C-H-π Interactions in 1-n-Butyl-3-methylimidazolium Tetraphenylborate Molten Salt: Solid and Solution Structures

Abstract

The crystal structure of 1-n-butyl-3-methylimidazolium tetraphenylborate molten salt (1) shows C-H-pi interactions between the hydrogens of the imidazolium cation and the phenyl rings of the tetraphenylborate anion. The imidazolium ring is surrounded by three tetraphenylborate anions that are connected with the same cation by C-H-pi (phenyl rings) interactions. The nearest inter-ion interaction is found between the N-CH-N proton of the cation and the B-phenyl centroid (2.349 A) with a nearly T-shaped geometry. The inter-ionic solution structure of 1 has been investigated by the detection of inter-ionic contacts in 1H NOESY NMR spectra between the protons of the cation and the anion. The 1H-NMR spectra of molten salt 1 is almost independent of its concentration in [D6]DMSO solution, the imidazolium proton chemical shifts are in the expected region and there are no observable NOE effects between the protons of the cation with those of the anion, indicating that 1 behaves in [D6]DMSO as a solvent-separated ion pair. In CDCl3 the 1H-NMR spectra of 1 are concentration dependent and all the imidazolium protons are shielded as compared with those observed in [D6]DMSO. Moreover, the 1H NOESY NMR spectra show all the peaks affected by the interaction between the protons of the imidazolium cation and those of the anion, indicating that in CDCl3 1 possesses a contact ion pair structure. The NCHN proton of the cation exhibits the greatest shielding (up to -4.5 ppm). an indication of the existence of C-H-pi interactions, even in solution. The calculated distance of this proton to the phenyl centroid is 2.3 A for a C-H -pi angle of 180 degrees. The apparent volumes for the cation and anion, calculated from the measured 13C-NMR relaxation times, increase from 38 and 140 A3 in [D6]DMSO to 360 and 600 A3 in CDCl3, respectively; this indicates the formation of floating aggregates of the type (1)(n) in CDCl3 via weak hydrogen bonds, with increasing concentration.

Structural characteristics of alkylimidazolium-based salts containing fluoroanions

“…Crystallographic structures of the two salts (MBz) 2 ImBF 4 and (MBz) 2 ImPF 6 are similar, where the cation is surrounded by six anions in both the salts. These structures resemble the BMImBPh 4 (BPh 4 = tetraphenylborate) structure [40]. As shown in Fig.…”

“…There appears to be no hydrogen bond interactions between the H atom of cation and the p-electron of BARF À . Unlike BMImBPh 4 , the bulky trifluoromethyl moieties prevent the anion and cation from coming into close enough proximity for such interactions to occur [40].…”

Synthesis of Co, Rh and Ir nanoparticles from metal carbonyls in ionic liquids and their use as biphasic liquid–liquid hydrogenation nanocatalysts for cyclohexene

“…Ionic liquids stabilize metal nanoparticles on the basis of their high ionic charge, high polarity, high dielectric constant and supramolecular network [24][25][26][27]. According to DLVO (DerjauginLandau-Verwey-Overbeek) theory [28,29], ILs should provide an electrosteric protection in the form of a ''protective shell" for MNPs, so that no extra stabilizing molecules or organic solvents are needed [30][31][32].…”

The liquid crystal state: An intermediate state to obtain crystal packing

“…These Hbonds induce structural directionality contrary to classical salts in which the cohesion is mainly due to ionic bonds (Fig. 1a) [25][26][27][28][29][30][31][32][33][34][35][36][37]. Moreover, complementary electrostatic, π-stacking and van der Waals interactions can also occur in these salts (Fig.…”