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