Dinitrogen chemical studies

The chemical reactivity of coordinated N2 has been extensively studied because of its potential relevance to the catalytic and biological fixation of N2 to NH3 (p. 1035). For other recent work on the reactions of coordinated dinitrogen see refs. 41-44... 

Figure 7.7 shows how the law of chemical equilibrium applies to one chemical system. Chemists have studied this system extensively. It involves the reversible reaction between two gases dinitrogen tetroxide, which is colourless, and nitrogen dioxide, which is dark brown. 

The first site of protonation in a dinitrogen complex, based on mechanistic studies of complexes, must be the dinitrogen itself, yielding the diazenido species (N2H). If protonation occurs at the metal then reaction proceeds no further, or results in the loss of coordinated dinitro-gen. The formal insertion of dinitrogen into a metal-hydride bond (a popular proposal in the early chemical nitrogen-fixation literature) is unknown. 

The catalyst used throughout the study was 0.66% w/w Pt/alumina (B.E.T. surface area 184 mV). Sample weights of between 0.20 g. and 0,55 g, were used and these were reduced at 873 K in a 2% dihydrogen in dinitrogen stream (GHSV = 2950 hr-1) for 30 min. before use. After reduction the catalysts were flushed with helium at the reduction temperature and the temperature adjusted to the reaction temperature in a helium flow. The labelled gases used in the course of the study were [2H]dihydrogen (Cambrian Chemicals pic), [2-13C]-propane and... 

A. Ozaki, K. Aika Catalytic Activadon of Dinitrogen The third chapter is a comprehensive and critical review of studies on the catalytic activation of dinitrogen, including chemisorption and coordination of dinitrogen, kinetics and mechanism of ammonia synthesis, chemical and instrumental characterization of active catalysts, and homogeneous activation of dinitrogen including metal complexes (353 references). 

Today we have sophisticated lab equipment to help us analyze the products of reactions. In the past, when such equipment was not available, chemists sometimes jeopardized their safety and health to determine the products of the reactions they studied. Sir Humphry Davy (1778-1829), a contributor to many areas of chemistry, thought nothing of inhaling the gaseous products of the chemical reactions that he carried out. He tried to breathe pure C02, then known as fixed air. He nearly suffocated himself by breathing hydrogen. In 1800, Davy inhaled dinitrogen monoxide, N20, otherwise known as nitrous oxide, and discovered its anaesthetic properties. What is nitrous oxide used for today ... 

Bis(i -cyclopentadienyl)titanium or titanocene, (Tj-C5H5)2Ti (1), and bis(i7-cyclopentadienyl)zirconium or zirconocene, (i7-C5H5)2Zr (2), although frequently referred to in the literature, have never actually been isolated as discrete chemical compounds. However, these molecules have been implicated as highly reactive intermediates in a wide variety of chemical reactions with olefins, hydrogen, carbon monoxide, and dinitrogen. In recent years some discrete, well-characterized bis(7j-cyclopenta-dienyl) and bis(Tj-pentamethylcyclopentadienyl) complexes of low-valent titanium and zirconium have been isolated and studied, and it has become possible to understand some of the reasons for the remarkable reactivity of titanocene- and zirconocene-related organometallics toward small unsaturated molecules. 

A study of chemically induced dynamic electron polarization, CIDEP (see Section 12.3.3) on F and G pairs of radicals formed under photolysis of a common termo- and photoinitiator 2,2 -azobis(2-methylpropionitrile) (AIBN) led to a tentative conclusion that initial spatial separation of 2-cyano-2-propyl radicals does not depend upon viscosity However, it is plausible that the diamagnetic dinitrogen molecule formed under photolysis of AIBN (and is invisible by ESR) separates further from a contact RP under photolysis in solvents of lower viscosity. The problem of initial spatial separation and mutual orientation ofradicals under photolysis still waits experimental elucidation. 

The study of physical equilibrium yields useful information, such as the equilibrium vapor pressure (see Section 11.8). However, chemists are particularly interested in chemical equilibrium processes, such as the reversible reaction involving nitrogen dioxide (NO2) and dinitrogen tetroxide (N2O4) (Figure 14.1). The progress of the reaction... 

Azidoiodanes 448 and 449 were proposed as reactive intermediates in synthetically useful azidation reactions involving the combination of PhIO, PhI(OAc)2, or PhICl2 with trimethylsilyl azide or NaNs [595-610]. Attempts to isolate these intermediates always resulted in rapid decomposition at -25 to 0 °C with the formation of iodobenzene and dinitrogen however, low-temperature spectroscopy and the subsequent chemical reactions in situ provided some experimental evidence for the existence of these species. A systematic study of structure and reactivity of these unstable azidoiodanes was attempted by Zbiral and coworkers in the early 1970s [595-598]. In particular, the IR spectroscopic measurements at-60 to 0 °C indicated the existence of PhI(Ns)2 and PhI(N3)OAc species in the mixture of PhI(OAc)2 with trimethylsilyl azide [595]. Interestingly, the absorption band of 2040 cm for the azido group in PhI(N3)OAc is very close to the absorption stretch of 2046-2048 cm in the stable, heterocyclic azidoiodanes 122-124 (Section 2.1.8.1.5). 

The nitrogenase enzymes are the sole mediators of biological nitrogen fixation, the microbial reduction of dinitrogen to ammonia. The enzymatic conversion entails the intervention of a set of biometalloclusters with distinctive, often unique, properties. The enzyme system is complex and particularly resistant to biochemical and biophysical analysis at the molecular level, and the clusters themselves have very limited synthetic precedent in form and reactivity. As a result, a comprehensive chemical understanding of the nitrogenase clusters remains elusive despite decades of study. 

---