Adjustable Receptor Based on a [3]Rotaxane Whose Two Threaded Rings Are Rigidly Attached to Two Porphyrinic Plates: Synthesis and Complexation Studies

The design and synthesis of a new type of receptor based on a [3]rotaxane, consisting of one thread and two threaded rings, is reported, as well as some of its complexing properties toward given guests. Two rings rigidly attached to porphyrins are threaded by a stiff rod incorporating two 2,2'-bipyridine-like chelates, the threading process being driven by two Cu(I) atoms acting as templates. A double-stoppering reaction based on click chemistry leads to the copper-complexed [3]rotaxane in which the rings are located close to the central part of the thread and the distance between the two porphyrin plates is short. Removal of the two Cu(I) cations releases the two rings which are now free to move along and around the thread. In these two states of the [3]rotaxane, free and complexed with copper, the two zinc(II) porphyrins attached to the rings can bind different ditopic guests bearing pyridyl groups or amines as terminal functions. UV-visible and NMR DOSY experiments were realized with guests of different sizes, and the association constants were determined. The free [3]rotaxane is both a strong and highly adaptable receptor with high stability constants for the host/guest complexes, log K being in the range of 6.3-7.5 for guests with a length varying between 2.8 and 18 angstrom. The copper-complexed [3]rotaxane is still a good receptor for small guests due to an entropic gain for this preorganized molecule compared to the free [3]rotaxane, but it is a less strong receptor for guests which do not fit the short distance between the two porphyrins.