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 = Mg1, L = NOTMP) is protonated to H-ML in a rapid equilibrium (log KH-MI, = 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 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.
|Original language||English (US)|
|Number of pages||7|
|State||Published - Dec 1 1996|
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry