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 [(Mn4O4)(V4O13)(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 [Mn2III Mn2IV ] to [Mn4IV ] showed that the change in oxidation state can indeed be tracked by both IR and UV/Vis spectroscopy.