Synthesis of [5]Rotaxanes Containing Bi- and Tridentate Coordination Sites in the Axis

• Published in: Chemistry : a European journal 2011-01, Vol.17 (3), p.947-957
• Main Authors: Collin, Jean-Paul, Durot, Stéphanie, Keller, Michel, Sauvage, Jean-Pierre, Trolez, Yann, Cetina, Mario, Rissanen, Kari
• Format: Article
• Language: English
• Subjects: Axles; Blocking; Chelating; Chemistry; click chemistry; copper; Diffraction; Fragmentation; Ligands; lithium; Precursors; rotaxanes; X-rays; zinc
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201002220

A new example of a linear [5]rotaxane has been synthesized by using the traditional “gathering‐and‐threading” approach but based on an unusual axle incorporating a symmetrical bis(bidentate) chelating fragment built on a 4,7‐phenanthroline core. The stoppering reaction is particularly noteworthy since, instead of using a trivial bulky stopper as precursor to the blocking group, two semistoppered copper‐complexed [2]pseudorotaxanes (namely [2]semirotaxanes) are used, which leads to the desired [5]rotaxane in good yield. The efficiency of the method relies on the use of “click” chemistry, with its very mild conditions, and on the protection by a transition‐metal (copper(I)) of the various coordinating groups present in the fragments to be interconnected (terpy and bidentate chelating groups), thus inhibiting potential detrimental side reactions during the copper‐catalyzed stoppering reaction. Since the external fragments and the central core of the system contain tri‐ and bidentate chelating units, respectively, the axle of the final [5]rotaxane incorporates two types of coordinating units: two external terpy groups (terpy: 2,2′:6′,2′′‐terpyridine) and two central bidentate ligands. Such a situation enables the system to tidy two different metals centers, and to localize them in a priori well‐defined positions. This is what was observed when mixing the free ligand with a mixture of Zn2+ and Li+: the zinc(II) ions were unambiguously shown to occupy the external sites, whereas the Li+ cations were found in the central part of the [5]rotaxane. An X‐ray diffraction study carried out on a [3]pseudorotaxane, the axis of which is similar to the central part of the [5]rotaxane axle, demonstrates that Zn2+ is clearly five‐coordinate, the fifth ligand being a counterion, even when the coordination site of the pseudorotaxane is designed for four‐coordinate metals, which is in marked contrast with copper(I) or Li+. Chemical tidying: A new example of a linear [5]rotaxane has been synthesized in good yield (see figure). The external fragments and the central core of the system contain tri‐ and bidentate chelating units, respectively. Such a situation enables the system to tidy four metal centres consisting of two different metallic ions, Li+ and Zn2+, and to localize them in well‐defined positions.