Tuning the structural and catalytic properties of copper(II)-based complexes containing pyridine-2,6-diimines

Abstract

Four mononuclear penta coordinated copper(II) chelates, [CuLⁿBr₂] nH₂O, containing the tridentate neutral ligands, pyridine-2,6-diimine (Lⁿ), were prepared via the template technique. Analytical and several physicochemical methods have been used to characterize the prepared metal chelates. Square-pyramidal stereochemistry was described to the current copper(II) complexes. DFT technique has been applied to optimize the structure of the running diimines and their corresponding copper-based compounds. Ligand substitution study performed to link the catalytic potency of the candidate oxidase mimics and their lability characters. Spectral investigations reveal that nature of substituents of the chelated ligands effectively tuning the Lewis acidity of copper(II) centers. Biomimetics of redox proteins specifically containing copper were examined towards the aerobic oxidation of polyphenol. Kinetic studies with the stopped-follow technique showed a close association between the Lewis acidity of the copper(II) nuclei of the prepared chelates and their oxidase-like activity. The catalytic activity of the natural enzyme (catechol oxidase from sweet potatoes) measured and compared with that for the present Cu^II chelates. The thermodynamic parameter drive force (ΔG° or λ) of the performed oxidation processes was determined from the values of redox potential of the chemical species involved in these catalytic reactions. The proposed catalytic reactions pathways have been discussed based on the outcomes of the kinetic investigations.Communicated by Ramaswamy H. Sarma.

Keywords: Copper(II); catalytic potential; catecholase; diimines; drive force; oxidase mimics; phenoxazinone synthase.