Decoding the CO<sub>2</sub> Reduction Mechanism of a Highly Active Organometallic Manganese Electrocatalyst: Direct Observation of a Hydride Intermediate and Its Implications
Sergio Fernández, Federico Franco, Marta Martı́nez Belmonte, Sofia Friães, Beatriz Royo, Josep M. Luis, Julio Lloret‐Fillol
Abstract
A detailed mechanistic study of the electrochemical CO 2 reduction catalyzed by the fac -[Mn I (CO) 3 (bis- Me NHC)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 concentrations, 1-MeCN + efficiently catalyzes CO 2 electroreduction with long catalyst durability and selectivity toward CO ( ca . 100%). The [Mn -I (CO) 3 (bis- Me NHC)] − anion ( 1 – ) and the tetracarbonyl [Mn I (CO) 4 (bis- Me NHC)] + complex ( 1-CO + ) are key intermediates of the catalytic CO 2 -to-CO mechanism due to their impact on 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 CO 2 . 1-H was also synthesized and thoroughly characterized, including by X-ray diffraction analysis. Stoichiometric reactivity studies of 1-H with CO 2 and labeled 13 CO 2 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 CO 2 due to the strong σ-donor properties of the bis- Me NHC moiety. However, the large potential required for the HCOO – release from 1-OCOH limits the overall catalytic CO 2 -to-HCOO – cycle. Moreover, the experimentally observed preferential selectivity for CO over formate is dictated by the shallow kinetic barrier for CO 2 binding to 1 – compared to the Mn–H bond formation. The detailed mechanistic study highlights the reduction potential, p K a, and hydricity of the metal hydride intermediate as crucial factors affecting the CO 2 RR selectivity in molecular systems.