Synthesis, spectral characterization and electrochemical studies of mixed-ligand complexes of platinum(II) with 2-(arylazo)pyridines and catechols. Single-crystal X-ray structure of dichloro{2-(phenylazo) pyridine}platinum(II)

Abstract

The reaction of 2-(arylazo)pyridines, RH 4C 6NNC 5H 4N (aap, 1 where R=H (pap), p-Me ( p-tap), p-Cl ( p-Clpap)) with K 2PtCl 4 in 1:2 boiling acetonitrile–water affords orange–red complexes of the type Pt(aap)Cl 2 ( 2). The IR spectrum shows two ν(PtCl) stretches suggesting cis-PtCl 2 configuration. The structural confirmation was carried out by an X-ray diffraction study of Pt(pap)Cl 2 ( 2a). The addition of catechols to a chloroform–methanol solution of Pt(aap)Cl 2 in the presence of Et 3N yielded green colored mixed-ligand complexes of the formula Pt(aap)( O, O) ( O, O=catecholate (cat) ( 3), 4- tert-butylcatecholate (tbcat), ( 4), 3,5-di- tert-butylcatecholate (dtbcat) ( 5) and tetrachlorocatecholate (tccat) ( 6)). The complexes were characterized by elemental analyses, IR, UV–Vis–NIR and 1H NMR spectral data. Electronic spectra exhibit a ligand–ligand charge transfer (LLCT) transition at NIR region; the position and symmetry of the band depend on the type of substituent on the catechol frame. This is qualitatively assigned as 3b 1(cat)→π*(aap) transition. Cyclic voltammogram of the complexes show four successive redox responses: two couples positive to SCE are referred to as oxidation of catechol to semiquinone and semiquinone to quinone, respectively and two couples negative to SCE correspond to azo reductions.