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 K_i^H) 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 (K_LnL) 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 K_LnL 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.