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.

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.
 * 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 VCl4 + 8 HBr  →  2 VBr3  +  8 HCl  +  Br2

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