Structural and Magnetic Studies of a Discrete Tetranuclear Complex Featuring an Oxalato-Bridged Ni₃^IICr^III Cluster

A tetranuclear oxalate-bridged complex was synthesized by a reaction between the tris(oxalato)chromium(III), [Cr(C2O4)3]3–, and a 3-fold excess of the Ni2+ ion capped with a tetradentate 1,4,7,10-tetraazacyclododecane (cyclen) ligand. The structure of [Ni3(cyclen)3Cr(μ-C2O4)3](ClO4)3 (1), established by X-ray diffraction, features a triangle of the Ni2+ ions centered by the [Cr(C2O4)3]3– anion through bridging oxalate ligands. The Ni2+ and Cr3+ ions exhibit oxalate-mediated ferromagnetic exchange coupling, which leads to the S = 9/2 ground state. Both simulation of the magnetic data and theoretical calculations show that the exchange coupling is isotropic. Although the study of a model mononuclear complex [Ni(cyclen)(MeCN)2]2+ by high-field EPR spectroscopy reveals a substantial zero-field splitting on the Ni2+ ion, complex 1 exhibits weak magnetic anisotropy within a giant spin model. This difference is attributed to dilution of the single-ion anisotropies of the Ni2+ ions due to their nearly symmetrical (trigonal) arrangement around the central Cr3+ ion. CASSCF calculations provided reasonable zero-field splitting parameters and Landé g-factors for the Ni2+ and Cr3+ ions in complex 1.