Closing the Loop on Transition-Metal-Mediated Nitrogen Fixation: Chemoselective Production of HN(SiMe3)2 from N2, Me3SiCl, and X-OH (X = R, R3Si, or Silica Gel)

Theoretical Investigations of Hydrolysis Mechanisms of N(SiMe3)3

Tris(trimethylsilyl)amine (N(SiMe3)3) is one of the most important intermediate products in the indirect synthesis of ammonia (NH3) from nitrogen (N2), which could be hydrolyzed to NH3 under mild conditions. Herein, the hydrolysis mechanism of N(SiMe3)3 has been systematically investigated using density functional theory (DFT) with explicit combined implicit water models. Under neutral conditions, the active barrier of the hydrolysis of N(SiMe3)3 is 17.6 kcal mol-1 in water solvent. The attacking of proton to N center and OH group to the Si atom from water is decoupled for the stabilization of OH group by solvent water molecules, which lower the hydrolysis energy barriers. Furthermore, under acid conditions, N(SiMe3)3 is easily coordinated with proton to form [NH(SiMe3)3]+, and the energy barrier of the hydrolysis reaction could be reduced to 11.5 kcal mol-1 of the first stage, making it being promoted according to the chemical equilibrium. Thus, the results provide an explanation for the possible mechanism of the quantitative conversion of N(SiMe3)3 to NH3 under mild conditions. The decoupled hydrolysis mechanism may play important role in other hydrolysis processes.

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.

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.

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.

Redox states of dinitrogen coordinated to a molybdenum atom

Chemical structures bearing a molybdenum atom have been suggested for the catalytic reduction of N₂ at ambient conditions. Previous computational studies on gas-phase MoN and MoN₂ species have focused only on neutral structures. Here, an ab initio electronic structure study on the redox states of small clusters composed of nitrogen and molybdenum is presented. The complete-active space self-consistent field method and its extension via second-order perturbative complement have been applied on [MoN]ⁿ and [MoN₂]ⁿ species (n = 0, 1±, 2±). Three different coordination modes (end-on, side-on, and linear NMoN) have been considered for the triatomic [MoN₂]ⁿ. Our results demonstrate that the reduced states of such systems lead to a greater degree of N₂ activation, which can be the starting point of different reaction channels.

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.

Synthesis of Dinuclear Mo-Fe Hydride Complexes and Catalytic Silylation of N2

Two transition-metal atoms bridged by hydrides may represent a useful structural motif for N₂ activation by molecular complexes and the enzyme active site. In this study, dinuclear Mo^IV -Fe^II complexes with bridging hydrides, Cp^R Mo(PMe₃ )(H)(μ-H)₃ FeCp* (2 a; Cp^R =Cp*=C₅ Me₅ , 2 b; Cp^R =C₅ Me₄ H), were synthesized via deprotonation of Cp^R Mo(PMe₃ )H₅ (1 a; Cp^R =Cp*, 1 b; Cp^R =C₅ Me₄ H) by Cp*FeN(SiMe₃ )₂ , and they were characterized by spectroscopy and crystallography. These Mo-Fe complexes reveal the shortest Mo-Fe distances ever reported (2.4005(3) Å for 2 a and 2.3952(3) Å for 2 b), and the Mo-Fe interactions were analyzed by computational studies. Removal of the terminal Mo-H hydride in 2 a-2 b by [Ph₃ C]⁺ in THF led to the formation of cationic THF adducts [Cp^R Mo(PMe₃ )(THF)(μ-H)₃ FeCp*]⁺ (3 a; Cp^R =Cp*, 3 b; Cp^R =C₅ Me₄ H). Further reaction of 3 a with LiPPh₂ gave rise to a phosphido-bridged complex Cp*Mo(PMe₃ )(μ-H)(μ-PPh₂ )FeCp* (4). A series of Mo-Fe complexes were subjected to catalytic silylation of N₂ in the presence of Na and Me₃ SiCl, furnishing up to 129±20 equiv of N(SiMe₃ )₃ per molecule of 2 b. Mechanism of the catalytic cycle was analyzed by DFT calculations.

