Sulfate-Incarcerating Nanojars: Solution and Solid-State Studies, Sulfate Extraction from Water, and Anion Exchange...

A series of 9 homologous sulfate-incarcerating nanojars [SO₄⊂{Cu(OH)(pz)}_n]^2- (Cu_n; n = 27-33; pz = pyrazolate), based on combinations of three [Cu(OH)(pz)]_x rings (x = 6-14, except 11)-namely, 6 + 12 + 9 (Cu₂₇), 6 + 12 + 10 (Cu₂₈), 8 + 13 + 8 (Cu₂₉), 7 + 13 + 9 (Cu₂₉), 8 + 14 + 8 (Cu₃₀), 7 + 14 + 9 (Cu₃₀), 8 + 14 + 9 (Cu₃₁), 8 + 14 + 10 (Cu₃₂), and 9 + 14 + 10 (Cu₃₃)-has been obtained and characterized by electrospray-ionization mass spectrometry (ESI-MS), variable-temperature ¹H NMR spectroscopy, and thermogravimetry. The X-ray crystal structure of Cu₂₉ (8 + 13 + 8) is described. Cu₃₂ and Cu₃₃, which are the largest nanojars in this series, are observed for the first time. Despite extensive overlap at a given temperature, monitoring the temperature-dependent variation of paramagnetically shifted pyrazole and OH proton signals in 60 different ¹H NMR spectra over a temperature range of 25-150 °C and a chemical shift range from 41 ppm to -59 ppm permits the assignment of individual protons in six different sulfate nanojars in a mixture. As opposed to ESI-MS, which only provides the size of nanojars, ¹H NMR offers additional information about their detailed composition. Thus, nanojars such as Cu₂₉ (8 + 13 + 8) and Cu₂₉ (7 + 13 + 9) can easily be differentiated in solution. High-temperature solution studies unveil a significant difference in the thermal stability of nanojars of different sizes obtained under kinetic control at ambient temperature, and aid in predicting the structure of the Cu₃₃ nanojar, as well as in explaining the absence of the Cu₁₁ ring from the Cu₆-Cu₁₄ series. Anion exchange studies using sulfate and carbonate reveal that, although each anion is thermodynamically preferred by a nanojar of a certain size, the exchange of an already incarcerated anion is hampered by a substantial kinetic barrier. The remarkably strong binding of anions by nanojars allows for the extraction of highly hydrophilic anions, such as sulfate and carbonate, from water into organic solvents, despite their very large hydration energies.