Two systems of [DabcoH_2]~(2+)/[PipH_2]~ (2+)-uranyl-oxalate showing reversible crystal-to-crystal transformations controlled by the diammonium/uranyl/oxalate ratios in aqueous solutions ([DabcoH_2]~(2+) = 1,4-diazabicyclo-[2.2.2]- octaneH_2 and [PipH_2]~(2+) = piperazineH_ 2)

摘要

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 piperazine (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][UO_ 2(C_2O_4)_2(H_2O)] ·2H_2O (dabco1) and [dabcoH_2][(UO_2)_ 2(C_2O_4)_3(H_2O)_ 2]·2H_2O (dabco2), and [pipH_2][UO_ 2(C_2O_4)_2(H_2O)] ·4H_2O (pip1) and [pipH_2][(UO_2)_ 2(C_2O_4)_3(H_2O)_ 2]·2H_2O (pip2). Besides the cations and lattice water, dabco1 and pip1 contain mononuclear anions of [(UO_2)(C_ 2O_4)_2(H_2O)]~(2-), whereas dabco2 and pip2 possess dinuclear anions of [(UO_2)_2(C_ 2O_4)_3(H_2O)_2]~ (2-), and in all structures, the uranium ion shows a pentagonal bipyramid environment made up of equatorial oxalate, water, and apical oxygen. The needle crystals of dabco1 belong to the chiral space group P6_522. In the structure, the [UO_2(C_2O_4)_ 2(H_2O)]~(2-) anions form anionic helixes along the 6_5 axis and they are further linked by N-H□O hydrogen bonds between the interhelix [dabcoH_2]~(2+) cation and oxalates of the anion. Disorder of lattice water and the [dabcoH_2]~(2+) cation is observed in dabco1. The thin plate crystals of pip1 in space group P1ì... possess a lamellar structure, with dense layers of [pipH_ 2]~(2+)·2[UO_2(C_2O_ 4)_2(H_2O)]~(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 [(UO_2)_2(C_2O_4)_ 3(H_2O)_2]~(2-) anion with one tetradentate oxalate bridge. The anions and ammonium cations form chains by N-H□O hydrogen bonds between them and then further form stacked layers via O-H□O hydrogen bonds among the coordinated and lattice water and the oxalate ligands. The ratio of diammonium/uranyl/oxalate in the reaction systems controlled the final outcomes and the occurrence of transformation, no transformation, and reverse transformation. The thermal stability; UV-vis, IR, and Raman spectra; and luminescence were also investigated.