Oxidative C-C bond cleavage of α-keto acids by cobalt(II) complexes of nitrogen donor ligands

Four cobalt(II) complexes, [(6Me₃TPA)Co^II(BF)] (BPh₄) (1), [(TPA)Co^II(BF)](BPh₄) (2), [{(6Me₃TPA)Co^II}₂(PP)](BPh₄) ₂ (3), and [(TPA)Co^II(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)Co^II}₂(oxalate)](BPh ₄)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)Co^III(PP)](BPh₄) (6), as established from the X-ray single-crystal structure. Cobalt(II) complexes of N₄ donor ligands oxidatively cleave the C-C bond of α-keto acids in the presence of dioxygen.