Advances in Inorganic Chemistry, Volume 41

Advances in INORGANIC CHEMISTRY Volume 41 ADVISORY BOARD I. Bertini Universita Degli Studi di Firenze Florence, lta...

Author: AG Sykes

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Advances in

INORGANIC CHEMISTRY

Volume 41

ADVISORY BOARD I. Bertini Universita Degli Studi di Firenze Florence, ltaly

A. H. Cowley University of Texas Austin, Texas

H. B. Gray California lnstitute of Technology Pasadena, California

M. L. H. Green University of Oxford Oxford, England

D. M. P. Mingos lmperial College of Science, Technology, and Medicine London, England

J. Reedijk Leiden University Leiden, The Netherlands

A. M. Sargeson The Australian National University Canberra, Australia

Y. Sasaki Hokkaido University Sapporo. Japan

0. Kahn

D. F. Shriver

Universite de Paris-Sud Orsay, France

Northwestern University Evanston, lllinois

A. Ludi Universitat Bern Bern, Switzerland

K. Wieghardt Ruhr-Universitat Bochum Bochum, Germany

Advances in

INORGANIC CHEMISTRY EDITED BY A. G. Sykes Department of Chemistry The University Newcastle upon Tyne United Kingdom

VOLUME 41

ACADEMIC PRESS San Diego New York Boston London Sydney Tokyo Toronto

This book is printed on acid-free paper.

@

Copyright 0 1994 by ACADEMIC PRESS, INC. All Rights Reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher.

Academic Press, Inc. A Division of Harcourt Brace & Company

525 B Street, Suite 1900, San Diego, California 92101-4495 United Kingdom Edition published by Academic Press Limited 24-28 Oval Road, London NW 1 7DX

International Standard Serial Number: 0898-8838 lnternational Standard Book Number:

0-12-02364 1-9

PRINTED IN THE UNITED STATES OF AMERICA 94 95 9 6 9 7 98 9 9 E B 9 8 7 6

5

4

3 2

I

CONTENTS The Coordination Chemistry of Technetium

JOHNBALDAS I. Introduction 11. Technetium(-I) 111. Technetium(0)

IV. V. VI. VII. VIII. IX. X. XI.

. . .

Technetium(1) . Technetium(I1) . Technetium(II1) . . Technetium(IV1 . . TechnetiumW . Technetium(V1) . . Technetium(VI1) . . Appendix: Abbreviations References . .

. . . .

. .

.

2 5 5 7 17 27 45 54

. . 80 . 94 . 99 . 101

.

Chemistry of Pentafluorosulfanyl Compounds

R. D. VERMA, ROBERTL. KIRCHMEIER, AND JEAN’NE M. SHREEVE . 126 I. Introduction . . 126 11. Pentafluorosulfanyl Halides . . 130 111. Pentafluorosulfanyl Hypohalites, SFSOX . . . IV. Pentafluorosulfanylalkanes, Alkenes, and Alkynes V. Sulfur Isocyanate Pentafluoride and Sulfur Isothiocyanate Pentafluoride . VI . Sulfur Cyanate Pentafluoride, SF50CN . . VII. Sulfur Cyanide Pentafluoride, SF&N VIII. Sulfur Isocyanide Pentafluoride, SF,NC . . IX. Pentafluorosulfanylamine and Other Derivatives X. Pentafluorosulfanyl N,N-Dichloroamine, SFSNC1z XI. Pentafluorosulfanyl N ,N Difluoramine, SFSNFz XII. Pentafluorosulfanyl Perfluoroalkylamines, SF,N(H)Rf XIII. SF,N(CF,)z . . XIV. SF5N(X)CF3(X=F, C1, Br, I) . . xv. SF,N(Cl)Rf (Rf=C2Fs,n-C3F7,n-C4F9) . XVI. Bis(pentafluorosulfany1)perfluoroalkylamines . XVII. Tris(pentafluorosulfanyl)amine,(SF5)3N . XVIII. Bis(pentafluorosulfanyl)bis(trifluoromethyUhydrazine, SF,(CF,)NN(CFz)SFS . XIX. Tetrakis(pentafluorosulfanyl)hydrazine,(SF&NN(SF& xx. Bis(pentafluorosulfanyl)amine,(SFS),NH . .

.

v

. 132 . 138 . 142

. . .

. .

143 143 144 145 146 146 147 147 149 149 150

. . .

150 150 151

.

. . . . .

vi

CONTENTS

XXI. XXII. XXIII. XXIV.

(SF&NX (X=F, C1) . N-Pentafluorosulfanyl Haloimines, F5SN=CX~ ( X = C1, F) . Pentafluorosulfanyliminodihalosulfanes,SF,N=SX2 ( X = F, C1) Pentafluorosulfanyl-P-Sultonesand Sulfonic Acids . References . .

. 151

. .

.

152 155 . 157 . 161

,

The Hunting of the Gallium Hydrides

ANTHONY J. DOWNSAND COLINR. PULHAM I. Introduction . 11. History and Chemical Background . 111. IV. V. VI. VII.

Conduct of the Hunt: Practical Methods of Attack . Toward Gallane: Preparations for the Hunt . Gallane a t Last! . . Hydrogen-Rich Derivatives of Gallane . Hydrides of the Other Group 13 Metals: Preliminaries and Prospects References .

172 173 177 188 196 211 221 . 228 ,

. . . . .

The Structures of the Group 15 Element(lll) Halides and Halogenoanions

GEORGEA. FISHERAND NICHOLAS c . NORMAN I. 11. 111. IV.

Introduction . Element Trihalides, EX3 Element(II1) Halogenoanions General Comments . References . . Note Added in Proof .

. 233 . 234

.

.

238

. 264 . 268 . 271

lntervalence Charge Transfer and Electro n Exchange Studies of Dinuclear Ruthenium Com plexes

ROBERTJ. CRUTCHLEY I. 11. 111. IV. V.

Introduction . Mixed-Valence Complexes . Electron Exchange . Future Studies . Glossary of Abbreviations and Ligand Structures References .

. . .

273 274 . 304 . 313 , 314 . 319

vii

CONTENTS

Recent Synthetic, Structural, Spectroscopic, and Theoretical Studies on Molecular Phosphorus Oxides and Oxide Sulfides

I. 11. 111. IV. V.

J. CLADE,F. FRICK, AND M. JANSEN Introduction . Molecular Phosphorus Oxides . Molecular Phosphorus Oxide Sulfides . Comparative Considerations . Concluding Remarks . . References .

.

327

. 329

.

364

. 381

. .

383 384

Structure and Reactivity of Transferrins

E. N. BAKER I. 11. 111. IV. V. VI. VII.

Introduction . Biological Roles . Transferrin Structure . Properties of the Metal and Anion Sites Mechanisms of Binding and Release Recombinant DNA Studies . Concluding Remarks . References .

INDEX . CONTENTS OF PREVIOUS VOLUMES .

. 389

.

391

. 393

.

419

. 445 . 452

.

455

. 456

.

. 465 . 477

This Page Intentionally Left Blank

ADVANCES I N INORGANIC CHEMISTRY, VOL.

41

THE COORDINATION CHEMISTRY OF TECHNETIUM JOHN BALDAS Australian Radiation Laboratory. Yallambie, Victoria 3085, Australia

I. Introduction 11. Technetium(-[)

111. Technetium(0) IV. Technetium(1) A. Carbonyl Complexes B. Cyclopentadienyl and Acene Complexes C. Cyano and Isonitrile Complexes D. Dinitrogen, Phosphine, Phosphite, and Related Complexes E. Complexes with Nitrogen Ligands F. Nitrosyl and Thionitrosyl Complexes V. Technetium(I1) A. Organometallic Complexes B. Halide Complexes and Clusters C. Complexes with Nitrogen Ligands D. Phosphine, Arsine, and Related Complexes E. Complexes with Sulfur Ligands F. Nitrosyl and Thionitrosyl Complexes VI. Technetium(II1) A. Carbonyl Complexes B. Cyclopentadienyl Complexes C. Cyano, Isonitrile, and Thiocyanato Complexes D. Aqua, Halide, and Related Dimeric Complexes E. Carboxylato and /3-Diketonato Complexes F. Complexes with Dioximes, Schiff Bases, and Other Nitrogen Ligands G. Complexes with Monodentate Phosphines and Related Ligands H. Complexes with Bidentate Phosphine, Arsine, and Related Ligands I. Complexes with Sulfur Ligands J. Nitrosyl and Thionitrosyl Complexes VII. Technetium(1V) A. Isonitrile and Thiocyanato Complexes B. Halide and Related Complexes C. Complexes with Oxygen Ligands and 0x0-Bridged Complexes D. Complexes with Schiff Base and Other Nitrogen Ligands E. Complexes with Phosphine and Arsine Ligands F. Complexes with Sulfur Ligands 1 Copyright 0 1994 by Academic Press, Inc. All rights of reproduction in any form reserved.

2

JOHN BALDAS

VIII. TechnetiumW A. Mononuclear [TcOI''' Complexes B. Complexes of the trans-[TcO(OH)1" and [TcO,]' Cores C. 0x0-Bridged lTc*0314*and Other Binuclear Complexes D. [TcS13* Complexes E. Nitrido Complexes F. Imido and Hydrazido Complexes G. Complexes Not Containing Multiply Bonded Ligands IX. Technetium(V1) A. 0x0 Complexes B. Nitrido Complexes C. Imido and Hydrazido Complexes D. Dithiolene and Related Complexes X. TechnetiumWII) A. 0x0 and Sulfido Complexes B. Nitrido and Imido Complexes C. Complexes Not Containing Multiply Bonded Ligands XI. Appendix: Abbreviations References

I. Introduction

Technetium, the ekamanganese of Mendeleev and the first of the artificially produced elements, was discovered in 1937 by Perrier and Segre in a molybdenum plate that had been bombarded with deuterons ( I , 2).The name technetium is derived from the Greek word for artificial. Twenty-one isotopes, all radioactive, of mass number 90-110 and several metastable isomers are known (3).Because the half-life of the longest lived isotope, 9 8 T ~is, 4.2 X l o 6 years, primordial technetium has long ceased to exist on earth but minute traces occur in nature (1ng of "Tc in 5.3 kg of pitchblende) as a result of the spontaneous fission of uranium ( 4 ) . The long-lived 99Tc[tuz = 2.11l(12) x lo5 years; p - decay energy = 293.6 keV] (5, 6) is produced in 6% yield from 235Ufission and is isolated in quantity from spent nuclear fuel (7). Technetium-99 is available commercially in gram quantities, usually as ammonium pertechnetate in aqueous solution. This is the only isotope used for macroscopic chemical studies and is here designated simply by the symbol Tc. The ground-state electronic configuration of the Tc atom is [Kr14d55s2with a 6S5/2(2s+1Sj) term symbol (8).Technetium metal dissolves in the oxidizing acids nitric, aqua regia, and concentrated sulfuric and in bromine water. Like rhenium, technetium dissolves in neutral and alkaline solutions of hydrogen peroxide to form the pertechnetate anion. In oxygen the metal burns to form the oxide Tc,O, (7). Apart from radioactivity considerations, the chemistry of

COORDINATION CHEMISTRY OF TECHNETIUM

3

technetium may be investigated by conventional synthetic and spectroscopic methods. Chemically, technetium resembles its third-row congener rhenium, but there are significant differences. In particular, there are the greater ease of reduction of the higher oxidation states of technetium and the greater substitution lability of the lower oxidation states compared with those of the rhenium analogs (9).The organometallic chemistry of technetium, however, rather closely resembles that of rhenium (10).Technetium complexes with the metal in oxidation states from -1 to + 7 are known but, although there is now much research activity in the area, the chemistry of technetium remains relatively undeveloped compared with that of manganese, rhenium, and the neighboring Group 6 and 8 transition metals. The results obtained to date are nonetheless very considerable and show the chemistry of technetium to be among the most varied and interesting of the transition metals. In the last 20 years or so the study of the coordination chemistry of technetium has assumed major practical importance due to the widespread use of the short-lived metastable isomer 9 9 m Tin~ diagnostic compound (radionuclear medicine (11-19). Generally, a 99mTc-labeled pharmaceutical) is injected intravenously into the patient and the in uiuo distribution determined by the use of scintillation techniques, including single photon emission computed tomography (SPECT) (15). The physical properties of 9 9 m Tare ~ near ideal. The gamma ray energy of 140 keV is sufficiently energetic to penetrate deeply seated tissue and is easily externally collimated and detected. The absence of a or /3 emission and the short half-life of 6.01 hr result in a low radiation dose to the patient and activities of up to 1 GBq may be administered. is usually obtained from a Technetium-99m in the form of NaggmTc04 99Mo/99mTc generator based on the decay scheme

Fission-produced 99Mo02- loaded onto an alumina column decays to 99mT~04-, which is conveniently eluted from the column by physiological saline (0.15 M NaCl) while the parent 99Mo0,2-is strongly retained (20,21). The generator eluate contains 9 9 m T~04and a variable quanmainly on the time interval since the previtity of 9 9 T ~ 0 4(depending ous elution) with a total Tc concentration in the range of to M (22,231.This mixture of 99mTc04-/99Tc04is referred to as “no carrier added” and is denoted simply as 99mT~04-. Radiopharmaceuticals are in the presence of a usually prepared by the reduction of 99mT~04-

4

JOHN BALDAS

ligand to give a Y y m complex T~ with the desired physiological behavior. A commonly used reducing agent is stannous tin. The 9 9 m Tradiophar~ maceutical is formed in high yield and radiochemical purity in aqueous solution at near-neutral pH and should be stable in the chemically aggressive in vivo environment at a Tc concentration of the order of 10-”M, which results from dilution by the blood volume (24,251. In a number of cases chromatographic comparisons have shown the structure of 9 9 m Tradiopharmaceutical ~ to be the same as the 99Tccomplex prepared at the macroscopic level but in others the structure and oxidation state are uncertain (19).”“‘Tc radiopharmaceuticals are now available for skeletal, myocardial, renal, hepatobiliary, thyroid, and lung imaging and for a variety of physiological function studies (15).Specific examples are described together with the 99Tcanalogs. The impact of technetium in medical diagnosis may be judged by the 1990 estimate that six t o seven million administrations of 9 9 m Tradio~ pharmaceuticals are performed annually in the United States (17 ) .As a result the study of technetium chemistry has to a degree been driven by the need to understand the chemistry of 9 9 m Tradiopharmaceuticals ~ and to develop new or improved organ-specific agents. Some of this chemistry is now being transferred to rhenium, whose high-energy pemitting lasRe and “‘Re radioisotopes show promise for the development of therapeutic radiopharmaceuticals (9). The aim of this chapter is to provide a fairly comprehensive overview of the status of technetium coordination chemistry up to the latter part of 1993. The term “coordination” is taken to include organometallic compounds. Binary halides are briefly described for the sake of completeness. The material is grouped into oxidation states, with the nitrosyl and thionitrosyl groups being treated as NO+ and NS’, the hydrido ligand as H-, and “noninnocent” ligands such as dithiolenes in the dianionic form. The literature of technetium chemistry consists of two now out-of-date books (26, 2 7 ) and a more recent Russian text ( 2 8 ) together with a comprehensive survey of the literature in two volumes of Gmelin published in 1982 and 1983 (29).Much information is to be found in three conference volumes (30-32) and there are numerous reviews of technetium chemistry (11-13,15,33-36). Specific areas to have been reviewed are crystal structures (37,381, EPR spectroscopy (39-41 1, cluster compounds (42),and analytical chemistry ( 4 3 )and a useful correlation chart of 99Tc NMR chemical shifts and oxidation states of technetium is available (441. Single-crystal X-ray diffraction has been particularly useful. The considerable fraction of technetium complexes to have been characterized by this method may be due to some extent to the difficulty in working with radioactive material but


5

is no doubt largely due to the recent development of the chemistry and the greater availability of crystallographic structure determination facilities. There is a vast literature developed in the search for potential 9 9 m Tradiopharmaceuticals. ~ In many cases the complexes are poorly, if at all, characterized, although the charge is usually determined by electrophoresis. Such complexes will, in general, be considered here only if there are points of specific chemical interest. “No carrier added” preparations will always be denoted as 99mT~. II. Technetium( - I)

This is the rarest oxidation state for technetium. The IR spectrum of a solution prepared by the addition of N a amalgam to [Tcz(CO)lolin THF showed two v(C0) bands at 1911 and 1865 cm-’, which were assigned to the carbonyl anion [Tc(CO)J by comparison with the spectra of [M(CO),]- (M = Mn, Re). Solutions of Na[Tc(CO),l in THF undergo the expected reactions, including the formation of volatile [HTc(CO),] on treatment with H,PO, (45).The [Tc(CO),]- anion has been used as a nucleophile for the preparation of mixed-metal decacarbonyls (46)by reactions such as

Photolysis of a mixture of [ T C ~ ( C Oand ) ~ ~[Fe(CO)J I in THF is reported to give NEt4[TcFe2(CO)lzl,where Tc(CO)~-replaces Fe(CO), the triangular structure of [Fe3(CO)121 (47). III. Tech netium(0)

Best known, and of great synthetic utility, is the colorless diamagnetic dimer [TC,(CO),~I(m.p., 159-160°C) (48,491, which may be prepared in up to 96% yield by the reaction of NH4Tc04with CO (90 atm initial pressure) in toluene at 200°C with a reaction time of 4 hr (50). The [Mz(CO)lol(M = Mn, Tc, Re) carbonyls are isomorphous (51). The structure of [ T C ~ ( C O (Fig. ) ~ ~ I 1) shows the Tc atoms octahedrally coordinated with a Tc-Tc single bond distance of 3.036(6) A and the equatorial carbonyl groups staggered (approximateDld symmetry) (51). The equatorial carbonyl groups on each Tc are bent away from the axial carbonyl toward the other half of the dimer. The greater n-acceptor character of the axial CO ligands is reflected in C-0 bond distances

6

JOHN BALDAS 05'

L.

c2n. 101'

01 I C

04 '- 04' FIG. 1. The structure of [Tc2(CO),,,](51 1.

0.09 A longer and Tc-C bond distances 0.10 A shorter than those of the equatorial ligands. The vibrational spectra of [M2(CO)lo](M = Mn, Tc, Re) have been extensively investigated and compared (521. For [TC,(CO),~]the equatorial and axial CO stretching force constants of 16.642 and 16.316 mdyn A-', respectively, again demonstrate the greater r-acceptor character of the axial CO ligands. The "Tc NMR spectrum of [Tc,(CO),,] consists of single sharp signal ( A V ~=, ~1.4 Hz) at -2477 ppm relative to Tc0,- (53).The [99"'T~(C0)51'radical is produced in the p- decay of [99Mo(C0)61and reacts with carrier [Mn(CO)J] to form [99"T~(C0)511(541. The heteronuclear carbonyls [M~TC(CO),~] and [TcRe(CO),,l have been prepared by the reaction of a carbonylate anion with a carbonyl halide and characterized by IR and mass spectrometry. The IR spectra of the six possible [M2(CO),,1 (M = Mn, Tc, Re) compounds are closely similar, with the three v(C0) peaks expected in local C,, symmetry (461,a point that emphasizes the general similarity of the structures of Group 7 carbonyls. A mixed cobalt carbonyl [(CO),COTC(CO)~I has also been reported (55)and the CO stretching force and interaction constants have been determined (56).A polymeric [Tc(CO),], , thought to be a trimer, has been claimed but remains inadequately characterized (57). Substitution of the CO ligands in [ T C ~ ( C O by ) ~ the ~ ] strong r-acceptor PF, is achieved either thermally or photolytically. In one study up to eight CO ligands were replaced to give a t least 24 [ T C ~ ( C O ) ~ ~ - ~ ( P F ~ ) , I


7

isomers, which were assigned on the basis of mass spectra, gas chromatographic retention times, and comparison with the rhenium analogs (58).The monosubstituted ~ ~ ~ - [ T C ~ ( C O )has ~ ( Pbeen F ~ )studied I by 99Tcand 19F NMR (59).Reaction of Tc vapor with PF3 at 77 K gives the volatile [ T c ~ ( P F ~ )~~ ] the basis of IR evidence, the formation (60). On of [ T C ~ ( C O ) ~ ( P Pand ~ , ) ][ T C ~ ( C O ) ~ ( P P has ~ ~been ) ~ I proposed in the reaction of [TC,(CO),~] with PPh, in decalin at 100-150°C (61). Photolysis of [ T C ~ ( C O )in ~ ~the ] presence of butadiene at -20°C gives [TC2(C0)&p-C4H6)],which is isomorphous with the Mn and Re analogs. The trans-butadiene ligand bridges the Tc atoms, which are separated by 3.117(1) A. The Tc-Cbutadiene bond distances (av., 2.389 are markedly longer than Tc-CO (av., 1.945 & (62).A dinitrogen complex originally reported as [Tc(N,)(dppe),I (63) has been shown to be the hydride [HTc’(N2)(dppe),l(641. IV. Technetiurn(1)

A notable feature of this oxidation state is that a considerable number of Tc and 99”’T~ complexes can be prepared in high yields in aqueous media 136). As a consequence the coordination chemistry of Tc(1) has been intensively investigated in the search for 9 9 m Tcationic ~ myocardial imaging agents. Tc(1)complexes have the low-spin d6configuration and are diamagnetic. The 18-electron rule is generally applicable and nicely explains the stability and the prevalence of six-coordinate complexes. A. CARBONYL COMPLEXES Complexes containing cyclopentadienyl and related ligands are considered in Section B.

1. Mononuclear Complexes Complexes containing from one to six carbonyl groups are known and all obey the 18-electron rule. The colorless salt [TC(CO)6]A1C14 is formed by the reaction of [Tc(CO),Cl] with AlCl, under 300 atm CO pressure and is soluble in THF, acetone, and methanol and stable in aqueous solution (65).The carbonyl halides [Tc(CO),X] (X = C1, Br, I) may be prepared by the reaction of the halogen with [TC~(CO),~I. Reaction with chlorine and bromine occurs readily at room temperature but reaction with iodine is extremely slow. The iodide has been prepared by the high-pressure carbonylation of [TC(CO)~II~ (45).An alternative

8

JOHN BALDAS

preparation of the carbonyl halides is by the reaction of K,[TcX,] with CO under pressure a t 230-250°C in the presence of Cu powder (65). The IR spectra of [Tc(CO),Xl (X = C1, Br, I) show the three v(C0) bands (2A, + E ) expected in C,, symmetry in the region 2153-1991 cm-' and a weak 13C0 isotope peak of the intense E mode (45).The ease of halide substitution in [Tc(CO),Xl (usually with the loss of one or more CO groups) makes these compounds key starting materials in technetium carbonyl chemistry (65). Simple substitution of X- occurs in the reaction of CF,COOAg with [Tc(CO),ClI to give [Tc(CO),(OOCCF3)1. The asymmetry introduced by the CF,COO- ligand results in the B , mode becoming IR active and four v(C0) bands are observed (66). Oxidation of [ T C ~ ( C O ) with ~ ~ I NOPF, in MeCN gives [Tc(CO),(CH3CN)]PF6in quantitative yield. This complex is a useful synthetic precursor for the preparation of cationic carbonyl complexes with a variety of ligands (67). The volatile, colorless hydride [HTc(CO),] is produced in only low yield by the reaction of [Tc(CO),]- with &PO, (45). Complexes based on the [Tc(CO),I core are [Tc(CO),(SzCNRz)I(R = Me, Et), the cationic [Tc(CO)4(PPh3)zlA1C14 (651, and [Tc(CO),(acac)l (68). The dithiocarbamato complexes are formed by the reaction of Na(SzCNR2) with [Tc(CO),Cl] in acetone or THF. Grinding of [Tc(CO),ClI with K(P-diketonate) under a layer of CCl, yields the unstable tetracarbonyl P-diketonates (68). A considerable number of complexes containing the [Tc(CO),] core have been prepared and a number of crystal structures have been reported. Cationic complexes are of the type [Tc(CO),L,]X, where L, represents three neutral monodentate ligands, a monodentate and bidentate neutral ligand, or a neutral tridentate ligand. Reaction of [Tc(CO),Brl with AgPF6 in MeCN gives a near quantitative yield of [Tc(C0),(MeCN),1PF6 and [Tc(C0),(MeCN)(PPh3),1PF, and [Tc(CO), (MeCN)(dppe)]PF, may be prepared by ligand exchange (69).Of particular interest in relation to potential 99mTcradiopharmaceuticals is the air-stable, water-soluble [Tc(CO)~(L~)]PF~ (L3 = tan; 1,4,7-trimethyltan; 1,4,7-trithiacyclononane) (67). Only one monoanionic ligand seems to be supported to give neutral complexes of the type [TcX(CO),L,I, where some examples are X = C1, Br, I, 02CR; L = PR, , AsR,, SbR,, P(OR),, py, MeCN, CNR, EtzNH; or Lz = bpy, phen, dppe, en (65,66, 68, 70-72); [Tc(CO),{HB(pz),}] (73). A novel preparative method with CO at atmospheric pressure yields 1(74). Fac and mer isomers may be distinguished by the IR spectrum; two v(C0) bands (A, + E in local CBUsymmetry for the CO groups) are expected for a fuc isomer and three (2A, + B , in local Cz, symmetry), for a mer isomer (67, 72). The "Tc NMR spectra of neutral complexes

9


:1;

CI

OC-Tc-CO

co 110°C N B u ~ ~ c O C I ~*]

PhMe/MeCN

I’

OG

PPh, (1)

95% yield

show chemical shifts of -940 to - 1820 ppm and those of cationic complexes, -2070 to -3520 ppm against TcO,- (70, 75). The crystal structure of 1 shows almost undistorted octahedral geometry with a P-Tc-P is distorted angle of 174.59(2)”(74),whereas that offac-[T~Br(CO)~(en)] with the Tc-C and C-0 bond distances the same for all three CO groups (76). A “piano stool” structure with CBVsymmetry is found for [Tc(CO),L] [L = HB(pz),, HB(3,5-Me2pz),l, which is isostructural with the Mn and Re analogs (73). An unusual complex is [TcBr(CO),(Ph-Pglup)], prepared by the reaction of [Tc(CO),Brl with a neutral chiral phosphinoglucose derivative (77). Complexes containing the [Tc(CO),] core may be prepared by substitution or carbonylation reactions. The thiolato complexes [Tc(CO),(PPh,),L] [L = SzCNEtz,S2COEt, SzP(OMe),l are formed on heating trans-[T~(CO),Cl(PPh,)~] with the ligand in acetone or THF (78). The } ~ formed ] C ~ O ~by the cis- and trans-isomers of [ T c ( C O ) ~ { P ( O E ~ ) ~ P ~are reaction of [TcClz{P(OEt)zPh}41C104 with CO (1atm) at 50°C (79).The cis-isomer is a distorted octahedron with the two Tc-CO bond distances both 1.90(2) A. mer-[TcX3(PMezPh),] (X = C1, Br) reacts with CO (1atm) in refluxing MeO(CH,),OMe containing added phosphine to ). A variety of [Tc(CO),(PPh3),Ll give only cis-[T~X(CO),(PMe~Ph)~3(72 complexes, where L is a carboxylato, mixed amido, or thiazolato ligand (80),and [Tc(C~)~L{P(OR)~}~]PF, (L = bpy, 4,4-Mezbpy) (67) have been prepared. Crystal structures of the Schiff base complex [Tc(CO),(PPh,), {(C,H,NS)N =CHC,H,O-o}] (81 and the pseudoallyl complexes [ T c ( C O ) , ( P M ~ , P ~ ) ~ ( ~ - M ~ C , H , N ~ N ~ N C , H[TC(CO),(PM~,P~)~ ~M~-~)], ( P h N q ( M e F N P h ) l (821, and [Tc(CO),(PPh,)2{SC(NHPh)S}l(83) show distorted octahedral geometry with the two CO ligands mutually cis and the PPh, ligands trans. Structurally characterized complexes with a tridentate ligand are ci~-[Tc(CO)~(PPh,)(tan)]Cland cis-[Tc(CO),(PPh,){HB(pz),}] (74). Electrochemical oxidation of [ T c ( C O ) ~ C ~ ( P M ~ results ~ P ~ ) , ]in the formation of [Tc~~’(CO)C~(M~CN),

10

JOHN BALDAS

(PMe,Ph),](ClO4), (841, an example of the oxidation of one 18-electron species to another. Reaction of [HTc(N,)(dppe),] with CO in benzene or with methanol in the presence of pyridine gives [HTc(CO)(dppe),I.In the latter reaction methanol serves as the source of CO. On reflux in MeCN, [HTc(CO)(dppe),] is converted to [Tc(CO)(MeCN)(dppe),1PF6(85). 2 . Dimeric and Polynuclear Complexes

The dimers [Tc(CO),X], (X = C1, Br, I) are formed by the reaction of the halogen with [ T C ~ ( C O (45). ) ~ ~ IThe ease of thermal decarbonylation of [Tc(C0I5X] in a n inert solvent or during vacuum sublimation increases in the order I < Br < C1 and decarbonylation proceeds in the sequence [Tc(CO),X] + [Tc(CO),Xl, + [Tc(CO),Xl, (86).Decarbonylation occurs more easily than that for the Mn or Re analogs. The presence of four u ( C 0 )bands and the TcCO bending region in the IR spectra is consistent with the D,, halide-bridged structure ( 2 )for the dimers ( 8 7 ) and has been confirmed crystallographically by the isostructural nature of [M(CO),BrI, (M = Tc, Re) (88).

oc .co

co X

'

3'

I ,.-'cp

'X (2)

(3)

Structure 3 (X = Br), consisting of a cube with F,-B~bridges, was assigned to the tetrameric [Tc(CO),Br], on the basis of X-ray diffraction data (88).This is confirmed by the single-crystal structure determination of 3 (X = Cl), which shows that the tetramer has crystallographic Tdsymmetry with bond distances Tc-C, 1.903(3) A; C-0, 1.128(4) A; and Tc-C1, 2.559(1) A. The Tc-Tc distance of 3.840(1) A shows the absence of a direct Tc-Tc interaction (76).The reaction of [Tc(CO),C1I4 with chlorine is reported to give the trimer [(OC),Tcl(p-C1),Tcl" (p-Cl),T~'(C0)~1 (89). The reaction of thiols, sulfides, diarsines, and Hacac with [Tc(CO),Xl gives the dimers [TC(CO)~(SP~)I, (651, [Tc(C0),C1(EPh2)I2(E = P, S, Se, As) (90,91), [Tc(CO),BrLI, (L = THF,

11


MeCN; for which the IR spectra are consistent with a centrosymmetric structure) (711, and [Tc(CO),(acac)l, (92).Extensive mass spectral data have been reported (93). Partial carbonylation of NaTcO, in methanol gives the unprece(4) dented cubane-type structure N ~ [ T C ~ ( C O ) ~ ( O M ~ )(Fig. , I 21, with each Tc atom obeying the 18-electron rule. The Na+ cation in 4 forms one corner of the cube with Na-OMe distances of ca. 2.38 and Na-OC interactions (ca. 2.51 A) with adjoining cubes completing the coordination octahedron. In solution, 4 exists as the cubane cluster and not the Na' salt. The [TC,(CO)~(OM~),]group may thus be likened to a n anionic crown ether with a high affinity for Na' (94). The reaction of KTc0, with HCOOH gives [Tc(CO),OH],, which is most likely the cubic tetramer (3) (X = OH) (95). The cubic structure of 3 has been established crystallographically for [M(CO),(p.,-OH)], (M = Mn, Re) (96). Reaction of [TC,(CO),~]with rneso-tetraphenylporphine(H,tpp) or mesoporphyrin IX dimethyl ester (H,mp) gives the unusual dimers [L{Tc(CO),},] (L = mp, tpp) (97, 98). These dimers are also formed by the thermal disproportionation of [(HL)Tc(CO),].The crystal structure of [tpp{Tc(CO),},] shows the two Tc(CO), moieties arranged in a tripod 020

>-

~. 021

i

'p

022

k'

c21

012

'--\

c12

032

010 i

011'

L-c2

030

FIG. 2. The structure of N ~ [ T C ~ ( C O ) ~ ( O M (4)~ )(94). .J

12

JOHN BALDAS

configuration, with one on each side of the porphine ring and each outof-plane Tc atom coordinated to three N atoms. The Tc.-Tc distance of 3.101 is somewhat long to constitute bonding, but is short enough to indicate some metal-metal interaction (97).The dark-red air-stable heteronuclear [mp{(OC),TcRe(CO),}] is formed on heating [Hmp{Re(CO),}] with [Tc,(CO),,l in decalin (98).

B. CYCLOPENTADIENYL AND ARENECOMPLEXES q5-Cyclopentadienyl complexes of the type [Cp’Tc(CO),l may be prepared by the reaction of TcCl,/CO/Cu or [Tc(CO),Xl with NaCp’ or LiCp’ (99,100).Crystal structures of [LTc(CO),I (L = C5Me5,C,Me,Et, indenyl) (101) and [{Me3N(CH,)3C,Me,}Tc(CO)311(100)show the piano stool arrangement ( 5 ) . [CpTc(CO),I undergoes acylation on reaction with PhCOCl to give the PhCOCp derivative (102).

Cp’2Fe+ [Mn(CO),I]

OC‘

-

+ 99mT~04-

oc

co

99m

co (6)

(5)

The 9 9 m Tcomplexes ~ (6) (R = N-methylpiperidine, quinuclidine) may be prepared in 30-90% radiochemical yield by the route shown on heating for 1h r in THF a t 150°C. These esters show high brain uptake in animals (103). Irradiation of [(C,M~,)TC(CO)~] in cyclohexane produces the $-C5Me, carbonyl-bridged dimers 7 and 8. The structure of 7 was established crystallographically and that of 8 was confirmed by spectroscopic comparison with the structurally characterized Re analog (104 1.

Q


13

The short Tc-Tc bond distance of 2.413(3) A in 7 corresponds to a triple bond and for 8 a Tc-Tc single bond has been proposed. These bond orders are those needed to satisfy the 18-electron rule. As expected, the v(C0) IR absorptions in 7 occur at 1821-1771 cm-', whereas for 8, which contains terminal and bridging CO groups, the range is 2012-1738 cm-'. The [Tc(arene),]PF, (arene = benzene, substituted benzene, aromatic hydrocarbon) complexes are formed by reaction of the arene with TcCl,/AlCl,/Al (105,106).The cations are stable to the air and to acids and bases. A large number of [99"Tc(arene)2]+complexes have been prepared and the structure has been demonstrated by HPLC comparisons with the 99Tccomplexes. The lipophilic 9 9 m Tcomplexes ~ of benzene substituted with four to six carbon atoms show promising myocardial uptake (107). C. CYANO AND ISONITRILE COMPLEXES Olive-green K,[TC(CN)~] has been prepared by the reduction of Tc0,with KICN- and shown to be isostructural with K,[M(CN)6] (M = Mn, Re) (108).The low CN force constant of 14.57 mdyn A-' indicates that cyanide is acting as a relatively strong n-acceptor (109).Following the discovery that the [99"Tc(CNtBu)6]+ cation is concentrated in the human myocardium, this class of complexes has been intensively investigated in the search for improved imaging agents (19, 110). The air- and water-stable [TC(CNR)~]X salts may be prepared by the reaction of [Tc"'(tu),]Cl, with RNC but a more convenient method is the reduction of TcO,- by Na2S204in aqueous ethanol in the presence of the ligand (111,112).The energy of the v(CN) IR absorption is 50-80 cm-' lower than that in the free ligand, consistent with extensive .rr-donation from Tc(1). Reversible one-electron oxidation occurs at 0.82-0.88 V vs SCE for alkyl derivatives, with the phenyl derivative more difficult to oxidize at 1.18 V vs SCE (112).The 99TcNMR spectra show a single signal at about - 1900 ppm relative to TcO,-, with small but significant chemical shift differences due to the substituents (44,113). The crystal structure of [TC(CN%U)~]PF, establishes that the geometry is octahedral with Tc-C bond distances of 2.029(5) A and that the complex is isomorphous with the Re analog (114).Systematic variation of the R group has led to the development of [99"T~(CNR)6]+, where CNR is (2-methoxy-2methylpropyl)isonitrile, as a radiopharmaceutical for myocardial imaging (19).I n uiuo, the methoxy groups are sequentially metabolized to hydroxy groups to give seven products of increasing hydrophilicity and the resulting desired faster blood and lung clearance in comparison

14

JOHN BALDAS

with [99mTc(CN'Bu)6]'(115).At high pH [Tc(CNCMe,COOMe),ICl undergoes random base-catalyzed ester hydrolysis of the coordinated ligands. The nine possible carboxylic acid products have been isolated and identified by HPLC, FABMS, IR, and "Tc NMR (116). Mixedligand complexes of the type [TC(CNR),(CNR')~_,I+ ( n = 0-6) and [Tc(CN'Bu), (PPh,),-,]PF6 ( n = 4, 5) have been prepared by synthesis with a mixture of ligands (117,1181, and truns-[Tc(dppe),(CN'Bu),]PF, (119)and [HTc(CNR)(dppe),l (85) have been prepared by substitution of [HTc(N,)(dppe),]. Photolysis of [TC(CNR)~]PF, in the presence of bpy, phen, or mixed ligand synthesis from Tc0,- gives a series of complexes of the type [Tc(CNR),L]PF,. The crystal structure of [Tc(CN'Bu),(bpy)]PF, shows that one of the isonitrile ligands is considerably bent, with a C=N-C angle of 148",suggesting a "pseudo" internal oxidation of Tc(1) to Tc(II1) (120). Oxidative addition of chlorine or bromine to [TC(CN'BU)~]PF, produces the seven-coordinate [Tc"'(CNLBu),X](PF6), in 75% yield (121 ).

D. DINITROGEN, PHOSPHINE, PHOSPHITE, AND RELATED COMPLEXES Crystallography and 'H NMR have confirmed the formula [HTc(N,)(dppe),] for the product of the reduction of [TcCl,(PPh,),] by Na amalgam under nitrogen in the presence of dppe. The Tc atom is octahedrally coordinated with the hydrido ligand trans to dinitrogen. The Tc-N and N-N distances are 2.05(1)and 0.98(1)8,, respectively, and the Tc-N-N angle is 178(1)" (64). The ease of substitution of dinitrogen and hydride makes this compound a versatile starting material for the preparation of Tc(1) mixed-ligand complexes (85). On UV irradiation [TC(CO),(HB(~,~-M~~~~)~}~ reacts with nitrogen to give the air-stable dinitrogen-bridged dimer [{(HB(3,5-Mezpz)3)Tc(CO)z}~(~-Nz)l. The N-N bond distance of dinitrogen is 1.160(3)A and the Tc-N-N angle is close to linear at 174". In the electronic spectrum a band at 21,552 cm-' ( E = 3175) has been assigned to a Tc + N2(.rr*)MLCT transition (122). Excess ligand serves as the reductant in the preparation of [Tc(dmpe)&F3S03 from Tc04- and dmpe. EXAFS analysis of the fluoride salt has established octahedral geometry with a Tc-P bond distance of 2.40 8, (123). The [TcL,]' (L = dmpe, depe) complexes undergo reversible electrochemical oxidation to [ T C ~ ~ L ~a Ireaction ~', that may be chemically produced by H,02(124).The Tc(II)/Tc(I)couple for depe as ligand is 164 mV more negative than that for dmpe, indicating that [Tc(depe),]' is considerably more easily oxidized than [Tc(dmpe),l+ . Thus, both oxidation states are air-stable for dmpe as ligand, whereas


15

[Tc(depe)J2+is air-stable but [Tc(depe),I+ must be prepared under airfree conditions. Pulse radiolysis studies show that the oxidation of I ~the + strong oxidant C1,- proceeds at, [Tc(dmpe),] to [ T ~ ( d r n p e ) ~by or near, the diffusion-controlled limit (k = 1 x lo9 M-' sec-') by an outer-sphere mechanism (125). The self-exchange rate of the [T~(dmpe),]+'~+ couple has been calculated by application of the Marcus theory to be 2 x lo6 it-' sec-' (126, 127). The diamagnetic mixed phosphine-phosphite complex [Tc(dppe)(tmp),]PF, has been prepared by substitution of [HTc(N,)(dppe)J and characterized by FABMS and 'H and 99TcNMR (128).A number of homoleptic phosphite, phosphonite, and phosphinite cationic complexes of the type [TcL6]X [L = tmp, PR(OMe)2,PEt,(OMe)] have been prepared, either from TcO,- or by reductive substitution of [Tc"'(tu),]C1,, and characterized by '9'12 and 31PNMR, FABMS, or X-ray photoelectron spectroscopy (129-132).The [99mTc(dmpe)31+ and [99mTc(tmp),]+cations proved disappointing as potential myocardial imaging agents in humans due to slow blood clearance, although the clearance in dogs was fast. This species difference is due to the strong binding of the cations to a plasma component present in human but not in dog blood (24). +

E. COMPLEXES WITH NITROGEN LIGANDS Electrochemical reduction of a mixture of Tc0,- and phen allowed the isolation of the purple crystalline [Tc(phen),]PF6. Conductivity measurements in MeCN confirmed a 1: 1 electrolyte and cerimetric titration confirmed the + 1 oxidation state (133).

F. NITROSYL AND THIONITROSYL COMPLEXES By the reduction of (NH,),[TcCl,] with NH,OH-HCl and addition of ammonia Eakins et al. obtained pink crystals, which they formulated as a hydroxylamine complex (1341,but which were later shown to be the diamagnetic nitrosyl complex trans-[T~(N0)(NH~)~(OH~)lCl~ (135). The ammine ligands are remarkably stable to substitution in acid solution and the nitrosyl group is stable to nucleophilic attack. The crystal structure reveals bond distances of 2.168(4) A for Tc-OH, and 2.164(5) A (av.) for Tc-NH, (136).The short Tc-NO distance of 1.716(4) A and the relatively long N-0 distance of 1.203(6) A together with the low v(N0) IR absorption at 1680 cm-' and the surprisingly acidic water (pK, = 7.3) indicate very strong back-donation from Tc to NO (135).The NO group has been estimated to carry a half-negative charge, which assigns an oxidation state of +2.5 to Tc rather than the + 1 based on the NO+ formalism (136).Oxidation gives the green trans-

16

JOHN BALDAS

[Tc~~(NO)(NH,),(OH,)]C~, with u(N0) at 1830 cm-' and a highly acidic trans water (pK, = 2.0) (135).The novel hydride [Tc(NO)(PP~,)~(H),I is formed by borohydride reduction of a mixture of [ T c ~ ~ ( N O ) ( P P ~ ~ ) ~ C I , I and PPh, (137).The TcH IR absorptions occur at 1733 and 1185 cm-', and v(N0) at 1636 cm-' is shifted to 1659 cm-' in the dideuterio complex, indicating a strong coupling of the nitrosyl and hydrido ligands. Reaction of [Tc(CNtBu),]N03with NOPF, or HN03/HOAcgives a high yield of [Tc(NO)(CN~BU),](PF,)~. The high value of 1865 cm-' for u ( N 0 ) is consistent with NO+ coordination (138). Similarly, [(CSMe5)Tc(CO),]undergoes substitution of CO by the isoelectronic NO+ to produce a good yield of [(C5MeS)Tc(NO)(C0)21PF,,for which v(N0) occurs a t the lower value of 1745 cm-' as a result of the increased back-bonding induced by the anionic Cp' ligand (100). NBu,[Tc"(NOIBr,] is reduced by CN'Bu to the neutral truns-[Tc'(NO)Br2(CNtBu),] [u(NO) a t 1755 cm-'I, for which the stereochemistry has been crystallographically established (138). Electrochemical or hydrazine reduction of the deep-purple (NBu,),[Tc"(NO)(NCS),] results in rustcolored crystals of (NBu,),[Tc(NO)(NCS),l (139).The crystal structure of [Tc(NO)Cl(dppe),]Cl.H,O has been briefly described (140).Reaction of AsPh4[TcVOC1,]with an excess of cyclooctane-1,2-dioxime (codoH,) yields brown crystals, shown by crystallography to be AsPh,[Tc(NO)Cl(codoH),].HC1(9) (141).The nitrosyl group appears to be derived from NH,OH formed by partial hydrolysis of the dioxime. The intense v(N0) IR absorption occurs a t 1701 cm-'.

S PMe,Ph N I ,.' , \ '

0.

'''.H.''

...o

(10)

Although nitrosyl complexes have been long known, the first thionitrosyl complex was reported only in 1974 (142).The Tc-N group shows a marked tendency to abstract sulfur to form Tc(NS) complexes in the +1, +2, and + 3 oxidation states. The Tc'(NS) complex (10) is prepared by the reaction of [TcVNC1,(PMe,Ph),l with 1 eq. of S2C1, (143) and


17

mer-[Tc(NS)Clz(pic),]by a remarkable reaction in which [TcVINC1,labstracts sulfur from the Sz042-anion (144 1. The 4NS) IR absorptions occur at 1177 [shifted to 1147 cm-' on I5N labeling (14511 and 1173 cm-' for 10 and the picoline complex, respectively. The nearly linear Tc-N-S angles of 177" and 176" are consistent with coordination by NS'. Structure 10 shows a small but distinct shortening of the Tc-C1 bond trans to NS relative to the cis bond, whereas for rner-[Tc(NS)Clz(pic),] the reverse is observed with NSTc-Cl,,,, 2.430(2) A, and trans, 2.443(1) 8, (143, 144).

V. Technetium( II)

Technetium(I1) complexes are paramagnetic with the d5 low-spin configuration. A characteristic feature is the considerable number of mixed-valence halide clusters containing Tc in oxidation states of + 1.5 to +3. This area has been reviewed (42).For convenience, all complexes, except those of [TcZl6+,are treated together here. EPR spectroscopy is particularly useful in both the detection of species in this oxidation state and the study of exchange reactions in solution. The nuclear spin of "Tc (I = $1 results in spectra of 10 lines with superimposed hyperfine splitting. The d5 low-spin system is treated as a d' system in the hole formalism (40). A. ORGANOMETALLIC COMPLEXES Although carbonyl and cyclopentadienyl complexes are well known for Tc(1) and TdIII), none appear to have been reported for Tc(I1). This may be ascribed to the tendency to follow the 18-electron rule, which, due to the odd number of electrons, would require dimer formation for compliance. Similarly, no Tc(I1) cyano or isonitrile complexes appear to have been isolated.

B. HALIDECOMPLEXES AND CLUSTERS 1. Mononuclear and Binuclear Complexes

The only monomeric complex is the tetrahedral [TcBr,I2-, identified crystallographically in the product (11) of the remarkable reaction NBu4[TcV1NBr41 b y

[TcVNBr(bpy)212[TcuBr41.

(11)

18

JOHN BALDAS

The mechanism of formation of 11 is unknown. The Br-Tc-Br bond angles in ITcBr,I2- are approximately tetrahedral, in the range 106.1-112.1", and the Tc-Br bond distances of 2.388-2.417 A are very short (146). From the reduction of KTcO,/HCl by hydrogen (30 atm at 140"C), crystals of the d5-d5 cluster K2[Tc"zC1,].2Hz0 were separated (147, 1481. Structural analysis shows a polymer of Tc2C12-units with strongly distorted D4dsymmetry, linked by bridging C1 in infinite zigzag chains, with the very short Tc-Tc bond distance of 2.044(1) A. On the basis of the short Tc-Tc distance a "quintuple" bond was suggested (148).The crystal structure has been reexamined at 15 and -53°C and Tc-Tc bond distances of 2.047(1) and 2.042(2) 8, found (149). These very short distances are not anomalous for a triple bond with the ~ ~ ~ 7 relectronic ~ 6 ~ t configuration i ~ ~ because, even though the 6 bond order is 0, the low oxidation state of the [Tc214+core strongly enhances the c and 7~ bonding (149).The bromo complex K,[Tc,Br6].2H,0 has also been prepared (147).Both complexes are diamagnetic (150). The turquoise-blue salts (NH,)3[Tc2C1,1~2H,0and Y [TczC1,1.9H,0 of the mixed-valence d4-d5 [Tc,C1,13- anion, with an average oxidation state of +2.5, were first prepared in 1963 by the reduction of [TcCl6l2with Zn/HCl (1341. Improved synthetic methods have been developed and a variety of [Tc2C1,I3- salts, with inorganic or organic cations, is now known (42, 1511. Crystal structures are available for (NH,),[Tc,C1,].2Hz0 (152)and the K+ (153,1541, Y3+(155),and pyH+ (156)salts. Also, the metal cation may be partially replaced by H30+,as in the structurally characterized K',K"3-,(H30), [TczC1,13-nH,0 (157). The [Tc,C~,]~-anion possesses virtual D4,,symmetry with the square-pyramidal end groups in the eclipsed conformation (Fig. 3). The short Tc-Tc distances of 2.117(2) for the K' (154)and 2.1185(5) A for the pyH+ salt (157)and the observed paramagnetism [peff= 1.78(3) BM for the NH,' and Y3+ salts (15811 are consistent with a strong metal-metal bond electronic configuration (1591,a conclusion order of 3.5 and a cz7r4i326*1 supported by the EPR spectra (158) and self-consistent field Xa scat~ ~the first species in tered-wave calculations (160). In fact, [ T c , C ~ , ]is which a bond order of 3.5 was recognized (159).In the electronic spectrum of K3[Tc,C1,], the main component of the 15,700-cm-' (638-nm) band has been assigned to the 6" + 7 ~ transition, * and the band originating at 5900 cm-' in the near IR, to the 6 -+ 6" transition (161). The [ T c , C ~ , ] ~ - / [ T C ~ Ccouple ~ ~ ] ~ - is electrochemically quasireversible in HCl/EtOH (158).In HC1 solution [TczC1813-undergoes hydrolysis, disproportionation, and oxidation by oxygen, with rupture of the Tc-Tc bond (162 1. At 280"C, anhydrous (NH,)3[Tc,C1,1 starts to disproportion-


19

CL

L3

1 bCL2,

CL33*

FIG.3. The structure of the [ T C ~ C I ~anion ] ~ - in K3[Tc2Cl~l.nH20(154).

ate to (NH4)2[TcC1,]and Tc metal (163).The [Tc2Br8I3-anion is rather less stable than the chloro analog but the gold-colored ( N B U , ) ~ [ T C ~ C ~ ~ ] may be obtained in 70% yield by the reduction of (NBu,),[Tc2Cl8] with BH4- in CH,Cl,. The Cs3[Tc,C18] salt a t 6 K shows a well-resolved vibronic structure of the 6 + 6* transition, with electronic origin a t about 5970 cm-’ (164). Reaction of K,[Tc,X8]~2H20 (X = C1, Br) with glacial acetic acid results in substitution to yield green crystals of [TC~(OAC)~X] (42,1651, and reaction of (NH,)3[Tc2C18]with molten 2-hydroxypyridine (Hhp) gives the dark-green [Tc,(hp),Cl] (166). Crystal structures of these three complexes reveal the familiar “lantern” arrangement, with the bidentate ligands bridging the two Tc atoms of each cluster and axial halide bridging the clusters. Infinite linear chains occur in [Tc,(hp),ClI [Tc-Tc, 2.095(1) A; Tc-C1, 2.679(1) A] and [Tc,(OAC)~B~I [Tc-Tc, 2.112(1) A; Tc-Br, 2.843(1) A] ( 1 6 7 ~complexes ) and zigzag chains with a Tc-C1-Tc angle of 120”for [Tc2(OAc),C11[Tc-Tc, 2.117(1) A1 (I67b).

