Iron–dinitrogen coordination chemistry: Dinitrogen activation and reactivity

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.

Synthesis and Characterization of a Copper Dinitrogen Complex Supported by a Weakly Coordinating Anion

We report the synthesis and full characterization of the copper dinitrogen complex [(η1-N2)Cu{Al(ORF)4}] 2 (RF=C(CF3)3) prepared by a cascade metathesis reaction of Ag[Al(ORF)4] with CuI-excess in iso-perfluorohexane (i-pfh) under N2 atmosphere. Title compound 2 features an extraordinarily high N2 stretching frequency at 2313/2314 cm-1 (IR/Raman) and was characterized by single-crystal and powder X-ray diffractometry. Quantum chemical charge displacement analysis based on natural orbitals of chemical valence (CD-NOCV) indicates that the copper-dinitrogen interaction is still governed by weak π-backdonation, but is significantly reduced compared to all literature-known transition metal dinitrogen complexes.

Formation of a diiron-(μ-η1:η1-CN) complex from acetonitrile solution

The activation of C-C bonds by transition-metal complexes is of continuing interest and acetonitrile (MeCN) has attracted attention as a cyanide source with comparatively low toxicity for organic cyanation reactions. A diiron end-on μ-η1:η1-CN-bridged complex was obtained from a crystallization experiment of an open-chain iron-NHC complex, namely, μ-cyanido-κ2C:N-bis{[(acetonitrile-κN)[3,3'-bis(pyridin-2-yl)-1,1'-(methylidene)bis(benzimidazol-2-ylidene)]iron(II)} tris(hexafluorophosphate), [Fe2(CN)(C2H3N)2(C25H18N6)2](PF6)3. The cyanide appears to originate from the MeCN solvent by C-C bond cleavage or through carbon-hydrogen oxidation.

The activating capture of N2 at the active site of Mo-nitrogenase

Dinitrogen is inherently inert. This report describes detailed density functional calculations (with a 485+ atom model) of mechanistic steps by which the enzyme nitrogenase activates unreactive N2 at the intact active site FeMo-co, to form a key intermediate with bound HNNH. This mechanism does not bind N2 first and then add H atoms, but rather captures N2 ('N2-ready') that diffuses in through the substrate channel and enters a strategic gallery of H atom donors in the reaction zone, between Fe2 and Fe6. This occurs at the E4 stage of the complete mechanism. Exploration of possible reactions of N2 in this space leads to the conclusion that the first reaction step is transfer of H on Fe7 to one end of N2-ready, soon followed by Fe-N bond formation, and then a second H transfer from bridging S2BH to the other N. Two H-N bonds and one or two N-Fe bonds are formed, in some cases with a single transition state. The variable positions and orientations of N2-ready lead to various reaction trajectories and products. The favourable products resulting from this capture, judged by the criteria of reaction energies, reaction barriers, and mechanistic competence for further hydrogenation reactions in the nitrogenase cycle, have Fe2-NH-NH bonding. The trajectory of one N2 capture reaction is described in detail, and calculations that separate the H atom component and the 'heavy atom' components of the classical activation energy are described, in the context of possible H atom tunneling in the activation of N2-ready. I present arguments for the activation of N2 by the pathway of concerted hydrogenation and binding of N2-ready, alternative to the commonly assumed pathway of binding N2 first, with subsequent hydrogenation. The active site of nitrogenase is well primed for the thermodynamic and kinetic advantages of N2 capture.

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.

Synthesis and Characterization of Bridging-Diazene Diiron Half-Sandwich Complexes: The Role of Sulfur Hydrogen Bonding

We report two bridging-diazene diiron complexes [Cp*Fe(8-quinolinethiolate)]2(μ-N2H2) (1-N2H2) and [Cp*Fe(1,2-Cy2PC6H4S)]2(μ-N2H2) (2-N2H2), synthesized by the reaction of hydrazine with the corresponding thiolate-based iron half-sandwich complex, [Cp*Fe(8-quinolinethiolate)]2 (1) and Cp*Fe(1,2-Cy2PC6H4S) (2). Crystallographic analysis reveals that the thiolate sites in 1-N2H2 and 2-N2H2 can engage in N-H···S hydrogen bonding with the diazene protons. 1-N2H2 is thermally stable in both solid and solution states, allowing for one-electron oxidation to afford a cationic diazene radical complex [1-N2H2]+ at room temperature. In contrast, 2-N2H2 tends to undergo N2H2/N2 transformation, leading to the formation of a Fe(III)-H species by the loss of N2. In addition to stabilizing HN=NH species through the hydrogen bonding, the thiolate-based ligands also seem to facilitate proton-coupled electron transfer, thereby promoting N-H cleavage.

