Tantalum complexes addition

Acetylenes 191 (R1 = C5H11, Me3Si or MeS R2 = C5H11 or C10H21) react with tantalum ) chloride and zinc to form tantalum complexes. Addition of the imine... 

Closely related mixed amido/imido/guanidinato tantalum complexes of the type Ta(NR R )[(R R2N)C(NR )2]( = NR ) (R R = Me, Et R = Cy, Pr R = Pr", BuO were synthesized by the insertion of carbodiimides into to tantalum-amide bonds in imidotantalum triamide precursors, and the effects of ligand substitution on thermal properties were studied by TGA/DTA measurements. In addition, selected compounds were pyrolyzed at 600 °C and the decomposition products were studied by GC-MS and NMR spectroscopy. ... 

The chemistry of tantalum is different. The tantalum pentaalkyl complex does not exists, because it transforms easily into a carbene complex by a-elimination. This complex reacts also with silica, leading to a supported tantalum complex.262,263 Their reaction proceeds first by the addition of the silanol OH group across the tantalum-carbon double bond followed by elimination of an alkane (Scheme 7.19). 

Probably the first isolated tungsten alkylidene complex active in metathesis and completely characterised is the one shown in Figure 16.10 reported by Wengrovius and Schrock the analysis included an X-ray structure determination by Churchill and co-workers [18], The alkylidene was transferred from a tantalum complex to yield the hexacoordinate tungsten complex containing two PEt3 ligands. One of these can be removed by the addition of half an equivalent of palladium chloride. The total turnover number of these catalysts with Lewis acids added was 50 in 24 hours. 

Addition of other nucleophiles or Lewis bases to neopentyltantalum complexes also generated alkylidene complexes, as shown in Scheme 13.2. Addition of slightly less than one molar equivalent of bis(neopentyl)zinc to tantalum pentachloride generated bis(neopentyl) tantalum trichloride, and addition of thalium cyclopentadienide to this intermediate generated the Cp-ligated tantalum alkylidene. Addition of dative ligands, such as THE, and monodentate or bidentate phosphines to the bis(neopentyl)tantalum complex generated the octahedral alkylidenes in Scheme 13.2. ... 

Tantalum and niobium are added, in the form of carbides, to cemented carbide compositions used in the production of cutting tools. Pure oxides are widely used in the optical industiy as additives and deposits, and in organic synthesis processes as catalysts and promoters [12, 13]. Binary and more complex oxide compounds based on tantalum and niobium form a huge family of ferroelectric materials that have high Curie temperatures, high dielectric permittivity, and piezoelectric, pyroelectric and non-linear optical properties [14-17]. Compounds of this class are used in the production of energy transformers, quantum electronics, piezoelectrics, acoustics, and so on. Two of... 

An increase in the Me F ratio leads to an increase in the acidity of the initial solution, whereas the acidity of alkali metals increases according to their molecular weight, from Li to Cs. Therefore the additives of fluorides of alkali metals having higher atomic weight provide formation of complex fluorides with lower coordination number of tantalum or niobium. 

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. 

Table 55 presents the results discussed above. Fluoride melts containing tantalum contain two types of complex ions, namely TaF6 and TaF72 . The equilibrium between the complexes depends on the concentration of fluoride ions in the system, but mostly upon the nature of the outer-sphere cations. The complex ionic structure of the melts can be adjusted by adding cations with a certain polarization potential. For instance, the presence of low polarization potential cations, such as cesium, leads primarily to the formation of TaF72 complexes, while the addition of cations with relatively high polarization potentials, such as lithium or sodium, shifts the equilibrium towards the formation of TaF6 ions. 

The scheme of the interaction mechanism (Equation 88) testifies to an electro-affinity of MeFe" ions. In addition, MeFe" ions have a lower negative charge, smaller size and higher mobility compared to MeF6X(n+1> ions. The above arguments lead to the assumption that the reduction to metal form of niobium or tantalum from melts, both by electrolysis [368] and by alkali metals, most probably occurs due to interaction with MeF6 ions. The kinetics of the reduction processes are defined by flowing equilibriums between hexa-and heptacoordinated complexes. 

The main problems encountered in the investigation of tantalum- and niobium-containing fluoride and oxyfluoride complexes are related to the tendency of the compounds to undergo hydrolysis, particularly at elevated/high temperatures. In addition, the interpretations of the observed effects are often nontrivial and unclear due to the relatively complicated inter-particular interactions and changes that occur under thermal treatment. From this point of view, vibration spectroscopy methods are of high importance due to the dependence of solid phase spectra on the temperature, which, above all, stems from the nature of such inter-ionic interactions [369]. 

Ammonium hydrofluoride is relatively stable, even in the molten state. In addition to being in contact with tantalum or niobium oxide, the compound will initiate the fluorination process yielding complex tantalum or niobium fluoride compounds. There is no doubt that thermal treatment of the hydroxides at high temperatures and/or at a high temperature rate leads to the enhancement of the defluorination processes, which in turn results in an increase in fluorine content of the final oxides. 

It is, therefore, required that all initial compounds be dried properly prior to performing the reduction. This procedure is not at all trivial and refers, first of all, to the diluent salts, and especially to potassium fluoride, KF, which is characterized by a strong hygroscopic property and a tendency to form stable crystal hydrates. The problem of contamination due to hydrolytic processes can usually be resolved in two manners. The first is to apply another tantalum-containing complex fluoride compound that does not undergo hydrolysis. The second involves the adjustment of the reduction process parameters and use of some additives that will "collect" the oxygen present, in the form of water, hydroxyl groups or other compounds. 

The reactivity of a remarkable electronically unsaturated tantalum methyli-dene complex, [p-MeCgH4C(NSiMe3)2]2Ta( = CH2)CH3, has been investigated. Electrophilic addition and olefination reactions of the Ta = CH2 functionality were reported. The alkylidene complex participates in group-transfer reactions not observed in sterically similar but electronically saturated analogs. Reactions with substrates containing unsaturated C-X (X = C, N, O) bonds yield [Ta] = X compounds and vinylated organic products. Scheme 117 shows the reaction with pyridine N-oxide, which leads to formation of a tantalum 0x0 complex. ... 

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