Ferromagnetic coupling in d(1)-d(3) linear oxido-bridged heterometallic complexes: ground-state models of metal-to-metal charge transfer excited states.

Convenient strategies have been developed to synthesize heterobi/trimetallic oxido complexes containing V(IV)-O-Cr(III), V(IV)-O-Cr(III)-O-Ti(IV) and V(IV)-O-Cr(III)-O-V(IV) cores. These compounds can serve as ground state models for probing the magnetic properties of metal-to-metal charge transfer excited states. Each of these complexes represents the first experimental demonstration of ferromagnetic coupling in a d(1)-d(3) oxido bridged compound, which confirms a long standing theoretical prediction for such a linkage. Structural characterization reveals a similar structure for each of the bi/trimetallic complexes with identical V[double bond, length as m-dash]O bond lengths (∼1.644 Å) and a linear V-O-Cr geometry. The Cr-O distances (1.943-1.964 Å) are significantly influenced by the ligands in the trans axial positions. Ferromagnetic coupling between the V(IV) and Cr(III) of V-O-Cr is measured by temperature-dependent magnetic susceptibility, showing J = +42.5 to +50.7 cm(-1) (H = -2JŜVŜCr). This is further supported by variable temperature X-band EPR. The values of J are found to be consistent with the function J = Ae(βr) (A = 9.221 × 10(8) and β = 8.607 Å(-1)), where r is the Cr-O bond distance. We propose a model that links either ferromagnetic or antiferromagentic exchange coupling with long excited state lifetimes in metal-to-metal charge transfer (MMCT) chromophores.