Coordination Polymers: Design, Analysis and Application

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Author: Stuart R. Batten | Suzanne M. Neville | David R. Turner

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Coordination Polymers Design, Analysis and Application

Coordination Polymers Design, Analysis and Application

Stuart R. Batten, Suzanne M. Neville and David R. Turner School of Chemistry, Monash University, Victoria, Australia

ISBN: 978-0-85404-837-3 A catalogue record for this book is available from the British Library r Stuart R. Batten, Suzanne M. Neville and David R. Turner, 2009 All rights reserved Apart from fair dealing for the purposes of research for non-commercial purposes or for private study, criticism or review, as permitted under the Copyright, Designs and Patents Act 1988 and the Copyright and Related Rights Regulations 2003, this publication may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in writing of The Royal Society of Chemistry or the copyright owner, or in the case of reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of the licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to The Royal Society of Chemistry at the address printed on this page. Published by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 0WF, UK Registered Charity Number 207890 For further information see our web site at www.rsc.org

Preface Some time ago, relatively recently, the field of coordination polymers exploded. Or rather, it underwent a ‘rapid expansion’, much like the Universe did after the Big Bang. Where once there was an occasional paper or two, there are now whole journals devoted to crystal engineering, of which coordination polymers are a major subdiscipline. No longer are these materials the vaguely defined ‘insoluble material’ that forms at the bottom of your vessel and spells death for your reaction. Rather, they are now the quarry that needs to be coaxed into crystalline form, or probed with X-rays and gases and magnets and lasers. They have gone from ‘polymeric rubbish of unknown composition’ to ‘materials of the future’. There are a number of reasons for being where we are now. Conceptual leaps were made in the early–mid-1990s in their deliberate design, allied with similar advances in related areas such as organic crystal engineering (aided by the blossoming of the Cambridge Structural Database as a research tool), and metallosupramolecular chemistry (and indeed supramolecular chemistry as a whole). Advances in X-ray diffraction equipment, and our ability to display these materials graphically, also helped; two-dimensional drawings became three-dimensional objects, and we learned to look beyond the local molecular connectivity to the crystal as a whole. Supramolecular chemistry taught us to be ambitious – no longer did we assemble things an atom at a time, but rather used whole molecules as our building blocks, and hoped that we had designed them as well as Nature designed her atoms so that they would self-assemble into the polymer we intended. Sometimes it worked, sometimes it didn’t; every time we learnt something. Indeed, the same holds true today. The next step, of course, was then to use that knowledge. New materials were made that had lots of holes in them, or were acentric or chiral or heterogeneous catalysts, or any combination of these. Magnetochemists had been using coordination polymers for some time, but even here they benefited from this new thinking. Soon Prussian Blue, cadmium cyanide, copper adiponitrile Coordination Polymers: Design, Analysis and Application By Stuart R. Batten, Suzanne M. Neville and David R. Turner r Stuart R. Batten, Suzanne M. Neville and David R. Turner, 2009 Published by the Royal Society of Chemistry, www.rsc.org

v

vi

Preface

nitrate and copper tetracyanotetraphenylmethane were just part of the cosmic background. So given that this is now a vast and important field, it seems timely for a book dedicated entirely to coordination polymers. This book does not seek to review the entire field comprehensively, in its vast breadth and depth. It simply could not, or at least not in a single volume. Rather, it is written with the eye of an eagle, taking in the whole landscape but also focusing in on particular areas that seem (to us) to be important at the time. The aim is to give the reader a flavour of each field, introduce them to the important concepts and developments, and to reinforce this with selected examples that are, in our subjective view, important, illustrative or occasionally even anomalous. The aim is to cover all aspects of coordination polymers, or as many as we can practicably in a forum such as this. For practical reasons, however, we have largely restricted ourselves to coordination polymers of transition metals (plus Zn, Cd and Hg) and lanthanoids and have ignored the s- and p-block metals. Similarly, we have chosen to include only ligands that are organic in nature (with a few notable exceptions, such as cyanide). More ‘inorganic’ materials such as those containing singleatom bridges (e.g. halides, oxides and sulfides) or e.g. phosphate-derived materials are largely ignored; sometimes this distinction is, unfortunately, subjective but nonetheless it is a necessary one. Exceptions will arise from time to time in our discussion (e.g. the chapter on organic–inorganic hybrids), but usually only when there is also a more ‘organic’ bridging ligand also present, or a point to be made by widening our metal or ligand vocabulary. The book has been subtly divided into sections. After the introductory chapter, there follows three chapters that deal with aspects of the design of coordination polymers, including nets, interpenetration and all the other factors that need to be taken into account (or explain why the design failed). Then follows a series of analysis chapters, where we survey the results reported to date for coordination polymers constructed using both transition metals and lanthanoids, and also the related areas of organometallic networks and organic–inorganic hybrid materials. Finally, we end with a series of chapters on the applications of these new materials, including long-range magnetism, spincrossover materials, porosity (including gas sorption), catalysis, non-linear optical activity and chirality. We hope that this book acts as both an important introduction to the field for graduate students and a valuable resource and overview for more experienced researchers. Where necessary, we have tried to take a tutorial approach, but we always urge the interested reader to consult the references closely; each is a treasure trove of information from which we have distilled only a portion. There exists such a vast field of knowledge that only selected examples can be mentioned that clearly demonstrate the principles and applications. We acknowledge the work of the many hundreds of researchers who we cannot explicitly mention in the space of these pages but who have contributed to the knowledge pool of the field.

Preface

vii

Finally, S.R.B. would like to thank his wife, Annmarie, and his children, Kira and Geordie, for putting up with their sometimes part-time husband and father during the writing of this book. He would also like to thank the ever patient Annie Jacob at the RSC, who must have thought this would never see the light of day. S.M.N. would like to thank those involved in coordinating and editing the book chapters for giving their valuable time. D.R.T. wishes to thank his partner, Jodie, for her patience after he said that it would be ‘a while’ before embarking on another large writing project. Sorry!

Contents Chapter 1

Introduction 1.1 1.2

Introduction Crystal Engineering, Supramolecular Chemistry, Metallosupramolecules 1.3 What is a Coordination Polymer? 1.4 Synthetic Techniques 1.5 Design, Analysis, Application References

Chapter 2

1 4 6 10 12 13

Nets: A Tool for Description and Design 2.1 2.2 2.3 2.4 2.5 2.6

Introduction Defining Networks Identifying and Naming Nets 1D Nets 2D Nets Common 3D Nets 2.6.1 3-Connected Nets 2.6.2 4-Connected Nets 2.6.3 5-Connected Nets 2.6.4 6-Connected Nets 2.6.5 Higher Connected Nets 2.6.6 Mixed Connectivity Nets 2.7 Rod Packings References

Coordination Polymers: Design, Analysis and Application By Stuart R. Batten, Suzanne M. Neville and David R. Turner r Stuart R. Batten, Suzanne M. Neville and David R. Turner, 2009 Published by the Royal Society of Chemistry, www.rsc.org

ix

19 20 25 28 29 36 36 39 45 46 47 48 52 53

x

Chapter 3

Contents

Interpenetration 3.1 3.2 3.3 3.4 3.5 3.6

Introduction Nomenclature 1D Interpenetration 2D Interpenetration 3D Interpenetration Interpenetration of Nets of Different Composition, Topology and Dimensions 3.7 Self-penetration 3.8 Other Entangled Systems References Chapter 4

84 87 89 89

Malleability of Coordination Polymers 4.1 Introduction 4.2 Supramolecular Isomerism 4.3 Malleability of Building Blocks 4.4 Synthetic Approach 4.5 Solvent Effects 4.6 Other Guests 4.7 Counterions 4.8 Weaker Interactions 4.9 Conclusion References

Chapter 5

59 67 69 70 77

96 98 107 111 115 119 120 124 134 134

Transition Metal Coordination Polymers 5.1 5.2

5.3

5.4

5.5

Introduction Pseudohalide Ligands 5.2.1 The Cyanide Ligand 5.2.2 The Azide Ligand 5.2.3 NCX Ligands (X ¼ O, S, Se) 5.2.4 Small Polynitrile Ligands (e.g. dca/tcm) Larger Nitrile Donor Ligands 5.3.1 Neutral Ligands 5.3.2 Anionic Nitrile Ligands Pyridyl Donor Ligands 5.4.1 2-Connecting Ligands 5.4.2 3-Connecting Ligands 5.4.3 4-Connecting Ligands Five-membered Ring Nitrogen Donor Ligands 5.5.1 2-Connecting Ligands 5.5.2 3-Connecting Ligands 5.5.3 Other Connectors

144 146 146 149 152 153 156 156 158 159 159 164 165 166 167 169 170

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Contents

5.6 5.7

N-Oxide Ligands Carboxylate Ligands 5.7.1 2-Connecting Ligands 5.7.2 3-Connecting Ligands 5.7.3 4-Connecting Ligands 5.8 Other Ligands 5.9 Mixed Donor Atom Ligands 5.10 Mixed Ligand Coordination Polymers References Chapter 6

Rare Earth Coordination Polymers 6.1 6.2 6.3 6.4 6.5 6.6

Introduction Cyanide/Nitrile and Pseudohalide Ligands Five-membered Ring Donors Pyridyl Donor Ligands N-Oxide Donors Rare Earth–Carboxylate Polymers 6.6.1 Monocarboxylate Ligands 6.6.2 Dicarboxylate Ligands 6.6.3 Tricarboxylate Ligands 6.6.4 Tetracarboxylate Ligands 6.7 Other Ligands 6.7.1 Oxalate Coordination Polymers 6.7.2 Chloranilic Acid and Dihydroxybenzoquinone 6.7.3 Polymers with Sulfonate-containing Ligands 6.7.4 Amide Ligands 6.7.5 Pyridylcarboxylate Ligands 6.8 Mixed 3d/4f Coordination Polymers 6.8.1 Cyanide-based Bimetallic Polymers 6.8.2 Carboxylate-based Bimetallic Polymers 6.8.3 Pyridylcarboxylate Ligands 6.8.4 Other Mixed O/N Donor Ligands 6.9 Actinide Coordination Polymers References

Chapter 7

170 172 173 175 177 178 180 181 183

191 192 195 196 196 202 203 205 210 212 212 212 215 217 217 219 221 221 224 224 229 229 231

Organometallic Networks 7.1 7.2

7.3

Introduction Large Aromatic Ligands 7.2.1 Cation–p Interactions in Ag(I)–PAH Systems 7.2.2 Cation–p Interactions with Cyclophanes 7.2.3 Non-aromatic Cation–p Interactions Organometallic Complexes as Ligands 7.3.1 Metallocene Bridging Ligands 7.3.2 Metal–Quinone Bridging Complexes

238 239 239 241 243 245 245 246

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Contents

7.4 7.5

Isocarbonyl Polymers R3Sn/R3Pb Systems 7.5.1 Metal–Cyanide Polymers 7.5.2 (Triorganostannyl)tetrazole Polymers References

Chapter 8

Inorganic–Organic Hybrids 8.1 8.2 8.3

Introduction 0D Metal–Oxide Substructures 1D Metal–Oxide Substructures 8.3.1 2D Polymers with 1D Metal–Oxide Substructures 8.3.2 3D Polymers with 1D Metal–Oxide Substructures 8.4 2D Metal–Oxide Substructures (Pillared Layers) References Chapter 9

248 251 251 252 254

257 259 260 261 263 264 271

Magnetism in Coordination Polymers 9.1 9.2

Introduction Long-range Magnetic Ordering 9.2.1 Introduction 9.2.2 Molecule-based Magnets 9.2.3 Single-chain Magnets 9.3 Spin Crossover 9.3.1 Introduction 9.3.2 Five-membered Heterocyclic Ring Bridging Ligands 9.3.3 Six-membered Ring Bridging Ligands 9.3.4 Cyanide Bridges in Hofmann Phases References

273 273 273 275 293 295 295 297 301 305 307

Chapter 10 Porous Coordination Polymers 10.1

Introduction 10.1.1 Terminology 10.2 Designing Permanent Porosity 10.2.1 Flexible Porosity 10.2.2 Rigid Porosity 10.3 Solvent Exchange 10.3.1 Flexible Guest Exchange 10.3.2 Rigid Guest Exchange 10.4 Gas Storage 10.4.1 Hydrogen Gas Storage 10.4.2 Methane Gas Storage

313 313 315 317 322 326 327 329 330 331 335

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Contents

10.4.3 Carbon Dioxide Gas Storage 10.4.4 Other Gases 10.5 Ion Exchange 10.6 Multifunctional Porous Materials 10.7 Conclusion References

336 337 338 338 340 341

Chapter 11 Acentric and Chiral Networks 11.1 11.2 11.3

Introduction Acentric Networks for Non-linear Optical Behaviour Chiral Networks 11.3.1 1D Helices from Achiral Components 11.3.2 2D Chiral Nets from Achiral Components 11.3.3 3D Chiral Nets from Achiral Components 11.3.4 Chiral Ligands 11.3.5 Applications References

345 346 352 352 354 355 361 367 369

Chapter 12 Reactive Coordination Polymers 12.1 Introduction 12.2 Topotactic Reactions 12.3 Chemically Reactive Ligands 12.4 Catalysis References

375 375 386 386 391

Chapter 13 Other Properties of Coordination Polymers 13.1 Introduction 13.2 Luminescence 13.3 Redox Activity 13.4 Conductivity 13.5 Negative Thermal Expansion 13.6 Multifunctional Materials References Subject Index

396 397 399 401 404 407 408 416

CHAPTER 1

Introduction 1.1 Introduction Sometime between 1704 and 1705, a Berlin colourmaker named Diesbach made a mistake.1 He was trying to make a red pigment known as cochineal red lake. The recipe was simple – iron sulfate and potash. But it turned out pale. Upon further concentration, it became deep blue! By using cheap potash, contaminated with animal oil made from ox blood, Diesbach had created Prussian Blue. This was the first man-made coordination polymer and in fact the first man-made coordination compound. It was also a valuable pigment; within a few short years it was being made commercially from a closely guarded recipe. It would, however, be another 372 years before the structure of Prussian Blue, Fe4[Fe(CN)6]3 xH2O, would be determined (Figure 1.1a).2 In the intervening years, little attention was paid to coordination polymers (certainly much less than their organic cousins received), with only a few scattered structural studies. The structures of Zn(CN)2 and Cd(CN)2 were reported by a Russian group in the depths of World War II.3 Powell and Rayner determined the structures of the Hofmann clathrate, [Ni(NH3)2Ni(CN)4] 2C6H6, shortly afterwards (Figure 1.1b);4 this work was later extensively followed up by Iwamoto’s group on related compounds.5 In 1959, a Japanese group reported, remarkably, that they had determined that the structure of [Cu(adiponitrile)]NO3 contained six interpenetrating diamond networks.6 The 1D chain structures of Ag(pyrazine)NO3 and Cu(pyrazine)(NO3)2 were reported in 1966 and 1970, respectively.7,8 The crystal structure of Co(pyrazine)2Cl2 was shown to have a square grid structure in 1971.9 As the 1980s came to a close, there was increasing interest in these materials,10 particularly in the field of molecule-based magnetic materials.11 However, it was not until a short communication in 1989,12 and a subsequent full paper in 1990,13 that interest really took off.


1

2

Figure 1.1

Chapter 1

The structures of (a) Prussian Blue2 and (b) the Hofmann clathrate.4

3

Introduction 14,15

In these and subsequent papers, Robson, Hoskins and co-workers outlined a net-based approach to the design of coordination polymers. They took the landmark work of Wells,16 which described crystal structures in terms of networks, and applied it to the design of new coordination polymers (Figure 1.2). Through this design approach, they proposed that new materials with interesting properties such as porosity and catalysis could be deliberately engineered. These ideas soon caught on, with other early groups in the field17–23 making important contributions that would ultimately lead to the explosion in research illustrated in Figure 1.3.

Figure 1.2

An early framework coordination polymer reported by Hoskins and Robson.13

Figure 1.3

The number of hits for ‘coordination polymer’ or ‘metal–organic framework’ in the Science Citation Index, by year.

4

Chapter 1

1.2 Crystal Engineering, Supramolecular Chemistry, Metallosupramolecules The development of coordination polymer research was reinforced by the growth of two other closely related areas: crystal engineering and supramolecular chemistry (particularly metallosupramolecular chemistry). Crystal engineering seeks to understand why molecules pack in the ways that they do and to use that knowledge to deliberately engineer the arrangements of molecules in new materials.24 This is important because the properties of materials are often governed by the way in which their constituent molecules are arranged. Control over this arrangement gives control over the properties. In ‘molecular’ (largely organic) crystal engineering, the interactions are weaker than coordination bonds and can range in strength from very strong hydrogen bonding to weak C–H A hydrogen bonds, halogen bonds, p interactions and, ultimately, van der Waals forces. The crystal engineer seeks to understand and harness all these interactions. However, despite the differences in the interactions, there is much that is common in these two areas. Indeed, coordination polymers, which essentially exist only in the solid state, should be considered as a subset of crystal engineering. Furthermore, the net-based approach for coordination polymers is equally valid for molecular species connected by well-defined interactions. For example, trimesic acid (benzene1,3,5-tricarboxylic acid) readily forms hexagonal sheets in which the molecules are connected by hydrogen bonding, as shown in Figure 1.4a.25 The large organic molecule shown in Figure 1.4b assembles, as one would predict, into seven interpenetrating diamond networks through hydrogen bonding between the peripheral functional groups.26 Many of the concepts and terminology in molecular crystal engineering also apply to coordination polymers. Interactions between molecules that direct their packing arrangements (such as the hydrogen bonding carboxylate dimer motif in Figure 1.4a) are known as supramolecular synthons;27 in coordination polymers, the main synthons are coordination bonds (although weaker synthons can also be important, as discussed in Chapter 4). The building blocks used to create the structure, such as the molecules shown in Figure 1.4, are called tectons;28 for coordination polymers, the tectons are metal ions and ligands. These two concepts are highlighted in Figure 1.5. The aim of supramolecular chemistry is similar: to create assemblies of molecules, that is, not to create structures an atom at a time, but to design molecules such that when combined they spontaneously self-assemble in a predetermined fashion into larger architectures.29 Thus crystal engineering can, in fact, be considered to be the supramolecular chemistry of the solid state. To quote Dunitz, ‘The crystal is, in a sense, the supramolecule par excellence . . . ’.30 The supramolecular chemist, like the crystal engineer, uses a range noncovalent intermolecular interactions, including hydrogen bonding and coordination bonds. Use of the later gives rise to metallosupramolecular chemistry, and much of the design and indeed the structures obtained have close relationships to coordination polymers. For example, the design and chemistry

Introduction

5

(a)

(b)

Figure 1.4

(a) The hydrogen-bonded sheets formed by trimesic acid24 and (b) a molecular tecton which assembles via hydrogen bonding into the diamond net.25

6

Chapter 1 H

O

O

H

O

O

H

O

O

O

O

H

O

O

H

O

O

Tectons

Ag

N

Figure 1.5

N

Ag N

H

Synthons

N

Ag N

N

Ag

Representative synthons and tectons for both organic hydrogen-bonded nets (top) and coordination polymers (bottom).

may be similar, except that the use of a convergent ligand building block will give a metallosupramolecule whereas a divergent one will generate a polymer (Figure 1.6a). Alternatively, even the same bridging ligand can be used, with construction of a metallosupramolecule being directed by the use of ‘capping’ chelating co-ligands on the metals [such as 2,2 0 -bipyridine, ethylenediamine (en), 1,10-phenanthroline, 1,4,7-triazacyclononane (TACN), cyclopentadiene]. In the absence of these capping groups, polymers are formed (Figure 1.6b). Despite the different products, both areas have the same modular approach to the design and synthesis and similar (or even the same) building blocks. Even the architectures achieved in the two fields can be similar. The supramolecule shown in Figure 1.7a, constructed from 4,4 0 -bipyridine (4,4 0 -bipy) and (en)PdII,31 has the same structure as the windows in the 2D coordination polymer obtained from reaction of the same ligand with ZnSiF6 (Figure 1.7b).32 The molecular cube in Figure 1.7c has the same connectivity as the cavities in Prussian Blue.33 Reaction of 2,4,6-tri(4-pyridyl)-1,3,5-triazine (tpt) with (en)PdII gives the discrete cages shown in Figure 1.7d;34 a similar reaction with CuI generates a 3D polymer which contains the same cages (Figure 1.7e).35 In the later structure there are no capping groups on the metal to terminate the structure and thus a polymer is generated in which the cages are connected by shared metal atoms.

1.3 What is a Coordination Polymer? A coordination polymer contains metal ions linked by coordinated ligands into an infinite array. This infinite net must be defined by coordination bonds and thus molecular species linked only by hydrogen bonding, such as the example shown in Figure 1.8,36 are elegant instances of molecular crystal engineering but are not coordination polymers. Similarly, a structure linked by coordination bonds in one direction and hydrogen bonds it two other directions is a 1D coordination polymer (although an overall 3D net may be defined by both sets of interactions).

7

Introduction

(a)

M Infinite 3D Polymer Bridging Ligand ML LM

ML ML

LM ML

LM LM (b)

Figure 1.6

ML

Discrete Supramolecular Cube

Direction of the formation of metallosupramolecules versus coordination polymers through use of (a) convergent versus divergent ligands and (b) use of terminal capping ligands.

We also exclude here more ‘inorganic’ materials, such as halides, oxides, hydroxides, alkoxides, sulfides and polyoxides (sulfates, phosphates, etc.), although we do include pseudohalides such as cyanide, azide and thiocyanate. Furthermore, for the purposes of this book, we largely ignore alkali and alkaline earth metals, which have more ionic bonding, and main group metals, in which the bonding is more covalent, and thus focus largely on the transition and lanthanoid ions. There are good reasons for doing this. One feature of the design of coordination polymers is that the strength and lability of the coordination bond are such that ordered materials can be readily synthesised because of the reversibility of these interactions. Unlike covalently bonded organic polymers, in which the

8

Figure 1.7

Chapter 1

(a) A molecular square formed from 4,4 0 -bipy and capping en ligands;30 (b) a 2D sheet formed by 4,4 0 -bipy;31 (c) a molecular metal cyanide cube containing capping TACN ligands;32 (d) a cage complex constructed with tpt and capping en ligands;33 (e) a coordination polymer containing analogous cages to part (d) linked through shared metal atoms.34

Introduction

Figure 1.8

9

A hydrogen-bonded network formed by a coordination complex, and not a coordination polymer.35

bonds are largely irreversible, errors in the assembly of a coordination polymer can be readily corrected during growth so that a periodic 3D structure with crystallographic order can be achieved. By contrast, in an organic polymer mistakes are ‘locked in’ once made, resulting in a material with much less periodic ordering. This ordering of coordination polymers allows detailed structural determination through X-ray crystallography and, through this, precise structure–property correlations. It is also important to the properties themselves, e.g. a regularity and consistency of pore size and environment that cannot be achieved with amorphous materials. On the other hand, the coordination bond is also strong enough to provide robust materials and good electronic and magnetic communication between metal centres. It is also directional, with generally predictable geometries around the metal centre (particularly for transition metals), allowing design to be attempted with some degree of confidence. A coordination polymer thus consists essentially of metal and ligands, although they often include guests and counterions. The metal ions, as discussed above, are usually transition metals and/or lanthanoids. For transition metals, the field is dominated by the first-row elements (plus Zn, Cd, Hg, Ag and, to a lesser extent, Au, Pd, Pt), due to their kinetic lability and ready availability and stability. Generally, transition metals have been more popular, due in part to the more predictable nature of their coordination geometries; however, lanthanoids have attracted increased attention recently, with their higher connectivity leading to interesting topologies, in addition to other inherent properties of

10

Chapter 1 N

R

R

N

N

O R

C

N

R O

N

N

Figure 1.9

N

N

Features of typical ligands used for coordination polymer construction, including (top) typical donor groups and (bottom) rigid versus flexible ligand choice.

interest (e.g. luminescence). These metals are commonly used as their halide, nitrate, perchlorate, tetrafluoroborate, hexafluorophosphate, hexafluorosilicate or triflate salts; ‘non-coordinating’ anions are usually preferred. Although the nature of metal salt chosen is important, the real variation in coordination polymers comes through the infinite variability and creativity of ligand design. Nonetheless, there are a number of features that are common to most ligands used (Figure 1.9). Typically, a ligand might have two divergent coordination sites (although there are many examples of higher connectivity), and these coordination sites are usually pyridyl, imidazole, nitrile or carboxylate functional groups (discounting the large body of pseudohalide work). These ligands can also range from the very rigid to the completely flexible, with the corresponding loss in predictability.

1.4 Synthetic Techniques One of the challenges of this research is to obtain single crystals suitable for detailed crystallographic analysis. Unlike molecular species, most coordination polymers are insoluble once synthesised (a property which is advantageous for other aspects) and so recrystallisation is not an option. If the polymers can be dissolved, it is usually through the use of strongly coordinating solvents, which are then likely to become part of the recrystallised species, which therefore becomes a different material to the original phase.

11

Introduction

Crystals are therefore usually obtained directly from the synthetic reaction mixtures. Although some species crystallise nicely from directly mixed solutions, for other systems the key to obtaining good crystals is to slow the precipitation down. This is most commonly done by allowing two separate solutions of metals and ligands to diffuse slowly into each other, and a number of different techniques have been established to this end (Figure 1.10). The simplest method is to layer carefully one solution on top of another in a small vial or tube. Often a buffer layer of pure solvent is layered between the two and the use of solvents with different densities (e.g. MeOH versus CHCl3) greatly aids separation. This layered solution should then be left so that the crystals can grow; typically this may take in the order of 2 weeks, although crystallisation can often take much longer (or shorter) times and so the reaction should be checked regularly, preferably without disturbing the crystal growth through handling. Regular inspection is important as crystals can come and go (for kinetic products) or become flawed, overgrown or otherwise deteriorate in quality over time. Other variations on this technique include locking one solution into a gel through the addition of a gelling agent such as tetramethoxysilane. The gel slows diffusion through reduction of convection and also provides a support for the growing crystals. Specially designed glassware such as H-tubes and U-tubes

Solution B

Solution B Buffer Layer

Gel Containing Solution A

Solution A

Solution A

H tube

Solution B

Solution B

Solution A U tube

Figure 1.10

Various methods for slow growth of coordination polymer crystals.

12

Chapter 1

(Figure 1.10) can also be used; often these can have a frit in the middle or (in the case of U-tubes) a separating gel plug can be created at the bottom first. As detailed in Chapter 4, there are a number of factors that contribute to stable crystalline packing arrangements. For the synthetic chemist, this means that there are therefore a number of other variables that can be adjusted to produce crystals. Variation of solvent, counterion or even metal choice can be explored, as can synthetic tweaks to the ligands. More recently, the use of solvothermal techniques has become increasingly popular, both as a method of obtaining good single crystals and as a means of obtaining phases which are unavailable through bench-top techniques. There is, overall, a large parameter space which can be explored in the quest for single crystals. However, one of the key reasons for obtaining crystal structures is to draw relationships between structures and properties and thus gain insights that can feed into the design of new materials. Therefore, it is important to recognise that the structures obtained from single crystals may be inherently unrepresentative (because the crystallographer chooses the best crystal available, for obvious reasons) of the bulk material upon which the properties are tested. Furthermore, reactions can often give more than one product. Hence it is important to check the correlation between the single crystals and the bulk product, and this is most easily achieved through the use of techniques such as powder diffraction or (less convincingly) infrared or Raman spectroscopy. Finally, although we largely focus in this book on materials characterised through single-crystal crystallography, the structures of some simple coordination polymers have been determined directly through powder diffraction.37 Powder diffraction can also be used to correlated known structures and new microcrystalline materials. These latter materials may be analogous to known structures but lacking in single crystals (e.g. the same structure but different metals) or synthesised using unusual techniques, such as solid-state decomposition or mechanochemical techniques.38 Furthermore, non-crystalline materials can also have very interesting properties even if the detailed structure is unknown. The compound V(tcne)2 0.5CH2Cl2, for example, has shown magnetic ordering above room temperature.39 This material has unfortunately only ever been obtained as an amorphous powder, so its structure, which is no doubt key to its magnetism, remains unknown. But it is still a magnet.

1.5 Design, Analysis, Application The rest of this book broadly follows the themes of design, analysis and application. Chapter 2 deals with design (nets), Chapter 3 deals with some of the consequences of nets (interpenetration) and Chapter 4 examines in detail the many other aspects that should be taken into account when designing or examining a structure. In the following four chapters, we provide an extensive analysis of reported coordination polymers and related areas such as organometallic networks and inorganic–organic hybrid materials. Finally, we look at the application of these materials to a number of fields, including magnetism

Introduction

13

(long-range ordering, spin crossover), porosity (gas storage, ion and guest exchange), non-linear optical activity, chiral networks, reactive networks, heterogeneous catalysis, luminescence, multifunctional materials and other assorted properties. The examples given throughout have been chosen based on their ability to illustrate a point, their historical significance or the fact that they are either typical or atypical (even exceptional) of a larger class of materials. In many cases, the choice is purely subjective and readers are directed to a long list of literature reviews available as additional complementary resources.15,40–96

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15

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17

59. U. Mueller, M. Schubert, F. Teich, H. Puetter, K. Schierle-Arndt and J. Pastre´, J. Mater. Chem., 2006, 16, 626. 60. R. Horikoshi and T. Mochida, Coord. Chem. Rev., 2006, 250, 2595. 61. A.Y. Robin and K.M. Fromm, Coord. Chem. Rev., 2006, 250, 2127. 62. (a) G.S. Papaefstathiou and L.R. MacGillivray, Coord. Chem. Rev., 2003, 246, 169; (b) A.N. Sokolov and L.R. MacGillivray, Cryst. Growth Des., 2006, 6, 2615. 63. A. Erxleben, Coord. Chem. Rev., 2003, 246, 203. 64. S. Decurtins, R. Pellaux, G. Antorrena and F. Palacio, Coord. Chem. Rev., 1999, 190–192, 841. 65. J.Y. Lu, Coord. Chem. Rev., 2003, 246, 327. 66. S.-L. Zheng, M.-L. Tong and X.-M. Chen, Coord. Chem. Rev., 2003 246, 185. 67. J.A.R. Navarro, E. Barea, M.A. Galindo, J.M. Salas, M.A. Romero, M. Quiro´s, N. Masciochi, S. Galli, A. Sironi and B. Lippert, J. Solid State Chem., 2005, 178, 2436. 68. (a) G.K.H. Shimizu, J. Solid State Chem., 2005, 178, 2519; (b) A.P. Coˆte´ and G.K.H. Shimizu, Coord. Chem. Rev., 2003, 245, 49. 69. (a) M. Kawano and M. Fujita, Coord. Chem. Rev., 2007, 251, 2592; (b) K. Biradha and M. Fujita, in Crystal Design: Structure and Function. Perspectives in Supramolecular Chemistry, Vol. 7, ed. G.R. Desiraju, Wiley, Chichester, 2003, Chapter 5. 70. K. Maeda, Micropor. Mesopor. Mater., 2004, 73, 47. 71. K. Pavani, A. Ramanan and M.S. Whittingham, J. Mol. Struct., 2006, 796, 179. 72. M. Hong, Cryst. Growth Des., 2007, 7, 10. 73. T.C.W. Mak and L. Zhao, Chem. Asian J., 2007, 2, 456. 74. A.K. Cheetham, C.N.R. Rao and R.K. Feller, Chem. Commun., 2006, 4780. 75. C.-L. Chen, B.-S. Kang and C.-Y. Su, Aust. J. Chem., 2006, 59, 3. 76. M.P. Suh and Y.E. Cheon, Aust. J. Chem., 2006, 59, 605. 77. C.N.R. Rao, S. Natarajan and R. Vaidhyanathan, Angew. Chem. Int. Ed., 2004, 43, 1466. 78. D.J. Collins and H.-C. Zhou, J. Mater. Chem., 2007, 17, 3154. 79. J. Ribas, A. Escuer, M. Monfort, R. Vincente, R. Corte´s, L. Lezama and T. Rojo, Coord. Chem. Rev., 1999, 193–195, 1027. 80. M. Verdaguer, A. Bleuzen, V. Marvaud, J. Vaissermann, M. Seuleiman, C. Desplanches, A. Scuiller, C. Train, R. Garde, G. Gelly, C. Lomenech, I. Rosenman, P. Veillet, C. Cartier and F. Villain, Coord. Chem. Rev., 1999, 190–192, 1023. 81. R. Lescoue¨zec, L.M. Toma, J. Vaissermann, M. Verdaguer, F.S. Delgado, C. Ruiz-Pe´rez, F. Lloret and M. Julve, Coord. Chem. Rev., 2005, 249, 2691. 82. R. Custelcean and B.A. Moyer, Eur. J. Inorg. Chem., 2007, 1321. 83. (a) J.A. Real, A.B. Gaspar, V. Niel and M.C. Munõz, Coord. Chem. Rev., 2003, 236, 121; (b) J. A. Real, in Transition Metals in Supramolecular

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Chemistry. Perspectives in Supramolecular Chemistry, Vol. 5, ed. J.-P. Sauvage, Wiley, Chichester, 1999, Chapter 2. (a) W. Mori, S. Takamizawa, C.N. Kato, T. Ohmura and T. Sato, Micropor. Mesopor. Mater., 2004, 73, 31; (b) W. Mori, T. Sato, T. Ohmura, C.N. Kato and T. Takei, J. Solid State Chem., 2005, 178, 2555. M. Ohba and H. Oˆkawa, Coord. Chem. Rev., 2000, 198, 313. (a) M.J. Hardie, R. Ahmad and C.J. Sumby, New J. Chem., 2005, 29, 1231; (b) M.J. Hardie, J. Chem. Crystallogr., 2007, 37, 69. C.M. Drain, I. Goldberg, I. Sylvain and A. Falber, Top. Curr. Chem., 2005, 245, 55. J. Larionova, S. Willemin, B. Donnadieu, B. Henner, C. Gue´rin, B. Gillon and A. Goujon, J. Phys. Chem. Solids, 2004, 65, 677. H. Zhou and A. Lachgar, Eur. J. Inorg. Chem., 2007, 1053. O.A. Efremova, Y.V. Mironov and V.E. Fedorov, Eur. J. Inorg. Chem., 2006, 2533. O. Kahn and C.J. Martinez, Science, 1998, 279, 44. L. O¨hrstro¨m and K. Larsson, Molecule-based Materials, the Structural Network Approach, Elsevier, Amsterdam, 2005. E.R.T. Tiekink and J.J. Vittal, Frontiers in Crystal Engineering, Wiley, Chichester, 2006. D. Braga and F. Grepioni, Making Crystals by Design, Wiley-VCH, Weinheim, 2007. R. W. Saalfrank and B. Demleitner, in Transition Metals in Supramolecular Chemistry. Perspectives in Supramolecular Chemistry, Vol. 5, ed. J.-P. Sauvage, Wiley, Chichester, 1999, Chapter 1. K. Kim, in Transition Metals in Supramolecular Chemistry. Perspectives in Supramolecular Chemistry, Vol. 5, ed. J.-P. Sauvage, Wiley, Chichester, 1999, Chapter 8.

CHAPTER 2

Nets: A Tool for Description and Design 2.1 Introduction One of the most powerful techniques in crystal engineering for both the analysis and design of solids is to reduce their crystal structures to networks (or nets). Networks can aid the description and understanding of complicated structures or provide a blueprint for the targeting of particular packing arrangements and their associated properties. An early leading figure in this approach was A. F. Wells, who, in a series of seminal books,1–3 described a number of molecular and polymeric structures in terms of networks and delineated a large number of possible networks, some already seen in real structures and others that, remarkably, were still theoretical at the time. It was not until a landmark paper in 1989, however, that the net approach was applied to the design of new coordination polymer structures.4 In this and a series of later papers,5–8 Robson, Hoskins and co-workers described a new approach for the crystal engineering of coordination polymers. In this approach, structures with particular topologies could be targeted through the use of metals and ligands with the appropriate coordination geometries. For example, a diamondlike network is the likely outcome of using tetrahedral metal ions (CuI, ZnII, CdII) and linear bridging ligands (cyanide) or both tetrahedral metal ions (CuI) and tetrahedral ligands (4,4 0 ,400 ,4000 -tetracyanotetraphenylmethane) (Figure 2.1). In the first case, the tetrahedral metal ions mimic the role of the tetrahedral carbon atoms in diamond and the linear cyanide bridges replace the C–C bonds. In the second case, the carbon nodes are replaced by both metal- and ligandbased tetrahedral nodes. Significantly, the increased distance between the nodes from 1.54 A˚ for diamond to 5.46 A˚ for Cd(CN)2 or 8.86 A˚ for CuI[C(C6H4CN)4]BF4 xC6H5NO2 leads to large increases in the sizes of the channels and Coordination Polymers: Design, Analysis and Application By Stuart R. Batten, Suzanne M. Neville and David R. Turner r Stuart R. Batten, Suzanne M. Neville and David R. Turner, 2009 Published by the Royal Society of Chemistry, www.rsc.org

19

20

Figure 2.1

Chapter 2

An adamantane cavity in the diamond-like structure of CuI[C(C6H4CN)4] BF4 xC6H5NO2.4

cavities in the structures. So much so, in fact, that there are two interpenetrating networks in Cd(CN)2 (Chapter 3), whereas in CuI[C(C6H4CN)4]BF4 xC6H5NO2 the ordered framework occupies only one-third of the crystal volume, with the rest of the volume occupied by essentially a liquid solution of counterions. Thus it was reasoned that this net-based approach could lead to fascinating new materials with applications such as high porosity (Chapter 10) and heterogeneous catalysis (Chapter 12), applications which have since been demonstrated repeatedly. Since this initial work, various jargon has developed. The net-based approach has been called reticular chemistry,9 and to borrow from network structures defined by weaker interactions such as hydrogen bonding, the intermolecular interactions used to create the network can be called supramolecular synthons,10 and the building blocks used to create the network (metals and ligands) are tectons.11 Nonetheless, the core concepts remain the same.

2.2 Defining Networks A good understanding of networks is therefore vital to the crystal engineer. But what is a net? For our purposes, a network is a polymeric collection of interlinked nodes; each link connects two nodes and each node is linked to three or more other nodes. A node cannot be connected to only two nodes; in this case it then becomes a link. Similarly, a link can only connect two nodes; if it connects more than two it is a node. And finally, since we are talking about crystal

Nets: A Tool for Description and Design

Figure 2.2

21

Two geometrically different but topologically identical nets.

structures here, the network must also have a repeating pattern and thus a finite number of unique nodes and links. A network is also a topological description and not a geometric one. For example, the two networks shown in Figure 2.2 are topologically identical despite the fact that they are geometrically very different. In both networks, the nodes are 3-connecting, although in one net the nodes are trigonal (leading to a hexagonal network) and in the other they are T-shaped (leading to a brickwork-like network). These networks are identical because one can be converted to the other by distortions that do not break links. By this reasoning, there is no topological difference between square-planar nodes and tetrahedral nodes – both are simply 4-connecting nodes. However, different nets are favoured by (and may even require) different node geometries. For example, (4,4) sheets are favoured by square-planar nodes, whereas the diamond net has, in its undistorted form, tetrahedral nodes. The PtS net has two different sorts of 4-connecting nodes; half are tetrahedral and half are squareplanar. Hence, although geometries are not strictly a topological feature, they can still be an important factor in network selection and design, particularly for the chemist who can provide a great deal of control over different nodal geometries. In practice, there is a considerable difference between square-planar and tetrahedral nodes and therefore we will often make distinctions between different nodal geometries here. Nonetheless, it is the connectivity of a network that defines it, not its geometry. Structures can be described as having a particular network topology even though they may be geometrically very different to the ‘ideal’ net. For example, the structure of Cu(CN)(4-cyanopyridine) contains three interpenetrating

22

Chapter 2

networks which have the diamond topology, even though half the links between the tetrahedral 4-connecting nodes (Cu atoms) are almost twice the length (9.54 A˚) of the other links (5.04 A˚).12 The connectivity of the node can also be very different to the local chemical geometry. This can manifest itself in a number of ways. Octahedral metals can act as 3-connecting nodes if they are bound by three chelating bridges, three monodentate bridges and three terminal (i.e. non-bridging) ligands or three pairs of monodentate bridging ligands which connect metals in pairs (Figure 2.3). It is common for square-planar nodes to be formed by octahedral metals, particularly when pyridyl donor ligands are used (it is sterically very difficult to fit six pyridyl donor groups around a first-row transition metal; more commonly four pyridyls occupy equatorial positions, while the axial positions are filled by sterically smaller terminal ligands, such as water or halide ions). Ligands with four coordination sites become simple links if they coordinate to only two metals or 3-connecting nodes if they only coordinate to three metals. Nodal geometries can also be very different to local chemical geometries. For example, tetrahedral metals can act as square-planar nodes if they are linked by bent ligands, such as in the structure of Zn(dca)2, where dca ¼ dicyanamide, 13 The reverse can also apply – non-linear linkers can N(CN) 2 (Figure 2.4). convert local square-planar geometries into tetrahedral nodal geometries.14 The next challenge for the net-based approach to coordination polymers (and crystal engineering in general) is to decide how the network is defined. That is, what are the nodes and links and which interactions are important and which are not? This sounds trivial, and often it is, but on other occasions it can be difficult and even subjective.

M L L

L M L

M L

L M

S S M

L M

L

L S

Figure 2.3

Three ways in which an octahedral metal ion can act as a 3-connecting node.


Figure 2.4

23

Sheet structure of Zn(dca)2 containing tetrahedral metals acting as squareplanar nodes.13

For example, metal–ligand cluster-like motifs are commonly observed in coordination polymers and it often simplifies the network description to regard these moieties [sometimes known as secondary building units (SBUs),15 depending on the nature of the species in question] as nodes rather than the individual metal atoms and ligands. For example, in the structure of Zn(BDC)(H2O) DMF, where BDC ¼ benzene-1,4-dicarboxylate,16 metal atoms are bridged into pairs by four carboxylate groups and these pairs are interconnected through the aromatic backbones of the BDC ligands to generate a sheet structure (Figure 2.5). Hence a simpler description of this structure is provided by defining the Zn2(O2C–)4 moieties as 4-connecting SBUs and the aromatic rings as simple linkers, rather than describing the network in terms of two different 4-connecting nodes – Zn atoms and BDC ligands. In the first case, the common and easily recognised (4,4) net is generated, whereas in the alternative description a more complicated and unusual binodal 4-connected network is obtained. Preformed clusters may also be used in the construction of coordination polymers. For example, the coordination polymer [Cu2(OAc)4]3(tpt)2 2MeOH, where tpt ¼ 2,4,6-(4-pyridyl)-1,3,5-triazine, was obtained from the reaction of Cu2(OAc)4(H2O)2 with tpt; the tpt acts as a 3-connecting node whereas the copper acetate dimer simply links the tpt nodes together (Figure 2.6).17 In the structure of Fe4[Re6Se8(CN)6]3 36H2O, both Re6Se8 metal clusters and individual Fe ions act as 6-connecting nodes to give an expanded Prussian Bluetype structure.18 Caution must be exercised, however, when choosing assemblies larger than just the metals and the ligands to be nodes. The composition of the chosen nodes should be clearly defined, as should the nature of the links between them. The nodes should also be chemically sensible subunits, such as well-defined clusters that have known molecular analogues, and the nodes and links should also describe the entire connectivity of the underlying structure. It is also important to remember that both metals and ligands can act as either nodes or links in a net (or even both if there are different types of

24

Figure 2.5

Chapter 2

A sheet structure containing Zn2(O2C–)4 dimers acting as 4-connecting SBUs.16

metals or ligands in the one structure). For example, in the 3D rutile (TiO2) net (see below), each Ti node is connected to 10 other Ti atoms via the oxygen atoms bonded to it. This is not, however, a 10-coordinate network, because the oxygen atoms connect to three Ti atoms each and thus are themselves 3-connecting nodes. Rutile is therefore a binodal 3,6-connected network. The same applies to more complicated coordination polymers – if a metal or ligand connects to three or more metals, then it is a node and cannot be simply classed as a link. Finally, care should be taken when choosing the interactions used to define the network. Usually this is relatively straightforward if only coordination bonds are to be used, although longer interactions can be easy to miss (e.g. Jahn–Teller distorted metals) or hard to separate from non-bonding interactions (e.g. some Ln structures). However, it is sometimes useful also to examine weaker interactions, such as metallophilic interactions, hydrogen bonds, p stacking interactions and so forth (Chapter 4). This can aid the understanding of the overall structure, although it should be remembered that, by definition, every crystal structure is 3D. However, if the network definition is to be extended to these interactions, then all interactions of similar strength should be used, e.g. all strong hydrogen bonds should be included rather than just selected interactions. Care should also be taken when representing the network graphically. The distances between some nodes which are not directly linked may be shorter than


Figure 2.6

25

The structure of [Cu2(OAc)4]3(tpt)2 2MeOH containing Cu2(OAc)4 clusters acting as bridges between the tpt nodes.17

or even exactly the same as the distances between nodes that truly are linked. This can often cause problems with many graphics programs and therefore the net should always be overlaid on the real structure to check that no links have been missed and that no ‘false’ links have been inadvertently created. Ultimately, the true test of the network approach is (a) does it increase the understanding of the structure and (b) does it correspond well to the chemical structure? If both of these criteria are not met then either the net is badly defined or this approach should not be used (some structures can be too complicated to reduce to a simple, easily understood net). A net should always simplify a structural description, not complicate it.

2.3 Identifying and Naming Nets Once the net has been defined, it needs to be identified and ascribed a name. There are a number of properties of a network that can aid this, particularly if the network has an unusual topology or is highly distorted (remember that the network can be very distorted from the ‘ideal’ net and still have the same topology). The most obvious information available is the number of different nodes and their connectivities. However, one should be aware that the number of topologically

26

Chapter 2

unique nodes in the underlying net can be much smaller than the number of crystallographically unique nodes in the actual structure; the crystal structure is a result of the connectivity, the geometry, and the shape and position of non-network elements (e.g. counterions, guests), whereas the underlying network is not. There are also a number of other mathematical properties that may be defined in a net. One of these is to determine the shortest circuits around each unique node. The shortest circuit is the number of nodes in the smallest loop in the net that can be constructed containing two given links from a given node. If all the possible unique pairs of links give the same sized circuits, then this is what Wells termed a uniform net.1,2 Furthermore, if all nodes have the same connectivity, then according to Wells it is a platonic uniform net and can be represented by the symbol (n,p), where n is the size of the shortest circuit and p is the connectivity of the nodes. An example of such a net is the 2D (6,3) net shown in Figure 2.7a – the nodes are 3-connecting and for each of the three unique pairs of links around any given node the shortest circuit is six-membered. Not all nets, however, conform to these conditions. The net shown in Figure 2.7b, for example, contains only one type of node but the three different pairs of links around the 3-connecting node form different sized rings. This is an important point: not all shortest circuits are the same size – each is only the shortest circuit for the associated pair of links. In this net, one pair is part of a four-membered ring whereas the other two pairs are parts of eight-membered rings. To describe this net we can use a Schla¨fli symbol. The short version of this simply lists the different sized rings; where more than one ring has the same size a superscript is used to indicate the number. Hence this net has the Schla¨fli symbol 4.82. Note that the connectivity of the nodes is not shown explicitly but can be derived simply by adding the superscripts (no superscript ¼ 1). This will give the number of shortest circuits, which for a p-connected node is p(p 1)/2. Hence for the 4.82 net, there are 1 + 2 ¼ 3 shortest circuits and therefore the node must be 3-connected [3(3 1)/2 ¼ 3]. If the node was 4-connecting there should be (4 3)/2 ¼ 6 shortest circuits and therefore the superscripts in the Schla¨fli symbol should add up to 6. The simple square grid net shown in Figure 2.7c illustrates this nicely – its Schla¨fli symbol is 44.62. Note that Wells labelled this net (4,4), choosing arbitrarily to ignore the six-membered circuits formed by the trans linkages, and this strictly incorrect notation is now used widely (as it will be here). The same situation applies to the net often referred to as the (3,6) net (Figure 2.7d). This net should not to be confused with the (6,3) net in Figure 2.7a or with a 3,6-connected net (i.e. a net with both 3-connecting and 6-connecting nodes), such as the one shown in Figure 2.7e. Finally, the Schla¨fli symbol can also be useful for nets with more than one type of node. The net in Figure 2.7f contains both 3-connecting and 4-connecting nodes, in a 4:1 ratio. Closer examination of this 3,4-connected binodal net shows that the Schla¨fli symbol is (4.62)4(64.102). Note that the connectivity of each node is listed, in parentheses, separately and that the subscript is used to indicate the relative ratio of the different nodes.

27


(a)

(c)

(e)

Figure 2.7

(b)

(d)

(f)

A selection of 2D nets: (a) (6,3); (b) 4.82; (c) (4,4); (d) (3,6); (e) (43)2(46.66.83); (f) (4.62)4(64.102).

A longer version of the Schla¨fli symbol is sometimes used (which is also referred to as the vertex symbol), which takes into account the number of shortest circuits for each pair of links and discounts circuits for which there is a ‘short cut’ back to the starting node.19 An entry into the mathematics of this and many other aspects of nets can be gained from the studies of O’Keeffe and co-workers.9,20–26

28

Chapter 2

Armed now with a graphical representation of the net, and also some mathematical information related to the net, the next step is to identify the network. There are a number of resources for doing this. First are the books by Wells,1–3 and later ones by O’Keeffe and Hyde26 and by O¨hrstro¨m and Larsson.27 There are also a number of recent reviews which illustrate a number of common nets.8,25,28,29 Finally, the searchable Reticular Chemistry Structure Resource (RCSR) website30,31 is a valuable source of network information (more than 1500 3D nets are contained in this database), including coordinates, cell parameters and space groups for each net, further mathematical information such as vertex symbols, genus and td10 values and diagrams of many of the nets. Recently, a number of computer programs have also been developed to aid in the identification of the nets and also to calculate the various mathematical descriptors of the net. These include Systre,32 OLEX33 and TOPOS.34 An unfortunate feature of this field is that there are a number of ways of naming the same net. As we have already seen, Wells used an (n,p) notation for a number of uniform nets that he identified. This notation, however, is not unique and therefore he added suffixes to distinguish between nets with the same symbol, e.g. (10,3)-a, (10,3)-b, (10,3)-c. The short Schla¨fli symbols can also be used to name the networks (e.g. the 4.82 net discussed above); however, these symbols are again not necessarily unique to a particular net. A way of naming nets that is unique is to name a simple, well-known structure that has the same topology – common examples include diamond, lonsdaleite, rutile, PtS, a-Po, SrAl2, sodalite, NbO, CdSO4 and ThSi2. Not all nets, however, have a simple, known prototype and a feature of the aforementioned RCSR database is the use of unique three-letter codes for each net (although in some cases the code is extended further),20 analogous to those used for zeolites. Thus diamond is given the dia symbol, whereas the (10,3)-b net, also known as the ThSi2 net and which has the Schla¨fli symbol 103, is assigned ths. This last example illustrates the fact that the same net can have many different names, all of which may be in common use; unfortunately, the crystal engineer needs to be familiar with them all. For the remainder of this chapter, we will examine a selection of the commonly encountered and fundamentally important networks. Many other networks have been reported and more continue to be reported, so we will necessarily focus on only a representative survey of the more basic and/or noteworthy nets and illustrate with chemical examples where appropriate.

2.4 1D Nets A number of 1D nets are shown in Figure 2.8. The simple linear chain shown in Figure 2.8a has a zigzag conformation, although it could be shown with a true linear geometry or a helical geometry and still be topologically the same. Figure 2.8b shows a net with loops rather than simple links connecting the nodes, whereas the net in Figure 2.8c contains alternating loops and links. In these and more complicated nets, the decision on whether to regard two links

29


(a)

(b)

(c)

(d)

(e)

Figure 2.8

A selection of 1D nets.

(or even more) connecting the same two nodes as either a loop or a single link can be rather subjective; if the double bridges in Figure 2.8b and c are treated as single links, then Figure 2.8a is generated. Particularly for higher dimension nets, this often leads to simpler network descriptions and can correspond to the actual chemical structure better. On other occasions, such as in systems where interpenetration occurs through the loops (Chapter 3),35 it can be better not to simplify the loops to simple links. By contrast, the ladder-like nets shown in Figure 2.8d and e cannot be reduced further to the linear chain. The two-legged ladder motif in Figure 2.8d is relatively common, however the three-legged ladder motif of Figure 2.8e is rather unusual, although not unknown.36 1D ladder motifs with more than three legs are also possible,37 as are numerous other more exotic topologies. If the multi-legged ladder nets are 1D nets which have some 2D aspect, then 1D nets with a 3D aspect would include a number of tube-like polymers which have been reported. The structure of a 1D tube formed by silver and 1,3,5triaminocyclohexane is shown in Figure 2.9a;38 both the ligand and the metal act as 3-connecting nodes. However, although the structure has a definite tubular geometry, the topology is in fact that of a ladder, as the side-on view of the structure clearly shows (Figure 2.9b); the tubular nature arises from distortions away from the ideal net. A more topologically definitive tubular net can be seen in the structure of M(dca)2(apym), where M ¼ Co, Ni and apym ¼ 2-aminopyrimidine.39 The topological net is shown in Figure 2.10. It consists of 3-connecting dca nodes and 5-connecting metal nodes (there are two sorts of dca ligands in the structure; the direct metal–metal links represent 2-connecting dca bridges). Interestingly, although not apparent in the network diagram in Figure 2.10, the tube has the same structure as the channels in the parent a-M(dca)2 compounds, which have the rutile topology (section 2.6.6).

2.5 2D Nets A number of planar 2D nets have already been shown in Figure 2.7, including the most commonly encountered (6,3) and (4,4) nets. Another noteworthy 2D

30

Figure 2.9

Figure 2.10

Chapter 2

A 1D tube network constructed from AgI and 1,3,5-triaminocyclohexane, viewed (a) end-on and (b) side-on.38

Schematic representation of the 1D tubes in M(dca)2(apym).39

topology is the Kagome´ lattice (Figure 2.11). This 4-connected net is of particular interest as it should lead to spin frustration for antiferromagnetically coupled spin sites.40 This is because of the three-membered rings – if the spin at once vertex of this ring is ‘up’ and at another vertex it is ‘down’ (i.e. antiferromagnetically coupled), then the third vertex does not know whether to be


Figure 2.11

31

The 2D Kagome´ lattice.

up or down to maintain the antiferromagnetic coupling. Obtaining a true Kagome´ lattice with coordination polymers has proved difficult to date, with only a few examples.41–43 One particularly significant compound, however, does show evidence of the expected spin-frustrated magnetic state and its magnetic properties are markedly different to those of a second phase obtained which is chemically identical but topologically different.41 One of the more unusual 2D nets reported is the trinodal 3,4-connected net formed in the structures of M(dca)2(H2O) phenazine, where M¼Co, Ni (Figure 2.12).44 Again, as for the M(dca)2(apym) tubes discussed above, the M(dca)2 sheets in this structure are identical to layers that can be identified in the parent rutile-like a-M(dca)2 compounds. Both the tubes and the sheets could be assembled, in a theoretical sense, to form the rutile network by converting the 2-connecting dca ligands (which provide the direct metal– metal links in the network diagrams shown here) into 3-connecting nodes by joining them to metal nodes in adjacent tubes or sheets (and vice versa). Hence although these three different dca compounds have different nets of different dimensions, they are all topologically (and also chemically) interrelated. An interesting aspect of this net is the occurrence of five-membered rings. At least two other 2D nets have been reported with five-membered rings. The structure of [Cu(4,4 0 -bipyridine)1.5(PPh3)]BF4 1.33THF 0.33CHCl3 contains 3-connecting Cu nodes and a sheet structure composed of five- and eightmembered rings (Figure 2.13).45 The topology is analogous to the sheet silicate mineral nekoite. The structure of [{Cu2(O2CEt)4}5(HMTA)3], where HMTA ¼ hexamethylenetetramine, is even more noteworthy in that the sheet is entirely tessellated by five-membered rings (Figure 2.14).46 Although a surface cannot be

32

Chapter 2

Figure 2.12

Schematic representation of the 3,4-connected sheet structure in M(dca)2(H2O) phenazine.44

Figure 2.13

Schematic representation of the sheet structure bipyridine)1.5(PPh3)]BF4 1.33THF 0.33CHCl3.45

in [Cu(4,4 0 -

covered completely by regular pentagons (unlike triangles, squares or hexagons), in this structure the rings are neither regular nor planar. The net is also binodal, containing both 3- and 4-connecting nodes. When discussing 1D nets, we mentioned that they can have a certain 2D (e.g. multi-legged ladders) or 3D character (e.g. tubes). The same applies to


Figure 2.14

33

The 2D sheets formed in [{Cu2(O2CEt)4}5(HMTA)3].46

2D nets – they can have a certain 3D aspect. By this we mean that even though they are polymeric in only two dimensions, they nonetheless have a certain ‘thickness’ that cannot be mapped upon a true 2D surface. Usually these nets are described best in terms of interconnected layers and many of them are interpenetrating (Chapter 3). For example, the 2D net in Ag(bpea)L1/2 H2O, where bpea ¼ 1,2-bis(4-pyridyl)ethane and L ¼ 4,4 0 -biphenylcarboxylate, contains layers of parallel rods;47 pairs of adjacent parallel layers are orientated in different directions and interconnected by further rods, to give a 2D net containing T-shaped 3-connecting nodes (Figure 2.15). Each bilayer interpenetrates with two other adjacent layers. An alternative method for generating a 2D bilayer is to crosslink pairs of 2D sheets rather than layers of rods. In Cd(isonicotinate)2(bpea)1/2(H2O), pairs of (4,4) sheets are crosslinked as shown in Figure 2.16a;48 this simple net may be considered to be a segment of the commonly observed 3D a-Po net (see section 2.6.4). The structure of [Ag(pyz)2][Ag2(pyz)5](PF6) 2G, where G ¼ guest solvent, is particularly remarkable as it contains two types of 2D nets – planar (4,4) sheets and bilayers of the same topology, as shown in Figure 2.16a.49 In the bilayers in these two structures, the individual pairs of sheets are interconnected by just one (shared) link per node, making a 5-connected net. However, in work detailing a series of coordination polymers in which lanthanoid metals are bridged by 4,4 0 -bipyridine-N,N 0 -dioxide ligands, bilayers were described in which (4,4) sheets are bridged such that each node is

34

Chapter 2

Figure 2.15

Schematic representation of the 2D bilayer structure in Ag(bpea)L1/2 H2O, where L ¼ 4,4 0 -biphenylcarboxylate.47

Figure 2.16

Three different bilayer structures formed by the crosslinking of pairs of (4,4) sheets.


35

connected not only to one node in the adjacent sheet, but also two, three or four others, leading to 5-, 6-, 7- and 8-connected 2D nets (Figure 2.16).50–52 Chemical examples of 5-, 6- and 7,8-connected nets were reported. In the structure of Cu4(dca)4(4,4 0 -bipy)3(MeCN)2, pairs of (6,3) sheets are bridged to create a bilayer, as shown in Figure 2.17.53 Only half of the trigonal nodes are bridged, creating a 3,4-connected net, but one could easily imagine an alternative topology in which all the 3-connecting (6,3) nodes are bridged to create a 4-connected net. There are only a few examples of multilayered 2D nets with more than two bridged layers. The structure of Cu3(bpee)(isonicotinate)6(H2O)2, where bpee ¼ 1, 2-bis(4-pyridyl)ethane, contains three interconnected layers – the outer two are (4,4) sheets and the inner one is a layer of rods (Figure 2.18).54 The remarkable structure of [Co5(bpea)9(H2O)8(SO4)4](SO4) 14H2O contains five interconnected layers of rods.55

Figure 2.17

A bilayer formed by the crosslinking of pairs of (6,3) sheets.

Figure 2.18

Schematic representation of the 2D trilayer structure of Cu3(bpee) (isonicotinate)6(H2O)2.54

36

Chapter 2

2.6 Common 3D Nets There are thousands of possible 3D nets, possibly more, yet a significant fraction of coordination polymers can be described by just a small number of simple nets. This has been illustrated by a recent survey of coordination polymer topologies from the Cambridge Database,30 and also in surveys of interpenetrating networks.56–58

2.6.1 3-Connected Nets The most commonly encountered 3-connected 3D nets are the (10,3) series (Figure 2.19) identified by Wells (by definition all have the Schla¨fli symbol 103).1,2 The most symmetrical of these is the cubic (10,3)-a net, also known as SrSi2 or srs net (Figure 2.19a). This net is noteworthy for the fact that it is inherently chiral, making it an attractive target for crystal engineers (Chapter 11). The structure contains fourfold helices along all axes that are all of the same

Figure 2.19

Four (10,3) nets: (a) (10,3)-a; (b) (10,3)-b; (c) (10,3)-c; (d) (10,3)-d.


37

hand. This chirality is undoubtedly a factor in the formation of chiral (10,3)-a coordination polymers templated by chiral cations such nets in M(oxalate)m 3 (Chapter 4).59–61 as M(2,2 0 -bipy)n+ 3 The other common variation of the (10,3) net is (10,3)-b, also known as the ThSi2 or ths net (Figure 2.19b). In its most symmetrical form, this net is characterised by crosslinked zigzag chains; however it is not unknown for the 3-connecting nodes to adopt a T-shaped geometry. Physical models of this net reveal a remarkable feature – it can collapse or expand like a wine rack. This is achieved by varying only the torsional angle around the bonds that crosslink the zigzag chains. This effect has been put to good use in the structure of (ZnI2)3(tpt)2 x(guest), in which the two interpenetrating (10,3)-b nets expand and contract depending on the presence or absence of guest molecules.62 Single (10,3)-b nets can also be formed in this system by changing the solvent medium from which the crystals grow (the solvent becomes clathrated within the structure). The (10,3)-c (or bto) and (10,3)-d (utp) nets are less common. The (10,3)-c net (Figure 2.19c) is similar to (10,3)-b, except that instead of the adjoining, connected zigzag chains being inclined at 901 to each other (in the most symmetrical form), they are inclined at 601, leading to three different chain directions instead of two. Similarly, the (10,3)-d net (Figure 2.19d) is reminiscent of (10,3)a, but there are fourfold helices of both handedness and the net is non-chiral. An example of a coordination polymer with this topology is the structure of Co(NO3)2L1.5 H2O, where L ¼ 1,4-bis(3-pyridyl)-2,3-diazabuta-1,3-diene, in which there are four interpenetrating nets.63 There are a number of other uniform (10,3) nets that were identified by Wells1,2 [(10,3)-e, -f and -g] or are listed on the RCSR (utk, utj, utm),31 but for which no coordination polymer has been reported. Structures displaying uniform nets with ring sizes other than 10 are also surprisingly rare, even though Wells identified a large number of (n,3) nets (n ¼ 7, 8, 9, 12). For example, the uninodal (8,3)-a (eta), (8,3)-b (etb) and (8,3)-d (etd) nets and the binodal (8,3)-c (etc) net are shown in Figure 2.20 (again, all are 83). Like the (10,3) nets, the (8,3)-a net is chiral whereas (8,3)-b is not. The (8,3)-d net is also chiral. The (8,3)-b net can be found in one pseudopolymorph of [Cu(5-methyltetrazolate)],64 and two interpenetrating (8,3)-c nets can be found in the inorganic compounds MTi2(PS4)3, where M¼Na, Ag.65 The remarkable (12,3) (or twt, 123) net, which is self-penetrating (Chapter 3), can be found in the structure of Ni(tpt)(NO3)2 (Figure 2.21).66 In the highest symmetry form of the net, the nodes are trigonal and all equivalent; in this coordination polymer there are two sorts of nodes – trigonal tpt ligands and T-shaped Ni nodes. There are, of course, many other possible 3-connected nets that are not uniform (i.e. have different sized shortest circuits) or have more than one type of node [such as (8,3)-c]. For example, a pseudopolymorph of [Cu(5-methyltetrazolate)] displays the (82.10)-a (LiGe, lig) net (Figure 2.22a).64 The related structures Cu(Rtz), where Rtz ¼ 3,5-dimethyl-1,2,4-triazole (mtz) or 3,5-dipropyl-1,2,4triazole (ptz), both show 3-connected nets, but the topologies vary with the alkyl

38

Figure 2.20

Chapter 2

Four (8,3) nets: (a) (8,3)-a (viewed from two different directions); (b) (8,3)-b; (c) (8,3)-c; (d) (8,3)-d.

chain.67 Cu(mtz) has an lvt-a (4.8.16) net, whereas Cu(ptz) has an nbo-a (4.122) net (Figure 2.22b and c). This last net is an interesting one to explore further. This net can be derived from the 4-connected NbO net (section 2.6.2) by replacing the four-connecting planar nodes with a ring of 3-connecting nodes. This process of deriving one net


Figure 2.21

39

The (12,3) net.

from another by replacing the nodes with a cluster of differently connected nodes is called decoration (if the replacement nodes have the same connectivity, it is known as augmentation).25 Hence a net is only as good as the nodes chosen – if Cu2(ptz)2 dimers were chosen as the nodes (which is perfectly reasonable), then an NbO net would result.

2.6.2 4-Connected Nets The most common 4-connected nets are those that can be constructed easily with tetrahedral and/or square-planar nodes. The most common net containing tetrahedral nodes is diamond (dia) (Figure 2.23a); it is, in fact, the most common 3D net of all. In its highest symmetry form it is cubic, and a feature of this net is the adamantane-like cavities, which are bounded by four six-membered rings in the chair conformation. A number of examples of this net were discussed in Section 2.1. The lonsdaleite (hexagonal diamond, lon) net is, surprisingly, as rare in coordination polymers in comparison with diamond as are the carbon prototypes. The net, shown in Figure 2.23b, is very closely related to diamond (it has the same 66 Schla¨fli symbol) – indeed, the two polymorphs of zinc sulfide display the two related nets – wurtzite has the lonsdaleite net whereas zinc blende displays the diamond net. The difference arises in the orientation of adjacent, linked tetrahedral nodes. In diamond, each pair of linked nodes has a staggered conformation, whereas in lonsdaleite one in four has the eclipsed conformation instead. This leads to two sorts of cavities in the net – a smaller

40

Figure 2.22

Chapter 2

The 3-connected nets (a) lig, (b) lvt-a and (c) nbo-a.

one bound by three six-membered rings with the boat conformation and a larger one bounded by five six-membered rings, three of which lie around the ‘equator’ of the cavity and have the boat conformation and two top and bottom which have the chair conformation. An example of the lonsdaleite net is the host framework of Cd(CN)2 0.5n-Bu2O 0.5H2O, where the lonsdaleite topology is labelled by the SiO2 polymorph having this topology (tridymite; the diamond polymorph of SiO2 is cristobalite).68 Another polymorph of SiO2 is quartz and this chiral net (qtz, 64.82, Figure 2.23c) is found in the structures of M[Au(CN)2]2, where M¼Zn, Co.69 Surprisingly, given that the nodes are rather distorted, the second most common net with ‘tetrahedral’ nodes is that displayed by SrAl2 (sra, 42.63.8), Figure 2.23d. This net can be described in terms of crosslinked corrugated


Figure 2.23

41

Selected 4-connected 3D nets containing tetrahedral nodes: (a) diamond; (b) lonsdaleite; (c) quartz; (d) SrAl2; (e) sodalite.

42

Chapter 2

ladders. The structure of Ag2L2(SbF6)2 4MeOH.CHCl3, where L ¼ 4,4 0 ,400 tricyanotriphenylmethanol, contains two interpenetrating sra nets.70 Conspicuously absent in this discussion are true zeolitic networks;71 given that one of the main interests in coordination polymers is to mimic the porous nature of zeolites, the low prevalence of coordination polymer topologies based on the many tetrahedral 4-connected zeolite nets is surprising. However, the fact that the majority of coordination polymers have linear links between the metal- and/or ligand-based nodes is no doubt a factor [in zeolites the bridges (e.g. Si–O–Al) are kinked]. One zeolite net that has been reported for a number of coordination polymers is that of sodalite (sod, 42.64, Figure 2.23e). A noteworthy example of this can be found in a (strictly inorganic) series of copper carbonates in which cations perform an important templating role.72 This topology is also shown by a series of coordination polymers obtained from the reaction of Cu(II) with two fused pyridazine derivatives.73 There have also been a few scattered reports of other zeolite topologies in coordination polymers, including zeolite MTN, ABW, DFT, GIS, LTA, ANA and RHO nets.74 Significantly, a lot of these coordination polymers have been constructed using imidazole-derived ligands, which mimic the kinked bridging nature of the zeolitic oxygen atoms. There is also a rich variety of 4-connected nets which can be constructed using planar nodes. The most common of these are the NbO (nbo, 64.82), CdSO4 (cds, 65.8) and lvt (42.84) nets (Figure 2.24a–c). In the NbO net, adjacent squareplanar nodes are all perpendicular to one another. By contrast, only half of the adjacent nodes in CdSO4 are perpendicular; the others are coplanar. In lvt, two of the nodes are coplanar and two are inclined but not perpendicular. The nodes are also planar but not ‘square’. The NbO net can be seen in the structure of Fe(NCS)2(tmbpz)2, where tmbpz ¼ 3,3 0 ,5,5 0 -tetramethyl-4,4 0 -bipyrazolyl,75 whereas the CdSO4 net can be found in SmL2(NO3)3 0.5H2O, where L ¼ 4,4 0 bipyridine-N,N 0 -dioxide.76 The structure of Cu(4,4 0 -bipy)2(CF3SO3)2 contains lvt nets.77 All three of these structures are doubly interpenetrating. A rare but fundamentally important net is the (8,4) (tcb, 86) net (Figure 2.24d).78 This platonic uniform net is the only uniform net known to contain only planar 4-connecting nodes and the only 4-connected uniform net not to have the Schla¨fli symbol 66. It is also the only known 4-connected net in which all the smallest rings are larger than seven-membered.27 The net contains two sorts of links and it is not possible for all the nodes to have 901 angles. Hence it cannot be described as ‘regular’ [all vertices, edges and angles equivalent (related by symmetry)] or ‘quasiregular’ [all vertices and edges (but not angles) equivalent].20 This net also does not represent a four-coordinate sphere packing; there are distances between nodes not directly connected that are shorter than those directly linked. Three other nets are shown in Figure 2.24 which have planar 4-connecting nodes. The quartz dual net (qzd, 75.9) can be found in the structure of a Cu–1, 2-bis(4-pyridyl)ethyne coordination polymer containing two chemically different (but topologically identical) nets,79 while the usf (65.8) net is named after the USF1 coordination polymer.80 Similarly, the mot [(66)(64.82)2] net is named after MOF112.81 This net is related to the NbO and CdSO4 nets but contains two different

Figure 2.24

Selected 4-connected 3D nets containing square-planar nodes: (a) NbO; (b) CdSO4; (c) lvt; (d) (8,4); (e) qzd; (f) usf; (g) mot.

44

Chapter 2

square-planar nodes. For one node all its connected neighbours are perpendicular, whereas for the other node half are perpendicular and half are coplanar. Notwithstanding the mot net (and a few selected others), we have to date largely ignored, for simplicity’s sake, multinodal nets. However, they warrant some further discussion here due to the two different 4-connected geometries commonly seen and the fact that a few significant binodal nets containing a tetrahedral node and a square-planar node have been observed. The first of these is the PtS (pts) net, which consists of equal numbers of tetrahedral and square-planar nodes (Figure 2.25a). Each node bonds only to nodes of the other type. The Schla¨fli symbol for this net is (42.84)(42.84), i.e. both nodes have the same Schla¨fli notation. This net is observed for a pair of Cu(I)–porphyrin coordination polymers, one of which is a single net (containing a pyridyl

Figure 2.25

Multinodal 4-connected nets: (a) binodal PtS (viewed from two different directions); (b) binodal moganite; (c) trinodal ptt.


45

functionalised porphyrin) and one which doubly interpenetrates (containing a benzonitrile functionalised porphyrin).7 The Cu(I) ions act as the tetrahedral nodes, whereas the porphyrins act as the square-planar nodes. The two different nodes in the moganite net (mog, Figure 2.25b) are in the ratio of 1:2 (square-planar:tetrahedral); the nodes do not strictly alternate as in PtS, but rather two of the links from each tetrahedral node go to other tetrahedral nodes. The Schla¨fli symbol is (42.62.82)(4.64.8)2. This net is displayed by the structure of Cd(CN)2 2/3H2O tBuOH,82 and also the more recent Cu coordination polymer of an in situ-generated tetrapyridyl ligand that acts both as a square-planar node and as a simple 2-connecting linker between the tetrahedral Cu nodes.83 The final net is the twisted pts net (ptt). It is similar in appearance to PtS and again there are equal numbers of square-planar and tetrahedral nodes, and each bonds only to the other (Figure 2.25c). However, there are two types of square-planar nodes and therefore the net is trinodal [Schla¨fli symbol (4.63.82)2(42.62.82)(62.84)]. This net is typified by a series of metal cyanide clathrates, [Cd(1,2-diaminopropane)(CN)4Ni] nG, where G ¼ guest.84

2.6.3 5-Connected Nets Nets with 5-connectivity are rare, no doubt due to the difficulty in generating 5-connecting metal centres and/or ligands. Two simple 5-connected nets are shown in Figure 2.26. These represent the two simplest ways to generate 5-connected nets – bridging 2D (6,3) sheets (boron nitride, bnn net, 46.64) and bridging (corrugated) (4,4) sheets (sqp net, 44.66). In the bnn net the nodes have trigonal bipyramidal geometry, whereas in the sqp net they have square-pyramidal

Figure 2.26

Two 5-connected nets: (a) boron nitride; (b) sqp.

46

Chapter 2

geometry. A distorted example of the bnn topology can be found in the structure of Mn[C(CN)3]2L, where L ¼ 1,2-bis(4-pyridyl)ethane-N,N 0 -dioxide.85

2.6.4 6-Connected Nets The overwhelmingly dominant 6-connected net is that of a-Po (also known as NaCl, ReO3 or pcu, 412.63), Figure 2.27a. This is the simplest net obtained from octahedral nodes and is typified by the structure of Prussian Blue.86 A less common geometry for 6-connecting centres is trigonal prismatic and a simple net formed from this geometry is the acs net (49.66), Figure 2.27b. This net has been observed in the structure of Eu[Ag(CN)2]3 3H2O.87 Combining both octahedral and trigonal prismatic nodes in a 1:1 ratio can result in the NiAs [nia, (412)(49.66)] net, Figure 2.27c. Each node in this net bonds only to nodes of the other type. This net can be found the structure of [dmenH2+ 2 ][M2(HCOO)6],

Figure 2.27

Selected 6-connected nets: (a) a-Po; (b) acs; (c) NiAs; (d) a self-penetrating 44.610.8 net.

Nets: A Tool for Description and Design II

II

47

dmenH221 ¼ N,N 0 -dimethylethylenediammonium.88

where M¼Mn , Co and This structure is clearly cation templated and remarkably, if the amine cation is changed, structures with acs89 or a-Po90 topology can be obtained! Another way of generating 6-connected nets is to crosslink (4,4) sheets in different ways. In a-Po the links are all parallel, but bridging the sheets in two inclined directions generates a new series of 6-connected nets. One such net is shown in Figure 2.27d. These nets are self-penetrating (Chapter 3) – in the net shown, which has 44.610.8 topology, each six-membered ring which includes a link within a layer and a link between the layers is penetrated by two other interlayer rods. This net is displayed by the structure of M(dca)2L, where L ¼ 1,2-bis(4-pyridyl)ethane-N,N 0 -dioxide.91 In other similarly constructed (but topologically different) nets, the equivalent six-membered rings are penetrated by one92–94 (48.66.8 topology) or three92 rods. The generation of these nets while retaining localised octahedral metal coordination is aided by corrugation of the sheets, kinked ligands, significant deviation from linearity in the angle around the donor atoms of the interlayer ligands or combinations of all these factors.

2.6.5 Higher Connected Nets Nets containing nodes with connectivities 46 are unusual due to the difficulty in coaxing first-row transition metals to take on such high coordination geometries, the difficulty in sterically fitting that many donors around such a metal and the synthetic challenge to produce organic ligands with such connectivities. For these reasons, most highly connected nets have as their nodes either heavier transition metals or lanthanoid metals (with ligands containing sterically small donor functionalities such as pyridyl-N-oxides or cyanides) or metal–ligand clusters. For example, a 7-connected wfq net has been reported for a 4,4 0 -bipyridine-N, 0 N -oxide coordination polymer of La.95 The net can be described in terms of (4,4) and (6,3) sheets that intersect at right-angles and share their nodes. Alternatively, (4,4) sheets are interconnected such that there is one intersheet bridge per node on one side of each sheet and two on the other. For 8-connected nets, the simplest is CsCl (bcu, 42464), and such an example was also reported in the same paper.95 This net can also be described in terms of bridged (4,4) sheets, where each node is connected to four others in adjoining sheets; two above and two below (Figure 2.28a). Furthermore and similarly to what was seen for 6-connected nets, a series of other 8-connected nets can be generated by bridging (4,4) sheets in different ways.52 This is often accompanied by a relative translation or rotation of adjoining sheets and can again lead to self-penetrating networks (e.g. a 424.5.63 self-penetrating net is shown in Figure 2.28b),96 but not always.50 Another way to generate 8-connected nets is to bridge (3,6) sheets, to give the hex (36.418.53.6) net. This net (Figure 2.28c) is found in the structure of [Co3(bpdc)3(4,4 0 -bipy)] 4DMF H2O, where bpdc ¼ biphenyldicarboxylate, in which a cobalt carboxylate trimer SBU acts as the 8-connecting node.97

48

Figure 2.28

Chapter 2

Three 8-connected nets: (a) CsCl; (b) a self-penetrating 424.5.63 net; (c) the hex net.

Finally, the 12-connected face-centred cubic lattice (fcu, 324.436.56, Figure 2.29) has been observed for two coordination polymers containing copper sulfide cluster SBUs.98,99

2.6.6 Mixed Connectivity Nets There are numerous nets with mixed connectivity nodes also worthy of analysis, starting with 3,4-connected nets. The Pt3O4 [pto, (83)4(86)3] net consists of alternating trigonal and square-planar nodes (Figure 2.30a) and has been observed in the structure of [Cu3L2(H2O)] 9DMF 2H2O, where L ¼ 4,4 0 ,400 -benzene-1,3, 5-triyltribenzoate.100 Another net with square-planar nodes is the twisted boracite [tbo, (63)4(62.82.102)3] net (Figure 2.30b). This net can be found in the structure of [Cu3(btc)2(H2O)3], where btc ¼ benzene-1,3,5-tricarboxylate.101


Figure 2.29

49

The 12-connected fcu net.

The boracite [bor, (63)4(62.84)3] net itself contains tetrahedral nodes rather than square-planar ones (Figure 2.30c). Both nets contain (3-c)4(4-c)6 cages (n-c ¼ n-connected node) which are connected together in an a-Po arrangement via sharing of the 4-c nodes. The cages have an octahedral shape, with 4-c nodes at the vertices and 3-c nodes on alternating faces; the linking of these by square-planar or tetrahedral nodes changes the orientation of the occupied faces of the neighbouring octahedra. Remarkably, the tpt ligand will form the boracite net [Cu3(tpt)4(ClO4)3.solv],102 the twisted boracite net {[Fe(SCN)2]3(tpt)4 solvent}103 and even discrete molecular cages ([{(en)Pd}6(tpt)4](NO3)12).104 Another form of [Cu3(tpt)4]31, formed from an ionic liquid, shows the aC3N4 [ctn, (83)4(86)3] net, Figure 2.30d.105 The b polymorph of C3N4 also forms an interesting net (Figure 2.30e), although it is trinodal (there are two different 3-connecting nodes) and there is, to date, no coordination polymer know with this net. The net can also be referred to as the (b-)Ge3N4, phenacite or sln net and has (63)3(83)(63.83)6 topology. In all these nets, 3-connecting nodes bond only to 4-connecting ones and vice versa. In the InS net [ins, (63)(65.8)], each 4-connected node bonds to three 3-connected nodes and one other 4-connected node (Figure 2.30e). In this net, found in the structure of Ag[C(CN)3](phenazine)1/2,106 there are (6,3) sheets in which every second node is tetrahedral and bonds to another 4-connected node in an adjacent sheet. In the tfz [(63)2(64.8.10)3] net, Figure 2.30f, there are also (6,3) sheets; however, in this case the midpoints of the links become 4-connecting square-planar nodes which crosslink the sheets. This net has

Figure 2.30

Selected 3,4-connected nets: (a) Pt3O4; (b) twisted boracite; (c) boracite; (d) a-C3N4; (e) b-C3N4 (or b-Ge3N4); (f) InS; (g) tfz.


Figure 2.31

51

The 3,5-connected hms net.

been seen in the structure of [Cu3(btc)2L3] 5H2O, where L ¼ N,N 0 -bis (4-pyridylformamide)-1,4-benzene.107 Nets with 3,5-connectivity are rare, but the hms [(63)(69.8)] net can be seen in the structures of Ag[C(CN)3]L, where L ¼ pyrazine, dabco, 4,4 0 -bipyridine.108 It consists of (6,3) sheets in which every second node is connected to identical nodes in adjoining sheets, becoming 5-connecting (Figure 2.31). There are an increasing number of 3,6-connected nets being reported,109 but the most common is the rutile net [TiO2, rtl, (4.62)2(42.61083)], Figure 2.32a. This structure is typified by the two series of compounds, M[N(CN)2]2110 (one net) and M[C(CN)3]2111 (two interpenetrating nets), where M ¼ first-row transition metal. Another polymorph of TiO2 is anatase and this net [ant, (42.6)2(44.62.88.10), Figure 2.32b] is observed in the citrate coordination polymer KCo3(C6H4O7)(C6H5O7)(H2O)2 8H2O.112 The sit net has the same Schla¨fli symbol as rutile, but the 6-connecting nodes in this net are trigonal prismatic rather than octahedral (Figure 2.32c). It is found in the structures of Zn3O(btc)2 2Et3NH and Zn3O(HBTB)2(H2O) 1/2DMF 3H2O, BTB ¼ benzenetribenzoate.113 The pyr [(63)2(612.83)] net (Figure 2.32d) is based on the structure of pyrites (FeS2), but with the S–S bonds removed. It can be found in the structure of Hg(tpt)2(ClO4)2 6C2H2Cl4.114 Lastly, a coordination polymer with the pentanodal 3,6-connected net qom has been reported,115 showing that even nets with simple nodal connectivity can be complicated. The final two nets we will examine are the 4,8-connected fluorite [flu, (46)2(412.612.84)] and scu [(44.62)2(416.612)] nets. The fluorite net, shown in Figure 2.33a, has tetrahedral 4-connecting nodes and is found in the structure of

52

Figure 2.32

Chapter 2

The 3,6-connected nets (a) rutile, (b) anatase, (c) sit and (d) pyr.

