Niobium V chlorides

Oxidation-Reduction Titrations. The extent of reduction resulting from reaction of niobium (V) chloride and bromide with pyridine was determined by indirect titration of crude reaction mixtures with standard ammonium tetrasulfato-cerate(IV) solution. Samples were stirred overnight in a stoppered flask with an excess of iron (III) ammonium sulfate. Any iron (II) formed by reaction with the niobium complex mixture was then titrated with the standard tetrasulfato-cerate(IV) solution using ferroin as indicator. Results of these determinations are given in Table III. 

When a sample of the niobium (V) chloride-pyridine or niobium(V) bromide-pyridine reaction mixture was dissolved in dilute acid, filtered to remove precipitated niobium oxides, and treated with concentrated sodium hydroxide solution, the same set of spectra were observed for the resulting solution as for the l-(4-pyridyl) pyridinium dihalides. The spectra before and after heating the solutions from the reaction mixtures are shown in Figure 2. Here also the peaks occurred at 432 and 365 m/x, with the 432-m, peak absent after heating. 

Samples of the niobium (V) chloride or niobium (V) bromide-pyridine reaction mixtures were hydrolyzed in concentrated hydrochloric acid. Aliquots were diluted and neutralized with sodium carbonate to a pH of approximately 8. Sodium tetraphenylborate(III) solution then was added and a precipitate of l-(4-pyridyl) pyridinium tetraphenylborate(III) was produced. The precipitate was filtered and extracted with concentrated hydrochloric acid. The ultraviolet absorption spectrum of the extract is shown in Figure 3 for comparison with the spectrum of a known sample of l-(4-pyridyl) pyridinium dichloride in dilute hydrochloric acid. 

Preparation of Dianiline Derivative of Glutaconaldehyde. A sample of the niobium(V) chloride-pyridine reaction mixture was hydrolyzed in dilute aqueous ammonia, and the solution was filtered to remove precipitated niobium (V) oxide. 

Reactions of Niobium(V) Chloride and Bromide with Pyridine. These reactions proceeded with an initial, rapid formation of a voluminous white solid in the case of the chloride and a maroon red solid in the case of the bromide, followed by a much slower reaction in each case. The solution and solid both became brown in the chloride reaction, while a bulky green solid separated from the red solution of the bromide. When the reaction was complete in the latter case, the solution above the green solid was almost colorless. 

Weight gain data and oxidation-reduction titers (Table II) for the niobium (V) chloride and bromide reaction products both indicated the reduction of niobium (V) to niobium (IV). The niobium (IV) adducts were separated by washing the crude reaction mixtures with anhydrous acetonitrile. This solvent removed the various organic products of the initial reaction plus any unreduced niobium compounds. Analysis of the washed products gave ratios of Nb X py of 1 4 2 and agreed with analyses for samples of NbCl py2 and NbBr4 py2 which... 

It was thus shown that the tetrahalodi(pyridine)niobium(IV) complexes were the reduction products of the reactions of niobium(V) chloride and niobium (V) bromide with pyridine. 

From these observations it was concluded that the major products of the reduction of niobium(V) chloride with anhydrous pyridine were tetrachlorodi-(pyridine)niobium(IV) and l-(4-pyridyl)pyridinium dichloride. Oxidation-reduction titrations indicated that this reduction accounted for approximately 70% of the reaction products. In view of the rapid reaction of tantalum(V) halides with pyridine to form 1 to 1 adducts, it was assumed that the remaining 30% of niobium (V) which was not reduced was present in the reaction mixture as pentachloro(pyridine)niobium(V). On this basis the following over-all reaction is proposed ... 

The method has also been examined for the production of other nonvolatile compounds. In these cases, the purification process involves subliming excess starting materials away from the product. For example, greater than 95 % yields of tungsten(IV) chloride from tungsten(VI) chloride and 85% yields of niobium-(IV) chloride from niobium(V) chloride can be obtained by using a 500-watt light bulb at 150°C. and a reaction period of 3 days. 

See Niobium(V) chloride and hexachloroniobates(V), synthesis 23 Potassium fluorosulfite, synthesis 28... 

Numerous methods have been described for the preparation of niobium(V) chloride, among them the reaction of niobium(V) oxide with thionyl chloride in a sealed system. In such a procedure some niobium(V) oxide trichloride, NbOCls, is almost always formed, and it is difficult to obtain the pentachloride completely free from this impurity, even by repeated sublimation. The simple, efficient method described here consists in allowing hydrous niobium(V) oxide to react with thionyl chloride at room temperature. Almost quantitative conversion is observed, the pentachloride dissolving in the thionyl chloride, from which it may be recovered, free of oxide trichloride, by vacuum evaporation... 

