Structure and Reactivity of a Mononuclear Non-heme Iron(III)–peroxo Complex (original) (raw)
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Structure and reactivity of a mononuclear non-haem iron (III)–peroxo complex
Nature, 2011
Oxygen-containing mononuclear iron species—iron (iii)–peroxo, iron (iii)–hydroperoxo and iron (iv)–oxo—are key intermediates in the catalytic activation of dioxygen by iron-containing metalloenzymes 1, 2, 3, 4, 5, 6, 7. It has been difficult to generate synthetic analogues of these three active iron–oxygen species in identical host complexes, which is necessary to elucidate changes to the structure of the iron centre during catalysis and the factors that control their chemical reactivities with substrates. Here we report the high-resolution ...
High-Valent Nonheme Iron Oxidants in Biology: Lessons from Synthetic Fe(IV)=O Complexes
Bulletin of Japan Society of Coordination Chemistry, 2013
Ever since Hayaishi and Mason established the existence of oxygenases in 1955, 1,2 chemists have been fascinated by the activation of dioxygen at biological metal centers that produce a powerful oxidant capable of cleaving strong C-H bonds. Within the last half century, the number of metalloenzymes found to activate O 2 has grown, with iron enzymes representing the largest subset of this class as well as the most versatile. 3 For many of these reactions, it is conjectured that O 2 binding to the metal center initiates a reaction sequence that leads to the cleavage of the O-O bond and formation of a high-valent metal-oxo species responsible for C-H bond functionalization. Dioxygen activating iron enzymes can be subdivided into two families, heme and nonheme. Due to their intense and characteristic chromophores, the heme enzymes were recognized earlier as a group, exemplified by the cytochromes P450 that play important roles in metabolism. 4 Based on detailed studies of the closely related peroxidases, a high-valent intermediate called Compound I was identified as the species responsible for the oxidative chemistry observed and subsequently described as an [Fe IV (O)(porphyrin cation radical)] species. However, owing to its high reactivity, the corresponding Compound I intermediate of cytochrome P450 proved elusive, and only recently were the appropriate conditions found that allowed its spectroscopic characterization. 5 The cytochromes P450 catalyze metabolic transformations that involve the cleavage of strong C-H bonds and were recently shown to be capable of carrying out the hydroxylation of light alkanes, including methane, in vitro. 6,7 Dioxygen activating enzymes with nonheme iron active sites emerged in the past 25 years as another family of enzymes capable of cleaving strong C-H bonds. There are two subclasses, one with diiron centers exemplified by soluble methane monooxygenase (sMMO) 8,9 and the other with monoiron sites represented by taurine:α-ketoglutarate dioxygenase (TauD). 10,11 For both subclasses, the iron centers are coordinated to a combination of imidazoles (from His residues), carboxylates (from Asp and Glu residues), and solvent-derived ligands. Reaction of the reduced enzymes with O 2 leads to the generation of high-valent intermediates that, like Compounds I in cytochromes P450, carry out the critical C-H cleavage step (Scheme 1). In the case of sMMO, this intermediate is called Q and is postulated on the basis of Mössbauer and EXAFS analysis to have an antiferromagnetically coupled high-spin diiron(IV) unit with an Fe 2 O 2 diamond core. 12,13 The corresponding TauD intermediate is called J and demonstrated to have a high-spin Fe IV =O unit. 11 Both intermediates were shown to be kinetically competent to oxidize their respective substrates
Inorg. Chem, 2003
