Biomimetic Oxygen Activation with Copper Complexes

 

Modelling of active sites of copper containing proteins contributes to a better understanding of structure-function relationship within these systems and is often aimed to design effective and environmental friendly oxidation catalysts. A key research objective of this project is to understand how supporting ligand structural features influence the relative stabilities against intramolecular oxygen attack and interconversions of copper-oxygen adduct species, and especially the effects which control the equilibrium between \(\mu\)-\(\eta\) ²: \(\eta\) ²-peroxo and bis(\(\mu\)-oxo) dicopper cores that are relevant to proposed metalloprotein active site intermediates. A novel approach to this topic in bioinorganic chemistry is the design and the synthesis of biomimetic hybridguanidine ligands followed by synthesis of the corresponding copper(I) complexes. Hybridguanidines combine the advantages of guanidines and amines by providing the ability to stabilise highly reactive Cu₂O₂ species and a spatially less demanding ligand environment favourable for the pre-coordination of substrate molecules and subsequent oxygen transfer. The generated Cu₂O₂ species are investigated experimentally by UV/Vis, resonance-Raman and EXAFS spectroscopy and theoretically by DFT analysis. Furthermore, they are evaluated towards their capability of transferring oxygen to specific organic substrates. The close interaction between experimental and theoretical methods is expected to lead to the most efficient oxygen-transferring system for use in oxidation catalysis.

Inspection of enzymatic reactions often provides chemists strategic direction to approach difficult molecular transformations. Paramount to efficiency is the mechanistic route, so attempting duplication of an enzymatic mechanism, while challenging, is a rational method of catalyst design. Tyrosinase, found in nearly all species of Life, catalyzes the aerobic oxidation of phenols to catechols through the intermediacy of a dioxygen-derived, side-on bonded peroxide coordinated to two copper centers. In a recent interdisciplinary study, we present a simple tyrosinase mimic capable of regioselective hydroxylation of a wide range of phenols, including substrates incompatible with the enzyme, in both stoichiometric and catalytic modes. We propose the efficiency of this synthetic binuclear copper catalyst is due to the stability of its oxygenated form under ambient conditions, rarely observed, and its operation through a mechanism demonstrated to parallel that accepted for tyrosinase, the natural world’s common method of phenolic hydroxylation.