Iron(iv) alkyl complexes: electronic structure contributions to Fe-C bond homolysis and migration reactions that form N-C bonds from N2.

High-valent iron alkyl complexes are rare, as they are prone to Fe-C bond homolysis. Here, we describe an unusual way to access formally iron(iv) alkyl complexes through double silylation of iron(i) alkyl dinitrogen complexes to form an NNSi2 group. Spectroscopically validated computations show that the disilylehydrazido(2-) ligand stabilizes the formal iron(iv) oxidation state through a strongly covalent Fe-N π-interaction, in which one π-bond fits an "inverted field" description. This means that the two bonding electrons are localized more on the metal than the ligand, and thus an iron(ii) resonance structure is a significant contributor, similar to the previously-reported phenyl analogue. However, in contrast to the phenyl complex which has an S = 1 ground state, the ground state of the alkyl complex is S = 2, which places one electron in the π* orbital, leading to longer and weaker Fe-N bonds. The reactivity of these hydrazido(2-) complexes is dependent on the steric and electronic properties of the specific alkyl group. When the alkyl group is the bulky trimethylsilylmethyl, the formally iron(iv) species is stable at room temperature and no migration of the alkyl ligand is observed. However, the analogous complex with the smaller methyl ligand does indeed undergo migration of the carbon-based ligand to the NNSi2 group to form a new N-C bond. This migration is followed by isomerization of the hydrazido ligand, and the product exists as two isomers that have distinct η1 and η2 binding of the hydrazido group. Lastly, when the alkyl group is benzyl, the Fe-C bond homolyzes to give a three-coordinate hydrazido(2-) complex which is likely due to the greater stability of a benzyl radical compared to that for methyl or trimethylsilylmethyl. These studies demonstrate the availability of a hydrocarbyl migration pathway at formally iron(iv) centers to form new N-C bonds directly to N2, though product selectivity is highly dependent on the identity of the migrating group.

Mössbauer and Nuclear Resonance Vibrational Spectroscopy Studies of Iron Species Involved in N-N Bond Cleavage.

Diketiminate-supported iron complexes are capable of cleaving the strong triple bond of N2 to give a tetra-iron complex with two nitrides (Rodriguez et al., Science, 2011, 334, 780-783). The mechanism of this reaction has been difficult to determine, but a transient green species was observed during the reaction that corresponds to a potential intermediate. Here, we describe studies aiming to identify the characteristics of this intermediate, using a range of spectroscopic techniques, including Mössbauer spectroscopy, electronic absorption spectroscopy, Raman spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and nuclear resonance vibrational spectroscopy (NRVS) complemented by density functional theory (DFT) calculations. We successfully elucidated the nature of the starting iron(II) species and the bis(nitride) species in THF solution, and in each case, THF breaks up the multiiron species. Various observations on the green intermediate species indicate that it has one N2 per two Fe atoms, has THF associated with it, and has NRVS features indicative of bridging N2. Computational models with a formally diiron(0)-N2 core are most consistent with the accumulated data, and on this basis, a mechanism for N2 splitting is suggested. This work shows the power of combining NRVS, Mössbauer, NMR, and vibrational spectroscopies with computations for revealing the nature of transient iron species during N2 cleavage.

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.

Redox-Induced Structural Reorganization Dictates Kinetics of Cobalt(III) Hydride Formation via Proton-Coupled Electron Transfer.

