Dinitrogen Complexes Research Articles

Two-step C-H, C-P bond activation at an α-diimine iron dinitrogen complex.

Reduction of 6-coordinate ((Ph2PPr)DI)FeBr₂ under N2 results in formation of the terminal dinitrogen complex, ((Ph2PPr)DI)FeN2. Heating this product to 75 °C allows for C-H and C-P activation of the chelate to generate the cisoid and transoid isomers of [(μ-PrPPh-κ(5)-P,N,N,Cγ,P-(Ph2PPr)DI(PrPPh))Fe]2. Mechanistic possibilities for this transformation are discussed.

Cobalt PNC(NHC) 'pincers': ligand dearomatisation, formation of dinuclear and N2 complexes and promotion of C-H activation.

Studies of ligand dearomatisation in cobalt phosphino-picoline N-heterocyclic carbene (PNC(NHC))-type pincers have revealed the formation of dinuclear cobalt complexes by either side-arm metalation or C-H activation at the NHC backbone; in the latter case, the product complex, featuring an 'anionic dicarbene', originated from a dearomatised Co(I)-N2 complex.

Rotational spectra of the Ne–N2 complex based on a new three-dimensional potential energy surface using neural networks

A new three-dimensional potential energy surface (PES) of the Ne–N2 van der Waals complex was constructed using the neural networks method based on ab initio data points at the CCSD(T) level. The aug-cc-pVQZ basis set was employed for all atoms, supplemented by midbond functions. The vibrationally averaged PES V00 is characterized by a global T-shaped minimum which occurs at R=3.385Å, θ=90.0° with a well depth of −49.202cm−1. Based on our three-dimensional PES, bound state calculations were performed for four isotopologues, i.e. 20Ne–14N2, 22Ne–14N2, 20Ne–15N2, 22Ne–15N2, and several intermolecular vibrational states were assigned by analyzing the wavefunctions. Moreover, the averaged structural parameters were determined and the pure rotational transition frequencies with J=0–5 are predicted. The spectroscopic constants were determined by fitting the rotational energy levels. The theoretical results are in good agreement with experimental data and this work gives more accurate results than those determined previously for the Ne–N2 complex.

A theoretical investigation of the energetics and spectroscopic properties of the gas-phase linear proton-bound cation-molecule complexes, XCH(+)-N2 (X = O, S).

The structural features, spectroscopic properties, and interaction energies of the linear proton-bound complexes of OCH(+) and its sulfur analog SCH(+) with N2 were investigated using the high-level ab initio methods MP2 and CCSD(T) as well as density functional theory with the aug-cc-pVXZ (X = D, T) basis sets. The rotational constants along with the vibrational frequencies of the cation-molecule complexes are reported here. A comparison of the interaction energies of the OCH(+)-N2 and SCH(+)-N2 complexes with those of the OCH(+)-CO and OCH(+)-OC complexes was also performed. The energies of all the complexes were determined at the complete basis set (CBS) limit. CS shows higher proton affinity at the C site than CO does, so the complex OCH(+)-N2 is relatively strongly bound and has a higher interaction energy than the SCH(+)-N2 complex. Symmetry-adapted perturbation theory (SAPT) was used to decompose the total interaction energies of the complexes into the attractive electrostatic interaction energy (E elst), induction energy (E ind), dispersion energy (E disp), and repulsive exchange energy (E exch). We found that the ratio of E ind to E disp is large for these linear proton-bound complexes, meaning that inductive effects are favored in these complexes. The bonding characteristics of the linear complexes were elucidated using natural bond orbital (NBO) theory. NBO analysis showed that the attractive interaction is caused by NBO charge transfer from the lone pair on N to the σ*(C-H) antibonding orbital in XCH(+)-N2 (X = O, S). The quantum theory of atoms in molecules (QTAIM) was used to analyze the strengths of the various bonds within and between the cation and molecule in each of these proton-bound complexes in terms of the electron density at bond critical points (BCP). Graphical Abstract Linear proton-bound complexes of OCH(+)-N2 and SCH(+)-N2. In these complexes, inductive effect is favored over dispersive effect. The attractive interaction is the NBO charge transfer from N-lone pair of N2 to CH σ* antibonding orbital of XCH(+) (X = O, S).

Electronic spectrum of the propargyl cation (H2C3H(+)) tagged with Ne and N2.

