Thermodynamics of Host–Guest Interactions between Fullerenes and a Buckycatcher

¹H NMR and isothermal titration calorimetry (ITC) experiments were employed to obtain reliable thermodynamic data for the formation of the 1:1 inclusion complexes of fullerenes C₆₀ and C₇₀ with the buckycatcher (C₆₀H₂₈). NMR measurements were done in toluene-<i>d</i>₈ and chlorobenzene-<i>d</i>₅ at 288, 298, and 308 K, while the ITC titrations were performed in toluene, chlorobenzene, <i>o</i>-dichlorobenzene, anisole, and 1,1,2,2-tetrachloroethane at temperatures from 278 to 323 K. The association constants, <i>K</i>_a, obtained with both techniques are in very good agreement. The thermodynamic data obtained by ITC indicate that generally the host–guest association is enthalpy-driven. Interestingly, the entropy contributions are, with rare exceptions, slightly stabilizing or close to zero. Neither Δ<i>H</i> nor Δ<i>S</i> is constant over the temperature range studied, and these thermodynamic functions exhibit classical enthalpy/entropy compensation. The Δ<i>C</i>_<i>p</i> values calculated from the temperature dependence of the calorimetric Δ<i>H</i> values are negative for the association of both fullerenes with the buckycatcher in toluene. The negative Δ<i>C</i>_<i>p</i> values are consistent with some desolvation of the host-cavity and the guest in the inclusion complexes, C₆₀@C₆₀H₂₈ and C₇₀@C₆₀H₂₈