Synthesis of mixed metal-organic pyrogallolarene nanocapsules and their host-guest chemistry
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] The formation of nanometer scale capsule (or capsule-like) assemblies continues to attract interest due to the potential application of such architectures as encapsulation tools or delivery vehicles, gas sequestration agents, and catalytic media, for example. C-alkylpyrogallolarenes have been utilized as effective molecular subunits for assembly into supra-molecules due to their bowl-shaped cavity and their ability to form complementary and structurally directing intermolecular interactions. We recently discovered that Ga ions form metal-organic hexameric nano-capsule (MONC) with various pyrogallolarene, in which 12 metal centers displaced only 36 of the 72 hydroxyl protons, resulting in distorted 'rugby-ball' like shapes and the incorporation of water molecules into what we have termed structural 'gates'. Herein we exploited this feature and tried to add various metal centers to 'fill up' the remaining potential binding sites. As a result, it is shown that the addition of different metal centers into this assembly results in the formation of a series of mixed metal MONCs with varied degrees of Ga composition and various shape change of the final nano-container. In addition to this, metal exchangeability also occurs among this family of mixed MONCs, which can be employed as a new route towards tailoring metal ratios within these large assemblies. Meanwhile, our experiments also show that metal ions (Ag, Cs or Tl) and some anions (nitrate and sulfate) can move through aqueous structural gates to the interior of pyrogallolarene metal-organic nano-capsules. Moreover, as we attempted to 'stitch up' the capsule seam with the presence of elusive guest Cs metal centres, we found both changing the anion in the potential Cs guest species and adding some cation into the solution of Ga-MONCs containing Cs species can help to 'lock' the tertiary metal on the capsule interior. Subsequent 'stitching up' of the capsule seam, combined with this locking process, facilitates the permanent Cs encapsulation and affords tri-metallic Ga/Zn/Cs MONCs. On the other hand, the following Zn 'stitch up' can lead to no Tl loss for the Tl overall assembly and we finally obtained another tri-metallic Ga/Zn/Tl MONC. This is assumed as the extremely high association constant between the Tl ion and pyrogallolarene molecules. Finally, the aggregate behaviors of Ga-MONCs and mixed Ga/Cu-MONCs have been studied in various solvent systems. Both spherical and hexagonal superstructures have been observed for a series of pyrogallarene MONCs. Furthermore, these spherical aggregates have shown a good stability in the air.
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