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

Abstract

The reaction of 1-ethyl-2-(arylazo)imidazoles (RaaiEt, 1) [R=H ( 1a), p-Me ( 1b), p-Cl ( 1c)] with K 2PtCl 4 in boiling acetonitrile–water (1:1) produces red–brown Pt(RaaiEt)Cl 2 ( 2) complexes. Addition of dioxolene in the presence of Et 3N to chloroform–methanol solution of Pt(RaaiEt)Cl 2 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)]·1/2H 2O ( 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 E 1/2 of the first oxidation and reduction process changes linearly with the LLCT transition.