Nitrogen Fixation via Splitting into Nitrido Complexes

Seven-Coordinate Group 6 Metal Hydrides Obtained by H2 Activation at B(C6F5)3 Adducts of N2 Complexes: Frustrated Lewis Pair-Type Reactivity of The B-N Linkage

The adducts 2M,R of general formula trans-[(L)M{R2P(CH2)2PR2}2{N2B(C6F5)3}] (L=ø or N2, M=Mo or W, R=Et or Ph), formed from Lewis acid-base pairing of B(C6F5)3 to a dinitrogen ligand of zero-valent group 6 bis(phosphine) complexes trans-[M{R2P(CH2)2PR2}2(N2)2] are shown to react with dihydrogen to afford hepta-coordinated bis(hydride) complexes [M(H)2{R2P(CH2)2PR2}{N2B(C6F5)3}] 3M,R which feature the rare ability to activate both dinitrogen and dihydrogen at a single metal center, except in the case where M=Mo and R=Ph for which fast precipitation of insoluble [Mo(H)4(dppe)2] (dppe=1,2-bis(diphenylphosphino)ethane) occurs. The frustrated Lewis pair (FLP)-related reactivity of the B-N linkage in compounds 3W,R was explored and led to distal N functionalization without involvement of the hydride ligands. It is shown in one example that the resulting bis(hydride) diazenido compounds may also be obtained through a sequence involving first FLP-type N-functionalization followed by oxidative addition of H2. Those oily compounds were found to have limited stability in solution or in their isolated states. Finally, treatment of 3W,Et with the Lewis base N,N-dimethylaminopyridine (DMAP) affords the simple but unknown bis(hydride)-dinitrogen species [W(H)2(depe)2(N2)] 11Et (depe=1,2-bis(diethylphosphino)ethane) which direct, selective formation from trans-[W(N2)2(depe)2] is not possible.

Predicting Activation of Small Molecules Including Dinitrogen via a Carbene with a σ0π2 Electronic Configuration

Although the main group species in the s and p blocks have begun to gain prominence in the field of dinitrogen (N2) activation in recent years, reports of carbene-mediated N2 activation remain particularly rare, especially for carbenes with a σ0π2 electronic configuration. Herein, we demonstrate examples of N2 activation initiated by a carbene with a σ0π2 electronic configuration and consequent N2 hydroboration reaction (with a reaction barrier as low as 19.9 kcal/mol) via density functional theory calculations. Meanwhile, the "push-pull" electronic effect upon introduction of a hydroborenium complex facilitates the generation of a thermodynamically and kinetically more stable product. In addition, such a σ0π2 carbene can also activate a series of H-X (X = H, CH3, or Bpin) bonds through an oxidative addition process with activation energies ranging from 6.0 to 18.0 kcal/mol. Our findings highlight the importance of σ0π2 carbenes in the field of small molecule activation, especially N2 activation.

Predicting Small Molecule Activations Including Dinitrogen Based on an Inorganic Benzene B4N2 Framework

Although main group species have emerged in the field of dinitrogen activation in recent years, the reported examples are particularly rare in comparison with transition metal complexes due to their significant challenges. Herein, we demonstrate a [4 + 2] cycloaddition reaction of N2 (with an activation energy as low as 12.5 kcal mol-1) initiated by an inorganic benzene via density functional theory calculations. Such N2 activation is supported by the elongated nitrogen-nitrogen bond distance (dNN), decreased vibration frequency (νNN), and weakened Wiberg bond index (WBINN). Subsequently, the "push-pull" electronic effect, formed by introducing a Lewis acid, HB(C6F5)2, facilitates the generation of thermodynamically more stable products. In addition, this inorganic benzene could also be used to activate a series of small molecules, including carbon dioxide, acetylene, ethylene, and acetonitrile with reaction barriers ranging from 4.7 to 11.6 kcal mol-1. Our findings provide an alternative approach to N2 activation and functionalization, theoretically validating the feasibility of the dual Lewis acid strategy for dinitrogen activation.

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.

Hydroamination of alkenes with dinitrogen and titanium polyhydrides

An ideal synthesis of alkyl amines would involve the direct use of abundant and easily accessible molecules such as dinitrogen (N2) and feedstock alkenes1-4. However, this ambition remains a great challenge as it is usually difficult to simultaneously activate both N2 and a simple alkene and combine them together through carbon-nitrogen (C-N) bond formation. Currently, the synthesis of alkyl amines relies on the use of ammonia produced through the Haber-Bosch process and prefunctionalized electrophilic carbon sources. Here we report the hydroamination of simple alkenes with N2 in a trititanium hydride framework, which activates both alkenes and N2, leading to selective C-N bond formation and providing the corresponding alkyl amines on further hydrogenation and protonation. Computational studies reveal key mechanistic details of N2 activation and selective C-N bond formation. This work demonstrates a strategy for the transformation of N2 and simple hydrocarbons into nitrogen-containing organic compounds mediated by a multinuclear hydride framework.

N2 Functionalization via Molybdenum-Nitride Complex: Stepwise BH Bond Additions

Reactivity of (triphosphine)MoIV-nitrido complex generated by N2 splitting, toward boranes is reported. The simple adduct Mo≡N→BH3 is observed with BH3.SMe2 while 1,2 addition is evidenced with 9-BBN leading to H-Mo=NBR2. A second addition of BH3.SMe2 is facile and forms an unprecedented complex featuring two bridging H between two B and the Mo centers. Addition of PMe3 or BH3.SMe2 promotes reductive elimination and N-H bond formation. The full sequence of functionalization at Mo≡N obtained after N2 splitting is therefore evidenced in this work.

Nitrogen Fixation by Manganese Complexes - Waiting for the Rush?

