Synthesis, spectral characterisation and electrochemical studies of dichloro-{1-benzyl-2-(arylazo)imidazole}platinum(II) complexes and their dioxolene derivatives

Abstract

1-Benzyl-2-(arylazo)imidazoles, p-RC₆H₄N=NC₃H₂-N₂1-CH₂Ph [RaaiBz (2); R=H(a), Me(b), Cl(c)], react with K₂PtCl₄ in boiling MeCN–H₂O (1:1 v/v) to give brownish-red Pt(RaaiBz)Cl₂ (3) complexes. Addition of dioxolene in the presence of Et₃N to a CHCl₃–MeOH solution of Pt(RaaiBz)Cl₂ yields green mixed complexes of composition [Pt(RaaiBz)(O,O)] [O,O = catecholate (cat) (4); 4-tert-butylcatecholate (tbcat), (5); 3,5-di-tert-butylcatecholate (dtbcat), (6); tetracholorocatecholate (tccat), (7)] which were characterised by elemental analyses, i.r., u.v.–vis.–near i.r. and ¹H-n.m.r. spectral data. The solution electronic spectra exhibit ligand-to-ligand charge-transfer (l.l.c.t) transitions in the red to near i.r. region; the position and symmetry of the band depend upon the substituent on the dioxolene and arylazoimidazole. This effect is qualitatively assigned as HOMO(dioxolene) → LUMO(RaaiBz). A cyclic voltammogram of the dioxolene complex reveals two consecutive oxidative couples corresponding to catechols to semiquinones and semiquinone to quinone, respectively and the reductive couples represent azo reductions.