Confined water in imidazolium based ionic liquids: a supramolecular guest@host complex case

Abstract

It is well known that the macroscopic physico-chemical properties of ionic liquids (ILs) are influenced by the presence of water that strongly interferes with the supramolecular organization of these fluids. However, little is known about the function of water traces within this confined space and restricted ionic environments, i.e. between cations and anions. Using specially designed ILs namely 1,2,3-trimethyl-1H-imidazol-3-ium imidazol-1-ide (MMMI·Im) and 3-n-butyl-1,2-dimethyl-1H-imidazol-3-ium imidazol-1-ide (BMMI·Im), the structure and function of water have been determined in condensed, solution and gas phases by X-ray diffraction studies, NMR, molecular dynamics simulations (MDS) and DFT calculations. In the solid state the water molecule is trapped inside the ionic network (constituted of contact ion pairs formed by π(+)-π(-) interaction) through strong H-bonds involving the water hydrogens and the nitrogens of two imidazolate anions forming a guest@host supramolecular structure. A similar structural arrangement was corroborated by DFT calculations and MDS. The presence of a guest@host species (H2O@ILpair) is maintained to a great extent even in solution as detected by (1)H-(1)H NOESY-experiments of the ILs dissolved in solvents with low and high dielectric constants. This confined water catalyses the H/D exchange with other substrates containing acidic-H such as chloroform.

Cation−Anion−CO2 Interactions in Imidazolium‐Based Ionic Liquid Sorbents

“…Similar result has just been published by Rezaeian and co‐workers, that observed by natural bond orbital (NBO) calculation an effective charge transfer from the amine group of the anions and CO 2 . Furthermore, we have demonstrated the difficulty to obtain dry ILs with basic anions . In fact, once again, for the ILs of group 2, the water can interact with CO 2 and form Main bicarbonate, as evidenced by the peak at 159.1–161.0 ppm in the 13 C NMR spectrum (Figures S5–S6).…”

Carbon Dioxide Capture by Aqueous Ionic Liquid Solutions

“…[26][27][28] Nonetheless,i th as been reported that although bicarbonate formation from CO 2 andw ater is conceivable, the levels of bicarbonate formed at such low CO 2 partial pressures are unlikelyt os ubstantiate the observed increase in CO 2 sorption with phenolate containing ILs. [36] While investigating the impact of water in the organization and reactivity of ILs, [35] we observed at hird type of sorption mechanism for CO 2 through the formation of bicarbonate species, which is probably more efficient and frequent than previously expected (Scheme 2). [8,12,15,18,19,[30][31][32] Althought he solubility of CO 2 in water [33] is well known, its contributiont ot he CO 2 solubility in wet ILs is usually not taken into account (see Ta ble S1).…”

“…Note that the 13 CNMR carbonyl resonance of zwitterionic species resulting from the reaction of amidines, such as 1,4,5,6-tetrahydropyrimidine, with CO 2 ,a ppears at around 151.0 ppm. [35] Althoughw ater is also trapped inside the IL supramolecular network of BMIm·PF 6 and BMIm·BF 4 ,t he nature of the contacts established with the cation and anion is markedlyd ifferent. [17,20,21,25,29] The most plausible pathway for the formation of bicarbonate involves the activation of water by contact ion pairs (imidazolium cation and the anion), acting as neutral bases,t og enerate hydroxide anionsa nd as protonb uffers to stabilize the bicarbonate thus formed (Schemes 2a nd 3).…”

“…[35] CO 2 quantification Carbon dioxide was supplied by Air Liquide with purity above 99.998 %. [35] CO 2 quantification Carbon dioxide was supplied by Air Liquide with purity above 99.998 %.…”

“…[24][25][26][27][28] Therefore, it can be envisaged that CO 2 could also be trapped into the IL by reactionw ith water to produce bicarbonate species. [35] Moreover,i tw as recently reported that confined water in nanoscopic environments promotes the hydrolysis of CO 3 2À to HCO 3 À and OH À . [29] It is important to note that usually only the "total" CO 2 sorptionw as measured and only in af ew cases the structures of CO 2 speciesi nI Ls was investigated by NMR and/orI Rs pectroscopy.…”