Hierarchy of Pyrophosphate-Functionalized Uranyl Peroxide Nanocluster Synthesis

Herein, we report a new salt of a pyrophosphate-functionalized uranyl peroxide nanocluster {U₂₄Pp₁₂} (1) exhibiting O_h molecular symmetry both in the solid and solution. Study of the system yielding 1 across a wide range of pH by single-crystal X-ray diffraction, small-angle X-ray scattering, and a combination of traditional ³¹P and diffusion-ordered spectroscopy (DOSY) NMR affords unprecedented insight into the amphoteric chemistry of this uranyl peroxide system. Key results include formation of a rare binary {U₂₄}·{U₂₄Pp₁₂} (3) system observed under alkaline conditions, and evidence of acid-promoted decomposition of {U₂₄Pp₁₂} (1) followed by spatial rearrangement and condensation of {U₄} building blocks into the {U₃₂Pp₁₆} (2) cluster. Furthermore, ³¹P DOSY NMR measurements performed on saturated solutions containing crystalline {U₃₂Pp₁₆} show only trace amounts (∼2% relative abundance) of the intact form of this cluster, suggesting a complex interconversion of {U₂₄Pp₁₂}, {U₃₂Pp₁₆}, and {U₄Pp_4-x} ions.