Catalytic promiscuity of a copper(II)-Mannich base complex having unprecedented radical pathway in catecholase activity

Abstract

A flexible single-side Mannich base ligand namely 4-(tert-butyl)-2-((ethyl(2-hydroxyethyl)amino)methyl)phenol (H2L) having O, N, O donor sites has been designed and synthesized using 4-(tert-butyl)phenol, formaldehyde and 2-(ethylamino)ethanol as precursors with the view to prepare polynuclear complex(s). In the first attempt a tetranuclear copper (II) complex, [Cu4(L)2(HL)2].(ClO4)2·H2O (1) has been synthesized employing H2L and copper(II) perchlorate with the aim to explore catalytic promiscuity of the species. Detailed experimental investigations suggest that in solution complex 1 exits as a dinuclear entity. The dinuclear species is a square-planar CuII complex as is evident from EPR and UV–Vis spectral studies. It exhibits excellent catalytic activities in catalyzing the oxidation of 3,5-di-tert-butyl catechol (3,5-DTBC) to 3,5-di-tert-butyl quinone (3,5-DTBQ); oxidation of o-aminophenol to phenoxazinone and PO bond hydrolysis of 4-nitrophenyl phosphate. Amazingly, complex 1 catalyses the aerobic oxidation of 3,5-DTBC not by well established metal centered redox participation, rather via a ligand centered radical pathway, a completely new finding in copper(II) based catechol oxidase modeling as are verified by experiments and seconded by DFT calculations.