FORMATION AND DISSOCIATION KINETICS OF THE MAGNESIUM(II) COMPLEX OF ZACYCLONONANE-1,4,7-TRIS(METHYLENEMETHYLPHOSPHINIC ACID)

The dissociation and the formation rate of the Mg-II complex of cyclon onane-1,4,7-tris(methylenemethylphosphinate) (NOTMP) have been studied by nonequilibrium potentiometry and P-31 NMR spectroscopy. The dissoc iation reaction was dominated by a proton-assisted pathway in which th e 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 x 10(-2) s(-1)). The formatio n reaction appeared to be faster, and the first part of the reaction w as 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 hit. The intermediate M-L was not observed directly , but likely involves partial coordination of M to L via the phosphina te 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 att ached to a ring nitrogen, while the metal is probably coordinated to t he phosphinate oxygens, similar to M-L. An overall reaction scheme was used to simulate all potentiometric pH curves and the NMR titration d ata. This model shows that, at equilibrium, (de)complexation is domina ted by the proton-assisted pathway at pH < 7.0, while above this pH th e spontaneous dissociation of ML and the formation of ML from M-L prev ail.