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

Jurriaan Huskens, A. Dean Sherry

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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 = 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 languageEnglish (US)
Pages (from-to)5137-5143
Number of pages7
JournalInorganic Chemistry
Volume35
Issue number18
StatePublished - 1996

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Magnesium
magnesium
dissociation
acids
Kinetics
Acids
protons
kinetics
Protons
potentiometric analysis
nuclear magnetic resonance
oxygen
titration
metal ions
Oxygen
nitrogen
ligands
1,4,7-triazacyclononane
1,4,7-triazacyclononane-1,4,7-tris(methylene methylphosphinic acid)
Complexation

ASJC Scopus subject areas

  • Inorganic Chemistry

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Formation and Dissociation Kinetics of the Magnesium(II) Complex of 1,4,7-Triazacyclononane-1,4,7-tris(methylenemethylphosphinic acid). / Huskens, Jurriaan; Sherry, A. Dean.

In: Inorganic Chemistry, Vol. 35, No. 18, 1996, p. 5137-5143.

Research output: Contribution to journalArticle

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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 = 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.",
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