Binary Mixtures of Ionic Liquids: A Joint Approach to Investigate their Properties and Catalytic Ability

Enhanced oxidative depolymerization of lignin in cooperative imidazolium-based ionic liquid binary mixtures

The aerobic oxidation of lignin model 2-phenoxyacetophenone (2-PAP) in cooperative ionic liquid mixtures (CoILs) with 1-ethyl-3-methylimidazolium acetate ([C₂C₁im]OAc) and 1-benzyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([B_ZC₁im]NTf₂) was investigated. Complete degradation of 2-PAP was achieved with [C₂C₁im]OAc/[B_ZC₁im]NTf₂ molar ratio (R_IL) of 1/1 and 1/2 at 100 °C for 2 h. The conversion and product yields from CoILs were higher than those in pure ILs, indicating the cooperative effects of [C₂C₁im]OAc/[B_ZC₁im]NTf₂ on cleaving aryl-ether bonds. [C₂C₁im]OAc promoted the catalytic cleavage of aryl-ether bonds and solvation, and [B_ZC₁im]NTf₂ induced the formation of alkyl radicals and enhanced the product selectivity. Accordingly, the highest conversion of alkali lignin (79.8%) was obtained with R_IL of 5/1 at 100 °C for 2 h, and phenol monomers (306 mg/g) were selectively produced. The CoILs exhibited good catalytic capacities for oxidative depolymerization of lignin, which strongly depends on the changes in intermolecular interactions and structural organization with varying R_IL.

Binary Mixtures of Imidazolium-Based Protic Ionic Liquids. Extended Temperature Range of the Liquid State Keeping High Ionic Conductivities

Binary mixtures based on the two protic ionic liquids 1-ethylimidazolium triflate ([C2HIm][TfO]) and 1-ethylimidazolium bis(trifluoromethanesulfonyl)imide ([C2HIm][TFSI]) have been investigated, with focus on phase behavior, ionic conductivity, and intermolecular interactions as a function of composition (χTFSI indicating the mole fraction of the added compound). It is found that on addition of [C2HIm][TFSI] to [C2HIm][TfO], the melting temperature is first decreased (0 <χ≤ 0.3) and then suppressed (0.3 <χ≤ 0.8) resulting in mixtures with no phase transitions. These mixtures display a wide temperature range of the liquid state and should be interesting for use in devices operating at extreme temperatures. The ionic conductivity does not vary significantly across the composition range analyzed, as evidenced in the comparative Arrhenius plot. The activation energy, Ea, estimated by fitting with the Arrhenius relation in a limited temperature range (between 60 and 140 °C) varies marginally and keeps values between 0.17 and 0.21 eV. These marginal differences can be rationalized by the initially very similar values of the two neat protic ionic liquids. Vibrational spectroscopy, including both Raman and infrared spectroscopies, reveals weakening of the cation–anion interactions for increasing content of [C2HIm][TFSI], which is reflected by the blue shift of the average N-H stretching mode and the red shift of the S-O stretching mode in the TfO anion. These trends correlate with the higher disorder in the mixtures observed by DSC and are evidenced by the decrease and suppression of the melting temperature as the amount of [C2HIm][TFSI] is increased.

Highly-fluorinated Triaminocyclopropenium Ionic Liquids

A series of highly‐fluorinated triaminocyclopropenium salts, with up to six fluorous groups, were prepared and their properties as ionic liquids investigated. Reaction of pentachlorocyclopropane or tetrachlorocyclopropene with bis(2,2,2‐trifluoroethyl)amine, HN(CH₂CF₃)₂, occurs in the presence of a trialkylamine, NR₃, to give cations with two fluorinated amino groups, [C₃(N(CH₂CF₃)₂)₂(NR₂)]⁺ (R=Et, Pr, Bu, Hex), with traces of [C₃(N(CH₂CF₃)₂)₃]⁺. Use of appropriate reagent ratios and reaction times and subsequent addition of a dialkylamine, HNR'R”, gives cations with one fluorinated amino group, [C₃(N(CH₂CF₃)₂)(NR₂)(NR'R”)]⁺ ((NR₂)(NR'R”)=(NBu₂)₂, (NEt₂)(NPr₂), (NBu₂)(NBuMe)). These cations were isolated as chloride salts and some of these were converted to bistriflamide, dicyanamide and triflate salts to provide ionic liquids. These salts were characterised by thermal (DSC and TGA) measurements and miscibility/solubility properties (determined in a range of solvents). Ionic liquids (ILs) were also characterised by density, viscosity and conductivity measurements where possible. X‐ray diffraction studies of chloride salts showed the formation of fluorous regions and more hydrophilic ionic regions in the solid state.

