Nitrogen activation

Key points: The commercial Haber process requires high temperatures and pressures to yield ammonia, which is a major ingredient in fertilizers and an important chemical intermediate. Nitrogen occurs in many compounds, but N₂ itself, with a triple bond between the two atoms, is strikingly unreactive. A few strong reducing agents can transfer electrons to the N₂ molecule at room temperature, leading to scission of the N-N bond, but usually the reaction needs extreme conditions. The prime example of this reaction is the slow reaction of lithium metal at room tempera ture, which yields Li₃N. Similarly, when Mg (the diagonal neighbor of Li) burns in air it forms the nitride as well as the oxide. The slowness of the reactions of N₂ appears to be the result of several factors. One is the strength of the N-N triple bond and hence the high activation energy required for breaking it. (The strength of this bond also accounts for the lack of nitro gen allotropes.) Another factor is the relatively large size of the HOMO LUMO gap in N₂ (Section 2.8b), which makes the molecule resistant to simple electron-transfer redox processes. A third factor is the low polarizability of N₂, which does not encourage the formation of the highly polar transition states that are often involved in electrophilic and nucleophilic displacement reactions. Cheap methods of nitrogen activation, its conversion into useful compounds, are highly desirable because they would have a profound effect on the economy, particularly in poorer agricultural economies. In the Haber process for the production of ammonia, H₂ and N₂ are combined at high tempera tures and pressures over an Fe catalyst, as we discuss in detail in Section 15.10. Much of the recent research aimed at achieving more economical ways of activating N₂ has been inspired by the way in which bacteria carry out the transformation at room temperature. Catalytic conversion of nitrogen to NH₄ involves the metalloenzyme nitrogenase, which occurs in nitrogen-fixing bacteria such as those found in the root nodules of legumes. The mechanism by which nitrogenase carries out this reaction, at an active site containing Fe, Mo, and S, is the topic of considerable research. In this connection, dinitrogen complexes of metals were discovered in 1965, at about the same time that it was realized that nitrogenase contains Mo (Section 27.13). These developments led to optimism that efficient homogeneous catalysts might be developed in which metal ions would coordinate to N₂ and promote its reduction. Many N₂ complexes have in fact been prepared, and in some cases the preparation is as simple as bubbling N₂ through an aqueous solution of a complex:

As with the isoelectronic CO molecule, end-on bonding is typical of N2 when it acts as a ligand (9, Section 22.17). The NN bond length in the Ru (II) complex is only slightly altered from that in the free molecule. However, when N₂ is coordinated to a more strongly reducing metal centre, this bond is consider ably lengthened by back-donation of electron density into the π* orbitals of N₂.

A recent advance has been the direct reduction of N₂ to ammonia at room temperature and atmospheric pressure with a molybdenum catalyst that contains a tetradentate triamidoamine ligand known as HIPTN₃ N. Nitrogen coordinates to the Mo centre and is converted to NH₃ on addition of a proton source and a reducing agent.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم شؤون المعارف ينظم دورة متقدمة في تحقيق المخطوطات لملاكاته

قسم الشؤون الفكرية يصدر العدد العشرين من مجلة دراسات إفريقية

المجمَع العلمي يكرم الأساتذة المشاركين في مشروع الدورات القرآنية الصيفية في كربلاء

العتبة العباسية المقدسة تختتم فعاليات البرنامج المركزي الأسبوعي لمنتسبيها

المزيد

الآخبار الصحية

هل من الآمن شرب القهوة على معدة فارغة؟

تحذير: مرض السكري يسرق طاقة قلبك

حقيقة صادمة.. لماذا يجب أن تتوقف عن شراء الجزر الصغير؟

بعد عمر الستين.. توقيت وجبة الإفطار مؤشر على صحتك

المزيد

الآخبار التكنلوجية

تركيب مقعد الطفل بأمان في سيارتك: دليل كامل وسهل

مليار مليار عملية في الثانية.. ألمانيا تدشن الحاسوب الفائق

أخيرا.. إنستغرام يصل رسميا إلى آيباد

حل يثير جدلا طبيا.. حقنة قادرة على إخفاء الشيب من رأسك ؟!

المزيد

مواضيع ذات صلة

Phosphine, arsane, and stibine

Hydrazine and hydroxylamine

Ammonia

Phosphides

Azides

Nitrides

Microporous solids

Aluminosilicates

Compounds with nitrogen

Oxides of germanium, tin, and lead

Compounds of carbon with oxygen and sulfur

Tin and lead halides