Cationic M2L4 cages for perchlorate removal from aqueous solutions and preferential perchlorate incorporation in hydrophilic solutions

Abstract

Four new cage-type compounds, [A ⊂ Cu₂(m-bbitrb)₄]A₃ (A = Cl⁻ (1a), Br⁻ (1b), m-bbitrb = 1,3-bis(benzimidazol-1-ylmethyl)-2,4,6-trimethylbenzene), and [SO₄ ⊂ Cu₂(m-bbitrb)₄]A₂ (A = Cl⁻ (2a), Br⁻ (2b)), were synthesized and characterized. These compounds have a M₂L₄-type cationic cage, incorporating an anion in the cage. Although the four compounds are insoluble in water, they exhibit ClO₄⁻ removal activity from aqueous solutions by exchange with anions located outside the cage. The [Cl ⊂ Cu₂(m-bbitrb)₄]³⁺ cage removes ClO₄⁻ from aqueous solutions preferentially. To study the selective removal process by the cationic cage, the affinity of the [Cu₂(m-bbitrb)₄]⁴⁺ cage for ClO₄⁻ was studied by acquiring the solid-state reflectance spectra, absorption spectra, and ESI-TOF mass spectra in solutions of MeCN and MeCN/H₂O for [SO₄ ⊂ Cu₂(m-bbitrb)₄](ClO₄)₂ (2d). The results indicated that the SO₄²⁻ in [SO₄ ⊂ Cu₂(m-bbitrb)₄]²⁺ was retained in the MeCN solution, whereas in the MeCN/H₂O solution the SO₄²⁻ was exchanged with the ClO₄⁻ located outside the cage. This is because an increase in the hydrophilicity of the solution promotes the incorporation of hydrophobic ClO₄⁻ into the hydrophobic cage, meaning that the preferential removal of ClO₄⁻ from aqueous solutions is due to the higher affinity of the hydrophobic space created outside the M₂L^B₄ cages in the solid state.