Second Coordination Sphere Effect Shifts CO2 to CO Reduction by Iron Porphyrin from Fe0 to FeI

• Published in: Angewandte Chemie International Edition 2024-01, Vol.63 (4), p.e202314439-n/a
• Main Authors: Amanullah, Sk, Gotico, Philipp, Sircoglou, Marie, Leibl, Winfried, Llansola‐Portoles, Manuel J., Tibiletti, Tania, Quaranta, Annamaria, Halime, Zakaria, Aukauloo, Ally
• Format: Article
• Language: English
• Subjects: Carbon Dioxide; Catalysis; Catalysts; Coordination; Electrocatalysis; Hydrogen bonding; Intermediates; Iron; Iron Porphyrin; Mechanism; Oxidation; Porphyrins; Protonation; Reduction; Spectroscopy; Urea; Valence
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.202314439

Iron porphyrins are among the most studied molecular catalysts for carbon dioxide (CO2) reduction and their reactivity is constantly being enhanced through the implementation of chemical functionalities in the second coordination sphere inspired by the active sites of enzymes. In this study, we were intrigued to observe that a multipoint hydrogen bonding scheme provided by embarked urea groups could also shift the redox activation step of CO2 from the well‐admitted Fe(0) to the Fe(I) state. Using EPR, resonance Raman, IR and UV‐Visible spectroscopies, we underpinned a two‐electron activation step of CO2 starting from the Fe(I) oxidation state to form, after protonation, an Fe(III)−COOH species. The addition of another electron and a proton to the latter species converged to the cleavage of a C−O bond with the loss of water molecule resulting in an Fe(II)−CO species. DFT analyses of these postulated intermediates are in good agreement with our collected spectroscopic data, allowing us to propose an alternative pathway in the catalytic CO2 reduction with iron porphyrin catalyst. Such a remarkable shift opens new lines of research in the design of molecular catalysts to reach low overpotentials in performing multi‐electronic CO2 reduction catalysis. Bio‐inspired second coordination sphere inducing a multipoint hydrogen bonding network triggers CO2 activation at an earlier stage of the reaction mechanism in CO2‐to‐CO electrocatalytic reduction by an iron porphyrin catalyst. Electrochemical and comprehensive spectroscopic investigations supported by DFT calculations revealed an unprecedented change in the active species’ formal oxidation state from Fe(0) to Fe(I).