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)

The reaction of 2-(arylazo)pyridines, RH4C6N=NC5H4N (aap, 1 where R=H (pap), p-Me (p-tap), p-Cl (p-Clpap)) with K2PtCl4 in 1:2 boiling acetonitrile-water affords orange-red complexes of the type Pt(aap)Cl-2 (2). The IR spectrum shows two v(Pt-Cl) stretches suggesting cis-PtCl2 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 Pl(aap)Cl-2 in the presence of Et3N 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, W-Vis-NIR and H-1 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)-->pi*(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. (C) 2001 Elsevier Science B.V. All rights reserved.