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.     

Thermal Methane Activation by La6O10− Cluster Anions

The first example of a metal oxide cluster anion, La₆O₁₀^? that can activate methane under ambient conditions is reported. This reaction is facilitated by the oxygen‐centered radical (O^??) and follows the hydrogen atom transfer mechanism. The La₆O₁₀^? has a high vertical electron detachment energy (VDE=4.06 eV) and a high symmetry (C_4v).     

A Cornucopia of Iridium Nitrogen Compounds Produced from Laser-Ablated Iridium Atoms and Dinitrogen

The reaction of laser-ablated iridium atoms with dinitrogen molecules and nitrogen atoms yield several neutral and ionic iridium dinitrogen complexes such as Ir(N₂), Ir(N₂)⁺, Ir(N₂)₂, Ir(N₂)₂⁻, IrNNIr, as well as the nitrido complexes IrN, Ir(N)₂ and IrIrN. These reaction products were deposited in solid neon, argon and nitrogen matrices and characterized by their infrared spectra. Assignments of vibrational bands are supported by ab initio and first principle calculations as well as ^14/15N isotope substitution experiments. The structural and electronic properties of the new dinitrogen and nitrido iridium complexes are discussed. While the formation of the elusive dinitrido complex Ir(N)₂ was observed in a subsequent reaction of IrN with N atoms within the cryogenic solid matrices, the threefold coordinated iridium trinitride Ir(N)₃ could not be observed so far.     

Methane Activation Mediated by a Series of Cerium–Vanadium Bimetallic Oxide Cluster Cations: Tuning Reactivity by Doping

The reactions of cerium–vanadium cluster cations Ce_xV_yO_z⁺ with CH₄ are investigated by time‐of‐flight mass spectrometry and density functional theory calculations. (CeO₂)_m(V₂O₅)_n⁺ clusters (m=1,2, n=1–5; m=3, n=1–4) with dimensions up to nanosize can abstract one hydrogen atom from CH₄. The theoretical study indicates that there are two types of active species in (CeO₂)_m(V₂O₅)_n⁺, V[(O_t)₂]^. and [(O_b)₂CeO_t]^. (O_t and O_b represent terminal and bridging oxygen atoms, respectively); the former is less reactive than the latter. The experimentally observed size‐dependent reactivities can be rationalized by considering the different active species and mechanisms. Interestingly, the reactivity of the (CeO₂)_m(V₂O₅)_n⁺ clusters falls between those of (CeO₂)_2–4⁺ and (V₂O₅)_1–5⁺ in terms of C?H bond activation, thus the nature of the active species and the cluster reactivity can be effectively tuned by doping.     

Dinitrogen Activation by Fryzuk’s [Nb(P2N2)] Complex and Comparison with the Laplaza–Cummins [Mo{N(R)Ar}3] and Schrock [Mo(N3N)] Systems

The reaction profile of N₂ with Fryzuk’s [Nb(P₂N₂)] (P₂N₂=PhP(CH₂SiMe₂NSiMe₂CH₂)₂PPh) complex is explored by density functional calculations on the model [Nb(PH₃)₂(NH₂)₂] system. The effects of ligand constraints, coordination number, metal and ligand donor atom on the reaction energetics are examined and compared to the analogous reactions of N₂ with the three‐coordinate Laplaza‐Cummins [Mo{N(R)Ar}₃] and four‐coordinate Schrock [Mo(N₃N)] (N₃N=[(RNCH₂CH₂)₃N]^3?) systems. When the model system is constrained to reflect the geometry of the P₂N₂ macrocycle, the N? N bond cleavage step, via a N₂‐bridged dimer intermediate, is calculated to be endothermic by 345 kJ mol^?1. In comparison, formation of the single‐N‐bridged species is calculated to be exothermic by 119 kJ mol^?1, and consequently is the thermodynamically favoured product, in agreement with experiment. The orientation of the amide and phosphine ligands has a significant effect on the overall reaction enthalpy and also the N? N bond cleavage step. When the ligand constraints are relaxed, the overall reaction enthalpy increases by 240 kJ mol^?1, but the N₂ cleavage step remains endothermic by 35 kJ mol^?1. Changing the phosphine ligands to amine donors has a dramatic effect, increasing the overall reaction exothermicity by 190 kJ mol^?1 and that of the N? N bond cleavage step by 85 kJ mol^?1, making it a favourable process. Replacing Nb^II with Mo^III has the opposite effect, resulting in a reduction in the overall reaction exothermicity by over 160 kJ mol^?1. The reaction profile for the model [Nb(P₂N₂)] system is compared to those calculated for the model Laplaza and Cummins [Mo{N(R)Ar}₃] and Schrock [Mo(N₃N)] systems. For both [Mo(N₃N)] and [Nb(P₂N₂)], the intermediate dimer is calculated to lie lower in energy than the products, although the final N? N cleavage step is much less endothermic for [Mo(N₃N)]. In contrast, every step of the reaction is favourable and the overall exothermicity is greatest for [Mo{N(R)Ar}₃], and therefore this system is predicted to be most suitable for dinitrogen cleavage.     

