Catalytic conversion of nitrogen to ammonia by an iron model complex

High-Spin [FeI3] Cluster Capable of Pnictogen Atom Capture

Using a new hexanucleating anildophosphine ligand tBuLH3 (1,3,5-C6H9(NHC6H3-5-F-2-P(tBu)2)3), the all-monovalent [FeI3] compound (tBuL)Fe3 (1) was isolated and characterized by X-ray diffraction analysis, SQUID magnetometry, 57Fe Mössbauer spectroscopy, and cyclic voltammetry. The molecular structure of 1 reveals very close Fe-Fe distances of 2.3825(7), 2.4146(8), and 2.3913(8) Å which results in significant Fe-Fe interactions and a maximum high-spin S = 9/2 spin state as determined by SQUID magnetometry and further supported by quantum chemical calculations. Compound 1 mediates the multielectron, oxidative atom transfer from inorganic azide ([Bu4N][N3]), cyanate (Na[NCO]), and phosphonate (Na(dioxane)2.5[PCO]) to afford the [Fe3]-nitrido (N3-) and [Fe3]-phosphido (P3-) pnictides, (tBuL)Fe3(μ3-N) (2) and [(tBuL)Fe3(μ3-P)(CO)]- (3), respectively. Compounds 1-3 exhibit rich electrochemical behavior with three (for 1), four (for 2) and five (for 3) distinct redox events being observed in the cyclic voltammograms of these compounds. Finally, the all-monovalent 1 and the formally FeII/FeII/FeI compound 3, were investigated by alternating current (ac) SQUID magnetometry, revealing slow magnetic relaxation in both compounds, with 3 being found to be a unique example of a [Fe3]-phosphido single-molecule magnet having an energy barrier relaxation reversal of U = 30.7(6) cm-1 in the absence of an external magnetic field. This study demonstrates the utility of an all low-valent polynuclear cluster to perform multielectron redox chemistry and exemplifies the redox flexibility and unique physical properties that are present in the corresponding midvalent oxidation products.

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.

Rational design of bimetallic MBene for efficient electrocatalytic nitrogen reduction

Electrocatalytic nitrogen reduction reaction (NRR) is one of the most promising approaches to achieving green and efficient NH3 production. However, the designs of efficient NRR catalysts with high activity and selectivity still are severely hampered by inherent linear scaling relations among the adsorption energies of NRR intermediates. Herein, the properties of ten M3B4 type MBenes have been initially investigated for efficient N2 activation and reduction to NH3via first-principles calculations. We highlight that Cr3B4 MBene possesses remarkable NRR activity with a record-low limiting potential (-0.13 V). Then, this work proposes descriptor-based design principles that can effectively evaluate the catalytic activity of MBenes, which have been further employed to design bimetallic M2M'B4 MBenes. As a result, 5 promising candidates including Ti2YB4, V2YB4, V2MoB4, Nb2YB4, and Nb2CrB4 with excellent NRR performance have been extracted from 20 bimetallic MBenes. Further analysis illuminates that constructing bimetallic MBenes can selectively tune the adsorption strength of NHNH2** and NH2NH2**, and break the linear scaling relations between their adsorption energies, rendering them ideal for NRR. This work not only pioneers the application of MBenes as efficient NRR catalysts but also proposes rational design principles for boosting their catalytic performance.

Enabling Sustainable Ammonia Synthesis: From Nitrogen Activation Strategies to Emerging Materials

Ammonia (NH3) is one of the most important precursors of various chemicals and fertilizers. Given that ammonia synthesis via the traditional Haber-Bosch process requires high temperatures and pressures, it is critical to explore effective strategies and catalysts for ammonia synthesis under mild reaction conditions. Although electrocatalysis and photocatalysis can convert N2 to NH3 under mild conditions, their efficiencies and production scales are still far from the requirements for industrialization. Thermal catalysis has been proven to be the most direct and effective approach for ammonia synthesis. Over the past few decades, significant efforts have been made to develop novel catalysts capable of nitrogen fixation and ammonia generation via thermal catalytic processes. In parallel with catalyst exploration, new strategies such as self-electron donation, hydride fixation, hydridic hydrogen reduction, and anionic vacancy promotion have also been explored to moderate the operating conditions and improve the catalytic efficiency of ammonia synthesis. In this review, the emergence of new materials and strategies for promoting N2 activation and NH3 formation during thermal catalysis is briefly summarized. Moreover, challenges and prospects are proposed for the future development of thermal catalytic ammonia synthesis.

Catalytic Nitrogen Fixation Using Well-Defined Molecular Catalysts under Ambient or Mild Reaction Conditions

Ammonia (NH3) is industrially produced from dinitrogen (N2) and dihydrogen (H2) by the Haber-Bosch process, although H2 is prepared from fossil fuels, and the reaction requires harsh conditions. On the other hand, microorganisms have fixed nitrogen under ambient reaction conditions. Recently, well-defined molecular transition metal complexes have been found to work as catalyst to convert N2 into NH3 by reactions with chemical reductants and proton sources under ambient reaction conditions. Among them, involvement of both N2-splitting pathway and proton-coupled electron transfer is found to be very effective for high catalytic activity. Furthermore, direct electrocatalytic and photocatalytic conversions of N2 into NH3 have been recently achieved. In addition to catalytic formation of NH3, selective catalytic conversion of N2 into hydrazine (NH2NH2) and catalytic silylation of N2 into silylamines have been reported. Catalytic C-N bond formation has been more recently established to afford cyanate anion (NCO-) under ambient reaction conditions. Further development of direct conversion of N2 into nitrogen-containing compounds as well as green ammonia synthesis leading to the use of ammonia as an energy carrier is expected.

Challenges and Advancements in the Electrochemical Utilization of Ammonia Using Solid Oxide Fuel Cells

Solid oxide fuel cells utilized with NH3 (NH3-SOFCs) have great potential to be environmentally friendly devices with high efficiency and energy density. The advancement of this technology is hindered by the sluggish kinetics of chemical or electrochemical processes occurring on anodes/catalysts. Extensive efforts have been devoted to developing efficient and durable anode/catalysts in recent decades. Although modifications to the structure, composition, and morphology of anodes or catalysts are effective, the mechanistic understandings of performance improvements or degradations remain incompletely understood. This review informatively commences by summarizing existing reports on the progress of NH3-SOFCs. It subsequently outlines the influence of factors on the performance of NH3-SOFCs. The degradation mechanisms of the cells/systems are also reviewed. Lastly, the persistent challenges in designing highly efficient electrodes/catalysts for low-temperature NH3-SOFCs, and future perspectives derived from SOFCs are discussed. Notably, durability, thermal cycling stability, and power density are identified as crucial indicators for enhancing low-temperature (550 °C or below) NH3-SOFCs. This review aims to offer an updated overview of how catalysts/electrodes affect electrochemical activity and durability, offering critical insights for improving performance and mechanistic understanding, as well as establishing the scientific foundation for the design of electrodes for NH3-SOFCs.

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.