Two-electron, one-proton reactions of a family of [CoCp(dxpe)(NCCH3)]2+ complexes (Cp = cyclopentadienyl, dxpe = 1,2-bis(di(aryl/alkyl)phosphino)ethane) form the corresponding hydride species [HCoCp(dxpe)]+ (dxpe = dppe (1,2-bis(diphenylphosphino)ethane), depe (1,2-bis(diethylphosphino)ethane), and dcpe (1,2-bis(dicyclohexylphosphino)ethane)) through a stepwise proton-coupled electron transfer process. For three [CoCp(dxpe)(NCCH3)]2+ complexes, peak shift analysis was employed to quantify apparent proton transfer rate constants from cyclic voltammograms recorded with acids ranging 22 pKa units. The apparent proton transfer rate constants correlate with the strength of the proton source for weak acids, but these apparent proton transfer rate constants curiously plateau (kpl) as the reaction becomes increasingly exergonic. The absolute apparent proton transfer rate constants across both these regions correlate with the steric bulk of the chelating diphosphine ligand, with bulkier ligands leading to slower kinetics (kplateau,depe = 3.5 × 107 M-1 s-1, kplateau,dppe = 1.7 × 107 M-1 s-1, kplateau,dcpe = 7.1 × 104 M-1 s-1). Mechanistic studies were conducted to identify the cause of the aberrant kPTapp-ΔpKa trends. When deuterated acids are employed, deuterium incorporation in the Cp ring is observed, indicating protonation of the CoCp(dxpe) species to form the corresponding hydride proceeds via initial ligand protonation. Digital simulations of cyclic voltammograms show ligand loss accompanying initial reduction gates subsequent PCET activity at higher driving forces. Together, these experiments reveal the details of the reaction mechanism: reduction of the Co(III) species is followed by dissociation of the bound acetonitrile ligand, subsequent reduction of the unligated Co(II) species to form a Co(I) species is followed by protonation, which occurs at the Cp ring, followed by tautomerization to generate the stable Co(III)-hydride product [HCoCp(dxpe)]+. Analysis as a function of chelating disphosphine ligand, solvent, and acid strength reveals that the ligand dissociation equilibrium is directly influenced by the steric bulk of the phosphine ligands and gates protonation, giving rise to the plateau of the apparent proton transfer rate constant with strong acids. The complexity of the reaction mechanism underpinning hydride formation, encompassing dynamic behavior of the entire ligand set, highlights the critical need to understand elementary reaction steps in proton-coupled electron transfer reactions.

Author Correction: Coupling dinitrogen and hydrocarbons through aryl migration.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Coupling dinitrogen and hydrocarbons through aryl migration.

The activation of abundant molecules such as hydrocarbons and atmospheric nitrogen (N2) remains a challenge because these molecules are often inert. The formation of carbon-nitrogen bonds from N2 typically has required reactive organic precursors that are incompatible with the reducing conditions that promote N2 reactivity1, which has prevented catalysis. Here we report a diketiminate-supported iron system that sequentially activates benzene and N2 to form aniline derivatives. The key to this coupling reaction is the partial silylation of a reduced iron-dinitrogen complex, followed by migration of a benzene-derived aryl group to the nitrogen. Further reduction releases N2-derived aniline, and the resulting iron species can re-enter the cyclic pathway. Specifically, we show that an easily prepared diketiminate iron bromide complex2 mediates the one-pot conversion of several petroleum-derived arenes into the corresponding silylated aniline derivatives, by using a mixture of sodium powder, crown ether, trimethylsilyl bromide and N2 as the nitrogen source. Numerous compounds along the cyclic pathway are isolated and crystallographically characterized, and their reactivity supports a mechanism for sequential hydrocarbon activation and N2 functionalization. This strategy couples nitrogen atoms from N2 with abundant hydrocarbons, and maps a route towards future catalytic systems.

The influences of carbon donor ligands on biomimetic multi-iron complexes for N2 reduction.

The active site clusters of nitrogenase enzymes possess the only examples of carbides in biology. These are the only biological FeS clusters that are capable of reducing N2 to NH4 +, implicating the central carbon and its interaction with Fe as important in the mechanism of N2 reduction. This biological question motivates study of the influence of carbon donors on the electronic structure and reactivity of unsaturated, high-spin iron centers. Here, we present functional and structural models that test the impacts of carbon donors and sulfide donors in simpler iron compounds. We report the first example of a diiron complex that is bridged by an alkylidene and a sulfide, which serves as a high-fidelity structural and spectroscopic model of a two-iron portion of the active-site cluster (FeMoco) in the resting state of Mo-nitrogenase. The model complexes have antiferromagnetically coupled pairs of high-spin iron centers, and sulfur K-edge X-ray absorption spectroscopy shows comparable covalency of the sulfide for C and S bridged species. The sulfur-bridged compound does not interact with N2 even upon reduction, but upon removal of the sulfide it becomes capable of reducing N2 to NH4 + with the addition of protons and electrons. This provides synthetic support for sulfide extrusion in the activation of nitrogenase cofactors.

Masked Radicals: Iron Complexes of Trityl, Benzophenone, and Phenylacetylene.

