Diiron System Research Articles

Predicting Mössbauer Parameters of Nonheme Diiron Complexes with Density Functional Theory.

Mössbauer spectroscopy provides significant insights into the electronic structure and environment of the metal centers. Herein, we investigate the electronic structures of a set of nonheme diiron complexes by evaluating two key parameters pertaining to Mössbauer spectroscopy, namely, the isomer shift (δ) and quadrupole splitting (|ΔEQ|), using different levels of density functional theory (DFT). The diiron systems investigated here span diverse oxidation states, bridging motifs, and spin coupling patterns, which present a challenging case for theoretical predictions. We demonstrate that the combination of B97-D3/def2-TZVP is an efficient approach in modeling both the δ and |ΔEQ| values with high accuracy for the representative nonheme diiron complexes. We also show that δ is accurately predicted irrespective of the choice of approximate density functional while the |ΔEQ| is sensitive to the level of theory employed. Further investigation shows that the present methodology assessed using synthetic nonheme diiron complexes could be extended to nonheme diiron enzyme active sites, featuring both ferromagnetic and antiferromagnetic coupling between the iron centers.

Self-adaptive Metal-Organic Framework Assembles Di-iron Active Sites to Mimic Monooxygenases.

Despite significant efforts, it remains a challenge to design artificial enzymes that can mimic both structures and functions of natural enzymes. Here, we report the post-synthetic construction of binuclear iron catalysts in MOF-253 to mimic natural di-iron monooxygenases. The adjacent bipyridyl (bpy) linkers in MOF-253 can freely rotate to form the [(bpy)FeIII(μ2-OH)]2 active site in a self-adaptive fashion. The composition and structure of the [(bpy)FeIII(μ2-OH)]2 active sites in MOF-253 were characterized by a combination of inductively coupled plasma-mass spectrometry, thermogravimetric analysis, X-ray absorption spectrometry, and Fourier-transform infrared spectroscopy. The MOF-based artificial monooxygenase effectively catalyzed oxidative transformations of organic compounds, including C-H oxidation and alkene epoxidation reactions, using O2 as the only oxidant, which indicates the successful recapitulation of the structure and functions of natural monooxygenases using readily accessible MOFs. The di-iron system exhibited at least 27 times higher catalytic activity than the corresponding mononuclear control. DFT calculations showed that the binuclear system had a 14.2 kcal/mol lower energy barrier than the mononuclear system in the rate-determining C-H activation process, suggesting the importance of cooperativity of the iron centers in the [(bpy)FeIII(μ2-OH)]2 active site in the rate-determining step. The stability and recyclability of the MOF-based artificial monooxygenase were also demonstrated.

Mimicking the Constrained Geometry of a Nitrogen-Fixation Intermediate.

Both biological and industrial nitrogen reduction catalysts activate N2 at multinuclear binding sites with constrained Fe-Fe distances. This contrasts with molecular diiron systems, which routinely form linear N2 bridges to minimize steric interactions. Model compounds that capture the salient geometric features of N2 binding by the nitrogenase enzymes and Mittasch catalysts would contribute to understanding their high N2-reduction activity. It is shown in the present study that use of a geometrically flexible, dinucleating macrocycle allows for the formation of a bridging N2 ligand with an unusual Fe-CtN2-Fe angle of 150° (CtN2 = centroid of N2), a geometry that approximates the α-N2 binding mode on Fe(111) surfaces that precedes N2 bond cleavage. The cavity size of the macrocycle prevents the formation of a linear Fe-N2-Fe unit and leads to orbital interactions that are distinct from those available to the linear configuration.

A New Route for Dioxygen Activation Uncovered from Quantum Mechanics Investigations of X‐Ray‐Diffraction‐Captured Intermediates of the Ferroxidase Reaction of Ferritins from Gram‐Negative Bacteria

AbstractFollowing the recent reassessment from X‐ray‐diffraction‐caught short‐path uptake to wide permeation of the S20A mutant Escherichia coli ferritin by ferrous ions, what happens when dioxygen gets to the diiron system is investigated. It is shown that the previously X‐ray‐caught situation of an unbound‐dioxygen diferrous state actually is an ephemeral state that changes into a P‐type peroxodiferric state, while a water molecule moves to bridge the two ferric ions, in what appears to be a remarkable redox process. All this stems from density functional theory (DFT)‐based quantum mechanics‐molecular mechanics and broken symmetry (BS) calculations that are extended to X‐ray‐diffraction‐caught oxodiferric and I‐type peroxodiferric systems from other subunits of the same crystal structure, validating them while determining antiferromagnetic coupling with all such diiron systems except the oxodiferric one. The latter converged, on BS, to ferromagnetic coupling under a wide variety of hybrid DFT functionals and basis sets. The trend of the Heisenberg J coupling reflects and substantiates the structures of the diiron core.

