Synthetic Models for Non-Heme Carboxylate-Bridged Diiron Metalloproteins: Strategies and Tactics

Synthesis, characterization, and preliminary oxygenation studies of benzyl- and ethyl-substituted pyridine ligands of carboxylate-rich diiron(II) complexes

In this study benzyl and ethyl groups were appended to pyridine and aniline ancillary ligands in diiron(II) complexes of the type [Fe(2)(mu-O(2)CAr(R))(2)(O(2)CAr(R))(2)(L)(2)], where (-)O(2)CAr(R) is a sterically hindered terphenyl carboxylate, 2,6-di(p-tolyl)- or 2,6-di(p-fluorophenyl)benzoate (R = Tol or 4-FPh, respectively). These crystallographically characterized compounds were prepared as analogues of the diiron(II) center in the hydroxylase component of soluble methane monooxygenase (MMOH). The use of 2-benzylpyridine (2-Bnpy) yielded doubly bridged [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(2-Bnpy)(2)] (1) and [Fe(2)(mu-O(2)CAr(4)(-)(FPh))(2)(O(2)CAr(4)(-)(FPh))(2)(2-Bnpy)(2)] (4), whereas tetra-bridged [Fe(2)(mu-O(2)CAr(Tol))(4)(4-Bnpy)(2)] (3) resulted when 4-benzylpyridine (4-Bnpy) was employed. Similarly, 2-(4-chlorobenzyl)pyridine (2-(4-ClBn)py) and 2-benzylaniline (2-Bnan) were employed as N-donor ligands to prepare [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(2-(4-ClBn)py)(2)] (2) and [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(2-Bnan)(2)] (5). The placement of the substituent on the pyridine ring had no effect on the geometry of the diiron(II) compounds isolated when 2-, 3-, or 4-ethylpyridine (2-, 3-, or 4-Etpy) was introduced as the ancillary nitrogen ligand. The isolated [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(2-Etpy)] (6), [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(3-Etpy)] (7), [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(4-Etpy)] (8), and [Fe(2)(mu-O(2)CAr(4)(-FPh))(2)(O(2)CAr(4)(-)(FPh))(2)(2-Etpy)(2)] (9) complexes all contain doubly bridged metal centers. The oxygenation of compounds 1-9 was studied by GC-MS and NMR analysis of the organic fragments following decomposition of the product complexes. Hydrocarbon fragment oxidation occurred for compounds in which the substrate moiety was in close proximity to the diiron center. The extent of oxidation depended upon the exact makeup of the ligand set.

Dioxygen-initiated oxidation of heteroatomic substrates incorporated into ancillary pyridine ligands of carboxylate-rich diiron(II) complexes

Progress toward the development of functional models of the carboxylate-bridged diiron active site in soluble methane monooxygenase is described in which potential substrates are introduced as substituents on bound pyridine ligands. Pyridine ligands incorporating a thiol, sulfide, sulfoxide, or phosphine moiety were allowed to react with the preassembled diiron(II) complex [Fe(2)(mu-O(2)CAr(R))(2)(O(2)CAr(R))(2)(THF)(2)], where (-)O(2)CAr(R) is a sterically hindered 2,6-di(p-tolyl)- or 2,6-di(p-fluorophenyl)benzoate (R = Tol or 4-FPh). The resulting diiron(II) complexes were characterized crystallographically. Triply and doubly bridged compounds [Fe(2)(mu-O(2)CAr(Tol))(3)(O(2)CAr(Tol))(2-MeSpy)] (4) and [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(2-MeS(O)py)(2)] (5) resulted when 2-methylthiopyridine (2-MeSpy) and 2-pyridylmethylsulfoxide (2-MeS(O)py), respectively, were employed. Another triply bridged diiron(II) complex, [Fe(2)(mu-O(2)CAr(4)(-)(FPh))(3)-(O(2)CAr(4)(-)(FPh))(2-Ph(2)Ppy)] (3), was obtained containing 2-diphenylphosphinopyridine (2-Ph(2)Ppy). The use of 2-mercaptopyridine (2-HSpy) produced the mononuclear complex [Fe(O(2)CAr(Tol))(2)(2-HSpy)(2)] (6a). Together with that of previously reported [Fe(2)(mu-O(2)CAr(Tol))(3)(O(2)CAr(Tol))(2-PhSpy)] (2) and [Fe(2)(mu-O(2)CAr(Tol))(3)(O(2)CAr(Tol))(2-Ph(2)Ppy)] (1), the dioxygen reactivity of these iron(II) complexes was investigated. A dioxygen-dependent intermediate (6b) formed upon exposure of 6a to O(2), the electronic structure of which was probed by various spectroscopic methods. Exposure of 4 and 5 to dioxygen revealed both sulfide and sulfoxide oxidation. Oxidation of 3 in CH(2)Cl(2) yields [Fe(2)(mu-OH)(2)(mu-O(2)CAr(4)(-)(FPh))(O(2)CAr(4)(-FPh))(3)(OH(2))(2-Ph(2)P(O)py)] (8), which contains the biologically relevant {Fe(2)(mu-OH)(2)(mu-O(2)CR)}(3+) core. This reaction is sensitive to the choice of carboxylate ligands, however, since the p-tolyl analogue 1 yielded a hexanuclear species, 7, upon oxidation.

