Tuning Second-Sphere Amine Basicity to Modulate Hydride Intermediates and Selectivity in Molecular Manganese Bipyridine CO2 Electroreduction

Manganese bipyridine tricarbonyl complexes represent a prominent class of molecular electrocatalysts for efficient and selective CO2-to-CO conversion. Recent efforts have sought to redirect product selectivity toward HCOOH by incorporating second-sphere amine proton relays. However, how the physicochemical properties of second-sphere amines govern catalytic activity and selectivity remains insufficiently understood. In this work, we report a new manganese bipyridine complex bearing 2-(pyrrolidin-1-ylmethyl)phenyl substituents at the 6,6′-positions and systematically evaluate its performance against the diethylamine- and morpholine-functionalized analogs to elucidate the role of second-sphere amine basicity on CO2 reduction. The pyrrolidine-functionalized catalyst achieves near-unity selectivity for HCOOH in acetonitrile with a Faradaic efficiency of 92%, outperforming the corresponding diethylamine (71%) and morpholine (23%) derivatives. This complex also displays outstanding catalytic activity with maximum turnover frequencies of 2300 s–1 and up to 70,000 s–1 under low- and high-overpotential conditions, respectively, ranking among the most active manganese bipyridine catalysts for CO2-to-HCOOH conversion. Mechanistic investigations reveal that the pyrrolidine groups function as proton relays, facilitating proton transfer to the manganese center and promoting formation of the key Mn-hydride intermediate for CO2-to-HCOOH conversion. Density functional theory calculations further demonstrate that the stronger basicity of the pyrrolidine functionality lowers the energy barriers for amine protonation and subsequent intramolecular proton transfer by 3–6 kcal mol–1 relative to the less basic diethylamine and morpholine analogs. Overall, this work establishes second-sphere amine basicity as a key molecular parameter for tuning both activity and product selectivity in the development of molecular CO2 reduction catalysts.