Columnar mesomorphism from hemi-disklike metallomesogens derived from 2,6-bis[3′,4′,5′-tri(alkoxy)phenyliminomethyl]pyridines (L):: Crystal and molecular structures of [M(L)Cl2 (M = Mn, Ni, Zn)

Four new series of non-disklike complexes of general formula [MCl2(L-n)] based upon substituted 2,6-bis(3',4',5'-trialkoxyphenyliminomethyl)- pyridine ligands (L-n) and with M = Zn-II, Co-II, Mu(II), and Ni-II have been prepared and examined for liquid crystallinity. A complete analysis of the thermal behavior by polarized-light optical microscopy, differential scanning calorimetry, and small-angle X-ray scattering revealed a rich and varied mesomorphism. Moreover, the high thermal stability of the compounds leads to rather extended mesomorphic ranges. The nature and thermal stability of each mesophase depend on both the length of the six terminal alkoxy chains, n (n = 8, 10, 12, 14, 16), and on the metal ions. As demonstrated by small-angle X-ray diffraction experiments, the mesomorphism of these complexes is solely of the columnar type. One compound shows an oblique columnar phase, while most of them show a hexagonal columnar phase, Col(h), and several types of rectangular columnar phase, Col(r). X-ray single-crystal structures obtained for three methoxy derivatives confirm the 1:1 metal-ligand stoichiometry of the complexes, in which the metal is pentacoordinate with a distorted, trigonal bipyramidal geometry. The crystalline structures also reveal the existence of some columnar organization in the solid state, the columns resulting from an alternated stacking of the complexes in one direction. By combining these results with those obtained from dilatometry experiments, a model for the molecular organization within the mesophases is proposed in which an antiparallel arrangement of the metallomesogens is retained in the mesophase.