{DOTA-bis(amide)}lanthanide complexes: NMR evidence for differences in water-molecule exchange rates for coordination isomers

Shanrong Zhang, Zoltan Kovacs, Shawn Burgess, Silvio Aime, Enzo Terreno, A. Dean Sherry

Research output: Contribution to journalArticle

64 Scopus citations

Abstract

Two derivatives of 1,4,7,10-tetraazacyclododecane with trans-acetate and trans-amide side-chain ligating groups have been prepared and their complexes with lanthanide cations examined by multinuclear NMR spectroscopy. These lanthanide complexes exist in aqueous solution as a mixture of slowly interconverting coordination isomers with 1H chemical shifts similar to those reported previously for the major (M) and minor (m) forms of the tetra-acetate ([Ln(dota)]-) and tetraamide ([Ln(dtma)]3+) complexes. As in the [Ln(dota)]- and [Ln(dtma)]3+ complexes, the m/M ratio proved to be a sensitive function of lanthanide size and temperature. An analysis of 1H hyperfine shifts in spectra of the Yb3+ complexes revealed significant differences between the axial (D1) and non-axial (D2) components of the magnetic susceptibility tensor anisotropy in the m and M coordination isomers and the energetics of ring inversion and m ⇔ M isomerization as determined by two-dimensional exchange spectroscopy (EXSY). 17O shift data for the Dy3+ complexes showed that both have one inner-sphere water molecule. A temperature-dependent 17O NMR study of bulk water linewidths for solutions of the Gd3+ complexes provided direct evidence for differences in water exchange rates for the two coordination isomers. The bound-water lifetimes (τ298M) in the M and m isomers of the Gd3+ complexes ranged from 1.4-2.4 μs and 3-14 ns, respectively. This indicates that 1) the inner-sphere water lifetimes for the complexes with a single positive charge reported here are considerably shorter for both coordination isomers than the corresponding values for the [Gd(dtma)]3+ complex with three positive charges, and 2) the difference in water lifetimes for M and m isomers in these two series is magnified in the [Gd{dota-bis(amide)}] complexes. This feature highlights the remarkable role of both charge and molecular geometry in determining the exchange rate of the coordinated water.

Original languageEnglish (US)
Pages (from-to)288-296
Number of pages9
JournalChemistry - A European Journal
Volume7
Issue number1
DOIs
StatePublished - Jan 5 2001

Keywords

  • Lanthanides
  • Macrocyclic ligands
  • NMR spectroscopy
  • Water exchange

ASJC Scopus subject areas

  • Catalysis
  • Organic Chemistry

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