Thermal degradation studies of alkyl-imidazolium salts and their application in nanocomposites

Increasing the thermal stability of organically-modified layered silicates is one of the key points in the successful technical application of polymer-layered silicate nanocomposites on the industrial scale. To circumvent the detrimental effect of the lower thermal stability of alkyl ammonium-treated montmorillonite, a series of alkyl-imidazolium molten salts were prepared and characterized by elemental analysis, thermogravimetry (TGA) and thermal desorption mass spectroscopy (TDMS). The effect of counter ion, alkyl chain length and structural isomerism on the thermal stability of the imidazolium salts was investigated. Alkyl-imidazolium-treated montmorillonite clays were prepared by ion exchange of the imidazolium salts with Na-montmorillonite. These organically-modified clays were characterized by X-ray diffraction (XRD), TDMS and thermogravimetry coupled with Fourier transform infrared spectroscopy (TGA-FTIR), and compared to the conventional quaternary alkyl ammonium montmorillonite. Results indicate that the counter ion has an effect on the thermal stability of the imidazolium salts, and that imidazolium salts with PF₆⁻, N(SO₂CF₃)₂⁻ and BF₄⁻ anions are thermally more stable than the halide salts. A relationship was observed between the chain length of the alkyl group and the thermo-oxidative stability; as the chain length increased from propyl, butyl, decyl, hexadecyl, octadecyl to eicosyl, the stability decreased. The results also show that the imidazolium-treated montmorillonite has greater thermal stability compared to the imidazolium halide. Analysis of the decomposition products by FTIR provides an insight about the decomposition products which are water, carbon dioxide and hydrocarbons.     

Continuous Synthesis of Peralkylated Imidazoles and their Transformation into Ionic Liquids with Improved (Electro)Chemical Stabilities

A versatile and efficient method to synthesize tetrasubstituted imidazoles via a one‐pot modified Debus–Radziszewski reaction and their subsequent transformation into the corresponding imidazolium ionic liquids is reported. The tetrasubstituted imidazoles were also synthesized by means of a continuous flow process. This straightforward synthetic procedure allows for a fast and selective synthesis of tetrasubstituted imidazoles on a large scale. The completely substituted imidazolium dicyanamide and bis(trifluoromethylsulfonyl)imide salts were obtained via a metathesis reaction of the imidazolium iodide salts. The melting points and viscosities are of the same order of magnitude as for their non‐substituted analogues. In addition to the superior chemical stability of these novel ionic liquids, which allows them to be applied in strong alkaline media, the improved thermal and electrochemical stabilities of these compounds compared with conventional imidazolium ionic liquids is also demonstrated by thermogravimetrical analysis (TGA) and cyclic voltammetry (CV). Although increased substitution of the ionic liquids does not further increase thermal stability, a definite increase in cathodic stability is observable.     

Hydrophobic, Highly Conductive Ambient-Temperature Molten Salts

New, hydrophobic ionic liquids with low melting points (<-30 degrees C to ambient temperature) have been synthesized and investigated, based on 1,3-dialkyl imidazolium cations and hydrophobic anions. Other imidazolium molten salts with hydrophilic anions and thus water-soluble are also described. The molten salts were characterized by NMR and elemental analysis. Their density, melting point, viscosity, conductivity, refractive index, electrochemical window, thermal stability, and miscibility with water and organic solvents were determined. The influence of the alkyl substituents in 1, 2, 3, and 4(5)-positions on these properties was scrutinized. Viscosities as low as 35 cP (for 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)amide (bis(triflyl)amide) and trifluoroacetate) and conductivities as high as 9.6 mS/cm were obtained. Photophysical probe studies were carried out to establish more precisely the solvent properties of 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)amide). The hydrophobic molten salts are promising solvents for electrochemical, photovoltaic, and synthetic applications.     

