H-Bonded metallomesogens derived from salicyladiminates

A series of new mesogenic compounds 1a-b (n=8, 10, 12, 14, 16) derived from salicyladimines and their palladium 2a, 2b, vanadyl 2a, and copper complexes 3a, 3b were prepared and their mesomorphic properties investigated by optical microscopy, differential scanning calorimetry and powder X-ray diffractometry. Pd²⁺ and VO²⁺ ions formed mononuclear complexes, whereas, Cu²⁺ ion formed binuclear complexes due to the relative acidic strength of Schiff base. Single crystallographic analysis of non-mesogenic compound 2a (n=8) confirmed its coordination geometry at Pd²⁺ as square plane. It crystallizes in a triclinic space group P−1 with a Z=1. As expected, the Pd²⁺ was coordinated via a trans-N₂O₂ donor set of phenolic-O and imine-N atoms, leaving two hydroxyl groups intact and uncoordinated. The two alkoxy chains, pointing to the opposite direction were parallel, and the molecule was considered as twisted Z-shaped. Both hydroxyl-OH groups attached on C₁₇ and C₁₈-Schiff imines participate in the H-bonds in the lattice. Interestingly, a pseudo polymeric structure was observed, in which H-bonded dimer was continuously extended by another H-bonded dimer in the lattice. Compounds 1 exhibited smectic A phases, and Pd and VO complexes and Cu complexes 3b exhibited smectic A or/and smectic X phases, however, Cu complexes 3a formed crystal phases. Intermolecular H-bonds might be attributed to the difference observed on the mesomorphic properties in these compounds. Copper complexes 2b were not active on ESR spectroscopy.