Tetranuclear iron(iii) complexes of an octadentate pyridine-carboxylate ligand and their catalytic activity in alkane oxidation by hydrogen peroxide

Reaction of the octadentate ligand 2,6-bis{3-[N,N-di(2-pyridylmethyl)amino]propoxy}benzoic acid (LH) with Fe(ClO4)3 leads to the formation of the tetranuclear complexes [Fe4(mu-O)2(LH)2(ClCH2-CO2)4](ClO4)4 (1), [{Fe2(mu-O)L(R-CO2)}2](ClO4)4 (2 R = C6H5-, 3 R = CH3-, 4, R = ClCH2-). The crystal structures of complexes 1 and 2 reveal that they consist of two Fe(III)2(mu-O)(mu-RCO2)2 cores that are linked via the two LH/L ligands to give a "dimer of dimers" structure. Complex assumes a helical shape, with protonated carboxylic acid moieties of the two ligands forming a hydrogen-bonded pair at the center of the cation. In complexes 2, 3 and 4, central carboxylates of the two ligands bridge the iron ions in each of the two Fe2O units, with an interdimer iron-iron separation of approximately 10 A and an intradimer separation of approximately 3.1 A. The second carboxylate bridge within the Fe2O units is defined by exogenous benzoate (2), acetate (3) or chloroacetate (4) ligands. The aqua complex [{Fe2(mu-O)L(H2O)2}2](ClO4)6 (5) is proposed to have a similar structure, but with the exogenous bridging carboxylates replaced by two terminal water ligands. These complexes exhibit electronic and Mössbauer spectral features that are similar to those of (mu-oxo)diiron(III) proteins as well as other related (mu-oxo)bis(mu-carboxylato)diiron(III) complexes. This similarity shows that these properties are not significantly affected by the nature of the bridging exogenous carboxylate, and that the octadentate framework ligand is essential in stabilizing the "dimer of dimers" structure. This structural feature remains in highly diluted solution (10(-5) M) as evidenced by electrospray ionization mass-spectroscopy (ES MS). Cyclic voltammetric studies of complexes 2 and 5 showed two irreversible two-electron reductions, indicating that the two Fe2O units of the tetranuclear complexes behave as distinct redox entities. Complexes 2, 3 and, especially, the aqua complex 5 are active alkane oxidation catalysts. Catalytic reactions carried out with alkane substrate molecules and hydrogen peroxide predominantly gave alcohols. High stereospecificity in the oxidation of cis-1,2-dimethylcyclohexane supports the metal-based molecular mechanism of O-insertion into C-H bonds postulated for non-heme iron enzymes such as methane monooxygenase.

Mono- and binuclear complexes of iron(ii) and iron(iii) with an N4O ligand: synthesis, structures and catalytic properties in alkane oxidation