Spin-State Ordering in Metal-Based Compounds Using the Localized Active Space Self-Consistent Field Method.

Quantitatively accurate calculations for spin-state ordering in transition-metal complexes typically demand a robust multiconfigurational treatment. The poor scaling of such methods with increasing size makes them impractical for large, strongly correlated systems. Density matrix embedding theory (DMET) is a fragmentation approach that can be used to specifically address this challenge. The single-determinantal bath framework of DMET is applicable in many situations, but it has been shown to perform poorly for molecules characterized by strong correlation when a multiconfigurational self-consistent field solver is used. To ameliorate this problem, the localized active space self-consistent field (LASSCF) method was recently described. In this work, LASSCF is applied to predict spin-state energetics in mono- and di-iron systems, and we show that the model offers an accuracy equivalent to that of CASSCF but at a substantially lower computational cost. Performance as a function of basis set and active space is also examined.

Mechanism of N-N Bond Formation by Transition Metal-Nitrosyl Complexes: Modeling Flavodiiron Nitric Oxide Reductases.

Nitric oxide (NO) has a number of important biological functions, including nerve signaling transduction, blood pressure control, and, at higher concentrations, immune defense. A number of pathogenic bacteria have developed methods of degrading this toxic molecule through the use of flavodiiron nitric oxide reductases (FNORs), which utilize a nonheme diiron active site to reduce NO → N2O. The well-characterized diiron model complex [Fe2(BPMP)(OPr)(NO)2]2+ (BPMP- = 2,6-bis[(bis(2- pyridylmethyl)amino)methyl]-4-methylphenolate), which mimics both the active site structure and reactivity of these enzymes, offers key insight into the mechanism of FNORs. Presently, we have used computational methods to elucidate a coherent reaction mechanism that shows how one and two-electron reduction of this complex induces N-N bond formation and N2O generation, while the parent complex remains stable. The initial formation of a N-N bond to generate hyponitrite (N2O22-) follows a radical-type coupling mechanism, which requires strong Fe-NO π-interactions to be overcome to effectively oxidize the iron centers. Hyponitrite formation provides the largest activation barrier with Δ G‡ = 7-8 kcal/mol (average of several functionals) in the two-electron, super-reduced mechanism. This is followed by the formation of a N2O22- complex with a novel binding mode for nonheme diiron systems, allowing for the facile release of N2O with the assistance of a carboxylate shift. This provides sufficient thermodynamic driving force for the reaction to proceed via N2O formation alone. Surprisingly, the one-electron "semireduced" mechanism is predicted to be competitive with the super-reduced mechanism. This is due to the asymmetry imparted by the BPMP- ligand, allowing a one-electron reduction to overcome one of the primary Fe-NO π-interactions. Generally, mediation of N2O formation by high-spin [{M-NO-}]2 cores depends on the ease of oxidizing the M centers and breaking of the M-NO π-bonds to formally generate a "full" 3NO- unit, allowing for the critical step of N-N bond formation to proceed (via a radical-type coupling mechanism).

Unprecedented (μ-1,1-Peroxo)diferric Structure for the Ambiphilic Orange Peroxo Intermediate of the Nonheme N-Oxygenase CmlI.

