Genqiang Xue

Million-fold activation of the [Fe2(μ-O)2] diamond core for C-H bond cleavage

In biological systems, the cleavage of strong C–H bonds is often carried out by iron centers – such as the methane monooxygenase in methane hydroxylation – through dioxygen activation mechanisms. High valent species with [Fe2(μ-O)2] diamond cores are thought to act as the oxidizing moieties, but the synthesis of complexes that cleave strong C–H bonds efficiently has remained a challenge. We report here the conversion of a synthetic complex with a valence-delocalized [Fe3.5(μ-O)2Fe3.5]3+ diamond core (1) into a complex with a valence-localized [HO-FeIII-O-FeIV=O]2+ open core (4), which cleaves C–H bonds over million-fold faster. This activity enhancement results from three factors: the formation of a terminal oxoiron(IV) moiety, the conversion of the low-spin (S = 1) FeIV=O center to a high-spin (S = 2) center, and the concentration of the oxidizing capability to the active terminal oxoiron(IV) moiety. This suggests that similar isomerization strategies might be employed by nonheme diiron enzymes.

A synthetic precedent for the [FeIV2(μ-O)2] diamond core proposed for methane monooxygenase intermediate Q

Intermediate Q, the methane-oxidizing species of soluble methane monooxygenase, is proposed to have an [FeIV2(μ-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°C on a valence-delocalized [FeIIIFeIV(μ-O)2(Lb)2]3+ complex (1b) (E1/2 = 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 λmax at 485 nm (9,800 M−1·cm−1) and 875 nm (2,200 M−1·cm−1). The 4.2 K Mössbauer spectrum of 2b exhibits a quadrupole doublet with δ = −0.04(1) mm·s−1 and ΔEQ = 2.09(2) mm·s−1, parameters typical of an iron(IV) center. The Mössbauer patterns observed in strong applied fields show that 2b is an antiferromagnetically coupled diiron(IV) center. Resonance Raman studies reveal the diagnostic vibration mode of the [Fe2(μ-O)2] core at 674 cm−1, downshifting 30 cm−1 upon 18O labeling. Extended x-ray absorption fine structure (EXAFS) analysis shows two O/N scatterers at 1.78 Å and an Fe scatterer at 2.73 Å. Based on the accumulated spectroscopic evidence, 2b thus can be formulated as [FeIV2(μ-O)2(Lb)2]4+, the first synthetic complex with an [FeIV2(μ-O)2] core. A comparison of 2b and its mononuclear analog [FeIV(O)(Lb)(NCMe)]2+ (4b) reveals that 4b is 100-fold more reactive than 2b in oxidizing weak CH bonds. This surprising observation may shed further light on how intermediate Q carries out the hydroxylation of methane.

Insights into the P-to-Q conversion in the catalytic cycle of methane monooxygenase from a synthetic model system

For the catalytic cycle of soluble methane monooxygenase (sMMO), it has been proposed that cleavage of the O–O bond in the (μ-peroxo)diiron(III) intermediate P gives rise to the diiron(IV) intermediate Q with an Fe2(μ–O)2 diamond core, which oxidizes methane to methanol. As a model for this conversion, (μ–oxo) diiron(III) complex 1 ([FeIII2(μ–O)(μ–O2H3)(L)2]3+, L = tris(3,5-dimethyl-4-methoxypyridyl-2-methyl)amine) has been treated consecutively with one eq of H2O2 and one eq of HClO4 to form 3 ([FeIV2(μ–O)2(L)2]4+). In the course of this reaction a new species, 2, can be observed before the protonation step; 2 gives rise to a cationic peak cluster by ESI-MS at m/z 1,399, corresponding to the {[Fe2O3L2H](OTf)2}+ ion in which 1 oxygen atom derives from 1 and the other two originate from H2O2. Mössbauer studies of 2 reveal the presence of two distinct, exchange coupled iron(IV) centers, and EXAFS fits indicate a short Fe–O bond at 1.66 Å and an Fe–Fe distance of 3.32 Å. Taken together, the spectroscopic data point to an HO-FeIV-O-FeIV = O core for 2. Protonation of 2 results in the loss of H2O and the formation of 3. Isotope labeling experiments show that the [FeIV2(μ–O)2] core of 3 can incorporate both oxygen atoms from H2O2. The reactions described here serve as the only biomimetic precedent for the conversion of intermediates P to Q in the sMMO reaction cycle and shed light on how a peroxodiiron(III) unit can transform into an [FeIV2(μ–O)2] core.

