Thermodynamics of host-guest interactions between methylpyridinium salts and phosphonate cavitands

In this work, the properties of complexation of tetraphosphonate cavitands towards methylpyridinium guests were investigated via isothermal titration calorimetry (ITC). For this purpose, Tiiii[C₃H₇, CH ₃, Ph], Tiiii[C₃H₇, H, Ph], TSiiii[C ₃H₇, H, Ph] hosts and three different methylpyridinium guests were synthesised. The role of the following parameters in the host-guest complexation was investigated: (i) solvation, (ii) nature of the guest counterion, (iii) presence of substituents in the apical positions of the receptor, and (iv) P=O versus P=S bridging units. The results showed that (i) switching from dichloroethane to methanol leads to a decrease of the association constant due to the competitive nature of the solvent, (ii) the guest counterion does not affect the thermodynamics of the process, (iii) the apical methyl groups enhance the binding affinity of the receptor and (iv) the comparison between phosphonate and thiophosphonate hosts clearly demonstrates that cation-dipole interactions are necessary for binding.