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

The unprecedented liquid-liquid extraction of the dinegative chromate ion (CrO₄^2-) 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 = 27-33; pz = pyrazolate anion; R = H or n-octyl) strongly bind the CrO₄^2- ion and efficiently transfer it from water into n-heptane or C₁₁ - C₁₃ 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 ¹H NMR spectroscopic, X-ray crystallographic, and thermal stability studies in solution and chemical stability studies toward NH₃, methanol, and Ba²⁺ ions are employed to explore the binding of the CrO₄^2- ion by nanojars. Titration of carbonate nanojars [CO₃ ⊂ {Cu(OH)(pz)}]^2- with H₂CrO₄ leads to anion exchange and the formation of chromate nanojars [CrO₄ ⊂ {Cu(OH)(pz)}]^2-. Details of chromate binding by H-bonding based on single-crystal structures of (Bu₄N)₂[CrO₄ ⊂ {Cu(OH)(pz)}₂₈], four pseudopolymorphs of (Bu₄N)₂[CrO₄ ⊂ {Cu(OH)(pz)}₃₁], and also the methoxy-substituted derivative (Bu₄N)₂[CrO₄ ⊂ {Cu₃₁(OH)₃₀(OCH₃)(pz)₃₁}] are presented.