Remarkable catalytic activity of dinitrogen-bridged dimolybdenum complexes bearing NHC-based PCP-pincer ligands toward nitrogen fixation

Theoretical Studies on the Reduction of N2 to NH3/N2H4 Catalyzed by Chromium Complexes

Nitrogen fixation catalyzed by transition metal complexes provides an attractive alternative to the Haber-Bosch process and has received widespread attention. In this work, the reaction pathways of N2 to NH3/N2H4 catalyzed by dinuclear and mononuclear Cr-N2 complexes bearing cyclopentadienyl-phosphine ligands and mediated by LutH+ (as proton donors) and CrCp*2 (as electron donors) were investigated systematically using density functional theory calculations. The key step of the reactions was clarified as the first hydrogenation. The effect of different combinations of six proton sources (three pyridine acids and three anilino acids) and three electron sources (CrCp*2, CoCp*2, and CoCp2) on the reduction of N2 to NH3/N2H4 was also explored by calculating the critical step of the reactions. Based on the calculations, the dinuclear Cr-N2 complex is expected to be an effective catalyst for the reduction of N2 to NH3/N2H4 when using the combinations of CrCp*2 with each of the six proton sources and of CoCp*2 with anilino acids. Our work provides insights into understanding and optimizing Cr catalytic systems for efficient dinitrogen fixations.

Theoretical Study of Imide Formation in Nitrogen Fixation Catalyzed by Molybdenum Complex Bearing PCP-Type Pincer Ligand with Metallocenes

Homogeneous catalysts using a mononuclear molybdenum nitride (Mo≡N) complex bearing PCP-type pincer ligands allow nitrogen fixation under very mild conditions. The catalytic cycle involves three hydrogenation processes yielding an Mo-ammine complex [MoI(NH3)(PCP)] from the Mo-nitride complex [MoI(N)(PCP)]. We primarily focused on the first hydrogenation step, forming an Mo-imide complex [MoI(NH)(PCP)] since previous experimental and theoretical studies suggest that imide formation is the rate-limiting step in the catalytic cycle. The choice of protonating agent and reductant strongly influences the catalytic reactivity in imide formation. In this computational quantum chemical study, 2,4,6-collidinium (ColH+) was employed as the protonation agent, while metallocenes Cp2MII and decamethylmetallocenes Cp*2MII (M = V, Cr, Mn, Fe, Co, and Ni) were employed as reductants. The reaction of ColH+ with the metallocenes yields protonated metallocenes, where a cyclopentadienyl ring of the metallocenes is protonated. Protonated Cp*2CrII and Cp*2CoII are potential proton-coupled electron transfer (PCET) mediators to facilitate the imide formation of [MoI(N)(PCP)] with low activation free energies. The concerted reaction mechanism was compared with the stepwise reaction, where ColH+ directly protonates [MoI(N)(PCP)], followed by reduction with the decamethylmetallocenes. Furthermore, we analyzed how proton transfer and electron transfer are concerted in the reaction of the PCET mediators with [MoI(N)(PCP)] by tracing electronic states along the reaction coordinates.

Dinitrogen reduction to ammonia with a pincer-Mo complex: new insights into the mechanism of nitride-to-ammonia conversion

The thioether-diphosphine pincer-ligated molybdenum complex (PSP)MoCl3 (1-Cl3, PSP = 4,5-bis(diisopropylphosphino)-2,7-di-tert-butyl-9,9-dimethyl-9H-thioxanthene) has been synthesized as a catalyst-precursor for N2 reduction catalysis with a focus on an integrated experimental/computational mechanistic investigation. The (PSP)Mo unit is isoelectronic with the (PNP)Mo (PNP = 2,6-bis(di-t-butylphosphinomethyl)pyridine) fragment found in the family of catalysts for the reduction of N2 to NH3 first reported by Nishibayashi and co-workers. Electrochemical studies reveal that 1-Cl3 is significantly more easily reduced than (PNP)MoCl3 (with a potential ca. 0.4 eV less negative). The reaction of 1-Cl3 with two reducing equivalents, under N2 atmosphere and in the presence of iodide, affords the nitride complex (PSP)Mo(N)(I). This observation suggests that the N2-bridged complex [(PSP)Mo(I)]2(N2) is formed and undergoes rapid cleavage. DFT calculations predict the splitting barrier of this complex to be low, in accord with calculations of (PNP)Mo and a related (PPP)Mo complex reported by Merakeb et al. Conversion of the nitride ligand to NH3 has been investigated in depth experimentally and computationally. Considering sequential addition of H atoms to the nitride through proton coupled electron-transfer or H-atom transfer, formation of the first N-H bond is thermodynamically relatively unfavorable. Experiment and theory, however, reveal that an N-H bond is readily formed by protonation of (PSP)Mo(N)(I) with lutidinium chloride, which is strongly promoted by coordination of Cl- to Mo. Other anions, e.g. triflate, can also act in this capacity although less effectively. These protonations, coupled with anion coordination, yield MoIV imide complexes, thereby circumventing the difficult formation of the first N-H bond corresponding to a low BDFE and formation of the respective MoIII imide complexes. The remaining two N-H bonds required to produce ammonia are formed thermodynamically much more favorably than the first. Computations suggest that formation of the MoIV imide is followed by a second protonation, then a rapid and favorable one-electron reduction, followed by a third protonation to afford coordinated ammonia. This comprehensive analysis of the elementary steps of ammonia synthesis provides guidance for future catalyst design.