Synthesis and properties of new tetrachlorocobaltate (II) and tetrachloromanganate (II) anion salts with dicationic counterions

New tetrachlorocobaltate (II) and tetrachloromanganate (II) ionic compounds containing various counterdications were synthesized and characterized. These salts are soluble in polar solvents such as methanol and water. Physical properties such as thermal stability, melting point, and magnetic susceptibility of these salts depend on the cation or anion structure. The thermal stability of the phosphinium or imidazolium based salts is higher than that of the pyridinium or triethylaminonium analogues. The melting point of these compounds is following the order of triphenylphosphinium > pyridinium > imidazolium dications, and symmetrical dicationic salts > unsymmetrical ones. The magnetic susceptibility (χ_MT values) of tetrachloromanganate (II) anions-based salts is higher than that of tetrachlorocobaltate (II) anions-based salts. These dicationic salts exhibit weak antiferromagnetic interactions and have higher magnetic susceptibility than that of the previously reported tetrachloromanganate (II) and tetrachlorocobaltate(II) salts with monocationic counterion.     

A comparative study of hydroxyl‐ and carboxylate‐functionalized imidazolium and benzimidazolium salts as precursors for N‐heterocyclic carbene ligands

The preparation of imidazolium and benzimidazolium salts with hydroxyl or carboxylate functions has been achieved using straightforward synthetic pathways. These salts in combination with palladium(II) acetate give active catalytic systems for Suzuki reaction. A comparative study has been performed, which has revealed that both the heterocycle and the functional group are important for the catalytic activity and stability of the catalyst. Copyright © 2015 John Wiley & Sons, Ltd.     

Amphiphilic properties of poly(oxyalkylene)amine-intercalated smectite aluminosilicates

Layered aluminosilicates, including synthetic fluorine mica and natural montmorillonite (MMT), were intercalated with poly(oxypropylene)-polyamine quaternary salts with a 230-5000 molecular weight range. The X-ray basal spacing of these silicates had been expanded from 13.5 to 83.7 A for the synthetic mica and to 92.0 A for MMT. The relative silicate dimensions (300-1000 nm for synthetic mica and 80-100 nm for MMT) were ascertained by direct TEM observations in the case of the co-intercalated synthetic mica and MMT mixtures with Mw = 2000 quaternary ammonium salts. The tailored organic incorporation of synthetic mica and MMT clays could alter these hydrophilic clays, making them amphiphilic, and enable the lowering of toluene/water interfacial tension to 2.0 mN/m at the critical concentration of 0.1 wt %.     

Syntheses and characterization of unsymmetric dicationic salts incorporating imidazolium and triazolium functionalities

A series of both imidazolium- and triazolium-based unsymmetric dicationic salts with alkyl and polyfluoroalkyl substituents were prepared and characterized. Most of them can be classified as ionic liquids (MP < 100 degrees C). Key physical properties, such as melting point, thermal stability, density, and solubility in common solvents were determined and were compared with those of the related monocationic imidazolium- or triazolium-based salts. The effects of anions and substituents bonded to the triazolium and imidazolium cations on these properties were examined. 1-(3-Butyl-imidazolium-1-yl)methylene-(4-butyl-1,2,4-triazolium diiodide) (2d), the precursor of 1-(3-butylimidazolium-1-yl)methylene-(4-butyl-1,2,4-triazolium) bi[bis(trifluoromethanesulfonyl)amide] (3d), reacted with Pd(OAc)(2) at 120 degrees C to generate a binuclear palladium(II) dicarbene complex. The palladium(II) complex was characterized by single-crystal X-ray diffraction analysis and was used as a catalyst precursor for palladium-catalyzed Heck cross-coupling reactions in 3d. Preliminary results show that 3d could serve as both the solvent and catalyst support in the catalytic reactions.     

Sulfur makes the difference: synthesis and mesomorphic properties of novel thioether-functionalized imidazolium ionic liquid crystals

Novel thioether-linked imidazolium ionic liquid crystals were synthesized starting from methyl 2-mercaptoacetate. The mesomorphic properties were determined by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction. All mesogens displayed smectic A mesophase geometries with strongly interdigitated bilayer structures. Comparison of the thioether-linked imidazolium salts with the corresponding amine- and amide-linked imidazolium salts as well as simple N-alkyl-imidazolium salts showed that both mesophase width and stability increased with increasing softness of the linking unit, thus indicating the beneficial effect of sulfur. Additionally, an increase of the length of the linking unit decreased the interdigitation of the alkyl chains.     

