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μ-Oxo Dimerization Effects on Ground- and Excited-State Properties of a Water-Soluble Iron Porphyrin CO₂ Reduction Catalyst

Follmer, Alec H. and Luedecke, Kaitlin M. and Hadt, Ryan G. (2022) μ-Oxo Dimerization Effects on Ground- and Excited-State Properties of a Water-Soluble Iron Porphyrin CO₂ Reduction Catalyst. Inorganic Chemistry, 61 (50). pp. 20493-20500. ISSN 0020-1669. doi:10.1021/acs.inorgchem.2c03215.

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Iron 5,10,15,20-tetra(para-N,N,N-trimethylanilinium)porphyrin (Fe-p-TMA) is a water-soluble catalyst capable of electrochemical and photochemical CO₂ reduction. Although its catalytic ability has been thoroughly investigated, the mechanism and associated intermediates are largely unknown. Previous studies proposed that Fe-p-TMA enters catalytic cycles as a monomeric species. However, we demonstrate herein that, in aqueous solutions, Fe-p-TMA undergoes formation of a μ-oxo porphyrin dimer that exists in equilibrium with its monomeric form. The propensity for μ-oxo formation is highly dependent on the solution pH and ionic strength. Indeed, the μ-oxo form is stabilized in the presence of electrolytes that are key components of catalytically relevant conditions. By leveraging the ability to chemically control and spectrally address both species, we characterize their ground-state electronic structures and excited-state photodynamics. Global fitting of ultrafast transient absorption data reveals two distinct excited-state relaxation pathways: a three-component sequential model consistent with monomeric relaxation and a two-component sequential model for the μ-oxo species. Relaxation of the monomeric species is best described as a ligand-to-metal charge transfer (τ₁ = ∼500 fs), an ionic strength-dependent metal-to-ligand charge transfer (τ₂ = 2–4 ps), and finally relaxation of a ligand field excited state to the ground state (τ₃ = 5 ps). Conversely, excited-state relaxation of the μ-oxo species proceeds via cleavage of an Fe^(III)–O bond to generate transient Fe^(IV)═O and Fe^(II) porphyrin species (τ₁ = 2 ps) that recombine to the ground-state μ-oxo species (τ₂ = ∼1 ns). This latter lifetime extends to timescales relevant for chemical reactivity. It is therefore emphasized that further consideration of catalyst speciation and chemical microenvironments is necessary for elucidating the mechanisms of catalytic CO₂ reduction reactions.

Item Type:Article
Related URLs:
URLURL TypeDescription
Follmer, Alec H.0000-0002-6244-6804
Luedecke, Kaitlin M.0000-0002-8163-9417
Hadt, Ryan G.0000-0001-6026-1358
Additional Information:The authors thank Dr. Paul H. Oyala for helpful EPR discussions. K.M.L. acknowledges support from the National Science Foundation Graduate Research Fellowship (NSF GRFP) under Grant no. DGE-1745301. This study is based on work performed by the Liquid Sunlight Alliance, which is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Fuels from Sunlight Hub under Award Number DE-SC0021266.
Group:Liquid Sunlight Alliance
Funding AgencyGrant Number
NSF Graduate Research FellowshipDGE-1745301
Department of Energy (DOE)DE-SC0021266
Issue or Number:50
Record Number:CaltechAUTHORS:20230125-514949300.25
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:118944
Deposited By: Research Services Depository
Deposited On:22 Feb 2023 19:25
Last Modified:22 Feb 2023 19:25

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