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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
journal contribution
posted on 2014-11-03, 00:00 authored by Cedric Colomban, Evgenij V. Kudrik, Valerie Briois, Janine
C. Shwarbrick, Alexander B. Sorokin, Pavel Afanasievμ-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 μ-carbido (1), μ-nitrido (2), and μ-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 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 1. According to the
X-ray emission spectroscopy, the μ-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 FeO 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 μ-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.