Catalytic, Spectroscopic, and Theoretical Studies of Fe₄S₄-Based Coordination Polymers as Heterogenous Coupled...

Iron-sulfur clusters play essential roles in biological systems, and thus synthetic [Fe₄S₄] clusters have been an area of active research. Recent studies have demonstrated that soluble [Fe₄S₄] clusters can serve as net H atom transfer mediators, improving the activity and selectivity of a homogeneous Mn CO₂ reduction catalyst. Here, we demonstrate that incorporating these [Fe₄S₄] clusters into a coordination polymer enables heterogeneous H atom transfer from an electrode surface to a Mn complex dissolved in solution. A previously reported solution-processable Fe₄S₄-based coordination polymer was successfully deposited on the surfaces of different electrodes. The coated electrodes serve as H atom transfer mediators to a soluble Mn CO₂ reduction catalyst displaying good product selectivity for formic acid. Furthermore, these electrodes are recyclable with a minimal decrease in activity after multiple catalytic cycles. The heterogenization of the mediator also enables the characterization of solution-phase and electrode surface species separately. Surface enhanced infrared absorption spectroscopy (SEIRAS) reveals spectroscopic signatures for an in situ generated active Mn-H species, providing a more complete mechanistic picture for this system. The active species, reaction mechanism, and the protonation sites on the [Fe₄S₄] clusters were further confirmed by density functional theory calculations. The observed H atom transfer reactivity of these coordination polymer-coated electrodes motivates additional applications of this composite material in reductive H atom transfer electrocatalysis.