Selective crystallization of sulfate with a simple bis-guanidinium ligand, self-assembled in situ from terephthalaldehyde and aminoguanidinium chloride, was employed as an effective way to separate the highly hydrophilic sulfate anion from aqueous solutions. The resulting bis-iminoguanidinium sulfate salt has exceptionally low aqueous solubility (Ksp =2.4×10-10 ), comparable to that of BaSO4 . Single-crystal X-ray diffraction analysis showed the sulfate anions are sequestered as [(SO4 )2 (H2 O)4 ]4- clusters within the crystals. Variable-temperature solubility measurements indicated the sulfate crystallization is slightly endothermic (ΔHcryst =3.7 kJ mol-1 ), thus entropy driven. The real-world utility of this crystallization-based approach for sulfate separation was demonstrated by removing up to 99 % of sulfate from seawater in a single step.
Keywords: anions; cluster compounds; crystallization; guanidines; self-assembly.
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