Catalytic NH3 Synthesis using N2 /H2 at Molecular Transition Metal Complexes: Concepts for Lead Structure Determination using Computational Chemistry

Role of Lewis acid/base anchor atoms in catalyst regeneration: a comprehensive study on biomimetic EP3Fe nitrogenases

In the quest for sustainable ammonia synthesis routes, biomimetic complexes have been intensively studied. Here we focus on the Peter's group Fe-nitrogenase catalyst with EPPP scorpionate ligands, and explore the effect of anchor atom selection (B, Al, Ga, N and P) and the impact of chloro substitution on the phenyl rings on nitrogen fixation. The reaction profiles of complexes with Lewis basic anchor atoms exhibited energy-demanding reduction steps, with more exergonic protonation steps compared to the smoother reaction profiles observed for catalysts with Lewis acid anchor atoms, also implying that catalyst regeneration is especially challenging for catalysts with Lewis basic anchor atoms. The binding affinities of N2 and H2 to the complexes suggest that the autocatalytic hydrogen evolution reaction (HER), which ultimately leads to consumption of reactants and catalyst deactivation, is likely to become more prevalent for heavier anchor atoms and be more significant for Lewis basic anchor atom complexes. Out of the studied complexes, boron showed the smoothest reaction profile and the smallest affinity for H2, which supports its superiour role as an anchor atom in accordance with experimental data.

An orbitally adapted push-pull template for N2 activation and reduction to diazene-diide

A Lewis superacidic bis(borane) C6F4{B(C6F5)2}2 was reacted with tungsten N2-complexes [W(N2)2(R2PCH2CH2PR2)2] (R = Ph or Et), affording zwitterionic boryldiazenido W(ii) complexes trans-[W(L)(R2PCH2CH2PR2)2(N2{B(C6F5)2(C6F4B(C6F5)3})] (L = ø, N2 or THF). These compounds feature only one N-B linkage of the covalent type, as a result of intramolecular boron-to-boron C6F5 transfer. Complex trans-[W(THF)(Et2PCH2CH2PEt2)2(N2{B(C6F5)2C6F4B(C6F5)3})] (5) was shown to split H2, leading to a seven-coordinate complex [W(H)2(Et2PCH2CH2PEt2)2(N2{B(C6F5)2}2C6F4)] (7). Interestingly, hydride storage at the metal triggers backward C6F5 transfer. This reverts the bis(boron) moiety to its bis(borane) state, now doubly binding the distal N, with structural parameters and DFT computations pointing to dative N→B bonding. By comparison with an N2 complex [W(H)2(Et2PCH2CH2PEt2)2(N2{B(C6F5)3}] (10) differing only in the Lewis acid (LA), namely B(C6F5)3, coordinated to the distal N, we demonstrate that two-fold LA coordination imparts strong N2 activation up to the diazene-diide (N22-) state. To the best of our knowledge, this is the first example of a neutral LA coordination that induces reduction of N2.

Recent Progress in Dinitrogen Activation by Gas-Phase Metal Species

Understanding the mechanisms to activate and functionalize dinitrogen (N₂) is of great importance for the rational design of nitrogen-fixation catalysts. Reactions of gas-phase species with N₂ are being actively studied to understand the bond activation and formation processes at a strictly molecular level. This Perspective provides an overview of the recent progress in combined experimental and theoretical studies on the activation and functionalization of N₂ by gas-phase metal species. New mechanistic insights into N₂ molecular adsorption, N≡N cleavage, and N-X (X = C, B, and H) formation have been introduced, in which the new reaction channels of ejecting neutral metal fragments and the coupling reactions of N₂ with other molecules are highlighted. Finally, the current challenges and outlooks of N₂ activation in the gas phase are discussed as well.

Synthetic ferripyrophyllite: preparation, characterization and catalytic application

Sheet silicates, also known as phyllosilicates, contain parallel sheets of tetrahedral silicate built up by [Si₂O₅]^2- entities connected through intermediate metal-oxygen octahedral layers. The well-known minerals talc and pyrophyllite are belonging to this group based on magnesium and aluminium, respectively. Surprisingly, the ferric analogue rarely occurs in nature and is found in mixtures and conglomerates with other materials only. While partial incorporation of iron into pyrophyllites has been achieved, no synthetic protocol for purely iron-based pyrophyllite has been published yet. Here we report about the first artificial synthesis of ferripyrophyllite under exceptional mild conditions. A similar ultrathin two-dimensional (2D) nanosheet morphology is obtained as in talc or pyrophyllite but with iron(iii) as a central metal. The high surface material exhibits a remarkably high thermostability. It shows some catalytic activity in ammonia synthesis and can serve as catalyst support material for noble metal nanoparticles.

