Synthesis of water-soluble palladium(ii) complexes with N-heterocyclic carbene chelate ligands and their use in the aerobic oxidation of 1-phenylethanol

Abstract

The synthesis of palladium(II) complexes containing N-heterocyclic carbene chelate ligands is reported. These chelate ligands have methylene or ethylene ring bridges and sulfonate, carbonate or ammonium functionalities to render the complexes soluble in water. Bis- [(bis-NHC)PdX₂] (X = Cl, Br or I) and tetra-carbene [(bis-NHC)₂Pd] complexes have been prepared and the crystal structures of two of them have been determined. The NHC ligand is coordinated to the metal center in normal (C2-bound) or abnormal (C4-bound) mode. Processes involving the formation of NHC-Pd bonds or the transfer of NHC ligands have been explored in water. The behavior of the bis-carbene complexes as catalysts for the aerobic oxidation of 1-phenylethanol is also discussed.