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 ) 4 Mo 4 O 16 ] (Cp E = C 5 (CH 3 ) 3 (COOC 2 H 5 ) 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 ) 4 Mo 4 O 16 ] showing the highest activity, followed by [(RhCp*) 4 Mo 4 O 16 ] (Cp* = C 5 (CH 3 ) 5 ), and then RhCl 3 combined with (NH 4 ) 6 [Mo 7 O 24 ]•4H 2 O. The catalyst prepared from [(RhCp E ) 4 Mo 4 O 16] effectively hydrogenates tertiary, secondary, and primary amides under mild conditions (0.8 MPa H 2 , 353−393 K), demonstrating a high activity and selectivity (conversion: 97%, selectivity: 76%) for primary amide hydrogenation under NH 3 -free conditions. Furthermore, we determine that carbonyl oxygen atoms in Cp E ligands contribute to the electrostatic interaction with Al 2 O 3 , leading to the high dispersibility of [(RhCp E ) 4 Mo 4 O 16 ] on the support. We conclude that the high efficiency of [(RhCp E ) 4 Mo 4 O 16 ] 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.