Tetraoxolene-bridged rare-earth complexes: A radical-bridged dinuclear Dy single-molecule magnet

Two families of neutral tetraoxolene-bridged dinuclear rare earth complexes of general formula [((HBpz)RE)(μ-tetraoxolene)] (RE = Y and Dy; HBpz = hydrotris(pyrazolyl)borate; tetraoxolene = fluoranilate (fa; 1-RE) or bromanilate (ba; 2-RE)) have been synthesised and characterised. In each case, the bridging tetraoxolene ligand is in the diamagnetic dianionic form and each rare earth metal centre has two HBpz ligands completing the coordination. Electrochemical studies on the soluble 2-RE family reveal a tetraoxolene-based reversible one-electron reduction. Bulk chemical reduction with cobaltocene affords the cobaltocenium (CoCp) salt of the 1e-reduced analogue: [CoCp][((HBpz)RE)(μ-ba)] (3-RE) that incorporates a radical trianionic form of the bromanilate bridging ligand. Alternating current (ac) magnetic susceptibility studies of 2-Dy reveal slow magnetic relaxation only in the presence of an applied magnetic field, but reduction to radical-bridged 3-Dy affords frequency-dependent peaks in the out-of-phase ac susceptibility in zero applied field. Exchange coupling between the Dy(iii) ions and the radical bridging ligand thus reduces zero-field magnetisation quantum tunnelling and confers single-molecule magnet status on the complex. Comprehensive analysis of the magnetic relaxation data indicates that a combination of Orbach, Raman and direct relaxation processes are required to fit the data for both dysprosium bromanilate complexes.