Niobium(V) chloride. Hydrous niobium(V) oxide (0.75 g. Nb) is precipitated from acid solution by the addition of ammonium hydroxide, thoroughly washed by centrifugation with water (two 15-ml. portions), 0.5 M nitric acid (two 10-ml. portions) to remove adsorbed ammonium ion, and acetone (three 20-ml. portions) and vacuum-dried at room temperature. If the initial hydroxide precipitation is carried out from hydrofluoric acid solution, an appreciable quantity of the hydrous oxide may dissolve in the nitric acid washes, presumably because of the presence of traces of fluoride. However, reprecipitation and treatment as above reduces losses at this stage. The dried hydrous oxide is placed in a 40-ml. centrifuge tube fitted with a standard-taper outer joint, and 10 to 15 ml. of freshly distilled thionyl chloride is added slowly, since the initial reaction may be vigorous. The vessel is stoppered loosely, and the reaction is allowed to go to completion at room temperature (24 to 48 hours). Any traces of undissolved hydrous oxide, usually very small, and any yellow crystalline compound (see Discussion) are removed by centrifugation, and the penta-chloride is isolated by vacuum evaporation of the thionyl chloride at room temperature and pumping for several hours at 10 mm. If necessary, the product is further purified by vacuum sublimation in a sealed tube ( 150°). The yield, based on dried hydrous oxide, is 90 to 95%. Anal. Calcd, for NbCU Nb, 34.39 Cl, 65.61. Found Nb, 34.27 Cl. 

The checker reports that standard filtration techniques can be substituted for the centrifugation steps. The reaction of hydrous niobium (V) oxide is done in a 100-ml. round-bottomed flask fitted with an outer standard-taper joint. The solution of niobium(V) chloride in thionyl chloride is filtered by attaching a filter tube (having a standard-taper inner joint at each end) with a medium-porosity sin-tered-glass filter to the neck of the reaction flask, attaching another 100-ml. round-bottomed flask to the opposite end of the filter tube, and inverting the assembly. The residue can be washed with small aliquots of thionyl chloride. With this kind of apparatus the synthesis can be scaled up by a factor of 10 or more. 

Cesium hexachloroniobate(V). A thionyl chloride solution (6 ml.) of niobium(V) chloride (1.5 g.) is added to a solution of cesium chloride (1 g.) in iodine(I) chloride (2 ml.) at room temperature. The reaction vessel, similar to the one described above, is stoppered, and the reaction mixture allowed to stand for 2 hours. The resulting precipitate, most of which forms immediately on mixing of the solutions, is isolated by centrifugation, washed with thionyl chloride (10-ml. portions) to remove all the iodine(I) chloride, and vacuum-dried at room temperature. After isolation, the bright yellow cesium hexachloroniobate(V) must be handled in a dry-box. The 3deld is 1.5 g. (62%). Anal. Calcd. for CsNbCle Cs, 30.32 Nb, 21.17 Cl, 48.50. Found Cs, 30.42 Nb, 21.12 Cl, 48.25. 

The properties of niobium (V) chloride are well known. Cesium hexachloroniobate(V) is a bright yellow, crystalline, moisture-sensitive compound. It is soluble in iodine (I) chloride, less so in iodine (I) chloride-thionyl chloride mixtures, and completely insoluble in thionyl chloride it is very slightly soluble in acetonitrile and nitromethane but insoluble in benzene, carbon disulfide, ethyl acetate, and diethyl... 

In addition to the obvious preparative advantages inherent in the pentachloride preparation, e.g., the low temperature of reaction and the possibility of performing the reaction without precautions against atmospheric moisture because of the protection afforded by thionyl chloride, the product is obtained free from oxide chloride. The major losses occur during the hydrous oxide precipitation and the nitric acid washings. The latter are essential to remove adsorbed ammonium ion, since, if this is not done, the reaction products will be niobium (V) chloride, in solution in thionyl chloride, and the bright yellow insoluble ammonium hexachloroniobate(V). In fact, the high purity of these two products in instances where complete removal of ammonium ion is not achieved shows clearly that the reaction of hydrous niobium (V) oxide with thionyl chloride is virtually quantitative. 

Niobium (V) chloride is quite soluble in the hot chloro-carbon mixture but forms light yellow needles in the reaction mixture when the solution is cooled. 

Tungsten (VI) chloride forms nearly black crystals under a dark brown solution when the mixture is cooled. As with molybdenum (V) chloride and niobium (V) chloride, the size of the crystals may be regulated by controlling the cooling rate. 

The properties of vanadium (III) chloride, niobium (V) chloride, molybdenum(V) chloride, and tungsten(VI) chloride have been previously described. 

In an inert-atmosphere dry-box, 2.36 g (0.01 mole) of niobium tetrachloride is suspended in 25 ml of acetonitrile in a 100-ml round-bottomed flask equipped with a magnetic stirring bar. Niobium(IV) chloride is conveniently prepared by reduction of niobium(V) chloride with aluminum metal. " This mixture is stirred for ca. 1 hr and then 5.84 g (0.06 mole) of potassium thiocyanate dissolved in 25 ml of acetonitrile is added. Stirring is continued for 4 hr to allow for complete reaction. The red solution is filtered through a medium-porosity sintered-glass frit, separating the insoluble potassium chloride from the soluble K2[Nb(NCS)e]. The KCl precipitate is washed with three 5-ml portions pf... 

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