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.