Degenerate [2]rotaxanes with electrostatic barriers

A synthetic approach to the preparation of [2]rotaxanes (1-5·6PF₆) 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 (CBPQT⁴⁺) ring has been developed. Since the π-electron-deficient bispyridinium units are introduced into the dumbbell components of the [2]rotaxanes 1-5·6PF₆, there are Coulombic charge-charge repulsions between these dicationic units and the CBPQT ⁴⁺ ring in the [2]rotaxanes. Thus, the CBPQT⁴⁺ rings in the degenerate [2]rotaxanes exhibit slow shuttling between two DNP recognition sites on the ¹H NMR time-scale on account of the electrostatic barrier posed by the bispyridinium units, as demonstrated by variable-temperature ¹H NMR spectroscopy. Electrochemical experiments carried out on the [2]rotaxanes 1·6PF₆ 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 CBPQT²⁽⁺⁾ diradical cationic ring. Under appropriate conditions, the ring components in the degenerate rotaxanes 1·6PF₆ and 2·6PF₆ can shuttle along the recognition sites - two DNP units and one-electron reduced bipyridinium radical cation - under redox control.