Formation and Dissociation Kinetics of the Magnesium(II) Complex of 1,4,7-Triazacyclononane-1,4,7-tris(methylenemethylphosphinic acid)

The dissociation and the formation rate of the Mg II complex of 1,4,7-triazacyclononane-1,4,7-tris(methylenemethylphosphinate) (NOTMP) have been studied by nonequilibrium potentiometry and 31 P NMR spectroscopy. The dissociation reaction was dominated by a proton-assisted pathway in which the complex ML (M = Mg II, L = NOTMP) is protonated to H−ML in a rapid equilibrium (log K H - ML = 5.2), which then dissociates to M and HL in a rate determining step ( k d,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 K M - HL = 1.79). This intermediate has a proton attached to a ring nitrogen, while the metal is probably coordinated to the phosphinate oxygens, similar to M−L. An overall reaction scheme was used to simulate all potentiometric pH curves and the NMR titration data. This model shows that, at equilibrium, (de)complexation is dominated by the proton-assisted pathway at pH < 7.0, while above this pH the spontaneous dissociation of ML and the formation of ML from M−L prevail.