In this project funded by the Chemical Catalysis Program of the Chemistry Division, Professor George Christou of the University of Florida synthesizes and studies compounds related to the oxygen-evolving complex (OEC). The OEC is found in plants and certain bacteria, and for over 2½ billion years has been converting water to oxygen gas using sunlight as an energy source. Hence the OEC is responsible for the production and maintenance of the atmospheric oxygen on which all respiring life depends. The OEC is too fragile to be isolated from natural sources; therefore researchers are seeking to synthesize the OEC in the laboratory to study its properties and how it makes oxygen. The Christou lab will synthesize model compounds for OEC and study their ability to catalyze conversion of water to oxygen. This research has broader impacts on current efforts to harness and store solar energy. It also creates opportunities for graduate students to acquire advanced training in synthesis and specialized techniques such as magnetochemistry and electrocatalysis of water oxidation.
The goal of the research is to synthesize and study Mn4Ca/oxo clusters as models of the OEC, using recently developed methodologies to make mixed Mn/Ca cubane compounds. Derivatives with chelate-induced, structural perturbations that mimic the effect of the peptide scaffold on the OEC will be synthesized, as will be derivatives with manganese in different oxidation states and with strontium instead of calcium, Sr being the only metal that can replace Ca in OEC with retention of significant activity. The characterization of the compounds by magnetic measurements, and by EPR and X-ray spectroscopy will provide benchmarks useful in the study of OEC. The proposed work will enable the testing of current hypotheses about the precise level of structural distortion of the OEC and its influence on the properties of OEC, about whether the Sr-substituted OEC is isostructural with the Ca form, and about the origin of the distinctly different, electronic properties of the OEC at a given oxidation level and depending on generation conditions. The OEC models and other synthetic Mn compounds will also be explored as functional models, namely as electrocatalysts for water oxidation to oxygen, building on the discovery of such activity with low overpotential by certain Mn carboxylate clusters.
