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 ≥ 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.

Two homologous series of N-alkylpyridinium salts, [Cn-Py] 2 [PdX 4 ] (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, and [C16-Py] 2 [PdBr 4 ] have been solved. All three complex salts crystallize in the triclinic space group P1 h. Crystal packing reveals layered structures with alternating polar and apolar sublayers, where each [PdX 4 ] 2anion 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 -[PdX 4 ] (n ) 16, 18; X ) Cl, Br) salts has been achieved also through variable-temperature powder X-ray diffraction. The [PdCl 4 ] 2salts 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-A d phases, respectively. On warming, the [PdBr 4 ] 2analogues only show the smectic-A d 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.

The 4,4‘-disubstituted azobenzene ligands HL a - d react with [(η3-C4H7)Pt(μ-Cl)]2 to give the dinuclear cycloplatinated complexes [(L a - d )Pt(μ-Cl)]2, which are easily converted to their mononuclear Pt(II) acetylacetonate derivatives [(L a - d )Pt(acac)]. Oxidative addition to the square-planar Pt(II) complexes [(L a - d )Pt(acac)] of electrophilic substrates such as I2 or CH3I (RI) eventually led to the corresponding octahedral Pt(IV) [(L a - d )Pt(acac)I2] and [(L a - d )Pt(acac)(CH3)I] products. Characterization by X-ray crystallography on model complexes [(L a )Pt(acac)], [(L a )Pt(acac)I2], and [(L a )Pt(acac)(CH3)I] has been carried out, showing that the I2 or CH3I ligands are bound to the Pt(IV) center at the apical positions. The presence of two ligands in apical position led to the loss of the short intermolecular Pt- - -Pt distance of 3.311(1) Å observed in the square-planar complex [(L a )Pt(acac)]. Thermotropic mesomorphism is observed for both the Pt(II) and Pt(IV) species with clearing temperatures, mainly lower than those of the corresponding organic ligands. These products are the first examples of Pt(IV) octahedral liquid crystalline species and suggest that oxidative addition to appropriate Pt(II) precursors should be a convenient synthetic procedure for new hexacoordinated Pt(IV) mesogenic materials.

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