Substrate-Triggered Activation of a Synthetic [Fe2(μ-O)2] Diamond Core for C–H Bond Cleavage

An [Fe(IV)(2)(μ-O)(2)] diamond core structure has been postulated for intermediate Q of soluble methane monooxygenase (sMMO-Q), the oxidant responsible for cleaving the strong C-H bond of methane and its hydroxylation. By extension, analogous species may be involved in the mechanisms of related diiron hydroxylases and desaturases. Because of the paucity of well-defined synthetic examples, there are few, if any, mechanistic studies on the oxidation of hydrocarbon substrates by complexes with high-valent [Fe(2)(μ-O)(2)] cores. We report here that water or alcohol substrates can activate synthetic [Fe(III)Fe(IV)(μ-O)(2)] complexes supported by tetradentate tris(pyridyl-2-methyl)amine ligands (1 and 2) by several orders of magnitude for C-H bond oxidation. On the basis of detailed kinetic studies, it is postulated that the activation results from Lewis base attack on the [Fe(III)Fe(IV)(μ-O)(2)] core, resulting in the formation of a more reactive species with a [X-Fe(III)-O-Fe(IV)═O] ring-opened structure (1-X, 2-X, X = OH(-) or OR(-)). Treatment of 2 with methoxide at -80 °C forms the 2-methoxide adduct in high yield, which is characterized by an S = 1/2 EPR signal indicative of an antiferromagnetically coupled [S = 5/2 Fe(III)/S = 2 Fe(IV)] pair. Even at this low temperature, the complex undergoes facile intramolecular C-H bond cleavage to generate formaldehyde, showing that the terminal high-spin Fe(IV)═O unit is capable of oxidizing a C-H bond as strong as 96 kcal mol(-1). This intramolecular oxidation of the methoxide ligand can in fact be competitive with intermolecular oxidation of triphenylmethane, which has a much weaker C-H bond (D(C-H) 81 kcal mol(-1)). The activation of the [Fe(III)Fe(IV)(μ-O)(2)] core is dramatically illustrated by the oxidation of 9,10-dihydroanthracene by 2-methoxide, which has a second-order rate constant that is 3.6 × 10(7)-fold larger than that for the parent diamond core complex 2. These observations provide strong support for the DFT-based notion that an S = 2 Fe(IV)═O unit is much more reactive at H-atom abstraction than its S = 1 counterpart and suggest that core isomerization could be a viable strategy for the [Fe(IV)(2)(μ-O)(2)] diamond core of sMMO-Q to selectively attack the strong C-H bond of methane in the presence of weaker C-H bonds of amino acid residues that define the diiron active site pocket.

Mössbauer and DFT Study of the Ferromagnetically Coupled Diiron(IV) Precursor to a Complex with an FeIV2O2 Diamond Core

Recently, we reported the reaction of the (mu-oxo)diiron(III) complex 1 ([Fe(III)(2)(mu-O)(mu-O(2)H(3))(L)(2)](3+), L = tris(3,5-dimethyl-4-methoxypyridyl-2-methyl)amine) with 1 equiv of H(2)O(2) to yield a diiron(IV) intermediate, 2 (Xue, G.; Fiedler, A. T.; Martinho, M.; Munck, E.; Que, L., Jr. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 20615-20). Upon treatment with HClO(4), complex 2 converted to a species with an Fe(IV)(2)(mu-O)(2) diamond core that serves as the only synthetic model to date for the diiron(IV) core proposed for intermediate Q of soluble methane monooxygenase. Here we report detailed Mossbauer and density functional theory (DFT) studies of 2. The Mossbauer studies reveal that 2 has distinct Fe(IV) sites, a and b. Studies in applied magnetic fields show that the spins of sites a and b (S(a) = S(b) = 1) are ferromagnetically coupled to yield a ground multiplet with S = 2. Analysis of the applied field spectra of the exchange-coupled system yields for site b a set of parameters that matches those obtained for the mononuclear [LFe(IV)(O)(NCMe)](2+) complex, showing that site b (labeled Fe(O)) has a terminal oxo group. Using the zero-field splitting parameters of [LFe(IV)(O)(NCMe)](2+) for our analysis of 2, we obtained parameters for site a that closely resemble those reported for the nonoxo Fe(IV) complex [(beta-BPMCN)Fe(IV)(OH)(OO(t)Bu)](2+), suggesting that a (labeled Fe(OH)) coordinates a hydroxo group. A DFT optimization performed on 2 yielded an Fe-Fe distance of 3.39 A and an Fe-(mu-O)-Fe angle of 131 degrees , in good agreement with the results of our previous EXAFS study. The DFT calculations reproduce the Mossbauer parameters (A-tensors, electric field gradient, and isomer shift) of 2 quite well, including the observation that the largest components of the electric field gradients of Fe(O) and Fe(OH) are perpendicular. The ferromagnetic behavior of 2 seems puzzling given that the Fe-(mu-O)-Fe angle is large but can be explained by noting that the orbital structures of Fe(O) and Fe(OH) are such that the unpaired electrons at the two sites delocalize into orthogonal orbitals at the bridging oxygen, rationalizing the ferromagnetic behavior of 2. Thus, inequivalent coordinations at Fe(O) and Fe(OH) define magnetic orbitals favorable for ferromagnetic ineractions.

