Niobium(V) chloride

Structure and properties

Niobium(V) chloride forms chloro-bridged dimers in the solid state (see figure). Each niobium centre is six-coordinate, but the octahedral coordination is significantly distorted. The equatorial niobium–chlorine bond lengths are 225 pm (terminal) and 256 pm (bridging), whilst the axial niobium-chlorine bonds are 229.2 pm and are deflected inwards to form an angle of 83.7° with the equatorial plane of the molecule. The Nb–Cl–Nb angle at the bridge is 101.3°. The Nb–Nb distance is 398.8 pm, too long for any metal-metal interaction. NbBr5, NbI5, TaCl5 TaBr5 and TaI5 are isostructural with NbCl5.

Preparation

Industrially, niobium pentachloride is obtained by direct chlorination of niobium metal at 300 to 350 °C: :2Nb + 5Cl2 → 2NbCl5

In the laboratory, niobium pentachloride is often prepared from Nb2O5, the main challenge being incomplete reaction to give NbOCl3. The conversion can be effected with thionyl chloride: It also can be prepared by chlorination of niobium pentoxide in the presence of carbon at 300 °C.

Uses

Niobium(V) chloride is the main precursor to the alkoxides of niobium, which find uses in sol-gel processing. It is also the precursor to many other Nb-containing reagents, including most organoniobium compounds.

In organic synthesis, NbCl5 is a very specialized Lewis acid in activating alkenes for the carbonyl-ene reaction and the Diels-Alder reaction. Niobium chloride can also generate N-acyliminium compounds from certain pyrrolidines which are substrates for nucleophiles such as allyltrimethylsilane, indole, or the silyl enol ether of benzophenone.

References

References

1. {{Cotton&Wilkinson4th
2. Cotton, F.A., P. A. Kibala, M. Matusz and R. B. W. Sandor. (1991). "Structure of the Second Polymorph of Niobium Pentachloride". [[Acta Crystallogr. C]].
3. {{cite encyclopedia. Joachim Eckert. (2005). Wiley-VCH
4. (1957). "Inorganic Syntheses".
5. (2005). "Studies on the Niobium Pentachloride-Mediated Nucleophilic Additions to an Enantiopure Cyclic N-acyliminium Ion Derived from (''S'')-malic acid". J. Braz. Chem. Soc..