The Ã(1)A1 ← X̃(1)A1 band system of the propargyl cation (H2C3H(+)) is measured over the 230-270 nm range by photodissociation of mass-selected H2C3H(+)-Ne and H2C3H(+)-N2 complexes in a tandem mass spectrometer. The band origin occurs at 37 618 cm(-1) for H2C3H(+)-Ne and 37 703 cm(-1) for H2C3H(+)-N2. Ground and excited state ab initio calculations for H2C3H(+) using the MCSCF and coupled-cluster (CC) response methods show that the ion has C2v symmetry in the ground X̃(1)A1 and excited Ã(1)A1 states and that the strong vibronic progression with a spacing of 630 cm(-1) is due to the C-C stretch vibrational mode, ν 5.

ChemInform Abstract: Recent Progress in Transition‐Metal‐Catalyzed Reduction of Molecular Dinitrogen under Ambient Reaction Conditions

AbstractReview: [recent progress in catalytic nitrogen fixation; 51 refs.

Heterobimetallic Ti/Co Complexes That Promote Catalytic N-N Bond Cleavage.

Treatment of the tris(phosphinoamide) titanium precursor ClTi(XylNP(i)Pr2)3 (1) with CoI2 leads to the heterobimetallic complex (η(2)-(i)Pr2PNXyl)Ti(XylNP(i)Pr2)2(μ-Cl)CoI (2). One-electron reduction of 2 affords (η(2)-(i)Pr2PNXyl)Ti(XylNP(i)Pr2)2CoI (3), which can be reduced by another electron under dinitrogen to generate the reduced diamagnetic complex (THF)Ti(XylNP(i)Pr2)3CoN2 (4). The removal of the dinitrogen ligand from 4 under vacuum affords (THF)Ti(XylNP(i)Pr2)3Co (5), which features a Ti-Co triple bond. Treatment of 4 with hydrazine or methyl hydrazine results in N-N bond cleavage and affords the new diamagnetic complexes (L)Ti(XylNP(i)Pr2)3CoN2 (L = NH3 (6), MeNH2 (7)). Complexes 4, 5, and 6 have been shown to catalyze the disproportionation of hydrazine into ammonia and dinitrogen gas through a mechanism involving a diazene intermediate.

N2 Reduction into Silylamine at Tridentate Phosphine/Mo Center: Catalysis and Mechanistic Study

Transition-metal-catalyzed formation of silylamine from N2 (i.e., dinitrogen reduction) is achieved here using a tridentate phosphine/Mo system. Starting from the Mo(0) dinitrogen complex, stepwise reactions with chlorosilanes allowed the synthesis of the Mo(IV) hydrazido complex. Further reaction with two electrons resulted in the N–N bond splitting and formation of bis-silylamide and Mo(IV) nitrido complex. Facile addition of chlorosilane on the nitrido complex was observed to lead to the Mo(IV) imido complex. The four complexes (i.e., Mo(0) dinitrogen, Mo(IV) hydrazido, nitrido, and imido) perform equally well in the catalytic process between stoichiometric amounts of K and chlorosilane under N2 and are thus likely catalytic intermediates.

Dioxygen Activation by Non-Adiabatic Oxidative Addition to a Single Metal Center.

A chromium(I) dinitrogen complex reacts rapidly with O2 to form the mononuclear dioxo complex [Tp(tBu,Me)Cr(V)(O)2] (Tp(tBu,Me) = hydrotris(3-tert-butyl-5-methylpyrazolyl)borate), whereas the analogous reaction with sulfur stops at the persulfido complex [Tp(tBu,Me)Cr(III)(S2)]. The transformation of the putative peroxo intermediate [Tp(tBu,Me)Cr(III)(O2)] (S = 3/2) into [Tp(tBu,Me)Cr(V)(O)2] (S = 1/2) is spin-forbidden. The minimum-energy crossing point for the two potential energy surfaces has been identified. Although the dinuclear complex [(Tp(tBu,Me)Cr)2(μ-O)2] exists, mechanistic experiments suggest that O2 activation occurs on a single metal center, by an oxidative addition on the quartet surface followed by crossover to the doublet surface.

