Collaborative Research: Electronic Structure and Mechanism of Oxygen Evolution from Synthetic Heterocubane Topological Mimics of the Oxygen-Evolving Complex of Photosystem II

During photosynthesis, plants and algae split water to generate oxygen using a protein called Photosystem II, which harbors a catalytic cluster of manganese and calcium atoms at its core. Understanding how this cluster performs water oxidation is a grand challenge of chemistry because it could guide the design of inexpensive, earth-abundant catalysts for hydrogen fuel production. In this project, Professor Michael Zdilla at Temple University and Professor Jose Mendoza-Cortes at Michigan State University are combining synthesis and atomistic quantum simulations to study structural models of this biological catalyst. The project bridges synthetic inorganic chemistry, advanced spectroscopy, and computational quantum theory, offering rich, vertically integrated training for undergraduate, graduate, and postdoctoral researchers. Engagement activities include undergraduate research opportunities open to all students, publicly accessible computational modules, and quantum-crystallography training available broadly to the scientific community. The oxygen-evolving complex (OEC) of Photosystem II is a CaMn4O5 cluster and the only known biological catalyst that oxidizes water to oxygen. Despite extensive synthetic efforts, manganese–oxo clusters that are simultaneously topologically analogous to the OEC and catalytically reactive remain rare, because strongly chelating ligands typically freeze the core geometry and suppress catalysis. Building on conformationally flexible manganese cubane scaffolds, this project pursues two objectives. Objective 1 will elucidate the mechanism of oxygen evolution from a biomimetic cubane bearing a pendant Mn(V)=O “dangler” unit that spontaneously evolves oxygen under mild conditions. Mechanistic interrogation will employ kinetics, isotopic double-crossover labeling, freeze-quench EPR, XAS/HERFD spectroscopy, and quantum-chemical calculations to determine reaction molecularity, the role of spin density on the terminal oxo, and the influence of water and calcium on rate and pathway. These studies directly test competing oxygen oxygen bond-forming hypotheses (nucleophilic attack vs. radical coupling) in a controlled molecular setting. Objective 2 will characterize the electronic structures of an isolable S3/S4 redox pair of model cubanes via quantum-chemical calculations, multi-edge spectroscopy, and quantum crystallography. Together, these objectives will establish spectroscopic benchmarks for high-oxidation-state intermediates relevant to the OEC catalytic cycle and clarify how flexible, lower-denticity scaffolds unlock reactivity inaccessible to traditional chelated models. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.