Tuning the structure and solubility of nanojars by peripheral ligand substitution, leading to unprecedented liquid–liquid extraction of the carbonate ion from water into aliphatic solvents

Abstract

Nanojars, a novel class of neutral anion-incarcerating agents of the general formula [Cu(II)(OH)(pz)]n (Cun; n = 27-31, pz = pyrazolate anion), efficiently sequester various oxoanions with large hydration energies from water. In this work, we explore whether substituents on the pyrazole ligand interfere with nanojar formation, and whether appropriate substituents could be employed to tune the solubility of nanojars in solvents of interest, such as long-chain aliphatic hydrocarbons (solvent of choice for large-scale liquid-liquid extraction processes) and water. To this end, we conducted a comprehensive study using 40 different pyrazole ligands, with one, two or three substituents in their 3-, 4- and 5-positions. The corresponding nanojars are characterized by single-crystal X-ray diffraction and/or electrospray-ionization mass spectrometry (ESI-MS). The results show that Cun-nanojars with various substituents in the pyrazole 4-position, including long chains, phenyl and CF3 groups, can be obtained. Straight chains are also tolerated at the pyrazole 3-position, and favor the Cu30-nanojar. Homoleptic nanojars, however, could not be obtained with phenyl or CF3 groups. Nevertheless, if used in mixture with the parent non-substituted pyrazole, sterically hindered pyrazoles do form heteroleptic nanojars. With 3,5-disubstituted pyrazoles, only heteroleptic nanojars are accessible. The crystal structure of novel nanojars (Bu4N)2[CO3⊂{Cu30(OH)30(3,5-Me2pz)y(pz)30-y}] (y = 14 and 15) is presented. We find that in contrast to the parent nanojar, which is insoluble in aliphatic solvents and water, nanojars with alkyl substituents are soluble in saturated hydrocarbon solvents, whereas nanojars based on novel pyrazoles, functionalized with oligoether chains, are readily soluble in water. Liquid-liquid extraction of carbonate from water under basic pH is presented for the first time.

Sulfate-bridged dimeric trinuclear copper(II)–pyrazolate complex with three different terminal ligands

“…A unique class of copper-pyrazolate complexes is defined by nanojars, based on a series of cyclic polymerization isomers, [cis-Cu II (-OH)(-pz)] n (pz = pyrazolate anion, n = 6-14, except 11), which incarcerate anions with large hydration energies (e.g., sulfate, phosphate, carbonate) with unprecedented strength (Fernando et al, 2012;Mezei, 2015;Ahmed, ISSN 2056-9890 Szymczyna et al, 2016) and permits the extraction of such anions from water into aliphatic solvents (Ahmed, Calco et al, 2016). Nanojars are obtained by self-assembly from a copper salt, pyrazole and a base (needed both for deprotonating pyrazole and as a hydroxide ion source) in the presence of an anion with large hydration energy, via a trinuclear intermediate, which is isolable and can be converted into nanojars by adding a base .…”

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 .…”

“…New evidence supporting the vulnerability of nanojars to acids emerges from an unexpected source. An attempt to grow single crystals from a solution of (Bu 4 N) 2 [{Cu(-OH)(-4-Ipz)} n CO 3 ] (n = 27-31) (Ahmed, Calco et al, 2016) in chloroform/1,4-dioxane provided, instead of the expected nanojars, crystals of (Bu 4 N) 2 [Cu 3 ( 3 -Cl) 2 (-4-I-pz) 3 Cl 3 ]Á0.5dioxane , accompanied by a color change of the solution from blue to green. The chloride ions originating from CHCl 3 is not surprising, as chloroform has long been known to slowly decompose in the presence of air and moisture producing HCl and phosgene (CHCl 3 + 1 2 O 2 !…”

“…The synthesis of (Bu 4 N) 2 [{Cu(-OH)(-4-I-pz)} n CO 3 ] (n = 27-31) was described earlier (Ahmed Calco & Mezei, 2016). Green plate-like crystals of the title compound were obtained by slow evaporation of a chloroform/1,4-dioxane (1 mL each) solution of (Bu 4 N) 2 [{Cu(-OH)(-4-I-pz)} n CO 3 ] (20 mg).…”

Doubling the Carbonate-Binding Capacity of Nanojars by the Formation of Expanded Nanojars

“…Indeed, an aqueous Ba 2+ solution is unable to precipitate the corresponding barium salt (e.g., BaSO 4 , K sp = 1.08 × 10 −10 at 25 °C in H 2 O) when stirred with a solution of the nanojars. We have also demonstrated that nanojars are able to transfer these anions, including one of the most hydrophilic ones, carbonate, from water into aliphatic solvents [ 11 ]. Thus, nanojars can be used as extraction agents for the removal of such anions from contaminated aqueous media by liquid–liquid extraction [ 12 ].…”