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

A potentiometric titration study of the complexion 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)₆NH ₂·(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.