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
Résumé
mu-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 mu-nitrido diiron phthalocyanines including initial [PcFe+3.5NFe+3.5Pc](0) and oxidized complexes [(PcFeNFePc)-N-IV-Pc-IV]+PF6 and [(PcFeNFeIV)-N-IV(Pc center dot+)]Br-2+(2) as model compounds for the intermediates in the catalytic cycle. These systems contain 3d(4) configuration of iron in high oxidation state Fe(IV). XES spectra of K beta 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 [(PcFeNFePc)-N-IV-Pc-IV]+PF6 (HS) and two electron oxidized cation radical species [(PcFeNFeIV)-N-IV(Pc center dot+)]Br-2+(2) (LS). Fe K-edge XANES spectra were recorded at the fluorescence energy of the main K beta(1.3) line or K beta' line in the K beta 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 [(PcFeNFeIV)-N-IV(Pc center dot+)(O)](0) to generate powerful oxidant able to oxidize methane via two-electron process. (C) 2011 Elsevier B.V. All rights reserved.