Magnetic anisotropy through cooperativity in multinuclear transition metal complexes: theoretical investigation of an anisotropic exchange mechanism

摘要

To be a candidate for a good single-molecule magnet, a compound should have a reasonably large zero-field splitting, a reasonably large total spin, and preferably axial symmetry.We investigate how this can be achieved in multi-heteronuclear d-Block transitionmetal complexes with both 3d and 5d centres.While spin-orbit coupling is large at the 5d centre, the 3d centres contribute many unpaired electrons. If the heavy-element building block does not have a single-ion anisotropy it might still transfer its spin-orbit effects to the 3d centre(s) through anisotropic exchange.We present spin-orbit configuration interaction calculations on a bi- and trinuclear compound with one Os~(III) and one or two Mn~(II) centres to demonstrate the mechanism. The interaction between the metal centres brings about zero-field splitting that is entirely due to anisotropic exchange. Because of strong spin-orbit coupling, the Os centre behaves as a pseudospin with S = (1/2) that shows antiferromagnetic and anisotropic exchange coupling with the Mn centres. The ground state of the binuclear complex has a pseudo S = 2 ground state with an axial zero-field splitting parameter D= +0.59 cm~(?1), while the trinuclear complex has a pseudo S = (9/2) ground state with a negative D = ?0.32 cm~(?1). Coordination of the second Mn centre thus increases total spin and magnetic anisotropy, and reverses the sign of D.