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.

Bimetallic H2 Addition and Intramolecular Caryl-H Activation Mediated by an Iron-Zinc Hydride

Heteronuclear Fe(μ-H)Zn hydride Cp*Fe(1,2-Cy2PC6H4)HZnEt (3) undergoes reversible intramolecular Caryl-H reductive elimination through coupling of the cyclometalated phosphinoaryl ligand and the hydride, giving rise to a formal Fe(0)-Zn(II) species. Addition of CO intercepts this equilibrium, affording Cp*(Cy2PPh)(CO)Fe-ZnEt that features a dative Fe-Zn bond. Significantly, this system achieves bimetallic H2 addition, as demonstrated by the transformation of the monohydride Fe(μ-H)Zn to a deuterated dihydride Fe-(μ-D)2-Zn upon reaction with D2.

The synthesis and characterization of an iron(VII) nitrido complex

Complexes of iron in high oxidation states are captivating research subjects due to their pivotal role as active intermediates in numerous catalytic processes. Structural and spectroscopic studies of well-defined model complexes often provide evidence of these intermediates. In addition to the fundamental molecular and electronic structure insights gained by these complexes, their reactivity also affects our understanding of catalytic reaction mechanisms for small molecule and bond-activation chemistry. Here, we report the synthesis, structural and spectroscopic characterization of a stable, octahedral Fe(VI) nitrido complex and an authenticated, unique Fe(VII) species, prepared by one-electron oxidation. The super-oxidized Fe(VII) nitride rearranges to an Fe(V) imide through an intramolecular amination mechanism and ligand exchange, which is characterized spectroscopically and computationally. This enables combined reactivity and stability studies on a single molecular system of a rare high-valent complex redox pair. Quantum chemical calculations complement the spectroscopic parameters and provide evidence for a diamagnetic (S = 0) d 2 Fe(VI) and a genuine S = 1/2, d 1 Fe(VII) configuration of these super-oxidized nitrido complexes.

Alkaline earth metal-assisted dinitrogen activation at nickel

Rare examples of trinuclear [Ni-N2-M-N2-Ni] core (M = Ca, Mg) with linear bridged dinitrogen ligands are reported in this work. The reduction of [iPr2NN]Ni(μ-Br)2Li(thf)2 (1) (iPr2NN = 2,4-bis-(2,6-diisopropylphenylimido)pentyl) with elemental Mg or Ca in THF under an atmosphere of dinitrogen yields the complex {iPr2NNNi(μ-N2)}2M (thf)4 (M = Mg, complex 2 and M = Ca, complex 3). The bridging end-on (μ-N2)2M(thf)4 moiety connects the two [iPr2NNNi]- nickelate fragments. A combination of X-ray crystallography, solution and solid-state spectroscopy have been applied to characterize complexes 2 and 3, and DFT studies have been used to help explain the bonding and electronic structure in these unique Ni-N2-Mg and Ni-N2-Ca complexes.

Combining Donor Strength and Oxidative Stability in Scorpionates: A Strongly Donating Fluorinated Mesoionic Tris(imidazol-5-ylidene)borate Ligand

