Mesomorphism of lanthanide-containing Schiff's base complexes with chloride counterions

Liquid crystalline complexes [Ln(LH)(3)Cl-3] have been synthesized, where Ln is a trivalent lanthanide ion (Pr-Lu, except Pm) and where LH is the Schiff's base ligand N-octadecyl-4-tetradecyloxysalicylaldimine. Although the ligand does not exhibit mesomorphism, the complexes do (SmA phase). The mesophase behaviour of these compounds has been investigated by polarizing optical microscopy, differential scanning calorimetry and high temperature X-ray diffraction. The lanthanide complexes have much higher melting and clearing points than comparable complexes with nitrate or dodecyl sulphate counterions. In addition, the transition temperatures are virtually independent of the type of lanthanide ion. This behaviour is opposite to that observed for similar complexes with nitrate counterions [Ln(LH)(3)(NO3)(3)]. The differences in temperature dependence can be related to structural differences. Whereas in the nitrate complexes the Schiff's base ligand binds in a zwitterionic form, two-dimensional H-1 NMR correlation spectroscopy (COSY) of [Lu(LH)(3)Cl-3] gives an indication that in the chloride complexes, besides coordination via the oxygen of molecules in the zwitterionic form, some of the Schiff's base ligands bind in a bidentate fashion (via the phenolic oxygen and the imine nitrogen).