title: 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 (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<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 approximately 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.
authors.affiliation: Department of Chemistry, The University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249-0698, United States of America.
authors.forename: Shourong
authors.initials: S
authors.lastname: Zhu
authors.forename: Karlos X
authors.initials: KX
authors.lastname: Moreno
authors.forename: Roxanne M
authors.initials: RM
authors.lastname: Jenkins
authors.forename: Judith A
authors.initials: JA
authors.lastname: Walmsley
mesh_terms: D018360:Crystallography, X-Ray; D006863:Hydrogen-Ion Concentration; D007202:Indicators and Reagents; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D009287:Naphthyridines / Q000737:chemistry*; D010165:Palladium / Q000737:chemistry*; D010984:Platinum / Q000737:chemistry; D011199:Potentiometry; D012996:Solutions; D021241:Spectrometry, Mass, Electrospray Ionization
chemical_list: D007202:Indicators and Reagents; D008024:Ligands; D009287:Naphthyridines; D012996:Solutions; C090639:palladium nitrate; D010984:Platinum; D010165:Palladium; C008756:palladium chloride