A solution and solid state study of niobium complexes University of ...

Chapter 2
Reactions of dialkylamides with alcohols: Dialkylamides, M(NR2)x (R = Me,
Et, SiMe3), react with alcohols according to the equation below:
2.7.2 Niobium alkoxides
M(NR2)x + x R’OH → M(OR’)x + x R2NH (2.12)
Knowledge of niobium and tantalum alkoxides is almost exclusively limited to those
in which the metal is in the +5 oxidation state. In solution these alkoxides exist in
equilibrium between the monomeric, M(OR)x, and dimeric ,M2(OR)10, forms with the
ratio of monomer to dimer depending on the solvent, temperature and on the R
group. A few Nb(IV) alkoxide complexes have been reported. Many of these are
products of the reduction of NbCl5 in alcohols, though several were prepared by the
reaction of alcohols with other Nb(IV) compounds. 73,74
The process of niobium and tantalum homoleptic alkoxides starts with the metal
chlorides. The reaction of niobium pentachloride with alcohol in the presence of
ammonia was described by Bradley et al. 75 Firstly, the NbCl5 is dissolved in an
organic solvent (benzene) then the alcohol is added, resulting in a vigorous reaction.
Secondly, an excess of ammonia is added to the reaction mixture. This forms the
desired niobium homoleptic alkoxide which is isolated by distillation after ammonium
chloride is removed by filtration.
Figure 2.13: Structure of [Nb(OMe)5]2.
The methoxides and ethoxides of tantalum are more volatile than the niobium
counterparts whereas for higher n-alkoxides, the reverse is true. With increasing
chain length the volatility of both niobium and tantalum alkoxides decrease.
73 F. A. Cotton, M. P. Diebold, W. J. Roth, Inorg. Chem., 26, 3323, 1987.
74 M. Melnik, P. Sharrock, Can. J. Chem., 63, 57, 1985.
75 D. C. Bradley, B. N. Chakravarti, W. Wardlaw, J. Chem. Soc., 2381, 1956.
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