Structural studies on layered alkylpyridinium iodopalladate networks

The reaction of alkylpyridinium (CnH2n+1NC5H5, hereafter Cn-Py) iodide salts in aqueous acetonitrile with a preformed palladium iodide precursor afforded two different types of organic-inorganic phases depending on the molar ratio. A 2:1 ratio yielded the phase [Cn-Py](2)[PdI](4) (3, n = 14, 16), which crystallized in the triclinic crystal system. The X-ray crystal structure of 3, (n = 14), refined in the space group <P(1)over bar> (a = 8.918(3) Angstrom, b = 9.894(3) Angstrom, c = 29.062(12) Angstrom, alpha = 93.51(3)degrees, beta = 94.17(3)degrees, gamma = 115.60(3)degrees and Z = 2), consists of interdigitated bilayers with a basal spacing of 29.0 Angstrom. The aliphatic chains of the cations, which run almost parallel to the stacking direction, are fully stretched between polar planes built on isolated [PdI4](2-) anions and cation headgroups. Changing the organic cation to palladium ratio to 1:1 led to a new phase [Cn-Py](2)[Pd2I6] (4, n = 14, 16), which crystallizes in the triclinic <P(1)over bar> space group (a = 9.399(4) Angstrom, b = 14.264(6) Angstrom, c = 29.415(13) Angstrom, alpha = 92.11(4)degrees, beta = 90.07(4)degrees, gamma = 104.53(3)degrees, Z = 3 for 4(n = 14); a = 9.417(2) Angstrom, b = 14.215(3) Angstrom, c = 31.552(6) Angstrom, alpha = 87.96(3)degrees, beta = 87.63(3)degrees, gamma = 75.67(3)degrees, Z = 3 for 4(n = 16)). The layered structure is basically of a continuously interdigitated single-layer type, with a bilayer sublattice superimposed. Isolated [Pd2I6](2-) anions contribute to the inorganic planes. A high degree of interdigitation and tilting of the aliphatic chains lead to basal spacings of 29.4 and 31.5 Angstrom for 4(n = 14) and 4(n = 16), respectively. The [Cn-Py](2)[PdI4] and [Cn-Py](2)[Pd2I6] phases were characterized by thermal analysis. Mesomorphic behavior was observed only for 3(n = 16), which was confirmed by variable-temperature powder XRD and optical microscopy.