Ligand design for stabilization of group 2 element compounds by intramolecular coordination

Aesthetically pleasing, interlocked ring systems (catenanes) accumulate cha rge, as they are prepared from neutral ligands and cations. A new approach to the fabrication of charge neutral catenanes, based on designed dianionic ligands and divalent cations is presented. Ba2L2(H2L = 3,5,25,27-tetraoxo- 6,7,12,15,18,21,24-heptaoxanonaeisane) represents a new member of the class of helical metal containing catenanes. Each ligand in Ba2L2 acts as a doub le-stranded heterotopic catenand. The two helicates are interlocked to form a [2] catenane. Ba2L2 is a rare example of a heterotopic, homodinuclear ca tenane. Most heterotopic catenates hold two different metal centers, the bi nding environments being tailored for each metal; e.g., a metal with prefer ence for 4-coordination binding two bipyridyls and one with preference for 6-coordination binding two terpyridyls. The coordinative flexibility of Ba2 + permits coordination both to the pseudo-crown ether portion of the catena nd, as well as to the anionic ketoester moieties, Ba2L2 also is a rare exam ple of an alkaline earth containing catenane, and is the first crystallogra phically characterized group 2 catenane.