20

JOHN BALDAS

Also isolated from the acetic acid reaction is K[Tc2(OAc),C1,1, the structure of which shows a distinctly longer Tc-Tc bond distance of 2.1260(5) A, with two axial chlorides at 2.589(1) ( 1 6 7 ~ The ) . effective magnetic moment for the three acetate dimers is 1.78 -+ 0.05 BM and the EPR spectra are consistent with the unpaired electron equally shared by the two Tc centers in the 6" ( b l u )antibonding molecular orbital (168,169). 2. Polynuclear Clusters

The development of this area has been entirely due to the work of Russian chemists (42). Reduction of HTcO, in concentrated HX (X = C1, Br, I) by hydrogen under pressure yields a mixture of products with average oxidation states of +1.5-2.0 for Tc and with varying H20 and H30' contents (42, 170, 171). Crystal structure determinations have identified three basic structural arrangements of the Tc atoms (157, 172,173),the trigonal prismatic ([Tc,X,(p-X),]X,)"- (X = C1, Br; n = 2,3), (NMe,),[Tc,Cl,(p-C1),] (172, 174, 175),the octahedral [Tc,Br6(p3-

0 Br

FIG.4. The structures of technetium bromo clusters. (a)Fragment of the structure of the trigonal prismatic clusters (NEt4)2[Tc6Br6(~-Br)61Brz and (NMe,),[Tc,Br,(p-Br),lBrz (175). (b) Fragment of the structure of the octahedral clusters ~ H 3 0 ~ H 2 0 ~ 3 1 2 ~ T c ~ B r 6 ~ ~ g - B r )equivalent ~l. pg-Br atoms are shown but the posiBr)J and ( N B ~ ~ ) ~ [ T c ~ B r s ( p ~Eight tions are not fully occupied (I72).(c)Fragment of the structure ofthe tetragonal prismatic O ~ zOl ~, T c ~ B r , ~ ~ and - B r ~[H(Hz0)212[TcaBr4(~~lBr, clusters [ T C ~ B ~ ~ ( ~ - B ~ ) ~[ ]HB~PH~z H Br)a]Br2(I 72 1.


b

21

Y

C

Br1,I2- (1 72, 1761, and the tetragonal prismatic {[Tc,X4(p-X)83X,}l"(X = Br, I; n = 1, x = 1 , O ; n = 2, x = 2; DZhsymmetry) (1 72,177-178). Structural examples are shown in Fig. 4 (172,175). The main structural features of the novel trigonal and tetragonal prismatic clusters are the presence of dimeric Tc-Tc units with strong, localized multiple bonds of the order of 3.0-4.0 and Tc-Tc bond distances in the range 2.16-2.19 A, forming the vertical edges and a system of formally single Tc-Tc bonds (2.51-2.70 A) delocalized along the metal skeleton. In the octahedral (NBU,)~[TC,B~,( P ~ - B ~cluster, ) ~ ] the Tc-Tc bond distances are

22

JOHN BALDAS

2.578(1)-2.609(1) 8, (172). The trigonal prismatic cluster chlorides readily undergo substitution by bromide in HBr at -140°C, with the preservation of the Tc framework. The reaction goes to completion and no mixed-ligand clusters can be isolated. In acetone, [H(H,O),l,[Tc,Br,(p-Br)JBr, undergoes partial substitution with HI at room temperature. Addition of NBu,' gives brown crystals shown by a n X-ray structure determination to be the mixed-ligand tetragonal prismatic cluster (NBu4),[Tc,(Br,,51,,5)4(~-Br~,5p-Io.5~~]Iz (179). X-ray photoelectron spectra (180),magnetic properties, and EPR studies of the Tc clusters have been reported and the mechanism of Tc-Tc bond formation has been discussed (1811. A molecular orbital analysis of the trigonal prismatic [ T C ~ C ~ , ,shows ] ~ - electron-rich c ~ ~ 7 r ~ triple 6 ~ 6 bonding *~ in each dimer unit, single bonding in the triangles, and two electrons in a net antibonding a," orbital (7r* with respect to the dimers) so that the number of framework bonding electrons is 30. This is very different from the magic numbers of 16,24, or 14 known for octahedra and 18 for trigonal prisms (182).Recently, crystal structures of six ternary chalcogenides of the type M4[TcGX,,]( n = 12 or 13; X = S or Se; M = K, Rb, or Cs) have revealed the presence of octahedral Tc clusters with Tc-Tc bond distances of 2.60-2.65 8, (183). c . COMPLEXES WITH NITROGEN LIGANDS The reaction of [ T C " * C ~ , ( C H ~ C N ) ( P P with ~ ~ )bpy, ~ ] phen, and terpy yields the blue-black Tc(I1) complexes [TcLJP+ (L = bpy, phen) and [Tc(terpy),]*+,which may be isolated as the BPh4- or PF,- salts. The crystal structure of [Tc(bpy),](PF,), shows exact D3 symmetry for the cation with all Tc-N bonds distances 2.077(10)A. The cyclic voltammogram of [Tc(bpy),](BPh,), indicates three diffusion-controlled reversible one-electron reduction processes at El,, -0.34, -1.36, and -1.70 V vs SCE, corresponding to successive reduction of Tc" + Tc' +. Tco +. Tc-' (184, 185). An EPR study of [Tc(bpy),](PF,), has shown that the unpaired electron occupies the dxyorbital and that extensive metal-ligand covalent interactions reduce the spin-orbit coupling to about 49% (186).The effective magnetic moment of [Tc(phen),I2+is 1.89 BM, in good agreement with the spin-only value for a Tc(I1) octahedral d5 configuration(1 33 1. A variety of mixed-ligand complexes, including [ T C C ~ , ( P M ~ ~ P(L~=) ~bpy, L I phen) and [TcCl(PMe,Ph),(terpy)]PF,, has been prepared and electrochemically investigated. Crystal structures have established the coordination arrangement for the distorted octahedral (12) and truns(P )-[TcBr(PMe2Ph),(terpy)lS03CF, (187).

23


PMe-Ph

n

N

N = phen

(13)

(12)

Crystallography, FABMS, and EPR spectroscopy have shown that the deep-purple major product isolated from the reaction of [Tc"'Cl,(PPh,),(MeCN)l with tris(2-aminoethy1)amine and 2-pyridinecarboxaldehyde in methanol is [Tc"(tren-py,)l(PF,>, [tren-py, (1311 (188).The mean Tc-imine and Tc-pyridine nitrogen bond distances are 2.071 and 2.109 A, respectively, and the Tc-+ertiary amine nitrogen distance is 2.933(7) A. The coordination geometry has been described as pseudoseven coordinate capped octahedral. It is arguable whether the long Tc-.Ntert distance constitutes coordination, but a Tc-.N interaction is indicated because the lone pair on this nitrogen is directed toward the Tc atom.

D. PHOSPHINE, ARSINE,AND RELATED COMPLEXES In view of the large number of mixed halide-phosphine complexes of the binuclear [Re,14' and [Re215+cores (1891,it is perhaps surprising that no such complex has been reported for technetium, but this may simply reflect the lack of work in this area. Mononuclear phosphine and arsine complexes are, however, common. The first Tc(I1) complex to be reported was trun~-[Tc(diars)~I~l in 1959 (190),followed by the chloro and bromo analogs in the next year (191). These complexes together with tr~ns-[Tc(dppe)~X~I (X = C1, Br) were prepared by SO2or BH4- reduction of the [Tc1''L2X21Xsalts and characterized by electronic spectroscopy, magnetic measurements (pefl= 2.05, 2.28 BM), and by being shown to be isostructural with the Re analogs (192).These conclusions have been confirmed by a crystal structure determination of truns[Tc(dppe),Cl,I (193).The rather long Tc-C1 bonds (av., 2.424 A) undergo a dramatic shortening of 0.105(2) A on oxidation to truns[Tc"'(dppe),Cl,I +,behavior consistent with a ligand that binds primarily by cr-donation. In contrast, the Tc-P bond lengthens by 0.072(2) A on oxidation from Tc(I1) to Tc(II1) due to r-back-bonding from Tc to P

24

JOHN BALDAS

being less favored in the higher oxidation state. In nonaqueous media truns-[Tc"(dppe),X,] is readily oxidized by a variety of one-equivalent oxidants. The rate of reduction of [(en),Co{S(CH,C,H,Me)CH,CH,NH2}I3+by [Tc(dppe),Cl,] in MeCN a t 25°C is rapid [it, = 3.0(7) x lo4 M -1 sec-'1 (193). Spectroelectrochemical studies of the structurally characterized truns-[T~(dppe),(NCS)~I show that the r-acceptor ability of N-bonded thiocyanate results in a marked stabilization of the lower oxidation state (194).The cationic complexes "I'cL~](PF,)~(L = dmpe, depe) are prepared by HzOz oxidation of [Tc'L,]PF,. The reduction of Tc0,- by excess dmpe gives a mixture of products in the +5, +3, and + 1 oxidation states, but neither truns-[T~~~(dmpe),Cl,I nor [Tc"(drnpe),l2+ appears to be a predominant component (124). Reaction of truns[TcvO2(dmpe),1 with halide may be used to prepare trans-[Tc(dmpe),X,] (1231. A series of cationic dithiocarbamato complexes [ T c ( S , C N R , ) ( ~ ~ ~ ~ )has ~ ] Precently F, been similarly prepared, and the crystal structure of [Tc(S,CNMe,)(depe),lPF,, determined (195).The dithiocarbamate is bidentate and the coordination geometry is octahedral. The E"' values of 0.298 to 0.312 and -0.517 to -0.544 V vs Ag/ AgCl for the Tc(III)/Tc(II)and Tc(II)/Tc(I) couples, respectively, show that dithiocarbamato ligands effectively stabilize the Tc(I1) oxidation state. A crystal structure determination has shown that the room temperature reduction of the tetramethylthiourea complex [TcvO(tmtU),](PF& by dppe in dmf solution is accompanied by a novel rearrangement to give the dithiocarbamato complex [Tc"(dppe),(S2CNMez)1PF6 (196).The mechanism of the rearrangement is uncertain, but the overall reaction is probably described by +

-

2(Me2N),CS+ HzO

Me2NCS2H+ (Me2N)&0 + MezNH

An interesting mixed-ligand complex is [Tc(dppe),(ox)l, prepared by the reduction of Tc04- by dppe in hot ethanol in the presence of oxalic acid, The average Tc-0 bond distances of 2.13(1) and 2.12(1)A in two independent molecules are somewhat longer than those found in other oxalato complexes (197).Reaction of (NH,),[TC'~C~,]with diethyl phenylphosphonite in the presence of BH,- yields the yellow octahedral truns-[Tc"{PhP(OEt),),C1,l, with Tc-P and Tc-Cl bond distances both 2.41(1) A. The low measured magnetic moment of 1.4 BM has been ascribed to partial decomposition (198).

E. COMPLEXES WITH SULFUR LIGANDS The reaction of truns-[TcVO(OH)(dmpe),12fwith excess 4-chlorobenzenethiol yields a mixture of the air-stable black cis- and red truns-


25

[Tc(SC6H,C1-p),(dmpe),I,which may be separated by fractional crystallization. Crystal structures have been determined for both complexes and show Tc-S-C angles of 114.0(3)" and 123.8(2)' for the cis- and trans-isomers, respectively. In CH2C12,the trans-isomer converts to the more stable cis-isomer with a half-life of about 74 min a t room may be temperature (199).Similarly, trans-[T~~O(OH)(diars)~](PF,), converted to [Tc**(SR),(diars),I(trans, R = Me, Bz; cis and trans, R = Ph), which can then be oxidized to the Tc(II1) complexes. The reversible Tc(III)/Tc(II) couple is in the range -0.32 to -0.47 vs Ag/AgCl. The crystal structure of trans-[Tc(SPh),(diars),] shows a Tc-S-C angle of 119.5(3)" (200). Reduction of NBu,TcO, by SnC1, in the presence of 1,4,74rithiacyclononane yields the dark-brown homoleptic complex [TcL,](BF,),.MeCN, with Tc coordinated by the two thioether ligands in a fac tridentate "sandwich" fashion. The three independent Tc-S bond distances are in the narrow range 2.372(3)-2.381(3) A. Electrochemical oxidation yields the yellow Tc(II1) complex and reduction yields the air-stable cherry-red Tc(1) complex (201). F. NITROSYL AND THIONITROSYL COMPLEXES The first Tc(I1) nitrosyl complex to be identified was the green trans[Tc(NO)(NH3),(OH,)]Cl3, prepared by Ce(1V) oxidation of the pink trans-[Tc1(NO)(NH3),(OH~)]Cl~ (135).From the reaction of Tc02.nH,0 with NO gas in 4 M HBr, red crystals of NBu,[Tc(NO)Br,I may be isolated (139).The NBu,[Tc(NO)X,l (X = C1, I) complexes are prepared by halide substitution (34, 202) and the ink-blue (NBu,),[Tc(NO)(NCS)5]is prepared by substitution with NCS- (139),whereas (ASP~,),[TC(NO)(NCS)~] may be prepared by the reduction of TcO,by NH,OH.HCl in the presence of NCS- (203). The preparation of (NBu4),[Tc(NO)C1,l has also been reported (202). The structure of (NBu,)trans-[Tc(NO)C14(MeOH)] shows a near-linear Tc-N-0 angle of 175.5(10)" and Tc-N and N-0 bond distances of 1.689(11) and 1.171(15) A, respectively. The Tc-O(H)Me distance is fairly short at 2.128(7) A (2041,a reflection of the wacceptor nature of the NO+ ligand. For the halide complexes v(N0) is observed in the IR spectrum a t -1800 cm-'. Reaction of [Tc(NO)Cl,]- with bpy or phen yields [Tc(NO)Cl,LI (205). Black-green crystals of [ T C ( N O ) C ~ , ( P M ~ ~ P ~ ) ~ ] (206) and dark-green [Tc(NO)Cl3(PPh3),1[v(NO) at 1805 cm-'1 (207) are formed by the reaction of NO with [TcCl,(PMe,Ph),] and [TcC1,(PPh3),(MeCN)1,respectively. Red (AsPh4)mer-[Tc(NO)C1,(acac)l (14) is formed when acacH is added to a solution of [Tc(NO)C1,XIn(X = C1, H,O), prepared by the addition of NH,OH.HCl to [TcC1,I2-.

26

JOHN BALDAS

The Tc-N-0 angle is substantially bent at 158.6(33)"and the Tc-O,,,, bond distances cis and trans to the NO+ ligand are not significantly for potendifferent at 2.06(1)and 2.08(1)8, (208,209). [99"T~(NO)C141tial radiopharmaceutical applications is formed in high yield by the addition of NH20H.HC1 to a previously heated 99"Tc0,-/HC1 solution and may be extracted by CH,Cl, after the addition of NBu,Cl (210).

1-

(14)

(15)

The AsPh,[Tc(NS)X,I (X = C1, Br) salts are prepared by the reaction of (ASP~,)~[TCX~I with (NSCl), and AsPh,[Tc(NS)(NCS),I is prepared by ligand exchange. In the case of [TcBr,12-, mixed-ligand complexes [Tc(NS)Cl,Br,-,l- ( n = 1-31 are formed and the addition of HBr is required to effect full substitution and to give red-brown crystals of AsPh,[Tc(NS)Br,]. The d N S ) IR absorptions occur a t 1232-1214 cm-', and for the thiocyanate complex the NCS deformation mode at 501 cm-' indicates that these ligands are N-bonded. In solution, [Tc"(NS)X,] readily loses sulfur to form the nitrido complex [TcV1NX41-(211). Treatment of [TcVNC1,(PMezPh),]with a n excess of S2C1, in CH2Clz under argon has been shown by FABMS and crystallography to yield not [Tc"(NS)C~,(PM~,P~)~I but the phosphine oxide complex (15) (2121,and presumably this also applies to the bromo complex prepared by ligand exchange in HBr (213).The source of oxygen for the oxidation of the phosphine in 15 is not clear. The v(NS) IR band at 1240 cm-' has been confirmed by a n isotope shift to 1206 cm-' on 15N labeling (145).On reaction with excess PMe2Ph, 15 undergoes sulfur abstraction to yield the starting material [TcVNC1,(PMe2Ph),]. The linear Tc-N-S [179.9(1)"]angle and the Tc-N and N-S bond distances of 1.7466)and 1.521(5) 8,, respectively, in 15 are consistent with NS' coordination (212).EPR spectroscopy has confirmed that the product of the reaction of [TcVNL2][L = N-(N"-morpholinylthiocarbonyl)benzamidinate~1-)1 with S2C12is [Tc"(NS)Cl,L] (214). The EPR spectra of low-spin 4d5 Tc(I1) nitrosyl and thionitrosyl complexes have been examined in detail and the results, reviewed (39-41 ). -


27

Complexes with the most n-bonding between Tc and the equatorial ligands show the largest glland smallest All, as seen by gll = 1.985, and All= 260 x cm-' for [Tc(NO)Cl,l2- andgll= 2.262, andAli= 155 x cm-' for [Tc(NO)I,]- (202, 215). In the case of [Tc(NO)(NH3)4(OH2)13+, for which there is no r-bonding in the equatorial plane, cm-' (216).EPR spectroscopy is particugll= 1.861 and Ail = 297 x larly useful for the study of ligand exchange such as that between [Tc(NO)Br,]- and [Tc(NO)C1,12- or [Tc(NO)141-(217).The g values and 99Tc hyperfine coupling constants are proportional to the spin-orbit coupling constants of the donor atoms and the composition of the mixedhalide coordination sphere may be unambiguously assigned (40). VI. Technetium( III)

The coordination chemistry of this oxidation state is rather more extensive and varied than that of Tc(I1). With appropriate ligands Tc(II1) is water- and air-stable, and cationic complexes with bidentate phosphine and arsine ligands have been extensively studied in the search for myocardial imaging agents. A marked difference between Tc and Re is the absence, at present, of any Tc analog of the extensive chemistry based on the trinuclear [Re3I9+core (189).A notable feature is that nearly all the seven-coordinate Tc complexes are found in this oxidation state. This is understandable in terms of the d4 electronic configuration of Tc(III), which requires seven singly bonded ligands to achieve an 18-electron count. A. CARBONYL COMPLEXES Yellow plates of the seven-coordinate [TcC1,(CO)(PMe2Ph),1~EtOH are formed by passing CO at atmospheric pressure through a refluxing solution of rner-[TcCl3(PMe2Ph),1in ethanol. The structure possesses approximate C,, symmetry and may be described as a distorted capped octahedron with the CO ligand inserted along the C3 axis on the phosphine face [Tc-C-0 angle, 178(2)"1(218).The pentagonal-bipyramidal complex (16) (Fig. 5 ) was unexpectedly prepared by the reduction of Tc04- with formamidinesulfinic acid [NH2(NH)CS02H]in the presence of Na(S2CNEt2).The CO stretch occurs as a n intense band at 1895 cm-' in the IR spectrum. The mechanism of the formation of the coordinated CO is unclear; a scheme that involves initial loss of SO2 from the coordinated sulfinic acid has been proposed. The CO ligand occupies an apical position with a near linear Tc-C-0 angle of 177.8(10)" and

28

JOHN BALDAS

J: w

C15

C

1

3

c12

FIG.5. The structure of [Tc(S,CNEt&CO)I (16)(219).

Tc-C and C-0 bond distances of 1.861(12)and 1.15(1)A, respectively. A comparison with the isostructural [Re(S2CNEt2),(CO)Iindicates that Tc(II1) is a poorer n-donor than is Re(II1) (219). Alkyl dithiocarbamate derivatives of 16 have been prepared by the above method [v(CO) a t 1907-1895 cm-'I and the CO ligand shown to be inert to substitution by EPh, (E = P, As) (220).The CO ligand is, however, readily substituted by the isoelectronic NO to give seven-coordinate [Tc(NO)(S2CNR2),l+cations (2211. A variety of [99mT~(SzCNR2)3(C0)1 complexes has been prepared by reduction of TcO,- with S2042-in the presence of CO and found to behave as hepatobiliary agents when injected into mice (222). A six-coordinate carbonyl complex is truns-mer-[TcCl3(PPh3)2(CO)I,prepared by the bubbling of CO through a solution of rner-[T~Cl,(PPh,)~(MeCN)l. The long Tc-CO bond of 1.985(9) A, the short C-0 bond of 1.12(1) A, and the high IR v ( C 0 ) absorption at 2054 cm-' indicate the absence of significant n-back-bonding in this complex (207). The bubbling of CO through a solution of [Tc(SAr),(MeCN),I (SAr = tmbt) yields orange crystals of [Tc(SA~),(CO)~]. One CO ligand may be displaced to give [Tc(SAr),(CO)(MeCN)]and [Tc(SAr),(CO)(py)l. Crystal structures show trigonal-bipyramidal coordination for both monocarbonyl complexes, with the three S atoms of the sterically hin+


29

dered thiolates occupying the equatorial plane and the CO and MeCN or pyridine ligands in the axial positions (223). B. CYCLOPENTADIENYL COMPLEXES The product of the BH,- reduction of TcCl,/NaCp in THF is the hydrido complex [HTcCp,], analogous to [HReCp,] and most likely with the same bent structure. The basic nature of [HTcCp,] is shown by the equilibrium

On addition of PF,- the rather insoluble [H,TcCp,IPF, salt precipitates (224).The TcCl,/KCp reaction in THF yields the diamagnetic airstable [TcCp,Cl], which on reaction with KCp gives the red diamagnetic [Tc(q5-Cp),(q'-Cp)], in which one ring is a-bonded (225).The structures are shown in Fig. 6. The oxidation of [(q5-C5Me5)Re1(C0),1by H202 yields the trioxo compound [(q5-C,Me5)ReV"O31,but with [(q5C,Me,)Tc'(CO),] the product has been assigned the polymeric structure (171,with Tc in the +3.5 oxidation state (226).

The planes of the C5Me5rings and the bridging oxygens are exactly parallel but the most striking feature is the exceptionally short Tc-Tc bond distance of 1.867(4) A. The Tc-Tc bonding hat3 been described as ~ r ~ ( , r r t 3 ) ~with 8 * , a net bond of approximate order 2.5 (227). IR absorptions at 909 and 880 cm-' have been assigned to v,,(TcO) and vWm (TcO), respectively (226).The crystal structure determination of 17 has, however, been questioned and it has been suggested that the (228).Treatment product obtained may in fact be [(q5-C5Me5)TcVI1O31 of [Tc1(C5Me5)(CO>,1 with Br,/CF,COOH gives [ T c ( C ~ M ~ ~ ) ( C Oas )~B~~I a mixture of cis- and trans-isomers (100).

30

JOHN BALDAS

c1

FIG.6.

C. CYANO, ISONITRILE, AND THIOCYANATO COMPLEXES The only Tc(II1) cyano complex is the seven-coordinate yellow-orange K,[TC(CN)~].~H~O prepared by the reaction of (NH,)2[Tc'VI,l with KCN in methanol under nitrogen. Raman and IR spectra indicate a pentago-


31

nal-bipyramidal structure symmetry) both in the solid state and in solution. In aqueous solution K4[Tc(CN),1.2H2Ois oxidized by air to [Tc"O(CN)J- (229).Seven-coordinate isonitrile complexes of the type [Tc(CNR)~X](PF~)~ (X = C1, Br) are formed by the oxidative addition of chlorine or bromine to the six-coordinate [Tc'(C"),]+ (1211. The reaction of (NH,),[TcX,I (X = C1, Br) with NH,NCS produces a mixture of the intensely purple ["I'C'"(NCS)~]~(Amm = 500 nm; E = 76,200) and = 400 nm) anions (230). the air-sensitive, yellow [Tc"'(NCS),I3- ,A( The redox couple [TC(NCS)~]~+ e [TC(NCS)~]~is electrochemically reversible (El,, = 0.18 V vs SCE) and the reduction of [Tc1"(NCS),l2is readily produced chemically by hydrazine. The crystal structure of (NBU,)~[TC(NCS)~] shows near-perfect octahedral geomery and establishes that thiocyanate is N-bonded with a n average Tc-N-C angle of 173(2)".

*

D. AQUA,HALIDE,AND RELATED DIMERIC COMPLEXES No mononuclear Tc(II1) halide is known. Thin-layer spectroelectrochemical techniques show that the [Tc'"X6I2- (X = C1, Br) complexes undergo a reversible one-electron reduction in HX/NaX aqueous media with the loss of 2.7 0.1 chloro ligands and 5.9 0.5 bromo ligands, respectively. These results indicate a low affinity of Tc(II1) for halide and the possibility of preparing [ T c ( O H ~ ) ~in] ~a+weakly coordinating aqueous medium (2311, The stability of the mixed-valence [ T C " ' ~ ' ' ~ C ~ ~ ] ~ and the apparent inability to prepare [ T c " ' ~ C ~ ~were I ~ - a puzzle for a number of years in view of the stability of [Re2C1,12- and the only fleeting existence of [RezC1813-(159). However, in 1980 the brightgreen (NBU,)~[TC~C~,] was prepared by the reduction of [TCCl6I2-with Zn/HC1 and converted to the carmine-red ( N B U ~ ) ~ [ T C ,by B ~ligand ~I by exchange with HBr (232). A recent synthesis of (NBU,)~[TC~C~,] reduction of NBu,[TcOCl,] with NBu,(BH4), followed by carefully controlled air oxidation of the initially formed brown product in CH2C1, in the presence of HC1 gas, gives yields of up to a 85% (164).The crystal structure of (NBu,),[Tc,Cl,] shows that the quadruple Tc-Tc a2n46 bond is, a t 2.147(4) A, about 0.04 A longer than the u2n46%*bond in [Tc,C~,]~-(bond order, 3.5)(233).This unexpected result is due to the greater influence of the change in the oxidation state of Tc than of the weak 6 bonding on the Tc-Tc bond distance (149). A number of complexes of the [Tc,]~' core have been prepared, either by oxidative substitution of [Tc2C1,I3- (234, 235), by substitution of [TC,C~,]~(236,2371, or by the reduction of Tc0,- by hydrogen in the presence of ligands (238). The crystal structure of the red [Tcz-

*

*

32

JOHN BALDAS

c4

FIG.7. The structure of cis-[T~~(OAc)~Cl,(dmaa)~] (237).

(00CCMe3)4C12]reveals a lantern structure, with the four pivalato ligands bridging the two Tc atoms in the eclipsed configuration (D4h symmetry), a Tc-Tc bond distance of 2.192(2) A, and axial chlorides with Tc-Cl bond distances of 2.408(4) A (234).Similar structures are found in [Tc,(OAC)~(TCO~)~I [Tc-Tc, 2.149(1) A1 (239) and K,[Tc, (SO,),(OH,),] [Tc-Tc, 2.155(1) A] (235,157).Reaction of [TczC1,12- with Ac20/HBF4yields czs-[Tcz(OAc)2C1,(OH,)21,in which the weakly bound axial water ligands are easily replaced by donor bases to give the green adducts cis-[Tc,(OAc),Cl,L,] (L = dmf, dmso, OPPh3, py). The structure of the dimethylacetamide (dmaa)adduct [Tc-Tc, 2.1835(7) A; Tc-Odmaa, av., 2.320 A] is shown in Fig. 7 (237). Orange-red [Tcz(OAc),Br,l is prepared by reaction of [Tc2BrsI2-with HOAc/Ac,O (236).A characteristic feature in the electronic spectra is the 6 6" transition at 600-700 nm (232,237).Normal coordinate analyses of [Tcz(OAc),Xzlgive Tc-Tc force constants of 4.08 and 3.99 mdyn for the chloro and bromo complexes, respectively (236). The Tc-Tc IR and Raman stretching frequencies of cis-[T~,(OAc)~Cl,L~l are lowered with increasing donor strength of the axial ligand L (240). Magnetic studies of [Tc2I6+complexes show only temperature-independent paramagnetism (150).

-

E. CARBOXYLATO AND P-DIKETONATO COMPLEXES Technetium(II1) complexes with aminocarboxylato ligands have been reported but none are well characterized (2411. 99mTc-iminodiacetate complexes formed with 2,6-alkylphenyl [ArNHCOCH2N(CH2C00)212ligands are used to image the hepatobiliary system. Studies with "Tc show evidence for [TC"~L,]-in the radiopharmaceutical preparations,


33

but [TcVOL,]- is also possible (19).A variety of Tc(II1) acac complexes and substituted analogs has been prepared by substitution/reduction of [TC'~X,(PR,),]and [TcrVX6Iz-or by S z 0 2 - reduction of Tc0,- in the presence of the ligand. These include [Tc(acac),l (2421; the dipivaloyl, trifluoro, and hexafluoro analogs (243); and [TcX(acac),(PPh,)I and [TcX,(a~ac)(PPh,)~l (X = C1, Br) (242).The S2O:- reduction method is suitable for the preparation of the neutral lipophilic 9 9 m Ttris ~ complexes, but these show little brain uptake (244).The cationic [Tc(acac),(MeCN),]ClO, is formed by the reaction of [Tc(acac),I with MeCN in the presence of HClO, (245).The crystal structure of [Tc(acac),I shows closely octahedral coordination, with a n average cis 0-Tc-0 angle of 90.2"and Tc-0 distances in the range 2.013(6)-2.030(6) (246).The structures of two crystalline forms of truns-[TcCl(acac),(PPh,)3,which show differences in the IR spectra, have been reported (247,248).The kinetics of ligand exchange of [Tc(acac),] have been studied by the use of 14C-Hacac, and the I, mechanism has been assigned to the rate determining formation of a n intermediate containing one monodentate acac and Hacac ligand (249).The base hydrolysis of [Tc(acac),I is kinetically more complex than that of [Ru(acac>,l (250).A variety of sixcoordinate tris complexes of monothio-P-diketonates has been prepared by substitution of [Tc(tu),]Cl, in refluxing methanol and characterized by IR, electronic, and mass spectrometry; 'H NMR; and, for the phenyl derivative [Tc{SC(Ph)CHC(Ph)O},],a crystal structure determination (251, 252).

F. COMPLEXES WITH DIOXIMES, SCHIFF BASES,AND OTHER NITROGEN LIGANDS An alternative approach to cationic myocardial imaging agents has been the development of neutral seven-coordinate Tc(II1) complexes based on 1,2-dioxime ligands (dioximeH,) with one end capped by a boronic acid derivative (19, 253). These complexes are generally referred to as BATOs (boronic acid udducts of technetium dioximes) and have the general structure [TcX(dioximeH),(dioxime)BR'](X = C1, Br; R' = alkyl) shown in 18. At the uncapped end the three dioxime oxygen atoms are intramolecularly bonded to two bridging protons. BAT0 complexes are prepared by template synthesis from TcO,- and Sn2+or from NBu,[TcOCl,], M,[TC&] (X = C1, Br) in the presence of the dioxime, HX, and the alkylboronic acid (254).The formation of BATOs from TcO,- and Sn2+ proceeds via several intermediates, including an Sn-monocapped [Tc"'(dioximeH),(p-OH)SnC13],which undergoes acid decomposition to give

34

JOHN BALDAS

R

the uncapped [Tc"'X(dioximeH),(dioximeHz)] complex. The uncapped tris complex is then monocapped by the boronic acid (255).Interestingly, although bidboron-capped) clathrochelates [M(dioxime),(BR),] have been known for a number of years for M = Co, Fe, and Ru, the BATOs are the first monocapped examples. Crystal structures have been reported for [TcBr(cdoH),(cdo)BRl (R = Me, Bu) and [TcBr(dmgH),(dmg)BRl (R = Me, Bu) (254);the structure of the n-butyl dimethylglyoxime complex is shown in Fig. 8. The six nitrogen atoms form a distorted trigonal prism monocapped by Br, which causes the two flanking dioximes to be spread away by about 20"toward the third dioxime ligand, thus probably precluding the addition of a second boron cap (2541. The cdoHz derivative [99mTcCl(cdoH)2(cdo)BMel is a radiopharmaceutical for differentiating normal from ischemic and infarcted myocardium (19).The axial chloride is labile to substitution and under physiological conditions is replaced by a hydroxy group with pK, between 7 and 7.4, which indicates that there may be a n equilibrium in uiuo between the neutral hydroxy and cationic aqua forms (24, 256, 257). The lability of the axial chloride is consistent with X-ray photoelectron spectra of "Tc BATOs, which show that the binding energy is between that for covalent and that for ionic bonds (258).The mechanism of chloride-hydroxide exchange has been shown to be S,l-CB, proceeding via a transient neutral six-coordinate complex (256).Electrochemically, chloro and bromo BATOs undergo a n irreversible twoelectron reduction that appears to be biologically inaccessible (259). The S- and N-bonded isomers [TcL(cdoH),(cdo)BMel (L = NCS, SCN) have been prepared. In solution, the S-isomer converts to the N-isomer when exposed to light (257).Cationic BATOs have also been prepared (260).The reaction

-

Tc04-+ 3 dioximeHz + 2 SnClZ

[Tc"'(dioximeH)3(p-OH)Sn1VC13] (19)


35

FIG.8. The structure of [TcBr(drngH),(dmg)BBu](254).

proceeds to completion. The oxygen bridge between SnIVand Tc"' is most likely in the hydroxyl form. Acid decomposition of 19 yields [TcCI (dioximeH),(dioximeH,~],which may be reconverted to 19 in 97% yield on reaction with SnC1, (255).When the crystal structure of [Tc(dmgH),(p-OH)SnC131~3H,0 was reported in 1976 the oxidation state of Tc was thought to be + 5 (2611,but a + 3 oxidation state is indicated by FABMS and the chemical behavior of 19 (255).The Sn" center is six coordinate, with the three chloro ligands in a fuc arrangement, and, in addition to the hydroxy bridge, two oxime oxygens complete the octahedral coordination sphere (2611. It may be noted that a + 3 oxidation for Tc is also consistent with an 18-electron seven-coordinate species. In the structurally characterized [T~~~~Cl(dmgH),(drngH~)l, the

36

JOHN BALDAS

four protons are shared by the three oxygens on each trigonal face and in the lH NMR spectrum appear as a broad singlet at 15.3 ppm (255). Crystallography has established that a by-product of the reaction of dmgH~/[TcC1,(MeCN)(PPh3),l/EtB(OH), is [TcCl(dmg)(dmgH)(butane2,3-dioneimineoxime)BEtl, where one of the uncapped C=NOH groups has been converted to C=NH and there is only one intramolecularly bound proton (262).A variety of seven-coordinate Re analogs of BATOs, uncapped [Re"'C1(cdoH),(cdoH~~],and monocapped [Re"'Cl(cdo)(cdoH),BR] has been prepared. Yields from Re0,- are low but [ReC13(MeCN)(PPh,),] is a suitable starting material. As with Tc, the biscapped Re complexes could not be prepared. Reaction of Mn(OAc),/ cdoH,/(OH),BPh/MeOH, however, gives a high yield of the biscapped six-coordinate [Mnll(cdo)(cdoH),{B(OMe)Ph}~l, in which each cap is covalently bonded to two oxime oxygen atoms (263). A series of cationic [Tc(L)(PR3),1PF,(PR3 = PEt3, PEtzPh, PEtPh, , PPh3)complexes of tetradentate (acac),en ligands and aromatic derivatives has been prepared by substitution/reduction of [TcOCl,I- (2641. The E for the reversible Tc(III)/Tc(II)couple is sensitive to the nature of the substituents on the Schiff base and the phosphine but is in the range -1.11 to -0.69 V vs Ag/AgCl (265). These cations are thus essentially biologically nonreducible and the 9 9 m Tcomplexes ~ are of interest as potential myocardial imaging agents. All complexes exhibit characteristic MLCT bands in the visible region, the energy of which correlates linearly with the potential of the Tc(IV)/Tc(III)and Tc(III)/ Tc(I1) couples (2641. The crystal structure of trans-[Tc{(acac),en}(PPh3),]PF6shows approximate octahedral coordination with the tetradentate Schiff base in the equatorial plane (264).The thio derivative [Tc{(sacac),en}(PPh,),]PF, has also been prepared from NBu,[TcOCl,I (266).A rather mixed coordination sphere is present in [Tc(quin)(PR,)L] (L = 20; E = 0, S; PR, = PMe,Ph, PEtzPh, PPh,). The crystal structure of [Tc(quin)(PEt,Ph)Ll (L = 20; E = 0)shows approximate octahedral O'

(E = 0, OphsalH,) (E = S, SphsalH,) (20)

P = PMe,Ph

(21)


37

geometry with the tridentate ligand L spanning three mer positions and the phosphine trans to the quinoline nitrogen (267). Crystallography has shown that the product of the reaction of a Schiff-base dithiocarbazate ester derivative (H,L) with [TcOCl,]-/PPh, is the octahedral cis(CZ)-truns(P)-[TcCl2(HL)(PPh3),1,where HL functions as a bidentate S,N-ligand (268). For [TcClL,(PMe,Ph)] (L = N-phenylsalicylidineiminate),the two bidentate chelate ligands are mutually orthogonal and the chloro and phosphine ligands, cis to each other (269).Novel complexes are [{TcL(PR,),},(p-O)] (L = 20; E = 0, S; PR3 = PMe,Ph, PPh,), which represent the only examples of Tc(II1) (21) the oxo-bridged dimers. In [(T~(Ophsal)(PMe~Ph)~},(p-O)] Tc-O-Tc angle is near-linear at 176.1(14)"and the Tc-Obridgedistances are 1.81(2) and 1.87(2) (270). The complexes rner-[TcC13L31(L = py, pic) and mer-[TcCl,(pic)(PMe,Ph),l are prepared from NBu4[TcOC1,1 dissolved in neat pyridine or picoline by the use of a phosphine as the oxygen acceptor. Linear correlations of reduction potentials in dmf with electrochemical ligand activity parameters are observed for the Tc(IV)/Tc(III), Tc(III)/Tc(II), and Tc(II)/Tc(I) couples. Crystal structures for mer-[T~Cl~(pic)~I and rner-[TcC13(pic)(PMe2Ph),lconfirm the expected octahedral geometry (2711. A variety of Tc(II1) complexes containing polypyridyl ligands has been prepared by substitution of [TcCl3(PR,R'),1 or [TC(tU)&13 or substitution/reduction of [TcC1,(PPh3),l (272, 273), and the electrochemical behavior has been studied (2741. Crystal structures have been reported for [TcCl,(PPh,)(bpy)I (2721, cis(CZ),truns(P)-[TcCl,(PMe,Ph),LlBPh, (L = bpy, phen), and ck(CZ),truns(P)-[TcCl,(PEtPh2)2(bpy)lCF3S03(273). The preparations of [TcCl,L(HB(pz),}] (L = PPh, ,OPPh, ,py), containing the tridentate HB(pz),- ligand, have been reported (275). Reaction of [Tc(tu),I3+ with phen is thought to give [Tc(phen),I(PF,), (276). The organohydrazine chemistry of Tc parallels that of Re (189).The air-stable biddiazenido) [TcC1(NzAr),(PPh3),lcomplexes are formed by the reaction of [TcVOC1,l- or [TcC~,(PP~,)~I with excess ArNHNH, in alcoholic solution or directly from TcO,- (277, 278, 140). The use of p-NO,C,H,NHNH, gives the lime-green monodiazenido complex [ T C C ~ , ( N N C , H , N O ~ - ~ ) ( Pin P ~high ~ ) ~ yield ] (140).Organodiazenido ligands most commonly bond in the singly bent, three-electron-donor mode with the doubly bent, one-electron-donor mode much less com(X = C1, Br) (22) mon. Crystal structures of [TcC1(NNC6H4X-p),(PPh3),l show trigonal-bipyramidal geometry with Tc-N-N angles of 166.2(6)" and 170.7(7)"for X = Br and the same essentially linear arrangement for X = C1 (277, 140).

38

JOHN BALDAS

PPh,

(22)

f

singly bent 3-electron donor

doubly bent l-electron donor

The bond distances indicate extensive delocalization and multiple bonding in the -NNAr moieties together with significant Tc backbonding. The diazenido ligands are thus singly bent, three-electron donors and [TcCl(NNAr),(PPh,),I complexes have a formal valence electron count of 18 (140). The cationic monodiazenido [TcCl(NNAr) (dppe),]+ complexes may be prepared by substitution of [TCCUNNA~)~ (PPh,),] or directly from Tc04- and isolated as the PF6- or BPh,- salts. The crystal structure of trans-[T~Cl(NNPh)(dppe~~IPF~~H~O shows slightly distorted octahedral geometry, a Tc-N-NPh angle of 163(2)",andTc-N and N-NAr bond distances of 1.917(19)and 1.25(4) A, respectively (140). Substitution of [TcC1(NNC6H,C1),(PPh,),1 with Na(S,CNMe,) in methanol yields the dark-orange [Tc(NNC6H4Cl)(S,CNMe2),(PPh,)]. The crystal structure shows distorted octahedral geometry, with the PPh, and diazenido ligands in cis positions and Tc-N-NAr and N-N-Ar angles of 178.6(4)"and 122.5(5)",respectively. The Tc-N and N-NAr bond distances are 1.763(3) and 1.236(4) A, respectively. The trans influence of the diazenido ligand is apparent because the trans Tc-S bond distance is longer [2.537(1) A1 than the other three Tc-S distances [2.412(2)-2.477(2) A] (279).

G . COMPLEXESWITH MONODENTATE PHOSPHINES AND RELATED LIGANDS The products of the reduction of Tc04- by phosphine/HX (X = C1, Br) depend on the nature of the phosphine and the reaction conditions. With PPh, only trans-[T~'~X,(PPh,),] is formed, whereas the more strongly reducing PR,Ph (R = Me, Et) gives the Tc(1V) complex at a Tc0,- : phoshine ratio of 1 :5 and rner-[T~"~X,(PR,Ph)~l at a ratio of 1 : 15 or higher (280). Alternatively, the Tc(II1) complexes may be prepared by the reduction of trans-LT~X,(PR,Ph)~l with excess PR2Ph (2811 or by the reaction of (NH,),[TcCl,] with PRzPh (282).The magnetic

39


moment of 2.8 BM for the Tc(II1) complexes is consistent with a t2: configuration in an octahedral environment (280).In MeCN solution, mer-[TcCl,(PMe,Ph),] may be electrochemically oxidized to [Tc'"Cl,(PMezPh)21or [Tc'VC13(PMe,Ph),]C104or reduced to Tc(I1) and Tc(1) phosphine complexes (2831. Reduction of NBu4[TcVOC141with PMe, yields mer-[TcC13(PMe3)31(284 and with PPh,/MeCN, [TcCI,(PPh,), (MeCN)] (207), whereas reduction of Tc0,- by PPh,/HCl/dmf yields 23 (L = dmf) (285). The MeCN complex (23) is a useful synthetic intermediate. On reaction with CO or NO only the MeCN ligand is substituted (207) but bpy and phen result in complete substitution, producing [TC"L,]~' salts (185).The crystal structures of mer-[TcCl, (PMe,),].(PhNCO), and mer-[TcCl,(PMe,Ph),] show a marked trans influence of the phosphine ligands with the Tc-C1 bond distances trans to P about 0.08-0.13 A longer than those trans to C1(284,282). Crystal structures have also been reported for 23.2PPh3 (L = dmf) (285)and truns-mer-[TcC1,(MeCN){P(m-MeCsH,),),l (185).The reaction of [Tc (S-tu),](PF,), with PMe, in methanol gives the hydrido complex [Tc(H){r)2-N,-S-NHC(NH2)S}(PMe3)41PFG (241, in which the thiourea ligand has undergone deprotonation and binds in the unusual $-N,-S mode. Structure 24 was established by crystallography, multinuclear NMR, and IR spectroscopy [v(TcH) at 1898 em-'] (284). CI

PMe,

l+

(24)

(L

MeCN, dmf)

(23)

A number of Tc(II1) phosphonite complexes of the type [TcX,{P(OEt)2Ph},]CIO, (X = C1, Br, I) have been prepared from (NH,),[TcX,]. The magnetic moments are in the range 2.3-2.6 BM (79).

H. COMPLEXES WITH BIDENTATE PHOSPHINE, ARSINE,AND RELATED LIGANDS The Tc(II1)complexes trans-[Tc(diars),X,]X (X = C1, Br, I) were first reported in 1959 (190) and the chemistry of the dppe analogs was described in detail later (192).These and related complexes have been

40

JOHN BALDAS

intensely investigated after it was shown by Deutsch et al. that the + 1 cation tr~ns-[~~"Tc(dmpe)~Cl~1+ accumulates in the heart (286). Complexes of the type trans-[TcL,X,IY (L = diars, depe, dmpe, dppe; X = C1, Br, I; Y = PF,, CF3S03, BPh,, BF,) are prepared by the reduction of TcO,-, [TcOX,I-, or [TCX,I2- with excess phosphine or arsine (287). Electrochemical and spectroelectrochemical studies have shown that the E "' value of the reversible Tc(III)/Tc(II)couple depends on the nature of X and of the bidentate ligand, with reduction being easier with the heavier halogen and also easier for dppe than for diars complexes (287,288). These effects result from the stabilization of the Tc(I1) d5 center over the Tc(II1) d4 center by a n increased ligand field. Typical E"' values are in the biologically accessible range of 100 to -250 mV vs Ag/AgCl. Under common laboratory conditions Tc(II1) is the stable state for the chloro and bromo complexes but when X = NCS, E"' is 390 mV and [Tc"(dppe),(NCS),] is the stable state (287). Comparison of tran~-[M"""L~X~]+'~ (M = Tc, Re) couples has shown that the Tc complex is always easier to reduce than the Re analog, with - E o f k )219 2 15mV (289).Thus, the significantly different biological behavior of [99"Tc(dmpe)2C1,1+and [ 1R6Re(dmpe),C12]appears to be due to the in uiuo reduction of the 9 9 m Tbut ~ not the '"Re complex (2901. Interestingly, pulse radiolysis studies have shown that in aqueous anionic surfactant media the [Tc(dmpe),Cl,] cation effectively partitions into the anionic micelles and is there relatively protected from the highly reactive negatively charged strong reductant eaq- and the strong oxidant C1,- (291). The chloro ligands in trans[Tc(dppe),Cl21 are rather unreactive to exchange (287). Reaction of tran~-[Tc(dppe)~Cl~l with LiAlH, yields yellow crystals of [Tc"'(H), (dppe),Cl] [v(TcH) at 1851 and 1775 cm-'1 (292). The electronic spectra of truns-[TcLzXz]+(X = C1, Br; L = diphosphine, diars) exhibit well-defined intense bands in the visible region (-20,000-23,000 cm-') that are 2500 ? 370 cm-' lower in energy than in the corresponding Tc(I1) complex (287). These bands have been assigned to X +. Tc LMCT transitions, and for Tc(II1) Cl/Br pairs the difference is about 1600 cm-'. All complexes are paramagnetic; the magnetic moment of trans-[Tc(dppe),Br,lBr is 2.47 BM (192, 287). FABMS has proven useful in the study of these Tc(II1) cationic complexes (293,294 ). Crystal structures for tr~ns-[Tc(diars)~Cl,]Y (Y = C1, ClO,) (295, 296), tr~ns-[Tc(dppe)~Br~lBF, (287), truns-[T~(dmpe)~Cl~] CF3S03 (I231, and tr~ns-[Tc(dppe)~C1,1NO,~HNO, (193)reveal the expected distorted octahedral geometry. The search for nonreducible Tc(II1)cations has led to the preparation of a variety of thiolato complexes of the type [TC(SR)~L,I+ (L = depe, +

+

+


41

dmpe, dppe, diars) (297, 298, 200). The geometry is generally trans, but for dmpe complexes with R being an aryl group the cis-isomer is formed (299).These complexes exhibit a reversible Tc(III)/Tc(II)couple for which E "' spans a range of values from - -200 to -600 mV vs Ag/ AgCl and are thus generally more difficult to reduce than the halide complexes (300). Again, the Re complexes are more difficult to reduce than the Tc analogs (301). A general method of synthesis is by the reaction of the thiolate with ~ ~ ~ ~ ~ - [ T C ~ O ( O H(297). ) L ~ FABMS ](PF~)~ (298,302) and spectroelectrochemical studies have been reported (3031. Crystal structures are available for trans-[Tc(SMe),L,]Y (L = depe, Y = PF,; L = dmpe, Y = CF3S03)(297), trans-[T~(SMe)~(diars)~]PF~ (2001, and cis-[Tc(SPh),(dmpe),IPF, (299). In the cis complex a trans influence is evident with the averaged Tc-P distance trans to P, 2.42(1) A, and that trans to S, 2.49(3) A. A crystal structure of the product of the reaction of tran~-[TcO(OH)(dmpe)~](PF,)~ with Na(S2CNEt2)has shown this to be trans-[Tc"*(scp),(dmpe),](PF,),, where scp represents the zwitterionic ligand -SCH,P+Me2(CH,),P(S)Me2.This unusual ligand appears to be formed by nucleophilic attack of a dmpe phosphorus center on the CS, elimination product of the dithiocarbamate, followed by a molecular rearrangement (300).Dithiolene ligands are well known to stabilize trigonal prismatic geometry, and the structure of the 3,4toluenedithiolato complex [Tc(tdt)(dmpe),]PF, (Fig. 9) shows a mean twist angle of 33(3)", which is about midway between the 60" of ideal octahedral geometry and the 0" of the ideal trigonal prism. Electrochemical and spectroelectrochemical studies show reversible Tc(III)/Tc(II) and Tc(II)/Tc(I)couples at -0.600 and -1.217 V, respectively, and a quasireversible Tc(IV)/Tc(III) couple at 0.680 V vs Ag/AgCl (304). A related cationic complex is [Tc(0-SC,H~0)(dmpe)~lBPh,, prepared by the reaction of 2-mercaptophenol with [Tc(dmpe),Cl,]Cl in ethanol. The geometry of the cation is described as distorted octahedral with a dihedral angle of 1 8 . ~ 3 )between " the TcOS plane and the "trans" TcPP plane containing one P atom from each ligand (305).An interesting series of Tc(II1) complexes based on the mixed bidentate ligands 25 and 26 has been prepared by the reduction of Tc0,- by the ligand and complex formation (306-309). Crystal structures have been reported for the octahedral phenolate [Tc(dppo),] (3071, thiolate [Tc(dppbt),] (307, 308), and propionate [Tc(dppp),].2dmso (306). In each case the three P atoms occupy mer positions. For the amine ligand (25) (R = NH,) an acid-base equilibrium is established and either the triply deprotonated [Tc(dppba),I or salts of the doubly deprotonated [Tc(dppba),(dppbaH)]+may be isolated depending on the pH. The crystal structure of [Tc(dppba),(dppbaH)I-

42

JOHN BALDAS

R = NH2 (dppbaH) R=SH (dppbtH) R=OH (dppoH) R = COOH

Ph2P(CH2),COOH n=l n=2 (dpppH)

E(CH2CH2PPh&

(26)

(27)

E=N,P

(25)

C10, again shows a mer arrangement for the phosphorus atoms. The proton is thought not to be delocalized over the three nitrogen atoms but to reside on the single nitrogen which corresponds to the longest of the 1.948(5)-, 1.979(5)-, and 2.04EK5I-A Tc-N bond distances (309).

FIG.9. The structure of the cation in [T~(tdt)(drnpe)~IPF~ (304).

