Cobalt(II) Citrate Cubane Single-Molecule Magnet Kyle W. Galloway, † Alexander M. Whyte, † Wolfgang Wernsdorfer, ‡ Javier Sanchez-Benitez, § Konstantin V. Kamenev, § Andrew Parkin, † Robert D. Peacock, † and Mark Murrie* ,† WestCHEM, Department of Chemistry, UniVersity of Glasgow, Glasgow G12 8QQ, U.K.; Institut Ne ´el, CNRS & UniVersity J. Fourier, BP 166, 38042 Grenoble, Cedex 9, France; School of Engineering and Electronics and Centre for Science at Extreme Conditions, UniVersity of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, U.K. Received May 16, 2008 An investigation of the magnetic properties of the cobalt(II) citrate cubane [C(NH 2 ) 3 ] 8 {Co 4 (cit) 4 }·4H 2 O reveals that the cluster is a new cobalt(II) single-molecule magnet, with an energy barrier to reorientation of the magnetization, ΔE/k B ) 21 K, and τ 0 ) 8 × 10 -7 s. The compound displays distinct, frequency-dependent peaks in the out- of-phase (′′) component of the ac magnetic susceptibility and magnetization versus field hysteresis loops that are temperature and sweep rate dependent. The hysteresis loops collapse at zero field due to very fast quantum tunneling of the magnetization (QTM). Introduction In the field of single-molecule magnets (SMMs), an objective of considerable importance is the synthesis of new clusters possessing greater barriers to the reorientation of the magnetization. 1 The height of the energy barrier (ΔE) varies with the square of the spin (S) and linearly with the axial zero-field splitting (ZFS) parameter (D < 0) via the equation ΔE ) S 2 |D|, or ΔE ) (S 2 - 1 / 4 )|D| for an integer or noninteger spin-ground state, respectively. Efforts to increase the barrier height have targeted the synthesis of clusters containing larger numbers of metal centers to increase S, the use of metals with a large anisotropy to increase D, or by using a directed synthesis approach toward specific cluster geometries. 2 A large number of metal centers do not guarantee a large molecular S: one approach is to have a cluster topology that provides ferromagnetic interactions such as a metal-oxo cubane structure that can promote ferro- magnetic interactions via superexchange. Metal-oxo cubane structures often appear as structural elements in clusters produced by self-assembly from complex inorganic reaction systems. This is illustrated by the work of Aromı ´ et al. 3 on the cobalt-pivalate system that offers a wide variety of products including Co 2 , Co 4 , Co 5 , Co 6 , Co 7 , and Co 14 clusters. The authors propose a reaction scheme based on the assembly of small binuclear units and stepwise addition of cobalt(II) ions to form cubane units. These can then be further enlarged, eventually allowing the formation * To whom correspondence should be addressed. E-mail: M.Murrie@ chem.gla.ac.uk. † University of Glasgow. ‡ Institut Ne ´el. § University of Edinburgh. (1) Milios, C. J.; Vinslava, A.; Wernsdorfer, W.; Moggach, S.; Parsons, S.; Perlepes, S. P.; Christou, G.; Brechin, E. K. J. Am. Chem. Soc. 2007, 129, 2754. (2) (a) Tassiopoulos, A. J.; Vinslave, A.; Wernsdorfer, W.; Abboud, K. A.; Christou, G. Angew. Chem., Int. Ed. 2003, 43, 2117. (b) Ako, A. M.; Hewitt, I. J.; Mereacre, V.; Clerac, R.; Wernsdorfer, W.; Anson, C. E.; Powell, A. K. Angew. Chem., Int. Ed. 2006, 45, 4926. (c) Brechin, E. K.; Boskovic, C.; Wernsdorfer, W.; Yoo, J.; Yamaguchi, A.; Sanudo, E. C.; Concolino, T. R.; Rheingold, A. L.; Ishimoto, H.; Hendrickson, D. N.; Christou, G. J. Am. Chem. 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