NMR and INDO study of the Ni(II) complexes of some bidentate ligands

Abstract

The joint use of NMR spectroscopy and quantum mechanical calculations proved a powerful tool for investigating the conformational and electronic properties of various ligands complexes to paramagnetic metal ions. It is well known that the spin–lattice relaxation rates cannot often be rationalized in terms of the Solomon-Bloembergen equation, in its original form, because of the important contribution arising from the unpaired spin density onto the ligand molecule [1]. Thus a modified S.B. equation, where both the metal-centered and ligand-centered dipolar contributions are taken into account, must be employed. In this work the Ni(II) complexes of dopamine [2], a molecule of biological relevance, phthalic acid [3], maleic and succinic acid [4] were examined. The common feature of these differently flexible ligands is a potential bidentate binding site. ▪ From the analysis of the proton and carbon paramagnetic shifts, induced on the nuclei of the different ligands bound to Ni 2+, a delocalization mechanism was hypothesized with the aid of an INDO M.O. analysis, performed on suitable radical models of the ligand. The radical model giving hyperfine coupling constants in the best agreement with the experimental shifts was then used to obtain a complexation model. The reliability of the model chosen was tested introducing the metal–nucleus distances in the reduced and modified S.B. equation and veryfying whether the calculated total unpaired spin densities and correlation times satisfy the experimental nuclear relaxations. In the case of ligands having a partially rigid skeleton (dopamine, phthalic and maleic acids) the approach led to a unique complexation model, consistent with the experimental data, while for the more flexible succinic acid the result were not so encouraging.