Samarium as a Catalytic Electron-Transfer Mediator in Electrocatalytic Nitrogen Reduction to Ammonia

Samarium diiodide (SmI2) exhibits high selectivity for N2R catalyzed by molybdenum complexes; however, it has so far been employed only as a stoichiometric reagent (0.3 equiv of NH3 per Sm) combined with coordinating proton sources (e.g., H2O, ROH). The latter inhibit catalytic turnover of SmIII owing to buildup of stable hydroxide/alkoxide sinks. Here, we report a tandem Sm/Mo-catalyzed eN2R system that achieves the lowest overpotential and highest Faradaic efficiency (82%) reported to date for nonaqueous eN2R at ambient pressure. Up to 8.4 equiv of NH3 is produced per Sm, representing a 25-fold increase over N2R with stoichiometric SmI2. A noncoordinating proton source enables electrochemical SmI3/SmI2 cycling at the applied potential of -1.45 V vs Fc+/0.

Chemocatalytic Conversion of Dinitrogen to Ammonia Mediated by a Tungsten Complex

Whereas molybdenum dinitrogen complexes have played a major role as catalytic model systems of nitrogenase, corresponding tungsten complexes were in most cases found to be catalytically inactive. Herein, we present a modified pentadentate tetrapodal (pentaPod) phosphine ligand in which two dimethylphosphine groups of the pentaPodMe (P5Me) ligand have been replaced with phospholanes (Pln). The derived molybdenum complex [Mo(N2)P5Pln] generates 22 and the analogous tungsten complex [W(N2)P5Pln] 7 equivalents of NH3 from N2 in the presence of 180 equivalents of SmI2(THF)2/H2O, rendering the latter the first tungsten complex chemocatalytically converting N2 to NH3. In contrast, the tungsten complex [W(N2)P5Me] generates ammonia from N2 only in a slightly overstoichiometric fashion. The reasons for these reactivity differences are investigated with the help of spectroscopic and electrochemical methods.

Synthesis and electrochemical properties of molybdenum nitrido complexes supported by redox-active NHC and MIC ligands

We report the synthesis of a series of molybdenum nitrido complexes supported by bis-phenolate N-heterocyclic and mesoionic carbenes (NHC & MIC). The reaction between MoN(OtBu)3 and the corresponding azolium salts [H3L1]Cl and [H3L2]Cl (with L1 = bis-phenolate triazolylidene and L2 = bis-phenolate benzimidazolylidene) gives clean access to the corresponding NHC/MIC complexes 1-Cl and 2-Cl. Electrochemical investigations of these complexes showed that they can be reversibly reduced at potentials of -1.13 and -1.01 V vs. Fc/[Fc]+ and the reduced complexes [1-Cl]- and [2-Cl]- can be cleanly isolated after chemical reduction with one equivalent of decamethylcobaltocene. Exchange of the halide atoms is furthermore reported to give a series of nitrido complexes supported by tert-butanolate (1-OtBu and 2-OtBu), perfluoro-tert-butanolate (1-OtBuF9 and 2-OtBuF9), tritylate (1-OCPh3 and 2-OCPh3), mesitolate (1-OMes and 2-OMes), thio-tert-butanolate (1-StBu), thiotritylate (1-SCPh3 and 2-SCPh3) and thiomesitolate complexes (1-SMes). The electrochemical properties of all complexes were evaluated and compared. All isolated complexes were characterized by multinuclear and multidimensional NMR spectroscopy and (if applicable) by EPR spectroscopy. Furthermore, the reactivity of 1-Cl and 2-Cl in the presence of protons and decamethylcobaltocene was investigated, which shows facile extrusion of ammonia, yielding diamagnetic bis-molybdenum(III) complexes 3 and 4.

