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.