X‑ray Absorption and Emission Spectroscopies of X‑Bridged Diiron Phthalocyanine Complexes (FePc)<sub>2</sub>X (X = C, N, O) Combined with DFT Study of (FePc)<sub>2</sub>X and Their High-Valent Diiron Oxo Complexes

μ-Nitrido diiron phthalocyanine [PcFe<sup>+3.5</sup>NFe<sup>+3.5</sup>Pc]<sup>0</sup> is an efficient catalyst, able to catalyze the oxidation of methane under near-ambient conditions. In this work, we compared the properties of structurally similar μ-carbido (<b>1</b>), μ-nitrido (<b>2</b>), and μ-oxo (<b>3</b>) dimers of iron phthalocyanine. The goal was to discern the structural and electronic differences between these complexes and to propose a rationale for the exceptional activity of <b>2</b>. Extended X-ray fine-structure spectroscopy, high-resolution X-ray emission spectroscopy, and resonant inelastic X-ray scattering were applied to study the geometry and electronic structure of iron species in the series <b>1</b>–<b>3</b>. The data provided by core hole spectroscopies were compared to the results of DFT calculations and found to coherently describe the structural and electronic properties of <b>1</b>–<b>3</b> as having equivalent iron centers with formal iron oxidation degrees of 3, 3.5, and 4 for the μ-oxo, μ-nitrido, and μ-carbido dimers, respectively. However, the bond length to the bringing atom changed in an unexpected sequence Fe–O > Fe–N < Fe–C, indicating redox non-innocence of the brigding μ-carbido ligand in <b>1</b>. According to the X-ray emission spectroscopy, the μ-nitrido dimer <b>2</b> is a low-spin compound, with the highest covalency in the series <b>1</b>–<b>3</b>. The DFT-calculated geometry and electronic structures as well as core hole spectra of hypothetical high-valent oxo complexes of <b>1</b>–<b>3</b> were compared, in order to explain the particular catalytic activity of <b>2</b> and to estimate the prospects of spectroscopic observation of such species. It appears that the terminal FeO bond is the longest in the oxo complex of <b>2</b>, due to the strong trans-effect of the nitrido ligand. The corresponding LUMO of the μ-nitrido diiron oxo complex has the lowest energy among the three oxo complexes. Therefore, the oxo complex of <b>2</b> is expected to have the highest oxidative power.