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

Abstract

We report IR and UV/Vis spectroscopic signatures that allow discriminating between the oxidation states of the manganese-based water oxidation catalyst [(Mn₄ O₄ )(V₄ O₁₃ )(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 ${}_2^{\mathrm{III}}$ Mn ${}_2^{\mathrm{IV}}$ ] to [Mn ${}_4^{\mathrm{IV}}$ ] showed that the change in oxidation state can indeed be tracked by both IR and UV/Vis spectroscopy.

Keywords: Density functional calculations; IR spectroscopy; Polyoxometalates; UV/Vis spectroscopy; Water splitting.