Nanojars are large (2 nm wide) anion-incarcerating coordination complexes of the composition [anion⊂{Cu(μ-OH)(μ-pz)}n] (n = 27-36), formed by the self-assembly of simple Cu(2+), HO(-), and pyrazolate (pz(-) = C3H3N2(-)) ions in the presence of certain anions with large hydration energy (e.g., CO3(2-), SO4(2-), PO4(3-), HPO4(2-)). Nanojars display spectacular chemical properties, such as unparalleled anion binding strength and, as shown herein, extraordinary resistance to extreme alkalinities (10 M NaOH). To shed light on the mechanism of the self-assembly process leading to these distinctive constructs, we employed an array of complementary techniques including mass spectrometry, pH titration, UV-vis and NMR spectroscopies, chemical synthesis, and single-crystal X-ray diffraction. In the reaction of Cu(NO3)2, pyrazole, NaOH, and Na2CO3 in tetrahydrofuran (THF), the first major intermediate is a trinuclear copper pyrazolate complex, [Cu3(μ3-OH)(μ-pz)3(NO3)2(H2O)], which was separately isolated and characterized. As the THF-insoluble NaOH slowly reacts, the nitrate ions are gradually precipitated out as NaNO3 and replaced by hydroxide ions. The resulting species, [Cu3(μ3-OH)(μ-pz)3(OH)x(NO3)3-x](-) (x = 1-3), have unstable terminal Cu-OH groups and react with each other to yield OH-bridged units, such as [Cu3(μ3-OH)(μ-pz)3(NO3)2]2(μ-OH) and then [{Cu3(μ3-OH)(μ-pz)3(μ-OH)2}x(NaNO3)y(Na2CO3)z] oligomers. The Cu3(OH)3(pz)3 repeating units of these oligomers have the same composition as the [Cu(OH)(pz)]n (n = 3x) nanojars and rearrange to the final products, Na2[CO3⊂{Cu(μ-OH)(μ-pz)}n] (n = 27, 29, 31), while eliminating the last amounts of NaNO3. pH titration, UV-vis monitoring, and chemical synthesis also confirm the formation of the trinuclear intermediate, followed by its clean transformation to nanojars. While displaying an unusual stability to high pH, nanojars are sensitive to acids stronger than water, a property exploitable for the recovery of the incarcerated anion. On lowering the pH, nanojars first break down to trinuclear complexes and finally to copper ions and pyrazole. This process is fully reversible, and nanojars are reassembled as pH is increased.

In one word,h ow would you describe your research? Spectacular!Did serendipity play apart in this work?Serendipity plays ac ontinuous role in our research, from the initial discovery of this novel class of extremely efficient anion incarcerating agents, which we termed nanojars, to the current results presented here.What was the biggest surprise (on the way to the results presented in this paper)?Given the structural complexity and novelty of nanojars, certainly nobody could have predicted the effect of ammonia on an anojar mixture. It was indeed ah uge surprise to find that one of the five homologous nanojars in the mixture is totally resistant to the action of NH 3 in solution, while the other four break down and rearrange into the NH 3 -resistant one.What was the biggest challenge( on the way to the results presented in this paper)?Although nanojars are readily synthesized, they always form as am ixture of nanojars of different sizes. Apart from crystallization that occasionally results in af ew small crystals, all common separation methods failed to provide larger samples of pure, individual nanojars. What aspects of this project do you find most exciting?This project involves ac omplex array of techniques, including covalent synthesis of new organic ligands, supramolecular/coordination self-assembly,X -ray crystallography,m ass spectrometry and nuclear magnetic resonance techniques, which all unveil unique, intriguing results about nanojars, and point to potential applications. The most exciting part of our research always is designing the next step, and awaiting for the results of the new experiments. Does the research open other avenues that you would like to investigate?One new avenue we are currently exploring is the selective extraction of anions from contaminated water,i ncluding highly toxic anions, such as arsenate, chromate and selenate, using nanojars.Invited for the cover of this issue is the team led by Gellert Mezeia tW estern Michigan University.T he image depicts "ammonia" selectively breakingd own nano jar mixtures into pure nanojar species, but one resists!R ead the full text of the arti-

Nanojars, a novel class of neutral anion-incarcerating agents of the general formula [Cu(II)(OH)(pz)]n (Cun; n = 27-31, pz = pyrazolate anion), efficiently sequester various oxoanions with large hydration energies from water. In this work, we explore whether substituents on the pyrazole ligand interfere with nanojar formation, and whether appropriate substituents could be employed to tune the solubility of nanojars in solvents of interest, such as long-chain aliphatic hydrocarbons (solvent of choice for large-scale liquid-liquid extraction processes) and water. To this end, we conducted a comprehensive study using 40 different pyrazole ligands, with one, two or three substituents in their 3-, 4- and 5-positions. The corresponding nanojars are characterized by single-crystal X-ray diffraction and/or electrospray-ionization mass spectrometry (ESI-MS). The results show that Cun-nanojars with various substituents in the pyrazole 4-position, including long chains, phenyl and CF3 groups, can be obtained. Straight chains are also tolerated at the pyrazole 3-position, and favor the Cu30-nanojar. Homoleptic nanojars, however, could not be obtained with phenyl or CF3 groups. Nevertheless, if used in mixture with the parent non-substituted pyrazole, sterically hindered pyrazoles do form heteroleptic nanojars. With 3,5-disubstituted pyrazoles, only heteroleptic nanojars are accessible. The crystal structure of novel nanojars (Bu4N)2[CO3⊂{Cu30(OH)30(3,5-Me2pz)y(pz)30-y}] (y = 14 and 15) is presented. We find that in contrast to the parent nanojar, which is insoluble in aliphatic solvents and water, nanojars with alkyl substituents are soluble in saturated hydrocarbon solvents, whereas nanojars based on novel pyrazoles, functionalized with oligoether chains, are readily soluble in water. Liquid-liquid extraction of carbonate from water under basic pH is presented for the first time.

A series of 9 homologous sulfate-incarcerating nanojars [SO⊂{Cu(OH)(pz)}] (Cu; n = 27-33; pz = pyrazolate), based on combinations of three [Cu(OH)(pz)] rings (x = 6-14, except 11)-namely, 6 + 12 + 9 (Cu), 6 + 12 + 10 (Cu), 8 + 13 + 8 (Cu), 7 + 13 + 9 (Cu), 8 + 14 + 8 (Cu), 7 + 14 + 9 (Cu), 8 + 14 + 9 (Cu), 8 + 14 + 10 (Cu), and 9 + 14 + 10 (Cu)-has been obtained and characterized by electrospray-ionization mass spectrometry (ESI-MS), variable-temperature H NMR spectroscopy, and thermogravimetry. The X-ray crystal structure of Cu (8 + 13 + 8) is described. Cu and Cu, 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 H 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,H NMR offers additional information about their detailed composition. Thus, nanojars such as Cu (8 + 13 + 8) and Cu (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 Cu nanojar, as well as in explaining the absence of the Cu ring from the Cu-Cu 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.