Nanojars have emerged as a class of anion sequestration agents of unparalleled efficiency. Dinegative oxoanions (e.g., carbonate, sulfate) template the formation of a series of homologous nanojars of the formula [anion{Cu(OH)(pz)}n] (Cun; 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). These supramolecular assemblies have been characterized by electrospray-ionization mass spectrometry (ESI-MS), variable-temperature 1H NMR spectroscopy, X-ray crystallography and thermogravimetry. 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 25−150 °C temperature range and a 41−(−59) ppm chemical shift range permits the assignment of individual protons in six different 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 selective extraction of highly hydrophilic anions, such as sulfate and carbonate, from water into organic solvents, in spite of their very large hydration energies.

Anion-Incarcerating nanojars: Solution and solid-state characterization and selective extraction of anions from water

253rd National Meeting (2017)

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