The previously unexplored noncovalent binding of the highly toxic tetrafluoroberyllate anion (BeF4 2–) and its extraction from water into organic solvents are presented. Nanojars resemble anion-binding proteins in that they also possess an inner anion binding pocket lined by a multitude of H-bond donors (OH groups), which wrap around the incarcerated anion and completely isolate it from the surrounding medium. The BeF4-binding propensity of [BeF4⊂{CuII(OH)­(pz)} n ]2– (pz = pyrazolate; n = 27–32) nanojars of different sizes is investigated using an array of techniques including mass spectrometry, paramagnetic 1H, 9Be, and 19F NMR spectroscopy, and X-ray crystallography, along with thermal stability studies in solution and chemical stability studies toward acidity and Ba2+ ions. The latter is found to be unable to precipitate the insoluble BaBeF4 from nanojar solutions, indicating a very strong binding of the BeF4 2– anion by nanojars. 9Be and 19F NMR spectroscopy allows for the unprecedented direct probing of the incarcerated anion in a nanojar and, along with 1H NMR studies, reveals the fluxional structure of nanojars and their inner anion-binding pockets. Single-crystal X-ray diffraction provides the crystal and molecular structures of (Bu4N)2[BeF4⊂{Cu­(OH)­(pz)}32], which contains a novel Cu x -ring combination (x = 9 + 14 + 9), (Bu4N)2[BeF4⊂{Cu­(OH)­(pz)}8+14+9], and (Bu4N)2[BeF4⊂{Cu­(OH)­(pz)}6+12+10] and offers detailed structural parameters related to the supramolecular binding of BeF4 2– in these nanojars. The extraction of BeF4 2– from water into organic solvents, including the highly hydrophobic solvent n-heptane, demonstrates that nanojars are efficient binding and extracting agents not only for oxoanions but also for fluoroanions.

By using nanojars as anion binding agents, selectivity for either carbonate or sulfate can be achieved upon rigidification of the nanojar outer-shell with tethers of varying lengths between pairs of pyrazole ligands.

Nanojars are a class of supramolecular anion-incarcerating coordination complexes that self-assemble from Cu2+ ions, pyrazole, and a strong base in the presence of highly hydrophilic anions. In this work, we show that if the strong base (e.g., NaOH or Bu4NOH) is replaced by a weak base such as a trialkylamine, capped nanojars of the formula [{Cu3(μ3-OH)­(μ-pz)3L3}­CO3⊂{Cu­(μ-OH)­(μ-pz)} n ] (pz = pyrazolate anion; L = neutral donor molecule; n = 27–31) are obtained instead of the conventional nanojars. Yet, to obtain capped nanojars, the conjugate acid side product originating from the weak base must be separated by transferring it to water either by precipitation of the water-insoluble capped nanojars or by liquid–liquid extraction. Full characterization using electrospray ionization mass spectrometry, UV–vis and variable-temperature 1H NMR spectroscopy in solution, and single-crystal X-ray diffraction, elemental analysis, and solubility studies in the solid state reveals similarities as well as drastic differences between capped nanojars and nanojars lacking the [Cu3(μ3-OH)­(μ-pz)3L3]2+ cap. Acid–base reactivity studies demonstrate that capped nanojars are intermediates in the pH-controlled assembly–disassembly of nanojars. During the self-assembly of capped nanojars, CO2 is selectively sequestered from air in the presence of other atmospheric gases and converted to carbonate, the binding of which is selective in the presence of NO3 –, ClO4 –, BF4 –, Cl–, and Br– ions.

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