 2]n m ¼ 2, 6, 9 NH(CH2)m2 oligomeric salt (FF)
X
[448]
S
[402]
H S, IR, molecular weight
[470] [449]
X
[448]
S
[402]
S S Raman
[482] [332,482–484] [485]
S Raman
[332,483] [485]
S S S MS S S
[486] [487–489] [487–490] [490] [331] [331]
S, MS
[490,491]
S, B S, MS S S S MS
[492,493] [490] [492,493] [493] [490,491] [490]
Lead 1-PbMe3 2]n R ¼ Me, Et, n-C4H7, 2 2[CB10H10C-PbR22 Ph (FF) {[(C2B10H11-1-C[O]O)(n-C4H9)2Pb]2O}2 anti-tumor compound (FF) [2 2O(O)C(CH2)m2 2NH(O)C2 2CB10H10C2 2C(O) NH(CH2)m2 2C(O)O-Pb2þ2 2]n m ¼ 2, 6, 9 oligomeric salt (FF) Arsenic 1-R-7-CHMe2 {R ¼ As[NEt2]2, AsCl2, AsO} 9-AsCl2 Antimony 9-SbCl2 Selenium Se2(7-RC2B10H10)2 R ¼ H, Me, n-C3H7, Ph (FF) 9-SeH Se2(9-C2B10H11)2 (FF) 9,10-(SeR)2 R ¼ H, Me 9,10-cyclo-[-Se-CRR0 -Se-] R ¼ H, Me; R0 ¼ Ph, Me HgSe(9-C2B10H11)2 (FF) Tellurium Te2(9-C2B10H11)2 (FF) 9-TeMe 9-TeX X ¼ Cl, Br HgTe(9-C2B10H11)2 (FF)
Exo-polyhedral transition metal derivatives Yttrium and lanthanide elements 2C(O)O2 2Y2 2]n (FF) S, IR, molecular weight, COND, TGA, DTA [2 2OC(O)2 2CB10H10C2 S, H, IR Cp*2Yb1,7-C2B10H12 inclusion compound (FF)
[477] [494] Continued
10.2 Synthesis and structure
571
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References
Titanium, zirconium, and hafnium 2CH2]2TiCp (FF) [Me—CB10H10C2 Cl2Ti(10-C2B10H10-9-I)2 (FF)
S S, B
[495] [496]
S S S, H MS (fragmentation study)
[497] [497] [498] [499]
S, ESR
[500]
S X S S S, S, S, S,
[501] [502] [503] [503] [416] [417] [504] [477]
Chromium, molybdenum, and tungsten 1-PhCr(CO)3-7-R R ¼ Me, Ph 1,7-[PhCr(CO)3]2 1-(Z6-C7H7)Cr(CO)3-7-Me 1-R-7-R0 R ¼ CH2PhCr(CO)3, PhCr(CO)3; R0 ¼ H, Ph, CH2Ph Manganese, technetium, and rhenium 1-[cyclo-(CO)2(CF3)M(CO)4]-7-Me M ¼ Mn, Re 1,9-cyclo-[N5 5NPh-Re(CO)4]-7-Me 1,3-cyclo-[(CH2NMe2)Re(CO)4]-7-Ph 9-Re(CO)5 9-C(O)Re(CO)5 9-Tl(Cl)M(CO)5 M ¼ Mn, Re 9-Tl(Cl)(C5H3R)Mn(CO)3 R ¼ H, CH2NMe2 9-(C5H4)Mn(CO)3 2C(O)O2 2Mn2 2]n (FF) [2 2OC(O)2 2CB10H10C2 Iron 1-Fe(CO)2Cp 1,7-[Fe(CO)2Cp]2 1-Fe(CO)2Cp-7-R R ¼ H, Me, Ph 1-CH(OH)(C5H4)FeCp 1-CH2(CO)Fe(CO)Cp-7-Me 1-CH2(CO)Fe(CO)Cp-7-Me-9,10-Br2 1-(Z6-naphthyl)FeCpþ 1-CH2C5H4FeCp-7-Me CFe(CO)2Cp 1-C 1-C(O)Fe(CO)2Cp 1-C(5 5CH2)C5H4FeCp 1,7-[C(5 5CH2)C5H4FeCp]2 2C(OH)MeC2B10H11]2 (FF) Fe[C5H42 2C(O)O2 2Fe2 2]n (FF) [2 2OC(O)2 2CB10H10C2 5CH2 2C6H4FeCp] 1-[C6H4-p-CH5 9-Fe(CO)2Cp
S, S, S S, S, S, S S S, S, S S, S S S S, S, S,
B, C, IR IR IR IR, molecular weight, COND, TGA, DTA
H, B, C, IR, MS, E H, IR E E X, H, B, C, IR, MS
ESR E H, IR
IR, molecular weight, COND, TGA, DTA UV, E, NLO [b (hyperpolarizability)] H, B, IR
[505] [506] [507] [508] [508] [509] [495] [495] [510] [511] [512] [506] [447] [447] [447] [477] [513] [514] Continued
572
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References
9-C(O)Fe(CO)2Cp 9-C5H4Fe(CO)2Br 9-(C5H4)Fe(CO)3
S, H, B, IR S, H, B, IR S, IR
[514] [514] [504]
S S, IR, UV S, IR, molecular weight, COND, TGA, DTA S, H, B, C, MS
[169] [257] [477] [515,516]
S, H, B, C, MS
[515]
S, H, IR, UV S S, H, B, P
[517] [518] [323]
S, H, IR S, H, IR S S H, IR S, H, IR
[519] [519] [520] [520,521] [521] [522]
S, H
[523]
S, H, IR
[521]
S, H, IR S, H, IR S, H, B, IR
[524] [524] [525]
S S S, IR, molecular weight, COND, TGA, DTA
[526] [495] [477]
S
[512]
S
[527]
Cobalt 9-CH2-C2Co2(CO)6 Co(phthalocyanine)(9-PhO2 2C2B10H10)4 (FF) 2C(O)O-Co2 2]n (FF) [2 2OC(O)2 2CB10H10C2 3-Co(1,2-C2B9H11)(1,2-C2B9H10)-82O(CH2)2-1-(1,7-C2B10H11) (FF) O(CH2)22 1,7-[(CH2)2-O-(CH2)22 2O-8-(1,2-C2B9H10)Co(1,2-C2B9H11)]2C2 B10 H10 2 (FF) Rhodium 1-Rh[PPh3]2-7-R R ¼ Me, Ph 1-Rh[PPh3]2-7-R R ¼ H, Me, Ph 2C2B10H10}þ {(C8H12)Rh(C20H12)2-O2P-S2 BF4 chiral thiophosphite (FF) Iridium 1-Ir(PPh3)2(CO)-7-R R ¼ H, Me, Ph 1-Ir(PPh3)2H2(CO)-7-R R ¼ H, Me, Ph 1-Ir(CO)(RCN)[PPh3]-7-Ph R ¼ Me, Ph 1-IrH2(CO)(RCN)(PPh3)-7-Ph R ¼ Me, Ph 1-Ir(H)(X)(CO)L2-7-R R ¼ H, Me,Ph; X ¼ Cl, Br, I; L ¼ PPh3, PMePh2 1-Ir(H)(CR5 5CHR)(CO)[PPh3]L-7-Ph L ¼ MeCN, PhCN 1-Ir(H)[CH2CH2C(O)OC(O)](CO)(MeCN) 2C] [PPh3]-7-Ph [C2 1-{Ir(H)(CO)(PhCN)[PPh3][CHCH2C(O)OC(O)]} 1-{Ir(H)(CO)2[PPh3][CHCH2C(O)OC(O)]} 2-IrHCl(EPh3)2 E ¼ P, As Nickel 1,7-Ni[PPh3]2 1-CH2Ni[PPh3]Cp-7-Me 2C(O)O2 2Ni2 2]n (FF) [2 2OC(O)2 2CB10H10C2 Palladium and platinum C-1,7-C2B10H11)2 M ¼ Pd, [Ph3P]2M(1-C Pt (FF) 1-CH2NR0 2Pd (Cl)(NC5H4-p-Me)-7-R [N-PdB] R ¼ H, Ph; R0 ¼ Me, Et
Continued
10.2 Synthesis and structure
573
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References
2EPh) E ¼ S, trans-ClPd(PhE2 2CB10H9C2 Se pincer complexes FF 1-cis-Pt[PEt3]2H 1-cis-Pt[PEt3]2H-7-R R ¼ Me, Ph 1-trans-Pt[PEt3]2H-7-Ph 1-cis-Pt[PEt3]2H-7-R R ¼ H, Me, Ph 1-cis-Pt[PPh3]2H-7-Ph 1-cis-[PPh2Me]2PtH-7-Ph 9-PtCl[PPh3]2 1-{Pt[PEt3][cyclo-Et2PCH2CH2]}-7-R R ¼ Me, Ph C2 C-Pt cyclo-{[Et3P]2Pt2 2C 2CB10H10C2 2C [PEt3]2}3½NC5 H42 2CðOÞ2 2NC5 H4 3 6þ hexagonal macrocycle (FF) cyclo-({C(O)O2 2Pt[PEt3]22 2OC(O)}C2B10H10)2 16-membered macrocycle (FF) 1,7-{trans-Pt[PEt3]2X}2 X ¼ I, ONO2 1,7-[cis-Cl2(NH3)Pt(NH2)(CH2)3]2 [1,7-[cis-Cl(NH3)2Pt(NH2) (CH2)3]2C2B10H10]2þ (FF) [1,7-[trans-Cl(NH3)2Pt(NH2) (CH2)3]2C2B10H10]2þ ½OSO2 CF3 2 (FF) 1,7-[(CH2)3SPt(terpyridyl)]2C2 B10 H10 2þ ½OSO3 CF3 2 (FF) 2C60]2 (FF) [(1,7-C2B10H11)[PPh3]2Pt2 1,7-[(CH2)3NH2PtCl2(NH3)]2 DNA in vitro binding for BNCT 1-(terpyridine)PtSCH22 2 C2 B10 H11 þ OSO2 CF3 intercalative DNA binding (FF)
S, X, H, B, Se, MS
[528]
S, S, S, S, S, S, S S,
H, P, IR
[529] [529] [529] [530] [530] [530] [531] [532]
S, H, C, IR
[533]
S, X, H, P
[534]
S, H, C, IR S, H, B, C, Pt, cytotoxicity studies S, H, B, C, Pt
[533] [535] [535]
S, H, B, C, Pt, cytotoxicity studies
[535]
S, H, B, C, Pt, MS, cell toxicity
[536]
S, ESR S
[537] [538]
S, H, B, C, Pt
[313]
S, X, H, B(2d), C, MS S, IR
[314] [467]
S, IR
[467]
S S S
[108] [512] [402]
S, COND, thermal and electrophysical properties
[403,404]
9-HgPt[PPh3]2Ge(C6F5)3-10-R R ¼ H, HgGe(C6F5)3 29-Hg2 2Pt[PPh3]2Ge(C6F5)2Ge C2B10H112 (C6F5)22 2Pt[PPh3]22 2Hg2 29-C2B10H11 (FF) Copper 1-Cu CCu 1-C 2CB10H10C2 2C(O) [2 2O(O)C(CH2)m-NH(O)C2 NH(CH2)m2 2C(O)O2 2Cu2þ2 2]n m ¼ 2, 6, 9 oligomeric salt (FF) 2C(O)O2 2Cu2þ2 2]n [2 21,7-O(O)C2 2CB10H102 oligomeric salt (FF)
H H H H, IR H, IR H, IR
Continued
574
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References
Silver CAg 1-C
S
[512]
S, X(Ph3P), H, P, IR, COND
[539]
S, COND, thermal and electrophysical properties
[403,404]
S
[402]
S, X, H
[280]
ESR Raman S, E S, H B (substituent effects), H, IR S S S H (Hg2 2CH coupling constants variation with substituents) S S S
[540] [541] [508] [542] [21] [543] [543] [187] [544]
S S MS (detailed) Raman S E S H, B, C, F, IR S S S S, IR S, IR
[546] [546] [547] [485] [548] [549] [147,179,548,550] [179] [551] [552] [553] [467] [467]
Gold (R3PAu)2C2B10H10 R3P ¼ Ph3P, Ph2MeP, Ph2(p-MeC6H4) (FF) Zinc and cadmium 2C(O)O2 2M2þ2 2]n [2 21,7-O(O)C2 2CB10H102 M ¼ Zn, Cd oligomeric salt (FF) 2NH(O)C2 2CB10 [2 2O(O)C(CH2)m2 H10C2 2C(O)NH(CH2)m2 2C(O)O2 2Cd2þ2 2]n m ¼ 2, 6, 9 oligomeric salt (FF) 9-CH2[20 -(bipyridyl)ZnCl2] Mercury Hg(C2B10H11}2• radicals (FF) 1-HgX X ¼ Cl, Br, I 1-HgCl-7-R R ¼ H, Me 1-HgMe 1,7-(HgMe)2 1-HgX-7-R X ¼ Cl, Br; R ¼ Me, Ph 2HgMe]2 biscarborane (FF) [CB10H10C2 1-HgCH5 5CMe2-7-R 1-HgGeEt3-7-R R ¼ H, Me, CH2Cl, Ph 1,7-(HgBr)2 (o-phenanthroline)2Hg[C(O)OCB10H10CH]2 (FF) MeCB10H10CHgCl•o-phenanthroline (FF) (MeCB10H10C)2Hg•o-phenanthroline (FF) 1-R-9-HgX X ¼ Cl, Br, I; R ¼ H, Me, Ph 9-HgR R ¼ Cl, Br, Et 9-HgCl 9-HgX 9-HgOC(O)CF3 B-[HgC(O)OCF3]n n ¼ 1-3 B-HgR R ¼ Et, I 9-HgCl•o-phenanthroline 9-HgGe(C6F5)3 9,10-[HgGe(C6F5)3]2
[465] [543] [545]
Continued
10.2 Synthesis and structure
575
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References
HCB10H10C-9-HgGe(C6F5)2Ge(C6F5)2-9CB10H10CH (FF) 9-HgPt[PPh3]2Ge(C6F5)3 9-HgPt[PPh3]2Ge(C6F5)3-10-HgGe(C6F5)3 HCB10H10C-9-HgGe(C6F5)2Ge(C6F5)2Pt [PPh3]2-9-CB10H10CH (FF) 1,7-R2-9-HgR0 R ¼ H, R0 ¼ Me Hg(R2C2B10H10)2 R ¼ H, Me, SiMe3 (FF) Hg(RC2B10H10)2 R ¼ H, SiMe3, GeMe3, SnMe3, CHSiMe2, HgMe (FF) Hg(RC2B10H10)2 R ¼ H, Me, Ph (FF) Hg(C2B10H9-9,10-Cl2)2 (FF) Hg(C2B10H11)2 (FF)
S, IR
[467]
S, IR S, IR S, IR
[467] [467] [467]
Hg Hg B (substituent effects), H, IR
[554] [554] [21]
S, E (pKa) S, E (pKa) Raman E (comparison with other R2Hg) E (electron affinity of B2 2Hg bond) MS (detailed) Raman S, E S
[555] [555] [541] [51] [556] [547] [485] [543] [543]
S S E S S S, MS S, H, B
[543] [219,557] [549] [558] [332] [490,491] [559]
S, S, S, S,
[496] [496] [496] [496]
Hg(C2B10H10-9-R)2 R ¼ H, Me (FF) Hg(n-C2B10H11)2 n ¼ 1, 9 (FF) Hg(RC2B10H11) 2 R ¼ Me, Ph (FF) R0 Hg(RC2B10H11) R ¼ Me, Ph; R0 ¼ Me, Ph (FF) (1,7-PhC2B10H10)Hg(1,2-PhC2B10H10) (FF) Hg(9-C2B10H11)2 (FF) Hg{9-[Me3Si]2C2B10H9]}2 (FF) HgS(9-C2B10H11)2 (FF) HgM(9-C2B10H11)2 M ¼ Se, Te (FF) (9-C2B10H11)Hg(10-nido-7,8-C2B9H10-7-R) R ¼ H, Ph, CHMe2 (FF) 9-Me-10-HgR R ¼ CF3C(O)O, Cl Hg(10-C2B10H10-9-X)2 X ¼ Br, I, Me (FF) C2B10H11-9-ClHg-C2B10H10-10-Me (FF) C2B10H11-9-CF3C(O)OHg-C2B10H1010-X X ¼ Cl, Br, I, Me (FF) C2B10H11-9-Sn{CH[MeC(O)O]2}2-Hg-Sn{CH [MeC(O)O]2}2-9-C2B10H11 (FF) cyclo-{[(t-C4H7)Me2Si]2C2B10H8}3Hg3 (FF) [(tert-C4H7)Me2Si]2C2B10H8-9,10-(HgX)2 [X ¼ Cl, OC(O)CF3] (FF) (HO)2Hg4(C2B10H10)2[CF3C(O)O]2 (FF) Detailed NMR Studies Parent
B B B B
S
[479]
S, X, H, B, C, Hg, MS S, H, B, C, Hg, MS
[560] [560]
S, X
[561]
H (variable T; quadrupole-induced spin relaxation) H (variable T; 1H—11B/10B quadrupolar-induced decoupling)
[562] [383] Continued
576
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References [563]
1-MMe3 M ¼ Si, Ge, Sn
H (proton spin-lattice relaxation time; phase transitions) B (nuclear quadrupole coupling) B (2d B2 2C correlated) B (solid), C (solid) C (2d B2 2C correlated) C (detailed assignments)
Other Experimental Studies 1-Li
Theoretical Studies Molecular and electronic structure calculations Parent
C2B10H11•, C2B10H10•• radicals X@C2B10H12 X ¼ Liþ, Be2 carboraneencapsulated ions (FF) RR0 C2B10H10n n ¼ 0, 2; R ¼ C7H6, B12H11; R ¼ C5H4, H) (FF) n-C(O)OH 1-(m-C6H4-OH)
[564–566] [567] [568] [567] [31]
Reaction with 1,2,4-triazine 4-oxides; nucleophilic substitution of H to form 1,2,4-triazin-5-yl derivatives OR ring transformation of the 1,2,4triazine ring into the triazoline ring
[569]
Intermolecular forces in solid Ab initio, stability Ab initio, thermodynamic parameters Ab initio structural, vibrational Free electron MO calculations MM3 force field Electron delocalization IR, vibrational frequencies C2 2H bond length compared with halomethanes Bond distances (Gaussian 98) Electron affinities, acidity Influence of charge, spin, substituents, and atom encapsulation on volume of cage DFT, Hartree-Fock, MPZ: correlation of frontier MOs and electrostatic potentials with reactivity DFT, isomer stabilities DFT, cage substitution DFT, CASPT2 DFT, stability
[570] [571–574] [575] [576] [577] [578] [579] [580] [581] [19] [571] [582]
[584] [585,586] [587] [588]
b (first hyperpolarizability); NLO
[589]
Electron density distribution from pKa measurements Computer-aided molecular design; atomic point charges; docking into active site of estrogen receptor
[590]
[583]
[591]
Continued
10.2 Synthesis and structure
577
Table 10-1 Selected 1,7-C2B10H12 Derivativesa—Cont’d Compoundb
Informationc
References
1,7-(OH)2 1-SH n-SH n ¼ 1, 4, 9 9-SCH2C(O)OH 1-Cl 1,7-Cl2 9-X X ¼ Cl, Br, I 1-Br 1,7-Br2, 9,10-Br2 H2C2B10X10 X ¼ Cl, Br (FF) 9,10-I2 1-I 9-MCl2 M ¼ As, Sb 9-HgR R ¼ Cl, Br, Et Hg(n-C2B10H11)2 n ¼ 1, 9 (FF) 1,7-[Au(PPh3)]2
DFT Charge distribution EHMO, NEMO; charge distribution EHMO, NEMO; charge distribution Dipole moment Ab initio structural Dipole moment Dipole moment Dipole moment Dipole moment Dipole moment Dipole moment Vibrational frequencies Raman (vibrational frequencies) Raman (vibrational frequencies) DFT
[170] [309] [176] [176] [344] [348] [363] [344] [68,363] [68,363] [363] [344] [485] [485] [485] [62]
Isomerization calculations Parent 1,7-Me2-Cl2 isomers
Isomerization from 1,2-C2B10H12 Cage isomerization via anticubeoctahedron
[583,592–595] [593]
C B (paramagnetic contribution) B2 2H coupling C 11 B shifts B (paramagnetic contribution)
[33] [596,597] [598] [170] [597] [597]
EHMO, NEMO; charge distribution EHMO, NEMO; charge distribution △Hformation, △Gformation
[176] [176] [246]
△Hformation, △Gformation
[246]
NMR calculations Parent
1,7-(OH)2 9,10-X2 X ¼ Cl, Br, I
Reactivity calculations n-OH n ¼ 1, 2, 4 n-C(O)OH n ¼ 1, 2, 4 CH-7-R R ¼ H, CHMe2 1-C(O)OOCMe2C peroxy alkynes C-CMe2OOCMe 3] 1-[C(O)OOCMe2C peroxy alkyne
a A more extensive listing can be found in Table 10-1 Extended at the website http://www.elsevierdirect.com/companion.jsp? ISBN=9780123741707. b Substituents on the carborane cage. “FF” indicates that the full formula of the compound is given. c S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; F, 19F NMR; P, 31P NMR; Li, 7Li NMR; Si, 29Si NMR; Pt, 195Pt NMR; Hg, 199Hg NMR; 2d, two-dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; UV, UV-visible data; E, electrochemical data; ESR, electron spin resonance data; MAG, magnetic susceptibility; COND, electrical conductivity; OR, optical rotation; NLO, nonlinear optical properties; DSC, differential scanning calorimetry; TGA, thermogravimetric analysis; BNCT, boron neutron capture therapy.
578
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-2 Selected 1,12-C2B10H12 Derivatives Synthesis and Characterization Compounda Non-transition metal derivatives Parent
Informationb
References
S, IR (actual spectrum) S [oxidation of M(C2B10H12)2 M ¼ Fe, Co, Ni] S (thermal isomerization of 1,2-C2B10H12) S (separation from o/m-C2B10H12 mixture via chlorination ! 1,12-C2B10H12 þ 1,7C2B10H112 29-Cl) S [isomer mixture from oxidation of CoðC2 B10 H12 Þ2 ] X, definitive, cocrystallized with [Me2N]3PO X, cocrystallized with H2 NP½NMe2 3 þ HCO3 X, 1:2 N(H)P[NMe2]3 adduct X, C2 2H X solid state interactions (X ¼ O, N, S, CR, Cp, arene) F. Cl, Br, I, C X (plastic crystals) ED H H (C2 2H shift, coupling constants) H (substituent effects) H (solid state spin-lattice relaxation, phase transitions) H (plastic phases) B (actual spectrum), MS B (comparison with 1,2- and 1,7-C2B10H12) B (substituent effects) B (nuclear quadrupole coupling) B B (spin-decoupled; interpreted) B, solid state JC,B, JC,C, JB,B NMR coupling constants C (C2 2H coupling, C hybridization) C(H) detailed assignments C, experimental and IGLO-calculated shifts C, solid state IR IR, solid state and plastic crystals IR, Raman Raman (variable temp and/or pressure) Raman (plastic phases) E (pKa)
[599,600] [10] [2,3] [601]
[11] [13] [14] [14] [15] [602] [17,19] [21] [24] [22,23] [603] [602,604] [599] [359] [21] [566] [24] [28] [568] [30] [35,36] [31,32] [33] [568] [21] [605] [40,45,605,606] [606–608] [602,604,607,609] [51] Continued
10.2 Synthesis and structure
579
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda
1,10 -(1,12-C2B10H11)2 (inter-cluster C2 2C bond 1.553 A˚) • [C2B10H12] radical anion (FF) 1-D 2H n ¼ 1–4 (FF) H2 2(CB10H10C)n2 Alkyl derivatives 1-Me
1,12-Me2 1-CHMe2 C2B10Me12 (FF) H2C2B10Me10 (FF) HC2B10Me11 (FF)
Informationb
References
pKa, metalation equilibrium constants pKa, acidity relative to 1,2- and 1,7-C2B10H12 MS (detailed study at 100-250 C) DSC (plastic phases) Dipole moment Heats of combustion, formation, and/or sublimation Heat of isomerization from 1,2-C2B10H12 Inner shell electron energy-loss spectrum (ISEELS) Molecular films, photoemission and inverse photoemission studies He photoelectron spectra, ionization potentials Luminescence, emission spectra Plastic phases, calorimetry Ionic fragmentation following photon-induced B 1s and C 1s excitation vs. energetics of decomposition ED
[53,54] [34] [49] [604] [67] [64,183,610]
MS (electron resonance capture mass spectra) IR, Raman S, B (n ¼ 1–3)
[72] [605] [613]
S H H (substituent effects) B C IR MS E (reduction; comparison with 1,2- and 1,7C2B10H12 derivatives) Heat of formation S ED B (substituent effects), H, IR S, X, H, C, B, MS S, X, H, C, B, MS S, H, C, B, MS
[614,615] [614,615] [22] [22,81,614] [80,81,614] [22] [614] [83]
[610] [59] [611] [58] [62] [602] [70]
[612]
[65,81,82] [615,616] [617] [21] [618] [93,618] [618] Continued
580
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda
Informationb
References
1-R R ¼ Me, n-C3H7, n-C4H9 1-R R ¼ n-C5H11, n-CH2CHMeEt 1,12-(n-C5H11)2 212-C5H4 1-C7H62 1-C7H7 (cycloheptatriene) 212-[cyclo-C5H42 21-OH-3,4-Me2] 1-C7H72 212-[cyclo-C5H32 23,4-Me2] 1-C7H72 2CB10H10C2 2CB10H10C2 2C7H15 C7H152 liquid crystal (FF) 2CB10H10C2 2(C6H4)2R 1-C5H112 R ¼ C8H17, OC8H17 (FF) 1-C5H112 2CB10H10C2 2C6H42 2C4N2H2R R ¼ C8H17, OC8H17 (FF) 2CB10H10C2 2C6H42 2C6H2F22 2OEt 1-C5H112 nematic liquid crystals (FF) 2CB10H10C2 2C6H42 2C6H2F3 1-C5H112 nematic liquid crystals C5H112 2CB10H10C2 2C6H42 2OC(O)2 2 C6H42 2O2 2C2H42 2C6X13 X ¼ H, F nematic ! smectic liquid crystals C5H112 2CB10H10C2 2C(O)O2 2C6H42 2 O2 2C3H6-R liquid crystals—smectic phase induction via fluorination (FF) 2O2 2C6H42 2CB10H10C2 2L2 2 C5H112 C6H42 2On2 2C5H11 L ¼ CH2CH2, C(O)O, CH5 5CHC(O)O, CH5 5N, CH5 5CH, C(O)NH, CH2CH2O(O)C n ¼ 0,1 liquid crystals—effect of linking group on mesogenic properties (FF) 2C(O)O2 2C6H42 2CB10H10C2 2 CnH2nþ12 2C(O)O2 2CnH2nþ1 effects of L2 2C6H42 phenylalkyl connecting groups on mesogenic properties Me2N2 2C6H42 2p-CB10H10C2 2p-C6H42 2NO2 2-Me 2,9-Me2 2-R R ¼ Et, CHMe2, n-C4H9, CH2CHMe2 Dipentyl derivatives
C(H) detailed assignments S, H, B, C, IR, MS S, H, B, C, IR, MS Hyperpolarizability; NLO S, X, H, B, C, IR, UV, MS, NLO S, H, B, C, IR, UV, MS, NLO S, X, H, B, C, IR, UV, MS, NLO S, X, H, B, C, UV, MS
[31] [619] [619] [90] [620] [620] [620] [621]
H, MS, DSC, polarizing microscopy, ferroelectric liquid crystal properties H, MS, DSC, polarizing microscopy, ferroelectric liquid crystal properties S, H, DSC, opto-electrical properties
[622]
[623]
S, H, DSC, opto-electrical properties
[623]
S, H, DSC, opto-electrical properties, phase transitions
[624]
S, H, phase transitions
[624]
S, H, C, MS, phase transition temperature
[625]
S
[626]
Dipole moment S, H, B, C, IR, MS S, H, B, C, MS S Molecular dynamics in liquid crystals
[627] [628] [629] [95] [630]
E (reduction; comparison with 1,2- and 1,7C2B10H12 derivatives) Dipole moment
[83]
Haloalkyl derivatives 1-CH2X X ¼ Cl, Br 1-CH2X
X ¼ Cl, Br, I
[622]
[344] Continued
10.2 Synthesis and structure
581
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda
Informationb
References
[Br(CH2)3]HC2B10Me10 (FF) [Br(CH2)n]HC2B10Me10 n ¼ 5, 6 (FF) 1-CH2Cl-12-R R ¼ H, Me, Ph, I, Cl, CH2Cl
S, H, B, C, MS S, H, B, C, MS E (reduction; comparison with 1,2- and 1,7C2B10H12 derivatives) S S S, H, B, C, MS S, H, B, C, MS S, X, H, B, C, MS S, MS
[106] [106] [104]
S X H C C (detailed assignments) B IR MS Friedel-Crafts acylation (triflic acid catalysis); Taft s constants. pKa S X B C IR MS UV S, B, C, IR, UV, MS S, B, C, IR, UV, MS S, B, C, IR, UV, MS S, B, C, IR, UV, MS S S, H, B, C, IR, MS S, H, B, C, IR, MS S, X, DSC S, X-ray powder, TGA
[108,109,632] [632] [632] [632] [32] [632] [632] [632] [633]
260 -Br) 1-(n-C6H122 240 -Br)2 1,12-(n-C4H92 1,12-(Cl2CHCH2)HC2B10Me10 (FF) 1,12-(Cl2CHCH2)2C2B10Me10 (FF) H2C2B10(CHCl2)10 (FF) C60(C2B10H11)nþ n ¼ 1,2 (FF) Aryl derivatives 1-Ph
1-Ph-12-R R ¼ H, Me
1,12-Ph2
1-p-C6H4Me 1,12-(p-C6H4Me)2 1-p-C6H4CF3 1,12-(p-C6H4CF3)2 1,12-(p-CH2C6H4-Me)2 1,3-(1-C2B10H11)2C6H4 (FF) 10 ,30 ,50 -(1-C2B10H11)3C6H3 (FF) 2C6H4]2 (FF) [p-Ph2 2CB10H10C2 2CB10H10C2 2C6H42 2]n (FF) [2 2p-C6H42
[102] [102] [153] [153] [631] [99]
[53] [634] [635] [634] [121,634] [634] [634] [634] [634] [634] [634] [634] [189] [632] [632] [636] [636] Continued
582
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda
Informationb
References
C2 2C 2C6H4)C2B10H10 1,12-(p-RC6H42 R ¼ H, 1,12-C2B10H11 (FF) 1-(p-C6H4Ph)-12-H 1,12-(p-C6H4Ph)2 1-C6H4R-12-R0 R ¼ NO2, CN; R0 ¼ H, (CH2)nOH, (CH2)nOSiMe3, C(O)OMe, C(O)OH 1-C6H4R-12-CH2OH R ¼ NO2, CN strong antiandrogenic activity; hydrophobic 1-(30 -C5H5N)-12-CH2OH androgen receptor antagonist 1-CH2OH-12-C6H4-p-cyclo-(CNHC(5 5S)ONC) oxadiazole; hydrophilic pharmacore of androgen receptor ligands 1-Ph-12-R R ¼ H, OH, C(O)OMe 1-m/p-C6H4NO22 212-R R ¼ H, OH, C(O)OMe 1-CH2C6H4R-12-R0 R ¼ H,NO2, CN; R0 ¼ OH, CH2OH, CH2OSi(Me)2CMe3 anti-androgenic activity 1-(p-C6H4OH)-12-(CH2)5CHMe2 incorporated into liposomes for BNCT 1,12-(C6H4-o/m/p-OR)2 R ¼ H, Me, n-C3H7, n-C5H11, OMe, OCSiMe3 1,12-[C13H9(C6H13)2Br]2 fluorenyl 2 2[C13H9(C6H13)2-CB10H10CC13H9(C6H13)2]n2 2 fluorenyl polymer 2-Ph
S, X, H, B, C, IR, MS
[637]
S, H, IR, UV, MS S, H, IR, UV, MS S, H, C, MS, binding affinity
[107] [107] [638]
S, H, C, MS, binding affinity
[638]
S
[639]
S
[640]
S, H, C, MS S, H, C, MS
[638] [638]
S, H, C, MS, transient transactivation assay
[641]
S, H, C, MS
[642]
S, H, C, binding affinity to estrogen receptor a
[643]
S, H, C, MS S, H, C, DSC, UV, fluorescence
[644] [644]
S H B C MS S S, X(OH), H
[95,127,629] [629] [629] [629] [629] [95] [645]
S (Pd-catalyzed cross-coupling), H, B, C S, X, H, B, C, MS S, H, B, C S, H, C, MS Pd-catalyzed demethylation with 2-MeOPhMgBr
[646] [629] [646] [647] [647]
2-R R ¼ CH2Ph, C6H4Me, C6H4F 2-(o-C6H4R) R ¼ H, OH, OMe proton-driven conformational change; intramolecular H bonding 2-(C6H4-p-naphthyl) 2,9-Ph2 2-C6H4-p-Ph 2-C6H4-o-OMe
Continued
10.2 Synthesis and structure
583
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda
Informationb
References
2-C6H4-o-OH
S (from Pd-catalyzed reaction of C2B10H112 22-I with 2-MeOPhMgBr), H, C, MS H (electron-accepting power of p-C2B10 cage) S, H, C, F, IR S, H, C, F, IR S, H, C, F, IR
[647] [648] [138] [138] [138]
S, H, B, C, MS S, H, B, C, MS
[619] [619]
S, H, B S, H, B
[649] [649]
S
[146]
S, H, B, C, IR, MS S, IR
[650] [473]
S, H, B, C, MS X, IR S, H, B, C, MS S, H, B, C, MS S
[153,650] [650] [153] [153] [651]
S
[95]
1-m/p-C6H4F 212-Ph 1-C6F52 p-(PhCB10H10C)2C6F4 (FF) 2C6F42 2[1,12p-[1,7-PhCB10H10C]2 CB10H10CH] (FF) 2C6H4F 1-CH2CH2C6H102 1-R-12-n-C5H11 R ¼ CH2CH2C6H10–C6H4F, p-C6H4Br 1,12-(p-C6H4Br)2 1-(p-C6H4Br)2–12-(C6H4-p-NC5H4-p-NC5H4) bipyridine 2-m/p-C6H4F Alkenyl derivatives 1-CH5 5CHCl 5CH2, CMe5 5CH2; 1-R-12-R0 R ¼ CH5 R0 ¼ SnMe3, SnEt3, Sn(n-C4H9)3 1,12-(CH5 5CHCl)2 5CH]C2B10Me10 (FF) 1,12-H[CH25 5CH]2C2B10Me10 (FF) 1,12-[CH25 2-CH5 5CHR R ¼ Ph, C6H4-Ph, C6H4Cl, C6H4Br, C6H4NO2, C6H4OMe, C6H4Me, 2-pyridine 2-CH2CH5 5CH2 Alkynyl derivatives CH 1-C CR R ¼ H, Ph 1-C CH)2 1,12-(C CR)2 R ¼ H, Ph 1,12-(C CR)2 R ¼ H, SiMe3 1,12-(C CSiMe3 1-C C2 1-C 2CMe2OH C2 1,12-[C 2CMe2OH]2 C2 C2 1-C 2C 2CMe2OH C)2 2C (HCB10H10C2 C2 1,12-[C 2SiMe3]2 C)2C2B10Me10 (FF) 1,12-(HC
S, S S, S S, S, S, S, S, S, S, S, S,
X, H, B, C, IR, MS, ED X, B, C, IR, MS H, B, C, IR, MS X, H, B, C, IR, MS, ED H, B, C, IR, MS H, B, C, IR, MS H, B, C, IR, MS H, B, C, IR, MS H, B, C, IR X, IR, UV H, B, C, MS
[160,652] [161] [653] [161] [650] [652] [652] [652] [652] [652] [653] [654] [153] Continued
584
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda
Informationb
References
C]2C2B10Me10 (FF) 2C 1,12-[Me3Si2 C]2C2B10Me10 (FF) 1,12-[Me(CH2)3OC(O)C C2 C2 2C 2C 2 1,12-HCB10Me10C2 CB10Me10CH (FF) C2 C2 2C 2C 2 1,12-RCB10Me10C2 CB10Me10CR0 R, R0 ¼ H, (CH2)2OH, CH2C(O) CH, C CC(O) H, CH2CHCl2, C(O)OMe, C CC(O)O(CH2)3Me, SiMe3 (FF) OMe, C C2 C2 2C 2 [1,12-RC 2CB10Me10C2 C2 C2 C 2CB10Me10C2 2C 2]2 R ¼ H, Me, C(O)O(CH2)3Me (FF) C2 C2 1,12-RCB10Me10C2 2C 2C 2 CB10Me10CR0 [R, R0 ¼ H, C(O)OMe] (FF) C2 2C 2C6H4)C2B10H10 1,12-(p-RC6H42 R ¼ H, 1,12-C2B10H11 (FF) C2 212-R R ¼ H, SiPh3 1-C 2C5H112 C2 2C 2CB10H10C2 2CB10H10C2 2 C5H112 CC5H11 liquid crystal (FF) C C2 2C 2C60)-7-Me 1-(C6H42
S, X, H, B, C, MS S, H, B, C, MS S, H, B, C, MS, UV
[153] [153] [153]
CC(O)OMe], H, B, C, MS S, X[R, R0 ¼ C
[153]
S, H, B, C, MS
[153]
S, H, B, C, MS
[153]
S, X, H, B, C, IR, MS
[637]
S, H, B, C, IR, MS S, X, H, B, C, UV, MS
[619] [621]
S, H, C, MS, UV, E, NLO (b hyperpolarizability; fluorescence) S, H, C, IR, MS
[164,165]
C2 C2 2C 2C6H3I2 2C 2 HCB10H10C2 CB10H10CH (FF) Thioxanthene(naphtha[2,1-b]-thiopyran)C2 C2 2CB10H10CH)2}2 (FF) {C 2C6H3(C motorized nanocar C)2C6H32 C2 2C 2C 2 (MeCB10H10C2 C]2C6H2[C C2 [C6H2(OC10H21)22 2C 2 C2 2C60]2 nanodragster C6H2(OCC10H21)2C C)2Me4C9N2BF2]2 {[(HCB10H10C C}2 nanocars C6H4C C)2Me4C9N2BF2]2{[(HCB10H10C C}22 C6H4C 2C6H2(OMe)2 nanocars C)2Me4C9N2BF2]2{[(HCB10H10C C}2C12N(C4H9) nanocars C6H4C C12N ¼ 9-carbazide C2 C2 2C 2C6H3(C 2 N2[C6H42 CB10H10CH)2]2 nanoworm C2 2C 2C6H3(OC3H7)22 2 [HCB10H10C2 C2 C 2C6H4]2N2 C2 HCB10H10C2 2C 2C6H32 2 C2 (C 2C6H42 2N5 5N2 2Ph)2 2 C2 C 2CB10H10CH C2 2C6H32 220 ,50 (C4H9)NC12H6[C (CB10H10CH)2]2 nanocar (FF)
[655]
S, H, C, IR, MS, kinetics of thermal isomerization
[655]
S, H, C, IR, MS
[656]
S, H, C, IR, UV, MS, fluorescence emission
[657]
S, H, C, IR, UV, MS, fluorescence emission
[657]
S, H, C, IR, UV, MS, fluorescence emission
[657]
S, H, C, IR, UV
[658]
S, H, C, IR, photo-isomerization
[659]
S, H, C, IR, photo-isomerization
[659]
S, H, C, IR, UV, fluorescence
[660] Continued
10.2 Synthesis and structure
585
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda C2 2C6H22 22 ,5 -(OC3H7)2]21 ,3 ,5 -{[C C2 C 2CB10H10CH}3C6H3 nanocar (FF) CR R ¼ H, Ph, SiMe3 2-C CR)2 R ¼ H, SiMe3 2,9-(C CSiMe3]2 2,9-[C C-2-H2C2B10H9 (FF) H2C2B10H9-2-C C2 C-2-H2C2B10H9 (FF) 2C H2C2B10H9-2-C C2 29-C 29-H2C2B10H82-I-H2C2B10H82 C2 2-C 29-H2C2B10H82 22-I (FF) C2 2C6H3-20 ,50 2,5-(MeO)2-1,4-[C C2 (C 2CB10H10CH)2]2C6H2 motorized nanocar (FF) C2 2,5-(C 2CB10H10CH)2-1,4C2 [C 2C6H32 220 ,50 (C C2 2CB10H10CH)2]2C6H2 nanocaterpillar (FF) 0
0
0
0
0
Alcohols and C- and B-hydroxy derivatives 1-OH
1,12-(OH)2
1-CH2OH (HOCH2)HC2B10Me10 (FF) (HOCH2)HC2B10HMe9-2-R R ¼ NHMe, NH2, CN (FF) 5NOH (FF) (HOCH2)HC2B10HMe9-2-CH5 5NO2 2]2 (FF) [(HOCH2)HC2B10HMe9-2-CH5 1-(CH2)3OH 1,12-(CH2OH)2 1-OH-12-CH(OEt)2 1,12-[C(OH)2]2 dihydrate 5S) 1-CH2OH-12-C6H4-p-cyclo-(CNHC(5 ONC) oxadiazole; hydrophilic pharmacore of androgen receptor ligands [HO(CH2)3]MeC2B10Me10 (FF) H2C2B10(OH)10 (FF)
Informationb
References
S, H, C, IR, UV, fluorescence
[660]
S, S, S, S, S, S,
[629] [629] [661] [661] [661] [661]
H, B, C, MS H, B, C, MS X, H, B, C, MS H, B, C, MS X, H, B, C, MS H, B, C, MS
S, H, C, IR, UV, fluorescence
[660]
S, H, C, IR, UV, fluorescence
[660]
S, H, B, C, IR, MS IR [hydrogen bonding with Et2O] Complex with a-cyclodextrin in aqueous solution; Ka (association constant) S, H, B, C, MS Complex with a-cyclodextrin in aqueous solution; Ka (association constant) S Heats of combustion and formation S, H, B, C, MS S, H, B, C, MS
[170–172] [181] [227]
S, S, S, S, S S, S, S
[663] [663] [664] [665] [662] [666] [667] [640]
X, H, B, C, MS X, H, B, C, MS H, B, C H, MS, aqueous solubility H, B, C X, H, B, C
S, H, B, C, MS S, X, B, MS
[170] [227] [662] [183] [663] [663]
[614] [668] Continued
586
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda
Informationb
References
n-p-C6H4OH n ¼ 1, 2
S, partition coefficients (log P)]; Hansch-Fujita hydrophobic parameters; drug design pKa, hydrophobicity, estrogen receptor binding affinity S S, B, C, IR, UV, MS S, B, C, IR, UV, MS S, B, C, IR, UV, MS
[669]
[188,189] [634] [634] [634]
Ka (association constants)
[195]
S S, H, MS
[670] [196]
S, H, MS
[196]
S, H, C, IR, MS
[671]
S, H, B, C, MS S, H, B, C, MS S, H, B, C, MS
[153] [153] [153]
S
[672]
S, H, B, C S, X, H, B pKa
[651] [673] [176,178]
S
[666]
S
[666]
S S(improved), H, C, MS C(H) detailed assignments S
[208,674] [205] [31] [208]
1,12-(CH2-p-C6H4OH)2 1-p-C6H4OH 1,12-(p-C6H4OH)2 (18-crown-6)þ [(p-C6H4OH)(p-C6H4OH) C2B10H10] (FF) 1-R-12-R0 R ¼ H, R0 ¼ OH, CH2OH, NH2; R ¼ R0 ¼ OH complexes with b-cyclodextrin 1-OH-12-(CH2)2C(O)OH propionic acid 1-(m-C6H4OH)-12-R R ¼ H, OH, (CH2)nOH n ¼ 1–3 estrogen agonists; hydrophobic pharmacore; estrogen receptor modulation; steroid receptors 1-(m-C6H4OMe)-12-R R ¼ H, OH, (CH2)nOH n ¼ 1-3 1-R-12-CH2SCMe3 R ¼ (CH2)3OH, CH2CH2C(O)OH 1,12-[HO(CH2)2]2C2B10Me10 (FF) 1,12-H[HO(CH2)2]C2B10Me10 (FF) C2 C2 2C 2C 2 1,12-RCB10Me10C2 CB10Me10CR0 [R, R0 ¼ H, (CH2)2OH] (FF) 1-{cyclo-[CHCH5 5CHC(O)CH2CH2]}2-CH2OH hydrophobic cage; binds to androgen receptor and shows anti-androgenic activity 2-OH
Alkoxy and aryloxy derivatives 1-B(OEt)2-12-R R ¼ CHEt2, C(O)H boronic acids 1-B[OCPhCH2CPhO]-12-R R ¼ CH(OEt)2, C(O)H boronic acids Aldehydes 1-C(O)H
1,12-[C(O)H]2
[191]
Continued
10.2 Synthesis and structure
587
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda
1-CH2C(O)H 1-CH(OEt)2 acetaldehyde diethyl acetal [HC(O)]HC2B10Me10 (FF) RR0 C2B10Me10 R ¼ HO(CH2)3, CH2SO2CF3; R0 ¼ H, CH2OH, CH2OSO2CF3, HO(CH2)3 (FF) (CF3SO2CH2)2C2B10Me10 (FF) 1-C(O)H-12-(p-C6H4-OC5H11) 1-C(O)H-12-R R ¼ B(OEt)2, B (OCPhCH2CPhO) boronic acid 1,12-(C(O)HCH2)HC2B10Me10 (FF) 1,12-(C(O)HCH2)2C2B10Me10 (FF) C2 C2 2C 2C 2 1,12-RCB10Me10C2 CB10Me10CR0 R, R0 ¼ H, CH2C(O)H (FF) Ketones 1-C(O)Ph 1,12-[C(O)Ph]2 Carboxylic acids and carboxylates 1-C(O)OH
1-C(O)OH-12-n-C5H11 1-C(O)OH-12-CH2CHMeEt 1-C(O)OH-12-Me 1-C(O)OH-12-NHC(O)OC(CH2)3 1-C(O)OH-12-R R ¼ NH2, NHC(O)OCMe3 2C(O)OH n ¼ 1-4 (FF) HO(O)C2 2(CB10H10C)n2 1-C(O)OH-12-p-C6H4OH estrogen agonists; hydrophobic pharmacore; estrogen receptor modulation; steroid receptors 1-C(O)OH-12-p-C6H4OMe 1-C(O)OH-12-(p-C6H4OR) R ¼ Me, H liquid crystals
Informationb
References
S(improved), H, C, MS S, H, B, C, MS S S S, H, B, C, MS S, H, B, C, MS
[205] [153] [211] [211] [614] [614]
S, X, H, B, C, MS S, dielectric constants, mesogenic properties S
[614] [675] [666]
S, H, B, C, MS S, H, B, C, MS S, H, B, C, MS
[153] [153] [153]
S C(H) detailed assignments S
[615] [31] [676]
S X, heat of formation Heat of combustion IR (actual spectrum) pKa pKa, E (half-neutralization potential) Complex with a-cyclodextrin in aqueous solution; ka (association constant) S, H, B, C, IR S, H, B, C, IR pKa, E (half-neutralization potential) S, H, C, IR S, H, B, C, IR S, B (n ¼ 1-3) S, H, MS
[600,677,678] [223] [65] [600] [176,224,615,678] [226] [227] [619,679] [619] [226] [234] [230] [613] [196]
S, H, MS S, H, DSC, mesogenic properties
[196] [680] Continued
588
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda
Informationb
References
1-CH2CH2C(O)OH-12-CH2SH 1-R-12-CH2SCMe3 R ¼ (CH2)3OH, CH2CH2C(O)OH n-SCH2C(O)OH n ¼ 1,2 1-C(O)OH-12-Me 1,12-[C(O)OH]2
S, H, C, IR, MS S, H, C, IR, MS
[671] [671]
MS, pKa S, IR (actual spectrum) S, H, B, C, IR
[176] [600] [52,230,615, 619,678] [681] [52,615,678] [227]
[HO(O)C]HC2B10Me10 (FF) 1-C(O)OH-12-N(boc)NH(boc) [boc ¼ tertbutyloxycarbonyl] 1-CH2C(O)OH 1-(CH2)2C(O)OH-12-OH propionic acid 1,12-(CH2-p-C6H4C(O)OH)2[HO(O)C]MeC2B10Me10 (FF) {1,12-[OC(O)]2C2B10H10}2 [Mo(N,N0 -di-panisylformamidinate)þ]2 (FF) 2-C(O)OH Ethers HCB10H10C-2-O-20 -CB10H10CH (FF) 2ðCH2 Þ4 O2 2B12 H11 2 R ¼ H, RCB10 H10 C2 Me (FF) water-soluble compounds for BNCT 2-OR R ¼ Me, Et, Ph, C6H4-p-Me, C6H3Me2, naphthyl (2 isomers), C6H4MeCl Esters and acyl halides 1-CH2C(O)Cl 1-C(O)Cl 1,12-[C(O)Cl]2 1-C(O)OEt 1,12-[C(O)OEt]2 1,12-[C(O)Cl]2 1-C(O)OMe-12-p-C6H4OMe 1-(p-C6H4OR)-12-R0 R ¼ Me, H; R0 ¼ H, C5H11 liquid crystals
X pKa Complex with a-cyclodextrin in aqueous solution; Ka (association constant) S, X pKa, E (half-neutralization potential) S, X, H, B, C, MS
[682] [226] [268]
pKa, E (half-neutralization potential) S S S, H, B, C, MS pKa S, X, H
[226] [670] [189] [614] [237] [683]
S
[236]
S, H, B, C, MS S, H, B, C, IR
[651] [256]
S, X(Me, C6H4-p-Me), H, B, C
[684]
35
[241] [685] [676,685] [685] [685] [676] [196] [680]
Cl NQR
S
S, H, MS S, H, DSC, mesogenic properties
Continued
10.2 Synthesis and structure
589
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda
Informationb
References
1-(p-C6H4-OC5H11)-12-(cycloCH2 2O2 2CH2CHRCH22 2O2 2) R ¼ CnH2nþ1, OCnH2nþ1, C6H4C nH2nþ1, n-C4H9 liquid crystals 1-C(O)OCHMeC6H13-2-OC(O)-p-C2H4C6X13 (X ¼ H, P) liquid crystals 5CH2]2 1-[C(O)OCH2CH5 [C(O)Cl]HC2B10Me10 (FF) 1-[CH2C6H4-p-OC(O)Me] 1-[(C6H4)2-p-OC(O)Me] 1,12-H[MeOC(O)]C2B10Me10 (FF) 1,12-[MeOC(O)]2C2B10Me10 (FF) 1-C(O)OCH2Ph-12-NHC(O)OC(CH2)3 1-C(O)OCH2Ph-12-NH2 1-CH2NH-CH[C(O)OMe]CH2CHMe2 1-CH5 5NCH[C(O)OMe]CH2CHMe2 2O2 2(C6H4)22 2O2 2C8H17] 1-C5H11-12-[C(O)2 liquid crystal 1-C5H11-12-[C(O)-O-C6H4-C6H3R]-cycloC6H10-cyclo-C5H11 liquid crystals—broad nematic phases 1-C5H11-12-[C(O)2 2O2 2C6H42 2CH2CH2]cyclo-C6H10-cyclo-C5H11 liquid crystals— broad nematic phases EtMeCHCH22 2CB10H10C2 2C(O)O2 2(C6H4)2cyclo-C5H11 optically active mesogenic esters (FF) 2CB10H10C2 2C6H42 2(C6H2-2,3-X2)C5H112 OnR X ¼ H, F; n ¼ 0, 1; R ¼ C6H13, C7H15, C8H17 liquid crystals (FF) 1-CH(OEt)2 1,12-[CH(OEt)2]2 212-X X ¼ Me3SiO, OH 1-CH(OEt)22 5CH-C(O)O(CH2)3 propylacrylate 1-CH25 MeCH2OC(O)CMe-[CH2CHC(O)O (CH2)3CB10H10CH]nBr (FF) poly(p-carborane) propyl acrylate p-C6H4OR diesters
S, dielectric constants, mesogenic properties
[675]
S, electrooptical properties
[686]
S S S S S, S, S, S, S, S, S,
[253] [682] [251] [251] [153] [153] [234] [234] [205] [205] [687]
1-p-C6H4OMe 1,12-(p-C6H4OMe)2 1-(p-C6F4OR)-12-Ph
H, H, H, H, H, H, H,
B, C, MS B, C, MS C, IR C, IR C, MS C, MS B, C, IR, MS, phase transition
S, thermal analysis, optical microscopy
[688]
S, thermal analysis, optical microscopy
[688]
S, H, DSC, OPT
[689]
S, DSC, polarizing microscopy; mesogenic and dielectric properties
[690]
S, S S, S, S,
[666] [666] [666] [664] [664]
H, B, C H(OH), B(OH), C(OH), H, B, C H, B, C
S, liquid crystalline behavior; conformational analysis S, B, C, IR, UV, MS S, B, C, IR, UV, MS S, X(Et), H, C, F, IR
[691] [634] [634] [138] Continued
590
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda
Informationb
References
1-(CH2)3OSiPh2CMe3-2-R R ¼ H, C(O)OH, (CH2)3OH, (CH2)2C(O)OH 2OC(O)(CH2)22 2CB10H10C2 2 CMe[C6H42 (CH2)2OSiPh2CMe3]3 (FF) 2-(C6H4-p-OMe) 2-CH2C(O)OEt Pentaerythritol-centered star dendrimers with 4, 8, and 16 p-carborane cages, for BNCT
S, H, MS, aqueous solubility
[665]
S, H, MS, aqueous solubility
[665]
S (Pd-catalyzed cross-coupling), H, B, C S, H, B, C, IR, MS S, H, MS, aqueous solubility
[646] [628] [665]
S, S, S, S S, S S, S, S,
[634] [648] [634] [261] [663] [676] [663] [663] [646]
Nitro and nitroso derivatives and nitrates 1-p-C6H4NO2 1,12-(p-C6H4NO2)2 1,12-[C(O)OC6H4-o-NO2]2 (ONOCH2)HC2B10Me10 (FF) 1-NO 5NOH (FF) (HOCH2)HC2B10HMe9-2-CH5 5NO2 2]2 (FF) [(HOCH2)HC2B10HMe9-2-CH5 2-p-C6H4NO2 Amines and imines 1-NH2
1-NH2-12-p-C6H4OH estrogen agonists; hydrophobic pharmacore; estrogen receptor modulation; steroid receptors 1-p-C6H4R R ¼ NMe2, NH2 1,12-(p-C6H4R)2 R ¼ NMe2, NH2 1-R-12-C(O)OH R ¼ NH2, NHC(O)OCMe3 1-NH2-12-C(O)OCH2Ph 2-NH2 1-(2-NC5H4) pyridyl 1,12-(2-NC5H4)2 pyridyl 2-R R ¼ furyl, 20 /30 -thienyl, methylindolyl, pyridyl, quinolyl, pyridylethynyl, quinolylethynyl 1-(C6H4-p-R) R ¼ NH2, NO2 self-assembled monolayers on gold surface 1-(C6H42 2N5 5N-C6H4-p-OH) R ¼ NH2, NO2 self-assembled monolayers on gold surface [ClNH3(CH2)n]2C2B10Me10 n ¼ 3, 4 (selfassembly into microrods via sonification) (FF)
B, C, IR, UV, MS H (Taft s constants) B, C, IR, UV, MS H, B, C, MS X, H, B, C, MS X, H, B, C, MS H, B, C
S, H, C, Ms, complex with a-cyclodextrin in aqueous solution; Ka (association constant) S S, H, MS
[676] [196]
S, B, C, IR, UV, MS S, B, C, IR, UV, MS S, H, B, C, IR S, H, C, IR pKa S, H, B, C, IR S, H, B, C, IR S, H, B, MS
[634] [634] [230] [234] [237] [279] [279] [281]
S, H, C, IR (FTIR-RA), refractive index
[692]
S, H, C, IR (FTIR-RA), refractive index
[692]
S, H, B, C, MS
[106]
[227]
Continued
10.2 Synthesis and structure
591
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda
Informationb
References
[H2N(CH2)3]HC2B10Me10 (FF) [H2N(CH2)n]HC2B10Me10 n ¼ 5, 6 (FF) (HOCH2)HC2B10HMe9-2-R R ¼ NHMe, NH2, CN (FF) 1-(p-C6H4Br)2-12-(C6H4-p-NC5H4-p-NC5H4) bipyridine 1,12-(C6H4-p-NC5H4-p-NC5H4)2 bipyridine 2-NC12H8 carbazole 2-N2C7H5 benzimidazole 2-N2C3H3 imidazole 2-NC8H6 indole 2-NHC6H4R R ¼ H, Cl, OMe 2CB10H10CH]4 (FF) Porphyrin[C(O)2 2CB10H10C2
S, H, B, C, MS S, H, B, C, MS S, H, B, C, MS
[106] [106] [663]
S, H, B
[649]
S, S, S, S, S, S, S,
H, B X, H, B X, H, B H, B X, H, B H, B H, MS
[649] [673] [673] [673] [673] [673] [693]
S, S, S, S, B S, S, S
X, H, B, C, IR, MS X, H, B, C, IR, MS H H, C, IR
[694] [694] [695] [230,234] [230] [234] [654] [696]
Amides and imides 1-NHC(O)H 1-NHC(O)OMe methyl carbamate [C(O)NHR]HC2B10Me10 R ¼ H, Ph (FF) 1-NHC(O)OCMe3-12-C(O)OH 1-NHC(O)OCMe3-12-C(O)OCH2Ph 1,12-[C(O)NH2]2 Ethylene oxide-modified 3-p-carboranyl thymidine derivatives for BNCT 2-NHC(O)R R ¼ Me,CH2Ph 2-cyclo-N(CH2)3C(O) 2-cyclo-N(CH2)5C(O) 2-[C6H4-p-NHC(O)Me]
H, C, IR IR, UV
S, H, B, C MS S, X, H, B, C MS S, H, B, C MS S (Pd-catalyzed cross-coupling), H, B, C
[288] [288] [288] [646]
Azides 1-C(O)N3 [N3(CH2)3]HC2B10Me10 (FF) 1-N(boc)NH(boc)-12-C(O)OH [boc ¼ tertbutyloxycarbonyl]hydrazino 1-NHNH2CF3C(O)OH-12-C(O)OH
S S, H, B, C, MS S, X, H, B, C, MS
[676] [106] [268]
S, H, B, C, MS
[268]
Nitriles and isonitriles 1,12-(CN)2 2-(C6H4-p-CN) (HOCH2)HC2B10HMe9-2-CN (FF) [NC(CH2)3]HC2B10Me10 (FF)
S, X, IR, UV S (Pd-catalyzed cross-coupling), H, B, C S, H, B, C, MS S, H, B, C, MS
[654] [646] [663] [106] Continued
592
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda Phosphorus derivatives 1-P(O)Me(OMe)-12-R R ¼ H, CMe5 5CH2 5CMe, Ph 1-PMe(OEt)-12-R R ¼ H, Me, CH25 1-[PMe(OEt)5 5N-C6H4-p-NO2]-12-R R ¼ H, 5CMe, Ph Me, CH25 1-PMe(OEt)R• R ¼ OCMe3, Me phosphoranyl radicals 1,12-(PPh2)2 1-P(O)(OEt)OCH5 5CCl2 1-(MePh2P)C2 B10 H11 þ I selective targeting of mitochondria for BNCT 2-P(O)(OMe)2 phosphonate
2O2 2CH2)3CH2Xþ X 1-PPh2CH2(CH22 X ¼ Br, water-soluble phosphonium salts for BNCT 2PPh22 2CH22 2 HCB10H10C2 (CH22 2O2 2CH2)32 2CH22 2PPh22 2 CB10H10CH2þ 2X X ¼ Br, I (FF) watersoluble phosphonium salts for BNCT 1,12-{P(O)[OCH2-cyclo-C5H5(OH)4O]2}2 water-soluble glycophosphonates for BNCT 1,12-[P(NMe2)2]2 Sulfur derivatives 1- SH 1-R R ¼ H, SH n-SH n ¼ 1,2 n-SCH2C(O)OH n ¼ 1,2 1-S(O)OH 1-SO2H sulfinic acid 1,12-[S(O)OH]2 1,12-[S(O)2OR]2 R ¼ H, Cl 1,12-(SH)2 1,12-(SH)2 attached to surface of Au microcrystals 1,12-(SPh)2 1,12-(SCl)2 1-CH2SH 1-CS2Me
Informationb
References
S S S
[697] [297] [297]
ESR
[298]
S, X, H, B, C S S, H, B, C, P, MS
[698] [699] [299]
S [UV irradiation of Hg(C2B10H11)2 in P(OMe)3] X S S, H, B, C, P, MS
[306,700] [700] [307] [701]
S, X, H, B, C, P, MS
[701]
S, H, B, C, P, MS, IR
[305]
S, X, H, B, C, MS, IR
[305]
S, H, B UV photoelectron pKa, MS, dipole moment pKa, MS, dipole moment S, H, B, C, IR, MS S S, H, B, C, IR, MS S, H, B, C, IR, MS S ED S, XPS, scanning electron microscopy, work functionsx S S S, H, C, IR, MS S, H, C, IR, MS
[308] [309] [176] [176] [702] [676] [702] [702] [315,676] [703] [704] [311] [315] [671] [671] Continued
10.2 Synthesis and structure
593
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda
Informationb
References
1-CH2SH-12-CH2CH2C(O)OH 1-CH2SCMe3 1-CH2SCMe3-12-R R ¼ (CH2)3OH, CH2CH2C(O)OH 1-SC4H9-12-R R ¼ H, SiPh3, CH2CHMeEt 1,12-(C4H3S)2 thiophene precursors to conducting polymers with high electrochemical and thermal resistance p-C6H4(CB10H10C2 2C4H3S)2 thiophene (FF) 1-S(O)2Ph-12-SPh sulfone 1,12-[S(O)2Ph]2 sulfone [S(O)2OMe]2C2B10(OMe)10 (FF) [S(O)2OR]2C2B10(OH)10 R ¼ H, Me (FF) [Mþ]2 {[S(O)2O]2C2B10(OH)10}2 M ¼ Na, K (FF) 1-CPhH(OSO3C6H4Me)-12-C6H4R R ¼ H, CF3, Me, OMe, NMe2 tosylates 1-benzyl p-toluenesulfonates (tosylates) 10 ,30 ,50 -{1,12-[Et2S(CH2)3Me2Si]C2B10H10}3C6H3 (FF) 10 ,30 ,50 -{1,12-[[Et2S(CH2)3]2MeSi]C2B10H10}3C6H3 (FF) 2-SH
S, H, C, IR, MS S, H, C, IR, MS S, H, C, IR, MS
[671] [671] [671]
S, H, B, C, IR, MS S, E, UV, TGA,
[619] [316]
S, S, S, S, S, S,
[317] [311] [311] [702] [702] [702]
Fluoro derivatives 2-F H2C2B10F10 (FF) 1,12-F2C2B10(CF3)10 (FF) 1,12-FHC2B10(CF3)10 (FF) 1,12-H2C2B10(CF3)10 (FF) Chloro derivatives 1-Cl
1-Cl-12-R0 R ¼ H, Me, Ph, I, Cl, CH2Cl 1-Cl-12-Me 1-Cl-12-R R ¼ H, Me
H, C, MS IR IR X, H, B, C, IR, MS X(H), H, B, C, IR, MS X, H, B, C, IR, MS
S, rate constants, electronic effects of substituents Hydrolysis kinetics S, H, B, C, IR, MS
[320] [322] [632]
S, H, B, C, IR, MS
[632]
S UV photoelectron
[327] [309]
S S (F2), IR S, X, B, C, F, IR, Raman, MS S, X, B, C, F, IR,MS S, X, B, C, F, IR, MS
[337,338] [38] [705] [705] [705]
S, IR (actual spectrum) H (C2 2H shift, coupling constants) E (reduction; comparison with 1,2- and 1,7C2B10H12 derivatives) E (reduction; comparison with 1,2- and 1,7C2B10H12 derivatives) S, IR (actual) 35 Cl NQR H (substituent effects)
[600] [24] [83] [104,400] [600] [345] [22] Continued
594
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda
1-Cl
2-Cl
1,12-Cl2
2,n-Cl2 n ¼ 3,4,7,9,10 2,9-Cl2 2,9,10/2,9,11-Cl3 2,10-Cl2 2,11-Cl2 B-Cln n ¼ 3, 4, 5 H2C2B10Cl10 (FF)
Bromo derivatives 1-Br
1,12-Br2
Informationb
References
B IR Dipole moment B IR S H (substituent effects) H H (C2 2H shift, coupling constants) B B (comparison with 1,2- and 1,7-C2B10H12) IR 35 Cl NQR, polarity of B2 2Cl bond pKa, acidity relative to 1,2- and 1,7-C2B10H12 S H (substituent effects) B IR (actual spectrum) IR ESR, dipole moment S S S H S S IR, Raman IR (C2 2H, H-bonding with solvents) 2H vibrational bands C, JCH correlation with C2 pKa, acidity relative to 1,2- and 1,7-C2B10H12
[22] [22] [344] [22] [22] [338,353,601,706] [23] [350] [24] [23] [359] [23] [362] [52] [600,677] [22] [22] [600] [22] [707] [601] [601] [706] [350] [601,706] [601] [45] [343] [34] [34,52]
S, IR (actual spectrum) S, H, B, C, IR, MS H (substituent effects) H (C2 2H shift, coupling constants) B IR E (reduction; comparison with 1,2- and 1,7C2B10H12 derivatives) Dipole moment S, H, B, C, IR, MS
[600] [652,677] [22] [24] [22] [22] [83] [344] [652] Continued
10.2 Synthesis and structure
595
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda
1-Br-12-Me
1-Br-12-R R ¼ H, Me, Ph, I, Cl, CH2Cl 2-Br
2,9-Br2 Iodo derivatives 1-I
1,12-I2
1-I-12-Me
1-I-12-R R ¼ H, Me, Ph, I, Cl, CH2Cl 2-I
Informationb
References
S H (substituent effects) B IR IR (actual spectrum) S IR (actual) H (substituent effects) E (reduction; comparison with 1,2- and 1,7C2B10H12 derivatives) S B (comparison with 1,2- and 1,7-C2B10H12) H (C2 2H shift, coupling constants) pKa, acidity relative to 1,2- and 1,7-C2B10H12 S
[600,677] [22] [22] [22] [600] [600,677] [600] [22] [104,400]
S, IR (actual spectrum) S H (C2 2H shift, coupling constants) Dipole moment H (substituent effects) B IR S H (substituent effects) B IR (actual spectrum) IR ED S IR (actual) H (substituent effects) E (reduction; comparison with 1,2- and 1,7C2B10H12 derivatives) S
[600] [677] [24] [344] [22] [22] [22] [600,676,677] [22] [22] [600] [22] [708,709] [600,677] [600] [22] [104,400]
X B (comparison with 1,2- and 1,7-C2B10H12) H (C2 2H shift, coupling constants) H B
[338,601,706] [359] [24] [52] [601]
[338,601,629, 646,706] [629] [359] [24] [629] [629] Continued
596
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda
2–125I 2-IPhþ X X ¼ BF4, Cl, I 2-IPhþ BF4 2,3-I2 2,7-I2 2,9-I2 H2C2B10I10 (FF) C-9-H2C2B10H8-22-I-H2C2B10H8-9-C C2 C 29-H2C2B10H8-2-I (FF)
Informationb
References
C MS E (reduction; comparison with 1,2- and 1,7C2B10H12 derivatives) 127 I NQR S (radiolabeling via Pd-catalyzed isotopic exchange) 127 I NQR quadrupole coupling S S, X, H, B, C, MS S, X, H, B, C, MS S H, B, C, MS S, B, IR, MS X S, H, B, C, MS
[629] [629] [400] [391,710] [392] [395] [711] [629] [629] [601,629] [629] [401] [629] [661]
Exo-polyhedral main-group metal and metalloid element derivatives Aluminum S (C2B10H11)nAlH4nM n ¼ 1, 2; M ¼ Li, Na (FF)
[406]
Thallium 2-Tl[C(O)OCF3]2 2-TlX2 X ¼ Cl, Br, SCN 1-R-12-R0 -2-Tl[C(O)OCF3] R, R0 ¼ H, Me, Ph
S S S
[712] [712] [413]
S B (substituent effects) C(H) detailed assignments S, H, B, C, MS S, H, B, C, IR, MS S, X(Me), H, B, C, MS S, H, B, C, IR, MS
[419] [21] [31] [153] [702] [614] [619]
S, S, S, S, S, S
[653] [654] [153] [713] [713] [666]
Silicon 1-SiMe3
1,12-(SiMe3)2 1-SiMePh2 1-SiPh3-12-R R ¼ H, Me 1-SiPh3-12-R R ¼ n-C5H11, EtCHMeCH2, C2 SC4H9, C 2C5H11, C6H42 2OC7H15, CH2CH2C6H10C6H4F C2 1,12-(C 2SiMe3)2 C)2C2B10Me10 (FF) 2C 1,12-(Me3Si2 1-Si(C6H13)3-12-C(CH2)3CI 2C(CH2)3C}2 (FF) {(C6H13)3Si-CB10H10C2 1-OSiMe3-12-CH(OEt)2
H, B, C, IR X, IR, UV X, H, B, C, MS H, B, C, IR, MS H, B, C, IR, MS
Continued
10.2 Synthesis and structure
597
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda
Informationb
References
Hg(CB10H10CR)2 R ¼ SiMe3, CHSiMe2 (FF) 2CB10H10C2 2SiR3 R ¼ n-C6H13, HCB10H10C2 n-C4H9, CMe3 (FF) 2(CB10H10C)22 2CB10H10C2 2 HCB10H10C2 SiR3 R ¼ n-C6H13, n-C4H9 (FF) C2 2-C 2SiMe3 C2 2,9-[C 2SiMe3]2 C2 2,9-[C 2SiMe3]2 B-CH2CH2SiMe3 B,B0 -(CH2CH2SiMe3]2 B-CH2CH2SiR3 R ¼ Cl, Me B,B0 -(CH2CH2SiR3)2 R ¼ Cl, Me n-CH2CH2SiCl3 n ¼ 1, B B-CH2CH2SiR3 R ¼ Me, Cl B,B0 -(CH2CH2SiR3)2 R ¼ Me, Cl 10 ,30 ,50 -{1,12-[[Et2S(CH2)3]2MeSi]]C2B10H10}3C6H3 (FF) 1,12-{Co2C2[SiMe3](CO)4[PPh2CH2PPh2]}2
B (substituent effects), H, IR S, H, B, C, IR, MS
[21] [714]
S, X (n-C4H9), H, B, C, IR, MS
[714]
S, H, B, C, MS S, H, B, C, MS S, X, H, B, C, MS MS (detailed) MS (detailed) S S MS (fragmentation patterns) IR (detailed study; inductive effect) IR (detailed study; inductive effect) S, H, B, C, IR, MS
[629] [629] [661] [461] [461] [462] [462] [715] [460] [460] [632]
S, X, H, B, IR, E
[716]
S B (substituent effects) C(H) detailed assignments B (substituent effects), H, IR
[419] [21] [31] [21]
S H, B (substituent effects), IR C(H) detailed assignments H (C2 2H shift), 119Sn (coupling constants) S, IR
[419,468] [21] [31] [24] [473]
Sn g-resonance spectra; quadrupole splittings; acceptor properties of cage S
[475] [466]
B (substituent effects), H, IR Mo¨ssbauer, pKa, E (half-neutralization potential)
[21] [226]
Mo¨ssbauer, pKa, E (half-neutralization potential)
[226]
S, H, B, IR
[525]
Germanium 1-GeMe3
2GeMe3]2 (FF) Hg[CB10H10C2 Tin 1-SnMe3
1-SnEt3 1-R-12-R0 [R ¼ SnMe3, SnEt3, Sn(n-C4H9)3]; R0 ¼ CH5 5CH2, CMe5 5CH2] 212-R 1-SnEt32 {2 2SnMe22 2[1,7-CB10H10C]2 2SnMe)22 21,12CB10H10C]}n polymer (FF) 2SnMe3]2 (FF) Hg[CB10H10C2 1-[(CH2)nC(O)O SnMe3 þ ]-12-R n ¼ 0, 1; R ¼ Me, Ph 1,12-[(CH2)nC(O)O SnMe3 þ ]2 n ¼ 0, 1 Arsenic 2-IrHCl(AsPh3)2
119
Continued
598
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda
Informationb
References
Exo-polyhedral transition metal derivatives Rhenium N2 Re½C 2CB10 H10 CH6 þ ðFFÞ
S, X, H, B, C, MS
[717]
S, ESR S, H, B, C, IR, E, MS, Raman, absorption and emission luminescence spectra S, H, B, C, IR, E (weak Fe-Fe coupling), MS Raman, absorption and emission luminescence spectra S, H, B, C, IR, Raman, E, MS, absorption and emission luminescence spectra X, Hg S, UV, E, NLO(b hyperpolarizability) S, X, H, B, C, IR, MS, E S, H, B, C, IR, MS, E
[510] [505,718] [718]
Iron 1-(Z6-naphthyl)FeCpþ 1-Fe(CO)2Cp
1,12-[Fe(CO)2Cp]2
[1-Fe(CO)2Cp-CB10H10C]2Hg (FF)
2p2 2CH5 5CHC6H4FeCp] 1-[C6H42 1-Fe(CO)PPh3 C]2 2C 1,12-[Cp(CO)2Fe2 Ruthenium 2NC5H32 2p2 2 (bpy)2Ru(NC5H42 2CB10H10C2 2p2 2C6H4Br)2þ C6H42 bpy ¼ bipyridine (FF) 2NC5H32 2p2 2 (bpy)2Ru(NC5H42 C6H42 2CB10H10C2 2p2 2C6H42 2 2NC5H4)RuðbpyÞ2 4þ bpy ¼ bipyridine NC5H32 (FF) photoluminescent complex C] 2C 1-[Cp*Ru(Ph2PCH2CH2PPh2)2 C]2 2C 1,12-[Cp*Ru(Ph2PCH2CH2PPh2)2 Cobalt 2C2B10H11)4 (FF) CpCo[Z4-C4(p-C6H42 1-Co2C2[SiMe3](CO)4[PPh2CH2PPh2] 1,12-{Co2C2[SiMe3](CO)4[PPh2CH2PPh2]}2 1,12-[(1,2-C2B9H11)Co(1,2-C2B9H10-8-O 2O(CH2)2]2C2 B10 H10 2 (FF) (CH2)22 [Co4(OH)2(O2C2 2CB10H10C2 2CO2)3(DMF)2]n polymers for CO2 adsorption and separation of CO2/CH4, CO2/N2, and O2/N2 mixtures Co(O2C2 2CB10H10C2 2CO2)(C5H5N)2(H2O) polymer for separation of CO2/CH4, CO2/N2, and O2/N2 mixtures
[505,718] [718] [505,718] [505] [513] [505] [505]
S, H, B, MS
[649]
S, H, B, MS,UV,E
[649]
S, X, H, C, B, P, IR, UV, E S, X, H, C, B, P, IR, UV, E
[719] [719]
S, S, S, S,
[637] [720] [716,720] [515,516]
X, H, C, B, IR, MS X, H, B, IR, E X, H, B, IR, E H, B, C, MS
S, TGA, variable-temp. X-ray diffraction, TGF, MAG
[721,722]
S, TGA, variable-temp. X-ray diffraction, TGF
[721]
Continued
10.2 Synthesis and structure
599
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda
Informationb
References
Iridium 2-IrHCl(EPh3)2 (E ¼ P, As)
S, H, B, IR
[525]
S, H, B, C, Pt S, H, B, C, Pt
[535] [535]
S, H, B, C, MS S, H, B, C, Pt, MS, cell toxicity
[314] [536]
S, X, H, C, IR S, X, H, C, IR S, H, C, F, IR
[533] [533] [533]
S, H, C, IR
[533]
S, H, C, IR
[533]
S, H, C, IR
[533]
S, H, C, IR
[533]
S, X, H, P
[534]
S, X, H, P
[534]
S, X, H, B, C, P, UV
[153]
S, H, B, C, P, UV
[153]
S
[108]
Platinum 1,12-[cis-Cl2(NH3)Pt(NH2)(CH2)3]2 [1,12-[trans-Cl(NH3)2Pt(NH2) (CH2)3]2C2B10H10]2þ ½OSO2 CF3 2 (FF) 1-CH2SPt(terpyridine)þ OSO2 CF3 1,12-[(CH2)3SPt(terpyridyl)]2C2 B10 H10 2þ ½OSO3 CF3 2 (FF) C2 1,12-(C 2C5H4N)2 1,12-{C C2 2C5H4N-trans-Pt[PEt3]2I}2 C2 1,12-[C 2C5H4N-trans-Pt [PEt3]2OSO2CF3]22 cyclo-½2; 9-C14 H8 3 fPt½PEt3 22 C2 2C 2CB10 H10 C2 2 NC5 H42 C2 C 2C5 H4 N2 2Pt½PEt3 2 g3 6þ (C14H8 ¼ phenanthrene) triangular macrocycle (FF) 2 cyclo-½1; 8-C14 H8 2 fPt½PEt3 22 C2 2C 2CB10 H10 C2 2 NC5 H42 C2 C 2C5 H4 N-Pt½PEt3 2 g2 4þ (C14H8 ¼ anthracene) rectangular macrocycle (FF) cyclo-f½Et3 P2 Ptg4 ðNC5 H42 2 C2 C2 C 2CB10 H10 C2 2C 2C5 H4 NÞ4 8þ square macrocycle (FF) C2 cyclo-fðCH2 Þ3 Pt½PEt3 2 g4 fC 2 C2 2Pt½PEt3 22 2C 2 C5 H4 N2 C2 CB10 H10 C2 2C 2Pt½PEt3 22 2 Cg4 8þ square macrocycle (FF) NC5 H42 2C cyclo-({C(O)O2 2Pt[PEt3]2-1,8-C14H8}C2B10H10)2 (C14H8 ¼ anthracene) rectangular macrocycle (FF) cyclo-({C(O)O-Pt[PEt3]2-2,9-C14H8}C2B10H10)2 (C14H8 ¼ phenanthrene) rhomboidal macrocycle (FF) C]2Pt[P(n-C4H9)3]2 [1,12-HCB10Me10C2 2C (FF) C2 C2 2C 2C 2 [1,12-HCB10Me10C2 C]2Pt[P(n-C4H9)3]2 (FF) CB10Me10C2 2C Copper 1-Cu
Continued
600
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda Gold 1-PPh2AuCl 2CB10 H10 C2 2PPh2 Þ2 þ Cl (FF) AuðPPh22 1,12-(PPh2AuCl)2 1,12-[Au(PPh3)]2 CAuL)2 {L ¼ PPh3, P(C6H4-41,12-(C OMe)3, C[NH(tert-C4H9)][NEt2], (tert-C4H9) NC} C2 CAu]x (FF) [AuC 2CB10H10C2 2C C2 C2 2C 2 [HC 2CB10H10C2 CH] (FF) C2 2C Au2 2C 2CB10H10C2 C2 C2 [HC 2CB10H10C2 2C 2 C2 C2 Au2 2C 2CB10H10C2 2C 2 C2 CH]3 (FF) Au2 2C 2CB10H10C2 2C Mercury 1-HgMe 1-HgX X ¼ Cl, Br, I 1-R-12-HgX X ¼ Cl, Br, I; R ¼ H, Me, Ph Hg(C2B10H11)2 (FF)
Hg(C2B10H11)2 OC4H8 (FF) 9-HgR R ¼ Cl, Br, Et 1-HgMe Hg(CB10H10CMe)2 (FF) Hg(CB10H10CPh)2 (FF) Hg(CB10H10CR)2 R ¼ H, SiMe3, GeMe3, SnMe3, CHSiMe2, HgMe (FF) 2CB10H10CH)2 (FF) Hg(CB10H10C-m-C6H42 2Ph)22,20 -bipyridine (FF) Hg(CB10H10C2 2Ph)2}22,20 -bipyridine (FF) {Hg(CB10H10C2 2Fe(CO)2Cp]2 (FF) Hg[CB10H10C2
2-R R ¼ HgOC(O)CF3, HgCl
Informationb
References
S, S, S, S, S,
[698] [698] [698] [62] [723]
H, B, C H, B, C H, B, C X, H, P, luminescence, emission spectra X[P(C6H4-4-OMe)3], H, B, C, IR
S, H, B, C, IR S, X, H, B, C, IR
[723] [723]
S, H, B, C, IR
[723]
S B (substituent effects), H, IR IR, Raman MS (detailed) S MS (detailed) E (electron affinity of B-Hg bond) E (comparison with other R2Hg) IR, Raman Raman (vibrational frequencies) ED S, X, H, B, C, IR Raman (vibrational frequencies) Raman (vibrational frequencies) MS (detailed) S, X, Hg B (substituent effects), H, IR
[676] [21] [541] [547] [615] [547] [556] [51] [541] [485] [724] [725] [485] [485] [547] [632] [21]
S, H, B, C, IR, MS S, X S, X S, H, B, C, IR, Raman, E, MS, absorption and emission luminescence spectra X, Hg S
[632] [632] [632] [505,718] [505] [548] Continued
10.2 Synthesis and structure
601
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda
Informationb
References
2-HgCl Hg(2-C2B10H11)2 (FF)
Raman (vibrational frequencies) Raman (vibrational frequencies) E
[485] [485] [549]
SCF Ab initio, stability Ab initio, thermodynamic parameters Ab initio structural, vibrational; IR Bond distances (Gaussian 98) C2 2H bond length compared with halomethanes Electron delocalization Charge distribution Ab initio electron-transfer [ET] properties CNDO, spherical electron distribution; comparison with cubane Molecular motion in 2 plastic phases; comparison with 1,2-C2B10H12 Atomic motion via classical force field and quantum mechanical calculations Isotopomers; nuclear spin weights Free electron MO calculations Computer-aided molecular design (CAMD) for modeling and docking carboranes for BNCT Intermolecular forces in solid MM3 force field Electron affinities, acidity Influence of charge, spin, substituents, and atom encapsulation on volume of cage DFT, cage substitution DFT, CASPT2 DFT, stability
[726] [571–574,703,727] [575] [576] [19] [581] [579] [309] [728] [729]
[585,586] [587] [588]
Ab initio electron-transfer [ET] properties Electron densities Ab initio; cluster parameters
[728] [634] [703]
b (first hyperpolarizability); NLO
[589]
Theoretical Studies Molecular and electronic structure calculations Parent
C2B10H11•, C2B10H10•• radicals X@C2B10H12 X ¼ Liþ, Be2 carboraneencapsulated ions ðCH2 Þ2 C2 B10 H10 þ (FF) 1,12-Ph2 1,12-X2 X ¼ Li, BeH, F, Cl, CN, Me, SiH3, OH, SH, H2, BH2 RR0 C2B10H10n n ¼ 0, 2; R ¼ C7H6, B12H11; R0 ¼ C5H4, H (FF)
[730] [731] [732] [577] [733] [570] [578] [571] [582]
Continued
602
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda
Informationb
References
1,12-(OH)2 n-SH n ¼ 1,2
DFT Charge distribution EHMO, NEMO; charge distribution EHMO, NEMO; charge distribution Dipole moments
[170] [309] [176] [176] [704]
Electron densities
[634]
Electron densities
[634]
b (first hyperpolarizability); NLO
[589]
b (first hyperpolarizability); NLO
[589]
Dipole moment Dipole moment Hybridization, bond order, comparison with Y derivatives of other clusters; electronic X cage-substituent interactions Ab initio enthalpies, free energies, phase transitions; ZINDO (UV-vis)] Dipole moment; electronic effects transmission EHMO, NEMO; charge distribution Conformational energies
[344] [344] [654]
Electron densities
[634]
EHMO, NEMO; charge distribution EHMO, NEMO; charge distribution C2 2C bond lengths
[176] [176] [153]
n-SCH2C(O)OH n ¼ 1,2 1,12-(SH)2/9,12-(SH)2 attached to surface of Au microcrystals 1-p-C6H4R R ¼ OMe, NO2, NMe2, CF3, OH, NH2, Me 1,12-(p-C6H4R)2 R ¼ OMe, NO2, NMe2, CF3, OH, NH2, Me 1-C7H6-12-R R ¼ C5H4, C5Me4, C5Et4, C5(CN)2Me2, C5(CN)2Ph2 2C7H6; RCB10H10CR2 R ¼ B12H11, B12H102 R0 ¼ H, C5H4, C7H6 (FF) 1-CH2X X ¼ Cl, Br, I 1-X X ¼ Cl, Br, I CSiMe3)2, C CR, C N, 1,12-R2 R ¼ (C O, N N C RCB10H10C2 2CB10H10C2 2R R ¼ C7H15, CC5H11 liquid crystal (FF) C Me2N2 2C6H4-p-CB10H10C2 2p-C6H42 2NO2 2-OH 2CB10H10C2 2C(O)O-(C6H4)2EtMeCHCH22 cyclo-C5H11 optically active mesogenic esters (FF) (18-crown-6)þ [(p-C6H4OH)(p-C6H4OH) C2B10H10] (FF) 1-C(O)OH n-SCH2C(O)OH n ¼ 1,2 5CHCl, 1,12-R2C2B10H10 R ¼ C(O)H, CH5 SiMe3 (FF) C2 C2 1,12-RCB10Me10C2 2C 2C 2 CB10Me10CR0 [R, R0 ¼ H, (CH2)2OH, CH2C CH, C CC (O)H, CH2CHCl2, C(O)OMe, C CC(O)O(CH2)3Me, SiMe3] (FF) (O)OMe, C C] 2C 1-[Cp*Ru(Ph2PCH2CH2PPh2)2 C]2 2C 1,12-[Cp*Ru(Ph2PCH2CH2PPh2)2 1,12-[Au(PPh3)]2
C2 2C bond lengths
[153]
DFT, electronic structure DFT, electronic structure DFT
[719] [719] [62]
Isomerization calculations Parent 1,12-Me2C2B10Cl2H8 (FF)
Cage isomerization of 1,2-C2B10H12 Cage isomerization of 1,2-Me2C2B10Cl2H8
[592–594] [593]
[621] [627] [176] [689]
Continued
10.2 Synthesis and structure
603
Table 10-2 Selected 1,12-C2B10H12 Derivatives—Cont’d Synthesis and Characterization Compounda
Informationb
References
C, IGLO B (paramagnetic contribution) C
[33] [596] [170]
Protonation Reduction to C2 B10 H12 2 isomers DFT (R ¼ OH, O, H2Oþ) proton-driven conformational change; intramolecular H bond
[734] [735] [645]
NMR calculations Parent
1,12-(OH)2 Reactivity calculations Parent
2-(o-C6H4R) a
Substituents on the carborane cage. “FF” indicates that the full formula of the compound is given. S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; F, 19F NMR; P, 31P NMR; Pt, 195Pt NMR; IR, infrared data; MS, mass spectroscopic data; UV, UV-vis data; E, electrochemical data; ESR, electron spin resonance; NLO, nonlinear optical properties; DSC, differential scanning calorimetry; BNCT, boron neutron capture therapy; ED, gas phase electron diffraction; MAG, magnetic susceptibility.
b
Thermal rearrangement of substituted derivatives of o-carborane can be conducted successfully provided the substituent groups can withstand the high temperatures required; for example, 1,2-MeC2B10H11 is converted to 1,7-MeC2B10H11 in 69% yield at 400 C, but 1,2-(HOCH2)2C2B10H10 decomposes [76]. A useful early discovery was the finding that 1-isopropyl-o-carborane at 450 C rearranges to parent m-carborane in >90% yield with loss of the alkyl group [8]. In C,C0 -disubstituted o-carboranes having bulky groups attached to the cage carbon atoms, the temperature required for cage rearrangement is lowered by steric repulsion between the substituents; for example, 1,2-(RPh2Si)2C2B10H10 (R ¼ Me, Cl) and 1,2-(ClMe2Si)2C2B10H10 isomerize to the corresponding m-carborane derivatives at 260 and 280 C, respectively [444,446]. The molecular geometry of m-carborane (Figure 1-2) was first deduced by 11B NMR evidence [27,28], supported by X-ray crystallographic investigations of 1,7-H2C2B10Cl10 [372] and other derivatives (Table 10-1) and established in the gas phase by electron diffraction data [17–19]. The solid state structure of the pure parent compound is complicated by carbon atom disorder, a problem that has been overcome by an X-ray diffraction study of a hexamethylphosphoramide (HMPA)-carborane cocrystallite in which an ordered structure is enforced by strong C2 2H O hydrogen bonding [13]. 2B and B2 2B bond lengths of 1.678(5)Precise gas-phase electron diffraction data [19] on 1,7-C2B10H12 yield C2 ˚ and 1.771(6)-1.801(5) A ˚ , respectively, distances that compare well with corresponding values obtained from 1.730(4) A X-ray crystallography [13] and ab initio calculations (Table 10-1).
10.2.2 p-Carborane The thermal rearrangement of 1,7- to 1,12-C2B10H12 takes place above 600 C with some decomposition to intractable products. Continuous passage of o-carborane vapor in an N2 atmosphere through a heated tube at 6623 C affords p-carborane in ca. 25% yield along with ca. 75% m-carborane; separation of the isomers is achieved on basic alumina [2,3]. Other than oxidation of metal complexes of nido-C2 B10 H12 2 (Chapter 11) to afford all three icosahedral C2B10H12 isomers [10,11], which is not practical as a synthetic approach, thermal isomerization remains the only available route to p-carborane.
604
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
The highly symmetrical D5d geometry of p-carborane was inferred very early from its 11B and 1H NMR spectra, which reveal a single BH environment, and its zero dipole moment; later gas-phase electron diffraction studies [17,19] and a solid-state X-ray investigation of its hydrogen-bonded HMPA-carborane cocrystallite [13] (vide supra) confirmed ˚ ], B(2)–B(3) [1.785(1) A ˚ ], and B(2)–B(7) [1.774(4) A ˚] the structure shown in Figure 1-2. The C2 2B [1.698(3) A bond lengths [19] determined by electron diffraction may be compared with those listed above for m-carborane. Additional crystallographic support is available from X-ray structural studies of a 1:2 p-carborane-N(H)P[NMe2]3 adduct and a pcarborane-H2 NP½NMe2 3 þ HCO3 cocrystallite [14], and an electron diffraction analysis of 1,10 -(1,12-C2B10H11)2 [612].
10.3 CAGE REARRANGEMENT MECHANISMS In the half-century following the discovery of o-carborane and its thermal isomerization to m- and p-carborane, elucidation of the mechanistic details of this process has been the target of a host of experimental and theoretical studies. As far back as the first edition of this book [738], this topic had already engendered a substantial number of papers and reviews in which a wide variety of possible mechanistic pathways was considered. The effort has continued to the present time, and with the aid of powerful theoretical tools developed in recent years the problem is close to a final resolution. The difficulties in establishing the operative mechanism, or mechanisms, for the isomerization are related in part to the high temperatures required, and also to the inability to isolate reaction intermediates in the unsubstituted carboranes. Accordingly, most experimental studies have utilized substituted derivatives whose attached groups serve as vertex labels, and in some cases also lower the energy requirement for rearrangement [446]. However, this introduces the complication that the substituents—halogens, for example—can themselves alter the rearrangement pathway through electronic and/or steric effects. Lipscomb and coworkers made the first proposal of a detailed mechanism [739–741], the cooperative diamond-square-diamond or DSD rearrangement, in which six diamond-like pairs of adjacent triangles in the icosahedron are converted to square faces by stretching the common edge in each pair, creating a cuboctahedral intermediate from which an icosahedron with separated carbons is easily generated as shown in Figure 10-1. The DSD mechanism was quickly seen to have two attractive features: it requires minimal bond-stretching and hence low activation energy, and it readily accounts for the 1,2- to 1,7-C2B10H12 isomerization in which adjacent carbon atoms become separated. However, there are problems. Although tensor harmonic studies have demonstrated that DSD processes in general are viable for polyhedral clusters [595], the DSD pathway in itself cannot generate 1,12-C2B10H12 from either the 1,2 or 1,7 isomers; moreover, experimental data on the thermal rearrangement of B-halo-o-carboranes give product distributions that are inconsistent with a simple DSD process [742]. This has led to other proposed mechanisms including rotation of two halves of the icosahedron (pentagonal pyramids) in opposite directions [75,354], rotation of triangular faces on the icosahedral surface [592,743], and combinations of these. For example, if one assumes a DSD process as in Figure 10-1 but allows rotation of triangular faces on the cuboctahedral intermediate, it is possible to account for the formation of 1,12-C2B10H12 as well the as the observed products of halo-o-carborane rearrangements [349]. Even in this case, some ad hoc assumptions are required in order to account for experimental results, e.g., preferential rotation
FIGURE 10-1 Proposed DSD rearrangement of o-carborane to m-carborane.
10.3 Cage rearrangement mechanisms
605
of triangles furthest from the carbon atoms, and a 9:1 ratio of conversion of the cuboctahedral intermediates to 1,2-C2B10 versus 1,7-C2B10 products [742]. In the m- to p-carborane isomerization, the experimental data suggest that rotations of B3 and B2C triangles occur to an equal extent [744]. Further studies on the isomerization of mono- and dihalo-o-carboranes yield similar product distributions, although the rotating-pyramidal mechanism is favored by these authors [745]. The latter process has also been invoked to explain the thermal conversion of the phosphacarborane 1,2-PCB10H10-12-Cl to the 1,7-isomer [746]. Over time, other proposals have been advanced, including extended triangle rotation (ETR) [747], cross-polyhedral binding [748], single-edge cleavage [749], and isomerization via anti-cuboctahedral [593,750], and 12-vertex nido-C2B10 intermediates, the last of these having been deduced from a 10B atom labeling investigation [751]. In recent years, additional experimental data and state-of-the-art computational studies [752,753] have shed new light on the rearrangement processes. A DFT investigation employing B3LYP methods for calculating vibrational frequencies and single-point electronic energies has given results that conform very closely with experimental findings [752]. These calculations indicate that the lowest free energy route from o-carborane to m-carborane involves a triangular face-rotation process with an activation energy of 207.6 kJ/mol, illustrated in Figure 10-2. This type of mechanism is equivalent to performing DSD operations on the quadrilateral faces of a cuboctahedral intermediate, shown as the middle structure in Figure 10-2. For the m-carborane to p-carborane rearrangement, the same study finds that the lowest energy process does not entail triangle rotations, but instead takes place via a series of open-faced intermediates. As shown in Figure 10-3, initial
FIGURE 10-2 Calculated mechanism for conversion of o-carborane to m-carborane via triangular face rotation.
•
⇒
• • 1,7-C2B10H12
⇒
• 10-1
•
⇒
⇒ •
• 10-3
⇒
• • 10-4
⇒
• • 1,12-C2B10H12
• 10-2
FIGURE 10-3 Calculated pathway for thermal isomerization of m-carborane to p-carborane via intermediates 10-1, 10-2, 10-3, and 10-4.
606
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
bond-breaking in m-carborane creates structures 10-1 and 10-2, both of which convert to an unstable closo species 10-3 which, it will be noted, has one carbon atom occupying a four-coordinate vertex that is adjacent to a six-coordinate boron. Rearrangement of this intermediate through successive DSD operations gives first 10-4 and then the final product, p-carborane [752].
10.4 “REVERSE ISOMERIZATION”: 1,12- ! 1,7- ! 1,2-C2B10H12 According to the polyhedral skeletal electron pair theory (PSEPT) described in Chapter 2, addition of two electrons to a 26electron closo-C2B10H12 cluster should open the cage to give a 28-electron nido-C2 B10 H12 2 species. Early molecular orbital calculations [739,754] suggested that dianions formed by reduction of o- and m-carborane, unlike the neutral isomers, would be of comparable stability, thereby creating a possible pathway for m- to o-carborane conversion based on the oxidation of a C2 B10 H12 2 dianion obtained from m-carborane. This prediction was verified, at first tentatively [75] and later conclusively [118,755–757], by the observation that alkali metals in liquid ammonia or THF react with 1,7RR0 C2B10H10 (R, R0 ¼ H, Me, or Ph) carboranes without evolution of H2 to generate RR0 C2 B10 H10 2 ions that on oxidation with O2 or permanganate yield 1,2-RR0 C2B10H10 products. Subsequently, it was found that p-carboranes on reduction form dianions that oxidize to yield m-carboranes [12,616,758,759] or, in some derivatives, both o- and m-carboranyl products [760]. The relationship of these species is summarized in Figure 10-4. As shown, two-electron reduction of either o- or m-carborane affords the same species, nido-7,9-C2 B10 H12 2 , which oxidizes to o-carborane. Reduction of p-carborane leads to a different dianion, nido-7,10-C2 B10 H12 2 , which on oxidation forms m-carborane; DFT calculations indicate that the reaction is complex and initially affords two unstable isomers that rearrange to the 7,9 and 7,10 forms [735]. In accordance with these findings, the reduction of C,C0 -diphenyl derivatives of o- and m-carborane [121] affords the same nido-7,9-Ph2 C2 B10 H10 2 dianion, and reduction of 1,12-diphenyl-p-carborane yields nido-7,10-Ph2 C2 B10 H10 2 . However, the latter species, in contrast to parent 7,10-C2 B10 H12 2 , undergoes a very facile rearrangement to 7,9Ph2 C2 B10 H12 2 [635]. The nature of these and other open-cage 12-vertex ions, including the protonated RR0 C2 B10 H11
Δ
Δ
1,7-C2B10H12
1,2-C2B10H12
1,12-C2B10H12
−
−2e
−2e−
2e−
2e−
2−
2−
2−
nido-7,9-C2B10H12
FIGURE 10-4 Interconversion of o-, m-, and p-carborane via nido dianions.
2e−
2− nido-7,10-C2B10H12
10.5 Substitution at carbon R R
R R
•
•
•
•
(1) 2n-C4H9Li
607
R
(2) 2
R
10-5
(1) 2n-C4H9Li (2) 2FeII, Cp−
R Fe
R
•
R
R Fe
•
R R
R R
(1) 2e− (2) −2e−
Fe
•
•
Fe
R2 = Me2, (CH2)5
10-7
10-6
FIGURE 10-5 Conversion of 1,7-[(C5H5)CR2]2C2B10H10 (10-5) to the 1,2 isomer 10-7 via reduction and reoxidation. The stretched cage C–C interaction in 10-7 is depicted as a dashed line.
monoanions, is discussed more fully in the following chapter, together with isoelectronic neutral C4B8 clusters and other related systems. An ingenious application of meta- to ortho-carborane isomerization via reduction and subsequent oxidation is seen in the synthesis of the 1,2-bis(ferrocenylmethylene)-o-carboranes 10-7 (Figure 10-5) [89]. The extremely bulky substituents on the adjacent cage carbon atoms preclude the preparation of these compounds directly from o-carborane as the introduction of the first group blocks entry of the second, but reduction of the easily accessible m-carboranyl analogues to a presumed open-cage species followed by reoxidation affords 10-7. The extraordinary steric crowding in this system ˚ in the (CH2)5-substituted is shown by the crystallographically determined cage carbon-carbon distance of 2.156(4) A species, by far the longest such interaction known in a true o-carborane derivative (the normal C2 2C separation is typi˚ ) [89]. cally about 1.63 A
10.5 SUBSTITUTION AT CARBON 10.5.1 Metallation with Group 1 and 2 metals The lower C2 2H acidity in m- and p-carborane relative to the ortho isomer, mentioned earlier, has been explored in detail and quantitative data are available for the three C2B10H12 isomers. Direct comparison between carborane systems is complicated by the fact that pKa is strongly influenced by substituents on the cage and also by the choice of solvent [52,761]. Nevertheless, competitive C-metallation studies via reaction with n-butyllithium in 1,2-dimethoxyethane (DME) at 25 C have yielded equilibrium acidities for the three parent carboranes and several derivatives [53]. Under these conditions the pKa values for o-, m-, and p-carborane, determined spectrophotometrically, are respectively 22.0, 25.6, and 26.8; in separate studies employing organopotassium metallation reagents in DME [54,55,762], the corresponding values are 23.3, 27.9, and 30.0, the higher numbers reflecting the fact that the carborane C2 2H bond is a stronger acid toward lithium aliphatic-aromatic reagents versus their potassium analogues [763]. The overall trend is clear: CH acidity decreases sharply in going from o-carborane to m-carborane, but the difference between m- and
608
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
p-carborane is small. These findings correlate with a decrease in C2 2H bond polarity in the order o- > m- > p-carborane, 2H spin coupling constant, and other properties [34]. All of as well as with decreasing C2 2H vibrational frequency, 13C2 these observations, in turn, can be seen as resulting from a more even electron density distribution in the 1,7- and 1,12C2B10H12 clusters versus the 1,2-C2B10H12 system, as supported by numerous theoretical studies cited in Tables 9-1, 10-1, and 10-2. Even the relatively weaker acids m- and p-carborane are much stronger proton donors than hydrocarbons; for example, the pKa of toluene under comparable conditions is 35 [53]. The electron-attracting power of the carborane cage is 2H bonds but nonetheless reacts with n-butylevident in 1-benzyl-2-methyl-o-carborane (pKa 19.5), which lacks cage C2 lithium, losing a proton exclusively from the methylene group [53]: C4 H9 Li
PhCH22 2CB10 H10 C2 2Me ! PhCH2 2CB10 H10 C2 2Me Liþ þ C4 H10 The effect on CH acidity of formally replacing one of the CH units in m-carborane with an isoelectronic P: or As: atom to form the heterocarboranes 1,7-PCB10H10 and 1,7-AsCB10H10 (Chapter 12) is seen in their pKa values of 24.1 and 24.7, respectively; for the p-carborane analogues 1,12-PCB10H10 and 1,12-AsCB10H10, the corresponding values are 25.9 and 26.8 [53]. As with o-carborane, C-metallation of m- and p-carborane can be achieved via treatment with Grignards [221,228,347] and other metal reagents including n-butyllithium [75–78,102,110,222,234, 268,600,615,677,678], MAlH4 (M ¼ Li, Na) [406], copper(I) halides [107,139,160,161], and sodium amide in liquid ammonia [87]. In general, 1:1 reactions of 1,7-C2B10H12 with metallating reagents afford primarily mono-C-metallated derivatives with smaller amounts of C,C0 -dimetallated species, indicating an equilibrium of the type 1; 7-LiC2 B10 H11 > 1; 7-Li2 C2 B10 H12 þ 1; 7-C2 B10 H12 with only a small fraction of the dilithio species present, as noted in Section 9.4. Similar equilibria are found in C-bromagnesio-m-carboranes. In competitive reactions of sodium amide with p-carborane, the ratio of mono- to disubstituted C-lithio derivatives is stoichiometry-controlled, but similar reactions with n-butyllithium are strongly solvent-dependent and tend to give nonstoichiometric monolithio- to dilithio-p-carborane product ratios [662,666].
10.5.2 Substitution via C-metallated carboranes 10.5.2.1 Addition of boron at carbon
Reactions of C-monolithio-m-carborane with phenylboron dichloride, or of C,C0 -dlithio-m-carborane with diphenylboron chloride, give respectively 10-8 and 10-9, paralleling the synthesis of the o-carboranyl phenylboron derivatives 9-15 and 9-16 described in Section 9.4 [129]. Ph B
C
C
C
C H
BPh2
Ph2B
C
C
10-8
H
10-9
The preparation of p-carboranyl boronic acid derivatives, (HO)2B2 2CB10H10C2 2R, (EtO)2B2 2C-B10H10C2 2R 2CB10H10C2 2R [R ¼ CH(OEt)2, C(O)H] via reactions of C-lithio [R ¼ CHEt2 or C(O)H], and [OCPhCH2CPhO]B2 species with trimethyl borate and subsequent steps, has been reported without details [666].
10.5 Substitution at carbon
609
10.5.2.2 Addition of aluminum and thallium at carbon m- and p-Carboranyl derivatives with C2 2Al bonds are accessible via treatment of parent 1,7- or 1,12-C2B10H11 with alkali metal aluminum hydrides in THF at 20 C [406]. THF
HCB10 H10 CH þ MAlH4 ! ðHCB10 H10 CÞn AIH4n Mþ
M ¼ Li; Na;
n ¼ 1; 2
Reactions of C-lithio-m-carboranes with thallium(III) chloride generate C-thallated derivatives [408]. LiCB10 H10 CR þ TlCl3 ! ðRCB10 H10 CÞ2 TlCl R ¼ H; Ph; CH2 Cl
10.5.2.3 Addition of silicon, germanium, tin, and lead at carbon The methods employed to prepare o-carboranyl Group 14 derivatives (Section 9.4) are generally applicable to the m- and p-carborane systems, except that the formation of exocyclic rings—common in o-carborane derivatives (see Figure 9-1)— is not seen. This is advantageous in the synthesis of m- and p-carboranyl silicon polymers (Chapter 14) as side reactions leading to cyclic monomers or dimers are avoided. Figure 10-6 outlines some of the main routes to C,C0 -disilyl m-carborane derivatives [420,435,446,466]. Reactions of C-monolithio- and C,C0 -dilithio m- or p-carboranes with trialkyl or triarylsilyl halides, alkoxychlorosilanes, or related compounds afford a range of C2 2Si derivatives, listed in Tables 10-1 and 10-2. Triphenylsilyl groups are particularly useful as protecting groups via attachment to one of the m- or p-carboranyl cage carbon atoms [421]; as C-lithiation is now restricted to the remaining CH vertex, this furnishes an efficient route to C-monosubstituted derivatives from which the SiPh3 group is removed on treatment with fluoride ion (Figure 10-7) [123,614,619,702]. Derivatives of m- and p-carborane featuring C2 2Ge, C2 2Sn, and C2 2Pb bonds (Tables 10-1 and 10-2) are similarly prepared from C-lithio carboranes and Group 14 halides; p-carboranyl C2 2Pb compounds have not been described. THF; 0 C
1; 7-Li2 C2 B10 H10 þ R2 MCl2 ! 2 2½CB10 H10 C2 2MR22 2n2 2 M ¼ Si; Ge; Sn; Pb R ¼ Me; Et; n-C4 H9 ; Ph
Li
Li C
C
SiR2Cl
CIR2Si C
R2SiCl2
C
H2NR2Si NH3
SiR2NH2 C
C
Et2O
MeSiCl3 Et2O
MeOH
R = Me, Ph
H2 O MeOR2Si SiMeCl2
Cl2MeSi C
SiR2OH
HOR2Si
C
C
C
C
C
H2O
(HO)2MeSi
SiMe(OH)2
FIGURE 10-6 Synthesis of 1,7-disilyl m-carborane derivatives from 1,7-Li2C2B10H10.