Three mononuclear iron complexes and one binuclear iron complex, [Fe(tpoen)Cl].0.5(Fe2OCl6) (1), [Fe(tpoen)Cl]PF6 (2), Fe(tpoen)Cl3 (3) and [[Fe(tpoen)]2(mu-O)](ClO4)4 (4) (tpoen = N-(2-pyridylmethoxyethyl)-N,N-bis(2-pyridylmethyl)amine), were synthesized as functional models of non-heme iron oxygenases. Crystallographic studies revealed that the Fe(II) center of 1 is in a pseudooctahedral environment with a pentadentate N4O ligand and a chloride ion trans to the oxygen atom. The Fe(III) center of 3 is ligated by three nitrogen atoms of tpoen and three chloride ions in a facial configuration. Each Fe(III) center of 4 is coordinated with four nitrogen atoms and an oxygen atom of tpoen with the Fe-O-Fe angle of 172.0(3) angstroms. Complexes 2, 3 and 4 catalysed the oxidation of cyclohexane with H2O2 in the total TNs of 24-36 with A/K ratios of 1.9-2.4. Under the same conditions they also catalysed both the oxidation of ethylbenzene to benzylic alcohol and acetobenzene with good activity (30-47 TN) and low selectivity (A/K 0.7), and the oxidation of adamantane with moderate activity (15-18 TN) and low regioselectivity (3 degrees/2 degrees 3.0-3.2). With mCPBA as oxidant the catalytic activities of 2, 3 and 4 increased 1.8 to 2.3-fold for the oxidation of cyclohexane and ethylbenzene and 6.3 to 7.5-fold for the oxidation of adamantane. Drastic enhancement of the regioselectivity was observed in the oxidation of adamantane (3 degrees/2 degrees 18.5-30.3).

Molecular oxygen and oxidation catalysis by phosphovanadomolybdates

The history of aerobic catalytic oxidation mediated by a subclass of polyoxometalates, the phosphovanadomolybdates of the Keggin structure, [PV(x)Mo(12-x)O40](3+x)-, is described. In the earlier research it was shown that phosphovanadomolybdates catalyze oxydehydrogenation reactions through an electron-transfer oxidation of a substrate by the polyoxometalate that is then reoxidized by oxygen. These aerobic oxidations are selective and synthetically useful in various transformations, notably diene aromatization, phenol dimerization and alcohol oxidation. Oxygen transfer from the polyoxometalate to arenes and alkylarenes was also discussed as a homogeneous analog of a Mars-van Krevelen oxidation. "Second generation" catalysts include binary complexes of the polyoxometalate and a organometallic compound useful, for example, for methane oxidation and nanoparticles stabilized by polyoxometalates effective for aerobic alkene epoxidation.

Aqueous Ferryl(IV) Ion: Kinetics of Oxygen Atom Transfer To Substrates and Oxo Exchange with Solvent Water

The aqueous iron(IV) ion, Fe(IV)(aq)O(2+), generated from O(3) and Fe(aq)(2+), reacts rapidly with various oxygen atom acceptors (sulfoxides, a water-soluble triarylphosphine, and a thiolatocobalt complex). In each case, Fe(IV)(aq)O(2+) is reduced to Fe(aq)(2+), and the substrate is oxidized to a product expected for oxygen atom transfer. Competition methods were used to determine the kinetics of these reactions, some of which have rate constants in excess of 10(7) M(-1) s(-1). Oxidation of dimethyl sulfoxide (DMSO) has k = 1.26 x 10(5) M(-1) s(-1) and shows no deuterium kinetic isotope effect, k(DMSO-d(6)) = 1.23 x 10(5) M(-1) s(-1). The Fe(IV)(aq)O(2+)/sulfoxide reaction is the product-forming step in a very efficient Fe(aq)(2+)-catalyzed oxidation of sulfoxides by ozone. This catalytic cycle, combined with labeling experiments in H(2)(18)O, was used to determine the rate constant for the oxo-group exchange between Fe(IV)(aq)O(2+) and solvent water under acidic conditions, k(exch) = 1.4 x 10(3) s(-1).

Tris(chloranilato)ferrate(III) Anionic Building Block Containing the (Dihydroxo)oxodiiron(III) Dimer Cation: Synthesis and Characterization of [(TPA)(OH)FeIIIOFeIII(OH)(TPA)][Fe(CA)3]0.5(BF4)0.5·1.5MeOH·H2O [TPA = tris(2-pyridylmethyl)amine; CA = chloranilate]

[(TPA)(OH)FeIIIOFeIII(OH)(TPA)][Fe(CA)3]0.5(BF4)0.5.1.5MeOH.H2O (1) which possesses both the [FeIII(CA)3]3- (CA= chloranilate) and hydroxooxoiron(III) ions has had its structure determined by single-crystal X-ray diffraction. The 2-300 K magnetic susceptibility of 1 provides the magnetic parameters, g = 2.07, J/kB = -165 K (115 cm-1), theta = -1 K, and the spin impurity, rho = 0.05, which indicates a strong antiferromagnetic interaction between iron(III) ions via the oxo anion.

