Linear Alkyl Diamine-Uranium-Phosphate Systems: U(VI) to U(IV) Reduction with Ethylenediamine

Mild-hydrothermal reactions in acidic medium using 1,3-diaminopropane, 1,4-diaminobutane, and 1,5-diaminopentane as structure directing agents led to three-dimensional (3D) uranyl phosphates (CH2)3(NH3)2{[(UO2)(H2O)][(UO2)(PO4)]4} (C3U5P4), (CH2)4(NH3)2{[(UO2)(H2O)][(UO2)(PO4)]4} (C4U5P4) and (CH2)5(NH3)2{[(UO2)(H2O)][(UO2)(PO4)]4} (C5U5P4). The structures of (C4U5P4) and (C5U5P4) were solved in the space group Cmc21 using single-crystal X-ray diffraction data. The compounds are isostructural to the corresponding uranyl vanadates and contain the same 3D inorganic framework built from uranyl-phosphate layers of uranophane-type anion topology pillared by [UO6(H2O)] pentagonal bipyramids. In neutral or basic medium the alkyl diamines decompose to give ammonium uranyl phosphate trihydrate. In the same conditions by using ethylenediamine, unexpected reduction of uranium(VI) to uranium(IV) occurs leading to the formation of (CH2)2(NH3)2[U(PO4)2] (C2UP2) single crystals. C2UP2 undergoes a reversible phase transition from triclinic to monoclinic symmetry at about 230 °C. The structure of the two forms results from the stacking of inorganic layers ∞1[U(PO4)2]2−, and organic layers containing ethylene diammonium ions, the two layers being linked by hydrogen bonds. Single crystals of (CH2)2(NH3)2[PO3OH] (C2HP) are formed by evaporation of the solution after filtering of C2UP2 single crystals. The structure of C2HP contains infinite ∞1[PO3OH]2− chains connected by (CH2)2(NH3)22+ ions through hydrogen bonds.以1,3-丙二胺、1,4-丁二胺与1,5-戊二胺作为结构导向剂（structure directing agents），在酸性介质中通过温和水热反应，制备得到三种三维（3D）磷酸铀酰（uranyl phosphate）化合物：(CH₂)₃(NH₃)₂{[(UO₂)(H₂O)][(UO₂)(PO₄)]₄}（记为C3U5P4）、(CH₂)₄(NH₃)₂{[(UO₂)(H₂O)][(UO₂)(PO₄)]₄}（记为C4U5P4）以及(CH₂)₅(NH₃)₂{[(UO₂)(H₂O)][(UO₂)(PO₄)]₄}（记为C5U5P4）。借助单晶X射线衍射（single-crystal X-ray diffraction）数据，解析获得C4U5P4与C5U5P4的晶体结构，其空间群（space group）均为Cmc2₁。该系列化合物与对应的钒酸铀酰（uranyl vanadates）同构，均构建有以铀酰-磷酸层为基础的三维无机骨架；该层具备钙铀云母型阴离子拓扑结构，并由[UO₆(H₂O)]五角双锥进行柱撑。在中性或碱性介质中，烷基二胺（alkyl diamines）会发生分解，生成三水合磷酸铀酰铵。在相同介质条件下，若使用乙二胺（ethylenediamine），则会发生意外的六价铀（U(VI)）还原为四价铀（U(IV)）的反应，生成(CH₂)₂(NH₃)₂[U(PO₄)₂]（记为C2UP2）单晶。C2UP2在约230 ℃时会发生从三斜晶系（triclinic）至单斜晶系（monoclinic）的可逆相变（reversible phase transition）。两种晶型的结构均由无机层∞₁[U(PO₄)₂]²⁻与含有乙二铵离子的有机层堆叠而成，两层之间通过氢键（hydrogen bonds）相互连接。过滤C2UP2单晶后，将母液蒸发可得到(CH₂)₂(NH₃)₂[PO₃OH]（记为C2HP）单晶。C2HP的结构中存在无限∞₁[PO₃OH]²⁻链，该链通过(CH₂)₂(NH₃)₂²⁺离子与氢键相连。