SiR2OMe C
C
610
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12 H
C
SiPh3
SiPh3
H
C
C
C
(1) n-C4H9
C
+
+
(2) Ph3SiCl
C
C
C
H
SiPh3
H
H
n-C4H9NF
(1) MeLi (2) MeI
SiPh3
H
C
C n-C4H9NF
C
C
Me
Me
FIGURE 10-7 Synthesis of 1,12-MeC2B10H11 via C-triphenylsilyl derivatives.
Reactions of 1,7-dilithio-m-carborane with tetrachlorogermane and dichlorodimethylgermane in ether [466] and THF [449,764] are typical, those with the latter reagent demonstrating a tendency to form short polymers. Et2 O
1; 7-Li2 C2 B10 H10 þ GeCl4 ! 1; 7-ðCl3 GeÞ2 C2 B10 H10 Et2 O
1; 7-Li2 C2 B10 H10 þ Me2 GeCl2 ! 1; 7-ðClMe2 GeÞ2 C2 B10 H10 ð20%Þ 2½CB10 H10 C2 2MR22 2n2 2 n 6ð80%Þ þ ClMe2 Ge2 With dichlorodiphenylstannane in ether, the main product is the monosubstituted derivative, but in toluene some polymers are generated [466]. Et2 O;25 C
1; 7-Li2 C2 B10 H10 þ Ph2 SnCl2 ! 1; 7-ClPh2 Sn2 2C2 B10 H11 Monomeric mono- and difunctional C-stannyl m-carboranes are obtainable from organochlorostannane reagents [187,472,474]. Et2 O
1; 7-Li2 C2 B10 H10 þ R3 SnCl ! 1; 7-ðR3 SnÞ2 C2 B10 H10 Et2 O
1; 7-Li-CB10 H10 C2 2R0 þ SnCl2 ! ðR02 CB10 H10 Þ2 Sn
R ¼ n-C4 H9 ; Ph
R ¼ Me; Ph
R0 ¼ Me; Ph
As in the case of o-carborane (Section 9.12), the introduction of stannyl groups at carbon in m-carborane and p-carborane can also be accomplished via direct reaction with stannylamines [469,473].
10.5 Substitution at carbon
611
120 C
R2 2CB10 H10 CH þ R03 Sn2 2NEt3 ! HCB10 H10 C2 2SnR03 þ R03 Sn2 2CB10 H10 C2 2SnR03 R ¼ H; Et3 Sn; CH5 5CH2 ; CMe5 5CH2 R0 ¼ Me; Et; n-C4 H9 140 C
R02 2CB10 H10 CH þ R2 SnðNEt3 Þ ! R02 2CB10 H10 C2 2SnR3 R ¼ Me; Et; n-C4 H9 R0 ¼ H; Me3 Sn; Et3 Sn; ðn-C4 H9 Þ3 Sn
10.5.2.4 Addition of nitrogen, phosphorus, arsenic, antimony, and bismuth at carbon The formation of Ccage2 2N bonds on m- and p-carborane is seldom achieved directly from their C-metallated derivatives, 2N(boc)NH(boc) hydrazines [R ¼ H, C(O)OH; boc ¼ terta rare example being the synthesis of 1-R2 2CB10H10C2 butyloxycarbonyl] from 1,7- or 1,12-LiC2B10H11 and N2H[C(O)OCMe3]2 [268]. Treatment of 1-lithio-p-carborane with NOCl [676] or methoxyamine followed by hydrolysis [230] affords 1,12-(ON)C2B10H11 or 1,12-(H2N)C2B10H11, respectively, but in low yields. Other routes to C2 2N derivatives have proved more generally useful. The m- and p-carboranyl C,C0 -dicarboxylic acids can be converted to heterobifunctional C-amino-C0 -carboxyl derivatives—important synthetic agents for boron neutron capture therapy (BNCT) and other biomedical applications—by reaction with diphenylphosphoryl azide (DPPA) and Et3N in alcoholic media. Acidification of the boc-protected amino group affords the NH2 derivative [230]. ðPhOÞ2 PON3 ð1Þ n-C4 H9 Li 2CB10 H10 C2 2CðOÞOH ! 1; 7- or 1; 12-HCB10 H10 CH ! HOðOÞC2 ð2Þ CO2 ;ð3Þ HCl HCl
TEA=Me3 COH
HOðOÞC2 2CB10 H10 C2 2NHOCMe3 ! OðOÞC2 2CB10 H10 C2 2NH2 Monofunctional m- and p-carboranyl C-amino and C-isocyanato derivatives can be efficiently produced from the carbonyl chloride via conversion to the azide, which undergoes a modified Curtius rearrangement to give the final product [234]. Me3 SiN3
2CðOÞCl ! ! HCB10 H10 C2 2NCO 1; 7- or 1; 12-HCB10 H10 C2 toluene reflux
1; 7- or 1; 12-HCB10 H10 C2 2CðOÞCl ! ! HCB10 H10 C2 2NHCðOÞOCMe3 Me3 COH reflux
Derivatives of m- and p-carborane with Ccage2 2P bonds (Tables 10-1 and 10-2) are typically prepared from mono- or dilithiocarboranes and halogenated reagents, as in the synthesis of phosphonous acid and phosphines [296,297,300,698]. As expected, cyclic products are not formed, in contrast to the extensive cyclization in the o2N and Ccage2 2P bonded m- and p-carboranes is carboranyl system shown in Figure 9-21. The chemistry of Ccage2 discussed in Section 10.14. ðEt2 NÞ2 PCl
HCl
H2 O
1; 7-LiC2 B10 H11 ! ðEt2 NÞ2 P2 2C2 B10 H11 ! Cl2 P2 2C2 B10 H11 ! ðOHÞ2 P2 2C2 B10 H11 Et2 O=C6 H6 reflux
PR2 Cl
1; 7- or 1;12-Li2 C2 B10 H10 ! ðR2 PÞ2 C2 B10 H10
R ¼ Cl; Ph
EtOPMeCl
1; 7- or 1;12-R2 2CB10 H10 C2 2Li ! R2 2CB10 H10 C2 2PMeðOEtÞ R ¼ H; Me; CH25 5CMe; Ph 2As bonded m-carboranyl compounds are extremely rare. The 1-(Et2N)2As-7-CHMe2 derivative, prepared Ccage2 2C2B10H11 on treatment with AsCl2; hydrolysis of the from 1,7-LiC2B10H11 and AsCl(NEt2)2, is converted to Cl2As2 2Sb or C2 2Bi bonds latter species affords OAs2 2C2B10H11 [482]. Derivatives of m-carborane having exopolyhedral C2 are unknown, as are C2 2As, C2 2Sb, and C2 2Bi derivatives of p-carborane.
612
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
10.5.2.5 Addition of oxygen at carbon The methods described in Section 9.4 for preparing C-hydroxy derivatives of o-carborane are, in general, applicable to m- and p-carborane as well. Most of these syntheses involve oxidation of C-lithio-carboranes with O2 [175], benzoyl peroxide [117,173,174], or trimethylsilyl peroxide [172], or reaction of the C-mono- or C,C,0 -dilithio derivative with trimethylborate followed by oxidation with H2O2 and acetic acid [170]. benzoyl peroxide
2Li ! Ph2 2CB10 H10 C2 2OH 1; 7-Ph2 2CB10 H10 C2 toluene; reflux
BðOMeÞ3
HOðOÞAc
Li2 2CB10 H10 C2 2Li ! ðMeOÞ2 B2 2CB10 H10 C2 2BðOMeÞ2 ! HO2 2CB10 H10 C2 2OH O2
An alternative route to the mono-C-hydroxy derivative is via diazotization of (H2N)C2B10H11 [171].
10.5.2.6 Addition of sulfur, selenium, and tellurium at carbon Sulfur-cage carbon bonds are generated in m- and p-carborane from C-metallated derivatives, as in the synthesis of C2 2S bonded o-carboranes (Section 9.4) [117,308,310–312,619,676,765]. 2RSSR
1; 7-Li2 2CB10 H10 C2 2Li ! RS2 2CB10 H10 C2 2SR R ¼ H; Et; C6 H4 Me 2RSLi
ð1Þ S;liq:NH3
1; 7- or 1; 12-R2 2CB10 H10 C2 2M þ! R2 2CB10 H10 C2 2SH ð2Þ H
M ¼ Li; Na; K
R ¼ H; Me; Ph
S8
1; 12-Li2 2CB10 H10 C2 2Li ! HS2 2CB10 H10 C2 2SH The thiols are reactive species that are easily converted to other derivatives as outlined in Section 10.16, and are of particular interest in applications involving attachment to gold surfaces (Chapter 17). Dilithio-p-carborane and SO2 combine to generate the C,C0 -bis(lithiosulfonato) derivative from which the bis(sulfinic acid) carborane is obtained on protonation [676,702]. Hþ
SO2
1; 12-Li2 2CB10 H10 C2 2Li ! LiO2 S2 2CB10 H10 C2 2SO2 Li ! HOðOÞS2 2CB10 H10 C2 2SðOÞOH C 6 H6
Treatment of the lithiosulfinato species with SO2Cl2 affords the bis(chlorosulfonyl) derivative, which on AlCl3promoted hydrolysis affords C,C0 -bis(sulfonic acid)-p-carborane [702]. SO2 Cl2
Hþ
1; 12-LiO2 S2 2CB10 H10 C2 2SO2 Li ! ClO2 S2 2CB10 H10 C2 2SO2 Cl ! HO3 S2 2CB10 H10 C2 2SO3 H AlCl3
Although a number of B2 2Se and B2 2Te m-carboranes are known (Section 10.6), the only reported m-carboranyl C2 2Se compounds are the dimers (1,7-RCB10H10C)2Se2 (R ¼ H, Me, n-C4H7, Ph), analogous to ocarboranyl species described in Section 9.4, that are generated from RCB10H10CNa and elemental selenium in liquid ammonia followed by air oxidation [486]. C2 2Te m-carboranes have not been characterized, nor have p-carboranyl selenium or tellurium derivatives of any type.
10.5.2.7 Addition of halogens at carbon Replacement of the C2 2H hydrogens in m-carborane by fluorine to form 1,7-F2C2B10H10 can be accomplished via reaction of the C,C0 -dilithio derivative with perchloryl fluoride [38]. (In contrast, as described below, treatment with elemental fluorine replaces only the B2 2H hydrogens, generating 1,7-H2C2B10F10.)
10.5 Substitution at carbon
613
The analogous p-carboranyl derivative 1,12-F2C2B10H10 has not been reported, but the attack of elemental fluorine on permethylated p-carborane results in formation of mono- and di-C-fluoro side products along with the per(trifluoromethyl) species which has been described as a perfluorinated nanosphere [705]. F2
1; 12-Me2 C2 B10 Me10 ! ðCF3 Þ2 C2 B10 ðCF3 Þ10 þ FðCF3 ÞC2 B10 ðCF3 Þ10 þ F2 C2 B10 ðCF3 Þ10 60 F
94%
0
C-chloro and C,C -dichloro m-carborane derivatives [347,377], the corresponding bromo and iodo compounds (Table 10-1) [347,378], and their p-carboranyl analogues (Table 10-2) [600,676,677], are prepared by reactions of C-lithio- or C,C0 -dilithiocarboranes with elemental halogens. In a different approach, parent m-carborane is converted to 1,7-Cl2C2B10H10 on treatment with 50% NaOH/CCl4 in the presence of Et3N(CH2Ph)þ Cl [346]. C-mono- and C,C0 -dibromo-p-carboranes have been obtained serendipitously, in good overall yield, in the reaction of the C,C0 -dilithio derivative with trimethylsilylbromoacetylene in the absence of a copper catalyst; when the catalyst is C)C2B10H11 (see below) is obtained instead [652]. present, 1,12-(Me3SiC Me3 SiCCBr
1; 12-Li2 C2 B10 H10 ! BrC2 B10 H11 ð25%Þ þ Br2 C2 B10 H10 ð57%Þ
10.5.2.8 Addition of zinc and mercury at carbon Derivatives having m- or p-carboranyl Ccage2 2Zn bonds are unknown. As with o-carborane (Section 9.4), C2 2Hg compounds (Tables 10-1 and 10-2) are readily obtained by treatment of C-lithiocarboranes with mercury(II) halides or organomercury reagents [187,542,543,615,725], for example, 2Li þ HgCl2 ! ðR2 2CB10 H10 CÞ2 Hg R ¼ Me; Ph 1; 7-R2 2CB10 H10 C2 1; 7- or 1; 12-HCB10 H10 C2 2Li þ MeHgCl ! HCB10 H10 C2 2HgMe 1; 7-Li2 2CB10 H10 C2 2CB10 H10 C2 2Li þ MeHgl ! MeHg2 2CB10 H10 C2 2CB10 H10 C2 2HgMe Binding of mercury to m-carboranyl carbons can also be accomplished by cleaving the Fe2 2C bond in 1,7Me2 2CB10H10C2 2Fe(CO)2Cp with HgCl2 as described in Section 9.4 for the o-carboranyl analogue [508]. Mercury-linked poly(p-carboranyl) chains, e.g., 10-10 and 10-11, have been prepared as connectors for construction of linear polymers [632]; a counterpart of 10-10 with CpFe(CO)2 units in place of the phenyl rings is described below. C H
C
1) Li[N(CHMe2)]2
C
C
2) HgCl2
C
C
Hg
10-10 H
H
C
H
C
C 1) (Me3C)OK, HgBr2 2) DMF
C
C C
C
H
C
C
C
Hg
C
C
10-11 Direct mercuration of carboranyl C2 2H bonds via reaction with organomercury hydroxides or acetates, which works well with o-carboranes (Section 9.4), reportedly fails with m- and p-carboranyl derivatives [766].
614
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
10.5.2.9 Addition of transition metals at carbon In comparison to their o-carboranyl counterparts, far fewer m-carboranyl derivatives featuring direct cage carbon-totransition metal bonds, and only a handful of p-carboranyl species of this type, have been characterized (Tables 10-1 and 10-2). However, this area is attracting increasing attention, especially directed toward linear chains in which p-carboranyl cages are connected by metal atoms. As in the mercury complexes noted earlier, such chains could form the basis for new types of electrically conducting polymers. Synthetic routes to carbon-bound m- and p-carborane transition metal derivatives parallel those employed for o-carborane compounds (Section 9.4), in most cases involving reactions of C-lithio- or C,C0 -dilithiocarboranes with organotransition metal halides [517–519,526,529,530,532,539] as illustrated by the platinum m-carboranyl complex [532] 10-12 and the m- and p-carboranyl iron compounds [505,506,718] 10-13 to 10-16. CH2 CH2
Pt
R C
trans-(Et3P)2PtCl2
C
PEt2
R = Me, Ph R
10-12
Li C
C
Fe
H
Cp(CO)2FeCl
C
(1) n-C4H7Li
CO
C
R=H
no reaction
(2) HgCl2
C O
10-13 O C H
C
C
Li
Cp(CO)2FeCl
H
C
C
Fe
CO
(1) n-C4H7Li
OC
Fe
C
C
Fe
(2) Cp(CO)2FeCl
CO C O
C O
10-14
10-15
O C
(1) n-C4H7Li OC
Fe
C
C
Hg
C
C
Fe
CO
(2) HgCl2 C O
10-16 In contrast to the o-carboranyl system, the separation of cage carbon atoms in m- and p-carborane prevents the for2C cycles as found in complexes such as 9-28 and 9-29 (Section 9.4). mation of intramolecular exo-polyhedral C2 2Xy2 The lithium in 1-lithio- or 1,7-dilithio-m-carborane can be replaced by copper via treatment with CuCl [108,507], affording C-cuprated derivatives that combine with organotransition metal reagents, for example, 2Cu þ CpFeðCOÞ2 Br ! CpðCOÞ2 Fe2 2CB10 H10 C2 2FeðCOÞ2 Cp 1; 7-Cu2 2CB10 H10 C2 A different sequence leading to metal addition at carbon is found in the reaction of m-carboranyl carboxylic acid chloride with NaFe(CO)2Cp to give the acyl derivative, which on heating loses carbon monoxide (see also Section 9.4) [506,508]: 2CðOÞCl þ NaFeðCOÞ2 Cp ! 1; 7-HCB10 H10 C2 D
HCB10 H10 C2 2CðOÞFeðCOÞ2 Cp ! HCB10 H10 C2 2FeðCOÞ2 Cp þ CO
10.5 Substitution at carbon
615
10.5.2.10 Addition of lanthanide series metals at carbon Derivatives of m- and p-carborane with direct bonds to lanthanide metals or yttrium have not been reported at this writing.
10.5.2.11 Organosubstitution at carbon Of the various methods described in Section 9.4 for introducing organic functional groups at o-carborane carbon atoms, only those involving C-metallation are generally applicable to the m- and p-carborane systems; for example, the useful species 1,2-dehydro-o-carborane, “carboryne,” has no counterpart in the meta and para isomers, and alkyne addition to B10H14 affords only o-carboranyl products. Alternatively, functionalized derivatives of m- and p-carborane that can survive the elevated temperatures required—for example, many C-alkyl, -alkenyl, -aryl, and -halo compounds—can be obtained by thermal isomerization of their o-carboranyl counterparts [75,85,100,115,116,118,135]. Yields, however, vary considerably and are often far from quantitative. C-metallation of m- and p-carborane by organolithium or other reagents, such as alkali metal amides in liquid ammonia, tends to proceed more slowly than with o-carborane under the same conditions, owing to weaker acidity of the C2 2H protons in the meta and para isomers as discussed in detail earlier in this section. The C-metallated products generally undergo transformations analogous to those described in Section 9.4 to give a range of organosubstituted derivatives listed in Tables 10-1 and 10-2. For example, m- and p-carboranyl C-lithio or C-bromomagnesio derivatives are easily converted to alkyl, alkenyl, alkynyl, and other compounds by reaction with the corresponding halides. As in the case of o-carborane (Section 9.4), the use of C-cuprated m- and p-carboranes is advantageous in the synthesis of certain products such as C-aryl [107,108,124,131,632,634,636,649], C-alkenyl [149,150,160,650], C-alkynyl [160,161,621,650,652], and other derivatives [279,671,713]. Phl
2Li þ CuCl ! HCB10 H10 C2 2Cu ! HCB10 H10 C2 2Ph HCB10 H10 C2 C2 C2 C2 Cu2 2CB10 H10 C2 2Cu þ Br2 2C 2R ! R2 2C 2CB10 H10 C2 2C 2R R ¼ H; Ph Cu2 2CB10 H10 C2 2Cu þ 2-Br2 2C5 H4 N ! NC5 H42 2CB10 H10 C2 2C5 H4 N Mel
HCB10 H10 C2 2Cu þ CS2 ! HCB10 H10 C2 2CS2 Me Novel, aesthetically appealing carboranyl-phenylene macrocycles have been prepared from 1,7-Cu2C2B10H10, which combines with m-diiodobenzene to give 10-17 [124] and with 1,2-(p-IC6H4)2C2B10H10 to form 10-18 [125], in both cases accompanied by other products. The structures shown have been confirmed by X-ray crystallography [124,125].
C
C
C
C
10-17
C
C
C
C
C
C
C
C
C
C
10-18
Like their o-carboranyl counterparts, copper derivatives of m-and p-carborane react with aryldiazonium tetrafluoroborates to yield 1-RC6H4-C2B10H11 derivatives and arylazocarborane side products [109,139]. Reactions of 1-Cu-1,7-
616
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
C2B10H11 with aryl iodides in pyridine give mixtures of mono- and diaryl products, in contrast to the analogous o-carborane processes which afford exclusively C-monoaryl products [107]. The synthetic utility of C-cupra-p-carboranes has been exploited in several interesting applications based on linear C-alkynyl- or C-aryl-p-carborane arrays, including liquid crystals [621], “carborarods,” [153], two-dimensional grid polymers [632,637], “nanoworms”[658,659] and “motorized nanocars,” [655,660], described in Chapter 17. Beyond the interactions with RX reagents to place R groups on the carboranyl carbon atom discussed earlier, other syntheses involving displacement of metals from m- and p-carboranyl cages have been demonstrated. For example, hexafluorobenzene and other polyfluorinated arenas combine with C-lithio- o-, m-, and p-carborane derivatives to generate perfluoroaryl-substituted products (see Section 9.4) [138,141]. Reactions of C-lithio- or bromomagnesiocarboranes with aldehydes [75,76,78,214,614,663], ketones [152,615], or CO2 [52,75,76,110,221,222,228,268,615,682] afford carbinols and carboxylic acids, respectively. B-permethyl-p-carboranes show similar chemistry [614]. ðCH2 OÞn
2Li þ! HOCH22 2CB10 H10 C2 2CH2 OH Li2 2CB10 H10 C2 H
MeOC6 H4 CHO
Ph2 2CB10 H10 C2 2Li þ! Ph2 2CB10 H10 C2 2CHðOHÞ2 2C6 H42 2OMe H
RCðOÞR
0
Li2 2CB10 H10 C2 2Li þ! HOCRR02 2CB10 H10 C2 2CRR0 OH H
R; R0 ¼ Me; CF3 ; CF2 Cl
CO2
2Li þ! HOðOÞC2 2CB10 H10 C2 2CðOÞOH Li2 2CB10 H10 C2 H
Mixed-cage water-soluble 1,2-, 1,7-, and 1,12-R2 2CB10 H10 C2 2ðCH2 Þ4 O2 2B12 H11 2 derivatives (R ¼ H, Me), synthe 2OðCH2 Þ4 [256], were mentioned in Section 9.15. sized from C-lithio-carboranes and B12 H112 Replacement of lithium by cycloheptatriene is accomplished by reacting C-lithio-p-carborane with 7methoxycycloheptatriene to form 10-19, which is converted to 10-20, a precursor to nonlinear optical (NLO) materials (Chapter 17) [620].
H
C
C
Li
(2) H+, H2O
Me
H
(1) C7H6OMe, C6H6 H
C
C
H
(1) n-C4H9Li
C
C
Me
(2)
O Me
10-19
OH
Me H
(3) H2O
H
−H2O Me
Me C
Me
C
H H
H
H
10-20
H
Δ
C
C
Me H
H
Treatment of 1,7- or 1,12-LiC2B10H11 or Li2C2B10H10 with methyl formate gives the 1-formyl or C,C0 -diformyl derivatives in high yield [205] (in contrast, as noted in Section 9.9, attempts to prepare 1,2-diformyl-o-carborane by this method gave only the cyclic ether 9-116). Similarly, as with their o-carboranyl analogues (Section 9.10), reactions of 2C(O)H LiC2B10H11 and PhOCH(OEt)2 generate acetals from which 1-formyl compounds of the type RCB10H10C2 (R ¼ H, Me, Ph), can be obtained by hydrolysis [208,211,212,666]. A direct route to 1-carboxymethyl-m-carborane (acetic acid) utilizes treatment with sodium in liquid ammonia [232].
10.6 Substitution at boron
617
ð1Þ NH3
HCB10 H10 C2 2Na þ BrCH2 CðOÞONa ! HCB10 H10 C2 2CH2 CðOÞOH ð2Þ HCl
0
In contrast to the action of phosgene on C,C -dilithio-o-carborane, which produces the cyclic diketone 9-120 (Section 9.10), the reaction with dilithio-m-carborane affords the diacid dichloride [239]. 2Li þ 2CðOÞCl2 ! ClðOÞC2 2CB10 H10 C2 2CðOÞCl Li2 2CB10 H10 C2
10.6 SUBSTITUTION AT BORON Boron-functionalized derivatives of m- and p-carborane are generally accessible by the same synthetic approaches used for their o-carboranyl analogues. Ortho-to-meta thermal cage isomerization, though viable for many C-substituted carboranes as noted earlier, is less commonly employed with boron-derivatized species other than in studies of cage rearrangement mechanisms (Section 10.3), as one is likely to obtain mixtures of two or more products. This is not a problem in B-peralkyl or B-perhalo derivatives such as 1,2-C2B10Cl12, which isomerize to their 1,7 counterparts on heating [370,371,374,377]. Direct replacement of B2 2H hydrogen atoms by organic groups is unusual, but the high temperature reaction of C6F6 with m-carborane affords 1,7-C2B10H11-B-C6F6 [767]. In p-carborane, boron substitution is simplified by the fact that all borons are chemically equivalent, restricting monosubstituted species to a single 1,12-C2B10H11-2-R isomer [353]; however, the high temperatures required for mto p-carborane cage rearrangement could lead to poly-B-substitution in some cases.
10.6.1 Boron insertion into nido-C2B9 dianions The addition of boron to 11-vertex cages to reconstitute the C2B10 icosahedron, described earlier in Sections 7.2 and 9.5, has been demonstrated in the m-carborane system via the reaction of RBX2 reagents (R ¼ F or NPh2) with nido-7,9C2 B9 H11 2 to form 1,7-C2B10H11-2-R in a manner analogous to the process shown in Figure 7-8 [155,156,186,273]. The synthetic advantage of this approach is that it affords 2(3)-substituted m-carboranes, which are difficult to prepare by other means; as discussed below, electrophiles attack preferentially at the most negative BH vertexes, that is, B (9,10), furthest removed from the cage carbon atoms.
10.6.2 Nucleophilic displacement As in the o-carborane system, (Section 9.5), certain boron-bound m- and p-carboranyl substituents such as amino, phenyldiazonium, thallium, and mercury can be replaced by other electron-donor groups, as in the conversion of 1,7- or 1,12H2C2B10H9-9-IPhþ to 1,7-H2C2B10H9-9-X or 1,12-H2C2B10H9-2-X, respectively (X ¼ F, Cl, Br), via reaction with halide ions [145,337,338,381]. In particular, 9-phenyliodonium-m-carboranyl cations, like their o-carboranyl counterparts discussed in the preceding chapter, react with nucleophiles to displace PhIþ and form the neutral B(9)-R species in good yield [365,394,396–398]. The same type of reaction by m-carboranyl bromonium salts yields B(9)-hydroxy, azido, and cyano derivatives [180]. Similarly, 1,7-R2C2B10H9-9-Tl[C(O)OCF3]2 (R ¼ H, Me) interacts with BF3OEt2 to give 1,7R2C2B10H9-9-F via F transfer [335]. Other reactions of the nucleophilic replacement type are noted in later sections of this chapter.
10.6.3 Electrophilic alkylation Treatment of m-carborane with methyl iodide over AlCl3 under Friedel-Crafts conditions [768] closely parallels the behavior of o-carborane (Section 9.5), affording the octamethyl derivative1,7-H2C2B10H2-4,5,6,8,9,10,11,12-Me8 in high yield [93,618]; the two borons [B(2) and B(3)] that are adjacent to both carbons, and carry the lowest negative
618
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
charge, remain unmethylated. However, the corresponding reaction of p-carborane or its C,C0 -dimethyl derivative results in quantitative formation of the decamethyl [93] or dodecamethyl [618] species 1,12-R2C2B10Me10 (R ¼ H, Me); radical chlorination of H2C2B10Me10 affords 1,12-H2C2B10(CHCl2)10 [631]. These compounds are representative of a new genre in organic chemistry, “spherical hydrocarbons” whose icosahedral C2B10 core is sheathed by a coating of alkyl substituents [769]. As will be seen later in this chapter, this structural motif has been extended to other classes of B-polyfunctional derivatives. Electrophilic alkylation with other alkyl halides has been reported, for example, Me2CHX (X ¼ Cl, Br) attacks mcarborane in CS2 over AlCl3 to give the 8- and 9-isopropyl and 8,9-diisopropyl derivatives [92]. Like the ortho isomer, 5CHSiCl3 over AlCl3 at 80-120 C to afford H2C2B10H10– both m- and p-carborane undergo silylation with CH25 2CH2CH2SiMe3 derivatives on treatment with MgI [462,770]. n(CH2CH2SiCl3)n products, which are converted to B2 m-Carborane is also alkylated by 4-chlorobutyric acid over AlCl3 in CS2 to 1,7-H2C2B10H9-9-(CH2)3C(O)OH [201], and by benzyl halides to form 9-benzyl derivatives [220]. Alkyl substitution at BH vertexes is often accomplished via metal-catalyzed reactions of B-halo-carboranes, as described in the following section.
10.6.4 Electrophilic halogenation 10.6.4.1 m-Carborane Reactions of 1,7-C2B10H10 and its derivatives with elemental Cl2, Br2, and I2, halomethanes, or other halogenating agents, facilitated by aluminum(III) or iron(III) salts or protonic acids, proceed similarly to those of o-carborane as elaborated in Section 9.5. As in electrophilic alkylation, the course of these reactions is dictated by charge distribution in the carborane framework, with halogen substitution occurring preferentially at the most electronegative BH vertexes [354,355,357,358,369,370,382,384,771–774], which in m-carborane are B(9,10) (the only borons not adjacent to carbon) followed by B(4,6,8,11) and then B(5,12) [754]. When conducted under less than forcing conditions, halogenation of mcarborane does not usually proceed beyond B(9,10) substitution, which parallels the action of o-carborane in that only those boron atoms not adjacent to carbon [B(8,9,10,12)] are attacked. The smaller inductive electron-withdrawing power (–I effect) of m- versus o-carborane is seen in a comparative rate investigation [356] which showed that electrophilic chlorination of m-carborane by Cl2 over AlCl3 in CH2Cl2 takes place more slowly than that of o-carborane by a factor of 3.5; with Br2 and I2 the corresponding ratios are 7:1 and 11.5:1, respectively. When conducted over many hours at 80 C over AlCl3, the reaction of m-carborane with Cl2 or I2 affords B-trihalo and B-tetrahalo derivatives [351,355,358], while Br2 under these conditions is reported to add up to six bromines [354,355]. As in the o-carborane system (Section 9.5), reactions of this type are highly solvent- and catalyst-dependent. Agents other than elemental halogens can be employed. m-Carborane reacts with CCl4 over AlCl3 to generate 9-chlorom-carborane easily and quantitatively [354,601,706]; with ClSO3H to give a mixture of 4-, 5-, and 9-chloro derivatives [352]; and with ICl over AlCl3 to give 9,10-diiodo-m-carborane in high yield [94]. 1,7-C2B10H9-9-I can be prepared via treatment with PhI(O2CCF3)2 in CCl4 [775] or, alternatively, with I2 and IO3 ion in acidic aqueous media at 80-100 C [382]. The corresponding reaction with bromine and bromate ion generates the 9-bromo and 9,10-dibromo compounds. 5ð1; 7-H2 C2 B10 H10 Þ þ 2Br2 þ BrO3 þ Hþ ! 5ð1; 7-H2 C2 B10 H9 -9-BrÞ þ 3H2 O Iodination via triflic acid and iodine monochloride at 120 C, which with o-carborane gives a high yield of a B-I8 product in which all boron atoms except B(3) and B(6) are halogenated (Section 9.5), is equally effective with m-carborane, producing 1,7-H2C2B10H2-4,5,6,8,9,10,11,12-I8 [401].
10.6.4.2 p-Carborane As the symmetry of this cluster places the same charge on all 10 boron atoms, there are no preferred sites for electrophilic (or nucleophilic) attack. Consequently, in contrast to o- and m-carborane, only one B-monohalo isomer is possible, but further halogenation will generate multiple isomers unless the substitution is limited by directive effects caused by
10.6 Substitution at boron
619
the presence of the first substituent. Experimentally, it is found that the interaction of p-carborane with Cl2 in refluxing CCl4 over AlCl3 for 24 h yields B-monochloro-, B-dichloro-, and B-trichloro products, but with Br2 and I2 only monohalo and dihalo products are isolated [3,706]. Some evidence suggests that there is a limited directive effect, with addition of a second chlorine to 2-chloro-p-carborane slightly favored at the meta (vs. the ortho and para) positions, suggesting inductive electron-withdrawal by the first chlorine [3]. Treatment of p-carborane with CCl4 over AlCl3 generates 2-chloro-p-carborane quantitatively, but the reaction is much slower than that of m-carborane; this property can be exploited to separate the para isomer from m-carborane via controlled chlorination with CCl4 in which only the latter is halogenated [601,706]. Further chlorination of p-carborane does not occur with CCl4, but Cl2 in the presence of AlCl3 affords the 2,10-dichloro derivative along with trichloro-, tetrachloro-, and pentachloro-p-carboranes. The reaction of p-carborane with Br2/AlCl3 forms the 2-bromo and 2,9-dibromo derivatives, respectively [601,706], and treatment with ICl over AlCl3 affords 2-iodo-p-carborane and three isomeric diiodo species, 1,12-H2C2B10H8-2,n-I2 (n ¼ 3, 7, 9) [629]. In contrast to the reaction of m-carborane with triflic acid and iodine monochloride at 120 C— which as noted above produces an octaiodo species but leaves the B(3,6) vertexes unaffected—similar treatment of p-carborane achieves full B-iodination to give 1,12-H2C2B10I10 [401].
10.6.5 Photochemical (radical) halogenation Reactions of Cl2 with m-carborane or its derivatives under ultraviolet light, like those of o-carborane described in Chapter 9, tend to be less regioselective than the electrophilic processes. Exposure of the parent carborane to an equimolar amount of Cl2 in CCl4 yields two mono-B-chloro derivatives, [5,776] but excess chlorine readily forms the B-decahalo compound [503,751,790]. 1; 7-H2 C2 B10 H10 þ 10Cl2 ! 1; 7-H2 C2 B10 Cl10 þ 10HCl The derivatives 1,7-(m/p-FC6H4)HC2B10Cl10 are similarly prepared from 1,7-(m/p-FC6H4)HC2B10H10 [142], while chlorination of 1,7-H2C2B10H8-9,10-Br2 yields H2C2B10Cl8-9,10-Br2 [523]. Photochemical reactions of m-carborane or its 1-phenyl derivative with Br2 and I2, like those of the o-carborane system, are sluggish compared to chlorination and afford only the 9-monohalo derivatives. As in the corresponding o-carboranyl system, the phenyl ring in 1,7-PhC2B10H11 is not halogenated [776]. Relative to its ortho and meta isomers, p-carborane is less susceptible to photochemical halogenation, but it reacts with Cl2 under ultraviolet light to form mono- to decachloro products, with complete conversion to 1,12-H2C2B10Cl10 in 6 h [3]. Exposure of p-carborane in CCl4 (without Cl2) to ultraviolet light for 50 h affords 1,12-C2B10Cl10 in 83% yield [601].
10.6.6 Fluorination Elemental fluorine reacts with m- and p-carborane in liquid hydrogen fluoride at 0 C to form the respective Bdecafluoro derivatives in high yield [38]. Under these conditions, no halogenation takes place on the carbon atoms, but perfluoro-m-carborane, 1,7-F2C2B10F10, can be obtained in 85% yield by direct reaction of the parent compound with F2 over 10 days [342] or alternatively by subjecting 1,7-F2C2B10H10 (mentioned earlier) to the F2/liquid HF treatment [38]. As was noted earlier, 1,12-F2C2B10F10 has not been reported, but the pertrifluoromethyl derivative (CF3)2C2B10(CF3)10 is formed on fluorination of 1,12-Me2C2B10Me10 [705]. Partially fluorinated m-carborane derivatives can be accessed in several ways, including nucleophilic displacement of amino or other groups with halide ions and insertion of boron into nido-7,9-C2 B9 H11 2 anions, as discussed earlier in this section. The action of SbF5 on m-carborane in fluorocarbon media generates the 9-F and 9,10-F2 derivatives, possibly via an SbF5-carborane intermediate as proposed for the analogous o-carborane reaction (Section 9.5). [339,340] In the p-carborane system, the only characterized B-fluoro species other than H2C2B10F10, mentioned earlier, is 1,12-H2C2B10H9-2-F, obtained by nucleophilic displacement of phenyliodonium with fluoride ion [337,338,381].
620
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
10.6.7 Addition of main-group elements and mercury at boron 10.6.7.1 B22Al, B22Ga, B22In, and B22Tl bond formation Derivatives of m- and p-carborane with aluminum, gallium, or indium directly linked to boron have not been characterized, but boron-thallated compounds are well known and are useful agents in the synthesis of thiocyanato and other derivatives as described in later sections of this chapter. Thallium(III) halides cleave the B2 2Hg bonds in m-carboranyl mercury derivatives to afford B-metallated products [412,777]: ð1; 7-or 1;12-H2 C2 B10 H9 Þ2 Hg þ TlX3 ! ðH2 C2 B10 H9 Þ2 TlX2
X ¼ Cl; Br
Tl2 2B bonded compounds can also be obtained from mercury trifluoroacteate m- or p-carborane derivatives in trifluoroacetic acid, or directly from m-carborane [409,410,413,414,712]. 1;7ð1;12Þ-R2 C2 B10 H9 -9-Hg½CðOÞOCF3 þ Tl½CðOÞOCF3 3 ! R2 C2 B10 H9 -9ð2Þ-Tl½CðOÞOCF3 R ¼ H; Me; Ph ð1;7-H2 C2 B10 H9 Þ2 Hg þ Tl½CðOÞOCF3 3 ! H2 C2 B10 H9 -9-Tl½CðOÞOCF3 ð1;12-H2 C2 B10 H9 Þ2 Hg þ Tl½CðOÞOCF3 3 ! H2 C2 B10 H9 -2-Tl½CðOÞOCF3 1;7-C2 B10 H12 þ Tl½CðOÞOCF3 3 ! 1; 7-RR0 C2 B10 H9 -9-Tl½CðOÞOCF3
10.6.7.2 B22Si, B22Ge, B22Sn, and B22Pb bond formation As in o-carborane chemistry, there are no known m- or p-carboranes having exo-polyhedral silicon–boron, germanium– boron, or lead–boron bonds. However, products with direct boron–tin connections are obtained on treatment of Bmercurio-m-carboranyl derivatives with tin(II) chloride [332,481]. D
ð1;7-H2 C2 B10 H9 -9-Þ2 Hg þ SnCl2 ! ðH2 C2 B10 H9 -9-Þ2 SnCl2 þ Hg D
1;7-R2 C2 B10 H9 -9-HgCl þ SnCl2 ! H2 C2 B10 H9 -9-SnCl3 þ Hg B2 2Sn derivatives can also be prepared via oxidative insertion of tin(II) acetylacetonate into m-carboranyl B2 2Hg bonds [479]. 1; 7-H2 C2 B10 H9 -9-HgR þ SnðCHAc2 Þ2 ! C2 B10 H11 -9-Sn fCH½MeCðOÞO2 g22 2Hg2 2Sn fCH½MeCðOÞO2 g2 -9-C2 B10 H11 The B2 2Sn bonds in m-carboranyl compounds are much more stable toward nucleophiles than are o-carboranyl B2 2Sn links, allowing extensive substitution chemistry to be conducted on the B-stannyl derivatives as is detailed in Section 9.12.
10.6.7.3 B22N, B22P, B22As, and B22Sb bond formation
Reactions that form m- or p-carboranyl boron–nitrogen bonds via direct replacement of B2 2H hydrogens have not been discovered, but B2 2N products are accessible via metal-catalyzed reactions of B-iodo-carboranes (see below). As is noted earlier in this chapter and in Sections 7.2 and 9.5, B2 2N bonded derivatives can also be generated via insertion of monoboron fragments into nido-7,9-C2 B9 H11 2 ions as well as by cage rearrangement of o-carboranyl species. In a few cases, boron–nitrogen bonds have been formed by nucleophilic displacement from bromonium or iodonium salts, as in the reaction of 1,7-H2C2B10H9-9-BrPhþ with sodium azide to give 1,7-H2C2B10H9-9-N3 [180]. Direct B2 2P bonds in m- and p-carborane are formed on UV irradiation of (1,7-H2C2B10H9-9-)2Hg or (1,12H2C2B10H9-2-)2Hg in P(OMe)3 to give the respective H2C2B10H9-9(2)-P(O)(OMe)2 phosphonates [306,700], and also via palladium-catalyzed cross-coupling of B-iodo derivatives as described below. No p-carboranes having bonds
10.6 Substitution at boron
621
between boron and the heavier Group 15 elements have been reported, but displacement of mercury from bis(9-mcarboranyl)mercury with AsCl3 or SbCl3 generates B(9)-substituted derivatives [332,482–484]: ð1; 7-C2 B10 H11 -9-Þ2 Hg þ MX3 ! 1; 7-C2 B10 H11 -9-MCl2
M ¼ As; Sb
10.6.7.4 B22O bond formation Oxidative hydroxylation of m-carborane with CrO3 in acetone and sulfuric acid forms 1,7-C2B10H11-9-OC(O)Me in moderate yield with smaller amounts of the 2-, 3-, and 4-OC(O)Me isomers along with an unspecified isomer of a B, B0 -diacetoxy derivative [178]. Removal of 9-BrPh from phenylboronium-m-carborane cations, a genre mentioned earlier, by reaction with NaNO2 yields 1,7-C2B10H11-9-OH [180]. (Other m-carboranyl B-hydroxy compounds are accessible via thermal rearrangement of o-carboranyl B-OH derivatives [177].) Oxidation of 1,12-(HOCH2)2C2B10H10 with 30% H2O2 affords the decahydroxy-p-carborane 1,12-H2C2B10(OH)10 in 80% yield [668]. 1,12-C2B10H11-2-OH and (1,12-C2B10H11-2-)2O have been identified as minor products, presumably from H2O contamination, in palladium-catalyzed nucleophilic substitutions (Heck reactions) on 2-iodo-p-carborane [651]. The 2-hydroxy derivative is also generated in 30% yield as a side product in the B-amination of 1,12-C2B10H11-2-I with aromatic amines in the presence of Pd(dba)2-BINAP-ButONa in dioxane at 100 C [673].
10.6.7.5 B22S, B22Se, and B22Te bond formation The methods described in Chapter 9 for binding sulfur to o-carboranyl boron atoms are also generally applicable to mand p-carborane. These include Friedel-Crafts sulfhydrylation via reaction with sulfur over AlCl3 at 100-130 C to form thiols [324,327,328] and refluxing with S2Cl2 or SCl2 over AlCl3 in CH2Cl2 to yield disulfur-linked biscarboranyl products analogous to 9-51 (Section 9.5) [326,333]. S8
1;7=1;12-R2 C2 B10 H0 þ ! 1; 7=1;12-R2 C2 B10 H9 -9ð2Þ-SH þ 1; 7-R2 C2 B10 H8 -5; 9-ðSHÞ2 AlCl3
AlCl3
1;7-R2 C2 B10 H0 þ S2 Cl2 ! R2 C2 B10 H92 2S2 2H9 B10 C2 R2
R ¼ H; Me
The latter species are converted to B(9)-thiols on reduction with zinc [326]. A disulfide is also obtained on displacement of the metal from bis(m-carboranyl)mercury [332]. 350 C
2S2 2S2 2H9 B10 C2 H2 ð1; 7-H2 C2 B10 H9 -9-Þ2 Hg þ S8 ! H2 C2 B10 H92 Derivatives of m-carborane containing B2 2Se or B2 2Te bonds are easily prepared, but analogous p-carboranes have 2Se2 2Se2 2H9B10C2H2 can be prepared similarly to the not been described. The diselenium-linked species H2C2B10H92 disulfide, from m-carborane and Se2Cl2/AlCl3 [331,488,489], or by heating m-carborane with selenium metal over AlCl3 in the absence of O2 [487]. Paralleling the behavior of its disulfide analogue, (1,7-H2C2B10H9)2Se2 is reduced to H2C2B10H9-9-SeH on reduction with zinc and HCl [489]. Selenium- and tellurium-linked biscarboranes are also generated from the elemental metal and bis(m-carboranyl) mercury [490,491]. ð1;7-C2 B10 H11 -9-Þ2 Hg þ M ! H2 C2 B10 H92 2M2 2Hg2 2Se2 2B10 H9 C2 H2 þ Hg M ¼ Se; Te Electrophilic addition of tellurium to m-carborane with tellurium(IV) chloride yields a trichlorotellurium product that forms a ditelluride on reaction with sodium sulfide [492,493]: AlCl3
Na2 S
1; 7-C2 B10 H12 þ TeCl4 ! C2 B10 H11 -9-TeCl3 ! H2 C2 B10 H112 2Te2 2Te2 2S2 2B10 H9 C2 H2
10.6.7.6 B22Hg bond formation Nucleophilic mercuration of m- and p-carborane with Hg[COC(O)CF3]2 in CF3C(O)OH forms 1,7-H2C2B10H9-9-HgOC (O)CF3 and 1,12-H2C2B10H9-2-HgOC(O)CF3, respectively, as in the preparation of the corresponding o-carboranyl
622
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
compound [548,550]. In the presence of excess Hg[COC(O)CF3]2, the reaction with m-carborane affords 1,7-H2C2B10H8-9,10-[HgOC(O)CF3]2 as well as a B,B0 ,B00 -[HgOC(O)CF3]3 derivative [551]. These B-mercurated compounds, like their o-carboranyl counterparts, are converted to B2 2HgX (X ¼ Cl, I) species on reaction with NaX [548,552]. Mercuration on B(9)-substituted m-carboranes, as expected for a nucleophilic process, takes place at B(10) to generate 1,7-H2C2B10H8-9-R-10-HgOC(O)CF3 products, analogous to the formation of 9-R-10-HgOC(O)CF3 o-carboranyl derivatives such as 9-48 (Section 9.5). As with the latter compounds, reduction of the m-carboranyl species with LiAlH4 forms Hg(H2C2B10H8-9-R)2 [496].
10.6.8 Addition of transition metals at boron Very few derivatives of m-carborane, and almost none of p-carborane, having exo-polyhedral transition metals (other than mercury) directly bonded to cage boron atoms have been isolated and characterized. The known compounds of this type, listed in Tables 10-1 and 10-2, are prepared by methods previously outlined in Section 9.5 for the corresponding ocarboranyl complexes. Reactions analogous to those discussed earlier include the thermal ejection of CO from 1,7H2C2B10H9-9-(CO)ML derivatives to give 1,7-H2C2B10H9-9-ML products where ML is Fe(CO)2Cp or Re(CO)5 [503,514] and the addition of iridium to m- and p-carborane via reaction with L2IrCl, where L is PPh3 or AsPh3, forming 1,7(1,12)-H2C2B10H9-2-IrL2HCl complexes [525]. Another approach utilizes the replacement of mercury by transition metals in B2 2Hg bonded derivatives, as in the interaction of 1,7-C2B10H11-9-HgCl with Pt(PPh3)3 in benzene to form 1,7-C2B10H11-9-PtCl(PPh)2 [531]. 2CB10H10C2 2Ph affords 10-21 [501], similar to the Cyclometalation of MeRe(CO)5 with 1,7-Me2 o-carboranyl complexes 9-54 and 9-55 discussed in Chapter 9.
C
N N
Ph
10-21 C
Re(CO)4
Me
10.6.9 Organosubstitution at boron 10.6.9.1 Metal-promoted cross-coupling of B-halo-m- and p-carboranes Highly successful methods, adapted from organic chemistry, for introducing organic functional groups at boron via transition metal-facilitated coupling to boron-halogenated o-carboranes are described in detail in Section 9.5. In general, this approach is also effective for m- and p-carborane although, at this writing, applications in the latter system have been limited. For m-carboranyl species, these include the formation of 1,7-R2C2B10H9-9-R derivatives by reactions of 9-iodo-m-carboranes with Grignards promoted by phosphino-palladium [94,95,98,167,200,202] or nickel [98] catalysts. ðPh3 PÞn MCl4n orðPh3 PÞ4 Pd
1; 7-H2 C2 B10 H9 -9-I þ RMgX ! 1; 7-H2 C2 B10 H9 -9-R M ¼ Pd; Ni n ¼ 2; 4 R ¼ Me; Et; n-C4 H9 ; CHMe2 ; Ph; CH2 Ph; m=p-C6 H4 Me; m=p-C6 H4 F; CH p-C6 H4 OH; p-C6 H4 OMe; C Analogous reactions of 2-iodo-p-carborane generate 1,12-R2C2B10H9-2-R products (Table 10-2) [95,281,629]. Difunctional 1,7-H2C2B10H8-9,10-R2 derivatives are similarly obtained from 9,10-diiodo-m-carboranes and Grignards in the presence of trans-(Ph3P)2PdCl2 [94,128]; similar treatment of 2,9-diiodo-p-carborane affords 1,12-H2C2B10H8-2,9R2 (R ¼ Me, Ph) [629]. As in the o-carboranyl case (Section 9.5), product yields are considerably improved when CuI is added as a cocatalyst [277,808]. The versatility of palladium-catalyzed cross-coupling can be extended by using
10.6 Substitution at boron
623
organozinc compounds in place of Grignards. In contrast to organomagnesium reagents, whose b-hydrogen atoms can reduce B-iodocarboranes to the parent species, no such side reactions occur with the organozinc compounds [96,97,281]. ðPh3 PÞ4 Pd
1; 7-H2 C2 B10 H9 -9-I þ RZnCl!1; 7-H2 C2 B10 H9 -9-R THF
CSiMe3 ; CH2 -1; 2-C2 B10 H11 ; 2-thienyl R ¼ Et; n-C4 H9 ; Ph; CH2 Ph; CH2 CH5 5CH2 ; Ph; C ðPh3 PÞ4 Pd
1; 7-H2 C2 B10 H8 -9; 12-I2 þ RZnCl ! 1; 2-H2 C2 B10 H8 -9; 12-R2
R ¼ Me; Ph; CH2 Ph
HF
A variety of heterocyclic-substituted m- and p-carborane derivatives can be accessed by this approach [281]. ½Pd
1; 12-H2 C2 B10 H9 -2-I þ ½Zn!1; 12-H2 C2 B10 H9 -2-R THF or dioxane
½Pd ¼ ðPh3 PÞ4 Pd; PdCl2 ðPPh3 Þ2 ; PdCl2 ðdppbÞ
O E
S [Zn] =
Zn
ZnCl
O
S Zn
2 2
E = O, S ZnCl N N
N
ZnCl
ZnCl
1,4-FC6H4Br undergoes (Ph3P)2PdCl2-catalyzed coupling to 9-iodo-m-carborane or 2-iodo-p-carborane in the absence of Grignard or organozinc reagents, forming 1,7-H2C2B10H9-9-C6H4F and 1,12-H2C2B10H9-2-C6H4F, respectively [146]. Likewise, palladium-facilitated reactions of trimethylvinylsilane with 9-halo-m-carboranes generate 9-vinyl-m-carborane [159], and the Heck reaction has been employed to prepare B-vinylated p-carboranes from 2-iodo-p-carborane and styrenes [651]. ½ðC3 H5 ÞPdCl2
5CH2 ! 1; 7-H2 C2 B10 H9 -9-CH5 5CH2 1; 7-H2 C2 B10 H9 -9-X þ Me3 SiCH5 Me3 SiX
X ¼ Br; I
Ag3 O4 ;DMF
1; 12-H2 C2 B10 H9 -2-I þ p-RC6 H4 CH5 5CH2 0! H2 C2 B10 H9 -9-trans-CH5 5CHC6 H4 R ½PdHermann s catalyst
R ¼ H; Me; Ph; Cl; Br; NO2 ; OMe CMgBr reagents (R ¼ Ph, SiMe3) without transition metal catalysts Conversely, 1,7-R2C2B10H9-9-I interacts with RC CR derivatives [168]. to afford B(9)-C Palladium-mediated reactions of 2-iodo-p-carborane and 9-iodo-m-carborane with amides have been used to prepare a range of B-amido derivatives (Tables 10-1 and 10-2) [288]. The m- and p-carboranyl phosphonates, 1,7-H2C2B10H9-9PO(OMe)2 and 1,12-H2C2B10H9-2-PO(OMe)2, are obtained from the corresponding B-iodo carboranes [307]. ðPh3 PÞ4 Pd
1; 7-or1; 12-H2 C2 B10 H82 2I þ HPOðOMeÞ2 ! H2 C2 B10 H82 2POðOMeÞ2 Et3 N
A closely related synthetic route employs the action of azoles and aromatic amines on 2-iodo-p-carborane, catalyzed by the system Pd(dba)2/BINAP/Me3C-ONa in dioxane (dba ¼ trans,trans-dibenzylideneacetone), to prepare a range of B-azole and B-amine derivatives (10-22) [673].
624
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12 H
C
H
C
I
PdL2I
L2Pd(0)
H
H
C
C
PdL2
RR⬘NH NRR⬘
base C
− base − L2Pd(0)
C
H
C
C
H
H
H
10-22
H
N
H
NRR⬘
− HI
H
N
H
N
N
N
N
RR⬘NH =
H H
N
H
N
N N
N
NH2
X N
O
N X = H, Cl, OMe
Another widely employed organometallic technique, palladium-catalyzed Suzuki-Miyuara coupling of boronic acids with organic halides, has proved unsuccessful when applied to 9-iodo-o-carborane, failing to give cross-coupled products (Section 9.5) [778]. PhB(OH)2 does interact with 9-iodo-m-carborane in benzene in the presence of Pd(PPh3)4 and aqueous Na2CO3 to afford 1,7-H2C2B10H9-9-Ph in 27% yield along with biphenyl and parent m-carborane, but the reaction is slow and the product distributions are atypical compared to those of organic Suzuki reactions [778]. In sharp contrast, 2-iodo-p-carborane combines with arylboronic acids in the presence of fluoride ion to generate B-aryl products in high yield. The function of the fluoride is suggested to be formation of an RB(OH)2F borate intermediate that promotes cross-coupling at the B2 2I bond [646]. Pd2 ðdbaÞ3 dppb
1;12-H2 C2 B10 H9 -2-I þ RBðOHÞ2 ! 1;12-H2 C2 B10 H8 -2-R CsF
R ¼ Ph; 1-naphthyl; 4-C6 H4 OMe; 3-C6 H4 NHCOMe; 4-C6 H4 CN; 2-C6 H4 NO2 ; 2-C6 H4 NO2 An important biomedical application of palladium-catalyzed chemistry is found in isotopic exchange reactions of m- and p-carboranyl B-iodo derivatives with 125I using Hermann’s catalyst, {Pd[P(o-C6H4Me)2C6H4CH2]}2(m-OCHMeO)2, in toluene at 100 C to obtain 125I-labeled 1,7-H2C2B10H9-9-I and 1,12-H2C2B10H9-2-I [392]. As is discussed in Chapter 16, these radioactively labeled compounds are of interest in pharmacokinetic investigations related to BNCT.
10.6.9.2 Other routes to B-organosubstituted m- and p-carboranes Several methods are available for appending organic functional groups at boron without the use of transition metal reagents. As in the corresponding o-carboranyl system (Section 9.5), treatment of bis(9-m-carboranyl)mercury with acetyl chloride over AlCl3 generates 9(12)-acetyl-o-carborane [218,219]. The acetyl derivative, in turn, is converted to the carboxylic acid on reaction with LiAlH4 in acetone followed by oxidation with CrO3 at 500 C [219] or, alternatively, via treatment with CrO3 and H2SO4 in acetic acid [236]. The latter method can also be used to prepare 2-carboxy-pcarborane from 1,12-H2C2B10H9-2-Me [236]. MeCðOÞCl
CrO3
ð1; 7-H2 C2 B10 H9 -9Þ2 Hg ! 1; 7-H2 C2 B10 H9 -9-CðOÞMe ! H2 C2 B10 H9 -9-CðOÞOH AlCl3
Insertion of :CHR carbenes (R ¼ H, COOEt) into B2 2H bonds occurs with both m- and p-carborane to form all possible B2 2CH2R isomers (four and one, respectively) [628], paralleling the reactions described in Section 9.5 for o-carborane in which ethyl diazoacetate is irradiated in C6F6 solution.
10.7 Alkyl, haloalkyl, and aryl derivatives
625
B(9)-aryl derivatives of m-carborane can be prepared from H2C2B10H9• radicals generated by ultraviolet photolysis of bis(m-carboranyl)mercury or thallium compounds in aromatic media [126]. hu
ð1; 7-H2 C2 B10 H9 -9Þ2 Hg þ RH ! 1; 7-H2 C2 B10 H9 -9-R
45-90% R ¼ Ph; C6 F5 ; p-C6 H3 Me2
While not practical as a method of synthesis, benzene and m-carborane at 630-750 C combine to form 1,7-H2C2B10H99-Ph and several other B-phenyl derivatives, the product distribution being dependent on the reaction temperature [127].
10.7 ALKYL, HALOALKYL, AND ARYL DERIVATIVES The routes to C- and B-substituted m- and p-carboranes that are outlined in the preceding sections of this chapter generally parallel those of o-carborane (Chapter 9) except that alkyne insertion into the decaborane cage affords only 1,2C2B10 products. On the other hand, many m- and p-carboranyl alkyl and aryl derivatives are sufficiently heat-resistant that they can be obtained by thermal rearrangement of their o-carborane analogues (Section 10.2). An unusual synthetic approach, thermal ejection of SO2 groups, has been employed to generate the 1,7-carboranophane 10-24 [86] from sulfur-containing cyclic compounds of the type discussed in Section 10.16. O S
CH2
CH2
O
CH2
C
CH2
S
CH2
CH2 CH2
C
CH2
[O]
CH2 C CH2 S
C
CH2 CH2 CH2 CH2
500 ⬚C
CH2 C
CH2 O
CH2
CH2
S O
CH2
C
CH2 CH2
CH
CH2
10-23
CH2
10-24
10.7.1 Properties of alkyl- and haloalkyl-substituted m- and p-carboranes The main trends in the electronic character of these compounds are discussed in Sections 9.6 and 10.5, and various electrochemical and other investigations are cited in Tables 10-1 and 10-2. A main point is that alkyl groups on the cage are electron-releasing and facilitate substitution at BH vertexes by electrophiles, except in the case of methyl groups bound to boron, which function as inductive electron-acceptors and inhibit electrophilic attack (Section 9.5). Like the corresponding o-carborane, 1-fluoromethyl-m-carborane undergoes Friedel-Crafts arylation with aromatic hydrocarbons [130]. AlCl3
1; 7-HCB10 H10 C2 2CH2 F þ arene ! HCB10 H10 C2 2CH2 R
R ¼ Ph; C6 H4 Me; C6 H3 Me2
Haloalkyl m-, p-, and o-carboranes can be employed in multistep syntheses, for example [103,105,106], ðCH2 Þ3 O
CBr4
2Li ! HCB10 Me10 C2 2ðCH2 Þ3 OH! 1; 7=1; 12-HCB10 Me10 C2 PPh3
NaN3
H2 =Pd
HCB10 Me10 C2 2ðCH2 Þ3 Br ! HCB10 Me10 C2 2ðCH2 Þ3 N3 ! HCB10 Me10 C2 2NH2
10.7.2 Properties of aryl-substituted m- and p-carboranes 10.7.2.1 Phenyl derivatives 1,7- and 1,12-C2B10H12 compounds bound to aromatic rings have special importance, both as vehicles for probing electronic interactions between the carborane and aryl groups and also as synthetic building blocks in a variety of applications. As benzene and the icosahedral C2B10 cage have similar spatial requirements and share the property of aromaticity, the
626
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
effects of replacing phenyl groups with carboranes are of interest, especially in materials and biochemical/medicinal applications. For example, arylcarboranes can be incorporated into biologically active molecules in which the steric and electronic properties of the carborane moiety are used to advantage, for example, in hydrophobic cores that exhibit antiandrogenic activity [638,639,641] or in liposomes for BNCT studies [642] (the emerging area of carborane-centered drug design is discussed in Chapter 16). Many experimental and theoretical investigations of the chemical and physical properties of arylsubstituted icosahedral carboranes have encompassed all three isomers, and are summarized in Section 9.6 while studies dealing with m- and p-carborane aryl derivatives are cited in Tables 10-1 and 10-2. Two important electronic effects mentioned in Chapter 9 are noteworthy. First, the phenyl ring in 1,2-PhC2B10H11 withdraws electron density from its attached cage carbon atom and also from the adjacent (unsubstituted) carbon, deshielding both. However, in 1,7-PhC2B10H11, 13C NMR spectra reveal that only the cage carbon bound to the phenyl group is deshielded [31,32]. Second, molecular orbital calculations and high-resolution X-ray diffraction data of 1-aryl o- and m-carboranes show that p-electron back donation from the ring into cage antibonding orbitals is significant, for example lengthening the cage C2 2C bond in the o-carborane system [112]. In p-carborane, 13C NMR and UV-vis spectra 0 2C6H42 2CB10H10C2 2C6H42 2X, where X is an electron acceptor or donor of C,C -diaryl derivatives of the type X2 such as NO2, CF3, Me, OMe, OH, NH2, or NMe2, show unequivocally that the 1,12-C2B10 cage transmits electronic effects between the attached substituents [634]. The evidence correlates well with a mechanism that is neither purely inductive nor resonance in nature, but resembles that found in chromophores connected by staffanes (saturated bicyclo-hydrocarbons) which involves p-s/s* orbital overlap between the donor or acceptor groups and the central cage [634]. These properties are reflected in the reactivity of arylcarboranes, and illuminate much of their chemistry. An example is the low reactivity of the phenyl ring in 1,2-, 1,7-, and 1,12-PhC2B10H11 toward electrophilic substitution, a consequence of the strong carboranyl-carbon –I effect [134,137,251]; all three compounds fail to undergo Friedel-Crafts acylation with acyl halides over aluminum chloride [147]. This reaction does, however, proceed in the presence of trifluoromethanesulfonic acid, with acylation taking place mainly at the para position of the phenyl ring in each case [633]. When an insulating group such as CH2 is present between the ring and cage carbon atom, the phenyl group undergoes normal, and fast, reactions with electrophiles [147]. On the other hand, if the phenyl is bonded to boron, the carboranyl group functions as an electron donor and facilitates acetylation, bromination, nitration, and mercuration with a strong meta-directive effect on the ring [147]. This underlines a point we have stressed earlier that bears repeating: the electronic behavior of o-, m-, or p-carborane cages toward attached groups is very different in carbon- versus boron-substituted derivatives. Aryl-substituted o-, m-, and p-carboranes have figured prominently in comparative studies of deboronation, that is, removal of boron from the C2B10 cage to generate nido-C2B9 anions. As is outlined in Chapter 7, the ease of deboronation by electron donors such as F decreases in the order o- > m- > p-carborane, reflecting lower polarity in the latter systems. A detailed multinuclear NMR study of the degradation of 1,2- and 1,7-HCB10H10C-p-C6H4F by fluoride ion in aqueous THF or MeCN to form the nido-7,8- and 7,9-(FC6H4)C2 B9 H12 ions, respectively, has shown that in both cases boron is extracted as a fluoroborate anion, HOBHF2 , and hence two F ions are required for each boron removed [133]. In the search for novel electronic conductors, aryl-substituted p-carboranes have been combined with acetylenic units C2 2C6H42 2C 2C6H42 2CB10H10C2 2H, [637] and other functionalities, as in rigid-rod chains such as H2 2CB10H10C2 described in the following section. The evolving use of arylcarboranes in molecular engineering, of which more will be said in later chapters, has generated interest in methods for controlling phenyl ring rotation. The compound 2-(o-hydroxyphenyl)-p-carborane (10-25, obtained by palladium-catalyzed coupling of o-MgBrC6H4OH with 2H O hydrogen bond, and experimental and theoretical studies show 1,12-C2B10H11-2-I has an intramolecular C2 that protonation of its hydroxyl group produces a reversible conformational change to form 10-26 [645]:
10.7 Alkyl, haloalkyl, and aryl derivatives
O
H
O−
O CF3SO3H
H
C
H
H
pyridine
H
H
H
C
C
C
C
H
H
C
10-27
10-25
10-26
627
10.7.2.2 Derivatives with C5 and C7 aromatic rings m-Carboranes with cyclopentadienyl substituents, prepared primarily for studies of ring-cage interactions, are obtained 2COCl with NaFe as described in Section 9.6 for their o-carboranyl counterparts, that is, via reaction of HCB10H10C2 2CO2 2Fe(CO)2Cp. The latter compound loses CO on heating to form (CO)2Cp to generate HCB10H10C2 2Fe(CO)2Cp, which has a direct iron-cage carbon bond [506]. Not surprisingly, there are no m-carboranyl HCB10H10C2 exocyclic metal complexes analogous to the previously discussed 9-63 and 9-64. As in the o-carboranyl compounds, the C5 ring carbons are strongly deshielded owing to electron attraction by the cage carbon nuclei. Derivatives of m- and p-carborane containing aromatic C7 rings have attracted attention because of their predicted, and in some cases experimentally confirmed, unusual electronic properties that might potentially be exploited in nonlinear optical (NLO) and other applications. The synthesis of tropenyliumyl derivatives, or ousenes, is outlined in Section 9.6 for o-carboranyl systems; corresponding m- and p-carboranyl derivatives such as 10-29 are similarly obtained, as shown [91]. Of special interest are target species in which a strong electron attractor and an electron donor are connected by a p-carborane cage, as in the linear systems 10-30 whose computed first hyperpolarizabilities (second-order response, b) have extremely high values 2C6 H42 2C5 H5 ([5.6.7] quinarexceeding 1000 1030 cm5 esu1. For comparison, the analogous hydrocarbon p-C7 H6 þ2 30 5 1 cm esu depending on orientation [90,589,620]. ene) has a calculated b ranging from 52 to 383 10 The fullerene-ethynylphenyl-p-carboranyl system 10-31 has been prepared and found to have a b value of 1189 1030 cm5 esu1, several times larger than the corresponding m- and o-carboranyl derivatives for which b is 483 and 386 1030 cm5 esu1, respectively [164,165]. Further discussion of projected NLO and related applications of such systems can be found in Chapter 17. Ph3C+
C
H C
−Ph3CH
+
C
H C
10-29
10-28 R R −
C
+
C
10-30
R R
R = H, Me, Et, CN
C H
C
C
C
CH3
10-31
628
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
10.8 ALKENYL AND ALKYNYL DERIVATIVES Carbon-substituted alkenyl and alkynyl m- and p-carboranes are conveniently prepared from haloalkenes or haloalkynes and C-metallated derivatives as described in Section 10.5, or by thermal isomerization of the corresponding o-carboranyl derivatives [158,163]. 1,7-Bis(isopropenyl)-m-carborane can be obtained in reactions of the C,C0 -bis(dimethylcarbinol) with AlCl3 or H2SO4 [152], and the pyrolysis of acetates of secondary alcohols affords C-vinyl products [193]. AlCl3
ðHOÞMe2 C2 2CB10 H10 C2 2CMe2 ðOHÞ ! H2 C5 5CMe2 2CB10 H10 C2 2CMeC5 5CH2 ðCH2 COÞ2 O
CH2 CHO
R2 2C2 2CB10 H10 C2 2Li ! R2 2CB10 H10 C2 2CHðOHÞMe ! D
R2 2CB10 H10 C2 2CHðOCOMeÞMe ! R2 2CB10 H10 C2 2CH5 5CH2 R¼H;Me
Another approach is via the dehydrohalogenation of alkenes with sodium amide in liquid ammonia, as in the formation CH from HCB10H10C2 2C 2CH5 5CXH (X 5 5 Cl, I) precursors [160]. B-alkenyl and of 1,7- or 1,12-HCB10H10C2 B-alkynyl m- and p-carboranes, of which relatively few have been characterized (see Tables 10-1 and 10-2), are generally synthesized via metal-catalyzed coupling, m- to p-carborane thermal isomerization, or boron insertion into nido cages (Section 10.6). The conversion of B-alkenyl to B-alkynyl o-carboranyl derivatives via bromination followed by treatment with sodium amide, described in Section 9.7, also works for m-carboranes, for example [169], Br2
1; 7-C2 B10 H11 -9-CH2 CH5 5CH2 ! CHCl3
NaNH2
Hþ
CH C2 B9 H11 -9-CH2 CHBrCH2 Br ! ! C2 B10 H11 -9-CH2 C NH3
10.8.1 Reactions and electronic properties 10.8.1.1 General reactivity The behavior of alkenyl- and alkynyl-functionalized m- and p-carborane derivatives toward halogens, acids, and other electrophiles generally follows the reactivity patterns of o-carboranyl compounds discussed earlier. In particular, directly C-bonded alkenyl derivatives are largely unreactive toward these reagents, but derivatives in which the alkene is bound to boron, or is separated from the carboranyl carbon atom by methylene or other groups, readily undergo nucleophilic attack by HCl, Br2, or similar reagents at the carbon–carbon double bond [158]. The course of reaction may be dictated 2CF5 5CF3 interacts with nucleophiles by the nature of the attacking and alkenyl groups; for example, 1,7-MeCB10H10C2 via both addition to the C5 5C group as well as substitution at boron [154].
10.8.1.2 Rigid rods
Y substituents directly linked to the cage carbon atoms (10-32) Derivatives of p-carborane having alkynyl or other X have interesting linear molecular architectures and have been investigated to determine the extent of p-interaction between the cage and the triple bond.
X
C
C
C
C
X
10-32
X = CH, N, CSiMe3, O
Evidence of such interaction can be seen in structural and spectroscopic studies of 1,12-diethynyl derivatives, CR (R ¼ H or SiMe3), in which the tropical B2 C2 2C 2B bonds and the intracage C C separation RC 2CB10H10C2 C)2C2B10H10, electron-withdrawal by the nitrile are lengthened compared to parent p-carborane [653,654]; in 1,12-(N groups shortens the cage C C and B2 2B distances relative to the diethynyl species. This work is complemented by
10.8 Alkenyl and alkynyl derivatives
629
electrochemical studies showing the transmission of electronic effects between p-carborane-bridged metal centers as in 10-15 and 10-16 mentioned earlier [718] and the mixed-cluster assembly 10-33 [716,720].
Me3Si
C
C
C
C
C
SiMe3
C
CO2(CO)6(dppm)
O C
O C
SiMe3
O C
CH2 P
C
C
C
C
P
C O
Co C
Co P
P
Co
C Co
CH2
O C
CO
C O
C O
SiMe3
10-33
Evidence from NMR and UV-vis spectra shows that electronic communication between the p-carborane cage and its substituents is weak, indeed substantially smaller than in analogous 6- and 10-vertex carborane systems [621,654,779]. Consequently, the C,C0 -diethynyl-p-carborane moiety appears less suited for NLO application than do electronic “push-pull” systems such as 10-30 and 10-31 discussed earlier. Nonetheless, the electron-delocalization and geometry of C-alkynyl m- and p-carboranes suggest their possible application as building blocks in nanoscale electronic systems. Several groups have explored the synthesis and properties of “carborarods” (a term coined by C, or other groups, or absent M. F. Hawthorne [153]) typified by 10-34, in which the linker X can be phenylene, C entirely (i.e., directly linked cages) [138,613,636,661,714].
H
X
C
C
X
H
10-34
n
Analogous systems have been constructed from the 10-vertex 1,10-C2B8H8 cage system (Chapter 6), for example, by coupling of 1,10-Cu2C2B8H8 [779]. An important building block for p-carborane-based carborarods, 1,12-diethynyl-p-carborane CH), has been obtained in low yield by desilylation of the bis(trimethylsilyl) derivative C2 2C (HC 2CB10H10C2 [650,653], but a recently developed multistep sequence affords this compound in 64% yield. The 1,12-diethynyl derivative can be trimethylsilylated if desired, regenerating 10-35 [153]. ðCH2 OÞn
SO2 py
Li2 2CB10 H10 C2 2Li ! HOCH22 2CB10 H10 C2 2CH2 OH ! DMSO
ðPh3 PCH2 ClÞCl
C4 H9 Li
OHC2 2CB10 H10 C2 2CHO ! CIHC5 5CH2 2CB10 H10 C2 2CH5 5CHCI ! KOCMe3 MeLi
C2 CH ! Me3 Si2 C2 C2 HC 2CB10 H10 C2 2C 2C 2CB10 H10 C2 2C 2SiMe3 Me3 SiCl
ð10:35Þ
Despite these synthetic advances, the utility of ethynyl-linked p-carborane rods is limited by their generally low solubility in organic solvents. This problem has been largely circumvented by employing as connectors boron-permethylated “camouflaged” carboranes, described earlier in this section, which are effectively “spherical hydrocarbons” that dissolve
630
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
easily in organic media and also impart enhanced thermal stability to their ethynyl-linked derivatives. Conversion of 1,12H2C2B10Me10 to the C-ethynyl species and then to the 1,10 -(1,4-butadiyne) carborarod 10-36 is accomplished in several steps [153]: ðCOClÞ2
C4 H9 Li
2H ! H2 2CB10 Me10 C2 2ðCH2 Þ2 OH ! H2 2CB10 Me10 C2 C2 H4 O
DMSO NaNH2
PCl5
H2 2CB10 Me10 C2 2CH2 CHO ! H2 2CB10 Me10 C2 2CH2 CHCl2 ! NH3
C H Li
CuCl;O ;py;
ClCðOÞOMe
DBU; 45 C; 2h
2 4 9 CH C2 H2 2CB10 Me10 C2 2C ! H2 2CB10 Me10 C2 2C 2CðOÞOMe !
C2 C2 H2 2CB10 Me10 C2 2C 2C 2CB10 Me10 C2 2H
ð10:36Þ
Functionalization of 10-36 at its antipodal carboranyl C2 2H bonds followed by CuCl-promoted linkage affords the C2 C2 C2 linear tetramer [HCB10Me10C2 2C 2C 2CB10Me10C2 2C 2]2 (10-37). While this compound can be lithiated at its carboranyl carbon atoms, its dilithio derivative is unreactive and hence cannot be coupled to form longer chains. C2 C2 2CB10Me10C2 2C 2C 2CB10Me10C2 2CH2OH However, by modifying the synthetic route using the diol HOCH22 C2 C2 C2 C2 2C 2C 2CB10Me10C2 2C 2]2 one obtains the ethynyl-terminated four-cage system [HC 2CB10Me10C2 (10-38), a promising building block for further oligomerizaton [153]. C2 C2 C2 2C 2C 2 Higher molecular weight oligomers have been generated from HC 2CB10Me10C2 CH by coupling with CuCl followed by oxidation to form a mixture of products formulated as H2 2C 2 CB10Me10C2 C2 C2 2C 2]n (10-39), where n is estimated from NMR peak area ratios to have an average value [C 2CB10Me10C2 ˚ and a molecular weight of 5285 Da [153]. In other variations of 16. A 16-cage chain has a calculated length of 156 A on this theme, metal-centered rods of the type 10-40 [153] and chains incorporating phenylene rings, for example, 10-41 [637], have also been prepared. P(n-C4H9)3 H C
C
C
C
C
C
C
C
C
Pt
C
C
C
C
C
C
C
C
C
C
C
H
P(n-C4H9)3
B−CH3
10-40
H
C
C
C
C
C
C
H
10-41 Although the linearity of p-carborane-based rods is an attractive feature for certain applications, m-carboranyl analogues such as the zigzag tetramer 10-42 employing 1,7-C2B10Me8H2 cages have been synthesized by similar methods and found to have solubility properties comparable to those of their p-carboranyl analogues [153].
H
C
C
C
C C
C
C
C
C C
C
C C C
C
C C
C B−CH3 B−H
10-42
C
C
H
10.9 Carboxylic acids and esters
631
10.9 CARBOXYLIC ACIDS AND ESTERS Like their o-carboranyl relatives, carboxyl-substituted m- and p-carboranes are important synthons that are finding increasing use in the designed synthesis of functionalized derivatives for BNCT, drug design, and other applications discussed in Chapters 16 and 17. For example, they are readily convertible into a wide variety of nitrogen- and phosphoruscontaining 1-carboxy-m-carboranyl salts that are potentially biologically active [238], and bifunctional amino acid carboranes are of interest in biomedical applications [234].
10.9.1 Synthesis The methods described in Section 9.8 for preparing C-carboxyl o-carborane derivatives are generally applicable as well 2(CH2)nC(O)OH to the m- and p-carboranyl systems (Tables 10-1 and 10-2). Most syntheses of 1,7- or 1,12-RCB10H10C2 (n ¼ 0-4) compounds employ reactions of C-lithio or C-MgBr derivatives with CO2 followed by acidification [75,76,110,221,222,228,233,600,677,678]; similar reactions of boron-permethylated p-carboranes give 1,122(CH2)nC(O)OH products [614]. 1,7-HCB10H10C2 2CH2C(O)OH can also be prepared by treatment of RCB10Me10C2 2Na with Na2 2CH2Br-C(O)O [232]. HCB10H10C2 Bifunctional amino acid derivatives such as 10-45 and its o- and p-carboranyl analogues, of interest in BNCT and other biomedical applications, are synthesized via a protection/deprotection strategy in which a carboxyl group is introduced onto the unsubstituted carbon atom of a boc-protected C-amino-carborane 10-43 (boc ¼ tert-butyloxycarbonyl); removal of the boc group gives the amino acid product [234].
O H
C
H
C
C
Cl
H
Me3SiN3
N C
C
O C
Δ, Me3COH
OCMe3
10-43 n-C4H9Li, CO2 H+ O
O NH2
C H
C
C
(1) CF3C(O)OH
C H
H
N C
C
(2) Et3N, CH2Cl2
10-45
O C OCMe3
10-44
As was noted in Section 10.6, B-carboxyl m-carboranes are accessible via CrO3 oxidation of B-acetyl derivatives or 2C(O)OH group, 1,12by boron insertion into nido-7,9-C2 B9 H11 2 . The only characterized p-carborane with a B2 H2C2B10H9-2-C(O)OH, has been prepared by chromic acid oxidation of 2-methyl-p-carborane [236].
632
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Paralleling o-carborane chemistry, carboxyalkyl and carboxyaryl derivatives of m- and p-carborane are readily accessible, as in the synthesis of benzoic acid derivatives via chromic acid treatment of C- or B-tolyl carboranes [115,189,780] and the electrophilic alkylation of m-carborane with chlorobutyric or chlorovaleric acid. The latter approach can also be used with benzyl halides to generate m-carboranyl B-p-carboxybenzyl products analogous to 9-103 (Section 9.8) [220]. CrO3
1; 7-HCB10 H10 C-m=p-C6 H4 Me ! HCB10 H10 C-m=p-C6 H4 CðOÞOH AlCl2
1; 7-H2 C2 B10 H10 þ ClðCH2 Þn CðOÞOH ! H2 C2 B10 H9 -9ð12Þ-ðCH2 Þn CðOÞOH CHCl2
n ¼ 3; 4
10.9.2 Properties of m- and p-carboranyl carboxylic acids 10.9.2.1 General trends Relative acid strengths and associated properties of o-, m-, and p-carboranyl acids have been extensively studied and are discussed in Section 9.8 and tabulated in Table 9-2. The principal findings are that (1) pKa values for the 2C(O)OH and HCB10H10C2 2C6H42 2C(O)OH cage isomers decrease in the order o- > m- > p-carborane, HCB10H10C2 reflecting weaker inductive electron-withdrawal by the carboranyl carbon atoms in the meta and para isomers; (2) acid strength is much lower in B- versus C-substituted acids owing to the fact that the carborane cage is electron-donating toward substituents attached to boron; and (3) effects of the carborane cages on pKa are considerably mitigated when alkyl or other insulating groups are present between the acid group and the cage. Although the reactivity of m- and p-carboranyl carboxylic acids largely resembles that of their o-carboranyl counterparts, there are notable differences. In most cases, these are attributable either to the electronic effects just noted, and/or to the separation of cage carbon atoms in the meta and para isomers, which prevents the formation of exo-polyhedral rings such as cyclic ketones and anhydrides from C,C0 -disubstituted derivatives.
10.9.2.2 Esterification In contrast to 1,2-[HO(O)C]2C2B10H10 (9-101, Section 9.8), which cannot be esterified, treatment of its m-carboranyl analogue with alcohols in acidic media readily yields the diester [75]. MeOH
1; 7-HOðOÞC2 2CB10 H10 C2 2CðOÞOH þ! MeOðOÞC2 2CB10 H10 C2 2CðOÞOMe H
A more generally applicable procedure involves treatment of m- and p-carboranyl C-carboxylates and C,C0 -dicarboxylates with PCl5 to generate the respective carbonyl chlorides [676,685] which in turn combine with alcohols to form esters [240,245,685], including unsaturated [253,781] and peroxy [197,252,781–783] compounds. ROH;Et3 N
1;7=1;12-ClðOÞC2 2CB10 H10 C2 2CðOÞCl ! ROðOÞC2 2CB10 H10 C2 2CðOÞOR CH R ¼ CH2 CH5 5CH2 ; CH2 C
THF;20 C
10.9 Carboxylic acids and esters
633
HOðCH2 Þn OOR0
1; 7-R-CB10 H10 C2 2CðOÞCl ! R002 2CB10 H10 C2 2CðOÞOðCH2 Þn OOR0 hexane;pyridine;10 C
R; R00 ¼ H; R0 ¼ CMe3 ; CMe2 Et; n ¼ 1; 2 R ¼ CðOÞCl; R0 ¼ CMe3 ; CMe2 Et; n ¼ 1; 2; R0 ¼ R0 OOðCH2 Þn OðOÞC pyridine
1;7-ClðOÞC2 2CB10 H10 CH þ Me3 CðOÞOOH ! Me3 CðOÞOO2 2CB10 H10 CH The same general approach affords esters of poly-B-methyl p-carborane from 1,12-[HO(O)C]2 2CB10Me10CH; the products are unreactive toward hydrolysis owing to steric hindrance from the methyls surrounding the carboxyl group [682]. 2CB10H10CH (10-46) and 1,7-RC(O)OCH22 2CB10H10C2 2CH2OC(O)R can be Esters of the type 1,7-RC(O)OCH22 obtained from RC(O)Cl and the corresponding m-carboranyl alcohol in the presence of phase-transfer catalysts [243]. O H
C
C
H
OH RC(O)Cl
C C
PhCH2NEt +3 Cl−
C
O
R
10-46
R = Et, Ph, CHMe2, o/m-C2B10H11
10.9.2.3 Decarboxylation
The C,C0 -dicarboxylic acid of m-carborane undergoes partial loss of C(O)OH on reaction with sodium ethoxide at 2CB10H10CH (30%) and parent m-carborane (6%) along with a trace of nido130-140 C, affording 1,7-Et(O)C2 C2 B9 H12 ; in contrast, 1,12-[HO(O)C]2C2B10H10 is unreactive under these conditions [685]. Unlike the o-carboranyl C,C-dicarboxylate, which has a strong tendency to form intramolecular exo-polyhedral rings (e.g., 9-104, 9-106, and 9-109), such behavior is precluded in the corresponding m- and p-carborane diacids which have nonadjacent carboxyl groups.
10.9.2.4 Dendrimers and polymers Acid-functionalized p-carborane derivatives have been employed to construct water-soluble polyester dendrimers that are designed to counter the inherent hydrophobicity of the carborane cage, which is a major obstacle in their use in cancer therapy as it limits the amount of boron that can be delivered to cells. In one strategy, acid 10-47, which contains a benzyl ether protecting group, is incorporated into a 16-cage dendrimer 10-48, whose benzyl units can be replaced by hydroxyls as shown to afford the water soluble 10-49. The polyhydroxyl species 10-49 can then be further esterified to generate higher-generation dendrimers [665,784].
634
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
O
EDC, (Me3C)Ph2SiCl
OH
O
3:2 CH2Cl2, pyridine
10-47
O O
O
O
O
O
O O
O
O
O
O O
O O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O O
O O
O O
O
O
O O
O
O
O O
O O
O
O
O
O
O
O
O
O
O
O
O
O
O
O O
O
10-48
O
O
O
O
O
O
Pd/C H2
O
O
O O O
OH HO
OH
EDC = I-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride HO
O
OH
O O
O
O
O
O OH
HO O
O
O
O O
O O
O O
O
O
HO
O
O
O
O O
O O
O O
O
O
O
O
O
O O
10-49
O
O O
O
O
OH O
O
O
O O
O
O
O O
O
HO
OH
O O
O
O
O O
O
HO
OH
OH HO
HO
10.10 Alcohols, hydroxy derivatives, and ethers
635
The hydrophobicity problem has also been attacked via construction of linear water-soluble polymers such as the p-carboranyl acrylate random copolymer 10-51, which is prepared from the monomer 10-50 via atom transfer radical polymerization (ATRP) in DMF solvent [664]. The polymer has a molecular weight of 27,600 g mol1 and a boron content of 7.4%, sufficient for emission of gamma radiation when irradiated with thermal neutrons, and hence potentially useful as an agent in neutron capture therapy. O O OH
Br
O
O
H
(1) n-C4H9Li (2)
O
O
C
C
C
Br
88 %
C
C
H
H
10-51
O
Cl
O
O
n
O
CuBr, PMDETA
C H
C
C
10-50
H
PMDETA = Me2NCH2CH2NMeCH2CH2NMe2
The developing role of polycarborane systems such as 10-49 and 10-51 in biomedical applications is further explored in Chapter 17.
10.10 ALCOHOLS, HYDROXY DERIVATIVES, AND ETHERS 10.10.1 Synthesis 10.10.1.1 Hydroxy-substituted derivatives General methods for preparing m- and p-carboranes with direct Ccage2 2OH and Bcage2 2OH links are outlined in Sections 10.5 and 10.6.
10.10.1.2 C-substituted alcohols C-hydroxyalkyl and -hydroxyaryl derivatives of m- and p-carborane are easily formed in reactions of C-metallated carboranes with aldehydes and ketones [75,76,184,187,198,614,662,663], as in the synthesis of bis(hydroxymethyl) derivatives from paraformaldehyde and their respective dilithio derivatives. ðCH2 OÞn
2CB10 R10 C2 2CH2 OH 1; 7=1; 12-Li2 C2 B10 R10 ! HOCH22
R ¼ H; Me
Similar treatment of 1-lithio-m-carborane affords a 1:1 mixture of 1,7-(HOCH2)C2B10H11 and 1,7-(HOCH2)2C2B10H10, but the former compound can be prepared more efficiently via the gas-phase thermal rearrangement of 1-acetoxymethylo-carborane to its m-carborane analogue followed by methanolysis of the latter [182]. Hþ
1; 7-HCB10 H10 C2 2CH2 OCðOÞMe þ MeOH ! HOCRR02 2CB10 H10 C2 2OH As was noted in Section 9.9 for the o-carboranyl system, monohydroxymethyl-m-carborane can be prepared in good yield via reduction of a trimethylsilyl-protected carboranyl methyl ester, with the SiMe3 group subsequently removed [204]. C-lithio-m- and p-carboranes combine with aliphatic, aromatic, and heterocyclic aldehydes to generate secondary alcohols [78,187,214], while ketones produce tertiary alcohols [152].
636
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12 Hþ
1; 7-Li2 2CB10 H10 C2 2Ph þ MeOC6 H4 CHO ! MeOC6 H4 CHðOHÞ2 2CB10 H10 C2 2Ph Hþ
1; 7-Li2 2CB10 H10 C2 2Li þ RCðOÞR ! HOCRR02 2CB10 H10 C2 2RR0 OH R; R0 ¼ Me; CF3 ; CF2 Cl Reactions of m-carboranyl aldehydes and ketones with organolithium or organomagnesium reagents form secondary and tertiary alcohols, respectively, in some cases accompanied by exo-polyhedral C2 2C bond cleavage [193,214]. This method has been employed to synthesize mixed o/m-carborane systems such as 10-52 [193]: H
OH
O C Me C
C
C
H
BrMg
Me
C
HCl
+
C C
C
C C
−MgBrCl H
10-52 In contrast to these findings, o-carboranyl ketones are cleaved by organolithium reagents, although Grignards do convert them to alcohols (Section 9.9). As we noted in the earlier discussion, this behavior implies that the alcoholate intermediates formed from m-carboranyl ketones are more stable than those of their o-carboranyl counterparts such as 9-112 [193,785]. C-lithio o- and m-carboranes both react with epoxides to form secondary alcohols, but the meta isomers are far less reactive. For example, while 1,2-LiMeC2B10H10 combines with epichlorohydrin to yield an epoxide and a secondary alcohol (Section 9.9), the same treatment of 1,7-LiMeC2B10H10 generates only the epoxide [78]. A related approach, widely employed [105,106,664,665], utilizes trimethylene oxide to form the 1-hydroxypropyl derivative. Accompanying formation of the C,C0 -dicarbinol can be blocked by placing a trimethylsilyl protecting group on one cage carbon [185]. ðCH2 Þ3 O
1;7=1;12-HCB10 H10 C2 2Li ! HCB10 H10 C2 2ðCH2 Þ3 OH ðn-C4 H9 Þ4 NF ð1Þ n-C4 H9 Li 2SiMe3 ! HOðCH2 Þ32 2CB10 H10 C2 2SiMe3 ! HOðCH2 Þ32 2CB10 H10 CH 1; 7-HCB10 H10 C2 ð2Þ ðCH2 Þ3 O
Pyridine-promoted coupling with C-cupra-m- and p-carboranes furnishes an efficient route to C-substituted hydroxyphenyl derivatives, which bind strongly to estrogen receptors and show promise as therapeutic agents (Chapter 16) [191,196,643,669]. ð1Þ CuCl
2Li ! 1;7=1;12-HCB10 H10 C2 ð2Þ p-MeOC6 H4 I;py BBr3 1;7=1;12-HCB10 H10 C2 2p-C6 H4 OMe ! 1;7=1;12-HCB10 H10 C2 2p-C6 H4 OH CH2 Cl2
10.10.1.3 B-substituted alcohols B-hydroxyalkyl and -hydroxyaryl derivatives of m-carborane, like their o-carboranyl cousins, can be obtained via reduction of ketone or carboxylic acid derivatives [199,203], or via metal-promoted cross-coupling (Section 10.6). For example, boron-bound m- and p-carboranyl phenols are accessible through palladium-catalyzed coupling of B-iodocarboranes with methoxyphenyl Grignard reagents in THF followed by demethylation with boron tribromide [669]. p-MeOPhMgBr BBr3 1;7=1;12-H2 C2 B10 H92 2I ! H2 C2 B10 H9 -p-C6 H4 OMe! ðPh3 PÞ2 PdCl2
1;7-H2 C2 B10 H9 -9-p-C6 H4 OH or 1;12-H2 C2 B10 H9 -2-p-C6 H4 OH
CH2 Cl2
10.10 Alcohols, hydroxy derivatives, and ethers
637
10.10.1.4 C-substituted ethers Carbon-bound m-carboranyl ethers can be prepared via conversion of alcohols using phase-transfer catalysis with organic halides (Section 9.9) [254,255]. Peroxy ethers are formed on treatment of carbonyl chlorides with peroxy alcohols or hydrogen peroxide [248,249]. pyridine
1; 7-HCB10 H10 C2 2CðOÞCl þ ROOH ! HCB10 H10 C2 2CðOÞOOR R ¼ H; CMe3 ; CMe2 Et; CðOÞMe; PhCH2 Application of this method to 1,7-[Cl(O)C]2C2B10H10 and (HOO)Me2CCH2CH2CMe2C(OOH) yields the cyclic ether 10-53, a rare example of a single-cage m-carborane derivative incorporating an exo-polyhedral ring [250]. Me2C O
CMe2 O
O C O
10-53
O
C
C
C O
Few p-carboranyl ethers have been prepared, a rare example being the previously cited mixed-cage system 2ðCH2 Þ4 O2 2B12 H11 2 (R ¼ H, Me) that is known for all three icosahedral carborane isomers (Section R2 2CB10 H10 C2 9.15).
10.10.1.5 B-substituted ethers B-alkoxy- and B-aryloxy-m-carboranes, for example, the 9-p-C6H4-OR and 9,10-(C6H4OR)2 (R ¼ H, Me) derivatives and others listed in Table 10-2, can be prepared by palladium-catalyzed cross-coupling of B-iodo-carborane derivatives with organomagnesium halides (Section 10.6) [128,202,257]. The approach described above for the preparation of p-carboranyl B-azole and B-amine derivatives, employing the catalyst Pd(dba)2/BINAP/Me3C-ONa in dioxane (dba ¼ trans,trans-dibenzylideneacetone), also affords boron-etherated p-carboranes via reactions of 1,12-C2B10H11-2-I with alkoxides and phenolates [684]. The bis(p-carboranyl) ether (1,12-C2B10H11-2-)2O mentioned earlier (Section 10.6) was serendipitously obtained during coupling reactions of styrenes with 1,12-C2B10H11-2-I [651].
10.10.2 Properties of m- and p-carboranyl alcohols 10.10.2.1 General observations Hydroxyalkyl and hydroxyaryl derivatives of m- and p-carborane, like their o-carborane counterparts (Section 9.9), undergo standard organic transformations such as oxidation to carboxylic acids or ketones, and oxidative cleavage of the exo-polyhedral C2 2C bond; often the course of reaction is measurably influenced by inductive electron-withdrawal by the carboranyl carbon atom. In the case of m- and p-carboranyl alcohols, as in other types of derivatives, the electronic effects tend to be mitigated by the weaker –I effect in these cages compared to the ortho isomer. In addition, as remarked earlier, the strong tendency of o-carborane diols to form exo-polyhedral rings is much less evident in the 1,7- and 1,12C2B10 systems owing to the separation of cage carbon atoms. For example, while 1,2-(HOCH2)2C2B10H10 loses H2O to 2O2 2CH2)C2B10H10 on heating with H2SO4 at 140 C [786,787], 1,7-(HOCH2)2C2B10H10 is unreacform 1,2-(cyclo-CH22 tive under these conditions; although sulfonation occurs at 175 C, no cyclic ether is formed [75].
638
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
10.10.2.2 Halogenation The reaction of 1,7-[(HO)Me2C]2C2B10H10 with bromine and sulfuric acid results in both cage bromination and cleavage of the exo-polyhedral C2 2C bonds. Replacement of the methyl groups with strongly electrophilic CF3 moieties leads to much higher acidity of the hydroxyl hydrogens [152]. Br2
1; 7-HOR2 C2 2CB10 H10 C2 2CR2 OH ! Br2 2CB10 Brn H10n C2 2Br R ¼ Me; CF3 H2 SO4
n ¼ 1; 2
Halogenation of m-carboranyl primary alcohols affords an efficient route to 1-haloalkyl derivatives [103,105]. PCl5 or Ph3 PBr2
1; 7-R2 2CB10 H10 C2 2CH2 OH ! R2 2CB10 H10 C2 2CH2 X
X ¼ Cl; Br;
R ¼ H; Me
CBr4
1; 7-HCB10 H10 C2 2ðCH2 Þ3 OH ! HCB10 H10 C2 2ðCH2 Þ3 X
10.10.2.3 Conversion to alkenes As is noted in Section 10.8, alcohols can serve as precursors to alkenyl and alkynyl m- or p-carboranes. For example, C-vinyl products can be prepared via pyrolysis of acetates of secondary alcohols [193] while the treatment of 1,7-[(HO)Me2C]2C2B10H10 with AlCl3 or H2SO4 gives the 1,7-bis(isopropenyl) derivative in good yield [152].
10.10.2.4 Oxidation and cleavage Secondary m-carboranyl alcohols are readily converted to ketones by chromic acid [78,214]. CrO3
1;7-Ph2 2CB10 H10 C2 2CHROH ! PhC2 2CB10 Brn H10n C2 2CðOÞR R ¼ Me; Et; Ph H2 SO4
Primary and secondary m-carboranyl alcohols, like the corresponding o-carboranyl compounds (Section 9.8), undergo oxidative cleavage on treatment with KMnO4 in basic media to give the parent carborane [78,788].
10.11 ALDEHYDES AND KETONES 10.11.1 Synthesis 10.11.1.1 Aldehydes Methods for preparing C-substituted o-carboranyl aldehydes, described in Section 9.10, are generally applicable to the m- and p-carborane systems (Tables 10-1 and 10-2). These include the ozonolysis of C-vinyl derivatives [193,206,225], the palladium-mediated hydrogenation of carboranyl acid chlorides [207,209,210,674], the oxidation of C-hydroxyalkylcarboranes [153,204], and the hydrolysis of acetals [208,211,212] (Section 10.5). In the last case, it was noted in Section 9.10 that although 1-formyl o-carborane can be prepared in 95% yield from C-lithio-o-carborane and methyl formate in a one-pot synthesis, when applied to 1,2-Li2C2B10H10 this approach affords only the cyclic ether 9-116 instead of the C,C0 -diformyl-o-carborane. However, with 1,7- and 1,12-Li2C2B10H10, this procedure readily generates the corresponding C,C0 -diformyl derivatives [205]. ð1Þ HCðOÞOMe
2Li ! 1; 7=1;12-HðOÞC2 2CB10 H10 C2 2CðOÞH 1;7=1;12-Li2 2CB10 H10 C2 ð2Þ HCl
Boron-bound m-carboranyl aldehydes are uncommon. B(2)-formyl-m-carborane can be generated by ozonolysis of 2vinyl-m-carborane [156], while the 9-pentanal 1,7-H2C2B10H9-9-CHMeCH2CH2C(O)H, like its o-carboranyl analogue 9-118 (Section 9.10), is formed on reduction of the ester H2C2B10H9-9-CHMeCH2CH2C(O)OEt with diisobutylaluminum hydride [201,213]. Boron-substituted p-carboranyl aldehydes have not been reported.
10.11 Aldehydes and ketones
639
10.11.1.2 Ketones Subject to the usual caveat that intramolecular exo-polyhedral cyclic structures such as 9-120, common in o-carboranyl chemistry, are generally not formed in the m- and p-carboranyl systems, the synthetic routes described in Section 9.10 can also be employed to generate ketones of the 1,7- and 1,12-C2B10H12 isomers. A common approach utilizes reactions of C-lithiocarboranes with acyl halides [615,676], as in the synthesis of the mixed-cage product 10-54 [129]. R
R O C C
Li
O
C
Cl
C C
O
C Cl
C C
C
R
O
C
C C
C
C
+
2
R = H, Me
10-54 As noted earlier in Section 10.10, m-carboranyl ketones are also generated on oxidation of secondary alcohols with chromic acid. Condensation of m- and p-carboranyl acyl chlorides with benzene over AlCl3 yields C-benzoyl derivatives [52,198], as in the conversion of 1,12-[Cl(O)C]2C2B10H10 to 1,12-bis(benzoyl)-p-carborane [717]. a,b-Unsaturated m-carboranylketones of the type 1,7-[C(O)CH5 5CHR]2 (R ¼ Ph, p-MeOC6H4, p-FC6H4, 2-furyl, 2-furylvinyl), like their o-carborane analogues (Section 9.10), can be prepared from C-acetyl o-carboranes and aldehydes in the presence of boric acid [216]: H3 BO3
2CðOÞMe þ R0 CHO ! R2 2CB10 H10 C2 2CðOÞ2 2CH ¼ CHR0 R2 2CB10 H10 C2 The 1,5-diketones 10-56 are generated quantitatively by oxidation of m-carboranyl pyrans 10-55, which in turn are obtained by reaction of C-lithiocarboranes with pyrilium perchlorate salts [217]. R⬘
O C
O R
Li C
C
R′
+
ClO4−
O
R⬘
R
R⬘
R C
C
R⬘
70 % HClO4−
C
C
R⬘ C O
R = H, Me, Ph R⬘ = CMe3, Ph, p-MeOC6H4
10-55
10-56
Boron-substituted m- and p-carboranyl ketones have been prepared via several routes, including the treatment of 1,7C2B10H9-2-C(O)OH with PCl5 followed by PhCH2Cl and AlCl3 to afford 1,7-C2B10H9-2-C(O)Ph [156,203,220]. Like its o-carboranyl counterpart (Section 9.10), 9(12)-acetyl-m-carborane is generated on reaction of bis(9-carboranyl)mercury with acetyl chloride over AlCl3 [773,774]. MeCðOÞCl
ð1; 7-H2 C2 B10 H9 -9Þ2 Hg ! 1; 7-H2 C2 B10 H9 -9-CðOÞMe AlCl3
Palladium-promoted cross-coupling of B-iodo m-carboranes with Grignards [200] or organozinc compounds [97] can also be used to produce B-benzoyl and other boron-bound ketone derivatives.
10.11.2 Properties Reactions of aldehyde and ketone derivatives of o-carborane are extensively treated in Section 9.10, and discussion of corresponding m- and p-carborane chemistry appears in earlier sections of this chapter. While the reactions of the o-carboranyl species are largely paralleled in the other isomers, the weaker –I electron-withdrawal of the cage carbon atoms in m- and p-carborane relative to the o-carborane species manifests itself in a greater resistance toward base-induced
640
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
cleavage of the Ccage2 2C(O)R bond. However, some cleavage occurs even in the 1,7- and 1,12-C2B10H12 systems [52,110,207,209,214]. For example, 1-formyl-o-carborane undergoes cleavage by ethanolic sodium ethoxide at room temperature, whereas 1-formyl-m-carborane is attacked only under reflux conditions [207,209]. Similarly, LiAlH4 cleaves (1,2-PhCB10H10C)2CO but only reduces (1,7-PhCB10H10C)2CO to (1,7-PhCB10H10C)2COH. Comparative studies show that, in line with their relative –I effects, carbon-bound m-carboranyl ketones are stronger bases than o-carboranyl ketones, although both are considerably weaker than benzophenone and acetophenone [215]. In m- and o-carboranyl ketones, the a-hydrogen atoms exhibit low proton mobility, as shown by the unreactivity of 1-acetyl-2-methyl-o-carborane toward Br2 in boiling CCl4 and the slow conversion of 1-acetyl-2-methyl-m-carborane to the 1-bromoacetyl derivative under these conditions [215].
10.12 SILICON, GERMANIUM, TIN, AND LEAD DERIVATIVES 10.12.1 Synthesis Methods for the preparation of C2 2M derivatives of m- and p-carborane in which M is a Group 14 element—aside from thermal rearrangement of o-carboranyl derivatives as described earlier in this chapter—are outlined in Section 10.5 and largely parallel those employed in o-carboranyl chemistry, described in Sections 9.11 and 9.12. The usual differences apply, for example, the lower acidity of m- and p-carboranyl versus o-carboranyl C2 2H bonds, and the absence of exo-polyhedral ring formation in the meta and para systems. The synthetic approach based on metathesis of C-lithiocarboranes is sufficiently versatile that mixed-cage o/mcarboranylsiloxanes such as 10-57 are easily accessible [438]. Reactions of this type, and others such as the interaction of m-carboranyl carbinols with alkylchlorosilanes (Section 9.11), have been utilized in the synthesis of siloxycarborane polymers as discussed in Chapter 14. H
C
Me
Me
Me
Me
Si
Si
Si
Si
C
C
O Me
O
C
Me
Me
H
C C
Me
10-57 Mercury-linked organogermyl compounds of the type R2 2CB10H10C2 2Hg2 2GeEt3 (R ¼ H, Me, CH2Cl, Ph) may be 2HgR0 (R0 ¼ Me, Ph, Cl) with Hg(GeEt3)2 [465]. obtained via treatment of R2 2CB10H10C2 Boron-substituted m- and p-carborane derivatives with direct B2 2M bonds, where M is a group 14 element, are limited to a few tin compounds (Section 10.6). However, B-organosilyl derivatives can be prepared similarly to the analogous o-carboranes (Sections 9.11 and 9.12), for example, via AlCl3-promoted reactions of vinyltrichlorosilane with parent 1,7- or 1,12-C2B10H12 [462] and by palladium-catalyzed cross-coupling of B-iodocarboranes with Grignards or organozinc reagents [97,463]. Derivatives featuring B2 2Hg2 2Ge links have been obtained in 70% yield from reactions of B-mercurated m-carboranes with digermanes or germanium hydrides [467]. ðC6 F5 Þ3 GeGeEt3
2GeðC6 F5 Þ3 1; 7-H2 C2 B10 H9 -9-HgX ! 1; 7-H2 C2 B10 H9 -9-Hg2 Et3 GeX
X ¼ Cl; OCðOÞCF3
ðC6 F5 Þ3 GeH
1; 7-H2 C2 B10 H9 -9-HgMe ! 1; 7-H2 C2 B10 H9 -9-Hg2 2GeðC6 F5 Þ3 MeH
Boron-substituted organotin derivatives, for example, 1,7-H2C2B10H9-2-CH2CH2SnEt3, are obtainable via hydrostannylation of B-vinyl-m-carboranes [478].
10.13 Nitrogen derivatives
641
10.12.2 Properties Reactions of C,C0 -disilyl m-carboranes with ammonia and other common reagents generally afford the expected C,C0 disubstituted products in high yield with no exo-polyhedral ring formation. Treatment of 1,7-(ClSiR2)2C2B10H10 (R ¼ alkyl or aryl) with ammonia generates (H2NSiR2)2C2B10H10 derivatives, while water and methanol yield (HOSiR2)2C2B10H10 and (MeOSiR2)2C2B10H10, respectively [435,446,466]. (In contrast, 1,2-(ClSiMe2)2C2B10H10 is unreactive toward water [435].) Although much of the study of C-silyl-m-carboranes has focused on polymer construction (Chapter14), development of its monomer chemistry has advanced. Reactions of C,C0 -bis(organosilyl) derivatives with carbinols afford alkoxyalkylsilyl products, which on hydrolysis form hydroxylalkylsilyl compounds with minor amounts of 1,7-Me2HSi2 2C2B10H11 and other species [441]. Hþ
ROH
1; 7-ðMe2 HSiÞ2 C2 B10 H10 ! ðROMe2 SiÞ2 C2 B10 H10 ! ðHOMe2 SiÞ2 C2 B10 H10
R ¼ Me; Et
Silylation of C,C0 disilyl-m-carboranes generates C,C0 -bis(disiloxanyl) derivatives, which are converted to hydroxysilyl products on hydrolysis, for example [440], Cl2 SiMeR
1; 7-ðHOMe2 SiÞ2 C2 B10 H10 ! ðR0 OMe2 SiÞ2 C2 B10 H10 R ¼ Me; Ph; CH2 ¼ CH R0 ¼ ClMe2 Si; ClMePhSi; ClMeðCH2 ¼ CHÞSi; Me2 SiH Hþ
ðR0 OMe2 SiÞ2 C2 B10 H10 ! ðHOMeSiROMe2 SiÞ2 C2 B10 H10 Monosubstituted silyl-m-carboranes have been found to undergo rearrangement to disubstituted products in the presence of catalytic KOH in ether [439]. cat:KOH
1; 7-ðRMe2 SiÞC2 B10 H11 ! ðRMe2 SiÞ2 C2 B10 H10 þ H2 C2 B10 H10
R ¼ H; OMe; OEt
A well-studied reaction of carborane derivatives of Group 14 elements, discussed in Section 9.12.1.3, is the cleavage of 2M bonds by nucleophiles. Alkaline solvolysis of HCB10H10C2 2MR3 (M ¼ Si, Ge, Sn) derivatives of o-, m-, and pCcage2 carborane is most facile when M is silicon and slowest for M ¼ Ge. If the Group 14 element is separated from the cage by a 2CH2MR3, the reactivity decreases in the order Sn > Ge > Si [419], reflecting the weakmethylene unit as in HCB10H10C2 ened –I effect of the cage carbon on the Group 14 atom. 2Sn bonds, as was noted in the earlier discussion, is extremely facile in o-carboranyl derivatives Cleavage of Ccage2 but is much slower in the less polar m- and p-carboranyl systems where the cage carbon atom electronegativity is considerably reduced [468,789–791]. Treatment of B-trichlorosilylalkyl m-carboranes with peroxy alcohols generates peroxy derivatives in high yield [258]. 2CðCH2 Þ2 SiCl3 þ ROOH ! 1; 7-H2 C2 B10 H92 2CðCH2 Þ2 SiðOORÞ3 1; 7-H2 C2 B10 H92 R ¼ CMe3 ; CMe2 Et; 2-cyclohexylisopropyl
10.13 NITROGEN DERIVATIVES In comparison to the extensively developed chemistry of o-carboranes with nitrogen-containing functional groups, outlined in Section 9.13, the corresponding m- and p-carboranyl derivatives have been somewhat less investigated although a number of them have been characterized (Tables 10-1 and 10-2). The lower polarity of 1,7- and 1,12-C2B10H12 is reflected in a generally reduced reactivity toward substitution at boron and carbon compared to 1,2-C2B10H12; for example, the 1,7- and 1,12 isomers are unreactive toward 100% nitric acid at room temperature, in contrast to 1,2-C2B10H12 which forms B-hydroxy and B-nitrato derivatives under these conditions.
