On the Mechanism of Light-Driven O₂ Evolution by the Mn(III) Complex [Mn(salpd)(OH₂)]⁺ and Quinone

Abstract

In this study, we apply TD-DFT and DFT calculations to explore the mechanistic details of O2 evolution in an artificial system that closely resembles Photosystem II (PSII). The reaction involves mononuclear Mn(III) complex [Mn(salpd)(OH2)]+ and p-benzoquinone under light-driven conditions. Our calculations reveal that the Schiff-base ligand salpd plays a crucial role in several key steps of the reaction, including the light-mediated oxidation of [Mn(salpd)(OH2)]+ to [Mn(salpd)(OH)]+ by p-benzoquinone, the subsequent oxidation of [Mn(salpd)(OH)]+ to the key Mn(V) intermediate [Mn(salpd)(O)]+, and the critical O-O bond formation step. This role is primarily due to the high propensity of the salpd ligand to undergo oxidation by one unit. This characteristic allows the salpd ligand to reduce Mn(IV) in the intermediate [Mn(salpd)(OH)]+ to Mn(III), triggering a Jahn-Teller effect that increases the ionic character of the hydroxide ligand. This transformation makes the resulting complex a strong nucleophile, facilitating O-O bond formation through a reaction between [Mn(salpd)(OH)]+ and [Mn(salpd)(O)]+ with a moderate overall activation free energy of 18.6 kcal/mol. The mechanistic insights presented in this study may provide a useful foundation for developing novel systems that catalyze water oxidation under light-driven conditions, mimicking Photosystem II, and could potentially contribute to advancements in sustainable energy generation.