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)CuII(Q2−)⇌(L)CuI(Qradical dot−) 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(I) 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 Å and a PCuP angle of 142.41(5)o, the ‘2+2’ coordination pattern could be reproduced by DFT calculations.