Coordination Chemistry Of Glyphosate - Structural And Spectroscopic Characterization Of Bis(Glyphosate)Metal(Iii) Complexes


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Glyphosate (N-(phosphonomethyl)glycine, PMG) is a widely used, commercially available herbicide which is degraded and immobilized upon contact with soil. Since metal ions are believed to play a role in this process, the coordination chemistry of glyphosate is of significant interest. Although several 1:1 metal complexes of glyphosate have been structurally characterized, no 2:1 complexes have been characterized other than by potentiometric titration. The present work describes the synthesis and crystal structure of Na3Co(PMG)2.11H2O as well as a spectroscopic study of the chromium complex. There are potentially eight geometric isomers for such a ML2 complex, all of which are formed in solution in the reaction of Na3Co(CO3)3 or CoCl2/H2O2 with PMG. The individual isomers are not separable due to rapid interconversion, despite the expected kinetic inertness of Co3+, and have been characterized by various 1D and 2D NMR methods as well as UV spectroscopy. The structurally characterized fac-RS-all-trans isomer produces all the other isomers upon dissolution in water. On the basis of kinetic data, a twist as well as an associative interchange mechanism is proposed for these isomerization processes. The most symmetric fac-RS-all-trans isomer crystallized as the trisodium salt from an equilibrium mixture of all isomers in H20/ethanol and was characterized by single-crystal X-ray diffraction. The compound Na3Co(PMG)2-11H2O crystallizes in the triclinic space group P1BAR with a = 10.131(2) angstrom, b = 11.316(3) angstrom, c = 11.911(4) angstrom, V = 1190.4(5) angstrom3, alpha = 97.21(3)degrees, beta = 109.47(3)degrees, gamma = 107.29(3)degrees, and Z = 2; the two metal complexes in the asymmetric unit are essentially identical. The symmetry of each complex anion is C(i), implying opposite configurations at the nitrogens of the glyphosate ligands and an achiral molecule. The structure was determined from 3099 unique data measured at -96-degrees-C. Refinement of 319 variables against 1615 observed data gave R = 5.2%, R(w) = 5.5%, and GOF = 1.506.
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