Water-soluble transition metal complexes of ruthenium(ii), osmium(ii), rhodium(iii) and iridium(iii) with chelating N-heterocyclic carbene ligands in hydrogenation and transfer hydrogenation catalysis

Abstract

The synthesis of novel Ru(II), Os(II), Rh(III) and Ir(III) mono-N-heterocyclic carbene (NHC) complexes with a pyridine substituent is reported. The reaction of the imidazolium salts bearing N-alkyl and sulfonated N-alkyl substituents with Ag₂O leads to the formation of the corresponding Ag(I) complexes. The metal complexes are available in good yields via transmetallation reactions from the corresponding silver complexes and [ArMCl₂]₂, where Ar = p-cymene or Cp* and M = Ru, Os, Rh or Ir. While N-alkyl substituted NHC complexes are almost insoluble in water (1.55 mg ml⁻¹), sulfonated N-alkyl substituted NHC complexes display good solubility in water (up to 400 mg mL⁻¹). All complexes were examined as catalysts in the transfer hydrogenation of acetophenone, which is quantitatively and highly selective reduced to 1-phenylethanol and 1-cyclohexylethanol. Additionally, the water-soluble complexes were examined in the complete hydrogenation of acetophenone with hydrogen in an autoclave, showing high conversions compared to literature-known systems.