Cd4(TCPM)2(DMF)4 4DMF 4H2O, where TCPM ¼ tetrakis(4-carboxyphenyl) methane,116 while the structure of Ca[Co(en)(oxalato)2]2 displays the scu net, which has square-planar 4-connecting nodes (Figure 2.33b).117

2.7 Rod Packings Sometimes a net is too complicated, or choosing nodes and links is too subjective, for the approach described above to be useful. For example, there are a large number of metal carboxylate polymers in which infinite rods of metals bridged by carboxylate groups are present; selecting which part of the rod to


Figure 2.33

53

The 4,8-connected nets (a) fluorite and (b) scu.

separate and call the node is difficult and subjective. In these situations, the concept of rod packings has been successfully applied to give useful topological information and categorisation.118

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61. S. Decurtins, H. Schmalle and R. Pellaux, New J. Chem., 1998, 22, 117. 62. K. Biradha and M. Fujita, Angew. Chem. Int. Ed., 2002, 41, 3392. 63. Y.-B. Dong, M.D. Smith and H.-C. zur Loye, Inorg. Chem., 2000, 39, 4927. 64. D. Li, W.J. Shi and L. Hou, Inorg. Chem., 2005, 44, 3907. 65. (a) X. Cieren, J. Angenault, J.-C. Courtier, S. Jaulmes, M. Quarton and F. Robert, J. Solid State Chem., 1996, 121, 230; (b) J. Angenault, X. Cieren, G. Wallez and M. Quarton, J. Solid State Chem., 2000, 153, 55. 66. B.F. Abrahams, S.R. Batten, M.J. Grannas, H. Hamit, B.F. Hoskins and R. Robson, Angew. Chem. Int. Ed., 1999, 38, 1475. 67. J.-P. Zhang, S.-L. Zheng, X.-C. Huang and X.-M. Chen, Angew. Chem. Int. Ed., 2004, 43, 206. 68. T. Kitazawa, T. Kikuyama, M. Takeda and T. Iwamoto, J. Chem. Soc., Dalton Trans., 1995, 3715. 69. (a) B.F. Hoskins, R. Robson and N.V.Y. Scarlett, Angew. Chem. Int. Ed. Engl., 1995, 34, 1203; (b) S.C. Abrahams, L.E. Zyontz and J.L. Bernstein, J. Chem. Phys., 1982, 76, 5458. 70. S. Ferlay, S. Koenig, M.W. Hosseini, J. Pansanel, A. De Cian and N. Kyritsakas, Chem. Commun., 2002, 218. 71. (a) Ch. Baerlocher, W.M. Meier and D.H. Olson, Atlas of Zeolite Framework Types, 5th edn, Elsevier, Amsterdam, 2001; (b) http:// www.iza-structure.org/databases. Accessed 18 March, 2008. 72. B.F. Abrahams, A. Hawley, M.G. Haywood, T.A. Hudson, R. Robson and D.A. Slizys, J. Am. Chem. Soc., 2004, 126, 2894. 73. P.V. Solntsev, J. Sieler, A.N. Chernega, J.A.K. Howard, T. Gelbrich and K.V. Domasevitch, Dalton Trans., 2004, 695. 74. (a) Y.-Q. Tian, Y.-M. Zhao, Z.-X. Chen, G.-N. Zhang, L.-H. Weng and D.Y. Zhao, Chem. Eur. J., 2007, 13, 4146; (b) Q. Fang, G. Zhu, M. Xue, J. Sun, Y. Wei, S. Qiu and R. Xu, Angew. Chem. Int. Ed., 2005, 44, 3845; (c) G. Fe´rey, C. Serre, C. Mellot-Draznieks, F. Millange, S. Surble´, J. Dutour and I. Margiolaki, Angew. Chem. Int. Ed., 2004, 43, 6296; (d) H. Hayashi, A.P. Cote, H. Furukawa, M. O’Keeffe and O.M. Yaghi, Nat. Mater., 2007, 6, 501; (e) X.D. Guo, G.S. Zhu, Z.Y. Li, Y. Chen, X. T. Li and S.L. Qiu, Inorg. Chem., 2006, 45, 4065; (f) X.-C. Huang, Y.-Y. Lin, J.P. Zhang and X.-M. Chen, Angew. Chem. Int. Ed., 2006, 45, 1557; (g) Y. Liu, V.C. Kratsov, R. Larsen and M. Eddaoudi, Chem. Commun., 2006, 1488. 75. P.V. Ganesan and C.J. Kepert, Chem. Commun., 2004, 2168. 76. D.-L. Long, A.J. Blake, N.R. Champness and M. Schroder, Chem. Commun., 2000, 1369. 77. L. Carlucci, N. Cozzi, G. Ciani, M. Moret, D.M. Proserpio and S. Rizzato, Chem. Commun., 2002, 1354. 78. M.-L. Tong, X.-M. Chen and S.R. Batten, J. Am. Chem. Soc., 2003, 125, 16170.


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79. L. Carlucci, G. Ciani, P. Macchi and D.M. Proserpio, Chem. Commun., 1998, 1837. 80. B. Moulton, H. Abourahma, M.W. Bradner, J. Lu, G.J. McManus and M.J. Zaworotko, Chem. Commun., 2003, 1342. 81. M. Eddaoudi, J. Kim, D. Vodak, A. Sudik, J. Wachter, M. O’Keeffe and O.M. Yaghi, Proc. Natl. Acad. Sci. USA, 2002, 99, 4900. 82. B.F. Abrahams, B.F. Hoskins and R. Robson, Chem. Commun., 1990, 60. 83. S. Hu, J.-C. Chen, M.-L. Tong, B. Wang, Y.-X. Yan and S.R. Batten, Angew. Chem. Int. Ed., 2005, 44, 5471. 84. (a) K.M. Park and T. Iwamoto, J. Chem. Soc., Chem. Commun., 1992, 72; (b) K.M. Park and T. Iwamoto, J. Chem. Soc., Dalton Trans., 1993, 1875. 85. H.-L. Sun, B.-Q. Ma, S. Gao and S.R. Batten, Cryst. Growth Des., 2005, 5, 1331. 86. H.J. Buser, D. Schwarzenbach, W. Petter and A. Ludi, Inorg. Chem., 1977, 16, 2704. 87. Z. Assefa, R.J. Staples and J.P. Fackler Jr, Acta Crystallogr. Sect. C, 1995, 51, 2527. 88. Z. Wang, X. Zhang, S.R. Batten, M. Kurmoo and S. Gao, Inorg. Chem., 2007, 46, 8439. 89. Z. Wang, B. Zhang, K. Inoue, H. Fujiwara, T. Otsuka, H. Kobayashi and M. Kurmoo, Inorg. Chem., 2007, 46, 437. 90. Z. Wang, B. Zhang, T. Otsuka, K. Inoe, H. Kobayashi and M. Kurmoo, Dalton Trans., 2004, 2209. 91. H.-L. Sun, S. Gao, B.-Q. Ma and S.R. Batten, CrystEngComm, 2004, 6, 579. 92. B.F. Abrahams, M.J. Hardie, B.F. Hoskins, R. Robson and E.E. Sutherland, J. Chem. Soc., Chem. Commun., 1994, 1049. 93. X.-J. Zhao, S.R. Batten and M. Du, Acta Crystallogr. Sect. E, 2004, 60, m1237. 94. D.-L. Long, R.J. Hill, A.J. Blake, N.R. Champness, P. Hubberstey, C. Wilson and M. Schro¨der, Chem. Eur. J., 2005, 11, 1384. 95. D.L. Long, A.J. Blake, N.R. Champness, C. Wilson and M. Schro¨der, Angew. Chem. Int. Ed., 2001, 40, 2444. 96. X.-L. Wang, C. Qin, E.-B. Wang, Z.-M. Su, L. Xu and S.R. Batten, Chem. Commun., 2005, 4789. 97. L. Pan, H. Liu, X. Lei, X. Huang, D.H. Olson, N.J. Turro and J. Li, Angew. Chem. Int. Ed., 2003, 42, 542. 98. X.-M. Zhang, R.-Q. Fang and H.-S. Wu, J. Am. Chem. Soc., 2005, 127, 7670. 99. D. Li, T. Wu, X.-P. Zhou, R. Zhou and X.-C. Huang, Angew. Chem. Int. Ed., 2005, 44, 4175. 100. B. Chen, M. Eddaoudi, S.T. Hyde, M. O’Keeffe and O.M. Yaghi, Science, 2001, 291, 1021. 101. S.S.Y. Chui, S.M.F. Lo, J.P.H. Charmant, A.G. Orpen and I.D. Williams, Science, 1999, 283, 1148.

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102. B.F. Abrahams, S.R. Batten, H. Hamit, B.F. Hoskins and R. Robson, Angew. Chem. Int. Ed. Engl., 1996, 35, 1690. 103. S. M. Neville, B. Moubaraki, K. S. Murray and C. J. Kepert, unpublished results. 104. M. Fujita, D. Oguro, M. Miyazawa, H. Oka, K. Yamaguchi and K. Ogura, Nature, 1995, 378, 469. 105. D.N. Dybtsev, H. Chun and K. Kim, Chem. Commun., 2004, 1594. 106. S.R. Batten, B.F. Hoskins and R. Robson, New J. Chem., 1998, 22, 173. 107. F. Luo, J.-M. Zheng and S.R. Batten, Chem. Commun., 2007, 3744. 108. B.F. Abrahams, S.R. Batten, B.F. Hoskins and R. Robson, Inorg. Chem., 2003, 42, 2654. 109. M. Du, Z.-H. Zhang, L.-F. Tang, X.-G. Wang, X.-J. Zhao and S.R. Batten, Chem. Eur. J., 2007, 13, 2578. 110. (a) S.R. Batten, P. Jensen, B. Moubaraki, K.S. Murray and R. Robson, Chem. Commun., 1998, 439; (b) M. Kurmoo and C.J. Kepert, New J. Chem., 1998, 22, 1515; (c) J.L. Manson, C.R. Kmety, Q. Huang, J.W. Lynn, G.M. Bendele, S. Pagola, P.W. Stephens, L.M. Liable-Sands, A. L. Rheingold, A.J. Epstein and J.S. Miller, Chem. Mater., 1998, 10, 2552. 111. (a) S.R. Batten, B.F. Hoskins and R. Robson, J. Chem. Soc., Chem. Commun., 1991, 445; (b) S.R. Batten, B.F. Hoskins, B. Moubaraki, K. S. Murray and R. Robson, J. Chem. Soc., Dalton Trans., 1999, 2977. 112. S.C. Xiang, X.T. Wu, J.J. Zhang, R.B. Fu, S.M. Hu and X.D. Zhang, J. Am. Chem. Soc., 2005, 127, 16352. 113. J. Kim, B. Chen, T.M. Reineke, H. Li, M. Eddaoudi, D.B. Moler, M. O’Keeffe and O.M. Yaghi, J. Am. Chem. Soc., 2001, 123, 8239. 114. S.R. Batten, B.F. Hoskins and R. Robson, Angew. Chem. Int. Ed. Engl., 1995, 34, 820. 115. H.K. Chae, D.Y. Siberio-Pe´rez, J. Kim, Y. Go, M. Eddaoudi, A.J. Matzger, M. O’Keeffe and O.M. Yaghi, Nature, 2004, 427, 523. 116. H. Chun, D. Kim, D.N. Dybstev and K. Kim, Angew. Chem. Int. Ed., 2004, 43, 971. 117. C. Borel, M. Ha˚kansson and L. O¨hrstro¨m, CrystEngComm, 2006, 8, 666. 118. N.L. Rosi, J. Kim, M. Eddaoudi, B. Chen, M. O’Keeffe and O.M. Yaghi, J. Am. Chem. Soc., 2005, 127, 1504.

CHAPTER 3

Interpenetration 3.1 Introduction The adage ‘Nature abhors a vacuum’ applies especially to crystals. Even the most porous framework is not synthesised empty. In the best-case scenario it contains lots of liquid solvent which can be easily displaced without destroying the crystal; in the worst-case scenario it contains multiple interpenetrating networks and/or guests that completely eliminate any potential pores. A true crystal engineer should seek to understand not just the topology of the designed network (as discussed in the previous chapter), but also the crystal structure as a whole, that is, what the consequences of large cavities in a single network are and how that space is used. A good appreciation of how the space within a crystal is filled is vital in designing a coordination polymer. There are three main ways in which a crystal can maximise its packing efficiency – intercalation, interdigitation and interpenetration.1 This is best illustrated by considering a series of three related structures. All contain 2D (4,4) sheets in which CuI ions are bridged in one direction by 2-connecting tricyanomethanide [tcm, C(CN) 3 ] anions and in the other direction by 2-connecting nitrogen-donor organic ligands, yet all three show a different method for filling the space. In the structure of [Cu(tcm)(bpee)] 0.25bpee 0.5MeCN, where bpee ¼ 1,2bis(4-pyridyl)ethene, the (4,4) sheets align to create rectangular channels which run perpendicular to the sheets (Figure 3.1a). Within these channels lie intercalated solvent molecules and uncoordinated bpee molecules. Intercalation is common in coordination polymers; usually it is solvent, which can be ordered (as it is in this structure) or it can be highly disordered and essentially liquid. In the structure of [Cu(tcm)(hmt)], where hmt ¼ hexamethylenetetramine, the highly corrugated sheets align themselves so that the uncoordinated tcm nitrile groups of each net project into the middle of the square windows of an adjoining net (Figure 3.1b). That is, they interdigitate. This mode of packing,


59

60

Figure 3.1

Chapter 3

Three ways of maximising packing efficiency: (a) intercalation; (b) interdigitation; (c) interpenetration.1

where nets intrude on the space occupied by each other, can occur for both 1D and 2D nets. The final packing mode, and the subject of most of this chapter, is interpenetration. This is displayed by the compound [Cu(tcm)(4,4 0 -bipy)], where 4,4 0 -bipy ¼ 4,4 0 -bipyridine. Two nets interweave as shown in Figure 3.1c; both

Interpenetration

61

nets occupy the same region of space, with each passing through the windows of the other. It is different from interdigitation in that the two nets could not be separated without breaking bonds. Indeed, this is how we define interpenetration: interpenetration occurs when two or more polymeric networks are not directly connected but cannot be separated topologically without the breaking of bonds. They are polymeric equivalents of molecular catenanes and rotaxanes.2 We can further explore this definition by looking at what is not an interpenetrating system. In the structure of HgI2L, where L ¼ 2,6-bis(4-pyridinylmethyl)-benzo[1,2-c:4,5-c 0 ]dipyrrole-1,3,5,7(2H,6H)-tetrone, the 1D chains interweave like threads in a cloth, as shown in Figure 3.2a.3 However, just like the cloth, single strands could (in a theoretical, topological sense) be pulled from the entanglement without the need for breaking bonds, and therefore this is not interpenetration. A number of coordination polymers have been constructed in which the bridging ligands between the metal nodes are rotaxanes, as shown schematically in Figure 3.2b.4 Although the network and the rotaxane rings cannot be separated, only the network is polymeric and so again this does not qualify. Finally, there are other ways in which nets can occupy the same space. We have already discussed interdigitation above, but 1D polymeric

Figure 3.2

Three entanglements that are not interpenetration: (a) woven 1D chains; (b) rotaxane-containing sheets; (c) 1D nets intercalated in 2D sheets.

62

Chapter 3

networks can also be intercalated into other nets without interpenetration. The example of [Mn(dca)2(H2O)2] H2O, where dca ¼ dicyanamide, N(CN)2, is shown in Figure 3.2c – (4,4) sheets stack to give square channels in which 1D polymers lie; both nets have the same formula.5 This, however, is intercalation, not interpenetration, because the 1D chains could (theoretically) be pulled out of the channels without breaking any bonds. Although the rings of the sheets are penetrated by the 1D chains, the 1D chains are not penetrated by any part of the 2D sheets – they are not interpenetrating. Interpenetration is now a widespread phenomenon, as shown by several extensive reviews on the subject2,6–14 and a website that lists and classifies all known examples.15 There are a number of consequences to the crystal engineer of interpenetration. The most obvious one is the reduction in porosity of the material, as the spaces in a single network are filled by further networks. This does not mean, however, that porosity is necessarily always eliminated. For example, the structure of Cu3(tpt)4(ClO4)3, where tpt ¼ 2,4,6-tri(4-pyridyl)1,3,5-triazine, (Figure 3.3) contains two interpenetrating networks which account for only ca. 65% of the crystal volume; the rest is occupied by essentially liquid solvent containing the highly disordered counterions.16 In this case, it is likely that the reason why a third network does not form has to do with (a) the way in which the nets interpenetrate makes it difficult for a third to form and (b) there is probably not enough room left for it to fit comfortably

Figure 3.3

Two interpenetrating nets in the structure of Cu3(tpt)4(ClO4)3.16

Interpenetration

Figure 3.4

63

The two nets in [{CuII(tcp)}CuI]BF4 17C6H5NO2.17

(especially given that these calculations have a significant margin or error). Nonetheless, this material undoubtedly has significant ‘open’ space within it. The porosity can even be optimised if the interpenetrating networks interpenetrate such that they do not maximise the distance between individual nets. For example, in the structure of [{CuII(tcp)}CuI]BF4 17C6H5NO2, where CuII(tcp) ¼ 5,10,15,20-tetrakis(4-cyanophenyl)-21H,23H-porphinecopper(II), the two interpenetrating PtS nets form such that one lies adjacent to the other rather than passing through the middle of its cavities and channels (Figure 3.4).17 This has been referred to elsewhere as ‘interweaving’,18 but this can be a somewhat subjective term and a topological analysis does not recognise geometric differences such as the distance between the interpenetrating nets. A more important point made in that paper,18 however, is the fact that interpenetration can be beneficial for the chemist seeking porous materials by structurally stabilising frameworks that would otherwise collapse upon solvent removal if they were a single net. Interpenetration can also create porous materials with interesting properties. In the structures of Fe(azpy)2(NCS)2 guest, where azpy ¼ trans-4,4 0 -azopyridine, there are two sets of (4,4) sheets that interpenetrate at an inclined angle (2D inclined interpenetration, see section 3.4).19 The presence and nature of the guest molecules induces structural changes in which the two sets of sheets move relative to each other in a scissors-like motion, changing the size and shape of the channels within the structure. This change is accompanied by a change in the spin crossover properties. Other materials have been reported which contain two interpenetrating 3D networks in which the pores between the nets adjust similarly upon exchange of counterions, leading to increased absorption of carbon dioxide.20 The same group has also reported a new class of materials in which a ‘gate-opening’ pressure is required before they will absorb gases.21 At this pressure, the interpenetrating networks within the structure move relative to each other to accommodate the incoming guests. Furthermore,

64

Figure 3.5

Chapter 3

A single closed-off cavity in the structure of Zn3(tpt)2(CN)3(NO3)3 solv.22

hysteresis can be observed when the gate-closing pressure for desorption is lower than the gate-opening pressure for absorption. The structure of Zn3(tpt)2(CN)3(NO3)3 solv is not actually porous at all, despite the fact that it has large cavities (Figure 3.5) containing up to 22 solvent molecules each. The two interpenetrating nets wrap around each other in such a way that cavities created are completely sealed off.22 Crystals of this material containing solvents such as chloroform and methanol are completely stable out of solution, even after several years, and yet the liquid solvent inside can be observed simply by putting the crystals in an ordinary solution NMR machine. In two recent reports, unusual magnetic properties have been ascribed as being due to the presence of two interpenetrating networks with the structures of the compounds.23,24 Furthermore, in the structures of Cu(dcnqi)2, where dcnqi ¼ dicyanoquinodiimine and its derivatives, there are seven interpenetrating diamond nets.25 The interpenetration results in the formation of infinite p-bonded stacks of dcnqi ligands which run through the structure and are responsible for the metal-like electrical conductivity. Nonetheless, there are many instances where it is desirable to reduce or even prevent interpenetration, and a number of strategies have been used to achieve this. One technique is to block off the spaces through which the nets interpenetrate. The structure of Cd(CN)2 contains two interpenetrating diamond

65

Interpenetration II

networks, but if half of the divalent Cd metal centres are replaced with monovalent CuI ions, counter cations are required to be included, which may block off some of the space required for the second network to form.26 Thus, the structure of [Me4N][CuICdII(CN)4] contains only one diamond network and only half the cavities are occupied by the counter cations, with the others empty. A similar single diamond network can be achieved by including CCl4 guest molecules.27 Another way to block off the room for further nets to form, which does not involve deliberate inclusion of guest molecules or ions in the cavities, is to adjust the net itself. For example, a metal–carboxylate coordination polymer has been reported which contains infinite metal–carboxylate rods which are crosslinked to give the net.28 In this structure, it is clear that the walls of the channels in the 3D net are too crowded to allow a second net to pass through them, even though the channels themselves are sizeable. Furthermore, it seems likely that some net topologies are more amenable to interpenetration that others. For example, in a recent survey it was found that 70% of interpenetrating networks have the diamond, SrSi2, a-Po or CdSO4 topology.10 These nets happen to be the only four nets known to be naturally self-dual nets.29 In simple terms, a net can be viewed as a ‘tiling’ of polyhedra (tiles) where the nodes (or vertices) and links (or edges) of the net occur where these polyhedra meet.30 For example, the diamond net can be described in terms of adamantane-shaped polyhedra. The dual of such a net is obtained by ‘putting new vertices in the center of the tiles and connecting such vertices by edges passing through common faces (each face of a tiling is common to two tiles) forming an interpenetrating net, so that all rings (faces) of the original structure are linked (catenated) to faces of the dual one and vice versa. When two dual nets are intergrown (interpenetrate) all rings of one net are linked with those of the other and we call the structure fully catenated. If two nets that are not duals interpenetrate, they are partly catenated.’30 The four self-dual nets are the only ones in which their duals have the same topology as the original nets, which perhaps explains the prominence of these topologies among interpenetrating systems (although diamond and a-Po are also the simplest topologies that contain tetrahedral and octahedral nodes). A further 11% of the interpenetrating structures are accounted for by just four more topologies – rutile, PtS, (10,3)-a and (10,3)-b.10 Finally, it has been reported that high dilution reaction conditions can favour the formation of single nets as opposed to interpenetrating structures.31 This is a technique that warrants considerable further investigation. A notable feature of interpenetrating networks is that they can interpenetrate in different ways. For example, two pairs of interpenetrating (4,4) sheets are shown schematically in Figure 3.6. The two nets in Figure 3.6a interpenetrate in the same way as the two [Cu(tcm)(4,4 0 -bipy)] nets do in Figure 3.1c. The two nets in Figure 3.6b, however, do not – they interpenetrate in a topologically different way. Examples are also presented later for different modes of interpenetration for 3D diamond nets and a-Po nets, and also less subtle variations in interpenetration modes, such as inclined versus parallel interpenetration of 1D and/or 2D nets.

66

Figure 3.6

Chapter 3

Two topologically different modes of interpenetration for (4,4) sheets.

So this leads to there being not only a topology of the individual networks, as discussed in the previous chapter, but also to there being a topology of interpenetration – an analysis required of the way in which the nets interpenetrate. Again, this has to do with the topological relationship of the nets and not the geometric relationship. Gaining an appreciation of this in addition to an understanding of the individual nets themselves is akin to the step taken from knowing the structures of the molecules to understanding how they pack together (which is, after all, the basis of crystal engineering). This chapter is

Interpenetration

67

devoted to understanding this topology of interpenetration and we will catalogue the interpenetration according to the topology of the individual nets as outlined in Chapter 2.

3.2 Nomenclature Given the importance of interpenetration topology, a nomenclature for interpenetrating networks is required. For 1D and 2D nets there are two important considerations: whether the interpenetrating nets are parallel or inclined and whether the overall entanglement is of the same dimension as the nets or higher. For example, consider the two types of interpenetration of 2D nets in Figure 3.7. In Figure 3.7a the mean planes of the two (6,3) nets are parallel (indeed, they have the same mean plane) and the overall entanglement is 2D. In Figure 3.7b, however, there are two sets of (4,4) sheets which pass through each other at an inclined angle. This leads to an overall entanglement that is 3D in nature even though the individual nets are only 2D (i.e. the interpenetration leads to a ‘dimensional increase’). Furthermore, 2D parallel nets can also form 3D entanglements if the mean planes of the nets are slightly offset (see section 3.4). In a general sense, if the interpenetrating nets share the same mean plane (or chains share the same direction of propagation), the overall assembly is

Figure 3.7

(a) 2D - 2D parallel interpenetration of (6,3) sheets; (b) 2D inclined interpenetration of (4,4) sheets.

68

Chapter 3

2D or 1D, respectively. If the planes or directions are parallel but not coincident, a dimensional increase usually occurs (although this does not necessarily have to be the case). If they are inclined, then dimensional increase must occur. As a result, a particularly useful nomenclature, particularly for 1D and 2D networks, takes the form mDð=nDÞ ! pD parallel=inclined interpenetration where m,n Z p.6,7 The examples discussed above would be defined as 2D - 2D parallel interpenetration, 2D inclined interpenetration [the ‘- 3D’ part is redundant, as inclined interpenetration of 2D nets (but not 1D nets) must result in a 3D entanglement] and 2D - 3D parallel interpenetration. The ‘/nD’ notation can be added when nets of different dimensions, topologies or chemical makeups are interpenetrating (examples are given in Section 3.6). This is the notation we will use here. For 3D nets (and for 1D/3D or 2D/3D interpenetration) there is not such an easy classification system, but nonetheless there are different ways in which these nets can interpenetrate; as mentioned earlier, we will see examples of this later for diamond and a-Po 3D nets. In an attempt to understand further and classify interpenetrating networks, other nomenclature has arisen. Ciani and co-workers introduced the terms polycatenation, polythreading and polyknotting.13 Polycatenated networks were defined as that subclass of interpenetrating 1D and 2D networks discussed above in which there is a ‘dimensional increase’, i.e. the overall entanglement is of higher dimension that the constituent nets. Polythreaded nets (‘polyrotaxanes’) are mostly those that contain polymeric nets entangled by only rotaxane interactions, although some examples, such as those mentioned earlier in which the links in a polymeric net are provided by rotaxanes, were also included even though there are catenane motifs present between the 0D ‘beads’ of the rotaxane and the rings of the polymeric net. Strictly, these are not interpenetrating as the individual nets could be separated (or are not polymeric); however, we will bend our definition momentarily to discuss a couple of significant polymeric examples in the 1D section (Section 3.3). Polyknotted networks are another term for selfpenetrating nets, which we will deal with in Section 3.7. Another notation that Ciani and co-workers introduced for polycatenated networks is the degree of catenation (Doc).13 When each net interpenetrates with a finite number of other nets (e.g. 1D - 2D parallel), it is defined as n, where n is the number of nets entangled to a single net (assuming all nets are equivalent). If each net interpenetrates with an infinite number of nets (e.g. 1D - 2D inclined), it is given the symbol (a/b/ . . . ), where a, b, . . . are the numbers of rings from other nets which are catenated with a single ring of the first, second, . . . net (in order of increasing value). Hence the 2D nets in Figure 3.7b have a Doc of (1/1). In their extensive crystal structure database reviews, the same group also introduced a number of other notations that related to the crystallographic

69

Interpenetration 10,11

relationships between the interpenetrating nets. These were very useful in the context of the database searches, but they will not be discussed in detail here because, as discussed in the previous chapter, there are many other factors which determine the symmetry of a crystal structure besides the topology. Finally, before we examine the different interpenetration topologies in detail, we should, as we did in the previous chapter, remind the reader once again to be cautious with how the networks are defined. This is particularly relevant here because there are very often significant interactions between interpenetrating networks. Even if the networks are well-defined coordination nets, adjacent nets can be linked by interactions such as hydrogen bonds or p stacking. For example, the ability of interpenetrating networks to form p stacking motifs between the networks has been discussed above and elsewhere.32 For networks defined by weaker interactions, the distinction between intranet and internet interactions can be subjective, so it is important to emphasise again that defining nets, even interpenetrating ones, is most useful when it aids the understanding of the overall structure. Furthermore, the crystal engineer should also keep in mind the interactions between the nets, which can be significant not only in their strength, but also in their importance in determining the overall structure.

3.3 1D Interpenetration Using the nomenclature discussed above, there are five possibilities for interpenetration of 1D nets, all of which are shown in Figure 3.8: 1D - 1D parallel, 1D - 2D parallel or inclined, 1D - 3D parallel or inclined. For 1D nets, parallel or inclined refers to the mean directions of propagation of the individual 1D nets rather than their orientations around these propagation directions, e.g. for planar ladders the orientations of the rungs can be perpendicular but the interpenetration still classed as parallel if the sides of the ladders still all run in the same direction. The same applies for 2D nets; it is the mean planes of the nets that matter and not the relative orientations of the interpenetrating nets within these planes. 1D - 1D parallel interpenetration has only been reported for hydrogen bonding networks.15 The topology shown in Figure 3.9a has been described previously as 1D - 1D parallel interpenetration;8,9 however, this is not strictly correct, as the two chains could be separated. Nonetheless, the entanglement motif (‘polythreading’) is remarkable. A similar argument applies to the chains in Ag2(bix)3(NO3)2, where bix ¼ 1,4-bis(imidazol-1-ylmethyl)benzene, which form only rotaxane-like interactions as shown in Figure 3.9b;33 although strictly not showing 1D - 2D inclined interpenetration,2,9 they are nonetheless intimately entangled and separation is a lot less trivial than for other systems which might be separated by a single pulling action (Section 3.8). 1D - 2D parallel interpenetration can be found in the structures of [Cu2L3(MeCN)2](X)2 solv, where L ¼ 1,4-bis(4-pyridyl)butadiyne and X ¼ PF6, BF4.34 Each 1D ladder in this structure interpenetrates with four others,

70

Figure 3.8

Chapter 3

Different modes of interpenetration for 1D nets: (a) 1D - 1D parallel interpenetration; (b) 1D - 2D parallel interpenetration; (c) 1D - 3D parallel interpenetration; (d) 1D - 2D inclined interpenetration; (e) 1D - 3D inclined interpenetration.

two on either side (Figure 3.10). 1D - 2D inclined interpenetration has been observed for a series of coordination polymers of 1-(1-imidazolyl)-4-(imidazol1-ylmethyl)benzene.35 1D - 3D inclined interpenetration has been observed for a number of structures with ladder-like networks. The structure of [CdL1.5](NO3)2, where L ¼ 1,4-bis(4-pyridylmethyl)benzene, is typical.36 The interpenetration is fivefold, i.e. each ring of each ladder is catenated with four other ladders (Figure 3.11). Finally, 1D - 3D parallel interpenetration has recently been observed in an organic–inorganic hybrid structure (Chapter 8) in which the individual nets are unusual ‘three-legged’ ladders.37

3.4 2D Interpenetration The structure of Ag(tcm) contains discrete layers composed of pairs of interpenetrating (6,3) nets showing 2D - 2D parallel interpenetration, as shown in

Interpenetration

Figure 3.9

71

Entanglement of 1D chains (strictly, not interpenetration) to generate overall (a) 1D and (b) 2D33 motifs.

Figure 3.12a.38 The structure of y-(BEDT-TTF)2Cu2(CN)(dca)2, where BEDTTTF ¼ bis(ethylenedithio)tetrathiafulvalene, similarly shows twofold 2D - 2D parallel interpenetration; however, the interpenetration topology of the two (6,3) nets is different (Figure 3.12b), as were the two (4,4) sheets shown schematically in Figure 3.6.39 The structure of CdL2, where L ¼ 3-[2-(4-pyridyl)ethenyl]benzoate, contains layers of three parallel interpenetrating (4,4) sheets as shown in Figure 3.12c.40 The current record for 2D - 2D parallel interpenetration is the sixfold interpenetrating structure of Ag(TEB)(CF3SO3), where TEB ¼ 1,3,5-tris(4-ethynlbenzonitrile)benzene.41 Although most cases of 2D - 2D parallel interpenetration are based on (6,3) or (4,4) nets which show subtle variations of interpenetration topology and numbers of nets, there are a few notable exceptions. The two 4.82 nets shown in Figure 3.13a belong to the structure of Cd2(dpt)2(NO3)4(MeCN), where dpt ¼ 2, 4-bis(4-pyridyl)-1,3,5-triazine.42 The structure of [Mn(p-XBP4)3](ClO4)2, where p-XBP4¼N,N 0 -p-phenylenedimethylenebis(pyridin-4-one), contains the two interpenetrating nets shown schematically in Figure 3.13b.43 This interpenetration contains a challenge in the topological description. In isolation, the

72

Figure 3.10

Chapter 3

1D - 2D parallel interpenetration of ladders in the structures of [Cu2L3(MeCN)2](X)2 solv, where L ¼ 1,4-bis(4-pyridyl)butadiyne and 34 X ¼ PF 6 , BF4 .

nets are best described as (4,4) sheets, with double bridges in one direction and single bridges in the other. However, this simplification subsequently causes problems in describing the interpenetration as they entangle in such a way that the single bridges pass through the loops created by the double bridges. In the (4,4) simplification this cannot be represented – links pass through the middle of links – and therefore the net is better described as a 6-connected net in which some of the shortest circuits are two-membered. An interesting way in which nets can interpenetrate is through ‘Borromean’ interpenetration.12 The Borromean ring motif contains three discrete rings which are entangled such that they cannot be separated without breaking one of the rings, but if any one ring is removed, the other two are no longer entangled (Figure 3.14a). That is, any two rings are themselves not catenated; it is only through the addition of the third that they become inextricably entangled. Similar motifs have been proposed for 1D nets;12 however, we shall illustrate the concept here with two examples involving 2D nets. The structure of [Cu(tmeda)2 {Ag(CN)2}3]ClO4, where tmeda ¼ N,N,N 0 ,N 0 -tetramethylethylenediamine, can be described in terms of three 2D - 2D parallel interpenetrating (6,3) nets (Figure 3.14b).44 However, although the three nets cannot be separated, removal

Interpenetration

Figure 3.11

73

1D - 3D inclined interpenetration of ladders in the structure of [CdL1.5](NO3)2, where L ¼ 1,4-bis(4-pyridylmethyl)benzene.36

of any one of the nets leaves the other two unconnected. A similar situation applies to 2D nets which can be defined in the structure of [Ag2L3]X2, where L ¼ N,N 0 -bis(salicylidene)-1,4-diaminobutane and X¼NO3, ClO4,45 which show 2D - 3D parallel interpenetration (Figure 3.14c). Each net is penetrated by two others, one above and one below, which are parallel but offset. Again, any two nets are not themselves interpenetrating and only the addition of subsequent nets creates an entanglement that cannot be undone. More conventional 2D - 3D parallel interpenetration still requires that the mean planes of the interpenetrating nets are parallel but not coincident, and in general it occurs for nets that are either highly corrugated and have some degree of ‘thickness’. For example, the structure of [Cu(bpee)1.5(PPh3)]PF6.1.5CH2Cl2 contains highly undulating (6,3) sheets.46 Each sheet interpenetrates with one above and one below it and the offset nature of the sheet planes leads to an overall 3D entanglement (Figure 3.15). In the remarkable structure of AgL(CF3SO3) 0.5H2O, where L ¼ 1,3,5-tris(4-cyanopenoxymethyl)-2,4,6-trimethylbenzene, there are layers of doubly interpenetrating 2D - 2D parallel (6,3) sheets.47 These layers then interpenetrate with adjacent parallel layers to give an overall 3D entanglement which might be termed 2D - 2D parallel 3D parallel interpenetration. 2D - 3D parallel interpenetration of ‘thick’ layers occurs in the structure of Cu4(dca)4(4,4 0 -bipy)3(MeCN)2.48 In this structure, thick bilayers are present

74

Figure 3.12

Chapter 3

(a) Twofold 2D - 2D parallel interpenetration of (6,3) sheets in Ag(tcm),38 viewed in two different directions; (b) topologically different twofold 2D - 2D parallel interpenetration of (6,3) sheets in the polymeric nets in y-(BEDT-TTF)2Cu2(CN)(dca);39 (c) threefold 2D - 2D parallel interpenetration of (4,4) sheets in CdL2, where L ¼ 3-[2-(4pyridyl)ethenyl]benzoate.40

which are composed of two crosslinked (6,3) sheets. Each of these layers interpenetrates with four others (two on each side) and the offset nature of their mean planes leads to an overall 3D entanglement (Figure 3.16). The other way to induce 3D motifs from 2D nets is through inclined interpenetration and, as for parallel interpenetration, there are different topological ways for nets of the same individual topologies to interpenetrate. For example, three different ways for inclined (4,4) sheets to interpenetrate have been defined.49 The structure of Co(dca)2(4,4 0 -bipy) 0.5H2O 0.5MeOH shows the ‘parallel/parallel’ mode (Figure 3.17a).50 Each window in each net is penetrated by a link from another inclined net. The ‘diagonal/diagonal’ mode can be found in the structures of [M(4,4 0 -bipy)2(H2O)2](SiF6), where M ¼ Zn, Cd, Cu; each

Interpenetration

Figure 3.13

75

(a) Twofold 2D - 2D parallel interpenetration of 4.82 sheets in the structure of Cd2(dpt)2(NO3)4(MeCN);42 (b) ‘(4,4)’ sheets interpenetrating in an unusual manner of [Mn(p-XBP4)3](ClO4)2.43

window contains a node of another inclined net (Figure 3.17b).51 In the ‘parallel/diagonal’ mode, the windows in one set of sheets contain links and in the other set of sheets they contain nodes (Figure 3.17c). These are relatively simple examples, but more complicated motifs have been reported. In the structure of CdL2(NO3)2, where L ¼ 1,2-bis(4-pyridyl)hexane, each window is penetrated by three other inclined sheets.52 [Ni(azpy)2 (NO3)2]2[Ni2(azpy)3(NO3)4] 4CH2Cl2 is even more complicated as it contains both (6,3) and (4,4) sheets.53 Each window in a (6,3) sheet is penetrated by two inclined (4,4) sheets, while each window in a (4,4) sheet is penetrated by only one (6,3) sheet. In the structure of Fe(bpb)2(NCS)2 0.5H2O, where

76

Chapter 3

Figure 3.14

(a) Borromean rings; (b) Borromean 2D - 2D parallel interpenetration in the structure of [Cu(tmeda)2{Ag(CN)2}3]ClO4;44 (c) Borromean 2D 3D parallel interpenetration observed in [Ag2L3]X2, where L ¼ N,N 0 bis(salicylidene)-1,4-diaminobutane, X ¼ NO3, ClO4.45

Figure 3.15

2D - 3D parallel interpenetration of undulating (6,3) sheets in the structure of [Cu(bpee)1.5(PPh3)]PF6 1.5CH2Cl2.46

Interpenetration

Figure 3.16

77

2D - 3D parallel interpenetration of ‘thick’ 2D sheets in the structure of Cu4(dca)4(4,4 0 -bipy)3(MeCN)2.48

bpb ¼ 1,4-bis(4-pyridyl)butadiene, there are three inclined stacks of (4,4) sheets interpenetrating in three mutually perpendicular directions.54 The structure of Co2(azpy)3(NO3)4 Me2CO 2H2O goes even further – there are four stacks of mutually inclined (6,3) sheets (Figure 3.18).55 The final example to mention here is the remarkable structure of [AgL2]SbF6, where L ¼ 3-cyanophenyl 4-cyanobenzoate.56 This contains layers of (4,4) sheets showing twofold 2D - 2D parallel interpenetration. These layers are then arranged in two inclined interpenetrating stacks, leading overall to 2D - 2D parallel - 3D inclined interpenetration!

3.5 3D Interpenetration As seen in the previous chapter, there are a myriad of different possible 3D network topologies and sometimes they interpenetrate. We will, for the sake of brevity, concentrate on selected examples of the simpler, more common nets here. For 3-connected nets, the two most commonly interpenetrating nets are (10,3)-a and (10,3)-b. As discussed in the previous chapter, the (10,3)-a net is chiral and as a result two possibilities have been observed to date – interpenetration of nets of all the same chirality (‘chiral interpenetration’) or interpenetration of equal numbers of nets of opposite handedness (‘racemic interpenetration’). The structure of Ag2(2,3-dimethylpyrazine)3(SbF6)2 contains two interpenetrating nets, one of each hand (Figure 3.19).57 In the structure of Zn(tpt)2/3(SiF6)(H2O)2(MeOH) there are eight interpenetrating nets, four each of either hand.58 An amazing hydrogen-bonded structure has been reported to contain 18 interpenetrating nets, nine of each hand!59 By

78

Figure 3.17

Chapter 3

Different interpenetration topologies for inclined (4,4) sheets: (a) parallel/ parallel; (b) diagonal/diagonal; (c) parallel/diagonal.49

contrast, in Ni3(btc)2(py)6(eg)6 B3eg B4H2O, where btc ¼ benzene-1,3,5-tricarboxylate, py ¼ pyridine and eg ¼ ethylene glycol, there are four nets which all have the same handedness.60 Interpenetrating (10,3)-b nets can be found in the structures of Zn3(tpt)2X6. guests, where X ¼ Cl, I (two nets; Figure 3.20),2,61 [Ag2(pyrazine)3](BF4)2 (three

Interpenetration

79

Figure 3.18

Four separate stacks of mutually inclined (6,3) sheets interpenetrating in the structure of Co2(azpy)3(NO3)4 Me2CO 2H2O.55

Figure 3.19

Two interpenetrating (10,3)-a nets in the structure of Ag2(2,3dimethylpyrazine)3(SbF6)2.57

nets)62 and [Cu(4,4 0 -bipy)1.5]NO3 1.25H2O (six nets).63 Four interpenetrating (10,3)-d nets have be reported in the structure of Co(NO3)2L1.5 H2O, where L ¼ 1,4-bis(3-pyridyl)-2,3-diazabuta-1,3-diene.64 Like the (10,3)-a net, (8,3)-a is chiral and interpenetration of four such nets all of the same hand has been seen in the solvated structure of [Cu2(tae)(4,4 0 -bipy)2] (NO3)2, where tae ¼ dibasic tetraacetylethane.65 The (12,3) net is self-penetrating

80

Figure 3.20

Chapter 3

Two interpenetrating (10,3)-b nets in the structure of Zn3(tpt)2X6.2,61

(Section 3.7), but even this net can interpenetrate, as seen in the structure of Co(L)(H2O)2, where L ¼ 2,2 0 -bipyridine-4,40 -dicarboxylate (two nets).66 The most common net to show interpenetration is the 4-connected diamond network. The archetypal examples of this are the structures of M(CN)2, where M¼Zn, Cd (Figure 3.21).26,67,68 The record number of interpenetrating nets observed to date for diamondoid coordination polymers is 10;69 for hydrogenbonded nets it is 11.70 Despite the variation in numbers of possible nets, nearly all interpenetrating diamond nets interpenetrate in the same fashion, which we shall call the ‘normal’ mode. A few structures, however, show ‘abnormal’ interpenetration. One such structure is that of b-Cu(dca)(bpee).48 We can illustrate this unusual interpenetration by examining the relationships between adamantane cavities within the five interpenetrating nets in the structure. First, an adamantane cavity in one net is chosen and then one of the four windows of this cavity is selected. Links from the other interpenetrating nets will penetrate this window; those links are selected and the nodes belonging to each of these links that lie inside the adamantane cavity of the first net are used to define unique adamantane cavities of the four other nets. Two of those adamantane cavities will

Interpenetration

Figure 3.21

81

Two interpenetrating diamondoid Cd(CN)2 nets.26,67,68

have the relationship with the first net’s cavity that is shown in Figure 3.22a. This is what is normally observed between interpenetrating diamond nets – a link from the internal node passes through each of the four windows of the first adamantane unit. However, the other two adamantane cavities have a different relationship with the first. Consider, for example, the two cavities shown in Figure 3.22b – this time one window has no links passing through it, while another has two! Hence the relationship is clearly topologically different. In this structure, the five nets are in fact equivalent and therefore the relationship described above is repeated throughout the structure with the nets swapping roles. In other structures any two nets interpenetrate in the normal fashion and it is only with the addition of subsequent nets that the interpenetration can become topologically different from normal. For example, the eight interpenetrating nets in [Ag(ddn)2]X, where ddn ¼ 1,12-dodecanedinitrile and X ¼ PF6, AsF6, have what has been classed a ‘4 + 4’ mode.69 There are two sets of four nets within which the normal mode is shown; replacement of any one of the nets within a set with one from the other set, however, induces a different interpenetration topology. In these ‘abnormal’ interpenetration modes, the role of the interactions between the nets can be critical – for example, in the structure of Cd(imidazole-4-acrylate)2.1.7H2O an abnormal mode is likely directed by hydrogen bonding interactions between the four interpenetrating nets.71

82

Figure 3.22

Chapter 3

Pairs of interpenetrating adamantane cavities in structure of b-Cu(dca) (bpee), showing (a) a ‘normal’ and (b) an ‘abnormal’ relationship.48

Other 4-connected nets to show interpenetration include the quartz net, found in the structures of M[Au(CN)2]3, where M ¼ Zn, Co.72 The quartz net is chiral; in these structures, all the six interpenetrating nets have the same hand. The structure of [Ag(sebn)2]XF6, where sebn ¼ sebaconitrile and X ¼ P, As, contains four interpenetrating SrSi2 nets,73 whereas [Cu(bpee)(H2O)(SO4)] has two interpenetrating CdSO4 nets.74 Interpenetrating NbO nets can be found in the structures of Cu2(OMe)2L2 0.69H2O, where HL ¼ 9-acridinecarboxylic

Interpenetration

Figure 3.23

83

Interpenetration of a-Po nets: (a) two nets in the structures of M(dca)2(pyrazine);77 (b) three nets in Rb[Cd{Ag(CN)2}3];78 (c) the abnormal twofold interpenetration seen for [Mn(bpea)(H2O)4] (ClO4)2(bpea)4.79

acid (two nets; Cu dimer nodes),75 and in FeL2[Ag(CN)2]2 2/3H2O, where L ¼ 3-cyanopyridine (three nets).76 Interpenetrating PtS nets are a feature of the copper porphyrin structure mentioned in Section 3.1.17 The second most common net topology to show interpenetration is the a-Po net. For example, the structures of M(dca)2(pyrazine), where M ¼ Mn, Fe, Co, Ni, Cu, Zn, contain two interpenetrating nets (Figure 3.23a),77 whereas Rb[Cd{Ag(CN)2}3] contains three nets (Figure 3.23b).78 Like diamond, there is a ‘normal’ mode of interpenetration. In the structure of [Mn(bpea)(H2O)4] (ClO4)2(bpea)4 the nets are defined by both coordination polymers and hydrogen bonds,79 but its unusual interpenetration (Figure 3.23c) is noteworthy as each cubic cavity interpenetrates ten cavities of the other net rather than the usual eight (as seen in Figure 3.23a). Other 6-connected nets are far less common; however, the structure of Eu[Ag(CN)2]3 3H2O has three interpenetrating acs nets (Figure 3.24).80 For other connectivities, the structure of Cu2(2,5-dimethylpyrazine)(dca)4 has two interpenetrating 5-connected nets,81 whereas the structures of M3(bpdc)3

84

Figure 3.24

Chapter 3

The three interpenetrating acs nets of Eu[Ag(CN)2]3 3H2O.80

(4,4 0 -bipy) solv, where M ¼ Co, Zn and bpdc ¼ biphenyldicarboxylate, contain pairs of 8-connected nets.82 There are a number of examples of interpenetrating nets containing differently connected nodes. We have already referred to the structure Cu3(tpt)4(ClO4)3 (Figure 3.3), which has two interpenetrating 3,4-connected boracite nets;16 two interpenetrating twisted boracite nets can be found in the structures of Cu3L2 (H2O)3, where L¼4,4 0 ,400 -s-triazine-2,4,6-triyltribenzoate or s-heptazine tribenzoate.83 The structure of Cu3(BTB)2(H2O)3 9DMF 2H2O, where H3BTB ¼ 4,4 0 ,400 -benzene-1,3,5-triyltribenzoic acid, contains two interpenetrating 3,4connected Pt3O4 nets.18 The structure of Ag(tcm)(phenazine)1/2 also has two 3, 4-connected nets (Figure 3.25a), whereas a change of ligands results in interpenetrating 3,5-connected nets in the structures of Ag(tcm)L, where L ¼ pyrazine, dabco, 4,4 0 -bipyridine (Figure 3.25b).38 The structures of M(tcm)2, where M ¼ Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, Hg, have interpenetrating 3,6connected rutile nets,84 and will be discussed in more detail in Section 3.7, whereas the two interpenetrating 3,6-connected nets in Zn4O(TCA)2 3DMF G, where TCA ¼ 4,4 0 ,400 -tricarboxtriphenylamine and G ¼ 3H2O, EtOH, have the pyr topology.85

3.6 Interpenetration of Nets of Different Composition, Topology and Dimensions We finished the previous section by looking at interpenetration of nets with different connectivity nodes. However, even more variety can be introduced into the interpenetration; nets of different composition, topology or even dimensions

Interpenetration

Figure 3.25

85

Twofold interpenetration of (a) 3,4-connected nets in Ag(tcm)(phenazine)1/1 and (b) 3,5-connected nets in Ag(tcm)L, where L ¼ pyrazine, dabco, 4,4 0 -bipyridine.38

can interpenetrate (‘heterogeneous interpenetration’). We have already seen one example for 2D nets – inclined interpenetration of (6,3) and (4,4) sheets in the structure of [Ni(azpy)2(NO3)2]2[Ni2(azpy)3(NO3)4] 4CH2Cl2.52 We will discuss further examples of heterogeneous interpenetration here. The compound [{Cu(bpey)2(H2O)2}{Cu(bpey)2(NO3)(H2O)}2][NO3]4 bpey 1.33H2O, where bpey ¼ 1,2-bis(4-pyridyl)ethyne, contains three interpenetrating qzd nets, one of which is chemically different to the other (two axial H2O ligands

86

Chapter 3 86

instead of one H2O and one nitrate ligand). In the inorganic structure of K2[PdSe10], the chemical difference between the two interpenetrating diamond nets is even more pronounced – in one the metals are bridged by Se42 links, in the other they are bridged by Se62 anions.87 Nets of different dimensions can also interpenetrate. In the structure of [Cu5(bpp)8(SO4)4(EtOH)(H2O)5](SO4) EtOH 255H2O, where bpp ¼ 1,3-bis (4-pyridyl)propane, (4,4) sheets interpenetrate with inclined 1D chains (Figure 3.26a), generating 1D/2D - 3D inclined interpenetration.88 Each sheet window has one chain through it, while each chain window has two sheets passing through it. 1D/3D interpenetration can be found in [Co(bix)2 (H2O)2](SO4) 7H2O (Figure 3.26b),89 which has 1D chains interpenetrating with a 3D CdSO4 net, whereas [Co(mppe)2(NCS)2] 2[Co(mppe)2(NCS)2] 5MeOH, where mppe ¼ 1-methyl-1 0 -(4-pyridyl)-2-(4-pyrimidyl)ethylene, has two 3D CdSO4 nets interpenetrating with 2D (4,4) sheets (2D/3D interpenetration) (Figure 3.26c).90 There are no coordination polymers examples of different topology 3D nets interpenetrating; however, there are a number of examples of inorganic

Figure 3.26

Interpenetration of different dimensionality nets: (a) 1D/2D - 3D interpenetration seen for [Cu5(bpp)8(SO4)4(EtOH)(H2O)5](SO4) EtOH 25.5H2O;88 (b) 1D/3D interpenetration seen for [Co(bix)2(H2O)2] (SO4) 7H2O;89 (c) 2D/3D interpenetration seen for [Co(mppe)2(NCS)2] 2[Co(mppe)2(NCS)2] 5MeOH.90

Interpenetration

87

structures or compounds containing both coordination polymer and hydrogenbonded nets.2,6,15

3.7 Self-penetration If interpenetrating nets are polymeric versions of molecular catenanes and polythreaded nets are polymeric versions of rotaxanes, then what is the polymeric version of a molecular knot? The answer is a self-penetrating net. This is a single net in which there are smallest circuits that are penetrated by rods of the same net. The smallest circuit stipulation is crucial, as one can find rods passing through rings in most nets if one makes the sizes of the rings large enough. Note also, however, that we mean shortest circuits in the topological sense (Chapter 2) – they may not be the smallest circuit that can be found in the structure. The relationship between self-penetration and interpenetration can be illustrated by a series of related compounds. The structures of M(dca)2 contain single rutile-related networks (Figure 3.27a).91 If the dca ligands are replaced by the similar but slightly larger tcm ligand, two interpenetrating rutile nets result for M(tcm)2 (Figure 3.27b).84 Note that the structure is not ‘fully catenated’,

Figure 3.27

Three related structures: (a) single rutile nets for M(dca)2;91 (b) interpenetrating nets for M(tcm)2;84 (c) a single self-penetrating net for (dca)(tcm).92

88

Chapter 3

i.e. the six-membered shortest circuits are catenated, but not the smaller four-membered ones. If only half of the dca ligands are replaced, then the resulting M(dca)(tcm) compounds form a single net that is closely related to rutile (indeed, it has the same Schla¨fli symbol), but is self-penetrating (Figure 3.27c).92 Rods pass through six-membered rings just as they do in the two interpenetrating nets of M(tcm)2; however, in this case the rods and rings are both part of the same net. A number of fundamental nets are in fact self-penetrating. A classic example is the (12,3) net (Figure 3.28a), found in the structure of Ni(tpt)(NO3)2.93 Two catenating 12-membered rings are highlighted in the figure (note that the Ni

Figure 3.28

The self-penetrating (a) (12,3) (undistorted)93 and (b) (8,4) nets.94

Interpenetration

89

compound has a distorted version of this net, where half the nodes are T-shaped). The (8,4) net is another simple example (Figure 3.28b).94 Nets as high as 8-connected have been reported to be self-penetrating.95

3.8 Other Entangled Systems Finally, although we have focused mainly on interpenetration (and self-penetration) here, there are other ways in which coordination polymers can entangle. These include motifs such as helices96,97 and braids,98 and also other arrangements which we have already mentioned – intercalation of nets (Figure 3.2c), interdigitation (Figure 3.1b) and cloth-like weaving of 1D chains (Figure 3.2a). We have also examined a few cases that are strictly examples of polythreading rather than interpenetration (Figures 3.2b and 3.9). Some further examples of these particularly stand out. In the structure of Zn(phen)L, where phen ¼ 1,10-phenanthroline and H2L ¼ trans-stilbene-4, 4 0 -dicarboxylic acid, the interwoven zigzag chains run in four different directions, giving an overall 3D entanglement.99 A remarkable helical structure has been reported in which nanotubes composed of five helical chains form and each tube is then entangled with four neighbouring tubes.100 Two compounds containing ‘railroad’ 1D nets (i.e. ladder motifs with sidearms projecting from either side) have shown fascinating entanglements whereby the side-arms of the railroads project into the windows of adjoining side-arms in a rotaxane-like fashion (although this should perhaps be classed as interdigitation rather than a polyrotaxane). In both structures the railroads all run in parallel directions, but in one structure the ‘plane’ of the railroads are parallel and produce a 2D entanglement,101 whereas in the other structure the planes are inclined and a 3D entanglement is produced.102 Finally, we shall finish with a series of structures that are interpenetrating. In these structures there are actual rotaxanes acting as bridges in the nets (as shown in Figure 3.2b). These nets then themselves interpenetrate, showing inclined interpenetration of (6,3) sheets,103 or twofold interpenetration of either a-Po nets or more complicated 6-connected nets.104

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CHAPTER 4

Malleability of Coordination Polymers 4.1 Introduction As outlined in earlier chapters, great progress has been made in the deliberate design of coordination polymers through crystal engineering. However, Chapter 2 perhaps gives an overly simplistic and optimistic view of the design process, and the challenges inherent in the crystal engineering of coordination networks should not be overlooked. Coordination networks can, in fact, be very malleable.1 For every example of a deliberately designed structure, there are numerous structures that did not form as expected and the final structures of coordination polymers can be influenced by numerous different factors.2,3 The challenge of deliberate design can be best illustrated by the formation of polymorphic structures. These are materials that contain exactly the same components but have different structures. For example, by variation of reaction time, temperature, concentration and additives (including anions and solvents), four different forms of Cu(2-pytz), where 2-Hpytz ¼ 3,5-di-2-pyridyl-1,2,4triazole can be formed.4 This is despite the fact that in all cases the CuI ions form the expected distorted tetrahedral geometry and the ligands each bond to two metal ions. One form is a molecular tetramer, whereas the other three contain 1D chain motifs. The formation of concomitant polymorphs (i.e. polymorphs which form together in the same reaction mixture) clouds the issue even further and clearly demonstrates the limitations of design: more than one product can be obtained using identical metals, ligands, solvents and reaction conditions. For example, the reaction of CuI, dicyanamide [N(CN)2–] (dca) and 1,2-bis(4-pyridyl)ethene (bpee) in acetonitrile results in the formation of both orange and red crystals.5 Both compounds have the same formula, Cu(dca)(bpee), and so are polymorphs. Coordination Polymers: Design, Analysis and Application By Stuart R. Batten, Suzanne M. Neville and David R. Turner r Stuart R. Batten, Suzanne M. Neville and David R. Turner, 2009 Published by the Royal Society of Chemistry, www.rsc.org

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Figure 4.1

97

(a) Two interpenetrating 2D (4,4) sheets in the structure of a-Cu(dca) (bpee); (b) one of the five interpenetrating diamond nets in the structure of b-Cu(dca)(bpee).5

The orange crystals (designated the a phase) contain pairs of (4,4) sheets showing parallel interpenetration (Figure 4.1a). The red crystals (b phase) contain five interpenetrating diamond nets (Figure 4.1b). To complicate matters further, a third (yellow) phase was formed but its structure could not be determined as the material was amorphous (although the IR spectrum showed the presence of both dca and bpee). Hence the ability to design these materials is clearly restricted by the fact that there are at least two different phases of Cu(dca)(bpee) which are stable and at least a third which also contains CuI, dca and bpee. In the case of Cd(NO3)2(pyrazine),6 the concomitant polymorphism is even more dramatic. Each ‘crystal’ is actually a multiple crystal, containing regions of two different polymorphs whose structures could only be determined by careful extraction of their individual diffraction patterns from the combined pattern measured. A number of these crystals were studied and although individual ones were composed mainly of one polymorph or the other, none was composed of only one phase. The structures of the two forms both contain chemically similar sheets, with the main difference being the stacking arrangement of these sheets.

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4.2 Supramolecular Isomerism Polymorphism, however, is only one part of a wider problem. Different structures can be obtained which are not strictly polymorphs (i.e. they have different formulae) but are nonetheless chemically similar. For example, reaction of Co(NO3)2 and 4,4 0 -bipyridine (4,4 0 -bipy) can result in 1D ladder polymers if the reaction is undertaken in either MeOH–CHCl3 or MeOH–MeCN (Figure 4.2a).7 The resulting compound has the formula Co2(NO3)4(4,4 0 -bipy)3 solv, where solv ¼ MeCN or CHCl3. However, if the same reaction is undertaken in CS2 or H2O, a different structure (a 2D bilayer) is obtained (Figure 4.2b).8 The formula

Figure 4.2

Three different structures formed from the reaction of Co(NO3)2 with 4,4 0 -bipy: (a) 1D ladders;7 (b) 2D bilayers;8 (c) 3D networks.9


99

of the framework remains the same, but in this case the guest solvents are different and so these are not true polymorphs of the first compounds. Furthermore, if the reaction is undertaken in a mixture of MeOH, pyridine and benzene, a third type of structure is obtained, composed of three interpenetrating 3connected 3D networks and intercalated benzene molecules (Figure 4.2c).9 Added to this are a number of other structural motifs which have been observed for metal nitrates and 4,4 0 -bipy, including other 3D architectures,10 and 2D herringbone11 and brick-wall12 networks. Another striking example is the reaction of copper acetate with 2-(2 0 hydroxyphenyl)-D2-thiazoline-4-carboxylic acid in a mixed methanol–ethanol solvent.13 Slow evaporation of this solution allows the successive isolation of four different crystalline solids containing both building blocks. To embrace the notion that a variety of different structures can be formed from similar or identical building blocks (even if the products may actually be chemically different), Zaworotko and co-workers proposed the concept of supramolecular isomers.14,15 This is defined as ‘the existence of more than one type of network superstructure for the same molecular building blocks’. The isomers may be due to differences in the way in which those building blocks interact, the presence or variation of other species (e.g. anions or intercalated solvents) within the structure that are not part of the network itself (but influence its architecture) or the fact that there are different possible network topologies for given types of nodes (architectural isomerism16). Four different types of supramolecular isomerism were identified: Structural. For these isomers the chemical components of the individual networks are identical but they form different networks in the different isomers. These can include true polymorphs, but also other networks where non-network components of the overall crystal structure may differ. Conformational. These are isomers brought about by the use of flexible ligands which can adopt a variety conformations and lead to different structures. Catenane. These isomers differ in the way that the networks interpenetrate (Chapter 3). This might be between structures that have interpenetrating networks and others that do not or between structures that have different numbers of interpenetrating nets or have nets that show different topologies of interpenetration. Optical. Some networks are chiral (Chapter 11) and therefore can form with either handedness. Schro¨der and Champness17 proposed a slightly different series of categories which give rise to different framework topologies (which do not include some other forms, such as conformational, catenane or optical supramolecular isomers): Class I – Polymorphic supramolecular isomers. These are structures containing the same components interacting in the same localised way (i.e. the same supramolecular synthons are present).

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Class II – Structural supramolecular isomers. These are structures containing the same components but which interact through different supramolecular interactions. Class III – Pseudo-polymorphic supramolecular isomers. These are frameworks made of the same components but containing different guest species (which are not part of the framework). These three classes were ‘related to previously used terminology: thus, Class I can be considered as polymorphism, Class II as structural isomerism and Class II as pseudo-polymorphism’.17 Pseudo-polymorphism is a concept which has been applied to molecular species: they are structures that differ chemically only by the nature of the included solvent in the host lattice (although the solvents can induce different arrangements of host molecules).18 The use of this term is still somewhat controversial,19 with the distinction between pseudo-polymorph and the previously used term solvate unclear. These different types of isomerism (and other allied considerations) are best illustrated through examples. The possibility that more than one structure is possible from any given building blocks should be immediately apparent from reading Chapter 2. For example, there are numerous networks containing 3-connecting nodes, including 1D ladders, 2D (6,3) sheets and 3D nets such as (8,3)-c, (10,3)-a, (10,3)-b, etc. Hence even absolute control over the geometry and connectivity of the nodes in a network does not give complete control over the final topology or even the dimensionality produced. For example, two polymorphs (or class I supramolecular isomers) of M(dca)2(pyrazine) have been reported.20 The a phase contains two interpenetrating a-Po networks, whereas the b phase consists of 2D (4,4) sheets (Figure 4.3). Hence the two phases have different topologies and dimensionalities, despite the fact that the bridging modes of the ligands and the metal coordination spheres (octahedral with two trans pyrazine ligands and four dca ligands) are exactly the same in both polymorphs. Interestingly, the a-Cu product can be converted to the b phase by suspension in water over a period of hours, indicating that the b phase is most likely the more thermodynamically favoured phase. Similar polymorphism is observed in the structures of the two phases of Cu(TCNQ), where TCNQ ¼ 7,7,8,8-tetracyanoquinodimethane.21 Both structures contain two interpenetrating 3D nets with similar local connectivities but different overall topologies. These different structures lead to changes in the crystal morphologies and magnetic and charge-transport properties. This elegantly emphasises the importance to the crystal engineer of understanding supramolecular isomerism. The two forms of Ag(4-cyanopyridine)2(BF4) can also be considered as class I supramolecular isomers.22 One form contains four interpenetrating diamondoid nets, in which the ligands bridge between tetrahedral AgI nodes (Figure 4.4a). In the other form, the silver atoms have distorted square-planar geometries and the ligands bridge these metals into 2D (4,4) sheets (Figure 4.4b). In this second form, not only are the metal geometries different from those in the first, but also


Figure 4.3

101

(a) Two interpenetrating 3D a-Po nets in the structure of a-M(dca)2 (pyrazine); (b) a 2D (4,4) sheet in the structure of b-M(dca)2(pyrazine).20

the Ag–nitrile interactions are much weaker. More will be discussed in Section 4.3 on the malleability of metal coordination environments. In the two forms of Cu(SCN)(dpt), where dpt ¼ 2,4-bis(4-pyridyl)-1,3,5triazine, the differences in the building blocks between the two isomers is even more pronounced.23 One form contains a 3D CdSO4-type net in which both the SCN anions and dpt ligands act as 2-connecting bridges (Figure 4.5a). The second form, however, is 1D, with each SCN anion connecting three metals and the dpt ligands being monodentate (Figure 4.5b). Thus these are class II structural supramolecular isomers. The reaction of CuI with bis(4-pyridyl) disulfide (bpds) is a fascinating one that illustrates many of the complexities of multiple product formation.24 Slow diffusion of CuI in MeCN or EtCN and bpds in CH2Cl2 results in four different

102

Figure 4.4

Chapter 4

Two forms of Ag(4-cyanopyridine)2(BF4): (a) diamond nets (an adamantane unit of one of the four interpenetrating nets shown); (b) (4,4) sheets.22

types of crystals. In the Cu-rich region, crystals of (CuI)2(bpds) 0.5 solv, where solv ¼ EtCN or MeCN, form. The networks in these two materials show different 1D topologies. In the EtCN solvate, 1D chains are produced, whereas the MeCN solvate has 1D nets with tubular motifs. These two structures were described by the authors as topological isomers, but were later classified as class III pseudo-polymorphic supramolecular isomers.17 In the ligand-rich region of the reaction, however, different products grew, namely CuI(bpds) solv, where solv ¼ MeCN or CH2Cl2. These two compounds have the same topology, a planar ribbon motif reported previously,25 despite that fact that the solvent varies as it does in the other compounds. Furthermore, another theme is illustrated by this system. Not only can polymorphs be obtained from the one


Figure 4.5

103

Two forms of Cu(SCN)(dpt): (a) 3D CdSO4 net with bridging dpt and m2 SCN ligands; (b) 1D chains with monodentate dpt and m3 SCN ligands.23

reaction system, but also chemically different compounds containing the same building blocks in different ratios and/or with different guest species. This can occur, as in this case, regardless of the ratio of the components used in solution. In fact, even when single products are formed, these can often contain the building blocks in different ratios to the actual reaction solution. Flexible ligands such as 1,2-bis(4-pyridyl)ethane (bpea) can induce supramolecular isomerism due to the variety of conformations that they can adopt. For example, reaction of Co(NO3)2 with bpea results in a series of compounds with the general formula Co2(bpea)3(NO3)4.guests.26 The ligand conformations change and generate different network topologies, depending on the reaction conditions (i.e. reaction solvents and the presence of other guests). A ladder motif is formed when all the ligands adopt the anti conformation, a 1D chain

104

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results from a 1:2 mixture of anti and gauche conformations and a 2D bilayer results from a 2:1 mixture of anti and gauche conformations. Zaworotko called this conformational isomerism;26 however, Schro¨der and Champness classified these as class III pseudo-polymorphic supramolecular isomers as each structure contains different guest molecules.17 They could also be classified as topological isomers. In these structures, the solvent molecules are guests. However, coordination of the solvent to the frameworks themselves can also result in multiple structures being formed. The reaction of CuI, dca and 4,4 0 -bipy in acetonitrile results in the formation of yellow–orange dichroic crystals of Cu2(dca)2(4,4 0 bipy)(MeCN)2 0.5(4,4 0 -bipy).5 The structure consists of 1D ladders with guest 4,4 0 -bipy molecules (Figure 4.6a). However, these crystals appear to be only a kinetic product of the reaction as they disappear after 1–3 days and are replaced by dark red crystals. This more thermodynamically favoured product, Cu4(dca)4(4,4 0 -bipy)3(MeCN)2, is composed of thick 2D bilayers than interpenetrate in a 2D - 3D parallel fashion (Figure 4.6b). What is significant here, in the context of this chapter, is that two products are formed, one kinetically favoured and the other thermodynamically favoured. Furthermore, the two products contain exactly the same ratio of the CuI, dca and 4,4 0 -bipy building blocks and differ in their formulae only in the ratio of coordinated solvent molecules (which, being the reaction medium, is in huge excess anyway). A similar situation is observed for two carboxylate structures, Co2(BPTC) (H2O)5 G (MOF-501) and Co2(BPTC)(H2O)(DMF)2 G (MOF-502), where BPTC ¼ biphenyl-3,3 0 ,5,5 0 -tetracarboxylate and G ¼ guest solvent molecules.27 The two compounds were obtained from solvothermal reactions in a mixture of DMF, EtOH and H2O. It was noted that the longer the reaction proceeded, the greater was the proportion of MOF-502 produced. Furthermore, crystals MOF501 could be directly converted into MOF-502 by re-immersion in the mother liquor and subjecting them to further solvothermal conditions. The structures of the two compounds were very different – MOF-501 has NbO topology, whereas MOF-502 shows PtS topology. It is not just the connectivity of the frameworks that can cause the formation of different structures – they can also differ in the way in which the individual polymeric nets interact. As discussed in the previous chapter, the (4,4) sheets of Cu(tcm)L, where tcm ¼ tricyanomethanide [C(CN)3] and L ¼ hexamethylenetetramine (hmt), bpee or 4,4 0 -bipy, show interdigitation, intercalation of solvent and uncoordinated ligands or interpenetration, respectively.28 Although the sheets in these structures are, obviously, chemically different, it would nonetheless have been difficult to predict how the sheets would arrange themselves even if the individual sheet topology had been predicted. An example of chemically identical networks giving different structures is Cd(CN)2 versus Cd(CN)2 CCl4. As described in the previous chapter, in the former structure there are two interpenetrating diamondoid nets,29 whereas in the latter there is only one net.30 Furthermore, the comparison between the structures of Cd(CN)2 and Cu[4,4 0 ,400 ,400 0 -tetracyanophenylmethane]BF4 xC6H5NO2 is remarkable. The former compound, with a node-to-node


Figure 4.6

105

(a) The ladder structure of Cu2(dca)2(4,4 0 -bipy)(MeCN)2 0.5(4,4 0 -bipy); (b) the 2D bilayer structure of Cu4(dca)4(4,4 0 -bipy)3(MeCN)2.5

distance of 5.46 A˚, has two interpenetrating nets, whereas the latter, with an internodal distance of 8.86 A˚, has only one net.29 These two Cd(CN)2-based structures are catenane isomers. Other examples include a series of a-Po-related carboxylate networks which occur both as highly porous single nets and as doubly interpenetrating structures.31 Ciani and coworkers reported a series of coordination polymers containing [Ag(ddn)]1, where ddn ¼ dodecane-1,12-dinitrile, diamond networks.32 With NO3 counterions, 10

106

Chapter 4

ClO4–

interpenetrating nets are formed, with four nets are formed and with PF6 or AsF6 eight interpenetrating nets with a different interpenetration topology are formed. This last point raises another form of catenane isomerism – structures can also differ in their topology of interpenetration. This was discussed extensively in the previous chapter and simple examples include 2D structures that show either parallel or inclined interpenetration or 3D diamond nets (as in the above example) which show different modes of interpenetration. Examples were also given in the previous chapter of another manifestation of the difficulties of deliberate design – compounds in which more than one type of polymeric net is formed in the same structure. For example, [Mn(dca)2 (H2O)2] H2O contains both 1D chains and 2D sheets which have exactly the same formula and form simultaneously within the one structure.33 The structure of Cu(4-pyrdpm)(hfacac), where 4-pyrdpm ¼ 5-(4-pyridyl)dipyrromethene and hfacac ¼ hexafluoroacetononate, contains both discrete molecular hexagons and helical 1D chains (Figure 4.7).34 Examples were given in Section 3.6 of interpenetrating nets with different compositions, topologies and dimensions.

Figure 4.7 The 1D chain and molecular ring motifs in the structure of Cu(4-pyrdpm) (hfacac).34


107

Finally, even structures that should be closely related can differ significantly. For example, given that the tcm anions in the structures of Cu(tcm)L, where L ¼ 4,4 0 -bipy, bpee, coordinate to only two metals each, one might expect the Cu(tcm)L structures to be very similar to the Cu(dca)L analogues.5,28 However, as described in this chapter, the structures formed are very different. So why do different structures form in similar or even identical reactions and what are the factors that can cause the final structure formed to differ from that targeted through the network approach described in Chapter 2 (or at least need to be taken into consideration)? The rest of this chapter will address these sometimes subtle influences on structure. These are sorted rather crudely by type; however, the boundaries often overlap between categories and more than one class of influence can act on a given structure. A number of these influences have also been encountered already in the discussion above.

4.3 Malleability of Building Blocks The network design approach assumes predictable metal and ligand geometries and connectivities; however, this is often not the case. Metals can have variable geometries, ligands can bind differently to expectation and flexible ligands can be hard to control. Metal geometries in particular can be very malleable, depending on the nature and oxidation state of the metal. An excellent guide to typical transition metal geometries can be found in a paper by Moore and co-workers35 and its associated website.36 This paper contains the results of a systematic survey of coordination geometries of transition metals, across their oxidation states, using the Cambridge Structural Database. It showed that metals adopt a variety of geometries, with some being more variable than others. For example, FeII (Figure 4.8a) overwhelmingly favours octahedral coordination geometry (ca. 74% have this geometry; 94% have either octahedral or tetrahedral geometry), whereas AgI (Figure 4.8b) contains a much greater variety of geometries, with the most common (linear) occurring in less that half of the structures (and four different geometries representing 91% of the total). One would expect that a similar survey of lanthanoid metal geometries would uncover an even greater variety. An example of this malleability of building blocks can be found in the structures of M(dca)2.37–41 There are two polymorphs formed. The a phase, formed by CrII, MnII, FeII, CoII, NiII and CuII, contains octahedral metals, 3-connecting dca ligands and 3D rutile network topologies (Figure 4.9a). The b phase, formed by CoII and ZnII, contains tetrahedral metals, 2-connecting dca anions and 2D (4,4) sheet topologies (Figure 4.9b). One can note a number of things about this system: 1. ZnII does not form the a phase like the rest of the metals, even though octahedral geometry is common for this ion. Furthermore, in the related M(tcm)2 structures, only one polymorph is formed and the metals are all octahedral, including in the ZnII derivative.

108

Figure 4.8

Chapter 4

The distribution of coordination geometries in the Cambridge Structural Database for (a) FeII and (b) AgI. Reproduced with permission from http:// sulfur.scs.uiuc.edu/OldPage/Intro%20Page/periodictable/pte.html.36

2. The CoII ion, for which octahedral and tetrahedral are common geometries, forms both isomers, although the b phase is only formed by depyridination of Co(dca)2(pyridine)2.39 3. Doping of CoII into the ZnII structure can be done with the resulting solid solutions retaining the b structure rather than converting to the a phase.39 However, only small levels of doping have been reported to date.


Figure 4.9

109

(a) Rutile structure of a-M(dca)2; (b) 2D sheet structure of b-M(dca)2.37–41

4. Not only is the metal geometry malleable, but so is the ligand connectivity. In one structure the dca binds three metals, in the other only two. The last point illustrates that even rigid ligands can adopt a variety of coordination modes. This can occur through mechanisms such as only some of the donor atoms coordinating or donor atoms possibly connecting to more than one metal. The dca anion can adopt a large number of coordination modes (Figure 4.10), although the m1,5 and, to a lesser extent, the m1,3,5 modes shown in the structures described above are the most common.42 The azide anion also adopts a number of coordination modes, most commonly the m1,3 and m1,1 bridging modes (among others). These two modes are particularly important as they lead to different magnetic interactions between the bridged metal ions (Chapter 9). The former mode generally produces antiferromagnetic interactions between metals, whereas the latter favours ferromagnetic exchange.43

110

Chapter 4

N N

N N

N

µ1

µ3

N

N

N

N

N

N

µ1,5

µ1,3

N

N

N

N

N

µ1,3,5

N

N N

µ1,1,3,5

N µ1,1,5

N

Figure 4.10

N

N

N µ1,1,3,5,5

The observed coordination modes of the dca ligand.42

The carboxylate functional group can also show a great variety of coordination modes, from binding in either a monodentate or chelating fashion to a single metal to connecting two or more metals in a variety of ways. Ligands can also, of course, be inherently flexible, and this obviously leads to the possibility of different structures because of different possible conformations of ligands. An example of this was discussed above in the context of conformational isomerism. However, even more rigid ligands can have different conformations of their donor atoms, depending on their geometry. Thus, whereas 4,4 0 -bipy has only one possible disposition of donor atoms, the related 3,3 0 -bipy can have a variety of different dispositions of the donor pyridyl nitrogens, depending on the torsion angle between the two pyridine rings. Such changes in confirmation have been observed in silver salts of 3,3 0 -dicyanodiphenylacetylene.44 Of course, there are a number of other considerations which need to be taken into account for the ligands. The donor ability of the binding sites of the ligand is important, as are their sterics – for example, it is very difficult sterically to fit six pyridyl donors around a transition metal,45 whereas nitrile donors do not have this problem. The structure formed by the reaction of Ag(tcm) with pyrazine is very different to that obtained with the bulkier phenazine.46 Even when the sterics do not directly affect the coordination geometry, they can still affect the structures. Ag(Mtta), where Mtta ¼ 5-methyltetrazolate, has a 2D (4.82) topology, whereas Ag(Etta), where Etta ¼ 5-ethyltetrazolate, has a 3D (10,3)-a topology


Figure 4.11

111

(a) The (4.82) 2D structure of Ag(Mtta); (b) the (10,3)-a 3D topology of Ag(Etta).47

(Figure 4.11).47 Even the size of non-bridging co-ligands can be important – Ni(phen)(oba) 0.5H2O, where phen ¼ 1,10-phenanthroline, oba ¼ 4,4 0 -oxybis (benzoate), contains 1D chains whereas Ni2(H2O)(TATP)2(oba)2 2H2O, where TATP ¼ 1,4,8,9-tetranitrogen-trisphene, contains 2D sheets.48

4.4 Synthetic Approach The synthetic approach towards coordination polymers can have a marked effect on the structure obtained. For example, slow diffusion of solutions of Cd(NO3)2 and pyrimidine (pym) resulted in crystals of [Cd(NO3)2(pym)2] and [Cd(NO3)2(pym)(MeCN)].49 However, when the reactions were repeated under rapid mixing and precipitation conditions, only the former was obtained {sometimes mixed with [Cd(NO3)2(pym)(H2O)]}; the latter could not be made

112

Chapter 4

under these conditions. A slow diffusion reaction of 1-methyl–1 0 ,2-bis(4pyridyl)ethane (mpe) and Co(NCS)2 gives crystals of one polymorph of Co(mpe)2(NCS)2 followed, after several days, by a second polymorph.50 The likelihood that the first product is a kinetically favoured one and the second is more thermodynamically stable was confirmed by a series of follow-up reactions. Crystals of the former could be made exclusively from high-concentration solutions diffused rapidly over a wide area, whereas the latter is the sole product of slowly diffused dilute solutions. In addition to concentration effects, reaction temperature can also be important. Three different polymorphs of Cu2(OAc)2(HQ) could be obtained from the reaction of copper acetate with hydroquinone (HQ), each made pure through variation of concentration and temperature.51 The reaction of silver salts with a flexible tetranitrile donor ligand favours structures containing the ligand in the trans conformation at 0 1C, but structures with cis conformation ligands at 30 1C.52 The effect of temperature can be even more significant if solvothermal techniques are used. In solvothermal reactions, the reaction mixture is usually sealed in a vessel and heated under autogenous pressure above the boiling point of the solvent, typically 120–260 1C for hydrothermal reactions.53–55 The properties of the solvent can also change markedly. For example, the viscosity of water decreases and the solubility of many materials increases significantly. Metastable kinetic phases can also be favoured under these conditions. There are a number of examples of solvothermal reactions giving different products at different temperatures or products different to bench-top reactions. Using these techniques, five different cobalt succinate phases can be obtained from reactions that range in temperature from room temperature to 250 1C.56,57 Hydrothermal reaction of NiCl2, 3-(3-pyridyl)acrylic acid (pyaraH) and NaOH at 180 1C for 24 h gives crystals of a-Ni(pyara)2(H2O)2, which has a 3D (8,4) net. However, if the reaction is repeated at only 150 1C, the b phase is obtained, which consists of 1D chains (Figure 4.12).58 Solvothermal reaction of nickel acetate and dicyclohexane-1,4-dicarboxylic acid results in different products at 120, 140 and 160 1C; higher temperatures appear to favour higher hydroxide contents in the products.59 A similar phenomenon occurred in the reaction of CoCl2 with 3,4-pyridinecarboxylic acid (3,4-pydaH) and NaOH.60 After 24 h at 150 1C, crystals of [Co(3,4-pyda)(H2O)2] H2O were formed. If the reaction was then continued at 180 1C for a further 24 h, [Co3(OH)2(3,4-pyda)2(H2O)2] formed. Repeating this reaction at either 155 or 175 1C resulted in only the former compound. As seen in the reaction of copper nitrate and N,N,N 0 ,N 0 tetrakis(4-carboxyyphenyl)phenylene-1,4-diamine in DMSO, as little as a 5 1C difference in reaction temperature can result in different products.61 The reaction of copper nitrate and dicyanonitrosomethanide (dcnm) is an interesting one as it illustrates nicely two important points about solvothermal reactions.62 First, room temperature reactions in methanol give the discrete complex Cu(mcoe)2(MeOH)2, where mcoe ¼ methyl 2-cyano-2-(hydroxyimino) ethanimidate, whereas solvothermal reactions (80 1C, 2 days) give the 2D polymer Cu(mcoe)2 (Figure 4.13). Thus different products are obtained for bench-top


Figure 4.12

113

(a) The 3D (8,4) structure of a-Ni(pyara)2(H2O)2; (b) the 1D chain structure of b-Ni(pyara)2(H2O)2.58

versus solvothermal reactions. Second, the mcoe ligand results from the nucleophilic addition of methanol to a nitrile group of the dcnm. Although in this example it occurs in both reactions, in situ synthesis of new ligands is a regular occurrence in solvothermal reactions, as shown by two recent reviews.63,64 For example, the conversion of nitrile groups into carboxylic groups occurs fairly readily under solvothermal conditions and this has been used to prepare a number of interesting coordination polymers containing pyridylcarboxylate ligands from pyridylnitrile precursors.65 Indeed, the authors list a number of reasons why this method is preferable to direct reactions with pre-made pyridylcarboxylates.66 Other authors have shown that nitriles give 1,2,4-triazoles in the presence of ammonia67 or tetrazoles in the presence of azide.68 Another factor which can affect the products is the pH of the reaction solution. In the solvothermal reaction of copper nitrate, 4,4 0 -bipy and benzene1,2,3-tricarboxylic acid (1,2,3-btcH3), different amounts of added base lead to the in situ generation of different decomposition products of 1,2,3-btcH3, which are subsequently incorporated into the products.69 No added base results in no ligand decomposition. However, even where there is no ligand decomposition, pH can be important, especially for e.g. carboxylate-based ligands.70 Another alternative synthetic route for the synthesis of coordination polymers is through the controlled decomposition of other polymers. For example, in Section 4.3 we discussed the synthesis of b-Co(dca)2 from loss of pyridine from Co(dca)2(pyridine)2.39 It is worth noting that (a) this route forms the

114

Figure 4.13

Chapter 4

(a) Molecular structure of Cu(mcoe)2(MeOH)2; (b) sheet structure of Cu(mcoe)2.62

b phase and not the a phase and (b) the b phase has yet to be synthesized by any other route. Furthermore, Na¨ther and co-workers have carried out extensive studies on the controlled decomposition of ligand-rich coordination polymers as a synthetic route to new ligand-poor phases which may be metastable.71 Another solid-state synthetic technique which as been applied is mechanochemical synthesis through e.g. ball milling or grinding of the solid precursors directly, in the absence of a solvent.72,73 For example, grinding together of silver acetate and 1,4-diazabicyclo[2.2.2]octane (dabco) results in the formation of the 1D polymer Ag(dabco)(OAc).74 The structure was confirmed by comparison of the powder diffraction pattern with that determined from solutiongrown single crystals. Reaction with zinc chloride instead of silver acetate, however, resulted in different phases being formed from solution versus mechanochemical approaches. These techniques have even been applied recently to the synthesis of microporous coordination polymers.75


115

Finally, one might expect that other, more unusual, synthetic approaches might result in different structures. For example, single crystals of a weakly bound organometallic polymer were recently obtained from the gas-phase reaction of 1,4-bis( p-tolylethyl)benzene and rhodium trifluoroacetate.76 In other work mercury coordination polymer ‘nanotubes’ have been synthesized at a water–chloroform liquid interface.77

4.5 Solvent Effects As we discussed above when talking about supramolecular isomerism and pseudo-polymorphism, the reaction solvent can have a great effect on the product formed. Usually this occurs in one of two ways: either the solvent coordinates to the metal and thus affects the number and disposition of coordination sites available for bridging ligands or it acts as a guest within the structure. The cadmium cyanide system is a nice illustrative example of these effects. As mentioned earlier, unsolvated Cd(CN)2 contains two interpenetrating diamond nets (Figure 4.14a),29 whereas the solvated Cd(CN)2 CCl4 contains only one diamond net,30 with the solvent molecules occupying all the adamantane cavities of the single net and thus preventing formation of the second net (Figure 4.14b). By contrast, in the structure of Cd(CN)2 0.5(n-Bu2O H2O), the cadmium cyanide framework adopts a lonsdaleite-like topology (Figure 4.14c).78 In Cd(CN)2 2/3H2O t-BuOH, one-third of the cadmium atoms have octahedral geometries (with two trans-coordinated waters), whereas the rest are tetrahedral.79 This leads to a 3D net with moganite topology (Figure 4.14d). In fact, the cadmium cyanide lattice shows a remarkable ability to adapt its structure depending on guest solvents (both coordinated and uncoordinated),80 with even a cursory search of the Cambridge Structural Database revealing at least 60 distinct Cd(CN)2 structures containing solvent molecules. The structural changes induced by solvents can vary according to the system. The reaction of 1,2-bis(4-pyridyl)ethane with Cu(PF2)2 in a variety of solvents gives a series of structures which all contain essentially similar 1D chain motifs.81 The chains, however, are flexible enough to allow different associations of the solvent, depending on their sterics. Solvents such as acetone, DMF and THF coordinate directly to the axial sites of the octahedral copper atoms, whereas in the THF, dioxane and 2-PrOH solvates water molecules occupy these sites and the other solvent molecules lie within the lattice. By contrast, in the metal–dicyanamide system, reaction of metal and ligand in water results in unsolvated 3D M(dca)2 networks [with the exception of a hydrated Mn phase, Mn(dca)2(H2O)2 H2O, also observed], whereas similar reactions in MeOH, DMF or pyridine result in the formation of 1D chains of M(dca)2(solv)2, where solv ¼ MeOH, DMF, pyridine.42 Reaction of ZnSiF6 and 4,4 0 -bipy in water results in a hydrated structure containing interpenetrating square grids,82 whereas when anhydrous DMF is used an unsolvated structure results in which the grids no longer interpenetrate

116

Figure 4.14

Chapter 4

(a) Two interpenetrating diamond nets of Cd(CN)2;29 (b) an adamantane unit of the single diamond net of Cd(CN)2 CCl4, with a disordered CCl4 guest shown;30 (c) the lonsdaleite cadmium cyanide net in Cd(CN)2 0.5(nBu2O H2O);78 (d) the moganite topology coordination net in the structure of Cd(CN)2 2/3H2O t-BuOH.79 Intercalated solvent molecules have been omitted for clarity in (c) and (d).

