Identifying Intermediates in Electrocatalytic Water Oxidation with a Manganese Corrole Complex

Abstract

Water nucleophilic attack (WNA) on high-valent terminal Mn-oxo species is proposed for O-O bond formation in natural and artificial water oxidation. Herein, we report an electrocatalytic water oxidation reaction with Mn-III tris(pentafluorophenyl)corrole (1) in propylene carbonate (PC). O-2 was generated at the Mn-V/IV potential with hydroxide, but a more anodic potential was required to evolve O-2 with only water. With a synthetic Mn-V(O) complex of 1, a second-order rate constant, k(2)(OH-), of 7.4 x 10(3) M-1 s(-1) was determined in the reaction of the Mn-V(O) complex of 1 with hydroxide, whereas its reaction with water occurred much more slowly with a k(2)(H2O) value of 4.4 x 10(-3) M-1 s(-1). This large reactivity difference of Mn-V(O) with hydroxide and water is consistent with different electrocatalytic behaviors of 1 with these two substrates. Significantly, during the electrolysis of 1 with water, a MnIV-peroxo species was identified with various spectroscopic methods, including UV-vis, electron paramagnetic resonance, and infrared spectroscopy. Isotope-labeling experiments confirmed that both O atoms of this peroxo species are derived from water, suggesting the involvement of the WNA mechanism in water oxidation by a Mn complex. Density functional theory calculations suggested that the nucleophilic attack of hydroxide on Mn-V(O) and also WNA to 1e(-)oxidized Mn-V(O) are feasibly involved in the catalytic cycles but that direct WNA to Mn-V(O) is not likely to be the main O-O bond formation pathway in the electrocatalytic water oxidation by 1.