Aliphatic C–C Bond Cleavage of α‑Hydroxy Ketones by Non-Heme Iron(II) Complexes: Mechanistic Insight into the Reaction Catalyzed by 2,4′-Dihydroxyacetophenone Dioxygenase

2,4′-Dihydroxyacetophenone dioxygenase (DAD) is a bacterial non-heme enzyme that carries out oxygenative aliphatic C–C bond cleavage of 2,4′-dihydroxyacetophenone (an α-hydroxy ketone) with the incorporation of both the oxygen atoms of dioxygen into the cleavage products. The crystal structure of the iron enzyme DAD has recently been determined, but very little is known about the mechanism of the C–C bond cleavage reaction. With the objective of gaining insights into the mechanism of the reaction catalyzed by DAD, six new biomimetic iron­(II)-α-hydroxy ketone complexes, [(Tp^Ph2)­Fe^II(PHAP)] (1), [(Tp^Ph2)­Fe^II(HCH)] (2), [(Tp^Ph2)­Fe^II(HBME)] (3), [(Tp^Ph2)­Fe^II(CHPE)] (4), [(6-Me₃-TPA)­Fe^II(PHAP)]⁺ (5), and [(6-Me₃-TPA)­Fe^II(HCH)]⁺ (6) (Tp^Ph2 = hydrotris­(3,5-diphenylpyrazol-1-yl)­borate, 6-Me₃-TPA = tris­(6-methyl-2-pyridylmethyl)­amine, PHAP-H = 2-phenyl-2-hydroxyacetophenone, HCH-H = 2-hydroxycyclohexanone, HBME-H = 2-hydroxy-1,2-bis­(4-methoxyphenyl)­ethanone, and CHPE-H = 1-(4-chlorophenyl)-2-hydroxy-2-phenylethanone), have been isolated and characterized. The single-crystal X-ray structure of 2 shows a five-coordinate iron­(II) complex with one tridentate facial ligand and a monoanionic bidentate α-hydroxy ketone, resulting in a distorted-square-pyramidal coordination geometry at the iron center. The iron­(II) complexes react with dioxygen to oxidatively cleave the aliphatic C–C bonds of the coordinated α-hydroxy ketones to afford 2 equiv of carboxylic acids. Mechanistic studies reveal that the C–C bond cleavage reaction proceeds through an intradiol pathway. Additionally, the coordinated α-hydroxy ketones in all of the complexes, except in complex 4, undergo two-electron oxidation to form the corresponding 1,2-diketones. However, the yields of 1,2-diketones are higher with the iron complexes of the tripodal N₄ ligand (6-Me₃-TPA) in comparison to the facial N₃ ligand (Tp^Ph2). These results strongly support the natural selection of a facial N₃ environment at the active site of the iron enzyme DAD.