Ionic liquids complex formation

MIXTURES OF SIMPLE IONIC LIQUIDS COMPLEX FORMATION... 

Mixtures of Simple Ionic Liquids Complex Formation. ... 

The formation of NHC complexes is clearly going to be important to the palladium chemistry seen in ionic liquids. The formation of such complexes has been seen to be advantageous (as in the Suzuki reaction) or disadvantageous (as in the telomerization of butadiene with methanol), depending on the particular system being investigated. What is yet to be seen is if the ionic liquid can be manipulated to generate NHC s with particular useful properties. 

Efforts have been made to explain the high rate acceleration of Mizoroki-Heck reactions in ionic liquids. The formation of the dialkylimidazol-2-ylidene palladium complex under conditions similar to those employed for the Mizoroki-Heck reaction has been studied. The C2-H proton of the imidazolium cation exhibits high acidity and can be deprotonated to form a carbene species, behaving as a good ligand for transition metals. Therefore, in the presence of a palladium salt and a base, [bmim][Br] formed the dimeric carbene complex 89, which further evolved to the monomeric c/x-90 and trans-9Q complexes. Each of these exists as an anti and a syn rotamer owing to the sterically demanding (V-alkyl substituents (Scheme 35 only the anti-90 rotamers are represented). 

Reichert WM, Holbrey JD, Vigour KB et al (2006) Approaches to crystallization from ionic liquids complex solvents-complex results, or, a strategy for controlled formation of new supramolecular architectures Chem Commun 4767—4779... 

A number of different methods to monitor the amount of methylimidazole left in a final ionic liquid are known. NMR spectroscopy is used by most academic groups, but may have a detection limit of about 1 mol%. The photometric analysis described by Holbrey, Seddon, and Wareing has the advantage of being a relatively quick method that can be performed with standard laboratory equipment [13]. This makes it particularly suitable for monitoring of the methylimidazole content during commercial ionic liquid synthesis. The method is based on the formation and colorimetric analysis of the intensely colored complex of l-methylimidazole with cop-per(II) chloride. 

The thermal behavior of many ionic liquids is relatively complex. For a typical IL, cooling from the liquid state causes glass formation at low temperatures solidifica-... 

In pyridinium chloride ionic liquids and in l,2-dimethyl-3-hexylimida2olium chloride ([HMMIMjCl), where the C(2) position is protected by a methyl group, only [PdClJ was observed, whereas in [HMIMjCl, the EXAFS showed the formation of a bis-carbene complex. In the presence of triphenylphosphine, Pd-P coordination was observed in all ionic liquids except where the carbene complex was formed. During the Heck reaction, the formation of palladium was found to be quicker than in the absence of reagents. Overall, the EXAFS showed the presence of small palladium clusters of approximately 1 nm diameter formed in solution. 

The first reaction pathway for the in situ formation of a metal-carbene complex in an imidazolium ionic liquid is based on the well loiown, relatively high acidity of the H atom in the 2-position of the imidazolium ion [29]. This can be removed (by basic ligands of the metal complex, for example) to form a metal-carbene complex (see Scheme 5.2-2, route a)). Xiao and co-workers demonstrated that a Pd imida-zolylidene complex was formed when Pd(OAc)2 was heated in the presence of [BMIMjBr [30]. The isolated Pd carbene complex was found to be active and stable in Heck coupling reactions (for more details see Section 5.2.4.4). Welton et al. were later able to characterize an isolated Pd-carbene complex obtained in this way by X-ray spectroscopy [31]. The reaction pathway to the complex is displayed in Scheme 5.2-3. 

Another means of in situ metal-carbene complex formation in an ionic liquid is the direct oxidative addition of the imidazolium cation to a metal center in a low oxidation state (see Scheme 5.2-2, route b)). Cavell and co-workers have observed oxidative addition on heating 1,3-dimethylimidazolium tetrafluoroborate with Pt(PPli3)4 in refluxing THF [32]. The Pt-carbene complex formed can decompose by reductive elimination. Winterton et al. have also described the formation of a Pt-car-bene complex by oxidative addition of the [EMIM] cation to PtCl2 in a basic [EMIM]C1/A1C13 system (free CP ions present) under ethylene pressure [33]. The formation of a Pt-carbene complex by oxidative addition of the imidazolium cation is displayed in Scheme 5.2-4. 

The use of imidazolium-based ionic liquids in Pd-catalyzed Heck reactions always carries with it the possibility of in situ formation of Pd-carbene complexes (for more details see Section 5.2.2.3). The formation of these under the conditions of the Heck reaction was confirmed by investigations by Xiao et al. [30], who described a significantly enhanced reactivity of the Heck reaction in [BMIM]Br in relation to the same reaction in [BMIM][Bp4] and explained this difference by the fact that formation of Pd-carbene complexes was observed only in the bromide melt. 

