Diborane: Chemical for Organic Synthesis

Jul 4, 2022 | SCIENCE, Chemistry, Pharmaceuticals

What Is Diborane? 

Diborane is a chemical compound with the formula B2H6. It is a colorless, toxic, highly flammable gas with a faintly sweet odor. Diborane is used in organic synthesis as a Lewis acid and dopant in semiconductor manufacturing. It is also used in rocket fuel mixtures.

Diborane

Diborane

 

History of Diborane

Diborane was initially synthesized in the 19th century through the hydrolysis of metal borides, but it was never a subject of research in the early days. Eventually, from 1912 to 1936, a study was conducted by Alfred Stock, a major researcher in boron hydrides chemistry, that resulted in methods for the synthesis and handling of the highly reactive, volatile boron hydrides that are toxic too. He proposed the first diborane – an ethane-like structure. S. H. Bauer’s electron diffraction measurements initially appeared to support his proposed structure. 

H.I. Schlessinger and A. B. Burg did not directly share 3-center 2-electron bonding in their review in the early 1940s due to some difference of opinion with L. Pauling (who supported the ethane-like structure). However, the review goes into detail about the ‘bridged D2h structure’.

H. Christopher Longuet-Higgins’ 1943 work on the structure and bonding of boron hydrides was a significant breakthrough in understanding these compounds. His paper, co-authored with his tutor R. P. Bell, reviews the subject’s history and confirms the structures deduced by Longuet-Higgins using infrared spectroscopy and K. Hedberg and V. Schomaker using electron diffraction in 1951.

In the 1950s, William Nunn Lipscomb Jr. used X-ray crystallography to confirm the understanding of boranes. He developed theories to explain their bonding and applied the same methods to related problems, such as carboranes’ structure. His work shed light on chemical bonding issues, earning him the Nobel Prize in Chemistry in 1976.

Structure and B-H Bonding

The molecule has D2h symmetry with four-terminal hydrogen atoms and two bridge bonds between boron atoms. The B-H bridge and terminal bonds have lengths of 1.33 and 1.19 angstroms, respectively. This length difference represents the relative strengths of the bonds, with B-H bridge bonds being weaker than B-H terminal bonds. The vibrational signatures of the B-H bridge bonds in the infrared spectrum, which are approximately 2100 and 2500 cm−1, indicate their weakness.

The molecular orbital theory describes a model where the bonds between boron and the terminal hydrogen atoms are the same as conventional 2-center, 2-electron covalent bonds. However, the bridging hydrogen atoms and bonding between the boron atoms differ from what we see in molecules such as hydrocarbons. 

Each boron bonds to the terminal hydrogen atoms with two electrons and has one valence electron left over for additional bonding. Each of the bridging hydrogen atoms provides one electron. Four electrons form two 3-center, 2-electron bonds that hold the B2H2 ring together, referred to as a banana bond.

Diborane has unusual bonding as its isoelectronic with C2H62+, which results from the diprotonation of the planar molecule ethylene. Other compounds with this type of bonding include digallane (Ga2H6) and Al2H6. Despite its instability, Al2H6 has been separated into solid hydrogen.

Synthesis 

Diborane is one of the most exciting compounds in organic chemistry, with extensive research that has resulted in the synthesis of many other compounds. The majority of preparations for diboration production involve hydride donors reacting directly or indirectly with boron halides and alkoxides. 

Industrially, it is synthesized by reducing BF3 with sodium hydride, lithium hydride, or lithium aluminum hydride. 

8 BF3 + 6 LiH → B2H6 + 6 LiBF4

In the laboratory, the synthesis process involves the reaction of boron trichloride with lithium aluminum hydride or boron trifluoride ether solution with sodium borohydride. Both methods produce up to 30 percent yield.

4 BCl3 + 3 LiAlH4 → 2 B2H6 + 3 LiAlCl4

4 BF3 + 3 NaBH4 → 2 B2H6 + 3 NaBF4

Conventional methods use the direct reaction of borohydride salts with a non-oxidizing acid, like phosphoric acid or dilute sulfuric acid.

2 BH4- + 2 H+ → 2 H2 + B2H6

Similarly, borohydride salt oxidation has been revealed and is still useful for small-scale preparations. As an example, consider using iodine as an oxidizer:

2 NaBH4 + I2 → 2 NaI + B2H6 + H2

Potassium hydroborate and phosphoric acid are also used for small-scale preparations. 

Reactions Using Diborane

  • It is a strongly reactive and versatile substance with numerous applications. Adduct formation with Lewis bases is one of its most common reaction patterns. These adducts frequently proceed to rapidly-produce other products.
  • It reacts with ammonia to form diammoniate of diborane (DADB) and smaller amounts of ammonia borane depending on the conditions. It also readily reacts to alkynes to yield substituted alkene products, which can further undergo addition reactions. 
  • Finally, it combines with water to form hydrogen and boric acid. As a result, this compound is a useful reagent for many different purposes.

B2H6 + 6 H2O → 2 B(OH)3 + 6 H2 

  • When combined with oxygen, it undergoes an exothermic reaction to produce boron trioxide and water.

2 B2H6 + 6 O2 → 2 B2O3 + 6 H2O 

  • When it reacts with methanol, it produces hydrogen and trimethoxyborate ester.

B2H6 + 6 MeOH → 2 B(OMe)3 + 6 H2

  • Diborane is treated with sodium amalgam to produce NaBH4 and Na[B3H8]. Lithium borohydride is formed when it is treated with lithium hydride in diethyl ether:

B2H6 + 2 LiH → 2 LiBH4

  • When it reacts with anhydrous hydrogen chloride or hydrogen bromide gas, a boron halohydride is formed:

B2H6 + HX → B2H5X + H2 (X = Cl, Br)

  • At 470K and 20bar, it reacts with carbon monoxide to form H3BCO.

A Reagent in Organic Synthesis

Diborane and its derivatives are essential reagents in hydroboration when alkenes are added across the B–H bonds to produce trialkyl boranes. These trialkyl boranes can then be further developed.

It is frequently used as a reducing agent due to its complementary reactivity to lithium aluminum hydride. It quickly reduces carboxylic acids to alcohols, whereas ketones react slowly.

Other Applications

Diborane is the essential compound in the borane family. It’s a rocket propellant and a precursor to metal boride films. It’s also been looked into for use in p-doping silicon semiconductors.

The molecule undergoes an exothermic reaction with oxygen, making it a potentially hazardous substance to work with. Although complete combustion is strongly exothermic, incomplete combustion can occur, resulting in carbon monoxide (CO) production as a byproduct. Because of its reactivity and flammability, this compound is also difficult to handle.

 

 

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