Di- and Trivalent Metal-Ion Solution Studies with the Phosphinate-Containing Heterocycle DEDA-(PO)

A 7-membered triprotic heterocycle, DEDA-(PO), was synthesized, characterized, and tested for its solution properties with three trivalent lanthanides (La³⁺, Gd³⁺, and Lu³⁺) and three biologically relevant divalent metal ions (Ca²⁺, Zn²⁺, and Cu²⁺). The ligand synthesis has been reported once before; however, the characterization results were previously misinterpreted to correspond to a larger, 14-membered heterocycle, TETA-(PO)₂. This manuscript serves to correct the original paper. Potentiometric titrations were carried out with each of the metal ions, and the thermodynamic stability values in terms of log β and log K_ML were calculated showing a 1:1 metal-to-ligand ratio preference for the divalent metal ions and a 1:2 ratio for the lanthanides. The stability of the 1:2 complexes decreased across the lanthanide series, presumed to be a steric effect. Further resolution to the potentiometry results was given via pH-dependent NMR spectrometry (with La³⁺) and pH-dependent UV-vis spectroscopy (with Cu²⁺), and the pM values were calculated for all metal ions. The solid-state structure of the 1:1 Cu²⁺-DEDA-(PO) complex was further characterized by X-ray crystallography.