(Figure 4.15).83 The Co–SCN–1-methyl-1 0 -(3-pyridyl)-2-(4-pyridyl)ethane system results in a number of different supramolecular isomers, where the variation in structure is proposed to be due to the coordination ability of the individual solvents and the formation of solvent-coordinated intermediates.84 The importance of the coordination ability of the solvent is reinforced by a study of the reaction of CdII and ZnII with 3,6-bis(pyridin-3-yl)-1,2,4,5-tetrazine in mixtures of CH2Cl2 and a variety of alcohols.85 In the structures formed, 2-PrOH does not coordinate, EtOH coordinated in the ZnII structure and MeOH coordinates in both. The solvent dependence can lead to interesting effects. For example, slow diffusion of an aqueous silver solution on one side of an H-tube with a THF


Figure 4.15

117

Solvent-dependent structures from the reaction of ZnSiF6 and 4,4 0 -bipy in (a) water (interpenetrating (4,4) sheets)82 and (b) anhydrous DMF (non-interpenetrating (4,4) sheets).83

solution of a dipyridyl ligand on the other side results in formation of two different unsolvated polymorphs of [AgL]ClO4.86 On the metal-rich side of the H-tube, the structure formed is a molecular ring, whereas on the ligand-rich side helical chains are formed. Although the formation of different structures is ascribed to concentration effects, it could well be due to the different solvent environments from which the two forms crystallise. The products obtained from the reaction of zinc nitrate and benzene-1,4-dicarboxylic acid (H2bdc) in DMF or DEF depend on the solvent purity.87 Fresh DEF resulted in the formation of [Zn4O(bdc)3] 3DEF, whereas reaction in DEF that had been in the laboratory for several weeks resulted in [NH2Et2]2[Zn3(bdc)4] 2.5DEF. The NH2Et+ 2 is a result of hydrolysis of DEF. A similar situation occurs for DMF: hydrolysis of the solvent over time leads to an increase in the concentration of the hydrolysis products and thus the formation of a different coordination polymer structure when used as the reaction medium.

118

Chapter 4

The effect of the solvent can also be important for the properties of the coordination polymers formed. For example, both the structures and properties of Mn(dcbp) 1/2DMF and Mn(dcbp) 2H2O, where H2dcbp ¼ 4,4 0 -dicarboxy-2,2 0 -bipyridine, differ significantly; the former binds DMF irretrievably, whereas the latter binds water reversibly while maintaining crystallinity.88 It is also worth mentioning that although the solvent can template the network structure, the reverse can also occur, that is, framework structures can template unusual solvent structures. For example, in the structure of [Co4 (dpdo)12][H(H2O)27(MeCN)12][PW12O40]3, where dpdo ¼ 4,4 0 -bipyridine-N,N 0 dioxide, there are large H1(H2O)27(MeCN)12 solvent clusters formed in the spaces within the 3D coordination polymer (Figure 4.16).89 A great variety of other interesting water clusters have been reported to form in crystal lattices.90 The solvent, however, does not necessarily have to be incorporated into the structure to have a directing effect. A recent study found that the particular unsolvated polymorph of Cu(2-ethylimidazolate) formed depended on the solvent polarity – polar solvents favoured the formation of helical chains whereas less polar solvents favoured chains with a zigzag conformation.91 One interesting class of solvents which has been underutilised in coordination polymer synthesis is ionic liquids.92 Use of these unusual solvents can result in unusual products. Reaction of CuI and 2,4,6-tri(4-pyridyl)-1,3,5-triazine in an ionic liquid results in two interpenetrating nets with a-C3N4 topology.93 The same reaction in CHCl3–C2H2Cl4–MeCN, however, results in pairs of

Figure 4.16

A large H1(H2O)27(MeCN)12 cluster observed in the structure of [Co4(dpdo)12][H(H2O)27(MeCN)12][PW12O40]3.89 The (H2O)26 cage contains a water in the middle and the water molecules hydrogen bonded to the acetonitrile molecules have half occupancy.

119

Malleability of Coordination Polymers 94

interpenetrating nets with the boracite topology. Two iron oxalatophosphates have been reported which have been synthesised from ionic liquids under ionothermal conditions but could not be replicated under hydrothermal conditions.95 Chiral ionic liquids have also been used to induce chirality in coordination polymers assembled from achiral components, even though the chiral anion component of the ionic liquid is not incorporated into the structure.96

4.6 Other Guests Not only can solvent molecules be included within the cavities and channels of a coordination polymer, but also other molecules can be trapped. These may be building blocks used to construct the framework or other neutral molecules. As an example of the former, the structure of [Cu(tcm)(bpee)] 0.25 bpee 0.5MeCN intercalates molecules of one of the framework building blocks (bpee), in addition to solvent molecules, within the channels created by the (4,4) sheets of the framework.28 Zaworotko and co-workers reported the inclusion of large aromatic species such as pyrene and naphthalene into metal–4,4 0 -bipyridine networks.97 The effect of intercalated species can be seen in the reaction of Cd(CF3SO3)2 and 4,4 0 -bipyridine.98 In the presence of 4-methoxy-2-nitroaniline, 2D (4,4) sheets are obtained which intercalate the guest molecules, whereas in the absence of this guest 1D chains are formed.

4.7 Counterions Many coordination polymers are charged, and these require counterions. These counterions, however, are not always innocent bystanders. They may direct the structural topology by occupying coordination sites on the metals or by acting as charged templates and lie in the cavities and channels of the structure. The properties of the counterions that may direct the structure include size, shape (geometry, symmetry), coordination ability, ability to hydrogen bond (and the strength of those interactions), ability to form other weak interactions (such as p–p interactions) and charge density. The ability of anions to template structures is well known in supramolecular chemistry,99 and coordination polymers show similar behaviour, even when the anions are not intended to be bridging building blocks (as are anions such as cyanide, azide and dicyanamide, which we will ignore here). For example, the reaction of AgBF4 or AgPF6 with 1,4-bis(4-pyridyl)-2,3,4,5-tetrazine (4pytz) results in solvated 1D chains which associate into parallel pairs via Ag Ag and p p interactions.100 However, reaction with AgNO3 generates a vastly different, unsolvated structure in which weak Ag ONO2 interactions connect adjacent inclined Ag(4pytz) chains to generate an overall 3D structure containing helical AgNO3 chains (Figure 4.17). A similar manifestation of the coordination ability of nitrate exists in the structures of AgLX, where L ¼ bis (3-pyridyl)dimethylsilane.101 For X ¼ CF3SO3 and PF6, the 1D AgL chains are

120

Figure 4.17

Chapter 4

The crosslinking of 1D Ag(4pytz) chains by AgNO3 helices to generate a 3D net in the structure of Ag(4pytz)NO3.100

linked into 2D sheets by argentophilic interactions, whereas for X ¼ NO3 the anion coordinates and thus disrupts the Ag Ag interactions. The anions, however, do not need to coordinate to affect the structure. Ciani and co-workers reported the effects of the anion choice on a number of systems.102 For coordination polymers of silver and 1,3-bis(4-pyridyl)propane, the structures obtained depended on the size and symmetry of the anions. Nitrate gave pairs of parallel interpenetrating (4,4) sheets, BF4 and ClO4 gave two interpenetrating diamond nets, PF6 and AsF6 gave layers of both single and doubly interpenetrating (4,4) sheets in the same structure, and SbF6 gave single (4,4) sheets. A similar variety of structures exist for Ag(sebn)2X, where sebn ¼ sebaconitrile (decane-1,10-dinitrile).103 Eight interpenetrating diamond nets are observed for BF4 and ClO4, four interpenetrating SrAl2 nets are seen for PF6 and AsF6, and SbF6 and CF3SO3 give 2D layers that show 2D - 3D parallel interpenetration. Even if the anion does not change the net topology, it can still alter the overall structure. The structures of [Ag(ddn)]X, where ddn ¼ dodecane-1, 12-dinitrile and X ¼ NO3, PF6, AsF6, ClO4, all contain diamondoid networks.104 The first anion, however, results in 10 interpenetrating networks, the second and third give eight interpenetrating nets and the last gives only four interpenetrating nets. Furthermore, the tenfold and fourfold interpenetrating nets show the normal mode of interpenetration for diamond nets (Chapter 3), whereas the eightfold interpenetrating system shows an abnormal ‘4 + 4’ mode. The variety of interpenetration numbers is related to the flexibility of the ligand. A similar situation occurs for [Ag2(1,4-dithiane)3]X2, where X ¼ BF4, CF3SO3, NO3.105 All structures contain 2D (6,3) nets, but the nitrate structure is the only one to show interpenetration (of the inclined variety).


Figure 4.18

121

(a) The chains formed by the reaction of di-3-pyridinylmethanone with Cd(BF4)2; (b) the sheets formed by reaction of the same ligand with Cd(ClO4)2 or Cu(X)2, where X ¼ BF4, ClO4.106 The trans-coordinated anions have been omitted for clarity.

The effect of the anions is not, however, always universal. In the structures obtained from the reaction of di-3-pyridinylmethanone with the ClO4 and BF4 salts of CuII and CdII, only the Cd(BF4)2 salt has 1D chains; the other three have (4,4) sheets (Figure 4.18).106 That is, the change in anion changes the structure for CdII but not CuII. On the other hand, the anion can be a very significant factor in the structure. Reaction of AgX, where X ¼ CF3SO3, NO3, ClO4, with ethylenediaminetetrapropionitrile results in three different structures.107 Interestingly, some structures can be converted from one to the other by immersion of the crystals in a solution of the appropriate anion. The CF3SO3 and NO3 structures can be reversibly interconverted from one to the other, but they themselves can only be converted into the ClO4 salt and not the reverse. The importance of anions to coordination polymer structure and stability can even be used to separate mixed anions from solution. Reaction of ZnII and the ligand N,N 0 -bis(m-pyridyl)urea, which contains hydrogen bond donor sites, in solutions containing mixtures of Cl, Br, I, SO4, NO3 and ClO4 produced solids containing only halide anions and no oxoanions.108 Furthermore, discrimination between the halides was also observed. Finally, in all these structures the anions are incorporated into the structure. However, the anions can even affect the structures of coordination polymers in which they are not included. Reaction of CuII with benzene-1,3,5-tricarboxylic

122

Chapter 4

acid in a series of ionic liquids results in different structures that depend on the anion of the ionic liquid, even through the anions are not incorporated into the structure.109 The bromide ionic liquid results in one structure, the bis(trifluoromethyl)sulfonylamide (Tf2N) ionic liquid results in two other structures and a fourth results when these two ionic liquids are used together in a 1:1 ratio. Cations can also readily template anionic coordination polymers. A classic example of this is the cation templation of metal oxalate networks.110 Tetraalkyl- or tetraarylammoniums or -phosphates generate 2D (6,3) sheets. However, when chiral [M(2,2 0 -bipy)3]21 metal complexes are used as the counter cations, chiral (10,3)-a 3D networks result. Both structure types show longrange magnetic ordering; however, the necessity for cations in the structure can also be used to incorporate other properties into the materials along with the magnetic ordering, through the careful choice of cation used. Properties include photochromism,111 non-linear optical activity,112 chirality,113 spin crossover behaviour114 and metallic conductivity.115 Anionic M(dca)3 networks are also templated by choice of cation.42 The cations Ph4P1 and Ph4As1 give 2D (4,4) anionic layers of M(dca)3, where M ¼ Mn, Fe, Co, Ni (Figure 4.19a).116–120 These cations lie in layers which alternate with the anionic coordination polymers sheets and pack such that there are supramolecular interactions known as ‘multiple phenyl embraces’ between them.121 In this particular case, the cations lie in columns within which each cation is engaged in an orthogonal fourfold phenyl embrace (O4PE) with each neighbour on either side. These O4PE interactions consist of a concerted array of four edge-to-face C–H p interactions between four phenyl groups, two from each adjoining cation. These supramolecular interactions are important to the stability of the overall structure. If one of the phenyl groups is replaced by a methyl group (as in MePh3P1), then completely different structures result for (MePh3P)[M(dca)3], where M ¼ Mn, Fe, Co, Ni.116,119 These now contain 3D M(dca)3 networks in which the cations lie in pairs in the cavities of the net (Figure 4.19b) and engage in moderate pseudo-sixfold phenyl embraces122 in their pairs. Thus a disruption to the supramolecular interactions between the cations (and also a reduction in their size) changes the anionic coordination polymer from a 2D to a 3D network. A number of other architectures have been observed for the M(dca)3 networks through variation of the cations. A systematic study of tetraalkylammonium salts revealed that the connectivity of the nets decreased with increasing cation size.123 Thus for tetrapropylammonium a 6-connected net was generated (a-Po), for tetrabutylammonium a 5-connected net resulted and for tetrapentylammonium the 4-connected diamond net was found. In all cases the metals remained octahedral and coordinated by six dca anions, but the proportion of single M(dca)M bridges to double M(dca)2M bridges decreased as the overall network connectivity decreased (Figure 4.19c–e). The a-Po net was also found for ferrocene and SPh+ counterions,124,125 and use of 3 21 0 M(2,2 -bipy)3 cations resulted in 2D (6,3) sheets (Figure 4.19f).126 Another system showing a great dependence on the nature of the counter cations is the coordination polymers of formate. When no suitable cations are


Figure 4.19

123

Cation templation of anionic M(dca) 3 nets: (a) (4,4) sheet structure obtained with Ph4E1, where E ¼ P, As;116–120 (b) two cations in a cavity of the 3D net obtained with MePh3P1;116,119 (c) the 6-connected a-Po net obtained from tetrapropylammonium;123 (d) the 5-connected net for tetrabutylammonium;123 (e) the 4-connected diamond net for tetrapentylammonium;123 (f ) the (6,3) 2D sheets obtained with M(2,2 0 bipy)321 cations.126 For clarity, only one cation is shown in (c)–(e), as is only one position of any disordered cations or dca ligands.

124

Chapter 4 127

present, diamond-like networks are generated. In the presence of suitable ammonium cations, however, 6-connected networks with either a-Po (412.63),128 NiAs (412.63)(49.66),129 or acs (49.66)130 topology are generated. Copper malonate polymers have also been shown to display cation-dependent structures.131 Finally, it is possible for structures to be templated by included ion pairs. Neutral Cu(2-pyrimidinolate)2 frameworks are able to include MNO3 ion pairs; the resultant structure depends on the nature of M.132 Furthermore, it is possible to interconvert between the different structures. The material is synthesised as a hydrated phase containing no ion pairs. This phase has rhombohedral symmetry and shows a distorted sodalite network. Immersion of this material in aqueous methanol solutions of MNO3, where M ¼ NH4, Li, results in absorption of MNO3 into the material and its conversion to a cubic phase with an undistorted sodalite network. Dehydration of the initial phase also results in conversion to the cubic phase; both processes are reversible. If, however, the initial phase is exposed to nitrate solutions of M ¼ Na, K, Rb, Tl, an orthorhombic phase containing 2D layers results. Removal of KNO3 or RbNO3 can occur through refluxing in a MeOH solution of 18-crown-6, to generate an ‘empty’ layered structure, which converts on exposure to water back to the original rhombohedral phase.

4.8 Weaker Interactions There are many other non-covalent interactions that contribute to the stabilisation of a particular crystal structure, including weak hydrogen bonds and electrostatic and van der Waals forces; however, perhaps the more easily quantified ones are (stronger) hydrogen bonds, p–p interactions and metallophilic interactions.133 We have already seen some examples of these weak interactions in action, with these interactions important in many of the examples discussed above of solvent, guest or counterion direction of structure. In more general terms, they can direct or stabilise structures through interactions within networks, between networks, between networks and guests and/or counterions, between guest and/or counterions themselves or combinations of all these. Hydrogen bonding interactions, both strong and weak,134 are used extensively in organic crystal engineering,135 and also in the linking of molecular metal complexes into networks.136 However, even for coordination polymers constructed using strong coordination bonds, the influence of hydrogen bonding can be important.133,137 The structure of [Cd(SCN)2(nic)2](nic), where nic ¼ nicotinic acid, contains 1D chains of Cd atoms bridged by double SCN bridges.138 The nic ligands bind via their pyridyl groups in a trans relationship and the uncoordinated carboxylate groups project outwards from the chains. The chains are aligned such that hydrogen bonds form between these carboxylate groups and either carboxylate groups of adjoining chains or those of intercalated nic molecules, generating an overall 2D network (Figure 4.20). In the structure of Zn(tph)(H2O)2, where


Figure 4.20

125

The hydrogen bonding of chains to other chains and intercalated nic molecules in the structure of [Cd(SCN)2(nic)2](nic).138

tph ¼ terephthalate, there is significant hydrogen bonding between the water ligands and the carboxylate groups of the terephthalate anions.139 If the water ligands are replaced by chelating ethylenediamine ligands, 1D zigzag chains are still formed, but this time the interchain hydrogen bonds are arranged differently due to the different orientations of the NH2 versus OH2 donors. The formation of hydrogen bonding interactions may be engineered by the deliberate use of bridging ligands containing hydrogen bonding sites. Hydrogen bonding interactions may be desirable as structural reinforcement, as a driving force for particular topologies or packing arrangements or to create cavities and channels within a structure with hydrophilic environments suitable for hydrogen bonding to particular guests. For example, the structure of Cd(imidazole-4-acrylate)2 1.7H2O shows a very unusual mode of interpenetration for its four diamond nets (Chapter 3).140 This is very likely driven by hydrogen bonding interactions between the nets. Two such ligands that contain hydrogen bonding sites are the dicarboxylates saccharate and mucate, which both contain two carboxylate groups separated by a C4 chain containing pendant alcohol groups. In the structure of Zn(saccharate)2 2H2O, each ligand bonds to four metal atoms, via both carboxylate and alcohol oxygen atoms.141 In turn, each metal coordinates to four ligands and an overall 3D network is formed. The network contains channels in which the sides are lined by ‘impenetrable walls’ of parallel ligands crosslinked by hydrogen bonds formed between uncoordinated carboxylate oxygens and coordinated alcohol groups (Figure 4.21a). Each ligand also contains two uncoordinated alcohol groups in its middle and these are arranged so that all these central alcohol groups in a ‘wall’ lie on only one side. These walls are in turn arranged so that there are two sorts of channels – hydrophilic ones in which the walls project only their alcohol-rich sides and hydrophobic ones into which there are no alcohol groups projected (Figure 4.21b). The former

126

Figure 4.21

Chapter 4

The structure of Zn(saccharate)2.2H2O: (a) the ‘impenetrable walls’ created by the hydrogen bonding of adjacent ligands; (b) the hydrophobic and hydrophilic channels, which both contain intercalated water molecules.141

channels are smaller (due to the presence of the hydroxide groups) but contain well-ordered water guest molecules. The latter channels are larger and contain more water, but this water is highly disordered. Furthermore, TGA indicates that the water from the hydrophobic channels is lost readily, whereas that in the hydrophilic channels is only lost at higher temperatures. The material was also found to retain its single-crystal nature upon desolvation, no doubt aided by the reinforcement of the structure by the hydrogen bonding within the network. The desolvated structure readily absorbs guests such


127

as I2, hydrocarbons, elemental sulfur, CCl4 and CI4 into the hydrophobic channels. Hence the alcohol groups in this structure play a central role in (a) directing the structure, (b) stabilising the structure to loss of solvent, (c) tailoring the chemical environment of the channels, (d) determining how strongly bound and ordered the guest water molecules are and (e) directing which channels are filled by incoming guests (according to their hydrophilic/hydrophobic nature). The mucate ligand forms two different types of polymers with lanthanoid ions, but again in these structures hydrogen bonding between the ligands plays a very important role in determining (and stabilising) the overall structure.142 The effect of hydrogen bonding on structure has been further investigated through the use of bis(pyridine) ligands containing amide or urea groups along their backbone.143 These are designed to form hydrogen bonding interactions between themselves or to included anions or guests. For example, a number of structures containing amide groups show the formation of (4,4) sheets that pack in such a way that b-sheet motifs are formed between the ligands of the different sheets.144 When a ligand containing two urea groups was reacted with silver nitrate, the 1D polymers formed showed a structural dependence on the solvent.145 If DMSO was used as the solvent, zigzag 1D chains were formed in which there were hydrogen bonds between urea groups and intercalated solvent molecules. If the solvent was changed to water, then the ureas instead hydrogen bond to the anion and intercalated water molecules, resulting in a change in conformation of the chain to a more linear motif. Thus the presence of the hydrogen bonding groups in the backbone of the ligand induced a solvent dependence in the structure. Amide-containing ligands have been used to create a series of materials with interesting porosity properties.137c,146,147 Two-dimensional sheets are formed in which their alignment is controlled by intersheet hydrogen bonding. These hydrogen bonding interactions perform a number of important roles. They direct the structure away from forming interpenetrating motifs, they stabilise the structure such that crystal-to-crystal transformations can take place upon removal of solvent and they hydrogen bond to guest solvent molecules. Furthermore, a selectivity in absorption has been shown in one structure, with guests such as cyclopentane, which has no hydrogen bonding capability, not being absorbed whereas the similarly sized THF is.147 Another structure, containing a trigonal amidopyridine ligand, shows absorption of n-butanol but not the similarly sized n-pentane or n-pentene.148 This same compound also shows selective catalysis of the Knoevenagel condensation reaction; the amide groups in the coordination polymer framework are also believed to be important in producing this reactivity. Another example of the physical properties of a coordination polymer being affected by the hydrogen bonding interactions is the compound [Fe(NCS)2 (bpbd)2].acetone, where bpbd ¼ 2,3-bis(4 0 -pyridyl)-2,3-diol.149 The structure consists of (4,4) sheets showing inclined interpenetration. Significantly, the alcohol groups form hydrogen bonding interactions between the interpenetrating sheets which are not present in the analogous structures formed by

128

Chapter 4

4,4 0 -azopyridine (azpy) or 1,2-bis(4-pyridyl)ethylene. This leads to a structure that is very stable towards desolvation and also one which shows a much more abrupt spin crossover transition, no doubt due to the increased cooperativity between the metal centres provided by the hydrogen bonding. Interestingly, in the azpy analogue guest molecules can hydrogen bond with the pendant thiocyanate groups and induce changes in both the structure and the spin crossover properties of the material.150 Changes in spin crossover properties are also seen in the compounds [Fe(pmd)(H2O){M(CN)2}2] H2O, where pmd ¼ pyrimidine and M ¼ Ag, Au.151 The interesting point about these structures for our discussion, however, is that the structures contain coordinated water molecules which hydrogen bond to nearby pyrimidine ligands. These water molecules, however, can be removed, resulting in a remarkable topotactic reaction occurring whereby the pyrimidine ligand moves to take the place in the Fe coordination sphere of the water molecule to which it was previously hydrogen bonded. Furthermore, this reaction is reversible. The spin crossover properties of the hydrated and dehydrated compounds differ significantly. Another series of compounds in which hydrogen bonding interactions can be replaced by coordination bonds in a topotactic fashion upon dehydration are [Zn2(Sala)2(H2O)2] 2H2O and [Cu2(Sala)2(H2O)], where H2Sala ¼ N(2-hydroxybenzyl)-L-alanine. The Zn compound actually consists of discrete dimers which are linked via hydrogen bonds into a 3D network.152 This network can be converted into a 3D coordination polymer by dehydration, whereupon Zn–OH2 O(carboxylate) intermolecular interactions are converted into Zn–O(carboxylate) coordination bonds. This process is aided by nearby NH O hydrogen bonds, which stabilise the correct orientation needed for this topotactic reaction. The Cu compound contains 1D helical coordination polymer chains, but again there is extensive hydrogen bonding, both within and between the chains.153 This compound also dehydrates to produce an overall 3D coordination polymer. Both dehydrations are irreversible. We discussed the moganite structure Cd(CN)2 2/3H2O t-BuOH earlier (Section 4.5);79 however, what we did not discuss at the time was the role of hydrogen bonding between the guest solvents and the framework in directing this structure. The structure contains hydrogen-bonded clusters consisting of two coordinated water molecules and three t-BuOH guest molecules which efficiently fill the cavity in which they are situated. Another series of structures that contain interesting hydrogen-bonded clusters between coordinated and guest solvent molecules are the Ln(dhbq)3 24H2O series, where Ln ¼ Y, La, Ce, Gd, Yb, Lu and dhbq ¼ dihydroxybenzoquinone.154 The lanthanoid ions coordinate to three dhbq ligands, which in turn each bridge to metal ions to generate 2D (6,3) sheets. Each metal, however, also coordinates to three water ligands and these ligands participate in the formation of a large hydrogen-bonded cage with 12 intercalated water molecules and an Ln(H2O)3 group of a second sheet. These Ln2(H2O)18 cages have a pentagonal dodecahedral arrangement (Figure 4.22). The Ln2(H2O)18 cages joining two sheets lie in the


Figure 4.22

129

The Ln2(H2O)18 pentagonal dodecahedron cluster formed in the structures of Ln(dhbq)3 24H2O.154

centre of the hexagonal cavity of a third sheet and thus they link every second sheet, generating two interpenetrating diamondoid nets. The cage also hydrogen bonds to the Ln6(dhbq)6 ring through which it passes. It is noteworthy that the same structure is observed across the entire lanthanoid series. By contrast, when the slightly larger dichloro derivative of dhbq, chloranilic acid, is used, different structures are obtained. For La (4,4) sheets are formed, Ce, Pr, Nd, Tb, Y and Eu form (6,3) sheets (but different to the dhbq analogues) and the Yb and Lu structures contain both 1D chains and discrete molecules. The structural variation of this series is attributed to the fact that the presence of the chlorine atoms in the ligand means that the hexagonal rings that contain the Ln2(H2O)18 clusters in the dhbq structures are not able to form, due to the smaller size of the ring (the chlorine atoms would project into the ring). Therefore, in the absence of the intricate hydrogen bonding motif seen in the dhbq compounds, the chloranilic acid structures are dominated by the size of the lanthanoid ions. Hydrogen bonding interactions between networks and counterions can also be important factors in determining structural topology. A remarkable example of this is two carbonate structures templated by the guanidinium cation.155 Reaction of Cu(NO3)2, KHCO3, K2CO3 and guanidinium results in the formation of [C(NH2)3]2[Cu(CO3)2]. This compound contains diamondoid networks in which the guanidinium cations lie in the six-membered windows of the adamantane cavities; each NH forms a hydrogen bond to a carboxylate oxygen (Figure 4.23a). Furthermore, a second, minor product was obtained from this reaction: K4[C(NH2)3]8[Cu6(CO3)12]. This second compound contains sodalite

130

Figure 4.23

Chapter 4

Hydrogen bonding cation templation of carboxylate nets: (a) a guanidinium cation in a six-membered ring of the diamond-like net of [C(NH2)3]2[Cu(CO3)2]; (b) two guanidinium cations in a six-membered ring [and a potassium cation (pale sphere) in a four-membered ring] of the sodalite net of K4[C(NH2)3]8[Cu6(CO3)12].155

copper carbonate networks which are, again, templated by the hydrogen bonding cations (and also the K1 ions, which also interact with the carbonate oxygens). A pair of guanidinium cations interacting with a six-membered window in this net are shown in Fig 4.23b. Further reactions have shown that the sodalite network can be templated for a range of divalent metals, and also with Na3(Me4N) or Li3(Me4N) in place of the potassium ions. The latter


131

compounds show a remarkable degree of crystal engineering and templation. In Li3[Me4N][C(NH2)3]8Cu6(CO)12 5H2O there are four types of cations and one type of anion all coming together to form the structure in a predictable fashion. The Li1 cations lie in the four-membered windows, the C(NH2)+ 3 cations lie in the hexagonal windows, the Me4N1 ions lie in the centre of the cavities and all help to template the formation of a sodalite network by the copper carbonate framework. The final example of the importance of hydrogen bonding is the compound [Cu(bpea)2(H2O)2][Cu(4-pySO3)4(H2O)2], where 4-pySO3H ¼ 4-pyridinesulfonic acid.156 The structure contains discrete [Cu(4-pySO3)4(H2O)2]2 anions intercalated into cationic (4,4) sheets of [Cu(bpea)2(H2O)2]21; both species are interconnected by extensive hydrogen bonding. What is remarkable about this structure in the context of hydrogen bonding structure direction is that neither these cationic sheets for the bpea ligand nor the discrete 4-pySO3 complex anions have been reported elsewhere. Interactions involving delocalised p systems also play an important role in crystal engineering, particularly p–p or C–H p interactions.157 We saw earlier in the cation templation of M(dca)3 networks that C–H p interactions between cations can be important. We also discussed the inclusion of large aromatic guests such as pyrene and naphthalene into metal–4,4 0 -bipyridine host frameworks; again C–H p interactions, both between the guests and between the guests and the coordination polymer frameworks, are important in directing these structures. The structure of Cu2(pyridine)2(5benzyloxybenzene-1,3-dicarboxylate)2 contains 2D polymers with Kagome´ lattice topology.158 Significantly, the pendant benzyloxy groups of the ligands project outwards from the sheets such that concerted arrays of C–H p interactions known as sextuplet phenyl embraces are formed between the nets.159 The aligning of pairs or even infinite stacks of aromatic rings is a feature of a considerable number of coordination polymers. It is likely that the large number of interpenetrating diamond networks is due, in part, to the ability of this motif to generate infinite p–p stacks of the ligands.160 A number of high-symmetry networks have been observed for coordination polymers of 2,4,6-tri(4-pyridyl)-1,3, 5-triazine (tpt) in which the formation of Piedfort pairs161 of aligned triazine rings is a feature (Figure 4.24a).162 The structure of [Cu2(O2CCH3)4]3(tpt)2 2MeOH contains (6,3) sheets of tpt ligands bridged by copper acetate dimers (Fig 4.24b).163 Despite the large size of their hexagonal windows, the sheets do not interpenetrate, but rather stack in an ABCA . . . fashion to give infinite p stacks, although in this case the closest contacts are between triazine rings and pyridine rings rather than pairs of triazine rings. The sheets are even somewhat corrugated to maximise the interactions within these p stacks. Although incorporation of chelating ligands such 2,2 0 -bipyridine or 1,10phenanthroline (1,10-phen) into coordination polymers usually has the effect of lowering the dimensionality of the network, they also provide opportunities for p–p interactions between nets. For example, the structure of Cu(eoba)(1,10phen), where eoba ¼ ethylenedi(4-oxybenzoate), contains zigzag chains which

132

Figure 4.24

Chapter 4

(a) Two tpt ligands showing the Piedfort pair arrangement;161,162 (b) p stacking of adjacent sheets in the structure of [Cu2(O2CCH3)4]3 (tpt)2 2MeOH.163

associate in pairs to create infinite p stacks of pendant 1,10-phen ligands (Figure 4.25).164 Another phenanthroline ligand, 4,7-phen, acts as a bridging ligand in the structure of [Cu(4,7-phen)(H2O)3](ClO4) 2(4,7-phen).165 1D chains with an undulating S-shape are formed and a 3D honeycomb structure is formed by the stacking of 4,7-phen ligands of adjacent chains. The structures of Ni(acac)2L solvent, where L ¼ 1,1-binaphthyl-6,6 0 -bipyridines, contain helical 1D chains that form ‘nanotubes’.166 Each nanotube is intricately entangled with four others, with this packing arrangement stabilised by considerable p–p interactions. The structures of [Cu4(4,4 0 -bipy)5(H2O)] X solv, where X ¼


Figure 4.25

133

The p stacking of 1,10-phen ligands from adjacent chains in the structure of Cu(eoba)(1,10-phen).164

(PF6)4 or (PF6)2(ClO4)2, contain thick 2D layers with monodentate 4,4 0 -bipy ligands projecting outwards on either side of the layer.167 These monodentate ligands form p–p interactions with those from adjoining layers, generating a porous 3D arrangement. The importance of p–p interactions can be seen in a study of the packing of linear AgL chains, where L ¼ bis(pyridine) bridging ligand.168 It was found, in general, that chains containing the 2,7-diazapyrene ligand, which is disc-shaped and has a large aromatic surface, align themselves to maximise p stacking arrangements, whereas the packing of chains containing the rod-shaped 1,4bis(4-pyridyl)butadiyne are dominated more by metal–anion interactions. It is also worth noting in the Ag(2,7-diazapyrene) chains that there are also significant metallophilic interactions between the chains. Metallophilic interactions are attractive interactions between closed d-shell ions such as CuI, AgI and AuI and the strengths of these interactions can rival those of hydrogen bonds.169 The structure of [Cu2(4,4 0 -bipy)(CN)2][Cu(SCN)] contains 2D (6,3) sheets which align to give channels in which lie 1D Cu(SCN) chains.170 These two species are connected by cuprophilic interactions, with the CuI CuI distance [2.651(4) A˚] being considerably shorter than the sum of the van der Waals radii (2.80 A˚) and comparable to the interatomic distance seen in metallic copper (2.556 A˚). The structure of [Ag(bpp)](CF3SO3), where bpp ¼ 1,3-bis(4-pyridyl)propane, contains 1D chains arranged into double helices; each pair of strands is connected by argentophilic interactions (Ag Ag ¼ 3.09 A˚) (Figure 4.26).171 Ag(4,4 0 -bipy)(NO3) contains 1D chains which run in two different directions and are connected by argentophilic interactions (Ag Ag ¼ 2.97 A˚) to generate three interpenetrating (10,3)-b networks.172 Two structures have been reported to show Borromean-type interpenetration of (6,3) sheets; in both structures argentophilic interactions are important in generating this unusual interpenetration topology.173,174 Finally, the structure of [Fe(pmd)(H2O) {Ag(CN)2}2] H2O, mentioned earlier in the discussion on hydrogen bonding, also contains significant aurophilic interactions [average Au Au ¼ 3.2901(4) A˚] between the three interpenetrating CdSO4 nets.151

134

Figure 4.26

Chapter 4

Two helical chains linked by argentophilic interactions in the structure of [Ag(bpp)](CF3SO3).171

4.9 Conclusion The issues discussed here perhaps cast an overly pessimistic view of the design of coordination polymers discussed in Chapter 2. It is important to emphasise that the net-based approach is a very important tool for the deliberate design of crystals and has been used extensively with great success. This is a very significant advance that should not be undervalued. Nonetheless, there are many other factors that can affect the structure obtained, as described above, and wherever possible these should be taken into account in the design process. The structures of coordination polymers are a lot more malleable that they may seem at first. Furthermore, more than one structure can be formed, even from the one reaction mixture, which underlines the need for powder diffraction and close examination of bulk samples to reinforce single-crystal structural analysis, so that true correlations between structures and properties can be drawn.

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CHAPTER 5

Transition Metal Coordination Polymers 5.1 Introduction Transition metals play an integral role in everyday life, from the natural biology in our bodies to large-scale infrastructure. A large range of accessible properties, of both a geometric and electronic nature, are displayed by transition metals and, because their chemistry is well documented, they can be conveniently incorporated into coordination polymers. Indeed, transition metal-containing coordination polymers are focused on more than any other type of metal–organic polymer for this reason. To this end, this chapter concerns this extensive area of coordination polymers that contain transition metal ions and general types of organic ligands that are employed with such metals. In terms of physically incorporating transition metals into coordination polymers, there are two approaches. First, one can use them as building blocks to direct a certain framework topology, and second, they can be chosen based on their electronic functionality, such as for magnetism or redox potential. Indeed, both of these approaches can be targeted together in the one material. The most commonly reported transition metals in coordination polymers are those which show labile metal–ligand bonds, such as manganese, iron, cobalt, nickel, copper, zinc, palladium, silver, cadmium, gold and mercury. A number of these metals exist in more than one oxidation state, i.e. Mn21/31, Fe21/31, Co21/31 and Cu1/21, so that an even larger range of options exist. However, although the expanse of possibilities is vast, the focus for coordination polymers generally seems to lie on first-row transition metals. Reasons for this possibly include cost, abundance and ease of chemistry. Typical coordination geometries that are seen in transition metals in coordination polymers include linear, bent, trigonal, square-planar, tetragonal, prismatic and octahedral (and distortions within each of these) (Figure 5.1). Coordination Polymers: Design, Analysis and Application By Stuart R. Batten, Suzanne M. Neville and David R. Turner r Stuart R. Batten, Suzanne M. Neville and David R. Turner, 2009 Published by the Royal Society of Chemistry, www.rsc.org

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Figure 5.1

145

Left: illustration of some transition metal coordination environments and common metals that form them. Right: example ligand functional groups which are used in coordination polymers with transition metals.

Thus, transition metals can be used as building blocks ranging from 2- to 6-connecting. For each metal ion and its respective oxidation state, the possible coordination geometries are well established so that particular ones can be targeted. For example, in coordination polymers where linear linking metals are required, Ag1, Au1 and Hg1 are often employed, whereas for squareplanar geometry Pt21 and Pd21 are common and for even greater coordination numbers Mn21/31, Fe21/31, Co21/31 and Ni21 can be can be targeted. Evidently, distortions and variations in particular cases are possible which cannot be engineered but this method allows some degree of design. An additional feature which is commonly used in coordination polymer chemistry to allow an even larger range of variations in transition metal coordination geometry is to use a mixture of bridging and terminal organic ligands such that some of the divergent metal sites are blocked. Further to this, rather than using transition metals as individual atom coordination nodes in framework materials, they may be incorporated into inorganic clusters, such as the basic zinc acetate cluster, which can act as an octahedral node. These inorganic clusters are termed secondary building units (SBUs) (Chapter 2). Within the range of transition metals observed, there is a vast range of electronic phenomena which may be harnessed through their incorporation into coordination polymer systems. Specific examples of this are described in Chapters 9–13. When choosing a transition metal for a coordination polymer, thought must also be given to the organic donor atoms which will form its coordination

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environment. Important here is the strength of metal–ligand interaction that is required. For example, when designing materials which show robust porosity (Chapter 10), strong bonding is required, whereas if post-synthetic exchange of terminal ligands is a target, e.g. for functionalising the pore lining, a weaker metal–ligand bond would be more useful. Evidently, the collection of coordination polymers containing transition metals is too vast to be discussed exhaustively in this chapter, so important examples will be highlighted. Examples will be discussed based on the bridging organic ligands’ coordinating functional group rather than by metal, namely pseudohalides, nitrile donors, pyridyl donors, five-membered ring donors, N-oxides, carboxylate donors, other selected donors and mixed donors (Figure 5.1). The properties of transition metal coordination polymers will be briefly introduced and discussed within particular examples, however they will be outlined in more detail in Chapters 9–13.

5.2 Pseudohalide Ligands Short bridging pseudohalide ligands such as cyanides, azides and nitrile donors provide a convenient way of connecting transition metals in the solid state. Importantly, pseudohalides are generally rigid with a somewhat defined directionality, so when combined with labile transition metals they are more disposed to form crystalline materials. In addition, their relatively short nature means that the voids formed are not large and thus the resultant frameworks and void areas are quite well ordered. Pseudohalides can bind to a range of transition metals and have a number of potential binding modes. This means that a range of structural diversity can be observed. In addition, mixed ligand pseudohalide-containing systems are commonly reported such that an even greater variety of 1D through to 3D topologies are possible. Due to the short nature of these type of ligands, they provide good exchange pathways for magnetic exchange and have been exploited extensively for this purpose, including the synthesis of room temperature magnets. Magnetic coordination polymers containing pseudohalide ligands are discussed in detail in Chapter 10.

5.2.1 The Cyanide Ligand One of the simplest examples of a pseudohalide is the cyanide ligand. In many reviews of coordination polymers, cyanides are discounted as true organic bridging ligands owing to the M–C bond necessary for them to act as bridges. They have been included here as an important class of bridging ligand, which provided many of the initial clues towards the strategic building of framework structures. Owing to the short, rigid nature of this ligand and its well-established transition metal coordination chemistry, cyanide-containing transition metal coordination polymers have found numerous further applications (e.g. longrange magnetic ordering, spin crossover, porosity, sensors).


147

The most recognized cyanide-containing materials are the Prussian Blue family, the first being the pigment Fe4[Fe(CN)6]3 15H2O.1,2 A large number of Prussian Blue analogues exist, with the general formula MAm[MB(CN)6]n nH2O, where octahedral [MB(CN)6] nodes are linked via octahedral metal centres (MA) (Figure 5.2a). These form simple a-Po-type networks which contain small channels often filled with water molecules. Depending on the relative valencies of MA and MB, some of the MB coordination sites can be occupied by bound water molecules. For example in CoII3[CoIII(CN)5]2, where for charge balance onethird of the cyanometallate sites must be vacant.3 This particular material is a porous magnet, showing both long-range magnetic ordering and stability of the framework structure in the empty host state. The bound water molecules can be thermally removed from Prussian Blue analogue structures, resulting in stable frameworks which contain vacant metal sites (Figure 5.2a).4 These vacant metal sites have implications in porous applications such as gas storage. For example, a recent investigation on defect analogues of the formula MII3 [CoIII(CN)6]2 (MII ¼ Mn, Fe, Co, Ni, Cu, Zn, Cd), showed hydrogen gas storage ability which is dependent on the MII ion employed.5 Many other hexacyanoferrate analogues exist which also show porous properties, such as the material K2Zn3[Fe(CN)6]2 nH2O, where octahedral [Fe(CN)6] nodes are linked via tetrahedral Zn21 centres. This material shows interesting adsorption characteristics of a range of gases (e.g. CO2, N2 and C2H4).6 In 1990, Hoskins and Robson realized that from a crystal engineering perspective cyanide ligands could be considered as rigid molecular rods which could be exploited to form different framework topologies based on the geometry of the metal ion, either tetrahedral or octahedral.7 The simplest

Figure 5.2

Simple cyanide containing frameworks. (a) Illustration of the cubic Prussian Blue structure with random site defects; (b) illustration of the templated diamond-like framework [N(CH3)4][CuZn(CN)4]. Part (a) reprinted with permission from J. Am. Chem. Soc., 125, 14590, copyright (2003).

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such example arises when only tetrahedral metal ions are utilised, such as Zn21 or Cd21. In this case, diamond-type framework topologies are formed of the formula Zn(CN)2 and Cd(CN)2; and [N(CH3)4][CuZn(CN)4] (Figure 5.2b). There are many other materials of the general formula Cd(CN)2 xG (G ¼ guest), where through variation of the guest molecules different topologies and distortions of topologies result.8 Other cyanide-containing coordination polymers may be generated through the use of linear metal–cyanide bridges, such as Au(CN)2 and Ag(CN)2. For example, when combined with tetrahedral Zn21 centres, a quartz-type structure forms, [ZnAu2(CN)4].9 In this case, as the Au(CN)2 units extend the length of the linear cyanide connection, larger cavities are generated which results in sixfold interpenetration of the nets. There are a multitude of other coordination polymers which contain cyanide and terminal or bridging co-ligands. A large series of materials called Hofmanntype clathrates exist where octahedral M21 ions and square planar M21 ions are linked into 2D sheets.10 The original material in this series was formed using [Ni(NH3)2]21 and [Ni(CN)]42 building blocks.11 Since that time the Ni21 ions have been replaced by a range of other M21 ions and the amine has been replaced by other terminal N-donor groups.10 Using the bidentate terminal co-ligand ethylene-1,2-diamine (en), the 3D coordination polymer [Mn(en)]3[Cr(CN)6]2 4H2O forms (Figure 5.3a).12 This material is comprised of [Cr(CN)6] units bridged by [Mn(en)] units and contains defect cube moieties of three Cr and three Mn atoms. In a further example, an intertwined helical 3D structure is generated through the use of D-/L-aminoalanine (NH2alaH) as a co-ligand (Figure 5.3b).13 Through bridging of [Cr(CN)6] hexacyanometallate units with (Mn-NH2ala) complexes, extended helical chains are linked into aggregates of three chains which are further bridged into a 3D network via the cyanide moieties, [(Cr(CN)6)(MnNH2ala)3] 3H2O. Additionally, porous structures may be generated using co-ligands, such as in the material [Ni(dipn)]2[Ni(dipn)(H2O)][Fe(CN)6]2 11H2O [dipn ¼ N,

Figure 5.3

Cyanide-containing coordination polymers that contain terminal co-ligands. (a) A 3D defect cubane structure, [Mn(en)]3[Cr(CN)6]2 4H2O; (b) a 3D structure comprised of helical 1D chains, [(Cr(CN)6)(Mn-NH2ala)3] 3H2O.


149

14

N-di(3-aminopropyl)amine]. This material is comprised of Fe–CN–Ni links which form 2D sheets which are bridged further into a 3D porous framework by additional Fe–CN–Ni links. This material shows a reversible crystal-toamorphous-like phase transformation associated with the removal and replacement of solvent molecules.

5.2.2 The Azide Ligand The azide ligand shows a number of binding modes and has been incorporated into an array of interesting 1D, 2D and 3D coordination polymers. The most common binding modes are end-to-end (EE, termed 1,3) or end-on (EO, termed 1,1) and can be observed uniquely in one structure or combined in the one structure (Figure 5.4). The type of binding observed plays are large role in magnetic coordination polymers, in particular, EO binding is sought for ferromagnetic coupling (see Chapter 9). Terminal binding of azides is also commonly observed but will not be discussed. In order to expand the range of framework topologies and their properties, bridging or terminal co-ligands can be used much as described above for cyanide-containing materials. Transition metals can be bridged by EE-bound azide ligands, through a single bridge or through a double bridge (Figure 5.4). Using a single azide bridge in the EE mode around an octahedral metal centre, a 3D perovskite structure forms.15 In particular, the material [N(CH3)4][Mn(N3)3] shows this structure, where each octahedral Mn21 ion is surrounded by six azide donors (Figure 5.5a). Small cavities are thus formed, which are filled with [N(CH3)4] anions. High-dimensional azide compounds such as this are important for the generation of long-range magnetically ordered materials. The use of a terminal co-ligand with EE azide binding, either single or double bridges, has been useful for the formation of 1D chain structures. Such an example has been reported for the material [Mn(pyOH)(N3)2] (pyOH ¼ 2-hydroxypyridine), where double bridging EE azide ligands are present (Figure 5.4).

Figure 5.4

Common binding modes of the azide ligand to transition metals.

150

Figure 5.5

Chapter 5

Coordination polymers containing EE binding of azide ligands. (a) [N(CH3)4][Mn(N3)3)]; (b) the 1D chain [Mn(pyOH)(N3)2].

Here, octahedral Mn21 ions are axial coordinated by monodentate pyOH ligands to form isolated 1D chains (Figure 5.5b).16 Importantly, through the use of bridging co-ligands such as pyrimidine (pm), 4,4 0 -bipyridine (4,4 0 -bipy) and 1,2-bis(4-pyridyl)ethane (bpe), extended frameworks of higher dimensionality may be generated, such as the materials [M(pm)(N3)2] (M ¼ Mn, Fe, Co, Ni) and [M(4,4 0 -bipy)(N3)2] (M ¼ Mn, Fe).17–20 In these materials, the azide ligands bridge in an EE fashion. For [M(pm)(N3)2], 2D sheets of [MN3]4 motifs are bridged by pm ligands to form a 3D pillaredlayered material.18 Through using a larger bridging ligand, 4,4 0 -bipy, a 3D network is formed where the octahedral M21 centres are linked axially by 2connecting 4,4 0 -bipy ligands into 1D chains.19,20 The coordination is completed by four N3 groups in the EE binding mode which bridge four separate Mn21 ions. Azide ligands which show the EO binding mode may bridge via single or double bridges where two metal centres coordinate to the same nitrogen donor atom (Figure 5.4). These systems form 1D chains when there are co-ligands present, such as in the material [Ni(en)(N3)2]. In this case, Ni21 centres are bridged by double EO azide ligands and bidentate diamine ligands which alternate above and below the chain.21 On the other hand, when a bridging co-ligand is employed, a higher dimensional structure can result. The 2-connecting bidentate ligand bipyrimidine (bipym) is useful for a short bridge in combination with azides. In particular, the material [Mn2(bipym)(N3)4] shows double EO azide bridging of metal centres which are further linked into a 2D honeycomb structure by bipym ligands (Figure 5.6a).22 In further examples using 4,4 0 -bipy, as for the EE binding mode examples where this ligand was used, 2D sheet structures are formed. This is nicely demonstrated in the material [Fe(N3)2(4,4 0 -bipy)], where octahedral Fe21 ions are doubly bridged by EO azide ligands forming 1D


Figure 5.6

151

Example of coordination polymers with EO binding of azide ligands. (a) The 2D honeycomb lattice of [Mn2(bipym)(N3)4]; (b) the 2D sheets of [Fe(N3)2(4,4 0 -bipy)].

chains (Figure 5.6b). These chains are further extended into a 2D sheet structure by 2-connecting 4,4 0 -bipy ligands.20 Further to this, the use of a longer, more flexible bridging co-ligand, 1,2-bis(imidazol-1-yl)ethane (bim), results in the formation of a 3D distorted diamond-type network.23 In [Mn(N3)2(bim)], 1D zigzag chains of double EO azide ligands are bridged by bim ligands in the anti conformation, resulting in a 3D rather than 2D topology. It is also common to observe the presence of both EE and EO binding of azide groups in the one material. This type of system is important magnetically for generating alternating antiferromagnetic–ferromagnetic chains (Chapter 9). Alternating binding has been observed in the material [Fe(N3)2(phen)] (phen ¼ 1,10-phenanthroline), which is a 1D chain structure where adjacent Fe21 ions are doubly bridged by alternating pairs of EE or EO azide ligands.17 The octahedral coordination around the metal centre is completed by bidentate phen ligands which alternate in an up–down fashion along the chain. The chains are held together by a series of p–p interactions through the phen ligands. A further example which exhibits an unusual alternating binding mode is the material [Zn(4,4 0 -bipy)(N3)2], which displays a 2D sheet structure. In this material, two Zn21 centres are doubly bridged equatorially via one EE and one EO azide ligand.24 The adjacent Zn21 centres are then doubly bridged by two EO azide ligands, thus the 1D chain continues to alternate in this fashion. The 1D chains are bridged into 2D sheets via 2-connecting 4,4 0 -bipy ligands. Interestingly, in the material [Mn(N3)2(L)] (L ¼ 4,5-diazafluoren-9-one azine), a triply interpenetrated (10,3) net is formed by using a Schiff base ligand.25 In this material, chains of Mn21 ions are alternately connected by two EO and EE azido bridges into dinuclear entities which are further bridged into a 3D framework. Importantly, this material shows spin-canted longrange ferromagnetic ordering arising from the alternating arrangement of azide binding modes.

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5.2.3 NCX Ligands (X = O, S, Se) Anionic ligands of the form NCX, where X ¼ O, S and Se, can be used in coordination polymers as terminal or bridging ligands. Focused on here are cases where these ligands act as bridges. These ligands have not been explored as much as the other pseudohalide ligands in terms of their bridging in coordination polymers. They have, however, been commonly used in combination with either terminal or bridging co-ligands. These ligands, as for azides, can act in both the EO and EE binding modes this can have implications for magnetic materials (Chapter 9). When there are no co-ligands present, a simple diamond-like 3D framework forms with tetrahedral metals and linear NCS ligands. There are a number of polymorphs of this type of material that have been reported. The b-CuNCS analogue consists of this simple diamond-like framework where the NCS ligands act as single EE bridges.26 A further example of the EE bridging mode of the SCN ligand is Co(bim)(SCN)2.23 This material displays a 1D chain structure in which metal centres are double bridged by SCN ligands and further bridged by looping bim ligands, such that a triple bridge results. This same EE binding of SCN ligands has also been observed in the material Co(bte)(SCN)2 [bte ¼ 1,2bis(1,2,4-triazol-1-yl)ethane].23 Here, the metal centres are doubly bridged by SCN ligands into 1D chains, which are further linked into 2D sheets via linear bte ligands. In general, the basic EE binding mode is the only one observed for NCS; however, in the materials AgSCN 2AgNO3 and AgSCN AgClO4 a number unusual binding modes are seen.27 In both of these materials the NCS ligands coordinate five metal ions, three from the S end and two from the N end of the ligand. Despite the same binding modes of NCS ligands, different structural topologies result in these two materials. In AgSCN 2AgNO3 a corrugated 2D layer structure is formed by the AgSCN binding but this is linked into a 3D framework by bridging NO3 anions. On the other hand, in AgSCN AgClO4 the ClO4 anions do not participate in the framework structure and a 3D structure comprised of hexagonal columns results. In the case of NCSe and NCO there are far fewer coordination polymers where they have been observed as a bridging ligand. Regarding NCSe, one such example includes the material [Cd2(dmen)2(SeCN)4] (dmen ¼ N, N-dimethylethylenediamine), which contains NCSe ligands linking Cd21 centres in an EE fashion (Figure 5.7a).28 The octahedral Cd21 centres are doubly bridged by NCSe ligands and bidentate amine ligands, such that a zigzag 1D chain structure results (Figure 5.7b). A further example includes the anionic 3D framework structure [Cu2(CN)2(SeCN)2]2, which contains [Cu(en)2(H2O)]21 complexes in the cavities.29 The framework is comprised of Cu21 centres which are doubly bridged by EE NCSe ligands and singly bridged by CN ligands. The NCO ligand has been seen to bridge metal centres in the EO mode in the material [Cd3(NCO)6(2-amp)3] [2-amp ¼ 2-(aminomethyl)pyridine]


Figure 5.7

153

Coordination polymers containing NCX bridging ligands. The zigzag chain structures of (a) [Cd2(dmen)2(SeCN)4 and (b) [Cd3(NCO)6(2-amp)3].

(Figure 5.7b).30 In this case, Cd21 centres are doubly bridged by EE NCO ligands through the nitrogen atom. The chelated aromatic amine ligands are arranged in an up–down fashion such that zigzag 1D chains result.

5.2.4 Small Polynitrile Ligands (e.g. dca/tcm) Short polynitrile ligands such as dicyanamide (dca) and tricyanomethanide (tcm) are very effective for the formation of coordination polymers with transition metals as they show a large range of binding modes. In particular, the dca ligand has been reported numerous times and is of interest in magnetic coordination polymers as it provides a good exchange pathway. As for the other pseudohalide ligands, the dca and tcm anions can act as either terminal or bridging ligands in coordination polymers; examples of their bridging in such materials will also be focused on here. The dca anion, for example, can act as a 2-connecting ligand through both nitrile nitrogen atoms (m1,5) or through one nitrile and the amide nitrogen atom (m1,3), or as a 3-connecting ligand through both nitriles and the amide nitrogen (m1,3,5) and as a more complicated 4-connecting ligand (m1,1,3,5) (Figure 5.8). The related tcm anion shows similar binding modes. There are three basic types of extended lattices containing dca and tcm: M(dca)2, M(dca)(tcm) and M(tcm)2. These materials form rutile-like 3D networks in most cases; however, other analogues exist. For example, a series of binary a-M(dca)2 (M ¼ Cr, Mn, Fe, Co, Ni, Cu) compounds exist that form single rutile-like 3D networks, where octahedral metal centres are bound by donors from six different dca ligands in the m1,3,5-bridging mode (Figure 5.9a).31 The coordination is comprised of four nitrile nitrogen atoms in the equatorial positions and two amide nitrogen atoms in the axial positions. Conversely, b-M(dca)2 (M ¼ Zn, Co) forms corrugated (4,4) grids where tetrahedral metal ions are bridged by m1,5-dca ligands (Figure 5.9b).32 A tcm analogue of the binary dca nets exists, M(tcm)2 (M ¼ Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, Hg), which also forms rutile-like networks (Figure 5.9c).33 Owing to the larger size of the tcm ligand, the nets are doubly interpenetrated such that a densely packed network results where there are close M M internetwork interactions.

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Figure 5.8

Selected common binding modes of dca to transition metals (M).

Figure 5.9

Coordination polymers containing dca and tcm. (a) The binary a-M(dca)2 rutile net; (b) the 2D sheet structure of b-M(dca)2; (c) the doubly interpenetrated nets of a-M(tcm)2.

Furthermore, when both dca and tcm are incorporated into the one material of formula M(dca)(tcm) (M ¼ Co, Ni, Cu), a 3D self-penetrated network forms which is structurally intermediate between the parent M(dca)2 and M(tcm)2 compounds.34 This network is closely related to the rutile net of M(dca)2 but


155

contains a helical sub-structure instead of the original square channels, as a result of the different sizes of the dca and tcm ligands. Networks containing dca and tcm can be modified by incorporating terminal or bridging co-ligands, such as pyridine or 4,4 0 -bipy, to form 1-, 2- or 3D structures of the general formula M(dca)2(L)n. These materials form a variety of topologies based on the co-ligand employed. The use of terminal ligands often results in lower dimensional structures, such as in the material M(dca)2 (bpym) H2O (bpym ¼ 2,2 0 -bipyrimidine), which consists of zigzag 1D chains.35 Each metal centre in this material is coordinated by a chelating bpym ligand, cis-m1,5-dca ligands, a monodentate dca and an H2O molecule. Additionally, the more commonly observed double bridging of dca ligands is seen in Mn(dca)2(py)2, which also contains axial terminal py ligands.36 Higher dimensional structures are possible when using terminal co-ligands. For example, 2D (6,3) sheets are generated in the material [Cu(dca)(MeCN)], which contains m1,5dca ligands,37 and a 3D diamond-like network is generated in the material [Cu(pn)2][Mn(dca)4] (pn ¼ 1,3-diaminopropane) (Figure 5.10).38 The use of bridging co-ligands is common in dca and tcm containing materials, such that 1D chains or 2D sheets may be bridged into more dimensions. For example, the material b-M(dca)2(pyz) forms 2D (4,4)-grids comprised of doubly bridged dca ligands linked by pyz ligands,39 whereas the material Cu(dca)2(4,4 0 -bipy) H2O shows a (4,4)-grid structure that is interpenetrated owing to the larger bridging ligand.40 Using the same ligands an a-Po structure can also be generated, Co(dca)2(4,4 0 -bipy), which is doubly interpenetrated (Figure 5.11b).40 Lower dimensional materials also exist, such as [M(dca)2(bpe)] which is comprised of doubly bridged dca ligands and looping bpe ligands (Figure 5.11a).41 Lastly, anionic networks of the type M(dca)3 and M(dca)42 can be templated through the use of organic or inorganic counter-cations. The framework

Figure 5.10

Coordination polymers containing dca and terminal coligands. (a) The (6,3)-sheet structure in [Cu(dca)(MeCN)]; (b) the diamond-like network of [Cu(pn)2][Mn(dca)4].

156

Figure 5.11

Chapter 5

Coordination polymers containing dca/tcm and bridging coligands. (a) The 1D chain structure of [Mn(dca)2(bpe)]; (b) a doubly interpenetrated a-Po topology in Co(dca)2(4,4 0 -bipy).

in this case can potentially be directed through the size, shape and charge of the chosen counter-cations. For example, the metallo-cation M(2,2 0 -bipy)3 (M21 ¼ Fe, Ni) has been used to template (6,3) sheets of Mn(dca)3.42 In addition, 2D (4,4) grids of Mn(dca)3 have been templated through the use of Ph4E1 (E ¼ P, As) cations.43,44

5.3 Larger Nitrile Donor Ligands 5.3.1 Neutral Ligands Neutral polynitrile ligands are discussed separately from those anionic pseudohalide ligands described in the previous section and anionic larger polynitrile ligands as they have different binding affinities to transition metals. Being weak donor ligands, neutral nitrile ligands tend to bind well to soft metals such as Cu1 and Ag1. As such, transition metal coordination polymers containing such ligands are dominated by Cu1 and Ag1. Indeed, many of the early examples of the deliberate design and construction of coordination polymer come from the nitrile donor class of ligand. In pioneering work by Hoskins and Robson, the rigid four connecting ligand 4,4 0 ,400 ,4000 -tetracyanophenylmethane was employed with Cu1, both of tetrahedral geometry, to form a diamond-like net, [CuI{C(C6H4CN4)}]1 (Figure 5.12a).45 This network utilizes the tetrahedral geometry known for Cu1 and that of the polynitrile unit to form deliberately a network isostructural to the simple diamond network. 3-connecting nitrile donor ligands have been very successful in the formation of coordination polymers. For example, the relatively small ligand 1,3,5-tricyanobenzene (TCB) forms a honeycomb network with Ag1.46 Further early examples of stable porous framework materials were reported using the analogous 3-connecting large nitrile ligands 1,3,5-tris(4-ethynylbenzonitrile)


Figure 5.12

157

Neutral nitrile ligand containing coordination polymers. (a) The 3D diamond-like net [CuI{C(C6H4 CN4)}]+; (b) the porous coordination polymer [Agl(TEBB)(CF3SO3)] 2C6H6 and (c) the zig-zag chains of [Ag2(L)(H2O)2(SO3CF3)2] 0.5C6H6.

benzene (TEB) and 1,3,5-tris(3-ethynylbenzonitrile)benzene (TEBB).46,47 In the material [Agl(TEBB)(CF3SO3)] 2C6H6, the Ag1 centres are in a trigonal pyramidal coordination environment, comprised of three nitrile donors and a triflate anion (Figure 5.12b).47 Large pores are generated which are filled with solvent benzene. In addition, the 2-connecting nitrile ligand 4,4 0 -biphenyldicarbonitrile (BPCN) was investigated with a range of Ag1 salts with the aim to form interpenetrated diamonoid frameworks.48 In these materials, tetrahedral Ag1 ions are bridged by BPCN ligands to form 3D networks with 9-fold interpenetration. In order to emphasise the possibility of incorporating ligands with multiple functionalities, a fulvene-type bidentate ligand was investigated.49 This ligand forms a zigzag 1D chain structure with Ag1, [Ag2(L)(H2O)2(SO3CF3)2] 0.5C6H6, where the chains are doubly bridged by coordinated water molecules and are further crosslinked into pseudo-2D sheets via hydrogen bonding interactions (Figure 5.12c). Interestingly, there is a large blue emission luminescent shift from the free ligand to that contained in the coordination polymer, leading to possibilities of luminescent sensing. The 4-connecting neutral polynitrile ligand 1,2,4,5-tetracyanobenzene (tcnb) is also effective for the formation of framework materials with transition metals. This ligand can act as a pseudo-square-planar node, much like that of tcne and tcnq discussed in the following section. When, for example, this ligand is combined with tetrahedral Cu1 ions, a PtS-like framework is formed,

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[Cu(tcnb)](PF6). The interpenetration of nets is inhibited by the presence of counterions in the channels. Furthermore, under modified synthetic conditions the 3D material [Cu(tcnb)(THF)](PF6) forms, where the tcnb ligands bind through only three of the four available sites. Notably, these two framework materials can be interchanged through solvent variation.

5.3.2 Anionic Nitrile Ligands The anionic radical ligands tetracyanoethylene (tcne) and an analogue 7,7,8,8tetracyano-p-quinodimethane (tcnq) have been utilised in coordination polymers owing to their interesting electronic properties. The magnetic properties of such materials are described in detail in Chapter 9. These ligands can bind in a number of different bridging modes such that they act as 2-, 3- or 4-connectors. However, considering this versatile binding ability, there are relatively few reported structural analyses of their incorporation into transition metal coordination polymers. A partial explanation for this may be that they tend to form powdered samples rather than single crystals, hence structural analysis is more difficult. In any case, examples of 1-, 2- and 3D networks exist. There are a number of different approaches have been reported for the incorporation of the tcne ligand. In terms of magnetic systems there are materials of the form [MIII(porphyrin)(tcne)] and MII(tcne)2 n(guest) (M ¼ V, Mn, Fe, Co, Ni; guest ¼ acetonitrile, THF, DCM). The former tend to produce 1D chain materials where the equatorial coordinated M–porphyrin moieties are connected axially via tcne ligands. In this case, the choice of porphyrin ligand substituent groups may lead to changes in the magnetic behaviour.51,52 In terms of MII(tcne)2 n(guest)-type materials, different structural motifs have been reported depending on both the metal ion and the synthetic conditions. For example, for V(tcne)2 a 3D network structure is proposed where each metal is surrounded by up to six tcne ligands.53 The tcne and tcnq ligands can also be viewed as a square-planar node for the deliberate construction of framework topologies. Thus, when combined with tetrahedral Ag1 ions results in the formation of a PtS-like network of [Ag(tcnq)] which is doubly interpenetrated.54 Further to this, in the mixed ligand coordination polymer [Zn(tcnq)(4,4 0 -bipy] 6MeOH, distorted octahedral Zn21 ions are bridged equatorially by 4-connecting tcnq ligands to form 2D sheets (Figure 5.13b).55 These sheets are further bridged into a 3D pillared-layered material via 2-connecting 4,4 0 -bipy ligands. Other examples exist where these ligands have been combined with the dinuclear metal centre complex M2(RCO2)4 (M ¼ Cu, Rh, Ru, Mo; R ¼ alkyl). For example, a 2D square grid structure forms with M ¼ Rh, comprised of planar 4-connected tcne ligands connected by linear Rh2(O2CCF3)4 units.56 The material [Rh2(OCCF3)4]2(tcne) (C6H6) contains square grid cavities which are lined with –CF3 groups. A similar approach has been used for the tcnq analogue to form 2D square grids of [Rh2(O2CCF3)4]2(tcnq) 3(C7H8) (Figure 5.13a).57


Figure 5.13

159

(a) 2D sheets of [Rh2(O2CCF3)4]2(tcnq)] 3(C7H8); (b) the pillaredlayered 3D material [Zn(tcnq)(4,4 0 -bpy)] 6MeOH.

Lastly, there are a number of derivatives of tcne and tcnq that have been used in coordination polymers. For example, the radical ion linear connector 2, 5-dimethyl-N,N 0 -dicyanoquinonediimine (DM-DCNQI) forms a diamond-like 3D structure when combined with tetrahedral Cu1 centres.58 This material shows high metallic conductivity. In addition, a fluorinated tcnq analogue exists, 2,3,5,6-tetrafuoro-7,7,8,8-tetracyanoquinodimethanide (tcnqf4), which forms a 3D interpenetrated structure with tetrahedral Ag1 centres.59

5.4 Pyridyl Donor Ligands The transition metal chemistry of pyridyl donor ligands is very well established and in terms of coordination polymers there are three general classes of these ligands: 2-, 3- and 4-connecting. Examples of higher coordination pyridyl ligands (i.e. 5- and 6-connecting) in coordination polymers exist but are rare.

5.4.1 2-Connecting Ligands Some of the earliest examples of transition metal coordination polymers focused on the simple 2-connecting ligand pyrazine (pyz). This ligand is the next longest after cyanide and also forms a linear connection with transition metals. An early example includes the material [CoCl2(pyz)2] which forms a simple (4,4) grid structure.60 Additionally, 1D zigzag chains are formed tetrahedral Zn21 in the material [ZnBr2(pyz)] and square grids with octahedral Zn21 in the material [ZnBr2(pyz)2] (Figure 5.14a and b).61 A more complex example includes the material [Zn(pyz)Cl2], which through connection of linear Zn– pyz–Zn chains with double m-Cl bridges forms a 2D layered material.62 Furthermore, as a result of an unusual trigonal coordination of Cu1 in the material [CuI2(pz)3(SiF6)], a 2D honeycomb framework forms.63 These 2D sheets are interpenetrated in a pseudo-3D material which is fairly densely packed.

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Notably, the ligand piperazine (ppz) may also act as a 2-connecting ligand, such as in the material [Ag(ppz)2](BF4) which forms undulating 2D sheets of tetrahedrally connecting Ag1 ions and linear ppz ligands.64 Pyrimidine, which acts as a bent connector, has been used in much the same way as pyrazine.65 Such ligands may also be functionalised such as in 2- and 4-hydroxypyrimidine (n-pymo), for interesting framework topologies. These ligands form 3D zeolitic sodalite networks of the form [M(n-pymo)2] (M ¼ Pd21, Cu21) (Figure 5.14c). Importantly, these materials show a remarkably robust nature to guest removal and show sorption of a range of gas molecules into the cage-like cavities.66 For longer linear pyridyl ligands, 4,4 0 -bipy is an obvious choice and has been reported in numerous coordination polymers. An early example of this

Figure 5.14

Example coordination polymers using the short bridging ligands pyrazine and pyrimidine and analogues therein. (a) zigzag chains of [ZnBr2(pyz)], (b) (4,4) grids of [ZnBr2(pyz)2] and (c) 3D zeolitic sodalite networks of [M(n-pymo)2] (M = Pd2+, Cu2+).


Figure 5.15

161

Coordination frameworks using 4,4 0 -bipy. (a) The bilayer of [Ni2(4,4 0 bipy)3(NO3)4] 6EtOH; (b) the 3D network [Zn(4,4 0 -bipy)2(SiF6)].

ligand was in the doubly-interpenetrated coordination polymer Zn(4,4 0 bipy)2SiF6 2H2O.67 The octahedral Zn21 centres are coordinated by transH2O molecules and four bridging 4,4 0 -bipy ligands to form infinite 2D grids. Since then, there have been a multitude of coordination polymers containing 4,4 0 -bipy, where depending on the transition metal chosen, it may form frameworks with topologies including simple 1D chains, 2D ladders, honeycomb nets, square grids, brick-wall structures and 3D networks. For example, a 1D chain structure is seen in [Ag(4,4 0 -bipy)](BF4), where the Ag1 ions are linearly connected by 4,4 0 -bipy ligands,68 whereas when an octahedral metal is used a ladder structure may result such as for [Co2(4,4 0 -bipy)3(NO3)4].69 Also more complex structures may result, such as in the material [Ni2(4,4 0 -bipy)3 (NO3)4] 6EtOH, where octahedral Ni21 centres are coordinated by two bidentate nitrate anions and three 2-connecting 4,4 0 -bipy ligands such that a Tshaped connector is generated (Figure 5.15a).70 This extends to form an interlocked bilayer structure held together by a network of hydrogen bonds. Such a T-shaped coordination of 4,4 0 -bipy ligands around a metal ion is observed in a different manner in the material [Ag(4,4 0 -bipy)] (NO3), where 1D chains are crosslinked through Ag. . .Ag interactions, resulting in a 3D framework.71 Furthermore, a 3D material is formed with 4,4 0 -bipy in the material [Zn(4,4 0 -bipy)2(SiF6)], where square grids of Zn(4,4 0 -bipy)2 are linked by SiF6 anions (Figure 5.15b).72,73 The Cu analogue of this material shows a large methane storage capacity owing to the presence of micropores. There is an extensive range of other bridging pyridyl ligands which have been designed and reported in transition metal frameworks. There are examples where features have been varied, such as aromaticity, ligand length, binding angle, flexibility and functionality. A few demonstrative examples of each of these features are now given.

5.4.1.1

Aromaticity

Through adding ligands with an aromatic nature, there is the potential for strong intramolecular interactions to occur to give a robust nature. One example which

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shows this property is [Cu(DAP)2](PF6) (DAP ¼ 2,7-diazapyrene), which is a triply interpenetrated adamantoid-type 3D network.74 Owing to the aromatic nature of the ligand, there is an array of p–p stacking between interpenetrated nets. Furthermore, the long aromatic ligand 4,4 0 -bis(4-pyridyl)biphenyl (bpbp) forms square grids with Ni(NO3)2 of the formula [Ni(NO3)2(bpbp)2] 4(o-xylene)].75 Two parallel packing modes of adjacent layers within this material exist based on the guests occupying the channels. Indeed, a crystal transformation is triggered where the layers slide such that larger pores are present with exchange from o-xylene to mesitylene. This flexible nature is enabled through the presence of weak interactions between adjacent layers.

5.4.1.2

Ligand Length

Through using ligands of varying length, different framework topologies can result, mainly concerning the degree of interpenetration of nets, where longer ligands may result in more nets interpenetrating. For example, when the shorter 2-connecting ligand bis-(4-pyridyl)ethane-1,2-diol (bped) is used, a (4,4) grid structure is formed where the Fe21 centres are axially coordinated by NCS ligands and equatorially coordinated by bped ligands.76 In this case, double interpenetration of the 2D sheets results. However, when the long ligand 1, 4-bis(4-pyridyl)butadiyne (bpb) is used in the material [Fe(NCS)2(bpb)2] 0.5 (MeOH), triple interpenetration of the (4,4) grids results.77 In a very different example, the 2-connecting porphyrin ligand 5,15-di(4pyridyl)-10,20-diphenylporphyrin with Zn21 forms a robust 3D network (Figure 5.16b).78 The Zn21 metal ion is located in the centre of the porphyrin and is bound axially by the pyridyl groups of two different porphyrins, such that it is self-complementary. This 3D network is stable to a range of different guest molecules where it undergoes single-crystal transformations.

5.4.1.3

Binding Angle

The binding angle in rigid 2-connecting ligands can be tuned relatively easily through variation of the n-pyridyl (n ¼ 2, 3, 4) coordination of the pyridyl group. For example, the ligands 3,6-bis(3 0 -pyridyl)-1,2,4,5-tetrazine and 3,6bis(4 0 -pyridyl)-1,2,4,5-tetrazine (3-pytz and 4-pytz) form 1D chains with Ag1 where the metal ions are linearly coordinated by 2-connecting ligands. When 4-pytz is used linear 1D chains result, whereas when 3-pytz is used zigzag chains result.68 Additionally, through using ligands which are conformationally flexible, i.e. they have a high degree of variability in their geometry in bridging metals, framework topologies may be varied. For example, the ligand 1,3-bis(4-pyridyl)propane (bpp) and other related ligands can exist in either the cis or trans conformation. When in the cis conformation the ligand is an angular connector, for the formation of 1D looping chain structures, and when in the trans conformation the ligand is a linear connector, for the formation of square grids.


Figure 5.16

163

Example 2-connecting pyridyl ligands and frameworks formed with transition metals. (a) The robust 3D network formed with 5,15-di(4-pyridyl)10,20-diphenylporphyrin and Zn21; (b) the chiral square grid framework formed with 9,9-bis[(S)-2-methylbutyl]-2,7-bis(4-pyridylethynyl)fluorine and Cu(NO3)2.

For example, in a series of 2-connecting pyridyl ligands where the central alkyl chain length was systematically varied, a variety of framework topologies were observed owing to the different possible ligand conformations.79 In the material [Zn3(OH)3(bpp)3](NO3)3 nH2O, which contains trinuclear Zn3(OH)3 units, a 2D interwoven structure results where the ligand is in the trans conformation, whereas when the longer ligand 1,7-bis(4-pyridyl)heptane (bph) is used a 1D chain results where dinuclear Zn2(OH) units are linked by looping ligands in the cis conformation, [Zn2(OH)(bph)4](ClO4)3 nH2O.

5.4.1.4

Functionality

Extended 2-connecting pyridyl ligands are prime candidates for incorporation of functionality, allowing variation in, for example, chirality, hydrogen bonding sites, a hydrophobic or hydrophilic nature and steric bulk. For example, a

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Chapter 5

chiral square grid framework is formed with the ligand 9,9-bis[(S)-2-methylbutyl]-2,7-bis(4-pyridylethynyl)fluorine and Cu(NO3)2 (Figure 5.16c).80 In this case, large square channels (25 25 A˚) form, which are lined with the chiral functional groups of the ligand. In a similar way, the chemical nature of the pore lining can be directed with choice of ligand such as in the (4,4)-grid structure [Co(L)2(H2O)2] [L ¼ 2,3-bis(2-hydroxyethoxy)-1,4-bis(4-pyridyl) benzene], which contains 1D chains of hydrogen bonds.81 Bidentate and tridentate bridging pyridyl ligands have also been reported, such as those containing 2,2 0 :6 0 ,2 0 0 -terpyridine as bridging groups. For example, in the 1D coordination polymer [Co(4-pyterpy)Cl2] nX [X ¼ H2O or MeOH, 4-pyterpy ¼ 4 0 -(4000 -pyridyl)-2,2 0 :6 0 ,2 0 0 -terpyridine], distorted octahedral CoCl2 centres are coordinated by three pyridyl groups of the terpy portion of the ligand and a pyridyl group from the other end of another ligand.82 The chains are oriented in a head-to-tail fashion where adjacent chains are involved in a series of p–p interactions. In a more recent example, this ligand has been incorporated into a bimetallic 1D chain, [{(H2O)(NO3)2CuFe(4-pyterpy)2}2 (NO3)4 nMeCN nH2O].83

5.4.2 3-Connecting Ligands A simple triazine (tri) ring can act as a 3-connecting ligand in coordination polymers. For example, in [Cu3X3(tri)] (X ¼ Br, I), 3D networks are formed by linking Cu(X) columns by tridentate tri ligands.84 The three connecting pyridyl-based ligand 2,4,6-tris(4-pyridyl)-1,3,5-triazine (tpt) has been very successful for the formation of framework materials containing transition metals with interesting 2D and 3D topologies. For example, a 2D honeycomb sheet structure is formed with Cu21 acetate dimers and tpt, [{(Cu2(OCCH3)4}3(tpt)2] 2MeOH (Figure 5.17a).85 In this material, although the windows formed in the sheet are large, interpenetration does not occur, due to favourable intramolecular interactions. On the other hand, higher dimensional materials may be targeted through careful choice of the metal, such as when the tetrahedral metal centre Cu1is combined with tpt, the material [Cu3(tpt)4](ClO4)3 forms, where each metal centre is surrounded by four 3-connecting tpt ligands (Figure 5.17a).86 This extends to form a doubly interpenetrated cubic (3,4) network involving corner sharing of octahedral cages. Notably, in the framework material [Hg(tpt)2] (ClO4)2 6C2H2Cl4 despite steric considerations, six pyridyl groups surround one metal centre to form a (6,3)-connected net.87 A unique mixed ligand 3D material, [Zn(CN)(NO3)(tpt)2/3] 18solv, which contains large pores, is generated with Zn21 and tpt (Figure 5.17a).88 This material contains [Zn(CN)]4 squares which are trans coordinated by 3-connecting tpt units into cages; these cages share the Zn4 faces. Intriguingly, the interpenetration of these nets results in sealed off pores filled with essentially liquid solvent. Many of the tpt-containing frameworks, owing to the large ligand size, show large pore volumes and may therefore show interesting guest exchange


Figure 5.17

165

3-connecting pyridyl based ligands and example frameworks formed with transition metals (a) tpt: [Zn(CN)(NO3)(tpt)2/3] 18solv and [{(Cu2(OCCH3)4})3(tpt)2] 2MeOH; (b) hat: [Ag(HAT)(ClO4)] 2CH3NO2.

behaviour. For example, a sponge-like guest exchange behaviour is observed in the (10,3)-b network [(ZnI2)3(tpt)2] 6C6H5NO2.89 Each Zn21 centre is tetrahedrally coordinated by two pyridyl groups and two I ions. The networks are doubly interpenetrated but 60% of the void volume is still occupied by solvent molecules. The framework remains intact with solvent removal but shrinks significantly; this process is reversible. In another approach to incorporating 3-connecting pyridyl-based ligands in coordination polymers, the ligand 1,4,5,8,9,12-hexaazatriphenylene (HAT) was investigated.90 This ligand provides three chelating binding sites at 1201 angles and forms a highly symmetrical (10,3)-a network of the formula [Ag(HAT) (ClO4)] 2CH3NO2 (Figure 5.17b). The chiral channels apparent in this material are filled with both ClO4 anions and nitromethane solvent. Exposure to air results in absorption of atmospheric water, resulting in a single-crystal transformation. This rigid ligand not only provides strong binding sites for a rigid nature, but also short communication pathways between bridged metal ions, which highlights the possibility of magnetic/electronic exchange in coordination polymers.

5.4.3 4-Connecting Ligands There are far fewer examples of 4-connecting pyridyl ligands than 2- and 3-connectors. Work on 4-connecting ligands in coordination polymers has been mainly focused on porphyrin analogues, but several other ligand examples exist. Four pyridyl ligands are generally large so they tend to produce

166

Figure 5.18

Chapter 5

Examples of coordination polymers containing four-connecting pyridyl donor ligands. (a) 3D PtS network of [CuII(tpp)CuI]; (b) 2D square grids of [Cu(O2CCH3)2(H2O)2] + 4,4 0 -tpcb.

frameworks with large cavities. An early example includes the material [CuII(tpp)CuI] [tpp ¼ 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine], where a combination of tetrahedral Cu1 centres and square-planar CuII(tpp) centres results in the formation of a PtS network (Figure 5.18a).91 This network has large channels which are filled with disordered solvent and comprise at least half of the crystal volume. Furthermore, this ligand has been used to form the 3D bimetallic framework material [Cd2(NO3)4(Pd(tpp))](H2O)4.92 In this material, the octahedral Cd21 centres act as either bent or linear bridges between the tpp units depending on the cis or trans coordination of the tcp pyridyl groups. The 3D structure is thus comprised of 1D chains of linear connected tpp units which are interconnected into a 3D framework. A 2D square grid structure is formed using the approximately square-planar ligand tetrakis(4-pyridyl)cyclobutane (4,4 0 -tpcb) and the Cu21 paddlewheel dimer [Cu2(O2CCH3)4(H2O)2] (Figure 5.18b).93 Whereas usually in a square grid arrangement the metal ions occupy the vertices, here the cyclobutane portion of the ligand is in this position and the Cu dimers form the linear connectors. This results in the channels being lined with methyl functional groups associated with the Cu dimers. This material is stable to guest removal and exchange.