Strongly donating scorpionate ligands support the study of high-valent transition metal chemistry; however, their use is frequently limited by oxidative degradation. To address this concern, we report the synthesis of a tris(imidazol-5-ylidene)borate ligand featuring trifluoromethyl groups surrounding its coordination pocket. This ligand represents the first example of a chelating poly(imidazol-5-ylidene) mesoionic carbene ligand, a scaffold that is expected to be extremely donating. The {NiNO}10 complex of this ligand, as well as that of a previously reported strongly donating tris(imidazol-2-ylidene)borate, has been synthesized and characterized. This new ligand's strong donor properties, as measured by the υNO of its {NiNO}10 complex and natural bonding orbital second-order perturbative energy analysis, are at par with those of the well-studied alkyl-substituted tris(imidazol-2-ylidene)borates, which are known to effectively stabilize high-valent intermediates. The good donor properties of this ligand, despite the electron-withdrawing trifluoromethyl substituents, arise from the strongly donating imidazol-5-ylidene mesoionic carbene arms. These donor properties, when combined with the robustness of trifluoromethyl groups toward oxidative decomposition, suggest this ligand scaffold will be a useful platform in the study of oxidizing high-valent transition-metal species.

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.

Metal Messengers: Communication in the Bacterial World through Transition-Metal-Sensing Two-Component Systems

Bacteria survive in highly dynamic and complex environments due, in part, to the presence of systems that allow the rapid control of gene expression in the presence of changing environmental stimuli. The crosstalk between intra- and extracellular bacterial environments is often facilitated by two-component signal transduction systems that are typically composed of a transmembrane histidine kinase and a cytosolic response regulator. Sensor histidine kinases and response regulators work in tandem with their modular domains containing highly conserved structural features to control a diverse array of genes that respond to changing environments. Bacterial two-component systems are widespread and play crucial roles in many important processes, such as motility, virulence, chemotaxis, and even transition metal homeostasis. Transition metals are essential for normal prokaryotic physiological processes, and the presence of these metal ions may also influence pathogenic virulence if their levels are appropriately controlled. To do so, bacteria use transition-metal-sensing two-component systems that bind and respond to rapid fluctuations in extracytosolic concentrations of transition metals. This perspective summarizes the structural and metal-binding features of bacterial transition-metal-sensing two-component systems and places a special emphasis on understanding how these systems are used by pathogens to establish infection in host cells and how these systems may be targeted for future therapeutic developments.

From Divalent to Pentavalent Iron Imido Complexes and an Fe(V) Nitride via N-C Bond Cleavage

As key intermediates in metal-catalyzed nitrogen-transfer chemistry, terminal imido complexes of iron have attracted significant attention for a long time. In search of versatile model compounds, the recently developed second-generation N-anchored tris-NHC chelating ligand tris-[2-(3-mesityl-imidazole-2-ylidene)-methyl]amine (TIMMN^Mes) was utilized to synthesize and compare two series of mid- to high-valent iron alkyl imido complexes, including a reactive Fe(V) adamantyl imido intermediate en route to an isolable Fe(V) nitrido complex. The chemistry toward the iron adamantyl imides was achieved by reacting the Fe(I) precursor [(TIMMN^Mes)Fe^I(N₂)]⁺ (1) with 1-adamantyl azide to yield the corresponding trivalent iron imide. Stepwise chemical reduction and oxidation lead to the isostructural series of low-spin [(TIMMN^Mes)Fe(NAd)]^0,1+,2+,3+ (2^Ad-5^Ad) in oxidation states II to V. The Fe(V) imide [(TIMMN^Mes)Fe(NAd)]³⁺ (5^Ad) is unstable under ambient conditions and converts to the air-stable nitride [(TIMMN^Mes)Fe^V(N)]²⁺ (6) via N-C bond cleavage. The stability of the pentavalent imide can be increased by derivatizing the nitride [(TIMMN^Mes)Fe^IV(N)]⁺ (7) with an ethyl group using the triethyloxonium salt Et₃OPF₆. This gives access to the analogous series of ethyl imides [(TIMMN^Mes)Fe(NEt)]^0,1+,2+,3+ (2^Et-5^Et), including the stable Fe(V) ethyl imide. Iron imido complexes exist in a manifold of different electronic structures, ultimately controlling their diverse reactivities. Accordingly, these complexes were characterized by single-crystal X-ray diffraction analyses, SQUID magnetization, and electrochemical methods, as well as ⁵⁷Fe Mössbauer, IR vibrational, UV/vis electronic absorption, multinuclear NMR, X-band EPR, and X-ray absorption spectroscopy. Our studies are complemented with quantum chemical calculations, thus providing further insight into the electronic structures of all complexes.

