Chemically Induced Contraction and Stretching of a Linear Rotaxane Dimer

Abstract

Copper(I)‐induced assembly of two self‐complementary identical units, which consist of a ring that incorporates a 1,10‐phenanthroline group attached to a small filament containing a second 1,10‐phenanthroline (phen) group, leads quantitatively to a doubly threaded complex. Each copper(I) center is four‐coordinate and is located inside a ring and bound to a phen from the macrocyle. The two other coordination sites are occupied by a phen from the filament connected to the other ring. An X‐ray structure of the dicopper(I) complex unambiguously demonstrates the doubly threaded nature of the system. The molecule has C₂ symmetry in the crystal. This is an extended form with a Cu⋅⋅⋅Cu separation of 18.3 Å and an overall length close to 40 Å. Further synthetic work, which utilizes the two terminal phenolic functions of the previous dicopper(I) complex, gives rise to a more complex system in which both filaments have been prolonged in opposite directions by 2,2′:6′,2″‐terpyridine (terpy) motifs and bulky stoppers. The organic backbone is that of a rotaxane dimer. Although redox cycling of Cu^I to Cu^II did not lead to intramolecular rearrangement, simple chemical reactions induced large conformational changes. The rotaxane dimer was set in motion as follows. The dicopper(I) complex, which is in an extended conformation, was demetallated by using KCN. From the free ligand, the dizinc complex was formed quantitatively at room temperature. ¹H NMR data show that a new conformation is obtained: each Zn^II is five‐coordinate (phen + terpy), and the molecule is in a contracted conformation. This process is reminiscent of biological muscles in the sense that the two filaments of this system can be moved along one another in a gliding motion that keeps the whole system together, but which converts a stretched compound (overall length ≈83 Å) into a contracted species (overall length ≈65 Å, according to CPK models). The motion is quantitatively reversed by the addition of an excess of copper(I) to the dizinc complex; this regenerates the extended starting form. Although the motivation of the present contribution was to illustrate that a musclelike molecule may be stretched or contracted using electrochemistry and coordination chemistry, the main body of the work is organic synthesis. This is testified by the fact that the dicopper(I) rotaxane dimer was obtained in 23 steps from commercially available compounds.