The final step in the biosynthesis of the antibiotic chloramphenicol is the oxidation of an aryl-amine substrate to an aryl-nitro product catalyzed by the N-oxygenase CmlI in three two-electron steps. The CmlI active site contains a diiron cluster ligated by three histidine and four glutamate residues and activates dioxygen to perform its role in the biosynthetic pathway. It was previously shown that the active oxidant used by CmlI to facilitate this chemistry is a peroxo-diferric intermediate (CmlIP). Spectroscopic characterization demonstrated that the peroxo binding geometry of CmlIP is not consistent with the μ-1,2 mode commonly observed in nonheme diiron systems. Its geometry was tentatively assigned as μ-η2:η1 based on comparison with resonance Raman (rR) features of mixed-metal model complexes in the absence of appropriate diiron models. Here, X-ray absorption spectroscopy (XAS) and rR studies have been used to establish a refined structure for the diferric cluster of CmlIP. The rR experiments carried out with isotopically labeled water identified the symmetric and asymmetric vibrations of an Fe-O-Fe unit in the active site at 485 and 780 cm-1, respectively, which was confirmed by the 1.83 Å Fe-O bond observed by XAS. In addition, a unique Fe···O scatterer at 2.82 Å observed from XAS analysis is assigned as arising from the distal O atom of a μ-1,1-peroxo ligand that is bound symmetrically between the irons. The (μ-oxo)(μ-1,1-peroxo)diferric core structure associated with CmlIP is unprecedented among diiron cluster-containing enzymes and corresponding biomimetic complexes. Importantly, it allows the peroxo-diferric intermediate to be ambiphilic, acting as an electrophilic oxidant in the initial N-hydroxylation of an arylamine and then becoming a nucleophilic oxidant in the final oxidation of an aryl-nitroso intermediate to the aryl-nitro product.

Redox Self-Adaptation of a Nitrene Transfer Catalyst to the Substrate Needs.

The development of iron catalysts for carbon-heteroatom bond formation, which has attracted strong interest in the context of green chemistry and nitrene transfer, has emerged as the most promising way to versatile amine synthetic processes. A diiron system was previously developed that proved efficient in catalytic sulfimidations and aziridinations thanks to an FeIII FeIV active species. To deal with more demanding benzylic and aliphatic substrates, the catalyst was found to activate itself to a FeIII FeIV L. active species able to catalyze aliphatic amination. Extensive DFT calculations show that this activation event drastically enhances the electron affinity of the active species to match the substrates requirements. Overall this process consists in a redox self-adaptation of the catalyst to the substrate needs.

Redox Self‐Adaptation of a Nitrene Transfer Catalyst to the Substrate Needs

AbstractThe development of iron catalysts for carbon–heteroatom bond formation, which has attracted strong interest in the context of green chemistry and nitrene transfer, has emerged as the most promising way to versatile amine synthetic processes. A diiron system was previously developed that proved efficient in catalytic sulfimidations and aziridinations thanks to an FeIIIFeIV active species. To deal with more demanding benzylic and aliphatic substrates, the catalyst was found to activate itself to a FeIIIFeIVL. active species able to catalyze aliphatic amination. Extensive DFT calculations show that this activation event drastically enhances the electron affinity of the active species to match the substrates requirements. Overall this process consists in a redox self‐adaptation of the catalyst to the substrate needs.

Formation and High Reactivity of the anti-Dioxo Form of High-Spin μ-Oxodioxodiiron(IV) as the Active Species That Cleaves Strong C-H Bonds.

Recently, it was shown that μ-oxo-μ-peroxodiiron(III) is converted to high-spin μ-oxodioxodiiron(IV) through O-O bond scission. Herein, the formation and high reactivity of the anti-dioxo form of high-spin μ-oxodioxodiiron(IV) as the active oxidant are demonstrated on the basis of resonance Raman and electronic-absorption spectral changes, detailed kinetic studies, DFT calculations, activation parameters, kinetic isotope effects (KIE), and catalytic oxidation of alkanes. Decay of μ-oxodioxodiiron(IV) was greatly accelerated on addition of substrate. The reactivity order of substrates is toluene<ethylbenzene≈cumene<trans-β-methylstyrene. The rate constants increased proportionally to the substrate concentration at low substrate concentration. At high substrate concentration, however, the rate constants converge to the same value regardless of the kind of substrate. This is explained by a two-step mechanism in which anti-μ-oxodioxodiiron(IV) is formed by syn-to-anti transformation of the syn-dioxo form and reacts with substrates as the oxidant. The anti-dioxo form is 620 times more reactive in the C-H bond cleavage of ethylbenzene than the most reactive diiron system reported so far. The KIE for the reaction with toluene/[D8 ]toluene is 95 at -30 °C, which the largest in diiron systems reported so far. The present diiron complex efficiently catalyzes the oxidation of various alkanes with H2 O2 .

