Iron-Substituted Polyoxotungstates as Inorganic Synzymes: Evidence for a Biomimetic Pathway in the Catalytic Oxygenation of Catechols

摘要

The oxygenase activity of natural metallo-enzymes offers a unique paradigm of what is the Holy Grail of oxidation chemistry.~[1–4] An innovative approach to the design of oxygenase functional mimics is the adoption of a totally inorganic ligand system derived from polyoxometalates (POMs),~[5–7] as an alternative to organic or organometallic coordination complexes.[8–9] Bio-inspired activity of transition-metal-substituted POMs (TMSP, with TM being Fe, Mn, Ru) has been proposed in the recent literature.~[10–14] However, due to the severe mechanistic complexity, which is frequently associated with metal-mediated aerobic oxidations, it is a major challenge to unravel the catalyst-s role along oxygen-transfer pathways. Therefore, in the realm of POM-based oxygenase mimics, the adherence to bio-inspired mechanistic features is often a matter of debate and retains a fundamental interest.~[15, 16] Among the class of Fesubstituted polyoxotungstates some structural types are known, where the coordination geometry of the iron center exhibits a striking oxygenase synzyme motif (Figure S1).~[17] In particular, the tetrasubstituted Krebs-type polyanions with general formula [Fe_4(H_2O)_(10)(β-XW_9O_(33))_2]~(n-) (Fe_4X_2W_(18), X=As~(III), Sb~(III) ; n=6; X=Se~(IV), Te~(IV); n=4), display two of the four iron sites with three terminal, substitution-labile coordination positions, typical of non-heme dioxygenase enzymes (Figure 1).~[18] The latter are responsible for the ca-tabolism/biodegradation of dihydroxylated aromatic compounds. While several functional models have been designed with coordination complexes using polydentate organic lig-ands,~[8] very few examples deal with POM-based systems, and FeIII framework-incorporated POMs as multi-turnover catalysts for the aerobic cleavage of catechols are unprecedented.~[19]