| Since their discovery, crown ethers have been applied in a variety of areas. By mimicking certain biological macrocyclic systems, they provide an excellent model, by which a better understanding of metal complexation in these environments can be achieved. Because of their ability to form complexes with ions and neutral molecules, crown ethers have been widely utilized in phase transfer catalysis as well as isotopic separation science. A structural survey of actinide/crown ether compounds was conducted in order to investigate the solid state chemistry of these complexes. Several parameters--the metal size, crown type, counterion, solvent systems and reaction and crystallization conditions--were varied to correlate their importance in complexation.;Under atmospheric conditions, two types of complexes were isolated, those containing only hydrogen-bonded crown interactions and instances where the crown interacts directly with the metal center. In both cases, water seems to play a very important role. When coordinated to the metal, water molecules exhibit the necessary donor properties required for the formation of hydrogen-bonded contacts. The water molecules also provide fierce competition with the crown ethers for metal-binding sites and in most cases prohibit the formation of complexes in which direct metal-ligand association exists. The results of this study indicate that direct interaction between the metal atoms and the crown ethers, in the presence of water, can only occur with polyether conformations which limit the steric repulsions within the metal coordination sphere. |