Evaluating the identity and diiron core transformations of a (μ-Oxo)diiron(III)complex supported by electron-rich tris(pyridyl-2-methyl) amine ligands

The composition of a (μ-oxo)diiron(III) complex coordinated by tris[(3,5-dimethyl-4-methoxy)pyridyl-2-methyl]amine (R(3)TPA) ligands was investigated. Characterization using a variety of spectroscopic methods and X-ray crystallography indicated that the reaction of iron(III) perchlorate, sodium hydroxide, and R(3)TPA affords [Fe(2)(μ-O)(μ-OH)(R(3)TPA)(2)](ClO(4))(3) (2) rather than the previously reported species [Fe(2)(μ-O)(OH)(H(2)O)(R(3)TPA)(2)](ClO(4))(3) (1). Facile conversion of the (μ-oxo)(μ-hydroxo)diiron(III) core of 2 to the (μ-oxo)(hydroxo)(aqua)diiron(III) core of 1 occurs in the presence of water and at low temperature. When 2 is exposed to wet acetonitrile at room temperature, the CH(3)CN adduct is hydrolyzed to CH(3)COO(-), which forms the compound [Fe(2)(μ-O)(μ-CH(3)COO)(R(3)TPA)(2)](ClO(4))(3) (10). The identity of 10 was confirmed by comparison of its spectroscopic properties with those of an independently prepared sample. To evaluate whether or not 1 and 2 are capable of generating the diiron(IV) species [Fe(2)(μ-O)(OH)(O)(R(3)TPA)(2)](3+) (4), which has previously been generated as a synthetic model for high-valent diiron protein oxygenated intermediates, studies were performed to investigate their reactivity with hydrogen peroxide. Because 2 reacts rapidly with hydrogen peroxide in CH(3)CN but not in CH(3)CN/H(2)O, conditions that favor conversion to 1, complex 1 is not a likely precursor to 4. Compound 4 also forms in the reaction of 2 with H(2)O(2) in solvents lacking a nitrile, suggesting that hydrolysis of CH(3)CN is not involved in the H(2)O(2) activation reaction. These findings shed light on the formation of several diiron complexes of electron-rich R(3)TPA ligands and elaborate on conditions required to generate synthetic models of diiron(IV) protein intermediates with this ligand framework.

Hydrogen-bonding effects on the reactivity of [X-Fe(III)-O-Fe(IV)═O] (X = OH, F) complexes toward C-H bond cleavage.

Complexes 1-OH and 1-F are related complexes that share similar [X-Fe(III)-O-Fe(IV)═O](3+) core structures with a total spin S of ½, which arises from antiferromagnetic coupling of an S = 5/2 Fe(III)-X site and an S = 2 Fe(IV)═O site. EXAFS analysis shows that 1-F has a nearly linear Fe(III)-O-Fe(IV) core compared to that of 1-OH, which has an Fe-O-Fe angle of ~130° due to the presence of a hydrogen bond between the hydroxo and oxo groups. Both complexes are at least 1000-fold more reactive at C-H bond cleavage than 2, a related complex with a [OH-Fe(IV)-O-Fe(IV)═O](4+) core having individual S = 1 Fe(IV) units. Interestingly, 1-F is 10-fold more reactive than 1-OH. This raises an interesting question about what gives rise to the reactivity difference. DFT calculations comparing 1-OH and 1-F strongly suggest that the H-bond in 1-OH does not significantly change the electrophilicity of the reactive Fe(IV)═O unit and that the lower reactivity of 1-OH arises from the additional activation barrier required to break its H-bond in the course of H-atom transfer by the oxoiron(IV) moiety.

Spectroscopic and theoretical investigation of a complex with an [O═Fe(IV)-O-Fe(IV)═O] core related to methane monooxygenase intermediate Q.

