Dimeric Iron Research Articles

Using PMHS for Catalytic Hydrosilylation with Dimeric Iron Complexes Featuring Mesoionic Carbene Ligands

Using PMHS for Catalytic Hydrosilylation with Dimeric Iron Complexes Featuring Mesoionic Carbene Ligands

Unveiling Stereo‐Electronic Effects in Homogeneous Catalysis Integrating Theory and Experiments: the Potential of Dimeric Iron(III) Salen Complexes in Methyl Levulinate Transesterification

AbstractBiomass represents an abundant and renewable source of raw materials that can be effectively converted into intermediates and base chemicals. Among these, levulinic acid has garnered significant attention due to its direct derivation from lignocellulosic biomass and the presence of two functional groups, a carboxylic acid, and a ketone, enabling diverse transformations into valuable products. Levulinic acid esters have already found applications in various industrial sectors such as cosmetics, plasticizers, and solvents. In this study, a broad array of alkyl levulinates has been successfully synthesized through homogeneous transesterification catalyzed by Lewis acids. A systematic screening of a library of μ‐oxo‐dinuclear Fe(III) salen complexes has been conducted, aiming to elucidate the impact of steric and electronic properties of their substituents. A complementary theoretical‐computational study based on quantum mechanics calculations and machine learning regression techniques has been employed to unravel the relationship between the structure of the catalysts and their corresponding properties, shedding light on the key factors influencing their catalytic performance.

N-Alkyl substituted triazenide-bridged homoleptic iron(II) dimers with an exceptionally short Fe-Fe bond.

Dinuclear transition metal complexes with direct metal-metal interactions have the potential to generate unique reactivities and properties. Using asymmetric triazine ligands HN3tBuR (R = Et, iPr, nBu) featuring different alkyl substituents at 1,3-N centers, we report here the first rational synthesis of 'tetragonal lantern' type Fe(II) triazenides [Fe2(N3tBuR)4] [R = Et (1), iPr (2), nBu (3)] having an exceptionally short Fe-Fe distance (2.167-2.174 Å). Unlike the previously reported lantern structures with related amidinate or guanidinate ligands, highly air-sensitive 1-3 show a lower spin ground state, as indicated by Mössbauer, 1H NMR and DFT studies.

Valence Delocalization and Metal-Metal Bonding in Carbon-Bridged Mixed-Valence Iron Complexes.

The carbide ligand in the iron-molybdenum cofactor (FeMoco) in nitrogenase bridges iron atoms in different oxidation states, yet it is difficult to discern its ability to mediate magnetic exchange interactions due to the structural complexity of the cofactor. Here, we describe two mixed-valent diiron complexes with C-based ketenylidene bridging ligands, and compare the carbon bridges with the more familiar sulfur bridges. The ground state of the [Fe2 (μ-CCO)2 ]+ complex with two carbon bridges (4) is S= , and it is valence delocalized on the Mössbauer timescale with a small thermal barrier for electron hopping that stems from the low Fe-C force constant. In contrast, one-electron reduction of the [Fe2 (μ-CCO)] complex with one carbon bridge (2) affords a mixed-valence species with a high-spin ground state (S= ), and the Fe-Fe distance contracts by 1 Å. Spectroscopic, magnetic, and computational studies of the latter reveal an Fe-Fe bonding interaction that leads to complete valence delocalization. Analysis of near-IR intervalence charge transfer transitions in 5 indicates a very large double exchange constant (B) in the range of 780-965 cm-1 . These results show that carbon bridges are extremely effective at stabilizing valence delocalized ground states in mixed-valent iron dimers.

