A Synthetic Single-Site Fe Nitrogenase: High Turnover, Freeze-Quench (57)Fe Mössbauer Data, and a Hydride Resting State

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.

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.

Mechanism of Nitrogen Reduction to Ammonia in a Diiron Model of Nitrogenase

Nitrogenase is a fascinating enzyme in biology that reduces dinitrogen from air to ammonia through stepwise reduction and protonation. Despite it being studied in detail by experimental and computational groups, there are still many unknown factors in the catalytic cycle of nitrogenase, especially related to the addition of protons and electrons and their order. A recent biomimetic study characterized a potential dinitrogen-bridged diiron cluster as a synthetic model of nitrogenase. Using strong acid and reductants, the dinitrogen was converted into ammonia molecules, but details of the mechanism remains unknown. In particular, it was unclear from the experimental studies whether the proton and electron transfer steps are sequential or alternating. Moreover, the work failed to establish what the function of the diiron core is and whether it split into mononuclear iron fragments during the reaction. To understand the structure and reactivity of the biomimetic dinitrogen-bridged diiron complex [(P2P'PhFeH)2(μ-N2)] with triphenylphosphine ligands, we performed a density functional theory study. Our computational methods were validated against experimental crystal structure coordinates, Mössbauer parameters, and vibrational frequencies and show excellent agreement. Subsequently, we investigated the alternating and consecutive addition of electrons and protons to the system. The calculations identify a number of possible reaction channels, namely, same-site protonation, alternating protonation, and complex dissociation into mononuclear iron centers. The calculations show that the overall mechanism is not a pure sequential set of electron and proton transfers but a mixture of alternating and consecutive steps. In particular, the first reaction steps will start with double proton transfer followed by an electron transfer, while thereafter, there is another proton transfer and a second electron transfer to give a complex whereby ammonia can split off with a low energetic barrier. The second channel starts with alternating protonation of the two nitrogen atoms, whereafter the initial double proton transfer, electrons and protons are added sequentially to form a hydrazine-bound complex. The latter split off ammonia spontaneously after further protonation. The various reaction channels are analyzed with valence bond and orbital diagrams. We anticipate the nitrogenase enzyme to operate with mixed alternating and consecutive protonation and electron transfer steps.

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.

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.

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.

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.

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 transfer hydrogenation of N2 to NH3 via a photoredox catalysis strategy

Inspired by momentum in applications of reductive photoredox catalysis to organic synthesis, photodriven transfer hydrogenations toward deep (>2 e^-) reductions of small molecules are attractive compared to using harsh chemical reagents. Noteworthy in this context is the nitrogen reduction reaction (N₂RR), where a synthetic photocatalyst system had yet to be developed. Noting that a reduced Hantzsch ester (HEH₂) and related organic structures can behave as 2 e^-/2 H⁺ photoreductants, we show here that, when partnered with a suitable catalyst (Mo) under blue light irradiation, HEH₂ facilitates delivery of successive H₂ equivalents for the 6 e^-/6 H⁺ catalytic reduction of N₂ to NH₃; this catalysis is enhanced by addition of a photoredox catalyst (Ir). Reductions of additional substrates (nitrate and acetylene) are also described.

Characterization of a Proposed Terminal Iron(III) Nitride Intermediate of Nitrogen Fixation Stabilized by a Trisphosphine-Borane Ligand

Terminal iron nitrides (Fe≡N) have been proposed as intermediates of Fe-mediated nitrogen fixation, and well-defined synthetic iron nitrides have been characterized in high oxidation states, including FeIV , FeV , and FeVI . This study reports the generation and low temperature characterization of a terminally bound iron(III) nitride, P3 B Fe(N) (P3 B =tris(o-diisopropylphosphinophenyl)borane), which is a proposed intermediate of iron-mediated nitrogen fixation by the P3 B Fe-catalyst system. CW- and pulse EPR spectroscopy (HYSCORE and ENDOR), supported by DFT calculations, help to define a 2 A ground state electronic structure of this C3 -symmetric nitride species, placing the unpaired spin in a sigma orbital along the B-Fe-N vector; this electronic structure is distinct for an iron nitride. The unusual d5 -configuration is stabilized by significant delocalization (≈50 %) of the unpaired electron onto the axial boron and nitrogen ligands, with a majority of the spin residing on boron.