Ionic liquids: "normal" solvents or nanostructured fluids?

Ionic liquids (ILs) are a class of non-conventional solvents, which, for almost two decades, have continued to generate burgeoning interest in different fields of present-day chemical research with few similar precedents. Among the various aspects related to ILs, a topic worthy of in-depth analysis is their influence on organic reactivity and reaction rates. In light of this, the present short review aims to provide an overview of the literature from 2010 to the present day that addresses this issue. In particular, we herein present two main different viewpoints by which the solvent effect of ILs is explained: the first is mainly based on considering the bulk polarity of ILs and linear solvation energy relationships, while the other treats ILs as nanostructured fluids. In both cases, studies dealing with IL mixtures are also covered. Finally, literature addressing the area of supramolecular catalysis "by" or "in" ILs is also reported. This is one of the few reviews covering these specific aspects, aiming to provide a useful framework to guide future research into the effects of ILs on organic reactivity.

Unexpected Substituent Effects in the Iso-Heterocyclic Boulton-Katritzky Rearrangement of 3-Aroylamino-5-methyl-1,2,4-oxadiazoles: A Mechanistic Study

The kinetics of the iso-heterocyclic mononuclear rearrangement of some 3-aroylamino-5-methyl-1,2,4-ozadiazoles was carefully examined under largely variable acidic or alkaline conditions. This reaction may proceed via two different mechanistic pathways (an uncatalyzed and a base-catalyzed one), as accounted for also by the evaluation of the relevant activation parameters. Substituent effects, as quantified by means of the Hammett's equation, appear relatively modest; however, they reveal some interesting anomalies, which enabled us to draw a very precise picture of the intimate reaction course.

Ultraslow Relaxation in Aprotic Double Salt Ionic Liquids

A mixture of two pure ionic liquids (ILs) or double salt ILs (DSILs) can push the limits of ILs in terms of unraveling their unique physicochemical properties and potential in clean technology. While the correlated ion dynamics and heterogeneity in the bulk of pure ILs have been reported, such a phenomenon at longer timescales in DSILs has never been elucidated. Here, a combination of temperature-dependent polarized dynamic light scattering and rheological measurements has been employed to reveal the presence of structural and ultraslow relaxation in three DSILs, each containing a 1-ethyl-3-methylimidazolium cation and two different anions. The slow relaxation caused by Brownian diffusion of cluster-like arrangements occurs at a timescale of a few to several hundred milliseconds; both the relaxation processes, nevertheless, are Arrhenius in nature. Notably, slow relaxation in the DSILs is much different compared to that in the pure ILs. The decay of intensity correlation functions (ICFs) and average hydrodynamic correlation length of the clusters and their response to temperature markedly vary with the nature of the two anions present in the DSILs. Stretched exponential analyses of the ICFs disclose the cluster-to-cluster transfer of ionic species as well as percolation dynamics among clusters. The identity of anions also governs whether the DSILs follow or violate the Stokes-Einstein relationship or not.

The physicochemical properties of a room-temperature liquidus binary ionic liquid mixture of [HNMP][CH3SO3]/[Bmim]Cl and its application for fructose conversion to 5-hydroxymethylfurfural

Via heating first and then cooling, binary ionic liquid (IL) mixture of N-methyl-2-pyrrolidonium methylsulfonate ([HNMP][CH3SO3]) and 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) could form a liquid at room temperature. The glass-transition temperature (T g) characterized by DSC depends on its composition with T g being as low as -63 °C. The physicochemical properties of the binary IL mixtures also vary with the composition. With the increase of the mole fraction of [Bmim]Cl, the hydrogen-bond accepting ability (β) of the binary IL mixture increases, but the hydrogen-bond donating ability (α) deceases. In this binary IL mixture, fructose could be effectively converted into 5-hydroxymethylfurfural (HMF) at room temperature. The HMF yields at a given time are found to be well correlated with the physicochemical properties of the binary mixture, especially the α and β values. Under specified conditions, the present IL mixture as medium for fructose dehydration into HMF is comparable to the medium formed by ILs and alcohol, where the alcohols have negative effect on the HMF formation.