Terminal N2 Dissociation in [(PNN)Fe(N2 )]2 (μ-N2 ) Leads to Local Spin-State Changes and Augmented Bridging N2 Activation

Nitrogen fixation at iron centres is a fundamental catalytic step for N₂ utilisation, relevant to biological (nitrogenase) and industrial (Haber-Bosch) processes. This step is coupled with important electronic structure changes which are currently poorly understood. We show here for the first time that terminal dinitrogen dissociation from iron complexes that coordinate N₂ in a terminal and bridging fashion leaves the Fe-N₂ -Fe unit intact but significantly enhances the degree of N₂ activation (Δν≈180 cm^-1 , Raman spectroscopy) through charge redistribution. The transformation proceeds with local spin state change at the iron centre (S= 1 / 2 ${{ 1/2 }}$ →S=³ /₂ ). Further dissociation of the bridging N₂ can be induced under thermolytic conditions, triggering a disproportionation reaction, from which the tetrahedral (PNN)₂ Fe could be isolated. This work shows that dinitrogen activation can be induced in the absence of external chemical stimuli such as reducing agents or Lewis acids.

Dinitrogen Binding and Activation: Bonding Analyses of Stable V(III/I)-N2-V(III/I) Complexes by the EDA-NOCV Method from the Perspective of Vanadium Nitrogenase

The FeVco cofactor of nitrogenase (VFe₇S₈(CO₃)C) is an alternative in the molybdenum (Mo)-deficient free soil living azotobacter vinelandii. The rate of N₂ reduction to NH₃ by FeVco is a few times higher than that by FeMoco (MoFe₇S₉C) at low temperature. It provides a N source in the form of ammonium ions to the soil. This biochemical NH₃ synthesis is an alternative to the industrial energy-demanding production of NH₃ by the Haber-Bosch process. The role of vanadium has not been clearly understood yet, which has led chemists to come up with several stable V-N₂ complexes which have been isolated and characterized in the laboratory over the past three decades. Herein, we report the EDA-NOCV analyses of dinitrogen-bonded stable complexes V(III/I)-N₂ (1-4) to provide deeper insights into the fundamental bonding aspects of V-N₂ bond, showing the interacting orbitals and corresponding pairwise orbital interaction energies (ΔE _orb(n)). The computed intrinsic interaction energy (ΔE _int) of V-N₂-V bonds is significantly higher than those of the previously reported Fe-N₂-Fe bonds. Covalent interaction energy (ΔE _orb) is more than double the electrostatic interaction energy (ΔE _elstat) of V-N₂-V bonds. ΔE _int values of V-N₂-V bonds are in the range of -172 to -204 kcal/mol. The V → N₂ ← V π-backdonation is four times stronger than V ← N₂ → V σ-donation. V-N₂ bonds are much more covalent in nature than Fe-N₂ bonds.

Spontaneous N2-diboranylation of [W(N2)2(dppe)2] with B2Br4(SMe2)2

The 1,3-bromoboration of [W(N₂)₂(dppe)₂] (dppe = 1,2-bis(diphenylphosphino)ethane) with B₂Br₄(SMe₂)₂ in the presence of various Lewis bases L yields diboranyldiazenido complexes, with L coordinating either at the terminal or internal boron atom. The 2 : 1 reaction of [W(N₂)₂(dppe)₂] and B₂Br₄(SMe₂)₂ yields a 1,2-bis(diazenido)diborane-bridged ditungsten complex with a fully planar π-conjugated BrWN₂B₂Br₂N₂WBr core.

Dehydrated UiO-66(SH)2 : The Zr-O Cluster and Its Photocatalytic Role Mimicking the Biological Nitrogen Fixation

This work reports the dehydrated Zr-based MOF UiO-66(SH)₂ as a visible-light-driven photocatalyst to mimic the biological N₂ fixation process. The ¹⁵ N₂ and other control experiments demonstrated that the new photocatalyst is highly efficient in converting N₂ to ammonia. In-situ TGA, XPS, and EXAFS as well as first-principles simulations were used to demonstrate the role of the thermal treatment and the changes of the local structures around Zr due to the dehydration. It was shown that the dehydration opened a gate for the entry of N₂ molecules into the [Zr₆ O₆ ] cluster where the strong N≡N bond was broken stepwise by μ-N-Zr type interactions driven by the photoelectrons aided by the protonation. This mechanism was discussed in comparison with the Lowe-Thorneley mechanism proposed for the MoFe nitrogenase, and with emphasis on the [Zr₆ O₆ ] cluster effect and the leading role of photoelectrons over the protonation. The results shed new light on understanding the catalytic mechanism of biological N₂ fixation and open a new way to fix N₂ under mild conditions.

