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

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 CuX2·nH2O (X = Cl, NO3, 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 (ΔG‡) [6.9 kcal/mol in Δ(PE + ZPE)‡ scale].

Graphical abstractA series of mononuclear copper complexes of sterically demanding [NNO] ligands display catecholase-like activity. Theoretical investigation revealed that an intramolecular hydrogen transfer is the critical rate-determining step of the catalytic cycle.Figure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights► Several Cu complexes as functional models of catechol oxidase have been synthesized. ► The Cu complexes carry out aerial oxidation of 3,5-DTBC to 3,5-DTBQ under ambient conditions. ► An intramolecular H-transfer is proposed to be the rate-determining step of the oxidation cycle.