Rh nanoparticles from thiolate dimers: selective and reusable hydrogenation catalysts in ionic liquids

Abstract

Thiolate-capped rhodium nanoparticles were synthesized by decomposition of the organometallic precursors [Rh(μ-SC₁₂H₂₅)(COD)]₂ (I) and [Rh(μ-SC₆H₁₁)(COD)]₂ (II) under hydrogen pressure, in imidazolium-based ionic liquids and organic solvent, without the external addition of ligands as stabilizers and were compared to RhNPs synthesized from [Rh(μ-OMe)(COD)]₂. The use of ionic liquids during the synthesis of rhodium nanoparticles resulted in homogeneously well-dispersed systems as observed by TEM, where the thiolate ligand remains intact as demonstrated by XPS analysis and gas chromatography. The thiolate-capped rhodium nanoparticles generated from II were active catalysts in hydrogenation reactions with high selectivity towards alkene, nitro and imine groups. The absence of thiolate ligand generated very active catalytic systems and the hydrogenation of carbonyl groups was possible. The effect of thiol stabilizer was studied in the reductive N-alkylation to produce N-benzylaniline through one pot synthesis from benzaldehyde and nitrobenzene, where only the thiolate-capped RhNPs showed complete conversion and high selectivity. Additionally, the thiolate-capped RhNPs permitted the reuse of the catalytic systems for ten catalytic cycles for selected hydrogenation reactions after product separation.