Magnetic dynamics and exchange coupling in dinuclear lanthanide complexes bridged by naphthoquinone and anthraquinone radicals

Abstract

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 Dy^III complexes, viz., [Dy₂(dhnq)(Tp)₄] (1), [Dy₂(dhaq)(Tp)₄] (2), {K(18-crown-6)}[Dy₂(dhnq)(Tp)₄] (3) and {K(18-crown-6)}[Dy₂(dhaq)(Tp)₄] (4) (Tp = tris(pyrazolyl)borate). In compounds 1 and 2, the Dy^III ions are bridged by the diamagnetic dianionic forms of dhnq²⁻ and dhaq², while complexes 3 and 4 are bridged by 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, U_eff, 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.