The Synthesis and Reactivity of Novel [Co(L)(PMG)]n+ Complexes
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Glyphosate (N-(phosphonomethyl)glycine) is the phytotoxic reagent in the widely used Roundup® herbicide. Its mode of action in plants is the disruption of the Shikimate pathway, part of an important route to the biosynthesis of essential aromatic amino acids. It is well documented that glyphosate can be degraded by soil microorganisms after contact of the herbicide solution with the soil. It is also accepted that glyphosate, an excellent ligand, is readily absorbed to metal ions, such as Fe(III), that can be abundant in soils. There have been many accounts on the microbial degradation of glyphosate, and several metal-glyphosate complexes have been synthesised and characterised. Surprisingly, given the degree of adsorption to metal ions in the soil, there have not, to date, been any reports in the literature on the reactivity of metal-glyphosate complexes. The behaviour of these types of complexes under various reaction conditions may give us an insight into the mechanisms present when glyphosate degrades. In order to explore the behaviour of metal-glyphosate adducts, we have prepared several new cobalt-PMG complexes in the lab. These complexes have been characterised by NMR, mass spectrometry, elemental analysis, and in some cases X-ray crystal structure determination. We chose to synthesise complexes where the PMG ligand is bidentate or tridentate, filling the remaining four or three (respectively) coordination sites with an ancillary, nitrogen-containing ligand. We have subjected the complexes to photolytic and basic conditions, as we are interested in ascertaining how coordinated PMG might behave when irradiated with UV light, and when it is C-deprotonated. Metal-glyphosate complexes in nature may be exposed to UV light, so we are concerned with how the coordinated ligand might react under these circumstances. We have found that the prepared cobalt-PMG complexes are reactive when exposed to UV light, and that this appears to result in the degradation of the complex, and in some cases, the PMG ligand itself. The reactivity of C-deprotonated PMG is also an area of interest to us. It is possible that elevations in soil pH can lead to C-deprotonation of glyphosate, then further reactivity that may contribute to the degradation of the compound. Furthermore, when the herbicide is held in the active site of an enzyme within a microbe, it may become deprotonated, and this may aid in its microbial degradation. We have found that, under basic conditions, the reactivity of the prepared PMG complexes depends on the ancillary ligand attached - π-acidic, pyridine-containing ancillary ligands appear to increase the reactivity of coordinated PMG. It seems that amine-containing ligands hinder the reactivity of the coordinated PMG such that the complex remains intact. It is hoped that the results of the research described in this thesis will assist in the future investigations into the reactivity of the herbicide glyphosate.