LIGAND-CONTROLLED OXIDATION-STATE AMBIVALENCE IN COPPER-QUINONE COMPLEXES - REPLACEMENT OF N-DONOR BY S-DONOR LIGANDS FAVORS THE COPPER(II)-SEMIQUINONE OVER THE COPPER(II)-CATECHOLATE FORM

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 Cu-63,Cu-65 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, pi-coordinated alkyne or alkene ligands.