Jackson D. Scholten

Carbon-Carbon Cross Coupling Reactions in Ionic Liquids Catalysed by Palladium Metal Nanoparticles

A brief summary of selected pioneering and mechanistic contributions in the field of carbon-carbon cross-coupling reactions with palladium nanoparticles (Pd-NPs) in ionic liquids (ILs) is presented. Five exemplary model systems using the Pd-NPs/ILs approach are presented: Heck, Suzuki, Stille, Sonogashira and Ullmann reactions which all have in common the use of ionic liquids as reaction media and the use of palladium nanoparticles as reservoir for the catalytically active palladium species.

Nanoscale Ru(0) particles: arene hydrogenation catalysts in imidazolium ionic liquids.

The reduction of [Ru(COD)(2-methylallyl) 2] (COD = 1,5-cyclooctadiene) dispersed in various room-temperature ionic liquids (ILs), namely, 1- n-butyl-3-methylimidazolium (BMI) and 1- n-decyl-3-methylimidazolium (DMI), associated with the N-bis(trifluoromethanesulfonyl)imidates (NTf 2) and the corresponding tetrafluoroborates (BF 4) with hydrogen gas (4 bar) at 50 degrees C leads to well-dispersed immobilized nanoparticles. Transmission electron microscopy (TEM) analysis of the particles dispersed in the ionic liquid shows the presence of [Ru(0)] n nanoparticles (Ru-NPs) of 2.1-3.5 nm in diameter. Nanoparticles with a smaller mean diameter were obtained in the ILs containing the less coordinating anion (NTf 2) than that in the tetrafluoroborate analogues. The ruthenium nanoparticles in ionic liquids were used for liquid-liquid biphasic hydrogenation of arenes under mild reaction conditions (50-90 degrees C and 4 bar). The apparent activation energy of E A = 42.0 kJ mol (-1) was estimated for the hydrogenation of toluene in the biphasic liquid-liquid system with Ru-NPs/BMI.NTf 2. TEM analysis of the ionic liquid material after the hydrogenation reactions shows no significant agglomeration of the [Ru(0)] n nanoparticles. The catalyst ionic liquid phase can be reused several times without a significant loss in catalytic activity.

Palladium nanoparticle catalysts in ionic liquids: synthesis, characterisation and selective partial hydrogenation of alkynes to Z-alkenes

The simple heating (120 °C) of Pd(OAc)2 in 1-butyronitrile-3-methylimidazolium-N-bis(trifluoromethane sulfonyl)imide ((BCN)MI·NTf2) under reduced pressure leads to the formation of stable and small-sized Pd(0)-NPs (diameter: 7.3 ± 2.2 nm). These metal nanoparticles were characterised by means of TEM, HRTEM and XPS analysis techniques. Moreover, the potential for partial hydrogenation of alkynes in multiphase systems was evaluated. The hydrogenation of internal alkynes at 25 °C and under 1 bar of hydrogen yields Z-alkenes (up to 98% selectivity). Application of higher hydrogen pressure (4 bar) in these reactions always led to the formation of alkanes without the detection of any alkenes. TOF values were attained up to 1282 h−1 with a good recyclability of the system which does not lose its activity for at least 4 runs.

Imidazolium ionic liquids as promoters and stabilising agents for the preparation of metal(0) nanoparticles by reduction and decomposition of organometallic complexes

The organometallic complexes ([Ru(COD)(2-methylallyl)2] and [Ni(COD)2] (COD=1,5-cyclooctadiene) dissolved in imidazolium ionic liquids (ILs) undergo reduction and decomposition, respectively, to afford stable ruthenium and nickel metal(0) nanoparticles (Ru(0)-NPs and Ni(0)-NPs) in the absence of classical reducing agents. Depending on the case, the reduction/auto-decomposition is promoted by either the cation and/or anion of the neat imidazolium ILs.