Construction and Function of Thiolate-Bridged Diiron NxHy Nitrogenase Model Complexes

ConspectusBiological nitrogen fixation mediated by nitrogenases has garnered significant research interest due to its critical importance to the development of efficient catalysts for mild ammonia synthesis. Although the active center of the most studied FeMo-nitrogenases has been determined to be a complicated [Fe7S9MoC] hetero-multinuclear metal-sulfur cluster known as the FeMo-cofactor, the exact binding site and reduction pathway of N2 remain a subject of debate. Over the past decades, the majority of studies have focused on mononuclear molybdenum or iron centers as potential reaction sites. In stark contrast, cooperative activation of N2 through bi- or multimetallic centers has been largely overlooked and underexplored, despite the renewed interest sparked by recent biochemical and computational studies. Consequently, constructing bioinspired bi- or multinuclear metallic model complexes presents an intriguing yet challenging prospect. In this Account, we detail our long-standing research on the design and synthesis of novel thiolate-bridged diiron complexes as nitrogenase models and their application to chemical simulations of potential biological N2 reduction pathways.Inspired by the structural and electronic features of the potential diiron active center in the belt region of the FeMo-cofactor, we have designed and synthesized a series of new thiolate-bridged diiron nitrogenase model complexes, wherein iron centers with +2 or +3 oxidation states are coordinated by Cp* as carbon-based donors and thiolate ligands as sulfur donors. Through the synergistic interaction between the two iron centers, unstable diazene (NH═NH) species can be trapped to generate the first example of a [Fe2S2]-type complex bearing a cis-μ-η1:η1-NH═NH subunit. Significantly, this species can not only catalyze the reductive N-N bond cleavage of hydrazine to ammonia but also trigger a stepwise reduction sequence NH═NH → [NH2-NH]- → [NH]2-(+NH3) → [NH2]- → NH3. Furthermore, an unprecedented thiolate-bridged diiron μ-nitride featuring a bent Fe-N-Fe moiety was successfully isolated and structurally characterized. Importantly, this diiron μ-nitride can undergo successive proton-coupled electron transfer processes to efficiently release ammonia in the presence of separate protons and electrons and can even be directly hydrogenated using H2 as a combination of protons and electrons for high-yield ammonia formation. Based on combined experimental and computational studies, we proposed two distinct reductive transformation sequences on the diiron centers, which involve a series of crucial NxHy intermediates. Moreover, we also achieved catalytic N2 reduction to silylamines with [Fe2S2]-type complexes by ligand modulation.Our bioinspired diiron cooperative scaffold may provide a suitable model for probing the potential N2 stepwise reduction pathways from the molecular level. Different from the traditional alternating and distal pathways dominated by mononuclear iron or molybdenum complexes, our proposed alternating transformation route based on the diiron centers may not involve the N2H4 intermediate, and the convergence point of the alternating and terminal pathways is imide, not amide. Our research strategy could inform the design and development of new types of bioinspired catalysts for mild and efficient nitrogen reduction in the future.

Heterobimetallic 3d-4f complexes supported by a Schiff-base tripodal ligand

Complexes featuring multiple metal centres are of growing interest regarding metal-metal cooperation and its tuneability. Here the synthesis and characterisation of heterobimetallic complexes of a 3d metal (4: Mn, 5: Co) and lanthanum supported by a (1,1,1-tris[(3-methoxysalicylideneamino)methyl]ethane) ligand is reported, as well as discussion of their electronic structure via electron paramagnetic resonance (EPR) spectroscopy, electrochemical experiments and computational studies. Competitive binding experiments of the ligand and various metal salts unequivocally demonstrate that in these heterobimetallic complexes the 3d metal (Mn, Co) selectively occupies the κ6-N3O3 binding site of the ligand, whilst La occupies the κ6-O6 metal binding site in line with their relative oxophilicities. EPR spectroscopy supported by density functional theory analysis indicates that the 3d metal is high spin in both cases (S = 5/2 (Mn), 3/2 (Co)). Cyclic voltammetry studies on the Mn/La and Co/La bimetallic complexes revealed a quasi-reversible Mn2+/3+ redox process and poorly-defined irreversible oxidation events respectively.

Toward Designing Reactive Metal Clusters for Dinitrogen Activation: A Guideline Based on N2 Initial Adsorption

Gas-phase metal clusters are ideal models to explore transition-metal-mediated N2 activation mechanism. However, the effective design and search of reactive clusters in N2 activation are currently hindered by the lack of clear guidelines. Inspired by the Sabatier principle, we discovered in this work that N2 initial adsorption energy (ΔEads) is an important parameter to control the N2 activation reactivity of metal clusters in the gas phase. This mechanistic insight obtained from high-level calculations rationalizes the N2 activation reactivity of many previously reported metal clusters when combined with the known factor determining the N≡N cleavage process. Furthermore, based on this guideline of ΔEads, we successfully designed several new reactive clusters for cleaving N≡N triple bond under mild conditions, including FeV2S2-, TaV2C2-, and TaV2C3-, the high N2 activation reactivity of which has been fully corroborated in our gas phase experiments employing mass spectrometry with collision-induced dissociation. The importance of ΔEads revealed in this work not only reshapes our understanding of N2 activation reactions in the gas phase but also could have implication for other N2 activation processes in the condensed phase. The more general establishment of this new perspective on N2 activation reactivity warrants future experimental and computational studies.

(Electro)chemical N2 Splitting by a Molybdenum Complex with an Anionic PNP Pincer-Type Ligand

Molybdenum(III) complexes bearing pincer-type ligands are well-known catalysts for N2-to-NH3 reduction. We investigated herein the impact of an anionic PNP pincer-type ligand in a Mo(III) complex on the (electro)chemical N2 splitting ([LMoCl3]-, 1 -, LH = 2,6-bis((di-tert-butylphosphaneyl)methyl)-pyridin-4-one). The increased electron-donating properties of the anionic ligand should lead to a stronger degree of N2 activation. The catalyst is indeed active in N2-to-NH3 conversion utilizing the proton-coupled electron transfer reagent SmI2/ethylene glycol. The corresponding Mo(V) nitrido complex 2H exhibits similar catalytic activity as 1H and thus could represent a viable intermediate. The Mo(IV) nitrido complex 3 - is also accessible by electrochemical reduction of 1 - under a N2 atmosphere. IR- and UV/vis-SEC measurements suggest that N2 splitting occurs via formation of an "overreduced" but more stable [(L(N2)2Mo0)2μ-N2]2- dimer. In line with this, the yield in the nitrido complex increases with lower applied potentials.

*H migration-assisted MvK mechanism for efficient electrochemical NH3 synthesis over TM-TiNO

The electrochemical NH3 synthesis on TiNO is proposed to follow the Mars-van Krevelen (MvK) mechanism, offering more favorable N2 adsorption and activation on the N vacancy (Nv) site, compared to the conventional associative mechanism. The regeneration cycle of Nv represents the rate-determining step in this process. This study investigates a series of TM (Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt)-TiNO to explore the *H migration (from TM to TiNO)-promoted Nv cycle. The screening results indicate that Ni-TiNO exhibits strong H2O decomposition for *H production with 0.242 eV and low *H migration resistance with 0.913 eV. Notably, *H migration from Ni to TiNO significantly reduces the Nv formation energy to 0.811 eV, compared to 1.387 eV on pure TiNO. Meanwhile, in the presence of *H, Nv formation takes precedence over Tiv and Ov. Lastly, electronic performance calculations reveal that the collaborative function provided by Ni and Nv enables highly stable and efficient NH3 synthesis. The *H migration-assisted MvK mechanism demonstrates effective catalytic cycle performance in electrochemical N2 fixation and may have potential applicability to other hydrogenation reactions utilizing water as a proton source.

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.

