Acid pK_a Dependence in O–O Bond Heterolysis of a Nonheme Fe^III–OOH Intermediate To Form a Potent Fe^V═O Oxidant with Heme Compound I-Like Reactivity

Abstract

Protons play essential roles in natural systems in controlling O−O bond cleavage of peroxoiron(III) species to give rise to the high-valent iron oxidants that carry out the desired transformations. Herein, we report kinetic and mechanistic evidence that acids can control the mode of O− O bond cleavage for a nonheme S = 1/2 Fe III −OOH species [(BnTPEN)Fe III (OOH)] 2+ (2, BnTPEN = N-benzyl-N,N′,N′tris(2-pyridylmethyl)-1,2-diaminoethane). Addition of acids having pK a values of >8.5 in CH 3 CN results in O−O bond homolysis, leading to the formation of hydroxyl radicals that give rise to alcohol/ketone (A/K) ratios of around 1 in the oxidation of cyclohexane. However, the introduction of acids with pK a values of <8.5 elicits a different outcome, namely the achievement of A/K ratios of as high as 9, the observation of rapid and catalytic hydroxylation of cyclohexane, and a million-fold acceleration in the decay rate of the Fe III −OOH intermediate at −40 °C. These results implicate the generation of a highly reactive Fe V O species via proton-assisted O−O bond heterolysis of the Fe III −OOH intermediate, which is unprecedented for nonheme iron complexes supported by neutral pentadentate ligands and serves as a nonheme analogue for heme enzyme compounds I.

Bio‐inspired Nonheme Iron Oxidation Catalysis: Involvement of Oxoiron(V) Oxidants in Cleaving Strong C−H Bonds

“…Two important papers related to the subject of this Review were published as this manuscript was undergoing review, editing and translation. In the first paper, Que and coworkers report that the addition of an equivalent of strong acid to the coordinatively saturated S = (N5)Fe III ‐OOH species supported by the pentadentate BnTPEN ligand (BnTPEN= N ‐benzyl‐ N , ′ , N ′‐tris(2‐pyridylmethyl)‐1,2‐diaminoethane) significantly facilitates O−O bond cleavage to form an oxidant capable of hydroxylating cyclohexane catalytically and with high A/K product ratios . This outcome represents quite an astonishing result as the [Fe III (BnTPEN)OOH] 2+ intermediate has long been shown to be a sluggish oxidant that inherently undergoes O−O bond homolysis to generate hydroxyl radicals .…”

Hydroxylation of Aromatics by H2O2 Catalyzed by Mononuclear Non‐heme Iron Complexes: Role of Triazole Hemilability in Substrate‐Induced Bifurcation of the H2O2 Activation Mechanism

“…To summarize, as decay rates of [(mtL 4 2 )Fe III (OOH)] 2 + are poorly dependent on benzene concentration, the substrate [48] 44 À89 [(TPA)Fe III (OOH)] 2 + + octene in MeCN [47] 45 À95 [(PyNMe 3 )Fe III (OOH)] 2 + in MeCN + H + [26] 46 À79 (BnTPEN)Fe III (OOH) 2 + + HOAc + HOTfi nMeCN [49] 30 À118 homolytic [(PyNMe 3 )Fe III (OOH)] 2 + + in MeCN [26] 43 À137 [(L 5 2 )Fe III (OOH)] 2 + in MeCN [13] 81 À1 [(TMC)Fe III (OOH)] 2 + in acetone/CF 3 CH 2 OH [16] 56 À75 [(N 4 Py)Fe III (OOH)] 2 + in acetone/CF 3 CH 2 OH [16] 53 À121 [(bppc)Fe III (OOH)] 2 + [28] 53 À68 does not react in the TS. However,g iven the differents ets of activation parameters obtainedi nt he presencea nd absence of benzene, it induces as witch in the mechanism.…”

The most reactive iron and manganese complexes with N-pentadentate ligands for dioxygen activation—synthesis, characteristics, applications

“…In first step of this reaction, the high-spin iron(II) complex reacts with dioxygen to form an iron(III)-superoxo adduct, which in further transformations gives the low-spin hydroperoxo complex of iron(III) [ 60 ]. The coordinative saturated S = 1/2 [(Bn-TPEN)Fe III (OOH)] 2+ ( 2′ ) species, can be produced, also in the reaction with excess hydrogen peroxide in hydroxylic solvents in the form of purple species stable for hours, named "sluggish oxidant" that undergoes via homolysis of the O–O bond [ 3 ]. Formation of ( 2′ ) in the presence of H 2 O 2 occurs, analogous to ( 2 ), in two stages.…”

“…Recently, synthesis of low-valent iron and manganese complexes capable of stabilizing intermediates produced in the oxygenation cycles, which show similarity to natural systems, is of great interest. Within the last two decades, a number of mononuclear complexes with oxoiron(IV) [ 3 – 20 ] and oxomanganese(IV) [ 9 , 11 , 18 , 21 ] units supported by polydentate non-heme ligands have been investigated. These complexes serve as models for high-valent intermediates in the catalytic cycles of enzymes that carry out a series of oxidative modifications.…”