Catalytic Dinitrogen Fixation to Form Ammonia at Ambient Reaction Conditions Using Transition Metal‐Dinitrogen Complexes

This paper presents recent progress in catalytic transformation of molecular dinitrogen into ammonia or its equivalents, such as silylamine, especially using transition metal‐dinitrogen complexes under ambient reaction conditions. Several catalytic systems have been recently established using molybdenum‐, iron‐, and cobalt‐dinitrogen complexes or their precursors as catalysts, providing new approaches to the development of novel nitrogen fixation under ambient reaction conditions.     

分子氮的电催化还原

对温和条件下分子氮的络合活化已有研究,但用修饰电极法电催化固氮成氨(或肼)尚未见报道,Shilov等曾发现在V(OH)_2-Mg(OH)_2的悬浮液中,V(Ⅱ)可起络合及还原作用,钒固氮酶在缺钼条件下也可活化分子氮,其活性中心可能与钼酶相似,也是通过有机硫配体而定位在蛋白质的肽链上,能否用含有机硫配体的钒表面配合物模拟钒酶,用电催化方法进行电子与能量的偶联从而固氮成氨?本文对此进行了研究。     

Catalytic Reduction of Molecular Dinitrogen to Ammonia and Hydrazine Using Vanadium Complexes

Newly designed and prepared vanadium complexes bearing anionic pyrrole‐based PNP‐type pincer and aryloxy ligands were found to work as effective catalysts for the direct conversion of molecular dinitrogen into ammonia and hydrazine under mild reaction conditions. This is the first successful example of vanadium‐catalyzed dinitrogen reduction under mild reaction conditions.     

Catalytic Reduction of Dinitrogen into Ammonia and Hydrazine by Using Chromium Complexes Bearing PCP-Type Pincer Ligands**

A series of chromium-halide, -nitride, and -dinitrogen complexes bearing carbene- and phosphine-based PCP-type pincer ligands has been newly prepared, and some of them are found to work as effective catalysts to reduce dinitrogen under atmospheric pressure, whereby up to 11.60 equiv. of ammonia and 2.52 equiv. of hydrazine (16.6 equiv. of fixed N atom) are produced based on the chromium atom. To the best of our knowledge, this is the first successful example of chromium-catalyzed conversion of dinitrogen to ammonia and hydrazine under mild reaction conditions.     

Tetrametallic Reduction of Dinitrogen: Formation of a Tetranuclear Samarium Dinitrogen Complex

The cooperative attack of four (dipyrromethanyl)Sm(II) units on dinitrogen resulted in a novel tetranuclear samarium dinitrogen complex (shown schematically). The presence of halogen atoms inhibited reactivity with dinitrogen through the assembly of divalent samarium clusters. dipyrr=diphenylmethyldipyrrolide dianion.     

Single-Atom Catalysis Using Chromium Embedded in Divacant Graphene for Conversion of Dinitrogen to Ammonia

Reduction of dinitrogen to ammonia under ambient conditions is a long-standing challenge. The few metal-based catalysts proposed have conspicuous disadvantages such as high cost, high energy consumption, and being hazardous to the environment. Single-atom catalysis has emerged as a new frontier in heterogeneous catalysis and metal atoms atomically dispersed on supports receive more and more attention owing to rapid advances in synthetic methodologies and computational modeling. Herein, we propose metal atoms embedded in divacant graphene as a catalyst for N₂ fixation based on density functional calculations. We systematically investigate the potential of using transition metal like Cr, Mn, Fe, Mo and Ru as catalysts and our study reveals that Cr embedded in graphene exhibit good catalytic activity for N₂ fixation. The synergy between the metal atoms and graphene surface provides a stable support to the metal center that has a high spin density to promote adsorption of N₂ and activation of its N≡N triple bond. Our study deciphers the mechanism of conversion of N₂ to ammonia following two possible reaction pathways, distal and enzymatic routes, via sequential protonation and reduction of activated N₂. The study provides a rational framework for conversion of dinitrogen to ammonia using single atom catalyst.     

