Oxygenation of a diiron(II) complex, [Fe(II)(2)(μ-OH)(2)(BnBQA)(2)(NCMe)(2)](2+) [2, where BnBQA is N-benzyl-N,N-bis(2-quinolinylmethyl)amine], results in the formation of a metastable peroxodiferric intermediate, 3. The treatment of 3 with strong acid affords its conjugate acid, 4, in which the (μ-oxo)(μ-1,2-peroxo)diiron(III) core of 3 is protonated at the oxo bridge. The core structures of 3 and 4 are characterized in detail by UV-vis, Mössbauer, resonance Raman, and X-ray absorption spectroscopies.
View Article and Find Full Text PDFOxoiron(V) species are postulated to be involved in the mechanisms of the arene cis-dihydroxylating Rieske dioxygenases and of bioinspired nonheme iron catalysts for alkane hydroxylation, olefin cis-dihydroxylation, and water oxidation. In an effort to obtain a synthetic oxoiron(V) complex, we report herein the one-electron oxidation of the S = 1 complex [Fe(IV)(O)(TMC)(NCCH(3))](2+) (1, where TMC is tetramethylcyclam) by treatment with tert -butyl hydroperoxide and strong base in acetonitrile to generate a metastable complex 2 at -44 °C, which has been characterized by UV-visible, resonance Raman, Mössbauer, and EPR methods. The defining spectroscopic characteristic of 2 is the unusual x/y anisotropy observed for the (57)Fe and (17)O A tensors associated with the high-valent Fe═O unit and for the (14)N A tensor of a ligand derived from acetonitrile.
View Article and Find Full Text PDFWith the goal of gaining insight into the structures of peroxo intermediates observed for oxygen-activating nonheme diiron enzymes, a series of metastable synthetic diiron(III)-peroxo complexes with [Fe(III)(2)(mu-O)(mu-1,2-O(2))] cores has been characterized by X-ray absorption and resonance Raman spectroscopies, EXAFS analysis shows that this basic core structure gives rise to an Fe-Fe distance of approximately 3.15 A; the distance is decreased by 0.1 A upon introduction of an additional carboxylate bridge.
View Article and Find Full Text PDFChem Commun (Camb)
May 2008
The stoichiometric formation of [FeIV(O)(TPA)(NCMe)]2+ (TPA = tris(2-pyridylmethyl)amine) from the reaction of [FeII(TPA)(NCMe)2]2+ with 1 equiv. peracetic acid exhibits more kinetic complexity than might be expected from the simple stoichiometry. A multiple-pathway mechanism with an FeIV-peracetic acid species, [(TPA)FeIV(O)((H)O3CR)]2+/+, as the primary oxidant is proposed.
View Article and Find Full Text PDFRecently, we reported the characterization of the S = (1)/ 2 complex [Fe (V)(O)B*] (-), where B* belongs to a family of tetraamido macrocyclic ligands (TAMLs) whose iron complexes activate peroxides for environmentally useful applications. The corresponding one-electron reduced species, [Fe (IV)(O)B*] (2-) ( 2), has now been prepared in >95% yield in aqueous solution at pH > 12 by oxidation of [Fe (III)(H 2O)B*] (-) ( 1), with tert-butyl hydroperoxide. At room temperature, the monomeric species 2 is in a reversible, pH-dependent equilibrium with dimeric species [B*Fe (IV)-O-Fe (IV)B*] (2-) ( 3), with a p K a near 10.
View Article and Find Full Text PDFProc Natl Acad Sci U S A
December 2007
Intermediate Q, the methane-oxidizing species of soluble methane monooxygenase, is proposed to have an [Fe(IV)(2)(mu-O)(2)] diamond core. In an effort to obtain a synthetic precedent for such a core, bulk electrolysis at 900 mV (versus Fc(+/0)) has been performed in MeCN at -40 degrees C on a valence-delocalized [Fe(III)Fe(IV)(mu-O)(2)(L(b))(2)](3+) complex (1b) (E(1/2) = 760 mV versus Fc(+/0)). Oxidation of 1b results in the near-quantitative formation of a deep red complex, designated 2b, that exhibits a visible spectrum with lambda(max) at 485 nm (9,800 M(-1).
View Article and Find Full Text PDFThe reactions of iron(II) complexes [Fe(T(pt-Bu,i-Pr))(OH)] (1a, Tp(t-Bu,i-Pr) = hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)borate), [Fe(6-Me2BPMCN)(OTf)2] (1b, 6-Me2BPMCN = N,N'-bis((2-methylpyridin-6-yl)methyl)-N,N'-dimethyl-trans-1,2-diaminocyclohexane), and [Fe(L8Py2)(OTf)](OTf) (1c, L8Py2 = 1,5-bis(pyridin-2-ylmethyl)-1,5-diazacyclooctane) with tert-BuOOH give rise to high-spin FeIII-OOR complexes. X-ray absorption spectra (XAS) of these high-spin species show characteristic features, distinct from those of low-spin Fe-OOR complexes (Rohde, J.-U.
View Article and Find Full Text PDFIron(V)-oxo species have been proposed as key reactive intermediates in the catalysis of oxygen-activating enzymes and synthetic catalysts. Here, we report the synthesis of [Fe(TAML)(O)]- in nearly quantitative yield, where TAML is a macrocyclic tetraamide ligand. Mass spectrometry, Mössbauer, electron paramagnetic resonance, and x-ray absorption spectroscopies, as well as reactivity studies and density functional theory calculations show that this long-lived (hours at -60 degrees C) intermediate is a spin S = 1/2 iron(V)-oxo complex.