Computational design of cooperatively acting molecular catalyst systems: carbene based tungsten- or molybdenum-catalysts with rhodium- or iridium-complexes for the ionic hydrogenation of N2 to NH3

This density functional theory (DFT) study explores the efficacy of cooperative catalytic systems in enabling the ionic hydrogenation of N2 with H2, leading to NH3 formation. A set of N-heterocyclic carbene-based pincer tungsten/molybdenum metal complexes of the form [(PCP)M1(H)2] (M1 = W/Mo) were chosen to bind N2 at the respective metal centres. Simultaneously, cationic rhodium/iridium complexes of type [Cp*M2{2-(2-pyridyl)phenyl}(CH3CN)]+ (Cp* = C5(CH3)5 and M2 = Rh/Ir), are employed as cooperative coordination partners for heterolytic H2 splitting. The stepwise transfer of protons and hydrides to the bound N2 and intermediate NxHy units results in the formation of NH3. Interestingly, the calculated results reveal an encouraging low range of energy spans ranging from ∼30 to 42 kcal mol-1 depending on different combinations of ligands and metal complexes. The optimal combination of pincer ligand and metal center allowed for an energy span of unprecedented 29.7 kcal mol-1 demonstrating significant potential for molecular catalysts for the N2/H2 reaction system. While exploring obvious potential off-cycle reactions leading to catalyst deactivation, the computed results indicate that no increase in energy span would need to be expected.

Rare-Earth Metalloligands for LowValent Cobalt Complexes: Fine Electronic Tuning via Co→RE Dative Interactions

Rare-earth metalloligand supported low-valent cobalt complexes were synthesized by utilizing a small-sized heptadentate phosphinomethylamine LsNH3 and a large-sized arene-anchored hexadentate phosphinomethylamine LlArH3 ligand precursors. The RE(III)-Co(-I)-N2 (RE = Sc, Lu, Y, Gd, La) complexes containing rare-earth metals including the smallest Sc and largest La were characterized by multinuclear NMR spectroscopy, X-ray diffraction analysis, electrochemistry, and computational studies. The Co(-I)→RE(III) dative interactions were all polarized with major contributions from the 3dz2 orbital of the cobalt center, which was slightly affected by the identity of rare-earth metalloligands. The IR spectroscopic data and redox potentials obtained from cyclic voltammetry revealed that the electronic property of the Co(-I) center was finely tuned by the rare-earth metalloligand, which was revealed by variation of the ligand systems containing LsN, LmN, and LlAr. Unlike the direct alteration of the electronic property of metal center via an ancillary ligand, such a series of rare-earth metalloligand represents a smooth strategy to tune the electronic property of transition metals.

Theoretical study of the nitrogen reduction reaction catalyzed by a B-doped MoO2 six-membered ring

In this study, two potential catalysts with double-B atom-doped atomic MoO2 (B2/MoO2) and single-B atom-doped atomic MoO2 (B/MoO2) were designed and constructed. The thermodynamics and selectivity of two catalysts in the nitrogen fixation reaction were analyzed by a DFT calculation method. The results show that B2/MoO2 shows better adsorption activation and reduction and can effectively activate nitrogen molecules by two adjacent boron atoms. It achieves an extremely low overpotential of -0.18 V and rapid NRR kinetics through an enzymatic mechanism. Therefore, B2/MoO2 is a very promising NRR candidate catalyst. This research shows that doping with diatomic B (as an active site) results in an excellent NRR catalytic activity, which provides a certain theoretical basis for the preparation of high-performance NRR catalysts.

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.

Crystal-Phase and Surface-Structure Engineering of Bi2O3 for Enhanced Electrochemical N2 Fixation to NH3

The nitrogen reduction reaction (NRR) for ammonia synthesis is hindered by weak N2 adsorption/activation abilities and the hydrogen evolution reaction (HER). In this study, αBi2O3 (monoclinic) and βBi2O3 (tetragonal) were first synthesized by calcination at different temperatures. Experiments and calculations revealed the effects of Bi2O3 with different crystal phases on N2 adsorption/activation abilities and HER. Then, αBi2O3-x and βBi2O3-x series catalysts with surface oxygen vacancies (OVs) and Bi0 active sites were synthesized through the partial in situ reduction method. The results demonstrate the following: (I) Tetragonal βBi2O3 can better adsorb N2 and cleave the N≡N bond, thereby obtaining a lower NRR rate-limiting energy barrier (*N≡N → *N≡N-H, 0.51 eV). Meanwhile, βBi2O3 can effectively suppress HER by limiting proton adsorption (H+ + e- → *H, 0.54 eV). Therefore, βBi2O3-x series catalysts exhibit higher NH3 yield and FE than αBi2O3-x. Meanwhile, in situ FTIR further confirms that βBi2O3 could better adsorb/activate N2, and the NRR distal mechanism occurs on the Bi2O3 surface. (II) The introduction of NaBH4 promotes the conversion of part of Bi3+ on the Bi2O3 surface into Bi0 and releases OVs. The additional active sites (OVs and Bi0) enhance the overall catalyst's adsorption/activation capacity for N2, further increasing the NH3 yield and FE. Meanwhile, semimetal Bi0 can effectively limit electron accessibility, thereby inhibiting the combination of charges and adsorbed protons, reducing the HER reaction and improving the FE of NRR. Therefore, the introduction of NaBH4 effectively improved the NH3 yield and FE of the αBi2O3-x and βBi2O3-x series catalysts. After optimization, the βBi2O3-0.6 catalyst has the best NRR performance (NH3 yield: 51.36 μg h-1 mg-1cat.; FE: 38.67%), which is superior to the majority of bismuth-based NRR catalysts. This work not only studies the effects of Bi2O3 with different crystal phases on N2 and HER reaction but also effectively regulates the active components of Bi2O3 surface, thereby realizing efficient NRR to NH3 reaction, which provide valuable insights for the rational design of Bi-based NRR electrocatalysts.

Electronic Structure and Ligand Effects on the Activation and Cleavage of N2 on a Molybdenum Center

Dinitrogen fixation under ambient conditions remains a challenge in the field of catalytic chemistry due to the inertness of N2. Nitrogenases and heterogeneous solid catalysts have displayed remarkable performance in the catalytic conversion of dinitrogen to ammonia. By introduction of molybdenum centers in molecular complexes, one of the most azophilic metals of the transitional metal series, moderate ammonia yields have been attained. Here, we present a combined multiconfigurational/density functional theory study that addresses how ligand fields of different strengths affect the binding and activation of dinitrogen on molybdenum atoms. First, we explored with MRCI computations the diatomic Mo-N and triatomic Mo-N2 molecular systems. Then, we performed a systematic examination on the stabilization effects introduced by external NH3 ligands, before we explore model neutral and charged complexes with different types of ligands (H2O, NH3, and PH3) and their consequences on the N2 binding and activation.

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.

Catalytic Reduction of Cyanide to Ammonia and Methane at a Mononuclear Fe Site

Nitrogenase enzymes catalyze nitrogen reduction (N2R) to ammonia and also the reduction of non-native substrates, including the 7H+/6e- reduction of cyanide to CH4 and NH3. CN- and N2 are isoelectronic, and it is hence fascinating to compare the mechanisms of synthetic Fe catalysts capable of both CN- and N2 reduction. Here, we describe the catalytic reduction of CN- to NH3 and CH4 by a highly selective (P3Si)Fe(CN) catalyst (P3Si represents a tris(phosphine)silyl ligand). Catalysis is driven in the presence of excess acid ([Ph2NH2]OTf) and reductant ((C6H6)2Cr), with turnover as high as 73 demonstrated. This catalyst system is also modestly competent for N2R and structurally related to other tris(phosphine)Fe-based N2R catalysts. The choice of catalyst and reductant is important to observe high yields. Mechanistic studies elucidate several intermediates of CN- reduction, including iron isocyanides (P3SiFeCNH+/0) and terminal iron aminocarbynes (P3SiFeCNH2+/0). Aminocarbynes are isoelectronic to iron hydrazidos (Fe═N-NH2+/0), which have been invoked as selectivity-determining intermediates of N2R (NH3 versus N2H4 products). For the present CN- reduction catalysis, reduction of aminocarbyne P3SiFeCNH2+ is proposed to be rate but not selectivity contributing. Instead, by comparison with the reactivity of a methylated aminocarbyne analogue (P3SiFeCNMe2), and associated computational studies, formation of a Fischer carbene (P3SiFeC(H)(NH2)+) intermediate that is on path for either CH4 and NH3 (6 e-) or CH3NH2 (4 e-) products is proposed. From this carbene intermediate, pathways to the observed CH4 and NH3 products (distinct from CH3NH2 formation) are considered to compare and contrast the (likely) mechanism/s of CN- and N2 reduction.

