Ferromagnetic coupling in d1–d3 linear oxido-bridged heterometallic complexes: ground-state models of metal-to-metal charge transfer excited states

Abstract

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 d¹–d³ 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 VO 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 = −2JŜ_VŜ_Cr). 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 × 10⁸ 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.