Synthesis and Reactivity of Iron- and Cobalt-Dinitrogen Complexes Bearing PSiP-Type Pincer Ligands toward Nitrogen Fixation

Dinitrogen silylation catalyzed by silylene cobalt(I) and silylene iron(I) chlorides

In this contribution, Co(PMe3)3Cl (1), bis(silylene) cobalt chlorides Co(LSi:)2(PMe3)2Cl (LSi: = {PhC(NtBu)2}SiCl (2); {p-CH3C6H4C(NtBu)2}SiCl (3); and {p-tBuC6H4C(NtBu)2}SiCl (4)) and bis(silylene) iron chlorides Fe(LSi:)2(PMe3)2Cl (LSi: = {PhC(NtBu)2}SiCl (5); {p-CH3C6H4C(NtBu)2}SiCl (6); {p-tBuC6H4C(NtBu)2}SiCl (7) and Fe(PMe3)2Cl2 (8)) were synthesized to study the effects of different metals and silylene ligands on the catalytic activity of complexes 1-8 in dinitrogen silylation reaction. The experimental results indicate that there is no substantial difference in catalytic activity between the phosphine cobalt complex 1 and the silylene cobalt chlorides 2-4 although the cobalt silylene complex 2 has slightly better catalytic activity than complexes 1, 3 and 4 in the dinitrogen silylation. Silylene iron complexes 5-7 are more active than the phosphine iron complex 8. Among the three silylene iron(I) chlorides 5-7, complex 5 is the most effective catalyst for dinitrogen silylation and 402 equiv. of N(SiMe3)3 could be obtained per Fe atom. In the dinitrogen silylation reaction catalyzed by iron complexes, the introduction of the silylene ligand made the silylene iron complexes 5-7 more active than the phosphine iron complex 8. In addition, iron chlorides 5-8 are more effective catalysts than cobalt(I) chlorides 1-4 for the dinitrogen silylation reaction. Complexes 3, 4, 6 and 7 were new complexes, and their molecular structures were determined by single crystal X-ray diffraction analysis.

X-type silyl ligands for transition-metal catalysis

Given the critical importance of novel ligand development for transition-metal (TM) catalysis, as well as the resurgence of the field of organosilicon chemistry and silyl ligands, to summarize the topic of X-type silyl ligands for TM catalysis is highly attractive and timely. This review particularly emphasizes the unique σ-donating characteristics and trans-effects of silyl ligands, highlighting their crucial roles in enhancing the reactivity and selectivity of various catalytic reactions, including small molecule activation, Kumada cross-coupling, hydrofunctionalization, C-H functionalization, and dehydrogenative Si-O coupling reactions. Additionally, future developments in this field are also provided, which would inspire new insights and applications in catalytic synthetic chemistry.

Synthesis of Bis(silylene) Iron Chlorides and Their Catalytic Activity for Dinitrogen Silylation

In this study, three tetracoordinated bis(silylene) iron(II) chlorides, namely, [SiCHRSi]FeCl2 (1) (R = H), (2) (R = CH3), and (3) (R = Ph), were synthesized through the reactions of the three different bis(silylene) ligands [LSiCHRSiL] (L = PhC(NtBu)2, L1 (R = H), L2 (R = CH3), L3 (R = Ph)) with FeCl2·(THF)1.5 in THF. The bis(silylene) Fe complexes 1-3 could be used as effective catalysts for dinitrogen silylation, with complex 3 demonstrating the highest turnover number (TON) of 746 equiv among the three complexes. The catalytic mechanism was explored, revealing the involvement of the pentacoordinated bis(dinitrogen) iron(0) complexes [SiCHRSi]Fe(N2)2(THF), (4)-(6), as the active catalysts in the dinitrogen silylation reaction. Additionally, the cyclic silylene compound 10 was obtained from the reaction of L1 with KC8. Single-crystal X-ray diffraction analyses confirmed the molecular structures of complexes 1-3 and 10 in the solid state.

