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

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 (1)H 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