Jozef Hodyl PhD Thesis - Theses - Flinders University

The full set of logK values obtained from inclusion studies with hydroxybenzoateguests and [Cd(TracHP12)](ClO 4 ) 2 , 49, is shown in Table 3.2.Table 3.2Binding constants (logK) for guest anion binding of hydroxy substituted benzoates with[Cd(TracHP12)](ClO 4 ) 2 , 49, determined by 1 H NMR monitored titrations in DMSO-d 6with additional binding constants for benzoate and 2,6-dihydroxybenzoate determinedin CD 3 OD. Both sets of logK values were obtained at 298 K.[Cd(TracHP12) 2+ , 49Guest species logK a logK b cpK abenzoate d 4.11 ± 0.06 3.96 ± 0.09 4.19p-hydroxybenzoate 4.09 ± 0.07 e 4.54m-hydroxybenzoate 4.16 ± 0.09 e 4.30o-hydroxybenzoate 3.08 ± 0.08 3.23 ± 0.03 2.972,6-dihydroxybenzoate 2.05 ± 0.03 e 1.05a Acquired in DMSO-d 6b Acquired in CD 3 OD c Values obtained in H 2 O from PHYSPROPdatabase for their respective conjugate acids. 202 d Guest stock solution 10% (v/v) D 2 O in DMSOd6 . e Not attempted.It is evident from the values that the trend of logK in which o-hydroxybenzoate >m-hydroxybenzoate > p-hydroxybenzoate is not seen here. Instead there is a clearrelationship between the logK values and the basicity of the guest species wherebythe more basic the guest species (indicated by higher pK a ) the higher the bindingconstant.It is interesting to note that o-hydroxybenzoate and 2,6-dihydroxybenzoate bothhave significantly lower pK a s and logKs. This is explained by the fact that bothspecies can engage in intramolecular hydrogen bonding not accessible to the m- andp-isomers. This internal hydrogen bonding is shown in Figure 3.15.One consequence of this intramolecular hydrogen bonding is a lowering of the203, 204pK a due to delocalisation of the negative charge on the carboxylate.67