Borohydride

Potassium borohydride, also known as potassium tetrahydridoborate and potassium tetrahydroborate,^[7] is an inorganic compound with the formula KBH₄ (sometimes written as K[BH₄]). It was discovered together with sodium borohydride by Hermann Irving Schlesinger and his team in 1940s.^[8]

Potassium borohydride

Identifiers
CAS Number	13762-51-1
3D model (JSmol)	Interactive image
ChemSpider	55580
ECHA InfoCard	100.033.949
EC Number	237-360-5
PubChem CID	4192641
UNII	0WS230DTGF
CompTox Dashboard (EPA)	DTXSID50929853
InChI InChI=1S/BH4.K/h1H4;/q-1;+1 Key: ICRGAIPBTSPUEX-UHFFFAOYSA-N
SMILES [BH4-].[K+]
Properties
Chemical formula	K[BH4]
Molar mass	53.94
Density	1.17 g/cm3
Melting point	585 °C (1,085 °F; 858 K)[1] (under H2 atmosphere)
Solubility in water	19 g/100 mL
Solubility	Very soluble in liquid ammonia; insoluble in acetonitrile, benzene, dioxane, diethyl ether, hexane, isopropylamine, and tetrahydrofuran.[2]
Structure[3][4]
Crystal structure	Cubic (NaCl), cF8
Space group	Fm3m, No. 225
Lattice constant	a = 0.671 nm
Lattice volume (V)	302.1 Å3
Thermochemistry[5]
Std enthalpy of formation (ΔfH⦵298)	-228.86 kJ·mol−1
Hazards
GHS labelling:Sigma-Aldrich Co., product no. 455571.
Pictograms
Signal word	Danger
Hazard statements	H260, H301, H314, H360FD
Precautionary statements	P231+P232, P260, P280, P303+P361+P353, P304+P340+P310, P305+P351+P338
NFPA 704 (fire diamond)	[6] 3 2 0 W
Autoignition temperature	above 300°C[1]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Potassium borohydride is a white crystalline powder, non-hygroscopic and stable in dry air. In water solution, it is stable at room temperature, but decomposes in boiling water, releasing hydrogen gas. Alkaline solutions are stable up to boiling point.^[1]

Synthesis

Potassium borohydride can be prepared by metathesis reaction from potassium hydroxide and sodium borohydride.^[2] This method is used on industrial scale.^[1]

KOH + NaBH₄ → KBH₄ + NaOH

H.I. Schlesinger et al prepared it by reacting diborane with potassium tetramethoxyborohydride, produced from methyl borate and potassium methoxide. Such two-step reaction is needed because diborane does not react with potassium methoxide directly.^[8]

KOCH₃ + B(OCH₃)₃ → KB(OCH₃)₄

KB(OCH₃)₄ + 2B₂H₆ → 3KBH₄ + 4B(OCH₃)₃

Reactions and applications

As other borohydrides, potassium borohydride is a stong reducing agent. It reduces carbonyl compounds to corresponding alcohols,^[2] nitroarenes to azoxybenzenes.^[9][10]

Potassium borohydride and iodine produce boron triiodide in situ. Such mixture can reduce nitroarenes to anilines.^[11]

System of potassium borohydride and copper(I) chloride can reduce nitroarenes, nitrosoarenes, azoarenes and azoxybenzenes to anilines, with yields close to 100%. It also reduces iodo-substituents, but leaves chloro- and bromo-groups untouched.^[12]

In organic chemistry, potassium borohydride is used not as widely as sodium borohydride, due to lower reactivity and insolubility in many common organic solvents. Ways to improve reactivity were proposed, such as adding lithium ions to solution to form lithium borohydride in situ or suspending potassium borohydride and basic alumina in wet THF.^[13]

In inorganic chemistry, potassium borohydride can be used, for example, for reduction of potassium vanadate to obtain nanocrystalline vanadium oxides,^[14] and reduction of copper(II) sulfate to produce copper nanoparticles.^[15]

