A2[MX4] copper(II) pyridinium salts.: From ionic liquids to layered solids to liquid crystals

The synthesis and characterization of a novel series of tetrachlorocuprate salts, based on N-alkylpyridinium cations ([Cn-Py](+)), are described. The [Cn-Py](2)[CuCl4] salts (n = 6, 9-18) were found to be crystalline solids except for the short-chain homologues (n = 6, 9, 10), which were obtained as viscous liquids. X-ray powder diffraction measurements showed that in the crystalline state the ionic species give rise to layered structures. An X-ray crystal structure study of [C15-Py](2)[CuCl4] also confirmed the lamellar packing and revealed the monolayer type of organization with strong interdigitation and canting of N-alkyl chains. Most solid salts (n greater than or equal to 12) are thermotropic liquid crystals, with hexagonal columnar, cubic, and smectic phases appearing in the order of increasing chain length and/or temperature. This unique phase behavior has been undoubtedly proved by optical, calorimetric, and diffractometric techniques. The details of the structural organization in both the columnar and cubic phases are not entirely defined yet.