New mononuclear copper(I) and copper(II) complexes containing N4 donors; crystal structure and catechol oxidase biomimetic catalytic activity

Abstract

The synthesis of new tetradentate Schiff-base containing N4 donors is described along with the preparation of a series of copper complexes derived from this ligand. These complexes have the form [CuLX]X′, X = OH−, X′ = (1) or X = X′ = Cl− (2); and [CuL]H2O(ClO4)2 (3), in addition to a copper(I) complex [Cu2L2](ClO4)2 (4). The synthesized compounds were spectroscopically characterized, showing the N4 donor ligand. The single-crystal X-ray structural analysis of 4 demonstrated the dimeric structure and univalent copper. This dimer consists of , two uncoordinated , and one acetonitrile. Each ligand is tetradentate via pyridyl and azomethine nitrogen atoms, providing a strongly distorted tetrahedron around copper(I) despite the pseudo-macrocyclic skeleton of the ligand. These complexes have been evaluated as functional model systems for catechol oxidase enzyme using 3,5-di-tert-butylcatechol (3,5-DTBC) as the test substrate. The catalytic performance of the air oxidation of 3,5-DTBC to the corresponding light absorbing 3,5-di-tert-butylquinone (3,5-DTBQ) at ambient conditions was studied using UV-Vis absorption spectra. Complex 4 exhibits the highest catalytic activity with turnover number of 33 h−1. A kinetic treatment on the basis of the Michaelis–Menten model was applied. Correlation among reactivity, binding constants, electrochemical properties, and the geometry was determined. These correlations showed that the rate of oxidation is linearly correlated with the binding constants for the five coordinate 1 and 2. The catalytic investigations demonstrate that geometrical effects are only one facet of the activity. The probable mechanistic implications of the catalyzed oxidation reactions are discussed.