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

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.