Single Iridium Pincer Complex for Roundtrip Electrochemical Conversion between Carbon Dioxide and Formate

An iridium (III) complex based on a PONOP pincer ligand (PONOP=2,6-bis(di-tert-butylphosphinito)pyridine) is a versatile catalyst for both electro-reduction of CO2 to formate and formate electro-oxidation to CO2. The electro-reduction of CO2 in acetonitrile is very efficient and selective, and the Faradaic efficiency for formate was 97 % and the turnover frequency (TOF) was as high as 67 s(-1). In addition, the complex is an effective electrocatalyst for formate electro-oxidation with apparent TOF of 4.8 s(-1) (0.06 M formate), and CO2 was the sole product. The catalyst was stable for 35 h of electrolysis. It can serve as a single catalyst for the roundtrip conversion between CO2 and formate with potential for electrochemical energy storage. Electrochemical and NMR spectroscopic studies suggested that the hydride species is critical for the bifunctional reactivity.