3-D water-soluble reversible neodymium(III) and lanthanum(III) coordination polymers

The formation of soluble supramolecular 3-D coordination polymers with Nd³⁺ and La³⁺ in aqueous solution has been studied for two bifunctional ligands consisting of two pyridine-2,6-dicarboxylate groups connected at the 4-position by O(CH₂CH₂O)₄ and O(CH₂CH₂O)₆ spacers (C4 and C6, respectively). Viscosity measurements were used to monitor the network formation as a function of the ligand concentration, and the ratio between metal ions and ligands. For corresponding conditions C4 solutions with Nd³⁺ always gave much higher viscosities than C6 solutions with Nd³⁺. C6 is long, and flexible enough to bind with both chelating groups to only one metal ion (ring formation). This causes the polymers to stop growing, resulting in smaller average sizes of the 3-D polymers. The ring-structures could be demonstrated by ¹H NMR spectroscopy using C6 and La³⁺ at low concentrations.