Unravelling the potential of low-valent tunable vanadium complexes in the nitrogen reduction reaction (NRR)

Density functional theory calculations have been carried out to investigate the potential of several hitherto unknown low-valent tripodal vanadium complexes towards conversion of dinitrogen to ammonia as a function of different equatorial (PiPr2 and SiPr) and bridgehead groups (B, C and Si). All the newly proposed vanadium complexes were probed towards understanding their efficiency in some of the key steps involved in the dinitrogen fixation process. They were found to be successful in preventing the release of hydrazine during the nitrogen reduction reaction. We have performed a comprehensive mechanistic study by considering all the possible pathways (distal, alternate and hybrid) to understand the efficiency of some of the proposed catalysts towards the dinitrogen reduction process. The exergonic reaction free energies obtained for some of the key steps and the presence of thermally surmountable barrier heights involved in the catalytic cycle indicate that these complexes may be considered as suitable platforms for the functionalization of dinitrogen.

Photodriven Ammonia Synthesis from Manganese Nitrides: Photophysics and Mechanistic Investigations

Ammonia synthesis from N,N,O,O-supported manganese(V) nitrides and 9,10-dihydroacridine using proton-coupled electron transfer and visible light irradiation in the absence of precious metal photocatalysts is described. While the reactivity of the nitride correlated with increased absorption of blue light, excited-state lifetimes determined by transient absorption were on the order of picoseconds. This eliminated excited-state manganese nitrides as responsible for bimolecular N-H bond formation. Spectroscopic measurements on the hydrogen source, dihydroacridine, demonstrated that photooxidation of 9,10-dihydroacridine was necessary for productive ammonia synthesis. Transient absorption and pulse radiolysis data for dihydroacridine provided evidence for the presence of intermediates with weak E-H bonds, including the dihydroacridinium radical cation and both isomers of the monohydroacridine radical, but notably these intermediates were unreactive toward hydrogen atom transfer and net N-H bond formation. Additional optimization of the reaction conditions using higher photon flux resulted in higher rates of the ammonia production from the manganese(V) nitrides due to increased activation of the dihydroacridine.

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

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

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.

New Pyridine Dicarbene Pincer Ligands with Ring Expanded NHCs and their Nickel and Chromium Complexes

The pincer complexes [NiIIBr(CNC)]Br (4), [CrIIIBr3(CNC)] (5 a) and [CrIIIBr2.3Cl0.7(CNC)] (5 b), where CNC=3,3'-(pyridine-2,6-diyl)bis(1-mesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene), were obtained from the novel ligand CNC, generated in situ from the precursor (CHNCH)Br2 and [NiIIBr2(PPh3)2] or from [CrII{N(SiMe3)2}2(THF)2] and (CHNCH)Br2 by aminolysis, respectively. The tetrahedrally distorted square planar (τ4≅0.30) geometry and the singlet ground state of Ni in 4 were attributed to steric constraints of the CNC backbone. Computational methods highlighted the dependence of the coordination geometry and the singlet-triplet energy difference on the size of the N-substituent of the tetrahydropyrimidine wingtips and contrasted it to the situation in 5-membered imidazolin-2-ylidene pincer analogues. The octahedral CrIII metal center in 5 a and 5 b is presumably formed after one electron oxidation from CH2Cl2. 4/MAO and 5 a/MAO were catalysts of moderate activity for the oligomerization and polymerization of ethylene, respectively. The analogous (CH^N^CH)Br2 precursor, where (CH^N^CH)=3,3'-(pyridine-2,6-diylbis(methylene))bis(1-mesityl-3,4,5,6-tetrahydropyrimidin-1-ium), was also prepared, however its coordination chemistry was not studied due to the inherent instability of the resulting free C^N^C ligand.

Iron promoted end-on dinitrogen-bridging in heterobimetallic complexes of uranium and lanthanides

