Borohydride refers to the anion , which is also called tetrahydroborate, and its salts. Borohydride or hydroborate is also the term used for compounds containing , where ''n'' is an integer from 0 to 3, for example
cyanoborohydride
Sodium cyanoborohydride is the chemical compound with the formula Na B H3 CN. It is a colourless salt, but commercial samples can appear tan. It is widely used in organic synthesis for the reduction of imines. The salt tolerates aqueous conditions ...
or cyanotrihydroborate and
triethylborohydride or triethylhydroborate . Borohydrides find wide use as
reducing agent
In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ).
Examples of substances that are commonly reducing agents include the Earth meta ...
s in
organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
. The most important borohydrides are
lithium borohydride
Lithium borohydride (LiBH4) is a borohydride and known in organic synthesis as a reducing agent for esters. Although less common than the related sodium borohydride, the lithium salt offers some advantages, being a stronger reducing agent and ...
and
sodium borohydride
Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate, is an inorganic compound with the formula Na BH4. This white solid, usually encountered as an aqueous basic solution, is a reducing agent that finds applica ...
, but other salts are well known (see Table).
Tetrahydroborates are also of academic and industrial interest in inorganic chemistry.
History
Alkali metal
The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
borohydrides were first described in 1940 by
Hermann Irving Schlesinger
Hermann Irving Schlesinger (October 11, 1882 – October 3, 1960) was an American inorganic chemist, working in boron chemistry.
He and Herbert C. Brown discovered sodium borohydride in 1940 and both were involved in the further development of bo ...
and
Herbert C. Brown
Herbert Charles Brown (May 22, 1912 – December 19, 2004) was an American chemist and recipient of the 1979 Nobel Prize in Chemistry for his work with organoboranes.
Life and career
Brown was born Herbert Brovarnik in London, to Ukrainian Jewis ...
. They synthesized lithium borohydride from
diborane
Diborane(6), generally known as diborane, is the chemical compound with the formula B2H6. It is a toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Diborane is a key boron compound with a variety of applications. It has attracte ...
:
:, where M = Li, Na, K, Rb, Cs, etc.
Current methods involve reduction of
trimethyl borate
Trimethyl borate is the organoboron compound with the formula B(OCH3)3. It is a colourless liquid that burns with a green flame. It is an intermediate in the preparation of sodium borohydride and is a popular reagent in organic chemistry. It is ...
with sodium hydride.
[
]
Structure
In the borohydride anion and most of its modifications, boron
Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...
has a tetrahedral
In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the ...
structure. The reactivity of the B−H bonds depends on the other ligands. Electron-releasing ethyl groups as in triethylborohydride render the B−H center highly nucleophilic. In contrast, cyanoborohydride is a weaker reductant owing to the electron-withdrawing cyano substituent. The countercation also influences the reducing power of the reagent.
Uses
Sodium borohydride is the borohydride that is produced on the largest scale industrially, estimated at 5000 tons/year in 2002. The main use is for the reduction of sulfur dioxide
Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic activ ...
to give sodium dithionite
Sodium dithionite (also known as sodium hydrosulfite) is a white crystalline powder with a sulfurous odor. Although it is stable in dry air, it decomposes in hot water and in acid solutions.
Structure
The structure has been examined by Raman s ...
:
:
Dithionite is used to bleach wood pulp.[ Sodium borohydride is also used to reduce aldehydes and ketones in the production of pharmaceuticals including ]chloramphenicol
Chloramphenicol is an antibiotic useful for the treatment of a number of bacterial infections. This includes use as an eye ointment to treat conjunctivitis. By mouth or by injection into a vein, it is used to treat meningitis, plague, cholera, a ...
, thiophenicol
Thiamphenicol (also known as thiophenicol and dextrosulphenidol) is an antibiotic. It is the methyl-sulfonyl analogue of chloramphenicol and has a similar spectrum of activity, but is 2.5 to 5 times as potent. Like chloramphenicol, it is insolubl ...
, vitamin A
Vitamin A is a fat-soluble vitamin and an essential nutrient for humans. It is a group of organic compounds that includes retinol, retinal (also known as retinaldehyde), retinoic acid, and several provitamin A carotenoids (most notably bet ...
, atropine
Atropine is a tropane alkaloid and anticholinergic medication used to treat certain types of nerve agent and pesticide poisonings as well as some types of slow heart rate, and to decrease saliva production during surgery. It is typically given i ...
, and scopolamine, as well as many flavorings and aromas.
Potential applications
Because of their high hydrogen content, borohydride complexes and salts have been of interest in the context of hydrogen storage
Hydrogen storage can be accomplished by several existing methods of holding hydrogen for later use. These include mechanical approaches such as using high pressures and low temperatures, or employing chemical compounds that release H2 upon demand ...
. Reminiscent of related work on ammonia borane, challenges are associated with slow kinetics and low yields of hydrogen as well as problems with regeneration of the parent borohydrides.
Coordination complexes
In its coordination complex
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
es, the borohydride ion is bound to the metal by means of one to three bridging hydrogen atoms.[ In most such compounds, the ligand is bidentate. Some ]homoleptic
In inorganic chemistry, a homoleptic chemical compound is a metal compound with all ligands identical. The term uses the " homo-" prefix to indicate that something is the same for all. Any metal species which has more than one type of ligand is he ...
borohydride complexes are volatile. One example is uranium borohydride.
Metal borohydride complexes can often be prepared by a simple salt elimination reaction:
:
Decomposition
Some metal tetrahydroborates transform on heating to give metal boride A boride is a compound between boron and a less electronegative element, for example silicon boride (SiB3 and SiB6). The borides are a very large group of compounds that are generally high melting and are covalent more than ionic in nature. Some bo ...
s. When the borohydride complex is volatile, this decomposition pathway is the basis of chemical vapor deposition
Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.
In typical CVD, the wafer (substra ...
(CVD), a way of depositing thin films of metal borides. For example, zirconium diboride
Zirconium diboride (ZrB2) is a highly covalent refractory ceramic material with a hexagonal crystal structure. ZrB2 is an ultra high temperature ceramic (UHTC) with a melting point of 3246 °C. This along with its relatively low density of ...
and hafnium diboride
Hafnium diboride belongs to the class of ultra-high-temperature ceramics, a type of ceramic composed of hafnium and boron. It has a melting temperature of about 3250 °C. It is an unusual ceramic, having relatively high thermal and electrical ...
can be prepared through CVD of the zirconium(IV) tetrahydroborate and hafnium(IV) tetrahydroborate :[
:
Metal diborides find uses as coatings because of their hardness, high melting point, strength, resistance to wear and corrosion, and good electrical conductivity.][
]
References
External links
{{Wikiquote
Sodium Tetrahydroborate
Anions