43


The corresponding Tc-N-C bond angles of 128.3(4)", 126.6(4)", and 129.1(4)",however, seem t o offer little distinction between neutral and anionic nitrogen. Cationic complexes of the type [TcCl2L1 have been prepared by the reaction of [TcCl,(PPh,),] with the tetradentate ligands (27) (310). +

I. COMPLEXES WITH SULFUR LIGANDS Only complexes in which sulfur ligands form the major part of the coordination sphere are discussed here; other sulfur complexes are described under various headings. Most important is the homoleptic orange-red thiourea complex "l"c(tu),]Cl,, which precipitates in high yield from a concentrated HCUethanol solution containing TcO,- and thiourea (111). The thiourea ligands are readily replaced, making this, and related complexes, valuable synthetic precursors for the preparation of Tc(II1) and other low-valent technetium complexes. Thus, for example, reaction of [Tc(~u)~I(PF,), with CNtBu results in reduction, (111).Crystal structures for giving a 62% yield of [TC'(CN~BU)~]PF, [ T c ( ~ u ) , ] C ~ (111 ~ . ~) H ~ ~[TcLG](PF6),(L = N-methylthiourea, N , N ' and dimethylthiourea) (311) establish approximate octahedral geometry with S-bonded thiourea ligands. Another complexes with a n all-sulfur coordination sphere is the cationic [TcL(SR)~]PF,,where L represents a linear tetradentate thioether. The crystal structure of [TcL(SP~)~]PF, (L = 5,8,11,14-tetrathiaoctadecane) shows the two benzenethiolato ligands to occupy cis positions with the thioether wrapped around the remaining four coordination sites to give strongly distorted octahedral geometry (312). Complexes of the type [TcL'L,], where L represents a dithiocarbamato or xanthato ligand, have been prepared by various routes. Crystal structures for [Tc(PMe2Ph)(S2CNEt2),1(313), [Tc(PMe2Ph)(S2COEt),1 (3141, and [Tc(PPh3)(S2COC4H9),1 (315)show pentagonal-bipyramidal geometry, similar to that of [ T c ( S ~ C N E ~ ~ ) ~ (in C OFig. ) ] 5 , with the monodentate ligand in an apical position. Reaction of NH4{S2P(OMe)2} (Me,dtp) with rner-[TcC1,(PMe2Ph),1 yields orange-red crystals, shown by crystallography to be the octahedral truns(CZ)-cis(P)-[TcC12 (PMe,Ph),(Me,dtp)l(314 1, whereas reaction with Na,(mnt) yields PPh4 [T~(PMe,Ph),(mnt)~1(316). The preparation of neutral [TcL,] complexes with bidentate N, N-substituted benzoylthiourea ligands has been reported (317). The reaction of the sterically hindered anions tmbt and 2,4,6-triisopropylbenzenethiolate (SAr) with [TcrVCl6I2-/L/Zndust (L = MeCN, py, PEt,) in the absence of air yields the diamagnetic [Tc(SAr),L21(318, 319). The crystal structure of the tetramethylben-

44

JOHN BALDAS

zenethiolato complex, [Tc(tmbt),(MeCN),], reveals trigonal-bipyramidal geometry, with the two MeCN ligands in the axial positions and the three S atoms in the equatorial plane, with the bulky aryl groups arranged two above and one below this plane. The orientation of the aryl rings observed in the crystal structure is shown by the 'H NMR spectra to persist in solution. The [Tc(tmbt),L,] (L = MeCN, py) complexes can be oxidized to TcW) 0x0 compounds by oxygen atom transfer from dmso and other oxygen donors, and [TcvO(tmbt),(py)] may be reduced to [Tc(tmbt),(PEt,),I by PEt3. In the oxidation of the MeCN complex, an intermediate Tc(II1) complex was isolated and shown by FABMS and crystallography to be [Tc(tmbt),(MeCN)(dmso)l. A catalytic amount of [TcvO(tmbt),(py)l results in the oxidation of PPh, to OPPh, by dmso via a n oxidative and reductive oxo-transfer cycle, with the catalyst still fully active after 500 turnovers (319). Reduction of Tc04- by S2042- in the presence of CN'Pr and the tetradentate "umbrella" ligand P(o-C6H4SH),(H,L) yields the trigonal-bipyramidal 14electron complex [TcL(CNiPr)l,with the isonitrile in a n axial position [Tc-CNR, 2.06" 8,l. In the presence of a large excess of the isonitrile, a sixth ligand is bound, giving the octahedral 16-electron cis-[TcL (CN'Pr),] [PTc-CNR, 2.058(8) 8,; STc-CNR, 2.081(7) A] (320).

J. NITROSYL AND THIONITROSYL COMPLEXES Reaction of NBu,[Tc"(NO)Cl41 with tmbtH yields orange crystals of the neutral [Tc11'(NO)Cl(tmbt)31. The nitrosyl and chloro ligands occupy the axial positions in the trigonal-bipyramidal structure. The Tc-N and N-0 bond distances are 1.767(6) and 1.150(7) A, respectively, and the Tc-N-0 angle is 176.8(6)".The 4 N O ) IR absorption occurs at 1798 cm-' (3211. A variety of seven-coordinate dithiocarbamato complexes [Tc(NO)(S,CNR,),]Y (Y = BF,, PF6,ClO,) is prepared by substitution of [Tc(S2CNR2),(CO)1with NOBF,. These complexes show 4 N O ) a t 1795-1771 cm-' (2211. The seven-coordinate [Tc(NS)X,(S~CNE~,)~] (X = C1, Br) is prepared by sulfur abstraction from S2C1, or SOC1, (X = C1) and SOBr, (X = Br) by [TcN(S2CNEt2),1.Absorptions at 1248 cm-' (X = C1) and 1250 cm-l (X = Br) in the IR spectra have been assigned to u(NS). Crystal structures for both complexes show pentagonal-bipyramidal coordination geometry with the NS ligand, one halide in the axial positions, and Tc-N-S angles of 177(2)" and 174(2)" for the two independent molecules of the chloro complex and 177.2(7)"for the bromo complex. In [Tc(NS)Br,(S2CNEt2),1the SNTc-Br,,,, bond distance of 2.595(1) 8, is lengthened by a small but significant amount over that of Tc-Br,,, [2.564(1) A1 (322, 323).


45

VII. Technetiurn(1V)

This oxidation state is intermediate between the low oxidation states stabilized by n-acceptor ligands and the high oxidation states stabilized by n-donor ligands. Thus, carbonyl complexes are unknown and, although bridging 0x0 groups are not uncommon, terminal 0x0 groups are at present unknown. The most useful starting material for the preparation of Tc(1V) complexes is [TCX,]~-(X = C1, Br), but TcO,- or [TcVOX,l- may also be used. A. ISONITRILE AND THIOCYANATO COMPLEXES Complexes ofthe type [TcX,L,] (X = C1, Br) are formed by the reaction of MeCN or CNR with TcX,. The IR spectra indicate that the yellow chloro and red bromo crystalline products are the &-isomers (324). The deep red-violet color (A,,, = 500 nm) produced when TcO,- is reduced in the presence of NCS- or by substitution of [TcX,I2- (X = C1, Br) is now known to be due to [TC'~(NCS),]~-. This anion is reduced by hydrazine to the yellow, air-sensitive [TC"'(NCS)~]~(Tc(IV)/Tc(III), 0.18 V vs SCE). The magnetic moment of 4.1 BM for the purple (ASP~,),[TC(NCS)~I is consistent with an octahedral d3 configuration (2301, and the presence of N-bonded thiocyanate is confirmed by the crystal structure of the octahedral (AsPh,) [Tc(NCS),]-CH,C1, . The Tc-N bond distances are 2.00(1) and 2.01(1) and the N-Tc-N angles are exactly 90". Two Tc-NCS groups are linear and for the remaining four the Tc-N-C and N-C-S angles are 175.9(9)" and 175.3(10)", respectively (325).

1

B. HALIDEAND RELATED COMPLEXES The highest binary chloride of technetium is the dark red TcCl,, formed as the major product of the chlorination of Tc metal (26,326). Crystallography reveals a polymeric chain structure of C1-bridged distorted octahedral TCC& units (327).Reaction of TcC1, with Me,SiBr yields "TcBr," (324).Of great importance and synthetic utility are the stable complex halides [TcX,I2-. Salts of the bright yellow chloro complex [TcC1,I2- are best prepared by prolonged reflux of Tc0,- in concentrated HC1 in order to ensure complete reduction of the initially formed [TcVOCl,l-. Concentrated HBr rapidly yields the red [TcBr,I2-, whereas the deep purple [TcI,12- may be prepared by ligand exchange of the chloro and bromo complexes with HI (26, 27, 328). The white fluoro complex KJTcF,] has been prepared by fusion of K,[TcBr,] with KHF,

46

JOHN BALDAS

(329);a convenient high-yield synthesis is by ligand exchange with AgF in 40% HF (330).All eight possible mixed [TcC1,Br6-,,]'- (n = 1-5) complexes have been separated by ion-exchange chromatography. A notable feature of this work is the use of the greater truns effect of Br compared with that of C1 in order to accomplish stereospecific synthesis. Ligand exchange of [TcBr,I2- with HC1 results in the cis/ fuc complexes for n = 2,3, and 4, whereas ligand exchange of [TcCl6I2with HBr yields the trunslmer isomers (3311. From the LMCT spectra an optical electronegativity value of 2.25 for Tc(IV) is indicated, compared with 2.05 for the less oxidizing Re(IV), and lODq is 28,400 cm-' for [TcF,I'- and 32,800 cm-' for [ReF6I2-(332,333).The force constants for all the [TcX,I'- complexes have been determined (334),and the IR and Raman spectra of the 10 [TcCl,,Br,-, 1'- ( n = 0-6) species, including the pure geometrical isomers, at 80 K have been completely assigned and supported by normal coordinate analysis. Due to the C1< Br truns influence, the force constants indicate that in asymmetric C1'-Tc-Br' axes the Tc-Br' bonds are strengthened by on average 6% and the Tc-C1' bonds weakened by 10% relative to symmetric Br-Tc-Br and C1-Tc-C1 axes, respectively (331). Luminescence spectra for mixed ClBr species have been reported (335,336).Recent peflvalues,utilizing diamagnetic corrections, are in the range 3.34-3.80 BM for M2[TcX,] (M = NH,, K; X = C1, Br, I), (NBU,)~[TCCI,],and (NEt4),[Tc16]a t 300 K (150).In general, EPR spectra are observed only at 4 for over 24 hr (357).

48

JOHN BALDAS

(28)

(29)

An unusual phosphine/diolato complex is 29, formed from [TcOCl,Iand o-(dipheny1phosphino)benzaldehyde.The Tc-0 bond distances are 1.95 8, (358).Reflux of (PPh,),[TcC1,] in salicylaldehyde yields PPh,[TcCl,(sal)] (per = 3.8 BM), for which the phenolic and aldehyde Tc-0 bond distances are 1.98(2) and 2.04(2) 8,, respectively (359).The pale yellow oxalato complex ( A ~ P ~ , ) , [ T c ( C ~ is O prepared ~)~I by substitution of [TcBr,]'- in oxalic acid solution. The crystal structure shows six oxygen atoms in distorted octahedral coordination with pseudo 0,symmetry and Tc-0 distances of 1.978(5)-2.001(4) 8, (360).The substitution of [TcX,]'- and the reduction of Tc0,- in the presence of carboxylic, hydroxycarboxylic, and aminocarboxylic acids has been extensively studied and it appears that, in general, the Tc(1V) species formed are dimeric (3611. The reaction of acacH with [TcX,l2- or [TcX,(PPh,),] (X = C1, Br) yields products depending on the reaction conditions, and PPh,[TcX4(acac)], [TcX,(acac),], and [TcBr,(acac)(PPh,)l have been isolated (2421. The [TcX,(acac),l complexes are stable to acid but in alkaline solution undergo loss of halide followed by loss of the acac anions (362).The cationic [Tc(acac),]BF, is formed by oxidation of [Tc(acac),] with [Fe(Cp),l+ (363). 2 . Binuclear Complexes

A novel series of p-0x0 complexes is formed when a starting material such as NBu,[TcOX,I, (NBu,),[TcX,I (X = C1, Br), truns-[TcO2(py),1C1, or TcO,-/BH,- reacts with pyridine or alkyl pyridines either in neat solution or in a noncoordinating solvent (364-367). Crystal structures of the picoline derivatives show that these mixed-valence Tc(III)/Tc(IV) complexes are of the asymmetric [X,L3TcOTcX3L21(30)and dissymmetric [XL,TcOTcX,LI (31) type.


f' ,'.'

L

L,, 0,

L

49

iiiCl 0

I ,..'

.CI

clz-cl L

(30)

L

=

picoline

(31)

The reaction of [TcOCl,]- with hot picoline results first in the formation of (30) and truns-[TcvO,(pic),]+,with the concentration of the latter remaining nearly constant; this species is not an immediate precursor of the dimeric forms. In the later stages of the reaction the asymmetric form (30)is converted to the dissymmetric form (31).The formation of picoline N-oxide indicates that oxygen atom transfer occurs in the reduction process. Both 30 and 31 are stable in organic solvents at room temperature but undergo equilibration on heating. In o-dichlorobenzene, the process is first order in C1- and requires the presence of free picoline to prevent decomposition (365).In the solid state both forms of pox0 pyridine derivatives have small magnetic moments of -0.9-1.3 BM and v,,(TcOTc) at 726-698 cm-' in the IR spectra. In the electronic spectra three relatively narrow intervalence CT bands appear at about 10,000 cm-' for both forms. X-ray photoelectron spectroscopic analysis indicates that the Tc ions differ by no more than a single oxidation state in both forms (366). The complexes [(T~X(bpy)~}~(p-O)lX~.bpy (X = C1, Br) and [{TcCl(phen),),(p0)]C12.4H20have been prepared and the crystal structures, determined (367). For these complexes the Tc-0-Tc bond angles of 171.6(9)"-173.0(3)"show a slight bending, whereas for 30 (asymmetric) the angle is 176.5(2)"(366)and for 31 (dissymmetric) the two independent molecules in the unit cell have angles of 175.7(9)"and 177.1(9)" (364). Reduction of Tc0,- in the presence of aminocarboxylic and carboxylic acids or substitution of [TcX,12- in aqueous solution leads to the formation of bis(p-0x0) Tc(IV/IV) or Tc(III/IV) dimers. Structural and IR data are listed in Table I and the structure of the oxalato complex is shown in Fig. 10. The four-membered Tc(p-O),Tc ring is near planar in all cases and the short Tc-Tc distances are consistent with a multiple

50

JOHN BALDAS

TABLE I STRUCTURAL AND IR DATAFOR T ~ ( p - 0 ) ~ TCOMPLEXES c Com p Iex

Tc-Tc

(A,

Tc-06, !A1 Tc-0-Tc I") iav.) 1av.l

K ~ I { T c ' " ( C ~ O ~ ~ ~ ~ ~ ~ ~ -2.361(11 O)~I~~H 1.913(1) ~O [{Tc'"iedtaH, l),!p-0)21.5H,O 2.331 Na21{Tc~~nta)t2(p-0),1.6H20 2.36312) Ba,I{Tc"' '"(tcta)t2I p-0)21CI04~9H~02.40211) (32)

1.913 1.919(2) 1.936(7)

75.7 75.2 76.0(1) 76.6(3)

i,(TcO,TcI (IR, cm-', asym, syml

Ref.

730, 401 725, 404 715, 410 734"

368 369, 368 370, 368 371

Raman spectrum of sodium salt.

bond. The edtaH, complex is diamagnetic and extended Huckel calculations have suggested the partly antibonding d r 2 6 * 2configuration of a Tc-Tc single bond for the six metal d electrons, rather than the triply bonded a2r26'configuration (369). This suggestion has been questioned, and it has been noted that a Tc-Tc triple bond with the 6 component weakened to such an extent that its contribution to the overall energy of the Tc-Tc bond is close to zero is consistent with the observed long bond distance (42). A characteristic feature of the electronic spectra of the Tc(IV/IV) dimers is an intense visible absorp-

013

01

FIG.10. The structure of the anion in K4[(Tc(C2O4)~}~(p-O)~I~3H~O (368).


51

tion at about 500 nm. In the IR spectra the asymmetric and symmetric oxygen stretches of the ring system occur at about 725 and 400 cm-', respectively. Treatment of a solution of [ T C ~ O ( O C H , C H ~ O ) ( ~ C ~ ~ ) ] ~ with BH,- yields, on heating, the deep-blue Tc(III/IV) dimeric anion (32)(3711. The EPR spectrum of the solid shows a [{T~(tcta)}~(p-O)~l~broad signal with the hyperfine splitting expected for a single electron coupled equally between two Tc atoms with spin 8. On oxidation by K2S208,blue 32 is converted to the pink Tc(IV/IV) dimer [{Tdtcta)}, (p-0)2]2-and the reaction is reversed on treatment of the pink dimer with hydrazine. For 32,the Tc-Tc bond distance of 2.402(1)8, is distinctly longer than the Tc-Tc distances of the Tc(IV/IV) dimers in Table I. A polynuclear Tc(1V) citrate complex of uncertain structure has been prepared by substitution of [TcBr6I2-(372). 3. Phosphonato Complexes

Of great clinical importance as skeletal imaging agents are the 9 9 m T ~ complexes of the phosphonates CH2(P0,H2)2(mdpH,) and RC(0H)(P03H2)2(R = H, Me), which localize in bone due to the affinity of the coordinated diphosphonate for calcium in actively growing bone. The radiopharmaceutical preparations appear to be a mixture of oligomers ~ but thought and polymers with the oxidation state of 9 9 m Tuncertain to be +4 (12,19). The crystal structure of the polymeric {[Li(OH,)31[Tc(OH)(mdp)l.jH20},, prepared by substitution of (NH,)2[TcBr61with mdpH,, is shown in Fig. 11.The structure consists of infinite polymeric

FIG.11. A portion of the {[Li(OH~)31[Tc(OH)(mdp)l.fH,0), structure (reproduced from Ref. 373 with permission).

52

JOHN BALDAS

chains, with each mdp ligand bridging two symmetry-related Tc atoms and each Tc atom bound to two symmetry-related mdp ligands. The bridging 0x0 ligand appears to be in the hydroxy form, consistent with a Tc(1V) oxidation state (373). An EXAFS study of the Tc-mdp form of the 99mTc-mdp bone seeking complex in solution indicates a Tc(1V) tetrameric structure, with each Tc having 1.5 +- 0.5 Tc neighbors and surrounded by six singly-bonded oxygen atoms from water or the diphosphonato ligands, and the absence of Tc=O groups (374).Raman spectroscopy of Tc-MeC(OH)(PO,), prepared by BH,- reduction, however, indicates the presence of T-0 and O=Tc=O cores (375) and thus of Tc(V) components in this preparation.

D.

COMPLEXES WITH SCHIFF BASEAND OTHER NITROGEN LIGANDS

A number of Schiff base complexes have been prepared by substitution of [TcCl6I2-or [TcCI,(PP~,)~] (376,269). The reaction of TcC1, with bpy yields [TcCl,(bpy)] (377) and thermolysis of (pyH),[TcCl,]] yields [TcCl,(py),], which has been suggested to be the cis-isomer on the basis of the far IR spectrum (378).Orange-colored [ T C C ~ ~ { H B ( ~isZformed )~)I by the reaction of [TcVOC1,]~/KHB(pz),/HC1 and has a magnetic moment of 3.7 BM, consistent with a d3 configuration (379). The reaction of [TcOCl,]- with aromatic amines and dppe in refluxing alcohols gives the purple air-stable imido complexes [TcCl(NAr)(dppe),]+ in good yield. Paramagnetism is evident in the broadened NMR spectra. The shows that the hycrystal structure of tr~ns-[TcCl(NNMe~)(dppe)~IPF~ drazido(2-) ligand is bonded as a linear four-electron donor (278).

E. COMPLEXES WITH PHOSPHINE AND ARSINE LIGANDS The emerald-green air-stable truns-[TcC1,(PPh3),lis readily prepared in high yield by the reduction of TcO,- with HCl/PPh3 (280). If the reaction is performed in acetone at room temperature, then the salts R[TcC1&PPh3)l(R = PPh,H, orange; AsPh, , yellow) are formed (380). In the case of the more highly reducing PMe,Ph and PEt,Ph, truns[TcCl,L,I is formed if the Tc: L ratio is 1: 5, and mer-[Tc"'Cl,L,] is formed at a 1: 15 ratio. On reflux in CCl,, the Tc(II1) complexes are oxidized to [TcCl,L,I (280). A number of bromo analogs and [TcC ~ , ( A S P ~ ,have ) ~ ] been reported (192, 280, 377). In all reactions of TcO,- with monodentate phosphines the intermediate oxidation states Tc(V1) and Tc(V) are not observed, whereas bidentate phosphines, in general, favor reduction to Tc(II1). The magnetic moments of 3.4-3.8 BM for [TC'~X,L~I are consistent with an octahedral d3 environment


53

(280). Crystal structures have been reported for (Ph3PCMe2CH2COMe)[TcCl,(PPh,)] (3801, (PEt,H)[TcCl,(PEt,)] (381), and trans-[TcC1,L2] [L = PMe, (3821, PMezPh (3831, PMePh,, PEt, (38111. The truns[TcCl,(PPh,),] complex is a useful starting material for Tc(1V) and Tc(II1) chemistry. On heating in coordinating solvents, such as dmso or pyridine, the PPh, ligands are displaced to yield [TcC1,L21 (L = dmso, py), whereas reaction with pyridine/PPh, results in reduction to "I'cCl&py),I (384 1.

F. COMPLEXES WITH SULFUR LIGANDS The reaction of [TcOCl,I- with a dithiocarbamate (S2CNC4H,0-) results in loss of the 0x0 group to give neutral [Tc(S2CNC,H80),].H20 (3851,and reaction with 2-mercaptopyrimidine (mcpH) gives the structurally characterized NBu,[TcCl,(mcp)l (386).A cationic complex is the blue-violet paramagnetic [Tc(S,CNEt,),(PMe2Ph)1PF,, formed by air oxidation of [TC~~'(S,CNE~,),(PM~,P~)] in the presence of HCl(387, 388). Substitution of [TcBr,]'- with Na,(mnt) in ethanol yields (AsPh,), [T~'~(rnnt),l. The crystal structure shows that Tc is coordinated to six S atoms with chelate twist angles of 32.6'-39.0', which are intermediate between the value of 60" for a regular octahedron and 0" for a trigonal prism (3891. Reduction of TcO,- by 1,2-benzenedithiol /HCl yields on standing the wine-red dimer [Tc,(bdt),] (390,391). The crystal structure (Fig. 12) shows each Tc atom coordinated to a trigonal prismatic array

FIG. 12. The structure of [Tcz(bdt)*I(391).

54

JOHN BALDAS

of six S atoms with a shared quadrilateral face and with the eight S atoms delineating a rhombohedral prism. An arrangement in which two bdt ligands span the rhombohedral faces and two span opposite edges is found rather than a "paddle wheel" with the bdt ligands spanning the four vertical edges. The Tc-Tc distance of 2.591(3) A and the d3-d3 configuration would seem to indicate a multiple bond, but any assignment needs to consider the noninnocent nature of the dithiolene ligands (3911. The dark-green dimer [Tc2(edt),(SCHCHS),1has been isolated from the reaction of 1,2-ethanedithiol with [TcC1,I2- (379).The structure is similar to that of [Tc,(bdt),l, with a Tc-Tc distance of 2.610(3) A. A novel feature is the dehydrogenation of (SCH,CH,S)2t o (SCHCHS)2-to give a mixed dithiolate-dithiolene coordination, with each dithiolene ligand coordinated to one Tc atom only and each S atom of the dithiolato ligands coordinated to both Tc atoms. VIII. Technetium(V)

The chemistry of this oxidation state is dominated by complexes containing oxygen and nitrogen multiple bonds. This is a reflection of both the tendency of high oxidation states to induce deprotonation of aqua or amine ligands and the ability of good n-donors such as O2 and N3- to stabilize high oxidation states. The greater ease of reduction of Tc in comparison with Re is seen for Tc in the absence of analogs of the large number of [Re0I3+complexes with monodentate phosphines of the type [ReOX3(PR3),](189). Otherwise, the chemistry of Tc(V) resembles that of Re(V), with the predominance of complexes based on the [Tc0l3', [TcO,]', [ T c , ~ , ] ~ 'and , [TcNl" cores. Tc(V) complexes not containing a multiply bonded oxygen or nitrogen ligand are relatively few. The only binary halide is the yellow TcF, (m.p., 50"C), formed as a by-product of the direct fluorination of Tc metal (392).The complex fluorides M[TcF,] (M = Na, K, Rb, Cs) have been prepared by the reduction of TcF, in the presence of MC1 and IF5 and the rhombohedral unit cell parameters determined (393). In NO[TcF,], the presence of the free NO+ cation results in an IR absorption at 2315 cm-' (394).

A. MONONUCLEAR [Tc013+COMPLEXES The structure and chemistry of the square-pyramidal five-coordinate and pseudo-octahedral six-coordinate [TcV0I3+complexes are dominated by the strong tetragonal distortion induced by the multiply bonded 0x0 ligand. The d orbital energy levels in C,, symmetry are in


55

the order b2 (dx.y)< e (dx,, dyz)< bl (dx2-y2)< a, (d,d (35,395).The d2 electrons are paired in the low-energy, essentially nonbonding, b2 (dxy) orbital, resulting in complexes with a 'A, ground state, which are either diamagnetic or show only temperature-independent paramagnetism (150).The [TcOI3+core may thus be regarded as a "closed shell" electronic configuration and complexes expected to be relatively kinetically inert to substitution, but this is dependent on the nature of the coordinated ligands (35).The TcO bond is formally triple with one cr and two 7~ (0 px, p,,/Tc d,,, dyz)components, but because of the unfavorable charge distribution in T c - d + , the bonding will be intermediate between triple and double. The strong trans influence of the 0x0 ligand results in the trans ligand being only weakly bound and often absent and the Tc atom being raised above the square basal or equatorial ligand plane. In complexes in which the trans ligand is present, the Tc-Lt,,, distance may be 0.1-0.2 A longer than that for the same ligand in a n equatorial position. An aqua cation of the type [TcO(OH2),13+,or of polymeric forms, is not found because [Tc0l3+is unstable to disproportionation to Tc(1V) and TcO,- (35).When the [TcO13+core is stabilized by suitable ligands, kinetically stable and substitution inert complexes result. A general route to [TcO13+complexes is by substitution of [TcOX,]- (X = C1, Br) (35,396).The NBu4[TcOC1,1salt is conveniently prepared in 99% yield from TcO,-/HCl and is readily soluble in polar organic solvents such as methanol, acetone, or MeCN (397).An alternative method is by the reduction of Tc0,- in the presence of the ligand(s1. A variety of reducing agents has been used, of which sodium dithionite is convenient and popular, but reduction to a lower oxidation state may also occur. The in uitro stability of [TcO13+complexes has been related to the solid angle factor sum of the coordinating atoms (398). 1 . Cyano and Thiocyanato Complexes

Green-yellow K2[TcVO(CN),l.4H20is formed in low yield from the reaction of Tc02.nH20with KCN or from aerobic crystallizations of K,[TC~~'(CN)~].~H,O. In the IR spectrum v(Tc0) occurs at the rather low value of 910 cm-' and three u(CN) absorptions are observed at 2095,2080, and 2035 cm-', which have been assigned to the 2Al + E modes in C,, symmetry. The lilac (NBu4),trans-[TcO(OMe)(CN),3is formed on substitution of NBu,[TcOCl,] with CN- in MeOH [u(TcO) at 932 cm-'I (229),and (NMe4)trans-[TcO(OH2)(CN)4].2H20 has been isolated from the protonation of [TcO2(CN),I3- (399).The strong trans labilizing effect of the 0x0 ligand is apparent in the rapid rate of exchange of the trans water for NCS-, for which the forward rate constant is 22 M-' sec-' at 25°C. The crystal structure of (bpyH)2trans-[TcO-

56

JOHN BALDAS

(NCS)(CN),] shows N-bonded thiocyanate and a short Tc=O bond dis(399). Substitution of [TcOCl,]- with NCS- gives tance of 1.612(8)i% a high yield of the bright-red (ASP~,)~[TCO(NCS)~I. In the presence of NCS- this complex is easily reduced to mixtures of [ T c ~ ~ ( N C S and )~]~[TC"1(NCS)6]3- (400).

2. Halide Complexes When TcO,- is added to concentrated HC1 at room temperature, a ~C formed l ~ l ~ and - , is yellow solution, thought to contain f u ~ - [ T c ~ ~ ~ O is then converted to an olive-green color on reduction to [TcVOC1,1- (35). If the solution is heated, the kinetically controlled product [TcOCl,]undergoes further reduction to the yellow thermodynamic product [TC'~C~,]~-. These steps are described by the equations Tc0,-

+ 3HC1

+ 3HCl + H30' [TcOCIJ + 3HC1-

[Tc03C1,]'-

-

[TcO3C1,I2-t H30+ [TcOCl41-t 3H20 t CI,

[TcClJ-

+ H30' + 112 Clp.

With concentrated HBr as the reductant, the preparation of [TcOBr,lis performed at, or below, 0°C to avoid reduction to [TCBr6I2- (396). The reduction of [TcOB~,,,I-'~-in 8.7 M HBr proceeds by a combination of first- and zero-order reactions (401). The product isolated on addition of cations to solutions of [TcOX,]- (X = C1, Br) in HX is dependent on the nature of the cation. Large cations such as NBu,' result in the small cations such precipitation of the five-coordinate R[TcOX,] (402); as NH,+, K+, or Cs+ result in the six-coordinate M2[TcOX,l (403,4041, and with NEt4+ the trans-aqua complex NEt,[TcO(OH2)Br,l has been isolated (405).These results show that the trans ligand is labile and indicate that crystal packing forces determine the composition of the solid form. In aqueous HX solution the most likely form is [TcO(OH,)X,]- (but is generally written simply as [TcOCl,l-). For [TcOCl,]- in 12 M HC1 the equilibrium

has been demonstrated by Raman spectroscopy and [TcO(OH,)C1,1was found to predominate by a factor of about 60. In CH2Cl2solution the equilibrium constant is ca. 400 times larger, indicating the equilibrium [TcOCl41- + C1-

[TcOC1,12-,

57


with the trans position in [TcOCl,]- either vacant or containing an only weakly interacting CH2C12molecule (404). In water, [TcOCl,Idisproportionates to Tc02.nH20and TcO,- in the reaction 3Tc(V) + 2Tc(IV) + Tc(VII1, whereas in 1M p-toluenesulfonic acid Cs2[TcOC1,] dissolves to give a brown Tc(1V)cation and TcO,- (35,406).This disproportionation is very slow in >2 M HC1 solutions (35). Salts such as NBu,[TcOX,] (X = C1, Br) may also be prepared directly from NBu4[Tc041and HX (407) and NBu4[TcO141by ligand exchange of NBu4[TcOC1,l with NaI in acetone (408). Structural and v(Tc0) data are listed in Table 11. The first structural characterization of the [TcOCl,I- anion in the [N(PPh3)21+salt showed only approximate C2, symmetry (402).This distortion is a consequence of the presence of the large cation in the crystal because in AsPh,[TcOX,] (X = C1, Br) the anions possess ideal C4,symmetry (409,410) and C,, symmetry for the anions is also indicated by the vibrational spectra of NBu4[TcOX41(X = C1, Br, I) (407,408). In the square-pyramidal five-coordinate [TcOX41complexes, the 0x0 ligand is in the apical position and the Tc=O bond distance is rather short at 1.60-1.62 A. Structurally, the square-pyramidal five-coordinateand octahedral six-coordinate complex anions are dominated by the trans influence of the 0x0 ligand, which results in the displacement of the Tc atom above the square basal or equatorial plane and, in six-coordinate complexes, the weakening of the bond trans to the 0x0 ligand. This trans bond weakening is indicated by the long Tc-OH, bond distance of 2.317(9) A in (NEt,)trans-[TcO(OH2)Br41 (405). In the five-coordinate complexes, the trans influence may be regarded as sufficiently large to prevent the bonding of a trans ligand. The TcO IR stretching frequency is sensitive to the presence and nature of the trans ligand. For [TcOX41- (X = C1, Br, I) this absorption TABLE I1 STRUCTURAL AND IR DATAFOR [Tc0l3+HALIDECOMPLEXES IAI

CampI e x

Tc=O 1b .

(NlPPh312tlTcOC141 AsPh,[TcOCI,l AsPh,lTcOBr,l NEt,[TcO(OH2)Br,l Cs21TcOCI5lb

1.610141 1.593(8) 1.61319) 1.618(9) 1.65

2.305 av. 2.309(21 2.460111 2.507(11 av. 2.36,iS

Cs,lTcOBrJ

1.66

2.50trnns 2.54cm 2.74hIlS

Tc-X

0-Tc-X

(A)

ulTcO) (cm-')

Ref.

("I

GTcX4'

103.2, 110.4

0.66 0.67 0.70

1016 1025

0.37

1000 954

402 409 410 405 404

952

404

106.8 106.6 99.5, 97.6

Displacement of Tc above the square basal or equatorial plane. Bond distances from solid-state EXAFS spectra.

-

58

JOHN BALDAS

occurs at 1025-1000 cm-', whereas the presence of trans halide in Cs,[TcOX,] (X = C1, Br) results in a substantial lowering in energy to 954 cm-' (404). The value of 992 cm-' reported for M2[TcOC1,] (M = NH,, K)in the solid state, however, indicates that the nature of the cation is important (4111. Normal coordinate analysis of NBu,[TcOX41 results in force constants of 8.41, 8.39, and 8.04 mdyn k' for X = C1, Br, and I, respectively, (412). These values, when compared with 8.61 and 8.55 mdyn k' for Ru=N in AsPh,[RuNX,l (X = C1, Br) (4131, indicate considerable triple bond character for [TcO13+.A bond order of 2.55-2.59 has been calculated for M2[TcOC1,1 (414)and the value will be higher for [TcOX41-. The NBu,[TcOX,l (X = C1, Br, I) and M,[TcOCl,] (M = NH,, K)salts are diamagnetic at 80-300 K, which is consistent with an 'A, (b;) electronic ground state (150,395).Three d-d bands in the electronic spectrum of (NH,),[TcOC1,1 in HC1 a t 10,700 ( E = 181, 16,700 (6), and 20,600 (24) cm-' have been assigned to ' E (b,e) t 'A,, 'A, (b,b,) + 'A,, and 'B2(b2al)+ 'A, transitions, respectively (414). Recent L-edge spectra of [MoO13' complexes, however, show that the assignment of 'B, + 'A, for the 20,600-cm-' peak in [TcOCl,12- is most likely incorrect and that this transition is more likely dxy+ dx2-y2in nature (415). Brown, thermally stable TcOC1, and grey-black TcOBr, have been prepared by chlorination or bromination of TcO, .The chloro compound is very readily hydrolyzed by water to Tc02-nHz0and Tc0,- in the ratio 2 : 1 (416). Water-sensitive [TcOX,(bpy)] (X = C1, Br), [TcOC13(phen)l.H,0 and [TcOCl,(OEt)(bpy)l are prepared by substitution of [TcOX,]- in ethanol/HX. The v(Tc0) IR absorptions of 910-850 cm-l for [TcOX,L] and 922 cm-' for the ethoxy complex indicate a somewhat lower TcO bond order than that for [TcOX,l- or [TcOX,12(417). Another example is [TcOCl,(terpy)], for which the terpy ligand is thought to be bidentate (418).Interesting related complexes are (AsPhJmer-[TcOX,(hbt)l (X = C1, Br), prepared by substitution of [TcOXJ [v(TcO)at 945 cm-', X = C1; 940 cm-', X = Brl. The structure

59


of the chloro complex (33) shows a Tc=O bond distance of 1.650(6)8, and a trans OTc-Ophenolic distance of 1.948(4)8, (419). 3. Complexes Based on the TcO{04}, TcO{S4}, TcO{O4-,,S,}, and TcO{Se4}Cores

Square-pyramidal complexes of the type [TcOL4]-,where L is an 0, S, or Se ligand, are readily prepared either by substitution of [TcOC14]or by the reduction of Tc04- in the presence of the ligand. A large number of complexes have been reported, mainly with bidentate ligands. Structural data and v(Tc0) values are summarized for representative complexes in Table 111. General features are square-pyramidal geometry with the 0x0 ligand in the apical position, e.g., 34, a T-0 bond distance of 1.63-1.67 A, and a considerable displacement of Tc by 0.70-0.88 8, above the square basal plane. The anionic ligands effectively neutralize the positive charge on [Tc0I3+and the position trans to the 0x0 ligand is usually vacant. The TcO{O,} complexes such as M[TCO(OCH~CH~O)~] (M= Na, NBu,) are relatively weak and hydrolyze in the absence of excess diol (420, 433).The catecholate complex [ T C O ( O ~ C ~ His, ~ )however, ~Imore stable and may be prepared by the addition of a stoichiometric amount of TABLE I11 STRUCTURAL AND IR DATAFOR [Tc0I3+C O M P L E X E S WITH 0,s, OR Se LICANDS Complex

Tc-L tAl iav.1

TcO 11%)

TcOiO,\ 1.64861 NBu,lTcOto-O,C,H,I,I N B u ~ 1 T c O l o - O ~ C ~ H ~ N O , ~ ~ I 1.634(4J NBu~[TcOlo-O,C,C1,l~1 1.646i5) iAsPh~~~1TcOiox~~lHox~l~3H~0 1.64016) TcOIS,j NBu,lTcOiSAr),l' AsPh~lTcOiedtl~l NBu,ITcO~SCH~COS)~I AsPh,lTcOtSCOCOSl,l AsPh41TcOlmntlpl NBu,lTcOLzl' A~Ph~lTcOIbdtl~l

1.659i11J 1.64(11 1.672181 1.646(4) 1.655(6) 1.672(61 1.658i5J

TcO{S,O,t AsPh,lTcOtSCH,CH,0121

1.662i5)

TcO{Se,t NEt,lTcOlSe,CCtCN),~~l

1.67(2)

_______~_____

1.957i3l 1.966 1.955 2.016cis

2.380

2.300 2.320(31 2.329ill 2.315(1J 2.316 2.315(2J

0 1.950(41 S 2.291(2) 2 47114) ~

Displacement of Tc above the square basal plane. Ar = 2.4.6-trimethylphenyl. 'L = SCHlCOOMelCH1COOMe)S.

6sbp'

IAJ

0.701 0.695 0.25

utTcOI iIR. cm-'I

Ref.

970 983 969 985

420 421 422 423

940

0.846 0.761 0.791 0.759 0.742 0.78 0.732

950 940 938

424 425 426 427 428 429 430

0.720

948

431

0.88

965

432

950

60

JOHN BALDAS

catechol to [TcOCl,]- (420).Substitution of [TcOCl,]- with oxalic acid yields pale-green crystals of the AsPh,+ salt of an oxalato complex with v(Tc0) at 963 cm-'. Recrystallization from acetone-water containing oxalic acid results in the isolation of emerald-green (AsPh,),[TcO(ox),(Hox)].3H20(35) with v(Tc0) at 985 cm-'. The structure (Fig. 13) is unusual, with a monodentate protonated oxalate coordinated cis to the 0x0 ligand (423).Also unusual is the absence of a significant trans influence of the 0x0 ligand, with the oxalate Tc-Otransdistance of 2.069(6)! Ibeing similar to 2.016 8, (av.)for Tc-OCis.The low susceptibility of oxalate to the trans influence has been noted for [MoO13+complexes but the reason is unclear (434). It is likely that the form in solution is the truns-aqua complex and that the crystallization of 35 is the result of crystal packing effects. The gluconate and heptagluconate complexes are of uncertain structure but thought to be [TcOL,I- from IR and Raman evidence (19).The 9 9 m Tcomplexes, ~ and 99mTc-diethylenetriaminepentaacetate [of unknown structure but the oxidation state is probably Tc(V)I,are useful as kidney and brain imaging agents (19). ,01

Y

c4

FIG. 13. The structure of the anion in (AsPh,),[TcO(ox)2(H~~)]~3Hz0 (36)(423).


61

The preparation of a variety of [TcO(P-diketonate),Cl] complexes has been reported (435). Pertechnetate is reduced by thiols in the presence of acid in a firstorder process in TcO,- and the thiol (436,437). The kinetic data for a series of p-substituted benzenethiols follow the Hammett relationship with a decrease in rate by more electron withdrawing substituents (438). TcO{S,} and related complexes are generally prepared from TcO,by the use of a reducing agent such as S2042-or by ligand exchange (439-443).There is now a considerable variety of TcO{S,} complexes; structurally characterized examples are listed in Table 111. Four thiolato ligands effectively satisfy the charge on the [TcO13' core to give square-pyramidal complexes that show little or no tendency to bind a sixth trans ligand. In general, these complexes are highly stable and substitution inert; for instance, [TcO(edt),I- is unaffected by PPh, in refluxing MeCN. Electrochemically, there is no tendency to oxidation to Tc(V1) (4411. The magnetic moments of a representative series have been shown to be field-strength dependent and lie in the region 0.1-1.5 BM. The frequency of v(Tc0) is -20 cm-' lower than that in the [Re0I3+ analog, and an LMCT band a t 330-450 nm in the electronic spectra of TcO{S,} complexes is at lower energy than that for Re (441).An interesting complex is NBu,[TcO(SCH2COS)21,prepared by use of commercial HSCH,COOH, indicating the presence of a significant content of HSCH,COSH as an impurity (441). The complex NB~,[TCO(S,MO)~] shows a low v(Tc0) at 895 cm-' and undergoes reduction by PPh, to give a product formulated as [Tc'V(PPh3)2(S4Mo)2(H20)l, but which is possibly a hydrate (444).Reduction of Tc04- by tetramethylthiourea/HCl yields [TcO(trntu),](PF,), , a labile complex useful for liFrom the reaction of [TcO(tmtu),](PF,), gand-exchange reactions (445). with dppe in dmf solution one of the products isolated has been shown by crystallography to be [TcO(~~~U)~(M~~NCS,)I(PF,)~, in which the dithiocarbamato ligand is presumably derived from tetramethylthiouA radiopharmaceutical rea by sulfur transfer and loss of NHMe, (196). for tumor imaging is [99mTcO(dmsa)21(dmsaH, = rneso-dimercaptosucThe 9 9 m Tcomplex ~ has been shown from 'H NMR and cinic acid) (446). chromatographic studies with [TcO(dmsa),l- and the dimethyl ester to be a mixture of three stereoisomers (446,447).The crystal structure of the ester NEt,[TcO{SCH(COOMe)CH(COOMe~S}21 has shown the product isolated in 21% yield to be the syn-endo form (429). The important 99mTc-dmsarenal agent is thought to contain Tc in a lower oxidation state, possibly Tc(III), but the structure is unknown (19, 446). The potential for different chemical behavior at the 9 9 m Tand ~ 99Tc concentration levels is illustrated by the mnt ligand, for which the

62

JOHN BALDAS

product at macroscopic concentration is [TcvO(mnt),1-, whereas the product at the 9 9 m Tlevel ~ is [99mTcrV(mnt)3]2~ (448).The preparations of a variety of [TcO13' complexes with -S(CH,),X(CH,),S- (X = 0, S)/ -SAr ligands (449-4501, of TcO-metallothioneins (451), and of the three possible NEt4[TcO{XYC=C(CN)2}21(X,Y = S or Se) complexes (452) have been reported. The 0x0 ligand in [TcO{S(CH2)20(CH2)2S}(SAr)l is removed by PPh, at room temperature (450). 4 . Complexes Based on TcO{N4},T c O { N 4 ~ , , 0 ,and } , TcO{N$,} Cores

The discovery that the neutral, lipophilic 9 9 m T ~complex 0 36, prepared by the reduction of TcO,- in the presence of the tetradentate propyleneamine oxime ligand, is able to cross the blood-brain barrier in both directions has stimulated much work in this area (19).

(36)

(37)(d, 1 )

A large number of variously substituted analogs were prepared and the d,Z stereoisomer (37) was found to be sufficiently retained in the brain due to transformation t o a more hydrophilic species, which is then unable to diffuse out of the brain. The 99mTccomplex (37) is now an important radiopharmaceutical for cerebral perfusion imaging and the evaluation of stroke (19).Crystal structures of 36 and meso-37 and a variety of analogs have been reported (453-455). The [TcO13+core is sufficiently electron deficient to deprotonate secondary aliphatic amines, and in 36 and analogs neutrality is achieved by loss of both amine protons and one oxime proton, with the remaining oxime proton being intramolecularly hydrogen bonded. Features of the structures are T-0 bond distances in the range 1.670(4)-1.682(5) A and the displacement of Tc above the N, plane, which for 36 is 0.678(1) A. Also, for 36 the distance between Tc and the deprotonated N (imino) atoms of 1.913 A (av.) is considerably shorter than the Tc-N(oxime) distance of 2.090 A (av.) (453).The u(Tc0) IR absorption in the rather


63

low range of 934-908 cm-' is consistent with the long TcO distances. A study of analogs of 36 with the aliphatic N(CH2)3Nchain replaced by two, four, and five carbons has shown that for the four- and fivecarbon chains both the five-coordinate monooxo and the six-coordinate truns-[TcO,]+ complexes are formed (455). Reduction of Tc04-/l,2diaminobenzene (pdaH,) by S2042- allows the isolation of diamagnetic NBu4[TcO(pda),](456).A single v(NH) confirms the deprotonated form of the ligands and the low v(Tc0) at 891 cm-' is consistent with coordination by four -NH groups with Tc-N 1.98 A. The Tc=O distance is 1.668(7)A and Tc lies 0.67 A above the N4 plane. Another complex of this group is [TcO(octaethylporphyrinate)]OAc (457). A variety of five- and six-coordinate structurally characterized complexes with mixed TcO{N,-, 0,) and related cores have been reported. The only seven-coordinate complex is [TcO(edta)l-, prepared by the reaction of [TcOCl,]- with edtaH, in anhydrous dmso. A crystal structure of the barium salt shows distorted pentagonal-bipyramidal geometry, with the 0x0 group and the two N atoms bound equatorially (458). The six-coordinate Schiff-base complex truns-[TcO(OH,){(acac),en}]X has distorted octahedral geometry, a Tc=O bond distance of 1.648(2) A, and Tc 0.39 A above the N202plane. As expected, the trans influence of the 0x0 ligand results in a long Tc-OH, bond distance of 2.282(2) A. Similarly, in truns-[TcO{(sal),en)C11 Tc-C1 is long at 2.527(4) A and Tc is displaced by -0.27 A (459). A structurally characterized sixcoordinate 8-quinolinolate complex is [TcO(Ophsal)(quin)](460). In the orange phenolic derivative NBu,[TcO(epa)l.H,O [epaH, = N,"-ethylenebis(2-phenoxyacetamide)], both the amide and the phenolic groups are deprotonated and v(Tc0) occurs at 925 cm-'. The average Tc-N distance is 1.977(6) A and Tc lies 0.65 A above the N202plane (4611.

(38)

The novel neutral complex 38 is formed by substitution of [TcO(OCH,CH,O),]- or [TcOCl,]- (462,463). Triple deprotonation of the starting ligand includes the loss of an amine and pyrrolic proton. The 99mTc-38 complex is undergoing clinical evaluation for efficacy in the detection and determination of the severity of stroke and illustrates the degree

64

JOHN BALDAS

of ligand design undertaken t o achieve the desired in vivo behavior. The Tc-Npyrrole bond distance of 1.993(4)8, is, as expected, rather longer than the TC-N,,,,~,,distance of 1.897(4)A. Other structurally characterized six-coordinated complexes are [TcOL] (L = ONNNO Schiff base) (464), the unusual [TcO(apa)l, where apa represents a pentadentate ONNNO Schiff-base ligand derived from dehydroacetic acid (4651, and [TcOL(sal)][L = N-salicylidine-D-glucosamine(2-)] (466).The last complex precipitates from a methanol solution of the glucose derivative and [TcOCl,]-. The presence of salicylaldehyde in the product does not appear to be the result of hydrolysis of the Schiff base prior to coordination. The bidentate coordination of the salicylaldehyde(-1) anion is unusual. The preparation of a variety of complexes with heterocyclic N,O and other ligands (467,468) and of salicylidine Schiff-base complexes with amino acids has been reported (469). Reaction of NBu,TcO, with 25 (R = NHJ under rigorously controlled conditions in ROH, to avoid further reduction to Tc(IIIj, yields [TcVO(OR)Lzl [R = Me, Et; L = 25(1-)].The crystal structure of the methoxy complex shows approximate octahedral geometry with an NNPP equatorial plane, a Tc=O bond distance of 1.700(8)A, a Tc-OMe bond distance of 1.999(8)8,, and an O=Tc-OMe angle of 158.3(3)".The trans alcoxy group accounts for the stability of these complexes and results in low v(Tc0) values of 878 (R = Me) and 857 cm-' (R = Et) (470).

5. Complexes Bused on TcO{N,-,S,} Cores The search for neutral, lipophilic 9 9 m Tcerebral ~ perfusion imaging agents has led t o the intensive investigation of the chemistry of [TcO13+ with bisaminedithiolato (BAT) ligands (17,191. The NzSzcoordination results in highly stable complexes in which neutrality is achieved by deprotonation of one of the amino groups. A common method of preparation is by the reduction of TcO,- with Sz02-in the presence of the 1igand . If one of the amino groups is substituted, then syn/unti isomerism is possible. For 39 (R1 = Me, Rz = HI, crystal structures of both isomers have been determined and the major product has been shown to be the syn form (with the methyl group pointing in the same direction as TcO). The Tc-N bond distance to the anionic nitrogen is 0.288(9) and 0.198(9) 8, shorter in the syn and anti forms, respectively, than the corresponding Tc-NMe bond distance (471). In syn-39 (R1 = Et, R2 = H), the Tc-N and Tc-NEt distances are 1.921(2)and 2.224(2) A, respectively (4721. A large number of variously imaginatively substituted BAT ligands have been synthesized and the "Tc and 9 9 m Tcomplexes, ~ prepared (473-478). In general, 'H and 13C NMR are useful for stereo-


(39)

65

(40)

chemical assignment of the 99Tccomplex and the stereochemistry is complex important in determining the level of brain uptake of the 99mT~ (477).Complexes include those for which a benzene ring forms part of the ligand skeleton (474)and for which R1 is a steroid moiety (475). An interesting example is 39 (R1= H, R2 = CH2-NC5HgPh),containing a pendant phenylpiperidine group for which the crystal structures of the syn and anti forms (with respect to R,) are available and differences in the brain uptake of the 9 9 m Tcomplexes ~ are found (477).Other crystal structures, including 40 (478),have been reported (474).The complexities of in uiuo behavior are illustrated by 99mTc-40,which is retained in the brain on trapping by enzymatic hydrolysis of one ester group to the free acid and the formation of a charged species. The hydrolysis is stereospecific and only the L,L enantiomer is trapped (19).Additionally, high brain uptake appears limited to humans and primates, presumably due to the high serum and lower brain esterase levels in lower animal species (17).Cationic BAT complexes have been prepared with or without alkylated amine groups and these are of interest as potential myocardial imaging agents (476).A number of crystal structures are available (476,479-481 1. Cationic NzSz complexes such as the six-coordinate tr~ns-[Tc0(0H,){(sacac)~en}]C1 are formed with imine nitrogen ligands. The v(Tc0) absorption in this complex occurs a t 964 cm-' and the Tc-OH, distance is quite long at 2.384(3) A (482). The greater acidity of the amide protons in diamidedithiols results in the loss of two amide protons, producing anionic complexes for which the 9 9 m Tpreparations ~ are of interest as renal agents (19). Yellow salts of the parent complex (41), and derivatives, may be isolated from the in situ hydrolysis of the S-protected ligand and Tc04-/Na2S204(483). Crystal structures of (AsMePh3)(4l)and the PPh4+salt of the butanediamine derivative show the usual square-pyramidal geometry, with the Tc atom displaced 0.771 and 0.67 A, respectively, above the N2S2 plane (484,485).The preparation of a variety of substituted analogs of 41 (and the monoamides) and crystal structures have been reported

66

JOHN BALDAS

(41)

(42)

(486-489). In a novel example the CH7CH7 - - bridge in 41 is replaced by a ribonucleoside (490). Crystallography and NMR have been used to assign stereoisomers (487, 491 1. In the reaction of [TcOCl,]with excess ligand, the blue lantern dimer (A~P~,),[(TCO)~(SCH,CONH(CH2)2NHCOCH2S},],with each Tc coordinated by four S atoms and an intramolecular Tc...Tc distance of 7.175 -81, is formed. In aqueous basic solution the dimer is immediately and quantitatively converted to 2 eq. of (AsPh4)(41)(492). If one of the thiolate groups in 41 is substituted by -CH,CH,-N-(piperidinyl), then a neutral complex is formed. This complex readily undergoes S-CH, bond cleavage in solution, assisted by neighboring group participation of the piperidine nitrogen, to reform 41 (493). The neutral six-coordinate D-penicillaminato (pen) complex (42) contains one bidentate ligand, one tridentate ligand with a Tc-Ocarboxylate distance of 2.214(4) A, and one free carboxyl group (494). The [TcO-(~-pen)(~-pen)lanion is fluxional in solution and racemizes by exchange of bonded and free carboxylate groups trans to the 0x0 ligand. Racemization of the Tc complex is faster than that for the Re analog (495). Other structurally characterized examples with bidentate NS ligands are NBu,[TcO(abt),l (496) and a cationic [TcOL,ICl [L = substituted (thiocarbamoyl)benzamidinate] (497).The preparations of a variety of Schiff-base dithiocarbazate derivative and N-heterocyclic thiolato complexes have been reported (498-500). An N3S complex is [TcO(MAG,)I- (431, for which the negative charge is achieved by deprotonation of the three amide groups of the mercaptoacetyltriglycinato ligand. In uiuo the carboxylic acid group is ionized and [99mT~O(MAG3)12is a n important radiopharmaceutical for the assessment of renal function. The presence of the uncoordinated carboxylate group in the dianion is important for efficient renal clearance (19).The crystal structure of AsPh4[TcO(MAGJ1 shows the carboxylic acid group to be distant from the Tc center, and two crystal forms of the methyl ester AsPh,[TcO(MAG,Me)l differ with the orientation of the carbomethoxy group being approximately parallel and perpendicular to the Tc=O bond (501).Calculations indicate that in solution

67


(43)

(44)

[TcO(MAG3)I2-is conformationally flexible (5021. A variation of N3S coordination is the inclusion of one pyridine nitrogen in the neutral 44. The Tc=O bond distance is 1.653(4) A, and Tc-Npyridine at 2.102(4) A is substantially longer than the Tc-Namid,distances [1.965(4)A (av.)l (488).A complex with NS3 coordination is [TcO(tmbt),(py)l (319). 6. Complexes Based on Other TcO Mixed Ligand Cores Numerous five- and six-coordinate TcO complexes containing mixed ligand atom coordination are known and many have been structurally characterized. Examples not containing sulfur are [TcO(OR)X2L,l(R = Me, Et; X = C1, Br; L = pyN02 or L2 = bpy, polypyridyl derivative) and [TcOC12(terpy)lTc04 (503-505, 418). For trans(N)-trans(Br)[ T ~ 0 ( 0 E t ) B r ~ ( p y N Othe ~ ) Tc-OEt ~], distance is short at 1.855(6)A and Tc=O is 1.684(6)A. The ethoxy group results in a low v(Tc0) at 938 cm-' (503).Reduction of Tc04- by HX /KBH4in the presence of HB(pz),yields the lipophilic [T~0Cl~(HB(pz)~}l (5061, and the bromo complex may be prepared from [TcOBr41-(396).In the chloro complex, the three N donor atoms span fac positions with OTc-N,, bond distances of 2.086(4) and 2.088(3) A, and OTc-N,,,, is markedly longer at 2.259(4) (506).The neutral six-coordinate [TcOL,Cl] (L = 2-methyl8-quinolinolate), prepared by substitution of [TcOClJ, is the cis-isomer and hence may be regarded as a TcO{N20C1} derivative. The Tc-Oquinolinolat bond distances cis and trans to the 0x0 group are 1.947(3) and 1.994(3) respectively, and Tc-C1 is 2.360(1) A (507).The chloro ligand in this and related complexes undergoes solvolysis in methanol (507,508).Electrochemical studies of [TcOClL,], where L is a bidentate N,O-Schiff base or 8-quinolinolate ligand, have shown reduction to Tc'"0 species (509).In [TcOC1(OCH2CH,0)(phen)l the C1 atom is also cis to the 0x0 group and the OTc-N bond distances are 2.173(4) (cis) and 2.268(4)A (trans) (505). Similar structurally characterized

d,

68

JOHN BALDAS

complexes are [TcOClL,] (L = N-phenylsalicylidineiminate)(510)and [TcOCl,L] (L = NNO Schiff base) (511).A five-coordinate example is [TcOCl(Ophsal)],in which Tc is displaced by 0.67 A above the ONOCl plane (512). Crystallography has shown that a product obtained from the reaction of 20 [E = S (SphsalH,)] with [TcOCl,I- is the octahedral [TcOCl(hbt),] with equatorial ONNCl coordination. The hbt ligand (see 33) is formed by an oxidative intramolecular ring closure (513).The five-coordinate [TcOCl(Sphsal)]has since been prepared by substitution of [TcOClJ, with a stoichiometric amount of the ligand and crystal structures of this complex (514)and the related [TcO(Sphsal)(SPh)l(515)reported. Crystal structures are also available for the dithiocarbazate derivative (45) (2681, the potential brain imaging agent (46) (5161, cis-[Tc0{8hydroxy-3,6-dithiaoctan-l-olate-(O,S,S, )}C121 (517), and the squarepyramidal AsPh,[TcO(MAG,)I [MAG2 = mercaptoacetylglycylglycinate(2-)],with the carboxylate group participating in the ONNS coordination (518).The preparation ofTcO complexes with a variety of dithiocarbazate derivatives (519, 520) and of tridentate Schiff bases with a thiolato coligand (521 has been reported. 0 II

An interesting structurally characterized complex is the distorted square-pyramidal [TcVO(SC,H2iPr,)2(PhNNCON2HPh)l (522),which has also been assigned the Tc(II1) oxidation state on the basis of structural and spectroscopic features (523).In view of the absence of any other Tc=O group in an oxidation state below Tc(V)and Holm's generalization that M=O groups are stabilized at metal centers with an oxidation state of no less than +4 (524),the TcW) assignment would seem preferable.

