In Situ Formation of Mixed Phosphine−Imidazolylidene Palladium Complexes in Room-Temperature Ionic Liquids

Ionic Liquids in Metal, Photo-, Electro-, and (Bio) Catalysis

Ionic liquids (ILs) have unique physicochemical properties that make them advantageous for catalysis, such as low vapor pressure, non-flammability, high thermal and chemical stabilities, and the ability to enhance the activity and stability of (bio)catalysts. ILs can improve the efficiency, selectivity, and sustainability of bio(transformations) by acting as activators of enzymes, selectively dissolving substrates and products, and reducing toxicity. They can also be recycled and reused multiple times without losing their effectiveness. ILs based on imidazolium cation are preferred for structural organization aspects, with a semiorganized layer surrounding the catalyst. ILs act as a container, providing a confined space that allows modulation of electronic and geometric effects, miscibility of reactants and products, and residence time of species. ILs can stabilize ionic and radical species and control the catalytic activity of dynamic processes. Supported IL phase (SILP) derivatives and polymeric ILs (PILs) are good options for molecular engineering of greener catalytic processes. The major factors governing metal, photo-, electro-, and biocatalysts in ILs are discussed in detail based on the vast literature available over the past two and a half decades. Catalytic reactions, ranging from hydrogenation and cross-coupling to oxidations, promoted by homogeneous and heterogeneous catalysts in both single and multiphase conditions, are extensively reviewed and discussed considering the knowledge accumulated until now.

Supported Ionic Liquid Phase (SILP) Allylic Alkylation of Amines in Continuous Flow

We present the use of Pd-complex-containing supported ionic liquid phases (SILPs) as a novel approach for continuous-flow allylic alkylation of N-nucleophiles. This immobilization strategy gave simple access to air-tolerating catalyst frameworks, providing rapid and convenient access to various achiral and chiral N-allylation products. Under optimized conditions, the flow-reaction could be maintained for 3.5 hours with constant product output; meanwhile, only a marginal 0.7 wt % of ionic liquid leaching and no detectable palladium-complex leaching could be observed.

Application of Coordination Compounds with Transition Metal Ions in the Chemical Industry-A Review

This publication presents the new trends and opportunities for further development of coordination compounds used in the chemical industry. The review describes the influence of various physicochemical factors regarding the coordination relationship (for example, steric hindrance, electron density, complex geometry, ligand), which condition technological processes. Coordination compounds are catalysts in technological processes used during organic synthesis, for example: Oxidation reactions, hydroformylation process, hydrogenation reaction, hydrocyanation process. In this article, we pointed out the possibilities of using complex compounds in catalysis, and we noticed what further research should be undertaken for this purpose.

Ionic liquids: a brief history

There is no doubt that ionic liquids have become a major subject of study for modern chemistry. We have become used to ever more publications in the field each year, although there is some evidence that this is beginning to plateau at approximately 3500 papers each year. They have been the subject of several major reviews and books, dealing with different applications and aspects of their behaviours. In this article, I will show a little of how interest in ionic liquids grew and developed.

Thermodynamic properties of selenoether-functionalized ionic liquids and their use for the synthesis of zinc selenide nanoparticles

Three selenoether-functionalized ionic liquids (ILs) of N-[(phenylseleno)methylene]pyridinium (1), N-(methyl)- (2) and N-(butyl)-N'-[(phenylseleno)methylene]imidazolium (3) with bis(trifluoromethanesulfonyl)imide anions ([NTf2]) were prepared from pyridine, N-methylimidazole and N-butylimidazole with in situ obtained phenylselenomethyl chloride, followed by ion exchange to give the desired compounds. The crystal structures of the bromide and tetraphenylborate salts of the above cations (1-Br, 2-BPh4 and 3-BPh4) confirm the formation of the desired cations and indicate a multitude of different supramolecular interactions besides the dominating Coulomb interactions between the cations and anions. The vaporization enthalpies of the synthesized [NTf2]-containing ILs were determined by means of a quartz-crystal microbalance method (QCM) and their densities were measured with an oscillating U-tube. These thermodynamic data have been used to develop a method for assessment of miscibility of conventional solvents in the selenium-containing ILs by using Hildebrandt solubility parameters, as well as for modeling with the electrolyte perturbed-chain statistical associating fluid theory (ePC-SAFT) method. Furthermore, structure-property relations between selenoether-functionalized and similarly shaped corresponding aryl-substituted imidazolium- and pyridinium-based ILs were analyzed and showed that the contribution of the selenium moiety to the enthalpy of vaporization of an IL is equal to the contribution of a methylene (CH2) group. An incremental approach to predict vaporization enthalpies of ILs by a group contribution method has been developed. The reaction of these ILs with zinc acetate dihydrate under microwave irradiation led to ZnSe nanoparticles of an average diameter between 4 and 10 nm, depending on the reaction conditions.

