Nuclear magnetic resonance and potentiometric studies of the protonation scheme of a triaza triacetic macrocycle and its complexes with lanthanum and lutetium

C. F G C Geraldes, M. C. Alpoim, M. P M Marques, A. D. Sherry, M. Singh

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Abstract

The protonation constants of the macrocyclic ligand 1,4,7-triazacyclononane-N,N′,N″-triacetic acid (NOTA) have been measured by potentiometry, and the protonation sequence of the various amino and carboxylate groups of NOTA has been studied in D2O as a function of pD from the chemical shifts of the nonlabile protons. Shielding constants for protonation of the amino groups were determined in a NMR study of the triaza macrocyclic amine, its trimethylated analogue, and NOTA and compared with values reported for linear polyamino polycarboxylates and cyclic tetraaza tetracarboxylate ligands. The results indicate that two nitrogens of NOTA are protonated at higher pH than the carboxylate groups. The last nitrogen is protonated only at very low pH. The sequence of protonation of NOTA supports the formation of hydrogen bonds between two protonated nitrogens and the adjacent two nonprotonated carboxylates. The 1H and 13C spectra of the La(NOTA) and Lu(NOTA) species were studied as a function of pH and temperature. The aqueous complexes show spectra characteristic of flexible triaza macrocycles, displaying fast interconversion between the two staggered σ and λ conformations of the ethylenediamine rings even at room temperature. Above pH 9.5, the La(NOTA) and Lu(NOTA) hydroxo complexes start to form and their spectra show evidence of greater rigidity as at room temperature the σ/λ ethylenediamine ring conformational interconversions in the Lu(NOTA) hydroxo complex are slow on the NMR time scale.

Original languageEnglish (US)
Pages (from-to)3876-3881
Number of pages6
JournalInorganic Chemistry
Volume24
Issue number23
StatePublished - 1985

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Lutetium
Lanthanum
lutetium
Protonation
lanthanum
Nuclear magnetic resonance
carboxylates
nuclear magnetic resonance
ethylenediamine
nitrogen
potentiometric analysis
ligands
rings
room temperature
rigidity
Nitrogen
shielding
chemical equilibrium
amines
analogs

ASJC Scopus subject areas

  • Inorganic Chemistry

Cite this

Nuclear magnetic resonance and potentiometric studies of the protonation scheme of a triaza triacetic macrocycle and its complexes with lanthanum and lutetium. / Geraldes, C. F G C; Alpoim, M. C.; Marques, M. P M; Sherry, A. D.; Singh, M.

In: Inorganic Chemistry, Vol. 24, No. 23, 1985, p. 3876-3881.

Research output: Contribution to journalArticle

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abstract = "The protonation constants of the macrocyclic ligand 1,4,7-triazacyclononane-N,N′,N″-triacetic acid (NOTA) have been measured by potentiometry, and the protonation sequence of the various amino and carboxylate groups of NOTA has been studied in D2O as a function of pD from the chemical shifts of the nonlabile protons. Shielding constants for protonation of the amino groups were determined in a NMR study of the triaza macrocyclic amine, its trimethylated analogue, and NOTA and compared with values reported for linear polyamino polycarboxylates and cyclic tetraaza tetracarboxylate ligands. The results indicate that two nitrogens of NOTA are protonated at higher pH than the carboxylate groups. The last nitrogen is protonated only at very low pH. The sequence of protonation of NOTA supports the formation of hydrogen bonds between two protonated nitrogens and the adjacent two nonprotonated carboxylates. The 1H and 13C spectra of the La(NOTA) and Lu(NOTA) species were studied as a function of pH and temperature. The aqueous complexes show spectra characteristic of flexible triaza macrocycles, displaying fast interconversion between the two staggered σ and λ conformations of the ethylenediamine rings even at room temperature. Above pH 9.5, the La(NOTA) and Lu(NOTA) hydroxo complexes start to form and their spectra show evidence of greater rigidity as at room temperature the σ/λ ethylenediamine ring conformational interconversions in the Lu(NOTA) hydroxo complex are slow on the NMR time scale.",
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T1 - Nuclear magnetic resonance and potentiometric studies of the protonation scheme of a triaza triacetic macrocycle and its complexes with lanthanum and lutetium

AU - Geraldes, C. F G C

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AU - Singh, M.

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N2 - The protonation constants of the macrocyclic ligand 1,4,7-triazacyclononane-N,N′,N″-triacetic acid (NOTA) have been measured by potentiometry, and the protonation sequence of the various amino and carboxylate groups of NOTA has been studied in D2O as a function of pD from the chemical shifts of the nonlabile protons. Shielding constants for protonation of the amino groups were determined in a NMR study of the triaza macrocyclic amine, its trimethylated analogue, and NOTA and compared with values reported for linear polyamino polycarboxylates and cyclic tetraaza tetracarboxylate ligands. The results indicate that two nitrogens of NOTA are protonated at higher pH than the carboxylate groups. The last nitrogen is protonated only at very low pH. The sequence of protonation of NOTA supports the formation of hydrogen bonds between two protonated nitrogens and the adjacent two nonprotonated carboxylates. The 1H and 13C spectra of the La(NOTA) and Lu(NOTA) species were studied as a function of pH and temperature. The aqueous complexes show spectra characteristic of flexible triaza macrocycles, displaying fast interconversion between the two staggered σ and λ conformations of the ethylenediamine rings even at room temperature. Above pH 9.5, the La(NOTA) and Lu(NOTA) hydroxo complexes start to form and their spectra show evidence of greater rigidity as at room temperature the σ/λ ethylenediamine ring conformational interconversions in the Lu(NOTA) hydroxo complex are slow on the NMR time scale.

AB - The protonation constants of the macrocyclic ligand 1,4,7-triazacyclononane-N,N′,N″-triacetic acid (NOTA) have been measured by potentiometry, and the protonation sequence of the various amino and carboxylate groups of NOTA has been studied in D2O as a function of pD from the chemical shifts of the nonlabile protons. Shielding constants for protonation of the amino groups were determined in a NMR study of the triaza macrocyclic amine, its trimethylated analogue, and NOTA and compared with values reported for linear polyamino polycarboxylates and cyclic tetraaza tetracarboxylate ligands. The results indicate that two nitrogens of NOTA are protonated at higher pH than the carboxylate groups. The last nitrogen is protonated only at very low pH. The sequence of protonation of NOTA supports the formation of hydrogen bonds between two protonated nitrogens and the adjacent two nonprotonated carboxylates. The 1H and 13C spectra of the La(NOTA) and Lu(NOTA) species were studied as a function of pH and temperature. The aqueous complexes show spectra characteristic of flexible triaza macrocycles, displaying fast interconversion between the two staggered σ and λ conformations of the ethylenediamine rings even at room temperature. Above pH 9.5, the La(NOTA) and Lu(NOTA) hydroxo complexes start to form and their spectra show evidence of greater rigidity as at room temperature the σ/λ ethylenediamine ring conformational interconversions in the Lu(NOTA) hydroxo complex are slow on the NMR time scale.

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