B. COMPLEXES OF THE tr~ns-[TcO(OH)1~' AND [TcO,l+ CORES Technetium, in common with rhenium (1891,forms a considerable number of cationic complexes containing the truns-[Tc021+core. The poor


69

ability of neutral cr- or weak 7r-donor equatorial ligands in truns[TcO(OH,)L,]~+t o neutralize the positive charge on the [TcO13' core results in enhanced acidity of the truns water and the following acid-base equilibria (525, 35): [0=Tc-0HJ3+

Ka 1

[O=Tc-OH]2t

__ K.2

[0=T~0]+.

Negatively charged ligands do not favor proton loss but an exception is CN-, which, although a good cr-donor, is also an effective n-acceptor. From neutral solution K,truns-[TcO2(CN),1 [v,,,(Tc02) at 785 cm-'] (PIC,, is isolated and acidification to pH 1yields trans-[TcO(OH2)(CN4)12.90) via the [TcO(OH)(CN),I2-intermediate. The 7r-acceptor nature of the equatorial cyanides in (NMe4)trans-[TcO(OH2)(CN),1~2H20 is apparent in the high value of 1029 cm-' for v(Tc0) (399).The dioxo complex is also formed by the hydrolysis of K2[TcO(CN),1 (229). In general, cationic truns-[TcO,]+ complexes are prepared by the reaction of Tc0,-/Na,S,O, (399) or [TcOX,I- (526, 527) and neutral nitrogen or N2S2cyclic thioether ligands. The truns-[Tc02(py),lC1complex is readily prepared by hydrolysis/oxidation of [TcCl6I2-in neutral conditions or by substitution of [TcOCl,]- in the presence of water and serves as a useful starting material for ligand exchange reactions (528,399). Crystal structures have been reported for [Tc02(cyclam)lC104~H20 (5251,truns-[TcO,(en),]X (X = C1, I) (529),[Tc02L,lC1~nH20 (L = imidazole, n = 2; L = 1-methylimidazole, n = 3) (530),[Tc02(4-tert-butylpyridine),]CF3S03.H20 (5261, [Tc0,(1,4-dithia-8,11-diazacyc1otetradecane)]PF, (5311, and a polymeric {Li[Tc02(1,4,8,11-tetraazaundecane)](CF3S03),},,(532). There is a brief mention of the structure of [TcO2(CN),I3-(35).Characteristic features illustrated by the structure of truns-[TcO,(en),]Cl are Tc=O bond distances of 1.752(1) and 1.741(1)A, a Tc-N bond distance of 2.158 A (av.) and an 0-Tc-0 angle of 178.6(3)" (529). Interestingly, although 36 exists as the monooxo form, an increase in the hydrocarbon chain to N(CH2I6Nresults in the formation of the neutral dioxo complex (no amine nitrogen deprotonated) with Tc=O, 1.745(3)A, and the 0-Tc-0 angle, 170.1(1)"(455). The long Tc=O distances and low asymmetric O=Tc-O IR stretching frequencies in the range 850-750 cm-' are indicative of a lower bond order than that in [Tc0l3' complexes. Group theoretical analysis presymmetry the maximum Tc-0 bond dicts that for truns-[TcO,]+ in order is 2 (533) and this is consistent with the bond order of 2.10 for indicated by the Tc=O stretching force constant of 6.23 mdyn truns-[TcO,(en),]C1(4141. Kinetic studies of pyridine exchange in truns[TcO,(py),]+ indicate a dissociative mechanism and the Tc complex has

70

JOHN BALDAS

been found to undergo exchange at ca. 8000 times the rate of the Re analog (534, 535). The cationic nature of [TcO,I+ complexes has attracted considerable radiopharmaceutical interest. A promising myocardial imaging agent that shows good blood and liver clearance is the diphosphine derivative ["""TcO,L,]+ [L = {CH2P(CH2CH,0Et),},]. The Tc=O bond distance in trans-[Tc02L,l[Cr(SCN),(NH3),] is 1.738(17) A (536). With the dmpe ligand the hydroxo complex trans-[TcO(OH)(dmpe),](CF3S03), has been isolated and Tc=O and Tc-OH bond distances of 1.66 and 1.96 A have been determined by EXAFS (123).The trans-[T~0(0H)(CN),]~species could not, however, be isolated due to rapid dimerization (399),but (NBu,),truns-[TcO(OMe)(CN4)], with a nonionizable methoxy group, may be regarded as a trapped form (229). The isolation of K,[TcO(OH)Cl,], with v(Tc0) at 900 cm-', has been claimed (537),but has not been substantiated. The formation of monoanionic TcO, complexes with Schiff bases has also been reported (538), but these may be the p-0x0 dimers. C. 0x0-BRIDGED[Tc2O3I4+ AND OTHERBINUCLEAR COMPLEXES Protonation of [TcO2(CN),I3- in acidic aqueous solution and rapid dimerization of the initially formed truns-[TcO(OH)(CN),I2~yield the purple p-0x0 dimer [TC,O~(CN)~]~(399). Generally, [Tc2O3I4 complexes are prepared from substitution reactions of [TcOCl,]-, for example, [{TcO(S,CNEt2),},(pL-0)1 (539),or from reduction of TcO,-. The reaction sequence is illustrated by truns-[TcOClL], prepared by substitution of [TcOCl,I- with (sacac),enH, (H,L) in dry solvents. In the presence of moisture, the labile chloride is replaced by water to give the cationic [TcO(OH,)LlCl, which then forms the p-0x0 dimer [{TcOL},(p-O)I by reaction of [TcOClLI with the intermediate hydroxy complex [TcO(OH)L1(540).Crystal structures of [{TcOL},(p-O)], where L represents a variety of tetradentate ONNO aminephenolato ligands, (e.g.,47) (541), the ONNO Schiff base ligands N,Nf-2-hydroxypropane1,3-bis(salicylideneiminate)(542) and N,N'-propane-l,3-diylbis(salicylideneiminate) (5431,and 48 (517 )have shown the presence of either a crystallographically imposed linear or a near-linear (167"- 173") Tc-0-Tc bridge with O=Tc-0 angles of 163"-171", giving an essentially linear [O=Tc-O-Tc=0I4+ core analagous to the [Re,O3l4+ core (189). Other features are Tc-Obridgebond distances of 1.90-1.92 A and the near-eclipsed arrangement of the donor atoms of the two Tc centers as shown in 48, in which the near-eclipsed atoms are C1 and S (517).The occurrence of linear d2-d2 [MV2O3l4+ (M = Tc, Re) cores is explained +

71


(47)

(48)

by MO theory. For a [O=ML,-0-L,M=O] complex in D4,, symmetry, the T interactions between the two tz9sets from the two metal atoms and the px,p,, sets from the three oxygen atoms give rise to two nonbonding molecular orbitals (b, + b,,), which do not correspond to any oxygen T linear combinations. A d2-d2 configuration corresponding to the occupation of these nonbonding orbitals satisfies the closed-shell electronic appears as a n configuration (544).In the IR spectra v,,,(Tc-0-Tc) intense broad band at 625-680 cm-' but v(Tc=O) is of variable intensity and may not be observed (541,543).A complex formulated as the mixed-valence K3[T~V'1V202C1al has been obtained by reflux of K,[Tc,. Cla].2H,0 in 2-butanone in air. The IR spectrum indicates the presence of both Tc=O (1020 cm-'1 and Tc-0-Tc (680 cm-'1, but the structure is uncertain (42). Formation of the novel dimer 49 on reaction of [TcOCl,I- with 1.5 eq. of edtH, may be viewed as the interaction of the Lewis base [TcO(edt),]- with the Lewis acid [TcO(edt)]+ (Fig. 14) (35, 545). On reaction with excess edtH,, 49 is converted quantitatively to [TcO(edt),]-, but [(TCO)~(S(CH~)~S}~] does not react with further amounts of ligand. Also, although a dimeric intermediate was found to form in the reaction of [TCO(SCH~CH~O)~]with 2 eq. of edtH, , no intermediate was detected in the substitution of [TcO(OCH2CHz0)21to [TcO(edt),]- (546).An interesting binuclear complex is [{TcO(OEt)Cl,},(p-L)], where L is a n N6 heterocyclic nitrogen ligand (547).Addition of NBu4[TcOC1,1 to ( N B U ~ ) ~ [ H ~ P Win ~ , MeCN O ~ ~ I yields purple crystals of the Tc-substituted Keggin polyoxotungstate derivative ( N B U ~ ) ~ [ P W ~ ~ TThe C Osilicon ~ ~ ] . derivative ( N B U ~ ) ~ [ S ~ W ~has ~TCO~~] been prepared by the addition of N~[TCO(OCH~CH,O)~] to a-KaSiWl,039~12Hz0 in sodium acetate buffer. Electrochemically, the Tc center of [PWl,TcO,O]"- appears to exhibit only three accessible oxidation states, Tc(1V) Tc(V) Tc(VI), in contrast to the five oxidation states, Re(II1)-Re(VII), accessible for the analogous Re cluster (548).

72

JOHN BALDAS

56

s4

TC2

c5-

- c3

c2

FIG.14. The structure of [ ( T ~ O ) ~ ( e d(49) t ) ~ l(545).

D. [TcS13' COMPLEXES at 520 The diamagnetic sulfido complex AsPh,[TcS(edt),] [ 4T-S) cm-'1 is formed from the reaction of [TcC1,12- with 1,2-ethanedithiol and [TCSC~,{HB(~~)~)I from the 0x0 complex by S atom transfer from B2S3 (379,549).The [TcS13' core is less stable than [Tc013+and readily undergoes replacement of the sulfido ligand by 0x0 in solution and under aerobic conditions. E. NITRIDOCOMPLEXES The nitrido ligand (N3-) is isoelectronic with the 0x0 ligand (02-) and is a powerful velectron donor that effectively stabilizes technetium in the + 5 to +7 oxidation states. The first complexes containing the [TcVNI2 core, [TcNCl2(PPh3),1and ITcN(S,CNEt,),I, were prepared from the reaction of TcO,- /ligand with NH2NH2.HC1as the reducing agent and source of the nitrido ligand (550,551), but this method is of limited applicability. Two general methods for the synthesis of [TcNI2+ complexes are by ligand exchange of [TcNCl2(PPh,),1 and by reduction/ exchange of [TcV'NX4]-(X = C1, Br) (551,552,220). A characteristic feature is the generally sharp v(TcN)IR absorption at 1100-1028 cm-', +


73

which is shifted by -30 cm-' on 15N labeling (145).The TcN bond is formally triple with one (T and two n components and quite short in the range 1.60-1.64A. The lower charge on [TcN12+in comparison with the isoelectronic [Tc0I3+results in little tendency to deprotonation of coordinated amine ligands. The TcN bond is very resistant to hydrolysis or removal by other reactions but readily reacts with active sulfur sources such as S2C1, to yield thionitrosyl complexes (322).In general, [TcNI2+complexes are not readily reduced and require agents such as chlorine for oxidation (553,554).Structurally, the strong trans influence of the nitrido ligand results in either five-coordinatesquare-pyramidal complexes or six-coordinate complexes with the trans ligand only weakly bound. The anion in (AsPh4),trans-[TcN(OH2)(CN),1~5H20 has distorted octahedral geometry with a TEN bond distance of 1.60(1)A, a very long NTc-OH, distance of 2.559(9)A, and the Tc atom 0.35 8, above the equatorial plane. A high v(TcN) at 1100 cm-' results from the n-acceptor property of the equatorial cyanides. The pK,, value of the coordinated water has not been determined but the long bond distance indicates very low acidity and high kinetic lability (555).In the case of [Re(E)(OH,)(CN),I"-(E = 0, N) the strong trans effect of the nitrido ligand is apparent in the pK,, values of 1.4 and 11.7 for the 0x0 and nitrido complexes, respectively, and a reaction rate constant some 9 x lo5times greater for the nitrido complex (556).The structurally characterized Cs2Na trans-[TcN(N3)(CN),I.2H2O is obtained by ligand exchange with N3- (5571, and Cs,K[TcN(CN),] may be isolated in the presence of cyanide (558).The IR spectrum of the latter complex shows three well-defined v(CN) absorptions (2A1+ E ) consistent with ClU symmetry. Reaction of NCS- with [TcNC1,(PPh3),1yields yellow [TcN(NCS),(PPh,),], which on reflux in MeCN is converted to orange-red crystals of trans,trans-[T~N(NCS)~(PPh~)~(MeCN)]4MeCN. The TEN bond distance is 1.629(4)A and the weak binding of the MeCN ligand is apparent in the long Tc-NCMe distance of 2.491(4)8, and the formation of the five-coordinate complex on dissolution in CHC13(559).The thiocyanato ligands areN-bonded, as is also the case for the structurally characterized (NEt,),[TcN(NCS),(MeCN)I, prepared by reaction of [TcNCl,]- with NCS- and crystallization from MeCN (560).The preparation of (AsPh,),[TcN(NCS),] has been reported (561) but crystallography shows the product crystallized from MeCN/EtOH is the trans-aqua complex (AsPh,)2[TcN(OH2)(NCS),l~EtOH (557).Spectroelectrochemical studies at -60°C show a reversible one-electron reduction of [Tc"'NX,]- (X = C1, Br) to [TcNX4I2-,but the colorless reduced species have not been isolated (562).

74

JOHN BALDAS

No TcvN(04) complex has yet been identified, but five-coordinate TcVN{S4}complexes are readily prepared by ligand exchange of [TCNC~,(PP~,)~] or reduction/substitution of [TcNClJ (552,5631. Key structural data are summarized in Table IV. Of particular interest is a comparison of the trans influence of the nitrido and 0x0 ligands in the same coordination environment. The three TcN/TcO pairs in Table IV show that, although the nitrido ligand exerts the greater trans influence, the 0x0 ligand exerts the greater structural steric effect (566). This is seen in the greater displacement of Tc above the square basal plane in the five-coordinate square-pyramidal complexes in Table IV and the correspondingly greater OTcL angles. The Tc-N bonds are shorter than the Tc=O bonds and it has been suggested that this may be largely accounted for by a-electron effects, with nitrogen utilizing an sp hybrid orbital and oxygen, an sp2 orbital in the TcN/O bond. The Tc-L distances are longer in the nitrido complexes, but this effect largely may be due to the lesser core charge on [TcNI2' (566). Reaction of [ T c N C ~ ~ ( P P ~with , ) ~ I K(S2COEt) and treatment with aqueous ethanol yield the dithiocarbonato complex K2[TcN(S2C0)21 on hydrolysis of the intermediate xanthate (564). The mixed-ligand complex AsPh4[TcN(S2CNEt2)(SCOCOS)I is prepared in a controlled fashion by the reduction/substitution of [TcV'NC1,(S2CNEt2)] with dithiooxalic acid (565).Reaction of thiourea with [TcNC14]-yields the orange [TcN(tu),Cl]Cl, which is a useful starting material for ligand exchange in aqueous solution (568).Mass spectrometry has shown that [TcN(SSeCNEtJ,] undergoes a thermally induced scrambling to give the S4, S,Se, and SSe, species (569).Chromatographic studies have shown the formation of [99"T~NL2]2(L = mnt, dto) on reaction of TABLE IV STRUCTURAL DATAFOR TcVN{S4}COMPLEXES AND TcVN{S,/Se4}/TcVO{S4/Se4} COMPLEXES WITH THE S A M E COORDINATION ENVIRONMENT hbp' Complex lTCNlS2CNEt2)2l K21TcN(S2CO)ZI.2H2O AsPh4[TcNC32CNEt211dto11 (AsPh4)p[TcN(dto121 AsPh,[TcO(dto),I tAsPh412[TcN(mnt)21 AsPh4[TcOlmnt121

INBu,)21TcN[Se2CC(CN12}21 NEt4[TcO{Se~CClCN12)~l

thl

TcL

1,604161 1.621(61 1.5412) 1.613(41 1.646141 1.5911) 1.655163 1.6111) 1.67(21

2.401 av. 2.390 av.

108.1 av. 107.3 av.

2.393 av.

106.0av.

2.378(21-2.391121 2.327(1)-2.330(1) 2.367(4)-2.41914) 2.31012)-2.320(21 2.508(21-2.52812) 2.463141-2.476141

105.4(2l-106.113) 108.6(21-109.9(2) 101.818)-106.8(81 107.4121-109,8121 106.1(4l-109.5(41 108.1(6l-112.416)

TcNlO

Displacement of Tc above the square basal plane.

th)

NTcLiOTcL ("I

(A)

Ref.

0.741(51 0.71 0.66 0.65 0.76 0.59 0.74 0.768(1) 0.88

550 564 565 427 427 566 428 567 432

75


[99mT~V1NC1,]with the ligands (448).Exchange of [99mT~N(dto)212(and the 99Tccomplex) with mnt occurs via an intermediate, presumably the mixed complex [99mTcN(dto)(mnt)12(570). The neutral [99mTcN{S2CNEt(OEt)},] is a promising neutral myocardial imaging agent. Cyclic voltammetry of the 99Tccomplex has shown no oxidation or reduction in the interval + 1.225 to - 1.75 V vs SCE, indicating that this complex should be stable in uiuo (571). A variety of TcN{N,} complexes has been prepared from [TcNCl,(PPh3)21or from [TcNCl,]- (usually in the presence of a n auxiliary reducing agent such as KBH4or PPh,) (572,573).The cationic ethyleneis typical, with v(TcN) a t diamine complex trans-[T~N(en)~Cl]BPh, 1085 cm-', distorted octahedral geometry, a TEN bond distance of 1.603(3)A, and the very marked trans influence of the nitrido ligand, resulting in a n NTc-Cl bond distance of 2.7320(8)8,. In trans-[TcN(tad)ClIBPh, (tad = 1,5,8,12-tetraazadodecane) the Tc-C1 distance is 2.663(2)A (572).Crystallography has shown that the product formed from [TcNC1,(PPhJ2] with excess diethylenetriamine in benzenelethano1 under aerobic conditions is the novel dicationic 50(BPh4),. The mechanism of formation of the zwitterionic NH, +CH2CH2NHCOO-carbamato ligand and the cleavage of the triamine to ethylenediamine is not clear. Under anhydrous conditions in an inert atmosphere, [TcNCl,(PPhJ2] is recovered unchanged, but 50 is readily isolated when a stream of C02is passed into the reaction mixture. The crystal structure shows that the zwitterionic ligand lies in a peculiar "transient state" and is stabilized by strong intramolecular hydrogen bonding (5741. H,

(50)

12+

(51)

Other related complexes are [TcN(cyclam)C1lC1and 51 (X = 01, for which neutrality is achieved by deprotonation of the two amide groups, with the resultant Tc-N bond distance of 2.051 8, significantly shorter than the Tc-NH distance of 2.126 8, (573).In the cationic 51C1 (X = H2)the NTc-OH2 bond is very long at 2.560(2) A (575).Crystallography

76

JOHN BALDAS

has shown the product of the reaction of [TcNBr,]- with bpy in ethanol to be {cis-[TcVNBr(bpy),l}z[TcllBr,]. The formation of the previously unknown [TcBr,],- under mild conditions is unprecedented. With methanol as the solvent the product is cis-[TcNBr(bpy),]BPh, (146,576).In cis-[TcNBrz((pyCHz)2NCHzCMezSBz}l, the Tc-N bond distance of the tertiary amine N atom coordinated trans to the nitrido ligand is 2.47(1) A and that of the pyridine N atoms coordinated cis is 2.141 A (av.). In solution there is a n equilibrium between the dibromo form and one in which a bromide ion is expelled, and the thioether sulfur is coordinated (577). Other structurally characterized complexes are cis-[TcNCl(phen),]PF, (which exhibits a pseudo-twofold symmetry axis that gives rise to reproducible enantiomeric disorder) and cis-[TcNCl(phen)zlC1.HzO(578). The preparations of [TcNL41Cl2(L = py,imidazole) (579) and [TcN(phthalocyanine)l (580) have been reported. The pyridine ligands in [TcN(OH)(py),]BPh, are labile and undergo exchange with pyridine in solution. The [TcN(OH)]+core shows similar "Tc NMR shifts to [TcO,I+. Reaction with tmbtH gives a quantitative yield of trans-[TcN(tmbt),(py),l (581). The most important phosphine complex is the synthetic intermediate [TcNCl2(PPh3),1,which may be prepared by a variety of routes, including hydrazine.HC1 reduction of Tc0,- in the presence of PPh, (5511, reductionlsubstitution of [TcNCl,]- (5521, and substitution of [TcN(tu),Cl]Cl (568). Reaction of PPh3 or AsPh, with [TcNX,]- gives the five-coordinate [TcNX,(EPh,),] (X = C1, Br; E = P, As) in high yield and with the sterically less demanding PMe,Ph, the six-coordinate cis[TCNX,(PM~,P~)~]. The presence of the trans halide ligand in the sixcoordinate complexes results in u(TcN) a t 1048 cm-' (X = C1) and 1028 cm-' (X = Br) compared with 1095-1090 cm-' for the five-coordinate complexes. All these complexes readily undergo ligand-exchange reactions (582). Tri(cyanoethy1)phosphine yields the anionic NBu,[TcNX3L] (583).The reduction of 9 9 m T ~ 0 in 4 - the presence of NH,-NR-C(=S)SMe/PPh,/HCl and addition of the ligand have been developed for the preparation of 99mT~N radiopharmaceuticals (584). The Tc=N bond distances in [ T C N C ~ ~ ( E P(E ~ , )=~ P, ] As) are 1.602(8) and 1.601(5) A, respectively, and the geometry may be regarded as intermediate between square-pyramidal and trigonal-bipyramidal, as shown in Fig. 15 for the arsine complex (585,586). The six-coordinate cis-mer4TcNC1,(PMe2Ph),1 shows NTc-Cl bond distances of 2.441(1) A (cis) and 2.665(1) A (trans) and a T c z N distance of 1.624(4)A (587). For the octahedral trans-[TcNCl(dmpe),]BPh, , a Tc=N bond distance of 1.853(6)A has been reported, but problems in the refinement were noted (576).This distance seems unreasonably long and is likely due

77


FIG.15. The structure of [TcNClz(AsPh&J (586)

to disorder between the trans nitrido and chloro ligands, giving rise to the crystallographic artifact of "distortional isomerism". Cationic [99mTcN(dppe)zC11+ and related complexes undergo in uiuo reduction and are then washed out of the myocardium (588).Cyclic voltammetry has shown that [TcNCl(dmpe),]+ undergoes reversible reduction with unexpected ease at -0.02 V vs SCE when compared with the irreversible reduction of [ReNCl(dppe),l+ at -1.8 V vs SCE (184). Trigonal-bipyramidal geometry is observed for 52,with Tc=N, 1.601(4) A, and a near-linear P-Tc-P angle of 176.5'. The ether oxygens cannot be regarded as coordinated with Tc-0 contact distances of 3.190(2) A. A related [TcNCl,Ll complex with L containing a tertiary amine brid e has square-pyramidal geometry and a Tc-.Naminedistance of 2.70(1) , indicative of incipient coordination (589).

1

COOEt

78

JOHN BALDAS

A variety of complexes containing 25 (R = SH), 27, or PPh, in a mixed coordination sphere has been reported, indicating the versatility of the [TcN],' core (3O8,310,590). The binuclear [{Tc,N2C1,L21(L = zPr2PCH,CH2PPr2)is thought to contain C1 bridges (576). Structurally characterized examples are 53 (5911, [TcNL(PPh,)l (L = tridentate S-methyl dithiocarbazate) (5921, and [TcNCl(PPh,){PhN=C(OEt)S}] (593).The TcN{02S2}core is found in thio-P-diketonato complexes (5941. Structurally characterized TcN{N,S,} complexes are [TcN(tox),] (tox = 8-quinolinethiolate) (5521, [TcN{(~acac)~en}] (540), and [TcN(Me, CNNC(S)SMe),] (592). A novel complex is [TcN(tmbt),L2], where L represents what is generally regarded as a "noncoordinating" proton sponge, 1,1,2,2-tetramethylganidine. The T e N bond distance and v(TcN) are unexceptional at 1.615(6) A and 1057 cm-l (581). Neutral bisaminedithiolato and N-(N"-morpholinylthiocarbony1)-N'-phenylbenzamidinato complexes have been prepared (481,595).

F. IMIDO AND HYDRAZIDO COMPLEXES The reaction of [TcOXJ (X = C1, Br) with ArNCO in toluene yields the moisture-sensitive blue-black [Tc(NAr)X,]- in high yield (278). Imido complexes containing phosphine ligands are formed from the reaction of [TcOC141~/organohydrazine or aromatic amine/phosphine (278, 523). Alternatively, TcO,- may be used as in the reaction with PPh,/PhNHNHCOMe in methanol containing a minimal amount of HC1 to give a good yield of the yellow-green octahedral imido complex [TcCl3(NPh)(PPh3),1, with the phosphine ligands in trans positions. The Tc=N bond distance of 1.704(4) A is longer than that in [Tc0I3+ or [TcN],' complexes, but the Tc=N-C bond angle of 171.8(4)"confirms that the imido(2-) ligand is in the linear triply bonded form. This bonding mode is also consistent with the v(TcN) IR absorption a t 1090 cm-'. Reaction with py/MeOH gives the mixed-ligand complex [TcC13(NPh)(PPh,)(py)] (523, 596). Similarly, in fuc-[TcCl,(NPh)(dppe)], v(TcN) occurs at 1110 cm-I and the Tc=N-C bond angle is 175.7(9)" (523). The hydrazido(2-) complex [TcCl3(NNMePh)(PPh3),1 is formed by reaction of [TcOC1,1-/NH2N(Me)Ph/PPh, in refluxing methanol. With less bulky phosphines, [TcCl2(NNMePh)(PMe2Ph),I+and the structurally characterized truns-[T~Cl(NNMe,)(dppe)~]PF, cations are formed (278). The hydrazido(2-) ligand in the dppe complex is coordinated in the linear (four-electron donor) mode. The reaction of MePhNNH,/dppe/[TcOCl,]- in methanol, however, yields the cationic oxoimido complex truns-[TcO(NH)(dppe),l , with marked asymmetry of +


79

imido complex tr~ns-[TcO(NH)(dppe)~] +,with marked asymmetry of the two axial ligands shown by crystallography (278). The structurally characterized trigonal prismatic diazene complex [Tc(HNNCSPh),(S,CPh)] (597) and the octahedral hydralazino complex [TcCl,(CaH5N4)(PPh3),l(598)were assigned the Tc(V) oxidation state but a n alternative assignment, that of Tc(1) and Tc(II1) species, respectively, has been proposed (523).The deep-green thiobenzoyldiazene complex NBu,[Tc(HNNCSPh),] is also likely to be trigonal prismatic (597). Addition of HC1 to [ T c ~ ~ ~ C ~ ( N N A ~ ) , ( Pyields P ~ , ) ,the ] neutral [TcCl,(NNAr)(NNHAr)(PPh,),] and addition of HBr yields the cationic doubly protonated [TcBr,(NNAr)(NHNHAr)(PPh,),]Br (277).These complexes and the diazene complex [TC(C~H,N,N=NH)~IBP~, have been assigned the Tc(1) oxidation state, but the 99TcNMR chemical shifts fall in the established TdV) region (277, 599). G. COMPLEXES NOTCONTAINING MULTIPLYBONDED LIGANDS Treatment of the 16-electron [T~~~'(diars),Cl,]Cl with chlorine results in oxidative addition, producing the brown 18-electron [Tcv(diars),Cl,]Cl with a magnetic moment of 0.9 BM (191). The crystal structure of [Tc(diars),Cl,]PF, shows DZddodecahedra1 eight-coordination geometry, with Tc-As bond distances of 2.578(2) and Tc-Cl bond distances of 2.442(4) (295).Reaction of NBu4[TcVO(abt),]with 12 M HC1 yields the blue NBu4[TcVCl,(abt)]by removal of the 0x0 ligand in a formally nonredox process. The crystal structure and the magnetic moment of 2.86 BM establish the presence of Tc(V) and thus that the abt ligand is in the doubly deprotonated form (600). The Tc=O bond in NBu4[Tc"O(abt),] is abnormally long [1.73(2) A] (496) and appears to be susceptible to protonation as indicated by 'H NMR evidence of an equilibrium between the anionic and the neutral species in wet CDC1, (600).Removal of the nitrido ligand in AsPh4[TcV1NC1,1by 1,2-benzenedithiol and reduction gives a low yield of AsPh,[TcV(bdt),]. The structure of the anion shows only small distortions from ideal trigonal prismatic geometry, with chelate twist angles for the three dithiolene ligands of 1.1",16.3", and 5.8", compared with the ideal value of 0" (360).This complex is more conveniently prepared in quantitative yield by the reduction of Tc0,- by bdtH, in refluxing EtOH/H20/HC1(558). The intermediate [TcvO(bdt),l- is the kinetically controlled product formed at room temperature. The thermodynamic product [Tc(bdt),]is then formed by removal of the 0x0 ligand in a formally nonredox process. A related complex is [Tc(abt),]- (601).

80

JOHN BALDAS

IX. Tech net i urn(V1)

The [Tcv1014+core is highly susceptible to hydrolysis and disproportionation and unlike [TcV0I3+is not readily stabilized by coordination. The nitrido ligand is, however, very effective in stabilizing Tc(V1) as [TcNI3'. A characteristic feature is the formation of dimeric [NTcOTcNI4+and [ N T C ( ~ - O ) ~ T Ccomplexes N ] ~ ~ that have no analogs for any other transition metal. Complexes not containing an 0x0, nitrido, or imido ligand are relatively few and confined to fluorides and complexes with dithiolene and other noninnocent ligands. Monomeric Tc(V1) (dl) is easily and reliably detected by EPR spectroscopy (401, but the dimeric species are EPR silent due to spin pairing (602).The only binary halide, and the highest fluoride for Tc, is the golden-yellow TcF,, prepared by the reaction of fluorine gas on the metal powder (603, 392). Reaction of TcF, with NOF and N02F yields (NO),[TcF,I and N02[TcF71,respectively. Magnetic moments of 1.72 and 1.67 BM confirm the +6 oxidation state (394).

A. 0x0 COMPLEXES The Tc0,2- and Re0,2- anions are rather less stable than Mn0,2-. Pulse radiolysis and cyclic voltammetry have shown that in alkaline aqueous solution Tc04,- has a lifetime of the order of milliseconds (604). In neutral solution Tc0:decays by a second-order process, about 100-foldmore slowly than Re0:- (605,606).The pK,, of H,TcO, is estimated to be 2-0.5 (607).The paramagnetic, violet (NMe4)2[T~041 [ p e K= 1.60 BM; v(Tc0) a t 780 cm-ll has been prepared by electrochemical reduction of Tc04- in MeCN with rigorous exclusion of air and water. The salt is extremely sensitive to air and atmospheric moisture, which cause rapid oxidation and disproportionation (608,609).Fluorination of Tc yields the blue [TcOF,] (m.p., 134°C)as a by-product (392). The blue monoclinic form is isostructural with ReOF,, the structure of which consists of infinite chains of F-bridged octahedra (610). A minor product, the green hexagonal form, is the F-bridged cyclic trimer with a Tc=O bond distance of 1.66(3) (611). The purple, light-sensitive [TcOCl,] has been prepared from the chlorination of Tc metal (326). Reduction of Tc04- by HC1 in concentrated HZSO4gives a deep-blue solution (Amax = 572 nm), shown by the EPR spectrum to be a Tc(V1) species, most likely [TcOClJ, although the presence of [TcOCl,] cannot be totally excluded. The EPR parameters are gll = 2.057, g, = 1.938, All = 230 x and A, = 96 x lop4 cm-'. After 1 hr the blue color vanishes and the EPR signal decreases (612).Unstable deep-blue


81

solutions containing Tc(V1)oxochloro complexes are also formed by the reduction of AsPh,TcO, in SOCl, or POCl, (613).The reaction of Tc207 with SnMe, gives the sublimable organometallic bis(p-oxo) dimer (54) (228).Coordination about each Tc atom is distorted square-pyramidal with Tc=O bond distances of 1.666(2) and 1.647(2)8, and Tc-Obridge distances of 1.900(2)-1.925(2) 8,. The dimer has been reported to be paramagnetic on the basis of the absence of "Tc NMR signals but the Tc-Tc distance of 2.5617(3) 8, would seem to indicate a single bond and consequent diamagnetism.

(54)

B. NITRIDOCOMPLEXES 1. Monomeric [TcN13+Complexes

The reaction of TcO,-/NaN, in refluxing HX (X = C1, Br) gives high yields of orange-red R[TcNCl,I and intensely blue R[TcNBr,] (R = AsPh,, NBu,] on precipitation with the organic cations (614). The structure of the square-pyramidal [TcNCl,I- is shown in Fig. 16. These

FIG.16. The structure of the [TcNCl41- anion in AsPh4[TcNCl41(614).

82

JOHN BALDAS

salts are air-stable and the remarkable resistance of the TcVINbond to acid hydrolysis is apparent from the method of preparation. Evaporation of the MeCN extract of the dried TcO,-/NaN,/HCl reaction mixture and dissolution of the residue in concentrated HC1 yield an orange-red solution that probably contains HTcNC1,. Addition of CsCl to this solution gives red crystals of the six-coordinate Cs2[TcNC1,] (5521, whereas NEt,Cl gives orange crystals of (NEt,)trans-[TcN(OH,)Cl,] (615). The aqua complex (NEt4)truns-[TcN(OH,)Br41may be prepared in high yield directly from the TcO,- /NaN3/HBr reaction. In concentrated HX solution the major species is most likely [TcN(OH2)X,](615). The [TcNClJ anion is also formed by the oxidation of TcVN species (554, 586, 616 ) and substitution/oxidation of [TcOCl,] with azide (617). The reaction of NH,0S03H with TcO,-/HCl also yields [TcNCl,]- and shows that a single amine nitrogen attached to a good leaving group may serve as an N3- precursor, but the product is contaminated with nitrosyl species and [TcCl6l2-(558).The structural features observed for the isoelectronic [TcVNI2+ /[TcV0I3+pairs are again apparent in a comparison of the [ T c ~ ' N I ~ ' / [ T c ~pairs ~I~+ in Table V. The nitrido ligand exerts the greater trans influence, in terms of the transNTc-OH, bond distance in the aqua complexes, but the OTcX angles are greater than the NTcX angles and the displacement of Tc above the square basal or equatorial plane is consequently greater for the 0x0 complexes. Also, the Tc-X bond distances are significantly greater

TABLE V STRUCTURAL AND IR DATAFOR [TcV1NI3'HALIDECOMPLEXESAND SOME [ T c " ~ ] ~ANALOGS ' Complex

Tc-NI=O iAi

AsPh4[TcNCI41 AsPh4lTcOCl41 AsPh4[TcNBrll AsPh,[TcOBr41 NEt41TcN10H21Br41

1.561i5l 1.593i6l 1.59616 ) 1.613i9) 1.559(9)

NEt4[TcOtOH21Br41

1.616191

[Rb(15-crown-5121 ITcNiOH,)C141 Cs?lTcNCI61

1.600i3) 1600'

Tc-X iA) 2.3220i9) 2.309i2) 2.4616i5) 2.46011) 2.510ill 2.518(11 2.50511I 2.506i 1I 2.320i2) 2.373(5),,, 2.740i511,,,

Tc-OH,

A)

2.44317, 2.317i9l 2.43i4)

OIN-Tc-X ("1 103.34(3) 106.6i1) 103.04(21 106.59(3) 97.212) 98.0i2l 97.6i2l 99.5i3) 94.5i2) 99.73i6l

6'

tA) 0.54

0.67 0.56 0.70 0.33 0.37

0.401

UiTcNIiiTcO) 1cm-l)

Ref.

1063

614 409 618 410 615

1000

405

1074

619

1027

615

1076 1025 1074

Displacement of Tc above the square basal or equatorial plane. The [TcYX,]- iY = N , 01 anions have ideal C,,, symmetry. Value fixed in the refinement due to the statistical disorder of the ligands in the cubic space group.


83

for the nitrido complexes. The v(TcN) IR absorption occurs at higher energy than that of v(Tc"0) but this difference may be accounted for, either partially or entirely, by the greater mass of the l60atom. Thus, the difference betwen v(Tc14N)at 1076 cm-' for AsPh4[TcNC1,1 and v(Tcl6O)at 1025 cm-' for AsPh,[TcOCl,I is less than the 61 cm-' calculated by the simple diatomic oscillator model. The presence of trans halide in Cs2[TcNX51(X = C1, Br) results in the decrease of z4TcN) to 1027 (X = C1) and 1028 cm-' (X = Br) from the values of 1076 and 1074 cm-' for AsPh,[TcNX,I (552, 614, 620). The NEt4[TcN(OH,)C1,1 complex undergoes complete dehydration to NEt,[TcNCl,I under vacuum and the aqua complex is reformed on exposure to atmospheric moisture. The small change in v(TcN) from 1065 to 1070 cm-' on removal of the trans water is indicative of very weak binding and of a Tc-OH, bond distance in the aquachloro complex that is longer than that in NEt,[TcN(OH,)Br,l, which does not undergo dehydration under the same conditions (615). The [Rb(15-crown-5),1[TcN(OH2~C141 salt contains [Rb(l5-crown-5),]+sandwich cations and isolated anions, with a n NTc-OH, bond distance of 2.43(4)A and v(TcN) at 1074 cm-'. Because this salt is prepared from SOC1, solution, the coordinated water presumably arises from atmospheric moisture (619). The 4d' (S = 1) configuration of Tc(V1) results in readily observed EPR spectra at temperatures of >77 K (40,41).The spectra of [TcNX,I(X = C1, Br) have been examined in detail (620,621 ), including singlecrystal EPR, electron nuclear double resonance (ENDOR), and electron spin echo envelope modulation (ESEEM)studies of 15N-enrichedAsPh,[TcNCl,] doped into the diamagnetic AsPh,[TcOC141 host (622, 623 1, as well as single-crystal EPR and 15N powder ENDOR studies of NBu,[TcNBr,]/[TcOBr4] (624). The molecular orbital of the unpaired electron is a combination of the Tc dx, and equatorial ligand p orbitals. Analysis of the hyperfine data for AsPh,[TcNCl,] indicates 20% of the spin density is localized in the 3p orbitals of the C1 atoms (622),but a polarized neutron diffraction study has shown exceptionally high covalence of the Tc-Cl bonds, with 46(5)%of the spin density located on the C1 atoms (625).Interestingly, AsPh4(CF3SO3)may also serve as a host lattice for AsPh4[TcNC141,giving an extremely well-resolved spectrum at 130 K (619). EPR spectroscopy has proven particularly useful for the identification of [TcNI3+species in solution and the monitoring of ligand-exchange reactions. Mixed-ligand [TcNBr,-, C1,I- ( n = 1-31 species have been identified in mixtures of [TcNCl,]- and [TcNBr,l- and equilibrium constants, determined (579, 626, 627). Mixed species are readily assigned because the EPR parameters are nearly linearly dependent on the spin-orbit coupling constants of the equato-

84

JOHN BALDAS

rial donor ligands (40,579,626,628). A 0.002 M solution of Cs,[TcNCl,] in 28.6 M HF shows the presence of the five [TcNF,-,Cl,I- (n = 0-4) species, presumably due to the low activity of fluoride ion in the solution. Partial removal of C1- by the addition of 1 eq. of AgF results in the disappearance of signals due to [TcNCl,I- and [TcNFCl,]-, and after the addition of 3 eq. of AgF only signals due to [TcNF,Cl]- and [TcNF,]- remain. The [TcNF,]- species may be prepared in solution by the dissolution of “TcN(OH),” in 50% HF but has not been isolated. The EPR parameters for [TcNF,]- are gll = 1.895, g , = 1.990, A,, = 377 x lo-,, and A, = 179 x lo-, cm-l(628). EPR studies of AsPh,[TcNX,] (X = C1, br) in organic solvents in the presence of a large molar ratio of X- and of Cs,[TcNX,] (X = C1, Br) in HX solution show no evidence for the equilibrium (620) [TcNXJ-

+ X-

[TcNXS]’-

The mixed-ligand species [TcNCl,(CN)I- and [ T c N C ~ ~ ( C N have ) ~ ] - been identified by EPR in the reaction of [ ( T C N ( C N ) ~ } ~ ( ~ - with O ) ~ IHCl ~(5651, [TcNBr,(NCS)]- and [TcNBr,(NCS),l- in the reaction of [TcNBr,]- with NCS- (5791, and [TcNX,(N,),-,,I- (X = C1, Br; n = 1-4) in the reaction of NBu,[TcNX,] with azide in acetone (629).Also, a variety of [TcNI3+ species such as [TcN(HSO,),I- and [TcN(H,PO,),I- have been identified in concentrated acid solution (630). The oxidation of [TcVNC1,(EPh3),l (E = P, As) to [TcNCl,I- by S0C12 has been shown by EPR to proceed via the Tc(V1) species [TcNCl,(EPh,)l (586). Substitution of R[TcNX,I in organic solvents occurs readily but generally results in reduction and the [TcVNI2+substituted product. Thus, reaction with PPh, , KNCS, Na(S2CNEt2),and 8-quinolinethiol yields [TcNCl,(PPh,),], (NEt,),[TcN(NCS),(MeCN)I, [TCN(S~CNE~,)~], and [TcN(C9H6NS),],respectively (552).Reduction also occurs upon substitution by nonreducing ligands such as bpy or phen (146,578). Substitution reactions in which the Tc(V1) oxidation state is retained are the reaction of AsPh,[TcNC1,3 with LiBr in acetone to give AsPh,[TcNBr,I and of NBu,[TcNBr,] with (sal)enH, to form [TcN{(sal)en}]C1(552,631). Attempts to prepare R[TcNI,l by ligand exchange with LiI in acetone result in oxidation of iodide to iodine. The ease of reduction of [TcNX,1(X = C1, Br) and the inability t o prepare [TcNI,I- may be understood in terms of the lowest energy n X + Tc LMCT transitions at 18,975 cm-’ (X = C1) and 13,100 cm-l (X = Br) for NBu4[TcNX41in MeCN (632). According to the theory of charge-transfer spectra and the optical electronegativity difference of 0.5 between C1- (n1 and I- (n), substitution of iodide for chloride is expected to result in a red shift of -15,000


85

cm-' for the lowest energy Laporte-allowed LMCT transition (633).An LMCT transition may be regarded as the transfer of an electron from a predominantly ligand orbital to a predominantly metal orbital and, if the energy difference is less than about 10,000 cm-', then, commonly, there will be total electron transfer, resulting in reduction of the metal and oxidation of the ligand (634). For [TcNI,l- in organic solvents the first LMCT band is calculated to be at -4000 cm-l and a facile redox reaction is apparent. The [TcNL,]- (L = HSO,, H2P04)species are colorless and show only an intense absorption at 33,800 and 37,600 cm-', respectively, which has been assigned to a TN += Tc LMCT transition (630). has The formation of [99"'TcNC1,]- on reaction of 99mTc0,-/NaN3/HC1 been established (635). After removal of HC1 the residue is stable t o oxidation under acidic conditions but undergoes oxidation to 99m T~04at pH > 4 (636). Addition of ligand solutions results in the formation of 99mTcNcomplexes with biological distributions different from those of 99mTccomplexes prepared from 99mT~04and the ligand by use of complexes are Sn2+or other reducing agents. In general, the 99mT~N cleared from the blood more slowly, indicating greater in uiuo reactivity and the exchange of 99mT~N with serum proteins. Although reaction of [99"'TcNC1,]- with thiolato ligands leads to reduction to Tc(V), the oxidation state with ligands such as gluconate or phosphonates is unclear (635). 2. Dimeric and Polymeric [TcN13+Complexes The very moisture-sensitive neutral TcNC1, may be prepared by the reaction of TcC1, with IN, or NBu,[TcNCl,] with GaCl, . The IR spectrum indicates a polymeric structure with TcNTc and TcC1,Tc bridges. TcNC1, is insoluble in CH,C1, but dissolves on addition of AsPh,Cl due to the formation of AsPh4[TcNC1,1 (637). Addition of 18-crown-6 to a suspension of Cs,[TcNCl,] in SOC1, results in the formation of an orange-red solution, which on slow evaporation of the solvent yields crystals of [Cs(18-crown-6)][TcNC14](619, 638). The structure consists of the unprecedented "infinite sandwich" M+/crown ether configuration with ordered and disordered infinite chains of [TcNClJ anions arranged in an antiparallel fashion (Fig. 17a). In the ordered TEN... Tc=N.-chain the TEN and N-Tc bond distances are 1.561(36) and 2.714(36)A, respectively. An unusual aspect of the structure is that the nearest neighbors of each Cs+cation are two Cs+cations at 4.275 A, whereas the Tc atoms of the four nearest anions are at 7.95 h; and the nearest Cs+...Clcontacts are 6.4-6.6 h;. Also, although each Cs+cation has neighbors a t 4.275,8.55, and 12.825 h; along each vertical column,


87

FIG.17b. A portion of the structure of [ C s ~ l 8 - c r o w n - 6 ~ 1 , [ ~ T c N C l ~ ~(639). ~(OH~~~]

in the horizontal plane the nearest Cs+ neighbors are at 11.23 A. The IR spectrum shows a single u(TcN) absorption at 1041 cm-', but partial 15N labeling results in complex behavior due to the coupling of TcN oscillators in the infinite [TcNCl,]- chains and is diagnostic for this arrangement. At a 12.5% 15N content the spectrum shows essentially two peaks, a t 1042 and 1025 cm-', with the latter peak being predominantly due to ''NTc.-15NTc.-'4NTc groupings. With 50% 15N content, the major "NTc absorption at 1015 cm-' is strong but the 14NTcabsorptions are reduced to two weak shoulders a t 1053 and 1045 cm-'. The EPR spectra over the temperature range 130-290 K indicate the presence of exchange interactions along the -.TcN.-TcN-- chains (619). Recrystallization of the infinite sandwich [Cs(18-crown-6)][TcNC14] from MeCN, acetone, or ethanol, or the reaction of Cs2[TcNCl51with with 18-crown-6 in 6 M HC1, yields [Cs(18-crown-6)14[(TcNC14),(OH2),l two u(TcN) absorptions at 1055 and 1046.5 cm-', which are shifted to 1023 and 1016 cm-' on 15N labeling. Ether diffusion into a n MeCN solution of the aqua complex may result in the crystallization of either the infinite sandwich or a mixture of the infinite sandwich and the

88

JOHN BALDAS

aqua complex. The crystal structure of the aqua complex shows dimeric [N&I’cC~,-.N+I’C(OH~)C~,]~units inside a square cage formed by four [Cs(18-crown-6)]+cations, with two monomeric [TcN(OH2)C1,1- units present in the lattice (Fig. 17b) (639).The [(H30)(18-crown-6)1,[(TcNC14)2(OHz)] complex has also been isolated and shown by crystallography to contain only dimeric [ N 9 c C 1 , ~ ~ ~ N 9 c ( O H 2 ) C l 4units I 2 - (639). Reaction of NBu4[TcNCl41with ( N B U ~ ) ~ [ H ~ P W in ~MeCN ~ O ~results ~I in the incorporation of TcN to give dark crystals of the Keggin polyoxotungstate derivative (NBu4)4[PWllTcN03gl,for which the Tc(V1) oxidation state is thought to be retained (548). A characteristic feature of the chemistry of [TcN13+is the formation of dimeric complexes based on the [NTc-O-TcNI4+and [NTC(~.-O),TCN]~+ cores (423,565, 630,640).Analogous nitrido complexes are not known for any other transition metal but the chemistry and structural aspects of the TcV*Ndimers parallel those of the well-known isoelectronic [OMoV-O-MoV014+ and [OMoV(pO)2MoV012+ dimers (640,641). Hydrolysis of Cs,[TcNCl,] in an ample quantity ofwater gives a brown precipitate of “TcN(OH),,”which has been formulated as the bis(p-0x0) dimer [(TCN(OH)(OH~)}~(~-O)~] (55) on the basis of its reactions and the presence of v(Tc0Tc) absorptions in the IR spectrum (628,640).This precipitate is the isoelectronic analog of “MoO(OH),,”a compound of unknown structure (641). Solutions of 55 in 7.5 MCF3S03H(avery weakly coordinating medium) are orange (A,,, = 474 nm) and EPR silent, showing the absence of monomeric species. The monomeric aqua cation [TcN(OH,),I3+ is thus not a viable species even in strongly acid solution and appears to spontaneously dimerize to the p-0x0 aqua cation [{TcN(OH2)4}2(p-0)14+ (56) (630). 14+

(56)

(57)

Solutions of 55 in 1 M p-toluenesulfonic acid, CF,S03H, or MeS03H are pale yellow and shown by paper electrophoresis to contain a single cationic species (630, 640).Also, dilution of a solution of 56 in 7.5 M CF3S03Hleads to the slow formation of the yellow species. That this species is the bis(p-oxo) aquanitrido cation [{TcN(OH,)~}~(~-O),]~+ (57) is indicated by the similarity of the electronic spectrum to that of the well-established [{MoO(OH2)3}2(p-0)212+ cation, the absence of EPR signals, and the isolation of [{TcN(S2CNEt2)},(p-O),1on reaction with


89

Na(S2CNEt2)(602). Structure 57 has been confirmed in solution by EXAFS studies, but the actual number of coordinated water molecules is uncertain (642). Addition of ethanol to a solution of 55 in aqueous CsOH precipitates the yellow Cs2[{TcN(OH)2}2(p-O)21 with u(TcN) at 1046 cm-' and v(Tc0Tc) at 734 cm-'. On treatment with HC1 this salt is converted to [TcNClJ (640). The only p-0x0 complex to have been isolated and structurally characterized is the cyclic tetramer (AsPh4)4[Tc4N4(0)2(ox)61 prepared by the reaction of AsPh,[TcNCl,] with oxalic acid in aqueous acetone (423). The centrosymmetric structure consists of two [(ox)TcN-0-TcN(ox)l units joined by two tetradentate oxalates (Fig. 18). The TEN bond distances are 1.639(17) and 1.606(17) A and the Tc-0-Tc bridges are only approximately linear,with angles of 150.4(8)"and Tc-Obridgedistances of 1.840(13) and 1.869(13) A. A marked asymmetry due to the trans influence of the nitrido ligand is apparent in the Tc-0 bond distances of the bridging oxalates, with NTc-Ot,,, , 2.410(11) and 2.369(12) A, and NTc-O,,,, 2.076(11)and 2.061(11) A. The p-0x0 structure of the oxalato complex with the nitrido ligands cis to the oxygen

FIG.18. The structure of the anion in (AsP~~)~[Tc~N~(O)~(OX)~] (423).

