High-Spin Iron(I) and Iron(0) Dinitrogen Complexes Supported by N-Heterocyclic Carbene Ligands

Synthesis of heteroleptic bis-phosphine bis-NHC iron (0) complexes: a strategy to enhance small molecule activation

We report the synthesis of heteroleptic iron complexes supported by both a bis-phosphine ligand (depe) and a bis-NHC ligand. The mixed ligand sets provide access to iron (0) adducts of N2 and CO that are highly activated, in comparison to homoleptic (i.e. Fe(depe)2L) variants. Computational and experimental studies revealed the mixed ligand set distorts the geometric and electronic structure to yield an unusually basic iron. Although protonation occurred at Fe, silylation of the Fe(0)N2 complex afforded a highly activated silyldiazenido [FeNNSiMe3]+ complex.

From d8 to d1: Iron(0) and Iron(I) Complexes Complete the Series of Eight Fe Oxidation States within the TIMMNMes Ligand Framework

Reduction of the ferrous precursor [(TIMMNMes)Fe(Cl)]+ (1) (TIMMNMes = tris-[(3-mesitylimidazol-2-ylidene)methyl]amine) to the low-valent iron(0) complex [(TIMMNMes)Fe(CO)3] (2) is presented, where the tris(N-heterocyclic carbene) (NHC) ligand framework remains intact, yet the coordination mode changed from 3-fold to 2-fold coordination of the carbene arms. Further, the corresponding iron(I) complexes [(TIMMNMes)Fe(L)]+ (L = free site, η1-N2, CO, py) (3) are synthesized and fully characterized. Complexes 1-3 demonstrate the notable steric and electronic flexibility of the TIMMNMes ligand framework by variation of the Fe-N anchor and Fe-carbene distances and the variable size of the axial cavity occupation. This is further underpinned by the oxidation of 3-N2 in a reaction with benzophenone to yield the corresponding, charge-separated iron(II) radical complex [(TIMMNMes)Fe(OCPh2)]+ (4). We found rather surprising similarities in the reactivity behavior when going to low- or high-valent oxidation states of the central iron ion. This is demonstrated by the closely related reactivity of 3-N2, where H atom abstraction with TEMPO triggers the formation of the metallacycle [(TIMMNMes*)Fe(py)]+ (5), and the reactivity of the highly unstable Fe(VII) nitride complex [(TIMMNMes)Fe(N)(F)]3+ to give the metallacyclic Fe(V) imido complex [(TIMMNMesN)Fe(NMes)(MeCN)]3+ (6) upon warming. Thus, the employed tris(carbene) chelate is not only capable of stabilizing the superoxidized Fe(VI) and Fe(VII) nitrides but equally supports the iron center in its low oxidation states 0 and +1. Isolation and characterization of these zero- and monovalent iron complexes demonstrate the extraordinary capability of the tris(carbene) chelate TIMMN to support iron in eight different oxidation states within the very same ligand platform.

Iron(I) Complex Bearing an Open-Shell Diazenido Ligand

Low-valent transition-metal diazenido species are important intermediates in transition-metal-mediated dinitrogen reduction reactions. Isolable complexes of the type unanimously feature closed-shell diazenido ligands. Those bearing open-shell diazenido ligands have remained elusive. Herein, we report the synthesis, characterization, and reactivity of a d7 iron(I) complex featuring an open-shell silyldiazenido ligand, [(ICy)Fe(NNSiiPr3)(η2:η2-dvtms)] (1, ICy = 1,3-dicyclohexylimidazole-2-ylidene, dvtms = divinyltetramethyldisiloxane). Complex 1 is prepared in good yield by silylation of the iron(-I)-N2 complex [K(18-crown-6)][(ICy)Fe(N2)(η2:η2-dvtms)] with iPr3SiOTf and has been fully characterized by various spectroscopic methods. Theoretical studies, in combination with characterization data, established an S = 1/2 ground spin-state for 1 that can best be described as a quartet iron(I) center featuring an antiferromagnetically coupled triplet silyldiazenido ligand. The diazenido and alkene ligands in 1 are labile, as indicated by the facile disproportionation reaction of 1 at ambient temperature to transform into the iron(II) bis(diazenido) species [(ICy)(NNSiiPr3)2Fe(dvtms)Fe(NNSiiPr3)2(ICy)] (2) and the iron(0) species [(ICy)Fe(η2:η2-dvtms)] and also the alkene-exchange reaction of 1 with PhCH═CHBC8H14 to form [(ICy)Fe(NNSiiPr3)(η2-trans-PhCH═CHBC8H14)] (3). Complex 1 is light-sensitive. Upon photolysis, it undergoes a SiiPr3 radical-transfer reaction to yield [(ICy)Fe(σ:η2-MeCHSiMe2OSiMe2CH═CHSiiPr3)] (4) and N2. The reactions of 1 with the trityl radical and organic bromides yield iron(II) complexes, which indicates its reducing nature. Moreover, 1 is a weak hydrogen-atom abstractor, as indicated by its inertness toward HSi(SiMe3)3 and cyclohexa-1,4-diene and the low calculated N-H bond dissociation energy (48 kcal/mol) of its corresponding iron(II) iso-hydrazenido species.

