Rotaxanes and pseudorotaxanes with Fe-, Pd- and Pt-containing axles. Molecular motion in the solid state and aggregation in solution

This article reviews our recent studies on structure and properties of rotaxanes and pseudorotaxanes with Fe-, Pd- and Pt-containing complexes as the axle component. Electrochemical oxidation of ferrocenylmethylamine in the presence of a hydrogen radical precursor induces formal protonation of the amino group and produces a pseudorotaxane of the resulting ammonium species with a crown ether. Single crystals of the ferrocene-containing pseudorotaxane undergo a thermal crystalline phase transition accompanied by changes in the optical properties of the crystals. X-Ray crystallographic studies of the low- and high-temperature phases revealed different intermolecular interactions and orientations of the aromatic rings in the crystalline state depending on the temperature. End-capping of the ferrocene-containing 2]pseudorotaxane using a cross-metathesis reaction yields 2]rotaxane under mild conditions. A rotaxane having a platinum-carboxylate complex as its axle is converted into related organic and inorganic rotaxanes by partial dissociation of the Pt-O bond. An N-alkylbipyridinium forms 3]pseudorotaxane with alpha-cyclodextrin (alpha-CD), and it reacts with platinum and palladium complexes to form the corresponding 5]rotaxanes containing four alpha-cyclodextrin moieties. Complexes without alpha-CD components form micelles in aqueous solution, while the addition of alpha-CD causes degradation of the micelles and the formation of rotaxanes.