Niobium reagents

The niobium imines can also be prepared by reaction of a niobium halo hydride with a nitrile. Thus a niobium reagent prepared from 1 and Bu3SnH reacts with nitriles to form a niobium-imido complex, which is hydrolyzed to a v/c-diamine. 

The niobium reagent [NbCl3(dme)] can be used to couple imines with carbonyl compounds giving amino alcohols such as 10, although generally lower stereoselectivity is observed. [6] In analogy to reactions with other early transition metals the mechanism is believed to proceed via a niobaziridine, which adds to the carbonyl functionality. 

The elements of Group 5 are in many ways similar to their predecessors in Group 4. They react with most non-metals, giving products which are frequently interstitial and nonstoichiometric, but they require high temperatures to do so. Their general resistance to corrosion is largely due to the formation of surface films of oxides which are particularly effective in the case of tantalum. Unless heated, tantalum is appreciably attacked only by oleum, hydrofluoric acid or, more particularly, a hydrofluoric/nitric acid mixture. Fused alkalis will also attack it. In addition to these reagents, vanadium and niobium are attacked by other hot concentrated mineral acids but are resistant to fused alkali. 

For a long period of time, molten salts containing niobium and tantalum were widely used for the production by electrolysis of metals and alloys. This situation initiated intensive investigations into the electrochemical processes that take place in molten fluorides containing dissolved tantalum and niobium in the form of complex fluoride compounds. Well-developed sodium reduction processes currently used are also based on molten salt media. In addition, molten salts are a suitable reagent media for the synthesis of various compounds, in the form of both single crystals and powdered material. The mechanisms of the chemical interactions and the compositions of the compounds depend on the structure of the melt. 

Commercial-scale application of solvents coming under the category of neutral reagents is largely found as applied to the nuclear industry materials, as in example, for the separation and refining of uranium, plutonium, thorium, zirconium, and niobium. A process flowsheet for extracting niobium and tantalum from various resources is shown in Figure 5.23. It will... 

Reagent scheme developed for beneficiation of niobium from feldspar-containing pegmatite ores. 

The results obtained indicated that cationic flotation of pyrochlore was not successful. Dispersant AQ4 has a pronounced effect on niobium metallurgical results. Dispersant/ depressant AQ4 is composed of the following individual reagents 60% orthodihydrox-ybenzene (Catacol), 30% low-molecular-weight acrylic acid (Accumer 2400) and 10% hexametapho sphate. 

Throughout the development testwork, emphasis was placed on finding an effective reagent scheme that would produce a bulk concentrate with satisfactory tantalum, niobium and zircon recoveries. The Ta/Nb-Zr separation study concentrated mainly on magnetic separation. The floatability of Ta/Nb and Zr depend on type of collector, modifier and depressant system used. 

Niobium(III) chloride-Dimethoxyethane, NbCl3 DME (1). This soluble Nb(III) reagent is obtained as a brick-red solid by reduction of NbCl> in DME with Bu,SnH. 

Niobium(IV) chloride, NbCl4(THF)2. This reagent (1) can be obtained1 by reduction of NbCl5 with aluminum powder in CH,CN followed by solvation with THF m.p. 110° dec., yield 50%. 

Precipitation inhibitors, dispersants contrasted, 3 686 Precipitation leachate procedure, synthetic, 25 868-869 Precipitation reactions, for niobium and tantalum determination, 27 142-143 Precipitation reagents, protein, 22 133 Precipitation with compressed antisolvent (PCA) process, 24 17, 18 Precipitator dust, in phosphorus manufacture, 19 12 Precipitators, electrostatic, 23 180 Precision agriculture, 23 328 26 269-270 Precision measurement techniques, noble gases in, 27 370 Precision scales, 26 245 Preconcentration, of uranium ores, 25 401 Pre-crosslinked polychloroprene grades, 19 852... 

The coordinated ethylene is readily expelled at 25 °C by the attack of a conjugated diene and the hydride is transferred to the sterically less crowded diene terminus. Thus the niobium hydrido-olefin complexes serve as convenient reagents for the preparation of 1,2- or 1,3-dialkyl-substituted allylniobium compounds starting from butadiene, (E,E) and (/i,Z)-2,4-hcxadicnc, (E)- and (Z)-l,3-pentadiene (equation 1), 3-methyl-l,3-pentadiene and isoprene (equation 2). All the allyl niobium compounds synthesized were isolated as air-sensitive pale-yellow crystals by crystallization from hexane. 

Reduction by NaAlH2Et2 was studied similarly, using an excess of the reagent (toluene solution, from Alfa) added to a benzene suspension of Cp2NbCl2-The niobium-containing product could not be separated from excess NaAlH2Et2, as they both appear to have very similar solubility properties. 

The strong Lewis acid niobium(V) pentachloride has been found to be an effective reagent for the monodealkylation of diethers with two alkyl-aryl linkages.128... 

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