Boranes

Boranes are boron-hydrogen compounds ( " hydrides " ) and their derivatives. The simplest compound of this class is the diborane B2H6. This is in equilibrium with monoborane that due to its dimerization Hypovalenz so that the equilibrium lies far to the side of the diborane.

History

About the first gaseous substance, which has been described as a hydride of boron and burned with a green flame, reported 1881 F. Jones and RL Tylor. This gas was formed by the action of hydrochloric acid on magnesium boride. In the further course of the investigations various formulas have been proposed for this compound, as BH3. Only Alfred Stock, who extended the borane chemistry in the years around the First World War, the first accurate studies conducted and published in 1912 the results indicate that the gas formed in the reaction of magnesium diboride with hydrochloric acid is a mixture of B4H10 and B6H12. Later, the B6H12 described by floor proved to be a mixture of B4H10, B5H9 and B6H10. It was not until 1964, by D. F. Gaines and R. Schaeffer pure B6H12 synthesized. During the Second World War, the research, in particular the synthesis possibilities of Hermann Irving Schlesinger and Herbert Charles Brown were operated increased because the boranes encountered in connection with the uranium enrichment (as U ( BH4) 4) and as rocket fuel on interest.

For a long time working on the structure of the diborane B2H6 problem and made ​​proposals for structural formulas of the classical valence theory. Final clarifications introduced in 1947 and 1951 examinations with a high-resolution infrared spectrometer or with X-ray structural analyzes with single crystals at low temperatures. These studies led to a break with the classical valence bond concept and incorporating the molecular orbital theory to the formulation as BHB three-center bond in addition to the terminal BH two-center bonds.

Properties

The chemistry of boranes and related Carbaboranes and metallaboranes is one of the most diverse areas of development in inorganic chemistry. Numerous neutral boranes BnHm, BnHmy borane anions and cations Bora BnHmx were synthesized.

Boranes are electron-deficient compounds, as more atoms are covalently linked as pairs of electrons are present. This leads to unusually high bond orders and coordination numbers. Are decisive for this multicenter bonds, mostly three-center bonds.

A distinction boranes on the stoichiometry and structure in hypercloso, closo-, nido-, arachno -, hypho, commo and conjuncto -boranes. The geometry of these borane structures determined by the ratio of the number of electrons to the number of skeleton framework atoms. The structures can be personalized with the Wade- rule for more complex, interlocking verquickte borane cluster (eg commo -boranes ) with the mno rule after Balakrishnarajan and for very large boranes with the (6m 2 n ) rule according to Paul determine from Ragué Schleyer.

In particular, the closo -boranes as B6H62, B9H92, B10H102, B12H122, B21H18 and B20H16 are very stable in comparison with simpler boranes such as B2H6 or B10H14. This stability is mainly due to steric effects, high symmetry and the fact that the closed cage structures of the closo- boranes are built without bridging hydrogens. As B12H122 - has the structure of the particularly stable B12 icosahedron, which forms the basis of the various boron modifications. William Lipscomb was awarded the Nobel Prize in Chemistry for his work on boranes and Carbaboranes 1976.

The reaction of diborane B2H6 with oxygen to Borsesquioxid B2O3 is one of the most exothermic reactions, which are well known. This aroused interest especially in the military because of the use as rocket fuel, so in this area intensive fundamental research has been done. After 15 years of military research was discontinued because of different problems: the substances participating in the reaction are unstable, smelly, toxic and especially sticky, which precludes use in engines.

The salts of boranes are known as borohydrides, borates or Hydro Hydridoboranate, the corresponding anions as borohydride ions ( Monoboranat / Tetrahydroboranat / Tetrahydridoboranat BH4, Diboranat B2H7 and B10H10 - Dekaboranat ). They are used as reduction and hydrogenation. Important examples are sodium borohydride and lithium borohydride. Sodium borohydride is formed by reaction of sodium hydride with diborane.

Production in the laboratory

Borane can be easily prepared from sodium borohydride and elemental iodine in situ as tetrahydrofuran complex ( BH3 · THF).

Of borane -tetrahydrofuran complex is commercially available.

Also, the display is possible by reaction of boron trifluoride with sodium hydride or lithium hydride:

Higher boranes are prepared from diborane B2H6 above reactions at elevated temperatures that are similar to the reforming process.

B12H122 - can be produced at 600-850 ° C in an autoclave from boron trioxide, sodium and hydrogen:

Use

In organic synthesis borane has two main applications. For the first discovered and explored by Herbert Charles Brown hydroboration, for the systematic investigation of 1979, he was awarded the Nobel Prize for Chemistry. In this reaction, water is added in an anti - Markovnikov addition to a carbon- carbon double bond formally. Another important use is borane in the reduction of carboxylic acids or their derivatives to give primary alcohols.

Further borane may be used by reacting with ammonia to produce ammonia-borane, which is discussed as a memory material for hydrogen gas for vehicles.