Structural and Computational Investigation of Halogen Bonding Effects on Spectroscopic Properties within a Series of Halogenated Uranyl Benzoates

The synthesis, structure, and spectroscopic characterization (luminescence, Raman, IR, and solid-state UV-vis) of five new uranyl (UO22+) complexes containing 2,2'-bipyridine and either benzoic acid (1), 4-fluorobenzoic acid (2), 4-chlorobenzoic acid (3), 4-bromobenzoic acid (4), and 4-iodobenzoic acid (5) are described. Single crystal X-ray diffraction analysis shows isostructural molecular complexes, with hexagonal bipyramidal uranyl metal centers coordinated by 2,2'-bipyridine and para-halobenzoate ligands. A notable halogen interaction to a uranyl yl oxo ligand is only present when the largest, most polarizable halogen is utilized with 4-iodobenzoate in complex 5. Inductive effects of increasingly larger halogens on equatorial ligands paired with the introduction of a halogen-oxo interaction produce red-shifts in both the luminescent and Raman spectra across the series of para-halogenated benzoates. Two uranyl Raman bands with relatively high intensity were observed in all compounds, and density functional theory calculations suggested a model consistent with anharmonic resonance coupling between the benzoate ligands and the uranyl yl stretch. Further, electrostatic surface potential maps and Hirshfeld surface analysis augment the discussion of noncovalent assembly and structure-property relationships by providing visualizations of the o -hole generated by polarized halogen atoms and the electrostatic potential of halogen-based acceptor-donor pairings.