Paola Vitale

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

The growing interest in the properties and applications of ionic liquids has recently led to research into the possibility of using their binary mixtures. This work reports on the effects of binary mixtures of ionic liquids on the outcome of organic reactions such as the mononuclear rearrangement of heterocycles and the solvatochromic behavior of Nile Red. Binary mixtures formed by ionic liquids differing in the structure of the cation and the anion are taken into account. In particular, ionic liquids such as 1-benzyl-3-butylimidazolium bis(trifluoromethanesulfonyl)imide, 1-(2,3,4,5,6-pentafluorobenzyl)-3-butylimidazolium bis(trifluoromethanesulfonyl)imide, and 1-benzyl-3-butylimidazolium tetrafluoroborate, are studied. To achieve a deep understanding of the properties of ionic-liquid binary mixtures, their three-dimensional organization was analyzed by a combination of resonance light scattering, UV/Vis spectroscopy, and (1)H and (19)F NMR spectroscopy. Data collected herein evidence that the most significant changes in the ionic lattice structure, and consequently the most pronounced effects exerted as solvent media, occur when the studied system involves a blend of different anions.

Determination of basic strength of aliphatic amines through ion pair formation in some ionic liquid solutions.

To have an evaluation of the basic strength of aliphatic amines in ionic liquid solution, the stability constants relevant to the formation of amine/p-nitrophenol ion pairs were determined in different ionic liquids at 298 K. In particular, aliphatic (pyrrolidinium) and aromatic (imidazolium) ionic liquids were used. Imidazolium ions, bmim(+) and bm(2)im(+), having different hydrogen bond donor abilities were taken into account. Anions were chosen ([BF(4)(-)], [PF(6)(-)], and [NTf(2)(-)]; where NTf(2) = bis(trifluoromethansulfonyl)imide) showing different shape, size, and coordination ability. Several primary, secondary (cyclic or not), and tertiary amines were used to study the effect of amine structure on the ion pair stability. The comparison between data collected in this work and those previously obtained in ordinary molecular solvents evidence that ionic liquids are solvent media able to exalt the amine basicity. Furthermore, the experimental trends obtained in different ionic liquids can only be rationalized considering the whole of parameters affecting their tridimensional structure.

Synthesis of aryl azides: A probe reaction to study the synergetic action of ultrasounds and ionic liquids

The combined effect of ultrasounds and ionic liquids was used to perform the synthesis of aryl azides by nucleophilic aromatic substitution in ionic liquid/[1-butyl-3-methylimidazolium][N3] binary mixtures. The ultrasounds efficiency was analyzed as a function of the substrate and of the ionic liquid structure. In the first case, both 6π and 10π electrons aryl halides were considered. As far as the ionic liquid structure is concerned, both aromatic and aliphatic ionic liquids were taken into account. Among aromatic cations, the effects due to different ability in giving hydrogen bond or π-π interactions were considered. The use of a geminal ionic liquid having an aromatic spacer was examined too. On the whole, collected data evidence an activating effect on the target reaction by the combined use of ultrasounds and ionic liquids. The structural order degree of the ionic liquid seems to be the main factor affecting the ultrasounds efficiency. Furthermore, the effects due to changes in the anion structure seem to be more significant than those due to changes in the cation structure.

Electronic and steric effects: how do they work in ionic liquids? The case of benzoic acid dissociation.

The need to have a measure of the strength of some substituted benzoic acids in ionic liquid solution led us to use the protonation equilibrium of sodium p-nitrophenolate as a probe reaction, which was studied by means of spectrophotometric titration at 298 K. In order to evaluate the importance of electronic effect of the substituents present on the aromatic ring, both electron-withdrawing and -donor substituents were taken into account. Furthermore, to have a measure of the importance of the steric effect of the substituents both para- and ortho-substituted benzoic acids were analyzed. The probe reaction was studied in two ionic liquids differing for the ability of the cation to give hydrogen bond and pi-pi interactions, namely [bm(2)im][NTf(2)] and [bmpyrr][NTf(2)]. Data collected show that benzoic acids are less dissociated in ionic liquid than in water solution. Furthermore, the equilibrium constant values seem to be significantly affected by both the nature of ionic liquid cation and the structure of the acid. In particular, the ortho-steric effect seems to operate differently in water and in the aromatic ionic liquid, determining in this solvent medium a particular behavior for ortho-substituted benzoic acids.