Manganese is currently experiencing a great deal of attention in homogeneous catalysis as a sustainable alternative to platinum group metals due to its abundance, affordable price and low toxicity. While homogeneous nitrogen fixation employing well-defined transition metal complexes has been an important part of coordination chemistry, manganese derivatives have been only sporadically used in this research area. In this contribution, the authors systematically cover manganese organometallic chemistry related to N2 activation spanning almost 60 years, identify apparent pitfalls and outline encouraging perspectives for its future development.

Dinitrogen activation at chromium by photochemically induced CrII-C bond homolysis

The synthesis of the organochromium(II) complexes [POCOPtBu]Cr(R) (R = p-Tol, Bn) is reported. Exposure of [POCOPtBu]Cr(Bn) to visible light promoted homolytic Cr-CBn bond cleavage and formed {[POCOPtBu]Cr}2(η1:η1μ-N2) via a putative [POCOPtBu]Cr(I) species.

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.

Molybdenum(VI) Nitrido Complexes with Tripodal Silanolate Ligands. Structure and Electronic Character of an Unsymmetrical Dimolybdenum μ-Nitrido Complex Formed by Incomplete Nitrogen Atom Transfer

In contrast to a tungsten nitrido complex endowed with a tripodal silanolate ligand framework, which was reported in the literature to be a dimeric species with a metallacyclic core, the corresponding molybdenum nitrides 3 are monomeric entities comprising a regular terminal nitride unit, as proven by single-crystal X-ray diffraction (SC-XRD). Their electronic character is largely determined by the constraints imposed on the metal center by the podand ligand architecture. 95Mo nuclear magnetic resonance (NMR) and, to a lesser extent, 14N NMR spectroscopy allow these effects to be studied, which become particularly apparent upon comparison with the spectral data of related molybdenum nitrides comprising unrestrained silanolate, alkoxide, or amide ligands. Attempted nitrogen atom transfer from these novel terminal nitrides to [(tBuArN)3Mo] (Ar = 3,5-dimethylphenyl) as the potential acceptor stopped at the stage of unsymmetric dimolybdenum μ-nitrido complex 13a as the first intermediate along the reaction pathway. SC-XRD, NMR, electron paramagnetic resonance, and ultraviolet-visible spectroscopy as well as magnetometry in combination with density functional theory allowed a clear picture of the geometric and electronic structure of this mixed-valent species to be drawn. 13a is formally best described as an adduct of the type [(Mo[O])+III-(μN)-III-(Mo[N])+VI], S = 1/2 complex with (Mo[O])+III in the low-spin configuration, whereas related complexes such as [(AdS)3Mo-(μN)-Mo(NtBuAr)3] (19; Ad = 1-adamantyl) have previously been regarded in the literature as mixed-valent Mo+IV/Mo+V species. The spin population at the two Mo centers is uneven and notably larger at the more reduced Mo[O] atom, whereas the only spin present at the (μN) bridge is derived from spin polarization.

Dinitrogen Cleavage and Multicoupling with Isocyanides in a Dititanium Dihydride Framework

Dinitrogen (N2) activation and functionalization through N-N bond cleavage and N-C bond formation are of great interest and importance but remain highly challenging. We report here for the first time N2 cleavage and selective multicoupling with isocyanides in a dititanium dihydride framework. The reaction of a dinitrogen dititanium dihydride complex [{(acriPNP)Ti}2(μ-η1:η2-N2)(μ-H)2] (1) with an excess (four or more equivalents) of p-methoxyphenyl isocyanide at room temperature gave a novel amidoamidinatoguanidinate complex [(acriPNP)Ti{NC(═NR)NC(═NR)CH2NR}Ti(acriPNP)(CNR)] (2, acriPNP = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide; R = p-MeOC6H4) through N2 splitting and coupling with three isocyanide molecules. When 1 equiv of p-methoxyphenyl isocyanide was used to react with 1 at -30 °C, the hydrogenation of the isocyanide unit by the two hydride ligands in 1 took place, affording an amidomethylene-bridged dititanium dinitrogen complex [{(acriPNP)Ti}2(μ-η1:η2-N2){μ-η1:η2-CH2N(p-MeOC6H4)}] (3), which upon reaction with another equivalent of p-methoxyphenyl isocyanide at room temperature gave an amidomethylene/nitrido/carbodiimido complex [(acriPNP)Ti(N═C═NR)(μ-N)(μ-η1:η2-CH2NR)Ti(acriPNP)] (4) through N2 cleavage and N═C bond formation. Further reaction of 4 with 1 equiv of p-methoxyphenyl isocyanide led to an unprecedented four-component (carbodiimido, nitrido, isocyanide, and amidomethylene) coupling, yielding an amidoamidinatoguanidinate complex [{(acriPNP)Ti}2{NC(═NR)NC(═NR)CH2NR}] (5), which on reaction with another equivalent of p-methoxyphenyl isocyanide afforded the isocyanide-coordinated analogue 2. The reaction of 1 with 2-naphthyl isocyanide also took place in a similar multicoupling fashion. Moreover, the cross-coupling reactions of the p-methoxyphenyl isocyanide-derived amidomethylene/nitrido/carbodiimido complex 4 with 2-naphthyl isocyanide, cyclohexyl isocyanide, and tert-butyl isocyanide were also achieved, which afforded the corresponding amidoamidinatoguanidinate products consisting of two different isocyanides. Density functional theory (DFT) calculations further elucidated the mechanistic details.

Catalytic dinitrogen reduction to hydrazine and ammonia using Cr(N2)2(diphosphine)2 complexes

The synthesis, characterization of trans-[Cr(N2)2(depe)2] (1) is described. 1 and trans-[Cr(N2)2(dmpe)2] (2) catalyze the reduction of N2 to N2H4 and NH3 in THF using SmI2 and H2O or ethylene glycol as proton sources. 2 produces the highest total fixed N for a molecular Cr catalyst to date.

