Asymmetric Elongation of Uranyl within a Silver Nanocluster

Confinement within nanoscale cavities can profoundly alter molecular structure and electronic properties, but this strategy has never been applied to inherently linear and symmetric uranyl cations (UO22+). Here, we report the first example of uranyl encapsulation within a self-assembled silver nanocluster, [UO2F5@Ag21(CCBut)14](BF4)3F (CCBut = 3,3-dimethyl-1-butynyl), abbreviated as UO2F5@Ag21. Single-crystal X-ray diffraction reveals asymmetric elongation of the axial O ≡ U≡O bonds from the ideal symmetric value of 1.77 Å to 1.788(9) and 1.837(9) Å, respectively. Raman and IR spectroscopy show extreme redshifts of both symmetric and asymmetric stretching vibrations (ν1 down to 765 cm-1). Transient absorption spectroscopy further reveals an excited-state quenching process of the (UO2F5)3- guest driven by unique confinement. High-energy-resolved fluorescence detected X-ray absorption near edge structure (HERFD-XANES) at the U M4 edge demonstrates re-engineered 5f electronic states, with a shifted and broadened 5fσ* feature indicating reduced U≡O covalency. This work establishes nanoconfinement as a powerful new route to reshape the electronic structure and property of actinide-based materials.