Reaction intermediates in the catalytic Gif-type oxidation from nuclear inelastic scattering

Abstract

Nuclear inelastic scattering (NIS) of synchrotron radiation, also known as nuclear resonant vibrational spectroscopy (NRVS), has been shown to provide valuable insights into metal-centered vibrations at Mössbauer-active nuclei. We present a study of the iron-centered vibrational density of states (VDOS) during the first step of the Gif-type oxidation of cyclohexene with a novel trinuclear Fe 3 (μ 3 -O) complex as catalyst precursor. The experiments were carried out on shock-frozen solutions for different combinations of reactants: Fe 3 (μ 3 -O) in pyridine solution, Fe 3 (μ 3 -O) plus Zn/acetic acid in pyridine without and with addition of either oxygen or cyclohexene, and Fe 3 (μ 3 -O)/Zn/acetic acid/pyridine/cyclohexene (reaction mixture) for reaction times of 1 min, 5 min, and 30 min. The projected VDOS of the Fe atoms was calculated on the basis of pseudopotential density functional calculations. Two possible reaction intermediates were identified as [Fe (III) (C 5 H 5 N) 2 (O 2 CCH 3 ) 2 ] + and Fe (II) (C 5 H 5 N) 4 (O 2 CCH 3 ) 2 , yielding evidence that NIS (NRVS) allows to identify the presence of iron-centered intermediates also in complex reaction mixtures.