Water-soluble transition metal complexes of ruthenium(), osmium(), rhodium() and iridium() 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 Ag 2 O 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 [ArMCl 2 ] 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. The synthesis of novel Ru( ii ), Os( ii ), Rh( iii ) and Ir( iii ) mono-N-heterocyclic carbene (NHC) complexes and their application in hydrogenation catalysis is reported.