Microwave Irradiation for the Facile Synthesis of Transition-Metal Nanoparticles (NPs) in Ionic Liquids (ILs) from Metal–Carbonyl Precursors and Ru-, Rh-, and Ir-NP/IL Dispersions as Biphasic Liquid–Liquid Hydrogenation Nanocatalysts for Cyclohexene

Stable chromium, molybde- num, tungsten, manganese, rhenium, ruthenium, osmium, cobalt, rhodium, and iridium metal nanoparticles (M- NPs) have been reproducibly obtained by facile, rapid (3 min), and energy- saving 10 W microwave irradiation (MWI) under an argon atmosphere from their metal-carbonyl precursors (Mx(CO)y) in the ionic liquid (IL) 1- butyl-3-methylimidazolium tetrafluoro- borate ((BMIm)A4)). This MWI syn- thesis is compared to UV-photolytic (1000 W, 15 min) or conventional ther- mal decomposition (180-2508C, 6- 12 h) of (Mx(CO)y) in ILs. The MWI- obtained nanoparticles have a very small (< 5 nm) and uniform size and are prepared without any additional stabilizers or capping molecules as long-term stable M-NP/IL dispersions (characterization by transmission elec- tron microscopy (TEM), transmission electron diffraction (TED), and dy- namic light scattering (DLS)). The ruthenium, rhodium, or iridium nano- particle/IL dispersions are highly active and easily recyclable catalysts for the biphasic liquid-liquid hydrogenation of cyclohexene to cyclohexane with activi- ties of up to 522 (mol product)A Ru) 1 h 1 and 884 (mol product)A Rh) 1 h 1 and give almost quantitative conversion within 2 h at 10 bar H2 and 908C. Catalyst poisoning experiments with CS2 (0.05 equiv per Ru) suggest a heterogeneous surface catalysis of Ru- NPs.