Highly Selective Nickel-Catalyzed Isomerization-Hydroboration of Alkenes Affords Terminal Functionalization at Remote C-H Position

We report herein the synthesis and characterization of nickel complexes supported by tridentate and bidentate phosphino(silyl) ancillary ligands, along with the successful application of these complexes as precatalysts for the hydroboration of terminal and internal alkenes using pinacolborane (HBPin). These reactions proceeded with low nickel loadings of 2.5-5 mol % in the absence of co-solvent, and in some cases at room temperature. Isomerization to afford exclusively the terminal hydroboration product was obtained across a range of internal alkenes, including tri- and tetra-substituted examples. This reactivity is unprecedented for nickel and offers a powerful means of achieving functionalization at a C-H position remote from the C=C double bond. Nickel-catalyzed deuteroboration experiments using DBPin support a mechanism involving 1,2-insertion of the alkene and subsequent chain-walking, which results in isotopic scrambling.

New cationic coinage metal complexes featuring silyl group functionalized phosphine: syntheses, structures and catalytic studies in alkyne-azide cycloaddition reactions

A series of coinage metal complexes bearing rarely explored ortho-silylated phosphine is reported. The treatment of diphenyl(2-(trimethylsilyl)phenyl)phosphine (1) with CuCl and [Cu(CH3CN)4]BF4 furnished the corresponding neutral [(1)CuCl]2 (2) and mono-cationic [(1)2Cu(CH3CN)]BF4 (3) complexes, respectively. The reactions of 1 with AgX (X = BF4-, NO3-) in 2 : 1 ratio furnished the corresponding mono cationic dicoordinate silver(I) complexes of the type [(1)2Ag]X (X = BF4- (4a), NO3- (4b)). The ortho-silylated phosphine ligand (1) was conveniently converted into the corresponding sulfide (5a) and selenide (5b) species, and their reactions with [Cu(CH3CN)4]BF4 yielded mono-cationic, homoleptic tris(silylphosphinochalcogenide)copper(I) complexes of the type [(5a/5b)3Cu]BF4 (6a/6b). The molecular structures of 2-4 and 6 were established by single-crystal X-ray diffraction analysis. The copper complexes 2, 3, and 6a were employed as catalysts in azide-alkyne cycloaddition reactions. Among these complexes, 3 was extensively used in the preparation of various mono- and bis-triazoles consisting of tolyl, benzyl, carbazolyl, and propargylic ether groups. Three sets of substituted triazole derivatives were achieved under mild conditions by employing copper(I) catalytic systems. The mechanistic studies indicated the formation of a heteroleptic copper(I) triazolide intermediate which was detected by high-resolution mass spectral analysis.

Synthesis and Reactivity of Cobalt-Dinitrogen Complexes Bearing Anionic PCP-Type Pincer Ligands toward Catalytic Silylamine Formation from Dinitrogen

A series of cobalt(I)-dinitrogen complexes bearing anionic 4-substituted benzene-based PCP-type pincer ligands are synthesized and characterized. These complexes work as highly efficient catalysts for the formation of silylamine from dinitrogen under ambient reaction conditions to produce up to 371 equiv of silylamine based on the cobalt atom of the catalyst.

Synthesis of silyl iron dinitrogen complexes for activation of dihydrogen and catalytic silylation of dinitrogen

Three novel iron dinitrogen hydrides, [FeH(ⁱPr-PSi^MeP)(N₂)(PMe₃)] (1), [FeH(ⁱPr-PSi^PhP)(N₂)(PMe₃)] (2), and [FeH(ⁱPr-PSi^Ph)(N₂)(PMe₃)] (3), supported by a silyl ligand are synthesized for the first time by changing the electronic effect and steric hindrance of the ligands through the reaction of ligands L1-L3 with Fe(PMe₃)₄ in a nitrogen atmosphere. The ligands containing an electron-donating group with large steric hindrance on the phosphorus atom are beneficial for the formation of dinitrogen complexes. A penta-coordinate iron hydride [FeH(ⁱPr-PSi^Ph)(PMe₃)₂] (4) was formed through the reaction of ligand L3 with Fe(PMe₃)₄ in an argon atmosphere under the same conditions. The reactions between complexes 1-3 with an atmospheric pressure of dihydrogen gas resulted in Fe(II) dihydrides, [(ⁱPr-PSi^Me(μ-H)P)Fe(H)₂(PMe₃)] (5), [(ⁱPr-PSi^Ph(μ-H)P)Fe(H)₂(PMe₃)] (6) and [(iPr-PSi^Ph(μ-H))Fe(H)₂(PMe₃)₂] (7), with an η²-(Si-H) coordination. The isolation of dihydrides 5-7 demonstrates the ability of the dinitrogen complexes 1-3 to realize the activation of dihydrogen under ambient temperature and pressure. The molecular structures of complexes 1-7 were elucidated by single crystal X-ray diffraction analysis. The iron dinitrogen hydrides 1-3 are effective catalysts for the silylation of dinitrogen under ambient conditions and among them 3 is the best catalyst.