642
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Methods for the synthesis of m- and p-carborane derivatives containing Ccage2 2N or Bcage2 2N bonds are described in Sections 10.5 and 10.6. Routes to other nitrogen-containing species are summarized here.
10.13.1 Nitro, nitrito, and related compounds Oxidation of 1-amino- or 1-nitroso-m-carboranes with chromium(III) oxide affords 1-nitro products in high yield; in acetic acid alone one obtains the 1-nitroso derivative [259]. CrO3
R2 2CB10 H10 C2 2NH2 ! R2 2CB10 H10 C2 2NO2 H2 SO4 =MeCðOÞOH CrO3
R2 2CB10 H10 C2 2NH2 ! R2 2CB10 H10 C2 2NO MeCðOÞOH
R ¼ H; Me
R ¼ H; Me
The oxidation of 1-amino- or 1-nitroso-p-carborane, in contrast, does not afford nitro derivatives [259]. As mentioned earlier (Section 10.5), 1-nitroso-p-carborane can be obtained by treatment of the 1-lithio derivative with NOCl. The C-nitritomethyl derivative of B-permethyl-p-carborane (10-59), prepared from 10-58 as shown, under ultraviolet irradiation undergoes a Barton transformation to a B-nitrosomethyl species which rearranges to the oxime 10-60 [663]. This reaction exploits the proximity of the nitrito group to the surrounding methyl C2 2H bonds, whose interaction generates NO and a CH2O radical leading to the observed products. OH
ONO
OH
OH NO
C
C B
hν
NOCl
OH
C
C
B
hν
CH=N
C6H5N, C6H6 C B−CH3
H
10-58
C
C
C
H
H
H
10-59
10-60
Compound 10-60 in turn undergoes reduction by LiAlH4 to form the 2-methylamino derivative 1,12-(HOCH2) HC2B10Me9-2-NHMe via a Beckmann rearrangement. Treatment of 10-60 with H2 in the presence of a palladium-carbon catalyst in ethanol affords the B-amino derivative with a B-cyano side product [663]. H2
ðHOCH2 ÞHC2 B10 Me9 -2-CH5 5NOH !ðHOCH2 ÞHC2 B10 Me-2-R Pd=C;HCl
R ¼ NH2 ; CN
The introduction of nitrophenyl groups at boron or carbon vertexes in m- and p-carborane is accomplished as described in Section 9.13 for the o-carborane system, for example, via the action of nitroarenediazonium salts [139] or nitroaryl iodides [107,634] on C-metallated carboranes [107] and the oxidation of C-aminophenyl derivatives [260]. Condensation of 1-COCl-m- or p-carborane with bis(4-hydroxy-3-nitrophenyl)methane forms the corresponding products (30 -NO2-40 -R-C6H3)2CH2 where R is 1,7- or 1,12-C2B10H11 [261]. The importance of nitroaryl-substituted derivatives in studies of the transmission of electronic effects in the p-carborane cage is discussed in Section 10.7.
10.13.2 Amines, azides, and diazonium salts 10.13.2.1 Synthesis
Like their o-carboranyl counterparts, C-amino- and C,C0 -diamino-m-carboranes are generated efficiently via hydrogenation of phenylazo derivatives over Raney nickel in ethanol [262].
10.13 Nitrogen derivatives
643
H2 =Ni
HCB10 H10 C2 2N5 5NPh ! HCB10 H10 C2 2NH2 H2 =Ni
PhN5 5N2 2CB10 H10 C2 2N5 5NPh ! H2 NC2 2B10 H10 C2 2NH2 As is noted in Section 10.7, C-haloalkyl-m-carboranes can be converted to to C-azidoalkyl derivatives such as 2(CH2)3N3 which in turn are reduced to amines, for example, HCB10Me10C2 2(CH2)3NH2 [105,106]. HCB10Me10C2 Other routes to amines, mentioned earlier, include the reduction of C-nitroso compounds [264] and the conversion of azides to isocyanates, which form amines on acidification [234,263]. Alkylamino- and arylamino-m- and p-carboranes are generated by reduction of nitriles [105], azides [106], and nitrophenyl derivatives [140,260] as well as by reactions of C-metallated carboranes, for example [265], 0
ClCH2 NR2
2Li ! R2 2CB10 H10 C2 2CH2 NR02 R2 2CB10 H10 C2 R ¼ H; CHMe2 ; R0 ¼ H; Me; Et Azides of m- and p-carborane are obtained by reacting the respective C2 2C(O)Cl derivatives with NaN3 in aqueous acetone [263,676]. All of these synthetic pathways parallel those described in Section 9.13 for the corresponding o-carboranyl species. Thermal isomerization of o-carboranyl to m-carboranyl amines is rarely employed, but the conversions of 1,2C2B10H11-3-NPh2 to 1,7-C2B10H11-2-NPh2 [273] and of 1,2-C2B10H11-3-NH2 to 1,7-C2B10H11-2/4-NH2 [274] have been reported. Low-temperature carborane cage rearrangement occurs in the reactions of 1,2-C2B10H11-3-X2 dianions (X ¼ H or NH2) with methyl halides, which afford B(3)-amino-m-carborane products (Section 9.5). Boron-substituted aminoaryl-m- and p-carboranes are accessible via palladium-catalyzed cross-coupling of B-iodocarborane substrates with Grignard reagents, as noted in Section 10.6, and C,C0 -bis(aminophenylcarbamoyl)-mcarboranes are generated in high yield from the respective bis(carbonyl chloride) derivatives [266]. ð1Þ p-HXC6 H4 NO2 ClðOÞC2 2CB10 H10 C2 2CðOÞCl ! H2 NC6 H4 XðOÞC2 2CB10 H10 C2 2CðOÞXC6 H4 NH2 ð2Þ cat:reduction
X ¼ NH; O
10.13.2.2 Properties The fact that the m- and p-carborane cages are more resistant than the ortho isomer to attack by bases, noted in earlier discussions in this chapter, is demonstrated by the inertness of m-carborane toward piperidine and n-propylamine in refluxing hexane [75]; o-carboranes, in contrast, are deboronated to nido-C2B9 species under these conditions. Amine derivatives are useful in the synthesis of derivatives designed for specific applications. For example, the amino acid-polyol 10-61 was prepared [277] as a highly water-soluble agent for potential use in boron neutron capture therapy. OH O OH OH
O O
OH C O C
OH
C NH2
10-61
644
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
The reduction of C-azidoalkyl derivatives of B-polymethylated “camouflaged” m- and p-carboranes can be used to 2(CH2)nNH3Cl (n ¼ 3, 4) and 1,12prepare C-amine hydrochloride salts formulated as 1,7-HCB10H2Me8C2 2(CH2)nNH3Cl (n ¼ 3-6). These compounds are amphiphilic, and upon sonification in solution selfHCB10H2Me8C2 assemble to form nanoscale rigid rods (carborarods) [106]. Aminophenol-m-carboranes are readily diazotized by nitrosylsulfuric acid in glacial acetic acid, forming diazonium salts that combine with halides to yield halophenyl-m-carboranes and with b-naphthol to form azo dyes (Section 9.13) [140].
10.13.3 Nitrogen heterocycles Pyridyl, bipyridyl, porphyrin, phthalocyanine, and other derivatives of 1,7- and 1,12-C2B10H12 (Tables 10-1 and 10-2) are accessible via reactions of C-metallated carboranes with halogenated organic substrates such as bromopyridine, as outlined earlier in Section 10.5. In some cases metallation is not required, as in the syntheses of isoxazole and C or CH25 5CH PhCNO) and PhCNO [286], or the preparation isoxazoline derivatives from RC2B10H11 (R ¼ HC of B-pyridyl-m-carboranes 10-62 and 10-63 via aza Diels-Alder reactions of 9-allyl-m-carborane with 1,2,4-triazines [280]:
H N
C R
N
Ph
N
CN
H
C Ph
R = p-tolyl
C
10-62
C
H
R
H N
N
N
CN
N
H
R
H
C
C
N
R R = Ph, p-tolyl C H
N
R
+ C
N
H
10-63
N N
N-Heterocyclic carboranes are of interest in the development of tumor-specific antigens, anticancer agents, and other biomedical applications (Chapter 16), and many have been prepared; for example, copper porphyrinates such as 10-64 exhibit cytotoxic behavior toward tumor cells [269,270]. Porphyrin derivatives having as many as eight attached p-carborane units have been synthesized [693].
10.13 Nitrogen derivatives
H
N
C
Cu
N
645
C C
N
OH
R
N
10-64 R = Me, CHMe2
In this context, investigators are interested to learn how the physical and biochemical properties of such derivatives vary with the choice of o-, m-, or p-carboranyl (or other) cage system. Accordingly, in many studies, derivatives of porphyrins and other heterocycles with all three icosahedral carboranes are prepared for direct comparison. The o-carboranyl systems that have been investigated are discussed in Section 9.13. Although the separation of the cage carbon atoms in the m-carborane cage inhibits formation of exo-polyhedral rings in single-cage derivatives (in contrast to the o-carboranyl system where they are common), macrocyclic structures incorporating two or more m-carborane clusters can be constructed, for example, the pyridine-linked compound 10-67 [123]. The nitrogen atoms in 10-67 are not quaternized by methylating agents, probably owing to electron-withdrawal by the carborane cages, but chloroperoxybenzoic acid in methylene chloride at room temperature forms the bis(N-oxide) carboracyclic derivative 10-68 [123]. H
C
C
N
Li H 1,3-(BrCH2)2NC5H3
C
Br
C
Et2O
H
1,7-Li2C2B10H10
N C
C
C
C
2:1 C6H6/Et2O
10-65 (1) n-C4H9Li (2) 1,3-(BrCH2)2NC5H3 THF
1) n-C4H9Li 2) 1,3-(BrCH2)2NC5H3 THF
N Br
1,7-Li2C2B10H10 2:1 C6H6/Et2O
N C
Br
C
N C
C
C
C N
10-66 10-67 C
N+
C
1-C(O)OOH-3-ClC6H4
O− O
C
N
−
+
C
10-68
H
646
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
In another application, p-carborane clusters serve as spacers in photoluminescent bipyridyl metal complexes such as 10-69 [649].
C
C N 2+ Ru (bpy)2
N
N
N
10-69
2+ Ru (bpy)2
10.13.4 Amides and imides C-amido derivatives of m- and p-carborane can be prepared from the acid chlorides via reactions with amines or ammonia [76,239,287] or by treatment of C-lithiocarboranes with aryl isocyanates [78,129]. 2NH3
ClðOÞC2 2CB10 H10 C2 2CðOÞCl ! H2 NðOÞC2 2CB10 H10 C2 2CðOÞNH2 As was mentioned in Section 10.6, B-amido-m- and p-carboranes are accessible via treatment of 1,7-C2B10H9-9-I or 1,12-C2B10H9-2-I with amides [288]; 1,7-C2B10H9-9-C(O)NMe2 has also been obtained from 1,7-C2B10H9-9-C(O)Cl and ammonia [287].
10.13.5 Nitriles, isonitriles, and isocyanates Nitrile and isonitrile derivatives of o- and m-carborane can be synthesized from C-lithio or Grignard reagents and cyanotosylates [293] or phenyl cyanate [294] (Section 9.13), as well as via treatment of C-bromoalkyl-m- and p-carboranes with sodium cyanide [106]: 2ðCH2 Þ3 Br þ NaCN ! 1;7=1;12-HCB10 Me10 C2 2ðCH2 Þ3 CN 1;7=1;12-HCB10 Me10 C2 Dehydration of carboxamides, a method described in Section 9.13 for o-carboranyl derivatives and employed to generate linear rods (Section 10.8), also works with p-carboranes [654]. 2CB10 H10 C2 2ðOÞNH2 þ ðMe3 SiOPO2 Þn ! NC2 2CB10 H10 C2 2CN 1;12-H2 NðOÞC2 Boron-connected m- and p-carboranyl nitriles are accessible by thermal rearrangement of o-carboranes [177] and via nucleophilic displacement of B(9)-iodonium or -bromonium substituents by cyanide ion [180] as noted in Section 10.6. A phthalodinitrile derivative, 1,7-C2B10H9-9-S-C6H3-30 ,40 -(CN)2, has been prepared from 9-mercapto-m-carborane and 4-nitrophthalodinitrile in dimethylformamide [295]. Methods for preparing C-substituted m- and p-carboranyl isocyanates parallel those of the analogous o-carboranes, and include the condensation of carboranyl diols with ClCN [290] and the reaction of carboxylic acid chlorides with azides [291,292]. In the latter process, discussed in Section 10.5, the initially formed C-carbonyl azide rearranges to 2NCO final product [234]. Conversion of B(9)-carboxyl-m-carborane to the B(9)-NCO derivative is the HCB10H10C2 effected by treatment with SOCl2 and NEt3 [292], but p-carboranyl boron-substituted isocyanates have not been described. Attempted preparation of the C-isonitrile derivative of p-carborane by dehydration of the formamide 10-70 with the Burgess reagent, methyl N-(triethylammoniumsulfonyl)carbamate—a method successfully employed with the o-carboranyl analogue (Section 9.13)—leads instead to the methyl formate 10-71, which apparently is produced by a reaction of methanol formed in the reaction with an isocyanate intermediate [694]. The reaction of 10-70 with SOCl2 and NEt3 regenerates C-amino-p-carborane almost quantitatively, but treatment with triphosgene generates the tetra(p-carboranyl)isonitrile 10-72, which on attempted complexation with ReðCOÞ3 Br3 2 affords the azetidine derivative 10-73 [694].
10.13 Nitrogen derivatives
647
O H
OMe
N O H
NH2
C H
N
10-71
Et3NSO2NC(O)OMe C
HC(O)OH
C C
Δ
H
SOCl2 NEt3, –78 ⬚C
C
C triphosgene
H
H
10-70
CH2Cl2
CB10H10CH
NEt3, –78 ⬚C H
CB10H10CH
N
C
C
N
N
H
C
C
O C
N
HCB10H10C
CB10H10CH
10-72
N N H
(NEt3)2Re(CO)3Br3
N N
C
C
H
AgPF6, THF
C
C
10-73
H
The contrast between this sequence and the behavior of o-carborane derivatives under similar conditions underlines the differences in electronic properties between the o- and p-carborane systems, principally the much reduced electronwithdrawing character in the para isomer.
10.13.6 Ureas Reactions of the B(9)-isocyanato-m-carborane with primary and secondary amines form asymmetrically substituted ureas, for example, 10-74 [292], which are of interest in the design of tumor-specific compounds for BNCT and other biomedical applications (Chapter 16). H
C
C
H
H
+
toluene HR
H
C
C
O NEt2
R=
reflux
N
NH
10-74 N
C
N O
N
H
O C R
648
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
Carbon-substituted m-carboranyl ureas have also been employed as building-blocks in macrocyclic systems such as 10-75 and 10-76 which are constructed from 1,7-bis(p-nitrophenyl)-m-carborane [289], e.g., ð1Þ H2 ;Pd=C;EtOH
triphosgene
ð2Þ HCOOH;Ac2 O ð3Þ BH3 THF
Et3 N
2CB10 H10 C2 2C6 H4 NO2 ! MeHN2 2CB10 H10 C2 2NHMe ! O2 NC6 H42 Et3 N
ClðOÞCMeNO2 2CB10 H10 C2 2NMeCðOÞCl !
10:75
D
O Me N
C
Me N
C
C C
C
C Me
Me N
N O
C
C
O O
Me
Me C
Me N
N
N
N
C
C
N
N
C
Me
Me
Me
C
O
C
C C
C
N
N
10-75 Me
C
10-76 Me
O
10.14 PHOSPHORUS DERIVATIVES 10.14.1 Synthesis Methods for binding phosphorus directly to the m- and p-carborane cages at carbon or boron are described in Sections 10.5 and 10.6. As was mentioned earlier, except for a few m-carboranyl C2 2As and C2 2Sb derivatives there are no known compounds in which the heavier Group 15 elements are linked to m- or p-carborane frameworks.
10.14.2 Properties 10.14.2.1 Oxyphosphorus derivatives Phosphinites and phosphonites, prepared from C-lithio-m- or p-carborane and EtOPMeCl or (EtO)2PCl, react like their o-carboranyl counterparts (Section 9.14). For example, they interact with p-nitroazidobenzene to give phosphazides [297], with chloral to afford alkenyl phosphates [699], and with methyl sulfate to effect transalkylation [697]. O2 NC6 H4 N3
R2 2CB10 H10 C2 2PMeðOEtÞ ! R2 2CB10 H10 C2 2PMeðOEtÞ5 5N2 2C6 H42 2NO2 R ¼ H; Me; Ph; CH2 ¼ CMe
10.14 Phosphorus derivatives
649
Cl3 CCðOÞH
1;12-HCB10 H10 C2 2PðOEtÞ2 ! HCB10 H10 C2 2PðOÞðOEtÞOCH5 5CCl2 Me2 SO4
1;12-HCB10 H10 C2 2PðOÞMeðOEtÞ ! HCB10 H10 C2 2PðOÞðOMeÞMe 130150 C
In a different approach, the reaction of phosphorus trichloride with an excess of 1-hydroxymethyl-m-carborane forms the tris(carboranylmethyl)phosphite; with a 2:1 ratio of carborane to PCl3 the phosphorochloridite and phosphorochloridate are obtained [182]. 2CH2 OÞ3 P ðHCB10 H10 C2
1=3PCl3
1=2PCl3
100 C
100 C
1; 7-HCB10 H10 C2 2CH2 OH !
ðHCB10 H10 C2 2CH2 OÞPCl2 þ ðHCB10 H10 C2 2CH2 OÞ2 PCl
10.14.2.2 Cage-opening reactions As with their o-carboranyl counterparts (Section 9.14), deboronation of methyldiarylphosphino m-carboranes with fluoride generates nido-carboranyl phosphonium salts; the corresponding p-carborane species, in contrast, are essentially impervious to base attack and are not deboronated [299]. MeI
CsF
1; 2-=1; 7-HCB10 H10 C2 2PPh2 ! HCB10 H10 C2 2PPh2 Meþ I ! 7; 8=7; 9-MePh2 Pþ C2 B9 H 11
10.14.2.3 Metal complexes Interactions of phosphino derivatives of m- and p-carboranes with transition metal centers have been explored to a limited extent, but this area has been slow to develop relative to the o-carboranyl systems described in Section 9.14. The steric properties of m- and p-carboranyl C,C0 -diphosphines are less favorable for metal chelation, and comparative studies of the properties of these species have not, in general, revealed any major advantages over their o-carboranyl counterparts. For example, chiral m-carboranyl B(9)-BINOL-derived phosphites exhibit enantioselectivity comparable to the corresponding o-carboranyl species such as 9-316 in the rhodium-catalyzed hydrogenation of dimethyl itaconite and methyl 2-acetamidoacrylate (up to 99.8% ee in CH2Cl2) [792,793]. Similar results are found in the isomeric C-substituted compounds 10-77a and 10-77b, which show only small differences in activity and selectivity toward asymmetric Rh-catalyzed hydrogenation [304].
C
R
C
R
O R=
C
P
R
O R
C a
O
10-77
b
The antipodal-carbon geometry of p-carborane has been used to advantage in the construction of linear goldphosphino complexes such as 10-78, obtained from 1,12-(Ph2P)2C2B10H10 and AuCl(SMe2) [698].
+ Ph2P
C
C
PPh2
Au 10-78
PPh2
C
C
PPh2
Cl−
650
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
The fact that stable p-carboranyl phosphino complexes are easily synthesized without cage degradation is notable, as PPh2 and similar nucleophiles readily extract boron from the o-carborane cage to generate nido-C2B9 products such as 9-318 [794,795], as described earlier in Sections 9.14 and 7.2. This contrasting behavior underlines the resistance of the 1,12-C2B10 cage to nucleophilic attack and its generally more robust nature compared to the 1,2-C2B10 framework.
10.15 SULFUR, SELENIUM, AND TELLURIUM DERIVATIVES 10.15.1 Synthesis The preparation of m- and p-carboranyl compounds having Group 16 elements directly bonded to cage boron or carbon atoms (Tables 10-1 and 10-2) is outlined in Sections 10.5 and 10.6. In the m-carborane system these include sulfur, selenium, and tellurium derivatives, but for p-carborane only those of sulfur are known; no p-carboranyl Se or Te compounds of any type have been reported. 2SH (R ¼ H, Me) are versatile synthons, undergoing oxidation to C-mercapto-m-carboranes, RCB10H10C2 2S2 2S2 2CB10H10CR and RCB10H10C2 2SO3H on reaction with H2O2 in acetic acid [312]. The dithiol RCB10H10C2 10-79 is easily converted to the bis(chlorosulfenyl) derivative 10-80, from which a wide variety of sulfur derivatives are obtainable via nucleophilic displacement of chlorine (Figure 10-8) [315]. The p-carboranyl analogue of 10-80, 1,12-(ClS)2C2B10H10, behaves similarly, for example, combining with aqueous NaOCl to give 1,12-(ClO2S)2C2B10H10. The reactions of 1,7- and 1,12-(ClS)2C2B10H10 with 1,7- and 1,12-Li2C2B10H10, 2]n2 2 where n 30 for the m-carboranyl product respectively, give linear polymers of the form 2 2[2 2S2 2CB10H10C2 [315,334]. HS
C
C
H2NS
SH Cl2 CCl4
NH3 ClS
10-79
C
C
SCl C6H6
KCN, H2O NCS
C
C
SCN Et2O
EtOS
C
C
SOEt
10-81
CCl4, H2O NaOCl
10-80
ClO2S
EtOH
Et3N EtOH
10-82
SNH2
C
C
ClS
C
C
10-83 FIGURE 10-8 Synthesis of C-substituted m-carboranes from 1,7-(ClS)2C2B10H10.
10-84
SOEt
C
C
10-85
SO2Cl
10.15 Sulfur, selenium, and tellurium derivatives
651
Like their o-carboranyl analogues, C-hydroxymethyl derivatives are precursors to m-carboranyl sulfur derivatives, as in the synthesis of methyl esters of sulfonic acids via reactions with sulfonyl chlorides [319]. ClSO2 Me
7-R2 2CB10 H10 C2 2CH2 OH ! HCB10 H10 C2 2CH2 O3 SMe
R ¼ H; Me
C 5 H5 N
Similarly, treatment of 1-hydroxymethyl-m-carborane with thionyl chloride in the presence of pyridine affords the stable bis(carboranylmethyl) sulfite [182]. SOCl2 ;C5 H5 N
1; 7-HCB10 H10 C2 2CH2 OH ! ðHCB10 H10 C2 2CH2 OÞ2 SO þ HCl CH2 Cl2
Under the same conditions, 1,2-HCB10H10C2 2CH2OH is converted to 1,2-HCB10H10C2 2CH2OCl (Section 9.9) [796]. Here again, stronger electron-withdrawal by the o-carboranyl versus the m-carboranyl carbon atom is demonstrated, in this case leading to chlorination of the sulfite intermediate in the former system. 2C2 2S linkages, as in Preparative methods have been developed for m- and p-carboranyl derivatives featuring Ccage2 10-86 - 10-87 [314] and 10-88 - 10-93 [671]. S C
Li
SMe
SH C
C
C (1) CuBr/LiBr
(1) Me2S•BH3 (2) HCl
(2) CS2 (3) MeI
C
C
C H
10-86 S
Li
C
C
H
H
10-87
C
H
H
10-88 (1) Hg(OAc)2 (2) PhOMe, CF3C(O)OH O
(3) HSCH2CH2OH, 70% CH3C(O)OH
C
C
H
H
10-90
10-89
SCMe3
SCMe3 C
C
C
10-93
cat. H2SO4
(2) HCl
C
C
isobutene
(1) Me2S•BH3
(2) CS2 (3) MeI
C
C
C
C (1) CuBr/LiBr
SH
SCMe3
SH
SMe
(C6H5NH+)2Cr2O72−
(1) n-C4H9Li (2) (CH2)3O
DMF C
C
O
OH
C OH
10-92
OH
10-91
Compounds of this type are of interest in biomedical and other applications; for example, methylsulfonate carbonyl p-carboranes derived from C-mercaptomethyl derivatives such as 10-89-10-93 could function as replacements for the Tyr(SO3H) residue in insect sulfakinin neuropeptide and other families [671]. The mercapto-m- and p-carboranes also have the ability to bind with platinum via S2 2Pt bonds, forming complexes analogous to 9-357 (Sections 9-15) that are potential candidates for DNA-binding agents for use in BNCT [313,314]. Like its o-carboranyl analogue, 1,12(HS)2C2B10H10 has found application as an agent for modifying the surfaces of gold microcrystals (Chapter 17) [704].
652
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
In a different area, m- and p-carboranyl thiophenes analogous to 9-353 are capable of electropolymerization to form thermal and electrochemically stable conductors [316,317]. Macrocycles 10-94 and 10-95 incorporating the m-carborane cluster are solids with melting points in the range 156-234 C that are prepared from C,C0 -bis(mercapto)-m-carborane [86]. As was mentioned in Section 10.7, oxidation of the sulfur atoms in 10-94 (n ¼ 8) forms a disulfone which on pyrolysis ejects the SO2 units to generate carboranophane 10-24 with a (CH2)8 exo-polyhedral chain. (CH2)n SH
S
S
C
C KOH
(CH2)n
Br(CH2)nBr
C
n = 6 0.5% n = 7 2% n = 8 4%
S
10-94
C
C
C
C
+
C
SH
S
n = 6 9% n = 7 14% n = 8 44%
S
S (CH2)n
10-95
10.15.2 Properties The hydrolysis of m- and p-carboranyl benzyl tosylates of the type R2 2C6H42 2CB10H10C2 2CH(Ph)OSO3 is markedly different from that of the corresponding o-carboranyl species. As previously noted (Section 9.15), good electron-donor R groups cause an increase in the rate for m- and p-carboranyl compounds but a corresponding decrease for the o-carboranyl systems, evidently as a result of cage-solvent interaction [320]. Other studies, also cited in the earlier discussion, show that the nature of the R substituent affects the retention of enantiomeric purity in the alcohol products, with strong donors inducing a change to an SN1 reaction mechanism [321]. Differences in electronic properties between the m- and p-carboranyl cages versus the ortho isomer are also seen in the behavior of the thioethers 10-96 and 10-99 toward hydrogen peroxide, both of which are easily oxidized to sulfones, unlike 1,2-(PhS)2C2B10H10 which fails to react at all (Section 9-15) [311]. Reduced electron-withdrawal by the less polar m- and p-carborane clusters increases the basicity of the sulfur atoms, and hence their ease of oxidation, relative to the ocarboranyl analogue.
O
O
S
S
S C
C
excess H2O2
30% H2O2 90% HC(O)OH reflux
C
O
C
O
O
C
C
S
S
10-96
10-97
S
10-98
O
O
O H2O2or O2N–C6H4–C(O)OH
S C
S
S O
O excess oxidant
C
10-100
O S
S
S
10-99
C
C
C
C
10-101
O
10.16 Halogen derivatives
653
Derivatives in which the substituent is bound to boron, whose o-, m-, or p-carboranyl cages usually function as electron donors rather than attractors, generally show smaller variation in the behavior of the three carborane cage isomers than do their carbon-bound counterparts. For example, the reaction of 1,7-H2C2B10H9-9-SCN with Kþ 2 C8 H8 2 proceeds like that of the corresponding o-carboranyl species (Section 9.15), with facile cleavage of the S2 2C bond but retention of 2S2 2S2 2B10H9C2H2 products [329]. Simithe B2 2S linkage, generating in both cases H2C2B10H9-9-SH and H2C2B10H92 larly, both 1,2- and 1,7-H2C2B10H8-B(9,12)-(EH)2 (E ¼ S or Se) are converted to the B(9,12)-(EMe)2 derivative by methyl iodide, and form cyclic heteroacetals such as 9-356 on treatment with benzaldehyde or dimethyl ether [331]. B(9)-mercapto derivatives of o- and m-carborane both combine with RR0 P(X)Cl reagents to form esters of pentavalent phosphorus acids [318] and are oxidized to sulfonic acids by H2O2 [325]. 1; 2=1; 7-H2 C2 B10 H9 -9-SH þ RR0 PðXÞCl ! H2 C2 B10 H9 -9-SPð¼ XÞRR0 R ¼ Me; Ph; EtO; R0 ¼ EtO; p-NO2 C6 H4 O; Ph; 1; 2-C2 B10 H11 X ¼ O; S HCðOÞOH
1; 2=1; 7-H2 C2 B10 H9 -9-SH þ H2 O2 ! H2 C2 B10 H9 -9-SO3 Hþ
10.16 HALOGEN DERIVATIVES 10.16.1 Synthesis and properties The preparation of B- and C-halogenated m- and p-carboranes, and many of their reaction types, are outlined earlier in this chapter (see especially Sections 10.5 and 10.6) and comparisons with o-carborane are examined in Chapter 9. Here we elaborate on the earlier discussions, with emphasis on differences between the properties of halogenated m- and p-carboranes versus their o-carboranyl analogues.
10.16.2 C-halo derivatives Like their o-carboranyl counterparts (Section 9.16), C-monobromo- and C-monoiodo-m-carboranes are dehalogenated in alcoholic base, while the C-monochloro derivatives undergo cage degradation with to nido-C2B9 products with evolution of H2 [378]. The C,C0 -dihalo m-carboranes, however, are stable in methanol and ethanol, unlike their o-carboranyl analogues which are deboronated in these solvents [347,378].
10.16.3 B-halo derivatives 10.16.3.1 Reactions with nucleophiles As is the case with their o-carboranyl analogues, boron-polyhalogenated m-carboranes of chlorine, bromine, and iodine are relatively unreactive toward water, amines, and other bases (although 1,7-H2C2B10Cl10, like its 1,2 isomer, is attacked by dimethyl sulfoxide with destruction of the cage and formation of boron oxychlorides [797]). The B-decafluoro isomers, in contrast, are far less robust. Both 1,2- and 1,7-H2C2B10F10 are rapidly hydrolyzed by water and more slowly in moist air, with replacement of the fluorine atoms by hydroxyl groups; further reaction leads to cage degradation and formation of boric acid [38]. H2 O
H2 O
1; 2=1; 7-H2 C2 B10 F10 ! H2 C2 B10 ðOHÞ10 ! BðOHÞ3 The p-carborane analogue 1,12-H2C2B10F10 appears unaffected by moist air, but in aqueous acetonitrile it, too, is hydrolyzed to boric acid [38]. The reactivity of m-carboranyl boron–halogen bonds toward nucleophilic substitution parallels that of the o-carboranyl compounds discussed in Section 9.16. For example, reaction with alkali metals in liquid ammonia effects the replacement of the halogen atoms by hydrogen, for example [114],
654
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12 Na
1; 7-H2 C2 B10 H8 -9;12-I2 ! H2 C2 B10 H10 liq:NH3
However, reaction rates with the m-carboranyl species are slower than those of the o-carboranyl isomers by a factor of 3 [114], reflecting the lower polarity of the meta cage system. Similar results are found with B-chloro and B-bromo derivatives.
10.16.3.2 Arylhalonium m- and p-carboranes As was briefly noted in Section 10.6, 9-iodo-m-carborane, like its o-carboranyl counterpart (Section 9.16), can be converted to phenyliodonium salts via oxidative condensation with benzene, or chlorinated to afford a dichloroiodonium product [393]: ð1Þ H2 SO4 ð2Þ HBF4
1; 7-H2 C2 B10 H9 -9-I ! H2 C2 B10 H9 -9-IPhþ BF 4 C6 H6 ;K2 S2 O8 Cl2
1; 7-H2 C2 B10 H9 -9-I ! H2 C2 B10 H9 -9-ICl2 CCl4
A p-carboranyl phenyliodonium salt has been similarly prepared in 80% yield [711]: ð1Þ H2 SO4 ð2Þ HBF4
1;12-H2 C2 B10 H9 -2-HgI ! H2 C2 B10 H9 -2-IPhþ BF 4 C 6 H 6 ; K2 S 2 O 8
Brominium salts can be prepared from bis(9-m-carboranyl)mercury [379]: BrF3
ð1; 7-H2 C2 B10 H9 -9-Þ2 Hg ! ð1; 7-H2 C2 B10 H9 -9-Þ2 Brþ BF 4 MeCN;Et2 OBF3 ;CH2 Cl2 ;70 C PhBrF2
ð1; 7-H2 C2 B10 H9 -9-Þ2 Hg ! 1; 7-H2 C2 B10 H9 -9-BrPhþ BF 4 MeCN;Et2 OBF3 ;CH2 Cl2 ;70 C
Boron-substituted o-, m-, and p-carboranyl phenylhalonium salts of the type H2C2B10H9-XPhþ BF 4 (X ¼ Cl, Br, I) react with bases (L) via three different modes [365,394,396–398,798]: •
• •
Nucleophilic substitution (SN mechanism) to generate H2C2B10H9-L and PhX (for X ¼ I, L ¼ N3 , OH–, F–, NO2 , C5H5N, and PPh3). The regioselectivity is surprisingly strong for the phenyliodonium species, and decreases in the order I > Br > Cl; for X ¼ I, substitution occurs only at boron, whereas for X ¼ Br or Cl the nucleophile adds both at boron and at a phenyl carbon atom. One-electron reduction involving cleavage of the halogen-phenyl bond to give H2C2B10H9-X and a C6H5 radical, regardless of the identity of X. Deboronation of the carborane cage to form nido-C2B9 species, a process that occurs most easily in the chloronium species and least (or not at all) in the iodonium systems.
Nucleophilic substitution and one-electron reduction often take place competitively, depending on the halogen involved. Alkaline hydrolysis of the m-carboranyl-9-phenyliodonium cation at 20 C operates exclusively via a radical process and rapidly (minutes) generates 9-iodo-m-carborane and benzene, but no H2C2B10H9-9-OH is formed [338]: 1; 7-H2 C2 B10 H9 -9-IPhþ þ OH ! H2 C2 B10 H9 -9-Ið78%Þ þ PhHð40%Þ þ PhIð10%Þ þ Ph2 ð3%Þ However, the corresponding treatment of 1,7-H2C2B10H9-9-BrPhþ is much slower (hours) and is competitive, with both extensive nucleophilic substitution at boron and one-electron reduction of the brominium ion occurring [365]: 1; 7-H2 C2 B10 H9 -9-BrPhþ þ OH ! H2 C2 B10 H9 -9-Brð50%Þ þ PhHð40%Þ þ PhBrð41%Þ þ ðH2 C2 B10 H9 Þ2 Oð25%Þ þ H2 C2 B10 H9 -9-OHð8%Þ þ Ph2 ð3%Þ þ PhOHð2%Þ
10.16 Halogen derivatives
655
The analogous reaction of phenylchloronium m-carborane, which also requires hours for completion, involves mainly nucleophilic substitution but it is less selective and takes place primarily at the phenyl carbon as shown by the high yield of phenol [365]: 1; 7-H2 C2 B10 H9 -9-ClPhþ þ OH ! H2 C2 B10 H9 -9-Clð63%Þ þ PhHð15%Þ þ PhOHð27%Þ þ PhClð36%Þ þ H2 C2 B10 H9 -9-OHð10%Þ þ H2 C2 B10 H9 -9-OPhð5%Þ þ ðH2 C2 B10 H9 Þ2 Oð5%Þ The nature of reactions of phenylhalonium-m-carborane ions varies considerably with the attacking base. For example, pyridine attacks 1,7-H2C2B10H9-9-IPhþ BF4 via a radical mechanism only, regardless of reaction conditions, affording 1; 7-H2 C2 B10 H9 -9-I; C5 H4 Nþ BF 4 , and phenylpyridine isomers; with the brominium and chloronium salts, the results are similar except that there is extensive cage degradation. Triphenylphosphine under UV irradiation reacts by a radical pathway, cleaving only the C2 2X bond, with progressively longer reaction times and more rigorous conditions required as one proceeds through the series I < Br < Cl [365,381,398,711]: the regiospecificity is maintained even in sterically hindered substrates such as the mesityliodonium-m-carborane 10-102, although the reaction rate is considerably reduced. Me
H
H
C
Me C
I+
Me
PPh3, hν, acetone
I +
C
Me
C Me
H
H
Me 82 %
50 %
10-102 Mechanistic studies indicate that one-electron reductions of H2C2B10H92 2X(arene)þ ions proceed via • 2I2 2aryl] intermediates, which undergo homolytic cleavage of the iodine-aryl link to form H2C2B10H9[H2C2B10H92 9-I and an aryl radical [398]. As in the behavior of diaryliodonium ions [799], such reactions are influenced by the presence of electron-donating or electron-withdrawing functional groups on the aryl ring. In nucleophilic substitution on H2C2B10H9-X(arene)þ ions, such substituents have no effect on regiospecificity. However, steric factors are important in nucleophilic attack involving sterically hindered arenes such as 10-102, whose reactions with Br– and NO2 are non2R regiospecific and afford all possible products, i.e., H2C2B10H9-9-I, Me3C6H3, H2C2B10H9-9-R, and Me3C6H32 (R ¼ Br or NO2) [398].
10.16.3.3 Acid strength Boron-polyhalogenated o-carboranes, as was noted in Chapter 9, are relatively strong C2 2H acids whose acidity tends to increase with the number of halogen substituents. In comparison, B-halo-m- and p-carboranes are less acidic, as can be seen from pKa values and in hydrogen bonding studies [51,343,371,373–376]. In comparing 1,7- and. 1,2-H2C2B10Cl10 the difference is striking: while the latter compound is easily deprotonated by cold aqueous base, the 1,7 isomer reacts only on heating [800]. The reduction in acidic character in going from the o- to m-carborane system is less pronounced for the B-bromo and B-iodo derivatives, which resemble halogenated o-carboranes in undergoing nucleophilic displacement of their halogen substituents by chlorine [801,802], for example, CuCl
1; 7-H2 C2 B10 H10n Xn ! H2 C2 B10 H10n Cln CuCl
1; 7-PhHC2 B10 H9 I ! PhHC2 B10 H9 Cl
n ¼ 1; 2
n ¼ 1; 2 X ¼ Br; I
As is the case with 1,2-H2C2B10H9-9-I, the analogous m-carboranyl derivative is easily deprotonated by sodium in liquid ammonia to afford the parent carborane [803].
656
CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
10.16.3.4 Cage degradation The base-promoted extraction of boron from C2B10 cages, a topic examined in Chapter 7 and revisited in Chapters 9 and 10, has been explored in detail with B-halogenated derivatives and affords an interesting comparison between the corresponding o- and m-carborane systems. The deboronation of 1,2-C2B10H11-9-I with fluoride ion, described in Section 9.16, results in the removal of the equivalent B(3)-H and B(6)-H vertexes to form a pair of nido-7,82I enantiomers as shown in Figure 9-26(A). However, the analogous reaction with 1,7-C2B10H11-9-X derivaC2B9H112 tives (X ¼ F, Cl, Br, or I) affords two anionic products, nido-7,8-C2B9H11-n-X (n ¼ 1,6), which are not enantiomers but distinct geometric isomers (Figure 10-9) obtained in a 2:1 ratio [179]. The difference in the products obtained in these reactions lies in symmetry considerations. In the o-carboranyl system, the extracted B(3)-H and B(6)-H units are geometrically equivalent, bear the same electric charge, and hence are extracted to an equal extent on base attack. In the m-carborane species, this is not the case, and the BH vertex nearest the halogen is preferentially removed in accordance with calculated Mulliken charges. −
C
C (n-C4H9)4N+ F−•H2O
C
−
C C
C
+
THF, 70 ⬚C C = CH X
X = F, Cl, Br, I
1,7-H2C2B10H9-9-X
X
X nido -7,9-C2B9H11-6-X−
2:1
nido -7,9-C2B9H11-1-X
FIGURE 10-9 Deboronation of 9-halo-m-carboranes.
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CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
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CHAPTER 10 Icosahedral carboranes: 1,7-C2B10H12 and 1,12-C2B10H12
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CHAPTER
Open 12-vertex and supra-icosahedral carboranes
11
11.1 OVERVIEW The thermodynamic stability of 12-vertex icosahedral boron clusters, which have 26 skeletal bonding electrons, is a major underlying theme—perhaps the predominant one—in boron cluster chemistry. The extraordinarily high resistance of the prototype B12 H12 2 anion toward thermal decomposition, oxidation, and hydrolysis, as noted in Chapter 2, is nearly matched by its isoelectronic carborane analogues, 1,2-, 1,7- and 1,12-C2B10H12 and CB11 H12 , whose chemistry has been described in the three chapters preceding this one. Moreover, the general stability associated with icosahedral clusters extends to 12-vertex metallacarborane and heterocarborane cage systems, discussed elsewhere in this book, and is evident in the presence of B12 icosahedra in all solid forms of elemental boron. Although some smaller carborane polyhedra (notably C2B5H7, C2B8H10, and many 7-vertex metallacarboranes) exhibit a stability that approaches that of the 12-vertex closo cages, these icosahedral systems have dominated research efforts and applications in the carborane field. These observations over several decades have led to the perception of an “icosahedral barrier” that seemed to preclude, or at least render difficult, the synthesis of individual polyhedral boron clusters of more than 12 vertexes (excluding multicage fused or linked systems) [1]. Theoretical calculations have suggested for years that no such barrier exists in principle and that supra-icosahedral Bn Hn q boron clusters, where n > 12 and 0 q 2, are capable of stable existence. Indeed, the “barrier” was surmounted long ago in transition metal metallacarboranes with the synthesis of 13- to 15-vertex clusters (Chapter 13), but it remains in place for all-boron polyhedra, as no Bn cluster larger than B12 H12 2 has been characterized. However, in the carborane field the 12-vertex limit has been decisively broken in the last decade with the synthesis and structural characterization of several 13-, 14-, and 15-atom CxBy cage systems that are described below in Section 11.5. We begin this chapter by examining the reductive cage-opening of the C2B10 icosahedron, a necessary first step in its expansion, followed by a discussion of the neutral C4B8 and its related cage systems that are isoelectronic analogues of the C2B10 anions. The final section outlines recent advances in the synthesis and characterization of supra-icosahedral carboranes.
11.2 OPEN-CAGE C2B10 ANIONS 11.2.1 Reduction of neutral C2B10 icosahedra As predicted by theory (Chapter 2), the addition of two electrons to a C2Bn2Hn closo-carborane opens the cage to form a nido-C2 Bn2 Hn 2 dianion that can be protonated to give a nido-C2 Bn2 22 Hnþ1 monoanion. The reversible reduction of icosahedral 1,2-, 1,7-, and 1,12-R2C2B10H10 (R ¼ H, alkyl, or aryl) to nido-R2 C2 B10 H10 2 dianions via reactions with alkali metals, as well as their protonation and subsequent rearrangement, are described in Chapter 10, and a special high-yield synthesis of nido-7,10-C2 B10 H13 (9-378) from B(3)-iodo-o-carborane is noted in Section 9.16. The “reverse Carboranes. DOI: 10.1016/B978-0-12-374170-7.00007-0 © 2011 Elsevier Inc. All rights reserved.
675
676
CHAPTER 11 Open 12-vertex and supra-icosahedral carboranes
isomerization” of p-carborane derivatives to the thermodynamically less favored m- and o-carboranes, via the rearrangement of 7,9-nido-R2 C2 B10 H10 2 dianions to their 7,10 isomers followed by their oxidation to neutral 1,7-R2C2B10H10, is discussed in Section 10.4. A listing of the characterized and theoretically investigated 12-vertex open-cage carboranes is given in Table 11-1.
TABLE 11-1 Supra-Icosahedral and Open-Cage 12-Vertex Carborane Derivatives Compound Synthesis and Characterization 12-Vertex Clusters nido-C2 B10 H12 2 derivatives C2 B10 H12 2 C2 B10 H12 2 isomers derived from 1,2-, 1,7-, 1,12-C2 B10 H12 2 C2 B10 H13 (2 isomers) C2 B10 H13 isomers C2B10H12-N(H)P(NMe2)3 Et2PHþ [(Me3C)2(n-C4H9)P]C2 B10 H12 OC2 B10 H11 1,10 -ð1; 2-C2 B10 H11 Þ2 2 Me2 C2 B10 H11 RPhC2 B10 H12 2 (R ¼ H, Ph) 1,3-[2,6-(Me2CH)2C6H3)NSiMe2]C2 B10 H11 3 (Me2HNþCH2CH2)C2 B10 H12 m-PhCH-PhCB10 H11 ½Ph2 C2 B10 H11 m-MeCH-MeCB10 H11 ½Me2 C2 B10 H11 PhC2B10H11-3-[-C-closo-(Ph)C2B10H10] Me2(s-C5H4)Si-C2 B10 H10 3 ðPhCH2 Þ2 C2 B10 H11 K(18-crown-6)þ (PhCH2)(PhCH)C2 B10 H11 K(18-crown-6)þ [C6H4(CH2)2]C2 B10 H11 Naþ3[(C5H4)CMe2-C2B10H11]3 NMe4 þ C2 B10 H13 LiðOEt2 Þ2 þ [SiMe2(C13H9)]C2 B10 H11 (H2N)C2 B10 H11 2 exo-(DME)3Ln-(PhCH2)2C2B10H10 (Ln ¼ Sm, Yb) (Ph3P)Au-7,9-Me2 C2 B10 H10 LnCl2(THF)5[m-CH-closo-C2B10H11-C2B10H11] (Ln ¼ Er, Y) (PhCH2)2C2 B10 H10 2 [MLþ]2 [ML ¼ Na(THF)3, K (dioxane), K(18-crown-6)] exo-{m(1,2)-[o-C6H4(CH2)2]-1,2C2B10H10}2K3ð18-crown-6Þ2 ð18-crown-6ÞKðMeCNÞ2 þ
Information
References
S (1,12-C2B10H12 þ Na in liquid NH3) S, H, B
[69] [70]
S, H, B S (high yield), B(2d), H(2d) S, X, H, B S, X, H, B, C, P S, H, B, C, IR X, H, B S, H, B, R X S (1,7-RPhC2B10H10 þ alkali metals) S, H, B, C, IR S, H, B, C, IR X X S, X, B S S, X, H, B, C, IR S, X, H, B, C, IR S, X, H, B, C, MS, IR S, H, B, C, IR S (high yield), H, B S, H, B, C, IR S (1,2-C2 B10 H12 2 þ Na, liquid NH3 ) S, X, H, B, C, MS, IR S, X, H, B, C, P S, X, H, B, C, IR
[70] [71] [72,73] [74] [75] [76] [70] [13] [77,78] [79] [80] [81] [82] [83] [8] [84] [84] [53] [7] [85] [86] [87] [88] [89] [90]
S, X, H, B, C, IR
[84]
S, X, H, B, C, IR
[10]
Continued
11.2 Open-cage C2B10 anions
677
TABLE 11-1 Supra-Icosahedral and Open-Cage 12-Vertex Carborane Derivatives—Cont’d Compound
[{m(1,2)-[o-C6H4(CH2)2]-1,2-C2 B10 H11 [Kþ (18-crown-6)]n [1,2-(cyclo-o-CH2-C6H4-CH2)C2B10H10]2MxLy (M ¼ Na, K; L ¼ THF, H2O) [1,2-(cyclo-o-CH2-C6H4-C6H4-CH2)C2B10H10]2MxLy (M ¼ Na, K; L ¼ THF, H2O) [1,2-(cyclo-o-CH2-C10H6-CH2)C2B10H10]2MxLy (M ¼ Na, K; L ¼ THF, H2O) [1,4-(m-o-CH2-C6H4-C6H4-CH2)C2B10H10]-M2(THF)3]n (M ¼ Li, Na) [{[(MeO(CH2)2]2C2B10H10]Na}Na(THF)]n polymer {1,3-[m-(CH2)n]C2 B10 H10 2 Na2 ðTHFÞ4 2þ }m (n ¼ 5, 6) polymer (1-n-C9H6)C2 B10 H10 3 Na3 ðTHFÞ5 3þ (n ¼ 10 , 20 ) arachno-C2 B10 H14 2 and C2 B10 H12 4 derivatives 8-NC-7,8-C2 B10 H14 m(1,2)-o-C6H4(CH2)2C2 B10 H10 4 M4 ðTHFÞ4 L2 4þ [M ¼ Li, Na; L ¼ THF, DME] [1,2-(cyclo-o-CH2-C6H4-CH2)C2B10H10]Li4(THF)6]2 2,3-(CH2)3C2 B10 H10 4 Li4 ðTHFÞ5 4þ [m-(CH2)n]C2 B10 H10 4 Li4 ðTHFÞ5 4þ (n ¼ 3, 4) 5CHCH2]C2 B10 H10 4 Li4 ðTHFÞ5 4þ [m-CH2CH5 nido-C4B8H12 derivatives Me4C4B8H8 Me4C4B8H7-4-C5H4FeCp Me4C4B8H8
Me2Et2C4B8H8 Me2(C4H9)2C4B8H8 Et2(C4H9)2C4B8H8 Et2(PhCH2)2C4B8H8 Et4C4B8H8
Et6C4B8H6 Me4C4B8H6(OEt)2 [EtC2B4H6-(CH2)6] 2Et2C4B8H8 [(CH2)6-Et2C4B8H8] 2 [EtC2B4H6-(CH2)6-Et2C4B8H8-]2 [(CH2)6]2Et2C4B8H8]3
Information
References
S, X, H, B, C, IR
[10]
S, X, H, B, C, IR
[9]
S, X, H, B, C, IR
[9]
S, X, H, B, C, IR
[9]
S, X, H, B, C, IR
[9]
S, X, H, B, C, IR S, X, H, B, C, IR
[91] [11]
S, X, H, B, C, IR
[6]
S, X, H, B, C, IR S, X, H, B, C
[50] [12]
S, S, S, S,
[9] [54] [11] [11]
X, X, X, X,
H, H, H, H,
B, B, B, B,
C, C, C, C,
IR IR IR, MS IR, MS
X S, X, H, B, MS S, H, B, C, (kinetics, thermodynamics of rearrangement) S, H, B, IR, MS, R S, H, B, C, MS S, H, B, C, MS S, H, B, C, MS S, H, B, C, MS S (detailed), B, IR, MS S, X, H, B, C, kinetics and thermodynamics of rearrangement S, H, B, IR, MS, UV S, H, B, IR S, H, B, MS, UV, IR S, H, B, MS, UV, IR S, H, B, MS, UV, IR S, H, B, MS, UV, IR
[38] [39] [40] [15] [36] [36] [36] [36] [22] [40] [20] [92] [19] [19] [19] [19] Continued
678
CHAPTER 11 Open 12-vertex and supra-icosahedral carboranes
TABLE 11-1 Supra-Icosahedral and Open-Cage 12-Vertex Carborane Derivatives—Cont’d Compound
Information
References
(PhCH2)6C4B8H6 (n-C3H7)4C4B8H8
S, H, B, IR, MS, UV S, H, B, C, kinetics and thermodynamics of rearrangement S, H, B, UV, IR, MS S, X, H, B, IR, MS S, MS S, X, H, B, C, IR, MS (n-C4H9) S, X, H, B, C, IR, MS S, X, H, B S, X, H, B, C, MS, IR, thermal rearrangement S, H, B, C, Si, IR, MS S, H, B, C, IR, MS Reaction with Cr(CO)6 Reaction with Cs S, B, IR, MS (positive, negative)
[20] [40] [18] [41] [21] [35] [35] [35] [42] [37] [35] [44] [49] [44]
S, S, S, S, S, S, S, S, S, S, S,
[43] [46] [50] [48] [49] [47] [47] [47] [47] [47] [47]
R4C4B8H8 R ¼ n-C4H9, i-C5H11, n-C6H13 (PhCH2)4C4B8H8 (indenyl-CH2)2C4B8H10 (Me3Si)2R2C4B8H8 (R ¼ n-C4H9, Me; C-adjacent) (Me3Si)4C4B8H8 (2 isom, C-apart) (Me3Si)2(n-C4H9)2C4B8H7Br (C-apart) (Me3Si)4C4B8H8 (2 isomers) (Me3Si)2C4B8H10 (Me3Si)2R2C4B8H8 (R ¼ n-C4H9, CMe3, Me; C-apart) (PhCH2)4C4B8H8 (Me3Si)4C4B8H8 [(CO)3Cr]2(PhCH2)4C4B8H8 (2 isomers) arachno-C4B8H14 and C4 B8 H12 2 derivatives 2Me4C4B8H8 CpCoC5H42 8-MeOC(O)-7,8,9,10-C4B8H13 ðSiMe3 Þ4 C4 B8 H9 2 [Cs(TMEDA)þCsþ(Me3Si)4C4 B8 H8 2 (THF)2Mg[(Me3Si)4C4B8H7Me] [exo-Cs(TMEDA)-Cs(Me3Si)4C4B8H8]n polymer M(THF)2(Me3Si)4C4B8H9 (M ¼ Li, Na, K) (THF)2Mg[(Me3Si)4C4B8H7Y] (Y ¼ CMe3, H) (TMEDA)Mg[(Me3Si)2(n-C4H9)2C4B8H8] exo-(m-H)3MgðTHFÞ3 2þ [(Me3Si)2Me2C4B8H8]2 arachno-C6B6H12 derivatives arachno-H6C6B6Et6 13-Vertex Clusters closo-C2B11H13 derivatives (CH2)3C2B11H11 C6H4(CH2)2C2B11H11 1,2-(cyclo-CH2SiMe2CH2)(Me2C2B11H11) 1,2 /1,6-Me2C2B11H11 [C6H4(CH2)2]C2B11H10-3-Ph 1,2-(CH2)3C2B11H10-3-R (R ¼ H, Ph, CEt5 5CHEt, CH5 5CHCMe3) (CH2)3C2B11H5X6 (X ¼ Me, Br, I) 1,2-(CH2)3C2B11H11 1,6-Me2C2B11H11
X H, B, IR, MS X, H, B, C, IR, MS X, H, C, B, Li, IR X, H, B, C, IR X, H, C, B, IR X, H, C, B, IR X, H, C, B, IR, ESR X, H, C, B, IR X, H, C, B, IR X, H, C, B, IR
S, H, B, C, MS
[51]
S, S, S, S, S, S,
[54] [55] [61] [61] [53] [55]
H, B, C, IR, MS X, H, B, C, IR, MS X, H, B, C, MS X(1,2), H, B, C, MS X, H, B, C, IR, MS X(Ph), H, B, C, IR, MS
S, X(Me, Br), H, B, C, IR Extrusion of C via reactions with nucleophiles Extrusion of C via reactions with nucleophiles
[55] [62] [62] Continued
11.2 Open-cage C2B10 anions
679
TABLE 11-1 Supra-Icosahedral and Open-Cage 12-Vertex Carborane Derivatives—Cont’d Compound
Information
References
S, X, H, B, C, IR, MS S, X(Me3CO, Me2N), H, B, C
[54,55] [68]
S, X, H, B, C S, X, E, UV, ESR
[68] [67]
S, H, B, MS
[52]
S, X, H, B, C, IR, MS S, X S, H, B, C, IR, MS X
[54] [57] [54,55] [55]
nido-C2 B12 H14 2 derivatives 2,3-ðCH2 Þ3 C2 B12 H12 2
S, H, B, C, IR, MS
[57]
nido-C4B10H14 derivatives Me4C4B10H10
S, MS
[52]
15-Vertex Clusters nido-C4B11H15 derivatives Me4C4B11H11 (2 isomers)
S, H, B, IR, MS
[52]
Oxidative isomerization Unreactive toward PhBCl2 Oxidation to 1,7-C2B10H12 Protonation
[93,94] [95] [69] [70]
Pyrolysis Thermal isomerization C C Facile isomerization to 7,9-Ph2 B10 H10 2 B, oxidative cage isomerization Oxidation to 1,2-RPhC2B10H10 Oxidation ! 1-Me-1,2-, 1,7-, and 1,12C2B10H11 þ 1,7-MeC2B10H10-7-Cl Formation of 1,7-C2B10H10-2-NH2-6-R (R ¼ H, Me) in liquid NH3
[70] [71] [96] [96] [3] [97] [77] [98]
2
nido-C2 B11 H13 derivatives ðCH2 Þ3 C2 B11 H11 2 (CH2)3C2B11H11-m(11,12)-BHR R ¼ MeO, Me3CO, Me2N ðCH2 Þ3 C2 B11 H12 1,2-Me2C2B11H11• Na[18-crown-6(THF)2]þ radical anion arachno-C4B9H13 derivatives Me4C4B9H11 14-Vertex Clusters closo-C2B12H14 derivatives 2,8-(CH2)3C2B12H12 2,3-(CH2)3C2B12H12 1,2-(CH2)3C2B12H12
Other Experimental Studies C2 B10 H12 2
C2 B10 H12 2 isomers derived from 1,2-, 1,7-, 1,12-C2 B10 H12 2 C2 B10 H13 (2 isomers) C2 B10 H13 isomers R2 C2 B10 H10 2 isomers (R ¼ Ph, Me) R2 C2 B10 H10 isomers (R ¼ H, Ph) 7,10-Ph2 B10 H10 2 RC2 B10 H11 2 isomers ( R ¼ H, Me) RPhC2 B10 H12 2 (R ¼ H, Ph) MeC2B10H10-12-Cl2 1,2-C2 B10 H11 -3-NH2 2
[99] Continued
680
CHAPTER 11 Open 12-vertex and supra-icosahedral carboranes
TABLE 11-1 Supra-Icosahedral and Open-Cage 12-Vertex Carborane Derivatives—Cont’d Compound 1,2-C2 B10 H12
2
1,2-C2B10H11-B-C6H4R2 (R ¼ H,F) 1,2-RR0 C2 B10 H10 2 RR0 C2 B10 H10 2 (R,R0 ¼ H, Me, Ph) 1,2-(C6H4-m, p-F)C2 B10 H11 2 , 1,2-(C6H4-m, p-F)C2 B10 H12 1,2-R2 C2 B10 H10 2 (R ¼ H, Ph) (H2N)C2 B10 H11 2 C2 B10 H12 2 derived from 1,12-C2B10H12 C2 B10 H12 2 derived from 1,12-C2B10H12 CoðC2 B10 H12 Þ2 Theoretical Studies Molecular and Electronic Structure Calculations C2B10H12n isomers (n ¼ 0, 1, 2) C2 B10 H12 2 C2 B10 H13 (2 isomers) C2 B10 H13 C2 B10 H12 2
C2 B10 H12 2 isomers OC2 B10 H11 C2B11H13 C2B12H14 C3 B9 H12 þ
Information
References
1,2-C2B10H10-n-p-C6H4F n ¼ 3, 4, 9 (all low yield) from (p-C6H4F)MgBr Oxidation to B-aryl-o-carborane derivatives Addition to CO group of ArCHO; similar to BH4 Amination with liq. NH3 ! 3-NH2 IR, F (electronic effects; ions formed from o- and m-carborane are identical) Oxidation, electron donation to electronacceptors; reaction with Ph2CO, p-benzoquinone, PhNO2], ESR Oxidation to 1-H2N-C2B10H11 No amination with Na þ liq. NH3 Oxidation to 1,7-C2B10H12 with Na þ liq. NH3 Oxidation ! o-, m-, p-C2B10H12 mixture; ratio is temperature-dependent
[100]
ab initio structures, electron affinity, acidities Stability Geometry MOs, etc ab initio; electron density localization; isomerization; oxidation, reduction; temperature effects Reduction of 1,12-C2B10H12 Optimized geometry Topology DFT, isomer stability Topology Hartree-Fock and B3LYP; cation isomer stabilities
nido-R4C4B8H8 (R ¼ H, Me) nido-C4B8H12 arachno-C4 B8 H12 2 CpCoC5H4-arachno-Me4C4B8H8 M(THF)2 arachno-(Me3Si)4C4B8H9 (M ¼ Li, Na, K) (THF)2Mg arachno-[(Me3Si)4C4B8H7Y] (Y ¼ CMe3, H) (THF)2Mg arachno-[(Me3Si)4C4B8H7Me] (TMEDA)Mg arachno-[(Me3Si)2(n-C4H9)2C4B8H8] exo-(m-H)3MgðTHFÞ3 2þ [arachno(Me3Si)2Me2C4B8H8]2
Topology Second moment Hu¨ckel
DFT DFT DFT DFT DFT
[101] [102] [103] [104] [105]
[87] [87] [87] [106]
[4] [107] [5] [108] [109]
[2] [75] [110] [59] [110] [14] [111] [110] [112] [111] [43] [47] [47] [47] [47] [47] Continued
11.2 Open-cage C2B10 anions
681
TABLE 11-1 Supra-Icosahedral and Open-Cage 12-Vertex Carborane Derivatives—Cont’d Compound
Information
References
NMR Calculations 7,9-C2 B10 H13 7,9-Me2 C2 B10 H11 C2 B10 H13 (2 isomers) C2B10H12-HNP(NMe2)3 nido-H4C4B8H8 (5 idealized structures)
GIAO (11B) GIAO (11B) IGLO (11B,13C) GIAO (11B) GIAO-ab initio 11B NMR
[113] [113] [5] [73] [35]
Reactivity Calculations 7,9-C2 B10 H13 7,9-Me2 C2 B10 H11
pKa pKa
[113] [113]
S, synthesis; X, X-ray diffraction; H, 1H NMR; B, 11B NMR; C, 13C NMR; F, 19F NMR; P, 31P NMR; 2d, two-dimensional (COSY) NMR; IR, infrared data; MS, mass spectroscopic data; ESR, electron spin resonance data.
Experimental and theoretical studies in recent years have considerably added to the understanding of these systems, leading to important new synthetic advances. It is clear that the nature of substituents attached to the cage carbon atoms can strongly influence their redox properties and the associated cage rearrangements of the kind outlined in Chapter 10. For example, the two-electron reduction of 1,12-diphenyl-p-carborane was anticipated to generate nido-7,10-Ph2 C2 B10 H10 2 , corresponding to the formation of nido-7,10-C2 B10 H12 2 from the parent 1,12-C2B10H12 (Figure 10-4). However, the dianion actually obtained, as shown from metal complexes generated from it, is nido-7,9-Ph2 C2 B10 H10 2 rather than the expected 7,10 isomer. Evidence suggests that the latter species probably forms initially but rapidly rearranges to the 7,9 cage (Figure 11-1) [2,3]. 2− Ph Ph
C
C
Ph
2e–
1,12-Ph2C2B10H10
C
C
2−
Ph
Ph
nido-7,10-Ph2C2B10H102–
Ph C
C
nido-7,9-Ph2C2B10H102–
FIGURE 11-1 Reductive cage-opening of 1,12-Ph2C2B10H10.
Although theoretical calculations on the parent dianions indicate that the nido-7,9-C2 B10 H12 2 geometry is, in fact, thermodynamically preferred [4,5], the 7,10 isomer is kinetically stable in solution and is converted to m-carborane (1,7C2B10H12) upon air oxidation, and is in contrast to the 7,9 parent dianion which oxidizes to o-carborane (1,2-C2B10H12) as shown in Figure 10-4. In the C,C-diphenyl anions, the rapid rearrangement of nido-7,10-Ph2 C2 B10 H10 2 to its 7,9 isomer shows that the aryl groups lower the activation energy for cage isomerization, for reasons that remain to be determined. The open faces on the 12-vertex dianions and tetraanions and the “reactive” monoanion isomer (7-4 in Section 7.2) readily bind to main-group and transition metal cations, generating stable 13-vertex or larger metallacarboranes that are described in Chapter 13. In many cases, elucidation of the anion structures by X-ray crystallography is dependent on their stabilization via complexation with Liþ, Naþ, or Csþ ions (Table 11-1). Additional versatility is afforded by appending a cyclopentadienide or indeneide [6] unit to the carborane, as in the nido-7,9-Me2(s-C5H4)E-C2 B10 H10 3 (E ¼ C, Si) trianions [7,8], that can complex with lanthanides and other trivalent metals (Chapter 13).
682
CHAPTER 11 Open 12-vertex and supra-icosahedral carboranes
11.2.2 Carbon-bridged clusters Some important synthetic pathways are based on o-carborane cages whose carbon atoms are linked by short exopolyhedral chains that constrain the ability of these carbons to migrate away from each other. For example, Xie, et al. have shown that the two-electron reduction of 11-1 with sodium or potassium metal generates the nido-dianion 11-2 that can coordinate an alkali metal ion to its open face [9] as in the bis(carboranyl)potassium complex {[o C6H4(CH2)2C2B10H10]2K}½Kð18-crown-6Þþ 2 [10]. C
excess M
C
THF
2− C C
M = Na, K
11-1
2+
Na2(THF)3
11-2
The enforced adjacent-carbon geometry renders the o-carborane cage more easily reducible, permitting the addition of four electrons to generate 12-vertex arachno tetraanions—something not possible in the unbridged derivatives. The reaction of carbon-linked o-carboranes with an excess of lithium metal yields open-cage products whose geometry depends on the length of the Cn linker [9,11,12]. When the chain is short (n ¼ 3 or 4) as in 11-1, 11-4, and 11-5, the cage carbons are retained in ortho positions in the arachno tetraanions 11-3, 11-6, and 11-7, respectively, which are isolated as dimers of their tetralithium salts. 4−
C
excess Li
C
THF
C C
4+
Li4(THF)5
11-1 11-3 4− C
excess Li
C
THF
C C
Li4(THF)54+
11-6
11-4
(CH2)n (CH2)n
C C
excess Li
C C
4−
Li4(THF)54+
THF n = 3, 4
11-5
11-7
The cages in the arachno tetraanions have 5- and 6-membered open faces that are coordinated to lithium ions in the crystal.
11.2 Open-cage C2B10 anions
683
When o-carboranes having longer exo-polyhedral chains, as in 11-8 - 11-10, are reduced, only two electrons are added to the cage regardless of reaction conditions; the cage C2 2C bond is cleaved and the nido dianions 11-11 - 11-13 are obtained, the first of which crystallizes as a polymeric chain linked by solvated sodium ions [9,11]. 2− C
excess Na C
C
C
2+
Na2(THF)4
THF
11-8
n
11-11 2− C
excess Na C
C
C
Na2(THF)42+
THF
11-9 11-12 2− C C
excess Na THF
2+
C
C
Na2(THF)4
11-10 11-13 The general pattern is clear: in 11-8, 11-9, and 11-10 the cage carbon atoms are relatively unconstrained and become separated on reduction, as they always do in the nonbridged o-carborane species that has been discussed in the earlier chapters. In contrast, the inability of the “short-leashed” carbon atoms, in 11-1, 11-4, and 11-5, to move apart prevents C2 2C cleavage and allows the cage to undergo four-electron reduction to form arachno species. A special situation arises in the case of 5-carbon connectors. When a flexible pentamethylene chain is present, as in 11-8, a nido cage is obtained, but a more rigid C5 link leads to an arachno tetraanion, as illustrated by the conversion of the naphthalene-bridged derivative 11-14 to 11-15 [9]. 4− C C
11-14
excess Li THF
4+
CC
Li4(THF)5
11-15
684
CHAPTER 11 Open 12-vertex and supra-icosahedral carboranes
From all of these findings it appears that the length of the exo-polyhedral chain is more important than rigidity in determining the reduction products of Cn-linked o-carboranes, with the latter property playing a role only in the borderline case of n ¼ 5. The carborane dianions and tetraanions formed in these reactions are thermally stable in the solid state but are easily oxidized in air, regenerating their neutral o-carborane derivatives [9]. Like other C2B10 anionic species (Section 7.2), the nido and arachno anions are strong Br nsted bases, reacting with water to acquire one or three protons, respectively, and forming the corresponding monoanions [9,13]. In the protonation of both the adjacent-carbon and the separated-carbon species, the initially formed kinetic product rearranges to a thermodynamically preferred isomer, as shown in Figure 11-2. These reactions illustrate the strong influence of cage carbon atom location on the geometry and relative stability of the monoanion products, a point that has been addressed in Chapter 7.
Δ
2−
C
−
−
H
H
H+
C
C
C
C
C
+
11-2
11-16
2−
− C
C R
11-18
H
R
C H+
C
11-17
−
R R
+
H
R C
C
R
7-4
Δ
7-5
FIGURE 11-2 Protonation of the carbons-adjacent dianion nido-(m2 2CH2C6H4CH2)C2 B10 H10 2 (11-2) to form the kinetic and thermodynamic isomers 11-16 and 11-17, and of the carbons-separated nido-dianion 11-18 (R ¼ H, alkyl) to generate the kinetic and thermodynamic monoanions 7-4 and 7-5.
11.3 NIDO- AND ARACHNO-C4B8 CARBORANES An alternative way of increasing the skeletal electron count in icosahedral clusters beyond 26, other than by reduction of neutral C2B10 species to anions, is to formally replace the boron atoms with carbon to create carbon-rich CnB12n cages in which n > 2. While there are no known methods for accomplishing such a replacement per se, several serendipitous discoveries have led to the isolation and characterization of a sizeable family of carbon-rich species. Although C3B9 clusters have not been found experimentally (C3 B9 H12 þ has been explored theoretically [14]), 12-vertex carboranes having 4-6 carbons now constitute a growing area of exploration. The currently known compounds are listed in Table 11-1.
11.3 Nido- and Arachno-C4B8 carboranes
685
11.3.1 Synthesis of Nido-C4B8 cages 11.3.1.1 Oxidative fusion of 2,3-R2C2B4H42 ligands The history of boron chemistry is full of unexpected twists, but particularly surprising was the 1974 discovery [15,16], that two pyramidal 2,3-R2 C2 B4 H4 2 ligands, when coordinated to a common transition metal cation, can undergo faceto-face binding with a net loss of four electrons, forming neutral R4C4B8H8 products that can be isolated as air-stable colorless solids. The tetramethyl compound is remarkably volatile, subliming easily at room temperature in air (in comparison, 1,2-C2B10H12 is nonvolatile under these conditions). The fusion reaction is an extremely facile process that occurs quantitatively in cold solvents, at temperatures as low as 30 C, with nido-C2B4 units [17–22] and, less cleanly, with 6-vertex MC2B3 nido-metallacarborane and 5-vertex MB4 nido-metallaborane cages [23]. In contrast, large clusters, such as nido-C2 B9 H12 , can form edge-linked dimeric structures as noted in Chapter 7, but do not undergo facial fusion. The synthesis of isomeric 28-skeletal electron R4C4B8H8 clusters from small-carborane metal sandwich complexes of 2,3-R2 C2 B4 H4 2 is schematically depicted in Figure 11-3. When R is methyl, ethyl, or n-propyl, the tetracarbon carboranes interchange reversibly in solution, reaching an equilibrium in which Keq ¼ [B]/[A] is dependent on the nature of the R substituents (see below).
C
C
R
R 2−
A
C C
C
R
C
R
R R
4+
[O] cat.
MHn C 2−
C
R
R C C MHn = FeH2, CoH
R R
R = alkyl, CH2-aryl
C C
B R
R
FIGURE 11-3 Conversion of (2,3-R2C2B4H4)2CoIIIH and (2,3-R2C2B4H4)2FeII(H)2 metallacarboranes to 12-vertex tetracarbon carborane isomers via ligand fusion. The metal-bound protons in the metal complexes are fluxional on the NMR time scale and migrate over M2 2B2 2B triangular faces.
Studies of R2 C2 B4 H4 2 fusion reactions and their tetracarbon carborane products have revealed much about these compounds, although some aspects still remain unclear. As this topic has been extensively reviewed [17,23–31], only the main points will be noted here. •
Fusion of the 2,3-R2 C2 B4 H4 2 dianions has been observed only in the presence of transition metals (however, as noted below, C4B8 cage products can be generated by heating neutral 2,3-(Me3Si)2C2B4H6 carboranes). As was noted in Chapter 4, the “carbons-separated” perethylated complex H2Fe(2,4-Et2C2B4Et4)2 is stable toward oxygen and does not undergo fusion [32].
686 •
•
•
CHAPTER 11 Open 12-vertex and supra-icosahedral carboranes
Solutions of the bright red (2,3-R2C2B4H4)2CoIIIH and (2,3-R2C2B4H4)2FeII(H)2 complexes are stable in deoxygenated nonpolar media such as diethyl ether and hexane, and no reaction is seen. In polar solvents, this is also the case with the cobalt complexes, but the iron compounds slowly convert to a purple paramagnetic species, about which more is said below. On exposure of the metal complex solutions to air, fusion occurs almost instantaneously to afford the colorless, airstable R4C4B8H8 carboranes. The reaction is intramolecular, as the oxidation of mixtures of (R2C2B4H4)2FeII(H)2 and (R0 2C2B4H4)2FeII(H)2, where R 6¼ R0 , afford only R4C4B8H8 and R0 4C4B8H8 with no R2R0 2C4B8H8 obtained. Evidence suggests that the fusion process involves the formation of a B2 2B interligand bond at an early stage, followed by an additional ligand–ligand linkage and the expulsion of the metal, either as a free element or as a solvated complex, depending on the solvent and other conditions. Although fusion may occur via more than one pathway, extremely oxygen-sensitive diiron complexes of the type (2,3-R2C2B4H4)2Fe2H2L2 (11-19) have been identified as intermediates in the fusion process that form from (Me2C2B4H4)2FeII(H)2 either on slow standing in polar solvents, or rapidly upon addition of a catalytic amount of FeCl3 [33]. X-ray crystallography of the tetramethyl/dimethoxyethane derivative of 11-19 has disclosed the structure shown below [34].
R
C C H H
C
R FeII
C
[O] cat.
R
C
L
FeII
L R R = alkyl, CH2-aryl
FeII C
C
R R C
R [O] cat.
R4C4B8H8 isomers
R
11-19
L = THF or 1/2 MeOC2H4OMe
ESR, multinuclear NMR, Mðssbauer spectra, and magnetic susceptibility measurements have established that 11-19 contains both low-spin Fe(II) and high-spin Fe(II) centers, the latter occupying the “outer” site where it binds to the ˚ is within normal bonding distance, the 2Fe separation of 2.41 A B2 2B edges on both C2B4 ligands; although the Fe2 metal centers are magnetically independent and may not be directly bonded to a significant extent [34]. The 11-19 complexes are indefinitely stable in solution under oxygen-free conditions, but are instantly converted to R4C4B8H8 on exposure to O2 or other oxidants. Further insight into metal-assisted cage fusion has been gleaned from the study of related reaction systems such as the conversion of nido-CoB4 H7 to Cp2Co2B8H12 metallaboranes and the synthesis of (ligand)2M2C4B6H6 clusters from (ligand)MC2 B3 H5 metallacarborane anions, which is discussed in Chapter 13.
11.3.1.2 Oxidative fusion of 2,3-(Me3Si)RC2B4H6nn anions Studies of the nickel-mediated fusion of trimethylsilyl-substituted nido-carborane mono- or dianions by Hosmane and coworkers [35] have given results that differ in interesting ways from the reactions just described for 2,3ðalkylÞ2 C2 B4 H4 2 ions. Figure 11-4 is a simplified representation of a fairly complex reaction system that illustrates two main points. First, the main process occurring is oxidative cage closure to form 6-vertex closo-1,2-R(Me3Si) C2B4H4 clusters (discussed earlier in Chapter 4), which is accompanied by fusion to generate C4B8 cages as minor products. Secondly, the fusion of ðMe3 SiÞRC2 B4 H5 monoanions mainly yields (Me3Si)2R2C4B8H8 isomers (e.g., 11-20 and 11-21) that feature cage C2 2C bonding, as in isomers A and B of the previously described R4C4B8H8 systems in which R ¼ Me, Et, or n-C3H7 (Figure 11-3). In contrast, the ðMe3 SiÞ2 C2 B4 H4 2 dianions fuse to give, predominantly, a different type of C4B8 cluster (11-22 and 11-23) whose carbon atoms are well separated. Inasmuch as 11-22 converts quantitatively to 11-23 upon mild heating, as occurs during the isolation of the products, it appears that the isomer initially
Me3Si 2 Li(TMEDA)+
C4H9
n−
C
C
+
C
C
R
NiCl2 / n-C6H12
C C
0 °C−
Ni0
11-21 4% SiMe3 Me3Si
C
C
C
C
SiMe3
+ C
32%
C4H9
SiMe3
+
C
C
C
C SiMe3
Me3Si
11-22 11%
SiMe3 100 °C C6D6
11-23 12%
FIGURE 11-4 Synthesis of carbons-adjacent R4C4B8H8 isomers 11-20 and 11-21 (R ¼ n-butyl) and carbons-separated R4C4B8H8 isomers 11-22 and 11-23 (R ¼ trimethylsilyl) via nickel-assisted fusion of nido-carborane anions.
11.3 Nido- and Arachno-C4B8 carboranes
C
C4H9
SiMe3
Me3Si C
Me3Si
C
11-20 37%
R = SiMe3 n=2 Me3Si
+
SiMe3
R = n-C4H9 n=1
n−
(Me3Si)RC2B4H6−n
C
C4H9
C 64%
Me3Si
C4H9
SiMe3
C
SiMe3
C
687
688
CHAPTER 11 Open 12-vertex and supra-icosahedral carboranes
generated by cage fusion is 11-22. Further discussion of these structures appears below. Although they have not been isolated, bis(carboranyl)nickel sandwich complexes analogous to the iron and cobalt species described above are assumed to be intermediates in these fusion reactions, and nickel metal is generated [35]. This pattern of reactivity is not rigidly followed, as minor amounts of carbons-adjacent C4B8 products are obtained from ðMe3 SiÞ2 C2 B4 H4 2 , while fusion of ðMe3 SiÞRC2 B4 H4 2 similarly generates low yields of carbons-separated C4B8 side products [35].
11.3.1.3 Fusion via thermal routes In a procedure that exploits the facile thermal displacement of cyclooctatriene from (R2C2B4H4)Fe(Z6-C8H10) metallacarboranes (Chapter 13), reactions of such complexes (R ¼ Et, n-C4H9, CH2Ph) with Naþ R02 C2 B4 H5 salts (R0 ¼ Me, Et, n-C4H9, CH2Ph) at 150 C, without solvent, afford mixtures of R2R20 C4B8H8 and R4C4B8H8 isomers whose cage geometries correspond to the previously described A and B forms of Me4C4B8H8 and Et4C4B8H8 [36]. This method furnishes a route to mixed-substituent R2R0 2C4B8H8 derivatives, albeit an inefficient one, since homosubstituted R4C4B8H8 products are obtained in comparable amounts. The only example of face-to-face fusion of two nido-C2B4 units in the absence of metals is a reported synthesis of (Me3Si)2C4B8H10 in 81% isolated yield from neutral (Me3Si)2C2B4H6 at 210 C in a sealed tube, with the loss of two equivalents of SiMe3H [37]. Although X-ray crystallographic data on the (Me3Si)2C4B8H10 product is not available, NMR evidence is consistent with a cage structure that is analogous to that of the A isomer of Me4C4B8H8 discussed above. In this unique reaction system, the net four-electron oxidation is accomplished via the removal of two Me3Si and two H atoms as trimethylsilane. The mechanism has not been established, but may involve an initial loss of Me3Si which abstracts a hydrogen atom from (Me3Si)2C2B4H6 to generate SiMe3H and a (Me3Si)C2B4H5 radical, two of which join to give (Me3Si)2C4B8H10 [37].
11.3.2 Structure and fluxional behavior of C4B8 clusters 11.3.2.1 R4C4B8H8 systems The cage geometries for isomers A and B depicted in Figure 11-3 are based on crystallographic studies of Me4C4B8H8 [38], Me4C4B8H7-C5H4FeCp [39], Et4C4B8H8 [40], and (PhCH2)4C4B8H8 [41] and on the solution NMR data for these and other R4C4B8H8 species in which R is n-propyl, isopentyl, n-hexyl, or indenylmethyl (Table 11-1) [18,19,21]. The ˚ ) that is absent in solid-state structures of A and B differ in that the former isomer features a central C2 2C bond (1.52 A B, whose C2B4 units are tilted away from each other (also see Chapter 1, Figure 1-3, bottom) [40]. While the geometry of A has been seen only in R4C4B8H8 carboranes, the geometry of B is observed in a variety of metallacarboranes in which one or more vertices are occupied by transition metals (Chapter 13). The cage architectures of both isomers reflect the distortion from the closo 12-vertex cage geometry that results from the addition of two skeletal electrons beyond the 26 allowed in a filled-shell icosahedral system. In A, this distortion takes the form of two quadrilateral open faces sharing a C2 2C edge, while B has a considerably more open-cage struc˚. ture with the central C2 2C distance opening to a nonbonding distance of 2.89 A An interesting property of the R4C4B8H8 clusters in which R is small (e.g., methyl, ethyl, propyl) is their fluxional behavior in solution, with the relative amounts of A and B being dependent on R. When crystals of Me4C4B8H8 are first placed in solution, multinuclear NMR spectra show that only the A isomer is present; however, in a matter of seconds, the isomer B appears, and within 20 min reaches equilibrium with Keq ¼ [B]/[A] ¼ 0.59, a value that persists even at 80 C and is essentially invariant with temperature. Et4C4B8H8 behaves in exactly the reverse sense, in that only isomer B is detected initially upon dissolution, with some A gradually forming; in the case of (n-C3H7)4C4B8H8, both A and B are seen immediately upon its entry into solution. The measured [B]/[A] equilibrium values of 2.2 and 1.9, respectively, for the tetraethyl and tetrapropyl derivatives demonstrate that the larger alkyl groups favor the more open B geometry [40]. Above room temperature, NMR data reveal additional fluxionality of the B isomer, which renders the four carbons equivalent on the NMR time scale, and is proposed to involve a cube-octahedral intermediate with all carbon atoms on
11.3 Nido- and Arachno-C4B8 carboranes
689
one face. The observed fluxional behavior of the R4C4B8H8 cages is quite unusual, with few other examples of reversible interconversion of neutral, nonmetallated carborane cage isomers under mild conditions. Also remarkable is what happens when the solvent is removed from solutions of these carboranes containing equilibrium mixtures of A and B. With Me4C4B8H8, evaporation of solvent affords purely isomer A, while for Et4C4B8H8, the residue contains only the B form; for (n-C3H7)4C4B8H8 both isomers are obtained. R4C4B8H8 and R4C4B8H6R2 derivatives bearing bulkier R groups such as benzyl [20,41] adopt only the open B form, both in solution and in the solid state. The behavior of the fluxional 28-skeletal electron C4B8 clusters implies that the observed A and B isomeric forms are nearly equivalent energetically, and that the activation energy for their interconversion must be extremely low despite the fact that the cleavage of the central C2 2C link is involved. This inference is reinforced by variable-temperature NMR rate measurements, which yield calculated values of DH for the A ! B interconversion of ca. 1.5, 1.8, and 2.3 kcal/mol for the tetramethyl, tetraethyl, and tetrapropyl compounds, respectively [40].
11.3.2.2 (Me3Si)2R2C4B8H8 systems
As mentioned earlier, the crystallographically determined cage structures of the “carbons-adjacent” species 11-20 and 11-21, containing two trimethylsilyl and two hydrocarbon substituents, closely correspond to those of isomers A and B in Figure 11-3, found in R4C4B8H8 compounds where R is a small alkyl group. The (Me3Si)4C4B8H8 systems 1122 and 11-23, on the other hand, have “carbons-separated” skeletal geometries that are unique in different ways [35,42]. The cage in 11-22 approximates a cuboctahedron, a geometric form that has been proposed as an intermediate in the diamond-square-diamond (DSD) rearrangement of o-carborane to m-carborane as discussed in Section 10.3. The open-cage carborane 11-23, obtained via a relatively facile thermal isomerization of 11-22, lacks any symmetry whatsoever and has a 6-membered open face that is typical of nido-carborane structures in general. This cage geometry is also found in the (Me3Si)2R2C4B8H8 species in which R is methyl, n-butyl, or tert-butyl. Multinuclear 1H, 11B, and 13C NMR studies of the trimethylsilyl-substituted tetracarbon carboranes show that, like their tetra-C-alkyl counterparts, they are fluxional in solution [35]. However, unlike the latter species, they have not been found to reversibly isomerize in solution, instead reverting to a single solid-state geometry on removal of solvent, as described above for Me4C4B8H8 and Et4C4B8H8. Ab initio molecular orbital calculations on C4B8H12 model compounds [35] allow the NMR spectra to be interpreted on the basis of cage fluxionality, and also support a plausible isomerization mechanism for the conversion of 11-22 to 11-23, which involves a fully symmetric cuboctahedral intermediate that has six square CBCB faces (also proposed earlier for the reversible rearrangement of R4C4B8H8 clusters [43]). However, the spectra of 11-20 and 11-21 have not been reconciled with the known solid-state structures [35].
11.3.3 Metal complexation of Nido-R4C4B8H8 carboranes The interaction of main-group and transition metals with C4B8 clusters affords exo-substituted derivatives and 13- to 15vertex metallacarboranes, in which the metal is incorporated into the cage framework as described in Chapter 13. Most of the known exo-metallated derivatives have been prepared from R4 C4 B8 H8 2 dianions, discussed below and in Chapter 13. However, neutral aryl-substituted compounds can form stable complexes via aryl Z6-coordination to s2d4 transition metals such as Cr(0) [44]. ðPhCH2 Þ4 C4 B8 H8 þ CrðCOÞ6 ! ½ðCOÞ3 Cr2 ðPhCH2 Þ4 C4 B8 H8 ð2 isomersÞ
11.3.4 Formation and properties of Arachno-R4C4B8H8n mono- and dianions 11.3.4.1 Reduction of Me4C4B8H8 The addition of two electrons to the neutral 12-vertex, 28 electron nido-C4B8 clusters generates 30-electron dianions that are expected to adopt arachno-class geometries. This is in fact observed, but the R4 C4 B8 H8 2 species have highly varied, fluxional structures with no single favored cage architecture. The reaction of neutral Me4C4B8H8 with sodium naphthaleneide in THF initially affords a green species that changes to a wine-colored, apparently, Me4 C4 B8 H9
690
CHAPTER 11 Open 12-vertex and supra-icosahedral carboranes
monoanion and finally to the yellow Me4 C4 B8 H8 2 dianion [45]. Although the NMR spectra of this ion suggest the presence of a twofold axis or a mirror plane, its structure in solution has not been determined. However, protonation of the dianion affords a Me4 C4 B8 H9 monoanion, whose geometry is known from the crystal structure of its zwitterionic 2(C5H4)CoCpþ (11-24) [43,46]. B-cobaltocenium derivative Me4 C4 B8 H9 2 R
Me H
C Me
C
−
R
C
C R = H, alkyl, aryl
Me
C
Me
C H
(C5H4)CoCp+
−
11-24
7-5
Consistent with its 2n þ 6 skeletal electron count, the C4B8 framework in 11-24 approximates that of an arachno fragment of a 14-vertex closo polyhedron (bicapped hexagonal antiprism) in which two adjacent vertexes are missing. The “extra” hydrogen is located on a cage carbon atom, as is also seen in the so-called reactive form of the nidoR2 C2 B10 H11 monoanion (7-5) that is discussed in Section 7.2. However, in 11-24, the methyl group is located over the open face and the proton attached to the carbon is directed away from it (probably reflecting the geometry of protonation), while in 7-5 it is the reverse. Also, in the nido cage 7-5 (R ¼ Me) the bridging MeCH unit is bound to a B2 2B edge, whereas in 11-24, an arachno system, it links nonvicinal boron and carbon atoms. The fact that protonation takes place at a carbon atom rather than at the B2 2B edge on the open face, can be understood as a way of relieving the valence angle strain on the bridging carbon. As noted earlier, NMR data suggest that the arachno-R4 C4 B8 H8 2 dianions, in general, are fluxional in solution. Reactions of these ions with transition metal ions produce a host of isolable 13- and 14-metallacarboranes whose cage geometries are remarkably varied, implying the existence of multiple isomers of the substrate anions in solution prior to metal complexation. The syntheses and structures of these metal derivatives are described in Chapter 13.
11.3.4.2 Reduction of (Me3Si)2R2C4B8H8 derivatives Treatment of trimethylsilyl-substituted C4B8 carboranes with magnesium metal in THF effects a two-electron reduction to give (Me3Si)2R2 C4 B8 H8 2 dianions (R ¼ SiMe3 or alkyl) that combine with the THF-solvated Mg2þ ions to generate a range of structurally diverse complexes [47,48]. Reaction of the “carbons-separated” isomer of (Me3Si)4C4B8H8 or its B-tert-methyl derivative with magnesium metal gives the complex 11-25 in good yield; similar treatment of carbonsseparated (Me3Si)2(n-C4H9)2C4B8H8 affords the same cage structure, despite the presence of two butyl groups in place of SiMe3 units. However, a magnesium complex with markedly different geometry, 11-26, is obtained from the B-methyl derivative (Me3Si)4C4B8H7Me. Me
SiMe3
SiMe3 SiMe3 C C BH HB
C Me3Si
C
R
11-25 HB
Me3Si
C
SiMe3
C
11-26
Mg THF
Me3Si THF
C
C
R = H, CMe3
Mg THF
THF
SiMe3
11.3 Nido- and Arachno-C4B8 carboranes
691
Yet another cage geometry, 11-27, is formed in 81% yield in the reaction of magnesium metal with the carbonsadjacent isomer of (Me3Si)2Me)2C4B8H8; in this case, the metal is bound not to the open face, but to a triangular B3 array 5CH via three B2 2H2 2Mg bridges. This dimethylethylene-bridged structure is very similar to that of H4C4B6H6-m(6,9)-HC5 (6-29), a derivative of the arachno-C4B6H12 (Section 6.2). Me
Me C
THF
11-27
THF THF
C SiMe3
H
C C
Mg H H
SiMe3
All of these magnesium-complexed clusters share the feature that some of the boron and/or carbon atoms reside outside the electron-delocalized framework and form what appear to be classically bonded (electron-precise) exo-polyhedral chain structures; similar features are seen in 10- to 12-vertex nido- and arachno-carboranes that have been described in earlier chapters. The observed cage structures are evidently strongly influenced by the choice of solvent and other factors involved in synthesis and workup; molecular orbital calculations show no significant differences in energy between the different forms [47]. Reactions with Group 1 metals take a different course from those of magnesium. Treatment of the carbons-separated isomer of (Me3Si)4C4B8H8 with Li, Na, K, or Cs metal first generates a paramagnetic, presumably monoanionic, species which slowly reacts with a second equivalent of metal to form a diamagnetic dianion. In reactions with Li, Na, or K, the dianion acquires a proton and the products are isolated as Mþ(Me3Si)4C4 B8 H9 salts; however, with cesium no protonation takes place and a polymer is formed consisting of (Me3Si)4C4 B8 H8 2 ions stitched together by Csþ ions [47,49]. M0
Hþ
M0
ðMe3 SiÞ4 C4 B8 H8 ! ½ðMe3 SiÞ4 C4 B8 H8 ! ðMe3 SiÞ4 C4 B8 H2 8 ! ðMe3 SiÞ4 C4 B8 H9 THF or TMEDA
11.3.5 Synthesis of neutral Arachno-C4B8 clusters In a process that extends the carbon-insertion approach discussed in Section 6.2, the incorporation of a C2 unit into the nido-5,6-C2 B8 H11 ion followed by acidification gives a product characterized as arachno-8-MeOC(O)-7,8,9,1028-C(O)OMe (11-28) [50]. This arachno geometry closely resembles a bicapped hexagonal antiprism C4B8H132 with two vertexes removed, and differs from 11-27 in that all four carbons appear to be fully integrated into the electron-delocalized cage. OMe H
H
C
O C C
11-28 C
11.3.6 Deboronation of Nido-C4B8 cages Although Me4C4B8H8 is not attacked by either triethylamine or THF under anaerobic conditions, it is converted by 95% aqueous ethanol to a CHMe-bridged 11-vertex C3B7 carborane (6-12) as described in Section 6.2, and its reversible proton-mediated cage rearrangement is shown in Figure 6-9. This and other 10- and 11-vertex carbon-rich carborane systems, synthesized in a variety of routes, are discussed in Chapters 6 and 7.
692
CHAPTER 11 Open 12-vertex and supra-icosahedral carboranes
11.3.7 General observations The cage structures and facile isomerizations of the carbon-rich 12-vertex carboranes described in this section differ strikingly from the 26-electron C2B10 icosahedral cage systems in one major respect. Isomerism in the latter species involves different heteroatom locations in the cage framework but the icosahedral cluster shape is maintained, as in 1,2- 1,7- and 1,12-C2B10H12 and their many metallacarborane derivatives in which one or more skeletal atoms are replaced by other elements. In the carbon-rich cages, in contrast, isomers often adopt dissimilar skeletal structures having different atom connectivities. This reflects a fact that has been noted earlier, that there is no dominant, favored geometry for 12-vertex cages having more than 26 skeletal electrons, and the shape of the cage in a given set of conditions is influenced by factors such as the choice of attached substituents, which in the 26-electron systems generally have little effect on structure. Consequently, the behavior of the carbon-rich systems is much less predictable than that of the CB11 and C2B10 carboranes and presents a more complex and difficult challenge to experimentalists.
11.4 ARACHNO-C6B6 CARBORANES Carborane cages with more than four carbon atoms incorporated into an electron-delocalized polyhedral framework are extremely rare, even if one stretches the definition of “framework” to include classically bonded bridging carbons as in 7-41 (Chapter 7). Excluding this latter class as marginal at best, the only known example of a nonmetallated CnBx polyhedron with n > 4 is the hexacarbon system arachno-H6C6B6Et6 (11-29) prepared by Wrackmeyer et al. via reaction of bis(diethylboryl)ethyne with tetraethyldiborane(6) [51]. The highly symmetric hexagonal-antiprismatic proposed geometry of 11-29 is based on multinuclear NMR spectroscopy and supported by DFT molecular orbital calculations, and is structurally related to known 14-vertex M2C2B10 and M2C4B8 bicapped hexagonal antiprismatic metallacarboranes, discussed in Chapter 13, in which metal atoms occupy the 6-coordinate apical vertices. C
C C C
C C
11-29
C = CH
A H6C6B6H6 cluster (6-29) mentioned in Chapter 6 has been described as a hexacarbon carborane but seems more appropriately viewed as an arachno-C4B6 cage with an ethylene bridge spanning its open face.
11.5 SUPRA-ICOSAHEDRAL CARBORANES Spectroscopically characterized products with the compositions Me4C4B9H11 and Me4C4B11H11, possibly representing 13- and 15-vertex clusters, respectively, were obtained as early as 1979 via iron-promoted oxidative fusion of nidoMe2 C2 B4 H4 2 and nido-Me2 C2 Bn Hn 2 dianions (n ¼ 5 or 7) in THF solution [52]; however, X-ray diffraction data have not been obtained for these compounds and their structures remain undefined. The first structurally characterized “supercarboranes” [1], CxBy clusters having more than 12 vertexes and lacking heteroatoms, are the dicarbon C2B11 and C2B12 species that are listed in Table 11-1 and described in the following sections.
11.5.1 13-vertex C2B11 and 14-vertex C2B12 clusters 11.5.1.1 Synthesis and characterization The true breakthrough in surmounting the “icosahedral barrier” cited at the beginning of this chapter came in the recognition by Welch and coworkers in 2002 [53] that boron can be inserted into carbon-tethered, 12-vertex nido-carborane
11.5 Supra-icosahedral carboranes
693
anions such as 11-2, discussed earlier, to form robust 13-vertex polyhedra; in contrast, unstable C2B11 cages appear to form initially upon reaction of R2 C2 B10 H12 2 ions with BR0 X2 reagents, but subsequently lose boron. Accordingly, the first characterized example of a 13-vertex carborane, nido-[C6H4(CH2)2]C2B11H10-3-Ph (11-30), was generated via the addition of boron to the nido-[C6H4(CH2)2]C2 B10 H11 2 dianion (obtained by deprotonation of the monoanion or, alternatively, by two-electron reduction of 1,2-[C6H4(CH2)2]C2B10H10) [53].
2− C
C
11-2
Ph C
PhBCl2
C
− 2 Cl−
11-30
X-ray crystallography of 11-30 has established the henicosahedral geometry shown, which features a trapezoidal C2 2C2 2B2 2B face. Density functional computations indicate that this cage structure is favored by 7.4 kJ mol1 over the fully triangulated docosahedron (as found in CpCo(C2B10H12, Chart 2-2), although the latter shape is calculated to be more stable for the as-yet unknown parent species C2 B11 H13 2 . In the case of 11-30, the preference of carbon for low-coordination sites is assumed to influence the observed structure [53]. Additional 13-vertex C2B11 carboranes similar to 11-30 and listed in Table 11-1 have been prepared by Xie et al. via boron insertions into other carbon-tethered nido-C2B10 dianions such as 1,2-(CH2)3C2 B10 H10 2 (analogous to 11-2) [54–56]. Arachno-carborane tetraanions like 11-3, 11-6, and 11-7, described in the previous section, also undergo boron insertion to generate both 13- and 14-vertex carboranes [54,55]. Treatment of 11-7 (n ¼ 3) with HBBr2SMe2 generates the 13-vertex species (CH2)3C2B11H11 (11-31), along with smaller amounts of the 14-vertex carborane 1,2-(CH2)3C2B12H12 (11-32) and a 12-vertex o-carborane derivative, 11-33. The last product evidently forms via cage oxidation of 11-7, which occurs as a competitive process during insertion and reflects the high reducing power of the anions [54].
4− CC
C
Li4(thf)5
4+
C
C
+
THF
11-7
CC
HBBr2•SMe2
+
C
11-31
11-32
11-33
32%
7%
2%
The solid-state 13-vertex cage geometries of 11-30 and 11-31 both feature several long, and presumably nonbonding, CB interactions that create open faces, but their cage connectivities differ slightly, reflecting the influence of their respective trimethylene versus o-xylyl tethering groups. In solution, however, both species exhibit simplified 11B and 1H NMR spectra even at low temperature, indicating rapid cage fluxionality [55]. The low-symmetry, 14-vertex, carborane 11-32 is converted to a more symmetric, fully triangulated isomer, 2,8-(CH2)3C2B12H12 (11-34), via reduction to its dianion followed by reaction with HBBr2SMe2 [54]. The crystallographically established bicapped hexagonal antiprismatic cage geometry of 11-34 (alternatively formulated [57] as 2,13-(CH2)3C2B12H12) corresponds to that calculated for the isoelectronic B14 H14 2 dianion [58,59].
694
CHAPTER 11 Open 12-vertex and supra-icosahedral carboranes
C C (1) Na, THF
C
(2) HBBr2•SMe2
C
11-32
11-34 37%
A third isomer of the 14-vertex system, 2,3-(CH2)3C2B12H12 (11-35), has been obtained from the reaction of (CH2)3C2B11H11)2[Naþ2(THF)4]n with HBBr2SMe2 and characterized by X-ray diffraction [57]. This species adopts the bicapped hexagonal prismatic cluster geometry with the framework carbon atoms occupying adjacent vertices in a C2B4 planar ring.
C C
11-35
An important recent advance in supra-icosahedral carborane chemistry is the discovery of a route to 13-vertex cages lacking exo-polyhedral tethers, which, in turn, can be thermally isomerized to separate the carbon atoms. As shown, the reduction of m-1,2-(CH2SiMe2CH2)C2B10H10 [60] to a dianion with sodium, followed by boron insertion, yields (CH2SiMe2CH2)C2B11H11 (11-36), which can be desilylated in silica gel to afford the untethered 13-vertex species Me2C2B11H11 (11-37); the latter compound, on heating in toluene, forms the carbons-separated isomer 11-38 [61]. Remarkably, 11-38 has also been generated in low yield from the o-carborane derivative 1,2-Me2C2B10H10 (11-39), a compound known for more than 50 years, via reduction to its dianion and addition of boron [61].
Si
Si
C
Me
Me
Me Me
C
C
(1) excess
C
C
Na0
C
silica
(2) HBBr2•SMe2
11-37
11-36 Δ C
C
C (1) excess
Na0 C
(2) HBBr2•SMe2
11-39
11-38
11.5 Supra-icosahedral carboranes
695
11.5.1.2 Cage carbon removal and polyhedral contraction Carbons-adjacent and carbons-separated 13-vertex C2B11 carboranes are attacked by electron donors to form, respectively, icosahedral CB11 H12 derivatives, 11-40 and 11-41, in which one of the cage carbon atoms is extruded to form part of an exo-polyhedral tetracarbon chain [62].
C
C
(1) MeOH (2) Me3NH+ Cl–
C
C R R = OMe–, PPh3
or PPh3
11-40
11-31
C
C
C
− OMe
C
(1) MeOH (2) Me3NH+ Cl–
11-41
11-38
Unlike the well-known cage-deboronation reactions discussed in the earlier chapters, the extraction of carbon atoms from carborane clusters is extremely rare, and the few known examples include the conversion of the aminocarborane 1-ðH2 NÞCB11 F11 to the cyanoborane dianion 3-ðNCÞB11 F10 2 (Chapter 8) [63] and the multistage degradation of 1,2-Me2C2B10H10 to CB9 or CB10 clusters, described in Chapters 6 and 7 [64,65]. In the present case, it has been speculated [62] that nucleophilic substitution at a cage carbon atom creates an electron-rich situation that induces the migration of that carbon outside the cage with concurrent closure of the CB11icosahedron. From 13C NMR and other evidence, it appears that the cage carbon atoms in the 13-vertex C2B11 carboranes are substantially more electron-poor than those in icosahedral C2B10 systems, an observation that may help to account for the major difference in their behavior toward nucleophiles [62].
11.5.1.3 Electrophilic alkylation and halogenation The reactivity of icosahedral carboranes toward electrophilic substitution, detailed in Chapters 8–10, is mirrored in the behavior of 13-vertex species, such as 11-31, which is readily methylated by iodomethane to give hexamethyl products. The same compound undergoes polyhalogenation by bromine and iodine in the presence of AlCl3 catalyst [55]. MeI=AlCl3
ðCH2 Þ3 C2 B11 H11 ! ðCH2 Þ3 C2 B11 H5 -8; 9; 10; 11; 12; 13-Me6 X2 =AlCl3
ðCH2 Þ3 C2 B11 H11 ! ðCH2 Þ3 C2 B11 H5 -8; 9; 10; 11; 12; 13-X6
X ¼ Br; I
As in the C2B10 and CB11 icosahedral systems, substitution takes place at the most negatively charged boron atom(s) (those furthest from carbon). While (CH2)3C2B11H5Me6 is stable toward air and water, the 13-vertex hexahalo derivatives, in contrast, are surprisingly reactive toward moisture, unlike the polyhalo derivatives of the 12-vertex carboranes. It may be that the open C2B2 trapezoidal faces in the (CH2)3C2B11H5X6 derivatives facilitate hydrolytic attack on the cage.
696
CHAPTER 11 Open 12-vertex and supra-icosahedral carboranes
11.5.1.4 Reductive cage-opening Like their 12-vertex counterparts, carbon-tethered 13-vertex closo-carboranes are reduced to open-cage dianions on treatment with Group 1 metals, with the metal-dianion salts (11-42) crystallizing in polymeric chains [55]. R
R C
C
excess M0
C
C
M = Li, Na R = o-C6H4, CH2
11-30, 11-31
ligand = THF, DME
M2(ligand)x
THF or DME
n
11-42
In contrast to the C2B10 dianions discussed at the start of this chapter, the 13-vertex species do not undergo further reduction to arachno cages, again illustrating the effect of the tethering groups on cage reactivity. The ðCH2 Þ3 C2 B11 H11 2 dianions (11-42, R ¼ CH2) are powerful reducing agents and are easily oxidized back to the neutral (CH2)3C2B11H11 (11-31) by many transition metal halides, thereby frustrating early attempts to create 14-vertex MC2B11 metallacarboranes via metal insertion. However, phosphine-metal reagents such as (Ph2PCH2CH2PPh2)NiCl2 have been successfully employed for this purpose, generating the desired metallacarboranes (Chapter 13) [55]. Reduction of 11-35 with excess sodium in THF yields a 14-vertex nido-ðCH2 Þ3 C2 B12 H12 2 dianion from which a 15-vertex metallacarborane has been prepared via metal insertion, as described in Section 13.5 [66].
11.5.1.5 Radical anion formation A dark brown intermediate species formed during the reduction of (CH2)3C2B11H11 (11-31) to the dianion has been identified as a radical monoanion, ðCH2 Þ3 C2 B11 H11 and isolated as a sodium crown ether salt (11-43) [67]. • C
C Na/THF
C
C Na+(CH2)12O6(THF)2
18-crown-6
11-31
11-43
This compound is notable as the first example of a 2n þ 3 skeletal electron boron cluster (midway between 2n þ 2 [closo] and 2n þ 4 [nido] systems) to be isolated and structurally characterized, although several o-carboranyl 2n þ 3-electron radical anions have been observed spectroscopically (Section 9.6). The crystallographic characterization of 11-43 has revealed a slightly more open cage structure than 11-31, with two trapezoidal faces. Like the CB11Me12 radical described in Chapter 8, 11-43 lacks an observable NMR spectrum but exhibits a clear ESR signal (g ¼ 1.994), both in the solid state and in THF solution [67]. Electrochemical one-electron reduction of 11-43 to the ðCH2 Þ3 C2 B11 H11 2 dianion (11-42) is straightforward [E1/2 (0/1) ¼ 1.28 V]. While it is possible that the exo-polyhedral trimethylene chain plays a role in stabilizing 11-43, this is unlikely inasmuch as the analogous 12-vertex species ðCH2 Þ3 C2 B10 H10 is unstable under the same conditions [67].
