Degenerate [2]rotaxanes with electrostatic barriers

By Li, Hao; Zhao, Yan-Li; Fahrenbach, Albert C.; Kim, Soo-Young; Paxton, Walter F. & Stoddart, J. Fraser
Published in Org. Biomol. Chem. The Royal Society of Chemistry 2011

Abstract

A synthetic approach to the preparation of [2]rotaxanes (1-5[middle dot]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 [small pi]-electron-deficient bispyridinium units are introduced into the dumbbell components of the [2]rotaxanes 1-5[middle dot]6PF6, there are Coulombic charge-charge repulsions between these dicationic units and the CBPQT4+ 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[middle dot]6PF6 and 2[middle dot]6PF6 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([radical dot]+) diradical cationic ring. Under appropriate conditions, the ring components in the degenerate rotaxanes 1[middle dot]6PF6 and 2[middle dot]6PF6 can shuttle along the recognition sites - two DNP units and one-electron reduced bipyridinium radical cation - under redox control.

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