We report the first Fe─CPh3 complex, and show that the long Fe─C bond can be disrupted by neutral π-acceptor ligands (benzophenone and phenylacetylene) to release the triphenylmethyl radical. The products are formally iron(I) complexes, but X-ray absorption spectroscopy coupled with density functional and multireference ab initio calculations indicates that the best description of all the complexes is iron(II). In the formally iron(I) complexes, this does not imply that the π-acceptor ligand has radical character, because the iron(II) description arises from doubly-occupied frontier molecular orbitals that are shared equitably by the iron and the π-acceptor ligand, and the unpaired electrons lie on the metal. Despite the lack of substantial radical character on the ligands, alkyne and ketone fragments can couple to form a high-spin iron(III) complex with a cyclized metalladihydrofuran core.

Isolation and characterization of a high-spin mixed-valent iron dinitrogen complex.

We report a rare example of a mixed-valence iron compound with an FeNNFe core, which gives insight into the structural, spectroscopic, and magnetic influences of single-electron reductions and oxidations. In the new compound, the odd electron is localized as judged from Mössbauer spectra at 80 K and infrared spectra at room temperature, and the backbonding into the N2 unit is intermediate between diiron(i) and diiron(0) congeners. Magnetic susceptibility and relaxation studies on the series of FeNNFe compounds show significant magnetic anisotropy, but through-barrier pathways enable fairly rapid magnetic relaxation.

Effects of N2 Binding Mode on Iron-Based Functionalization of Dinitrogen to Form an Iron(III) Hydrazido Complex.

Distinguishing the reactivity differences between N2 complexes having different binding modes is crucial for the design of effective N2-functionalizing reactions. Here, we compare the reactions of a K-bridged, dinuclear FeNNFe complex with a monomeric Fe(N2) complex where the bimetallic core is broken up by the addition of chelating agents. The new anionic iron(0) dinitrogen complex has enhanced electron density at the distal N atoms of coordinated N2, and though the N2 is not as weakened in this monomeric compound, it is much more reactive toward silylation by (CH3)3SiI (TMSI). Double silylation of N2 gives a three-coordinate iron(III) hydrazido(2-) complex, which is finely balanced between coexisting S = 1/2 and S = 3/2 states that are characterized by crystallography, spectroscopy, and computations. These results give insight into the interdependence between binding modes, alkali dependence, reactivity, and magnetic properties within an iron system that functionalizes N2.

High-Frequency Fe-H Vibrations in a Bridging Hydride Complex Characterized by NRVS and DFT.

High-spin iron species with bridging hydrides have been detected in species trapped during nitrogenase catalysis, but there are few general methods of evaluating Fe-H bonds in high-spin multinuclear iron systems. An 57 Fe nuclear resonance vibrational spectroscopy (NRVS) study on an Fe(µ-H)2 Fe model complex reveals Fe-H stretching vibrations for bridging hydrides at frequencies greater than 1200 cm-1 . These isotope-sensitive vibrational bands are not evident in infrared (IR) spectra, showing the power of NRVS for identifying hydrides in this high-spin iron system. Complementary density functional theory (DFT) calculations elucidate the normal modes of the rhomboidal iron hydride core.

ENDOR characterization of an iron-alkene complex provides insight into a corresponding organometallic intermediate of nitrogenase.

A bio-organometallic intermediate, denoted PA, was previously trapped during the reduction of propargyl alcohol to allyl alcohol (AA) by nitrogenase, and a similar one was trapped during acetylene reduction, representing foundational examples of alkene binding to a metal center in biology. ENDOR spectroscopy led to the conclusion that these intermediates have η2 binding of the alkene, with the hydrogens on the terminal carbon structurally/magnetically equivalent and related by local mirror symmetry. However, our understanding of both the PA intermediate, and of the dependability of the ENDOR analysis on which this understanding was based, was constrained by the absence of reference iron-alkene complexes for EPR/ENDOR comparison. Here, we report an ENDOR study of the crystallographically characterized biomimetic iron(i) complex 1, which exhibits η2 coordination of styrene, thus connecting hyperfine and structural parameters of an Fe-bound alkene fragment for the first time. A tilt of the alkene plane of 1 from normal to the crystallographic Fe-C2-C1 plane causes substantial differences in the dipolar couplings of the two terminal vinylic protons. Comparison of the hyperfine couplings of 1 and PA confirms the proposed symmetry of PA, and that the η2 interaction forms a scalene Fe-C-C triangle, rather than an isosceles triangle. This spectroscopic study of a structurally characterized complex thus shows the exceptional sensitivity of ENDOR spectroscopy to structural details, while enhancing our understanding of the geometry of a key nitrogenase adduct.

