Malcolm A. HalcrowEDITOR

SPIN-CROSSOVER MATERIALSPROPERTIES AND APPLICATIONS

Spin-Crossover Materials

Spin-Crossover Materials

Properties and Applications

Edited by

MALCOLM A. HALCROWSchool of Chemistry, University of Leeds, UK

A John Wiley & Sons, Ltd., Publication

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Contents

List of Contributors xv

Preface xvii

1 The Development of Spin-Crossover Research 1Keith S. Murray1.1 Introduction 11.2 Discrete Clusters of SCO Compounds 4

1.2.1 Dinuclear FeII - FeII SCO Clusters 61.2.2 Tri-, Tetra-, Penta- and Hexa-nuclear FeII SCO Clusters 18

1.3 1D Chains of FeII SCO Materials 221.4 1D Chains of FeIII SCO Materials 281.5 2D Sheets of FeII SCO Materials 291.6 3D Porous SCO Materials 301.7 Some Recent Developments in Mononuclear SCO FeII, FeIII and CoII Compounds 33

1.7.1 Iron(II) and Iron(III) 331.7.2 Cobalt(II) 35

1.8 Multifunctional/Hybrid SCO Materials 371.8.1 SCO and Porosity 381.8.2 SCO and Electrical Conductivity 381.8.3 SCO and (i) Short-Range Exchange Coupling or (ii) Long-Range Magnetic Order 381.8.4 SCO and Liquid Crystals 391.8.5 SCO and Gels 391.8.6 SCO and NLO 39

1.9 Developments in Instrumental Methods in Spin-Crossover Measurements 401.10 Applications of Molecular Spin-Crossover Compounds 411.11 Summary 42Acknowledgements 42References 43

2 Novel Mononuclear Spin-Crossover Complexes 55Birgit Weber2.1 Introduction and General Considerations 552.2 Novel Coordination Numbers (CN), Coordination Geometries and Metal Centres 57

2.2.1 Coordination Number 7 572.2.2 Coordination Number 6 582.2.3 Coordination Number 5 602.2.4 Coordination Number 4+1 622.2.5 Coordination Number 4 63

vi Contents

2.3 Iron Complexes with Novel Ligand Donor Atoms and New Ligand Systems 652.3.1 N6 Coordination Sphere 652.3.2 N4O2 Coordination Sphere 66

2.4 Other Examples 702.5 Conclusion and Outlook 72References 72

3 Spin-Crossover in Discrete Polynuclear Complexes 77Juan Olguın and Sally Brooker3.1 Introduction 773.2 Dinuclear Iron(II) Complexes 79

3.2.1 Supramolecular Approach 793.2.2 ‘Controlled/Designer-Ligand’ Approach 843.2.3 Ligands with Two Isolated Binding Pockets 843.2.4 Ligands with Potential for Communication between Binding Pockets 91

3.3 Higher Nuclearity Iron(II) Compounds 983.3.1 Trinuclear Iron(II) Complexes 983.3.2 Tetranuclear Iron(II) Complexes 1003.3.3 Higher Nuclearity Mixed Metal/Valent Iron(II) Complexes 103

3.4 Iron(III) 1043.4.1 Dinuclear Iron(III) Complexes 1043.4.2 Mixed Metal Iron(III) Complexes 1083.4.3 Mixed Valence Iron(II)/(III) Complexes 108

3.5 Cobalt(II) 1093.5.1 Dinuclear Cobalt(II) Complexes 1093.5.2 Trinuclear Cobalt(II) Complexes 110

3.6 Dinuclear Chromium(II) Complex 1113.7 Concluding Remarks 112References 113

4 Polymeric Spin-Crossover Materials 121M. Carmen Munoz and Jose Antonio Real4.1 Introduction 1214.2 One-Dimensional SCO-CPs 121

4.2.1 Triazole Based Bridges 1214.2.2 Tetrazole Based Bridges 1244.2.3 Bis-Monodentate Pyridine-Like Bridges 1244.2.4 Polydentate Chelate Bridges 1264.2.5 Anionic Bridging Ligands 127

4.3 Two Dimensional SCO-CPs 1284.3.1 Neutral Organic Bridging Ligands 1284.3.2 Dicyanometalate [MI(CN)2]– Bridging Ligands (MI = Cu, Ag, Au) 1304.3.3 Tetracyanometalate [MII(CN)4]2– Bridging Ligands (MII = Ni, Pd, Pt) 132

4.4 Three-Dimensional SCO-CPs 1334.4.1 Neutral Organic Bridging Ligands 1334.4.2 Dicyanometalate [MI(CN)2]– Bridging Ligands 134

Contents vii

4.4.3 Tetracyanometalate [MII(CN)4]2– Bridging Ligands 1364.4.4 Hexa- and Octacyano-metallate Bridging Ligands 137

4.5 Conclusion 138Acknowledgement 138References 139

5 Structure:Function Relationships in Molecular Spin-Crossover Materials 147Malcolm A. Halcrow5.1 Introduction 1475.2 Molecular Shape 150

5.2.1 Molecular Shape Inducing Cooperativity 1535.2.2 Molecular Shape Inhibiting Spin-Crossover 154

5.3 Crystal Packing 1555.3.1 Short Intermolecular Contacts 1565.3.2 Inhibition of Spin-Crossover by Steric Congestion 157

5.4 Cooperativity Mediated by Disorder 1585.5 Compounds Showing Wide Thermal Hysteresis 158

5.5.1 Compounds with Symmetric Hysteresis Loops 1595.5.2 Compounds with Structured Hysteresis Loops 161

5.6 Other Noteworthy Compounds 1625.6.1 Iron(II) Triazole Coordination Polymers 1625.6.2 Cooperative Complexes of Other Metal Ions 163

5.7 Conclusions 164References 164

6 Charge Transfer-Induced Spin-Transitions in Cyanometallate Materials 171Kim R. Dunbar, Catalina Achim and Michael Shatruk6.1 Introduction 1716.2 Characterization of CTIST Compounds 1736.3 CTIST in Coordination Polymers 174

6.3.1 Co-Fe Prussian Blue Analogs 1746.3.2 Other Prussian Blue Analogs 1836.3.3 Coordination Polymers Based on Octacyanometallates 185

6.4 CTIST in Nanoscale Materials 1896.4.1 Thin Films 1896.4.2 Nanoparticles 192

6.5 CTIST in Polynuclear Transition Metal Complexes 1956.6 Summary and Outlook 198Acknowledgement 199References 199

7 Valence Tautomeric Transitions in Cobalt-dioxolene Complexes 203Colette Boskovic7.1 Introduction 2037.2 Induction of Valence Tautomeric Transitions 205

7.2.1 Thermally Induced Valence Tautomerism 2057.2.2 Pressure Induced Valence Tautomerism 205

viii Contents

7.2.3 Light Induced Valence Tautomerism 2077.2.4 Magnetic Field Induced Valence Tautomerism 2087.2.5 X-Ray Induced Valence Tautomerism 209

7.3 Other Factors that Contribute to the Valence Tautomeric Transition 2107.3.1 Ancillary Ligand Effects 2107.3.2 Counterion and Solvation Effects 2107.3.3 Cooperativity 2127.3.4 Valence Tautomerism in Solution 214

7.4 Polynuclear Valence Tautomeric Complexes 2147.4.1 Dinuclear Valence Tautomeric Complexes 2147.4.2 Polymeric Valence Tautomeric Complexes 217

7.5 Bifunctional Valence Tautomeric Complexes 2187.6 Concluding Remarks 220Acknowledgements 221References 221

8 Reversible Spin Pairing in Crystalline Organic Radicals 225Jeremy M. Rawson and John J. Hayward8.1 Introduction 2258.2 Radical Pairs: Solution and Gas Phase Studies 226

