Synthesis and characterization of piano-stool ruthenium complexes with N,N′-pyridine imine bidentate ligands and their application in styrene oxidation

• New arene ruthenium(II) pyridine-imine complexes have been prepared.
• Their crystal structure show the pseudo-octahedral piano stool geometry.
• The complexes were applied in the oxidation of olefins in a H2O/t-butanol mixture.
• IR and UV-vis studies suggest the mechanism involves a Ru(IV) oxo species.

The new complexes [(η6-arene)RuCl(C5H4N-2-CHN-R)]PF6 (arene = C6H6 with R = 4- flourophenyl (1), 4-chlorophenyl (2), 4-bromophenyl (3), 4 -iodophenyl (4), 2, 5-dichlorophenyl (5) or p-cymene with R = 4-flourophenyl (6), 4-chlorophenyl (7), 4-bromophenyl (8), 4- iodophenyl (9), 2, 5 - dichlorophenyl (10)) have been synthesized by reacting the ruthenium arene precursors [(η6-arene)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, thermal analysis, IR spectroscopy and single crystal x-ray structures for compounds 7 and 10. The single crystal structures confirmed coordination of the ligand to the ruthenium(II) centre. The Ru(II) centre has a pseudo-octahedral three legged piano stool geometry in which the arene ring occupies the apex of the stool and the ruthenium is coordinated to the N,N′-bidentate ligand and a chloride ligand at the base of the stool. Two polymorphs of 7 were identified. The synthesized Ru(II) complexes were tested as catalysts for the oxidation of styrene to benzaldehyde using NaIO4 as a co-oxidant. All complexes were active catalysts for styrene oxidation and they gave high yields of benzaldehyde as the major product.

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