Trapping of a Terminal Intermediate in the Boron-Mediated Dinitrogen Reduction: Mono-, Tri-, and Tetrafunctionalized Hydrazines in Two Steps from N2

The addition of chlorotrimethylsilane to a boron-mediated, transition-metal-free N2 activation reaction leads to the isolation of multiple potassium boryl(silyl)hydrazido species, likely trapping products of a terminal dinitrogen complex of boron. One of these silylated N2 species can be protonated or methylated, providing access to mono- to tetrafunctionalized hydrazines in two steps from N2 and in the absence of transition metals.

Oxidation-induced ambiphilicity triggers N-N bond formation and dinitrogen release in octahedral terminal molybdenum(v) nitrido complexes

Coupling of octahedral, terminal d1 molybdenum(v) nitrido complexes supported by a dianionic pentadentate ligand via N-N bond formation to give μ-dinitrogen complexes was found to be thermodynamically feasible but faces significant kinetic barriers. However, upon oxidation, a kinetically favored nucleophilic/electrophilic N-N bond forming mechanism was enabled to give monocationic μ-dinitrogen dimers. Computational and experimental evidence for this "oxidation-induced ambiphilic nitrido coupling" mechanism is presented. The factors influencing release of dinitrogen from the resulting μ-dinitrogen dimers were also probed and it was found that further oxidation to a dicationic species is required to induce (very rapid) loss of dinitrogen. The mechanistic path discovered for N-N bond formation and dinitrogen release follows an ECECC sequence (E = "electrochemical step"; C = "chemical step"). Experimental evidence for the intermediacy of a highly electrophilic, cationic d0 molybdenum(vi) nitrido in the N-N bond forming mechanism via trapping with an isonitrile reagent is also discussed. Together these results are relevant to the development of molecular catalysts capable of mediating ammonia oxidation to dihydrogen and dinitrogen.

Synthesis of Collidine from Dinitrogen via a Tungsten Nitride

The synthesis of pyridines from dinitrogen in homogeneous solution is known to be challenging considering that an N2 cleavage step needs to be combined with two N-C coupling steps. Herein, a tungsten complex bearing a tailor-made 2,2'-(tBu2As)2-substituted tolane ligand scaffold was shown to split N2 to afford the corresponding tungsten nitride, which is not the case for the corresponding (iPr2As)2-substituted derivative. The former nitride was then reacted with 2,4,6-trimethylpyrylium triflate, which led to the formation of a tungsten oxo complex, along with collidine. Over the course of this reaction, the O atom of the pyrylium starting material was replaced with an N atom via a hitherto unprecedented skeletal editing process.

Unusual Electronic Structures of an Electron Transfer Series of [Cr(μ-η1 : η1 -N2 )Cr]0/1+/2

In dinitrogen (N2 ) fixation chemistry, bimetallic end-on bridging N2 complexes M(μ-η1  : η1 -N2 )M can split N2 into terminal nitrides and hence attract great attention. To date, only 4d and 5d transition complexes, but none of 3d counterparts, could realize such a transformation. Likewise, complexes {[Cp*Cr(dmpe)]2 (μ-N2 )}0/1+/2+ (1-3) are incapable to cleave N2 , in contrast to their Mo congeners. Remarkably, cross this series the N-N bond length of the N2 ligand and the N-N stretching frequency exhibit unprecedented nonmonotonic variations, and complexes 1 and 2 in both solid and solution states display rare thermally activated ligand-mediated two-center spin transitions, distinct from discrete dinuclear spin crossovers. In-depth analyses using wave function based ab initio calculations reveal that the Cr-N2 -Cr bonding in complexes 1-3 is distinguished by strong multireference character and cannot be described by solely one electron configuration or Lewis structure, and that all intriguing spectroscopic observations originate in their sophisticate multireference electronic structures. More critical is that such multireference bonding of complexes 1-3 is at least a key factor that contributes to their kinetic inertness toward N2 splitting. The mechanistic understanding is then used to rationalize the disparate reactivity of related 3d M(μ-η1  : η1 -N2 )M complexes compared to their 4d and 5d analogs.

Studies on the chemical reduction of polynuclear titanium(IV) nitrido complexes

The redox chemistry of cube-type titanium(IV) nitrido complexes [{Ti4(η5-C5Me5)3(R)}(μ3-N)4] (R = η5-C5Me5 (1), N(SiMe3)2 (2), η5-C5H4SiMe3 (3), and η5-C5H5 (4)) was investigated by electrochemical methods and chemical reactions. Cyclic voltammetry studies indicate that 1-4 undergo a reversible one-electron reduction at ca. -1.8 V vs. ferrocenium/ferrocene. Thus, complex 1 reacts with sodium sand in tetrahydrofuran to produce the highly reactive ionic compound [Na(thf)6][{Ti(η5-C5Me5)}4(μ3-N)4] (5). The treatment of complexes 1-4 in toluene with one equivalent of [K(C5Me5)] in the presence of macrocycles (L) leads to C10Me10 and the formation of more stable derivatives [K(L)][{Ti4(η5-C5Me5)3(R)}(μ3-N)4] (R = η5-C5Me5, L = 18-crown-6 (6), crypt-222 (7); R = N(SiMe3)2, L = 18-crown-6 (8), crypt-222 (9); R = η5-C5H4SiMe3, L = 18-crown-6 (10), crypt-222 (11); R = η5-C5H5, L = crypt-222 (12)). However, the analogous reaction of 4 with [K(C5Me5)] and 18-crown-6 affords [{(18-crown-6)K}2(μ-η5:η5-C5H5)][{Ti4(η5-C5Me5)3(η5-C5H5)}(μ3-N)4] (13) via abstraction of one cyclopentadienide group from a putative intermediate [(18-crown-6)K(μ-η5:η5-C5H5)Ti4(η5-C5Me5)3(μ3-N)4]. In contrast to the cube-type nitrido systems 1-4, the cyclic voltammogram of the trinuclear imido-nitrido titanium(IV) complex [{Ti(η5-C5Me5)(μ-NH)}3(μ3-N)] (14) does not reveal any reversible redox event and 14 readily reacts with [K(C5Me5)] to afford C5Me5H and the diamagnetic derivative [{K(μ4-N)(μ3-NH)2Ti3(η5-C5Me5)3(μ3-N)}2] (15). The treatment of 15 with two equiv. of 18-crown-6 polyethers produces the molecular species [(L)K{(μ3-N)(μ3-NH)2Ti3(η5-C5Me5)3(μ3-N)}] (L = 18-crown-6 (16), dibenzo-18-crown-6 (17)). Complex 17 further reacts with one equiv. of dibenzo-18-crown-6 to yield the ion-separated compound [K(dibenzo-18-crown-6)2][Ti3(η5-C5Me5)3(μ3-N)(μ-N)(μ-NH)2] (18) similar to the ion pair [K(crypt-222)][Ti3(η5-C5Me5)3(μ3-N)(μ-N)(μ-NH)2] (19) obtained in the treatment of 15 with cryptand-222.

