Precursors ionic liquids

These results suggest that by altering the acidity of the hydrogen bonding partner of the carbene, or by altering the basicity of the anion in imidazolium salts (like Im-ILs), the system can be switched between the two alternative structures in Fig. 8, and carbene precursor ionic liquids can be designed. 

Depending on the coordinative properties of the anion and on the degree of the cation s reactivity, the ionic liquid can be regarded as an innocent solvent, as a ligand (or ligand precursor), as a co-catalyst, or as the catalyst itself... 

Ionic liquids formed by treatment of a halide salt with a Lewis acid (such as chloro-aluminate or chlorostannate melts) generally act both as solvent and as co-catalyst in transition metal catalysis. The reason for this is that the Lewis acidity or basicity, which is always present (at least latently), results in strong interactions with the catalyst complex. In many cases, the Lewis acidity of an ionic liquid is used to convert the neutral catalyst precursor into the corresponding cationic active form. The activation of Cp2TiCl2 [26] and (ligand)2NiCl2 [27] in acidic chloroaluminate melts and the activation of (PR3)2PtCl2 in chlorostannate melts [28] are examples of this land of activation (Eqs. 5.2-1, 5.2-2, and 5.2-3). 

Ionic liquid as solvent and ligand/ligand precursor... 

Both the cation and the anion of an ionic liquid can act as a ligand or ligand precursor for a transition metal complex dissolved in the ionic liquid. 

With respect to the ionic liquid s cation the situation is quite different, since catalytic reactions with anionic transition metal complexes are not yet very common in ionic liquids. However, an imidazolium moiety as an ionic liquid cation can act as a ligand precursor for the dissolved transition metal. Its transformation into a lig-... 

This preliminary study has recently been extended to these and other ionic liquids, using CuCl as the catalyst precursor [46]. Some relevant results are gathered in Table 6. 

Dobbs, W., Suisse, J.-M., Douce, L. and Welter, R. (2006) Electrodeposition of Silver Partides and Gold Nanopartides from Ionic Liquid-Crystal Precursors. Angewandte Chemie (International Edition in English), 45, 4179-4182. 

Kramer, J., Redel, E., Thomann, R. and Janiak, C. (2008) Use of ionic liquids for the synthesis of iron, ruthenium, and osmium nanopartides from their metal carbonyl precursors. Organometallics,... 

Estruga, M., Domingo, C., Domenech, X., and Ayllon, J.A. (2010) Zirconium-doped and silicon-doped Ti02 photocatalysts synthesis from ionic-liquid-like precursors. Journal of Colloid and Interface Science, 344 (2), 327-333. 

Scheme 7.1. Activation of a neutral catalyst precursor by chloroaluminate ionic liquids...

The reaction of 3,4-diacyl-l,2,5-oxadiazole 2-oxides (furoxans) with activated nitriles in ionic liquids and in ethanol unexpectedly resulted in 3-acyl-4-acylamino-l,2,5-°xadiazoles (furazans) <2003MC230>. 3-Formyl-4-phenyl-l,2,5-oxadiazole Ar-oxide 105 is a good precursor for the synthesis of functional substituted furoxans (Scheme 28) <1999JME1941, 2000MOL520, 2000JFA2995>. 

Much emphasis has been placed in recent times on easily recoverable liquid bi-phasic catalysts, including metal clusters in nonconventional solvents. For instance, aqueous solutions of the complexes [Ru3(CO)12.x(TPPTS)x] (x=l, 2, 3 TPPTS = triphenylphosphine-trisulfonate, P(m-C6H4S03Na)3) catalyze the hydrogenation of simple alkenes (1-octene, cyclohexene, styrene) at 60°C and 60 bar H2 at TOF up to 500 h 1 [24], while [Ru i(CO)C (TPPMS) >,] (TPPMS = triphenylphos-phine-monosulfonate, PPh2(m-C6H4S03Na) is an efficient catalyst precursor for the aqueous hydrogenation of the C=C bond of acrylic acid (TOF 780 h 1 at 40 °C and 3 bar H2) and other activated alkenes [25]. The same catalysts proved to be poorly active in room temperature ionic liquids such as [bmim][BF4] (bmim= Tbutyl-3-methylimidazolium). No details about the active species involved are known at this point. 

Recently, Dupont and coworkers described the use of room-temperature imi-dazolium ionic liquids for the formation and stabilization of transition-metal nanoparticles. The potential interest in the use of ionic liquids is to promote a bi-phasic organic-organic catalytic system for a recycling process. The mixture forms a two-phase system consisting of a lower phase which contains the nanocatalyst in the ionic liquid, and an upper phase which contains the organic products. Rhodium and iridium [105], platinum [73] or ruthenium [74] nanoparticles were prepared from various salts or organometallic precursors in dry 1-bu-tyl-3-methylimidazolium hexafluorophosphate (BMI PF6) ionic liquid under hydrogen pressure (4 bar) at 75 °C. Nanoparticles with a mean diameter of 2-3 nm... 

Table 41.6 Hydrogenation of styrene to ethylbenzene using [H3Os4(CO)12] as a catalyst precursor in various ionic liquids [72],...

A comparison of hydrogenation activity in a biphasic ionic liquid-containing system and a homogeneous media was carried out by Dupont et al. [99]. These authors used a [RuC12-(S)-BINAP]2 NEt3 catalyst precursor dissolved in [BMIM][BF4]. This... 

The SILP carbonylation catalyst was prepared by one-step impregnation of sihca support using a methanohc solution of the ionic liquid [BMIMjl and the dimer [Rh(CO)2l]2- The use of the dimeric precursor complex allowed formation of the catalyst anion [Rh(CO)2l2] directly during catalyst preparation without formation of contaminating byproducts in the ionic hquid catalyst solution. 

Scheme 15.3 Preparation of soluble iridium nanoparticles from in situ reduction of the organometallic precursor [ir(COD)Cl]2 in imidazolium ionic liquids.

The Mizoroki-Heck reaction in liquid imidazolium salts as the solvent is a special case of an in situ system Under the reaction conditions NHC complexes of palladium are formed as the active catalyst from the solvent and the ligand-free palladium precursor. In general, ionic liquids are novel reaction media for homogeneous catalysis. They allow easy separation of product and catalyst after the reaction. ... 

Cholin chloride, shown in Scheme 1, was recently shown to be a versatile cationic precursor for the preparation of a wide variety of ionic liquids (48). This precursor is particularly interesting, as cholin is a vitamin B4 additive to animal food. Therefore, it is not expected to be restricted in applications by health-related concerns, whereas a number of other ionic liquids remain to be fully evaluated for their environmental and health effects, including the ones based on imidazolium ions. 

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