IR and EXAFS spectroscopic studies of glyphosate protonation and copper(II) complexes of glyphosate in aqueous solution

The varying degrees of protonation of N-(phosphonomethyl)glycine (PMG, glyphosate) were investigated with infrared (IR) spectroscopy and ab initio frequency calculations. The zwitterionic nature of PMG in solution was confirmed, and intramolecular hydrogen bonding was identified. Successive protonation of the PMG molecule follows the order amine, phosphonate, carboxylate. Intramolecular hydrogen bonding is indicated to exist at all stages of protonation: between both RCO(2-) and RNH(2)(+) and RPO(3)(2-) and RNH(2+) in HL(2)(-) (where L represents the ligand PMG); between RCO(2)(-) and RNH(2)(+) in H(2)L(-); predominantly between RPO(3)(2-) and RNH(2)(+) in H(3)L. There are strong indications that the zwitterion is intact throughout the pH range investigated. Results from IR and extended X-ray absorption fine structure (EXAFS) spectroscopies provide new evidence for structures of N-(phosphonomethyl)glycinecopper(II) complexes. The structures of 1:1 complexes, CuL(-) and CuHL, are essentially the same, differing only in protonation of the phosphonate group. Copper(II) lies at the center of a Jahn-Teller distorted octahedron with all three donor groups (amine, carboxylate, phosphonate) of PMG chelating with copper(II) to form two five-membered chelate rings oriented in the equatorial plane. EXAFS indicates that oxygen (most likely a water molecule) is a fourth ligand, which would thus occupy the fourth corner in the equatorial plane of the elongated octahedron. CuL(2)(4-) most probably forms an isomeric mixture in solution, and there are indications that this mixture is dominated by complexes where two PMG ligands are bound to copper(II) via equatorial and axial positions, with both phosphonate and carboxylate donor groups responsible for chelation at axial positions.