Synthetic Molecules and Materials as Models of the Oxygen Evolving Complex of Photosystem II
Efficient catalytic water oxidation is an important reaction for the development of solar hydrogen as a source of green energy. Extraction of high energy electrons from water for the preparation of fuel leaves O2 as a byproduct. Nature boasts the only truly effective catalyst for this reaction, the oxygen evolving complex (OEC), a tetramanganese-calcium-oxo cluster. Though decades of research has gone to the synthesis of biologically-inspired Mn-O clusters, these systems fall short in biomimetic reactivity in comparison to other metal systems (Ru, Co, Ni, and Cu for example), which begs the question: Why have we been unable to mimic the biological chemistry with manganese?
Geometric rearrangements, and changes in coordination number are known to occur in the OEC, but current model chemistry relies heavily on chelation to stabilize synthetic clusters, rendering them inert, and without open coordination sites. Our group at Temple is exploring a new approach to manganese cluster synthesis, using sterics of bridging and terminal ligands to direct sterics and coordination number. Through our approach, we are able to access biologically relevant geometries, and prepare unchelated clusters with significant reactivity seen rarely (or never) in decades of chelate clusters. These reactivities include N-N and C-N bond cleavage, hydrogen atom transfer, ligand exchange, reductive elimination, and cluster rearrangement.
A second topic is the exploration of solid-state layered manganese oxides (termed birnessite) in the chemical and electrochemical oxidation of water. The birnessite phase of MnO2 is typically viewed as a poor water oxidation catalyst, but modification of the material by the introduction of highly active “defect” sites turns this material from a poor catalyst into an active catalyst for water oxidation. The studies being undertaken may shed light on the identity and structure of these sites and aid the development of rubust and affordable catalysts for this important reaction.