Evidence for the spontaneous formation of N-heterocyclic carbenes in imidazolium based ionic liquids

We present a study of the reactions of aldehydes in ionic liquids which gives evidence for the spontaneous formation of N-heterocyclic carbenes in ionic liquids based on 1,3-dialkyl substituted imidazolium cations from the lack of a deuterium isotope effect on the reaction of these ionic liquids with aldehydes.

Metal Nanoparticles in Ionic Liquids

During the last years ionic liquids (ILs) were increasingly used and investigated as reaction media, hydrogen sources, catalysts, templating agents and stabilizers for the synthesis of (monometallic and bimetallic) metal nanoparticles (M-NPs). Especially ILs with 1,3-dialkyl-imidazolium cations featured prominently in the formation and stabilization of M-NPs. This chapter summarizes studies which focused on the interdependencies of the IL with the metal nanoparticle and tried to elucidate, for example, influences of the IL-cation, -anion and alkyl chain length. Qualitatively, the size of M-NPs was found to increase with the size of the IL-anion. The influence of the size of imidazolium-cation is less clear. The M-NP size was both found to increase and to decrease with increasing chain lengths of the 1,3-dialkyl-imidazolium cation. It is evident from such reports on cation and anion effects of ILs that the interaction between an IL and a (growing) metal nanoparticle is far from understood. Factors like IL-viscosity, hydrogen-bonding capability and the relative ratio of polar and non-polar domains of ILs may also influence the stability of nanoparticles in ionic liquids and an improved understanding of the IL-nanoparticle interaction would be needed for a more rational design of nanomaterials in ILs. Furthermore, thiol-, ether-, carboxylic acid-, amino- and hydroxyl-functionalized ILs add to the complexity by acting also as coordinating capping ligands. In addition imidazolium cations are precursors to N-heterocyclic carbenes, NHCs which form from imidazolium-based ionic liquids by in situ deprotonation at the acidic C2-H ring position as intermediate species during the nanoparticle seeding and growth process or as surface coordinating ligand for the stabilization of the metal nanoparticle.

Highly efficient and recyclable copper based ionic liquid catalysts for amide synthesis

Copper based ionic liquids have been successfully prepared for the synthesis of primary, secondary and tertiary amides under mild reaction conditions.

Directly probing the effect of the solvent on a catalyst electronic environment using X-ray photoelectron spectroscopy

The anion of an ionic liquid can significantly influence the electronic environment of a metal centre, and thus impact upon reaction performance in a model Suzuki cross coupling reaction.

Opening an Aladdin's cave: the Suzuki coupling in a room-temperature ionic liquid

This viewpoint will discuss the impact of the 2000 Chem. Commun. report that the palladium-catalysed Suzuki coupling can be performed quickly and easily in a room-temperature ionic liquid. This work has helped fuel what have now become whole areas of chemistry of their own: the use of ionic liquids as reaction media for catalytic transformations, and the application of palladium complexes bearing N-heterocyclic carbene (NHC) ligands in the Suzuki coupling and beyond.

Charge-tagged ligands: useful tools for immobilising complexes and detecting reaction species during catalysis

A critical overview is presented on the use of charged tagged ligands (CTLs) as immobilising agents in organometallic catalysis and as probes for studying mechanisms through electrospray ionisation mass spectrometry (ESI-MS) based on the most recent literature.

In recent years, charge-tagged ligands (CTLs) have become valuable tools in organometallic catalysis. Insertion of an ionic side chain into the molecular skeleton of a known ligand has become a useful protocol for anchoring ligands, and consequently catalysts, in polar and ionic liquid phases. In addition, the insertion of a cationic moiety into a ligand is a powerful tool that can be used to detect reaction intermediates in organometallic catalysis through electrospray ionisation mass spectrometry (ESI-MS) experiments. The insertion of an ionic tag ensures the charge in the intermediates independently of the ESI-MS. For this reason, these ligands have been used as ionic probes in mechanistic studies for several catalytic reactions. Here, we summarise selected examples on the use of CTLs as immobilising agents in organometallic catalysis and as probes for studying mechanisms through ESI-MS.

Catalyst life in imidazolium-based ionic liquids for palladium-catalysed asymmetric allylic alkylation

Highlighting the reactivity of palladium species in catalytic asymmetric allylic alkylation using imidazolium-based ionic liquids.

