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.