Dioxygen Activation by Non‐Adiabatic Oxidative Addition to a Single Metal Center

AbstractA chromium(I) dinitrogen complex reacts rapidly with O2 to form the mononuclear dioxo complex [TptBu,MeCrV(O)2] (TptBu,Me=hydrotris(3‐tert‐butyl‐5‐methylpyrazolyl)borate), whereas the analogous reaction with sulfur stops at the persulfido complex [TptBu,MeCrIII(S2)]. The transformation of the putative peroxo intermediate [TptBu,MeCrIII(O2)] (S=3/2) into [TptBu,MeCrV(O)2] (S=1/2) is spin‐forbidden. The minimum‐energy crossing point for the two potential energy surfaces has been identified. Although the dinuclear complex [(TptBu,MeCr)2(μ‐O)2] exists, mechanistic experiments suggest that O2 activation occurs on a single metal center, by an oxidative addition on the quartet surface followed by crossover to the doublet surface.

Theoretical studies of three-dimensional potential energy surfaces using neural networks and rotational spectra of the Ar–N2 complex

ABSTRACTA new three-dimensional potential energy surface (PES) of the Ar–N2 van der Waals complex is constructed using the neural network method based on ab initio data points at the CCSD(T) level. The aug-cc-pVQZ basis set is employed for all atoms with midbond functions. The vibrationally averaged PES V00 is characterised by a global T-shaped minimum which occurs at R = 3.715 Å, θ = 90.0° with a well depth of 98.779 cm−1. Based on our three-dimensional PES, bound-state calculations are performed for three isotopomers of Ar–14N2, Ar–15N2, and Ar–14N15N, and several intermolecular vibrational states are assigned by analysing the wavefunctions. Moreover, the averaged structural parameters are calculated and the pure rotational transition frequencies with J = 0--6 are predicted. The spectroscopic constants are determined by fitting the rotational energy levels. The theoretical results are in good agreement with experimental data and this work gives more accurate results than those determined previously for the Ar–N2 complex.

ChemInform Abstract: Molybdenum‐Catalyzed Reduction of Molecular Dinitrogen into Ammonia under Ambient Reaction Conditions

AbstractReview: 47 refs.

A Terminal N2 Complex of High-Spin Iron(I) in a Weak, Trigonal Ligand Field.

The role of Fe in biological and industrial N2 fixation has inspired the intense study of small molecule analogues of Fe-(NxHy) intermediates of potential relevance to these processes. Although a number of low-coordinate Fe-(N2) featuring varying degrees of fidelity to the nitrogenase active site are now known, these complexes frequently feature strongly donating ligands that either enforce low- or intermediate-spin states or result in linear Fe-(N2)-Fe bridging motifs. Given that the nitrogenase active site uses weak-field sulfide ligands to stabilize its reactive Fe center(s), N2 binding to high-spin Fe is of great interest. Herein, we report the synthesis and characterization of the first terminal N2 complex of high-spin (S = 3/2) Fe(I) as well as a bridging Fe-(N2)-Fe analogue. Electron paramagnetic resonance and solution magnetic moment determination confirm the high-spin state, and vibrational experiments indicate a substantial degree of activation of the N≡N bond in these complexes. Density functional theory calculations reveal an electronic structure for the terminal adduct featuring substantial delocalization of unpaired spin onto the N2 ligand.

N2 activation by an iron complex with a strong electron-donating iminophosphorane ligand.

A new tridentate cyclopentane-bridged iminophosphorane ligand, N-(2-diisopropylphosphinophenyl)-P,P-diisopropyl-P-(2-(2,6-diisopropylphenylamido)cyclopent-1-enyl)phosphoranimine (NpNPiPr), was synthesized and used in the preparation of a diiron dinitrogen complex. The reaction of the iron complex FeBr(NpNPiPr) with KC8 under dinitrogen yielded the dinuclear dinitrogen Fe complex [Fe(NpNPiPr)]2(μ-N2), which was characterized by X-ray analysis and resonance Raman and NMR spectroscopies. The X-ray analysis revealed a diiron complex bridged by the dinitrogen molecule, with each metal center coordinated by an NpNPiPr ligand and dinitrogen in a trigonal-monopyramidal geometry. The N–N bond length is 1.184(6) Å, and resonance Raman spectra indicate that the N–N stretching mode ν(14N2/15N2) is 1755/1700 cm–1. The magnetic moment of [Fe(NpNPiPr)]2(μ-N2) in benzene-d6 solution, as measured by 1H NMR spectroscopy by the Evans method, is 6.91μB (S = 3). The Mössbauer spectrum at 78 K showed δ = 0.73 mm/s and ΔEQ = 1.83 mm/s. These findings suggest that the iron ions are divalent with a high-spin configuration and that the N2 molecule has (N═N)2– character. Density functional theory calculations performed on [Fe(NpNPiPr)]2(μ-N2) also suggested that the iron is in a high-spin divalent state and that the coordinated dinitrogen molecule is effectively activated by π back-donation from the two iron ions (dπ) to the dinitrogen molecule (πx* and πy*). This is supported by cooperation between a large negative charge on the iminophosphorane ligand and strong electron donation and effective orbital overlap between the iron dπ orbitals and N2 π* orbitals supplied by the phosphine ligand.

