Boranes

Diborane

Boranes are hydrogen boride compounds ("boron hydrides") and their derivatives . The simplest compound in this class of substances is diborane B ₂ H ₆ . This is in equilibrium with monoborane, which dimerizes due to its hypovalency, so that the equilibrium is far on the side of the diborane.

${\displaystyle \mathrm {\ B_{2}H_{6}\ \rightleftharpoons \ \ 2\ BH_{3}} }$

history

The first gaseous substance, which was described as the hydride of boron and which burned with a green flame, was reported by F. Jones and RL Tylor in 1881. This gas was created by the action of hydrochloric acid on magnesium boride. In the further course of the investigation, various formulas were proposed for this compound, according to BH ₃ . It was not until Alfred Stock , who developed borane chemistry in the years around the First World War , carried out the first more detailed investigations and published the results in 1912 that the gas formed during the reaction of magnesium diboride with hydrochloric acid was a mixture of B ₄ H ₁₀ and B. ₆ H ₁₂ is. The B ₆ H ₁₂ described by Stock later turned out to be a mixture of B ₄ H ₁₀ , B ₅ H ₉ and B ₆ H ₁₀ . It was not until 1964 that DF Gaines and R. Schaeffer synthesized pure B ₆ H ₁₂ . During the Second World War, research, especially the synthesis possibilities, was intensified by Hermann Irving Schlesinger and Herbert Charles Brown , because the boranes aroused interest in connection with uranium enrichment (as U (BH ₄ ) ₄ ) and as rocket fuel .

For a long time, the structural problem of the diborane B ₂ H _{6 was} worked on and proposals for structural formulas were made using classical valence theory. Final clarifications were made in 1947 and 1951 by investigations with a high-resolution infrared spectrometer and with X-ray structure analyzes with single crystals at low temperatures. These investigations led to a break with the classic valence stroke concept and, taking into account the molecular orbital theory , the formulation as a BHB three -center bond in addition to the terminal BH two-center bonds.

properties

The chemistry of boranes and related carbaboranes as well as metallaboranes is one of the most diverse fields of development in inorganic chemistry. Numerous neutral boranes B _n H _m , borane anions B _n H _m ^y−, and borane cations B _n H _m ^{x +} were synthesized.

Boranes are electron deficiency compounds because there are more atoms covalently linked than there are electron pairs. This leads to unusually high bond orders and coordination numbers. Multicenter bonds , mostly three- center bonds , are decisive for this .

According to stoichiometry and structure, the boranes are divided into hypercloso-, closo-, nido-, arachno-, hypho-, commo- and conjuncto-boranes. The geometry of these borane structures is determined by the ratio of the number of framework electrons to the number of framework atoms. The structures can be determined with the Wade rule , for more complicated, intermingled borane clusters (e.g. commo-boranes) with the mno rule according to Balakrishnarajan and for very large boranes with the (6m + 2n) rule according to Paul determined by Ragué Schleyer .

closo-dodeca boranate anion B ₁₂ H ₁₂ ²⁻ icosahedron

In particular the closo-boranes such as B ₆ H ₆ ²⁻ , B ₉ H ₉ ²⁻ , B ₁₀ H ₁₀ ²⁻ , B ₁₂ H ₁₂ ²⁻ , B ₂₁ H ₁₈ ^- and B ₂₀ H ₁₆ are in comparison with simpler boranes like B ₂ H ₆ or B ₁₀ H ₁₄ very stable. This stability is mainly based on steric effects, high symmetry and the fact that the closed cage structures of the closo-boranes are built without bridging hydrogen. So has B ₁₂ H ₁₂ ^2- , the structure of particularly stable B ₁₂ - icosahedron , which forms the basis of the various boron modifications. William Lipscomb received the Nobel Prize in Chemistry in 1976 for his work on boranes and carbaboranes .

The conversion of diborane B ₂ H ₆ with oxygen to boron trioxide B ₂ O ₃ is one of the strongest exothermic reactions known. This especially aroused interest in the military because of its use as a rocket fuel, so that intensive basic research was carried out in this area. After 15 years, military research was discontinued due to various problems: the substances involved in the reaction are unstable, strong odor, poisonous and, above all, sticky, which rules out their use in engines.

The salts of boranes are referred to as boranates , hydroborates or hydridoboranates, the corresponding anions as boranate ions (monoboranate / tetrahydroboranate / tetrahydridoboranate BH ₄ ^- , diboranate B ₂ H ₇ ^- and decaboranate B ₁₀ H ₁₀ ^- ). They are used as reducing and hydrogenating agents. Important representatives are sodium boranate and lithium boranate . Sodium borate is produced by the reaction of sodium hydride with diborane.

${\displaystyle \mathrm {2\ NaH\ +\ B_{2}H_{6}\longrightarrow \ 2\ NaBH_{4}} }$

Manufacture in the laboratory

Borane can easily be produced in situ as a tetrahydrofuran complex (BH ₃ · THF) from sodium boranate and elemental iodine .

${\displaystyle \mathrm {2\ NaBH_{4}\ +\ I_{2}\ {\xrightarrow[{}]{[THF]}}2\ NaI\ +\ H_{2}\ +\ 2\ BH_{3}-THF} }$

The borane-tetrahydrofuran complex is commercially available.

It can also be represented by reacting boron trifluoride with sodium hydride or lithium hydride :

${\displaystyle \mathrm {2\ BF_{3}\ +\ 6\ NaH\ \longrightarrow \ \ B_{2}H_{6}\ +\ 6\ NaF} }$

Higher boranes are produced from the diborane B ₂ H ₆ via reactions at higher temperatures, which are similar to the reforming process .

B ₁₂ H ₁₂ ²⁻ can be produced from boron trioxide, sodium and hydrogen at 600–850 ° C in an autoclave:

${\displaystyle \mathrm {6\ B_{2}O_{3}\ +\ 2\ Na\ +24\ H_{2}\ \longrightarrow \ Na_{2}B_{12}H_{12}\ +\ 18\ H_{2}O} }$

use

In the organic synthesis borane has two main applications. The first was hydroboration , discovered and researched by Herbert Charles Brown , for whose systematic investigation he received the 1979 Nobel Prize in Chemistry. In this reaction, water is formally added to a carbon-carbon double bond in an anti-Markovnikov addition . Borane is also used in the reduction of carboxylic acids or their derivatives to primary alcohols .

Furthermore, borane can be used by reacting with ammonia to produce ammonium borane , which is being discussed as a storage substance for hydrogen gas for vehicles.

literature

• Peter Paetzold : News from Boron and its Compounds, In: Chemistry in our time , 9th year 1975, No. 2, p. 73, ISSN  0009-2851
• Georg Süss-Fink : From the enfant terrible to the model boy: Die Borane, In: Chemistry in our time , 20th year 1986, No. 3, pp. 90-100, ISSN  0009-2851

Web links

Commons : Borane  - collection of images, videos and audio files

• HC Brown, Nobel Prize Lecture (PDF; 342 kB)
• Seminar University of Siegen (PDF; 93 kB)

Individual evidence

1. ↑ ASB Prasad, JVB Kanth, M. Periasamy, Tetrahedron 1992 , 48 , 4623-4628.