Glyphosate complexation to aluminium(III). An equilibrium and structural study in solution using potentiometry, multinuclear NMR, ATR–FTIR, ESI-MS and DFT calculations

The stoichiometries and stability constants of a series of Al3+–N-phosponomethyl glycine (PMG/H3L) complexes have been determined in acidic aqueous solution using a combination of precise potentiometric titration data, quantitative 27Al and 31P NMR spectra, ATR–FTIR spectrum and ESI-MS measurements (0.6 M NaCl, 25 °C). Besides the mononuclear AlH2L2+, Al(H2L)(HL), Al(HL)2- and Al(HL)L2−, dimeric Al2(HL)L+ and trinuclear Al3H5L42+ complexes have been postulated.1H and 31P NMR data show that different isomers co-exist in solution and the isomerization reactions are slow on the 31P NMR time scale. The geometries of monomeric and dimeric complexes likely double hydroxo bridged and double phosphonate bridged isomers have been optimized using DFT ab initio calculations starting from rational structural proposals. Energy calculations using the PCM solvation method also support the co-existence of isomers in solutions.