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)(2)O(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 similar to 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.