Hydroxylation of Aromatics by H2 O2 Catalyzed by Mononuclear Non-heme Iron Complexes: Role of Triazole Hemilability in Substrate-Induced Bifurcation of the H2 O2 Activation Mechanism.

Abstract

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-FeV (O)(OH) moiety. Molecular models allow the generation of a FeIII (OOH) species with H2 O2 , to yield a FeV (O)(OH) species with tetradentate ligands, or {FeIV (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 towards aromatic substrates was studied in the presence of H2 O2 . Spectroscopic and kinetic studies reflect that triazole is bound at the FeII state, but decoordinates in the FeIII (OOH). The resulting [(mtL4 2 )FeIII (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 is proposed to follow a side-on pathway leading to a putative (N4 )FeV (O)(OH), while in the presence of aromatics it switches to an end-on homolytic pathway yielding a {(N5 )FeIV (O); OH. } reactive species, through recoordination of triazole. This switch significantly impacts the reaction regioselectivity.