In-situ spectroscopic observation of dynamic-coupling oxygen on atomically dispersed iridium electrocatalyst for acidic water oxidation

Uncovering the dynamics of active sites in the working conditions is crucial to realizing increased activity, enhanced stability and reduced cost of oxygen evolution reaction (OER) electrocatalysts in proton exchange membrane electrolytes. Herein, we identify at the atomic level potential-driven dynamic-coupling oxygen on atomically dispersed hetero-nitrogen-configured Ir sites (AD-HN-Ir) in the OER working conditions to successfully provide the atomically dispersed Ir electrocatalyst with ultrahigh electrochemical acidic OER activity. Using in-situ synchrotron radiation infrared and X-ray absorption spectroscopies, we directly observe that one oxygen atom is formed at the Ir active site with an O-hetero-Ir-N₄ structure as a more electrophilic active centre in the experiment, which effectively promotes the generation of key *OOH intermediates under working potentials; this process is favourable for the dissociation of H₂O over Ir active sites and resistance to over-oxidation and dissolution of the active sites. The optimal AD-HN-Ir electrocatalyst delivers a large mass activity of 2860 A g_metal^-1 and a large turnover frequency of 5110 h^-1 at a low overpotential of 216 mV (10 mA cm^-2), 480-510 times larger than those of the commercial IrO₂. More importantly, the AD-HN-Ir electrocatalyst shows no evident deactivation after continuous 100 h OER operation in an acidic medium.

Ammonia Formation Catalyzed by a Dinitrogen-Bridged Dirhenium Complex Bearing PNP-Pincer Ligands under Mild Reaction Conditions*

A series of rhenium complexes bearing a pyridine-based PNP-type pincer ligand are synthesized from rhenium phosphine complexes as precursors. A dinitrogen-bridged dirhenium complex bearing the PNP-type pincer ligands catalytically converts dinitrogen into ammonia during the reaction with KC₈ as a reductant and [HPCy₃ ]BAr^F ₄ (Cy=cyclohexyl, Ar^F =3,5-(CF₃ )₂ C₆ H₃ ) as a proton source at -78 °C to afford 8.4 equiv of ammonia based on the rhenium atom of the catalyst. The rhenium-dinitrogen complex also catalyzes silylation of dinitrogen in the reaction with KC₈ as a reductant and Me₃ SiCl as a silylating reagent under ambient reaction conditions to afford 11.7 equiv of tris(trimethylsilyl)amine based on the rhenium atom of the catalyst. These results demonstrate the first successful example of catalytic nitrogen fixation under mild reaction conditions using rhenium-dinitrogen complexes as catalysts.

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.

Halogen Bonding Involving I2 and d8 Transition-Metal Pincer Complexes

We systematically investigated iodine–metal and iodine–iodine bonding in van Koten’s pincer complex and 19 modifications changing substituents and/or the transition metal with a PBE0–D3(BJ)/aug–cc–pVTZ/PP(M,I) model chemistry. As a novel tool for the quantitative assessment of the iodine–metal and iodine–iodine bond strength in these complexes we used the local mode analysis, originally introduced by Konkoli and Cremer, complemented with NBO and Bader’s QTAIM analyses. Our study reveals the major electronic effects in the catalytic activity of the M–I–I non-classical three-center bond of the pincer complex, which is involved in the oxidative addition of molecular iodine I2 to the metal center. According to our investigations the charge transfer from the metal to the σ* antibonding orbital of the I–I bond changes the 3c–4e character of the M–I–I three-center bond, which leads to weakening of the iodine I–I bond and strengthening of the metal–iodine M–I bond, facilitating in this way the oxidative addition of I2 to the metal. The charge transfer can be systematically modified by substitution at different places of the pincer complex and by different transition metals, changing the strength of both the M–I and the I2 bonds. We also modeled for the original pincer complex how solvents with different polarity influence the 3c–4e character of the M–I–I bond. Our results provide new guidelines for the design of pincer complexes with specific iodine–metal bond strengths and introduce the local vibrational mode analysis as an efficient tool to assess the bond strength in complexes.

