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

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.