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

Mononuclear bipyridine (bpy)-ligated Co( ii ) chlorodiketonate complexes [(bpy) 2 Co(R-PhC(O)C(Cl)C(O)R-Ph)]ClO 4 (R = -H ( 8 ), -CH 3 ( 9 ), and -OCH 3 ( 10 )), were prepared, characterized and investigated for O 2 -dependent aliphatic C-C bond cleavage reactivity. Complexes 8-10 have a distorted psuedo-octahedral geometry. 1 H NMR spectra of 8-10 in CD 3 CN show signals for the coordinated diketonate moiety, and signals suggesting ligand exchange reactivity leading to the formation of a small amount of [(bpy) 3 Co](ClO 4 ) 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 18 O 2 results in a high level of 18 O incorporation (>80%) in the benzoate anion. The product mixture, high level of 18 O 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. Illumination of aerobic acetonitrile solutions of bipyridine-ligated Co( ii ) chlorodiketonate complexes results in O 2 -dependent aliphatic C-C bond cleavage with high 18 O incorporation.