EDA-NOCV analysis of carbene-borylene bonded dinitrogen complexes for deeper bonding insight: A fair comparison with a metal-dinitrogen system

Binding of dinitrogen (N₂ ) to a transition metal center (M) and followed by its activation under milder conditions is no longer impossible; rather, it is routinely studied in laboratories by transition metal complexes. In contrast, binding of N₂ by main group elements has been a challenge for decades, until very recently, an exotic cAAC-borylene (cAAC = cyclic alkyl(amino) carbene) species showed similar binding affinity to kinetically inert and non-polar dinitrogen (N₂ ) gas under ambient conditions. Since then, N₂ binding by short lived borylene species has made a captivating news in different journals for its unusual features and future prospects. Herein, we carried out different types of DFT calculations, including EDA-NOCV analysis of the relevant cAAC-boron-dinitrogen complexes and their precursors, to shed light on the deeper insight of the bonding secret (EDA-NOCV = energy decomposition analysis coupled with natural orbital for chemical valence). The hidden bonding aspects have been uncovered and are presented in details. Additionally, similar calculations have been carried out in comparison with a selected stable dinitrogen bridged-diiron(I) complex. Singlet cAAC ligand is known to be an exotic stable species which, combined with the BAr group, produces an intermediate singlet electron-deficient (cAAC)(BAr) species possessing a high lying HOMO suitable for overlapping with the high lying π*-orbital of N₂ via effective π-backdonation. The BN₂ interaction energy has been compared with that of the FeN₂ bond. Our thorough bonding analysis might answer the unasked questions of experimental chemists about how boron compounds could mimic the transition metal of dinitrogen binding and activation, uncovering hidden bonding aspects. Importantly, Pauling repulsion energy also plays a crucial role and decides the binding efficiency in terms of intrinsic interaction energy between the boron-center and the N₂ ligand.

A Parent Iron Amido Complex in Catalysis of Ammonia Oxidation

Parent amido complexes are crucial intermediates in ammonia-based transformations. We report a well-defined ferric ammine system [Cp*Fe(1,2-Ph₂PC₆H₄NH)(NH₃)]⁺ ([1-NH₃]⁺), which processes electrocatalytic ammonia oxidation to N₂ and H₂ at a mild potential. Through establishing elementary e^-/H⁺ conversions with the ferric ammine, a formal Fe(IV)-amido species, [1-NH₂]⁺, together with its conjugated Lewis acid, [1-NH₃]²⁺, was isolated and structurally characterized for the first time. Mechanism studies indicated that further oxidation of [1-NH₂]⁺ induces the reaction of the parent amido unit with NH₃. The formation of hydrazine is realized by the non-innocent nature of the phenylamido ligand that facilitates the concerted transfer of one proton and two electrons.

Bonding and stability of dinitrogen-bonded donor base-stabilized Si(0)/Ge(0) species [(cAACMe-Si/Ge)2(N2)]: EDA-NOCV analysis

