Anion-controlled supramolecular crystal structures and ionic liquids from fatty acid-substituted ethyl-nicotinate ionic compounds

Abstract

A series of bio-inspired fatty acid-substituted ethyl-nicotinate ionic compounds, 1-(5-carboxypentyl)-3-(ethoxycarbonyl)pyridin-1-ium ionic compounds ([Nia-OEt-C5COOH] X, X = Br⁻ (1), B(Ph)₄⁻ (2), NO₃⁻ (3), BF₄⁻ (4), PF₆⁻ (5), and OTf⁻ (6)) were synthesized. The supramolecular crystal structures of 1 and 2 were characterized by single crystal X-ray diffraction. The Nia-OEt-C5COOH cations in the two crystal structures formed dimerized secondary rectangular motifs with weak C–H…OC hydrogen bonding which extended into a lipid-like bilayer and a β-sheet-like tape stacking, respectively. Four acidic ionic liquids (AILs) were formed by the other anions (3–6) because these anions and the two carbonyl O atoms of the Nia-OEt-C5COOH cation have small hydrogen bonding acceptor ability differences. In addition, the flexible ester group of Nia-OEt-C5COOH has a low barrier to the rotation of the C–O–C bonds, making it less rigid and it has a lower melting point. The four AILs were characterized by DSC thermal analysis, infrared spectroscopy and ionic conductivity.