Host-guest complexation of aromatic carboxylic acids and their conjugate bases by b-cyclodextrins mono-substituted at C(6) by linear and cyclic polyalkyltri-amines in aqueous solution

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 6A-[2-(2-aminoethylamino)ethylamino]-, 6A-[3-(3-aminopropylamino)propylamino]-, 6A-(1,4,7-triazacyclononan-1-yl)-, and 6A-(1,5,9-triazacyclododecan-1-yl)-6A-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.