Ligand-Functionalized\nOrganometallic Polyoxometalate\nas an Efficient Catalyst Precursor for Amide Hydrogenation

Amide hydrogenation is an important process for producing\namines,\nwith the development of efficient heterogeneous catalysts relying\non the creation of bimetallic active sites where the two components\ninteract synergistically. In this study, we develop a method for preparing\ncatalysts using ligand-functionalized organometallic polyoxometalates\nby synthesizing a Rh–Mo organometallic polyoxometalate, [(RhCp<sup>E</sup>)<sub>4</sub>Mo<sub>4</sub>O<sub>16</sub>] (Cp<sup>E</sup> = C<sub>5</sub>(CH<sub>3</sub>)<sub>3</sub>(COOC<sub>2</sub>H<sub>5</sub>)<sub>2</sub>), with Rh–O–Mo interfacial structures\nand ethoxycarbonyl-functionalized ligands as a catalyst precursor.\nThe activity of supported Rh–Mo catalysts for amide hydrogenation\ndepend on the precursor used, with [(RhCp<sup>E</sup>)<sub>4</sub>Mo<sub>4</sub>O<sub>16</sub>] showing the highest activity, followed\nby [(RhCp*)<sub>4</sub>Mo<sub>4</sub>O<sub>16</sub>] (Cp* = C<sub>5</sub>(CH<sub>3</sub>)<sub>5</sub>), and then RhCl<sub>3</sub> combined\nwith (NH<sub>4</sub>)<sub>6</sub>[Mo<sub>7</sub>O<sub>24</sub>]·4H<sub>2</sub>O. The catalyst prepared from [(RhCp<sup>E</sup>)<sub>4</sub>Mo<sub>4</sub>O<sub>16</sub>] effectively hydrogenates tertiary,\nsecondary, and primary amides under mild conditions (0.8 MPa H<sub>2</sub>, 353–393 K), demonstrating a high activity and selectivity\n(conversion: 97%, selectivity: 76%) for primary amide hydrogenation\nunder NH<sub>3</sub>-free conditions. Furthermore, we determine that\ncarbonyl oxygen atoms in Cp<sup>E</sup> ligands contribute to the\nelectrostatic interaction with Al<sub>2</sub>O<sub>3</sub>, leading\nto the high dispersibility of [(RhCp<sup>E</sup>)<sub>4</sub>Mo<sub>4</sub>O<sub>16</sub>] on the support. We conclude that the high\nefficiency of [(RhCp<sup>E</sup>)<sub>4</sub>Mo<sub>4</sub>O<sub>16</sub>] as a catalyst precursor originates from the effective formation\nof Rh/Mo interfacial active sites, which is assisted by the electrostatic\ninteraction between the Cp<sup>E</sup> ligands and support.