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 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, pK a , and hydricity of the metal hydride intermediate as crucial factors affecting the CO 2 RR selectivity in molecular systems.

Electrocatalytic CO2 Reduction: Monitoring of Catalytically Active, Downgraded, and Upgraded Cobalt Complexes

“…Interestingly, the potential ramping experiment done with the TPACo complex revealed that the dimeric hydride-carbonyl species dominated the mass spectra at potentials negative to −2 V, substantiating that this HER pathway becomes dominant at highly negative potentials (Figures b and Figure S26). Nevertheless, detecting cobalt hydride intermediate by EC-ESI-MS is remarkable as the direct detection and full characterization of hydride intermediates during the electrocatalytic reactions is rarely reported …”

“…Homogeneous electrocatalytic CO 2 reduction is being extensively explored to achieve reaction selectivity and serves as a mechanistic model for improved large-scale CO 2 electroreduction processes. − Numerous molecular catalysts, based on the transition metal complexes, have been proposed for CO 2 reduction, generally exhibiting good product selectivity. , However, they often display mediocre electrocatalytic performance, possibly due to the poisoning or degradation of the catalyst or undesirable side reactions. ,, The in-depth investigation of the electrolytic CO 2 reduction reaction mechanism is thus imperative for developing efficient and robust molecular catalysts. Although several remarkable mechanistic studies have been reported, − the fate of the catalysts during the electrocatalytic CO 2 reduction with a molecular-level insight remains under-investigated. This is primarily attributed to the limited availability of suitable analytical tools.…”

“Anion-Switchable” Pincer-Mn(I) Catalyst for the Reductive N-Methylation of Amines with Formic Acid and CO2

“…7 Soon thereafter, Royo and Lloret disclosed the extraordinary activity and selectivity of a Mn-based catalyst bearing a bidentate methylene-bridged bis-imidazolylidene ligand toward the electroreduction of CO 2 to CO, 8 and unveiled the factors that govern its reactivity and selectivity. 9 This Mn(I)-bisNHC complex was also found to be active in the reductive Nformylation and N-methylation of secondary amines using either CO 2 or formic acid as C 1 building blocks, showing remarkable selectivity to the N-methylated products. 10 Luca and co-workers showed that pincer Mn(I) complexes were particularly effective in the electrocatalytic reduction of CO 2 , 11 and performed detailed mechanistic studies.…”

CO2 Reduction by an Iron(I) Porphyrinate System: Effect of Hydrogen Bonding on the Second Coordination Sphere

“…The cumulative emission of CO 2 is raising environmental concerns for its role in global warming, and consequently CO 2 reduction is an urgent global issue. In the face of these serious environmental problems, the capture of CO 2 has become a hot issue in recent years. − However, apart from CO 2 capture and storage, research has started into the utilization of CO 2 for the synthesis of valuable chemicals and materials. − Because CO 2 is a cheap, abundant, and nontoxic C 1 feedstock, it can be converted into various carbon resources such as alcohols, acids, esters, and hydrocarbons. − This approach has 2-fold advantages, namely, CO 2 reduction in the atmosphere as well as reduction in the use of unrenewable fossil fuels for synthesis of these compounds. Therefore, transforming CO 2 into valuable chemicals may provide a solution to not only environmental problems caused by the emission of CO 2 but also the anticipated depletion of fossil resources.…”