Preparation of zinc borohydride solution

[demorol]

Sodium borohydride (5.58g 95%, 0.14 mole, 16.6% excess) and 300mL of THF were introduced in a 500mL oven-dried RB flask, fitted with a rubber cap on the side arm, a magnetic stirring bar, and a reflux condenser connected to a mercury bubbler. To the slurry solution 100mL of THF solution of zinc chloride (8.1774g, 0.6 mole) was added slowly with stirring by means of a hypodermic syringe. The solution was stirred for 3 days, and then allowed to stand at room temperature to permit the sodium chloride precipitate to settle down. Clear chloride-free supernatant solution was visualized after 2-3 h and an aliquot was removed by a syringe and analyzed for hydride gasometrically.

The slight excess (16.6%) of sodium borohydride greatly faciliates the quantitative precipitation of chloride as sodium chloride. The hydride concentration of the solution obtained was 1.37 M, which is larger by 14% than calculated value, 1.2 M based on zinc chloride added. This means that excess amount of sodium borohydride was dissolved in this hydride solution as Na[Zn(BH₄)₃] whereas sodium borohydride alone hardly dissolve in THF.

The solution was stored under positive nitrogen pressure without filtering out the precipitate and did not show any measerable change of the concentration over 30 days.

Reference: Journal of the Korean Chemical Society, Vol. 20, No. 1, 1976

[Lego]

A variation of the preparation

J. Org. Chem. 1995, 60, 5314-5315

Preparation of Zinc Borohydride.

A predried 500 mL side arm flask fitted with a reflux condenser and connected to a mercury bubbler was charged with 20.8 g (165 mmol) of freshly fused ZnCl₂ and 12.9 g (330 mmol) of NaBH₄, followed by 250 mL of dry THF using a double-ended needle. The contents were stirred  using a magnetic stirrer for 24 h at room temperature. The active hydride content of the supernatant solution was estimated  by quenching aliquots with 2 N H₂SO₄ and estimating the hydrogen evolved using a gas burette. The supernatant solution  which was 0.66 M in Zn(BH₄)₂ was used as such for further reactions.



Lego's voice: Impatient bees can use ZnCl₂ straight from the package and filter the whole suspension after stirring for 24 h to get the solution of  Zn(BH₄)₂ in THF (slightly less than decanting it). The filter residue should bee carefully quenched with MeOH.

[Rhodium]

Zinc Borohydride^{JACS 82, 6074 (1960)}

The ether-soluble reducing agent zinc borohydride can conveniently be prepared from sodium borohydride and zinc chloride. Zinc borohydride has been obtained before from lithium borohydride and zinc chloride^1,2 and from zinc hydride and diborane³.

Experimental:

A mixture of Reagent grade anhydrous zinc chloride (4.0g, 0.029 mole) with 50 ml of anhydrous ether was boiled until most of the solid had dissolved. The mixture was allowed to stand, and the supernatant liquid was carefully decanted from 0.4g of insoluble material. The ethereal zinc chloride solution was added dropwise a t room temperature to a stirred suspension of 2.7 g of 96% sodium borohydride (0.069 mole) in 150 ml of absolute ether. Stirring was continued overnight. The solids were allowed to settle, and the liquid was removed by decantation and if necessary clarified by centrifugation. This ethereal solution of zinc borohydride was stored in a stoppered bottle at 5°C. An aliquot treated first with dilute nitric acid and then with silver nitrate solution gave no precipitate of silver chloride.
 
[1]: Z. Naturforsch., 76, 579 (1952)
[2]: Angew. Chem., 66, 16 (1953)
[3]: J. Am. Chem. Soc. 73, 4585 (1951)

[Rhodium]

Zinc Borohydride (0.5 M in DME)
 
To sodium Borohydride (1.95 g) in redistilled DME (50 ml) was added recently fused zinc chloride (3.4 g). The mixture was stirred overnight at 0-5°C. After filtration under nitrogen, the clear solution (ca. 0.5M) was used immediately.

DME = Dimethoxyethane, MeO-CH₂CH₂-OMe

Ref: J. Chem. Soc. Perkin Trans. 1, 814 (1973)