Thermodynamic and NMR Studies on the Complexation of Lanthanoid(III) Ions with <i>o</i>-Phenylenediamine-<i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetraacetate in Aqueous Solutions

Abstract

Abstract The complex formation of o-phenylenediamine-N,N,N′,N′-tetraacetate (PhDTA, H4L) with trivalent lanthanoid ions (M3+) in aqueous solutions has been studied at a temperature of 25.0 ± 0.2 °C and an ionic strength of 1.0 M in NaClO4 by potentiometry, calorimetry, and 1H and 139La NMR. The formation constant, KML, of 1 : 1 PhDTA complexes increases monotonically with the atomic number of the central metal ions (log KML = 11.39 for La, and 16.26 for Lu), showing the tetrad effect and a linear free energy relationship with the formation constant of CyDTA complexes. The change in entropy increases gradually with increasing atomic number, as is the case for EDTA. The large change in entropy ( ≈ 257–329 J K−1 mol−1) overweighs the endothermicity ( = 5.4–14.6 kJ mol−1) of the complexation. The NMR study indicated that the La–PhDTA complex has four acetate groups with an identical magnetic environment and a lanthanum nucleus with higher electric symmetry in comparison with the corresponding EDTA and CyDTA complexes.