Co-electrocatalytic CO2 reduction involving a molecular chromium complex mediated by a dibenzophosphole oxide

The exacerbating circumstances of the geochemical landscape have driven innovations in the development of green strategies and technologies. An area of importance is the generation of feedstock chemicals via the utilization of anthropogenic carbon dioxide (CO₂). Specifically, the reduction of CO₂ to carbon monoxide (CO) requires the use of electrocatalysts to facilitate chemical conversion. Our research group focuses on studying the behavior of catalytic molecular systems. Presented herein is a new example of a mediated co-electrocatalytic system selective for the reduction of CO₂ to CO, comprised of a previously reported molecular Cr complex and 5-phenylbenzo[b]phosphindole-5-oxide (PhBPO) as a redox mediator. Under protic conditions, a rate enhancement of the co-electrocatalytic system is observed and attains a turnover frequency (TOF) of 15 s^–1. It is proposed that, in the reduced states, PhBPO coordinates with the Cr metal center in an axial position trans to an intermediate hydroxycarbonyl species (M–CO₂H). The strength of the axial bonding interaction between RM and catalyst is responsible for mediating electron transfer to the catalyst and lowering the barrier for C–OH bond cleavage in comparison to the previously observed pancake bonding phenomenon in analogous co-electrocatalytic systems. It is thought that the incorporation of more electron-poor PhBPO derivatives with greater aromaticity may further enhance catalytic activity. The implication that axial coordination strength must be balanced against the contribution of pancake bonding for co-catalytic activity will be discussed.