Ligand-Functionalized Organometallic Polyoxometalate as an Efficient Catalyst Precursor for Amide Hydrogenation.

Abstract

Amide hydrogenation is an important process for producing amines, with the development of efficient heterogeneous catalysts relying on the creation of bimetallic active sites where the two components interact synergistically. In this study, we develop a method for preparing catalysts using ligand-functionalized organometallic polyoxometalates by synthesizing a Rh-Mo organometallic polyoxometalate, [(RhCp^E)₄Mo₄O₁₆] (Cp^E = C₅(CH₃)₃(COOC₂H₅)₂), with Rh-O-Mo interfacial structures and ethoxycarbonyl-functionalized ligands as a catalyst precursor. The activity of supported Rh-Mo catalysts for amide hydrogenation depend on the precursor used, with [(RhCp^E)₄Mo₄O₁₆] showing the highest activity, followed by [(RhCp*)₄Mo₄O₁₆] (Cp* = C₅(CH₃)₅), and then RhCl₃ combined with (NH₄)₆[Mo₇O₂₄]·4H₂O. The catalyst prepared from [(RhCp^E)₄Mo₄O₁₆] effectively hydrogenates tertiary, secondary, and primary amides under mild conditions (0.8 MPa H₂, 353-393 K), demonstrating a high activity and selectivity (conversion: 97%, selectivity: 76%) for primary amide hydrogenation under NH₃-free conditions. Furthermore, we determine that carbonyl oxygen atoms in Cp^E ligands contribute to the electrostatic interaction with Al₂O₃, leading to the high dispersibility of [(RhCp^E)₄Mo₄O₁₆] on the support. We conclude that the high efficiency of [(RhCp^E)₄Mo₄O₁₆] as a catalyst precursor originates from the effective formation of Rh/Mo interfacial active sites, which is assisted by the electrostatic interaction between the Cp^E ligands and support.