Origin of Dissimilar Single-Molecule Magnet Behavior of Three MnII(2)MoIII Complexes Based on [MoIII(CN)7]4- Heptacyanomolybdate: Interplay of MoIII-CN-MnII Anisotropic Exchange Interactions.

Abstract

The origin of contrasting single-molecule magnet (SMM) behavior of three MnII2MoIII complexes based on [MoIII(CN)7]4– heptacyanomolybdate is analyzed; only the apical Mn2Mo isomer exhibits SMM properties with Ueff = 40.5 cm(-1) and TB = 3.2 K, while the two equatorial isomers are simple paramagnets [Qian, K.; J. Am. Chem. Soc. 2013, 135, 13302]. A microscopic theory of anisotropic spin coupling between orbitally degenerate [MoIII(CN)7](4-) complexes (pentagonal bipyramid) and bound MnII ions is developed. It is shown that the [MoIII(CN)7](4-) complex has a unique property of uniaxial anisotropic spin coupling in the apical and equatorial MoIII-CN-MnII pairs, H̑eff = -Jxy(SMoxSMnx + SMoySMny) - JzSMozSMnz, regardless of their actual low symmetry. The difference in the SMM behavior originates from a different ratio between the anisotropic exchange parameters Jz and Jxy for the apical and equatorial Mo-CN-Mn groups. In the apical Mn2Mo isomer, an Ising-type anisotropic spin coupling (Jz = -34, Jxy = -11 cm(-1)) produces a double-well potential of spin states resulting in SMM behavior. Exchange anisotropy of an xy-type (|Jz| < |Jxy|) in the equatorial Mn2Mo isomers results in a single-well potential with no SMM properties. The prospects of anisotropic uniaxial spin coupling in engineering of high Ueff and TB values are discussed.