Phase equilibrium and physical properties of biobased ionic liquid mixtures

Protic ionic liquid crystals (PILCs) obtained from natural sources are promising compounds due to their peculiar properties and sustainable appeal.

Iso-solvation effects in mixtures of ionic liquids on the kinetics of a model SNAr reaction

The S_NAr reaction between 1-chloro-2,4-dinitrobenzene and morpholine was used as a model system to study solvation effects in a series of mixtures involving imidazolium based ionic liquids.

Ionic liquid binary mixtures: how different factors contribute to determine their effect on the reactivity

We studied how mixing ionic liquids affected the rate of the Diels Alder reaction between 9-anthracenemethanol and N-ethylmaleimide.

A structural investigation of ionic liquid mixtures

The role of hydrogen bonding, π⁺–π⁺ stacking and anion–π⁺ interactions on the structure of ionic liquid mixtures has been elucidated through a combined theoretical and experimental approach.

Probing structural patterns of ion association and solvation in mixtures of imidazolium ionic liquids with acetonitrile by means of relative1H and13C NMR chemical shifts

Competition between ion solvation and association in mixtures of imidazolium ionic liquids and molecular solvents can be systematically addressed by the analysis of relative chemical shift variation with mixture composition.

Properties modification by eutectic formation in mixtures of ionic liquids

The composition and temperature of three eutectic mixtures of ionic liquids and their physical properties (density, viscosity, and surface tension) are presented. Melting temperature depressions of up to ca. 50 K were found with regard to those of the parent ionic liquids.

The effect of an ionic liquid on the rate of reaction at a phosphorus centre

Rate constant of a substitution at phosphorus varies with amount of ionic liquid present; enthalpic benefit offsets entropic cost.

Ionic liquids: anion effect on the reaction of O,O-diethyl O-(2,4-dinitrophenyl) phosphate triester with piperidine

The ionic liquids can be considered as designer solvents in the titled reaction because by an appropriate choice of the anion it is possible to steer the selectivity of this reaction.

Changes in the SNAr reaction mechanism brought about by preferential solvation

For model S_NAr reactions in mixtures of acetonitrile and water, we found preferential solvation in favor of the aqueous phase.

π-Conjugated diimidazolium salts: rigid structure to obtain organized materials

The photophysical properties and aggregation ability of π-conjugated diimidazolium salts were studied in solution and in the solid state. Experimental investigations were supported by DFT calculations.

Imidazolium salts with varying anions as charge carriers for detection of neutral bis(triphenylphosphine)palladium(II) dichloride in electrospray ionization mass spectrometry

RATIONALE

While electrospray ionization is a popular technique for mass analysis, without a charged species it is ineffective. This coupled with solvent restrictions hinders the analysis of organometallic complexes. Detecting neutral species whose solubility is limited to nonconventional solvents is a problem that can be overcome with the right charge carrier, which is described in this study.

METHODS

Ionic liquids were synthesized and analyzed by electrospray ionization quadrupole ion trap mass spectrometry. The neutral palladium complex was also analyzed using different imidazolium salts as the charge carrier with the same method and instrumentation. Theoretical complements were also performed using Gaussian 09 at the density functional theory levels, using B3LYP functionals and the 6-31 g (d,p) basis set for geometry optimizations.

RESULTS

Low concentration imidazolium salts in methanol showed aggregation behavior of the ionic liquid, where the cation peak and [cation](n+1)[anion]n peaks were observed in positive mode, while the [cation]n[anion](n+1) peaks were seen in negative mode. The unbound anion was observed in all the negative mode spectra except for the salt with the SCN anion when in THF. Solutions of PdCl2(PPh3)2 and a small amount of ionic liquid in THF showed the palladium complex adducted with the imidazolium cation for each of the ionic liquids studied.

CONCLUSIONS

A charge carrier for a neutral organometallic complex was found in imidazolium salts, where the cation was observed as the ionizing agent. Differing ion intensities of the complex-adduct peak resulted from the anions ability to dissociate from the cation.