Nanojars are a class of neutral, nanosized toroidal copper(II)-hydroxide/pyrazolate assemblies comprised of a series of homologous oligomerization isomers of the formula [anionÌ{CuII(μ-OH)(μ-pz)}n] (Cun; n = 26−36; pz = pyrazolate, C3H3N2-), which strongly bind hydrophilic anions such as carbonate, sulfate and phosphate. A combination of single-crystal X-ray crystallography, electrospray-ionization mass spectrometry (ESI-MS) and H-NMR spectroscopy demonstrated that the various Cun nanojars are all based on a combination of three or four [Cu(OH)(pz)]x rings (x = 6−14, except 11), tightly held together by a network of hydrogen bonds and axial Cu–O interactions. The incarcerated anion guest plays a crucial role in the formation of the nanojar host, as the empty nanojar lacking a central anion cannot be obtained. Apparently, the anion is sequestered in-situ during the formation of the host-guest complex by self-assembly. We have demonstrated that nanojars are able to sequester CO2 directly from the atmosphere by conversion into CO32- and incarceration of the carbonate ion in the center of the complex. Moreover, a second CO32- ion is bound if 1,10-phenanthroline is added to the reaction mixture. The resulting capped-nanojars, [{Cu3(μ3-OH)(μ-pz)3}CO3Ì{Cu(μ-OH)(μ-pz)}27] (1) and [{Cu2(phen)2CO3}CO3Ì{Cu(μ-OH)(μ-pz)}27] (2) have been characterized by single-crystal X-ray crystallography and electrospray-ionization mass spectrometry (ESI-MS). Complex 1 provides a new intermediate in the pH-dependent, step-wise transformation of Cu2+, OH- and pyrazolate ions into [CO3Ì{CuII(μ-OH)(μ-pz)}n]2- nanojars. The mechanism of this transformation, as well as the reversible binding of CO2 by nanojars will also be discussed.

Atmospheric CO2 sequestration by binding one or two CO32− ions in nanojars

ACS Fall 2021

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