Macrocycles and their selectivity for metal ions on the basis of size

The metal ion size selectivity of the oxygen— and nitrogen— donor macrocycles is examined. It is shown that the presence of the neutral oxygen donor in ligands leads to stronger complexation of large metal ions, with less favourable complexation of small, irrespective of whether the oxygen donor is part of a macrocyclic ring or not. Molecular Mechanics calculations indicate that the size—dependence of the complexing of metal ions by the neutral oxygen donors is controlled by a balance between the steric strain produced by the group bearing the oxygen donor, and its inductive effects. For tetraazamacrocycles the stability is controlled by the size of the chelate ring formed on complex—formation. Larger chelate rings lead to greater complex stabilisation for small metal ions, while larger metal ions show progressively greater complex destabilisation with larger chelate rings. This apparent paradox is also examined using molecular mechanics calculations. The use of neutral oxygen donors and chelate ring size to control metal ion size selectivity in ligand design is discussed.