Borohydride sublimation

Purification of the desired product is accomplished by sublimation where only the unreacted A1(BHi+)3 and AnCBH ) are volatile. The large difference in volatilities of these compounds permit easy separation. ThCBHit) and PaCBHit) are obtained on a 0° cold finger by heating the solid reaction mixture to 120° and 55°, respectively. Uranium, neptunium, and plutonium borohydrides sublime at room temperature and are collected in a dry ice trap through which the A1(BHi+)3 passes into a liquid nitrogen trap. 

Although the lUPAC has recommended the names tetrahydroborate, tetrahydroaluminate, etc, this nomenclature is not yet ia general use. Borohydrides. The alkaU metal borohydrides are the most important complex hydrides. They are ionic, white, crystalline, high melting soHds that are sensitive to moisture but not to oxygen. Group 13 (IIIA) and transition-metal borohydrides, on the other hand, are covalendy bonded and are either Hquids or sublimable soHds. The alkaline-earth borohydrides are iatermediate between these two extremes, and display some covalent character. 

The compounds are isolated by sublimation from the reaction mixture. Perhaps surprisingly the compounds fall into two quite distinct classes. Those of Np and Pu are unstable, volatile, monomeric liquids which at low temperatures crystallize with the 12-coordinate structure of Zr(BFl4)4 (Fig. 21.7, p. 969). The borohydrides of Th, Pa and U, on the other hand, are thermally more stable and less reactive solids. They possess a curious helical polymeric structure in which each An is surrounded by 6 BFI4 ions, 4 being bridging groups attached by 2 FI atoms and... 

The only examples of compounds of this type are the borohydrides, MIV(BH4)4 (MIV = Th-Pu) and MIV(MeBH3)4 (MIV = Th, U, Np). These compounds are conveniently prepared by reaction, for example, of the metal tetrafluoride with A1(BH4)3 in a sealed tube, followed by vacuum sublimation of the product.159 The neptunium and plutonium compounds are liquids at room temperature and are more volatile than the thorium, protactinium or uranium analogues. 

A1(BH4)3, Ti(BH4)3, Zr(BH4)4 and Hf(BH4)4 are sublimate borohydrides with low melting temperatures. The melting and boiling temperatures are shown in Table 15.8, and the vapor pressure is shown in Table 15.3. It has been reported that these M(BH4) decompose very slowly to form hydrogen and non-volatile compounds of indefinite composition. The decomposition reaction of A1(BH4)3 was reported to be as follows ... 

Similar to aluminum borohydride, but less volatile. Spon -taneously flammable. Sublimes at 913. Dec above 123"... 

The reduction of molybdenum and tungsten hexacarbonyls by sodium in liquid ammonia to form the [M(CO)s] (M = Mo, W) anions is less satisfactory, because of the tendency for side reactions to occur in these systems leading to an impure product. An alternative route to the molybdenum derivative (65) involves first the reduction of molybdenum hexacarbonyl with sodium borohydride in liquid ammonia at — -40°C to produce the [Mo2(CO)io] anion. This [Mo2(CO)jq] anion may then be treated with carbon monoxide under pressure at 150° C to form an equimolar mixture of [Mo(CO)s] and molybdenum hexacarbonyl, the latter being readily separated by sublimation... 

Two approaches for the synthesis of nanostructured M50 type steel (composed of 4.0% Cr, 4.5% Mo, 1.0% V, 0.8% C and balance Fe) powders and their consolidation are reported in this chapter. One approach involved the sonochemical decomposition of organometallic precursors and the other involved the reduction of the metal halides with lithium triethyl borohydride followed by vaccum sublimation of the powders to remove lithium chloride. The as-synthesized powders are amorphous by X-ray diffraction (XRD) but the peaks corresponding to bcc a-Fe are observed in the compacts. The morphology and composition of the powders synthesized by both techniques, as well as the compacts, were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Hardness, density, particle size and impurity contents were also determined for the compacts. In addition, pure nanosized iron particles obtained by the ultrasound decompositon of iron pentacarbonyl were consolidated and the properties of the latter were studied. 

Vinyl Ruthenocene, VR. Ruthenocene, 5.00g was converted to acetylruthenocene, mp 110-111 , in 89% yield by the procedure of Hill and Richards.The advantage of this procedure over conventional acylation with acetyl chloride and aluminum chloride is that very little unreacted ruthenocene or 1,1 - diacetylruthenocene are found in the product. Acetylruthenocene was reduced with sodium borohydride in ethanol to hydroxyethylruthenocene which was converted to vinyl ruthenocene, mp 51-52° in 40-50% overall yield by vacuum sublimation from alumina at 150°. " ... 

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