Recognition, selective binding and transport of small inorganic anions by molecular receptors are essential features of several natural and industrial processes, such as anion transport in biological systems, removal of unwanted anions and recovery of valuable anions from aqueous environments. We developed a class of neutral, nano-sized toroidal copper(II)-hydroxide/pyrazolate assemblies, termed nanojars, comprised of a series of homologous anion-incarcerating oligomerization isomers of the formula [anion{Cu(OH)(pz)}n] (n = 26−33; pz = pyrazolate, C3H3N2–) which bind small inorganic anions (including carbonate, sulfate, phosphate, arsenate) with extraordinary strength. Nanojars are a unique class of anion-binding agents set apart from other artificial anion encapsulating agents by a unique combination of useful properties, including strong and selective binding of hydrophilic anions, exceptional stability at high pH, remarkable thermal stability, straightforward preparation and recyclability. We have demonstrated that nanojars are selective for trigonal planar/pyramidal and tetrahedral anions that carry a 2– or 3– charge (anions with large hydration energies, ΔGhyd &gt; –700 kJ/mol) over similarly shaped anions that carry a 1– charge (anions with small hydration energies, ΔGhyd &lt; –500 kJ/mol). In fact, nanojars do not assemble at all with anions such as nitrate or perchlorate (ΔGhyd = –289 and –229 kJ/mol, respectively). Although nanojars based on the parent pyrazole ligand (pz–) do not show significant selectivity among anions with comparable hydration energies, such as carbonate and sulfate (ΔGhyd = –1324 and –1064 kJ/mol, respectively), nanojars obtained with ethylene-tethered pyrazole ligands (–pzCH2CH2pz–) are totally selective for carbonate over sulfate. Pure carbonate-incarcerating nanojars can also be obtained easily with pz– if carbonate is the only anion source. In contrast, pure nanojars that incarcerate anions with significantly lower hydration energies (selenate, chromate, molybdate, tungstate, hydrogen phosphate) could not be obtained due to the presence of small but significant amounts of carbonate that accompanies salts of the aforementioned anions (even in high-purity reagents). We find that interference from carbonate can be eliminated by assembling the nanojars in an acidified reaction medium. Nevertheless, the acidity must be carefully controlled, as all nanojars are acid-sensitive and decompose at pH &lt; 6.

Tuning the anion binding selectivity of nanojars by ligand-shell rigidification and controlled acidification

2019 Fall National Meeting

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