Magnetic properties and pH-controlled reversible interconversion of µ-oxido into µ-hydroxido in oxo-carboxylato bridged iron(III) dimers: Theoretical and experimental insights

Three dinuclear complexes of formula [FeIII2(µ-O)(µ-OOCR)(L)2](ClO4)·G, with R/G=H/(EtOAc)2 (1·2EtOAc), CH3/(Et2O)2 (2·2Et2O) and C6H5/(CH3OH)2(CH3CN)2 (3·2CH3OH·2CH3CN), [where HL = 2-((((1-methylbenzimidazol-2-yl)methyl)(pyridin-2-ylmethyl)amino)methyl)phenol] have been prepared. The ligand provides a N,N,N,O donor set in its deprotonated form. X-ray diffraction analyses revealed that the complexes have a similar oxido-carboxylato-bridged structure where each metal ion has a tertiary amine, one imidazolyl N atom, one pyridyl N atom and a phenolate O atom as terminal coordination. Thus, each metal centre assumes a FeIIIN3O3 coordination in an octahedral environment. Addition of acid lead to the generation of similar dinuclear complexes formulated as [FeIII2(µ-OH)(µ-OOCR)(L)2](ClO4)2, with R=H (1H), CH3 (2H) and C6H5 (3H) in solution. This protonation of the µ-O bridge is reversible and addition of base regenerates the original complexes (1–3). Magnetic measurements showed intramolecular antiferromagnetic Fe···Fe interactions in compounds 1–3 with J = ‒276(1), ‒197(1) and ‒190(1) cm−1, respectively. DFT/B3LYP calculation helped us to explicitly state the sign and magnitude of the magnetic coupling constants along with an approaching to the spin density distribution.

Dynamic effects on ligand field from rapid hydride motion in an iron(ii) dimer with an S = 3 ground state.

Hydride complexes are important in catalysis and in iron-sulfur enzymes like nitrogenase, but the impact of hydride mobility on local iron spin states has been underexplored. We describe studies of a dimeric diiron(ii) hydride complex using X-ray and neutron crystallography, Mössbauer spectroscopy, magnetism, DFT, and ab initio calculations, which give insight into the dynamics and the electronic structure brought about by the hydrides. The two iron sites in the dimer have differing square-planar (intermediate-spin) and tetrahedral (high-spin) iron geometries, which are distinguished only by the hydride positions. These are strongly coupled to give an S total = 3 ground state with substantial magnetic anisotropy, and the merits of both localized and delocalized spin models are discussed. The dynamic nature of the sites is dependent on crystal packing, as shown by changes during a phase transformation that occurs near 160 K. The change in dynamics of the hydride motion leads to insight into its influence on the electronic structure. The accumulated data indicate that the two sites can trade geometries by rotating the hydrides, at a rate that is rapid above the phase transition temperature but slow below it. This small movement of the hydrides causes large changes in the ligand field because they are strong-field ligands. This suggests that hydrides could be useful in catalysis not only due to their reactivity, but also due to their ability to rapidly modulate the local electronic structure and spin states at metal sites.

In Ukrainian

Metal composites modified with various heteroatoms, such as N, B, Si, are used to obtain matrix composites with specified parameters with the strongest adhesive-cohesive bonds between metal atoms and a carbon nanoparticle. Such carbon nanoparticles functionalized with heteroatoms are promising for many metal composites. One of the interesting and promising metals as a matrix for such research work is iron. To predict the specifics of the interaction of iron with the surface of carbon nanomaterials supplemented with heteroatoms of different chemical structure, it is advisable to model such processes using quantum chemistry methods. The aim of the work was to find out the effect of temperature on the chemical interaction of iron clusters with native, boron-, silicon-, and nitrogen-containing graphene-like planes (GLP). The results of the calculations show that the highest value of the energy effect of the chemical interaction for the native graphene-like plane is +204.3 kJ/mol, in the case of calculations both by the B3LYP/6-31G(d,p) method and by the MP2/6-31G(d, p) (+370.7 kJ/mol). The lower value of the energy effect is found in the presence of nitrogen atoms in the composition of the graphene-like plane. This value is even lower for the interaction of iron dimers with a silicon-containing carbon nanocluster. The lowest values of the energy effect, calculated by both methods, are characteristic of the boron-containing graphene-like plane. In particular, for the B3LYP/6-31G(d,p) method, the value of the energy effect of the reaction is ‑210.5 kJ/mol, and for the MP2/6-31G(d,p) method this value is +16.6 kJ/mol. The presence of boron atoms in the composition of the nanocarbon matrix best contributes to the interaction with the iron nanocluster, regardless of the chosen research method. The dependence curves of the Gibbs free energy of the interaction of iron dimers with a graphene-like plane and its derivatives in all cases qualitatively correlate with similar energy effects. In addition, in all cases, the values of the Gibbs free energy increase with increasing temperature.

