Structural, Thermodynamic, and Mesomorphic Consequences of Replacing Nitrates with Trifluoroacetate Counteranions in Ternary Lanthanide Complexes with Hexacatenar Tridentate Ligands

The promesogenic hexacatenar tridentate ligands L3Cn (I shape) and L4Cn (V shape) react with trivalent lanthanide
trifluoroacetates, Ln((CF3CO2)3, to give either monometallic [Ln(LiCn)(CF3CO2)3] or trifluoroacetato-bridged bimetallic
[Ln(LiCn)(CF3CO2)3]2 complexes in the solid state, as exemplified by the crystal structures of [Lu(L4C0)(CF3CO2)3(H2O)], [Lu(L4C0)(CF3CO2)3]2, and [La(L3C4)(CF3CO2)3]2. Although the dimerization process is influenced by the
competiting complexation of anions or solvent molecules, the coordination of CF3CO2- instead of NO3- to Ln(III)
produces a significant lengthening of the Ln−N(ligand) bond distances. This translates into a considerable decrease
of the affinity of the LiC12 (i = 3, 4) ligands for Ln(CF3CO2)3 in solution, thus leading to significant dissociation of
the [Ln(LiC12)(CF3CO2)3] complexes at millimolar concentrations. The thermal properties of these complexes also
suffer from their limited thermodynamic stability, and the thermotropic liquid crystalline phases produced at high
temperatures reflect mixtures of different species. However, a hexagonal columnar organization characterizes the
main component in the mesophases obtained with [Ln(L3C12)(CF3CO2)3] at high temperature. A tentative interpretation
of the small-angle X-ray scaterring (SAXS) profiles suggests that disklike dimers of [Ln(L3C12)(CF3CO2)3]2 are packed
along the columnar axes. For [Ln(L4C12)(CF3CO2)3], SAXS profiles are compatible with a lamellar organization in
the mesophases originating from the existence of rodlike dimers of [Ln(L4C12)(CF3CO2)3]2 as the major component
in the liquid-crystal state.