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

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.