Aliphatic Carbon−Carbon Bond Cleavage Reactivity of a Mononuclear Ni(II) cis-β-Keto−Enolate Complex in the Presence of Base and O₂:  A Model Reaction for Acireductone Dioxygenase (ARD)

The synthesis, characterization, and reactivity properties of a mononuclear Ni(II) cis-β-keto−enolate complex, [(6-Ph₂TPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO₄ (1) (6-Ph₂TPA = N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine) are reported. Complex 1 was characterized by X-ray crystallography, elemental analysis, ¹H NMR, and electronic absorption and infrared spectroscopy. Treatment of 1 with 1 equiv of Me₄NOH·5H₂O in the presence of O₂ results in oxidative carbon−carbon bond cleavage and the formation of a new Ni(II) dicarboxylate complex, [(6-Ph₂TPA)Ni(O₂CPh)₂(H₂O)] (2). Complex 2 has been characterized by X-ray crystallography, ¹H NMR, UV−vis, IR, and elemental analysis. Use of ¹⁸O₂ in the reaction of 1 to produce 2 results in the incorporation of one ¹⁸O atom per carboxylate ligand in the majority of the sample. Production of CO in the oxidative conversion of 1 to 2 was detected using the palladium chloride method. This is the first functional model system of relevance to acireductone dioxygenase (ARD), a novel Ni(II)-containing enzyme that catalyzes a reaction that is a shunt out of the methionine salvage pathway in K. pneumoniae.