Low-Valent Transition Metalate Anions in Synthesis, Small Molecule Activation, and Catalysis

This review surveys the synthesis and reactivity of low-oxidation state metalate anions of the d-block elements, with an emphasis on contributions reported between 2006 and 2022. Although the field has a long and rich history, the chemistry of transition metalate anions has been greatly enhanced in the last 15 years by the application of advanced concepts in complex synthesis and ligand design. In recent years, the potential of highly reactive metalate complexes in the fields of small molecule activation and homogeneous catalysis has become increasingly evident. Consequently, exciting applications in small molecule activation have been developed, including in catalytic transformations. This article intends to guide the reader through the fascinating world of low-valent transition metalates. The first part of the review describes the synthesis and reactivity of d-block metalates stabilized by an assortment of ligand frameworks, including carbonyls, isocyanides, alkenes and polyarenes, phosphines and phosphorus heterocycles, amides, and redox-active nitrogen-based ligands. Thereby, the reader will be familiarized with the impact of different ligand types on the physical and chemical properties of metalates. In addition, ion-pairing interactions and metal-metal bonding may have a dramatic influence on metalate structures and reactivities. The complex ramifications of these effects are examined in a separate section. The second part of the review is devoted to the reactivity of the metalates toward small inorganic molecules such as H2, N2, CO, CO2, P4 and related species. It is shown that the use of highly electron-rich and reactive metalates in small molecule activation translates into impressive catalytic properties in the hydrogenation of organic molecules and the reduction of N2, CO, and CO2. The results discussed in this review illustrate that the potential of transition metalate anions is increasingly being tapped for challenging catalytic processes with relevance to organic synthesis and energy conversion. Therefore, it is hoped that this review will serve as a useful resource to inspire further developments in this dynamic research field.

Untangling ancillary ligand donation versus locus of oxidation effects on metal nitride reactivity

We detail the relative role of ancillary ligand electron-donating ability in comparison to the locus of oxidation (either metal or ligand) on the electrophilic reactivity of a series of oxidized Mn salen nitride complexes. The electron-donating ability of the ancillary salen ligand was tuned via the para-phenolate substituent (R = CF3, H, tBu, OiPr, NMe2, NEt2) in order to have minimal effect on the geometry at the metal center. Through a suite of experimental (electrochemistry, electron paramagnetic resonance spectroscopy, UV-vis-NIR spectroscopy) and theoretical (density functional theory) techniques, we have demonstrated that metal-based oxidation to [MnVI(SalR)N]+ occurs for R = CF3, H, tBu, OiPr, while ligand radical formation to [MnV(SalR)N]+˙ occurs with the more electron-donating substituents R = NMe2, NEt2. We next investigated the reactivity of the electrophilic nitride with triarylphosphines to form a MnIV phosphoraneiminato adduct and determined that the rate of reaction decreases as the electron-donating ability of the salen para-phenolate substituent is increased. Using a Hammett plot, we find a break in the Hammett relation between R = OiPr and R = NMe2, without a change in mechanism, consistent with the locus of oxidation exhibiting a dominant effect on nitride reactivity, and not the overall donating ability of the ancillary salen ligand. This work differentiates between the subtle and interconnected effects of ancillary ligand electron-donating ability, and locus of oxidation, on electrophilic nitride reactivity.

Insight into Controllable Metal-Support Interactions in Metal/Metal Electrocatalysts for Efficient Energy-Saving Hydrogen Production

Controllable metal-support interaction (MSI) modulations have long been studied for improving the performance of catalysts supported on metal oxides. However, the corresponding in-depth study for metal1-metal2 (M1-M2) composited configurations is rarely achieved due to the lack of reliable models and manipulation mechanisms of MSI modifications. We modeled ruthenium on copper support (Ru-Cu) metal catalysts with negligible interfacial contact potential (e0.06 V) and investigated MSI-dependent hydrogen evolution reaction (HER) catalysis kinetics induced by an electronic hydroxyl (HO-) modifier. Comprehensive simulations and characterizations confirmed that adjusting the HO- coverage can readily realize the tailorable improvement of MSI, facilitating charge migration at the Ru-Cu interface and optimizing the overall HER pathway on active Ru. As a result, a 5/10 monolayer (ML) HO-modified catalyst (5/10 ML) exhibits superior HER activity and durability owing to the relatively stronger MSI. This catalyst also ensured sustainable and efficient hydrogen generation in a urea electrolyzer with significant energy savings. Our work provides a valuable reference for optimizing the MSI-activity relationship in M1-M2 catalysts that target more than just HER.

The impact of Lewis acid variation on reactions with di-tert-butyl diazo diesters

Reactions of (tBuO2CN)2 with Lewis acids and FLPs have previously been shown to prompt the formation of diazene compounds. In this work, we show that the reaction of (tBuO2CN)2 with 9-BBN leads to a bicyclic heterocyclic product (tBuOCO(BBN)CN)21. In contrast, the reactions of (tBuO2CN)2 with BF3 or [Et3Si][B(C6F5)4] lead to the isolation of [tBuNHNH2tBu][BF4] 2 and [tBuN(H)NtBu][B(C6F5)4] 3, respectively. The mechanism for the formation of 2 is probed computationally, demonstrating that steric and electronic considerations of the Lewis acid impact the reaction pathway.

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.

Controllable Electrocatalytic to Photocatalytic Conversion in Ferroelectric Heterostructures

Photocatalytic and electrocatalytic reactions to produce value-added chemicals offer promising solutions for addressing the energy crisis and environmental pollution. Photocatalysis is driven by light excitation and charge separation and relies on semiconducting catalysts, while electrocatalysis is driven by external electric current and is mostly based on metallic catalysts with high electrical conductivity. Due to the distinct reaction mechanism, the conversion between the two catalytic types has remained largely unexplored. Herein, by means of density functional theory (DFT) simulations, we demonstrated that the ferroelectric heterostructures Mo-BN@In2Se3 and WSe2@In2Se3 can exhibit semiconducting or metallic features depending on the polarization direction as a result of the built-in field and electron transfer. Using the nitrogen reduction reaction (NRR) and hydrogen evolution reaction (HER) as examples, the metallic heterostructures act as excellent electrocatalysts for these reactions, while the semiconducting heterostructures serve as the corresponding photocatalysts with improved optical absorption, enhanced charge separation, and low Gibbs free energy change. The findings not only bridge physical phenomena of the electronic phase transition with chemical reactions but also offer a new and feasible approach to significantly improve the catalytic efficiency.