Synthesis, structure, and spectroscopy of an oxodiiron(II) complex

Bridging oxo species are important in the synthetic and biological chemistry of iron, and are found with iron oxidation states from +2 to +4. We report the first oxodiiron(II) complex that has been crystallographically characterized. It has been examined by NMR, IR, and Mössbauer spectroscopies as well as density-functional calculations.

Synthesis of diethynyltriptycene-linked dipyridyl ligands

[reaction: see text] An efficient route to a new family of dinucleating ligands has been developed. A convergent strategy to these ligands involved dual Sonogashira cross-coupling of 2,3-diethynyltriptycene with a variety of functionally diverse 5-bromopyridines. The resultant ligands were accessed in four steps and 40-50% overall yield from 1,2,4,5-tetrabromobenzene. Synthesis of an imidazole and a quinoline derivative by this method is also described.

High-valent nonheme iron. Two distinct iron(IV) species derived from a common iron(II) precursor

The reaction of [Fe(II)(beta-BPMCN)(OTf)2] (1, BPMCN = N,N'-bis(2-pyridylmethyl)-N,N'-dimethyl-trans-1,2-diaminocyclohexane) with tBuOOH at low-temperature yields alkylperoxoiron(III) intermediates 2 in CH2Cl2 and 2-NCMe in CH3CN. At -45 degrees C and above, 2-NCMe converts to a pale green species 3 (lambda(max) = 753 nm, epsilon = 280 M(-1) cm(-1)) in 90% yield, identified as [Fe(IV)(O)(BPMCN)(NCCH3)]2+ by comparison to other nonheme [Fe(IV)(O)(L)]2+ complexes. Below -55 degrees C in CH2Cl2, 2 decays instead to form deep turquoise 4 (lambda(max) = 656, 845 nm; epsilon = 4000, 3600 M(-1) cm(-1)), formulated to be an unprecedented alkylperoxoiron(IV) complex [Fe(IV)(BPMCN)(OH)(OOtBu)]2+ on the basis of Mössbauer, EXAFS, resonance Raman, NMR, and mass spectral evidence. The reactivity of 1 with tBuOOH in the two solvents reveals an unexpectedly rich iron(IV) chemistry that can be supported by the BPMCN ligand.

Octahedral Non-Heme Non-Oxo Fe(IV) Species Stabilized by a Redox-Innocent N-Methylated Cyclam−Acetate Ligand

The ligand 1,4,8-tri-N-methyl-1,4,8,11-tetraazacyclotetradecane-11-acetic acid (Me3cyclam-acetic acid) has been synthesized by Eschweiler-Clarke methylation of cyclam-acetic acid, and the iron(III) complex [(Me3cyclam-acetate)FeN3]PF6, 1, has been synthesized, which has been found to have significantly different properties than its unmethylated analogue, [(cyclam-acetate)FeN3]PF6, 2. Whereas the iron ion in 2 is low spin with S = 1/2, 1 is found to be high spin at temperatures above 100 K, though low-spin species are observed at lower temperatures, indicating a spin crossover phenomenon. The iron(II) species 1red is electrochemically more accessible than 2red since the Fe2+/3+ redox wave in 1 appears approximately 350 mV more positive than the corresponding wave in 2. Also, 1 displays a reversible Fe3+/4+ redox wave, which is irreversible in 2, denoting that the Fe(IV) species 1ox is kinetically stable. 1red and 1ox have been generated electrochemically in solution and studied spectroscopically. Mössbauer spectroscopy has confirmed that, in both reduction and oxidation, iron is the redox center, that 1red is high spin (S = 2), and that 1ox is low spin (S = 1), in contrast to 2red which is low spin and 2ox which could not be isolated.

Artificial diiron proteins: from structure to function

De novo protein design provides an attractive approach for the construction of models to probe the features required for the function of complex metalloproteins. These minimal models contain the essential elements believed necessary for activity of the protein. In this article, we summarize the design, structure determination, and functional properties of a family of artificial diiron proteins.