Dinitrogen Fixation: Rationalizing Strategies Utilizing Molecular Complexes

Dinitrogen (N2 ) is the most abundant gas in Earth's atmosphere, but its inertness hinders its use as a nitrogen source in the biosphere and in industry. Efficient catalysts are hence required to ov. ercome the high kinetic barriers associated to N2 transformation. In that respect, molecular complexes have demonstrated strong potential to mediate N2 functionalization reactions under mild conditions while providing a straightforward understanding of the reaction mechanisms. This Review emphasizes the strategies for N2 reduction and functionalization using molecular transition metal and actinide complexes according to their proposed reaction mechanisms, distinguishing complexes inducing cleavage of the N≡N bond before (dissociative mechanism) or concomitantly with functionalization (associative mechanism). We present here the main examples of stoichiometric and catalytic N2 functionalization reactions following these strategies.

The Effect of Substituents on the Formation of Silyl [PSiP] Pincer Cobalt(I) Complexes and Catalytic Application in Both Nitrogen Silylation and Alkene Hydrosilylation

Four different [PSiP]-pincer ligands L1-L4 ((2-Ph₂PC₆H₄)₂SiHR (R = H (L1) and Ph (L2)) and (2-ⁱPr₂PC₆H₄)₂SiHR' (R' = Ph (L3) and H (L4)) were used to investigate the effect of substituents at P and/or Si atom of the [PSiP] pincer ligands on the formation of silyl cobalt(I) complexes by the reactions with CoMe(PMe₃)₄ via Si-H cleavage. Two penta-coordinated silyl cobalt(I) complexes, (2-Ph₂PC₆H₄)₂HSiCo(PMe₃)₂ (1) and (2-Ph₂PC₆H₄)₂PhSiCo(PMe₃)₂ (2), were obtained from the reactions of L1 and L2 with CoMe(PMe₃)₄, respectively. Under similar reaction conditions, a tetra-coordinated cobalt(I) complex (2-ⁱPr₂PC₆H₄)₂PhSiCo(PMe₃) (3) was isolated from the interaction of L3 with CoMe(PMe₃)₄. It was found that, only in the case of ligand L4, silyl dinitrogen cobalt(I) complex 4, [(2-ⁱPr₂PC₆H₄)₂HSiCo(N₂)(PMe₃)], was formed. Our results indicate that the increasing of electron cloud density at the Co center is beneficial for the formation of a dinitrogen cobalt complex because the large electron density at Co center leads to the enhancement of the π-backbonding from cobalt to the coordinated N₂. It was found that silyl dinitrogen cobalt(I) complex 4 is an effective catalyst for catalytic transformation of dinitrogen into silylamine. Among these four silyl cobalt(I) complexes, complex 1 is the best catalyst for hydrosilylation of alkenes with excellent regioselectivity. For aromatic alkenes, catalyst 1 provided Markovnikov products, while for aliphatic alkenes, anti-Markovnikov products could be obtained. Both catalytic reaction mechanisms were proposed and discussed. The molecular structures of complexes 1-4 were confirmed by single-crystal X-ray diffraction.

