Hydroxylation of Aromatics by H 2 O 2 Catalyzed by Mononuclear Non‐heme Iron Complexes: Role of Triazole Hemilability in Substrate‐Induced Bifurcation of the H 2 O 2 Activation Mechanism
Résumé
Rieske dioxygenases are metalloenzymes capable of achieving cis-dihydroxylation of aromatics under mild conditions using O2 and a source of electrons. The intermediate responsible for this reactivity is proposed to be a cis-Fe V (O)(OH) moiety. Molecular models allow generating Fe III (OOH) species with H2O2, to yield a Fe V (O)(OH) species with tetradentate ligands, or {Fe IV (O)/OH•} pairs with pentadentate ones. We have designed a new pentadentate ligand mtL4 2 bearing a labile triazole, to generate an "in-between" situation. Two iron complexes [(mtL4 2)FeCl](PF6) and [(mtL4 2)Fe(OTf)2]) were obtained and their reactivity studied towards aromatic substrates in the presence of H2O2. Spectroscopic and kinetic studies reflect that triazole is bound at the Fe II state, but decoordinates in the Fe III (OOH). The resulting [(mtL4 2)Fe III (OOH)(MeCN)] 2+ then lies on a bifurcated decay pathway (end-on homolytic vs side-on heterolytic) depending on the addition of aromatic substrate: in the absence of substrate, it follows a side-on pathway leading to a putative (N4)Fe V (O)(OH), while in the presence of aromatics it switches to an end-on homolytic pathway yielding a {(N5)Fe IV (O);OH•} reactive species, through recoordination of triazole This switch significantly impacts the reaction regioselectivity.
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