Structural analysis of mono-substituted N-butyl-pyridinium salts: in search of ionic liquids

Abstract

Four mono-substituted N-butylpyridinium salts, 1-butyl-4dimethylaminopyridinium chloride [b4dmapy]Cl, 1-butyl-4-methylpyridinium bromide [b4mpy]Br, 1-butyl-4-methylpyridinium hexafluorophosphate [b4mpy][PF 6 ], and 1-butyl-3-methylpyridinium hexafluorophosphate [b3mpy][PF 6 ] were synthesized and characterized using single crystal X-ray diffraction. The crystal structures were examined with the intent of identifying ion interactions leading to higher melting points of the halide salts with respect to the [PF 6 ]salts. The changes in hydrogen bonding, C-HÁÁÁp, and van der Waals interactions have been analyzed with respect to anion, functional groups, and the symmetry of the cation to establish interdependence with the compound's physicochemical properties. It has been observed that the cation-anion interactions are represented by highly directional hydrogen bonds and show strong preference to positions of interaction depending on the anion. The cations of the halide salts show strong tendency towards higher dimensional formations, while those of the [PF 6 ]salts prefer low dimensional assemblies both being based mainly on the weaker van der Waals interactions. These interactions depend on the shape of the cation but may offer certain structure-ordering rigidity accommodating variable anions.

Insights into Ionic Liquids: From Z-Bonds to Quasi-Liquids

“…In ice, linear HBs always form between different water molecules. However, in IL crystals, the HB substantially deviates from a linear geometry and induces the imidazolium cation to form a zigzag packing chain motif, where the anion localizes between these zigzag chains. ,− These unique HBs link the cation and anion (Figure ), forming distinct zigzag configurations in crystal ILs, including EmimPF 6 , EmimBF 4 , 1,3-dimethylimidazolium chloride, 1-butyl-3-methylimidazolium chloride (BmimCl), and 1,2,4-triazolium perfluorobutanesulfonate (TAZPfBu). Physically, the formation of zigzag chains is mainly attributable to the following reasons: (1) the coupling interactions between the HB and electrostatic interactions, where the strong electrostatic attractive forces correspond to a bent HB bond, and (2) stabilization of the entire system or minimization the total potential energy, for which HBs should form between the cation and anion to the maximum extent.…”

Hydro-ionothermally Generated Paramagnetic Fe State-Embedded Ferromagnetic Iron Oxide with a 2D Hexagonal Nanostructure: A Conceptual Intervention on the Methylene Blue Degradation Mechanism

“…In an IL-assisted hydrothermal synthesis, this can be brought about by the combined effect of the transition-metal ion ratio (here, the Fe 2+ /Fe 3+ ratio) and the selection of suitable ILs. Recently, a structural investigation of mono-substituted N -butyl pyridinium salts by Kelley et al reported the conservation of a distinctly non-symmetrical L-shape among butyl pyridinium cations, forming extended chains of the network that bulwark the mutual anion–anion repulsions . The anions of these ILs would dominate their interactions with solutes .…”

“…Recently, a structural investigation of mono-substituted N-butyl pyridinium salts by Kelley cations, forming extended chains of the network that bulwark the mutual anion−anion repulsions. 10 The anions of these ILs would dominate their interactions with solutes. 11 The cations with halide anions displaying greater plasticity during crystallization form a uniformly weak acidic surface.…”

“…11 The cations with halide anions displaying greater plasticity during crystallization form a uniformly weak acidic surface. 10 Therefore, it is likely that the weakly solvated anions coordinate with the Fe 2+/3+ metal ions, which pulls them together toward the pyridinium cation supramolecular templates. In the case of hydrophilic pyridinium and ILs with longer alkyl chain lengths, such as N-dodecyl pyridinium perchlorate [C 12 Py] + [ClO 4 ], the cations showed the tendency to complex with Fe 2+ ions.…”

“…The anions of these ILs would dominate their interactions with solutes . The cations with halide anions displaying greater plasticity during crystallization form a uniformly weak acidic surface . Therefore, it is likely that the weakly solvated anions coordinate with the Fe 2+/3+ metal ions, which pulls them together toward the pyridinium cation supramolecular templates.…”