Host–guest complexation of aromatic carboxylic acids and their conjugate bases by 6A-(ω-aminoalkylamino)-6A-deoxy-β-cyclodextrins† in aqueous solution

Abstract

A potentiometric titration study of the complexation of the guests benzoic acid, 4-methylbenzoic acid and (RS)-2-phenylpropanoic acid and their conjugate bases by host 6^A-(ω-aminoalkylamino)-6^A-deoxy-β-cyclodextrins, where the ω-aminoalkylamino groups are -NH(CH₂)_nNH₂ (n = 2, 3, 4 and 6), is reported. Of the 64 host–guest complexes whose formation is statistically possible over the pH range 2.0–12.0 studied, 35 were detected. Their stability constants range from 140 ± 35 dm³ mol^–1 for the [βCDNH(CH₂)₂NH₃·4-methylbenzoate] complex to 1760 ± 150 dm³ mol^–1 for the [βCDNH(CH₂)₆NH₂·(S)-2-phenylpropanoate]^– complex at 298.2 K and I = 0.10 mol dm^–3 (NaClO₄). The charge and hydrophobicity of both host and guest appear to be significant factors in the variation of host–guest complex stability. Qualitative structural information on the host–guest complexes obtained from 600 MHz ¹H NMR ROESY spectroscopy is generally consistent with the structures generated by molecular modelling.