Interplay of magnetic states and hyperfine fields of iron dimers on MgO(001)

Individual nuclear spin states can have very long lifetimes and could be useful as qubits. Progress in this direction was achieved on MgO/Ag(001) via detection of the hyperfine interaction (HFI) of Fe, Ti and Cu adatoms using scanning tunneling microscopy. Previously, we systematically quantified from first-principles the HFI for the whole series of 3d transition adatoms (Sc-Cu) deposited on various ultra-thin insulators, establishing the trends of the computed HFI with respect to the filling of the magnetic s- and d-orbitals of the adatoms and on the bonding with the substrate. Here we explore the case of dimers by investigating the correlation between the HFI and the magnetic state of free standing Fe dimers, single Fe adatoms and dimers deposited on a bilayer of MgO(001). We find that the magnitude of the HFI can be controlled by switching the magnetic state of the dimers. For short Fe-Fe distances, the antiferromagnetic state enhances the HFI with respect to that of the ferromagnetic state. By increasing the distance between the magnetic atoms, a transition toward the opposite behavior is observed. Furthermore, we demonstrate the ability to substantially modify the HFI by atomic control of the location of the adatoms on the substrate. Our results establish the limits of applicability of the usual hyperfine hamiltonian and we propose an extension based on multiple scattering processes.

Insights into Fe Species Structure‐Performance Relationship for Direct Methane Conversion toward Oxygenates over Fe‐MOR Catalysts

AbstractDirect oxidation of methane to value‐added products under mild conditions remains a grand challenge due to the problem of selective CH4 activation. Here we report a Fe‐MOR‐F catalyst prepared via freeze‐drying method, indicating excellent catalytic performance for the direct partial oxidation of methane using H2O2 under mild conditions. Various characterizations including XRD, NH3‐TPD, H2‐TPR, UV‐Vis, XPS, and EPR studies revealed that Fe‐MOR‐F catalyst possesses more highly dispersed monomeric and dimeric iron species, which is strongly correlated to its high catalytic activity for direct methane oxidation. The DFT calculations indicate the highly dispersed monomeric and dimeric iron species can effectively homogenize H2O2 into active hydroxyl groups, while iron clusters can decompose H2O2 into inactive O2. The EPR experiments show more ⋅OH was generated on Fe‐MOR‐F catalyst, which is in line with DFT calculation results. These structural characteristics endow Fe‐MOR‐F catalyst with superior catalytic performance for direct methane oxidation in terms of oxygenates yield (9.8 mol kgcat−1 h−1) and selectivity (91.8 %) at 80 °C. This work not only highlights the excellent catalytic performance of methane activation on the highly dispersed monomeric and dimeric iron species, but also promotes the potential application by freeze‐drying method for the preparation of the highly dispersed active species.

Macroheterocyclic μ-Nitrido- and μ-Carbido Dimeric Iron and Ruthenium Complexes as a Molecular Platform for Modeling Oxidative Enzymes (A Review)

Macroheterocyclic μ-Nitrido- and μ-Carbido Dimeric Iron and Ruthenium Complexes as a Molecular Platform for Modeling Oxidative Enzymes (A Review)

An Iron-Mesoionic Carbene Complex for Catalytic Intramolecular C-H Amination Utilizing Organic Azides.

