Vanadium tetrachloride

Vanadium tetrachloride is the chemical compound with the formula VCl₄. This bright red liquid is an important starting reagent in the preparation of vanadium compounds. It forms adducts with many donor ligands, for example, VCl₄(THF)₂. It is also the precursor to vanadocene dichloride.

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With one more valence electron than diamagnetic TiCl₄, VCl₄ is a paramagnetic liquid. Few chemical compounds are both liquid (at room temperature) and paramagnetic.

VCl₄ is prepared by chlorination of vanadium metal. Notice that VCl₅ cannot be prepared under normal conditions: Cl₂ lacks the oxidizing power to attack VCl₄. In contrast, the heavier analogues NbCl₅ and TaCl₅ are stable and not particularly oxidizing. Of course, VF₅ does exist, reflecting the increased oxidizing power of F₂ vs Cl₂. Indicative of its oxidizing power, VCl₄ releases Cl₂ at its boiling point (standard pressure) to afford vanadium(III) chloride.

Applications

• In organic synthesis, VCl₄ is used to couple phenols. For example, it converts phenol into 4,4'-biphenol.^[1]

2 C₆H₅OH + 2 VCl₄ → HOC₆H₄-C₆H₄OH + 2 VCl₃ + 2 HCl

This reaction highlights the oxidizing ability of VCl₄, consistent with it being resistant to further oxidation by Cl₂.

• VCl₄ is a catalyst for the polymerization of alkenes, especially those useful in the rubber industry. The underlying technology is related to Ziegler-Natta catalysis, which involves the intermediacy of vanadium alkyls.
• Further reinforcing its high oxidizing powers, VCl₄ reacts with HBr to produce VBr₃. The reaction proceeds via VBr₄, which releases Br₂ at room temperature.^[2]

2 VCl₄ + 8 HBr → 2 VBr₃ + 8 HCl + Br₂

Safety considerations

VCl₄ is a volatile, aggressive oxidant that readily hydrolyzes to release HCl.

References

1. ^ M. K. O’Brien, B. Vanasse, “Vanadium(IV) Chloride” in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York.
2. ^ Calderazzo, F.; Maichle-Mossmer, C.; Pampaloni, G. and Strähle, J., "Low-temperature Syntheses of Vanadium(III) and Molybdenum(IV) Bromides by Halide Exchange", Journal of the Chemical Society, Dalton Transactions, 1993, pages 655-8.