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

Some halogenometalate species have been observed to have formed spontaneously during spectroelectrochemical studies in ionic liquids. For example, [MoCl ] (which is hydrolyzed in water, is coordinated by solvent in polar solvents, and has salts that are insoluble in non-polar solvents) can only be observed in basic (X(A1C13) < 0.5 chloroaluminate ionic liquids [1]. FFowever, this work has been directed at the measurement of electrochemical data, rather than exploitation of the ionic liquids as solvents for synthesis [2]. It has been shown that the tetrachloroa-luminate ion will act as a bidentate ligand in acidic X(A1C13) > 0.5 chloroaluminate ionic liquids, forming [M(AlCl4)3] ions [3]. This was also the result of the spontaneous formation of the complexes, rather than a deliberate attempt to synthesize them. 

The only reports of directed synthesis of coordination complexes in ionic liquids are from oxo-exchange chemistry. Exposure of chloroaluminate ionic liquids to water results in the formation of a variety of aluminium oxo- and hydroxo-contain-ing species [4]. Dissolution of metals more oxophilic than aluminium will generate metal oxohalide species. FFussey et al. have used phosgene (COCI2) to deoxochlori-nate [NbOa5] - (Scheme 6.1-1) [5]. 

Olefin dimerisation with Ni-NHC complexes became a topic of interest following reports of Ni(II) phosphine complexes being employed in imidazolium-based ionic liquid solvents [23, 24]. It had previonsly been established that aIkyl-Ni(II) complexes containing NHC ligands can rapidly decompose via imidazolium formation (Scheme 4.1) [5], and it was thus of interest to explore the effect that an excess of the imidazolinm cation would have on this reaction. 

A typical special feature of the melts of ionic crystals (ionic liquids) are their high concentrations of free ions, of about 25 M. Because of the short interionic distances, considerable electrostatic forces act between the ions, so that melts exhibit pronounced tendencies for the formation of different ionic aggregates ion pairs, triplets, complex ions, and so on. 

Besides the advantage of recyclability, reactions in ionic liquids are generally faster and are run under milder conditions than reactions with conventional solvents. Further activation may come from ultrasonic agitation.520 Since the majority of ionic liquids used are imidazolium salts, the effect of these solvents can be at least partly attributed to the in situ formation of carbene complexes (Section 9.6.3.4.10).521 Cross-coupling of ArB(OH)2 can also be efficiently performed in ionic liquids based on long-chain tetraalkylphosphonium salts, in which case aryl bromides and some aryl chlorides can be processed in the presence of the trivial ligand PPh3.522... 

The possibility of adjusting acidity/coordination properties opens up a wide range of possible interactions between the ionic liquid solvent and the dissolved transition metal complex. Depending on the acidity/coordination properties of the anion and on the reactivity of the cation (the possibility of carbene ligand formation from 1,3-dialkylimidazolium salts is of particular importance here [37]), the ionic liquid can be regarded as an innocent solvent, as a ligand precursor, as a co-catalyst or as the catalyst itself. 

Recently, van Leeuwen and co-workers provided support for the similarity between the active catalytic species in ionic liquids and in organic solvents by spectroscopic investigations [80]. These authors compared the complex formation of [Rh(acac)(CO)2] in the presence of 4 equivalents of sulfoxantphos ligand dissolved in [BMIM][PF6] and... 

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. 

The previous chapters have demonstrated that liquid-liquid extraction is a mass transfer unit operation involving two liquid phases, the raffinate and the extract phase, which have very small mutual solubihty. Let us assume that the raffinate phase is wastewater from a coke plant polluted with phenol. To separate the phenol from the water, there must be close contact with the extract phase, toluene in this case. Water and toluene are not mutually soluble, but toluene is a better solvent for phenol and can extract it from water. Thus, toluene and phenol together are the extract phase. If the solvent reacts with the extracted substance during the extraction, the whole process is called reactive extraction. The reaction is usually used to alter the properties of inorganic cations and anions so they can be extracted from an aqueous solution into the nonpolar organic phase. The mechanisms for these reactions involve ion pah-formation, solvation of an ionic compound, or formation of covalent metal-extractant complexes (see Chapters 3 and 4). Often formation of these new species is a slow process and, in many cases, it is not possible to use columns for this type of extraction mixer-settlers are used instead (Chapter 8). 

A limitation of aluminum-containing ionic liquids arises from their moisture sensitivity. Moreover, most transition metal complexes and organic reactants are not unreactive in the presence of the chloroaluminate compounds. These ionic liquids react with water in a highly exothermic manner, with the formation of hydrogen chloride and a white precipitate. 

The strong affinity of ionic liquids for aromatics has been attributed to the formation of liquid clathrates (90-92). Liquid clathrates (93) are semi-ordered liquids containing complex salt hosts. They are formed by associative interactions between aromatic molecules and salt ions, which separate cation-anion-packing interactions to a sufficient degree that localized cage structures are formed. Although the aromatic compounds are highly soluble in the ionic liquid phase, the... 

The carbene complexes can also be formed by direct oxidative addition of ze-rovalent metal to an ionic liquid. The oxidative addition of a C-H bond has been demonstrated by heating [MMIM]BF4 with Pt(PPh3)4 in THF, resulting in the formation of a stable cationic platinum carbene complex (Scheme 15) (189). An effective method to protect this carbene-metal-alkyl complex from reductive elimination is to perform the reaction with an imidazolium salt as a solvent. 

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