Recently, dinitrogen (N₂) binding and its activation have been achieved by non-metal compounds like intermediate cAAC-borylene as (cAAC)₂(B-Dur)₂(N₂) [cAAC = cyclic alkyl(amino) carbene; Dur = aryl group, 2,3,5,6-tetramethylphenyl; B-Dur = borylene]. It has attracted a lot of scientific attention from different research areas because of its future prospects as a potent species towards the metal free reduction of N₂ into ammonia (NH₃) under mild conditions. Two (cAAC)(B-Dur) units, each of which possesses six valence electrons around the B-centre, are shown to accept σ-donations from the N₂ ligand (B ← N₂). Two B-Dur further provide π-backdonations (B → N₂) to a central N₂ ligand to strengthen the B–N₂–B bond, providing maximum stability to the compound (cAAC)₂(B-Dur)₂(N₂) since the summation of each pair wise interaction accounted for the total stabilization energy of the molecule. (cAAC)(B-Dur) unit is isolobal to cAAC–E (E = Si, Ge) fragment. Herein, we report on the stability and bonding of cAAC–E bonded N₂-complex (cAAC–E)₂(N₂) (1–2; Si, Ge) by NBO, QTAIM and EDA-NOCV analyses (EDA-NOCV = energy decomposition analysis coupled with natural orbital for chemical valence; QTAIM = quantum theory of atoms in molecule). Our calculation suggested that syntheses of elusive (cAAC–E)₂(N₂) (1–2; Si, Ge) species may be possible with cAAC ligands having bulky substitutions adjacent to the C_cAAC atom by preventing the homo-dimerization of two (cAAC)(E) units which can lead to the formation of (cAAC–E)₂. The formation of E Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> E bond is thermodynamically more favourable (E = Si, Ge) over binding energy of N₂ inbetween two cAAC–E units.

Dinitrogen (N₂) binding and its activation have been achieved by non-metal compounds like intermediate cAACborylene with the general formula of (cAAC)₂(B-Dur)₂(N₂) [cAAC = cyclic alkyl(amino)carbene; Dur = aryl group, 2,3,5,6-tetramethylphenyl; B-Dur = aryl-borylene].

Estimations of Fe0/−1–N2 interaction energies of iron(0)-dicarbene and its reduced analogue by EDA-NOCV analyses: crucial steps in dinitrogen activation under mild conditions

Metal complexes containing low valence iron atoms are often experimentally observed to bind with the dinitrogen (N₂) molecule. This phenomenon has attracted the attention of industrialists, chemists and bio-chemists since these N₂-bonded iron complexes can produce ammonia under suitable chemical or electrochemical conditions. The higher binding affinity of the Fe-atom towards N₂ is a bit ‘mysterious’ compared to that of the other first row transition metal atoms. Fine powders of α-Fe⁰ are even part of industrial ammonia production (Haber–Bosch process) which operates at high temperature and high pressure. Herein, we report the EDA-NOCV analyses of the previously reported dinitrogen-bonded neutral molecular complex (cAAC^R)₂Fe⁰–N₂ (1) and mono-anionic complex (cAAC^R)₂Fe⁻¹–N₂ (2) to give deeper insight of the Fe–N₂ interacting orbitals and corresponding pairwise intrinsic interaction energies (cAAC^R = cyclic alkyl(amino) carbene; R = Dipp or Me). The Fe⁰ atom of 1 prefers to accept electron densities from N₂via σ-donation while the comparatively electron rich Fe⁻¹ centre of 2 donates electron densities to N₂via π-backdonation. However, major stability due to the formation of an Fe–N₂ bond arises due to Fe → N₂ π-backdonation in both 1 and 2. The cAAC^R ligands act as a charge reservoir around the Fe centre. The electron densities drift away from cAAC ligands during the binding of N₂ molecules mostly via π-backdonation. EDA-NOCV analysis suggests that N₂ is a stronger π-acceptor rather than a σ-donor.

The stable Fe–N₂ bond of stable complex should have a sufficiently high interaction energy.

EDA-NOCV Calculation for Efficient N2 Binding to the Reduced Ni3S8 Complex: Estimation of Ni-N2 Intrinsic Interaction Energies

The binding of the dinitrogen molecule to the metal center is the first and crucial step toward dinitrogen activation. Favorable interaction energies are desired by chemists and biochemists to study model complexes in the laboratory. An electrochemically reduced form of a previously isolated sulfur-bridged Ni₃S₈ complex is inferred to bind N₂ at multiple Ni centers, and this bonded N₂ undergoes reductive protonation to produce hydrazine (N₂H₄) as the product in the presence of a proton donor. Density functional theory (DFT) calculations and quantum theory of atoms in molecules (QTAIM) analysis have been carried out to shed light on the nature of N₂ binding to an anionic trinuclear Ni₃S₈ complex. Additionally, energy decomposition analysis with the combination of natural orbital for chemical valence (EDA-NOCV) analysis has been performed to estimate the pairwise interaction energies between the Ni center and the N₂ molecule under experimental conditions.

