{DOTA-bis(amide)}lanthanide Complexes: NMR Evidence for Differences in Water-Molecule Exchange Rates for Coordination Isomers
✍ Scribed by Shanrong Zhang; Zoltan Kovacs; Shawn Burgess; Silvio Aime; Enzo Terreno; A. Dean Sherry
- Book ID
- 101368946
- Publisher
- John Wiley and Sons
- Year
- 2001
- Tongue
- English
- Weight
- 246 KB
- Volume
- 7
- Category
- Article
- ISSN
- 0947-6539
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✦ Synopsis
Two derivatives of 1,4,7,10tetraazacyclododecane 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 1 H chemical shifts similar to those reported previously for the major (M) and minor (m) forms of the tetraacetate ([Ln(dota)] À ) and tetraamide ([Ln(dtma)] 3 ) complexes. As in the [Ln(dota)] À and [Ln(dtma)] 3 complexes, the m/M ratio proved to be a sensitive function of lanthanide size and temperature. An analysis of 1 H hyperfine shifts in spectra of the Yb 3 complexes revealed significant differences between the axial (D 1 ) and non-axial (D 2 ) components of the magnetic susceptibility tensor anisotropy in the m and M coordination isomers and the energetics of ring inversion and m D M isomerization as determined by twodimensional exchange spectroscopy (EXSY). 17 O shift data for the Dy 3 complexes showed that both have one inner-sphere water molecule. A temperature-dependent 17 O NMR study of bulk water linewidths for solutions of the Gd 3 complexes provided direct evidence for differences in water exchange rates for the two coordination isomers.
The bound-water lifetimes (t 298
M in the M and m isomers of the Gd 3 complexes ranged from 1.4 ± 2.4 ms 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.