8.2.1 Radical Dimerisation in Solution 2268.2.2 Computational Studies on Dimerisation 226

8.3 Dimerisation in the Solid State 2298.3.1 Structural Studies 2298.3.2 Electronic Structure and Bonding 2298.3.3 Thermally Accessible Triplet States 2308.3.4 Spin-Transition Radical Dimers 2308.3.5 Trithiatriazinyl, TTTA: A Case Study 233

8.4 Summary and Future Perspectives 234Acknowledgements 235References 235

9 Breathing Crystals from Copper Nitroxyl Complexes 239Victor Ovcharenko and Elena Bagryanskaya9.1 Introduction 2399.2 Structural and Magnetic Anomalies 2419.3 Relationship between the Chemical Step and the Physical Property 2459.4 Relationship between the Thermally Induced Reorientation of Aromatic Solvate

Molecules and the Character of the Magnetic Anomaly 2519.5 EPR Study of Breathing Crystals 255

9.5.1 General Trends of EPR of Strongly Exchange-coupled Spin Triads 2569.5.2 Predominant Population of the Ground Multiplet 2579.5.3 Dynamic Spin Exchange Processes 259

9.6 Classification of Spin-Transitions in Breathing Crystals and Correlationswith Magnetic Susceptibility 261

Contents ix

9.7 The Detailed Magnetic Structure of Breathing Crystals 2669.7.1 EPR Measurements of Temperature Dependence of Intra-cluster

Exchange Interaction 2669.7.2 EPR Measurement of Dipole–Dipole Interaction and Inter-cluster

Exchange Interaction 2689.8 EPR-detected LIESST on Breathing Crystals 2729.9 Conclusion 275References 276

10 Spin-State Switching in Solution 281Matthew P. Shores, Christina M. Klug and Stephanie R. Fiedler10.1 Introduction and Scope 28110.2 Spin-Crossover: Solid State Versus Solution 28210.3 Practical Considerations 283

10.3.1 NMR Characterization 28310.3.2 SQUID Magnetometry 28510.3.3 Electronic Absorption Spectroscopy 285

10.4 Spin-Crossover in Solution 28510.4.1 Solution Characterization 28510.4.2 Solvent Effects 28710.4.3 Substituent Effects 288

10.5 Ligation Changes Driving Spin-State Switching in Solution 28810.5.1 Solvent Exchange/Loss 28810.5.2 Anion Exchange/Loss 28910.5.3 (Photo)Isomerization 29010.5.4 Encapsulation 291

10.6 Second Coordination Sphere Triggers for Spin-State Switching 29110.6.1 External Anion-Dependent Spin Switching 29310.6.2 Using Ligand Fields to Tune Anion Triggered Spin-State Switching

in Solution 29310.7 Challenges and Opportunities 294

10.7.1 New Opportunities for Anion Reporting in Solution 29410.7.2 MRI Contrast 295

10.8 Conclusions/Outlook 295Acknowledgement 295Abbreviations 295References 296

11 Multifunctional Materials Combining Spin-Crossover with Conductivityand Magnetic Ordering 303Osamu Sato, Zhao-Yang Li, Zi-Shuo Yao, Soonchul Kang and Shinji Kanegawa11.1 Introduction 30311.2 Spin-Crossover and Conductivity: Spin-Crossover Conductors 303

11.2.1 Conclusions 30811.3 Spin-Crossover and Magnetic Interaction: Spin-Crossover Magnets 308

11.3.1 Hybrid Spin-Crossover Cation and Anionic Magnetic Framework 308

x Contents

11.3.2 Incorporation of Spin-Crossover Sites in a Magnetic Framework 31011.3.3 Conclusion 316

References 316

12 Amphiphilic and Liquid Crystalline Spin-Crossover Complexes 321Shinya Hayami12.1 Introduction 32112.2 Unique Magnetic Properties of SCO Cobalt(II) Compounds with Long Alkyl Chains 322

12.2.1 Reverse Spin-Transition for Cobalt(II) Compounds 32212.2.2 Re-Entrant Spin-Transition for Cobalt(II) Compounds 324

12.3 Liquid Crystalline SCO Compounds 32512.3.1 Metallomesogens with SCO Property 32612.3.2 Synchronization of SCO and Liquid Crystal Transition 327

12.4 Langmuir–Blodgett Films and Amphiphilic SCO Compounds 33112.4.1 SCO Langmuir–Blodgett Films 33212.4.2 Amphiphilic SCO Compounds 333

12.5 Conclusion and Outlook 339References 340

13 Luminescent Spin-Crossover Materials 347Helena J. Shepherd, Carlos M. Quintero, Gabor Molnar, Lionel Salmonand Azzedine Bousseksou13.1 General Introduction 34713.2 Introduction to Luminescent Materials and Luminescence Energy Transfer 348

13.2.1 Photoexcitation of Luminescent Materials 34913.2.2 Return to the Ground State 351

13.3 Electronic Transitions and Optical Properties of Spin-Crossover Complexes 35813.4 Materials with Combined Spin-Crossover and Luminescent Functionalities 361

13.4.1 General Considerations 36113.4.2 Examples of Luminescent Spin-Crossover Compounds (Ligands,

Counterions) 36213.4.3 Luminescent Doping 366

13.5 Concluding Remarks 371Acknowledgements 372References 372

14 Nanoparticles, Thin Films and Surface Patterns from Spin-Crossover Materials andElectrical Spin State Control 375Paulo Nuno Martinho, Cyril Rajnak and Mario Ruben14.1 Introduction 37514.2 Nanoparticles and Nanocrystals 376

14.2.1 Reverse Micelle (Microemulsion) Technique 37614.2.2 Sol-Gel Techniques 386

14.3 Thin Films 38714.3.1 Langmuir–Blodgett Deposition 38714.3.2 Surface-Assisted Molecular Self-assembly 39014.3.3 Diverse Techniques 390

Contents xi

14.4 Surface Patterns 39314.4.1 Surface Patterns of Spin-Crossover 393

14.5 Electrical Spin State Control 39614.6 Conclusion 399References 400

15 Ultrafast Studies of the Light-Induced Spin Change in Fe(II)-Polypyridine Complexes 405Majed Chergui15.1 Introduction 40515.2 Properties of Fe(II) Complexes 406

15.2.1 Electronic Structure 40615.2.2 Molecular Structure 40715.2.3 Vibrational Modes 40715.2.4 Kinetics of Ground State Recovery 408

15.3 From the Singlet to the Quintet State 40815.3.1 Departing from the MCLT Manifold 40915.3.2 Arrival into the HS State 41215.3.3 Vibrational relaxation of the HS State 414

15.4 Ultrafast X-Ray Studies 41515.5 Summary and Outlook 417Acknowledgements 419References 420

16 Real-Time Observation of Spin-Transitions by Optical Microscopy 425Francois Varret, Christian Chong, Ahmed Slimani, Damien Garrot, Yann Garcia and Anil D. Naik16.1 Introduction 42516.2 Experimental Aspects 426

16.2.1 Single Crystals 42616.2.2 The Sample Cell 42616.2.3 Cryostat, Objective, Camera 42716.2.4 Setting of the Cryostat 42716.2.5 Observation Modes 428

16.3 Selected Investigations 42916.3.1 The Interplay between Structure and Spin Transformations:

[Fe(bbtr)3](ClO4)2 42916.3.2 Colorimetric Investigation of [Fe(ptz)6](BF4)2 43016.3.3 The Transformation Front in [Fe(btr)2(NCS)2]·H2O Crystals 43316.3.4 The Evolution of the Frontline in [Fe(bbtr)3](ClO4)2 Crystals 43616.3.5 An Example of a Robust Crystal: [Fe(btr)3](ClO4)2 437

