Mixed-valent diruthenium (II,III) long-chain carboxylates. 1. Molecular design of columnar liquid-crystalline order

Six series of binuclear mixed-valent diruthenium tetracarboxylates of formula Ru-2(O2CR)(4)X, containing different combinations of equatorial carboxylates (RCO2-) and anions (X-) have been synthesized and characterized, and their liquid-crystalline properties have been examined by optical microscopy under polarized light, differential scanning calorimetry, and powder X-ray diffraction studies. Three series derived from linear aliphatic carboxylates, Ru-2(O2C-(CH2)(n-2)-CH3)(4)X with X = Cl (n = 5-9, 12, 16), X = O2C-R (R = (CH2)(n-2)-CH3, n = 6-12, 14, 15, 16), and X = DOS (DOS = dodecyl sulfate, n = 6, 8, 9, 12, 16, 18), were studied. The nature of the counterion X strongly affects the appearance of the liquid-crystalline order: the chloro complexes are not mesomorphic, whereas the complexes with RCO2 or DOS anions exhibit a columnar hexagonal mesophase above ca. 150 degrees C. A structural model for the mesophase of the Ru-2(O2C-(CH2)(n-2)-CH3)(5) series is proposed. Three other types of Ru-2(O2CR)(4)X complexes were obtained with a 3,4-dialkoxybenzoate as equatorial ligand (RCO2 = 3,4-(CH3(CH2)(n-1)O)(2)PhCO2) and with X = Cl (n = 12, 15, 16), X = RCO2 (n = 12, 15, 16) and X = DOS (n = 12, 15, 16). All complexes of the three latter types are mesogenic, with transition temperatures lower than those found for the aliphatic analogues, and the pentakis(dialkoxybenzoate) derivatives are liquid crystalline at room temperature, indicating that the nature of the equatorial ligands is also a determinant of the mesogenic character. The results are interpreted in terms of the ability of the axial and equatorial ligands to efficiently fill the interdimeric space, thereby creating adequate conditions for columnar order.