THE EFFECT OF LIGAND RIGIDITY ON THE STABILITY OF EUROPIUM(III) COMPLEXES WITH SUBSTITUTED DIGLYCOLIC ACIDS

The stability of europium(III) complexes with substituted diglycolic acids is altered by changing the ligand rigidity. The diglycolate analog tetrahydrofuran-2,3,4,5-tetracarboxylic acid (THFTCA) achieves nearly an order of magnitude stronger binding of europium(III) in dilute acidic solutions, partly as a result of favorable complexation geometry forced by the tetrahydrofuran ring. The analogous aromatic ring systems furan-2,3,4,5-tetracarboxylic acid and furan-2,5-dicarboxylic acid, which force the carboxylate groups to assume a planar configuration, exhibit much lower affinity for europium(III) in dilute acid. The observed increase in stability for the THCA complex implies that one or two of the six geometric isomers of THFTCA must be dominant.