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

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The newly synthesized palladium complex [(Azo-6)Pd(μu-C1)]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°C (nematic) and above 220 until 235°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) Å. Moreover the molecules of 1 are arranged in pairs with an intermolecular Pd-Pd non-bonded interaction distance of 3.668(1) Å. 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)⇌E(single), on heating; N(pair)→SE(pair), at constant temperature) are suggested.