pH-Dependent Selective Transfer Hydrogenation of α,β-Unsaturated Carbonyls in Aqueous Media Utilizing Half-Sandwich Ruthenium(II) Complexes

Half-sandwich ruthenium(II) PTA complexes bearing the 1,2-dihydropentalenyl (C₈H₉^-, Dp) and indenyl (C₉H₇^-, Ind) ancillary ligands have been synthesized and characterized using multinuclear NMR spectroscopy and X-ray crystallography. The complexes DpRu(PTA)(PPh₃)Cl, DpRu(PTA)₂Cl, IndRu(PTA)(PPh₃)Cl, and [IndRu(PTA)₂(PPh₃)]Cl were obtained in good to excellent yields. The solid-state structures of these compounds exhibit piano stool geometries with η⁵-coordination of the indenyl and dihydropentalenyl moieties. DpRu(PTA)₂Cl is water-soluble (S_25°C = 43 mg/mL), while the mixed phosphine compounds are slightly soluble in acidic solutions. The Ru−H complexes, Cp‘Ru(PTA)(PPh₃)H (Cp‘ = Ind, Cp), have been synthesized in good yield and spectroscopically and structurally characterized. The ruthenium hydrides undergo an H/D exchange reaction with CD₃OD with relative rates CpRu(PTA)(PPh₃)H ≫ IndRu(PTA)(PPh₃)H > CpRu(PTA)₂H. The air-stable Cp‘Ru(PTA)(PR₃)Cl complexes (Cp‘ = Cp, Dp, Ind; PR₃ = PPh₃ or PTA) exhibit activity in the regioselective transfer hydrogenation of α,β-unsaturated carbonyls in aqueous media with HCOONa, HCOOH, or isopropanol/Na₂CO₃ serving as the hydrogen source. They were found to be effective in the selective reduction of the carbonyl functionality of cinnamaldehyde and the CC bond of benzylidene acetone and chalcone. IndRu(PTA)(PPh₃)Cl was less active than Cp‘Ru(PTA)(PPh₃)Cl (Cp‘ = Cp or Dp). Results of the transfer hydrogenation of unsaturated substrates using CpRu(PTA)(PPh₃)H are also reported.