11.5.1.6 Reactions of 2,3-(CH2)3C2B12H12 with nucleophiles The chemistry of 14-vertex carboranes is in an early stage of exploration, but studies have already uncovered some remarkable findings that differentiate these systems from the 12-vertex C2B10 cages that have been described in Chapter 9 and the 13-vertex carboranes discussed in this section. Nucleophilic attack on the C,C0 -trimethylene derivative 11-35 with methanol generates derivatives of the closo-1-CB11H12 anion (11-44 to 11-46) via an unusual process in which a
11.5 Supra-icosahedral carboranes
697
vertex carbon atom is converted to an exo-polyhedral role [68]. Conversely, 11-35 reacts with other bases such as LiNMe3 to form boron-bridged 13-vertex nido-8,9-(CH2)3C2B11H11-m(11,12)-BHR anions (11-47, R ¼ MeO, Me3CO, Me2N) that regenerate 11-35 on treatment with HCl. Reaction of 11-47 with excess LiNMe3 removes the bridging BHR group to give the nido-8,9-ðCH2 Þ3 C2 B11 H12 anion (11-48), which is also formed directly from 11-35 by reaction with CsF or piperidine [68]. − C C C
OMe
MeOH
11-35
LiNMe3, THF
OMe −
+
11-44
C
− C
+
11-45 NMe2 −
H
HCl
11-46
H
B C C
LiNMe3
−
C C
CsF
11-47
11-48
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11.5 Supra-icosahedral carboranes
699
[72] Batsanov, A. S.; Copley, R. C. B.; Davidson, M. G.; Fox, M. A.; Hibbert, T. G.; Howard, J. A. K.; et al. J. Cluster. Sci. 2006, 17, 119. [73] Davidson, M. G.; Fox, M. A.; Hibbert, T. G.; Howard, J. A. K.; Mackinnon, A.; Neretin, I. S.; et al. Chem. Commun. 1999, 1649. [74] Charmant, J. P. H.; Haddow, M. F.; Mistry, R.; Norman, N. C.; Orpen, A. G.; Pringle, P. G. Dalton. Trans. 2008, 1409. [75] Zharov, I.; Saxena, A.; Michl, J.; Miller, R. D. Inorg. Chem. 1997, 36, 6033. [76] Getman, T. D.; Knobler, C. B.; Hawthorne, M. F. J. Am. Chem. Soc. 1990, 112, 4593. [77] Zakharkin, L. I.; Kalinin, V. N.; Podvisotskaya, L. S. Izv. Akad. Nauk. SSSR, Ser. Khim. 1967, 2212 [Russian p. 2310]. [78] Zakharkin, L. I.; Podvisotskaya, L. S. Zh. Obshch. Khim. 1967, 37, 475 [Russian p. 506]. [79] Wang, J.; Zhu, Y.; Li, S.; Zheng, C.; Maguire, J. A.; Hosmane, N. S. J. Organomet. Chem. 2003, 680, 173. [80] Cheung, M.-S.; Chan, H.-S.; Xie, Z. Organometallics 2005, 24, 3037. [81] Tolpin, E. I.; Lipscomb, W. N. Inorg. Chem. 1973, 12, 2257. [82] Churchill, M. R.; DeBoer, B. G. Inorg. Chem. 1973, 12, 2674. [83] Zakharkin, L. I.; Zhigareva, G. G.; Polyakov, A. V.; Yanovskii, A. I.; Struchkov, Yu. T. Izv. Akad. Nauk. SSSR., Ser. Khim. 1987, 872 [Russian]. [84] Chui, K.; Li, H.-W.; Xie, Z. Organometallics 2000, 19, 5447. [85] Vin˜as, C.; Barbera`, G.; Teixidor, F. J. Organomet. Chem. 2002, 642, 16. [86] Wang, S.; Li, H.-W.; Xie, Z. Organometallics 2001, 20, 3842. [87] Brattsev, V. A.; Stanko, V. I. Zh. Obshch. Khim. 1969, 39, 2144 [Russian]. [88] Xie, Z.; Liu, Z.; Yang, Q.; Mak, T. C. W. Organometallics 1999, 18, 3603. [89] Jeffery, J. C.; Jelliss, P. A.; Stone, F. G. A. Inorg. Chem. 1993, 32, 3382. [90] Wang, S.; Yang, Q.; Mak, T. C. W.; Xie, Z. Organometallics 1999, 18, 4478. [91] Cheung, M.-S.; Chan, H.-S.; Xie, Z. Organometallics 2004, 23, 517. [92] Maxwell, W. M.; Wong, K.-S.; Grimes, R. N. Inorg. Chem. 1977, 16, 3094. [93] Stanko, V. I.; Gol’tyapin, Yu. V.; Brattsev, V. A. Zh. Obshch. Khim. 1969, 39, 1142 [Russian p. 1175]. [94] Stanko, V. I.; Brattsev, V. A.; Gol’tyapin, Yu. V. Zh. Obshch. Khim. 1969, 39, 2686 [Russian]. [95] Leyden, R. N.; Hawthorne, M. F. Inorg. Chem. 1975, 14, 2018. [96] Zakharkin, L. I.; Kalinin, V. N.; Antonovich, V. A.; Rys, E. G. Izv. Akad. Nauk. SSSR, Ser. Khim. 1976, 1009 [Russian p. 1036]. [97] Stanko, V. I.; Babushkina, T. A.; Brattsev, V. A.; Klimova, T. P.; Alymov, A. M.; Vasil’ev, A. M.; et al. J. Organomet. Chem. 1974, 78, 313. [98] Stanko, V. I.; Anorova, G. A. Zh. Obshch. Khim. 1974, 44, 2112 [Russian]. [99] Stanko, V. I.; Gol’tyapin, Yu. V. Zh. Obshch. Khim. 1982, 52, 78 [Russian]. [100] Kalinin, V. N.; Kobel’kova, N. I.; Astakhin, A. V.; Zakharkin, L. I. Izv. Akad. Nauk. SSSR, Ser. Khim. 1977, 2376 [Russian]. [101] Kalinin, V. N.; Kobel’kova, N. I.; Astakhin, A. V.; Gusev, A. I.; Zakharkin, L. I. J. Organomet. Chem. 1978, 149, 9. [102] Kalinin, V. N.; Zakharkin, L. I. Izv. Akad. Nauk. SSSR, Ser. Khim. 1971, 2353 [Russian]. [103] Zakharkin, L. I.; Kalinin, V. N. Izv. Akad. Nauk. SSSR, Ser. Khim. 1967, 2471 [Russian p. 2585]. [104] Zakharkin, L. I.; Kalinin, V. N.; Kvasov, B. A.; Snyakin, A. P. Zh. Obshch. Khim. 1971, 41, 1726 [Russian]. [105] Zakharkin, L. I.; Kalinin, V. N. Izv. Akad. Nauk. SSSR., Ser. Khim. 1970, 2386 [Russian]. [106] Brattsev, V. A.; Stanko, V. I. J. Organomet. Chem. 1973, 55, 205. [107] McKee, M. L. J. Am. Chem. Soc. 1992, 114, 5856. [108] Hall, J. H., Jr; Dixon, D. A.; Kleier, D. A.; Halgren, T. A.; Brown, L. D.; Lipscomb, W. N. J. Am. Chem. Soc. 1975, 97, 4202. [109] Ionov, S. P.; Kuznetsov, N. T. Russ. J. Coord. Chem. 2001, 27, 605. [110] King, R. B. J. Organomet. Chem. 2007, 692, 1773. [111] Lipscomb, W. N. Inorg. Chem. 1979, 18, 901. [112] Lee, S. Inorg. Chem. 1992, 31, 3063. [113] Farras, P.; Teixidor, F.; Branchadell, V. Inorg. Chem. 2006, 45, 7947.
CHAPTER
Heteroatom carboranes of the main group elements
12
12.1 OVERVIEW The 1965 landmark synthesis of metallacarboranes of the transition metals, described in the following chapter, was followed in short order by the discovery that main group elements such as phosphorus, nitrogen, sulfur, and beryllium can play the same role [1]. To date, carborane cages incorporating heteroatoms spanning almost the entire periodic table of elements, excluding only groups 17 and 18, have been isolated and characterized (see Tables 12-1–12-5). As these compounds are found in a wide range of polyhedral sizes, structures, number and location of skeletal heteroatoms, and exo-polyhedral substituents, in many cases involving more than one type of heteroatom in the same cage, the scope and versatility of this chemistry far exceeds that of any other class of molecular clusters or organic heterocycles. Moreover, as much of this area is thinly developed, with the chemistry of many species unexamined beyond their original characterization, its potential is virtually limitless and currently is almost completely untapped. For example, at present nearly all known practical applications of carboranes utilize the icosahedral C2B10H12 and CB11 H12 systems or their transition metal and organosubstituted derivatives; one can only speculate what further advances will be possible as heterocarboranes are brought into the picture.
12.2 HETEROCARBORANES OF THE GROUP 1 AND 2 ELEMENTS 12.2.1 Lithium, sodium, potassium, and rubidium Salts of alkali and alkaline earth metal cations with nido-carborane dianions such as 7-vertex R2 C2 B4 Hn n6 (n ¼ 4 or 5) and 12-vertex R2 C2 B9 Hn n11 (n ¼ 9 or 10) have long been employed as precursors to transition metal complexes and many other types of derivatives, as described in preceding chapters of this book. For years, the Group 1 salts received little attention, in part because their reactions in solution tend to yield the same products regardless of the identity of the metal cation. However, the general assumption that the metal-cage interactions are weak, predominantly electrostatic, and uninteresting, has been challenged by detailed crystallographic and NMR studies in recent years [2–4]. Experimental NMR data on Mþ 2 C2 B9 H11 2 salts (M ¼ Li, Na, or K) in a variety of solvents, supported by DFT calculations on these species and on several Mþ 2 R2 C2 B4 H4 2 systems, show marked differences in their spectroscopic behavior as the cation is changed, indicating strong ion-pairing or covalent interactions between the metal ions and carborane substrates [5]. This finding is buttressed by numerous X-ray crystallographic studies (Table 12-1) of lithium, sodium, potassium, and cesium salts 2BH edge. in which the metal variously binds to a C2B3 open face, a B3 triangular face, or an HB2 The deprotonation of neutral nido-2,3-R2C2B4H6 carboranes is discussed in Chapter 4. Regardless of the solvent employed, sodium hydride or potassium hydride removes only one of the B-H-B bridging hydrogens, affording a R2 C2 B4 H5 monoanion; even with an excess of NaH or KH in hot solution, no dianion is obtained [6]. In contrast, Carboranes. DOI: 10.1016/B978-0-12-374170-7.00006-9 © 2011 Elsevier Inc. All rights reserved.
701
702
CHAPTER 12 Heteroatom carboranes of the main group elements
Table 12-1 Heterocarboranes of the Group 1 and 2 Elements Synthesis and Characterization Compounda
Information
References
LITHIUM 7-vertex LiC2B4 clusters 1,2,4-(THF)2Li[(SiMe3)2C2B4H5] 1,2,4-(THF)2(m-H)2Li2[(SiMe3)2C2B4H4] 1,2,4-(Me2NC2H4NMe2)2(m-H)2Li2[(SiMe3)2C2B4H4] (L)Li{[(SiMe3)RC2B4H4]-(m-H)2-exo-Li(L)} [L ¼ (Me2N)2C2H4, none; R ¼ SiMe3, Me, H) LiðTMEDAÞ2 þ 1,2,3-Li½ðSiMe3 Þ2 C2 B4 H5 2
S, H, B, C, IR S, X, H, B, C, Li, IR, MS S, X, H, B, C, Li, IR S, X[SiMe3, (Me2N)2C2H4], H, B, C, Li, IR S, X, H, B, C, Li, IR
[9] [12] [12] [10]
12-vertex LiC2B9 clusters [Liþ]2 7,8-/7,9-/2,9-C2 B9 H11 2
S, H, B, C
[5]
7-vertex LiC3B3 clusters [(MeCH)3(BMe)3Li]4
S, X, H, B
[19]
SODIUM 7-vertex NaC2B4 clusters 1,2,3-(TMEDA)Na[Me(SiMe3)C2B4H4]-m(4,5)-H2Na(TMEDA)2 1,2,3-LNa[(SiMe3)RC2B4H5] [L ¼ THF, (Me2N)2C2H4; R ¼ SiMe3, Me, H] 1,2,3-(THF)Na[(SiMe3)C2B4H4-exo-Li(THF)] (R ¼ SiMe3, Me, H) 1,2,4-(THF)2Na[(SiMe3)2C2B4H5] 1,2,4-(THF)(TMEDA)Na[(SiMe3)2C2B4H5]
S, H, B, C, IR S, X [(Me2N)2C2H4, SiMe3; THF, SiMe3], H, B, C, IR S, H, B, C, IR S, H, B, C, IR S, X
[13] [10]
12-vertex NaC2B9 clusters [Naþ]2 7,8-/7,9-/2,9-C2 B9 H11 2 (THF)Na[(PhCH2)2C2B10H10]
S, H, B, C S, X, H, B, C, IR
[5] [14]
POTASSIUM 12-vertex KC2B9 clusters [Kþ]2 7,8-/7,9-/2,9-C2 B9 H11 2
S, H, B, C
[5]
S, X, H, B, C, IR S, X, H, B, C, IR S, X, H, B, C, IR*
[14] [15] [17]
S, X, H, B, C, IR*
[17]
BERYLLIUM 12-Vertex BeC2B9 clusters 3,1,2-LBe(C2B9H11) (L ¼ Et2O, Me3N)
S, H, B
[24]
MAGNESIUM 7-vertex MgC2B4 clusters 1,2,3-[(Me2N)2C2H4]Mg[(Me3Si)2C2B4H4]
S, X, H, B, C, IR
[25]
13-vertex KC2B10 clusters {KðTHFÞ2 þ (dioxane)K[(PhCH2)2C2B10H10]}n 4,1,8-(18-crown-6)K(C2B10H4Me8) exo-{m(1,2)-[o-C6H4(CH2)2]-1,2-C2B10H10}2 K3 ð18-crown-6Þ2 ð18-crown-6ÞKðMeCNÞ2 þ [{m(1,2)-[o-C6H4(CH2)2]-1,2-C2 B10 H11 [Kþ(18-crown-6)]n
[7]
[10] [9,11] [9]
Continued
12.2 Heterocarboranes of the group 1 and 2 elements
703
Table 12-1 Heterocarboranes of the Group 1 and 2 Elements—Cont’d Synthesis and Characterization Compounda
Information
References
{1,2,3-Mg[(Me3Si)2C2B4H4]2} {1,2,4-Mg[(Me3Si)2C2B4H4]2}2 1,2,3-(TMEDA)Mg[(Me3Si)2C2B4H4] 1,2,4-(TMEDA)Mg[(Me3Si)2C2B4H4] 1,2,4-(THF)3Mg[(Me3Si)2C2B4H4]
S, S, S, S, S,
[25,26] [25,26] [26] [26] [27]
13-vertex MgC4B8 clusters (THF)2Mg[(Me3Si)4C4B8H7Me] arachno-(THF)2Mg[(Me3Si)4C4B8H8]
S, X, H, B, C, IR S, X, H, B, C, IR
[16] [207]
CALCIUM 13-vertex CaC2B10 clusters (MeCN)4Ca(C2B10H12)
S, X, H, B, IR
[28]
STRONTIUM 13-vertex SrC2B10 clusters [(MeCN)3Sr(C2B10H12)]n (polymer) (THF)3Sr(C2B10H12)
S, X S, H, B
[29] [29]
BARIUM 7-vertex BaC2B4 clusters arachno-1,2,3-(THF)2Ba[(SiMe3)C2B4H5]2
S, X, H, B, C, Si, IR, MS
[30]
GIAO Molecular conformation DFT NMR: MP2 covalence or strong ion pairing between M and anions
[9] [69] [16] [5]
B B B B B B NMR: MP2 covalence or strong ion pairing between M and anions
[26] [26] [26] [26] [26] [26] [5]
2
Theoretical Studies Molecular and electronic structure calculations 1,2,4-(THF)2M[(SiMe3)2C2B4H5] (M ¼ Na, Li) 1,2,3-Mg(C2B4H6) (THF)2Mg[(Me3Si)4C4B8H7Me] [Mþ]2 7,8-/7,9-/2,9-C2 B9 H11 2 (M ¼ Li, Na) NMR calculations 1,2,n-Li(C2B4H6) (n ¼ 3, 4) 5,6-(m-H2)2-1,2,3-Li(C2B4H6) 1,2,3-(en)Mg(C2B4H6) 1,2,4-(en)Mg(C2B4H6) 1,2,3-MgðC2 B4 H6 Þ2 2 1,2,4-MgðC2 B4 H6 Þ2 2 [Mþ]2C2 B9 H11 2 (M ¼ Li, Na)
X, H, B, C, IR H, B, C, IR X, H, B, C, IR H, B, C, IR X, H, B, C
S ¼ synthesis, X ¼ X-ray diffraction, H ¼ 1H NMR, B ¼ 11B NMR, C ¼ 13C NMR, F ¼ 19F NMR, P ¼ 31P NMR, Li ¼ 7Li NMR, 2d ¼ twodimensional (COSY) NMR, IR ¼ infrared data, MS ¼ mass spectroscopic data, UV ¼ UV-visible data, E ¼ electrochemical data, ESR ¼ electron spin resonance data, MAG ¼ magnetic susceptibility, COND ¼ electrical conductivity, OR ¼ optical rotation a Metals and other heteroatoms (other than carbon) incorporated into the cluster framework are in boldface.
704
CHAPTER 12 Heteroatom carboranes of the main group elements
lithium alkyls easily remove the remaining B-H-B proton to give the dianion. This behavior is understandable, given the solid state structures of the respective salts. Treatment of the ðMe3 SiÞ2 C2 B4 H5 monoanion (12-1) with n-butyllithium generates the dianion, whose LiðTMEDAÞ2 þ complex (12-2) has one metal in a capping position on the C2B3 face while the other is coordinated to the cage via two B-H-Li bridges—a recurring pattern in Group 1 C2B4 metallacarboranes [3,4,6]. Slow sublimation of the monoanion lithium salt affords the bis(carboranyl)lithium anion 12-3 which retains the bridging protons in face-capping roles [7]. − Li+
− Li+ C
C
C
SiMe3
sublimation
Me3Si
H
SiMe3
12-1
H
Me3Si
C
SiMe3
Li H SiMe3
C C
1) n-C4H9Li
12-3
2) TMEDA
N H
Li N
12-2
SiMe3
Li N
SiMe3
C
C
H
N
Unlike the reactions with alkyllithium, the deprotonation of 2,3-R2C2B4H6 carboranes by NaH or KH in THF or dioxane is a heterogeneous process occurring on the metal hydride surface [8]. As revealed in the X-ray structures [9–11] of the TMEDA-solvated salt 12-4 and its THF analogue, the sodium complexes crystallize as dimers in which the remaining bridging proton on each cage is inaccessible to further attack by solid NaH or KH. Similar findings were made for the corresponding salts of the 2,4-R2 C2 B4 H5 and 2,4-R2 C2 B4 H4 2 ions, whose cage carbon atoms are separated (Chapter 4) [9,12].
N H
N Na Me3Si Me3Si
C C
C
H
C
H H
SiMe3
SiMe3
H
Na H
12-4
N N
In the case of the more fully solvated salt 1,2,3-(TMEDA)Na[Me(SiMe3)C2B4H4]-m(4,5)-H2Na(TMEDA)2, whose Na(TMEDA)2 unit does not occupy a capping position as in 12-4 but instead coordinates to the cage via exopolyhedral bonds, the bridge hydrogen is unprotected and hence is removable by attack of H ion to form the carborane dianion [13].
12.2 Heterocarboranes of the group 1 and 2 elements
705
Similarly, sandwich complexes of the heavier Group 1 metals with 12-vertex nido- and arachno-carborane anions exist in the solid state, as in the ½o- C6 H4 ðCH2 Þ2 C2 B10 H10 2 2 ½Kþ 3 , ½ðMe3 SiÞ4 C4 B8 H8 2 2 ½Csþ 4 , and similar systems described in Chapter 11 and listed in Table 11-1 [14–18]. Despite the metal-to-open face coordination in their crystalline states, it is problematic whether these compounds merit description as “potassacarboranes” and “cesacarboranes” inasmuch as the metal ions are easily displaced in solution by transition metals and other entities that bind much more strongly to the carborane cage. With some exceptions, the term metallacarborane is normally reserved for clusters whose metal atoms remain tightly bound into an electron-delocalized boron-carbon skeletal framework under a range of conditions. A unique type of lithiacarborane with the composition [(MeCH)3(BMe)3Li]4 (12-5) has been prepared via the reaction of lithium metal with hexamethyltriboracyclohexane [19]. In this compound, each monomer unit is coordinated via B2 2B and B2 2C edges to a lithium ion, which serves to link four such entities into a cyclic tetramer in the crystal. As the binding in 12-5 is nonclassical, involving a combination of two- and three-center electron pair bonds, this system is appropriately viewed as a lithium–carborane complex rather than as an organoborane.
Me Me B C H
B
H
C
B
Me Li0, Et2O
B
C Me
Me
Me
Me
Me
THF
Me
C H
H
Me B
B
C C
Li
Me
H
Me
H
4
12-5
12.2.2 Beryllium, magnesium, calcium, strontium, and barium Only one beryllacarborane has been reported, which may be surprising considering the strategic location of element number 4 in the periodic table adjacent to boron, and its general propensity to form strong covalent bonds; indeed, several beryllaboranes lacking framework carbon atoms have been prepared by Gaines and coworkers [20–23]. The dangers attendant to working with volatile compounds of this metal have no doubt been a deterrent to studies in this area, but intrepid explorers in the future are likely to be rewarded with a large and structurally diverse family of beryllacarboranes. In any case, the one known example, an icosahedral BeC2B9 cluster, was obtained as a white crystalline solid triethylamine adduct via beryllium insertion into a neutral nido-dicarbaundecaborane cage [24]. ð1Þ C6 H6 =Et2 O
7;8-C2 B9 H13 þ BeMe2 2ðOEtÞ2 ! 3;1;2-ðEt3 NÞBeC2 B9 H11 þ 2CH4 ð2Þ Et3 N
Interestingly, the reaction of BeCl2 with 7,8-C2 B9 H11 2 (dicarbollide ion) failed to give a beryllacarborane, as a Lewis base adduct is evidently required in order to stabilize the product [24]. Magnesacarboranes are currently limited to 7-vertex MgC2B4 and 13-vertex MgC4B8 systems; oddly, no icosahedral MgC2B9 cluster (or indeed any 12-vertex metallacarborane of a Group 2 metal other than Be) has been reported. Displacement of sodium from Naþ ½ðMe3 SiÞRC2 B4 H5 salts by magnesium leads to edge-metallated (12-6), half-sandwich (12-7) or full-sandwich (12-8) magnesacarborane products as shown [25,26].
706
CHAPTER 12 Heteroatom carboranes of the main group elements
Me3Si
C C
H
R H
R
N
−2 NaBr −2 MgH R = Me
H
C Me3Si
Mg
2 MeMgBr −78-25 °C, C6H6
N
H
C
H
N
2 MeMgBr −78-25 °C, C6H6
R
C
C
SiMe3
Na
−2 NaBr −2 MgH
N
C
C
SiMe3
Mg N
SiMe3
N
R = SiMe3
12-7 12-6
(n-C4H9)2Mg
2−
0-25 °C, C6H6 R = SiMe3 C Me3Si
−2 n-C4H10
C
Mg
C
SiMe3
SiMe3
+ 2Li(TMEDA)2
12-8
C Me3Si
Treatment of (TMEDA)2[Liþ]2[2,4-ðMe3 SiÞ2 C2 B4 H4 2 ] (the carbons-separated analogue of 12-2) with MgBr2 in benzene generates 1,2,4-(TMEDA)Mg[(Me3Si)2C2B4H4] and 3-Mg[2,4-ðMe3 SiÞ2 C2 B4 H4 2 2 whose cage structures correspond to 12-7 and 12-8, respectively, except that the carbon atoms occupy nonadjacent vertexes [26]. Similarly, 1,2,4(THF)3Mg[(Me3Si)2C2B4H4] was obtained from (THF)2[Naþ]2[2,4-ðMe3 SiÞ2 C2 B4 H4 2 ] via reaction with MeMgBr in diethyl ether [27]. All of these 1,2,3- and 1,2,4-MgC2B4 magnesacarborane clusters are obtained as air-stable colorless crystals. The only known examples of larger magnesium-carborane complexes are those derived from arachno-ðMe3 SiÞ2 R2 C4 B8 H8 2 dianions and described in Chapter 11, a structurally diverse group that includes species in which the metal is clearly a part of the cage system (11-25) and others in which it is attached solely through exo-polyhedral bonds (11-26, 11-27). Metallacarboranes of the heavier Group 2 metals are similarly few in number, and at this writing are confined to a handful of 7- and 13-vertex species. The disodium salt of nido-7,9-C2 B10 H12 2 reacts with CaI2 in THF solution at room temperature to form a 13-vertex cluster which is recrystallized from MeCN/Et2O as 1,2,4-(MeCN)4Ca(C2B10H12) (12-9) [28]. L
L
2−
L
C Ca
CaI2/THF
C
MeCN 2–
nido-7,9-C2B10H12
C C
C = CH
L
12-9 L = MeCN
Similar treatment of the same carborane anion with strontium diiodide in THF affords a product characterized as 1,2,4(THF)3Sr(C2B10H12) (12-10) which on recrystallization from acetonitrile gives a spiral-chain crystalline polymer, [1,2,42H2 2B and Sr2 2H2 2C bonds [29]. When redis(MeCN)3Sr(C2B10H12)]n (12-11) whose units are linked by inter-cage Sr2 solved in THF, 12-11 reverts to 12-10.
12.3 Heterocarboranes of the group 13 elements
L
Sr H
H
L
L H
H
Sr
C
L
H
C
12-11
C
Sr C
L = MeCN C = CH
H
L
707
L
The metal-cage interaction in these calcium and strontium complexes, all of which are extremely air and moisture sensitive compounds, is believed to be largely ionic in nature on the basis of their IR spectra and other evidence [29]. The only barium-containing metallacarborane reported to date is the bis(nido-dicarbahexaboranyl) species 12-12, obtained nearly quantitatively from the reaction between 2,3-(Me3Si)2C2B4H6 and a bis(trimethylsilyl)zicate reagent in THF [30]. Me3Si
C C
Me3Si Me3Si Me3Si
(THF)4Ba[(μ-CH2SiMe3)2Zn(CH2SiMe3)]2
C C H
H
H
H
12-12
THF Ba
–2 Zn(CH2SiMe3)2 Me3Si
C Me3Si
THF H
C
In this complex, X-ray diffraction data reveal that the barium ion is bound to one carborane unit via Ba2 2H2 2B2 and Ba2 2C interactions, and to the other through two Ba2 2H2 2B bonds, as shown.
12.3 HETEROCARBORANES OF THE GROUP 13 ELEMENTS As congeners of boron having three valence electrons, the Group 13 metals are of special interest in the context of boron cluster chemistry, especially given the fact that the four heavier members of this group have far less proclivity toward forming electron-delocalized polyhedral clusters than has boron itself. For example, the possibility that stable molecular AlxHy clusters analogous to the boranes might exist has been explored experimentally and theoretically [31,32], though none has thus far been isolated and characterized. Nevertheless, each of the Group 13 elements has been incorporated into 7- to 12-vertex metallacarborane frameworks (Table 12-2).
Table 12-2 Heterocarboranes of the Group 13 Elements Synthesis and Characterization Compounda
Information
References
ALUMINUM 7-vertex AlC2B4 clusters nido-6,3,4-(Et3N)(H)Al(Et2C2B4H4) nido,nido-2,3-Et2C2B4H5-m,6-Al(NEt3)-3,4-Et2C2B4H4
S, H, B, IR, MS S, H, B, IR, MS
[34] [34] Continued
708
CHAPTER 12 Heteroatom carboranes of the main group elements
Table 12-2 Heterocarboranes of the Group 13 Elements—Cont’d Synthesis and Characterization Compounda 12-vertex AlC2B9 clusters 3,1,2-RAl(C2B9H11) R ¼ Me, Et 3,1,2-EtAl(C2B9H11) 2,1,7-EtAl(C2B9H11) 3,1,2-EtAl(C2B9H11)(OC4H8)2 3,1,2-EtAl(Me2C2B9H9) 3,1,2-RAl(C2B9H11)L R ¼ Et, Me2CHCH2; L ¼ OEt2, NMe3 3,1,2-MeAl[R(Me2N)C2B9H9] R ¼ H, Me 3,1,2-MeAl{[R(PhCH)2N(CH2)2]C2B9H9} R ¼ H, Me 3-Al (1,2-C2B9H11) (1,2-C2B9H9)-8,9-(m-H)2AlEt2 3-Al(1,2-C2 B9 H11 Þ2 3,1,2-MeAl[R(CH2NMe2)C2B9H9] R ¼ H, Me (Al-N) constrained geometry complex nido-(L)EtAl(Me2C2B9H9) L ¼ Et2O, OC4H8
Information
References
S, X S, S, S, S S, S, S, S, S,
[35] [36] [35] [35,37] [38] [40] [43] [43] [39,208] [39,209] [44]
H, B, IR H, B, IR, MS IR H, B, C, Al X(H), H, B, C, IR, MS X(H, Me), H, B, C, IR, MS X, H, B, C, IR X, H, B, IR X, H, B, C
S, H, B, C, Al
[38]
GALLIUM 7-vertex GaC2B4 clusters 1,2,3-MeGa(C2B4H6) 1,2,4-(Me3C)Ga[(SiMe3)2C2B4H4] {1,2,4-Ga[(SiMe3)2C2B4H4]}2 (Ga-Ga) 1,2,3-(tert-C4H9)Ga[(SiMe3)RC2B4H4] R ¼ SiMe3, Me, H {1,2,3(tert-C4H9)Ga[(SiMe3)RC2B4H4]}2(m-C8H6N4) R ¼ SiMe3, Me, H 1,2,3-(TMEDA)ClGa[(SiMe3)RC2B4H4] 1,2,4-(TMEDA)ClGa[(SiMe3)RC2B4H4] 1,2,3-Ga½ðSiMe3 Þ2 C2 B4 H4 2 1,2,4-Ga½ðSiMe3 Þ2 C2 B4 H4 2 1,2,3-(2,20 -bipyrimidine)3(tert-C4H9)Ga[(SiMe3)2C2B4H4] 1,2,4-(Me3C)Ga[(SiMe3)RC2B4H4] R ¼ SiMe3, Me 1,2,3-(Me3C)(2,20 -bipyridine)Ga[(SiMe3)2C2B4H4] R ¼ SiMe3, Me 1,2,4-(Me3C)(2,20 -bipyridine)Ga[(SiMe3)RC2B4H4] R ¼ SiMe3, Me 1,2,4-(Me3C)(L)Ga[(SiMe3)2C2B4H4] L ¼ C8H6N4, C15H11N3
S, S, S, S, S, S, S, S, S, S, S, S, S, S,
X, H, B, IR, MS H, B, C, MS X, H, B, C, MS X(SiMe3), H, B, C, IR, MS X(SiMe3), H, B, C, IR, MS X, H, B, C, IR X, H, B, C, IR X, H, B, C, IR X, H, B, C, IR X H, B, C, IR, MS X, H, B, C, IR, MS X(SiMe3), H, B, C, IR, MS X(C8H6N4), H, B, C, IR, MS
[45,46] [50] [50] [48] [48] [49] [49] [49] [49] [53] [52] [52] [52] [52]
12-vertex GaC2B9 clusters 3,1,2-EtGa(C2B9H11) 3,1,2-MeGa[R(Me2N)C2B9H9] R ¼ H, Me 3,1,2-MeGa{[R(PhCH)2N(CH2)2]C2B9H9} R ¼ H, Me 2,1,7-EtGa(C2B9H11) Tlþ[3,1,2-Ga(C2B9H11)2]
S, S, S, S, S,
H, B, C, IR, MS X(H), H, B, C, IR, MS X(H), H, B, C, IR, MS H H, X, B, C, IR, MS
[35] [43] [43] [35] [39,54]
INDIUM 7-vertex InC2B4 clusters 1,2,3-MeIn(C2B4H6)
S, H, B, IR, MS
[46] Continued
12.3 Heterocarboranes of the group 13 elements
709
Table 12-2 Heterocarboranes of the Group 13 Elements—Cont’d Synthesis and Characterization Compounda
Information
References
{1,2,3-(Me2CH)In[(Me3Si)2C2B4H4]}2 “nido”-1,2,3-(Me2CH)In[(SiMe3)RC2B4H4] R ¼ SiMe3, Me, H “nido-”1,2,3-(2,20 -bipyrimidine)(Me2CH)In[(SiMe3)RC2B4H4] R ¼ SiMe3, Me, H 1,2,4-(Me2CH)In[(SiMe3)RC2B4H4] R ¼ SiMe3, Me 1,2,4-(Me2CH)(L)In[(SiMe3)C2B4H5] L ¼ 2,20 -bipyridine, 2,20 -bipyrimidine
S, H, X, B, C, IR, MS S, H, X(SiMe3), B, C, IR, MS S, H, X(SiMe3), B, C, MS
[51,74] [51] [51]
S, H, X(SiMe3), B, C, IR, MS S, H, X(2,20 -bipyridine), B, C, IR, MS
[52] [52]
12-vertex InC2B9 clusters 3,1,2-MeIn[R(Me2N)C2B9H9] R ¼ H, Me nido-LInðC2 B9 H11 Þ2 2 L ¼ S(CH2)2S(CH2)2S, (PhS)2, S(CH2)2S
S, H, B, C, IR, MS S, X, H, B, IR
[43] [55]
THALLIUM 7-vertex TlC2B4 clusters 1,7,2,4,5-CpCoTl(Me2HC3B2Me2)
S, X, H, B, MS
[56]
3,1,2-Tl(MeC2B9H10) 3,1,2-Tl(Me2C2B9H9) Tlþ 3,1,2-Tl(Me2C2B9H9) Cs[Tl3(7,9-[PhNC(O)]2C2B9H9] Tl2(7,9-C2B9H11)
S, S, S, S X B S S S, S, S,
[62] [62] [62] [41,61] [59–61] [61] [41] [41] [58] [63] [63]
12-vertex TlAsC2B8 clusters TlAs(C2B8H11)
S, B
[57]
DFT (overlap populations; ligand-ligand interactions) SCF, GIAO 11B NMR DFT; stability versus 7-vertex icosahedral cages DFT, structure
[210]
12-vertex TlC2B9 clusters 3,1,2-Tl(C2B9H12) 3,1,2-Tl(MeC2B9H11) 3,1,2-Tl(Me2C2B9H10) 3,1,2-Tl(C2B9H11)
Theoretical Studies Molecular and electronic structure calculations ALUMINUM 3-Al(1,2-C2 B4 H6 Þ2 3-Al(1,2-C2 B9 H11 Þ2 3,1,2-LAl(C2B9H11) L ¼ Me, Et 2N) 3,1,2-MeAl[R(CH2NMe2)C2B9H9] R ¼ H, Me (Al2 constrained geometry complex
H, B, IR, MS H, B, IR H, B, IR
X, H, B IR, X-ray fluorescence IR
[42] [211] [44] Continued
710
CHAPTER 12 Heteroatom carboranes of the main group elements
Table 12-2 Heterocarboranes of the Group 13 Elements—Cont’d Synthesis and Characterization Compounda GALLIUM 1,2,3-MeGa(C2B4H6) 1,2,3-LGa(R2C2B4H4) L ¼ Me, CMe3; R ¼ H, SiMe3 1,2,3/1,2,4-(C10N2H8)(Me3C)Ga[(Me3Si)2C2B4H4] 1,2,4-(C8N4H6)(Me3C)Ga[(Me3Si)2C2B4H4] 1,2,3/1,2,4-Cl(Me2NCH2)2Ga[(Me3Si)2C2B4H4] 1-Ga[2,3/2,4-ðMe3 SiÞ2 C2 B4 H4 2 2 2Ga) 1-{Ga[2,4-(Me3Si)2C2B4H4]}2 (Ga2 3,1,2-EtGa(C2B9H11) 3-Ga(1,2-C2 B4 H6 Þ2 INDIUM 1,2,3-MeIn(C2B4H6) 1,2,3/1,2,4-(Me2CH)In[(Me3Si)2C2B4H4] 1,2,3/1,2,4-(C10N2H8)(Me2CH)In[(Me3Si)2C2B4H4] 1,2,3-(C8N4H6)(Me2CH)In[(Me3Si)2C2B4H4]
THALLIUM 3,1,2-Tl(C2B9H11)
Information
References
SCF, structure DFT; stability versus 12-vertex icosahedral cages DFT; stability versus 12-vertex icosahedral cages DFT; stability versus 12-vertex icosahedral cages DFT; stability versus 12-vertex icosahedral cages DFT; stability versus 12-vertex icosahedral cages DFT; stability versus 12-vertex icosahedral cages DFT; stability versus 7-vertex icosahedral cages DFT (overlap populations; ligand-ligand interactions)
[47] [211]
DFT; stability versus icosahedral cages DFT; stability versus icosahedral cages DFT; stability versus icosahedral cages DFT; stability versus icosahedral cages
[211] [211] [211] [211] [211] [211] [210]
12-vertex
[211]
12-vertex
[211]
12-vertex
[211]
12-vertex
[211]
DFT; stability versus 7-vertex icosahedral cages
[211]
S ¼ synthesis, X ¼ X-ray diffraction, H ¼ 1H NMR, B ¼ 11B NMR, C ¼ 13C NMR, IR ¼ infrared data, MS ¼ mass spectroscopic data. a Heteroatoms (other than carbon) incorporated into the cluster framework are in boldface.
12.3.1 Aluminum 12.3.1.1 Synthesis and structure Several aluminum-containing metallacarboranes are known. As was noted in Section 4.5, nido-2,3-C2B4H7-m(4,5)-AlL3 (L ¼ Me, PPh3) complexes are generated via insertion of an AlL3 unit into a basal B2 2B edge of the nido-2,3-C2 B4 H7 anion (Section 4.5) [33]. In a different approach, the reaction of neutral nido-2,3-Et2C2B4H6 with triethylaminealane under a variety of conditions affords the small aluminacarborane clusters 12-13 to 12-15 in good yield. The structures of 12-14 and 12-15, based on spectroscopic characterization, are notable as rare examples of nido 7-vertex boron clusters [34].
12.3 Heterocarboranes of the group 13 elements
C C
C
C
Et3N AlH3
12-13
0 C H
711
H
Al
H
H
H
2,3-Et2C2B4H6
NEt3 50 C
Et3N AlH3
50 C
70 C
H
C C C
C
Al
NEt3
C
70 C
H
Al
C
2,3-Et2C2B4H6
NEt3
12-15
12-14
Large aluminacarboranes are accessible via metal insertion into nido-7,8-C2B9H13 using aluminum alkyls, as in the syntheses of the metal-bridged species 12-16, which exist in tautomeric equilibria in solution, and the icosahedral compounds 12-17 which are generated on heating 12-16 [35] and have been crystallographically characterized [36]. (An earlier report [37] of the preparation of 12-17 (R ¼ Et) from the reaction of EtAlCl2 with ðNaþ Þ2 C2 B9 H11 2 lacks supporting data other than elemental analysis.) High-temperature rearrangement of 3,1,2-EtAl(C2B9H11) to the 2,1,7 isomer 12-18 parallels the conversion of 1,2- to 1,7-C2B10H12 (o- to m-carborane) discussed in Chapter 10. R
R R
H
H
C
−RH
C
C
R = Me, Et C = CH
7,8-C2B9H13
C
H
H
AlR3
C
C
H
H
H
H
R
Al
Al
12-16
R
R
Al
Al
C
410 C C
12-18
C
−RH
79 C C6H6
C
12-17
The synthesis of 12-16 and 12-17 from a neutral carborane precursor is unusual, as metallacarboranes are typically prepared via interactions of cationic metal reagents with carborane anions (see the following chapter). Other examples of metal insertion into neutral carboranes can be cited, for example, the gallacarborane syntheses described below, and a few metallacarborane preparations described in Chapter 13, but they are rare.
712
CHAPTER 12 Heteroatom carboranes of the main group elements
In sharp contrast to the analogous B(3)-organosubstituted o-carborane derivatives 1,2-C2B10H11-3-R (R ¼ alkyl or aryl), which are resistant to removal of boron by attacking bases (Section 7.2), compounds 12-16 to 12-18 are demetallated by moist air, forming Et3 NHþ C2 B9 H11 salts, and are explosively hydrolyzed on contact with water [35]. This behavior reflects the strongly electrophilic character of the aluminum atom, which can be seen in the reaction of the 12-17 analogue 3,1,2-EtAl(Me2C2B9H9) (12-19) with mild Lewis bases to form adducts of type 12-20 in which the cage is partially opened. Stronger bases such as 2,20 -bipyridine remove the aluminum from the cage to generate nido-Me2 C2 B9 H9 2 , while other reagents effect the replacement of aluminum with boron, germanium(II), or tin(II) to afford, respectively, 12-21, 12-22, or 12-23 [38]. When SnCl4 is employed, the result is not formation of a tin(IV) stannacarborane but oxidative closure of the carborane moiety to produce closo-2,3-Me2C2B9H9. L Al
Al
2− C
Me
Me
C
2,2bipyridine
C Me
Me L = Et2O, THF
12-19
MeBBr2 Me
SnCl4
12-20
SnCl2
GeI2
Me
Ge C
C
C
Me
C
Me Me
Me
Me
Sn
C
C
Me
C Me
7,8-Me2C2B9H92−
B
C
L
C
C
Me
C Me
12-21
2,3-Me2C2B9H9
12-22 12-23
The formation of Lewis base adducts of (alkyl)Al(R2C2B9H9) aluminacarboranes has also been explored with amines, phosphines, and other electron donors [39,40]. With excess triphenylphosphine the product is an Z1-metallated cage, nido-7,8-C2B9H12-10-endo-AlEt(PEt3)2 (12-24) in which the carborane evidently binds to aluminum via a twoelectron B2 2Al bond [39]. Me3P Al Me3P C C
12-24 C = CH
The structure of 12-24 can be viewed as an extremely slip-distorted AlC2B9 complex (where slip-distortion is a measure of the departure from “idealized” Z5-metal-to-cage coordination, as discussed in Chapter 13), or alternatively as an endo-substituted derivative of nido-7,8-C2 B9 H12 [39]. Lesser degrees of slip-distortion are seen in other Group 13
12.3 Heterocarboranes of the group 13 elements
713
metallacarborane complexes, especially in certain gallium systems discussed below; however, 12-17 and its analogues are essentially undistorted. A different facet of the reactivity of 3-EtAl(C2B9H11) (12-17) is seen in its catalytic behavior toward the exchange of hydrogen between B-H and arene C-H units, and in the oligomerization of alkenes under mild conditions without transition metals [39]. Further discussion of this area of interest appears in Chapter 17. Bis(dicarbollyl)aluminum sandwiches, which are counterparts of aluminum-arene sandwich complexes, have been prepared. Heating 12-17 in aromatic media forms the charge-compensated species 12-25, while reactions of suspensions of the insoluble thallacarborane salt Tlþ ½TlC2 B9 H11 [41] with alkylaluminum reagents generate the monoanion 12-26. The zwitterionic complex 12-25 exhibits dynamic behavior in solution on the NMR time scale, indicating that the Et2Al moiety migrates over the surface of both polyhedral cages [39]. − Tl+ Al
C C
C
toluene
C
80 - 90 C
12-17
C = CH
Et Et
C
C 2 Tl+[TlC2B9H11] −
Al H
C
Al
4 AlR2Cl
Al
toluene C
C
C
–Tl[AlR2Cl2] –2 Tl[AlR3Cl] R = Me, Et, i-C4H9
H
12-25
12-26
Ab initio GIAO-SCF 11B NMR calculations show that the aluminum center in 12-26 is highly deshielded compared to Cp*2Al, implying that the diamagnetic ring current in the carborane ligands is relatively weak [42]. A different genre of aluminum-carborane complexes that are not strictly speaking metallacarboranes is exemplified by AlðC2 B6 H8 Þ2 (5-12), described in Chapter 5, and nido-6,9-C2B8H10-m(6,9)-AlEt(OEt2) (6-2) and Alðnido-6; 9-C2 B8 H10 Þ2 (Chapter 6). In these species, the essentially sp3-hybridized Al atoms are classically bound to the carborane 2C two-electron bonds, and hence do not units, bridging the open faces of nido-C2B6 or nido-C2B8 clusters via Al2 participate in electron-delocalized cage bonding.
12.3.1.2 Constrained-geometry complexes The well-known ability of aluminum to bind cyclopentadienyl rings in a variety of modes ranging from Z1 to Z5 coordination has inspired the synthesis of dicarbollyl-Al systems that can exploit this versatility. Of particular interest are analogues of tethered s,p-cyclopentadienyl titanium complexes such as [Me2Si(Me4C5)(Me3CN)]TiCl2, that are effective olefin polymerization catalysts. Accordingly, the C-aminodicarbollyl compounds 12-27 interact with trimethylaluminum to generate methane and form isolable “s, s” species 12-28 which are Z1-coordinated to the carborane ligand; further CH4 elimination yields the closo-aluminacarborane 12-29 which features novel Z1:Z5 (s,p) coordination [43,44]. + HN
Me Me
Me Me
N Me
Al
Me
H
N Me
Al
H
C C
12-27
Me
Me
C
AlMe3 R
C
–CH4 C = CH R = H, Me
12-28
R
C –CH4
12-29
C
714
CHAPTER 12 Heteroatom carboranes of the main group elements
The corresponding dimethylene-tethered constrained-geometry complex 12-31 does not undergo conversion to the Z1:Z5 species 12-32, but the latter compound can be obtained via trans-metallation of the titanium complex 12-33 with AlMe3 [43]. CH2Ph
PhCH2
+ HN
PhCH2
CH2Ph
CH2Ph
N Me
Al H
CH2Ph N
Al
H
C C
C
AlMe3 R
C
–CH4
C C
R
C = CH R = H, Me
12-30
12-32
12-31 PhCH2 Ti(NMe2)4
CH2Ph
AlMe3 −Ti(NMe2)2Me2
N Me2N
−2 NMe2H
Me2N
R
Ti C C
R
12-33
The experimental observation that Z1:Z5 coordination is achievable in the aluminum-dicarbollyl system, but is not observed in organoaluminum systems, is illuminated by SCF calculations showing that the dicarbollyl-amino ligands furnish stronger metal-ligand p-interaction than is possible in cyclopentadienyl complexes [43].
12.3.2 Gallium and indium 12.3.2.1 Seven-vertex cages The 1969 synthesis [45] of 1,2,3-MeGa(C2B4H6) (12-34) in 20-30% yield from nido-C2B4H8 and GaMe3 in the gas phase—the first confirmed insertion of a Group 13 metal into a carborane framework—was followed by the preparation of the analogous indacarborane 1,2,3-MeIn(C2B4H6) (12-35) via a similar reaction with InMe3 at 95-110 C [46]. An X-ray structural analysis of 12-34 [46] disclosed that the Ga-CH3 bond is tilted away from the vector perpendicular to the C2B3 plane by 23 , a feature attributed by MO calculations [47] to maximization of overlap between the bonding orbitals of the carborane ligand and the GaMe fragment. Me
Ga
H
C
H
C
GaMe3 215 C 2,3-C2B4H8
C = CH
C C
12-34
12.3 Heterocarboranes of the group 13 elements
715
The gallium and indium compounds are both thermally stable below 100 C but decompose at high temperatures to nidoC2B4H8 and other products. Anhydrous HCl attacks 12-34 to generate nido-C2B4H8 in 40-50% yield, but the indacarborane and HCl afford only solid products and H2. Both metallacarboranes undergo complete cage degradation on reaction with Br2 at room temperature [46]. Trimethylsilyl-substituted gallacarboranes related to 12-34 are prepared from the disodium or dilithium salts of the nido-2,3-ðMe3 SiÞ2 C2 B4 H4 2 dianion (Chapter 4), which reacts with trichlorogallane to give the half-sandwich and fullsandwich species 12-36 and 12-37, respectively; reaction stoichiometry dictates which product predominates [48,49]. −
+ Li(TMEDA)2 C N
Cl
Ga
C N
2 GaCl3 0 °C
Me3Si
(ML)2[(Me3Si)2C2B4H6] L = TMEDA
M = Li
C Me3Si
C6H6
GaCl3 0 °C
SiMe3
Ga
C6H6
Me3Si C
M = Na
C
SiMe3
Me3Si
C
12-36
12-37
The carbons-separated isomers, 12-38 and 12-39 are generated in a similar manner from [Na(TMEDA)þ]2[nido-2,4ðMe3 SiÞ2 C2 B4 H4 2 ], which is obtained via cage-opening of closo-1,2-(Me3Si)2C2B4H4 with lithium as described in Chapter 4 (see Figure 4–7). N Cl
+ Na(TMEDA) N
− Me3Si
C C
Ga 2 GaCl3 Me3Si
0 C C6H6
C
(NaL)2[2,4-(Me3Si)2C2B4H6] L = TMEDA
GaCl3
SiMe3
Ga
0 C C6H6 C
C
Me3Si
SiMe3
C
Me3Si
12-38
12-39
In each of these 7-vertex gallacarborane cages, the metal exhibits slip-distortion of the kind noted earlier for 12-24 (and in some transition metal complexes discussed in Chapter 13) wherein the metal is displaced away from the cage carbon ˚ longer than the Ga2 atoms so that the Ga2 2C distances are ca. 0.1 A 2B bonds. The slippage is greatest in the half-sandwich species 12-36 and 12-38, in which the metal-cage coordination approximates Z3 and Z2 binding, respectively [49]. Treatment of the carbons-separated dianion with tert-butyldichlorogallane dimer in THF affords the expected product 12-40 along with the unusual dimeric species 12-41 which contains a direct Ga2 2Ga bond [50]. Me3C SiMe3
4 [Na(THF)]2[2,4-(Me3Si)2C2B4H6]
Ga
2 (Me3C–GaCl2)2 0 C
THF
−C2Me6 −8 NaCl
C Me3Si
Me3Si
C SiMe3
C Ga
+
Ga
C 2:1
12-40
C SiMe3
C
12-41
SiMe3
716
CHAPTER 12 Heteroatom carboranes of the main group elements
Both gallacarboranes exhibit slip-distorted structures with slightly longer Ga2 2C than Ga2 2B distances, and the metal˚ in 12-41 is one of the shortest known Ga2 metal interaction of 2.343(2) A 2Ga bonds. Reaction of the dilithiocarborane salt [Li(THF)þ]2[nido-2,4-ðMe3 SiÞ2 C2 B4 H4 2 ] with the same gallium reagent forms only the monomer 12-40 via a simple metal-for-metal replacement; no 12-41 is obtained and there is no loss of the tert-butyl unit [50]. Similarly, lithium or sodium salts of the adjacent-carbon dianion nido-2,3-ðMe3 SiÞRC2 B4 H4 2 (R ¼ H, Me, or SiMe3) combine with (Me3GaCl2)2 to afford only 1,2,3-(Me3C)Ga[(Me3Si)RC2B4H4] products analogous to 12-40 [48]. Bis(trimethylsilyl) 7-vertex closo-indacarboranes that are structural counterparts of 12-34 and 12-40 have been prepared from the nido-2,3- and 2,4-carborane dianions [51], but no bis(carboranyl)indium sandwiches analogous to 12-37 and 12-39 have been reported. ðMe2 CHÞInI2
Liþ Naþ ðTHFÞ2 ½2;3-ðMe3 SiÞRC2 B4 H4 2 ! 1; 2; 3-ðMe2 CHÞIn½ðMe3 SiÞRC2 B4 H4 THF; 0 C
R ¼ H; Me; SiMe3
ðMe2 CHÞInI2
2 Liþ ! 1; 2; 4-ðMe2 CHÞIn½ðMe3 SiÞRC2 B4 H4 R ¼ Me; SiMe3 2 ðTHFÞ4 ½2;4-ðMe3 SiÞRC2 B4 H4 THF; 0 C
Both 1,2,3- and 1,2,4-(Me2CH)In[(Me3Si)2C2B4H4] (12-42) crystallize as dimers (unlike their gallium counterparts) and show even more pronounced slip-distortion than is seen in the corresponding gallium systems [52]. N N Me2HC H
In C
In Me3Si
N N In
SiMe3
H
C
Me2HC
C
H
Me3Si
SiMe3
C
H
12-43
12-42 CHMe2
Me3Si
C
C
Me3Si
Lewis bases such as bipyrimidine and bipyridine readily complex with the small galla- and indacarboranes, producing extremely distorted cages as in 12-43, in which electron donation from the base induces the metal to move out of bonding range of the cage carbons [48,51–53]. Molecular orbital calculations [51] indicate that this configuration increases the overlap of electron-pair donor orbitals on the nitrogen atoms with gallium acceptor orbitals, while at the same time increasing Ga2 2B bonding and decreasing the interaction of the metal with antibonding orbitals on the cage carbon atoms.
12.3.2.2 Twelve-vertex cages The synthesis and chemistry of icosahedral galla- and indacarboranes largely, though not entirely, parallel that of the AlC2B9 clusters described above. Triethylgallium and nido-7,8-C2B9H13 combine at elevated temperature to yield nido-7,8-C2B9H12-m-GaEt, a counterpart of 12-16, which like the latter species is fluxional in solution and on heating in the solid state is converted to a closo product, 3,1,2-EtGa(C2B9H11) [35]. However, the latter compound, in contrast to its aluminum analogue 12-17, does not cleanly isomerize on pyrolysis; at 350 C it is unchanged, and at 400 C it largely decomposes to C2B9H13 with a small amount of a product that is apparently 2,1,7-EtGa(C2B9H11) [35]. The reaction of Tlþ ½TlC2 B9 H11 with excess GaCl3 in toluene affords a commo-3-Ga(1,2-C2 B9 H11 Þ2 anion (12-44) [39,54] whose structure resembles that of its aluminum counterpart 12-26 but differs in two respects: unlike the latter compound, 12-44 is slip-distorted like the 7-vertex GaC2B4 systems just discussed, such that the gallium is bound primarily to
12.3 Heterocarboranes of the group 13 elements
717
the three boron atoms on the C2B3 face of each carborane ligand; moreover, in 12-44 the two ligands are in an eclipsed conformation while in 12-26 they are staggered [39]. Several thiolate-bridged indium-dicarbollyl dimers (Table 12-2) have been synthesized from Tlþ ½TlC2 B9 H11 and InCl3 or InX(SR2) reagents [55]. The molecular structures of these compounds follow the general pattern of 12-45, in which the metal is extremely slip-distorted and binds to only one boron atom; a full octet of electrons on each indium center is achieved through dimerization as shown. SPh In Tl+[Tl(C2B9H11)]−
C C
In(SPh)2I, (Ph3P)2NCl S S
THF C
In
C
12-45
PhS
C = CH
Constrained-geometry galla- and indacarborane aminomethyl-tethered complexes of the kind described above for aluminacarboranes have been prepared by similar routes, the main difference being that in the gallium and indium systems the reactions directly afford Z1:Z5 (s, p) products analogous to 12-29; although s,s-type intermediates corresponding to 12-28 are detected in NMR spectra, they are not isolable [43]. The carborane sandwich compounds of aluminum, gallium, and indium are notable in that the metal in nearly all cases is in a formal þ3 oxidation state, in contrast to most main group organometallic sandwiches such as Cp2MII in which the metal is formally þ2. In a simple qualitative sense, this testifies to the high degree of electron-delocalized covalent binding in the metallacarboranes, which is distinct from the predominantly ionic metal-ligand binding in cyclopentadienyl metal sandwiches. Consequently, it is entirely realistic to view ligand-metal units such as EtAl as formally replacing BH vertexes in heteroborane clusters as in the Wade-Mingos model outlined in Chapter 2.
12.3.3 Thallium With two exceptions, all known and characterized thallacarboranes are 12-vertex TlC2B9 systems. The diborolyl–cobalt sandwich complex 12-46 combines with Tl(C5H5) in refluxing THF to generate in 64% yield 12-47, in which a bare thallium atom occupies one apex of a pentagonal bipyramidal Tl(Me2HC3B2Me2)CoCp cluster [56]. Crystallographic analysis reveals that the thallium in this case is displaced away from the ring centroid toward the unique ring carbon located between the borons, which is the site of highest electron density.
Co
Me
12-46
C
B H
C
B
C
Me
THF
Co
Me
Tl(C5H5)
Me
C
B H
C
B
Me C
12-47 Me
Me Me
Tl
The other reported nondicarbollyl thallacarborane is TlAsC2B8H11, obtained in low yield from the reaction of nido-AsC2B8H11 with aqueous TlOH and presumed to have closed 12-vertex cage geometry, although structural characterization is lacking [57]. The icosahedral 3,1,2-Tl(C2B9H11) ion has been mentioned several times earlier in this section as a starting reagent in the synthesis of other metallacarboranes, a role in which it is particularly useful as its resident thallium atom is easily displaced
718
CHAPTER 12 Heteroatom carboranes of the main group elements
by other metals and nonmetals. This ion and its C-alkyl derivatives were originally prepared as Tlþ[Tl(RR0 C2B9H9)] (R¼H, 5Me) salts via the reaction of thallium(I) acetate with nido-7,8-RR0 C2 B9 H10 ions in aqueous alkaline soluR¼Me; R5 5R05 tion [41], and can also be obtained from nido-RR0 C2B9H11 derivatives and thallium ethoxide [58]. X-ray diffraction analyses of Ph3MePþ[3,1,2-Tl(C2B9H11)] [59,60] and Tlþ[3,1,2-Tl(Me2C2B9H11)] [58] confirm the distorted icosahedral geometry analogous to 12-17 and other closo-3,1,2-C2B9 cage systems described earlier. In the crystal structure of (Ph3P)2NþTl(C2B9H11) (12-48), the thallacarborane units form a dimer in which each thallium is 2H2 2B bridges to the other [61]. located over the C2B3 face of one dicarbollide ion while binding via a pair of Tl2 2-
H
C Tl
C
H
12-48 C = CH
H
Tl
C
C H
˚ , contrasting sharply with (Ph2PMe)2NþTl(C2B9H11) in which it is In this dimer the Tl—Tl separation is only 4.24 A ˚ 7.97 A, demonstrating how the choice of cation can exert a strong influence on structure. ˚ , respectively) exceed the calculated In all of these structures the long Tl2 2B and Tl2 2C distances (>2.6 and >2.9 A sum of covalent radii, suggesting that the Tl(C2B9H11) cluster is best viewed as a Tlþ C2 B9 H11 2 ion pair rather than as a covalently bound system [58,60]. This description is in accord with the facile displacement of thallium mentioned above; however, NMR data suggest that there is, in fact, a strong intramolecular interaction between the thallium ion and dicarbollide anion in MeCN solution [61]. Pyrolysis of Tlþ[3,1,2-Tl(RR0 C2B9H9)] salts in vacuum at 100 C affords mainly thallium metal and colorless, air-stable Tl(RR0 C2B9H10) products that are presumed to be nido-cage dimers linked by B-H-B bridges [62]. On further heating at higher temperatures, closo-RR0 C2B9H9 carboranes are generated. Strong protic acids decompose the Tlþ[3,1,2-Tl(RR0 C2B9H9)] salts to give nido-7,8-RR0 C2B9H11 and 2 Tlþ, but weak acids such as acetic generate Tl(RR0 C2B9H10) identical to that obtained on pyrolysis [62]. Tlþ ½3;1;2-TlðRR0 C2 B9 H9 Þ þ RCðOÞOH ! TlðRR0 C2 B9 H10 Þ þ RCðOÞOTl Only one report has appeared on thallium complexes of the “carbons-separated” dicarbollide ion, nido-7,9C2 B9 H11 2 . Treatment of Me3NHþ 7,9-C2 B9 H12 with aqueous KOH followed by thallium acetate at 20 C affords yellow, unstable Tl2[7,9-C2B9H11] which has not been characterized. However, the reaction of its C,C0 -dianilide derivative with thallium ethoxide gives a stable product containing three thallium atoms, in which it is presumed that the NH protons in the anilide groups are replaced by thallium [63]. THF; Et2 O
Csþ 7; 9-½PhNHðCOÞ2 C2 B9 H 10 þ TlOEt ! CsTl3 f7; 9-½PhNðCOÞ2 C2 B9 H9 g
12.4 HETEROCARBORANES OF THE GROUP 14 ELEMENTS A curious aspect of Group 14 heterocarborane chemistry is that silicon, aside from carbon the lightest and most abundant of these elements, was the last to be fully integrated into a carborane cage system. Even now, the chemistry of silicacarboranes is less developed than that of its congeners germanium and tin, as will be apparent from the listings in Table 12-3. Much of the interest in the Group 14 heterocarboranes derives from comparisons of their structure and reactivity with those of closely related organometallic sandwich compounds such as Cp2Si, Cp2Ge, and Cp2Sn.
12.4 Heterocarboranes of the group 14 elements
719
Table 12-3 Heterocarboranes of the Group 14 Elements Synthesis and Characterization Compounda
Information
References
1,2,3-Si[(Me3Si)2C2B4H4] 1-Si[2,3-(Me3Si)MeC2B4H4]2 1-Si[2,3-(Me3Si)C2B4H5]2 nido?-1,2,3-H(Cl)Si[(Me3Si)MeC2B4H4] nido?-1,2,3-H2Si[(Me3Si)MeC2B4H4]
S S, S, S, S, S, S,
[66,67] [66] [66] [67] [67] [67] [67]
12-vertex SiC2B9 clusters 3-Si(1,2-C2B9H11)2 nido-(C2B9H10)-Si(C2B9H11)-10-BH(PMe3)2 3,1,2-Me(m-OCH2)Si(MeC2B9H9)
S, X, H, B, C, IR, MS S, X S, H, B, C, IR
[71] [71] [75]
S, S, S, S, S, S,
X, H, B, C, Si, IR, MS H, B, C, Si, IR, MS H, B, C, Si, IR, MS X, H, B, C, IR X(Me3Si), H, B, C, IR X(Me3Si), H, B, C, IR
[76,77] [77] [77] [78] [78] [78]
S, S, S, S, S, S, S, S, S,
H, B, C, IR H, B, C, IR B X, H, B, C, Si, IR, MS X, H, B, C, Si, IR, MS X, H, B, C, Si, IR, UV, MS X, H, B, C, Si, IR, MS X(Me3Si, Me), H, B, C, MS X(Me3Si), H, B, C, IR
[78] [78] [78] [212,213] [79] [213] [213] [100] [101]
SILICON 7-vertex SiC2B4 clusters 1-Si[(Me3Si)2(2,3-C2B4H4]2
GERMANIUM 7-vertex GeC2B4 clusters 1-Ge[2,3-(Me3Si)2C2B4H4]2 1-Ge[2,3-R(Me3Si)C2B4H4]2 R ¼ H, Me 1-Ge[2,3-Me(Me3Si)C2B4H4]2 25-GeCl3] R ¼ SiMe3, Me, H 1,2,3-Ge[(Me3Si)RC2B4H32 25-GeCl3] R ¼ SiMe3, Me 1,2,3-(C10H8N2)Ge[(Me3Si)RC2B4H32 25-GeCl3]}2 m-C8H6N4-{1,2,3-Ge[(Me3Si)RC2B4H32 R ¼ SiMe3, Me 25-GeCl3] R ¼ SiMe3, Me 1,2,3-(C15H11N3)Ge[(Me3Si)2C2B4H32 1,2,3-CpFe(C5H4)CH2NMe2-Ge[(Me3Si)2C2B4H3-5-NMe2] 1,2,3-Ge[(Me3Si)RC2B4H4] R ¼ SiMe3, Me, H 1,2,3-(2,20 -bipyridine)[Ge(Me3Si)2C2B4H4] 1,2,3-Ge[(Me3Si)2C2B4H3]-GeCl3 1,2,3-(2,20 -bipyridine)Ge[(Me3Si)RC2B4H4] R ¼ Me, H 1,2,3-Ge[(Me3Si)RC2B4H4] R ¼ Me3Si, Me, H 1,2,3-(2,20 -bipyrimidine)Ge[(Me3Si)RC2B4H4] R ¼ Me3Si, Me, H 1,2,3-[CpFe(C5H4)CH2NMe2]-Ge[(Me3Si)RC2B4H4] R ¼ Me3Si, Me, H
X, H, B, C, Si, IR, MS H, B, C, IR, MS X, H, B, C, Si, IR, MS H, B, C, Si, IR, MS H, B, C, Si, IR, MS H, B, C, Si, IR, MS
8-vertex GeC2B5 clusters Ge(C2B5H7-nMen) n ¼ 2-6 Wedged-GeFe(Me2C2B4H4)2
S, MS S, H, B, IR, MS
[80] [81]
12-vertex GeCB10 clusters 1,2-Ge(CB10H11) 1,2-MeGe(CB10H11)
S, H, B, IR S, H, B, C, IR, MS
[90] [90] Continued
720
CHAPTER 12 Heteroatom carboranes of the main group elements
Table 12-3 Heterocarboranes of the Group 14 Elements—Cont’d Synthesis and Characterization Compounda
Information
References
1,2-EtGe(CB10H11) 1,2-(CO)5M]-Ge(CB10H11) M ¼ Cr, Mo, W 1,2-Cp(CO)2Fe-Ge(CB10H11) 1,2-(Z7-C7H7)(CO)2Mo-Ge(CB10H11)
S, S, S, S,
H, B, IR B, IR B, H, IR B, H, IR
[90] [90] [91] [91]
12-vertex GeC2B9 clusters 3,1,2-Ge(C2B9H11) 2,1,7-Ge(C2B9H11) 3,1,2-Ge(MeRC2B9H9) R ¼ H, Me 3,1,2-Ge(Me2C2B9H9) 3,1,2-ClGe(Me2C2B9H9) 3,1,2-ClGe(Me2C2B9H9)2 3,1,2-Me(m-OCH2)Ge(MeC2B9H9)
S, S, H S, S, S, S,
H, B, IR, MS H, B, IR, MS
[85] [87] [85] [38,88] [88] [88] [75]
12-vertex GePCB9 and GeAsCB9 clusters 3,1,2-GeE(CB9H10) E ¼ P, As 2,1,7-GeE(CB9H10) E ¼ P, As
S, H, B, IR S, H, B, IR
[92] [92]
S, MS S, H, B, IR, MS S, H, B, C, Si, Sn, Mo¨ssbauer, IR, MS X S, H, B, C, Si, Sn, Mo¨ssbauer, IR, MS X S, X S, X, H, B, C, Sn, IR, MS S, H, B, C, Sn, IR, MS S, H, B, C, Sn, IR, MS S, X(Me3Si), H, B, C, Sn, IR, MS S, X(Me3Si), H, B, C, Sn, IR, MS
[93] [93] [94] [214] [94] [215] [95] [105] [97] [97] [97] [97]
S, H, B, C, Sn, IR, MS S, X, H, B, C, Si, Sn, IR, MS S, H, B, C, Si, Sn, IR, MS S, H, B, C, Si, Sn, IR, MS X S, H, B, C, Si, Sn X, S, H, B, C, Si, Sn S, X, H, B, C, Si, Sn, IR, MS S, X(Me3Si), H, B, C, Si, Sn, IR, MS
[97] [98] [98] [98] [216] [98] [98] [99,217] [99,217]
TIN 7-vertex SnC2B4 clusters 1,2,3-Sn(C2B4H6) 1,2,3-Sn(Me2C2B4H4) 1,2,3-Sn[(Me3Si)2C2B4H4] 1,2,3-Sn[(Me3Si)MeC2B4H4] 1-Sn[(Me3Si)2(2,3-C2B4H4]2 1-Sn[2,3-(Me3Si)MeC2B4H4]2 1,2,4-Sn(RR0 C2B4H4) R, R0 ¼ H, Me3Si 1,2,4-(2,20 -bipyridine)Sn[(Me3Si)2C2B4H4] 1,2,4-(2,20 -bipyrimidine)Sn(RR0 C2B4H4) R,R0 ¼ Me, Me3Si 1,2,4-[CpFe(C5H4)CH2NMe2]-Sn(RR0 C2B4H4) R,R0 ¼ Me, Me3Si 1,2,4-(terpyridine)Sn[(Me3Si)2C2B4H4] 1,2,3-(2,20 -bipyridine)Sn[(Me3Si)2C2B4H4] 1,2,3-(2,20 -bipyridine)Sn[(Me3Si)HC2B4H4] 1,2,3-(2,20 -bipyridine)Sn[(Me3Si)MeC2B4H4] 1,2,3-(THF)2Sn[(Me3Si)RC2B4H4] R ¼ Me, Me3Si 1,2,3-Sn[(Me3Si)HC2B4H4] 1,2,3-(2,20 -bipyrimidine)Sn[(Me3Si)2C2B4H4] 1,2,3-(2,20 -bipyrimidine)Sn[(Me3Si)RC2B4H4] R ¼ H, Me, Me3Si
H, B, C, IR X, H, B, C, P X, H, B, C, P H, B, C, IR
Continued
12.4 Heterocarboranes of the group 14 elements
721
Table 12-3 Heterocarboranes of the Group 14 Elements—Cont’d Synthesis and Characterization Compounda
Information
References
1,2,3-(CpFeC5H4-CH2NMe2)-Sn[(Me3Si)RC2B4H4] R ¼ H, Me, Me3Si 1,2,3-(terpyridine)Sn[(Me3Si)RC2B4H4] R ¼ Me, Me3Si
S, X(Me3Si), H, B, C, Sn, MS
[99]
S, X(Me3Si), H, B, C, Sn, UV, MS
[102]
7-vertex open SnC2B4 clusters “nido/arachno”-1,2,3-(terpyridine)Sn[(Me3Si)MeC2B4H4]
X
[218]
8-vertex SnMC2B4 clusters CpCoSn(Me2C2B4H4)
S, H, B, IR, MS
[93]
8-vertex open SnMC2B4 clusters Wedged-SnFe(Me2C2B4H4)2
S, H, B, IR, MS
[81]
11-vertex SnC2B8 clusters 1,2,3-Sn(C2B8H10) 1,2,3-Sn(RC2B8H9) R ¼ Me, Ph 1,2,3-Sn(Me2C2B8H8) R ¼ Me, Ph 1,2,3-R2Sn(C2B8H10) R ¼ Me, Ph 1,2,3-Me2Sn(C2B8H9)-2-Me 1,2,3-Sn(C2B8H10) arachno-1,2,3-Me2Sn(C2B8H10)
S, H, B(2d) S, H, B(2d) S, H, B(2d) H, B H, B H, B H, B
[111] [111] [111] [110] [110] [110] [109]
12-vertex closo-SnCB10 clusters 2,1-Sn(CB10H11) (Ph3P)4Au4(SnCB10H11)2 (Ph3P)2Rh(SnCB10H11)3
S, X, H, B(2d), C, Sn, Mo¨ssbauer S, X, B, P, Sn S, X, B, P, Sn
[112] [112] [112]
Mo¨ssbauer S, H, B, IR, MS S (mechanico-chemical) Doping in pseudoisocynanine solid films S, H, B, C UV Sn S, H, B, C, Sn S, H, B, C, Sn S, H, B, C, Sn Raman
[116,117] [85] [219] [220] [88,115] [115] [88] [38] [88] [88] [221]
S, H, B, C, UV S, X, UV
[115] [115]
12-vertex closo-SnC2B9 clusters 3,1,2-Sn(C2B9H11) 3,1,2-Sn(C2B9H11)
3,1,2-Sn(Me2C2B9H9)
3,1,2-ClSn(Me2C2B9H9) 3-Sn(1,2-Me2 C2 B9 H9 Þ2 2
12-vertex nido-SnC2B9 clusters nido-1,2,3-(pyridine)2Sn(Me2C2B9H9) nido-1,2,3-(2,20 -bipyridine)Sn(Me2C2B9H9)
Continued
722
CHAPTER 12 Heteroatom carboranes of the main group elements
Table 12-3 Heterocarboranes of the Group 14 Elements—Cont’d Synthesis and Characterization Compounda
Information
References
nido-1,2,3-(o-phen)Sn(Me2C2B9H9) nido-1,2,3-(TMEDA)Sn(Me2C2B9H9) nido-1,2,3-(THF)Sn(Me2C2B9H9) nido-1,2,3-(THF)2Sn(Me2C2B9H9) nido-1,2,3-(Ph3P)Sn(Me2C2B9H9) nido-1,2,3-(MeCN)nSn(Me2C2B9H9) nido-1,2,3-Me2Sn(Me2C2B9H9)
S, S, S, S, S, S, S,
[115] [115] [115] [115] [115] [115] [38]
13-vertex SnC2B10 clusters 4,1,2-Sn{[o-C6H4(CH2)2]C2B10H10} 4,1,2-LSn{[o-C6H4(CH2)2]C2B10H10} L ¼ MeCN, C4H8O,2 2OCH2CH2O2 2 4,1,2-LSn[(CH2)3C2B10H10] L ¼ 2,20 -bipyr, 1,10phenanthroline, 4,40 -diphenylpyridine 4,1,2-(bipyr)Sn{[C6H4(CH2)2]C2B10H10} 4,1,6-Sn(R2C2B10H10) R ¼ H, Me 4,1,6-L2Sn(Me2C2B10H10) L ¼ 2,20 -bipyridine, 1,10phenathroline, 4,40 -dimethylbipyridine, 4,40 -diphenylbipyridine 4,1,10-Sn(Me2C2B10H10) 4,1,10-(bipyr)Sn(Me2C2B10H10) 4,1,12-Sn(Me2C2B10H10) 4,1,12-(bipyr)Sn(Me2C2B10H10) LEAD 7-vertex PbC2B4 clusters 1,2,3-Pb(R2C2B4H4) R ¼ H, Me 1,2,3-Pb[(Me3Si)2C2B4H4] 1,2,3-Pb[(Me3Si)(R)C2B4H4] R ¼ H, Me, Me3Si 1,2,3-(2,20 -bipyridine)Pb[(Me3Si)RC2B4H4] R ¼ Me3Si, Me, H 1,2,3-(2,20 -bipyridine)Pb[(Me3Si)MeC2B4H4] m-(2,20 -bipyrimidine)Pb[(Me3Si)RC2B4H4]2 R ¼ Me3Si, Me, H 2Pb[(Me3Si)RC2B4H4] 1,2,3-[CpFe(C5H4)CH2NMe2]2 R ¼ Me3Si, Me, H 2Pb[(Me3Si)MeC2B4H4] 1,2,3-[CpFe(C5H4)CH2NMe2]2 [1,2,4-(TMEDA)Pb[(Me3Si)2C2B4H4]2 12-vertex PbC2B9 clusters 1,2,3-Pb(C2B9H11) Theoretical Studies Molecular and electronic structure calculations closo-X2B10H10, nido-X2B9H11 X ¼ CH, SiH, GeH, SnH, PbH
UV H, B, C, UV X, H, B, C, UV H, B H, B, C, UV H, B H, B, C, Sn, MS
S, H, B, C, Sn, IR S, X, H, B, C, Sn, IR
[119] [119]
S, X, H, B
[120]
S, X, H, B S, X, H, B, C, Sn, MS S, X, H, B, C, IR, MS
[120] [118] [122]
S, S, S, S,
X, H, B X, H, B H, B X, H, B
[121] [121] [121] [121]
S, S, S, S, X S, S,
H, B, IR, MS X, H, B, C, Pb, IR, MS X(Me, Me3Si), H, B, C X(Me3Si), H, B, C
[93] [222] [96] [96] [223] [100] [101]
X(Me, Me3Si), H, B, C, MS X(Me3Si), H, B, C, IR
X S, X, H, B, C, IR
[224] [95]
S, H, B, IR, MS
[85]
DFT, isomer energies
[225] Continued
12.4 Heterocarboranes of the group 14 elements
723
Table 12-3 Heterocarboranes of the Group 14 Elements—Cont’d Synthesis and Characterization Compounda
Information
References
Si(CB3H5), Si(CB4H6), Si(CB5H7) Si(CB5H7) 2Si(CB5H6) (CB5H6)Si2 1,2,3-Sn(Me2C2B4H4) R ¼ H, Me, SiMe3 1,2,3-(20 ,200 -bipyridine)Sn(C2B4H6) 1,2,3-(20 ,200 -bipyridine)Sn(Me2C2B4H4) 1,2,3-H2Si(C2B4H6)
Ab initio DFT, bonding and geometry DFT, bonding and geometry MNDO MNDO Structural distortions Molecular conformation Heat of formation MNDO-SCF Ab initio Stability, heat of formation MNDO-SCF Ab initio, bond lengths DFT: relative energies; energy penalties DFT: relative energies; energy penalties DFT, bonding and geometry DFT, bonding and geometry Ab initio Ab initio, bond lengths Extended Hu¨ckel DFT: relative energies; energy penalties DFT: relative energies; energy penalties DFT, bonding and geometry DFT, bonding and geometry Ab initio MNDO-SCF
[127] [226] [226] [104] [104] [103] [68,69] [68] [73] [227] [68] [73] [228] [229] [229]
MNDO DFT: relative energies; energy penalties DFT: relative energies; energy penalties DFT, Sn-cage bonding
[97] [229]
[120]
DFT, Sn-cage bonding DFT (fluxionality) DFT
[120] [118] [122]
1-Si(2,3-C2B4H6)2 1-Si(2,3-RR0 C2B4H4)2 R, R0 ¼ SiMe3, Me Si2(C2B2H4) 3-Si(1,2-C2B9H11)2 1,2-Si(CB10H12) nido-7,8,9/7,8,11/7,8,10/7,9,10-Si(C2B8H10) nido-7,8,9/7,8,11/7,8,10/7,9,10-HSi(C2B8H10) Ge(CB5H7) 2Ge(CB5H6) (CB5H6)Ge2 Ge(RR0 C2B4H4)2 R, R0 ¼ SiMe3, Me 1,2-Ge(CB10H)12 Wedged-GeFe(Me2C2B4H4)2 nido-7,8,9/7,8,11/7,8,10/7,9,10-Ge(C2B8H10) nido-7,8,9/7,8,11/7,8,10/7,9,10-HGe(C2B8H10) Sn(CB5H7) 2Sn(CB5H6) (CB5H6)Sn2 Sn(RR0 C2B4H4)2 R, R0 ¼ SiMe3, Me 1,2,3-(C15H11N3)Sn[(Me3Si)RC2B4H4] R ¼ Me, Me3Si; C15H11N3 ¼ terpyridine) 1,2,4-Sn(RR0 C2B4H4) R, R0 ¼ H, SiMe3 nido-7,8,9/7,8,11/7,8,10/7,9,10-Sn(C2B8H10) nido-7,8,9/7,8,11/7,8,10/7,9,10-HSn(C2B8H10) 4,1,2-LSn[(CH2)3C2B10H10] L ¼ 2,20 -bipyr, 1,10-phenanthroline, 4,40 -diphenylpyridine 4,1,2-(bipyr)Sn{[C6H4(CH2)2]C2B10H10} 4,1,6-Sn(C2B10H12) 4,1,6-L2Sn(C2B10H12) L ¼ 2,20 -bipyridine, 1, 10-phenathroline, 4,40 -dimethylbipyridine, 4,40 -diphenylbipyridine
[226] [226] [227] [228] [230] [229] [229] [226] [226] [227] [102]
[229]
Continued
724
CHAPTER 12 Heteroatom carboranes of the main group elements
Table 12-3 Heterocarboranes of the Group 14 Elements—Cont’d Synthesis and Characterization Compounda
Information
References
Ab initio Fenske-Hall
[227] [101]
Donor-acceptor properties 2,1-Sn(CB10H11), 3,1,2-Sn(C2B9H11)
DFT
[114]
NMR calculations 1,2,3-Ge(C2B4H5)-5-X X ¼ H, GeCl3 1,2,3-(H3N)Ge(C2B4H4)-5-X X ¼ H, GeCl3
B (GIAO) B (GIAO)
[78] [78]
0
0
Pb(RR C2B4H4)2 R,R ¼ SiMe3, Me 2Pb[(Me3Si)RC2B4H4] 1,2,3-[CpFe(C5H4)CH2NMe2]2 R ¼ Me3Si, Me, H
S ¼ synthesis, X ¼ X-ray diffraction, H ¼ 1H NMR, B ¼ 11B NMR, C ¼ 13C NMR, F ¼ 19F NMR, P ¼ 31P NMR, Si ¼ 29Si NMR, Li ¼ 7Li NMR, 2d ¼ two-dimensional (COSY) NMR, IR ¼ infrared data, MS ¼ mass spectroscopic data, UV ¼ UV-visible data, E ¼ electrochemical data, ESR ¼ electron spin resonance data, MAG ¼ magnetic susceptibility, COND ¼ electrical conductivity, OR ¼ optical rotation. a Heteroatoms (other than carbon) incorporated into the cluster framework are in boldface.
12.4.1 Silicon A class of derivatives that are not silacarboranes per se, in that silicon is not part of the electron-delocalized framework, consists of the nido-2,3-RR0 C2B4H5-m(4,5)-MR00 3 species (M ¼ Si, Ge, Sn, Pb) described in Chapter 4 and depicted in Figure 4-5, that features three-center, two-electron B-M-B binding at a boron–boron edge [64]; a related compound in which silicon bridges two carborane units is m, m0 -SiH2(C2B4H4)2 [65]. On mild heating, the silicon- and germaniumbridged 2,3-C2B4H7-m(4,5)-MMe3 complexes rearrange to their thermodynamically preferred terminally bound B(4)-MMe isomers, but the tin- and lead-bridged species do not, even under forcing conditions [64,65]. In contrast to a structurally similar iron-bridged complex C2B4H7-m(4,5)-Fe(CO)2Cp (4-22), which easily converts to a 7-vertex FeC2B4 closo cluster, the Group 14-bridged species show no such behavior. True silacarborane clusters, obtained from the reaction of Naþ Liþ ½ðMe3 SiÞRC2 B4 H4 2 salts with SiCl4 in THF, include the 7-vertex commo-metallacarboranes SiIV[(Me3Si)RC2B4H4] (12-49) and a single-cage minor product SiII[(Me3Si)2C2B4H4] (12-50) isolated in 1% yield, in which the formal oxidation state of silicon is þ4 and þ2, respectively [66,67]. X Me3Si R
12-49 R = H, Me, SiMe3
C C
Si
Si
Si
Me3Si C C
R SiMe3
C
C
Me
C
H
C
Me3Si
Me3Si
12-50
12-51 X = Cl 12-52 X = H
The interaction of Naþ Liþ ½ðMe3 SiÞMeC2 B4 H4 2 with SiH2Cl2 affords a silicon(IV) product, Cl(H)Si[(Me3Si)2C2B4H4] (12-51) which combines with NaH in THF to give (H)2Si[(Me3Si)2C2B4H4] (12-52), both clusters 2H2 2B bridging [67]. The structures in 12-51 and 12-52 have not exhibiting 29Si NMR spectra that suggest possible Si2 been established, and the skeletal electron-counting and assignment of silicon oxidation state are uncertain. If the SiHX (X ¼ Cl or H) groups in these species are regarded as two-electron, two-orbital donors to cage bonding (i.e., the Si2 2H
12.4 Heterocarboranes of the group 14 elements
725
and Si2 2Cl bonds are strictly exo-polyhedral), then these clusters are 7-vertex 16-electron (2n þ 2) systems which would be expected to have closo geometry according to the PSEPT rules discussed in Chapter 2. However, if an Si2 2H hydrogen and its associated two electrons are assimilated into the cage, the SiHX unit becomes a formal four-electron, three-orbital donor to skeletal bonding and presumably affords a 7-vertex nido-SiC2B4 cage. Although the latter model appears more consistent with NMR evidence, ab initio [68] and molecular mechanics calculations [69] on parent H2SiC2B4H6 favor a closo structure with two terminal Si-H bonds. Conceivably, the apparent discrepancy might be explained by effects of the SiMe3 and Me groups in 12-52 that stabilize a nido cage geometry [68]. The bonding is clearer in the commo-silacarboranes 12-49 [67] and in the analogous 12-vertex silicon sandwich 3-Si (1,2-C2B9H11)2 (12-53), which is prepared from SiCl4 and 7,8-C2 B9 H11 2 ion in refluxing benzene [70,71]. In each of these complexes, the C2B3 faces coordinated to silicon are essentially parallel, as is also the case in the silocenes, Si(Z5-C5R5)2 [72]; like the latter species, MNDO-SCF studies show that the silacarborane sandwiches are bound mainly via overlap between the p orbitals on silicon with p molecular orbitals on the ligands [73]. There are, however, significant differences: unlike the silocenes, the silacarboranes are slip-distorted with Si closer to boron than carbon atoms in the bonding faces, and silicon adopts a þ4 oxidation state in contrast to the Si(Z5-C5R5)2 complexes in which it is þ2. Both the slip-distortion and the stable Si(IV) oxidation state in 12-49 and 12-53 are accounted for in terms of LUMO antibonding interactions that are not present in the corresponding silocene HOMOs [74].
Me C
M
C Si C
C
O
12-53 C = CH
CH2 C C
12-54 Me
M = Si, Ge
Spectroscopically characterized constrained-geometry complexes 12-54 have been constructed from the nido-7,8ðOCH2 ÞMeC2 B9 H9 3 trianion and MeMCl3 (M ¼ Si, Ge) reagents [75].
12.4.2 Germanium 12.4.2.1 Seven-vertex cages
The reaction of Liþ[2,3-(Me3Si)2C2B4H5] with GeCl4 in a 2:1 ratio in THF yields a mixture of the commogermacarborane 1-GeIV[2,3-(Me3Si)2C2B4H4]2 (12-55) and the half-sandwich 1,2,3-GeII[(Me3Si)2C2B4H4] (12-56), which are structural and electronic analogues of 12-49 and 12-50, respectively [76]. An alternative solvent-free approach employing displacement of tin from 1,2,3-Sn[(Me3Si)RC2B4H5] substrates affords only the bis(carboranyl) product 12-55 and derivatives thereof [77]. 150-160 C 1; 2; 3-Sn½ðMe3 SiÞRC2 B4 H4 þ GeCl4 ! 1-GeIV ½2; 3-ðMe3 SiÞRC2 B4 H4 2 no solvent
R ¼ H; Me; SiMe3
The crystallographically determined structure of 12-55 is very similar to that of its silicon counterpart 12-49, except that 12-55 exhibits greater slip-distortion of the metal away from carbon and toward the boron atoms. In contrast, treatment of the same stannacarborane reagents with a slight excess of GeCl4 forms the GeCl3-substituted closo-germacarboranes 1,2,3-GeII[(Me3Si)RC2B4H3]-5-GeIVCl3 (12-56), in which the cage germanium atom in all three derivatives is located directly over the ring centroid [78,79]. The absence of slip-distortion in these clusters has been attributed to weakened cage Ge-B(5) bonding as a consequence of electron withdrawal by the GeIV-Cl3 moiety [79].
726
CHAPTER 12 Heteroatom carboranes of the main group elements Ge GeCl3 C
Me3Si
12-56
C
R = H, Me, SiMe3
R
The presence of both Ge(II) and Ge(IV) centers in 12-56 leads to some interesting chemistry involving adduct formation with Lewis bases. Interaction with 2,20 -bipyridine, 2,20 -bipyrimidine, terpyridine, and other bases results in complexation exclusively at the divalent germanium atom in the carborane cage to give structures such as 12-57 and 12-58 [78].
N
Cl3Ge
Me3Si N
N C
Ge
Ge GeCl3
GeCl3 C R
Ge N
C
Me3Si
N N
C
R
C SiMe3
R
C
12-57
R = H, Me, SiMe3
12-58
In these complexes, as in their counterparts lacking an external GeCl3 substituent (Table 12-3), the coordination of the Lewis base to the metal weakens the binding of germanium to the cage carbons trans to the electron-donor nitrogen atoms, resulting in extreme slip-distortion and effective Z3 metal-carborane coordination, a situation not uncommon in base adducts of Group 13 and 14 metallacarboranes (e.g., 12-36, 12-38, and 12-43). Despite the presence of a lone pair of electrons on the Ge(II) atoms in 12-57, 12-58, and other such complexes, base coordination takes place only at this metal center; the GeIVCl3 group, while not itself complexed, is considered to enhance the Lewis acidity at the apical Ge(II) via electron withdrawal from the cage [78].
12.4.2.2 Eight-vertex cages Reactions of Me2GeH2 have given evidence of methyl derivatives of GeC2B5H7 which were not characterized [80]. An unusual heterobimetallic complex, GeIIFeII(2,3-Me2C2B4H4)2 (12-59) containing germanium in a wedged position between two carborane ligands in a bis(carboranyl)iron sandwich, has been prepared from the reaction of GeI2 with 1-HFeII(2,3-Me2 C2 B4 H4 Þ2 in THF at 0 C [81]. Me Me
C C
Fe
Me
C Me
C
Ge
12-59
12.4 Heterocarboranes of the group 14 elements
727
This species and its Fe-Sn counterpart have been spectroscopically characterized as structural analogues of the diiron complex 11-19 (Chapter 11) [82], a manganese-wedged complex, (TMEDA)Mn2[(SiMe3)2C2B4H4]2 [83], and an iron–cobalt system, CpCoIIIFeII(2,3-Me2C2B4H4)2 (Chapter 13), in which one boron atom occupies a wedging location [84], all of which have been structurally established by X-ray crystallography. NMR data suggest that 12-59 is fluxional in solution, with the germanium atom migrating between equivalent FeB2 faces on the NMR time scale [81]. This system is isoelectronic with its precursor H2FeII(2,3-Me2C2B4H4)2 if one assumes a net two-electron contribution to skeletal bonding from Ge, which replaces the electrons supplied by the two metal-bound hydrogen atoms.
12.4.2.3 Twelve-vertex cages The first clear examples of heterocarboranes containing heavier Group 14 elements in the cage framework were the icosahedral 3,1,2-M(C2B9H11) series (M ¼ Ge, Sn, or Pb), synthesized by Rudolph and coworkers in 1969 via reactions of nido-7,8-C2 B9 H11 2 dianions with metal dihalides [85,86]. The 2,1,7-Ge(C2B9H11) isomer with nonadjacent carbon atoms is similarly prepared from nido-7,8-C2 B9 H11 2 and GeI2 [87]. An alternative approach involving replacement of aluminum in 3,1,2-EtAl(Me2C2B9H9) by germanium to form 3,1,2-Ge(Me2C2B9H9) (12-22) [38] was described earlier in this section. In each of these systems, a bare metal atom formally replaces a BH unit in a C2B10 cage, donating two of its four valence electrons and thus maintaining a 26 skeletal electron closo 12-vertex cage structure. The thallium reagent Tlþ[3,1,2-Tl(Me2C2B9H11)] mentioned in the previous section can also be employed to make 12-22 via reaction with GeCl2. The triphenylmethylphosphonium salt, however, on treatment with GeCl2 or SnCl2 generates the metallacarborane anions ClGe(Me2C2B9H11) (12-60) and ClSn(Me2C2B9H11) (12-61), the first of which has been shown crystallographically to be slip-distorted as in 12-57 with the metal binding essentially only to the boron atoms [88]. Cl M
Ph3MeP+ −
Ph3MeP+ Tl(Me2C2B9H9)−
MCl2 −TlCl
C C
Me Me
12-60 M = Ge 12-61 M = Sn
Another class of 12-vertex GeC2B9 compounds, the constrained-geometry complexes 12-54, was mentioned earlier in this section. Shortly after the original syntheses of Group 14 MC2B9 clusters, Todd and coworkers inserted germanium into the nido-CB10 H11 anion to obtain a closed 12-vertex MeGe(CB10H11) system in which the MeGe moiety supplies three electrons to the cage, thereby functioning as a surrogate for CH [89,90]. Treatment of this product with piperidine removes the methyl group to generate the 1,2-Ge(CB10H11) anion, each of these species preserving the icosahedral 26-electron cage architecture. An alternative approach employing GeCl4 followed by reaction with alkyl halides affords a higher yield of 1,2-MeGe(CB10H11), as well as its EtGe analogue [90]. MeGeCl3 Na3[CB10H11(THF)2] ⎯⎯⎯→ 1,2-MeGe(CB10H11)
NC5H10
1,2-Ge(CB10H11)−
MeI
The Ge(CB10H11) anion combines with cationic transition-metal complexes to generate exo-metallated species such as 1,2-[(CO)5M]-Ge(CB10H11) (M ¼ Cr, Mo, W) and others listed in Table 12-3 [90,91]. Still another facet of maingroup heterocarborane chemistry, illustrating its extraordinary versatility, is the synthesis of phosphagerma- and arsagermacarboranes 3,1,2 and 2,1,7-GeE(CB9H9) (12-62 to 12-65, E ¼ P, As) from the nido-7,8- and 7,9-ECB9 H9 2 anions described in the following section [92].
728
CHAPTER 12 Heteroatom carboranes of the main group elements Ge 2–
7,8-ECB9H10 E = P, As
GeI2
H
C
12-62 E = P 12-63 E = As
E
C6H6 reflux
Ge 2–
7,9-ECB9H10 E = P, As
GeI2
H
C
E
C6H6
12-64 E = P 12-65 E = As
reflux
These compounds, isolated as air-stable crystalline solids and characterized spectroscopically, are structural analogues of the dicarbon clusters 3,1,2- and 2,1,7-Ge(C2B9H9) with P or As formally replacing an isoelectronic CH unit. Pyrolysis of 12-64 affords still other unidentified cage isomers [92].
12.4.3 Tin and lead To a considerable extent the syntheses, structures, and reactions of the stannacarboranes parallel those of the germanium systems, although notable differences do arise, especially in the synthetic routes employed, where the tendency of Sn(II) halides to undergo reduction to metallic tin is often seen. A general observation one can make is that the usual variation in properties within the members of a given main group of the Periodic Table, familiar to generations of general chemistry students, tends to be considerably mitigated when these elements are incorporated into electron-delocalized polyhedral boron clusters. The large degree of similarity between sila-, germa-, stanna-, and plumbacarboranes is a case in point, and an even more striking example is seen in the Group 15 heterocarboranes described in the following section.
12.4.3.1 Seven- and eight-vertex cages
Small stanna- and plumbacarboranes were first prepared in low yield by treatment of nido-RRC2 B4 H5 ions (R ¼ H or Me) with SnCl2 or PbBr2 in THF solution to afford closo-1,2,3-M(R2C2B4H4) (M ¼ Sn, Pb; R ¼ H, Me), characterized by NMR, IR, and mass spectroscopy as closo 7-vertex cages that are isostructural with the silacarborane 12-50 [93]. (The corresponding reactions with GeI2 give unstable brownish-red products whose mass spectra correspond to GeC2B4 clusters but have not been characterized.) The colorless tin compounds and yellow lead species are air-sensitive solids; the former are thermally stable at room temperature, but the lead clusters slowly decompose on standing in vacuo. Insertion of cobalt into Sn(Me2C2B4H4) via reaction with CpCo(CO)2 generates the 8-vertex cobaltastannacarborane CpCoSn(Me2C2B4H4) (12-66), a red solid whose proposed structure is based on NMR and other spectroscopic data. Although it is air-stable in the solid state, in solution 12-66 loses tin to form the known cobaltacarborane 1,2,3-CpCo (Me2C2B4H4) (Chapter 13) [93].
Sn Co
CpCo(CO)2 Me
C
C
150 C
C
Sn
12-66
Me Me
C
Me
Co
−Sn
Me
C Me
C
12.4 Heterocarboranes of the group 14 elements
729
The reaction of Pb(Me2C2B4H4) with CpCo(CO)2 affords only CpCo(Me2C2B4H4), with no apparent formation of a cobalt-lead analogue of 12-66. However, the tin-wedged 8-vertex cluster SnFe(Me2C2B4H4)2, an analogue of GeFe (Me2C2B4H4)2 (12-59), has been prepared [81]. Reactions of C-trimethylsilyl nido-carborane anions with SnCl2 [94,95] or PbCl2 [95,96] give closo-1,2,3-Sn (RR0 C2B4H4) and closo-1,2,3-Pb(RR0 C2B4H4) compounds (R ¼ H, Me, or SiMe3). “Carbons-separated” isomers are uncommon, but several 1,2,4-Sn(RR0 C2B4H4)-Lewis base adducts [97] and a single plumbacarborane of this genre, closo-1,2,4-(TMEDA)PbII[(Me3Si)2C2B4H4] [95], have been characterized. X-ray crystal structures of many of the 7-vertex species (Table 12-3) reveal closo cage geometries that exhibit slight to moderate slip-distortions with the metal residing closer to the boron than carbon atoms in the C2B3 ring, as in analogous silicon and germanium systems discussed above. Mo¨ssbauer spectra of the 7-vertex stannacarboranes are consistent with the presence of an outward-directed lone pair of electrons on the Sn(II) center [94], but the metal nevertheless functions as a strong electron-acceptor. Like their germacarborane counterparts described earlier, the SnC2B4 and PbC2B4 clusters form a variety of Lewis base adducts of the type LnSn[(Me3Si)RC2B4H4], where Ln is a mono-, di-, or tridentate ligand and R is H, Me, or SiMe3 (Table 12-3) [96–102]. The crystallographically established structures of these complexes have the same essential features found in the base adducts of 7-vertex heterocarboranes of gallium, indium, and germanium described above (e.g., 12-36, 12-38, 12-42, 12-43, 12-57, 12-58): typically the metal is displaced away from the cage carbon atoms and toward the borons, and the base ligand is tilted away from the metal-ring centroid axis. As in the previously discussed systems, MNDO-SCF and Fenske-Hall molecular orbital studies [103,104] indicate that these distortions can be accounted for in terms of maximizing the bonding overlap between metal and ring orbitals and minimizing ligand-ligand repulsions.
12.4.3.2 Commo-stannacarboranes In general, carborane anions tend not to form bis(carboranyl) sandwiches with tin or lead. As already noted, SnCl2 and PbCl2 afford single-cage MC2B4 clusters, and even the reaction of SnCl4 with ðMe3 SiÞ2 C2 B4 H4 2 gives only SnII[(Me3Si)2C2B4H4] [77]; similar behavior is seen with C2 B9 H11 2 , as discussed below. This finding is generally ascribed to a reduced ability of these metals to achieve higher oxidation states [3,4]. However, a tin(IV) sandwich, commo-1-SnIV[2,3(Me3Si)MeC2B4H4]2 (12-67), has been prepared by Hosmane and coworkers via oxidation of SnII[(Me3Si)MeC2B4H4] with titanium tetrachloride in THF [105].
+ 2TiCl4 C Me3Si
C
C
Sn
C
THF, C6H6
Sn
25 °C
Me C Me3Si
SiMe3
Me
C
+ 2TiCl3(THF)3 + SnCl2 Me
12-67
In this interesting bent structure, the first reported example of a tin(IV) sandwich, the ring centroids of the carborane ligands form an angle of 142.5 with the tin center, similar to the corresponding angles in Cp2SnII and Cp*2SnII [106,107]. However, in these stannocenes and in other divalent Group 14 sandwiches, the bending has been ascribed to a lone pair of electrons on tin [108], whereas the metal in 12-67 is in a þ4 oxidation state and presumably has no lone pair, nor is there any evidence of hydride ligands in the vicinity of tin that would account for its bent shape [105]. Given the uniqueness of this species, the only structurally characterized example of a tin or lead bis(carboranyl) sandwich, it is difficult to judge whether the bending arises from factors specific to this system or may prove to have more general significance.
730
CHAPTER 12 Heteroatom carboranes of the main group elements
12.4.3.3 Eleven-vertex cages As Group 14 atoms are easily introduced into nido-C2B4 and nido-C2B9 dianions via reactions with the metal dihalides, similar insertions of these elements should be possible with other carborane substrates, but few examples have been reported. Treatment of nido-6,9-RC2 B8 H9 2 (R ¼ H, Me, Ph) or nido-6,9-Me2 C2 B8 H8 2 ions with SnCl2 in THF affords the respective 11-vertex closo-1,2,3-Sn(RR0 C2B8H8) stannacarboranes (12-68). NMR and mass spectroscopic data favor the fully closed cage structure over more open architectures in which the tin is less fully incorporated into the cluster framework [109–111]. Sn
12-68 C
C
R R
R = R = H, Me R = H, R = Me, Ph
12.4.3.4 Twelve-vertex cages
Insertion of tin into the nido-CB10 H13 anion (Chapter 7) forms the monocarbon stannacarborane Sn(CB10H11) (12-69) [112], an isoelectronic analogue of the stannaborane [113] SnB11 H11 2 . In contrast to the dicarbon SnC2B4 and SnC2B9 clusters, 12-69 exhibits no Lewis acidity toward electron donors, instead acting as a nucleophile in forming transition metal complexes such as 12-71 and the remarkable gold cluster 12-70 [112]. − Sn
− H
H
C
(1) n-C4H9Li, −78 C, THF
C
(2) SnCl2, −78 C, THF
C Au(PPh3)Cl Sn
C = CH
CH2Cl2
12-69 −
Rh(PPh3)3Cl CH2Cl2 −PPh3, −Cl−
C
Ph3P
Au
Au
PPh3
Ph3P
Au
Au
PPh3
Sn Sn C Ph3P
C
Sn
Rh
PPh3
Sn
C
12-71 12-70
12.4 Heterocarboranes of the group 14 elements
731
The 119Sn Mo¨ssbauer spectrum of 12-69 shows an isomer shift intermediate between those of SnB11 H11 2 and Sn(C2B9H11) but closer to the former, indicating greater covalent character in the tin-cage bonding in 12-69 than in Sn(C2B9H11) [112]. In agreement with experiment, DFT calculations on 12-69 show that its s-donor and p-acceptor properties are intermediate between those of its isoelectronic counterparts SnB11 H11 2 , a very strong donor, and 3,1 2-Sn(C2B10H11), a poor s-donor but strong p-acceptor [114]. The lower HOMO and LUMO energies of the stannacarboranes are a consequence of replacing one or two boron atoms with relatively more electronegative carbons. All three species are found to resemble SnCl3 in their HOMO and LUMO energies and orbital shapes. The long-known 3,1,2-Sn(C2B9H11) system and its C,C0 -dimethyl derivative 3,1,2-Sn(Me2C2B9H9) are prepared from dicarbollide anions [85,86,115], 3,1,2-EtAl(Me2C2B9H9) [38], or Tl(Me2C2B9H11)] [88] via reaction with SnCl2 as described earlier. Although these icosahedral stannacarboranes are well characterized spectroscopically, there are surprisingly no reported X-ray crystal structures (other than of Lewis base adducts) and hence no direct information on the degree of slip-distortion. The same is true of the lone example of a 12-vertex plumbacarborane, 3,1,2-Pb(C2B9H11), on which there has apparently been no further study since its original synthesis [85] (an isoelectronic heteroborane, PbB11 H11 2 , has been prepared [113]). As mentioned earlier, the open-cage monoanion ClSn(Me2C2B9H11) (12-61) is generated via displacement of thallium from Tl(Me2C2B9H11)] by reaction with SnCl2 [88]. From the similarity of its NMR spectra to those of its crystallographically characterized germanium counterpart 12-60, a similar open-cage structure is assumed for 12-61. Studies of 3,1,2-Sn(C2B9H11) by 119Sn Mo¨ssbauer spectroscopy support the assignment of an Sn(II) valence state and suggest that the electron lone pair has less directional character than in Cp2Sn [116]; this is seen experimentally in the failure of the stannacarborane to polymerize as stannocene does. The high Mo¨ssbauer chemical shift has been taken as an indication that tin does not bond via the lone pair, instead employing its 5p orbitals with some 5d contribution. Moreover, the large quadrupole splitting implies unequal participation of the tin 5px and 5py,z electrons in binding to the carborane ligand [117]. Like its germanium analogue, 3,1,2-Sn(Me2C2B9H9) coordinates to a variety of electron donors including pyridine, 2,20 -bipyridine, o-phenanthroline, TMEDA, triphenylphosphine, and THF, generating adducts of the general formula (ligand)Sn(Me2C2B9H9) [115]. Crystallographic structures of the bipyridine and THF complexes show slip-distorted structures analogous to 12-57 in which the tin is essentially Z3-coordinated to the carborane cage, binding only to the boron atoms. The same geometry is assigned from NMR evidence to the dimethyl adduct Me2Sn(Me2C2B9H9), prepared via a metal-exchange reaction of Me2SnCl2 with EtAl(Me2C2B9H9) (12-19) [38].
12.4.3.5 Thirteen-vertex cages Although supraicosahedral clusters are well established in transition metal metallacarborane chemistry and in several 13- to 15-vertex carboranes (Chapters 13 and 11), the only main-group heterocarborane systems in this class are 13-vertex 4,1,6-Sn(R2C2B10H10) stannacarboranes (12-72), first prepared by Welch and coworkers in 2002 by metal insertion into nido-7,9-R2 C2 B10 H10 2 dianions (R ¼ H, Me) [118]. Subsequently Wong, Chan, and Xie reported the characterization of the o-xylyl adjacent-carbon species 4,1,2-Sn{[o-C6H4(CH2)2]C2B10H10} (12-73) by an analogous route [119], and 4,1,2-LSn{[R(CH2)2]C2B10H10} Lewis base adducts (Table 12-3) have been studied in detail by the Welch group [120]. A third cage isomer, 4,1,10-Sn(Me2C2B10H10) has been prepared by reduction of 1,12-dimethylp-carborane (1,12-Me2C2B10H10) with sodium followed by reaction with SnCl2 in THF; this species in refluxing toluene converts to yet another isomer, 4,1,12-Sn(Me2C2B10H10) [121]. The Lewis acidity of the smaller stannacarboranes extends to the supraicosahedral systems, with 12-72 and its 4,1,10 and 4,1,12 isomers forming adducts having slip-distorted structures (12-74) [121,122] and 12-73 generating complexes with acetonitrile, THF, and dimethoxyethane [119]. The degree of distortion varies in the latter species, with the DME and THF complexes showing very weak Sn-cage carbon interactions similar to 12-74, whereas in the adduct with the weaker base MeCN (12-75) the tin is essentially Z6-coordinated to the carborane ligand.
732
CHAPTER 12 Heteroatom carboranes of the main group elements
L
4
C R
L
Sn
2− R SnCl2
C
THF R = H, Me
6
R
1
3
7
10
C 13
Sn
R L2
2
8 9
12
C
toluene 5
R L2 = bipyridine, o-phenanthroline
11
12-72 C
11-2
MeCN
C
THF
12-74
Sn
C
SnCl2
C
C
MeCN
Sn
2−
R
C
THF
C C
12-75
12-73
12.5 HETEROCARBORANES OF THE GROUP 15 ELEMENTS Given the relative electron-richness of the elements to the right of carbon in the Periodic Table, it is not surprising that their incorporation into carborane frameworks often results in open-cage nido, arachno, or even hypho clusters. For nitrogen this is the case exclusively; there are no known closo-azacarboranes (although closo-azaboranes— formally related to carboranes via replacement of CH units by N atoms—are well known [123–126]). The heavier Group 15 elements, which are substantially less electronegative, do form 12-vertex closo-heterocarboranes, but their smaller homologues are virtually all open-cage systems.
12.5.1 Nitrogen 12.5.1.1 Six-vertex cages Closo-NCBnHnþ1 (n ¼ 3-5) azacarboranes are predicted from ab initio MO calculations to be stable [127] but have not yet been found experimentally. However, the nido-azadicarbahexaborane derivatives 12-77 and 12-78 are formed on hydroboration of substituted azadiborolanes (12-76) with BF3THF [128]; other derivatives of this system are obtained via reaction of 12-76 (R ¼ Cl) with lithium borates [129]. These reactions are proposed to involve bridging hydroboration of the double bonds in 12-76 to give bicyclic heteroazaborane intermediates which rearrange to the final products [128]. H
C C
CMe3
R
B B B
N
B
B Me
BH3·THF R = 1,2,4,5C6Me4H
N
Me
BH3·THF R = CMe3
B
R C C
B B
N
Me
H
R
12-77
12-76
12-78
R = H, CMe3
12.5.1.2 Eight- to twelve-vertex cages In general, the larger azacarboranes are prepared by insertion of nitrogen into highly reactive open-cage carboranes, often via amine-bridged intermediates. The reaction of arachno-6,7-C2B7H13 (5-20, Section 5.6; formerly numbered
12.5 Heterocarboranes of the group 15 elements
733
5,6-C2B7H13) with n-butylnitrite in ether generates the 10-vertex isomers arachno-6,5,9- and 6,5,10-NC2B7H12 (12-79, 12-80) along with an 8-vertex monocarbon cluster, endo-8-Me-hypho-7,8-NCB6H11 (12-81) [130,131]. H H
H
C
H
H
H
n-C4H9ONO, Et2O
H
C
H H
C
5-20
12-79 6%
H
C
Me
+
C
N
H
H
H
H
H
= BH
H
H
N
+
C
r.t.
