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

mu-Nitrido diiron phthalocyanine [PcFe+3.5NFe+3.5Pc](0) 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 mu-carbido (1), mu-nitrido (2), and mu-oxo (3) 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 2. 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 1-3. 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 1-3 as having equivalent iron centers with formal iron oxidation degrees of 3, 3.5, and 4 for the mu-oxo, mu-nitrido, and mu-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 mu-carbido ligand in 1. According to the X-ray emission spectroscopy, the mu-nitrido dimer 2 is a low-spin compound, with the highest covalency in the series 1-3. The DFT-calculated geometry and electronic structures as well as core hole spectra of hypothetical high-valent oxo complexes of 1-3 were compared, in order to explain the particular catalytic activity of 2 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 2, due to the strong trans-effect of the nitrido ligand. The corresponding LUMO of the mu-nitrido diiron oxo complex has the lowest energy among the three oxo complexes. Therefore, the oxo complex of 2 is expected to have the highest oxidative power.