Photooxidation by a long-lived photoexcited state of a triflic acid-bound Mn(IV)-oxo complex with the highest quantum efficiency

Abstract

The photoexcited state of high-valent metal-oxo complexes binding protons would show the much enhanced oxidizing reactivity, providing a new way to achieve a photochemical oxidation of thermally inert substrates. However, there has been only one example for the long-lived photoexcited state of a MnIV-oxo complex binding scandium ion that is a rare-earth metal element. We report herein the formation of a long-lived photoexcited state of a MnIV-oxo complex binding triflic acid, composed of only earth-abundant elements, which exhibits the highest oxidizing reactivity among the high-valent metal-oxo species, including the excited state, reported so far. Photoexcitation of a triflic acid-bound MnIV-oxo complex (1, [(N4Py)MnIV(O)]2+–(HOTf)2) in a deaerated trifluoroethanol/acetonitrile resulted in the formation of a long-lived photoexcited state with a lifetime of 21 μs, which oxidized toluene to produce benzyl alcohol that was further oxidized to benzaldehyde. The driving force dependence of the rate constant of the photoinduced electron transfer from electron donors to the long-lived 2E excited state of 1 afforded a small reorganization energy of electron transfer (λ = 0.53 eV), since the 2E excited state of 1 may be composed of the ligand-to-metal charge-transfer (LMCT) state and the efficient electron transfer to the excited sate of 1 occurs at the ligand center. The quantum efficiency of 200% was achieved for the photochemical four-electron oxidation of toluene by 1, being the highest among various photooxidation reactions reported so far. The photodynamics were measured by laser-induced transient absorption spectroscopy to clarify the photooxidation mechanism of 1. © 2023 Elsevier B.V.