Crystal architecture and mesophase structure of long-chain N-alkylpyridinium tetrachlorometallates

The N-hexadecylpyridinium derivatives [C16Py]2[MCl4] (M=Cd (1), Zn (2)) were prepared and structurally characterized by X-ray diffraction methods. The crystal structure of 1 reveals a layered arrangement with alternation of monolayers of interdigitated cations with layers of isolated [CdCl4]2− anions. CH⋯ClCd interactions reinforce the association between anions and cations in the polar region of the supramolecular structure. Both 1 and 2 display a thermotropic smectic A phase in addition to some high-temperature crystalline modifications. The thermal polymorphism of the salts was studied by X-ray powder diffraction, differential scanning calorimetry (DSC), and thermal optical microscopy. The structural and mesomorphic characteristics of 1 and 2 are compared with those of the analogous salts along the series [C16Py]2[MCl4] (M=Co, Ni, Cu, Zn, Pd, Cd).