Water Affects the Stereochemistry and Dioxygen Reactivity of Carboxylate-Rich Diiron(II) Models for the Diiron Centers in Dioxygen-Dependent Non-Heme Enzymes

Carboxylate-bridged high-spin diiron(II) complexes with distinctive electronic transitions were prepared by using 4-cyanopyridine (4-NCC(5)H(4)N) ligands to shift the charge-transfer bands to the visible region of the absorption spectrum. This property facilitated quantitation of water-dependent equilibria in the carboxylate-rich diiron(II) complex, [Fe(2)(mu-O(2)CAr(Tol))(4)(4-NCC(5)H(4)N)(2)] (1), where (-)O(2)CAr(Tol) is 2,6-di-(p-tolyl)benzoate. Addition of water to 1 reversibly shifts two of the bridging carboxylate ligands to chelating terminal coordination positions, converting the structure from a paddlewheel to a windmill geometry and generating [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(4-NCC(5)H(4)N)(2)(H(2)O)(2)] (3). This process is temperature dependent in solution, rendering the system thermochromic. Quantitative treatment of the temperature-dependent spectroscopic changes over the temperature range from 188 to 298 K in CH(2)Cl(2) afforded thermodynamic parameters for the interconversion of 1 and 3. Stopped flow kinetic studies revealed that water reacts with the diiron(II) center ca. 1000 time faster than dioxygen and that the water-containing diiron(II) complex reacts with dioxygen ca. 10 times faster than anhydrous analogue 1. Addition of {H(OEt(2))(2)}{B}, where B(-) is tetrakis(3,5-di(trifluoromethyl)phenyl)borate, to 1 converts it to [Fe(2)(mu-O(2)CAr(Tol))(3)(4-NCC(5)H(4)N)(2)](B) (5), which was also structurally characterized. Mossbauer spectroscopic investigations of solid samples of 1, 3, and 5, in conjunction with several literature values for high-spin iron(II) complexes in an oxygen-rich coordination environment, establish a correlation between isomer shift, coordination number, and N/O composition. The products of oxygenating 1 in CH(2)Cl(2) were identified crystallographically to be [Fe(2)(mu-OH)(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(4-NCC(5)H(4)N)(2)].2(HO(2)CAr(Tol)) (6) and [Fe(6)(mu-O)(2)(mu-OH)(4)(mu-O(2)CAr(Tol))(6)(4-NCC(5)H(4)N)(4)Cl(2)] (7).

Hydroxylation of C-H bonds at carboxylate-bridged diiron centres

Nature uses carboxylate-bridged diiron centres at the active sites of enzymes that catalyse the selective hydroxylation of hydrocarbons to alcohols. The resting diiron(III) state of the hydroxylase component of soluble methane monooxygenase enzyme is converted by two-electron transfer from an NADH-requiring reductase into the active diiron(II) form, which subsequently reacts with O2 to generate a high-valent diiron(IV) oxo species (Q) that converts CH4 into CH3OH. In this step, C-H bond activation is achieved through a transition state having a linear C...H...O unit involving a bound methyl radical. Kinetic studies of the reaction of Q with substrates CH3X, where X=H, D, CH3, NO2, CN or OH, reveal two classes of reactivity depending upon whether binding to the enzyme or C-H bond activation is rate-limiting. Access of substrates to the carboxylate-bridged diiron active site in the hydroxylase (MMOH) occurs through a series of hydrophobic pockets. In the hydroxylase component of the closely related enzyme toluene/o-xylene monooxygenase (ToMOH), substrates enter through a wide channel in the alpha-subunit of the protein that tracks a course identical to that found in the structurally homologous MMOH. Synthetic models for the carboxylate-bridged diiron centres in MMOH and ToMOH have been prepared that reproduce the stoichiometry and key geometric and physical properties of the reduced and oxidized forms of the proteins. Reactions of the diiron(II) model complexes with dioxygen similarly generate reactive intermediates, including high-valent species capable not only of hydroxylating pendant C-H bonds but also of oxidizing phosphine and sulphide groups.

