Liquid-Crystalline Materials Based on Rhodium Carboxylate Coordination Polymers:  Synthesis, Characterization and Mesomorphic Properties of Tetra(alkoxybenzoato)dirhodium(II) Complexes and Their Pyrazine Adducts

Abstract

Tetra(alkoxybenzoato)dirhodium(II) complexes of the formula Rh2(x,y,z-BmOCn)4 (where B is the benzoate group; m is the number of alkoxy chains on the aromatic ring; x, y, and z are their positions of anchoring; and n is the number of carbon atoms in each alkoxy chain) and the corresponding pyrazine adducts have been synthesized and characterized. The pyrazine adducts were prepared to have a polymeric structure via connected metallic centers. The complexes synthesized are Rh2(3,4,5-B3OCn)4 for n = 8, 10, 14, and 18; Rh2(3,4-B2OCn)4 for n = 9, 14, and 18; and Rh2(3,5-B2OCn)4 for n = 9 and 18, as well as the corresponding pyrazine adducts [Rh2(3,4,5-B3OCn)4pz]x for n = 10, 14, and 18; [Rh2(3,4-B2OCn)4pz]x for n = 14 and 18; and [Rh2(3,5-B2OCn)4pz]x for n = 9 and 18. Their liquid-crystalline (LC) properties have been analyzed. Most of them exhibit LC columnar and cubic mesophases with melting transition temperatures close to or even below room temperature. The mesomorphic character of the pyrazinic compounds, compared to the nonmesomorphic aliphatic carboxylate analogues, can be interpreted in terms of the ability of the equatorial ligands to fill the interdimeric space efficiently. Some internal characteristics about the arrangement of the molecules inside the columns are discussed.