Gemini imidazolium amphiphiles for the synthesis, stabilization, and drug delivery from gold nanoparticles

Gold nanoparticles (AuNPs) are considered useful vehicles for medical therapy and diagnosis. Despite the progress made in this field, there is need to find direct, reliable, and versatile synthetic procedures for their preparation as well as new multifunctional coating agents. In this sense, we have explored the use of imidazolium amphiphiles to prepare new AuNPs designed for anion recognition and transport. Thus, in this work we describe (a) the synthesis, by a phase transfer method, of new gold nanoparticles using gemini-type surfactants as ligands based on imidazolium salts, those ligands acting as transfer agents into organic media and also as nanoparticle stabilizers, (b) the examination of their stability in solution, (c) the chemical and physical characterization of the nanoparticles, using a variety of techniques, including UV-visible spectroscopy (UV-vis), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), (d) toxicity data concerning both the imidazolium ligands and the imidazolium coated nanoparticles, (e) the assessment of their molecular recognition ability toward molecules of biological interest, such as anions and carboxylate containing model drugs, such as ibuprofen, (f) the study of their toxicity and those of their coating ligands, as well as their ability for cell internalization, and (g) the study of their ability for delivering anionic pharmaceuticals. The structurally governed triple role of those new gemini-type surfactants is responsible for the preparation, remarkable stability, and delivery properties of these functional AuNPs.     

Effects of organoclays on the thermal processing of pe/clay nanocomposites

Nanocomposites containing both polyethylene and montmorillonite clay organically modified with four different types of quaternary ammonium salts were obtained via direct melt intercalation. Thus, the main purpose of this work was to evaluate the effect of the organoclay on the thermal stability of polyethylene. The organoclays were characterized by XRD, FTIR, DSC and TG. The polyethylene/organoclay nanocomposites were studied by XRD, TEM, TG, besides an evaluation of their mechanical properties. The results showed that the salts were incorporated by intercalation between the layers of the organoclay and, apparently that the nanocomposites were more thermally stable than pure polyethylene.     

Click ionic liquids: a family of promising tunable solvents and application in Suzuki-Miyaura cross-coupling

A series of click ionic salts 4?a-4?n was prepared through click reaction of organic azides with alkyne-functionalized imidazolium or 2-methylimidazolium salts, followed by metathesis with lithium bis(trifluoromethanesulfonyl)amide or potassium hexafluorophosphate. All salts were characterized by IR, NMR, TGA, and DSC, and most of them can be classified as ionic liquids. Their steric and electronic properties can be easily tuned and modified through variation of the aromatic or aliphatic substituents at the imidazolium and/or triazolyl rings. The effect of anions and substituents at the two rings on the physicochemical properties was investigated. The charge and orbital distributions based on the optimized structures of cations in the salts were calculated. Reaction of 4?a with PdCl(2) produced mononuclear click complex 4?a-Pd, the structure of which was confirmed by single-crystal X-ray diffraction analysis. Suzuki-Miyaura cross-coupling shows good catalytic stability and high recyclability in the presence of PdCl(2) in 4?a. TEM and XPS analyses show formation of palladium nanoparticles after the reaction. The palladium NPs in 4?a are immobilized by the synergetic effect of coordination and electrostatic interactions with 1,2,3-triazolyl and imidazolium, respectively.     

Synthesis and X-ray crystal structure of fluorous imidazolium salts

Synthesis of two salts involving CH₂O spacer between the imidazole nitrogen and hexafluoroisopropyl group in the fluorous imidazolium cations is reported. Such an insertion would result in the formation of ??-ammonium ether. The two fluorous imidazolium salts involve one or two -CH₂OCH(CF₃)₂ groups attached to the imidazole nitrogen atoms. These products were synthesized from the reaction between methyimidazole and imidazole as nucleophiles and sevochlorane, ClCH₂OCH(CF₃)₂, as electrophile, in different molar ratios. The resulting products have been characterized by ¹H, ¹³C, and ¹⁹F NMR and FTIR spectroscopy. Also, the single crystal X-ray diffraction analysis for the symmetrically substituted imidazolium product is presented. The preliminary animal tests indicated no anesthetic property but the two tested salts were found to behave as calmative.     