Intermediates in the oxygenation of a nonheme diiron(II) complex, including the first evidence for a bound superoxo species

The 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. A doublet peak pattern for the (16)O(18)O isotopomer of 2 in mixed-isotope Raman experiments strongly suggests that the superoxide ligand of 2 is bound end-on. This first example of a nonheme iron-superoxo intermediate exhibits the highest frequency nu(O-O) yet observed for a biomimetic metal-dioxygen adduct. The bound superoxide of 2, unlike the bound peroxide of 4, is readily reduced by 2,4-di-tert-butylphenol via a proton-coupled electron-transfer mechanism, emphasizing that metal-superoxo species may serve as oxidants in oxygen activation mechanisms of metalloenzymes. The discovery of intermediates 2 and 3 allows us to dissect the initial steps of dioxygen binding at a diiron center leading to its activation for substrate oxidation.

Betaine-Induced Assembly of Neutral Infinite Columns and Chains of Linked Silver(I) Polyhedra with Embedded Acetylenediide

Ten polymeric silver(I) double salts containing embedded acetylenediide: [(Ag2C2)2(AgCF3CO2)9(L1)3] (1), [(Ag2C2)2(AgCF3CO2)10(L2)3]H2O (2), [(Ag2C2)(AgCF3CO2)4(L3)(H2O)]0.75 H2O (3), [(Ag2C2)(1.5)(AgCF3CO2)7(L4)2] (4), [(Ag2C2)(AgCF3CO2)7(L5)2(H2O)] (5), [(Ag2C2) (AgC2F5CO2)7(L1)3(H2O)] (6), [(Ag2C2)(AgCF3CO2)7(L1)3(H2O)]2 H2O (7), [(Ag2C2)(AgC2F5CO2)6(L3)2] (8), [(Ag2C2)2(AgC2F5CO2)12(L4)2(H2O)4]H2O (9), and [(Ag2C2)(AgCF3CO2)6(L3)2(H2O)]H2O (10) have been isolated by varying the types of betaines, the perfluorocarboxylate ligands employed, and the reaction conditions. Single-crystal X-ray analysis has shown that 1-4 all have a columnar structure composed of fused silver(I) double cages, with C2(2-) species embedded in its stem and an exterior coat comprising anionic and zwitterionic carboxylates. For 5 and 6, single silver(I) cages are linked into a beaded chain through both types of carboxylate ligands. In 7, two different coordination modes of L1 connect the silver(I) polyhedra into a chain. For 8, the mu(2)-O,O' coordination mode of L3 connects the silver(I) double cages into a chain. Compound 9 exhibits a two-dimensional architecture generated from the cross-linkage of double cages by C2F5CO2-, L4, and [Ag2(C2F5CO2)2] units. Similar to 9, 10 is also a two-dimensional structure, which is formed by connecting the chains of linked double cages through [Ag2(CF3CO2)2] bridging.

Origin of the Unusual Kinetics of Iron Deposition in Human H-Chain Ferritin

From microorganisms to humans, ferritin plays a central role in the biological management of iron. The ferritins function as iron storage and detoxification proteins by oxidatively depositing iron as a hydrous ferric hydroxide mineral core within their shell-like structures. The mechanism by which the mineral core is formed has been the subject of intense investigation for many years. A diiron ferroxidase site located on the H-chain subunit of vertebrate ferritins catalyzes the oxidation of Fe(II) to Fe(III) by molecular oxygen. A previous stopped-flow kinetics study of a transient mu-peroxodiFe(III) intermediate formed at this site revealed very unusual kinetics curves, the shape of which depended markedly on the amount of iron presented to the protein. In the present work, a mathematical model for catalysis is developed that explains the observed kinetics. The model consists of two sequential mechanisms. In the first mechanism, turnover of iron at the ferroxidase site is rapid, resulting in steady-state production of the peroxo intermediate with continual formation of the mineral core until the available Fe(II) in solution is consumed. At this point, the second mechanism comes into play whereby the peroxo intermediate decays and the ferroxidase site is postulated to vacate its complement of iron. The kinetic data reveal for the first time that Fe(II) in excess of that required to saturate the ferroxidase site promotes rapid turnover of Fe(III) at this site and that the ferroxidase site plays a role in catalysis at all levels of iron loading of the protein (48-800 Fe/protein). The data also provide evidence for a second intermediate, a putative hydroperoxodiFe(III) complex, that is a decay product of the peroxo intermediate.