H
H
H
H
N
C
12-81 (exo) 5%
12-80 20%
Similarly, treatment of the workhorse nido-7,8-C2 B9 H12 anion with aqueous nitrite in benzene affords the 11-vertex azacarboranes arachno-1,8,11-NC2B8H13 (12-82) and nido-10,7,8-NC2B8H11 (12-83) [131–133]; conversion of 12-82 to 12-83 can be effected via deprotonation with proton sponge [1,8-bis(dimethylamino)naphthalene] and acetone [133]. Interestingly, the reaction of 12-82 with sodium hydride followed by iodine and water forms the endo isomer of 12-81, evidently by elimination of two adjacent BH units and conversion of a framework carbon into a methyl group [130]. H
−
H
H
N
H
C C
NaNO2 H
H
H
C
H2O, C6H6
12-82 H
N
12-83
35%
1) NaH 2) I2, H2O
H
Me
H
H
15%
1) PS 68% 2) Me2CO
C
12-81 (endo)
H
H
7,8-C2B9H−12
H
C C
N
+
C
= BH
H
H
22%
The nido cluster 12-83 can be deprotonated with sodium hydride or proton sponge to give the nido-10,7,8NC2 B8 H10 ion, whose electron lone pair on nitrogen is readily alkylated [133,134]: PhCH2 NC2 B8 H10
PhCH2 Br
Me2 SO4
NC2 B8 H 10 ! MeNC2 B8 H10
A series of open-cage azacarboranes has been generated by Sneddon and coworkers via nitrile insertion into carborane and boron hydride anions [135]. The nido-5,6-C2 B8 H11 ion (Section 6.2) attacks MeCN at the nitrile carbon, with N unit to form the cyano-bridged cluster anion 12-84. Curiously, protonation of this subsequent hydroboration of the C compound does not give the anticipated neutral counterpart of 12-84; instead, it leads to incorporation of the nitrile carbon into the cage and formation of the known tricarbon carborane nido-5,6,9-MeC3B7H10 (6-18). H
Me
−
C
MeCN
C H
H
N
C
H
−
H
C C
12-84 65%
H
C
H
C C
H
5,6-C2B8H−11
Me H2SO4
H
6-18 42%
However, treatment of 5,6-C2 B8 H11 with tert-butyl isonitrile in CH2Cl2 generates neutral arachno-(Me3C)NC3B8H12 (12-85), a cage isomer of 12-84, in low yield as a byproduct of the synthesis of the tricarbon carborane nido-7,8,92C3B8H10 (7-29, Section 7.2) [136]. (Me3C)H2N2
734
CHAPTER 12 Heteroatom carboranes of the main group elements
H
CMe3
H
− C
C N
H
H
Me3CNC
C
H
H
H
5,6-C2B8H−11
+
C
C
H
C NH2CMe3 C H
C
12-85 2%
7-29 70%
−H+ H
−
C
C
H
H
H
H
C
H2SO4
C
C
C
H
H
12-86 97%
12-87 48%
Deprotonation of 12-85 with proton sponge quantitatively removes the Me3C2 2NBH fragment to give the parent carborane anion 5,6,9-C3 B7 H10 (12-86); protonation of this species affords the neutral carborane 5,6,9-C3B7H11 (12-87), the parent form of 6-18 [136]. Nitrile addition to the decaborane anion B10 H13 affords the bridged species 12-88 which, like the closely related structure 12-85, can be regarded either as a CN-bridged decaborane, or alternatively as an arachno-12,7NðRÞCB10 H13 azacarborane [135]. In the latter description, the cage skeleton is viewed as a fragment of a 14-vertex bicapped hexagonal antiprism with two vacant vertexes, consistent with its arachno (2n þ 6) skeletal electron count. Protonation of 12-88 results in loss of boron and formation of the hypho-13,12-N(R)CB9H15 cluster (12-89) in which the nitrogen is bonded to two cage boron atoms; deprotonation of this system with proton sponge affords an anion 12-89 which rearranges at room temperature to a different structure, hypho-13,12-NðRÞCB9 H14 (12-90). Protonation of this anion, in turn, leads to further boron loss, forming hypho-13,8-N(R)CB8H14 (12-91); this species can be reversibly deprotonated to generate the anion 12-91. These structures have been established from X-ray diffraction data and/or from multinuclear NMR data supported by IGLO/NMR calculations [135]. R
−
H H
H
R
H
C
N H
RCN
H
H
−
C H2SO4
N H H
H
H
H
R = CN, CH2Ph B10H−13
−H+
12-88 H
R C
N
C
− −H
+
H H
H
H2SO4 H
C
−
N
H
H2SO4
H
R
H
R
H
12-89
12-89-
H
N
H H
H
H H
H
12-91-
12-91
12-90
Another monocarbon azacarborane system, arachno-9,6-NCB8H13 (12-92), has been synthesized from arachnoCB8H14 and sodium nitrite [137]
12.5 Heterocarboranes of the group 15 elements
735
H
H
N
H
C
H
12-92
In a different approach to azacarborane synthesis, the attack of ammonia or amines on closo-1,2-C2B8H10 opens the cage and inserts nitrogen to form arachno-1,6,9-RR0 NC2B8H11 clusters (12-93) [138,139], whose cage geometry is identical with 6-9, described earlier in Section 6.2 (This type of compound, viewed as NRR0 -bridged derivatives of arachno-5,10-C2B8H14, is alternatively formulated as arachno-5,10-C2B8H11-m(6,9)-NRR0 .) [140]. The monoalkylamine compounds of type 12-93 are isolated in syn and anti forms, with the latter geometry favored as it reduces steric interaction between the R0 group and the bridging hydrogen. As shown, the syn species is convertible to the anti form via deprotonation to the 12-93 anion followed by reprotonation [138]. R
C
H
C = CH
1,2-C2B8H10
R = H; R = H, Me, CMe3 R = R = Et
R
H
N −
B C
12-93−
syn-12-93−
R
H
R
C
N
−
C
C
H
H+
C
anti-12-93−
R = Me, CMe3
R
H
N
N −
C
C
C
Et2O
12-93
H
H
−
NaH
C
C
= BH
R
N
N RRNH
C
H
R
R
C
C
anti-12-93
Azacarborane chemistry is potentially capable of expansion into entirely new directions by introducing metals into the cage framework, but few examples are known at present. A hint of the possibilities inherent in this area is given by the isolation of arachno-(C6Me6)Ru(MeNCB9H12-OMe) (12-94) from the reaction of the ruthenaborane nido-6(C6Me6)RuB9H12-8-OMe with MeNC [141]. H
12-94
C N Me
H H
Ru
MeO
12.5.2 Phosphorus Phosphacarboranes are of interest in large part because the P atom is isoelectronic with CH and hence PCBnHx1 clusters are analogues of C2BnHx carboranes. A wide variety of phosphacarboranes has been characterized, ranging from 5to 12-vertex clusters and including both open- and closed-cage systems, in many cases containing heteroatoms of the main-group or transition elements in addition to phosphorus (Table 12-4). As compared to the azacarboranes described above, phosphacarborane chemistry is considerably more varied in terms of both molecular structure and synthesis; there are many different ways to incorporate phosphorus into carborane cages [142].
736
CHAPTER 12 Heteroatom carboranes of the main group elements
Table 12-4 Heterocarboranes of the Group 15 Elements Synthesis and Characterization Compounda
Information
References
S, H, B, C, MS S, H, B, C, MS
[128] [129]
S, X, H, B, C, MS
[128]
8-vertex NCB6 clusters hypho-7,8-NCB6H11-8-exo/endo-Me
S, X, H, B, MS
[130]
10-vertex NCB8 clusters arachno-9,6-NCB8H13 hypho-13,8-N(R)CB8H14 R ¼ Me, CH2Ph hypho-13,8-NðRÞCB8 H13 R ¼ Me, CH2Ph
S, X, H, B S, H, B, IR, MS S, H, B, IR
[137] [135] [135]
10-vertex NC2B7 clusters nido-6,8,9-NC2B7H10 arachno-6,5,9/6,5,10-NC2B7H12
S, H, B, MS S, H, B, MS
[130] [130]
11-vertex NCB9 clusters hypho-13,12-N(R)CB9H15 R ¼ Me, CH2Ph hypho-13,11-NðRÞCB9 H14 R ¼ Me, CH2Ph
S, H, X(CH2Ph), B(2d), C, IR, MS S, H, X(Me), B, C, IR
[135] [135]
S, H, B, IR S, B H(2d), B(2d), MS S, X S, H(2d), B(2d), IR, MS S, H(2d), B(2d), IR, MS S, B, IR H(2d), B(2d), IR, MS S, H, B(2d), MS S, X(CMe3, H), H, B(2d), C, MS
[131,133] [132] [133] [134] [133] [133] [132,133] [133] [139] [138]
12-vertex NCB10 clusters arachno-12,7-NðRÞCB10 H13 R ¼ Me, CH2Ph
S, X(Me), H, B(2d), C, IR
[135]
12-vertex NMCB9 clusters arachno-(C6Me6)Ru(MeNCB9H12)
S, X, H, B, MS
[141]
12-vertex NC3B8 clusters arachno-12,7-NðMeÞC3 B8 H11 arachno-7,5,1,12-(Me3CN)NC3B8H12
S, H, B(2d), C, IR S, H, B, C
[135] [136]
NITROGEN 6-vertex NC2B3 clusters nido-2,4,5-MeN[(CHMe2)2C2B3H]-1-CMe3-3-R R ¼ H, CMe3 nido-2,4,5-MeN[(CHMe2)2C2B3H2]-3-R R ¼ H, 1,2,4,5-C6Me4H [duryl] nido-2,4,5-MeN[(CHMe2)2C2B3H]-3,6-R2 R ¼ H, 1,2,4,5C6Me4H [duryl]
11-vertex NC2B8 clusters nido-10,7,8-NC2B8H11
nido-10,7,8-(PhCH2)NC2B8H10 nido-10,7,8-NC2 B8 H10 nido-10,7,8-RNC2B8H10 R ¼ Me, PhCH2 arachno-NC2B8H13 arachno-11,5,10-(CMe3)HNC2B8H11, (2 isomers) arachno-1,6,9-RR0 NC2B8H11 (R, R0 ¼ H, H; H, Me; Me, H; CMe3, H; Et, Et)
Continued
12.5 Heterocarboranes of the group 15 elements
737
Table 12-4 Heterocarboranes of the Group 15 Elements—Cont’d Synthesis and Characterization Compounda
Information
References
12-vertex NC4B7 clusters arachno-N{[1,2,3,4-CpFeC3B7H9]-2-CH2}C4B7H10-5-CH2CN
S, X, H, B, MS
[231]
PHOSPHORUS 5-vertex PCB3 clusters nido-1,2-P[(CMe3)CB3H5]
S, H, B, P
[143]
7-vertex PC2B4 clusters 1,2,3-[(CMe3)3C6H2]P[(Me3Si)2C2B4H4] nido-6,3,4-R0 R2PC2B4H4 (R0 ¼ Ph, CMe3, Me; R ¼ Et, CH2Ph) arachno-PhEt2 PC2 B4 H4 2
S, H, B, C, P, IR, MS S, H, B, C, P, IR, MS S, B, P
[144] [145] [145]
8-vertex P2C2B4 clusters arachno-4,5,7,8-(Ph2Et)2P2C2B4H4
S, B, MS
[145]
10-vertex PCB8 clusters 2,1/6,1-PCB8H9 2,1-PCB8H9 PSHþ nido-6,9-(exo-R)PCB8 H9 R ¼ Ph, Me arachno-6,7-(endo-X)(exo-Ph)PCB8 H11 X ¼ O, S, BH3, Br arachno-6,7-[cis-(Ph3P)2PtCl](exo-Ph)PCB8H11 arachno-6,7-[exo-Fe(CO)2](endo-Ph)PCB8H11 arachno-6,7-[endo-Fe(CO)2](exo-Ph)PCB8H11 arachno-6,7-[exo-Mn(CO)5](endo-Ph)PCB8H11 arachno-6,7-(exo-R)PCB8H12 R ¼ Ph, Me arachno-6,7-(exo-R)PCB8 H11
S, H, B(2d), C, P, IR, MS S S, X, H(2d), B(2d), P, IR, MS S, X(S), H, B, P, IR, MS S, X, B, IR S (photolytic isomerization), B, IR, MS S, X, B, IR, MS S, X, B, IR S, H(2d), B(2d), P, IR, MS S, H(2d), B(2d), P, IR, MS
[146] [148] [150] [150] [151] [151] [151] [151] [150] [150]
cis-[arachno-6,5,7-PhPC2B7H11]2MBr2 M ¼ Pd, Pt trans-[arachno-6,5,7-PhPC2B7H11]2PdBr2
S, X S, X S, S,
[168,169] [168] [168,169] [168] [168] [168]
11-vertex PCB9 clusters nido-7,9-PCB9H12 nido-7,8-PCB9 H11 nido-7,n-PCB9 H11 n ¼ 8, 9 nido-7,8-MePCB9H11 n ¼ 8, 9 nido-7,9-MePCB9H11 n ¼ 8, 9 nido-7,9-EtPCB9H11
S, P S, S, S, S,
10-vertex PC2B7 clusters arachno-6,8,9-RPC2B7H11 R ¼ Ph, Me arachno-6,5,7-RPC2B7H11 R ¼ Ph, Me
H, B(2d), C, P, IR, MS H, B(2d), C, P, IR, MS X, H, B, C, P, MS X, H, B, C, P, MS
H, B, IR H, H, H, H,
B, B, B, B,
IR UV, MS P, IR, MS MS
[153] [157] [153] [153] [153] [156] Continued
738
CHAPTER 12 Heteroatom carboranes of the main group elements
Table 12-4 Heterocarboranes of the Group 15 Elements—Cont’d Synthesis and Characterization Compounda
Information
References
nido-7,8-(PhCH2)PCB9H11 nido-7,9-(PhCH2)PCB9H11 nido-7,9-(CH2CHCH2)PCB9H11 nido-7,9-[Ph(CH)2CH2]PCB9H11 2PCB9 H11 n ¼ 8, 9 nido-7,n-ðCOÞ5 Cr2 2PCB9 H11 n ¼ 8, 9 nido-7,n-ðCOÞ5 Mo2 2PCB9 H11 nido-7,8-ðCOÞ5 Mo2 7 2PCB9H11 nido-(Z -C7H7)(CO)2Mo2 2PCB9 H11 n ¼ 8, 9 nido-7,n-ðCOÞ5 W2 2PCB9H11 nido-Cp(CO)2Fe2
S, H, MS S, H, B, MS S, H, B, MS S, H, B, MS S, H, B, IR S, H, B, IR COND S, H, B, IR S, H, B, IR S, H, B, IR
[156] [156] [156] [156] [157] [157] [157] [91] [157] [91]
11-vertex P2CB8 clusters nido-7,8,9-P2CB8H10 nido-7,8,n-P2 CB8 H9 n ¼ 9, 10
S, H, B(2d), C, P S, H, B(2d), C, P
[148] [148]
11-Vertex MPCB8 clusters 1,2,3-CpFe[RPCB8H9] R ¼ Ph, Me nido-6,7-(Ph3P)2Pt(PhPCB8H10]
S, X, H, B, IR S, X, B, IR
[151] [151]
11-vertex PC2B8 clusters nido-PC2B8H11 nido-7,10-11-RPC2B8H10 R ¼ Me, Ph nido-7,8,11-PC2B8H11 nido-7,8,11-PhPC2B8H10 nido-7,8,11-PC2 B8 H10 nido-7,8,9-PC2B8H10-10-R R ¼ H, Cl nido-7,8,9-PC2B8H9-10-R R ¼ H, Cl 2PC2B8H10 nido-7,8,9-Cp(CO)2Fe2 2PC2B8H10 nido-7,9,10-Cp(CO)2Fe2 nido-7,8,9-PC2B8H10-10-X X ¼ Cl, Br, I nido-7,8,9-PC2B8H9-10-X X ¼ Cl, Br, I
S, S, S, S, S, S, S, S, S, S, S,
B, MS B(2d), H, C, P, IR, PS X, H, B(2d), P, MS X, H, B(2d), P, MS X, H, B(2d), P B(2d), H, C, P X(Cl), B(2d), H, C, P H, B(2d), C, IR, MS H, B(2d) H, B(2d), P, MS H, B(2d), P, MS
[57] [170] [172] [172] [172] [171] [171] [174] [174] [148] [148]
11-vertex MPC2B7 clusters nido-11,7,9,10-(Ph3P)2Pt(PhPC2B7H9) nido-11,7,9,10-CpCo(PhPC2B7H9) nido-11,7,9,10-Ni(PhPC2B7H9)2
S, X, H, B, C, P, IR, MS S, X, H, B, C, P, IR, MS S, X, H, B, C, P, IR, MS
[168] [168] [168]
11-vertex P2C2B7 clusters nido-7,8,9,11-P2C2B7H8-3-R R ¼ H, Cl nido-7,8,9,10-P2C2B7H9 nido-7,9,8,10-P2C2B7H9 nido-7,8,9,10-P2C2B7H8-n-Cl n ¼ 4, 11
S, S, S, S,
[176] [176] [176] [176]
X(Cl), H, B, P, IR, MS X, H, B, P, IR, MS H, B, P, IR, MS H, B, P, IR, MS
Continued
12.5 Heterocarboranes of the group 15 elements
739
Table 12-4 Heterocarboranes of the Group 15 Elements—Cont’d Synthesis and Characterization Compounda
Information
References
nido-7,8,9,11-P2C2B7H8-10-X X ¼ Cl, I nido-7,8,9,11-P2C2B7H7-5,10-X2 X ¼ Cl, I nido-7,8,9,11-P2C2B7H8-3-Br nido-7,8,9,11-P2C2B7H7-2,3-Br2
S, S, S, S,
[177] [177] [177] [177]
11-vertex P3CB7 clusters nido-7,8,9,10-P3CB7H8 nido-7,8,9,10-P3CB7H7-4-Me-11-R R ¼ H, Cl
S, H, B(2d), C, P S, B(2d), H, P, MS
[148] [149]
S, IR, UV, MS H B C C (C-H coupling, C hybridization) P P (P-C coupling) IR, Raman E He photoelectron spectra Molecular refractivity, dielectric constant Dipole moment Molecular films, photoemission and inverse photoemission studies B, dipole moment S S S, H, IR, MS S, IR, MS H B C P S, IR, MS S, B, IR, UV, MS H IR, Raman B C (C-H coupling, C hybridization) P (P-C coupling) E
[153] [153,179,232] [153,155,232] [232] [233] [232] [233] [228,234] [154,235] [236] [237] [179,237] [238]
12-vertex PCB10 clusters 1,2-PCB10H11
1,2-PCB10H10Cl (3 isomers) 1,2-PCB10H10X X ¼ Cl, Br, I 1,2-PCB10H9X2 X ¼ Cl, Br, I 1,2-PCB10H10Br 1,2-PCB10H9Br2
1,2-PCB10H8Br3 1,7-PCB10H11
B(2d), B(2d), B(2d), B(2d),
H, H, H, H,
P P P P
[155] [163] [163] [153] [153] [153,232] [153,232] [232] [232] [153] [153] [153,179] [234] [155] [233] [233] [235] Continued
740
CHAPTER 12 Heteroatom carboranes of the main group elements
Table 12-4 Heterocarboranes of the Group 15 Elements—Cont’d Synthesis and Characterization Compounda
Information
References [237] [239] [240] [179,237] [238]
1,12-P(MeHg)CB10H10 1,12-PCB10H10Cl (2 isomers) 1,12-PCB10H10X X ¼ Cl, Br, I 1,12-PCB10H9X2 X ¼ Cl, Br, I nido-1,12-PCB10 H11 2
Molecular refractivity, dielectric constant pKa Kinetic C-H acidity Dipole moment Molecular films, photoemission and inverse photoemission studies S, H S S B, dipole moment X S, X, H, B, C IR S S, IR S, H, IR, MS S, IR, MS S S, B, IR, UV, MS H B C (C-H coupling, C hybridization) P (P-C coupling) IR, Raman E ED pKa Kinetic C-H acidity Molecular refractivity, dielectric constant Dipole moment S, H B, dipole moment S S S
12-vertex M2PCB8 clusters Cp2Fe2(MePCB8H9)
S, X, MAG, IR, MS
[151]
12-vertex MPCB9 clusters 3,1,2-GePCB9H10
S, H, B, IR
[92]
1,7-P(MeHg)CB10H10 1,7-PCB10H10X X ¼ Cl, Br, I 1,7-PCB10H9X2 X ¼ Cl, Br, I 1,7-PCB10H10-Cl (6 isomers) 1,7-PCB10H9-9,10-Cl2 1,7-PCB10H9-9,10-I2 1,7-PDCB10H10 1,7-PMeCB10H10 1,7-PCB10H10C-C(O)-1,7-AsCB10H10 1,7-PCB10H10Br 1,7-PCB10H9Br2 nido-1,7-PCB10 H11 2 1,12-PCB10H11
[162] [163] [163] [155] [155] [164] [234] [152] [180] [153] [153] [154] [153] [153,179] [155] [233] [233] [234] [235] [241] [239] [240] [237] [237] [162] [155] [163] [163] [154]
Continued
12.5 Heterocarboranes of the group 15 elements
741
Table 12-4 Heterocarboranes of the Group 15 Elements—Cont’d Synthesis and Characterization Compounda
Information
References
2,1,7-GePCB9H10 2,1,7-(Et3P)(CO)2Mn(MePCB9H10) 2,1,7-(CO)3Mn(MePCB9H10) 2,1,7-ðCOÞ3 MnPCB9 H10 3,1,2-FeðPCB9 H10 Þ2 2 3,1,2-Fe(MePCB9H10)2 2,1,7-Fe(MePCB9H10)2 (2 isomers)
2,1,7-(Ph3P)BrNi(MePCB9H10) 2,1,7-(Ph3P)2Rh(MePCB9H10)
S, H, B, IR S, IR S, IR, UV S, IR, UV S, H, IR, UV, E S, H, IR, UV, E S, H, B, IR, UV, E X S, H, IR, E S, H, B, IR S, H, IR, UV, MS S, H, IR, UV, E B, IR, UV, MAG S, H, IR, UV, E, MAG S, H, IR, UV B, H, IR, UV S, H, IR, MS S, H, IR, UV, E, MAG E S S, H, IR, MS, UV S, H, IR, MS, UV S S
[92] [242] [158] [158] [158] [158] [158] [243] [158] [161] [158] [158] [158] [158] [158] [158] [158] [158] [244] [243] [245] [245] [246] [246]
12-vertex PC2B9 clusters nido-ClP(Me2C2B9H9) E ¼ P, As nido-(CHMe2)P(Me2C2B9H9) nido-XP(Me2C2B9H9)•AlCl3 X ¼ F, Cl
S, X, H, B, C, P, MS S S
[167] [167] [167]
S, X, H S, H, B(2d), C, IR S, H, B(2d), C, P, IR, MS S, X(H), H, B, P, E(H), R(thermal rearrangement) S, H, B, P, E(Cl) S, X(H), H, B, P, E S, X, H, B, P, E S, H, P, E S, H, B, P
[174] [174] [175] [159]
(1,7-MePCB9H10)Fe(1,7-PCB9H10) 2,1,7-Fe½ðCOÞ5 M PCB9 H10 2 2 M ¼ Cr, Mo, W 2,1,7-CpFe(MePCB9H10) 2,1,7-FeðPCB9 H10 Þ2 2 2,1,7-FeðPCB9 H10 Þ2 3,1,2-Co(PCB9H10)2 3,1,2-CoðPCB9 H10 Þ2 2,1,7-CoðPCB9 H10 Þ2 (1,7-MePCB9H10)Co(1,7-PCB9H10) 2,1,7-Co(MePCB9H10)2 (2 isomers) 2,1,7-Ni(MePCB9H10)2 2,1,7-LNi(MePCB9H10) (L ¼ C3H5, MeC3H4, Ph3C3, NO)
12-vertex MPC2B8 clusters 1,2,4,5-CpFe(PC2B8H10) 1,2,4,8-CpFe(PC2B8H10) CpFe(PC2B8H10) (3 isomers) 1,2,3,4-(C4Me4)Co(PC2B8H9-5-R) (R ¼ H, Cl) 1,2,3,6-(C4Me4)Co(PC2B8H9-5-R) (R ¼ H, Cl) 1,2,4,8-(C4Me4)Co(PC2B8H9-5-Cl) 1,2,4,5-(C4Me4)Co(PC2B8H10) 1,2,3,5-(C4Me4)Co(PC2B8H10) 1,2,3,10-(C4Me4)Co(PC2B8H9-5-Cl)
[159] [159] [159] [159] [159] Continued
742
CHAPTER 12 Heteroatom carboranes of the main group elements
Table 12-4 Heterocarboranes of the Group 15 Elements—Cont’d Synthesis and Characterization Compounda
Information
References
closo-MP2CB8 clusters 1,2,3,n-CpFe(P2CB8H9) (n ¼ 4, 5) 1,2,3,5-(C4Me4)Co(P2CB8H9-5-Cl)
S, X(n ¼ 5), H, B(2d), C, P, IR, MS S, H, B, P
[160] [159]
nido-7,9-AsCB9 H11 nido-7,9-MeAsCB9H11 nido-7,8-ðCOÞ5 MoAsCB9 H11 nido-7,9-ðCOÞ5 MAsCB9 H11 M ¼ Cr, Mo, W
S, B S, S, S, S,
[179] [157] [179] [179] [157] [157]
11-vertex AsC2B8 clusters nido-AsC2B8H11
S, B, MS
[57]
11-vertex As2C2B7 clusters nido-As2C2B7H9 nido-As2C2B7H9 (3 isomers) nido-As2C2B7H8-3-X X ¼ Cl (2 isomers), I
S, H, B, IR S, H, B, IR, MS S, X(I), H, B, IR, MS
[181] [182] [182]
S C(C-H coupling, C hybridization) H, B, MS IR Raman E He photoelectron spectra Dipole moment S, H, B, IR, MS C(C-H coupling, C hybridization) IR, Raman E pKa Kinetic C-H acidity Dipole moment S S S S S
[179,185] [233] [179] [179,234] [228,234] [154,235] [236] [247] [179] [233] [234] [235] [239] [240] [247] [180] [180] [180] [180] [180]
ARSENIC 11-vertex AsCB9 clusters nido-7,8-AsCB9 H11
12-vertex AsCB10 clusters 1,2-AsCB10H11
1,7-AsCB10H11
1,7-As(LiCB10H10) 1,7-As{[C(O)OR]CB10H10} R ¼ H, Cl, Et 1,7-AsCB10H10-C(O)-1,7-CB10H10CMe 1,7-AsCB10H10-C(O)-1,7-CB10H10P 1,7-As[(PhCO)CB10H10]
B, IR H, H, H, H,
IR IR B, IR B, IR
Continued
12.5 Heterocarboranes of the group 15 elements
743
Table 12-4 Heterocarboranes of the Group 15 Elements—Cont’d Synthesis and Characterization Compounda
Information
References
1,7-As[(MeHg)CB10H10] 1,7-As[(D)CB10H10] 1,12-AsCB10H11
1,12-As[(MeHg)CB10H10] nido-1,2-AsCB10 H11 2
S, H IR S, H, B, MS C(C-H coupling, C hybridization) ED IR, Raman E pKa Kinetic C-H acidity Dipole moment S, H S
[162] [234] [179] [233] [241] [234] [235] [239] [240] [247] [162] [154]
12-vertex MAsCB9 clusters 3,1,2-GeAsCB9H10 2,1,7-GeAsCB9H10 2,1,7-Fe½ðCOÞ5 Cr AsCB9 H10 2 2 3,1,2-CpCoAsCB9H10
S, S, S, S,
H, B, IR, MS H, B, IR, MS H, B, IR X
[92] [92] [161] [179]
12-vertex AsC2B9 clusters MeAsC2B9H11 MeAs(Me2C2B9H9) RAsC2B9H11 R ¼ Ph, Br, n-C4H9 nido-PhAsC2B9H11 nido-PhAsMe2C2B9H9 nido-ClAs(Me2C2B9H9) E ¼ P, As nido-(CHMe2)As(Me2C2B9H9) nido-ClAs(Me2C2B9H9)AlCl3 nido?-(Me2As)2C2B9H11 (structure uncertain) nido?-(Me2As)2Me2C2B9H9 (structure uncertain) nido?-Me2AsC2B9H11-OEt (structure uncertain)
S, S, S, S, S S, S S S, S, S,
H, H, H, H,
[183] [183] [183] [248] [248] [167] [167] [167] [183] [183] [183]
12-vertex AsTlC2B8 clusters AsTlC2B8H11
B
[57]
ANTIMONY 11-vertex SbCB9 clusters nido-7,8-SbCB9 H11
IR
[179]
S, H, B, IR, MS, dipole moment B(2d), H(2d)
[179] [249]
12-vertex SbCB10 clusters 1,2-SbCB10H11
B, B, B, B,
IR, MS IR IR MS
X, H, B, C, P, MS
H, B, IR, MS H, B, IR, MS B, MS
Continued
744
CHAPTER 12 Heteroatom carboranes of the main group elements
Table 12-4 Heterocarboranes of the Group 15 Elements—Cont’d Synthesis and Characterization Compounda
Information
References
1,7-SbCB10H11
He photoelectron spectra S, H, B, IR, MS
[236] [179]
Ab initio Ab initio
[127] [250] [251] [252] [252] [127] [253]
Theoretical Studies Molecular and electronic structure calculations NCB3H4, NCB4H5, NCB5H6 arachno-6,9-NCB8H13 nido-10,7,8-NC2B8H11 nido-7,9,10-NC2 B8 H10 nido-7,9,10-HNC2B8H10 PCB3H4, PCB4H5, PCB5H6 P2CB2H4 nido-RR0 P(R00 CB4H8) PSHþ nido-6,9-(exo-R)PCB8 H9 R ¼ Ph, Me arachno-6,7-(exo-R)PCB8H12 R ¼ Ph, Me arachno-6,7-(exo-R)PCB8 H11 arachno-6,7-(endo-X)(exo-Ph)PCB8 H11 X ¼ O, S, BH3, Br arachno-6,8,9-RPC2B7H11 R ¼ Ph, Me arachno-6,5,7-RPC2B7H11 R ¼ Ph, Me cis-[arachno-6,5,7-PhPC2B7H11]2MBr2 M ¼ Pd, Pt trans-[arachno-6,5,7-PhPC2B7H11]2PdBr2 nido-7,10-11-RPC2B8H10 R ¼ Me, Ph nido-7,9,10-MePC2B8H10 nido-7,8,10-PhPC2B8H10 nido-7,8,9-PC2B8H10-10-R, nido-7,8,9-PC2B8H9-10-R R ¼ H, Cl nido-7,8,9-P2CB8H10 nido-7,8,9/7,8,10/7,10,8/7,9,8-P2 CB8 H9 nido-7,8,9,11-P2C2B7H8-3-R R ¼ H, Cl nido-7,8,9,10-P2C2B7H9 nido-7,9,8,10-P2C2B7H9 nido-7,8,9,10-P2C2B7H8-n-Cl (n ¼ 4, 11) nido-7,8,9,10-P3CB7H7-4-Me-11-R R ¼ H, Cl 1,2-PCB10H11 1,2-AsCB10H11 1,2-SbCB10H11 Isomerization calculations 1,2-PCB10H10-12-Cl
DFT, isomer stability DFT, isomer stability Ab initio Ab initio, DFT (distorted trigonal bipyramid) MNDO-SCF DFT DFT DFT DFT DFT DFT DFT DFT DFT DFT DFT Ab initio (geometry)
[254] [150] [150] [150] [150,168] [168,169] [168,169] [168] [168] [170] [170] [255] [171]
DFT, geometry DFT, geometry RMP2 RMP2 RMP2 RMP2 RMP2 Ab initio, bond lengths Ab initio, bond lengths MNDO MNDO
[148] [148] [176] [176] [176] [176] [149] [228] [228] [236] [236]
Cage rearrangement to 1,7-PCB10H109-Cl
[206] Continued
12.5 Heterocarboranes of the group 15 elements
745
Table 12-4 Heterocarboranes of the Group 15 Elements—Cont’d Synthesis and Characterization Compounda NMR calculations arachno-6,9-NCB8H13 1,12-NCB10H11 nido-6,8,9-NC2B7H10 nido-10,7,8-NC2B8H11 nido-10,7,8-NC2 B8 H10 nido-10,7,8-RNC2B8H10 R ¼ Me, PhCH2 nido-7,8,9/7,8,11/7,8,10/7,9,10-NC2 B8 H10 nido-7,8,9/7,8,11/7,8,10/7,9,10-HNC2B8H10 arachno-NC2B8H13 arachno-6,5,9/6,5,10-NC2B7H12 hypho-7,8-NCB6H11-8-exo/endo-Me nido-1,2-P(CMe3)CB3H5 nido-6,5,7-PC2B4H7 nido-6,3,4-R0 R2PC2B4H4 R0 ¼ Ph, CMe3, Me; R ¼ Et, CH2Ph nido-7,8,9/7,8,11/7,8,10/7,9,10-PC2 B8 H10 nido-7,8,9/7,8,11/7,8,10/7,9,10-HPC2B8H10 arachno-[PhEt2PC2B4H4]2 arachno-4,5,7,8-P2C2B4H8 2,n,1-PCB8H9 (n ¼ 1, 6) arachno-6,7-(endo-X)(exo-Ph)PCB8 H11 X ¼ O, S, BH3, Br nido-7,8,9-P2CB8H10 nido-7,8,9/7,8,10/7,10,8/7,9,8-P2 CB8 H9 arachno-6,5,7-RPC2B7H11 R ¼ Ph, Me arachno-6,8,9-RPC2B7H11 R ¼ Ph, Me cis-[arachno-6,5,7-PhPC2B7H11]2MBr2 M ¼ Pd, Pt trans-[arachno-6,5,7-PhPC2B7H11]2PdBr2 nido-7,8,11-PC2B8H11 nido-7,8,11-PhPC2B8H10 nido-7,8,11-PC2 B8 H10 nido-7,10-11-RPC2B8H10 R ¼ Me, Ph nido-7,9,10-MePC2B8H10 nido-7,8,10-PhPC2B8H10 nido-7,8,9-PC2B8H10-10-R, nido-7,8,9-PC2B8H9-10-R R ¼ H, Cl nido-11,7,9,10-(Ph3P)2Pt(PhPC2B7H9) nido-11,7,9,10-Ni(PhPC2B7H9)2 nido-11,7,9,10-CpCo(PhPC2B7H9) nido-As2C2B7H8-3-X X ¼ H, Cl [2 isomers], I
Information
References
B, ab initio GIAO-SCF B (IGLO) B (IGLO) B (IGLO) DFT: relative energies; energy penalties DFT: relative energies; energy penalties B (IGLO) GIAO-SCF GIAO-SCF GIAO IGLO IGLO
[250] [251] [130] [133] [133] [133] [229] [229] [133] [130] [130] [143] [145] [145]
DFT: relative energies; energy penalties DFT: relative energies; energy penalties IGLO IGLO GIAO GIAO 1 H, 11B, 13C NMR (GIAO) 1 H, 11B, 13C NMR (GIAO) GIAO GIAO GIAO GIAO DFT/GIAO DFT/GIAO DFT/GIAO GIAO GIAO GIAO (rotamers and 11B spectrum) GIAO
[229] [229] [145] [145] [146] [168] [148] [148] [168] [168] [168] [168] [172] [172] [172] [170] [170] [255] [171]
DFT/GIAO DFT/GIAO DFT/GIAO DFT
[168] [168] [168] [182] Continued
746
CHAPTER 12 Heteroatom carboranes of the main group elements
Table 12-4 Heterocarboranes of the Group 15 Elements—Cont’d Synthesis and Characterization Compounda
nido-7,8,9/7,8,11/7,8,10/7,9,10-AsC2 B8 H10 nido-7,8,9/7,8,11/7,8,10/7,9,10-AsC2B8H10 nido-7,8,9/7,8,11/7,8,10/7,9,10-SbC2 B8 H10 nido-7,8,9/7,8,11/7,8,10/7,9,10-HSbC2B8H10
Information
References
DFT: relative energies; energy penalties DFT: relative energies; energy penalties DFT: relative energies; energy penalties DFT: relative energies; energy penalties
[229] [229] [229] [229]
S ¼ synthesis, X ¼ X-ray diffraction, H ¼ 1H NMR, B ¼ 11B NMR, C ¼ 13C NMR, F ¼ 19F NMR, P ¼ 31P NMR, 2d ¼ two-dimensional (COSY) NMR, IR ¼ infrared data, MS ¼ mass spectroscopic data, UV ¼ UV-visible data, E ¼ electrochemical data, MAG ¼ magnetic susceptibility, COND ¼ electrical conductivity. a Heteroatoms (other than carbon) incorporated into the cluster framework are in boldface.
12.5.2.1 Five- to eight-vertex phosphacarboranes In a reaction reminiscent of the insertion of acetylene into B4H10 to form nido-C2B3H7 (Section 4.3), the same boron C2 hydride combines with the phosphaalkyne P 2CMe3 to give nido-1,2-P(Me3C)CB3H5 (12-95), the smallest known heterocarborane [143]. The structure of this compound, based on NMR data and DFT optimized-geometry calculations, is analogous to that of nido-C2B3H7 with P formally replacing the apical CH unit. Like the latter carborane, 12-95 decomposes in air, and in vacuo in the liquid state, but is stable in the gas phase at room temperature. Not surprisingly, the 31P NMR shift in this compound is found at high field, but its extreme value of 501 ppm in CS2 solution (vs. 488 for P4) is the most negative value recorded for any known compound. P C
B B H
CMe3
12-95
B
B = BH
H
Insertion of phosphorus into the nido-2,3-ðMe3 SiÞ2 C2 B4 H4 2 dianion via reaction with 2,4,6-(Me3C)3C3H2PCl2 affords the proposed 7-vertex cluster 12-96, isolated in 38% yield as an air-sensitive solid and characterized from NMR and other spectroscopic data [144]. The requirement of 16 skeletal electrons for a 7-vertex closed polyhedron according to PSEPT rules (Chapter 2) implies that the phosphorus in this cluster donates only two electrons to the cage, in which case there must be an exo-polyhedral lone pair in addition to the aryl substituent. One expects that such a structure would be considerably distorted from the idealized geometry depicted here, but an X-ray study to resolve this question has not yet appeared. While 7-vertex closo-heterocarboranes of Group 1, 2, 13, and 14 elements are known, as discussed earlier in this chapter, 12-96 is currently the only Group 15 system in this class excluding transition metal complexes. C6H2(CMe3)3
••
P
12-96 C
C
SiMe3
Me3Si
An apparently similar approach involving reactions of nido-2,3-R2 C2 B4 H4 2 dianions (R ¼ Et or CH2Ph) with R0 PCl2 reagents (R0 ¼ Me, Ph, CMe3) nevertheless generates rather different products, namely 7-vertex nido-R0 PR2C2B4H4 phosphacarboranes (12-97) whose proposed cage geometry is inferred from NMR evidence [145]. Reduction of
12.5 Heterocarboranes of the group 15 elements
747
nido-PhPEt2C2B4H4 with lithium metal or naphthaleneide ion forms the arachno dianion 12-972 for which structure (a) is favored over (b) on the basis of NMR data and ab initio/IGLO/NMR calculations. Treatment of the dianion with PhPCl2 yields the 8-vertex diphosphacarborane 12-98 whose proposed structure is similar to the R4C4B4H4 system described in Chapter 5 (see Figure 5-9) [145]. R
P
R
C C
2e−
R
Et
Ph
C C
P
Ph Ph
or
12-972-
(a)
R = Et, CH2Ph R = Me, Ph, CMe3
C
P C
Et
R = Et, R = Ph
12-97
Et
Et
Ph
(b)
Et P P
C C
PhPCl2 Et
12-98
12.5.2.2 Large monocarbon phosphacarboranes The insertion of phosphorus into highly reactive 9-vertex open-cage carborane anions (a genre discussed in Chapter 5) is a very productive route to phosphacarboranes, as in the reaction of nido-1-CB8 H11 (generated in situ by deprotonation of the neutral carborane with proton sponge) with PCl3 to give closo-6,1-PCB8H9 (12-99) [146,147]. The same treatment of arachno-4-CB8 H13 affords closo-2,1-PCB8H9 (12-100) as well as nido-7,8,9-P2CB8H10 (12-101) (the major product) and nido-7,8,9,10-P3CB7H8 (12-102) [147,148]. Additionally, B-methyl and B-methyl-B0 -chloro derivatives of the latter compound (Table 12-4) have been prepared from the reaction of CB7H10Me2 (formed in situ from C2 B7 H12 and NaH) with proton sponge and excess PCl3 [149]. The bridging proton in 12-101 can be reversibly removed to generate the monoanion 12-103, whose tetraphenylphosphonium salt isomerizes at elevated temperature to afford PPh4 þ nido7,8,10-P2 CB8 H9 (12-104) [148]. C H
H H
C
H
PCl3
H
P
P
+
PS 4-CB8H14
12-100
C
P
12-101
15%
P
+
P C
P
12-102
34%
5%
C = CH H H
H
C
−
−
PCl3 PS
H+
−H+
C
P
P
P
C
350 C
P
C P
1-CB8H12
12-99
27%
12-103
93%
12-104
86%
The structures depicted, deduced from multinuclear NMR data and ab initio calculations, all conform to the PSEPT electron-counting rules outlined in Chapter 2, if one assumes that each phosphorus atom donates three electrons to framework binding and has an exo-polyhedral lone pair. Although these compounds are air-stable solids, no X-ray diffraction studies have been reported.
748
CHAPTER 12 Heteroatom carboranes of the main group elements
Deprotonation of arachno-4-CB8H14 with an excess of proton sponge followed by reaction with RPCl2 (R ¼ Me or Ph) generates two products, PSHþnido-6,9-RPCB8 H9 (12-105) and arachno-6,7-RPCB8H12 (12-106), the relative yields varying with conditions and the nature of R [147,150]. The conversion of the thermodynamically disfavored 12-106 structure to the more stable 12-105 has been explored via DFT calculations. R
C
H H
H H
−
P
RPCl2
R
C
+
excess PS
−2PSH+Cl− 4-CB8H14
H
C
12-106
12-105
R = Ph, Me C = CH
H H
P
In an alternative route, treatment of arachno-4-CB8 H12 2 (a fluxional species discussed in Section 5.6) with RPCl2 affords 12-106 in quantitative yield [150]. Removal of the endo proton on phosphorus yields the 12-106 anion, whose electron lone pair on phosphorus is very reactive with electrophiles and is easily metallated to form products such as 12-107– 12-109 and similar complexes listed in Table 12-4 [151]. The reaction of 12-106 (R ¼ Me) with FeCl2 and NaþCp affords the P-Me analogue of 12-108 along with an icosahedral diferraphosphacarborane, 2,3,1,7-Cp2Fe2(MePCB8H9). Nonmetallic reactants such as O2, S8, BH3 THF, and Br2 similarly combine with 12-106 (R ¼ Ph) to give the corresponding arachno-6,7(endo-X)P(exo-Ph)CB8H11 products (X ¼ O, S, BH3, or Br) that are structurally analogous to 12-108 [150]. Photolysis of 12-108 converts it to the less crowded isomer 12-110 in which the metal group occupies an exo site. R R RPCl 2
2− 4-CB8H12
H
H H
P
R −H+
C
−2Cl− R = Ph, Me
P
(CO)5Mn
H H
H+
H H
P Mn(CO)5Br
C
C
12-106-
12-106
C = CH
−
12-107 29% CpFe(CO)2I R = Ph
OC
Ph Fe OC
C O
H H
P C
12-110 76%
hn
OC Ph
Fe
Fe H H
P C
12-108 58%
+
Ph
P C
12-109 2%
The most intensively studied phosphacarboranes (and the first to be prepared) are the icosahedral closo-PCB10H11 clusters discovered by Todd and coworkers in the 1960s [152] and the 11-vertex nido-PCB9 H10 anions derived from them. Much of the chemistry of these species parallels that of their isoelectronic C2B10H12 analogues, as seen in their thermal isomerization, cage degradation to 11-vertex nido anions, and reactions with nucleophiles and electrophiles. The prototype compound 1,2-PCB10H11 (12-111), an air-stable white sublimable crystalline solid, is obtained in 40-50% yield by reacting nido-CB10 H11 3 with PCl3; at 500 C it rearranges to the 1,7 isomer 12-112 and at higher temperatures generates the para isomer 1,12-PCB10H11 (12-113) [153]. Very similarly to the thermal rearrangement of 1,2-C2B10H12 (o-carborane), the formation of the 1,7 isomer is a high-yield, relatively clean process, but the 1,12 species is obtained only as a mixture with 1,7-PCB10H11. Reduction of 12-113 with sodium naphthaleneide forms the 1,12-PCB10 H11 2 dianion which rearranges to the 1,7 isomer; oxidization of the latter species by CuCl2 yields neutral 1,7-PCB10H11 (12-112) [154].
12.5 Heterocarboranes of the group 15 elements
749
As in the case of the 1,7- to 1,12-C2B10H12 rearrangement discussed in detail in Section 10.3, a simple diamondsquare-diamond mechanism cannot produce 12-113 from 12-111 or 12-112, implying a more complex process. From thermal rearrangement studies on chlorinated derivatives, it has been suggested that the conversion of 1,2-PCB10H1012-Cl to 1,7-PCB10H10-9(10)-Cl may take place via mutual rotation of the two pentagonal pyramidal halves of the icosahedron [155]. However, the usual caveat in such studies applies, i.e., one cannot exclude the possibility that the halogen substituent itself influences the rearrangement mechanism.
3C
PCl3
P
P
P
650 C
500 C
C
(1) Na0, C10H8 (2) CuCl2
C
C = CH
12-112
12-111
piperidine
piperidine H
C
12-113
-
H
H
P
-
+
H
H
C
P P
C
C RX
12-115
12-114
12-116 H
P RX
R = Me, Et, PhCH2, CH2=CHCH2, PhCH=CHCH2
H
C
R
C
X = Br, I
P
12-117
R
12-118 Again reminiscent of the C2B10 carboranes, piperidine attacks 12-111 and 12-112 to extract the B(3)-H unit and form the corresponding nido-7,8- and 7,9-PCB9 H11 anions 12-114 and 12-115, which are, of course, analogues of the C2 B9 H11 2 (dicarbollide) ions. Protonation of 12-115 on an ion exchange column takes place exclusively at phosphorus, forming neutral nido-7,9-HPCB9H11 (12-116); similarly, alkylation of both anions affords the P-alkyl derivatives 12-117 and 12-118 respectively [156]. In similar fashion, the PCB9 H11 anions combine photochemically with Group 6 metal 2PCB9H11 complexes (M ¼ Cr, Mo, W) containing M-P s bonds [157], carbonyls to give nido-7,8- and 7,9-(CO)5M2 2PCB9H11 and with the cations nido-(Z7-C7H7)(CO)2Moþ and nido-Cp(CO)2Feþ to form the corresponding L(CO)2M2 compounds [91]. Removal of the bridge proton from nido-7,8- and 7,9-PCB9 H11 and the corresponding RPCB9H11 clusters affords open-cage PCB9 H10 2 and RPCB9 H10 anions, respectively. Like their dicarbollide cousins described in the following chapter, these species form Z5-coordinated main group [92] and transition metal [158,159] phosphametallacarboranes such as 12-119 and 12-120 as well as their 7,8 (C and P adjacent) isomers, FeP2CB8 cages [160], and others listed in Table 12-4.
750
CHAPTER 12 Heteroatom carboranes of the main group elements 2− 2−
Ge P
C
GeI2
P
C
FeCl2
P
C
C P
12-119
C = CH
P
C
M(CO)6
Fe
THF
C6H6
2−
M(CO)5
Fe
hν −CO M= Cr, Mo, W
(CO)5M
12-120
P
C
12-121
Interestingly, the phosphorus atoms in 12-120 retain their ability to form s-bonded derivatives via coordination of their electron lone pairs to acceptor groups, as in photochemical reactions with Group 6 metal carbonyls to form complexes of type 12-121 [161]. The icosahedral phosphacarborane 12-111, like its counterpart o-carborane, can be metallated at the carbon atom by reaction with n-butyllithium and then treated with methyl iodide to form the C-alkylated species 1,2-P(Me)CB10H10 [152]. Similarly, lithiation of the 1,7- and 1,12 isomers 12-112 and 12-113 followed by reaction with methylmercury bromide forms the C-mercurated derivatives 1,7- and 1,12-P(MeHg)CB10H10 [162]. Also bearing a close resemblance to the behavior of o-, m-, and p-carborane is the reactivity of the closo-PCB10H11 isomers toward halogens. Both 12-111 and 12-112 undergo AlCl3-promoted bromination with Br2 to form mono- di-, and tri-B-bromo derivatives [153], and a more extensive study has shown that all three isomers are halogenated at boron under both electrophilic and photochemical conditions [163]. Electrophilic halogenation is easiest with 1,2-PCB10H11 and is progressively more difficult with the 1,7 and 1,12 systems, with up to two chlorine or iodine atoms and up to four bromines added, as expected, at the most electronegative BH vertexes, that is, those furthest from the P and C atoms (Table 12-4) [163]. Few X-ray structures have been reported, but those of 1,7-PCB10H9-9,10-Cl2 [155] and 1,7-PCB10H9-9,10-I2 [164] (12-122, X ¼ Cl, I) are available. Photochlorination of all three PCB10H11 isomers, as with the C2B10H12 carboranes discussed in Sections 9.5 and 10.6, is much less selective and produces all possible halogen-substituted products [163]. P
P X2 C
X C
AlCl3 X
X = Cl, Br, I
12-112
12-122
C = CH
In at least one important respect, the behavior of the closo-PCB10H11 isomers departs significantly from the C2B10H12 carboranes. Zakharkin and Kyskin found that 1,2-PCB10H11 (12-111) is more susceptible to base attack than are its carborane counterparts, with n-butyl llithium in ether-benzene solution at 30 C, or EtMgBr in THF at reflux, extracting the phosphorus atom from the cage to generate nido-CB10 H13 quantitatively [165]. −
P H
12-111
C
base
H
C
The same result is obtained with alcoholic KOH, but the reaction with 1,7-PCB10H11 (12-112) is slower and affords mainly nido-PCB9 H11 with only a 10% yield of nido-CB10 H13 . In contrast, 1,12-PCB10H11 is relatively unreactive toward nucleophiles.
12.5 Heterocarboranes of the group 15 elements
751
As in the icosahedral carboranes, lithiation of 1,7-PCB10H11 at carbon is efficiently achieved with n-butyllithium in ether, enabling the synthesis of C-substituted organic derivatives including the carboxylic acid as well as amides, ketones, and other products [166]. ð1Þ PCl5
CO2
2CPB10 H11 ! H2 NðOÞC2 2CPB10 H11 1; 7-PðLiÞCB10 H11 ! HOðOÞC2 ð2Þ NH3
12.5.2.3 Large dicarbon phosphacarboranes
Closo-PC2B9H11 phosphacarboranes—of special interest as isoelectronic analogues of the still-unknown closo-C3 B9 H12 þ system—are important synthetic targets but have not yet been found. However, the preparation of the chlorine-substituted cluster nido-3,1,2-ClPMe2C2B9H9 (12-123) from the reaction of the nido-7,8-Me2 C2 B9 H9 2 dianion with PCl3 comes close [167]. From the crystallographically established structure of its arsenic analogue [167], 12-123 is presumed to have the open geometry shown, consistent with its 28 skeletal electrons (Cl:P is assumed to donate four electrons to cage bonding). Cl P Me
C C
12-123
Me
A number of 10- and 11-vertex nido-PC2Bn (n ¼ 7, 8) clusters have been characterized. The arachno-6,7- and 6,8C2B7H13 isomers (Section 5.6) can be converted to dicarbon phosphacarboranes via the same approach used for the syntheses of monocarbon clusters from arachno-4-CB8H14 as described above. Accordingly, deprotonation of 6,7C2B7H13 and addition of RPCl2 (R ¼ Ph, Me) produce arachno-6,8,9-RPC2B7H11 (12-124) while the same treatment of 6,8-C2B7H13 affords arachno-6,5,7-RPC2B7H11 (12-125) [168,169]. Though structurally similar, the phosphorus centers in these isomers exhibit notably different base-donor properties. The lone pair in 12-125 coordinates readily to Lewis acids, generating endo-complexes of type 12-126, and also binds to electron-accepting metals forming endometallated derivatives such as the trans-PdBr2 and cis-PtBr2 species 12-127 and 12-128 [168]. Conversely, isomer 12-124 exhibits no such donor properties. It does however incorporate late transition metals to form Z4-coordinated metal complexes (Table 12-4) either via its mono-deprotonated 6,8,9-RPC2 B7 H10 anion or, as shown, via direct oxidative addition of cobalt to form nido-11,7,9,10-CpCo(PC2B7H9) (12-129). This structure, an 11vertex 26-electron cobaltaphosphacarborane, may be compared with the iron cluster 12-109 mentioned earlier, which has two fewer electrons and adopts closo geometry.
H H
H H
(1) RPCl2, 3 PS (2) HCl
C
−3PSH+Cl−
C
6,7-C2B7H13
R = Ph, Me C = CH = BH
R P
H
C C
12-124
Co
H
CpCo(CO)2 −2 CO, H2
Ph P
C C
R = Ph
12-129
752
CHAPTER 12 Heteroatom carboranes of the main group elements
H H
X H
H H
RPCl2 2 PS
C H C
R
P
H H
C C
−2PSH+Cl−
L = BH3•THF, S8, H2O2
12-125
12-126 X = BH3, HS, O
PdBr2
PtBr2 Ph
Ph
C C
Pd
Ph
P
P
Br
P
C C
Pt
C C Br
Br
C C Br
P Ph
Ph
C C
R = Ph
6,8-C2B7H13
HH
P
L
HH
HH
12-127
HH
12-128
Although DFT calculations show a large HOMO component at the endo-phosphorus position in both 12-125 and 12-124, the much stronger electron-donor character of the former isomer is attributed to greater delocalization of electron density from phosphorus into three nearby electron-deficient boron atoms in 12-124, as contrasted with 12-125 in which P is adjacent to only one boron [168]. It is also possible that the relatively close bridging hydrogen in 12-124 inhibits interaction of phosphorus with incoming electron-acceptors. Eleven-vertex phosphadicarbollide clusters are generated by inserting phosphorus into nido-C2 B8 H10 2 cages, as in the synthesis of the adjacent-carbon nido-7,8,9-RPC2B8H10 derivatives (12-130) via treatment of nido-5,6-C2B8H12 (Chapter 6) with proton sponge followed by RPCl2 (R ¼ Me or Ph) [170]. A similar procedure employing phosphorus trichloride affords nido-7,8,9-PC2B8H11 (12-131) [57] whose bridging proton is removable to give the monoanion 12-131 [147,171]. P
RPCl2
C
R
C
PS H H
C
C = CH R = Me, Ph
12-130
C
-
H
5,6-C2B8H12
PCl3
P C
C
-H+
P C
C
PS
12-131
12-131-
12.5 Heterocarboranes of the group 15 elements
753
Corresponding reactions of the nido-6,9-C2 B8 H10 2 dianion generate the carbons-separated phosphacarboranes nido7,8,10-PhPC2B8H10 (12-132) and nido-7,8,11-PC2B8H11 (12-133) whose structures are deduced from NMR data and DFT/GIAO calculations [147,172].
H
2− C
C
2− 6,9-C2B8H10
C
PhPCl2
C
P
C Ph
P
C
+
12-132
12-133
64%
PCl3
−
14% H
C
H+
C P
C
C
−H+
12-133-
P
12-133 35%
As in much of heterocarborane cluster chemistry, these phosphorus-insertion reactions offer rich synthetic possibilities with the potential to generate a multitude of novel structures; however, the ability to direct this chemistry toward specific target molecules will be largely a hit-and-miss proposition until a better understanding of the processes involved is gained. As expected, the chemistry of nido-7,8,9-PC2B8H11 (12-131) to a large extent parallels that of the dicarbollide systems. Electrophilic halogenation takes place almost exclusively at B(10), the most electronegative site, with chlorine, bromine, or iodine in a variety of solvents, forming the neutral species 12-134 that are easily deprotonated with proton sponge to give the anion [173].
−
H
H
P C
C
CCl4, Br2, or I2
X
P C
C
C = CH X = Cl, Br, I
X
P C
C
PS
AlCl3
12-131
−H+
12-134
12-134-
In a departure from dicarbollide metal chemistry, the electron lone pair on phosphorus allows a stepwise route to iron-phosphacarborane sandwich complexes, as seen in the reaction of 12-131 with [CpFe(CO)2]2 in refluxing benzene which affords the s-complex 12-135. On reflux at higher temperature, this compound isomerizes to 12-136 and also loses CO to give the closed polyhedral clusters 1,2,4,5- and 1,2,4,8-CpFe(PC2B8H10) (12-137 and 12-138) [174]. The latter two compounds are also formed on refluxing 12-136 in toluene.
754
CHAPTER 12 Heteroatom carboranes of the main group elements
H
P C
Fe
P
[CpFe(CO)2]2
C
C
C
C O
C O
C6H6, reflux
xylene reflux
C = CH
12-131
12-135 38%
Fe
Fe C
Fe
P
+
C
C
C O
C O
+
P
P C
C
C
12-136 28%
12-137 30%
12-138 5%
Thermal isomerization of 12-137 at 180-350 C generates still other isomers, some of which are unexpected and reveal pathways that are notably different from those encountered in MC2B9 cage rearrangements (Chapter 13) [175]. For example, in the conversion of 12-137 to 12-139 one carbon migrates toward phosphorus, while the formation of 12-140 requires movement of phosphorus away from the iron center. Moreover, 12-138 is converted to 12-142 in 51% yield on heating at 350 C [175].
C
Fe
Fe
Fe P
mesitylene
C
P
reflux
C P
C
C = CH
12-137
C
+
C
12-140 13%
12-139 15%
350 °C Fe
Fe P C
P
+ C
C
C
12-141 52%
12-142 23%
Clearly, driving forces other than just C-C separation are at work here. A triangle-rotation process (Section 10.30) would be consistent with these findings, but other mechanisms cannot be excluded. These observations underline not only the seemingly unbounded versatility of metallacarborane chemistry but also its truly daunting complexity. The introduction of phosphorus or other heteroatoms to the already complicated problem of metal-carborane cage isomerization ensures that considerable work lies ahead to untangle the mechanisms and truly understand these processes.
12.5 Heterocarboranes of the group 15 elements
755
Dicarbon-diphosphorus 11-vertex nido clusters that are formally related to the P3CB7H10 species 12-102 via replacement of one P vertex by CH (and are also isoelectronic with the C4B7H11 carboranes described in Chapter 7) have been prepared by phosphorus insertion into arachno-6,8- and 6,7-C2B7H13 following deprotonation [147,176]. The former carborane affords nido-7,8,9,11-P2C2B7H9 (12-143) accompanied by a B-chlorinated derivative; when triethylamine is used instead of proton sponge as a deprotonating agent, an additional isomer, nido-7,9,8,10-P2C2B7H9 (12-145) is obtained in low yield. Treatment of 6,7-C2B7H13 with proton sponge and PCl3 yields yet another isomer, nido7,8,9,10-P2C2B7H9 (12-146) along with two B-chloro derivatives. 11
C
7
PCl3 , PS H H
CH2Cl2
P
8
P
2
6
P
C 5
3
H
C H C
10 9
C P
C
+ Cl
4 1
C = CH
12-143 54%
12-144 7% P
PCl3 , NEt3
6,8-C2B7H13
12-143 (28%)
+
12-144 (15%)
+
C C
P
12-145
CH2Cl2 H H
3%
H
C
H
PCl3 , PS
C
CH2Cl2
P
P C
C
P
+
Cl
P C
C
P
+
P C
C
Cl 6,7-C2B7H13
12-146 21%
12-147
13%
12-148 1%
Halogenation studies on 12-143 show that electrophilic conditions favor substitution of chlorine or iodine at locations furthest from phosphorus but adjacent to the cage carbon atoms, i.e., at the B(5) and B(10) vertexes [177]. This finding departs from the usual behavior of carboranes, which generally undergo electrophilic attack at the boron vertexes most removed from carbon. Bromination of 12-143 with N-bromosuccinimide in CH2Cl2 proceeds differently, giving mainly the 3-bromo derivative with a small yield of the 2,3-Br2 species, a result consistent with a radical mechanism.
12.5.3 Arsenic and antimony As the listings in Table 12-4 attest, carboranes incorporating arsenic in the cage skeleton are fairly numerous, but it will be noted that all of the characterized species are either nido 11-vertex or closo 12-vertex systems. Only two antimonycontaining heterocarboranes are known, and none of bismuth have been reported (although BiEB10H10 [E ¼ P, As, Sb, Bi] clusters have been prepared [178]). While the structures of the heterocarboranes of the heavier Group 15 elements generally parallel those of their phosphorus counterparts, different synthetic approaches are required in some cases.
12.5.3.1 Monocarbon cages As in the case of phosphorus, the first arsa- and stibacarboranes were prepared by Todd and coworkers employing a procedure similar to the synthesis of 1,2-PCB10H11 (12-111) as described above. Accordingly, the reaction of Na3[CB10H11] (THF)2 with PCl3 in THF at room temperature forms white, sublimable 1,2-AsCB10H11 (12-149) in 25% yield, but the corresponding treatment with SbCl3 fails to give stibacarborane products. However, SbI3 and the carborane substrate in cold THF afford 1,2-SbCB10H11 (12-150) in 41% yield [179]. NMR and other spectroscopic data support icosahedral geometry analogous to 12-111 for these clusters. While their structures are not in doubt, the absence of X-ray crystallographic data for any closo-MCB10 Group 15 heterocarborane is a hindrance to gaining a more detailed understanding of their skeletal bonding (an X-ray study of the related 3,1,2-CpCo(PCB10H10) cluster has been reported [179]).
756
CHAPTER 12 Heteroatom carboranes of the main group elements
Thermal rearrangement of 12-149 to 1,7-AsCB10H11 (12-151) occurs at 495 C in a sealed tube, and heating at 575 C for 13 h generates the 1,12 isomer (12-152) together with 12-151 in a 1:1 mixture, as in the corresponding isomerization of the phosphorus analogue 12-111 described earlier [179]. The rearrangement of 1,2-SbCB10H11 to its 1,7 and possibly 1,12 isomers proceeds similarly at 500 C. “Reverse isomerization” of 1,12-AsCB10H11 to its 1,7 isomer, as in the previously discussed case of 1,12-PB10H11, is achieved by reduction with sodium naphthaleneide to the dianion followed by rearrangement at room temperature to 1,7-AsCB10 H11 2 and oxidation of the latter to give neutral 1,7-AsCB10H11 [154]. The m-arsacarborane 12-151, like its phosphorus counterpart, is lithiated by n-butyllithium in ether, and 1,7-As(Li) CB10H10 in turn affords an entry to C-organosubstituted derivatives including the C-carboxylic acid 12-153 and ketones such as 12-155 [180]. Cleavage of the latter compound with sodium ethoxide occurs primarily at the phosphacarboranecarbonyl linkage, demonstrating that the PCB10 cage is the stronger electron-acceptor. Further evidence of this comes from reactions of the C-lithioderivatives of 1,7- and 1,12-AsCB10H11 and their phosphorus counterparts with CH3HgBr to form the respective MeHg-CEB10H11 derivatives (E ¼ As, P). Detailed 1H NMR studies of these compounds show that the electron-attracting power in this family decreases in the order 1,7-PCB10H11 > 1,7-AsCB10H11 > 1,7-C2B10H10 > 1,12-PCB10H11 > 1,12-AsCB10H11 > 1,12-C2B10H10 [162]. These findings are as one would expect on the basis of the decreasing electronegativities of C, P, and As and the well-known order of CH acid strength in the C2B10H10 isomers discussed in Section 9.8.
As
As
As
(1) PCl5
(1) CO2 C Li
C
(2) H+, H2O
C
O
C
(2) C6H6,
C
AlCl3
P
O As
P
C C
Cl
12-154
H
12-153
O
C
C
12-155
Li
NaOEt 15%
85% O As
O P
C C
Na
+
H
C
As
C
H
+
C Na
P C
Paralleling phosphacarborane chemistry, piperidine deboronates 12-149 and 12-151 to generate the respective nido-7,8- and 7,9-AsCB9 H11 anions 12-156 and 12-157, and similarly converts 12-150 to nido-7,8-SbCB9 H11 . As in dicarbollide chemistry, deprotonation of the arsacarbollide anions followed by metal insertion affords icosahedral metallaarsacarboranes such as 12-158 [179] and the previously mentioned GeAsCB9H9 clusters 12-63 and 12-65 [92].
12.5 Heterocarboranes of the group 15 elements
As H
C
piperidine
−
Co
As
(1) Et3N
C
As
(2) CoCl2
C = CH
12-149
757
C
12-156
12-158
The closo-ferraarsacarborane 3-Fe(1,7-PCB9H10)2, like its phosphorus analogue 12-119, reacts with Group 6 metal carbonyls under ultraviolet light to form M2 2As bonded derivatives similar to 12-121 in which the electron lone pair on arsenic functions as a nucleophile [161]. Methylation of 7,9-AsCB9 H11 via reaction with methyl iodide proceeds as expected at the arsenic site to give 7,9-MeAsCB9H11, but the same treatment of 7,8-AsCB9 H11 has given inconclusive results, and no reaction is observed with 7,8-SbCB9 H11 [179]. Both isomers of nido-AsCB9 H11 undergo photochemical reactions with Group 6 metal carbonyls in the same manner as the nido-PCB9 H11 anions described above, to form the s-bonded 2AsCB9H11 (M ¼ Cr, complex nido-7,8-(CO)5Mo-AsCB9H11 (12-159) and the isostructural products 7,9-(CO)5M2 Mo, W) [157]. O C H
12-159
As C
Mo
O C CO
C O C O
12.5.3.2 Dicarbon cages A 1979 report describes the synthesis of nido-7,8,9-AsC2B8H11 (12-160) from the reaction of AsCl3 with nido-5,6C2B8H12 [57], a procedure analogous to the preparation of nido-7,8,9-PC2B8H11 (12-131) mentioned earlier. Although the structural characterizations of these clusters as 11-vertex nido cages with As or P adjacent to carbon on the open face seem reasonable, no further studies of either compound have been published. Deprotonation of arachno-6,8- and 6,7-C2B7H13 followed by treatment with AsCl3 or AsI3 yields results that closely mimic the corresponding reactions with PCl3 described previously. From 6,8-C2B7H13 one obtains 7,8,9,11As2C2B7H9 (12-161) [181,182] which is isostructural with 12-143, while 6,7-C2B7H13 affords 7,8,9,10-As2C2B7H9 (12-162) [182], an analogue of 12-146; in both cases, a B(3)-Cl or B(3)-I derivative is also isolated as a side product [182]. The assigned structures are supported by NMR spectra and DFT calculations, and by an X-ray crystallographic study on the B(3)-iodo derivative 12-163 [182] which confirms the structure of the parent compound 12-161 proposed 23 years earlier [181]. As
I
C As
C
12-163 C = CH
The attempted insertion of arsenic into nido-7,8-C2 B9 H11 2 (dicarbollide) ion by reaction with MeAsBr2 gives only intractable polymers, but treatment of the previously described thallium salt TlþTl(C2B9H11) (Section 12.3) with RAsX2 reagents yields 12-vertex clusters characterized from spectroscopic data as RAsC2B9H11 (12-164) [183]. In more recent work, ClAsMe2C2B9H9 derivatives have been obtained from nido-7,8-Me2 C2 B9 H9 2 and AsCl3 [167]. As in
758
CHAPTER 12 Heteroatom carboranes of the main group elements
cases discussed earlier involving phosphacarboranes, the RAs group could conceivably donate two electrons to the cage while retaining two electrons in a nonbonded lone pair (as suggested for 12-96), or alternatively could function as a fourelectron donor, creating a 28-electron 12-vertex system that would have an open structure. In fact, the latter model is supported by a crystallographic study of 12-164 (R ¼ Cl, R0 ¼ Me) that reveals an open geometry, a structure also assigned to the phosphorus analogue 12-123 [167]. Br As R Tl+ TlC2B9H−11
RAsX2
As
BBr3
C
C
R R
Me2C2B9H92–
H
C C
H
R = Ph, R = H
AsCl3 or AsBr3
12-165 Me2CH
12-164 R = Me, Ph, n-C4H9; R = H
Me2CHMgCl
As
R = Cl, R = Me
R = Cl, R = Me
C
Me
C Me
12-166 Substitution at arsenic is achieved by reacting 12-164 (R ¼ Ph) with BBr3 to give the BrAs product 12-165 [183] and by treating the As-chlorinated cluster with Me2CHMgCl to give the isopropyl derivative 12-166 [167]. The arsacarboranes of type 12-164 in which R0 ¼ H are reported to decompose in air, although 12-165 is notably more stable than the others; all of them are completely converted to nido-7,8-C2 B9 H12 by ethanolic KOH [183]. The reaction of Me2AsBr with 7,8-C2 B9 H12 generates a diarsenic species characterized as (Me2As)2C2B9H11, for which a nido structure containing two AsMe2 groups has been proposed. NMR data do not distinguish between two possibilities, both containing one externally bonded AsMe2 group; the other AsMe2 moiety may occupy a bridging location on the open face or, alternatively, could reside in a 12-vertex nido-AsC2B9 cage framework [183].
12.6 HETEROCARBORANES OF THE GROUP 16 ELEMENTS Sulfur, selenium, and tellurium, with their six valence electrons, are capable of donating two to four electrons to skeletal bonding orbitals. As four-electron donors, neutral S, Se, or Te atoms can formally replace BH2 units, as in the known thiaborane clusters closo-1-SB9H9 [184] and closo-SeB11H11 [185] which are isoelectronic analogues of B10 H10 2 and B12 H12 2 , respectively. In neutral carborane cages, such replacements would lead to cations like closo-SC2 B9 H11 2þ or closo-SCB10 H11 þ , which would be 26-electron systems isoelectronic with C2B10H12 and CB11 H12 , but no such species have been characterized. At this writing, known Group 16 heterocarboranes are limited to 6- to 11-vertex thiacarboranes and selenacarboranes, listed in Table 12-5. Again, while carbon-free tellurium clusters such as TeB11H11 [185] have been prepared, no telluracarboranes have been reported.
12.6 Heterocarboranes of the group 16 elements
759
Table 12-5 Heterocarboranes of the Group 16 Elements Synthesis and Characterization Compounda
Information
References
SULFUR 6-vertex SC2B3 clusters nido-2,4,5-S[(CHMe2)2C2B3H]-1-R-4-R0 R ¼ H, Ph; R0 ¼ H, Ph, C6Me4H nido-2,4,5-S[(CHMe2)2C2B3H]-3,6-[C6H-2,3,5,6-Me4]2
S, X(H, C6Me4H), H, B, C, MS
[186]
S, X, H, B, C, MS
[128]
S, H(2d), B(2d) S, H(2d), B(2d), MS
[189,203] [189]
S, H, B, UV S, B(2d), MS
[187] [188]
S, MS S, H, B, IR, MS B, Raman
[186] [190] [191]
S, H, B, MS
[203]
arachno-6,9-SCB8 H11
S, H, B(2d) S, H, B S, H(2d), B(2d) ED He photoelectron spectrum S, H(2d), B(2d)
[189] [137] [189] [250] [199] [189]
10-vertex SC2B7 clusters arachno-5,6,9-SC2B7H11
S, H, B, MS
[192]
10-vertex PtSCB7 clusters arachno-9,6,8-(PMe2Ph)2Pt(SCB7H9) arachno-9,6,8-(PMe2Ph)(Me2P-o-C6H4-10-)Pt(SCB7H8)
S, H, B, MS S, H, B
[205] [205]
10-vertex Ir2SCB6 clusters nido-2,7,8,6-Cp*2Ir2(SCB6H7-9-R) (R ¼ H, Cl)
S, X, H, B, MS
[204]
11-vertex SCB9 clusters nido-7,8-S(Me3CNH2)CB9H9 nido-7,8-S(NH3)CB9H9
S, H, B, IR, MS S, H, B, IR, MS
[194] [194]
8-vertex SCB6 clusters hypho-7,8-SCB6 H11 hypho-7,8-MeSCB6H11 9-vertex SCB7 clusters arachno-6,8-SCB7H11
9-vertex SC2B6 clusters “nido”-S(CHMe2)2C2B6H6 arachno-4,6,8-SC2B6H10
9-vertex MSCB6 clusters arachno-5,4,6-Cp*M(SCB6H10) (M ¼ Rh, Ir) 10-vertex SCB8 clusters nido-6,9-SCB8 H9 arachno-6,9-SCB8H12
Continued
760
CHAPTER 12 Heteroatom carboranes of the main group elements
Table 12-5 Heterocarboranes of the Group 16 Elements—Cont’d Synthesis and Characterization Compounda
Information
References
nido-7,8-SR(CB9H9) (R ¼ Me3N, Me2BuN) nido-7,8-S(C6H11NR2)CB9H9 (R ¼ H, Me) nido-7,9-SCB9H11 nido-7,9-SCB9 H10 2NH2)CSB9H9 nido-7,8-(Me3C2 2NH2)CSB9H9 nido-7,9-(Me3C2
S, S, S, S, S S,
[194] [194] [189] [189] [195] [195]
11-vertex SC2B8 clusters nido-7,9,10-SC2B8H10
H, B B H, B(2d), MS H, B(2d) X, H, B
nido-7,9,10-SC2B8H8-8,11-(HS)2 þ 3 isomers nido-7,10,11-SC2B8H10 arachno-1,6,7-SC2 B8 H11
S, B S, MS He photoelectron spectrum ED S, H, B, MS S, H, B(2d), C, IR, MS S, H(2d), B(2d), C, IR, MS
[132] [187] [199] [198] [200] [170] [201]
11-vertex RhSCB8 clusters 1,2,3-(Ph3P)2RhSCB8H9 nido-8,7,9-(Ph3P)2(H)RhSCB8H10
S, B, H, B, P S, B, H, B, P
[202] [202]
SELENIUM 10-vertex SeC2B7 clusters arachno-SeC2B7H11
S, H, B, MS
[193]
11-vertex SeCB9 clusters nido-7,9-Se[(Me3N)CB9H9] nido-7,8-Se[(C6H11NH2)CB9H9]
S, H, B, IR, MS S, H
[196] [194]
11-vertex SeC2B8 clusters nido-10,7,8-SeC2B8H10
S, B, MS
[185,197]
Theoretical Studies Molecular and electronic structure calculations arachno-6,9-SCB8H12 nido-10,7,8-SC2B8H10 nido-10,7,8-SC2B8H10 nido-2,8/7,8/7,9-SCB9H11 nido-1,7/7,1/7,8/7,9-SCB9 H10 nido-7,1,9/7,8,9/7,8,11/7,8,10/7,9,10/8,2,10-SC2B8H10 nido-1,7/2,4/7,8/7,9-SeCB9H11 nido-7,1/7,8/7,9-SeCB9 H10 nido-7,1,9/7,8,9/7,8,11/7,8,10/7,9,10-SeC2B8H10
Ab initio MNDO Ab initio DFT: relative DFT: relative DFT: relative DFT: relative DFT: relative DFT: relative
[250] [199] [198] [229] [229] [229] [229] [229] [229]
energies; energies; energies; energies; energies; energies;
energy energy energy energy energy energy
penalties penalties penalties penalties penalties penalties
Continued
12.6 Heterocarboranes of the group 16 elements
761
Table 12-5 Heterocarboranes of the Group 16 Elements—Cont’d Synthesis and Characterization Compounda
Information
References
NMR calculations 6,8,4-S(R2C2B6H6) (R ¼ H, CHMe2) nido-5,2,3-SC2B3H5 nido-SC2B6H8 arachno-6,9-SCB8H12 nido-7,10-11-SC2B8H10 arachno-4,6,8-SC2B6H10 arachno-5,6,9-SC2B7H11 arachno-1,6,7-SC2 B8 H11
IGLO IGLO IGLO Ab initio, B GIAO/DFT IGLO GIAO/IGLO GIAO
[186] [186] [186] [250] [170] [191] [192] [201]
S ¼ synthesis, X ¼ X-ray diffraction, H ¼ 1H NMR, B ¼ 11B NMR, C ¼ 13C NMR, F ¼ 19F NMR, P ¼ 31P NMR, Li ¼ 7Li NMR, 2d ¼ twodimensional (COSY) NMR, IR ¼ infrared data, MS ¼ mass spectroscopic data, UV ¼ UV-visible data, E ¼ electrochemical data, ESR ¼ electron spin resonance data, MAG ¼ magnetic susceptibility, COND ¼ electrical conductivity, OR ¼ optical rotation. a Heteroatoms (other than carbon) incorporated into the cluster framework are in boldface.
12.6.1 Sulfur and selenium 12.6.1.1 Six- to ten-vertex cages The smallest known thiacarboranes, organosubstituted derivatives of nido-2,4,5-SC2B3H5 (12-168), are obtained by hydroboration of the thiadiborolanes 12-167 and 12-169 [128,186]. R
Cl B
BCl2 (Me2Si)2S −2 Me3SiCl
BCl2
R
B
2 Li(RBH3) S
−2 LiCl
B
R = H, Ph,
Cl
2,3,5,6-C6HMe4
12-167
C C
B B
S
H
12-168 R = H, Ph R = H, Ph, 2,3,5,6-C6HMe4
R
H
H
B
B
B S
R
BH3•THF B
B B R
12-169
R = 2,3,5,6-C6H2Me4
R
R C C
B B
S
S R
12-168 R = 2,3,5,6-C6H2Me4
The nido 6-vertex cage architecture of 12-168, an isoelectronic cousin of the CnB6nH10n (n ¼ 1-5) nido-carboranes discussed in Chapter 4, is supported by spectroscopic data and an X-ray structure determination of the B(3,6)(C6HMe4)2 disubstituted derivative [128].
762
CHAPTER 12 Heteroatom carboranes of the main group elements
A side product of these reactions, S(Me2C)2C2B6H6 (12-170) has not been isolated but is of interest in terms of skeletal electron-counting theory. Its 1H and 11B NMR spectra, supported by an ab initio IGLO/NMR analysis, are consistent with a tricapped trigonal prismatic cage as shown. However, if the sulfur donates four electrons to cage bonding, the framework will contain 22 bonding electrons, corresponding to a nido system. The calculations imply that the sulfur atom has only one exo-polyhedral lone pair and hence is a four-electron donor, and reveal two long, presumably non˚ , consistent with the expected cage-opening [186]. bonding B2 2B interactions of 2.23 A S B B
B B
C
12-170 B = BH
C
B B
In contrast to the syntheses just described, most thiacarboranes have been prepared by insertion of sulfur into carborane cages, often accompanied by loss of boron and/or carbon from the cluster framework. Czech workers found that treatment of the “reactive” form of the nido-C2 B10 H13 anion (Chapter 7) with thiosulfate salts generates arachno6,8-SCB7H11 (12-171) in 45% yield [187,188]. H
−
H
C
HSO−3
C
H
H
H H
H
C
S
C2B10H−13
12-171
The highly reactive 9-vertex carborane arachno-4-CB8H14 (Chapter 5), from which aza- and phosphacarboranes have been synthesized as described earlier, reacts with sulfur in the presence of triethylamine to form arachno-6,9SCB8H12 (12-172) [137,189]. Deprotonation of this compound affords the monoanion, which is converted by acetone to nido-6,9-SCB8 H9 (12-173). Acidification of the latter species with concentrated HCl removes a boron vertex to afford the previously mentioned species 12-171, which is further degraded by hydride ion to an 8-vertex cage, hypho7,8-SCB6 H11 (12-174). Methylation of this cluster affords hypho-7,8-MeSCB6H11 (12-175) [189]. Although no X-ray structural analyses have been reported for any of these thiacarboranes, their assigned structures are strongly supported by two-dimensional (COSY) 1H and 11B NMR data. −
H
H
H
H
C
H H
H
S, Et3N
H
S
H
C
H
C
−H+
H
H
H
S
CHCl3
Me2CO
12-172-
12-172
CB8H14
−
H
C
H H
−
H
H
C S H
12-175
MeI Me
H H
H
H
C S
NaH
H H
H
C S
H
12-174
12-171
H
HCl
12-173
S
12.6 Heterocarboranes of the group 16 elements
763
Another example of degradative sulfur insertion is seen in the reaction of the nido-7,9-C2 B9 H12 anion with sodium sulfite and aqueous HCl, which gives the dicarbon species arachno-4,6,8-SC2B6H10 (12-176) in low yield together with the known carborane arachno-6,8-C2B7H13 (5-19, Chapter 5) [190,191]. Notably, the adjacent-carbon isomer nido-7,8C2 B9 H12 under the same conditions affords only 6,8-C2B7H13, underlining again the difference in reactivity between the two nido-C2 B9 H12 isomers that was discussed in Chapter 7. (As noted below, 7,8-C2 B9 H12 does react with NaHSO3 to afford 11-vertex nido-SC2B8H10.) H
C
C
H
H
S Na2SO3
C
H
H
C
+
C
dilute HCl
H
H H
H
H
H
12-176
7,9-C2B9H-11
H
C
6,8-C2B7H13
Similarly, treatment of 5,6-C2B8H12 with sulfur and triethylamine generates 6,8,9-SC2B7H11 (12-177), a dicarbon analogue of 12-173 [192]. A related selenacarborane, SeC2B7H11, has been obtained from the reaction of arachnoC2B7H13 with sodium polyselenide in aqueous base followed by acidification [193]. The characterization of this compound via NMR and mass spectroscopy does not yield a unique assignment of the cage structure, which may resemble that of 12-177 but with the carbon atoms occupying nonadjacent 7 and 9 vertexes rather than 8 and 9 as in the latter species. H
H H
C
H
S, Et3N
H
C
S
C
C H
H
12-177
5,6-C2B8H12
12.6.1.2 Eleven-vertex cages Monocarbon nido-SCB9 clusters have been prepared by diverse routes. Todd and coworkers obtained nido-S(R)CB9H9 and the corresponding selenium compounds via carbon insertion into 6-SB9H11 or 6-SeB9H11 employing alkyl isocyanides [194]. On the basis of NMR spectra and the assumption that no cage rearrangement occurred under the mild reaction conditions, they assigned the structure as 7,8-E(R)CB9H9 (E ¼ S or Se) (12-178). However, later work by Czech and Russian workers on the reaction of tert-butyl isocyanide with 6-SB9H11 disclosed that 12-178 (E ¼ S) does in fact isomerize at room temperature to 7,9-S(R)CB9H9 (12-179), whose structure was proved by X-ray crystallography [195]. H H
E
E
S C
RNC
C
r.t.
NH2-alkyl
NH2-alkyl
6-EB9H11
E = S,Se
12-178
12-179
In a different approach, a selenacarborane of the 12-179 class, 7,9-Se(Me3N)CB9H9, has been synthesized via treatment of (Me3N)CB9H11 with sodium polyselenide [196]. The thiacarborane 12-179 is obtained in parent form (12-180) together with 12-171 by thermolysis of 12-172 followed by protonation [189]. The thermolysis reaction evidently occurs via comproportionation in which boron is transferred between 12-172 cages with loss of H2.
764
CHAPTER 12 Heteroatom carboranes of the main group elements
H
H
C
-
-
H
S
S
Δ
C
-H2
12-172-
-
H H H
+
12-180-
12-171-
H+
H+
H
H
H H
S C
H
C S
H
12-180 62%
H
C S
12-171 17%
The 11-vertex dicarbon cluster nido-7,9,10-SC2B8H10 (12-181) is generated by degradative sulfur insertion into the nido-7,8-C2 B9 H12 anion [132], a process that has been shown to be extremely pH sensitive. In aqueous HCl, there is no reaction, but treatment of the carborane anion with K2S2O5 in water, followed by extraction of the product in ether and workup in hexane, affords 12-181 in 41% yield [187]. The selenium counterpart of 12-181, 7,9,10-SeC2B8H10, has been prepared by the reaction of 7,8-C2 B9 H12 with aqueous NaHSeO3 [185] or Na2SeO3 [197]. H
C
NaHSO3
C
S
C
C
C = CH
12-181
7,8-C2B9H-12
The structure of 12-181 is well established from gas-phase electron diffraction [198] and UV-photoelectron/ab initio [199] investigations. Sulhydrylation of this compound via reaction with sulfur over aluminum chloride at 120 C yields 7,9,10-SC2B8H8-8,11-(SH)2 and several unidentified isomers of that derivative [200]. A different 11-vertex dicarbon thiacarborane, nido-7,10,11-SC2B8H10 (12-182) has been prepared in 77% yield by sulfur insertion, without boron loss, into nido-5,6-C2B8H12 in the presence of proton sponge and structurally characterized from NMR spectra and DFT/GIAO calculations [170]. H H
C C
5,6-C2B8H12
SCl2, PS
C
C
S
−2 PSH+Cl− C = CH 2
12-182
Addition of elemental sulfur to the nido-5,6-C2 B8 H10 dianion in THF affords the structurally unique arachno-11 vertex species, 7,1,6-SC2B8H11 (12-183) in 60% yield [201]. As with most thiacarboranes, X-ray crystallography has been precluded by the difficulty of obtaining suitable crystals, but once again the ab initio/GIAO/NMR technique provides a reliable structural assignment. The 12-283 cage geometry can be viewed as a fragment of a 13-vertex closo polyhedron derived by removal of adjacent 6- and 5-coordinate vertexes; as expected from the rules outlined in Chapter 2, the sulfur and carbon atoms occupy low-coordinate vertexes [201].
12.6 Heterocarboranes of the group 16 elements
765
H H
2-
H
C
C
C
H
S S
C
12-183 H
25,6-C2B8H10
12.6.1.3 Metal sandwich complexes Given the high reactivity of open-cage carboranes and heterocarboranes toward transition metal ions, it is not surprising that the thiacarboranes exhibit similar behavior. The base-promoted reaction of arachno-6,9-SCB8H12 (12-172) with (Ph3P)3RhCl generates 11-vertex nido-(Ph2P)2HRh(SCB8H10) (12-184), which loses hydrogen on heating to form closo-(Ph2P)2Rh(SCB8H9) (12-185) [202]. In contrast to the formation of 12-184, which entails metal insertion with no loss of boron, reactions of 12-172 with (Cp*MCl2)2 (M ¼ Rh, Ir) give 9-vertex arachno-Cp*M(SCB6H10) products (12-186) [203]. The latter species can also be prepared by treatment of the hypho-7,8-SCB6 H11 anion (12-174) with the same rhodium and iridium reagents [203]. In the presence of a strong base, the reaction of 12-174 with (Cp*IrCl2)2 affords a diiridium-thiacarborane cluster, Cp*2Ir2(SCB6H8) (12-187), whose geometry has been crystallographically confirmed [204]. PPh3
Ph3P
PPh3
Ph3P
H H
H
C
Rh
H
H
S
C
(Ph3P)3RhCl
H
H
Δ
EtOH, base
12-172
12-184
12-185
98%
100%
M = Rh, Ir H
H H
[Cp*MCl2]2
H
C S
M = Rh, Ir
12-186 M
− H H
10 - 20%
H
C S H
[Cp*IrCl2]2 TMED
12-174 12-187
H H
TMED = N,N,N,N-
S
C
-H2
[Cp*MCl2]2
H
Rh
H
S
H
Ir
S
C
tetramethylnaphthalenediamine Ir
35%
766
CHAPTER 12 Heteroatom carboranes of the main group elements
In similar chemistry, arachno-6,8-SCB7H11 (12-171) combines with cis-PtCl2(PMe2Ph)2 to generate 10-vertex arachno(Me2Ph)2Pt(SCB7H9) (12-188). At 480 C this compound loses H2, but in contrast to 12-184, which forms a closed polyhedral cage, 12-188 undergoes ortho-cycloboronation to give 12-189 [205]. PMe2Ph
PMe2Ph H
H
H H
C S
Pt
H
S cis-(Me2PPh)2PtCl2
H
PMe2Ph
Δ
C
S
-H2
Pt
H H C
PMe2
TMED
12-171
12-188 41%
12-189 51%
Complexation of the thiacarbollide anion 12-179 , obtained by deprotonation of 12-179, with iron or cobalt reagents under visible light produces the monocarbon closo-thiametallacarboranes 12-190 and 12-191. At 110 C in toluene, the latter compound surprisingly undergoes polyhedral contraction to the 11-vertex species 12-192 via loss of a boron atom; the B(OH)2 group is proposed to form during workup of the reaction products in air [195].
Co S
+ C R (C4Me4)Co(C6H6) hν
12-190
71%
− S
Fe
CpFe(C6H6)+
C R
12-179R = NHCMe3
hν
S
C R
110 C
Fe S
C
B(OH)2
toluene (H2O)
12-191
76%
12-192
44%
The metallathiacarboranes, metallaphosphaboranes, and other transition metal-main group heterocarboranes add a new dimension to the already large field of metallacarborane chemistry outlined in the following chapter. Particularly intriguing is the question of what changes in structure and chemistry will be found as skeletal boron atoms are progressively replaced by metal and main group heteroatoms. Certainly the presence of main group elements in the cage framework can be expected to modify the properties of metallacarborane systems, especially in their behavior as catalysts and electronic conductors; however, the development of controlled, efficient synthetic routes to targeted cluster systems of this type presents a major challenge.
References [1] Wesemann, L. In: Crabtree, R. H., Mingos, D. M. P., Eds.; Comprehensive organometallic chemistry III; Elsevier: Oxford, England, 2007; Chapter 3.03 [Review]. [2] Hosmane, N. S. Pure Appl. Chem. 2003, 75, 1219 [Review]. [3] Hosmane, N. S.; Maguire, J. A. Organometallics 2005, 24, 1356 [Review]. [4] Saxena, A. K.; Maguire, J. A.; Hosmane, N. S. Chem. Rev. 1997, 97, 2421. [5] Fox, M. A.; Hughes, A. K.; Johnson, A. L.; Paterson, M. A. J. J. Chem. Soc. Dalton Trans. 2002, 2009. [6] Onak, T.; Dunks, G. B. Inorg. Chem. 1966, 5, 439. [7] Hosmane, N. S.; Yang, J.; Zhang, H.; Maguire, J. A. J. Am. Chem. Soc. 1996, 118, 5150. [8] Fessler, M. E.; Whelan, T.; Spencer, J. T.; Grimes, R. N. J. Am. Chem. Soc. 1987, 109, 7416. [9] Ezhova, M. B.; Zhang, H.; Maguire, J. A.; Hosmane, N. S. J. Organomet. Chem. 1998, 550, 409. [10] Hosmane, N. S.; Saxena, A. K.; Barreto, R. D.; Zhang, H.; Maguire, J. A.; Jia, L.; et al. Organometallics 1993, 12, 3001. [11] Hosmane, N. S.; Siriwardane, U.; Zhang, G.; Zhu, H.; Maguire, J. A. J. Chem. Soc. Chem. Commun. 1989, 1128. [12] Zhang, H.; Wang, Y.; Saxena, A. K.; Oki, A. R.; Maguire, J. A.; Hosmane, N. S. Organometallics 1993, 12, 3933. [13] Hosmane, N. S.; Jia, L.; Wang, Y.; Saxena, A. K.; Zhang, H.; Maguire, J. A. Organometallics 1994, 13, 4113.
12.6 Heterocarboranes of the group 16 elements [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57]
767
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12.6 Heterocarboranes of the group 16 elements
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CHAPTER
Carborane polymers and dendrimers
14
14.1 OVERVIEW The synthesis of high molecular weight films, coatings, elastomers, fibers, hard plastics, and oils from monomeric precursors was an early focus of research in carborane chemistry—indeed its principal motivator—as investigators discovered that the unique combination of steric and electronic properties afforded by carborane cages can lead to polymeric materials of unmatched thermal and oxidative stability. Most of these studies have centered on the electronic consequences of formally replacing aryl, olefinic, or other connecting groups in conventional polymers with carboranyl units while retaining the same general structures. Until a few years ago, relatively little attention was directed to the possibilities of creating architecturally novel three-dimensional polymers that exploit the unique steric properties of the carborane cages (for example, by employing boron-bound as well as carbon-bound connectors), although the conversion of carborane polymers to ceramics has attracted interest. Some aspects of carborane polymer chemistry have been mentioned briefly in previous chapters, for example the polymerization of alkenyl- alkynyl-, and carbinol-o-carboranes and other derivatives (Sections 9.7, 9.9, and 9.10), and several types of m- and p-carborane-based polymers (Sections 10.5, 10.9, 10.12, and 10.15). Although remarkable materials such as siloxy-linked polymers were the first carborane products to find commercial application, practical uses for carborane derivatives have long since expanded well beyond polymers per se, and now extend across a broad spectrum of interests, as the following chapters of this book demonstrate. Nevertheless, carborane polymers remain an area of active interest, both academically and in terms of practical use. The present chapter is an attempt to summarize the main compound types and to indicate the current state of the art in this area.
14.2 POLYMERS OF SUBICOSAHEDRAL CLOSO-CARBORANES In contrast to their larger homologues, the smallest carboranes polymerize under some conditions without the need for linking groups. As noted in Chapter 4, nido-1,2-C2B3H7 in the liquid state, or even in concentrated solutions, spontaneously forms a polymeric solid of the composition (C2B3H7)n that has not been structurally characterized [1], while liquid closo-1,5-C2B3H5 slowly forms a dimer and higher polymers [2]. The larger, much more stable cluster 2,4-C2B5H7 (Chapter 5) shows no such behavior, but its C-monolithio derivative polymerizes in ether-hexane [3]. As described in Section 5.3, solid wax siloxane-linked chains of the type 2CB5H5C2 2SiMe22 2O2 2]n2 2 (5-2) and similar 2,4-C2B5H7-based polymers have been synthesized [4–11]; 2 2[2 2SiMe22 however, in contrast to the C2B10-siloxane products described below, the lack of an economic synthetic route to C2B5H7 has prevented the C2B5 polymers from reaching commercial application despite their attractive properties. A few polymeric systems incorporating larger subicosahedral carboranes have been prepared. As is noted in Section 6.3, Kþ and Rbþ salts of the 10-vertex cluster closo-1-CB9H5-6,7,8,9,10-Br5 (6-31) form coordination polymers with bowl-shaped cyclotriveratrylene (CTV) hosts [12], while Ag(toluene)þ1-CB9 H5 Br5 crystallizes as a zigzag chain [13]. Siloxy-bridged polymers of closo-1,10-C2B8H10 have been synthesized [4]. Carboranes. DOI: 10.1016/B978-0-12-374170-7.00004-5 © 2011 Elsevier Inc. All rights reserved.
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1016 CHAPTER 14 Carborane polymers and dendrimers
14.3 POLYMERS OF OPEN-CAGE CARBORANES Owing to their generally higher reactivity, nido- and arachno-carborane cages are less attractive than closo clusters as units for incorporation into polymeric chains. However, the open faces on some such systems can be used to advantage to create novel cross-linked structures. For example, nido-7-(H3N)CB10H12 and a,o-dihaloalkanes interact to produce 2(CH2)n2 2}x copolymers of high molecular weight in which CB10H12 clusters are pendant to the chain {2 2NH(CB10H12)2 (Section 7.2); the carborane cages can be deprotonated and coordinated to metal cations, forming metallacarborane5CMe)C2 B9 H11 anion with methyl methcross-linked polymers such as 7-8 [14]. Co-polymerization of the nido-(CH25 acrylate generates linear chains with appended open-cage C2B9 units that are amenable to metal complexation as described in Section 14.6 [15,16]. Many nido- and arachno-carborane anions crystallize as polymeric chains linked by Group 1, Group 2, or lanthanide 2O2 2CH2CH22 2O2 2 metal cations, cited in Chapters 7, 11, and 12, as found for example in {Agþ[nido-m(SCH2CH22 2O2 2CH2CH2S)-7,8-C2B9H10]}n [17], in salts of C2 B10 H12 2 (12-11) [18], (CH2)nC2 B10 H10 2 (11-11) CH2CH22 [19,20], (Me3Si)4C4 B8 H8 2 [21,22], and (CH2)3C2 B11 H11 2 (11-42) [23], in metal-diborole polydecker compounds (13-183, 13-184) [24], in the spiral polymer [4,1,6-(MeCN)3Eu(C2B10H12)]1 (Section 13.6) [25,26], and in other transition metal, actinide, and lanthanide C2 B10 H12 2 polymers noted in Chapter 13.
14.4 POLYMERS OF CLOSO-CB11 CARBORANES The role of monocarbon closo-carboranes in polymer chemistry has been limited thus far, but two modes of interaction 5CH-(CH2)n2]CB11 Me11 salts have been demonstrated. Michl and coworkers found serendipitously that Liþ[CH25 þ 2[CH22 2CH(CB10Me10)2 2CH2)2 2 hydrocarbon (Chapter 8) undergo Li -catalyzed radical polymerization to afford 2 chains containing closo-CB10Me10 units as pendant groups [27]. The process takes place in the solid state or in solution and is initiated by O2, azoisobutylnitrile, or di-tert-butylperoxide [27–29]. The three-dimensionally aromatic CB11 Me12 radical has been suggested as a possible synthon for conducting polymers [30]. A different genre of extended system incorporating closo-CB11 clusters is that of coordination polymers featuring interactions between metal cations and negatively charged substituents on the carborane cage [31]. An example is the (bipyridine)Agþ PhCB11 H5 I6 salt shown in Figure 8-2, whose units are linked via Agþ-I and Agþ-pyridine binding [32]. A related system that does not involve metal ions is the extremely acidic species Hþ HCB11 Cl11 , the strongest currently known isolable acid (Chapter 8), whose solid state structure consists of a Cl2 2H Cl bridged linear polymer [33].
14.5 POLYMERS OF CLOSO-C2B10 CARBORANES Most current interest in carborane polymers is centered on the commercially available 1,2-, 1,7-, and 1,12-C2B10H12 (o-, m-, and p-carboranes) and their metal and nonmetal derivatives. Of the two main types, i.e., polymers in which the carborane cage is incorporated into the main chain (labeled Class I in the first edition of Carboranes [34]) and those having the cage pendant to the chain (Class II), the former has received more attention over the years because of the general assumption that the carborane must be part of the backbone in order to have a major influence on polymer properties. However, recent studies have altered this view to some extent, and Class II polymers are receiving closer attention. Moreover, it is now known that in some cases desired properties can be efficiently introduced into conventional polymers by introducing carboranes in small quantities as additives or dopants. A number of references to polymers of various types that are cited in earlier chapters of this book are extended and developed in the following discussion.
14.5 Polymers of Closo-C2B10 carboranes 1017
14.5.1 Systems with carborane units in the polymeric chain 14.5.1.1 Single atom-linked polymers Linear carborane polymers with single-atom connectors tend to be hard thermoplastic materials whose properties can be tailored by varying the metal, substituent groups, and reaction conditions. As noted in Chapter 10, reactions of C,C0 dilithio derivatives of m- and p-carborane with silicon, germanium, tin, and lead reagents generate short polymers along with monomeric products [35–37]. (The corresponding reactions of dilithio-o-carborane invariably lead to exocyclicsubstituted monomeric products, as discussed in Chapter 9.) THF; 0 C
2½CB10 H10 C2 2MR22 2n2 2 1; 7-Li2 C2 B10 H10 þ R2 MCl2 ! 2 M ¼ Si; Ge; Sn; Pb; R ¼ Me; Et; n-C4 H9 ; Ph In general, p-carboranyl polymers of this type remain hard at higher temperatures (300-400 C) than do their m-carboranyl analogues, which soften at ca. 250 C. [36,37] Cage substituents can have a marked influence; for example, replacement of the B(9)2 2H hydrogen atom by bromine in the SnMe2-linked polymer raises the softening point by nearly 100 C [38]. The choice of linking group is also crucial: while Ph2SnCl2 and p-Li2C2B10H10 form a polymer 2[CB10H10C2 2SnMe22 2]n2 2 that is insolaveraging 11 repeating units, Me2SnCl2 generates a material formulated as 2 uble in organic solvents and is air-stable up to 425 C [38]. Heteropolymers containing both m- and p-carboranyl units or combining germanium and tin linkers have also been obtained. [38] 1; 7-Li2 C2 B10 H10 þ 1; 12-Li2 C2 B10 H10 þ Me2 SnCl2 ! ½1; 7-CB10 H10 C2 2SnMe22 2n ½1; 12-CB10 H10 C2 2SnMe22 2n
1; 7=1; 12-Li2 C2 B10 H10 þ Me2 GeCl2 þ Me2 SnCl2 ! ½2 2CB10 H10 C2 2GeMe22 2n½2 2CB10 H10 C2 2SnMe22 2n Phosphorus- and sulfur-linked chains are similarly prepared from dilithiocarboranes; for example, 1,7-Li2C2B10H10 and PCl3 afford a low molecular weight polymer [39], while somewhat longer m- and p-carborane sulfur-carborane polymers can be produced via reactions with bis(chlorosulfenyl) carboranes [38,40,41]. 1; 7-Li2 C2 B10 H10 þ 1; 7-ðClSÞ2 C2 B10 H10 ! Cl2 2½2 2CB10 H10 C2 2S2 2n2 2Cl;
n ffi 30;
1; 12-Li2 C2 B10 H10 þ 1; 12-ðClSÞ2 C2 B10 H10 ! Cl2 2½2 2CB10 H10 C2 2S2 2n2 2OH;
mp 231-233 C mp > 410 C
The propensity of mercury to form stable R2 2Hg2 2R arrays suggests that this metal might be a good linker for carboranyl groups. Mercury-linked polymers are known, but rare; the reaction of C,C0 -dilithio-m-carborane with HgCl2 reportedly gives a product with a molecular weight of 10,000 [42]. 1; 7-Li2 C2 B10 H10 þ HgCl2 ! 2 2½CB10 H10 C2 2Hg2 2n2 2 Aryl-p-carborane units linked by mercury atoms, as in the monomeric species 10-10 and 10-11 [43] (Chapter 10), are potential synthons for long-chain polymers although none has been reported.
14.5.1.2 Alkynyl-linked polymers The inclusion of acetylene connectors in linear carborane polymers promotes thermal cross-linking, leading to inorganic/ organic “hybrid” elastomers and ceramics that exhibit extremely high resistance to thermal and oxidative degradation [44]. Some of these, mentioned in Chapter 9, are of the pendant type and are described below. In recent work, the phenylacetylene-terminated m-carboranyl polymer 14-1 has been prepared from 1,7-Li2C2B10H10 and SiHMeCl2 and found to have exceptional thermal and oxidative stability and solubility in common organic solvents; it can be converted at high temperature to a black ceramic that undergoes no weight loss on further heating [45].
1018 CHAPTER 14 Carborane polymers and dendrimers
H
C
C
Me
Me
Si
C
C
H
Si
C
Δ
C
thermoset
1000 ⬚C
ceramic
n
14-1 Similar desirable properties are found in hybrid m-carboranyl diacetylene-siloxane-linked polymers of the composition C2 C2 C2 C2 f½2 2C 2C 2SiMe22 2A2 2SiMe22 2x ½2 2C 2C 2SiMe22 2O2 2SiMe22 2CB10 H10 C2 22 2SiMe22 2O2 2SiMe22 2y gn
(A ¼ p-C6H4; x ¼ 0-100%; y ¼ 0-100%), which are prepared by polycondensation of 1,4-dilithiobutadiyne with 1,7-[Cl (SiMe2O]2C2B10H10 and p-C6H4(SiMe2Cl)2 in varying ratios [46–48]. Insertion of m-carboranyl cross-linkers into diethynylbenzene-silicon chains results in products having the architecture of 14-2 in which structural changes occurring on thermal curing have been investigated in detail [49,50]. C
Si
Si C
C
C
C
R2HSi
Si n
C
C
SiHR2
R C
C
C
Si
R
C
+
C
R
C
Si
R
C
C
C
R = Me, Ph
C
C
C
14-2
n
C n
Carborane-siloxane-acetylene polymers of several types have been investigated, including thermosetting materials that form surface layers of boron and silicon oxides at 400 C, and when heated at 900 C develop a coating of silica that retards oxidation of the bulk sample [51]. Alternating m-carboranyl-trisiloxane-diacetylene-linked copolymers of C2 C2 2CB10H10C2 2(SiMe2O)2SiMe22 2]n and block copolymers formulated as the type [2 2C 2C 2(SiMe2O)2SiMe2 2 n o C2 Cx2 ½ðSiMe2 OÞ2 SiMe22 2C 2C 2ðSiMe2 OÞ2 SiMe22 2½CB10 H10 C2 2ðSiMe2 OÞ2 SiMe22 2CB10 H10 Cy2 2 n
form elastomeric networks upon thermal curing. In the latter systems, the properties vary as the x:y ratio is changed, so that elastomers having desired characteristics can be obtained by varying the copolymer sequence [52–54]. In another variation, incorporation of ferrocenyl units into a m-carboranyl-siloxane-diacetylene chain yields a highly oxidationresistant polymer (14-3) that shows weight retention of 78% at 1000 C [55]; in comparison, other siloxyl-ferrocenylene polymers exhibit ca. 50% weight retention under similar treatment [56]. Me
H
Si
H
Si O
Me
Me C
C
H
H
Me
Si
Si O
Fe
14-3
Me
H
Si
H
Me
Si O
C
Me
H
C
Si
H
Si O
Me C
C
C
C n
14.5 Polymers of Closo-C2B10 carboranes 1019 Systems containing both alkynyl and aryl groups in the main chain form another important class of carborane polymers, discussed in the following subsection.