5.5 Five-membered Ring Nitrogen Donor Ligands Five-membered ring nitrogen donor (N-donor) ligand groups include imidazole, triazole, tetrazole and pyrazole. Ligands containing these groups have been used in much the same way as pyridyl donor ligands as they bind with a similar affinity to transition metals. Due to the possibility of multiple metal binding sites for triazole, tetrazole and pyrazole rings, a greater diversity of network topologies exists. This multiple binding possibility also allows for a more rigid coordination, much like metal carboxylate cluster chemistry, compared with single atom binding with pyridyl ligands.


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5.5.1 2-Connecting Ligands Where bridging 2-connecting five-membered N-donor ligands are incorporated into coordination polymers, they may occupy all the coordination sites around the metal or contain terminal coordinated solvent or anions, generally forming 3D or 2D networks, respectively. A simple 2-connecting ligand is presented in 1D chain materials which contain a single triazole (trz) ring. A basic example is the material [CuCl2(trz)], where octahedral Cu21 ions are bridged by a trz ligand and doubly bridged by Cl anions.94 In such a manner, a basic 1D chain structure is formed. Furthermore, a series of 1D chain materials exist where the counterions do not participate in the metal coordination. In this case the metal centres are triply bridged by triazole ligands to form materials of the general formula Fe (4-Rtrz)3]A2 nH2O (A ¼ anion; 4-Rtrz ¼ triazole ligands with various substituents in the 4-position).95 These materials have important implications in the field of polymeric spin crossover materials (Chapter 9). This triply bridged chain structure is also observed when single pyrazolate (pz) ligands are employed with Fe21 and Co21.96 In a similar fashion, single imidazole (im) rings have been used for the synthesis of a 3D framework of the formula [Co5(im)10] mb (mb ¼ 3-methylbutan-1-ol) (Figure 5.19a).97 Here, the Co21 centres are coordinated by four im ligands to form essentially 4-connecting tetrahedral units. These units are joined by Co21 atoms to form a 3D network with a topology similar to that of zeolitic neutral silicates. Additionally materials of the formula [M(im)2 xG] have been reported which also form zeolitic frameworks.98,99 Furthermore, short benzimidazolate type ligands have been used to form a series of zeolitelike framework topologies.100 The short bridging 4,4 0 -bitriazolyl- and 4,4 0 -bipyrazolyl-type ligand have been successful in the formation of a number of framework materials with

Figure 5.19

Examples which contain five-membered N-donor ligands. (a) The 3D zeolitic framework [Co5im10] 2mb; (b) the 2D polyrotaxane structure of [Zn(bix)2(NO3)2] 4.5H2O.

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Chapter 5

interesting topologies. The simple 2-connecting ligand 4,4 0 -bis-1,2,4-triazole (btr) has been incorporated into the 3D framework material [Fe(btr)3](ClO4)2, where each Fe21 centre is surrounded by six ligands.101 This material shows interesting magnetic properties. A more complicated 3D CsCl-type structure is formed in the material [Cu3Cl2(H2O)2(btr)4]2(ZnCl4)2(Cl3ZnOZnCl3), which contains trinuclear CuCl moieties linked by btr ligands bound in a similar fashion to the [M(4-Rtrz)3]A2 nH2O 1D chain materials.102 Furthermore, a similar ligand, 3,3 0 ,5,5 0 -tetramethyl-4,4 0 -bipyrazolyl, which is twisted owing to the steric constraint of the methyl groups, forms 3D frameworks with a number of different transition metals. For example, a doubly interpenetrated NbO network is formed with Fe(NCS)2.103 This material contains hexagonal solventfilled channels and is stable to guest removal and replacement. Further, examples exist where longer bridging ligands, such as 1,4-bis(tetrazol-1-yl)butane (btzb), have been incorporated into 3D networks.104 Longer imidazolate ligands have been used for the construction of interesting topologies. In particular, the ligand 1,4-bis(imidazol-1-ylmethyl)benzene (bix) has been reported in the 2D polyrotaxane structure [Zn(bix)2(NO3)2] 4.5H2O (Figure 5.19b).105 The structure consists of two independent sheets where macrocyclic rings of [Zn2(bix)2] centres are linked together by rods of bix. The polyrotaxane structure is generated when the macrocyclic rings of independent sheets are threaded by rings of the other sheet. In a different approach, the linear 2-connecting tetrazolate ligand 1,4-benzeneditetrazolate (BDT2) has been incorporated in to a number of 2D and 3D materials. The concept applied in this series of materials is to connect rigid clusters with diazolate ligands in an effort to make materials that exhibit stable porosity.106 This approach is much like that used in metal carboxylate chemistry where stable clusters are utilized as a building block in the formation of porous materials.107 The materials Mn3(BDT)2Cl2(DEF)6 and Mn4(BDT)3 (NO3)2(DEF)6 (N,N-diethylformamide) form 2D sheets. In the former material, 4-connecting Mn3 units are linked via BDT2 ligands to produce a neutral 2D framework, whereas in the latter, there are Mn2 paddlewheel units in which the metals are connected by three BDT2 ligands. Through variation of the synthetic conditions with this ligand, the 3D materials Zn3(BDT)3(DMF)4(H2O)2 3.5CH3OH, Mn3(BDT)3(DMF)4(H2O)2 Mn2(BDT)Cl2(DMF)2 1.5CH3OH H2O and 3CH3OH 2H2O DMF, Cu(BDT) (DMF) CH3OH 0.25DMF are generated.106 The first two materials are isostructural and consist of linear M3 units linked via BDT2 bridges to form neutral 3D frameworks. The third material consists of 1D helical chains of alternating Mn21 and Cl ions, which are connected through bridging BDT2 ligands into an (8,3)-b type of network with large channels. The last material is composed of infinite 1D chains of Cu21 ions linked by BDT2 ligands to form a 3D framework with square channels. These materials have been investigated for their hydrogen storage capability. As for 2-connecting pyridyl donor ligands, bidentate and tridentate ligands are possible which contain five-membered N-donor rings. For example, the double-ended terpy-style ligand 1,4-bis(1,2 0 :6 0 ,1 0 ’-bispyrazolylpyridine-4 0 -yl)

Transition Metal Coordination Polymers 21 108

benzene forms 1D chains with Fe . behaviour near room temperature.

169

This material shows magnetic switching

5.5.2 3-Connecting Ligands There are far fewer examples of 3-connecting five-membered N-donor ligands than for that of pyridyl donors. The examples that exist are dominated by imidazole donor groups. For example, the ligand 1,3,5-tris(imidazol-1-ylmethyl)2,4,6-trimethylbenzene (titmb) forms a honeycomb 2D sheet structure with octahedral Zn21, [Zn(titmb)2](NO3)2 2MeOH.109 Each Zn21 centre is coordinated by six imidazole ligands, which act as 3-connectors. Other examples of this ligand also exist, such as 1D chains where the ligand acts as only a 2-connector.109 Furthermore, the similar 3-connecting ligand 1,3-bis(1-imidazolyl)-5-(imidazol-1-ylmethyl)benzene (bimb) has been incorporated into the 2D structure [Mn(bimb)2](ClO4) (Figure 5.20a).110 In this material, each Mn21 ion is coordinated by six imidazole groups. Each ligand acts as a 3-connector and, owing to the flexible third arm of the bimb ligand, forms an unusual double 2D sheet structure. A recent example includes the 3-connecting benzenetristetrazolate ligand, which has been combined with Mn21 to form an interesting 3D framework topology (Figure 5.20b).111 In this material, the Mn21 centres form [Mn4Cl]61 clusters, which coordinate eight tetrazolate groups that are bound through two nitrogen atoms. The 3D material [Mn(DMF)6]3[(Mn4Cl)3(BTT)8(H2O)12]2 42DMF 11H2O 20CH3OH, which is described as a 3,8-connected net, is comprised of octahedral cages that are corner shared. Importantly, this material is stable in the absence of guest molecules and shows a large uptake of hydrogen gas.

Figure 5.20

Coordination polymers containing 3-connecting imidazole ligands. (a) The double 2D sheets of [Mn(bimb)2](ClO4); (b) the porous 3D network of [Mn (DMF)6]3[(Mn4Cl)3(BTT)8(H2O)12]2 42DMF 11H2O 20CH3OH.

170

Figure 5.21

Chapter 5

The 6-connecting ligand hexakis(imidazo1-1-ylmethy1)benzene (hkimb) which forms an a-Po type network with Cd2+.

5.5.3 Other Connectors The 6-connecting imidazole ligand hexakis(imidazo1-1-ylmethy1)benzene (hkimb) has been used for the construction of a 3D network (Figure 5.21).112 This ligand was chosen to demonstrate the possibility of extending bridging ligands beyond 2–4-connecting. The imidazole groups are small enough to allow six-coordination around the octahedral metal ion Cd21. Indeed, an a-Potype network is formed where six imidazole groups are coordinated to each Cd21 centre. The imidazole groups in each ligand are in an unusual conformation, where instead of being in an alternating ‘up–down’ fashion, in one group three are ‘up’ and the other three are ‘down’.

5.6 N-Oxide Ligands Apart from carboxylate ligands, O-donor ligands have received far less attention, in particular N-oxide ligands. Examples of N-oxide ligands are dominated by those that are bifunctional, such that one part of the ligand binds via an N-oxide but other binding groups are present, e.g. pyridyl or carboxylate groups. However, examples of purely N-oxide-containing ligands exist. The ligand 4,4 0 -bipyridine N,N 0 -oxide (O-4,4 0 -bipy) is an N-oxide analogue of 4,4 0 -bipyridine which shows a number of interesting binding modes with transition metals. In the material [Zn(MeOH)2(O-4,4 0 -bipy3)](SiF6) 3MeOH there are octahedral Zn21 centres which are coordinated by O-atoms from four O-4,4 0 -bipy ligands and two MeOH molecules (Figure 5.22a).113 Some of the O-4,4 0 -bipy ligands act as two-connectors via direct coordination to metal centres and other O-4,4 0 -bipy ligands bridge via hydrogen bonding with the bound MeOH molecules. Thus, if the hydrogen-bonded ligands are taken as 2-connected bridges, a 3D (6,3) net topology is formed. This ligand has further been used in the construction of a doubly interpenetrated coordination polymer

Transition Metal Coordination Polymers 21

21

171

114

with Co and Ni ions. In the material [M(NO3)2(H2O)4] (O-4,4 0 -bipy)2, an ReO3-like topology is observed through the linking of metal centres via hydrogen-bonded 2-connected O-4,4 0 -bipy ligands. The ligand 1,3-bis(4-pyridyl)propane N,N 0 -dioxide (O-bpp), a longer version of O-4,4 0 -bipy, has also been reported in a series of coordination polymers. In the materials [M(O-bpp)(H2O)(SCN)2]H (M ¼ Co, Cd, Mn, Zn), [M(O-bpp)2 (SCN)2]H (M ¼ Co, Cd) and [M(O-bpp)2(H2O)2(SCN)2] (M ¼ Mn, Zn), the O-bpp ligand acts in a number of different binding modes.115 For example, in the Co21 analogue [Co(O-bpp)(H2O)(SCN)2]H the metal centre is coordinated by cis-NCS anions, one water molecule and three fac-O-atoms from O-bpp ligands (Figure 5.22b). The O-bpp ligands are bound such that one end bridges two metals and the other end is unidentate; this is termed the m3-O,O,O 0 -mode. This extends to form undulating 2D sheets, whereas in the analogue [Co(Obpp)2(SCN)2]H the NCS ligands are now trans coordinated and there are no bound solvent molecules. Here, the O-bpp ligands all act as mononuclear nodes bridging between two metal ions only. A (4,4)-grid topology results in this case. In terms of mixed functional group N-oxide ligands, examples include pyridine carboxylate N-oxide (INO) and nicotinate N-oxide (NNO). In materials which contain these ligands other co-ligands have been employed, such as in the materials [M(L)(N3)(H2O)] (M ¼ Mn, Co, Ni) and [Cu(L)(N3)(H2O)0.5] (L ¼ INO or NNO).116 In each of these examples, the azide ligand is bound in the EO mode, such that M–N3/COO chains are generated. In the former example, the chains are linked into 2D layers by the pyridyl N-oxide of the ligand and the latter into a 3D structure (Figure 5.23). Other more complex mixed functional group N-oxide ligands exist which contain multiple carboxylate groups, such as pyridine-2,6-dicarboxylic acid N-oxide (pydco) and 2,2 0 -bipyridine-3,3 0 -dicarboxylate 1,1 0 -dioxide (bpdado).117,118 With pydco, two coordination polymers have been reported, [Zn(pydco)(H2O)] and

Figure 5.22

Coordination polymers which contain N-oxide ligands. (a) The material Zn(MeOH)2(O-4,4 0 -bipy3)](SiF6) 3MeOH, which shows a (6,3) topology; (b) the undulating 2D sheets of [Co(O-bpp)(H2O)(SCN)2]H.

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Figure 5.23

Chapter 5

Coordination polymers containing mixed functional group N-oxide ligands. (a) [Co(INO)(N3)(H2O); (b) [Cu(INO)(N3)(H2O)0.5].

[Cu(pydco)2]; the former results in 1D double helical chains linked in to 2D by water molecules and the latter forms 1D infinite ladders.118 In both cases the carboxylate and groups act in a monodentate fashion. However, in the Zn analogue the N-oxide is monodentate and in the Cu analogue the N-oxide bridges two metal atoms. In the case of bpdado, a 1D tubular structure is formed.117 In [Cu2(bpdado)2(H2O)2] H2O the bpdado ligands coordinate through both N-oxide groups and monodentate via each carboxylate group.

5.7 Carboxylate Ligands Examples of transition metal-containing framework materials with carboxylate ligands are very abundant due to both the strong bonds that they form and the wide range of such possible ligands. The carboxylate group can bind in either a monodentate or a bidentate fashion to a metal ion and hence there are two approaches to their incorporation into coordination polymers. Where monodentate binding is apparent, carboxylate ligands are used as basic geometric connectors as for pyridyl donor ligands which can only bind in a monodentate fashion. Such M–O bonds are strong but not as strong as when bidentate binding occurs. The bidentate binding of carboxylate ligands often results in the in situ formation of inorganic clusters within coordination polymers. These inorganic clusters are termed structural building units (SBUs) and when linked via the bridging carboxylate ligands tend to form rigid metal–organic frameworks (MOFs). In this way, a series of isostructural materials (IRMOFs) have


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been generated using varying lengths and functional groups on bridging carboxylate ligands. There are three common types of inorganic cluster groups which have been incorporated into MOFs: the paddlewheel, which has four binding sites and two terminal sites, the octahedral ‘zinc acetate’ node and the trigonal prismatic trimer with an oxo centre, which has three terminal ligand sites. These clusters each act as rigid nodes and in the cases where there are terminal ligands, such as water, attached to the cluster the number of nodes may be varied through ligand exchange of the terminal groups for bridging groups. In this same manner, the terminal groups may be exchanged for other terminal groups to functionalise the lining of the framework. The types of inorganic clusters are reviewed in more detail in Chapter 9. Additionally, a series of hybrid open-framework materials have been constructed from inorganic chains linked by carboxylate ligands.

5.7.1 2-Connecting Ligands There are numerous examples of 2-connecting carboxylate ligands used in coordination polymers. Indeed, 2-connecting carboxylate and 2-connecting pyridyl ligands are by far the most commonly reported types of ligands. Both monodentate and bidentate binding modes have been observed and in some cases both in the one material. Selected examples will be given here which show the scope of binding modes and additional functional groups that can be explored in such materials. The 2-connecting ligand benzene-1,4-dicarboxylic acid (1,4-bdc) is very common in coordination polymer chemistry. This ligand is readily available and binds well to transition metals. For example, it has been used in the 3D coordination polymer [Cd(1,4-bdc)(py)], where the Cd21 centres are coordinated by two bidentate ligands and two monodentate ligands (Figure 5.24a).119 Interestingly, this material shows a significant enhancement of the fluorescence signal compared with the free ligand. This ligand has also been used in the material [FeIII 2 O(OCCH3)2(1,4bdc)] 2(MeOH) (MIL-85), which is built up from Fe31 octahedra linked by 1,4-bdc ligands and acetate groups.120 Helical 1D chains are thus formed which are linked into 3D via 1,4-bdc ligands. Furthermore, a Cr31 dicarboxylate material has been reported which consists of metal octahedra in 1D chains linked by 1,4-bdc ligands into a 3D structure (MIL-53).121 Also in this series of materials is the 3D material Cr3F(H2O)2O(1,4-bdc)3 nH2O (MIL-101), which is built from trimeric building blocks chelated by 1,4-bdc ligands.122 This material forms a zeolitic-like cubic structure which contains very large pores. The angular version of 1,4-bdc is 1,3-bdc and it has been used in much the same way. For example, it forms the 3D material [KCo7(OH)3(1,3bdc)6(H2O)4] 12H2O, which shows interesting magnetic behaviour dependent on the presence or absence of guest molecules (Figure 5.24a).123 This material is comprised of Co7 clusters of octahedral Co21 atoms connected by a combination of ligands (bi- and tridentate), m3-OH groups and water molecules. Each

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Chapter 5

Co7 cluster is joined to six other units via 1,3-bdc ligands to form an a-Po-type network. As for pyridyl 2-connecting ligands, chiral 2-connecting carboxylate ligands have also been explored in order to line the pore surface with such functionalities. This is nicely shown with the ligand 6,6 0 -dichloro-2,2 0 -diethoxy-1,1 0 binaphthylene-4,4 0 -dicarboxylic acid (bda), which forms a (4,4) grid structure with a range of divalent transition metal salts of the general formula [M2(mH2O)(bda)2(py)3(dmf)] (DMF) (H2O) (M ¼ Mn, Co, Ni) (Figure 5.24b).124 The manganese structure consists of dimetallic carboxylate units which are bridged by bda ligands, two which are monodentate and two bidentate. In this way, 2D grids are formed where the 1,1 0 -bi-2-naphthol functional groups are located in the pores. Unfortunately, offset stacking of parallel grids inhibits porosity in this material for further chiral applications.

Figure 5.24

2-connecting carboxylate ligands in coordination polymers. (a) The material [Cd(1,4-bdc)(py)] which forms a 3D network; (b) the (4,4) grid structure of [M2(m-H2O)(bda)2(py)3(dmf)] (DMF) (H2O).


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In a different approach, the ligand D-saccharate (C6H8O8), an aldaric acid produced readily from D-glucose, contains carboxylate groups at each end. This ligand has been used to form the 3D coordination polymer [Zn(C6H8O8)] 2H2O, where both of the carboxylate groups are bound in a monodentate fashion and are further chelated by hydroxyl groups. In this way, each ligand coordinates four metal centres and each metal centre coordinates four ligands, such that a 3D 4-connected net is formed.125 Interestingly, in this material there are two types of square channels, one being hydrophobic and the other hydrophilic in nature. With regard to 2-connecting carboxylate ligands in conjunction with inorganic clusters, there are a number of interesting series which have been compiled using strategic design principles. Importantly, an isoreticular series of porous materials containing the basic zinc acetate SBU and dicarboxylate ligands exists. These materials all have the general formula ZnO4(L)3 and form cubic structures such that each metal cluster is surrounded by six carboxylate groups from the bridging ligands. This basic unit extends to form 3D networks of CaB6 topology (Figure 5.25a).107 The original example in the series contains the ligand 1,4-bdc (MOF-5) and formed a network which contained 80% void volume (Figure 5.25a).126 From this point, a whole range of other dicarboxylate ligands were used to form isostructural materials in which the ligand length and functional groups were varied. Ligand examples include naphthalene-2,6-dicarboxylate (IRMOF-8), pyrene-2,7-dicarboyxlate (IRMOF-12) and terphenyl-4,4 0 0 -dicarboxylate (IRMOF-16) (Figure 5.25b).107 Notably, a few members of the series, mostly where the ligands are longer, form interpenetrated nets of the same topology; however, non-interpenetrated analogues of these materials can be formed also through variations in the concentrations used in the synthesis.107 The dinuclear copper paddlewheel cluster is also useful for the formation of 2D and 3D rigid frameworks and has the added advantage that the terminal axial ligands may be replaced in situ for functionalisation. This cluster can act as a square-planar node such as in the material [Cu2(o-Br-bdc)2(H2O)2] (DMF)8 (H2O)2, in which, owing to a 901 twist of the carboxylate groups at each end of the ligand forced by the bromine groups, an NbO-type 3D structures results (MOF-101).127 This network is non-interpenetrating, which is surprising considering the large open voids which account for 79% of the crystal space. Further to this, a series of paddlewheel cluster-containing materials exist where the cluster acts as an octahedral node.128–130 In this manner, square grids of 2-connecting carboxylate ligands (e.g. fumarate, terephthalate, styrene dicarboxylate and 4,4 0 -biphenyl dicarboxylate) linked via paddlewheel clusters are extended into 3D using diamine pillars.

5.7.2 3-Connecting Ligands The 3-connecting ligand btc, when bound in a monodentate fashion through the carboxylate groups, has been used to form 2D and 3D structures. For

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Figure 5.25

Chapter 5

(a) General cubic 3D framework formed in the MOF series and representation of MOF-5; (b) example 2-connecting carboxylate ligands that are employed in this series. Reprinted with permission from Science, 295, 469–472, copyright 2002 AAAS.

example, when an octahedral transition metal is axially bound by planar btc ligands, a 2D (6,3) net structure results, with the general formula [Ni3(btc)2].131 Alternatively, in the material [Ni3(btc)2(eg)6(py)6] 4H2O 3eg (eg ¼ ethane-1,2diol), where octahedral Ni21 centres are axially coordinated by 3-connecting btc ligands which are twisted rather than planar across the metal atom, a 3D structure results (Figure 5.26a).132 This particular material extends to form a four-fold interpenetrated chiral (10,3)-a net. In another approach, a bismacrocyclic nickel complex, [Ni2(C26H52N10)(Cl)4] H2O, was used in combination with btc to form a pillared-layered bilayer type.133 In this material, each Ni macrocycle is trans-coordinated to btc ligands and each btc acts as a 3-connector. In this way, 2D brick layer motifs are formed which are bridged by the bismacrocycle xylyl groups. Furthermore, both mono- and bidentate binding of carboxylate groups have been observed in the one framework material such as in the sheet structure of [Co(btc)3(py)2].134,135 Importantly by combining 3-connecting carboxylate ligands and the paddlewheel inorganic cluster can lead to interesting 3D metal–organic frameworks. A well-known example of this type of material is the Pt3O4-type 3D framework [Cu3(btc)2] 3H2O (HKUST-1).136 In this material, the btc ligands are bound in a bidentate fashion within the inorganic paddlewheel clusters. This material contains large voids which are filled with disordered solvent. In a similar way, a larger analogue of btc, 4,4 0 ,400 -benzene-1,3,5-triyltribenzoic acid (btb), has been


177

combined with the Cu paddlewheel cluster to form a 3D framework. The material [Cu3(btb)2(H2O)3] (DMF)9 (H2O)2 also shows a Pt3O4-type structure and in this case, owing to the larger size of the ligand, it is doubly interpenetrating.137 Furthermore, 3-connecting carboxylate ligands have been used in combination with the basic zinc acetate cluster to form 3D structures. For example, the ligand benzene-1,3,5-tribenzoate (btb) has been used in the material [Zn4O(btb)2] (MOF-177), where each cluster is linked to six btb ligands (Figure 5.26b);138 a remarkably large surface area results. In another example, the 3-connecting ligand 4,4 0 ,4 0 0 -tricarboxytriphenylamine (tca) has been used with the zinc acetate cluster to form the material [Zn4O(tca)2] (DMF)3(H2O)3 (MOF-150). Here, four [ZnO4] tetrahedra which share a common corner are linked by the 3-connecting tca ligands to form a 3D pyrite-type structure (FeS2).139 Materials which contain the oxo trimer cluster have been reported, such as the material [Cr3F(H2O)3O(btc)2] nH2O (MIL-100), which contains the 3-connecting ligand btc and forms a stable 3D zeolite topology.140

5.7.3 4-Connecting Ligands There are a number of examples of 4-connecting carboxylate ligands that have been incorporated into coordination polymers with transition metals. The ligand benzene-1,2,4,5-tetracarboxylate (btec), which is the 4-connecting analogue of btc, is one such example. This ligand has been incorporated into 3D structures including the material [Zn(btec)12], which shows an unusual (4,8)connected topology.141 The structure is build up from tetrahedral Zn21 centres which are bridged by four bidentate btec ligands. In this way, Zn–CO2 2D

Figure 5.26

Example coordination polymers using 3-connected carboxylate ligands. (a) the 4 x (10,3)-a net of [Ni3(btc)2(eg)6(py)6] 4H2O 3eg (single net shown); (b) the 3D network of [Zn4O(btb)2] (MOF-177).

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layers are formed which are linked into 3D via the btec benzene groups, somewhat like a pillared-layered material. The ligand tetrahydrofuran-2,3,4,5-tetracarboxylic acid (thftca) has been incorporated into the porous network [Zn3(thftca)(OH)2(H2O)1.33] 3H2O.127 This structure is built from ZnO6 clusters connected by thftca ligands which actually act as nine-dentate ligands. Large channels are generated along one direction. This structure also contains lone ZnO6 octahedra in the channels, which undergo an interesting reversible translation with desolvation. The paddlewheel cluster has also been combined with four connecting carboxylate ligands, e.g. the material [Cu2(H2O)2(adip)] 2DMF [H4adip ¼ 5,5 0 (anthracene-9,10-diyl)diisophthalate], which contains large cubo-octahedral cages linked into a 3D framework.143 Furthermore, the 4-connecting ligand biphenyl-3,3 0 ,5,5 0 -tetracarboxylic acid (bptc) has been used in a similar fashion to form a 3D framework with NbO topology, [Cu2(bptc)(H2O)2(dmf)3(H2O)] (MOF-505).144 In another study, the length of 4-connecting carboxylate ligands of this type was varied by extending the central phenyl portion of the ligands to form materials of the type [Cu2(L)(H2O)2] (Figure 5.27).145 In such a way, analogous NbO networks were formed with increasingly larger pore diameters. Additionally, with the paddlewheel SBU, the 4-connecting ligands adamantane-1,3,5,7-tetracarboxylate (atc) and the longer version adamantanetetrabenzoate (atb), PtS-type networks have been formed.146,147

5.8 Other Ligands There are many other ligands in transition metal coordination polymers which do not fall into the general classes described above. A few interesting examples will be presented here which are either used extensively or show an interesting approach. First, the oxalate dianion ligand has been observed in many transition metal coordination polymers and can act as a 2-connecting bidentate bridging ligand. In this way, there are two general families of materials which contain the oxalate dianion which may form 2D or 3D materials. Anionic layered 2D structures can be formed of the general formula [MIIMIII(ox)3] when cations of the type [XR4]1 are used (X ¼ N, P and R ¼ alkyl group). The materials [Fe(Cp*)2][MIIMIII(ox)3] (MII ¼ Mn, Fe, Co, Ni, Cu, Zn; MIII ¼ Cr, Fe) also form 2D structures which consist of honeycomb layers of [MIIMIII(ox)3] with [Fe(Cp*)2] cations in the channels.148 On the other hand, a 3D structure may be generated, [M2II(ox)3]2, [MIMIII(ox)3]2 or [MIIMIII(ox)3]n, when a tris-chelated transition metal diimine cation is used, e.g. [MII/III (bpy)3]m1. The material rad[MnIICr(ox)3] [rad1 ¼ 2-(1-methylpyridinium-4-yl)4,4,5,5-tetamethylimidazolin-1-oxyl-3-oxide], has a dimetallic 3D network formed by Cr(ox)3 units linked by Mn(ox)3(H2O) units. The radicals are located in zigzag chains within the network.149 The oxalate ligand is also used extensively in the formation of mixed ligand coordination polymers.


Figure 5.27

179

4-Connecting carboxylate ligands. (a) The 3D structure of [Cu2(L)(H2O)2]; (b) the ligands used in its analogues.

Second, the simple, short 4-connecting ligand hexamethylenetetramine (hmt), with four tetrahedrally arranged amine nitrogen donors, has been used extensively in coordination polymers. For example, in the 2D structure [(HgCl2)2(hmt)], the hmt ligand connects four HgCl2 centres into puckered (4,4) grids (Figure 5.28a).150 The presence of Hg Cl interactions between adjacent sheets gives this material a pseudo-3D character. Also, this ligand has been used with Ag1 to form a square grid structure, [Ag2(Tos)2(hmt)].151 In this example, the Ag1 ions act as bent rather than linear connectors and, as observed above, the hmt ligand acts as a 4-connector. On the other hand, in the related material [Ag3(hmt)2(H2O)4](PF6)3 a hexagonal sheet structure is formed as both the hmt ligands and Ag1 ions act as 3-connectors.151 In this structure, a series of hydrogen bonding interactions between sheets results in a pseudo-3D array. The hmt ligand has also been incorporated into an undulating 2D sheet structure which contains copper carboxylate dimers as linkers.152 Also, an interesting cationic 3D structure exists with this ligand, [Ag(hmt)] (PF6) H2O,153 where trigonal Ag1 centres are linked by tridentate hmt ligands. Third, the formate ligand, which can act as a 3-connector, has been used in much the same way as azide in magnetic materials. The formate ligand can link two or three metal ions and despite its short nature can be used for the formation of porous framework materials. For example, a 3D diamond-like framework has been reported, [Mn3(HCOO)6] (MeOH) (H2O), which is built up from MnMn4 tetrahedral units which have an Mn21 core connected to four other Mn21 ions via six formate ligands.154 Additionally, another 3D structure containing formate, Mn(HCOO)3, which contains octahedrally coordinated Mn31 ions acts as a host to carbon dioxide molecules.155 There are a range of other ligands that have been reported that contain donor groups which have not been greatly explored in coordination polymer chemistry. One such class of material is those containing pyridone units.

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For example, the 2-connecting pyridone ligand N,N 0 -o-phenylenedimethylenebis(pyridin-4-one) (o-XBP4) has been used to form a sheet structure, [Mn (o-XBP4)(H2O)2(NO3)](NO3). In this material, Mn(H2O)2(NO3) units are linked by o-XBP4 ligands into 1D chains, which are further joined by nitrates.156 Additionally, the potentially 6-connecting ligand hexakis(N-pyridin-4-onemethyl)benzene (HPMB) has been reported. This ligand forms the 3D structure [Co(HPMB)](BF4)2 8MeOH3.5 H2O where all oxygen atoms on the ligand are coordinated to Co21 ions.157 Additionally, phosphane ligands have been used in coordination polymers. For example, the bulky triphosphane ligand 1,3,5-tris(diphenylphosphanyl) benzene (tdpp) forms a 2D hexagonal layer structure with Ag1.158 In particular, the material [Ag4(tdpp)3(OTf)4] forms through the coordination of trigonal planar metal centres by 3-connecting ligands (Figure 5.28c). There are therefore very wide channels generated and, owing to the bulky nature of the phenyl rings, interpenetration of nets does not occur. Additionally, a number of large organic radical ligands based on the PMT (polychlorinated triphenylmethyl) radical have been used to construct porous framework materials. In this novel approach, a 3-connecting polychlorinated triphenylmethyltricarboxylic acid radical ligand, PTMTC, was incorporated into a 2D honeycomb structure, [Cu3(PTMTC)2(py)6(EtOH)2(H2O)] (MOROF-1) (Figure 5.28b).159 The square-pyramidal Cu21 ions are coordinated by two monodentate carboxylate groups, two terminal py groups and solvent molecules. Thus as each metal is 2-connecting and each ligand is 3connecting, 2D sheets of (6,3) nets are formed which contain large pores. This ligand has also been used for the construction a (6,3) helical structure in the material [Co6(PTMTC)4(py)17(H2O)4(EtOH)] (MOROF-3), which also contains large channels.160

5.9 Mixed Donor Atom Ligands Ligands which contain both pyridyl and carboxylate functional groups have been very successful for the formation of porous framework materials, for example pyrazine-2-carboxylate (2-pyc) and the longer version 4-(pyridin-4-yl) pyridine-2-carboxylic acid (ppca). A 3D cuboid framework is formed using 2pyc, [Cu(2-pyc)2HgI2] HgI2 (Figure 5.29a).161 This material contains large square channels which encapsulate linear HgI2 molecules. The larger ligand ppca is used in a similar fashion to create the mixed metal framework material [Co2(ppca)2(H2O)(V4O12)0.5] nH2O (Figure 5.29b).162 In this structure, mixed metal 1D chains are bridged to form a 3D structure. This material undergoes an interesting single-crystal transformation associated with the dehydration process. A more complex example includes the material [Zn3(m3-O)(L)] 2H3O 12H2O (D-POST), where L is a chiral pyridyl carboxylate ligand.163 The concept in this material was to use the carboxylate portion of the ligand to form oxo-bridged trinuclear clusters which could be linked together via the pyridyl


Figure 5.28

181

Other examples of ligands incorporated into coordination polymers. (a) [(HgCl2)2(hmt)]; (b) [Cu3(PTMTC)2(py)6(EtOH)2(H2O)] (MOROF-1); (c) [Ag4(tdpp)3(OTf)4].

groups. Indeed, a 3D material forms, which contains large chiral channels. Importantly, this material shows enantioselective inclusion of metal complexes in its pores.

5.10 Mixed Ligand Coordination Polymers There are many coordination polymers which have been designed and synthesised utilising the properties of more than one bridging ligand. This approach also has benefits when using a combination of bridging ligands and termini to block certain metal coordination sites, and some examples have already been discussed throughout this chapter. Of more interest here are systems where a combination of bridging ligands is used in the one material. This approach has been used regularly in the pseudohalide class of ligands, which are short and rigid. In combination with longer bridging ligands, materials may form with interesting topologies and the possibility of achieving larger voids than may have been seen using purely these short ligands. This approach also allows the pore linings to be tuned as required through the more versatile nature of larger bridging ligands such as pyridyl and carboxylate donors. A nice example of this is metallocyanide sheets bridged by pyridyl ligands, such as in the 3D material [Fe(pyrazine)2M(CN)4] 4H2O (M ¼ Ni, Pt, Pd).164 In this material, the square-planar M(CN)4 centres are connected into (4,4) grid sheets by Fe21 atoms. These grids are further bridged into 3D via 2connecting pyrazine ligands. There are a number of such materials, which are called Hoffmann clathrate-type compounds, where both the bridging pyridyl

182

Figure 5.29

Chapter 5

Examples of pyridyl carboxylate containing coordination polymers. (a) [Cu(2-pyc)2HgI2] HgI2; (b) [Co2(ppca)2(H2O)(V4O12)0.5] nH2O.

group and the connectivity of the metal–cyanide portion have been modified. For example, using the linear 2-connecting unit [Cu(CN)2], instead of a squareplanar group, linked by pyrimidine groups, results in the formation of a 2D sheet structure, Fe(pyrimidine)2[Cu(CN)2]2.165 These materials have implications in cooperative spin crossover behaviour. Furthermore, a large series of pillared-layered materials exist where the mixed functional group ligand pyrazine-2,3-dicarboxylate (pzdc) forms 2D layers of the type Cu(pzdc) (Figure 5.30). In an innovative approach, these layers can be systematically linked into 3D structures using 2-connecting pyridyl donor ligands,166 e.g. when pyz is used a small interlayer space is generated, and thus smaller cavities, whereas when 4,4 0 -bipy is used a larger, more spacious interlayer space results. A good example of where the pillar ligand induces a specific chemical environment is [Cu2(pzdc)2(dpyg)] 8H2O [dpyg ¼ 1,2-di(4-pyridyl)glycol], where the


Figure 5.30

183

Representation of the pillared-layered materials [Cu2(pzdc)2(L)] where the pillared (L) can be modified as indicated.

pillar ligand dpyg contains –OH groups.167 These groups are directed into the channels, hence providing a hydrophilic environment. Indeed, solvent water molecules are located in the pores and hydrogen bond to the bpyg ligands. A similar situation is observed when the ligand n-(4-pyridyl)isonicotinamide is incorporated into a pillared-layered material.166 Other analogues of the pillared-layered materials exist where the pillars do not connect in a linear fashion but bridge in a cross manner. For example, when the ligand 4,4 0 -azpy is used this arrangement results when there are significant hydrogen bonding interactions between adjacent pillars.168

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CHAPTER 6

Rare Earth Coordination Polymers 6.1 Introduction The previous chapter demonstrated the myriad of ways in which transition metals can be incorporated into coordination polymers. The regular and predictable geometries of transition metals have been a major influence on their dominance of network design. Polymers containing metals of the lanthanoid series, which along with yttrium, scandium and lanthanum are termed the rare earth metals, have received considerably less attention although there has been an increasing number of occurrences in the literature in recent years.1,2 The rare earth metals have several properties that make them more difficult to form ‘predictable’ coordination polymers. The major reason is their lack of predictable coordination geometries. Whereas coordinative bonding in d-block metals is governed by the electrons in the d-orbitals, giving rise to rigid polyhedra, the bonding of the f-block elements is predominantly ionic in nature, leading to highly unpredictable coordination geometries.3,4 It is rare to see examples of regular geometries, such as octahedral or tetrahedral, for rare earth metals and such geometries tend to arise due to the steric demands of the ligands rather than the electronic effects of the metal ion. A further complication with rare earth ions is their size, which, coupled with their lack of rigid coordination geometries, leads to high coordination numbers. Typically, the rare earths have coordination numbers in the range 7–10, although complexes are known in the range 2–12. Very low coordination numbers are only observed in complexes containing extremely bulky ligands and conversely those with very high coordination numbers are found in complexes with sterically undemanding ligands. The lanthanoid contraction also has an effect on the unpredictability of network structures, with the size of the lanthanoid ions decreasing substantially across the period and therefore resulting in decreasing coordination numbers. Coordination Polymers: Design, Analysis and Application By Stuart R. Batten, Suzanne M. Neville and David R. Turner r Stuart R. Batten, Suzanne M. Neville and David R. Turner, 2009 Published by the Royal Society of Chemistry, www.rsc.org

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Structures obtained using one metal may not be reproducible with a different element, with gadolinium, in the centre of the period, often acting as the ‘watershed’ between differing structures of the heavy and light lanthanoid elements. A further complication of rare earth metals can be their oxophilic nature, making aqueous synthesis challenging due to their affinity for aqua ligands. Similarly, there exists an affinity towards strongly coordinating oxo anions such as nitrate. Although sometimes problematic, their oxophilicity can also be advantageous when preparing heterometallic complexes that incorporate d-block metals as different binding sites can be constructed to account for the relative affinities of the different metal ions. Another result of the oxophilic nature of rare earth metals is that a significant portion of synthetic work is conducted under inert atmospheres to prevent the incorporation of oxide/ hydroxide into the complexes, and in many cases the final products can also display sensitivity to oxygen and/or water. Despite the challenges inherent to the design and synthesis of lanthanoid coordination polymers, there are considerable rewards from the construction of such systems.5 Many of the rare earth metals have unique properties, especially in terms of luminescent activity, that make their complexes attractive synthetic targets. In particular, complexes of terbium and erbium are known to display significant luminescent properties that may be exploited in deliberately designed materials. There is increasing interest in the magnetic properties of the rare earth metals, particularly in those with a large number of unpaired electrons such as gadolinium and samarium. For example, known magnetic materials include a number of mixed 3d/4f alloys. There also exists the possibility of functional networks that can utilise the different oxidation states of the rare earth metals, which does not affect the coordination geometry to the same extent as for transition metals. For purely aesthetic reasons, rare earth coordination polymers can also be unusual due to their unique coordination geometries. The high coordination numbers can give rise to unusual network topologies with 7- or 8-connecting nodes not uncommon in such systems. Rare earth coordination polymers have also shown promise in the construction of non-linear optical (NLO) materials and porous networks. Due to their oxophilic nature, the bulk of work on rare earth polymers has been carried out using carboxylate derivatives; however, there are a significant number of interesting systems that use nitrogen donor systems. Discussion in this chapter will be divided by ligand systems, examining the influence of the ligand binding mode on the structure of the coordination polymers.

6.2 Cyanide/Nitrile and Pseudohalide Ligands Although rare earth elements are oxophilic and largely form coordination polymers with oxygen-donor ligands, there is a significant body of research examining the use of nitrile-based ligands to form functional lanthanoid networks. Typically, nitrile-based lanthanoid coordination networks are synthesised under rigorously anhydrous conditions using non-coordinating solvents or

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at least solvents that cannot act as oxygen-donor ligands. Such work incorporates simple cyanide ligands or cyanates (commonly grouped as pseudohalide ligands) with larger polynitrile ligands such as dicyanamide or tricyanomethanide and tetracyanoethylene (TCNE). Polycyano species such as [Fe(CN)6]3 have also been combined into polymers with rare earths, although these are discussed separately alongside other mixed 3d/4f polymers (see Section 6.8). Small, linear ligands such as azide and (thio)cyanate are often used in transition metal polymers, but occur infrequently in those containing rare earth elements. One of the few rare earth–azide polymers known, presumably due to their explosive nature, is (NH3CH2CH2NH3)[Nd2(N3)8].6 The anionic framework contains three distinct azide binding modes: terminal, bridging via a single nitrogen atom or bridging between three Nd atoms with one end of the ligand Z1 and the other being m2 (Figure 6.1a). Isocyanate ligands have been observed to form bridging binding modes between 4f/4f and 3d/4f atoms pairs via the terminal nitrogen atom in the polymer [Eu2Ni(OCN)8(DMF)6] (Figure 6.1b).7 Dicyanamide (dca) and tricyanomethanide (tcm) have been extensively used in transition metal chemistry to synthesise novel architectures and to provide magnetic communication between metals (see Section 5.4). Rare earth dicyanamide complexes have been prepared via ion exchange and structural analysis

Figure 6.1

(a) The anionic polymer [Nd2(N3)8]2 contains m3 and m2 azide ligands (m2 shown in light grey, terminal ligands not shown for clarity), whereas (b) isocyanate ligands are observed to bridge only in an ‘end-on’ fashion in a mixed 3d/4f polymer.

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has been carried out by powder diffraction. The anhydrous 3D polymers [Ln(dca)3] were found to be structurally related to PuBr3. The hydrated structure of [Gd(dca)3(H2O)2] was recently determined from single-crystal data and also found to form a 3D polymer, and shows conversion to the dehydrated phase at 1301C.9 With 1,10-phenanthroline as a neutral co-ligand, dicyanamide only forms hydrogen-bonded cationic (4,4)-sheets of [Gd(dca)2(1,10-phen)2 (H2O)3]1 with lattice dca– anions to counter the charge (Figure 6.2a).10 Less common polynitrile ligands, such as [C(CONH2)(CN)2] (cdm), can also form

Figure 6.2

(a) Hydrogen-bonded cationic (4,4)-sheet of [Gd(dca)2(1,10-phen)2 (H2O)3]1. (b) The mixed nitrile–amide donor ligand cdm in the 2D polymer {[Eu(cdm)3(H2O)3] H2O}.


Figure 6.3

195

The layered structure of {[Gd2(TCNQ)5(H2O)9][Gd(TCNQ)4(H2O)3]} 4H2O consists of alternate (a) cationic and (b) anionic layers (terminal TCNQ ligands not shown for clarity).

polymeric rare earth complexes. The 2D polymer {[Eu(cdm)3(H2O)3] H2O} demonstrates strong second harmonic generation.11 The anion coordinates through both the amide oxygen atom, as may be expected, and through the one of the nitrile arms (Figure 6.2b). In addition to the dca and tcm anions, there has also been research into rare earth coordination polymers containing the larger radical polynitrile ligands tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ), which have also been used in a large number of transition metal polymers (see Chapter 5). One such compound is the unusual layered solid {[Gd2(TCNQ)5(H2O)9] [Gd(TCNQ)4(H2O)3]} 4H2O (Figure 6.3).12 The structure consists of alternating cationic and anionic sheets composed of different Gd:TCNQ ratios with both containing nine-coordinate gadolinium. Gadolinium atoms in both layers are bridged by combinations of m2 binding modes using either the nitrile arms on one side of the central ring or two arms on opposing sides of the ring. Stabilisation of the structure comes from hydrogen bonding between aqua ligands and noncoordinating nitrile arms of the TCNQ ligands. The magnetic behaviour of the material was tested and was found to be overall ferrimagnetic, although the pathways through which this occurs are not trivial, with both direct exchange and superexchange in addition to interaction between layers. Antiferromagnetic ordering has been observed in polymers of [Ln(TCNE)3(MeCN)x] (where Ln ¼ Dy or Gd), although the compound have not been characterised crystallographically.13

6.3 Five-membered Ring Donors Heterocyclic ligands such as pyrazole, triazole and tetrazole have been widely studied in transition metal complexes (see Section 5.5). In rare earth coordination polymers, these heterocyclic ligands are often studied under strictly air- and moisture-sensitive conditions and in the absence of competing oxygen-donor solvents. A more recently developed synthetic approach is to carry out reactions in melts using the conjugate acid of the ligand which is deprotonated in situ by the elemental rare earth. Both pyrazolate (pz) and

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Chapter 6

1,2,4-triazolate (tz) coordination polymers have been synthesised using this method. The solvent-free species [Yb(tz)3] and the mixed-valent Eu(II)/Eu(III) species [Eu2(tz)5(tzH)2] were the first reported three-dimensional 4f networks with only nitrogen donor groups.14 The nine-coordinate Yb atoms are bound by three Z1 ligands and three Z2 triazolates and the resulting 3D network can be viewed as distorted octahedral (reducing the Z2 coordination point to a single vertex) that are linked in six directions by bridging Z2:Z1 triazolate ligands (Figure 6.4a). 1,2,3-benzotriazolate forms solvent-free 1D polymers in which the triazolate anions are Z2:Z1 bridging between adjacent metal atoms in addition to terminal Z2 ligands (both the anion and the neutral, protonated triazole).15 The pyrazole-based complex [Ho(Pz)3(PzH)3] forms a 1D polymer with bridging Z1:Z1 pyrazolate ligands and terminal PzH ligands (Figure 6.4b).16 Interestingly, the larger rare earth metal neodymium produces discrete complexes under similar conditions with the pyrazolate ligands Z2 coordinated to the metal.

6.4 Pyridyl Donor Ligands As mentioned previously, lanthanoids exhibit a strong preference for oxygen donor ligands. Because of this, pyridyl-based bridging ligands such as 4,4 0 bipydine and phenazine, which are widely used in transition metal chemistry, occur very infrequently in rare earth coordination polymers. The N-oxide derivatives and substituted pyridines (such as pyridylcarboxylates) are more utilised (see Sections 6.5 and 6.8, respectively). 4,4 0 -Bipyridine has been observed in 1D and 3D rare earth polymers, although in both cases there are also carboxylate co-ligands. The polymeric chains in [Ln2(crot)6(H2O)2(4,4 0 bipy)] (Ln ¼ Nd, Gd, Ho, Er and Y) contain Ln dimers bridged by two m2-Z2:Z1 crotonate ligands (crot) and with two chelating carboxylates and an aqua ligand coordinated to each metal.17 The dimers are bridged by the bipyridine ligand to form infinite 1D chains (Figure 6.5a). A similar anionic 1D chain has also been reported in which 1,2-bis(4-pyridyl)ethane ligands bridge between monomeric [La(NO3)4] groups.18 The green luminescent 3D polymer [Tb2(O2CPh)6(4,4 0 -bipy)] contains parallel 1D chains of carboxylate-bridged terbium (see Section 6.6).19 These chains are linked by 4,4 0 -bipyridine ligands to form square channels in which the benzoate groups protrude (Figure 6.5b).

6.5 N-Oxide Donors Although a few rare earth polymers that contain pyridyl derivatives have been reported, their number is limited owing to the lower affinity of lanthanoids for nitrogen donor ligands. However, pyridyl ligands may be used once converted to their N-oxide derivatives (usually by oxidation using H2O2). Phosphorus


Figure 6.4

197

(a) The complex [Yb(tz)3] was the first example of a 3d-4f polymer with only nitrogen donor atoms. Reproduced with permission from Chem. Commun., 2006, 2060. (b) The 1D chain [Ho(pz)3(pzH)3] contains N,N 0 -bridging pyrazolate ligands (The pz/pzH positions are disordered).

oxides may also be used.20 A significant amount of work has been carried out using 4,4 0 -bipyridine-N,N 0 -dioxide (bipyO2) and derivatives, particularly by Champness, Schro¨der and co-workers.21,22 The 4,4 0 -bipyridine ligand is a mainstay of transition metal coordination polymers, acting as a simple linear bridge between metals, and the incorporation of its dioxide derivative into lanthanoid frameworks provides an ideal contrast between the behaviour of d-block and f-block systems. The versatility afforded by the lanthanoid coordination sphere can be observed in a series of terbium structures of bipyO2 in which changing method of crystallisation and solvents for the reaction between Tb(NO3)3 and

198

Figure 6.5

Chapter 6

(a) The repeating unit of the 1D polymer {[Ln(crot)3(H2O)(4,4 0 -bipy)1/2]2} and (b) the 3D polymer [Tb2(O2CPh)6(4,4 0 -bipy)] (phenyl rings, positioned in the channels, are omitted for clarity).

the dioxide ligand yielded very different products, including 1D chains of [Tb(bipyO2)(CH3OH)(NO3)3] and both ladder-type chains and 4.82 sheets with the composition [Tb(bipyO2)1.5(NO3)3] (Figure 6.6).23 In the 1D chains, the nine-coordinate Tb atoms are coordinated by three bidentate nitrate ligands, one methanol ligand and two bridging N-oxide ligands, therefore the coordination sphere consists of only oxygen donors. The zigzag chains form a 3D hydrogen-bonded network due to interactions between the coordinated methanol and a nitrate ligand of an adjacent chain. The terbium atoms in the laddered chains of [Tb(bipyO2)1.5(NO3)3] are also nine-coordinate; however, there is no coordinated solvent which is replaced by a third bridging N-oxide ligand. The terbium atoms act as T-shaped connectors in the ladder and voids are filled by solvent molecules. An analogous thallium complex is also known.24 Under different reaction conditions, a polymeric product of the same composition as the ladder forms that adopts a 2D sheet topology. The terbium atoms again act as 3-connecting nodes with the same coordinated species; however, the relative orientation of the ligands around the metal is different and a 4.82


Figure 6.6

199

Different coordination polymers form using Tb(NO3)3 and 4,4 0 -bipyridine-N,N 0 -dioxide depending on the crystallisation conditions used: (a) 1D chain; (b) ladder-type chain; (c) 4.82 sheet.

network is formed. Parallel sheets are staggered such that an eight-membered ring is situated above a four-membered ring and small channels in the lattice hold solvent molecules. Lanthanum, a larger rare earth than terbium, has been used with the bipyO2 ligand to produce two 3D networks that both contain unusual five-connecting nodes.25 The two related materials were formed from the reaction of LaCl3, NaBPh4 and bipyO2 in methanol; [La4L10(CH3OH)10Cl3]Cl(BPh4)8 and [LaL2.5(CH3OH){Ph2B(OMe)2}](BPh4)2. The latter product, obtained as the minor component, contains a single crystallographically unique eight-coordinate lanthanum atom and has a 466282 network topology. This is similar to the network formed in the major product which contains both eight- and ninecoordinate lanthanum atoms, both acting as 5-connecting nodes, and forms a 4466 topology. Both 5- and 6-connecting nodes have also been used to synthesise bilayered sheets in which (4,4) nets are joined by further bridging ligands.26 The (4,4) nets, or square grids, can be formed using rare earth metals such as [La(bipyO2)2(NO3)3] (Figure 6.7a). In this polymer, the lanthanum is 10-coordinate and acts as a distorted square-planar node with four bridging dioxide ligands and the remaining coordination sites occupied by chelating nitrate anions. If the nitrate anions are removed from the reaction mixture and replaced with non-coordinating anions such as triflate, perchlorate or tetraphenylborate, then more coordination sites will become available to which the bridging ligand may coordinate. For example, the complex {[Yb(bipyO2)3(H2O)2](CF3SO3)3} contains eight-coordinate square-antiprismatic ytterbium atoms with six bridging N-oxide ligands and two terminal aqua ligands

200

Figure 6.7

Chapter 6

(a) The (4,4)-sheet structure of the polymer [La(bipyO2)2(NO3)3]; (b) the coordination environment of the ytterbium atom in {[Yb(bipyO2)3(H2O)2](CF3SO3)3}; (c) the resultant bilayered sheet.

(Figure 6.7b). Four of the bridging ligands form a (4,4) sheet analogous to that in [La(bipyO2)2(NO3)3] whereas the remaining two bridge between these sheets to form the bilayered architecture (Figure 6.7c). N-Oxide bridging ligands have been used in rare examples of 8-connected topologies that defy the stereotypic body-centred cubic topology of CsCl.27 Whereas CsCl adopts a 42464 topology, the complex {[La(bipyO2)4](ClO4)3} contains eight-coordinate lanthanum atoms and forms a 3D network with a 334155862 topology that contains both trigonal and pentagonal subunits. A compositionally related compound, {[Yb(bipyO2)4](CF3SO3)3}, is binodal, with the nodes present in a 2:1 ratio, and both are eight-coordinate, and with a unique (3541459)(35413510)2 topology. {[La(bipyO2)4](CF3SO3)3} forms a more standard CsCl framework, demonstrating the difference that can result from utilising different f-block metals.28 The smaller bridging ligand pyrazine-N,N 0 -dioxide has also been demonstrated to form a CsCl network topology.29 As with transition metal frameworks of 4,4 0 -bipyridine, it has been shown that rare earth polymers of its N-oxide derivative can give rise to interpenetrating structures.30,31 Longer derivatives of bipyO2 have also given rise to network materials such as the interpenetrating a-Po nets of


201

{[Ln(L)3](ClO4)3(H2O)}, which contain the 1,2-bis(4-pyridyl)ethane-N,N 0 dioxide ligand and interpenetrating (6,3) sheets using the same ligand in the complex {[Ln(NO3)3(L)1.5] H2O} (Ln ¼ Er or Yb in both cases).32 There have been some reports in which the dioxide ligand adopts unusual bridging modes with the terminal oxygen atoms bridging between two lanthanoids giving an overall m3-O,O,O 0 or m4-O,O,O 0 ,O 0 binding mode.33,34 For example, the complex {[La(m1L)2(m3L)(H2O)2](CF3SO3)3}, which contains both terminal and bridging 1,2-bis(4-pyridyl)ethane-N,N 0 -dioxide ligands, forms a 3D coordination polymer with bridging solely via tridentate N-oxide ligands (Figure 6.8).33 Perhaps the most interesting lanthanoid coordination polymer that has been reported using N-oxide ligands is the 3D polyrotaxane synthesised by Hoffart and Loeb.35 The bipyridinium ligand shown in Figure 6.9a is combined with

Figure 6.8

The tetradentate binding mode of the 1,2-bis(4-pyridyl)ethane-N,N 0 -dioxide ligand is observed in the 3D polymer {[La(m1L)2(m3L)(H2O)2](CF3SO3)3}.

202

Figure 6.9

Chapter 6

(a) The formation of an N,N 0 -dioxide bipyridinium pseudorotaxane; (b) the coordination environment around a single metal atom in a [2]rotaxane polymer.

a crown ether to produce a pseudorotaxane that has terminal N-oxide functionalities. Combining this pseudorotaxane with Ln(CF3SO3)3 (Ln ¼ Sm, Eu, Gd, Tb) yields a coordination polymer in which each lanthanoid atom acts as a 6-connecting node with each bridge being a [2]rotaxane. The lanthanoid atoms are eight-coordinate with the remaining two coordination sites occupied by an aqua ligand and a terminal triflate anion. The resulting distorted a-Po nets have an Ln–Ln separation of approximately 23.5 A˚, leading to large cavities in a single network; however, these gaps are filled by a second interpenetrating net. The use of a smaller lanthanoid, ytterbium, results in a very different network in which seven-coordinate Yb acts as a 6-connecting node (with a triflate anion in the remaining coordination site) in which all bridges are again [2]rotaxanes. In this latter case, sheets comprising alternating triangles and squares are bridged by pillar ligands into a 3D network. Dicarboxylate ligands have also been used to form polyrotaxane frameworks using cucurbituril as the cyclic species.36

6.6 Rare Earth–Carboxylate Polymers The largest class of rare earth polymers are those that contain carboxylatebased ligands.37 Carboxylates cater for the oxophilic nature of 4f metal ions and their ability to adopt many different binding modes fits well with the metals’ lack of defined coordination geometry. Carboxylate groups can coordinate in binding modes ranging from the relatively rare Z1 mode to simultaneous chelating and bridging in a m2-Z2:Z1 or m3-Z1:Z2:Z1 mode (Figure 6.10). The number of reports of Ln–RCO2 polymers is too large to cover comprehensively in a book such as this, and only an overview of some general


Figure 6.10

203

Binding modes observed for carboxylate ligands with rare earth metals (from top left), Z1, Z2, Z,Z-m2-Z1:Z1, m2-Z2:Z1, m3-Z1:Z2:Z1, Z,E-m2-Z2:Z1 and E,E-m2-Z2:Z1.

structural types and common ligands can be given alongside some more unusual examples. Monocarboxylate ligands, such as acetate and benzoate derivatives, form 1D polymers almost exclusively, albeit with a variety of different bridging modes between adjacent metals. For this reason, less time is devoted to examining these species, with the emphasis below being on polycarboxylate ligands and the interesting structural variety that can be seen for their polymeric complexes.

6.6.1 Monocarboxylate Ligands Simple monocarboxylate ligands of the form RCO2 tend to form 1D polymeric chains where the alkyl or aryl R groups are orientated away from the Ln–CO2 backbone. A notable exception to this is the formate anion, HCO2, which more readily forms 3D structures as the lack of steric bulk (R ¼ H) permits an E,E-m2-Z2:Z1 binding mode more readily than with other R substituents (i.e. the carboxylate groups can coordinate ‘backwards’ in the direction of the substituent). An example of a simple rare earth–carboxylate chain is the cerium acetate monohydrate structure, [Ce(C2H3O2)3(H2O)] (Figure 6.11a).38 The ninecoordinate cerium atoms are bridged to two others by a combination of bidentate and tridentate bridging carboxylates. The aqua ligands form both intra- and inter-chain hydrogen bonds, a common motif for such polymers. Acetate is also observed in 2D polymers such as {[Pr(C2H3O2)3(H2O)] H2O}, which forms a (6,3) network (Figure 6.11b)39 and in the anhydrous 3D polymer [La(C2H3O2)3] (Figure 6.11c), which contains some of the acetate ligands in the more unusual tri-bridging m3-Z1:Z2:Z1 binding mode.40 The acetate ligand can also form dimeric complexes with terminal, chelating anions with smaller rare earth elements or in a situation where more solvent molecules coordinate to the metal, reducing the number of free coordination sites (note that the 3D acetate network features no solvent, allowing for more coordination from the acetate ligands).

204

Figure 6.11

Chapter 6

Rare earth–acetate coordination polymers: (a) 1D chain in [Ce(C2H3O2)3 (H2O)]; (b) 2D sheets in {[Pr(C2H3O2)3(H2O)] H2O}; (c) the 3D anhydrous network [La(C2H3O2)3].

The use of larger carboxylate ligands discourages the formation of 2D and 3D polymers and instead chains or dimers become the predominant species. Studies of polymers containing benzoate-derived ligands with lanthanum and terbium have shown that substituents on the phenyl rings, particularly in the


205

ortho-positions, can have an influence on the metal coordination environment, alter the bridging binding modes of the carboxylates and in some cases prevent the formation of a polymeric chain and yield dimeric products, such as with 2,4,6-trimethylbenzoate.41,42 A notable exception to the formation of polymeric chains was observed in one of the first examples of a rare earth carboxylate, tris(propynato)scandium.43 The anions are all in bridging Z1:Z1, with both E,E and Z,Z orientations (see Figure 6.10) between octahedral scandium atoms. The resulting structure is an a-Po network in which the ethyne groups cover the faces of the cubic structure.

6.6.2 Dicarboxylate Ligands Dicarboxylate ligands are commonly used to synthesise lanthanoid coordination polymers and range from relatively rigid ligands, such as benzene or naphthalene derivatives, to more flexible species with alkyl chains between terminal carboxylate groups. The use of flexible ligands provides a significant contrast between structures depending on the length of the alkyl chain, with polymers known that contain [O2C–(CH2)n–CO2]2 ligands, where n ¼ 1 (malonate), 2 (succinate), 3 (glutarate) or 4 (adipate), in addition to other straight-chain anions such as fumarate and tartrate. The ability of the carboxylate groups both to chelate and to bridge, however, means that the ligand often cannot be treated as a simple linear connector (such as 4,4 0 -bipyridine) and coordination networks are correspondingly more complicated. The oxalate anion, which is essentially two fused carboxylates, adopts a different coordination mode to the carboxylate group and is dealt with elsewhere (see Section 6.7.1). Benzene-1,4-dicarboxylate (terephthalate, 1,4-bdc) is a widely used ligand due to the rigidity provided by the benzene backbone. The para-substitution pattern also facilitates polymer formation (it is a divergent ligand) and discourages the formation of discrete complexes. An example of a 3D polymer containing the 1,4-bdc ligand is {[Gd2(1,4-bdc)3(DEF)2] H2O} (DEF ¼ N,N-diethylformamide) in which carboxylate chains, similar to those observed for monocarboxylate ligands, are connected by the 1,4-bdc linker with the DEF ligands projecting into the 1D channels (Figure 6.12a).44 The complex [Eu2(BDC)3(DMF)2(H2O)2], with the smaller DMF ligand, forms a 3D network that displays twofold interpenetration (Figure 6.12b).45 A thermally stable, porous 3D network based around chains of octahedral scandium, [Sc2(1,4-bdc)3], with a very different network structure, is also known and shows high thermal stability and nitrogen adsorption (Figure 6.12c).46 Neutral co-ligands, solvents and the size of the rare earth coordination sphere can all play a role in determining the polymeric structure, with 1D chains having been reported for the lutetium complex {[Lu2(1,4-bdc)3(H2O)8] 2H2O}.47 Terephthalate polymers have been synthesised that form microporous frameworks with an a-Po or distorted a-Po topology and can show uptake of carbon dioxide.48 Removal of aqua ligands from the structure does not affect the network and creates accessible metal sites within the channels.49 The use of longer anions to expand these networks

206

Figure 6.12

Chapter 6

(a) View along the rare earth chains in the 3D polymer {[Gd2(1,4bdc)3(DEF)2] H2O} (DEF ligands in channels removed for clarity); (b) one independent 3D network from the twofold interpenetrating structure of [Eu2(1,4-bdc)3(DMF)2(H2O)2]; (c) [Sc2(1,4-bdc)3].

resulted in two interpenetrating a-Po nets, although the structures retained significant solvent- and/or gas-accessible voids.50 Substituted terephthalate anions, such as 2-aminoterephthalate (atpt), have been used to add further functional groups to the interior of any channels,


207

promoting more hydrogen bonding with guest species. For example, the 3D polymer {[Ln2(atpt)3(H2O)2] 0.5(4,4 0 -bipy)(H2O)} (Ln ¼ Eu, Gd or Yb) contains a hydrogen-bonded bipy–2H2O chain running through the channels with both p. . .p and hydrogen bonding interactions between the guests and the host lattice.51 Larger, rigid dicarboxylate ligands have been extensively used by O’Keeffe, Yaghi and co-workers to demonstrate the synthesis of metal–organic frameworks (MOFs) and to examine the packing of infinite rod-shaped building blocks, i.e. the infinite Ln–carboxylate chains, showing a preference towards 3- and 4-connecting rods.52 Although the linear 1,4-benzene dicarboxylate ligand is the most often used isomer, the 1,2- and 1,3-isomers (phthalate and isophthalate, respectively) have also attracted considerable attention, although they tend to contain fewer channels due to the closer proximity of the binding carboxylate groups around the ligand.53 The complex [Tb4(1,3-bdc)6(1,10-phen)], for example, forms a 3D network with alternate channels filled by either the phenanthroline co-ligands or the benzene rings of the 1,3-bdc ligands with no uncoordinated guest species (Figure 6.13a).54 This lack of guest species is not always the case, as demonstrated by the anionic 3D network {[Ln4(1,3-bdc)7(H2O)2}]2 (Ln ¼ Nd, Sm or Eu) that contains [CuI(2,2 0 -bipy)2]1 cations within large cavities.55 Two Cu(I) cations are p-stacked within each cavity (Figure 6.13b). In some cases, carboxylate groups are responsible for the formation of secondary building units (SBUs), such as Ln2 dimers, which are then linked into a network structure; for example, the complex [Ln2(2,2 0 -bipy)2(1,3-bdc)3] (Ln ¼ Gd, Dy, Y) contains dimeric SBUs which form a 2D square lattice with two types of bridging environment between the SBU nodes.56 Polymers containing phthalate are less likely to form 3D polymers due to the tight angle between the adjacent carboxylate groups and both 1D57 and 2D58 polymers have been reported. A variation on the use of benzenedicarboxylate ligands is the utilisation of naphthalene derivatives. Naphthalene-2,6-dicarboxylate acts as an extended version of terephthalate and can be used to construct open-framework materials with the ligands bridging between lanthanoid–carboxylate chains.59 4,4 0 -Biphenyldicarboxylate can also act as an extended analogue of terephthalate and can yield structures that have alternating Ln–carboxylate and organic layers.60 The naphthalene-1,4-dicarboxylate ligand also forms structures similar to those of terephthalate, with the carboxylate groups on opposite side of the same ring, although the extra ring allows for significant p-interactions inside the resultant channels.61 More flexible, alkyl-based ligands adopt a variety of different structures although the general trend of chains or clusters of Ln–carboxylates, as with the above benzoate derivatives, is common to all. The shortest alkyldicarboxylate, malonate, has been observed to bind in a number of ways, including chelation to a metal using oxygen atoms from opposing ends of the ligand (to produce a six-membered chelate ring). Malonate polymers have been reported that undergo a temperature-dependent phase transition62 and that show ferromagnetic coupling between gadolinium atoms with m2-Z2:Z1 carboxylate groups.63 The longer succinate ligand, in most cases, does not form a chelate

208

Figure 6.13

Chapter 6

(a) The 3D network [Tb4(1,3-bdc)6(1,10-phen)] in which channels are occupied by phenyl groups of the ligands; (b) the cation templated {[Ln4(1,3-bdc)7(H2O)2]}{[CuI(2,2 0 -bipy)2]2}.

ring analogous to that observed for malonate polymers and instead bridges between Ln–carboxylate chains in a more ‘conventional’ manner; however, there are examples of the seven-membered chelate ring being formed.64 Flexible bridges are observed in complexes such as [Sc2(suc)2.5(OH)], which is able to act as a Lewis acid catalyst. Dioctahedral scandium units (with a single corner-sharing hydroxide bridge) are joined into a 3D structure by succinate anions, with each oxygen atom of the ligand coordinated to a different scandium atom (Figure 6.14a).65 The succinate anion has also been observed to act as a 5- or 6-connecting ligand. A similar length ligand is


Figure 6.14

209

(a) The 3D succinate framework [Sc2(suc)2.5(OH)]; (b) {[Sm2(fumarate)3(H2O)4] 3H2O} with guests and aqua ligands omitted for clarity; (c) large guest-containing channels in the glutarate polymer {[Pr2(glu)3(H2O)4] 10.5H2O}; (d) the hexyl-spaced [La2(suberate)3(H2O)2].

fumarate, trans-O2CCHCHCO2, although the presence of the double bond means that the geometry of the ligand is more rigid. For instance, the 3D polymer {[Sm2(fumarate)3(H2O)4] 3H2O} contains channels with hexameric water clusters that can be removed, giving a second crystalline phase (Figure 6.14b).66 Rehydration yields the original product, as shown by X-ray powder diffraction studies. The fumarate ligands again bridge between Sm–carboxylate chains with a variety of chelating and bridging binding modes of the terminal carboxylate groups. Further increases in the length of the anion to glutarate result in larger spaces between Ln–CO2 chains, allowing the inclusion of guest molecules inside channels, such as in the 3D network {[Pr2(glu)3(H2O)4] 10.5H2O} (Figure 6.14c).67 Longer alkyl chains have also been used, such as adipate, and there has

210

Chapter 6

recently been a report of suberate (hexane-1,6-dicarboxylate) as a ligand in the complex [La2(suberate)3(H2O)2] with parallel La–carboxylate chains separated by the distance of the hexyl chains (Figure 6.14d).68 Although attention above has been focused on the main classes of alkyl and aryl dibenzoates, there are a number of other, less common ligands that have been incorporated into rare earth polymers. Cyclohexane-1,4-dicarboxylate has been observed to form 1D, 2D and 3D polymers, the last two from pH-dependent syntheses, with the carboxylate groups able to adopt a variety of different relative conformations around the flexible core.69,70 In the cis,cis form, the cyclohexane-1,4-dicarboxylate anion is able to form an eight-membered chelate ring to a rare earth metal and simultaneously acts as both a convergent and a divergent ligand. This behaviour is also seen with the dianion of itaconic acid, or methylenesuccinic acid, in which the ethene group fixes the relative conformation of the acid substituents such that a seven-membered chelate ring can form more readily than in the flexible succinate analogue.71

6.6.3 Tricarboxylate Ligands The most reported tricarboxylate ligand is benzene-1,3,5-tricarboxylate (trimesate, TMA), although polymers are also known that contain the 1,2,4isomer or cyclohexane derivatives. The number of carboxylate groups, combined with the number of possible coordination modes to lanthanoids, can conceivably lead to a great deal of structural diversity, and this was shown by the reaction of ErCl3 with Na3(TMA) which, with concentration variations, leads to three different crystalline phases.72 The different phases contain 1D chain polymers, 2D sheets or, most interestingly, 1D tubular polymers. The 2D sheets are isomorphous with those of the gadolinium complex {[Gd(TMA)(H2O)3] 1.5H2O}, which has a double-layer (6,3) sheet structure in which all carboxylate groups are Z2 chelating and both the metals and the ligands act as three-connecting nodes (Figure 6.15a).73 Each sheet is closely hydrogen bonded to another with the aqua ligands from one passing through the hexagonal holes of the other. These ‘bilayers’ are then further stacked to create the long-range structure. The complex {[Nd(TMA)(H2O)4] H2O} forms similar sheets.74 The tubular polymer has the formula {Er3(TMA)3(H2O)8 4H2O} and contains a similar honeycomb motif to the 2D sheets except that they are rolled into a tube (Figure 6.15b). The TMA ligand binds in a similar manner (i.e. with three Z2 interactions) and the lanthanoids are again 3-connecting, but one of the three unique erbium atoms has only two aqua ligands. There is extensive hydrogen bonding between adjacent chains and to the water molecules that reside within the tubes. Trimesate polymers synthesised from non-aqueous (i.e. less competitive) media have been shown to adopt very different coordination modes, leading to a 3D network structure similar to that of the zeolite ABW topology.75 Both the ligand and metal in the complexes [Ln(TMA)(DMF)(DMSO)] (Ln ¼ Tb, Ho, Er, Yb, Y) act as 4-connecting nodes with the ligand adopting a m4-Z2:Z2:Z1:Z1 coordination mode (Figure 6.15c). Benzene-1,2,4-tricarboxylate


Figure 6.15

211

Ln–tricarboxylate polymers. Similar honeycomb motifs are observed in (a) a single (6,3) net from the double-layered sheet of {[Gd(TMA)(H2O)3] 1.5H2O} and (b) the tubular polymer {Er3(TMA)3(H2O)8 4H2O}. Reproduced from Inorg. Chim. Acta, 2000, 304, 161 with permission from Elsevier. (c) The zeolite ABW topology of the binodal 4-connecting net in [Ln(TMA)(DMF)(DMSO)]. Reproduced with permission from Inorg. Chem., 2006, 45, 4065.

adopts very different binding modes to trimesate as the ligand can chelate to the metal atom using one oxygen atom from each of the adjacent carboxylate groups (i.e. those in the 1,2-positions). The ligand has been observed to act as both a 5- and 6-connecting node in 3D polymers.76 The larger, more flexible

212

Chapter 6

analogue of trimesate, benzene-1,3,5-triacetic acid, has been shown to form 3D polymers with significant cavities, also binding in a five- or six-coordinate manner.77 Cyclohexane-1,3,5-tricarboxylate has been observed to form polymers with high connectivity including the complex [Ln(C9H9O6)] (Ln ¼ Er or Tb), which contains the lanthanoid in an unusual octahedral coordination geometry, binding Z1 to six ligands which are also 6-connecting. The resulting 3D network is thermally stable to 600 1C.78

6.6.4 Tetracarboxylate Ligands Benzene-1,2,4,5-tetracaboxylate (btec) has been used in a number of rare earth coordination polymers and can adopt a number of different binding modes, although it is usually 4- or 6-connecting. The complex {[Yb4(btec)3(H2O)8] 6H2O} is a 3D network that contains 4-connecting btec anions (with each carboxylate Z2 chelating) in layers with 6-connecting ligands also in the layer and bridging between layers to generate the 3D structure.79 The layers are stacked in a manner that gives solvent-containing channels through the network (Figure 6.16a). Nanoporous structures have also been reported that incorporated both the btec4 ligand and the partially deprotonated H2btec2 species.80 Unusual tetracarboxylate ligands have also been used in rare earth polymers. Recently, complexes containing the free-base tetra(4-carboxyphenyl)porphyrin were reported.81 In one of these 3D polymers, praseodymium chains are bridged by porphyrin ligands to give a 3D network with channels between the parallel porphyrin ligands (Figure 6.16b). The porphyrin ligands can be viewed as forming planes through the structure and the carboxylate groups coordinate in bridging m2-Z1:Z1 binding modes with four carboxylates bridging pairs of Pr atoms. The lanthanoid chains have alternating (CO2)4 and oxalate bridges between adjacent metal atoms. A 3D network that contains the cyclobutane-1,2,3,4-tetracarboxylate anion has also been synthesised in which the ligand adopts a number of different binding modes, including a m5 mode (Figure 6.16c).82 Further to the use of tetracarboxylate ligands are reports of benzenehexacarboxylate incorporated into polymeric materials which has been observed to act an 8-connecting ligand.83

6.7 Other Ligands The previous sections have dealt with relatively common ligands classes that are used in lanthanoid coordination polymers, but there are also a number of other ligands that have received less attention. This section contains a brief overview of some of these lesser studied, yet still very interesting, systems.

6.7.1 Oxalate Coordination Polymers Oxalate, O2CCO2, has been used to synthesise a number of rare earth coordination polymers. The standard coordination mode that is observed for


Figure 6.16

213

Ln–tetracarboxylate polymers: (a) {[Yb4(btec)3(H2O)8] 6H2O}; (b) porphyrin-based 3D network; (c) unusual coordination mode of the cyclobutane-1,2,3,4-tetracarboxylate ligand.

the oxalate anion is a bridging mode between metal atoms with five-membered chelate rings (it is also possible for the oxalate anion to form four-membered chelate rings similarly to carboxylate ligands). Rare earth–oxalate polymers have been known since the 1960s with the synthesis of (NH4)[Y(C2O4)2(H2O)]84 and [Er(C2O4)(HC2O4)] 3H2O.85 More recent studies on similar systems have shown that 3D framework structures can be formed with counter-cations templating the structure and residing in the cavities. For example, guanidinium and tetramethylammonium cations have been used to template 3D networks via hydrothermal synthesis.86 The cations, along with water molecules, reside in the channels of the framework in which each Nd atom is linked to four others by bis-chelating oxalate ligands. The framework can be viewed as honeycomb sheets that are connected by a further oxalate ligand to adjacent layers (Figure 6.17a). A similar polymeric species, [NMe4]{[Nd2(H2O)3](C2O4)3.5} 4H2O, can also be viewed as layers with oxalate bridges although the

214

Figure 6.17

Chapter 6

Oxalate-based 3D networks: (a) the guanidinium-templated framework in (CN3H6)2{[Nd(H2O)]2(C2O4)4} 3H2O; (b) the tetramethylammoniumtemplated [NMe4][Yb(C2O4)2]; (c) the (4,4)-sheet structure of (CN3H6)[La(C2O4)2(H2O)].

structure of the network is significantly different. The same study showed that tetramethylammonium cations can template the formation of significant channels in the 3D network [NMe4][Yb(C2O4)2] (Figure 6.17b). In this case, the ytterbium atoms are eight-coordinate with the coordination sphere only


215

containing chelating oxalate ligands in a roughly tetrahedral geometry. Guanidinium counter-cations have also been used in the construction of 2D oxalate polymers under hydrothermal conditions (Fig 6.17c).87 The complex (CN3H6)[La(C2O4)2(H2O)] contains nine-coordinate lanthanum atoms that form a (4,4) sheet network with bridging, bis-chelating oxalate ligands with an aqua ligand in the remaining coordination site. Although the metal atoms are four connecting, as in the previous [NMe4][Yb(C2O4)2] complex, the geometry of the oxalate anions around the larger lanthanum atom is approaching squareplanar (if the aqua ligand is ignored). The guanidinium cations reside between the anionic layers. A similar (4,4) network has been reported in the mixedligand compound (NH4)2[Nd2(C2O4)3(CO3)(H2O)] H2O in which oxalate ladders are connected by m2(O,O) carbonate ligands to form the 2D sheet.88 A (6,3) sheet structure has also been reported in the compounds {[Ln2(C2O4)3(H2O)6] 3H2O 0.5HNO3} (Ln ¼ La and Pr) in which the oxalate anions arise from atmospheric CO2 and nitric acid is present as a guest species.89 The smaller rare earth metals have also been used in oxalate coordination polymers, such as the 3D structure of [C6N2H16]0.5[Y(H2O)(C2O4)2] 2H2O that contains hydrothermally synthesised amine molecules, derived from 1,2diaminopropane, in the lattice.90 This framework also shows reversible adsorption of both water and methanol. Similar compounds have been reported in which the organoamine molecules undergo reactions under the hydrothermal conditions and become incorporated into the lattice, such as in [OC(CH3)NCH2CH(CH3)NH3][Nd(C2O4)2] H2O, where the N-(2-aminopropylacetimide) cations arise from reaction with acetic acid.91 Channels in the structure are filled with these pendant ligands, adding chemical functionality to the structure, while the 3D framework is largely composed of the bridging oxalate ligands.

6.7.2 Chloranilic Acid and Dihydroxybenzoquinone Robson and co-workers have reported a number of coordination polymers that utilise the dianions of chloranilic acid (can) and dihydroxybenzoquinone (dhbq) as bridging ligands.92,93 These anions are able to act as m2-Z2:Z2 bridges with five-membered chelate rings at both ends, similar to those found in oxalate polymers, although with a larger separation between metal atoms. The isomorphous series [Ln2(dhbq)3(H2O)6] 18H2O (Ln ¼ Y, La, Ce, Gd, Yb and Lu) are 2D coordination polymers in which the (6,3) coordination framework is templated by lanthanoid–water cages formed from interstitial water molecules and aqua ligands from layers above and below the hexagonal cavities (Figure 6.18a). This series is unusual, with isomorphous structures covering the whole size range of the rare earth group from lanthanum to yttrium. Each lanthanoid atom is nine-coordinate with three chelating dhbq dianions and three aqua ligands in the coordination sphere. Each metal atom acts as a 3-connecting node in the (6,3) sheet with tetradentate dhbq acting as the sole bridging species. The Ln2(H2O)18 cages that exist within the hexagonal gaps have volumes around

216

Figure 6.18

Chapter 6

(a) The (6,3) net in the structure of [Ln2(dhbq)3 24H2O and (b) the Ln2(H2O)18 cage that exists in the hexagonal gaps. (c) Chloranilate forms different structures depending on the size of the rare earth ion, such as {[La2(can)3] 13H2O}. Reproduced with permission from J. Chem. Soc., Dalton Trans., 2002, 1586.

41 A˚3, smaller than those of the related (H2O)20 cages that are found in some clathrate hydrates (Figure 6.18b). Coordination polymers of deprotonated chloranilic acid (can) are different from both the dhbq complexes, and from each other, and are dependent on the size of the metal ion (and therefore its decreasing coordination number across the lanthanoid group).94,95 For the most part these polymeric complexes are 2D sheets, such as {[La2(can)3] 13H2O} (Figure 6.18c), although there is significant structural variation across the period.


