Interaction of niobium and tantalum pentahalides with O-donors: coordination chemistry and activation reactions

The chemistry of niobium and tantalum pentahalides, MX(5), with oxygen compounds is reviewed herein. The polynuclear structure of MX(5) is readily broken by addition of oxygen-containing organic molecules, L, to give either mononuclear or ionic dinuclear coordination adducts. Then activation of the organic ligand may take place favoured by several factors, i.e. low M-X bond energy, high temperature, presence of more than one oxygen function within L, L/M molar ratio ≥ 2. The activation reactions are often uncommon in the context of metal halides; they include the cleavage of C(sp3)-O, C(sp2)-O, C-H and C-C bonds, and eventual successive rearrangements proceeding with C-O or C-C couplings. The recently elucidated reactivity of MX(5) with limited amounts of oxygen compounds will be presented, and possible connections with the relevant MX(5)-directed syntheses reported in the literature will be outlined.     

Reactivity of niobium and tantalum pentahalides with cyclic ethers and the isolation and characterization of intermediates in the polymerization of tetrahydrofuran

The complexes MX5(THF) (M = Nb, X = Cl, 2a; M = Ta, X = F, 2c, X = Cl, 2d) and [MX4(THF){O(CH2)4O(CH2)3CH2)}][MX6] (M = Nb, X = Cl, 3a; M = Ta, X = Cl, 3d, X = Br, 3e, X = I, 3f) result from reactions of MX5 with 0.5 and 1.5 equiv of THF, respectively. Compounds 3 contain the unprecedented 4-(tetrahydrofuran-1-ium)-butan-1-oxo ligand and are likely to play a role in the course of THF polymerization catalyzed by MX5. The addition of L (L = 2,5-dimethyltetrahydrofuran, tetrahydropyran, 1,4-dioxane) to MX5 results in the formation of the hexacoordinated complexes MX5(L). The molecular structures of 2d, 3d, and NbCl5(dioxane), 6a, have been ascertained by X-ray diffraction studies.     

Infrared spectra,vibrational assignment,normal coordinate analysis and thermodynamic functions of tetracyanomethane

The infrared spectrum of tetracyanomethane in acetonitrile solution has been recorded and the full set of fundamental frequencies assigned. A normal coordinate analysis has been carried out and the values of the calculated force constants are compared with those of related molecules. Some thermodynamic functions have also been computed.     

Infrared study on niobium pentahalides and oxytrihalides trapped in cryogenic matrices

The i.r. spectra of Ar and N₂ matrices trapping vapours coming from solid NbX₅, NbOX₃ and mixtures NbX₅/Nb₂O₅ (X = Cl, Br, I) have been investigated. Differences in the spectra of NbCl₅ taken at different vaporization temperatures point to the presence of polymeric species in the vapour. The bands due to monomeric NbCl₅, at 443 cm⁻¹ (the E′ NbCl stretch) and at 395 cm⁻¹ (the A^″₂ NbCl stretch), can be recognized since they are more intense for matrices trapping vapours taken at higher furnace temperature. Annealing experiments also help in assigning monomer absorptions since they decrease in intensity under diffusion. It is remarkable, however, that the polymeric species formed in the matrix on annealing are different from that present in the vapour. The furnace temperature does not affect significantly the spectrum of NbBr₅ and the only two observed absorptions, at 315 cm⁻¹ (the E′ NbBr stretch) and at 288 cm⁻¹ (the A^″₂ NbBr stretch) are referred to the monomer. Here, also, the assignment is supported by the decrease in intensity undertaken by these two absorptions on annealing the matrix.Changes in furnace temperature do not give rise to detectable changes in the spectrum of matrices trapping Nb oxytrihalides, NbOX₃. In this case, the absorptions of monomers have been recognized on the grounds of the tendency to decrease under annealing. The A₁ NbO stretch is observed at 993 (X = Cl), 989 (X = Br) and 977 (X = I) cm⁻¹. The E NbX stretch is at 443 (X = Cl) and 339 (X = Br) cm⁻¹. The other A₁ NbX stretch has never been observed. The spectrum is complicated by band multiplicity probably due to multiple site trapping.     

Normal coordinate analysis of trans-stilbene and tolane

The vibrational frequencies and modes of trans-stilbene and tolane have been calculated using a simplified overlay valence force field previously developed for a series of smaller conjugated molecules. The force constants have been directly transferred assuming that the large molecules can be regarded as weakly coupled systems. Several unexpected changes in frequencies and modes on isotopic substitution are discussed, the most interesting ones occurring in the Raman spectrum of α,α′-¹³C substituted trans-stilbene. On the basis of the calculations many revisions in the assignments have been made. The rms frequency deviations are 8.0 cm⁻¹ and 6.9 cm⁻¹ for trans-stilbene and tolane, respectively.     

Normal coordinate analysis of 1,4-diethynylbenzene

Normal coordinate analysis has been carried out for the 1,4-diethynyl benzene molecule on the basis of experimental frequencies from the natural compound and its deuterated derivatives. A canonic force field has been set up by transferring force constants from related molecules. This force field has been refined to the experimental frequencies. A reassignment of the vibrational spectrum of 1,4-diethynyl benzene has been carried out.     

