Lewis-base binding properties of free-base β-octahalotetraphenylporphyrins

Symmetrically substituted perhaloporphyrins, 2,3,7,8,12,13,17,18-octahalo (bromo and chloro)-5,10,15,20-tetraphenylporphyrins (H2OBTPP and H2OCTPP) have been examined as receptors for binding to Lewis bases. The kinetics of base binding to haloporphyrins showed an enhanced binding constants for H2OBTPP relative to H2OCTPP and follows a linear trend with increase in pKa values of the bases. The binding of Lewis-bases to perhaloporphyrin core is largely influenced by the extent of nonplanarity of the porphyrin core than the electron-withdrawing ability of the substituents. The higher binding constants of H2OBTPP relative to H2OCTPP have been interpreted in terms of greater nonplarity of the former than the latter.