217

6.7.3 Polymers with Sulfonate-containing Ligands The oxophilicity of the rare earth elements has already been mentioned, particularly with the plethora of carboxylate complexes that have been reported. A lesser studied class of rare earth polymers are those in which sulfonate functionalities are used as the coordinating species. An example is the solvothermally prepared complex {[Eu2(1,4-BDS)(4-SB)2] 3H2O}, which contains both 4-sulfobenzoate (4-SB) ligands and in situ-synthesised benzene-1,4-disulfonate (1,4-BDS) ligands.96 The 3D network has a pillar-type structure in which carboxylate–sulfonate bridged layers are separated by benzene spacers (Figure 6.19a). The dehydrated complex shows remarkable thermal stability up to 5731C and the original product can be obtained by re-absorption of water. A larger disulfonate ligand, naphthalene-1,5-disulfonate (NDS), has been used to synthesise a functional material with a CdI2-type topology in which the disulfonate ligand coordinates to six lanthanoid atoms. The complexes [Ln(OH)(NDS)(H2O)] (Ln ¼ La, Pr and Nd) were synthesised hydrothermally using Na2(NDS) and Ln(NO3)3.97 The nine-coordinate metal atoms are coordinated by two bridging hydroxide anions, five NDS ligands and a terminal aqua ligand. The metal coordination spheres are joined into dimers by sharing an OH–OH edge (Figure 6.19b). The complexes were tested for their catalytic ability as they contain both active metal sites and acidic groups. They were found to catalyse efficiently the conversion of linalool, the oxides of which are fragrances found in natural systems, to hydroxyl ethers. Sulfonate groups have also been incorporated into a number of ligands that contain more than one functional, coordinating group, such as 5-sulfosalycylate98 and 5-sulfoisophthalate.99 An interesting example of a bifunctional sulfonate polymer is the mixed metal complex {Ba2(H2O)4[EuL3(H2O)2](H2O)5.44Cl}, where L ¼ 4,4 0 -disulfo-2,2 0 -bipyridine-N,N 0 -dioxide.100 The binding of the ligand is such that the rare earth is surrounded solely by N-oxide donors and aqua ligands whereas the sulfonate groups coordinate exclusively to the barium atoms. The aqua ligands are orientated into channels that run through the 3D lattice and contain water molecules and chloride anions (Figure 6.19c). Dehydration of the material (both interstitial and coordinated water) leads to an amorphous material, i.e. the framework structure is not retained; however, 95% of the original water content can be reversibly readsorbed. Sulfonatocalixarenes have also been observed to form coordination polymers with rare earth ions.101,102

6.7.4 Amide Ligands Amide groups are able to act as oxygen donor ligands and are therefore suited to use in rare earth complexes; however, there are few example of polymeric complexes using solely amide-based ligands. An example of an amide-based coordination polymer is that made containing a cyclic amide ligands derived from 4-(methylamino)benzoic acid.103 The complexes {[La(L)(H2O)6] 3(CF3SO3)} and {[Yb(L)(H2O)3(CF3CO3)] 2(CF3SO3)} contain a similar 2D

218

Figure 6.19

Chapter 6

(a) Partial structure of the pillared complex {[Eu2(1,4-BDS)(4-SB)2] 3H2O}; (b) the 3D network of the naphthalene-1,5-disulfonate polymer [Ln(OH)(NDS)(H2O)]; (c) the structure of the mixed-metal compound {Ba2(H2O)4[EuL3(H2O)2](H2O)5.44Cl} (L ¼ 4,4 0 -disulfo-2,2 0 -bipyridineN,N 0 -dioxide).

polymer in which both the amide ligand and the rare earth atom act as 3-connecting nodes (Figure 6.20). For the lanthanum complex, the large trigonal holes are aligned above each other to create channels running through the structure. More flexible amide ligands, based around a trisubstituted benzene or methane core, have also been successfully incorporated into


Figure 6.20

219

The 2D polymer containing a cyclic amide ligand packs so that the triangular holes stack to form channels.

polymeric structures with unusually distorted (10,3)-a topologies. However, the flexibility of the ligands leads to more densely packed structures with no significant channels or pores.104

6.7.5 Pyridylcarboxylate Ligands Although there are few rare earth polymers with ligands that contain mixed donor groups, the largest single class is pyridylcarboxylates. Although it was demonstrated in Section 6.3 that multi-pyridyl donors do not often form polymeric products with f-block metals, the incorporation of carboxylate functionalities does lead to a significant number of instances in which pyridyl nitrogen atoms coordinate to the rare earth metal, often as part of a chelating coordination mode. Perhaps the most obvious chelating pyridylcarboxylate ligand is 2,6-pyridinedicarboxylate (PDC), which is able to coordinate in an Z3 fashion to rare earth atoms via the pyridine nitrogen atom and one oxygen atom from each carboxylate group. The remaining oxygen atoms are available to bridge to adjacent metals. For example, hydrothermal reactions using pyridine-2, 6-dicarboxylic acid have been shown to yield 1D, 2D and 3D praseodymium polymers depending on the reaction conditions and reactant stoichiometries used.105 The 1D polymer (Figure 6.21a) demonstrates the manner in which the ligand is able to chelate to the metal and bridge to adjacent metal atoms. In this

220

Figure 6.21

Chapter 6

Examples of pyridyl-2,6-dicarboxylate polymers. (a) The 1D chain {[Pr(PDC)(HPDC)(H2O)2] 4H2O}. (b) The 2D sheet {[Pr3(PDC)4(HPDC)(H2O)8] 8H2O}. (c) The 3D network {[La(PDC)(H2O)4] Cl} (chloride guests in channels not shown).

case, the non-coordinating oxygen atoms are involved in hydrogen bonding with lattice water molecules. The 2D polymer {[Pr3(PDC)4(HPDC)(H2O)8] 8H2O} is a square grid with eight praseodymium atoms comprising one window (Figure 6.21b). There are two distinct metal coordination environments in the structure, one of which is nine-coordinate with three tris-chelating ligands whereas the other is coordinated by monodentate carboxylates and aqua ligands. The related complex {[Pr2(PDC)3(H2O)3] H2O}


221

contains larger windows with 10 Pr atoms each and the 2D networks interpenetrate to give a 3D structure. The 1D polymer was found to be a helical chain. A 3D polymer has been synthesised with lanthanum, {[La(PDC)(H2O)4] Cl}, that contains edge-sharing nanotubes that contain chloride anions (Figure 6.21c).106 A novel microporous structure that has been synthesised using the PDC2 anion is [{La(H2O)5(PDC)}{[La(H2O)(PDC)}]2{Mo8O26} 10H2O, which contains molybdate clusters between cationic La–PDC sheets.107 The structure is related to inorganic–organic hybrid pillared complexes (see Chapter 8) and the water molecules that reside in the channels can be removed with no loss of structural integrity. Polymeric complexes with the isomeric ligand pyridine2,5-dicarboxylate form very different structures that display rutile-related 3D topologies.108 The more highly substituted ligand pyridine-2,4,6-tricarboxylate (PTC) has also been observed in 3D coordination polymers and can act as a 5-connecting ligand (i.e. with the same tri-bridging mode as the 2,6-dicarboxylate analogue but with an additional O,O 0 -bridging carboxylate in the para-position).109 Pyridylcarboxylates in which no N,O-chelation can occur rarely form coordination polymers with N–Ln interactions, although there are exceptions to this, such as the 2D coordination polymer [Ln(3,4-PDC)(3,4-HPDC)].110 Although pyridyl derivatives form the bulk of mixed N/O donor ligands in rare earth polymers, derivatives of other heterocyclic rings have also been used; for example, imidazole-4,5-dicarboxylic acid 111 and pyrazole-3,5-dicarboxylic acid have both been used in rare earth coordination networks.112

6.8 Mixed 3d/4f Coordination Polymers The use of heterometallic networks is increasing with the growing understanding of how monometallic coordination polymers can be constructed and of their properties.113,114 A combination of 3d and 4f metals into a single framework has the potential to lead to new functional materials with properties that cannot be obtained from the use of a single metal element alone. In particular, it is anticipated that novel magnetic properties can be harnessed by creating coupling pathways between heterometals and that novel optical properties can be targeted. There are a few classes of ligands that have been used effectively in synthesising 3d/4f polymers, most notably polycyanometallates, carboxylates and pyridylcarboxylates, which are dealt with separately below. That is not to say that these are the only ligands that have been used. Ligands as diverse as glycine,115 nitrilotriacetate,116 Schiff-base ligands117 and oxalate derivatives118 have been used to form mixed-metal coordination polymers; however, discussion here is limited to the main ligand classes.

6.8.1 Cyanide-based Bimetallic Polymers Coordination polymers that contain cyanide ligands are common among transition metals, especially within materials that have bulk magnetic

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Chapter 6

properties (see Chapter 9). The incorporation of cyano ligands into bimetallic 3d/4f complexes is achieved by the use of polycyanometallate anions as starting materials, such as [W(CN)8]3 or [Fe(CN)6]3, and discrete complexes can be formed in addition to polymeric cyano-bridged polymers.119 A similar class of compounds can be formed using isocarbonyl ligands (see Section 7.4). The variable nature of rare earth coordination spheres coupled with the ability of polycyanometallates to act as 6- or 8-connecting nodes can give rise to unusual topologies with high connectivity, although simpler polymeric compounds are also known. An example of a 1D heterometallic polymer is {[Ln(2,2 0 -bipy)(H2O)4M(CN)6] 4H2O 1.5(2,2 0 -bipy)}, where Ln ¼ Sm–Yb and M ¼ Fe(III) or Co(III) (Figure 6.22a).120,121 The cyanometallate groups bridge in a trans manner whereas the cyano groups coordinate to the rare earth atom in mutually cis positions. The Fe(III) polymers with gadolinium, dysprosium and terbium display antiferromagnetic exchange coupling. A similar structural motif is observed in the polymers [Ln(terpy)(DMF)4][WV(CN)8] 6H2O (Ln ¼ Gd or Sm) in which the octacyanotungstate anion acts as a linear bridge.122 Also 1D polymers can be synthesised that are trimetallic using bimetallic molecular starting materials. An example of this approach is the stepwise synthesis of the polymer [Cu(salpn)Gd(H2O)3{Fe(CN)6}] 4H2O, in which the Cu(II) complex of a Schiff-base ligand (salpn) is first reacted with Gd(NO3)3 before displacement of the nitrate ligands by [Fe(CN)6]3 anions.123 The gadolinium and copper atoms are bridged by two m2-alkoxy oxygen atoms whereas a 1D ladder is generated by the linking of three gadolinium atoms by the polycyanometallate groups (Figure 6.22b), and although there is no magnetic communication between adjacent dinuclear Cu–Gd, there are ferromagnetic interactions between these and the [Fe(CN)6]3 groups. The 2D coordination polymer [NdFe(bpym)(H2O)4(CN)6] 3H2O (bpym ¼ 2,2 0 -bipyrimidine) displays ferromagnetic interactions, although there is no long-range ordering.124 The 2D structure of this polymer is unusual with both 3-connecting nodes (Nd) and 4-connecting nodes (Fe) and the bipyrimidine ligands serve only as terminal chelating species. The complex [Gd(DMF)2(H2O)3Cr(CN)6] also forms a 2D sheet; however, both the rare earth and the transition metal act as three connecting nodes giving a binodal (6,3) net in which cyano ligands are again the sole bridging species.125 The polymer {[Ru(phen)(CN)4]3[Gd(phen)(H2O)3]2 6H2O} also forms corrugated (6,3) sheets; however, only the gadolinium atoms act as three-connecting nodes with the ruthenium anions acting as bridging species through cis-cyanide ligands (Figure 6.22c).126 Other ruthenium species have also been used as 2-connecting bridges, such as [Ru(acac)2(CN)2], which acts as a linear bridge with equatorially coordinated acac ligands.127 An interesting example of a ruthenium–rare earth polymer is the 3D network of {Nd2[{Ru(CN)4}3(m3-HAT)] 23H2O}, which is synthesised using the unusual [{Ru(CN)4}3(m3-HAT)]6 anion (HAT ¼ hexaazatriphenylene) with 12 externally directed cyanide ligands.128 The complex anion acts as an 8-connecting node in the network with six in-plane interactions and two to adjacent


Figure 6.22

223

Example of heterometallic coordination polymers that contain polycyanometallate subunits. (a) 1D chain of {[Ln(2,2 0 -bipy)(H2O)4M(CN)6] 4H2O 1.5(2,2 0 -bipy)}; (b) ladder-type polymer of [Cu(salpn)Gd(H2O)3{Fe(CN)6}] 4H2O; (c) the 2D sheet {[Ru(phen)(CN)4]3[Gd(phen)(H2O)3]2 6H2O} (with phen ligands partially omitted for clarity); (d) a sheet within the 3D network {Nd2[{Ru(CN)4}3(m3-HAT)] 23H2O}.

layers (Figure 6.22d). The neodymium atoms act as 4-connecting nodes (three in-plane, one perpendicular). The Ru - Nd energy transfer in the network results in sensitised near-IR luminescence. A mixed 3d/4f coordination polymer has also been reported that adopts a anionic sodalite-type structure, [{Nd(CH3OH)4MoIV(CN)8}3]3, which is charge balanced by interstitial [Nd(H2O)8]31 cations.129 The molybdate groups act as 4-connecting nodes (with four of the cyano ligands being terminal) and the neodymium atoms are

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Chapter 6

also 4-connecting. Methanol molecules reside inside the channels of this zeolitetype material. While the above example deals with high-connectivity transition metal nodes, it should be noted that networks with lower connectivity are also available. Dicyanoaurate and dicyanoargentate can be used as linear connecting species between rare earth atoms so that the network topology is defined solely by the coordination environment of the lanthanoid element, giving an unusual 49.66 acs topology. For example, the complexes [La(Au(CN)2)3(H2O)3] and [La(Ag(CN)2)3(H2O)3] are isostructural and complexes can be obtained with mixed Au–Ag positions that show the existence of heterometallic Au–Ag interactions.130

6.8.2 Carboxylate-based Bimetallic Polymers Section 6.6 demonstrated that the largest studied class of rare earth coordination polymers are those containing carboxylate ligands, from simple acetate complexes to complex ligands with multiple carboxylate groups. There are also a number of heterometallic polymers that use carboxylate groups to bridge between heterometallic elements. For example, the 2D polymers [Eu2M(H2O)4][O2C(CH2)3CO2]4 2H2O (where M ¼ Mn, Fe, Co or Ni) contain the glutarate dianion bridging between heterometallic chains of alternating 3d and 4f metals with subtle structural variations depending on the nature of the 3d metal (Figure 6.23a).131 The dicarboxylate adopts two different binding modes, heterometallic variations of those seen in Section 6.6. The ligand bridges between three europium atoms and one 3d metal with either a m2:Z1(O)–Z1(O 0 ) mode at either end or a m2:Z1(O)– Z2(O,O 0 ) mode in which the transition metal is the recipient of a single interaction whereas the rare earth is chelated. Each 3d metal is coordinated to four glutarate anions via oxygen atoms while the remaining axial sites of the distorted octahedron contain aqua ligands, whereas the europium atoms are surrounded by six anions in a mixture of chelating and monodentate binding modes in addition to a single water molecule. The polymers show antiferromagnetic coupling between the 3d/4f metals. Recently, three series of heterometallic coordination polymers were reported that contain isophthalic acid.132 The polymer [Cu3Eu2(ip)6] (ip¼isophthalate), for example, forms a 3D network which can be viewed as a (6,3) substructure in which the metal atoms are linked by the carboxylate groups (Figure 6.23b). The europium atoms act as the 6-connecting nodes in the substructure with the copper atoms lying midway along the edges. The sheet substructure is linked by the isophthalate ligands into an overall 3D structure. Isophthalate complexes in which 2,2 0 -bipyridine was present in the reaction mixture have a dramatically different network structure and are based on complex SBUs such as the linear {Er6Cu2(2,20 -bipy)2(O2C)11} units in the a-Po network [Cu(2,2 0 -bipy)Eu3(ip)5(Hip)(H2O)].

6.8.3 Pyridylcarboxylate Ligands Heterometallic coordination polymers containing pyridylcarboxylate ligands typically coordinate to a transition metal through the pyridyl nitrogen atom


Figure 6.23

225

(a) One sheet of the polymer [Eu2Ni(H2O)4][O2C(CH2)3CO2]4 2H2O with glutarate ligands represented by straight lines between the metal polyhedra; (b) (6,3) substructure in the 3D polymer [Cu3Eu2(ip)6]. Reproduced with permission of the Royal Society of Chemistry from Dalton Trans., 2005, 2603 and from Wiley-VCH Verlag GmbH & Co. KGaA from Chem. Eur. J., 2007, 13, 4948, respectively.

and to 4f metals via the carboxylate oxygen atoms. Both monocarboxylato- and dicarboxylatopyridine anions have been used in such polymers. Picolinic acid (pyridine-2-carboxylic acid), is a widely used bridging ligand as it has the ability to form an N,O-chelate to one metal while the remaining carboxylate oxygen atom can bridge to a second metal atom. This coordination mode allows for transition metal metalloligands, such as [Cu(pic)2], to be preformed before reacting them with rare earth salts. This approach has been successfully employed to give a range of 1D and 2D polymers depending on the

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Chapter 6 133

mole ratios of [Cu(pic)2] and Ln(ClO4)3 that were used. These complexes include the 1D polymer {[LnCu2(pic)4(H2O)6](ClO4)3 . H2O} (Ln ¼ Sm, Nd or Pr) and the 2D network {[Ln2Cu5(pic)10(H2O)8](ClO4)6 . 2H2O} (Ln ¼ Gd–Yb) using 1:2 or 1:3 ratios of [Cu(pic)2]: Ln(ClO4)3, respectively (Figure 6.24). The cationic 1D polymer contains both bridging and terminal [Cu(pic)2] metalloligands, with each rare earth coordinated to three ligands (two bridging, one terminal) and six water molecules (Figure 6.24a). The 3d and 4f metals are

Figure 6.24

Structures of heterometallic polymers containing the [Cu(pic)2] metalloligand: (a) the 1D chain in {[LnCu2(pic)4(H2O)6](ClO4)3 . H2O} (with zigzag chain propagation highlighted); (b) the 2D sheet in {[Ln2Cu5(pic)10 (H2O)8](ClO4)6 . 2H2O} (with terminal [Cu(pic)2] groups omitted for clarity).


227

separated by a three-atom O–C–O bridge. The 1D chain has a zigzag structure with the bridging metalloligands coordinated to the rare earth atoms at mutually cis positions and the terminal ligands participate in inter-chain hydrogen bonding. The 2D polymer [Ln2Cu5(pic)10(H2O)8](ClO4)6 . 2H2O also contains a terminal [Cu(pic)2] species, with one coordinated to each rare earth atom, although there are now three bridging metalloligands that act as the edges of a (6,3) sheet in which the lanthanoid is the 3-connecting node (Figure 6.24b). Both the 1D and 2D polymers show antiferromagnetic behaviour at low temperatures when appropriate lanthanoid atoms are used. A 2D samarium–nickel polymer using picolinate has also been reported which contains the transition metal with an octahedral coordination geometry surrounded by three N,O-chelating picolinate ligands [unlike the square-planar Cu(I) in the previous example]. The compound, {[SmNi(pic)3(H2O)5](ClO4)2 3H2O} is binodal with both 3d and 4f metals acting as alternating 3-connecting nodes in a 4.82 net and antiferromagnetic behaviour is again observed at low temperatures.134 The isomeric isonicotinic acid (pyridine-4-caroboxylic acid) has been used in Ag(I)–Ln polymers with either acetate or benzene-1, 3-dicarboxylate as co-ligands.135,136 In these cases the 4d/4f heterometallic elements are well separated, with the Ag(I) coordinating to the pyridyl nitrogen atom in most cases and the rare earth element polymerising with the carboxylate termini of the ligand. Pyridyldicarboxylate ligands have also been used in 3d/4f coordination polymers. Pyridine-2,3-dicarboxylate (quinolate, QA) is able to adopt a variety of binding modes; it is able to chelate in a similar manner to picolinate and can also bridge via one or both carboxylate groups. The gadolinium–cobalt complex [Gd(H2O)3Co(QA)3] demonstrates these binding modes well, in contrast to the picolinate polymers discussed above (Figure 6.25a).137 Three dicarboxylate anions are N,O-chelated to the octahedral cobalt atoms with the m-carboxylate groups of each ligand bound Z2 to nine-coordinate gadolinium atoms (which have aqua ligands filling the coordination sphere). The structure is a (6,3) sheet with alternating Co–Gd 3-connecting nodes and transforms to a perovskite oxide, GdCoO3, at moderate temperatures (ca. 700 1C). The non-coordinating oxygen atom of the o-carboxylate group is involved in hydrogen bonding with the aqua ligands of the gadolinium. Another quinolate framework has been described that adopts a 3D structure, [Ln2(H2O)4M2(H2O)2(QA)5] (Ln/M ¼ Gd/Co, Gd/Ni or Dy/Co), in which the quinolate ligands adopt three different binding modes (Figure 6.25b).138 In two of the ligand binding modes the quinolate is m3 bridging, using all available donor atoms. The m2 bridging mode differs from that in the previously discussed complex in that the remaining free oxygen atom belongs to the m-carboxylate. The channels in the network hold 6–7 water molecules per repeating formula unit, although powder diffraction suggests that the network partially collapses once these have been removed by heating. Whereas picolinate and quinolate are able to act as bidentate chelating ligands towards transition metals, pyridine-2,6-dicarboxylate acts as a tridentate ligand (using the pyridine nitrogen atom and one oxygen from each carboxylate). This binding mode allows for two ligands to coordinate to an

228

Figure 6.25

Chapter 6

(a) The (6,3) sheet structure of [Gd(H2O)3Co(QA)3]. (b) Binding modes of the quinolate anion in the 3D network [Ln2(H2O)4M2(H2O)2(QA)5]. Reproduced with permission from Inorg. Chem., 2005, 44, 5241.

octahedral metal with two remaining oxygen atoms potentially free for bridging to a heterometal. In the complex {[Ln4Cu2(pydc)8(H2O)8] . 18H2O} (pydc¼ pyridine-2,6-dicarboxylate, Ln ¼ La or Pr) the ligands adopt a m2-Z3(Cu): Z1(Ln) binding mode with one oxygen atom of the ligand not coordinated to a metal. The complex forms a 1D polymer with two [Cu(pydc)]2– bridges between adjacent lanthanoid atoms. The structure contains extensive hydrogen bonding involving the non-coordinating oxygen atoms with both ligated and interstitial water molecules. The pydc2 ligand has been reported to form different structures under hydrothermal conditions, including an Mn–Yb nanoporous material that reversibly holds water in a channel of diameter 7.3 A˚.139


229

6.8.4 Other Mixed O/N Donor Ligands The iminodiacetate anion (ida) acts in a similar manner to pyridine-2,6-dicarboxylate as it is able to act as a tris-chelating ligand with free oxygen atoms for bridging to heterometals. A nanoporous Ln–Cu complex has been reported in which the iminodiacetate ligands form a [Cu(ida)2]2– metalloligand in which the ligands are both Z3 binding (cf. Figure 6.24a), which then bridges between four rare earth atoms via the remaining oxygen atoms.140 The resulting structure contains large channels that contain water, and once the solvent has been thermally removed the materials are stable to 300 1C (Figure 6.26a). The dehydrated samples have the water guests reintroduced, although the resulting materials do not have as high a degree of solvation as the originally prepared product. The more flexible ligand nitrilotriacetate (nta) has also been used in heterometallic polymers. For example, the complexes M[Cu(nta)Cl]2Ln(H2O)4, where M ¼ Na or K and Ln ¼ Gd or La, form 2D sheets with alternating Cu–Ln centres that are connected by the alkali metal cations.141 In the gadolinium complexes, antiferromagnetic coupling is observed between the 3d/4f metals. Another mixed N/O donor ligand is glycinate, which forms an unusual polymer containing Ln6Cu24 clusters, acting as four connecting nodes, and trans-Cu(gly)2 bridges.142

6.9 Actinide Coordination Polymers Actinide coordination polymers are rarer than their lanthanide counterparts, not helped by the radioactivity of the majority of the 5f elements. Examples of actinide polymers are therefore restricted primarily to uranium species with

Figure 6.26

Nanoporous Ln–Cu complex using iminodiacetate (guests not shown for clarity).

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carboxylate ligands. Although the actinide elements are larger and carry more charge than the lanthanides, they share a preference for oxygen donor ligands. An oxalate polymer demonstrates their similarity by doping U(IV) into an Ln(III) framework with counter-anions from channels removed to accommodate the charge difference.143 A simple example is the 2D polymer [UO2(O2C(CH2)3CO2)], in which the glutarate anions bridge between uranyl dimers (Figure 6.27a).144 Longer chain dicarboxylates have also been used to

Figure 6.27

Actinide polymers. (a) [UO2(O2C(CH2)3CO2)]. (b) {[(Th2F5)(3,5-pdc)2(H2O)][NO3]}. Reproduced with permission from J. Am. Chem. Soc., 2003, 125, 12688. (c) [(UO2)Cu(pzdc)2(H2O)2].

231

Rare Earth Coordination Polymers 145

create different networks using the uranyl cation. Citrate has also recently been shown to form a 3D polymer with uranyl and the anion has been widely studied with regard to its complexation properties for decontamination use.146 Pyridinecarboxylate species have been widely used in 5f polymers, as they have for rare earth polymers (see Section 6.7). Pyridine-3,5-dicarboxylate (3,5pdc) has been reported to form a polymer that contains thorium–fluoride chains and bears a passing resemblance to lanthanoid–carboxylate networks. However, the pyridyldicarboxylate in the complex {[(Th2F5)(3,5-pdc)2(H2O)] [NO3]} is in a m2 bridging mode, with one oxygen atom from each carboxylate group coordinated to a Th–F chain (Figure 6.27b).147 The nitrogen atom of the pyridine is non-coordinating and pores in the lattice are filled by nitrate counter-anions. Other pyridyldicarboxylates have been used in both uranium and mixed 3d/5f coordination polymers.148 Pyrazole-3,5-dicarboxylate (pzdc) can also be utilised in mixed 3d/5f polymers. For example, the complex [(UO2)Cu(pzdc)2(H2O)2] contains [Cu(pzdc)2] groups that bridge between uranyl cations via Z1 carboxylates (Figure 6.27c).149

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CHAPTER 7

Organometallic Networks 7.1 Introduction Most coordination polymers contain ‘conventional’ ligands with ‘normal’ coordinative interactions between the organic ligand and the metal cation; however, there are a significant number of networks that are held together by virtue of organometallic interactions. Such interactions include direct metal–carbon bonds, with ligands such as cyanides and carbonyls, and also metal–p interactions from electron-rich aromatic systems.1 Organometallic networks (OMNs) offer a variety of unique properties due to the bonding modes that they contain, particularly in terms of their electrical properties. A significant portion of organometallic networks contain aromatic ligands, including cyclopentadienyl (cp) rings. Such a classification therefore incorporates ligands that contain transition metal metallocenes within them, with a sizeable number being ferrocene-based bridging ligands. In addition, larger aryl groups, such as naphthalene and anthracene, also form novel networked structures.2 Networks also exiwzm t that contain metal interactions to linear psystems such as terminal ethenes. Organometallic molecules may be used as bridging species between other metals, giving metal–organometallic networks (MOMNs) with particular emphasis on benzoquinone derivatives, incorporating an organic redox-active moiety into framework materials.3 Bridging metallocene ligands are also a significantly researched area. In principle, the use of organometallic bridges allows for the possibility of metal–metal interactions within heterometallic systems. Smaller ligands, such as carbonyls and isocyanides, have been utilised to great effect in coordination networks. The short distance between adjacent metal ions (e.g. a two-atom bridge of carbonyl) allows for a significant degree of communication, thereby assisting in the construction of solids that are magnetically ordered.


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Organometallic Networks

239

7.2 Large Aromatic Ligands 7.2.1 Cation–p Interactions in Ag(I)–PAH Systems Interactions between electron-rich aromatic groups and metal cations are well documented, with the most obvious examples being the cyclopentadienyl sandwich complexes such as ferrocene and cobaltocenium. Larger aromatic systems, those with fused rings or covalently linked phenyl groups, are also able to form metal–p interactions. The larger size of such ligands allows them to bridge between multiple metal cations and therefore form organometallic coordination networks. A significant amount of work has been carried out by Munakata and co-workers examining organometallic networks containing Ag(I) bridged by large polyaromatic ligands. Ag(I) is used due to its flexible coordination environment and diverse stereochemistry (thereby allowing the ligands to be predominant in determining the network structure). The soft metal nature of Ag(I) also lends itself to the formation of interactions with electron-rich p-systems. Ag(I)–aromatic complexes were first postulated as early as 1921 (with benzene),4 and there are now around 80 known examples of systems containing polycyclic aromatic hydrocarbons (PAHs).5 Several generic network topologies have been shown to recur with different ligands (Figure 7.1), in addition to some examples of discrete complexes that can be formed by ligands surrounding the metal ion. Networks are typically obtained by the crystallisation of AgClO4 or AgCF3SO3 in the presence of the organic aromatic molecule. Organic molecules that have been studied include linear polyaromatics (benzanthracene, pentacene),6 non-linear polyaromatics (pyrene, perylene)7 and non-planar systems such as hexaphenylbenzene,8 triptycene 9,10 and [2.2]paracyclophane.11 Typically, all of the above ligand classes form Z2 interactions with Ag(I) although there are

Figure 7.1

Ag(I)–aromatic networks adopt a variety of different structures and have also been observed to form discrete complexes. Reproduced by permission of the Royal Society of Chemistry, from Org. Biomol. Chem., 2005, 3, 407.5

240

Figure 7.2

Chapter 7

The compound [Ag2(dibenz[a,h]anthracene)(ClO4)2] forms a ‘W-type’ 1D coordination polymer with four Z2 cation–p interactions per ligand.

exceptions to this, such as Z1 coordination observed in the Ag(I) complex of hexaphenylbenzene.8 An example of the coordination mode observed in Ag(I)–PAH networks is shown in Figure 7.2. The compound [Ag2(dibenz[a,h]anthracene)(ClO4)2] forms 1D coordination polymers that adopt a ‘W-type’ architecture. Each of the organic ligands is involved in four Z2 cation–p interactions with Ag(I) ions and bridges between two Ag2 dimers (in addition to bridging with the dimers). The Ag(I) cations adopt non-standard five- and six-coordinate geometries. Although many PAHs form similar network topologies (Figure 7.1), there are some structures that are considerably different from others. An interesting example is the double-stranded helical Ag(I) complex that is formed with

241

Organometallic Networks 12

benzo[e]acephenanthylene. The structure contains a helical strand of AgClO4 around which is wound a second strand of the organic ligand. The 1D chains are joined into a 2D sheets by further interactions with the PAH molecules. The compound also shows semiconducting behaviour. In addition to being structurally interesting, Ag(I)–PAH compounds have also demonstrated some interesting physical properties and behaviour. Guest desorption and absorption have been observed in the Ag(I)–PAH complex of triptycene (tpty), [Ag3(tpty)3 (ClO4)3] 2(toluene).9 The organometallic framework contains channels of approximately 3.1 4.0 A˚, which are lined by aromatic ligands and aryl CH groups. The size of the pores is a nearly ideal fit for aromatic solvents, benzene and toluene, aided by the favourable hydrophobic interior environment. The complex is synthesised from toluene, which can be removed by heating under argon at 150 1C. Subsequent immersion of the resulting powder in toluene or benzene results in the absorption of the solvent into the lattice. One of the largest PAH ligands reported is C30H12 (hemibuckminsterfullerene), which has been incorporated into a twofold interpenetrating 3D diamondoid organometallic framework {[Rh2(O2CCF3)4]3(C30H12)} (Figure 7.3a).13 The network consists of rhodium–carboxylate clusters that act as linear connectors with hemifullerene ligands as the 4-connecting nodes. Of the four rhodium atoms that coordinate to each hemifullerene, via carbon atoms at the rim of the molecule, three have an Z2 coordination mode and are exo-coordinated, whereas the fourth is endo-coordinated and has only an Z1 interaction (Figure 7.3b). Other bowlshaped PAHs have also been shown to form organometallic coordination polymers.14 Whilst the focus above has been placed on Ag(I) complexes, for which there is the most substantial body of work, it should be noted that other metal ions are also observed to form network solids through cation–p interactions. For example, the three-dimensional network [NaYb(C5H5)3] is held together entirely by Z5 interactions between both Na1 and Yb21 and the cyclopentadienyl anions.15 Other examples include [BiCl3(trimethylbenzene)] (containing Bi–arene–Bi Z6 linkages) 16 and the mixed manganese–caesium network Cs[MnCp3] that forms a (10,3)-a network that displays racemic twofold interpenetration.17

7.2.2 Cation–p Interactions with Cyclophanes Cyclophanes are non-planar, macrocyclic aromatic systems and therefore give access to a range of coordination networks that differ significantly from those that contain planar aromatic systems. The most readily available cyclophanes, [2.2]paracyclophane and [2.2.2]paracyclophane, are able to act as 2- and 3-connecting nodes, respectively, by coordination of metals to the exterior surface of the aryl groups. Munakata and co-workers have shown that coordination complexes containing Ag(I) can be prepared by solution methods;18,19 however, a recent study by Petrukhina et al. adopted a different synthetic approach by using gas-phase deposition to synthesise Rh-based networks.20 As part of this work,

242

Figure 7.3

Chapter 7

(a) The diamondoid hemibuckminsterfullerene network {[Rh2(O2CCF3)4]3 (C30H12)}; (b) the ligand coordinates to the metals via a combination of Z1 (Rh4) and Z2 interactions.


243

[Rh2(O2CCF3)4] paddlewheel complexes were shown to act as linear connectors between cyclophane ligands, forming 1D chains with the 2-connecting [2.2]cyclophane molecule and (6,3) sheets with the 3-connecting [2.2.2]cyclophane ligand (Figure 7.4). The coordination mode between the cyclophanes and Rh complexes in both cases is by Z2 interactions. These products were not exclusive, with [2.2.2]cyclophane also giving rise to the discrete complex [(Rh2(O2CCF3)4 (cyc)2] and a 1D chain when [(Ru2(O2CCF3)2(CO)4] was used as the metal-containing species. Despite the apparent porous nature of the (6,3) sheet compound (with aligned, vacant cyclophane rings), no significant gas adsorption was observed.

7.2.3 Non-aromatic Cation–p Interactions Organometallic coordination networks containing cation–p interactions are not restricted to aromatic ligands, with several examples of ethene- and ethynebased ligands used to form polymeric complexes. The coordination of conjugated p-systems to transition metals is generally expected to result in polymers with conductive or non-linear optical (NLO) properties (see Chapter 13). As with the PAH systems, multi-dimensional polymers are often created using polyethene ligands, such as with 1,8-diphenylocta-1,3,5,7-tetraene (dpot), which forms 2D networks of the composition [Ag2(dpot)(ClO4)2].21 The metal–ligand interactions themselves form 1D chains which are then bridged by the perchlorate counter-anions to generate a 2D sheet with a (4.62)2(42.62.82) topology. Coordination to the metal affects the planarity of the dpot molecule. A similar synthesis using the shorter 1,4-diphenylbuta-1,3-diene ligand results in compounds that do not contain any ethene–metal interactions (only Z2 interactions from the phenyl rings).22 Terminal acetylene functionalities can also form sideon interactions with metal ions and bis(acetylene) ligands have been used to form polymeric organometallic complexes.23–25 Some results, which at first seem surprising, have also been obtained that incorporate p-donor systems. For example, the solvothermal reaction of 4-pyridylacrylic acid (4-pyaH), 2,2 0 -bipyridine and [Cu(MeCN)4](BF4) yields the 1D polymeric complex {[(2,2 0 -bipy)(4-pyaH)Cu][BF4]}n in which the 4-pyaH ligand coordinates to Cu(I) via the ethene, when it would reasonably be expected that a carboxylate would be involved in metal coordination (Figure 7.5a). The carboxylic acid instead forms an array of hydrogen bonds between parallel polymeric strands. The complex is thermally stable at above 200 1C and exhibits a strong red fluorescent emission.26 Other Cu(I)– alkene complexes are also known that are stabilised by chelating ligands such as pyrazolylborates,27 1,10-phenanthroline 28 and di-2-pyridylamine.29 Analogous complexes to {[(2,2 0 -bipy)(4-pyaH)Cu][BF4]}n are known with 1,10-phenanthroline and using the 3-pyaH ligand, with the resulting structure in the latter case also incorporating uncoordinated water in the lattice.30 If the carboxylic acid group is omitted, i.e. 4-vinylpyridine (4-vp) is used, then 2D coordination polymers are formed when hydrothermally reacted

244

Figure 7.4

Chapter 7

[Rh2(O2CCF3)4] paddlewheel complexes are connected by (a) [2.2]paracyclophane and (b) [2.2.2]paracyclophane to form 1D and 2D coordination polymers, respectively.


245

Figure 7.5 Alkene coordination polymers (a) 1D {[(2,2 0 -bipy)(4-pyaH)Cu][BF4]} (anions not shown for clarity) and (b) a section of the 2D sheet [Cu (4-vp)Cl]n.

with CuCl (Figure 7.5b).31 The 2D polymer [Cu(4-vp)Cl]n contains holes of approximately 10 7 A˚ and has dichloride bridges in addition to bridging 4-vp ligands.

7.3 Organometallic Complexes as Ligands 7.3.1 Metallocene Bridging Ligands The incorporation of organometallic species into a network as bridging ligands between other metals gives rise to what have been termed metal–organometallic networks (MOMNs). A common way in which organometallic components are included in coordination networks is by the use of bridging ligands that contain metallocenes, typically ferrocene or cobaltocenium which under standard conditions contain Fe(II) or Co(III) centres, respectively. Symmetrically

246

Chapter 7

substituted metallocenes, those with appended functional groups on both cp rings, are able to act as linear connectors between metal nodes and introduce a redox-active centre into the resulting network. The coordination geometry around the metallocene itself is very flexible, with the cp rings able to rotate to any angle with respect to the other,32 although sterically there is often a preference for the functional groups to be 1801 removed from each other. An example of such a system is the 2D Cu(II) network {[Cu(ccdc)2] 2MeOH} that incorporates cobaltocenium dicarboxylate (ccdc) as the bridging ligand (Figure 7.6a).33 The network is a (4,4) sheet with the carboxylate ligands Z1 coordinated to the square-planar Cu(II). The chemistry of the ccdc ligand compared with the ferrocene analogue is significantly different due to the change in the charge of the ligand, giving rise to 2:1 or 1:1 ligand–metal complexes, respectively.34 The redox potential of the ccdc ligand shows a negative shift of 295 mV upon inclusion in the coordination network (compared with solution-state measurements of the free ligand). Whereas the use of short functional groups results largely in linear bridging ligands, more extended and flexible groups can result in different ligand geometries and therefore impact on the resulting networks. An example of this behaviour is work using a ferrocene ligand with triazolylmethyl pendant groups, 1,1 0 -bis[(1H-1,2,4-triazol-1-yl)methyl]ferrocene (btmf).35 This ligand has been shown to form 1D chains with either one or two ligands bridging between adjacent metals. The bistriazole ligand demonstrates that metallocene-based ligands are able to adopt different geometries, with the functional groups adopting a cisoid geometry in the octahedral metal complex [CdCl2(btmf)2] and a transoid geometry in the tetrahedral metal complex [ZnCl2(btmf)] (Figures 7.6b and c, respectively). Solid-state electrochemistry of the coordination networks shows quasi-reversible wave couples in both cases.

7.3.2 Metal–Quinone Bridging Complexes The previous section discussed the use of sandwich complexes as bridging ligands. It is also possible to form coordination networks using piano-stool complexes as the bridging species, with the most prevalent examples being those in which Z4 benzoquinone complexes are used (note that the protonated hydroquinone species display Z6 coordination).36–38 Piano-stool complexes can be synthesised using 1,4-hydroquinone, 1,3-hydroquinone (resorcinol) or 1,2hydroquinone (catechol). Quinone complexes are obtained by deprotonation of the hydroquinone followed by coordination to a semi-protected metal fragment, typically Ru(COD) or Mn(CO)3. Metal complexes of 1,2-hydroquinone typically form discrete complexes as the ortho-substitution pattern lends itself to metal chelation. The most useful complexes for forming network solids are those containing p-benzoquinone, such as the stable anionic species [(Z4benzoquinone)Mn(CO)3] (p-QMTC), which represents an organometallic complex with 1801 disposed terminal oxygen atoms that are available for


Figure 7.6

247

Metallocene coordination networks. (a) A (4,4) sheet containing cobaltocenium dicarboxylate. (b, c) Cadmium and zinc 1D chains, [CdCl2(btmf)2] and [ZnCl2(btmf)], containing a ferrocene-based triazole ligand in cis and trans geometries, respectively.

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Chapter 7

coordination to other metals. Although the p-QMTC ligand would be expected to form a linear bridge, the benzoquinone ring is frequently observed not to be coplanar with metal atoms when in a coordination network. Synthesis of the benzoquinone networks is typically by using the semiquinone complex as a precursor and deprotonating this complex in situ with the desired metal acetate salt. Reaction with octahedral metals results in 1D chains of the type [M(p-QMTC)2L2] (where L is a neutral, terminal ligand such as DMSO or pyridine in an axial position and M ¼ Co or Cd) (Figure 7.7a).39 Each pair of adjacent metals is bridged by two p-QMTC ligands, which are partially eclipsed and have an interplanar separation of only 3.3 A˚, indicative of p–p stabilisation. If the synthetic conditions are altered to have a higher metal ion concentration, then an isomeric 2D network is preferentially formed.40 The incorporation of bridging organic ligands, such as 4,4 0 -bipyridine, into reaction mixtures that would yield 1D and 2D networks result in the formation of frameworks with 2D and 3D topologies, respectively, in which the bridging organic ligands replace the previous terminal axial ligands (Figure 7.7b), leaving porous channels in some cases. When combined with tetrahedral Zn(II), the p-QMTC ligand generates two interpenetrating diamondoid networks.39 Recent work with this ligand has resulted in a 3D lanthanoid framework with the composition [Eu(p-QMTC)3] which contains octahedral Eu(III) nodes (Figure 7.7c).41 A variety of other complexes containing p-QMTC are also known, for which the reader is directed to perspective articles by Sweigart and co-workers.36,37

7.4 Isocarbonyl Polymers Another class of organometallic coordination polymers is those that contain bridging m2(C,O) carbonyl ligands – termed ‘isocarbonyl’ in this coordination mode. Such complexes are known for heterometallic 3d/4f polymers.42,43 Two types of 3d/4f bimetallic carbonyl polymers are known: those containing direct metal–metal bonds and those in which all adjacent metals are CO bridged. The former type is synthesised using very nucleophilic carbonylate anions, such as [Fe(CO)4]2. The anions are generated in situ by the reduction of Fe3(CO)12 with an elemental rare earth, such as ytterbium, in liquid ammonia, yielding [(NH3)2YbFe(CO)4]. Dissolving this complex in acetonitrile displaces the ammonia and the polymeric complexes {[(CH3CN)3YbFe(CO)4]2 CH3CN} and {(CH3CN)3YbFe(CO)4} can be isolated depending on the conditions used (Figure 7.8).44 In both cases the iron is five-coordinate, including one bond to an ytterbium which is of a comparable length to those in the YbFe2 alloy. The difference between the two structures lies in the ratio of isocarbonyl bridges to terminal carbonyl ligands: 2:2 in the 1D chain and 3:1 in the 2D polymeric sheet. Cyanides can also be used to form heterometallic networks (see Section 6.8.1). More recent 3d/4f isocarbonyl research has produced polymers that contain transition metal clusters, such as {(Et2O)3Yb[Co4 (CO)11]} (Figure 7.8c).45 The tetrahedral [Co4(CO)11]2 clusters are generated


Figure 7.7

249

(a) 1D chain of [M(p-QMTC)2(DMSO)2] has two bridging organometallic groups. (b) The organometallic ligand [(Z4-benzoquinone)Mn(CO)3] can form 1D and 2D networks when reacted with salts of octahedral metals and 2D/3D networks when organic bridging ligands are used simultaneously and (c) forms a 3D network with octahedral Eu(III). (a) and (b) Reprinted with permission from Acc. Chem. Res., 2004, 37, 1.36 Copyright 2004, American Chemical Society.

250

Figure 7.8

Chapter 7

Carbonyl coordination polymers (a) 1D {[(CH3CN)3YbFe(CO)4]2 CH3CN} and (b) 2D {(CH3CN)3YbFe(CO)4}. (c) Polymer containing transition metal–carbonyl clusters, {(Et2O)3Yb[Co4(CO)11]}.45


251

in situ. Three isocarbonyl ligands bridge from the tetrahedral clusters with both the cluster and the rare earth acting as 3-connecting nodes in the 2D sheet.

7.5 R3Sn/R3Pb Systems 7.5.1 Metal–Cyanide Polymers An extensively studied class of organometallic 3D coordination polymers is those with the general formula [(R3M)3Md(CN)6], where M ¼ Pb or Sn and Md is either cobalt or iron (Figure 7.9a).46–48 The networks are generated by the

Figure 7.9

Structures of the 3D organometallic polymers (a) [(Me3Sn)3Fe(CN)6] and (b) [(Me3Sn)4W(CN)8] (guest species removed for clarity in both cases).

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Chapter 7

reaction of R3MX (where X is a halide) with the respective hexacyanide ion. Each transition metal is bridging by CN–(R3M)–NC chains with the metal (Sn or Pb) adopting a distorted trigonal bipyramidal geometry. Only one-third of these five-atom bridges are linear (reminiscent of an extended Prussian Blue-type arrangement). The networks can act as host species for a variety of neutral guests as they contain large channels (ca. 9.5 A˚) which are lined by the alkyl groups. Furthermore, this class of compound can oxidise the species within the lattice using the d-transition metal as the oxidising agent. An example of oxidising behaviour is the charge-transfer (CT) complex formed by the intercalation of pyridine derivatives within the [(Me3Pb)3Fe(CN)6] framework.49 Reduction of the FeIII centres to FeII occurs with a notable colour change and the resulting CT complexes act as weak semiconductors. Other species can also be oxidised within the host lattice, such as metallocenes and metal iodides.50,51 Once FeIII has been reduced, the [(R3M)3Fe(CN)6] network then carries a negative charge and can be written as [(Me3MIV)3FeII(CN)6](Gn1)1/n (where G is a cationic guest). The negatively charged networks can act as hosts for a variety of different cationic guests, such as tetraalkylammonium cations or cobaltocenium. The variety of such compounds can be further extended by using Cu(I) in place of Fe/Ru, yielding the complex [(Et3Sn)2Cu(CN)4](n-Bu4N).52 The CuI polymer is tetragonally distorted from the expected diamondoid structure, with n-Bu4N cations providing the correct size-fit for the pores and preventing interpenetration of multiple networks (a common occurrence in ‘super-diamondoid’ networks). Molybdenum and tungsten have also been incorporated into Me3Sn polymers, yielding complexes of the formula [(Me3Sn)4M(CN)8] (Figure 7.9b).53 A more recent advance in this class of materials has been the use of thiocyanate ligands instead of cyanide. The complex [(Me3Sn)3RhIII(SCN)6] displays a ‘super Prussian Blue’ structure in which two 3D frameworks interpenetrate with SCN–Sn–NCS spacers between the Rh atoms.54 The Rh sites are pseudo-octahedral and the Sn linkers deviate slightly from linearity. Rh was employed for this purpose owing to its affinity for the soft sulfur site of SCN– (contrasting with the preference of Sn for the harder nitrogen donor).

7.5.2 (Triorganostannyl)tetrazole Polymers A smaller class of organotin polymers that have been reported are those by Molloy and co-workers that use tetrazole ligands to bridge between R3Sn species to form 1D or 2D polymers.55–57 A variety of tetrazole-based ligands, incorporating one to four tetrazole moieties have been used. The tetrazole species are synthesised by the reaction of the appropriate azide derivative, R3SnN3, with a polynitrile, such as (CN)4C6H2. The multiple coordination sites of the tetrazole functionality allow for a wide diversity in its coordination modes, and therefore in the nature of the products obtained. In the case of 1,2,4,5-tetrakis(triethylstannyltetrazolyl)benzene, a 3D polymeric net is formed


Figure 7.10

253

(a) The 3D network of the hydrated 1,2,4,5-tetrakis(triethylstannyltetrazolyl) benzene (ethyl groups omitted for clarity). (b) The helical 1D chain that forms using a tetrazolylpyridine ligand.

in which four of the five crystallographically unique tin species bridge between tetrazole groups (the fifth has a coordinated water molecule).56 The N–Sn–N angle is close to linear in all cases (Figure 7.10a). While the tetrakis(tetrazole) yields a 3D network, reducing the number of potential coordination sites by using tetrazolylpyridines results in either discrete species or, in one case, a helical 1D polymer.57 The helical structure of 4-[2-(triethylstannyl)tetrazol-5yl]pyridine is shown in Figure 7.10b and it can be seen again that the Et3Sn species acts as a nearly linear bridge.

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References 1. A.S. Abd-El-Aziz, in Encyclopedia of Supramolecular Chemistry, ed. J.W. Steed and J.L. Atwood, Marcel Dekker, New York, 2004, p. 1014. 2. M. Munakata, L.P. Wu and G.L. Ning, Coord Chem. Rev., 2000, 198, 171, and references therein. 3. J.A. Reingold, S.U. Son, S.B. Kim, C.A. Dullaghan, M. Oh, P.C. Frake, G.B. Carpenter and D.A. Sweigart, Dalton Trans., 2006, 2385. 4. A.E. Hill, J. Am. Chem. Soc., 1921, 43, 254. 5. E.L. Elliott, G.A. Hernańdez, A. Linden and J.S. Siegel, Org. Biomol. Chem., 2005, 3, 407. 6. G.L. Ning, L.P. Wu, K. Sugimoto, M. Munakata, T. Kuroda-Sowa and M. Maekawa, J. Chem. Soc., Dalton Trans., 1999, 2529. 7. M. Munakata, L.P. Wu, T. Kuroda-Sowa, M. Maekawa, Y. Suenaga and K. Sugimoto, Inorg. Chem., 1997, 36, 4903. 8. G.L. Ning, M. Munakata, L.P. Wu, M. Maekawa, Y. Suenaga, T. Kuroda-Sowa and K. Sugimoto, Inorg. Chem., 1999, 38, 5668. 9. M. Munakata, L.P. Wu, K. Sugimoto, T. Kuroda-Sowa, M. Maekawa, Y. Suenaga, N. Maeno and M. Fujita, Inorg. Chem., 1999, 38, 5674. 10. M. Wen, M. Munakata, Y. Suenaga, T. Kuroda-Sowa and M. Maekawa, Inorg. Chim. Acta, 2002, 340, 8. 11. S.Q. Liu, H. Konaka, T. Kuroda-Sowa, M. Maekawa, Y. Suenaga, G.L. Ning and M. Munakata, Inorg. Chim. Acta, 2005, 358, 919. 12. M. Munakata, G.L. Ning, Y. Suenaga, K. Sugimoto, T. Kuroda-Soma and M. Maekawa, Chem. Commun., 1999, 1545. 13. M.A. Petrukhina, K.W. Andreini, L. Peng and L.T. Scott, Angew. Chem. Int. Ed., 2004, 43, 5477. 14. M.A. Petrukhina, Coord. Chem. Rev., 2007, 251, 1690. 15. C. Apostolidis, G.B. Deacon, E. Dornberger, F.T. Edelmann, B. Kanellakopulos, P. MacKinnon and D. Stalke, Chem. Commun., 1997, 1047. 16. A. Schier, J.M. Wallis, G. Mu¨ller and H. Schmidbaur, Angew. Chem. Int. Ed. Engl., 1986, 25, 757. 17. S. Kheradmandan, H.W. Schmalle, H. Jacobsen, O. Blacque, T. Fox, H. Berke, M. Gross and S. Decurtins, Chem. Eur. J., 2002, 8, 2526. 18. M. Munakata, L.P. Wu, G.L. Ning, T. Kuroda-Sowa, M. Maekawa, Y. Suenaga and N. Maeno, J. Am. Chem. Soc., 1999, 121, 4968. 19. S.Q. Liu, H. Konaka, T. Kuroda-Sowa, M. Maekawa, Y. Suenaga, G.L. Ning and M. Munakata, Inorg. Chim. Acta, 2005, 358, 919. 20. M.A. Petrukhina, A.S. Filatov, Y. Sevryugina, K.W. Andreini and S. Takamizawa, Organometallics, 2006, 25, 2135. 21. J.C. Zhong, M. Munakata, T. Kuroda-Sowa, M. Maekawa, Y. Suenaga and H. Konaka, Inorg. Chim. Acta, 2001, 322, 150. 22. J.C. Zhong, M. Munakata, M. Maekawa, T. Kuroda-Sowa, Y. Suenaga and H. Konaka, Inorg. Chim. Acta, 2003, 342, 202.


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23. Y.-B. Dong, Q. Zhang, L. Wang, J.-P. Ma, R.-Q. Huang, D.-Z. Shen and D.-Z. Chen, Inorg. Chem., 2005, 44, 6591. 24. L. Zhao and T.C.W. Mak, J. Am. Chem. Soc., 2005, 127, 14966. 25. T.C.W. Mak and L. Zhao, Chem. Asian J., 2007, 2, 456. 26. J. Zhang, R.-G. Xiong, J.-L. Zuo and X.-Z. You, Chem. Commun., 2000, 1495. 27. J.S. Thompson and J.F. Whitney, J. Am. Chem. Soc., 1983, 105, 5488. 28. H. Masuda, N. Yamamoto, T. Taga, K. Machida, S. Kitagawa and M. Munakata, J. Organomet. Chem., 1987, 322, 121. 29. J.S. Thompson and R.M. Swiatek, Inorg. Chem., 1985, 24, 110. 30. J. Zhang, R.-G. Xiong, J.-L. Zuo, C.-M. Chi and X.-Z. You, J. Chem. Soc., Dalton Trans., 2000, 2898. 31. J. Zhang, R.-G. Xiong, X.-T. Chen, Z. Xue, S.-M. Peng and X.-Z. You, Organometallics, 2002, 21, 235. 32. G. Dong, L. Yu-Ting, D. Chun-Ying, M. Hong and M. Qing-Jin, Inorg. Chem., 2003, 42, 2519. 33. M. Kondo, Y. Hayakawa, M. Miyazawa, A. Oyama, K. Unoura, K. Kawaguchi, T. Naito, K. Maeda and F. Uchida, Inorg. Chem., 2004, 43, 5801. 34. G. Dong, M. Hong, D. Chun-Ying, L. Feng and M. Qing-Jin, J. Chem. Soc., Dalton Trans., 2002, 2593. 35. Y. Gao, B. Twamley and J.M. Shreeve, Organometallics, 2006, 25, 3364, and references therein for more ferrocenyl ligands. 36. M. Oh, G.B. Carpenter and D.A. Sweigart, Acc. Chem. Res., 2004, 37, 1. 37. J.A. Reingold, S.U. Son, S.B. Kim, C.A. Dullaghan, M. Oh, P.C. Frake, G.B. Carpenter and D.A. Sweigart, Dalton Trans., 2006, 2385. 38. J.A. Reingold, S.U. Son, G.B. Carpenter and D.A. Sweigart, J. Inorg. Org. Polym. Mater., 2006, 16, 1. 39. M. Oh, G.B. Carpenter and D.A. Sweigart, Angew. Chem. Int. Ed., 2001, 40, 3191. 40. M. Oh, G.B. Carpenter and D.A. Sweigart, Angew. Chem. Int. Ed., 2002, 41, 3650. 41. J.A. Reingold, M. Jin and D.A. Sweigart, Inorg. Chim. Acta, 2005, 359, 1983. 42. C.E. Plee`nik, S. Liu and S.G. Shore, Acc. Chem. Res., 2003, 36, 499. 43. S.G. Shore, D.W. Knoeppel, H.B. Deng, J.P. Liu, J.P. White and S.H. Chun, J. Alloys Compd., 1997, 249, 25. 44. H. Deng, S.-H. Chun, P. Florian, P.J. Grandinetti and S.G. Shore, Inorg. Chem., 1996, 35, 3891. 45. C.E. Plee`nik, S. Liu, X. Chen, E.A. Meyers and S.G. Shore, J. Am. Chem. Soc., 2004, 126, 204. 46. K. Yu¨nlu¨, N. Ho¨ck and R.D. Fischer, Angew. Chem. Int. Ed. Engl., 1985, 24, 879. 47. U. Behrens, A.K. Brimah, T.M. Soliman, R.D. Fischer, D.C. Apperley, N.A. Davies and R.K. Harris, Organometallics, 1992, 11, 1718.

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48. T.M. Soliman, S.E.H. Etaiw, G. Fendesak and R.D. Fischer, J. Organomet. Chem., 1995, 415, C5. 49. A.M.A. Ibrahim, S.E.H. Etaiw and T.M. Soliman, J. Organomet. Chem., 1992, 430, 87. 50. P. Brandt, A.K. Brimah and R.D. Fischer, Angew. Chem. Int. Ed., 1988, 27, 1521. 51. S. Eller, P. Brandt, A.K. Brimah, P. Schwarz and R.D. Fischer, Angew. Chem. Int. Ed., 1989, 28, 1263. 52. A.K. Brimah, E. Siebel, R.D. Fischer, N.A. Davies, D.C. Apperley and R.K. Harris, J. Organomet. Chem., 1994, 475, 85. 53. J. Lu, W.T.A. Harrison and A.J. Jacobson, Angew. Chem. Int. Ed., 1995, 34, 2557. 54. E. Siebel and R.D. Fischer, Chem. Eur. J., 1997, 3, 1987. 55. S. Bhandari, M.F. Mahon, J.G. McGinley, K.C. Molloy and C.E.E. Roper, J. Chem. Soc., Dalton Trans., 1998, 3425. 56. S. Bhandari, M.F. Mahon and K.C. Molloy, J. Chem. Soc., Dalton Trans., 1999, 1951. 57. S. Bhandari, C.G. Frost, C.E. Hague, M.F. Mahon and K.C. Molloy, J. Chem. Soc., Dalton Trans., 2000, 663.

CHAPTER 8

Inorganic–Organic Hybrids 8.1 Introduction The majority of chapters in this book deal with coordination polymers that are composed of metal ions linked by organic bridging ligands. This chapter discusses a different class of coordination complexes – those which contain inorganic clusters or polymeric subunits that are bridged by organic ligands to form structures with a greater dimensionality than that of the inorganic framework alone.1–6 The inorganic components within these hybrid polymeric materials are often metal–oxide or metal–sulfide species, typically vanadate-, molybdate- or tungstate-based materials. In theory, the inorganic components of hybrid materials can be of any size, from discrete clusters, through chains and sheets, to complete 3D networks; however, in practice, the most commonly observed motif is that of 2D inorganic sheets that are bridged by organic ligands into what is known as a ‘pillared-layer’ structure. The hybrid approach to the construction of coordination polymers can be seen as altering the microstructure of traditional inorganic solids to produce new materials.7 Traditional inorganic oxides display a variety of useful properties, for example, as molecular sieves (zeolites), catalysts (Cr2O3/Al2O3 for alkene polymerisation) and magnetic storage (CrO2). Modifying inorganic materials by incorporating organic ligands is one way in which novel materials can be generated while hopefully retaining, or improving upon, some of the properties of the parent structures. The synthesis of inorganic–organic hybrid materials is frequently carried out under hydrothermal conditions, similar to the synthesis of zeolites, yielding the thermodynamically favoured product under the conditions used. The metal– oxide complexes are obtained using either a neutral or charged oxide precursor, e.g. V2O5 or NaVO3. Organic species and often heterometallic precursors are also used in the reaction mixture. The heterometallic elements may cap the polyoxometallate and then bridge to the organic spacer, or may also bridge


257

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Figure 8.1 Various structural roles that organic co-ligands can exert on metal–oxide structures: (a) templating; (b) bound between oxide species; (c) bound to oxides via a secondary heterometal. Reprinted from Coord. Chem. Rev., 190–192, 737–770. Copyright 1999, with permission from Elsevier.

directly between the inorganic clusters (thereby leading to inorganic substructures with higher dimensionality). It should be noted that, as with zeolite synthesis, many phases form within a very narrow window of conditions and adjusting one parameter (e.g. temperature, concentrations or reaction time) can result in the formation of a completely different network. Minor changes in the ligand used, for example the addition of a methyl group, may also have pronounced effects on the final structure.8 Organic species can influence the inorganic framework in a number of different ways (Figure 8.1).9 Discrete organic species or metal–ligand complexes can acts as templates around which the inorganic framework grows, although this does not yield hybrid frameworks and is therefore not discussed here (Figure 8.1a). Alternatively, the ligands can interact with, and bridge between, inorganic substructures. This can occur by a direct interaction between the ligand and the oxide (Figure 8.1b) or by the use of an intermediary heterometal (Figure 8.1c). Organic groups that are used for such applications are usually bridging organodiamine molecules such as 4,4 0 -bipyridine or DABCO. Chelating, capping ligands can also alter the structure of metal oxides, by limiting their growth in one or more directions, but are not topologically part of the framework net. Discussion of various structural types is categorised below according to the dimensionality of the inorganic substructures from clusters (0D) to 2D sheets. By the ‘inorganic substructure’ we refer to the inorganic fragments that would remain should the organic components be theoretically removed. The dimensionality of the hybrid coordination polymers is typically related to that of the inorganic framework. 1D inorganic chains most frequently form 2D or 3D hybrid polymers with organic bridges in a either one or two

Inorganic–Organic Hybrids

259

directions, respectively, perpendicular to the chains. Similarly, 2D inorganic sheets form 3D networks, often referred to as pillared layers due to their appearance.

8.2 0D Metal–Oxide Substructures Most examples of metal–oxide–organic hybrid materials contain infinite oxide substructures, however, a small number contain discrete metal–oxide clusters, i.e. they have 0D inorganic substructures. When bridged by organic ligands the resultant networks may contain the clusters acting as the topological nodes in the network (assuming the bridging ligands are linear) or the clusters themselves may act as simple bridging species. A very simple example of a metal–oxide cluster being incorporated into a hybrid coordination polymer is [{Cu(4,4 0 -bipy)2}(Mo2O7)].10 The inorganic cluster is a small [Mo2O7]2 species that bridges between trigonal planar Cu(I) atoms that form part of a 1D [Cu(4,4 0 -bipy)] chain (Figure 8.2a). The end result of this inorganic bridging is a bridged, double-chain (or ladder) structure. An example of a significantly larger metal–oxide cluster acting as a simple bridging species is within the complex [{Ni(H2O)2(4,4 0 -bipy)2}2Mo8O26] (Figure 8.2b).11 The oxide cluster connects to the Ni–bipy framework by bonding between a terminal oxo group and a Ni(II) site, producing an overall 2D network. The ellipsoidal cluster represents a good size-fit for the rectangular channels created by the Ni(II) framework. Mo8O26 clusters have also been observed acting as two-connecting bridges in the tetra(2-pyridyl)pyrazine (tpyprz) complex [{Ag4(tpyprz)2(H2O)}Mo8O26], in which the anionic cluster bridges between {Ag4(tpyprz)2(H2O)41} chains to create a 2D network.12 Zubieta and co-workers have shown that more than one type of bridging ligand can be incorporated into hybrid structures that contain 0D inorganic components.13–15 Hybrid chains can be constructed in which capped {Mo5O15} units are bridged by organic diphosphonate ligands, O3P(CH4)2PO3 (Figure 8.3a). The capping species used were metal–ligand subunits of Cu(II) and 2,2 0 -bipyridine. When the capping bipyridine ligands were replaced in the synthetic mixture with the bridging ligand tetra(2-pyridyl)pyrazine (tpypyz) and a (CH2)2 bridged diphosphonate, a different structure resulted. Similar 1D chains are present; however, {Cu2(tpypyz)(H2O)2} units bridge between these chains to create a 2D network in which the molybdate clusters act as 4-connecting nodes (Figure 8.3b). The shortened alkyl spacer allows for the clusters to be directly bridged by a Cu(II) atom, however, giving a 1D metal-oxide substructure. Metal–oxide clusters have been combined with a variety of different bridging ligands to form heterometallic Cu/Mo 2D polymers, for example, 1,2-bis(4-pyridyl)ethylene,16 3,3 0 -bipyridine17,18 and 4,7-phenanthroline.19 Phosphomolybate clusters have also attracted significant attention as the inorganic components in hybrid materials.20 Another example of a 2D network that contains discrete inorganic clusters is [{Cu(tpyprz)}2V4O12] (Figure 8.4).21 Using Co(II) or Ni(II) results in the

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Figure 8.2 (a) The simple ladder-type structure of [{Cu(4,40 -bipy)}2(Mo2O7)] with dimolybdate ‘rungs’; (b) the crystal structure of [{Ni(H2O)2(4,40 -bipy)2}2Mo8O26] with [Mo8O26]4 units incorporated into the Ni–bipy lattice giving a 2D network. (terminal 4,40 -bipy ligands removed for clarity).

formation of 1D inorganic substructures due to the geometric coordination preferences of the transition metal.

8.3 1D Metal–Oxide Substructures Although there are several examples of 0D metal–oxide clusters incorporated into hybrid organic–inorganic polymers, it is more common to find infinite inorganic substructures, i.e. chains or sheets. In the case of 1D inorganic chains that are bridged by organic ligands, the resulting polymer can be of the same dimensionality (i.e. ladder-type chains) or the dimensionality can be increased to 2D or 3D coordination polymers. These cases are dealt with separately below.


Figure 8.3

261

Structures containing {Mo5O15} units (a) bridged by O3P(CH2)4PO3 ligands into 1D chains (with Cu–2,2 0 -bipy capped clusters) and (b) bridged by O3P(CH2)2PO3 ligands and {Cu2(tpypyz)(H2O)2} units to form a 2D (4,4) sheet hybrid polymer.

8.3.1 2D Polymers with 1D Metal–Oxide Substructures 2D polymeric structures containing 1D inorganic substructures can be created when the organic linking component runs perpendicular to the inorganic chain in a single direction. If the organic linkers run in two mutually perpendicular directions to the inorganic chain, then a 3D hybrid polymer will be formed (see Section 8.3.2). A simple example of a 2D hybrid polymer is (4,4 0 -H2bipy)[V2(HPO4)4(4,4 0 bipy)2], shown in Figure 8.5.22 Adjacent octahedral vanadium atoms are doubly bridged by hydrogenphosphate groups, leading to the formation of 1D inorganic chains. The vacant trans sites of the vanadium are bridged by 4,4 0 -bipyridine ligands to arrange the chains into anionic 2D sheets. The sheets themselves are connected by hydrogen bonding

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Figure 8.4

[{Cu(tpyprz)}2V4O12] (tpyprz ¼ tetra(2-pyridyl)pyrazine).

Figure 8.5

The 2D anionic polymer [V2(HPO4)4(4,4 0 -bpy)2] in which 4,4 0 -bipyridine ligands bridge between the 1D inorganic substructure (phosphate depicted as dark polyhedra).


263

between the HPO4 groups and the counter-cations form hydrogen-bonded chains between the layers in a perpendicular direction to the bridging bipyridine ligands. A more complex heterometallic example of a 2D hybrid polymer with a 1D inorganic substructure, despite its deceptively simple empirical formula, is the complex [Cu(dpa)VO3] [dpa ¼ di(4-pyridyl)amine].23 The compound contains 1D vanadate chains that are connected via trigonal planar copper atoms to a parallel Cu(dpa) chain at every fifth vanadium site (Figure 8.6a). On the opposite side of the chains are tetrahedrally disposed Cu(II), which join to dipyridylamine ligands that run in chains at an angle of 731 to the inorganic chains, leading to the formation of a 2D polymer (Figure 8.6b). Much larger inorganic components are observed in the complex [{Cu2(4,7-phen)(4,7phenH)2}Mo12AsO40] 2.66H2O, in which the Keggin-type clusters are linked into a 2D sheet (Figure 8.6c).24

8.3.2 3D Polymers with 1D Metal–Oxide Substructures One of the examples given in the previous section showed how di(4-pyridyl)amine could be used to construct 2D hybrid polymers with 1D inorganic substructures (Figure 8.6). In this case, copper was used as a heterometal, providing tetrahedral and trigonal planar metal sites. Di(4-pyridyl)amine has also been observed in a nickel–molybdate heterometallic material in which the 3D hybrid contains 1D inorganic subunits. [Ni(dpa)2(MoO4)] contains 1D chains of [Ni(MoO4)] consisting of corner-sharing molybdate tetrahedra and octahedral Ni(II) (Figure 8.7a). The remaining four equatorial sites of the Ni octahedral are coordinated to the dipyridylamine ligands that form a 2D sheet perpendicular to the inorganic chains (Figure 8.7b), thereby generating an overall 3D hybrid network. Another example of a 3D coordination network that contains 1D inorganic strands is [M(Cr2O7)(4,4 0 -bipy)2], where M ¼ Cu or Ni.25 The inorganic component of these structures is a chain comprised of the dichromate anions and M21 cations. That chains are similar to those in [Ni(dpa)2(MoO4)], although the M(II) atoms are further removed due to the presence of dichromate instead of molybdate. The bridging 4,4 0 -bipyridine ligands form a 2D sheet parallel to the inorganic chains, with each M(II) acting as a square-planar node with the dichromate anions in the axial octahedral positions (Figure 8.8a). The overall 3D network topology is a-Po and there are two interpenetrating networks in the structure with the metal–oxide chains running through the square channels that the bipyridine sheets create (Figure 8.8b). A more complex example of a 3D coordination polymer that contains 1D inorganic substructures is [Cu2(Mo4O13)(Mepyz)2 3H2O] (Mepyz ¼ 2-methylpyrazine) (Figure 8.9).26 The inorganic chains in this polymer are based around (b-Mo8O26)4 clusters which are bridged by the organic ligands. The copper heterometal atoms link between the clusters and the methylpyrazine ligands.

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Figure 8.6

Chapter 8

(a) The inorganic vanadate chain in [Cu(dpa)VO3] (shown as light tetrahedra) has parallel dipyridylamine ligands attached at every fifth vanadium site; (b) the inorganic chains are formed into a 2D hybrid polymer by chains of dpa ligands at 731 to the vanadate chains (viewed along the inorganic chains); (c) [{Cu2(4,7-phen)(4,7-phenH)2}Mo12AsO40] 2.66H2O.

8.4 2D Metal–Oxide Substructures (Pillared Layers) Hybrid polymers that contain 2D inorganic substructures generally form 3D structures in which the organic ligands bridge between the sheets. Such structures are commonly referred to as pillared layers, as when viewed parallel to the


Figure 8.7

265

(a) The [Ni(MoO4)] inorganic substructure in [Ni(dpa)2(MoO4)] runs perpendicular to (b) sheets of dipyridylamine ligands that bridge between the nickel octahedral (shown in dark grey).

plane of the inorganic substructure the organic ligands appear to form pillars between the sheets.27 In a certain orientation the materials therefore have an alternating inorganic–organic composition. This class of hybrid material is the most heavily reported in the literature and is of great interest due to the large channels that can be formed. An example of a 3D network that uses 3,3 0 -bipyridine as the bridging ligand is shown in Figure 8.10.28 The compound [Cu(3,3 0 -bipy)0.5MoO4] contains bimetallic oxide layers with molybdate tetrahedral and square-pyramidal Cu(II) atoms (Figure 8.10a). These layers are bridged by the bipyridine ligands that coordinate to one of the basal positions of the Cu(II). The copper sites share an O–O edge and therefore immediately adjacent Cu(II) atoms are bridging to neighbouring inorganic sheets in opposing directions (i.e. ‘above’ and ‘below’, Figure 8.10b). The copper–bipy chains can be viewed as having the formula {Cu2(3,3 0 -bpy)}n4n1 (i.e. one bridging bipyridine per copper ‘dimer’). In terms of magnetic properties, the compound only shows antiferromagnetically coupled dimers (with the sole magnetic activity coming from the d9 copper). The magnetic properties of materials with a pillared-layer architecture are potentially interesting as the inorganic layers are insulated from each other by the organic layers. The structure obtained when using the asymmetric bridging ligand 3,4 0 bipyridine under similar synthetic conditions is significantly different, although

266

Figure 8.8

Chapter 8

(a) The network [M(Cr2O7)(4,4 0 -bipy)2] contains parallel inorganic [MII(Cr2O7)] chains perpendicular to a sheet comprising 4,4 0 -bipyridine ligands resulting in (b) an a-Po network, two of which interpenetrate (only one shown for clarity viewed along the inorganic chains).

it is still a 3D network containing a 2D inorganic substructure (Figure 8.11). The inorganic substructure is a layer of corner-sharing Mo tetrahedral and square-pyramidal Cu(II) (Figure 8.11a). Each copper atom is coordinated to two bipyridine ligands which extend to neighbouring sheets in opposing directions (Figure 8.11b). The magnetic behaviour of the 3,4 0 -bipyridine material is different from that of the 3,3 0 -bipyridine complex, reflecting the difference in the inorganic substructure, and shows behaviour suggesting ferromagnetic chains that couple antiferromagnetically at very low temperatures.


Figure 8.9

267

Views of the 1D inorganic substructure in the 3D polymer [Cu2(Mo4O13)(Mepyz)2 3H2O]. (a) along the 1D substructure and (b) perpendicular to the chain.

The main features of interest with pillared solids are their guest exchange properties and magnetic interactions within the inorganic substructures.29 A series of compounds containing cobalt hydroxysulfates have been reported that display both of these properties.30 The frameworks [Co4(SO4)(OH)6(L)0.5] (where L ¼ ethylenediamine or DABCO) are synthesised with enclathrated water and display different thermal stabilities due to the difference in flexibility of the pillar ligand (Figure 8.12a). The more rigid DABCO ligand results in a higher thermal stability and the framework maintains its structural integrity upon removal of the guest solvent. The complicated magnetic behaviour of the materials shows zero-field antiferromagnetism, driven by interlayer exchange

268

Figure 8.10

Chapter 8

(a) The 2D inorganic substructure of the compound [Cu(3,3 0 -bpy)0.5MoO4]; (b) the pillared layered structure arising from the bridging bipyridine ligands between the heterometallic oxide sheets (molybdate tetrahedra are displayed in light grey).

that is mediated by the organic pillars, although this is sufficiently weak to undergo a field induced transition to a metamagnetic state. The related transcyclohexane-1,4-dicarobxylate (chdc) complex [Co5(OH)8(chdc)] has also been reported.31 The material dehydrates in two distinct steps and the final anhydrous structure has no vacant voids due to tilting of the framework and a 9% reduction in the inter-layer spacing (Figure 8.12b). The material, in both its fully hydrated and fully dehydrated phases, shows ferrimagnetic behaviour. Related cobalt phosphate layered compounds with fluoride bridges have also been shown to display ferrimagnetic behaviour.32 A further potential use of layered solids is the chemical activity that can be accessed by incorporating coordinatively unsaturated metal sites into the framework at positions that are accessible to guest species. Pyrazine has been used in many pillared systems with its opposing nitrogen coordination


Figure 8.11

269

(a) The 2D inorganic substructure of the pillared material [Cu(3,4 0 bipy)MoO4]; (b) the overall 3D network (molybdate tetrahedra are displayed in light grey).

sites; however, in a recent study, pyrazine-2-carboxylate (pzc) was incorporated into framework materials, adding extra functionality to the interior of the channels. The complex, [Cu2(pzc)2(H2O)2ReO4], contains CuReO4 layers that are bridged by [Cu(pzc)2(H2O)2] pillars.33 The ligated water molecules are removable by heating in a reversible process. Once removed, there are coordinatively unsaturated metal sites within the channels of the structure. Although attention has been devoted to transition metal pillared structures, primarily due to their more interesting magnetic potential, there are also examples of similar structures with rare earth metals. For example, rare earth

270

Figure 8.12

Chapter 8

(a) Structure of the pillared layered complex [Co4(SO4)(OH)6 (DABCO)0.5]. Reproduced with permission from J. Am. Chem. Soc., 2001, 123, 10584. (b) The change in the framework of [Co5(OH)8(chdc)] upon dehydration. Reproduced with permission from Inorg. Chem., 2003, 42, 6709.


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hydroxide layers bridged by the disulfonate ligands naphthalene-2,6-disulfonate and anthraquinone-2,6-disulfonate have been shown to form layered complexes with porous channels.34 Many rare earth dicarboxylate complexes show similar structures, although these are not strictly hybrid materials and are discussed in Chapter 7.

References 1. D.J. Chesnut, D. Hagrman, P.J. Zapf, R.P. Hammond, R. LaDuca Jr., R.C. Haushalter and J. Zubieta, Coord. Chem. Rev., 1999, 190–192, 737. 2. P.J. Hagrman, D. Hagrman and J. Zubieta, Angew. Chem. Int. Ed., 1999, 38, 2638. 3. R.C. Finn, E. Burkholder and J. Zubieta, in Crystal Design: Structure and Function ed. G.R. Desiraju, Wiley, Chichester, 2003, 241–274. 4. C. Robl, in Chemistry at the Beginning of the Third Millennium ed. L. Fabbrizzi and A. Poggi, Springer, Berlin, 2000, p. 279. 5. D. Hagrman and J. Zubieta, Solid State Chem. Cryst. Chem., 2000, 3, 231. 6. N. Guillou, C. Livage and G. Fe´rey, Eur. J. Inorg. Chem., 2006, 4963. 7. P. Day, J. Chem. Soc. Dalton Trans., 2000, 3483. 8. Y.-P. Ren, X.J. Kong, X.Y. Hu, M. Sun, L.S. Long, R.B. Huang and L.S. Zheng, Inorg. Chem., 2006, 45, 4016. 9. P.J. Hagrmann, R.C. Finn and J. Zubieta, Solid State Sci., 2001, 3, 745. 10. W.S. You, E.B. Wang, L. Xu, C.-W. Hu and G.-Y. Luan, Acta Crystallogr., Sect. C, 2000, 56, 289. 11. D. Hagrman, C. Zubieta, D.J. Rose, J. Zubieta and R.C. Haushalter, Angew. Chem. Int. Ed. Engl., 1997, 36, 873. 12. E. Burkholder and J. Zubieta, Solid State Sci., 2004, 6, 1421. 13. R.C. Finn, E. Burkholder and J. Zubieta, Chem. Commun., 2001, 1852. 14. E. Burkholder, V. Golub, C.J. O’Connor and J. Zubieta, Inorg. Chem., 2003, 42, 6729. 15. E. Burkholder, V. Golub, C.J. O’Connor and J. Zubieta, Inorg. Chem., 2004, 43, 7014. 16. D. Hagrman, C. Sangregorio, C.J. O’Connor and J. Zubieta, J. Chem. Soc., Dalton Trans., 1998, 3707. 17. R.S. Rarig Jr. and J. Zubieta, Polyhedron, 2003, 22, 177. 18. R.L. LaDuca Jr., M. Desciak, M. Laskoski, R.S. Rarig Jr. and J. Zubieta, J. Chem. Soc., Dalton Trans., 2000, 2255. 19. D. Hagrman, P.J. Zapf and J. Zubieta, Chem. Commun., 1998, 1283. 20. N.G. Armatas, E. Burkholder and J. Zubieta, J. Solid State Chem., 2005, 178, 2430. 21. W. Ouellette, E. Burkholder, S. Manzar, L. Bewley, R.S. Rarig and J. Zubieta, Solid State Sci., 2004, 6, 77. 22. C.-H. Huang, L.-H. Huang and K.-H. Lii, Inorg. Chem., 2001, 40, 2625. 23. R.L. LaDuca Jr, R. Finn and J. Zubieta, Chem. Commun., 1999, 1669.

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24. T. Soumahoro, E. Burkholder, W. Ouellette and J. Zubieta, Inorg. Chim. Acta., 2005, 358, 606. 25. A.L. Kopf, P.A. Maggard, C.L. Stern and K.R. Poeppelmier, Acta. Crystallogr., Sect. C, 2005, 61, m165. 26. R.N. Devi and J. Zubieta, Inorg. Chim. Acta., 2002, 332, 72. 27. S. Kitagawa and R. Kitaura, Comments Inorg. Chem., 2002, 23, 101. 28. R.S. Rarig Jr, R. Lam, P.Y. Zavalij, J.K. Ngala, R.L. LaDuca Jr., J.E. Greedan and J. Zubieta, Inorg. Chem., 2002, 41, 2124. 29. D. Maspoch, D. Ruiz-Molinaa and J. Veciana, Chem. Soc. Rev., 2007, 36, 770. 30. A. Rujiwatra, C.J. Kepert, J.B. Claridge, M.J. Rosseinsky, H. Kumagai and M. Kurmoo, J. Am. Chem. Soc., 2001, 123, 10584. 31. M. Kurmoo, H. Kumagai, S.M. Hughes and C.J. Kepert, Inorg. Chem., 2003, 42, 6709. 32. W.-K. Chang, R.-K. Chiang, Y.-C. Jiang, S.-L. Wang, S.-F. Lee and K.-H. Lii, Inorg. Chem., 2004, 43, 2564. 33. P.A. Maggard, B. Yan and J. Luo, Angew. Chem. Int. Ed., 2005, 44, 2553. 34. F. Gańdara, J. Perles, N. Snejko, M. Iglesias, B. Go´mez-Lor, E. Gutie´rrez-Puebla and M. A´. Monge, Angew. Chem. Int. Ed., 2006, 45, 7998.

CHAPTER 9

Magnetism in Coordination Polymers 9.1 Introduction There are two classes of magnetic coordination polymers that will be discussed: those displaying long-range ordering and those that display a spin crossover (SCO). Both of these phenomena involve the ordering of electrons through an external stimulus and are of great importance to the data storage and electronic industries. Recent interest has been devoted towards the construction of multifunctional magnetic materials, for example nanoporous magnets and chiral magnets for advanced functions. This chapter has been divided into the two sections outlined above and contains a brief introduction on each. Both of these areas are extensive and this chapter focuses on key examples within each that give a clear view of the state-of-the-art with a somewhat tutorial approach.

9.2 Long-range Magnetic Ordering 9.2.1 Introduction The generation of materials that order magnetically is an important goal within materials chemistry, with the main aim being to establish long-range magnetic ordering. Cooperative magnetism requires an interaction (coupling) between the spins of paramagnetic centres. A goal that has seen concerted attention is the generation of molecule-based materials with residual permanent magnetization (M) at zero field (H) with a high critical temperature (Tc). This requires a structure that allows for coupling of parallel spins, termed ferromagnetism or the antiparallel coupling of unequal spins, termed ferrimagnetism, of neighbouring paramagnetic spin carriers so that a non-zero spin of the bulk material Coordination Polymers: Design, Analysis and Application By Stuart R. Batten, Suzanne M. Neville and David R. Turner r Stuart R. Batten, Suzanne M. Neville and David R. Turner, 2009 Published by the Royal Society of Chemistry, www.rsc.org

273

274

Figure 9.1

Chapter 9

Illustration of the spin alignment in (a) a ferromagnet, (b) an antiferromagnet, (c) a ferrimagnet and (d) a spin-canted ferromagnet.

results (Figure 9.1). When antiparallel coupling of equal spins occurs, termed antiferromagnetism, no residual spin of the bulk material is observed. An additional arrangement within these classes is termed spin canting, which occurs when the local arrangement of spins differ in magnitude such that the moment is reduced but non-zero. Other terms that are used to describe magnets are metamagnets, which show a magnetic field-dependent transformation from antiferromagnetic to ferromagnetic behaviour, and spin glass, which is a material that shows short-range magnetic ordering but no long-range ordering. The nature of the magnetic ordering can be determined by both temperaturedependent magnetic susceptibility measurements and the field dependence of the magnetization below Tc. The molar magnetic susceptibility (wM) of a compound in a magnetic field is the sum of its diamagnetic (repulsive) and paramagnetic (attractive) components. Ferromagnets show a temperaturedependent molar magnetic susceptibility, wMT, that increases continuously as the temperature is lowered to Tc (Tc is the Curie temperature). Ferrimagnets show a broad minimum in the wMT values in the paramagnetic range above Tc. The Neél temperature, TN, is the critical temperature in this case. Additionally, the coercive field Hc is the reverse magnetic field required to reduce the magnetisation to zero. A hard magnet is characterised by a large Hc value (4100 Oe) and a soft magnet is characterised by a small Hc value (o10 Oe). Hard magnets are required for magnetic data storage purposes. A long-range ordered magnet is not only characterized by its critical temperature but also by field-dependent hysteresis loops. When present, hysteresis loops confer a memory effect on the system and are one of the attractive features of long range ordered materials for application in magnetic data storage. AC susceptibilities, wM 0 (in-phase), wM00 (out-of-phase), are also used to detect long-range ordering. It is difficult to synthesise materials which order in all three dimensions. Lowdimensional materials typically show Tc values below 10 K. Intramolecular interactions (i.e. bonds) are much more efficient than intermolecular interactions (i.e. through space) at communicating magnetic information. Hence there is great interest in magnetic coordination polymers, which connect the magnetic centres through direct coordinative links. In addition, such materials have the real possibility of displaying high critical temperatures, particularly in the case of 3D ferromagnets.

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Although the research field of coordination polymers is fairly well established, the problem with incorporating magnetic centres into such materials lies in the general inability of the long organic linking ligands typically employed in coordination chemistry to communicate magnetic information effectively. In order to achieve strong coupling between the spin centres, short bridges are needed, such as oxo, cyano or azido ligands. Indeed, one of the most common examples within the study of magnetic materials is metal cyanides such as the Prussian Blue family, which can show spontaneous magnetization at high Tc values. The oxalate dianion and the azide anion are also commonly employed into many 2D and 3D networks that display ferro- and ferri- long-range ordering. Another approach is to use mixed-ligand systems containing both short and long bridging ligands. Thus magnetic ordering may be attained in addition to interesting higher dimensional framework topologies. Spin carriers do not exclusively need to be metal centres; it is also possible to communicate magnetic information via radical organic ligands within coordination polymers. Indeed, this opens the door for magnetic communication using longer ligands to generate further framework topologies which may even display a porous nature. Furthermore, the synergy of magnetism and porosity within one material opens an avenue for the development of advanced multifunctional materials. In particular, low-density magnetic materials and molecular magnetic sensors provide future exciting possibilities.

