Investigations into the metal species of the homogeneous iron(iii) catalyzed Michael addition reactions

Abstract

An investigation into the species formed in the first step of the solvent free homogeneous Michael reaction of α,β-unsaturated ketones with 2-oxocyclopentanecarboxylate (1) is presented. This reaction is catalyzed by FeCl₃·6H₂O (2) and Fe(ClO₄)₃·9H₂O (3). EXAFS, XANES, Raman and UV-Vis studies were carried out to explain the experimentally found higher catalytic activity of Fe(ClO₄)₃·9H₂O (3) compared to FeCl₃·6H₂O (2). A very intense pre-edge peak is found for a 1.6 mol% solution of FeCl₃·6H₂O (2) in 1, suggesting a tetrachloroferrate(III) compound to be present in this solution. This is proved by UV-Vis and Raman spectroscopy. The counterion of this anionic complex is an octahedral [Fe^III(1–H)₂(H₂O)₂]⁺ complex with two deprotonated 2-oxocyclopentanecarboxylate (1) as the chelating ligand, (1–H)⁻, as suggested by the examination of the XANES region, the obtained coordination numbers from the EXAFS analysis and by UV-Vis and Raman spectroscopies. In summary, the anion–cation species [Fe^IIICl₄]⁻[Fe^III(1–H)₂(H₂O)₂]⁺ is formed with FeCl₃·6H₂O (2), whereas in the case of Fe(ClO₄)₃·9H₂O (3) XAFS, Raman and UV-Vis investigations suggest the presence of a complex of the form [Fe^III(1–H)₂(H₂O)₂]⁺[ClO₄]⁻. The obtained results are discussed to explain the reduced catalytic activity of FeCl₃·6H₂O (2) in comparison to Fe(ClO₄)₃·9H₂O (3).