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, ν 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 ν 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 (η 1-OOH)] 2+ (1a) ion in this series (N4Py) N,N-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 (η 2-O 2)] + (1b). A detailed analysis of 1a and 1b by EPR and Mössbauer spectroscopy provides insights into their electronic properties. The orientation of the observed 57 Fe A-tensor of 1a 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 45°rotated relative to the octahedral field. EXAFS studies of 1a and 1b reveal the first metrical details of the iron−peroxo units in this family of complexes: [(N4Py)Fe III (η 1-OOH)] 2+ has an Fe−O bond of 1.76 Å, while [(N4Py)Fe III (η 2-O 2)] + has two Fe−O bonds of 1.93 Å, values which are in very good agreement with results obtained from DFT calculations. Mononuclear iron(III) peroxide species are implicated as intermediates in the mechanisms of oxygen activating biomolecules such as cytochrome P450, 1 heme oxygenase, 2 the antitumor drug bleomycin, 3 and Rieske dioxygenases, 4,5 as well as superoxide reductases from anaerobic bacteria. 6-9 Experimental evidence for some of these intermediates has
Angewandte Chemie International Edition, 2008
Dioxygen activation by mononuclear iron oxygenases in general requires two electrons and protons to facilitate the reductive cleavage of the O-O bond and formation of a high-valent iron oxidant.[1,2] For enzymes with an iron(III) resting state, the oxidant is postulated to have a formally Fe V oxidation state, e.g. Fe IV (O)(porphyrin radical) for cytochrome P450[i] and Fe V (O)(OH) for the Rieske dioxygenases.[ii] On the other hand, enzymes with an iron(II) resting state often require a tetrahydropterin or an α-keto acid cofactor to form an Fe IV (O) intermediate.[2] Such intermediates have recently been trapped and characterized for several enzymes.[iii] In model nonheme iron systems, there has been significant recent progress in the generation and characterization of Fe IV (O) complexes, most of which were prepared by reaction of iron(II) precursors with oxygen-atom donors (e.g. peroxides, peroxyacids and ArIO).[iv] The one exception has been the formation of [Fe IV (O)(TMC)(CH 3 CN)] 2+ (1),[v] reported by Nam and co-workers in the reaction of its iron(II) precursor with O 2 in the presence of alcohols or ethers.[vi] The mechanism for the formation of 1 under these conditions is not well established, but was postulated to result from O-O bond homolysis of a (μ-1,2peroxo)diiron(III) intermediate, resembling the mechanism postulated for [Fe IV (O)(TPP)] formation by the oxygenation of [Fe II (TPP)].[vii] In the latter case, coordination of imidazole trans to the peroxo ligand promoted oxoiron(IV) formation. By extension, it seems plausible that binding of the added alcohol or ether to the iron may also promote the formation of 1, as Fe II (TMC)(OTf) 2 in the absence of such additives is air-stable.[6] In the course of our work, we appended a pyridine moiety to the TMC framework to obtain the pentadentate ligand TMC-py (Scheme 1). Its iron(II) and oxoiron(IV) complexes were ** This work is the result of equal efforts from AT and JE.
Journal of Chemical Theory and Computation, 2012
Iron(III)-superoxo intermediates are believed to play key roles in oxygenation reactions by non-heme iron enzymes. We now report that a non-heme iron(II) complex activates O 2 and generates its corresponding iron(IV)-oxo complex in the presence of substrates with weak C-H bonds (e.g., olefins and alkylaromatic compounds). We propose that a putative iron(III)-superoxo intermediate initiates the O 2 -activation chemistry by abstracting a H atom from the substrate, with subsequent generation of a high-valent iron(IV)-oxo intermediate from the resulting iron(III)-hydroperoxo species.