Alkali Metal Variation and Twisting of the FeNNFe Core in Bridging Diiron Dinitrogen Complexes.

Alkali metal cations can interact with Fe–N2 complexes, potentially enhancing back-bonding or influencing the geometry of the iron atom. These influences are relevant to large-scale N2 reduction by iron, such as in the FeMoco of nitrogenase and the alkali-promoted Haber–Bosch process. However, to our knowledge there have been no systematic studies of a large range of alkali metals regarding their influence on transition metal–dinitrogen complexes. In this work, we varied the alkali metal in [alkali cation]2[LFeNNFeL] complexes (L = bulky β-diketiminate ligand) through the size range from Na+ to K+, Rb+, and Cs+. The FeNNFe cores have similar Fe–N and N–N distances and N–N stretching frequencies despite the drastic change in alkali metal cation size. The two diketiminates twist relative to one another, with larger dihedral angles accommodating the larger cations. In order to explain why the twisting has so little influence on the core, we performed density functional theory calculations on a simplified LFeNNFeL model, which show that the two metals surprisingly do not compete for back-bonding to the same π* orbital of N2, even when the ligand planes are parallel. This diiron system can tolerate distortion of the ligand planes through compensating orbital energy changes, and thus, a range of ligand orientations can give very similar energies.

Role of the Metal in the Bonding and Properties of Bimetallic Complexes Involving Manganese, Iron, and Cobalt

A multidentate ligand platform is introduced that enables the isolation of both homo- and heterobimetallic complexes of divalent first-row transition metal ions such as Mn(II), Fe(II), and Co(II). By means of a two-step metalation strategy, five bimetallic coordination complexes were synthesized with the general formula M1M2Cl(py3tren), where py3tren is the triply deprotonated form of N,N,N-tris(2-(2-pyridylamino)ethyl)amine. The metal-metal pairings include dicobalt (1), cobalt-iron (2), cobalt-manganese (3), diiron (4), and iron-manganese (5). The bimetallic complexes have been investigated by X-ray diffraction and X-ray anomalous scattering studies, cyclic voltammetry, magnetometry, Mössbauer spectroscopy, UV-vis-NIR spectroscopy, NMR spectroscopy, combustion analyses, inductively coupled plasma optical emission spectrometry, and ab initio quantum chemical methods. Only the diiron chloride complex in this series contains a metal-metal single bond (2.29 Å). The others show weak metal-metal interactions (2.49 to 2.53 Å). The diiron complex is also distinct with a septet ground state, while the other bimetallic species have much lower spin states from S = 0 to S = 1. We propose that the diiron system has delocalized metal-metal bonding electrons, which seems to correlate with a short metal-metal bond and a higher spin state. Multiconfigurational wave function calculations revealed that, indeed, the metal-metal bonding orbitals in the diiron complex are much more delocalized than those of the dicobalt analogue.

Diiron species containing a cyclic PPh2NPh2diphosphine related to the [FeFe]H2ases active site

A new dissymmetrically disubstituted diiron dithiolate species, [Fe(2)(CO)(4)(κ(2)-P(Ph)(2)N(Ph)(2))(μ-pdt)] (pdt = S(CH(2))(3)S), was prepared by using a flexible cyclic base-containing diphosphine, 1,3,5,7-tetraphenyl 1,5-diaza-3,7-diphosphacyclooctane (P(Ph)(2)N(Ph)(2) = {PhPCH(2)NPh}(2)). Preliminary investigations of proton and electron transfers on the diiron system have been done.

ChemInform Abstract: Current Challenges of Modeling Diiron Enzyme Active Sites for Dioxygen Activation by Biomimetic Synthetic Complexes

AbstractReview: 72 refs.

Current challenges of modeling diiron enzyme active sites for dioxygen activation by biomimetic synthetic complexes

This tutorial review describes recent progress in modeling the active sites of carboxylate-rich non-heme diiron enzymes that activate dioxygen to carry out several key reactions in Nature. The chemistry of soluble methane monooxygenase, which catalyzes the selective oxidation of methane to methanol, is of particular interest for (bio)technological applications. Novel synthetic diiron complexes that mimic structural, and, to a lesser extent, functional features of these diiron enzymes are discussed. The chemistry of the enzymes is also briefly summarized. A particular focus of this review is on models that mimic characteristics of the diiron systems that were previously not emphasized, including systems that contain (i) aqua ligands, (ii) different substrates tethered to the ligand framework, (iii) dendrimers attached to carboxylates to mimic the protein environment, (iv) two N-donors in a syn-orientation with respect to the iron-iron vector, and (v) a N-rich ligand environment capable of accessing oxygenated high-valent diiron intermediates.

