Kinetics and intermediates in the autoxidation of synthetic, non-porphyrin iron(II) dioxygen carriers

Abstract

Studies are reported on the autoxidation, in the solvent system 3:1:1 acetone/pyridine/water (APW), of a series of iron(II) cyclidenes, of the general formula [Fe(R* 123,R2,m-xyl)Cl]+ with R3 = Me, Ph and R2 = Me, Bz, which have high relevance as dioxygen carrier model systems. Autoxidation rates can be modeled by an electron-transfer mechanism involving a parallel dioxygen adduct equilibrium, with the autoxidation reaction being driven by a subsequent reaction of the primary superoxide product. The deduced rate constants k' for the electron-transfer process decrease in the sequence MeMe > MeBz > PhMe > PhBz, which is the order of increasing electronwithdrawing power and/or steric bulk of the substituents. Products of the reaction were identified explicitly for one of the complexes (R2 = R3 = Me), employing mainly UV-vis and ESR spectroscopy. A remarkable product of the autoxidation of iron(II) cyclidenes in this specific solvent system, is a peroxoiron(III) species, which can even be observed at ambient temperatures. Its ESR properties (low-spin iron g = 2 signal with an extremely low anisotropy) are in close similarity to those of natural systems analogues, and in contrast to the first reported model systems. This species can be generated by five independent routes (autoxidation; Fe02 + e~; Fe(II) + K02; Fe(III) + H202; and Fe(III) + C6H5IO) in the basic medium, emphasizing its relevance both as a cytochrome P450 model and as example of 0-0 bond formation.Non-Porphyrin Iron(II) Dioxygen Carriers

Characterization and Properties of Non-Heme Iron Peroxo Complexes

“…Complexes of PMAH [16] and Me,Me,m-xyl ligands [17] 15,18]. Information on equilibria in solution has also been obtained as well as evidence for ligand degradation [9,10] and Fe(IV)O species [11,18].…”

Models and mechanisms of O‐O bond activation by cytochrome P450

“…Notably, investigations with a superoxide‐generating system ruled out O 2 ·− itself to function as the active intermediate, so that superoxide was invoked to serve as a source for production of the ultimate oxygenating species, presumably Fe(III)‐OOH, catalyzing attack on the electron‐rich nitrogen centre of the tertiary arylamine [126–128]. The active oxidant thus was anticipated to arise from interaction, in the presence of protons, of newly generated O 2 ·− with either ferrous or oxyferrous [Fe(III)‐O 2 ·− ] P450, as given in [129–132]. That Fe(III)‐OOH generated in this way would only serve as a precursor in the transformation to: iron‐oxene as the actual catalyst could be discounted on kinetic grounds.…”

“…•) ] P450, as given in Eqns 1 and 2 [129][130][131][132]. That Fe(III)-OOH generated in this way would only serve as a precursor in the transformation to:…”

“…Similarly, water has been advocated to play a decisive role in regeneration, through reaction with (R ·+ )MP8Fe(IV) = O, of the active oxidant operative in the microperoxidase/H 2 O 2 ‐driven hydrocarbon oxygenation in bi‐ and tricyclic aromatic compounds [189] or oxidative aromate dehalogenation [185,190](Scheme 5). It should be emphasized that reaction of iodosylbenzene with purified CYP2B1 [191] or nonporphyrin iron(III) chelates in basic media [132,192] has been detected to prompt O‐O bond formation at the iron centres. Alkoxylating dehalogenation of halophenols, carried out by MP8/H 2 O 2 in alcoholic solvents, has been hypothesized to implicate a mechanism, in which the iron‐oxene resonance form reacts with alcohol to generate an Fe(III)‐OOR intermediate [193].…”

“…Noteworthy, experiments on BLM‐mediated epoxidation of cis ‐stilbene, employing iodosylbenzene as the oxygen donor in the presence of O, disclosed the epoxide oxygen to derive primarily from labeled water [229,245]. As pre‐equilibration of the oxidant with water prior to activation has been demonstrated not to result in exchange with H 2 O [229,246,247], this observation strongly suggests O = Fe(IV)BLM ·+ to be capable of undergoing exchange with solvent to regenerate Fe(III)‐OOH as the epoxidizing species [132]. This view is substantiated by studies with the iron complex of the BLM mimic depicted in Fig.…”