Novel cationic ruthenium half-sandwich complexes of 1-substituted-4-pyridyl-1H-1,2,3-triazole were found to be efficient catalyst precursors for transfer hydrogenation reactions. Based on several ...experimental observations, a plausible mechanism for the process is proposed.
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A series of novel cationic ruthenium half-sandwich complexes, bearing 1-substituted-4-pyridyl-1H-1,2,3-triazole ligands, were synthesized and fully characterized by a range of analytical techniques. The complexes were found to be efficient catalyst precursors for transfer hydrogenation reactions using ketones as substrates. We demonstrated that the complexes could hydrogenate acetophenone in excellent conversions (∼75 %) within 3 h, employing a low concentration of base of only 2 mol %. Extending the reaction time to 6 h gave near quantitative conversions for both catalysts employed. In addition to this, the transfer hydrogenation of acetophenone was also found to proceed even at low catalyst loadings (0.025–0.05 mol%) and low base concentrations (0.25–1.0 mol%). Under these conditions substrate conversion was moderate (22–60 %). The catalytic efficiency of the most effective catalyst was then evaluated in the transfer hydrogenation of a small library of aromatic and aliphatic ketones. It was shown that more challenging substrates such as benzophenone and 2-octanone could be hydrogenated to the corresponding secondary alcohols in conversions > 90 %. Finally, based on several experimental observations, a possible mechanism for the process is proposed.
Transfer hydrogenation of aromatic ketones catalyzed by eight cyclometallated ruthenium half-sandwich complexes, including three new complexes, was examined. The catalytic process was studied using ...different ratios of substrate to base and base to catalyst and using a deuterated reductant. Optimum conditions for catalysis were shown to be in the presence of higher amounts of base in refluxing isopropanol. Under these conditions, the complexes were reduced in situ to give Ru(0) nanoparticles invisible to the naked eye. The nanoparticles were characterized by TEM, DLS and XPS. The catalytic transfer hydrogenation, under conditions in which nanoparticles were generated, was found to be far greater than the transfer hydrogenation by the molecular catalyst. Complete characterization of the three new complexes, including the X-ray crystallographic characterization of these complexes was carried out.
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•Cyclometallated Ru half-sandwich complexes catalyse transfer hydrogenation.•Ru half-sandwich complexes are reduced in situ to form Ru(0) nanoparticles.•The catalytic activity is essentially due to Ru(0) nanoparticles.•Efficient transfer hydrogenation of 15 aromatic ketones is demonstrated.
