Probing the catalytic properties of copper(II) complexes of appended cyclams : Correlations between catalysis and stability constants or electrochemical properties

Copper(II) complexes were prepared from the dinucleating octadentate ligand tetra-1,4,8,11-(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane (tpmc). The five-coordinate complexes [Cu2(tpmc)X](ClO4)3 (with X = F, Cl, Br, or I) were characterized by chemical analysis, absorption spectroscopy and cyclic voltammetry. The data show the dinculear Cu(II) complexes to be five-coordinate with a bridging halide donor. All display chemically reversible redox behavior, with two one-electron reductions observed in acetonitrile at glassy carbon. The values for E0′ range from 118 to - 126 mV vs SCE for the first reduction, and from - 122 to - 241 mV for the second reduction. Catalysis of the oxidation of 3,5-di-tert-butylcatechol to 3,5-ditert-butylquinone using these complexes was studied. Correlations among reactivity, binding constants, and reduction potentials were determined. These correlations show that the rate of the oxidation reaction is linearly correlated to the binding of the halide bridge; the most reactive complex ([Cu2(tpmc)I](ClO4)3) is the one with the smallest binding constant while the least reactive ([Cu2(tpmc)Cl](ClO4)3) has the largest binding constant. Electrochemical data showed a non-linear relationship between the complexes' ability to oxidize 3,5-di-tert-butylcatechol and their reduction potentials. Comparisons to the naturally occurring enzyme tyrosinase are made. © 1998 Elsevier Science Ltd. All rights reserved.