Heterometallic UO₂²⁺/Ag⁺ Complexes: Structural Design and Luminescence Properties

Reported are the synthesis, structural characterization, and luminescence properties of 11 novel UO₂²⁺/Ag⁺ heterometallic complexes. Halogenated benzoic acids (2,6-dihalobenzoic acid (halo = F, Br), 3,5-dichlorobenzoic acid, and 3-halobenzoic acid (halo = Br, I)) and N-donor polycyclic ligands (2,2′-bipyridine, 2,2’;6′,2″-terpyridine, 1,10-phenanthroline, 2,2′-bipyrimidine) were employed to synthesize a set of compounds and induce structural diversity. The primary mode of coordination with the uranyl cation is hexagonal bipyramidal monomeric units with three halobenzoate ligands in the equatorial plane, though 1-D chains with pentagonal bipyramidal uranyl centers also form. The Ag⁺ cations coordinate preferentially to the N-donor ligands and serve as counter-cations for the anionic uranyl motifs. The soft ligand character of the N-donor molecules is found to be a requirement for the inclusion of the Ag⁺ cation into the structures. Anionic uranyl units and cationic silver units assemble via noncovalent interactions between π systems on adjacent rings and between halogens (when Br and I are present). Solid-state emission spectra display the usual uranyl band with superimposed vibronic fine structure, except for that of compound 1, which shows emission from the 2,2′-bipyridine center. This family of compounds represents a substantial contribution to the already rich library of UO₂²⁺/Ag⁺ compounds, and the synthetic parameters discussed within reveal a platform for the design of new heterometallic uranyl-containing materials.