From neutral to ionic species: amine–p-tert-butylcalix(n)arene (n= 6, 8) interaction. Electrochemical, thermodynamic and structural studies in benzonitrile

Abstract

Solubilities and derived Gibbs energies, enthalpies and entropies of solution of p-tert-butylcalix(n)arenes (n= 4, 6, 8) in benzonitrile at 298.15 K are reported. An important consequence of amine–calixarene interactions in solution is the generation of new electrolytes. Thus, using two different calixarenes (cyclic hexamer and octamer) and various amines, conductance measurements are performed in order to derive equilibria data for the process involving two neutral species to give an electrolyte, as a result of a proton-transfer reaction from the calixarene to the amine. The results obtained by conductivity are checked by using a combination of pK_a values for p-tert-butylcalix(n)arene and various amines in benzonitrile calculated from potentiometric data. Thermodynamic parameters for these processes obtained from titration calorimetry reflect the distinctive properties of cryptands relative to other cyclic and aliphatic amines, suggesting that cryptand 222 is likely to host the proton released by the cyclic octamer in its cavity. ¹³C NMR studies on these systems revealed that the charge on the calixarene anion is delocalised as a result of a dynamic interchange of the remaining protons among the oxygen atoms present in the oligomeric molecule. Finally, the extraction of amines from aqueous solutions by calixarenes is analysed by taking into account the individual parameters that contribute to the overall process. It is shown that the transfer of amines from water to benzonitrile is favoured in the presence of calixarenes. Although the main contribution to the extraction process results from ion-pair formation in solution, the selective extraction of these amines by calixarenes is largely controlled by the transfer Gibbs energy of the amine from water to the non-aqueous phase.