Directing a Non‐Heme Iron(III)‐Hydroperoxide Species on a Trifurcated Reactivity Pathway

Abstract

The reactivity of [Fe (tpena)] (tpena=N,N,N'-tris(2-pyridylmethyl)ethylenediamine-N'-acetate) as a catalyst for oxidation reactions depends on its ratio to the terminal oxidant H O and presence or absence of sacrificial substrates. The outcome can be switched between: 1) catalysed H O disproportionation, 2) selective catalytic oxidation of methanol or benzyl alcohol to the corresponding aldehyde, or 3) oxidative decomposition of the tpena ligand. A common mechanism is proposed involving homolytic O-O cleavage in the detected transient purple low-spin (S=1/2 ) [(tpenaH)Fe O-OH] . The resultant iron(IV) oxo and hydroxyl radical both participate in controllable hydrogen-atom transfer (HAT) reactions. Consistent with the presence of a weaker σ-donor carboxylate ligand, the most pronounced difference in the spectroscopic properties of [Fe(OOH)(tpenaH)] and its conjugate base, [Fe(OO)(tpenaH)] , compared to non-heme iron(III) peroxide analogues supported by neutral multidentate N-only ligands, are slightly blue-shifted maxima of the visible absorption band assigned to ligand-to-metal charge-transfer (LMCT) transitions and, corroborating this, lower Fe /Fe redox potentials for the pro-catalysts.

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

“…Thisc onstituteso ne of the rare examples of as ubstrate-inducedm echanism bifurcation. [59] Consequently,t he regioselectivity obtained with [(mtL 4 The hemilability of triazole allowed us to accumulate an Fe III (OOH)i ntermediatea nd fully study the mechanism of aromatic hydroxylation by (N 5 )Fe non-heme complexes under catalytic conditions (MeCN) whichw as previously impossible with parentsystems based on TPEN or L 5 2 ligands. Finally,t his study gives further insight into how ligand denticity affects the nature of the Fe III…”

Catalytic Alkyl Hydroperoxide and Acyl Hydroperoxide Disproportionation by a Nonheme Iron Complex

“…In this latter group one specific pyridyl arm is uncoordinated and protonated in all crystal structures. [8][9][10]12,14 This allows for the creation of another important biomimetic motifa second coordination sphere base.…”

“…For example, [Fe(tpena)] 2+ catalyses highly effective alcohol oxidation by H2O2 and in the absence of alcohol or another substrate, H2O2 disproportionation. 8 By contrast the N5/N6 ligand supported [Fe(Rtpen)] 2+ systems neither catalyse H2O2 disproportionation or alcohol oxidation by H2O2. In fact, methanol can be used as a solvent for observing transient [Fe III OOH(Rtpen)] 2+ where half-lives of min to h (rt) are found for these complexes.…”

“…The solid state precursor for the solution state chemistry described below is the dark brown [8][9][10]12,14 In these, the glycyl donor is located cis to the exogenous ligand which in turn is trans to the amine donor associated with the dipyridylamine moiety. (Figure 2a).…”

“…3 Despite these biological precedents, reports of biomimetic terminal oxidant activation using iron complexes of multidentate ligands containing a carboxylato donor remain relatively few by comparison to the considerable volume of work over the last three decades for iron complexes based on neutral aminopyridyl N-donor only ligands. [4][5][6] Our efforts to fill this gap, have revealed that iron complexes of glycyl substituted tetra, penta and hexadentate aminopyridyl ligands activate a range of oxidants including O2, 7 H2O2, 7,8 PhIO, 9 methylmorpholine-N-oxide, 9,10 with reactivity patterns that are distinct from those found for counterpart iron complexes based on neutral N4, N5 and N6-donor only ligands. In addition, for these systems, water can be primed to act as the O atom donor for the production of oxidizing iron(IV)oxo complexes by electrochemical 11 or Ce(IV) 12 activation in aqueous solutions.…”