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Infrared Spectroelectrochemistry of Iron-Nitrosyl Triarylcorroles. Implications for Ligand Noninnocence.

ABSTRACT: Recent DFT calculations have suggested that iron nitrosyl triarylcorrole complexes have substantial {FeNO}⁷-corrole^•2- character. With this formulation, reduction of Fe(C)(NO) complexes, where C = triarylcorrole, should be centered on the corrole macrocycle rather than on the {FeNO}⁷ moiety. To verify this proposition, visible and infrared spectroelectrochemical studies of Fe(C)(NO) were carried out and the results were interpreted using DFT (B3LYP/STO-TZP) calculations. The first reduction of Fe(C)(NO) led to significant changes in the Soret and Q-band regions of the visible spectrum as well as to a significant downshift in the ν_NO and changes in the corrole vibrational frequencies. DFT calculations, which showed that the electron was mostly added to the corrole ligand (85%), were also able to predict the observed shifts in the ν_NO and corrole bands upon reduction. These results underscore the importance of monitoring both the corrole and nitrosyl vibrations in ascertaining the site of reduction. By contrast, the visible spectroelectrochemistry of the second reduction revealed only minor changes in the Soret band upon reduction, consistent with the reduction of the FeNO moiety.

SUBMITTER: Rahman MH

PROVIDER: S-EPMC7997370 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:We use quantitative experimental and theoretical approaches to characterize the vibrational dynamics of the Fe atom in porphyrins designed to model heme protein active sites. Nuclear resonance vibrational spectroscopy (NRVS) yields frequencies, amplitudes, and directions for 57Fe vibrations in a series of ferrous nitrosyl porphyrins, which provide a benchmark for evaluation of quantum chemical vibrational calculations. Detailed normal mode predictions result from DFT calculations on ferrous nitrosyl tetraphenylporphyrin Fe(TPP)(NO), its cation [Fe(TPP)(NO)]+, and ferrous nitrosyl porphine Fe(P)(NO). Differing functionals lead to significant variability in the predicted Fe-NO bond length and frequency for Fe(TPP)(NO). Otherwise, quantitative comparison of calculated and measured Fe dynamics on an absolute scale reveals good overall agreement, suggesting that DFT calculations provide a reliable guide to the character of observed Fe vibrational modes. These include a series of modes involving Fe motion in the plane of the porphyrin, which are rarely identified using infrared and Raman spectroscopies. The NO binding geometry breaks the four-fold symmetry of the Fe environment, and the resulting frequency splittings of the in-plane modes predicted for Fe(TPP)(NO) agree with observations. In contrast to expectations of a simple three-body model, mode energy remains localized on the FeNO fragment for only two modes, an N-O stretch and a mode with mixed Fe-NO stretch and FeNO bend character. Bending of the FeNO unit also contributes to several of the in-plane modes, but no primary FeNO bending mode is identified for Fe(TPP)(NO). Vibrations associated with hindered rotation of the NO and heme doming are predicted at low frequencies, where Fe motion perpendicular to the heme is identified experimentally at 73 and 128 cm-1. Identification of the latter two modes is a crucial first step toward quantifying the reactive energetics of Fe porphyrins and heme proteins.

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Infrared Spectroelectrochemistry of Iron-Nitrosyl Triarylcorroles. Implications for Ligand Noninnocence.

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