Molecular Electrochemical Reductive Splitting of Dinitrogen with a Molybdenum Complex

Nitrogen reduction under mild conditions (room T and atmospheric P), using a non-fossil source of hydrogen remains a challenge. Molecular metal complexes, notably Mo based, have recently been shown to be active for such nitrogen fixation. We report electrochemical N2 splitting with a MoIII triphosphino complex [(PPP)MoI3 ], at room temperature and a moderately negative potential. A MoIV nitride species was generated, which is confirmed by electrochemistry and NMR studies. The reaction goes through two successive one electron reductions of the starting Mo species, coordination of a N2 molecule, and further splitting to a MoIV nitride complex. Preliminary DFT studies support the formation of a bridging MoI N2 MoI dinitrogen dimer evolving to the Mo nitride via a low energy transition state. This example joins a short list of molecular complexes for N2 electrochemical reductive cleavage. It opens a door to electrochemical proton-coupled electron transfer (PCET) conversion studies of N2 to NH3 .

Sulfonium cations as versatile strongly π-acidic ligands

More than a century old, sulfonium cations are still intriguing species in the landscape of organic chemistry. On one hand they have found broad applications in organic synthesis and materials science, but on the other hand, while isoelectronic to the ubiquitous tertiary phosphine ligands, their own coordination chemistry has been neglected for the last three decades. Here we report the synthesis and full characterization of the first Rh(i) and Pt(ii) complexes of sulfonium. Moreover, for the first time, coordination of an aromatic sulfonium has been established. A thorough computational analysis of the exceptionally short S-Rh bonds obtained attests to the strongly π-accepting nature of sulfonium cations and places them among the best π-acceptor ligands available today. Our calculations also show that embedding within a pincer framework enhances their π-acidity even further. Therefore, in addition to the stability and modularity that these frameworks offer, our pincer complexes might open the way for sulfonium cations to become powerful tools in π-acid catalysis.

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.

Facile conversion of ammonia to a nitride in a rhenium system that cleaves dinitrogen

Rhenium complexes with aliphatic PNP pincer ligands have been shown to be capable of reductive N₂ splitting to nitride complexes. However, the conversion of the resulting nitride to ammonia has not been observed. Here, the thermodynamics and mechanism of the hypothetical N–H bond forming steps are evaluated through the reverse reaction, conversion of ammonia to the nitride complex. Depending on the conditions, treatment of a rhenium(iii) precursor with ammonia gives either a bis(amine) complex [(PNP)Re(NH₂)₂Cl]⁺, or results in dehydrohalogenation to the rhenium(iii) amido complex, (PNP)Re(NH₂)Cl. The N–H hydrogen atoms in this amido complex can be abstracted by PCET reagents which implies that they are quite weak. Calorimetric measurements show that the average bond dissociation enthalpy of the two amido N–H bonds is 57 kcal mol⁻¹, while DFT computations indicate a substantially weaker N–H bond of the putative rhenium(iv)-imide intermediate (BDE = 38 kcal mol⁻¹). Our analysis demonstrates that addition of the first H atom to the nitride complex is a thermochemical bottleneck for NH₃ generation.

Rhenium–PNP complexes split N₂ to nitrides, but the nitrides do not give ammonia. Here, the thermodynamics of the hypothetical N–H bond forming steps are evaluated through the reverse reaction, showing that the first H addition is the bottleneck.

Synthesis and Reactivity of Manganese Complexes Bearing Anionic PNP- and PCP-Type Pincer Ligands toward Nitrogen Fixation

A series of manganese complexes bearing an anionic pyrrole-based PNP-type pincer ligand and an anionic benzene-based PCP-type pincer ligand is synthesized and characterized. The reactivity of these complexes toward ammonia formation and silylamine formation from dinitrogen under mild conditions is evaluated to produce only stoichiometric amounts of ammonia and silylamine, probably because the manganese pincer complexes are unstable under reducing conditions.

