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

<jats:title>Abstract</jats:title><jats:p>Stable chromium, molybdenum, 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 [M<jats:sub><jats:italic>x</jats:italic></jats:sub>(CO)<jats:sub><jats:italic>y</jats:italic></jats:sub>] in the ionic liquid (IL) 1‐butyl‐3‐methylimidazolium tetrafluoroborate ([BMIm][BF<jats:sub>4</jats:sub>]). This MWI synthesis is compared to UV‐photolytic (1000 W, 15 min) or conventional thermal decomposition (180–250 °C, 6–12 h) of [M<jats:sub><jats:italic>x</jats:italic></jats:sub>(CO)<jats:sub><jats:italic>y</jats:italic></jats:sub>] 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 electron microscopy (TEM), transmission electron diffraction (TED), and dynamic light scattering (DLS)). The ruthenium, rhodium, or iridium nanoparticle/IL dispersions are highly active and easily recyclable catalysts for the biphasic liquid–liquid hydrogenation of cyclohexene to cyclohexane with activities of up to 522 (mol product) (mol Ru)<jats:sup>−1</jats:sup> h<jats:sup>−1</jats:sup> and 884 (mol product) (mol Rh)<jats:sup>−1</jats:sup> h<jats:sup>−1</jats:sup> and give almost quantitative conversion within 2 h at 10 bar H<jats:sub>2</jats:sub> and 90 °C. Catalyst poisoning experiments with CS<jats:sub>2</jats:sub> (0.05 equiv per Ru) suggest a heterogeneous surface catalysis of Ru‐NPs.</jats:p>