An All-Inorganic, Polyoxometalate-Based Catechol Dioxygenase That Exhibits >100 000 Catalytic Turnovers

Following a critical analysis of the dioxygenase literature and injection of the insights therein into the development of new dioxygenase catalysts, two new, of four total exemplary, polyoxoanion precatalysts have been synthesized, characterized, and then discovered to exhibit record catalytic lifetime 3,5-di-tert-butylcatechol (DTBC; 1) dioxygenase activity using molecular oxygen as the terminal oxidant. A total of 24 additional polyoxoanion and other precatalysts have also been surveyed for their DTBC dioxygenase activity. The four exemplary precatalyst complexes are the trivanadium(V)-containing, orange-red parent polyoxoanions (n-Bu₄N)₇[SiW₉V₃O₄₀], I, and (n-Bu₄N)₉[P₂W₁₅V₃O₆₂], II, and their previously unknown polyoxoanion-supported, dark green iron complexes (n-Bu₄N)₅[(CH₃CN)_xFe·SiW₉V₃O₄₀], III, and (n-Bu₄N)₅[(CH₃CN)_xFe·P₂W₁₅V₃O₆₂], IV. Careful, high (95 ± 5%) mass balance studies are reported, studies rare in the dioxygenase literature, but studies made possible in the case of I−IV by their high activity and long lifetimes which yielded sizable amounts of isolable, and thus unequivocally characterizable, products (the characterization of products 2, 3, 4, and 6 includes single-crystal X-ray crystallography structures):  3,5-di-tert-butyl-1-oxacyclohepta-3,5-diene-2,7-dione (muconic acid anhydride), 2; 4,6-di-tert-butyl-2H-pyran-2-one, 3; a new, previously unidentified product (once misidentified in the literature), spiro[1,4-benzodioxin-2(3H), 2‘-[2H]pyran]-3-one, 4‘,6,6‘,8-tetrakis(1,1-dimethylethyl), 4; 3,5-di-tert-butyl-5-(carboxymethyl)-2-furanone, 5; and the autoxidation product, 3,5-di-tert-butyl-1,2-benzoquinone, 6. Quantitative yields for each of the above products are also reported. Solvent effects on the dioxygenase reaction are evaluated by survey studies in 5 solvents; the highest yields are observed in non-coordinating solvents such as 1,2-dichloroethane. Oxygen uptake studies are reported; the results confirm the 1 O₂:1 DTBC stoichiometry which defines a catechol dioxygenase and address, for the first time, the details of how this ∼1:1 stoichiometry actually arises from the linear combination of the individual stoichiometries of the five, formally parallel, major reactions yielding the five major products. Initial kinetic studies are also reported; the O₂ uptake kinetics reveal a novel product and catalyst evolution mechanism consisting of an A → B induction period, followed by an A + B → 2 B autocatalytic step for complexes I and III, where A = O₂ and B is a product of the DTBC plus O₂ reaction. Catalyst lifetime experiments with (n-Bu₄N)₅[(CH₃CN)_xFe·SiW₉V₃O₄₀], III, as a prototype precatalyst reveal a DTBC dioxygenase catalytic lifetime of >100 000 total catalytic turnovers (TTOs), a record compared to any reported dioxygenase, man-made or enzymic. A Summary and Conclusions section is presented, as is a list of the needed additional, in-progress, kinetic, mechanistic, and catalyst isolation and characterization studies. The long-term goal of such studies is the development of even longer-lived, more selective dioxygenase catalysts able to oxygenate the full range of interesting substrates of enzymic dioxygenases, as well as abiological substrates such as propene.