Nanojars are neutral, nano-sized anion-incarcerating agents of the general formula [CuII(µ-OH) (µ-pz)]n (n = 26 – 36), formed by the reaction of Cu2+, HO– and pyrazolate ions in the presence of oxoanions with large hydration energy (e.g. CO32–, SO42–, PO43–, HPO42–). Nanojars bind oxoanions with unparalleled strength and are stable to highly alkaline (10 M NaOH) solutions. To understand the self-assembly process and the extraordinary chemical properties of nanojars, we carried out a mechanistic study by using various techniques including mass spectrometry, pH titration, UV–vis and NMR spectroscopies, chemical synthesis, and single-crystal X-ray diffraction. [Cu3(µ3-OH)(µ-pz)3(NO3)2(H2O)], a trinuclear species, is the first major intermediate from the reaction of Cu(NO3)2, pyrazole, NaOH and Na2CO3 in THF. The nitrate ions then gradually precipitate out as NaNO3 and are replaced by hydroxide ions originating from the slowly reacting, THF-insoluble NaOH pellets. 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 form 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. In these oligomers, the Cu3(OH)3(pz)3 repeating units have the same composition as the [Cu(OH)(pz)]n (n = 3x) nanojars and are rearranged to the final products, Na2[CO3{Cu(µ-OH) (µ-pz)}n] (n = 27, 29, 31), while eliminating the last amounts of NaNO3. Although nanojars show high stability at extremely basic conditions, acids stronger than water are able to destroy nanojars, a property exploitable for the recovery of the incarcerated anions.

Mechanistic study of the pH-dependent assembly/disassembly of nanojars

254th National Meeting (2017)

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