Catalytic Conversion of Dinitrogen into Ammonia under Ambient Reaction Conditions by Using Proton Source from Water

Molybdenum‐catalyzed conversion of molecular dinitrogen into ammonia under ambient reaction conditions has been achieved by using a proton source generated in situ from the ruthenium‐catalyzed oxidation of water in combination with visible light and a photosensitizer. The preset reaction system is considered as a new model for the nitrogen fixation by photosynthetic bacteria.     

Selective Dinitrogen Conversion to Ammonia Using Water and Visible Light through Plasmon‐induced Charge Separation

The generation of ammonia from atmospheric nitrogen and water using sunlight is a preferable approach to obtaining ammonia as an energy carrier and potentially represents a new paradigm for achieving a low‐carbon and sustainable‐energy society. Herein, we report the selective conversion of dinitrogen into ammonia through plasmon‐induced charge separation by using a strontium titanate (SrTiO₃) photoelectrode loaded with gold nanoparticles (Au‐NPs) and a zirconium/zirconium oxide (Zr/ZrO_x) thin film. We observed the simultaneous stoichiometric production of ammonia and oxygen from nitrogen and water under visible‐light irradiation.     

Cycling between Molybdenum-Dinitrogen and -Nitride Complexes to Support the Reaction Pathway for Catalytic Formation of Ammonia from Dinitrogen

Cycling between molybdenum(I)-dinitrogen and molybdenum(IV)-nitride complexes was investigated under ambient reaction conditions. A kinetic study of the second-order reaction rate for the conversion of the molybdenum-dinitrogen complex into the molybdenum-nitride complex indicates that the formation of the dinitrogen-bridged dimolybdenum complex is involved in the rate-determining step. DFT calculations indicate that the molybdenum-dinitrogen complex transforms into the molybdenum-nitride complex via direct cleavage of the nitrogen-nitrogen triple bond of the bridging dinitrogen ligand of the dinitrogen-bridged dimolybdenum complex. The corresponding reaction of the molybdenum-nitride complex transforming into the molybdenum-dinitrogen complex proceeds via the ligand exchange of ammonia for dinitrogen at the dinitrogen-bridged dimolybdenum complexes. A new modified reaction pathway has been proposed based on the findings of our experimental and theoretical results.     

Dinitrogen complexes of sulfur-ligated iron

We report a unique class of dinitrogen complexes of iron featuring sulfur donors in the ancillary ligand. The ligands utilized are related to the recently studied tris(phosphino)silyl ligands (2-R(2)PC(6)H(4))(3)Si (R = Ph, iPr) but have one or two phosphine arms replaced with thioether donors. Depending on the number of phosphine arms replaced, both mononuclear and dinuclear iron complexes with dinitrogen are accessible. These complexes contribute to a desirable class of model complexes that possess both dinitrogen and sulfur ligands in the immediate iron coordination sphere.     

Conversion of Dinitrogen into Acetonitrile under Ambient Conditions

About 20 % of the ammonia production is used as the chemical feedstock for nitrogen‐containing chemicals. However, while synthetic nitrogen fixation at ambient conditions has had some groundbreaking contributions in recent years, progress for the direct conversion of N₂ into organic products remains limited and catalytic reactions are unknown. Herein, the rhenium‐mediated synthesis of acetonitrile using dinitrogen and ethyl triflate is presented. A synthetic cycle in three reaction steps with high individual isolated yields and recovery of the rhenium pincer starting complex is shown. The cycle comprises alkylation of a nitride that arises from N₂ splitting and subsequent imido ligand centered oxidation to nitrile via a 1‐azavinylidene (ketimido) intermediate. Different synthetic strategies for intra‐ and intermolecular imido ligand oxidation and associated metal reduction were evaluated that rely on simple proton, electron, and hydrogen‐atom transfer steps.     

