122 Structure and Bonding

Series Editor: D. M. P.Mingos

Editorial Board: P.Day·T.J.Meyer·H.W.Roesky·J.-P.Sauvage Structure and Bonding Series Editor: D. M. P.Mingos Recently Published and Forthcoming Volumes

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With contributions by G. Aromí · B. Barbara · E. K. Brechin · A. Caneschi R. Clérac · A. Cornia · A. F. Costantino · C. Coulon J. Curély · D. Gatteschi · T. Mallah · M. Mannini E. J. L. McInnes · H. Miyasaka · J.-N. Rebilly R. Sessoli · L. Zobbi

123 The series Structure and Bonding publishes critical reviews on topics of research concerned with chemical structure and bonding. The scope of the series spans the entire Periodic Table. It focuses attention on new and developing areas of modern structural and theoretical chemistry such as na- nostructures, molecular electronics, designed molecular solids, surfaces, metal clusters and supra- molecular structures. Physical and spectroscopic techniques used to determine, examine and model structures fall within the purview of Structure and Bonding to the extent that the focus is on the scientific results obtained and not on specialist information concerning the techniques themselves. Issues associated with the development of bonding models and generalizations that illuminate the reactivity pathways and rates of chemical processes are also relevant. As a rule, contributions are specially commissioned. The editors and publishers will, however, always be pleased to receive suggestions and supplementary information. Papers are accepted for Structure and Bonding in English. In references Structure and Bonding is abbeviated Struct Bond and is cited as a journal.

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Preface

In some ways the story of single-molecule magnets (SMMs) starts with work performed in the Christou group during the 1980s. The research was dedi- cated to the synthesis of model compounds of the oxygen-evolving complex in Photosystem II. The work resulted in a very large number of polymetallic mixed-valent manganese complexes being made—many having rather more metal centres than are strictly speaking required for an accurate representation oftheactivesiteoftheenzyme. The work had produced many beautiful model compounds and was very well reviewed at the time [1], but lacked a model for the highest oxidation state of the biological cycle. In attempting to make such a model by oxi- dation of manganese acetate with permanganate, a compound of formula [Mn12O12(O2CPh)16(H2O)4] was isolated [2]. This was a new compound, but rather surprisingly it had a very close precedent: in 1980 Lis had made [Mn12O12(O2CMe)16(H2O)4] and published the crystal structure [3]. Lis also included preliminary magnetic measurements from 4–300 K, but did not in- terpret them. Even more surprisingly the first proposal of a dodecanuclear manganese complex from this type of reaction was made in 1921 by Weinland and Fischer [4], although given the equipment available at the time Wein- land and Fischer did not get the metal-to-ligand stoichiometry correct. The rediscovery by the Christou group was therefore serendipitous. Boydet al. measuredthemagnetic propertiesof [Mn12O12(O2CPh)16(H2O)4] and deduced a spin ground state of S =14[2].Thisisincorrect,butthemis- take is unsurprising in that the behaviour of this compound is unlike that of previously prepared high-spin molecules. At that time, the highest spin known for a molecule was S = 12, for a molecule reported by Gatteschi’s group [5]. As a result of the paper by Boyd et al. the Gatteschi group reinvestigated the Lis compound by high-field magnetisation and high-frequency EPR spectroscopy. The resulting paper, published by Caneschi et al. in 1991 [6], describes the freez- ing of the magnetisation at low fields, and also explains this behaviour—which is analogous to that of superparamagnets—due to the negative axial anisotropy of the high-spin ground state of the compound. The ground state was found to be S = 10, and later measurements show that [Mn12O12(O2CPh)16(H2O)4] also has an S = 10 ground state [7, 8]. The paper by Caneschi et al. [6] misses one trick: the term single-molecule magnet is not used, however this is now X Preface how these molecules are universally known. Despite the lack of a snappy title this paper introduces many of the basic ideas underlying the physics of SMMs. These initial discoveries have provided physicists with ideal objects for studying quantum phenomena, such as tunnelling of magnetisation. This was first reported by Barbara and co-workers [9], and simultaneously by Friedman et al. [10]. Many following studies have been designed to understand these phenomena and the mechanism of tunnelling. The advantage in studying the physics of SMMs over particulate supraparamagnets is that coordination chemists can vary the physical properties of the quantum objects using the skills of a synthetic chemist. The result has been a remarkably large number of derivatives of “Mn12”. Reviews of the physics and the quantum phenomena of SMMs have been published previously [11, 12], therefore here we have concentrated on aspects that have not been reviewed. Aromi and Brechin explore synthetic routes to SMMs, covering the literature exhaustively until May 2005. McInnes has reviewed the spectroscopic studies of SMMs published until mid-2005. Mallah has reviewed the use of metallocyanates in making SMMs, and Cornia has reviewed the growing field of hybrid materials featuring SMMs. The review by Clérac, Coulon and Miyasaka covers the more recently discovered phenomena of single-chain magnets. Underlying all this chemistry is the physics; Curély and Barbara provide an article on the general theory of superexchange in molecular species. Probably the most disappointing aspect of research into SMMs has been the stubborn fact that the highest energy barrier to reorientation of magneti- sation remained at around 60 K for a dozen years after the phenomenon was discovered. Recent developments have moved this barrier to 90 K, albeit for amononuclear cobalt complexthat cannot beisolatedinthesolidstate[13].The work by Koga and co-workers suggests a new route towards SMMs, involving photoactivated heterospin systems. The other route that is becoming popular is to incorporate lanthanides and other heavier metal ions in an attempt to in- crease the anisotropy of the system. While this could raise the energy barrier, a significant breakthrough is required before we can consider making SMMs that store magnetic information at room temperature. Regardless of whether information storage in molecules at applicable temperatures is possible, the quantum phenomena that have been discovered and investigated over the last dozen years are fascinating. It is also possible that these phenomena may offer an alternate route to applications.

