Structural determination of Zn and Cd–DTPA complexes: MS, infrared, 13C NMR and theoretical investigation

The joint application of MS, infrared and 13C NMR techniques for the determination of metal–DTPA structures (metal = Zn and Cd; DTPA = diethylenetriaminepentacetic acid) is reported. Mass spectrometry allowed determining the 1:1 stoichiometry of the complexes, while infrared analysis suggested that both nitrogen and carboxyl groups are sites for complexation. The 13C NMR spectrum for the cadmium-containing complex evidenced the existence of free and complexed carboxyl groups, due to a straight singlet at 179.0 ppm (free carboxylic 13C) and to two broad singlets or a broad doublet at 178.3 ppm (complexed carboxylic 13C, 2JCd–C(=O) = 45.2 Hz). A similar interpretation might be given for the zinc derivative and, with the aid of DFT calculations, structures for both complexes were then proposed.