Oxyfluorides formation

Tantalum Compounds. Potassium heptafluorotantalate [16924-00-8] K TaF, is the most important tantalum compound produced at plant scale. This compound is used in large quantities for tantalum metal production. The fluorotantalate is prepared by adding potassium salts such as KCl and KF to the hot aqueous tantalum solution produced by the solvent extraction process. The mixture is then allowed to cool under strictiy controlled conditions to get a crystalline mass having a reproducible particle size distribution. To prevent the formation of oxyfluorides, it is necessary to start with reaction mixtures having an excess of about 5% HF on a wt/wt basis. The acid is added directiy to the reaction mixture or together with the aqueous solution of the potassium compound. Potassium heptafluorotantalate is produced either in a batch process where the quantity of output is about 300—500 kg K TaFy, or by a continuously operated process (28). 

Ammonium complex fluorometalates are generally susceptible to hydrolysis by water formed in the course of the fluorination process (see 25). Such interactions, resulting in the formation of oxyfluoride metalates can be described as follows ... 

Predominant formation of either complex fluoride or complex oxyfluoride depends on the interaction rates ratio of processes (25) and (26). The relatively high interaction rates of (27) and (28) lead to the synthesis of simple fluorides or oxyfluorides, respectively. With the availability of two or more cations in the system, the ammonium complex fluorometalates interact forming stable binary fluorides or oxyfluorides or mixtures thereof. 

The steric similarity of oxygen and fluorine ions enables the formation of coordination-type structures in some tantalum and niobium oxyfluoride compounds. 

The spectra of the initial saturated solution, with a F Nb of approximately 6, are of particular interest because of the presence of a weak band at about 900-930 cm 1. This band can be attributed to NbO bonds in oxyfluoride complexes. Even small additions of HF lead to the disappearance of the above effect. This can be explained based on a complex solvatation model. In solutions with a F Nb ratio of about 6, hexafluoroniobate complex, NbF6, initiates the formation of HF that interacts with complex ions as a solvate. This process is called autosolvatation and is represented by two interactions. The first is a hydrolysis process that leads to the formation of HF ... 

Based on Equation (55), it was concluded/assumed that niobium-containing oxyfluoride melts are characterized by the formation of complex ions NbOF63. ... 

The proposed model of the structure of oxyfluoride melts corresponds with the conductivity results shown in Fig. 69. The specific conductivity of the melt drops abruptly and asymptotically approaches a constant value with the increase in tantalum oxide concentration. This can be regarded as an additional indication of the formation of oxyfluorotantale-associated polyanions, which leads to a decrease in the volume in which light ions, such as potassium and fluorine, can move. The formation of the polyanions can be presented as follows ... 

Analysis of the physicochemical properties of fluoride and oxyfluoride melts reveals that the complex ions are characterized by coordination numbers that do not exceed seven. Fluoride melts consist of the complex ions MeF72 and MeFe. Molten chloride-fluoride systems initiate the formation of heteroligand complexes of the form MeFgCl2 . Oxyfluoride and oxyfluoride-chloride melts can contain oxyfluoride complexes MeOF63 at relatively low concentrations. The behavior of the more concentrated melts can be attributed to the formation of oxyfluorometalate polyanions. 

The formation of complexes in fluoride and oxyfluoride melts containing tantalum and niobium will be discussed later on in detail. 

Adding potassium hydroxide, KOH, to a melt containing KF and a 0.1 mol fraction of K2TaF7 leads to the appearance of an additional band at 900 cm 1, as shown in Fig. 79 [342]. This band corresponds to TaO bond vibrations in TaOF63 complex ions. Interpretation of IR spectra obtained from more concentrated melts is less clear (Fig. 80). The observed absorption in the range of 900-700 cm 1 indicates the formation of oxyfluoride polyanions with oxygen bridges. ..OTaO. The appearance of a fine band structure could be related to very low concentrations of some isolated components. These isolated conditions prevent resonance interaction between components and thus also prevent expansion of the bands by a mechanism of resonance [362]. 

