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 (C<sub>8</sub>H<sub>9</sub><sup>-</sup>, Dp) and indenyl (C<sub>9</sub>H<sub>7</sub><sup>-</sup>, Ind) ancillary ligands have been synthesized and characterized using multinuclear NMR spectroscopy and X-ray crystallography. The complexes DpRu(PTA)(PPh<sub>3</sub>)Cl, DpRu(PTA)<sub>2</sub>Cl, IndRu(PTA)(PPh<sub>3</sub>)Cl, and [IndRu(PTA)<sub>2</sub>(PPh<sub>3</sub>)]Cl were obtained in good to excellent yields. The solid-state structures of these compounds exhibit piano stool geometries with η<sup>5</sup>-coordination of the indenyl and dihydropentalenyl moieties. DpRu(PTA)<sub>2</sub>Cl is water-soluble (<i>S</i><sub>25</sub><sub>°</sub><sub>C</sub> = 43 mg/mL), while the mixed phosphine compounds are slightly soluble in acidic solutions. The Ru−H complexes, Cp‘Ru(PTA)(PPh<sub>3</sub>)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 CD<sub>3</sub>OD with relative rates CpRu(PTA)(PPh<sub>3</sub>)H ≫ IndRu(PTA)(PPh<sub>3</sub>)H > CpRu(PTA)<sub>2</sub>H. The air-stable Cp‘Ru(PTA)(PR<sub>3</sub>)Cl complexes (Cp‘ = Cp, Dp, Ind; PR<sub>3</sub> = PPh<sub>3</sub> or PTA) exhibit activity in the regioselective transfer hydrogenation of α,β-unsaturated carbonyls in aqueous media with HCOONa, HCOOH, or isopropanol/Na<sub>2</sub>CO<sub>3</sub> 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)(PPh<sub>3</sub>)Cl was less active than Cp‘Ru(PTA)(PPh<sub>3</sub>)Cl (Cp‘ = Cp or Dp). Results of the transfer hydrogenation of unsaturated substrates using CpRu(PTA)(PPh<sub>3</sub>)H are also reported.