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

Abstract

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

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

“…Noteworthy here is the complete disappearance of the peak at m / z 2318, corresponding to the carbonate nanojar Cu 31 . It had been shown earlier that the Cu 31 nanojar easily exchanges carbonate for sulfate because the larger size of sulfate is more complementary to the cavity size of the Cu 31 nanojar …”

“…Once self-assembled, the nanojar wraps around the incarcerated anion using a multitude of hydrogen bonds and completely isolates it from the surrounding medium. Consequently, the anion (carbonate, sulfate, − phosphate, and arsenate) is bound extremely strongly, so that an aqueous Ba 2+ solution is unable to precipitate the corresponding highly insoluble barium salt (e.g., BaSO 4 ; K sp = 1.08 × 10 –10 ) when stirred with a solution of the nanojars. Owing to this exceptionally strong binding affinity, nanojars are able to transfer even the most hydrophilic anions, such as carbonate, from water into aliphatic solvents and have been developed into extraction agents for the removal of such anions from contaminated aqueous media by liquid–liquid extraction .…”

Pyrene-Functionalized Fluorescent Nanojars: Synthesis, Mass Spectrometric, and Photophysical Studies

“…Increasing temperatures lead to the sharpening of these peaks as the magnetic moment of the copper complex decreases and reveal a new peak at ∼34.0 ppm (Cu 9 -ring) at 50 °C which was too broad to be observable at 25 °C. As in the [CO 3 ⊂{Cu(OH)(pz)} n ] 2− (n = 27, 29) NJs studied before, 4,5 the pyrazole protons in the Cu 9 -ring of the Cu 27 NJ are the most sensitive to temperature changes as the corresponding peak shifts upfield by over 5 ppm units on going from 25 to 140 °C in a DMSO-d 6 solution. Meanwhile, the peaks corresponding to the Cu 6 -and Cu 12 -rings of the Cu 27 NJ, as well as the Cu 8 -and Cu 13 -rings of the Cu 29 NJ experience much smaller shifts of less than 0.6 ppm units.…”

“…The 1 H NMR spectrum of the NJ mixture displays peaks in the 20−40 ppm window that are consistent with previous observations for NJs. 4,5 The signals of the chemically equivalent pyrazole protons in the 3-and 5-positions of both Cu 27 (Cu 6 +Cu 12 +Cu 9 ) and Cu 29 (Cu 8 +Cu 13 +Cu 8 ) NJs are significantly downfield shifted compared to free pyrazole, due to the presence of paramagnetic Cu 2+ ions. The extent of these shifts is temperature-dependent, as confirmed by variable temperature 1 H NMR studies in the 25−140 °C range (Figure S10).…”

Halogen-bonded network of trinuclear copper(II) 4-iodopyrazolate complexes formed by mutual breakdown of chloroform and nanojars

“…Nanojars, supramolecular coordination complexes of the formula [{Cu(-OH)(-pz)} n anion] (pz = pyrazolate anion; n = 27-36), have emerged as a new class of anion encapsulation agents of unparalleled efficiency, which allow the extraction of anions with large hydration energies, such as phosphate, carbonate and sulfate, from water into organic solvents (Mezei, Baran et al, 2004;Fernando et al, 2012;Mezei, 2015;Ahmed, Szymczyna et al, 2016;Ahmed, Calco & Mezei, 2016;Ahmed, Hartman & Mezei, 2016). Trinuclear copper pyrazolate complexes have been identified as key intermediates in the self-assembly mechanism of nanojars from copper(II) nitrate, pyrazole and NaOH (1:1:2 molar ratio) in the presence of carbonate .…”