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

Four cobalt(II) complexes, [(6Me 3 TPA)Co II (BF)](BPh 4 ) ( 1 ), [(TPA)Co II (BF)](BPh 4 ) ( 2 ), [{(6Me 3 TPA)Co II } 2 (PP)](BPh 4 ) 2 ( 3 ), and [(TPA)Co II (PPH)](BPh 4 ) ( 4 ) [where 6Me 3 TPA = 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 [{(6Me 3 TPA)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)](BPh 4 ) ( 6 ), as established from the X‐ray single‐crystal structure.