Research Projects

Unlike noble metals such as ruthenium or iridium, manganese remains underexplored in synthetic water oxidation catalysis, despite being the native metal in the oxygen-evolving complex (OEC) of Photosystem II (PS-II). In the OEC, a [Mn₄CaO₅] cluster performs the remarkable feat of water oxidation under mild conditions, where high-valent Mn=O species and Mn–O–Ca bridges are proposed to play key roles in O–O bond formation. Yet, despite extensive modeling efforts, well-defined synthetic analogues of Mn(V)=O species and Mn(V)(O)(μ-O)LA (LA = Lewis acid) remain scarce, and the precise role of redox-inactive Lewis acids like Ca²⁺ is still poorly understood. Our group previously reported the [(TPA)Mn(IV)(O)(μ-O)Ce(IV)(NO₃)₃]⁺ complex as one of the closest synthetic mimics of the proposed Mn(V)(O)(μ-O)Ca(II)(H₂O) intermediate in PS-II. While structurally relevant, this complex was formed via a rapid, one-pot oxidation with ceric ammonium nitrate (CAN), offering limited mechanistic insight and no access to the higher Mn(V) oxidation state. Moreover, the stoichiometric use of CAN generates Ce(III) waste, limiting sustainability and atom economy. Addressing these limitations, the current proposal aims to develop sustainable and mechanistically controlled routes to Mn(V)(O)(μ-O)LA complexes, integrating both structural fidelity and functional reactivity akin to the native OEC. Preliminary work in our lab has revealed the sequential generation of Mn(III)–O–LA, Mn(IV)(O)(μ-O)LA, and potentially Mn(V)(O)(μ-O)LA species from Mn(II) precursors supported by neutral and anionic polypyridyl ligands. Distinct spectroscopic signatures, such as UV-vis absorptions and isosbestic transitions, suggest controlled pathways toward these high-valent species. Notably, we have demonstrated the substitution of Ce⁴⁺ with redox-inactive Sc³⁺, hinting at the possibility of mimicking the Ca²⁺ role in PS-II and expanding the versatility of Lewis acid partners. A central innovation of this project lies in coupling electrochemistry with redox-active Lewis acids like Ce. Instead of relying on excess CAN, we propose to regenerate Ce(IV) in situ from Ce(III) electrochemically, providing a sustainable, atom-efficient oxidation strategy. This approach not only reduces chemical waste but also allows precise control over redox inputs, crucial for probing high-valent Mn species. The central hypothesis of this proposal is that strategically designed ligands, i.e., neutral and anionic, can stabilize elusive Mn(V)=O species and enable the assembly of Mn(V)(O)(μ-O)LA motifs. We further hypothesize that Lewis acid coordination, combined with electrochemical oxidation, will modulate the electronic structure and reactivity of these manganese centers, facilitating pathways relevant for catalytic water oxidation and other oxidative transformations, including C–H bond activation.