Two- and Three-Dimensional Europium−Organic Assemblies with the all-cis and all-trans Isomers of 1,2,3,4,5,6-Cyclohexanehexacarboxylic Acid

Abstract

When reacted with europium nitrate under mild conditions, all-cis 1,2,3,4,5,6-cyclohexanehexacarboxylic acid (H6Lcis) retains its configuration and gives the complexes [Eu(H3Lcis)(H2O)4]·xH2O with x = 3 (1) or 6 (2). In both cases, the three carboxylate groups are in the equatorial position and the carboxylic groups are axial. The carboxylate groups are all chelating in 1, thus giving a hexagonal, two-dimensional network; one group is chelating, one bridging bidentate and the other monodentate in 2, and the resulting two-dimensional assembly displays a tetragonal arrangement. Under hydrothermal conditions with prolonged heating at 180 °C, H6L undergoes isomerization into the all-trans form, resulting in the formation of [Eu2(Ltrans)(H2O)8]·4H2O (3). All six carboxylate groups are in equatorial position, four of them being chelating and the other two monodentate; the former gives rise to planar ribbon-like subunits, which are assembled into a three-dimensional framework of rutile-type topology by the latter. Complex 3 was also obtained from in situ hydrolysis, oxidation, and isomerization of bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride. Compounds 1−3 are the first lanthanide ion complexes with the ligand H6−xLx−.