Host-guest complexation of aromatic carboxylic acids and their conjugate bases by beta-cyclodextrins monosubstituted at C 6 by linear and cyclic alkyl triamines in aqueous solution

A pH titrimetric study of the complexation of the guests benzoic acid, 4-me thylbenzoic acid and (R)- and (S)-2-phenylpropanoic acids and their conjuga te bases by the host 6(A)-[2-(2-aminoethylamino)ethylamino]-, 6(A)-[3-(3-am inopropylamino)propylamino]-, 6(A)-(1,4,7-triazacyclononan-1-yl)-, and 6(A) -(1,5,9-triazacyclododecan-1-yl)-6(A)-deoxy-beta-cyclodextrins (beta CDdien , beta CDdipn, beta CDtacn and beta CDtacdo, respectively) is reported. Ove r the pH range 3.0-11.0, 49 host-guest complexes were detected. Their stabi lity constants (K) range from 220+/-50 dm(3) mol(-1) for the beta CDdienH(2 )(2+). benzoate(-) complex to 48000+/-11000 dm(3) mol(-1) for the beta CDdi pnH(3)(3+).(S)-2-phenylpropanoic acid complex at 298.2 K and I = 0.10 mol d m(-3) (NaClO4). The latter K value is among the highest reported for a comp lex of a simple carboxylic acid with a substituted beta-cyclodextrin. The c harge, hydrophobicity and stereochemistry of both host and guest appear to be significant factors in the variation of host-guest complex stability. H- 1 ROESY n.m.r. studies of some of the complexes formed are also reported.