Mesomorphic behaviour, single crystal and low angle variable temperature X-ray diffraction studies of the chloro-bridged cyclopalladated dimer obtained from 4,4'-hexyloxyazobenzene

The newly synthesized palladium complex [(Azo-6)Pd(mu-Cl)](2), 1, [H(Azo-6)=4,4'-hexyloxyazobenzene] has been investigated by optical observations, calorimetric measurements, and single crystal and low angle variable temperature X-ray diffraction techniques. Two different mesophases appear from 210 to 220 degrees C (nematic) and above 220 until 235 degrees C (smectic E) where the compound eventually decomposes. The nematic phase is not in thermodynamic equilibrium and when subjected to an annealing process transforms into a smectic phase. The molecular structure of the dinuclear cyclopalladated complex 1 has an intramolecular Pd-Pd distance of 3.528(1)Angstrom. Moreover the molecules of 1 are arranged in pairs with an intermolecular Pd-Pd non-bonded interaction distance of 3.668(1)Angstrom. On the basis of this structural feature and of the variable temperature low-angle XRD studies, transitions involving dissociation of the molecular pairs (e.g. N(pair)reversible arrow S-E(single), on heating; N(pair)-->S-E(pair), at constant temperature) are suggested.