9.2.2 Molecule-based Magnets Molecule-based magnets (MBMs) are a class of magnetic materials composed of molecular components (molecules or molecular ions) as opposed to inorganic network solids or metallic lattices, which are constructed from single atoms. Generally, MBMs are formed by transition metals, rare earth ions, free radicals or diamagnetic ligands. The focus here is on coordination polymers which contain transition metals acting as the spin carrier, but examples containing free radical ligands with metal ions will also be presented. Molecule-based magnets include zero-dimensional isolated molecules or 1D chains, 2D layers and 3D structures where the magnetic centres are linked directly. Such materials are commonly prepared by low-temperature, solutionbased methods, rather than the high-temperature methodologies often employed in inorganic network solid synthesis (e.g. metal oxides). The advantage of this design principle is that significantly more control is attained through using molecular building blocks of defined direction and connectivity where the critical temperature is potentially tuneable. Polymeric MBMs will now be described and discussed in terms of the organic bridging ligand which provides a potential magnetic exchange pathway, beginning with shorter, rigid ligands and moving to longer and more complex examples. There are many systems which contain two or more organic ligands of this type; they will also be discussed briefly where appropriate. More recent

276

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focus in the literature has been on the additional functionalities that magnetic coordination polymers can provide, such as porosity and chirality. As there are many very good review articles and books available on the historical aspects of magnetic coordination polymers,1,2 this chapter will focus on the more recent investigations into polymeric MBMs but will also give a brief account of important past achievements.

9.2.2.1

Cyanides

The Prussian Blue family is very large and refers to materials with the general formula Ak[B(CN)6]l nH2O, where A ¼ HS and B ¼ LS. These materials form 3D network structures where, depending on the k:l ratio, different phases can form, i.e. when k ¼ l face-centred cubic structures form and when k 4 l some B(CN) sites are missing and are often occupied by water molecules, this type being termed defect phases (Chapter 5). Cyanide-containing materials in general show fascinating magnetic behaviours with ordering either ferro- or ferrimagnetically and some with very high ordering temperatures. The first ever magnetic measurements on a Prussian Blue were for FeIII4 [FeII(CN)6]3 15H2O, which exhibits long-range ferromagnetic ordering at Tc ¼ 5.6 K.3 This low ordering temperature is a consequence of the weak exchange coupling due to the presence of diamagnetic low-spin Fe21 centres. By replacing diamagnetic centres by paramagnetic centres, higher Tc Prussian Blue magnetic analogues can be attained due to stronger magnetic coupling. Further to this, by using B metal ions with filled p-orbitals, which can communicate via the empty p-orbitals of the CN ligand, magnetic pathways can be enhanced further. For example, in the Prussian Blue analogues CsNiII[Cr(CN)6] 2H2O, which is a ferromagnet (orthogonal orbitals), and CsMnII[Cr(CN)6] H2O, which is a ferrimagnet (non-orthogonal orbitals), higher ordering temperatures were attained, Tc ¼ 90 K.4,5 For many years, magnetic ordering temperatures for Prussian Blue analogues did not exceed this value. In order to increase the Tc even further, it was realised that the ferromagnetic contributions from the A ion need to be minimised. This may be achieved by incorporating an A ion which has minimal eg electrons (i.e. Mn21, Cr21 and V21). Indeed, for example, a vanadium-containing analogue, VII0.42VIII0.58[CrIII(CN)6]0.86 2.8H2O, is a ferrimagnet above room temperature, Tc ¼ 315 K,6 and by increasing the relative amount of VII in the sample the analogous material K0.058VII/III[Cr(CN)6]0.79 (SO4)0.058 0.93H2O was prepared, which shows an even higher Tc of 375 K.7 Additionally, the crystalline material KIVII [CrIII(CN)6] 2H2O shows a Tc of 376 K.8 More recently, in a further application of Prussian Blue analogues, solvatomagnetic effects were investigated in a number of materials. Such materials commonly show drastic colour changes associated with the guest exchange chemistry and so have further application as magnetic colour sensors. For example, with solvent exchange to ethanol the defect material CoII1.5[CrIII(CN)6] 7.5(H2O) shows a variation in Tc from 25 to 18 K, along with a colour change from peach to deep blue.9 Similarly, the as-synthesised material


277

Co[Cr(CN)6]2/3 nH2O with dehydration shows a change from ferromagnetically coupled, Tc ¼ 28 K, to antiferromagnetically coupled, Tc ¼ 22 K.10 This material also shows a distinct colour change from pink to blue. The colour and magnetic changes in these two materials are largely concerned with changes in the number and type of bound solvent molecules. Furthermore, another member of this family, K0.2Mn1.4Cr(CN)6 6H2O, shows an increase in Tc from 66 to 99 K with desolvation.11 Hexacyanometallate anions may also be used as building blocks to incorporate into a range of other bimetallic molecular compounds with interesting framework topologies, especially those with lower symmetry, e.g. the material [Ni(en)2]3[Fe(CN)6]2.H2O, which is comprised of zigzag chains of alternating [Fe(CN)6]3 and cis-[Ni(en)2]21. These chains are bridged by trans-[Ni(en)2]21 units, resulting in a 2D sheet. This material shows ferromagnetic ordering, Tc ¼ 18.6 K.12 In another example, a 3D network structure is seen in the material [Mn(en)]3[Cr(CN)6]2 4H2O, which shows a magnetic ordering at Tc ¼ 69 K.13 There are many other co-ligands that have been used in such a way, which have produced an array of 1D to 3D assemblies with interesting magnetic properties.14 There are too many such materials to discuss comprehensively here; notably, however, the use of metal cyanides with co-ligands has also led to the development of porous magnetic systems whose magnetic properties are sensitive to hydration and dehydration. For example, the 2D honeycomb coordination polymer [Ni(cyclam)]3[W(CN)8]2 (cyclam ¼ 1,4,8,11-tetraazacyclotetradecane) reversibly absorbs water molecules (Figure 9.2).15 The hydrated material shows a canted antiferromagnetic behaviour which is switched to ferromagnetic with dehydration. This magnetic sensing behaviour is further demonstrated in the 2D MnIICrIII ferrimagnet [Mn(NNdmenH)(H2O)][Cr(CN)6] H2O (NNdmenH ¼ N, N-dimethylethylenediamine), which contains a diamine co-ligand in addition to terminal water molecules.16 This material shows a reversible structural conversion upon dehydration which results in the generation of a 3D pillared-sheet

Figure 9.2

The 2D honeycomb coordination polymer [Ni(cyclam)]3[W(CN)8]2 which shows a magnetic sensing behaviour with hydration/dehydration. Reproduced with permission from Inorg. Chem., 46, 8123, copyright (2007), ACS.

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framework. The 2D layered material shows ferromagnetic ordering at 35.2 K, which is increased to 60.4 K in the 3D material.

9.2.2.2

Azides

Pseudohalides are excellent ligands for obtaining low- to high-dimensional systems. In particular, the azide ligand forms a good bridge between divalent metal ions (e.g. Cu, Ni, Mn) and, due to its versatile coordinating modes, endto-end (EE) or end-on (EO), an array of interesting 1D, 2D and 3D coordination polymers have been observed (Chapter 5). Important for magnetic exchange, the EO binding mode generally shows ferromagnetic coupling and the EE mode generally shows antiferromagnetic coupling. However, this can vary depending on the M–N–M bond angle, especially when the angle is large for the EE binding mode. An alternative magnetic outcome is that varied bonding modes and M–N–M angles are observed in one structure, resulting in ‘alternating magnetic systems’. Hence it is important to direct the binding mode, using known synthetic conditions, depending on the magnetic result required. Additionally, the formation of 1D, 2D or 3D networks can be influenced via judicious co-ligand choice. Many structures have been reported which contain only the EE binding mode. In this case, metals can be linked by one or two azido ligands. For a single bridge, either cis or trans conformations of adjacent ligands are possible and, generally, the trans conformation shows greater antiferromagnetic coupling than the cis conformation. 1D chains have been observed where two bidentate ligands or one tetradentate ligand are employed. Higher dimensional materials can be obtained using monodentate terminal ligands or through bridging 2D materials with bidentate ligands or without any blocking co-ligands, e.g. the 2D material [Mn(4-acpy)2(N3)2] (4-acpy ¼ 4-acetylpyridine), which shows longrange ordering below Tc ¼ 28 K due to spin canting.17 A related 3D material, [Mn(py)2(N3)2], which has a trans arrangement of pyridine ligands, also shows long-range ordering (Tc ¼ 40 K) due to spin canting.18 Polymeric systems which contain both EE and EO binding modes have also been observed. This type of material is very interesting especially when alternating antiferromagnetic and ferromagnetic coupling is observed in the one structure. For example, the 1D chain material [Mn(bpy)(m1,1-N3)(m1,3-N3)] shows such alternating magnetic behaviour.19 Additionally, the 2D material [Ni(Me2tn)(N3)2] which is comprised of azide ligands which show both EO binding, coordinating two metal centres, and EE binding at one end, shows interesting, alternating, ferromagnetic and antiferromagnetic behaviour and also spin canting at low temperatures.20,21 Lastly, it is possible to have only the EO binding mode in polymeric systems. As for the single EE and mixed systems, the framework dimensionality may be controlled by the number and type of co-ligands employed. For example, the 1D chain material [Ni(en)(N3)2] is obtained using the bidentate ligand ethylenediamine and, as expected due to the presence of only EE binding, this material shows ferromagnetic coupling.22


9.2.2.3

279

Other Pseudohalides

Compared with azides, polymeric complexes containing other pseudohalides (e.g. XCN, where X ¼ O, S and Se) are less abundant. This is largely due to their less versatile binding nature and their less effective magnetic exchange pathway. Commonly, these ligands bind terminally and therefore do not provide an exchange pathway in coordination polymers. However, they have been reported to bridge metal ions via the EE binding mode. Additionally, the ligand OCN has been observed in the EO mode. Generally, the EE mode for OCN and SCN can act both antiferromagnetically and ferromagnetically and for SeCN generally ferromagnetic coupling is observed. Commonly, these ligands are incorporated into coordination polymers with coligands, either bridging or terminal. For example, Co(bim)(SCN)2 [bim ¼ 1,2-bis (imidazol-1-yl)ethane] is a 1D chain material where the SCN ligands are in the EE binding mode and doubly bridge two metal centres (Figure 9.3a).23 The metal centres are then further bridged by bim ligands, such that a triply bridged 1D chain results. Magnetically, this material shows ferromagnetic interaction between Co21 ions with no long-range ordering to low temperature. In a related material, Co(bte)(SCN)2 [bte ¼ 1,2-bis(1,2,4-triazol-1-yl)ethane] the metal centres are doubly bridged by EE SCN ligands into 1D chains, which are then linked into 2D sheets via linear bte ligands (Figure 9.3b).23 Magnetically this material is a metamagnet below Tc ¼ 2.9 K as a result of competition between weakly antiferromagnetic coupling of ferromagnetic [Co(SCN)2] chains.

9.2.2.4

Dicyanamide (dca) and Tricyanomethanide (tcm)

Although the bridges discussed above are evidently efficient for magnetic coupling interactions, inherently they lack a diversity of bridging modes, so limited framework topologies are formed. Magnetic coupling between long bridges, over four single bonds, is generally very weak. When long bridges are used effectively for magnetic coupling, they are always conjugated. Thus intermediate spin carriers such as the dicyanamide [dca, N(CN)2] and tricyanomethanide [tcm, C(CN)3] anions, which may act as 3-connectors or longer and exhibit a range of coordination bridging modes with transition metals, are ideal candidates. The dca anion can coordinate to a metal ion such that it is monodentate through one of the nitrile nitrogen atoms, bidentate through both nitrile nitrogen atoms, bidentate through one nitrile and the amide nitrogen atom, tridentate through both nitriles and the amide nitrogen or a more complicated tetradentate bridging mode (Chapter 5). The related species tcm shows similar binding modes. Generally, magnetic ordering can occur through the M–NCN–M pathway when dca is present and the M–NC–C–CN–M pathway when tcm is present. A range of different structures types have been reported which contain the dca [N(CN)2] and tcm [C(CN)3] anions; however, only a few display cooperative magnetic properties. Extended lattices of the general formula M(dca)2, M(dca) (tcm) and M(tcm)2 exist, all of which can show long-range magnetic ordering

280

Figure 9.3

Chapter 9

(a) 1D triply-bridged Co(bte)(SCN)2.

material

Co(bim)(SCN)2;

(b)

2D

sheet

when the correct exchange pathway is present. The M–dca networks can be modified by incorporating terminal or bridging co-ligands, such as pyridine or 4,4 0 -bipyridine, to form 1D, 2D or 3D structures of the formula M(dca)2(L)n. In addition, anionic networks of the type M(dca)3 and M(dca)42 can be formed via templation around organic or inorganic counter-cations. Binary a-M(dca)2 compounds (M ¼ Cr, Mn, Fe, Co, Ni, Cu) are comprised of single rutile-like 3D networks (Figure 9.4a). Six different dca ligands surround each metal centre, consisting of four nitrile nitrogen atoms in the equatorial positions and two amide nitrogen atoms in the axial positions. Each dca anion is involved in a m1,3,5-bridging mode. All the metal analogues display long-range magnetic ordering; specifically the Cr, Mn and Fe analogues act as canted-spin antiferromagnets and the Co, Ni and Cu analogues act as ferromagnets.24–27


Figure 9.4

281

(a) Single rutile network of a-M(dca)2 (M = Cr, Mn, Fe, Co, Ni, Cu); (b) tube-like structure of M(dca)2(2-aminopyrimidine) (M = Co, Ni); (c) doubly interpenetrating a-Po network of a-M(dca)2(pyrazine) (M = Mn, Fe, Co, Ni, Cu, Zn).

A second form of M(dca)2 (M ¼ Zn, Co) has also been observed, termed the b-form, such that tetrahedral metal ions are bridged by m1,5-dca ligands to form corrugated (4,4) grids.25,28 The Co analogue is a spin-canted antiferromagnet.25,27 Similarly, tcm-containing materials of the formula M(tcm)2 (M ¼ Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, Hg) also form rutile-like networks.19–23 However, the larger ligand, compared with dca, results in doubly interpenetration of the nets. These networks are tightly packed and highlighted by significant network distortion and close internetwork M M distances. These materials do not show long-range ordering, with the exception of a few analogues (Cu, Cr) that order at very low temperatures. When both dca and tcm are incorporated into the one material of formula M(dca)(tcm) (M ¼ Co, Ni, Cu), a 3D self-penetrated network results. These materials are intermediate both structurally and magnetically between the parent M(dca)2 and M(tcm)2 compounds. In all cases, both the dca and tcm anions are 3-connected about the octahedral metal centres. This self-penetrated network is closely related to the rutile net of M(dca)2 but contains a helical substructure instead of the original square channels, as a result of the different sizes of the dca and tcm ligands. Magnetically, the Co and Ni analogues show ferromagnetic ordering (Tc ¼ 3.5 and 8.0 K, respectively).29 In order to vary the framework topologies and to increase the numbers of observed examples of dca binding modes, co-ligands have been introduced; these co-ligands may be bridging or terminal in nature. Terminal or nonbridging examples include 1D, 2D and 3D networks; however, magnetic exchange between metal centres is very weak and antiferromagnetic, if present at all. This is largely due to the dca anion bridging modes observed in such materials being largely dominated by the long m1,5-dca bridges not conducive to magnetic exchange. On the other hand, when the m1,3,5-bridging mode is observed, which is a sufficient magnetic exchange pathway, such as in the tubelike structure M(dca)2(2-aminopyrimidine) (M ¼ Co, Ni) (Figure 9.4b) and the

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Chapter 9

2D sheet-like structure M(dca)2(H2O) phenazine, ferromagnetic interactions are observed.30,31 When bridging co-ligands are introduced, instead of terminal ones, the framework dimensionality may be increased which magnetically is preferable for potential long-range magnetic coupling. However, in practice the magnetic exchange within these materials is generally very weak except when the bridging co-ligands provide better exchange pathways than the dca anions. For example, the Mn analogue of the interpenetrated 3D a-Po network a-M(dca)2(pyrazine) (M ¼ Mn, Fe, Co, Ni, Cu, Fe), which is formed by the bridging of 2D (4,4) sheets of metals atoms bridged by m1,5-dca anions by pyrazine ligands, shows weak antiferromagnetic coupling (TN ¼ 2.5 K) (Figure 9.4c).32 Another approach to generating dca structures is to form anionic networks by the use of templating organic or inorganic counter-cations. The chosen size, shape and charge of the counter-cations play a large role in the framework formed. For example, 2D (4,4) nets of Mn(dca)3 form when Ph4E1 (E ¼ P, As) is used; the Ph4E cations are located between the layers. In this case, the dca anions bridge in a m1,5-fashion, resulting in weak antiferromagnetic coupling.33,34 Other examples exist which contain M(2,2 0 -bipy) cations (M ¼ Fe, Ni), resulting in (6,3) sheets of the formula M(dca)3 (M ¼ Mn), but do not show any magnetic interactions.35

9.2.2.5

Tetracyanoethylene (tcne) and 7,7,8,8-Tetracyanop-quinodimethane (tcnq)

The organic radical anion tetracyanoethylene (tcne) and the analogue 7,7,8,8tetracyano-p-quinodimethane (tcnq) show a number of metal binding modes, where either two or four metals are bridged. These ligands are strong electron acceptors and show interesting magnetic properties in coordination polymers. There are two general types of magnetic systems which contain tcne: [MIII (porphyrin)(tcne)] and MII(tcne)2 n(guest) (M ¼ V, Mn, Fe, Co, Ni; guest ¼ acetonitrile, THF, DCM). The porphyrin-containing 1D coordination polymer [MnIII(tpp)][tcne] 2PhMe (H2tpp ¼ meso-tetraphenylporphyrin) is a ferrimagnet with a Tc of 14 K.36 The MnII ions are equatorially bound to tpp and axially bound to tcne ligands to form 1D chains. Variation of the tpp ligand substituent groups leads to changes in the magnetic behaviour, which led to studies on the importance of 1D versus 3D structure–magnetic behaviour. An example is [MnOEP][tcne], which shows weak ferromagnetic coupling of the parallel 1D chains.37 The chains are non-uniform for this material due to the presence of two different tcne orientations, thus leading to the realisation that chain interactions are important for long-range ordering. Within the series MII(tcne)2 n(guest) there are a range of reported magnetic properties depending on the preparative conditions and a lack of structural certainty due to the precipitation of powders and the extreme air sensitivity of certain analogues.38 Within this series is the material V(tcne)2 12CH2Cl2, which,


283

although the structure is still unknown, is proposed to consist of a 3D network where each metal is surrounded by up to six tcne ligands.39 Importantly, this material is a magnet above room temperature. Additionally, the material Fe(tcne)2 shows an ordering temperature of Tc ¼ 100 K.40 Further studies on other metal analogues have led to the development of tuneable magnetism within this series. For example, a CoII analogue does not show magnetic ordering, hence solid solutions of the formula VxCo1x(tcne)2 zCH2Cl2 allow fine tuning of the magnetic properties.41 An analogue of the M(tcne) family of magnets containing the ligand tcnq exists, Fe(tcnq)2 nCH2Cl2. This material shows a reduction in Tc but still orders at Tc ¼ 35 K.40 More recently, tcnq has been incorporated into a 1D chain system which contains three types of spin carriers, [{Cu(L)}2Gd(tcnq)2] tcnq solvent [L ¼ N,N 0 -propylenebis(3-methoxysalicylideneiminato)].42 This material shows ferromagnetic coupling between the metal ions, but no contribution from the tcnq radicals. Furthermore, a polymeric material comprised of alternating cationic and anionic layers orders magnetically at 3.5 K.43

9.2.2.6

Other Radical Ligands

One approach to overcoming the problem of insufficient communication for long-range magnetic ordering in coordination polymers which contain long bridging ligands is presented by organic radical ligands. Such ligands, when incorporated into coordination polymers, may result in larger magnetic couplings as the organic radicals can act as a magnetic relay. This has been reported for molecule-based systems and also more recently for porous coordination polymers, where the radical ligands are bound to the metal ions within the framework as either terminal or bridging (e.g. tcne or tcnq described above) ligands. Nitronyl nitroxide-based radical cations have been used extensively in metal– radical systems with the aim of understanding such magnetic materials. For example, the radical ligand tempol (4-hydroxy-2,2,6,6-tetramethylpiperidinylN-oxyl) and hexafluoroacetylacetonate (hfac) forms 1D chains of the formula Cu(hfac)2 (tempol), which show alternating ferromagnetic and antiferromagnetic exchange.44 Further derivatives of nitronyl nitroxide radicals have been reported which can coordinate more strongly, thus forming more complex framework structures. In particular, the pyridyl-substituted analogue has been successful for the construction of magnetic coordination polymers which show long-range ordering.45 Further to this, nitronyl nitroxide radical cations have been used in combination with pseudohalides, such as azide and dca, to form other polymeric systems. For example the 3D framework [MnII(NIT-tz)(dca)2] [NIT-tz ¼ 2-(2-thiazole)4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxy-3-oxide].46 The radical ligand NIT-tz is bidentate and the dca units, bound in the EE mode, extend the structure which contains diamond-shaped channels. This material shows moderate antiferromagnetic interactions between the MnII and the radical and weak antiferromagnetic interactions mediated by the dca ligand.

284

Figure 9.5

Chapter 9

(a) The 2D honeycomb (6,3)-net of [Cu3(PTMTC)2(py)6(EtOH)2(H2O)] (MOFOF-1); (b) magnetic measurements for the solvated (filled circles) and desolvated (open circles) materials. Reprinted with permission from Nature Publishing Group, Nat. Mat., 2, 190, copyright (2003).

In order for organic radical ligands to form porous frameworks with the potential for guest-induced magnetic properties, larger ligands must be designed. A polychlorinated triphenylmethyltricarboxylic acid radical ligand, PTMTC, an extended 3-connecting organic ligand based on the PMT (polychlorinated triphenylmethyl) radical, is the first such ligand to be used to form a porous framework. This ligand has been incorporated into a 2D honeycomb (6,3) net, [Cu3(PTMTC)2(py)6(EtOH)2(H2O)] (MOFOF-1), with large pores (Figure 9.5).47 Magnetically, this material acts as a ferrimagnet with a low Tc. Notably, this material shows a reversible ‘shrinking–-breathing’ process with ethanol guest removal and replacement. This guest-induced structural transformation is accompanied by subtle changes in the magnetic properties; the wT minimum is displaced by 20 K and the magnetic response at low temperature is reduced. Thus the solvation-desolvation process can be monitored entirely by magnetic measurements. This organic radical approach has been further demonstrated in the Co21containing material [Co6(PTMTC)4(py)17(H2O)4(EtOH)] (MOROF-3), which consists of a (6,3)-helical structure with large channels. This material shows bulk magnetic ordering below 1.8 K with mixed ferromagnetic and antiferromagnetic interactions.48 Additionally, a mixed-ligand system containing the radical PTMTC ligand exists, Co(PTMTC)(4,4 0 -bipy)(H2O)3 6EtOH 2H2O, which shows a (3,5)-connected network topology and antiferromagnetic interactions.49

9.2.2.7

Oxamato Systems

The oxamide ligand provides a very good exchange pathway for metal ions to communicate magnetic information. The bimetallic chain compound [MnCu(obbz)] H2O [obbz ¼ oxamidobis(benzonato)] shows ferromagnetic


285

behaviour below 14 K and a field-dependent hysteresis loop at 4.2 K characteristic of a soft magnet.50 In a further example, the zigzag chain material [MnCu(opba)(DMSO)3] [opba ¼ o-phenylenebis(oxamato)] shows 1D ferromagnetic behaviour.51 In an attempt to increase the dimensionality of magnetic systems further, a magnetic cation was used in this type of system, resulting in the 2D material (rad)2-Mn2[Cu(opba)]3(DMSO)2 2H2O [rad1 ¼ 2-(4-N-methylpyridinium)4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide], which contains three kinds of spin carriers: metal ions (Mn21, Cu21), an oxamato bridge (opba) and an organic radical cation (rad1).45 The structure consists of the interpenetration of 2D hexagonal honeycomb nets of [Cu(opba)]2 linked by octahedral Mn21 ions. The interlocked nets are further bridged via the rad1 cations through the Cu ions to form a 3D structure. Magnetically, the oxamato groups efficiently transmit an antiferromagnetic interaction between metal ions while the rad1 cations which connect the 2D layers favour ferromagnetic coupling, resulting in an overall Tc ¼ 22.5 K. One of the highly acclaimed aspects of oxamide containing coordination polymers is the first introduction of the concept of ‘magnetic molecular sponges’.52 In particular, this concept was demonstrated on the material [CoCu(obbz) (H2O)4] 2H2O [obbz ¼ N,N 0 -bis(2-carboxyphenyl)oxamido], which shows reversible hydration–dehydration (two steps) accompanied by dramatic magnetic changes. The as-synthesised non-magnetic material is mononuclear and the first dehydration step allows a transformation to a 1D structure which acts as a ferrimagnet. The second dehydration step results in the formation of a 2D (or 3D) material, which now behaves as a ferrimagnet with an increased Tc of 30 K.

9.2.2.8

Oxalate Ligand and Analogues

The bis-bidentate oxalate bridge is a good mediator for both ferromagnetic and antiferromagnetic interactions and has been utilised extensively for the construction of 2D and 3D polymeric molecule-based magnets. There are two general families of materials containing the oxalate dianion where the formation of 2D or 3D structures is dependent on the type of counterion employed. Layered 2D structures containing [MIIMIII(ox)3] are obtained when the cation [XR4]1 is used (X ¼ N, P and R ¼ alkyl group), whereas 3D structures of the formula [M2II(ox)3]2–, [MIMIII(ox)3]2 or [MIIMIII(ox)3]n, form when the cation is a tris-chelated transition metal diimine, e.g. [MII/III(bpy)3]m1. The 2D structures consist of extended metallo-oxalate bridged layers containing two types of metal atoms. The layers are separated by the counterion which templates the interlayer separation and general net structure. The metals in the oxalate layer play a large role in the magnetic properties observed. For example in the series of 2D layered materials [Fe(Cp*)2][MIIMIII(ox)3] (M21 ¼ Mn, Fe, Co, Ni, Cu, Zn; MIII ¼ Cr, Fe), which contain a magnetically interesting organometallic cation, all are magnets which display spontaneous

286

Figure 9.6

Chapter 9

(a) The layered magnet [Fe(Cp*)2][MIIMIII(ox)3] and (b) representative magnetic properties of the CrIIICuII derivative (ac susceptibility versus temperature and hysteresis loop at 5 K). From Chem. Comm., 1997, 1727. Reproduced by permission of The Royal Society of Chemistry.

magnetization below Tc (5.3–43.3 K) and hysteresis loops (Figure 9.6).53 The CrIIIMII series are ferromagnets which have ferromagnetic interactions between the Cr31 and M21 ions mediated through the oxalate bridge. On the other hand, the FeIIIMII series show antiferromagnetic coupling, which gives rise to ferrimagnets (for M21 ¼ Fe, Co) or spin-canted antiferromagnets (for M21 ¼ Mn). This is a common trend in other [XR4]1 derivatives. It appears that the cations are not directly involved in the long-range magnetic ordering, which is largely controlled by the bimetallic oxalate layers. Alternatively, free radicals as counterions within the oxalate lattice are effective for templating the formation of 3D networks, as seen in the material rad[MnIICr(ox)3] [rad1¼ 2-(1-methylpyridinium-4-yl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide], where the radicals are located in zigzag chains within the 3D dimetallic network (Figure 9.7a).54 The 3D network is comprised of Cr(ox)3 units linked by Mn(ox)3(H2O) units of the opposite chirality to form a (10,3) net. The oxalate bridge that links Cr and Mn is unusual as it is bidentate towards Cr and monodentate towards Mn. Magnetically, there are antiferromagnetic interactions at low temperatures transmitted through the unusual oxalate binding mode. This


Figure 9.7

287

(a) Schematic view of the (10,3) network of rad[MnIICr(ox)3] 2H2O; (b) View of 2D coordination polymers of the formula [M(ox)(4,4 0 -bipy)].

288

Chapter 9

system also shows the presence of a low-dimensional system with dominant ferromagnetic interactions. In addition, 1D, 2D and 3D materials can be achieved using mixed bischelating ligands such as oxalate and 4,4 0 -bipyridine in the 2D coordination polymers of the general formula [M(ox)(4,4 0 -bipy)] (MII ¼ Fe, Co and Ni).55 This family of materials consist of 1D chains of M(ox) linked by 4,4 0 -bipy ligands to generate an open framework with rectangular channels (Figure 9.7b). Magnetic studies revealed spontaneous antiferromagnetic behaviour with certain canting structures, which is attributed to strong exchange interactions between the oxalate-bridged metals. In a further example, the 1D chain material [FeII(D)FeII(L)(ox)2(phen)2], which acts more as a 2D sheet structure due to p–p interactions between phen groups on adjacent chains, shows strong ferromagnetic ordering at low temperature. This ordering reflects the 2D sheet structure and also displays intra-chain antiferromagnetic coupling.56 Lastly, oxalate analogues have been successfully incorporated into coordination polymers such as the dithiooxalato analogue in [N-n-Pr4][FeIIFeIII(C2O2S2)3], which displays ferromagnetic ordering below 6 K.57

9.2.2.9

Carboxylate Ligands

Both short and long carboxylate linking ligands have been successful in forming polymeric materials with interesting magnetic properties. Shorter carboxylate ligands have been reported in magnetic coordination polymers, providing interesting magnetic properties due to the increased magnetic exchange coupling. In particular, the formate ligand, which can act as a 3-connector, has been used in much the same way as azide in magnetic materials. The formate ligand shows bridging modes similar to the azide ligand and can link two or three metal ions. Indeed, antiferromagnetic or ferromagnetic coupling is observed in similar binding modes to azides. However, there have been far fewer magnetic studies on formato systems than those containing azide. For example, antiferromagnetic ordering is observed in M(HCOO)2 2H2O (M ¼ Mn, Fe, Co; Tc ¼ 3.47–15.5 K) and an unusual weak ferrimagnetism in Ni(HCOO)2 2H2O.58 Additionally, the combination of formate and azide in the one system has been achieved in [(CH3)2NH2][M(N3)2(HCOO)] (M ¼ Fe, Co), which consists of chains of [M(N3)2(HCOO)] separated by cations.59 Metamagnetism is observed below 10 K due to antiferromagnetic coupling of ferromagnetic chains. Even though the formate ligand is short, it can still result in systems which contain pores that can be exploited for guest-sensing magnetism. For example, the 3D diamond framework [Mn3(HCOO)6] (MeOH) (H2O) contains bridging formate ligands and exhibits 1D channels filled with solvent (Figure 9.8).60 Each node is occupied by MnMn4 tetrahedral units which have an Mn21 core connected to four other Mn21 ions via six formate ligands. This material is thermally robust to 260 1C and to guest removal and exchange. Both the


Figure 9.8

289

Representation of the diamondoid coordination polymer [Mn3(HCOO)6] (MeOH) (H2O) and wT versus temperature plot of the solvated (1) and desolvated (2) materials. From Chem. Comm., 2004, 416. Reproduced with permission of The Royal Society of Chemistry.

as-synthesised and empty frameworks show ferromagnetic behaviour and longrange magnetic ordering (Tc ¼ 4.8–9.7 K). Importantly, the Tc can be perturbed from between 5 and 10 K with guest exchange to a large range of guests, thus acting as a magnetic molecular sensor. There are also many examples of magnetic coordination polymers which contain longer carboxylate ligands. Although long organic linker ligands are not likely to communicate magnetic information, the linking of metal clusters with such ligands provides a possible route to generating high-dimensional magnetic coordination polymers which are potentially porous. Carboxylate ligands provide a unique means to achieving such materials as their chemistry is well established. Although the magnetic properties within this class of materials are not astounding, they provide a means potentially to perturb magnetic interactions through solvent molecule removal–exchange. An early example of magnetic ordering in a 3D framework material is in [Cu3(btc)2(H2O)3] (HKUST-1, btc ¼ benzene-1,3,5-tricarboxylate), which is comprised of paddlewheel Cu dimers linked by btc ligands.61,62 This material contains large channels (9 9 A˚) filled with water molecules. The magnetic characterization of this material revealed a strong antiferromagnetic interaction within each Cu dimer and weak ferromagnetic interactions between Cu dimers. This material also shows gas storage capability and can exchange the bound water molecules for other organic ligands such as pyridine (Chapter 10), opening the possibility of magnetic perturbation by guest inclusion–exchange. The use of Cu paddlewheel dimers in magnetic framework materials has also been reported for [(Cu2(py)2(1,3-bdc)2)3] and [(Cu2(py)2(1,3-bdc)2)4] (1,3-bdc ¼ benzene-1,3-dicarboxylate) which differ in triangular- or square-shaped bowl topologies, respectively (Figure 9.9).63,64 The materials both contain large cavities that are stable to the removal of guest molecules. Magnetically,

290

Figure 9.9

Chapter 9

Illustration of (a) [(Cu2(py)2(1,3-bdc)2)3] and (b) [(Cu2(py)2(1,3-bdc)2)4].

[(Cu2(py)2(1,3-bdc)2)3] shows a strong intradimer antiferromagnetic interaction, typical of discrete Cu dimers, and weaker interdimer antiferromagnetic interactions. Notably, there is an unusual remnant magnetization for [(Cu2(py)2(1,3-bdc)2)3], arising from the triangular lattice arrangement. On the other hand, [(Cu2(py)2(1,3-bdc)2)4], with a square arrangement, does not show any remnant magnetization. Furthermore, the 3D coordination polymer [VIII(H2O)]3O(O2CC6H4CO2)3 (Cl 9H2O), which is made from octahedral vanadium trimers connected by anionic linkers, shows a magnetically frustrated framework. The frustration in this case arises from the triangular topology of the VIII ions in the metal clusters, which also results in a low magnetic ordering temperature.65 There are a number of other 3D frameworks containing long carboxylate ligands which show magnetic ordering, but usually at low temperatures, such as the 3D material K[M3(btc)3] 5H2O (M ¼ Co, Fe, MIL-45), which is comprised of undulated octahedral chains linked by btc ligands, and exhibits ferromagnetism below 10 K for the Co analogue and 20 K for the mixed-metal analogue.66 A complex 3D Ni–succinate material formed under hydrothermal conditions orders ferrimagnetically below 20 K.67

9.2.2.10

Imidazolates

There have been a number of magnetic studies of imidazole-bridged 1D, 2D and 3D metal complexes, revealing that this linkage is efficient for magnetic exchange. Most recently, a family of Co21-related imidazolate 3D coordination polymers were found to show a diversity of magnetic behaviours, including antiferromagnetic and ferromagnetic ordering (Figure 9.10).68 These materials have the general formula [Co(im)2]n (n=2 or 4), where rational choice of solvent and counter-ligand results in five polymorphic structures which show silica-like frameworks. The materials [Co(im)2 0.5py] and [Co(im)2 0.5Ch]


Figure 9.10

291

Stick representations of (a) diamond-like nets of [Co(im)2 0.5py] and [Co(im)2 0.5Ch]; (b) the polymorphs [Co(im)2]4 and (c) [Co(im)2]2; (d) the zeolite topology of [Co(im)2 0.4Mb]5.

(Ch ¼ cyclohexanol) are isostructural and form infinite diamond-like nets. Two materials of formula [Co(im)2] show different framework structures but are generally comprised of chains connected into a dense 3D framework. The material [Co(im)2 0.4 Mb]5 (Mb ¼ 3-methylbutan-1-ol) shows a zeolitic topology comprised of a number of Co(im) rings. All of these polymorphic materials show antiferromagnetic coupling between the Co21 ions, transmitted through the imidazole ligand. However, stoichiometric and topological variances within this series result in uncompensated antiferromagnetic coupling, which leads to spin canting. In particular, [Co(im)2 0.5Ch] shows weak ferromagnetism (o15 K), one polymorph of [Co(im)2]n shows fairly strong ferromagnetism (o11.5 K) and the other polymorph displays stronger ferromagnetism (Tc ¼15.5 K); finally, [Co(im)2 0.4Mb]5 is a hidden canted antiferromagnet (Tc ¼ 10.6 K).

292

9.2.2.11

Chapter 9

Chiral Magnets

Introducing the extra property of chirality into magnetic materials lends to the possibility of magneto-optical phenomena such as non-linear magneto-optical effects and magneto-chiral dichroism. To obtain such magnetic materials, the chiral structure must be retained in the entire crystal, not just the molecular structure. This is rather difficult to achieve, hence there are only a handful of examples of chiral polymeric magnets. In general, there are two approaches to introducing chirality into coordination polymers, first through the use of a chiral ligand and second through intrinsic chirality in the structure, such as helices. Much greater control is attained through using chiral ligands and as such chiral magnets are generally targeted through this means. For example, there are a handful of examples where chiral nitroxide radical ligands have been used to form chiral 1D chains. In addition, chiral bidentate diamine ligands have been used in Prussian Blue analogue materials to form chiral 2D and 3D networks. First, in the 1D helical chain coordination polymer [Mn(hfac)2-(R)-3MLNN], where (R)-3MLNN ¼ (R)-methyl[3-(4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxy-3-oxide)phenoxy]-2-propionate, the chirality in the ligand induces a chiral configuration in the Mn21 coordination sphere (Figure 9.11).69 This material shows a ferromagnetic phase transition at 3 K and shows field-dependent hysteresis. In a further related material using a chiral bisnitroxide ligand with [Mn(hfac)2], a 1D polymer is formed.70 While the ligand contains an S-configured chiral carbon centre, an R-configured helical structure forms. Magnetically, this material shows metamagnetic behaviour. The use of chiral ligands in the synthesis of cyanide-bridged coordination polymers is a good strategy for developing chiral magnetic materials with high ordering temperatures. By using organic ligands which take up some of the coordination sites around the metal, the standard cubic bimetallic cyanide network is changed. Examples include the 2D material [Cr(CN)6][MnII (S or R)-pnH(H2O)](H2O) (pn ¼ 1,2-diaminopropane),71 which forms bimetallic 2D sheets and the related 3D material K0.4[Cr(CN)6][MnII-(S)-pn]-(S)-pnH0.6, which forms a helical dimetallic loop structure.72 The former shows ferrimagnetic behaviour at 38 K and the later 3D ferrimagnetic ordering at 53 K. Through changing to a different chiral amine, D- or L-aminoalanine, 3D materials of the formula [{Cr(CN)6}(MnII-(D or L)-NH2ala)3] 3H2O are formed which are comprised of helical chains linked together into a 3D network.73 These materials order ferrimagnetically at 35 K. Additionally, chiral magnetic materials have been generated using hexacyanoferrate building blocks and chiral diamines such as trans-cyclohexane-1,2-diamine, e.g. the 2D bimetallic complex [Ni(trans-(1S,2S)-chxn)2]3[Fe(CN)6]2 2H2O, (chxn=trans-cyclohexane-1,2-diamine) which shows ferromagnetic ordering at 13.8 K.74 Furthermore, the chiral alanine derivative N-(2-hydroxybenzyl)-L-alanine (H2Sala) has been used to form a 3D material with a very high ordering


Figure 9.11

293

(a) The chiral 1D chain material [Mn(hfac)2(R)-3MLNN] which (b) orders as a ferrimagnet at 4.6 K and shows field dependent hysteresis. Reproduced with permission from Angew. Chem. Int. Ed., 41, 586, copyright 2002, Wiley-VCH, Verlag GmbH and Co., KG Weinheim.

temperature.75 Initially, the material [Cu2(Sala)2H2O] is formed, which consists of dimers linked into helical 1D chains. Through thermal treatment of this material, the bound water molecule can be removed to form [Cu2(Sala)2], which is a chiral 3D polymer. The hydrated sample has a magnetic ordering temperature of 300 K and remarkably the dehydrated material shows an ordering temperature of 435 K.

9.2.3 Single-chain Magnets Whereas long-range ordering in magnetic materials is useful for applications in magnetic data storage, the isolation of single-chain magnets (SCM) which show magnetic ordering along the chain only may possibly be used as 1D magnetic

294

Chapter 9

nanowires for data storage. The general concept is to form isolated 1D chains which can be individually magnetised. While purely 1D systems are known to have no long-range ordering, the combination of large uniaxial anisotropy and large magnetic interactions between magnetic units within the chain results in long relaxation times. Thus, importantly, hysteresis or memory effects should be observed in these systems, even though 3D ordering is not present. In practice, it is difficult to obtain true SCMs which show characteristic slow magnetic relaxation times. Indeed, although the theory of SCM behaviour has long been known, it was only in 2001 that the first fully characterised 1D material displaying SCM behaviour was reported. The material CoII(hfac)2(NITPhOMe); NITPhOMe ¼ 40 -methoxy-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide), comprised of Co(hfac)2 and the organic radical NITPhOMe connected into helical chains,76 shows slow relaxation and hysteresis effects not associated with 3D order. There is antiferromagnetic coupling within the chain and due to non-compensation of the magnetic moments of the CoII centre and the radical 1D ferrimagnetic behaviour is apparent. In a further example, the bimetallic 1D chain material [Mn2(saltmen)2 Ni(pao)2(py)2](ClO4)2 [saltmen ¼ N,N 0 -(1,1,2,2-tetramethylethylene)bis (salicylideneiminate); pao ¼ pyridine-2-aldoximate], which shows good magnetic isolation, displays ferromagnetic coupling with slow relaxation and large field hysteresis loops.77 In order to have magnetic interactions in one direction and inhibit them in other directions, the use of short bridging ligands which will communicate effectively, in combination with bulky co-ligands to isolate the chains, has been another successful approach to developing SCMs. An example is the helical chain compound [Co(N3)2(bt)] (bt ¼ 2,2 0 -bithiazoline), which contains double end-on azido bridges and a bulky co-ligand that sufficiently isolates the chains.78 There are strong ferromagnetic interactions along the chain due to the end-on binding mode of the azido ligand and no long-range ordering due to the weak interchain coupling provided by the bt ligand. Indeed, evidenced by the slow magnetisation relaxation and hysteretic effect, this material shows SCM behaviour with a blocking temperature of 5 K and an energy barrier of 90 K. More recently, in a different approach, the material [FeII(ClO4)2{FeIII (bpca)2}](ClO4) [bpca ¼ bis(2-pyridylcarbonyl)amine] was reported, which is comprised of alternating chains of Fe21 and Fe31 metal ions connected through bpca ligands (Figure 9.12).79 This material shows SCM behaviour induced by a ferromagnetic arrangement of metal ions of HS Fe21 and LS Fe31 and the elongated geometries of the Fe21 ions. Additionally, magnetic 1D chains can be isolated by linking them into higher dimensions with long bridging ligands. This method was effective in [Co(N3)2 (H2O)2](bpeado) [bpeado ¼ 1,2-bis(4-pyridyl)ethane-N,N 0 -dioxide], where the bpeado ligand does not bind but rather hydrogen bonds to coordinated water and effectively isolates the chains.80 This material shows the slow magnetisation relaxation and large hysteresis loops characteristic of SCMs.


Figure 9.12

295

The 1D chain material [FeII(ClO4)2{FeIII(bpca)2}](ClO4) and the magnetic data characteristic of a SCM. Reproduced with permission from J. Am. Chem. Soc., 127, 10150, copyright 2005, ACS.

9.3 Spin Crossover 9.3.1 Introduction When a transition metal ion (d4–d7) is in an octahedral environment, there is a competition between the spin pairing energy (P) and the energy gap (D) of the eg and t2g metal orbitals. The magnitude of the energy gap is determined by the ligand field strength of the metal coordination environment. In the case where D is greater than P then the d-electrons remain paired (where possible) and fill first the lowest energy, t2g, orbitals, then the required eg orbitals. The metal ground spin state for this case is defined as low spin (LS). When D is less than P then the d-electrons can unpair and distribute to fill both the t2g and eg orbitals. This is called the high-spin (HS) state. On the other hand, when the magnitudes of D and P are approximately equal, a switching between the LS and HS states can occur via an external perturbation; most commonly via temperature variation but also pressure or light irradiation (Figure 9.13). This phenomenon is called spin crossover (SCO), and the consequences of its incorporation into coordination polymers is the focus of the second half of this chapter. The fundamental aspects of SCO will be only briefly addressed through key examples. For detailed explanations on this subject, the reader is referred to specialised molecular magnetism books and reviews.1,81 A final notable point is that is possible to observe SCO in solutions of monomers where intermolecular

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Chapter 9

effects are eliminated. As it is not possible to obtain 1D, 2D or 3D species in the solution phase, only solid-state SCO will be presented here. While SCO is possible in d4–d7 transition metals, it is most commonly reported for Fe21, Fe31 and Co21 metal ion-containing materials and of these three Fe21 is by far the most common. Extensive studies over the past few decades, mainly on mononuclear complexes, have revealed target coordination environments that provide sufficient ligand field strength for SCO to occur, such as [Fe(NCX)2(py)4] (where X ¼ S, Se, BH3 and py is a pyridyl donor ligand) and [Fe(tri)3]A (where tri is a triazole donor ligand and A is an anion). However, there are other factors outside the inner metal coordination environment which also influence the presence of a spin transition and the SCO character observed. In particular, the presence of intermolecular interactions such as hydrogen bonding, p-stacking and van der Waals forces plays a large role, in addition to cation, anion and solvent effects. These kinds of interactions are largely governed by crystal packing and are difficult to control. Associated with the transition from HS to LS states, there is a change in the relative number of unpaired electrons; thus SCO is most easily detected by magnetic susceptibility measurements versus temperature. The magnetic consequences are most commonly plotted as the temperature-dependent molar susceptibility, wMT (cm3 mol1 K), versus temperature. Historically the SCO curve was expressed in terms of the effective magnetic moment [meff ¼ O(7.997wMT) (BM)] because of recognised meff values, but this is less common now since meff is a derived unit dependent on the applied field (H), especially on high H. The temperature at which a spin transition occurs is often referred to as a T12 value and is defined as the temperature where half of the metal centres are in the HS state. There are various types of SCO transitions that can be observed: full, half, incomplete, two-step, multiple-step and variations between (Figure 9.13). In addition, spin transitions can show thermal hysteresis loops such that the heating

Figure 9.13

Octahedral crystal field splitting diagram showing the high-spin and lowspin states for Fe21 (left). The general spin transition types (HS fraction versus temperature) for gradual, abrupt, two-step and with hysteresis (right).


297

and cooling modes of the magnetic susceptibility measurements do not follow the same path. This hysteretic behaviour, termed bistability, provides a memory where at one temperature two different states can potentially exist in the one material. It is for this reason that SCO materials are of interest worldwide for possible use in binary switching and electronic devices. Note that this thermal hysteresis is different in origin to the magnetic hysteresis described in Section 9.2. Further to this, as a consequence of the increased occupancy of the antibonding eg orbitals in the HS state, there are measurable changes in the metal–ligand bond lengths over the SCO. In particular, for Fe21, there is commonly an average 0.2 A˚ variation over a full spin transition, very much in the range detectable by single-crystal diffraction. Indeed, it is common for the structures of both the HS and LS materials to be reported. An SCO often also results in a striking thermochromic effect, for Fe21, commonly from white in the HS state to purple in the LS state. For this reason, SCO materials may also be used in display devices and sensors. There are many other techniques that are used to detect and follow SCO (e.g. Mo¨ssbauer spectroscopy, infrared spectroscopy and optical reflectivity) but these will not be considered here. The important question then arises of why to incorporate SCO centres into coordination polymers. Traditionally, SCO material research has focused on mononuclear Fe21 materials towards attaining hysteretic spin transitions centred at about room temperature. The cooperativity between these mononuclear metal–ligand centres is controlled by the intermolecular lattice effects described above. However, when connecting SCO centres by the use of organic bridging ligands to form infinite networks, theoretically, cooperativity should be propagated through the framework more efficiently than in discrete systems. Indeed, there are a number of reports of SCO in polymeric materials which demonstrate this enhanced cooperative effect. There are however, also now covalently bridged coordination polymer examples that do not show enhanced cooperativity. Furthermore, through incorporating SCO into porous frameworks there is the additional possibility of perturbing SCO centres through guest molecule interactions, thus producing molecular sensing devices created through the synergy of the host framework and guest molecules. At the fundamental electronic/structural level, the incorporation of SCO centres into clusters, such as dinuclear systems, leads to microstates such as HS–HS, HS–LS and LS–LS, the HS–LS being of particular interest. Likewise in 1D chains, one can envisage HS–HS–HS–HS or LS–LS–LS–LS or HS–LS–HS–LS ordering. Examples where SCO centres have been successfully included in coordination polymers are now presented. These examples have been divided into sections based on the ligands utilised in their metal coordination environment.

9.3.2 Five-membered Heterocyclic Ring Bridging Ligands Five-membered ring examples, e.g. triazole, tetrazole and pyrazole, have featured significantly in extended polymeric SCO systems. Furthermore, such studies have

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Chapter 9

contributed greatly to the fundamental understanding of the SCO phenomenon. There are two general formulas into which these type of material fall, Fe(L)2(NCX)2 and [Fe(L)3]A, where X ¼ S, Se and BH3, L ¼ five-membered ring donor and A ¼ anion. Generally, the former materials form 2D networks, as a result of the terminating anion coordination, and the latter 3D networks.

9.3.2.1

Triazoles

A widely acclaimed series of 1D SCO chains containing triazole-type ligands exist in which the Fe21 centres are triply bridged by three triazole ligands through the nitrogen atoms in the 1- and 2-positions; however, no crystal structure exists to date. The nature of the spin transition in this series, of the general formula [Fe(4-Rtrz)3]A2 nH2O, where A is an anion, can be modulated depending on the substituent in the 4-position, the anion, the solvent and the degree of solvation (Figure 9.14). Within this series, very abrupt spin transitions have been observed, with hysteresis loops of up to 35 K width, and they generally display high spin transition temperatures. For example, [Fe(Htrz)3](ClO4)2 shows an abrupt SCO with hysteresis (T1/2k ¼ 296 K and

Figure 9.14

(a) Representation of triazole 1D chain of the general formula [Fe(4Rtrz)3]A2 nH2O where the (b) R-group on the ligand has been varied; (c) wMT versus T plot for (Fe(Htrz)2.85(NH2trz)0.15](ClO4)2 nH2O; (d) a display device showing number 7. From Science, 1998, 279, 44. Reprinted with permission from AAAS. Reproduced with permission from J. Am. Chem. Soc., 115, 9810, copyright 1993, ACS.


299

T1/2m ¼ 313 K). The SCO temperature of this material can be tuned by partially substituting Htrz by 4-NH2trz (4-amino-1,2,4-triazole), such as in the ‘ligand alloy’ species [Fe(Htrz)2.85(NH2trz)0.15](ClO4)2 nH2O that displays a spin transition with a wide hysteresis loop centred around room temperature (Figure. 9.14c).82 In a further example, [Fe(NH2trz)3](tosylate) 2H2O, the SCO behaviour can be altered significantly through a dehydration–rehydration process.83 The hydrated phase undergoes a very abrupt SCO, T1/2 ¼ 361 K, and the dehydrated phase undergoes a smoother transition with hysteresis whereby the transition temperature is lowered (T1/2k ¼ 279 K and T1/2m ¼ 296 K). These materials have been tested as contrast agents for magnetic resonance imaging and for the construction of thermal display devices (Figure 9.14d).84,85 The coordination polymer [Fe(btr)2(NCS)2] H2O (btr ¼ 4,4 0 -bis-1,2,4triazole) represents the first 2D polymeric SCO material reported.86 The structure consists of octahedral Fe21 centres surrounded by trans-thiocyanate ligands and four btr ligand triazole groups. These btr ligands link Fe21 centres to form infinite 2D grids, a recurring motif throughout polymeric SCO materials. These 2D layers are stacked and interact via van der Waals forces. Magnetically this material shows a very abrupt SCO centred around 134 K with a wide hysteresis loop, ca. 21 K. Initially it was suggested that the cooperative SCO nature was triggered by a crystalline phase change, but was later revealed to be associated with strong inelastic interactions between the SCO centres allowed by the rigid btr ligands. As will be highlighted in further examples, in particular throughout the pyridyl ligand section, short and rigid bridging ligands appear to enhance communication compared with flexible ligands. Whereas there are several examples of 1D and 2D SCO polymeric materials, there are few examples of 3D SCO coordination polymers which contain triazole ligands. The first example was [Fe(btr)3](ClO4)2, which structurally consists of six btr ligands surrounding each Fe21 centre. Each metal centre is then linked to six others through the btr ligand to form a 3D net. This material shows a two-step SCO, with the step at lower temperature being more abrupt with a 3 K hysteresis loop. This two-step SCO can be accounted for by the presence of two crystallographically distinct Fe21 sites within the lattice, where one is in a sufficiently different environment to undergo SCO at a different temperature.87

9.3.2.2

Tetrazoles

Tetrazole ligands have been used in much the same way as triazoles, where Fe21 materials of the general formula [Fe(L)3]A have been prepared and analysed. The tetrazole ligand and its derivatives provide a stronger ligand field than the triazole analogues and thus are expected to show spin transitions which are higher in temperature. The 3D coordination polymers [Fe(btzb)3](A) solvent [btzb ¼ 1,4-bis (tetrazol-1-yl)butane; A ¼ ClO4 or PF6] show a similar structure to the triazole-containing material [Fe(btr)3](ClO4) (Figure 9.15).88 However, as the

300

Chapter 9

Figure 9.15

The 3D coordination polymer [Fe(btzb)3]A (solvent), A = ClO4 or PF6 and the SCO behaviour (wMT versus T) of the PF6 analogue with solvent methanol (J) compared to ethanol (). Reproduced with permission from Inorg. Chem., 46, 4220, copyright 2007, ACS.

btzb ligand is significantly longer than btr, a triply interpenetrated net is formed. The two analogues of this material, where the anion is varied, show different SCO behaviours. The ClO4– analogue undergoes an incomplete but cooperative SCO, whereas the PF6– analogue shows a two-step cooperative transition. In SCO materials where long flexible bridging ligands are used, cooperativity is not expected; however, here, the close-packed nature of the interpenetrating nets provides sufficient rigidity to the system. Further reports on the PF6 material showed a solvent-assisted variation in magnetic behaviour between the methanol and ethanol synthesised materials.89 Further to this, a systematic study of the influence of ligand length on SCO behaviour was carried out with a 3D series of bis-tetrazole ligands where the alkane spacer length was varied. Materials of the general formula [Fe(nditz)3](ClO4)2 (n ¼ 4–9) were synthesised and magnetic studies revealed a range of SCO behaviours where generally increasing the spacer length raises the SCO temperature.90 Systematic studies such as this are important especially for the future generation of SCO coordination polymers as they reveal types of ligands to target.

9.3.2.3

Pyrazole Chelates Linked by Aromatic Spacers

Recently, a 1D chain material using a double-ended terpy-style ligand was reported using the ligand 1,4-bis(1,2 0 :6 0 ,100 -bispyrazolylpyridine-4 0 -yl)benzene, L (Figure 9.16).91 The material [Fe(L)](BF4)2 exhibits a SCO above room temperature (323 K) with a hysteresis loop of 10 K. Compared with a mononuclear material with the same metal coordination environment, both the transition temperature and hysteresis loop are increased; thus highlighting that


Figure 9.16

301

The 1D coordination polymer [Fe(1,4-bis(1,2 0 :6 0 ,100 -bispyrazolylpyridin4 0 -yl)benzene)](BF4)2 (top) and plot of wT versus T. From Chem. Comm., 2007, 2636. Reproduced with permission of The Royal Society of Chemistry.

incorporating SCO centres into polymeric arrays is indeed a means to achieve greater cooperativity between metal sites.

9.3.3 Six-membered Ring Bridging Ligands Following from SCO coordination polymer examples containing five-membered ring ligands, examples containing six-membered ring ligands are of the general formula Fe(NCX)2(L)2 (X ¼ S, Se), where the Fe21 centres are surrounded by trans-chalcogenide ligands and four pyridyl groups from bismonodentate ligands. Such materials generally form square/rhombic grid structures where, depending on the ligand length/flexibility, they can be arranged in parallel stacks or interpenetrate to form pseudo 3D networks. The first example of this type was provided by the 2D interpenetrated structure [Fe(NCS)2 (tvp)2] MeOH (tvp ¼1,2-di-(4-pyridyl)-ethylene).92 This material displays a gradual SCO around 215 K which was sensitive to the sample preparation conditions, in particular showing larger residual HS fractions at low temperature depending on the particle size and potentially the degree of solvation.

302

Figure 9.17

Chapter 9

Network representation of interpenetrated 2D sheets of [Fe2(NCS)4 (azpy)4] (guest] (top left) and the hydrogen-bonding interactions between the ethanol and thiocyanate ligands (top right). Plot of magnetic moment versus T for the ethanol and propan-1-ol solvated materials and the desolvated material.

This example highlights the possibility within this class of material of tuning the SCO properties via solvent guest exchange studies. This was nicely demonstrated in the related material [Fe2(NCS)4(azpy)4](guest) (trans-4,4 0 azopyridine, guest ¼ methanol, ethanol, propan-1-ol), which also shows an interpenetrated structure (Figure 9.17).93 The as-synthesised material where the 1D channels are filled with ethanol molecules undergoes a one-step half SCO. Structurally there are two inequivalent Fe21 sites within this material, differentiated by hydrogen bonding interactions between the ethanol molecules and the S atom of the thiocyanate ligands of the framework. The Fe21 centre involved in hydrogen bonding interactions which undergoes the spin transition; the other remains HS. Importantly, when the solvent is removed from this material it remains crystalline and undergoes a crystallographic phase change such that all the Fe21 centres become equivalent. Now that there are no host– guest hydrogen bonding interactions, the material remains HS over all temperatures, thus acting as a solvent sensor. Furthermore, the solvent ethanol can be exchanged for methanol or propan-1-ol and the magnetic properties altered; the propan-l-ol case, for example, shows a two-step SCO.


303

In a further related example, [Fe(NCS)2(bpbd)2] (acetone) [bpbd ¼ 2,3-bis(4 0 -pyridyl)-2,3-butanediol], it was shown that via incorporation of hydrogen bonding donors intrinsic to the framework, namely the diol groups, it is possible to retain the spin transition in the desolvated state,94 thus providing a new means to study the effect of solvent perturbation on SCO coordination polymers in a uniquely robust system. In addition to using monodentate bridging pyridyl-based ligands, bidentate ligands such as those based on the 2,2 0 -dipyridylamine ligand have been successfully incorporated into framework materials. In particular, 1D SCO polymers have resulted from the use of the 2- and 3-connecting ligands cddt {2-chloro-4,6-bis(di-pyridin-2-ylamino)-1,3,5-triazine} and dpyatriz {2,4,6tris(di-pyridin-2-ylamino)-1,3,5-triazine}. Chain materials of the formula Fe(NCX)2(cddt) n(guest) (X ¼ S, Se; guest ¼ MeOH or CHCl3) show spin transitions which are sensitive to axial ligand variation and intermolecular interactions.95 Notably, three analogues of the thiocyanate-containing material were reported where only one undergoes a spin transition owing to a network of p–p interactions between adjacent chains. In a further example, [Fe3(dpyatriz)2(CH3CH2CN)4(BF4)2](BF4)4 4CH3CH2CN is synthesised in two steps with the initial isolation of a dinuclear species, which is further reacted with excess Fe21 salt to form 1D chains (Figure 9.18).96 Importantly, the crystal packing of these 1D chains leads to channel-like cavities which can host various guest molecules. The Fe21 centres in the dinuclear portion of the chains in the as-synthesised material undergo a spin transition at room temperature. This SCO can be modulated via guest molecule exchange; specifically, the spin transition is lost in the water solvated and propan-2-ol solvated analogues and is shifted lower in temperature (ca. 20 K) with acetonitrile solvation. This provides the first example of an SCO material capable of room temperature guest molecule sensing. Thus far, the focus in this section has been on SCO systems containing Fe21 centres. However, although limited, examples also exist for Co21 and Fe31. For example, the 1D chain material [Co(pyterpy)Cl2] nX (X ¼ H2O or MeOH) exhibits an SCO [S ¼ 1/2 to 3/2 (d7)] sensitive to guest molecules (Figure 9.19).97 The ligand pyterpy is based on a combination of pyridine and terpyridine units where metal binding can occur at both ends. The distorted octahedral Co21 centres are coordinated by three pyridyl groups of the terpy portion of the ligand, a pyridyl group from the other end of another ligand and finally two trans-chloride atoms. The chains are held together by p–p interactions in a head-to-tail fashion and although this is an unusual (N4Cl2) coordination environment for a Co21 SCO system, a one-step SCO is observed for the watersolvated material with a small hysteresis loop (T1/2k ¼ 222 K and T1/2m 223 K). Interestingly, the methanol-solvated material remains HS (S ¼ 3/2) over all temperatures. Furthermore, the 1D chain material [Fe(acacen)(bpyp)](BPh4) [acacen ¼ N,N 0 -bis(acetylacetonato)ethylenediamine; bpyp ¼ 1,3-bis(4-pyridyl)propane],

304

Figure 9.18

Chapter 9

The 1D chain material Fe3(dpyatriz)2(CH3CH2CN)4(BF4)2](BF4)4 4CH3CH2CN comprised of dinuclear building blocks linked by Fe21. The SCO is sensitive to guest exchange (acetonitrile, propionitrile, water). Reproduced with permission from Adv. Mater., 19, 1397, copyright 2007, Wiley-VCH, Verlag GmbH and Co., KG Weinheim.


Figure 9.19

305

(a) The 1D chain material [Co(pyterpy)Cl2] nX (X = H2O or MeOH); (b) plot of the solvent sensitive magnetism (wMT versus T). Reproduced with permission from Inorg. Chem., 43, 4124, copyright 2004, ACS.

containing the [FeIIIN4O2] coordination environment, one used extensively in mononuclear Schiff-base Fe31 chemistry, shows a gradual SCO characteristic of the S ¼ 1/2 to S ¼ 5/2 spin change in d5 Fe31.98

9.3.4 Cyanide Bridges in Hofmann Phases Hofmann-type clathrate compounds have been known for a long time and consist of 2D metal cyanide sheets and aromatic molecules located between the sheets. The 2D sheets are comprised of alternating square-planar and octahedral M21 centres linked through cyanide bridges. In Hofmann pyridine complexes, the coordination at each octahedral M21 centre is completed by pyridyl groups protruding up and down. More recently, 3D Hofmann-type materials have been reported through using bridging pyridyl ligands. Having short bridges connecting potential SCO centres, these 2D and 3D materials show a range of SCO behaviours which are cooperative, high in transition temperature and often perturbable by solvent–host interactions.

306 (a) The 2D Hofmann-type network [Fe(py)Ni(CN)4] (1a), the analogous 3D network [Fe(pyz)Ni(CN)4] 2H2O (2a) and SCO behaviours (wMT versus T) of 1a and 2a; (b) the doubly interpenetrated networks {Fe(4,4 0 -bipy)2[Ag(CN)2]2} and {Fe(bpe)2[Ag(CN)2]2}. Reproduced with permission from Inorg. Chem., 40, 3838, copyright 2001, ACS.

Chapter 9

Figure 9.20


307

The first example of a 2D Hofmann-type material to display SCO was Fe(py)2[Ni(CN)4], where 2D extended metallocyanide sheets are offset stacked with pyridyl ligands in the interlayer space (Figure 9.20a). This material undergoes a cooperative SCO between 210 and 170 K.99 The M ¼ Pd and Pt analogues have also been reported and show similar magnetic behaviour to the Ni derivative. Through the use of pyrazine in place of pyridine, the dimensionality can be increased from 2D to 3D, resulting in materials of the general formula Fe(pyrazine)[MII(CN)4] (M ¼ Ni, Pd, Pt) (Figure 9.20a).100 Such materials generally display more cooperative spin transitions with higher transition temperatures than their 2D analogues. For instance, when the magnetic behaviour of the 2D material Fe(py)2[Ni(CN)4] is compared with that of the analogous 3D material Fe(pyrazine)[Ni(CN)4] 2(H2O), there is a ca. 100 K increase in spin transition temperature (Figure 9.20). This study provided the first clear illustration of the magnetic consequence of increasing the framework dimensionality. An alternate approach is to use linear metallocyanide building blocks such as [MI(CN)2] (M ¼ Cu, Ag, Au) combined with Fe21 and pyridyl ligands. A large range of 1D to 3D frameworks of this type has been reported and the magnetic properties are equally as interesting as the above Hofmann-type systems. For example, the 2D CdCl2-type network structure in Fe(pyrimidine)2[Cu(CN)2]2 is formed by the equatorial coordination of cyanide groups of [Cu(CN)2] anions to form linear, looping 1D chains. These 1D chains are linked by the axial coordination of pyrimidine ligands, which act as bridges between adjacent chains to form 2D layers.101 Magnetically, this material displays a complete one-step SCO with a small hysteresis gap (T1/2k¼ 132 K and T1/2m ¼ 142 K). Likewise, 3D Hofmann-like networks can be formed by the incorporation of bridging ligands such as 4,4 0 -bipy and bpe. For example, the two isostructural materials of formula Fe(L)2[Ag(CN)2]2, where L ¼ 4,4 0 -bipy or bpe, consist of doubly interpenetrated 3D networks (Figure 9.20c,d).102 These materials are constructed from 2D corrugated sheets of Fe4[Ag(CN)2]4 bridged via either 4,4 0 -bipy or bpe ligands through both the Fe21 and Ag1 atoms. Despite these two materials forming essentially the same network topology, they show remarkably different SCO behaviours, where the 4,4 0 -bipy analogue remains HS and the bpe analogue shows a cooperative SCO with a large hysteresis loop (ca. 95 K); this is one of the largest hysteresis loops reported for SCO compounds. This spin transition becomes less complete with repeated cycling of the heating and cooling modes, eventually resulting in 50% conversion between HS and LS forms (Figure 9.20).

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77. R. Cle´rac, H. Miyasaka, M. Yamashita and C. Coulon, J. Am. Chem. Soc., 2002, 124, 12837. 78. T.-F. Liu, D. Fu, S. Gao, D.S. Gao, Y.-Z. Zhang, H.-L. Sun, G. Su and Y.-J. Liu, J. Am. Chem. Soc., 2003, 125, 13976. 79. T. Kajiwara, M. Nakano, Y. Kaneko, S. Takaishi, T. Ito, M. Yamashita, A. Igashira-Kamiyama, H. Nojiri, Y. Ono and N. Kojima, J. Am. Chem. Soc., 2005, 127, 10150. 80. X.-Y. Wang, Z.-M. Wang and S. Gao, Chem. Commun., 2008, 281. 81. P. Gu¨tlich and H.A. Goodwin, Top. Curr. Chem., 2004, 233, 1. 82. J. Kro¨ber, E. Codjovi, O. Kahn, F. Grolie`ne and C. Jay, J. Am. Chem. Soc., 1993, 115, 9810. 83. E. Codjovi, L. Sommier, O. Kahn and C. Jay, New J. Chem., 1996, 503. 84. O. Kahn and C.J. Martinez, Science, 1998, 279, 44. 85. R.N. Muller, L.V. Elst and S. Laurent, J. Am. Chem. Soc., 2003, 125, 8405. 86. W. Vreugdenhil, J.H. Van Diemen, R.A.G. De Graaff, J.G. Haasnoot, J. Reedijk, A.M. Van Der Kraan, O. Kahn and J. Zarembowitch, Polyhedron, 1990, 9, 2971. 87. Y. Garcia, O. Kahn, L. Rabardel, B. Chansou, L. Salmon and J.P. Tuchagues, Inorg. Chem., 1999, 38, 4663. 88. P.J. van Koningsbruggen, Y. Garcia, H. Kooijman, A.L. Spek, J.G. Haasnoot, O. Kahn, J. Linares, E. Codjovi and F. Varret, J. Chem. Soc. Dalton Trans., 2001, 466. 89. M. Bartel, A. Absmeier, G. Jameson, F. Werner, K. Kato, M. Takata, R. Boca, M. Hasegawa, K. Mereiter, A. Caneschi and W. Linert, Inorg. Chem., 2007, 46, 4220. 90. A. Absmeier, M. Bartel, C. Carbonera, G. Jameson, P. Weinberger, A. Caneschi, K. Mereiter, J.-F. Le´tard and W. Linert, Chem. Eur. J., 2006, 12, 2235. 91. C. Rajadurai, O. Fuhr, R. Kruk, M. Ghafari, H. Hahn and M. Ruben, Chem. Commun., 2007, 2636. 92. J.A. Real, E. Andre´s, M.C. Munõz, M. Julve, T. Granier, A. Bousseksou and F. Varret, Science, 1995, 268, 265. 93. G.J. Halder, C.J. Kepert, B. Moubaraki, K.S. Murray and J.D. Cashion, Science, 2002, 298, 1762. 94. S.M. Neville, B. Moubaraki, K.S. Murray and C.J. Kepert, Angew. Chem. Int. Ed., 2007, 46, 2059. 95. S.M. Neville, B.A. Leita, D.A. Offermann, M.B. Duriska, B. Moubaraki, K.W. Chapman, G.J. Halder and K.S. Murray, Eur. J. Inorg. Chem., 2007, 1073. 96. M. Quesada, V.A. Penã-O’Shea, G. Aromı´ , S. Geremia, C. Massera, O. Roubeau, P. Gamez and J. Reedijk, Adv. Mater., 2007, 19, 1397. ¯ kawa, Y. Maeda, 97. S. Hayami, K. Hashiguchi, G. Juha´sz, M. Ohba, H. O K. Kato, K. Osaka, M. Takata and K. Inoue, Inorg. Chem., 2004, 43, 4124.

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CHAPTER 10

Porous Coordination Polymers 10.1 Introduction Porosity is one of the most widely studied individual properties within the field of coordination polymers. This is not so surprising considering the widespread industrial application of synthetic and natural zeolites, examples including petrochemical cracking, ion exchange, gas separation and purification. Like zeolites, coordination polymers consist of infinite networks comprised of interconnected structural units which have the potential to house nanometer-sized spaces. Importantly, owing to the unique rational design features of coordination polymers (Chapter 2) and the recent advances in obtaining robust or flexible porous systems, novel porous applications are emerging, beyond the scope of zeolites. These applications have sparked commercial interest in areas such as molecular separations, solvent and gas storage and heterogeneous catalysis. More advanced functions are also being realised through incorporating additional functions into porous coordination polymers, such as magnetic molecular sensing, gas sensing and chiral separations. Key illustrative examples of the methods for obtaining porous coordination polymers, the types of porous coordination polymers and the applications they find are given within this chapter.

10.1.1

Terminology

Before moving to examples and applications of porous framework materials, a few terms must be explicitly defined, in particular the term porosity, thus far employed rather loosely in this rapidly expanding field of porous coordination polymers.1 This lack of strict definition is largely due to the term porous, suggesting having pores, hence leading to many coordination polymers being reported as ‘porous’ based entirely on structural evidence of spaces filled with solvent and/or counterions. Although visual evidence of pores does not preclude the existence of porosity, it does not demonstrate it exclusively. Coordination Polymers: Design, Analysis and Application By Stuart R. Batten, Suzanne M. Neville and David R. Turner r Stuart R. Batten, Suzanne M. Neville and David R. Turner, 2009 Published by the Royal Society of Chemistry, www.rsc.org

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Pores are defined as ‘minute openings through which fluids or gases can pass’.1 Therefore, to be termed a porous material, proof of porosity beyond structural evidence must be demonstrated; often this is via gas adsorption– desorption isotherm studies. Other parallel studies are equally valid, such as comparison of solvated and desolvated powder X-ray diffraction patterns, thermogravimetric analysis and infrared spectroscopy. Importantly, isotherm studies provide evidence of permanent porosity, defined as the retention of structural integrity during the adsorption and desorption of guest species from the host material. Permanent porosity may also be demonstrated directly through structural characterisation, i.e. powder or single-crystal X-ray diffraction studies, of the empty host material (termed the apohost), but should be confirmed through parallel gas sorption studies (Figure 10.1a). Pores within framework materials have a uniform size and shape distribution compared with other porous materials and thus can be divided into classes based entirely on their pore size.2 Macroporous materials have pore diameters Z50 nm, mesoporous materials have pore diameters between 2 and 50 nm and microporous materials have pore diameters o2 nm. The majority of porous coordination polymers fall into the mesoporous and microporous pore size range; however, the term nanoporous, i.e. containing pores in the nanoporous range (0.2–10 nm), is often generically used within this field to describe ‘small pores’, but should be used with care (Figure 10.1b). Gas sorption isotherm studies are essential for determining the properties of porous coordination polymers, providing information on the guest molecule–surface interactions, pore size and topology. Characteristic gas sorption isotherm shapes are obtained for each type of pore and give information on

Figure 10.1

(a) Schematic representation of permanent porosity, where the guest molecules located in the pores of the host material can be removed reversibly to generate a stable apohost. (b) Scale of micro-, meso- and macropore sizes. Cross symbols indicate the nanoporous region.

Porous Coordination Polymers

Figure 10.2

315

General shape of isotherm classifications Type I-VI. Reprinted from J. Solid State Chem., 178, 2491–2510, copyright (2005), with permission from Elsevier.

how the host and guest interact. There are six general isotherm types (Types I– VI). Microporous materials generally show Type I behaviour, which is characterised by a sharp increase in mass at low relative pressure where essentially all the pore volume is filled, followed by a plateau. Types II, III and VI are generally indicative of meso- or macroporous materials and Types IV and V are indicative of mesoporous materials (Figure 10.2). The reader is referred to specialised books and reviews on porous materials for further information outside the scope of this chapter.2–5

10.2 Designing Permanent Porosity Nature generally tries to avoid empty space; any voids or cavities which are generated in the synthesis of coordination polymers are commonly filled with guest solvent molecules and/or counterions. An alternative outcome is that a framework forms which is close packed such that there is no apparent accessible void volume or that interpenetration of nets occurs such that any potential voids are filled (see Chapter 3). Many early examples within the field of porous coordination polymer chemistry revealed that guest removal from the host structure commonly led to

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collapse of the framework structure. However, in more recent times reports of truly porous materials have become more commonplace. This step towards reliably attaining permanent porosity has been largely enabled through the identification of a range of specific framework components and topologies which are conducive to allowing stability of the empty host. A key step in designing permanent porosity is through initially targeting either a rigid framework, where essentially no changes are observed between the full and empty host structures, or a flexible framework, where substantial movement of the host structure may occur with desolvation but without the loss of porosity. Through careful selection of framework components which are predisposed to form certain topologies, porous materials may be obtained (Chapter 2). In terms of synthetic design, flexible versus rigid coordination polymers differ mainly in the general components from which they are designed and the relative inter- and intramolecular interactions imparted on the resultant framework. To aid in describing flexible through to rigid frameworks, seven very general subclasses will be described: (1) 1D chains, (2) 2D stacked layers, (3) interdigitated 2D layers, (4) ‘pseudo’-3D materials, (5) 3D pillared layered, (6) 3D interpenetrated and (7) 3D networks (Figure 10.3). These divisions are useful, first, to show the diversity within the areas of flexible through to rigid frameworks, second, to highlight that it is possible to obtain porosity in 1D, 2D and

Figure 10.3

Seven general classes of coordination polymers encompassing flexible through to rigid frameworks. The classes are generally divided based on network dimensionality.


317

3D materials, and third, to show that there is some degree of overlap between the two types. Demonstrative examples of each will be presented. Notably, from a historical perspective, the first generation of porous coordination polymers consist of those which contain pores sustained only in the presence of guest molecules, the second generation of porous coordination polymers consist of stable and robust frameworks in the absence of guest molecules and the third generation are those which have flexible and dynamic frameworks that change their porous nature in response to external stimuli.6

10.2.1

Flexible Porosity

Examples of porous coordination polymers are dominated by rigid systems, largely due to the ease of characterising the empty host, but more recent times have seen numerous examples of flexible systems complete with structural characterisation in the desolvated state. Flexible/dynamic porous materials can be generated through the linking of flexible moieties with strong bonds (organic or inorganic) together with weaker bonds (i.e. p–p stacking, hydrogen bonding and van der Waals interactions). The concept is that weaker interactions, such as p–p stacks, hydrogen bonds and van der Waals forces provide stability intrinsic to retaining porosity with an inbuilt flexibility allowing concerted structural modifications to occur with solvent removal and replacement. Thus a ‘soft’ framework is generated. In many cases, the added weak bonds increase the dimensionality of the framework such that a ‘pseudo’-2D or -3D character is attained; this is important as higher dimensional structures generally show a more robust nature. Furthermore, materials which show dynamic porosity may show new modes of guest adsorption/desorption where a specific ‘gate-opening’ pressure is required before guest molecules may enter.7 Importantly, for dynamic porous materials hysteresis is possible due to differing ‘gate-opening’ and ‘gate-closing’ pressures, thus opening the door for new applications such as switches, sensors and separating devices based on this property.

10.2.1.1

1D Chain Materials

Individually 1D chains would not be considered as potential porous candidates; however, examples exist where small voids are created through their packing in the solid state. Not all 1D chain materials will show a porous nature; generally, the weak interactions introduced above must be present. A simple 1D chain example is [Rh2(O2CPh)4(pyz)] (pyz ¼ pyrazine), which is readily constructed from rhodium(II) benzoate units bridged by pyz.8 The solidstate packing of this material is defined by benzene and pyz interactions resulting in efficient high-density packing, but with narrow micropores. This material shows interesting gas sorption properties and importantly represents the first example of in situ single-crystal diffraction of adsorption of a gas. A more complex example of 1D chains is provided by the homochiral material [Cd(L)2(ClO4)2] 11EtOH 6H2O [L ¼ (S)-2,2 0 -diethoxy-1,1 0 -binaphthyl-6,

318

Figure 10.4

Chapter 10

(a) The 1D chain material, [Cd(L)2(ClO4)2] 11EtOH 6H2O (L ¼ (S)2,2 0 -diethoxy-1,1 0 -binaphthyl-6,6 0 -bis(4-vinylpyridine), (b) the 2D stacked grid material, [Co(NCS)2(4-peia)2] 4Me2CO, (c) the interdigitated 2D material, [Ni2(4,4 0 -bipy)3 (NO3)4 EtOH and (d) the ‘pseudo’ 3D interpenetrated material [Fe2(NCS)4(4,4 0 -azpy)4] (guest).

6 0 -bis(4-vinylpyridine)], which displays robust porosity.9 Adjacent metal centres are linked by two ligands to form a linear looping chain structure. The chains lie parallel and their packing is strongly directed by p–p stacking interactions. Adjacent layers are rotated by 1201 such that large chiral hexagonal channels are apparent. Permanent porosity of this material was confirmed by CO2 gassorption isotherms, and further guest exchange experiments (water and benzene) revealed a single-crystal structural transformation process (Figure 10.4a).

10.2.1.2

2D Stacked Layers

As with 1D chains, this type of material may initially be overlooked as potentially showing porosity. However, through parallel packing, voids may be created within or between the layers depending on their packing efficiency. There are many examples of stacked layer coordination polymers, but one recurring motif is stacked (4,4) grids, where octahedral metals are equatorially linked by bridging monodentate ligands. The offset stacked (4,4) grids of [Co(NCS)2 (4-peia)2] 4Me2CO [4-peia ¼ N-(2-pyridin-4-ylethyl)isonicotinamide] provide a nice example of designing ligands with inbuilt hydrogen bonding character for robust flexibility.10 The octahedral Co(NCS)2 centres are equatorially bridged by


319

4-peia ligands to form 2D layers with large square cavities. Generally, with the use of a long bridging ligand, interpenetration of grids would occur; however, the ligand design in this material, which induces complementary hydrogen bonds, inhibits this. This material shows an interesting amorphous to crystalline behaviour with solvent acetone desorption and resorption (Figure 10.4b). Other 2D layer topologies are possible, such as the (6,3) honeycomb structure [{Cu2(O2CCH3)4}3(tpt)2] 2MeOH [tpt ¼ 2,4,6-tris(4-pyridyl)-1,3,5-triazine].11 These layers contain large hexagonal windows which, rather than interpenetrating, as one would expect, stack in an ABC fashion. This stacking is directed by p–p interactions with the tpt ligands of adjacent sheets.

10.2.1.3

2D Interdigitated

These materials include stacked species which are not simple layers, such as (4,4) grids, held together by weak forces. It is the interdigitation of layers which contain complementary contours that define the channels. The first example of a single-crystal to single-crystal transformation with desolvation comes from this class, highlighting that a more 3D nature is imparted through interdigitation than stacking. Specifically, the coordination polymer [Ni2(4,4 0 -bipy)3 (NO3)4] EtOH (4,4 0 -bipy ¼ 4,4 0 -bipyridine), which is comprised of interlocked bilayers formed by the T-shaped coordination of ligands around the metal ion, is arranged in a ‘tongue-and-groove’-type fashion with hydrogen bonding apparent between alternate layers.12 The interdigitation defines 1D channels filled with solvent ethanol, which can be removed with heating and the single crystallinity retained. The flexibility of this framework is evident by a subtle scissoring motion within the bilayers, which is reversible with solvent removal and replacement (Figure 10.4c). Another example includes the mutually interdigitated framework [Cu(dhbc)2 (4,4 0 -bipy)] H2O (dhbc ¼ 2,5-dihydroxybenzoic acid), which consists of 1D chains of Cu(4,4 0 -bipy) linked by dhbc.7 The dhbc benzene planes are upright such that interdigitation occurs, with p–p stacking between adjacent layers. Solvent-filled 1D channels are defined by the interdigitation. The host is stable to desolvation as followed by powder X-ray diffraction (PXRD) and shows a small shift in the relative positions of the layers. Interestingly, nitrogen isotherm studies on the desolvated material showed that a gate-opening pressure is required before the pores allow molecule entry, which is facilitated by the flexible nature of the p–p interactions. Further to this, there is hysteresis in the adsorption and desorption isotherms.

10.2.1.4

‘Pseudo’-3D Materials

These materials consist of lower dimensional coordination polymers which through interpenetration form ‘pseudo’-3D materials. Such materials are not truly 3D but, through weak interactions defined by their interpenetration or via the interactions intrinsic to their mutual interpenetration, they take on a 3D

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character. The extent and strength of these interactions greatly influence the degree of flexibility or rigidity observed in the framework. In the framework material [Fe2(4,4 0 -azpy)4(NCS)4] (EtOH), the doubly interpenetrated (4,4) grids are held together via van der Waals interactions.13 Upon solvent removal from the 1D channels defined by the interpenetration, the grids move by a scissoring action such that there is essentially no void volume. This flexible single-crystal transformation process is fully reversible (Figure 10.4d). Further to this, when stronger interactions exist between interpenetrated grids, a more rigid robust nature can be observed. This is seen in the interpenetrated (4,4) grid material Fe(NCS)2(bpbd)2 (acetone) [bpbd ¼ 2,3-bis (4 0 -pyridyl)-2,3-butanediol], which contains a bridging pyridyl ligand with inbuilt hydrogen bonding sites.14 Hydrogen bonds exist between interpenetrated grids and provide sufficient stability that with solvent removal and replacement essentially no changes to the framework occur. The interesting magnetic properties of this and the former material are outlined in Chapter 9. Many other, more complicated, examples of interpenetrated ‘pseudo’-3D materials have been reported and are outlined in Chapter 3.

10.2.1.5

3D Pillared Layered

The pillared layered approach to building porous coordination polymers has been very successful as simple modification of the pillars can control both the size and chemical environment of the pores.15 A number of layers have been utilised for which the conditions for synthesis are now established. This means that a wide range of pillars can be used in combination with these layers to access infinite possibilities for this type of material (Figure 10.5). For example, the [Cu(pzdc)] (pzdc ¼ pyrazine-2,3-dicarboxylate) layer is commonly employed with 2-connecting pyridyl-type pillar ligands.15 A simple example is the material [Cu2(pzdc)2(pyz)], for which pyz forms the pillar. As this pillar is short, the resultant channels are equally small. On the other hand, when a

Figure 10.5

Schematic representation of pillared layered motifs where the layer remains the same and the pillar can be modified from short (pyz), to longer (4,4 0 -bipy, 4,4 0 -azpy), to functional (pia).