Density Functional Calculations for Prediction of 57Fe Mössbauer Isomer Shifts and Quadrupole Splittings in ß-Diketiminate Complexes.

The relative ease of Mössbauer spectroscopy and of density functional theory (DFT) calculations encourages the use of Mössbauer parameters as a validation method for calculations, and the use of calculations as a double check on crystallographic structures. A number of studies have proposed correlations between the computationally determined electron density at the iron nucleus and the observed isomer shift, but deviations from these correlations in low-valent iron ß-diketiminate complexes encouraged us to determine a new correlation for these compounds. The use of B3LYP/def2-TZVP in the ORCA platform provides an excellent balance of accuracy and speed. We provide here not only this new correlation and a clear guide to its use but also a systematic analysis of the limitations of this approach. We also highlight the impact of crystallographic inaccuracies, DFT model truncation, and spin states, with intent to assist experimentalists to use Mössbauer spectroscopy and calculations together.

Synthesis and Mechanism of Formation of Hydride-Sulfide Complexes of Iron.

Iron-sulfide complexes with hydride ligands provide an experimental precedent for spectroscopically detected hydride species on the iron-sulfur MoFe7S9C cofactor of nitrogenase. In this contribution, we expand upon our recent synthesis of the first iron sulfide hydride complex from an iron hydride and a sodium thiolate ( Arnet, N. A.; Dugan, T. R.; Menges, F. S.; Mercado, B. Q.; Brennessel, W. W.; Bill, E.; Johnson, M. A.; Holland, P. L., J. Am. Chem. Soc. 2015 , 137 , 13220 - 13223 ). First, we describe the isolation of an analogous iron sulfide hydride with a smaller diketiminate supporting ligand, which benefits from easier preparation of the hydride precursor and easier isolation of the product. Second, we describe mechanistic studies on the C-S bond cleavage through which the iron sulfide hydride product is formed. In a key experiment, use of cyclopropylmethanethiolate as the sulfur precursor leads to products from cyclopropane ring opening, implicating an alkyl radical as an intermediate. Combined with the results of isotopic labeling studies, the data are consistent with a mechanism in which homolytic C-S bond cleavage is followed by rebound of the alkyl radical to abstract a hydrogen atom from iron to give the observed alkane and iron-sulfide products.

Correction to "Density Functional Calculations for Prediction of 57Fe Mossbauer Isomer Shifts and Quadrupole Splittings in ß-Diketiminate Complexes".

[This corrects the article DOI: 10.1021/acsomega.7b00595.].

Alkali-Controlled C-H Cleavage or N-C Bond Formation by N2-Derived Iron Nitrides and Imides.

Formation of N-H and N-C bonds from functionalization of N2 is a potential route to utilization of this abundant resource. One of the key challenges is to make the products of N2 activation reactive enough to undergo further reactions under mild conditions. This paper explores the strategy of "alkali control," where the presence of an alkali metal cation enables the reduction of N2 under mild conditions, and then chelation of the alkali metal cation uncovers a highly reactive species that can break benzylic C-H bonds to give new N-H and Fe-C bonds. The ability to "turn on" this C-H activation pathway with 18-crown-6 is demonstrated with three different N2 reduction products of N2 cleavage in an iron-potassium system. The alkali control strategy can also turn on an intermolecular reaction of an N2-derived nitride with methyl tosylate that gives a new N-C bond. Since the transient K(+)-free intermediate reacts with this electrophile but not with the weak C-H bonds in 1,4-cyclohexadiene, it is proposed that the C-H cleavage occurs by a deprotonation mechanism. The combined results demonstrate that a K(+) ion can mask the latent nucleophilicity of N2-derived nitride and imide ligands within a trimetallic iron system and points a way toward control over N2 functionalization.

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 ß-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.

Stepwise N-H Bond Formation From N2-Derived Iron Nitride, Imide and Amide Intermediates to Ammonia.

