Mechanism of a strongly anisotropic Mo-III-CN-Mn-II spin-spin coupling in molecular magnets based on the [Mo(CN)(7)](4-) heptacyanometalate: A new strategy for single-molecule magnets with high blocking temperatures
Journal of the American Chemical Society vol:125 issue:32 pages:9750-9760
Unusual spin coupling between Mo-III and Mn-II cyano-bridged ions in bimetallic molecular magnets based on the [Mo-III(CN)(7)](4-) heptacyanometalate is analyzed in terms of the superexchange theory. Due to the orbital degeneracy and strong spin-orbit coupling on Mo-III, the ground state of the pentagonal-bipyramidal [Mo-III(CN)(7)](4-) complex corresponds to an anisotropic Kramers doublet. Using a specially adapted kinetic exchange model we have shown that the Mo-III-CN-Mn-II superexchange interaction is extremely anisotropic: it is described by an Ising-like spin Hamiltonian JS(Mo)(z) S-Mn(z) for the apical pairs and by the J(z)S(Mo)(z) S-Mn(z) + J(xy)(S-Mo(x) S-Mn(x) + S-Mo(y) S-Mn(y)) spin Hamiltonian for the equatorial pairs (in the latter case J(z) and J(xy) can have opposite signs). This anisotropy resulted from an interplay of several Ising-like (Sm'o Sm'n) and isotropic (S-Mo(z) S-Mn(z)) ferro- and antiferromagnetic contributions originating from metal-to-metal electron transfers through the pi and sigma orbitals of the cyano bridges. The Mo-III-CN-Mn-II exchange anisotropy is distinct from the anisotropy of the g-tensor of [Mo-III(CN)(7)](4-); moreover, there is no correlation between the exchange anisotropy and g-tensor anisotropy. We indicate that highly anisotropic spin-spin couplings (such as the Ising-like JS(Mo)(z) S-Mn(z) combined with large exchange parameters represent a very important source of the global magnetic anisotropy of polyatomic molecular magnetic clusters. Since the total spin of such clusters is no longer a good quantum number, the spin spectrum pattern can differ considerably from the conventional scheme described by the zero-field splitting of the isotropic spin of the ground state. As a result, the spin reorientation barrier of the magnetic cluster may be considerably larger. This finding opens a new way in the strategy of designing single-molecule magnets (SMM) with unusually high blocking temperatures. The use of orbitally degenerate complexes with a strong spin-orbit coupling (such as [Mo-III(CN)(7)](4-) or its 5d analogues) as building blocks is therefore very promising for these purposes.