Enabling Nucleophilic Reactivity in High-Spin Fe(II) Imido Complexes: From Elementary Steps to Cooperative Catalysis

ConspectusTransition metal complexes featuring an M═NR bond have received great attention as critical intermediates in the synthesis of nitrogen-containing compounds. In general, the properties of the imido ligand in these complexes are dependent on the nature of the metal center. Thus, the imido ligand tends to be nucleophilic in early transition metal complexes and electrophilic in late transition metal complexes. Nonetheless, the supporting ligand can have a dramatic effect on its reactivity. For example, there are sporadic examples of nucleophilic late transition metal imido complexes, often based on strongly donating supporting ligands. Building on these earlier works, in this Article, we show that the imido ligand in a low-coordinate high-spin bis(carbene)borate Fe(II) complex is able to access previously unknown reaction pathways, ultimately leading to new catalytic transformations. We first focus on the synthesis, characterization, and stoichiometric reactivity of a highly nucleophilic Fe(II) imido complex. The entry point for this system is the intermediate-spin three-coordinate Fe(III) imido complex, which is generated from the reaction of an Fe(I) synthon with an organic azide. Alkali metal reduction leads to a series of M+ (M = Li, Na, K) coordinated and charge-separated (M = K(18-C-6)) high-spin Fe(II) imido complexes, all of which have been isolated and fully characterized. Combined with the electronic structure calculations, these results reveal that the alkali ions moderately polarize the Fe═N bond according to K+ ≈ Na+ < Li+. As a result, the basicity of the imido ligand increases from the charged separated complex to K+, Na+, and Li+ coordinated complexes, as validated by intermolecular proton transfer equilibria. The impact of the counterion on imido ligand reactivity is demonstrated through protonation, alkylation, and hydrogen atom abstraction reactions. The counterion also directs the outcome of [2 + 2] reactions with benzophenone, where alkali coordination facilitates double bond metathesis. Building from here, we describe how the unusual nucleophilicity of the high-spin Fe(II) imido complex revealed in stoichiometric reactions can be extended to new catalytic transformations. For example, a [2 + 2] cycloaddition reaction serves as the basis for the catalytic guanylation of carbodiimides under mild conditions. More interestingly, this complex also exhibits the first ene-like reactivity of an M═NR bond in reactions with alkynes, nitriles, and alkenes. These transformations form the basis of catalytic alkyne and nitrile α-deuteration and pKa-dictated alkene transposition reactions, respectively. Mechanistic studies reveal the critical role of metal-ligand cooperativity in facilitating these catalytic transformations and suggest the new avenues for transition metal imido complexes in catalysis that extend beyond classical nitrene transfer chemistry.

Biomass directional pyrolysis based on element economy to produce high-quality fuels, chemicals, carbon materials - A review

Biomass is regarded as the only carbon-containing renewable energy source and has performed an increasingly important role in the gradual substitution of conventional fossil energy, which also contributes to the goals of carbon neutrality. In the past decade, the academic field has paid much greater attention to the development of biomass pyrolysis technologies. However, most biomass conversion technologies mainly derive from the fossil fuel industry, and it must be noticed that the large element component difference between biomass and traditional fossil fuels. Thus, it's necessary to develop biomass directional pyrolysis technology based on the unique element distribution of biomass for realizing enrichment target element (i.e., element economy). This article provides a broad review of biomass directional pyrolysis to produce high-quality fuels, chemicals, and carbon materials based on element economy. The C (carbon) element economy of biomass pyrolysis is realized by the production of high-performance carbon materials from different carbon sources. For efficient H (hydrogen) element utilization, high-value hydrocarbons could be obtained by the co-pyrolysis or catalytic pyrolysis of biomass and cheap hydrogen source. For improving the O (oxygen) element economy, different from the traditional hydrodeoxygenation (HDO) process, the high content of O in biomass would also become an advantage because biomass is an appropriate raw material for producing oxygenated liquid additives. Based on the N (nitrogen) element economy, the recent studies on preparing N-containing chemicals (or N-rich carbon materials) are reviewed. Moreover, the feasibility of the biomass poly-generation industrialization and the suitable process for different types of target products are also mentioned. Moreover, the enviro-economic assessment of representative biomass pyrolysis technologies is analyzed. Finally, the brief challenges and perspectives of biomass pyrolysis are provided.

μ2 -η1 :η1 -N2 Bridged Bimetallic Dinitrogen Complexes: Geometry of the First Excited State in Connection to N2 π-Photoactivation

Bimetallic end-on μ2 -η1 :η1 -N2 bridging dinitrogen complexes have served as the platform for photochemical N2 activation, mainly for the N-N cleavage. However, the alternate N-N π-photoactivation route has remained largely unexplored. This study strengthens the notion of weakening the N-N bond through the population of π* orbital upon electronic excitation from the ground to the first excited state using four prototypical complexes based on Fe (1), Mo (2), and Ru (3,4). The complexes 1-4 possess characteristic N-N π* based LUMO (π*-π*-π*) centered on their M-N-N-M core, which was earlier postulated to play a central role in the N2 photoactivation. Vertical electronic excitation of the highest oscillator strength involves transitions to the N-N π*-based acceptor orbital (π*-π*-π*) in complexes 1-4. This induces geometry relaxation of the first excited metal-to-nitrogen (π*) charge transfer (1 MNCT) state leading to a "zigzag" M-N-N-M core in the equilibrium structure. Obtaining the equilibrium geometry in the first excited state with the full-sized complexes widens the scope of N-N π-photoactivation with μ2 -η1 :η1 -N2 bridging dinitrogen complexes. Promisingly, the elongated N-N bond and bent ∠MNN angle in the photoexcited S1 state of 1-4 resemble their radical- and di-anion forms, which lead toward thermodynamically feasible N-N protonation in the S1 excited state.

Dinitrogen Activation and Functionalization Affording Chromium Diazenido and Hydrazido Complexes

ConspectusThe activation and functionalization of N2 to form nitrogen-element bonds have long posed challenges to industrial, biological, and synthetic chemists. The first transition-metal dinitrogen complex prepared by Allen and Senoff in 1965 provoked researchers to explore homogeneous N2 fixation. Despite intensive research in the last six decades, efficient and quantitative conversion of N2 to diazenido and hydrazido species remains problematic. Relative to a plethora of reactions to generate N2 complexes, their functionalization reactions are rather rare, and the yields are often unsatisfactory, emphasizing the need for systematic investigations of the reaction mechanisms.In this Account, we summarize our recent work on the synthesis, spectroscopic features, electronic structures, and reactivities of several Cr-N2 complexes. Initially, a series of dinuclear and trinuclear Cr(I)-N2 complexes bearing cyclopentadienyl-phosphine ligands were accessed. However, they cannot achieve N2 functionalization but undergo oxidative addition reactions with phenylsilane, azobenzene, and other unsaturated organic compounds at the low-valent Cr(I) centers rather than at the N2 unit. Further reduction of these Cr(I) complexes leads to the formation of more activated mononuclear Cr(0) bis-dinitrogen complexes. Remarkably, silylation of the cyclopentadienyl-phosphine Cr(0)-N2 complex with Me3SiCl afforded the first Cr hydrazido complex. This process follows the distal pathway to functionalize the Nβ atom twice, yielding an end-on η1-hydrazido complex, Cr(III)═N-N(SiMe3)2. In contrast, upon substitution of the phosphine ligand in the Cr(0)-N2 complex with a N-heterocyclic carbene (NHC) ligand, the corresponding reaction with Me3SiCl proceeds via the alternating pathway; the silylation occurs at both Nα and Nβ atoms and generates a side-on η2-hydrazido complex, Cr(III)(η2-Me3SiN-NSiMe3). Both silylation reactions are inevitably accompanied by the formation of Cr(III) hydrazido complexes and Cr(II) chlorides with a 2:1 ratio. These processes exhibit a peculiar '3-4-2-1' stoichiometry (i.e., treating 3 equiv of Cr(0)-N2 complexes with 4 equiv of Me3SiCl yields 2 equiv of Cr(III) disilyl-hydrazido complexes and 1 equiv of Cr(II) chloride). Upon replacing the monodentate phosphine and/or NHC ligand with a bisphosphine ligand, a monodinitrogen Cr(0) complex, instead of the bis-dinitrogen Cr(0) complexes, is obtained; consequently, the silylation reactions progress via the normal two-electron route, which passes through Cr(II)-N═N-R diazenido species as an intermediate and furnishes [Cr(IV)═N-NR2]+ hydrazido as the final products. More importantly, this type of Cr(0)-N2 complex can be not only silylated but also protonated and alkylated proficiently. All of the second-order reaction rates of the first and second transformations are determined along with the lifetimes of the intervening diazenido species. Based on these findings, we have successfully carried out nearly quantitative preparations of the Cr(IV) hydrazido species with unmixed or hybrid substituents.The studies of Cr-N2 systems provide effective approaches for the activation and functionalization of N2, deepening the understanding of N2 electrophilic attack. We hope that this Account will inspire more discoveries related to the transformation of gaseous N2 to high-value-added nitrogen-containing organic compounds.

