Probing the Influences of a Lewis Acid on Mn (IV)-Oxo Bonds: A Guide Towards O-O Bond Formation

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

References

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.