16.4 Conclusions and Prospects 439Acknowledgements 439References 440

17 Theoretical Prediction of Spin-Crossover at the Molecular Level 443Robert J. Deeth, Christopher M. Handley and Benjamin J. Houghton17.1 Introduction 44317.2 Beginnings: Valence Bond and Ligand Field Theories 443

xii Contents

17.3 Quantum Chemistry 44617.4 Empirical Methods 449

17.4.1 Semi-Empirical MO Theory 44917.4.2 Ligand Field Molecular Mechanics 449

17.5 Conclusions 452References 452

18 Theoretical Descriptions of Spin-Transitions in Bulk Lattices 455Cristian Enachescu, Masamichi Nishino and Seiji Miyashita18.1 Introduction 45518.2 Elastic Interaction Models for Spin-Crossover Systems 457

18.2.1 Thermal Expansion of Volume and Pressure-Induced Transitions 45918.2.2 Long-Range Interactions and Nucleation Features 461

18.3 Mechano-Elastic Model 46518.4 Conclusions 471References 471

19 Optimizing the Stability of Trapped Metastable Spin States 475Jean-Francois Letard, Guillaume Chastanet, Philippe Guionneauand Cedric Desplanches19.1 Introduction 47519.2 Light-Induced Excited Spin-State Trapping (LIESST) Effect 476

19.2.1 LIESST Effect 47619.2.2 Variable Temperature Fourier Transform Infrared Spectroscopy (VTFTIR) 47719.2.3 The Low-Energy Gap 478

19.3 The T(LIESST) Approach: The Case of Mononuclear Compounds 47919.3.1 Principle of the T(LIESST) Measurement 47919.3.2 The T(LIESST) Database 48219.3.3 Parameters Affecting the T0 Factor 48419.3.4 The T(LIESST) Approach to Fe(III) Metal Complexes 486

19.4 The T(LIESST) Approach: An Extension to Polynuclear Iron(II) Complexes 48819.4.1 Binuclear Compounds 48819.4.2 Trinuclear/Tetranuclear Complexes 49219.4.3 Hexanuclear Complexes 49319.4.4 Polymeric Complexes 49319.4.5 Nanoparticles 494

19.5 Simulation and Extrapolation of a T(LIESST) Experiment 49519.5.1 Simulation of T(LIESST) Curve 49519.5.2 Simulation and Extrapolation 497

19.6 Conclusions 500Acknowledgements 500References 500

20 Piezo- and Photo-Crystallography Applied to Spin-Crossover Materials 507Philippe Guionneau and Eric Collet20.1 Introduction 50720.2 Spin-Crossover and Piezo-Crystallography 507

20.2.1 Pressure-Induced SCO: Expectation Versus Observation 508

Contents xiii

20.2.2 Piezo-Crystallography and SCO: Investigations 50920.2.3 Piezo-Crystallography and SCO: Challenges 512

20.3 Crystallography of Photoexcited SCO Materials 51220.3.1 Photo-Crystallography of SCO: Probing the Change of Molecular Structure 51320.3.2 Light-Induced Broken Symmetry: Reaching New States by Laser Excitation 51420.3.3 Photoswitching between Different Excited States 51520.3.4 Slow Phase Nucleation Dynamical Process and Hysteretic Behaviour 51620.3.5 Ultrafast Time-Resolved Crystallography of SCO Photoswitching Dynamics 517

Acknowledgements 519List of Abbreviations 519References 520

21 Spin-Transitions in Metal Oxides 527Jean-Pascal Rueff21.1 Introduction 527

21.1.1 CEF Approach to Spin State Stability 52821.1.2 Stoner Criterion for Itinerant Magnetism 52821.1.3 Probes of the Spin-Transitions 529

21.2 RIXS: A Probe of the 3d Electronic Properties 53021.2.1 Overview of the RIXS Process 53021.2.2 X-Ray Emission as a Probe for the Spin State 53021.2.3 Direct View of the 3d: Pre-Edge Features at the Metal K-Edge by RIXS and

PFY-XAS 53121.3 Experimental Results 533

21.3.1 High Pressure Magnetic Collapse 53421.3.2 Application to Geophysics 53621.3.3 Occurrence of Intermediate Spin State in Cobaltates 53721.3.4 Photoexcited Spin-Transition in Crossover Compounds and ps Dynamics 538

21.4 Conclusions and Perspectives 538References 540

Index 543

List of Contributors

Catalina Achim, Department of Chemistry, Carnegie Mellon University, USA.

Elena Bagryanskaya, International Tomography Center, Siberian Branch of the Russian Academy of Sci-ences, Russian Federation and N.N. Vorozhtov Novosibirsk Institute of Organic Chemistry, SiberianBranch of the Russian Academy of Science, Russia Federation.

Colette Boskovic, School of Chemistry, University of Melbourne, Australia.

Azzedine Bousseksou, Laboratoire de Chimie de Coordination, CNRS & Universite de Toulouse (UPS,INPT), France.

Sally Brooker, Department of Chemistry and The MacDiarmid Institute for Advanced Materials and Nan-otechnology, University of Otago, New Zealand.

Guillaume Chastanet, CNRS, Universite de Bordeaux, ICMCB, France.

Majed Chergui, Ecole Polytechnique Federale de Lausanne, Laboratoire de Spectroscopie Ultrarapide(LSU), ISIC, Faculte des Sciences de Base, BSP, Switzerland.

Christian Chong, GEMAC, Universite de Versailles, CNRS UMR 8635, France.

Eric Collet, Institut de Physique de Rennes, Universite de Rennes, 1-CNRS 35042 Rennes Cedex, France.

Robert J. Deeth, Inorganic Computational Chemistry Group, Department of Chemistry, University ofWarwick, UK.

Cedric Desplanches, CNRS, Universite de Bordeaux, ICMCB, France.

Kim R. Dunbar, Department of Chemistry, Texas A&M University, USA.

Cristian Enachescu, Department of Physics, Faculty of Physics, Alexandru Ioan Cuza University, Romania.

Stephanie R. Fiedler, Department of Chemistry, Colorado State University, USA.

Yann Garcia, Institute of Condensed Matter and Nanosciences, MOST – Inorganic Chemistry, UniversiteCatholique de Louvain, Belgium.

Damien Garrot, GEMAC, Universite de Versailles, CNRS UMR 8635, France.

Philippe Guionneau, CNRS, Universite de Bordeaux, ICMCB, F-33608 Pessac, France.

Malcolm A. Halcrow, School of Chemistry, University of Leeds, UK.

Christopher M. Handley, Inorganic Computational Chemistry Group, Department of Chemistry, Universityof Warwick, UK.

Shinya Hayami, Kumamoto University, Department of Chemistry, Japan.

John J. Hayward, Department of Chemistry and Biochemistry, The University of Windsor, Canada.

Benjamin J. Houghton, Inorganic Computational Chemistry Group, Department of Chemistry, Universityof Warwick, UK.

Shinji Kanegawa, Kyushu University, Institute for Materials Chemistry and Engineering, Japan.

Soonchul Kang, Kyushu University, Institute for Materials Chemistry and Engineering, Japan.

xvi List of Contributors

Christina M. Klug, Department of Chemistry, Colorado State University, USA.

Jean-Francois Letard, CNRS, Universite de Bordeaux, ICMCB, France.

Zhao-Yang Li, Kyushu University, Institute for Materials Chemistry and Engineering, Japan.

Paulo Nuno Martinho, Institute of Nanotechnology, Karlsruhe Institute of Technology, Germany.

Seiji Miyashita, Department of Physics, Graduate School of Science, The University of Tokyo, Japan, andCREST, JST, Japan.

