Cationic pyridyl(benzoazole) ruthenium(II) complexes: Efficient and recyclable catalysts in biphasic hydrogenation of alkenes and alkynes

• Water soluble cationic (pyridyl)benzoazole Ru(II) complexes have been prepared.
• The complexes form active catalysts for alkene hydrogenation under mild conditions.
• The catalysts are highly recyclable and exhibit good activities for six cycles.
• The aqueous/organic volume ratio affects the recovery of the active species.

The synthesis, structural characterization of cationic 2-(2-pyridyl)benzoazole)ruthenium(II) complexes and their applications in biphasic hydrogenations of alkenes is reported. Reactions of 2-(2-pyridyl)benzoimidazole (L1), 2-(2-pyridyl)benzothiazole (L2) and 2-(2-pyridyl)benzoxazole (L3) with [η6-(2-phenoxyethanol)RuCl2]2 produced the corresponding cationic complexes [η6-(2-phenoxyethanol)RuCl(L1)]Cl (1), [η6-(2-phenoxyethanol)RuCl(L2)]Cl (2) and [η6-(2-phenoxyethanol)RuCl(L3)]Cl (3) in good yields. Solid state structures of 1–3 confirmed the bidentate coordination modes of L1–L3 and formation of cationic species through displacement of one chloride ligand from Ru(II) coordination sphere. Complexes 1–3 produced active catalysts for high pressure hydrogenation of alkenes both in methanol and biphasic conditions. Relatively lower activities were observed in the hydrogenation of terminal alkynes giving a mixture of alkane and alkene products. Complexes 1–3 were recyclable under biphasic conditions and retained significant catalytic activities in six cycles. Reaction parameters such as substrate/catalyst ratio, temperature, and aqueous/organic ratio affected the catalytic trends.

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