Mechanisms of Electrochemical N2 Splitting by a Molybdenum Pincer Complex

Molybdenum complexes supported by tridentate pincer ligands are exceptional catalysts for dinitrogen fixation using chemical reductants, but little is known about their prospects for electrochemical reduction of dinitrogen. The viability of electrochemical N₂ binding and splitting by a molybdenum(III) pincer complex, (^pyPNP)MoBr₃ (^pyPNP = 2,6-bis(^tBu₂PCH₂)-C₅H₃N)), is established in this work, providing a foundation for a detailed mechanistic study of electrode-driven formation of the nitride complex (^pyPNP)Mo(N)Br. Electrochemical kinetic analysis, optical and vibrational spectroelectrochemical monitoring, and computational studies point to two concurrent reaction pathways: In the reaction-diffusion layer near the electrode surface, the molybdenum(III) precursor is reduced by 2e^- and generates a bimetallic molybdenum(I) Mo₂(μ-N₂) species capable of N-N bond scission; and in the bulk solution away from the electrode surface, over-reduced molybdenum(0) species undergo chemical redox reactions via comproportionation to generate the same bimetallic molybdenum(I) species capable of N₂ cleavage. The comproportionation reactions reveal the surprising intermediacy of dimolybdenum(0) complex trans,trans-[(^pyPNP)Mo(N₂)₂](μ-N₂) in N₂ splitting pathways. The same "over-reduced" molybdenum(0) species was also found to cleave N₂ upon addition of lutidinium, an acid frequently used in catalytic reduction of dinitrogen.

Synergistic Catalysis of the Synthesis of Ammonia with Co-Based Catalysts and Plasma: From Nanoparticles to a Single Atom

In this study, a series of Co nanoparticles (NPs) with different sizes and Co single-atom catalysts (SACs) with different cobalt-nitrogen coordination numbers (Co-N2, Co-N3, and Co-N4) were synthesized and applied to the synthesis of ammonia catalyzed by plasma at low temperatures and atmospheric pressures. Under the same reaction conditions, the yield of nitrogen obtained from the reduction to ammonia over a series of Co NP catalysts varies with the Co particle size. The smaller the size of the Co NPs, the greater the number of exposed active centers, and the catalytic activity is higher. Among the Co SACs, the best catalyst was Co-N2 with two coordinated nitrogen atoms, and the ammonia yield was 181 mg·h-1·gcat-1. The experimental and theoretical calculations were consistent in that a low Co-N coordination number was beneficial to the adsorption and dissociation of N2, thereby enhancing the reduction activity of N2 and promoting the increase of ammonia production.

A comparative analysis of the mechanisms of ammonia synthesis on various catalysts using density functional theory

In this review, we present the recent progress in ammonia synthesis research using density functional theory (DFT) calculations on various industrial catalysts, metal nitrides and nano-cluster-supported catalysts. The mechanism of ammonia synthesis on the industrial Fe catalyst is generally accepted to be a dissociative mechanism. We have recently found, using DFT techniques, that on Co₃Mo₃N (111) surfaces, an associative mechanism in the synthesis of ammonia can offer a new low-energy pathway that was previously unknown. In particular, we have shown that metal nitrides that are also known to have high activity for ammonia synthesis can readily form nitrogen vacancies which can activate dinitrogen, thereby promoting the associative mechanism. These fundamental studies suggest that a promising route to the discovery of low-temperature ammonia synthesis catalysts will be to identify systems that proceed via the associative mechanism, which is closer to the nitrogen-fixation mechanism occurring in nitrogenases.

Reduction of highly bulky triphenolamine molybdenum nitrido and chloride complexes

Transition metal nitrides are key intermediates in the catalytic reduction of dinitrogen to ammonia. To date, transition metal nitride complexes with the triphenolamine (TPA) ligand have not been reported and the system with the ligand has been much less studied for ammonia formation compared with other systems. Herein, we report a series of molybdenum complexes supported by a sterically demanding TPA ligand, including a nitrido complex NMo(TPA). We achieved the stoichiometric conversion of the nitride moiety into ammonia under ambient conditions by adding proton and electron sources to NMo(TPA). However, the catalytic turnover for N₂ reduction to ammonia was not observed in the triphenolamine ligand system unlike the Schrock system-triamidoamine ligand. Density functional theory calculation revealed that the molybdenum center favors binding NH₃ over N₂ by 16.9 kcal mol^-1 and the structural lability of the trigonal bipyramidal (TBP) molybdenum complex seems to prevent catalytic turnover. Our systematic study showed that the electronegativity and bond length of ancillary ligands determine the preference between N₂ and NH₃, suggesting a systematic design strategy for improvement.