14.5.1.3 Arylene-linked polymers As with acetylenic linkers, the introduction of aromatic rings into the backbones of carborane polymers has a distinct stabilizing effect, with the added advantage that one can fine-tune the properties of the polymer by varying the nature and location of substituents on the aryl groups [57–62]. Heating aromatic compounds with icosahedral carboranes above 400 C generates B-arylated monomers and aryl-crosslinked three-dimensional polymers [57]. Alternatively, linear p-conjugated aryl-ethynyl-carborane polymers that exhibit intense blue luminescence can be obtained via polycondensation of bis(p-iodophenyl)carboranes with p-dialkynylarenes [63]. C2 CH ! ½2 C2 C2 2Ar2 2C 2C 2C6 H42 2CB10 H10 C2 2C6 H42 2C 2n 1; 7ðIC6 H4 Þ2 C2 B10 H10 þ HC Ar ¼ 1; 4-C6 H6 -3;6-R2 ðR ¼ C16 H33 ; C6 H13 ; CF3 Þ; 9;9-dihexylfluorene A polymer of the same type containing an axially chiral binaphthyl unit in the chain (14-4) shows similar behavior [64]. The optical properties of these species are attributable to the absence of a direct C2 2C bond in the m-carborane cage; the corresponding o-carboranyl polymers display no such luminescence, but do exhibit aggregation-induced emission. [64,65] C8H17O
C C
OC8H17
C C n
14-4
C C
Fluorene polymers incorporating o- or p-carboranyl cages in the main chain are photoluminescent under UV irradiation, and clearly reflect the participation of the boron clusters in the chain conjugation. While the p-carboranyl compound shows a strong blue emission typical of polyfluorenes [66], the o-carborane system (14-5) exhibits interesting orange fluorescence in CHCl3 arising from a combination of blue emission from the fluorine units and a lower energy emission from the o-carboranyl cages. In the solid state, 14-5 is a pure green emitter [67]. Br
H
n
Ni0, bpy C
C
1.5 COD toluene, DMF
C
C
14-5
1020 CHAPTER 14 Carborane polymers and dendrimers Other varieties of arylene-linked polymers include m-carboranyl polyazomthines [68], poly(dihydrobenzothia2CB10H10C2 2C6H42 2C(O)O2 2C6H42 2CB10H10C2 2O2 2]n [70], and zoles) [69], polyarylates such as [2 2C(O)2 2C6H42 2O2 2C6H42 21,2-CB10H10C2 2C6H42 2O2 2C6H42 2C6H3R2 2]n (R ¼ Ph, aryl ether systems of the type [2 2C6H42 C6H4OPh) that can be thermally cross-linked at 400 C to give three-dimensional solids [71]. The polyarylates have attracted sufficient interest in applications such as liquid crystals (Chapter 17) to warrant extensive studies of their thermal, electronic, dielectric, and other properties [72–76]. Also worthy of note is a carborane-acrylolylferrocene copolymer obtained by the reaction of o-carboranylstyrene with acrylolylferrocene in the presence of di-tert-butyl peroxide [77]. 2CB10H10C2 2C6H42 2C(O)2 2C6H42 2 and 2 2CB10H10C2 2C6H42 2O2 2C6H42 2 C(O)2 2C6H42 2 Polyketones having 2 2C6H42 repeating units (derived from 9-126 to 9-127, Chapter 9) [78], or similar ones [79–83], can be linked in different ratios to afford o-, m-, and p-carboranyl polymers of varying solubility, heat resistance, and other properties. In particular, o- and m-carboranyl poly(aryletherketone)s having high molecular weights (up to 150,000) and exhibiting very high mass retention at 850 C even in air have been prepared [81].
14.5.1.4 Carborane polyesters Reactions of o-carboranyl diols with organic diacids to generate moderately high molecular weight ester-linked polymers are described in Section 9.9. The relatively simple syntheses and potentially useful properties of these materials (which include the polyarylates mentioned above, as well as polyester dendrimers [84] and acetylene peroxyesters [85]) have stimulated studies on molecular mobility [86], structure-reactivity correlation [87], polydispersity [88], crystallinity [89], and other properties [90]. For example, films exhibiting high metallic conductivity have been obtained by pyrolytically induced crosslinking in dihydroxyphenyl oxide polyesters [91]. As was noted in Chapter 9, thermolysis of bis(carboxyphenyl)-o-carboranyl polyesters results in extensive crosslinking, unlike conventional organic polyesters which decompose on similar treatment [92]. Carborane polyesters of the pendant type are described later in this section.
14.5.1.5 Carborane-siloxane polymers The stabilizing effect of carboranes on siloxane and silsesquioxane polymers was a very early empirical discovery, ascribed to the extremely high thermal and oxidative stability of the carborane cages and their ability to function as electron sinks; more recent work reveals that thermal crosslinking above 400 C also contributes to the oxidative stability of the polymeric products [93]. The original work on development of siloxy-m-carborane polymers in the 1960s [34,38,94–100] led to the first commercially available carborane products, stationary phases for high-temperature gas chromatography marketed as DEXSIL and UCARSIL. The basic process employed FeCl3-catalyzed condensation polymerization of 1,7-[(MeO) Me2Si]2C2B10H10 with bis(chlorosilyl)-m-carboranes, alkylchlorosilanes, or alkylchlorosiloxanes to form copolymers 2(SiMe2O)m2 2]n having molecular weights of about 10,000. Although this method prosuch as [2 2SiMe2-1,7-CB10H10C2 duces undesirable FeCl3-induced cross-linking which inhibits the formation of higher molecular weight linear polymers [101], the problem can be alleviated by using phenyl-substituted dimethoxy-m-carboranes in FeCl3-catalyzed condensation with SiR2Cl2 monomers to give desirable elastomers [102,103] or via reaction of 1,2-(HOCH2)2C2B10H10 with 5CH2) [104]. Cl2MeSi(CH25 Alternative approaches that avoid metal catalysts have been explored [105], including the condensation of 1,7-[(HO) Me2Si]2C2B10H10 with SiR2L2 reagents where R is an amino, carbamate, or ureido group and L is NMe2 or an amido unit [106–111]. With bis(ureido)silanes, linear polymers having molecular weights above 250,000 have been obtained. In an 21,7/1,12-CB10H10C2 2 alternative approach, m- and p-carboranyl-siloxane linear copolymers of the formula [2 2SiR22 2]n are obtained on reaction of the C,C-dilithiocarborane with Cl(SiR2O)2SiR2Cl (R ¼ Me, Ph) [110]. (SiR2O)32 More recently, elastomeric network polymers containing m-carboranyl-siloxane crosslinks, for example, 14-6, have been synthesized via rapid hydrosilation of bis(vinylsiloxy)- or bis(ethynylsiloxy) carboranes with reactive polysiloxanes in the presence of the Karstedt catalyst (platinum divinyltetramethyldisiloxane). These materials are characterized by oxidative stability up to 300 C and resistance to degradation below 500 C [112].
14.5 Polymers of Closo-C2B10 carboranes 1021
Me
Me
Me
Si O
Me
Me
Si C
Me
C
Me
Me
Si
H
Me
Si
Si
O
Me Me
O Si
Me
+
C
Si(OSiMe2H)4 C
Karstedt catalyst r.t., air
Me
Si
Me
O Me
Si
Me
H
Si Me
Me Me
Si O
Me
Me
Si
Si
C
C
Me Me
O
O
Me
Si
Si
H
H
Me
Si
O
Me
O
Me Me
Si
O
Si O
Me
Me
Me
Me
Si
C
Me
Si
C
Me
Me
Si
O
H
H
Me
Si H
Me
Si Me
O Me
Si
14-6
Me
C
C
Me
Si H
O
Me
Si H
H
Related, but different, reactions employing chloroplatinic acid (Speier’s catalyst) proceed much more slowly (days) and afford crosslinked polymers of a different structure (14-7) that are hard plastic materials [113]. H
Me
H
Si
Me
H
Si
H
Si
C
C
O
Me
Me
H
Si
Me
H
Si
O
Si
O
+
H
Me
Si O
n H2PtCl6 catalyst r.t., air
O
O
Me
Si
H
O
Me
H
H
Si Si
O
Me Me
Si O
H
H
C
C
Me
n
Me
Si
H
Si
Me
O
Si O
Me
Si O
14-7
n
Me
1022 CHAPTER 14 Carborane polymers and dendrimers Thermally stable networks incorporating o-carboranyl units have been prepared via hydrolytic and non-hydrolytic sol-gel processes [114], as illustrated by the synthesis of 14-8 from a bis(trichlorosilyl)carborane precursor. O
1,2-(Cl3Si)2C2B10H10 +
Si
C
C
Si
H2O, THF
O O
n
14-8 Hydrolytic sol-gel methods have also been employed to generate o-carboranyl xerogels of types 14-9 and 14-10 (the latter of which are pendant [Class II] polymers) whose properties are very dependent on the choice of catalyst and other conditions of synthesis [115]. SiO1.5 SiO1.5 C
14-9
14-10
C
C C
SiO1.5 n
Si
R
R = Me, Ph
SiO1.5 SiO1.5 n
In the xerogels of type 14-8, the carborane is retained in the product, which is a nonporous solid. In contrast, xerogels of type 14-9 undergo nucleophilic cleavage of the C2 2Si bonds by F or OH ions (a well-known reaction [116,117]) affording porous structures in which the carborane functions as a template that is removed from the material and completely recovered [115]. Recovery of the expensive carborane is an important development [118] that significantly enhances the economic viability of carborane-based materials [110]. As the state of the art in synthesis [52,110,114] and structural characterization [119] of carborane-siloxane copolymers has advanced, the synthesis of materials targeted for specific purposes has increased accordingly. Much activity has centered on peroxide-vulcanized materials, often reinforced by fillers such as high-surface silicas that have outstanding properties including retention of elastomeric behavior after long treatment at high temperature, resistance to flammability, and resistance to organic solvents [110]. Application of siloxane polymers in high-temperature gas chromatography, previously mentioned, continues as an area of interest [120–126], as does their use as precursors to ceramics [79,127– 132] and as oxidation-resistant coatings for carbon fibers [133] and other substrates (Chapter 17).
14.5.1.6 Class I polymers with other linking groups Incorporation of carborane cages into the backbones of conjugated polymers containing thiophene, pyrrole, or other heterocycles strongly affects the electronic properties of these materials. As mentioned in Chapter 9, thiophene derivatives such as 9-345 and 9-353 can be electropolymerized to give o-carboranyl conducting polymers exhibiting superior thermal and electrochemical stability relative to conventional thiophene polymers [134–136]. Electrochemical, UVvisible, and atomic force field microscopic studies of polymers obtained via electropolymerization of the o-carboranyl thiophene 9-353 and the m- and p-carboranyl species 14-12 and 14-13 show that the o-carboranyl system affords the most conjugated and highly conductive polythiophene [134]. DFT molecular modeling studies indicate that 9-353 undergoes an intramolecular b-b0 cyclization leading to a more planar conjugated polymer backbone than is obtained from 14-12 to 14-13 [134].
14.5 Polymers of Closo-C2B10 carboranes 1023 S S S
S
C
C
C
S C
C
C
14-12
C C
S
S C
C S
9-353
14-11
14-13
Thiophene- and pyrrole-based polymers containing pendant carborane cages are described below. A number of polyamides and polyimides incorporating m-carborane in the chain [107,109,137–142] including an electrically conducting phthalic anhydride-oxydianiline copolymer [143] have been reported, and Schiff base polymers have been obtained via condensation of 1,7-bis(amninophenylcarboxy)- or 1,7-bis(aminophenylamido)-m-carborane derivatives with dialdehydes [144]. Polyformals generated via condensation of m-carboranyl diols with formaldehyde have been described [145], and cross-linking with peroxy-substituted carboranes has been found to considerably enhance the oxidative thermal stability of low-density polyethylene [146].
14.5.2 Polymers with pendant carboranes Pendant-type (Class II) systems generated from alkenyl o-carborane derivatives, mentioned in Section 9.7, were among the first types of carborane polymers to be investigated, but the recognition that greater stabilization is achieved by incorporating the cages into the polymer backbone, as in Class I polymers, shifted research interest strongly in the latter direction. However, in recent years, there is renewed interest in Class II materials. Pendant carborane groups have been found to impart useful properties such as the stability of annealed films [147], and carborane-substituted monomers are employed as model compounds in studies of the thermosetting mechanism of phenylvinylsilylene-ethynylene polymers [148].
14.5.2.1 Hydrocarbon chains Linear polymers with attached o-carboranyl substituents, for example, 2 2[CH22 2CH(C2B10H10)2 2]n2 2 (9-91), are generated via g-irradiation of 1-alkenyl-o-carboranes [149–157]. Co-polymerization of 1-alkenyl-o-carboranes with styrene and methylmethacrylate has been employed to give products bearing ferrocenylcarbonyl groups at the C(2) cage location [158,159]. A number of references to polymers with appended carboranyl cages are cited in Chapters 9 and 10. As is noted in Section 9.7, diethynylbenzene-phenylacetylene copolymers exhibiting high oxidative thermal stability are obtained from nickelcatalyzed cyclotrimerization of 1-(p-ethynylphenyl)-o-carborane (9-97) and diethynylbenzene [160]; also mentioned is the copolymerization of 9-97 with p-diethynylbenzene to generate thermosetting polymers [161]. The designed syntheses of water-soluble p-carboranyl macromolecules including the dendrimer 10-49 [162,163] and the acrylate random copolymer 10-51 [164] are described in Section 10.9. Other recently explored pendant-o-carboranyl systems include the polyfluorene- [147] and oxynorbornene-derived [165] linear polymers 14-14 and 14-15, respectively, both of which can be copolymerized with their corresponding organic monomers to give products of higher molecular weight. In both cases, the carborane cages significantly improve the stability of the polymers compared to that of their non-carborane counterparts. The fluorescence emission of the polyfluorene polymer 14-14 [147] lacks the distinctive pattern observed in the related Class I fluorene system 14-5 discussed above, which arises from the conjugation of the carborane and fluorene moieties; in 14-14, such interaction is blocked by the dimethylene spacers.
1024 CHAPTER 14 Carborane polymers and dendrimers O n
O
n
O
N
R = SiMe2CMe3 R
R
R C
C
C
C
C
C
14-15
14-14
14.5.2.2 Pyrrole polymers Widespread interest in electrically conducting polypyrrole films has stimulated investigation of carboranyl-substituted polymers obtained from monomers such as 14-16 via electropolymerization, which is a fast, convenient method for depositing films of uniform thickness on electrode surfaces. Both 14-16 and 14-17 generate conducting polymers that are superior to unsubstituted polypyrroles in thermal and electrochemical stability, although the dimethylene-linked compound (14-17), with a lower oxidation potential, is more efficiently electropolymerized than is 14-16 [166,167]. Bis(carboranyl) mono- and dimethylene-linked pyrroles (14-18) do not generate polymer films under any of the conditions examined [168]. N Me
N
Me
C
C
C
N Me
Me
C
C C
C
n = 1, 2
nC
n
14-18
14-17
14-16
14.5.2.3 Phosphazene polymers Allcock and coworkers have shown that thermolysis of cyclic o-carboranyl phosphazene derivatives such as 9-290, discussed in Chapter 9, results in linear chain polymers that are rendered stable to hydrolysis by replacing the chlorine substituents with trifluoroethoxide groups to form products of type 14-19. An alternative approach involving treatment of poly(dichlorophosphazene) with C-lithio-o-carboranes results in polymers similar to 14-19 but containing three or more carborane cages per repeating unit. Both types have molecular weights of ca. 1 106 (14-19) [169].
C C
R
C Cl
N P
P
N
N
Cl P
9-290 R = Me, Ph
Cl
Cl
Cl
R⬘
R⬘
C
R
1) 250 ⬚C 2) NaOCH2CF3 −NaCl
N
P
N
R⬘
14-19
Metallacarborane analogues of 14-19 have also been synthesized (Section 14.6).
P R⬘
N
P R⬘
R⬘ = OCH2CF3
n
14.5 Polymers of Closo-C2B10 carboranes 1025 Similar carborane-phosphazene polymers have been obtained by Korshak et al. via reactions of 1,2-(HOCH2) C2B10H10 with poly(dichlorophosphazenes) and hexachlorocyclophosphazene [170,171] and by thermal polymerization of the spirobicyclic monomer 14-20 [172]. Cl
Cl C
O
N
P
N
P
P C
O
Cl
N
14-20
Cl
14.5.2.4 Dendritic systems Another area of current interest in the development of novel inorganic hybrid materials centers on two- or threedimensional networks constructed from carboranyl and multifunctional dendrimer building blocks [173]. Such systems can be regarded as conceptual extensions of monomolecular dendrimers such as the polysilyl system 9-174 and others discussed in Section 9.11, as well as the 12-cage o-carboranyl derivative 2C CH2 CH2 CH22 2CB10 H10 C2 2CH2 CH2 CH2 CðCH2 CH2 CH2 OCH2 PhÞ3 4 C½ðCH2 Þ82 2O2 2(terpyridyl)3Ru2 2 CB10H10C2 2SiMe2CMe3]8þ reported by Newcome and coworkers [174,175] and a four-cage C[CH22 4 complex prepared by Armspach et al. [176]. An interesting example of a dendritic network is provided by the designed synthesis of the siloxy-linked assembly 14-23 from a vinyl-terminated m-carborane derivative (14-21) and polyhedral oligomeric silsequioxane (POSS) clusters (14-22) via Karstedt-catalyzed hydrosilylation [177]. Materials such as 14-23, and reinforced versions of it having additional 2 2Si2 2O-Si2 2 linkages, have thermal-oxidative properties far excelling those of POSS networks lacking carborane clusters.
Si
O
Si
C
C
Si
Si
O
14-21 + H H
H Si
Si
O O Si O O Si Si O O O Si O Si O O O O Si H Si Si O O Si O Si O O O O Si
H
O
H
Si
Si
14-22
Si H
= Si8O12 H
14-23
= CB10H10C
A different type of dendronized polymer is illustrated by 14-24, in which fourth-generation polyester dendrimers are appended to a hydrocarbon polymer backbone via benzyl-p-carboranyl linking units; as expected, such materials exhibit high water solubility, facilitating potential biomedical applications [178,179]. Dendrimers containing metallacarborane appendages are described below.
1026 CHAPTER 14 Carborane polymers and dendrimers
n
14-24 Cl
OH
OH
C
HO
OH
O
O C
O
O
O
O
OH
O O
O HO
O
O
O
HO
O
O
O
O
O O O
O O
O
O
O
HO
O O
HO HO
OH
O OH
O
OH
O OH
HO OH
14.5.2.5 Metal coordination polymers of C2B10 carboranes The linkage of suitably functionalized o-, m-, or p-carboranes via metal ions, given the huge array of accessible derivatives of these clusters, opens the way to a potentially almost unlimited range of extended systems. (Compounds of this type, in which the metal ions reside outside the carborane cages, differ from polymers incorporating metallacarborane clusters, described in the following section.) Those investigated so far include porphyrin assemblies [180] such as Cu[porphyrin(4-C5H5N)]4[Cp*Ir(m-S)2(C2B10H10)]4(THF)2 and {Zn[porphyrin(C5H5N)]4[Cp*Ir(m-S)2(C2B10H10)]2(CHCl3)6}n and metal-organic frameworks (MOFs), for example, {Zn3(OH)[1,12-(CO2)2C2B10H10]2.5(DEF)4}n [181] and 2CO2)3(DMF)2]n [182] (DEF ¼ diethylformamide). In systems of the latter type, variation [Co4(OH)2(p-O2C-CB10H10C2 of reaction conditions leads to very different morphologies whose capacities to absorb gases such as H2, N2, and CO2 vary widely [182]. Solid-state metal-linked chains of open-cage carborane anions, mentioned in earlier chapters, are noted in Section 14.3
14.6 METALLACARBORANE POLYMERS The concept of incorporating metallacarboranes into films, coatings, and bulk polymers is a highly attractive idea for a number of reasons. In general, the metals are covalently bound into specified cage locations, and such materials can be expected to undergo reversible redox processes and survive exposure to extreme temperatures and reactive chemicals without degradation, and may be tailored to function as catalytic centers. Moreover, the extensively developed synthetic and cage-substitution chemistry of metallacarboranes discussed in Chapter 13, along with decades of experimental and theoretical elucidation of their physicochemical properties, opens the way to the design and synthesis of some truly novel metallopolymer systems.
14.6 Metallacarborane polymers 1027 Essentially, there are three ways of achieving inclusion of metallacarboranes in polymeric materials: covalent attachment of metal-containing units onto existing polymers, polymerization of metallacarborane monomers, and doping polymers with metallacarborane additives. All three approaches have been explored to a limited extent.
14.6.1 Metallation of Nido-carboranyl polymers Coordination of Co3þ ions to the carborane open faces in the linear polymer {2 2NH(CB10H12)2 2(CH2)n2 2}x to form a dark red cobaltacarborane-cross-linked network (7-8), in which the linking groups are Co-(CB10H12)2 sandwiches, is described in Section 7.2. Conversely, if one treats polymers bearing pendant nido-carboranes with transition-metal organometallic reagents, the result is not cross-linking but rather capping of the cage units to give closo-metallacarborane units, as seen in the synthesis of the phosphazene-based rhodacarborane system 14-25 and analogous polymers bearing 2 2CH2(C2B9H9)M(CO)3 pendant groups where M is molybdenum or tungsten [183]. PPh3
C C
− C H
H
N
Cl
P Cl
C H PPh 3 C
C
Cl C5H10NH
P
N 2
R⬘
N
−HCl
Cl
P Cl
n
H
Rh
N 2
(Ph3P)3RhCl
P R⬘
N
P Cl
n
N 2
P R⬘
n
14-25 Similar treatment of the copolymer obtained from a C-isopropenyl-nido-carborane and methyl methacrylate, mentioned earlier, is reported to generate the rhodacarborane polymer 14-26 [184,185]. PPh3
MeO
C
O
C
C CH2
C Me
M = K, Cs, NMe
CH2 x
−
M+
MeO
H
O
C
C (Ph3P)3RhCl
C Me
C
EtOH, 78 ⬚C y
x = 80-93, y = 20-7
CH2
C Me
CH2 x
Rh
H
H
PPh3
C Me
y
14-26
14.6.2 Metallacarborane dendrimers Given the rising interest in metallodendrimers [186–189], stimulated by their potential function as electrophores, chromophores, or catalysts in designed materials, the high stability and tailorability of metallacarboranes offer special advantages in this area. However, as with their carborane-based counterparts mentioned earlier, few metallacarborane dendrimers have been characterized to date. The first reported examples were the diaminobutane (DAB)-based species DAB-dend-{[nido-(Z5-C5H4NHC(O)]Co(Et2C2B3H5)}n (n ¼ 16 and 32 [14-27]), prepared in 76-79% yield as air-stable yellow solids [190]. As is found in cobaltocenium- and ferrocenyl-bearing dendrimers [191–195], there is no evidence of metal-metal interaction, and electrochemical reduction occurs simultaneously at all cobalt centers [190].
1028 CHAPTER 14 Carborane polymers and dendrimers B H C B H C B
H
H
H H
C
B B H C C B
H
B B C CB
B B C B B
Co
B B C C
Co
O H
C
B
Cl
H
C
C
C O
HN
HN
HN
Co
Co
O
O
C
H
B B C B C
H
Co
H
B B C B C Co
H
N
N
N O H C N
H
N
B H C B H C B
C O
N C O N C
N
14-27
N
N
N
H
N C
H
N
O
N
N
N
C N
N
H
N
N
Co
H
O
B B C C B
N
C NH
H
C NH
Co B C B C H B
H
O C
H
O
O
B C C B B
H
H
NH O C
C
C
Co
B C Co B C H B
NH
NH
O
Co
H
B C C B B
H
B C Co B C H B
H
H
NH C
O
Co
B
B C C H B B
H
C C B B
Co
O
B C Co B C H B
H
Co C C B B B
O
C
C CB B B H
C
NH C
NH
Co
O
H
N
N
NH
Co
N
N
N
NH
C B C B H B H
O C
N
O C
Co
C
H
N
N
N
C B C B H B H
O
H
H
Co
N C
H
O
C B C B H B
N
C N
B B C H C B
Co
O
H
H
C B C B H B H
O
N
O
Co
Co
C
H
N
B B C B C
O
N
N
O C N
Co
B C H C B B H
H
H
H
Co
H
N
N
H
B B B C C
B H C C B B H
Co
H
N
N
N
H
C O
HN
N
N
H
H
B = BH Ccarborane = C–Et
B H C B CB
Co
O
HN
HN
C
DAB-32 Et3N CH2Cl2
Co
N H
C O
Co
C
O C O
H
B B C C
O
B B H C C B
H
C B Co C B
Co
Co
Co
H
H
H
B B H C C B
H
B
H
H
H
H
Metallodendrimers containing the ubiquitous bis(dicarbollyl)cobalt anion have been synthesized from poly(vinylsilanes) to give products such as 14-28 and similar molecules having cyclo-Si4O4 cores [196], designed as water-soluble agents for delivering boron to cells for BNCT (boron neutron capture therapy) and other biomedical applications (Chapter 16). An unusual feature of these compounds, in contrast to many other bis(dicarbollyl)cobalt derivatives that have been prepared for various applications, is their attachment to the framework via the cage carbon rather than boron atoms. 8−
Co CC Si
CC
CC
Si
Si
Co Si
C
C
Si
THF, 50 ⬚C Si
C
C
Si C
Co
Si
C C
Si
Si
Si
Si
CC
Co C
Si
C
CC
Si
Si
Co C
Si
Co CC
Si
Si
Karstedt cat.
Si
Si Si
C
CC
Si Si
Si
H
Co
C C
Si Si C
14-28
C
Co C = C, CH
C C
Si C C
CC
Co
C
14.6 Metallacarborane polymers 1029 Other metallacarborane-containing dendrimers include star-shaped aryl ether systems such as, 14-29, in which, interestingly, the luminescence of the nonmetallated molecule is quenched by the introduction of three to nine m(1,10 )-SiMeH-3-Co(1,2-C2B9H10)2 units [197]. Such findings increase the prospects for successfully tailoring metallacarborane-based materials for application as optical sensors and other purposes.
Co
O
C C
O
6−
C C
CC
O
O
Si
Si
Co C
CC
Si
Co
O
C
O
H
O
C
Karstedt cat.
Co
THF, 50 ⬚C
C C
C C
Si O
O
O
O
C
O
O
C
Si
O O O
C
Co CC
O
O
C = C, CH
Si
14-29
C C
Co
Si C C
C C
C C
Co
14.6.3 Polymerization of metallacarborane monomers Several methods have been demonstrated for generating polymers from metallacarborane starting materials, many of them making use of the commercially available 3-Co(1,2-C2B9H11) 2 anion or its substituted derivatives. Copolymerization of deprotonated 8,8’-(m-NH2)-3-CoIII(1,2-C2B9H10)2 with 1,10-dibromodecane forms the pendant-cobaltacarborane product 14-30 with a molecular weight of 5660 [14]. H
H
N
+ n Br(CH2)10Br
Co C C
C C
NaH/THF N
−2n HBr C = CH
n
Co C C
C C
14-30
The same reaction sequence, when applied to NiIV[(H3N)CB10H10]2, produces the Class I polymer 14-31 of indeterminate molecular weight in which nickelacarborane sandwich units are incorporated into the main chain [14].
C Ni H3N
C
NH3
+ n Br(CH2)10Br
C NaH/THF
NH2
Ni H2N
−2n HBr
14-31
C n
1030 CHAPTER 14 Carborane polymers and dendrimers Another system of this class, brown solid 14-32, is obtained on condensation polymerization of the hexamethylene diamine salt of 3-Co(1,2-C2B9H10)2-m(8,8)-[C(O)OH]2 [198]. 2−
O CC
C OH OH
Co C
C
C
H2N(CH2)6NH2
CO2
C
Co
C
C
O
CO2
C
H
30-300 ⬚C
O
−2H2O
C
CC
C
Co C
N
O
H
C
14-32
C = C, CH
H2N(CH2)6NH2+ 2
N
n
Bis(dicarbollyl)cobaltate units functionalized with polythiophene substituents (14-33) can be electropolymerized to give highly conjugated poly(bisdicarbollylcobalt-thiophene) films that efficiently catalyze the reduction of protons to H2 [199]. − S CC
S
n
C
electropolymerization
n
Co C
semiconducting films
n = 1-3
14-33 Remarkable examples of self-assembled metallacarborane polymers are the ruthenacarborane-mercaptane complex 14-34 and its 3,1,2-CpCo(C2B9H10-8-SH) analogue, which have been shown by X-ray crystallography to consist of networks held together by C2 2H S2 2H H2 2B interactions involving both hydrogen and dihydrogen bonding [200,201].
Ru H
C
H
H
C
H
H
H
Ru
S C
H
S C H
H H H
H
H
CC
S
H
Ru H
H
CC
S Ru
14-34 A different type of coordination polymer is formed by Agþ CoðC2 B9 H11 Þ2 and a derivatized form of CTV, a rigid bowl-shaped molecule whose host-guest compounds with 1-CB9 H5 Br5 (6-31) are described in Section 6.3. The onedimensional cobaltacarborane-CTV complex is converted into a 2D network via addition of acetonitrile, which induces dimerization of the host CTV entities to form a structure that features both clathrate inclusion and specific host-guest interactions [202].
14.6 Metallacarborane polymers 1031 Other macrocyclic hosts can be employed, as seen in a complex consisting of layers of the nickel(II) N4-grafted crown ether 14-35 separated by layers of Csþ CoðC2 B9 H11 Þ2 units. Here again, dihydrogen bonding plays a role in stabilizing the structure, as adjacent cobaltacarborane units are held in place in a helical arrangement via B2 2H H2 2B interactions [203]. O
O
O O
N
N
O
Ni N
14-35
N O
O
O
O
These examples of metallacarborane crystal engineering likely represent the tantalizing emergence of a new subfield of nanotechnology that is sure to grow over time. By combining suitably functionalized metallacarboranes with well-chosen host molecules, one expects that a wide range of self-assembled two- and three-dimensional metallacarborane-based networks having desired electronic, optical, magnetic, or other properties can be constructed.
14.6.4 Metallacarboranes as dopants and inclusion compounds in polymeric materials Modification of polymer properties via introduction of small quantities of metallacarboranes into existing polymers generally offers less control over the product than one has in stoichiometric syntheses such as those described above. However, this approach offers the advantages of simplicity of preparation as well as economy, as in many cases, small amounts of the metallacarborane additive suffice to achieve the desired properties. Most reported materials of this type utilize the readily available CoðC2 B9 H11 Þ2 ion or its B-halogenated derivatives. Among those described are polymer membranes impregnated by CoðC2 B9 H8 Cl3 Þ2 that are employed as electrochemical sensors for Pb2þ ion [204] or lanthanide metals [205]; hydrogen-sensing polypyrrole microelectrodes [206,207] doped with CoðC2 B9 H11 Þ2 ; and a polymer consisting of poly(3,4-ethylenedioxy-thiophene) with non-extrudable entrapped CoðC2 B9 H11 Þ2 ions [208]. 2C(O)NH-pIn a very different approach, treatment of the m-carboranyl polyamide [2 2C(O)2 2CB10H10C2 2NH2 2]n with Cr(CO)6 affords a product exhibiting higher tensile strength, a higher melting point, and lower elonC6H42 gation than the unmodified polymer [209]. Although there is evidence for Cr-polyamide bonds in this material, the nature of the chromium interaction with the polymer has not been established.
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1036 CHAPTER 14 Carborane polymers and dendrimers [182] Farha, O. K.; Spokoyny, A. M.; Mulfort, K. L.; Galli, S.; Hupp, J. T.; Mirkin, C. A. Small 2009, 5, 172. [183] Allcock, H. R.; Scopelianos, A. G.; Whittle, R. R.; Tollefson, N. M. J. Am. Chem. Soc. 1983, 105, 1316. [184] Kalinin, V. N.; Mel’nik, O. A.; Sakharova, A. A.; Frunze, T. M.; Zakharkin, L. I.; Borunova, N. V.; Sharf, V. Z. Izv. Akad. Nauk. SSSR, Ser. Khim. 1984, 1966 [Russian p. 2151]. [185] Kalinin, V. N.; Mel’nik, O. A.; Sakharova, A. A.; Frunze, T. M.; Zakharkin, L. I.; Borunova, N. V.; Sharf, V. Z. Izv. Akad. Nauk. SSSR, Ser. Khim. 1985, 2442 [Russian]. [186] Astruc, D.; Blais, J.-C.; Cloutet, E.; Djakovitch, L.; Rigaut, S.; Ruiz, J.; Sartor, V.; Vale´rio, C. In Topics in Current Chemistry; Vo¨gtle, F., Ed.; Springer: Berlin, Germany, 2000; vol. 210, p 229. [187] Juris, A.; Venturi, M.; Ceroni, P.; Balzani, V.; Campagna, S.; Serroni, S. Collect. Czech. Chem. Commun. 2001, 66, 1. [188] Kreiter, R.; Kleij, A. W.; Klein Gebbink, R. J. M.; van Koten, G. In Topics in Current Chemistry; Vo¨gtle, F., Schalley, C. A., Eds.; Springer: Berlin, Germany, 2001; vol. 217, p 163. [189] van Manen, H.-J.; van Veggel, F. C. J. M.; Reinhoudt, D. N. In Topics in Current Chemistry; Vo¨gtle, F., Schalley, C. A., Eds.; Springer: Berlin, Germany, 2001; vol. 217, p 121. [190] Yao, H.; Grimes, R. N.; Corsini, M.; Zanello, P. Organometallics 2003, 22, 4381. [191] Casado, C. M.; Gonzales, B.; Cuadrado, I.; Alonso, B.; Moran, M.; Losada, J. Angew. Chem. Int. Ed. Engl. 2000, 39, 2135. [192] Gonzales, B.; Casado, C. M.; Alonso, B.; Cuadrado, I.; Moran, M.; Wang, Y.; Kaifer, A. E. Chem. Commun. 1998, 2569. [193] Gonza´les, B.; Cuadrado, I.; Casado, C. M.; Alonzo, B.; Pastor, C. J. Organometallics 2000, 19, 5518. [194] Vale´rio, C.; Fillaut, J.-L.; Ruiz, J.; Guittard, J.; Blais, J.-C.; Astruc, D. J. Am. Chem. Soc. 1997, 119, 2588. [195] Vale´rio, C.; Ruiz, J.; Fillaut, J.-L.; Astruc, D. C. R. Acad. Sci. II 1999, 79. [196] Jua´rez-Pe´rez, E. J.; Vin˜as, C.; Teixidor, F.; Nu´n˜ez, R. Organometallics 2009, 28, 5550. [197] Jua´rez-Pe´rez, E. J.; Vin˜as, C.; Teixidor, F.; Santillan, R.; Farfa´n, N.; Abreu, A.; Ye´pez, R.; Nu´n˜ez, R. Macromolecules 2010, 43, 150. [198] Fino, S. A.; Benwitz, K. A.; Sullivan, K. M.; LaMar, D. L.; Stroup, K. M.; Giles, S. M.; Balaich, G. J.; Chamberlin, R. M.; Abney, K. D. Inorg. Chem. 1997, 36, 4604. [199] Fabre, B.; Hao, E.; LeJeune, Z. M.; Amuhaya, E. K.; Barriere, F.; Garno, J. C.; Vicente, M. G. H. ACS Appl. Mater. Interfaces 2010, 2, 691. [200] Planas, J. G.; Vin˜as, C.; Teixidor, F.; Comas-Vives, A.; Ujaque, G.; Lledos, A.; Light, M. E.; Hursthouse, M. B. J. Am. Chem. Soc. 2005, 127, 15976. [201] Planas, J. G.; Vin˜as, C.; Teixidor, F.; Hursthouse, M. B.; Light, M. E. Dalton. Trans. 2004, 2059. [202] Hardie, M. J.; Sumby, C. J. Inorg. Chem. 2004, 43, 6872. [203] Malic, N.; Nichols, P. J.; Raston, C. L. Chem. Commun. 2002, 16. [204] Kirsanov, D. O.; Mednova, O. V.; Pol’shin, E. N.; Legin, A. V.; Alyapyshev, M. Yu.; Eliseev, I. I.; Babain, V. A.; Vlasov, Yu. G. Russ. J. Appl. Chem. 2009, 82, 247. [205] Kirsanov, D. O.; Legin, A. V.; Babain, V. A.; Vlasov, Y. G. Russ. J. Appl. Chem. 2005, 78, 568. [206] Masalles, C.; Borros, S.; Vinas, C.; Teixidor, F. Adv. Mater. 2002, 14, 449. [207] Zine, N.; Bausells, J.; Teixidor, F.; Vinas, C.; Masalles, C.; Samatier, J.; Errachid, A. Mater. Sci. Eng. C Biomim. Supramol. Syst. 2006, 26, 399. [208] David, V.; Vinas, C.; Teixidor, F. Polymer 2006, 47, 4694. [209] Komarova, L. G.; Krivykh, V. V.; Kats, G. A.; Babchinitser, T. M.; Bekasova, N. I. Vysokomol. Soedin., Ser. B. 1990, 32, 346 [Russian].
CHAPTER
Carboranes in catalysis
15
15.1 OVERVIEW The discovery of metallacarboranes in the 1960s soon sparked efforts to harness the chemistry of this new field for practical applications, with perhaps the most obvious potential lying in the area of transition-metal catalysis. Not only is there a direct analogy with metallocene-based catalytic systems via formal replacement of cyclopentadienyl ligands with carboranyl units, but the much greater versatility and robustness of the metallacarboranes suggest a huge potential for creating catalysts tailored to specific needs. Although catalytic roles for nonmetallated carboranes have been explored to a very limited extent as described in the following section, most attention has centered on metal-containing homogeneous (and to a much lesser extent heterogeneous) catalysis of organic processes. As in other carborane applications, their stability under a wide range of reaction conditions, coupled with their facile tunability via choice of substituents, makes their use in place of conventional catalysts attractive for certain purposes, especially when they can be recovered intact.
15.2 NON-METALLATED CARBORANES AS CATALYTIC AGENTS Most reports of catalysis by neutral carboranes in the absence of metals involve extreme conditions in which the cage disintegrates, as in the use of 1-n-hexyl-o-carborane [1] and methyl(tris-o-carboranyl)methyl perchlorate [2] to promote combustion of rocket and jet fuels, or the graphitization of polyacetylene resins in the presence of parent o-carborane [3]. An unusual case in which o-carborane is reported to play a catalytic role while presumably remaining intact (although the mechanism is not known) involves the thermal oligomerization of isophthalonitrile to form triazines [4].
15.3 CLOSO-CB11 ANIONS IN CATALYSIS
The HCB11 H11 cluster and its B-substituted derivatives are among the weakest known Br nsted bases, and as such, have found extensive use in the stabilization of very strong acids (Chapter 8) and as counterions to catalytically active cations, which is the topic of discussion here.
15.3.1 Salts with main group metal cations Although most applications employ its polyhalogenated derivatives, the unsubstituted closo-HCB11 H11 ion forms a low-melting (19 C) N-methylpyridiunium salt that serves as an ionic liquid medium for palladium-catalyzed dehalogenation of aromatic halides [5], and the same anion stabilizes the diorganotin cation Ph[2,6-(MeOCH2)2C6H3]Sn(mCH2)2Sn[2,6-(MeOCH2)2C6H3]Ph2þ, which catalyzes the acetylation of alcohols [6]. As has been demonstrated by Michl, the nearly noncoordinating CB11 Me12 ion, in combination with the lithium cation, affords Liþ in an extremely Carboranes. DOI: 10.1016/B978-0-12-374170-7.00003-3 © 2011 Elsevier Inc. All rights reserved.
1037
1038 CHAPTER 15 Carboranes in catalysis active form that catalyzes the rearrangement of pericyclic hydrocarbons [7] (Section 8.2) as well as the radical polymerization of terminal alkenes [8] and isobutylene [9]. Earlier, Todd and coworkers had found that Liþ CoðC2 B9 H11 Þ2 catalyzes the conjugate addition of silyl ketene acetals to hindered a,b-unsaturated carbonyls [10] as well as nucleophilic substitution on allylic acetates [11]. Also effective in catalysis are salts of closo-HCB11 Cl11 , the strongest known superacid [12–14] and other B-perhalo derivatives, as seen in several examples cited in Chapter 8, e.g., the hydrodefluorination of perfluoroalkanes by SiR3 þ HCB11 H5 X6 (X ¼ H, Cl, or Br) salts [15,16] and the ring-opening polymerization of cyclo-N3P3Cl6 [17]. Dialkylaluminum salts such as Et2 Alþ HCB11 H5 X6 (X ¼ Cl, Br) are effective catalysts for ethylene oligomerization [18], alkylation of aliphatic C2 2F bonds [19], and the polymerization of cyclohexene oxide [18].
15.3.2 Salts with transition metal-containing cations The ability of monocarbon carborane anions to enhance the catalytic activity of cationic transition-metal species is evident in a variety of reaction systems, especially those employing rhodium, iridium, or silver. As shown by Weller and mentioned in Section 8.2, ðPPh3 ÞAgþ CB11 H6 Br6 effectively catalyzes a hetero Diels-Alder reaction, affording the product quantitatively with a catalyst loading of only 0.1% [20,21]. Interestingly, this reaction requires the presence of water, implying a water-silver interaction in the active catalyst. Crystallographic and NMR studies reveal significant interaction between the silver ion and the carborane cage in both the solid state (15-1) and solution. In comparison with analogous (PPh3)AgþX catalysts in which X is BF4 , OTf, or ClO4 , 15-1 performs significantly better [21]. H
C Br
15-1
Br
Br Ag Br
PPh3
Br
Br
In a different application, the silver salt Agþ HCB11 H11 activates dihalogeno(diphosphane) metal complexes of cobalt, nickel, and palladium as catalysts for the vinyl addition and polymerization of norbornene [22]. The hydrogenation of alkenes is catalyzed by (norbornadiene)ðPh3 PÞ2 Rhþ HCB11 H5 X6 (X ¼ H, Br) and ðR3 PÞ2 H2 Irþ CB11 H5 X6 salts [23,24], while the unsubstituted CB11 H11 anion partnered with RhCl(C8H12)þ performs a similar role in the addition of arylboronic acids to aldehydes [25]. RhClðC8 H12 Þþ HCB11 H 11
C10 H72 2CHOþðHOÞ2 B2 2C6 H42 2OMe ! C10 H72 2CðOHÞ2 2C6 H42 2OMe 80 C
Other reaction types that exploit the near-noncoordination of CB11 counterions include the catalytic dehydrogenation of cyclohexane to benzene by ðZ4 -C6 H8 ÞðPh3 PÞ2 Rhþ HCB11 H5 Br6 [26] and the hydrosilylation of alkenes, catalyzed by the silylene-iridium salt 15-2 [27]. PR2
C
H
N
Ir
−
H
+
SiPh2 SiHPh2
15-2
H Br
PR2
Br
Br Br
Br Br
R = CHMe2
15.4 Exo-metallated carboranes in catalysis 1039
15.4 EXO-METALLATED CARBORANES IN CATALYSIS A more direct role for carboranes as catalytic agents is found in derivatives featuring exo-polyhedral substituents containing catalytically active metal centers. In these systems, the carborane cage (usually o-carborane) serves as a platform for attachment of the metal-containing group whose solubility and other properties can be adjusted by appropriate selection of substituents. Some exo-metallated o-carboranyl catalysts can be viewed as structural analogues of corresponding metallocene species [28] with the carborane cage formally replacing a cyclopentadienyl ring; others have no metallocene counterparts, and are designed to exploit the icosahedral C2B10 architecture for advantage in catalysis. A few selected systems of the latter type are presented here for illustration.
15.4.1 Closo-carborane systems 15.4.1.1 Complexes of early transition metals and lanthanide elements Much attention has been directed to the development of carborane-based olefin polymerization precatalysts that improve upon commercially developed organometallics, such as the cyclopentadienyl-bridged constrained-geometry complexes [29–32]. In the o-carboranyl systems, a few species utilize only one cage vertex as in 15-3 [33], but the more common architecture is of the exocyclic constrained-geometry type [34] utilizing both cage carbons, as found in 15-4 [35,36], 15-5 [37],and 15-6 [38]. All of them catalyze the polymerization of ethylene in the presence of MAO (methylalumoxane), the last two being particularly effective. In complexes of type 15-4, the species in which M ¼ Zr and E ¼ B is superior to the others, demonstrating that catalytic performance is strongly affected by choice of metal and bridging unit [36]. Me2HC
CHMe2 N
Cl
C M
R M = Ti, Zr R = H, Me
15-3
C C
15-4
Me
Zr
C Zr
C
Cl
Me
E Me
15-5
Me
E = C, Si
Me
Me
E
Si
C
C
C
Zr Me
NMe2
Me2N
Cl Cl
Cl
M = Ti, Zr, Hf E = B, P
Me
Me
C
C C
C
15-6
E E
M
C
Me Me
Cl
Me Al(NMe2)2
Zr
Me2N
15-7
C
Zr
C C
NMe2
E = C, Si
15-8
Me
Me
Other o-carboranyl zirconium complexes are effective in the catalytic conversion of CH25 5CHCN to poly(acrylonitrile) and the trimerization of PhNCO (promoted by 15-7) [39], as well as in the synthesis of syndiotactic poly(methyl methacrylate) from methyl methacrylate catalyzed by 15-8 in the absence of a cocatalyst [40]. The latter reaction is also catalyzed by lanthanide complexes having the geometry of 15-4 with E ¼ B, M ¼ Nd, Er, or Y, and a cyclopentadienyl ring in place of the indenyl group [41].
1040 CHAPTER 15 Carboranes in catalysis 15.4.1.2 Complexes of middle and late transition metals Exo-metallated o-carborane derivatives of the more electron-rich transition elements are catalytically active in a wide range of reactions. Both the iridium compound 15-9 and the square-planar bicyclic nickel complex 15-10 are active cocatalysts with MAO in the polymerization of olefins, the former affording up to 370 kg mol1h1 of polyethylene from C2H4 [42] while the latter species polymerizes up to 3000 kg mol1h1 of norbornene [43]. Dichalcogen-bridged CR (R ¼ Ph or C(O)OH) at 70 C to give rhodium complexes of type 15-11 catalyze the cyclotrimerization of HC equimolar mixtures of the corresponding 1,3,5- and 1,2,4-trisubstituted benzene products [44].
C C C
C C
E
C
Ni
Rh C
N
C
Ir
E
Cl
15-11
15-10
15-9
E = S, Se
The catalytic activity of a few exo-metallated m- and p-carboranes has been explored; for example, 1-(CO)(PhCN)(Ph3P) Ir-7-PhC2B10H10 promotes the hydrogenation of terminal olefins and alkynes under mild conditions [45]. In general, however, m- and p-carboranyl complexes have not been found to offer synthetic advantages over their o-carboranyl counterparts. As carborane chemists have become more adept at designing and synthesizing metal complexes to order, increasingly sophisticated systems have been devised, especially in the area of optically active catalysts. The synthesis of pure enantiomers such as the P-chiral phosphino complexes 9-308 and 9-309 and the diferrocenyl compound 9-310 are mentioned in Chapter 9. Reagents of the B(9)-BINOL-derived phosphite 9-316, described in Section 9.14 and its mcarboranyl analogue (10-77, Chapter 10) [46,47] promote the rhodium-catalyzed hydrogenation of dimethyl itaconite with up to 99.8% enantioselectivity. Similarly, multi-carboranyl chiral BINOL diphosphites such as 15-12 are effective in the palladium-catalyzed allylic amination of (E)-1,3-diphenylallyl acetate with pyrrolidine and di-n-propylamine, giving high enantioselectivitiy [48]. H
RO P RO
C O
C
15-12
R= RO
P
O
RO
Chiral phosphinooxazoline-substituted o-carboranes have been employed in iridium- and rhodium-catalyzed hydrogenation of olefins, affording very high ee values [49]. The diamidophosphite 15-13, employed as a ligand (L) in the complex (Z3-C3H5)PdL2, catalyzes the allylic alkylation of (E)-1,3-diphenylallyl acetate with dimethylmalonate in up to 95% ee [50]. Ph Me
Me
O C C
N P N
15-13
15.4 Exo-metallated carboranes in catalysis 1041 One can also append metal-containing moieties to chiral nido-C2B9 anions, affording complexes such as 9-317 (Section 9.14) that can enantioselectively catalyze the hydrosilylation of acetophenone and the hydrogenation of acetamidocinnamic acid, among other reactions. Asymmetric catalysis with phosphites can be employed to synthesize chiral carboranes, as in the allylic alkylation of 1-(2-phenylacetyl)-2-phenyl-o-carborane to give 15-14 [51] and similar reactions [52], and in the use of binaphthylphosphite derivatives 1,2-R2–9-OPO2(C20H12) (R ¼ H, Me) in the rhodium-catalyzed asymmetric hydrogenation of prochiral olefins [53].
RO
Ph
Ph
OMe
C
C
C
O Ph
P
N
Ph
N
C
[Pd(allyl)Cl]2
C
OC(O)OMe BSA, KOAc
OMe C
*
O
15-14
Ph
Phosphino-o-carboranes have been investigated as ligands in Suzuki-Miyaura palladium-catalyzed cross-coupling reactions, in which the steric bulk and tailorability of carborane ligands allow the design of catalysts having optimal properties for specific applications. In the example shown in Figure 15-1, cross-coupling of phenylboronic acid with p-bromotoluene is catalyzed by a palladium-(1-diphenylphosphino)-o-carboranyl complex formed in situ, affording the coupled product in 93% yield [54]. In this case, both the steric bulk and the electron-withdrawing nature of the cage are involved; for example, replacement of the 1-Ph2PCH2-C2B10H10 ligand by the less electrophilic 9-Ph2PCH2-C2B10H10 or the less sterically demanding 1-Et2PCH2-C2B10H10 ligands results in much lower catalytic efficiency. C
Me
C
B(OH)2
+ Br
PPh2
R
R
R = H, Me
Pd(OAc)2
FIGURE 15-1 Phosphino-o-carborane-promoted Suzuki-Miyaura coupling.
Similarly, 1,2-(Ph2P)2C2B10H10 has been shown to function as a dual-mode ligand in the palladium-catalyzed conversion of aryl iodides to allenes via the reaction 1;2ðPh2 PÞ2 C2 B10 H10
C2 ! RHC5 5C5 5CH2 R2 2I þ HC 2CH2 NðC6 H11 Þ2 Pdð0ÞLn
in which the carborane ligand functions in both the Sonogashira coupling and hydride transfer reactions [55]. Other reported catalytic applications of phosphino-o-carboranes include the hydrosilylation of olefins by 1,2(Ph2P)2C2B10H10 complexed to NiCl2 [56]; the carbonylation of methanol to acetic acid by the mono- and dinuclear complexes 15-15 and 15-16 (which, for M ¼ Rh, is a far more effective catalyst than RhI2 ðCOÞ2 ) [57–59]; and the hydrogenation of cyclohexane promoted by the N,P-chelated rhodium and iridium complexes 15-17, which takes place with 100% efficiency with the rhodium catalyst [59].
1042 CHAPTER 15 Carboranes in catalysis
O C
P
C
Ph
Ph
Ph
Ph
Me Me
O C
P
N
M
C
S
M C
C
PPh2
S
15-15
M = Rh, Ir
S
C P
C O
15-16
C
C
M
15-17
Ph
Ph
BF4-
+ M
C P
Ph
Ph
An example of an o-carborane-based catalytic system designed for a specific application is the ruthenium carbene 15-18, which can be deboronated to give the nido-C2B9 anion 15-19 [60]. Both compounds, modeled after the conventional catalysts of type 15-20, are highly effective in promoting ring-closing metathesis, but their solubility properties are complementary: 15-18 is highly soluble in nonpolar media while 15-19 dissolves easily in polar solvents such as acetone. Compound 15-19 is recyclable from ionic liquids and is suitable for attachment to cationic resins. (n-C4H9)4N+ H − C C
O C O
C H
Cl
15-18
L
Ru
L
O Ru
H
O
Ru
Cl
L=
Cl
15-19 N
L
O
Cl
Cl
Cl
15-20
N
Among other recently discovered catalytic roles for o-carborane-based complexes, nickel-silyl and -germyl complexes of the type (Et3P)2Ni[Me2E)2C2B10H10 (E ¼ Si, Ge) are reported to be effective in the catalysis of double silylation of unsaturated hydrocarbons, nitriles, and aldehydes [61]. A different, and mechanistically interesting, finding is the magnesium-assisted demethylation of 3-(o-methoxyphenyl)-o-carborane during palladium-catalyzed coupling of 1,2C2B10H11-3-I with the methoxyphenyl Grignard reagent to generate the 3-phenol derivative 15-21 [62]. In this process the o-carboranyl C2 2H hydrogens interact with the methoxy group via C2 2HO hydrogen bonds, affording an example in which both the cage geometry and its electron-withdrawing property influence the course of the reaction. ArMgBr Me
H
H
C C
C H
I
2-MeOPhMgBr
C H
H H
O
C H+
C
O
H
[Pd]
15-21
15.4.2 Nido-carborane systems Complexes of the exo-nido type, in which a transition metal is coordinated to the outer surface of a nido-C2B9 cluster, have played an important role in studies of metallacarborane catalysis since Hawthorne’s original investigations of rhodiumdicarbollide systems. In that work [63,64], the catalytic activity of icosahedral rhodacarboranes in the hydrogenation and isomerization of alkenes, and in the hydrogenolysis and hydrisilanolysis of alkenylcarboxylates, was shown to involve
15.4 Exo-metallated carboranes in catalysis 1043 an equilibrium between the RhIIIC2B9 cluster and an exo-nido Rh(I) tautomer (15-22). The latter species in turn undergo reversible regiospecific oxidative addition to a B2 2H bond, forming an exo-Rh(III) complex which is the active catalyst [65,66]. L
H
L
R
C
−
−
Rh
H
H
R C
C
C
R H
R
R
+ RhI H
R = H, alkyl, aryl
C
R
L L
C
15-22
H
+ RhIII
L
L
The exo-nido form can be stabilized via use of bulky R groups on the cage carbon atoms [67]. Although one might assume that higher catalytic activity would be achieved by forcing the equilibrium to the right, increasing the concentration of the open-cage form 15-22, this is not always the case. In some reactions, for example, the hydrosilanolysis of alkenyl acetates [68,69], use of a “forced” exo-nido complex (in which rearrangement to the closo tautomer is inhibited) actually reduces the catalytic effectiveness relative to the closo form, and in other cases there is little difference between the closo and exo-nido catalysts. As is noted in the following section, closo-MC2B9 clusters are superior to the exo-nido species in some catalytic applications. Teixidor et al. have explored the use of forced exo-nido complexes such as 15-23 to 15-25 that are catalyst precursors in the hydrogenation of terminal alkenes, but not internal alkenes. These species have stable Rh2 2L2 2C linkages which prevent rearrangement to the closo tautomers, as well as M2 2H2 2B three-center bonds. The latter feature is required [68] for catalytic activity in the hydrogenation of 1-hexene at 25 C and P ¼ 1 atm; isomers in which the metal is bound to the carborane only via the cage carbon vertexes are inactive [70–72]. Under these conditions, complex 15-23 (L ¼ SPh) gives by far the best performance, with evidence that dissociation of PPh3 is involved in the catalytic cycle, while the ruthenium complexes 15-24 are also effective. The rhodium-cyclooctadiene species 15-25 with L ¼ SR are essentially inactive in the hydrogenation of terminal olefins, but interestingly, they rearrange in solution to closorhodacarboranes that catalyze the hydrogenation of cyclohexene in high yield [73] (see following section).
PPh3 Rh
H
H
C Me
15-23
C
PPh3
PPh3 H
L
H
C
C
PPh3
Ru L
Me
C
L
Me
H
15-24 L = PEt2, PPh2, SEt, SPh
C
X
Rh
H
H
15-25
R = Et, Ph
X = H, Cl
Exo-nido metal complexes of these and similar types are active precatalysts in other reactions. Both 15-23 and 15-25 with L ¼ PPh2 are highly efficient in catalyzing the cyclopropanation of styrene and other alkenes with ethyl diazoacetate (EtDA), as are the ruthenium compounds 15-24, although with lower selectivity [71,74,75]. A designed bimetallic complex (15-26), whose rhodium centers are held in rigid proximity to each other with the aim of promoting cooperative action, catalyzes the hydrogenation of 1-hexene an order of magnitude faster than does Cp*2Rh2Cl4, and is also much more efficient than the latter compound in the cyclopropanation of alkenes with EtDA [76].
1044 CHAPTER 15 Carboranes in catalysis
+ Rh Cl Rh Cl
15-26
Ph Ph
H
S
H
C C
H
C S
Ph
C
P
O
P
O
Rh PPh2 Ph
15-27
Ph
Complexes such as 15-23 to 15-25, with their asymmetrical metal-to-cage coordination, offer possibilities for chiral catalysis. As is pointed out in Section 9.14, pure enantiomers of phosphino-substituted nido-carboranes are now accessible and these can be converted to metal complexes such as 9-317 which are effective hydrosilylation and hydrogenation catalysts. In another example, the chiral species 15-27 has been shown to catalyze the hydrogenation of acetamidocinnamic acid and the hydrosilylation of acetophenone, with up to 80% enantioselectivity in the former case but only 27% in the latter [77]. In other applications of exo-nido metallated carboranes, ruthenium complexes have been employed as catalyst precursors in the radical polymerization of styrene and methyl methacrylate [78–80], and the effectiveness of the ruthenium carbene 15-18 in catalyzing ring-closing metathesis was mentioned earlier in this chapter. Nido-carborane-methacrylate copolymers, cited in Chapter 14, can be complexed with rhodium and other metals to give polymer-bound catalysts incorporating exo-metallated or closo-metallacarborane cages (e.g. 14-26) for the hydrogenation of olefins and alkynes [81,82]. In the area of early transition metal catalysis, exo-nido complexes of the type L2MeZr-(7,8-C2B9H12) (L ¼ C5Me5, C5Me4Et) have been shown to polymerize ethylene at 60 C and 300 psi [83].
15.5 METALLACARBORANES IN CATALYSIS The defining characteristic of transition-metal metallacarboranes—the presence of one or more metal centers covalently bound into a carborane cage—together with the extensive development of synthetic methodology in this field (Chapter 13) makes metallacarborane catalysts designed for specified purposes a very attractive target, especially in applications where they outperform conventional catalysts and can be recycled. Consequently, considerable effort has been directed to the design, synthesis, and study of metallacarborane catalyst precursors. As in other areas of carborane research, interest has focused mainly on 12-vertex icosahedral systems because of the commercial availability of the C2B10 isomers.
15.5.1 Subicosahedral clusters As was noted in Section 13.3, 6-vertex nido-MC2B3 and 7-vertex closo-MC2B4 metallacarboranes are close structural analogues of metallocenes, yet are generally far more stable than the latter group and can be fused and linked in a variety of ways to form extended polymetallic systems; moreover, in many cases their metal centers can undergo reversible redox processes. While there appears to be considerable potential for developing catalytic applications of the smaller metallacarboranes [84], very few reports of their implementation have appeared. Aside from the 10-vertex clusters 2,1,6-(Ph3P)2Ru(RR0 C2B7H9) (13-271) mentioned in Chapter 13, which are excellent precatalysts for the homogeneous hydrogenation of terminal alkenes [85], the only subicosahedral metallacarboranes (in which the metal is bound into the cage framework) whose catalytic application has been demonstrated are the 7-vertex clusters 15-28 and variants bearing different phosphino ligands [86]. These compounds, when activated by MAO, catalyze the Ziegler-Natta polymerization of ethylene at 1 atm and 25 C. The complex having L ¼ Me and M ¼ Ti is particularly effective, affording 1890 kg of polyethylene per mole of 15-28 per hour and requiring only 500 moles of MAO per mole of metallacarborane; the polydispersity is low (PDI ¼ 2.29) indicating a relatively uniform polymeric product [86].
15.5 Metallacarboranes in catalysis 1045
Et Et
C C
15-28 M
L
P
L
M = Ti, Zr L = Cl, Me
P
15.5.2 Icosahedral clusters 15.5.2.1 Early transition metal complexes
The well-known effectiveness of Cp2MIVRþ cations (where M is a Group 4 metal) as Ziegler-Natta olefin polymerization catalysts has stimulated extensive research on neutral isoelectronic analogues of the type CpLMIV(R2C2B9H9) in which R2 C2 B9 H9 2 formally replaces a Cp ligand. The metallacarboranes offer the advantages of stability, tailorability via introduction of substituents on the cage, and solubility in a wide range of organic solvents, and are very similar electronically to the metallocene systems. Theoretical studies indicate that the catalytic activity of bent-metallocene analogues such as CpHZr(C2B9H11) arises, as in their Cp2M(H)þ counterparts, from two empty low-energy molecular orbitals that facilitate M2 2H2 2C, M2 2H2 2B, and M-Lewis base interactions [87]. In some of the early experimental work in this area, Jordan and coworkers found that 3,1,2-CpMeZr(C2B9H11) and related species are active catalysts in ethylene polymerization [87], as are other Group 4 MC2B9 clusters, especially those of zirconium [88,89]. Co-polymerization of ethylene with norbornene [90,91] and terminal olefins [92] has been achieved with similar zirconium systems. A kinetic study of the catalysis of copolymerization of C2H4 with 1-hexene or 1-octene by [Cp*MeZr(C2B9H11)]n with MAO or alkylaluminum cocatalyst revealed a complex mechanism that is strongly dependent on temperature and the type and concentration of the cocatalyst [92]. In a somewhat different application of metallacarboranes in olefin polymerization, bis(dicarbollyl) MðC2 B9 H11 Þ2 anions (M ¼ Fe, Co, Ni) have been employed as weakly coordinating counterions in the Cp2ZrMeþ-promoted copolymerization of ethylene with a-olefins [93]. As mentioned in Section 15.3, a similar passive role is played by the CoðC2 B9 H11 Þ2 anion in its lithium salt, affording a highly activated form of Liþ that functions as a powerful catalyst [10]. The dimeric hafnium complex 15-29 catalyzes the hydrogenation of internal alkynes to cis-alkenes, apparently via its mononuclear form 15-30 [94]. A different dihafnium species (15-31) promotes the regioselective dimerization of terminal alkynes while preventing formation of trimers or other oligomers, owing to the intramolecular cyclization of a mononuclear intermediate in the reaction [95].
H
C C
C C
H2
Hf
− H2
Hf H
C
C
2
C
C Hf
H
C C
C = C, CH
15-29
H
Hf
15-30
Hf H H
15-31
Constrained-geometry complexes of the type 3,1,2-Cl2Ti[R(Me2NCH2)C2B9H9] (R ¼ H, Me) [96], which feature Ti2 2N2 2C2 2cage linkages, and the very similar bent-metallocene systems Cl2M[(C5H4-CMe2)C2B9H10] (13-364 and 13-365, M ¼ Zr or Hf) [97] mentioned in Section 13.5, are outstanding catalysts for ethylene polymerization in the presence of MAO. The titanacarboranyl amide 15-32 prepared by Shen and Xie is effective in the catalytic transamination of guanidines, a potentially useful application in the synthesis of pharmaceuticals [98,99].
1046 CHAPTER 15 Carboranes in catalysis
R⬘ R
N
N H
R⬘2NH + 5-10 mol % 15-32
R⬘ N
C6D6 or C6H5Me 110-115 ⬚C
R
Me
R⬘NH2 + 5-10 mol % 15-32
Me N
R
N H
N
R
R⬘ R
C6D6 or C6H5Me 110-115 ⬚C
R = CHMe2, cyclo-C6H11 R⬘ = alkyl, cycloalkyl, aryl
N H
N N H
R
NMe2 N Me2N O
N Ti
15-32 C
C
15.5.2.2 Rhodium complexes The catalytic properties of metallacarboranes of Group 8-10 metals have been extensively studied, in particular those of rhodium and ruthenium. In many of these systems, the active catalyst is an exo-nido tautomer generated in solution, as in the rhodium system 15-22 discussed above; however, there are exceptions. For example, in the hydrosilanolysis of alkenyl acetates by triethylsilane, the closo form of 3,1,2-(PPh3)2HRh(C2B9H11) is a far more effective precatalyst than is its exo-nido tautomer [69]. The 3,1,2-, 2,1,7-, and 2,1,12-(PPh3)2H-Rh(C2B9H11) isomers, and various substituted derivatives, are active catalyst precursors in alkene isomerization and hydrogenation [68] The 3,1,2 and/or 2,1,7 isomers of this rhodacarborane have been found by Zakharkin and coworkers to be catalytically active in the hydrosilylation of alkenes [100], alkynes [100], and ketones [101], in the isomerization of unsaturated alcohols to aldehydes and ketones [102], and in the alcoholysis of silyl hydrides to silyl ethers [103]. The last-mentioned reaction, common in organic synthesis as a method for protecting OH groups, is also catalyzed 5CH2)C2 B9 H10 ] by 3,1,2-(Z4-C8H12)Rh[Me2C-C2B9H10] and the zwitterionic complex 3,1,2-(Z3-C8 H13 þ )Rh[(MeC5 3 [104]. Closely related to the latter species is a family of RS-substituted Z -cyclooctenylrhodacarboranes, 15-33, that are formed on room-temperature rearrangement of the previously mentioned exo-nido complexes 15-25.
15-25
C R
C
Rh
Rh
H
H
CHCl3
S R⬘
R = Me, Ph R⬘ = Me, Et, Ph
C R
C
15-33
S R⬘
Notably, the closo compounds 15-33 are more effective precatalysts in the hydrogenation of cyclohexene than are their exo-nido precursors [73]. In general, the effects of cage substitution on the catalytic properties of (PPh3)2HRh(C2B9H11) have been little investigated, although dynamic NMR data on several C-substituted derivatives reveals that the metal vertex undergoes hindered rotation on the five-membered face of the carborane ligand [105]. A study by Balagurova and coworkers on the polyfluorinated derivatives (PPh3)2XRh(C2B9H11nFn) (X ¼ H, Cl; n ¼ 1-3) found differences in selectivity, compared to parent (PPh3)2HRh(C2B9H11), in the catalysis of the hydrosilylation of styrene and phenylacetylene; for example, the formation of trans-PhHC5 5CHSiMe2Ph from styrene is favored by the fluorinated catalysts. Overall, the fluoro species are less selective than the unsubstituted complex [106]. Catalytic activity has also been demonstrated in polymer-bound rhodacarboranes. The polyphosphazenerhodacarborane system 14-25, described in Chapter 14, and its monomeric form both catalyze the hydrogenation of
15.5 Metallacarboranes in catalysis 1047 1-hexene at 2.5 atm and 25 C) [107], and the polyamide {3,1,2-H(PPh3P)2Rh[HNPhNHC(O)](CO)C2B9H10}n is effective in the hydrosilylation of 1-hexene (as is its monomer) [108]. Poly[3,1,2-(Ph3P)2HRh(C2B9H10)-4-styrylmethyl] promotes the isomerization of 1-octene to cis- and trans-2-octene in 20% and 65% yields, respectively) and catalyzes the reduction of blocked alkenes to alkanes [109]; an ESCA study of this system found identical electronic environments in the rhodium centers of the monomer and polymer [109]. Other applications of rhodacarboranes in catalysis have been explored. Neutral norbornadienyl (Z2,3-C7H7CH2)Rh (C2B9H11) rhodacarboranes function as precatalysts in the synthesis of doxycycline B, a tetracycline antibiotic widely employed in chemotherapy, via hydrogenation of methacycline [110,111]. In an elegant example of designed synthesis, Hawthorne et al. prepared the 1-n-butenyl-substituted rhodacarborane 15-34 which can be irreversibly hydrogenated to afford the 1-n-butyl derivative 15-35, whose rhodium center has a vacant coordination site and is one of the most active known catalysts for alkene hydrogenation [112]. A C,C0 -di(n-butenyl) version of 15-34, in which the two alkenyl groups link to form an exo-polyhedral methylated C7 chain coordinated to rhodium, is not hydrogenated by H2 but is moderately active in the catalysis of alkene isomerization [113]. H
PPh3
H
Rh C
15-34
PPh3
Rh H2
C
C
H
15-35
C
H
In a subsequent application of this work, the asymmetric (R)-BINAP complex 15-36, prepared in ionic liquid media, has been shown to quantitatively hydrogenate prochiral ketones at 50 C and 12 atm of H2 with very high ee values [114]. In this system, the catalytically active species is the Rh(III) complex 15-38, formed by reversible oxidative addition of Rh(I) to the B2 2H bonds in 15-37. Of the ionic liquids examined in this reaction, the most efficient is (N-nbutylpyridinium)þCB11 H12 , whose closo-carborane anion may interact with the rhodium center in 15-37 to generate species such as 15-39, which in turn may form catalytically active Rh(I) tautomers (e.g., 15-40) [114].
L = electron pair donor H
PPh3
H
C H
BINAP C
−PPh3
C
CC
− H
Rh
Rh
− H
C C
C
+ Rh
− 2L
H
15-35
15-37
15-36 (n-C4H9)C2B9H11-
H
L
15-38
− (n-C4H9)C2B9H11-
− C
+ Rh
CB11H12-
− CB11H12-
H
L
2L
H
H
H
H
H
+ Rh
H
15-39
H
− 2L
L
−
2L
+ C
Rh H
15-40
L
1048 CHAPTER 15 Carboranes in catalysis 15.5.2.3 Ruthenium complexes In principle, the use of less expensive ruthenium-based catalysts in place of rhodium compounds is desirable. However, while some catalytic functions of rhodacarboranes are also performed by ruthenium systems, in general the latter group affords no particular advantages in synthesis. Nonetheless, several specific ruthenacarborane-based applications have been explored. Charge-compensated complexes of the type 3,1,2-(Ph3P)2HRu(RR0 C2B9H8–8SR00 R000 ) [R, R0 ¼ H, Me; R00 , R000 ¼ Me, Et, Ph, (CH2)4] are catalyst precursors for the cyclopropanation of alkenes with diazoesters [74,115] and for the radical polymerization of styrene and n-butyl acrylate [116]. In the latter type of reaction, catalytic activity is considerably increased by replacing the two triphenylphosphine groups with a bidentate bisphosphino ligand as in paramagnetic 3,1,2-[Ph2P(CH2)nPPh2]ClRu(C2B9H11) complexes (n ¼ 2-4) [117]. Ph3P PPh3 H
Ru
15-41
H
C H
C
Ph2P Cl
Cl
PPh2
Ru Ph2P(CH2)2(PPh2)2 C6H6, 80 ⬚C
H
C H
15-42 C
The species for which n ¼ 2 (15-42), prepared from diamagnetic 15-41, is reported by Chizhevsky, Grishin, and coworkers to be one of the most efficient catalytic agents known for the controlled radical polymerization of vinyl monomers; unlike cyclopentadienyl-ruthenium polymerization catalysts such as Cp*RuCl(PPh3)2 and (Z5-C9H7)RuCl (PPh3)2, cocatalysts are not required with the ruthenacarborane systems [117]. Ruthenacarboranes of the 15-42 type function as chain growth regulators, allowing the controlled polymerization of methyl methacrylate (as do exo-nido ruthenium-carboranes mentioned earlier) [80]. Their performance is improved by 1-2 orders of magnitude in the presence of trialkylamine activators, which increase the reaction rate and the maximum polymer yield, while also affording a much narrower range of product molecular weights [78]. A related area of investigation concerns the Kharasch addition of CCl4 to alkenes, a reaction that operates via an atom transfer radical mechanism and has assumed new importance in the development of atom transfer radical polymerization [118]. Compounds in the 3,1,2-(Ph3P)2HRu(HRC2B9H8–8-SR0 R00 ) family [R ¼ H, Me; R0 , R000 ¼ Me, Et, Ph, (CH2)4] are reportedly excellent precatalysts for the Kharasch reaction, surpassing the performance of the best available cyclopentadienyl- and indenyl-ruthenium complexes for the addition of CCl4 to methyl methacrylate and styrene under mild conditions [119–121]. As with metallacarboranes in general, the ability to modulate the catalytic behavior via cage substitution is a further advantage afforded by these complexes.
15.5.2.4 Other late transition-metal complexes
Polystyrene-bound (Ph3P)2Ni(RC2B9H9) clusters (R ¼ H, Me), prepared from the corresponding [CH22 2CHPh 2]n polymer by BH extraction in ethanolic KOH and reaction with (Ph3P)2NiCl2, promote the polymeri(RC2B10H10)2 zation of substituted olefins without the need for a cocatalyst [122]. The same nickelacarboranes supported on singlewalled carbon nanotubes catalyze the polymerization of ethylene and vinyl chloride in the presence of MAO, with higher activity than is found for the unsupported monomeric complexes; it has been suggested that interaction of the metallacarborane cluster with the delocalized nanotube p-electron system may account for the improved performance [123]. Indirect roles of icosahedral metallacarboranes in catalysis have been described, e.g., the action of bis(dicarbollyl) complexes on the combustion of propellants [124] and the catalysis of phenylboration by iridium nanoparticles generated from (Ph3P)2HIr(C2B9H11) by reduction [125]. Also in this general category is the use of the weakly coordinating 3-Co(1,2-C2 B9 H11 Þ2 sandwich as a counterion for lithium in Liþ-catalyzed reactions, mentioned in Section 15.3.
15.5 Metallacarboranes in catalysis 1049
15.5.3 Supraicosahedral clusters While there is no apparent reason that metallacarboranes larger than 12 vertexes should not be as capable of catalytic action as their smaller cousins, such systems have been virtually unexplored. At this writing, the only reported example is the disilazane-bridged sandwich 1-TaN[2,4-(SiMe2)RC2B10H10]2 (13-418, R ¼ Me, Ph) mentioned in Section 13.6, which promotes alkene polymerization in the presence of MAO.
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1050 CHAPTER 15 Carboranes in catalysis [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [67] [68] [69] [70] [71] [72] [73] [74] [75] [76] [77] [78]
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15.5 Metallacarboranes in catalysis 1051 [79] Grishin, I. D.; Kolyakina, E. V.; Cheredilin, D. N.; Chizhevsky, I. T.; Grishin, D. F. Polym. Sci. Ser. A 2007, 49, 1079. [80] Kolyakina, E. V.; Grishin, I. D.; Cheredilin, D. N.; Dolgushin, F. M.; Chizhevsky, I. T.; Grishin, D. F. Russ. Chem. Bull. 2006, 55, 89. [81] Kalinin, V. N.; Mel’nik, O. A.; Sakharova, A. A.; Frunze, T. M.; Zakharkin, L. I.; Borunova, N. V.; Sharf, V. Z. Izv. Akad. Nauk. SSSR Ser. Khim. 1984, 1966 [Russian p. 2151]. [82] Kalinin, V. N.; Mel’nik, O. A.; Sakharova, A. A.; Frunze, T. M.; Zakharkin, L. I.; Borunova, N. V.; Sharf, V. Z. Izv. Akad. Nauk. SSSR Ser. Khim. 1985, 2442 [Russian]. [83] Hlatky, G. G.; Turner, H. W.; Eckman, R. R. J. Am. Chem. Soc. 1989, 111, 2728. [84] Colacol, T. J.; Hosmane, N. S. Z. Anorg. Allg. Chem. 2005, 631, 2659 [review]. [85] Jung, C. W.; Baker, R. T.; Hawthorne, M. F. J. Am. Chem. Soc. 1981, 103, 810. [86] Dodge, T.; Curtis, M. A.; Russell, J. M.; Sabat, M.; Finn, M. G.; Grimes, R. N. J. Am. Chem. Soc. 2000, 122, 10573. [87] Crowther, D. J.; Jordan, R. F. Makromol. Chem. Macromol. Symp. 1993, 66, 121. [88] Saccheo, S.; Gioia, G.; Grassi, A.; Bowen, D. E.; Jordan, R. F. J. Mol. Catal. 1998, 128, 111. [89] Zhu, Y.; Zhong, Y.; Carpenter, K.; Maguire, J. A.; Hosmane, N. S. J. Organomet. Chem. 2005, 690, 2802. [90] Altamura, P.; Grassi, A. Macromolecules 2001, 34, 9197. [91] De Rosa, C.; Corradini, P.; Buono, A.; Auriemma, F.; Grassi, A.; Altamura, P. Macromolecules 2003, 36, 3789. [92] Kim, I.; Ha, C. S. Appl. Catal. A 2003, 251, 167. [93] Hlatky, G. G.; Eckman, R. R.; Turner, H. W. Organometallics 1992, 11, 1413. [94] Yoshida, M.; Crowther, D. J.; Jordan, R. F. Organometallics 1997, 16, 1349. [95] Yoshida, M.; Jordan, R. F. Organometallics 1997, 16, 4508. [96] Kim, D.-H.; Won, J. H.; Kim, S.-J.; Ko, J.; Kim, S. H.; Cho, S.; Kang, S. O. Organometallics 2001, 20, 4298. [97] Wang, Y.; Liu, D.; Chan, H.-S.; Xie, Z. Organometallics 2008, 27, 2825. [98] Shen, H.; Chan, H.-S.; Xie, Z. Organometallics 2006, 25, 5515. [99] Shen, H.; Xie, Z. Organometallics 2008, 27, 2685. [100] Zakharkin, L. I.; Agakhanova, T. B. Izv. Akad. Nauk. SSSR Ser. Khim. 1980, 1208 [Russian]. [101] Zakharkin, L. I.; Agakhanova, T. B. Izv. Akad. Nauk. SSSR Ser. Khim. 1978, 2151 [Russian]. [102] Zakharkin, L. I.; Agakhanova, T. B. Izv. Akad. Nauk. SSSR Ser. Khim. 1978, 2833 [Russian]. [103] Zakharkin, L. I.; Zhigareva, G. G. Izv. Akad. Nauk. SSSR Ser. Khim. 1992, 41, 1284 [English]. [104] Zhigareva, G. G.; Podvisotskaya, L. S. Russ. J. Gen. Chem. 1994, 64, 564 (Zh. Obshch. Khim. 1994, 64, 619) [Russian]. [105] Marder, T. B.; Baker, R. T.; Long, J. A.; Doi, J. A.; Hawthorne, M. F. J. Am. Chem. Soc. 1981, 103, 2988. [106] Lebedev, V. N.; Balagurova, E. V.; Dolgushin, F. M.; Yanovskii, A. I.; Zakharkin, L. I. Russ. Chem. Bull. 1997, 46, 550. [107] Allcock, H. R.; Scopelianos, A. G.; Whittle, R. R.; Tollefson, N. M. J. Am. Chem. Soc. 1983, 105, 1316. [108] Kats, G. A.; Komarova, L. G.; Rusanov, A. L. Izv. Akad. Nauk. SSSR Ser. Khim. 1991, 855 [Russian p. 960]. [109] Sosinsky, B. A.; Kalb, W. C.; Grey, R. A.; Uski, V. A.; Hawthorne, M. F. J. Am. Chem. Soc. 1977, 99, 6768. [110] Felekidis, A.; Goblet-Stachow, M.; Liegeois, J. F.; Pirotte, B.; Delarge, J.; Demonceau, A.; et al. J. Organomet. Chem. 1997, 536-537, 405. [111] Pirotte, B.; Felekidis, A.; Fontaine, M.; Demonceau, A.; Noels, A. F.; Delarge, J.; Chizhevsky, I. T.; Zinevich, T. V.; Pisareva, I. V.; Bregadze, V. I. Tetrahedron Lett. 1993, 34, 1471. [112] Delaney, M. S.; Knobler, C. B.; Hawthorne, M. F. Inorg. Chem. 1981, 20, 1341. [113] Delaney, M. S.; Teller, R. G.; Hawthorne, M. F. J Chem Soc Chem Commun 1981, 235. [114] Yinghuai, Z.; Carpenter, C.; Bun, C. C.; Bahnmueller, S.; Ke, C. P.; Srid, V. S.; Kee, L. W.; Hawthorne, M. F. Angew. Chem. Int. Ed. 2003, 42, 3792. [115] Tutusaus, O.; Delfosse, S.; Demonceau, A.; Noels, A. F.; Nunez, R.; Vinas, C.; Teixidor, F. Tetrahedron Lett. 2002, 43, 983. [116] Tutusaus, O.; Delfosse, S.; Simal, S.; Demonceau, A.; Noels, A. F.; Nu´n˜ez, R.; Vin˜as, C.; Teixidor, F. Inorg. Chem. Commun. 2002, 5, 941. [117] Cheredilin, D. N.; Dolgushin, F. M.; Grishin, I. D.; Kolyakina, E. V.; Nikiforov, A. S.; Solodovnikov, S. P.; Il’in, M. M.; Davankov, V. A.; Chizhevsky, I. T.; Grishin, D. F. Russ. Chem. Bull. 2006, 55, 1163. [118] Severin, K. Curr. Org. Chem. 2006, 10, 217 [review]. [119] Simal, F.; Sebille, S.; Demonceau, A.; Noels, A. F.; Nu´n˜ez, R.; Abad, M.; Teixidor, F.; Vin˜as, C. Tetrahedron Lett. 2000, 41, 5347. [120] Tutusaus, O.; Delfosse, S.; Demonceau, A.; Noels, A. F.; Vinas, C.; Teixidor, F. Tetrahedron Lett. 2003, 44, 8421.
1052 CHAPTER 15 Carboranes in catalysis [121] Tutusaus, O.; Vin˜as, C.; Nu´n˜ez, R.; Teixidor, F.; Demonceau, A.; Delfosse, S.; Noels, A. F.; Mata, I.; Molins, E. J. Am. Chem. Soc. 2003, 125, 11830. [122] Yinghuai, Z.; Parthiban, A.; Kooli, F. B. H. Catal. Today 2004, 96, 143. [123] Yinghuai, Z.; Sia, S. L. P.; Carpenter, K.; Kooli, F.; Kemp, R. A. J Phys Chem Solids 2006, 67, 1216. [124] Nechai, G. V.; Sokolovskii, F. S.; Chuiko, S. V. Russ J Phys Chem 2009, 3, 458. [125] Yinghuai, Z.; Chenyan, K.; Peng, A. T.; Emi, A.; Monalisa, W.; Louis, L. K.-J.; Hosmane, N. S.; Maguire, J. A. Inorg. Chem. 2008, 47, 5756.
CHAPTER
Carboranes in medicine
16
16.1 OVERVIEW The enormous scope of carborane chemistry practically assures significant applications of this field to human needs, no doubt in some ways not predictable . . . It is conceivable that carborane-based . . . drugs may one day be important commercial products (Ref. [1], p. 181). If this 1970 prediction from the first edition of Carboranes has not yet been quite fulfilled, there has certainly been considerable movement in that direction. Of all the applications of carborane chemistry currently under development, none has stimulated wider interest, or holds potentially greater impact, than those centered on diagnostic and therapeutic medicine. In recent years, a number of reviews have appeared on various facets of this topic [2–21] that provide a level of detail well beyond what can be presented here. This chapter summarizes the main lines of investigation and actual applications accompanied by illustrative examples, with no attempt at comprehensive coverage. Indeed, what is written here can be only a snapshot of a very rapidly developing field in which any current summary is certain to be outdated in short order. One caveat: biomedical applications of noncarborane clusters such as B12 H12 2 and its substituted derivatives, while historically important and still under exploration to a limited extent, are basically outside the scope of this discussion. However, they are well covered in many of the reviews cited above. A number of general properties of carboranes lend themselves to exploitation in medicine: • • • • • •
Their thermal and chemical stability in a variety of environments allows them to survive in many biological systems without degradation, and they tend to show low toxicity. Their generally lipophilic and strongly hydrophobic character, arising from the hydridic nature of the B2 2H bonds, is advantageous in a variety of biomedical applications. Their high boron content—much higher than that of classical organoboron compounds—affords an efficient means of delivering the 10B isotope to target cells for boron neutron capture therapy. Controlled deboronation of closo-C2B10 cages generates water-soluble nido-C2B9 anions, which in turn can be converted to metallacarboranes via metal insertion. Well-developed methodologies for attaching functional groups to carborane cages, outlined in earlier chapters of this book, allow the synthesis of libraries of compounds designed for specific roles in therapy and imaging. Significant pharmacological activity has been discovered in a number of carborane and metallacarborane derivatives, opening the way to the creation of novel and potentially revolutionary classes of drugs.
16.2 CARBORANES IN DRUG DEVELOPMENT 16.2.1 Carboranes as pharmacophores For years, research on biochemically active carborane derivatives has been driven primarily by a search for more efficient ways to load boron into tumor cells for boron neutron capture therapy (BNCT), an important effort that continues to this day as described in the following section. More recently, the recognition that boron clusters have remarkable Carboranes. DOI: 10.1016/B978-0-12-374170-7.00002-1 © 2011 Elsevier Inc. All rights reserved.
1053
1054 CHAPTER 16 Carboranes in medicine properties that can be exploited in drug design, along with the discovery of antitumor behavior and other kinds of biological activity in some carboranes and metallacarboranes, has accelerated interest in their potential pharmacological value, quite apart from their utility as BNCT agents. Although the current high cost of carboranes would appear to limit their potential as pharmaceuticals, an important stimulus to research in this area is the need to “think outside the box” in drug design. For example, the increasingly urgent need for new classes of antibiotics is driving biomedical investigators to explore the use of novel molecular scaffolds [22]. If carborane-based agents can be developed that outperform conventional drugs for specific purposes (some already exist, as will be seen), the cost issue may be largely neutralized.
16.2.1.1 Design of hydrophobic carborane pharmacophores A special attribute of icosahedral C2B10 cages, important in drug design, is their nearly spherical shape with a spatial requirement only slightly larger than that of a rotating phenyl ring. In principle, this allows replacement of aryl rings with carboranyl units in biologically active molecules while substantially retaining their general properties. Moreover, one can fine-tune the behavior of such species by varying the type of carborane system and by introducing one or more metal centers, either as part of a metallacarborane cage or as exo-polyhedral substituents. Another very important property of C2B10 clusters is their previously noted hydrophobicity, which increases in the order o- > m- > p-carborane (with decreasing dipole moment) and prevents classical hydrogen bonding but allows dihydrogen bonding such as B2 2HH2 2X where X is N, C, or S. These properties have led investigators to synthesize an impressive range of carborane analogues of biologically relevant molecules, starting in the late 1960s with the preparation of racemic o-carboranylalanine [23,24]. Subsequent decades have seen the enantioselective syntheses of the L- and D-alanine isomers [25–28] and the preparation of carboranyl derivatives of tyrosine [29] and other amino acids [30–41], peptides [42–47], nucleic acids [12,13,19,48,49], sugars [50–52], and other compound types [41,53,54], as well as others mentioned in later sections of this chapter. As interest in the potential of carborane-based therapies has advanced, the early strategy of attaching carborane cages as substituents on biological molecules such as chlorophyll has given way to a conceptually different approach: the engineering of novel systems that interact with enzymes and other biological assemblies via the carborane cage itself. Among several recent studies focusing on drug design, the complexation of carboranes with cyclodextrins has been explored [55,56] and it is found that 1,2-R(HO)C2B10H10 molecules (R ¼ H, OH) bind strongly with b-cyclodextrin (Ka > 1 106 M1) [56] but are too large to fit into the hydrophobic cavity of a-cyclodextrin [57]. NMR studies of the interaction of the CoðC2 B9 H11 Þ2 ion with a-, b-, and g-cyclodextrins reveal three different patterns reflecting differences in the inner cavity size: with a-CD a 1:1 complex is formed, b-CD gives both 1:1 and 2:1 complexes, and g-CD produces more than two complex types [58]. Complementing the experimental work, state-of-the-art computational methods have been applied to drug design via, for example, molecular dynamics simulations of the CB10 H11 anion with biomolecules [59] and docking studies of closo- and nido-carboranyl antifolates with the active site of a human dihydrofolate reductase [60]. While much of this effort has centered on potential BNCT application, the pharmacological properties of these compounds have drawn increasing attention. Significantly, o-carboranyl-L-alanine is found to be more lipophilic than L-phenylalanine and L-adamantylalanine [61], while the introduction of carborane units into a variety of peptides in many cases substantially enhances their biological activity, in some cases by an order of magnitude or more [11]. For example, replacement of the Phe or Tyr residue in the insect pheromone pyrokinin by an o-carboranyl-amino acid group leads to a 10-fold increase in activity in in vivo measurements and a 30-fold increase in in vitro studies [46]. As carborane interactions with biological systems are understood in greater detail, exploration of their use as pharmacophores is growing accordingly. Some areas of current research interest are outlined here.
16.2.1.2 Estrogen receptor (ER) agonists and antagonists An important advance in carborane-based drug design was made in 1999 by Endo and coworkers [62–65], who showed that hydrophobic C2B10 cages can bind effectively to the very important ERs. In this and subsequent work, the nonaromatic portion of 17b-estradiol (16-1) was replaced by a m- or p-carborane cluster as in, for example, 16-2 and 16-3, creating entities possessing the necessary features for effective binding, i.e., a hydrophobic core, an attached phenolic group
16.2 Carboranes in drug development 1055 that can hydrogen-bond to nearby amino acid moieties, and a suitably located amino or other polar functional group. In vitro testing of these compounds showed that the estrogenic activities of the p-carboranyl species (16-3) are higher than those of the analogous m-carboranyl systems (16-2); compound 16-3 (R ¼ CH2OH) is 10 times as active as the estradiol 16-1 [66]. R H
OH
C R
OH
C
C
H H
C
H
16-1
HO
16-3
16-2
R = H, CH2OH, C(O)OH, NH2
OH
Further investigation of the structure-activity relationship in 16-2 and 16-3, involving varying the locations of the OH groups, the introduction of methylene spacers between the phenol and the carborane cage, and other modifications, established that 16-3 with R ¼ CH2OH is the most potent of all compounds in this group; in vivo studies in estrogendeficient mice show that this compound is as effective as the natural estradiol 16-1 in arresting bone loss [8,67,68]. Why this is so became clear from computational docking studies that revealed the interaction between this molecule and the ER receptor to be very similar to that of 16-1. More recent investigation has disclosed that the bisphenol 2CB10H10C2 2C6H4-o/m/p-OH exhibit similar properties, showing strong affinp-carborane derivatives 1,12-p-HOC6H42 ity for the ER a, but the location of the OH group is crucial, with the p-OH compound found to be 1000 times as potent in estrogenic activity as the m-OH isomer [69]. Other research has focused on estrogen antagonists, leading to the synthesis of carborane analogues of tamoxifen [70–72], a well-known anti-estrogen used in breast cancer therapy, and of selective estrogen receptor modulators (SERMs). Compounds of the latter type function both as agonists and antagonists, for example promoting bone retention while inhibiting the proliferation of cancer cells. A series of phenol-substituted o- and m-carboranes was found to exhibit a wide range of SERM behavior toward the ER, with the m-carboranyl derivatives 16-4 having the best agonist-antagonist balance [73]. In more recent work in the Endo laboratory, 16-5 has been demonstrated to be a highly selective ER modulator that inhibits bone loss without the risk of uterine cancer that accompanies conventional treatments for postmenopausal osteoporosis [74]. Significantly, compound 16-6, which is isostructural with 16-4, has virtually no estrogenic activity owing to the presence of acidic CH hydrogen atoms which prevent the cage from entering the hydrophobic pocket on the receptor [73]. RO
OH HO C
OH
C C
C
16-4 R=
N
,
16-5
N
H
HO
H
HO
C
C
C
16-6
H
N
O
OH
16-7
C
X X = O, S R = Me, Et, cyclohexyl
R
1056 CHAPTER 16 Carboranes in medicine The carbamate and thiocarbamate derivatives 16-7 bind strongly to ERa, with their NH protons interacting with the amino acid groups of the receptor; the thiocarbamates in particular exhibit potent estrogenic activity. In tests with human breast cancer cells, these compounds are all agonists, with none showing antagonistic behavior [75].
16.2.1.3 Androgen receptor (AR) antagonists A corresponding strategy has been employed to design and synthesize carborane-based nonsteroidal antagonists for androgen receptors [76–80] such as testosterone (16-8) [81]. Compounds 16-10 and 16-11 are particularly effective, rivaling the well known anti-androgen flutamide (16-9) in potency and exceeding the potency of hydroxyflutamide, an agent employed against prostate cancer [76,79]. The evidence suggests that the hydrophobic carborane pharmacophore has a very high AR binding affinity, which may lead to development of even more effective AR antagonists of this class [79]. OH C
C
OH
NO2
16-8
O
16-10
H
H
OH
R
N
C
C
O R = 2-CN, 4-CN
O2N CF3
16-9
16-11
16.2.1.4 Retinoid agonists and antagonists Advances have also been made in the design of carborane-based retinoic acid receptor modulators in which the carborane once again functions as a hydrophobic pharmacophore [82–87].Compound 16-14, a mimic of retinoid agonists such as 16-13, has potency comparable to that of the native trans-retinoic acid 16-12 in its ability to differentiate human promyelelocytic leukemia HL-60 cells. Studies show that the planarity of the phenylamine group plays a crucial role in the biological activity [83]. O C OH
16-12 H
R
N
N OH
R
16-13
C R = H, Me
O
OH
C
C
C
O
16-14
16.2.1.5 Transthyretin amyloid inhibitors Nonsteroidal anti-inflammatory drugs (NSAIDS) such as 16-15 and 16-16 inhibit aggregation of the transthyretin protein (TTR) by binding to its hydrophobic channels; however, they also inhibit cyclooxygenase (COX) enzymes, leading to gastrointestinal side effects, an effect ascribed to p-p stacking of the aryl rings [88]. Julius, Hawthorne, and coworkers
16.2 Carboranes in drug development 1057 prepared a series of NSAID carborane analogues designed to bind effectively with TTR while avoiding the unwanted p-stacking [89,90]. Of the compounds examined, the most effective were found to be 16-17 and 16-18, which largely block fibril formation (in the case of 16-18, almost completely) while retaining inhibition of TTR but without inhibition of COX enzymes, in experiments employing a 2:1 carborane-TTR ratio. As we have seen in previously cited examples of carborane-based pharmacophores, closely related derivatives are considerably less effective, showing again that even minor modifications in structure typically have major consequences in biological activity. OH
O
C
H
O
N
C N
F3C
C
OH
C
H
16-17
16-15
CF3
O C
O
OH
C F
OH
16-16
C
F
OH
C
16-18
F
Other carborane-based NSAIDs have been reported, including an analogue of aspirin in which the benzene ring is replaced with an o-carborane cage with apparently little change in pharmacological properties [91].
16.2.2 HIV Protease inhibitors A potentially important biomedical application of carborane chemistry has emerged from the finding that certain functionalized derivatives of the bis(dicarbollyl)cobalt anion [CoIII(C2B9H11)2] [92–100] (and an earlier report involving o-carboranyl porphyrins [101]) are potent inhibitors of HIV-1 protease. As drug resistance is a serious problem with current anti-HIV treatments, boron cluster compounds offer an alternative approach that is well worth pursuing. Cooperative studies by several laboratories in the Czech Republic have demonstrated that sodium salts of water-soluble anions containing pairs of ether-linked bis(dicarbollyl) sandwiches, such as 16-19 and related species (similar to 13-370, Chapter 13), are effective inhibitors of HIV-1 protease variants that are resistant to conventional drugs. Moreover, the metallacarborane salts exhibit low toxicity and high stability in biological systems. Crystallographic data on the complex formed by the parent anion (R ¼ R0 ¼ H) and the HIV protease revealed the manner of binding of the anion to hydrophobic pockets formed by side chains of the HIV protease. On the basis of this information combined with computational studies [93,94], the hydrophobic interaction was increased further by substitution on the cages, resulting in even higher levels of selectivity and inhibition [98]. 2− O
O CC Co C
R
CC
O
N
O
R⬘
Co C
C C = CH
16-19
R, R⬘ = H, alkyl, aryl, carbinol, C3H6C(O)O−, C2H4SO3-, 7-CB10H11-, 11-CB11H11, SO2C6H5-3-Me
C
1058 CHAPTER 16 Carboranes in medicine A related but different class of HIV-1 inhibitors contain tetraphenylporphyrin cores with either one or four appended [O(CH2)2O(CH2)2O-(C2B9H10)Co(C2B9H11)] anionic substituents [102]. These compounds exhibit interesting properties in solution, being monomeric in methanol and forming triplet states and singlet oxygen on excitation, but quickly forming aggregates in aqueous media. Both compounds are effective noncompetitive HIV-1 protease inhibitors with high specificity; in addition, like other porphyrin-based derivatives, they may also find possible use in boron neutron capture and photodynamic therapies (BNCT and PDT, described later in this chapter) and hence could conceivably function in both capacities.
16.2.3 Anticoagulants A particularly elegant and state-of-the-art example of directed synthesis is the preparation of the permethylated p-carborane derivative 16-21, designed via computational docking as an inhibitor of a-human thrombin, an agent involved in blood coagulation [103]. Compound 16-21, synthesized from the bisimido derivative 16-20 as shown, functions as a size-selective hydrophobic pharmacophore that binds at the main S1-S3 active site of the thrombin, at the same time attacking the Ser195 hydroxyl unit via an attached acylating group. The versatility of 16-20 should allow construction of compounds designed to interact more effectively with the thrombin active site [103]. MeSO2
O
H
N O Me
C
H
Br
C N O
16-20
Me
C
C
H
N O = B-Me
SO2Me N
LHMDS, THF, −78 ° LHMDS = lithiumhexamethyldisilazide
N
N
O
O
H
16-21
16.2.4 Antitumor agents In contrast to the decades-long effort to develop BNCT as a viable cancer treatment (described in the following section), interest in the pharmacological anticancer properties of carboranes has received far less attention, but there is nonetheless activity in this area. In general, the mechanisms involved are not well defined, and most studies of anticancer behavior thus far have entailed mainly empirical screening to identify useful compound types. However, rational drug design is receiving increasing attention, as in the development of carboranyl derivatives of nucleosides.
16.2.4.1 Antiviral and anticancer nucleosides Considerable research has been directed to the synthesis of nucleosides covalently linked to carborane or metallacarborane units (e.g., the bis(adenosine diphosphate) derivative 9-285 cited in Section 9.14) [104], with the goal of achieving selective accumulation of boron in tumor cells, and thereby greatly enhancing the effectiveness of BNCT as discussed below. Cytotoxicity studies accompanying these investigations show selective antiviral and/or antitumor action in some compounds that may allow them to function both as cytotoxic agents and as boron carriers for BNCT. Among the many nucleosides examined, those bearing carborane-substituted uridine or uracil groups have some interesting properties. For example, 20 -O-(o-carboranymethyl)uridine (16-22) has been shown by Barth and Soloway to accumulate in F98 glioma tumors in rats at a concentration 13 times higher than in normal brain tissue [105,106]. Anticancer assays of the 50 -ocarboranyluracil nucleosides 16-23 and 16-24 reveal high toxicity (IC50 ¼ 2-4 mM) toward murine leukemia P-388, L1210, MBL-2, melanoma B16, and Meth A sarcoma cells; 16-23 is nontoxic toward rat 9L and human U-251 glioma cells [107]. In vivo cytotoxicity studies of 16-23 and 50 -o-carboranylxylofuranosyluracil (16-25) show high tolerance in mice [108,109], and a pharmacokinetic study of 16-23 indicates that it accumulates in high levels in the brain, with its distribution restricted by plasma protein binding [110].
16.2 Carboranes in drug development 1059 H
H
N
O
H
O
HO
HO O
N
O
O O
N
O
C
C
O
HO
C
O
H
C
N
OH
C
R
OH
16-23 R = H 16-24 R = OH
16-22
N
O HO
C
N
H
HO
H
16-25
Other reports of pharmacological behavior of carboranes include findings that o-carborane-bearing benzolactams bind strongly to protein kinase C [111] and that o-carboranyl phthalimide derivatives effectively modulate tumor necrosis factor alpha (TNF-a) [112]. In very recent work, the phenoxyanilide 16-26 has been found to be a potent inhibitor of hypoxia-induced (HIF)-1a protein accumulation in HeLa cells [113], blocking tumor growth without affecting the expression of HIF-1 mRNa, suggesting that compounds of this genre may have potential as antitumor agents. OH O HO
O B
N
OH
H
H
C C
16-26 Carboranyl analogues of a well-known antibacterial antifolate trimethoprim (TMP, 16-27), have been evaluated for activity against human, rat liver, and bacterial dihydrofolate reductase (DHFR), and also against human tumor cell lines [114]. In comparison with TMP, 16-28 and 16-29 show modest antibacterial and cytotoxic activity, with 16-28 exhibiting about a tenfold increase over TMP. Aside from the prospects of identifying specific compounds for therapeutic use, studies of this type are important for the insights they afford into structure-activity relationships (in particular, the effects of incorporating boron cages). NH2 N
NH2
NH2
N
N
H2N OMe
N
N H
H2N
H2N
OMe
16-28
+
− H
C C
C
16-27
NH
C
16-29
OMe
16.2.4.2 Exo-metallated carborane derivatives In the search for effective antitumor agents, a prime area for exploration lies in carborane-based metal complexes, given the interest in metal-containing antitumor drugs that began with Rosenberg’s discovery of cisplatin (cisdichlorodiammineplatinum[II]) in 1969 [115]. As summarized in recent reviews [4,15], cytotoxity toward tumor cells has been demonstrated both in clusters that incorporate metal centers in the cage framework (metallacarboranes) as well
1060 CHAPTER 16 Carboranes in medicine as in derivatives with exo-polyhedral metal-containing groups. Among the latter category are carborane-substituted porphyrins, which have been of interest primarily as boron carriers for BNCT and photodynamic therapy (PDT) as described below, but which also in some cases manifest toxicity toward cancer cells. Such compounds include watersoluble copper complexes of the type CsþCu(porphyrin)Ph4[CH(OH)(CB11H11] which kill human tumor cells, e.g., K562 leukemia and MCF-7 breast carcinoma which are resistant to conventional drugs [116,117]. Other o-carboranyl copper porphyrins have been subjected to in vivo studies of their biodistribution in mice bearing human glioblastoma tumors [118]. Similar m-carboranyl porphyrinates such as 10-64 (Section 10.13) [116,117] and its o-carboranyl analogues (Section 9.13) have been cited earlier.
16.2.4.3 Tin complexes The well-documented antitumor activity of many organotin derivatives [119,120] has encouraged studies of carboranyl counterparts such as 16-30 and 16-31 having direct B2 2Sn bonds, prepared by methods described in Sections 9.12 and 10.6. Compound 16-30 (a m-carboranyl analogue of 9-219, Section 9.12), 16-31, and some related tin complexes show stronger in vitro cytotoxicity toward a number of tumor cell lines including MCF-7 and WiDr breast cancer cells, than do the clinical drugs cisplatin and doxorubicin. The m-carboranyl counterpart of 16-31 is comparable to control drugs in in vivo tests of L1210 leukemia in mice [121]. H C
C H H C
H
H
C H
C
C
O
Sn
16-30
O Sn
R = CH2CH2[OCH2CH2]2OMe R R O O C C Ph C C C C Sn O O
16-32
C
Cl Cl − + Na
N O
C H
C O
C H
16-31
Ph Ph
Other o- and m-carboranyl tin derivatives, such as [(1,2-HCB10H10C-9-C(O)O]2Sn(n-C4H9)2 and similar species, exhibit high in vitro activity against seven different cancer cell types, outperforming cisplatin, carboplatin, and 5-fluorouracil by wide margins [122,123]. Of this group, by far the highest cytotoxicity is shown by the highly watersoluble anionic complex 16-32 [124].
16.2.4.4 Platinum complexes Special interest attaches to carboranyl-platinum compounds as antitumor agents because of the well-known effectiveness of cisplatin and related drugs in clinical use. Despite its success against certain types of cancer, cisplatin has undesirable side effects and encounters resistance from some types of tumor cells, which might be avoided in alternative platinum compounds [125,126]. As is noted in Chapter 9, Rendina and coworkers have prepared terpyridine-platinum complexes such as 9-357 (Section 9.15) and 16-33, and demonstrated their cytotoxicity toward human ovarian cancer [49,127–131]. Although both forms of 16-33 have IC50 values close to that of cisplatin, that in which L ¼ Cl is more effective, a fact ascribed to its ionic nature which not only increases water solubility compared to the neutral species, but also very likely promotes binding to DNA sites in the plasmid. NH3 Cl
Pt L
NH3 NH2
C
C
NH2
Pt L
Cl
16-33 L = NH3, Cl−
16.2 Carboranes in drug development 1061 16.2.4.5 Complexes of other transition metals The well-known cytotoxic behavior exhibited by several types of metallocene sandwich complexes of the types Cp2MX2 and Cp2Feþ X [132] suggests that analogous metallacarboranes might show similar behavior, and in fact may offer advantages in tailorability, resistance to degradation, and solubility. The metallacarboranes screened for antitumor properties thus far fall mainly into two classes: transition metal complexes of nido-R2 C2 B4 H4 2 ligands (e.g., 16-34 to 16-40), which are isoelectronic with C5 H5 and similar in size [133,134], and 11-vertex MC3B7 clusters such as 16-41 to 16-43, whose tricarbon carborane ligands are monoanions and can formally replace C5 H5 in metallocenes without changing the metal oxidation state [135,136]. In vitro studies of these compounds, which are prepared by methods described in Chapter 13, reveal potent cytoxicity (ED50 < 1 mgml1 in many cases) toward a variety of common tumor cell types, including murine P388 lymphocytic growth, Tmolt3 leukemia, L-1210 lymphoid leukemia, Sk-2 melanoma, HeLa-S3 human uterine carcinoma, and lung broncogenic MB-9812. The activity toward solid tumors is more selective; of the small metallacarboranes tested, only 16-35 is effective against lung cancer A549, but 16-34, 16-35, 16-36, 16-39, and 16-40, and others not shown, are active toward lung carcinoma MB9812 [133]. Melanoma Sk-2 growth is retarded by 16-36, 16-39, and a B-iodo derivative of 16-35, while osteosarcoma bone growth is inhibited by 16-37 and 16-39, and similar selectivity is exhibited toward other solid tumor cell lines. None of these compounds showed activity toward human HeLa carcinoma. X
C C
Cl
Ta
16-34
C
C
C
Mo
Mo
C C C C O OO O
C C
Co
Co
16-37 X = H, Y = Br 16-38 X = Br, Y = H
16-36
16-35
Br Br
W
Y
Fe
Fe
Cl
C C
C C
C C
Br
O O O C C O C C
16-39
Me
C
Br Fe Me
C
C C
Cl
V Ph
C
C
C
Me
C
C C
Nb
C
C C
C = C, CH
16-40 16-41
16-42
16-43
Mode of action studies on 16-35, 16-36, 16-39, and related compounds against P388 lymphocytic leukemia [133], and of 16-42 and 16-43 versus human HL-60 leukemia [137], indicate that the synthesis of DNA and RNA is preferentially suppressed in the tumor cells, not unlike the mode of cytotoxic action of ferrocenium salts and other metallocenes [132]. The sulfur-bridged exo-metallated ruthenium-arene complex 16-44 is cytotoxic toward both normal cells and embryonic lung fibroplast (HELF) and SMMC-7721 hepatocellular carcinoma cells, although the effect is highly concentrationdependent [138]. The carboxyl-substituted complex 16-45 is much more selective, attacking the cancer cells preferentially via a different mechanism from that of 16-44, again underlining the potential for developing carborane-based drugs targeted at specific tumor cell lines.
