Interlacing molecular threads on transition metals
Copper(1) is ideally adapted to a three-dimensional template effect allowing interlacing of 1,lO-phenanthroline based molecular threads. By using appropriate cyclization procedures, interlocked macrocyclic ligands, the catenands, were prepared via the intermediate synthesis of their corresponding copper(1) complexes, the catenates. Systems containing two and three interlocked rings were made. Catenands display very special coordinating properties, allowing in particular to strongly stabilize low formal oxidation states (for instance, a Ni(1) catenate is air stable) and photo-excited states. A [3 ]-catenand (consisting of three interlocked coordinating rings) contains two potential complexing sites. Hetero-dinuclear compounds have been made : Cu(I)-Ag(I) and Cu(1)-Co(I1). Electrochemical studies show the two complex subunits to strongly interact, the mode of interaction probably involving intramolecular stacking between aromatic rings of the ligands rather than direct communication between the metals. If directed rings are employed, the catenands and catenates synthesized present novel properties : the systems obtained are topologically chiral. By using two copper(1) centers as templating species and by entwining two bis-chelating threads around them in the proper fashion, a double helix is obtained. It leads after cyclization to a dicopper complex molecular trefoil knot which can be demetalated to a knotted free macrocyclic ligand. The synthesis of catenanes (interlocked rings) and knots has been a highly challenging problem to chemists for several decades (ref. 1,2). Such molecular systems (Figure 1) are fascinating both for their aesthetic aspect and for their intriguing topological properties. Chemical topology was initiated in 1961 by the publication of an important article in which the notion of topological isomers was introduced (ref. 1). The archetypal system of topological isomerism is that of the trefoil knot and the unknotted ring, as depicted in Fig. 2. Catenane Knot Unknotted ring Knots Fig. 1 Fig. 2. Topological stereoisomers. A s discussed more recently (ref. 3 ) , the three hypothetical molecules of Fig. 2 are