Rare Earth Metal Complexes Supported by a Tripodal Tris(amido) Ligand System Featuring an Arene Anchor

A new tripodal tris(amido) ligand system featuring an arene anchor was developed and applied to the coordination chemistry of rare earth metals. Two tris(amido) ligands with a 1,3,5-triphenylbenzene backbone were prepared in two steps from commercially available reagents on a gram scale. Salt metathesis and alkane elimination reactions were exploited to prepare mononuclear rare earth metal complexes in moderate to good yields. For salt metathesis reactions, while metal tribromides yielded neutral metal tris(amido) complexes, metal trichlorides led to the formation of ate complexes with an additional chloride bound to the metal center. The new compounds were characterized by X-ray crystallography, elemental analysis, and ¹H and ¹³C nuclear magnetic resonance spectroscopy. The rare earth metal complexes exhibit a trigonal planar coordination geometry for the [MN₃] fragment in the solid state rather than a trigonal pyramidal geometry, commonly observed for rare earth metal tris(amido) complexes such as M[N(SiMe₃)₂]₃. Moreover, the arene anchor of the tripodal ligands is engaged in a nonnegligible interaction with the rare earth metal ions. Density functional theory calculations were performed to gain insight into the bonding interactions between the tripodal ligands and the rare earth metal ions. While LUMOs of these rare earth metal complexes are mainly π* orbitals of the arene with a minor component of metal-based orbitals, HOMO-15 and HOMO-16 of a lanthanum complex show that the arene anchor serves as a π donor to the trivalent lanthanum ion.

Synthesis of tris(3-pyridyl)aluminate ligand and its unexpected stability against hydrolysis: revealing cooperativity effects in heterobimetallic pyridyl aluminates

We report the elusive metallic anion [EtAl(3-py)₃]^- (3-py = 3-pyridyl) (1), the first member of the anionic tris(3-pyridyl) family. Unexpectedly, the lithium complex 1Li shows substantial protic stability against water and alcohols, unlike related tris(2-pyridyl)aluminate analogues. This stability appears to be related to the inability of the [EtAl(3-py)₃]^- anion to chelate Li⁺, which precludes a decomposition pathway involving Li/Al cooperativity.

Fixation of Dinitrogen at an Asymmetric Binuclear Titanium Complex

A new type of dititanium dinitrogen complex supported by a triphenolamine (TPA) ligand is reported. Analysis by single-crystal X-ray diffraction and Raman and NMR spectroscopy reveals different coordination geometries for the two titanium centers. Hence, coordination of TPA and a nitrogen ligand results in trigonal-bipyramidal geometry, while an octahedral titanium center is obtained upon additional coordination of an ethoxide generated upon C-O bond cleavage in a diethyl ether solvent molecule. The titanium complex successfully generates ammonia in the presence of an excess amount of PCy₃HI and KC₈ in 154% yield (per titanium atom). A titanium complex with a bulkier TPA does not form a dinitrogen complex, and mononuclear titanium dinitrogen complexes were not accessible, presumably because of the high tendency of early transition metals to form binuclear dinitrogen complexes.

Structure, reactivity, and spectroscopy of nitrogenase-related synthetic and biological clusters

The reduction of dinitrogen (N2) is essential for its incorporation into nucleic acids and amino acids, which are vital to life on earth. Nitrogenases convert atmospheric dinitrogen to two ammonia molecules (NH3) under ambient conditions. The catalytic active sites of these enzymes (known as FeM-cofactor clusters, where M = Mo, V, Fe) are the sites of N2 binding and activation and have been a source of great interest for chemists for decades. In this review, recent studies on nitrogenase-related synthetic molecular complexes and biological clusters are discussed, with a focus on their reactivity and spectroscopic characterization. The molecular models that are discussed span from simple mononuclear iron complexes to multinuclear iron complexes and heterometallic iron complexes. In addition, recent work on the extracted biological cofactors is discussed. An emphasis is placed on how these studies have contributed towards our understanding of the electronic structure and mechanism of nitrogenases.

Comprehensive Understanding of the Thriving Ambient Electrochemical Nitrogen Reduction Reaction

The electrochemical method of combining N₂ and H₂ O to produce ammonia (i.e., the electrochemical nitrogen reduction reaction [E-NRR]) continues to draw attention as it is both environmentally friendly and well suited for a progressively distributed farm economy. Despite the multitude of recent works on the E-NRR, further progress in this field faces a bottleneck. On the one hand, despite the extensive exploration and trial-and-error evaluation of E-NRR catalysts, no study has stood out to become the stage protagonist. On the other hand, the current level of ammonia production (microgram-scale) is an almost insurmountable obstacle for its qualitative and quantitative determination, hindering the discrimination between true activity and contamination. Herein i) the popular theory and mechanism of the NRR are introduced; ii) a comprehensive summary of the recent progress in the field of the E-NRR and related catalysts is provided; iii) the operational procedures of the E-NRR are addressed, including the acquisition of key metrics, the challenges faced, and the most suitable solutions; iv) the guiding principles and standardized recommendations for the E-NRR are emphasized and future research directions and prospects are provided.

