NHC-stabilised Rh nanoparticles: Surface study and application in the catalytic hydrogenation of aromatic substrates

- Synthesis and characterisation of small RhNPs stabilised by N-heterocyclic carbenes.
- Evidence of the location of the ligands on the faces, edges and apexes of the NPs.
- Hydrogenation of aromatic ketones, phenols and N-heteroaromatic substrates.
- Tuning of the selectivity as a function of the reaction conditions.
- Full reduction of quinoline under mild conditions with total selectivity.

New Rh-NPs stabilised by N-Heterocyclic Carbenes (NHC) were synthesized by decomposition of [Rh(η3-C3H5)3] under H2 atmosphere and fully characterized. Surface studies by FT-IR and NMR spectroscopy employing isotopically labelled ligands were also performed. The Rh0.2 NPs are active catalysts in the reduction of various aromatic substrates. In the reduction of phenol, high selectivities to cyclohexanone or cyclohexanol were obtained depending on the reaction conditions. However, this catalytic system exhibited much lower activity in the hydrogenation of substituted phenols. Pyridine was easily hydrogenated under mild conditions and interestingly, the hydrogenation of 4-methyl and 4-trifluoromethylpyridine resulted slower than that of 2-methylpyridine. The hydrogenation of 1-(pyridin-2-yl)propan-2-one provided the β-enaminone 13a in high yield as a consequence of the partial reduction of the pyridine ring followed by isomerization. Quinoline could be either partially hydrogenated to 1,2,3,4-tetrahydroquinoline or fully reduced to decahydroquinoline by adjusting the reaction conditions.

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