Gabor Molnar, Laboratoire de Chimie de Coordination, CNRS & Universite de Toulouse (UPS, INPT),France.

M. Carmen Munoz, Departamento de Fısica Aplicada, Universitat Politecnica de Valencia, Spain.

Keith S. Murray, School of Chemistry, Monash University, Australia.

Anil D. Naik, Institute of Condensed Matter and Nanosciences, MOST – Inorganic Chemistry, UniversiteCatholique de Louvain, Belgium.

Masamichi Nishino, Computational Materials Science Centre, National Institute for Materials Science,Japan.

Juan Olguın, Department of Chemistry, University of Otago, New Zealand.

Victor Ovcharenko, International Tomography Center, Siberian Branch of the Russian Academy of Sciences,Russian Federation.

Carlos M. Quintero, Laboratoire de Chimie de Coordination, CNRS & Universite de Toulouse (UPS, INPT),France.

Cyril Rajnak, Institute of Nanotechnology, Karlsruhe Institute of Technology, Germany.

Jeremy M. Rawson, Department of Chemistry and Biochemistry, The University of Windsor, Canada.

Jose Antonio Real, Instituto de Ciencia Molecular (ICMol)/Departamento de Quımica Inorganica, Univer-sidad de Valencia, Spain.

Mario Ruben, Institute of Nanotechnology, Karlsruhe Institute of Technology, Germany.

Jean-Pascal Rueff, Synchrotron SOLEIL, France.

Lionel Salmon, Laboratoire de Chimie de Coordination, CNRS & Universite de Toulouse (UPS, INPT),France.

Osamu Sato, Kyushu University, Institute for Materials Chemistry and Engineering, Japan.

Michael Shatruk, Department of Chemistry and Biochemistry, Florida State University, USA.

Helena J. Shepherd, Laboratoire de Chimie de Coordination, CNRS & Universite de Toulouse (UPS, INPT),France.

Matthew P. Shores, Department of Chemistry, Colorado State University, USA.

Ahmed Slimani, GEMAC, Universite de Versailles, CNRS UMR 8635, France.

Francois Varret, GEMAC, Universite de Versailles, CNRS UMR 8635, France.

Birgit Weber, Inorganic Chemistry II, Universitat Bayreuth, Germany.

Zi-Shuo Yao, Kyushu University, Institute for Materials Chemistry and Engineering, Japan.

Preface

The spin-crossover process involves the rearrangement of electrons in a metal ion, from a high spin to alow spin state. These correspond to the distributions of electrons within the metal orbital energy levels thatyield the maximum and minimum number of unpaired electrons respectively. The phenomenon is particularlyprevalent in iron chemistry and can occur in any phase of matter, although it is most often studied inthe solid state. Spin-crossover has a large impact on the physical properties of a solid material, includingits magnetic moment, colour, dielectric constant and electrical resistance. Moreover some spin-crossovermaterials show pronounced hysteresis, which often reflects a structural phase change during the transition.Within the hysteresis loop, the materials are genuinely bistable switches that can be either high or low spindepending on their history.

Several practical applications of spin-crossover materials have been demonstrated that make use of theirswitching properties. They include: display and memory devices, with pixels of a spin-transition materialwhose colour or dielectric constant is switched by spot-heating and cooling; electrical and electroluminescentdevices, where changes in the electrical resistance of a spin-crossover thin-film can be detected, or usedto quench light emission; and, using the switchable paramagnetism of a spin-crossover compound in atemperature-sensitive MRI contrast agent. Switchable liquid crystals, nanoparticles and thin films of spin-crossover materials have also been achieved, that function almost as well as the same materials in the bulkphase. Notably, most of these application studies have been carried out using just two materials, whosespin-transitions show thermal hysteresis of an appropriate width (30–50 K) that spans room temperature.The production of new switchable spin-crossover materials with technologically useful properties by design,rather than by trial and error, remains a problem of crystal engineering that is only now beginning to beaddressed. This combination of technical challenge and practical application explains why an effect that wasfirst observed in the early 1930s continues to be heavily studied by groups around the world.

For the past eight years, the “bible” in the field has been the three-book set from the Topics in CurrentChemistry monograph series, edited by Philipp Gutlich and Harold Goodwin and published in 2004.1 Thisbook is intended to complement that earlier work, and concentrates on aspects of spin-crossover researchthat have developed since then, or are otherwise covered in less detail in the Topics in Current Chemistryvolumes. Articles from the Topics in Current Chemistry series are cited in this book where appropriate, andshould be referred to by the reader.

The first four chapters present an overview of the development of spin-crossover research (Murray), andmore detailed surveys of the mononuclear (Weber), polynuclear (Olguın and Brooker) and polymeric (Munozand Real) spin-crossover complexes that have been discovered since 2004. The structures of these solidcompounds are then examined, to describe the state of play in the crystal engineering of spin-transitionmolecular materials (Halcrow). As before, these first chapters are intended to supplement those in the Topicsin Current Chemistry volumes,1 which give a more comprehensive survey of the types of compounds that areknown to exhibit spin-crossover.

The next chapters cover alternative types of spin state transition found in molecule-based materials, whosechemistry has developed particularly rapidly since 2004. These include two different types of charge-transfer-induced spin-transition, based on electron transfer between different metal ions (Dunbar et al.), and betweena metal and coordinated ligand (Boskovic). Other chapters cover spin-transitions based on reversible spin

xviii Preface

pairing between organic radical centres (Rawson and Hayward), and magnetic transitions associated withJahn–Teller switching in copper/radical coordination polymers (Ovcharenko and Bagryanskaya). The physicalcharacteristics of these different types of transition show many similarities to metal ion spin-crossover,including examples of thermal hysteresis and excited spin-state trapping at low temperatures.

The following chapter by Shores et al., updates the chemistry of spin-crossover in solution. The measure-ment of the thermodynamics and kinetics of spin-transitions in solution is well-established. However, therehas been a recent recognition that spin-crossover is also subject to supramolecular influences in solution, andcan be responsive to host–guest binding interactions.

The next topic to be discussed is the application of spin-crossover compounds, in multifunctional switchablematerials and in nanotechnology. This is covered in chapters describing materials combining spin-crossoverwith conductivity and magnetic ordering (Sato et al.), with liquid crystallinity and amphiphilic behaviour(Hayami), and with fluorescence (Bousseksou et al.). Several of these properties have been exploited tomake functional or multifunctional nanoparticles, thin films and surface patterns, or even in switchablesingle-molecule junctions. These aspects are brought together by Ruben et al.

The next set of chapters describes advances in the physical and theoretical methods for studying spin-crossover materials. Coverage is limited to methods that have grown in importance since 2004, and the readeris referred back to the Topics in Current Chemistry series for a more comprehensive treatment of the topic.1

The chapter by Chergui covers ultrafast measurements of high→low-spin switching, that have deconvolutedthe electron redistribution and molecular structure changes that take place during a spin-transition. Next, Varretet al. describe the use of optical microscopy to monitor spin-crossover in single crystals at the macroscopiclevel. This is followed by two chapters describing advances in the theoretical description of spin-crossover,in single molecules (Deeth et al.) and in bulk lattices (Enachescu et al.). Last are discussions of advances inthe study of light- or pressure-induced spin-state trapping phenomena, in bulk materials (Letard et al.) and insingle crystals (Guionneau and Collet).

In the final chapter, Rueff describes the importance of pressure-induced spin-crossover to geology. A largeproportion of the Earth’s mantle contains iron-containing oxide materials, which undergo spin-crossover athigh pressures in the laboratory. This has been intensely researched during the last eight years, to determinewhether these spin state changes also occur in the mantle, and whether they can explain certain anomalies inits physical properties.