Mo Cluster Support on C2N as a Highly-efficient Catalyst for Electrocatalytic Nitrogen Reduction Reaction

The conversion of nitrogen to ammonia by electrocatalysis under mild conditions is a valuable research direction, which has been a sustainable alternative to the traditional Haber-Bosch method. However, the conversion remains a huge challenge in chemistry at this time. In this work, the density functional theory (DFT) is used to study the electrocatalytic nitrogen reduction reaction (NRR) performance of Mo₁₂ clusters on C₂N monolayer (Mo₁₂−C₂N). It is found that the diversity of active sites of the Mo₁₂ cluster provides favorable reaction paths for intermediates, which reduces reaction barrier of NRR. Mo₁₂−C₂N shows excellent NRR performances with limiting potentials of −0.26 V vs. reversible hydrogen electrode (RHE).     

Mechanism of [3+2] Cycloaddition of Alkynes to the [Mo3S4(acac)3(py)3][PF6] Cluster

A study, involving kinetic measurements on the stopped‐flow and conventional UV/Vis timescales, ESI‐MS, NMR spectroscopy and DFT calculations, has been carried out to understand the mechanism of the reaction of [Mo₃S₄(acac)₃(py)₃][PF₆] ([ 1 ]PF₆; acac=acetylacetonate, py=pyridine) with two RC?CR alkynes (R=CH₂OH (btd), COOH (adc)) in CH₃CN. Both reactions show polyphasic kinetics, but experimental and computational data indicate that alkyne activation occurs in a single kinetic step through a concerted mechanism similar to that of organic [3+2] cycloaddition reactions, in this case through the interaction with one Mo(μ‐S)₂ moiety of [ 1 ]⁺. The rate of this step is three orders of magnitude faster for adc than that for btd, and the products initially formed evolve in subsequent steps into compounds that result from substitution of py ligands or from reorganization to give species with different structures. Activation strain analysis of the [3+2] cycloaddition step reveals that the deformation of the two reactants has a small contribution to the difference in the computed activation barriers, which is mainly associated with the change in the extent of their interaction at the transition‐state structures. Subsequent frontier molecular orbital analysis shows that the carboxylic acid substituents on adc stabilize its HOMO and LUMO orbitals with respect to those on btd due to better electron‐withdrawing properties. As a result, the frontier molecular orbitals of the cluster and alkyne become closer in energy; this allows a stronger interaction.     

CH Bond Activation by Early Transition Metal Carbide Cluster Anion MoC3−

Although early transition metal (ETM) carbides can activate C?H bonds in condensed‐phase systems, the electronic‐level mechanism is unclear. Atomic clusters are ideal model systems for understanding the mechanisms of bond activation. For the first time, C?H activation of a simple alkane (ethane) by an ETM carbide cluster anion (MoC₃^?) under thermal‐collision conditions has been identified by using high‐resolution mass spectrometry, photoelectron imaging spectroscopy, and high‐level quantum chemical calculations. Dehydrogenation and ethene elimination were observed in the reaction of MoC₃^? with C₂H₆. The C?H activation follows a mechanism of oxidative addition that is much more favorable in the carbon‐stabilized low‐spin ground electronic state than in the high‐spin excited state. The reaction efficiency between the MoC₃^? anion and C₂H₆ is low (0.23±0.05) %. A comparison between the anionic and a highly efficient cationic reaction system (Pt⁺+C₂H₆) was made. It turned out that the potential‐energy surfaces for the entrance channels of the anionic and cationic reaction systems can be very different.     

