Nanojars are a class of supramolecular coordination complexes that self-assemble from Cu2+, pyrazolate (C3H3N2-), and HO- in the presence of certain anions with large hydration energy. The self-assembled nanojars consist of three [(Cu(OH)(C3H3N2)]x rings (x = 6-14, except 11) combined in a jar-shaped structure that incarcerates an anion in the center. Nanojars have the general formula [A{Cu(OH)(C3H3N2)}n]; (A = anion and n = 27-33). Nanojars based on 1H-pyrazole ligands are neutral with total selectivity towards doubly charged anions such as carbonate (CO32-) and sulfate (SO42-) in the presence of singly charged anions such as nitrate (NO3-) and perchlorate (ClO4-). Carbonate nanojars form as a mixture of four different sizes [CO3{Cu(OH)(C3H3N2)}n] (n = 27, 29, 30, 31). In this work, pyrazole ligand exchange in the peripheral structure of carbonate nanojars is investigated. A series of substituted pyrazoles, such as 4-methylpyrazole, 3-methylpyrazole, 3,5-dimethylpyrazole, and 4-nitropyrazole, were introduced in various equivalents to 1H-pyrazole-based carbonate nanojars in solution, and the ligand exchange was monitored by electrospray ionization-mass spectrometry (ESI-MS). The ESI-MS spectra showed the gradually increasing ligand exchange in carbonate nanojars as more equivalents of the substituted pyrazole ligand is added. When excess 4-methylpyrazole is added to the carbonate nanojars, 100% ligand exchange occurred to the four sizes of carbonate nanojars. However, when excess 3-methylpyrazole is added to the carbonate nanojars, only 52%, 59%, 57%, and 68% ligand exchange was observed for carbonate nanojars with n = 27, 29, 30, and 31, respectively. Similarly, the substitution of 1H-pyrazole in carbonate nanojars was observed when 3,5-dimethylpyrazole or 4-nitropyrazole is used. Additionally, stanozolol (a pyrazole-containing steroid), 4-methylimidazole, and 4-methyltriazole were explored for pyrazole ligand exchange in carbonate nanojars. The ESI-MS spectra showed that 1H-pyrazole ligands in carbonate nanojars were replaced by stanozolol ligands. No pyrazole ligand exchange in carbonate nanojars was observed when 4-methylimidazole or 4-methyltriazole was used. These results reveal the ease of functionalization of nanojars through pyrazole ligand exchange and demonstrate a pathway for partial or complete post-synthetic modifications of nanojars.

Pyrazole ligand exchange in nanojars monitored by electrospray ionization mass spectrometry

ACS Fall 2021

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