Guionneau and Collet have dedicated their chapter to Andres Goeta. Andres was one of the leaders of ateam at the University of Durham who pioneered the study of excited spin states in spin-transition materialsby photo-crystallography. Andres had been due to contribute to this book but passed away suddenly in July2011. I would like to express my appreciation to Drs Guionneau and Collet, for stepping into the breach andproviding a chapter on this important topic at short notice. But, more importantly, I also dedicate this bookas a whole to Andres’ memory.

Malcolm A. HalcrowLeeds, UKJuly 2012

1. Gutlich, P., Goodwin, H. A. (Eds) (2004) Spin Crossover in Transition Metal Compounds I–III. Top. Curr. Chem.,vols. 233–235. Springer Verlag, Berlin/Heidelberg, Germany.

1The Development of

Spin-Crossover Research

Keith S. Murray

School of Chemistry, Monash University, Australia

Dedicated to my good friend, the late Hans Toftlund who was a fund of knowledge on spin-crossoverand many other inorganic chemistry topics.

1.1 Introduction

The approach to this chapter is a personal one and treats the topics in some depth rather than attemptingto provide a compendium of all that has been published in this vast field. So the author apologises inadvance to those whose contributions are not included. The subject of spin-crossover (SCO), (or spin-equilibrium or spin-transition (ST)), in d-block metal complexes spans some nine decades and is one ofthose intriguing areas of inorganic research that has had a number of quiet times and rebirths, not unlike thesubject of magnetochemistry. The oft quoted work of Cambi and Szego initiated the subject (Fig. 1.1). It wascarried out in the institute of industrial chemistry of the University of Milan in 1931, and showed some 16tris(N,N-disubstituted dithiocarbamate) iron(III) derivatives, [Fe(R2NCS2)3], in Table 3 of their iconic paper,with anomalous magnetic susceptibilities relative to that of the high spin (HS) d5 value for the O-bonded[Fe(acac)3].1 These compounds will be discussed further, later. A present day ‘Googling’ of spin-crossover inWikipedia reveals a brief and useful survey of development in the subject, finishing with efforts (ongoing) atcommercial applications of these molecular magnetic ‘switching’ materials. The Google search shows manyhundreds of hits for spin-crossover. Between 1931 and 2011 there has been, in the author’s view, a number ofbroadly distinguishable periods. Those interested in this topic and in the history of science may well disagreewith the definition of such periods. But here we go.

Spin-Crossover Materials: Properties and Applications, First Edition. Edited by Malcolm A. Halcrow.© 2013 John Wiley & Sons, Ltd. Published 2013 by John Wiley & Sons, Ltd.

2 Spin-Crossover Materials

Figure 1.1 Extract from the Cambi and Szego paper on FeIII tris-dithiocarbamate compounds. Note that magneticmoments (p in Table III) are in Weiss magnetons, which are ∼5 x Bohr magneton values. Adapted with permissionfrom [1]. Copyright Wiley-VCH Verlag GmbH & Co., 1931.

Between 1931 and the 1960s the subject lay largely dormant, with the Second World War probably playingsome part in the lack of activity, at least as far as publications were concerned. However, coordinationchemistry, and associated magnetochemical studies of the d-block complexes prepared, continued during the1940s and 1950s in Europe,2 the USA,3 Japan4 and Australia – in the latter country by the likes of Burrows,5

Nyholm6 and Dwyer,7 but spin-crossover did not feature. SCO was recognised by Pauling in regard to FeII

heme oxygenation in 1936.8

The 1960–80 period can be labelled ‘the renaissance in mononuclear SCO compounds’ and there wasgreat activity occurring in many research groups worldwide. Not only did Martin, Ewald and group,9 thenFiggis10 in Australia, reinvestigate the [Fe(R2NCS2)3] family, including the first applied pressure work onSCO materials; on the other side of the world in Russia, Zelentsov and Gerbeleu and co-workers11 developedbis-tridentate thiosemicarbazone FeIII complexes of types [Fe(5-X-thsa)2]− and [Fe(5-X-thsa)(5-X-tshaH)]having FeN2S2O2 coordination spheres, the complexes often producing sharper and more hysteretic spin-transitions than the [Fe(R2NCS2)3] compounds. We will see later that the thsa-FeIII materials have beenreceiving recent attention in other laboratories.12 Iron(III) SCO monomers containing N4O2 or N3O3 liganddonor combinations, commonly from Schiff-base chelators, began to emerge from work in the USA,13 Japan14

and Australia,15 with a report by Hendrickson and group catching the eye in which they found that the natureof the spin-transition (shape and T1/2

) was found to depend on the size of crystallites and how finely thecrystallites were ground.16 Such nonligand-field/noncovalent ‘supramolecular’ and physical effects continueto intrigue studies of cooperativity in crystalline SCO samples.

The first iron(II) d6 SCO monomers were discovered in the mid-1960s and this led to an explosionin studying the kinds of N-donor ligand combinations that would yield SCO behaviour, a pursuit thatcontinues today. The first examples, by Konig and Madeja,17 were of the type cis-[Fe(NCS)2(1,10-phen)2]and the 2,2′-bipy analogue, with a FeIIN6 mixed heterocyclic/pseudohalide(N) donor set providing theappropriate ligand-field. There are many such related pyridine-containing ligand systems that make up theFeN6 chromophore, including the tetradentate tripyridylmethylamine compounds, [Fe(NCS)2(TPA)],18 andthe bis-dipyridylamine [Fe(NCS)2(DPA)2]19 complexes or congeners thereof, such as the DPA-substitutedtriazines to be discussed later. Hexakis-tetrazole complexes such as [Fe(1-propyl-tetrazole)6]2+, discoveredby Haasnoot et al.,20 provided a great vehicle for the detailed study of its abrupt spin-transition by Gutlich and

The Development of Spin-Crossover Research 3

co-workers by use of magnetic, Mossbauer spectral, structural and thermodynamic methods.21 It also providesa very good student demonstration of its rapid colour change, from colourless to violet, and vice versa, uponcooling in liquid nitrogen, then rewarming above its T1/2

of 130 K. The tris-chelated picolylamine family,[Fe(2-pic)3](Cl)2·solvate, was likewise much studied in this early period,22 and has proved recently to yielddetailed synchrotron X-ray structural information on intermediate phases (IPs) existing at temperatures wheresteps/inflections occur along the complex thermal spin-transition (see the section below on mononuclear SCOmaterials).23

Other azole N-donors, such as the ubiquitous tris-(1-pyrazolyl)-hydridoborate (‘scorpionate’) facial chela-tors, led Trofimenko and Jesson to study the spin-crossover properties of [Fe(HB(pz)3)2] and substituted-pyrazole analogues.24 Interestingly, it took another 30-plus years to see the SCO properties in the cationictris-(1-pyrazolyl)methane analogues, [Fe(HC(pz)3)2](anion)2.25–27 Goodwin et al. made extensive studies onthe synthesis, structure, Mossbauer spectra and magnetism of a variety of pyrazolyl-pyridine chelates ofFe(II)28 and these have led to further recent advances by Halcrow,29 Letard,30 Ruben and co-workers.31

Cobalt(II) d7 SCO complexes were some of the earliest to be investigated, by Baker et al. and Martinet al., the [Co(terpy)2](anion)2 systems showing gradual spin-transitions that were sensitive to changes inanion.32, 33

1980–2012 period. Following a lessening of interest, or perhaps a slowing in the frenetic activity expendedon monomeric species during 1960–1980, the subject received renewed and continuing interest primarily,but not solely, because of the challenges in polynuclear iron(II and III) and cobalt(II) SCO chemistry. Thefundamental reason was to see if covalent bridging between SCO metal centres, in crystals, would influencethe degree of cooperativity and thermal hysteresis loop widths when compared to monomeric analogues.34, 35

Supramolecular bridging interactions were of similar importance. In other words, the question was posed as towhether the spin-transitions on individual metal centres would occur sequentially or simultaneously. Multisteptransitions could well occur, as had already been seen in the form of 2-step transitions in some monomericcompounds. More on this will be discussed later. A second fundamental question was to investigate whetherany synergy occurred between spin-crossover and spin-spin magnetic exchange, the latter originating betweenparamagnetic single-ion centres (e.g. HS–HS FeII; S = 2 : S = 2 coupling), via superexchange interactionsacross bridging groups, in discrete clusters, 1D chains, 2D sheets and 3D frameworks.34, 35 The other majorimpetus for renewed interest in SCO compounds was the possibility of producing electronically useful ‘newage’ materials for use in displays, sensors and memories.36 In Europe, a network of SCO researchers, ‘TOSS’(thermally and optical spin state switching) was formed and lasted many years, to be replaced, in part, by‘MAGMANet’– the latter, ongoing network includes researchers working in all areas of molecular magneticmaterials.

