The process of photosynthesis has been intensely studied since its discovery in the late 1720’s.1 Understanding how plants oxidize water to O2 is of relevant importance due to its potential use in fuel cell technology. Water splitting cells generate H+ and electrons for the fuel cell to use. Inside the chloroplast, plants contain the oxygen evolving complex (OEC) a Mn4O5Ca cluster that is responsible for O2 evolution.2 It has been demonstrated that the OEC goes through four intermediate states before oxygen evolution; however, the exact mechanism of O2 evolution is yet to be determined.3 There are two proposals for O–O bond formation through (i) radical coupling between two oxygens, or (ii) H2O/OH– nucleophilic attack on a Mn(V)-oxo.4 While Ca2+ is essential, its direct role remains unclear.5-7 Therefore it is important to assess the role of redox-inactive Lewis acids and their influence on the reactivity of Mn(IV)-oxo through low molecular weight synthetic models.
The work presented in this seminar examines the potential role of Ca2+ in the OEC by studying the structural/spectroscopic/electronic properties resulting from LA interactions with well-defined L-Mn=O complexes.
Incorporating a variety of Lewis acids bound to Mn (IV)-oxo moieties leads to changes in the basicity of the Mn (IV)-oxo as observed from studies: electron transfer (ET), oxygen atom transfer (OAT) and hydrogen atom transfer (HAT). The results indicate that increasing Lewis acidity leads to a more electrophilic Mn (IV)-oxo. This favors O-O bond formation, as indicated by the ET and OAT reactivity studies.6 Moreover, HAT rates were found to be inversely related to OAT and ET thus providing further evidence that the increase in Lewis acidity leads to a more electrophilic Mn (IV)-oxo. Furthermore, it was discovered that there is a linear correlation between the Raman shifts and the basicity of the Mn (IV)-oxo.7
1. Krogmann, D., Discoveries in oxygenic photosynthesis (1727–2003): a perspective. Photosynthesis Research 2004, 80 (1), 15-57.
2. Shevela, D.; Kern, J. F.; Govindjee, G.; Whitmarsh, J.; Messinger, J., Photosystem II. eLS 2021, 2, 1-16.
3. Kern, J.; Renger, G., Photosystem II: Structure and mechanism of the water: plastoquinone oxidoreductase. Photosynthesis research 2007, 94 (2), 183-202.
4. McAlpin, J. G.; Stich, T. A.; Casey, W. H.; Britt, R. D., Comparison of cobalt and manganese in the chemistry of water oxidation. Coordination Chemistry Reviews 2012, 256 (21-22), 2445-2452.
5. Leeladee, P.; Baglia, R. A.; Prokop, K. A.; Latifi, R.; De Visser, S. P.; Goldberg, D. P., Valence tautomerism in a high-valent manganese–oxo porphyrinoid complex induced by a lewis acid. Journal of the American Chemical Society 2012, 134 (25), 10397-10400.
6. Sankaralingam, M.; Lee, Y.-M.; Pineda-Galvan, Y.; Karmalkar, D. G.; Seo, M. S.; Jeon, S. H.; Pushkar, Y.; Fukuzumi, S.; Nam, W., Redox reactivity of a mononuclear manganese-oxo complex binding calcium ion and other redox-inactive metal ions. Journal of the American Chemical Society 2018, 141 (3), 1324-1336.
7. Karmalkar, D. G.; Seo, M. S.; Lee, Y.-M.; Kim, Y.; Lee, E.; Sarangi, R.; Fukuzumi, S.; Nam, W., Deeper Understanding of Mononuclear Manganese (IV)–Oxo Binding Brønsted and Lewis Acids and the Manganese (IV)–Hydroxide Complex. Inorganic Chemistry 2021, 60 (22), 16996-17007.