Synthesis and Characterization of a Cu14 Hydride Cluster Supported by Neutral Donor Ligands

The copper hydride clusters [Cu₁₄H₁₂(phen)₆(PPh₃)₄][X]₂ (X=Cl or OTf; OTf=trifluoromethanesulfonate, phen=1,10‐phenanthroline) are obtained in good yields by the reaction of [(Ph₃P)CuH]₆ with phen, in the presence of a halide or pseudohalide source. The complex [Cu₁₄H₁₂(phen)₆(PPh₃)₄][Cl]₂ reacts with CO₂ in CH₂Cl₂, in the presence of excess Ph₃P, to form the formate complex [(Ph₃P)₂Cu(κ²‐O₂CH)], along with [(phen)(Ph₃P)CuCl].     

An electronic perspective on the reduction of an n=n double bond at a conserved dimolybdenum core

Density functional theory has been used to assess the role of the bimetallic core in supporting reductive cleavage of the N=N double bond in [Cp2Mo2(mu-SMe)3(mu-eta1:eta1-HN=NPh)]+. The HOMO of the complex, the Mo-Mo delta orbital, plays a key role as a source of high-energy electrons, available for transfer into the vacant orbitals of the N=N unit. As a result, the metal centres cycle between the Mo(III) and Mo(IV) oxidation states. The symmetry of the Mo-Mo delta "buffer" orbital has a profound influence on the reaction pathway, because significant overlap with the redox-active orbital on the N=N unit (pi* or sigma*) is required for efficient electron transfer. The orthogonality of the Mo-Mo delta and N-N sigma* orbitals in the eta1:eta1 coordination mode ensures that electron transfer into the N-N sigma bond is effectively blocked, and a rate-limiting eta1:eta1-->eta1 rearrangement is a necessary precursor to cleavage of the bond.     

Thermal Ethane Activation by Bare [V2O5]+ and [Nb2O5]+ Cluster Cations: on the Origin of Their Different Reactivities

The gas‐phase reactivity of [V₂O₅]⁺ and [Nb₂O₅]⁺ towards ethane has been investigated by means of mass spectrometry and density functional theory (DFT) calculations. The two metal oxides give rise to the formation of quite different reaction products; for example, the direct room‐temperature conversions C₂H₆→C₂H₅OH or C₂H₆→CH₃CHO are brought about solely by [V₂O₅]⁺. In distinct contrast, for the couple [Nb₂O₅]⁺/C₂H₆, one observes only single and double hydrogen‐atom abstraction from the hydrocarbon. DFT calculations reveal that different modes of attack in the initial phase of C?H bond activation together with quite different bond‐dissociation energies of the M?O bonds cause the rather varying reactivities of [V₂O₅]⁺ and [Nb₂O₅]⁺ towards ethane. The gas‐phase generation of acetaldehyde from ethane by bare [V₂O₅]⁺ may provide mechanistic insight in the related vanadium‐catalyzed large‐scale process.     

Ruthenium‐Catalyzed Oxidative Coupling of Primary Amines with Internal Alkynes through CH Bond Activation: Scope and Mechanistic Studies

The oxidative coupling of primary amines with internal alkynes catalyzed by Ru complexes is presented as a general atom‐economy methodology with a broad scope of applications in the synthesis of N‐heterocycles. Reactions proceed through regioselective C?H bond activation in 15 minutes under microwave irradiation or in 24 hours with conventional heating. The synthesis of 2,3,5‐substituted pyridines, benzo[h]isoquinolines, benzo[g]isoquinolines, 8,9‐dihydro‐benzo[de]quinoline, 5,6,7,8‐tetrahydroisoquinolines, pyrido[3,4g]isoquinolines, and pyrido[4,3g]isoquinolines is achievable depending on the starting primary amine used. DFT calculations on a benzylamine substrate support a reaction mechanism that consists of acetate‐assisted C?H bond activation, migratory‐insertion, and C?N bond formation steps that involve 28–30 kcal mol^?1. The computational study is extended to additional substrates, namely, 1‐naphthylmethyl‐, 2‐methylallyl‐, and 2‐thiophenemethylamines.     

