Calcium bis tetrahydroborate

The tetrahydroborate salts of alkali metals, M[BH4] (M = Li, Na, K),1 are important because they serve as starting materials for the preparation of other boron hydrides2,3 and because they are used frequently as reducing agents.4 The lithium and sodium salts are prepared on a technical scale.5 9 The tetrahydroborate salts of the alkaline earth metals, M[BH4]2 (M = Mg, Ca, Sr, Ba), have not as yet been used extensively however, calcium bis[tetrahydroborate(l-)], Ca(BH4)2,10 is very soluble in tetrahydrofuran (THF) and it therefore has considerable potential application as a substitute for the lithium and sodium salts. 

Calcium bis[tetrahydroborate(l-)] may be prepared from sodium tetra-hydroborate( 1 -) and calcium dichloride by cation exchange11 in a suitable solvent such as dimethylformamide,12 an amine13,14 or an alcohol.13,14 A particularly good preparation of very pure Ca[BH4]2 involves the reaction of calcium dihydride with the triethylamine-borane adduct.15,16 This method may also be used for the preparation of other tetrahydroborates of alkali and alkaline earth metals. The triethylamine-borane adduct17 may be synthesized in a variety of ways, e.g., from triethylamine, sodium tetrahydro-borate(l —), and trichloroborane 18 from a trialkoxyborane, aluminum metal, and hydrogen in the presence of triethylamine 19 or by hydrogenation under pressure of a mixture of triethylborane and triethylamine.20 The triethylamine-borane adduct is a colorless liquid (mp — 2°). It is stable to air and moisture at room temperature and it is easily purified by vacuum distillation (bp 95-96°/12 torr). 

The preparation of calcium bis[tetrahydroborate(l —)] described here is recommended because high yields of pure product are usually obtained without difficulty. If the material should be contaminated by impurities from the calcium hydride, then it may be extracted with THF. Needles of Ca[BH4]-2THF11,21,22 may be obtained from this solution. The THF can then be readily removed by heating and pumping. 

Caution. Calcium dihydride and calcium bis[tetrahydroborate( 1 —)] react vigorously with water, alcohols, and other compounds which contain acidic hydrogen atoms, sometimes with spontaneous ignition. The reaction should be carried out in an efficient hood and the reagents and products should be handled under an inert gas. 

Calcium bis[tetrahydroborate(l —)] is obtained as a fine powder which must be stored in well-sealed containers. It is stable in dry air at room temperature but traces of moisture decompose it with liberation of hydrogen. The gas evolution is quantitative when dilute hydrochloric acid is added. The compound decomposes at ca. 260°. The solvate Ca[BH4]2 -2THF loses THF on heating to 240° in vacuo. Calcium bis[tetra-hydroborate(l —)] dissolves exothermally in THF and crystalline Ca[BH4]2 -2THF may be isolated from the solution. The Ca[BH4]2 2THF is very soluble in dimethyl sulfoxide, in hexamethylphosphoric triamide, and in bis(2-methoxyethyl) ether (diglyme). 

Boron Triiodide. Boron ttiiodide is not manufactured on a large scale. Small-scale production of BI from boron and iodine is possible in the temperature range 700—900°C (70—72). Excess I2 can be removed as Snl by reaction with Sn, followed by distillation (71). The reaction of metal tetrahydroborates and I2 is convenient for laboratory preparation of BI (73,74). BI can also by synthesized from B2H and HI in a furnace at 250°C (75), or by the reaction of B with excess Agl or Cul between 450—700°C, under vacuum (76). High purity BI has been prepared by the reaction of I2 with mixtures of boron carbide and calcium carbide at elevated temperatures. 

REDUCTION, REAGENTS Bis(triphenyl-phosphine)copper tetrahydroborate. Borane-Pyridine. Calcium-Methylamine/ ethylenediaminc. Chlorobis(cyclopenta-dienyl)tetrahydroboratozirconium(IV). Chromium(II)-Amine complexes. Copper(0)-lsonitrile complexes. 2,2-Dihydroxy-l, 1-binaphthyl-Lithium aluminum hydride. Di-iododimethylsilane. Diisobutyl-aluminum 2,6-di-/-butylphenoxide. Diisobutyl aluminum hydride. Dimethyl sulfide-Trifluoroacetic anhydride. Disodium tetracarbonylferrate. Lithium-Ammonia. Lithium-Ethylenediamine. Lithium bronze. Lithium aluminum hydride. Lithium triethylborohydride. Potassium-Graphite. 1,3-Propanedithiol. Pyridine-Sulfur trioxide complex. 

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