Niobium complexes reduction products

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. 

Reactions of Tantalum(V) and Niobium(V) Iodides with Pyridine. Both niobium(V) and tantalum(V) iodide were reduced in pyridine. In each case the di-adduct of pyridine with the metal tetraiodide was produced, along with elemental iodine as its pyridine complex. The two reduction products were identified by analysis after washing with chloroform to remove the iodine liberated in the reaction. Further identification of the tantalum product was provided by x-ray diffraction data, which compared favorably to those obtained for samples of the tetraiododi(pyridine)niobium (IV) as shown in Table V. 

Thus, the niobium hydrides in fact do exhibit the desired hydridic character they can reduce acetone but cannot react with CO reduction products. From the color, the absence of any NMR, and the formation of 1 mol of free cyclo-pentadiene, the niobium-containing product appears to be a monocyclopenta-dienyl Nb(IV) complex. The rather facile removal of one of the Cp groups appears somewhat surprising but has been observed in related systems. For example, alcoholysis of Cp2TiCl2 gives CpTiC OEt) plus cyclopentadiene, even at room temperature (26). 

The first tris(arene) niobium complex, [Nb(l-4-jj -anthracene)3], prepared by the Na or K anthracene reduction of NbCl4(THF)2, undergoes facile anthracene displacement in the presence of CO to afford [K(18-crown-6)(THF)2][Nb(CO)6]. Reduction of TaCls by sodium naphthalene provides [Na(THF)][Ta(jj -naphthalene)3], the first homoleptic naphthalene complex of a third row transition metal. This complex reacts with CO and anthracene to give [Ta(CO)e] and [Ta(l-4-jj -anthracene)3], respectively. The latter product reacts with cyclooctatetraene (COT) to give [Ta(COT)3]-.==i= ... 

Vanadium, Niobium and Tantalum An entry into Cp 2Ta chemistry has been developed with the reaction of Cp TaCl3PMe3 with LiCp. The complex Cp 2Nb(BH4) provides an appropriate entry into Cp 2NbH3 chemistry,A study of the electrochemistry of Cp2NbCl2 has led to the identification of the one-electron reduction product (Cp2NbCl3NbCp2l and (Cp2NbCl]2 on the way to niobocene. Reactions of mono and dihydrides of Cp2TaH(C0)... 

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. 

Apparently the dissociation is enhanced in pyridine, owing to formation of the stable complexes of both niobium (IV) iodide and elemental iodine. Although some reduction of tantalum (V) iodide also occurred in pyridine, the analytical data suggest that this reaction was not complete, and that some unreduced tantalum (V) iodide remained in the washed product. By comparison with niobium (V) iodide, the smaller extent of reduction of tantalum (V) iodide may be accounted for in terms of the greater stability of the latter toward the dissociation shown in Equation 8. 

Numerous complexes of niobium(IV) and niobium(V) halides with various nitrogen-donor ligands have been reported in the literature. The products obtained from these reactions are critically dependent upon the reaction conditions. " Tetrakis(isothiocyanato)bis(2,2 -bipyridine)niobium(IV) can be prepared directly from the hexakis(isothiocyanato)niobate(IV) complex or by the reduction of the hexakis(isothiocyanato)niobate(V) ion according to the published method. This method can also be extended to the preparation of other analogous complexes. 

Tetrakis(isothiocyanato)bis(2,2 -bipyridine)niobium(IV) is a medium-brown crystalline solid. It is only very slightly soluble in 1,2-dichloroethane, dichlo-romethane, and acetonitrile. These extremely dilute solutions of the complex readily decompose in the presence of oxygen or moisture however, in the solid state the complex appears to be air-stable. It does not melt or decompose below ca. 275°. The infrared spectrum, ultraviolet spectrum, magnetic moment, and x-ray powder diffraction pattern of the product obtained by this procedure are essentially identical to those obtained for the product prepared by the reduction of the hexakis(isothiocyanate)niobate(V) complex. ... 

Very recently, the groups of Bergman and Arnold reported that a niobium-imido complex catalyzes an efficient and selective partial hydrogenation of 1-phenyl-1-propyne to Z-p-methylstyrene under H2/CO mixtures (Fig. 14) [36], An Nb(V) metallacyclopropene complex similar to the previous Ti system was proposed, which was followed by a-bond metathesis with H2 and subsequent reductive elimination to yield the Z-alkene. An excess of CO is required not only for catalyst stability but also for achieving catalyst turnover by replacing the product from the Nb complex. However, only one substrate was included in this report. 

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