Reduction of N2 to ammonia in nature and in electrocatalysis takes place through 1-proton/1-electron steps, motivating efforts to experimentally study the steps during proton/electron transfer to well-characterized N2-derived species with bridging nitrides. We report here the protonation and reduction reactions of an N2-derived iron bis(nitride) complex (Rodriguez et al., Science, 2011, 334, 780). We isolate and definitively characterize triiron imido and amido intermediates that lie along the path to ammonia formation, and Mössbauer spectroscopy shows the oxidation level of iron atoms in these mixed-valence clusters. The first two H atoms add to one of the two nitrides of the bis(nitride) complex, and the proton-coupled electron transfer in the second step can be concerted or stepwise depending on the sources of protons and electrons. The characterization of partially protonated nitrides and their mechanisms of formation are expected to guide efforts to convert N2 to ammonia with mild acids.

Dinitrogen binding and cleavage by multinuclear iron complexes.

The iron-molybdenum cofactor of nitrogenase has unprecedented coordination chemistry, including a high-spin iron cluster called the iron-molybdenum cofactor (FeMoco). Thus, understanding the mechanism of nitrogenase challenges coordination chemists to understand the fundamental N2 chemistry of high-spin iron sites. This Account summarizes a series of studies in which we have synthesized a number of new compounds with multiple iron atoms, characterized them using crystallography and spectroscopy, and studied their reactions in detail. These studies show that formally iron(I) and iron(0) complexes with three- and four-coordinate metal atoms have the ability to weaken and break the triple bond of N2. These reactions occur at or below room temperature, indicating that they are kinetically facile. This in turn implies that iron sites in the FeMoco are chemically reasonable locations for N2 binding and reduction. The careful evaluation of these compounds and their reaction pathways has taught important lessons about what characteristics make iron more effective for N2 activation. Cooperation of two iron atoms can lengthen and weaken the N-N bond, while three working together enables iron atoms to completely cleave the N-N bond to nitrides. Alkali metals (typically introduced into the reaction as part of the reducing agent) are thermodynamically useful because the alkali metal cations stabilize highly reduced complexes, pull electron density into the N2 unit, and make reduced nitride products more stable. Alkali metals can also play a kinetic role, because cation-π interactions with the supporting ligands can hold iron atoms near enough to one another to facilitate the cooperation of multiple iron atoms. Many of these principles may also be relevant to the iron-catalyzed Haber-Bosch process, at which collections of iron atoms (often promoted by the addition of alkali metals) break the N-N bond of N2. The results of these studies teach more general lessons as well. They have demonstrated that N2 can be a redox-active ligand, accepting spin and electron density in complexes of N2(2-). They have shown the power of cooperation between multiple transition metals, and also between alkali metals and transition metals. Finally, alkali metal based cation-π interactions have the potential to be broadly useful for bringing metals close together with sufficient flexibility to allow multistep, multielectron reactions. At the same time, the positive charge on the alkali metal cation stabilizes charge buildup in intermediates.

Mechanistic elucidation of the stepwise formation of a tetranuclear manganese pinned butterfly cluster via N-N bond cleavage, hydrogen atom transfer, and cluster rearrangement.

A mechanistic pathway for the formation of the structurally characterized manganese-amide-hydrazide pinned butterfly complex, Mn4(µ3-PhN-NPh-κ(3)N,N')2(µ-PhN-NPh-κ(2)-N,N')(µ-NHPh)2L4 (L = THF, py), is proposed and supported by the use of labeling studies, kinetic measurements, kinetic competition experiments, kinetic isotope effects, and hydrogen atom transfer reagent substitution, and via the isolation and characterization of intermediates using X-ray diffraction and electron paramagnetic resonance spectroscopy. The data support a formation mechanism whereby bis[bis(trimethylsilyl)amido]manganese(II) (Mn(NR2)2, where R = SiMe3) reacts with N,N'-diphenylhydrazine (PhNHNHPh) via initial proton transfer, followed by reductive N-N bond cleavage to form a long-lived Mn(IV) imido multinuclear complex. Coordinating solvents activate this cluster for abstraction of hydrogen atoms from an additional equivalent of PhNHNHPh resulting in a Mn(II)phenylamido dimer, Mn2(µ-NHPh)2(NR2)2L2. This dimeric complex further assembles in fast steps with two additional equivalents of PhNHNHPh replacing the terminal silylamido ligands with η(1)-hydrazine ligands to give a dimeric Mn2(µ-NHPh)2(PhN-NHPh)2L4 intermediate, and finally, the addition of two additional equivalents of Mn(NR2)2 and PhNHNHPh gives the pinned butterfly cluster.