Crystal engineering with the uranyl cation and amino acids

Uranyl hybrid materials attract interest owing to promise of synthesizing functional materials, but typically experience limitations in extending dimensionality. This is due to the tendency of the uranyl cation to oligomerize along its equatorial plane, leading to the formation of flat secondary building units. One way to overcome these limitations is to utilize weak interactions to hold a structure together. This can be achieved through using ligands to build secondary building units through strong coordinative bonds that simultaneously provide supramolecular interactions as a means to extend dimensionality in the structure. We examined amino acids as a ligand choice because of its dual features of having a carboxyl group for coordination to the uranyl cation and an amino group that can be protonated to provide charge-assisted hydrogen bonding between to secondary building units in the structure.

Aqueous benchtop chemistry in ambient conditions were used to synthesize and crystallize thirteen uranyl-glycine coordination compounds whose structures were elucidated with single crystal X-ray diffraction. Under these conditions, 1D coordination polymers form. The structural features in these compounds were varied to investigate their effects on the hydrogen bonding, including the presence/absence of metal center hydrolysis, the presence of other H-bond accepting carboxylate ligands, the use of dicarboxylic acid ligands to connect uranyl centers, and the addition of a secondary metal. The compounds provide insight into how the charge-assisted hydrogen bonding provided by zwitterionic amino acids is a viable means to extending the dimensionality of uranyl hybrid materials in a variety of chemical systems.