Bimetallic cobalt-dinitrogen complexes: impact of the supporting metal on N2 activation.

Expanding a family of cobalt bimetallic complexes, we report the synthesis of the Ti(III) metalloligand, Ti[N(o-(NCH2P((i)Pr)2)C6H4)3] (abbreviated as TiL), and three heterobimetallics that pair cobalt with an early transition metal ion: CoTiL (1), K(crypt-222)[(N2)CoVL] (2), and K(crypt-222)[(N2)CoCrL] (3). The latter two complexes, along with previously reported K(crypt-222)[(N2)CoAlL] and K(crypt-222)[(N2)Co2L], constitute an isostructural series of cobalt bimetallics that bind dinitrogen in an end-on fashion, i.e. [(N2)CoML](-). The characterization of 1-3 includes cyclic voltammetry, X-ray crystallography, and infrared spectroscopy. The [CoTiL](0/-) reduction potential is extremely negative at -3.20 V versus Fc(+)/Fc. In the CoML series where M is a transition metal, the reduction potentials shift anodically as M is varied across the first-row period. Among the [(N2)CoML](-) compounds, the dinitrogen ligand is weakly activated, as evidenced by N-N bond lengths between 1.110(8) and 1.135(4) Å and by N-N stretching frequencies between 1971 and 1995 cm(-1). Though changes in νN2 are subtle, the extent of N2 activation decreases across the first-row period. A correlation is found between the [CoML](0/-) reduction potentials and N2 activation, where the more cathodic potentials correspond to lower N-N frequencies. Theoretical calculations of the [(N2)CoML](-) complexes reveal important variations in the electronic structure and Co-M interactions, which depend on the exact nature of the supporting metal ion, M.

Metal-Mediated Production of Isocyanates, R3 EN=C=O from Dinitrogen, Carbon Dioxide, and R3 ECl.

A highly efficient and versatile chemical cycle has been developed for the production of isocyanates through the molecular fixation of N2 , CO2 and R3 ECl (E=C, Si, and Ge). Key steps include a 'one-pot' photolytic N-N bond cleavage of a Group 6 dinuclear dinitrogen complex with in situ trapping by R3 ECl to provide a metal terminal imido complex that can engage in simultaneous nitrene-group transfer and oxygen-atom transfer to generate an intermediate metal terminal oxo complex with release of the isocyanate product. Reaction of the oxo complex with additional equivalents of R3 ECl regenerates a metal dichloride that is the precursor for dinuclear dinitrogen starting material.

Formation and Subsequent Reactivity of a N2-Stabilized Cobalt–Hydride Complex

The reduced heterobimetallic Co/Zr complex N2Co(iPr2PNMes)3Zr(THF) (1) has been previously reported to react with the C═O bonds of CO2 and benzophenone to generate Zr/Co μ-oxo complexes OC-Co(iPr2PNMes)2(μ-O)Zr(iPr2PNMes) (1-CO2) and Ph2C═Co(iPr2PNMes)2(μ-O)Zr(iPr2PNMes) (1-Ph2CO), respectively. Herein, we report a similar reaction of 1 with pyridine-N-oxide to form an analogous complex (pyridine)Co(iPr2PNMes)2(μ-O)Zr(iPr2PNMes) (2) with a more labile ligand bound to cobalt. Much like 1-CO2 and 1-Ph2CO, compound 2 reacts with Ph3SiH via formation of a Si–O linkage to form (N2)(H)Co(iPr2PNMes)3ZrOSiPh3 (5). The dinitrogen ligand in 5 is weakly bound and can be readily removed in vacuo or displaced by other L-type ligands. This allows complex 5 to undergo insertion reactions with unsaturated substrates, including diphenyldiazomethane, CO2, benzonitrile, and phenylacetylene to give hydrazonato (Ph2C═NNH)Co(iPr2PNMes)3ZrOSiPh3 (7), formate (OC(H)O)Co(iPr2PNMes)3ZrOSiPh3 (8), ketimide (PhHC═N)Co(iPr2PNMes)3ZrO...