Manchester, February 2006 Richard Winpenny Preface XI References

1. Christou, G (1989) Acc Chem Res 22:328 2. Boyd PDW, Li Q, Vincent JB, Folting K, Chang H, Streib W, Huffman JC, Christou G, Hendrickson DN (1988) J Am Chem Soc 110:8537 3. Lis T (1980) Acta Cryst B 36:2042 4. Weinland RF, Fischer G (1921) Z Anorg Allg Chem 120:161 5. Caneschi A, Gatteschi D, Laugier J, Rey P, Sessoli R, Zanchini C (1988) J Am Chem Soc 110:2795 6. Caneschi A, Gatteschi D, Sessoli R, Barra AL, Brunel LC, Guillot M (1991) J Am Chem Soc 113:5873 7. Sessoli R, Gatteschi D, Caneschi A, Novak MA (1993) Nature 365:141 8. Sessoli R, Tsai HL, Schake AR, Wang SY, Vincent JB, Folting K, Gatteschi D, Christou G, Hendrickson DN (1993) J Am Chem Soc 115:1804 9. Thomas L, Lionti F, Ballou R, Gatteschi D, Sessoli R, Barbara B (1996) Nature 383:145 10. Friedman JR, Sarachik MP, Tejada J, Ziolo R (1996) Phys Rev Lett 76:211 11. Gatteschi D, Sessoli R, Cornia A (2004) Comp Coord Chem 7:779 12. Gatteschi D, Sessoli R (2003) Angew Chem Int Ed 42:268 13. Koga N, Karasawa S (2005) Bull Chem Soc Jpn 78:1384

Contents

Synthesis of 3d Metallic Single-Molecule Magnets G.Aromí·E.K.Brechin...... 1

Spectroscopy of Single-Molecule Magnets E.J.L.McInnes...... 69

Synthesis of Single-Molecule Magnets Using Metallocyanates J.-N.Rebilly·T.Mallah...... 103

PreparationofNovelMaterialsUsingSMMs A. Cornia · A. F. Costantino · L. Zobbi · A. Caneschi · D. Gatteschi · M.Mannini·R.Sessoli...... 133

Single-Chain Magnets: Theoretical Approach and Experimental Systems C.Coulon·H.Miyasaka·R.Clérac...... 163

General Theory of Superexchange in Molecules J.Curély·B.Barbara...... 207

Author Index Volumes 101–122 ...... 251

Subject Index ...... 261 Contents of Volume 117

Magnetic Functions Beyond the Spin-Hamiltonian Volume Editor: D. Michael P. Mingos ISBN: 3-540-26079-X

Magnetic Parameters and Magnetic Functions in Mononuclear Complexes Beyond the Spin-Hamiltonian Formalism R. Boˇca 1Introduction 2 Energy Levels of Multiterm Systems 3 Modeling the Magnetic Parameters 4 Calculations of Energy Levels and Magnetic Parameters 5 Empirical Magnetic Parameters 6Conclusions Appendix A Spectroscopic Constants, Coefficients, and Matrix Elements B Irreducible Tensors and Tensor Operators C Classification of Crystal-Field Terms and Multiplets D Calculated Energy Levels and Magnetic Parameters References