Reactivity of a heterobinuclear heme–peroxo–Cu complex with para-substituted catechols shows a pKa-dependent change in mechanism

Abstract

In biological systems, heme–copper oxidase (HCO) enzymes play a crucial role in the oxygen reduction reaction (ORR), where the pivotal O–O bond cleavage of the (heme)Fe^III–peroxo–Cu^II intermediate is facilitated by active-site (peroxo core) hydrogen bonding followed by proton-coupled electron transfer (PCET) from a nearby (phenolic) tyrosine residue. A useful approach to comprehend the fundamental relationships among H-bonding/proton/H-atom donors and their abilities to induce O–O bond homolysis involves the investigation of synthetic, bioinspired model systems where the exogenous substrate properties (such as pK_a and bond dissociation energy (BDE)) can be systematically altered. This report details the reactivity of a heme–peroxo–copper HCO model complex (LS-4DCHIm) toward a series of substituted catechol substrates that span a range of pK_a and O–H bond BDE values, exhibiting different reaction mechanisms. Considering their interactions with the bridging peroxo ligand in LS-4DCHIm, the catechol substrates are importantly capable of one or two (i) H-bonds, (ii) proton transfers, and/or (iii) net H-atom transfers, thereby making them attractive, yet complex candidates for studying the redox chemistry of the metal-bound peroxide. A combination of spectroscopic studies and kinetic analysis implies that the suitable modulation of pK_a and O–H bond BDE values of catechols result in either double proton transfer with the release of H₂O₂ or double PCET resulting in reductive O–O bond rupture. The distinguishing role of substrate properties in directing the mechanism and outcome of O₂ protonation/reduction reactions is discussed in terms of designing O₂-reduction catalysts based on biological inspiration.