Glymes alkali metal complexes

Other approaches to self-assembling receptors have been reported in recent years. A self-assembling, trimeric palladium complex based on the bis(benzimidazole) ligand (17) was designed by Williams and coworkers [4]. The complex contains a hydrophobic cavity that in the X-ray structure has included a molecule of acetonitrile. In a different context, Schepartz and McDevitt [70] have used the chelation of nickel(n) by A,7V -bis(salicylaldehy-de)ethylenediamine (salen) derivatives to control the position of K -binding glyme chains, and it has been shown that these self-assembled ionophores influence alkali metal transport across liquid membranes [71]. Also, Shinkai and coworkers [72] and Schneider and Ruf [73] have used metal chelation to induce an allosteric effect on binding at a second site. 

The simplest chelates of this type, ethyleneglycol (eg) and 1,2-dimethoxyethane (glyme-2), give complexes with alkali and alkaline earth metal cations. The complexes MX2-2eg = Mg, Ca X = NOi-), MXz-Seg (MJ = Mg, Ca X = Cr, Br-), CaX2-4eg (X = Cr, Br ) and SrBr2 2eg have been isolated and IR studies suggest that the majority are likely to contain monodentate The Sr complex is, however, believed to have bidentate eg present. 

These results clearly indicate that the chelate ligation is driven primarily by the enthalpic factor and the entropy plays merely a trivial role in determining the complex stability. This is quite reasonable since the structures of these chelate complexes are strictly defined by the number and direction of the coordination sites of given heavy/transition metal ions, and therefore there is little room for the entropic term to adjust flexibly the complex structure and stability. On the contrary, alkali and alkaline earth metal ions also have the formal coordination numbers, but the actual number and direction of ligand coordination are highly flexible in the weak interaction-driven ligation by hard donors like glyme and crown ether. 

---