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.

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.

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.

Multimetallic Uranium Nitride Cubane Clusters from Dinitrogen Cleavage

Dinitrogen cleavage provides an attractive but poorly studied route to the assembly of multimetallic nitride clusters. Here, we show that the monoelectron reduction of the dinitrogen complex [{U(OC6H2-But3-2,4,6)3}2(μ-η2:η2-N2)], 1, allows us to generate, for the first time, a uranium complex presenting a rare triply reduced N2 moiety ((μ-η2:η2-N2)•3-). Importantly, the bound dinitrogen can be further reduced, affording the U4N4 cubane cluster, 3, and the U6N6 edge-shared cubane cluster, 4, thus showing that (N2)•3- can be an intermediate in nitride formation. The tetranitride cluster showed high reactivity with electrophiles, yielding ammonia quantitatively upon acid addition and promoting CO cleavage to yield quantitative conversion of nitride into cyanide. These results show that dinitrogen reduction provides a versatile route for the assembly of large highly reactive nitride clusters, with U6N6 providing the first example of a molecular nitride of any metal formed from a complete cleavage of three N2 molecules.

Dinitrogen cleavage by a dinuclear uranium(iii) complex

Understanding the role of multimetallic cooperativity and of alkali ion-binding in the second coordination sphere is important for the design of complexes that can promote dinitrogen (N2) cleavage and functionalization. Herein, we compare the reaction products and mechanism of N2 reduction of the previously reported K2-bound dinuclear uranium(iii) complex, [K2{[UIII(OSi(OtBu)3)3]2(μ-O)}], B, with those of the analogous dinuclear uranium(iii) complexes, [K(2.2.2-cryptand)][K{UIII(OSi(OtBu)3)3}2(μ-O)], 1, and [K(2.2.2-cryptand)]2[{UIII(OSi(OtBu)3)3}2(μ-O)], 2, where one or two K+ ions have been removed from the second coordination sphere by addition of 2.2.2-cryptand. In this study, we found that the complete removal of the K+ ions from the inner coordination sphere leads to an enhanced reducing ability, as confirmed by cyclic voltammetry studies, of the resulting complex 2, and yields two new species upon N2 addition, namely the U(iii)/U(iv) complex, [K(2.2.2-cryptand)][{UIII(OSi(OtBu)3)3}(μ-O){UIV(OSi(OtBu)3)3}], 3, and the N2 cleavage product, the bis-nitride, terminal-oxo complex, [K(2.2.2-cryptand)]2[{UV(OSi(OtBu)3)3}(μ-N)2{UVI(OSi(OtBu)3)2(κ-O)}], 4. We propose that the formation of these two products involves a tetranuclear uranium-N2 intermediate that can only form in the absence of coordinated alkali ions, resulting in a six-electron transfer and cleavage of N2, demonstrating the possibility of a three-electron transfer from U(iii) to N2. These results give an insight into the relationship between alkali ion binding modes, multimetallic cooperativity and reactivity, and demonstrate how these parameters can be tuned to cleave and functionalize N2.

Syntheses and Characterizations of Hetero-Bimetallic Chromium-Dinitrogen Transition-Metal Complexes

In the domain of N2 activation, hetero-bimetallic dinitrogen complexes are garnering substantial interest due to their potential to induce polarization in nonpolar N2 gas. Herein, we present the syntheses and characterizations of three novel hetero-multimetallic dinitrogen complexes: Cp*Cr(depe)N2V(depe)Me[O, P, O] 5, Cp*Cr(depe)N2V(depe)Tipp[O, P, O] 6, and [Cp*Cr(depe)N2]2TiTipp[O, P, O] 7. These complexes were synthesized via a transmetalation process involving the treatment of [Cr0-N2]- complex 4 with vanadium and titanium chloride complexes bearing alkyl or aryl substituted bis(o-hydroxyphenyl)-phenyl phosphine R[O, P, O] ligand (alkyl = methyl, aryl = 2,4,6-tri-isopropylbenzene). X-ray analysis shows that complexes 5 and 6 exhibit heterodinuclear structures, while complex 7 exhibits a heterotrinuclear core with two N2 ligands concurrently coordinated to two chromium and one titanium atoms. Raman spectroscopic data show that the N-N stretching vibration of the N2 moiety is clearly downshifted relative to free N2 and to mononuclear [Cr0-N2]- complex 4.

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.

