MULTI-DENTATE ACYCLIC NEUTRAL LIGANDS AND THEIR COMPLEXATION

The properties of cyclic crown ethers are approximated by acyclic neutral ligands (popdands), which are compared and contrasted with open‐chain bioionophores and acidic chelating agents in this article. Variations of the endo‐polarophilicity/exo‐lipophilicity balance, complex stability, ion slectivity can often be accomplished more easily, with greater versatility, and at less expense with acyclic polyethers than with their cyclic counterparts; complexation and decomplexation are generally faster in acyclic systems; and the pseudocavity usually has greater conformational flexibility. Acyclic crown ethers and open‐chain cryptands stiffened by rigid “terminal groups” containing donor atoms readily form crystalline complexes of alkali and alkaline earth metals. Some oppen‐chain neutral ligands form helical conformations in their crycstalin complexes. The observed coordination numbers and geometries are of theoretical interst. Attractive terminal group interactions lead to pseudocyclic species occupying a position intermediate between cyclic and acyclic ligands. It has recently proved possible to isolate crystalline complexes of alkali and alkaline earth metal ions with weakly donating oligo(ethylene glycol ethers) and with glycols; such complexes have also been obtained with sugars. Acyclic neutral ligands can serve as simple models of nigericin‐type bioionophores and be used analytically in microelectrodes. The recently discoverd crystalline stoichiometric complexes formed by some acyclic neutral ligands with guest molecules such as urea, thiourea, and water provide a fresh insight into weak interactions between neutral molecules and for the development of urea receptors. Copyright © 1979 by Verlag Chemie, GmbH, Germany