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 HBpz(3)(-) (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)(2)Cr(ox)Ln(HBpz(3))(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, beta = 93.71(2) degrees, 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.