Isolation of a Bimetallic Cobalt(III) Nitride and Examination of Its Hydrogen Atom Abstraction Chemistry and Reactivity toward H2

Room temperature photolysis of the bis(azide)cobaltate(II) complex [Na(THF)_x][(^ketguan)Co(N₃)₂] (^ketguan = [(tBu₂CN)C(NDipp)₂]^-, Dipp = 2,6-diisopropylphenyl) (3a) in THF cleanly forms the binuclear cobalt nitride Na(THF)₄{[(^ketguan)Co(N₃)]₂(μ-N)} (1). Compound 1 represents the first example of an isolable, bimetallic cobalt nitride complex, and it has been fully characterized by spectroscopic, magnetic, and computational analyses. Density functional theory supports a Co^III═N═Co^III canonical form with significant π-bonding between the cobalt centers and the nitride atom. Unlike other group 9 bridging nitride complexes, no radical character is detected at the bridging N atom of 1. Indeed, 1 is unreactive toward weak C-H donors and even cocrystallizes with a molecule of cyclohexadiene (CHD) in its crystallographic unit cell to give 1·CHD as a room temperature stable product. Notably, addition of pyridine to 1 or photolyzed solutions of [(^ketguan)Co(N₃)(py)]₂ (4a) leads to destabilization via activation of the nitride unit, resulting in the mixed-valent Co(II)/Co(III) bridged imido species [(^ketguan)Co(py)][(^ketguan)Co](μ-NH)(μ-N₃) (5) formed from intermolecular hydrogen atom abstraction (HAA) of strong C-H bonds (BDE ∼ 100 kcal/mol). Kinetic rate analysis of the formation of 5 in the presence of C₆H₁₂ or C₆D₁₂ gives a KIE = 2.5 ± 0.1, supportive of a HAA formation pathway. The reactivity of our system was further probed by photolyzing benzene/pyridine solutions of 4a under H₂ and D₂ atmospheres (150 psi), which leads to the exclusive formation of the bis(imido) complexes [(^ketguan)Co(μ-NH)]₂ (6) and [(^ketguan)Co(μ-ND)]₂ (6-D), respectively, as a result of dihydrogen activation. These results provide unique insights into the chemistry and electronic structure of late 3d metal nitrides while providing entryway into C-H activation pathways.

Terminal uranium(V)-nitride hydrogenations involving direct addition or Frustrated Lewis Pair mechanisms

Despite their importance as mechanistic models for heterogeneous Haber Bosch ammonia synthesis from dinitrogen and dihydrogen, homogeneous molecular terminal metal-nitrides are notoriously unreactive towards dihydrogen, and only a few electron-rich, low-coordinate variants demonstrate any hydrogenolysis chemistry. Here, we report hydrogenolysis of a terminal uranium(V)-nitride under mild conditions even though it is electron-poor and not low-coordinate. Two divergent hydrogenolysis mechanisms are found; direct 1,2-dihydrogen addition across the uranium(V)-nitride then H-atom 1,1-migratory insertion to give a uranium(III)-amide, or with trimesitylborane a Frustrated Lewis Pair (FLP) route that produces a uranium(IV)-amide with sacrificial trimesitylborane radical anion. An isostructural uranium(VI)-nitride is inert to hydrogenolysis, suggesting the 5f¹ electron of the uranium(V)-nitride is not purely non-bonding. Further FLP reactivity between the uranium(IV)-amide, dihydrogen, and triphenylborane is suggested by the formation of ammonia-triphenylborane. A reactivity cycle for ammonia synthesis is demonstrated, and this work establishes a unique marriage of actinide and FLP chemistries.

Despite their importance as mechanistic models for Haber Bosch ammonia synthesis from N₂ and H₂, high oxidation state terminal metal-nitrides are notoriously unreactive towards H₂. Here, the authors report hydrogenolysis of a uranium(V)-nitride, which can occur directly or by Frustrated Lewis Pair chemistry with a borane ancillary.

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.