90

JOHN BALDAS

bridge may be contrasted to the linear (or near linear) [OTcV-0-T~V014td2-d2 dimers, in which the 0x0 ligands are trans to the oxygen bridge. This difference in geometry is explained in terms of closed-shell electronic structures. In the syn conformer (Czvsymmetry) shown in 56 and 60 [and also for an anti conformer (C,,, symmetry)], only one molecular orbital is available for the metal d electrons and the Tc(V1)d'-d' configuration will thus satisfy the closed-shell requirement (544). Addition of AsPh,Cl and then HCl to a solution of Cs,[TcNCl,] in water with sufficient MeS0,H added to dissolve the initial precipitate gives a high yield of the yellow (AsPh4)z[(TcNC12),(p-O)21, and the bromo complex may be similarly prepared (6431. Dithiocarbamato, cyano, and ethanedithiolato complexes have been prepared by the addition of the ligand to solutions of Cs2[TcNCl51in aqueous Na4P2O7(565). The structure of the [ ( T ~ N C l ~ ) ~ ( p - 0anion ) ~ 1 ~(643) in Fig. 19 shows the features of the Tc0,Tc ring system and structural data are given in Table VI. The geometry of the [Tc2N2O2I2+ complexes may be described as two square pyramids sharing the bridging oxygens to give a bent TczOzring. Each Tc atom is displaced above the plane of the four basal donor atoms by 0.50-0.67 A. A comparison of the isostructural [{TcN~S2CNEtz~}z~p-0~21 and [ ( M O O ( S ~ C N E ~ ~ ) } ~(644) (~-O)~I again shows the greater effect of the 0x0 ligand, with the Mo atoms displaced by 0.73 A above the square basal planes compared with 0.65 A for the Tc atoms (565).The Tc-Tc distances of 2.542(2)-2.591(1) A correspond to a d,..'-d~,,l single bond and account for the absence of EPR spectra. The two nitrogen atoms in complexes are N

N

FIG. 19. The structure of the [ ( T C N C I ~ ) ~ ( ~ -anion O ) ~ ](643). ~-

91


TABLE VI STRUCTURAL DATAFOR Complex

T-N

th

[NTC(~-O),TCN]~' DIMERS

Tc-Tc

(A)

T d b &

"4)

6-sbp0 ~

AI

Ref.

~~

I AsPh,)zl{TcNClzjzlp-OlZl

1.648(81 1.65018) 1.6101131

2.5791 11

1.896(71-1.953(5)

2.57511I

IAsPh,121{TcN~CN12}2~p-OIZl 1.70111 I(TcNIS2CNEt2it2( p-0121

tAsPh4121{TcNBr2t21 p-0l21

I{TcN1S~CNC,H,1)21/~-O121

1.62314) 1.624(4) 1.6512) 1.59121

632

1.9%11-1.95( 1)

0.54(11 0.5211) 0.59(11

2.560121

1.921(91-1.924191

0.50

557

2.543(11

1.935131-1.942(3)

565

2.542121

1.934(131-1.9471121

0.65111 0.65(1) 0.65111 0.67111

632

565

Displacement of Tc atom above the square basal plane

bent back from each other to a n N.-N contact distance of 3.3-3.5 A. In the absence of this bending the N-N distance would be the same as the Tc-Tc bond distance and rather shorter than the van der Waals contact distance of about 3.1 A (632).The Tc0,Tc ring system is readily detected in the IR spectrum by the presence of a strong asymmetric stretching mode a t 710-700 cm-' and a weaker symmetric mode a t 515-450 cm-'. These assignments have been confirmed by l80labeling (565, 632). All dimers have the syn stereochemistry shown in Fig. 19 and show two v(TcN) absorptions as a result of the in-phase and outof-phase vibration of the coupled TcN oscillators. For (AsPh,), [(TcNX2)2(p-0)2] these absorptions occur a t 1063 and 1054 cm-' (X = C1) and 1059 and 1051 cm-' (X = Br) (632).Surprisingly, (AsPh,),[{TcN(CN)2}2(p-O)21 does not show significant 4 C N ) IR absorptions (565). The crystal structure, however, shows the CN bond distances to be normal, at 1.12-1.18 A (643). The formation of bis(p-0x0) dimers greatly reduces the susceptibility with of TcVINto reduction. Thus, reaction of (AsPh,),[(T~NCl~)~(p-0)~1 in good yield, Na(S2CNEt2) in MeCN gives [{TcN(S~CNE~~)},(~-O)~I whereas reaction of [TcNCl,]- in the same solvent gives only the reduced [ T C ~ N ( S ~ C N E ~ ~ ) The ~ I Tc02Tc bridge is readily cleaved (565). by HCl in organic solvents. This reaction allows the preparation of Tc(V1) species such as [TcNC12(S2CNEt2)1, which are not accessible by partial substitution of [TcNClJ. The electronic spectra of the bis(p0x0) dimers do not show pronounced visible absorptions (565, 632). The interconversions and equilibria of [TcNI3' species in solutions of inorganic and organic acids have been studied by UV-visible and EPR spectroscopy (630,643,645)and are described by Scheme 1.Monomeric species are identified by their EPR spectra. The pox0 dimers

92

JOHN BALDAS

2[TcNL,

SCHEME 1. L

=

1-

monoanionic ligand.

are readily distinguished from bis(p-oxo) dimers by the intense visible absorption at 470-580 nm, which arises from a transition in the threecenter Tc"O2Tc .rr-bond system. High acidity and the presence of coordinating anions such as C1- favor the monomeric species. The reaction sequence may occur in either direction, depending on whether 55 or Cs,[TcNCl,] is dissolved in the acid. Solutions of Cs2[TcNC1,] in 3.33 M HC1 show only the presence of [TcNCl,]- whereas in 0.5 M HC1 a pink species (A,, = 538 nm) is formed. If Cs,[TcNCl,] is first hydrolyzed and then HC1 added to 3.33 M ,an intensely blue species (Ama = 566 nm) that is converted to [TcNCl,]- by first-order kinetics is formed. The pink and blue species have been formulated as 59 and 60 (L = Cl), respectively (632, 643). At low acid concentrations the interconversions are slow and paper electrophoresis is a useful technique for the separation of species. Thus, when Cs,[TcNCl,] is dissolved in 0.5 M H2S04an orange anionic species and a colorless slow moving cationic species (with some S02- coordination) may be separated (630). The presence of pox0 and bis(p-oxo)dimers in HC1 and H2S04solutions has been confirmed by EXAFS studies (642). C. IMIDOAND HYDRAZIDO COMPLEXES The reaction of [ T c ( N A ~ )with ~ I ] Na/THF at room temperature yields the homoleptic diamagnetic dimer [Tc,(NAr),I (Ar = 2,6-diisopropylphenyl). The dimer is air-stable in solution and adopts an unprecedented "ethane-like" structure (61),with the six imido ligands symmetry equivalent in a staggered arrangement.


(61)

93

(62)

The unsupported Tc-Tc single bond distance is 2.744(1) 8, and the Tc=N bond distances are 1.758(2) 8, (646).Reduction of [Tc(NAr),II (Ar = 2,6-dimethylphenyl) with 1 eq. of Na gives the dimer [Tc,(NAr),(p-NAr),I, which on reaction with MeMgCl undergoes the unprecedented substitution of imido by methyl groups to give successively [TcM~~(NA~)(~-NA~)~Tc(NA~),~ and tetramethyl derivative (62) (647). The Tc-Tc bond distances in the dimethyl and tetramethyl derivatives of 2.673(2) and 2.733(1) 8,, respectively, are similar to the bond distance in 61 and consistent with a d'-d' single bond. For the tetramethyl derivative only the "Z-type" isomer (62) is observed in the solid state. Reaction of the tetrachlorocatecholate complex NBU,[TCO(C~C~,O~)~] with NHzNPhzin CH2C12followed by addition of methanol yields purple crystals of the unusual paramagnetic Tc(V)/(VI) mixed-valence complex N B ~ , [ T C ~ ( N N P ~ ~ ) ~ ( C ~ C ~ ~ ~The ~ )crystal ~I~CH~C~~ structure of the dimeric anion shows the presence of bridging hydrazido(2-) ligands and a Tc-Tc bond distance of 2.612(2) 8,. The complex is EPR silent in various solvents to 196 K but at lower temperatures shows a broad line centered at g = 2.015. In the electronic spectrum a weak absorption a t 12,000 cm-' is consistent with an intervalence charge-transfer band (4221.

D. DITHIOLENE AND RELATED COMPLEXES Green [Tc(tdt),] is formed in about 5% yield from the reaction of TcO,- with Zn(tdt) in 7.5 M H2S04. Electrochemically, [Tdtdt),] is oxidized to [TcV**(tdt),1+ and readily reduced to [Tcv(tdt),1- and with more difficulty to [ T ~ ' ~ ( t d t ) , ](648). ~The deep-green [Tdbdt),] may be prepared quantitatively by the oxidation of [Tc"(bdt),]- with iodine (558).The green [Tc(abt),I complex is formed on allowing a mixture of TcO4-/2-aminobenzenethiol/HC1 to stand overnight (649). The coordination sphere is a tapered trigonal prism, with the three N and S atoms occupying the triangular faces and Tc-N and Tc-S bond distances of 1.982(9)-2.004(8) 8, and 2.339(3)-2.359(3) A, respectively (650). The EPR spectrum of [Tc(abt),] has been analyzed (649, 651)

94

JOHN BALDAS

and the effect of concentration and solvent composition on the spectra of frozen solutions, interpreted in terms of the breakdown of molecular aggregates to the monomeric species (651). Dark-blue [Tc(dbcat),I (A,, = 594 nm; E = 19,000) is formed in high yield from NH4Tc04and 3,5-di-tert-butylcatechol (dbcatH,) in methanol. A well-resolved 10-line EPR spectrum is observed in solution at room temperature. Reversible electrochemical oxidation yields the Tc(VI1) species with surprising ease and there are two reversible reductions to Tc(V)and Tc(1V)species. The coordination geometry of [Tc(dbcat),l is approximately octahedral with the twist angle of 41.7" much closer to the ideal octahedral value of 60" than to the ideal trigonal primatic value of 0". The Tc-0 bond distances are in the range 1.945(6)-1.974(6) A (652). X. Technetium(VI1)

The aqueous solution chemistry of Tc(VI1)is dominated by the stability of the TcO,- anion. Strong oxidizing agents such as HN03 or H202 ultimately, but at varying rates, oxidize all technetium compounds to TcO,- (12).Technetium, unlike rhenium, does not form a heptafluoride (7). The coordination chemistry of Tc(VI1)has been regarded as rather limited but recent results show it to be potentially extensive and novel.

A. 0x0 AND SULFIDO COMPLEXES The only product formed when Tc metal is burned in an excess of oxygen at 500°C is the volatile, crystalline, yellow Tc207(m.p. 119.5"C) (7).In the solid state the structure of Tcz07consists of isolated centrosymmetric molecules with tetrahedral coordination about Tc and a linear Tc-O-Tc bridge with Tc-Obridgebond distances of 1.840 A (653). The oxide dissolves in water t o give a colorless solution of the strong acid HTcO, . Concentrated solutions of the acid are red and on evaporation dark-red crystals of hygroscopic anhydrous HTcO, are obtained (7). The Tc0,- anion absorbs strongly in the UV region a t 244 and 287.5 nm (654) and the red color of HTcO, is thought to be due to a disturbance from tetrahedral symmetry, resulting in the movement of the edge of the 287-nm absorption band into the visible region (27). The alkali metal salts are very stable; KTcO, sublimes at about 1000°C without decomposition (7). A considerable number of Tc0,- salts have been prepared and structurally characterized (655-659). In KTcO the TcO,- anion is tetrahedral with Tc-0 bond distances of 1.711(3) (or 1.724 A if corrected for librational oscillation) (659)but in NMe,TcO,


95

the anion has approximate CSusymmetry with Tc-0 bond distances of 1.589(11)A and 1.696(6)-1.719(9)A (656). The TcO,- anion shows no tendency to form polyanions and, unlike ReO,- (6601,appears to have little tendency to act as a ligand, although it may be present as a counteranion. The E" values for the M04-/M02couples of 1.695,0.738, and 0.510 V for Mn, Tc, and Re, respectively, show that TcO,- is a stronger oxidizing agent than ReO,-, but very much weaker than MnO,- (654). Many technetium complexes in lower oxidation states may be prepared directly from TcO,- in the presence of the ligand and a suitable reducing agent. Key starting materials such as [TcOCl,]and [TcC1612-are readily prepared by the reduction of TcO,- by 12 M HCl in the cold and under reflux, respectively, and [TcNClJ by HCl reduction in the presence of azide (35, 614). A kinetic study of the reduction of Tc0,- by HBr shows that the first-step Tc0,- + [TcVOBr,]is a pseudo-first-order process and the second-step [TcOBr,]- + [TcBr612-is a combination of a first-order with a zero-order process (661). Pertechnetate is an effective catalyst in the oxidation of hydrazine by NO3- or ClO,- (662). A unique property of TcO,- is the remarkable inhibition of the corrosion of soft iron or carbon steels at concentrations as low as 5 x M.The ReO,- anion is inactive in this respect (654). Brown-black Tc2S7may be prepared by H2Sprecipitation from 2-4 M HC1 or H2S04 (7). There is some evidence for the presence of the [TcO,S]- anion in solution (663) but the formation of thiopertechnetates needs further investigation. Reaction of Tc207 with SnMe, yields MeTcO,, the dimer [(Me2(54), TCO)~(~-O ) ~ ]and the polymeric ester {Me3SnOTc0,}, (228, 664). The structure of the polymer consists of infinite zigzag chains with Tc=O bond distances of 1.655(13) and 1.676(15) A and a Tc-Obridge distance of 1.72(1)A (664).The oxide MeTcO, is a much stronger Lewis acid than MeReO, and reacts with olefins such as cyclohexene to form a Tc(V) glycolato complex, which decomposes in the presence of water and acids to stereospecifically produce the cis-diol and the disproportionation products Tc02.nH,0 and TcO,- (228). The ester [TcO,(OSiMe,)] is a useful synthetic intermediate (665, 666). Yellow crystals of [Tc03Fl(m.p., 18.3"C)are formed from the reaction of fluorine with TcO, at 150°C (667)or by the dissolution of NH,TcO, in anhydrous HF (668).In the presence of water, [TcO,F] hydrolyzes to TcO,- and HF (667). The pale-yellow liquid [Tc03C11(b.p., 25°C) is formed quantitatively on heating TcC1, in oxygen at 450°C (326).The vibrational spectra of [TcO,X] (X = F, C1) have been assigned in CSu symmetry (669, 670). The equilibrium Tc03F + HF Tc03+ + HF2has been demonstrated by 99Tcand 170NMR and confirmed by the

*

96

JOHN BALDAS

addition of AsF, to a solution of Tc0,- in HF. NMR has also identified the species [Tc2O5F41 and [TcO,F,I in the reaction of XeF, with [TcO,F]/ HF (671).Pure [Tc02F31(m.p., 200 + l°C) has been isolated from the reaction of Tc,O,/HF/XeF, and consists of open chains of F-bridged cis-Tc02F4octahedral units, with Tc=O bond distances of 1.646(9) A, a Tc-F terminal bond distance of 1.834(7) L$, and a bridging bond distance of 2.080(5) L$ (672). The yellow transitory intermediate formed on addition of TcO,- to 12 M HC1 is thought, by analogy with the ~1~ attempts to isolate the reaction of Re04-, to be f a c - [ T ~ ~ " O ~ C 1but NBu,+ salt result in reduction to [TcOCl,J- or hydrolysis to Tc04- (35). The presence of choline chloride appears to stabilize [TcO3Cl3I2-and the solution remains bright yellow for several hours (673).In the presence of bpy or phen the reaction of TcO,- with ethanolic 12 M HC1 yields a yellow precipitate of [TcO,ClLI and with HBr yields orange [TcO,Br(bpy)l. These complexes are hydrolyzed by water to TcO,- and are reduced by reflux in ethanolic HX to [TcVOX3Ll.In the IR spectra there are three v(Tc0) bands in the range 910-850 cm-' (417). Slurries of [TcO,ClLI (63) (L = phen, bpy, Me4-phen, NO2-phen) in acetone or CH2C12cleanly oxidize olefins at 22°C to give high yields (>70%) of the stereospecific TcVOdiolato complexes (64).

Hydrolysis of 64 with concentrated HCl yields [TcOCl,L] and the stereospecific diol. Thus, reaction of cis-4-octene with [TcO,Cl(phen)l and hydrolysis gives only the meso-diol, whereas trans-4-octene gives 80% of the DL and 20% of the meso isomers, indicating some racemization (L = during the hydrolysis process (674).The binuclear [(TCO,X)~(~.-L)I polynitrogen heterocycle; X = C1, OR) has been prepared from Tc04or [TcOClJ (547). Reaction of TcO,-, the tripodal ligand [(~5-Cp)Co{PO(OR)2},l-, and concentrated HNO, gives [LTc03]in 97%yield. This complex may also be prepared, but in low yield, from the oxidation of [LTcOCl,] (675). The structure of the rhenium analog indicates that in [LTcO,] the geometry is distorted octahedral with coordination by three facial oxygens from the tripodal ligand and three technetyl 0x0 ligands (676). Similarly, HNO, oxidation of [TcVOC12{HB(pz),}]yields [TcO,{HB-

97


(pz),}], which may also be prepared from TcO,-/HB(pz),- in ethanol containing concentrated HzS04.Bubbling ethylene through a CH2Clz solution of [Tc03{HB(pz)3}lyields the glycolato complex [TcVO(OCH, CHzO){HB(pz)3}l.Notably, [ReO,{HB(pz),}I does not react with ethylene due to the greater difficulty of reducing Re(VI1) to Re(V) (677). B. NITRIDOAND IMIDO COMPLEXES In view of the stability of the Tc=N bond and the preparation of

Kz[Re(N)O,], it would seem likely that nitridotechnetic(VI1) acid [Tc(N)O3Hz1,or its salts, could be prepared from the reaction of Tc207 with liquid ammonia or NHz-/NH3,but these reactions have not been attempted (640).Slow evaporation of a solution of Cs,[TcNCl,] in 10% HzOzyields yellow-orange crystals of the explosive nitridoperoxo complex C ~ [ T C N ( O ~ )(Fig. ~ C ~20). I The coordination geometry is a distorted pentagonal pyramid with the nitrido ligand in the apical position [Tc=N, 1.63(2)A] and q 2 peroxo ligands with 0-0 bond distances of 1.41(2) and 1.46(2)A (678).The AsP~,[TcN(O,)~X] (X = C1, Br) complexes are prepared from AsPh,[TcNX41/Hz02 and are thermally more stable. Addition of bpy, phen, or oxalic acid to the pale-yellow solution of “TcN(OH),” in 10% H202yields [TcN(O,),LI (L = bpy, phen) and (679). The crystal structure of the dimeric (AsPh4)z[{TcN(0,)2}2(ox)l the oxalate dimer shows the anion to consist of two TcN(02), units bridged by a tetradentate sideways-bound oxalate with distorted penN

CL

FIG.20. The structure of the anion in CS[TCN(O~)~CI] (678).

98

JOHN BALDAS

tagonal-bipyramidal geometry about each Tc atom (680). In the IR spectra of the nitridoperoxo complexes, 4TcN) occurs a t 1069-1035 cm-', 40-0) at 912-894 cm-'; and u,,(Tc02) at 665-647 cm-' (679). These complexes are the only examples of nitridoperoxo complexes and rare examples of peroxo complexes of a metal in the + 7 oxidation state. The [TcN(O,),] core is isoelectronic with the well-known [MO(O,),] (M = Cr, Mo, W) cores and emphasizes the analogy between isoelectronic [MOO]and [TcNl complexes noted for [TcV'NI3+dimeric species. Oxidation of [TcOCl,]- by H202gives TcO,-, with no evidence for the formation of transitory peroxo species (678). The reaction of NBu,[TcOCl,] with Ph2NNH2and 2,4,6-triisopropylbenzenethiol yields yellow crystals of the novel nitrido-hydrazido(2-), formally Tc(VII), binuclear complex 65.0.5Et20. The geometry about each Tc atom is distorted square-pyramidal with long Tc= NNPh, bond distances of 1.88(1)A, Tc-N bond distances of 1.64(1) A, and Tc-N-NPh2 angles of 140.2(11)"and 141.7(11)".The Tc-S (bridging) distance of 2.470(7) A is significantly longer than the average Tc-S (terminal) distance of 2.379(6) A. The nitrido ligands result from N-N bond cleavage of the organohydrazine (6811.

Ar

H Tc \ ArN411 'NAr

A rS

SAr (65)

N ..

Ar

(66)

The reaction of ArNCO (Ar = 2,6-dimethyl- or 2,6-diisopropylphenyl) with [TcO,(OSiMe,)] yields the imido complex [Tc(NAr),(OSiMe,)l. Tetrahydrofuran solutions of [Tc(NAr),(OSiMe,)] react readily with Grignard reagents to form deep blue-green [Tc(NAr),R] (R = Me, Et, qlallyl) and with F - to give the oxoimido complex [(Ph,P),NI[TcO(NAr),l. Reaction with ISiMe, in toluene yields [Tc(NAr),I]. Crystal structures of [Tc(NAr)&OSiMe,)l and [Tc(NAr),I] (Ar = 2,6-diisopropylphenyl) show approximate tetrahedral geometry and, for the iodo complex, Tc=N bond distances of 1.740(7)-1.763(6) A and Tc-N-C bond angles of 164.8(6)"-169.4(6)". The presence of three imido ligands imparts a high degree of stability to the Tc(VI1) center. Electrochemical studies show that the complexes are difficult to reduce and are also moderately air stable. NMR spectra indicate free rotation about the N-C(Ar) bonds (666).The reaction of KCp with [Tc(NAr),I] rapidly forms the green,


99

air- and water-stable r)'-Cp complex (66).Two of the Tc=N bond distances in 66 are similar, a t 1.748(2) and 1.753(2) A, but the third is significantly longer, a t 1.761(2) A. With a n excess of KCp, the airsensitive K[Cp,Tc(NAr),] is formed and a symmetrical structure is indicated by the 'H NMR spectrum (682). C. COMPLEXES NOTCONTAINING MULTIPLY BONDEDLIGANDS Treatment of NH,TcO, with K/en/EtOH yields the classic hydrido complex K,[TcH,I, isostructural with K,[ReH,l (683).The structure of the [TcH,I2- anion is thus a trigonal prism capped on the three rectangular faces (684).The chemical behavior of [ReHglZpand [TcHg12is similar but the Tc complex is more reactive. In solution, [TcHg12has been shown by 'H and "Tc NMR to be stereochemically nonrigid (6711. The preparation of [TcH,(PEt,Ph),J has been reported (685).The green [Tc(pda),]TcO, is formed on reflux of a solution of Tc0,- and 1,2 diaminobenzene (pdaH,) in methanol. The geometry of the [Tc(pda),l cation is trigonal prismatic with the pda ligands in the paddle wheel arrangement and the six Tc-NH bond distances in the range 1.98(1)-2.03(2) A. The presence of a single dNH) IR absorption at 3235 cm-' confirms that the ligands are in the deprotonated dianionic form (456). +

XI. Appendix: Abbreviations

abtH acacH (acac),enH, AcO atm av. bdtH, bPY Bu 'Bu Bz cdoH, CP 15-crown-5 18-crown-6 cyclam depe

2-aminobenzenethiol acetylacetone N , N'-ethylenebis(acety1acetoneimine) acetate atmosphere average value l,2-benzenedithiol 2,2'-bipyridine n-butyl tert-butyl benzyl cyclohexane-1,2-dioxime cyclopentadienyl 1,4,7,10,13-pentaoxacyclopentadecane 1,4,7,10,13,16-hexaoxacyclooctadecane 1,4,8,11-tetraazacyclotetradecane 1,2-bis(diethylphosphino)ethane

100 diars dmf dmgH2 dmpe dmso dPPe dtoH, &

edtH, edtaH, en

EPR Et EXAFS FABMS HB(pz), hbt HPLC LMCT Me MLCT mntH, ntaH, OphsalH, ox Ph phen pic Pr PY quinH (sacac),enH, salH (sal),enH, SphsalH, tan tctaH, tdtH terPY THF tmbtH tmP tmtu tu

JOHN BALDAS

1,2-phenylenebis(dimethylarsine) dimethylformamide dimethylglyoxime 1,2-bis(dimethylphosphino)ethane dimethylsulfoxide 1,2-bis(diphenylphosphino)ethane dithiooxalic acid molar extinction coefficient (M-’ cm-’) 1,2-ethanediethiol ethylenediaminetetraacetic acid 1,2-ethanediamine electron paramagnetic resonance ethyl extended X-ray absorption fine structure fast atom bombardment mass spectrometry hydrotris(pyrazo1-1-yllborate(1-1 2-(2-hydroxyphenyl)benzothiazolate( 1-) high-performance liquid chromatography ligand-to-metal charge transfer methyl metal-to-ligand charge transfer maleonitriledithiol nitrilotriacetic acid N -(2-hydroxyphenyl)salicylideneimine oxalate(2-) phenyl 1,lO-phenanthroline 4-methylpyridine is0 - propyl pyridine 8-hydroxyquinoline N,” -ethylenebis( thioacetylacetoneimine) salicylaldehyde bis(salicy1idine)ethylenediamine N - (2-sulfidophenyl)salicylideneimine 1,4,7-triazacyclononane 1,4,7-triazacyclononane-N,”,N’’-triacetic acid 3,4-toluenedithiol 2,2 : 6’,2”-terpyridine tetrahydrofur an 2,3,5,64etramethylbenzenethiol trimethylphosphite tetramethylthiourea thiourea


101

ACKNOWLEDGMENTS

I thank Dr. S. F. Colmanet for preparing the structural figures.

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954 (1991). 639. Baldas, J., Colmanet, S. F., Craig, D. C., Rae, A. D., and Williams, G. A,, unpub-

lished results. 640. Baldas, J., Boas, J. F., Bonnyman, J., Colmanet, S. F., and Williams, G. A., Znorg.

Chcm. Acta 179, 151 (1991). 641. Stiefel, E. I., Prog. Inorg. Chem. 22, l(1977). 642. Williams, G. A., and Martin, L. J., personal communication. 643. Baldas, J., Boas, J. F., Colmanet, S. F., Ivanov, Z., and Williams, G. A.,Radiochirn.

Acta, 63,111 (1993). Ricard, L., Martin, C., Wiest, R., and Weiss, R., Inorg. Chem. 14,2300 (1975). Baldas, J.,and Boas, J. F., J.Chem. SOC.,Dalton Trans., 2585 (1988). Burrell, A. K., and Bryan, J. C., Angew. Chem. Int. Ed. Engl. 32,94 (1993). Burrell, A. K., and Bryan, J. C., Organometallics 12,2426 (1993). Kawashima, M., Koyama, M., and Fujinaga, T.,J.Znorg.Nucl. Chem. 38,801(1976). Kirmse, R.,Stach, J., and Spies, H., Inorg. Chim. Acta 45, L251 (1980). Baldas, J., Boas, J., Bonnyman, J., Mackay, M. F., and Williams, G. A,, Aust. J . Chem. 35, 2413 (1982). 651. Baldas, J., Boas, J. F., Bonnyman, J., Pilbrow, J. R., and Williams, G. A,, J.A m . Chem. SOC.107, 1886 (1985). 652. delearie, L. A., Haltiwanger, R. C., and Pierpont, C. G., J . A m . Chem. SOC.111, 4324 (1989). 653. Krebs, B., Z . Anorg. Allg. Chem. 380,146 (1971). 654. Boyd, G. E., J. Chem. Ed. 36,3 (1959). 655. Faggiani, R., Lock, C. J. L., and Poce, J., Acta Cryst. B 36, 231 (1980). 656. German, K. E., Grigor’ev, M. S., Kuzina, A. F., and Spitsyn, V. I., Russ. J.Inorg. Chem. (Engl. Transl.)32,667 (1987). 657. Rochon, F. D., Kong, P. C., and Melanson, R., Acta Cryst. C 46, 8 (1990). 658. Keller, C.,and Kanellakopulos, B., Radiochim. Acta 1, 107 (1963). 659. Krebs, B., and Hasse, K.-D., Actu Cryst. B 32, 1334 (1976). 660. Chakravorti, M. C., Coord. Chem. Rev. 106,205(1990). 661. Truffer-Caron, S.,Ianoz, E., and Lerch, P., Znorg. Chim. Actu 149, 119 (1988). 662. Kemp, T. J., Thyer, A. M., and Wilson, P. D., J . Chem. SOC.,Dalton Trans., 2601 (1993);2607 (1993). 663. Miiller, A., Krebs, B., and Diemann, E., 2. Anorg. Allg. Chem. 353, 259 (1967). 664. Kanellakopulos, B., Raptis, K., Nuber, B., and Ziegler, M. L.,2.Naturforsch. B 46, 15 (1991). 665. Nugent, W. A.,Inorg. Chem. 22, 965 (1983). 666. Bryan, J. C., Burrell, A. K., Miller, M. M., Smith, W. H., Burns, C. J., and Sattelberger, A. P., Polyhedron 12,1769 (1993). 667. Selig, H., and Malm, J. G., J.Znorg. Nucl. Chem. 25,349 (1963). 668. Binenboyn, J.,El-Gad, U., and Selig, H., Znorg. Chem. 13,319 (1974). 669. Guest, A,, Howard-Lock, H. E., and Lock, C. J. L., J . Mol. Spectrosc. 43,273 (1972). 670. Baran, E.J., Spectrosc. Lett. 8, 599 (1975). 671. Franklin, K. J., Lock, C. J. L.,Sayer, B. G.,and Schrobilgen, G. J.,J . A m . Chem. SOC.104,5303 (1982).

644. 645. 646. 647. 648. 649. 650.


123

672. Mercier, H. P. A,, and Schrobilgen, G. J., Znorg. Chem. 32,145 (1993). 673. Aronson, F. L., Hwang, L. L.-Y., Ronca, N., Solomon, N. A., and Steigman, J., Znorg. Chem. 24, 541 (1985). 674. Pearlstein, R. M., and Davison, A., Polyhedron 7, 1981 (1988). 675. Thomas, J. A., and Davison, A.,Znorg. Chem. 31, 1976 (1992). 676. Banbery, H. J., Hussain, W., Evans, I. G., Hamor, T. A., Jones, C. J., McCleverty, J . A,, Schulte, H.-J., Engles, B., and Klaui, W., Polyhedron 9,2549 (1990). 677. Thomas, J. A., and Davison, A., Znorg. Chim. Acta 190,231 (1991). 678. Baldas, J., Colmanet, S. F., and Mackay, M. F., J . C h e n . Soc., Chem. Comm., 1890 (1989). 679. Baldas, J., and Colmanet, S. F., Znorg. Chim.Acta 176, 1 (1990). 680. Baldas, J., Colmanet, S. F., and Williams, G. A., J . Chem. Soc., Dalton Trans., 1631 (1991). 681. Abrams, M. J., Chen, Q . , Shaikh, S. N., and Zubieta, J . , Znorg. Chim. Acta 176, 11 (1990). 682. Burrell, A. K., and Bryan, J . C., Organometallics 11, 3501 (1992). 683. Ginsberg, A. P., Znorg. Chem. 3, 567 (1964). 684. Abrahams, S.C., Ginsberg, A. P., and Knox, K., Znorg. Chem. 3,558 (1964). 685. Kaden, L., Lorenz, B., and Wahren, M., Zsotopenpraxis 18,400(1982).


ADVANCES IN INORGANIC

CHEMISTRY,VOL. 41

CHEMISTRY OF PENTAFLUOROSULFANYL COMPOUNDS R. D. VERMA,’ ROBERT L. KIRCHMEIER, AND JEAN’NE M. SHREEVE Department of Chemistry, University of Idaho, Moscow, Idaho 83843

I. Introduction 11. Pentafluorosulfanyl Halides 111. Pentafluorosulfanyl Hypohalites, SFSOX A. SFSOF B. SF50CI IV. Pentafluorosulfanylalkanes, Alkenes, and Alkynes V. Sulfur Isocyanate Pentafluoride and Sulfur Isothiocyanate Pentafluoride VI. Sulfur Cyanate Pentafluoride, SF50CN VII. Sulfur Cyanide Pentafluoride, SF5CN VIII. Sulfur Isocyanide Pentafluoride, SF5NC IX. Pentafluorosulfanylamine and Other Derivatives X. Pentafluorosulfanyl N,N-Dichloroamine, SF5NCI2 XI. Pentafluorosulfanyl N,N-Difluoramine, SF5NF2 XII. Pentafluorosulfanyl Perfluoroalkylamines, SF5N(H)& XIII. SFSN(CF3)Z XIV. SF5N(X)CF3(X = F, C1, Br, I) x v . SF5N(CI)Rf(R,= CzF5, n-C3F7,n-C4F9) XVI. Bis(pentafluorosu1fanyl)perfluoroalkylamines XVII. Tris(pentafluorosulfanyl)amine,(SF5I3N SF5(CF3)NN(CF3)SF5 XVIII. Bis(pentafluorosulfanyl)bis(trifluoromethyl)hydrazine, XIX. Tetrakis(pentafluorosulfanyl)hydrazine,(SF&NN(SF& x x . Bis(pentafluorosulfanyl)amine,(SF&NH XXI. (SF5)zNX(X = F, C1) XXII. N-Pentafluorosulfanyl Haloimines, F5SN=CX2 (X = C1, F) A. SF5N=CCI2 B. SF5N=CFz XXIII. Pentafluorosulfanyliminodihalosulfanes,SF5N=SX2 (X = F, C1) A. SF,N=SFz B. SF,N=SClz XXIV. Pentafluorosulfanyl-P-sultones and Sulfonic Acids A. ~ ~ - 0 2

B. F50-2 References Visiting Professor (1992), Punjab University, Chandigarh, India. 125 Copyright 0 1994 by Academic Press, Inc. All rights of reprcduction in any form reserved.

126

VERMA. KIRCHMEIER, AND SHREEVE

I. Introduction

Fluorinated compoundscontaining five- and six-coordinatesulfur are of considerable interest. They include those with sulfur as the central atom surrounded by five or six ligands as well as many with six-coordinate sulfur as a functional group, such as pentafluorosulfanyl, SF,. Compoundsin which this group is present are of special interest because they often possess the advantageous properties of the parent compound, SF6,among which are a high group electronegativity, large steric bulk, a nonfunctional hexacoordinate stereochemistry, and high thermal and hydrolytic stability. These properties are manifested in various potential applications such as their use as solvents for polymers, as perfluorinated blood substitutes, as surface active agents, as fumigants, and as thermally and chemically stable systems (11. This chapter gives the reader a broad picture of the synthesis and chemistry of the various classes of pentafluorosulfanyl compounds, many of which are the subject of much ongoing research. I I . Pentafluorosulfanyl Halides

The two compounds of this class that are known, SF,Cl and to a lesser extent SF,Br, are important intermediates in the preparation of derivatives that contain the SF, group. The chloro derivative, SF,Cl, was first prepared in 1960 (2)as a minor product ofthe reaction between SC1, and HF-free F2diluted with nitrogen at - 10°C. Other methods of preparation include chlorination of S2FIo(3),electrolysis of an SC12/HF mixture (41, reaction of C1F with SF, ( 5 , 6 )and KSCN (7), and reaction of chlorine, CsF, and SF, (8). High yields are obtained as shown below (9, 10): SF,

+ CIF

CsF

SF5C1(92%).

25'C. 0.5 hr

A mixture of S2FIoand Br, at 140-150°C results in the formation of SF5Br (11, 12). An alternate method is the reaction of SF, with BrF (or a mixture of BrF, and Br,) at 90-100°C (13). It is reported that SF5C1 (m.p., -64°C; b.p., -21°C) is stable up to ca. 400°C in inert vessels, but decomposes at substantially lower temperature in the presence of Cu/Hg or in ultraviolet light. It is not hydrolyzed by water or aqueous acids, but is rapidly decomposed by aqueous alkali (14): SF5Cl + 7 0 H -

-

+ 5F- + C1- + H' + 3H20.

PENTAFLUOROSULFANYL COMPOUNDS

127

The bromo derivative, SF,Br (m.p., -78°C; b.p., 31"C), is less stable thermally, with decomposition starting at 150°C. The products of the thermal decomposition of SF,X (X = C1, Br) are SF6,SF,, and X, , The products of UV photolysis of SF5Br are SF,, SF,, S2Flo,and Br, (15). The vacuum-UV photolysis of SF,Br in a n Ar matrix a t 8 K provides a convenient method of generating BrF (16). The vapor-phase Raman spectrum of SF5Cl (17 ) ,the argon-matrix Raman and infrared spectra of SF,Cl and SF,Br (181, and the vaporphase infrared and liquid-phase Raman spectra of SF,Br (191,as well as photoelectron diffraction (20)and microwave spectra of SF5C1 (21) and SF,Br (22) have been reported. The ionization potential of SF, (9.65 eV) has been measured by photoionization mass spectrometry of SF,Cl (23). Phosphorus(II1) compounds are oxidatively fluorinated by SF,C1; e.g., C6H,PC1,, (C6H5),PC1, and CH3PC12 form the fluorophosphoranes C,H,PF,, (C,H,),PF,, and CH,PF,, respectively (24). Reaction of SF5C1 with methylamine yields CH,N=SF, (25). The first known metal-pentafluorosulfanyi complex, {PtC1(SF,)[P(C,H,),]2} is synthesized by the reaction of trans-stilbenebis(tripheny1phosphine)platinum(0) with SF,C1 in benzene (26). Under photochemical conditions SF5Cl can be reacted with some simple substrates. For example, with H2 it gives S2F1,(27);with 0 2 , the products are SFBOOSF,and SF,OSF, (28); and with N2F4, SF5NF, is obtained (29).The ability of SF5X to form the stable SF5. radical is a n important feature of its chemistry. In some instances, fluorination takes place with breakdown of the SF, group. Fluorination occurs in the gas-phase photochemical reaction between SF5C1 and SO,, which gives SOF,, S02F2, S02C1,, S2FI0, SF,OSO2F, and SF50SF5(30).Similarly, the photochemical reaction with CO yields COClF, COF,, SF,, S2FI0,SF,, and COS (31). When SF5C1 reacts with trimethylsilylcyanide, (CH3),SiCN, at -1O"C, a white sublimable solid, S(CN), (32),is formed. On reaction of SF5C1with (CF3I2NCNunder photolytic conditions, (CF,),NC(Cl)= NSF, is obtained (33),and with (CF3),NCF2N(C1)CF,, (CF,),NCF,N(SF5)CF3is produced in 50% yield. This product is characterized by spectral (IR, NMR, mass) data (34). The fluorine atoms of SF5X are substituted in reactions with nucleophiles such as dimethylaminotrimethylsilane,(CH3),NSi(CH3)3,and lithium hexafluoropropylidenimine, LiN=C(CF3), ; e.g., SF,X with (CH,),NSi(CH,), at -78°C gives (CH,),NSF,X (35, 36). The equivalence of four fluorine atoms in equatorial positions is supported by 19F NMR of the products (35).The replacement of F rather than C1 or Br in SF5X is probably favored because of the higher SiF bond energy.

128

VERMA, KIRCHMEIER, AND SHREEVE

The reaction of LiN=C(CF,), with SF,X leads to stepwise replacement of fluorine with concomitant fluoride ion migration as shown (37-39 ): SFSX + LiN=C(CF&-

SF3X[=NCF(CF&1 + LiF

SF3X[=NCF(CF3)z] + LiN=C(CF&-

SFX[=NCF(CF3)zlz + LiF.

Both SF,Cl and SF,Br react with unsaturated organic compounds. There are numerous examples of the addition of an SF, group and a halogen atom across a C=C, C=O, C=C, or C=N bond, although under certain conditions fluorination also occurs. The photochemical reactions of SF5C1 with olefins have been studied in detail and are believed to involve a chain reaction as shown below (40).

Similar reactions take place with other olefins (propene, cyclohexene, butadiene, and vinyl chloride). These reactions may also be initiated thermally (90-100°C). Some polymerization is observed. In fact, it is found that with isobutene and styrene only polymerization occurs (41 1. With less reactive fluoroolefins, addition of SF,Cl takes place only under photolytic conditions or by using a radical initiator such as benzoyl peroxide. Trifluoro-, tetrafluoro-, and chlorotrifluoroethylene as well as hexafluoropropylene give mainly 2-chloropolyfluoroalkylsulfur pentafluoride. With excess fluoroolefin, telomers of the type SF,(CF,),Cl are obtained ( 4 2 4 4 ) .Acrylic acid derivatives, CH,=CHR (R = CO,Me, CN), react with SF5C1 in Freon-113 at 120°C to give 32-42% SF,CH,CHClR (45). Addition of SF5X (X = C1, Br) to olefins gives rise to a variety of pentafluorosulfanylalkanes ( 4 6 ) .


129

CH,0CF=CF2 G O ) , CHF=CHF (511, (CH3)3SiCH=CH2 (52), CH,=CFCI, CH2=CHCF3, CHFECHCl, CHF=CFCI (53), CF,=CHCI (54),

X

= C1(40,43,48,49,50,52), X = Br (47,51,52,54)

As shown, the reactions of SF5Br with olefins are the same as those described for SF5C1,but, in general, take place under milder conditions in keeping with the greater reactivity of the bromo compound. Thus, C2H4 and SF,Br give SF,CH,CH,Br at room temperature and addition to the halogenated olefins, CH2=CHF, CH,=CF, , CHF=CF, , and CClF=CF2, occurs without irradiation or use of a catalyst (47, 51 ). Fluorinated polymers are formed when SF,Br reacts with the appropriate fluoroolefins at 90 ? 5°C and autogenous pressures of up to 90 atm for periods of 4 days to 2 weeks (53).

+ CH,=CHF SF,Br

+ CHZ=CFz + CFH=CF2

+ CF2=CF2

(excess)

-

SF5(CH2CHF),,Br

SF5(CH2CF2IsBr SF5(CFHCF2)30Bro.7 SF5(CF2CF2)16Bro,3

With C2H,, no polymer is obtained. In addition to the above polymers, the adducts SF,RBr (R = CH,CHF, CH2CF2, CFHCF,, CFzCFz, CH,CFCl, CH2CHCF3,CHFCHCI, CHFCFCI, CF2CC1H) are also formed (53,54).The monomeric adducts, with the exception of SF,CF2CF,Br, are reported (47).Polymers and telomers containing SF, are found in the patent literature (55)and elsewhere (56). The reaction between SF5X and acetylene is similar to that with olefins (57,58).The addition product SF,CH=CHX can be converted to SF,C=CH [X = C1 (11%)(57), X = Br (-50%) (58)l.Reactions between CH,C=CH or CF3C-CH and SF,Br give SF,CH=C(CH,)Br in 30%yield (b.p., 109 ? l°C) and SF5CH=C(CF3)Brin 58% yield (b.p., 93 ? l"C), respectively. The products are clear, colorless, hydrolytically stable liquids and are identified from their IR, NMR, and mass spectra (59).The reaction of SF5C1with CH3C=COR gives an 85% yield of ethylidenesulfur tetrafluoride, CH3CH=SF4 (60).A trigonal bipyramidal structure is proposed based on NMR studies. The addition of SF5C1to a C=O group occurs less readily than addition to C=C bonds. With ketene the product of reaction at 25°C in a pressure vessel is SF,CH,C(O)Cl. This acid chloride is a useful precursor to the strong acid SF,CH,COOH (45).

130


Addition of SF,C1 to the -C=N functionality is a reaction of considerable preparative significance (8). t ClCN

-

SFSN=CClp

SFSC1 t CFQCN__* SFSN=C(CF3)CI

+ C3FTCN

SF,N=C(C3F,)Cl

It is possible to fluorinate the products using NaF in tetramethylene sulfone. The compound thus obtained from SF5N=CC1,, i.e., SF5N= CF, , forms a mercurial, Hg[NCF3(SF6)I2,with HgF,, and on heating isomerizes to SF4=NCF3. With (C2N2)two molecules of SF5C1add to give SF,N=C(Cl)C(Cl)=NSF,. With methylamine SF,C1 gives CH3N=SF, (61). Irradiation of a mixture of SF&l and (CF3),NC1gives SF,N(CF3)2(62). When SF,Br reacts with pentafluorosulfanyl(fluorosulfuryllketene F5S(S02F)C=C=0, only BrF addition takes place, to give F,SCBr(SO,F)C(O)F (63).Sulfur tetrafluoride is the other product. Pentafluorothiophenyllithium readily attacks SF,Br at - 78°C to form bis(pentafluoropheny1)trisulfane and bis(pentafluoropheny1)disulfane (64).However, in its reaction with SF5C1,C6F,SC1 is produced in addition to the di- and trisulfanes (64).Regardless of the molar ratio of the reactants, these are the only products obtained. Similar behavior is observed in the nucleophilic reactions of trifluoromethylthiolithium. Reaction of SF5X(X= C1, Br) with C6F5Liforms an unstable intermediate, (C,F,),SXF, which is hydrolyzed t o (C6F,),SO. The mechanism for the formation of the compounds isolated is suggested (64). 111. Pentafluorosulfanyl Hypohalites, SF,OX

A. SF50F The first of these compounds to be isolated was SF,OF, formed in low yield by fluorination of SO, or SOF, with F, at 200°C using AgF, as a catalyst (65). It is also obtained in >90% yield by the reaction of F2 with SOF4 at 25°C in the presence of CsF in a static system (66, 67). The salt CsOSF, is believed to be an intermediate, analogous to the report that the salt CsOCF, is an intermediate in the reaction between F2 and COF2in the presence of CsF (68).Pentafluorosulfanyl hypofluorite is also obtained in the reaction between CsOSF, and FS020F at 100°C (69). Although SF50F(b.p., -55.1"C) is thermally stable to about 200"C, at higher temperatures it decomposes to SF6 and 0,(70). Photolysis


131

gives a low yield of the peroxide, (SF&O2, which is consistent with cleavage of the O-F bond on irradiation. That this bond also breaks upon thermolysis is indicated by the formation of SF,0NF2 when the compound is heated with N2F4 (71-73). When SF50Fis reacted with SO2 in the liquid phase, SF,, S02F2,SO,, and SF,0S02F are formed (74). With SF, it gives SF,OSF,, SF500SF5,and SF50SF40SF5.A similar reaction in the presence of O2 gives SF,OSF,OOSF, and SF,OSF,OOSF,OSF, as additional products. These new products react with benzene to give C6H,0SF,0SF,. The reaction of CF,OF with SF, gives CF30SF, as the only product. Trifluoromethyl hypofluorite, sulfur(1V) fluoride, and oxygen react to give CF,OSF,OSF, , CF,OSF,OOSF, , and a compound believed to be CF30SF,00SF,0CF,. A reaction mechanism is proposed (74). Other reactions of SF50Fthat have been investigated include reactions with CO a t 165°C to form COF2 and SOF, and those with CCl, in UV light, giving COF,, C12, and SF,OSF, (75). With NO2, SOF, and N02F are formed (75). Photolytic reaction of SF,OF with oxalyl chloride gives F,SOC(O)F (76). Pentafluorosulfanyl hypofluorite adds readily to a number of alkenes to give only one product, containing the components SF,O and F (77, 78). Because of its facile synthesis, some use is made of SF,OF in electrophilic fluorination (79). The gas-phase structure of SF50F is reported (80). B. SF,OCl This hypochlorite is considerably less stable than the hypofluorite. The hypobromite and hypoiodite have not been prepared. Pentafluorosulfanyl hypochlorite is synthesized from C1F and SOF, in the presence of CsF at -20°C (81, 82). Pentafluorosulfanyl hypochlorite (b.p., 8.9"C) is thermally stable up to about 20"C, a t which temperature it decomposes to SOF, and C1F. Upon photolysis, the peroxide (SF,)2O2 is formed in high yield (83-85) via cleavage of the 0-C1 bond. The yield of the peroxide is much greater than that observed when SF50F is irradiated. Photolysis of SF,OCl with N2F4 (84) and CO (86) gives SF,0NF2 and SF,OC(O)Cl, respectively. Because of the partial positive charge associated with chlorine in the hypochlorite, SF,OCl reacts readily with molecules containing negative chlorine. Seppelt exploited this property (87) and reacted SF,OCl with HC1 at - 95°C to obtain the unstable pentafluoroorthosulfuric acid, SF,OH, which decomposes via elimination of HF at -60°C. Al-

132


though the decomposition of CF,OH is thermodynamically more favored, its greater stability is attributed to a longer intramolecular H-F distance compared with that in SF50H (88,89). A low-temperature addition reaction occurs between SF50C1 and symmetric fluoroolefins to form pentafluorosulfanylalkyl ethers in nearly quantitative yields (78,90-92). It is found that with unsymmetrical olefins the chlorine atom of the hypochlorite most often bonds to the olefinic carbon atom with higher electron density. The reactions of SF50C1and SF,OF with fluorinated ethylenes are used to prepare new SF,O-substituted fluorocarbons in 44-77% yield (93). SFSOC1 t CF,=CFH

+SFSOCF2CFHCI

(44%)

SF5OCI t CF,=CFClSF,OCl

+ CF,=CFBr

SFSOCFClCF2Cl (50%)

-

SF,OCFBrCF,CI

+ SF50CF2CFBrC1 (77%)

The compounds are characterized by "F and 'H NMR, IR, and Raman spectral studies. Compounds of the type SF50Rf are thermally very stable. They do not yield the perfluorovinyl derivative, SF50CF=CF2, on dehalogenation as is the case when CF,OCF=CF, is formed from CF30CFC1CF2C1(94). At room temperature, SF,OCl adds quantitatively to C3F7NCto give C,F7N=C(C1)OSF5. The IR, NMR, and mass spectra are compatible with the structure (95). IV. Pentafluorosulfanylalkanes, Alkenes, and Alkynes

Several perfluoroalkylpentafluorosulfur(V1) compounds, &SF, (R,= CF,, C2F5, n-C3F7,i-C3F7,n-C4Fg,sec.-C4Fg,n-C5F11, i-C5Fll), are reported. When CF,ClSCl is passed over AgF, at 60-70°C with a residence time of 10 min, a 30% yield of SF,CF,, together with CF2C1SF3 (40%) and CF,SF, (30961, is obtained (96). Fluorination of CF,SSCF, with COF, provides SF5CF3in high yield (97). The reaction of CzF5SSC2F5with ClF a t 25°C (10 hr) yields SF5C2F5(17.1%). When RfSCl (Rf = C2F5,n-C3F7,n-C,Fg) is treated with C1F at 25°C (10 hr) small amounts of RfSF5are formed (98).The physical properties and spectral data (IR, "F NMR, and mass) are reported (96,98,99), as is the gasphase electron diffraction structure of SF,CF, (100). Electrofluorination of alkane thiols in AHF leads to variable yields of perfluoroalkylpentafluorosulfur(V1)compounds (101, 102). Dithiols

133


and sulfides also yield SF5Rf. Dithiols give both cyclic and acyclic sulfur(V1) derivatives (1031,e.g., HS(CH2),SH-

ECF

SF5(CF2),,SF5+ ( m F 4

n=4,5

Electrochemical fluorination of sulfur dissolved in CS2in the presence of chlorine gives several products, including SF5CF3,SF5CF2SF5,and CF2C1SF,(104).The reaction of elemental fluorine with branched alkyl mercaptans or sulfides gives SF, organofluorine compounds (105). FZ/He

(CH,)3CCHzSH

- 120°C to rwm temperature

FZ/He

(CH,),CSH

* SFSCF2C(CF3)2F

- 120°C to rmm temperature

Fz/He

(CH,),CSC(CH,),

'SFSCF2C(CF3)3

- 120°C to rwm temperature

SFSCF2C(CF3)2F + C(CF3)3F

F,/He

(CH,)*HCSCH(CH3)2

SFSCF2CFzCF3 - 120°C to rwm temperature

Direct fluorination of CS2with F2diluted with He at - 120°C (3 days), followed by warming to -80°C (3 days), gives a 15% yield of (SF5),CF2 (106,107).The IR spectrum of this product is in agreement with that provided earlier (108). The gas-phase structure of (SF5l2CF2is reported (109). The compound SF,CH, can be obtained from SF5CH2COOAgby the following sequence of reactions. SF5CH2COOAg+ X

2

A SFSCHZX

X2 = Br2 (110,111), 12 (112) SF5CH2X

SFSCH3

Similarly, SF,CH2F (113)and SF5CHF2(114) are also formed. The parent acid is obtained by the reaction of SF5C1with the ketene, CH2= C=O, followed by hydrolysis of the resulting acid chloride (110-112). SFSCI + CHZ=C=O-

SF5CH2C(O)Cl-

HZ0

SFSCHZCOOH

.

