Light-induced O2-dependent aliphatic carbon–carbon (C–C) bond cleavage in bipyridine-ligated Co(ii) chlorodiketonate complexes

Abstract

Mononuclear bipyridine (bpy)-ligated Co(II) chlorodiketonate complexes [(bpy)₂Co(R–PhC(O)C(Cl)C(O)R–Ph)]ClO₄ (R = –H (8), –CH₃ (9), and –OCH₃ (10)), were prepared, characterized and investigated for O₂-dependent aliphatic C–C bond cleavage reactivity. Complexes 8–10 have a distorted psuedo-octahedral geometry. ¹H NMR spectra of 8–10 in CD₃CN show signals for the coordinated diketonate moiety, and signals suggesting ligand exchange reactivity leading to the formation of a small amount of [(bpy)₃Co](ClO₄)₂ (11) in solution. While 8–10 are air stable at room temperature, illumination at 350 nm results in oxidative cleavage reactivity within the diketonate moiety leading to the formation of 1,3-diphenylpropanetrione, benzoic acid, benzoic anhydride, and benzil. Illumination of 8 under ¹⁸O₂ results in a high level of ¹⁸O incorporation (>80%) in the benzoate anion. The product mixture, high level of ¹⁸O incorporation, and additional mechanistic studies suggest a reaction sequence wherein light-induced reactivity leads to the formation of a triketone intermediate that undergoes either oxidative C–C bond cleavage or benzoyl migration promoted by a bipyridine-ligated Co(II) or Co(III) fragment.