NMR, relaxometric, and structural studies of the hydration and exchange dynamics of cationic lanthanide complexes of macrocyclic tetraamide ligands

Silvio Aime, Alessandro Barge, James I. Bruce, Mauro Botta, Judith A K Howard, Janet M. Moloney, David Parker, Alvaro S. De Sousa, Mark Woods

Research output: Contribution to journalArticle

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Abstract

The solution structure and dynamics of metal-bound water exchange have been investigated in a series of lanthanide complexes of primary, secondary, and tertiary tetraamide derivatives of 1,4,7,10-tetraazacyclododecane. In the gadolinium complexes at ambient pH, water exchange lifetimes (τ(m)) determined by 17O NMR were sufficiently long (19 μs for [Gd · 2]3+, 298 K, 17 μs for [Gd · 3]3+, and 8 μs for [Gd · 4]3+) to limit the measured relaxivity. Direct 1H NMR observation of the bound water resonance is possible for the corresponding Eu complexes at low temperature in CD3CN, and the rate of water proton exchange is about 50 times faster in the twisted square antiprismatic isomer (m) than in the isomeric square antiprismatic (M) complex. The ratio of these two isomers in solution is sensitive to the steric demand of the amide substituent, with m/M = 2 for [Eu · 4]3+, but 0.25 for [Eu · 2]3+. The slowness of coordinated water exchange has allowed the rate of prototropic exchange to be studied: in basic media deprotonation of the bound water molecule or of proximate ligand amide NH protons leads to relaxivity enhancements, whereas in acidic media, hydration around the strongly ion-paired complexes is perturbed, facilitating water exchange. The X-ray crystal structure of ligand 3 reveals a hydrogen-bonded structure with two pairs of ring N-substituents related in a trans arrangement, contrasting with the structure of diprotonated DOTA in which the ligand is predisposed to bind metal ions. In the dysprosium complex [Dy · 3 · OH2](PF6)3, the metal ion adopts a regular monocapped square antiprismatic coordination geometry, with a water Dy-O bond length of 2.427(3) Å, and a PF6 counterion is strongly hydrogen-bonded to this bound water molecule.

Original languageEnglish (US)
Pages (from-to)5762-5771
Number of pages10
JournalJournal of the American Chemical Society
Volume121
Issue number24
DOIs
StatePublished - Jun 23 1999
Externally publishedYes

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Lanthanoid Series Elements
Rare earth elements
Hydration
Ion exchange
Ligands
Nuclear magnetic resonance
Water
Metals
Ions
Amides
Isomers
Metal ions
Protons
Hydrogen
Dysprosium
Deprotonation
Molecules
Gadolinium
Bond length
Crystal structure

ASJC Scopus subject areas

  • Chemistry(all)

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NMR, relaxometric, and structural studies of the hydration and exchange dynamics of cationic lanthanide complexes of macrocyclic tetraamide ligands. / Aime, Silvio; Barge, Alessandro; Bruce, James I.; Botta, Mauro; Howard, Judith A K; Moloney, Janet M.; Parker, David; De Sousa, Alvaro S.; Woods, Mark.

In: Journal of the American Chemical Society, Vol. 121, No. 24, 23.06.1999, p. 5762-5771.

Research output: Contribution to journalArticle

Aime, Silvio ; Barge, Alessandro ; Bruce, James I. ; Botta, Mauro ; Howard, Judith A K ; Moloney, Janet M. ; Parker, David ; De Sousa, Alvaro S. ; Woods, Mark. / NMR, relaxometric, and structural studies of the hydration and exchange dynamics of cationic lanthanide complexes of macrocyclic tetraamide ligands. In: Journal of the American Chemical Society. 1999 ; Vol. 121, No. 24. pp. 5762-5771.
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abstract = "The solution structure and dynamics of metal-bound water exchange have been investigated in a series of lanthanide complexes of primary, secondary, and tertiary tetraamide derivatives of 1,4,7,10-tetraazacyclododecane. In the gadolinium complexes at ambient pH, water exchange lifetimes (τ(m)) determined by 17O NMR were sufficiently long (19 μs for [Gd · 2]3+, 298 K, 17 μs for [Gd · 3]3+, and 8 μs for [Gd · 4]3+) to limit the measured relaxivity. Direct 1H NMR observation of the bound water resonance is possible for the corresponding Eu complexes at low temperature in CD3CN, and the rate of water proton exchange is about 50 times faster in the twisted square antiprismatic isomer (m) than in the isomeric square antiprismatic (M) complex. The ratio of these two isomers in solution is sensitive to the steric demand of the amide substituent, with m/M = 2 for [Eu · 4]3+, but 0.25 for [Eu · 2]3+. The slowness of coordinated water exchange has allowed the rate of prototropic exchange to be studied: in basic media deprotonation of the bound water molecule or of proximate ligand amide NH protons leads to relaxivity enhancements, whereas in acidic media, hydration around the strongly ion-paired complexes is perturbed, facilitating water exchange. The X-ray crystal structure of ligand 3 reveals a hydrogen-bonded structure with two pairs of ring N-substituents related in a trans arrangement, contrasting with the structure of diprotonated DOTA in which the ligand is predisposed to bind metal ions. In the dysprosium complex [Dy · 3 · OH2](PF6)3, the metal ion adopts a regular monocapped square antiprismatic coordination geometry, with a water Dy-O bond length of 2.427(3) {\AA}, and a PF6 counterion is strongly hydrogen-bonded to this bound water molecule.",
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T1 - NMR, relaxometric, and structural studies of the hydration and exchange dynamics of cationic lanthanide complexes of macrocyclic tetraamide ligands