End-on binding of dinitrogen to low valent metal centres is common in transition metal chemistry but remains extremely rare in f-elements chemistry. In particular, heterobimetallic end-on N2 bridged complexes of lanthanides are unprecedented despite their potential relevance in catalytic reduction of dinitrogen. Here we report the synthesis and characterization of a series of N2 bridged heterobimetallic complexes of U(iii), Ln(iii) and Ln(ii) which were prepared by reacting the Fe dinitrogen complex [Fe(depe)2(N2)] (depe = 1,2-bis(diethylphosphino)-ethane), complex A with [MIII{N(SiMe3)2}3] (M = U, Ce, Sm, Dy, Tm) and [LnII{N(SiMe3)2}2], (Ln = Sm, Yb). Despite the lack of reactivity of the U(iii), Ln(iii) and Ln(ii) amide complexes with dinitrogen, the end-on dinitrogen bridged heterobimetallic complexes [{Fe(depe)2}(μ-η1:η1-N2)(M{N(SiMe3)2}3)], 1-M (M = U(iii), Ce(iii), Sm(iii), Dy(iii) and Tm(iii)), [{Fe(depe)2}(μ-η1:η1-N2)(Ln{N(SiMe3)2}2)], 1*-Ln (Ln = Sm(ii), Yb(ii)) and [{Fe(depe)2(μ-η1:η1-N2)}2{SmII{N(SiMe3)2}2}], 3 could be prepared. The synthetic method used here allowed to isolate unprecedented end-on bridging N2 complexes of divalent lanthanides which provide relevant structural models for the species involved in the catalytic reduction of dinitrogen by Fe/Sm(ii) systems. Computational studies showed an essentially electrostatic interaction of the end-on bridging N2 with both Ln(iii) and Ln(ii) complexes with the degree of N2 activation correlating with their Lewis acidity. In contrast, a back-bonding covalent contribution to the U(iii)-N2Fe bond was identified by computational studies. Computational studies also suggest that end-on binding of N2 to U(iii) and Ln(ii) complexes is favoured for the iron-bound N2 compared to free N2 due to the higher N2 polarization.

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.

To Split or Not to Split: [AsCCAs]-Coordinated Mo, W, and Re Complexes and Their Reactivity toward Molecular Dinitrogen

Molybdenum, tungsten, and rhenium halides bearing a 2,2'-(iPr2As)2-substituted diphenylacetylene ([AsCCAs], 1-As) were prepared and reduced under an atmosphere of dinitrogen in order to activate the latter substrate. In the case of molybdenum, a diiodo (2-As) and a triiodo molybdenum precursor (5) were equally suited for reductive N2 splitting, which led to the isolation of [AsCCAs]Mo≡N(I) (3-As) in each case. For tungsten, [AsCCAs]WCl3 (6) was reduced under N2 to afford {[AsCCAs]WCl2}2(N2) (7), which is best described as a dinuclear π8δ4-configured μ-(η1: η1)-N2-bridged dimer. Attempts to reductively cleave the N2 unit in 7 did not lead to the expected tungsten nitride (8), which had to be prepared independently via the treatment of 7 with sodium azide. To arrive at a π10δ4-configured N2-bridged dimer in a tetragonally distorted ligand environment, [AsCCAs]ReCl3 (9) was reduced in the presence of N2. As expected, a μ-(η1: η1)-N2-bridged dirhenium species, namely, {[AsCCAs]ReCl2}2(N2) (10), was formed, but found to very quickly decompose (presumably via loss of N2), not only under reduced pressure, but also upon irradiation or heating. Hence, an alternative synthetic route to the originally envisioned nitride, [AsCCAs]Re≡N(Cl)2 (11), was developed. While all the aforementioned nitrides (3-As, 8, and 11) were found to be fairly robust, significantly different stabilities were noticed for {[AsCCAs]MCl2}2(N2) (7 for M = W, 10 for M = Re), which is ascribed to the electronically different MN2M cores (π8δ4 for 7 vs π10δ4 for 10) in these μ-(η1: η1)-N2-bridged dimers.

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.

Benchmark Study of Density Functional Theory Methods in Geometry Optimization of Transition Metal-Dinitrogen Complexes

The current benchmark study is focused on determining the most precise theoretical method for optimizing the geometry of transition metal-dinitrogen complexes. To accomplish this goal, seven density functional (DF) methods from five distinct classes of density functional theory (DFT) have been selected, including B3LYP-D3(BJ), BP86-D3(BJ), PBE0-D3(BJ), ωB97X-D, M06, M06-L, and TPSSh-D3(BJ). These DFs will be utilized with the Karlsruhe basis set (def2-SVP). To carry out this benchmark study, a total of forty-two structurally diverse transition metal-dinitrogen compounds with experimentally known X-ray data have been selected from the Cambridge Crystallographic Data Centre (CCDC). Based on a comparison of the theoretical data with experimental values (X-ray) of the selected transition metal-dinitrogen compounds, statistical parameters such as root-mean-square deviation (RMSD) and N-N and M-N bond lengths are obtained to evaluate the performance of the seven chosen DFs. According to the obtained results, among all DFT methods used in the study, Minnesota functionals (M06 and M06-L) and TPSSh-D3(BJ) show good performance, with lower RMSD values. This suggests that these three methods are the most reliable for optimizing the geometry of transition metal-dinitrogen complexes. Based on the absolute errors of the N-N and M-N bond lengths relative to the X-ray data, further analysis is conducted, and it is determined that M06-L is the best functional for optimizing the geometry of transition metal-dinitrogen compounds. Additionally, the influence of using a high-level basis set (def2-TZVP) compared to def2-SVP on the calculated RMSD among the seven chosen methods is found to be negligible.

