Functional mimics of catechol oxidase by mononuclear copper complexes of sterically demanding [NNO] ligands

Abstract

Several mononuclear copper complexes 1( a– b) and 2( a– b) supported over sterically demanding [NNO] ligands namely, N-(aryl)-2-[(pyridin-2-ylmethyl)amino]acetamide [aryl = 2,6-diethylphenyl ( 1) and mesityl ( 2)], exhibit catecholase-like activity in performing the aerial oxidation of 3,5-di- t-butylcatehol (3,5-DTBC) to 3,5-di- t-butyl-catequinone (3,5-DTBQ) under ambient conditions. The 1( a– b) and 2( a– b) complexes were directly synthesized from the reaction of the respective ligands 1– 2 with CuX 2· nH 2O (X = Cl, NO 3, n = 2, 3) in 55–85% yield. Mechanistic insights on the catalytic cycle as obtained by density functional theory studies for a representative complex 1a suggest that an intramolecular hydrogen transfer, from a catechol-OH moiety to a copper bound superoxo moiety, form the rate-determining step of the oxidation process, displaying an activation barrier of 18.3 kcal/mol (Δ G ‡) [6.9 kcal/mol in Δ(PE + ZPE) ‡ scale].