Computational Mechanistic Study of Electro-Oxidation of Ammonia to N2 by Homogenous Ruthenium and Iron Complexes

A comprehensive DFT study of the electrocatalytic oxidation of ammonia to dinitrogen by a ruthenium polypyridyl complex, [(tpy)(bpy)Ru^II(NH₃)]²⁺ (a), and its NMe₂-substituted derivative (b) is presented. The thermodynamics and kinetics of electron (ET) and proton transfer (PT) steps and transition states are calculated. NMe₂ substitution on bpy reduces the ET steps on average 8 kcal/mol for complex b as compared to a. The calculations indicate that N-N formation occurs by ammonia nucleophilic attack/H-transfer via a nitrene intermediate rather than a nitride intermediate. Comparison of the free energy profiles of Ru-b with its first-row Fe congener reveals that the thermodynamics are less favorable for the Fe-b model, especially for ET steps. The N-H bond dissociation free energies (BDFEs) for NH₃ to form N₂ show the following trend: Ru-b < Ru-a < Fe-b, indicating the lowest and most favorable BDFEs for Ru-b complex.

Ta2 +-mediated ammonia synthesis from N2 and H2 at ambient temperature

In a full catalytic cycle, bare Ta₂ ⁺ in the highly diluted gas phase is able to mediate the formation of ammonia in a Haber-Bosch-like process starting from N₂ and H₂ at ambient temperature. This finding is the result of extensive quantum chemical calculations supported by experiments using Fourier transform ion cyclotron resonance MS. The planar Ta₂N₂ ⁺, consisting of a four-membered ring of alternating Ta and N atoms, proved to be a key intermediate. It is formed in a highly exothermic process either by the reaction of Ta₂ ⁺ with N₂ from the educt side or with two molecules of NH₃ from the product side. In the thermal reaction of Ta₂ ⁺ with N₂, the N≡N triple bond of dinitrogen is entirely broken. A detailed analysis of the frontier orbitals involved in the rate-determining step shows that this unexpected reaction is accomplished by the interplay of vacant and doubly occupied d-orbitals, which serve as both electron acceptors and electron donors during the cleavage of the triple bond of N≡N by the ditantalum center. The ability of Ta₂ ⁺ to serve as a multipurpose tool is further shown by splitting the single bond of H₂ in a less exothermic reaction as well. The insight into the microscopic mechanisms obtained may provide guidance for the rational design of polymetallic catalysts to bring about ammonia formation by the activation of molecular nitrogen and hydrogen at ambient conditions.

Catalytic Dinitrogen Reduction to Ammonia at a Triamidoamine-Titanium Complex

Catalytic reduction of N2 to NH3 by a Ti complex has been achieved, thus now adding an early d-block metal to the small group of mid- and late-d-block metals (Mo, Fe, Ru, Os, Co) that catalytically produce NH3 by N2 reduction and protonolysis under homogeneous, abiological conditions. Reduction of [TiIV (TrenTMS )X] (X=Cl, 1A; I, 1B; TrenTMS =N(CH2 CH2 NSiMe3 )3 ) with KC8 affords [TiIII (TrenTMS )] (2). Addition of N2 affords [{(TrenTMS )TiIII }2 (μ-η1 :η1 -N2 )] (3); further reduction with KC8 gives [{(TrenTMS )TiIV }2 (μ-η1 :η1 :η2 :η2 -N2 K2 )] (4). Addition of benzo-15-crown-5 ether (B15C5) to 4 affords [{(TrenTMS )TiIV }2 (μ-η1 :η1 -N2 )][K(B15C5)2 ]2 (5). Complexes 3-5 treated under N2 with KC8 and [R3 PH][I], (the weakest H+ source yet used in N2 reduction) produce up to 18 equiv of NH3 with only trace N2 H4 . When only acid is present, N2 H4 is the dominant product, suggesting successive protonation produces [{(TrenTMS )TiIV }2 (μ-η1 :η1 -N2 H4 )][I]2 , and that extruded N2 H4 reacts further with [R3 PH][I]/KC8 to form NH3 .