321

longer bridging ligand such as 4,4 0 -bipy is used, an isostructural material is formed but with larger channels. Furthermore, ligands may be chosen which contain functional groups such that the pore surface properties can be tuned, e.g. the ligands n-(4-pyridyl) isonicotinamide (pia) and 1,2-di(4-pyridyl)-glycol (dpyg), which have specific hydrogen bonding sites provided by the amide group and hydroxyl groups, respectively.15,16 The robust porous nature of each of these materials has been examined by methane gas adsorption and showed larger uptakes for materials containing the longer ligands. Other more complicated examples of pillared layered materials exist, such as [Cd(pzdc)(4,4 0 -azpy)] 2(H2O), for which the 4,4 0 -azpy ligands bridge the layers in a criss-cross fashion.17 There is an extensive network of p–p interactions between 4,4 0 -azpy ligands in the pillar region. This material is robust to guest removal and shows interesting expanding behaviour in this state which facilitates uptake of a large range of solvent guest molecules. It should be noted that pillared layered inorganic–organic hybrid materials exist, such as cobalt hydroxide layers linked by organic ligands such as DABCO or cyclohexane dicarboxylate.18,19 These types of material show interesting porous and magnetic properties and are reviewed in Chapter 8.

10.2.1.6

3D Interpenetrated

Although a 3D framework may be rigid, when interpenetration of nets occurs a degree of flexibility can be observed with relative motion of the nets. A good example of this is the doubly interpenetrated 3D network structure [(ZnI2)3(tpt)2] 6C6H5NO2.20 This material shows a dynamic nature whereby it shrinks when guest molecules are removed and swells when they are returned. In this example, both the framework and relative distance of interpenetrated nets are distorted in the transformation (Figure 10.6). Another example of a robust but flexible 3D material is the doubly interpenetrated a-Po network which contains ZnO4 units bridged by a tetracarboxylate porphyrin (PIZA-4).21 Whereas this material shows a robust nature in the absence

Figure 10.6

Schematic representation of the flexible porous 3D interpenetrated network [(ZnI2)3(tpt)2] 6C6H5NO2. This material undergoes a guest induced modification.

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of guest molecules via N2 sorption isotherm studies, PXRD studies show a reversible movement of the interpenetrated nets towards one another with desolvation. The movement of interpenetrated but rigid nets shown in both this and the former example is a promising way to control the size of pores for size and shape selectivity of guests. On the other hand, 3D interpenetration may lead to robust coordination polymers due to the strength of intermolecular interactions between nets. For example, the material [Cu3(BTB)2(H2O)3] (DMF)9 (H2O)2 (BTB ¼ 4,4 0 ,400 -benzene-1, 3,5-triyltribenzoic acid) shows a Pt3O4-type net structure which is doubly interpenetrating.22 This interpenetration results in stability of the empty framework which may not be otherwise observed. Additionally, the doubly interpenetrated NbO-type net [Fe(NCS)2(tmbpz)2] nMeCN (tmbpz ¼ 3,3 0 ,5,5 0 tetramethyl-4,4 0 -bipyrazolyl) highlights the stability that inter-framework hydrogen bonds provide.23 This material exhibits robust stability upon desorption of the guest molecules, largely due to interactions between the methyl ligand and thiocyanate ligand, thus not only preventing collapse of the framework but also inhibiting slippage of the frameworks relative to each other. This example also highlights the difference in having weaker interactions (i.e. hydrogen bonding) between interpenetrated 3D nets compared with 2D nets where flexibility is still allowed.

10.2.2 10.2.2.1

Rigid Porosity 3D Networks

Rigid pores can be generated through the use of secondary building units (SBUs) (Chapter 2), which are rigid and have defined bonding directions such that specific framework topologies are targeted. Importantly, for a successful self-assembly process, the rigidity and directionality of the SBU must be maintained during synthesis. Thus, the design features of rigid frameworks compared with flexible frameworks differ greatly in the reduced degree of freedom allowed in the synthetic process. There are two commonly reported SBU types: inorganic clusters and rigid organic ligands (Figure 10.7). Ways of incorporating them into extended networks with representative examples now follow. One further point of relevance is that polymeric materials which show strong bonding to permit a robust nature, linking units which can be modified by organic synthesis, and distinct topologies, such as those containing inorganic cluster SBUs, are referred to as metal–organic frameworks (MOFs).24 In essence, MOFs are a subclass of coordination polymers which display welldefined properties. First, SBUs may take the form of inorganic clusters, which are well established in the field of molecular coordination chemistry. There are three clusters which are commonly reported in MOFs (Figure 10.7a–c): the paddlewheel, which has four binding sites and two terminal sites, the octahedral ‘zinc acetate’ node and the trigonal prismatic trimer with an oxo centre, which has three


323

Figure 10.7 Examples of inorganic and organic secondary building units (SBUs). The three common types of inorganic clusters: (a) the paddlewheel; (b) the basic zinc acetate; (c) the oxo-centred trimer. (d) Representative examples of organic SBUs. Reprinted from Microporous Mesoporous Mater., 73, 3–14, copyright (2004).

terminal ligand sites. These clusters are not preformed but made in situ during the self-assembly process. The general conditions required for their synthesis are known from molecular coordination chemistry. The inorganic clusters are most often linked with linear organic ligands, commonly 2-connecting carboxylic acid ligands, and more recently higher connecting ligands. The clusters may also be linked through replacement of the terminal groups with bridging ligands. A very important series of MOFs exist which contain the ‘basic zinc acetate’ inorganic cluster SBU. These materials are all of the formula ZnO4(L)3 (L ¼ linear carboxylate linker) and all form the same cubic topology. The first example within this series contains the ligand benzene-1,4-dicarboxylate (1,4-bdc, MOF-5) and is stable up to 400 1C, where importantly there are no changes to the framework with solvent removal, shown by single-crystal analysis.25 This remarkable thermal stability is largely provided by the strength of the metal–carboxylate bond and makes such materials ideal candidates for molecule storage applications, such as gas storage. In addition, the ability for such materials to be regenerated may times over, as they are so rigid, adds to their value for these applications. Further to this, an isoreticular series (IRMOF) has been designed using a range of other linking carboxylate ligands of varying length, shape and aromaticity.24 In this way, a range of materials with increased and indeed tuneable pore size, shape and chemical nature have been generated (Figure 10.8). This

324

Figure 10.8

Chapter 10

Representation of IRMOF-1, -8, -12 and -16, highlighting the pore size increase with larger linear linking ligands. Reprinted with permission from Science, 295, 469–472, copyright 2002 AAAS.

ability to tune the physical and chemical nature of the pores is important for further systematic exploration in terms of gas and solvent storage and other applications, where we are only just beginning to understand the optimal characteristics required for each application.26,27 Examples of other linear carboxylate linkers include naphthalene-2, 6-dicarboxylate (IRMOF-8), pyrene-2,7-dicarboxylate (IRMOF-12) and terphenyl-4,400 -dicarboxylate (IRMOF-16).24 The latter forms a structure with remarkably large pores accounting for 91.1% of the crystal volume. Other linear carboxylate ligands have been used in an attempt to functionalise further the pore surface with groups such as –Br and –NH2 and alkyl chains of various lengths.27,28 More recently, trigonal ligands, such as benzene-1,3,5tribenzoate (MOF-177), and other more complex 3-connecting ligands, have been used with the basic zinc acetate cluster to form 3D materials of a different topology.29,30 Notably, MOF-177 shows one of the highest surface areas recorded.31 The paddlewheel bimetallic cluster (M21 ¼ Cu, Zn, Fe, Mo, Rh, Ru) has also been very successful in producing MOFs of a robust and rigid nature. An important feature of the paddlewheel cluster itself is the ability to remove or replace the terminal axial ligands without collapse of the framework structure. This property has been shown to be important for increased hydrogen storage as bare metal sites are produced which may have an enhanced hydrogen binding affinity. Additionally, the terminal axial ligand can be exchanged for other terminal organic ligands for chemical functionalisation of the pores32 or exchanged for bridging organic ligands such that the paddlewheel motif becomes an octahedral node.33–35 As for the ‘basic zinc acetate’ cluster materials, a range of ligand shapes and geometries have been used to link the paddlewheel clusters. An early example of a robust network containing this SBU is the Pt3O4-type material [Cu3(btc)2(H2O)3] (btc ¼ benzene-1,3,5-tricarboxylate) (Figure 10.9a).32 It contains square-shaped pores (9 9 A˚) and is stable to high temperatures. Importantly, the Cu atoms in the solvated phase contain a coordinated solvent water molecule which can be removed, resulting in coordinatively unsaturated


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Figure 10.9 3D networks which incorporate inorganic and organic SBUs. (a) [Cu3(btc)2(H2O)3]; (b) a network containing the 3-connecting ligand 1,3,5-benzenetristetrazolate; (c) PIZA-1, containing a 4-connecting porphyrin based ligand. Reprinted with permission from Nature Publishing Group, Nat. Mater., 1, 118–121, copyright (2002).

Cu21 sites. These labile sites can also be readily exchanged for other ligands, such as pyridine, which acts to change the pore chemical environment. Furthermore, a recent example containing a 4-connecting anthracene carboxylate derivative was reported (PCN-14).36 The 3D material contains cages which exhibit the highest methane gas uptake to date. Additionally, a series of materials have been reported where the paddlewheel SBU acts as an octahedral unit. In these materials square grids of copper dicarboxylate (e.g. fumarate, terephthalate, styrenedicarboxylate and 4,4 0 biphenyldicarboxylate) are linked axially with diamines, such as DABCO or triethylenediamine (TED), to form 3D networks.33–35 Within this isoreticular series, the linear linker lengths have been varied, leading to different pore sizes but also resulting in materials with interpenetrated nets. Some of these materials have been shown to have ideal pore sizes for methane adsorption.33–35 The third type of inorganic cluster SBU, the oxo-centred trimer, has also been incorporated into porous framework materials. An isoreticular series of porous 3D materials (MIL-88) have been synthesised, via either low- or high-temperature

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methods, using M (M ¼ Fe, Cr) and linear carboxylate ligands such as 1,4-bdc, naphthalene-6-dicarboxylic acid and biphenyl-4,4 0 -dicarboxylic acid.37 These materials contain large bipyramidal cages which show pore diameters ranging between 14 and 28 A˚ depending on the ligand size. As for other materials containing inorganic clusters, these compounds are thermally stable to high temperatures, but show a slightly flexible nature in the empty state. Another series of MIL materials which contain the trimer SBU have been reported, MIL-100 and MIL-101, which show giant pore sizes (ca. 25–34 A˚).38,39 The material MIL-100 contains the three connecting ligand btc and forms a stable 3D zeolite topology and MIL-101 contains the two connecting ligand 1,4-bdc. These materials show interesting hydrogen storage capability.40 Finally, SBUs may be in the form of rigid organic ligands which can coordinate in more than two directions. This differentiation of connecting in more than two directions distinguishes rigid networks from flexible pillared layered and related materials. Additionally, the increased connectivity ensures a rigid resultant framework. Of course, the ligand must in itself be rigid. Indeed, many of the organic SBU ligands which have been successfully incorporated into rigid framework materials contain a central core, such as a porphyrin or adamantane unit, which provides much of the stability.41,42 For example the tritopic bridging ligand benzene-1,3,5-tristetrazolate (BTT3) forms a stable cubic coordination polymer with manganese.43 In this anionic 3D network, the rigid BTT3 ligands are joined by [Mn4Cl]71 units and the charge is balanced with [Mn(DMF6)]21 cations within a cavity (Figure 10.9b). This material is thermally stable to high temperatures and shows a large capacity for hydrogen storage. In a further example, a porphyrin-based 4-connecting carboxylate ligand forms a rigid 3D framework joined by trinuclear clusters (PIZAF-1) (Figure 10.9c).42 The extended structure contains large oval-shaped channels which are stable to high temperatures and show size and shape selectivity to a range of guest molecules. Notably, the amount of water this material can adsorb is more than the molecular sieve zeolite 4A.

10.3 Solvent Exchange Solvent exchange properties can also be divided into examples of robust and of flexible materials. The importance of these two types is evident by their different potential applications. Robust systems, by their nature, have pores of a defined size and shape that do not change with or without solvent. Thus rigid systems are more suited for applications such as gas storage and catalysis. On the other hand, flexible systems may modify pore size and shape, influenced by the presence or absence of solvent guest molecules. Hence flexible systems find different applications, such as size and shape selectivity, sensing, and gas entrapment. Of course, there is overlap between the two areas. Solvent exchange can be quantitatively measured by vapour adsorption measurements and thermogravimetric analysis, and monitored via powder


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diffraction and NMR techniques. Whereas the interpretation of isotherms of rigid systems is generally straightforward, for flexible systems, which may not undergo the same solvation and desolvation processes, the interpretation becomes more difficult. Indeed, isotherm shapes have been reported which show sorption behaviours only at higher pressures and which show hysteresis in adsorption–desorption profiles.16 Importantly, guest removal and exchange experiments have developed remarkably, with reports of continuous monitoring of the structure during guest desorption and sorption, thus providing mechanical information intrinsic to the desolvation process, in particular for flexible frameworks where large structural changes occur. Such studies are useful for identifying key components that will aid in further designing robust porous coordination polymers.

10.3.1

Flexible Guest Exchange

Flexible or dynamic guest exchange is conceptually a way to accommodate and separate molecules with a selective recognition process. This process is much like the way that molecular recognition works in bioenzymes and metalloproteins. A proposed set of dynamic pore classes includes induced fit, breathing and deformation (and healing-type pores) (Figure 10.10).44 Each of these types essentially is aimed at increasing the surface contact/interactions between the

Figure 10.10

Dynamic contrivances for guest removal and uptake. (a) Induced fit; (b) breathing; (c) deformation. Host represented as thick line surrounding guest (shape) where appropriate.

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host and guest, which, as the following examples highlight, can lead to dramatic structural consequences. Healing pores have been omitted as they essentially result from a lack of porosity to a gain in porosity, which is outside the scope defined here. To accommodate the guest molecules better, porous coordination polymers may shrink, or induce-fit, their pore size so as to increase the contact between host and guest, such as in the pillared layered material [Cu2(pzdc)2 (4,4 0 bpy)] xH2O, which shows reversible structural change with sorption/desorption.15 Upon exposure to benzene, the b-axis contracts by 6.8% to suit the benzene shape better. In a related 3D pillared layered material, [Cd(pzdc)(bpee)] 1.5H2O [bpee ¼ 1,2-di(4-pyridyl)-ethene], selective absorption of H2O and MeOH is seen over larger solvents. This selectivity is due to the small aperture of the channels, where upon desolvation the unit cell contracts, inhibiting the entry of large guests. This is in contrast the 4,4 0 -azpy analogue, which expands upon desolvation and can adsorb a large range of solvents.17 Interesting guest-induced fit behaviour is observed in the pillared layered material [Cu2(pzdc)2(dpyg)] 8H2O, which contains a pore system lined with OH groups from the dpyg ligand.16 Whereas the adsorption isotherm of this material shows no uptake of methane, a significant uptake of MeOH, which is of a similar size, is observed. Furthermore, a hysteretic profile for MeOH adsorption is seen which is associated with a framework transformation to allow guest entry. This selective guest behaviour and unusual hysteretic uptake are due to the favourable hydrogen bonding interactions between the methanol and host, which are less efficient with methane (Figure 10.11a). In contrast, when the pore size is too small to accommodate a guest, an expansion of the pore size may occur so as to house the guest adequately. The stacked square grid material [Ni(NO3)2(L)2] (guest) [L ¼ 4,4 0 -di(4-pyridyl)biphenyl], which is comprised of rectangular channels defined by the stacking of the layers, shows sliding of the stacked layers with guest exchange such that the channel dimensions are increased.45 In particular, the as-synthesised material contains o-xylene and MeOH guest molecules, which can be removed with heating and the framework remains stable. Immersion in a new solvent, mesitylene, results in an obvious colour change and this structural modification. A pronounced guest-induced single-crystal to single-crystal transformation is seen in the interpenetrated 3D network [(ZnI2)3(tpt)2] (guest).20 Upon exposure to the atmosphere, the solvent (nitro- or cyanobenzene) is lost from the pores and the crystals change from colourless to yellow. A crystal transformation occurs, resulting in a 20% reduction in void volume as the interpenetrated nets shrink to compensate for the empty void. The original ‘expanded’ material may be regenerated with reintroduction of the solvent nitrobenzene. Also important in the scheme of guest exchange is determining the exact behaviour of guest molecules as they enter and process through the pores. The 2D interdigitated bilayer material [Ni2(4,4 0 -bipy)3(NO3)4] (EtOH) was first described as a robust framework material capable of stability in the absence of


Figure 10.11

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(a) Methanol solvent sorption and desorption isotherm of the inducedfit dynamics within the pillared-layered coordination polymer [Cu2(pzdc)2(dpyg)] 8H2O, showing hysteresis. (b) Reversible ethanol removal from [M2(4,4 0 -bipy)3(NO3)4].n(EtOH) (M ¼ Ni, Co). Reproduced with permission from Angew. Chem. Int. Ed., 41, 133–135, copyright 2002 Wiley-VCH, Verlag GmbH and Co., KG Weinheim.

ethanol guest molecules (Figure 10.11b).12 Further work on the Co21 analogue, [Co2(4,4 0 -bipy)3(NO3)4] (guest) (guest ¼ EtOH, MeOH, Me2CO, MeCN, THF, CH2Cl2), which is more symmetrical than the Ni21 analogue, thus simplifying crystallography, investigating the inclusion properties of other guest molecules with varying shape and size showed the system to be fairly flexible.46 Detailed in situ single-crystal diffraction studies revealed intra- and inter-bilayer flexibility to allow inclusion of this range of guests.

10.3.2

Rigid Guest Exchange

Guest exchange in rigid frameworks is very much less complicated than that of flexible systems, as such materials generally show simple adsorption/desorption behaviours that can be easily correlated with structural features. In some respects rigid materials provide an easier path to tailoring guest exchange than dynamic systems, as particular pore size and character can be tailored with the knowledge that these properties will not change with guest influences. A key report on the guest exchange and selectivity within a rigid porous framework was on the Co–metalloporphyrin 3D material PIZAF-1 (Figure 10.9c).42

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The selectivity with regard to hydrophobicity, size and shape of a large range of guests was probed in this system via thermogravimetric monitoring of the desolvation process. Different guests were exposed to the empty host material for periods up to 1 week. The guests that showed the most uptake were water, amines and alcohols. Remarkably, it was demonstrated that the extremely fast response to water compared with other common organic solvents, such as benzene and toluene, results in this material acting as a better desiccant in 1 h than Zeolite 4A in 1 day. In general, for selectivity with regard to alcohols and amines, the longer chain and more bulky analogues showed lower uptake than shorter, less sterically hindered types. Additionally, vapour sorption isotherm measurements on rigid frameworks can aid in the estimation of pore volumes and window sizes. For example, MOF-5 showed uptake of organic vapours of CH2Cl2, CHCl3, CCl4, C6H6 and C6H12 with isotherm shapes reminiscent of zeolites.25 In a novel means of following guest uptake, crystalline samples of MOF-177, which contains a trigonal carboxylate ligand and shows a wide thermal stability window between 100 and 350 1C, were saturated in large polycyclic organic dyes and the colour saturation throughout the crystal was monitored.31 This material contains voids which comprise 81% of the crystal volume; however, some guest selectivity between the dyes was observed. In particular, the largest dye is seen to only penetrate the outer part of the crystal. Additionally, the uptake of C60 was monitored via Raman spectroscopy, which showed a broadening of the fullerene bands due to interactions with the framework surface. This concept of following guest exchange via immersion in coloured solutions was also nicely demonstrated in a 3D coordination polymer derived from a sugar derivative, Zn(D-saccharate).47 This material contains two very different pore environments; one hydrophilic the other hydrophobic. When exposed to I2 vapour, the crystals become deep red and single-crystal analysis revealed that the I2 molecules displace the water molecules located in the hydrophobic channels.

10.4 Gas Storage The safe storage and transport of gases in high concentrations requires compression at high pressures for the gases of interest (e.g. CH4, H2) at room temperature. This process consumes a great deal of energy and does not make for safe conveyance. A solution to this is the use of solid adsorbents as carriers and is useful even at low gas pressures. Porous materials such as zeolites and activated carbons have been used thus far, but each shows limitations. Porous coordination polymers which have a large percentage of uniform microporosity (required for high efficiency) and a degree of tuneability have emerged as excellent alternative candidates for solid gas adsorbents. Gas storage potential is measured through sorption isotherm studies, but interest in developing single-crystal adsorbents has also led to structural studies of gas loaded framework materials. Such structural studies are also important for realising preferential binding sites for the various gas molecules. These will


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be outlined below, but first a few important terms that reoccur throughout this section are presented. When a gas molecule is transferred from the gas phase by a solid surface (including pore surfaces) and adheres to the surface, gas adsorption is said to have occurred. This is in contrast to gas absorption, where the gas molecules are taken into the solid phase (also termed occlusion). There are two ways in which a gas can interact with a solid: first, chemisorption, where gas molecules form chemical bonds with the solid, and second, physisorption, where gas molecules interact with the surface of the solid without forming covalent bonds. The chemisorption process allows very high volume uptakes of gaseous molecules under ambient conditions, but requires energy input for gas release. On the other hand, as the interactions between gas and solid in the physisorption process are weak, the volumes of uptake are general low and occur only at low temperatures. Hence both processes have their drawbacks and indeed, to achieve optimum gas adsorption within solid adsorbents, porous materials are sought which show intermediate strengths of interactions (‘pseudo’-chemisorbents). The degree of interaction between the framework and gas molecule can be quantified by determination of the heat/enthalpy of adsorption, DHads. This value is routinely calculated via collecting adsorption data at two temperatures (e.g. 77 and 87 K) and applying the Clausius–Clapeyron equation. Materials which have strong chemisorption interactions, such as metal hydrides for hydrogen storage, have high DHads values, whereas materials which have weaker physisorption interactions, such as activated carbons and zeolites, show low DHads values. Generally, when the enthalpy of adsorption is 420 kJ mol1 then chemical adsorption is occurring, but this is not a strict value.2 In the search for materials which adequately store gas molecules, intermediate DHads values are sought, such that they adsorb and release gas molecules at workable temperatures and pressures. Indeed, porous coordination polymers have emerged as perfect candidates for the storage and separation of a range of gases, with DHads values nearing the targeted range and showing respectable volumetric uptake. In comparison with other porous materials, the inner surfaces of porous coordination polymers are rich in hydrocarbons and aromatic groups, which are known to attract guest molecules. Additionally, the tuneability of pore character allows optimum properties to be readily targeted. This is particularly important as the gases of interest (i.e. H2, CH4, CO2, C2H4, O2) differ in size, shape and chemical nature, and as such the required pore size, shape and character of the solid adsorbent will also require variation. Examples, divided according to specific gas molecules, will now be outlined, including characteristics identified as important towards attaining greater gas storage and the target parameters for each gas.

10.4.1

Hydrogen Gas Storage

The US Department of Energy has set a short-term design target of 6.0 wt% and 45 g L1 for hydrogen storage materials to be used in automobile fuelling by

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Figure 10.12

Chapter 10

Heat of adsorption values observed for materials which undergo physisorption and those which undergo chemisorption. The target range for useful hydrogen storage is between 10 and 50 kJ mol1.

2010, and a long-term goal of 9.0 wt% and 81 g L1 by 2015.48 An additional goal is to store these amounts of hydrogen near ambient temperatures and reasonable pressures (o100 bar). From a practical perspective, for incorporation into vehicles, potential materials must also be of minimal weight and volume. Of equal importance to volume weight percentage uptake are the kinetics and thermodynamics of hydrogen release and recharge, i.e. the enthalpy of adsorption values (Figure 10.12). Materials such as metal hydrides, which undergo chemisorption of hydrogen gas, generally show DHads values above 50 kJ mol1, which is too high for efficient gas release, whereas materials such as carbon nanotubes, zeolites and activated carbon, which undergo physisorption of hydrogen gas, generally show DHads values below 10 kJ mol1, which means that hydrogen adsorption can only occur at very low temperatures. Porous coordination polymers fall into a class of materials which undergo physisorption of hydrogen molecules; however, owing to the exceptionally high porosity, uniform pore size and hydrogen occupation sites which provide ‘pseudo’-chemisorptiontype interactions, these materials provide perfect candidates for hydrogen storage and release with intermediate DHads values. Indeed, hydrogen storage capability for coordination polymers was first reported in 2003,26 and although these initial findings showed low weight percentage uptake, they suggested good future potential. The first report of a porous MOF which adsorbed hydrogen was Zn4O(1,4bdc) (MOF-5), which showed an uptake of 4.5 wt% at 77 K (pressure o1 atm) and 1.0 wt% at room temperature (20 bar).26,28 Since that time, a multitude of porous coordination polymers have been examined for hydrogen adsorption, with a focus on a complete mechanistic understanding to target greater uptakes and optimal DHads values. The characteristics of importance thus far reported are (1) a large pore volume and surface area for higher loadings, (2) multiple sites for hydrogen–framework contacts such as aromatics and (3) bare metal sites for stronger gas binding. While discussion is still continuing regarding the relative importance of each of these features, key studies have recently been reported which are identifying trends. Such studies and trends will now be addressed. Many studies have been directed towards designing porous frameworks with exceptionally high void volumes that could hold hydrogen gas. Examples include the IRMOF series, which show some of the largest pores reported, and the MIL series, consisting of giant pore systems.40,49 Indeed, many of these


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materials adsorb large amounts of hydrogen and show a good correlation between pore volume and hydrogen uptake capacity.49,50 In particular, a study on members of the MOF series where the pore volume and size were varied with ligand and also the SBUs, showed that hydrogen saturation uptake correlates well with surface area. Although large pores are evidently effective for allowing large volumes of gas to reside, small pores (in the range 4.5–5 A˚) have been shown to provide a better medium for hydrogen to interact efficiently with the surface of the pores. This increased interaction is important for release and uptake at ambient temperatures. One convenient way of generating small pores is through interpenetration of networks. For example, a fourfold interpenetrated framework material containing Zn4O and a binaphthylcarboxylate shows small pores and a surface area of only 502 m2 g1.51 Although this material shows a hydrogen adsorption of only 1.12 wt% at room temperature (48 bar), the surface area is acting much more efficiently compared with materials with a larger surface area. Also important in this study was the recognition that interactions between aromatic rings of the framework pore surface and hydrogen molecules are favourable for greater uptake. This theory of larger areas of aromatic surfaces for increased hydrogen uptake was also investigated in a study of five members of the IRMOF series. In this report, the internal surface area and number of rings in the organic linker were varied systematically and showed a distinct correlation between H2 uptake and aromaticity.28 In a similar report, the pore surface of IRMOFs was further functionalised with groups such as –Br and –NH2 for greater attraction of hydrogen molecules. However, surprisingly, there does not appear to be any direct correlation or enhanced uptake with these groups.24 Whereas initial reports of hydrogen storage in framework materials proposed that it was purely aromatic surface areas towards which hydrogen molecules have an affinity, further structural studies of the real-time loading of hydrogen into a porous coordination polymer have revealed that a preferred loading site at extremely low pressures is a bare metal site.43,52 Importantly, such coordinatively unsaturated metal sites should favour higher binding enthalpies. Additionally, the ability of hydrogen to bind to metal atoms should allow more efficient packing of molecules and thus increased storage capacities. Neutron powder diffraction studies of deuterium loading into the 3D coordination polymer [Cu3(btc)2](H2O)3] (HKUST-1) were carried out to investigate the preferred binding sites directly (Figure 10.13).52 Bare Cu sites in this material are attained via heating, i.e. thermal activation, thus removing the terminal water molecules bound to the paddlewheel cluster. This study showed the most favourable site for D2 occupation near the bare Cu sites. Notably, the deuterium is 2.39 A˚ from the Cu centre, indicating that there is significant interaction. The next favoured site identified was that associated with the aromatic rings in the smaller of the pores, followed by other loadings in the larger pores. In a more recent report on a manganese benzenetristetrazolate coordination polymer, which also contains unsaturated metal sites when thermally activated, a DHads value of 10.1 kJ mol1 was achieved. This value is the highest value

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Figure 10.13

Chapter 10

(a) D2 site occupation in [Cu3(btc)2(H2O)3] determined via neutron diffraction powder diffraction showing D2 as spheres. (b) The location of D2 at a distance of 2.39 A˚ from the axial bare metal sites.

thus far for porous coordination polymers. This material also shows a high uptake of 6.9 wt% at 77 K and 90 bar. This value exceeds the US Department of Energy 2010 target.43 Neutron powder diffraction studies on this material using D2 in place of H2 showed binding to the unsaturated metal sites but also at further non-metal sites within the pores. It is important for further applications that significant amounts of hydrogen are adsorbed at ambient temperatures. There are no porous coordination polymers which retain a sufficient amount of hydrogen under these conditions. A number of materials do show some uptake at room temperature and all of these contain coordinatively unsaturated metal sites. However, the pseudo-chemisorption of hydrogen, in conjunction with ideal pore size and surface area, appears to be the best means for achieving storage at near-ambient temperatures. This is highlighted elegantly in a report on two chromium carboxylate framework materials, which contain btc or 1,4-bdc (MIL-100 and MIL-101, respectively).40 Both materials contain large pore volumes and in the thermally activated state contain unsaturated metal sites. Notably, the surface area of MIL-101 is approximately twice that of MIL-100. The hydrogen uptake profiles for both materials are characteristic of physisorption and show maximum uptakes of 3.3 wt% (MIL-100) and 6.1 wt% (MIL-101) at moderate pressures and 77 K. Furthermore, MIL-101 shows a 0.43 wt% uptake under ambient conditions compared with 0.15 wt% for MIL-100. The DHads values for MIL100 and MIL-101 are 6 and 10 kJ mol1, respectively, the latter being one of the highest reported. While the larger uptake observed in MIL-101 is consistent with the larger pores when compared with MIL-100 and the small uptake at ambient temperature for both is consistent with the presence of bare metal sites, the uptake at ambient temperature is significantly greater for MIL-101.


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Importantly, MIL-101 contains micropores which could house hydrogen with better surface contact than MIL-100, which does not possess such pores, thus probably being the cause of improved hydrogen storage under ambient temperatures. Significantly, although definite trends are appearing for improved hydrogen storage, there are further examples with contradictory results, particularly as the reports of structural results obtained often vary with technique. For example, structural studies of the loading of hydrogen into MOF-5 have been carried out numerous times and show varied results. In one study using inelastic neutron scattering, two sites were identified, namely interactions with the Zn atoms and interactions with the ligand linker molecules. In a further report, sites were identified associated with the ZnO4 clusters only. Variable-temperature single-crystal Laue neutron diffraction studies then revealed congregation of the hydrogen gas in the vicinity of the framework nodes, but in a different position to previous reports.53 Furthermore, differential pair distribution function (PDF) analysis of highenergy X-ray and neutron scattering results on the Prussian Blue analogue MnII3[CoIII(CN)6]2 revealed no evidence of preferential binding to bare metal sites.54 Indeed, this study revealed the hydrogen density to be in the centre of the small pores likely to maximise interactions with the overall surface. Hence care must be taken in experimental procedures and analysis in this relatively new emerging area.

10.4.2

Methane Gas Storage

The use of methane as a potential clean fuel is an attractive prospect which has shown longstanding challenges. The storage of methane on adsorbents is of interest as an alternative to high-pressure compressed gas storage. The volume of methane adsorbed per unit volume of adsorbent is the value of interest for commercial viability. Indeed, activated carbons have achieved the target set by the US Department of Energy of 180 v/v under 35 bar and near ambient temperature. However, they are not commercially feasible owing to refuelling issues. In addition, the energy density of adsorbed gas must be comparable to that of compressed natural gas used currently.55 Initial studies on conventional adsorbents revealed that to achieve high adsorption capacities micropores must be present; the presence of meso- or macropores does not contribute to pore filling. Hence porous coordination polymers for which the pore size can be controlled represent a new future for methane storage. Indeed, early examples of methane storage in porous frameworks showed enhanced capacity compared with conventional materials such as zeolite 5A (83 cm3 g1 at 298 K and 35 bar) and the high surface area activated carbon AX-21. Since then, many other examples have been reported with even greater storage capacities where work has been focused towards designing the ideal pore size for the greatest methane framework interactions. There have been far fewer studies on methane storage in framework materials than hydrogen

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storage; however, a few trends are starting to emerge regarding the conditions required for high uptake. Methane storage was studied in [CuSiF6(4,4 0 -bipy)2], a pillared layered system where square grids of [Cu(4,4 0 -bipy)2] are bridged by SiF2 6 ions to form a 3D network.56 This material contains micropores of dimensions 8 8 A˚, which in the as-synthesised phase are filled with water guest molecules. This material is stable in the empty state. At high pressures, the methane adsorption (134 cm3 g1 at 298 K and 35 bar) is much higher than that of zeolite 5A. Further to this, the Cu(terephthalate) dimer, when bridged with triethylenediamine, results in a 3D material with uniform micropores.33 This material shows a high methane uptake of 212 cm3 g1 (at 298 K and 34 bar); higher than the theoretical uptake of activated carbons. The series of IRMOFs have also emerged as excellent candidates for high storage volumes of methane. Central to their use as methane storage vessels, IRMOFs are thermally stable to high temperatures and remain crystalline in the desolvated state. IRMOF-6, which contains a cyclobutane–bdc derivative linker, shows an exceptionally high surface area of 2630 m2 g1 and an uptake of 240 cm3 g1 of methane at 298 K and 35 bar.24 This high methane uptake capacity exceeds that of zeolite 5A and other porous frameworks. At this pressure, this material shows 70% of the amount stored in standard compressed methane cylinders – a remarkable advance. Finally, it has been proposed that large aromatic surface areas will promote greater interaction between methane and the framework. This was investigated in the 3D network material which contains the highly aromatic ligand 5,5-(anthracene-9,10-diyl)diisophthalate and the copper paddlewheel SBU.36 This material shows an absolute methane-adsorption capacity of 230 v/v within the cage-like pores. Importantly, this material shows a heat of adsorption of 30 kJ mol1, which indicates a strong methane–framework interaction and suggests that the large aromatic content may contribute to this property.

10.4.3

Carbon Dioxide Gas Storage

The development of materials which store carbon dioxide is directed towards capturing the gas from industrial exhausts, such as plant flues. This would have a direct impact on reducing emissions. Current techniques used for CO2, such as chilling and pressurising the exhaust, are not cost effective and are inefficient. Alternative porous materials have also been investigated, such as silicates and carbons, but as they involve chemisorption of CO2 molecules they do not have long-term viability. For CO2 storage to be achieved, materials which reversibly store large volumes must be developed. Yet again, porous coordination polymers provide this alternative path to CO2 storage and show great promise already. In situ single-crystal diffraction studies of gas sorption were carried out on the 1D coordination polymer [Rh2(O2CPh)4(pyz)].57 There are no channels defined by the packing of these chains, only empty cages which are linked by narrow gaps (1 A˚). However, in the presence of CO2, the crystal undergoes a


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phase change to reduced symmetry, resulting in the generation of 1D channels. Although this structure is low-dimensional, this CO2 adsorbed state is stabilised by interactions between the gas and channel surface. The amount of CO2 gas adsorbed at ambient temperature is 23.7 cm3 (STP) g1 [37.4 cm3 (STP) cm3], which is equivalent to 0.81 CO2 molecules per Rh2 unit. As for hydrogen and methane storage, MOFs show the ability to store large amounts of carbon dioxide. In particular, the 4,40 ,400 -benzene-1,3,5-triyltribenzoic acid-containing MOF (MOF-177) shows a high capacity of 33.5 mmol g1 (147 wt%).58 This material has a large surface area (4500 m2 g–1). This uptake equates to a ninefold increase in CO2 uptake in a container where MOF-177 is present compared with when there is no solid adsorbent. Further comparison of these values with the benchmark materials zeolite 13X and MAXSORB, showing uptakes of 7.4 and 25 mmol g1, respectively, reveals significantly increased uptakes.

10.4.4

Other Gases

The trapping of oxygen molecules in porous coordination polymers is interesting as their magnetic and redox properties may be altered in the confined space. An early example of direct structural observation of O2 molecules physisorbed in channels is the pillared layered material [Cu2(pzdc)2(pyz)].59 In situ high-resolution synchrotron powder X-ray diffraction measurements revealed 1D ladder structure arrays of O2 molecules with distances close to those for solid O2. In addition, the 1D chain system [Rh2(O2CPh)4(pyz)], in addition to showing CO2 adsorption, shows O2 adsorption properties. Indeed, single-crystal analysis of the O2 sorbed material below 90 K revealed single-molecule chain oxygen within the host lattice.60 At higher temperatures, the position of oxygen could not be determined due to the increased thermal motion of the guest molecules. Further slight modification of the channel surface through incorporation of a methyl-substituted pyrazine group resulted in structural characterisation of the oxygen even at room temperature, probably due to enhanced contact between the host and gas molecules. Another gas of interest for adsorption into framework materials is acetylene, owing to its high reactivity and inability to be compressed above 0.2 MPa. Indeed, the material [Cu2(pzdc)2(pyz)] also shows a fast and high uptake of acetylene of 42 cm3 (at STP).61 Structural analysis of this material with adsorbed acetylene molecules using PXRD methods showed one gas molecule per pore nicely separated in 1D channels. Further to this, the possibility of selectively including gases is an exciting possibility for framework material where such materials may act as gas sensors or be used to purify gases. Indeed, whereas [Cu2(pzdc)2(pyz)] shows a fast uptake of acetylene, it shows a gradual adsorption of CO2).61 The enthalpy of adsorption values for C2H4 compared with CO2 uptake indicate a better interaction with the former, thus suited for gas separations. Such separations between these two gases are important in order to purify the acetylene, which is often contaminated by CO2.

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Other examples of coordination polymers which selectively separate gases are [Er2(pda)3(H2O)2] (pda ¼ phenylene-1,4-diacetic acid), which absorbs CO2 but not Ar or N2, and [Mn(HCO2)2] 1/3(dioxane), which adsorbs H2 and CO2 but not N2 and other large diameter gases.62,63 There are a handful of other examples not mentioned here; however, gas separation experiments are certainly an area which deserves more attention in the future for porous coordination materials.

10.5 Ion Exchange Outlined in Chapter 5 is the early work by Hoskins and Robson on the diamond-like net [CuI{C(C6H4.CN4)}]1.64 The initially synthesized material contains BF4– anions in the large pores; they were unable to be located in crystal structure analysis but their presence was confirmed by IR spectroscopy. In stipulating the use of coordination polymers for zeolite-like applications, such as ion exchange, the crystals were soaked in a solution containing PF 6 anions and showed complete ion exchange, confirmed also via IR spectroscopy. Many more examples of ion exchange followed this first example where the sample was immersed in solution. Further reports on the 1D chain material [Ag(4,4 0 bipy)(NO3)], where anion exchange was shown via immersion in aqueous solutions containing PF6, BF4 and SO42, revealed a solvent-mediated process rather than a solid-state mechanism.65 This highlights that care needs to be taken with this approach so that dissolution of one crystalline phase and precipitation of a new phase do not occur. In another example, the anion-exchange process was monitored via singlecrystal analysis. The 1D chain material [Ag(N,N 0 -bis(3-pyridinecarboxamine)1,6-hexane] (ClO4), whose chains are held together in the solid state via complementary amide hydrogen bonds to afford ‘pseudo’-2D sheets, shows selective anion exchange.66 This material, which was also synthesised with nitrate or triflate anions, shows a highly selective conversion between only the NO 3 and CF3SO 3 to ClO4 analogues. This selectivity is described based on the hydration energy of each anion and the energy required for the structural reorganisations. In a novel approach, the bimodal porous coordination polymer [Ni(bpe)2{N(CN)2}][N(CN)2] 5(H2O) uses anion exchange as a means to control sorption properties.67 The robust interpenetrated 3D a-Po-type framework houses two distinct pores; one where the water is located and one where the free dicyanamide anion is located. The free N(CN)2 anions can be selectively exchanged for smaller N3 anions, resulting in an increase in effective pore size as the interpenetrated nets shift in relative position. The N2-exchanged network adsorbs more CO2 than the as-synthesised material (Figure 10.14).

10.6 Multifunctional Porous Materials Although materials that display a porous nature can be utilised specifically for this property alone, i.e. gas and solvent storage, the incorporation of additional


Figure 10.14

339

Illustration of pore size variation with selective anion exchange from N(CN)2 to N3 in [Ni(bpe)2(N(CN)2][N(CN)2] 5(H2O) and CO2 isotherm comparison of the as-synthesised and anion exchanged materials. Reprinted with permission from Nature Publishing Group, Nat. Mater., 6, 142–148, copyright (2007).

functionality may lead to physical properties which can be influenced based on guest molecule inclusion and exclusion. A few examples of multifunctional materials which include porosity will now be presented; see Chapter 13 for further discussion of multifunctional materials. The following few examples highlight the magnetic molecular sensing capabilities of porous multifunctional materials. Although such behaviour has been reported for discrete systems, the robust nature of porous coordination polymers, allowing them to house a range of guests, makes them improved candidates (Chapter 9). In the nanoporous coordination polymer [Fe2(4,4 0 -azpy)4(NCS)4] (EtOH), consisting of a doubly interpenetrated (4,4) net, the presence or absence of specific solvent molecules in the 1D channels dramatically influences the magnetic properties.13 The iron(II) spin crossover centres in the framework can be switched from ‘on’ to ‘off’ purely via guest desorption and can be tuned via guest exchange. Additionally, a 3D coordination polymer constructed from Cr31, bridging terephthalate and doubly bridging hydroxides, contains 1D channels filled with water.68 Upon guest removal and uptake, there is an expansion and shrinking of the pores driven by solvent–host and host–host hydrogen bonding interactions. This material acts as a magnetic sensor with shift in the Neél temperature from 65 K in the solvated state to 55 K in the desolvated state. Other physical properties, such as thermochromism and luminescence, may be modified via guest exchange, resulting in an alternative path to producing molecular sensing materials. This concept is demonstrated nicely in the doubly interpenetrated (3,6)-connected network [Zn4O(NTB)2] 3DEF EtOH (NTB ¼ nitrilo-4,4 0 ,400 -trisbenzoic acid), which shows guest-dependent blue luminescence.69 The host material, which is stable in the empty state to high

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temperatures, displays photoluminescence which is sensitive to the presence or absence of guest molecules. Furthermore, the photoluminescence of this material is sensitive to a range of different guest molecules. The difference in luminescence spectra is most likely a result of variations in the p–p interactions of the various solvated materials. In a further example, the IRMOF containing trans-stilbene-4,4 0 -dicarboxylic acid shows an emission spectrum which is sensitive to solvent exchange.70 Specifically, the emission spectra show a reversible increase in peak maxima from hexane to chloroform to toluene. Additionally, the 3D network described above, [Ni(bpe)2(N(CN)2)](N(CN)2) 5(H2O), which displays selective anion exchange, shows a colour change from violet to green during the exchange process.67 As the selective anion exchange results in a concomitant increase in pore volume, there is potential for applications in gas separation and sensing. Lastly, the framework of a pillared layered bilayer material comprised of a bismacrocyclic nickel complex and btc, can undergo a redox reaction when the crystals are reacted with iodine.71 The redox reaction, involving a single-crystal transformation, results in a positively charged framework of Ni21 and Ni31 and channels that include both I2 and I3. Photographs of the crystal during this process were taken, showing distinct colour and morphology changes. The retention of single crystallinity after a chemical reaction has occurred is rare and important for device development.

10.7 Conclusion The field of porous coordination polymer chemistry has come a long way in the past two decades, which is self-evident from the increasing numbers of reports and ways of consistently obtaining permanent porosity, be it flexible or rigid in nature. One key factor that gives porous frameworks strength over other porous materials is the building block method by which they are constructed. The selfassembly of discrete components allows the synthetic chemist to add the required attributes directly and be fairly assured that they will be retained in the final product. This approach has led to a number of important series of porous materials where the pore size, shape and chemical nature can be systematically tuned; a very important step for the second and third generation of materials which are focused on porous applications. Coordination polymers in such porous applications as gas storage and molecular separations have extended past the level at which other porous materials held the records, and in some cases outreached government initiatives set for future years. Indeed, within the wide range of porous applications which coordination polymers now find, there are examples that show true possible future applications within the biological world, e.g. for targeted drug delivery, and the physical world, e.g. for molecular sensing, gas separations and electronic devices.72–74 Further to this, the large number of early examples of ‘porous’ coordination polymers, which have not truly been examined, will certainly provide interesting results upon reinvestigation.


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It is important to realise that we have only touched the surface of the scope of porous metal–organic frameworks and are still discovering new facets with each new example reported. The amazing interest which this field has created will certainly result in new and remarkable applications and examples outwith the reaches of our current imagination.

References 1. L.J. Barbour, Chem. Commun., 2006, 1163–1168. 2. S. Takamizawa, Making Crystals by Design: Nanoporosity, Gas Storage, Gas Sensing, Wiley-VCH, Weinheim, 2007. 3. M.J. Rosseinsky, Microporous Mesoporous Mater., 2004, 73, 15–30. 4. J.L.C. Rowsell and O.M. Yaghi, Microporous Mesoporous Mater., 2004, 73, 3–14. 5. A.J. Fletcher, K.M. Thomas and M.J. Rosseinsky, J. Solid State Chem., 2005, 178, 2491–2510. 6. S. Kitagawa and M. Kondo, Bull. Chem. Soc. Jpn., 1998, 71, 1739. 7. R. Kitaura, K. Seki, G. Akiyama and S. Kitagawa, Angew. Chem. Int. Ed., 2003, 42, 428. 8. S. Takamizawa, T. Hiroki, E. Nakata, K. Mochizuki and W. Mori, Chem. Lett., 2002, 1208–1209. 9. C.-D. Wu and W. Lin, Angew. Chem. Int. Ed., 2005, 44, 1958–1961. 10. K. Uemura, S. Kitagawa, K. Fukui and K. Saito, J. Am. Chem. Soc., 2004, 126, 3817–3828. 11. S.R. Batten, B.F. Hoskins, B. Moubaraki, K.S. Murray and R. Robson, Chem. Commun., 2000, 1095–1096. 12. C.J. Kepert and M.J. Rosseinsky, Chem. Commun., 1999, 375–376. 13. G.J. Halder, C.J. Kepert, B. Moubaraki, K.S. Murray and J.D. Cashion, Science, 2002, 298, 1762–1765. 14. S.M. Neville, B. Moubaraki, K.S. Murray and C.J. Kepert, Angew. Chem. Int. Ed., 2007, 46, 2059–2062. 15. M. Kondo, T. Okubo, A. Asami, S.-I. Noro, T. Yoshitomi, S. Kitagawa, T. Ishii, H. Matsuzaka and K. Seki, Angew. Chem. Int. Ed., 1999, 38, 140–143. 16. R. Kitaura, K. Fujimoto, S. Noro, M. Kondo and S. Kitagawa, Angew. Chem. Int. Ed., 2002, 41, 133–135. 17. T.K. Maji, K. Uemura, H.-C. Chang, R. Matsuda and S. Kitagawa, Angew. Chem. Int. Ed., 2004, 43, 3269–3272. 18. M. Kurmoo, H. Kumagai, S.M. Hughes and C.J. Kepert, Inorg. Chem., 2003, 42, 6709–6722. 19. A. Rujiwatra, C.J. Kepert, J.B. Claridge, M.J. Rosseinsky, H. Kumagai and M. Kurmoo, J. Am. Chem. Soc., 2001, 123, 10584. 20. K. Biradha and M. Fujita, Angew. Chem. Int. Ed., 2002, 41, 3392–3395. 21. D.W. Smithenry, S.R. Wilson and K.S. Suslick, Inorg. Chem., 2003, 42, 7719–7721.

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CHAPTER 11

Acentric and Chiral Networks 11.1 Introduction One property of a crystal structure that should lend itself well to crystal engineering is its symmetry. Structures which are acentric, i.e. do not contain a centre of symmetry, are of particular interest. This symmetry (or rather, lack of symmetry) is important as the following properties require acentric materials:1 Second-order non-linear optical (NLO) behaviour Application of a laser to a material that shows second-harmonic generation (SHG) results in the doubling of the frequency of that light (or a halving of its wavelength). A typical example is the shining of invisible 1064 nm light from an Nd:YAG laser on a sample, resulting in its conversion to visible (green) 532 nm light. Piezoelectricity In these materials, application of a mechanical force induces electrical polarisation or, alternatively, application of an electrical field causes macroscopic strain. These materials are used in a wide range of applications to convert electrical energy into mechanical energy or vice versa. Pyroelectricity A pyroelectric material is one in which there is a change in the spontaneous polarisation with temperature. For this property, the structure must not just be acentric, but also possess a polar space group. Ferroelectricity Ferroelectric materials are pyroelectric materials in which the polarisation is reversible. These materials must not only be polar, but also contain a permanent dipole moment capable of being reversed upon application of a voltage. Chirality Some space groups are not just acentric, but chiral. Materials with these properties have a number of applications which will be discussed in Section 11.3. Of all these properties, the most work has been done with NLO and chiral materials, and these will form the basis of the discussion that follows. One exception is the structure of KCo[Au(CN)2]3, which crystallises in the space Coordination Polymers: Design, Analysis and Application By Stuart R. Batten, Suzanne M. Neville and David R. Turner r Stuart R. Batten, Suzanne M. Neville and David R. Turner, 2009 Published by the Royal Society of Chemistry, www.rsc.org

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group P312, contains three interpenetrating a-Po networks and shows piezoelectric properties.2 The 3D polymer [Cu(NH3)2]2[Mo(CN)8] has been reported to be pyroelectric,3 while ferroelectric properties have been reported for a handful of coordination polymers.4 The response of the dielectric properties of a porous material to a range of guests has also been examined.5

11.2 Acentric Networks for Non-linear Optical Behaviour Non-linear optical materials are of significant interest for applications in photonic technology.6 For this reason, considerable research has been undertaken on new organic-based molecular NLO materials. Ideally, for a good SHG response a material needs to be non-centrosymmetric and have a large polarisability. Hence the organic materials best suited for this purpose have an electron donor and acceptor separated by a conjugated bridge. This, however, makes the molecules highly dipolar and leads to centrosymmetric packing of molecules as they tend to align with their dipoles antiparallel. Organic materials can also suffer from chemical or thermal instability. Thus commercially used materials remain inorganic compounds such as potassium dihydrogenphosphate (KDP) and lithium niobate. Coordination polymers, however, provide significant advantages for the design of new SHG active materials.7 The structural direction provided by a network of coordination bonds can overcome unfavourable dipole–dipole interactions and coordination polymers can display high thermal and chemical robustness (KDP, for example, is hygroscopic). The network approach to design also permits the targeting of specific acentric topologies, as described below. SHG response is usually measured (and quoted) relative to existing standards such as quartz, urea or KDP. Care should be exercised in interpreting these values as they differ according to the standard used. For example, the efficiency of KDP is around 10 times that of a-quartz. Evans and Lin have outlined a very effective strategy for the design of suitable acentric materials based on the diamond net.7 The diamond net contains non-centric tetrahedral nodes and is therefore not predisposed to the formation of centrosymmetric structures, especially if acentric ligands are used in their assembly (as is likely for SHG materials in which polarisable components are desirable). The diamond net is also a common network topology observed and is therefore fairly predictable. It is also likely to form with metals such as ZnII and CdII, which have the added advantage of filled d shells, meaning that optical losses from d - d transitions are prevented. One problem with this design strategy, however, is the formation of interpenetrating nets. Although individual nets may be acentric, interpenetration may produce pairs of nets that are related by a centre of symmetry. Thus an even number of interpenetrating nets may produce an overall centrosymmetric structure and therefore it is desirable to target structures with odd numbers of interpenetrating nets. Fortunately, one of the main determining factors for the

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O L5

O

O

N L1

O

O N L6

N

O

O L2

O O

O O

N

N L3

L7

O

O N

O L4

S N

O

L8

O N N

O L9

Figure 11.1

O

A series of asymmetric pyridylcarboxylate ligands used to construct SHG active coordination polymers.7

number of nets formed is the length of the bridging ligand and hence the degree of interpenetration can be controlled by varying the ligand length. To exploit this approach, Lin’s group embarked on a study of ZnII and CdII coordination polymers of linear pyridylcarboxylate ligands, such as those shown in Figure 11.1.7 The ligands chosen were rigid, to minimize alternative packing arrangements (Chapter 4), were fully conjugated and unsymmetrical and therefore were excellent building blocks for the desired products. Significantly, many of these materials were synthesized indirectly, through hydrothermal in situ conversion of the nitrile ligand analogues into the carboxylate ligands before growth of the crystals (Chapter 4). As a typical example, reaction of 4-cyanopyridine with zinc perchlorate in ethanol–water under solvothermal conditions (130 1C) for 48 h results in the formation of colourless crystals of Zn(L1)2.8 The structure forms the expected diamond nets (Figure 11.2) and the odd number of interpenetrating nets means that it crystallises in a non-centric (and chiral) space group (P212121). Furthermore, the compound shows moderate SHG activity (1.5 times that of quartz). Analogous reactions with CdII, however, result in the formation of ethanol solvates in which only two interpenetrating nets are formed. Similarly, the

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Figure 11.2

Chapter 11

An adamantane cavity of one of the three interpenetrating diamond nets in Zn(L1)2.8

presence of pyrazine in the reaction mixture also results in two nets. The nets in both structures are related a centre of symmetry and therefore the overall structures have the centrosymmetric space group Pbca and hence no SHG activity.9 Longer ligands generate higher degrees of interpenetration. The ligand L2 with ZnII or CdII gives five interpenetrating nets in the space group Cc; SHG activities are 126 and 18 times that of quartz, respectively.10 Reaction of ligand L3 with ZnII results in five interpenetrating nets; however, in this case the nodes are Zn2(m-OH) dimers and two of the links between the nodes are each provided by pairs of ligands related by an inversion centre.10 As a result, the structure is centrosymmetric, despite the odd number of interpenetrating nets. By contrast, reaction of the same ligand with CdII gives seven interpenetrating nets, the space group Ia and an SHG activity 310 times that of quartz. Use of the even longer ligand L4 gives eight interpenetrating diamond networks, with the ZnII centres adopting both tetrahedral and octahedral geometries and the CdII centres adopting only octahedral geometry.11 Despite the even degree of interpenetration, the structures adopt the chiral C2 space group and display SHG activities 310 and 345 times that of quartz, respectively. Although individual results vary, it is noteworthy that the SHG efficiency, in general terms, seems to increase with increase in the number of interpenetrating networks. Lower dimensional acentric structures can also be formed, using angular m-pyridylcarboxylates.7 Reaction of the ligands L5–L9 results in a series of structures containing (4,4) 2D sheets. While a number of these are non-centric, control over this property is more limited than it is for the 3D diamondoid structures. While individual nets may be acentric, adjacent or interpenetrating nets may be related by inversion centres.

Acentric and Chiral Networks

Figure 11.3

349

Three interpenetrating (4,4) sheets in the structure Cd(L7)2.9

Reaction of L5 with ZnII gives chiral sheets which show a moderate SHG response.12 By contrast, CdII gives a centrosymmetric 3D structure, showing that sheets are not guaranteed.13 Reaction of L7 with ZnII results in a remarkable structure containing four crystallographically independent grids.13 Two interpenetrate and the other two are single sheets. The structure crystallises in the space group Cc and shows high SHG efficiency (400 times that of quartz). Even more remarkable is the compound obtained from the reaction of this ligand with CdII.12 It contains (4,4) sheets (Figure 11.3) in the space group Fdd2 which show threefold parallel 2D - 2D interpenetration. It shows an SHG efficiency 800 times that of quartz. By comparison, the commercially important LiNbO3 shows a relative efficiency of only 600 times that of quartz. A number of other examples of SHG response from coordination polymers of pyridylcarboxylates have also been reported.14 A related structure with ligand L4 has been shown to be the first chiral octupolar coordination polymer.15 It contains Cd3(m3-OH) trimers linked by the ligands into a 2D (3,6) net (Figure 11.4) and shows a powder SHG intensity of 130 times that of quartz. Hydrothermal synthesis has also been used to synthesise in situ novel tetrazole ligands. These then give diamond or (4,4) sheet networks with zinc, with the best response being a sheet structure containing a multicentre acceptor–donor ligand, which gives an SHG response of ca. 50 times that of urea (or ca. 500 times that of KDP).16 A similar ligand has given 1D chains with an SHG response 80 times that of urea.17 The same group has also reported

350

Chapter 11

Figure 11.4

The (3,6) sheet structure of [Cd(m3-OH)(pyridine)6(L4)3](ClO4)2.12 The L4 ligands are badly disordered and therefore for clarity they are just represented as a link between the nitrogen and carboxylate donors.

Figure 11.5

The acentric Cd(SCN)3 1D chain motif.17

a 2D structure with 3,5-nitrotyrosine with an SHG response six times that of urea.18 Despite the example above, the formation of acentric structures from 1D polymers is even harder to control than 2D or 3D assemblies, although some strategies have been explored with success.19 The targeting of 1D Cd(SCN)3 chains has been explored for SHG active materials.20 The chains consist of Cd atoms linked by triple M(SCN)3M bridges (Figure 11.5) and are inherently asymmetric as the metal atoms have CdS3N3 coordination environments, with like donors in a facial arrangement. The chains also contain an extended p-conjugated system, with highly polarisable metals and ligands, which makes them attractive targets. However, the relative alignments of the chains are dependent on the counter cation and in particular its shape and symmetry. With [(18C6)K]1 (18C6 ¼ 18-crown–6), the chains align parallel to give an acentric structure with SHG activity 200 times that of quartz.21 With


Figure 11.6

351

An adamantane cavity in the diamond like net of ZnCd(SCN)4.20

[(18C6)2Na2(H2O)2]21 cations, antiparallel alignment of the chains is observed, with generation of a centrosymmetric structure. Other thiocyanate structures have also been observed to give SHG active materials. CdHg(SCN)4, which has a 3D network, shows an SHG response comparable to that of LiNbO3.22 ZnCd(SCN)4 has an analogous structure (Figure 11.6) and shows an SHG response similar to that of urea.23 The related selenocyanate structures ZnCd(SeCN)4 and CdHg(SeCN)4 show activities 45 and 25 times that of urea, respectively.24 Finally, another small cyano anion, carbamyldicyanomethanide [cda, C(CN)2(CONH2)] has been shown to form an SHG active complex with Eu. The compound Eu(cda)3(H2O)3.H2O has a 2D structure and shows a response 16.8 times that of urea.25 Another way of generating acentric structures is the inclusion of acentric guests or counterions into the structure. Inclusion of the NLO-active chromophore 2-nitroaniline into a cadmium 4,4 0 -bipyridine framework results in the formation of an SHG active material.26 Incorporation of a series of stilbazoliumshaped cationic chromophores into anionic metal oxalate frameworks results in materials that are not only NLO active, but also ferromagnetically ordered.27 Third -order NLO activity has also been studied for coordination polymers.28 However, as these measurements are generally done by dissolving the polymers in DMF, it is difficult to draw correlations between the solid-state structures and the behaviour of solution species, especially in a strongly coordinating solvent such as DMF. Very large solid-state third-order NLO responses have, however, been reported for a series of 1D halide-bridge Ni polymers.29

352

Chapter 11

11.3 Chiral Networks Chirality is of immense interest to many chemists and the crystal engineer is no exception.30–34 Chiral networks have possible applications as enantioselective catalysts or for the separation of enantiomers from racemic solutions through enantioselective adsorption. Coordination polymers are particularly attractive for these applications as there are only two chiral zeolites known (zeolite b and titanosilicate ETS-10) and they are difficult to isolate enantiomerically pure.35,36 Such materials can be realised from either achiral components or, more reliably, through the use of chiral ligands. For achiral components, chirality can be induced by the geometry of the polymer, the arrangement of asymmetric ligands or by the formation of an inherently chiral network topology. However, one of the challenges for this approach (and for the use of racemic mixtures of chiral ligands) is the formation of bulk samples of crystals in which the polymers are all of the same hand, or the macroscopic resolution of crystals of the same hand (usually racemic mixtures of crystals are formed). Crystals may also be twinned, containing equal amounts of either hand within each individual crystal.

11.3.1

1D Helices from Achiral Components

The most obvious chiral 1D arrangement is the helix, either single or multiply stranded. However, with low-dimensional chiral systems, as discussed above for other acentric structures, the formation of an overall chiral structure requires that all polymers (helices) are of the same hand. Thus the interactions between the helices should favour a homochiral packing arrangement rather than a heterochiral one. In the structure of Ni(4,4 0 -bipy)(C6H4CO2)2(MeOH)2, where 4,4 0 -bipy ¼ 4, 0 4 -bipyridine, there are helical 1D chains which twist around 41 or 43 helices (Figure 11.7).37 The helices pack in such a way that C–H p interactions are formed between the chains. This is an important contributing factor in the formation of helices of only one hand within the structure. Analysis of several crystals, however, revealed that the product contains a random mixture of both left- and right-handed crystals. Large chiral cavities are formed between the helices which can contain a variety of aromatic guest molecules. Similarly, hydrogen bonding between chains is an important factor in inducing homochiral packing of 1D chains of dimers and trimers bridged by azide ligands.38 Chirality is induced in the chains through the twisted conformations of diazine co-ligands which also help generate the dimeric and trimeric clusters. The two forms of [AgL](CF3SO3), where L ¼ 2-pyridinyl-3-pyridinylmethanone, illustrate the uncertainty of using achiral components.39 One form contains 21 helices of opposite chirality, to give an overall racemate. The other form contains 41 helices of only a single hand, with argentophilic interchain interactions playing an important role in the homochiral assembly (but not in the racemate). The formation of the two supramolecular isomers is dependent on the reaction


Figure 11.7

353

The 1D helical chains of Ni(4,4 0 -bipy)(C6H4CO2)2(MeOH)2; to highlight the helix, only metal atoms and 4,4 0 -bipy ligands are shown.34

conditions, including the solvent medium chosen. Once again, however, individual crystals of the chiral structure randomly display either enantiomer. One remarkable system that does show macroscopic resolution of chiral crystals constructed from achiral components is the structure of CdL(NO3)2 H2O EtOH, where L ¼ 5-(9-anthracenyl)pyrimidine.40 The crystal structure consists of 1D chains of Cd atoms bridged by the pyrimidine ligands; the chirality is induced by the twisting of the pyrimidine rings about the Cd–Cd connection (Figure 11.8). The helices are reinforced by intrastrand hydrogen bonding and homochiral packing of helices is also stabilised by interstrand hydrogen bonding. Furthermore, it appears that individual colonies of crystals all display the same chirality, as shown by circular dichroism (CD) studies. All crystals from a single crystallisation experiment show the same CD signal. Since the crystals grow rapidly as a single colony, it is thought that they all take the handedness of the first appearing nucleus (the handedness of which is governed by chance). This is further confirmed by seeding experiments which induce the formation of only crystals of the same chirality as the chosen seed. In the absence of ethanol in the reaction, an achiral structure, containing non-helical zigzag chains, is formed. This achiral structure can also be obtained by leaving the chiral structure in the open atmosphere for 3–4 days, resulting in a loss of ethanol ligand and absorption of water molecules. Interestingly, if this achiral sample is then placed in the presence of ethanol vapour, the chiral structure is regenerated and it has the same handedness as the original sample. A similar homochiral crystal growth was observed for crystals of helical chains formed by reaction of 2,5-diphenyl-3,4-di(3-pyridyl)cyclopenta-2,

354

Chapter 11

Figure 11.8 The helical chain formed in the structure of CdL(NO3)2 H2O EtOH,

where L ¼ 5-(9-anthracenyl)pyrimidine.37 Water, ethanol and nitrate ligands are omitted to highlight the helical core.

4-dien-1-one with ZnCl2 or HgCl2.41 Again, the system shows spontaneous chiral resolution, with colonies of homochiral crystals being formed. This was confirmed by the Cotton effects in their CD spectra.

11.3.2

2D Chiral Nets from Achiral Components

Formation of bulk homochiral samples from achiral components has also been observed for 2D networks. The compound Co(PDC)(H2O)2.H2O, where H2PDC ¼ pyridine-2,5-dicarboxylic acid, contains two types of chains which are linked together to form a 2D sheet.42 The chains are of the opposite hand but chemically different and thus an overall chiral sheet is formed. Hydrogen bonding between the layers communicates the chirality of an individual sheet and therefore the overall structure is also chiral. Furthermore, bulk samples are chiral rather than racemic, as evidenced by vibrational CD. Reaction of 2,5-bis(2-pyridyl)-3,4-diazahexa-2,4-diene with MnII and azide in methanol results in two sorts of crystals.38,43 Both contain chiral layers, but


Figure 11.9

355

The chiral (6,3) sheets formed in the structure of [Cu(PPh3)L1.5]ClO4 CHCl3, where L ¼ N,N-(2-pyridyl)(4-pyridylmethyl)amine.41

in the unsolvated form adjacent layers are related by an inversion centre, resulting in an overall racemic structure. The other crystals, however, contain methanol and adjacent layers are related by a 31 screw. Hence the structure is overall chiral. The structure of [Cu(PPh3)L1.5]ClO4 CHCl3, where L ¼ N,N-(2-pyridyl) (4-pyridylmethyl)amine, contains 2D (6,3) sheets.44 The ligands have a kinked, asymmetric bridging mode which results in the formation of large chiral cavities (Figure 11.9). Finally, a series of CuI polymers containing cyanide or iodide and the asymmetric ligand 2-methylpyrazine display chiral 2D or 3D structures.45 Use of the chiral ligand (R)-2-methylpiperazine in analogous reactions also, unsurprisingly, results in a chiral 3D network.

11.3.3

3D Chiral Nets from Achiral Components

As we saw in our discussion on nets (Chapter 2), some network topologies are inherently chiral – for example, (8,3)-a (which contains three-, four- and sixfold helices), (8,3)-d (four-, six- and 12-fold helices), (12,3) (four- and sixfold single helices and sixfold double helices), (10,3)-a (three-, four- and eightfold helices) and quartz (threefold single and sixfold double helices) (Figure 11.10). There are no examples of the first two topologies that we are aware of at present, but the structure of Ni(tpt)(NO3)2, where tpt ¼ 2,4,6-tri(4-pyridyl)-1,3,5-triazine, has a (12,3) net,46 and there are a number of examples of the last two (discussed below and in Chapter 2). Hence achiral, even highly symmetrical ligands can be

356

Figure 11.10

Chapter 11

Inherently chiral 3D nets (from Chapter 2): (a) (8,3)-a (with threefold helices visible); (b) (8,3)-d (four-, six- and 12-fold helices visible; (c) (12,3) (sixfold double helices visible; (d) (10,3)-a (four- and eightfold helices visible); (e) quartz (threefold single and sixfold double helices visible). Note that in each structure each type of helix is all of the same hand.

used to create chiral 3D nets without the need for the formation of local chiral geometries. The 3D nature of the net also allows greater control over the overall chirality of the crystal, although interpenetration means that, as seen for 1D and 2D systems, individual network chirality can negated by the formation of equal numbers of chiral nets of opposite hands. This possibility of a ‘3D racemate’ was first noted by Wells in relation to interpenetrating (10,3)-a nets.47 Of course, the interpenetrating nets could all have the same hand, as observed in some of the examples discussed below, or odd numbers of nets could interpenetrate, resulting in an overall chirality (analogous to the desirability discussed earlier of odd numbers of interpenetrating diamond nets for the formation of acentric structures). Odd numbers of interpenetrating nets, however, have not been


Figure 11.10

Continued.

357

358

Chapter 11 48

observed to date for (10,3)-a nets, and only one example has been reported for quartz (in which the three interpenetrating nets all have the same hand anyway).49 Other notable examples of interpenetrating quartz nets, in which all of the interpenetrating nets have the same hand, include M[Au(CN)2]2, where M ¼ Zn, Co (sixfold homochiral interpenetration),50 and Zn(isonicotinate)2 2H2O and InH(terephthalate)2 (twofold homochiral interpenetration).51 The most commonly encountered chiral topology, however, is the (10,3)-a net. As described above, however, this net can interpenetrate in either a homochiral or heterochiral fashion. For example, the structure of [Zn(tpt)2/3(SiF6)(H2O)2 (MeOH)] contains eight interpenetrating nets, but four are of opposite hand to the other four and thus a racemate is formed.52 A similar situation exists for the two interpenetrating nets in Ag2(2,3-dimethylpyrazine)3(SbF6)2 (Figure 11.11a).53 By contrast, the structure of Ag(hat)ClO4 solvent, where hat ¼ 1,4,5,8,9, 12-hexaazatriphenylene, contains only one net and is therefore chiral.54 The structure of Ag(hmt)](PF6) H2O, where hmt ¼ hexamethylenetetramine, contains two interpenetrating nets of opposite chirality; however, they are chemically different – one is a coordination polymer and the other is a hydrogen-bonded net 55 between intercalated water molecules and PF 6 anions. As a result, this structure is also chiral. Homochiral interpenetration of (10,3)-a nets occurs in a series of related structures with the basic formula Ni3(btc)2(L)6(ROH)x guests, where btc ¼ benzene1,3,5-tricarboxylate, L ¼ pyridine, 3-picoline or 4-picoline, ROH ¼ ethylene glycol, propane-1,2-diol, butane-2,3-diol, butane-1,4-diol, hexane-1,2,6-triol, glycerol, x ¼ usually 3, 4 or 6.33,56–59 Depending on the choice of pyridine and alcohol (which can be either monodentate, bidentate chelating or bidentate bridging), two (Figure 11.11b) or four nets with the same hand are formed. In the ethylene glycol structure, the diols are monodentate and four nets are formed with 28% solvent-accessible volume between the nets. Use of propane1,2-diol instead of ethylene glycol results in the replacement of two monodentate ligands with a single chelating one and the formation of only two nets. This generates 51% solvent-accessible volume in the structure. Furthermore, propane-1,2-diol is chiral and the use of chirally resolved diols allows direction of the handedness of the structures formed. Reactions containing only the (S)-diol result in the formation of networks with anticlockwise helices, whereas the (R)-diol generates the clockwise helical topology. These porous structures also show moderate chiral selectivity towards guests which is size dependent. Thus 1,1 0 -bi-2-naphthol is enantioselectively adsorbed with 8.3% enantiomeric excess (ee), whereas smaller guests such as ethyl 3-hydroxybutyrate and fenchone show no enantioselectivity. When butane-1,4-diol is used in the reaction, four (10,3)-a nets are generated in which pairs of nets are crosslinked by bridging diols, which thus stabilise the structure and generate two chiral 3-connected (62.10)(6.122) nets. Another way to engineer chiral nets is through the use of chiral counterions. This has been shown for the formation of (10,3)-a metal oxalate networks.30,60 When chiral cations such as M(2,2 0 -bipy)321, where 2,2 0 -bipy ¼ 2,2 0 -bipyridine,


Figure 11.11

359

Schematic representations of pairs of (10,3)-a nets showing (a) heterochiral50 and (b) homochiral57 interpenetration.

are used, (10,3)-a nets are formed in which only one enantiomer of the cation is included in each crystal [and thus templates the chirality of the (10,3)-a coordination polymer net].61 These materials also behave as magnets, with ordering temperatures up to 20 K.61–63 Of course, the use of racemic mixtures of starting materials results in a racemic mixture of crystals. Homochiral products, however, can be produced in

360

Chapter 11 30

two ways. Chirally resolved metal complex cations can be used; inert RuII complexes are particularly effective.64–67 Alternatively, resolved tris(oxalato)metalates can be used,66,67 as in the following reaction: 2ðracÞ-½Ruð2; 20 -bipyÞ3 2þ þ D-½CrðoxÞ3 3 þ Liþ ! ½D-Ruð2; 20 -bipyÞ3 ½D-LiD-CrðoxÞ3 þ L-½Ruð2; 20 -bipyÞ3 2þ This strategy has even been used to resolve [Ru(2,2 0 -bipy)2(ppy)]1, where ppy ¼ 2-phenylpyridine, where the classic ‘tartrate method’ failed.66 The other predominant topology for oxalate networks is the (6,3) 2D sheet topology. Unlike (10,3)-b, in this topology adjacent metal centres have different hands. This means that a homometallic network is achiral. Heterometallic networks, however, are chiral. As for the other 2D systems discussed in the previous section, the overall chirality still depends on the relative conformations of adjoining sheets. Achiral structures are common, with D-M1L-M2(ox)3 layers alternating with L-M1D-M2(ox)3 layers. As before, however, reaction of chirally resolved, kinetically inert L- or D-[CrIII(ox)3]3 building blocks with MnII or FeII results in a chiral structure in which all sheets are of the same hand (Figure 11.12).68 A number of other chiral networks have been reported. The structure of Cd(tcm)[B(OMe)4] 1.6MeOH, where tcm ¼ tricyanomethanide, C(CN)3, contains sixfold helices of Cd[B(OMe)4] linked by the tcm ligands.69 All helices are of the same hand and they contain similarly sixfold helical hydrogenbonded chains of intercalated methanol molecules. The structure of

Figure 11.12

A chiral M1M2(oxalate)3 (6,3) sheet.68


361

[Zn(DPT)2(H2O)2][Zn(DPT)2(MeCN)2](ClO4)4 2MeCN, where DPT ¼ 2,4di(4-pyridyl)-1,3,5-triazine, contains two interpenetrating 3D nets of opposite hand.70 However, although the nets are topologically the same, they are chemically different and an overall chiral structure results. A chiral 3D nickel glutarate has been reported that is both porous and shows ferromagnetism.71 Finally, as discussed in Chapter 4, the use of an enantiopure chiral ionic liquid as a reaction medium induces the formation of a chiral structure from achiral components, even though the chiral anionic component of the ionic liquid is not included in the structure.72 The bulk samples appear to be homochiral and the use of the opposite handed ionic liquid induces the formation of the oppositely handed framework.

11.3.4

Chiral Ligands

The most reliable way of obtaining chiral networks is through the use of chiral ligands. The most obvious building blocks are the building blocks of life – amino acids. The structure of Co(L-Glu)(H2O) H2O, where L-Glu ¼ L-glutamate, contains a chiral 3D network with water-filled channels.73 Reaction of L-aspartate with NiCl2 and base under hydrothermal conditions (150 1C for 2 days) results in the formation of homochiral chains of Ni2O(L-Asp) (H2O)2 4H2O.74 If the same reaction is performed with a racemic mixture of L- and D-Asp, conglomerate crystallisation of the L- and D-forms of the polymer results. Furthermore, if a reaction containing only the L-isomer of aspartate is heated at 170 1C for 7 days, thermal racemisation of the aspartic acid occurs and a centrosymmetric 1D structure containing both hands of the amino acid is obtained. Reaction of L-tyrosine with CuII gives Cu(L-Tyr)2, which contains right-handed helical 1D chains.75 An analogous reaction in the presence of 4,4 0 bipyridine also gives helical chains, but in this case they are bridged by the 4,4 0 bipy ligands to give a chiral 2D sheet. Amino acids can also be used as a chiral foundation for larger ligands. Reaction of D- or L-aminoalanine with Cr(CN)63 and MnII results in the formation of chiral 3D networks which show long-range magnetic ordering.76 The copper and zinc 1D coordination polymers of 4-sulfo-L-phenylalanine are soluble in water and the zinc complex shows SHG activity comparable to urea.77 Hydrothermal reaction of (S)-3-cyanophenylalanine with azide and ZnCl2 or CdCl2 results in the formation of chiral 3D SrAl2 topology networks containing the (S)-5-(3-tetrazoyl)phenylalanine ligand.78 The zinc polymer of (S)-1-phophonomethylproline contains a chiral 3D open network.79 Reaction of N-(2-hydroxybenzyl)-L-glutamic acid with NiII gives tube-like 1D helices which contain hydrogen-bonded helices of water molecules, whereas the 1D CuII polymer has a zigzag geometry.80 The synthetic amino acid derivative 6-methoxy-(8S,9R)-cinchonan-9-ol-3-carboxylic acid gives 1D chains with ZnCl2 that show an SHG response 20 times that of KDP. Furthermore, measurements on the dielectric properties showed a dipolar chain relaxation process and a high dielectric constant.81

362

Chapter 11 II

The reaction of N-(2-hydroxybenzyl)-L-alanine (H2sala) with Zn generates a structure containing hydrated dimers connected by hydrogen bonding.82 Dehydration of this material generates a chiral 3D coordination polymer. The CuII salt of this ligand contains chiral 1D chains, but again desolvation of this initial hydrated phase generated an anhydrous chiral 3D network.83 Finally, further derivatisation of the sala ligand with chloro or methyl groups in the 5-position of the aromatic ring again results in dimers hydrogen bonded into a 3D net which can be converted into a coordination polymer upon desolvation.84 Unlike the previous examples, however, this process is reversible. A number of chiral carboxylate ligands have also been used to generate chiral coordination polymers. Reaction of S-(–)-lactate with ZnII generates (4,4) sheets containing chiral cavities (Figure 11.13).85 Addition of isonicotinic acid to the reaction generates a different 2D sheet which shows an SHG response 1.2 times that of urea.86 Reaction of tartaric acid with molybdate and LnCl3 generates 1D double helices of molybdate–tartaric acid in which the two strands are interconnected by Ln ions.87 L-Tartaric acid gives left-handed helices, whereas D-tartaric acid generates right-handed helices. In addition to being chiral, these materials are also semiconductors. L-Tartrate was also shown to survive hydrothermal reactions up to 160 1C, to generate robust enantiopure 3D networks with LnIII.88 A related material can also be made

Figure 11.13

The chiral (4,4) sheet structure of Zn[S-(–)-lactate]2.85


363

containing bridging succinate co-ligands. Interestingly, use of racemic D/Ltartrate results in the formation of a different, more condensed structure that contains both ligand enantiomers. A similar situation is observed for InIII polymers of this ligand: In(L-tartrate)H2O 0.5H2O has a 2D structure, whereas In(OH)(D/L-tartrate) 2H2O has a 3D structure.89 Finally, reaction of D-saccharate with ZnII generates a chiral 3D network that is porous and contains both hydrophobic and hydrophilic channels.90 A large and varied series of chiral ligands have been used which are based on the chiral 1,1 0 -bi-2-naphthol (BINOL) core (Figure 11.14).31 These show a variety of substitution positions and coordinating functional groups. Reaction of L10 with AgNO3 or AgClO4 results in the formation of luminescent 2D sheets containing helical chains.91 Reaction of the same ligand with Ni(acac)2(H2O)2 gives triple helices that are further assembled into layers by p p stacking interactions between the naphthol moieties (Figure 11.15a).92 Reaction with CdII generates chiral 1D nets with either a zigzag geometry or a chain containing linked 38-membered rings.93 The ligand L11 gives either 1D chains or 2D (4,4) nets (Figure 11.15b) with ZnII, CdII or NiII; the 2D sheet structures are porous.94 A number of fascinating structures have been obtained with L12. Highlights include 1D nets that show single-crystal to single-crystal transformations upon solvent exchange95 or gas sorption.95,96 The most topologically interesting structures, however, come from the reaction of L12 (or L13) with Ni(acac)2(H2O)2.97 Fivefold helices are formed around 41 axes in which each of the five strands is parallel and the fivefold helix forms a tube-like motif. Furthermore, each ‘nanotube’ interlocks with four of its neighbours, with formation of significant p p interactions. For the crown ether derivative L13 the crowns lie inside the nanotubes. In addition to these pyridyl ligands, carboxylic acid derivatives of BINOL can also be synthesised. With transition metals the ligand L14 gives 2D (4,4) sheets with metal dimer nodes,98 whereas with lanthanoids 3D nets with 6-connected 49.66 topology are obtained (again, the nodes are metal dimers).99 With L15, the ethoxy or methoxy groups of the previous ligands have been replaced with alcohol groups. As a result, the 1D chains or 2D sheets this ligand forms with metal ions are further crosslinked by hydrogen bonding involving these alcohol groups.100,101 This increases the dimensionality of the networks formed and stabilises the structures towards loss of solvent. Finally, three other BINOL-derived ligands have been used to generate chiral chains with interesting structures. The chains formed by reaction of L16 with silver salts run in three different directions in an intimately entangled fashion.102 The silver triflate polymer of L17 contains four different types of chains (supramolecular isomers).103 Two types run along the a axis, one runs along the b axis and one runs along the c axis. Reaction of L18 with lanthanoids results in the formation of chiral 2D layers in which the crown ethers project into the interlayer space.104 A number of other chiral ligand types have also been used to generate interesting chiral materials. Water-soluble silver coordination polymers of

N N

N

N

Cl

O

O

O

O

O

O

O

O O

O

O O

Cl L11

L10

L12

N

L13 N

N N

N

CO2H Cl

Br

HO2C O O

OH OH

Cl

O O

HO2C L

14

O O O

NH

Py

NH

Py

O

Br

CO2H

L15

L16

L17

N

N

HO HO

O P

O

O O HO HO

P O

O O

O EtO P HO

O HO P HO

Cl

O

Cl

O

O

O

O

L19

O

O HO EtO P O

HO HO P

L18

O

L20

L21

N OH HO O P Ph

Ph RuL Cl P X Ru P X Cl Ph Ph HO

P

O OH

O HO P HO

22

HO

H2N X2 = 2dmf or

Ph

H2N

Ph

HO

P O

Ph Ph RuL23 Cl P X Ru P X Cl Ph Ph H2N X2 = 2dmf or H2N

Ph Ph

N

HO

Cl

HO OH OH

L25 R

R = linking group

Cl

L24

OH OH

N

Figure 11.14

A series of chiral BINOL derived ligands used to construct chiral coordination polymers.


Figure 11.15

L-histidine

365

(a) The triple helix formed from the reaction of L10 with Ni(acac)2 (H2O)2;92 (b) the (4,4) sheets formed from the reaction of L11 with CdII.94

and (R)-(+)- and (S)-()-2-pyrrolidone-5-carboxylic acid have been reported and their antifungal and antibacterial properties investigated.105 A (4,4) sheet structure has been reported for a long bis(pyridine) ligand containing a chiral fluorene core; large windows of dimensions 25 25 A˚ are created (Figure 11.16a).106 Aromatic N-heterocyclic ligands containing fused chiral bornane groups produce a series of 1D polymers with silver (Figure 11.16b),107 whereas homochiral triple helices are observed from the reaction of HgCl2 with a bis(pyridine) ligand containing an isomannide core (Figure 11.16c).108 Lee and co-workers have reported a series of coordination polymers containing silver and large, trigonal trinitrile ligands.109 These ligands can be functionalised with pendant groups and thus addition of the chiral myrtanol moiety resulted in a chiral (6,3) network. Finally, a bis (pyridine) ligand containing two chiral centres has been used to produce a

366

Figure 11.16

Chapter 11

(a) Large (4,4) sheets formed by a long bis(pyridine) ligand containing a chiral fluorene core;106 (b) chains formed from the reaction of silver with an aromatic N-heterocyclic ligand containing fused chiral bornane groups;107 (c) triple helices formed by a bis(pyridine) ligand containing a chiral isomannide core.108


367

(centrosymmetric) sheet structure that contains all three diastereomers of the ligand, (R,R), (S,S) and (R,S), in the one structure.110

11.3.5

Applications

As discussed earlier, the main applications of chiral networks are in chiral catalysis and separation of enantiomers through enantioselective absorption. We saw earlier an example of chiral (10,3)-a nets being used to resolve enantiomers of guests and coordinating diols.33,56–59 The chiral (6,3) net shown in Figure 11.9 is also constructed from achiral components and will selectively intercalate enantiomers when crystallized from racemic butan-2-ol.111 The problem with achiral building blocks, of course, is that racemic mixtures of crystals of either hand are usually produced and thus in the second example resolution of the butan-2-ol was achieved only through manual separation of the crystals of either hand with the aid of CD spectra. Use of chiral ligands is a more effective strategy. Thus a 2D sheet organometallic structure containing the chiral 4-vinylbenzylcinchonidinium ligand shows selective intercalation of (R)-butan-2-ol from a racemic mixture, with an ee of 25%.112 The related ligand quitenine [6 0 -methoxy-(8S,9R)cinchonan-9-ol-3-carboxylic acid], prepared from the antimalarial alkaloid quinine, gives a chiral diamond net that shows enantioselective intercalation of (S)-butan-2-ol with an ee value of 98.2%.113 Reaction of quinine itself with CuI halides generates chiral 2D nets which show no enantioselective separation of racemic sec-butylamine, but do show significant ferroelectric properties.114 A number of porous frameworks constructed from the ligands shown in Figure 11.14 also show enantioselective catalysis and absorption. The ligand L19 gives a threefold interpenetrating 3D network with CdII that shows chirally preferential absorption of 1-phenylethanol, but with only a modest enantioenrichment of 6%.115 The phosphate ligands L20 and L21 give a series of 1D, 2D and 3D networks with transition metals116 and lanthanoids.117 The lanthanoid 2D complexes of L20 show catalysis of a number of organic reactions, including cyanosilylation of aldehydes and ring opening of meso-carboxylic anhydrides. Unfortunately, however, the ee values for these reactions were very low (o5%). Similarly, enantioselective absorption experiments with trans-1,2diaminocyclohexane also gave low enantio-enrichments. More significantly, reaction of the ruthenium complexes RuL22 and RuL23 with Zr(O-t-Bu)4 gives two coordination polymers in which it is postulated that the Ru complex lies pendant to the zirconium phosphonate backbones (the crystal structures are unknown).118,119 The materials are porous (as shown by gas sorption experiments) and show very high activity and enantioselectivity for the hydrogenation of b-keto esters and aromatic ketones. The ee values shown are up to 99.2% and in some cases are higher than the corresponding homogeneous catalysts. The catalysts have also been shown to be reusable over several cycles without loss of enantioselectivity.

