Receptor versatility of tris(pyridin-1-ium-2-ylmethyl)amine in anion binding through hydrogen bonding

Abstract

The receptor ability of tris(pyridin-1-ium-2-ylmethyl)amine (triprotonated tris(2-pyridylmethyl)amine, H₃TPA³⁺) toward inorganic anions such as PF₆⁻, CF₃SO₃⁻, Br⁻, and Cl⁻ was investigated. Several spectroscopic studies revealed that H₃TPA³⁺ offered characteristic receptor selectivity in the anion complexation. Structural analysis of the receptor-anion complexes [H₃TPA(PF₆)](PF₆)₂, [H₃TPA(CF₃SO₃)](CF₃SO₃)(PF₆), [H₃TPA(Br)](PF₆)₂, and [H₃TPA(Cl)](PF₆)₂ indicated that the H₃TPA³⁺ receptor nicely caught each anion guest (X⁻) in its three dimensional cavity via hydrogen bonding (X⋯H⋯N) with pyridinium groups. Treatment of the [H₃TPA(X)]²⁺ (X = PF₆⁻ or CF₃SO₃⁻) complex with a large excess of KBr in the solid state led to the replacement of PF₆⁻ or CF₃SO₃⁻ with Br⁻ to give [H₃TPA(Br)]²⁺, whereas Cl⁻ in the [H₃TPA(Cl)]²⁺ complex was not replaced. The PF₆⁻ anions located in and out of the H₃TPA³⁺ cavity were replaced stepwise with the added Cl⁻ or Br⁻anion. The strength of the hydrogen bonding increased as the pK_b value of the anion decreased in the series Cl⁻ > Br⁻ > CF₃SO₃⁻ > PF₆⁻.