Spectral Signatures of Oxidation States in a Manganese-Oxo Cubane Water Oxidation Catalyst.

We report IR and UV/Vis spectroscopic signatures that allow discriminating between the oxidation states of the manganese-based water oxidation catalyst [(Mn ₄ O ₄ )(V ₄ O ₁₃ )(OAc) ₃ ] ^3- . Simulated IR spectra show that V=O stretching vibrations in the 900-1000 cm ^-1 region shift consistently by about 20 cm ^-1 per oxidation equivalent. Multiple bands in the 1450-1550 cm ^-1 region also change systematically upon oxidation/reduction. The computed UV/Vis spectra predict that the spectral range above 350 nm is characteristic of the managanese-oxo cubane oxidation state, whereas transitions at higher energy are due to the vanadate ligand. The presence of absorption signals above 680 nm is indicative of the presence of Mn ^III atoms. Spectroelectrochemical measurements of the oxidation from [Mn 2 III Mn 2 IV ] to [Mn 4 IV ] showed that the change in oxidation state can indeed be tracked by both IR and UV/Vis spectroscopy.
(© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)