Magnetic Dynamics and Exchange Coupling in Dinuclear Lanthanide Complexes Bridged by Naphthoquinone and Anthraquinone Radicals

The use of the bis(bidentate) quinoid ligands 5,8-Dihydroxy-1,4-naphthoquinone (dhnqH2) and 1,5-Dihydroxyanthraquinone (dhaqH2) in Ln chemistry has afforded four new dinuclear DyIII complexes, viz., [Dy2(dhnq)(Tpz)4].6CH2Cl2 (1.6CH2Cl2), [Dy2(dhaq)(Tpz)4].6CH2Cl2 (2.6CH2Cl2), {K(18-crown-6)}[Dy2(dhnq)(Tpz)4].2THF (3.2THF) and {K(18-crown-6)}[Dy2(dhaq)(Tpz)4].2THF (4.2THF) (Tpz = tris(pyrazolyl)borate). In compounds 1 and 2 the DyIII ions are bridged by the diamagnetic dianionic forms of dhnq2- and dhaq2, while complexes 3 and 4 are bridged the one-electron reduced, radical forms of these ligands. The presence of the ligand-centered radical has been confirmed by X-ray crystallography and SQUID magnetometry. Alternating current (ac) magnetic susceptibility studies revealed that the relaxation dynamics of 1 and 2 are primarily governed by fast quantum tunneling of magnetization (QTM). Conversely, the radical-bridged complexes 3 and 4 behave as single-molecule magnets (SMMs) with energy barriers for the magnetization reversal, Ueff, of 24.17 K and 16.70 K, respectively, in the absence of a direct current (dc) applied field. The strong ferromagnetic Dy-radical interactions, computed using ab initio POLY_ANISO calculations, led to coupling constants of J = +5.0 and +1.2 cm⁻¹ for 3 and 4, respectively, which explains the SMM behavior in these complexes.