The synthesis of N-heterocycles is of paramount importance for the pharmaceutical industry. They are often synthesized through atom economic and environmentally unfriendly methods, generating significant waste. A less explored, but greener, alternative is the synthesis through the direct intramolecular C-H amination utilizing organic azides. Few examples exist by using this method, but many are limited due to the required use of stoichiometric amounts of Boc2O. Herein, we report a homoleptic C,O-chelating mesoionic carbene-iron complex, which is the first iron-based complex that does not require the addition of any protecting groups for this transformation and that is active also in strong donor solvents such as THF or even DMSO. The achieved turnover number is an order of magnitude higher than any other reported catalytic system. A variety of C-H bonds were activated, including benzylic, primary, secondary, and tertiary. By following the reaction over time, we determined the presence of an initiation period. Kinetic studies showed a first-order dependence on substrate concentration and half-order dependence on catalyst concentration. Intermolecular competition reactions with deuterated substrate showed no KIE, while separate reactions with deuterium-labeled substrate resulted in a KIE of 2.0. Moreover, utilizing deuterated substrate significantly decreased the initiation period of the catalysis. Preliminary mechanistic studies suggest a unique mechanism involving a dimeric iron species as the catalyst resting state.

Exchange constants for local spin Hamiltonians from tight-binding models

We consider the mapping of tight-binding electronic structure theory to a local spin Hamiltonian, based on the adiabatic approximation for spin degrees of freedom in itinerant-electron systems. Local spin Hamiltonians are introduced in order to describe the energy landscape of small magnetic fluctuations, locally around a given spin configuration. They are designed for linear response near a given magnetic state and in general insufficient to capture arbitrarily strong deviations of spin configurations from the equilibrium. In order to achieve this mapping, we include a linear term in the local spin Hamiltonian that, together with the usual bilinear exchange tensor, produces an improved accuracy of effective magnetic Weiss fields for non-collinear states. We also provide examples from tight-binding electronic structure theory, where our implementation of the calculation of exchange constants is based on constraining fields that stabilize an out-of-equilibrium spin configuration. We check our formalism by means of numerical calculations for iron dimers and chains.

RADICAL INITIATED COPOLYMERIZATION OF METHYL METHACRYLATE AND ACRYLONITRILE IN PRESENCE OF IRON COMPLEXES

This article represents data on the effect of organometallic iron complexes: ferrocene, dicarbonyl dimer of cyclopentadienyl iron and tricarbonyl cyclooctatetraene iron on the copolymerization of methyl methacrylate (MMA) and acrylonitrile (AN) initiated by benzoyl peroxide. It is shown that the introduction of metal complexes and their structure affect the initial rate of copolymerization, the form of the diagrams of the composition of the obtained copolymers, and the values of the effective constants of the relative activities of the comonomers in the copolymerization of methyl methacrylate and acrylonitrile (system metallocomplex of iron – peroxide benzoyl: ferrocene – peroxide benzoyl: rММА = 1.58; rАН= 0.08 in 60 °С; rММА = 1.30; rАН = 0.05 in 50 °С; dicarbonyl dimer cyclopentadienyl iron–peroxide benzoyl: rММА = 1.36; rАН = 0.06 in 60 °С; rММА = 1.09; rАН = 0.14 in 50 °С; tricarbonyl cyclooctatetraene iron -peroxide benzoyl: rММА = 1.08; rАН = 0.15 in 60 °С; rММА = 1.14; rАН = 0.05 in 50 °С; peroxide benzoyl: rММА = 1.11; rАН = 0.07 in 60 °С; rММА = 1.11; rАН = 0.07 in 50 °С). The proportions of triadic sequences of units in copolymers, experimentally determined from 1H NMR spectroscopy, are given, as well as the calculated proportions of dyads. The presence of iron metal complexes affects the distribution of the proportions of the triad and dyad sequences of units, as well as their microstructure in copolymers. These changes are explained by the formation of macromolecules, both with the participation of free radicals and the stereospecific coordination active centers of polymerization that are formed in the presence of iron complexes.