1062 CHAPTER 16 Carboranes in medicine
R H
16-44 R = H, R⬘ = C5H4FeCp 16-45 R = C(O)OH, R⬘ = H
Ru
C C S
R⬘ S
C
C
16.3 BORON NEUTRON CAPTURE THERAPY 16.3.1 Background For many decades, interest in medical application of boron chemistry has centered primarily on the BNCT approach to cancer treatment, which exploits a remarkable and unique property of the 10B nucleus. This isotope, which accounts for 20% of boron in nature, is itself nonradioactive but combines with low-energy thermal neutrons to generate high-energy 2.31 and 2.79 MeV a particles (helium nuclei) that are capable of destroying cellular structures within a radius of 5-9 mm, corresponding to approximately one cell diameter (also formed are a Li3þ ion and a 480 KeV photon). This reaction, first discovered in 1935 [139], soon came to the attention of a physician, G. L. Locher [140], who recognized its potential value as a treatment for inoperable tumors in which two ostensibly benign agents, boron and slow neutrons, are brought together within tumor cells that are then selectively destroyed while leaving neighboring healthy tissue intact.1 However, at that time and for years thereafter, boron compounds with the required combination of low toxicity, solubility, and stability in aqueous media, and high boron content did not exist, and remained unknown until the discovery of the extraordinarily stable polyhedral B12 H12 2 anion by Pitochelli and Hawthorne in 1960 [141] as recounted in Chapter 1. With the availability of these ions along with appropriately functionalized carborane derivatives and other boron compounds, a serious research effort to harness BNCT as a viable treatment for cancer began in the United States and other countries, and has continued to the present time. However, the story has seen many twists and turns over the years, and BNCT has yet to receive United States or European government approval for general clinical application. Some success was reported in the treatment of brain cancer patients in Japan by H. D. Hatanaka, a physician with access to a nuclear reactor, who employed BNCT using the mercapto-substituted anion B12H11SH2 against surgically exposed gliomas [142,143]. However, an independent analysis of some of these cases suggested that the survival rate was not significantly different from that expected for untreated patients [144]. Larger scale clinical trials in the U.S., Japan, and Europe have given results that are comparable to those seen with conventional radiation therapy [3]. However, these studies have been severely limited in two respects. First, they have been largely confined to patients with high-grade brain tumors such as glioblastoma multiforme (GBM), an invariably fatal cancer for which no effective treatment is known; except for limited studies on melanoma and sarcoma [3], BNCT remains essentially untested clinically against most other forms of cancer. The other issue is that BNCT trials on human patients have been restricted to just three compounds dating to the 1960s or earlier, none specifically designed for use against the tumors of interest, consisting of a monoboron species, l-p-hydroxyborylphenylalanine (BPA), and the disodium salts of B12H11SH2 (BSH) and B10 H10 2 . While the first two show some selectivity toward tumor cells and, in the case of BPA, an ability to pass the blood-brain barrier, in truth these compounds are relics of an earlier, primitive era in BNCT research. Despite the current availability of whole families of carborane and metallacarborane derivatives Although 10B has a high neutron capture cross-section (3.8 103 barns), the cross-sections of a few other nuclei, for example, 157Gd, Gd, 151Eu, 149Sm, and 113Cd, are even higher. However, none of these elements approaches boron in its ability to form stable covalently bonded compounds. Fortuitously, the nuclei of elements commonly found in human tissues (H, C, N, O, S, P) have cross-sections orders of magnitude lower and do not interact significantly with thermal neutrons. 1
155
16.3 Boron neutron capture therapy 1063 designed specifically for BNCT application, some of which have given dramatic results in animal studies, inexplicably none has been sanctioned for study in humans. Even as current clinical trials are mired in the last century, research on BNCT and associated techniques such as boron neutron capture synvectomy (BNCS) nonetheless continues at a rapid pace, as will be seen in the discussion following. A new thermal neutron beam facility at the University of Missouri, designed explicitly for BNCT and coordinated with animal studies on site [145], is anticipated to advance this technology to a new level and, finally, realize the tremendous potential of this unique weapon in the war on cancer. Many excellent reviews that have appeared in recent years afford a reasonably current picture of this field [2,3,5,6,9,15–21,146].
16.3.2 Approaches to BNCT The successful application of boron neutron therapy requires a minimum concentration of 20-35 micrograms of 10B nuclei per gram of tumor (corresponding to about a billion (109) 10B atoms per cell), with ideally no boron present in healthy tissue (in fast-neutron therapy, which employs precisely targeted neutron beams in conjunction with high boron concentration, the latter requirement is less important). The implementation of BNCT therefore presents the twin challenge of delivering sufficient boron to the tumor and doing it selectively, so that the noncancerous cells are excluded. In addition, there is the obvious requirement of low chemical toxicity on the part of the delivery agents. Over the course of BNCT studies, the strategies employed have grown increasingly sophisticated. Compounds employed in very early work in the 1950s such as boric acid, which showed low retention in tumor tissue and almost no selectivity, were later replaced by the so-called second generation agents that include the three substances used in clinical trials mentioned above, which have low toxicity and tumor:brain boron ratios exceeding 1. Third-generation agents, at the forefront of current BNCT laboratory research, are species designed to accumulate in the vicinity of the tumor or to bind to a specific location in the tumor cell. These, in turn, can be categorized in two groups, small molecules and high molecular-weight agents, whose interaction with cancer cells is mechanistically different.
16.3.3 Small-molecule boron delivery agents A wide variety of carboranes and metallacarboranes functionalized to achieve tumor selectivity have been prepared as candidates for BNCT, including amino acids, carboxylic acids, carbohydrates, porphyrinates, ureas, polyphosphates, and others mentioned earlier in this and/or previous chapters. For agents to be administered by injection, good water solubility is required, but in order to cross the blood-brain barrier one also needs liphophilicity; consequently, amphiphilic compounds combining both properties are desirable (this requirement is less important for species designed for attachment to liposomes or other biological transport vehicles, described below). The general principles involved in the design of molecules targeted for interaction with biological systems are discussed above in Section 16.2. Here, we outline briefly some compound types of special importance in BNCT studies.
16.3.3.1 Amino acids, nucleosides, and nucleotides The importance of carboranyl amino acids, including derivatives of L- and D-alanine (e.g., 9-227), tyrosine, amino acid polyols (10-61), and others in the development of BNCT and other medical applications was noted earlier. Considerable attention has also been directed to carboranyl derivatives of nucleic acid precursors including purines, pyrimidines, nucleosides, and nucleotides in the search for agents capable of selectively delivering boron to tumor tissues. In addition to the o-carboranyl nucleosides mentioned earlier such as 16-22 to 16-25, several groups have synthesized nucleoside derivatives of carboranes [147–149] and metallacarboranes [100,150–152], in many cases employing cyclic ether ring-opening reactions of the kind developed by Plesˇek and coworkers [153–155] for polyhedral borane derivatives. This general approach has been used by Lesnikowski and coworkers to generate nucleoside-metallacarborane conjugates that are candidates for BNCT, base-specific metal labeling of DNA, and other applications [13,151,156–160]. Examples include the bis(dicarbollyl)cobalt and -iron deoxyadenosine derivatives 16-46 that can be converted to DNA-dinucleotides [161].
1064 CHAPTER 16 Carboranes in medicine − O H
CC
O
N
M N
DMTO
N O
PO3H2
N
C
C
N
16-46
M = Fe, Co C = CH
The tumor-selectivity of certain carborane- and metallacarborane-bearing nucleosides has been explored in detail, as, for example, carborane-substituted thymidine analogues whose phosphate derivatives accumulate in cancer cells via kinase-mediated trapping [19,162–171].The uptake of carborane-containing thymidine nucleosides by T cells of the immune system with no adverse effects on cell function has also been reported [172]. In another example, the nucleotide base 5-o-carboranyluracil has been found to exhibit some selectivity in targeting human prostate tumor cells [173]. Aside from BNCT applications, nucleotides are under exploration in the development of novel biomaterials as described in Chapter 17. A complicating factor in designing carborane-substituted nucleosides for biomedical application is their tendency to aggregate in aqueous solution under certain conditions, a phenomenon measurable by light scattering [174–176]. Aggregation is most pronounced in nucleosides whose carborane substituents are electrically neutral or have a charge distributed over a large cluster system, as in CoðC2 B9 H11 Þ2 derivatives; it is essentially absent in derivatives containing charged nido-carborane substituents, which greatly promote solubility in water. The problem can be overcome via use of surfactants or copolymers such as polyethylene glycol or human serum albumen as deaggregation agents [97].
16.3.3.2 Porphyrins, phthalocynanines, and related derivatives The potential for medical application of porphyrin-like compounds is of interest for several reasons in addition to the cytotoxicity toward tumor cells mentioned in the previous section: they are readily taken up in cells via binding of their planar aromatic systems to DNA; they can form stable complexes with a variety of metal ions; and their fluorescence makes them ideal for spectroscopic detection and imaging. Appending polyhedral boron clusters to such molecules adds even greater versatility, opening up possible applications in BNCT and related areas [3,5,99,146,177–185]. The pharmacological properties of carboranyl porphyrins, including inhibition of HIV-1 protease and cytotoxicity toward tumor cells, were noted earlier in this chapter, and the utilization of compounds of this type in BNCT and PDT has been extensively studied; complexes such as the m-carboranyl porphyrinate 10-64 and its o-carboranyl counterpart have been cited earlier (Sections 10.13 and 9.13, respectively). The past development of this area has been thoroughly documented in the reviews cited earlier, but some recent findings serve to illustrate current research trends. For example, the well-known protonated porphyrin equipped with four ocarboranyl clusters, known as BOPP (16-47), has excellent selectivity for the mitochondrial and lysosomal areas of tumor cells, but not the nucleus [186,187]. Kahl et al. have prepared a conjugate of the m-carboranyl analogue of this molecule with a peptide nuclear localization sequence (NLS) that associates with lipoproteins, thereby offering a possible approach for delivering boron to cancer cell nuclei [188]. The same group has found that a “cocktail” of BOPP together with the previously mentioned BPA (L-p-hydroxyborylphenylalanine, an agent employed in clinical BNCT trials) significantly improves the uptake of boron in human tumors [189]; Pisarev and associates similarly observed that administration of BOPP 5-7 days before BPA resulted in substantially increased accumulation of boron in undifferentiated thyroid sarcoma cells in animals, with high rates of tumor remission following BNCT [190]. A different study found that uptake of 16-47 in cancer cells is enhanced via cooperativity with low-density lipoprotein receptors, suggesting yet another possible approach to boron delivery [191].
16.3 Boron neutron capture therapy 1065 H H
C C
C
H
O
C
O
O
C
C
O
C
H
C H
O O N
O
N
Me O
C
C
OR
Me
O H
O
C
NH
OR
M
RO
N
N
C
N N
O
HN
Me
Me
C OK
KO
H
RO
C
C
O
O
O
C H
16-47
16-48
R = H, Me; M = 2H, Cu
Other porphyrins having multiple carborane substituents are mentioned in Section 16.2. Tetrakis(o-carboranylether) compounds such as 16-48 exhibit high selectivity, with tumor-to-blood and tumor-to-brain boron concentration ratios exceeding 80:1 and low toxicity in in vivo tests; of these, complex 16-48 (R ¼ Me, M ¼ Cu) appears an especially promising candidate as a delivery agent for BNCT [192]. Analogous compounds having eight carborane cages per molecule (16-48 with R ¼ C2B10H11 and M ¼ 2H or Cu) have also been investigated [192], as have ionic porphyrinates with 4 or 8 attached nido-carborane cages. In general, the latter type shows reduced accumulation in tumor cells owing to high solubility in water [193], but meso-tetra(4-nido-carboranylaryl)porphyrins show strong affinity for murine melanotic melanoma [194,195] or human glioblastoma T98G [196] cells and are effective in BNCT treatment of mice. Dicarbollylcobalt-substituted porphyrins have also received much attention as possible boron-delivery agents for BNCT and PDA. Vicente and coworkers have investigated derivatives having up to eight attached [O(CH2)2]2O(C2B9H11)Co(C2B9H10) units (16-49 to 16-52) and demonstrated their localization in HEp2 human carcinoma cells, as well as their low toxicity under both dark and light conditions [197]. Of this group, the highest specificity, which occurs via aggregation primarily in the tumor lysosomes, is found for the two- and three-cobaltacarborane species 16-50 and 16-51. Conjugation of complexes of this type with the HIV-1 Tat 48-60 peptide sequence leads to enhanced cellular lysosomal uptake [99]. R
R⬘
R⬘
R
16-49 16-50 16-51 16-52
R = L; R⬘, R⬙, R = H R, R⬘ = L; R⬙, R = H R, R⬘, R⬙ = L; R = H R, R⬘, R⬙, R = L
N H
N
L=
N
-
H
N
K+
O
R
R⬙
C C
O
O
M C = CH C R
R⬙
C
1066 CHAPTER 16 Carboranes in medicine The toxicity of carboranyl porphyrins, obviously an important issue for agents designed to deliver large amounts of boron to tissues, has been investigated in a number of studies, and in general is found to be negligible to low [198–202]. Observed toxic effects in mice and rats were found in most cases to be transient [181,203]. Like the porphyrins, carborane derivatives of structurally related molecules such as phthalocynanines [5,204–208] and chlorins [14,183,209–217] can perform the dual functions of delivering boron to tumors and absorbing near-IR photons, and hence are good candidates for both BNCT and PDT application. Typical phthalocynanines that have been investigated are the zinc complexes 9-244 (Section 9.13) [205], 16-53, and 16-54 [206]. The role of chlorin derivatives in PDT is discussed below. RO
OL
N
N
N
Zn
16-53 R = L 16-54 R = Me
N
−
N
O N
N
CC
O
N
L=
K+
M C = CH
C
C
16.3.3.3 Other small-molecule boron delivery agents In addition to the types noted above, a wide range of carborane derivatives are candidates for BNCT application by virtue of their ability to selectively transport boron into cancer cells; many of these have also demonstrated pharmacological antitumor properties as discussed in the previous section. These include a variety of ureas, carbohydrates, phosphates, DNAbinding agents, uridines, and others. Carbohydrates are of interest because they can bind selectively to lectin receptors, which are involved in the conversion of normal cells to cancerous ones, and oligosaccharide-carborane glycoconjugates are under investigation as vehicles for targeted boron delivery to tumors [3,15,218–231]. For example, highly water-soluble unprotected glycosides such as 16-55 and 16-56 display very low cytotoxicity and are good candidates for BNCT [231]. OH O HO HO
C C
HO HO
OH
H
O HO
16-55
OH
R⬘
O HO HO
C C
OH
16-56
R⬘ = H, Me, OH O HO O HO OH
Other small-molecule carboranes that have shown good tumor uptake and relatively low toxicity include, among others, cholesterol esters [232,233], oligomeric phosphate diesters [234], piperidines [235], quinazolines [236,237], acyloxy-amides [238], deoxyuridines [239], triazines [240], cyanocobalamine (vitamin B12) conjugates [241], glycophosphonates [242,243], closo-carboranyl Nile blue derivatives [244], and salts of dequalinium, rhodamine-123, and tetraphenylphosphonium cations with nido-carboranyl anions [244].
16.3.4 High molecular weight boron delivery agents A different strategic concept for selectively introducing boron to tumor cells, extensively reviewed [3,15,17,146,245,246], entails attaching multiple borane or carborane cages to large entities such as monoclonal antibodies, dendrimers, vascular and epidermal endothelial growth factors, dextrans, liposomes, or other high molecular weight species that can be targeted to tumor cells. Boron-functionalized antibodies have been explored as BNCT carriers for decades, but a fundamental problem arises: in order to meet the requirement of ca. 109 10B nuclei per tumor cell noted earlier, each antibody molecule must carry an estimated 1000 such atoms [247], equivalent to 100 C2B10 or 90 CB11 cages. However, such major alteration very
16.3 Boron neutron capture therapy 1067 often modifies the receptor properties of the antibody, reducing its tumor specificity [248]. To circumvent this problem, other approaches have been pursued in recent years, the most prominent of which will be briefly summarized.
16.3.4.1 Dendrimers and other polymers One way of attaching large numbers of boron clusters to biological receptor-targeting agents while retaining their essential properties is to link the agent to a single macromolecular substituent containing multiple cages. This has been done, for example, by functionalizing the polyamine polylysine with B12H11(NCO)2 units that furnish ca. 2700 boron atoms, and attaching it to the anti-B16 melanoma antibody IB16-6; in this case 58% of the immunoreactivity of the native antibody was retained [249]. Similar bioconjugates have been found to have more than 1000 boron atoms and to retain 40-90% of their immunoreactivity; one such modified antibody has over 6000 boron atoms with no loss of immunoreactivity [250]. However, the heterogeneity arising from compositional variation in these large polymers is a drawback in their use. Dendrimers of several varieties have been discussed in earlier chapters, some of which appear suitable for use in boronrich antibodies, for example, the star-shaped 16-cage polyhydroxylated molecule 10-49 (Section 10.9) [251,252], the fourth-generation polyester system 14-24 (Section 14.5) [253], and the polysilyl CoðC2 B9 H11 Þ2 -functionalized dendrimer 14-28 (Section 14.6) [254]. Azoborane-substituted polyamidoamino “starburst” dendrimers conjugated to monoclonal antibodies have been employed against murine B16 melanoma and other tumor strains [255,256]. Other examples in which dendrimers are appended to large monoclonal antibodies or other high-molecular weight entities such as vascular endothelial growth factor, folic acid, epidermal growth factor, and others, have been cited in recent reviews [3,15,17]. PEGylated polyester dendrimers carrying the drug doxorubicin are found to have a long circulation half-life in mice and to show higher tumor selectivity than Doxil, a PEGylated liposome that is currently in clinical use [257]. Particularly innovative dendritic species are the 132-boron nanospherical “closomers” (16-57) that consist of an icosahedral B12 core tethered to 12 o-carboranyl cages by n-hexyl ether connecting links [258]. Deboronation of all 12 carborane units by strong bases affords water-soluble (Naþ)14 B12[(OCH2)6(nido-7,8-RC2B9H8)]1214 salts. The latter compound with R ¼ Me showed low toxicity in mice and good selectivity for tumor uptake in murine biodistribution studies.
C C
R C
C
R
C C
R
R C
C
O
C
O O
R
C
O
O
O
O
C
O
R
C
O
O
C
R
O
C
O
R C
C C
R
R C
R
R
C C
C
C
C
C
16-57 R = H, Me
1068 CHAPTER 16 Carboranes in medicine Nondendritic boronated polymers are also candidates for tumor targeting. The water-soluble p-carboranyl acrylate copolymer 10-51 [259], mentioned in Section 10.10, has a molecular weight of 27.6 kg mole1 and is sufficiently boronrich to be a viable agent for BNCT. Other viable possibilities include dextrans, glucose polymers, and similar longchain molecules. Strategies for directing boronated polymers into specific tumor tissues have been reviewed by Soloway et al. [17] Linkage of o-carborane cages to peptides is another avenue for exploration, as in the preparation of Tyr3-octreotate conjugates [260] containing up to 60 boron atoms per peptide which retain strong selectivity for tumor receptor sites [261].
16.3.4.2 Lipoproteins Low-density lipoproteins, or LDLs, are metabolized at different rates by tumor and normal cells because of the increased need of the former for cholesterol in building new membranes, leading to LDL accretion in tumor cells. This suggests that boronated cholesterol mimics such as the ester 16-58 can be used to selectively introduce boron into cancerous tissue, a concept proposed and subsequently explored by Kahl [188,262–265] and by other groups [266–271]. In vitro studies have demonstrated good tumor cellular uptake for many of these agents. However, radiolabeling experiments showing an inability to cross the blood-brain barrier [272,273] suggest that they may not be effective agents against cancers of the central nervous system.
C H
16-58
O
C O
16.3.4.3 Liposomes A powerful technology for tumor-specific delivery of boron is centered on liposomes, which are biodegradable, self-assembled spherical vesicles of 50-100 nm diameter consisting of a phospholipid bilayer surrounding an aqueous core, that are widely used in drug delivery. Liposomes in general are efficiently taken up by cancer tissues (the enhanced permeability and retention or EPR effect) owing to the large pore sizes of the latter compared to normal tissues, and can also be attached to molecules that bind to specific receptor sites; consequently they can deliver high concentrations of boron to tumors without the need for specific targeting by the boron compounds per se. Following the first use of liposomes as boron carriers by the groups of Hawthorne [274] and Yanagie [275] in the early 1990s, considerable effort has been directed to refining and extending this technique as outlined in a number of reviews [3,17,146,246,276–280]. Boranes and carboranes can be loaded into liposomes either by encapsulation within the aqueous core or by attachment to the lipid bilayer. Problems encountered with the first approach, such as leakage of the boron compounds on storage, low efficiency of encapsulation, and altered liposome behavior, have led investigators to turn increasingly to the latter method, and a variety of liposome conjugates with closo-carborane, nido-carborane, or dodecaborate cages has been prepared. In these systems, the boron cluster is typically appended to a phospholipid, cholesterol, or other functionality that binds to the liposome bilayer [266,281–289]. By attaching a boron-containing ligand to the surface of a liposome that encapsulates anticancer drugs in its inner cavity, one has, in principle, a dual agent capable of both BNCT and chemotherapy functions. This may be achievable with species such as the double-tailed nido-carborane lipid 16-59, which forms a stable liposome conjugate at 25% molar ratio toward distearoylphosphatidylcholine (DSCP) with cholesterol, and can be coupled to transferrin for selective delivery to Tf receptor sites in tumor tissue [290].
16.3 Boron neutron capture therapy 1069
O
O
- H C C
O
16-59
C = C, CH
In recent work, liposomes bearing poly(carboranylalkylthio)porphyrazines (16-60, 16-61) [291–294] have been shown to be effective against carcinoma and melanoma in BCNT studies, and in addition are intense photoreceptors, suggesting a potentially dual role in cancer therapy.
C R
C
C
C
R
R S
C
S
C
C
C
N
S
R
16-60 R = H, M = Mg 16-61 R = Me, M = 2H
S
M
N
N
S
S
R
N
C
C
C S
C
S
R
C C
R
C
R C
16.3.4.4 Nanoparticles and nanotubes The techniques that have been explored for loading boron into tumors for BNCT and other applications as outlined above are hybrid in character, involving the coupling of inorganic (e.g., boron cage) units to bioorganic moieties to achieve the desired objective. Extending this concept are some intriguing new approaches based on novel inorganic platforms, one example being the 13-cage nanoscale closomers 16-57 mentioned earlier. In a different strategy, watersoluble silver nanoparticles (25 nm diameter) are functionalized with carboranes and a solvating thiosuccimidyl group (16-62) along with antibodies targeted to anti-EGFR antibodies on the surfaces of cancer cells [295]. This technique delivers approximately 4.5 108 boron atoms per cell as determined by surface-enhanced Raman scattering (SERS), close to the level required for effective 10B neutron capture therapy, and may have potential as an imaging technique in addition to its therapeutic function.
1070 CHAPTER 16 Carboranes in medicine
O C
N
O
C
H
N
16-62
O
H C
S
Ab
16-63
O
C S
R
C
N S
Ag
C R R = Me, Ph
Another type of boron delivery vehicle that has been investigated is the single-walled carbon nanotube (SWCNT), a material of interest for biomedical and other applications [296–298]. Observations that these particles can easily pass through membranes [299] and appear largely nontoxic have led investigators to explore their possible use as BNCT agents. Following their successful insertion into SWCNTs and direct imaging via electron microscopy [300], o-carborane cages have been covalently attached by nitrene cycloaddition to afford the species 16-63 [301,302]. Deboronation of the carborane cages in ethanolic KOH gives water-soluble nido-RC2 B9 H11 -substituted nanotubes, which show some selectivity toward cancer cells in in vivo testing in mice. In a different study, it has been demonstrated via spectroscopic and gel electrophoresis methods that o-carborane can be physisorbed onto the surfaces of lyso-phosphadylcholine-functioanlized SWCNTs in aqueous dispersions without affecting their membrane-penetrating properties [303].
16.4 BORON NEUTRON CAPTURE SYNOVECTOMY Like BNCT, BNCS is a binary therapy utilizing the 10B neutron capture reaction to target and destroy specific tissue types, in this case directed to rheumatoid arthritis, an autoimmune disease affecting millions of patients. BNCS entails introduction of a boronated compound into the inflamed joint, and offers a noninvasive alternative to surgery or to radionuclide synvectomy, a therapy currently in use that involves direct injection of b emitters into the affected joint. An advantage of BNCS over the latter method is that it mitigates the problem of leakage of radionuclides into healthy tissues that is encountered in practice. While BNCS has been explored by Yanch and coworkers via animal studies employing K2B12H12 [304], Hawthorne et al. [305] used liposomes containing a nido-C2 B9 H12 derivative in the aqueous core and Na3[B20H17NH2CH2CH2NH2] in the lipid bilayer, as an effective method for introducing boron via intravenous injection into laboratory rats. In this study, selective delivery of boron to the arthritic tissue along with rapid blood clearance demonstrated the viability of BNCS as a treatment for rheumatoid arthritis. A few agents designed specifically for BNCS have been reported, for example, corticosteroid ester derivatives of o-carborane [306], and the method has been tested in arthritic rabbits using K2B12H12 with a good response and no adverse effects [307].
16.5 PHOTODYNAMIC THERAPY PDT, a cancer treatment that does not require boron but resembles BNCT and BNCS in the sense that the interaction of benign agents is employed to destroy targeted tumor cells, is an established clinical procedure for treatment of tumors accessible by light beams. In this procedure, a photosensitizer (usually a porphyrin or phthalocyanine) intercepts
16.6 Carboranes in molecular imaging and radiotherapy 1071 low-energy photons in the 630-750 nm (red light) wavelength range to generate an agent, typically singlet oxygen, which is cytotoxic to the cell within which it is produced, a process similar to the action of alpha particles in BNCT. Although boron per se plays no role in PDT, the general inertness, ready tailorability, and other properties of carboranes make them excellent platforms for attached photosensitizers; moreover, they open the possibility of synthesizing dualaction compounds capable of both PDT and BNCT application. A number of porphyrin [99,116,117,181,194,196,308], phthalocyanine [204], and chlorin [14,183,209–217] derivatives, several of which have been mentioned in this and earlier chapters, for example, 10-64, 16-53, 16-54, and others, have been investigated as PDT (as well as BNCT) agents. As was noted earlier, animal studies on many of these compounds show generally good selectivity toward tumor cells as well as low toxicity. The previously cited porphyrin derivative BOPP (16-47), though it does not deliver therapeutically useful concentrations of boron to cancer tissues for BNCT, nonetheless appears a good candidate for PDT [309], and the mechanism of singlet oxygen generation has been explored via measurement of phosphorescence induced by polychromatic excitation [310].
16.6 CARBORANES IN MOLECULAR IMAGING AND RADIOTHERAPY Labeling of carboranes and other polyhedral boranes with radionuclides has long been employed as a means of tracking the distribution of boron in biological systems [10], and the development in recent years of powerful techniques including positron emission tomography (PET) and single photon emission computed tomography (SPECT) has enabled whole-body imaging of live animals. Beyond imaging per se, radionuclide-labeled boron compounds that can be selectively introduced into tumor cells provide a vehicle for targeted radionuclide therapy. This particular application of carboranes in medicine differs in one major respect from BNCT and other techniques described above: it requires delivery of only very small amounts of the agent into tumor tissues—orders of magnitude less than the concentration needed in the other protocols. As radiolabeling normally involves only minor modification of tumor-specific molecules, their biophysical properties are essentially unchanged, which affords a major advantage. The marrying of well-established PET and SPECT clinical techniques with polyhedral boron chemistry is another example of a cutting-edge interdisciplinary research area that is unfolding at a rapid pace.
16.6.1 Radiohalogenation The most commonly employed radioisotopes in current use are halogens, because of their useful chemical properties that simplify incorporation into molecules combined with the availability of a wide range of specific radiohalogen isotopes with varying radioactive decay modes and half-lives. Some of these are positron emitters suitable for PET imaging, for example, 18F, 75Br, 76Br, and 124I, while others such as 77Br and 123I decay via electron capture and can be employed for SPECT imaging. Still others such as 125I and 131I are used in therapy. As in other biomedical applications of carboranes, their stability over a wide range of conditions makes them wellsuited as scaffolds for attaching radiolabels [10,20,311] and affords an advantage over many radiohalogenated organic molecules such as peptides, which in biological environments tend to undergo cleavage of their weak carbon-halogen bonds. Many of the tumor-specific species developed for BNCT as described above can be equipped with radioisotopes and employed in radioimaging and therapy. As one illustration, a nido-carborane derivative, 7-(4-SCNC6H4)7,8-C2 B9 H11 , prepared by Hawthorne and coworkers in 1985 and coupled to the tumor antigen IgG for BNCT [312], was later labeled with 76Br and conjugated to an anti-HER2 antibody for use in breast cancer therapy [313]. Significantly, the radiolabel remained intact with no diminution of immunoreactivity of the antibody following labeling. Similar studies are detailed in recent reviews [2,15]. As an example of more recent work, a conjugate of nido-7,8-[H2N(CH2)3]C2 B9 H12 with dextran (an enzymeresistant polysaccharide) was labeled with 125I and evaluated in rat liver, where 70% of the iodine was retained after 24 h, exceeding the stability of labeled albumin used as a control [314]. Mention has been made of radiohalogen labeling throughout this volume, often in reference to the readily available 125I or 131I isotopes as in the syntheses of iodinated derivatives of nido-7-RCB10H12 for BNCT-related biodistribution studies in mice (Section 7.2) [315,316], of labeled
1072 CHAPTER 16 Carboranes in medicine o-carborane derivatives via isotopic exchange reactions with 125I or 131I (Section 9.5), [317], and of analogous m- and p-carboranes (Section 10.6) [317]. In a different application of radiolabeling, the rhenacarborane 16-64 was administered into the bloodstream and, notwithstanding its large molecular weight (742.91 Da), was observed to cross the blood-brain barrier by transmembrane diffusion [318]. The uptake was rapid and amounted to approximately 0.1% per gram of brain, suggesting that such compounds may be useful as targeted therapeutic agents for the central nervous system. 131I
O N
N
N
Re
16-64
C H H
Radioiodination has been applied to other biological systems such as carborane-protein conjugates [319] and derivatives of biotin (vitamin H), for example 16-65. In the latter case, the strong affinity of biotin for streptavidin is exploited to introduce therapeutic doses of radioiodinated biotin molecules into tumor cells that have been pretargeted via introduction of a cancer-seeking monoclonal antibody-streptavidin conjugate [320,321]. O
OH
O
S
X
S N
N
H HN
−
O
H
NH
N
N
H
H
C
C
16-65 X = 125I 16-66 X = 211At
O
16.6.2 Radioastatination Astatine-211, a heavier congener of iodine that emits a particles at energies lethal to cells, has attracted interest in recent years as a therapeutic tool, but has the drawback that direct labeling of proteins with this element forms weak At2 2C bonds that are subject to cleavage under clinical conditions. This problem is lessened in the 211At-labeled carboranes such as derivatives of biotin (16-66), human epidermal growth factor, and others, which exhibit generally higher stability in vivo than their noncarborane analogues, although some loss of astatine is still observed [319–323]. However, proteins bearing astatine-labeled B10 H10 2 cages are stable in vivo [319], as are the astatinated Venus flytrap complexes 16-67 [323], a genre introduced by Hawthorne in the 1980s (see 7-27, Chapter 7).
H
C C − 211At
16-67
− C
C
R
R = H, DTPA
H
DTPA = diethylenetriaminepentaacetate
In general, astatination of molecules has been found to have little effect on their biological activity.
16.6 Carboranes in molecular imaging and radiotherapy 1073
16.6.3 Radiometal labeling An alternative to halogen isotopes as agents for molecular imaging and therapy is provided by radioactive metals, of which the most common in use today is technetium-99m, a 140-keV g-ray emitter. Its short half life of 6 h and commercial availability via portable generators that produce aqueous 99mTcO4 make technetium attractive for clinical application. However, a difficulty that arises with technetium as well as other metals is that the metal chelate complexes used for in vivo and in vitro imaging tend to dissociate, often via competitive binding with serum transferrin, and do not remain bound to protein sites. Once again, this problem seems made to order for metallacarboranes with their typically very robust metal-ligand binding. Pursuing this idea, Valliant and coworkers have developed a high-yield microwave-assisted synthesis of dicarbollyl complexes of rhenium (a heavier congener of technetium) formulated as 2,1,8- and 3,1,7-(CO)2LRe(RR0 C2B9H9), (L ¼ CO, NO; R, R0 ¼ H, Ph, CH2Ph, p-C6H4OH), and used this method to prepare tracer-level quantities of their 99m Tc analogues [324,325]. The rhenium species are luminescent, allowing in vitro monitoring, while the technetium compounds are suitable for in vivo radioimaging via the SPECT method mentioned earlier [326]. In addition, several of the rhenacarboranes show high affinity for the estrogen receptor; 2,1,8-(CO)99m 3 Tc[(p-HOC6H4)HR’C2B9H9] can be generated in high radiochemical yield and is a viable candidate for use in cancer and osteoporosis therapy [327]. Other 99mTc-carborane-labeled biomolecules currently under study include derivatives of thymidine [328], carbohydrates [329], WAY 100635 (a serotonin 5HT1A receptor) [2], and others [330], as well as species labeled with 188Re, a therapeutically useful radionuclide [2]. Carboranes and metallacarboranes labeled with radioisotopes of other metals have also been investigated. The first example of this approach was the trapping of 57Co (a g emitter with a half-life of 271 days) in a pyrazole-bridged Venus flytrap complex (7-25, Section 7.2) by Hawthorne et al., who conjugated it to the anti-carcinoembryonic monoclonal antibody T84.66 and conducted biodistribution studies in mice, showing strong localization in tumor cells and low accumulation in the liver [331]. Carboranes incorporating other isotopes include 68Ga- and 111In-containing thymidine derivatives [332] and recombinant streptavidin labeled with 213Bi, a short-lived a emitter [320].
16.6.4 Gadolinium in imaging and therapy Element 64 is of special interest because its two main isotopes, 155Gd and 157Gd, have even higher neutron capture cross-sections than 10B, that of 157Gd being 85 times larger, and in addition are effective image-enhancing agents in magnetic resonance imaging (MRI). Although a few 7-vertex, 12-vertex, and 13-vertex gadolinium-carborane sandwich complexes have been characterized (Chapter 13 and Tables 13-1, 13-3, and 13-4), biomedical applications have not been reported for these compounds. However, Italian chemists have synthesized the bifunctional complex 16-68 which bears a palmityl chain for binding to the LDL receptors that are overexpressed on several tumor cells, as well as an attached Gd (III)-DOTA (1,4,7,10-tetraazacyclododecane-N, N00 , N00 , N000 -tetraacetic acid) complex which allows quantitative determination of the boron concentration reaching the tumor cells [333]. Such dual-purpose agents have the capability of enhancing the effectiveness of BNCT via the presence of gadolinium while simultaneously assaying the biodistribution of the complex in the system. O −
O H
N
C C
N
N H
O
− O N
C15H31 O
N
O
N
O −
Gd3+
O
N
O
16-68
Gadolinium-labeled tetracarboranylporphyrin complexes have been prepared and employed as MRI contrast agents in studies of human leukemia, melanoma, and breast cancer cells in mice, with up to 35% of the gadolinium administered found to localize in the tumors, and only minimal amounts in the liver, kidney, and spleen [334].
1074 CHAPTER 16 Carboranes in medicine
16.6.5 Other carborane-based imaging methods 16.6.5.1 Secondary ion mass spectrometry An imaging technique that does not involve radiation is secondary ion mass spectrometry (SIMS), in which an ion microscope is employed to image cryogenically prepared fast-frozen freeze-dried cells with a resolution of 500 nm. By imaging human glioblastoma T98G cell loaded with BNCT agents such as o-carboranyl-substituted thymidine analogues, the boron has been found to be distributed throughout the interphase and mitotic cells, including the chromosomes [335]. This finding may be an indication of incorporation of the carborane-thymidine moiety into DNA, which if true would have important implications for BNCT and other therapies.
16.6.5.2 Electron microscopy The fact that individual molecules of o-carborane enclosed in SWCNTs can be observed via high-resolution transmission electron microscopy, mentioned above, suggests the possibility of imaging carborane cages in biological systems [227]. This has been accomplished in some boron-containing tissues [336–338], though not yet for carboranes [227], via application of energy-filtering transmission electron microscopy (EFTEM), a method in which inelastically scattered electrons are filtered in order to map a specific element.
16.6.5.3 Fourier transform infrared labeling
The characteristic B-H absorption band at ca. 2400-2650 cm1 wavelength, a region that is nearly transparent in nucleic acids, can serve as a label in such molecules bearing carborane or other boron-containing substituents. The viability of this approach has been demonstrated in an FTNMR study employing o-carborane or CoðC2 B9 H11 Þ2 units as labels in nucleosides and DNA-oligonucleotide conjugates [339].
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CHAPTER
Carboranes in other applications
17
17.1 OVERVIEW The three preceding chapters describe ways in which carboranes and metallacarboranes have been put to work in polymer science, catalysis, and medicine. In these well-established fields, the underlying rationale is that properly designed carborane derivatives, by virtue of their enhanced stability, tailorability, electronic properties, and other attributes, can provide performance superior to that of conventional organic or inorganic systems. However, the potential for application of carborane chemistry extends well beyond those specific areas. Some of the most intriguing advances have been those that “break the mold” with novel molecular architectures or properties that are simply not encountered, or perhaps even possible, in the absence of boron. A number of examples cited earlier fit this description—for example, the macromolecular “closomers” mentioned in Chapter 16—but these offer just a tantalizing taste of what lies ahead. In this chapter, we outline some relatively recent and very interdisciplinary efforts that impact on a number of diverse areas of technology. The classic 1992 review by Plesˇek [1] sets a benchmark by which to measure the advance of carborane-based technologies since that time. This chapter presents a short survey of current and projected carborane applications beyond those involving polymers, catalysis, or medicine, including several, such as single-molecule nanocars, that have emerged only recently.
17.2 METAL ION EXTRACTION The processing of nuclear fuel generates liquid waste containing radionuclides with long half-lives, especially 90Sr (29 years) and 140Cs (30 years) along with lanthanides and actinides, and their handling and storage presents a major challenge. Among the more effective agents for recovery of these metals from radioactive waste, in largescale use for decades in eastern Europe, are substituted derivatives of the bis(dicarbollyl)cobaltate(1) ion 3-Coð1; 2-C2 B9 H11 Þ2 (13-366) whose synthesis and chemistry are described in Section 13.5. The solubility behavior of this extremely hydrophobic sandwich complex, whose sodium salt resembles NaCl in its electrolytic properties, is extraordinary: it is quantitatively transferred from a 0.5 M aqueous solution to diethyl ether on shaking with an equal volume of the latter solvent [1]. The anion is often employed as a nearly noncoordinating counterion, as, for example, in a study of dendridic species in a water-dichloroethane system [2]. Parent 13-366 and its derivatives were first applied to solvent extraction of radionuclides in the 1970s by Czech workers, who found that that the hexachloro derivative 3-Coð1; 2-C2 B9 H8 Cl3 Þ2 (17-1) is stable toward 3 M HNO3 (while the parent ion is not), allowing its use in industrial-scale removal of nuclides such as 140Csþ and 90Sr2þ [1,3–8].
Carboranes. DOI: 10.1016/B978-0-12-374170-7.00001-X © 2011 Elsevier Inc. All rights reserved.
1083
1084 CHAPTER 17 Carboranes in other applications
Cl
−
Cl Cl
CC
17-1
CoIII Cl
C
C = CH
C
Cl Cl
The liquid-liquid extraction of cations of Group 1, Group 2, lanthanide and actinide elements [3,9–17], as well as Pb [15,18], Zn [15,19], Ag [20,21], and other metals, employing 17-1 and its derivative anions together with crown ethers, calix[n]arenes, polyethylene glycol, polyethers, and other complexing agents, has been extensively investigated. A favored solvent is nitrobenzene because it promotes selectivity toward desired metal cations [22–24]; it is, however, ecologically unfriendly. Teixidor and coworkers have explored the radionuclide-extraction properties of analogous bis(dicarbollyl) complexes of other metals including iron and nickel [23], as well as functionalized derivatives of 13-366 having aromatic, ether, or other substituents on the cage [22,24]. More recently, attention has turned to synergistic approaches that utilize bis(dicarbollyl)cobaltate anions in concert with other effective complexing agents such as ethylbis(triazinyl)pyridine (Et-BTP) [25], bis(tetraazolyl)pyridines (ATP) [26], valinomycin [15,19], and carbamoyl methyl phosphane oxide (CMPO) [27], as in the Universal Extraction (UNEX) process developed jointly by U.S. and Russian chemists that employs CMPO [28]. Bifunctional extraction agents that incorporate cobalt sandwiches and CMPO-type groups in the same molecule have been synthesized, including the calix[4]arenesupported system 17-2 [29] and the boron-functionalized sandwich 17-3 [30]. These complexes, and others of similar design, are highly efficient in extracting lanthanides and actinides from nuclear waste even in strongly acidic media. −
− C C
C C
−
Co
O
Co O CC O
Ph2P
O C
PPh2 O
C
O
CC
O NH
O HN
CC Co
O
C
(CH2)n (CH2)n
C = C, CH
O
17-2 n = 2-7
O
O N
P
C (CH2)n
C
Ph R
17-3 n = 1, R = Ph, n-C8H17 n = 2, R = Ph
Cobalt bis(dicarbollide)-based extraction of metals from nuclear waste, the largest-scale current application of carborane chemistry in industry, is the focus of continuing research and development aimed at optimizing its selectivity and efficiency. A recent review examines the effects of radiation on the chemistry of the extraction processes [31]. The hydrophobic nature of the hexachloro anion 17-1 has interesting consequences in two-phase water-supercritical CO2 systems, where it promotes the formation of ion pairs of cryptate anions with Csþ and Kþ that enables their efficient extraction to the CO2 phase [32,33]. Other anions examined, including picrate and perfluorooctanoate, do not exhibit this property.
17.3 Carborane-based materials 1085
17.3 CARBORANE-BASED MATERIALS 17.3.1 Ionic liquids Although room-temperature ionic liquids have attracted increasing interest as reaction media for a number of years, relatively few with boron cluster anions have been reported until fairly recently. A few examples having application in catalytic systems include N-alkylpyridinium [34,35], N0 -dialkylimidazolium [36], and BINAP [37] salts of closoRCB11 H11 anions (Section 15.3 and Table 8-1). Indeed, most ionic liquid carboranes employ HCB11 H12 or its derivatives, well known for their least-coordinating properties (Chapter 8), although the o-carboranyl imidazolium salts RR0 N2 C3 H3 þ C2 B10 H11 (R ¼ Me, Et; R0 ¼ Et, n-C4H9) are similarly low-melting [38]. A timely survey of this area is provided by Nieuwenhuyzen et al. [39]. Metallacarborane-based ionic liquids are quite rare, the only reported examples at this writing being 1-alkyl-3methyl-imidazolium salts of 3-Coð1; 2-C2 B9 H11 Þ2 (13-366) mentioned in Chapter 13, which are liquids at room temperature and have very low glass transition points; the low melting points in these salts are attributed to poor crystal packing [39].
17.3.2 Liquid crystals Materials that behave as fluids at room temperature but are partially ordered and show birefringence, known as liquid crystals or mesogens, have important applications in electro-optical displays [40]. Mesogenic behavior is achieved by combining a rigid central unit with flexible alkenyl, alkynyl, or other groups to produce rod-like molecules that typically align in a nematic or smectic phase (Figure 17-1). Owing to their chemical stability, rigid cage architecture, and s-aromatic delocalized electronic structures, closocarboranes and polyhedral BnHn2 anions are attractive candidates as core units for liquid crystals, especially in combination with organic rings, and have been extensively explored as such by Kaszynski and others [41–49]. The carboranes of main interest have been the 10-vertex 1,10-C2B8H10 and 1-CB9 H10 clusters and the 12-vertex (icosahedral) 1,12-C2B10H10 (p-carborane) and 1-CB11 H12 cages, all of which can be derivatized by appending organic functional groups to their antipodal 1,10, or 1,12 cage vertexes as described in Chapters 6, 8, and 10. In these studies, derivatives of the commercially available p-carborane have been utilized more extensively than those of the somewhat less accessible 10-vertex species. Considerable progress has been achieved in tailoring the properties of carborane-based liquid crystals by appropriate selection of organic functional groups and connecting units. For example, in the 17-4 series, the stability based on choice of X increases in the order CH2CH2 < C(O)O < OCH2 < O(O)C, regardless of the carborane present in the chain [44]. Liquid crystals such as 17-5 and 17-6, with partially fluorinated arene rings, a compound type that is important in flat panel electro-optical displays, are polar nematic materials having low viscosity and high resistivity [50].
Nematic
FIGURE 17-1 Common liquid crystal phases.
Smectic
1086 CHAPTER 17 Carboranes in other applications
X
C3H7
O
O
O
C
C
O
X
C3H7
17-4 X = OCH2, CH2CH2, C(O)O, O(O)C
= C
C
,
C
C
F OEt
C5H11
17-5
F
C5H11
F
F
17-6
F
Ionic liquid crystals (ILCs) are of increasing interest and importance for applications employing ion conductors, such as batteries and solar cells, in part because of the anisotropic ion mobility exhibited by some ILCs [51,52]. In most ILCs studied so far, the cation—usually ammonium, imidazolium, or pyridinium—is responsible for the mesogenic behavior. However, the nature of the anion is also important, not only in determining ion conductivity but also in terms of its photolytic and other properties. The anionic closo-carboranes 1-CB9 H10 and 1-CB11 H12 are of special interest because of their charge delocalization and nearly noncoordinating character, and liquid crystals such as 17-7 and 17-8 exhibit interesting properties; for example, the esters 17-7 show soft crystalline polymorphism with mutually perpendicular layers, and the diazene 17-8 is photolytically active [52]. −
O C C5H11
C
C
O
L
C6H13
C
C7H15O
+ N
C4H9
n
17-7 17-8
L = O(O)C, n = 0, 1 L = NKN, n = 1
Other liquid crystal systems based on well-characterized amino acid and dinitrogen acid precursors of the types 2CB9H10-10-N2 (Chapter 6) are under investigation [53]. Polar zwitter1-OðOÞC2 2CB9 H10 -10-NH3 and 1-HO(O)C2 2CB9 H9 -10-C6 H13 (L ¼ C5H10S, C7H12N) have ionic sulfonium and quinuclidium derivatives of the types C5 H11 -L2 been shown to enhance the mesogenicity of nematic hosts [54]. Metallacarboranes offer intriguing possibilities for the design and synthesis of liquid crystals, given the wide range of variable features such as the metals, metal oxidation states, cage composition and size, and location of cage heteroatoms, but at present this area is virtually unexplored. A rare example is the bis(tricarbollide)iron(II) system 17-9 which shows strong UV absorption and a preference for the nematic phase [55]. C CHKN
CnH2n+1O
C C
17-9
C C
Fe
NKCH
OCnH2n+1
C
C = C, CH
17.3.3 Nonlinear optical materials Substances that manifest nonlinear optical (NLO) activity—in which an applied electromagnetic field is altered nonlinearly, changing its frequency and other properties—are of interest for application in areas such as optical switching, data storage, image processing, communications, and protection of optical sensors from high-energy laser beams [56–58].
17.3 Carborane-based materials 1087 Few molecules have the requisite properties for second-order nonlinear behavior (the most commonly encountered type), i.e., extensive electron delocalization, a large difference between ground- and excited-state dipole moments, a large transition state dipole moment, and adoption of a noncentrosymmetric space group in the crystalline state. A widely studied class of organic molecules designed to meet these requirements are the “push-pull” systems in which electron-attracting and electron-donating groups are connected by a p-delocalized bridge. As seen in other applications, the replacement of aryl rings by electron-withdrawing boron clusters in these molecules can have significant consequences, especially in a greatly enhanced first hyperpolarizability (b), a measure of the NLO response [59–63]. The nearly spherical 1,12-C2B10H12 (p-carborane), CB10 H11 , and B12 H12 2 clusters serve as connecting units in a variety of systems, several of which have been discussed earlier, e.g., the proposed tropylium-p2C5 R5 (10-30) and the known [64] fullerene carboranyl-cyclopentadienyl system [56,57] p-C7 H6 þ -1;12-CB10 H10 C2 2C C2 2C60H (10-31) (Section 10.7). Both of these molecules have extremely high Me–1,12-CB10H10C-p-C6H42 calculated b values exceeding 1000 1030 cm5 esu1, far exceeding that of the benzene-bridged analogue 2C5 H5 for which b is calculated to lie in the range 52–283 1030 cm5 esu1 [56,57,65]. The synthp-C7 H6 þ -C6 H42 eses and electronic structures of tropylium-substituted carboranes are discussed in Sections 8.2, 9.6, and 10.7. The m- and o-carboranyl analogues of 10-31, whose carborane units are more polar and less electron-delocalized than p-carborane, have far lower b values (386 and 483 1030 cm5 esu1, respectively) than the p-carboranyl systems [64,66]. In 10-31 the fullerene cage, interestingly, is calculated to be an electron donor [64]. A similar situation is found in the conjugated ferrocene-p-carborane dyads 17-10, which show high hyperpolarizabilites (exceeding 100 1030 cm5 esu1) that slightly exceed those of their o- and m-carborane counterparts [67]. Photochemical and electrochemical data show that the carborane cage acts as an electron-acceptor, a consequence of local depletion of electron density at the carbon vertexes and not an erroneously invoked “electron deficiency” of the entire cluster (see Chapter 2). As expected, the o-carboranyl cluster is a stronger electron-attractor than the less polar m- and p-carboranyl isomers. H
C C
C
E
Fe
17-10
H
E = CH, N
Although published experimental and theoretical studies of carborane-based NLO compounds are almost entirely restricted to the 12-vertex C2B10H12 and CB10 H11 cages, the second-order NLO properties of the 5- to 7-vertex carbor2C2BnHn2 2C7H7 (n ¼ 3-5) have been explored by DFT B3LYP calculations at the 6-31G* level [68]. ane systems C5H42 The calculated b values are comparable to those of analogous species bridged by p-carboranyl clusters; interestingly, the most stable of these molecules are calculated to be those having 1,5-C2B3H3 and 1,6-C2B4H4 connectors. Similar calculations on 7-vertex CpCo(C2B3H5) cobaltacarboranes bearing p-nitrophenylethenyl substituents (17-11 to 17-16) revealed some notable trends [69]. R
R
H
C
C H
R
H
C C
H
Co
H
C C
C
R
H C
Co
Co
H
H
C
H C
H
C Co
Co
C Co
R
17-11
17-12
17-13
17-14
R = CHKCHJC6H4-p-NO2
R
17-15
17-16
1088 CHAPTER 17 Carboranes in other applications Consistent with their isoelectronic relationship with ferrocene, these sandwiches have slightly smaller b values than 2CH5 5CH2 2C6H42 2NO2, but replacement of the NO2 groups with NH2 in the cobaltacarborane complexes CpFe(C5H4)2 yields calculated polarizabilities very close to that of the ferrocenyl analogue. Depending on the location of the substituent, the cobaltacarborane entity can function either as an electron attractor, as in 17-15 (which has an extremely electron-poor apical carbon atom), or an electron donor, as found for all of the other species shown. Other than the work just cited, few experimental or theoretical investigations of metallacarborane NLO properties have appeared, and despite the potentially large advantages afforded by metallacarboranes for NLO application this area has been slow to develop. Salts of 3-MIII ð1; 2-C2 B9 H11 Þ2 (M ¼ Fe, Co) exhibit weak second-order harmonic generation [70], as does the exo-metallated dithioplatino-o-carborane 17-17 [71]. However, the heterobimetallic complex 17-18 is an effective third-order NLO chromophore, whose hyperpolarizability (g) of 2.60 1029 esu is an order of magnitude larger than that of other reported dithiolene complexes [72].
O
O
C C
S
S
Ir Mo
S
Pt C
N
17-17
C
Ir
N
C C
S
S S
C C
17-18
17.3.4 Electroactive systems Electronic communication between the metal centers in polymetallacarborane systems has been a question of longstanding interest, in part because of the possibility of creating new types of tailored electrical conductors or semiconductors (“molecular wires”) and other devices. As discussed in Chapter 13, detailed studies of several types of linked 6- and 7-vertex metallacarborane assemblies, as well as some larger ones, have revealed widely varying behavior, ranging from fully localized (Robin-Day Class I) to fully delocalized (Class III) systems. Multidecker sandwiches, which can be viewed as apically fused pentagonal bipyramidal MC2B3 or MC3B2 clusters, similarly exhibit different degrees of electron delocalization depending on the metals, metal oxidation sates, and other variables (Section 13.3); the polydecker sandwiches 13-183 (nickel) and 13-184 (rhodium) are, respectively, a semiconductor and an insulator. The construction of useful carborane- and metallacarborane-based electroactive materials, and especially the tailoring of their properties for specific applications, has been a theme of many reviews and proposals over the years. However, the implementation of these ideas has only recently begun to attract wider attention as the ability of carboranes to address some long-standing problems gains visibility. A recent case in point is the demonstration of the bis (dicarbollyl)nickel(III/IV) couple (17-19), a long-known metallacarborane sandwich system [73], as an improved redox shuttle for dye-sensitized solar cells [74]. Unlike the widely employed I= I3 couple, 17-19 does not lead to adsorption on TiO2 photoanodes and is superior to the ferrocene-ferrocenium (Fc/Fcþ) couple in two respects: it exhibits power conversion efficiencies 90 times higher, and avoids the problem of interception of injected electrons by Fcþ.
17.3 Carborane-based materials 1089 −
C
17-19
C
0
− e−
C
C
NiIII C
C
+ e− C = CH or CR
NiIV C
C
The structurally similar bis(dicarbollyl)cobaltate ion 3-Coð1; 2-C2 B9 H11 Þ2 and its derivatives, a well-known family with increasingly broad application (e.g., in radionuclide extraction and other technologies discussed earlier), are also finding a role in the development of electronic materials and devices. Salts of the parent anion with the planar radical cations tetrathiafulvalene (TTFþ), bis(methylenedilithio)tetrathiofulvalene (BMDT-TTFþ), and tetramethyltetraselenafulvalenium (TMTSFþ) are semiconductors with room temperature conductivities between 104 and 15 Ohm1 cm1 and crystallize in layered structures that are amenable to crystal engineering via introduction of substituents on the cages [75]. The electropolymerization of bis(thiophene) derivatives of 3-Coð1; 2-C2 B9 H11 Þ2 to generate highly conjugated semi2C2B9H10)Co(B9H10C22 2C4H4)2 2]n films that efficiently catalyze the reduction of protons to H2 conducting [2 2(SC4H42 [76] is noted in Chapter 14. Doping of polypyrrole films with 3-Coð1; 2-C2 B9 H11 Þ2 has some remarkable effects [77–81]. As shown by the Teixidor group, such treatment renders the polypyrrole more resistant to overoxidation, a degradative process that leads to decreases in conjugation and electrical conductivity, by 300-500 mV. Moreover, the doped material is self-recovering, with its electroactivity restored after a few hours even if the overoxidation limit is reached; grafting the cobaltate anion to the polypyrrole chain permits the use of much higher voltages than is otherwise possible [77,79]. Thiophene-based conducting polymers incorporating o-, m-, or p-carborane in the main chain have been explored fairly extensively. As is noted in Chapter 14, electropolymerization of monomers such as 1,2-, 1,7-, and 1,12(C4H4S)2C2B10H10 (9-353, 14-12, and 14-13) affords polymers that are far more electrochemically and thermally stable than other polythiophenes [82–84]. Of these materials, the most highly conjugated and conductive polymer is obtained from the 1,2-(C4H4S)2C2B10H10 isomer 9-353, as shown by electrochemical, UV-visible, atomic force field microscopic measurements [82] and DFT molecular modeling studies [82]. A different potential application of carboranes in microelectronics involves the controlled, tunable modification of metal surfaces (usually gold or silver) by bound carborane molecules, usually thiol derivatives that have a strong affinity for these metals. Detailed studies by Czech scientists employing X-ray photoelectron spectroscopy (XPS) along with scanning electron, atomic force, and transmission electron microscopies, and other techniques, show that the 1,2- and 9,12(HS)2 derivatives of o-carborane (9-25 and 17-20), in both neutral and deprotonated (dithiolate) forms, are immobilized on flat gold surfaces or gold nanoparticles [85,86]. As shown in Figure 17-2, coordination of these species occurs via both thiol groups, while the p-carboranyl thiolate 17-21 necessarily binds through just one sulfur; the packing of molecules on the surface is dense, with approximately one carborane per eight gold atoms. The electron distributions and dipole moments of 9-25 and 17-20 are essentially the same in the monolayer as in the free molecules. In related work, [HO(O) 2C6H42 2(OCH2CH2)4S-]2 molecules arrayed on the surface of gold nanoparticles have been employed C2 2CB10H10C2 to generate high density DNA microarrays [87]. Russian workers have reported the creation of a single-electron transistor consisting of a Langmuir-Blodgett monomolecular film of stearic acid and 1,7-Me2C2B10H9-OC(O)CF3 deposited on highly ordered pyrolytic graphite. Control of the tunneling current was achieved through an STM (single-electron tunneling microscopy) tip [88,89]. Metallacarboranes also can be attached to solid surfaces, thereby broadening the scope of their potential applications: the Teixidor group has anchored cobalt bis(dicarbollide) complexes to TiO2 particles via P(O)(OH) or O2P(O)(OH) substituents that bridge the carborane ligands between B(8) and B(80 ) as in the sulfate-bridged species 13-367 (Chapter 13) [90]. An entirely different, albeit indirect, role of carboranes in electronic devices exploits the extremely low nucleophilicity of the closo-CB11 H12 anion and its derivatives, a topic discussed at length in Chapter 8. In this type of
1090 CHAPTER 17 Carboranes in other applications
C C
SH
HS
SH
HS
17-21-
H
C
C
C
H
17-20
9-25 C C
S−
−S
S−
−S
9-252-
SH
C
C C
H
S
C
C S
S
H
17-202-
FIGURE 17-2 Modes of attachment of o-carborane dithiolates 9-252 and 17-202, and the p-carborane thiolate 17-21 to gold surfaces.
application, the monocarbon carborane anions are essentially noncoordinating counterions for positively charged electron-delocalized organic cations, and the near-absence of interaction with the anions allows cation-cation interactions to form electroactive structures. An example is the cation-radical salt TTFþ CB11 Me12 (17-22), whose TTFþ• cations undergo both spontaneous self-association to form diamagnetic ðTTFÞ2 2þ species and cross-association with TTF itself to generate mixed-valence TTFþ•TTF cation-radicals. The latter species form p-doped stacked arrays that exhibit Robin-Day Class II (partially delocalized) behavior [91]. Me Me
−
C
Me
17-22
Me
S
S
Me
S
S
+•
Me
Me Me
Me Me
The same carborane anion forms highly soluble salts with p-phenylene-2,6-diphenylpyridinium cation oligomers (17-23) [92] that form linear rods with lengths up to 9 nm. Owing to highly twisted orientations of the connecting phenylene rings, the oliogomeric chains interact only weakly with each other. Of all anions investigated in this system, CB11 Me12 was found to afford the best solubility characteristics, suggesting that extension of this synthetic strategy to true “molecular wires” may be feasible.
Me Me
−
C
Me
Me
2
R Me
Me
+ N
+ N
R⬘
Me
Me
Me Me
17-23
n R, R⬘ = H, NH2, NHC(O)Me
n = 1-5
17.3 Carborane-based materials 1091 In a similar application, the hexachloro CB11 H6 Cl6 anion is employed as a counterion in p- and n-dopable films containing electrochemically generated C60 þ cations [93]. 2CB8H8C2 2CH22 2) DFT calculations on the conductivity of the 1,10-dimethylene-1,0-dicarbadecaborane (2 2CH22 unit as a bridge between gold electrodes indicate significant electron transport, with strong coupling between the carborane and gold molecular orbital systems [94].
17.3.5 Networks and supramolecular assemblies The intrinsic appeal of carboranes as building-blocks for architecturally novel macromolecular or supramolecular systems has spurred extensive experimental and theoretical studies. Many types of covalently linked multicage species, prepared by stepwise construction from small molecules, are described in preceding chapters of this book; examples include dendrimers, carborarods, macrocycles, mercuracarborands, and similar systems based on icosahedral clusters (Chapters 8–10 and 14), linked and stacked metallacarboranes (Chapter 13), and numerous others. Mass spectroscopic observation of “superclusters” in the gas phase in reactions of C2B10H12 carboranes in an ion trap has been reported [95]. The controlled assembly of carborane-containing molecules into ordered extended structures (crystal engineering) [96–102] is a major synthetic challenge, not surprising considering the inherent difficulty of manipulating the generally weak bonding interactions involved. Nevertheless, modern computational methods and careful synthetic work have led to some recent advances in this area. One-, two- and three-dimensional hydrogen-bonded networks are known; for example, Ln(4,40 -bipyridine)3þ salts of 3,1,2-CpCo(C2B9H11)– (Ln ¼ Gd, Tb, Ho) are 3D systems [103], while 2H N and C2 2H N interactions [103]. The dinuclear ð4; 40 -bphÞ3 ðbphÞþ CB11 H11 forms a 1D ladder featuring N2 copper complexes 17-24 self-assemble to form 2D supramolecular networks held together by C2 2H H2 2B dihydrogen bonds involving B(4)-H and isopropyl CH groups (Figure 17-3) [104]. Dihydrogen bonds of several types are featured in
H
C
P P
H
X
Cu
C
Cu
X
H
C H
P
P
C
H
H
P P
H
Cu
C H
H H
P P
H
Cu
C C
X X
H
H
C
H
C
Cu
H
P
P
H
H
P P
H
Cu
C H
C
17-24 X = Cl, Br, I
X X
C C
Cu P
C P
H
H
C
Cu
X
P
C
H
X
H
P
H
FIGURE 17-3 2D supramolecular dihydrogen-bonded network of Cu2(m-X)2{[(Me2CH)2P]2C2B10H10}2 complexes.
H
1092 CHAPTER 17 Carboranes in other applications other carborane assemblies; for example, 3,1,2-(Z6-C6H6)Ru(C2B9H10-8-SH) which features C2 2H S-H H2 2B interactions [105], a grafted crown ether-N4 -3-Coð1; 2-C2 B9 H11 Þ2 macrocycle [106], and 8,90 -[3,1,2-CpCo(C2B9H10)]2 [107], the latter two structures having intermolecular C2 2H H2 2B bonds. Supramolecular assemblies of 10-vertex FeC2B7 clusters held together by O H interactions have been explored by Stone [98]. A detailed investigation by Vin˜as and associates [108] of the crystal packing in a wide range of 2H I2 2B hydrogen bonds play B-iodinated o-carboranes having one to 10 iodo substituents has disclosed that Ccage2 a major role in the structures of partially iodinated species such as 1,2-H2C2B10H9-3-I [Figure 17-4(A)]. In contrast, in the decaiodo derivative 1,2-H2C2B10I10, the CH hydrogens are sterically prevented by adjacent iodines from participating in hydrogen bonds; instead a 3D network held together by B2 2I I2 2B interactions is formed [Figure 17-4(B)]. Information of this kind, developed through experimental and computational studies, should help to bring closer the goal of controlled synthesis of extended carborane-based materials having desired architectures and properties.
C
C
C H
H
I H
C
H
H
I
I
H
C
H
I
I H
H
C
C
H
H
C
C
C
A I
I
I
I
H
I
C
I I
B
I
H
I I
H
I
I
H
I I
I I H
I
C
C
I
C
I
H
I I I
H
C
I
I
I
C
I
C
H
H
I
C
I
I I I
I
I C
I
H
C
I
I
I
H
C
I I
I
I
C H
I
FIGURE 17-4 Intermolecular interactions in 1,2-H2C2B10H9-3-I (A) and 1,2-H2C2B10I10 (B).
In another example of supramolecular synthesis, the tris(o-carboranyl-rhodium)triazine complex 17-25 crystallizes with toluene to form an infinite helical assembly connected via Cp*-toluene-Cp* p-stacking. The p-stacking, in ˚ ) that bends the individual 17-25 complex turn, is made possible by B2 2Hd pyridyldþ hydrogen bonding (3.1-3.3 A units into a bell-shaped conformation by pulling the pyridyl rings to one side of the triazyl plane, as shown in 17-26 [109].
17.3 Carborane-based materials 1093
C
C
H
H E
E
N
Rh
C
C
N
S
17-25
N
N
E = S, Se
S
S
N S
N
E
N
N
S
17-26
N Rh
N H
H
S N
C C
E
Rh
E C
E
C
∞
∞
In other extended solid state structures, carboranes and metallacarboranes play a more passive role, for example as inclusion species in cyclotriveratrylene (CTV) host-guest complexes like CB9 H5 Br5 (Chapter 6, 6-31) and (CTV)2(1,2C2B10H12) [110]. Similarly, m-carborane serves as a filler in a bimodal poly(dimethylsiloxane) network [111], and 3-Coð1; 2-C2 B9 H11 Þ2 functions as a counterion in the platinum-mediated self-assembly of supramolecular truncated tetrahedral pyridylethynylbenzene structures [112].
17.3.6 Gas separation and storage materials A potentially significant emerging area of interest centering on carborane-based metal-organic frameworks (MOFs) is illustrated by the synthesis of {Zn3(OH)[1,12-(CO2)2C2B10H10]2.5(DEF)4}n (17-27) (DEF ¼ dimethylformamide) [113], designed as a carborane analogue of the well-known cubic framework material [Zn4O(bdc)3]n (bdc ¼ benzene1,4-dicarboxylate) (MOF-5). In designing this material, replacement of the benzene rings in MOF-5, an effective storage material for H2, with 1,12-bis(carboxyl)-p-carboranyl cages was anticipated to produce smaller pores and hence higher heats of adsorption. Although 17-27 was found to have a different composition and structure from MOF-5, on heating under vacuum at 300 C it loses all of its DEF to form an as yet structurally undefined material that exhibits high uptake of H2 (2.1% at 1 atm) at 77 K, exceeding that of most MOFs [113]. A similar compound (17-28), prepared using dimethylformamide (DMF) in place of DEF, on evacuation at 300 C loses all of its DMF with formation of uncoordinated zinc sites. The latter material is highly effective at selectively absorbing CO2 from CO2-CH4 mixtures, evidently as a consequence of the polarity of the former versus the nonpolarity of methane [114]. The adsorption characteristics of 17-28 compare favorably with those of zeolites, which require higher temperatures for regeneration. 2CB10H10C2 2CO2)3(DMF)2]n (17-29), prepared by solCobalt-containing p-carboranyl MOFs, [Co4(OH)2(O2C2 vothermal reaction of 1,12-[HO(O)C]2C2B10H10 with Co(NO3)2, can be obtained in several forms having different morphologies and gas adsorption properties merely by varying the solvent and reaction temperature [115,116]. All of these materials, in which the solvent is DMF, pyridine, or diethyl ether, crystallize as 3D networks with large open 2CB10H10C2 2CO2)(C5H5N)2(H2O) effichannels between the carborane cages; 17-29 and the related polymer Co(O2C2 ciently separate CO2/CH4, CO2/N2, and O2/N2 mixtures [115]. Given the large variation in their capacity and selectivity toward H2, N2, and CO2, tailoring MOFs of this type for optimal performance of specific tasks seems eminently feasible.
1094 CHAPTER 17 Carboranes in other applications
O C
C
O
Co
O O
Co O
O O
17-29
O C
C
(solvent molecules not shown)
C C O
O O O
Co
O O
O C
Co
C O
The hydrogen-storage capability of multidecker complexes consisting of titanium atoms sandwiched between planar C3B2H5 rings has been explored computationally using DFT theory [117]. The results suggest that such materials, having the empirical formula Tin(C3B2H5)n1, can accommodate up to 5.1% of H2 with an average binding energy of 0.58 eV per H2 molecule and would be ideal hydrogen storage materials if they can be synthesized. A main attribute in the proposed compounds is that the metal atoms will remain separated, in contrast to Tin(C5H5)n complexes which tend to cluster, reducing their ability to take up H2. Given that many well-characterized multidecker sandwiches incorporating C3B2H5 or C2B3H5 rings have been prepared, as described in Section 13.3, there is reason to believe that the proposed titanium multideckers can be prepared and their hydrogen storage capability explored. Other theoretical studies suggest that transition metal carboride nanostructures might be efficient storage media for H2, although no synthetic route to such materials is offered [118].
17.3.7 Films and monolayers A number of developing applications employ carboranes attached as monomolecular layers or thin films to metal or other surfaces. Deposition of o- or m-carboranyl thiols [1,2- or 1,7-(HS)C2B10H11] on a Au[111] surface affords self-assembled monolayers (SAMs) with dissimilar properties [119]. The more polar o-carborane molecules have a higher mutual attraction and form a more tightly interconnected layer than do the m-carboranes; however, the m-carboranyl SAM is more hydrophilic and hence more easily wetted, because of its outward dipole orientation. As revealed by scanning tunneling ˚ ) and lack the much larger protrumicroscopy, both SAMs are remarkably smooth, with only small depressions (ca. 1 A sions and domain boundaries found in n-alkanethiolate and diamondoid SAMs [119]. Carboranes also offer advantages in lithography, both in the etch resistance they provide via formation of nonvolatile oxides on treatment with an oxygen plasma, and in their UV transparency. Polymeric resists for extreme UV lithography, in which pendant carborane cages are attached to polystyrene-butadiene copolymers, are found to have excellent etch resistance [120,121]. These properties have also been exploited in nanoimprint lithography (NIL), a nanopatterning technique employing polymers that harden under UV light while attached to a template. In one such system, 10% by 2CB10H10C2 2(CH2)3OC(O)CH5 5CH2, was added to siliweight of a silyl-o-carborane acrylate monomer, Me3CSiMe22 con wafers coated with propylene glycol methyl ethyl acetate (PGMEA), affording outstanding resistance to etching by an oxygen plasma. In turn, this enables effective image transfer to the underlying lift-off layer, allowing the fabrication of metallic interdigitated electrode patterns [122].
17.3 Carborane-based materials 1095
17.3.8 Carboranes in biomaterials The introduction of carboranes and metallacarboranes into nucleic acids and other biological molecules for use in boron neutron capture therapy (BNCT) is described in Chapter 16 (for example, 16-46). However, a different kind of application entails the use of DNA, quite apart from its usual biological role, as a basis for new materials with potentially novel properties. As discussed in a recent review by Lesnikowski [123], the underlying concept is that the inherent information storage and self-assembly capabilities of DNA can in principle be exploited in devices such as electrochemical sensors and luminescent probes, as in artificial nucleases and other applications. Metal-labeled DNA is of particular interest, for example, as sensors for the electrochemical detection of DNA hybridization. Ferrocenyl oligonucleotides have been extensively studied because of the reversible FeII/FeIII couple and other desirable qualities, but other metallocenes lack the air-stability of ferrocene and are less useful. Carboranes and metallacarboranes, in contrast, offer favorable redox properties, stability, and tailorability. Addition of a bridging o-carboranyl group allows electronic tuning of complex modeling of the active site of an Fe2 2Fe hydrogenase active site [124], and carborane-modified DNA-oligonucleotides such as 17-30 to 17-32 are amenable to the introduction of metal centers by standard methods, described in Chapter 13 [123]. O H
N
O
Me
OH
H
O P
OH O
C C
O
O
17-30
H
C C
O
O
O H
N O
O
5⬘-CGC
N
C H
C
N
O O
O P
P
OH
OH
O
O
TTTTTTTTTTT-3⬘
H
N
O
N
O O
H
TTTTTTTTTTT-3⬘
17-31
CGTTTGGCTG-3⬘
17-32
This methodology has been put to use in the electrochemical detection of DNA hybridization, in which C2 B9 H11 labeled DNA in nanomolar concentration is used as a probe for targeting or signaling, monitored via differential pulse voltammetry; this technique permits the detection of single-base mismatch [125]. Metallacarborane labeling has been proposed as a means of multipotential electrochemical coding of DNA, on the basis of studies of 2-deoxyadenosineCrð1; 2-C2 B9 H11 Þ2 conjugates that are analogues of the ferra- and cobaltacarborane conjugates (16-46) described in the preceding chapter [126,127]. With the availability of well-developed synthetic routes to such conjugates, this area is likely to attract increasing interest.
17.3.9 Carborane-based ceramics An indirect area of application of carboranes—so labeled here because the cage structure is typically destroyed in the process—involves their conversion at high temperatures to solid state boron-carbon materials (in some cases with incorporation of other elements) for a variety of purposes. As carboranes are not inexpensive, materials obtained in this way must be special indeed, with properties not obtainable from more conventional sources. Carboranes can serve as singlesource precursors to ceramics with controlled composition, and covalently bound polymers are particularly favored for this purpose, given their highly ordered structures. Pyrolysis of sllyl- or siloxycarborane polymers at temperatures of 1000 C or higher has been employed to generate a variety of ceramics with desired properties; examples of such precursors include the disilylcarborane 14-1 and ferrocenyl-siloxy-carboranyl polymers (14-1 and 14-3) described in Chapter 14, and a number of others [128–135]. Siloxy-C2B10H10-acetylene polymers have also been converted to elastic coatings for carbon fibers [136].
1096 CHAPTER 17 Carboranes in other applications Monomeric o-carborane has found application in the synthesis of superhard B4C coatings as a precursor to boron carbide coatings for tokomak fusion reactors [137–144] via pulsed-laser chemical vapor deposition (CVD) [145], helium glow discharge [144], or plasma [139,146] methods. Semiconducting boron carbide films prepared from all three C2B10H12 isomers [147] and semiconducting carbon-boron-metal films obtained from o-carborane and ferrocene, cobaltocene, or nickelocene via plasma CVD have been investigated [148–150]. Studies employing photoionization mass spectrometry, combined with DFT calculations, indicate that dehydrogenation of the carboranes via loss of H2 occurs preferentially at the B2 2H bonds furthest from the carbon atoms [151]. Deposition of semiconducting boron films onto silicon wafers by generation of o-carborane beams from a Bernas ion source has been demonstrated, with results superior to those obtained with B10H14 owing to the higher thermal stability of the carborane [152]. In other applications, o- and m-carboranes have been employed as flame-retardant coatings [153] and as additives in polyurethane adhesives [154]. Three-dimensional solid-state materials have also been explored, as in the formation of 10-30 nm diameter (C2B4H2)n and (C2B10H4)n nanoparticles by gas phase pyrolysis of 1,6-C2B4H6 and of 1,2-C2B10H12, respectively, at 1150-2000 K [155]. Plasma-enhanced CVD of 10B-enriched o-carborane has been used to fabricate semiconducting B2 2C alloys for use as solid-state neutron detectors [156], and pyrolysis of a molybdenum-containing carboranylsiloxane at 1000 C affords a superconducting mixture of carbon nanotubes and Mo2C [157].
17.4 MOLECULAR MACHINES Mechanical devices on a molecular scale are a developing technology commanding considerable interest but also presenting formidable challenges to their synthesis, study, and practical implementation [158–160]. One synthetic approach, the “top-down” concept, entails the construction of macroscopic objects on progressively smaller scales by techniques such as photolithography, but this method is reaching its practical limits. In the alternative “bottom-up” approach, individual molecules are designed to function as nanoscale machines. Carboranes were introduced conceptually into this general area by Hawthorne and his coworkers, who noted that the 3-NiIII ð1; 2-R2 C2 B9 H9 Þ2 =3-NiIV ð1; 2-R2 C2 B9 H9 Þ2 0 couple (17-19), discussed earlier, involves a change in rotational conformation: the Ni(III) and Ni(IV) species are respectively transoid and cisoid, owing to the fact that different minimum-energy geometries are adopted depending on whether the LUMO is occupied or unoccupied [161]. This presents the possibility of effecting reversible partial rotation (up to 4p/5) by redox or photolytic action on the complex; if the molecule is rendered optically active [162] by appropriate substitution on the cage, rotation in one direction would be favored for particular enantiomers. Such complexes might function as molecular oscillators capable of altering the electronic states of covalently attached molecules or surfaces [161]. A different type of molecular device, the nanocar, has been investigated and experimentally implemented by Tour et al. [163]. These workers have synthesized a family of molecular vehicles whose “engines” are organic groups that collect light, thermal, or electrical energy and convert it to rotary motion of attached “wheels.” Originally employing C60 fullerenes as wheels, the motion of nanocars on gold surfaces was detectable by an STM tip, which induced motion by placing the tip in front of the nanocar. However, it was found that p-carboranyl wheels afford advantages over fullerenes, including higher solubility in organic solvents and the fact that fullerenes, unlike carborane cages, rapidly absorb light energy and hence are unsuited for use in light-powered nanovehicles [164,165]. A variety of carborane-wheeled molecules has been synthesized via standard organic methods combined with organometallic metal-catalyzed coupling reactions, including the nanocar 17-33, the nanocaterpillar 17-34, and the trigonal nanocar 17-35 [166]. Attachment of a dye label (tetramethylrhodamine isothiocyanate) to these nanovehicles has enabled measurement of their motion on a glass surface by single molecule fluorescence imaging, showing that they reach speeds of up to 4.1 nm s1 over distances as much as 2.5 mm [167]. This approach has been improved upon by building nanocars that incorporate fluorescent groups in the chassis [164], or even better the axles [168], thereby eliminating the need for a tag which might interfere with movement of the vehicle. In 17-36 and 17-37, for example, the difluoroboradiazaindacene (BODIPY) functionalities, which exhibit a strong S0 ! S1 p-p* transition, absorb photons from a 514 nm laser with high quantum yields (45-49%) and transmit energy efficiently to the carborane wheels [168].
17.4 Molecular machines 1097
H
direction of motion
HC
C
C
C
C
direction of motion
CH
OC3H7 C3H7O R
R 2
17-35 HC
C
C
17-33 R = OMe
17-34 R =
OC3H7
OC3H7 2
CH
C C
CH
2 OC3H7
OC3H7
CH
HC
F
HC
F
HC
N
C
N
F B
N
C
C
CH
C
CH
F B
N
C
C
CH
OMe
17-36
HC
17-37
C N
B F F
N
C
CH
HC
MeO
C N B
N
F F
That the carborane cages in these molecules do in fact rotate on their axles, so that the nanocars actually roll on surfaces as opposed to hopping or sliding, has been verified by kinetic [164] and single molecule fluorescence imaging studies [167] and through control analogues in which the carborane wheels are replaced by tert-butyl groups [168]. At this writing, the variety of nanovehicles continues to expand, with the synthesis of nanotrucks (containing a porphyrin chassis that could transport a metal atom or other load) [169], nanodragsters (sporting large fullerene rear wheels and C2 2C 2p-C6H4smaller p-carborane front wheels) [170], and nanoworms in which two HCB10H10C2 C2 2N5 5N2 2C6H4 chromophore whose cis-trans photoisomerization C 2CB10H10CH axles are connected by a C6H42 is expected to induce an inchworm-like crawling motion [171]. Alternative types of energy generation are also being explored, for example, a dipolar nanocar whose chassis has a dimethylamino unit at one end and a nitro group at the other; its large dipole moment may lead to induced motion over a surface in an applied electric field [172].
1098 CHAPTER 17 Carboranes in other applications
17.5 CARBORANES AS NONCATALYTIC SYNTHETIC AGENTS The role of carboranes and metallacarboranes in the synthesis of organic (and nonboron inorganic) compounds is normally associated with catalysis, as discussed in Chapter 15. In contrast, the use of carboranes as stoichiometric reagents in organic synthesis is comparatively rare, with one major exception: the exploitation of closo-CB11 H12 anions and their derivatives as nearly noncoordinating counterions enabling the isolation of a broad array of long-sought organic and inorganic cationic species, a topic treated in Chapter 8. A further example is afforded by the synthesis of a HCB11 H5 Br6 salt of the bis(thioxanthene) dication 17-38, whose central carbon atom is hexacoordinate and apparently hypervalent as well, on the basis of X-ray crystallographic evidence and DFT calculations that indicate donation of four oxygen electron lone pairs into empty p* orbitals on the central carbon [173]. Me S
Me Me
O
O
C
Me
Me + S
+
O O
Me Me
MeI Ag+HCB11H5B6−
+S
Me Me
O O
C
S
O O
Me
Me Me
Me
Me
2+
C
O O
Me Me
S
+S
Me
O O
17-38
Me
In a similar vein, benchtop syntheses of slightly water-soluble HCB11 H12 salts of the triazolium and imidazolium cations have been developed [174], and Csþ HCB11 Br6 H5 is used as a noncoordinating supporting electrolyte in studies of intervalence charge transfer in mixed-valence ruthenium and osmium complexes [175]. The same anion is employed as the counterion for extremely electrophilic catecholatoborane C4H6O2Bþ transient species which are powerful reagents for intermolecular electrophilic borylation [176]. There are other situations in which carboranes have been employed as noncatalytic reagents to achieve specific synthetic goals. For example, by using o-carborane as a template, Scheer et al. constructed the multilayer cluster C2B10H12@{[Cp*Fe(Z5-P5)]12(CuCl)20} (17-39), an analogue of Ih-C80 in which a Cu20 dodecahedron surrounds an icosahedral array of 12 pentaphosphaferrocene units, which itself encapsulates an icosahedral C2B10H12 cage [177].
17-39
Do and coworkers employ o-carboranyl substituents in triarylboranes such as Ph2BF-p-C6H4-CB10H10CPh as a way to substantially increase the Lewis acidity of the boron atom, and hence its affinity for fluoride ion. From spectroscopic data and DFT calculations, the effect is ascribed to a contribution from the C2B10 cage to the LUMO coupled with a strong inductive effect [178]. In another area of organic synthesis, the alteration of triplet carbene properties via substitution with C- and B-bonded o-carboranyl units is discussed in Section 9.7.
17.6 Carboranes in analysis 1099 In a development of potential significance for organic synthesis, Macgregor, Welch, and coworkers have found that cleavage of an arene C2 2C bond—previously known only at high temperatures—occurs upon reduction of 1,10 (C2B10H11)2 with Ru(p-cymene)2þ fragments at room temperature [179]. In this reaction, conducted in an attempt to prepare a bis(ruthenacarborane) via insertion of Ru(cymene) unit into each cage of the biscarborane, the product obtained instead has two such units incorporated into a single cage, with one of the cymenes having a ruptured C2 2C bond. DFT calculations suggest that the bond-breaking occurs as the cymene is reduced while trapped between two metal centers on the carborane cage—yet another example of chemistry made possible through the unique steric and other properties afforded by carborane systems.
17.6 CARBORANES IN ANALYSIS 17.6.1 Gas chromatography As is recounted in Chapter 14, the earliest commercially marketed carborane products were siloxy-m-carborane polymers developed for use as liquid phases in high temperature gas chromatography. Decades later, modified carboranesiloxane copolymers remain the products of choice for this purpose. Interest in this area continues, with the further development of this technology [180,181] and its applications, which include the analysis of polybrominated and polychlorinated ethers and biphenyls and chlorinated pesticides in human serum and milk [182], biodiesel [183], and other materials [184]. Curiously, only recently have the structures of the DEXSIL siloxycarborane polymers been elucidated in detail via 29Si NMR spectroscopy [185].
17.6.2 Ion-selective electrodes Another area in which carboranes have demonstrated superior performance and versatility is in the development of ionsensitive electrodes (ISEs), important analytical tools that are especially useful in clinical diagnosis and environmental monitoring. Among the widely used types of ISEs are liquid-polymer membranes (usually polyvinyl chlorides or PVCs, that contain ion-selective species capable of complexing with target anions or cations) combined with lipophilic ions that can detect specific ions. Tetraphenylborates, employed as cation detectors for many years [186], are susceptible to hydrolysis in the presence of oxidizing agents, acids, or light [187,188] and carborane anions are drawing increasing interest as alternative agents for this purpose [22–24,187,189–194]. The most commonly employed are the polyhalogenated species CB11 X11 (X ¼ Cl, Br, or I) and the cobalt bis(dicarbollide) anion 3-Coð1; 2-C2 B9 H11 Þ2 , both types being well suited for ISEs by virtue of their chemical inertness and strongly lipophilic character. The CB11 I11 ion, for example, exhibits much higher selectivity and lower detection limits than tetraphenylborates [187,195], as do BðC6 H3 RR0 R00 Þ4 þ CoðC2 B9 H11 Þ2 salts in which R, R0 , or R00 ¼ H, Cl, CF3, or C(CF3)2OMe) [189]. The 3-Coð1; 2-C2 B9 H11 Þ2 ion, a very lipophilic, low charge-density species whose volume is less than a third of that 2H H2 2B and NH H2 2B dihydrogen hydrogen bonds with the membrane, is also used as a of BPh4 and forms Ccage2 doping agent in ion-selective microelectrodes based on conducting polypyrrole films. In these “intelligent membranes,” whose cation sensitivity is controlled by an applied potential, the 3-CoðC2 B9 H11 Þ2 anion enhances resistance of the polypyrrole to overoxidation [191,194]. Systems of this kind are effective sensors for lanthanides [196,197], Hþ [198], Pb2þ [199], and Naþ [200] and used in conjunction with crown ethers for Kþ ion [201]. PVC membranes containing 3-Mð1; 2-C2 B9 H11 Þ2 anions, where M is Fe, Co, or Ni, function as ISEs for the detection of Csþ, Sr2þ, and Eu2þ in nuclear waste [23]. In a very recent advance, stepwise chlorination of 3-CoðC2 B9 H11 Þ2 has been shown to linearly reduce the redox potential by 0.1 V per added chlorine [202]. Efficient ISEs for analysis of the anti-tuberculosis antibiotics isoniacid (17-40) and pyrazinamide (17-41) have been obtained via formation of ion-pair complexes of the type ½cation-NHþ x ½CoðC2 B9 H11 Þ2 y in PVC membranes [203]. In another medicinal application, Stoica et al. have prepared enantiomer-selective membrane electrodes capable of detecting optically active amino acids, by inclusion of the 3-Coð1;2-C2 B9 H11 Þ2 anion in PVC membranes containing enantiomerically pure D- or L-tryptophan or D- or L-histidine [204].
1100 CHAPTER 17 Carboranes in other applications N
O
NH2
O N
17-40
17-41 N
NH2 N
In comparison to cation detectors, ionophores for anion-selective sensors are more difficult to implement [205], but anticrown “mecurocarborands,” described in Section 9.17, contain electrophilic mercury centers that make them excellent agents for trapping halide ions as in 9-388 [206,207]. The trimercury complex Hg3(1,2-C2B9H10)3 (9-386) can detect I at nanomolar concentration [190] (even lower if embedded in a rotating disc [208]) and has been fabricated into polymer films [205] and phospholipid membranes of fluorescence-sensing lipobeads [209], for effective detection of Cl levels in blood and other biological fluids. Methylation of the carborane cages in 9-386 has been shown to increase the ionophoric sensitivity of this mecurocarborand [210].
17.7 INTO THE FUTURE The wide range of applications outlined in this and earlier chapters guarantees that carboranes will have a place in the technology of the twenty-first century. There are clearly specific tasks for which carboranes are better suited than any known alternatives—for example, in the stabilization of species requiring nearly noncoordinating, oxidatively stable counterions, the construction of extremely temperature- and oxidation-resistant polymers, and the extraction of metals from radioactive waste—and others with huge but still as-yet unfulfilled potential, such as BNCT and other medical applications. Continuing development of these areas, and numerous others discussed throughout this book, may well lead to a significant expansion of the role of carborane-based materials and methods in addressing practical needs. However, the really intriguing notion is the possibility that the unique combination of properties offered by carboranes, coupled with their virtually unmatched capacity for tailoring to specific purposes, will lead eventually to some truly revolutionary, game-changing advance. Among other possibilities, the field of nanoelectronics comes especially to mind as a place where this might happen. One thing is clear, and this could not have been said just a few years ago: carboranes are no longer exotic species unknown to most scientists. They have taken their place as versatile synthetic tools, building block units, electroactive substituents, complexing (and almost-noncomplexing) agents, 10B carriers for BNCT, active pharmacophores, radionuclide extractors, and even, as we have seen, wheels on molecular cars. The small community of boron chemists who study the fundamental properties of carboranes is now surrounded by a much larger population of organic and inorganic chemists, physicists, engineers, medical technologists, materials scientists, and others with little or no background in boron chemistry, who are increasingly drawn to these extraordinary molecules with their seemingly limitless possibilities.
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Index Note: Page numbers followed by f and t indicate figures and tables, respectively.