Efficient Nitrogen Fixation via a Redox-Flexible Single-Iron Site with Reverse-Dative Iron → Boron σ Bonding

Model systems of the FeMo cofactor of nitrogenase have been explored extensively in catalysis to gain insights into their ability for nitrogen fixation that is of vital importance to the human society. Here we investigate the trigonal pyramidal borane-ligand Fe complex by first-principles calculations, and find that the variation of oxidation state of Fe along the reaction path correlates with that of the reverse-dative Fe → B bonding. The redox-flexibility of the reverse-dative Fe → B bonding helps to provide an electron reservoir that buffers and stabilizes the evolution of Fe oxidation state, which is essential for forming the key intermediates of N₂ activation. Our work provides insights for understanding and optimizing homogeneous and surface single-atom catalysts with reverse-dative donating ligands for efficient dinitrogen fixation. The extension of this kind of molecular catalytic active center to heterogeneous catalysts with surface single-clusters is also discussed.

A Ruthenium Hydrido Dinitrogen Core Conserved across Multielectron/Multiproton Changes to the Pincer Ligand Backbone

A series of ruthenium(II) hydrido dinitrogen complexes supported by pincer ligands in different formal oxidation states have been prepared and characterized. Treating a ruthenium dichloride complex supported by the pincer ligand bis(di-tert-butylphosphinoethyl)amine (H-PNP) with reductant or base generates new five-coordinate cis-hydridodinitrogen ruthenium complexes each containing different forms of the pincer ligand. Further ligand transformations provide access to the first isostructural set of complexes featuring all six different forms of the pincer ligand. The conserved cis-hydridodinitrogen structure facilitates characterization of the π-donor, π-acceptor, and/or σ-donor properties of the ligands and assessment of the impact of ligand-centered multielectron/multiproton changes on N₂ activation. Crystallographic studies, infrared spectroscopy, and ¹⁵N NMR spectroscopy indicate that N₂ remains weakly activated in all cases, providing insight into the donor properties of the different pincer ligand states. Ramifications on applications of (pincer)Ru species in catalysis are considered.

Dinitrogen photoactivation: status quo and future perspectives

New perspectives for dinitrogen activation: an overview of photochemical pathways to cleave the strong N–N bond.

Nitrogen Reduction to Ammonia on a Biomimetic Mononuclear Iron Centre: Insights into the Nitrogenase Enzyme

Nitrogenases catalyse nitrogen fixation to ammonia on a multinuclear Fe‐Mo centre, but their mechanism and particularly the order of proton and electron transfer processes that happen during the catalytic cycle is still unresolved. Recently, a unique biomimetic mononuclear iron model was developed using tris(phosphine)borate (TPB) ligands that was shown to convert N₂ into NH₃. Herein, we present a computational study on the [(TPB)FeN₂]⁻ complex and describe its conversion into ammonia through the addition of electrons and protons. In particular, we tested the consecutive proton transfer on only the distal nitrogen atom or alternated protonation of the distal/proximal nitrogen. It is found that the lowest energy pathway is consecutive addition of three protons to the same site, which forms ammonia and an iron‐nitrido complex. In addition, the proton transfer step of complexes with the metal in various oxidation and spin states were tested and show that the pK_a values of biomimetic mononuclear nitrogenase intermediates vary little with iron oxidation states. As such, the model gives several possible NH₃ formation pathways depending on the order of electron/proton transfer, and all should be physically accessible in the natural system. These results may have implications for enzymatic nitrogenases and give insight into the catalytic properties of mononuclear iron centres.

Hydrazine Capture and N-N Bond Cleavage at Iron Enabled by Flexible Appended Lewis Acids

Incorporation of two 9-borabicyclo[3.3.1]nonyl substituents within the secondary coordination sphere of a pincer-based Fe(II) complex provides Lewis acidic sites capable of binding 1 or 2 equiv of N₂H₄. Reduction of the 1:1 Fe:N₂H₄ species affords a rare Fe(NH₂)₂ complex in which the amido ligands are stabilized through interactions with the appended boranes. The NH₂ units can be released as NH₃ upon protonation and exchanged with exogenous N₂H₄.

Activation of Dinitrogen as A Dipolarophile in 1,3-Dipolar Cycloadditions: A Theoretical Study Using Nitrile Imines as "Octet" 1,3-Dipoles

Theoretical calculations at the G4MP2 level of theory demonstrate that it is possible to activate dinitrogen to make it react in dipolar cycloadditions using neutral beryllium derivatives and other neutral metallic compounds. For the particular case of beryllium, the barrier decreases more than 40 kJ·mol^–1 with respect to the non-catalysed reaction. The activation achieved is lower than using diazonium salts (models of protonated N₂), but still in a range that can be experimentally attainable.