Identifying Intermediates in Electrocatalytic Water Oxidation with a Manganese Corrole Complex

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 × 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 (H 2 O) value of 4.4 × 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 Mn IV -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 .