The Use of Imidazolium Ionic Liquids for the Formation and Stabilization of Ir0 and Rh0 Nanoparticles: Efficient Catalysts for the Hydrogenation of Arenes

Stable transition‐metal nanoparticles of the type [M0]n are easily accessible through the reduction of IrI or RhIII compounds dissolved in “dry” 1‐n‐butyl‐3‐methylimidazolium hexafluorophosphate ionic liquid by molecular hydrogen. The formation of these [M0]n nanoparticles is straightforward; they are prepared in dry ionic liquid whereas the presence of the water causes the partial decomposition of ionic liquid with the formation of phosphates, HF and transition‐metal fluorides. Transmission electron microscopy (TEM) observations and X‐ray diffraction analysis (XRD) show the formation of [Ir0]n and [Rh0]n nanoparticles with 2.0–2.5 nm in diameter. The isolated [M0]n nanoparticles can be redispersed in the ionic liquid, in acetone or used in solventless conditions for the liquid–liquid biphasic, homogeneous or heterogeneous hydrogenation of arenes under mild reaction conditions (75 °C and 4 atm). The recovered iridium nanoparticles can be reused several times without any significant loss in catalytic activity. Unprecedented total turnover numbers (TTO) of 3509 in 32 h, for arene hydrogenation by nanoparticles catalysts, have been achieved in the reduction of benzene by the [Ir0]n in solventless conditions. Contrarily, the recovered Rh0 nanoparticles show significant agglomeration into large particles with a loss of catalytic activity. The hydrogenation of arenes containing functional groups, such as anisole, by the [Ir0]n nanoparticles occurs with concomitant hydrogenolysis of the CO bond, suggesting that these nanoparticles behave as “heterogeneous catalysts” rather than “homogeneous catalysts”. Ir0 and Rh0 nanoparticles of 2–3 nm diameter with a narrow size distribution can be synthesized in a straightforward manner by using a one‐component liquid phase, namely 1‐n‐butyl‐3‐methylimidazolium hexafluorophosphate ionic liquid, by reduction with molecular hydrogen of standard metal salts. The nanomaterials formed efficiently promote the homogeneous, liquid–liquid two‐phase or “solventless” hydrogenation of arenes under mild reaction conditions.