Influence of gem-Dimethyl substitution on the stability, kinetics and relaxometric properties of PDTA complexes

Attila Forgács, Giovanni B. Giovenzana, Mauro Botta, Erno Brücher, Imre Tóth, Zsolt Baranyai

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Abstract

The protonation constants of DMPDTA (H 4DMPDTA = 2,2-dimethylpropylenediamine-N,N,N',N'-tetraacetic acid) and the stability and protonation constants of its Ln 3+ and some divalent metal complexes have been determined by pH potentiometry and spectrophotometry (Cu 2+) and compared with the corresponding properties of the complexes formed with PDTA (H 4PDTA = propylenediamine-N,N,N',N'-tetraacetic acid). The log K 2 H value of DMPDTA is lower by 1.5 log K units than that of PDTA. The stability constants (log K ML) of the Ln 3+ complexes formed with DMPDTA are lower by 1.0-1.5 log K units than those of PDTA. The kinetics of the transmetallation reactions of Gd(DMPDTA) - and Gd(PDTA) - with Cu 2+ and Eu 3+ have been studied by spectrophotometry. The reactions with Cu 2+ and Eu 3+ occur predominantly by spontaneous and proton-assisted dissociation of the Gd(DMPDTA) - and Gd(PDTA) - complexes. The rates of the metal-exchange reactions of Gd(DMPDTA) - are significantly lower than those of Gd(PDTA) -. The presence of the two methyl groups on the ligand backbone increases the kinetic inertness of Gd(DMPDTA) - due to the higher conformational rigidity of DMPDTA. Temperature-dependent 17O NMR spectra and 1/T 1 1H nuclear magnetic relaxation dispersion profiles of the Gd 3+ complexes were measured and analyzed to obtain the parameters that influence the water exchange rate and rotational dynamics. The introduction of a gem-dimethyl group on the backbone of the ligand leads to a significant variation in the properties of the corresponding complexes, which originate from a different conformational behaviour.

Original languageEnglish
Pages (from-to)2074-2086
Number of pages13
JournalEuropean Journal of Inorganic Chemistry
Issue number12
DOIs
Publication statusPublished - Apr 1 2012

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Keywords

  • Kinetics
  • Lanthanides
  • Relaxation
  • Thermodynamics

ASJC Scopus subject areas

  • Inorganic Chemistry

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