A transition metal containing rotaxane in motion: Electrochemically induced pirouetting of the ring on the threaded dumbbell

In this work we describe a rotaxane in which a new type of motion, pirouett ing of the wheel around its axle, can be electrochemically triggered. The r otaxane was synthesized by using the three-dimensional effect of copper(I). It incorporates both a hetero-biscoordinating ring (containing both 2,9-di phenyl-1,10-phenanthroline and 2,2':6',2"-terpyridine units) and a molecula r string which contains only one 2,9-diphenyl-1,10-phenanthroline. Both end s of the string are functionnalized by a bulky stopper ( tris(p-tert-butylp henyl) (4-hydroxypbenyl)methane). Large molecular motions have been induced electrochemically in this molecule and were detected by using cyclic volta mmetry. The driving force of the two rearrangement processes observed is th e high stability of two markedly different coordination environments for co pper(I) and copper(II) ions. In the copper(I) state, two phenanthroline uni ts tone of the ring, one of the string) interact with the metal in a tetrah edral geometry (Cu-(4)(I)), whereas in the copper(II) state the five-coordi nate geometry (Cu-(5)(II)) is due to the phenanthroline of the string and t o the terpyridine of the ring. The kinetic rate constants of the two molecu lar motion processes (from Cu-(5)(I)) to Cu-(4)(II)) and fom Cu-(4)(II)) to Cu-(5)(II))) have been determined and both rates are much faster than the ones in previously studied analoguous systems. In addition, the rate of the pirouetting motion depends greatly on the copper oxidation state. The diva lent four-coordinate copper complex (Cu-(4)(II))) rearranges in tens of sec onds, whereas the monovalent five-coordinate copper system leads to the fou r-coordinate complex in the millisecond time scale.