Arylazoimidazoleplatinum(II) complexes and their dioxolene derivatives: single crystal X-ray structure of (catecholato){1-ethyl-2-(p-tolylazo)-imidazole}platinum(II)

The reaction of 1-ethyl-2-(arylazo)imidazoles (RaaiEt, 1) [R=H (1a), p-Me (1b), p-Cl (1c)] with K2PtCl4 in boiling acetonitrile–water (1:1) produces red–brown Pt(RaaiEt)Cl2 (2) complexes. Addition of dioxolene in the presence of Et3N to chloroform–methanol solution of Pt(RaaiEt)Cl2 has yielded green-coloured mixed-ligand complexes [Pt(RaaiEt)(O,O)] [O,O=catecholate (cat) (3), 4-tert-butylcatecholate (tbcat) (4), 3,5-di-tert-butylcatecholate (dtbcat) (5), tetrachlorocatecholate (tccat) (6)]. The structure of [Pt(p-MeaaiEt)(cat)]·1/2H2O (3b) was confirmed by X-ray diffraction. Electronic spectra exhibit ligand-to-ligand charge transfer transition (LLCT) in the VIS–NIR region; the position and symmetry of the band depend on the substituent type on the dioxolene and arylazoimidazole frames. This is qualitatively assigned as a HOMO(cat)→LUMO(RaaiEt) transition. Cyclic voltammograms of the complexes show four successive redox responses: two couples at positive to saturated calomel electrode (SCE) correspond to catechol to semiquinone and semiquinone to quinone oxidations, and two couples at negative to SCE correspond to the azo reductions. The difference in the potential E1/2 of the first oxidation and reduction process changes linearly with the LLCT transition.