Synthesis, equilibrium and NMR studies of lanthanide(III) complexes of the N-mono(methylamide) and N ′-mono(methylamide) derivatives of diethylenetriamine-N,N,N ′,N ″,N ″-pentaacetic acid†

Abstract

Two derivative ligands of diethylenetriamine-N,N,N′,N′′,N′′-pentaacetic acid (H5DTPA; H5L1) were synthesized: the symmetric and asymmetric mono(methylamides) DTPA-N′-MA (H4L3) and DTPA-N-MA (H4L4). The protonation constants (log KiH) of L3 and L4 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 protonation 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 (KLnL) of the complexes of these ligands with lanthanide(III) ions were determined by direct pH-potentiometry and competition titration. The stability constants decrease in the sequence LnL1 > LnL3 > LnL4. The log KLnL values of the complexes LnL3 and LnL4 increase with increasing atomic number of Ln3+ from La to Gd, then remain roughly unchanged up to Er and subsequently decrease slightly to Lu. The 1H and 13C NMR spectra reveal the presence of 2 isomers for the complexes LnL3 and 4 isomers for LnL4 at around 0 °C. The AB multiplets of the acetate methylene protons demonstrate the long lifetime of the Ln3+–N bonds. The coupling observed between the methyl and methylene protons of the methylamide group in the 2-D COSY spectra indicates partial double bond character of the C–NMe bond.