Crystal structures, thermal analysis and electrochemical behaviors of functionalized pyridinium ionic liquids comprising one 1-ethyl acetate group

Abstract

Pyridinium ionic liquids(ILs, 1-ethyl acetate pyridinium hexfluorophosphate[EAPy][PF6] and 1-ethyl acetate-3-methyl pyridinium hexfluorophosphate[EAMPy][PF6]), were synthesized by a two-step process involving introduction of one ethyl acetate group and anion metathesis. Colorless single crystals of the two ILs were initially obtained using the solvent-evaporation method in mixed solvents. Single-crystal X-ray diffraction was used to determine the crystal structures. [EAPy][PF₆] crystallize in the monoclinic space group C2/c with a=2.2748(16) nm, b=0.6204(4) nm, c=1.8552(12) nm and Z=8, whereas [EAMPy][PF₆] crystallizes in the orthorhombic space group P2₁2₁2₁ with a=0.7126(17) nm, b=1.2792(3) nm, c=1.5327(3) nm and Z=4. The structure of [EAPy][PF₆] contains double zigzag chains formed by alternately pairing large organic cations with the octahedral anions of [P1F6]–or [P2F₆]^–. The [P1F₆]^– and [P2F₆]^– anions occupy respectively two distinct crystallographic sites in crystal packing models. The structure of [EAMPy][PF₆] includes ladder-type chains constructed through pairing pyridinium cations with inorganic anions of [PF₆]^–. The [PF₆]^– anion in [EAMPy][PF₆] shows a distorted octahedron structure and is sandwiched by ethyl acetate groups in crystallographic stacking. This study reveals the influence of chemical modification involving the methyl group(CH₃) onto crystallographic structure of pyridinium ILs. Thermal analysis indicates that the difficult crystallization of the two ILs is related to the low void filling of ion pairs in crystal structure, leading to relatively low melting point and evident supercooling during the cooling process. Additionally, the experimental results indicate that the two ILs have electrochemical activity. The ethyl acetate group also allows downward shifting of electrochemical windows to less negative positions and the ionic conductivities of the two ILs follow an Arrhenius-type behavior.