Synthesis of Dinuclear Mo-Fe Hydride Complexes and Catalytic Silylation of N2

Two transition-metal atoms bridged by hydrides may represent a useful structural motif for N₂ activation by molecular complexes and the enzyme active site. In this study, dinuclear Mo^IV -Fe^II complexes with bridging hydrides, Cp^R Mo(PMe₃ )(H)(μ-H)₃ FeCp* (2 a; Cp^R =Cp*=C₅ Me₅ , 2 b; Cp^R =C₅ Me₄ H), were synthesized via deprotonation of Cp^R Mo(PMe₃ )H₅ (1 a; Cp^R =Cp*, 1 b; Cp^R =C₅ Me₄ H) by Cp*FeN(SiMe₃ )₂ , and they were characterized by spectroscopy and crystallography. These Mo-Fe complexes reveal the shortest Mo-Fe distances ever reported (2.4005(3) Å for 2 a and 2.3952(3) Å for 2 b), and the Mo-Fe interactions were analyzed by computational studies. Removal of the terminal Mo-H hydride in 2 a-2 b by [Ph₃ C]⁺ in THF led to the formation of cationic THF adducts [Cp^R Mo(PMe₃ )(THF)(μ-H)₃ FeCp*]⁺ (3 a; Cp^R =Cp*, 3 b; Cp^R =C₅ Me₄ H). Further reaction of 3 a with LiPPh₂ gave rise to a phosphido-bridged complex Cp*Mo(PMe₃ )(μ-H)(μ-PPh₂ )FeCp* (4). A series of Mo-Fe complexes were subjected to catalytic silylation of N₂ in the presence of Na and Me₃ SiCl, furnishing up to 129±20 equiv of N(SiMe₃ )₃ per molecule of 2 b. Mechanism of the catalytic cycle was analyzed by DFT calculations.

Bimetallic iron–tin catalyst for N2 to NH3 and a silyldiazenido model intermediate

A new tin-supported iron complex catalyzes N₂ fixation. The role of this heavy main group element in the catalysis is evaluated.

Reductive Silylation Using a Bis-silylated Diaza-2,5-cyclohexadiene

1,4-Bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene, 1, was tested as a reagent for the reductive silylation of various unsaturated functionalities, including N-heterocycles, quinones, and other redox-active moieties in addition to deoxygenation of main group oxides. Whereas most reactions tested are thermodynamically favorable, based on DFT calculations, a few do not occur, perhaps giving limited insight on the mechanism of this very attractive reductive process. Of note, reductive silylation reactions show a strong solvent dependence where a polar solvent facilitates conversions.

Iridium complexes featuring a tridentate SiPSi ligand: from dimeric to monomeric 14, 16 or 18-electron species

Dimer [Ir{P(o-C₆H₄CH₂SiMe₂)₂Ph}(μ-Cl)]₂ featuring tricoordinate phosphinodisilyl ligands dissociates in solution to the reactive 14 electron monomer [Ir(SiPSi)Cl].

Enhanced Activation of Coordinated Dinitrogen with p-Block Lewis Acids

This Concept article highlights recent research on Lewis acid adducts of dinitrogen complexes, including our contributions. After a reminder of the early works, it is demonstrated that such kind of species offers a new platform for dinitrogen functionalization as well as valuable models for the understanding of elementary steps of (bio)catalytic cycles. When possible, parallels regarding this mode of activation from the orbital point of view are drawn between the different systems discussed herein.

Development of catalytic nitrogen fixation using transition metal-dinitrogen complexes under mild reaction conditions

This paper describes our recent progress in catalytic nitrogen fixation using transition metal-dinitrogen complexes as catalysts. Our research group has recently developed novel reaction systems for the catalytic transformation of molecular dinitrogen into ammonia and hydrazine using molybdenum-, iron-, cobalt- and vanadium-dinitrogen complexes under mild reaction conditions. The new findings presented in this paper may provide a new approach to the development of economical nitrogen fixation to replace the energy-consuming Haber-Bosch process.

Fe-Catalyzed Conversion of N2 to N(SiMe3)3 via an Fe-Hydrazido Resting State

The catalytic conversion of N₂ to N(SiMe₃)₃ by homogeneous transition metal compounds is a rapidly developing field, yet few mechanistic details have been experimentally elucidated for 3 d element catalysts. Herein we show that Fe(PP)₂(N₂) (PP = R₂PCH₂CH₂PR₂; R = Me, 1^Me; R = Et, 1^Et) are highly effective for the catalytic production of N(SiMe₃)₃ from N₂ (using KC₈/Me₃SiCl), with the yields being the highest reported to date for Fe-based catalysts. We propose that N₂ fixation proceeds via electrophilic N_β silylation and 1e^- reduction to form unstable Fe^I(NN-SiMe₃) intermediates, which disproportionate to 1^Me/Et and hydrazido Fe^II[N-N(SiMe₃)₂] species (3^Me/Et); the latter act as resting states on the catalytic cycle. Subsequent 2e^- reduction of 3^Me/Et leads to N-N scission and formation of [N(SiMe₃)₂]^- and putative anionic Fe imido products. These mechanistic results are supported by both experiment and DFT calculations.

