The reduction of atmospheric nitrogen and carbon dioxide plays a pivotal role in global material cycles. The corresponding biological processes employ transition metal-sulfur (M-S) clusters as active-site cofactors of enzymes. In this context, synthetic M-S clusters have been explored as models of enzymatic cofactors over the past 50 years (1). However, in terms of reducing inert small molecules, catalytic reactions by synthetic clusters are scarce and lag far behind biological systems. We have been developing a stable and modifiable M-S platform capable of capturing and even catalytically reducing such molecules.
Our recent discovery of cubic Mo-Fe-S clusters that catalytically reduce nitrogen to silylamine (2) prompted us to investigate their reactivity toward carbon dioxide. A robust Mo-Fe-S cube with cyclopentadienyl-derivative ligands exhibited excellent stability under reducing conditions and catalyzed the reduction of carbon dioxide to methane in the presence of samarium diiodide (as a reductant) and water (as a proton source). The apparent turnover number for methane production exceeded 10,000 in large-scale experiments. While hydrogen evolution occurred due to water reduction, its extent and impact on methane selectivity will be discussed. Interestingly, a significantly smaller amount of carbon monoxide (CO) than methane was observed as a byproduct, suggesting minimal release of CO from the reaction site during the stepwise reduction of carbon dioxide to CO and CO to methane. These results demonstrate the robustness and unique reactivity of our cuboidal Mo-Fe-S cluster. The reaction mechanism will be discussed further in this talk, along with experimental data on kinetics and intermediate detection.