Jozef Hodyl PhD Thesis - Theses - Flinders University

Table 6.2Percent inclusion data for hydroxybenzoate guest species with Si-GPS-[Cd(Trac)](ClO 4 ) 2 , 94, conducted in aqueous media with a guest concentration of 5 x 10 -3 mol dm -3at pH 7 in 0.01 mol dm -3 HEPES buffer at 298 K and, logK values for [Cd((S)-athppc)](ClO 4 ) 2 , 29, in 20% aqueous dioxane and model receptor [Cd(TracHP12)](ClO 4 ) 2 , 49, inDMSO-d 6 also at 298 K.Receptor94Receptor29Receptor49Guest species % Inclusion a logK b logK c pK adbenzoate 30 2.3 ± 0.1 4.11 ± 0.06 4.19p-hydroxybenzoate 32 4.5 ± 0.3 4.09 ± 0.07 4.54m-hydroxybenzoate 48 5.3 ± 0.5 4.16 ± 0.09 4.30o-hydroxybenzoate 81 7.1 ± 0.5 3.08 ± 0.08 2.972,6-dihydroxybenzoate 95 7.5 ± 0.9 2.05 ± 0.03 1.05a Conducted in aqueous solution. b Obtained by fluorescence monitored titrations in 20% aqueousdioxane. c Conducted in DMSO-d 6 . d Values obtained in H 2 O from PHYSPROP database for theirrespective conjugate acids. 202Rationalisation of the trend in DMSO-d 6 with receptor 49 was explained by theinability of three guest species, benzoate, p-hydroxybenzoate and m-hydroxybenzoate to participate in intramolecular hydrogen bonding and, therefore, inDMSO-d 6 they exhibited significantly larger binding constants than o-hydroxybenzoate and 2,6-dihydroxybenzoate, which are both known to form stronghydrogen bonds between their phenolic and carboxylate groups in that medium. 227,228 The trend observed in aqueous conditions with receptor 94 appears to result fromweaker intramolecular hydrogen bonds within these two ortho-substituted guestmolecules due to competition with water molecules. Weakening intramolecularhydrogen bonding within these molecules strengthens the O-H ... O hydrogen bondingand also allows for non-classical hydrogen bonding interactions to occur between theguest hydroxy groups and aromatic π-systems of the host pendant arms as shown inFigure 6.1.98