Journal of the American Chemical Society, Vol.140, No.42, 13827-13834, 2018
Neighboring Component Effect in a Tri-stable Rotaxane
The redox properties of cyclobis(paraquat-p-phenylene)cyclophane (CBPQT(4+)) render 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(center dot+))). Although it is expected that the fully reduced neutral state (CBPQT((0))) might behave as a pi-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 tri-stable rotaxane in which a CBPQT(4+) ring is mechanically interlocked with a dumbbell component containing five recognition sites-(i) a bipyridinium radical cation (BIPY(center dot+)) 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(center dot+)) ring and the reduced BIPY(center dot+) unit in the dumbbell component, investigations in solution have now confirmed the presence of additional non-covalent bonding interactions between the CBPQT((0)) ring, acting as a donor in its neutral state, and the two tetrafluorophenylene acceptors in the dumbbell component. The unveiling of this piece of molecular recognition in a 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.