Copper complexes with non-innocent ligands:: probing CuII/catecholato-CuI/o-semiquinonato redox isomer equilibria with EPR spectroscopy

The temperature-dependent intramolecular metal/ligand electron transfer equilibrium (L)Cu-II(Q(2-))reversible arrow(L)Cu-I(Q(.-)) has been quantitatively analyzed by EPR as a function of the non-innocent o-quinonoid ligand Q and of the co-ligand L. Suitable co-ligands were 1-methyl-(2-methylthiomethyl)-1H-benzimidazole and 1-diphenylphosphino-2-dimethylaminoethane (dde) which can tolerate both copper oxidation states. EPR hyperfine data were extracted for a variety of alkyl- and methoxy-substituted o-semiquinones and their copper(l) complexes in solution. A close match of Q and Cu redox orbitals is essential for observing the redox isomer equilibrium which has been similarly reported for aminoxidase enzymes. Efforts to isolate the mixed-ligand compounds from solution failed due to coordination disproportionation. The new [Cu(dde)(2)](ClO4) was structurally characterized with comparable Cu-N and Cu-P distances of 2.24-2.27 Angstrom and a P-Cu-P angle of 142.41(5)degrees, the '2 + 2' coordination pattern could be reproduced by DFT calculations. (C) 2002 Elsevier Science B.V. All rights reserved.