Atomic level N-coordinated Fe dual-metal embedded in graphene: An efficient double atoms catalyst for CO oxidation

Double atoms catalysts (DACs) emerged very recently as excellent materials and were used in various catalytic reactions due to their excellent catalytic activity. Although active research on the design DACs has been ongoing for almost a decade, little attention has been paid to the detailed catalytic mechanisms. Based on density functional theory (DFT) analysis, the iron dimer embedded in N-containing graphene (FeFe@N6) has the most excellent adsorption properties of O2 and CO among DACs systems (M1M2@N6, M1/M2 = Fe, Co, Ni). We considered the influence of N atoms concentration and position on the adsorption performance and stability of the iron dimer, FeFe@N6 was screened as a potential DAC. We systematically studied the catalytic mechanisms of CO oxidation on FeFe@N6 and its monometallic counterpart (VFe@N6). CO oxidation on FeFe@N6 along the Eley–Rideal (ER) and termolecular Eley–Rideal (TER) mechanisms needs to overcome the low energy barriers of 0.513 eV and 0.299 eV, which is smaller than the maximum energy barrier (0.584 eV) of CO oxidation along the Langmuir–Hinshelwood (LH) mechanism on VFe@N6. Therefore, FeFe@N6 has high activity for CO oxidation, which can provide useful guidance for the development of low-cost and efficient green DACs.

Copper−iron dimer for selective C–C coupling in electrochemical CO2 reduction

The electrochemical CO2 reduction to fuels and chemicals using renewable electricity provides a promising strategy for resolving energy environmental crisis and achieving carbon neutral. Progress has been made on catalyst design for CO2 conversion to high valued products. However, the efficient production of multi−carbon compounds is very challenging due to low selectivity and high overpotential. Understanding of catalytic mechanism at the atom level is the key to developing high−performance CO2 reduction catalysts. Herein, employing comprehensive density functional theory computations, we systematically investigated the structures, reaction intermediates, CO2 reduction mechanisms, and the selectivity of state−of−art catalysts—heteronuclear CuFe dimers anchored on nitrogenated carbon monolayers as the CO2 reduction electrocatalysts. The results show that the strong binding between cooperative CuFe dimer and nitrogenated carbon matrix not only prevents the metal from clustering but also dictates favorable electronic structures, that explains their superior CO2 reduction activity, high C2H5OH selectivity and the mechanism of hydrogen evolution inhibition.

Effect of 3d/4p Mixing on 1s2p Resonant Inelastic X-ray Scattering: Electronic Structure of Oxo-Bridged Iron Dimers.

1s2p resonant inelastic X-ray scattering (1s2p RIXS) has proven successful in the determination of the differential orbital covalency (DOC, the amount of metal vs ligand character in each d molecular orbital) of highly covalent centrosymmetric iron environments including heme models and enzymes. However, many reactive intermediates have noncentrosymmetric environments, e.g., the presence of strong metal-oxo bonds, which results in the mixing of metal 4p character into the 3d orbitals. This leads to significant intensity enhancement in the metal K-pre-edge and as shown here, the associated 1s2p RIXS features, which impact their insight into electronic structure. Binuclear oxo bridged high spin Fe(III) complexes are used to determine the effects of 4p mixing on 1s2p RIXS spectra. In addition to developing the analysis of 4p mixing on K-edge XAS and 1s2p RIXS data, this study explains the selective nature of the 4p mixing that also enhances the analysis of L-edge XAS intensity in terms of DOC. These 1s2p RIXS biferric model studies enable new structural insight from related data on peroxo bridged biferric enzyme intermediates. The dimeric nature of the oxo bridged Fe(III) complexes further results in ligand-to-ligand interactions between the Fe(III) sites and angle dependent features just above the pre-edge that reflect the superexchange pathway of the oxo bridge. Finally, we present a methodology that enables DOC to be obtained when L-edge XAS is inaccessible and only 1s2p RIXS experiments can be performed as in many metalloenzyme intermediates in solution.

