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.

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

Franco, F
;
2023-01-01

Abstract

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.
2023
25-lug-2023
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11368/3067162
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