Dinitrogen activation by titanium sandwich complexes

The activation of dinitrogen by titanium sandwich complexes of the general form (eta5-C5Me4R)2Ti (R = CHMe2, CMe3, SiMe3) has been systematically investigated. Low-temperature, in situ, solution infrared spectroscopy has allowed detection of monomeric bis-dinitrogen complexes of titanium that are isostructural with more familiar dicarbonyl derivatives. One example, (eta5-C5Me4CHMe2)2Ti(N2)2, has also been characterized by X-ray diffraction and reveals weakly activated dinitrogen ligands. From the solution IR data, the relative azophilicity of the titanium sandwich complexes has been established and increases with smaller cyclopentadienyl substituents.     

Molybdenum-catalyzed reduction of molecular dinitrogen into ammonia under ambient reaction conditions

Synthesis of transition metal–dinitrogen complexes and stoichiometric transformations of their coordinated dinitrogen into ammonia and hydrazine have so far been well investigated in order to achieve a novel nitrogen fixation under ambient conditions. As an extension of our study, the dimolybdenum–dinitrogen complex bearing PNP pincer ligands has been found to work as an effective catalyst for the formation of ammonia from dinitrogen, where 52 equiv of ammonia are produced based on the catalyst (26 equiv of ammonia are produced based on the molybdenum atom of the catalyst). This is the most effective catalytic reaction system for the formation of ammonia from molecular dinitrogen catalyzed by transition metal–dinitrogen complexes as catalysts under ambient reaction conditions. Herein, we describe recent results concerning the catalytic reaction, including the proposed reaction pathway.     

Trichloro(Dinitrogen)Platinate(II)

Zeise's salt, [PtCl₃(H₂C=CH₂)]⁻_, is the oldest known organometallic complex, featuring ethylene strongly bound to a platinum salt. Many derivatives are known, but none involving dinitrogen, and indeed dinitrogen complexes are unknown for both platinum and palladium. Electrospray ionization mass spectrometry of K₂[PtCl₄] solutions generate strong ions corresponding to [PtCl₃(N₂)]⁻, the identity of which was confirmed through ion-mobility spectrometry and MS/MS experiments that proved it to be distinct from its isobaric counterparts [PtCl₃(C₂H₄)]⁻ and [PtCl₃(CO)]⁻. Computational analysis established a gas-phase platinum–dinitrogen bond strength of 116 kJ mol⁻¹, substantially weaker than the ethylene and carbon monoxide analogues but stronger than for polar solvents such as water, methanol and dimethylformamide, and strong enough that the calculated N−N bond length of 1.119 Å represents weakening to a degree typical of isolated dinitrogen complexes.     

固氮酶活性中心FeMo-Cofactor对N2活化方式的探讨

结合实验和理论计算的结果, 讨论了固氮酶的活性中心铁钼辅基(FeMo-co-facto r) 对N₂ 的各种活化方式, 并在此基础上提出了一种新模型, 即N₂ 在FeMo2cofacto r 的内部以“4Fe 端基配位+2Fe 侧基配位”的方式被活化,N₂ 的三重键完全断裂, 断裂产生的两个含N 的碎片分别偏向两侧的“窗口”, 再在H 的进攻下被还原为NH₃, 并分别从两侧的“窗口”离去。     

Probing the Potential of Hitherto Unexplored Base-Stabilized Borylenes in Dinitrogen Binding

Computational investigations were carried out to probe the potential of several dicoordinate, singly base-stabilized borylenes of the form [L→BR] (L=neutral Lewis base) in dinitrogen binding. The calculated reaction free energies and activation barriers associated with the formation of mono- and diborylene-N₂ adducts suggest the presence of thermally surmountable kinetic barriers towards their possible isolation. Our results show that the exergonicity of dinitrogen activation and fixation is linearly dependent on the natural charge at the boron center, which can be tuned to design novel boron-based compounds with potential applications to small-molecule activation. EDA-NOCV analysis reveals strong binding of dinitrogen to these base-stabilized borylenes.     

