A Propeller-like Uranyl Metallomesogen Thomas Cardinaels, † Jan Ramaekers, † Daniel Guillon, ‡ Bertrand Donnio,* ,‡ and Koen Binnemans* ,† Department of Chemistry, Katholieke UniVersiteit LeuVen, Celestijnenlaan 200F, 3001 LeuVen, Belgium, and Groupe des Mate ´ riaux Organiques, Institut de Physique et Chimie des Mate ´ riaux de Strasbourg, UMR 7504 CNRS-UniVersite ´ Louis Pasteur, BP43, 23 rue du Loess, F-67034 Strasbourg Cedex 2, France Received September 28, 2005; E-mail: Koen.Binnemans@chem.kuleuven.be; bdonnio@ipcms.u-strasbg.fr The attractiveness of metal-containing liquid crystals (metal- lomesogens) is that coordination of the mesogenic ligands to the metal center allows the building of molecular edifices with geometries that are impossible to achieve by all-organic liquid crystals. 1 Moreover, induction of liquid-crystalline behavior can be achieved in nonmesomorphic ligands upon coordination to metal ions. Thanks to intensive research efforts in the field of metal- lomesogens during the last two decades, nearly every metal of the periodic system has been incorporated into liquid crystals. Although lyotropic uranium-containing liquid crystals have been known for a long time, 2 it was not until recently that examples of thermotropic uranium-containing metallomesogens have been reported. Sinn and co-workers described calamitic uranyl complexes of -diketonate 3 and tropolonate ligands, 3,4 whereas Sessler’s mesomorphic uranium- alaskaphyrin complexes are the first examples of discotic uranium- containing metallomesogens. 5 Very recently, Aiello et al. reported on mesomorphic uranyl Schiff base complexes. 6 In all these thermotropic liquid crystals, the uranium is present in the hexavalent oxidation state as the dioxouranium(VI) or uranyl cation. The classic design strategy to obtain columnar phases for metallomesogens is to incorporate the metal ion into the central cavity of a flat macrocyclic ligand with peripherally attached alkyl chains (e.g., liquid-crystalline metallophthalocyanines). 1 However, tris--diketonate complexes with an octahedral metal center and an overall propeller-like molecular shape can exhibit columnar mesophases, as well. 7,8 Here, we present a new approach to the design of propeller-like metallomesogens, which is based on the ability of the linear uranyl cation to form complexes by coordination of ligands in the equatorial plane. The idea was to replace the 1,10-phenanthroline ligands in the previously reported 9,10 [UO 2 (phen) 3 ][OTf] 2 complex (where OTf ) CF 3 SO 3 or triflate) by an imidazo[4,5-f]-1,10- phenanthroline moiety bearing three long alkoxy chains. This ligand was prepared by first oxidizing 1,10-phenanthroline to 1,10- phenanthroline-5,6-dione, 11 followed by reaction with 3,4,5-tris- (tetradecyloxy)benzaldehyde and ammonium acetate in hot glacial acetic acid. 12 The uranyl complex was synthesized by reaction between the ligand and uranyl triflate (3:1 molar ratio) in ethanol. The structure of the uranyl complex is shown in Figure 1. Although we were not able to obtain single crystals of the uranyl complex, it is reasonable to assume that its first coordination sphere is comparable with that observed for the [UO 2 (phen) 3 ][OTf] 2 complex, with an equal population of right-handed and left-handed helices. 9,10 The coordination polyhedron can be described as a bi-end-capped trigonal antiprism. The choice of the triflate ion as counterion is not arbitrary because weakly coordinating anions are required to avoid competition of the anion with the phenanthroline ligands for binding to the uranyl ion. The liquid-crystalline properties of the complex were examined by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction on a powder sample. The ligand is not mesomorphic, whereas the uranyl complex melts at 95 °C into a mesophase (∆H ) 69.5 kJ mol -1 ) and clears into the isotropic liquid at 181 °C(∆H ) 2.8 kJ mol -1 ). The fluid and birefringent optical texture of the uranyl metallomesogen (Figure 2, POM) confirms mesomorphism, but the mesophase could not be characterized only on this basis. The X-ray pattern of the uranyl complex (Figure 3) is charac- teristic of a hexagonal columnar phase (Col h ). Two Bragg reflections are observed in the small angle range: an intense and sharp reflection at 2θ 1 ) 2.31° (d 1 ) 38.2 Å) and another small signal at 2θ 2 ) 4.012° (d 2 ) 22.0 Å). The reciprocal d spacings are in the † Katholieke Universiteit Leuven. ‡ Institut de Physique et Chimie des Mate ´riaux de Strasbourg. Figure 1. Structure of the uranyl complex. The two noncoordinating triflate counterions have been omitted. Figure 2. Natural optical texture of the uranyl complex at 170 °C (500× magnification). Published on Web 11/24/2005 17602 9 J. AM. CHEM. SOC. 2005, 127, 17602-17603 10.1021/ja056664w CCC: $30.25 © 2005 American Chemical Society