Ligand-controlled oxidation state ambivalence in copper–quinone complexes. Replacement of N-donor by S-donor ligands favours the copper(I)–semiquinone over the copper(II)–catecholate form

Abstract

Paramagnetic complexes between chelating o-quinone derivatives Q^n–(Q = 3,5-di-tert-butyl-o-benzoquinone) and coordinatively unsaturated copper compounds (L)Cu have been obtained and studied by EPR spectroscopy. With nitrogen donor co-ligands, L, e.g. saturated polyamines or a bis(imidazole) chelate ligand, the Cu^II(Q^2–) form was observed. In contrast, tetrahydrothiophene and bi-, tri- or tetra-dentate chelating thioethers favour the Cu^I(Q^.–) form as ground state as evident from small ^63,65Cu coupling (<1.5 mT), from semiquinone hyperfine splitting and from g factors (<2.010) which are relatively close to the free-electron value. However, the thioether co-ligands generally effect higher metal-coupling constants and g factors in (L)Cu^I(Q^.–) systems than triorganophosphine, triorganoarsine, carbonyl, π-coordinated alkyne or alkene ligands.