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

We report IR and UV/Vis spectroscopic signatures that allow discriminating among 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 900-1000cm - 1 region shift consistently by about 20cm - 1 per oxidation equivalent. Multiple bands in the 1450-1550cm - 1 region also change systematically upon oxidation/reduction. The computed UV/Vis spectra predict that the spectral range above 350nm 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 680nm is indicative of the presence of Mn III atoms. Spectroelectrochemical measurements of the oxidation from [Mn III 2 Mn IV 2 ] to [Mn IV 4 ] showed that the change in oxidation state can indeed be tracked by both IR and UV/Vis spectroscopy.