7-Dialkylamino-1-alkylquinolinium salts: highly versatile and stable fluorescent probes

7-Dialkylamino- and 7-alkylsulfenyl-1-alkylquinolinium salts have been synthesized using a novel synthetic approach. The key intermediate, 7-fluoro-1-methylquinolinium iodide, was shown to possess high reactivity toward nitrogen and sulfur nucleophiles, and the kinetics of this nucleophilic aromatic substitution reaction was investigated. A wide variety of compounds were synthesized and characterized spectroscopically. High fluorescence quantum yields were observed, and this was attributed to the rigid molecular architecture. The thermal and photochemical stability of a number of compounds was investigated, and it was demonstrated that 7-dialkylamino-1-methylquinolinium salts have superior stability compared to a number of hemicyanine dyes and rigid charge-transfer probes. Based on the high quantum yields, the large Stokes shifts, and in particular, the high thermal and photochemical stability, it is concluded that 7-dialkylamino-1-methylquinolinium salts are excellent color-shifting, mobility-sensitive fluorescent probes for polymer characterization and other demanding applications.     

Electrostatic and Non‐covalent Interactions in Dicationic Imidazolium–Sulfonium Salts with Mixed Anions

A series of thioether‐functionalised imidazolium salts have been prepared and characterized. Subsequent reaction of the thioether‐functionalised imidazolium salts with iodomethane affords imidazolium–sulfonium salts composed of doubly charged cations and two different anions. Imidazolium–sulfonium salts containing a single anion type are obtained either by a solvent extraction method or by anion exchange. The imidazolium–sulfonium salts undergo a methyl‐transfer reaction on exposure to water, giving rise to a new, singly charged imidazolium salt with iodide introduced at the 2‐position of the imidazolium ring. Crystal structures of some of the imidazolium–sulfonium salts were determined by X‐ray crystallography providing the topology of the interactions between the dications and the anions. Electrospray ionization mass spectrometry and quantum‐chemical calculations were used to rationalise the relative strength of these interactions.     

Synthesis and characterization of ionic liquids incorporating the nitrile functionality

A series of imidazolium salts with the nitrile functional group attached to the alkyl side chain, viz. [CnCNmim][X] (where CnCNmim is the 1-alkylnitrile-3-methylimidazolium cation and Cn= (CH2)(n), n = 1-4; X = Cl, PF(6), and BF(4)) and [C3CNdmim][X] (where CnCNdmim is the 1-alkylnitrile-2,3-dimethylimidazolium cation and C(n) = (CH2)(n), n = 3; X = Cl, PF(6), and BF(4)), have been prepared and characterized using spectroscopic methods. The majority of the nitrile-functionalized imidazolium salts can be classed as ionic liquids since they melt below 100 degrees C. Four of the imidazolium salts have been characterized in the solid state using single-crystal X-ray diffraction analysis to reveal an extensive series of hydrogen bonds between H atoms on the cation and the anion. The relationship between the solid-state structure and the melting point is discussed. Key physical properties (density, viscosity, and solubility in common solvents) of the low melting ionic liquid have been determined and are compared with those of the related 1-alkyl-3-methylimidazolium and 1-alkyl-2,3-dimethylimidazolium ionic liquids. It was envisaged that these ionic liquids could act as both solvent and ligand for catalyzed reactions, and this application is demonstrated in hydrogenation reactions, which show that retention of the catalyst in the ionic liquid during product extraction is extremely high.     

Exfoliation of smectite clays by branched polyamines consisting of multiple ionic sites

Exfoliation through an ionic exchange reaction of layered silicate clays, including synthetic fluorinated mica (Mica) and natural montmorillonite (MMT), were achieved by using polyvalent amine salts as the intercalating agents. The requisite polyamine was synthesized from the epoxy/amine coupling reaction, involving a trifunctional poly(oxypropylene)-triamine (ca. 440 g/mol M_w) and diglycidyl ether of bisphenol-A. The polyamine was a mixture of oligomeric adducts consisting of multiple amine functionalities and a branched backbone. Partial acidification by HCl addition generated a series of amine salts that affected the intercalation and the expansion of the silicate interlayer in the range of 15.2-60.0 Å XRD d spacing. At the specific acidified ratio (H⁺/amine = 1/3 equiv ratio), the polyamine salts rendered the clay’s layered structure into randomization. The result was confirmed by using XRD and transmission electronic microscopy (TEM). The hybrids of polyamines and Mica or MMT were blended into epoxy resins and cured into nanocomposites, which exhibited the improvements of thermal stability and hardness.     

