Supramolecular architecture elucidation of the room temperature columnar mesophases exhibited by mixed-valent diruthenium alkoxybenzoates

Abstract

Three series of mesogenic coordination polymers based on mixed-valent diruthenium tetracarboxylates, namely Ru₂(3,4-B2OCn)₄Cl, Ru₂(3,5-B2OCn)₄Cl and Ru₂(3,4,5-B3OCn)₄Cl [3,4-B2OCn = 3,4-(C_nH_2n+1O)₂C₆H₃CO₂⁻, 3,5-B2OCn = 3,5-(C_nH_2n+1O)₂C₆H₃CO₂⁻ and 3,4,5-B3OCn = 3,4,5-(C_nH_2n+1O)₃C₆H₂CO₂⁻] have been synthesized and characterized, with the aim of obtaining columnar liquid crystalline phases. These equatorial BmOCn ligands have been selected under the assumption that they will efficiently fill the intermolecular space around the axial Cl atoms, thus warranting a parallel arrangement of the polymeric ⋯Ru₂–Cl–Ru₂–Cl⋯ strands in the crystalline phase, which should facilitate the occurrence of Col mesophases. The liquid crystalline character of the synthesized compounds has been studied by means of polarized optical microscopy, differential scanning calorimetry, and variable temperature X-ray diffraction. They all exhibit thermotropic Col_H mesophases, the 3,4-B2OCn derivatives also exhibit a lamellar mesophase. Many of the studied compounds exhibit room temperature mesomorphism. The thermal stability domain of the Col_H mesophases of the 3,5-B2OCn and 3,4,5-B3OCn derivatives is remarkably wide, from below 0 °C to ca. 300 °C in some cases. Based on pieces of structural information coming from XRD experiments, dilatometry, and local probes (magnetic susceptibility measurements, resonance Raman and IR spectroscopies, and Exafs) we suggest a model for the structural arrangement of the polymeric strands in the mesophase, in which the central polar backbone of the polymeric strands is in a zig-zag conformation, surrounded by a continuum of molten aliphatic chains. A crystallographic study of the non-mesogenic light homologue Ru₂(3,4,5-B3OC2)₄Cl confirms these aspects of the model, and shows the predicted preorganization of the polymeric strands in the crystalline phase.