Expanding the Utility of β-Diketiminate Ligands in Heavy Group VI Chemistry of Molybdenum and Tungsten

We report the synthesis of 17 molybdenum and tungsten complexes supported by the ubiquitous BDI ligand framework (BDI = β-diketiminate). The focal entry point is the synthesis of four molybdenum and tungsten(V) BDI complexes of the general formula [MO(BDI^R)Cl₂] [M = Mo, R = Dipp (1); M = W, R = Dipp (2); M = Mo, R = Mes (3); M = W, R = Mes (4)] synthesized by the reaction between MoOCl₃(THF)₂ or WOCl₃(THF)₂ and LiBDI^R. Reactivity studies show that the BDI^Dipp complexes are excellent precursors toward adduct formation, reacting smoothly with dimethylaminopyridine (DMAP) and triethylphosphine oxide (OPEt₃). No reaction with small phosphines has been observed, strongly contrasting the chemistry of previously reported rhenium(V) complexes. Additionally, the complexes 1 and 2 are good precursors for salt metathesis reactions. While 1 can be chemically reduced to the first stable example of a Mo(IV) BDI complex 15, reduction of 2 resulted in degradation of the BDI ligand via a nitrene transfer reaction, leading to MAD (4-((2,6-diisopropylphenyl)imino)pent-2-enide) supported tungsten(V) and tungsten(VI) complexes 16 and 17. All reported complexes have been thoroughly studied by VT-NMR and (heteronuclear) NMR spectroscopy, as well as UV-vis and EPR spectroscopy, IR spectroscopy, and X-ray diffraction analysis.

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.

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.

In Silico Partial N2 to NH3 Conversion with a Light Atom Molecule

N₂ can be stepwise converted in silico into one molecule NH₃ and a secondary amide with a bond activator molecule consisting only of light main group elements. The proposed N₂ -activating pincer-related compound carries a silyl ion (Si⁽⁺⁾ ) center as well as three Lewis acidic (-BF₂ ) and three Lewis basic (-PMe₂ ) sites, providing an efficient binding pocket for gaseous N₂ within the framework of intramolecular frustrated Lewis pairs (FLP). In addition, it exhibits supportive secondary P-B and F⋅⋅⋅B contacts, which stabilize the structure. In the PSi⁽⁺⁾ -N-N-BP environment the N≡N triple bond is extended from 1.09 Å to remarkable 1.43 Å, resembling a N-N single bond. The strongly activated N-N-fragment is prone to subsequent hydride addition and protonation steps, resulting in the energy efficient transfer of two hydrogen equivalents. The next hydride added causes the release of one molecule NH₃ , but leaves the ligand system as poisoned R₃ Si⁽⁺⁾ -NH₂ -PMe₂ or R₃ Si⁽⁺⁾ -NH₃ dead-end states behind. The study indicates that approximately tetrahedral constrained SiBP₂ -pockets are capable to activate N₂ , whereas the acid-rich SiB₃ - and SiB₂ P-pocktes, as well as the base-rich SiP₃ -pockets fail, hinting towards the high relevance of the acid-base proportion and relative orientation. The electronic structure of the N₂ -activated state is compared to the corresponding state of a recently published peri-substituted bond activator molecule featuring a PSi⁽⁺⁾ -N-N-Si⁽⁺⁾ P site (S. Mebs, J. Beckmann, Physical Chemistry Chemical Physics 2022, 24, 20953-20967).

Selective alkane hydroxylation and alkene epoxidation using H2O2 and Fe(II) catalysts electrostatically attached to a fluorinated surface

Fe(II) complexes with pentadentate ligands, including N-heterocyclic carbene moieties, were prepared and electrostatically attached onto the perfluorinated surface of a mesoporous aluminosilicate. The heterogeneous catalysts were applied to the catalytic oxidation of cyclohexane and cyclohexene using H₂O₂ as an oxidant in CH₃CN, demonstrating high performance and selectivity in alkane hydroxylation and cyclohexene epoxidation.

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.

