Platinum complexes of diazoligands. Studies of regioselective aromatic ring amination, oxidative halogen addition and reductive halogen elimination reactions

Abstract

2-(Arylazo)pyridine ligands, L^1a–1c react with the salt K₂[PtCl₄] to give the mononuclear complexes [PtCl₂(L¹)] (1), which readily react with ArNH₂ to yield the monochloro complexes of type [PtCl(L²)] (HL² = 2-[(2-(arylamino)phenyl)azo]pyridine) (2) via regioselective ortho-amine fusion at the pendent aryl ring of coordinated L¹. Oxidative addition of the electrophiles Y₂ (Y = Cl, Br, I) to the square-planar platinum(II) complex, 2 has led to syntheses of the corresponding octahedral platinum(IV) complexes, [PtY₃(L²)] (3) in high yields. Ascorbate ion reductions of the platinum(IV) complexes, 3, resulted in reductive halogen elimination to revert to the platinum(II) complexes 2 almost quantitatively. Isolation of products and X-ray structure determination of the representative complexes followed all these chemical reactions. In crystal packing, the compound [PtCl₂(L^1c)] (1c) forms dimeric units with a Pt⋯Pt distance of 3.699(1) Å. In contrast, the crystal packing of 2b revealed that the molecules are arranged in an antiparallel fashion to form a noncovalent 1D chain to accommodate π(aryl)–π(pyridyl) and Pt–π(aryl) interactions. Notably, the oxidation of [Pt^IICl(L^2a)] (2a) by I₂ produced a mixed halide complex [Pt^IVClI₂(L^2a)] (5), which, in turn, is reduced by ascorbate ion to produce [Pt^III(L^2a)] with the elimination of ClI. All the platinum(II) complexes are brown, the platinum(IV) complexes, on the other hand, are green. Low-energy visible range transitions in the complexes of the extended ligand [L²]⁻ are ascribed to ligand based π–π* transitions. Cyclic voltammetric behaviour of the complexes is reported.