Decoding the CO2 Reduction Mechanism of a Highly Active Organometallic Manganese Electrocatalyst: Direct Observation of a Hydride Intermediate and Its Implications

Abstract

A detailed mechanistic study of the electrochemical CO2 reduction catalyzed by the fac‐[MnI(CO)3(bis‐MeNHC)MeCN]+ complex (1-MeCN+) is reported herein by combining in situ FTIR spectroelectrochemistry (SEC), synthesis and characterization of catalytic intermediates and DFT calculations. Under low proton concentration, 1-MeCN+ efficiently catalyzes CO2 electroreduction with excellent catalyst durability and selectivity towards CO (ca. 100%). The [Mn-I(CO)3(bis‐MeNHC)]- anion (1-) and the tetracarbonyl [MnI(CO)4(bis‐MeNHC)]+ complex (1-CO+) are key intermediates of the catalytic CO2-to-CO mechanism, due to their impact in the selectivity and the reaction rate, respectively. Increasing the proton concentration increases formate production (up to 15 % FE), although CO remains the major product. The origin of formate is ascribed to the competitive protonation of 1- to form a Mn(I)-hydride (1-H), detected by SEC in the absence of CO2. 1-H was also synthesized and thoroughly characterized, including by X-ray diffraction analysis. Stoichiometric reactivity studies of 1-H with CO2 and labeled 13CO2 indicate a fast formation of the corresponding neutral Mn(I)-formate species (1-OCOH) at room temperature. DFT modeling confirms the intrinsic capability of 1-H to undergo hydride transfer to CO2 due to the strong σ-donor properties of the bis-MeNHC moiety. However, the large potential required for the HCOO- release from 1-OCOH limits the overall catalytic CO2-to-HCOO- cycle. Moreover, the experimentally observed preferential selectivity for CO over formate is dictated by the shallow kinetic barrier for CO2 binding to 1- compared to the Mn-H bond formation. The detailed mechanistic study highlights the reduction potential, pKa, and hydricity of the metal-hydride intermediate as crucial factors affecting the CO2RR selectivity in molecular systems