Structure-Property Relations in Ionic Liquids: 1,2,3-Trimethyl-imidazolium and 1,2,3-Trimethyl-benzimidazolium bis-(trifluorsulfonyl)imide

Vaporization enthalpies for a pairs of the ionic liquids (ILs) 1,2,3-trimethyl-imidazolium and 1,2,3-trimethyl-benzimidazolium bistrifluorsulfonylimide and the corresponding molecular liquids 1,2-dimethyl-imidazole and 1,2-dimethyl-benzimidazole were determined using a quartz crystal microbalance method and TGA (for ILs) and the transpiration method (for molecular compounds). Differences in vaporization enthalpies in molecular and ionic liquids are discussed in context of structure-property relations.

Effect of ionic liquid impurities on the synthesis of silver nanoparticles

Imidazolium-based ionic liquids have been widely utilized as versatile solvents for metal nanoparticle synthesis; however, reactions to synthesize silver nanoparticles that are performed identically in different commercially obtained lots of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF(4)) give divergent results. This suggests that impurities in these nominally identical solvents play an important role in the resulting silver nanoparticle quality. To test the effect that impurities have on the quality of silver nanoparticles synthesized in BMIM-BF(4), silver nanoparticles were synthesized in carefully prepared and purified BMIM-BF(4) and compared against silver nanoparticles that were synthesized in the purified BMIM-BF(4) that had been spiked with trace amounts of water, chloride, and 1-methylimidazole. It was clearly demonstrated that trace amounts of these common ionic liquid impurities cause significant deviation in size and shape (creating polydisperse and irregularly shaped ensembles of both large and small particles), and also negatively impact the stabilization of the resulting silver nanoparticles.

Pyridine Mediated Palladium Acetate-Catalysed Dehalogenation of Biaryl Halides in Ionic Liquid System

The palladium acetate-catalysed dehalogenation of biaryl halides has been investigated in room temperature ionic liquids with pyridine acting as a co-solvent. The significant effect of pyridine in conjunction with Pd(OAc)2 in [bmim][PF6] gave excellent results in the dehalogenation of biaryl iodines and bromides. The pyridine activated the C2-H of the imidazolium salt to be the source of hydrogen. The separation of desired product was easily performed by extraction with diethyl ether and the Pd(OAc)2–[bmim][PF6] system could be recycled five times with only a small decrease in activity. This procedure represents an environmentally friendly method for the preparation of biaryl compounds without the use of complex ligands.

Palladium containing periodic mesoporous organosilica with imidazolium framework (Pd@PMO-IL): an efficient and recyclable catalyst for the aerobic oxidation of alcohols

The application of a novel palladium containing ionic liquid based periodic mesoporous organosilica (Pd@PMO-IL) catalyst in the aerobic oxidation of primary and secondary alcohols under molecular oxygen and air atmospheres is investigated. It was found that the catalyst is quite effective for the selective oxidation of several activated and non-activated alcoholic substrates. The catalyst system could be successfully recovered and reused several times without any significant decrease in activity and selectivity. Moreover, the hot filtration test, atomic absorption spectroscopy (AA) and kinetic study with and without selective catalyst poisons showed that the catalyst works in a heterogeneous pathway without any palladium leaching in reaction solution. Furthermore, nitrogen-sorption experiment and transmission electron microscopy (TEM) image proved the superior stability of high-ordered PMO-IL mesostructure during reaction process. TEM image also confirmed the presence of well-distributed Pd-nanoparticles in the uniform mesochannels of the material. These observations can be attributed to the ionic liquid nature of PMO-IL mesostructure which facilitates the reaction through production, chemical immobilization and stabilization of active palladium nanoparticles, as well as preventing Pd-agglomeration during overall process.

Metal Complexes of an Ionic Liquid-Derived Carbene

A range of metal carbene complexes containing the ionic liquid-derived N-heterocyclic carbene (NHC) 1-nbutyl-3-methylimidazol-2-ylidene (IBuMe, 1) have been prepared by (i) direct ligand substitution using the free NHC ([Mo(CO)5(IBuMe)] 2), (ii) transmetallation using the silver salt [AgCl(IBuMe)] (3) ([RhCl(NBD)(IBuMe)] (4) and [IrCl(COD)(IBuMe)] (5), NBD = 2,5-norbornadiene, COD = 1,5-cyclooctadiene) and (iii) direct reaction of a metal acetate with the hydrochloride salt of 1 (trans-[PdCl2(IBuMe)] (6)). The dicarbonyl cis-[RhCl(CO)2(IBuMe)] (7) has been prepared by diene substitution under a carbon monoxide atmosphere. The molecular structures of 2, 4, 5 and 6 are reported and the sigma donation and steric properties of 1 are discussed relative to those of common imidazol-2-ylidene ligands.

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.