Screening Carbon-Boron Frustrated Lewis Pairs for Small-Molecule Activation including N2 , O2 , CO, CO2 , CS2 , H2 O and CH4 : A Computational Study

Dinitrogen (N₂ ) activation is particularly challenging under ambient conditions because of its large highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap (10.8 eV) and high bond dissociation energy (945 kJ mol^-1 ) of the N≡N triple bond, attracting considerable attention from both experimental and theoretical chemists. However, most effort has focused on metallic systems. In contrast, nitrogen activation by frustrated Lewis pairs (FLPs) has been initiated recently via theoretical calculations. Here we perform density functional theory (DFT) calculations to screen a series of experimentally viable FLPs for small-molecule activation including N₂ , O₂ , CO, CO₂ , CS₂ , H₂ O and CH₄ . In addition, aromaticity is found to play an important role in most of these small-molecule activation. The particularly thermodynamic stabilities of the activation products and low reaction barriers could be a step forward for the development of FLP towards small-molecule activation including N₂ , inviting experimental chemists' verification.

Near ambient N2 fixation on solid electrodes versus enzymes and homogeneous catalysts

The Mo/Fe nitrogenase enzyme is unique in its ability to efficiently reduce dinitrogen to ammonia at atmospheric pressures and room temperature. Should an artificial electrolytic device achieve the same feat, it would revolutionize fertilizer production and even provide an energy-dense, truly carbon-free fuel. This Review provides a coherent comparison of recent progress made in dinitrogen fixation on solid electrodes, homogeneous catalysts and nitrogenases. Specific emphasis is placed on systems for which there is unequivocal evidence that dinitrogen reduction has taken place. By establishing the cross-cutting themes and synergies between these systems, we identify viable avenues for future research.

Vanadium-Catalyzed Dinitrogen Reduction to Ammonia via a [V]═NNH2 Intermediate

The catalytic transformation of N₂ to NH₃ by transition metal complexes is of great interest and importance but has remained a challenge to date. Despite the essential role of vanadium in biological N₂ fixation, well-defined vanadium complexes that can catalyze the conversion of N₂ to NH₃ are scarce. In particular, a V(N_xH_y) intermediate derived from proton/electron transfer reactions of coordinated N₂ remains unknown. Here, we report a dinitrogen-bridged divanadium complex bearing POCOP (2,6-(^tBu₂PO)₂-C₆H₃) pincer and aryloxy ligands, which can serve as a catalyst for the reduction of N₂ to NH₃ and N₂H₄. Low-temperature protonation and reduction of the dinitrogen complex afforded the first structurally characterized neutral metal hydrazido(2-) species ([V]═NNH₂), which mediated ¹⁵N₂ conversion to ¹⁵NH₃, indicating that it is a plausible intermediate of the catalysis. DFT calculations showed that the vanadium hydrazido complex [V]═NNH₂ possessed a N-H bond dissociation free energy (BDFE_N-H) of as high as 59.1 kcal/mol. The protonation of a vanadium amide complex ([V]-NH₂) with [Ph₂NH₂][OTf] resulted in the release of NH₃ and the formation of a vanadium triflate complex, which upon reduction under N₂ afforded the vanadium dinitrogen complex. These transformations model the final steps of a vanadium-catalyzed N₂ reduction cycle. Both experimental and theoretical studies suggest that the catalytic reaction may proceed via a distal pathway to liberate NH₃. These findings provide unprecedented insights into the mechanism of N₂ reduction related to FeV nitrogenase.

Catalytic nitrogen fixation using visible light energy

The synthesis of ammonia from atmospheric dinitrogen, nitrogen fixation, is one of the essential reactions for human beings. Because the current industrial nitrogen fixation depends on dihydrogen produced from fossil fuels as raw material, the development of a nitrogen fixation reaction that relies on the energy provided by renewable energy, such as visible light, is an important research goal from the viewpoint of sustainable chemistry. Herein, we establish an iridium- and molybdenum-catalysed process for synthesizing ammonia from dinitrogen under ambient reaction conditions and visible light irradiation. In this reaction system, iridium complexes and molybdenum triiodide complexes bearing N-heterocyclic carbene-based pincer ligands act as cooperative catalysts to activate 9,10-dihydroacridine and dinitrogen, respectively. The reaction of dinitrogen with 9,10-dihydroacridine is not thermodynamically favoured, and it only takes place under visible light irradiation. Therefore, the described reaction system is one that affords visible light energy-driven ammonia formation from dinitrogen catalytically.