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24

120,121

Reaction of Cd with L gives a series of 2D or 3D structures. For the 3D structures Ti(O-i-Pr)4 can be diffused into the framework and reacted with the two pendant alcohol groups of the ligand. This generates chiral (BINOL)Ti(O-i-Pr)2 groups, which project into the network cavities and act as catalytic sites for the reaction of ZnEt2 to aromatic aldehydes to give chiral secondary alcohols. The ee values are comparable to those of the homogeneous analogues. The fact that the catalysis takes place within the channels of the structure was demonstrated through the use of dendric aldehydes – yields dropped off significantly with increasing dendrimer size, with the largest dendrimer showing no reaction at all. By contrast, the homogenous catalysts showed no size selectivity. Furthermore, the 2D structures show inclined interpenetration and the fact that catalytic materials are not formed when they are reacted with Ti(O-i-Pr)4 is ascribed to the interpenetration causing the alcohol groups of the BINOL ligands being sterically inaccessible. Ligands containing two linked BINOL groups (of the type L25) have also been shown to give effective heterogeneous catalysts.122,123 The detailed structures of these polymers have, again, not been characterised crystallographically. However, the TiIV complexes show catalysis of the carbonyl–ene reaction with ee values of up to 96%, while the AlIII polymers show similar maximum ee values for the catalysis of Michael reactions. Separation and testing of the supernatant solutions from these reactions showed no activity, confirming the heterogeneous nature of the catalysis. A remarkable structure is obtained from the reaction of a D-tartaric acidderived pyridylcarboxylate ligand with ZnII.124 It contains metal carboxylate trimers linked by pyridyl groups into a 2D network (Figure 11.17). There are a

Figure 11.17

A 2D (6,3) sheet structure formed from the reaction of a D-tartaric acidderived pyridylcarboxylate ligand with ZnII.124


369

number of features of this compound which are of interest. There are free pyridyl groups that project into channels within the structure. These are protonated in the as-synthesised material, but the protons can be exchanged for alkali metal ions such as Na1, K1 and Rb1. Significantly, the material will also exchange the protons for [Ru(2,2 0 -bipy)3]21 in an enantioselective fashion; 80% of the protons are exchanged and the Ru complex is absorbed with an ee value of 66%. The pyridyl groups can also be N-alkylated through reaction of e.g. iodomethane, meaning that the pore size can be deliberately tailored. The material also shows catalysis of a transesterification reaction of an ester in 77% yield, but with a modest ee value of only 8%. The catalysis is also size selective. Finally, reaction of L-lactic acid, benzene-1,4-dicarboxylic acid and ZnII results in a chiral 3D network which is porous (as shown by hydrogen sorption and solvent and guest exchange).125 This material showed very significant sizeselective sorption of small sulfoxides. It also showed highly size- and chemoselective catalytic oxidation of thio ethers to sulfoxides by urea hydroperoxide or H2O2, with yields (with H2O2) being in some cases quantitative. At least 30 catalytic cycles can be performed without loss of oxidation selectivity. Unfortunately, no enantioselectivity was observed in the catalysis, but enantiomerically enriched mixtures could still be obtained due to the enantioselective absorption of reaction products by the polymer.

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CHAPTER 12

Reactive Coordination Polymers 12.1 Introduction Most of the applications discussed to date have been physical properties, where the framework, by and large, is chemically unresponsive (with some notable exceptions). However, coordination polymers can also be chemically reactive; they are not always rigid, chemically inert monoliths. This may take the form of frameworks that undergo reversible or irreversible chemical changes themselves or materials that act as heterogeneous catalysts.

12.2 Topotactic Reactions Structural changes within a framework can take place as a result of a number of stimuli, including loss or exchange of solvents or other included species, heating or cooling and irradiation of light.1 We have already seen a number of instances in previous chapters (e.g. Chapters 4 and 10) where loss or exchange of included species induces a change within the structure. These species (usually solvents) may be coordinated to the metal ions or lie within the lattice and template the structure. Indeed, desolvation or guest exchange is, in itself, a chemical change. For example, the structure of [ZnCu(2,4-pydca)2(H2O)3(DMF)] DMF will exchange both the coordinated and guest DMF molecules with water, to give a chemically different material.2 Like many such experiments, however, it is achieved by immersion in the incoming solvent and therefore it is difficult to say whether it is a direct exchange or a solvent-mediated process aided by localized partial or even complete dissolution followed by recrystallisation of the new phase.3 Similar considerations apply to frameworks which show anion4 or even ion-pair exchange.5 Rather than extensively review these types of exchange, we will instead focus here on a few representative examples in which the coordination polymer framework itself is changed significantly by the change in solvation. The structures Coordination Polymers: Design, Analysis and Application By Stuart R. Batten, Suzanne M. Neville and David R. Turner r Stuart R. Batten, Suzanne M. Neville and David R. Turner, 2009 Published by the Royal Society of Chemistry, www.rsc.org

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II

of [ML2(H2O)2](NO3)2 1.5H2O (M ¼ Co , Ni ; L ¼ a bipyridyl ligand with ethylene glycol side-chains) contain 2D (4,4) sheets in which the metal atoms are coordinated to four pyridyl ligands and two trans water ligands (Figure 12.1a).6 Heating at 150 1C for 24 h leads to the yellow Co crystals turning red but retaining their crystallinity. The crystal structure of this red product reveals that the coordinated water has been driven out and replaced in

Figure 12.1

Change in the coordination environment of the metal atom in the structure of [ML2(H2O)2](NO3)2 1.5H2O (M¼CoII, NiII; L ¼ a bipyridyl ligand with ethylene glycol side-chains) (a) before and (b) after dehydration.6

Reactive Coordination Polymers

377

the coordination sphere by nitrate anions (Figure 12.1b). Subsequent exposure of this material to the atmosphere leads to reabsorption of water and regeneration of the original yellow crystals. The Ni compound undergoes an analogous transformation but without the same dramatic colour change. A similar reversible dehydration (upon heating at 300 1C for 2 h) and rehydration (upon exposure to the air or immersion in water) also occurs for the structure of [Co2(ppca)2(H2O)(V4O12)0.5] 3.62H2O, where ppca ¼ 4-(pyridin-4-yl)pyridine2-carboxylate.7 In this system, the colour of the crystals changes between red (hydrated) and brown (dehydrated). The retention of single crystallinity upon desolvation allowed for crystal structure analysis, which showed that the coordination sphere of the CoII centre changes from octahedral to trigonal bipyramidal upon loss of the coordinated water molecule. In the structure of [Zn3L(OH)2(H2O)1.33] 3H2O, where L ¼ tetrahydrofuran2,3,4,5-tetracarboxylate, there is a complicated 3D network.8 Remarkably, upon dehydration by heating under vacuum, some of the zinc atoms move from one part of the structure to another. The dehydration results in these metal atoms having coordination numbers as low as two and so they move to another part of the network, ca. 3 A˚ away, which provides for higher coordination numbers. Rehydration of the structure in air over 2 days results in about twothirds of the zinc atoms moving back to their original position. An even more dramatic single-crystal to single-crystal transformation takes place for Cd(CN)2 2/3H2O t-BuOH.9 The structure contains a 3D moganite framework which contains sizable hexagonal channels (Figure 12.2a). When the crystals and a small volume of mother liquor were placed on a porous tile in a sealed container containing a saturated atmosphere of chloroform vapour (supplied by a reservoir of liquid chloroform which contacts neither the crystals nor tile), the crystals transform into a new phase while retaining their shape and transparency. In this new phase, Cd(CN)2 CHCl3, the water and tert-butanol molecules are replaced by chloroform molecules and the coordination framework becomes a single diamond net (Figure 12.2b). The mechanism of this dramatic change in topology is ascribed to shearing and repositioning of certain planar sections of the moganite structure. Both compounds decompose upon loss of solvent to the twofold interpenetrating diamond structure of Cd(CN)2. The structure of [Ag6Cl(atz)4]OH 6H2O, where Hatz ¼ 3-amino-1,2,4-triazole, contains five interpenetrating 3D nets with 4.142 topology.10 The nets are crosslinked by Ag Cl interactions. Remarkably, upon partial dehydration, a structure containing six interpenetrating 4.142 nets is produced. The fivefold interpenetrating structure is regenerated upon exposure to saturated water vapour. A similar dramatic structural reorganisation occurs upon dehydration of a cobalt thiocyanate polymer of 1-methyl-1 0 ,2-bis(4-pyridyl)ethane.11 A hydrated phase containing two 3D CdSO4 nets interpenetrating with (4,4) sheets is converted to a phase containing only (4,4) sheets, which show inclined interpenetration. Vittal and co-workers reported a series of structures in which hydrogen bonding links between dimers and chains are replaced by coordination bonds upon dehydration. The structure of [Zn(sala)(H2O)2]2 2H2O, where

378

Chapter 12

Figure 12.2 (a) The moganite structure of Cd(CN)2 2/3H2O t-BuOH and (b) the diamond network of Cd(CN)2 CHCl3.9 For clarity, only the cadmium cyanide framework is shown.

H2sala ¼ N-(2-hydroxybenzyl)-L-alanine, contains discrete dimers in which each metal ion has a bound water ligand (Figure 12.3a).12 Adjacent dimers are bridged into a 3D net by hydrogen bonding interactions between these water ligands and uncoordinated carboxylate oxygen atoms of adjacent dimers; this is reinforced by further interdimer hydrogen bonding between the other (coordinated) carboxylate oxygen and the NH of the sala ligand. Dehydration by heating yields an anhydrous compound in which the Zn-(H2O) OC(O)R hydrogen bonding


Figure 12.3

379

The conversion of hydrogen bond interactions to coordination bonds in the structure of [Zn(sala)(H2O)2]2 2H2O, where H2sala ¼ N(2-hydroxybenzyl)-L-alanine, (a) before and (b) after dehydration.12 (c) The hydrogen bond interactions between adjacent zigzag chains in the structure of the related CuII compound (only one dimer subunit is shown for the left chain).13 Striped bonds represent hydrogen bonds and for clarity only water and amine protons are shown.

interaction is replaced by a Zn–OC(O)R coordination bond (Figure 12.3b), generating a 3D coordination polymer. The related CuII complexes contain similar dimers (Figure 12.3c); however, in this structure one of the two metal atoms in the dimer already has a bound

380

Chapter 12

carboxylate oxygen from an adjacent dimer (and therefore has no bound water).13 As a result, a 1D chain is produced, but these chains are again crosslinked into a 3D net by hydrogen bonding between water ligands and uncoordinated carboxylate oxygens of adjacent chains (the orientation is slightly different in this case and the NH of the ligand hydrogen bonds also to the uncoordinated carboxylate oxygen rather than the bound one as is the case for Zn). Again, dehydration results in the formation of a 3D coordination polymer; both the Zn and Cu dehydrated compounds have the diamond topology. Significantly, both the hydrated and the dehydrated Cu phases also show canted antiferromagnetism, with ordering temperatures of 400 and 435 K, respectively.14 In both of these structures, the dehydration is irreversible. However, when analogous Zn compounds are made with chloro or methyl substituents in the 5-position of the aromatic ring of the sala ligand, it is found that only ca. 90% of the water can be removed below 110 1C.15 Furthermore, the dehydration is reversible. Detailed structural analysis suggested that although these compounds and the Zn–sala parent compound are isomorphous, the additional R groups occupy the channels in the structure and also produce repulsive interactions in the dehydrated phases. A similar direction of new coordination bonds by outgoing water ligands occurs in the structures of [Fe(pmd)(H2O){M(CN)2}2] H2O, where pmd ¼ pyrimidine and M ¼ Ag, Au.16 The structures contain Fe atoms with two trans waters, each of which hydrogen bonds to a nitrogen of an adjoining pyrimidine ligand. Upon dehydration, these interactions are replaced by Fe–pmd coordination bonds and the structure is converted from three interpenetrating 3D CdSO4 coordination polymer nets into a single 3D net. Furthermore, the hydrated phases show spin crossover with hysteresis, while the dehydrated Ag phase shows a lower temperature crossover with greater hysteresis and the dehydrated Au phase shows no crossover. The dehydration/rehydration is reversible. The compound Fe(3-cyanopyridine)2(MeOH)2/3[Au(CN)2]2 also shows spin crossover.17 The structure consists of 2D (4,4) sheets which are crosslinked by hydrogen bonds between the coordinated methanol molecules and dangling, terminal Au(CN) 2 ligands, to give three interpenetrating 3D NbO nets. These NbO nets are converted to coordination polymers by desolvation of the methanol, leading to the formation of new coordination bonds between the Au(CN) 2 ligands and the Fe atoms. The desolvated material also shows spin crossover, but at a lower temperature.

CO2H CO2H

CO2H hv solid

Figure 12.4

Photodimerisation in the solid state of b-cinnamic acid.

CO2H


381

A very early use of the phrase ‘crystal engineering’ was by Schmidt in relation to deliberately constructing crystals containing cinnamic acids aligned perfectly for photodimerisation reactions (Figure 12.4).18 This followed the observation that some polymorphs of cinnamic acid showed photodimerisation whereas others did not; the difference was in how the molecules were aligned in the crystal structure. In particular, the molecules should be parallel and separated by o4.2 A˚. Similar photodimerisation and polymerisation reactions have been observed for coordination polymers of alkene- and alkyne-containing ligands.19 In these materials, the metal ions align the double and triple bonds into close proximity and parallel for the photodimerisation of alkenes. For the polymerization of alkynes, adjacent triple bonds are more commonly inclined to each other rather than parallel. These reactions are usually induced by heating or irradiation by UV light or 60Co g-rays. MacGillivray and co-workers used dinuclear Schiff base ligands to create ladder-like 1D coordination polymers containing bridging pairs of 1,4-bis(4-pyridyl)ethene (bpee) ligands.20 The two metal atoms in the Schiff base are separated by 3.19 A˚ and therefore the two CQC bonds of the bpee ligands are parallel and separated by only 3.71 A˚. UV irradiation of the polymer results in solid state [2 + 2] photodimerisation. The irradiation also leads to decomposition of the single crystals; however, this work is an extension of earlier work on a molecular species containing pairs of the same Schiff base linked by a single pair of bpee bridges (rather than a pair on each side of each Schiff base, as is the case in the polymer).21 These dimers show similar photodimerisation reactions, but with retention of single crystallinity. The structure of the dimer before and after irradiation is shown in Figure 12.5. Interestingly, this group has also reported a number of coordination polymers constructed with ligands previously synthesised in high yield in solid-state photodimerisation reactions. In these reactions, the precursors are brought into alignment in the solid by hydrogen bonding organic templates.22 They also reported the photodimerisation of trans-1-(4-pyridyl)-2-(phenyl)ethylene, with the ligands held in close proximity by pairs of silver atoms (which also show argentophilic interactions between them).23 Upon UV irradiation, the ligands dimerise, the silver atoms move ca. 1.16 A˚ and the dimers are converted into a 1D polymer by Ag C(phenyl) interactions. The bpee ligand also shows [2 + 2] photodimerisation in the structure of [Zn(O2CCH3)(O2CCF3)]2(bpee)2.24 Pairs of 1D chains are bridged by acetate anions, which lead to the bpee ligands being aligned side-by-side and the appropriate distance for dimerisation (Figure 12.6a). Irradiation leads, as expected, to the formation of tetrakis(4-pyridyl)cyclobutane, with retention of single crystallinity (Figure 12.6b). Similar alignment of chains directed by acetate-derived bridges occurs for [Ag(O2CCF3)]2(bpee)2 H2O and again the bpee ligands are photoreactive.25 In the structure of [Ag(bpee)(H2O)](CF3CO2) MeCN, however, silver atoms coordinate to the water molecules rather than to bridging trifluoroacetate anions and therefore the packing of the chains is different. They are no longer aligned, but rather are staggered, with the closest distance between

382

Figure 12.5

Chapter 12

Photodimerisation of bpee ligands in a molecular species (a) before and (b) after irradiation.21

the ethylenic carbons being 5.15 A˚. Desolvation, however, does produce a phase which is photoactive; it is proposed that the chains align themselves again, with formation of trifluoroacetate bridges, as the water ligands are removed. Two other coordination polymers containing the tetrakis(4-pyridyl)cyclobutane ligand have been reported from reaction mixtures of metals and bpee.26,27 In these compounds, however, the analogous crystal structures containing the undimerised bpee ligands were not observed and it was postulated that the dimerisation occurs during crystal growth. The structure of cadmium fumarate contains metal dimers bridged into (4,4) sheets by the dicarboxylate ligands.28 Irradiation leads to topochemical [2 + 2] cycloaddition between pairs of adjacent layers, along with partial dehydration. Complete dehydration leads to material which is not photoreactive. The compound bis(but-3-enoato)zinc is also photoreactive, but in this case polymeric species are formed.29 The structure contains 2D (4,4) sheets of metal ions


383

Figure 12.6 Photodimerisation of bpee ligands in the 1D chain complex [Zn(O2CCH3) (O2CCF3)]2(bpee)2 (a) before and (b) after irradiation.24

Figure 12.7

The 2D sheet structure of bis(but-3-enoato)zinc.29 Only the ligands on the top of the sheet are close enough to show interligand reactivity (interaction marked by striped bond).

bridged by carboxylate groups (Figure 12.7). The ‘tails’ of the ligands project either side of the sheets, but are orientated in different ways. Only on one side of the sheet are they suitably arranged for polymerisation, hence only ca. 50% conversion is observed.

384

Figure 12.8

Chapter 12

The structure of Cd(CN)2(bpb), with criss-crossing butadiene groups.30

Acetylene-derived and related ligands can also lead to coordination polymers in which polymerisation of the organic components can occur. The structure of Cd(CN)2(bpb), where bpb ¼ 1,4-bis(4-pyridyl)butadiyne, contains undulating cadmium cyanide layers bridged bpb ligands (Figure 12.8).30 The bpb moieties bridge in an inclined fashion such that they criss-cross each other, with their central butadiyne groups in close proximity. Exposure to daylight leads to the colourless crystals turning a dark blue–purple colour and solution of the parent crystal structure required data collection on several crystals due to decomposition in the X-ray beam. This suggested the likelihood of diacetylene polymerisation. Irradiation of tris(propynoato)scandium(III) with 60Co g-rays also resulted in polymerisation.31 The structure has a 3D a-Po topology in which the metal atoms are, as for the earlier zinc compound, bridged by carboxylate groups. The acetylene ‘tails’ of the ligands project into the cavities, again in a criss-cross fashion (Figure 12.9). Finally, lanthanoid and cadmium polymers of acetylenedicarboxylate have been shown to undergo polymerisation of the carbon triple bonds upon heating.32,33 Another series of compounds showing photoinduced reactivity have been reported.34–40 These contain thienyl derived ligands in which a reversible internal ring opening and closing reaction is controlled by irradiation of different wavelength light. This is often accompanied by a change in colour of the material irradiated. For example, a series of silver coordination polymers have been constructed with the ligand cis-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl) ethane, which is in the ‘open’ form (Figure 12.10).34 Irradiation with 450 nm light induces the cyclisation reaction to the ‘closed’ form, with a change in crystal colour of orange or red to yellow. The ring-opening reaction can then be induced with 560 nm light, with a return to the original crystal colours. Finally, a number of other structures have been shown to undergo temperaturedependent topotactic reactions. The structure of [CuBr(dpds)]2, where

385


Figure 12.9

The 3D a-Po structure of tris(propynoato)scandium(III).31

Ag

Ag N

Ag

N C

Ag N

C

N C

C

450 nm 560 nm S

S

S

S

Ag

Ag

Ag

Ag

Figure 12.10

Photoactivated ring opening and closing of cis-1,2-dicyano-1,2-bis(2,4,5trimethyl-3-thienyl)ethane observed in its silver coordination polymers.34

dpds ¼ 2,2 0 -dipyridyl disulfide, contains discrete dimers which are converted into a 2D sheet structure upon heating.41 Similarly, the structure of NiBr2[P(CH2CH2CN)3]2 also undergoes a conversion from monomers to polymers in the solid state; the nitrile groups coordinate to the Ni atoms of adjacent monomers,

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Chapter 12

changing their coordination geometries from square-planar to octahedral and the colour from red to blue.42 The reaction occurs at room temperature, but is accelerated by heating. The structure of ZnCl2(4,4 0 -bipy), where 4,4 0 -bipy ¼ 4,4 0 bipyridine, contains zigzag chains of tetrahedral zinc ions connected by bridging 4,4 0 -bipy ligands.43 Upon cooling to temperatures below 130 K, a conversion to a 2D sheet structure occurs. The chloride ions, terminal in the high-temperature phase, move to coordinate also to the zinc atoms in adjoining chains, such that infinite chains of zinc atoms bridged by double chloride bridges are formed. The Zn(4,4 0 -bipy) chains become linear and the zinc atoms change from tetrahedral to octahedral geometry. Interestingly, this change is reversible but shows a large hysteresis – conversion back to the chains does not occur until above 360 K.

12.3 Chemically Reactive Ligands We have detailed above a number of structures containing photoactive ligands. However, a series of related compounds have also been reported which show different types of reactivity. In this series, reported by Lee and co-workers, large trigonal benzonitrile-donor ligands are reacted with silver to give either (6,3) or (10,3)-b networks.44–48 Significantly, the organic ligands can be functionalised with side-chains while still producing the large, hexagonal channel-containing structures. These side-chains can be varied to control the size and chemical environment of the channels or can be reactive towards incoming guests. For example, guests such as di-tert-butylsilyl bis(trifluoromethanesulfonate) will react with alcohol side-chains from the coordination framework to produce polymers through the creation of O–Si–O bonds. These can increase the robustness of the material while retaining its porosity. Similarly, trifluoroacetic anhydride guests will react with pendant alcohols of the coordination polymer host to produce esters. A final example of reactive ligands can be found in a chiral zinc coordination polymer discussed in the previous chapter.49 The structure contains D-tartaric acid-derived pyridylcarboxylate ligands. Uncoordinated pyridyl groups from these ligands project into the spaces within the porous structure and can be N-alkylated through reaction with reagents such as iodomethane.

12.4 Catalysis The majority of interest in reactive coordination polymers lies, of course, in their application towards heterogeneous catalysis. Such materials need to be porous and contain unsaturated metal atoms or suitably active ligands to act as reactive sites. The metals can either be the nodal metals used to construct the coordination polymer or metals contained within bridging ligands. Advantages of these materials include improved catalyst recovery, enhanced stability and size or shape selectivity. We have already explored the synthesis and reactivity of chiral coordination polymers in the previous chapter and the reader is directed to this discussion for


387

those materials. A notable illustrative example, not specifically discussed in Chapter 11, is a structure constructed with chiral Mn(salen)-derived ligands.50 These are based on the well-known homogeneous Jacobsen-type catalysts, but are functionalised with peripheral pyridyl groups to allow assembly into a polymeric species. In this case, they act as pillars between layers of zinc atoms bridged by dicarboxylate ligands. The reactive Mn(salen) groups lie exposed to the channels created in the structure and the material acts as an asymmetric catalyst for alkene epoxidation. Enantiomeric excesses observed are comparable to those with the molecular analogues. Kitagawa and co-workers also described the incorporation of (achiral) Schiff bases into porous coordination polymers, but no catalytic properties were reported.51 This same group, however, reported catalysis of Knoevenagel condensation reactions by a cadmium coordination polymer of the 3-connecting benzene-1,3,5-tricarboxylic acid tris[N-(4-pyridyl)amide] ligand.52 The metal ions are fully saturated (they are surrounded by six pyridyl donors) and the reaction is base catalysed by the amide groups of the ligands and is size selective. For example, the reaction of benzaldehyde with malononitrile showed 98% conversion, whereas the equivalent reaction with ethyl cyanoacetate showed only 7% conversion and no reaction was observed for cyanoacetic acid tert-butyl ester. One of the earliest reports of heterogeneous catalysis by coordination polymers was for the (4,4) grid structure of Cd(4,4 0 -bipy)2(NO3)2.53 This material shows catalysis for the cyanosilylation of aldehydes, with shape specificity. Moderate yields (40%) were obtained for 2-tolualdehyde cyanosilylation, whereas poorer yields (18%) were obtained for 3-tolualdehyde. It was found that a- and b-naphthaldehyde reacted well whereas the larger 9-anthraldehyde did not. Later work on this material showed that it also catalysed the cyanosilylation of imines; however, absorption studies showed no detectable uptake of imines, suggesting that the catalysis is surface based (although it was noted that the reactivity is remarkably high if this is the case).54 The structure of Cu3(btc)2(H2O)3, where btc ¼ benzene-1,3,5-tricarboxylate, was reported to contain a porous twisted boracite network (Figure 12.11);55 however, it was only in subsequent work by other groups that its catalytic properties were reported.56,57 Dehydration leads to copper-based Lewis acid sites and catalysis of the cyanosilylation of benzaldehyde occurs with reasonable yields (50–60%) and high selectivity. It also catalyses the isomerisation of terpene derivatives, such as the rearrangement of a-pinene oxide to campholenic aldehyde and the cyclisation of citronellal to isopulegol. The 3D succinate polymers of a series of rare earths have also been shown to act as Lewis acids, catalysing the acetalization of aldehydes and acting as redox agents in the oxidation of sulfides.58 A series of lanthanide disulfonates of general formula Ln(OH)(NDS)(H2O), where NDS ¼ naphthalene-1,5disulfonate, have been shown to be selective bifunctional catalysts for oxidation and epoxide ring-opening reactions.59 Epoxide ring opening and alcoholysis have also been shown to be catalysed by a number of CuII-based coordination polymers.60,61 The same group also reported the transesterification of esters with methanol by ZnII coordination polymers.62–64

388

Figure 12.11

Chapter 12

The porous structure of Cu3(btc)2(H2O)3.55

A series of CuII and FeII polymers have been shown to catalyse the oxidation of benzyl alcohol to benzaldehyde with H2O2 in water, with high selectivity (up to almost 100% for benzaldehyde) and conversion (up to 87% for benzyl alcohol).65 Photocatalysed degradation of pollutants is of obvious environmental interest and a uranium–nickel–organic hybrid material has been demonstrated to oxidize photocatalytically a model pollutant, Methyl Blue.66 The structure of [Cu(mipt)(H2O)](H2O)2, where mipt ¼ 5-methylisophthalate, consists of (4,4) sheets of paddlewheel Cu2 dimers connected by the bent mipt ligands (Figure 12.12).67 The sheets pack in an eclipsed fashion to give two types of channels, one of which is lined with accessible copper sites. Dehydration results in a material which catalyses the oxidation of CO to CO2. Similar catalysis is also observed for another compound in which nickel clusters are linked into a porous 3D network by alkali metal ions.68 Another reaction involving gases is the decomposition of NO to N2 and O2; this is catalysed by [Cu(bpee)2]NO3, which contains five interpenetrating diamond nets.69 This material also catalyses the reduction of NO with n-hexane in the presence of O2. The coordination polymer examples given above have all had their structures well characterised crystallographically; however, a number of amorphous coordination polymers have also shown significant catalytic activity. Reaction of anthracenebisresorcinol with La(O-i-Pr)3 results in a material that catalyses typical base-promoted reactions such as Michael, nitroaldol and aldol reactions.70 The same tetraphenol ligand reacted with Zr(O-i-Bu)4 gives a heterogeneous


Figure 12.12

389

The 2D (4,4) sheet structure of [Cu(mipt)(H2O)](H2O)2.67

catalyst for the Diels–Alder reaction of acrolein with cyclohexa1,3-diene.71 This reaction can even be undertaken with this catalyst in a flow system. The amorphous PdII coordination polymer of a large trigonal phosphine ligand showed catalysis of the Suzuki–Miyaura reaction under atmospheric conditions in water.72 Mori and co-workers reported a series of Ru2 and Rh2 dicarboxylate polymers and Rh2–metalloporphyrintetracarboxylate compounds which are proposed to have 2D sheet structures.73–78 These show catalysis of the hydrogenation of alkenes (ethane, propene and but-1-ene) and for dissociation of hydrogen. For the bimetallic Rh2–metalloporphyrin compounds, it is proposed that the hydrogen molecules are activated at the Rh atoms to form hydrides whereas the metal centres in the porphyrins bind to the alkene molecules. The hydrides are then transferred to the alkenes to form the alkanes. By contrast, a recent series of indium coordination polymers containing both carboxylate and pyridyl donor ligands have been well characterized crystallographically and display selective catalysis of acetalization of aldehydes.79 A related compound, In2(OH)3(bdc)1.5, where bdc ¼ benzene-1,4-dicarboxylate, shows reduction of nitroaromatics and selective oxidation of organic sulfides.80 However, due to the dense structure of this material, the catalysis is believed to be a surface phenomenon. The Lewis acid catalyst properties of a scandium succinate polymer, as demonstrated by Friedel–Crafts acylation reactions of anisol with acetic anhydride and also by acetalisation of aldehydes, are also believed to be surface based.81 A number of coordination polymers have also been shown to catalyse polymerisation reactions. A titanium polymer prepared from Ti(O-i-Pr)4 and butane-1,4-diol has been shown to be a highly effective initiator for the

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Chapter 12 82

ring-opening polymerisation of cyclic esters. Similarly prepared Ti polymers containing 2,7-dihydroxynaphthalene act as Ziegler–Natta catalysts; however, reactivity is mediocre, with polydisperse, linear polyethylene and atactic polypropylene polymers resulting.83 Cyanide polymers such as the 3D compounds Zn3[M(CN)6]2, where M ¼ Co, Fe,84 and the 2D compounds Co(H2O)2 [M(CN)4] 4H2O, where M ¼ Ni, Pd, Pt,85 catalyse the polymerization of propylene oxide to poly(propylene oxide) and propylene oxide and CO2 to give poly(propylene oxide-co-propylene carbonate). The spaces within coordination polymers can also act as product-directing containers. For example, an acetylene, methyl propiolate, can be polymerised within the channels of Cu2(pzdc)2(4,4 0 -bipy), where pzdc ¼ pyrazine-2,3dicarboxylate; the narrow pore sizes of the channels favour trans addition polymerisation.86 The reaction is promoted by the carboxylate groups lining the channels and the polymer can be extracted with DMF. The radical polymerisation of styrene has been observed in the channels of M2(bdc)2(triethylenediamine), where M ¼ Zn, Cu; the resultant polystyrene polymers can be obtained by dissolution of the framework materials by NaOH.87 Finally, a ‘ship-in-a-bottle’ synthesis has been reported in the compound [Co3(bpdc)3(4,4 0 -bipy)] 4DMF H2O, where bpdc ¼ biphenyldicarboxylate.88

Figure 12.13

The 1D channels in the structure of [Co3(bpdc)3(4,4 0 -bipy)] 4DMF H2O.88


391

The structure contains two interpenetrating 8-connected nets in which 1D channels are observed (Figure 12.13). Molecules such as o-methyldibenzyl ketone can thus be absorbed into the structure and then subsequently, due to the confined space within the framework, show shape selectivity in its photolysis, to give a 100% yield of the a-cleavage products.

References 1. J.J. Vittal, Coord. Chem. Rev., 2007, 251, 1781. 2. S. Noro, S. Kitagawa, M. Yamashita and T. Wada, Chem. Commun., 2002, 222. 3. (a) A.N. Khlobystov, N.R. Champness, C.J. Roberts, S.J.B. Tendler, C. Thompson and M. Schro¨der, CrystEngComm, 2002, 426; (b) C. Thompson, N.R. Champness, A.N. Khlobystov, C.J. Roberts, M. Schro¨der, S.J.B. Tendler and M.J. Wilkinson, J. Microsc., 2004, 214, 261. 4. (a) K.S. Min and M.P. Suh, J. Am. Chem. Soc., 2000, 122, 6834; (b) S. Noro, R. Kitaura, M. Kondo, S. Kitagawa, T. Ishii, H. Matsuzaka and M. Yamashita, J. Am. Chem. Soc., 2002, 124, 2568. 5. E. Barea, J.A.R. Navarro, J.M. Salas, N. Masciocchi, S. Galli and A. Sironi, J. Am. Chem. Soc., 2004, 126, 3014. 6. (a) K. Takaoka, M. Kawano, M. Tominaga and M. Fujita, Angew. Chem. Int. Ed., 2005, 44, 2151; (b) K. Takaoka, M. Kawano, T. Hozumi, S. Ohkoshi and M. Fujita, Inorg. Chem., 2006, 45, 3976. 7. C.-L. Chen, A.M. Goforth, M.D. Smith, C.-Y. Su and H.-C. zur Loye, Angew. Chem. Int. Ed., 2005, 44, 6673. 8. K. Hanson, N. Calin, D. Bugaris, M. Scancella and S.C. Sevov, J. Am. Chem. Soc., 2004, 126, 10502. 9. B.F. Abrahams, M.J. Hardie, B.F. Hoskins, R. Robson and G.A. Williams, J. Am. Chem. Soc., 1992, 114, 10641. 10. J.-P. Zhang, Y.-Y. Lin, W.-X. Zhang and X.-M. Chen, J. Am. Chem. Soc., 2005, 127, 14162. 11. D.M. Shin, I.S. Lee, D. Cho and Y.K. Chung, Inorg. Chem., 2003, 42, 7722. 12. J.D. Ranford, J.J. Vittal and D. Wu, Angew. Chem. Int. Ed., 1998, 37, 1114. 13. J.D. Ranford, J.J. Vittal, D. Wu and X. Yang, Angew. Chem. Int. Ed., 1999, 38, 3498. 14. X. Yang, L. Si, J. Ding, J.D. Ranford and J.J. Vittal, Appl. Phys. Lett., 2001, 78, 3502. 15. J.J. Vittal and X. Yang, Cryst. Growth Des., 2002, 2, 259. 16. V. Niel, A.L. Thompson, M.C. Munõz, A. Galet, A.E. Goeta and J.A. Real, Angew. Chem. Int. Ed., 2003, 42, 3760. 17. A. Galet, M.C. Munõz and J.A. Real, Chem. Commun., 2006, 4321. 18. G.M.J. Schmidt, Pure Appl. Chem., 1971, 27, 647. 19. I.G. Georgiev and L.R. MacGillivray, Chem. Soc. Rev., 2007, 36, 1239.

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20. G.S. Papaefstathiou, I.G. Georgiev, T. Friscie and L.R. MacGillivray, Chem. Commun., 2005, 3974. 21. G.S. Papaefstathiou, Z. Zhong, L. Geng and L.R. MacGillivray, J. Am. Chem. Soc., 2004, 126, 9158. 22. (a) G.S. Papaefstathiou, T. Griscic and L.R. MacGillivray, J. Am. Chem. Soc., 2005, 127, 14160; (b) G.S. Papaefstathiou and L.R. MacGillivray, Angew. Chem. Int. Ed., 2002, 41, 2070. 23. Q. Chu, D.C. Swenson and L.R. MacGillivray, Angew. Chem. Int. Ed., 2005, 44, 3569. 24. N.L. Toh, M. Nagarathinam and J.J. Vittal, Angew. Chem. Int. Ed., 2005, 44, 2237. 25. M. Nagarathinam and J.J. Vittal, Angew. Chem. Int. Ed., 2006, 45, 4337. 26. A.J. Blake, N.R. Champness, S.S.M. Chung, W.-S. Li and M. Schro¨der, Chem. Commun., 1997, 1675. 27. J.Y. Lee, S.J. Hong, C. Kim and Y. Kim, Dalton Trans., 2005, 3716. 28. A. Michaelides, S. Skoulika and M.G. Siskos, Chem. Commun., 2004, 2418. 29. M.J. Vela, V. Buchholz, V. Enkelmann, B.B. Snider and B.M. Foxman, Chem. Commun., 2000, 2225. 30. B.F. Abrahams, M.J. Hardie, B.F. Hoskins, R. Robson and E.E. Sutherland, J. Chem. Soc., Chem. Commun., 1994, 1049. 31. J.S. Brodkin and B.M. Foxman, J. Chem. Soc., Chem. Commun., 1991, 1073. 32. A. Michaelides and S. Skoulika, Cryst. Growth Des., 2005, 5, 529. 33. S. Skoulika, P. Dallas, M.G. Siskos, Y. Deligiannakis and A. Michaelides, Chem. Mater., 2003, 15, 4576. 34. H. Konaka, L.P. Wu, M. Munakata, T. Kuroda-Sowa, M. Maekawa and Y. Suenaga, Inorg. Chem., 2003, 42, 1928. 35. J. Han, M. Maekawa, Y. Suenaga, H. Ebisu, A. Nabei, T. Kuroda-Sowa and M. Munakata, Inorg. Chem., 2007, 46, 3313. 36. M. Munakata, J. Han, A. Nabei, T. Kuroda-Sowa, M. Maekawa, Y. Suenaga and N. Gunjima, Polyhedron, 2006, 25, 3519. 37. J. Han, A. Nabei, Y. Suenaga, M. Maekawa, H. Isihara, T. Kuroda-Sowa and M. Munakata, Polyhedron, 2006, 25, 2483. 38. J. Han, H. Konaka, T. Kuroda-Sowa, M. Maekawa, Y. Suenaga, H. Isihara and M. Munakata, Inorg. Chim. Acta, 2006, 359, 99. 39. M. Munakata, J. Han, M. Maekawa, Y. Suenaga, T. Kuroda-Sowa, A. Nabei and H. Ebisu, Inorg. Chim. Acta, 2007, 360, 2792. 40. M. Munakata, J. Han, A. Nabei, T. Kuroda-Sowa, M. Maekawa, Y. Suenaga and N. Gunjima, Inorg. Chim. Acta, 2006, 359, 4281. 41. S. Kubo, T. Nishioka, K. Ishikawa, I. Kinoshita and K. Isobe, Chem. Lett., 1988, 1067. 42. K. Cheng and B.M. Foxman, J. Am. Chem. Soc., 1977, 99, 8102. 43. C. Hu and U. Englert, Angew. Chem. Int. Ed., 2005, 44, 2281. 44. Z. Xu, Y.-H. Kiang, S. Lee, E.B. Lobkovsky and N. Emmott, J. Am. Chem. Soc., 2000, 122, 8376.


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CHAPTER 13

Other Properties of Coordination Polymers 13.1 Introduction This chapter deals with the miscellaneous other properties of coordination polymers not covered in the previous chapters. The more notable of those properties are discussed separately in the sections below, but some of the more unusual properties observed include adsorption of hydrochloride gas,1 emission of sound during precipitation (‘ . . . precipitation is accompanied by a rather strong cracking sound. This cracking is heard again when the mixture is shaken after some hours.’)2 and the application of Gd coordination polymer nanoparticles to contrast enhancing agents.3 They have also been used as useful precursors for metal oxide materials.4 Although we have largely discussed crystalline materials through this book, coordination polymers will also form gels. These gels may have reversible sol–gel transitions controlled by stimuli such as counterion exchange5 or photoisomerisation of ligands,6 or act as templates for porous organic polymers,7 catalysts8 or sensors.9 Coordination polymers can also show colour changes upon change of solvent molecules incorporated into the structure.10 For example, two Co coordination polymers of the [Re6S8(CN)6]4– cluster contain water ligands coordinated to the cobalt atoms and show a vapochromic response to a number of organic solvents.11 Tetrahydrofuran changes the colour of the material from orange to either purple or green; with diethyl ether it becomes blue. The material is thus acting as a sensor. The colour changes are attributed to the incoming solvent, leading to displacement of the water ligands on the cobalt atoms and a change in their geometry from octahedral to tetrahedral (leading to the observed colour changes).


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Figure 13.1

397

Luminescence occurs by either singlet–singlet decay (fluorescence) or triplet–singlet decay (phosphorescence). Reproduced by permission of the Royal Society of Chemistry from Chem. Soc. Rev., 2005, 34, 1048.

13.2 Luminescence Luminescence arises due to an electronic transition from an excited state, caused by photoexcitation, to the ground state, resulting in the emission of light.12,13 Depending on the nature of the excited state, luminescence is divided into two categories: fluorescence and phosphorescence (Figure 13.1). Fluorescence occurs rapidly with some energy dissipation, meaning that the emitted light is red shifted compared with that which was absorbed. Phosphorescence occurs more slowly from a triplet excited state to a singlet ground state and is the effect observed in materials that glow in the dark. Luminescent compounds usually require organic chromophoric ligands which absorb light and then pass the excitation energy to the metal ion. These ‘antenna ligands’ must possess an excited state that is capable of sensitising the metal ion emission. The photoexcitation in this case is referred to as a ligand-to-metal charge-transfer process (LMCT). The reverse case is also possible, whereby the charge is transferred to the ligand (MLCT). In some cases the metal ion is not involved in the luminescence event with intraligand processes, such as p–p* transitions, responsible for fluorescence. Although organic polymers and discrete molecules can also display luminescence behaviour, coordination polymers are possibly the most versatile materials as there is the potential to couple the emission properties with guest exchange or other physical attributes. Coordination polymers can also be more thermally stable than organic species alone, making them more useful in many

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applications. A significant amount of research has been focused on the use of the lanthanoids.14,15 Lanthanoid ions are widely used in photonic applications such as LEDs and lasers. Although the majority of the 4f elements are luminescent, europium and terbium are significantly more emissive, i.e. they have larger energy gaps, and have therefore attracted most attention. Gadolinium also has a large energy gap; however, it emits in the UV region, rather than the visible region of Eu and Tb, and is therefore less useful as a probe. Other lanthanoids emit in the near-IR range with potential uses in telecommunications devices. Although much work is centred around 4f metals, there are also examples with Zn(II) and Cd(II)16 and other d10 transition metal ions such as Cu(I) and Ag(I). An example of a coordination polymer that exhibits guest-dependent fluorescence is the microporous 3D network [Eu(btc)] (btc ¼ benzene-1,3,5-tricarboxylate).17 The compound is synthesised with one aqua ligand per Eu(III) and lattice water, although both can be removed at 150 1C to yield a guest-free framework that is stable to 500 1C and has vacant metal coordination sites within porous channels. The emission spectrum of [Eu(btc)] shows no emission from the carboxylate ligand due to energy transfer to the metal ion. The photoluminescence spectrum of the material is sensitive to guest solvents that are introduced to the dehydrated material, with DMF showing the most significant fluorescence enhancement and acetone showing a dramatic quenching effect. Rare earth carboxylate frameworks often display luminescent behaviour, for example those containing benzenedicarboxylate18 and adipate,19 although guest-dependent behaviour is infrequently observed. Another example of guest-influenced fluorescence is a terbium–mucate network that has recently been reported and shows a dependence of the fluorescence on the concentration of carbonate.20 Luminescent carboxylate-based coordination polymers have also been constructed using d-block metal ions. Cadmium–benzenetricarboxylate polymers, both 2D and 3D, have been reported that display strong fluorescence emission resulting from ligand-to-metal charge transfer (LMCT).21 The incorporation of DABCO co-ligands in one of the polymeric species results in a bathochromic shift of the emission band which is postulated to be caused by s-donations from the neutral co-ligand. Zinc complexes with benzenecarboxylate ligands also show emission due to an LMCT process.22 The widely used framework MOF-5 (see Chapter 10), which contains Zn4O13 nodes, shows an emission band due to an O2Zn21 - OZn1 LMCT process, meaning that the cluster behaves as a ZnO quantum dot in addition to the organic bridges acting as photon antennae.23 An interesting recent example ion-dependent fluorescence is a zinc–carboxylate polymer that contains additional bis-Schiff base binding pockets (Figure 13.2).24 The metal ion in the binding pocket can be exchanged, resulting in the fluorescence being switched off. Transition metals can be used in coordination polymers in a purely structural role, i.e. they do not partake in absorption or emission. An example of such a system is the 2D network [Zn(1,2-bdc)(H2O)] (1,2-bdc ¼ benzene-1,2-dicarboxylate) in which fluorescence arises from intraligand p–p* transitions with a bathochromic shift compared with the free ligand.25 Carboxylate-based ligands are not the only bridging groups that can be used in luminescent materials, although they do account for the bulk of research in


399

Figure 13.2 Ion exchange in a carboxylate polymer with Schiff base binding pockets results in the fluorescence being switched off. Reproduced with permission from Angew. Chem. Int. Ed., 2006, 45, 5492. Copyright Wiley-VCH Verlog GmbH & Co. KGaA.

the area. Examples have been reported of d-block coordination polymers with a variety of different ligand types such as poly(pyridyl) ligands,26,27 bis(imidazolyl) ligands28 and bis(sulfonate) ligands.29 A Cu(I)–alkene polymer, containing an Z2-olefinic coordination mode, displays red fluorescence due to an MLCT process.30 Red fluorescence is also observed from Cu(I)–halide clusters, occurring via a combination of halide-to-metal charge transfer (XMCT) and d-s transitions from Cu–Cu interactions.31 Polymeric complexes of silver are known to luminesce at low temperatures, although examples that have significant emission at room temperature are rarer. Phosphorescence in such cases does not have to result from LMCT processes. For example, the 2D polymer [Ag2(bpyz)(NO3)2] (bpyz ¼ 2,2-bipyrazine) displays strong room temperature emission that is attributed to Ag–Ag interactions.32 Carboxylate-based Ag(I) polymers, such as [Ag4(Hbtc)2],33 are known to show intense blue emission. Although discussion above has focused on the properties of the metal–ligand frameworks, a recent report has shown that it is possible for guest–ligand interactions to result in fluorescence emission. A 1D Zn–4,4 0 -bipyridine polymer, with pyrene guest molecules within the lattice, shows strong fluorescence due to a bipyridine–pyrene exciplex.34 This work demonstrates the potential that coordination frameworks have to act as solid-state sensor species.

13.3 Redox Activity A number of redox-active coordination polymers have been reported.35 As these are solid-state materials, designing redox activity can be a challenge – any

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change in oxidation state must be compensated by either a corresponding opposite change elsewhere in the structure (either in the framework or by guests) or the absorption/desorption of the appropriate counterions. For example, reaction of 3,6-bis(4 0 -pyridyl-1 0 -pyridinio)pyridazine dichloride with CuCl2 results in a 1D chain structure in which the chains pack in a ‘logstacking’ type of arrangement.36 Irradiation of the material results in a 15% increase in room temperature magnetic susceptibility and the crystals take on a blue tinge, indicating photoreduction of the ligand to a radical species. To balance the charge, Cl ions within the structure are oxidised to Cl2. That is, there is a photoinduced electron transfer from the Cl to the ligand. The porous metal carboxylate polymer [FeIII(OH)0.8F0.2(O2CC6H4CO2)] shows reversible electrochemical uptake of lithium, which is of interest for applications in Libased batteries.37 The lithium is oxidised to Li1 and the FeIII is reduced to FeII in the ‘charging’ cycle, and the reverse occurs upon discharge. Suh and co-workers have reported a series of structures containing cyclambased NiII complexes bridged by polycarboxylate ligands. In these structures, oxidation to NiIII is observed, induced by, for example, intercalation of I2 and 38 Immersion of other such materials in solutions of partial reduction to I 3. AgNO3 or NaAuCl4 results in the formation of Ag and Au nanoparticles.39,40 The NiII is oxidised to NiIII and NO3 and Cl counterions are absorbed into the structure. The structure of CoIII(pyrazine)(3,6-DBSQ)(3,6-DBCat) (3,6-DBSQ and 3, 6-DBCat are the semiquinonate and catecholate forms of 3,6-di-tert-butyl-1, 2-benzoquinone) contains 1D chains.41 Irradiation with a tungsten–halogen lamp results in the transformation of the chains to CoII(pyrazine)(3,6-DBSQ)2, with a transfer of electrons from the catecholate ligands to the cobalt atoms. The Co–N(pyrazine) bond lengths are ca. 0.2 A˚ longer for CoII than for CoIII, which translates to 0.06 mm per millimetre along the polymer propagation direction. A noticeable distortion is thus observed in some crystals upon irradiation. A number of charge–transfer systems have also been reported.42 For example, the molecule 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) is well known as a good electron donor or acceptor, depending on its oxidation state.43 In [Zn(TCNQ)(4,4 0 -bipy)] 6H2O, where 4,4 0 -bipy ¼ 4,4 0 -bipyridine, the TCNQ is present as the dianion and aromatic species can be included into the porous structure (Figure 13.3).44 The material changes colour, specific to the aromatic inclusion species, due to the formation of charge-transfer interactions between the guests and the framework TCNQ2 ligands. A series of cadmium cyanide structures containing methylviologen dications have been reported and a number of these show colour changes from colourless to blue upon irradiation with UV light.45 It is proposed that this is due to reduction to the methylviologen radical cation; however, the nature of the electron donor is unclear. Finally, a number of other redox-active centres can be deliberately introduced into coordination polymers. One well-known redox species is the ferrocene moiety; ligands containing this moiety have been incorporated into a number of nets, which have then been shown to be redox active.46 One such species is shown in Figure 13.4.


401

Figure 13.3 The porous structure of [Zn(TCNQ)(4,4 0 -bipy)] 6H2O.44

Figure 13.4

A 1D silver coordination polymer of a redox-active ferrocene-containing ligand.46

13.4 Conductivity One of the most notable examples of conductivity in coordination polymers are the compounds Cu(R1,R2-DCNQI)2, where R1, R2 ¼ H, Cl, Br, I, Me, OMe and DCNQI ¼ N,N 0 -dicyanoquinonediimine.47 The structures contain seven interpenetrating diamond nets, with infinite stacks of DCNQI radical anions created (Figure 13.5). The copper atoms have an average oxidation state of +1.33 and the series show metal-like conductivities. The dimethyl derivative, for example, has a conductivity of 1000 S cm1 at room temperature, which rises to

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Figure 13.5

The seven interpenetrating diamond nets of Cu(Me2-DCNQI)2, which shows metal-like conductivity.47

500 000 S cm1 at ca. 10 K, comparable to metallic copper when the number of mobile electrons per unit volume is taken into account. The electron transport occurs both along the p-stacked radical ligands and between the anions and the mixed-valence cations. Other salts (Li, Na, K, Rb, Ag, Tl, NH4) have similar structures but generally behave as semiconductors because mixed-valence metal sites are not present. The structure of Cu(TCNQ) also contains radical polynitrile anions. This material is of considerable interest as it was shown that films of this material can undergo electric field-induced bistable switching,48 that is, it switches from a high resistance to a low resistance state at a critical threshold potential. Furthermore, there was a memory effect – the material remains in the low resistance state for a short period after the field is shut off. The mechanism proposed involved partial electron transfer according to the following reaction (with the high-resistance state on the left-hand side): ½Cuþ ðTCNQ Þn Ð½Cu0 x þ ½TCNQ0 x þ ½Cuþ ðTCNQ Þnx This effect, however, remained controversial49 as difficulties were experienced in repeating it in a consistent fashion. Dunbar and co-workers, however, showed that two polymorphs of Cu(TCNQ) are formed.50 In phase I the TCNQ radicals are arranged in p-stacking arrangements with separations of ca. 3.24 A˚, whereas in phase II, which has two interpenetrating PtS nets, the closest approach of the

403


TCNQ rings is ca. 6.8 A˚. Furthermore, it was shown that phase I (the kinetic phase) is a good semiconductor (conductivity 0.25 S cm1 at room temperature), whereas phase II (the more thermodynamically stable phase) is a very poor semiconductor (conductivity 1.3 105 S cm 1). It was therefore proposed that the variable results observed were due to different mixtures of the two phases in different samples and that the switching behaviour involves a phase transformation from phase II to phase I. This was later supported by other detailed experiments.51 A series of structures containing ‘shish kebab’ motifs have been synthesised.52 These contain metal macrocycles (such as porphyrins and phthalocyanines) bridged in the axial positions by linear bis(pyridyl)-type bridges to give 1D chains (Figure 13.6). Good semiconductive properties (sRT up to ca. 0.1 S cm1) have been observed for both the undoped materials and others oxidatively doped with iodine. In addition to iodine intercalation, a good way to introduce conductive properties into coordination frameworks is through the use of radical cations such as bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET). A series of ‘organic’ superconductors have been reported with layers of these cations sandwiched between layers of either discrete or polymeric anions. For the polymeric anions, a number of different motifs have been observed, although it is the packing arrangement within the layers of cations which determines if they are superconductors or not. The electrooxidation of ET in the presence of cyanide, CuI and dicyanamide [dca, N(CN)2] results in the formation of three different products.53,54 In k-(ET)2Cu(CN)(dca) the anions are composed of chains of copper atoms bridged by cyanide ligands, with pendant dca anions (Figure 13.7a).53–57 The Tc of this compound is 11.2 K. The structure of k 0 -(ET)2Cu2(CN)3 (Tc ¼ 3.8 K) contains 2D (6,3) anionic sheets (Figure 13.7b),53,54,56–59 whereas y(ET)2Cu2(CN)(dca)2 contains layers of doubly interpenetrating (6,3) sheets (Figure 13.7c) but is a semiconductor with no superconducting properties observed.53,60,61 The highest superconducting Tc values for this series, however,

N

N

N

N

N

N

N

N N

N

N

Fe

N

N

N

N

Fe

N N

N

N

Figure 13.6

N

N N

Fe

N

N N

N

N

N

N N

N

N

Schematic representation of a typical ‘shish kebab’ polymer, Fe(Pc)(pyz), where Pc ¼ phthalocyanine and pyz ¼ pyrazine.52

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Chapter 13 62

were seen for k-(ET)2Cu(dca)X (Tc ¼ 12.8 K at 0.3 bar for X ¼ Cl and Tc ¼ 11.8 K at ambient pressure for X ¼ Br63–65). These compounds have 1D chains of copper atoms bridged by dca ligands, with pendant halide anions (Figure 13.7d). The structure of (ET)2Ag4(CN)5 was reported as being the first example of a 3D donor-network organic conductor.66 The ET cations have a 3D arrangement (rather than the 2D arrangements in the above compounds), although the anions again form polymeric 2D layers (Figure 13.7e). Other derivatives of ET have also been used to give conducting coordination polymer structures. The oxygen derivative bis(ethylenedioxy)tetrathiafulvalene (BEDO-TTF) is contained in the superconducting materials (bm-BEDOTTF)3Cu2(SCN)3 (Tc ¼ 1.06 K).67 The anion layers contain 2D (6,3) sheets (Figure 13.7f). The structure of k-(BETS)2[FeIII(C2O4)Cl2] contains the selenium derivative bis(ethylenedithio)tetraselenafulvalene and shows metallic behaviour from room temperature (conductivity at 300 K is 85 S cm1) down to 4.2 K.68 The anions are composed of 1D chains of oxalate-bridged metal atoms. There are numerous other systems for which semiconductive behaviour has been observed.69,70 These include a number of networks formed between silver and aromatic hydrocarbons,71 bridging ligands based on tetrathiafulvalene (TTF),72 complexes with Ag. . .Ag interactions,73,74 coordination polymers of pyridylthiol ligands74,75 and many other varied systems.76 Finally, we shall mention two other unusual compounds. The structure of [Rh2(acam)4]2I 6H2O, where Hacam ¼ acetamide, contains a porous diamond net composed of m4-I atoms bridged by 2-connecting mixed-valence Rh2(acam)4 moieties.77 The structure has a large degree of hydration and shows an electrical conductivity that varies by a factor of 105 during dehydration–rehydration cycles. The compound Cu(H2dtoa), where H2dtoa ¼ dithiooxamide anion, has a 2D structure and shows proton conduction similar to the proton-exchange membrane Nafion.78

13.5 Negative Thermal Expansion Negative thermal expansion (NTE) materials shrink with increasing temperature rather than expand.79,80 Among the more significant materials to show this property are the coordination polymers M(CN)2, where M ¼ Zn, Cd, ZnxCd1x.81–83 These show coefficients of thermal expansion of up to a ¼ 20.4 106 K1 (a ¼ dl/ldT). The structures contain two interpenetrating diamond nets; however, it is the cyanide bridges that are important for generating the observed NTE. It is proposed that the NTE is due to the thermal vibrations of the bridging cyanide ligands around the metal–metal axis (Figure 13.8). As the temperature increases, the off-axis vibration increases and the bridged metals are drawn closer (even though the individual ‘true’ bonds increase in size). This is because the further away from the metal–metal axis the cyanide atoms vibrate to, the shorter are the ‘apparent’ bond lengths (which is derived from the average positions of the atoms).


405

Figure 13.7 The anion coordination nets found in a series of salts of the radical cations

ET and derivatives: (a) [Cu(CN)(dca) ]; (b) [Cu2(CN)3]; (c) [Cu2(CN) (dca)2]; (d) [Cu(dca)X], X ¼ Cl, Br; (e) [Ag4(CN)5]; (f) [Cu2(SCN)3].

406

Figure 13.8

Chapter 13

Transverse vibration of a two atom bridge, leading to NTE. Reprinted with permission from J. Am. Chem. Soc., 2005, 127, 15630. Copyright 2005 American Chemical Society.81

Other phases have been shown to exhibit either NTE or ZTE (zero thermal expansion), including Prussian Blue-type structures84–86 and linear chains [in the latter case the NTE is anisotropic; in two of the three cell directions positive thermal expansion (PTE) is observed].87 The thermal expansion properties have also been found to be dependent on guest species.88 For example, the compound ErIII[CoIII(CN)6] 4H2O shows PTE in the hydrated form and NTE in the dehydrated form.89 The change is attributed to the thermal dependence of the kinetic volume of bound and unbound water molecules in the structure. The structure of Cd(CN)2 CCl4 contains a single diamond net with intercalated CCl4 molecules and shows PTE with a ¼ +10.0(2) 106 K1.90 However, this material can be partially or completely desolvated and as the degree of solvation drops the material changes from showing PTE to showing NTE, with the fully desolvated material having a ¼ 33.5(5) 106. Large NTE behaviour has also been calculated for a range of porous metal carboxylate-based coordination polymer frameworks, with experimental evidence drawn from the literature to support the calculations.91


407

13.6 Multifunctional Materials A significant advantage of the modular approach to coordination polymer design is the fact that materials may be engineered with multiple properties. We have already seen some examples throughout this book – for example, chiral magnets were discussed in Section 9.2.2.11.92 Chiral catalysts were discussed in Section 11.3.5 – these materials need to have at least three different properties: chirality, porosity and chemical reactivity. Materials that act as chemical sensors also need to combine porosity with a ‘reporting’ function such as magnetic or luminescent (Section 13.2) properties. A number of examples of the combination of porosity with other properties were discussed in Section 10.6. For example, the material Fe2(azpy)4(NCS)4 (guest), where azpy ¼ trans-4,4 0 -azopyridine, combines porosity with guest-responsive spin crossover properties.93 This material contains 2D sheets showing inclined interpenetration; a 1D chain material has also been reported to show guestresponsive crossover properties.94 Magnetically ordered porous materials are a particular challenge as the spin centres must be close enough to show magnetic communication while arranging themselves to provide open spaces within the framework.95 This has been achieved with a series of metal formates of general formula M3(HCOO)6, where M ¼ Mn, Fe, Co, Ni.96–98 The structures contain diamond-like arrangements of metal atoms bridged by the formate ligands (Figure 13.9) and the magnetic ordering temperatures are dependent on the nature of the exchangeable guests. Guest-dependent magnetic behaviour was also observed in a porous cobalt isophthalate magnet.99 A vanadium terephthalate has been shown to be an antiferromagnet below TN ¼ 95 K,100 while a

Figure 13.9

The porous structure of the M3(HCOO)6 magnets.97

408

Chapter 13 101

nickel succinate shows ferromagnetic ordering below 20 K. Both structures are also porous. A nickel glutarate has been found to be a chiral, porous ferromagnet.102 Prussian Blue-type metal cyanide magnets have also been shown to be porous.103 Finally, a copper coordination polymer of a large, trigonal radical anion ligand has been reported to show magnetic ordering and contains very large pores (2.8–3.1 nm).104 The presence and nature of solvent included in the structure induces structural changes in the framework which affect the magnetic properties. Another significant body of work has sought to combine magnetic properties with conductivity.105 A very successful strategy for this has been employed by Coronado’s group, which is to intercalate layers of ET radical cations between 2D (6,3) metal oxalate sheets. The ET layers provide conductivity, while the metal oxalate sheets provide magnetic ordering; the two sublattices effectively show no interaction. The first example of this was the compound [ET]3[MnCr(C2O4)3].106 This material is a magnet below Tc ¼ 5.5 K, shows room temperature conductivity of ca. 250 S cm1 and shows metallic behaviour down to at least 2 K. When ET is replaced with its selenium analogue, Tc remains essentially the same (5.3 K), but the material is a semiconductor.107 The compound [ET]x[MnRh(C2O4)3] CH2Cl2, x ¼ 2.526(1), shows high room temperature conductivity (13 S cm–1) and metallic behaviour, but is not magnetic due to the diamagnetic low-spin RhIII.108 This strategy has been further extended to other cations and metals.109,110 The metal oxalate–cation system is, in fact, a rather versatile one for combining magnetism (metal oxalate nets) with other properties (cation induced).111 Use of tris(chelate)metal cations can induce chirality (see Section 11.3.3), optical activity, photophysical or luminescent properties.111,112 Other cations have been used to introduce photoreactivity,113 redox activity,114 non-linear optical activity115 or spin crossover properties.116 A number of other materials have been reported to combine properties such as anion-dependent magnetism and photoreactivity,117 luminescence and conductivity,118 catalysis and luminescence,119 and sorption and optoelectronic properties.120 Finally, a chiral structure has been shown to be porous, to act as a catalyst, show anion exchange (with enantioselectivity) and undergo N-alkylation of pyridyl groups within the pores.121

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Subject Index Note: Page numbers in italic refer to figures. -Po net 46–7, 83 acentric networks 345–51 acetate ligands 203–4 acetylene gas storage 337 actinide polymers 229–31 adamantane cavity 20 adsorption/desorption 314–15, 327, 331 Ag(I)–aromatic complexes 239–41 alkene/alkyne-based ligands 243–5, 384 see also tetracyanoethylene ligand amide ligands 217–19 amino acids 361–2 amorphous polymers 12, 388–9 anatase net 51, 52 anion templation 119–22 antiferromagnetism 274 and Kagomé latticee 30–2 apohost defined 314 architectural isomerism 99 aromatic polycyclic ligands 239–41 augmentation 39 azide ligand magnetic ordering 109, 278, 288 rare earth polymers 193 transition metal polymers 146, 149–51

benzene-1,3,5-tribenzoate ligand 177, 324 benzene-1,3,5-tristetrazolate ligand 169, 325, 333–4 benzene-1,4-disulfonate ligands 217 benzoquinones 246, 248 1,1 -bi-2-napththol 363–4 bilayer 2D structures 33–5 bimetallic polymers carboxylate-based 224 cyanide-based 221, 221–4, 277 metal–oxide 259–71 see also heterometallic BINOL ligands 363, 364, 368 3,3 -bipyridine ligand 265–6 3,4 -bipyridine ligand 266–7 4,4 -bipyridine ligands 160–1, 196 Hofmann-like nets 307 metal–oxide substructures 258, 261–3 N-oxide donors 170–1, 196–202 bistable switching 402 bonding/non-bonding interations 24 boracite nets 49, 50 interpenetrating 84 Borromean interpenetration 72–3, 76

ball milling 114 BEDT-TTF radical cation 403–4 benzene carboxylate ligands 23, 173, 175–7, 205–7, 210–12, 289–90 see also trimesic acid

cadmium cyanide lattice 115 capping ligands 6, 7, 8 carbon dioxide absorption 63–4, 205 carbon monoxide oxidation 388 carbonyl bridging ligands 238, 248–51

Subject Index

carboxylate ligands bimetallic polymers 224 chiral 174–5, 362–3 isomeric polymer 104 and luminescence 398–9 magnetic ordering 288–90 and porous polymers 324–6 rare earth polymers 202–3, 398 monocarboxylate 203–5 dicarboxylate 205–10, 229–30 tricarboxylate 210–12 tetracarboxylate 212, 213 transition metal polymers 23, 107, 172–3 2-connecting 173–5 3-connecting 175–7 4-connecting 177–8 see also pyridylcarboxylate catalytic activity 13, 20, 368–9, 386–91 catenane isomerism 99, 105 catenated structure 65 cation templation 122–4, 129–31 cation– interactions cyclophanes 241–3 non-aromatic 243–5 chalcogenide ligands 301 charge–transfer processes luminescence 397–9 redox 400 chemically reactive ligands 386 chemisorption 331 chiral 1D chains 317–18 chiral ligands 174–5, 362–3 amino acid-based 363, 365–7 BINOL derived 363–4 Mn(salen)-derived 387 and molecular magnets 292–3 pyridylcarboxlylate 180–1 chiral networks 37, 40, 345, 345–6, 352 applications 367–9 from achiral components 1D helices 352–4 2D nets 354–5 3D nets 355–61 from chiral ligands 361–7 interpenetrating 78–80

417 chloranilic acid 215–16 clathrates 1, 2, 181–2, 216, 305–7 clusters hydrogen bonded 128–9 inorganic 172–3, 175, 257 metal–ligand 23, 25 metal–oxide 258–61 and rigid porosity 324–6 cobaltocenium dicarboxylate 246, 247 computer programs 28 conductivity 64, 401–4 conformational isomers 99, 103–4 connectivity 20-5, see also networks convergent ligands 7 cooperative magnetism 273 coordination bond 7, 9–10 coordination geometries rare earth metals 191–2 transition metals 107–9, 144–5 coordination polymers, introduction to coordination polymers defined 6–10 design, analysis and applications 12–13 development and research 1–6 synthetic techniques 10–12 counterions anion templation 119–22 cation templation 122–4, 129–31 crystal engineering 4–6 crystallization techniques 10–12 cyanate ligand molecular ordering 279 rare earth polymers 193 transition metal polymers 152–3 cyanide ligands bimetallic polymers 221–4 magnetic ordering 276–8 organotin polymers 251–2 rare earth polymers 192–5 spin crossover 305–7 transition metal polymers 146–9 cyanosilation 387 cyclohexane-1,3,5-triacetate ligand 212, 388-9 cyclohexane-1,4-dicarboxylate ligand 210, 321

418

cyclopentadyenyl ions 238, 241 cyclophanes 241–3 DABCO ligand 258, 267, 321 DCNQI ligand 401–2 decomposition, controlled 113–14 decoration 39 delocalised systems 131–3 design 3, 12, 32 and coordination bond 7, 9–10 and malleability 96–7 see also nets diamond-like networks 19–20, 80–1 dicyanamide ligand coordination modes 109–10 magnetic ordering 279–82 rare earth polymers 193–4 and solvent effects 115–16 transition metal polymers 153–6 dicyanonitrosomethanide ligand 112–13 N,N-dicyanoquinonediimine 401–2 Diels-Alder catalysis 389 dihydroxybenzoquinone 215–16 divergent ligands 7 dynamic guest exchange 327–9 enantioselective properties 180–1, 367–9 end-on/end-to-end binding 149–52, 278–9 entangled systems 61, 89 epoxidation reactions 387 ET radical cation 403–4, 405, 408 ferrimagnetism 273–4 ferrocene 238, 245–6, 400, 401 ferroelectricity 345 ferromagnetism 273–4 five-membered rings networks 31–2 nitrogen donor ligands 166–9 flexible guest exchange 327–9 flexible vs rigid polymers 316–17 see also porous polymers fluorescence defined 397 see also luminescence fluorite net 51, 53 formate ligand 179, 203, 288–9, 407

Subject Index

Friedel-Crafts catalyst 389 gas sorption 327, 331 isotherms 314–15 gas storage 147, 330–1 acetylene 337 hydrogen 331–5 methane 335–6 oxygen 337 gate opening/closing 63–4, 317 gels 396 glutamate ligand 361 glutarate ligand 209–10, 230, 408 glycinate ligand 229 graphical representation 28 guanidinium cation 129–31 guest molecules 375 flexible exchange 327–9 gate opening/closing 63–4, 317 intercalated 119 rigid exchange 329–30 see also hydration/dehydration hard magnets 274 hemibuckminsterfullerene 241, 242 heterogenous interpenetration 84–7, 106 heterometallic (3d/4f) polymers 221–9 carboxylate-based ligands 224 cyanide-based ligands 221–4 glycinate and iminodiacetate ligands 229 isocarbonyl polymers 248–51 pyridylcarboxylate ligands 224–8 hexaazatriphenylene ligand 165 hexamethylenetetramine ligand 179 hexaphenylbenzene 239–40 Hofmann clathrates see clathrates homochiral systems see chiral networks hybrid structures see inorganic–organic hydration/dehydration 128, 268, 376–80, 404 hydrogen bonding 5, 124–31 hydrogen phosphate groups 262–3 hydrogen storage 331–5 hydrogenation 389 hydroquinones 246

419

Subject Index

imidazole ligands 166–70 magnetic ordering 290–1 inclined interpenetration 69–70 induced fit 327–8 inorganic clusters 172–3, 175, 257 rigid porosity 175, 322–6 inorganic–organic hybrids 257–9 0D metal–oxide substructures 259–60, 261 1D metal–oxide substructures 260 2D metal–oxide substructures 264–71 1D metal–oxide substructures 2D polymers 261–3 3D polymers 263–4 interactions, network 24, 124–34 see also cation– intercalation 59, 60, 104, 119 iodine 403 of networks 62 interconnected layers 33 interdigitation 59, 61, 89, 104 and permanent porosity 319 interpenetration 59–95 definition 61–2 entangled systems 61, 89 heterogeneous 84–7, 106 and isomerism 104–6 and magnetic properties 64 modes of 1D 69–70 2D 70–7 3D 77–84 nomenclature 67–9 and porosity 62–4, 319–20, 321–2, 333 reducing 64–5 self-penetration 37, 39, 79–80, 87–9, 281 topography of 65–7 interweaving 61, 63 iodine doping 403 ionic liquids 118–19 IRMOFs 172–3, 175, 323–4, 336, 340 isocarbonyl polymers 248–51 isocyanate ligand 193 isomerism, supramolecular 98–106 isoreticular metal–organic frameworks see IRMOFs

isostructural materials 172–3 isotherms, gas sorption 314–15 Kagomé lattice 29–31 Knoevenagel condensation 387 ladder-like nets 29, 69, 70, 72, 73 lanthanoids see rare earth ligands aromaticity 162–3 binding angle 162–3 capping 6, 7, 8 coordination modes 109–10 divergent vs convergent 6, 7 functionality 163–4 ligand length 162 linear 19 rigid vs flexible 10 steric effects 110–11 tetrahedral 19 see also under rare earth metal; transition metal links, network 20–5 londalite net 39–40, 41 long-range magnetic ordering see magnetic ordering luminescence 157, 192, 196, 339, 397–9 macroporous defined 314 magnetic data storage 293 magnetic molecular sponge 285 magnetic ordering background and terminology 273–5 molecule-based magnets 275–6 azides 278 carboxylate ligands 288–90 chiral magnets 292–3 cyanates/thiocyanates/selenocyanates 279 cyanides 276–8 dicyanamide 279–82 imidazolates 290–1 organic radical ligands 283–4 oxalate ligands and analogues 285–8 oxamato systems 284–5 tetracyano-p-quinodimethane 382–3

420

tetracyanoethylene 382–3 tricyanomethanide 279–82 single-chain magnets 293–4 magnetic sensing 229 magnetism see magnetic ordering; spin crossover magneto-optical effects 292 malleability and design 96–7 counter ions 119–24 metal and ligand geometries 107–11 solvent effects 115–19 supramolecular isomerism 98–107 and synthetic approach 112–15 weaker/non-covalent interactions 124–34 malonate ligand 207–9 mechanochemical synthesis 114 mesoporous defined 314 metal ions common 9–10 octahedral 22, 22 tetrahedral 19, 23 see also rare earth metal; transition metal metallocene bridging ligands 238, 245–6, 400, 401 metallocyanide sheets 181 metallophilic interations 124, 133–4 metallosupramolecules 4, 6, 7 metal–ligand clusters see clusters metal–organic frameworks see MOFs metal–organometallic networks 238, 245–6 metal–oxide substructures see under inorganic–organic hybrids metal–quinone bridging complexes 246, 248 metamagnet defined 274 methane storage 335–6 microporous defined 314 microporous polymers 114 MIL materials 325–6, 332, 334–5 mixed 3d/4f polymers see heterometallic mixed connectivity nets 48–52 mixed donor atom ligands 180–1

Subject Index

mixed ligand polymers 181–3 MOFs 172–3 isomerism 104 rigid porosity 322–4, 330, 337 see also IRMOFs moganite net 44, 45 molecular-based magnets see under magnetic ordering molybdate-based materials 257, 259–60, 261 pillared-layers 265–70 mucate ligand 125–6 multifunctional materials 13, 396, 407–8 multilayered 2D structures 35 multinodal nets 44–5 N-oxide ligands rare earth polymers 197–202 transition metal polymers 170–2, 294 nanoporous defined 314 nanotubes 89, 115, 132 nanowires 294 naphthalene-1,5-disulfonate ligands 217 naphthalene-2,6-dicarboxylate ligand 107 negative thermal expansion 404, 406 networks background/introduction 3, 19–20 1D nets 28–9, 30 2D nets 27, 29–35 3D nets 36 3-connected 36–9 4-connected 39–45 5-connected 45–6 6-connected 46–7 7-connected 47 8-connected 47, 48 12-connected 48, 49 mixed connectivity 48–52 defining features of 20–5 flexible vs rigid 316–17 identifying and naming 25–8 rod packings 52–3 see also chiral networks; interpenetration nicotinate ligand 124–5 N-oxide 171

Subject Index

NITphOMe ligand 294 nitrile ligands 158–9 rare earth polymers 192–5 transition metal polymers 146, 156–9 nitronyl nitroxide-based radicals 283 nitroxide ligands 292 nodes and nodal geometries 20–5 nomenclature, network 25–8 interpenetrating 67–9 non-covalent interactions 124–33 non-crystalline materials 12 non-linear optical activity 192, 345–51 occlusion 331 OLEX program 28 optical activity see non-linear optical activity optical isomers 99 organolead polymers 251–2 organometallic networks 238 cation– interactions Ag(I)-PAH systems 239–41 alkene/alkyne based ligands 243–5 cyclophanes 241, 243, 244 isocarbonyl polymers 248, 250–1 metallocene bridging ligands 245–6, 247 metal–quinone bridging complexes 246, 248, 249 organotin/organolead systems metal–cyanide polymers 251–2 tetrazole polymers 252–3 oxalate ligands 178 magnetic ordering 285–8, 408 rare earth polymers 205, 212–15 oxamide ligand 284–5 oxidation , catalysis of 388, 389 oxo-centred trimer 322–3, 325 packing modes 59–61 paddlewheel structures and catalysis 388 and cylcophanes 243, 244 and molecular ordering 289–90 and porosity 323, 324–5 transition metal polymers 166, 173, 175, 176, 178

421 parallel interpenetration 69–70 permanent porosity defined 314 designing 315–17 phosphane ligand 180 phosphomolybdate clusters 259 phosphorescence defined 397 see also luminescence photodimerisation/polymerisation 381–4 physisorption 331 –cation interactions with cyclophanes 241–3 non-aromatic 243–5 - interactions 124, 131–3, 288 stacking and porosity 317, 318 piano-stool complexes 246 picolinic acid 225–7 Piedfort pairs 131, 132 piezoelectricity 345 pillared-layers 159, 182–3, 259, 264–71 magnetic interactions 265–8 and permanent porosity 320–1 redox reaction 340 piperazine ligand 160 PIZAF-1 321, 325, 326, 329–30 platinum see Pt platonic uniform net 26 polychlorinated carboxylate ligands 180, 284 polycyclic aromatic ligands 239–41 polymerisation catalyst 389–90 polymorphism 96, 98–104, 291 and metal geometries 107–9 polynitrile ligands rare earth polymers 195 transition metal polymers 153–9 porosity defined 313–14 and interpenetration 62–4, 321–2, 333 porous polymers 127, 147, 148–9 applications 313 gas storage 330–8 ion exchange 338–40 multifunctional materials 338–40 solvent exchange 326–30 background and terminology 313–15

422

designing permanent porosity 315–17 flexible systems 316–17 1D chain materials 317–18 2D stacked layers 318–19 guest exchange 327–9 pseudo-3D materials 319–20 3D interpenetrated 321–2 3D pillared layered 320–1 rigid 3D systems 166, 175, 316, 322–6 guest exchange 329–30 inorganic cluster units 175, 322–6 rigid organic ligands 326 powder diffraction 12 prismatic trimer 173 Prussian Blue 6, 147 discovery and structure 1, 2, 46 magnetism 276 ‘super’ Prussian Blue 252 pseudohalide ligands magnetic ordering 278–9 rare earth polymers 192–5 transition metal polymers 146–56 pseudorotoxane 201–2 psuedo-polymorphism 100, 102–4 Pt2O4 net 48, 50 PtS net 65, 83 pyrazine carboxylate ligands 182, 268 pyrazine ligand 159 pyrazole ligands rare earth polymers 195–6 spin crossover 300–1 transition metal polymers 166–9 pyridine-2,6-dicarboxylic acid N-oxide 171 pyridone ligand 180 pyridyl ligands rare earth polymers 196 spin crossover materials 306–7 transition metal polymers 160–1 2-connecting 159–64 3-connecting 164–5 4-connecting 165–6 pyridylcarboxylate ligands actinide polymers 231 and enantioselective properties 368–9 heterometallic (3d/4f) polymers 224–8

Subject Index

rare earth polymers 219–21 transition metal polymers 180–1, 182, 196 pyrimidine ligand 160 pyroelectricity 345 quartz nets 40, 41, 82–3 quinones 215–16, 246, 248 racemization 367 radical ligands 283–4, 286 BEDT-TTF/ET 403–4, 405 DCNQI/TCNQ 401–3 rare earth polymers 191–2 actinide polymers 229, 229–31 amide ligands 217–19 carboxylate ligands 202-3 monocarboxylate 203–5 dicarboxylate 205–10, 230 tricarboxylate 210–12 tetracarboxylate 212 chloranilate ligand 215–16 coordination geometies 191–2 cyanide/nitrile/pseudohalide ligands 192–5 dihydrobenzoquinone ligand 215–16 heterocyclic five-membered ring donors 195–6 hydrogen bonded clusters 128–9 N-oxide donors 196–202 oxalate ligand 212–15, 230 pyridyl donors 296 pyridylcarboxylate ligands 219–21 sulfonate-containing ligands 217, 387 see also heterometallic (3d/4f) polymers reactive polymers catalysis 386–91 topotactic reactions 375–86 redox activity 399–401 reticular chemistry 20 Structure Resource 28 reversibility of coordination 7–8 rod packing 52–3 rotoxanes 61, 89, 201–2 rutile nets 31, 51, 52 interpenetrating 84, 87

Subject Index

saccharate ligand 125–6, 175 sandwich complexes see metallocenes Schläfli symbol 26–8 second-harmonic generation 345, 346–51 secondary building units 23, 24, 172–3 and rigid porosity 322–6 see also clusters self-dual nets 65 self-penetrating nets 37, 39, 79–80, 87–9, 281 semiconductor properties 403–4 sheet structures 23, 24, 31, 32, 33 interpenetrating 65–6, 67, 74–9 SHG response 345, 346–51 ship-in-a-bottle 390–1 shish kebab structures 403 shortest circuit 26 silver–aromatic complexes 239–41 single-chain magnets 293–4 solvents effects 115–19, 302–3 guest exchange flexible/dynamic 326–9 rigid 326, 329–30 hydration/dehydration 128, 268, 376–80, 404 solvothermal techniques 12, 112–13 spin canting 274, 281 spin coupling 273–4 spin crossover 63, 128 background/introduction 295–7 cyanide bridges in Hofmann phases 305–7 five-membered bridging ligands 297–8 pyrazole chelates with aromatic spacers 300–1 tetrazoles 299–300 triazoles 298–9 guest-responsive 407 six-membered bridging ligands 301–5 spin frustration 30–1 spin glass 274 spin states 295–7 structural building units see secondary building units structural isomers 99, 101, 103

423 SUBs see secondary building units sulfonate-containing ligands 217, 387 supramolecular chemistry 4–6 supramolecular isomerism 98–106 synthetic techniques 10–12 synthons 4, 6, 20 Systre program 28 TCNQ ligand 402–3 tectons 4, 6, 20 temperature dependent reactions 384–6 terephthalate ligand 23, 173, 205–6 tetracyano-p-quinodimethane ligand 158–9, 195 magnetic ordering 382–3 tetracyanoaryl ligands 157–8 tetracyanoethylene ligand magnetic ordering 382–3 rare earth polymers 193, 195 transition metal polymers 158–9 tetrahedral ligands 19 tetrazole ligands 166–9 spin crossover 299–300 thermochromism 229 thiocyanate ligands 152, 252 magnetic ordering 279 tin see organotin topological isomers 102 topology of networks 21 TOPOS program 28 topotactic reactions 375–86 chemically reactive ligands 386 photoactive effects 381–4 solvation effects 128, 375–81 temperature dependent 384–6 transition metal polymers 9–10, 144–90 common ligands carboxylate 172–8 five-membered ring nitrogen donor 166–70 larger nitrile donor 156–9 miscellaneous 178–80 mixed donor atom 180–1 N-oxide 170–1 pseudohalide 146–56 pyridyl donor 159–66

424

coordination geometries 107–9, 144–5 mixed ligand polymers 181–3 see also heterometallic (3d/4f) polymers triazole ligands metallocene bridging 246, 247 rare earth polymers 196 spin crossover 297–9 transition metal polymers 166–9 tricyanobenzene ligand 156–7 tricyanomethanide ligand magnetic ordering 279–82 rare earth polymers 193–4 transition metal polymers 153–6 trilayer 2D structures 35 trimesate ligand 175–6, 210–21 trimesic acid 4, 5

Subject Index

(triorganostannyl)tetrazole polymers 252–3 tryptycene 239, 241 tubular nets 29, 30, 89, 115, 132 tungstate-based materials 257 uniform net 26 vanadate-based materials 257 vertex symbol 27 W-type architecture 240 weak interactions 124–33 woven structure 61, 63, 89 zeolite nets 42 zeolites 313 zinc acetate cluster 173, 175, 176, 322, 323

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