Nuclear magnetic resonance investigation of intramolecular processes in metal polyaminepolycarboxylates Ethylenebis(oxyethyleneamino)tetraacetic acid complexes

Abstract

Abstract The kinetics of the acetate scrambling processes occurring in some diamagnetic complexes formed by ethylenebis(oxyethyleneamino)tetraacetic acid (EGTA) has been investigated in D2O solutions by performing computer line shape analysis of the NMR spectra recorded at different temperatures in the working range 0–100°C. The NMR patterns obtained suggest ether oxygen coordination in the scandium(III) and lutetium(III) complexes, but not in those formed with the magnesium(II), calcium(II), strontium(II), barium(II), yttrium(III), lanthanum(III), zinc(II) and lead(II) ions, and allow one to calculate the activation parameters for the LuEGTA and LaEGTA systems. But only qualitative conclusions can be drawn for the other chelates. Twist mechanisms and water assisted displacements are discussed in reaching a suitable reaction pathway for the intramolecular rearrangements in both groups of complexes. The possibility of acetate scrambling occurring either by only breaking MO bonds (Δ⇌Λ conversion) or also by rupture of MN ones (N inversion) seems valid since nitrogen atoms and acetate arms are the only coordinating groups. An intramolecular change not involving MN bond breaking can be ruled out when the ether oxygen atoms behave as donor units as well.