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

Rieske dioxygenases are metalloenzymes capable of achieving cis-dihydroxylation of aromatics under mild conditions using O and a source of electrons. The intermediate responsible for this reactivity is proposed to be a cis-Fe (O)(OH) moiety. Molecular models allow the generation of a Fe (OOH) species with H O , to yield a Fe (O)(OH) species with tetradentate ligands, or {Fe (O); OH } pairs with pentadentate ones. We have designed a new pentadentate ligand, mtL , bearing a labile triazole, to generate an "in-between" situation. Two iron complexes, [(mtL )FeCl](PF ) and [(mtL )Fe(OTf) ]), were obtained and their reactivity towards aromatic substrates was studied in the presence of H O . Spectroscopic and kinetic studies reflect that triazole is bound at the Fe state, but decoordinates in the Fe (OOH). The resulting [(mtL )Fe (OOH)(MeCN)] 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 is proposed to follow a side-on pathway leading to a putative (N )Fe (O)(OH), while in the presence of aromatics it switches to an end-on homolytic pathway yielding a {(N )Fe (O); OH } reactive species, through recoordination of triazole. This switch significantly impacts the reaction regioselectivity.