Alkylpyridinium Tetrahalidometallate Ionic Liquids and Ionic Liquid Crystals: Insights into the Origin of Their Phase Behavior

Six N ‐alkylpyridinium salts [C n Py] 2 [MCl 4 ] ( n = 4 or 12 and M = Co, Cu, Zn) were synthesized, and their structure and thermal properties were studied. The [C 4 Py] 2 [MCl 4 ] compounds are monoclinic and crystallize in the space group P 2 1 / n . The crystals of the longer chain analogues [C 12 Py] 2 [MCl 4 ] are triclinic and crystallize in the space group P 1 . Above the melting temperature, all compounds are ionic liquids (ILs). The derivatives with the longer C12 chain exhibit liquid crystallinity and the shorter chain compounds only show a melting transition. Consistent with single‐crystal analysis, electron paramagnetic resonance spectroscopy suggests that the [CuCl 4 ] 2– ions in the Cu‐based ILs have a distorted tetrahedral geometry.