Tuning the structure, reactivity and magnetic communication of nitride-bridged uranium complexes with the ancillary ligands

The reactivity of the nitride ligand is increased in complexes of uranium(iv) when bound by the OSi(O^tBu)₃ ligand as opposed to N(SiMe₃)₂, but magnetic exchange coupling is decreased.

Molecular uranium nitride complexes were prepared to relate their small molecule reactivity to the nature of the U Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> U bonding imposed by the supporting ligand. The U⁴⁺–U⁴⁺ nitride complexes, [NBu₄][{((^tBuO)₃SiO)₃U}₂(μ-N)], [NBu₄]-1, and [NBu₄][((Me₃Si)₂N)₃U}₂(μ-N)], 2, were synthesised by reacting NBu₄N₃ with the U³⁺ complexes, [U(OSi(O^tBu)₃)₂(μ-OSi(O^tBu)₃)]₂ and [U(N(SiMe₃)₂)₃], respectively. Oxidation of 2 with AgBPh₄ gave the U⁴⁺–U⁵⁺ analogue, [((Me₃Si)₂N)₃U}₂(μ-N)], 4. The previously reported methylene-bridged U⁴⁺–U⁴⁺ nitride [Na(dme)₃][((Me₃Si)₂)₂U(μ-N)(μ-κ²-C,N-CH₂SiMe₂NSiMe₃)U(N(SiMe₃)₂)₂] (dme = 1,2-dimethoxyethane), [Na(dme)₃]-3, provided a versatile precursor for the synthesis of the mixed-ligand U⁴⁺–U⁴⁺ nitride complex, [Na(dme)₃][((Me₃Si)₂N)₃U(μ-N)U(N(SiMe₃)₂)(OSi(O^tBu)₃)], 5. The reactivity of the 1–5 complexes was assessed with CO₂, CO, and H₂. Complex [NBu₄]-1 displays similar reactivity to the previously reported heterobimetallic complex, [Cs{((^tBuO)₃SiO)₃U}₂(μ-N)], [Cs]-1, whereas the amide complexes 2 and 4 are unreactive with these substrates. The mixed-ligand complexes 3 and 5 react with CO and CO₂ but not H₂. The nitride complexes [NBu₄]-1, 2, 4, and 5 along with their small molecule activation products were structurally characterized. Magnetic data measured for the all-siloxide complexes [NBu₄]-1 and [Cs]-1 show uncoupled uranium centers, while strong antiferromagnetic coupling was found in complexes containing amide ligands, namely 2 and 5 (with maxima in the χ versus T plot of 90 K and 55 K). Computational analysis indicates that the U(μ-N) bond order decreases with the introduction of oxygen-based ligands effectively increasing the nucleophilicity of the bridging nitride.

Steric Switching From Photochemical to Thermal N2 Splitting: A Computational Analysis of the Isomerization Reaction {(Cp*)(Am)Mo}2(μ-η1:η1-N2) → {(Cp*)(Am)Mo}2(μ-N)2

A μ-η1:η1-N2-bridged Mo dimer, {(η5-C5Me5)[N(Et)C(Ph)N(Et)]Mo}2(μ-N2), cleaves dinitrogen thermally resulting in a crystallographically characterized bis-μ-N-bridged dimer, {(η5-C5Me5)[N(Et)C(Ph)N(Et)]Mo}2(μ-N)2. A structurally related Mo dimer with a bulkier amidinate ligand, ([N(iPr)C(Me)N(iPr)]), is only capable of photochemical dinitrogen activation. These opposing reactivities were rationalized as steric switching between the thermally and photochemically active species. A computational analysis of the geometric and electronic structures of intermediates along the isomerization pathway from Mo2(μ-η1:η1-N2) to Mo2(μ-η2:η1-N2) and Mo2(μ-η2:η2-N2), and finally Mo2(μ-N)2, is presented here. The extent to which dispersion affects the thermodynamics of the isomers is evaluated, and it is found that dispersion interactions play a significant role in stabilizing the product and making the reaction exergonic. The concept of steric switching is further explored with theoretical models with sterically even less demanding ligands, indicating that systematic ligand modifications could be used to rationally design the N2 activation energy landscape. An analysis of electronic excitations in the computed UV-vis spectra of the two complexes shows that a particular type of asymmetric excitations is only present in the photoactive complex.

