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Spin-Controlled Binding of Carbon Dioxide by an Iron Center: Insights from Ultrafast Mid-Infrared Spectroscopy.

ABSTRACT: The influence of the spin on the mode of binding between carbon dioxide (CO₂ ) and a transition-metal (TM) center is an entirely open question. Herein, we use an iron(III) oxalato complex with nearly vanishing doublet-sextet gap, and its ultrafast photolysis, to generate TM-CO₂ bonding patterns and determine their structure in?situ by femtosecond mid-infrared spectroscopy. The formation of the nascent TM-CO₂ species according to [L₄ Fe^III (C₂ O₄ )]⁺ + h? ? [L₄ Fe(CO₂ )]⁺ + CO₂ , with L₄ =cyclam, is evidenced by the coincident appearance of the characteristic asymmetric stretching absorption of the CO₂ -ligand between 1600?cm^-1 and 1800?cm^-1 and that of the free CO₂ -co-fragment near 2337?cm^-1 . On the high-spin surface (S=5/2), the product complex features a bent carbon dioxide radical anion ligand that is O-"end-on"-bound to the metal. In contrast, on the intermediate-spin and low-spin surfaces, the product exhibits a "side-on"-bound, bent carbon dioxide ligand that has either a partial open-shell (for S=3/2) or fully closed-shell character (for S=1/2).

SUBMITTER: Straub S

PROVIDER: S-EPMC7898313 | biostudies-literature | 2021 Feb

REPOSITORIES: biostudies-literature

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Spin-Controlled Binding of Carbon Dioxide by an Iron Center: Insights from Ultrafast Mid-Infrared Spectroscopy.

Straub Steffen S Vöhringer Peter P

Angewandte Chemie (International ed. in English) 20201127 5

The influence of the spin on the mode of binding between carbon dioxide (CO2 ) and a transition-metal (TM) center is an entirely open question. Herein, we use an iron(III) oxalato complex with nearly vanishing doublet-sextet gap, and its ultrafast photolysis, to generate TM-CO2 bonding patterns and determine their structure in situ by femtosecond mid-infrared spectroscopy. The formation of the nascent TM-CO2 species according to [L4 FeIII (C ...[more]

PMID: 33022879

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Project description:Phototropins, major blue-light receptors in plants, are sensitive to blue light through a pair of flavin mononucleotide (FMN)-binding light oxygen and voltage (LOV) domains, LOV1 and LOV2. LOV2 undergoes a photocycle involving light-driven covalent adduct formation between a conserved cysteine and the FMN C(4a) atom. Here, the primary reactions of Avena sativa phototropin 1 LOV2 (AsLOV2) were studied using ultrafast mid-infrared spectroscopy and quantum chemistry. The singlet excited state (S1) evolves into the triplet state (T1) with a lifetime of 1.5 ns at a yield of approximately 50%. The infrared signature of S1 is characterized by absorption bands at 1657 cm(-1), 1495-1415 cm(-1), and 1375 cm(-1). The T1 state shows infrared bands at 1657 cm(-1), 1645 cm(-1), 1491-1438 cm(-1), and 1390 cm(-1). For both electronic states, these bands are assigned principally to C=O, C=N, C-C, and C-N stretch modes. The overall downshifting of C=O and C=N bond stretch modes is consistent with an overall bond-order decrease of the conjugated isoalloxazine system upon a pi-pi* transition. The configuration interaction singles (CIS) method was used to calculate the vibrational spectra of the S1 and T1 excited pipi* states, as well as respective electronic energies, structural parameters, electronic dipole moments, and intrinsic force constants. The harmonic frequencies of S1 and T1, as calculated by the CIS method, are in satisfactory agreement with the evident band positions and intensities. On the other hand, CIS calculations of a T1 cation that was protonated at the N(5) site did not reproduce the experimental FMN T1 spectrum. We conclude that the FMN T1 state remains nonprotonated on a nanosecond timescale, which rules out an ionic mechanism for covalent adduct formation involving cysteine-N(5) proton transfer on this timescale. Finally, we observed a heterogeneous population of singly and doubly H-bonded FMN C(4)=O conformers in the dark state, with stretch frequencies at 1714 cm(-1) and 1694 cm(-1), respectively.

Dataset Information

Spin-Controlled Binding of Carbon Dioxide by an Iron Center: Insights from Ultrafast Mid-Infrared Spectroscopy.

Publications

Spin-Controlled Binding of Carbon Dioxide by an Iron Center: Insights from Ultrafast Mid-Infrared Spectroscopy.

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