Dioxygen Activation and Catalytic Aerobic Oxidation by a Mononuclear Nonheme Iron(II) Complex

We have used dioxygen, not artificial oxidants such as peracids, iodosylarenes, and hydroperoxides, in the generation of a mononuclear nonheme oxoiron(IV) complex, [Fe(IV)(TMC)(O)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), from its corresponding Fe(II) complex, [Fe(TMC)(CF3SO3)2]. The formation of oxoiron(IV) species by activating dioxygen was markedly dependent on iron(II) complexes and solvents, and this observation was interpreted with the electronic effect of iron(II) complexes on dioxygen activation to form oxoiron(IV) species. A catalytic aerobic oxidation of organic substrates was demonstrated in the presence of the [Fe(TMC)]2+ complex. By carrying out 18O-labeled water experiment, we were able to conclude that the oxidation of organic substrates was mediated by an oxoiron(IV) intermediate, not by a radical type of autoxidation process.

Water-dependent reactions of diiron(II) carboxylate complexes

Carboxylate-rich diiron(II) compounds with varying numbers of water ligands have been characterized, including the first complex with a {Fe2(mu-OH2)2(mu-O2CArTol)}3+ unit. The isolation of these complexes reveals how water can alter the structural properties of carboxylate-bridged diiron(II) core similar to those that occur in a variety of dioxygen-activating metalloenzyme cores. Mössbauer and variable temperature, variable field magnetic susceptibility experiments indicate that the compound [Fe2(mu-OH2)2(mu-O2CAr4F-Ph)(O2CAr4F-Ph)3(THF)2(OH2)] has a high-spin diiron(II) core with little significant magnetic exchange coupling.

A Late-Transition Metal Oxo Complex: K 7 Na 9 [O=Pt IV (H 2 O)L 2 ], L = [PW 9 O 34 ] 9-

Terminal mono-oxo complexes of the late transition metal elements have long been considered too unstable to synthesize because of repulsion between the oxygen electrons and the mostly filled metal d orbitals. A platinum(IV)-oxo compound flanked by two polytungstate ligands, K7Na9[O=Pt(H2O)L2], L = [PW9O34(9-)], has now been prepared and isolated at room temperature as air-stable brown crystals. X-ray and neutron diffraction at 30 kelvin revealed a very short [1.720(18) angstrom] Pt-O bond and no evidence of a hydrogen atom at the terminal oxygen, ruling out a better precedented Pt-OH complex. Density functional theory and spectroscopic data account for the stability of the Pt(IV)-oxo unit by electron withdrawal into delocalized orbitals of the polytungstates.

Reactivity of Aqueous Fe(IV) in Hydride and Hydrogen Atom Transfer Reactions

Oxidation of cyclobutanol by aqueous Fe(IV) generates cyclobutanone in approximately 70% yield. In addition to this two-electron process, a smaller fraction of the reaction takes place by a one-electron process, believed to yield ring-opened products. A series of aliphatic alcohols, aldehydes, and ethers also react in parallel hydrogen atom and hydride transfer reactions, but acetone and acetonitrile react by hydrogen atom transfer only. Precise rate constants for each pathway for a number of substrates were obtained from a combination of detailed kinetics and product studies and kinetic simulations. Solvent kinetic isotope effect for the self-decay of Fe(IV), kH2O/kD2O = 2.8, is consistent with hydrogen atom abstraction from water.

Reactivity of a (μ-Oxo)(μ-hydroxo)diiron(III) Diamond Core with Water, Urea, Substituted Ureas, and Acetamide

