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

Abstract

Stable transition-metal nanoparticles of the type [M(0)](n) are easily accessible through the reduction of Ir(I) or Rh(III) compounds dissolved in "dry" 1-n-butyl-3-methylimidazolium hexafluorophosphate ionic liquid by molecular hydrogen. The formation of these [M(0)](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 [Ir(0)](n) and [Rh(0)](n) nanoparticles with 2.0-2.5 nm in diameter. The isolated [M(0)](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 degrees 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 [Ir(0)](n) in solventless conditions. Contrarily, the recovered Rh(0) 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 [Ir(0)](n) nanoparticles occurs with concomitant hydrogenolysis of the C-O bond, suggesting that these nanoparticles behave as "heterogeneous catalysts" rather than "homogeneous catalysts".

Catalytic Applications of Metal Nanoparticles in Imidazolium Ionic Liquids

“…It is worth mentioning that the purity, in particular the water and halide contents, [21] of the imidazolium ILs play important role in the MNP chemistry, since these impurities may influence the stability and catalytic properties of the material. [22] Preparation and Characterization of MNPs in ILs Transition-metal NPs are easily prepared by: 1) controlled decomposition of organometallic compounds in the formal zero oxidation state such as [Pt 2 A C H T U N G T R E N N U N G (dba) 3 ], [23] [24] or [NiA C H T U N G T R E N N U N G (cod) 2 ] [25] (dba = dibenzylideneacetone, cod = 1,5-cyclooctadiene, and cot = 1,3,5-cyclooctatriene) dispersed in ILs; 2) chemical reduction (usually with hydrogen or a hydride source) of transition-metal compounds such as [Pd- [27] and RuO 2 [28] dispersed in the ILs and 3) simple transfer of the freshly prepared MNP in water or "classical" organic solvents to the ILs. [29] Alternatively, in situ laser radiation may be used to induce the fragmentation of relatively large MNPs dispersed in ILs into smaller particles with a narrow size distribution.…”

Nanoscale Ru(0) Particles: Arene Hydrogenation Catalysts in Imidazolium Ionic Liquids

“…29 In most of the cases, the catalytic properties (activity and selectivity) of these soluble metal nanoparticles are similar to those observed with classical heterogeneous catalysts; that is, they possess a pronounced surface like (multisite) rather than a single site like catalytic properties. 30,31 We report herein a simple organometallic approach 32 for the synthesis and catalytic application of Ru(0) nanoparticles in imidazolium ionic liquids (Chart 1) using a clean straightforward hydrogenation route with the readily available versatile ruthenium precursor [Ru(COD)(2-methylallyl) 2 ] (Chart 1). [33][34][35][36] In most cases, the Ru-NPs are well dispersed and efficiently immobilized in the imidazolium ionic liquid and do not separate from the liquid phase.…”

Recyclable nanoscale copper(i) catalysts in ionic liquid media for selective decarboxylative C–C bond cleavage

“…organo-boron, organo-tin, organo-zinc) reagents. 6 Nowadays metal nanoparticles, such as ruthenium, [7][8][9][10][11][12][13][14][15] platinum, 16 iridium, [17][18][19] palladium, 5,[20][21][22][23][24][25][26][27][28][29][30][31] cobalt, 32 and iron, 33 have great potential for application 34 in homogeneous and heterogeneous catalysis, including hydrogenation, 7 hydrogenolysis, 35 Fischer-Tropsch reactions 32,36 and cross-coupling reactions. 4,5,21,23 Copper 37 and especially copper(I)oxide 38 nanoparticles combine the high catalytic activity of precious metals with easy availability and low cost.…”