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Complexation of Imidazopyridine-Based Cations with a 24-Crown‑8 Ether Host: [2]Pseudorotaxane and Partially Threaded Structures

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journal contribution
posted on 01.11.2013, 00:00 by Surisadai I. Moreno-Olivares, Ruy Cervantes, Jorge Tiburcio
A new series of linear molecules derived from 1,2-bis­(imidazopyridin-2-yl)­ethane can fully or partially penetrate the cavity of the dibenzo-24-crown-8 macrocycle to produce a new family of host–guest complexes. Protonation or alkylation of the nitrogen atoms on the pyridine rings led to an increase in the guest total positive charge up to 4+ and simultaneously generated two new recognition sites (pyridinium motifs) that are in competition with the 1,2-bis­(benzimidazole)­ethane motif for the crown ether. The relative position of the pyridine ring and the chemical nature of the N-substituent determined the preferred motif and the host–guest complex geometry: (i) for linear guests with relatively bulky groups (i.e., a benzyl substituent), the 1,2-bis­(benzimidazole)­ethane motif is favored, leading to a fully threaded complex with a [2]­pseudorotaxane geometry; (ii) for small substituents, such as −H and −CH3 groups, regardless of the guest shape, the pyridinium motifs are preferred, leading to external partially threaded complexes in a 2:1 host to guest stoichiometry.