Terminal end-on coordination of dinitrogen versus isoelectronic CO: A comparison using the charge displacement analysis

The metal dinitrogen bonding in a wide series of terminal end-on dinitrogen complexes is investigated with the charge displacement analysis based on natural orbitals of chemical valence (CD-NOCV). The effect of the σ donation and π backdonation on the NN bond are discussed and compared with the observations for a series of carbonyl complexes, published in 2016 by Tarantelli et al. The σ donation is relative invariant over the series of dinitrogen complexes and has no significant effect on the NN bond strength, whereas the π backdonation causes a considerable elongation of the NN bond. Some uncommon examples of weakly bound dinitrogen with blue-shifted stretching frequency compared to free N2 (ν = 2330 cm-1 ) are known. The dinitrogen bonding in these complexes is simulated with a point charge. Apparently, electrostatics account for the shortened N─N bond in these systems.

Disarming the alkoxide trap to access a practical FeCl3 system for borrowing-hydrogen N-alkylation

Disarming the alkoxide trap using an in situ reduction strategy to access a practical FeCl3 and N-heterocyclic carbene system for borrowing-hydrogen N-alkylation.

Computational Study of CO2 Reduction Catalyzed by Iron(I) Complex at Different Spin States: Cooperativity of Hydrogen Bonding and Auxiliary Group Effect

To explore alternative approaches to the CO₂ reduction to formate and provide an insight into the spin state effect on the CO₂ reduction, we theoretically designed a kind of low-valence iron(I) model complex, whose doublet, quartet, and sextet states are denoted as ² Fe(I), ⁴ Fe(I), and ⁶ Fe(I), respectively. This complex is featured with an iron(I) center, which bonds to a 1,2-ethanediamine (en) and a 2-hydroxy-biphenyl group. Reaction mechanisms for the CO₂ reduction to formate catalyzed by this iron(I) model complex were explored using density functional theory (DFT) computations. Studies showed that the univalent iron(I) compound can efficiently fix and activate a CO₂ molecule, whereas its oxidized forms with trivalent iron(III) or bivalent iron(II) cannot activate CO₂. For the iron(I) compound, it was found that the lowest spin state ² Fe(I) is the most favorable for the CO₂ reduction as the reactions barriers involving ² Fe(I), ⁴ Fe(I), and ⁶ Fe(I) are 25.6, 37.2, and 35.9 kcal/mol, respectively. Yet, a photosensitizer-free visible-light-mediated high-low spin shift from ⁴ Fe(I) and ⁶ Fe(I) to ² Fe(I) is likely through the reverse intersystem crossing (RIC) because the ⁴ Fe(I) and ⁶ Fe(I) compounds have strong absorption in the visible-light range. Notably, the synergistic interaction between the hydrogen bonding from the auxiliary hydroxyl group in the 2-hydroxy-biphenyl moiety to CO₂ and an intermediate five-membered ring promotes the proton transfer, leading to the formation of the -COOH moiety from CO₂ and the Fe-O bond. With the addition of H₂, one H₂ molecule is split by the Fe-O bond and thus serves as H atom sources for both the CO₂ reduction and the recovery of the auxiliary hydroxyl group. The present theoretical study provides a novel solution for the challenging CO₂ reduction, which calls for further experimental verifications.

Systematic Design of a Frustrated Lewis Pair Containing Methyleneborane and Carbene for Dinitrogen Activation

Activation of atmospherically abundant dinitrogen (N₂) by metal-free species under mild reaction conditions has been one of the most challenging areas in chemistry for decades. Very recent but limited progress in N₂ activation by boron species, including two-coordinated borylene and methyleneborane and three-coordinated borole and borane, has been made toward metal-free N₂ activation. Here, we systematically probe an experimentally viable frustrated Lewis pair (FLP) containing two moieties (methyleneborane and carbene) for N₂ activation via density functional theory (DFT) calculations, which has proven to be an efficient approach for N₂ activation in a thermodynamically and kinetically favorable manner. Aromaticity is found to play a crucial role in stabilization of the product. This study could be a valuable alternative for the development of metal-free N₂ activation chemistry, highlighting great potential of FLP for N₂ activation and functionalization.

