Interactions of 1,5-naphthyridine with Pd(en)Cl2 or [Pd(en)(H2O)2](NO3)2 in aqueous solution

Abstract

The nature of the complexes formed in aqueous solution between either Pd(en)Cl₂ or [Pd(en)(H₂O)₂](NO₃)₂ and 1,5-naphthyridine (1,5-NAP), where en is ethylenediamine, have been investigated by 1-D and 2-D ¹H NMR spectroscopy and potentiometric titration. Above pH 5.0, two major complexes have been identified with the stoichiometries of 2:1 and 1:1 (M:L ratio) as well as small amounts of a 1:2 complex and/or oligomer. The 2:1 complex consisted of a Pd(en)²⁺ moiety symmetrically bonded to each of the nitrogen atoms of the 1,5-NAP, as indicated by the presence of just three ¹H NMR resonances in the aromatic region. The 1:1 complex had six resonances as a result of only one 1,5-NAP nitrogen atom being bonded to a Pd(en)²⁺ group. At pH <5, the uncomplexed nitrogen of the 1:1 and other singly bonded 1,5-NAP species became protonated and resulted in the formation of a large number of complexes. Job's method plots at pH 6 showed that the 1:1 complex is stable over a large concentration range. Above pH ∼6 the 1:1 complex can dimerize via deprotonation of a water ligand on the Pd(en)²⁺ to form an hydoxo-bridged or oxo-bridged species. Evidence for this was observed in the upfield shifts of the resonances as the pH increased. The species distribution curve from potentiometric titrations and the NMR data were in good agreement at concentrations of 1–4 mM. NOESY data indicated that free 1,5-NAP ligand was exchanging with that in the 1:1 complex. In order to interpret the en region of the ¹H NMR spectra, the spectra of [Pd(en)(H₂O)₂](NO₃)₂ in D₂O at various pD were obtained.