A Acid(s). See also Carboxylic acids alkenyl derivative reactions with, 412–413 B-substituted o-carboranyl, 422t o-carboranyl phosphinic acids and phosphonic acids, 479f CH acidity of o-carborane derivatives, 377, 425; of m- and p-carborane derivatives, 607–608, 655 chloroplatinic, 1021 C-substituted o-carboranyl, 422t monocarborane, proton-donating capability of, 289–290 reactions of o-carboranyl alkenyl derivatives with, 412–413 Sulfinic acids, o-carboranyl, 386; m- and p-carboranyl, 612 Acid-base properties of CB11H12 , 289–290 of H2C2B10Cl10, 502 Actinide metals in 12-vertex metallacarboranes, 883t in supraicosahedral metallacarboranes, 966t Acyl halide derivatives 1,2-C2B10H12, 322t 1,7-C2B10H12, 552t 1,12-C2B10H12, 588t Agonists estrogen receptor, 1054–1056 retinoid receptor, 1056 Alcohols bis(o-carboranyl), 427 1,2-C2B10H12, 427–435 B-substituted alcohols, 430 C- and B-hydroxy derivatives, 430–431 C-substituted, 427–430 derivatives, 430–431, 317t 1,7- and 1,12-C2B10H12 B-substituted, 636 C-substituted, 635–636 derivatives, 549t, 585t m- and p-carboranyl, properties of, 637–638 synthesis, 635–637 m- and p-carboranyl converted to alkenes, 638 general observations, 637 halogenation, 638 oxidation and cleavage, 638 properties, 637–638 o-carboranyl, 394, 420, 421, 427, 428, 434–435 reactions, 434–435 synthesis, 427–434, 635–637 Aldehydes 1,2-C2B10H12 derivatives, 319t, 435–442
B-substituted, 435 o-carboranyl, reactions of, 440–442 cleavage, 440–441 C-substituted, 435 electronic effects, 441 steric effects, 441 synthesis, 435 1,7-C2B10H12 derivatives, 550t, 638–640 properties, 639–640 synthesis, 638–639 1,12-C2B10H12 derivatives, 586t, 638–640 properties, 639–640 synthesis, 638–639 reactions of o-carboranyl, 428, 429, 440–442 Alkenyl derivatives 1,2-C2B10H12, 313t reactions with acids, 412–413 amide addition, 416 coupling and cyclization reactions, 415 reactions with halogens, 411–412 hydrogenation, 413 hydrostannylation, 456 metallation, 414–415 reactions with oxidants and radicals, 413–414 polymerization of 1-alkenyl-o-carboranes, 416–417 properties, 411–417 reactions with other electrophiles, 413 synthesis and properties of o-carboranyl carbenes, 416 1,7-C2B10H12, 548t, 628–630 1,12-C2B10H12, 583t, 628–630 m- and p-carboranyl alcohols converted to, 638 o-carboranyl carbene synthesis and, 416 rigid rods, 628–630 Alkenylpentaboranes, synthesis of, 42 Alkoxy derivatives 1,2-C2B10H12, 318t 1,7-C2B10H12, 550t 1,12-C2B10H12, 586t Alkoxysilanes derivatives, 452–454 Alkyl derivatives 1- and 2-CB9H10 , 170 1,2-C2B10H12, 305t, 406–410 1,7-C2B10H12, 543t, 625 1,12-C2B10H12, 579t, 625 CB11H11 , mixed alkyl-halo, 288 halides, 390–391, 397 Alkylation, 55, 55f 2,4-C2B5H7, 109–110 CB11H12 , poly-B-alkylation, 287–288
1107
1108 Index Alkylation (Continued) nido-C2B9H11 2 , 229–231 nido-7,8-C2B9H11 2 , 230 nido-7,9-C2B9H11 2 , 231 nido-C2B9H12 , 229–231 nido-C2B9H13, 229–231 electrophilic 1,2-C2B10H12, 396–397 1,7-C2B10H12, 617–618 1,12-C2B10H12, 617–618 13-vertex C2B11 and 14-vertex C2B12 clusters, 695 metal-free, 395 Alkyl-substituted m-carboranes, 625 Alkyl-substituted o-carboranes, 396 Alkyl-substituted p-carboranes, 625 Alkyne(s) in alkyne-ferraborane photolysis, 117f in alkyne-pentaborane reactions, 52 in borane-alkyne reactions gas phase, 21 importance of, 21 in solution, 22 nido-2,3,4,5-C4B2H6 derivatives from, 67f in carborane-alkyne reactions, 22 insertion, 32f reagents in C2B10H12 synthesis, 375–376 Alkynyl derivatives 1,2-C2B10H12, 302t, 411–421 addition to triple bond, 417 polymerization, 418 properties, 417–418 proton donation, 417 1,7-C2B10H12, 548t, 628–630 reactivity, 628 1,12-C2B10H12, 583t, 628–630 reactivity, 628 rigid rods, 628–630 Alkynyl-linked polymers, 1017–1019 Allenes, nido-2,3,4,5-C4B2H6 derivatives from, 67f Aluminum addition at carbon, in 1,2-C2B10H12 derivatives, 382; in 1,7- and 1,12-C2B10H12 derivatives, 609 addition at boron, in 1,2-C2B10H12 derivatives, 402; in 1,7- and 1,12-C2B10H12 derivatives, 620 constrained-geometry complexes, 713–714 derivatives 1,2-C2B10H12, 346t 1,7-C2B10H12, 565t 1,12-C2B10H12, 596t in heteroatom carboranes, 707t, 710–714 Amide derivatives 1,2-C2B10H12, 330t, 416, 471 1,7-C2B10H12, 556t, 646 1,12-C2B10H12, 591t, 646
Amine derivatives 1,2-C2B10H12, 327t, 463–467 1,7-C2B10H12, 554t, 642–644 1,12-C2B10H12, 590t, 642–644 heterocyclic, 329t Amino acids introduced to 1- and 2-CB9H10 , 170 as small-molecule boron delivery agents, 1063–1064 Analysis, carboranes in, 1099–1100 gas chromatography, 1099 ion-selective electrodes, 1099–1100 Androgen receptor (AR) antagonists, 1056 Anions nido-C2B9, versatility of, 236–237 C2B10, open-cage, 675–684 carbon-bridged clusters, 682–684 reduction of neutral C2B10 icosahedra, 675–681 CB11 anions in catalysis, 1037–1038 salts with main-group metal cations, 1037–1038 salts with transition metal-containing cations, 1038 C2B11 radical anions, formation of, 696 Antagonists androgen receptor, 1056 estrogen receptor, 1054–1056 retinoid, 1056 Anticancer nucleosides, 1058–1059 Anticoagulants, 1058 Antimony addition at carbon, in 1,2-C2B10H12 derivatives, 383–385; in 1,7- and 1,12-C2B10H12 derivatives, 611 addition at boron, in 1,2-C2B10H12 derivatives, 402–403; in 1,7- and 1,12-C2B10H12 derivatives, 620–621 1,2-C2B10H12 derivatives, 353t, 385, 475 1,7-C2B10H12 derivatives, 570t in heteroatom carboranes, 743t, 755 Antitumor agents antiviral and anticancer nucleosides, 1058–1059 development of, 1058–1062 exo-metallated carborane derivatives, 1059–1060 other transition metal complexes, 1061–1062 platinum complexes, 1060 tin complexes, 1060 Antiviral nucleosides, 1058–1059 Applications, carborane, 1. See also Medicine, carboranes in analysis, 1099–1100 gas chromatography, 1099 ion-selective electrodes, 1099–1100 carborane-based materials, 1085–1096 biomaterials, 1095 ceramics, 1095–1096 electroactive systems, 1088–1091 films and monolayers, 1094 gas separation and storage materials, 1093–1094 ionic liquids, 1085
Index 1109 liquid crystals, 1085–1086 networks and supramolecular assemblies, 1091–1093 nonlinear optical materials, 1086–1088 future of, 1100 metal ion extraction, 1083–1084 molecular machines, 1096–1097 noncatalytic synthetic agents, 1098–1099 overview of, 1083 AR antagonists. See Androgen receptor antagonists Arsenacarborane, 153 Aromatic rings C5, in 1,2-C2B10H12 derivatives, 410; in 1,7- and 1,12-C2B10H12 derivatives, 627 C7 and larger, in 1,2-C2B10H12 derivatives, 410; in 1,7- and 1,12-C2B10H12 derivatives, 627 Aromaticity cluster, 16–17 Aromatics, in carborane-aromatic hydrocarbon analogy, 53–54 Arsenic addition at carbon, in 1,2-C2B10H12 derivatives, 383–385; in 1,7- and 1,12-C2B10H12 derivatives, 611 addition at boron, in 1,2-C2B10H12 derivatives, 402–403; in 1,7- and 1,12-C2B10H12 derivatives, 620–621 1,2-C2B10H12 derivatives, 353t, 385, 475 1,7-C2B10H12 derivatives, 570t 1,12-C2B10H12 derivatives, 597t in heteroatom carboranes, 742t, 755–758 Aryl derivatives 1,2-C2B10H12, C-aryl derivatives, 308t, 406–410 B-aryl derivatives, 410 with C5 aromatic rings, 410 with C7 and larger aromatic rings, 409–410 electronic structure, 406–408 metallation of C-benzyl derivatives, 408 properties, 406–410 reactivity, 406–408 reduction to anionic species, 408–409 1,7-C2B10H12, 545t, 625–627 1,12-C2B10H12, 581t, 625–627 properties of alkyl- and haloalkyl-substituted m- and p-carboranes, 625 properties of aryl-substituted m- and p-carboranes, 625–627 Aryldiazonium tetrafluoroborates, 380 Arylene-linked polymers, 1019–1020 Arylhalonium m- and p-carboranes, 654–655 Aryloxy derivatives 1,2-C2B10H12, 550t 1,7-C2B10H12, 550t 1,12-C2B10H12, 586t Aryl-substituted m- and p-carboranes derivatives with C5 and C7 aromatic rings, 627 phenyl derivatives, 625–627 properties of, 625–627 Aryl-substituted o-carboranes, electrophilic halogenation, 399 Astatination, 1072
Asymmetric catalysis, 1041 Azides 1,2-C2B10H12 derivatives, 331t, 463–467 1,7-C2B10H12 derivatives, 556t, 642–644 1,12-C2B10H12 derivatives, 591t, 642–644 Azo derivatives, 332t
B B5H9, supply of, 52–53 B10H14 1,2-C2B10H12 synthesis from, 301–376 B12H12 2 dianion, 267 Barene, 2 Barium in heteroatom carboranes, 707, 703t B-decahalo-o-carboranes, properties of, 501–502 Benzene-centered multicage pinwheel complexes, 820–821 Benzocarborane, 390–391 Benzo-o-carboranes, 393 Benzyne, 392 Beryllium in heteroatom carboranes, 702t, 705 B–Ga bond formation, 402, 620 B–Ge bond formation, 402, 620 BH4 salts, B10H14 synthesis from, 376 B-hydroxy derivatives 1,2-C2B10H12, 317t, 401, 430–431 1,7-C2B10H12, 549t, 612, 617, 621 1,12-C2B10H12, 585t, 621 B-I9 and B-I10 o-carborane derivatives, 399f Bifunctional extraction agents, 1084 Biomaterials, carborane-based, 1095 Bipyridine derivatives, 467–468 Bis(o-carboranyl) alcohol, 427 Bis(dicarbollide)-based extraction of metals, cobalt, 1084 Bis(carboranyl) carbinols, 428 Biscarborane, 381 Bis(o-carboranyl) derivatives, 384–385 Bis(2,3-dicarbahexaboranyl) metal complexes, 56 Bismuth 1,2-C2B10H12 derivatives, 353t, 384; 1,7- and 1,2-C2B10H12 derivatives, 611 BNCS. See Boron neutron capture synvectomy BNCT. See Boron neutron capture therapy Bond formation, boron B–Al, 402, 620 B–As, 402–403, 620–621 B–Ga, 402, 620 B–Ge, 402, 620 B–Hg, 401, 621–622 B–In, 402, 620 B–N, 402–403, 620–621 B–O, 430, 621 B–P, 620–621
1110 Index Bond formation, boron (Continued) B–Pb, 402, 620 B–S, 403, 621 B–Sb, 402–403, 620–621 B–Se, 403, 621 B–Si, 402, 620 B–Sn, 402, 620 B–Te, 403, 621 B–Ti, 402, 620 Bonding, structure and cage rearrangement, 16 cluster aromaticity, 16–17 delocalized, 15–16, 18 dicarbon clusters, 953, 965 “electron deficiency” in polyhedral boron clusters, 17–18 electron-counting in classically bonded clusters, 15–16 electron-pair bonding in nido-2,3-C2B4H8, 9f extensions of electron-counting rules, 14–15 general perspective, 7 isomer stability, 16 localized-bond approach, 8 nomenclature, 7–8 numbering, 7–8 structural patterns in boron clusters, 9–14 Borane-alkyne reactions gas phase, 21 importance of, 21 in solution, 22 Borane-based aircraft and rocket fuels, 1–2 Borderline structures, 59–60 B-organosilyl derivatives, 454 B-organosubstituted m- and p-carboranes, 622–625 Boron added at carbon, 381–382, 608 added to 11-vertex nido-carboranes, 376 1,2-C2B10H12 derivatives, 346t 1,7-C2B10H12 derivatives, 557t via C-metallated carboranes, 381–382 by electrophilic reagents, 284–285 extraction, mechanism of, 225–226 fluorination, 619 halogenation, 109 insertion, 97f into nido-C2B9 dianions, 617 synthesis of B-substituted derivatives via, 286 polyalkylation at, 287–288 polyhalogenation at, 286–287 polyhedral clusters, “electron deficiency” in, 17–18 small-molecule boron delivery agents amino acids, nucleosides, and nucleotides, 1063–1064 in BNCT, 1063–1066 other agents, 1066 porphyrins, phthalocynanines, and related derivatives, 1064–1066 structural patterns in clusters, 9–14
substituents on, 164t, 179–180 transition metals added at, 404–405, 622 Boron, substitution at, 24–25 closo-2,4-C2B5H7, 109 closo-C2B9H11, 253–254 1,2-C2B10H12, 395–406 1,7- and 1,12-C2B10H12, 617–625 boron insertion into nido-C2B9 dianions, 617 electrophilic alkylation, 617–618 electrophilic halogenation, 618–619 fluorination, 619 main-group elements added at boron, 620–622 mercury added at boron, 620–622 nucleophilic displacement, 617 organosubstitution at boron, 622–625 photochemical halogenation, 619 transition metals added at boron, 622 organosubstitution, 622–625 metal-promoted cross-coupling of B-halo-m and p-carboranes, 622–624 photochemical, 400–401 thermal, 400–401 132-Boron nanospherical closomers, 1067 Boron neutron capture synvectomy (BNCS), 1063, 1070 Boron neutron capture therapy (BNCT), 24–25, 231–232, 1053–1054 approaches, 1063 background, 1062–1063 carboranes in medicine, 1062–1070 high molecular weight boron delivery agents, 1066–1070 small-molecule boron delivery agents, 1063–1066 B–P bond formation, 620–621 B–Pb bond formation, 402, 620 BrCB11Br11 , 287 Breast cancer, 1055, 1060 Brellochs reaction, 145, 226–227 Bridge hydrogen placement, 114 Bridge insertion, metals, 54f Bromo derivatives 1,2-C2B10H12, 342t 1,7-C2B10H12, 562t 1,12-C2B10H12, 594t B–S–P systems, 478 B-stannyl derivatives, 460 B-substituted acids, 423t B-substituted alcohols 1,2-C2B10H12, 430 1,7- and 1,12-C2B10H12, 636 B-substituted aldehydes, 435 B-substituted derivatives amines, azides, and diazonium salts, 466–467 indirect routes to, 395–396 synthesis from B10H14 derivatives, 395 synthesis from C2B9 and C2B10 dianions, 396 nitrato, nitro, and related compounds, 462–463
Index 1111 synthesis, via boron insertion, 286 thiols, thioethers, disulfides, and related compounds, 495 thiophosphites and thiophosphates, 482–483 B-substituted ethers 1,2-C2B10H12, 434 1,7- and 1,12-C2B10H12, 637 B-substituted ketones, 439–440 B-substituted phosphines, 478 B-substitution electrophilic, 285 photochemical, 110 B–Te bond formation, 403, 621 B(9)-thiophosphinite, 482–483 B3LYP methods, 605 B–Ti bond formation, 402, 620 tert-butyldimethylsilyl group, 378 B(3)-vinyl derivatives, 416f
C C2B2Hx, 27–28, 28t closo-C2B3H5 bonding, 39–40 derivatives, 35t, 38, 39 physical properties, 40–41 pyrolysis of, 40 reactivity, 40–41 structure, 39–40 synthesis, 33–39 closo-1,5-C2B3H5, 33–41 1,5-C2B3H5 peralkyl derivatives, 161f nido-1,2-C2B3H7, 29–32 closo-1,2-C2B4H6 derivatives, 79t introduction of substituents, 82–83 physical properties, 80–82 reactions, 82 structure, 80–82 synthesis, 78–80, 79t closo-1,6-C2B4H6 cage linkage, 82 derivatives, 75t introduction of substituents, 82–83 physical properties, 80–82 polyhedral expansion, 82 reactions, 82 structure, 80–82 synthesis, 75t, 78–80 nido-2,4-C2B4H7, 58 nido-2,3-C2B4H8, 8, 9f bridge deprotonation of, 53f bridge insertion of, 54f characterization, 45t derivatives, 45–53, 45t
properties, 53–56 structure, 53–56 synthesis, 45–53, 45t nido-2,4-C2B4H8 characterization, 57t derivatives, 57t properties, 59 structure, 59 synthesis, 56–59 arachno-C2B4Hx clusters synthesis and characterization, 59t hypho-C2B4Hx clusters synthesis and characterization, 59t closo-2,3-C2B5H7 derivatives, 98t synthesis and characterization, 98t closo-2,4-C2B5H7 B-alkylation, -alkenylation and, 109–110 cage coupling, 110–111 cage opening, 110 cage rearrangement, 110 characterization, 100t derivatives, 100t electronic properties, 107 photochemical B-substitution, 110 polyhedral expansion, 110 structure, 107 substitution at boron, 109 substitution at carbon, 107–108 synthesis, 99–107, 100t nido-C2B5H8 derivatives, 94t as intermediate, 93–94 synthesis, 93f closo-1,7-C2B6H8 derivatives, 122t properties, 123–124 structure, 123–124 synthesis, 121–123 nido-C2B6H8 2 , 114 nido-C2B6H10 open-cage derivatives, 112t synthesis and characterization, 112t arachno-4,8-C2B6H11, 136–137 arachno-C2B6H12 synthesis, 115f hypho-C2B6H13 synthesis, 116f closo-4,5-C2B7H9, 134–135 characterization, 135t derivatives, 135t properties, 135–137 reactivity, 136–137 structure, 135–137 synthesis, 134–135, 135t
1112 Index nido-C2B7H9, 127 nido-C2B7H11, 127, 129t 6,8-C2B7H12, 123f, 128–132, 128f, 160, 160f 6,7-C2B7H13, 133, 159f 6,8-C2B7H13, 128–132 arachno-C2B7H13 characterization, 129t derivatives, 129t properties, 128–132, 133 structure, 128–132, 133 synthesis, 127–128, 129t hypho-C2B7H15, 129t C2B7Hx, 129t arachno-C2B8 clusters, 156t closo-1,2-C2B8 clusters, 177 closo-1,6-C2B8 clusters, 177 closo-1,10-C2B8 clusters, 177 C2B8H10 cage degradation, 180 cage expansion, 180 cage rearrangement, 178 characterization, 171t, 172t, 173t, 176t derivatives, 171t, 172t, 173t, 176t electronic properties, 178 introduction of substituents on boron, 179–180 introduction of substituents on carbon, 178–179 structures, 178 synthesis, 171t, 172t, 173t, 176t, 177 1,2-C2B8H10, 171t 1,6-C2B8H10, 172t 1,10-C2B8H10, 173t nido-6,9-C2B8H10 2 , 150, 153–154, 155 nido-5,6-C2B8H11 , 153–154 5,6-C2B8H115-R, 154f 5,6-C2B8H116-R, 154f arachno-6,9-C2B8H11X2–, 155 nido-C2B8H12 cage rearrangement, 154–155 characterization, 151t converted to other cluster systems, 154–155 deprotonation, 153–154 derivatives, 151t heteroatom insertion, 153–154 introduction of substituents, 154 structure, 153 synthesis, 149–152, 149f, 151t nido-5,6-C2B8H12, 150f, 154 arachno-5,6-C2B8H13-6(9)-L, 158 arachno-5,10-C2B8H13-9-Ly, 158 arachno-C2B8H14 characteristics, 156t circular dichroism, 158 derivatives, 155f, 156t enantiomeric resolution, 158 introduction of substituents, 157–158
structure, 157 synthesis, 155–157, 156t arachno-5,6-C2B8H14, 155f arachno-5,10-C2B8H14, 155f, 157 arachno-6,9-C2B8H14, 157, 158 nido-C2B9 anionic, versatility of, 236–237 cage systems, structural studies of, 228–229 dianions, boron insertion into, 617 nido-2,7-, 2,8-, 2,9-C2B9 derivatives, 223t 2,3-C2B9H9-10-X, 249f closo-C2B9H11 cage isomerization, 253 cage opening, 253 derivatives, 249t structure, 252 substitution at boron, 253–254 synthesis and characterization, 248–251, 249t nido-C2B9H11 2 alkylation, 229–231 cage rearrangement, 229–231 controlled deboronation, 197–226 derivatives, 198t, 219t, 235–237 deuteration, 229 dimers and, 235–237 functional groups of main-group elements introduced to, 231–235 halogenation, 229 introduction of substituents, 229 properties, 239 protonation, 229–231 reboronation, 238 structural studies, 228–229 synthesis, 228 transition metal complexes, 238 nido-7,8-C2B9H11 2 alkylation, 230 derivatives, 198t nido-7,9-C2B9H11 2 alkylation/protonation, 231 derivatives, 219t nido-7,9-C2B9H11-8-SMe3, 232–233 nido-7,8-C2B9H11-9-SMe3, 232–233 nido-C2B9H12 alkylation, 229–231 cage rearrangement, 229–231 controlled deboronation, 197–226 derivatives, 198t, 219t, 235–237 deuteration, 229 dimers and, 235–237 functional groups of main-group elements introduced, 231–235 halogenation, 229 introduction of substituents, 229 properties, 239 protonation, 229–231
Index 1113 reboronation, 238 structural studies, 228–229 transition metal complexes, 238 nido-7,8-C2B9H12 , 198t, 226–227 nido-7,9-C2B9H12 , 219t, 227 nido-C2B9H13 alkylation, 229–231 cage rearrangement, 229–231 controlled deboronation, 197–226 derivatives, 198t, 219t, 235–237 deuteration, 229 dimers and, 235–237 functional groups of main-group elements introduced, 231–235 halogenation, 229 introduction of substituents, 229 properties, 239 protonation, 229–231 reboronation, 238 structural studies, 228–229 synthesis, 227 transition metal complexes, 238 nido-7,8-C2B9H13, 198t nido-7,9-C2B9H13, 219t C2B10 anions, open-cage, 675–684 carbon-bridged clusters, 682–684 reduction of neutral C2B10 icosahedra, 675–681 closo-C2B10 carboranes, 1016–1026 C2B10 carboranes, metal coordination polymers of, 1026 C2B10 dianions, 396 1,2-C2B10H12, 387, 399–400 acyl halide derivatives, 322t alcohols and hydroxy derivatives, 427–435 alcohols, 317t, 430–431 B-substituted alcohols, 430 B-substituted ethers, 434 C- and B-hydroxy derivatives, 430–431 C-substituted alcohols, 430–431 C-substituted ethers, 431–434 reactions, 434–435 synthesis, 427–434 aldehydes, 319t, 435–442 B-substituted, 435 o-carboranyl, reactions of, 440–442 cleavage, 440–441 C-substituted, 435 electronic effects, 441 steric effects, 441 synthesis, 435 alkenyl derivatives, 313t, 411 acid reactions with, 412–413 addition of amides, 416 coupling and cyclization reactions, 415 halogen reactions with, 411–412
hydrogenation, 413 metallation, 414–415 oxidant and radical reactions with, 413–414 polymerization of 1-alkenyl-o-carboranes, 416–417 properties, 411–417 reactions with other electrophiles, 413 synthesis and properties of o-carboranyl carbenes, 416 alkoxy derivatives, 318t alkyl derivatives, 406–410 alkynyl derivatives, 316t, 411–418 addition to triple bond, 417 polymerization, 418 properties, 417–418 proton donation, 417 aluminum derivatives, 346t aryl derivatives, 308t, 406–410 B-aryl derivatives, 410 derivatives with C5 aromatic rings, 410 derivatives with C7 and larger aromatic rings, 409–410 electronic structure, 406–408 metallation of C-benzyl derivatives, 408 properties, 406–410 reactivity, 406–408 reduction to anionic species, 408–409 bromo derivatives, 342t cage synthesis from B10H14, 301–376 from BH4 salts, 376 boron addition to 11-vertex nido-carboranes, 376 other o-carborane-forming reactions, 376 carboxylic acids, 321t, 418–427 acid strength, 425 B-carboxyl derivatives, 422–424 C-carboxyl derivatives, 418–424 decarboxylation, 425–427 ionization constants of, 422t o-carboranyl, properties of, 425–427 synthesis, 418–424 direct substitution, on o-carboranyl boron atoms, 396–403 B–Al, B–Ga, B–In, and B–Ti bond formation, 402 electrophilic alkylation, 396–397 electrophilic halogenation, 398–399 fluorination, 400 nucleophilic displacement, 396 photochemical halogenation, 399–400 thermal and photochemical organosubstitution, 400–401 thermal iodination, 399 esters, 322t, 418–427 ethers, 324t, 431–434 germanium, tin, and lead derivatives, 351t, 454–461 B-stannyl derivatives, 460 C-germyl and C-stannyl derivatives, 383f, 454–459 C-plumbyl derivatives, 461 C-stannyl and C-germyl metal chelate complexes, 457–458 hydrostannylation of alkenylcarboranes, 456
1114 Index 1,2-C2B10H12 (Continued) reactivity of o-carboranyl C-Sn bonds, 459 haloalkyl derivatives, 406–410 halogen derivatives B-decahalo-o-carboranes properties, 501–502 C-halomethyl-o-carborane properties, 506 C-halo-o-carborane properties, 505–506 deboronation, 504 halogen migration, 504–505 partially B-halogenated o-carborane properties, 502–505 synthesis, 501–506 imines, 302t indirect routes to B-substituted derivatives, 395–396 synthesis from B10H14 derivatives, 395 synthesis from C2B9 and C2B10 dianions, 396 ketones, 319t, 435–442 B-substituted, 439–440 o-carboranyl, reactions of, 440–442 cleavage, 440–441 C-substituted, 436–439 electronic effects, 441 steric effects, 441 synthesis, 436–440 mercury derivatives, 369t, 506–507 metallation with Group 1 and 2 metals, 377–381 metal-promoted cross-coupling of B-halo-o-carboranes, 405–406 nitrogen derivatives, 461–474 amides and imides, 330t, 471 amines, azides, and diazonium salts, 327t, 331t, 463–467 carbamates and ureas, 302t, 473–474 nitrato, nitro, and related compounds, 461–463 nitriles, isonitriles, and isocyanates, 302t, 471–473 nitrogen heterocycles, 467–470 overview, 301 parent, 377 phosphorus, arsenic, antimony, and bismuth derivatives, 322t, 353t, 475–488 cage-opening reactions, 487–488 cyclotriphosphazenes, 483 metal complexes (exo-polyhedral), 484–487 phosphates, phosphites, and related derivatives, 478–481 phosphino and related derivatives, 475–478 polyphosphate derivatives, 480–481 thiophosphites and thiophosphates, 481–483 properties, 376–377 in reverse isomerization, 606–607 silicon derivatives, 442–454 alkoxysilanes, 452–454 B-organosilyl derivatives, 454 C-silyl derivatives, 443–454 cyclization reactions, 443–447 dendrimers, 448–450 metal chelate complexes, 447–448 silicon-linked mixed-carborane derivatives, 450 silyl as protecting group in synthesis, 450–451
structure, 376–377 substitution at boron, 395–406 substitution at carbon, 377–395 substitution via C-metallated carboranes, 381–393 addition of aluminum, gallium, indium, and thallium at carbon, 382 addition of boron at carbon, 381–382 addition of halogen at carbon, 387 addition of lanthanide series metals at carbon, 390 addition of nitrogen, phosphorus, arsenic, antimony, and bismuth at carbon, 383–385 addition of oxygen at carbon, 385 addition of silicon, germanium, tin, and lead at carbon, 383 addition of sulfur, selenium, and tellurium at carbon, 386–387 addition of transition metals at carbon, 388–390 addition of zinc and mercury at carbon, 387–388 without metallation, 394–395 organosubstitution at carbon, 390–393 sulfur, selenium, and tellurium derivatives, 336t, 353t, 354t, 488–500 metal complexes (exo-polyhedral), 495–500 thiols, thioethers, disulfides, and related compounds, 488–500 transition metals added at boron, 404–405 1,7-C2B10H12 acyl halide derivatives, 552t alcohols, 585t, 635–638 B-substituted, 636 C-substituted, 635–636 properties of m- and p-carboranyl alcohols, 637–638 synthesis, 635–637 aldehydes, 550t, 638–640 properties, 639–640 synthesis, 638–639 alkenyl derivatives, 548t, 628–630 alkoxy derivatives, 550t alkyl, haloalkyl, and aryl derivatives, 543t, 545t, 625–627 properties of alkyl- and haloalkyl-substituted m- and p-carboranes, 625 properties of aryl-substituted m- and p-carboranes, 625–627 alkynyl derivatives, 542t, 628–630 aluminum derivatives, 542t antimony derivatives, 542t cage rearrangement mechanisms, 604–606 carboxylic acids and esters, 551t, 631–635 properties, 632–635 synthesis, 631–632 ethers, 553t, 635–638 B-substituted, 637 C-substituted, 637 synthesis, 635–637 halogen derivatives, 653–656 B-halo, 653–656 C-halo, 653 properties, 653 synthesis, 653
Index 1115 hydroxy derivatives, 549t, 635–638 imines, 554t ketones, 550t, 638–640 properties, 639–640 synthesis, 638–639 nitrogen derivatives, 641–648 amides and imides, 556t, 646 amines, azides, and diazonum salts, 554t, 642–644 nitriles, isonitriles, and isocyanates, 556t, 646–647 nitro, nitrito, and related compounds, 554t, 642 nitrogen heterocycles, 644–646 ureas, 556t, 647–648 overview, 541 phosphorus derivatives, 557t, 648–650 properties, 648–650 synthesis, 648 silicon, germanium, tin, and lead derivatives, 565t, 568t, 569t, 570t, 640–641 properties, 641 synthesis, 640 structure, 541 substitution at boron, 617–625 boron insertion into nido-C2B9 dianions, 617 electrophilic alkylation, 617–618 electrophilic halogenation, 618–619 fluorination, 619 main-group elements added at boron, 620–622 mercury added at boron, 620–622 nucleophilic displacement, 617 organosubstitution, 622–625 photochemical halogenation, 619 transition metals added at boron, 622 substitution at carbon, 607–617 metallation with Group 1 and Group 2 metals, 607–608 substitution via C-metallated carboranes, 608–617 substitution via C-metallated carboranes, 608–617 aluminum added at carbon, 609 boron added at carbon, 608 halogens added at carbon, 612–613 lanthanide series metals added at carbon, 615 nitrogen, phosphorus, arsenic, antimony, and bismuth added at carbon, 611 organosubstitution at carbon, 615–617 oxygen added at carbon, 612 silicon, germanium, tin, and lead added at carbon, 609–611 sulfur, selenium, and tellurium added at carbon, 612 thallium added at carbon, 609 transition metals added at carbon, 614 zinc and mercury added at carbon, 613 sulfur, selenium, and tellurium derivatives, 558t, 570t, 650–653 properties, 652–653 synthesis, 650–652 1,12-C2B10H12 acyl halide derivatives, 552t
alcohols, 585t, 635–636 properties of, 637–638 aldehydes, 586t, 638 properties, 639–640 synthesis, 638–639 alkenyl and alkynyl derivatives, 583t, 628–630 alkoxy derivatives, 586t alkyl, haloalkyl, and aryl derivatives, 625–627 properties, 625–627 aluminum derivatives, 596t amine derivatives, 590t, 642–644 cage rearrangement mechanisms, 604–606 p-carborane synthesis, 603–604 carboxylic acids, 587t, 631–635 properties, 632–635 synthesis, 631–632 esters, 588t, 631–635 properties, 632–635 synthesis, 631–632 ethers, 588t, 637 haloalkyl derivatives, 580t, 625 halogen derivatives, 653–656 B-halo, 653–656 C-halo, 653 properties, 653 synthesis, 653 hydroxy derivatives, 585t imines, 590t ketones, 587t, 638–640 properties, 639–640 synthesis, 638–639 nitrogen derivatives, 641–648 amides and imides, 591t, 646 amines, azides, and diazonum salts, 590t, 642–644 nitriles, isonitriles, and isocyanates, 591t, 646–647 nitro, nitrito, and related compounds, 590t, 642 nitrogen heterocycles, 644–646 ureas, 647–648 overview, 541 phosphorus derivatives, 592t, 648–650 properties, 648–650 synthesis, 648 silicon, germanium, tin, and lead derivatives, 596t, 597t, 640–641 properties, 641 synthesis, 640 structure, 604 substitution at boron, 617–625 boron insertion into nido-C2B9 dianions, 617 electrophilic alkylation, 617–618 electrophilic halogenation, 618–619 fluorination, 619 main-group elements added at boron, 620–622 mercury added at boron, 620–622 nucleophilic displacement, 617 organosubstitution at boron, 622–625
1116 Index 1,12-C2B10H12 (Continued) photochemical halogenation, 619 transition metals added at boron, 622 substitution at carbon, 607–617 metallation with Group 1 and Group 2 metals, 607–608 substitution via C-metallated carboranes, 608–617 substitution via C-metallated carboranes, 608–617 aluminum added at carbon, 609 boron added at cabon, 608 halogens added at carbon, 612–613 lanthanide series metals added at carbon, 615 nitrogen, phosphorus, arsenic, antimony, and bismuth added at carbon, 611 organosubstitution at carbon, 615–617 oxygen added at carbon, 612 silicon, germanium, tin, and lead added at carbon, 609–611 sulfur, selenium, and tellurium added at carbon, 612 thallium added at carbon, 609 transition metals added at carbon, 614 zinc and mercury added at carbon, 613 sulfur, selenium, and tellurium derivatives, 592t, 650–653 properties, 652–653 synthesis, 650–652 nido-C2B10H13 , 194 nido-6-C2B10H13 , 147f C2B11 clusters cage carbon removal and polyhedral contraction, 695 electrophilic alkylation and halogenation, 695 radical anion formation, 696 reductive cage-opening, 696 synthesis and characterization, 692–694 C2B12 clusters cage carbon removal and polyhedral contraction, 695 electrophilic alkylation and halogenation, 695 radical anion formation, 696 reactions of 2,3-(CH2)3C2B12H12, 696–697 reductive cage-opening, 696 synthesis and characterization, 692–694 closo-C3B2H5 þ , 41 nido-C3B2 clusters, 32–33 nido-2,3,4-C3B3H7 characterization, 61t derivatives, 61t properties, 62 structure, 62 synthesis, 60–61, 61t nido-2,3,5-C3B3H7 properties, 64 structure, 64 synthesis, 62–64, 63f C3B4 clusters, 111 nido-C3B5H9, 116 C3B6 derivatives, 131t hypho-C3B6H13 , 133
hypho-C3B6H14, 131t C3B7 clusters, 131t nido-C3B7H11, 131t properties, 161 structure, 161 synthesis, 158–161 arachno-C3B7H13, 131t properties, 161 structure, 161 synthesis, 158–161 hypho-C3B8 clusters, 240t nido-C3B8 clusters, 240t nido-C3B8H11 hypho-C3B8H15 and, 244 cage isomerization, 242–244 derivatives, 240t reactions, 244 structures, 244 synthesis, 239–241, 240t 7,8,9-C3B8H12, 241f nido-C3B8H12 hypho-C3B8H15 and, 244 cage isomerization, 242–244 derivatives, 240t reactions, 244 structures, 244 synthesis, 239–241, 240t hypho-C3B8H15 , 244 synthesis, 245f nido-2,3,4,5-C4B2H6 characterization, 65t derivatives, 64–71, 65t, 67f, 68f properties, 69–71 structure, 69–71 synthesis, 64–69, 65t, 70f, 71f nido-C4B4H8 characterization, 118t properties, 118–120 structure, 118–120 synthesis, 116–118, 117f, 118t nido-C4B6H10, 118t properties, 162 structure, 162 synthesis, 161–162 arachno-C4B6H12, 118t, 162 nido-C4B7H11, 118t synthesis, 244–245 arachno-C4B7H13, 118t arachno-C4B7H13, 245–246 nido- and arachno-C4B8 carboranes, 684–692 deboronation of nido-C4B8 cages, 691 formation and properties of arachno-R4C4B8H8n– mono- and dianions, 689–691 general observations, 692 metal complexation of nido-R4C4B8H8, 689
Index 1117 structure and fluxional behavior of, 688–689 (Me3Si)2R2C4B8H8 systems, 689 R4C4B8H8 systems, 688–689 synthesis of neutral arachno-C4B8 clusters, 691 synthesis of nido-C4B8 cages, 685–688 arachno-C4B8 clusters, 691 C4B8 clusters, structure and fluxional behavior of nido- and arachno-C4B8 carboranes, 688–689 (Me3Si)2R2C4B8H8 systems, 689 R4C4B8H8 systems, 688–689 (Me3Si)2C4B8H8 derivatives, reduction of, 690–691 nido-C4Bn derivatives, 118t nido-2,3,4,5,6-C5BH6þ characterization, 72t derivatives, 72t properties, 73 structure, 73 synthesis, 71f, 72, 72t arachno-C6B6 carboranes, 692 C7 aromatic rings, 409–410, 627 Cadmium derivatives of 1,7-C2B10H12, 574t Cage(s) coupling, 110–111 deboronation, 485, 691 degradation B-halo derivatives, 656 C2B8H10, 180 dicarbon base-promoted cage-opening and oxidative closure, 875 8- to 11-vertex metallacarboranes and, 757–758, 868–877 fusion, metal-promoted, 23 nido-MC2B3 and closo-M2C2B3 cage linkage, 828–829 general synthetic methods, 828 metallacarboranes of transition and lanthanide elements, 828–833 multidecker sandwich synthesis, 829 multi-stack assemblies, 831 closo-MC2B4 cages, 815 nido-MC3B2 and closo-M2C3B2 cages diborolyl pentadecker and hexadecker sandwiches, 837 hybrid multidecker sandwiches, 836–837 metallacarboranes of transition and lanthanide elements and, 833–838 polydecker sandwich systems, 838 synthesis and reactivity, 833 thiadiborolyl sandwiches, 836 methylene-bridged C3B7, 245–246 monocarbon antimony and arsenic, 755–757 carbon insertion into metallacarboranes, 865–866 8- to 11-vertex metallacarboranes, 839–868 insertion into closo-carborane anions, 861
insertion into open-cage carboranes, 862–863 metal extraction from, 866–867 metal insertion into, 866 substitution at, 867 numbering changes in, publications and, 124 for closo-carboranes, 2, 3f, 4f opening bases promoting, 875 closo-2,4-C2B5H7, 110 1,2-C2B10H12, 487–488 phosphorus derivatives, 487–488, 649 reactions, 487–488 reductive, 681f, 696 rearrangement closo-2,4-C2B5H7, 110 C2B8H10, 178 nido-C2B8H12, 154–155 nido-C2B9H11 2 , 229–231 nido-C2B9H12 , 229–231 nido-C2B9H13, 229–231 1,2- and 1,7-C2B10H12, 604–606 metal-induced, 243f polyhedral, 23 in substituted derivatives, 110 tricarbon 8- to 11-vertex metallacarboranes and, 878–880 reactions of closo-MC3B7 clusters, 879 synthetic routes, 878 Cage synthesis 1,2-C2B10H12 from B10H14, 301–376 from BH4– salts, 376 boron addition to 11-vertex nido-carboranes, 376 other o-carborane-forming reactions, 376 nido-C4B8 cages, 685–688 nido-MC2B3 and closo-M2C2B3, 829 closo-MC2B4 cages, 815 nido-MC3B2 and closo-M2C3B2 cages, 833 tricarbon cages, 878 Calcium 1,2-C2B10H12 derivatives, 346t 1,7-C2B10H12 derivatives, 565t in heteroatom carboranes, 703t, 705–707 Cancer. See also Antitumor agents; Boron neutron capture synvectomy; Boron neutron capture therapy; Photodynamic therapy anticancer nucleosides for, 1058–1059 breast, 1055, 1060 GBM, 1062–1063 lung, 1061 melanoma, 1061 ovarian, 1060 uterine, 1055, 1061
1118 Index Capped polyhedra, paradigms satisfied by, 15 Capping principle, 11, 14f Carbamates, 473–474 Carbon aluminum added at, 382, 609 antimony added at, 383–385, 611 arsenic added at, 383–385, 611 bismuth added at, 383–385, 611 boron added at, 381–382, 608 electron withdrawal at, 377 extraction, from dicarbon species, 120 gallium added at, 382 germanium added at, 383, 609–611 halogens added at, 387, 612–613 indium added at, 382 insertion into 6,8-C2B7H12, 123f, 128f, 160f via cyano- and isocyanoboranes, 22 into metallacarboranes, 865–866 lanthanide series metals added at, 390, 615 lead added at, 383, 609–611 mercury added at, 387–388, 613 nitrogen added at, 383–385, 611 oxygen added at, 385, 612 phosphorus added at, 383–385, 611 removal, from cages, 695 selenium added at, 386–387, 612 short-leashed, 683 silicon added at, 383, 609–611 substituents introduced on, 178–179 sulfur added at, 386–387, 612 tellurium added at, 386–387, 612 thallium added at, 382, 609 tin added at, 383, 609–611 transition metals added at, 388–390, 614 zinc added at, 387–388, 613 Carbon vertex, substitution at, 24–25 closo-2,4-C2B5H7, 107–108 1,7- and 1,12-C2B10H12, 607–617 metallation with Group 1 and Group 2 metals, 607–608 substitution via C-metallated carboranes, 608–617 1,2-C2B10H12 and, 377–395 CB11H12, 283–284 without metallation, 394–395 organosubstitution, 390–393, 615–617 5-carbon connectors, 683 Carbon-bridged clusters nido-CB10, 195–196 open-cage C2B10 anions, 682–684 Carbon-rich 13- and 14-vertex clusters supraicosahedral metallacarboranes, 981–984 tetracarbon clusters, 981–982 tricarbon clusters, 981 Carboracycles, synthesis of, 452f Carborane(s). See also specific carboranes
anions, in metal systems, 290 defined, 1, 27 electronic behavior of carborane clusters toward substituents, 407–408 first reported in journal literature, 2 general preparative routes to borane-alkyne reactions in solution, 22 carbon insertion via cyano-and isocyanoboranes, 22 carborane-alkyne reactions, 22 gas phase borane-alkyne reactions, 21 from organoboranes, 22 overview of, 21–22 as ligands, 25, 231–232 neutral, metals inserted into, 809, 810f open-cage, 2, 4f pentacarbon, 246 polyesters, 1017–1019 polyhedral, 9 prochiral, 487 sea urchin, 15 as substituents, 25 variations in, 2 closo-Carborane anions, insertion into, 861 Carborane applications, 1. See also Medicine, carboranes in, and Catalysis analysis, 1099–1100 gas chromatography, 1099 ion-selective electrodes, 1099–1100 carborane-based materials, 1085–1096 biomaterials, 1095 ceramics, 1095–1096 electroactive systems, 1088–1091 films and monolayers, 1094 gas separation and storage materials, 1093–1094 ionic liquids, 1085 liquid crystals, 1085–1086 networks and supramolecular assemblies, 1091–1093 nonlinear optical materials, 1086–1088 metal ion extraction, 1083–1084 molecular machines, 1096–1097 noncatalytic synthetic agents, 1098–1099 overview of, 1083 Carborane polymers and dendrimers metallacarborane polymers, 1026–1031 metallacarborane dendrimers, 1027–1029 metallacarboranes as dopants and inclusion compounds in polymeric materials, 1031 metallation of nido-carboranyl polymers, 1027 polymerization of metallacarborane monomers, 1029–1031 overview of, 1015 polymers of closo-C2B10 carboranes, 1016–1026 polymers with pendant carboranes, 1023–1026 systems with carborane units in the polymeric chain, 1017–1023
Index 1119 polymers of closo-CB11 carboranes, 1016 polymers of open-cage carboranes, 1016 polymers of subicosahedral closo-carboranes, 1015 closo-Carborane systems early transition metal and lanthanide element complexes, 1039 exo-metallated carboranes in catalysis, 1039–1042 middle and late transition metal complexes, 1040–1042 nido-Carborane systems, exo-metallated carboranes in catalysis and, 1042–1044 Carborane-alkyne reactions, 22 Carborane-aromatic hydrocarbon analogy, 53–54 Carborane-based materials, 1085–1096 biomaterials, 1095 ceramics, 1095–1096 electroactive systems, 1088–1091 films and monolayers, 1094 gas separation and storage materials, 1093–1094 ionic liquids, 1085 liquid crystals, 1085–1086 networks and supramolecular assemblies, 1091–1093 nonlinear optical materials, 1086–1088 closo-Carboranes, numbering, 2, 3f, 4f m-Carboranes. See 1,7-C2B10H12 o-Carboranes. See 1,2-C2B10H12 p-Carboranes. See 1,12-C2B10H12 Carborane-siloxane polymers, 1018, 1020–1022 Carborane-siloxane-acetylene polymers, 1018 o-Carboranyl Grignards, 378–379, 380 nido-Carboranyl polymers, metallation of, 1027 Carboranyl porphyrins, 1066 Carborod geometry, 284–285 Carbosilane frameworks, multi-branched, 448–449 Carboxamides, dehydration of, 472 Carboxylate derivatives, 578t Carboxylic acids 1,2-C2B10H12, 321t, 418–427 acid strength, 425 B-carboxyl derivatives, 422–424 C-carboxyl derivatives, 418–424 decarboxylation, 425–427 ionization constants of, 422t 1,7-C2B10H12, 551t, 631–635 acid strength, 425, 632 properties, 632–635 synthesis, 631–632 1,12-C2B10H12, 631–635 acid strength, 425, 632 derivatives, 578t properties, 632–635 synthesis, 631–632 Catalysis asymmetric, 1041 closo-CB11 anions in catalysis, 1037–1038 salts with main-group metal cations, 1037–1038 salts with transition metal-containing cations, 1038
metallacarboranes in catalysis, 1044–1049 icosahedral clusters, 1045–1048 subicosahedral clusters, 1044–1045 supraicosahedral clusters, 1049 exo-metallated carboranes in catalysis closo-carborane systems, 1039–1042 nido-carborane systems, 1042–1044 non-metallated carboranes as catalytic agents, 1037 overview of, 1037 phase-transfer, 432 Catalytic agents, non-metallated carboranes as, 1037 Cations inserted into open-cage carborane anions, 773–808 interaction with CB11H12–, 267–268 main-group metal, salts with, 1037–1038 transition metal, salts with, 1038 CB3Hx, 27–28, 28t arachno-CB4 systems, 28–29, 58–59 nido-CB4 systems, 28–29 CB4Hx derivatives, 34t closo-1-CB5H7 and 1-CB5H6 derivatives, 74t structure and properties, 73–75 synthesis, 73 nido-2-CB5H9 characterization, 42t derivatives, 41–43, 42t isoelectronic and isostructural, 41f properties, 44 structure, 44 synthesis, 41–43 hypho-CB5H13, 44–45 closo-2-CB6H7 characterization, 95t derivatives, 95t reactivity, 96 synthesis, 95t, 96f closo-CB7H8 structure, 120 synthesis, 120 arachno-CB7H10Me2–, 111 arachno-CB7H13, 111, 111f CB7H12 , 111 4-CB8H9 structure and properties, 134 synthesis, 134 closo-CB8H9 , 125t nido-CB8H12 derivatives, 125t properties, 126–127 structure, 126–127 synthesis, 124–125, 125t
1120 Index nido-1-CB8H12, 126 arachno-CB8H13 derivatives, 125t properties, 126–127 structure, 126–127 synthesis, 124–125, 125t arachno-CB8H14 derivatives, 125t properties, 126–127 structure, 126–127 synthesis, 124–125, 125t arachno-4-CB8H14, 126, 127 CB9 open-cage derivatives, 146t nido-9-(Me3CNH2)CB9H8-8-CN-conjuncto-B8H10, 148 1- and 2-CB9H10 alkyl, amino, and other organic groups introduced, 170 characterization, 164t introduction of substituents, 169–170 properties, 169 structure, 169 synthesis, 163–168, 164t transition metal complexes, 170–171 closo-CB9H10 derivatives, 164t nido-CB9H12 derivatives, 146t properties, 148–149 structure, 148–149 synthesis, 145–148, 146t nido-6-CB9H12 , 145, 148 arachno-6-CB9H12-L compounds, 147 arachno-CB9H14 derivatives, 146t properties, 148–149 structure, 148–149 synthesis, 145–148, 146t nido-CB10 clusters, carbon- and phosphorus-bridged, 195–196 closo-CB10H11 derivatives, 247t structure, 247 synthesis, 246–247, 247t nido-CB10H11 3 derivatives, 188t 7-CB10H11-9-Me-10-OH–, 194 nido-CB10H12 2 derivatives, 188t 1,12-E(H)CB10H13 , 193 7-CB10H13 , 193, 194 nido-CB10H13 cage expansion reactions, 196 carbon-bridged nido-CB10 clusters, 195–196 derivatives, 188t halogenation, 193–194 phosphorus-bridged nido-CB10 clusters, 195–196
polymerization, 196–197 preparation routes, 187 structure, 193 substitution at boron, 193 synthesis, 187–193, 188t closo-CB11 anions in catalysis, 1037–1038 salts with main-group metal cations, 1037–1038 salts with transition metal-containing cations, 1038 closo-CB11 carboranes, polymers of, 1016 closo-CB11 clusters overview, 267 parent CB11H12 acid-base properties, 289–290 cation interactions with, 267–268 derivatives, 268t metal complexation, 290–292 mixed alkyl-halo derivatives, 288 overview, 267 polyalkylation at boron, 287–288 polyhalogenation at boron, 286–287 reactivity, 268t special properties, 293–294 structure, 267–282 substitution at boron, 284–285 substitution at carbon, 283–284 synthesis, 267–282, 268t, 286 structure, 267–294 synthesis, 267–294, 268t I-(H2N)CB11F11 , 287 CB11H11-12-R derivatives, 285f CB11H12 , parent acid-base properties, 289–290 cation interactions with, 267–268 derivatives, 268t metal complexation, 290–292 mixed alkyl-halo derivatives, 288 overview, 267 polyalkylation at boron, 287–288 polyhalogenation at boron, 286–287 reactivity, 268t special properties, 293–294 structure, 267–282 substitution at boron, 284–285 substitution at carbon, 283–284 synthesis, 267–282, 268t, 286 (Me2NRþ)CB11H11 compounds, 283 CB11Me12, 293–294 C-benzyl derivatives, 408 C-carboxyl derivatives, 418–424 C,C’-diphenyl derivatives, 606–607 C,C’-disilacyclobutane-o-carborane, 444f Ceramics carborane-based, 1095–1096 hybrid, 1017–1018
Index 1121 C-ethynyl derivatives, 380 C-germyl derivatives, 383f, 454–459 C-germyl metal chelate complexes, 457–458 CH acidity, 377, 607–608 C-halo derivatives, 284, 653 C-halomethyl-o-carboranes, 506 C-halo-o-carboranes, 505–506 Charge-compensated carborane ligands, 231–232 Chelate complexes, 484 C-stannyl and C-germyl, 457–458 silicon, 447–448 Chiral bis(aminohalophosphine) derivatives, 477 Chlorination, 169, 387 electrophilic, 400 Friedel-Crafts, 83, 193–194 Chloro derivatives 1,2-C2B10H12, 340t 1,7-C2B10H12, 542t 1,12-C2B10H12, 578t Chromium 1,2-C2B10H12 derivatives, 356t 1,7-C2B10H12 derivatives, 571t in 6- and 7-vertex metallacarboranes, 774t in 12-vertex metallacarboranes, 883t in 8- to 11-vertex metallacarboranes, 839t in supraicosahedral metallacarboranes, 966t C-hydroxy derivatives 1,2-C2B10H12, 317t, 430–431 1,7-C2B10H12, 549t 1,12-C2B10H12, 585t ionization constants, 431t Cisplatin, 1060 Classically bonded clusters, electron-counting in, 15–16 Cleavage 1,2-C2B10H12 aldehydes and ketones, 440–441 m- and p-carboranyl alcohols, 638 disulfide, 482 C-lithiation, 179, 180f, 377 C-lithiocarboranes, 378 C-lithio-o-carboranes, 385, 436 Closomers, 132-boron nanospherical, 1067 Cluster aromaticity, 16–17 Cluster binding, 10, 11f Cluster building-block fragments, 10t Cluster geometry, 10, 11–14, 39 Cluster vertexes, numbering of, 7–8 C–M bonded transition metal complexes, 390 C-mercapto derivatives, 386 C-metallated carboranes 1,2-C2B10H12 substitution via, 381–393 addition of aluminum, gallium, indium, and thallium at carbon, 382 addition of boron at carbon, 381–382 addition of halogen at carbon, 387 addition of lanthanide series metals at carbon, 390
addition of nitrogen, phosphorus, arsenic, antimony, and bismuth at carbon, 383–385 addition of oxygen at carbon, 385 addition of silicon, germanium, tin, and lead at carbon, 383 addition of sulfur, selenium, and tellurium at carbon, 386–387 addition of transition metals at carbon, 388–390 addition of zinc and mercury at carbon, 387–388 without metallation, 394–395 organosubstitution at carbon, 390–393 1,7- and 1,12-C2B10H12 substitution via, 608–617 aluminum added at carbon, 609 boron added at cabon, 608 halogens added at carbon, 612–613 lanthanide series metals added at carbon, 615 nitrogen, phosphorus, arsenic, antimony, and bismuth added at carbon, 611 organosubstitution at carbon, 615–617 oxygen added at carbon, 612 silicon, germanium, tin, and lead added at carbon, 609–611 sulfur, selenium, and tellurium added at carbon, 612 thallium added at carbon, 609 transition metals added at carbon, 614 zinc and mercury added at carbon, 613 C-monoaryl products, 380 C-monolithio derivative, 391 C-monolithio-o-carborane, 450 Cobalt bis(dicarbollide)-based extraction, 1084 derivatives 1,2-C2B10H12, 360t 1,7-C2B10H12, 572t 1,12-C2B10H12, 598t in 6- and 7-vertex metallacarboranes, 785t in 12-vertex metallacarboranes, 911t in 8- to 11-vertex metallacarboranes, 849t in supraicosahedral metallacarboranes, 971t Condensed-cluster structures, 14–15 Constrained-geometry complexes, 713–714 Contraction, polyhedral interconversion reactions, 23–24 metallacarboranes of transition and lanthanide elements and, 811–812 13-vertex C2B11 and 14-vertex C2B12 clusters, 695 Controlled deboronation, 197–226 Coordination polymers, 1016 Copolymers, 108, 196 Copper derivatives 1,2-C2B10H12, 367t 1,7-C2B10H12, 573t 1,12-C2B10H12, 599t 8- to 11-vertex metallacarboranes, 860t 12-vertex metallacarboranes, 943t Coupling. See also Cross-coupling alkenyl derivatives and, 415
1122 Index Coupling (Continued) cage, 110–111 oxidative, 232 Suzuki, 405–406 COX enzymes. See Cyclooxygenase enzymes C–P derivatives, 384f C-plumbyl derivatives, 461 Cross-coupling metal-promoted of B-halo-m and p-carboranes, 622–624 of B-halo-o-carboranes, 405–406 palladium-cross-coupling reactions, 284 Crystal engineering, 290, 291f, 500, 1031 C–S and C–Se derivatives, 488–491 C-siloxy derivatives, 453 C-silyl derivatives, 383f, 443–454 C-sodio-o-carboranes, 428 C-stannyl derivatives, 383f, 454–459 C-stannyl metal chelate complexes, 457–458 C-substituted acids, 422t C-substituted alcohols 1,2-C2B10H12, 427–430 1,7- and 1,12-C2B10H12, 635–636 C-substituted aldehydes, 435 C-substituted derivatives amines, azides, and diazonium salts, 463–465 nitrato, nitro, and related compounds, 461–462 thiophosphites and thiophosphates, 481–482 C-substituted ethers 1,2-C2B10H12, 431–434 1,7- and 1,12-C2B10H12, 637 C-substituted ketones, 436–439 C-substituted oxyphosphorus compounds, 478–480 C-substituted phosphines, 475–478 C–S–X–S–C rings, 490 C-triethylgermylmercury derivatives, 456 CTV. See Cyclotriveratrylene C–X–S and C–X–Se derivatives, 491–494 Cyanoboranes, carbon insertion via, 22 Cyanoethylation, 395 Cyanoolefins, 132 Cyanotosylate, 471 Cyclic ethers, 432, 433, 433f, 434 Cyclic ketones, 438 Cyclic o-carboranyl phosphine syntheses, 476f Cyclic triphosphazenes, 483 Cyclization reactions, 415, 443–447 Cycloheptatriene, 616 Cyclooctadiene-diborolyl sandwich complexes, 835–836 Cyclooxygenase (COX) enzymes, 1056–1057 Cyclopentadienyl rings, 713 Cyclopentadienylsilyl-substituted carboranes, 448 Cyclotriphosphazenes, 483 Cyclotriveratrylene (CTV), 169
D Deamination, 239 Deboronation base-induced, 226 cage, 485, 691 controlled, 197–226 of halogen derivatives, 504 Decarboxylation, 420 1,2-C2B10H12 carboxylic acids, 425–427 m-carboranyl carboxylic acids, 633 p-carboranyl carboxylic acids, 633 o-carboranyl carboxylic acids, 425–427 Dehalogenation, 117f, 229 Dehydration, of carboxamides, 472 Dehydrohalogenation, 503 1,2-Dehydro-o-carborane, 391–392, 392f Delocalized structure, 15–16, 18 Deltahedral clusters, 8 Dendrimers 1,2-C2B10H12 silicon derivatives, 448–450 m- and p-carboranyl carboxylic acids, 633–635 as high molecular weight boron delivery agents, 1067–1068 metallacarborane, 1027–1029 Dendrimers, carborane polymers and metallacarborane polymers, 1026–1031 metallacarborane dendrimers, 1027–1029 metallacarboranes as dopants and inclusion compounds in polymeric materials, 1031 metallation of nido-carboranyl polymers, 1027 polymerization of metallacarborane monomers, 1029–1031 overview of, 1015 polymers of closo-C2B10 carboranes, 1016–1026 with pendant carboranes, 1023–1026 systems with carborane units in the polymeric chain, 1017–1023 polymers of closo-CB11 carboranes, 1016 polymers of open-cage carboranes, 1016 polymers of subicosahedral closo-carboranes, 1015 Dendritic systems, polymers with pendant carboranes and, 1025–1026 Deprotection, 464–465 Deprotonation bridge, 53f nido-2,3-C2B4H8, 53f 5,6-C2B8H115-R, 154f nido-C2B8H12, 153–154 arachno-5,6,10-Me3C3B7H9-m-(6,9)-CHMe, 158, 158f nido-2,3-R2C2B4H6, 701–704 DHFR. See Dihydrofolate reductase Dialkyl sulfides, 233 Dialkylation, 55f Diamond-square-diamond (dsd) sequence, 81–82, 99, 99f, 123f, 242, 604–605, 604f
Index 1123 Diazonium salts 1,2-C2B10H12, 463–467 1,7-C2B10H12, 642–644 1,12-C2B10H12, 642–644 properties, 643–644 synthesis, 642–643 Diboraalkenes, nido-2,3,4,5-C4B2H6 derivatives from, 67f Diborapentafulvene, nido-2,3,4,5-C4B2H6 derivatives from, 67f Diborolyl pentadecker and hexadecker sandwiches, 837 Diborolyl-cobalt sandwich complex, 717 Diborolyl-ruthenium complexes, 834 Dicarbollide anions, 225f, 228 Dicarbollide ions, 238f Dicarbollide salts, 234 Dicarbollylcobalt-substituted porphyrins, 1065 Diels-Alder chemistry, 392, 393 Dihydrofolate reductase (DHFR), 1059 Diselenium exocyclic compounds, 447 Disilacyclobutane derivatives, 444, 444f Disulfides B-substituted derivatives, 495 as 1,2-C2B10H12 derivatives, 488–500 cleavage, 482 C–S and C–Se derivatives, 488–491 C–X–S and C–X–Se derivatives, 491–494 Divergent approach, 448–449 DMF. See Dimethylformamide Dopants, metallacarboranes as, 1031 Double boron swing mechanism, 154f Double silylation, 445 Doxorubicin, 1060 Drug development agonists estrogen receptor, 1054–1056 retinoid, 1056 antagonists androgen receptor, 1056 estrogen receptor, 1054–1056 retinoid, 1056 antiviral and anticancer nucleosides, 1058–1059 carboranes in, 1053–1062 anticoagulants, 1058 antitumor agents, 1058–1062 carboranes as pharmacopheres, 1053–1057 HIV protease inhibitors, 1057–1058 hydrophobic carborane pharmacopheres, 1054 exo-metallated carborane derivatives, 1059–1060 NSAIDS, 1056–1057 platinum complexes, 1060 SERMs, 1055 tamoxifen, 1055 tin complexes, 1060 transthyretin amyloid inhibitors, 1056–1057 Dsd sequence. See Diamond-square-diamond sequence
E Early transition metal complexes closo-carborane systems, 1039 icosahedral clusters and, 1045–1046 8- to 11-vertex metallacarboranes, 839–881 dicarbon cages, 757–758, 868–877 monocarbon cages, 839–868 synthesis and characterization, 839t tetracarbon clusters, 880–881 tricarbon cages, 878–880 Elastomers, hybrid, 1017–1018 Electroactive systems, 1088–1091 Electrodes, ion-selective, 1099–1100 “Electron deficiency,” in polyhedral boron clusters, 17–18 Electron microscopy, 1074 Electron withdrawal, at carbon, 377 Electron-counting in classically bonded clusters, 15–16 rules, extensions of, 14–15 Electron-delocalization, 377 Electron-delocalized cluster architectures, 18 Electronic conductors, 626–627 Electronic effects, 1,2-C2B10H12, 441 Electrophiles, alkenyl derivative reactions with, 413 Electrophilic alkylation, 230f 1,2-C2B10H12, 244 1,7-C2B10H12, 617–618 1,12-C2B10H12, 617–618 13-vertex C2B11 and 14-vertex C2B12 clusters, 695 Electrophilic B-substitution, 285 Electrophilic chlorination, 400 Electrophilic ethylation, 397f Electrophilic halogenation, 157, 286 1,2-C2B10H12, 398–399 1,7- and 1,12-C2B10H12, 618–619 m-carborane, 618 p-carborane, 618–619 13-vertex C2B11 and 14-vertex C2B12 clusters, 695 Electrophilic iodination, 194 Electrophilic reagents, substitution at boron by, 284–285 Electrophilic substitution, dicarbon cage, 875 Enantiomeric resolution, 158 Epithiopropane derivatives, 493 Epoxides, 542t ER agonists and antagonists. See Estrogen receptor agonists and antagonists Esterification, of m- and p-carboranyl carboxylic acids, 632–633 Esters 1,2-C2B10H12, 322t, 418–427 1,7- and 1,12-C2B10H12, 552t, 588t, 631–635 properties, 632–635 synthesis, 631–632 Estrogen receptor (ER) agonists and antagonists, 1054–1056
1124 Index Ethers 1,2-C2B10H12 B-substituted, 434 C-substituted, 431–434 1,7- and 1,12-C2B10H12 B-substituted, 637 C-substituted, 637 cyclic, 432, 433, 433f, 434 multicage extended, 432 solvent mixtures containing, 378 synthesis, 427–434, 635–637 Ethyl iodide, 502 Ethylation, electrophilic, 397f Expansion, polyhedral, 107 closo-1,6-C2B4H6, 82 closo-2,4-C2B5H7, 110 interconversion reactions, 23–24 metallacarboranes of transition and lanthanide elements and, 811–812
F Ferraborane, in alkyne-ferraborane photolysis, 117f Ferracarborane sandwiches, 53–54 Films, 1094 Fischer carbene complexes, 160f Flash thermolysis, 41–42 Flow pyrolysis, of alkenylpentaboranes, 42 Fluorene polymers, 1019 Fluorination, 169, 288 direct substitution on o-carboranyl boron atoms, 400 substitution at boron, 619 Fluoro derivatives 1,7-C2B10H12, 559t, 619 1,12-C2B10H12, 593t, 619 1,2-C2B10H12 derivatives, 340t, 387, 400 Fluxional behavior, of C4B8 clusters, 688–689 (Me3Si)2R2C4B8H8 systems, 689 R4C4B8H8 systems, 688–689 Fluxional cage interconversion of 5,6-C2B8H11 anion, 154f Fourier transform infrared labeling, 1074 Friedel-Crafts chlorination, 83, 193–194 Friedel-Crafts substitution, 194 Fullerenes, 9, 290 Fusion metal-promoted, 23 oxidative metallacarboranes of transition and lanthanide elements and, 812–813 of 2,3-R2C2B4H42– ligands, 685–686 of 2,3-(Me3Si)RC2B4H–nn– anions, 686–688 via thermal routes, 688
G Gadolinium, in imaging and therapy, 1073 Gallium addition at carbon, in 1,2-C2B10H12 derivatives, 382
addition at boron, in 1,2-C2B10H12 derivatives, 402; in 1,7- and 1,12-C2B10H12 derivatives, 620 1,2-C2B10H12 derivatives, 346t 1,7-C2B10H12 derivatives, 565t in heteroatom carboranes, 708t, 714–717 12-vertex cages, 716–717 Gas chromatography, 1099 Gas phase boron hydride-alkyne reactions, 21 Gas separation and storage materials, 1093–1094 GBM. See Glioblastoma multiforme General preparative routes to carboranes borane-alkyne reactions in solution, 22 carbon insertion via cyano-and isocyanoboranes, 22 carborane-alkyne reactions, 22 carboranes from organoboranes, 22 gas phase borane-alkyne reactions, 21 overview of, 21–22 Germanium added at carbon, 383, 609–611 added at boron, 402, 620 1,2-C2B10H12 derivatives, 351t, 454–461 C-germyl metal chelate complexes, 457–458 1,7-C2B10H12 derivatives, 568t, 640–641 properties, 641 synthesis, 640 1,12-C2B10H12 derivatives, 597t, 640–641 properties, 641 synthesis, 640 in heteroatom carboranes, 719t, 725–728 Glioblastoma multiforme (GBM), 1062–1063 Gold 1,2-C2B10H12 derivatives, 368t 1,7-C2B10H12 derivatives, 574t 1,12-C2B10H12 derivatives, 600t in 8- to 11-vertex metallacarboranes, 860t in 12-vertex metallacarboranes, 944t in supraicosahedral metallacarboranes, 974t Grignard reactions, 419 Grignard reagents, 378–379, 380, 429, 436–437 Group 1 and Group 2 elements 1,7- and 1,12-C2B10H12, 607–608 heteroatom carboranes of, 701–707 beryllium, magnesium, calcium, strontium, and barium, 705–707 lithium, sodium, potassium, and rubidium, 701–705 synthesis and characterization, 702t metallation with, 377–381, 607–608 Group 13 elements heteroatom carboranes of, 707–718 aluminum, 707t, 710–714 gallium and indium, 714–717 synthesis and characterization, 707t thallium, 717–718 special interest in, 707 Group 14 elements, heteroatom carboranes of, 718–732 germanium, 725–728
Index 1125 silicon, 724–725 synthesis and characterization, 719t tin and lead, 728–732 Group 15 elements, heteroatom carboranes of, 732–758 antimony, 755–758 arsenic, 755–758 nitrogen, 732–735 phosphorus, 735–755 synthesis and characterization, 736t Group 16 elements, heteroatom carboranes of, 758–766 metal sandwich complexes, 765–766 sulfur and selenium, 761–766 synthesis and characterization, 759t
H Hafnium 1,2-C2B10H12 derivatives, 355t 1,7-C2B10H12 derivatives, 571t in 6- and 7-vertex metallacarboranes, 774t in 12-vertex metallacarboranes, 886t in supraicosahedral metallacarboranes, 967t Halides acyl halide derivatives, 302t 1,7-C2B10H12, 542t 1,12-C2B10H12, 578t alkyl, 390–391, 397 aluminum halide catalysts, 56 Haloalkyl derivatives, 406–410 1,7-C2B10H12, 545t 1,12-C2B10H12, 580t 1,2-C2B10H12 derivatives, 307t Haloalkyl-substituted m- and p-carboranes, 625 Haloaryl derivatives, 313t, 547t Halogen added at carbon, 387, 612–613 alkenyl derivative reactions with, 411–412 migration, 504–505 substitution, effects of, 502–504 Halogen derivatives 1,2-C2B10H12 B-decahalo-o-carboranes properties, 501–502 C-halomethyl-o-carborane properties, 506 C-halo-o-carborane properties, 505–506 deboronation, 504 halogen migration, 504–505 partially B-halogenated o-carborane properties, 502–505 synthesis, 501–506 1,7- and 1,12-C2B10H12, 653–656 B-halo, 653–656 C-halo, 653 properties, 653 synthesis, 653 deboronation, 504 halogen migration, 504–505 synthesis, 501–506, 653
Halogenation, 54, 56 nido-CB10H13 , 193–194 nido-C2B9H11 2 , 229 nido-C2B9H12 , 229 nido-C2B9H13, 229 of m- and p-carboranyl alcohols, 638 electrophilic, 157, 286 1,2-C2B10H12, 398–399 1,7- and 1,12-C2B10H12, 618–619 13-vertex C2B11 and 14-vertex C2B12 clusters, 695 introduction of substituents to 1- and 2-CB9H10 and, 169–170 photochemical, 399–400, 619 polyhalogenation, 169–170, 286–287 radiohalogenation, 1071–1072 substitution at boron and, 109 Halomethanes, 398 1-halo-o-carborane, 380 Halopyridines, 467 HCB11F11 , 287 HCB11Me11 , 293–294 HCB11X11 , 287 Heteroatom carboranes of main-group elements of Group 1 and Group 2 elements, 701–707 beryllium, magnesium, calcium, strontium, and barium, 705–707 lithium, sodium, potassium, and rubidium, 701–705 synthesis and characterization, 702t of Group 13 elements, 707–718 aluminum, 707t, 710–714 gallium and indium, 714–717 synthesis and characterization, 707t thallium, 717–718 of Group 14 elements, 718–732 germanium, 725–728 silicon, 724–725 synthesis and characterization, 719t tin and lead, 728–732 of Group 15 elements, 732–758 antimony, 755–758 arsenic, 755–758 nitrogen, 732–735 phosphorus, 735–755 synthesis and characterization, 736t of Group 16 elements, 758–766 metal sandwich complexes, 765–766 sulfur and selenium, 761–766 synthesis and characterization, 759t overview of, 701 Heterocycles, nitrogen, 467–470, 644–646 Heterometallic cages, 871 Heterometallic monocarbon clusters, 949 Hexadecker sandwiches, 837 High molecular weight boron delivery agents in BNCT, 1066–1070 dendrimers and other polymers, 1067–1068 lipoproteins, 1068
1126 Index High molecular weight boron delivery agents (Continued) liposomes, 1068–1069 nanoparticles and nanotubes, 1069–1070 Highest atomic number rule, 8 HIV protease inhibitors, 1057–1058 Hybrid elastomers and ceramics, 1017–1018 Hybrid multidecker sandwiches, 836–837 Hydrazine derivatives, 542t Hydrazo derivatives, 302t Hydroboration, 114, 117f, 180 Hydrocarbon chains, polymers with pendant carboranes and, 1023–1024 Hydrocarbon substituents, 231 Hydrocarbons, inserted into metallaboranes, 809–811 Hydrogen, fluxional tautomerizing behavior of, 233 Hydrolysis acid, 193, 419–420 sol-gel processes, 1022 Hydrophobic carborane pharmacophores, design of, 1054 Hydrophobicity, 633–634, 635 Hydrostannylation, of alkenylcarboranes, 456 Hydroxy derivatives. See also C-hydroxy derivatives 1,2-C2B10H12, 317t, 427–435 1,7-C2B10H12, 549t, 635–638 1,12-C2B10H12, 585t Hypercloso clusters, 14
I –I effect, 396, 397 Icosahedral barrier, 675 ILCs. See Ionic liquid crystals Imides 1,2-C2B10H12, 330t, 471 1,7-C2B10H12, 542t, 646 1,12-C2B10H12, 578t, 646 Imines 1,2-C2B10H12, 330t 1,7-C2B10H12, 556t 1,12-C2B10H12, 591t Inclusion compounds, metallacarboranes in, 1031 Indium addition at carbon, in 1,2-C2B10H12 derivatives, 382 addition at boron, in 1,2-C2B10H12 derivatives, 402; in 1,7- and 1,12-C2B10H12 derivatives, 620 1,2-C2B10H12 derivatives, 346t 1,7-C2B10H12 derivatives, 565t in heteroatom carboranes, 708t, 714–717 Inductive electron withdrawal, 413, 425 Iodination, 286 electrophilic, 194 radioiodination, 1072 thermal, 399 Iodo derivatives 1,2-C2B10H12, 344t 1,7-C2B10H12, 563t
1,12-C2B10H12, 595t Ionic liquid crystals (ILCs), 1086 Ionic liquids, 1085 Ion-selective electrodes (ISEs), 1099–1100 Iridium 1,2-C2B10H12 derivatives, 361t 1,7-C2B10H12 derivatives, 572t 1,12-C2B10H12 derivatives, 599t in 6- and 7-vertex metallacarboranes, 799t in 8- to 11-vertex metallacarboranes, 855t in 12-vertex metallacarboranes, 935t in supraicosahedral metallacarboranes, 973t Iron 1,2-C2B10H12 derivatives, 357t 1,7-C2B10H12 derivatives, 571t 1,12-C2B10H12 derivatives, 598t in 6- and 7-vertex metallacarboranes, 777t in 12-vertex metallacarboranes, 900t in 8- to 11-vertex metallacarboranes, 842t in supraicosahedral metallacarboranes, 966t ISEs. See Ion-selective electrodes Isocloso clusters, 14 Isocyanates 1,2-C2B10H12 derivatives, 331t, 471–473 1,7-C2B10H12 derivatives, 556t, 646–647 1,12-C2B10H12 derivatives, 646–647 Isocyanoboranes, carbon insertion via, 22 Isoelectronic 6-vertex nido-2-CB5H9 clusters, 41f Isolobal principle, 10 Isonitriles 1,2-C2B10H12 derivatives, 332t, 471–473 1,7-C2B10H12 derivatives, 556t, 646–647 1,12-C2B10H12 derivatives, 591t, 646–647 Isostructural 6-vertex nido-2-CB5H9 clusters, 41f Isoxazole derivatives, 470 Isoxazoline derivatives, 470
K Ketones B-benzyl ketone derivatives, 439 bis(o-carboranyl), 436 1,2-C2B10H12, 319t, 435–442 B-substituted, 439–440 o-carboranyl, reactions of, 440–442 cleavage, 440–441 C-substituted, 436–439 electronic effects, 441 steric effects, 441 synthesis, 436–440 1,7-C2B10H12, 550t, 638–640 synthesis, 638–639 1,12-C2B10H12, 587t, 638–640 properties, 639–640 synthesis, 638–639 o-carboranyl, 429, 436, 440–442
Index 1127 C-substituted, 436–439 cyclic, 438 polyketones, 1020 Kinetically stabilized cage structures, 16
L Lanthanide elements, metallacarboranes of, 804t, 883t, 966t Lanthanide elements, 1,2-C2B10H12 derivatives, 355t; 1,7-C2B10H12 derivatives, 570t Lanthanide metal o-carboranyl-tetramethylcyclopentadienylsilyl chelate complexes, 448f Lanthanide series metals, added at carbon, 390, 615 Lanthanon complexes of nido-C2B9 clusters, 215t Lanthanum in LnC2B9 metallacarboranes, 883t LDLs. See Low-density lipoproteins Lead added at carbon, 383, 609–611 bond formation, 402, 620 C-plumbyl derivatives, 461 1,7-C2B10H12 derivatives, 570t, 640–641 in heteroatom carboranes, 728–732 Lewis bases adducts, 301 derivatives, 124 reactions with, 233f Ligands carboranes as, 25, 231–232 charge-compensated, 231–232 exo-polyhedral, properties of, 876 substitution reactions, 949 transition metal groups, 10–11 Line formulas, 8 Lipoproteins as high molecular weight boron delivery agents, 1068 LDL, 1068 Liposomes, as high molecular weight boron delivery agents, 1068–1069 Liquid crystals, 1085–1086 Lithiation C-lithiation, 179, 180f, 377 in dimethoxyethane, 378 1-lithio-o-carborane, 380 Lithium in heteroatom carboranes, 701–705 Lithography, 1094 Localized-bond approach, 8 Low-density lipoproteins (LDLs), 1068 Lung cancer, 1061
M Magnesium 1,2-C2B10H12 derivatives, 346t 1,7-C2B10H12 derivatives, 565t in heteroatom carboranes, 705–707
Main-group elements. See also Heteroatom carboranes, of main-group elements added at boron, 620–622 functional groups of, 231–235 Main-group metal cations, 1037–1038 Manganacarborane sandwich complex, 62 Manganese 1,2-C2B10H12 derivatives, 357t 1,7-C2B10H12 derivatives, 571t in 6- and 7-vertex metallacarboranes, 776t, 816 in 12-vertex metallacarboranes, 882, 896t in 8- to 11-vertex metallacarboranes, 840t, 867, 879 in supraicosahedral metallacarboranes, 969t nido-MC2B3 cages cage linkage, 828–829 general synthetic methods, 828 metallacarboranes of transition and lanthanide elements and, 828–833 multidecker sandwich synthesis, 829 multi-stack assemblies, 831 Medicine, carboranes in, 231–232 BNCS, 1063, 1070 BNCT, 1062–1070 approaches, 1063 background, 1062–1063 high molecular weight boron delivery agents, 1066–1070 small-molecule boron delivery agents, 1063–1066 drug development, 1053–1062 anticoagulants, 1058 antitumor agents, 1058–1062 carboranes as pharmacopheres, 1053–1057 HIV protease inhibitors, 1057–1058 molecular imaging and radiotherapy, 1071–1074 electron microscopy, 1074 Fourier transform infrared labeling, 1074 gadolinium in, 1073 other methods, 1074 radioastatination, 1072 radiohalogenation, 1071–1072 radiometal labeling, 1073 secondary ion mass spectometry, 1074 overview of, 1053 PDT, 1059–1060, 1070–1071 properties ideal for, 1053 Melanoma, 1061 Mercuracarborand macrocycles, 507 Mercury added at boron, 620–622 added at carbon, 387–388, 613 bond formation, 401, 621–622 derivatives 7,8-C2B9H13, 234–235 1,2-C2B10H12, 369t, 506–507 1,7-C2B10H12, 574t, 613
1128 Index Mercury (Continued) 1,12-C2B10H12, 600t, 613 in 12-vertex metallacarboranes, 954 Metal(s). See also specific metals actinides in 12-vertex metallacarboranes, 883t in supraicosahedral metallacarboranes, 966t atom reactions, 814 cations inserted into open-cage carborane anions, 773–808 salts with, 1037–1038 chelate complexes, 484 C-stannyl and C-germyl, 457–458 silicon, 447–448 coordination polymers, 290, 1026 displacement and transfer of, 813–814 inserted into monocarbon cages, 866 inserted into neutral carboranes, 809, 810f metal-metal communication, 822 systems, carborane anions in, 290 Metal complexation cage deboronation accompanying, 485 CB11H12–, 290–292 1,2-C2B10H12 phosphorus derivatives, 484–487 of nido-R4C4B8H8, 689 sulfur, selenium, and tellurium 1,2-C2B10H12 derivatives, 495–500 Metal ion extraction carborane applications, 1083–1084 from monocarbon metallacarboranes, 866–867 from nuclear waste, 1084 Metal sandwich complexes heteroatom carboranes of Group 16 elements, 765–766 transition metal, 6-7-vertex, 773, 775t–777t, 781t–782t, 784t, 785t, 787t, 792t–806t, 807, 808, 813, 815–817, 822, 828–831, 833–838; 8-11 vertex, 840t, 843t, 847t, 849t, 853t, 854t, 857t, 859t, 861t, 863, 866, 871, 875, 880; 12-vertex, 937t, 939t, 949, 951, 952, 955–957, 961, 963; 13-vertex, 965, 975–977, 984 Metal-free alkylation, 395 Metal-induced cage rearrangement, 243f Metallaboranes hydrocarbons inserted into, 809–811 in isocyanide-metallaborane approach, 948 Metallacarboranes carbon insertion into, 865–866 in catalysis, 1044–1049 icosahedral clusters, 1045–1048 subicosahedral clusters, 1044–1045 supraicosahedral clusters, 1049 dendrimers, 1027–1029 as dopants and inclusion compounds in polymeric materials, 1031 metal insertion into, 866 monocarbon, metal extraction from, 866–867 monomers, 1029–1031 polymers, 1026–1031
metallacarborane dendrimers, 1027–1029 metallation of nido-carboranyl polymers, 1027 polymerization of metallacarborane monomers, 1029–1031 tailored derivatives, 958–959 Metallacarboranes, of transition and lanthanide elements 8- to 11-vertex metallacarboranes, 839–881 dicarbon cages, 868–877 monocarbon cages, 839–868 synthesis and characterization, 839t tetracarbon clusters, 880–881 tricarbon cages, 878–880 general synthetic methods, 773–815 insertion of hydrocarbons into metallaboranes, 809–811 insertion of metal cations into open-cage carborane anions, 773–808 insertion of metals into neutral carboranes, 809, 810f metal atom reactions, 814 metal displacement and transfer, 813–814 other routes, 814–815 oxidative fusion, 812–813 polyhedral expansion and contraction, 811–812 overview of, 773 6- and 7-vertex metallacarboranes, 815–838 closo six-vertex cages, 815 functionalization and linkage of closo-LMC2B4 and nidoLMC2B3 clusters, 817–821 nido-MC2B3 and closo-M2C2B3 cages, 828–833 nido-MC3B2 and closo-M2C3B2 cages, 833–838 metal-metal communication in linked MC2B4 clusters, 822 small metallacarboranes in organic synthesis, 823–827 synthesis of closo-MC2B4 cages, 815 thermal rearrangement of 7-vertex metallacarboranes, 822–823 transition element synthesis, 774t supraicosahedral metallacarboranes carbon-rich 13- and 14-vertex clusters, 981–984 dicarbon 13-vertex clusters, 965–980 synthesis and characterization, 966t of transition and lanthanide elements, 965–984 12-vertex metallacarboranes, 882–965 dicarbon clusters, 951–961 monocarbon clusters, 882–951 pentacarbon clusters, 963–965 synthesis and characterization, 883t tetracarbon clusters, 963–965 tricarbon clusters, 961–963 exo-Metallated carboranes in catalysis closo-carborane systems, 1039–1042 nido-carborane systems, 1042–1044 anticancer agents, 1059–1060 Metallation alkenyl derivatives of 1,2-C2B10H12, 414–415 1,7- and 1,12-C2B10H12, 607–608 of nido-carboranyl polymers, 1027
Index 1129 of C-benzyl derivatives, 408 with Group 1 and 2 metals, 377–381, 607–608 substitution at carbon without, 394–395 Metallocenes, 952 Metal-metal communication, in linked MC2B4 clusters, 822 Metal-methyl group interaction, 293 Metal-organic frameworks (MOFs), 1093 Metal-promoted cage fusion, 23 Metal-promoted cross-coupling of B-halo-m and p-carboranes, 622–624 of B-halo-o-carboranes, 405–406 Methylation of C2B9 anions, 231f, 232f Microelectronics, 493–494, 1089 Migration, halogen, 504–505 Mixed alkyl-halo derivatives, 288 Mixed-carborane derivatives, silicon-linked, 450 MNDO. See Modified neglect of differential overlap Mno rule, 14–15 Modified neglect of differential overlap (MNDO), 107 MOFs. See Metal-organic frameworks Molecular imaging and radiotherapy, 1071–1074 electron microscopy, 1074 Fourier transform infrared labeling, 1074 gadolinium in, 1073 other methods, 1074 radioastatination, 1072 radiohalogenation, 1071–1072 radiometal labeling, 1073 secondary ion mass spectometry, 1074 Molecular machines, 1096–1097 Molecular orbitals (MOs), 107 Molecular wires, 294 Molybdenum 1,2-C2B10H12 derivatives, 356t 1,7-C2B10H12 derivatives, 571t in 6- and 7-vertex metallacarboranes, 776t in 12-vertex metallacarboranes, 888t in 8- to 11-vertex metallacarboranes, 840t in supraicosahedral metallacarboranes, 968t Monocarbon clusters heterometallic, 949 ligand substitution reactions, 949 synthetic routes, 882, 947–948, 949 12-vertex metallacarboranes, 882–951 Monocarbon phosphacarboranes, large, 747–751 Monocarbon carborane acids, proton-donating capability of, 289–290 Monocarboranyl thiophosphite, 482–483 Monocarboxylic acid derivatives, 391 Monodentate systems, 495–496 Monolayers, 1094 Monomers, metallacarborane, 1029–1031 MOs. See Molecular orbitals Multi-branched carbosilane frameworks, 448–449 Multicage derivatives, 198t, 235–237, 249t Multicage extended ethers, 432
Multi-carboranyl ureas, 474 Multidecker sandwiches, 829–838 hybrid, 836–837 Multi-stack assemblies, 831
N Nanoparticles, 1069–1070 Nanotubes as high molecular weight boron delivery agents, 1069–1070 SWCNT, 1070 Networks and supramolecular assemblies, 1091–1093 Neutral C2B10 icosahedra, reduction of, 675–681 Neutral carboranes, metals inserted into, 809, 810f Neutral multi-cage derivatives, 198t Neutral single-cage derivatives, 198t (Ph3P)2Ni(C2B10H10), 389 Nickel 1,2-C2B10H12 derivatives, 364t 1,7-C2B10H12 derivatives, 572t in 6- and 7-vertex metallacarboranes, 800t in 8- to 11-vertex metallacarboranes, 857t in 12-vertex metallacarboranes, 936t in supraicosahedral metallacarboranes, 966t NICS. See Nuclear independent chemical shift Nido dianions, interconversion via, 606f Niobium in 6- and 7-vertex metallacarboranes, 775t in 12-vertex metallacarboranes, 887t in 8- to 11-vertex metallacarboranes, 840t Nitrates 1,2-C2B10H12 derivatives, 326t 1,7-C2B10H12 derivatives, 554t 1,12-C2B10H12 derivatives, 590t Nitrato, nitro, and related compounds B-substituted derivatives, 462–463 as 1,2-C2B10H12 nitrogen derivatives, 461–463 C-substituted derivatives, 461–462 Nitriles, 158–160, 471–473 addition of, 734 1,2-C2B10H12 derivatives, 332t, 646–647 1,7-C2B10H12 derivatives, 556t, 646–647 1,12-C2B10H12 derivatives, 591t, 646–647 insertion of, 733 Nitro esters, 462 Nitrogen added at carbon, 383–385, 611 bond formation, 402–403 in heteroatom carboranes, 736t, 732–735 heterocycles, 467–470, 644–646 Nitrogen-containing derivatives 1,2-C2B10H12, 461–474 amides and imides, 471 amines, azides, and diazonium salts, 463–467, 327t, 331t carbamates and ureas, 473–474 nitrato, nitro, and related compounds, 461–463
1130 Index Nitrogen-containing derivatives (Continued) nitriles, isonitriles, and isocyanates, 332t, 471–473 nitrogen heterocycles, 467–470 1,7-C2B10H12, 641–648 amides and imides, 646 amines, azides, and diazonum salts, 554t, 556t, 642–644 nitriles, isonitriles, and isocyanates, 542t, 646–647 nitro, nitrito, and related compounds, 642 nitrogen heterocycles, 644–646 ureas, 647–648 1,12-C2B10H12, 641–648 amides and imides, 646 amines, azides, and diazonum salts, 590t, 591t, 642–644 nitriles, isonitriles, and isocyanates, 578t, 646–647 nitro, nitrito, and related compounds, 642 nitrogen heterocycles, 644–646 ureas, 647–648 isonitriles, 471–473, 646–647 Nitronyl nitroxide mono- and biradicals, 462 Nitrophenyl groups, 461 Nitroso derivatives 1,2-C2B10H12, 326t 1,7-C2B10H12, 554t 1,12-C2B10H12, 590t NLO. See Nonlinear optical materials NMe4 þ salt, 244 Nomenclature, 7–8 Noncatalytic synthetic agents, 1098–1099 Nonclassical clusters, 18 Nondendritic boronated polymers, 1068 Nonlinear optical materials (NLO), 1086–1088 Non-metallated carboranes, as catalytic agents, 1037 Nonsteroidal anti-inflammatory drugs (NSAIDS), 1056–1057 NSAIDS. See Nonsteroidal anti-inflammatory drugs Nuclear independent chemical shift (NICS), 17 Nuclear waste, metals extracted from, 1084 Nucleophiles, B-halo derivative reactions with, 653–654 Nucleophilic displacement, 396, 617 Nucleophilic substitution, 70 Nucleosides, 1063–1064 Nucleotides, 1063–1064 Numbering cage changes in, publications and, 124 for closo-carboranes, 2, 3f, 4f
O Oligomers, staircase, 831–832 Oligophosphates, 480–481 One-electron donors, 10t One-electron electrolytic reduction, 254 Open 12-vertex and supraicosahedral carboranes nido- and arachno-C4B8 carboranes, 684–692 deboronation of nido-C4B8 cages, 691
formation and properties of arachno-R4C4B8H8 n mono- and dianions, 689–691 general observations, 692 metal complexation of nido-R4C4B8H8, 689 structure and fluxional behavior of, 688–689 synthesis of nido-C4B8 cages, 685–688 synthesis of neutral arachno-C4B8 clusters, 691 arachno-C6B6 carboranes, 692 derivatives, 676t open-cage C2B10 anions, 675–684 carbon-bridged clusters, 682–684 reduction of neutral C2B10 icosahedra, 675–681 supraicosahedral carboranes, 692–697 thermodynamic stability of, 675 13-vertex C2B11 and 14-vertex C2B12 clusters, 692–697 Open-cage C2B3 clusters, 32 Open-cage C2B3Hx derivatives, 30t Open-cage C2B10 anions, 675–684 carbon-bridged clusters, 682–684 reduction of neutral C2B10 icosahedra, 675–681 Opening, cage base-promoted, 875 closo-2,4-C2B5H7, 110 1,2-C2B10H12, 487–488 phosphorus derivatives, 487–488, 649 reactions, 487–488 reductive, 681f, 696 Organic synthesis, small metallacarboranes in, 823–827 Organoboranes, 1, 22 Organosubstitution at boron, 622–625 metal-promoted cross-coupling of B-halo-m and p-carboranes, 622–624 other routes, 624–625 photochemical, 400–401 thermal, 400–401 at carbon, 390–393, 615–617 m- and p-carboranes, 622–625 Osmium 1,2-C2B10H12 derivatives, 359t in 6- and 7-vertex metallacarboranes, 785t in 12-vertex metallacarboranes, 911t in 8- to 11-vertex metallacarboranes, 848t Ousenes, 409 Ovarian cancer, 1060 Oxadisilacyclohexane, 445 Oxidants, alkenyl derivative reactions with, 413–414 Oxidative closure, 248, 875 Oxidative coupling, 232 Oxidative fusion metallacarboranes of transition and lanthanide elements and, 812–813 of 2,3-R2C2B4H4 2 ligands, 685–686 of 2,3-(Me3Si)RC2B4Hn n anions, 686–688
Index 1131 Oxygen added at carbon, 385, 612 B-O bond formation, 621 Oxyphosphorus derivatives, 648–649
P Palladium 1,2-C2B10H12 derivatives, 365t 1,7-C2B10H12 derivatives, 572t in 6- and 7-vertex metallacarboranes, 803t in 8- to 11-vertex metallacarboranes, 858t in 12-vertex metallacarboranes, 940t in supraicosahedral metallacarboranes, 973t Palladium cross-coupling reactions in CB11H12 derivatives, 279t, 284 in 1,2-C2B10H12 derivatives, 334t, 405, 434, 439, 454, 469, 503, 504 in 1,7-C2B10H12 derivatives, 544t, 546t, 582t, 590t, 591t, 620, 622, 624, 636, 637, 639, 640, 643 in metallacarboranes, 954 PDT. See Photodynamic therapy Pendant carboranes, polymers with, 1023–1026 dendritic systems, 1025–1026 hydrocarbon chains, 1023–1024 metal coordination polymers of C2B10 carboranes, 1026 phosphazene polymers, 1022–1023 pyrrole polymers, 1024 Pentaborane-alkyne reactions, 52 Pentacarbon carboranes, 246 Pentacarbon clusters, 963–965 Pentadecker sandwiches, 837 Peptides, o-carborane cages linked to, 1068 Perhalogenated species, 286–287 Peroxide derivatives, 542t PET. See Positron emission tomography Pharmacophores androgen receptor antagonists, 1056 carboranes as, 1053–1057 design of hydrophobic carborane pharmacopheres, 1054 estrogen receptor agonists and antagonists, 1054–1056 retinoid agonists and antagonists, 1056 transythretin amyloid inhibitors, 1056–1057 Phase-transfer catalysis, 432 Phosphacarboranes 5- to 8-vertex, 746–747 large dicarbon, 751–755 large monocarbon, 747–751 Phosphates, 478–481 Phosphazene moieties, 483 Phosphazene polymers, 1022–1023 Phosphines B-substituted, 478 C-substituted, 475–478 derivatives, 195 trimethyl, 109
Phosphino derivatives B-substituted phosphines, 478 1,2-C2B10H12, 475–478 C-substituted phosphines, 475–478 Phosphites, 478–481 Phosphonites, 480 Phosphorus added at carbon, 383–385, 611 bond formation, 402–403 5- to 8-vertex phosphacarboranes, 746–747 in heteroatom carboranes, 735–755, 737t large dicarbon phosphacarboranes, 751–755 large monocarbon phosphacarboranes, 747–751 synthesis and, 736t Phosphorus derivatives 1,2-C2B10H12, 332t, 475–488 cage-opening reactions, 487–488 cyclotriphosphazenes, 483 metal complexes, 484–487 phosphates, phosphites, and related derivatives, 478–481 phosphino and related derivatives, 475–478 polyphosphate derivatives, 480–481 thiophosphites and thiophosphates, 481–483 1,7-C2B10H12, 557t, 648–650 properties, 648–650 synthesis, 648 1,12-C2B10H12, 592t, 648–650 properties, 648–650 synthesis, 648 Phosphorus-bridged nido-CB10 clusters, 195–196 Photochemical bromination, 400 Photochemical B-substitution, 110 Photochemical halogenation, 399–400, 619 Photochemical organosubstitution, 400–401 Photodynamic therapy (PDT), 1059–1060, 1070–1071 Photolysis, 56 alkyne-ferraborane, 117f cophotolysis, 110 Phthalimido derivatives, 465 Phthalocynanines, as small-molecule boron delivery agents, 1064–1066 Pinwheel complexes, 820–821 Platinum 1,2-C2B10H12 derivatives, 365t 1,7-C2B10H12 derivatives, 572t 1,12-C2B10H12 derivatives, 599t complexes, as antitumor agents, 1060 oxidative addition of, 499 6- to 7-vertex metallacarboranes, 803t in 8- to 11-vertex metallacarboranes, 858t in 12-vertex metallacarboranes, 942t in supraicosahedral metallacarboranes, 973t Polyalkylation, at boron, 287–288 Polydecker sandwich systems, 838 Polyesters carborane, 1017–1019
1132 Index Polyesters (Continued) moderate chain length, 434 Polyhalogenation, 169–170, 286–287 Polyhedral boron clusters, “electron deficiency” in, 17–18 Polyhedral carboranes, 9 Polyhedral contraction interconversion reactions, 23–24 metallacarboranes of transition and lanthanide elements and, 811–812 13-vertex C2B11 and 14-vertex C2B12 clusters, 695 Polyhedral expansion, 107 closo-1,6-C2B4H6, 82 closo-2,4-C2B5H7, 110 interconversion reactions, 23–24 metallacarboranes of transition and lanthanide elements and, 811–812 exo-polyhedral ligands, properties of, 876 Polyhedral rearrangement, 23 exo-polyhedral rings, 443 Polyhedral skeletal electron pair theory (PSEPT), 10, 11–16, 606 Polyhedral valence electrons (PVE), 15–16 Polyketones, 1020 Polymer-bound rhodacarboranes, 1046–1047 Polymeric materials, metallacarboranes as dopants and inclusion compounds in, 1031 Polymerization alkenyl derivatives, 416–417 alkynyl derivatives, 418 nido-CB10H13–, 196–197 of metallacarborane monomers, 1029–1031 Polymers. See also Carborane polymers and dendrimers alkynyl-linked, 1017–1019 arylene-linked, 1019–1020 of closo-C2B10 carboranes, 1016–1026 carborane-siloxane, 1018, 1020–1022 carborane-siloxane-acetylene, 1018 m- and p-carboranyl carboxylic acids, 633–635 closo-CB11 carborane, 1016 class I polymers with other linking groups, 1022–1023 coordination, 1016 copolymers, 108, 196 fluorene, 1019 as high molecular weight boron delivery agents, 1067–1068 metal coordination, 290, 1026 metallacarborane, 1026–1031 metallacarboranes as dopants and inclusion compounds in polymeric materials, 1031 metallation of nido-carboranyl polymers, 1027 polymerization of metallacarborane monomers, 1029–1031 nondendritic boronated, 1068 with pendant carboranes, 1023–1026 dendritic systems, 1025–1026 hydrocarbon chains, 1023–1024 metal coordination polymers of C2B10 carboranes, 1026 phosphazene polymers, 1022–1023 pyrrole polymers, 1024
single atom-linked, 1017 thiophene-based conducting, 1089 Polyphosphate derivatives, 480–481 Porphyrins carboranyl, 1066 dicarbollylcobalt-substituted, 1065 as small-molecule boron delivery agents, 1064–1066 Positron emission tomography (PET), 1071 Potassium in heteroatom carboranes, 701–705 reduction, of thiadiborolenes, 117f synthesis and, 702t Prochiral carboranes, 487 Propargyl bromide, 230 Propiolic acid, 375 Proton donation alkynyl derivatives and, 417 by monocarborane acids, 289–290 Proton sponge, 111, 160, 244 Protonation nido-C2B9H11 2 , 229–231 nido-7,9-C2B9H11 2 , 231 nido-C2B9H12 , 229–231 nido-C2B9H13, 229–231 cage isomerization induced by, 957 fullerenes and, 290 PSEPT. See Polyhedral skeletal electron pair theory Publications, numbering changes and, 124 PVE. See Polyhedral valence electrons Pyridine, 380 Pyridyl derivatives, 467 Pyrrole polymers, 1024 Pyrroles, 468, 469
R Radical anion formation, 696 Radical halogenation. See Photochemical halogenation Radicals, alkenyl derivative reactions with, 413–414 Radioastatination, 1072 Radiohalogenation, 1071–1072 Radioiodination, 1072 Radiometal labeling in medicine, 1073 technetium in, 1073 Radiotherapy and molecular imaging, 1071–1074 electron microscopy, 1074 Fourier transform infrared labeling, 1074 gadolinium in, 1073 other methods, 1074 radioastatination, 1072 radiohalogenation, 1071–1072 radiometal labeling, 1073 secondary ion mass spectometry, 1074
Index 1133 Reactions alkenyl derivatives with acids, 412–413 1,7 and 1,12-C2B10H12, 628–630 with halogens, 411–412 with other electrophiles, 413 with oxidants and radicals, 413–414 B-halo derivative, with nucleophiles, 653–654 borane-alkyne gas phase, 21 importance of, 21 in solution, 22 Brellochs, 145, 226–227 closo-1,2-C2B4H6, 82 closo-1,6-C2B4H6, 82 1,2-C2B10H12 alcohols, hydroxy derivatives, and ether reactions, 434–435 of 1,2-C2B10H12 ketones, 440–442 of 2,3-(CH2)3C2B12H12, 696–697 nido-C3B8H11 , 244 nido-C3B8H12, 244 cage-opening, 487–488 carborane-alkyne reactions, 22 o-carboranyl aldehyde, 440–442 o-carboranyl ketone, 440–442 cyclization, 415, 443–447 interconversion, 23–24 of 7,8,9-C3B8H12 derivatives, 241f metal-promoted cage fusion, 23 polyhedral expansion and contraction, 23–24 polyhedral rearrangement, 23 with Lewis bases, 233f ligand substitution reactions, 949 of closo-MC3B7 clusters, 879 metal atom reactions, 814 sealed-tube, 287 Reactivity. See also Synthesis and reactivity alkenyl and alkynyl derivatives, 628 of aromatic rings, 407–408 aryl derivatives, 406–408 closo-4,5-C2B7H9, 136–137 closo-2-CB6H7, 96 CB11H12 , 268t of closo-C2B3H5, 40–41 nido-MC3B2 and closo-M2C3B2 cages, 833 of o-carboranyl C-Sn bonds, 459 Reagents alkyne, 375–376 electrophilic, 284–285 Grignard, 378–379, 380, 429, 436–437 iminophosphorane, 226 mercury, 234–235 RBX2, 286 RX, 616 Venus flytrap, 451
Rearrangement, cage closo-2,4-C2B5H7, 110 C2B8H10, 178 nido-C2B8H12, 154–155 nido-C2B9H11 2 , 229–231 nido-C2B9H12 , 229–231 nido-C2B9H13, 229–231 1,2- and 1,7-C2B10H12, 604–606 metal-induced, 243f polyhedral, 23 in structure and bonding, 16 in substituted derivatives, 110 Reboronation, 238 Redox, cage isomerization induced by, 957 Reductive cage-opening, 681f, 696 Regioselective substitution, 818f Reprotonation, 154f, 158, 158f Retinoid agonists and antagonists, 1056 Reverse isomerization, 606–607 1,2-(Ph3P)2Rh–CB10H10C–Ph, 388–389 Rhenium 1,2-C2B10H12 derivatives, 357t 1,7-C2B10H12 derivatives, 571t 1,12-C2B10H12 derivatives, 598t in 12-vertex metallacarboranes, 897t in 8- to 11-vertex metallacarboranes, 841t in supraicosahedral metallacarboranes, 970t Rhodacarboranes, polymer-bound, 1046–1047 Rhodium 1,2-C2B10H12 derivatives, 361t 1,7-C2B10H12 derivatives, 572t in 6- and 7-vertex metallacarboranes, 798t in 12-vertex metallacarboranes, 928t in 8- to 11-vertex metallacarboranes, 853t in supraicosahedral metallacarboranes, 972t Rigid rods, 628–630 Ring substitution, 955 Rubidium, 701 Ruthenium 1,2-C2B10H12 derivatives, 359t 1,12-C2B10H12 derivatives, 598t in 6- and 7-vertex metallacarboranes, 782t in 8- to 11-vertex metallacarboranes, 847t in 12-vertex metallacarboranes, 906t in supraicosahedral metallacarboranes, 970t
S Salts amine-hydrochloride, 472–473 BH4 salts, 376 diazonium B-substituted derivatives, 466–467 1,7-C2B10H12, 542t, 642–644 1,12-C2B10H12, 578t, 642–644 as 1,2-C2B10H12 nitrogen derivatives, 302t, 463–467
1134 Index Salts (Continued) C-substituted derivatives, 463–465 properties, 643–644 synthesis, 642–643 dicarbollide, 234 with main-group metal cations, 1037–1038 nido-Me5C5BXþ, 73f with transition metal-containing cations, 1038 Scandium in metallacarboranes, 883t Sea urchin carboranes, 15 Sealed-tube reactions, 287 Secondary ion mass spectometry, 1074 Selective estrogen receptor modulators (SERMs), 1055 Selenium added at carbon, 386–387, 612 bond formation, 403, 621 1,2-C2B10H12 derivatives, 353t, 488–500 metal complexes, 495–500 thiols, thioethers, disulfides, and related compounds, 488–500 1,7-C2B10H12 derivatives, 570t, 650–653 properties, 652–653 synthesis, 650–652 in heteroatom carboranes, 760t, 761–766 metal complexation, 495–500 bidentate systems, 496–500 monodentate systems, 495–496 SEP. See Skeletal electron pair SERMs. See Selective estrogen receptor modulators Silicon added at carbon, 383, 609–611 1,2-C2B10H12 derivatives, 347t, 442–454 alkoxysilanes, 452–454 B-organosilyl derivatives, 454 C-silyl derivatives, 443–454 cyclization reactions, 443–447 dendrimers, 448–450 metal chelate complexes, 447–448 silicon-linked mixed-carborane derivatives, 450 silyl as protecting group in synthesis, 450–451 1,7-C2B10H12 derivatives, 565t, 640–641 properties, 641 synthesis, 640 1,12-C2B10H12 derivatives, 596t, 640–641 properties, 641 synthesis, 640 in heteroatom carboranes, 719t, 724–725 synthesis and, 719t Silicon-linked mixed-carborane derivatives, 450 Siloxanes, 1018, 1020–1022 Silver 1,2-C2B10H12 derivatives, 368t 1,7-C2B10H12 derivatives, 574tt in 8- to 11-vertex metallacarboranes, 860t in 12-vertex metallacarboranes, 944t Silyl, as protecting group in synthesis, 450–451
Silylium derivatives, 291 Single atom-linked polymers, 1017 Single photon emission computed tomography (SPECT), 1071 Single-cage derivatives, 198t, 249t Single-walled carbon nanotube (SWCNT), 1070 Skeletal electron contributions, 10, 10t Skeletal electron pair (SEP), 10, 11–16 Slip-distortion in metallacarborane complexes, 715 Small carboranes CB3Hx and C2B2Hx, 27–28, 28t 5-vertex closo clusters, 33–41 CB4Hx, 33 closo-C2B3H5, 33–41 closo-C2B3H5 þ , 41 5-vertex open clusters, 28–33 arachno-CB4 systems, 28–29, 58–59 nido-1,2-C2B3H7, 29–32 nido-C3B2 clusters, 32–33 nido-CB4 systems, 28–29, 58–59 open-cage C2B3 clusters, 32 4-vertex open clusters, 27–28 overview of, 27 6-vertex closo clusters, 73–83 1-CB5H7 and 1-CB5H6 , 73–75 closo-1,2-C2B4H6, 75–83 closo-1,6-C2B4H6, 75–83 6-vertex open clusters, 41–73 arachno-C2B4Hx clusters, 59–60 nido-2,3-C2B4H8, 45–56 nido-2,4-C2B4H8, 56–59 hypho-C2B4Hx clusters, 59–60 nido-2,3,4-C3B3H7, 60–62 nido-2,3,5-C3B3H7, 62–64 nido-2,3,4,5-C4B2H6 derivatives from, 64–71 nido-2,3,4,5,6-C5BH6 þ , 72–73 nido-2-CB5H9, 41–44 hypho-CB5H13, 44–45 Small metallacarboranes, in organic synthesis, 823–827 Small-molecule boron delivery agents amino acids, nucleosides, and nucleotides, 1063–1064 in BNCT, 1063–1066 other agents, 1066 porphyrins, phthalocynanines, and related derivatives, 1064–1066 Sodium in heteroatom carboranes, 701–705, 702t Sol-gel processes, hydrolytic, 1022 Solvent mixtures, ether-containing, 378 SPECT. See Single photon emission computed tomography Speier’s catalyst, 1021 Staircase oligomers, 831–832 commo-Stannacarboranes, 729 Steric crowding, 607 Steric effects, 1,2-C2B10H12, 441 Stock, Alfred, 17, 18
Index 1135 Strontium in heteroatom carboranes, 703t, 705–707 Structural patterns based on 5- to 10-vertex polyhedra, 12f based on 11- to 15-vertex polyhedra, 13f in boron clusters, 9–14 Structure bonding and cage rearrangement, 16 cluster aromaticity, 16–17 delocalized, 15–16, 18 dicarbon clusters, 953, 965 “electron deficiency” in polyhedral boron clusters, 17–18 electron-counting in classically bonded clusters, 15–16 electron-pair bonding in nido-2,3-C2B4H8, 9f extensions of electron-counting rules, 14–15 general perspective, 7 isomer stability, 16 localized-bond approach, 8 nomenclature, 7–8 numbering, 7–8 structural patterns in boron clusters, 9–14 closo-C2B3H5, 39–40 1,2-C2B4H6, 80–82 1,6-C2B4H6, 80–82 closo-1,2-C2B4H6, 80–82 closo-1,6-C2B4H6, 80–82 nido-2,3-C2B4H8, 53–56 nido-2,4-C2B4H8, 59 closo-2,4-C2B5H7, 107 closo-1,7-C2B6H8, 123–124 closo-4,5-C2B7H9, 135–137 6,7-C2B7H13, 133 arachno-C2B7H13, 128–132, 133 C2B8H10, 178 nido-C2B8H12, 153 arachno-C2B8H14, 157 closo-C2B9H11, 252 nido-C2B9H11 2 , 228–229 nido-C2B9H12 , 228–229 nido-C2B9H13, 228–229 1,2-C2B10H12, 376–377 1,7- and 1,12-C2B10H12, 541–604 nido-2,3,4-C3B3H7, 62 nido-2,3,5-C3B3H7, 64 nido-C3B7H11, 161 arachno-C3B7H13, 161 nido-C3B8H11–, 244 nido-C3B8H12, 244 nido-2,3,4,5-C4B2H6, 69–71 nido-C4B4H8, 118–120 nido-C4B6H10, 162 C4B8 clusters nido- and arachno-C4B8 carboranes, 688–689 (Me3Si)2R2C4B8H8 systems, 689
R4C4B8H8 systems, 688–689 nido-2,3,4,5,6-C5BH6 þ, 73 1-CB5H7 and 1-CB5H6 , 73–75 closo-CB7H8 , 120 4-CB8H9 , 134 nido-CB8H12, 126–127 arachno-CB8H13 , 126–127 arachno-CB8H14, 126–127 1- and 2-CB9H10 , 169 nido-CB9H12 , 148–149 arachno-CB9H14 , 148–149 closo-CB10H11 , 247 nido-CB10H13 , 193 closo-CB11 clusters, 267–294 CB11H12 , 267–294 dicarbon 13-vertex clusters, 953, 965 wedged, 871 Subicosahedral clusters, 1044–1045 Substituents carboranes as, 25 electronic behavior of carboranes toward, 407–408 Substituted derivatives, cage rearrangement in, 110 Substitution. See also Boron, substitution at; Carbon, substitution at; Organosubstitution via C-metallated carboranes, 1,2-C2B10H12 addition of aluminum, gallium, indium, and thallium at carbon, 382 addition of boron at carbon, 381–382 addition of halogen at carbon, 387 addition of lanthanide series metals at carbon, 390 addition of nitrogen, phosphorus, arsenic, antimony, and bismuth at carbon, 383–385 addition of oxygen at carbon, 385 addition of silicon, germanium, tin, and lead at carbon, 383 addition of sulfur, selenium, and tellurium at carbon, 386–387 addition of transition metals at carbon, 388–390 addition of zinc and mercury at carbon, 387–388 without metallation, 394–395 organosubstitution at carbon, 390–393 via C-metallated carboranes, 1,7- and 1,12-C2B10H12, 608–617 aluminum added at carbon, 609 boron added at cabon, 608 halogens added at carbon, 612–613 lanthanide series metals added at carbon, 615 nitrogen, phosphorus, arsenic, antimony, and bismuth added at carbon, 611 organosubstitution at carbon, 615–617 oxygen added at carbon, 612 silicon, germanium, tin, and lead added at carbon, 609–611 sulfur, selenium, and tellurium added at carbon, 612 thallium added at carbon, 609 transition metals added at carbon, 614 zinc and mercury added at carbon, 613 direct, on o-carboranyl boron atoms, 396–403 B–Al, B–Ga, B–In, and B–Ti bond formation, 402 B–Hg bond formation, 401
1136 Index Substitution (Continued) B–N, B–P, B–As, and B–Sb bond formation, 402–403 B–S, B–Se, and B–Te bond formation, 403 B–Si, B–Ge, B–Sn, and B–Pb bond formation, 402 electrophilic alkylation, 396–397 electrophilic halogenation, 398–399 fluorination, 400 nucleophilic displacement, 396 photochemical halogenation, 399–400 synthesis of B–OH and B–ONO2 derivatives via o-carborane oxidation, 401 thermal and photochemical organosubstitution, 400–401 thermal iodination, 399 electrophilic B-substitution, 285 dicarbon cages, 875 Friedel-Crafts, 194 halogen, 502–504 ligands substitution reactions, 949 nucleophilic, 70 photochemical B-substitution, 110 regioselective, 818f ring, 955 Sulfur added at carbon, 386–387, 612 bond formation, 403, 621 11-vertex cages, 763–765 in heteroatom carboranes, 759t, 761–766 metal complexation, 495–500 bidentate systems, 496–500 monodentate systems, 495–496 6- to 10-vertex cages, 761–763 synthesis and, 759t Sulfur derivatives bidentate systems, 496–500 1,2-C2B10H12 derivatives, 336t, 488–500 metal complexes, 495–500 thiols, thioethers, disulfides, and related compounds, 488–500 1,7-C2B10H12 derivatives, 558t, 650–653 properties, 652–653 synthesis, 650–652 1,12-C2B10H12 derivatives, 592t, 650–653 properties, 652–653 synthesis, 650–652 monodentate systems, 495–496 Sulfylhydrylation, 403 Superaromatic clusters, 16–17 Supraicosahedral and open 12-vertex carboranes nido- and arachno-C4B8 carboranes, 684–692 deboronation of nido-C4B8 cages, 691 formation and properties of arachno-R4C4B8H8n– mono- and dianions, 689–691 general observations, 692 metal complexation of nido-R4C4B8H8, 689 structure and fluxional behavior of, 688–689
synthesis of neutral arachno-C4B8 clusters, 691 synthesis of nido-C4B8 cages, 685–688 arachno-C6B6 carboranes, 692 derivatives, 676t open-cage C2B10 anions, 675–684 carbon-bridged clusters, 682–684 reduction of neutral C2B10 icosahedra, 675–681 supraicosahedral carboranes, 692–697 thermodynamic stability of, 675 13-vertex C2B11 and 14-vertex C2B12 clusters, 692–697 Supraicosahedral clusters, metallacarboranes in catalysis and, 1049 Supraicosahedral metallacarboranes, of transition and lanthanide elements carbon-rich 13- and 14-vertex clusters, 981–984 dicarbon 13-vertex clusters, 965–980 synthesis and characterization, 966t of transition and lanthanide elements, 965–984 Supramolecular assemblies, 1091–1093 Suzuki coupling, 405–406 SWCNT. See Single-walled carbon nanotube Synthetic agents, noncatalytic, 1098–1099
T Tamoxifen, 1055 Tantalum in 6- and 7-vertex metallacarboranes, 775t in 12-vertex metallacarboranes, 887t in 8- to 11-vertex metallacarboranes, 840t in supraicosahedral metallacarboranes, 968t Technetium 1,2-C2B10H12 derivatives, 357t 1,7-C2B10H12 derivatives, 571t in radiometal labeling, 1073 in 12-vertex metallacarboranes, 897t Tellurium added at carbon, 386–387, 612 bond formation, 403, 621 metal complexation, 500 bidentate systems, 496–500 1,2-C2B10H12 dertivatves, 354t, 302t, 488–500 metal complexes, 495–500 1,7-C2B10H12 dertivatves, 570t, 650–653 properties, 652–653 synthesis, 650–652 Terpyridine derivatives, 467–468 nido-Tetracarbadecaborane architecture, 162 nido-Tetracarbaoctoranes, 116–120 nido-Tetracarbaoctaboranes, 244–245 Tetracarbon clusters carbon-rich 13- and 14-vertex clusters, 981–982 8- to 11-vertex metallacarboranes, 880–881 14-vertex, 981–982 12-vertex metallacarboranes, 963–965 Tetradecker sandwiches, 830, 834–835 Tetrahedron, as special case, 14
Index 1137 Thallation, 402 Thallium addition at carbon, in 1,2-C2B10H12 derivatives, 382; in 1,7- and 1,12-C2B10H12 derivatives, 609 addition at boron, in 1,2-C2B10H12 derivatives, 402; in 1,7- and 1,12-C2B10H12 derivatives, 620 1,2-C2B10H12 derivatives, 347t 1,7-C2B10H12 derivatives, 565t 1,12-C2B10H12 derivatives, 596t in heteroatom carboranes, 709t, 717–718 Thermal fusion, 688 Thermal iodination, 399 Thermal isomerization, 16 Thermal organosubstitution, 400–401 Thermal rearrangement of 12-vertex cobaltacarboranes, 956 Thermal rearrangement, of 7-vertex metallacarboranes, 822–823 Thermodynamic stability, of open 12-vertex and supraicosahedral carboranes, 675 Thiadiborolenes, potassium reduction of, 117f Thiadiborolyl sandwiches, 836 Thioethers B-substituted derivatives, 495 as 1,2-C2B10H12 derivatives, 488–500 C–S and C–Se derivatives, 488–491 C–X–S and C–X–Se derivatives, 491–494 Thiols applications, 612 B-substituted derivatives, 495 as 1,2-C2B10H12 derivatives, 488–500 C–S and C–Se derivatives, 488–491 C–X–S and C–X–Se derivatives, 491–494 Thiophene-based conducting polymers, 1089 Thiophenes, 493–494 Thiophosphates B-substituted derivatives, 482–483 C-substituted derivatives, 481–482 phosphorus, arsenic, antimony, and bismuth derivatives, 481–483 Thiophosphites bis(o-carboranyl) thiophosphite, 483 B-substituted derivatives, 482–483 C-substituted derivatives, 481–482 monocarboranyl thiophosphite, 482–483 phosphorus, arsenic, antimony, and bismuth derivatives, 481–483 Thorium, in metallacarboranes, 883t Three-dimensional s-aromaticity, 377 Tin added at carbon, 383, 609–611 bond formation, 402, 620 complexes, as antitumor agents, 1060 in heteroatom carboranes, 720t, 728–732 commo-stannacarboranes, 729 1,2-C2B10H12 derivatives, 351t, 454–461 B-stannyl derivatives, 460 C-stannyl derivatives, 454–459
C-stannyl metal chelate complexes, 457–458 hydrostannylation of alkenylcarboranes, 456 reactivity of o-carboranyl C-Sn bonds, 459 1,7-C2B10H12 derivatives, 569t, 640–641 properties, 641 synthesis, 640 1,12-C2B10H12 derivatives, 597t, 640–641 properties, 641 synthesis, 640 Titanium 1,2-C2B10H12 derivatives, 355t 1,7-C2B10H12 derivatives, 571t in 6- and 7-vertex metallacarboranes, 774t in 12-vertex metallacarboranes, 884t in supraicosahedral metallacarboranes, 967t Topological resonance energy (TRE), 17 Transition elements, metallacarboranes of 8- to 11-vertex metallacarboranes, 839–881 dicarbon cages, 868–877 monocarbon cages, 839–868 synthesis and characterization, 839t tetracarbon clusters, 880–881 tricarbon cages, 878–880 general synthetic methods, 773–815 insertion of hydrocarbons into metallaboranes, 809–811 insertion of metal cations into open-cage carborane anions, 773–808 insertion of metals into neutral carboranes, 809, 810f metal atom reactions, 814 metal displacement and transfer, 813–814 other routes, 814–815 oxidative fusion, 812–813 polyhedral expansion and contraction, 811–812 overview of, 773 6- and 7-vertex metallacarboranes, 815–838 closo six-vertex cages, 815 functionalization and linkage of closo-LMC2B4 and nidoLMC2B3 clusters, 817–821 nido-MC2B3 and closo-M2C2B3 cages, 828–833 nido-MC3B2 and closo-M2C3B2 cages, 833–838 metal-metal communication in linked MC2B4 clusters, 822 small metallacarboranes in organic synthesis, 823–827 synthesis of closo-MC2B4 cages, 815 thermal rearrangement of 7-vertex metallacarboranes, 822–823 supraicosahedral metallacarboranes carbon-rich 13- and 14-vertex clusters, 981–984 dicarbon 13-vertex clusters, 965–980 synthesis and characterization, 966t of transition and lanthanide elements, 965–984 12-vertex metallacarboranes, 882–965 dicarbon clusters, 951–961 monocarbon clusters, 882–951
1138 Index Transition elements, metallacarboranes of (Continued) pentacarbon clusters, 963–965 synthesis and characterization, 883t tetracarbon clusters, 963–965 tricarbon clusters, 961–963 Transition metal(s) added at boron, 404–405, 622 added at carbon, 388–390, 614 antitumor agents and, 1061–1062 nido-C2B9H112– complexes, 238 nido-C2B9H12– complexes, 238 nido-C2B9H13 complexes, 238 1- and 2-CB9H10– complexes, 170–171 C–M bonded complexes, 390 derivatives, exo-polyhedral, 302t early transition metal complexes closo-carborane systems, 1039 icosahedral clusters and, 1045–1046 late transition metal complexes closo-carborane systems, 1040–1042 icosahedral clusters and, 1048 ligand groups, 10–11 middle transition metal complexes, 1040–1042 salts with transition metal-containing cations, 1038 sandwich complexes, importance of, 773 Transthyretin amyloid inhibitors, 1056–1057 Transthyretin protein (TTR), 1056–1057 TRE. See Topological resonance energy Triangular face rotation, 605f Triazine derivatives, 467–468 Tricarbollide ions, 239 Tricarbon 8-vertex carboranes, 116 Tricarbon cages 8- to 11-vertex metallacarboranes and, 878–880 reactions of closo-MC3B7 clusters, 879 synthetic routes, 878 Tricarbon clusters carbon-rich 13- and 14-vertex clusters, 981 12-vertex metallacarboranes, 961–963 Triethylamine, 52, 96 Triflates, 493 Trigonal pyramidal cluster geometry, 39 Trimethyl phosphine, 109 Trimethylamine, 109 Triphenylsilyl groups, 609, 610f Triphosphazenes, 483 Triple-decker sandwiches, 829–830 Triple-triple-decker system, 831–832 Tropenyliumyl-substituted o-carboranes, 409 TTR. See Transthyretin protein Tungsten 1,2-C2B10H12 derivatives, 356t 1,7-C2B10H12 derivatives, 571t in 6- and 7-vertex metallacarboranes, 776t
in 12-vertex metallacarboranes, 891t in supraicosahedral metallacarboranes, 969t 12-vertex carboranes, open nido- and arachno-C4B8 carboranes, 684–692 deboronation of nido-C4B8 cages, 691 formation and properties of arachno-R4C4B8H8n– mono- and dianions, 689–691 general observations, 692 metal complexation of nido-R4C4B8H8, 689 structure and fluxional behavior of, 688–689 synthesis of neutral arachno-C4B8 clusters, 691 synthesis of nido-C4B8 cages, 685–688 arachno-C6B6 carboranes, 692 derivatives, 676t open-cage C2B10 anions, 675–684 carbon-bridged clusters, 682–684 reduction of neutral C2B10 icosahedra, 675–681 supraicosahedral carboranes, 692–697 thermodynamic stability of, 675 13-vertex C2B11 and 14-vertex C2B12 clusters, 692–697 12-vertex metallacarboranes, 882–965 dicarbon clusters, 951–961 monocarbon clusters, 882–951 pentacarbon clusters, 963–965 synthesis and characterization, 883t tetracarbon clusters, 963–965 tricarbon clusters, 961–963 Two-electron donors, 10t
U Uranium in metallacarboranes, 883t, 967t Ureas 1,7-C2B10H12, 556t, 647–648 1,12-C2B10H12, 647–648 1,2-C2B10H12 derivatives, 332t, 473–474 multi-carboranyl, 474 Uterine cancer, 1055, 1061
V Vanadium in 6- and 7-vertex metallacarboranes, 775t in 8- to 11-vertex metallacarboranes, 839t in supraicosahedral metallacarboranes, 968t Vapor phase boron hydride-alkyne reactions, 21 Venus flytrap reagents, 451 Vinyl groups, 291 1-Vinyl-o-carborane, 399 Vinyltitanium(V)-C2B4 complexes, 117f
W Wade-Mingos rules, 9, 10. See also Polyhedral skeletal electron pair theory Wedged structures, 871
X Xerogels, 1022
Index 1139
Y Ylide group, 116 Yttrium 1,2-C2B10H12 derivatives, 355t 1,7-C2B10H12 derivatives, 570t in 6- and 7-vertex metallacarboranes, 774t in 12-vertex metallacarboranes, 883t in supraicosahedral metallacarboranes, 966t
Z Zinc added at carbon, 387–388, 613 1,2-C2B10H12 derivatives, 369t
1,7-C2B10H12 derivatives, 574t in 7-vertex metallacarboranes, 805t in 12-vertex metallacarboranes, 944t in 12-vertex metallacarboranes, 883t Zirconium 1,2-C2B10H12 derivatives, 355t 1,7-C2B10H12 derivatives, 571t in 6- and 7-vertex metallacarboranes, 774t in 12-vertex metallacarboranes, 885t in supraicosahedral metallacarboranes, 967t Zwitterionic copolymers, 196
