Rieske dioxygenases are metalloenzymes capable of achieving
cis-dihydroxylation of aromatics under mild conditions using O
2 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 the generation of a Fe
III(OOH) species with H
2O
2, 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, mtL
42, bearing a labile triazole, to generate an “in-between” situation. Two iron complexes, [(mtL
42)FeCl](PF
6) and [(mtL
42)Fe(OTf)
2]), were obtained and their reactivity towards aromatic substrates was studied in the presence of H
2O
2. Spectroscopic and kinetic studies reflect that triazole is bound at the Fe
II state, but decoordinates in the Fe
III(OOH). The resulting [(mtL
42)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 is proposed to follow 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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