Pb-Oxo Interactions in Uranyl Hybrid Materials: A Combined Experimental and Computational Analysis of Bonding and...

Reported are the syntheses and characterization of six new heterometallic UO₂²⁺/Pb²⁺ compounds. These materials feature rare instances of M-oxo interactions, which influence the bonding properties of the uranyl cation. The spectroscopic effects of these interactions were measured using luminescence and Raman spectroscopy. Computational density functional theory-based natural bonding orbital and quantum theory of atoms in molecules methods indicate interactions arise predominantly through charge transfer between cationic units via the electron-donating uranyl O sp lone pair orbitals and electron-accepting Pb²⁺ p orbitals. The interaction strength varies as a function of Pb-oxo interaction distance and angle with energy values ranging from 0.47 kcal/mol in the longer contacts to 21.94 kcal/mol in the shorter contacts. Uranyl units with stronger interactions at the oxo display an asymmetric bond weakening and a loss of covalent character in the U═O bonds interacting closely with the Pb²⁺ ion. Luminescence quenching is observed in cases in which strong Pb-oxo interactions are present and is accompanied by red-shifting of the uranyl symmetric Raman stretch. Changes to inner sphere uranyl bonding manifest as a weakening of the U═O bond as a result of interaction with the Pb²⁺ ion. Comprehensive evaluation of the effects of metal ions on uranyl spectra supports modeling efforts probing uranyl bonding and may inform applications such as forensic signatures.