Decomposition of Formic Acid Catalyzed by a Phosphine‐Free Ruthenium Complex in a Task‐Specific Ionic Liquid

The dehydrogenation of formic acid is effectively catalyzed by the Ru complex [{RuCl2(p‐cymene)}2] dissolved in the ionic liquid (IL) 1‐(2‐(diethylamino)ethyl)‐3‐methylimidazolium chloride at 80 °C without additional bases. This catalytic system gives TOF values of up to 1540 h−1. Preliminary kinetic insights show formal reaction orders of 0.70(±0.15), 0.78(±0.03) and 2.00(±0.17) for the Ru catalyst, IL 1, and formic acid, respectively. The apparent activation energy of this process is estimated to be (69.1±7.6) kJ mol−1. In addition, dimeric Ru hydride ionic species involved in the reaction, such as [{Ru(p‐cymene)}2{(H)μ‐(H)‐μ‐(HCO2)}]+ and [{Ru(p‐cymene)}2{(H)μ‐(Cl)μ‐(HCO2)}]+, are identified by mass spectrometry. The presence of water in large amounts inhibits higher conversions. Finally, a remarkable catalytic activity is observed during recycles, indicating this system’s potential for hydrogen gas production.

Application of Chiral Ionic Liquids for Asymmetric Induction in Catalysis

Here we present the state-of-the-art for asymmetric catalysis using chiral ionic liquids (CILs) as source of chiral information. The current review covers reactions using typical homogeneous catalysts e.g. organocatalysts, transition metal complexes and solid catalysts for heterogeneous catalysis in solvent-systems with chiral ionic liquids. INTRODUCTION Most of the chemical production processes in industry involve catalysts (>90%) and the majority are heterogeneous processes [1]. This is due to practical aspects like purification of products or intermediates. However, solid catalysts are disadvantageous if asymmetric reactions such as asymmetric hydrogenation need to be carried out. In that case, chiral organometallic complexes or organocatalysts acting in homogeneous manner are clearly superior to the heterogeneous type of catalyst. Further attempts for asymmetric induction in principle use substrates, reagents and auxiliaries bearing the chiral information successfully [2], or chiral reaction media, the latter one less successfully [3]. Other investigations involved asymmetric induction by chiral particles of achiral substances, like chiral quartz (SiO2) or glycine crystals, for example in asymmetric photochemical reactions in the solid state [4, 5]. In the absence of chiral molecules also circularly polarized light can be used as chiral force such as the synthesis of hexahelicene with left or right circularly polarized light [4, 5]. There are reports though, which are describing the immobilization of chiral homogeneous catalyst on solid support [6]. However, in most cases homogeneous catalysts do not fulfill the necessary requirements (catalyst stability, catalyst recycling, separation from product and intermediates) for industrial processes like continuous-flow processes. For years, the industrial adaptation only seemed practical if homogeneous catalysts are immobilized on solid supports or soluble polymers which can be easily separated from solution [6]. Combining the advantages of solid catalysts (stability, recyclable, purification, long life-time) [6, 7] and the advantages of homogeneous catalysts (chemo-, regioand asymmetric reaction controls) [6, 7] lead to chiral ionic liquids (CILs) as an reaction media for the immobilization of the catalyst. Nowadays, CILs seem to be the most adequate *Address correspondence to this author at the Universidade Federal do Rio Grande do Sul (UFRGS), Institute of Chemistry Campus do Vale, Laboratory of Molecular Catalysis, Avenida Bento Gonçalves 9500 (P.O. Box 15003), Porto Alegre RS – Brazil, CEP 91501-970, Brazil; Fax: +49-32124702238; E-mail: martin-prechtl@gmx.net materials potentially fulfilling all the desired requirements for a heterogeneous-type solid catalyst as well as typical molecular complex catalysts or organocatalysts for asymmetric catalysis. Recently, their unique material and solvent properties and the growing interest in a sustainable, “green” chemistry has led to an amazing increase in interest in such salts [8]. Ionic liquids (ILs) are well-organized three-dimensional media. It has been shown that intramolecular C-H••• interactions results in a well-organized 3D structure where ionic channels are formed by cations and anions [9]. This supramolecular arrangement render them in a plethora of applications and opportunities [10]: a media for metal nanoparticles formation and stabilization [11], the formation of organized liquid clathrates [12], and in many different physicochemical processes [13]. There are many different reviews on the subject of ILs that can attend many readers with distinctive needs [14]. Not