新颖五核Mo/Cu/S簇合物[MoOS3Cu4(α-MePy)6Br2]的合成及晶体结构

杂核金属含硫簇合物具有丰富的结构,在生物化学、催化和非线性光学等方面显示了诱人的应用前景.近年来,我们主要从事用[MS4 ]2 - 和[Cp* MS3]- ( M=Mo,W)和Cu( ) ,Ag( )反应合成Mo( W) /Cu( Ag) /S簇合物并研究其非线性光学性质[1,2 ] .鉴于用硫代金属酸盐作前驱体合成M- Cu- Ag- S杂三核金属簇合物的工作鲜有报道[3] ,我们尝试用( NH4 ) 2 Mo OS3和Cu Br,Ag Br在α- Me Py中反应,希望得到Mo- Cu- Ag- S杂三核金属簇合物.但上述反应却形成了1个仅含Mo和Cu( )的五核簇合物[Mo OS3Cu4 (α- Me Py) 6 Br2 ],其所含的Mo S3…     

Activation of Methane Promoted by Adsorption of CO on Mo2C2− Cluster Anions

Atomic clusters are being actively studied for activation of methane, the most stable alkane molecule. While many cluster cations are very reactive with methane, the cluster anions are usually not very reactive, particularly for noble metal free anions. This study reports that the reactivity of molybdenum carbide cluster anions with methane can be much enhanced by adsorption of CO. The Mo₂C₂^? is inert with CH₄ while the CO addition product Mo₂C₃O^? brings about dehydrogenation of CH₄ under thermal collision conditions. The cluster structures and reactions are characterized by mass spectrometry, photoelectron spectroscopy, and quantum chemistry calculations, which demonstrate that the Mo₂C₃O^? isomer with dissociated CO is reactive but the one with non‐dissociated CO is unreactive. The enhancement of cluster reactivity promoted by CO adsorption in this study is compared with those of reported systems of a few carbonyl complexes.     

Redox‐Neutral Rhodium‐Catalyzed CH Functionalization of Arylamine N‐Oxides with Diazo Compounds: Primary C(sp3)H/C(sp2)H Activation and Oxygen‐Atom Transfer

An unprecedented rhodium(III)‐catalyzed regioselective redox‐neutral annulation reaction of 1‐naphthylamine N‐oxides with diazo compounds was developed to afford various biologically important 1H‐benzo[g]indolines. This coupling reaction proceeds under mild reaction conditions and does not require external oxidants. The only by‐products are dinitrogen and water. More significantly, this reaction represents the first example of dual functiaonalization of unactivated a primary C(sp³)? H bond and C(sp²)? H bond with diazocarbonyl compounds. DFT calculations revealed that an intermediate iminium is most likely involved in the catalytic cycle. Moreover, a rhodium(III)‐catalyzed coupling of readily available tertiary aniline N‐oxides with α‐diazomalonates was also developed under external oxidant‐free conditions to access various aminomandelic acid derivatives by an O‐atom‐transfer reaction.     

RhIII‐Catalyzed C(sp3)H Bond Activation by an External Base Metalation/Deprotonation Mechanism: A Theoretical Study

The C(sp³)?H bond activation of 8‐methylquinoline followed by alkyne insertion catalyzed by a Rh^III complex has been studied by using density functional theory (DFT) calculations. Contrary to common belief, the C?H bond activation of methylquinoline does not occur by the traditional intramolecular concerted metalation/deprotonation (CMD) mechanism but by an external base CMD mechanism. The use of free acetate or copper(II) acetate as base permits the C?H activation step, as observed experimentally. However, the following insertion is possible only if copper(II) acetate is used. The insertion followed by metathesis occurs via a cationic Rh^III complex and is irreversible, which ensures the efficiency of the entire process. Therefore the use of copper is crucial for completing the catalytic cycle. The present work should help to rationalize the origins of the experimental results described in the literature.     

Mechanistic Study on Rh‐Catalyzed Stereoselective CC/CH Activation of tert‐Cyclobutanols

A mechanistic study was performed on the Rh‐catalyzed stereoselective C?C/C?H activation of tert‐cyclobutanols. The present study corroborated the previous proposal that the reaction occurs by metalation, β‐C elimination, 1,4‐Rh transfer, C?O insertion, and a final catalyst‐regeneration step. The rate‐determining step was found to be the 1,4‐Rh transfer step, whereas the stereoselectivity‐determining step did not correspond to any of the aforementioned steps. It was found that both the thermodynamic stability of the product of the β‐C elimination and the kinetic feasibility of the 1,4‐Rh transfer and C?O insertion steps made important contributions. In other words, three steps (i.e., β‐C elimination, 1,4‐Rh transfer, and C?O insertion) were found to be important in determining the configurations of the two quaternary stereocenters.