Capped Nanojars: Synthesis, Solution and Solid-State Characterization, and Atmospheric CO2 Sequestration by Selective Binding of Carbonate

Abstract

Nanojars are a class of supramolecular anion-incarcerating coordination complexes that self-assemble from Cu2+ ions, pyrazole, and a strong base in the presence of highly hydrophilic anions. In this work, we show that if the strong base (e.g., NaOH or Bu4NOH) is replaced by a weak base such as a trialkylamine, capped nanojars of the formula [{Cu3(μ3-OH)(μ-pz)3L3}CO3⊂{Cu(μ-OH)(μ-pz)}n] (pz = pyrazolate anion; L = neutral donor molecule; n = 27-31) are obtained instead of the conventional nanojars. Yet, to obtain capped nanojars, the conjugate acid side product originating from the weak base must be separated by transferring it to water either by precipitation of the water-insoluble capped nanojars or by liquid-liquid extraction. Full characterization using electrospray ionization mass spectrometry, UV-vis and variable-temperature 1H NMR spectroscopy in solution, and single-crystal X-ray diffraction, elemental analysis, and solubility studies in the solid state reveals similarities as well as drastic differences between capped nanojars and nanojars lacking the [Cu3(μ3-OH)(μ-pz)3L3]2+ cap. Acid-base reactivity studies demonstrate that capped nanojars are intermediates in the pH-controlled assembly-disassembly of nanojars. During the self-assembly of capped nanojars, CO2 is selectively sequestered from air in the presence of other atmospheric gases and converted to carbonate, the binding of which is selective in the presence of NO3-, ClO4-, BF4-, Cl-, and Br- ions.