The Neighboring Component Effect in a Tristable [2]Rotaxane

The redox properties of cyclobis(paraquat-p-phenylene) cyclophane (CBPQT^(4+)) renders it a uniquely variable source of recognition in the context of mechanically interlocked molecules, through aromatic donor-acceptor interactions in its fully oxidized state (CPBQT^(4+)) and radical- pairing interactions in its partially reduced state (CBPQT^(2(•+))). Although it is expected that the fully reduced neutral state (CBPQT^((0))) might behave as a p-donating recognition unit, resulting in a dramatic change in its binding properties when compared with the other two redox states, its role in rotaxanes has not yet been investigated. To address this challenge, we report herein the synthesis of a tristable [2]rotaxane in which a CBPQT^(4+) ring is mechanically interlocked with a dumbbell component containing five recognition sites—(i) one, a bipyridinium radical cation (BIPY^((•+))) located centrally along the axis of the dumbbell, straddled by (ii) two tetrafluorophenylene units linked to (iii) two triazole rings. In addition to the selective recognition between (iv) the CBPQT^(4+) ring and the triazole units, and (v) the CBPQT^(2(•+)) ring and the reduced BIPY^((•+)) unit in the dumbbell component, investigations in solution have now confirmed the presence of additional noncovalent bonding interactions between the CBPQT^((0)) ring, acting as a donor in its neutral state towards the two tetrafluorophenylene acceptors in the dumbbell component. The unveiling of this piece of molecular recognition in a [2]rotaxane is reminiscent of the existence in much simpler, covalently linked, organic molecules of neighboring group participation (anchimeric assistance giving way to transannular interactions) in small-, medium-, and large-membered rings.