Structural and Functional Analogues of the Active Sites of the [Fe]-, [NiFe]-, and [FeFe]-Hydrogenases

This article sets out to review the chemistry relating to the synthesis of structural and functional analogues of the three classes of hydrogenases. This chemistry has grown explosively over the last 10 or so years since the first X-ray structures of [NiFe]- and [FeFe]-hydrogenase systems were published. There are now some 400 papers covering structural and functional aspects, with the majority of these associated with the di-iron system. As much emphasized in earlier papers

Influence of a Pendant Amine in the Second Coordination Sphere on Proton Transfer at a Dissymmetrically Disubstituted Diiron System Related to the [2Fe]H Subsite of [FeFe]H2ase

Studies of the protonation of [Fe2(CO)4(kappa2-PNP)(mu-pdt)] (1; PNP = (Ph2PCH2)2NCH3) by HBF4.Et2O showed that the nature of the reaction product depends on whether the reaction is conducted in acetone or in dichloromethane. In acetone, an N-protonated form, 2, is isolated. Tautomerization of 2 in CH2Cl2 gives rise to a mu-hydride species 3. Variable-temperature NMR experiments have been performed to clarify the processes involved.

Efficient enhancement of DNA cleavage activity by introducing guanidinium groups into diiron(III) complex

Inspired by the structures of natural nucleases, guanidinium groups were introduced into binuclear iron(III) systems. Compared with the corresponding analogue without guanidinium groups, the new diiron(III) system led to considerable rate enhancement on DNA cleavage. The cooperativity between metal ions and guanidine groups was evidenced by the fact that no significant cleavage was observed after incubating pBR322 plasmid DNA with non-metalated ligands or free Fe3+ ion. DNA binding experiments indicated that introduction of positively charged guanidinium groups can obtain more than one order of magnitude enhancement in the affinity of complex with DNA.

Magnetic Exchange Interactions in Oxo-Bridged Diiron(III) Systems: Density Functional Calculations Coupling the Broken Symmetry Approach

The calculations on magnetic exchange interaction of Cl3FeOFeCl32- and the related modeling compounds were performed by using the density functional theory coupling the broken symmetry approach. The calculated results show the absence of a direct Cl3Fe---FeCl3 magnetic coupling and the effect of the terminal Cl ligands on magnetic exchange interaction in [Fe−O−Fe]4+, while the protonation of μ-oxo bridge reduces significantly the magnetic coupling constant J value. On the other hand, the J value is insensitive to variation of the Fe−O−Fe angle; however, the dependence of the J value on the Fe−O distance can be expressed as an exponential function, while the J value keeps a constant to the variation of the O−H distance in the μ-hydroxo and μ-aqua bridges. Molecular orbital interaction is applied to explain the magnetic exchange interaction in μ-oxo bridged iron(III) dimers. The validity of qualitative magneto−structural correlation for the models used is further confirmed by full geometry optimization.

Investigation of the reaction product of 1,4,7,10-tetraphenyl-cyclododecahexa-1,2,3,7,8,9-ene with dodecacarbonyl-triiron by X-ray diffraction and MS: comparative structural study of the hexacarbonyl-μ2-(1η2-1,2-ene-2η3-3,4,5-allyl-1σ1-5)diiron(FeFe) system

Reaction of dodecacarbonyl-triiron with the 12-membered bis(butatriene) results in phenyl ring metallation and an intramolecular crosslinking of the macrocycle with the formation of a system of three condensed rings. The central core of the complex is formed by Fe2(CO)6 unit bridged by an organic ligand which is η3:η1-bonded by the π-allylic system and η2-bonded by the olefinic group. The molecular structure of the complex was determined by X-ray diffraction. Differences in fragmentation patterns in MS (EI and LSIMS) were examined. Comparative studies of the geometry of the complex obtained and a series of compounds with isostructural core have been carried out.