NMR and X‐ray Crystallographic Analysis of Thermodynamically Stable Tetraphenylporphyrin‐Stoppered Rotaxanes

Abstract

AbstractTetraphenylporphyrin‐stoppered rotaxanes, in which tetraphenylporphyrin rhodium(III) chloride serves for end‐capping of the thread molecule through axial coordination, were prepared from the inclusion complexes formed between crown ether and secondary ammonium cation derivatives. Two types of the thread molecules, with different molecular lengths, were used for the rotaxane preparation. The obtained porphyrin‐stoppered rotaxanes proved to be thermodynamically stable in their pure forms at room temperature. 1H NMR measurements indicated that the protons of the thread molecule and the crown ether are considerably affected by the shielding effect of the terminal porphyrin. We confirmed that the intensity of the shielding effect is applicable as a probe for estimation of the location of the crown ether. We found different chemical shifts for individual members of pairs of methylene protons on the same crown ether carbon when the relatively shorter thread molecule was used, whereas we observed similar chemical shifts for those in the rotaxane with longer thread molecule. This result indicates that the stronger shielding effect of the porphyrin exposes the two faces of the crown ether to different magnetic environments in the case of the relatively shorter thread molecule. X‐ray crystallography for the porphyrin‐stoppered rotaxane with the relatively shorter thread molecule revealed that the crown ether forms many hydrogen bonds with the thread molecule. We confirmed that not only the hydrogen atoms of the secondary ammonium ion and the neighboring methylene groups, but also the β‐hydrogen atoms of the axially coordinated pyridine moiety participate in hydrogen bond formation with the crown ether oxygen atoms. It is likely that the catechol rings of the crown ether interact with the phenyl groups of the terminal porphyrin. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)