Boron Insertion into the N≡N Bond of a Tungsten Dinitrogen Complex

The 1,3-addition of 1,2-diaryl-1,2-dibromodiboranes (B2Br2Ar2) to trans-[W(N2)2(dppe)2] (dppe = κ2-(Ph2PCH2)2), which is accompanied by a Br-Ar substituent exchange between the two boron atoms, is followed by a spontaneous rearrangement of the resulting tungsten diboranyldiazenido complex to a 2-aza-1,3-diboraallenylimido complex displaying a linear, cumulenic B=N=B moiety. This rearrangement involves the splitting of both the B-B and N=N bonds of the N2B2 ligand, formal insertion of a BAr boranediyl moiety into the N=N bond, and coordination of the remaining BArBr boryl moiety to the terminal nitrogen atom. Density functional theory calculations show that the reaction proceeds via a cyclic NB2 intermediate, followed by dissociation into a tungsten nitrido complex and a linear boryliminoborane, which recombine by adduct formation between the nitrido ligand and the electron-deficient iminoborane boron atom. The linear B=N=B moiety also undergoes facile 1,2-addition of Brønsted acids (HY = HOPh, HSPh, and H2NPh) with concomitant Y-Br substituent exchange at the terminal boron atom, yielding cationic (borylamino)borylimido tungsten complexes.

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.

Cryo IR spectroscopy and cryo kinetics of dinitrogen activation and cleavage by small tantalum cluster cations

We investigate small tantalum clusters Tan+, n = 2-4, for their capability to cleave N2 adsorption spontaneously. We utilize infrared photon dissociation (IR-PD) spectroscopy of isolated and size selected clusters under cryogenic conditions within a buffer gas filled ion trap, and we augment our experiments by quantum chemical simulations (at DFT level). All Tan+ clusters, n = 2-4, seem to cleave N2 efficiently. We confirm and extend a previous study under ambient conditions on Ta2+ cluster [Geng et al., Proc. Natl. Acad. Sci. U. S. A. 115, 11680-11687 (2018)]. Our cryo studies and the concomitant DFT simulations of the tantalum trimer Ta3+ suggest cleavage of the first and activation of the second and third N2 molecule across surmountable barriers and along much-involved multidimensional reaction paths. We unravel the underlying reaction processes and the intermediates involved. The study of the N2 adsorbate complexes of Ta4+ presented here extends our earlier study and previously published spectra from (4,m), m = 1-5 [Fries et al., Phys. Chem. Chem. Phys. 23(19), 11345-11354 (2021)], up to m = 12. We confirm the priory published double activation and nitride formation, succeeded by single side-on N2 coordination. Significant red shifts of IR-PD bands from these side-on coordinated μ2-κN:κN,N N2 ligands correlate with the degree of tilting towards the second coordinating Ta center. All subsequently attaching N2 adsorbates onto Ta4+ coordinate in an end-on fashion, and we find clear evidence for co-existence of end-on coordination isomers. The study of stepwise N2 adsorption revealed adsorption limits m(max) of [Tan(N2)m]+ which increase with n, and kinetic fits revealed significant N2 desorption rates upon higher N2 loads. The enhanced absolute rate constants of the very first adsorbate steps kabs(n,0) of the small Ta3+ and Ta4+ clusters independently suggest dissociative N2 adsorption and likely N2 cleavage into Ta nitrides.

Isonitrile μ2-carbido complexes

The μ-carbido complex [WPt(μ-C)Br(CO)2(PPh3)2(Tp*)] (Tp = hydrotris(dimethylpyrazolyl)borate) undergoes substitution of one phosphine ligand with isonitriles to afford complexes [WPt(μ-C)Br(CNR)(CO)2(PPh3)(Tp*)] (R = tBu, C6H3Me2-2,6, C6H2Me3-2,4,6). For aryl but not alkyl isocyanides disubstitution follows to afford [WPt(μ-C)Br(CNR)2(CO)2(Tp*)] (R = C6H2Me2-2,6, C6H2Me3-2,4,6). The bis(isonitrile) derivatives, including [WPt(μ-C)Br(CNtBu)2(CO)2(Tp*)], may also be prepared from the reactions of triangulo-[Pt3(CNR)6] with [W(CBr)(CO)2(Tp*)]. Bis- and tris(dimethylpyrazolyl)borate pro-ligand salts replace the bromide and one phosphine in [WPt(μ-C)Br(CNC6H2Me3)(CO)2(PPh3)(Tp*)] or the bromide and one isonitrile in [WPt(μ-C)Br(CNC6H2Me3)2(CO)2(Tp*)] to afford [WPt(μ-C)(CNC6H2Me3)(CO)2(Tp*)(L)] (L = κ2-Tp*, dihydrobis(pyrazolyl)borate). Structural, spectroscopic and computational data for the complexes are discussed to interrogate the nature of the WC-Pt carbido bridge by analogy with a range of other sp-C1 and sp-B1 ligands (CN, CCH, CP, CAs, CSb, CNO, BO, BNH and BCH2).

An Experimental and Computational Investigation Rules Out Direct Nucleophilic Addition on the N2 Ligand in Manganese Dinitrogen Complex [Cp(CO)2 Mn(N2 )]

We have re-examined the reactivity of the manganese dinitrogen complex [Cp(CO)2 Mn(N2 )] (1, Cp=η5 -cyclopentadienyl, C5 H5 ) with phenylithium (PhLi). By combining experiment and density functional theory (DFT), we have found that, unlike previously reported, the direct nucleophilic attack of the carbanion onto coordinated dinitrogen does not occur. Instead, PhLi reacts with one of the CO ligands to provide an anionic acylcarbonyl dinitrogen metallate [Cp(CO)(N2 )MnCOPh]Li (3) that is stable only below -40 °C. Full characterization of 3 (including single crystal X-ray diffraction) was performed. This complex decomposes quickly above -20 °C with N2 loss to give a phenylate complex [Cp(CO)2 MnPh]Li (2). The latter compound was erroneously formulated as an anionic diazenido compound [Cp(CO)2 MnN(Ph)=N]Li in earlier reports, ruling out the claimed and so-far unique behavior of the N2 ligand in 1. DFT calculations were run to explore both the hypothesized and the experimentally verified reactivity of 1 with PhLi and are fully consistent with our results. Direct attack of a nucleophile on metal-coordinated N2 remains to be demonstrated.

