A self-assembled, redox-responsive receptor for the selective extraction of LiCl from water

There is considerable interest in synthetic ionophores with high affinity and selectivity for Li(+). But so far, compounds that selectively bind Li(+) in the presence of other alkali and alkaline earth metal ions are rare and current approaches toward this goal are often accompanied with substantial synthetic efforts. Here we describe a trinuclear ruthenium metallamacrocyclic complex (1) that was obtained by self-assembly of ruthenium halfsandwich complexes and 3-hydroxy-2-pyridone ligands. This complex was shown to be an extremely potent receptor for LiCl with an affinity high enough to extract LiCl from water. The selectivity of this receptor is exceptional: even in the presence of a large excess of Na(+), K(+), Cs(+), Ca(2+), and Mg(2+), Li(+) was extracted exclusively. The Li(+)/Na(+) selectivity ratio was determined to be higher than 1,000:1. Compared with other synthetic ionophores, the receptor 1 offers two additional advantages: (i) the synthesis can be accomplished in one step by using simple starting materials; and (ii) the presence of lithium ions can be detected electrochemically. Complex 1 is therefore a very attractive candidate for the construction of a Li(+)-specific chemosensor.