Estimations of Fe-N2 Intrinsic Interaction Energies of Iron-Sulfur/Nitrogen-Carbon Sites: A Deeper Bonding Insight by EDA-NOCV Analysis of a Model Complex of the Nitrogenase Cofactor

The MoFe₇S₉C^1- unit of the nitrogenase cofactor (FeMoco) attracts chemists and biochemists due to its unusual ability to bind aerial dinitrogen (N₂) at ambient condition and catalytically convert it into ammonia (NH₃). The mode of N₂ binding and its reaction pathways are yet not clear. An important conclusion has been made based on the very recent synthesis and isolation of model Fe(I/0)-complexes with sulfur-donor ligands under the cleavage of one Fe-S bond followed by binding of N₂ at the Fe(0) center. These complexes are structurally relevant to the nitrogenase cofactor (MoFe₇S₉C^1-). Herein, we report the EDA-NOCV analyses and NICS calculations of the dinitrogen-bonded dianionic complex Fe⁰-N₂ (1) (having a C_Ar ← Fe π-bond) and monoanionic complex Fe^I-N₂ (2) (having a C_Ar-Fe σ-bond) to provide a deeper insight into the Fe-N₂ interacting orbitals and corresponding pairwise interaction energies (EDA-NOCV = energy decomposition analysis coupled with natural orbital for chemical valence; NICS = nucleus-independent chemical shifts). The orbital interaction in the Fe-N₂ bond is significantly larger than Coulombic interactions, with major pairwise contributions coming from d(Fe) orbitals to the empty π* orbitals of N₂ (three Fe → N₂). ΔE _int values are in the range of -61 to -77 kcal mol^-1. Very interestingly, NICS calculations have been carried out for the fragments before and after binding of the N₂ molecule. The computed σ- and π-aromaticity values are attributed to the position of the Fe atoms, oxidation states of Fe centers, and Fe-C bond lengths of these two complexes.

Comprehensive insights into synthetic nitrogen fixation assisted by molecular catalysts under ambient or mild conditions

N2 is fixed as NH3 industrially by the Haber-Bosch process under harsh conditions, whereas biological nitrogen fixation is achieved under ambient conditions, which has prompted development of alternative methods to fix N2 catalyzed by transition metal molecular complexes. Since the early 21st century, catalytic conversion of N2 into NH3 under ambient conditions has been achieved by using molecular catalysts, and now H2O has been utilized as a proton source with turnover frequencies reaching the values found for biological nitrogen fixation. In this review, recent advances in the development of molecular catalysts for synthetic N2 fixation under ambient or mild conditions are summarized, and potential directions for future research are also discussed.

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.

Ligand Tailoring Toward an Air-Stable Iron(V) Nitrido Complex

A new supporting ligand, tris-[2-(3-mesityl-imidazol-2-ylidene)methyl]amine (TIMMN^Mes), was developed and utilized to isolate an air-stable iron(V) complex bearing a terminal nitrido ligand, which was synthesized by one-electron oxidation from the iron(IV) precursor. Single-crystal X-ray diffraction analyses of both complexes reveal that the metal-centered oxidation is escorted by iron nitride (Fe≡N) bond elongation, which in turn is accompanied by the accommodation of the high-valence iron center closer to the equatorial plane of a trigonal bipyramid. This contrasts with the previous observation of the only other literature-known Fe(IV)≡N/Fe(V)≡N redox pair, namely, [PhB(^tBuIm)₃FeN]^0/+. On the basis of ⁵⁷Fe Mössbauer, EPR, and UV/vis electronic absorption spectroscopy as well as quantum chemical calculations, we identified the lesser degree of pyramidalization around the iron atom, the Jahn-Teller distortion, and the resulting nature of the SOMO to be the decisive factors at play.

