Sulfate-Incarcerating Nanojars: Solution and Solid-State Studies, Sulfate Extraction from Water, and Anion Exchange with Carbonate

A series of 9 homologous sulfate-incarcerating
nanojars [SO4⊂{Cu­(OH)­(pz)}n]2– (Cun; n = 27–33;
pz = pyrazolate), based on combinations of three [Cu­(OH)­(pz)]x rings (x = 6–14,
except 11)namely, 6 + 12 + 9 (Cu27), 6 + 12 + 10
(Cu28), 8 + 13 + 8 (Cu29), 7 + 13 + 9 (Cu29), 8 + 14 + 8 (Cu30), 7 + 14 + 9 (Cu30), 8 + 14 + 9 (Cu31), 8 + 14 + 10 (Cu32), and
9 + 14 + 10 (Cu33)has been obtained and characterized
by electrospray-ionization mass spectrometry (ESI-MS), variable-temperature 1H NMR spectroscopy, and thermogravimetry. The X-ray crystal
structure of Cu29 (8 + 13 + 8) is described. Cu32 and Cu33, 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 1H 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, 1H NMR offers additional information about their detailed composition.
Thus, nanojars such as Cu29 (8 + 13 + 8) and Cu29 (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 Cu33 nanojar, as well as in explaining the absence of the Cu11 ring from the Cu6–Cu14 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.

一系列由九个同源硫酸盐包裹型纳米笼[SO4⊂{Cu-(OH)-(pz)}n]2–（Cun；n = 27–33；pz = 咪唑）构成，这些纳米笼基于三种[Cu-(OH)-(pz)]x环（x = 6–14，除11外）的组合，具体包括6 + 12 + 9（Cu27）、6 + 12 + 10（Cu28）、8 + 13 + 8（Cu29）、7 + 13 + 9（Cu29）、8 + 14 + 8（Cu30）、7 + 14 + 9（Cu30）、8 + 14 + 9（Cu31）、8 + 14 + 10（Cu32）和9 + 14 + 10（Cu33）。这些纳米笼通过电喷雾电离质谱法（ESI-MS）、变温1H核磁共振波谱法以及热重分析法进行了获得和表征。Cu29（8 + 13 + 8）的X射线晶体结构得到了描述。Cu32和Cu33，作为本系列中最大的纳米笼，首次被观察到。尽管在特定温度下存在广泛的重叠，但通过监测60种不同的1H核磁共振波谱中反磁性移位的咪唑和OH质子信号的温度依赖性变化，在25–150 °C的温度范围和41 ppm至−59 ppm的化学位移范围内，可以实现对混合物中六个不同硫酸盐纳米笼中个别质子的归属。与仅提供纳米笼尺寸的ESI-MS不同，1H核磁共振提供了关于其详细组成的额外信息。因此，如Cu29（8 + 13 + 8）和Cu29（7 + 13 + 9）这样的纳米笼可以在溶液中轻易区分。高温溶液研究揭示了在室温下动力学控制下获得的尺寸不同的纳米笼的热稳定性存在显著差异，有助于预测Cu33纳米笼的结构，并解释Cu11环在Cu6–Cu14系列中的缺失。使用硫酸盐和碳酸盐进行的阴离子交换研究表明，尽管每个纳米笼在热力学上对特定尺寸的阴离子有偏好，但已囚禁的阴离子的交换受到显著的动力学障碍。纳米笼对阴离子的强结合能力使得可以从水中提取高度亲水的阴离子，如硫酸盐和碳酸盐，进入有机溶剂，尽管它们的亲水化能非常大。