A series of iron(III) complexes of the tetradentate ligand BPMEN (N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)ethane-1,2-diamine) were prepared and structurally characterized. Complex [Fe(2)(mu-O)(mu-OH)(BPMEN)(2)](ClO(4))(3) (1) contains a (mu-oxo)(mu-hydroxo)diiron(III) diamond core. Complex [Fe(BPMEN)(urea)(OEt)](ClO(4))(2) (2) is a rare example of a mononuclear non-heme iron(III) alkoxide complex. Complexes [Fe(2)(mu-O)(mu-OC(NH(2))NH)(BPMEN)(2)](ClO(4))(3) (3) and [Fe(2)(mu-O)(mu-OC(NHMe)NH)(BPMEN)(2)](ClO(4))(3) (4) feature N,O-bridging deprotonated urea ligands. The kinetics and equilibrium of the reactions of 1 with ligands L (L = water, urea, 1-methylurea, 1,1-dimethylurea, 1,3-dimethylurea, 1,1,3,3-tetramethylurea, and acetamide) in acetonitrile solutions were studied by stopped-flow UV-vis spectrophotometry, NMR, and mass spectrometry. All these ligands react with 1 in a rapid equilibrium, opening the four-membered Fe(III)(mu-O)(mu-OH)Fe(III) core and forming intermediates with a (HO)Fe(III)(mu-O)Fe(III)(L) core. The entropy and enthalpy for urea binding through oxygen are DeltaH degrees = -25 kJ mol(-1) and DeltaS degrees = -53.4 J mol(-1) K(-1) with an equilibrium constant of K(1) = 37 L mol(-1) at 25 degrees C. Addition of methyl groups on one of the urea nitrogen did not affect this reaction, but the addition of methyl groups on both nitrogens considerably decreased the value of K(1). An opening of the hydroxo bridge in the diamond core complex [Fe(2)(mu-O)(mu-OH)(BPMEN)(2)] is a rapid associative process, with activation enthalpy of about 60 kJ mol(-1) and activation entropies ranging from -25 to -43 J mol(-1) K(-1). For the incoming ligands with the -CONH(2) functionality (urea, 1-methylurea, 1,1-dimethylurea, and acetamide), a second, slow step occurs, leading to the formation of stable N,O-coordinated amidate diiron(III) species such as 3 and 4. The rate of this ring-closure reaction is controlled by the steric bulk of the incoming ligand and by the acidity of the amide group.

Oxidation of Sulfide, Phosphine, and Benzyl Substrates Tethered to N-Donor Pyridine Ligands in Carboxylate-Bridged Diiron(II) Complexes

Substituted pyridines were employed to prepare a series of terphenylcarboxylate-bridged diiron(II) compounds to mimic aspects of the chemistry at the active sites of bacterial multicomponent monooxygenases, including soluble methane monooxygenase (sMMO) and toluene monooxygenase (ToMO). Complexes of general formula [Fe2(O2CArTol)4L], L = 2, 3, or 4-pyridyldiphenylphosphine, 2-pyridylphenylsulfide, or 2-benzylpyridine and ArTol = 2,6-di(p-tolyl)benzoate, were synthesized and characterized by X-ray crystallography. Upon exposure of these compounds to dioxygen, ligand oxidation ensued and, in one case, proceeded catalytically.

Enhanced selectivity in non-heme iron catalysed oxidation of alkanes with peracids: evidence for involvement of Fe(iv)O species

Catalytic alkane oxidation with high selectivity using peracids and an (N4Py)Fe complex is presented and the role of [(N4Py)Fe(IV)=O]2+ species, molecular oxygen and hydroxyl radicals in the catalysis is discussed.

Mechanistic studies on oxidation of hydrazine by a µ-oxo diiron(iii,iii) complex in aqueous acidic media—proton coupled electron transfer

[Fe2(mu-O)(phen)4(H2O)2]4+ (1), one of the simplest mu-oxo diiron(III) complexes, quantitatively oxidises hydrazine to dinitrogen and itself is reduced to two moles of ferroin, [Fe(phen)3]2+ in presence of excess phenanthroline. The weak dibasic acid, 1 (pKa1= 3.71 +/- 0.05 and pKa2= 5.28 +/- 0.10 at 25.0 degrees C, I= 1.0 mol dm(-3)(NaNO3)) and its conjugate bases, [Fe2(mu-O)(phen)4(H2O)(OH)]3+ (2) and [Fe2(mu-O)(phen)4(OH)2]2+ (3) are involved in the redox process with the reactivity order 1 > 2 > 3 whereas N2H4 and not N2H5+ was found to be reactive in the pH interval studied 3.45-5.60. Cyclic voltammetric studies indicate poor oxidizing capacity of the title substitution-labile diiron complex, yet it oxidizes N2H4 with a moderate rate--a proton coupled electron transfer (1e, 1H+) drags the energetically unfavourable reaction to completion. The rate retardation in D2O media is substantially higher at higher pH due to the increasing basicity of the oxo-ligand in the order 3 > 2 > 1. Marcus calculations result an unacceptably high one-electron self-exchange rate for the iron center indicating an inner-sphere nature of the electron-transfer.