Modeling Spin Interactions in a Cyclic Trimer and a Cuboidal Co 4 O 4 Core with Co(II) in Tetrahedral and Octahedral Environments John F. Berry, ² F. Albert Cotton,* ,² Chun Y. Liu, ² Tongbu Lu, ²,§ Carlos A. Murillo,* ,² Boris S. Tsukerblat,* ,‡ Dino Villagra ´ n, ² and Xiaoping Wang ² Contribution from the Department of Chemistry and Laboratory for Molecular Structure and Bonding, P.O. Box 30012, Texas A&M UniVersity, College Station, Texas 77842-3012, Department of Chemistry, Ben-Gurion UniVersity of the NegeV, P.O. Box 653, Beer-SheVa 84105, Israel, and School of Chemistry & Chemical Engineering, Sun-Yat Sen UniVersity, Guangzhou 510275, P. R. China Received September 24, 2004; E-mail: cotton@tamu.edu; murillo@tamu.edu; tsuker@bgumail.bgu.ac.il Abstract: The X-ray crystallographic structures, the magnetic susceptibilities from 2 to 300 K, and a theoretical analysis of the magnetism for a triangular and a tetranuclear molecule consisting of linked high- spin cobalt(II) centers are described. The interpretation of the magnetic data for the triangular compound [Co(depa)Cl]3 (depa is the anion of 2,2′-(bis-4-ethylpyridyl)amine), which has tetrahedrally coordinated Co 2+ ions, entails isotropic antiferromagnetic exchange interaction and antisymmetric exchange acting within the two low-lying spin doublets. Two strong isotropic ferromagnetic interactions have been modeled in the cuboidal compound Co4(DPM)4(CH3O)4(CH3OH)4 (DPM represents the anion of dipivaloylmethane), which has octahedral coordination, and the system can be approximately considered as two weakly coupled S ) 3 species. Introduction The discovery of the single molecular magnetism phenom- enon 1 has created a strong impetus for the study of the exchange interactions in clusters with a large number of coupled para- magnetic centers, such as the so-called Mn 12 cluster (Mn 12 O 12 - (OAc) 16 (H 2 O) 4 ). 1d,e In the past few years there has been also renewed interest in smaller metal clusters, like triangular systems 2 in which interplay between isotropic and anisotropic exchange interactions 3 gives rise to a number of interesting peculiarities in the magnetic properties, including EPR. These relatively simple systems are helpful for testing models of exchange interaction and provide a good basis for the study of the magnetic anisotropy. Examples are the studies of associated Co(II) atoms joined by appropriate linkers giving dinuclear, 4 tetranuclear, 5 and larger clusters 6 in which there may be ferromagnetic or antiferromagnetic coupling between cobalt atoms. However, it should be kept in mind that the magnetic behavior of high-spin cobalt(II) complexes has long been known to provide many traps for the unwary because of the great variety of magnetic behaviors they exhibit depending on factors such as the type and number of ligands that can give rise to low- 7 or high-spin 8 tetrahedral or octahedral complexes or those with other geometries. 9 Even for a given geometry such as in the tetrahedral CoCl 4 2- ions, commonly found distortions have ² Texas A&M University. ‡ Ben-Gurion University of the Negev. § Sun-Yat Sen University. (1) (a) Gatteschi, D.; Sessoli, R. Angew. Chem., Int. Ed. 2003, 42, 268. (b) Christou, G.; Gatteschi, D.; Hendrickson, D. N.; Sessoli, R. MRS Bull. 2000, 25, 66. (c) Awschalom, D. D.; Di Vencenzo, D. P. Phys. Today 1995, 43. (d) Sessoli, R.; Tsai, H.-L.; Schake, A. R.; Wang, S.; Vincent, J. B.; Folting, K.; Gatteschi, D.; Christou, G.; Hendrickson, D. N. J. Am. Chem. Soc. 1993, 115, 1804. 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