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{DOTA‐bis(amide)}lanthanide Complexes: NMR Evidence for Differences in Water‐Molecule Exchange Rates for Coordination Isomers
Author(s) -
Zhang Shanrong,
Kovacs Zoltan,
Burgess Shawn,
Aime Silvio,
Terreno Enzo,
Sherry A. Dean
Publication year - 2001
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20010105)7:1<288::aid-chem288>3.0.co;2-6
Subject(s) - chemistry , lanthanide , coordination sphere , nuclear magnetic resonance spectroscopy , amide , molecule , isomerization , cis–trans isomerism , crystallography , coordination complex , aqueous solution , spectroscopy , structural isomer , stereochemistry , metal , ion , organic chemistry , physics , quantum mechanics , catalysis
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 1 H chemical shifts similar to those reported previously for the major ( M ) and minor ( m ) forms of the tetraacetate ([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 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 ⇔ M isomerization as determined by two‐dimensional 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 ( τ $\rm{_{M}^{298}}$ ) in the M and m isomers of the Gd 3+ 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.