Metal ion recognition in aqueous solution by highly preorganized non-macrocyclic ligands

The metal ion complexing properties of highly preorganized non-macrocyclic ligands in aqueous solution are discussed and contrasted with those of less preorganized analogues that have simple ethylene bridges between the donor atoms. High levels of preorganization can be achieved using cyclohexenyl bridges between ligand donor atoms, use of reinforced bridges such us bispidines, or by use of extended aromatic systems as bridges, such as those found in 1,10-phenanthroline (1,10-phen). Cyclohexenyl groups increase thermodynamic stability of metal ion complexes, as indicated by logK1 values (formation constants), that increase by between 1 and 5log units compared to less preorganized analogues. The way in which such bridges alter selectivity in the direction of smaller metal ions is discussed. Rigid bridges such as those provided by bispidine are discussed in terms of increased logK1 values, and sharply increased selectivity for smaller metal ions. Ligands derived from 1,10-phen by placing donor groups at the 2- and 9-positions are discussed, including examples with acetates (PDA), pyridyls (DPP) and phenolates (DPHP). The five-membered chelate rings of PDA lead to strong selectivity for larger metal ions, including Cd(II), La(III), and Gd(III). Possible uses of PDA type ligands for Gd(III)-based MRI agents are discussed. The remarkably high stability of complexes of PDA is discussed in terms of the role of H-bonding with the solvent in stabilizing complexes with metal ions, and the very high level of preorganization of the ligand.