References

1. Rittmeyer, P.; Wietelmann, U.; Felderhoff, M. "Hydrides". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. pp. 12, 18–19. doi:10.1002/14356007.a13_199.pub2. ISBN 978-3527306732.
2. Douglas Banus, M.; Bragdon, Robert W.; Hinckley, Alfred A. (July 1, 1954). "Potassium, Rubidium and Cesium Borohydrides". Journal of the American Chemical Society. 76 (14): 3848–3849. doi:10.1021/ja01643a077.
3. Luck, R. L.; Schelter, E. J. (1999). "Potassium borohydride". Acta Crystallographica Section C Crystal Structure Communications. 55 (12): IUC9900151. Bibcode:1999AcCrC..55C0151L. doi:10.1107/S010827019909842X.
4. Kim, Ki Chul; Sholl, David S. (2010). "Crystal Structures and Thermodynamic Investigations of LiK(BH₄)₂, KBH₄, and NaBH₄ from First-Principles Calculations". The Journal of Physical Chemistry C. 114: 678–686. doi:10.1021/jp909120p.
5. Johnson, Walter H.; Schumm, Richard H.; Wilson, Isa H.; Prosen, Edward J. (1961). "Heats of hydrolysis and formation of potassium borohydride". Journal of Research of the National Bureau of Standards Section A: Physics and Chemistry. 65A (2): 97–99. doi:10.6028/jres.065a.011. PMC 5287272. PMID 32196230.
6. "SAFETY DATA SHEET - Potassium borohydride". ThermoFisher Scientific.
7. B. D. James; M. G. H. Wallbridge (1 January 1970). "Metal Tetrahydroborates". Progress in Inorganic Chemistry. Vol. 11. pp. 112, 136–139. doi:10.1002/9780470166123.ch3. ISBN 978-0-471-54081-6.
8. Schlesinger, H. I.; Brown, Herbert C.; Hoekstra, Henry R.; Rapp, Louis R. (1953). "Reactions of Diborane with Alkali Metal Hydrides and Their Addition Compounds. New Syntheses of Borohydrides. Sodium and Potassium Borohydrides". Journal of the American Chemical Society. 75: 199–204. doi:10.1021/ja01097a053.
9. Shine, H. J.; Mallory, H. E. (1962). "The Reduction of Aromatic Nitro Compounds by Potassium Borohydride". The Journal of Organic Chemistry. 27 (7): 2390–2391. doi:10.1021/jo01054a025.
10. Liu, Yufang; Liu, Bo; Guo, Ailing; Dong, Zhenming; Jin, Shuo; Lu, Yun (2011). "Reduction of Nitroarenes to Azoxybenzenes by Potassium Borohydride in Water". Molecules. 16 (5): 3563–3568. doi:10.3390/molecules16053563. PMC 6263276. PMID 21527883.
11. Ćorković, Andrej; Chiarella, Thomas; Williams, Florence J. (2023). "Boron Triiodide-Mediated Reduction of Nitroarenes Using Borohydride Reagents". Organic Letters. 25 (49): 8787–8791. doi:10.1021/acs.orglett.3c03257. PMC 10729015. PMID 38052021.
12. He, Yun; Zhao, He; Pan, Xinfu; Wang, Shaofei (1989). "Reduction with Metal Borohydride-Transition Metal Salt System. I. Reduction of Aromatic Nitro Compounds with Potassium Borohydride-Copper(I) Chloride". Synthetic Communications. 19 (17): 3047–3050. doi:10.1080/00397918908052699.
13. Darin, Vitor Andre; Neto, Alberto Federman; Borges, Aurea Donizete Lanchote; Miller, Joseph (1999). "Potassium Borohydride: Improved Use as a Reducing Agent for Organic and Organometallic Ketones". Main Group Metal Chemistry. 22 (8). doi:10.1515/MGMC.1999.22.8.505.
14. Tsang, Chi Fo; Kim, Jaekook; Manthiram, Arumugam (1998). "Synthesis of reduced vanadium oxides in aqueous solutions". Journal of Materials Chemistry. 8 (2): 425–428. doi:10.1039/A706273G.
15. Zhang, Qiu-li; Yang, Zhi-mao; Ding, Bing-jun; Lan, Xin-zhe; Guo, Ying-Juan (2010). "Preparation of copper nanoparticles by chemical reduction method using potassium borohydride". Transactions of Nonferrous Metals Society of China. 20: s240–s244. doi:10.1016/S1003-6326(10)60047-7.