Degenerate [2]rotaxanes with electrostatic barriers

A synthetic approach to the preparation of [2]rotaxanes (1-5.6PF(6)) incorporating bispyridinium derivatives and two 1,5-dioxynaphthalene (DNP) units situated in the rod portions of their dumbbell components that are encircled by a single cyclobis(paraquat-p-phenylene) tetracationic (CBPQT(4+)) ring has been developed. Since the pi-electron-deficient bispyridinium units are introduced into the dumbbell components of the [2]rotaxanes 1-5.6PF(6), there are Coulombic charge-charge repulsions between these dicationic units and the CBPQT(4+) ring in the [2] rotaxanes. Thus, the CBPQT(4+) rings in the degenerate [2]rotaxanes exhibit slow shuttling between two DNP recognition sites on the (1)H NMR time-scale on account of the electrostatic barrier posed by the bispyridinium units, as demonstrated by variable-temperature (1)H NMR spectroscopy. Electrochemical experiments carried out on the [2]rotaxanes 1.6PF(6) and 2.6PF(6) indicate that the one-electron reduced bipyridinium radical cation in the dumbbell components of the [2] rotaxanes serves as an additional recognition site for the two-electron reduced CBPQT(2)((.+)) diradical cationic ring. Under appropriate conditions, the ring components in the degenerate rotaxanes 1.6PF(6) and 2.6PF(6) can shuttle along the recognition sites - two DNP units and one-electron reduced bipyridinium radical cation - under redox control.