Chromate Incarceration by Nanojars and Its Removal from Water by Liquid–Liquid Extraction

Abstract

The unprecedented liquid−liquid extraction of the dinegative chromate ion (CrO 4 2− ) from neutral aqueous solutions into aliphatic hydrocarbon solvents using nanojars as extraction agents is demonstrated. Transferring chromate from water into an organic solvent is extremely challenging due to its large hydration energy (ΔG h °= −950 kJ/mol) and strong oxidizing ability. Owing to their highly hydrophilic anion binding pockets lined by a multitude of hydrogen bond donor OH groups, neutral nanojars of the formula [cis-Cu II (μ-OH)(μ-4-Rpz)] n (n = 27−33; pz = pyrazolate anion; R = H or n-octyl) strongly bind the CrO 4 2− ion and efficiently transfer it from water into n-heptane or C 11 − C 13 isoalkanes (when R = n-octyl). The extracted chromate can easily be recovered from the organic layer by stripping with an aqueous acid solution. Electrospray ionization mass spectrometric, UV−vis and paramagnetic 1 H NMR spectroscopic, X-ray crystallographic, and thermal stability studies in solution and chemical stability studies toward NH 3 , methanol, and Ba 2+ ions are employed to explore the binding of the CrO 4 2− ion by nanojars. Titration of carbonate nanojars [CO 3 ⊂ {Cu(OH)(pz)} n ] 2− with H 2 CrO 4 leads to anion exchange and the formation of chromate nanojars [CrO 4 ⊂ {Cu(OH)(pz)} n ] 2− . Details of chromate binding by H-bonding based on single-crystal structures of (Bu 4 N) 2 [CrO 4 ⊂ {Cu(OH)(pz)} 28 ], four pseudopolymorphs of (Bu 4 N) 2 [CrO 4 ⊂ {Cu(OH)(pz)} 31 ], and also the methoxy-substituted derivative (Bu 4 N) 2 [CrO 4 ⊂ {Cu 31 (OH) 30 (OCH 3 )(pz) 31 }] are presented.

Supramolecular Binding of Phosphonate Dianions by Nanojars and Nanojar Clamshells

“…The UV–vis spectra of Cu n RPO 3 ( n = 27–31; R = Me, Et, n Pr, n Bu, Bn, and Ph) in THF are virtually identical and display two peaks with absorption maxima at 345–349 and 602–608 nm, corresponding to charge-transfer and d – d transitions, respectively (with extinction coefficients of ε 347 nm = 2 × 10 4 L mol –1 cm –1 and ε 605 nm = 2 × 10 3 L mol –1 cm –1 ) (Figure ). The λ max values for the phosphonate nanojars are similar to the ones measured for the analogous nanojar mixtures with carbonate, sulfate, and chromate (λ max = 350–351 and 599–602 nm). ,, Thus, the entrapped anion has minimal effects on the UV–vis absorption of the nanojar framework.…”

“…Small amounts of formate-substituted analogues, in which one or more pz – moieties are substituted by HCOO – , are also observed in most spectra at 11 m / z units less than the parent peak. Nanojars are extremely sensitive to even traces of formic acid, which is either present in the mass spectrometer due to its common use as an additive to improve peak shapes and to promote ionization by producing [M + H] + ions or could result from the degradation of formaldehyde-based resins used in vial caps. , …”

“…Given the inability of using host–guest titration (because a guest-free nanojar host cannot be obtained), the binding strength of phosphonates by nanojars was assessed similarly to other anions by using competitive binding experiments with Ba 2+ . ,,, Neither in the case of heterogeneous conditions, when aqueous Ba­(NO 3 ) 2 and water-immiscible 2-methyltetrahydrofuran (2-MeTHF) solutions of Cu n RPO 3 nanojars ( n = 27–31, R = Me or Bn) were vigorously stirred together, nor in the case of homogeneous conditions using 2-MeTHF-soluble barium dioctyl sulfosuccinate, Ba­(DOSS) 2 , was any precipitate of a barium salt or a nanojar degradation product observed. Furthermore, ESI-MS analyses of the 2-MeTHF solutions after Ba 2+ treatment show no nanojar degradation products either nor any significant changes in nanojar composition, confirming the strong binding of phosphonates by the different nanojar species.…”

“…Nanojars are extremely sensitive to even traces of formic acid, which is either present in the mass spectrometer due to its common use as an additive to improve peak shapes and to promote ionization by producing [M + H] + ions or could result from the degradation of formaldehyde-based resins used in vial caps. 41 , 42 …”

“…Differences in structural details of the various Cu x rings in the Cu 27 –Cu 31 nanojars were analyzed using the dihedral angles and the component fold and twist angles between the mean planes of pyrazolate moieties and adjacent Cu–O–Cu units (as defined earlier). 42 , 44 Tables S10 and S11 illustrate that in 1 – 10 the average fold angles in the larger, flatter central rings (Cu 12 –Cu 14 ) are quite consistent at 41.0(6)–43.2(2)°, whereas in the two smaller, more puckered side rings, the angles vary more widely: 37.9(2)–51.8(5)° (Cu 9 or Cu 10 ) and 45.6(4)–56.4(2)° (Cu 6 –Cu 8 ). The corresponding average twist angles are 3.4(2)–6.2(4)° in the central rings (Cu 12 –Cu 14 ), and 2.1(2)–5.2(3)° (Cu 9 or Cu 10 ) and 0.9(2)–5.2(2)° (Cu 6 or Cu 8 ) in the two side rings.…”