Synthesis and characterization of water-soluble arene-ruthenium complexes and their application in biphasic olefin oxidation

Abstract

New water-soluble complexes [(η6-C6H6)RuCl(C5H4N-2-CH = N-R)]Cl (1) (with R = 4-hydroxymethylphenyl (a), 2,4-dichlorophenyl (b), 2-fluorophenyl (c), 3-carboxyphenyl (d)) have been synthesized by reacting [(η6-C6H6)Ru(μ-Cl)Cl]2 with the N,N′-bidentate ligands in a 1:2 ratio. Full characterization of all complexes was accomplished using 1H and 13C NMR, elemental analyses, UV-Vis spectroscopy, IR spectroscopy and single crystal X-ray crystallography for determination of the structure of 1d, as 1d·4H2O. The single crystal structure confirmed coordination of the ligand to the ruthenium(II) center leading to a structure commonly described as a pseudo-octahedral, three-legged piano stool. The geometry around the Ru(II) center is such that the arene ring occupies the apex of the stool while the N,N′-bidentate ligand and a chloride occupy the base of the stool. The synthesized Ru(II) complexes were tested as catalysts for oxidation of styrene using NaIO4 as a co-oxidant in a biphasic system. All complexes were active, giving good yields of benzaldehyde. Catalyst 1c was later investigated for olefin oxidation and gave high yields of the corresponding aldehydes as the major products in all cases.