Effects of organization of zinc porphyrin hosts on binding enhancements and recognition of axial ligands
Two zinc porphyrin hosts, 2 and 3, containing an insufficiently preorganized binding pocket above each porphyrin plane have been synthesized and characterized. The binding behaviour of these hosts with various amine ligands as guests were examined spectrophotometrically in CHCl3 and were compared with that of a ‘bis-roof’ porphyrin zinc(II) complex. 1, which has a similar but fully preorganized binding pocket. The binding enhancements and shape selectivity revealed by 1 for saturated amines were almost diminished in 2 and 3. Proton NMR data showed that an induced fit of 2 for amine guests was incomplete probably owing to a weak driving force by non-polar host–guest interactions. Therefore, preorganization of the porphyrin hosts is indispensable to show recognition ability for amines in non-polar environments. The binding enhancements that provide the recognition phenomenon for amine guests are attributed to the dispersion force between the porphyrin hosts and the amine guests.