Borohydride refers to the anion [BH4]−, which is also called tetrahydridoborate, and its salts.[1] Borohydride or hydroborate is also the term used for compounds containing [BH4−nXn]−, where n is an integer from 0 to 3, for example cyanoborohydride or cyanotrihydroborate [BH3(CN)]− and triethylborohydride or triethylhydroborate [BH(CH2CH3)3]−. Borohydrides find wide use as reducing agentsinorganic synthesis. The most important borohydrides are lithium borohydride and sodium borohydride, but other salts are well known (see Table).[2] Tetrahydroborates are also of academic and industrial interest in inorganic chemistry.[3]
In the borohydride anion and most of its modifications, boron has a tetrahedral structure.[6] 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/year in 2002. The main use is for the reduction of sulfur dioxide to give sodium dithionite:
Dithionite is used to bleach wood pulp.[2] 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.[8] 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.
Some metal tetrahydroborates transform on heating to give metal borides. When the borohydride complex is volatile, this decomposition pathway is the basis of chemical vapor deposition (CVD), a way of depositing thin films of metal borides.[12] For example, zirconium diborideZrB2 and hafnium diborideHfB2 can be prepared through CVD of the zirconium(IV) tetrahydroborate Zr[BH4]4 and hafnium(IV) tetrahydroborate Hf[BH4]4:[12]
M[BH4]4 → MB2 + B2H6 + 5 H2
Metal diborides find uses as coatings because of their hardness, high melting point, strength, resistance to wear and corrosion, and good electrical conductivity.[12]
^Schlesinger, H. C.; Brown, H. R. (1940). "Metallo Borohydrides. III. Lithium Borohydride". J. Am. Chem. Soc. 62 (12): 3429–3435. doi:10.1021/ja01869a039.
^Schlesinger, H. C.; Brown, H. R.; Hoekstra, L. R. (1953). "Reactions of Diborane with Alkali Metal Hydrides and Their Addition Compounds. New Syntheses of Borohydrides. Sodium and Potassium Borohydrides". J. Am. Chem. Soc. 75: 199–204. doi:10.1021/ja01097a053.
^Hutchins, Robert O.; Hutchins, MaryGail K.; Crawley, Matthew L. (2007). "Sodium Cyanoborohydride". Encyclopedia of Reagents for Organic Synthesis. John Wiley & Sons. doi:10.1002/047084289X.rs059.pub2. ISBN978-0471936237.
^Jaroń, Tomasz; Wegner, Wojciech; Grochala, Wojciech (17 August 2018). "M[Y(BH4)4] and M2Li[Y(BH4)6−xClx] (M = Rb, Cs): new borohydride derivatives of yttrium and their hydrogen storage properties". Dalton Transactions. 42 (19): 6886–93. doi:10.1039/C3DT33048F. PMID23503711.
^Marks, T. J.; Kolb, J. R. (1977). "Borohydride". Chem. Rev. 77: 263. doi:10.1021/cr60306a004.
^Besora, M.; Lledós, A. (2008). "Coordination Modes and Hydride Exchange Dynamics in Transition Metal Tetrahydroborate Complexes". Structure and Bonding. 130: 149–202. doi:10.1007/430_2007_076. ISBN978-3-540-78633-7.
^Franz, H.; Fusstetter, H.; Nöth, H. (1976). "Borohydride". Z. Anorg. Allg. Chem. 427: 97–113. doi:10.1002/zaac.654270202.
^ abcJensen, J. A.; Gozum, J. E.; Pollina, D. M.; Girolami, G. S. (1988). "Titanium, Zirconium, and Hafnium tetrahydroborates as "tailored" CVD precursors for metal diboride thin films". J. Am. Chem. Soc. 110 (5): 1643–1644. doi:10.1021/ja00213a058.
