Transition-metal-complexed catenanes and rotaxanes: from dynamic systems to functional molecular machines

Transition metal-based catenanes and rotaxanes constitute a specific class of mechanically interlocked molecules whose metal centers are essential both as templates in the construction of the compounds and for their ability to induce large-amplitude motions. In the present chapter we will first present a historical perspective of the field of interlocking compounds in general, in relation to molecular machines, starting with old work dating back to the 1980s and 1990s. Copper was shown many years ago to be the metal of choice for synthesizing the compounds via a template approach and for setting the molecules in motion using a redox signal (Cu(II)/Cu(I)). In a second paragraph, we will discuss various rotaxanes able to undergo a pirouetting motion of the axis within the threaded ring. Two families of such molecules will be mentioned: (1) a porphyrin-containing [2]rotaxane whose pirouetting motion is induced by a chemical reaction and (2) electrochemically driven systems. In this second category of [2]rotaxanes, the rate of motion could be dramatically increased by gradually modifying structural parameters and, in particular, by making the metal center less and less hindered by its surrounding ligands. The third section will be devoted to molecular shuttles and muscles, both families of compounds being reminiscent of linear machines such as biological muscles. By replacing the classical 2,9-diaryl-1,10-phenanthroline chelate (highly shielding and hindering) used by our group since the 1980s by an endocyclic but non-sterically hindering 3,3'-biisoquinoline derivative, the shuttling rate was increased in spectacular fashion, demonstrating the importance of steric factors in transition metal-based molecular machines. The same 3,3'-biisoquinoline motif was also used in the elaboration of a three-station shuttle, leading to long-distance (>20 Å) transport of a ring along the axis on which it is threaded. Finally, porphyrin-containing [3]rotaxanes and [4]rotaxanes, the latter displaying an overall cyclic structure, will be discussed and shown to behave as adjustable and switchable receptors. The synthesis of such compounds is a particularly challenging task in itself. In addition, the new receptors display fascinating properties such as, in particular, their ability to compress various guests and to expel them from their binding site using a chemical signal.