The Stability of the Complex and the Basicity of the Anion Impact the Selectivity and Affinity of Tripodal Gadolinium Complexes for Anions

The affinities and selectivities of lanthanide complexes with open coordination sites for anions vary considerably with the chelate. In order to determine the effect of the stability of a lanthanide complex on its affinity for anions, five different complexes featuring different bidentate chelating moieties were synthesized, and their affinity for anions in water at neutral pH were evaluated by longitudinal relaxometry measurements. The chelates comprise both oxygen and nitrogen donors including maltol, 1,2-hydroxypyridinone, hydroxamic acid, pyridin-2-ylmethanol, and carbamoylmethylphosphonate diester. They were chosen to span a range of basicities all the while maintaining a similar tripodal tris-bidentate architecture, thereby allowing for a direct study of the role of the coordinating motif on the supramolecular recognition of anions by the corresponding Gd-III complex. Overall, for ligands containing the same number of protonation steps, and therefore the same charge at neutral pH, the lower the acidity of the chelate (higher Sigma pK(a)'s), the less stable the corresponding Gd m complex, and the higher its affinity for anions. Regardless of the number of protonation steps, the more stable Gd-III complexes form ternary or quaternary assemblies with coordinating anions. In contrast, the same anions readily displace the chelate of the least stable complexes, resulting instead in the formation of Gd-III.anion precipitates. Irrespective of the chelate, in the absence of steric hindrance at the open coordination site, the affinity of Gd-III complexes for anions follows the order phosphate > arsenate > bicarbonate > fluoride. Hence, the selectivity and affinity of Gd-III complexes of tripodal tris-bidentate chelates for anions is a function of the stability of the Gd-III complex and the basicity of the anion.