Ammonia from dinitrogen at ambient conditions by organometallic catalysts

Fixation of atmospheric dinitrogen in plants by [Mo-Fe] cofactor of nitrogenase enzyme takes place efficiently under atmospheric pressure and normal temperature. In search for an alternative methodology for the highly energy intensive Haber-Bosch process, design and synthesis of highly efficient inorganic and organometallic complexes by mimicking the structure and function of [Mo-Fe] cofactor system is highly desirable for ammonia synthesis from dinitrogen. An ideal catalyst for ammonia synthesis should effectively catalyse the reduction of dinitrogen in the presence of a proton source under mild to moderate conditions, and thereby, significantly reducing the cost of ammonia production and increasing the energy efficacy of the process. In the light of current research, it is evident that there is a plenty of scope for the development and enhanced performance of the inorganic and organometallic catalysts for ammonia synthesis under ambient temperature and pressure. The review furnishes a comprehensive outlook of numerous organometallic catalysts used in the synthesis of ammonia from dinitrogen in the past few decades.

Catalytic Ammonia Synthesis Mediated by Molybdenum Complexes with PN3P Pincer Ligands: Influence of P/N Substituents and Molecular Mechanism

Three molybdenum trihalogenido complexes supported by different PN³P pincer ligands were synthesized and investigated regarding their activity towards catalytic N₂-to-NH₃ conversion. The highest yields were obtained with the H-PN³P^tBu ligand. The corresponding Mo(V)-nitrido complex also shows good catalytic activity. Experiments regarding the formation of the analogous Mo(IV)-nitrido complex lead to the conclusion that the mechanism of catalytic ammonia formation mediated by the title systems does not involve N-N cleavage of a dinuclear Mo-dinitrogen complex, but follows the classic Chatt cycle.

Terminal N2 Dissociation in [(PNN)Fe(N2 )]2 (μ-N2 ) Leads to Local Spin-State Changes and Augmented Bridging N2 Activation

Nitrogen fixation at iron centres is a fundamental catalytic step for N₂ utilisation, relevant to biological (nitrogenase) and industrial (Haber-Bosch) processes. This step is coupled with important electronic structure changes which are currently poorly understood. We show here for the first time that terminal dinitrogen dissociation from iron complexes that coordinate N₂ in a terminal and bridging fashion leaves the Fe-N₂ -Fe unit intact but significantly enhances the degree of N₂ activation (Δν≈180 cm^-1 , Raman spectroscopy) through charge redistribution. The transformation proceeds with local spin state change at the iron centre (S= 1 / 2 ${{ 1/2 }}$ →S=³ /₂ ). Further dissociation of the bridging N₂ can be induced under thermolytic conditions, triggering a disproportionation reaction, from which the tetrahedral (PNN)₂ Fe could be isolated. This work shows that dinitrogen activation can be induced in the absence of external chemical stimuli such as reducing agents or Lewis acids.

In silico activation of dinitrogen with a light atom molecule

The NN triple bond can be cleaved in silico with a light atom molecule containing only the earth abundant elements C, H, Si, and P. Extensive density functional theory (DFT) computations on various classes of peri-substituted scaffolds containing Lewis acidic and basic sites in the framework of frustrated Lewis pairs (FLP) indicate that the presence of two silyl cations and two P atoms in a flexible but not too flexible arrangement is essential for energy efficient N₂-activation. The non-bonding lone-pair electrons of the P atoms thereby serve as donors towards N₂, whereas the lone-pairs of N₂ donate into the silyl cations. Newly formed lone-pair basins in the N₂-adducts balance surplus charge. Thereby, the N-N bond distance is increased by astonishing 0.3 Å, from 1.1 Å in N₂ gas to 1.4 Å in the adduct, which makes this bond prone to subsequent addition of hydride ions and protonation, forming two secondary amine sites in the process and eventually breaking the NN triple bond. Potential formation of dead-end states, in which the dications ("active states") aversively form a Lewis acid (LA)-Lewis base (LB) bond, or in which the LA and LB sites are too far away from each other to be able to capture N₂, are problematic but might be circumvented by proper choice of spacer molecules, such as acenaphthalene or biphenylene, and the ligands attached to the LA and LB atoms, such as phenyl or mesityl, and by purging the reaction solutions with gaseous N₂ in the initial reaction steps. Charge redistributions via N₂-activation and splitting were monitored by a variety of real-space bonding indicators (RSBIs) derived from the calculated electron and electron pair densities, which provided valuable insight into the bonding situation within the different reaction steps.

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.

Tungsten and Molybdenum Dinitrogen Complexes Supported by a Pentadentate Tetrapodal Phosphine Ligand: Comparative Spectroscopic, Electrochemical and Reactivity Studies

The tungsten dinitrogen complex [W(N2)(PMe2PPPh2)] (2) (PMe2PPPh2 = [2-({bis[3-(diphenylphosphino)propyl]-phosphino}methyl)-2-methylpropane-1,3-diyl]bis(dimethylphosphine)]) is synthesized and characterized by X-ray diffraction as well as IR and NMR spectroscopies, showing strong analogies to its molybdenum analogue...

