Origin of Dissimilar Single-Molecule Magnet Behavior of Three Mn2IIMoIII Complexes Based on [MoIII(CN)7]4- Heptacyanomolybdate: Interplay of MoIII-CN-MnII Anisotropic Exchange Interactions

The origin of contrasting single-molecule magnet (SMM) behavior of three (Mn2MoIII)-Mo-II complexes based on [Mo-III(CN)(7)](4-) heptacyanomolybdate is analyzed; only the apical Mn2Mo isomer exhibits SMM properties with U-eff = 40.5 cm(-1) and T-B = 3.2 K, while the two equatorial isomers are simple paramagnets [Qian, K.; et al. J. Am. Chem. Soc. 2013, 135, 13302]. A microscopic theory of anisotropic spin coupling between orbitally degenerate [Mo-III(CN)(7)](4-) complexes (pentagonal bipyramid) and bound Mn-II ions is developed. It is shown that the [Mo-III(CN)(7)](4-) complex has a unique property of uniaxial anisotropic spin coupling in the apical and equatorial Mo-III-CN-Mn-II pairs, H-eff = -J(xy)((SMoSMnx)-S-x + (SMoSMny)-S-y) - J(z)S(Mo)(z)S(Mn)(z), regardless of their actual low symmetry. The difference in the SMM behavior originates from a different ratio between the anisotropic exchange parameters J(z) and J(xy) for the apical and equatorial Mo-CN-Mn groups. In the apical Mn2Mo isomer, an Ising-type anisotropic spin coupling (J(z) = -34, J(xy) = -11 cm(-1)) produces a double-well potential of spin states resulting in SMM behavior. Exchange anisotropy of an xy-type (vertical bar J(z)vertical bar < vertical bar J(xy)vertical bar) 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 U-eff and T-B values are discussed.