ENTHALPICALLY DRIVEN CYCLOPHANE ARENE INCLUSION COMPLEXATION - SOLVENT-DEPENDENT CALORIMETRIC STUDIES

The thermodynamic quantities characterizing tight apolar complexation processes have been investigated in a calorimetric study with cyclophanes as the receptors and benzene derivatives as well as pyrene as the substrates. In water, the inclusion complexation of benzene derivatives is strongly exothermic, accompanied by an unfavorable entropic term. A large part of the favorable enthalpy change results from solvent-specific contributions. Enthalpic data obtained from the van't Hoff analysis of H-1 NMR titrations are qualitatively in good agreement with the DELTA-H-degrees values measured directly by calorimetry. Larger uncertainties in the van't Hoff data are a result of changes in the heat capacity DELTA-C(p)-degrees. Negative heat capacity effects are characteristic for all inclusion processes in water and in methanol investigated in this study. The largest negative DELTA-C(p)-degrees values are measured for the complexation of benzene derivatives that possess a molecular dipole and hydroxy substituents and therefore interact strongly with their solvent cages. A calorimetric study in 12 solvents of different polarities shows that water is not special in providing an enthalpic driving force for apolar complexation. In all the solvents studied, the formation of a cyclophane-pyrene inclusion complex is enthalpically driven. The exothermicity generally increases from apolar solvents, to dipolar aprotic solvents, to protic solvents. A strong isoequilibrium relationship (R = 0.954) correlates the enthalpy and entropy for pyrene complexation in the different environments. A closer analysis reveals that an even stronger dual isoequilibrium relationship (R = 1.00 and 0.979) is expressed in the plot of enthalpy change against change in entropy. Stronger dual isoequilibrium relationships are also seen in the plots of the complexation free energy against the enthalpy and entropy changes, compared to single isoequilibrium relationships covering all solvents.