Decoding the CO2 Reduction Mechanism of a Highly Active Organometallic Manganese Electrocatalyst: Direct Observation of a Hydride Intermediate and Its Implications
A detailed mechanistic study of the electrochemical CO2 reduction catalyzed by the fac-[Mn-I(CO)(3)(bis-(NHC)-N-Me)MeCN](+) complex (1-MeCN+ ) is reported herein by combining in situFTIR spectroelectrochemistry(SEC), synthesis and characterization of catalytic intermediates,and DFT calculations. Under low proton concentrations, 1-MeCN+ efficiently catalyzes CO2 electroreductionwith long catalyst durability and selectivity toward CO (ca. 100%). The [Mn-I(CO)(3)(bis-(NHC)-N-Me)](-) anion (1(-) ) and the tetracarbonyl [Mn-I(CO)(4)(bis-(NHC)-N-Me)](+) complex (1-CO+ )are key intermediates of the catalytic CO2-to-CO mechanismdue to their impact on the selectivity and the reaction rate, respectively.Increasing the proton concentration increases formate production (upto 15% FE), although CO remains the major product. The origin of formateis ascribed to the competitive protonation of 1(-) to form a Mn(I) hydride (1-H), detected by SECin the absence of CO2. 1-H was also synthesizedand thoroughly characterized, including by X-ray diffraction analysis.Stoichiometric reactivity studies of 1-H with CO2 and labeled (CO2)-C-13 indicate a fast formationof the corresponding neutral Mn(I) formate species (1-OCOH) at room temperature. DFT modeling confirms the intrinsic capabilityof 1-H to undergo hydride transfer to CO2 dueto the strong & sigma;-donor properties of the bis-(Me)NHCmoiety. However, the large potential required for the HCOO- release from 1-OCOH limits the overall catalytic CO2-to-HCOO- cycle. Moreover, the experimentallyobserved preferential selectivity for CO over formate is dictatedby the shallow kinetic barrier for CO2 binding to 1(-) compared to the Mn-H bond formation.The detailed mechanistic study highlights the reduction potential,pK (a), and hydricity of the metal hydrideintermediate as crucial factors affecting the CO2RR selectivityin molecular systems
