In nature, photosystem II carries out water splitting as part of photosynthesis to fuel cell growth and produce the oxygen all aerobic life depends on. Within photosystem II, the rate-limiting water oxidation reaction is catalyzed by the oxygen-evolving complex, a highly efficient tetramanganese enzyme that has inspired many attempts to develop synthetic catalysts of similar structure.[1] Schwarz et al. have proposed a tetramanganese-polyoxovanadate water oxidation catalyst, [Mn4V4O17(OAc3)3- (Mn4V4), that shows considerable activity (TON≈12000, TOF≈3,6s-1 under photocatalytic conditions.[2,3] However, Mn4V4 still falls short of the activity of the oxygen evolving complex by orders of magnitude.[4] Detailed mechanistic understanding of the reactivity of Mn4V4 is required to design improved water oxidation catalysts. To this end, we have carried out a comprehensive theoretical investigation of the water oxidation mechanism on Mn4V4 using density functional theory. Having optimized and compared intermediates across various catalyst oxidation and protonation states, we propose a direct coupling mechanism for the formation of O2 consisting of a series of (proton-coupled) electron transfer steps. Furthermore, we have studied the effects of Jahn-Teller distortions on the catalytic activity of Mn4V4. Using the insights gained through our theoretical simulations, we hope to unravel the reactivity of Mn4V4 and thereby open the way for the rational design of improved water oxidation catalysts.

References:
[1]: Umena, Y.; Kawakami, K.; Shen, J.-R.; Kamiya, N. Nature 2011, 473 (7345), 55–60, http://doi.org/10.1038/nature09913
[2]: Schwarz, B.; Forster, J.; Goetz, M. K.; Yücel, D.; Berger, C.; Jacob, T.; Streb, C. Angew. Chem. Int. Ed. 2016, 55 (21), 6329–6333, https://doi.org/10.1002/anie.201601799
[3]: Huber, L.; Amthor, S.; Schwarz, B.; Mizaikoff, B.; Streb, C.; Rau, S. Sustainable Energy Fuels 2018, 2 (9), 1974–1978, https://doi.org/10.1039/C8SE00328A
[4]: Dau, H.; Limberg, C.; Reier, T.; Risch, M.; Roggan, S.; Strasser, P. ChemCatChem 2010, 2 (7), 724–761, https://doi.org/10.1002/cctc.201000126