Borohydride

Borohydride refers to the anion BH−4 and its salts. Borohydride is also the term used for compounds containing BH4−nX−n, for example cyanoborohydride (B(CN)H−3) and triethylborohydride (B(C2H5)3H−). Borohydrides find wide use as reducing agents in organic synthesis. The most important borohydrides are lithium borohydride and sodium borohydride, but other salts are well known (see Table). Tetrahydroborates are also of academic and industrial interest in inorganic chemistry. Borohydride refers to the anion BH−4 and its salts. Borohydride is also the term used for compounds containing BH4−nX−n, for example cyanoborohydride (B(CN)H−3) and triethylborohydride (B(C2H5)3H−). Borohydrides find wide use as reducing agents in organic synthesis. The most important borohydrides are lithium borohydride and sodium borohydride, but other salts are well known (see Table). Tetrahydroborates are also of academic and industrial interest in inorganic chemistry. Alkali metal borohydrides were first described in 1940 by Hermann Irving Schlesinger and Herbert C. Brown. They synthesized lithium borohydride (LiBH4) from diborane (B2H6): Current methods involve reduction of trimethyl borate with sodium hydride. In the borohydride anion and most of its modifications, boron has a tetrahedral 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. Sodium borohydride is the borohydride that is produced on the largest scale industrially, estimated at 5000 tons/y in 2002. The main use is for the reduction of sulfur dioxide to give sodium dithionite: Dithionite is used to bleach wood pulp. Sodium borohydride is also used to reduce aldehydes and ketones in the production of pharmaceuticals including chloramphenicol, thiophenicol, vitamin A, atropine, and scopolamine, as well as many flavorings and aromas. Because of their high hydrogen content, borohydride complexes and salts have been of interest in the context of hydrogen storage. 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. In its coordination complexes, the borohydride ion is bound to the metal by means of one to three bridging hydrogen atoms. In most such compounds, the BH−4 ligand is bidentate. Some homoleptic borohydride complexes are volatile. One example is uranium borohydride. Metal borohydride complexes can often be prepared by a simple salt elimination reaction: