Gas phase chemical equilibrium in dinitrogen trioxide and dinitrogen tetroxide

The ideal gas chemical thermodynamic properties for NO, NO₂, N₂O₃, and N₂O₄ for the temperature range 50 to 5000 K were evaluated by the statistical thermodynamic method using the most recent molecular parameters. In the calculations for NO and NO₂, the effects of anharmonicity and vibration—rotation interaction were included. The contributions due to centrifugal distortion were also included for NO₂. For evaluation of the thermodynamic properties for N₂O₃ and N₂O₄ molecules, the rigid-rotor and harmonic-oscillator model were adopted. A free internal rotation was assumed for N₂O₃ and an internal rotation barrier height (V₂) of 1.58 kcal mol⁻¹ was derived for N₂O₄. The thermodynamic properties due to hindered internal rotation were clculated using a partition function formed by summation of internal rotation energy levels. The thermodynamic properties for two equilibrium mixtures: NO₂---N₂O₄ and N₂O₃---NO---NO₂---N₂O₄ were also calculated. The effects of temperature and pressure on heat capacities and compositions of these two mixtures are illustrated graphically and the calculated heat capacities and equilibrim constants are in good agreement with available experimental values.     

Photochemical transformations of dinitrogen tetraoxide in a frozen glassy matrix

The presence of minima on the potential energy surface was shown by semiempirical calculations. These minima correspond to five isomers of dinitrogen tetraoxide: two symmetrical structures of the O₂N-NO₂ type, two nonsymmetrical structures of the O₂N-ONO type, and nitrosonium nitrate NO⁺NO₃ ^–. Quantum yields of photochemical reactions of dinitrogen tetraoxide in a matrix of glassy methylcyclohexane (at 77 K) were determined experimentally to be equal to l, 0.15, and 10^–5 for photoisomerization of symmetrical N₂O₄ into the nonsymmetrical isomer, reverse photoisome rization, and photodissoeiation to form the stabilized pair of NO₂ fragments, respectively. Measurements of the degree of orientation of the products of photochemical transformations performed by photolysis with polarized light show that the photoisomerization occurs via the intramolecular mechanism in a matrix cage, and the photodissoeiation is associated with considerable migration of the molecular fragments formed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 854–858, April, 1996.     

Selective electroreduction of dinitrogen on a rotating copper amalgam electrode

On an amalgamated rotating copper disk electrode in an alkaline methanol solution in the presence of mixed Ti(OH)₃–Mo(III) hydroxide, dinitrogen is shown to be selectively electroreduced to hydrazine in the potential range –1.6 to –1.9 V (with respect to the saturated calomel electrode).

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, Ti(OH)₃–Mo(III) –1,6+ –1,9 ( ) .

     

Studies of low-coordinate iron dinitrogen complexes

Understanding the interaction of N2 with iron is relevant to the iron catalyst used in the Haber process and to possible roles of the FeMoco active site of nitrogenase. The work reported here uses synthetic compounds to evaluate the extent of NN weakening in low-coordinate iron complexes with an FeNNFe core. The steric effects, oxidation level, presence of alkali metals, and coordination number of the iron atoms are varied, to gain insight into the factors that weaken the NN bond. Diiron complexes with a bridging N2 ligand, L(R)FeNNFeL(R) (L(R) = beta-diketiminate; R = Me, tBu), result from reduction of [L(R)FeCl]n under a dinitrogen atmosphere, and an iron(I) precursor of an N2 complex can be observed. X-ray crystallographic and resonance Raman data for L(R)FeNNFeL(R) show a reduction in the N-N bond order, and calculations (density functional and multireference) indicate that the bond weakening arises from cooperative back-bonding into the N2 pi orbitals. Increasing the coordination number of iron from three to four through binding of pyridines gives compounds with comparable N-N weakening, and both are substantially weakened relative to five-coordinate iron-N2 complexes, even those with a lower oxidation state. Treatment of L(R)FeNNFeL(R) with KC8 gives K2L(R)FeNNFeL(R), and calculations indicate that reduction of the iron and alkali metal coordination cooperatively weaken the N-N bond. The complexes L(R)FeNNFeL(R) react as iron(I) fragments, losing N2 to yield iron(I) phosphine, CO, and benzene complexes. They also reduce ketones and aldehydes to give the products of pinacol coupling. The K2L(R)FeNNFeL(R) compounds can be alkylated at iron, with loss of N2.     

Reaction of thebaine with dinitrogen tetroxide

Treatment of thebaine with dinitrogen tetroxide gave a mixture of 14β-nitrocodeinone (23%) and 8-nitrothebaine (7%).     

Organic chemistry of dinitrogen and related ligands. Part 2. Non-radical acylation of ligating dinitrogen

Summary Ligating dinitrogen, in the complexes,trans-M(N₂)₂(Ph₂PCH₂CH₂PPh₂)₂ (where M=Mo or W) may be acylated by trifluoroacetic anhydride, to give trifluoroacetyldiazenido complexes in high yield. Unlike corresponding reactions with acyl halides, trifluoroacetylation proceeds under nonradical conditions, and a mechanism analogous to that of protonation is suggested.Dinitrogen-derived ligands may also be acylated by (CF₃CO)₂O, so that the methyldiazenido ligand is converted into a methyl (trifluoroacetyl) hydrazido (2-) ligand, and the acetyldiazenido ligand undergoes transacylation to give a trifluoroacetyl diazenido derivative.New complexes were characterised by i.r.,¹⁹F and¹H n.m.r. spectra, and by elemental analysis.Part 1 is ref. 10     

Nitride formation by thermolysis of a kinetically stable niobium dinitrogen complex

The reduction of [P(2)N(2)]NbCl (where [P(2)N(2)] = PhP(CH(2)SiMe(2)NSiMe(2)CH(2))(2)PPh) with KC(8) under a dinitrogen atmosphere generates the paramagnetic dinuclear dinitrogen complex ([P(2)N(2)]Nb)(2)(mu-N(2)) (2). Complex 2 has been characterized crystallographically and by EPR spectroscopy. Variable-temperature magnetic susceptibility measurements indicate that 2 displays antiferromagnetic coupling between two Nb(IV) (d(1)) centers. A density functional theory calculation on the model complex [(PH(3))(2)(NH(2))(2)Nb](2)(mu-N(2)) was performed. Thermolysis of ([P(2)N(2)]Nb)(2)(mu-N(2)) in toluene generates the paramagnetic bridging nitride species where one N atom of the dinitrogen ligand inserts into the macrocycle backbone to form [P(2)N(2)]Nb(mu-N)Nb[PN(3)] (3) (where [PN(3)] = PhPMe(CHSiMe(2)NSiMe(2)CH(2)P(Ph)CH(2)SiMe(2)NSiMe(2)N)). Complex 3 has been characterized in the solid state as well as by variable-temperature magnetic susceptibility measurements. The reaction of ([P(2)N(2)]Nb)(2)(mu-N(2)) with phenylacetylene displaces the dinitrogen fragment to generate a paramagnetic eta(2)-alkyne complex, [P(2)N(2)]Nb(eta(2)-HCCPh) (4).     

A kinetic study of methane conversion by a dinitrogen microwave plasma

Conversion of CH₄ with a N₂ microwave plasma (2.45 GHz) is studied. The experiments cover the absorbed microwave power range 300–700 W with 17–62% of methane in the gas mixture, with pressures of 10–40 mbar and flow rates of 140–650 ml· min^–1. The yields of C₂ hydrocarbons and dihydrogen are analyzed by gas chromatography. The distance of methane addition downstream of the plasma plays an important role on the composition and the concentration of the products obtained. This distance mainly determines the energy concentrated in the active species of the plasma when they react with methane. Different behaviors for acetylene formation, on the one hand, and for ethane and ethene formation, on the other hand, have been observed, and this finding allows us to propose a kinetic mechanism for the decay of methane and for the formation of C₂ hydrocarbons.     

Activation of dinitrogen at binuclear sites

TMC Literature Highlights-22,Transition Met. Chem.,15, 251 (1990).     

Revisiting mechanistic studies on dinitrogen reduction to ammonia by an iron dinitrogen complex as nitrogenase mimic

Conversion of free nitrogen to ammonia is a required chemical reaction for both biologically and industrially but their mechanism, specifically the attachment of electron and proton transfer during the cycle, is still doubtful. In this view, a thorough knowledge of the mechanism is crucial. In this article, we employ a density functional method on [(TPB)FeN₂]⁻, the iron-dinitrogen complex carrying the tris(phosphine)borone (TPB) ligand, for the ammonia production with the inclusion of electrons and protons. The electronic structures, reactivity, and mechanistic possibilities have been extensively explored using the B3LYP functional. Both asymmetric and symmetric pathways in addition to the possible intermediates species and transition states are considered here. Our results conclude tremendously small energy barrier of 3.5 kJ/mol for the first protonation (S = 1/2) for the N─H bond activation by the [(TPB)FeN₂]⁻ species. However, high activation barrier for the third protonation was estimated to be 78.5 kJ/mol, which is explained by the high energy of the unoccupied δ_x²_-y² orbital in ¹ts4 species. The computed spectroscopic parameters such as absorption, electron paramagnetic resonance, and Mössbauer also established the electronic structure details of the species. The calculated parameters are compatible with the experimental results.     

Reactions of water, dihydrogen, dinitrogen, dinitrogen oxide, methane over illuminated ultraclean graphite

Some photocatalytic reactions of different gases on ultraclean graphite have been studied by means of quadrupole mass spectrometry. With water vapor, dihydrogen, dinitrogen and dinitrogen oxide, the UV-illuminated graphite acts as a reducing agent. TPD measurements indicate that photocatalytic processes are strongly affected by adsorption-desorption equilibria.     

The conversion of ligating dinitrogen into amines

Treatment of organohydrazido(2?)-complexes of molybdenum and tungsten with lithium aluminum hydride gives good yields of amines. Simple acid treatment gives essentially none, and base distillation yields around 40%.     

A rare terminal dinitrogen complex of chromium

Cis and trans-Cr-N(2) complexes supported by the diphosphine ligand P(Ph)(2)N(Bn)(2) have been prepared. Positioned pendant amines in the second coordination sphere influence the thermodynamically preferred geometric isomer. Electronic structure calculations indicate negligible Cr-N(2) back-bonding; rather, electronic polarization of N(2) ligand is thought to stabilize Cr-N(2) binding.     

Advances in molecular electrochemical activation of dinitrogen

Nitrogen fixation is an important and challenging transformation in chemistry. It is currently achieved by the Haber–Bosch process that reduces N₂ to NH₃ on large scale but at the cost of a huge environmental footprint. In the search for greener processes, electrochemically driven N₂ reduction might become a sustainable answer. The development of molecular homogeneous catalysis has gained momentum over the last decades, and we focus in this review on the emerging electrochemical systems based on molecular catalysts for N₂ fixation.     

Kinetic effects in heterometallic dinitrogen cleavage

The rhenium(I) dinitrogen complex (PhMe2P)4ClRe(N2) reacts with [Mo2(S2CNEt2)6](OTf)2 (6) to give the N(2)-bridged complex [(PhMe2P)4ClRe(mu-N2)Mo(S2CNEt2)3]OTf ([7]OTf). Spectroscopic (nu(NN) = 1818 cm(-1)) and structural data [d(NN) = 1.167(6) A] indicate that the bridging N(2) moiety in 7+ is slightly activated relative to free N2 or to the mononuclear Re complex. However, the complex is stable with respect to N2 cleavage. The putative products of such a cleavage, the known (Et2NCS2)3Mo(N) (5) and the newly prepared [(PhMe2P)(4)ClRe(N)]OTf ([9]OTf), are stable compounds that do not react with each other to give products of nitride coupling. Thus, the failure of 7+ to interconvert with 5 and 9+ is due not to the thermodynamic stability of the NN bond but rather to kinetic factors that disfavor N2 cleavage and nitride coupling. Implications of this result for using polar effects to facilitate N2 cleavage to nitrides as a strategy for nitrogen fixation are discussed.     

New avenues in dinitrogen fixation research

The present article focuses on a recent finding concerning dinitrogen fixation by using titanium oxide/conducting polymer composite systems and its comparison with an earlier fixation method (Schrauzer process) that makes use of a powdered titanium oxide. Both processes work under the stimulus of light at room temperature and pressure, but dinitrogen is fixed to a solid ammonium salt crystal in the former and to a gaseous ammonia molecule in the latter process. Differences in the physicochemical concepts between the two processes are discussed.