LUMINESCENCE STUDIES OF THE STEREOSELECTIVITY IN THE MIXED-LIGAND COMPLEXES FORMED BY TERBIUM(III) WITH ENANTIOMERICALLY RESOLVED 1,2-PROPANEDIAMINETETRAACETIC ACID AND ALPHA-HYDROXYPHENYLCARBOXYLIC ACIDS

It has been found that the binding of mandelic acid (MAN) at the inner coordination sphere of Tb(propanediaminetetraacetate) is profoundly a ffected by the enantiomeric identity of the MAN ligand, but that such stereoselective effects are absent in the formation of analogous terna ry complexes containing phenyllactic acid (PLA). For Tb(PDTA) (MAN) co mplex species, the largest formation constants were obtained when the PDTA and MAN ligands were of the opposite absolute configuration, and the smallest were obtained when the two bound ligands were of the same configuration. The formation constants of all Tb(PDTA) (PLA) complexe s were found to be equivalent to within experimental error, indicating no stereoselectivity in the formation of these ternary complexes. The circularly polarized luminescence within the Tb(III) emission bands i ndicated that steric interactions accompanied the binding of MAN by a Tb(PDTA) complex, which were deduced to be associated with perturbatio ns in ligand conformations. No such effects were noted between bound P DTA and PLA ligands, where the observed CPL intensities of the ternary complexes could be calculated by a simple addition of PDTA and PLA co ntributions. Determinations of metal ion hydration revealed that the f ully formed Tb(PDTA)(MAN) complexes were essentially anhydrous, while the fully formed Tb(PDTA)(PLA) to complexes still contained residual w ater. This finding indicated that the binding of MAN a Tb(PDTA) comple x is accompanied by a total expulsion of all coordinated water molecul es, implying the existence of a tight fit by the MAN ligand in the res idual inner coordination sphere of a Tb (PDTA) complex. The lack of an y demonstrable steric interactions between bound PDTA and PLA ligands implies that the PLA ligand has a smaller steric requirements than doe s the MAN ligand. This is thought to be due to the additional methylen e group of PLA, which provides an additional spacing between the coord inating hydroxycarboxylate group and the sterically demanding phenyl g roup, and which appears to be sufficient to move the phenyl group out to a distance which does not require its interaction with the PDTA fun ctionality. (C) 1997 Wiley-Liss, Inc.