Oxidative C–C Bond Cleavage of α‐Keto Acids by Cobalt(II) Complexes of Nitrogen Donor Ligands

Four cobalt(II) complexes, [(6Me3TPA)CoII(BF)](BPh4) (1), [(TPA)CoII(BF)](BPh4) (2), [{(6Me3TPA)CoII}2(PP)](BPh4)2 (3), and [(TPA)CoII(PPH)](BPh4) (4) [where 6Me3TPA = tris(6-methyl-2-pyridylmethyl)amine, TPA = tris(2-pyridylmethyl)amine, BF = monoanionic benzoylformate, PP = dianionic phenylpyruvate, and PPH = monoanionic phenylpyruvate], of α-keto acid derivatives have been isolated to show their versatile reactivity with dioxygen. The X-ray crystal structure of 2 suggests a five-coordinate cobalt(II) center coordinated by a monodentate benzoylformate and a tetradentate nitrogen-donor supporting ligand. Conversely, complex 3 is a dinuclear cobalt complex where two cobalt(II) centers are bridged by PP. While complex 1 is unreactive towards dioxygen, 2 reacts slowly with oxygen to exhibit quantitative decarboxylation of coordinated benzoylformate to benzoate. An active cobalt-oxygen intermediate, intercepted by external substrates, is proposed to initiate the decarboxylation reaction. Complex 3 also reacts with dioxygen but to cleave the C2–C3 bond of PP with concomitant formation of benzaldehyde and an oxalate-bridged dicobalt(II) complex [{(6Me3TPA)CoII}2(oxalate)](BPh4)2 (5). The mononuclear PPH-cobalt(II) complex (4), unlike 2 and 3, does not undergo oxidative decarboxylation or C–C bond cleavage of PPH. In the reaction with dioxygen, 4 is oxidized to a PP-cobalt(III) complex, [(TPA)CoIII(PP)](BPh4) (6), as established from the X-ray single-crystal structure.