Two Systems of [DabcoH2]2+/[PipH2]2+-Uranyl-Oxalate Showing Reversible Crystal-to-Crystal Transformations Controlled by the Diammonium/Uranyl/Oxalate Ratios in Aqueous Solutions ([DabcoH2]2+=1,4-Diazabicyclo-[2.2.2]-octaneH2 and [PipH2]2+ = PiperazineH2)

We present here two systems of [dabcoH(2)](2+)-uranyl-oxalate and [pipH(2)](2+)-uranyl-oxalate in which [dabcoH(2)](2+) and [pipH(2)](2+) are cations of doubly protonated 1,4-diazabicyclo-[2.2.2]-octane (dabco) and peperazine (pip), respectively. Each system yielded two different crystals and showed the reversible crystal-to-crystal transformations between them in aqueous solutions, controlled by the ratio of reactants or building blocks in the reaction systems. The four compounds in pairs are [dabcoH(2)][UO2-(C2O4)(2)(H2O) (dabco1) and [dabcoH(2)][(UO2)(2)(C2O4)(3)-(H2O)(2)]center dot 2H(2)O (dabco2), and [pipH(2)][UO2(C2O4)(H2O)]center dot 4H(2)O (pip1) and [pipH(2)][(UO2)(2)(C2O4)(3)(H2O)(2)]center dot 2H(2)O (pip2). Besides the cations and lattice water, dabco1 and pip1 contain mononuclear anions of [UO2-(C2O4)(2)(H2O)](2-), whereas dabco2 and pip2 posses dinuclear anions of [(UO2)(2)(C2O4)(3)(H2O)](2-), and in all structures, the uranium ion shows a pentagonal bipyramid environment made up of equatorial oxalate, water, and apical oxygen. The needle crystal of sabco1 belong to the chiral space group P6(5)22. In the structure, the [UO2(C2O4)(2)(H2O)](2-) anions form anionic helixes along the 6(5) axis and they are further linked by N-H center dot center dot center dot O hydrogen bonds between the interhelix [dabcoH(2)](2+) cation and oxalates of the anion. Disorder of lattice water and [dabcoH(2)](2+) cation is observed in dabco1. The thin plate crystals of pip1 in space group P (1) over bar possess a lamellar structure, with dense layers of [pipH(2)](2+)center dot 2[CO(C2O4)(2)(H2O)]2- pillared by other crystallographically unique [pipH(2)](2+) cations, and the structure contains lattice water forming branched zigzag water chains. Block crystals of dabco2 and pip2, in monoclinic space groups C2/c and P2(1)/c, respectively, are of similar layer-like structures, possessing the dinuclear [(UO2)(2)(C2O4)(3)(H2O)(2)](2-) anion with one tetradentate oxalate bridge. The anions and ammonium cations form chains by N-H center dot center dot center dot O hydrogen bonds between them and then further form stacked layers via O-H center dot center dot center dot O hydrogen bonds among the coordinated and lattice water and the oxalate ligands. The ratio of diammonium/uranyl/oxalate in the reaction systems controlled transformation. The thermal stability; UV-vis, IR, and Raman spectra; and luminescence were also investigated.