134


The silver salt, SF5CH2COOAg,is formed by reacting the acid with silver carbonate (50, 111, 112). When SF5CH2Bris lithiated at -llo"C, LiF is lost on warming to leave the remarkably stable methylene sulfur tetrafluoride, CH2=SF4 (111).It is also synthesized by the reaction of O=C(C1)CH,SF5 with [Mn(CO),]- (115,116). Pentafluorosulfanylalkanesare also obtained by saturation of SF5containing olefins. Dehydrohalogenationof SF,-alkanes results in the formation of SF,-alkenes, which can be converted to additional SF,alkanes. SFSCHFCF2X-

SF&HFCHFBr

KOH

SF,CF=CFz ( 4 3 , 4 7 )

reflux

KOH

(X = Br, CI)

SF,CF=CHF (51)

retlux

A variety of materials add across the double bond in the resulting vinyl-SF, compounds (117), e.g., SF5CH=CH2

70°C

+ SFSBr

SFSCHBrCHzSF5.

8 hr

Attempts to dehydrobrominatethis bis-SF, compound result in the loss of SF, and HF to give SF5CBr=CH2. Indirect "HF" addition (via KF-formamide) to pentafluorosulfur olefins yields hydrylpentafluorosulfur-F-alkanes. SFSCX=CFZ

+ HF

-

SFSCH(XKF3

X = H, F, CF3 (118)

Reaction of SF,CF=CF, with a mixture of I, and IF, gives SF,CFICF, (119).The presence of H in the hydrylpentafluorosulfur-F-alkanesprovides a site to introduce additional functional groups (120).Reaction of SF5CHXCF3with S206F,gives the corresponding fluorosulfate (118, 121).Similarly (CF3),CHSF5with S206Fzat 80°C (25 hr) forms (CF3)ZC(OSO,F)SF, (66%yield) (118).Both (CF3),N0 and C1, add as well. SFSCH=CHX X

=

H, Y

+Y

-

SFSCHYCHXY

(CF3)ZNO;X = CH3, Y

= C1z (121)


135

In some cases, cyclization occurs (122).

SF5CF=CF2

+ CHz=CHCH=CH,

-

CHZ=CHdHCF&H2(!XF)SFS (56)

Trifluoroethenylpentafluorosulfur(VI), SF,CF=CF, , is obtained in 80% yield by debromination of SF,CFBrCF,Br with Cu at 188°C (0.5 hr, 3 Torr). Irradiation of SF,CFBrCF,Br in a silica tube in the presence of mercury gives SF5CF=CF2 in 67% yield (56).Trifluoroethenylpentafluorosulfur(V1) is stable in air up to 300°C. It decomposes at 380°C, giving SF, and unidentified fluorocarbons (56).In perfluorobutylamine SF5CF=CF2 reacts with 0,- or 0, at 25°C (12 hr) to give, respectively, 72 or 52% yields of FC(0)CF20SF5,as well as CF3C1, SOF,, S02F,, and (SF5)20.The IR and 19FNMR spectra of FC(0)CF20SF5are reported (123). When SF5CF=CF2 reacts with NaOCH, in CH,OH a t 60°C (20 hr), SF,CHFCF20CH3 is formed (90% yield; b.p., 73.4"C) (56). In aqueous solution in the presence of K,S,O,-Na,SO,, copolymerization of CF2=CFSF5 with CH2=CF2 at 85°C (20 hr) gives [(CH2CF2)xCF,CF(SF,)], [68% yield; v SF = 867 (vs, br) cm-'I (56).With KF and formamide, CF3CHFSF5 is obtained (86% yield; b.p., 26.9"C) (118). Under similar conditions SF5C(CF3)=CF2 gives a 91% yield of (CF3),CHSF, (b.p., 52.2"C) (118). The former is obtained in -60% yield by shaking CF,CH(SF,)CF,Cl with excess KOH at 20°C for 1hr. The SF, monomer, 2-chloro-3-(pentafluorosulfur)tetrafluoropropene[SF5CF2C(Cl)CF,], is prepared in 20% yield by the addition of SF5Cl to CF2= C=CF, in a metal vessel at 100°C (124). A mixture of AgF in acetonitrile reacts with excess SF5CF=CF2 in 2 2 : 1 molar ratio at 25°C and with stirring to yield AgCF(CF,)SF, that is isolated as the acetonitrile solvate. The solvate is stable up to 50°C. Thermolysis at 80-90°C gives CF,CF(SF,)CF(SF,)CF, as the major product. Other products include C2F5CF(CF3)SF5,SF,, S2F1,, SOF,, CF3CF=CFCF3, and Ag. The solvate reacts with HX (X = C1, Br, OH) at 20°C (1hr) to give CF3CHFSF5and traces of CF3CF(SF5)CF(SF,)CF,. With CH,I at 20°C (12 hr), SOF,, SiF,, and CF3CHFSF, are formed (125). Bromination of AgCF(CF3)SF5 with dry Br, over the temperature range from - 196 to 20°C (12 hr) gives CF,CFBrSF, (45% yield; b.p., 57.2"C) and (CF,CFSF,),. The latter compound is unstable and decomposes to S2F1,and CF3CF=CFCF3 a t 100°C (125).The IR, 19F NMR, and mass spectral data of CF,CF(SF,)CF(SF,)CF, are reported (125). A mixture of SF5CF=CF2 and Br, a t 20°C (20 days) in the dark

136


gives only traces of CF,BrCFBrSF,. Yields of -91% of CF,BrCFBrSF, are obtained on UV irradiation of the same mixture for 20 hr in a silica tube. The photochemical reaction between SF5CF=CF2 and HBr also give CF,BrCFBrSF, in 17.5% yield. An equilibrium mixture of SF,CF=CF2, C1, , and Br, in CH,C1, gives a mixture of CF,BrCFClSF, (9.5%), CF2C1CFC1SF, (l%), CF,ClCFBrSF, (6.5%), and CF,BrCFBrSF, (62%).The same reaction at 80°C (22 hr) gives CF2BrCFC1SF, (13%), CF2C1CFC1SF5(l%), CF2C1CFBrSF, (15%), and CF,BrCFBrSF, (51%) (56). Spectral data (19FNMR) for CF,BrCFBrSF,, CF,BrCFClSF,, and CF,ClCFBrSF, are available (99). Irradiation (UV) of a mixture of SF5CF=CF2 and CF,I in a sealed silica tube gives a 9% yield of CF,CF,CFISF, (56).An equimolar mixture of S2Floand ICF2CF21when pressurized with CF,=CF2 (150 psi) and heated to 150°C for 4 hr yields ICF,CF,SF, (126). Heating the mixture to 150°C with intermittent injection of CF2=CF2 leads to the formation of a product mixture, which on distillation gives three main fractions identified as SF5CF2CF21,SF5(CF2CF2)21, and SF,(CF,CF,),I. The mass spectrum of the residue shows parent ions corresponding to SF5(CF2CF2),I,n = 4-9. With slow warming, a mixture of S2FI0,12, and C2F, (pressurized) (from 20 to 150°C) gives a violet liquid with a n estimated molecular composition of SF,CF2CF21(34%), SF,(CF2CF2)21 (35%),SF5(CF2CF2)31 (14%),SF5(CF2CF2)41 (5%), and SF,(CF,CF,),I (2%) (125, 126). An interesting reaction involving iodo-SF, alkanes is oxidative fluorination by ClF3 (127).

With NaOC1, SF,CF=CF, ,in Freon-113 and in the presence of a phase0

I\

transfer catalyst [N(n-C8H1,),CH3+C1-1, forms the epoxide SF,CFCF2 (128).The epoxide is decomposed by ether and reacts with CsF to give primarily SF4and CF,C(O)F. Carbonyl fluoride reacts with SF5CF=CF2 in the presence of CsF and acetonitrile to give SF,CF(CF,)C(O)F, which reacts further with NH,, CH,OH, and H 2 0 to give, respectively, SF,CF(CF,)C(O)NH,, SF5CF(CF3)C(0)OCH3,and SF5CF(CF3)C(0)OH (129).On dehydration with P4010,the amide, SF,CF(CF,)C(O)NH,, gives the nitrile SF,CF(CF,)CN. All of these compounds, with the exception of the amide, are colorless, stable liquids. The amide is a stable, white solid (m.p., 32-34°C). Trialkylphosphites react with F-alkenes, e.g., CF,CR’=CF,, to give the phosphonates, CF,CR’=CFP(O)(OR), (R = Et, ‘Pr; R’ = F, CF,),

137


and alkyl fluorides (RF) via an Arbuzov-type reaction (130).The use of trimethylsilylphosphites, (RO),POSiMe, (R = Et, SiMe,), is advantageous because of their greater nucleophilicity and the ease of formation of trimethylsilylfluoride (131). Trimethylsilylphosphites react with SF,CF=CF, to give alkenylphosphonates SF5CF=CFP(0)(OR)2 (R = Et, SiMe,) (132).Only (E) isomers are formed. These compounds are colorless, moisture-sensitive liquids. The SF,-substituted iodoperfluoroalkene, SF,CF=CFI(E), is obtained in 24% yield (133)via the reaction oftrimethylphosphine, iodine, and SF,CF=CF,. It is a colorless liquid and is characterized by spectral data (133). The substituted acetylene, SF5C=CH, is obtained in 11%yield from the reaction of SF,C1 with acetylene (134).It is also formed in -9% yield in a four-step synthesis from the reaction of SF,Br with C2H, (135). SF5Br + C H E C H

-

SF5CH=CHBr

SF5CH=CHBr + Br,

SF,CHBrCHBr

F5SCBrCHBr

SF,CHBrCHBr,

%CO, 25°C

Zddiglyme

+

/"=%H Br

/"=%Br

Br

* F,SC E C H

This SF,-acetylene can also be obtained in -50% yield by dehydrobromination of SF,CH=CHBr (135).The gas-phase electron diffraction structure of SF,C=CH is reported (136).In the 'H NMR spectrum, the acetylenic proton resonates at a more shielded position and appears as a pentet with JF-H = 3 Hz (134). This suggests that it couples significantly only with the four equatorial S-F atoms and not the axial S-F atom. Of particular interest are comparative 19F NMR spectral studies of F,SC=CH and other saturated hydrocarbons/fluorocarbons containing the SF, group (137). Pentafluorosulfanyl acetylene is a useful starting reagent for the synthesis of a variety of SF, derivatives of saturated ethers, vinyl ethers, pyrazoles, cyclic alkenes, and alkyl-substituted phenyl-sulfur pentafluorides (134).It is also used for the preparation of a number of F,S-containing alkenes and alkynes (138,139).

+ SF5C=CH SF5Br

+ RC=CH

110°C

3 hr

(R = CH3, CF3)

-

SFSCBr=C(H)SF5

F,SCH=C(Br)R-

KOH

KOH

FSSC~CSFS

F5SC=CR

138


Mono- and bis(pentafluorosulfur)diacetylenes, F,SC=C-C=CH and F,SC=C-C=CSF,, are obtained by the addition of F,SBr to diacetylene followed by dehydrobromination. These monomers yield interesting polymers (140). A variety of uses are proposed for several of these pentafluorosulfur-containing alkanes and alkenes, e.g., as dielectric insulators (122, 141 ), elastomer precursors (142), blood substitutes (143), fumigants (144), and insecticides (145). V. Sulfur isocyanate Pentafluoride and Sulfur lsothiocyanate Pentafiuoride

Pentafluorosulfanyl isocyanate, SF5NC0 and SF5NCS,were first reported in 1964 by Tullock et al. (8).They are obtained from the reaction of pentafluorosulfanyl(trifluoromethy1)aminewith benzoic acid and thiobenzoic acid, respectively. An alternative preparative method of F,SNCS involves thiolysis of F,SN=CC12 with H2S in the presence of NaF.

Hydrolysis of SF,N=CCl, gives only traces of SF,NCO. Another route for the preparation of SF,NCO is found in the reaction of NSF,, COF2, and AHF (146). Reaction of N,N'-bis(pentafluorosulfanyl)urea,(SF,NH),CO, with a slight excess of COF, at 100°C (12 hr) gives essentially pure SF,NCO (147).When COC1, is used instead of COF2,the reaction proceeds much less cleanly. Nevertheless, the infrared spectrum of the product mixture suggests that the reaction proceeds through the formation of a n intermediate cyclic compound, which is not isolated.

With N,N' -bis(trifluoromethyl)urea, the corresponding cyclic intermediate has been isolated and characterized (148). The preparation and purification of SF,NCO are greatly simplified by first preparing SF,NHC(0)F from a n equimolar reaction mixture of NSF,, COF,, and AHF (149), followed by dehydrofluorination (150, 151 >. Although reaction of SF,NH, with COF, gives SF,NCO in good yield


139

(146), the analogous reaction of SF5NH2with either CSClF or CSCl, fails to produce more than 1-2% of SF,NCS (150).Two methods that often produce isothiocyanates in high yield involve the reactions of iminodichloromethanes with either Na2S or P2S5 (152).Reaction of dichloro(pentafluorosulfany1imino)methanewith P2S5 in refluxing toluene gives SF,NCS in 70% yield (150).It is also obtained in -40% yield when SF,N=CCl2 reacts with triphenylphosphine in benzonitrile for 1day (153). Both F,SN=C=O (b.p., 5-55°C) and SF,N=C=S (b.p., 47-48°C) are well characterized by their elemental analyses and IR, 19F, and 13C NMR spectral studies (8, 146, 150). The gas-phase structure of SF,NCO obtained by electron diffraction and microwave spectroscopy is reported (154). The isocyanate, SF,NCO, is easily hydrolyzed to SF,NH, and COP, whereas SF5NCS is hydrolytically very stable. Both compounds undergo addition reactions with substrates containing easily replaceable hydrogen atoms, i.e., alcohols, thiols, and amines. With alcohols (thiols) the isocyanate and isothiocyanate give urethanes and thiourethanes, respectively. SFSNCO + ROH

-

SFSNHC(0)OR

R = CH3, CH2CH2C(O)C(O)CNHSFS,CsH5, I-C6H,C(O)C(O)CNHSF,, (8)~ 4-C6H,OH (15O),C ~ H S C H

SFSNCS + CH30H SFSNCO + RSH SFSNCS + CHSSH

SFSNHC(S)OCH,

SFSNHC(0)SR R

=

CH3, CsH5 (150)

SFSNHC(S)SCHz

The urethanes are stable indefinitely in aqueous solution, but are decomposed by aqueous alkali. The dithiourethanes are, however, unstable and decompose readily at room temperature. With PCl, the urethane SF5NHC(0)CH3gives SF5NC0 as the major product (1551, whereas the fluorosulfonylurethanesgive the corresponding sulfonylchloroimines (156).The thiourethanes, SF,NHC(O)SR, give SF,NCO as well as the imine (155).The crystal structure of SF5NHC(0)SCH3 is reported (157). On treatment of SF,NCO with polynitroalcohols, carbamates are produced. They are potentially energetic and have densities of -2 g/cm3(158). SFSNCO

+ R(N02),CH20H

-

SFSNHC02CH2R(N02),

140


Such carbamates can be further nitrated, e.g., N-pentafluorosulfanyl3,3,3-trinitropropyl carbamate is nitrated with trifluoroacetyl nitrate [a mixture of (CF3CO)20+ 100% HNO31to produce N-pentafluorosulfanyl-N-nitro-3,3,3-trinitropropyl carbamate in 43% yield (159). iCF,COI,O + HNO,

SFSN(NOZ)COzCHZCH2C(N02)3

SF,NHCO,CHzCHzC(N02)3

These nitro compounds, which have a pentafluorosulfanyl group attached, exhibit increased density, decreased shock sensitivity, and good thermal stability and release considerable energy upon detonation (160). Pentafluorosulfanyl isocyanate reacts with ammonia and primary, secondary, and tertiary amines to form a variety of substituted ureas (150). + RNH,

-

SF,NHC ( 0 )NHR

R = H, CH, CH,CH,NHC ( 0 )NHSF,, C,H,,

SF,NCO

+ RR NH

-

4-C6H,CH,C,H,NHC

SF,NHC (0)N R R

R =R

+ R,N

( 0 )NHSF,

= C,H,,C,H,

to)N+R,

SF,N-C

N+R, = +NQ '

, N+H:,

The thioisocyanate, SF,NCS, reacts with aniline to give SF,NHC(S)NHCGH,. The substituted ureas undergo thermal decomposition. The zwitterionic derivatives, SF,N-C(O)N+R, , are far less stable thermally than the analogous fluorosulfonyl derivatives (1611. The reaction of SF,NCO with triphenylphosphine gives some evidence for a zwitterionic compound, but the compound could not be isolated (1611. On the other hand, fluorosulfonyl isocyanate does react with tertiary phosphines, producing the corresponding adducts in high yield (162).The IR, NMR, and mass spectral data for these substituted ureas are reported (150). Dimethylsulfoxide, aldehydes, and formamides react with SF,NCO to give imines and amidines (147,150).

+ (CH,),SO

+ RC(O)H SFSNCO

-co,

SFSN=S(CH,)z

-coy

SFSN=C(H)R

(R = CsH5,4-CGH4CH,, 4-CsH4OCH3)

141


-co,

+ RR'NC(0)H

SFSN=C(H)NRR'

(R= R'

SFSNCO

+ (CH3)2NC(O)CH,

= CH3; R = CH3, R' = CsH5)

-co,

SF,N=C(CH3)N(CH3)2

Certain electron-deficient isocyanates are known to react with organic carbonyls and sulfoxides to yield the corresponding imines (163-165). With acetylacetone, SF,NCO gives the N-(pentafluorosulfanyl) amide of diacetoacetic acid, which enolizes to give two enolic products in CDC13. In (CD,),SO, however, only one enolic form and a keto form is observed. The structures of these tautomers are supported by 'H, 19F,and 13C NMR spectroscopy (150). Analogous reactions with both chloro- and fluorosulfonyl isocyanates are also reported (166).Reactions of SF,NCO with trimethylorthoformate, HC(OMe), , give SF,NHC(0)C(OMe)3 and SF,N(CH,)C(O)OMe (150).The formation of the latter probably takes place via the same mechanism as has been proposed for a similar reaction with chlorosulfonyl isocyanate (167). When SF,N(SiMe,)C(O)OMe is reacted with CsF in the presence of 18-crown-6,Cs(18-crown-6),[SF5NC(0)0Me] is formed. Decomposition occurs to give [Cs(18-crown-6)z][SF,l,which is characterized by singlecrystal X-ray analysis. The SF, anion is described as naked SF,; i.e., the distance between Cs+ and SF,-F is >7 A (167b). At 60-80"C, SF,NCO reacts with PCl, to give SF,N=CCl, (147). The product is identified by comparison of spectral data obtained with those reported earlier (8). Amides containing SF, form when SF5NC0 reacts with carboxylic acids (148). The reaction is believed to pass through a mixed acid anhydride intermediate that loses COz to give the corresponding N-pentafluorosulfanyl amide in good yield. No reaction is observed with carboxylic acids having electron-deficient carboxylate groups. SFSNCO t RCOOH

-

-co,

[SF,NHC(O)OC(O)R]

SFSNHC(0)R

R = CH3 (98%); CH=CH2 (35%)

With (dimethylamino)triethylsilane,fluorine loss occurs via formation of (CH3),SiF (168).

iaopentane

SF5NC0 + (CH3)3SiN(CZH5)2

-196 to -3o'C

SF4=NC(0)N(C2H5)2

142


On reaction with trimethyl(methoxy)silane, cis-CH30SF,NC0 is formed (169). The reaction appears to proceed as shown below, with subsequent intramolecular migration of the methoxy group. SF,NCO

.

F

I NCO I .OCH,

F

S

F'

F,S,

+ (CH,),SiOCH,

,C

( 0 )OCH,

N

I

-(CH,),SiF

TCH3),

F

On further reaction with methanol, the methoxy compound gives a urethane derivative. cZS-CH,OSF~NCOt CH30H

-

c~s-CH~OSF~NHC(O)OCH~

VI. Sulfur Cyanate Pentafluoride, SF,OCN

Seppelt et al. (170)reported the first synthesis of SF50CN by the sequence of reactions below. SF,OCI

+ Cl,C=NCl

Freon-114 -120 to -70°C

-

(16%)

F,SO Hg, -20°C 4 hr stirring

SF50CC12NC1'2

-c1,

SF,OCC1,NCl2

\

c1I"="\c1 (i) (60-90%)

(i)

Hg, 25°C ultrasound

F,SO

+

\

F'

/C=N CI (ii) (10-409)

SF,OCN (10%)

The identifi ation of isomers i and ii is made on the basis of NMR data (170). Only isomer i undergoes further chlorine elimination in the presence of mercury to afford the desired product in 10% yield. Isomer ii under similar conditions decomposes to give SOF, and ClCN. The IR and 14N NMR spectra (170, 172) are used to differentiate between SF5 cyanate and SF, isocyanate.


143

The cyanate, SF,OCN (b.p., 5°C; m.p., -SOT), undergoes rearrangement at high temperature to give SF,N=C=O. The IR, Raman, 19F and 14NNMR, and mass spectral data for SF,OCN are reported (170). Its geometric structure has been determined by gas-phase electron diffraction spectroscopy (173)and the results have been compared with Ab initio calculations (173) the gas-phase structure of SF,NCO (154). are consistent with the experimental geometries of both isomers.

VII. Sulfur Cyanide Pentafluoride, SF,CN

The synthesis of SF,CN by fluorination of methyl thiocyanate, CH,SCN, was first claimed in 1959 (174),but the claim was in error. The actual product isolated was the isomer CF3N=SF2 (175).Various other attempts to synthesize SF,CN, such as by reaction of (FCN), with SF, in the presence of CsF (176),the photolytic reaction between S,F~,, and (CN)2(111, and metathesis between CsSF, and BrCN (177), failed. It was presumed that SF,CN, if formed, isomerized to CF,N=SF, (I 77). The first successful synthesis of SF&N (5%yield) by fluorination of (SCN), in FC1,CCF2C1 with elemental fluorine diluted with N, (1: 10) at -20°C is reported by Losking and Willner (178).It is a stable, colorless gas at room temperature (b.p., -25°C). It does not isomerize as suggested in the earlier literature (177).It does not decompose even on pyrolysis at 350°C. The IR and 19FNMR spectral data and molecular weight data are consistent with the structure (178).The molecular structures obtained by gas-phase electron diffraction and microwave spectroscopy concur (179).

VIII. Sulfur lsocyanide Pentafluoride, SF,NC

The preparation of SF,NC in about 5% yield by the following sequence of reactions is reported by Thrasher (180). SFhN=CClz

+ 3HF

-

SFSNHCF, + 2HC1

SF5NHCF3+ BBr3

SF5N(CF3)BBr2

-HBr

SF,N(CF3)BBrz SFSN=CBr2

SF5N=CBr2

-BF3

MgiTHF

-MgBr,

SF5NC

144


A similar sequence of reactions gives CF,NC in 65-90% yield (1811. Reaction of SF5N=CBr, with lithium alkyls/aryls also produces SF5NC in low yield (-5%) (180).However, SF5N=CC1, under similar reaction conditions produces only traces of the isocyanide. Pentafluorosulfanyl isocyanide is a colorless gas and slowly isomerizes to SF5CN at ambient temperature. The IR and lgFNMR spectral data of SF5NC are reported (180). IX. Pentafluorosulfanylarnine and Other Derivatives

Pentafluorosulfanylamine is prepared in -34% yield by the addition of AHF to NSF3 (182). NSF3

+ 2HF

-

FSSNH2

It is a volatile white solid (m.p., 43°C). The vapor pressure is given by the expression log P,, = -209612' + 9.145. It is soluble in ether even at -78°C. Dissociation into NSF, and HF increases rapidly with temperature and the presence of moisture. It is stable when stored at -78°C and can be handled in a dry glass vacuum system. In aqueous base, hydrolysis occurs. SFSNHz + 6 0 H -

-

SO3NH-

+ 5F- + 3Hz0

Some important reactions of SF5NH, are summarized below.

+ SFI + SOFI + Fz

SFSNHZ + COF2

+ SCI, + PC15

+ SOCI, + Clp-

SFSN=SF*

(183,184)

SFSN=SOFz (185) SFSNFz

(186)

SF5NC0

(146)

SFSN=SClp (187,188) SFSN=PClS (147) SFSN=SC12 (189)

With BF3 and PF,, SF5NH, forms 1:1 adducts (183).Reaction occurs at room temperature between SF5NH2and various acid chlorides and fluorides containing electron-deficient carbonyl groups to produce N-pentafluorosulfanyl amides, F5SNHC(0)R(R = F, CF3, CH,) (149). The reaction of SF5NC0with certain carboxylic acids at room temperature provides a n alternate route for the preparation of amides, SF,-


145

NHC(0)R (149). Malonic acid reacts with SF,NCO a t 60°C to give both the amide acid, SF,NHC(O)CH,COOH, and the diamide, SF,NHC(O)CH2C(0)NHSF5(149). This diamide is also obtained from the reaction of SF,NH, with carbon suboxide (149). 2SF5NHz + C3O2

-

SFSNHC(O)CH2C(O)NHSF,

In water, SF,NHC(O)F gives the urea derivative,. (SF,NH),CO (190). Reaction of the amide SF,NHC(O)R with PCl, gives the corresponding pentafluorosulfanylimines.

-

SFSNHC(0)R+ PC15

60-100°C

CCI,

SFSN=C(Cl)R + POCl3 + HCI

Similar reactions are employed for the synthesis of chlorimines from amides (191 and N-fluorosulfonylimines (192, 193). Acylation of SF,NH, with oxalyl chloride produces the corresponding diamide; SF,NHC(O)C(O)NHSF,, in 78% yield, whereas acylation by fluorosuccinyl chloride yields both the diamide, [SF,NHC(O)CF,I,, and the cyclic succinimide, SF5k(O)CF2CF26O.The identity of all these compounds is confirmed by IR, NMR, and mass spectrometry (149). Nucleophiles such as H20, NH,, and CH,OH open the ring of the cyclic imide to give products such as SF,NHC(O)CF,CF,C(O)X (X = OH, NH,, OCH,) (190).Mono- and disubstituted products, SF,NHC(O)(CF,),,,C(O)F and SF,NHC(0)(CF2)3,4C(O)NHSF5, are formed by the reaction of SF5NH2 with perfluoroglutaryl chloride and perfluoroadipoyl fluoride, respectively (190). The amide acid fluorides are hydrolyzed by atmospheric moisture to the amide acid, SF5NHC(0)(CF2)3,,C(O)OH.The amide, SF,NHC(O)NHSF, (1461, reacts with PCl, to produce the carbodiimide, SF,N=C=NSF, (149). The latter is also obtained by the reaction of SF,NH, with SF,N=CCl2 (147). X. Pentafluorosulfanyl N,N-Dichloroamine, SF,NCI,

Chlorine monofluoride reacts with NSF, at -78°C to give SF,NC12 in 25-32% yield (194, 195). It is also prepared by the reaction of C1, with NSF, in the presence of HgF, (196). NSFB + ClF NSF3 + 2C12

-78°C HgFz

FSSNC12 FSSNC12

146


Pentafluorosulfanyl N,N-dichloroamine is a light yellow volatile liquid [b.p., 64°C (extrapolated; m.p., -120°C]. It is sensitive to mechanical shock and is thermally unstable at 80"C, giving SF,Cl, N,, and Cl,. It is hydrolyzed slowly to give SF,N(H)Cl and finally SF5NH2.It reacts slowly with mercury, producing NSF, . It reacts with PC13 and Se2C12 or Se at low temperature to give SF5N=PC13 and SF5N=SeC1,, respectively, in >80% yield (197). On warming to room temperature, SF,N=PCl, decomposes, giving PF, , Cl, , and (NSCl), .By comparison, SF5N=SeC12 decomposes to give NSF3, SeF, , and SeC1,. The dichloroamine reacts with SC1, or S&l, to give SF,N=SCl,, which is also unstable. With SF5N=SC12, SF5NC12reacts to give SF,N=S=NSF, (198).

With HC1, SF5NC12forms a n adduct, SF,NH,.HCl, which decomposes giving NSF, , HF, and HC1. Analogous reactions of anhydrous HC1 with RfNC1, are well known (199-201 ).

XI. Pentafluorosulfanyl N,N-Difluorarnine, SF,NF,

In 1963 three groups reported the synthesis of SF5NF2(29,202,203). The best preparative method is the UV irradiation of SF, or SF,Cl with N,F,. It is a colorless gas (b.p., -17.5"C) and can be stored in steel cylinders at room temperature. It slowly decomposes on heating to SF, and NF,. The IR, NMR, and mass spectral data (29,202) and the gas-phase structure are reported (204).

XII. Pentafluorosulfanyl Perfluoroalkylarnines, SF,N(H)R,

A 75%yield of SF,N(H)CF, is obtained by the reaction of AHF with perfluoroazomethine, SF,N=CF, (8).Reaction of SF,N=CCl2 and HF also gives the same product. SF,N=CF2

+ HF

-

SFSN(HEF3


147

This amine is a thermally stable liquid (b.p., 28.5-31°C). Although it does not attack glass, it is completely hydrolyzed by aqueous alkali. Its IR, NMR, and mass spectral data are reported (8). The higher homologue, SF5N(H)C2F,,is obtained as one of the products during the reaction of HF with SF5N=C(C1)CF3 (8).It is a stable liquid (b.p., 45.5-47°C). The reaction of HF with SF5N=C(C1)C3F, does not give SF5N(H)C4Fg,but rather SF,N=CFC3F7 (62% yield), which apparently results from the rapid loss of HF by SF5N(H)C4Fg.

XIII. SF,N(CF,),

Dobbie (208) reported the preparation of SF,N(CF3)2 (-10% yield) by prolonged irradiation of a mixture of SF5C1/SF4and (CF3I2NC1.It is a stable compound (b.p., 33°C) and is unaffected by acid or alkali at room temperature. The compound SF5N(CF3)CzF5is also reported (2091.

XIV. SF,N(X)CF, (X = F, CI, Br, I)

The fluorination of sulfur difluoride imides gives SF5N(F)Rf.

When Rf = CF, or CzF6, CsF is used as a catalyst (2051,but when Rf = S02F,the presence of CsF is not necessary (206). Roberts (207) suggests CF3N(F)SF5as the probable structure for the product of fluorination of methyl thiocyanate with elemental fluorine, although the actual workers (174)proposed the alternate structure, SF5CF2NF2. The reaction between SF5N=CC12 and HgFz gives the mercurial, Hg[N(CF3)SF512,in almost quantitative yield (8,210).

Reactions of the mercurial with halogens or interhalogens lead to the formation of a series of pentafluorosulfanyl N-halo(trifluoromethy1) amines, SF,N(X)CF, (X = F, Br, C1, I) (210).

148


2FZ

2SF,N(F)CF,

-HgF2

-

(54%)

PClz

PSF,N(Cl)CF,

-HgCI,

(98%)

H~[N(CF,)SFSI~

2Br2

-HgBr, 21CI

2SF5N(Br)CF3 (80%)

2SFSN(I)CF, (unstable)

When a mixture of SF5NHCF3,AgF,, and C1, is heated, SF,N(C1)CF3 is obtained (8).The N-fluoro derivative is also reported from the direct fluorination of both CF3N=SF2 (205)and SF5N=CF2 (222 ). Halogens do not react with the mercurial Hg[N(SF5),I2(212)or Hg[N(SO2CF3),1 (213). Instead, these N-haloamines are formed either by alternate methods (214,215)or with polar halogenides such as BrOS0,F (213). On the other hand, all of the N-halobis(trifluoromethy1)amineswith the exception of N-fluoro derivatives are obtained from the reaction of Hg[N(CF3),I2with halogens (216).Mews (212) has attributed the lack of reactivity of Hg[N(SF5),l2with halogens to greater N-X bond polarity in the N-haloamines, which would result from the greater group electronegativity of N(SF,), (3.2-3.45) compared with (CF,),N (2.85-3.0). It is also possible that the greater steric bulk of the SF, groups lowers the reactivity of this mercurial compound relative to Hg[N(CF3)2]2.The spectroscopic data [IR, NMR, mass] of the N-halo derivatives are reported (210). Methyl iodide reacts with the mercurial Hg[N(CF3)SF5I2to give a n N-methyl derivative in 33% yield. Hg[N(CFs)SFS]+ 2CH3I

-

2SFSN(CHS)CF3+ HgIz

It is a clear liquid and is identified by spectral analysis (210). The N-bromoamine, SF,N(Br)CF3, adds to the alkenes C2H4 and C,F, to give SF5N(CF3)CH2CH2Br(85%) and the mixture of isomers SF5N(CF3)CF2CF(Br)CF3 and SF,N(CF,)CF(CF,)CF,Br (53%),respectively. These formulations are supported by spectral analysis (210). Similarly, the N-chloroamine, SF,N(Cl)CF,, also reacts with C2H4 and C2F4, giving SF,N(CF,)CH,CH,Cl (88%yield; b.p., 100°C) and SF5N(CF,)CF,CF,Cl (52%).Both are colorless liquids and are characterized from their spectroscopic (IR, NMR, and mass) data (217). The preparation and characterization of CF,N(SF,)TeF, are also reported (218).


149

XV. SF,N(CI)R, (R, = C,F,, n-C,F,, n-C,F,)

The pentafluorosulfanyl N-chloroperfluoroalkylamines, SF5N(Cl)Rf (RF = C2F5, n-C3F7, n-C4F9), are prepared (217) by the reaction of chlorine monofluoride with fluorimines, SF,N=C(F)Rf. The latter are obtained by the reaction of SF5Cl with nitriles (8,217). SF,N=C(F)&

+ CIF-

SF,N(CI)CF,&

& = CzF5 (87%);C3F7(89%);n-C4F9(64%)

TheN-chloroamines, SF,N(C1)C2F5and SF5N(C1)C4Fg, react with C2H4 to give SF,N(C,F5)CH2CH2C1and SF,N(C,Fg)CH2CH,C1, respectively.

XVI. Bis(pentafluorosulfanyl)perfluoroalkylamines

The tertiary amine bis(pentafluorosulfanyl)trifluoromethylamine, (SF&NCF,, is formed in over 90% yield from the gas-phase UV photolysis of SF,N(C1)CF3 (210).

A small amount of hydrazine, SF,(CF3)NN(CF3)SF5,is also produced The tertiary amine, (SF,),NCF,, is a liquid (b.p., in the reaction (8). 72-74°C) and is characterized by IR, NMR, and mass spectroscopy (210).

The tertiary amines bis(pentafluorosulfany1)perfluoroethylamine and bis(pentafluorosulfany1)perfluoropropylamineare prepared by the following sequence of reactions (217).

The yields of (SF5l2NC2F5and (SF,),NC,F, are 58 and 8%,respectively (217).The overall yield decreases with the increased chain length of

150


the perfluoroalkyl group. This decreased yield is attributed to p-elimination of a perfluoroalkyl radical from the perfluoroalkyl chain

Analogous p-eliminations during photolysis of perfluoro-N-chloramines have been observed by Shreeve et al. (219,220). Addition of C1F to unsaturated systems such as N=S (221,222) and N=C (219, 223, 224) is known to produce highly fluorinated N-chloramines. The spectroscopic data (IR, NMR, and mass) of (SF,),NC,F, and (SF,),NC,F, are reported (217).

XVII. Tris(pentafluorosuIfanyl)arnine,(SF,),N

(SF,)3N is obtained in over 90% yield by the UV photolysis of (SF5I2NC1(217,225). These SF,-containing tertiary amines are likely to find commercial applications.

XVIII. Bis(pentafluorosuIfanyl)bis(trifluoromethyl)hydrazine, SF, (CF,)NN(CF,)SF,

The hydrazine SF,(CF,)NN(CF,)SF, is obtained in 62% yield by the reaction of AgF, with F,SN(H)CF, a t 100°C (8). BSF,N(H)CF3 + 2AgFz

-

SFS(CF3)NN(CF3)SFS+ 2AgF

+ 2HF

A similar reaction with (CF3),NH is described (226).A small amount of SF5(CF3)NN(CF3)SF5 is obtained during the photolysis of SF,N(Cl)CF, (210). A hydrolytically stable liquid, SF5(CF3)NN(CF3)SF5,boils a t 103-104°C. It is not attacked by aqueous alkali at 100°C. It reacts with chlorine to give the chloramine ClN(SF,)(CF,), which adds to perfluoropropylene in a fashion similar to that of BrN(CF,), (227).

XIX. Tetrakis(pentafIuorosuIfanyl)hydrazine, (SF,),NN(SF,),

The preparation of (SF,),NN(SF,), (a white solid; m.p., 44.5-46°C) is claimed from the UV photolysis of SF5NClzand SF,CI (228). However, it was later demonstrated that the white solid is (SF,),N (225).


151

XX. Bis(pentafluorosulfanyl)amine, (SF,),NH

When NSFBis fluorinated at low temperature with elemental fluorine, N-(pentafluorosulfany1)iminosulfur tetrafluoride is formed in -50% yield (229). 3NSF3 + 3F2

- 196°C to

SFSN=SF,

-20°C

+ SF, + N2

It is a stable, colorless liquid (b.p., 49°C; m.p., 2) (35-37).

175

GALLIUM HYDRIDES

All these vicissitudes must, we feel, call in question the preparation of anything approaching pure gallane in the period before 1989. Monosubstituted derivatives of gallane, [H,GaX],, have been no less elusive. The displacement reaction Me,N.GaH,Cl(c)

-

-1 [H2GaCll,(1) + Me3N.BF3(c)

+ BF,(g)

(5)

was reported to afford a thermally unstable oil, believed to be monochlorogallane, [H,GaClI, (38).Little headway was made, however, with the characterization of the product, and, in the light of the doubts cast on the analogous preparation of gallane itself and of more recent studies (to be described) (36, 37, 39, 401, this claim too must be treated with caution. By contrast, the same period had witnessed the convincing authentication of several disubstituted derivatives of gallane. These included compounds of the type [HGaX212,where X = C1 or Br, prepared by metathesis (41, 42): 2MeaSiH + GazXs

253 K

[HGaXz12+ 2Me3SiX.

(6)

The chloride forms white crystals that melt at 29°C with decomposition and, when heated to 150”C, decompose quantitatively into gallium “dichloride,” Ga[GaCl,], and hydrogen. Several diorganogallanes were also described. Among these were [Et,GaHI, (43-45) and [Bu’,GaHI, (46)prepared, for example, by reactions such as (7) and (8):

-

Et3Ga + NaGaH, 1 - [Bu’,GaCl],

2

+ LiH

70°C

EtpO. 40-45°C

1 [Et2GaHln+ NaGaH3Et n

1 - [Bu’,GaHI,

(7)

+ LICI.

Only recently, however, with the isolation and more detailed interrogation of the compounds [Me,GaH], (47) and HGa(BH4)2(48-50), has it been possible to discover with any sureness the properties-including the structures-of such compounds. Insofar as these properties have a bearing on the hunting of other gallium hydrides, it will be better that we elaborate on them in due course (in Section IV).

B. COMPLEXES OF GALLANE The confusion besetting the status of gallane itself has certainly not extended to coordinated derivatives, usually of the type LaGaH, ,where

t

/ I

-

c

0

c

GALLIUM HYDRIDES

177

L is a basic species like Me3N or H- (12, 32). Prepared by methods along the lines indicated in Scheme 1, many of these have been known for some years, in several cases as compounds long-lived a t ambient temperatures. Numerous neutral 1: 1 complexes have been prepared with stabilities varying in the sequences Me2NH > Me3N > C5H5N> Et3N > PhNMe, s=Ph3N; Me,N = Me3P > Me2PH; and R3N > RzO or R,S. More or less tetrahedral coordination of the gallium center is the norm, as illustrated in Fig. 1 for the gaseous molecules Me3N.GaH3 (511 and [Me2NGaH212(52),whose structures have been determined by electron diffraction. Measuring about 150 pm, the Ga-H, bonds are somewhat shorter here than in the monohydride GaH [re= 166.21 pm ClS)], but comparable with Ge'"-H and As"'-H bonds. The Ga-H, stretching modes of L.GaH3 moieties have wave numbers that vary with the nature of L (and with the medium) in the range 1720-1880 cm-' (32,5345). Partial replacement of the hydrogen by more electronegative substituents like chlorine shifts v(Ga-H,) to higher energy, as exemplified by the mean wave numbers (in cm-') in the following series: Me3N.GaH3, 1836; Me3N.GaH,Cl, 1902; and Me,N-GaHCl,, 1959. Ill.

Conduct of the Hunt: Practical Methods of Attack

A. HANDLING The gallium hydrides known prior to 1989 are, without exception, unusually susceptible to attack by air or moisture. None of them is thermally robust and some evidently decompose a t subambient temperatures. Of necessity, therefore, the compounds are nearly always handled in uacuo. However, because they tend to dissolve in, or react with, vacuum greases, it is usually a minimum requirement that they be handled in apparatus incorporating greaseless valves and unions with vacuum seals made in a material like Teflon. Yet even these measures are inadequate sometimes to guard against decomposition or reaction with impurities adsorbed on the surfaces of the apparatus. To succeed with the synthesis and manipulation of such hydrides has demanded scrupulous attention to practical techniques, including the development of special procedures (37,56).There follows a brief outline indicating some of the practical and strategic considerations. 1. Vapor Transfer and Sampling In the pursuit of gallane and related hypersensitive compounds, we have found that operations must be carried out at pressures mm

a

b

as deduced from the electron-diffraction patterns FIG.1. Molecular structures of the gallane derivatives (a)Me3N.GaH, and (b) [Me2NGaH2I2 of the vapors (reproduced with permission from Refs. 51 and 52).

GALLIUM HYDRIDES

179

Hg in all -glass apparatus individually constructed and incorporating appropriately sited break-seals and constrictions (37, 56). Figure 2a illustrates a typical assembly. Such apparatus possesses two crucial advantages. In the first place, cooling of all parts can be effected by blowing a stream of cold gas over the exposed surfaces. Second, the apparatus can be rigorously preconditioned by heating under continuous pumping to eliminate volatile impurities adsorbed on the inner walls. Distillation trains are kept short, each trap normally being equipped (a)with constrictions to permit isolation of a sample by sealing under vacuum and (b) with a break-seal permitting access to the contents to be regained under equally rigorous conditions. Even so, the properties of some gallium hydrides impose severe limitations on what

i1 Break-seal ampoule containing gallane

flCold nitrogen gas

containing

FIG.2. Pyrex glass apparatus used (a) for the synthesis and sampling of a base-free gallane and (b) for the admission of the gallane vapor to the chamber of the electrondiffraction apparatus. In (a) A is a sample of [H,GaCI],; B,, B,, and B3 are greaseless valves; C is freshly prepared LiGaH,, LiBH4,or [Bu",NI[B,H,]; D,,Dz,and D3are U-tube traps for fractionation of the volatile components of the reaction mixture; and E is an NMR tube (reproduced with permission from Ref. 56; copyright 1991, American Chemical Society).

180

DOWNS AND PULHAM

can be realistically achieved by way of physical and chemical characterization. Thus, the proclivity to decompose means that vapor pressures must be kept low and this tends to thwart a variety of potentially informative studies involving, for example, the use of Raman or NMR spectroscopy to investigate the vapor. The problems are less acute with more sensitive techniques, like modern FTIR spectroscopy offering sampling times in the order of 1-2 min, and the lifetime of the vapor sample can be extended by going to lower pressures with a compensating increase in path length (as in a multiple-reflection cell) (37, 56). There is also scope, as yet unexploited, for introducing the molecules into a supersonic jet and investigating their microwave, rovibrational spectra, or both, taking advantage of the enhanced signal strength and spectroscopic resolution made possible by the internal cooling induced under these conditions (57-59). Without rigorous sampling techniques, however, there is the risk of confusion caused by reactions of the hydride with adventitious impurities; such is the case, for example, with conventional mass spectrometric analysis. Accordingly, for electron-diffraction measurements it has been necessary to construct a special allglass inlet assembly (37, 56). As illustrated in Fig. 2b, this provides for the direct injection of the gallium hydride vapor from a storage ampoule into the chamber of the diffraction apparatus via a glass channel that can be suitably passivated and then cooled to whatever temperature may be needed to forestall thermal decomposition. But problems, like sorrows, “come not single spies.” The strongly reducing vapor of the gallium hydride is liable to react with the emulsion of the photographic plates used to record the electron-diffraction patterns; the resulting fogging effects can be alleviated, but not eliminated, by various measures (37, 56).

2. Trapping Experiments Such is the thermal instability of the gallium hydrides that meaningful studies of the condensed phases are largely confined to samples at low temperatures. In these circumstances, the tracking and identification of the compounds have called for trapping. This may be achieved physically by quenching the vapor on a cold surface either alone or with an excess of a suitable diluent [as in matrix isolation (SO)];questions of identity and likely structure are then addressed by reference to the infrared or Raman spectrum of the deposit, the attribution of the features being checked by examining the effects of isotopic enrichment of the sample. Alternatively the experimenter may resort to chemical trapping of the hydride by treating it with a compound likely to undergo a fast and quantitative reaction, yielding a known product. Trime-

GALLIUM HYDRIDES

181

thylamine is such a compound and the discovery that the addition reactions (9)and (10) take place cleanly at 178 K has provided irresistible proof of the identities of the relevant gallanes: 1

2 [H,GaC112 + mNMe3 1

- IGaH,],

+ mNMe,

178 K

178 K

H2ClGa(NMe3), (36,39,40)

H3Ga(NMe3), (37,56,61).

(9) (10)

3. Chemical Analysis

To determine the composition of a compound that is more or less short-lived at normal temperatures, it is necessary to devise a method of chemical analysis that can be carried out on site, that is, with the aid of a glass vacuum line. The prime desideratum is to find a suitable reaction of the compound that can be engineered to proceed quantitatively in an evacuated, preconditioned glass ampoule to deliver stable products that are amenable to direct assay, for example, by weighing, tensimetric measurements, or elemental analysis. A good illustration is provided by hydridogallium bidtetrahydroborate) (see Section IV.A), which decomposes at or just above room temperature in accordance with Eq. (11)(48-50):

In this case a Toepler pump can be used to remove the volatile products derived from a known mass of the gallane, and an efficient trap cooled to 77 K to separate the condensable B,H, from the noncondensable H2; the two fractions are then assayed tensimetrically. The mass balance is completed simply by weighing the residue of elemental gallium. Similar measures can be adopted to determine the stoicheiometry of a reaction engaging a gallane with another compound, e.g., NH, ,NMe, , or HC1.

B. PHYSICAL METHODSOF DETECTIONAND ANALYSIS It will be evident from the preceding account that not all physical methods lend themselves equally well to the detection and specification of the gallium hydrides. There are, for example, obvious difficulties in trying to grow single crystals of a low-melting, thermally unstable compound, and, although the case of hypofluorous acid (62)shows what can be achieved ultimately, X-ray methods do not obviously commend

182

DOWNS AND PULHAM

themselves at the first sighting of the molecular quarry. In any case, there is ample circumstantial evidence that aggregation is a primary motif in gallium hydride chemistry and that the form of the compound may well vary from one phase to another. Three methods have in the event formed the spearhead of the present hunt, namely, (a) vibrational spectroscopy; (b) NMR spectroscopy; and (c) electron diffraction of the vapor. Some of the more robust molecules, like HGa(BH,), (48-50) and H2GaB3H,(63), can be identified by their mass spectra, and UV photoelectron spectroscopy has also been turned to account in studies of gaseous tetrahydroborate molecules like ANBH,), (64 and HGa(BH,), (501, notably for the light it sheds on the mode of coordination of the BH, ligand. Compared with the three primary methods, however, such sources of supplementary information are barely in the running. 1. Vibrational Spectroscopy

Identification of a gallium hydride sample has been accomplished in the first place more often than not by reference to its infrared spectrum, supported sometimes by its Raman spectrum. To test and elaborate upon the inferences drawn from such a spectrum, we can appeal to various particulars; most informative of these are (i) signs of rotational fine structure associated with the infrared absorptions of the vapor, (ii) the response to isotopic changes, and (iii) analogies with the vibrational properties of related, authenticated molecules. As with the characterization of metal carbonyls, it is the stretching vibrations that by their energies and intensities offer the most telling commentary on the molecular identity. Where v(C-0) modes differentiate terminal from bridging carbonyl functions, so v(M-H) modes differentiate terminal from bridging M-H functions in hydride derivatives of a Group 13 metal, M. Studies carried out in Oxford supplement earlier measurements in establishing, for example, that the stretching modes of terminal Ga-H bonds give rise to strong infrared absorptions in the range 1720-2050 cm-' (Refs. 32,37, 38, 41, 42,48-50, 54-56, 63, and 65, for example). This contrasts with bridging Ga-H-Ga units for which v(Ga-H) occurs with variable intensity in the range 900-1720 cm-' (37,47,56,66,67). A similar distinction can be drawn between terminal Al-H and bridging A1-H-A1 moieties (12, 47, 68, 69), just as the number, energies, and intensity patterns of v(B-H) modes are familiar signatures for the BH, ligand in different types of coordination (70, 71 ). For an M-H-M bridge there are two stretching vibrations depicted

183

GALLIUM HYDRIDES

4 Ga. . .

. ..Ga

Ga . .

.....Ga

(lb)

(la)

schematically in l a and lb, which may be described as antisymmetric bas) and symmetric (us), respectively. The wave numbers of these two vibrations are linked via the simple relationship (12) to the M-H-M bond angle 8: uas/us= tan(8/2).