Research Progress in Composite Materials for Photocatalytic Nitrogen Fixation

Ammonia is an essential component of modern chemical products and the building unit of natural life molecules. The Haber-Bosch (H-B) process is mainly used in the ammonia synthesis process in the industry. In this process, nitrogen and hydrogen react to produce ammonia with metal catalysts under high temperatures and pressure. However, the H-B process consumes a lot of energy and simultaneously emits greenhouse gases. In the "double carbon" effect, to promote the combination of photocatalytic technology and artificial nitrogen fixation, the development of green synthetic reactions has been widely discussed. Using an inexhaustible supply of sunlight as a power source, researchers have used photocatalysts to reduce nitrogen to ammonia, which is energy-dense and easy to store and transport. This process completes the conversion from light energy to chemical energy. At the same time, it achieves zero carbon emissions, reducing energy consumption and environmental pollution in industrial ammonia synthesis from the source. The application of photocatalytic technology in the nitrogen cycle has become one of the research hotspots in the new energy field. This article provides a classification of and an introduction to nitrogen-fixing photocatalysts reported in recent years and prospects the future development trends in this field.

Design, synthesis and reactivity of dimolybdenum complex bearing quaterphenylene-bridged pyridine-based PNP-type pincer ligand

Dimolybdenum complexes bearing 3,3'''-(1,1':3',1'':3'',1'''-quaterphenylene)-bridged pyridine-based PNP-type pincer ligand are designed and prepared according to DFT calculations on the cleavage step of dinitrogen-bridged dimolybdenum complexes bearing polyphenylene-bridged pyridine-based PNP-type pincer ligands. The dimolybdenum complexes are found to work as effective catalysts toward ammonia formation from dinitrogen with samarium diiodide as a reductant and water as a proton source under ambient reaction conditions.

Electrocatalytic Reduction of Dinitrogen to Ammonia with Water as Proton and Electron Donor Catalyzed by a Combination of a Tri-ironoxotungstate and an Alkali Metal Cation

The electrification of ammonia synthesis is a key target for its decentralization and lowering impact on atmospheric CO2 concentrations. The lithium metal electrochemical reduction of nitrogen to ammonia using alcohols as proton/electron donors is an important advance, but requires rather negative potentials, and anhydrous conditions. Organometallic electrocatalysts using redox mediators have also been reported. Water as a proton and electron donor has not been demonstrated in these reactions. Here a N2 to NH3 electrocatalytic reduction using an inorganic molecular catalyst, a tri-iron substituted polyoxotungstate, {SiFe3W9}, is presented. The catalyst requires the presence of Li+ or Na+ cations as promoters through their binding to {SiFe3W9}. Experimental NMR, CV and UV-vis measurements, and MD simulations and DFT calculations show that the alkali metal cation enables the decrease of the redox potential of {SiFe3W9} allowing the activation of N2. Controlled potential electrolysis with highly purified 14N2 and 15N2 ruled out formation of NH3 from contaminants. Importantly, using Na+ cations and polyethylene glycol as solvent, the anodic oxidation of water can be used as a proton and electron donor for the formation of NH3. In an undivided cell electrolyzer under 1 bar N2, rates of NH3 formation of 1.15 nmol sec-1 cm-2, faradaic efficiencies of ∼25%, 5.1 equiv of NH3 per equivalent of {SiFe3W9} in 10 h, and a TOF of 64 s-1 were obtained. The future development of suitable high surface area cathodes and well solubilized N2 and the use of H2O as the reducing agent are important keys to the future deployment of an electrocatalytic ammonia synthesis.

Revisiting the electronic structure of N2-bound cAAC-borylene at the CASSCF level: a detailed bonding picture of borylene-N2 interaction

A base-trapped borylene species featuring a cyclic-(alkyl)(amino)carbene (cAAC) has shown unique bonding interactions with dinitrogen, thereby, opening a new avenue for N2 activation by main-group compounds. The detailed electronic structure and qualitative bonding picture between cAAC-trapped borylene and N2 remain to be fully understood. This work presents a multiconfigurational complete active space self-consistent field (CASSCF)-based electronic structure investigation on the N2-bound cAAC-borylene species (1) isolated by Braunschweig et al. Specifically, the synergistic bonding between the borylene units and N2 involving the donation from the N-N σ to the unoccupied orbital of borylene and back-donation from the occupied orbital of borylene to the N-N π* has been unequivocally established using CASSCF-derived natural orbitals and electronic configuration. Bonding interactions between the HOMO of the borylene units and the N-N π* (HOMOcAAC-B + π*NN) and the LUMO of the borylene units and the N-N σ (LUMOcAAC-B + σNN) in 1 were apparent through the CASSCF-derived natural orbitals. The unique bonding of the B-N-N-B core in 1 and the resulting geometry have also been compared with the M-N-N-M core of a prototypical transition metal(M)-N2 complex. Finally, the change in the electronic structure and geometry of the N2-bound borylene species 1 on two-electron reduction has been investigated in the context of N2 activation.

Cleavage of Carbon Dioxide C=O Bond Promoted by Nickel-Boron Cooperativity in a PBP-Ni Complex

The synthesis and characterization of (tBu PBP)Ni(OAc) (5) by insertion of carbon dioxide into the Ni-C bond of (tBu PBP)NiMe (1) is presented. An unexpected CO2 cleavage process involving the formation of new B-O and Ni-CO bonds leads to the generation of a butterfly-structured tetra-nickel cluster (tBu PBOP)2 Ni4 (μ-CO)2 (6). Mechanistic investigation of this reaction indicates a reductive scission of CO2 by O-atom transfer to the boron atom via a cooperative nickel-boron mechanism. The CO2 activation reaction produces a three-coordinate (tBu P2 BO)Ni-acyl intermediate (A) that leads to a (tBu P2 BO)-NiI complex (B) via a likely radical pathway. The NiI species is trapped by treatment with the radical trap (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) to give (tBu P2 BO)NiII (η2 -TEMPO) (7). Additionally, 13 C and 1 H NMR spectroscopy analysis using 13 C-enriched CO2 provides information about the species involved in the CO2 activation process.

