Equilibria and formation kinetics of some cyclen derivative complexes of lanthanides

Erika Szilágyi, Éva Tóth, Zoltán Kovács, Johannes Platzek, Bernd Radüchel, E. Brücher

Research output: Contribution to journalArticle

41 Citations (Scopus)

Abstract

The kinetics of the formation reactions between the lanthanide(III) ions Ce3+, Eu3+ and Yb3+ and four cyclen derivative ligands, DO3A-B, DO3A-ME, DO2A and DO2A-2B, were studied by spectrophotometry and a stopped-flow method at 25°C in 1.0 M KCl solutions. The reactions were found to be of first order, which was interpreted in terms of the formation of a diprotonated intermediate, Ln(H2L)+. The formation of products occurs via deprotonation and rearrangement of the intermediate, characterised by the rate constant, k(r). The rate law obtained, k(r) = k(OH)[OH-], is similar to those obtained for the formation reactions of DOTA and DOTA derivative complexes. The rate constants, k(OH), decrease with decrease in the number of charged carboxylate functional groups in the ligands; the lowest rates were found for the formation of the DO2A complexes. The formation rates increase significantly from Ce3+ to Yb3+. The direct proportionality between the formation rates and [H+]- was interpreted by assuming the equilibrium formation of a monoprotonated intermediate, Ln(HL), which undergoes deprotonation in a slow, rate-determining step. The validity of general base catalysis was detected in the formation reactions, which supports the assumption of the rate-controlling role of deprotonation of the monoprotonated intermediate. The protonation constants of the ligands DO3A-ME and DO2A-2B and the stability constants of their complexes were also determined. The ligand DO2A forms the usual complexes Ln(DO2A)+ and the unusual species Ln(DO2A)2 -. In the complexes Ln(DO2A)2 -, the two ligands exhibit different coordination modes which were demonstrated by 1H NMR spectroscopy. (C) 2000 Elsevier Science S.A.

Original languageEnglish
Pages (from-to)226-234
Number of pages9
JournalInorganica Chimica Acta
Volume298
Issue number2
DOIs
Publication statusPublished - Feb 15 2000

Fingerprint

Lanthanoid Series Elements
Rare earth elements
Ligands
Deprotonation
Derivatives
Kinetics
kinetics
ligands
Rate constants
Spectrophotometry
Protonation
Catalysis
Functional groups
Nuclear magnetic resonance spectroscopy
Magnetic Resonance Spectroscopy
cyclen
Ions
spectrophotometry
carboxylates
catalysis

Keywords

  • Cyclen complexes
  • Lanthanide complexes
  • Stability constants

ASJC Scopus subject areas

  • Biochemistry
  • Inorganic Chemistry
  • Physical and Theoretical Chemistry
  • Materials Chemistry

Cite this

Equilibria and formation kinetics of some cyclen derivative complexes of lanthanides. / Szilágyi, Erika; Tóth, Éva; Kovács, Zoltán; Platzek, Johannes; Radüchel, Bernd; Brücher, E.

In: Inorganica Chimica Acta, Vol. 298, No. 2, 15.02.2000, p. 226-234.

Research output: Contribution to journalArticle

Szilágyi, Erika ; Tóth, Éva ; Kovács, Zoltán ; Platzek, Johannes ; Radüchel, Bernd ; Brücher, E. / Equilibria and formation kinetics of some cyclen derivative complexes of lanthanides. In: Inorganica Chimica Acta. 2000 ; Vol. 298, No. 2. pp. 226-234.
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AU - Szilágyi, Erika

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AU - Radüchel, Bernd

AU - Brücher, E.

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AB - The kinetics of the formation reactions between the lanthanide(III) ions Ce3+, Eu3+ and Yb3+ and four cyclen derivative ligands, DO3A-B, DO3A-ME, DO2A and DO2A-2B, were studied by spectrophotometry and a stopped-flow method at 25°C in 1.0 M KCl solutions. The reactions were found to be of first order, which was interpreted in terms of the formation of a diprotonated intermediate, Ln(H2L)+. The formation of products occurs via deprotonation and rearrangement of the intermediate, characterised by the rate constant, k(r). The rate law obtained, k(r) = k(OH)[OH-], is similar to those obtained for the formation reactions of DOTA and DOTA derivative complexes. The rate constants, k(OH), decrease with decrease in the number of charged carboxylate functional groups in the ligands; the lowest rates were found for the formation of the DO2A complexes. The formation rates increase significantly from Ce3+ to Yb3+. The direct proportionality between the formation rates and [H+]- was interpreted by assuming the equilibrium formation of a monoprotonated intermediate, Ln(HL), which undergoes deprotonation in a slow, rate-determining step. The validity of general base catalysis was detected in the formation reactions, which supports the assumption of the rate-controlling role of deprotonation of the monoprotonated intermediate. The protonation constants of the ligands DO3A-ME and DO2A-2B and the stability constants of their complexes were also determined. The ligand DO2A forms the usual complexes Ln(DO2A)+ and the unusual species Ln(DO2A)2 -. In the complexes Ln(DO2A)2 -, the two ligands exhibit different coordination modes which were demonstrated by 1H NMR spectroscopy. (C) 2000 Elsevier Science S.A.

KW - Cyclen complexes

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KW - Stability constants

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