Complexation of Eu(III) in Aqueous Solution: Glyphosate vs. N-Methylglyphosate

Abstract

N-Methylation represents a subtle structural modification of glyphosate that may alter the coupling between ligand protonation and metal coordination, yet its influence on lanthanide complexation remains insufficiently understood. In this study, the protonation and Eu(III) complexation behaviors of glyphosate (H3A) and its N-methylated derivative, N-methylglyphosate (H3B), in aqueous solution were systematically compared using potentiometry, NMR spectroscopy, ESI-MS, and Eu(III) luminescence measurements. Potentiometric and 31P NMR analyses showed that N-methylation primarily affects the first protonation equilibrium while preserving preferential initial protonation at a phosphonate oxygen. Potentiometric analysis supported a Eu(III) complexation model restricted to a maximum 1:2 metal-to-ligand stoichiometry, containing EuLH, EuL, EuL2H2, and EuL2, with H3B exhibiting higher cumulative formation constants than H3A. ESI-MS provided complementary support for the predominant 1:1/1:2 stoichiometric framework, while no definitive evidence for 1:3 species was obtained under the measurement conditions. La(III) NMR and Eu(III) luminescence measurements further revealed distinct local coordination and hydration responses for the two ligands. Together, these results demonstrate that N-methylation modifies both the thermodynamics and local coordination behavior of glyphosate-type aminophosphonate ligands.