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]
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Identifiers | |||
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3D model (JSmol) |
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ChemSpider | |||
ECHA InfoCard | 100.033.949 | ||
EC Number |
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PubChem CID |
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CompTox Dashboard (EPA) |
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Properties | |||
K[BH4] | |||
Molar mass | 53.94 | ||
Density | 1.17 g/cm3 | ||
Melting point | 585 °C (1,085 °F; 858 K)[1] (under H2 atmosphere) | ||
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] | |||
Cubic (NaCl), cF8 | |||
Fm3m, No. 225 | |||
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. | |||
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Danger | |||
H260, H301, H314, H360FD | |||
P231+P232, P260, P280, P303+P361+P353, P304+P340+P310, P305+P351+P338 | |||
NFPA 704 (fire diamond) | |||
above 300°C[1] | |||
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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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]