Dalton Transactions

The complexation and selective transport of alkaline earth metal cations in general and Ba in particular have been thoroughly investigated for a long time. Important applications for Ba extraction include the purification of spent nuclear fuel as well as sulfur analytics. Various binding domains have been successfully established, e.g. calixarenes, cyclic oligopeptides or substituted crown ethers. Especially the use of the lipophilic dicarbollyl-cobaltate anion in combination with a wide variety of donor ligands was extensively studied by Makrlík et al. Over the past two decades we have been investigating the well-established norbornadiene-quadricyclane system regarding the feasibility of active ion transport through artificial lipophilic membranes. The light induced [2 + 2]-cycloaddition or cycloreversion results in a change of the size of the ligand’s cavity, which subsequently affects the complexation properties for metal ions. We synthesized a wide variety of symetrically four-fold-substituted norbornadiene and quadricyclane derivatives. Esters, polyethers as well as amides were considered as suitable binding domains mainly for alkali metal cations. The tetrakis(diisopropyl amide) substituted isomers 1 and 2 proved to be the most promising candidates in terms of active ion transport. The amides 1 and 2 showed remarkably differing association constants for Li, Na and K and were successfully tested in passive transport experiments (Scheme 1).