Use of Charge-Assisted Hydrogen Bonding in the Supramolecular Assembly of Hybrid Uranyl Materials

Supramolecular assembly
of U­(VI) materials can be limited by the
passivation of the uranyl oxo group and the propensity of the metal
center to hydrolyze, resulting in the formation of extended two-dimensional
(2D) structures. To overcome these barriers, the use of charge-assisted
H-bonding was explored using amino acids (glycine [Gly] and l-alanine [Ala]), resulting in the formation of three novel compounds
{[(UO2)3(Gly)2(O)2(OH)2]­(H2O)6 (1), [(UO2)5(Gly)4(O)3(OH)3]­(NO3)­(H2O)12 (2), and [(UO2)3(Ala)2O­(OH)3]­(NO3)­(H2O)3 (3)} that have been characterized
by X-ray diffraction, elemental analysis, TGA, and vibrational spectroscopy.
Hydrolysis of the uranyl cation (UO22+) chelated
by bridging zwitterionic amino acids results in the formation of infinite
chains when synthesized from mildly acidic aqueous solutions. While
positively charged chains form densely packed structures, the neutral
UO2-glycine chains support a nanoporous (internal diameter
∼1.35 nm) supramolecular architecture through multifurcated
charge-assisted hydrogen bonding. These interactions occur directly
between the protonated amine of glycine and the uranyl’s oxo
moiety, representing a unique supramolecular synthon for the assembly
of hybrid porous uranyl materials. The zwitterionic glycine ligands
also assist in the helical assembly of water molecules that are hydrogen
bonded to the interior walls of the nanopores, resulting in the formation
of an empty 0.85 nm channel through the pore space.

铀(VI)材料的超分子组装（supramolecular assembly）常受限于铀酰氧基团（uranyl oxo group）的钝化作用以及金属中心的水解倾向，最终会生成延伸型二维（2D）结构。为克服上述限制，本研究采用氨基酸（甘氨酸[Gly]与L-丙氨酸[Ala]）构建电荷辅助氢键（charge-assisted H-bonding），成功合成了三种新型化合物：{[(UO₂)₃(Gly)₂(O)₂(OH)₂]·(H₂O)₆ (1)、[(UO₂)₅(Gly)₄(O)₃(OH)₃]·(NO₃)·(H₂O)₁₂ (2) 以及 [(UO₂)₃(Ala)₂O(OH)₃]·(NO₃)·(H₂O)₃ (3)}，并通过X射线衍射（X-ray diffraction）、元素分析、热重分析（TGA）与振动光谱学（vibrational spectroscopy）对其进行了表征。由桥联两性离子氨基酸螯合的铀酰阳离子（uranyl cation，UO₂²⁺）在弱酸性水溶液中发生水解反应时，会生成无限长链结构。带正电的链会形成紧密堆积的结构，而中性铀酰-甘氨酸链则通过多分叉电荷辅助氢键构建出内径约为1.35 nm的纳米多孔超分子架构（supramolecular architecture）。这类相互作用直接发生在甘氨酸的质子化氨基与铀酰的氧代基团之间，代表了一种用于构建杂化多孔铀酰材料的独特超分子合成子（supramolecular synthon）。两性离子甘氨酸配体还能促进水分子形成螺旋组装体，这些水分子通过氢键与纳米孔内壁结合，最终在孔道内形成直径为0.85 nm的中空通道。