Effect of ligand donors on the catalytic properties of metal complexes. Copper(II) complexes as catalysts for the oxidation of 3,5-di-tert-butylcatechol

Abstract

Copper(II) complexes 1–5 with the tetracentate ligand L 1 = N, N-bis(3,5-dimethylpyrazol-1-ylmethyl)-1-hydroxy-2-aminoethane (pzmhe) and 6–10 with the analogous N 3O benzimidazole ligand L 2 = N, N-bis(2-benzimidazolylmethyl)-1-hydroxy-2-aminoethane (bbmhe) were prepared. The five-coordinate complexes [Cu(L)X]Y 1.2 (with 1, 6: X = H 2O, Y = BF 4; 2, 7: X = NO 3, Y = NO 3; 3, 8: X = Br, Y = Br; 4, 9: X = Cl, Y = Cl; 5, 10: X = N 3, Y = BF 4) were characterized by chemical analysis, absorption and EPR spectroscopies and cyclic voltammetry. The data show the complexes have square pyramidal geometry. All display chemically reversible one-electron redox behavior in acetonitrile. The E° t values are more positive with L 1 than with L 2 by 144–234 mV. Oxidation of 3,5-di-tert-butylcatechol to 3,5-di-tert-butylquinone by these complexes was studied. The reaction rate depends on the nature of the heterocyclic donor, its basicity, steric effects, reduction potentials, and the type of exogenous donor present. Large rate variations were observed with the most effective catalysts those with pzmhe; the [Cu(pzmhe)N 3]BF 4 complex was most active with [Cu(bbmhe)Br]Br and [Cu(bbmhe)Cl]Cl complexes relatively inactive. Electrochemical data showed a non-linear relationship between their ability to oxidize catechols and their reduction potentials. The results point to the bulky ligand environment provided by the benzimidazoles as being responsible for the diminution of their rates of catalysis compared to the pyrazole complexes.