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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.

The reaction of CpRu(PTA)2Cl (1) with KOH in methanol afforded the water-soluble hydride CpRu(PTA)2H (2) in good yield. The solid-state structures of both CpRu(PTA)2H and the previously reported CpRu(PTA)2Cl are described and exhibit classic piano stool geometries. Complex 2 is one of a very few structurally characterized CpRu(PR3)2H compounds known and the only one that is water-soluble. The ruthenium hydride is stable and soluble (S 25 ° C = 20 mg/mL) in deoxygenated water; however, the complex does react with chlorinated solvents to yield 1. Complex 2 undergoes H/D exchange with D2O, and the kinetics of this process were monitored by 31P NMR spectroscopy as a function of temperature. Activation parameters for the reaction of 2 with D2O were obtained:  ΔH ⧧ = 68 ± 2 kJ/mol; ΔS ⧧ = −94 ± 7 J/mol·K. A normal kinetic isotope effect of 7.9 was calculated for the H/D exchange reaction described here. These kinetic parameters suggest an associative mechanism with little Ru−H (or Ru−D) bond cleavage at the transition state. The reaction is postulated to occur via protonation of the hydride ligand of 2 by water followed by deprotonation by the resultant hydroxide. Finally DFT calculations were performed on all complexes and are consistent with the observed solid-state structures of 1 and 2.

The hydrogenation of benzylidene acetone has been accomplished under biphasic conditions (water/diethyl ether) utilizing CpRu(PTA) 2 H as the catalyst under mild conditions. The results show high selectivity toward hydrogenation of the olefin in benzylidene acetone with 4-phenylbutan-2-one as the major product. The effects of pH, hydrogen pressure, and salts on the reduction are described. The presence of anions such as BF 4and PF 6significantly improve catalytic activity, while Clshuts down catalysis due to the formation of the catalytically inactive CpRu(PTA) 2 Cl. The aforementioned factors do not influence the selectivity of the reaction. Spectroscopic studies of the nature of CpRu(PTA) 2 H under the reaction conditions enabled the identification of mono-and dihydride intermediates, [CpRu-(PTA)(PTAH)H] + and [CpRu(PTA) 2 (H) 2 ] + , respectively. The distribution of these intermediates as a function of pH and the effect on reaction rates are presented. Reaction rate is found to increase with an increase in concentration of [CpRu(PTA)(PTAH)H] + , which is the presumed active catalytic species. Based on the data presented a 1,4-conjugate addition mechanism is proposed. Finally, the solid-state structures of the mono-and diprotonated complexes of CpRu(PTA) 2 Cl, [CpRu(PTA)(PTAH)Cl](PF 6 ), and CpRu(PTAH) 2 Cl](PF 6 ) 2 are reported.

A new synthetic method for annulating hydroquinones to Fe 2 S 2 (CO) 6 moieties is reported. Piperidine catalyzed a multistep reaction between Fe 2 (μ-SH) 2 (CO) 6 and quinones to afford bridged adducts in 26-76% yields. The hydroquinone adducts undergo reversible two-electron reductions. In the presence of acetic acid, hydrogen is produced catalytically with these adducts at potentials more negative than that of the initial reversible reduction. Spectroscopic studies suggest the presence of intramolecular hydrogen bonding between the phenolic OH groups and the adjacent sulfur atoms. Computations, which are in good agreement with the electrochemical studies and spectroscopic data, indicate that the hydrogen bonding is most important in the reduced forms of the catalysts. This hydrogen bonding lowers the reduction potential for catalysis but also lowers the basicity and thereby the reactivity of the catalysts.