The development of catalysts for the transformations of strong bonds is of critical importance in the creation of a sustainable energy future. Organometallic complexes have been explored that are capable of reducing water or carbon dioxide to potential fuels, both of which involve multiple proton and electron transfers. Proton transfers are coupled to the strength of the acids and bases involved, and the acidity and basicity scales in common electrochemical solvents like acetonitrile are vastly different from aqueous pH scales. Rhenium-based catalysts for CO2 reduction are canonical examples of effective catalysts, but there are still gaps in understanding of what makes a good catalyst compared to a bad one. In examining the behavior of a relatively poor CO2 reduction catalyst, Re(PyBimH)(CO)3Cl (PyBimH = 2-(2- pyridyl)benzimidazole), we discovered that the acidic proton on the PyBimH ligand is electrochemically removed, generating an anion that is not easily further reduced. Our studies explain this complex's ineffectiveness for CO2 reduction compared to similar complexes without an acidic proton on the ligand. In addition, we developed a method for estimating the pKa of a proton in acetonitrile using cyclic voltammetry, and used it to measure the pKa of Re(PyBimH)(CO)3Cl. This method for pKa measurement will be very useful for those studying chemistry that involves proton transfers in acetonitrile, such as electrocatalytic CO2 reduction, and is likely applicable to other solvent systems as well.

Lessons from a bad catalyst: Developing an electrochemical method for measuring pKa values in acetonitrile

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.