Unusual Hydrogenation Reactivities of a Thiolate-Bridged Dicobalt μ-Nitride Featuring a Bent {CoIII-N-CoIII} Core

Transition metal nitrides have received considerable attention owing to their crucial roles in nitrogen fixation and nitrogen atom transfer reactions. Compared to the early and middle transition metals, it is much more challenging to access late transition metal nitrides, especially cobalt in group 9. So far, only a handful of cobalt nitrides have been reported; consequently, their hydrogenation reactivity is largely unexplored. Herein, we present a structurally and spectroscopically well-characterized thiolate-bridged dicobalt μ-nitride [Cp*CoIII(μ-SAd)(μ-N)CoIIICp*] (2) featuring a bent {CoIII(μ-N)CoIII} core. Remarkably, complex 2 can realize not only direct hydrogenation of nitride to amide but also stepwise N-H bond formation from nitride to ammonia. Specifically, 2 can facilely activate dihydrogen (H2) at mild conditions to generate a dicobalt μ-amide [Cp*CoII(μ-SAd)(μ-NH2)CoIICp*] (4) via an unusual mechanism of two-electron oxidation of H2 as proposed by computational studies; in the presence of protons (H+) and electrons, nitride 2 can convert to dicobalt μ-imide [Cp*CoIII(μ-SAd)(μ-NH)CoIIICp*][BPh4] (3[BPh4]) and to CoIICoII μ-amide 4, and finally release ammonia. In contrast to 2, the only other structurally characterized dicobalt μ-nitride Na(THF)4{[(ketguan)CoIII(N3)]2(μ-N)} (ketguan = [(tBu2CN)C(NDipp)2]-, Dipp = 2,6-diisopropylphenyl) (e) that possesses a linear {CoIII(μ-N)CoIII} moiety cannot directly react with H2 or H+. Further in-depth electronic structure analyses shed light on how the varying geometries of the {CoIII(μ-N)CoIII} moieties in 2 and e, bent vs linear, impart their disparate reactivities.

Probing Electronic Effects in Tridentate Copper(I) Complexes by CIVP Spectroscopy

Ligand electronic effects play an important role in catalysis, where small changes to ligand structure can bring about large changes in catalytic activity. Therefore, accurate experimental quantification of ligand electronic properties plays a crucial role in understanding and tuning chemical reactivity. In this work, we used cryogenic ion vibrational predissociation (CIVP) spectroscopy to experimentally quantify electronic effects in terpyridine ligands, as simple model systems, by measuring CIVP spectra of their copper complexes tagged by N2 molecules. We used the N2 stretching vibration as a reporter chromophore to probe electronic effects of the investigated ligands and employed quantum chemical calculations to better understand how different substituents influence the vibrational frequencies of the stretching vibration of the chromophore. Our data show that the electronic character, as well as position and number of substituents, can affect the N≡N vibrational frequency, and that the N≡N bond serves as a sensitive probe for electronic and steric effects.

Binding of Nitriles and Isonitriles to V(III) and Mo(III) Complexes: Ligand vs Metal Controlled Mechanism

The synthesis and structures of nitrile complexes of V(N[^tBu]Ar)₃, 2 (Ar = 3,5-Me₂C₆H₃), are described. Thermochemical and kinetic data for their formation were determined by variable temperature Fourier transform infrared (FTIR), calorimetry, and stopped-flow techniques. The extent of back-bonding from metal to coordinated nitrile indicates that electron donation from the metal to the nitrile plays a less prominent role for 2 than for the related complex Mo(N[^tBu]Ar)₃, 1. Kinetic studies reveal similar rate constants for nitrile binding to 2, but the activation parameters depend critically on the nature of R in RCN. Activation enthalpies range from 2.9 to 7.2 kcal·mol^-1, and activation entropies from -9 to -28 cal·mol^-1·K^-1 in an opposing manner. Density functional theory (DFT) calculations provide a plausible explanation supporting the formation of a π-stacking interaction between a pendant arene of the metal anilide of 2 and the arene substituent on the incoming nitrile in favorable cases. Data for ligand binding to 1 do not exhibit this range of activation parameters and are clustered in a small area centered at ΔH^‡ = 5.0 kcal·mol^-1 and ΔS^‡ = -26 cal·mol^-1·K^-1. Computational studies are in agreement with the experimental data and indicate a stronger dependence on electronic factors associated with the change in spin state upon ligand binding to 1.

Progress in the chemistry of molecular actinide-nitride compounds

The chemistry of actinide-nitrides has witnessed significant advances in the last ten years with a large focus on uranium and a few breakthroughs with thorium. Following the early discovery of the first terminal and bridging nitride complexes, various synthetic routes to uranium nitrides have since been identified, although the range of ligands capable of stabilizing uranium nitrides still remains scarce. In particular, both terminal- and bridging-nitrides possess attractive advantages for potential reactivity, especially in light of the recent development of uranium complexes for dinitrogen reduction and functionalization. The first molecular thorium bridged-nitride complexes have also been recently identified, anticipating the possibility of expanding nitride chemistry not only to low-valent thorium, but also to the transuranic elements.

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.