Functionalized Ionic (Poly)Styrenes and their Application as Catalysts in the Cycloaddition of CO2 to Epoxides

Valorization of CO₂, an abundant C(1) synthon, into platform chemicals is of growing interest. In this context, a key reaction is the cycloaddition of CO₂ to epoxides to form carbonates (CCE). A series of organocatalysts for this reaction, based on imidazolium salts modified with a styrene functional group and a second variable functional group, were prepared and characterized. Both the ionic salts and ionic polymers derived from the salts were evaluated in this reaction. In general, the monomers perform slightly better than the polymers; however, the latter are considerably more convenient to use. Of the catalysts studied, the ionic polystyrene imidazolium salt with an ammonium functionality shows highest activity, tolerates a range of epoxides, and can be recycled multiple times without loss of activity.     

Synthesis and application of chiral N-heterocyclic carbene-oxazoline ligands: iridium-catalyzed enantioselective hydrogenation

Two libraries of enantiomerically pure imidazolium salts bearing an oxazoline unit were synthesized. Deprotonation of the imidazolium salts and complexation of the resulting oxazoline-carbene ligands to iridium(I) was achieved in one step by mixing the imidazolium salts with NaOtBu and [(eta(4)-cod)IrCl](2) in THF at room temperature. The air-stable complexes were purified by flash chromatography. All complexes were analyzed by two-dimensional (2D) NMR methods and one compound from each family was characterized by X-ray structure analysis. The two libraries of iridium complexes were successfully tested in the asymmetric hydrogenation of unfunctionalized and functionalized olefins. Enantioselectivities of up to 90 % ee were obtained with trans-alpha-methylstilbene. Upon complexation of imidazolium salt 15 p with R(1) = phenyl, C-H bond activation of the phenyl ring gave rise to iridium(III) complex 17, which was fully characterized by NMR spectroscopy and X-ray structure analysis. Complex 17 proved to be catalytically inactive in the hydrogenation.     

β-环糊精功能化石墨烯的制备及热稳定性的增强

采用2种有机合成路线制备了结构不同的环糊精共价修饰的功能化石墨烯纳米材料,并利用FTIR,XRD,TEM,SEM和TG分析等技术对产物的结构和性能进行了表征.结果表明,2种石墨烯基纳米材料由于合成策略的不同导致溶剂分散性能存在一定的差别,但它们均可均匀分散于N,N-二甲基甲酰胺(DMF)、二甲基亚砜(DMSO)和乙二醇中.同时,环糊精的引入使其热学稳定性显著提高.该材料在阻燃型复合材料等领域中具有一定的潜在应用价值.     

Flexible,dicationic imidazolium salts for in situ application in palladium‐catalysed Mizoroki–Heck coupling of acrylates under aerobic conditions

The synthesis, characterization and in situ catalytic performance of new unsymmetric N,N′‐disubstituted imidazolium‐based dicationic salts in Mizoroki–Heck coupling of acrylates with aryl bromides under aerobic conditions are described. A series of flexible dicationic salts with varying steric and electronic properties were synthesized in good to excellent yields. All the salts were well characterized using spectroscopic techniques. X‐ray diffraction analysis of two salts with the same dicationic backbone and different counter anions shows that the ligand adopts two different conformations which are influenced by the nature of the anion. Thus, the ligand is capable of changing its conformation according to the change in environment due to its flexible nature. All the synthesized imidazolium salts were found to be active in in situ palladium‐catalysed Mizoroki–Heck coupling under aerobic conditions. Amongst the salts, the hydroxyl‐functionalized imidazolium salt, incorporating the features of both bidentate chelating O,O ligand and carbene, shows the maximum catalytic activity. A variety of aryl and heteroaryl methyl and ethyl cinnamates were synthesized using these imidazolium salts as preligands. In addition, NMR studies confirm in situ generation of normal N‐heterocyclic carbenes from the C‐2 position of imidazol‐2‐ylidene ring. The mercury poisoning test was also performed to ascertain the nature of catalytically active palladium species. Aerobic conditions, low catalytic loading (0.5 mol%), shorter reaction times, broad functional group tolerance and good to excellent isolated yields are some of the significant features of the novel catalytic systems described here. Copyright © 2016 John Wiley & Sons, Ltd.