X-Ray absorption in homogeneous catalysis research: the iron-catalyzed Michael addition reaction by XAS, RIXS and multi-dimensional spectroscopy

Abstract

A survey over X-ray absorption methods in homogeneous catalysis research is given with the example of the iron-catalyzed Michael addition reaction. A thorough investigation of the catalytic cycle was possible by combination of conventional X-ray absorption spectroscopy (XAS), resonant inelastic X-ray scattering (RIXS) and multi-dimensional spectroscopy. The catalytically active compound formed in the first step of the Michael reaction of methyl vinyl ketone with 2-oxocyclopentanecarboxylate (1) could be elucidated in situ by RIXS spectroscopy, and the reduced catalytic activity of FeCl₃·6H₂O (2) compared to Fe(ClO₄)₃·9H₂O (3) could be further explained by the formation of a [Fe^IIICl₄⁻]₃[Fe^III(1-H)₂(H₂O)₂⁺][H⁺]₂ complex. Chloride was identified as catalyst poison with a combined XAS-UV/vis study, which revealed that Cl⁻ binds quantitatively to the available iron centers that are deactivated by formation of [FeCl₄⁻]. Operando studies in the course of the reaction of methyl vinyl ketone with 1 by combined XAS-Raman spectroscopy allowed the exclusion of changes in the oxidation state and the octahedral geometry at the iron site; a reaction order of two with respect to methyl vinyl ketone and a rate constant of k = 1.413 min⁻² were determined by analysis of the CC and CO vibration band. Finally, a dedicated experimental set-up for three-dimensional spectroscopic studies (XAS, UV/vis and Raman) of homogeneous catalytic reactions under laboratory conditions, which emerged from the discussed investigations, is presented.