Design and Implementation of a CO2 Reduction Catalyst with an Internal Electron Transfer Mediator: Improving Turnover Frequency by More than 10-Fold

The electrochemical upconversion of carbon dioxide (CO2) is an extant chemical problem. To that end, iron porphyrins are known to readily convert CO2 to carbon monoxide (CO). Herein, we use semiclassical electron transfer (ET) theory to design new catalysts that can supply electrons to active sites with increased rates, thereby improving CO2-to-CO conversion kinetics. Specifically, we report a new iron tetraphenylporphyrin complex that is modified a pyrenyl redox mediator situated ca. 12 Å from the iron ion. We demonstrate that the pyrene-based redox events occur at potentials slightly less reducing than the formal FeI/0 redox event that affords entry to the established iron porphyrin CO2 reduction scheme. The iron porphyrin-pyrene molecular catalyst shows CO2 reduction rates that are between 10 and 100 times larger than related iron porphyrin electrocatalysts. We propose that the small, uphill intramolecular ET event is the origin of the substantial increase in observed CO2 reduction rate constants. These findings show that modest uphill intramolecular ET steps can offer a promising new way to design CO2 reduction catalysts that have improved performance. In conjunction with other leading design elements (e.g., proton delivery), the addition of redox mediators offers a strategy to further improve CO2 reduction electrocatalyst systems.