Four-electron reduction of O2 over multiple CuI centers: Quantum theory

Abstract

A series of copper complexes were tested theoretically for oxygen electro reduction, beginning with bare Cu and Cu+ centers, with and without N-bearing ligands, and ending with one, two, and three H2N–Cu–Imidazole CuI centers. Adsorption energies for reaction intermediates formed during the four one-electron reduction steps to water were calculated, using B3LYP hybrid density functional theory, and then used in a linear free energy relationship to predict the reversible potential for each step. CuI sites not poisoned by O(ads), OH(ads), and H2O were sought, and so initial screening was based on the predicted reversible potentials for O(ads) reduction to OH(ads) and OH(ads) reduction to H2O and the adsorption bond strengths of O2 and H2O. The CuI centers in H2N–Cu–Imidazole and H2N–Cu–Imidazole had the best properties in this screening. The former was used to model four-electron reduction by copper laccases, some of which have very little overpotential. On the basis of calculations on a model for 3-CuI catalytic sites composed of three H2N–Cu–Imidazole in loose association, the following conclusions were reached: (i) the reduction potential for OH bonded to the model is higher than calculated for Pt, which is consistent with the higher observed overpotentials for Pt compared to lacasses; (ii) H2O bonds weakly to the CuI centers and does not poison them; (iii) model-dependent heat losses were calculated for non-electron transfer steps and it is shown how they contribute to the overpotential for the overall four-electron reduction. Finally, it is shown that CH3S–Cu–(Imidazole)2 possesses the electron donor–acceptor properties that allow it to be an intermediate in electron transfer to the catalytic site. This study shows that loosely coordinated CuI centers present opportunities for four-electron oxygen reduction at low overpotential.