MESOMORPHIC BEHAVIOR, 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(A zo-6)=4,4'-hexyloxyazobenzene] has been investigated by optical observ ations, calorimetric measurements, and single crystal and low angle va riable temperature X-ray diffraction techniques. Two different mesopha ses appear from 210 to 220 degrees C (nematic) and above 220 until 235 degrees C (smectic E) where the compound eventually decomposes. The n ematic phase is not in thermodynamic equilibrium and when subjected to an annealing process transforms into a smectic phase. The molecular s tructure of the dinuclear cyclopalladated complex 1 has an intramolecu lar Pd-Pd distance of 3.528(1)Angstrom. Moreover the molecules of 1 ar e arranged in pairs with an intermolecular Pd-Pd non-bonded interactio n distance of 3.668(1)Angstrom. On the basis of this structural featur e and of the variable temperature low-angle XRD studies, transitions i nvolving dissociation of the molecular pairs (e.g. N(pair)reversible a rrow S-E(single), on heating; N(pair)-->S-E(pair), at constant tempera ture) are suggested.