before 1999 the first CIL, an imidazolium salt with a lactate as chiral anion was published by Seddon, and in the following years most of the CILs had chiral cations derived from the “chiral pool” [15]. Their chirality origin is from axial, central or planar chirality. Additionally, it can be found in both cation, and the anion – the so called doubly chiral IL. As a consequence of a very well organized 3D structure, CILs have an intrinsic potential for enantioselective reactions, especially because in a well organized media it is reasonable to expect some chiral transmission. Whether considering that ILs can participate in the transition state or stabilizing charged (or polar) intermediates, it is expected that chiral induction takes place in that media. The chiral transmission using CILs as the single source of chirality may take place through two different pathways: (1) the CILs indeed participate in the reaction intermediates or transition states [16], e.g. some proline TSCILs (taskspecific CIL) and (2) through ion-pairing formation [17]. The attempts to use chiral solvents as source of chirality goes back to the 1970s, when Seebach and Oei performed electrochemical reduction of ketones in a chiral amino ether which led to a rather low enantioselectivity of ~24% ee [18]. Further investigations in the past decades gave also comparably low asymmetric induction which resulted in the ac1260 Current Organic Chemistry, 2009, Vol. 13, No. 13 Prechtl et al. cepted conclusion that chirality transfer from chiral solvents is rather low [3]. Later on, it was not necessary to wait again several decades for further advantages in catalysis using chiral solvent. This conclusion seemed to be unchanged at least until middle of 2004. Then, the first motivating result was published by Vo-Thanh and coworkers [19]. The middle of May 2006 can be dated as the breakthrough of catalysis using CIL as single source of chirality. Almost parallel Leitner [20], Afonso [21], Malhotra [22], Cheng [23], and all their respectively coworkers presented their contribution to this research field with remarkably high enantiomeric excesses in different reactions. Nowadays, many applications of CILs in different areas of research may be found in the open literature. CILs are used for enantioselective separation of pharmaceutical products by capillary electrophoresis,[24] to tune some kinetics aspects on the asymmetric hydrogenation of some specific compounds [25], chiral stationary phase for the liquid chromatographic resolution [26], enantiomeric recognition properties [27], influence on the excitedstate properties [28], and others. Furthermore, the importance of CILs are reflected in various publications showing simply their syntheses, characterizations and properties [29, 30, 31] Several reviews can be found describing the development in the syntheses and applications of CILs [15]. In the present review we focus on the application of CILs in asymmetric catalysis and not on the synthesis/design of CILs which are more intensively discussed in previous reviews [15]. 1. ORGANOCATALYSIS 1.1. Baylis-Hillman Reaction Vo-Thanh and coworkers presented the pioneering work about a DABCO (1,4-diazabicyclo[2.2.2]octane) catalyzed asymmetric Baylis-Hillman reaction between benzaldehyde and methyl acrylate 3 (Scheme 1, Table 1). Enantioselectivities up to 44% ee were obtained using a chiral cation, based on ephedrine, as source of chirality [19].

In situ generated palladium nanoparticles in imidazolium-based ionic liquids: a versatile medium for an efficient and selective partial biodiesel hydrogenation

An important drawback to be overcome in biodiesel technology is its low oxidative stability. One approach to improve the oxidative stability of soybean oil biodiesel is the partial hydrogenation of double bonds. In the current work, an efficient two-phase catalytic system using palladium acetate dissolved in BMI·BF4 ionic liquid to an in situ generation of palladium nanoparticles was developed in order to promote a selective hydrogenation reaction. Upon using this catalytic system it was possible to partially hydrogenate biodiesel into mono-hydrogenated compounds avoiding the formation of saturated compounds. The nanoparticulate system was compared with the traditional heterogeneous Pd/C system and gave far higher selectivity. It was possible to recover and reuse the ionic phase containing the catalyst up to three times without significant loss in its catalytic performance. Indeed, atomic absorption spectroscopy showed an excellent reclaim of the catalyst, which stayed in the ionic phase. Several parameters, such as temperature, hydrogen pressure, metal concentration and reaction time, were also evaluated.