Electrochemical N2 fixation to NH3 under ambient conditions: porous LiFe5O8 nanoparticle–reduced graphene oxide as a highly efficient and selective catalyst

In this work, porous LiFe₅O₈–rGO achieves a high NH₃ yield of 36.025 mg h⁻¹ mg_cat.⁻¹ and a high faradaic efficiency of 13.08% at −0.2 V vs. the reversible hydrogen electrode in 0.1 M HCl.

Functionalization of N2 via Formal 1,3-Haloboration of a Tungsten(0) σ-Dinitrogen Complex

Boron tribromide and aryldihaloboranes were found to undergo 1,3-haloboration across one W-N≡N moiety of a group 6 end-on dinitrogen complex (i.e. trans-[W(N2 )2 (dppe)2 ]). The N-borylated products consist of a reduced diazenido unit sandwiched between a WII center and a trivalent boron substituent (W-N=N-BXAr), and have all been fully characterized by NMR and IR spectroscopy, elemental analysis, and single-crystal X-ray diffraction. Both the terminal N atom and boron center in the W-N=N-BXAr unit can be further derivatized using electrophiles and nucleophiles/Lewis bases, respectively. This mild reduction and functionalization of a weakly activated N2 ligand with boron halides is unprecedented, and hints at the possibility of generating value-added nitrogen compounds directly from molecular dinitrogen.

Lithium Iron Oxide (LiFeO2) for Electroreduction of Dinitrogen to Ammonia

Electrochemical nitrogen fixation offers a promising route for sustainable NH₃ production, while the rational design of effective and durable electrocatalysts is urgently required for an effective nitrogen reduction reaction (NRR) process. Herein, we explore lithium iron oxide (LiFeO₂) as a potential NRR catalyst. The developed LiFeO₂/reduced graphene oxide (rGO) delivered a combination of both a high NH₃ yield (40.5 μg h^-1 mg^-1) and high Faradaic efficiency (16.4%), exceeding those of nearly all the previously reported Li- and Fe-based catalysts. Theoretical computations showed that Fe and Li atoms on the LiFeO₂ (111) facet synergistically activated N₂ while Fe atoms served as the key active centers. Meanwhile, the undesired HER can be well impeded on both Fe and Li atoms to enable a high NRR selectivity.

Opening up the Valence Shell: A T-Shaped Iron(I) Metalloradical and Its Potential for Atom Abstraction

A thermally stable, T-shaped, d7 high-spin iron(I) complex was obtained by reduction of a PNP-supported ferrous chloride. Paramagnetic NMR spectroscopy combined with DFT modeling was used to analyze the electronic structure of the coordinatively highly unsaturated complex. The metalloradical character of the compound was demonstrated by the formation of a benzophenone ketyl radical complex upon addition of benzophenone. Furthermore, the compound displays a rich chemistry as an oxygen-atom abstractor from epoxides, yielding a dinuclear, diferrous [Fe2 O] complex.

Catalytic N2-to-NH3 (or -N2H4) Conversion by Well-Defined Molecular Coordination Complexes

Nitrogen fixation, the six-electron/six-proton reduction of N₂, to give NH₃, is one of the most challenging and important chemical transformations. Notwithstanding the barriers associated with this reaction, significant progress has been made in developing molecular complexes that reduce N₂ into its bioavailable form, NH₃. This progress is driven by the dual aims of better understanding biological nitrogenases and improving upon industrial nitrogen fixation. In this review, we highlight both mechanistic understanding of nitrogen fixation that has been developed, as well as advances in yields, efficiencies, and rates that make molecular alternatives to nitrogen fixation increasingly appealing. We begin with a historical discussion of N₂ functionalization chemistry that traverses a timeline of events leading up to the discovery of the first bona fide molecular catalyst system and follow with a comprehensive overview of d-block compounds that have been targeted as catalysts up to and including 2019. We end with a summary of lessons learned from this significant research effort and last offer a discussion of key remaining challenges in the field.

Dinitrogen activation by a penta-pyridyl molybdenum complex

A new dinitrogen (N₂) molybdenum(0) complex supported exclusively by pyridine ligands was synthesized.

Where silylene–silicon centres matter in the activation of small molecules

Small molecules such as H₂, N₂, CO, NH₃, O₂ are ubiquitous stable species and their activation and role in the formation of value-added products are of fundamental importance in nature and industry.