Catalytic Silylation of Dinitrogen by a Family of Triiron Complexes

A series of triiron complexes supported by a tris(β-diketiminate)cyclophane (L ^3- ) catalyze the reduction of dinitrogen to tris(trimethylsilyl)amine using KC₈ and Me₃SiCl. Employing Fe₃Br₃ L affords 83 ± 7 equiv. NH₄ ⁺/complex after protonolysis, which is a 50% yield based on reducing equivalents. The series of triiron compounds tested evidences the subtle effects of ancillary donors, including halides, hydrides, sulfides, and carbonyl ligands, and metal oxidation state on N(SiMe₃)₃ yield, and highlight Fe₃(μ₃-N)L as a common species in product mixtures. These results suggest that ancillary ligands can be abstracted with Lewis acids under reducing conditions.

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.

Dinitrogen functionalization at a ditantalum center. Balancing N2 displacement and N2 functionalization in the reaction of coordinated N2 with CS2

The reaction of carbon disulfide (CS2) with the side-on end-on dinitrogen complex ([NPNSi]Ta)2(μ-η1:η2-N2)(μ-H)2 (1) (where [NPNSi] = [PhP(CH2SiMe2NPh)2]) has been studied and shown to generate two products, the ratio of which depends on the concentration of added carbon disulfide. At high concentrations of CS2, N-N bond cleavage and functionalization occur to generate a ditantalum complex with an isothiocyanato ligand N-bound to Ta along with bridging sulfido and nitrido moieties. At lower concentrations of CS2, less dinitrogen functionalization is observed; instead, N2 is displaced and the CS2 molecule is completely disassembled to generate a ditantalum derivative with bridging methylene and two sulfide moieties. Kinetic and DFT studies have been performed and provide clues about the product ratio and mechanistic information and shed light on why N2 functionalization is challenging.

Catalytic Silylation of N2 and Synthesis of NH3 and N2H4 by Net Hydrogen Atom Transfer Reactions Using a Chromium P4 Macrocycle

We report the first discrete molecular Cr-based catalysts for the reduction of N₂. This study is focused on the reactivity of the Cr-N₂ complex, trans-[Cr(N₂)₂(P^Ph₄N^Bn₄)] (P₄Cr(N₂)₂), bearing a 16-membered tetraphosphine macrocycle. The architecture of the [16]-P^Ph₄N^Bn₄ ligand is critical to preserve the structural integrity of the catalyst. P₄Cr(N₂)₂ was found to mediate the reduction of N₂ at room temperature and 1 atm pressure by three complementary reaction pathways: (1) Cr-catalyzed reduction of N₂ to N(SiMe₃)₃ by Na and Me₃SiCl, affording up to 34 equiv N(SiMe₃)₃; (2) stoichiometric reduction of N₂ by protons and electrons (for example, the reaction of cobaltocene and collidinium triflate at room temperature afforded 1.9 equiv of NH₃, or at -78 °C afforded a mixture of NH₃ and N₂H₄); and (3) the first example of NH₃ formation from the reaction of a terminally bound N₂ ligand with a traditional H atom source, TEMPOH (2,2,6,6-tetramethylpiperidine-1-ol). We found that trans-[Cr(¹⁵N₂)₂(P^Ph₄N^Bn₄)] reacts with excess TEMPOH to afford 1.4 equiv of ¹⁵NH₃. Isotopic labeling studies using TEMPOD afforded ND₃ as the product of N₂ reduction, confirming that the H atoms are provided by TEMPOH.

Preparation and reactivity of iron complexes bearing anionic carbazole-based PNP-type pincer ligands toward catalytic nitrogen fixation

Newly prepared iron complexes bearing carbazole-based PNP-type pincer ligands are found to work as catalysts toward nitrogen fixation.