Dinitrogen functionalization with bis(cyclopentadienyl) complexes of zirconium and hafnium

The rich chemistry of substituted bis(cyclopentadienyl)zirconium and hafnium complexes bearing side-on coordinated dinitrogen ligands is highlighted in this Perspective. Our studies in this area were initially motivated by the desire to understand side-on vs. end-on dinitrogen coordination in bimetallic zirconocene and hafnocene N2 compounds. In the cases where eta2,eta2-dinitrogen compounds were isolated, both structural and computational data have established significant imido character in the metal-nitrogen bonds. This additional bonding interaction, which is diminished in end-on complexes bearing both terminal and bridging N2 ligands, facilitates dinitrogen functionalization by non-polar reagents including dihydrogen, carbon-hydrogen bonds and weak Br?nsted acids such as water and ethanol. In hafnocene chemistry, where unwanted side-on, end-on isomerization is suppressed, cycloaddition of phenylisocyanate to coordinated N2 has also been accomplished. For N-H bond forming reactions involving H2, kinetic measurements, in addition to isotopic labelling and computational studies, are consistent with dinitrogen functionalization by 1,2-addition involving a highly ordered, four-centred transition structure.     

Predicting Dinitrogen Activation via Transition-Metal-Involved [4+2] Cycloaddition Reaction

As the strongest triple bond in nature, the N≡N triple bond activation has always been a challenging project in chemistry. On the other hand, since the award of the Nobel Prize in Chemistry in 1950, the Diels-Alder reaction has served as a powerful and widely applied tool in the synthesis of natural products and new materials. However, the application of the Diels-Alder reaction to dinitrogen activation remains less developed. Here we first demonstrate that a transition-metal-involved [4+2] Diels-Alder cycloaddition reaction could be used to activate dinitrogen without an additional reductant by density functional theory calculations. Further study reveals that such a dinitrogen activation by 1-metalla-1,3-dienes screened out from a series of transition metal complexes (38 species) according to the effects of metal center, ligand, and substituents can become favorable both thermodynamically (with an exergonicity of 28.2 kcal mol⁻¹) and kinetically (with an activation energy as low as 13.8 kcal mol⁻¹). Our findings highlight an important application of the Diels-Alder reaction in dinitrogen activation, inviting experimental chemists’ verification.     

Catalytic Ammonia Oxidation to Dinitrogen by a Nickel Complex

We report a nickel complex for catalytic oxidation of ammonia to dinitrogen under ambient conditions. Using the aryloxyl radical 2,4,6-tri-tert-butylphenoxyl (^tBu₃ArO⋅) as a H atom acceptor to cleave the N−H bond of a coordinated NH₃ ligand up to 56 equiv of N₂ per Ni center can be generated. Employing the N-oxyl radical 2,2,6,6-(tetramethylpiperidin-1-yl)oxyl (TEMPO⋅) as the H-atom acceptor, up to 15 equiv of N₂ per Ni center are formed. A bridging Ni-hydrazine product identified by isotopic nitrogen (¹⁵N) studies and supported by computational models indicates the N−N bond forming step occurs by bimetallic homocoupling of two paramagnetic [Ni]−NH₂ fragments. Ni-mediated hydrazine disproportionation to N₂ and NH₃ completes the catalytic cycle.     

CO-Induced Dinitrogen Fixation and Cleavage Mediated by Boron

The boron atoms react with carbon monoxide and dinitrogen forming the end-on bonded NNBCO complex in solid neon or in nitrogen matrices. The NNBCO complex rearranges to the (η²-N₂)BCO isomer with a more activated side-on bonded dinitrogen ligand upon visible light excitation. (η²-N₂)BCO and its weakly CO-coordinated complexes further isomerize to the NBNCO and B(NCO)₂ molecules with N−N bond being completely cleaved under UV light irradiation. The geometries, energies and vibrational spectra of the molecules are calculated with quantum chemical methods and the electronic structures are analyzed with charge- and energy-partitioning methods.