N‐Heterocyclic Carbene (NHC)‐Stabilized Ru0 Nanoparticles: In Situ Generation of an Efficient Transfer Hydrogenation Catalyst

Tethered and untethered ruthenium half‐sandwich complexes were synthesized and characterized spectroscopically. X‐ray crystallographic analysis of three untethered and two tethered Ru N‐heterocyclic carbene (NHC) complexes were also carried out. These RuNHC complexes catalyze transfer hydrogenation of aromatic ketones in 2‐propanol under reflux, optimally in the presence of (25 mol %) KOH. Under these conditions, the formation of 2–3 nm‐sized Ru0 nanoparticles was detected by TEM measurements. A solid‐state NMR investigation of the nanoparticles suggested that the NHC ligands were bound to the surface of the Ru nanoparticles (NPs). This base‐promoted route to NHC‐stabilized ruthenium nanoparticles directly from arene‐tethered ruthenium–NHC complexes and from untethered ruthenium–NHC complexes is more convenient than previously known routes to NHC‐stabilized Ru nanocatalysts. Similar catalytically active RuNPs were also generated from the reaction of a mixture of [RuCl2(p‐cymene)]2 and the NHC precursor with KOH in isopropanol under reflux. The transfer hydrogenation catalyzed by these NHC‐stabilized RuNPs possess a high turnover number. The catalytic efficiency was significantly reduced if nanoparticles were exposed to air or allowed to aggregate and precipitate by cooling the reaction mixtures during the reaction. N‐Heterocyclic carbene (NHC)‐stabilized Ru0 nanoparticles (NPs) were generated in situ from tethered and untethered ruthenium half‐sandwich complexes. These catalytically active Ru0 NPs were explored for transfer hydrogenation of aromatic ketones in 2‐propanol. The transfer hydrogenation catalyzed by these NPs possess a high turnover number. The catalytic efficiency was significantly reduced if these NPs were exposed to air or allowed to aggregate and precipitate by cooling the reaction mixtures.