Induced Fit Process in the Selective Distal Binding of Imidazoles in Zinc(II) Porphyrin Receptors

The respective affinities of various imidazole derivatives, imidazole (ImH), 2-methylimidazole (2-MeImH), 2-phenylimidazole (2-PhImH), N-methylimidazole (N-MeIm), 2-methylbenzimidazole (2-MeBzImH), and 4,5-dimethylbenzimidazole (4,5-Me(2)BzImH), for two phenanthroline (Phen) strapped zinc(II) porphyrin receptors porphen-Zn 1-Zn and 2-Zn have been studied. The formation of a supplementary H-bond considerably enhances the affinity of the zinc(II)-porphen receptor for imidazoles unsubstituted on the pyrrolic nitrogen (ImH) versus N-substituted imidazoles such as N-MeIm. The ImHs subset porphen-Zn complexes are formed with association constants up to 4 orders of magnitude superior to those measured either for N-MeIm as substrate or TPP-Zn as receptor. Distal or proximal binding of the substrates was determined by (1)H NMR measurements and titration. In two cases, the very high stability of the inclusion complex enabled the use of 2D NMR techniques. Excellent correlation between solution and solid-state structures has been obtained. A total of six X-ray structures are detailed in this article showing that the evolution of the shape of the zinc(II) receptor is mostly dependent on the steric constraints induced by the substitution on the imidazole. Hindered guests also progressively induce considerable mobility restrictions and severe distortions on the receptor, especially in the case of 2-MeBzImH and 2-PhImH.