How Does the Solvent Modulate Shuttling in a Pillararene/Imidazolium [2]Rotaxane? Insights from Free Energy Calculations

Pillararene-based [2]­rotaxanes have gained notoriety since the synthesis of the first pillar[5]­arene in 2008. The marked propensity of pillararenes to bind cationic groups is often utilized to prepare functional host–guest complexes. Interestingly enough, the interaction of pillararenes with cationic groups is modulated by the nature of the solvent. The molecular mechanism that underlies binding, examined experimentally, remains, however, partially understood. In the present contribution, the solvent-controlled motion in a [2]­rotaxane composed of a 1,4-diethoxy­pillar[5]­arene (P[5]) ring threaded onto an hydrogen-bond donor imidazolium axle was investigated in eight different solvents. Apart from the polarity, the hydrogen-bond-accepting ability of the solvent was considered with particular care. In environments featuring hydrogen-bond acceptors, the P[5] tends to include the alkyl chain at one end of the axle, staying away from the cationic imidazolium unit at the other end of it. Inclusion is primarily driven by the favorable interaction of the alkyl chain with the P[5], alongside the hydrogen-bonding interaction of the imidazolium moiety with the solvent. However, in a low-polarity solvent, devoid of hydrogen-bond acceptors, the P[5] binds favorably the imidazolium moiety and the neighboring methylene groups, resulting in hydrogen bonds established between the imidazolium moiety and the P[5], and the unusual C–H···π interaction of the methylene groups adjacent to the imidazolium moiety with the benzene rings of the P[5]. The present results have important bearings on the design of artificial molecular machines formed by pillararenes and cationic moieties.