Prospects and challenges for nitrogen-atom transfer catalysis

Conversion of C-H bonds to C-N bonds via C-H amination promises to streamline the synthesis of nitrogen-containing compounds. Nitrogen-group transfer (NGT) from metal nitrenes ([M]-NR complexes) has been the focus of intense research and development. By contrast, potentially complementary nitrogen-atom transfer (NAT) chemistry, in which a terminal metal nitride (an [M]-N complex) engages with a C-H bond, is underdeveloped. Although the earliest examples of stoichiometric NAT chemistry were reported 25 years ago, catalytic protocols are only now beginning to emerge. Here, we summarize the current state of the art in NAT chemistry and discuss opportunities and challenges for its development. We highlight the synthetic complementarity of NGT and NAT and discuss critical aspects of nitride electronic structure that dictate the philicity of the metal-supported nitrogen atom. We also examine the characteristic reactivity of metal nitrides and present emerging strategies and remaining obstacles to harnessing NAT for selective, catalytic nitrogenation of unfunctionalized organic small molecules.

Visible Light-Induced Oxidation of Alcohols by a Luminescent Osmium(VI) Nitrido Complex: Evidence for the Generation of PhIO+ as a Highly Active Oxidant in the Presence of PhIO

Although alcohols are readily oxidized by a variety of oxidants, their oxidation by metal nitrido complexes is yet to be studied. We report herein visible-light-induced oxidation of primary and secondary alcohols to carbonyl compounds by a strongly luminescent osmium(VI) nitrido complex (OsN). The proposed mechanism involves initial rate-limiting hydrogen-atom transfer (HAT) from the α-carbon of the alcohol to OsN*. Attempts to develop catalytic oxidation of alcohols by OsN* using PhIO as the terminal oxidant resulted in the formation of novel osmium(IV) iminato complexes in which the nitrido ligand is bonded to a δ-carbon of the alcohol. Experimental and theoretical studies suggest that OsN* is reductively quenched by PhIO to generate PhIO⁺, which is a highly active oxidant that readily undergoes α- and δ-C-H activation of alcohols.

Achieving Control over the Reduction/Coupling Dichotomy of N2 by Boron Metallomimetics

We report a detailed computational and experimental study of the fixation and reductive coupling of dinitrogen with low-valent boron compounds. Consistent with our mechanistic findings, the selectivity toward nitrogen fixation or coupling can be controlled through either steric bulk or the reaction conditions, allowing for the on-demand synthesis of nitrogen chains. The electronic structure and intriguing magnetic properties of intermediates and products of the reaction of dinitrogen with borylenes are also elucidated using high-level computational approaches.

Synthesis of highly twinned ZnSe nanorods for enhancing N2 electrochemical conversion to NH3

In this work, we report an atomistic understanding of the hydrogenation behavior of a highly twinned ZnSe nanorod (T-ZnSe) with a large density of surface atomic steps and the activation of N₂ molecules adsorbed on its surface. Theoretical calculations suggest that the atomic steps are essential for the hydrogenation of T-ZnSe, which greatly enhances its catalytic activity. As a result, the T-ZnSe nanorods exhibit a significantly enhanced NH₃ production rate of 13.3 μg h^-1 mg^-1 and faradaic efficiency of 5.83% towards the NRR compared with the pristine ZnSe nanorods. This report offers an important pathway for the development of efficient catalysts for the NRR, and a versatile anion-exchange strategy for efficiently manipulating materials' functionalities.

Dinitrogen Functionalization Affording Chromium Diazenido and Side-on η2-Hydrazido Complexes

Isolation of key intermediate complexes in dinitrogen functionalization is crucial for elucidating the mechanistic details and further investigation. Herein, the synthesis and characterization of (μ-η¹:η¹-N₂)(η¹-N₂)-Cr(I) 3 and (η¹-N₂)₂-Cr(0) complexes 4 supported by Cp* (Cp* = C₅Me₅) and NHC ligands were reported. Further functionalization of Cr(0)-N₂ complex 4 with silyl halides delivered the key intermediates in the alternating pathway, the chromium diazenido complex 5 and the chromium side-on η²-hydrazido complex 6. Protonation of 6 led to the quantitative formation of N₂H₄. Moreover, the [η²-Me₃SiNNSiMe₃]^2- unit in 6 enabled N-C bond formation reactions with CO₂ and ^tBuNCO, giving the corresponding N,O-chelating hydrazidochromium complexes 7 and 8, respectively.

Conceptual and Practical Aspects of Metal-Organic Frameworks for Solid-Gas Reactions

The presence of site-isolated and well-defined metal sites has enabled the use of metal-organic frameworks (MOFs) as catalysts that can be rationally modulated. Because MOFs can be addressed and manipulated through molecular synthetic pathways, they are chemically similar to molecular catalysts. They are, nevertheless, solid-state materials and therefore can be thought of as privileged solid molecular catalysts that excel in applications involving gas-phase reactions. This contrasts with homogeneous catalysts, which are overwhelmingly used in the solution phase. Herein, we review theories dictating gas phase reactivity within porous solids and discuss key catalytic gas-solid reactions. We further treat theoretical aspects of diffusion within confined pores, the enrichment of adsorbates, the types of solvation spheres that a MOF might impart on adsorbates, definitions of acidity/basicity in the absence of solvent, the stabilization of reactive intermediates, and the generation and characterization of defect sites. The key catalytic reactions we discuss broadly include reductive reactions (olefin hydrogenation, semihydrogenation, and selective catalytic reduction), oxidative reactions (oxygenation of hydrocarbons, oxidative dehydrogenation, and carbon monoxide oxidation), and C-C bond forming reactions (olefin dimerization/polymerization, isomerization, and carbonylation reactions).