(12)

The relation is subject to certain approximations, namely (a) that the mass of M is effectively infinite compared with that of hydrogen; (b) that the angle-bending force constant, k,, is much smaller than the bond-stretching force constant, k,, and that neither u, nor u,, experiences significant mixing with any other motion of the molecule at large; and (c) that the stretch-stretch interaction constant, k,, is negligible (72). As 8 approaches go", so, according to Eq. (12), the ratio u,,/u, approaches unity; i.e., the separation between u,, and u, tends to zero, and, as 0 approaches 180",the ratio and frequency separation increase. There is a concomitant change in the relative intensities of the two infrared absorptions corresponding to these modes, such that the ratio Ia,/I, runs from near unity to infinity as 0 ranges from 90 to 180". Such considerations have been invaluable in the first sighting of hydrogenbridged molecules like dimethylgallane, [MezGaH], (471, and gallane itself, [GaH,], (56). At the same time, a relatively simple molecule like Ga2H6(37,561 or GaBH6(37,651 may be expected to betray its presence in the gas phase by infrared absorptions modulated by rotational structure. Scrutiny of this structure, preferably at high resolution, opens up the prospect of determining the symmetry properties of the associated vibrational transitions and of securing at least a rough estimate of one or more of the rotational constants. Thus, GazH6with a diborane-like structure (2) featuring two massive gallium atoms on a common symmetry axis H

184

DOWNS AND PULHAM

has a momenta1 ellipsoid approximating to that of a highly prolate symmetric top with ZA G ZB = I,. Such a molecule may be expected to show infrared absorptions of two kinds according to whether the dipole change associated with the vibrational transition is parallel or perpendicular t o the Ga-Ga axis. A parallel-type band should be dominated by more or less structured P- and R-branches flanking a single, very narrow Q-branch, whereas a perpendicular-type band should be dominated not by the P- and R-branches, which form an unresolved or only partially resolved background, but by a regular series of Q-subbranches with a spacing approximating to 2(A’ (where the rotational constants A ’ and B ’ refer to the u = 1 vibrational state and B ’ is the average of the two constants B’ and (2’). Under modest resolution a parallel-type feature is expected to show a P-R branch separation, AuPR,to which we may appeal, through another approximate relation, viz. Eq. (131, for an estimate of the mean rotational constant B:

a’)

Because the magnitude ofB is governed mainly by the Ga...Ga distance, it is thus possible to gauge this distance to a useful approximation. A similar approach was used to arrive at the first estimate of the Xe-F bond length in the linear molecule XeFz (73).How the results of such studies in practice point irresistibly to the fugitive species GazH6 is more aptly treated later (Section V.B.2).

2 . NMR Spectroscopy The normal edge that NMR measurements enjoy in so many matters of detection and characterization has been somewhat blunted where gallium hydrides are involved. The studies carried out to date have been concerned exclusively with hydrocarbon solutions of the compounds, sometimes at room temperature, more often at low temperatures (down to 190 K). One immediate problem is whether the compound has the same form under these conditions as in the vapor at low pressure or in the solid phase. The signs are that aggregation becomes a serious issue in solution where a given gallium hydride often exists in more than one oligomeric form. Further problems stem from the quadrupolar character of the two naturally occurring gallium isotopes, 69Ga (60.108%, Z = I) and 71Ga(39.892%, Z = $1 ( l o ) ,which causes marked broadening of the ‘H resonances originating in hydrogen atoms bound t o gallium. The ‘H signals do sharpen when the solution is cooled, but

GALLIUM HYDRIDES

185

the spectra hold relatively little structural information, beyond the finding that terminal hydrogen atoms have chemical shifts in the range 6 4.0-5.5, whereas bridging hydrogen atoms resonate at lower frequency (6 1-3.1) (32, 36, 37). The 71Ganucleus (which is superior in its receptivity and width factor to the more abundant 69Ga)has featured in numerous NMR experiments (74,751, but the line widths associated with gallium atoms in less than symmetrical environments make it relatively unattractive as a marker for a gallium hydride. Rather more instructive are the NMR spectra of mixed gallium boron hydrides like GaBH, and GaB,H,, , with which “B measurements help to build up a fuller picture. Yet further complications are in store. First, there is the susceptibility of any hydrogen-bridged framework to undergo facile exchange. Fluxionality is a familiar phenomenon in metal tetrahydroborates, most investigations implying magnetic equivalence of the four protons of each BH4 ligand irrespective of its coordination mode and of the temperature (71,76).Compounds containing a metal coordinated to an octahydrotriborate group, B,H,, show a wider repertoire of properties, ranging from the essentially rigid behavior of Mn(B,H&CO), to the rapid exchange of boron and hydrogen atoms in Be(B,H,)(C,H,) and M+[B,H,l- salts (63, 77-78). A rigid system involving bidentate ligation of the B,H, group, with the adoption of a B4Hl,-like structure, gives a distinctive “B NMR spectrum featuring two resonances with intensities in the ratio 2 : 1 and that exhibit characteristic chemical shifts. On the other hand, coupling patterns frequently belie the simplicity of their appearance, being invariably subject to second-order effects. Another potential source of confusion comes from relaxation phemomena. This problem is exemplified by the effects of cooling a metal tetrahydroborate sample, resulting typically in the collapse of the multiplet structures of the ‘H and “B resonances. Such a change reflects the decrease in the spin-lattice relaxation time T,brought about by the increased viscosity of the sample at low temperatures; the accelerated interconversion between nuclear spin states can lead ultimately to decoupling of the ‘H and “B nuclei, a phenomenon referred to as “thermal” or “correlation-time” decoupling (711. 3. Electron Diffraction Electron diffraction needs no introduction as a relatively simple and direct tool for determining the interatomic distances within a molecule in the vapor state, in which it is free from the constraints and perturbations of the condensed phases (79,801. In an ideal situation, the method is capable of defining quite precisely the positions of hydrogen atoms

186

DOWNS AND PULHAM

because the electron scattering, unlike X-ray scattering, originates from close to the nuclei and the intensity is determined by the product of scattering factors for the components of a given atom pair. There is scope to determine the structures of quite complicated molecules, always provided that the inherent shortcomings of the method are recognized. One problem arises from the low resolution of electron-diffraction measurements. The experimental results, after various preliminary manipulations, can be presented as a plot of scattering intensity against scattering angle, which contains one-dimensional information only. Fourier transformation of the complex wave form gives, as in Fig. 3, g radial distribution curve that contains one peak for each distance in the molecule. If two or more of these distances are similar, the corresponding peaks will overlap and it may be impossible to resolve them. In these circumstances the geometric and vibrational parameters used to specify the molecular model tend to act in concert or to be subject to correlation, and, unless one or more of the parameters can be constrained on the basis of additional, independent information, there must be a degree of uncertainty about the structure analysis. The problem is clearly exacerbated for a relatively weakly scattering atom pair (like Ga-HI, which has a separation more or less coincident with that of a more strongly scattering pair (like Ga-N), to the detriment of the accuracy with which the separation of the first pair can be determined. Sometimes, too, the scattering pattern may fail to distinguish clearly between different model structures for a particular compound. This has proved to be the case, for example, with gaseous Be(BH,),, the nature of which has still to be resolved (79, 80). The fortunes of Be(BH4I2illustrate inter alza the potential mischief that impurities may cause, particularly if the impurity molecules carry interatomic distances similar t o those in the sample molecule. Understandably, therefore, it is imperative that pure samples be used and that steps be taken to prevent decomposition or contamination of the sample en route to the diffraction chamber. Another problem arises from the fact that each distance determined by electron diffraction is averaged over the vibrations of the molecule. The effect is most acute for a molecule undergoing one or more vibrations of large amplitude, as with the bending vibration of linear C1-Hg-C1 or the out-of-plane puckering vibration of the central fourmembered ring of Cl,Ga(p-Cl),GaCl, ; certain nonbonded distances then appear on average to be appreciably shorter than expected. Unless a correction is made for the so-called “shrinkage”effect (ideally through calculations requiring a detailed knowledge of the vibrational proper-

187

GALLIUM HYDRIDES

I , Scattering

0 .

I

'

I

40

3;io

intensity

slnm-1

Fourier transformation

Ga-H, C-N

I

--. L

L

a

Radial

l o Ga. . H ,

c

distribution curve

r Ipm

N FIG.3. The molecularscattering intensity pattern for the Me3N.GaH3molecule related by Fourier transformationto the corresponding radial distribution curve. The difference between the observed and calculated radial distribution curves is also shown (adapted from Ref. 51 ).

188

DOWNS AND PULHAM

ties of the molecule), the analysis may give one to believe that a linear molecule is bent or that a planar one is nonplanar. Shrinkage need not be a problem with a molecule whose vibrational properties are well characterized or when there is independent evidence from some other source affording a realistic estimate of the magnitude of the effect. Otherwise, though, there is no escaping the uncertainty implicit in the vibrational-averaging of interatomic distances. IV. Toward Gallane: Preparations for the Hunt

A. DIMETHYLGALLIUM TETRAHYDROBORATE One of the first compounds containing a Ga-H bond to receive detailed attention was the mixed gallium boron hydride Me,GaBH,. First reported in 1943 as the product of the interaction of Me3Ga with B2H6 (811, the compound is more satisfactorily prepared by the reaction of Me,GaCl with freshly recrystallized lithium tetrahydroborate in the absence of a solvent at 258 K (see Scheme 2)(82). The preferred method is significant in pointing the way to what has generally proved to be the most productive and dependable strategy for the synthesis of hydrides of the heavier Group 13 elements. Avoidance of a solvent is dictated by the need to minimize the risks of contamination of the free hydride. Nevertheless, a relatively efficient reaction can be engineered, with yields depending chiefly on the precise state and purity of the solid reagents. A similar approach has been adopted previously for making volatile tetrahydroborates of other metals, e.g., AUBH,), (83) and Zr(BH,), (84 1. Dimethylgallium tetrahydroborate melts at ca. 274 K and has a vapor pressure of 13-14 mm Hg at 273 K. It decomposes at ambient temperatures; at a pressure of 10 mm Hg the vapor has a halflife in the order of 3 hr, decomposing to give elemental gallium and hydrogen together with a mixture of methylated boranes. The vapor is composed of the diborane-like molecules Me2Ga(pH),BH2,in which fourfold coordination of the gallium is completed by bidentate ligation of the BH4 group (82,851. The resemblance to diborane is underlined by the response to nitrogen bases. Thus, ammonia induces heterolytic cleavage of the Me,Ga(p-H),BH2 skeleton with the formation of the salt-like product [Me2Ga(NH,),1+BH4-. By contrast, trimethylamine forms a molecular adduct Me,N.GaMe,(BH,) at low temperatures and this appears to undergo homolytic cleavage at temperatures above 228 K to give Me,N-BH, with gallium metal and dihydrogen, as well as traces of an unstable intermediate containing Ga-H bonds.

189

GALLIUM HYDRIDES Ga + H2

+ methylboranes

Me2GaBH4

Me2GaCl + +2H6

+ H2

Me3N.!3H3

+ Ga + H2

SCHEME 2. Formation and some reactions of dimethylgallium tetrahydroborate, Me2GaBH4(82).

B. HYDRIDOGALLIUM ~~~(TETRAHYDROBORATE) The reaction with Me2GaC1suggests that lithium tetrahydroborate should also undergo metathesis with gallium(II1) chloride, as it does with aluminium(II1) chloride (83),to give the tris(tetrahydrob0rate) derivative of the Group 13 metal. In fact, it was found in 1976 that the powdered solids react at ca. 228 K to afford not Ga(BH& but the hydridogallium compound HGa(BH,), (48);the same product is formed when dichlorogallane, [HGaCl2I2(41,42 1, is substituted for the trichloride (Scheme 3). Hydridogallium bis(tetrahydrob0rate) melts at about

190

DOWNS AND PULHAM Me3N-BH3 + Me3N.GaH3 Ga + B2H6 + -$I2 3

Me2GaBH4 + MeHGaBH4

Excess Me3N

+a+[GaC14]-

+ B2H6 + $H2

t

293 K

[HGa(NH3) 3 ] 2+[BH43-2

SCHEME 3. Preparation and some reactions of hydridogallium bis(tetrahydroborate1, iIGa(BH& (48-50).

200 K t o give a colorless, relatively mobile liquid that has a vapor pressure of ca. 10 mm Hg at 228 K. It decomposes to gallium metal, dihydrogen, and B,H6 at room temperature [see Eq. (ll)];at a pressure of ca. 10 mm Hg the vapor has a half-life on the order of 10 min. The compound is a rare example of an M3H9species involving one or more Group 13 elements, M, and for which theoretical studies predict a cyclic structure (3) with the three M atoms linked in pairs through single hydrogen bridges (86).On the evidence of the density, mass and vibrational spectra, and electron-diffraction pattern, however, the vapor consists of molecules in the form not of 3 but of HGa[(p-H),BH212,with a single terminal Ga-H bond and two bidentate BH, groups making

GALLIUM HYDRIDES

191

up a five-coordinated gallium center (see Fig. 4 and Table I) (48-50). On the other hand, the physical properties of the compound imply that the monomer is subject to loose aggregation in the condensed phases; in particular, the ‘H and IIB NMR spectra of C6D5CD, solutions at temperatures in the range 190-270 K suggest that the monomer and an oligomer [HGa(BH,),l, (where n = 2 in all probability) coexist in equilibrium. Noteworthy chemical features revealed in Scheme 3 include the behaviors of bases as varied as CO, NMe,, and NH,, which bring about homolytic or heterolytic fission of the Ga(p-H),BH, fragments. The reaction with CO provides a link with the thermally less

FIG.4. Structureof the gaseous molecule HGa(BH& as determinedby electron diffraction. H atoms are numbered 1, 2, 2’, 3, 3’, 4, 4’, 5, and 5’ (48-50).

192

DOWNS AND PULHAM

stable gallaborane, H2GaBH4, whose detailed characterization postdates that of HGa(BH,), by some 14 years (see Section V1.A). Of Ga(BH4)3,however, there is still no vestige, even a t low temperatures. These findings appear to emphasize the comparative reluctance of gallium, compared with aluminium, to rise to coordination numbers greater than four (10).

C. DIMETHYLGALLANE After earlier sightings of what was probably the impure compound, dimethylgallane, [Me2GaH], , was prepared relatively efficiently in 1984-1986 by way of the exchange reaction between trimethylgallium and freshly prepared lithium or sodium tetrahydrogallate (Scheme 4) (47). Careful fractionation in U ~ C U Ogives a viscous liquid (m.p., ca. 273 K; v.p. at 293 K, ca. 1mm Hg) that decomposes in a matter of days at room temperature. The compound resembles physically the product reported some 40 years previously (27-30) as having been isolated from the discharge reaction between Me,Ga and H2 [Eq. (311.It resembles too the corresponding aluminium compound, [Me2A1Hl,, which exists in the vapor and solution phases as a mixture of oligomers with n = 2 and, probably, 4 (35, 69, 87-89). On the evidence of its mass and infrared spectra and electron-diffraction pattern (471, the vapor of dimethylgallane at low pressures and near-ambient temperatures consists predominantly of the dimeric molecule (4) (see Fig. 3 and Table I). This represented, therefore, the first gallium hydride to be shown positively to contain a Ga(p-H),Ga bridging unit in a structure closely resembling that of the corresponding alane, [Me2A1H], (87-89). At 261.0 pm, the Ga-SGa separation is comparable with the shorter Gaa-Ga distances (247-307 pm) displayed by elemental gallium in its different guises (101, and the Ga-H bond, measuring 170.8 pm, is notable for being about 20 pm longer than the terminal Ga-H bonds of molecules like Me,N.GaH, (51)and HGa(BH,), (49, 50). The infrared spectrum

(4)

of the vapor includes, in addition to the bands associated with internal motions of the Me,Ga groups, two conspicuous features at 1290 and 1185 cm-’, which shift to 971 and 893 cm-l, respectively, when the hydrogen bound to gallium is replaced by deuterium. These must repre-

GALLIUM HYDRIDES

193

2

Me 2GaBH4

SCHEME4. Preparation and some reactions of dimethylgallane, [Me2GaH], (reproduced with permission from Ref. 47).

sent to a fair approximation the antisymmetric (la;v,,, 1290/971cm-'1 and symmetric (lb; v,, 1185/893 cm-') stretching vibrations of the central Ga(p-H/D),Ga moiety of 4. The Ga-H-Ga bond angle, 8, is estimated on the basis of Eq.(12) to be ca. 95"; the electron-diffraction measurements imply a value of 99.6" 2 1.4". Condensation of the dimethylgallane at low temperatures gives a solid with a significantly different pattern of infrared absorptions attributable to v,, and v,; vas now gives rise to an intense, broad band centered near 1700/1250 cm-' and v, to a much weaker feature near 965/850 cm-'. The obvious inference is that the degree of aggregation has increased on condensation with the formation, possibly, of a trimer akin to 3 or, more likely, the

194

DOWNS AND PULHAM

(5)

tetramer (5) akin to [Me2A1H14(89).In either case the Ga-H-Ga bridge angle has opened out to at least 120".As we shall see shortly, dimethylgallane foreshadows in its structural and spectroscopic properties the most distinctive features of gallane itself, [GaH,], ,

D. MONOCHLOROGALLANE The next step involves not so much a logical progression as a stroke of luck. We have noted already that dichlorogallane, [HGaCl,], , is produced by metathesis between gallium(II1) chloride and trimethylsilane, in accordance with Eq. (6) (41,421. In an experiment designed to prepare dichlorogallane by this route, an excess of trimethylsilane was inadvertently taken. Exchange between the neat reagents took place, as expected, at 250-258 K, but fractional condensation of the volatile products in uucuo gave not a white solid, which dichlorogallane is known to be (41,421, but a viscous, colorless liquid freezing a t ca.183 K and with a vapor pressure at ambient temperatures on the order of 0.5 mm Hg (36, 39, 40). The compound was found to decompose over a period of days at room temperature with the quantitative formation of equimolar proportions of H, and an involatile white solid having the composition GaC1. Hence exchange with an excess of the silane has given rise to monochlorogallane, which, on the evidence of its infrared and Raman spectra, exists as the dimer [H2GaC11,in both the condensed and vapor phases: Ga,CI, + 4Me3SiH

-

[H,GaCI],

IA

+ 4Me3SiC1.

2Hz + 2GaCI

(14)

The electron-diffraction pattern of the vapor can be satisfactorily interpreted, like the vibrational properties, on the basis of a molecule H,Ga(p-Cl),GaH, with D,, symmetry, as illustrated in Fig. 3. Here

GALLIUM HYDRIDES

195

the Ga-H distance is back to the 150-pm mark that typifies terminal Ga-H bonds and the central Ga(p-Cl),Ga unit is comparable in its dimensions to those in [Me,GaC112 (47) and Ga2C16(90). Quantum mechanical methods confirm that chlorine-bridging takes precedence over hydrogen-bridging and predict dimensions and vibrational properties in keeping with the experimental findings (91, 92). More recently the monomeric molecules HGaC1, (93) and H2GaC1 (94)have both been prepared by quite a different approach. This involves the photolytically induced addition of HC1 or H2 to molecular GaCl isolated in a n argon matrix: GaCl + HX

-

HXGaCl (X = C1 or H).

(15)

Identification rests firmly on the infrared spectra of the natural and deuteriated molecules, the results being borne out by ub initio calculations. Whether this method would lend itself to the synthesis of monochlorogallane on the larger scale remains to be seen. With the unearthing of monochlorogallane good fortune has delivered into the stalkers’ hands the most important agent yet in the pursuit of hydrogen-rich gallanes. Scheme 5 illustrates some of its more important reactions (36,37,39,40).Its decomposition is intriguing in offering a low-temperature route to what appears to be metastable gallium(1) chloride (10).Independent studies have shown that GaCl, maintained in a mixed toluene/diethyl ether solvent a t subambient temperatures, can be used as a source of other gallium(1) compounds, e.g., C,H,Ga (95, 961, and the decomposition of a monosubstituted gallane opens up further synthetic possibilities. Predictably bases cleave the Ga(pCl),Ga skeleton of monochlorogallane. For example, trimethylamine forms the 2 : 1adduct (Me,N),GaH2C1at low temperatures; this dissociates at room temperature to the 1: 1 adduct Me,N.GaH,Cl (38) and free NMe, . Ammonia, by contrast, brings about heterolytic cleavage with the generation of the salt-like product [H,Ga(NH,),]+Cl-. The ability of Ga-H bonds to add t o the double bonds of alkenes has also been shown to extend from dichlorogallane (41,42)to monochlorogallane, ethene reacting in stages to give first cis- and trans-[EtHGaCl], and then [Et2GaC11,. Most compelling of all, though, is the role of monochlorogallane as a precursor to other derivatives of the type [H,GaX], , typically through the interaction with a salt of the X- anion under solvent-free conditions. From this point the hunt for gallane itself is truly entered.

196

DOWNS AND PULHAM

Et

NH,.

[Me3N]2.GaH2CI

19) K Room

temperacure

Me3N.EaH2CI

+

Me3N

2 [H2Ga ( NH3 )2] +CI

SCHEME 5. Preparation and reactions of [H2GaC112(reproduced with permission from Ref. 10). V. Gallane at Last!

A. PRELIMINARIES Earlier attempts to prepare gallane in the Oxford laboratory (35,361 compassed a variety of potential routes, viz., (a) displacement reactions involving an adduct of GaH,, e.g., Me3N.GaH, or MGaH, (M = Li or Na), and an acid, e.g., BF, or HCl; (b) gas-phase pyrolysis or matrix photolysis of an adduct of GaH,; and (c) the interaction of a tetrahydrogallate MGaH, with a gallium compound, e.g., Ga,Cl,. The only one of these to give any encouragement was the interaction of gallium(II1) chloride with a tetrahydrogallate, the solid mixture yielding under

197

GALLIUM HYDRIDES

solvent-free conditions at ambient temperatures small amounts (in the order of 1-2 mg or less) of a volatile, thermally unstable product, in addition to substantial quantities of elemental gallium and hydrogen. The condensate formed by quenching the vapor of the product on a CsI window held at 77 K was typically characterized by the infrared spectrum reproduced in Fig. 5, with three main absorptions a t 1978 (s), 1705 (s,br),and 550 cm-' (s,vbr) (97).The spectrum differs significantly from the one reported by Greenwood and Wallbridge (33)for the product they identified as free gallane for it includes a prominent, broad absorption near 1700 cm-', as well as the much sharper feature near 1980 cm- The vibrational properties of known gallium hydrides, as outlined in earlier sections, prompt the bands at 1978 and 1705 cm-' most plausibly to be identified with v(Ga-H,) and v(Ga-Hb-Ga) fundamentals, respectively, in an aggregate with a comparatively wide Ga-Hb-Ga bridge angle [cf. [Me,GaHI, (47).Irrespective of the conditions of the experiment, however, it proved impossible to isolate a product that was entirely free from chloride. Thus, chemical analysis showed the proportion Ga:C1 to be typically 5:1, and the 'H NMR spectrum of C6D,CD3 solutions at low temperatures confirmed that

'.

2000

1800

1600

1400

1200

1000

800

600

400

20

2000

1800

1600

1400

1200

1000

800

600

400

20

;/cm-' FIG.5. The IR spectrum of the annealed solid film formed by condensing the volatile products of the reaction between solid gallium(II1)chloride and (i) NaGaHl or (ii)NaGaDl on a CsI window held at 77 K (reproducedwith permission from Ref. 56; copyright 1991, American Chemical Society).

198

DOWNS AND PULHAM

the product contained more than one gallium hydride derivative. It appears, therefore, that a hydride-rich product including species like [GaH,], is formed through the interaction of neat gallium(II1) chloride and a tetrahydrogallate, but only in very low yields (

42s 560 576 719

7

5

VS W

S

vw

762(?) (7X1.2)'

missing

(954. I )*

inactive

>

vw

niisbing

F2
C104- > HP20:- > ATP3- > C1-, whereas other anions (HPO:-, AMP2-, SO4’-, F-, BF4-, HC03-) had negligible effects. The order SCN- > C104- > C1- follows the lyotropic series for the binding of anions t o positively charged sites on proteins (196) but the binding appears to be relatively specific, rather than simply chaotropic, at least for a given transferrin. Two C1- ions are bound to each lobe of human transferrin, giving four in all. Moreover they are bound in pairwise fashion, with strong positive cooperativity. Two possible sites for the binding of nonsynergistic anions to transferrins may be identified from the crystal structures (Fig. 271, although there could be others given the large number of positively charged residues (lysine and arginine) on such large proteins. The first is the “essential” arginine at the synergistic anion site of each lobe. Binding to the other side of this arginine could perturb the interactions it makes with the synergistic anion and thus indirectly perturb the metal site. The second may involve the positively charged side chains behind the iron site, near the hinge region. In the N-lobes of serum transferrin and ovotransferrin, a pair of lysines are hydrogen bonded ( 77,811,and disruption of this unusual arrangement by anion binding could perturb the iron site (811. If this is the secondary anion site, however, it must be differentbetween different transferrins and between N- and C-lobes, because of the sequence differences that exist (Section 1II.C).

FIG.27. Possible sites for the binding of secondary, nonsynergistic anions, which may perturb the iron site and modulate release. Residue 121 is the “essential” arginine. Residues 210 and 301 are located behind the iron site, near the Tyr ligands and the hinge region. Numbering is as for the N-lobe of lactoferrin. The identities of these residues, which vary in the two lobes and between different transferrins, can be seen in Table 111.

TRANSFERRIN STRUCTURE AND REACTIVITY

439

An effect that is almost certainly related is that of salts on the relative stabilities of the two metal binding sites and on the kinetics of metal ion release. Increasing concentrations of salts, such as NaF, NaC1, NaBr, NaI, NaN03, Na2S04,and NaClO,, increase the stability of the N-terminal site relative to the C-terminal site of human transferrin (197).In the presence of an accepting chelator, salts also accelerate iron release from the C-site (197,198),thus reversing the normal order (N-site faster than C-site). This is discussed further in Section V.B. Caution should be exercised, however, in extrapolating results from one transferrin to another, however, because sequence differences are likely to alter these weak, secondary binding sites. 4 . Conformational Differences Associated with Anion Binding

Spectroscopic studies have consistently demonstrated the existence of multiple conformational states for the metal sites in transferrins, especially when using metal ions other than Fe3+ and anions other The differences are not necessarily related to intrinsic than CO:-. geometrical differences between the two sites in each molecule, but also reflect changes dependent on pH, the nature of the synergistic anion, or salt effects. For Co2+-substitutedovotransferrin, for example, not only are the N- and C-sites distinguishable by CD spectroscopy, when oxalate is the anion, but 'H NMR spectra reveal the existenceof conformers(139). For V02+-substitutedtransferrin, the EPR spectra were examined using 16 different anions (185),and the resultant spectra could be grouped into two classes, A and B, which were anion dependent. Anions with one carboxylate and a nonionized electron donor group L gave only class B spectra, whereas dicarboxylate anions gave both class A and B spectra. For the latter anions, transition between class A and class B spectra was associated with ionization of a protein group of pK -10.0. A pH-dependent change is also seen in the EPR spectrum of Cu2+substituted ovotransferrin, with carbonate as the associated anion, this time associated with ionization of a group ofpK -9.5 (157).EPR spectra of monoferric transferrins have also shown that each site (N or C) exhibits two types of spectrum and that the equilibrium between the two is affected by added NaCl (191); this equilibrium is presumably the cause of the salt-induced EPR perturbations noted by Folajtar and Chasteen (195). Some tentative conclusions about the nature of these conformational differences may be drawn from the crystallographic studies of Cu2+ and oxalate-substituted lactoferrins (26,192,193). Anions which gave class A spectra with V02+-substitutedtransferrins are those that can

440

E. N. BAKER

interact with the “essential” arginine as in Fig. 26, i.e., dicarboxylate anions. It may therefore be the ionization of this arginine that determines the conformer, perhaps by determining the degree of asymmetry in the bidentate anion binding. In the Cu2+complexes, the geometry in the N-lobe of copper-lactoferrin suggests that either the anion or Tyr 92 is protonated (Fig. 21), and it may be the loss of this proton at higher pH that causes the change to a bidentate carbonate, as seen in the C-lobe, and gives the EPR change seen for Cu2+-ovotransferrin (157). In general, given that symmetric bidentate, asymmetric bidentate, and even monodentate anion configurations are seen in the various lactoferrin structures, it seems likely that it is changes of this nature that are detected spectroscopically.

D. DIFFERENCESBETWEEN THE Two SITES Proteins of the transferrin family share a common evolutionary history, which has resulted in the presence of two homologous halves to each molecule (Sections 1II.A and III.B.l). Except for the two outliers, melanotransferrin and M. sextu transferrin, each also has two metal binding sites. This raises a number of questions, which have been the subject of much debate over the years (1,3,16,17). Why are there two sites? How similar are they, and do the differences between them have any functional or physiological significance?Is there any cooperativity between them?

1. Structural Comparison The four metal-binding amino acid residues (2 Tyr, 1Asp, 1His) are present in both N- and C-sites of all transferrins so far sequenced, apart from melanotransferrin and the insect proteins (Table 111).The same is true of the anion-bindingArg and Thr residues, and the residues at the N-terminus of the anion-binding helix are also strongly conserved. Superposition of the 81 common atoms of these residues, plus metal and anion, shows that their rms deviation in the N- and Csites of diferric human lactoferrin is only 0.3 A. This close structural similarity is reflected in their spectroscopic properties. Where these have been compared, with the “physiological” Fe3+ and C032- ions bound, they are so similar as to be virtually identical (107, 56, 199). Nevertheless, there are a number of factors that can potentially lead to inequivalence in properties: (i) Outside the immediate binding site there are sequence differences between the two lobes, e.g., in the basic residues behind the metal site,


44 1

in the residues that line the binding cleft beyond the “essential” arginine (Fig. 71, and in the pattern of disulfides. (ii) The “front-to-back”packing of the two lobes (Fig. 3) means that the two binding clefts have different environments with respect to the molecule as a whole (the N-terminal cleft is more exposed and accessible). (iii) The fact that each binding site is created by closure of two proteins domains over metal and anion and that there is considerable “empty space” in the interdomain cleft (i.e., filled only by solvent) gives potential for 3D structural differences, especially when different metal ions and anions are bound. 2 . Differences in Properties

Where chemical or physical differences can be detected between the two sites, there remains the problem of distinguishing which site is which. For serum transferrin this is helped immensely by the ability to prepare monoferric forms, loaded in either the N- or C-site (198, 2001, and to be able to separate them by electrophoresis in 6 M urea, the Makey-Seal method (201).This enabled the so-called A and B sites, differentiated in earlier studies, to be identified with the C-and N-terminal sites, respectively (2021. Comparisons of the diferric proteins with N- and C-loaded monoferric transferrins or (more recently) recombinant half-molecules have by now revealed a number of inequivalences. Both kinetic and thermodynamic effects differentiate the two lobes of transferrins. Aisen et al. (107) have shown that C-terminal site of transferrin binds iron more strongly than the N-terminal site, with their effective binding constants differing by a factor of about 20. The C-terminal site also appears to be the more strongly binding site for other metal ions, for example, in lanthanide binding (149, 150).Iron release also differs, with the rate of iron release being faster for the N-terminal site (108). These two effects, tighter binding and slower release from the C-lobe, may be linked to its lesser flexibility (85),as seen in thermodynamic measurements (108)and inferred from the “one opedone-closed” apolactoferrin structure (80, 82) (see Section III.B.5). The reduced flexibility of the C-lobe may arise from the presence of an extra disulfide bridge [483-677, lactoferrin numbering, or number 7 in the nomenclature of Williams (8711. This disulfide, which has no equivalent in the N-lobe of any transferrin, adds an extra constraint between the two domains of the C-lobe (Fig. 10). Predictions (80) that it would inhibit opening of the C-lobe have been born out by the lesser opening of this lobe seen in the fully open apolactoferrin structure (109).

442

E. N. BAKER

The two sites also differ in their pH stability towards iron release. Experiments on serum transferrin showed that one site loses iron at a pH near 6.0, and the other at a pH nearer 5.0 (203,2041, giving a distinctly biphasic pH-induced release profile (Fig. 28). The acid-stable A site was later shown to be the C-terminal site (202).It is this differential response to pH, together with kinetic effects (below),that enables N-terminal and C-terminal monoferric transferrins to be prepared (200). Although the N-terminal site is more labile, both kinetically and to acid, the reasons are not necessarily the same; the acid stability may depend on the protonation of specific residues (Section V.B) and is likely to differ somewhat from one transferrin to another in response to sequence changes. The biphasic acid-induced release of iron seen for transferrin is not shared by lactoferrin. Although biphasic release from lactoferrin, in the presence at EDTA, has been reported (2051, under most conditions both sites release iron essentially together at a pH(2.5-4.0) several units lower than that for transferrin (Fig. 28). The two sites (in transferrin, at least) also show differences in iron loading behaviour. In uitro, when Fe3+is added as a chelate complex, there are differences in which site is preferentially loaded, depending on the nature of the chelate ligand; these differences are apparently kinetically determined and differ from one transferrin to another (17).

P

5 n 4)

LL

hp

i

"

2.0

3.0

4.0

5.0

6.0

7.0

8.0

PH FIG.28. The pH dependence of iron release from human serum transferrin (TO, human lactoferrin (Lf),and the recombinant N-terminal half-molecule of human lactoferrin (LfN).Also shown is a plot (dashed line) for the release of cerium from Ce4+-substituted lactoferrin, demonstrating the increased difference between the two sites for metal ions other than Fe3+.


443

In uiuo, a study of fresh serum showed that the N-site of human transferrin was more highly occupied, the average distribution in 22 samples being 39% apo-Tf, 23% Fe,-Tf, 11%Fe,-Tf, and 27% Fe2-Tf(206). 3. Metal and Anion Substitution

Differences between the two sites become more pronounced for metal ions other than Fe3+ and anions other than C032-. The differences are most pronounced for larger metal ions such as lanthanides. For transferrin some of the larger lanthanides appear to bind in only one of the two sites (Section IV.B.3), and for lactoferrin, although binding occurs in both sites, the second metal ion binds much more weakly, as shown by the curvature of the UV difference titration graph (Fig. 18); the biphasic release of Ce4+from lactoferrin contrasts with that of Fe3+ (Fig. 28). Even metal ions of the first transition series, of similar size to Fe3+,enhance the differences between the two sites. When Cr3' is bound to either transferrin (134) or lactoferrin (1541, different EPR signals are seen for the two sites, and one C P ion is much more readily displaced by Fe3+than the other. Likewise, the EPR spectra of V02+substituted transferrin indicate different metal configurations in the two sites (2071,as do 13CNMR studies of Co2+-substitutedovotransferrin (139).In these cases one metal ion is also released much more readily than the other as the pH is lowered. The crystal structure of copper-lactoferrin (26)shows the kind of differences that may occur. In one site the coordination geometry is six-coordinate, distorted octahedral, whereas in the other it is fivecoordinate, square pyramidal. One could suggest that the sites are and that there is an element optimized for the binding of Fe3+and C0:of misfitting when a different metal ion, with different size, stereochemical requirements, or both, is bound. The sites can adjust, with small movements, but these are different in the two sites. O:are Distinct differences are also seen when anions other than G used. The crystal structure of oxalate-substituted diferric lactoferrin shows differences in the anion binding in the two sites; in the C-site the oxalate is symmetric bidentate, whereas in the N-site it is asymmetric (193). When Cu2+is the metal ion the oxalate binding differences become even more pronounced. Copper-transferrin binds oxalate only in its N-terminal site (91). Copper-lactoferrin and copper-ovotransferrin each bind two oxalate ions but binding occurs preferentially in the C-lobe (157,192).These different affinitiesmean that hybrid complexes can be prepared with oxalate in one site and carbonate in the other (92, 157, 192).The use of oxalate as synergistic anion gives rise to spectroscopically distinct sites for other metal ions also (1711.

444

E. N.BAKER

The X-ray structural studies on lactoferrin show that it is not simply a question of how much room there is for a larger anion or cation in a given site. The N-terminal site in lactoferrin apparently has more room than the C-terminal site, yet it is the C-terminal site that is preferentially occupied by oxalate (I92); perhaps the explanation is that the more favorable square pyramidal copper geometry in the Nsite (with monodentate anion) makes it less amenable to substitution of oxalate. In general, the two sites have enough flexibility that the precise structure adopted depends on the particular metal and anion, emphasizing the synergistic relationship between the two and making the result of a given substitution rather hard to predict. 4 . Functional Aspects

The small but significant difference in iron release from the two sites of transferrin led Fletcher and Huehns (208) to suggest that they might have different biological functions. It was suggested that one site might be involved in iron transport and release and the other, more as a storage site (for antibacterial or iron defense purposes). Although the idea has remained controversial (e.g., see discussion in Ref. 31, several recent observations have led to renewed interest. The apolactoferrin structure (80) suggested a distinct difference in flexibility between the two lobes, which should also apply to transferrin. Studies of transferrin-receptor interactions (209) have also shown that the receptor specifically acts on the C-lobe, prying it open to release the iron, whereas the N-lobe loses iron by the long-proposed acid-mediated release (20). This neatly explains the observation that circulating transferrin has more iron in its N-lobe, whereas the less facile C-lobe release would have been expected to lead to a buildup in the C-site. These observations come together with the evolutionary comparisons, which show that the N-lobe is highly conserved, through all species, whereas the C-lobe has become diversified. In serum transferrins, ovotransferrins, and lactoferrins it releases iron less readily than the N-lobe, because of its lesser flexibility, whereas in melanotransferrin and hornworm transferrin it no longer binds iron at all. Perhaps C-site binding has only remained where a receptor mechanism exists to extract iron from this site? Finally the question of whether there is any cooperativity between the sites remains to be addressed. Although evidence of cooperativity from solution studies has mostly been equivocal (31,there are certainly structural interactions between the two lobes, involving helices from each (78, 851, and studies of a half-molecule fragment of lactoferrin have shown that separating the N-lobe from the C-lobe gives it altered


445

properties of iron release (49). Thermodynamic studies have shown that binding at one site is signaled to the other, presumably through changes in interlobe interactions (210) and it seems likely that the binding properties of each lobe are modified by the presence of the other. V. Mechanisms of Binding and Release

The most striking feature of transferrin chemistry is that iron is bound with extraordinary avidity, yet it can be released without any denaturation and the protein can be recycled through many cycles of uptake and release. The mechanisms by which this is done are of fundamental importance to understanding biological transport processes. A. UPTAKEOF IRON 1. Mechanism of Binding

In uiuo uptake of iron by transferrins usually involves its addition as a ferric-chelate complex, to prevent hydrolytic attack on the ferric ion (2111. Complexes such as ferric citrate and ferric nitrilotriacetate are commonly used. Under these conditions, kinetic schemes for the uptake of iron by transferrins have identified five steps in the formation of the specific metal-anion-transferrin ternary complex (120). These may be summarized as follows. 1. Binding of the (bilcarbonate anion to apotransferrin. 2. Detachment of one or more ligands from the added metal chelate.

3. Formation of a quaternary transferrin-anion-metal-chelate complex. 4. Loss of the chelate ligands(s1. 5. Conformational change to the final specific transferrin complex.

Evidence that the anion binds first comes from kinetic data (119) and from spectroscopic results, in which both 'H NMR (118) and UV difference (177) spectra indicate that the anion binds to the apoprotein. Strong support comes from the 3D structural data; the positive charge at the anion site should deter metal binding until it is neutralized by a suitable anion ( 78,85). Suggestions that nitrilotriacetate transiently occupies the anion site when iron is added as a Fe3+-NTA complex (212) may imply that the early steps can vary depending on the form of the added iron, but the key point probably remains that the anion site must be occupied as a first step. Spectroscopic evidence for the

446

E. N. BAKER

quaternary complex envisaged in Step 3 has been obtained from studies using ferric-acetohydroxamate in iron uptake experiments (120). The above mechanism is totally consistent with the crystallographic results from the various forms of lactoferrin and transferrin (Section 1II.B).These lead to a structural model of binding shown pictorially in Fig. 29. In the first step the synergistic anion (usually carbonate) is bound in the specific site on domain 2 of each lobe. Binding may be preceded by electrostatic attraction from the exposed helix N-termini and several basic sidechains in the open interdomain cleft. With the anion bound, four of the six iron ligands are in place on

+o

+a

FIG.29. A structural model of the steps involved in the in uitro uptake of iron by transferrins, shown for one lobe. (0)Iron; (A)carbonate; Y, Tyr ligands; H, His ligand; D, Asp ligand; (0) chelate ligands. The positive charge at the anion site is due to the helix N-terminus and the Arg side chain. (Note that this is for the case in which Fe3+ is added as a chelate complex.)


447

domain 2 (two carbonate oxygens and two Tyr sidechains); the metal then binds to these groups, possibly with some of its chelating ligands still attached, to give the quaternary complex. A model for such an intermediate is provided by the 18-kDa domain 2 fragment of duck ovotransferrin, whose crystal structure has been determined by Lindley et al. (76). In this structure the iron atom is bound to the bidentate carbonate ion and both Tyr residues, with the remaining two coordination sites occupied by a non-protein ligand, possibly a glycine molecule (Fig. 14). The final step in binding involves the closure of the two domains over the metal ion. Any remaining chelate groups are expelled as the metal ion binds t o the Asp and His ligands t o complete its coordination. The closed configuration is locked together by the Asp ligand, which plays a critical role in the metal-bound structure (78). Not only does the Asp carboxylate bind to the metal ion, but also it is involved in a strong hydrogen bonding interaction between the two domains. Its nonligated carboxylate oxygen atom receives a hydrogen bond from the NH group of residue 122 (466 in the C-lobe) in domain 2 as well as from the NH of residue 62 (397) in domain 1.The strength of these interactions is probably enhanced by the fact that both NH groups are at positively charged helix N-termini, helix 3 and helix 5 (Fig. 9a), and their importance is emphasized by the fact that when the Asp ligand is mutated to Ser which has only a single hydroxyl group, no stable closed structure appears to be formed (106). 2 . Importance of Dynamics

Protein dynamics clearly plays a crucial role in metal binding and release. With respect to metal binding, the open structure described in Section III.B.5 should not be taken to imply that in the absence of a bound metal ion the cleft is always open. In fact the closed but metalfree C-lobe seen for one form of apolactoferrin (80) suggests that in the absence of a metal ion very little energy separates the open and closed states, and in fact there may be a dynamic equilibrium between them (82); at the very least the closed configuration may now and again be sampled. Similar conclusions have been drawn for bacterial binding proteins for which closed but ligand-free structures have also been observed (213). The importance of this model is that in the intermediate in which the metal ion is bound to domain 2 (step 3 in Fig. 29) it would be some 8 to 9 away from the Asp and His ligands on domain 1, as judged by the extent of opening of the apolactoferrin (80) and C-terminal monoferric transferrin (110) structures. How then does it “find” these

448

E. N.BAKER

two ligands to complete its coordination? Only if the dynamics of an equilibrium allows it to explore the closed structure, at which time the ligands will be able to lock on to the metal. Such dynamics are common in proteins, especially where the movement of domains (214)or flexible “lids” or “flaps” (215) are concerned. Finally, the Asp ligand has been described as a “trig er” associated with domain closure (106).However, at a distance of 9 from a metal ion bound to domain 2 it is hard to see how it could induce closure. Rather, it should be seen as a lock that holds the closed structure in place once the protein dynamics have brought the domains close together.

1

B. RELEASE OF IRON Iron release can be stimulated by a number of factorsthat can operate individually or together. In uitro, these include reduction of Fe3+to the much more weakly bound Fe2+,the use of competitive chelators, and the acid liability of the two sites, which results in iron release at low pH. In uiuo, receptor interactions are of fundamental importance. Added to these are the modulatory effects of ionic strength (216)and nonsynergistic anions such as C1- and C10,- (198). 1 . Kinetics

Most kinetic studies of iron release have focused on pathways involving the use of chelate ligands such as EDTA (2171, pyrophosphate (218-2201, phosphonates (220,221 1, catecholates (108,2161, hydroxamates (1201, and nitrilotriacetate (2211. In many cases, simple saturation kinetics are observed, and interpreted in terms of the formation (120,122 ). The of a quaternary complex, 1igand-Fe-tran~ferrin-CO~~failure t o observe this complex spectroscopically [in contrast to iron uptake studies (120)] has been explained in terms of a rate-limiting conformational change, giving a basic three-step mechanism, which is essentially the reverse of that given for iron uptake (Section V.A.l). 1. A rate-limiting conformational change of the diferric protein from closed to open configuration. 2. Rapid attack of the chelator to give a quaternary complex. 3. Rapid decay of the quaternary complex to products. The kinetics are, however, considerably more complex. Both pH and salt affect the two sites differently (Section IV.D.2), so that iron release kinetics are very different at pH 6 compared with those at pH 7.4, for example. The kinetics are also dependent on the particular chelator


449

used; this is not surprising because most of the ligands used are anionic, many of them able to bind quite strongly to transferrins even in the absence of iron (177). There is now accumulating evidence of more than one pathway for release. Harris and coworkers (221,223,224) identified two parallel pathways, operative at either site, one being the saturation pathway envisaged above, the other being first order with respect to chelate concentration.Their relative importance can depend both on the nature of the chelator and on concentrations. Using pyrophosphate, iron removal from the N-site follows only the saturation pathway, whereas for the C-site both pathways operate (223);using nitrilotris(methy1enephosphonate) (NTP)the saturation pathway is favored,but using nitrilotriacetate (NTA)the first-order pathway operates at both sites (2211. The first-order pathway may be associated with interactions involving the chelate anion; it could either substitute for carbonate in the specific site (223) or bind to some other allosteric effector site, such as that occupied by C1- or C10,- (219). Much attention has been paid to the significance of salts (217) and nonsynergistic anions (195,198) in promoting or modulating iron release, especially given the important observation that iron release extrapolates to zero as the ionic strength of the medium nears zero (216). A variety of anions accelerate the first-orderpathway (2201,with Clodbeing the most active species; this has been attributed to the presence of cationic groups near the metal binding site, which form a specific effector or allosteric anion-bindingsite (219,220). The complex kinetics may result from competition between chelate anions and effector anions for such sites. Egan et al. (225) have further analyzed the release of iron from C-terminal monoferric transferrin to pyrophosphate in terms of the existence of a kinetically significant anion binding (KISAB) site. Here it is envisaged that two pathways operate, with either pyrophosphate or an added anion occupying this site. Comparisons of release from free and receptor-complexed transferrin also show that the release-promoting effects of the receptor and of the anion (in this case C1- are independent of each other (226). Many studies have noted weak cooperativity between the sites during iron release (3). One recent analysis used mixed-metal transferrins, with kinetically inert Co3+in one site and Fe3+in the other (221,224). With pyrophosphate, release of iron from the C-site was accelerated by the presence of a metal in the N-site, but no corresponding effect was seen for iron release from the N-site. The cooperative effects were also weaker and somewhat different for different chelators (221). Finally it is important to realize that most studies of iron release

450

E. N. BAKER

have focused on human serum transferrin. Many of the finer details may be dependent on the interactions of chelators, salts, etc., with residues that are within the binding cleft but outside the immediate iron site; these residues tend to vary from one species to another and from one transferrin to another, and it is likely that kinetic details will also.

2 . Structural Aspects of Iron Release The conformational change from closed to open configuration is a key feature of models for iron release. The nature of this change can now be inferred from crystallographic and solution studies (Section III.B.5). What is less clear is how the conformational change is triggered. In uiuo, receptor binding is clearly involved, but pH and salt effects can also play a part. In uitro, reduction of the pH is in itself sufficient. In attempting to understand iron release in structural terms one must look for potential protonation and anion binding sites and seek more knowledge of the interactions made with transferrin receptors. The effect of pH differs for the two sites of transferrin and differs between transferrin and lactoferrin. When titrated with acid, in the absence of chelators, serum transferrin loses iron over the pH range 6.0 to 4.0; release is biphasic (Fig. 281, with iron lost from the more acid-labile N-lobe site first (Section IV.D.l). Lactoferrin, on the other hand, is distinctly more stable in acid, with release occurring 2 pH units lower, over the pH range 4.0 to 2.5, and the two sites losing iron essentially together. Several explanations for the effect of pH on iron removal have been put forward. Protonation of the carbonate ion could cause repulsion between it and the anion-binding Arg residue (121 in the N-lobe, 465 in the C-lobe), or promote a change from bidentate to monodentate coordination, as seen in the N-lobe of copper-lactoferrin (26). Either effect could then be the first step in the breakup of the Fe3+-transferrin complex. An alternative site where protonation could stimulate iron release is at the back of the iron site, in the hinge region. It is here that distinct differences between lactoferrin and transferrin, involving ionizable residues, are found (Fig. 30). In the N-lobes of both rabbit transferrin (81) and chicken ovotransferrin (771, a pair of lysine residues (206 and 296, transferrin numbering) are in hydrogen bonded contact, implying that one is in its neutral form; protonation of this lysine would break this interdomain interaction and could destabilize the closed structure. The pair of lysines has been referred to as a “dilysine trigger” ( 77). In the C-lobe of transferrin a different combination of charged residues is found (a salt bridge Lys * * * Asp * - Arg), which


45 1

.. ..

