Electron Spin Resonance Study of Copper Ion Complexation by Glyphosate and Related Ligands

Abstract

AbstractGlyphosate (N‐phosphonomethylglycine) forms chelates with multivalent cations in aqueous solution, yet no detailed electron spin resonance (ESR) study of Cu‐glyphosate complexation has been conducted to date. In this study, the ESR spectra of Cu2+ complexes with glyphosate, iminodiacetic acid, and aminomethylphosphonic acid were obtained for aqueous solutions at room temperature and liquid N2 temperature in order to detail the coordination chemistry of these complexes. The Cu2+/ligand mole ratios as well as pH were varied to determine the effect on the ESR spectrum. These spectroscopic results were compared with speciation calculations based on stability constants from published potentiometric data in order to make reasonable assignments of the ESR spectra to particular Culigand complexes. The degree of complexation of aqueous Cu2+ estimated by ESR was in approximate agreement with that calculated from the published stability constants. The most reasonable interpretation of the ESR spectra pointed to the existence of both 1:1 and 2:1 ligand‐Cu complexes in which the amine N occupied one and two equatorial coordination positions, respectively. The carboxylate group complexed in preference to phosphonate in equatorial positions, and coordination at the axial positions in the 2:1 complexes was suggested. The spectra also suggested that the protonated 1:1 Cu‐ligand complexes were bonded via the amine N, indicating that proton addition was at carboxylate or phosphonate groups. The results indicated that Cu2+ bonded strongly to the amine groups, and by weaker forces to the carboxylate and phosphonate groups. Glyphosate was able to mobilize exchangeable Cu2+ on clays, forming uncharged or anionic complexes that diffused into solution.