Inept N2 Activation of Tri-Nuclear Nickel Complex with Labile Sulfur Ligands Facilitates Selective N2 H4 Formation in Electrocatalytic Conversion of N2

Conversion of N2 to the energy vector N2 H4 under benign conditions is highly desirable. However, such N2 fixation processes are extremely rare. It has been recently reported that N2 to N2 H4 conversion can be achieved electrochemically by using a trinuclear [Ni3 (S2 C3 H6 )4 ]2- complex (named as [Ni3 S8 ]2- ). There are hardly any precedents of Nitrogen Reduction Reaction (NRR) by molecular catalysts having Ni and the highly unusual selectivity for N2 H4 over NH3 makes this electrochemical reduction unique. A systematic theoretical study employing calibrated Density Functional Theory to unearth the mechanisms of NRR (4e- /4H+ ) and Hydrogen Evolution Reaction (2e- /2H+ ) was conducted for the aforementioned trinuclear Ni complex. Our findings unravel a curious case of ligand lability working in tandem with metal centers in facilitating this unprecedented electrocatalytic activity. Furthermore, it is shown that the poor N-N bond activation property of Ni is responsible for this unusual selectivity. Additionally, the Hydrogen Evolution Reaction (HER) mechanistic pathways have also been delineated in this report. The mechanistic intricacies thus unearthed in this study may assist in developing more efficient electrocatalysts for N2 H4 production through NRR.

Advancing electrocatalytic nitrogen fixation: insights from molecular systems

Nitrogen fixation has a rich history within the inorganic chemistry community. In recent years attention has (re)focused on developing electrocatalytic systems capable of mediating the nitrogen reduction reaction (N2RR). Well-defined molecular catalyst systems have much to offer in this context. This personal perspective summarizes recent progress from our laboratory at Caltech, pulling together lessons learned from a number of studies we have conducted, placing them within the broader context of thermodynamic efficiency and selectivity for the N2RR. In particular, proton-coupled electron transfer (PCET) provides an attractive strategy to achieve enhanced efficiency for the multi-electron/proton reduction of N2 to produce NH3 (or NH4+), and electrocatalytic PCET (ePCET) via an ePCET mediator affords a promising means of mitigating HER such that the N2RR can be achieved in a catalytic fashion.

Highly Selective Fe-Catalyzed Nitrogen Fixation to Hydrazine Enabled by Sm(II) Reagents with Tailored Redox Potential and pKa

Controlling product selectivity in multiproton, multielectron reductions of unsaturated small molecules is of fundamental interest in catalysis. For the N₂ reduction reaction (N₂RR) in particular, parameters that dictate selectivity for either the 6H⁺/6e^- product ammonia (NH₃) or the 4H⁺/4e^- product hydrazine (N₂H₄) are poorly understood. To probe this issue, we have developed conditions to invert the selectivity of a tris(phosphino)borane iron catalyst (Fe), with which NH₃ is typically the major product of N₂R, to instead favor N₂H₄ as the sole observed fixed-N product (>99:1). This dramatic shift is achieved by replacing moderate reductants and strong acids with a very strongly reducing but weakly acidic Sm^II-(2-pyrrolidone) core supported by a hexadentate dianionic macrocyclic ligand (Sm^II-PH) as the net hydrogen-atom donor. The activity and efficiency of the catalyst with this reagent remain high (up to 69 equiv of N₂H₄ per Fe and 67% fixed-N yield per H⁺). However, by generating N₂H₄ as the kinetic product, the overpotential of this Sm-driven reaction is 700 mV lower than that of the mildest reported set of NH₃-selective conditions with Fe. Mechanistic data support assignment of iron hydrazido(2-) species FeNNH₂ as selectivity-determining: we infer that protonation of FeNNH₂ at N_β, favored by strong acids, releases NH₃, whereas one-electron reduction to FeNNH₂^-, favored by strong reductants such as Sm^II-PH, produces N₂H₄ via reactivity initiated at N_α. Spectroscopic data also implicate a role for Sm^III-binding to anionic FeN₂^- (via an Fe-N₂- -Sm^III species) with respect to catalytic efficacy.

Dinitrogen Activation by Heteronuclear Bimetallic Cluster Anion FeV- in the Gas Phase

Nitrogen activation is a significant but difficult project in the chemical area. Photoelectron spectroscopy (PES) and calculated results are used to investigate the reaction mechanism of the heteronuclear bimetallic cluster FeV^- toward N₂ activation. The results clearly show that N₂ can be fully activated by FeV^- at room temperature, forming the FeV(μ₂-N)₂^- complex with the totally ruptured N≡N bond. Electronic structure analysis reveals that the activation of N₂ by FeV^- is achieved by the electron transfer of bimetallic atoms and electron back-donation to the metal core, which demonstrates that heteronuclear bimetallic anionic clusters are very important to nitrogen activation. This study provides important information for the rational design of synthetic ammonia catalysts.

Cyclopentadienyl ring activation in organometallic chemistry and catalysis

The cyclopentadienyl (Cp) ligand is a cornerstone of modern organometallic chemistry. Since the discovery of ferrocene, the Cp ligand and its various derivatives have become foundational motifs in catalysis, medicine and materials science. Although largely considered an ancillary ligand for altering the stereoelectronic properties of transition metal centres, there is mounting evidence that the core Cp ring structure also serves as a reservoir for reactive protons (H⁺), hydrides (H^-) or radical hydrogen (H^•) atoms. This Review chronicles the field of Cp ring activation, highlighting the pivotal role that Cp ligands can have in electrocatalytic H₂ production, N₂ reduction, hydride transfer reactions and proton-coupled electron transfer.

Snapshots of Early-Stage Quantitative N2 Electrophilic Functionalization

Electrophilic functionalization of N₂ moieties in metal dinitrogen complexes typically initiates the catalytic synthesis of N-containing molecules directly from N₂. Despite intensive research in the last six decades, how to efficiently and even quantitatively convert N₂ into diazenido and hydrazido species still poses a great challenge. In this regard, systematic and comprehensive investigations to elucidate the reaction intricacies are of profound significance. Herein, we report a kinetic dissection on the first and second electrophilic functionalization steps of a new Cr⁰-N₂ system with HOTf, MeOTf, and Me₃SiOTf. All reactions pass through fleeting diazenido intermediates and furnish long-lived final hydrazido products, and both steps are quantitative conversions at low temperatures. All of the second-order reaction rates of the first and second transformations were determined as well as the lifetimes of the intervening diazenido species. Based on these findings, we succeeded in large-scale and near-quantitative preparation of all hydrazido species.

Syntheses, Characterizations, Crystal Structures, and Protonation Reactions of Dinitrogen Chromium Complexes Supported with Triamidoamine Ligands

A novel dinitrogen-dichromium complex, [{Cr(L^Bn)}₂(μ-N₂)] (1), has been prepared from reaction of CrCl₃ with a lithiated triamidoamine ligand (Li₃L^Bn) under dinitrogen. The X-ray crystal structure analysis of 1 revealed that it is composed of two independent dimeric Cr complexes bridged by N₂ in the unit cell. The bridged N-N bond lengths (1.188(4) and 1.185(7) Å) were longer than the free dinitrogen molecule. The elongations of N-N bonds in 1 were also supported by the fact that the ν(N-N) stretching vibration at 1772 cm^-1 observed in toluene is smaller than the free N₂. Complex 1 was identified to be a 5-coordinated high spin Cr(IV) complex by Cr K-edge XANES measurement. The ¹H NMR spectrum and temperature dependent magnetic susceptibility of 1 indicated that complex 1 is in the S = 1 ground state, in which two Cr(IV) ions and unpaired electron spins of the bridging N₂^2- ligand are strongly antiferromagnetically coupled. Reaction of complex 1 with 2.3 equiv of Na or K gave chromium complexes with N₂ between the Cr ion and the respective alkali metal ion, [{CrNa(L^Bn)(N₂)(Et₂O)}₂] (2) and [{CrK(L^Bn)(N₂)}₄(Et₂O)₂] (3), respectively. Furthermore, the complexes 2 and 3 reacted with 15-crown-5 and 18-crown-6 to form the respective crown-ether adducts, [CrNa(L^Bn)(N₂)(15-crown-5)] (4) and [CrK(L^Bn)(N₂)(18-crown-6)] (5). The XANES measurements of complexes 2, 3, 4, and 5 revealed that they are high spin Cr(IV) complexes like complex 1. All complexes reacted with a reducing agent and a proton source to form NH₃ and/or N₂H₄. The yields of these products in the presence of K⁺ were higher than those in the presence of Na⁺. The electronic structures and binding properties of 1, 2, 3, 4, and 5 were evaluated and discussed based on their DFT calculations.

