Palladium(II) complexes bearing the terpyridine-type tridentate ligand with benzo[b]-1,5-naphthyridin-2-yl groups

New types of tridentate ligands, 2-(benzo[b]-1,5-naphthyridin-2-yl)-6-(quinolin-2-yl)-4-tert-butylpyridine (bnqp) and 2,6-bis(benzo[b]-1,5-naphthyridin-2-yl)-4-tert-butylpyridine (bbnp) that are able to accommodate and release two and four electrons, respectively, were synthesized. The palladium(II) complexes having the ligand, [PdCl(bnqp)](PF6) (1) and [PdCl(bbnp)](PF6) (2), were also prepared. The molecular structure of 2 was determined by a X-ray diffraction study, where the Pd–Cl coordination bond deviates from a square planner geometry with a N(2)–Pd(1)–Cl(1) angle of 166.6(1)° because of a steric hindrance of the hydrogen atom at the 10-position of benzo[b]-1,5-naphthyridin-2-yl groups. UV–Vis absorption spectra of 1 and 2 in DMSO did not show any interactions with HClO4, whereas the same acid significantly influenced the patterns of the ligand localized redox reaction in the cyclic voltammograms of those complexes. On the other hand, chemical reduction of 1 and 2 using Na2S2O3 or Na2S2O4 in CH3CN/H2O resulted in deposition of metallic palladium(0) with liberating the ligand probably due to the intramolecular electron transfer from the reduced ligand to the palladium(II) center.

Graphical abstractTwo palladium(II) complexes with new types of tridentate ligands containing one and two benzo[b]-1,5-naphthyridin-2-yl groups were synthesized and characterized. In these complexes, ligand localized reductions took place prior to the metal centered one, and protons participated in those reductions under aqueous conditions. Electron transfer from the reduced forms of the ligand to the metal center caused deposition of metallic palladium.Figure optionsDownload full-size imageDownload as PowerPoint slide