Shuttles and muscles: Linear molecular machines based on transition metals

Transition-metal-containing rotaxanes can behave as linear motors at the mo lecular level. The molecules are set into motion either by an electrochemic al reaction or using a chemical signal. In a first example, a simple rotaxa ne is described that consists of a ring threaded by a two-coordination-site axle. The ring contains a bidentate ligand, coordinated to a copper center . The axle incorporates both a bidentate and a terdentate ligand. By oxidiz ing or reducing the copper center to Cu(II) or Cu(I) respectively, the ring glides from a given position on the axle to another position and vice vers a. By generalizing the concept to a rotaxane dimer, whose synthesis involve s a quantitative double-threading reaction triggered by copper(I) complexat ion, a molecular assembly reminiscent of a muscle is constructed. By exchan ging the two metal centers of the complex (copper(I) / zinc(III)), a large- amplitude movement is generated, which corresponds to a contraction/stretch ing process. The copper(I)-containing rotaxane dimer is in a stretched situ ation (overall length similar to8 nm), whereas the zinc(II) complexed compo und is contracted (length similar to6.5 nm). The stretching/contraction pro cess is reversible and it is hoped that, in the future, other types of sign als can be used (electrochemical or light pulse) to trigger the motion.