In the discrete cluster class, the first dinuclear Fe(II) SCO complexes studied were by Kahn, Real et al.34 oftype [Fe(NCX)2(bidentate)(μ-bipyrimidine)Fe(NCX)2(N,N-bidentate)], bidentate = 2,2′-bipy, bis-thiazoline,etc., and the first dinuclear Co(II) complex, by Kahn and Zarembowitch,37 was of the binucleating Schiffbase fsaen type with N,O donor groups. The Leiden group of Reedijk, Haasnoot et al. reported trinuclear1,2,4-triazole-bridged Fe(II) complexes in which the central ion showed the spin-transition while the terminalFe atoms remained HS.38, 39 Exchange coupling between neighbouring atoms was generally weak, a few cm−1

at most. In a CoII2 macrocyclic derivative simultaneous crossover and exchange was observed by Brooker

et al.40

Lehn, Ruben et al.41 reported the first tetranuclear 2×2 ‘grid’ FeII4L4 SCO complex that showed rather

broad spin-transitions. Other recent FeII4 examples are described later and include crystal structures and

physicochemical proof of the various spin state combinations. Dunbar and co-workers described a trigonal-pyramidally shaped FeII

5 SCO cluster.42 The first hexanuclear SCO ‘nanoball’, containing six FeIIN6 chro-mophores held within ditopic scorpionate-pyridyl ligands that also had eight CuI centres in the hydridopyra-zolylborate N3 ‘inner-pockets’, was made by Batten, Duriska et al.43

4 Spin-Crossover Materials

This chapter will now cover recent developments in polynuclear and mononuclear SCO materials, withupdates on theory, then briefer sections will cover recent advances in multifunctional materials, instrumenta-tion/measurement and, finally, applications.

1.2 Discrete Clusters of SCO Compounds

There have been a number of synthetic and physicochemical challenges in polynuclear cluster materials thatcontain SCO centres and many, but not all, of these have been overcome in the last decade. In the area ofsynthesis and design the key challenges have included:

� The design of bridging and terminal ligands and the coordination environment around constituent FeII (orFeIII or CoII) centres that yield a spin-transition.

� The design and isolation of tri-, tetra- and higher nuclearity SCO clusters.� The aim of observing and understanding synergy between exchange coupling and SCO between, and

within, nearest neighbour ions. Why is exchange coupling often negligible?

The coordination environment that yields the correct ligand-field and hence induce spin-crossover at FeII

centres is commonly made up of six pyridyl- or azole-N donors or combinations of these with (usually) twoNCX− ligands, where X = S, Se, BH3; or N(CN)2

− (dicyanamide, dca−).21, 44 Chelating ligands containingthese donors are commonly used as terminal, and sometimes bridging, ligands. Recently, combinations of N,Odonors, from Schiff base ligands have proved successful in FeII systems.45, 46 The latter donors also inducespin-transitions in six-coordinate FeIII compounds, both mononuclear,12–14, 16, 47 dinuclear and trinuclear,48

and in six- or five-coordinate CoII compounds.37, 49 However, six S-donors, such as in the abovementionedtris-dithiocarbamates are commonly employed in FeIII compounds.1, 9, 10 N,N,N-tridentate chelating ligandssuch as terpy have been long known to yield CoII SCO mononuclears50 (more recent aspects on these aregiven later) and one wonders if, when combined with bridging 4,4′-dipyridine type and nonbridging NCX−

ligands, they will yield new CoII dinuclears as has been found to be the case for FeII and FeIII.47, 51 Care isalways needed with the M:L:NCX− stoichiometry employed in such reactions so that a dinuclear rather thana mononuclear product ML3 is obtained.

Designing larger SCO clusters requires, for instance in FeII4 species, the proper combination of termi-

nal chelating groups and bridging 2-connectors. In the case of squares, a flexible tetradentate L such astrimethylpyridylamine (TPA) can be combined with linear CN− or 4,4′-bipy to yield [FeII

4L4(bridge)4] SCOspecies. Cubane FeII

4L4 SCO clusters require facial-tridentate terminal ligands, such as the scorpionatesHB(pz)3

− or HC(pz)3 combined with three two-connectors of the CN− or 4,4′-bipy types. The SCO prop-erties are discussed later. Triangular or rectangular FeII clusters can be obtained that don’t possess CN− or4,4′-bipy bridges by using an appropriate polypodal central linker, such as a 1,3,5-tri-tris(pyrazolyl)methane-substituted benzene, together with a FeII(HC(X-pz)3) terminal group, which yields a triangle of widely spaced[FeN6] SCO centres.52, 53 The early, linear FeII

3-1,2,4-triazole SCO compounds (spin change at only one Fe),that could have yielded 1D [Fe(1,2,4-triazole)3]2+(anion)2 chains, required the correct mole ratio of reagentsbe used, and some luck.38, 54 When the anion is NCS−, [Fe2L5(NCS)4] dinuclears are obtained, that, apartfrom the first example by Reedijk and Haasnoot et al.,55 have invariably remained HS–HS irrespective of thetriazole used; more on this below.

Designing higher nuclearity SCO clusters, such as an octahedrally disposed FeII6 aggregate in a 14-metal

FeII6CuI

8 ‘nanoball’ required the skills and ‘3D vision’ of those, like Batten,43 designing self-assembledmetallosupramolecular polyhedra (Archimedean and Platonic), here of the pseudo-spherical type. A Tp4-py

scorpionate ligand was employed to make the nanoball with tetrahedral CuI ions in the inner coordination

The Development of Spin-Crossover Research 5

H

B

N

N

N

N

N

N

NN N

(a) bi-functional scorpionate ligand[Tp4-py]-

(b) preformed molecular buildingblock: [Cu'(Tp4-py)(MeCN)]

Fe"

+ Fe"(NCS)2+ Cu'

Cu'

(c) supramolecular nanoball (d) polyhedral representation

Figure 1.2 Formation and structure of FeII6CuI

8 ‘nanoball’ showing (a); the bifunctional Tp4-py ligand, (b); theCuI(Tp4-py)(MeCN) building block, (c); the supramolecular nanoball, (d); the polyhedral representation andpacking motif. Reproduced with permission from [43]. Copyright Wiley-VCH Verlag GmbH & Co., 2009.

