Synthesis, Structure, and Thermotropic Mesomorphism of Layered N-Alkylpyridinum Tetrahalopalladate(II) Salts

Two homologous series of N-alkylpyridinium salts, [Cn-Py]2[PdX4] (n = 12, 14, 16, 18; X = Cl, Br), have been synthesized, and their thermal behavior has been studied by differential scanning calorimetry and hot-stage polarizing optical microscopy. Except for the substances with n = 12, all the materials show thermotropic polymorphism. The crystal structures of [C12-Py]2(PdCl4], [C16-Py]2[PdCl4, and [C16-Py]2[PdBr4] have been solved. All three complex salts crystallize in the triclinic space group P1. Crystal packing reveals layered structures with alternating polar and apolar sublayers, where each (PdX4)2- anion is sandwiched between two antiparallel pyridinium cations. Alkyl chains are highly interdigitated and tilted with respect to the layer normal. The structural characterization of the [Cn-Py]2- [PdX4] (n = 16, 18; X = Cl, Br) salts has been achieved also through variable-temperature powder X-ray diffraction. The [PdCl4]2- salts undergo transitions to a second crystalline phase and then to an ordered and a disordered smectic phase. The latter are assigned to single-layered smectic-E and partially bilayered smectic-Ad phases, respectively. On warming, the [PdBr4]2- analogues only show the smectic-Ad phase, preceded by a crystalline phase very similar to the room-temperature solid phase. The relative importance of hydrogen bonding throughout the sequence of structural changes is briefly addressed.