Multinuclear NMR and DFT studies of the structure and fluxionality for MIII-ethylenediamine-tetraacetate complexes (M(EDTA)−, M = Al, Ga and In) in solution

Multinuclear NMR studies of M(EDTA)− complexes (M = Al, Ga and In) by 1H, 13C and 27Al NMR in solution while varying the temperature, pH or using different solvents have been done. The features of the high-field NMR spectra show that the complexes have similar structure in solution and solid state, i.e., the Al(III) and Ga(III) are octahedrally coordinated by the hexadentate EDTA ligand, whilst In(III) is seven-coordinated in a trigonal prismatic fashion by a hexadentate EDTA and one water molecule. M(EDTA)(OH)2− complexes (M = Ga, In) have also been detected, pK values in D2O are 6.08 ± 0.06 and 9.17 ± 0.07, respectively. In the case of the octahedral Al(III) and Ga(III) complexes, the fluxional rearrangement of the chelate rings can be followed by 1H NMR, but In(H2O)(EDTA)− does not show fluxionality for structural reasons. The exchange between the axial and equatorial acetate arms of Al(EDTA)−, located at different N-atoms of the ligand, could be a proton-catalyzed reaction assisted with water molecule(s). Density functional theory (DFT) calculations have been carried out to reveal the effect of protonation and the role of solvent.