title: Degenerate [2]rotaxanes with electrostatic barriers.
abstract: 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 π-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.
authors.affiliation: Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, USA.
authors.forename: Hao
authors.initials: H
authors.lastname: Li
authors.forename: Yan-Li
authors.initials: YL
authors.lastname: Zhao
authors.forename: Albert C
authors.initials: AC
authors.lastname: Fahrenbach
authors.forename: Soo-Young
authors.initials: SY
authors.lastname: Kim
authors.forename: Walter F
authors.initials: WF
authors.lastname: Paxton
authors.forename: J Fraser
authors.initials: JF
authors.lastname: Stoddart