Facile N-functionalization and strong magnetic communication in a diuranium(v) bis-nitride complex

Uranium nitride complexes are of high interest because of their ability to effect dinitrogen reduction and functionalization and to promote magnetic communication, but studies of their properties and reactivity remain rare. Here we have prepared in 73% yield the diuranium(v) bis-nitride complex [K2{[U(OSi(O t Bu)3)3]2(μ-N)2}], 4, from the thermal decomposition of the nitride-, azide-bridged diuranium(iv) complex [K2{[U(OSi(O t Bu)3)3]2(μ-N)(μ-N3)}], 3. The bis-nitride 4 reacts in ambient conditions with 1 equiv. of CS2 and 1 equiv. of CO2 resulting in N-C bond formation to afford the diuranium(v) complexes [K2{[U(OSi(O t Bu)3)3]2(μ-N)(μ-S)(μ-NCS)}], 5 and [K2{[U(OSi(O t Bu)3)3]2(μ-N)(μ-O)(μ-NCO)}], 6, respectively. Both nitrides in 4 react with CO resulting in oxidative addition of CO to one nitride and CO cleavage by the second nitride to afford the diuranium(iv) complex [K2{[U(OSi(O t Bu)3)3]2(μ-CN)(μ-O)(μ-NCO)}], 7. Complex 4 also effects the remarkable oxidative cleavage of H2 in mild conditions to afford the bis-imido bridged diuranium(iv) complex [K2{[U(OSi(O t Bu)3)3]2(μ-NH)2}], 8 that can be further protonated to afford ammonia in 73% yield. Complex 8 provides a good model for hydrogen cleavage by metal nitrides in the Haber-Bosch process. The measured magnetic data show an unusually strong antiferromagnetic coupling between uranium(v) ions in the complexes 4 and 6 with Neel temperatures of 77 K and 60 K respectively, demonstrating that nitrides are attractives linkers for promoting magnetic communication in uranium complexes.

Catalytic Silylation of N2 and Synthesis of NH3 and N2H4 by Net Hydrogen Atom Transfer Reactions Using a Chromium P4 Macrocycle

We report the first discrete molecular Cr-based catalysts for the reduction of N₂. This study is focused on the reactivity of the Cr-N₂ complex, trans-[Cr(N₂)₂(P^Ph₄N^Bn₄)] (P₄Cr(N₂)₂), bearing a 16-membered tetraphosphine macrocycle. The architecture of the [16]-P^Ph₄N^Bn₄ ligand is critical to preserve the structural integrity of the catalyst. P₄Cr(N₂)₂ was found to mediate the reduction of N₂ at room temperature and 1 atm pressure by three complementary reaction pathways: (1) Cr-catalyzed reduction of N₂ to N(SiMe₃)₃ by Na and Me₃SiCl, affording up to 34 equiv N(SiMe₃)₃; (2) stoichiometric reduction of N₂ by protons and electrons (for example, the reaction of cobaltocene and collidinium triflate at room temperature afforded 1.9 equiv of NH₃, or at -78 °C afforded a mixture of NH₃ and N₂H₄); and (3) the first example of NH₃ formation from the reaction of a terminally bound N₂ ligand with a traditional H atom source, TEMPOH (2,2,6,6-tetramethylpiperidine-1-ol). We found that trans-[Cr(¹⁵N₂)₂(P^Ph₄N^Bn₄)] reacts with excess TEMPOH to afford 1.4 equiv of ¹⁵NH₃. Isotopic labeling studies using TEMPOD afforded ND₃ as the product of N₂ reduction, confirming that the H atoms are provided by TEMPOH.

Dinitrogen photoactivation: status quo and future perspectives

New perspectives for dinitrogen activation: an overview of photochemical pathways to cleave the strong N–N bond.