Abstract

The dissociation and the formation rate of the MgII complex of 1,4,7-triazacyclononane-1,4,7-tris(methylenemethylphosphinate) (NOTMP) have been studied by nonequilibrium potentiometry and 31P NMR spectroscopy. The dissociation reaction was dominated by a proton-assisted pathway in which the complex ML (M = MgII, L = NOTMP) is protonated to H−ML in a rapid equilibrium (log KH-ML = 5.2), which then dissociates to M and HL in a rate determining step (kd,H-ML = 1.4 × 10-2 s-1). The formation reaction appeared to be faster, and the first part of the reaction was dominated by a pathway in which the metal ion rapidly forms a weak complex with the non-protonated ligand (M−L) that slowly rearranges to the final complex ML. The intermediate M−L was not observed directly, but likely involves partial coordination of M to L via the phosphinate oxygens. Below pH 7, a proton-assisted pathway prevailed involving a species M−HL (log KM-HL = 1.79). This intermediate has a proton attached to a ring nitrogen, while the metal...

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