Probing the Influence of N-Donor Capping Ligands on Supramolecular Assembly in Molecular Uranyl Materials

The syntheses and crystal structures of six new compounds containing the UO22+ cation, 3,5-dichlorobenzoic acid, and a chelating N-donor [2,2-bipyridine (bipy), 1,10-phenanthroline (phen), 4,7-dimethyl-1,10-phenanthroline (dimethylphen), 2,2:6,2-terpyridine (terpy), 4-chloro-2,2:6,2-terpyridine (Cl-terpy), or 2,4,6-tris(2-pyridyl)-s-triazine (TPTZ)] are reported. Single-crystal X-ray diffraction analysis of these materials enabled the exploration of the structural relationship between the benzoic acids and the chelating N-donor as well as providing a platform to evaluate the effects of ligand choice on uranyl hydrolysis and subsequent oligomerization. At an unadjusted pH (ca. 3), a mix of uranyl monomers and dimers are observed, dimer formation resulting from both bridging carboxylate linkers and hydroxo bridges. Assembly by halogen- and hydrogen-bonding interactions as well as - interactions was observed depending on the experimental conditions utilized. Further, spectroscopic characterization (both vibrational and luminescence) of complexes 1, 4, and 5 to explore the effects of the electron-donating ability of the capping ligand on the corresponding uranyl luminescence and vibrational spectra suggests that there is a relationship between the observed bathochromic shifts and the electron-donating ability of the capping ligands.