Enthalpic and entropic contributions to the enhanced binding of pyridineligands by cyclic metalloporphyrin hosts

Abstract

The binding constants between the pyridine ligands 1–3 and the metalloporphyrin hosts 4, 6, and 7 were measured at various temperatures in toluene. The binding constants and the thermodynamic parameters found for the various complexes indicate that the cyclic nature of hosts 6 and 7 enhances binding of the substituted pyridine ligands 1 and 2, due to enthalpic factors with the smaller host 6, or entropic factors with the larger host 7. This enhanced binding is likely to lead to an increase in the reactive complex concentration in host-induced accelerated reactions; these cavity effects, which are not present in linear templates, could contribute to the greater efficiency of cyclic hosts. A plot of the enthalpic energetic gain vs. the entropic energetic cost measured for binding ligands 1–3 to hosts 4, 6 and 7 shows an enthalpy–entropy compensation effect.