Dual Active Centers Bridged by Oxygen Vacancies of Ru Single Atoms Hybrids Supported on Molybdenum Oxide for Photocatalytic Ammonia Synthesis

Photocatalytic synthesis of ammonia (NH 3 ) holds significant potential compared with the Haber-Bosch process. However, the reported photocatalysts suffered from low efficiency owing to localized electrons deficiency. Here, Ru-SA (single atoms)/H x MoO 3-y hybrids with abundant of Mo n+ (n < 6) species neighboring oxygen vacancies (O V ) are synthesized via a H-spillover process. Detailed characterizations demonstrate that Ru-SA/H x MoO 3 y hybrids can quantitatively produce NH 3 from N 2 and H 2 by the synergetic effect of dual active centers (Ru SA and Mo n+ ). That is, Ru SA boost the activation and migration of H 2 , and Mo n+ species act as the trapping sites of localized electrons and the adsorption and dissociation sites of N 2 , finally leading to NH 3 synthesis on Mo n+ -OH. The NH 3 generation rate is as high as 4.0 mmol h -1 g -1 , accompanied by an apparent quantum efficiency over 6.0% at 650 nm. Our finding may open up a new strategy for acquiring a better NH 3 synthesis approach under mild conditions.

Molybdenum-Mediated N2 -Splitting and Functionalization in the Presence of a Coordinated Alkyne

A new [PCCP]-coordinated molybdenum platform comprising a coordinated alkyne was employed for the cleavage of molecular dinitrogen. The coordinated η2 -alkyne was left unaffected during this reduction. DFT calculations suggest that the reaction proceeds via an initially generated terminal N2 -complex, which is converted to a dinuclear μ-(η1 :η1 )-N2 -bridged intermediate prior to N-N bond cleavage. Protonation, alkylation and acylation of the resulting molybdenum nitrido complex led to the corresponding N-functionalized imido complexes. Upon oxidation of the N-acylated imido derivative in MeCN, a fumaronitrile fragment was built up via C-C coupling of MeCN to afford a dinuclear molybdenum complex. The key finding that the strong N≡N bond may be cleaved in the presence of a weaker, but spatially constrained C≡C bond contradicts the widespread paradigm that coordinated alkynes are in general more reactive than gaseous N2 .

Stannylenes based on pyrrole-phosphane and dipyrromethane-diphosphane scaffolds: syntheses and behavior as precursors to PSnP pincer palladium(II), palladium(0) and gold(I) complexes

2-Ditertbutylphosphanylmethylpyrrole (H₂pyrmP^tBu₂) and 2,2'-bis(diisopropylphosphanylmethyl)-5,5'-dimethyldipyrromethane ((HpyrmPⁱPr₂)₂CMe₂) have been used to synthesize new P-donor-stabilized stannylenes in which the Sn atom is attached to one, SnCl(HpyrmP^tBu₂) (1) and Sn{N(SiMe₃)₂}(HpyrmP^tBu₂) (2), or two pyrrolyl-phosphane scaffolds, Sn(HpyrmP^tBu₂)₂ (3), or to a dipyrromethane-diphosphane scaffold, Sn(pyrmPⁱPr₂)₂CMe₂ (4). It has been found that stannylenes 3 and 4 are excellent precursors to transition metal complexes containing PSnP pincer-type ligands. Their reactions with chlorido transition metal complexes have afforded [PdCl{κ³P,Sn,P-SnCl(HpyrmP^tBu₂)₂}] (6), [PdCl{κ³P,Sn,P-SnCl(pyrmPⁱPr₂)₂CMe₂}] (7) and [Au{κ³P,Sn,P-SnCl(HpyrmP^tBu₂)₂}] (8), which contain a PSnP pincer-type chloridostannyl ligand. While complexes 6 and 7 are square-planar palladium(II) complexes, compound 8 is an uncommon gold(I) complex having a T-shaped coordination geometry with a very long Sn-Au bond (3.120 Å). The T-shaped palladium(0) complex [Pd{κ³P,Sn,P-Sn(pyrmPⁱPr₂)₂CMe₂}] (9), which contains an unprecedented PSnP pincer-type stannylene that behaves as a Z-type (σ-acceptor) ligand, has been prepared from 4 and [Pd(η³-C₃H₅)(η⁵-C₅H₅)].