Aggregation Processes of Perylene Bisimide Diimidazolium Salts

The supramolecular aggregation of three diimidazolium-functionalized perylene bisimides, differing in the alkyl chain length was investigated. These salts form aggregates in solvents like chloroform, dichloromethane, and glycerol. Solvent-, concentration-, and temperature-dependent spectroscopic studies were carried out, evidencing the occurrence of an isodesmic, enthalpy-driven aggregation process, underpinned by π-π stacking and hydrogen bonding. Moreover, dynamic light scattering (DLS) measurements and SEM images revealed that these salts aggregate in chloroform into elongated structures.

The effects of structural changes on the anti-microbial and anti-proliferative activities of diimidazolium salts

An array of diimidazolium salts has been synthesized and used to investigate their anti-microbial and anti-proliferative activities. In particular, salts based on the 3,3′-di-n-alkyl-1,1′-(1,n-phenylenedimethylene)diimidazolium cation and differing in the alkyl chain length on the imidazolium ion, the isomeric substitution on the aromatic spacer and in the anion nature were used. The anti-proliferative activity was evaluated against cervical (HeLa), colon adenocarcinoma (HT-29) and breast (SKBR3) cancer cell lines. In the latter case, also a morphological assessment after treatment with salts was performed. All salts were tested for their hemolytic activity against human erythrocytes. On the other hand, the anti-microbial activity was investigated using both Gram-negative (Escherichia coli) and Gram-positive (Kokuria rhizophila, Staphilococcus aureus and Bacillus subtilis) strains. On the whole, the data collected demonstrate that biological activity is the result of the combined action of both cation and anion structures. In general, the cation hydrophobicity plays the most significant role with structural features of the anion becoming more relevant in the presence of a shorter alkyl chain on the cationic head.

Supramolecular Hydro- and Ionogels: A Study of Their Properties and Antibacterial Activity

Diimidazolium-based organic salts, bearing peptides or amino acids as anions have been synthesised and tested for their gelling ability in biocompatible solvents. These low molecular weight salts were successfully used as gelators in phosphate buffered saline (PBS) solution and ionic liquids. Then, the properties of the obtained soft materials were analysed in terms of melting temperature and gel strength as accounted for by rheological investigations. The gel-phase formation was studied by using UV/Vis and resonance light scattering measurements, whereas the morphology of the soft materials was analysed by using polarised optical microscopy and scanning electron microscopy. To get information about the organisation of the gelator in the gelatinous matrix, X-ray diffraction measurements were performed both on the neat gelators and their gels. The results collected show that the properties of the gel phases, like the thermal stability, the self-repairing ability, the resistance to flow as well as the morphology, are dependent on the nature of the anion. Furthermore, bioassays revealed that the obtained diimidazolium organic salts possessed antimicrobial activity, against gram-negative and gram-positive tester strains. In particular and noteworthy, the diimidazolium organic salts exert a bactericidal capability, which was retained even if they are included in the gel phase. Thus, a novel kind of bioactive soft material was obtained that could be fruitfully employed as a non-covalent coating exerting antibacterial capability.

Supramolecular complexes formed by dimethoxypillar[5]arenes and imidazolium salts: a joint experimental and computational investigation

To gain more insights into the factors acting on the stability of complexes formed by pillar[5]arenes, we investigated the interaction of dimethoxypillar[5]arene with some 1-(n-alkyl)-3-(9-anthracenylmethyl)-imidazolium based organic salts. In particular, we used salts differing in not only the anion nature ([Cl−], [BF4−], [NTf2−] and [SbF6−]) and alkyl chain length (butyl, octyl and dodecylimidazolium derivatives), but also the presence of a hydrogenated or fluorinated tail (octyl and perfluorooctylimidazolium derivatives). Host–guest systems were analysed in solution, solid state and gas phase, using a combined approach of different techniques like fluorescence, 1H NMR, ESI-MS, DSC and DFT calculations. Data collected show that the complex stability is heavily affected by solvent nature. Furthermore, solvent being the same, it increases with the decrease of the alkyl chain length and the anion coordination ability. The joint information obtained from 1H NMR and DFT calculations allows drawing a picture of the complex geometry. On the other hand, fluorescence investigation on the solid state evidences the occurrence of an aggregation enhanced emission process for these systems.