Do secondary and tertiary ammonium cations act as structure-directing agents in the formation of layered uranyl selenites?

Two new layered uranyl selenites, [C4H12N 2]0.5[UO2(HSeO3)(SeO3)] (1) and [C6H14N2]0.5[UO 2(HSeO3)(SeO3)]· 0.5H 2O·0.5CH3CO2H (2), have been isolated from mild hydrothermal reactions. The preparation of 1 was achieved by reacting UO2(C2H3O2)2· 2H2O with H2SeO4 in the presence of piperazine at 130 °C for 2 d. Crystals of 2 were synthesized by reacting UO 2(C2H3O2)2·2H 2O, H2SeO4, and 1,4-diazabicyclo[2.2.2]octane at 150 °C for 2 d. The structure of 1 consists of UO2 2+ cations that are bound by bridging HSeO3- anions and chelating/bridging SeO32- anions to yield UO7 pentagonal bipyramids. The joining of the uranyl moieties by the hydrogen selenite and selenite anions creates two-dimensional ∞2[UO2(HSeO3)(SeO 3)]- layers that extend in the bc-plane. The stereochemically active lone pair of electrons on the HSeO3 - and SeO32- anions align along the a-axis making each layer polar. The ∞2[UO 2(HSeO3)(SeO3)]- layers and piperazinium cations stack in a AA′BAA′B sequence where two layers stack on one another without intervening piperazinium cations. While each ∞2[UO2(HSeO3)(SeO 3)]- layer is polar, in the AA′ stacking, the polarity of the second sheet is reversed with respect to the first, yielding an overall structure that is centrosymmetric. The structure of 2 is constructed from uranyl cations that are bound by three bridging SeO3 2- and two bridging HSeO3- anions to create UO7 pentagonal bipyramids. The linking of the uranyl cations by the HSeO3- and SeO32- anions creates ∞2[UO2(HSeO3)(SeO 3)]- layers that extend in the ac-plane. In 1 and 2, the organic ammonium cations form hydrogen bonds with the anionic uranyl selenite layers. Crystallographic data: 1, monoclinic, space group P21/c, a = 10.9378(5) Å, b = 8.6903(4) Å, c = 9.9913(5) Å, β = 90.3040(8)°, Z = 4; 2, orthorhombic, space group Pnma, a = 13.0858(8) Å, b = 17.555(1) Å, c = 10.5984(7) Å, Z = 8.