AU - Aime, Silvio

AU - Barge, Alessandro

AU - Bruce, James I.

AU - Botta, Mauro

AU - Howard, Judith A K

AU - Moloney, Janet M.

AU - Parker, David

AU - De Sousa, Alvaro S.

AU - Woods, Mark

PY - 1999/6/23

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N2 - The solution structure and dynamics of metal-bound water exchange have been investigated in a series of lanthanide complexes of primary, secondary, and tertiary tetraamide derivatives of 1,4,7,10-tetraazacyclododecane. In the gadolinium complexes at ambient pH, water exchange lifetimes (τ(m)) determined by 17O NMR were sufficiently long (19 μs for [Gd · 2]3+, 298 K, 17 μs for [Gd · 3]3+, and 8 μs for [Gd · 4]3+) to limit the measured relaxivity. Direct 1H NMR observation of the bound water resonance is possible for the corresponding Eu complexes at low temperature in CD3CN, and the rate of water proton exchange is about 50 times faster in the twisted square antiprismatic isomer (m) than in the isomeric square antiprismatic (M) complex. The ratio of these two isomers in solution is sensitive to the steric demand of the amide substituent, with m/M = 2 for [Eu · 4]3+, but 0.25 for [Eu · 2]3+. The slowness of coordinated water exchange has allowed the rate of prototropic exchange to be studied: in basic media deprotonation of the bound water molecule or of proximate ligand amide NH protons leads to relaxivity enhancements, whereas in acidic media, hydration around the strongly ion-paired complexes is perturbed, facilitating water exchange. The X-ray crystal structure of ligand 3 reveals a hydrogen-bonded structure with two pairs of ring N-substituents related in a trans arrangement, contrasting with the structure of diprotonated DOTA in which the ligand is predisposed to bind metal ions. In the dysprosium complex [Dy · 3 · OH2](PF6)3, the metal ion adopts a regular monocapped square antiprismatic coordination geometry, with a water Dy-O bond length of 2.427(3) Å, and a PF6 counterion is strongly hydrogen-bonded to this bound water molecule.

AB - The solution structure and dynamics of metal-bound water exchange have been investigated in a series of lanthanide complexes of primary, secondary, and tertiary tetraamide derivatives of 1,4,7,10-tetraazacyclododecane. In the gadolinium complexes at ambient pH, water exchange lifetimes (τ(m)) determined by 17O NMR were sufficiently long (19 μs for [Gd · 2]3+, 298 K, 17 μs for [Gd · 3]3+, and 8 μs for [Gd · 4]3+) to limit the measured relaxivity. Direct 1H NMR observation of the bound water resonance is possible for the corresponding Eu complexes at low temperature in CD3CN, and the rate of water proton exchange is about 50 times faster in the twisted square antiprismatic isomer (m) than in the isomeric square antiprismatic (M) complex. The ratio of these two isomers in solution is sensitive to the steric demand of the amide substituent, with m/M = 2 for [Eu · 4]3+, but 0.25 for [Eu · 2]3+. The slowness of coordinated water exchange has allowed the rate of prototropic exchange to be studied: in basic media deprotonation of the bound water molecule or of proximate ligand amide NH protons leads to relaxivity enhancements, whereas in acidic media, hydration around the strongly ion-paired complexes is perturbed, facilitating water exchange. The X-ray crystal structure of ligand 3 reveals a hydrogen-bonded structure with two pairs of ring N-substituents related in a trans arrangement, contrasting with the structure of diprotonated DOTA in which the ligand is predisposed to bind metal ions. In the dysprosium complex [Dy · 3 · OH2](PF6)3, the metal ion adopts a regular monocapped square antiprismatic coordination geometry, with a water Dy-O bond length of 2.427(3) Å, and a PF6 counterion is strongly hydrogen-bonded to this bound water molecule.

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