Synthesis and Reactivity of Iron Complexes with a Biomimetic SCS Pincer Ligand

Recent experimental evidence suggests that the FeMoco of nitrogenase undergoes structural rearrangement during N₂ reduction, which may result in the generation of coordinatively unsaturated iron sites with two sulfur donors and a carbon donor. In an effort to synthesize and study small-molecule model complexes with a one-carbon/two-sulfur coordination environment, we have designed two new SCS pincer ligands containing a central NHC donor accompanied by thioether- or thiolate-functionalized aryl groups. Metalation of the thioether ligand with Fe(OTf)₂ gives 6-coordinate complexes in which the SCS ligand binds meridionally. In contrast, metalation of the thiolate ligand with Fe(HMDS)₂ gives a four-coordinate pseudotetrahedral amide complex in which the ligand binds facially, illustrating the potential structural flexibility of these ligands. Reaction of the amide complex with a bulky monothiol gives a four-coordinate complex with a one-carbon/three-sulfur coordination environment that resembles the resting state of nitrogenase. Reaction of the amide complex with phenylhydrazine gives a product with a rare κ¹-bound phenylhydrazido group which undergoes N-N cleavage to give a phenylamido complex.

Two-State Reactivity in Iron-Catalyzed Alkene Isomerization Confers σ-Base Resistance

A low-coordinate, high spin (S = 3/2) organometallic iron(I) complex is a catalyst for the isomerization of alkenes. A combination of experimental and computational mechanistic studies supports a mechanism in which alkene isomerization occurs by the allyl mechanism. Importantly, while substrate binding occurs on the S = 3/2 surface, oxidative addition to an η¹-allyl intermediate only occurs on the S = 1/2 surface. Since this spin state change is only possible when the alkene substrate is bound, the catalyst has high immunity to typical σ-base poisons due to the antibonding interactions of the high spin state.

Iron N-heterocyclic carbene complexes in homogeneous catalysis

This review article summarizes recent development of homogeneous iron N-heterocyclic carbene catalysts.

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.

High-spin diiron alkylidenes give insight into the electronic structure and functional relevance of carbon in the FeMoco active site of nitrogenase.

Nitrogenase-Relevant Reactivity of a Synthetic Iron-Sulfur-Carbon Site

Simple synthetic compounds with only S and C donors offer a ligation environment similar to the active site of nitrogenase (FeMoco) and thus demonstrate reasonable mechanisms and geometries for N₂ binding and reduction in nature. We recently reported the first example of N₂ binding at a mononuclear iron site supported by only S and C donors. In this work, we report experiments that examine the mechanism of N₂ binding in this system. The reduction of an iron(II) tris(thiolate) complex with 1 equiv of KC₈ leads to a thermally unstable intermediate, and a combination of Mössbauer, EPR, and X-ray absorption spectroscopies identifies it as a high-spin (S = 3/2) iron(I) species that maintains coordination of all three sulfur atoms. DFT calculations suggest that this iron(I) intermediate has a pseudotetrahedral geometry that resembles the S₃C iron coordination environment of the belt iron sites in the resting state of the FeMoco. Further reduction to the iron(0) oxidation level under argon causes the dissociation of one of the thiolate donors and gives an η⁶-arene species which reacts with N₂. Thus, in this system the loss of thiolate and binding of N₂ require reduction beyond the iron(I) level to the iron(0) level. Further reduction of the iron(0)-N₂ complex gives a reactive, formally iron(-I) species. Treatment of the putative iron(-I) complex with weak acids gives low yields of ammonia and hydrazine, demonstrating that these nitrogenase products can be generated from N₂ at a synthetic Fe-S-C site. Catalytic N₂ reduction is not observed, which is attributed to protonation of the supporting ligand and degradation of the complex via ligand dissociation. Identification of the challenges in this system gives insight into the design features needed for functional biomimetic complexes.

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.

An arene-tethered silylene ligand enabling reversible dinitrogen binding to iron and catalytic silylation

A silylene–iron(0) dinitrogen complex enabled the catalytic silylation of N₂ with high activity.

Triphos–Fe dinitrogen and dinitrogen–hydride complexes: relevance to catalytic N2 reductions

Mechanistic investigations of the catalyzed reduction of N₂ with a rare (tridentate phosphine)Fe(0) bis dinitrogen complex.

Reversible dinitrogen binding to [Cp′Fe(NHC)] associated with an N2-induced spin state change

Reversible N₂ coordination to [Cp′Fe(IiPr₂Me₂)] induces a spin-state change.