A Spontaneous Structural Transition of {U₂₄Pp₁₂} Clusters Triggered by Alkali Counterion Replacement in Dilute Solution

Abstract

At ransition between two isomeric clusters involving the change of the main skeleton structure of aw ell-defined,r igid molecular cluster [(UO 2 ) 24 (O 2 ) 24 (P 2 O 7 ) 12 ] 48À ,{ U 24 Pp 12 }, is achieved by simply introducing propera lkali cationsi nto its dilute aqueouss olution. While the unique structuralt ransition can be triggered by introducing any of the Na + /K + /Rb + /Cs + alkali ions, the two isomers, Li/Na-{U 24 Pp 12 }a nd Na/K-{U 24 Pp 12 }, as typical macroions,c an accurately choose among different alkali counter-cations based on their hydrated sizes, and the ion selectivity process clearlys howed endothermic features.T he preferred K + and Rb + ions have suitable sizes to be incorporated into the properw indows on {U 24 Pp 12 }n anocapsules,a ss upported by the transition points in both ITC studies and IR measurements.Isomerism is commonly seen in macromolecular systems from biomacromolecules to inorganic clusters such as polyoxometalate (POM) clusters.[1] For well-defined POM molecular clusters, understanding the stability and transition issues will underpin its applicationsinb iochemistry, [2] drug delivery, [3] catalysis [4] etc. As an example, the geometrical isomers of the Kegginf amily, depending on the successive 608-rotations of the trimeric group, were postulated by Baker and Figgis in 1970.[5] The unusual structuralf lexibility has led to the efforts for rationalizing the dynamics and stabilityo fK eggin anions.[6] The five anionic isomers, with the same formula, demonstrated different redox properties that vary from each other,w hich made the early investigationst owards triggering the isomeric transition mainly concentrateo nc hanging their redoxs tates, for example, by heatingo ra pplying reductant. [1b, 6b] Furthermore, Kim and Pope reported that the positions of different coordinated cations sandwiched between two A-type R-PW9 Keggins were not alwayst he same, while the structure of the two individual clusters remained untouched.[7] Herein we report an ovel type of isomeric structural transition of the uranyl peroxide clusters, which is triggered and affected by its ion exchange and ion selectivity properties. To the best of our knowledge,t his is, so far,t he mildest and most convenient way of changing the main skeleton of ap olyoxoanion.Counterionsp lay an essential role in regulating the behaviors of soluble macroions and charged nanoparticles in solution.[8] The demands for precise guidance on ions within inorganic nanopores have drawn substantial attention.[9] Exploring the nature of ion selectivity and its consequent effect on macroions, especially their stability, remainsachallenge.T he actinyl peroxide nanocages are ideal modelsf or investigating the macroion-counterion interaction in solution owing to their rigid andp orous structures, good stability, and suitable charge density.[10] The developmento fu ranium-based nanomaterials can also impact the management of nuclear wastes. [11] We probe the ion selectivity and the transition of two isomeric uranyl...

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

“…For comparison, a prior isothermal titration calorimetry study produced the same ratio for Rb + as the titrant but a higher 1:8 ratio for K + . 22 2). The crystallographic results, including unit cell parameters, and structure descriptions are available in the Supporting Information.…”

“…For comparison, a prior isothermal titration calorimetry study produced the same ratio for Rb + as the titrant but a higher 1:8 ratio for K + . 22 Slow evaporation of solutions titrated with KNO 3 and RbNO 3 (for K + /Rb + mol equiv ≥ 3.1) resulted in the formation of crystals that were suitable for single-crystal X-ray diffraction studies. Diffraction results confirmed the proposed D 4h molecular symmetry of { U 2).…”

“…The Pp-functionalized uranyl peroxide nanocluster {U 24 Pp 12 } is one of the most studied actinide–peroxide complexes, for reasons discussed above. − Prior findings demonstrated that {U 24 Pp 12 } can assume at least two unique conformations corresponding to an idealized O h (Li/Na salt, δ = 3.80 ppm) or D 4 h (Na/K salt, δ = 3.45/4.28 ppm) molecular symmetry. , Herein, solutions containing solvated Li/Na salt of {U 24 Pp 12 } ([{U 24 Pp 12 }] = 1.6 mM) were titrated using 0.25 M KNO 3 , RbNO 3 , and CsNO 3 solutions. 1D 31 P NMR spectra representing different stages of the titration process are summarized in Figures (KNO 3 and RbNO 3 ) and S1 (CsNO 3 ) and Table .…”

“…20 A subsequent isothermal titration calorimetry study noted that the introduction of K + or Rb + into the O h {U 24 Pp 12 } system led to a strong endothermic process that was interpreted to indicate binding of these cations within the cluster's cavity and a change to the D 4h molecular symmetry. 22 Because no bonds are broken, this change in symmetry is subsequently referred to as conformational rearrangement.…”

“…NMR experiments demonstrated that the two chemically unique Pp environments are present in its D 4 h conformation transfer magnetization dynamically. − Single-crystal diffraction studies of {U 24 Pp 12 } showed that different conformations of this cluster ( O h vs D 4 h ) are likely associated with the presence of different alkali-metal cations in their cavities . A subsequent isothermal titration calorimetry study noted that the introduction of K + or Rb + into the O h {U 24 Pp 12 } system led to a strong endothermic process that was interpreted to indicate binding of these cations within the cluster’s cavity and a change to the D 4 h molecular symmetry . Because no bonds are broken, this change in symmetry is subsequently referred to as conformational rearrangement .…”