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

The joint application of MS, infrared and C-13 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 C-13 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 C-13) and to two broad singlets or a broad doublet at 178.3 ppm (complexed carboxylic C-13, (2)J(Cd-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. (c) 2007 Elsevier B.V. All rights reserved.