Dynamics of Cation-Induced Conformational Changes in Nanometer-Sized Uranyl Peroxide Clusters

Conformational changes of the pyrophosphate (Pp)-functionalized uranyl peroxide nanocluster [(UO2)(24)(O-2)(24)(P2O7)(12)](48-)({U(24)Pp(12)}), dissolved as a Li/Na salt, can be induced by the titration of alkali cations into solution. The most symmetric conformer of the molecule has idealized octahedral (O-h) molecular symmetry. One-dimensional P-31 NMR experiments provide direct evidence that both K+ and Rb+ ions trigger an O-h-to-D-4h conformational change within {U(24)Pp(12)}. Variable-temperature P-31 NMR experiments conducted on partially titrated {U(24)Pp(12)} systems show an effect on the rates; increased activation enthalpy and entropy for the D-4h-to-O-h transition is observed in the presence of Rb+ compared to K+. Two-dimensional, exchange spectroscopy P-31 NMR revealed that magnetization transfer links chemically unique Pp bridges that are present in the D-4h conformation and that this magnetization transfer occurs via a conformational rearrangement mechanism as the bridges interconvert between two symmetries. The interconversion is triggered by the departure and reentry of K (or Rb) cations out of and into the cavity of the cluster. This rearrangement allows Pp bridges to interconvert without the need to break bonds. Cs ions exhibit unique interactions with {U(24)Pp(12)} clusters and cause only minor changes in the solution P-31 NMR signatures, suggesting that O-h symmetry is conserved. Single-crystal X-ray diffraction measurements reveal that the mixed Li/Na/Cs salt adopts D-2h molecular symmetry, implying that while solvated, this cluster is in equilibrium with a more symmetric form. These results highlight the unusually flexible nature of the actinide-based {U(24)Pp(12)} and its sensitivity to countercations in solution.