A fast-moving copper-based molecular shuttle: synthesis and dynamic properties.

Fast-track changes: The synthesis of a new copper-based molecular shuttle is described, with a coordinating macrocycle based on a nonhindering but endocyclic ligand (see scheme), which makes the ligand exchange easier, and thus the motions of the ring along the thread faster.The present report deals with the synthesis of a two-station [2]rotaxane consisting of a dpbiiq-incorporating macrocycle (dpbiiq: 8,8'-diphenyl-3,3'-biisoquinoline) threaded by a coordinating fragment whose complexing units are a dpp and a terpy ligand (dpp: 2,9-diphenyl-1,10-phenanthroline; terpy: 2,2',6',2"-terpyridine). The [2]rotaxane was prepared in 11 steps from commercially available or easy-to-make molecules, without taking into account the preparation of the dpbiiq-containing 39-membered ring, which was available in our group. The ring-incorporated bidentate chelate is at the same time endocyclic and sterically nonhindering, which is a specific property of the dpbiiq-coordinating unit. This unique feature has a profound influence on the rate of the ring-and-copper translation motion between the two stations of the axle. Based on an analogous multistep strategy, a related molecular shuttle has also been prepared that contains exactly the same axle and stoppers as the first compound but whose threaded ring incorporates the sterically hindering dpp chelate. The translation motions of this other system are several orders of magnitude slower than the corresponding movements of the dpbiiq-based compound. The motion corresponding to the rearrangement of the unstable five-coordinate copper(I) form of the compounds is relatively fast for both shuttles; the half lifetime of the five-coordinate Cu(I) species being below 20 ms for the dpbiiq-containing system and below 1 s for the dpp-based molecule. The reverse motion corresponding to the rearrangement of the four-coordinate copper(II) complexes is much slower, especially for the dpp-based system. It is of the order of several hours for the dpp-based shuttle and only one second or less for the dpbiiq system, under exactly the same conditions. The remarkable difference between the motion rates for the two two-station shuttles demonstrates that the use of a very open chelate such as dpbiiq is extremely beneficial in the context of fast-moving molecular machines.