Direct CO₂ Reduction to CO with an Fe₄S₄-Based Coordination Polymer

Fe₄S₄ clusters play essential roles in nature, classically in electron transport but increasingly in newly discovered reactivity or catalysis. These roles have spurred interest in developing synthetic Fe₄S₄ systems and while several molecular and material systems built from Fe₄S₄ clusters have been developed, comparatively few examples of synthetic Fe₄S₄ cluster-based catalysts exist. Herein, we present the use of an Fe₄S₄-based coordination polymer as a catalyst for the direct and selective electroreduction of CO₂ to CO. Computational studies suggest that the reaction proceeds through CO₂ binding to a reduced Fe₄S₄ cluster, followed by a series of protonation, reduction, and H₂O loss steps to yield a CO-bound cluster that can finally exchange with CO₂ to restart the catalytic cycle. CO bound clusters are predicted to be thermodynamically stable, suggesting that carbonyl species might be off-cycle intermediates. Mechanistic CV studies as well as in situ studies by IR spectroscopy provide evidence for carbonyl-ligated clusters, supporting these compounds as unusual examples of small molecule binding to Fe₄S₄ clusters. This work establishes Fe₄S₄ cluster-based coordination polymers as direct electrocatalysts for CO₂ reduction and provides mechanistic insights into how these species mediate catalytic conversions of small molecules.