Vanadium(III) Chloride

Vanadium(III) chloride describes the inorganic compound with the formula VCl₃ and its hydrates. It forms a purple anhydrous form and a green hexahydrate Cl₂(H₂O)₄l·2H₂O. These hygroscopic salts are common precursors to other vanadium(III) complexes and is used as a mild reducing agent.

Structure and electronic configuration

VCl₃ has the common layered BiI₃ structure, a motif that features hexagonally closest-packed chloride framework with vanadium ions occupying the octahedral holes. VBr₃ and VI₃ adopt the same structure, but VF₃ features a structure more closely related to ReO₃. The V³⁺cation has a ''d''² electronic configuration with two unpaired electrons, making the compound paramagnetic.Greenwood, N. N. and Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. p. 990 . VCl₃ is a Mott insulator and undergoes an antiferromagnetic transition at low temperatures.

Solid hexahydrate, Cl₂(H₂O)₄l·2H₂O, has a monoclinic crystal structure and consists of slightly distorted octahedral ''trans''- Cl₂(H₂O)₄sup>+ centers as well as chloride and two molecules of water of crystallization. The hexahydrate phase loses two water of crystallization to form the tetrahydrate if heated to 90 °C in a stream of hydrogen chloride gas.

Uses

Solutions of vanadium(III) chloride in sulfuric acid and hydrochloric acid are used as electrolytes in vanadium redox batteries. It is also used as a mild Lewis acid in organic synthesis. One example of such is its use as a catalyst in the cleavage of the acetonide group. Another example of the use of VCl₃ as a reducing agent is shown in the determination of nitrate and nitrite concentration in water, where VCl₃ reduces nitrate to nitrite. This method is a safer alternative to the cadmium column method.

Preparation

VCl₃ is prepared by heating VCl₄ at 160–170 °C under a flowing stream of inert gas, which sweeps out the Cl₂. The bright red liquid converts to a purple solid.

The vanadium oxides can also be used to produce vanadium(III) chloride. For example, vanadium(III) oxide reacts with thionyl chloride at 200 °C:
:V₂O₃ + 3 SOCl₂ → 2 VCl₃ + 3 SO₂
The reaction of vanadium(V) oxide and disulfur dichloride also produces vanadium(III) chloride with the release of sulfur dioxide and sulfur.

The hexahydrate can be prepared by evaporation of acidic aqueous solutions of the trichloride.

Reactions

Heating of VCl₃ decomposes with volatilization of VCl₄, leaving VCl₂ above 350 °C. Upon heating under H₂ at 675 °C (but less than 700 °C), VCl₃ reduces to greenish VCl₂.
:: 2 VCl₃ + H₂ → 2 VCl₂ + 2 HCl

Comproportionation of vanadium trichloride and vanadium(V) oxides gives vanadium oxydichloride:
:V₂O₅ + VOCl₃ + 3 VCl₃ → 6 VOCl₂

The heating of the hexahydrate does not give the anhydrous form, instead undergoes partial hydrolysis and forms vanadium oxydichloride at 160 °C. In an inert atmosphere, it forms a trihydrate at 130 °C and at higher temperatures, it forms vanadium oxychloride.

Vanadium trichloride catalyses the pinacol coupling reaction of benzaldehyde (PhCHO) to 1,2-diphenyl-1,2-ethanediol by various reducing metals such as zinc:
:Zn + 2 H₂O + 2 PhCHO → (PhCH(OH))₂ + Zn(OH)₂

Complexes

VCl₃ forms colorful adducts and derivatives with a broad scale of ligands. VCl₃ dissolves in water to give the aquo complexes. From these solutions, the hexahydrate Cl₂(H₂O)₄l^.2H₂O crystallizes. In other words, two of the water molecules are not bound to the vanadium, whose structure resembles the corresponding Fe(III) derivative. Removal of the two bound chloride ligands gives the green hexaaquo complex (H₂O)₆sup>3+.

With tetrahydrofuran, VCl₃ forms the red/pink complex VCl₃(THF)₃, and zinc-powder reduction of the latter gives Caulton's reagent, V(THF)₃)₂Cl₃sub>2 n₂Cl₆

Vanadium(III) chloride reacts with acetonitrile to give the green adduct VCl₃(MeCN)₃. When treated with KCN, VCl₃ converts to (CN)₇sup>4− (early metals commonly adopt coordination numbers greater than 6 with compact ligands). Complementarily, larger metals can form complexes with rather bulky ligands. This aspect is illustrated by the isolation of VCl₃(NMe₃)₂, containing two bulky NMe₃ ligands. Vanadium(III) chloride is able to form complexes with other adducts, such as pyridine or triphenylphosphine oxide.

Organometallic derivatives

Vanadium(III) chloride as its thf complex is a precursor to V(mesityl)₃.
:: VCl₃(THF)₃ + 3 LiC₆H₂-2,4,6-Me₃ → V(C₆H₂-2,4,6-Me₃)₃(THF) + 3 LiCl

References

{{DEFAULTSORT:Vanadium(Iii) Chloride
Vanadium(III) compounds
Chlorides
Metal halides