Novel two-dimensional uranyl–organic assemblages in the citrate and d(–)-citramalate families

Abstract

Uranyl nitrate reacts with D(–)-citramalic acid (H₃citml) under mild hydrothermal conditions to give the two-dimensional polymer [UO₂(Hcitml)] 1, in which each ligand chelates one metal atom through its hydroxyl and α-carboxylate groups and binds to three others in monodentate fashion. The resulting neutral layers display isolated uranyl pentagonal bipyramidal polyhedra. Whereas citric acid (H₄cit) has been shown previously to give various three- and monodimensional uranyl–organic assemblages, complexation under hydrothermal conditions in the presence of either NaOH/NEt₄Cl or pyridine yields the complexes [NEt₄]₂[(UO₂)₃(cit)₂(H₂O)₂]·2H₂O 2 and [Hpy]₂[(UO₂)₃(cit)(Hcit)(OH)] 3, respectively, which both crystallize as two-dimensional frameworks. The layers are either planar and separated by the counter ions in 2 or corrugated and hydrogen bonded to one another in 3. In both 2 and 3, [UO₂(cit)]₂^4– dimeric subunits with edge-sharing pentagonal bipyramidal uranium coordination polyhedra are present but, in both cases and in contrast with previous structures containing [UO₂(Hcit)]₂^2– dimers, the carboxylate group not involved in the dimer formation is coordinated to another uranyl unit, which is part of either a centrosymmetric hexagonal bipyramidal bis-aquated group or a different, [(UO₂)₂(Hcit)(OH)] dimer. These examples of two-dimensional assemblages further illustrate the variety of architectures which can be obtained with citric and related acids and the important structure-directing effects of the counter ions.