Structural and catalytic aspects of copper(II) complexes containing 2,6-bis(imino)pyridyl ligands

Abstract

The synthesis, structure, and ligand substitution mechanism of a new five-coordinate trigonal-bipyramidal copper(II) complex, [CuII(py tBuMe2N3)Cl2] (1), with a sterically constrained py tBuMe2N3 chelate ligand, py tBuMe2N3 = 2,6-bis-(ketimino)pyridyl, are reported. The kinetics and mechanism of chloride substitution by thiourea, as a function of nucleophile concentration, temperature, and pressure, were studied in detail and compared with an earlier study reported for the analogous complex [CuII(py tBuN3)Cl2] (2) [py tBuN3 = 2,6-bis-(aldimino)pyridyl]. Catalysis of the oxidation of 3,5-di-tert-butylcatechol to 3,5-di-tert-butylquinone by 1 and 2 was studied. Correlations between the reactivity, chloride substitution behavior, and reduction potentials of both complexes were made. These show that the rate of oxidation is independent of the rate of chloride substitution, indicating that the substitution of chloride by catechol as substrate occurs in a fast step. Spectral data show a non-linear relationship between the ability of the complexes to oxidize 3,5-DTBC and the Lewis acidity of their copper(II) centers. Electrochemical data demonstrate that the most effective complex 1 has a E 0 value that approaches the E 0 value of the natural tyrosinase enzyme.