Ammonia Formation Catalyzed by a Dinitrogen-Bridged Dirhenium Complex Bearing PNP-Pincer Ligands under Mild Reaction Conditions*

A series of rhenium complexes bearing a pyridine-based PNP-type pincer ligand are synthesized from rhenium phosphine complexes as precursors. A dinitrogen-bridged dirhenium complex bearing the PNP-type pincer ligands catalytically converts dinitrogen into ammonia during the reaction with KC₈ as a reductant and [HPCy₃ ]BAr^F ₄ (Cy=cyclohexyl, Ar^F =3,5-(CF₃ )₂ C₆ H₃ ) as a proton source at -78 °C to afford 8.4 equiv of ammonia based on the rhenium atom of the catalyst. The rhenium-dinitrogen complex also catalyzes silylation of dinitrogen in the reaction with KC₈ as a reductant and Me₃ SiCl as a silylating reagent under ambient reaction conditions to afford 11.7 equiv of tris(trimethylsilyl)amine based on the rhenium atom of the catalyst. These results demonstrate the first successful example of catalytic nitrogen fixation under mild reaction conditions using rhenium-dinitrogen complexes as catalysts.

Comprehensive insights into synthetic nitrogen fixation assisted by molecular catalysts under ambient or mild conditions

N2 is fixed as NH3 industrially by the Haber-Bosch process under harsh conditions, whereas biological nitrogen fixation is achieved under ambient conditions, which has prompted development of alternative methods to fix N2 catalyzed by transition metal molecular complexes. Since the early 21st century, catalytic conversion of N2 into NH3 under ambient conditions has been achieved by using molecular catalysts, and now H2O has been utilized as a proton source with turnover frequencies reaching the values found for biological nitrogen fixation. In this review, recent advances in the development of molecular catalysts for synthetic N2 fixation under ambient or mild conditions are summarized, and potential directions for future research are also discussed.

Enhanced Activity of Titanocene Complex for Electrocatalytic Nitrogen Reduction Reaction

Enhanced titanocene (Cp2TiCl2) based electrocatalytic system for nitrogen reduction was shown, comprising glassy carbon electrode, high level of the catechol redox mediator, optimized binary THF/MeOH solvent and unique design of the reactor having ammonia permeable membrane at the outlet, which allowed constant nitrogen flow through the working solution during entire electrolysis without risk of evaporation of the solvent. Catalytic activity was observed in the potential range of (−1.5)–(−2.3) V, reaching TON of 2.83%, corresponding to the production of 0.566 μmol NH3 (9.64 μg) in 24 h hydrolysis at −2.3 V using 0.02 mmol TiCp2Cl2 (5 mg).

Dinitrogen Fixation: Rationalizing Strategies Utilizing Molecular Complexes

Dinitrogen (N2 ) is the most abundant gas in Earth's atmosphere, but its inertness hinders its use as a nitrogen source in the biosphere and in industry. Efficient catalysts are hence required to ov. ercome the high kinetic barriers associated to N2 transformation. In that respect, molecular complexes have demonstrated strong potential to mediate N2 functionalization reactions under mild conditions while providing a straightforward understanding of the reaction mechanisms. This Review emphasizes the strategies for N2 reduction and functionalization using molecular transition metal and actinide complexes according to their proposed reaction mechanisms, distinguishing complexes inducing cleavage of the N≡N bond before (dissociative mechanism) or concomitantly with functionalization (associative mechanism). We present here the main examples of stoichiometric and catalytic N2 functionalization reactions following these strategies.

Catalytic conversion of nitrogen molecule into ammonia using molybdenum complexes under ambient reaction conditions

Nitrogen fixation using homogeneous transition metal complexes under mild reaction conditions is a challenging topic in the field of chemistry. Several successful examples of the catalytic conversion of nitrogen molecule into ammonia using various transition metal complexes in the presence of reductants and proton sources have been reported so far, together with detailed investigations on the reaction mechanism. Among these, only molybdenum complexes have been shown to serve as effective catalysts under ambient reaction conditions, in stark contrast with other transition metal-catalysed reactions that proceed at low reaction temperature such as -78 °C. In this feature article, we classify the molybdenum-catalysed reactions into four types: reactions via the Schrock cycle, reactions via dinuclear reaction systems, reactions via direct cleavage of the nitrogen-nitrogen triple bond of dinitrogen, and reactions via the Chatt-type cycle. We describe these catalytic systems focusing on the catalytic activity and mechanistic investigations. We hope that the present feature article provides useful information to develop more efficient nitrogen fixation systems under mild reaction conditions.