ISSN:
0899-0042
Keywords:
terbium(III)
;
mandelic acid
;
circularly polarized luminescence
;
1,2-propanediaminetetraacetic acid
;
phenyllactic acid
;
stereoselectivity
;
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
It has been found that the binding of mandelic acid (MAN) at the inner coordination sphere of Tb(propanediaminetetraacetate) is profoundly affected by the enantiomeric identity of the MAN ligand, but that such stereoselective effects are absent in the formation of analogous ternary complexes containing phenyllactic acid (PLA). For Tb(PDTA)(MAN) complex 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) complexes 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 indicated that steric interactions accompanied the binding of MAN by a Tb(PDTA) complex, which were deduced to be associated with perturbations in ligand conformations. No such effects were noted between bound PDTA and PLA ligands, where the observed CPL intensities of the ternary complexes could be calculated by a simple addition of PDTA and PLA contributions. Determinations of metal ion hydration revealed that the fully formed Tb(PDTA)(MAN) complexes were essentially anhydrous, while the fully formed Tb(PDTA)(PLA) to complexes still contained residual water. This finding indicated that the binding of MAN a Tb(PDTA) complex is accompanied by a total expulsion of all coordinated water molecules, implying the existence of a tight fit by the MAN ligand in the residual inner coordination sphere of a Tb(PDTA) complex. The lack of any demonstrable steric interactions between bound PDTA and PLA ligands implies that the PLA ligand has a smaller steric requirements than does the MAN ligand. This is thought to be due to the additional methylene group of PLA, which provides an additional spacing between the coordinating hydroxycarboxylate group and the sterically demanding phenyl group, and which appears to be sufficient to move the phenyl group out to a distance which does not require its interaction with the PDTA functionality. Chirality 9:583-592, 1997. © 1997 Wiley-Liss, Inc.
Additional Material:
11 Ill.
Type of Medium:
Electronic Resource
