Binding Thermodynamics of Ferrocenyl Ammoniums by Sulfonated Calix[4]arenes

The binding constants (Ka), standard molar enthalpy changes (Delta H degrees), and entropy changes (T Delta S degrees) between two sulfonated calix[4]arene hosts including rigid p-sulfonatocalix[4]arene (SC4A) and flexible p-sulfonatothiacalix[4]arene (STC4A), and three ferrocenyl ammonium guests modified by different aliphatic chains were determined in aqueous solution (pH 7.0 and pH 2.0) by isothermal titration microcalorimetry (ITC). The gained data indicated that for the flexible host STC4A, the host-guest binding affinities increased with the increase of length of the aliphatic chain of guests whereas the rigid host SC4A behaved almost oppositely. Compared with those at pH 2.0, the binding affinities between the hosts and the guests were much stronger at pH 7.0. This should be attributed to the higher pi-electron density and the extra negative charge of calixarene cavities owing to one of the phenolic hydroxyl groups is deprotonated at pH 7.0, thus affording stronger pi MIDLINE HORIZONTAL ELLIPSIS pi, C-HMIDLINE HORIZONTAL ELLIPSIS pi and electrostatic interactions. Job plot and ITC confirmed that the molecular binding were stoichiometric 1 : 1 between hosts and guests. NMR experiments showed that the signals of the guest protons with addition of hosts underwent pronounced upfield shifts suggesting the effective binding between hosts and guests. In addition, the molecular selective binding behavior was discussed from the perspective of thermodynamics. Thermodynamically, all the complexation between SC4A or STC4A and the ferrocenyl ammonium guests are entirely enthalpically driven, along with unfavorable or negligible favorable entropy changes. Combining the thermodynamic parameters and the binding modes deduced by NMR spectra, the host-guest binding abilities greatly depend on the size, electron density and rigidity/flexibility of the calixarene cone, guest substituent, as well as pH.image