Synthesis and Characterisation of Fluorescent Carbon Nanodots Produced in Ionic Liquids by Laser Ablation

Carbon nanodots (C-dots) with an average size of 1.5 and 3.0 nm were produced by laser ablation in different imidazolium ionic liquids (ILs), namely, 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMI.BF4 ), 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMI.NTf2 ) and 1-n-octyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (OMI.NTf2 ). The mean size of the nanoparticles is influenced by the imidazolium alkyl side chain but not by the nature of the anion. However, by varying the anion (BF4 vs. NTf2 ) it was possible to detect a significant modification of the fluorescence properties. The C-dots are much probably stabilised by an electrostatic layer of the IL and this interaction has played an important role with regard to the formation, stabilisation and photoluminescence properties of the nanodots. A tuneable broadband fluorescence emission from the colloidal suspension was observed under ultraviolet/visible excitation with fluorescence lifetimes fitted by a multi-exponential decay with average values around 7 ns.

Hybrid tantalum oxide nanoparticles from the hydrolysis of imidazolium tantalate ionic liquids: efficient catalysts for hydrogen generation from ethanol/water solutions

The reaction of equimolar amounts of 1-n-butyl-3-methylimidazolium chloride (BMI·Cl) or 1-n-decyl-3-methylimidazolium chloride (DMI·Cl) with TaCl5 affords imidazolium tantalate ionic liquids (ILs) BMI·TaCl61 and DMI·TaCl62. The hydrolysis of ILs 1 and 2 yields hybrid-like tantalum oxide nanoparticles (NPs) with size distribution dependent on the nature of the IL used (3.8–22 nm from IL 1 and 1.5–6 nm from 2). A significant aggregation/agglomeration of the particles was observed after the removal of the IL content of the hybrid material by calcination, forming predominantly large particles (mainly bulk tantalum oxides). These new hybrid-like Ta2O5/IL NPs are highly active photocatalyst nanomaterials for hydrogen production by reforming of ethanol at ambient temperature. Hydrogen evolution rates up to 7.2 mmol H2 g−1 h−1 and high apparent quantum yields up to 17% were measured. The hybrid-like Ta2O5/IL NPs sputtered-decorated with ultra-small Pt NPs (1.0 ± 0.3 nm) as co-catalysts reached activities leading to even higher hydrogen production (9.2 H2 mmol g−1 h−1; apparent quantum yield of 22%). The calcined materials (with or without Pt NPs) showed much lower photocatalytic activity under the same reaction conditions (up to 2.8 mmol g−1 of H2). The remarkable activity of the hybrid-like Ta2O5/IL NPs may be related to the presence of the remaining IL that provides hydrophilic regions, facilitating the approach of polar molecules (water and alcohol) to the semiconductor active photocatalytic sites.

Mesoporous Foam TiO2 Nanomaterials for Effective Hydrogen Production

Hydrolysis of TiCl4 in a diether-functionalized imidazolium ionic liquid (IL), namely 1-methyl-3-[2-(2-methoxy(ethoxy)ethyl]imidazolium methane sulfonate (M(MEE)I⋅CH3 SO3 ), results in a heterostructured organic/inorganic and sponge-like porous TiO2 material. The thermal treatment (300 °C) followed by calcination (500 °C) affords highly porous TiO2 . The characterization of the obtained samples (with and without IL, before and after calcination) by XRD, SEM, and TEM reveals TiO2 anatase crystalline phases and irregular-shaped particles with different porous structures. These hierarchical-structured mesoporous TiO2 nanomaterials were employed as efficient photocatalysts in the water-splitting process, yielding up to 1304 μmol g(-1) on hydrogen production.