The formation of oxide compounds as a result of the thermal treatment of oxyfluorides is due to high temperature hydrolysis and reduction-oxidation processes. 

Thus, in cubic oxyfluorides of niobium and tantalum with rock-salt (NaCl) crystal structures, the formation and extinction of spontaneous polarization occurs due to polar ordering or disordering of Li+ - Nb5+(Ta5+) dipoles. 

Potassium heptafluorotantalate, K2TaF7, or as it is called by its commercial name K-salt, is a starting material for tantalum metal production. K-salt is produced by adding potassium fluoride, KF, or potassium chloride, KC1, to a tantalum strip solution that results from a liquid-liquid extraction process. In order to prevent hydrolysis and co-precipitation of potassium oxyfluoro-tantalate, a small excess of HF is added to the solution [24]. Another way to avoid the possible formation and co-precipitation of oxyfluoride phases is to use potassium hydrofluoride, KHF2, as a potassium-containing agent. The yield of the precipitation depends mostly on the concentration of the potassium-containing salt and is independent of the HF concentration [535]. 

Numerous reactions have been reported in which FCIO2 is formed as a product. Most of these involve the interaction between a chlorine fluoride or oxyfluoride with an oxide or hydroxide. The oxidation state of the chlorine fluoride is not important since +1, -l-III, and - -V compounds all yield FCIO2 owing to the tendency of the lower oxyfluorides, such as FCIO, to disproportionate. The presence of excess chlorine fluoride is important to avoid formation of chlorine oxides. The following equations are typical examples for these types of reactions ... 

Chlorine monofluoride oxide, 18 328-330 force field of, 18 329, 330 infrared spectrum of, 18 328, 329 stretching force constants for, 18 330 synthesis of, 18 328 Chlorine nitrate fluorination of, 18 332 preparation of, 5 54 Chlorine oxides, 46 109-110, 158 fluorination of, 18 348 Chlorine oxyfluorides, 18 319-389, see also specific compounds adduct formation, 18 327, 328 amphoteric nature of, 18 327, 328 bond lengths, 18 326 bond strengths, 18 323-327 geometry of, 18 320-323 ligand distribution, 18 323 reactivity of, 18 327, 328 stretching force constants, 18 324-327 Chlorine pentafluoride oxide, 18 345, 346 Chlorine trifluoride, reaction with difluoramine, 33 157... 

Double Fluorides of Niobium Pentafluoride.—Niobium pentafluoride shows a strong tendency to form stable double fluorides with the fluorides of other metals. These are conveniently prepared by the action of carbonates of the metals on solutions of niobium pentoxide in a large excess of hydrofluoric add, or by the addition of a large excess of hydrofluoric acid to solutions of the oxyfluorides of the metals. In the absence of excess of hydrofluoric add hydrolysis takes place as usual with the formation of niobium oxytrifluoride, NbOF3. The precipitation of these double fluorides indicates the probable existence in solution of niobium pentafluoride stability is imparted by the formation of complex anions containing several fluorine atoms. 

The norbelladine derivative 408, which served as the starting material for the synthesis of ( )-oxocrinine (415) (Scheme 35), may be readily prepared from the reductive animation of piperonal with tyramine followed by acylation with trifluoroacetic anhydride (191,192). When the N-acylated monophenol 408 was treated with excess thallium tris(trifluoroacetate) in methylene chloride, the di-enone 412 was obtained in 19% yield (191), whereas use of the oxidant vanadium oxyfluoride in trifluoroacetic acid/trifluoroacetic anhydride afforded 412 in 88% yield (192). Base-induced N-deacylation of 412 was accompanied by spontaneous cyclization to furnish racemic oxocrinine (415). Attempts to oxidize the free amine derived from 408 led to the formation of a number of products, some of which resulted from oxidation at nitrogen. 

---