Host-Guest Complexation of Aromatic Carboxylic Acids and their Conjugate Bases by β-Cyclodextrins Monosubstituted at C 6 by Linear and Cyclic Alkyl Triamines in Aqueous Solution

Abstract

A pH titrimetric study of the complexation of the guests benzoic acid, 4-methylbenzoic acid and (R)- and (S)-2-phenylpropanoic acids and their conjugate bases by the host 6^A-[2-(2-aminoethylamino)ethylamino]-, 6^A-[3-(3-aminopropylamino)propylamino]-, 6^A-(1,4,7-triazacyclononan-1-yl)-, and 6^A-(1,5,9-triazacyclododecan-1-yl)-6^A-deoxy-β-cyclodextrins (βCDdien, βCDdipn, βCDtacn and βCDtacdo, respectively) is reported. Over the pH range 3.0–11.0, 49 host–guest complexes were detected. Their stability constants (K) range from 220±50 dm³ mol^–1 for the βCDdienH₂²⁺ ·benzoate^– complex to 48000±11000 dm³ mol^–1 for the βCDdipnH₂²⁺·(S)-2-phenylpropanoic acid complex at 298.2 K and I = 0.10 mol dm^–3 (NaClO₄). The latter K value is among the highest reported for a complex of a simple carboxylic acid with a substituted β-cyclodextrin. The charge, hydrophobicity and stereochemistry of both host and guest appear to be significant factors in the variation of host–guest complex stability. ¹H ROESY n.m.r. studies of some of the complexes formed are also reported.