Effect of a Hydrogen Bond Accepting Microenvironment on Electrocatalytic CO₂ Reduction to Formate via Hydride...

The rate-determining step in electrochemically driven C-H bond formation with CO₂ is often hydride transfer (HT) to the substrate, which is a chemical step that follows the electron transfer (ET) and proton transfer (PT) elementary steps that comprise hydride formation. In the HT step, reaction with protons to give H₂ or reaction with CO₂ to give formate can occur. The properties of the catalyst-hydride intermediate, along with the characteristics of the reaction solution, will determine both the product selectivity and the overall rate of catalysis. Herein, we characterize and discuss the role of H-bond accepting functional groups on HT. The well-characterized HT catalyst Na(diglyme)₂[Fe₄N(CO)₁₂] was used for this study along with six derivatives of the cluster that contain H-bond accepting functional groups in the secondary coordination sphere (SCS). We demonstrate that the rate of HT to a substrate, H⁺ or CO₂, directly controls the selectivity for formate vs H₂ formation, that there is no direct correlation of the HT rate with H-bond accepting features of the microenvironment, and that the proton source in the reaction solution modulates the observed effects of H-bond accepting SCS. These results illustrate that studies of the H-bond accepting properties of a catalyst microenvironment require consideration of many factors.