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

  • Sergio Fernández
    Sergio Fernández
    Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Avinguda Països Catalans 16, 43007 Tarragona, Spain
  • Federico Franco*
    Federico Franco
    Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Avinguda Països Catalans 16, 43007 Tarragona, Spain
    *Email: [email protected]
  • Marta Martínez Belmonte
    Marta Martínez Belmonte
    Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Avinguda Països Catalans 16, 43007 Tarragona, Spain
  • Sofia Friães
    Sofia Friães
    Instituto de Tecnologia Química e Biológica António Xavier (ITQB), Nova University of Lisbon, Av. da República, 2780-157 Oeiras, Portugal
  • Beatriz Royo
    Beatriz Royo
    Instituto de Tecnologia Química e Biológica António Xavier (ITQB), Nova University of Lisbon, Av. da República, 2780-157 Oeiras, Portugal
    More by Beatriz Royo
  • Josep M. Luis*
    Josep M. Luis
    Institut de Química Computacional i Catèlisi (IQCC) and Departament de Química, Universitat de Girona, Campus Montilivi, Girona, E-17003 Catalonia, Spain
    *Email: [email protected]
  • , and 
  • Julio Lloret-Fillol*
    Julio Lloret-Fillol
    Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Avinguda Països Catalans 16, 43007 Tarragona, Spain
    Catalan Institution for Research and Advanced Studies (ICREA), Passeig Lluïs Companys, 23, 08010 Barcelona, Spain
    *Email: [email protected]
Cite this: ACS Catal. 2023, 13, 15, 10375–10385
Publication Date (Web):July 25, 2023
https://doi.org/10.1021/acscatal.3c01430
Copyright © 2023 American Chemical Society

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    Abstract

    Abstract Image

    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 concentrations, 1-MeCN+ efficiently catalyzes CO2 electroreduction with long catalyst durability and selectivity toward 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 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 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.

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    Supporting Information

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    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscatal.3c01430.

    • General methods and materials, synthesis and characterization of complexes, electrochemistry, SEC, estimation of thermodynamic parameters, and computational details with optimized DFT structures (PDF)

    • Crystallographic information of [1-CO]BF4 (CIF)

    • Crystallographic information of 1-H (CIF)

    • Crystallographic information of 1-Br (CIF)

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    Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

    Cited By

    This article is cited by 2 publications.

    1. Chengxu Zhu, Carmine D’Agostino, Sam P. de Visser. CO2 Reduction by an Iron(I) Porphyrinate System: Effect of Hydrogen Bonding on the Second Coordination Sphere. Inorganic Chemistry 2024, 63 (10) , 4474-4481. https://doi.org/10.1021/acs.inorgchem.3c04246
    2. Abhinav Bairagi, Aleksandr Y. Pereverzev, Paul Tinnemans, Evgeny A. Pidko, Jana Roithová. Electrocatalytic CO2 Reduction: Monitoring of Catalytically Active, Downgraded, and Upgraded Cobalt Complexes. Journal of the American Chemical Society 2024, 146 (8) , 5480-5492. https://doi.org/10.1021/jacs.3c13290

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