Study of N-bridged diiron phthalocyanine relevant to methane oxidation: Insight into oxidation and spin states from high resolution 1s core hole X-ray spectroscopy

Abstract

μ-Nitrido diiron phthalocyanine [PcFe+3.5NFe+3.5Pc]0 is a highly efficient catalyst, able to oxidize methane under near-ambient conditions. In this work, high resolution X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) were applied to study iron species in the series of μ-nitrido diiron phthalocyanines including initial [PcFe+3.5NFe+3.5Pc]0 and oxidized complexes [PcFeIVNFeIVPc]+PF6 and [PcFeIVNFeIV(Pc+)]2+Br2 as model compounds for the intermediates in the catalytic cycle. These systems contain 3d4 configuration of iron in high oxidation state Fe(IV). XES spectra of Kβ line are sensible to the local iron spin density and show unexpected difference in the spin state between the initial [PcFe+3.5NFe+3.5Pc]0 (LS), one-electron oxidized [PcFeIVNFeIVPc]+PF6 (HS) and two electron oxidized cation radical species [PcFeIVNFeIV(Pc+)]2+Br2 (LS). Fe K-edge XANES spectra were recorded at the fluorescence energy of the main Kβ1,3 line or Kβ′ line in the Kβ emission spectrum. The conclusions of RIXS study corroborated XES data. Our study suggests that the main catalytic pathway of oxidation includes two-electron transformation from the LS initial complex to the LS two–electron oxidized [PcFeIVNFeIV(Pc+)(O)]0 to generate powerful oxidant able to oxidize methane via two-electron process.