View Article and Find Full Text PDFOxygenation of [Cu(I)(6-PhTPA)](SbF(6)) in acetone at -90 degrees C produces a short-lived Cu(III)(2)(mu-O)(2) intermediate that exhibits an oxygen-isotope-sensitive nu(Cu-O) mode at 599 cm(-1) and an overtone at 1192 cm(-1). The formation of this intermediate is very fast and is second-order in copper(I) complex, implying that two copper-containing species interact in the rate-limiting step or in pre-equilibrium steps prior to the rate determining step. The decay of this intermediate was facile even at -90 degrees C but did not afford any arene hydroxylation product.
View Article and Find Full Text PDFDensity functional theory (DFT) calculations have been carried out for a series of Cu(I) complexes bearing N-hexadentate macrocyclic dinucleating ligands and for their corresponding peroxo species (1c-8c) generated by their interaction with molecular O2. For complexes 1c-7c, it has been found that the side-on peroxodicopper(II) is the favored structure with regard to the bis(mu-oxo)dicopper(III). For those complexes, the singlet state has also been shown to be more stable than the triplet state.
View Article and Find Full Text PDFHigh valent iron-oxo species are often invoked as the key oxidizing agents in the catalytic cycles of oxygen activating nonheme iron enzymes, and three of these intermediates have in fact been characterized. To gain further insight into such species, a number of biomimetic complexes have been designed and investigated as functional models for these enzymes. Progress since 2000 is summarized in this review.
View Article and Find Full Text PDFThe reaction of [Fe(2)(mu-OH)(2)(6-Me(3)-TPA)(2)](2+) (1) [6-Me(3)-TPA, Tris(6-methyl-2-pyridylmethyl)amine] with O(2) in CH(2)Cl(2) at -80 degrees C gives rise to two new intermediates, 2 and 3, before the formation of previously characterized [Fe(2)(O)(O(2))(6-Me(3)-TPA)(2)](2+) (4) that allow the oxygenation reaction to be monitored one electron-transfer step at a time. Raman evidence assigns 2 and 3 as a diiron-superoxo species and a diiron-peroxo species, respectively. Intermediate 2 exhibits its nu(O-O) at 1,310 cm(-1) with a -71-cm(-1) (18)O isotope shift.
View Article and Find Full Text PDFA series of complexes with [Fe(II)(2)(mu-OH)(2)] cores has been synthesized with N3 and N4 ligands and structurally characterized to serve as models for nonheme diiron(II) sites in enzymes that bind and activate O(2). These complexes react with O(2) in solution via bimolecular rate-limiting steps that differ in rate by 10(3)-fold, depending on ligand denticity and steric hindrance near the diiron center. Low-temperature trapping of a (mu-oxo)(mu-1,2-peroxo)diiron(III) intermediate after O(2) binding requires sufficient steric hindrance around the diiron center and the loss of a proton (presumably that of a hydroxo bridge or a yet unobserved hydroperoxo intermediate).
View Article and Find Full Text PDFTransient mononuclear low-spin alkylperoxoiron(III) and oxoiron(IV) complexes that are relevant to the activation of dioxygen by nonheme iron enzymes have been generated from synthetic iron(II) complexes of neutral tetradentate (TPA) and pentadentate (N4Py, Bn-TPEN) ligands and structurally characterized by means of Fe K-edge X-ray absorption spectroscopy (XAS). Notable features obtained from fits of the EXAFS region are Fe-O bond lengths of 1.78 A for the alkylperoxoiron(III) intermediates and 1.
View Article and Find Full Text PDFCompounds that contain the anion [MeReO(edt)(SPh)](-) (3-) were synthesized with the countercations 2-picolinium (PicH+3-) and 2,6-lutidinium (LutH+3-), where edt is 1,2-ethanedithiolate. Both PicH+3- and MeReO(edt)(tetramethylthiourea) (4) were crystallographically characterized. The rhenium atom in each of these compounds exists in a five-coordinate distorted square pyramid.
View Article and Find Full Text PDFFour new methyloxorhenium(V) compounds were synthesized with these tridentate chelating ligands: 2-mercaptoethyl sulfide (abbreviated HSSSH), 2-mercaptoethyl ether (HSOSH), thioldiglycolic acid (HOSOH), and 2-(salicylideneamino)benzoic acid (HONOH). Their reactions with MeReO(3) under suitable conditions led to these products: MeReO(SSS), 1, MeReO(SOS), 2, MeReO(OSO)(PAr(3)), 3, and MeReO(ONO)(PPh(3)), 4. These compounds were characterized spectroscopically and crystallographically.
View Article and Find Full Text PDFFour new methyloxorhenium(V) complexes were synthesized: MeReO(PA)(2) (1), MeReO(HQ)(2) (2), MeReO(MQ)(2) (3), and MeReO(diphenylphosphinobenzoate)(2) (4) (in which PAH = 2-picolinic acid, HQH = 8-hydroxyquinoline, and MQH = 8-mercaptoquinoline). Although only one geometric structure has been identified crystallographically for 1, 2, and 3, two isomers of 3 and 4 in solution were detected by NMR spectroscopy. These compounds catalyze the sulfoxidation of thioethers by pyridine N-oxides and sulfoxides.
View Article and Find Full Text PDFChelating dithiolate ligands--e.g., mtp from 2-(mercaptomethyl)thiophenol, edt from 1,2-ethanedithiol, and pdt from 1,3-propanedithiol--stabilize high-valent oxorhenium(V) against hydrolytic and oxidative decomposition.
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