Lewis Structures and the Bonding Classification of End-on Bridging Dinitrogen Transition Metal Complexes

The activation of dinitrogen by coordination to transition metal ions is a widely used and promising approach to the utilization of Earth's most abundant nitrogen source for chemical synthesis. End-on bridging N₂ complexes (μ-η¹:η¹-N₂) are key species in nitrogen fixation chemistry, but a lack of consensus on the seemingly simple task of assigning a Lewis structure for such complexes has prevented application of valence electron counting and other tools for understanding and predicting reactivity trends. The Lewis structures of bridging N₂ complexes have traditionally been determined by comparing the experimentally observed NN distance to the bond lengths of free N₂, diazene, and hydrazine. We introduce an alternative approach here and argue that the Lewis structure should be assigned based on the total π-bond order in the MNNM core (number of π-bonds), which derives from the character (bonding or antibonding) and occupancy of the delocalized π-symmetry molecular orbitals (π-MOs) in MNNM. To illustrate this approach, the complexes cis,cis-[(^iPr4PONOP)MCl₂]₂(μ-N₂) (M = W, Re, and Os) are examined in detail. Each complex is shown to have a different number of nitrogen-nitrogen and metal-nitrogen π-bonds, indicated as, respectively: W≡N-N≡W, Re═N═N═Re, and Os-N≡N-Os. It follows that each of these Lewis structures represents a distinct class of complexes (diazanyl, diazenyl, and dinitrogen, respectively), in which the μ-N₂ ligand has a different electron donor number (total of 8e^-, 6e^-, or 4e^-, respectively). We show how this classification can greatly aid in understanding and predicting the properties and reactivity patterns of μ-N₂ complexes.

Dinitrogen Activation and Addition to Unsaturated C-E (E=C, N, O, S) Bonds Mediated by Transition Metal Complexes

Dinitrogen (N₂ ) activation and functionalization is of fundamental interest and practical importance. This review focuses on N₂ activation and addition to unsaturated substrates, including carbon monoxide, carbon dioxide, heteroallenes, aldehydes, ketones, acid halides, nitriles, alkynes, and allenes, mediated by transition metal complexes, which afforded a variety of N-C bond formation products. Emphases are placed on the reaction modes and mechanisms. We hope that this work would stimulate further explorations in this challenging field.

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 Synthesis at Room Temperature and Atmospheric Pressure from N2 : A Boron-Radical Approach

Ammonia, NH3 , is an essential molecule, being part of fertilizers. It is currently synthesized via the Haber-Bosch process, from the very stable dinitrogen molecule, N2 and dihydrogen, H2 . This process requires high temperatures and pressures, thereby generating ca 1.6 % of the global CO2 emissions. Alternative strategies are needed to realize the functionalization of N2 to NH3 under mild conditions. Here, we show that boron-centered radicals provide a means of activating N2 at room temperature and atmospheric pressure whilst allowing a radical process to occur, leading to the production of borylamines. Subsequent hydrolysis released NH4 + , the acidic form of NH3 . EPR spectroscopy supported the intermediacy of radicals in the process, corroborated by DFT calculations, which rationalized the mechanism of the N2 functionalization by R2 B radicals.

Facile Addition of B-H and B-B Bonds to an Iron(IV) Nitride Complex

The nitride ligand in the iron(IV) complex PhB(ⁱPr₂Im)₃Fe≡N reacts with boron hydrides to afford PhB(ⁱPr₂Im)₃FeN(B)H (B = 9-BBN (1), Bpin (2)) and with (Bpin)₂ to afford PhB(ⁱPr₂Im)₃FeN(Bpin)₂ (3). The iron(II) borylamido products have all been structurally and spectroscopically characterized, demonstrating facile insertion into B-H and B-B bonds by PhB(ⁱPr₂Im)₃Fe≡N. Density functional theory (DFT) calculations reveal that the quintet state (S = 2) is significantly lower in energy than the singlet (S = 0) and triplet (S = 1) states for all products. Stoichiometric reaction with (Bpin)₂ does not produce the mono-borylated iron imido species PhB(ⁱPr₂Im)₃FeN(Bpin). DFT calculations suggest that this is because PhB(ⁱPr₂Im)₃FeN(Bpin) is unstable toward disproportionation to the starting iron(IV) nitride and PhB(ⁱPr₂Im)₃FeN(Bpin)₂. Attempts at B-C bond insertion using phenyl- and benzyl-pinacol borane were unsuccessful, which we attribute to unfavorable kinetics.

Catalytic Nitrous Oxide Reduction with H2 Mediated by Pincer Ir Complexes

Reduction of nitrous oxide (N2O) with H2 to N2 and water is an attractive process for the decomposition of this greenhouse gas to environmentally benign species. Herein, a series of iridium complexes based on proton-responsive pincer ligands (1-4) are shown to catalyze the hydrogenation of N2O under mild conditions (2 bar H2/N2O (1:1), 30 °C). Among the tested catalysts, the Ir complex 4, based on a lutidine-derived CNP pincer ligand having nonequivalent phosphine and N-heterocyclic carbene (NHC) side donors, gave rise to the highest catalytic activity (turnover frequency (TOF) = 11.9 h-1 at 30 °C, and 16.4 h-1 at 55 °C). Insights into the reaction mechanism with 4 have been obtained through NMR spectroscopy. Thus, reaction of 4 with N2O in tetrahydrofuran-d8 (THF-d8) initially produces deprotonated (at the NHC arm) species 5NHC, which readily reacts with H2 to regenerate the trihydride complex 4. However, prolonged exposure of 4 to N2O for 6 h yields the dinitrogen Ir(I) complex 7P, having a deprotonated (at the P-arm) pincer ligand. Complex 7P is a poor catalytic precursor in the N2O hydrogenation, pointing out to the formation of 7P as a catalyst deactivation pathway. Moreover, when the reaction of 4 with N2O is carried out in wet THF-d8, formation of a new species, which has been assigned to the hydroxo species 8, is observed. Finally, taking into account the experimental results, density functional theory (DFT) calculations were performed to get information on the catalytic cycle steps. Calculations are in agreement with 4 as the TOF-determining intermediate (TDI) and the transfer of an apical hydrido ligand to the terminal nitrogen atom of N2O as the TOF-determining transition state (TDTS), with very similar reaction rates for the mechanisms involving either the NHC- or the P-CH2 pincer methylene linkers.