SER

FIG. 30. Residues at the back of the iron site, near the hinge region, that may be implicated in the stimulation or modulation of iron release. The interactions present in human lactoferrin and rabbit transferrin are compared. Where the conformations are different, lactoferrin residues are shown with solid bonds, transferrin, with open bonds. Where the residues differ in identity or number, those for transferrin are in parentheses.

would be less easily protonated. In the N-lobe of human lactoferrin the interactions are different again; one Lys is changed to Arg (Table 111) and a conformational difference leads to a Glu - - - Lys ion pair in place of the Lys . - .Lys pair. This could certainly account for the greater acid stability of the lactoferrin N-lobe site. (The situation may not be so simple, however, because the sequences of both bovine and porcine lactoferrins have both lysines.) Crystallographicand mutagenesis studies will be required to disentangle these effects. Studies of half-molecule fragments also suggest that the region a t the back of the iron site, near the hinge in the interdomain connecting strands, could be the site where protonation stimulates release. The recombinant N-terminal half-molecule of human lactoferrin releases iron over the pH range 6.0 to 4.0 (Fig. 281, approximately 2 pH units higher than that of intact lactoferrin, but very similar t o transferrin (49,205).The crystal structure shows that this decreased acid stability is associated with the loss of stabilizing contacts normally made by the C-lobe, leading to unwinding of a helix at the back of the iron site and

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E. N. BAKER

increased solvent exposure of both this region and the hinge (75). A proteolytic fragment of lactoferrin that lacks even more of the structure in this region is correspondingly more acid labile (227). Potential anion-binding sites that could stimulate iron release have been discussed in Section IV.C.3 and are shown in Fig. 27. Binding to sites near the hinge, such as the Lys . . . Lys pair in the N-lobe of serum transferrin, has the potential either t o promote domain opening, by disrupting interdomain interactions or the nearby hinge, or to perturb the iron site via the iron ligands; the Lys * Lys pair in transferrin and Arg 210 in lactoferrin are hydrogen bonded to the Tyr ligands (75, 77, 78). Binding to the “essential” Arg residue could disrupt the metal-synergistic anion interaction, leading to a change to monodentate coordination or complete displacement of the synergistic anion. Defined structural pathways for iron release may exist. If some chelate anions bind to cationic groups on the protein first, as suggested (2211, the metal could be passed to this site en route to the outside. If binding was to the “essential” arginine, which also helps hold the synergistic anion, the metal might just transfer from one to the other. An intriguing observation concerns interactions with cyanide. The iron in transferrin is usually high-spin Fe3+,but it can be converted to low spin in the presence of CN- (228); flash-freezing then traps an intermediate in which CN- is exchanged for some of the normal transferrin ligands in the C-lobe. X-ray absorption studies of this intermediate indicate that it involves at least two Tyr ligands, but that at least one of them may be different from those normally ligated (81). This could implicate a Tyr-mediated pathway. Studies of the transferrin receptor indicate that it acts preferentially on the less flexible C-lobe, to stimulate release from this site (209), and that the receptor and anion-binding effects are independent (226). The interdomain strands that contain the C-lobe hinge are highly exposed to the external environment and are rich in charged residues; they could be involved in binding the receptor, simple anions (such as C1-), or both. The problem at present is that so many potential sites exist. What is required is detailed knowledge of transferrin-receptor interactions, either through the crystallographic studies that have already been initiated (229)or by mutagenesis. VI. Recombinant DNA Studies

The past six years have been an explosion of new results from X-ray crystallographic studies, which have added a new dimension to our


453

understanding of transferrin chemistry. The next few years should increasingly see another powerful approach, that of recombinant DNA studies, brought to bear. These techniques allow the production of mutant transferrins, in which single, selected amino acids are changed. Alternatively, chimeric transferrins in which part of one molecule is substituted into another can be constructed. Mutagenesis can be carried out either on the whole transferrin molecules or on half-molecules, because half-molecules of defined length can be made. The first successful expression of any transferrin was reported in 1990 when Funk et al. (48)isolated the cDNA for human serum transferrin and introduced a stop signal following the codon for Asp 337; the resulting DNA construct thus coded for the N-terminal half-molecule. This recombinant human transferrin N-terminal half-molecule, hTf/2N, was expressed in a cell culture system incorporating baby hamster kidney cells, after unsuccessful attempts at expression in Escherichia coli. Similar methods have also been used to obtain expression of full-length human serum transferrin (230) and both the N-terminal half-molecule (49) and the whole molecule (231) of human lactoferrin. The use of an animal cell culture expression system has advantages in ensuring correct folding of the recombinant proteins and allowing glycosylation to occur, but is more laborious and gives lower levels of expression than are usually possible withE. coli. All the same, expression levels of 20-40 mg per liter of culture medium have been reported (48,49),ample quantities for characterization. (The recombinant proteins are secreted into the culture medium.) A recent report of the expression of human serum transferrin in E.coli (232)suggests that higher levels may be attainable although little characterization of the protein obtained has yet been carried out. The recombinant whole molecules are both expressed in glycosylated form, although the glycosylation patterns differ from the proteins isolated from natural sources. The recombinant human transferrin binds to receptors both in its glycosylated form and as a nonglycosylated mutant, showing that the carbohydrate is not required for receptor binding (230).Recombinant human lactoferrin shows identical spectroscopic properties and shows an identical profile of pH-dependent iron release when compared with human milk lactoferrin (231). The half-molecules differ somewhat, although this is not unexpected, because the properties of the whole molecules are an amalgam of those of their two slightly different sites. Moreover, the lactoferrin halfmolecule shows that its iron release properties are changed as a result of the loss of interactions from the other lobe (Ref. 49; see also Section V.B.2 and Fig. 28). The visible A,, for the transferrin half-molecule

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E. N . BAKER

is increased from 465 nm (native transferrin) to 473 nm (2331,whereas that for the lactoferrin half-molecule is reduced from 465 to 454 nm. The reasons are not clear, but presumably have to do with the fine detail of the two metal sites, at a level that probably even high resolution X-ray analyses may not explain. A number of site-specificmutants have already been prepared, both of transferrin (233)and of lactoferrin (234).In all cases the mutations have been made in transferrin or lactoferrin half-molecules and the targets have been amino acids in and around the metal binding sites. Characterization has been limited so far to their visible spectra and some measures of iron binding properties; for lactoferrin the pH dependence of iron release has been determined (2341, whereas for transferrin the strength of iron binding has been inferred from the migration of the mutant proteins on urea gels (233).In neither case have binding constants yet been determined. Already some intriguing observations have resulted, however, giving a glimpse of what may be to come with fuller characterization. Most mutations alter the value of A,, (Table VIII). This is true even of residues outside the immediate binding site, such as Lys 206 and His 207 in transferrin (233), showing that even changes some distance away can perturb the metal site. Lys 206 is one of the two lysines that are hydrogen bonded together behind the iron site and that are potential sites for protonation and salt effects (Section V.B.2).Interestingly, mutations of Lys 206 to Gln and of His 207 to Glu, both mutations that reduce the positive charge, appear to increase the strength of iron binding. Mutation of the Asp ligand to Ser, as in the C-lobe of melanotransferTABLE VIII PROPERTIES OF RECOMBINANT TRANSFERRINS~ Transferrim* N-lobe (Tf,) Asp 63 Ser-TfN Asp 63 Cys-TfN Gly 65 Arg-TfN Lys 206 Gh-TfN His 207 Glu-TfN

,,,A 473 420 440 468 460 484

Iron binding Strong Weak Weak Weak Strong Strong

Lactoferrins N-lobe (LfN) Asp 60 Ser-LfN Arg 121 Ser-LfN Arg 121 ASp-LfN Asp 60 Ser Arg 121 Ser

ILfN

hmax

Iron release (pH)

454 434 454 472

5.5-4.0 7.0-5.0 5.5-4.0 >7.0

472

>7.0

Notation for mutants: Asp 63 Ser means that Asp 63 is mutated to Ser. Asp 63 in transferrin corresponds to Asp 60 in lactoferrin, and similarly Gly 65, to Gly 62; Arg 124, to Arg 121; Lys 206, to Arg 210; and His 207, to Glu 211.


455

rin, does not abolish iron binding, but reduces A,, considerably (the protein is yellow) and weakens binding considerably. The transferrin mutant (Asp 63 Ser) loses iron on urea gels, traveling as the apoprotein (2331, and low-angle X-ray solution scattering studies (106)suggest that it does not form the usual closed structure when iron is bound. The lactoferrin mutant (Asp 60 Ser) loses iron below pH 7, considerably more readily than the wild-type protein. Mutations of Asp 63 to Cys in transferrin, and of the nearby Gly 65 to Arg, have similar effects, again possibly inhibiting domain closure. Mutation of the anion-binding Arg 121 to Ser in lactoferrin does not alter Amax or significantly perturb the pH dependence of iron release, implying that the Arg residue is not essential for anion binding and arguing against a repulsion between it and a protonated anion as an important factor in iron release. Intriguingly, the double mutation Asp 60 to Ser and Arg 121 to Ser in the lactoferrin half-molecule (matching mutations in the C-lobe of melanotransferrin) weakens but does not abolish iron binding. VII. Concluding Remarks

Historically, our understanding of transferrin chemistry has depended to a large extent on spectroscopic and other physicochemical approaches. Crystallographic studies over the past few years have added a new structural dimension. The iron ligands have been established definitively and the open and closed forms of the protein have been defined. Other groups in proximity to the metal and anion sites, with possible modulatory roles, can also be identified. It is thus an opportune time to reexamine the complementary role of spectroscopy. For example, UV difference spectra principally monitor the binding of the Tyr ligands to a metal ion, and the same is true, to a first approximation, of the visible charge transfer spectra. Because the Tyr ligands are associated with only one domain (domain 21, these techniques indicate only binding to this domain and do not show whether an open or a closed structure is adopted for a particular metal ion (or anion). Binding to the ligands of the other domain may be indicated by techniques sensitive to the His ligand, e.g., NMR. Thus combinations of approaches, including techniques such as low-angle solution scattering, and the ultimate power of X-ray crystallography can be used to address important questions. Does binding a particular metal ion or anion result in an open or closed structure? If so, what are the implications for the transport of such species?

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A second major challenge and opportunity for bioinorganic chemists is to use the power offered by recombinant DNA technology. To be able to compare spectroscopicand other results from mutants in which single amino acids have been changed gives a unique possibility to really understand such complex systems. This will also require complementary X-ray structure analyses in order to disentangle the effects of structure and chemistry. (Interpretations of mutagenesis experiments are seldom straightforward.) At the functional and physiological levels, perhaps the greatest need is a better understanding of transferrin-receptor interactions. Mutagenesis experiments again will contribute to this, but the crystal structure of a transferrin-receptor complex would be the ultimate prize. Although this chapter has concentrated on structure, as well as metal and anion binding properties, in the end it is because of their physiological roles, actual or possible, that we study transferrins and find such fascination in their chemistry. ACKNOWLEDGMENTS

I thank Heather Baker, Catherine Day, Rob Evans, Peter Lindley, Clyde Smith, John Tweedie, and Harmon Zuccola for access to their unpublished data; Clyde Smith for help with illustrations; and Heather Baker and Andrew Brodie for their critical reading of the manuscript. I owe a particular debt to Peter Lindley, Rob Evans, and members of the Birkbeck College transferrin group for the free and rewarding collaborative interactions we have always had and to Phil Aisen for being a constant source of inspiration to all in the transferrin field. I gratefully acknowledge financial support for my own research on lactoferrin, over a number of years, from the U S . National Institutes of Health (Grant HD-20859), the Wellcome Trust, the Health Research Council of New Zealand, the New Zealand Dairy Research Institute, and Massey University. I am grateful also to the Howard Hughes Medical Institute for an International Research Scholar award, which has helped support this work.

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11660 (1991). Bates, G. W., and Schlabach, M. R., J . Biol. Chem. 248,3228 (1973). Bates, G. W., and Wernicke, J., J . Biol. Chem. 246,3679 (1971). Mowbray, S.,unpublished data. Bennett, W. S . , and Huber, R., Crit. Rev. Biochem. 15,291 (1984). Gerstein, M., and Chothia, C., J. Mol. Biol. 220, 133 (1991). Kretchmar, S.A., and Raymond, K. N., Inorg. Chem. 27, 1436 (1988). Baldwin, D. A., Biochim. Biophys. Actu 623, 183 (1980). Cowart, R. E., Swope, S., Loh, T. T., Chasteen, N. D., and Bates, G. W., J . Biol. Chem. 261,4607 (1986). Bertini, I., Hirose, J., Luchinat, C., Messori, L., Piccioli, M., and Scozzafava, A., Inorg. Chem. 27,2405 (1988). Harris, W. R., and Bali, P. K., Inorg. Chem. 27,2687 (1988). Bali, P.K., Harris, W. R., and Nesset-Tollefson, D., Inorg. Chem. 30, 502 (1991). Carrano, C. J., and Raymond, K. N., J . A m . Chem. SOC.101,5401 (1979). Harris, W. R., Rezvani, A. B., and Bali, P. K., Inorg. Chem. 26, 2711 (1987). Bali, P. K., and Harris, W. R., J. Am. Chem. SOC.111, 4457 (1989). Egan, T. J., Ross, D. C., Purves, L. R., and Adams, P. A,, Inorg. Chem. 31, 1994 (1992). Egan, T. J., Zak, O., and Aisen, P., Biochemistry 32,8162 (1993). Legrand, D., Mazurier, J., Colavizza, D., Montreuil, J., and Spik, G., Biochem. J. 266,575 (1990). Swope, K. S., Chasteen, N. D., Weber, K. E., and Harris, D. C., J.A m . Chem. SOC. 110,3835 (1988). Borhani, D. W., and Harrison, S. C., J . Mol. Biol. 218, 685 (1991). Mason, A. B., Miller, M. K., Funk, W. D., Banfield, D. K., Savage K. J., Oliver, R. W. A., Green, B. N., MacGillivray, R. T. A., and Woodworth, R. C., Biochemistry 32,5472 (1993). Stowell, K. M., Rado, T. A., Funk, W. D., and Tweedie, J. W., Biochem. J. 276, 349 (1991). Ikeda, R. A., Bowman, B. H., Yang, F., and Lokey, L. K., Gene 117,265 (1992). Woodworth, R. C., Mason, A. B., Funk, W. D., and MacGillivray, R. T. A., Biochemistry 30, 10824 (1991). Day, C. L., Baker, E. N., and Tweedie, J. W., unpublished data.


INDEX

A Absorption, transient, class I1 mixed-valence complexes, 303 Absorption spectra, see also Visible absorption spectra; X-ray absorption spectroscopy class I1 mixed-valence complexes, 294-295 Actinides, transferrin binding, 428-429 Aluminum, transferrin binding, 426 Aluminum hydrides, 221-226 IR spectra, 223 matrix isolation, 222 synthesis, 222 vibrational spectra, 223-224 Amide protons, in diamidedithiols, 65 Anions binding, transferrins, 406 conformational differences associated with, 439-440 interlocking sites model, 434-435 metal-anion interactions, 433-437 nonsynergistic anions, 437-439 Schlabach-Bates model, 436-437 synergistic anions, 431-433 nonsynergistic, 437-439 sites lactoferrin, 418 transferrins, 403-407 sequence similarities, 412-414 synergistic, characteristics, 431-432 Antiferromagnetic exchange, charge transfer model, 305-307 Aqua complexes, technetium(III), 31 Arene complexes, technetium(I1, 13 Arsine technetium(I1) complexes, 23-24 technetium(II1) complexes, 39-43 technetium(1V) complexes, 52-53 [As2Br812-,241 [As2Br913-,249-250 Ad,, 236

[ A s ~ I ~ ~253 I~-, [As&I4-, 260 AsPh4[TcNC14],83, 85-86, 84-85 (ASP~~)~[TC~N~(O)~(OX)~I, 89 Auger electron spectroscopy molecular phosphorus oxides, 360-362 molecular phosphorus oxide sulfides, 380

B Bacterial binding properties, similarities with transferrins, 416-418 BH4 SOUP, 214-215 [BiBr41-,264 [BiBr6I3-,248-249 [Bi2Brl1I5-,255-256 [Bi4Brl6I4-,244 BiCI3, 235,237 [BiC14]-,264 [{BiC14},l"-, 242 [BiC16I3-, 266-267 [Bi4CIl8l6-,255-256 [Bi8C13016~-, 260-261 BiF,, 234-235 BiI,, 236-238 [BiI6I3-,266 [Bi3Il2I3-,253 Bismuth halides, 267 Bismuth tribromide, 235-237 Bispentafluorosulfanylamine, 151 Bis(pentafluorosulfanyl)bis(trifluoromethyl)hydrazine, 150 Bis(pentafluorosulfany1)perfluoroalkylamines, 149-150 Bleaney-Bowers expression, 308 [Bu4"NI~[Sb2ClJ,241 C

Carbonyl complexes technetium(I), 7-12 technetium(III), 27-29

465

466

INDEX

Carboxylato complexes, technetium(III),

32-33 Charge transfer model, antiferromagnetic exchange, 305-307 Charge-transfer transition dipole moment, 276 [CSH,HN]JSb&lg], 251-252 C=N, functionality, 130 C - 0 bond, 28 Comproportionation constants, class I1 mixed-valence complexes, 290-292 Comproportionation equilibrium,

280-281 [Co(NH&I[Sb2Fgl, 250 Copper, transferrin binding, 424-425 Creutz-Taube ion, 281-282 derivatives, 286 physical characterization, 288-289 [Cs(l8-cro~n-6)[TcNC1~1, 85-87 Cs[TcN(02)2CI],97 Cyano complexes mononuclear [Tc0I3+,55-56 technetium(I), 13-14 technetium(II), 30-31 Cyclopentadienyl complexes, technetium(I), 12

D P-Diketonato complexes, technetiumUII),

33 Dimethylgallane, 192-194 Dimethylgallium tetrahydroborate,

188-189 Dinitrogen complexes, technetium(I), 14 Dioximes, technetium(II1) complexes,

33-36 Disulfide bonding, transferrins, 402-403 Disulfide bridges, transferrins, 415-416 Dithiolene complexes, technetium(VI),

93-94 DNA, recombinant, transferrin studies,

452-455

E Electroabsorption spectroscopy, 279 class I1 mixed-valence complexes, 289,

291,294-297 [ { ( N H ~ ) ~ R U } ~ ( ~ -294, ~ Y Z296 )I~',

Electron diffraction gallaborane, 214 gallane vapor, 204-207 2-galla-arachno-tetraborane, 218 gallium hydrides, 185-188 Electron exchange, 304-313 charge transfer model, antiferromagnetic exchange, 305-307 constant J, experimental evaluation,

308 exchange coupled ruthenium dinuclear complexes, 308-313 magnetic, 304-305 superexchange, 307-309 Electronic coupling, between donor and acceptor wave functions, 278 Electron paramagnetic resonance spectra, see EPR spectra Element(II1) halogenoanions, 238-264 cis E-X bonds, 267 coordination geometries, 265-266 [EX,]-, 239-240 [{EX4},]"-,242-244 [EX5]'-, 246 l{EX,},lz"-, 247-248 [EX,13-, 248-249 [{E,X,},l"-, 260-261 [E2Xs]", 240-241 [EZXgl3-,249-251 [{E2Xg},I3''-, 251-252 [EzXioI4-, 247 [EzX11]5-,255-256 [{E3X1o},In-, 261-264 [E3XLIl2-,256-257 [E3XI2l3-,252-254 [E,X,,},I"-, 263-264 [E4XI6l4-,244-246 [E4X1J-, 255-256 [E5XI8l3-,254-255 [E&2I4-, 257-258 [E8XzsI4-,258-260 [EBX3Ol6-, 260-261 extended Huckel molecular orbital analysis, 266-267 Jahn-Teller distortion, 266-267 potential energy diagram, 278 structural chemistry, 264 VSEPR theory, 265-266 Element trihalides, 234-238 primary and secondary bond, 236-237

467

INDEX

EPR spectra, diferric lactoferrin,

433-434

trapping experiments, 180-181 vapor transfer and sampling, 177,

179-180 history and chemical background,

F

173-177 hydridogallium bis(tetrahydroborate),

First transition series, substitution, transferrins, 423-426 Fluorescence quenching, metal binding of transferrins, 419-420 FSSCHCFZOSO,, 159-161

G

189-192 monochlorogallane, 194-196 physical methods of detection and analysis, 181-188 electron diffraction, 185-188 NMR spectroscopy, 184-185 vibrational spectroscopy, 182-184 Group 13 metals, transferrin binding,

426-427 Ga2D6,vibrational properties, 203,205 Ga2H6,183-184 vibrational properties, 203,205 Ga-H bond, 210,214 Gallaborane, 211-216 electron diffraction, 214 infrared spectra, 212-213 synthesis, 215 Gallane, 172-173,196-198 chemical analysis, 199,201 chemical properties, 208-209 complexes, 175-178 electron diffraction, 204-207 gallaborane, 211-216 2-galla-arachno-tetraborane,216-220 'H NMR spectrum, 207-208 IR spectrum, 200-202 physical properties, 199 search for, 173-175 synthesis, 198-199 vibrational spectra, 200-215 2-Galla-arachno-tetraborane, 216-220 llB NMR spectrum, 217,219-220 decomposition of vapor, 216-217 electron diffraction, 218 structure, 217-218 Gallium, transferrin binding, 426 Gallium hydrides, 171-227;see also Pentafluorosulfanyl hypohalites dimensions, 203-204 dimethylgallane, 192-194 dimethylgallium tetrahydroborate,

188-189 handling, 177,179-181 chemical analysis, 181

H Hafnium, transferrin binding, 428 Halide complexes and clusters mononuclear [Tc013', 56-59 [TcN13-, 82 technetium(II), 17-22 mononuclear and binuclear, 17-20 polynuclear, 20-22 technetium(III), 31-32,45-47 HGa(BH&, 191 Histidine, involvement in transferrin iron binding, 403-404 H3P.GaH3, 208,210 [HTcCpzI, 29 Hiickel molecular orbital analysis, extended dinuclear ruthenium complexes,

311-312 element(II1) halogenoanions, 266-267 Hush model, 274-280 class I1 mixed-valence complexes, parameters, 293 intervalence band properties relationships, 276 wave functions, 277 Hydrazido complexes technetium(V), 78-79 technetium(VI), 92-93 Hydridogallium bidtetrahydroborate),

189-192 Hydrogen, bonding interactions, in transferrin binding site, 404-405

468

INDEX

2-Hydroxyl-l-(pentafluoro-h6-sulfanyl)1,2,2-trifluoroethanesulfonic acid sultone. 157-158

I Imido complexes technetium(V), 78-79 technetium(VI1, 92-93 technetium(VII), 97-99 Indium, transferrin binding, 427 Indium hydrides, 225-227 Infrared spectra aluminum hydrides, 223 gallaborane, 212-213 gallane, 200-202 Interlocking sites model, 434-435 Iron coordination, transferrins, 403-405 release, transferrins functional aspects, 444 kinetics, 448-450 pH dependence, 442,450 structural aspects, 450-452 transferrin uptake binding mechanism, 445-447 dynamics importance, 447-448 Isonitrile complexes technetium(I), 13-14 technetium(II), 31 technetium(III), 45

J Jahn-Teller distortion, element(II1) halogenoanions, 266-267

K

L Lactoferrin, 390 anion sites, 418 biological role, 392-393 conformational change, 407-411 connecting peptide, 415 Cuzt-substituted, 439-440 metal and anion binding, 429-430

diferric, EPR spectra, 433-434 domain organization, 398-400 half-molecules, 411-412, 453-454 iron release, 450-452 lanthanide binding, 427-428 oxalate binding, 435-436 polypeptide folding, 417-418 pattern, 400-401 proteolytic fragments, 396 recombinant, 453 ribbon diagram, 399 sequence identity, 393-394 site-specific mutants, 454 three-dimensional structure, 397 visible absorption spectra, 423-424 Lanthanides, transferrin binding, 427-428

M Magnetic susceptibility, temperature dependence, 310 Mass spectrometry, molecular phosphorus oxides, 362 Melanotransferrin, 390-391 biological role, 393 recombinant DNA studies, 454-455 sequence identity, 394 Me,N-GaH, molecule, molecular scattering intensity pattern, 186-187 Metal-anion interactions, transferrins, 433-437 Metal-to-metal charge transfer, 275 antiferromagnetically exchange coupled system, 306-307 Creutz-Taube ion, 286 class I1 M, 290-292 Ru(II)-Ru(III) dinuclear complexes, 313 solvent effects, class I1 mixed-valence complexes, 297-300 thermochromism, 300 Metal-metal coupling class I11 mixed-valence complexes, 284 crown ether addition, 286-288 dependence on bridging ligand size, 302 Metals binding, transferrins, spectroscopic monitors, 419-420

469

INDEX

carried by transferrins, 392 site, transferrins, 403-407 design, 406-407 sequence similarities, 412-414 M-H-M bridge, 182-183 Microwave spectrum, molecular phosphorus oxides, 362 Minitransferrins, 411-413 Mixed-valence complexes, 274-304 class 11, 289-304 activation free energy for energy transfer, 301-302 bridging ligand nature, 301-303 comproportionation constants,

290-292 distance between ruthenium ions,

300-302 electroabsorption spectroscopy, 289,

291,294-297 Hush model parameters, 293 metal-to-metal charge transfer,

290-292 solvent effects, 297-300 molecular electronics and materials,

303-304 Raman and electronic absorption spectroscopy, 297 transient absorption studies, 303 class I11 bridging ligand nature, 284-286 Creutz-Taube ion, 288-289 criteria, 282-284 metal-metal coupling, 284 ruthenium ion nature, 286-288 comproportionation equilibrium,

280-281

[{(NH,),RU}~(~L-PYZ)I~~, 288-289 absorption and electroabsorption spectra, 294-296 "H&[SbCI51, 246 "H,I2[SbCl,F21, 248 Nitrido complexes technetium(V), 72-78 technetium(VI), 81-92 dimeric and polymeric [TcN13', 85-92 monomeric [TcN13', 81-85 technetium(VII), 97-99 Nitrogen ligands technetium(1) complexes, 15 technetium(I1) complexes, 22-23 technetium(II1) complexes, 33-38 technetium(1V) complexes, 52 Nitrosyl complexes technetium(I), 15-16 technetium(II), 25-27 technetium(III), 44 NMR spectroscopy IlB, 2-galla-aruchno-tetraborane, 217, 219-220 gallium hydrides, 184-185 'H, gallane, 207-208 31P

molecular phosphorus oxides, 349,

351-352 molecular oxide phosphorus sulfides,

374-377 solid-state molecular phosphorus oxides,

352-354 molecular phosphorus oxide sulfides,

375-378

Hush model, 274-280 PKS model, 281 potential energy-configuration diagram, 275 potential energy curves, distortion, 275 Molecular electronics, class I1 mixed-valence complexes, 303-304 Monochlorogallane, 194-196

N Na[SbF41,243 [{(NH3)5R~}2(pDi~yd)14t, 312-313

[ N T c ( ~ - O ) ~ T Cdimers, N ] ~ ~ 90-91

0 Organometallic complexes, technetium(II), 17 Oscillator strength, theoretical expression, 276 Ovotransferrin, 390 biological role, 393 CO'+-substituted, 439 half-molecules, 396,411 quarter-molecule, 412-413

470

INDEX

Oxalate binding, lactoferrin, 435-436 0x0-bridged complexes technetium(IV), 47-52 binuclear complexes, 48-51 mononuclear complexes, 47-48 phosphato complexes, 51-52 technetium(VI), 80-81 technetiumWII), 94 Oxygen ligands, technetium(1V) complexes, 47-52 binuclear complexes, 48-51 mononuclear complexes, 47-48 phosphato complexes, 51-52

P [PBrJ, 239-240 [PCIJ, 239 Pentafluorosulfanylalkanes, 128-129, 132-138 Pentafluorosulfanylalkenes, 132-138 Pentafluorosulfanylalkynes, 132-138 Pentafluorosulfanylamine, 144-145 N-Pentafluorosulfanyl chloroimine, 152-154 Pentafluorosulfanyl compounds, 125-161 bispentafluorosulfanylamine, 151 bis(pentafluorosulfanyl)bis(trifluoromethyl)hydrazine, 150 bis(pentafluorosulfany1)perfluoroalkylamines, 149-150 FSSCHCFzOS02, 159-161 halides, 126-130 2-hydroxyl-1-(pentafluoro-h6-sulfanyl)1,2,2-trifluoroethanesulfonic acid sultone, 157-158 pentafluorosulfanylalkanes,alkenes, and alkynes, 132-138 pentafluorosulfanylamine,144-145 pentafluorosulfanyl N,N-dichloroamine, 145-146 pentafluorosulfanyl N, N-difluoramine, 146 N-pentafluorosulfanyl haloimines, 152-154 pentafluorosulfanyl hypohalites, 130-132 pentafluorosulfanyliminodihalosulfanes, 155-157

pentafluorosulfanyl perfluoroalkylamines, 146-147 pentafluorosulfanyl-p-sultones and sulfonic acids, 157-161 SFsN(CF312, 147 SFSN(Cl)RI, 149 (SFS)ZNX, 151-152 SFSN(X)CF3, 147-148 sulfur cyanate pentafluoride, 142-143 sulfur cyanide pentafluoride, 143 sulfur isocyanate and isothiocyanate pentafluorides, 138-142 sulfur isocyanide pentafluoride, 143-144 tetrakis(pentafluorosulfany1 )hydrazine, 150 tris(pentafluorosulfanyl)amine,150 Pentafluorosulfanyl N,N-dichloroamine, 145-146 Pentafluorosulfanyl N,N-difluoramine, 146 N-Pentafluorosulfanyl fluoroimine, 154 Pentafluorosulfanyl halides, 126-130 N-Pentafluorosulfanyl haloimines, 152-154 Pentafluorosulfanyl hypohalites, 130-132 SFEOF, 130-131 Pentafluorosulfanyliminodichlorosulfane, 155-157 Pentafluorosulfanyliminodifluorosulfane, 155 Pentafluorosulfanyliminodihalosulfanes, 155-157 Pentafluorosulfanyl perfluoroalkylamines, 146-147 [PhCH2Me2N.AlH,12,225-226 Phosphine complexes bidentate, technetium(III), 39-43 monodentate, technetiumUII), 38-39 technetium(I1, 14-15 technetium(II), 23-24 Phosphine ligands, technetium(1V) complexes, 52-53 Phosphito complexes, technetium(I), 15 Phosphorus molecular oxides bonding features, 363-364 compared to molecular phosphorus oxide sulfides, 381-383

471

INDEX

crystal and molecular structures, 337-345 bond angles, 343 comparison of molecular structures, 340-343 crystallographic data, 345 in gaseous state, 337-338 group-subgroup relationships, 344 molecular packing, 343-345 in solid state, 337-345 mass spectrometric, 362 microwave spectrum, 362 molecular packings, 382 photoelectron and auger electron spectroscopy, 360-362 31PNMR spectroscopy, 349, 351-352 solid-state NMR spectroscopy, 352-354 structure, 328 synthesis, 329-336 phosphorus (IIIN) oxides, 334-336 phosphorus pentoxide, 330-331 phosphorus trioxide, 331-333 theoretical studies, 362-364 vacuum-ultraviolet spectrum, 362 vibrational spectroscopy, 346-349 X-ray absorption spectroscopy, 355-359 molecular oxide sulfides bond angles, 368 compared to molecular phosphorus oxides, 381-383 crystal and molecular structures, 366-370 force constants, 372 molecular geometries, 368 molecular packings, 382 solid state, 369-370 photoelectron and Auger electron spectroscopy, 379-380 31PNMR spectroscopy, 374-375 solid-state NMR spectroscopy, 375-378 synthesis, 364-366 theoretical studies, 380-381 vibrational spectroscopy, 370-374 X-ray absorption spectroscopy, 378-379 Phosphorus(III/V) oxides crystal structure, 339

IR and Raman absorbances, 351 molecular structure, 328 31PNMR spectroscopy, 351-352 solid-state NMR spectroscopy, 352-353 synthesis, 334-336 vibrational spectroscopy, 349-351 Phosphorus pentoxide bonding features, 363 core binding energies, 361-362 crystal structure, 337-338 solid-state NMR spectroscopy, 352-353 synthesis, 330-331 vibrational spectroscopy, 346-348 X-ray absorption spectroscopy, 355-359 Phosphorus trioxide bonding features, 363 core binding energies, 361-362 crystal structure, 339 ionization energy data, 360-361 31PNMR spectroscopy, 349, 351 solid-state NMR spectroscopy, 352 synthesis, 331-333 vibrational spectroscopy, 346-348 X-ray absorption spectroscopy, 355-359 Photoelectron spectroscopy molecular phosphorus oxides, 360-362 molecular phosphorus oxide sulfides, 379-380 PKS model, 281 P407,crystal structure, 340-341 P409crystal structure, 339 Polymerization, pentafluorosulfanyl halides, 128 Polypeptide chain, transferrins, 397-398 folding, 400-402, 417-418 [Pr4"N12[Sb2C181, 241

R Raman spectroscopy, class I1 mixed-valence complexes, 297 Reorganization parameter, inner and outer sphere, 279 Ruthenium, dinuclear complexes, 273-319; see also Mixed-valence complexes antiferromagnetic superexchange, 309 electron exchange, constant J, 308

472

INDEX

exchange coupled, 308-313 extended Huckel molecular orbital calculations, 311-312 future studies, 313-314 ligand structures, 314-319 magnetic electron exchange, 304-305 magnetic susceptibility, temperature dependence, 310 properties, 273-274 superexchange, 307-309 Ruthenium (III), coordination sphere, 299-300 Ruthenium(III/II) couples, 282-283 Ruthenium ions, 286-288 distance between, 300-302

S

/

/

SbBr3, 235 SbC13, 235 [SbCIJ, 264 [SbCI5l2-,247-248 [Sb4C116I4-,244-245 [SbzC13F613-,252 SbF3, 234 [Sb*FTI-, 251 [{Sb,Fl,},]"-, 262-263 [(Sb4F13},In-,263-264 [Sb4F16I4-,245-246 SbIB, 236 [{Sb217}n]"-,260-261 [Sb2Iz2l4-, 265 [{Sb311O},]"-, 261-262 [Sb,I1,I2-, 256-257 [Sb5Il8I3-,254, 254-255 [Sb&2I4-, 257-258 [Sb81z814-, 258-259 Schiff base technetium(II1) complexes, 36-37 technetium(1V) complexes, 52 Schlabach-Bates model, 436-437 Serum transferrins, 390 biological role, 391-392 half-molecules, 396 recombinant, 453 structure, 397 SF5Br, 126-130 S F S C g H , 137 S F S C M F Z , 135-136 SFSCH2COOAg, 133-134

SFSCI, 126-130 SFSN(CF&, 147 SF,N(Cl)R,, 149 (SFS)zNX, 151-152 SFSN(X)CF3, 147-148 SFSOC1, 131-132 SF,OF, 130-131 Solution X-ray scattering measurements, transferrins, 409-410 Stark effect, 279 class I1 mixed-valence complexes, 289, 291,294-297 Sulfido complexes, technetiumWII), 94 Sulfur, electrochemical fluorination, 133 Sulfur cyanate pentafluoride, 142-143 Sulfur cyanide pentafluoride, 143 Sulfur isocyanate pentafluoride, 138-141 Sulfur isocyanide pentafluoride, 143-144 Sulfur isothiocyanate pentafluoride, 140-142 Sulfur ligands technetium(I1)complexes, 24-25 technetium(II1) complexes, 43-44 technetium(1V) complexes, 53-54 99mT~ physical properties, 3-4 use in diagnostic nuclear medicine, 3-4

T [Tcz(bdt),I, 53-54 [TcBr6I2-,45-47, 47-48 [T~Br(dmgH)~(dmg)BBu], 34-35 Tc-CI, 83 TcCl,, 45-46 [TCC16]2-,48, 69 [T~*C181~-, 18-19 [TC~(CO)~OI, 5-7 dimeric carbonyl complexes, 11-12 [TC(CO)~] core, carbonyl complexes, 9-10 [TdCO),] core, carbonyl complexes, 8-9 [Tc(CO),X12, dimeric and polynuclear carbony1 complexes, 10-11 [TcH,12-, 99 Tc-N, bond distance, 22-23, 37-38, 75-76, 93 T c N , 85 bond distance, 73

INDEX

[TcNI3~,technetium(V1) complexes dimeric and polymeric, 85-92 monomeric, 81-85 [Tc(NAr),Il, 98-99 [TcNClJ, 73-74, 74-76, 81-82,85 [TcNCl,(AsPh,),l, 76-77 [{T~NC12}2(/~-0)2]~-, 90 [ T C N C ~ ~ ( Pcomplexes, P ~ ~ ) ~ ] 74-75 [TcN12-core, 72-73 TcN{N4}complexes, technetium(V),75 T c - N - 0 angle, 25-26 [{TcN(OH~)~}~(/L-O)~]~*, 88-89 TcVN{S4} complexes, 74 Tc-O,66-67,80-81 bond distances, 47-48, 94-95 TFO, 69-70 [Tc0l3-, mononuclear complexes, 54-68 based on other TcO mixed ligand cores, 67-68 based on TcO{N4},TcO{N,-,,O,,},and Tc){N2P2} cores, 62-64 based on TcO{N,_,S,} cores, 64-67 based on TcO(04},TcO{S4},TcO{04_,,Sn},and TcO{Se,} cores, 59-62 cyano and thiocyanato, 55-56 halide, 56-59 truns-[TcO2I+cores, technetium(V) complexes, 68-70 TcO,-, 80 Tc(p-02)Tccomplexes, 49-50 [ T C ~ O ~ 0x0-bridged, ]~', technetiumW complexes, 70-72 [TcOBr,]-, 57-58 [TcOC14]-,48-49, 53, 56-58, 66, 67-68, 69-71,78 [TcOCI~L],96 [ ( T ~ O ) ~ ( e d71-72 t)~l, [TcO,F], 95-96 Tc--OH2, distance, 83 [TcOI,]-, 57-58 [TcO(MAG3)lZ-,66-67 TcO{N,}, mononuclear [Tc013' complexes, 62-63 TcO{N,-,O,}, mononuclear [Tc013+complexes, 63-64 TcO{N2P2},mononuclear [Tc013+complexes, 64 TcO{N,-,S,} cores, mononuclear [Tc013+ complexes, 64-67 Tc0{0,}, mononuclear [Tc0I3 complexes, 59-61 +

473

tr~ns-[TcO(OH)1~+ cores, technetium(V1 complexes, 68-70 TcO{O,_,S,}, mononuclear [TcO13*complexes, 59-61 TcO(S4},mononuclear [Tc0l3+complexes, 59, 61 TcOISe,}, mononuclear [Tc0I3' complexes, 59, 62 Tc-0-Tc bridge, 70-71, 94 TcOzTc bridge, 91 Tc-P, bond distances, 23-24 Tc-S, bridging distance, 98 T& angles, 25 [TcS13+complexes, technetium(V), 72 Tc-Tc bond, 2 9 , 5 0 4 1 , 54 bond distance, 18-21, 31-32, 90-91, 93 [Tdtdt),], 93 [T~(tdt)(dmpe)~IPF~, 41-42 Technetium, 1-99 isotopes, 2 Technetium(-l), 5 Technetium(O), 5-7 TechnetiudI), 7-17 carbonyl complexes, 7-12 dimeric and polynuclear, 10-12 mononuclear, 7-10 cyano and isonitrile complexes, 13-14 cyclopentadienyl and arene complexes, 12-13 dinitrogen, phosphine, phosphito, and related complexes, 14-15 nitrogen ligand complexes, 15 nitrosyl and thionitrosyl complexes, 15-17 Technetium(II), 17-27 complexes nitrogen ligands, 22-23 sulfur ligands, 24-25 halide complexes and clusters, 17-22 binuclear, 18-20 mononuclear, 17-18 polynuclear, 20-22 nitrosyl and thionitrosyl complexes, 25-27 organometallic complexes, 17 phosphine, arsine, and related complexes, 23-24 TechnetiumUII), 27-44 aqua, halide, and related dimeric cornplexes, 31-32

474

INDEX

carbonyl complexes, 27-29 carboxylato and P-diketonato complexes, 32-33 complexes bidentate phosphine, arsine, and related ligands, 39-43 dioximes, Schiff bases and other nitrogen ligands, 33-38 monodentate phosphines and related ligands, 38-39 sulfur ligands, 43-44 cyano, isonitrile, and thiocyanato complexes, 30-31 cyclopentadienyl complexes, 29-30 nitrosyl and thionitrosyl complexes, 44 nonreducible cations, 40-41 organohydrazine chemistry, 37 Technetium(IV), 45-54 complexes oxygen ligands and 0x0-bridged complexes, 47-52 binuclear complexes, 48-51 mononuclear complexes, 47-48 phosphato complexes, 51-52 phosphine and arsine ligands, 52-53 Schiff base and other nitrogen ligands, 52 sulfur ligands, 53-54 halide and related complexes, 45-47 isonitrile and thiocyanato complexes, 45 Technetium(V), 54-79 complexes not containing multiply bonded ligands, 79 fr~ns-[TcO(OH)1~' cores, 68-70 imido and hydrazido complexes, 78-79 mononuclear [Tc013+complexes, 54-68 based on other TcO mixed ligand cores, 67-68 based on TcO{N4},TcO{04-"On},and TcO{N2P2}cores, 62-64 based on TcO{N4-"Sn}cores, 64-67 based on TcO{04},TcO{S4},TcO{04-"Sn},and TcO{Se,} cores, 59-62 cyano and thiocyanato, 55-56 halides, 56-59 nitrido complexes, 72-78 0x0-bridged [Tc2O3I4+ and other binuclear complexes, 70-72

synianfz isomerism, 64-65 [TcS13+complexes, 72 Technetium(VI), 80-94

dithiolene and related complexes, 93-94 imido and hydrazido complexes, 92-93 nitrido complexes, 81-92 dimeric and polymeric [TcNI3+, 85-92 monomeric [TcNI3', 81-85 0x0 complexes, 80-81 Technetium(VI1). 94-99 complexes not containing multiply bonded ligands, 99 nitrido and imido complexes, 97-99 0x0 and sulfido complexes, 94-97 Tetrakis(pentafluorosulfanyl)hydrazine, 150 Thallium hydrides, 225-227 Thermochromism, metal-to-metal charge transfer, 300 Thiocyanato complexes mononuclear [Tc013-,55-56 technetium(II), 31 technetium(III),45 Thionitrosyl complexes technetium(I), 16-17 technetium(II), 26-27 technetium(III), 44 Transferrins, 389-456; see also specific transferrins

anion binding conformational differences associated with, 439-440 interlocking sites model, 434-435 metal-anion interactions, 433-437 nonsynergistic anions, 437-439 Schlabach-Bates model, 436-437 synergistic anions, 431-433 biological roles, 391-393 diferric complexes, visible absorption spectra, 433 differences between metal and anion sites, 440-445 differences in properties, 441-443 functional aspects, 444-445 structural comparison, 440-441 substitution, 443-444 iron release kinetics, 448-450 structural aspects, 450-452

475

INDEX

iron uptake binding mechanism, 445-447 dynamics importance, 447-448 lactoferrin, 390 melanotransferrin, 390-391 metal binding, spectroscopic monitors, 419-420 metal substitution and spectroscopy, 420-431 actinides and other metal ions, 428-429 binding constants, 422-423 first transition series, 423-426 group 13 metals, 426-427 lanthanides, 427-428 structural aspects, 429-431 ovotransferrin, 390 possible evolutionary development, 395 properties, 390-391 proteins, 390 recombinant DNA studies, 452-455 serum, see Serum transferrins site-specific mutants, 454 solution X-ray scattering measurements, 409-410 structure, 393-418 conformational change, 407-41 1 disulfide bonding, 402-403 domain organization, 397-398 general organization, 397-400 half- and quarter-molecules, 395-396,411-412 metal and anion sites, 403-407 polypeptide folding, 400-402, 417-418 primary, 393-396 similarities with bacterial binding proteins, 416-418 three-dimensional, 396-412 variations among, 412-416 UV difference spectra, 419, 421

Transient absorption studies, class I1 mixed-valence complexes, 303 Trapping experiments, gallium hydrides, 180-181 Tris(pentafluorosulfanyl)amine,150 Tyrosine, involvement in transferrin iron binding, 403-404

U UV difference spectra, transferrins, 419, 42 1

V Vacuum-ultraviolet spectrum, molecular phosphorus oxides, 362 Vanadium, transferrin binding, 423-424 Vapor transfer, gallium hydrides, 177, 179-180 Vibrational spectroscopy aluminum hydrides, 223-224 gallane, 200-215 gallium hydrides, 182-184 molecular phosphorus oxides, 346-349 Visible absorption spectra diferric transferrin complexes, 433 lactoferrin, 423-424 VSEPR theory, element(II1) halogenoanions. 265-266

W Wave functions, 217 A

X-ray absorption spectroscopy molecular phosphorus oxides, 355-359 molecular phosphorus oxide sulfides, 378-379


CONTENTS OF PREVIOUS VOLUMES

VOLUME 31 Preparation and Purification of Actinidine Metals J . C. Spirlet, J R . Peterson, and L . B . Asprey Astatine: Its Organonuclear Chemistry and Biomedical Applications J . Brown Polysulfide Complexes of Metals A . Miiller and E . Diemann Iminoboranes Peter Paetzold Synthesis and Reactions of' Phosphorus Rich Silphosphanes G . Fritz INDEX

VOLUME 32 Dynamics of Spin Equibria in Metal Complexes James K Beattie Hydroxo-Bridged Complexes of Chromium(III), Cobalt(II), Rhodium(III), and Iridium(II1) Johan Springborg Catenated Nitrogen Ligands Part 11 Transition Metal Derivatives of Trizoles, Tetrazoles, Pentazoles, and Hexazine David S Moore and Stephen D Robinson

The Redox Chemistry of Nickel A . Graham Lappin and Alexander McAuley Nickel in Metalloproteins R . Cammack Nitrosyl Complexes of Iron-Sulfur Clusters Anthony R . Butler, Christopher Glidewell, and Min-Hsin Li INDEX

VOLUME 33 1.6-Disubstituted Triptycenes Alan G . Massey Cysteine-Containing Oligopeptide Model Complexes of Iron-Sulfur Proteins Akira Nakamura and Norikazu Ueyama Reduction Potentials Involving Inorganic Free Radicals in Aqueous Solution Daijid M . Stanbury The Nitrogen Fluorides and Some Related Compounds H. J . Emeleus. Jean'ne M. Shreeve, and R . D. Verma Higher Oxidation State Manganese Biomolecules John B . Vincent and George Chrisfou Double Bonds between Phosphorus and Carbon R . Appel and F. Knoll INDEX

417

478


VOLUME 3 4

VOLUME 36

Homoleptic Complexes of 2,2'-Bipyridine E . C CoristablP

Inorganic Chemistry and Drug Design Peter J . Satiler

Compounds of Thorium and Uranium in Low (c:IVl Oxidation States Isabel Santos. A . Pirrs de Matos, and Alfred G . Maddork

Lithium and Medicine: Inorganic Pharmacology N. .J. Birch and J . D . Phillips

Leaving Groups on Inert Metal Complexes with Inherent or Induced Liability Geoffrey A Lnwrancr The Coordination of Metal Aquaions G. W . Neilson and I E .Enderbv An Appraisal of Square-Planar Substitution Reactions R . J Cross Transition Metal Nitrosyl Complexes D . Michael P. Mingos and Durren J . Shermari

The Mo-. V-, and Fe-Based Nitrogenase Systems of Azobacter Robert R . Eadv T h e Extraction of Metals from Ores IJsing Bacteria U . Keith E w a r f and Martin N Hughes

Sol id -State Bioi norganic Chemist rq' : Mechanisms and Models of Biomineralization Stephen Mann and Carole C . Per? Magnetic Circular Dichroism of Hemoproteins M R . Cheesman. C . Greenic~ood.arid A . J Thomson

INDEX

VOLUME 35 Chemistry of Thioether Macrocyclic Complexes Alexander J . Blak<Jand Martin Schrdder Vanadium: A Biologically Kelevant Element Ron Wever and Kenneth K i d i n Structure, Reactivity, Spectra, and Redox Properties of CobaltlIII) Hexaarnines Philip Hendry and Andreas 1,udi The Metallic Face of Boron Thomas P. Fehlner Developments in Chalcogen-Halide Chemistry Burnt Krebs and Frank-Peter Ahler-s Interaction between Optical Centers and Their Surroundings: An Inorganlc Chemist's Approach G. Blasse INI)EX

Flavocytochrome b, Stephen K . C h a p m a n , Scott A . White. and Graeme A . Reid X-Ray Absorption Spectroscopy and t h e Structures of Transition Metal Centers in Proteins C David Garner Direct Electrochemistry of Proteins and Enzymes Liang-Hong Guo and H . Allen 0 . Hill Active-Site Properties o f te Blue Copper Proteins A . G. Sykes The Uptake, Storage, and Mobilization of Iron and Aluminum in Biology S . Jemil A . Faterni, Fahini H A . Kadir, and D U LJ .~Williamson, mid Geoffrey R Moore Probing Structure-Function Relations in Ferritin and Bacterioferritin P . M . Harrison, S . C . Andres. P . J . A r t y m i u k , G. C . Ford, J . R . Guest, J . Hirzmann. D M L O U J S O I I .


479

Dynamic Electrochemistry of Iron-Sulfur Proteins Frascr A . Armstrong VOLUME 37 On the Coordination Number of the Metal in Crystalline Halogenocuprates(1)and Halogenoargentates(1) Susan Jagner and Goran Helgesson Structures of Organonitrogen-Lithium Compounds Recent Patterns and Perspectives in Organolithium Chemistry Karma Gregorv. Paul uon RagiLe Schleyer. and Ronald Snaith Cubane and Incomplete Cubane-Type Molybdenum and Tungsten 0 x 0 1 Sulfido Clusters Takashi Shibahara Interactions of Platinum Amine Compounds with Sulfur-Containing Biomolecules and DNA Fragments Edwin L M Lempers and J a n Reedijk Recent Advances in Osmium Chemistry Peter A Lay and W Dean Harman Oxidation of Coordinated Diimine Ligands in Basic Solutions of Tristdiimine)iron(III), ruthenium( 1111, and -0smiurntIII1 0 M ~ n s t e dand G Nord INIIEX

EPR Spectroscopy of Iron-Sulfur Proteins Wilfred R . Hagen Structural and Functional Diversity of Ferredoxins and Related Proteins Hiroshi Matsubara a n d Kazuhlko Saeki Iron-Sulfur Clusters in Enzymes: Themes and Variations Richard C a n m a c k Aconitase: An Iron-Sulfur Enzyme Mary Claire Kennedy a n d C . David stout Novel Iron-Sulfur Centers in Metalloenzymes and Redox Proteins from Extremely Thermophilic Bacteria Michael W . W . A d a m s Evolution of Hydrogenase Genes Gerrit Voordoww Density-Functional Theory of Spin Polarization and Spin Coupling in Iron-Sulfur Clusters Louis Noodleman and David A . Case INDEX

VOLUME 39

VOLUME 38

Synthetic Approach to t h e Structure and Function of Copper Proteins Nobumasa Kitajima

Trinuclear Cuboidal and Heterometallic Cubane-Type Iron-Sulfur Clusters New Structural and Reacticity Themes i n Chemistry and Biology R H Holm

Transition Metal and Organic RedoxActive Macrocycles Designed to Electrochemically Recognize Charged and Neutral Guest Species Paul D . Beer

Replacement of Sulfur by Selenium in Iron-Sulfur Proteins Jacques Meyer, Jean-Marc Moulis. Jacques Gaillurd. and Marc Lutz

Structures of Complexes in Solution Derived from X-Ray Diffraction Measurements Georg Johanssan

480


High-Valent Complexes of Ruthenium and Osmium Chi-Ming Che arid Vivian Wing W a h Yam Heteronuclear Gold Cluster Compounds D Michael P Mingos and Michael J Watson Molecular Aspects on the Dissolution and Nucleation of Ionic Crystals in Water Hitoshr Ohlakc INDEX

VOLUME 40 Bioinorganic Chemistry of Pterin-Containing Molybdenum and Tungsten Enzymes ./ohti H . Etteirtark a i d Charles G . Yo/tr/g

Structure and Function of Nitrogenase Douglas C . Rees. Michael K . Chan. arid Jorlgsurl Ki1,l Blue Copper Oxidases A . Messerschrnidl Quadruply Bridged Dinuclear Coniplexes of Platinum, Palladium. and Nickel Keisrike Uiiinkoshi arid Yoichi Sasaki Octacyano and 0x0- and Nitridotetracyano Complexes of Second and ‘Third Series Early Transition Metals Johatiri G . Leipoldt. Stephen S . Bassotr. and Aitdreas Rood1 Macrocyclic Complexes a s Models for Nonporphine Metalloproteins Vichie McKee Complexes of Sterically Hindered Thiolate Ligands J . R . Dilic,orth and J . H u INDEX

Advances in Inorganic Chemistry, Volume 41

Advances in Protein Chemistry Volume 41

Advances in Heterocyclic Chemistry, Volume 41

Advances in Protein Chemistry Volume 41

Advances in Clinical Chemistry Volume 41

Advances in Inorganic Chemistry, Volume 53

Advances in Inorganic Chemistry, Volume 49

Advances in inorganic Chemistry, Volume 52

Advances in Physical Organic Chemistry, Volume 41

Advances in Catalysis, Volume 41

Advances in Genetics Volume 41

Advances in Chromatography: Volume 41

Advances in Immunology Volume 41

Advances in Computers, Volume 41

Advances in Agronomy, Volume 41

Advances in Organic Chemistry, 50 (Advances in Inorganic Chemistry)

Advances in Parasitology Volume 41

Advances in Parasitology Volume 41

Advances in Organometallic Chemistry, Vol. 41

Advances in Carbohydrate Chemistry and Biochemistry, Volume 41

Advances in Carbohydrate Chemistry and Biochemistry, Volume 41

Advances in Inorganic Chemistry Volume 46 (Including Bioinorganic Studies)

Advances in applied mechanics. Volume 41

Advances in Applied Mechanics, Volume 41

Advances in Ecological Research, Volume 41

Advances in Microbial Physiology Volume 41

Advances in Botanical Research, Volume 41

Advances in Microbial Physiology Volume 41

Advances in Cancer Research Volume 41

Annual Reports in Medicinal Chemistry, Volume 41

Inorganic Chemistry in Tables

Advances in Inorganic Chemistry, Volume 41

Advances in Protein Chemistry Volume 41

Advances in Heterocyclic Chemistry, Volume 41

Advances in Protein Chemistry Volume 41

Advances in Clinical Chemistry Volume 41

Advances in Inorganic Chemistry, Volume 53

Advances in Inorganic Chemistry, Volume 49

Advances in inorganic Chemistry, Volume 52

Advances in Physical Organic Chemistry, Volume 41

Advances in Catalysis, Volume 41

Advances in Genetics Volume 41

Advances in Chromatography: Volume 41

Advances in Immunology Volume 41

Advances in Computers, Volume 41

Advances in Agronomy, Volume 41

Advances in Organic Chemistry, 50 (Advances in Inorganic Chemistry)

Advances in Parasitology Volume 41

Advances in Parasitology Volume 41

Advances in Organometallic Chemistry, Vol. 41

Advances in Carbohydrate Chemistry and Biochemistry, Volume 41

Advances in Carbohydrate Chemistry and Biochemistry, Volume 41

Advances in Inorganic Chemistry Volume 46 (Including Bioinorganic Studies)

Advances in applied mechanics. Volume 41

Advances in Applied Mechanics, Volume 41

Advances in Ecological Research, Volume 41

Advances in Microbial Physiology Volume 41

Advances in Botanical Research, Volume 41

Advances in Microbial Physiology Volume 41

Advances in Cancer Research Volume 41

Annual Reports in Medicinal Chemistry, Volume 41

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