Metal-centered X-ray absorption and emission spectroscopy of iron corroles: implications for ligand non-innocence

Abstract

Determining the electronic structure of transition metal complexes with non-innocent ligands is challenging, both experimentally and theoretically. In this study, we investigate the electronic structure of iron corrole nitrosyl (Fe[TPC](NO), TPC = meso-triphenylcorrole) using a combination of Fe L-edge and K pre-edge X-ray absorption spectroscopy (XAS), Kβ X-ray emission spectroscopy (XES), and multiconfigurational calculations. A key debate revolves around the distribution of radical character on the ligand and the spin/oxidation state of the iron center. The experimental spectra reveal that the Fe center adopts a low-spin configuration, characterized as either Fe^II or Fe^III with strong π back-bonding and little spin polarization. Calculations identified Fe[TPC](NO) primarily as {FeNO}⁶ with a corrole³⁻, where the {FeNO}⁶ unit exhibits Fe^III character. However, no localized hole was found in the iron t_2g orbital due to strong covalent mixing with NO π* orbitals. The occupation of the Fe 3d_z² orbital, and thus the radical character on the corrole ligand, is highly sensitive to the axial ligand environment. This was supported by wavefunction analysis over varying Fe–NO distances and comparison with iron corrole chloride (Fe[TPC]Cl), which displayed significant radical character on the corrole ligand due to the weak axial ligand. These findings provide critical insights into ligand non-innocence and covalency in metal corrole systems, offering a foundation for understanding other highly covalent transition metal systems.