Previous efforts to model the diiron(IV) intermediate Q of soluble methane monooxygenase have led to the synthesis of a diiron(IV) TPA complex, 2, with an O=Fe(IV)-O-Fe(IV)-OH core that has two ferromagnetically coupled Sloc = 1 sites. Addition of base to 2 at -85 °C elicits its conjugate base 6 with a novel O═Fe(IV)-O-Fe(IV)═O core. In frozen solution, 6 exists in two forms, 6a and 6b, that we have characterized extensively using Mössbauer and parallel mode EPR spectroscopy. The conversion between 2 and 6 is quantitative, but the relative proportions of 6a and 6b are solvent dependent. 6a has two equivalent high-spin (Sloc = 2) sites, which are antiferromagnetically coupled; its quadrupole splitting (0.52 mm/s) and isomer shift (0.14 mm/s) match those of intermediate Q. DFT calculations suggest that 6a assumes an anti conformation with a dihedral O═Fe-Fe═O angle of 180°. Mössbauer and EPR analyses show that 6b is a diiron(IV) complex with ferromagnetically coupled Sloc = 1 and Sloc = 2 sites to give total spin St = 3. Analysis of the zero-field splittings and magnetic hyperfine tensors suggests that the dihedral O═Fe-Fe═O angle of 6b is ∼90°. DFT calculations indicate that this angle is enforced by hydrogen bonding to both terminal oxo groups from a shared water molecule. The water molecule preorganizes 6b, facilitating protonation of one oxo group to regenerate 2, a protonation step difficult to achieve for mononuclear Fe(IV)═O complexes. Complex 6 represents an intriguing addition to the handful of diiron(IV) complexes that have been characterized.

Mössbauer, Electron Paramagnetic Resonance, and Density Functional Theory Studies of Synthetic S = 1/2 FeIII−O−FeIV═O Complexes. Superexchange-Mediated Spin Transition at the FeIV═O Site

Previously we have characterized two high-valent complexes [LFe(IV)(μ-O)(2)Fe(III)L], 1, and [LFe(IV)(O)(μ-O)(OH) Fe(IV)L], 4. Addition of hydroxide or fluoride to 1 produces two new complexes, 1-OH and 1-F. Electron paramagnetic resonance (EPR) and Mössbauer studies show that both complexes have an S = 1/2 ground state which results from antiferromagnetic coupling of the spins of a high-spin (S(a) = 5/2) Fe(III) and a high-spin (S(b) = 2) Fe(IV) site. 1-OH can also be obtained by a 1-electron reduction of 4, which has been shown to have an Fe(IV)═O site. Radiolytic reduction of 4 at 77 K yields a Mössbauer spectrum identical to that observed for 1-OH, showing that the latter contains an Fe(IV)═O. Interestingly, the Fe(IV)═O moiety has S(b) = 1 in 4 and S(b) = 2 in 1-OH and 1-F. From the temperature dependence of the S = 1/2 signal we have determined the exchange coupling constant J (ℋ = JS(a)·S(b) convention) to be 90 ± 20 cm(-1) for both 1-OH and 1-F. Broken-symmetry density functional theory (DFT) calculations yield J = 135 cm(-1) for 1-OH and J = 104 cm(-1) for 1-F, in good agreement with the experiments. DFT analysis shows that the S(b) = 1 → S(b) = 2 transition of the Fe(IV)═O site upon reduction of the Fe(IV)-OH site to high-spin Fe(III) is driven primarily by the strong antiferromagnetic exchange in the (S(a) = 5/2, S(b) = 2) couple.

Cyclohexene as a substrate probe for the nature of the high-valent iron-oxo oxidant in Fe(TPA)-catalyzed oxidations

Cyclohexene is shown to be a versatile substrate probe in shedding light on the nature of the high-valent oxidant generated by bio-inspired nonheme iron catalysts. Cyclohexene provides the oxidant a choice to attack an allylic C–H bond or a CC bond, and the selectivity observed should depend on the electronic properties of the oxidant. Allylic oxidation products are predominantly observed in the reaction with [FeIV(O)(TPA)(NCCH3)]2+ (TPA = tris(2-pyridylmethyl)amine), while epoxide and cis-diol are the major products found in the [FeII(TPA)(NCCH3)2]2+-catalyzed reaction with H2O2. This difference suggests that the oxidant generated in the latter case must be distinct from [FeIV(O)(TPA)(NCCH3)]2+ and supports an oxoiron(V)-hydroxo species that has been proposed as the oxidant.