Columnar Liquid Crystals Derived from Long-Chain Carboxylates of Transition Metal Ions

Copper^II complexes of the fatty acids exhibit a thermotropic liquid crystalline phase of the columnar type. X-ray diffraction studies of this mesophase indicate that the spine of a column is made of regularly stacked dicopper tetracarboxylate cores, which are surrounded by disordered, liquid-like alkyl chains. In turn, the column axes occupy the nodes of a two-dimensional hexagonal lattice. Examination of the local environment of the copper atoms by Cu K_α EXAFS spectroscopy reveals insignificant changes in bond lengths within the binuclear core upon the transition from the lamellar crystalline phase to the columnar mesophase. However, a slight, but sharp, decrease of the magnetic moment of the dicopper unit is observed at the solid-mesophase transition temperature (Δ μ _eff≈ -0.04 μ _B). The columnar mesophase, and the crystalline phase alike, obey a Bleaney-Bowers relation. The singlet-triplet gap is higher in the former (-2J = 310-330 cm^-1 against 290-300 cm^-1). Dirhodium^{II, II} complexes of selected fatty acids were prepared, and their thermal behaviour was investigated. They exhibit the same thermotropic columnar mesophase as their Cu^II congeners, reflecting the presence of isostructural dimetallic cores in both series. The metal-metal single bond that is present in these dirhodium^{II, II} mesogens is Raman active (ν _Rh-Rh≈ 350 cm^-1); it is a potential probe of the change in molecular and supramolecular structures of this class of compounds at their phase transition. Diruthenium^{II, II} complexes of fatty acids were obtained by Cr^II reduction of the mixed-valence chlorodiruthenium^{II, III} tetrabutyrate, followed by ligand exchange with the appropriate carboxylic acid. Magnetic susceptibility studies indicate that these diruthenium^{II, II} carboxylates have an M_s = 0 ground state and a thermally accessible M_s = ± 1 excited state, suggesting a zero-field split (D ≈ 300 cm^-1) σ ²π ⁴δ ²(δ ^*π ^*)⁴ configuration. The formation of a columnar liquid-crystalline phase is reflected by a sharp increase in the magnetic moment of these complexes at the phase transition (Δ μ _eff≈ + 0.15 μ _B).