Pb-oxo Interactions in Uranyl Hybrid Materials: a Combined Experimental and Computational Analysis of Bonding and Spectroscopic Properties

Reported are the syntheses and characterization of six new heterometallic UO22+/Pb2+ compounds. These materials feature rare instances of M-oxo interactions, which influence bonding properties of the uranyl cation. The spectroscopic effects of these interactions were measured using diffuse reflectance, luminescence and Raman spectroscopy. Computational density functional theory (DFT) based natural bonding orbital (NBO) and quantum theory of atoms in molecules (QTAIM) methods indicate interactions arise predominantly through charge transfer between cationic units via the electron donating uranyl O spx 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 ranging from 0.47 kcal/mol in the longer contacts to 21.94 kcal/mol in the shorter contacts. Uranyl units with stronger interactions 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 where strong Pb-oxo interactions are present, and is accompanied by significant 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.