Separation of niobium and tantalum with cupferron

An attempt to separate niobium and tantalum by cupfcrron was only moderately successful at pH 4.5 to 5.5 in the presence of a magnesia mixture as a coagulating agent. A more satisfactory separation of niobium and tantalum from each other, tried out up to ratios of 30:1 and 1.30, is effected with Sn⁺² or Sn⁺⁴ as a co-precipitating agent under the conditions described niobium can be separated, in the presence of complexone III, from almost all the ions except U, Be, Ti and PO₄^-3. Iron and other tervalent elements, when present in 100 fold excess with respect to niobium, require double precipitation The method gives highly satisfactory results when applied to the analysis of niobium in niobium-molybdenum stainless steel.The use of titanium as a co-precipitant is less successful than that of tin     

Formation of niobium and tantalum ylide complexes

The reactions of tri(bis(ethyl)amino)phosphorus ylide (Et₂N)₃PCH₂ with cyclopentadienyl (Cp) metal (V) tetrachloride CpMCl₄ (M = Nb 1; Ta 3) and pentamethylcycopentadienyl (Cp^∗) metal (V) tetrachloride Cp^∗MCl₄ (M = Nb 2; Ta 4) were investigated. The hexa-coordinate ylide adducts complexes 5 (CpNbCl₄(H₂CP(NEt₂)₃)), 6 (Cp^∗NbCl₄(H₂CP(NEt₂)₃)) and 8 (Cp^∗TaCl₄(H₂CP(NEt₂)₃)) with pseudo-octahedral geometry were structurally analyzed with X-ray diffraction. Compound 4 (Cp^∗TaCl₄) reacted with three molar equivalent of phosphorus ylide to form one ionic complex 9 ([H₃C-P(NEt₂)₃][Cp^∗TaCl₅]) which was also structurally analyzed with X-ray diffraction. The possible formation mechanism of compound 9 has been discussed.     

Separation of niobium and tantalum with n-benzoyl-n-phenylhydroxylamine

The use of N-benzoyl-N-phenylhydroxylamine for the separation of niobium and tantalum, allows a satisfactory estimation of niobium from a tartrate solution at an acidity of 2.0N. The pH range for complete precipitation can be extended to 6.5. For tantalum precipitation, the pH of the solution should be below 1.5 and the acidity may even be above 2.0N. At pH 3.5–6.5, niobium is completely precipitated and tantalum remains in solution; the latter is precipitated by lowering the pH. Niobium and tantalum in ratios of 1:16 to 100:1 can be separated by a single precipitation, in the case of a ratio of 1:100 precipitation must be carried out twice. Titanium, zirconium, vanadate and molybdate interfere with the determination of niobium though other ions have no effect in the presence of complexone III and tartaric acid. The precipitates are granular and easy to filter and wash. The time taken for a complete analysis is much less than that of other methods     

2-Methoxyethanol complexes of niobium and tantalum

The reactions of Et₄NMCl₆ and MCl₅ (M  Nb, Ta) with 2-methoxyethanol have been studied and complexes MCl₃(OCM₂CH₂OMe)₂ and Et₄N TaCl_5−x (OCH₂CH₂OMe)_x (x = 1,2) have been isolated, and characterised by i.r. and NMR spectra. The reactions of Et₄NMCl₆ with MeOH are also reported.     

The selenide systems of niobium and tantalum

The selenide systems of niobium and tantalum were investigated under high pressures in the composition limits of MX₂ to MX₃. Two distinct phases, NbSe₂ and NbSe₃, were obtained in the niobium system. In the tantalum system, there were three phases, TaSe₂, “TaSe₃,” which has been thought of as a triselenide, and stoichiometric TaSe₃. The ratio of $\text{Se}\text{Ta}$ in “TaSe₃” was 2.8. Conversions of the triselenides under high pressure to NbSe₂ or TaSe₂ were observed at the starting composition of $\text{Se}\text{M}\text{= 2.0}$ and to “TaSe₃” at $\text{Se}\text{Ta}\text{= 2.8}$.     

Normal coordinate analysis of cis-1,4-polybutadiene

The fundamental vibrational frequencies of an isolated chain of cis-1,4-polybutadiene have been calculated. Infrared and polarized Raman data are used in the force constant refinement routine. The assignments of the vibrational frequencies are discussed in terms of the potential energy distribution.     

Normal coordinate analysis of the indole ring

Normal coordinate calculations have been performed for indole and 11 deuterated indoles. The set of valence force constants obtained is able to reproduce i.r. and Raman frequencies with an average error of about 6 cm⁻¹. Extension of the force field to skatole has also been made and the calculated frequency shifts on isotopic substitutions are compared with the experimental ones of tryptophan. The general agreement between the calculation and experiment suggests that the vibrational modes obtained here are of practical use in the interpretation of Raman spectra of proteins containing tryptophan residues.