Advances in iron-based electrocatalysts for nitrate reduction

Excessive nitrate has been a critical issue in the water environment, originating from the burning of fossil fuels, inefficient use of nitrogen fertilizers, and discharge of domestic and industrial wastewater. Among the effective treatments for nitrate reduction, electrocatalysis has become an advanced technique because it uses electrons as green reducing agents and can achieve high selectivity through cathode potential control. The effectiveness of electrocatalytic nitrate reduction (NO₃RR) mainly lies in the electrocatalyst. Iron-based catalysts have the advantages of high activity and low cost, which are well-used in the field of electrocatalytic nitrates. A comprehensive overview of the electrocatalytic mechanism and the iron-based materials for NO₃RR are given in terms of monometallic iron-based materials as well as bimetallic and oxide iron-based materials. A detailed introduction to NO₃RR on zero valent iron, single-atom iron catalysts, and Cu/Fe-based bimetallic electrocatalysts are provided, as they are essential for the improvement of NO₃RR performance. Finally, the advantages of iron-based materials for NO₃RR and the problems in current applications are summarized, and the development prospects of efficient iron-based catalysts are proposed.

Dinitrogen Complexes of Cobalt(-I) Supported by Rare-Earth Metal-Based Metalloligands

Sequential reactions of heptadentate phosphinoamine LH₃ with rare-earth metal tris-alkyl precursor (Me₃SiCH₂)₃Ln(THF)₂ (Ln = Sc, Lu, Yb, Y, Gd) and a low-valent cobalt complex (Ph₃P)₃CoI afforded rare-earth metal-supported cobalt iodide complexes. Reduction of these iodide complexes under N₂ allowed the isolation of the first series of dinitrogen complexes of Co(-I) featuring dative Co(-I) → Ln (Ln = Sc, Lu, Yb, Y, Gd) bonding interactions. These compounds were characterized by multinuclear NMR spectroscopy, X-ray diffraction analysis, electrochemistry, and computational studies. The correlation of N-N vibrational frequencies with the pK_a of [Ln(H₂O)₆]³⁺ showed that strongest activation of N₂ was achieved with the least Lewis acidic Gd(III) ion. Interestingly, these Ln-Co-N₂ complexes catalyzed silylation of N₂ in the presence of KC₈ and Me₃SiCl with turnover numbers (TONs) up to 16, where the lutetium-supported Co(-I) complex showed the highest activity within the series. The role of the Lewis acidic Ln(III) was crucial to achieve catalytic turnovers and tunable reactivity toward N₂ functionalization.

N2 Reduction versus H2 Evolution in a Molybdenum- or Tungsten-Based Small-Molecule Model System of Nitrogenase

Molybdenum dinitrogen complexes have played a major role as catalytic model systems of nitrogenase. In comparison, analogous tungsten complexes have in most cases found to be catalytically inactive. Herein, a tungsten complex was shown to be supported by a pentadentate tetrapodal (pentaPod) phosphine ligand, under conditions of N₂ fixation, primarily catalyzes the hydrogen evolution reaction (HER), in contrast to its Mo analogue, which catalytically mediates the nitrogen-reduction reaction (N₂ RR). DFT calculations were employed to evaluate possible mechanisms and identify the most likely pathways of N₂ RR and HER activities exhibited by Mo- and W-pentaPod complexes. Two mechanisms for N₂ RR by PCET are considered, starting from neutral (M(0) cycle) and cationic (M(I) cycle) dinitrogen complexes (M=Mo, W). The latter was found to be energetically more favorable. For HER three scenarios are treated; that is, through bimolecular reactions of early M-N_x H_y intermediates, pure hydride intermediates or mixed M(H)(N_x H_y ) species.

ortho-Phenylene-bridged phosphorus/silicon Lewis pairs

A series of five ortho-phenylene-bridged phosphorus-silicon Lewis pairs was synthesized, with phosphorus bearing isopropyl groups while the substituents at the silicon atom vary (-CH₃, -Cl, -F). Possible interactions between Lewis acid and base were investigated both experimentally (NMR, XRD) and theoretically to determine the influence of the different substituents. Calculated ortho-interaction energies (OIEs) show a stabilizing interactions between the acidic and basic units which were also found for the meta- and para-interaction energies (MIEs and PIEs, respectively), indicating stabilization resulting not from direct acid-base interaction but from electronic interactions through the ring. Further spectroscopic (NMR, XRD) and theoretical (NBO, QTAIM, SAPT) investigations confirmed the absence of direct interactions between silicon and phoshorus.

Light Alters the NH3 vs N2 H4 Product Profile in Iron-catalyzed Nitrogen Reduction via Dual Reactivity from an Iron Hydrazido (Fe=NNH2 ) Intermediate

Whereas synthetically catalyzed nitrogen reduction (N2 R) to produce ammonia is widely studied, catalysis to instead produce hydrazine (N2 H4 ) has received less attention despite its considerable mechanistic interest. Herein, we disclose that irradiation of a tris(phosphine)borane (P3 B ) Fe catalyst, P3 B Fe+ , significantly alters its product profile to increase N2 H4 versus NH3 ; P3 B Fe+ is otherwise known to be highly selective for NH3 . We posit a key terminal hydrazido intermediate, P3 B Fe=NNH2 , as selectivity-determining. Whereas its singlet ground state undergoes protonation to liberate NH3 , a low-lying triplet excited state leads to reactivity at Nα and formation of N2 H4 . Associated electrochemical and spectroscopic studies establish that N2 H4 lies along a unique product pathway; NH3 is not produced from N2 H4 . Our findings are distinct from the canonical mechanism for hydrazine formation, which proceeds via a diazene (HN=NH) intermediate and showcase light as a tool to tailor selectivity.

Sulfur-Ligated [2Fe-2C] Clusters as Synthetic Model Systems for Nitrogenase

Metal clusters featuring carbon and sulfur donors have coordination environments comparable to the active site of nitrogenase enzymes. Here, we report a series of di-iron clusters supported by the dianionic yldiide ligands, in which the Fe sites are bridged by two μ2-C atoms and four pendant S donors.The [L2Fe2] (L = {[Ph2P(S)]2C}2-) cluster is isolable in two oxidation levels, all-ferrous Fe2II and mixed-valence FeIIFeIII. The mixed-valence cluster displays two peaks in the Mössbauer spectra, indicating slow electron transfer between the two sites. The addition of the Lewis base 4-dimethylaminopyridine to the Fe2II cluster results in coordination with only one of the two Fe sites, even in the presence of an excess base. Conversely, the cluster reacts with 8 equiv of isocyanide tBuNC to give a monometallic complex featuring a new C-C bond between the ligand backbone and the isocyanide. The electronic structure descriptions of these complexes are further supported by X-ray absorption and resonant X-ray emission spectroscopies.