Journal of the American Chemical Society, 1994
A series of monomeric carboxylate ferrous complexes with a tripodal N3 ligand HB(3,5-iPrzpz)3 has been synthesized and characterized to model the iron site in non-heme iron proteins which bind or activate dioxygen. The structures of Fe(OAc)(HB(3,54Przpz)3) (2), Fe(OBz)(MeCN)(HB(3,5-iPrzpz)3) (3), and Fe(OOCtBu)(HB(3,5-iPrzpz)s) (13) were determined by X-ray crystallography. The five-coordinate complex Fe(OBz)(HB(3,54Pr~pz)~) (4) was found to bind a variety of a-donating ligands such as dimethyl sulfoxide and pyridine at the open coordination site. The reaction between the ferrous complexes and dioxygen has been explored. Fe(OBz)(HB(3,5-iPr~pz)p) (4) was found to bind dioxygen to form an adduct which is reasonably stable below-20 OC. The dioxygen adduct wascharacterized by dioxygen uptake measurement, and UV-vis, resonance Raman, 'H-NMR, and X-ray absorption spectroscopy. On the basis of these results, the Fe:02 stoichiometry of 2:1, an intense absorption band at 682 nm, u (0-0) at 876 cm-1, and F w F e separation of 4.3 A estimated from EXAFS, the dioxygen adduct was identifed as a p-peroxo dinuclear ferric complex. The variable temperature magnetic susceptibility measurement of the isolated p-peroxo complex indicates that the complex is antiferromagnetically coupled with J =-33 cm-l, which is weaker than those known for other p-peroxo dinuclear ferric complexes. This characteristic feature may be associated with the structurally unique Fe-O-GFe frame, which is discussed on the basis of the extended Hiickel calculations. Above-20 OC, the reaction of 4 with dioxygen causes irreversible oxidation, resulting in formation of a trimeric ferric complex (HB(3,5-iPr2pz)~)Fe(OBz)~(0)Fe(OH)(OBz)~Fe(HB(3,5-iPr~pz)~) (16). This assignment is consistent with the magnetic property, Massbauer spectrum, and X-ray analysis.
Inorganic Chemistry, 2004
Density functional calculations using the B3LYP functional have been used to study the reaction mechanism of [Fe(Tp Ph2)BF] (Tp Ph2) hydrotris(3,5-diphenylpyrazol-1-yl)borate; BF) benzoylformate) with dioxygen. This mononuclear non-heme iron(II) complex was recently synthesized, and it proved to be the first biomimetic complex reproducing the dioxygenase activity of R-ketoglutarate-dependent enzymes. Moreover, the enthalpy and entropy of activation for this biologically interesting process were derived from kinetic experiments offering a unique possibility for direct comparison of theoretical and experimental data. The results reported here support a mechanism in which oxidative decarboxylation of the keto acid is the rate-limiting step. This oxygen activation process proceeds on the septet potential energy surface through a transition state for a concerted O−O and C−C bond cleavage. In the next step, a high-valent iron−oxo species performs electrophilic attack on the phenyl ring of the Tp Ph2 ligand leading to an iron(III)−radical σ-complex. Subsequent proton-coupled electron-transfer yields an iron(II)−phenol intermediate, which can bind dioxygen and reduce it to a superoxide radical. Finally, the protonated superoxide radical leaves the first coordination sphere of the iron(III)−phenolate complex and dismutates to dioxygen and hydrogen peroxide. The calculated activation barrier (enthalpy and entropy) and the overall reaction energy profile agree well with experimental data. A comparison to the enzymatic process, which is suggested to occur on the quintet surface, has been made.
Activation of Dioxygen by Iron and Manganese Complexes: A Heme and Nonheme Perspective
Journal of the American Chemical Society, 2016
The rational design of well-defined, first-row transition metal complexes that can activate dioxygen has been a challenging goal for the synthetic inorganic chemist. The activation of O2 is important in part because of its central role in the functioning of metalloenzymes, which utilize O2 to perform a number of challenging reactions including the highly selective oxidation of various substrates. There is also great interest in utilizing O2, an abundant and environmentally benign oxidant, in synthetic catalytic oxidation systems. This Perspective brings together recent examples of biomimetic Fe and Mn complexes that can activate O2 in heme or nonheme-type ligand environments. The use of oxidants such as hypervalent iodine (e.g., ArIO), peracids (e.g., m-CPBA), peroxides (e.g., H2O2) or even superoxide is a popular choice for accessing well-characterized metal-superoxo, metal-peroxo, or metal-oxo species, but the instances of biomimetic Fe/Mn complexes that react with dioxygen to yie...