‘pockets’ and with the outer 4-pyridyl groups doing just what they were intended to do, rather than leadingto polymeric alternatives (Fig. 1.2). Choice of co-ligands was, of course, important to create the crossoverligand-field at each Fe(py)4 centre; viz. NCS− and NCMe in trans positions. Other important features suchas porosity in crystals of these nonframework nanoball materials, and the effect of guest sorption on thespin-transition, provided an added bonus. The chemistry and functions of metallosupramolecular polyhedraare receiving much current interest from a number of groups including those of Fujita,56 MacGillivray,57

Ward58 and Stang.59

The physicochemical, electronic, magnetic and theoretical challenges in polynuclear SCO clusters included:

� Understanding 1-step, vs. 2-step, vs. multistep spin-transitions.� Understanding cooperativity (thermal hysteresis) within and between SCO clusters and relating to struc-

ture.� Observing and understanding photomagnetic LIESST effects in polynuclear clusters and comparing

results to monomers.� Gaining a theoretical understanding of SCO in clusters and of synergy with exchange coupling.

Answers to such challenges, or challenges that still remain, will be discussed when describing the variousmolecular cluster classes, below.

6 Spin-Crossover Materials

Recent advances in dinuclear and polynuclear SCO compounds are described in other chapters in this book,and a number of reviews on the topic are available.35, 44, 45, 60–66 Aspects that are relevant to the developmentof SCO research are now given without attempting to be fully comprehensive of all such reports. Thetimeline dating from the Topics in Current Chemistry’s three volumes on ‘Spin-crossover in transition metalcompounds’ (2004) is largely followed.67 Many of the present subsections have related chapters within thesethree volumes.

1.2.1 Dinuclear FeII - FeII SCO Clusters

Kahn, Real and co-workers first reported their bipyrimidine(bpym)-bridged FeII SCO complexes, of type[(NCX)2(2,2′-bipy)Fe(μ-bpym)Fe(2,2′-bipy)(NCX)2] in 1987 and posed some of the challenges shown above,such as is there synergy between exchange and SCO?34 The X and 2,2′-bipy groups were systematicallyvaried and it was noted that, even when all seemed to be in place to achieve the crossover ligand-field insuch dinuclear complexes, the spin state HS–HS was stabilised at all temperatures, without SCO occurring.Many of us since then have experienced similar, rather frustrating, HS–HS results, when using other ligandcombinations.68 Effects other than the ligand-field can, of course, influence whether or not SCO occurs.There was, nevertheless, a good ‘spin-off’ for Kahn et al. in that the weak antiferromagnetic exchangecoupling (2J = –4.1 cm−1 from −2JS1·S2 Hamiltonian) between the HS FeII centres could be observed andquantified from χMT vs. T plots. The low temperature part of such χMT plots was also seen in ‘half’ crossoverμ-bpym examples, that had a ‘HS–LS’ plateau above this region, and could be extrapolated to the hightemperature HS–HS data, above T1/2

. Later, the application of pressure to the bpym/NCS HS–HS exampleyielded SCO.60, 62 One of the important properties displayed by the [(NCX)2(2,2′-bipy)Fe(μ-bpym)Fe(2,2′-bipy)(NCX)2] family is that of reversible spin switching.63, 69

Some 10 years later, we64 and others70 began exploring other dinuclear FeIIFeII SCO compounds, using avariety of terminal and bridging groups, to create FeN6 or, more recently, FeN4O2 donor sets at each FeII.45

The reasons for this rekindling of interest were many, some given above as bullet points, while others includedquestions such as ‘do other dinuclear systems exist to compare with the bipyrimidine family?’; ‘do thesedinuclears display 1-step HS–HS to LS–LS transitions or 2-step HS–HS to HS–LS to LS–LS transitionswith decreasing temperature?’; ‘is it possible to isolate the “HS–LS” molecule that gives the χMT plateauthat occurs between the HS–HS and LS–LS states, and are these individual HS–LS molecules or 50:50HS–HS:LS–LS molecular mixtures?’.

Bridging groups employed include pyrazolates, triazolates, triazoles, pyrimidines, 4,4′-bipy (and similar2-connecting dipyridyls) or N(CN)2

−, the first three forming part of polytopic chelating ligands (Fig. 1.3).The terminal groups are generally pyridine-derived chelates sometimes in combination with NCX−, carefullychosen to create 6-coordination at each FeII. Dinuclear helicates of stoichiometry Fe2L3

2+ have also beeninvestigated where L is a ditopic N,N-chelator (Fig. 1.4).71

Key discoveries to emerge, usually supported by structures, magnetism, Mossbauer spectra and DFTcalculations, were (i) the observation of full, 1-step HS–HS to LS–LS transitions in pyrazolate-bridgedcompounds,64, 65, 70 (ii) 2-step HS–HS to ‘HS–LS’ to LS–LS transitions in pyrazolate,64 triazolate,72 pyrim-idine73 and 4,4′-bipy51, 74 (and related) bridged compounds, where ‘HS–LS’ was usually found to be aHS–LS molecule but examples of 50:50 HS–HS:LS–LS were also found, (iii) ‘half’ SCO examples thatwere ‘trapped’ in the HS–LS form following the HS–HS to HS–LS spin-transition, in trihelicates71 and intriazole-bridged molecules with the latter HS–LS molecules of Brooker et al. being the first such HS–LSform to be structurally characterised,75, 76 (iv) the LIESST properties of dinuclear species,60, 64, 69 (v) the DFTcalculations that predicted/rationalised the 2-step transitions,77 (vi) the very weak to zero HS–HS exchangecoupling even in dinuclears that contained bridges capable of transmitting stronger exchange.34, 35, 44, 60–64

The Development of Spin-Crossover Research 7

Figure 1.3 Various bridging groups used in dinuclear FeII SCO compounds.

In general, these studies of dinuclear systems that followed from the bipyrimidine-bridged work confirmedmany of the findings of Kahn et al.34 as well as making significant advances in our fundamental knowledgeof such covalently-bridged SCO molecules.

Here, current examples containing 4,4′-bipy-type bridging, [[Fe(dpia)(cis-NCS)2]2(μ-L)] (L = 4,4′-bipy78

or bpe,79 Fig. 1.5) are described, first, in some detail, giving emphasis to modern developments. The synthesisrequired a tridentate chelating ligand, dpia (di(2-picolyl)amine), to make the FeII centres 6-coordinate withFeN6 donor sets. Care had to be taken not to make the homoleptic bis-monomer, [Fe(dpia)2]2+. A simi-lar compound employing L = 3-bpp, viz. [[Fe(3-bpp)(trans-NCS)2]2(μ-4,4′-bipy)]·2MeOH, with unusual

8 Spin-Crossover Materials

(a) (b)

N17

N10

N16

N6 N4

N5O1

O3

N3

N9N2

O2

N8

N1

N7

N13N15

N14

8

7

6

5

µ eff

/ µB

4

30 50 100 150

T / K

200

time

1st run

250 300

Fe2

N18N11

N12Fe1

Figure 1.4 (a) Structure of an Fe2L32+ triple helicate by Kruger et al. (b) Magnetic properties as a function of

time. Reproduced from [71] with permission of The Royal Society of Chemistry, 2011.

structural nuances that have been recently described,51 was reported a little earlier. [[Fe(dpia)(cis-NCS)2]2(μ-bpe)] forms two polymorphs and a pseudopolymorph [[Fe(dpia)(cis-NCS)2]2(μ-bpe)]·2MeOH all showingquite different χMT vs. T plots: the polymorph 1 shows a 2-step gradual spin-transition, polymorph 2 remainedHS–HS, and the methanol adduct showing an abrupt 1-step (full HS–HS to LS–LS) transition; Fig. 1.5).79

There was no thermal hysteresis in any of the spin-transitions. Such differences in susceptibilities as these arenot unusual in SCO chemistry and the authors were, of course, keen to find out why such differences occurred.The crystal structures of the dinuclear molecules, and how they packed in the crystal, were discussed in detail,as were the octahedral distortion parameters, � (sum of deviations of the 12 cis N-Fe–N angles from 90◦) forthe HS–HS structures, around each FeII centre, with polymorph 2 having two distinct Fe sites even at 293 K.The 2-step example 1 did not show structurally distinct Fe sites within each binuclear molecule at 183 K, thetemperature (inflection point) at which [HS–LS] molecules would exist, but displayed a similar (averaged)structure to the [HS–HS] form, however with shorter Fe–N lengths. [LS–LS] Fe–N lengths were observed

