Synthesis, equilibrium and NMR studies of lanthanide(III) complexes of theN-mono(methylamide) and N '-mono(methylamide) derivatives of diethylenetriamine-N,N,N ',N '',N ''-pentaacetic acid
L. Sarka et al., Synthesis, equilibrium and NMR studies of lanthanide(III) complexes of theN-mono(methylamide) and N '-mono(methylamide) derivatives of diethylenetriamine-N,N,N ',N '',N ''-pentaacetic acid, J CHEM S DA, (20), 2000, pp. 3699-3703
Citations number
47
Categorie Soggetti
Inorganic & Nuclear Chemistry
Journal title
JOURNAL OF THE CHEMICAL SOCIETY-DALTON TRANSACTIONS
Two derivative ligands of diethylenetriamine-N,N,N',N ",N "-pentaacetic aci
d (H(5)DTPA; H5L1) were synthesized: the symmetric and asymmetric mono(meth
ylamides) DTPA-N'-MA (H4L3) and DTPA-N-MA (H4L4). The protonation constants
(log K-i(H)) of L-3 and L-4 were obtained by pH-potentiometric titration:
10.04, 8.41, 2.73, 1.94 and 10.18, 6.19, 3.55, 2.0, respectively. The proto
nation constants and the sites of protonation were interpreted on the basis
of the pH dependence of the chemical shifts of the non-labile protons. The
stability constants (K-LnL) of the complexes of these ligands with lanthan
ide(III) ions were determined by direct pH-potentiometry and competition ti
tration. The stability constants decrease in the sequence LnL(1) > LnL(3) >
LnL(4). The log K-LnL values of the complexes LnL(3) and LnL(4) increase w
ith increasing atomic number of Ln(3+) from La to Gd, then remain roughly u
nchanged up to Er and subsequently decrease slightly to Lu. The H-1 and C-1
3 NMR spectra reveal the presence of 2 isomers for the complexes LnL(3) and
4 isomers for LnL(4) at around 0 degrees C. The AB multiplets of the aceta
te methylene protons demonstrate the long lifetime of the Ln(3+)-N bonds. T
he coupling observed between the methyl and methylene protons of the methyl
amide group in the 2-D COSY spectra indicates partial double bond character
of the C-NMe bond.