[2]Catenanes Built Around Octahedral Transition-Metal Complexes that Contain Two Intertwined Endocyclic but Non-sterically Hindering Tridentate Ligands

Sterically hindering bidentate chelates, such as 2,9-diphenyl-1,10-phenanthroline, form entwined complexes with copper(I) and other tetrahedrally coordinated transition-metal centres. To prepare octahedral complexes containing two entwined tridentate ligands and thus apply a strategy similar to that used for making catenanes with tetrahedral metal centres, the use of the classical terpy ligand (terpy=2,2':6',2''-terpyridine) appears to be attractive. In fact, 6,6''-diphenyl-2,2':6',2''-terpyridine (dp-terpy) is not appropriate due to strong pinching of the organic backbone by coordination to the metal and thus stable entwined complexes with this ligand cannot be obtained. Herein, we report the synthesis and coordination properties of a new family of tridentate ligands, the main features of which are their endocyclic nature and non-sterically hindering character. The coordinating fragment consists of two 8'-phenylisoquinolin-3'-yl groups attached at the 2 and 6 positions of a pyridine nucleus. Octahedral complexes containing two such entangled ligands around an octahedral metal centre, such as FeII, RuII or CoIII, are highly stable, with no steric congestion around the metal. By using functionalised ligands bearing terminal olefins, double ring-closing metathesis leads to [2]catenanes in good yield with FeII or CoIII as the templating metal centre. The X-ray crystallography structures of the FeII precursor and the FeII catenane are also reported. These show that although significant pinching of the ligand is observed in both FeII complexes, the system is very open and no steric constraints can be detected.