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The reactivity of [Fe III(tpena)] 2+ (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 2O 2 and presence or absence of sacrificial substrates. The outcome can be switched between: 1) catalysed H 2O 2 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 IIIO−OH] 2+. 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)] 2+ 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 III/Fe II redox potentials for the pro-catalysts.

The Fe III/Fe II redox potentials for [Fe(tpen)] 2+/3+, [Fe(tpena)] +/2+, and [Fe(tpenO)] +/2+ ( N-R- N, N' ,N'-tris(2-pyridylmethyl)ethane-1,2-diamine, where R = CH 2C 6H 4N, CH 2COO -, CH 2CH 2O -, respectively) span 470 mV with the oxidation potentials following the order [Fe II(tpenO)] + (MeOH) < [Fe II(tpena)] + (MeCN) < [Fe II(tpen)] 2+ (MeCN). In their +3 oxidation states the complexes react with 1 equiv of H 2O 2 to give the purple [Fe III(OOH)(HL)] n+ ( n = 2 for L = tpena, tpenO; n = 3 for L = tpen). A pyridine arm is decoordinated in these complexes, furnishing a second coordination sphere base which is protonated at ambient pH. The lifetimes of these transient species depend on how readily the substrate (sometimes the solvent) is oxidized and reflect the trend in both the O-O bond lability and oxidizing potency of the putative iron-based oxidant derived from the iron(III) peroxides. In methanol solution, [Fe III(tpenO)] 2+ and [Fe III(tpena)] 2+ exist in their Fe(III) states and hence the formation of [Fe III(OOH)(Htpena)] 2+ and [Fe III(OOH)(HtpenO)] 2+ is instantaneous. This is in contrast to the short lag time that occurs before adduct formation between [Fe II(tpen)] 2+ and H 2O 2 due to the requisite prior oxidation of the solution-state iron(II) complex to its iron(III) state. Stabilization of the +3 iron oxidation state in the resting state catalysts affords complexes that activate H 2O 2 more readily with the consequence of higher yields in the oxidation of the C-H bonds using H 2O 2 as terminal oxidant. The presence of a cis monodentate carboxylato donor increases the rate of oxidation by hydrogen atom transfer in comparison to the systems with an alkoxo or pyridine in this position. Competing with substrate oxidation is the oxidative modification of the alkoxido group in [Fe III(tpenO)] 2+, converting it to a carboxylato group in the presence of H 2O 2: in effect, transforming tpenO to tpena.

Mononuclear iron(III) species with end-on and side-on peroxide have been proposed or identified in the catalytic cycles of the antitumor drug bleomycin and a variety of enzymes, such as cytochrome P450 and Rieske dioxygenases, Only recently have biomimetic analogues of such reactive species been, generated and characterized at low temperatures. We report the synthesis and characterization of a series of iron(II) complexes with pentadentate N5 ligands that react with H(2)O(2) to generate transient low-spin Fe(III)-OOH intermediates. These intermediates have low-spin iron(III) centers exhibiting hydroperoxo-to-iron(III) charge-transfer bands in the 500-600-nm region. Their resonance Raman frequencies, nu(O-O), near 800 cm(-1) are significantly lower than those observed for high-spin counterparts. The hydroperoxo-to-iron(III) charge-transfer transition blue-shifts and the nu(O-O) of the Fe-OOH unit decreases as the N5 ligand becomes more electron donating. Thus, increasing electron density at the low-spin Fe(III) center weakens the O-O bond, in accord with conclusions drawn from published DFT calculations, The parent [(N4Py)Fe(III)(eta(1)-OOH)](2+) (1a) ion in this series (N4Py = NN-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) can be converted to its conjugate base, which is demonstrated to be a high-spin iron(III) complex with a side-on peroxo ligand, [(N4Py)Fe(III)(eta(2)-O(2))](+) (1b). A detailed analysis of 1a and 1b by EPR and Mossbauer spectroscopy provides insights into their electronic properties. The orientation of the observed (57)Fe A-tensor of 1 a can be explained with the frequently employed Griffith model provided the rhombic component of the ligand field, determined by the disposition of the hydroperoxo ligand, is 45degrees rotated relative to the octahedral field. EXAFS studies of la and 1b reveal the first metrical details of the iron-peroxo units in this family Of complexes: [(N4Py)Fe(III)(eta(1)-OOH)](2+) has an Fe-O bond of 1.76 Angstrom, while [(N4Py)Fe(III)(eta(2)-O(2))]- has two Fe-O bonds of 1.93 Angstrom, values which are in very good agreement with results obtained from DFT calculations.