Heterodinuclear Complexes Containing d- and f-block Elements: Synthesis, Structural Characterization, and Metal-Metal Interactions of Novel Chromium(III)-Lanthanide(III) Compounds Bridged by Oxalate

The reaction of Ln(III) ions with a tripodal ligand HBpz₃^- (hydrotris(pyrazol-1-yl)borate) and a "complex ligand" [Cr(acac)₂(ox)]^- (acac^- = acetylacetonate, ox^2- = oxalate) in aqueous solution afforded a series of the novel 3d-4f heterodinuclear complexes [(acac)2Cr(ox)Ln(HBpz₃)₂] (Ln = Eu (1), Gd (2), Tb (3), Yb (4), Lu (5)). The crystal structure of 4 has been determined by X-ray diffraction. Complex 4 crystallizes in monoclinic space group P2/n, of which the cell parameters are a = 8.594(3) Å, b = 18.538(4) Å, c = 12.093(2) Å, β = 93.71(2)°, and Z = 2. The Yb atom has an eight-coordinate distorted square antiprismatic coordination geometry. The intramolecular Cr⋯Yb distance is 5.631(1) Å. The magnetic susceptibility data for complex 2 showed that the Cr^III-Gd^III interaction is weakly antiferromagnetic with an exchange coupling constant J_CrGd = -0.09 cm^-1. The luminescence measurements demonstrated the energy transfers for both Ln(III) → Cr(III) and Cr(III) → Ln(III), of which the degree of emission quenching depends on the energy gap of the excited levels in two metal centers. These results reveal that the metal-metal interactions between Cr(III) and Ln(III) are very weak in magnetic interaction but are strong from the viewpoint of energy transfer.