Dinitrogen binding and cleavage by multinuclear iron complexes.

ConspectusThe iron–molybdenum cofactor of nitrogenase has unprecedented coordination chemistry, including a high-spin iron cluster called the iron-molybdenum cofactor (FeMoco). Thus, understanding the mechanism of nitrogenase challenges coordination chemists to understand the fundamental N2 chemistry of high-spin iron sites. This Account summarizes a series of studies in which we have synthesized a number of new compounds with multiple iron atoms, characterized them using crystallography and spectroscopy, and studied their reactions in detail. These studies show that formally iron(I) and iron(0) complexes with three- and four-coordinate metal atoms have the ability to weaken and break the triple bond of N2. These reactions occur at or below room temperature, indicating that they are kinetically facile. This in turn implies that iron sites in the FeMoco are chemically reasonable locations for N2 binding and reduction.The careful evaluation of these compounds and their reaction pathways has taught important lessons about what characteristics make iron more effective for N2 activation. Cooperation of two iron atoms can lengthen and weaken the N–N bond, while three working together enables iron atoms to completely cleave the N–N bond to nitrides. Alkali metals (typically introduced into the reaction as part of the reducing agent) are thermodynamically useful because the alkali metal cations stabilize highly reduced complexes, pull electron density into the N2 unit, and make reduced nitride products more stable. Alkali metals can also play a kinetic role, because cation−π interactions with the supporting ligands can hold iron atoms near enough to one another to facilitate the cooperation of multiple iron atoms. Many of these principles may also be relevant to the iron-catalyzed Haber–Bosch process, at which collections of iron atoms (often promoted by the addition of alkali metals) break the N–N bond of N2.The results of these studies teach more general lessons as well. They have demonstrated that N2 can be a redox-active ligand, accepting spin and electron density in complexes of N22–. They have shown the power of cooperation between multiple transition metals, and also between alkali metals and transition metals. Finally, alkali metal based cation−π interactions have the potential to be broadly useful for bringing metals close together with sufficient flexibility to allow multistep, multielectron reactions. At the same time, the positive charge on the alkali metal cation stabilizes charge buildup in intermediates.

Molybdenum-catalyzed reduction of molecular dinitrogen into ammonia under ambient reaction conditions

Synthesis of transition metal–dinitrogen complexes and stoichiometric transformations of their coordinated dinitrogen into ammonia and hydrazine have so far been well investigated in order to achieve a novel nitrogen fixation under ambient conditions. As an extension of our study, the dimolybdenum–dinitrogen complex bearing PNP pincer ligands has been found to work as an effective catalyst for the formation of ammonia from dinitrogen, where 52 equiv of ammonia are produced based on the catalyst (26 equiv of ammonia are produced based on the molybdenum atom of the catalyst). This is the most effective catalytic reaction system for the formation of ammonia from molecular dinitrogen catalyzed by transition metal–dinitrogen complexes as catalysts under ambient reaction conditions. Herein, we describe recent results concerning the catalytic reaction, including the proposed reaction pathway.

Synthesis and Protonation of N-Heterocyclic-Carbene-Supported Dinitrogen Complexes of Molybdenum(0)

The N-heterocyclic-carbene-supported dinitrogen (N2) complexes of molybdenum(0) trans-Mo(N2)2(LR)4 (LR = 1,3-R2-4,5-dimethylimidazol-2-ylidene; 2a: R = Me, 2b: R = Et) and mer-Mo(N2)3(LR)3 (3: R = iPr) were synthesized from MoCl4(THF)2 and KC8 in the presence of LR under an atmosphere of N2. In agreement with the characteristically strong σ-donation from the N-heterocyclic carbenes to molybdenum, complexes 2a, 2b, and 3 exhibited lower N–N stretching frequencies in their IR spectra and displayed longer N–N as well as shorter Mo–N bond lengths in their molecular structures relative to corresponding phosphine analogues. Although protonation of the molybdenum-bound N2 ligands in these phosphine complexes with H2O is unprecedented, it was accomplished for 2a, 2b, and 3 under concomitant release of ammonia.