Methane Conversion over C2N-Supported Fe2 Dimers

Methane is a vast hydrocarbon resource around the globe that has the potential to replace petroleum as a raw material and energy source. Therefore, the catalytic conversion of methane into high value-added chemicals is significantly important for the utilization of this hydrocarbon resource. However, this is a great challenge due to the high-energy input required to overcome the reaction barrier. Herein, a highly active catalytic conversion process of methane on an iron dimer anchored on a two-dimensional (2D) C2N monolayer (Fe2@C2N) is reported. Density functional theory calculations reveal that the superior properties of Fe2@C2N can be attributed to the formation of the Fe-O-Fe intermediate with H2O2 as the O-donor molecule, which facilitates the formation of methyl radicals and promotes the conversion of methane. This finding could pave the way toward highly efficient non-precious metal catalysts for methane oxidation reactions.

Fabricating Dual‐Atom Iron Catalysts for Efficient Oxygen Evolution Reaction: A Heteroatom Modulator Approach

AbstractUnderstanding the thermal aggregation behavior of metal atoms is important for the synthesis of supported metal clusters. Here, derived from a metal–organic framework encapsulating a trinuclear FeIII2FeII complex (denoted as Fe3) within the channels, a well‐defined nitrogen‐doped carbon layer is fabricated as an ideal support for stabilizing the generated iron nanoclusters. Atomic replacement of FeII by other metal(II) ions (e.g., ZnII/CoII) via synthesizing isostructural trinuclear‐complex precursors (Fe2Zn/Fe2Co), namely the “heteroatom modulator approach”, is inhibiting the aggregation of Fe atoms toward nanoclusters with formation of a stable iron dimer in an optimal metal–nitrogen moiety, clearly identified by direct transmission electron microscopy and X‐ray absorption fine structure analysis. The supported iron dimer, serving as cooperative metal–metal site, acts as efficient oxygen evolution catalyst. Our findings offer an atomic insight to guide the future design of ultrasmall metal clusters bearing outstanding catalytic capabilities.

Fabricating Dual-Atom Iron Catalysts for Efficient Oxygen Evolution Reaction: A Heteroatom Modulator Approach.

Understanding the thermal aggregation behavior of metal atoms is important for the synthesis of supported metal clusters. Here, derived from a metal-organic framework encapsulating a trinuclear FeIII 2 FeII complex (denoted as Fe3 ) within the channels, a well-defined nitrogen-doped carbon layer is fabricated as an ideal support for stabilizing the generated iron nanoclusters. Atomic replacement of FeII by other metal(II) ions (e.g., ZnII /CoII ) via synthesizing isostructural trinuclear-complex precursors (Fe2 Zn/Fe2 Co), namely the "heteroatom modulator approach", is inhibiting the aggregation of Fe atoms toward nanoclusters with formation of a stable iron dimer in an optimal metal-nitrogen moiety, clearly identified by direct transmission electron microscopy and X-ray absorption fine structure analysis. The supported iron dimer, serving as cooperative metal-metal site, acts as efficient oxygen evolution catalyst. Our findings offer an atomic insight to guide the future design of ultrasmall metal clusters bearing outstanding catalytic capabilities.

Nitrogen reduction reaction on small iron clusters supported by N-doped graphene: A theoretical study of the atomically precise active-site mechanism

Nonprecious metal catalysts are known of significance for electrochemical N2 reduction reaction (NRR) of which the mechanism has been illustrated by ongoing investigations of single atom catalysis. However, it remains challenging to fully understand the size-dependent synergistic effect of active sites inherited in substantial nanocatalysts. In this work, four types of small iron clusters Fen (n = 1–4) supported on nitrogen-doped graphene sheets are constructed to figure out the size dependence and synergistic effect of active sites for NRR catalytic activities. It is revealed that Fe3 and Fe4 clusters on N4G supports exhibit higher NRR activity than single-iron atom and iron dimer clusters, showing lowered limiting potential and restricted hydrogen evolution reaction (HER) which is a competitive reaction channel. In particular, the Fe4-N4G displays outstanding NRR performance for “side-on” adsorption of N2 with a small limiting potential (−0.45 V). Besides the specific structure and strong interface interaction within the Fe4-N4G itself, the high NRR activity is associated with the unique bonding/antibonding orbital interactions of N-N and N-Fe for the adsorptive N2 and NNH intermediates, as well as relatively large charge transfer between N2 and the cluster Fe4-N4G.