Acireductone Dioxygenase- (ARD-) Type Reactivity of a Nickel(II) Complex Having Monoanionic Coordination of a Model Substrate: Product Identification and Comparisons to Unreactive Analogues

A mononuclear Ni(II) complex ([(6-Ph2TPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO4 (1)), supported by the 6-Ph2TPA chelate
ligand (6-Ph2TPA = N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine) and containing a cis-β-keto-enolate ligand having a C2 hydroxyl substituent, undergoes reaction with O2 to produce a Ni(II) monobenzoate
complex ([(6-Ph2TPA)Ni(O2CPh)]ClO4 (3)), CO, benzil (PhC(O)C(O)Ph), benzoic acid, and other minor unidentified
phenyl-containing products. Complex 3 has been identified through independent synthesis and was characterized
by X-ray crystallography, 1H NMR, FAB-MS, FTIR, and elemental analysis. A series of cis-β-keto-enolate Ni(II)
complexes supported by the 6-Ph2TPA ligand ([(6-Ph2TPA)Ni(PhC(O)CHC(O)Ph)]ClO4 (4), [(6-Ph2TPA)Ni(CH3C(O)CHC(O)CH3)]ClO4 (5), and [(6-Ph2TPA)Ni(PhC(O)CHC(O)C(O)Ph) (6)) have been prepared and characterized.
While these complexes exhibit structural and/or spectroscopic similarity to 1, all are unreactive with O2. The results
of this study are discussed in terms of relevance to Ni(II)-containing acireductone dioxygenase enzymes, as well
as in the context of recently reported cofactor-free, quercetin, and β-diketone dioxygenases.