(a) (b) 8

7

6

5

4

3

2χ MT

(cm

3 m

ol–1

K)

1

00 50 100 150

T (K)200 250 300

S2A

S2B

C2B

C2A

C3 C15 C16

C18C17 C20

C4

C5C7

C8C9

C10

C11C12

C13

C14

C19

Fe1

C6

N3

N4

N5

S1

C1

N2 N1

N6

Figure 1.5 (a) Structure of [[Fe(dpia)(NCS)2]2(μ-bpe)]. (b) Magnetic data for various forms of[[Fe(dpia)(NCS)2]2(μ-bpe)]; � [[Fe(dpia)(NCS)2]2(μ-bpe)] polymorph 1, � [[Fe(dpia)(NCS)2]2(μ-bpe)] poly-morph 2, � [[Fe(dpia)(NCS)2]2(μ-bpe)]·2MeOH, ◦ [[Fe(dpia)(NCS)2]2(μ-4,4′-bipy)]. Reproduced from [79] withpermission of The Royal Society of Chemistry, 2011.

The Development of Spin-Crossover Research 9

at 90 K with lower octahedral distortions (lower �) than in the [HS–HS] form. No crystallographic phasechange occurred between 300 and 90 K.

Comparisons of core geometries, Fe..Fe separations and octahedral distortions in the three bpe-bridgedspecies were made to those in [[Fe(dpia)(cis-NCS)2]2(μ-4,4′-bipy)]78 and in other dinuclear SCO compoundshaving bipyrimidine, pyrazolate, triazolate and dicyanamide bridges.64 The ‘take home’ message was thatthe nature of the spin-transition (full 1-step; full 2-step; half crossover) in dinuclear FeII SCO compoundswas primarily related to the degree of the octahedral distortions at the FeN6 cores, that is intramoleculareffects, these being influenced by packing and ligand strain arising from terminal and/or bridging ligands.Inter-dinuclear interactions, viz. H-bonding, π -stacking, van der Waals, were deemed to be responsible for thediffering cooperativity, highlighted by the abrupt (more cooperative) transition in [[Fe(dpia)(cis-NCS)2]2(μ-bpe)]·2MeOH. A strong distortion, having a {higher �/weaker ligand field} in the starting [HS–HS] form wasfelt to stabilise the HS state, whatever the temperature. Then the relative degree of distortion of FeN6 sites inthe [HS–HS] form was felt to be responsible for whether the ‘half’ transition [HS–HS] to [HS–LS] occurred,with a large distortion on the HS site preventing it going on to form LS, or whether the 2-step [HS–HS] ↔[HS–LS] ↔ [LS–LS] transition occurred. A mild distortion was present in the HS site in the [HS–LS] formof the latter. Similar conclusions have recently been obtained for two new alkyne-linked dipyridyl bridgedanalogues, [[Fe(dpia)(NCS)2]2(bpac)]·nCH3OH [n = 0 (1) and 2 (2), bpac = 1,2-bis(4-pyridyl)ethyne].80

The related compound [[Fe(3-bpp)(trans-NCS)2]2(μ-4,4′-bipy)]·2MeOH51, 74 was not included in the com-parative magnetostructural discussions given in the phia papers.79, 80 It showed three crystallographic phasesas the temperature was lowered: phase 1, 300–161 K, P21/n; phase II, 151–113 K, Cc; phase III, 115–30 K,P1(and photoexcited phase III∗, 30 K, P1). These corresponded to the spin states [HS–HS], [HS–HS] and[HS–LS] for I to III, respectively, in agreement with magnetic data for the ‘half’ spin-transition, T1/2

∼114 K(Fig. 1.6). Thermal hysteresis in the cell volume was shown in the warming mode, with �T ∼ 4 K. The phasechange I to II could not be seen in the χMT plot but it could be clearly seen in heat capacity data. It wouldbe interesting to see the corresponding evolution of synchrotron PXRD cell data between 300 and 30 K asdescribed for the 3D framework SCO systems and some of our other complexes, described later. All Fe siteswere identical in phase I, while there are four different Fe sites (2 per dinuclear) in phases II and III. OneFe site (type 1) underwent SCO while the other (type 2) did not. In phase III, all Fe–N lengths, volumes ofFe octahedra and � values pointed to two HS sites and two LS sites (2 different dinuclear molecules). Thusthe partial crossover to form [HS–LS] molecules was confirmed and the LS sites were clearly identified inthese [HS–LS] forms. A thermal and light-induced structural and spin state diagram is given in Figure 1.7.

00

2

4

6

8

100 200T (K)

χT (

cm3 .

Km

ol–1

)

Figure 1.6 Structure and magnetism of [[Fe(mer-3-bpp)(trans-NCS)2]2(μ-4,4′-bipy)]·2MeOH. Reproduced from[79] with permission of The Royal Society of Chemistry, 2011.

10 Spin-Crossover Materials

40

2170

2160

2150

2140

2130

2120102 105 108 111 114 117 120 123

2100

2120

2140

2160

2180

2200

2220

60 80 100 120 140 160 180

Temperature (K)

Temperature (K)

Uni

t cel

l vol

ume

(Å3 )

Uni

t cel

l vol

ume

(Å3 )

phase III phase II phase I

(a)

(c)phase III – 30 K phase II – 140 K

phase III* – 30 Kphase I – 293 K

T ~ 117 K

T ~ 161 K

T ~ 113 K

P1-

P1- P21/n

HS

HS

LS

LS

(b)

200 220 240 260 280 300

HS

Cc

HS

HS

HS

HS

hν

HS

HS*

HS*HS

Figure 1.7 (a) Cell volume vs. temperature for phases of [[Fe(mer-3-bpp)(trans-NCS)2]2(μ-4,4′-bipy)]·2MeOH.,

(b) Thermal hysteresis in the cell volume. (c) Structural and spin state diagram. Reproduced from [74] withpermission of The Royal Society of Chemistry, 2010.

The interdinuclear π–π and solvent ineractions were discussed in detail, as well as a ‘switching’ role beinginvoked for the planar to non-planar 4,4′-bipy geometric change occurring between phase I and II. Finally,this thorough study showed that the [HS–HS] forms of phases I and (photoexcited/metastable) III∗ weredifferent. However, the distortion arguments given for the phia analogues78–80 apply generally to this 3-bppcompound.

Another 2-step dinuclear SCO complex, [[Fe(ddpp)(NCS)2]2]·4CH2Cl2, with ddpp-(N-py) bridging(ddpp = 2,5-di(2′,2′′-dipyridylamino)pyridine), provided the first structural characterisation of a ‘ordered’[HS–LS] molecule existing at the plateau temperature. The Fe sites were also structurally inequivalent inthe [HS–HS] and [LS–LS] forms. No crystallographic phase change occurred between 250 K (HS–HS) and25 K (LS–LS). Octahedral distortion parameters, �, were significantly different for the two Fe sites, at alltemperatures.64, 73, 81

One of the first reported dinuclear compounds was by Haasnoot et al. using L = 1,2,4(N-p-tolyl) triazole,viz. [FeII

2(L)5(cis-NCS)4]. Three of the triazoles bridged via N1,N2, while the terminal ones coordinated byone of these N atoms, with two NCS per Fe, cis disposed. Two dinuclears formed a H-bonded ‘pentamer’ byencompassing one [Fe(L)2(NCS)2(H2O)2 monomer.82 The dinuclear moieties showed SCO with T1/2

= 111 K,