NMR Studies of Microscopic Sites of Protonation and Metal-Binding with Multidentate Complexing Agents

Abstract

NMR techniques have been used to indicate the time-average location of protons on multidentate complexing agents. For example, a proton spends an equivalent time on each of the nitrogens in symmetrical compounds such as ethylenediamine, EDTA, EGTA, etc. This conclusion is reached from the fact that the chemical shifts of -CH₂- protons adjacent to the N-ligand atom are equal. In a species such as DTPA (Y^5- -), where the “middle” acetate -CH₂- protons are distinguishable from the other four -CH₂- acetate protons, the relative extent of chemical shifts downfield suggests that the first proton added (forming HY^4-) spends more of its time on the central N-atom. After a second equivalent of acid has been added (forming H₂Y^3-), the middle acetate -CH₂- proton peak shifts upfield, very close to the field at which it occurred for the completely non-protonated basic form (Y^5-). The downfield shift of all the remaining acetate -CH₂ - protons then shows that the H₂Y^3-species is chiefly protonated on the two end nitrogen atoms. Similar results have been obtained with the polyamines. Quantitative evaluation of the percent protonation of the various ligand atoms has been made through computer computations using substituent constants obtained from model compounds.