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 [VCl2(H2O)4]Cl·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 d2 electronic configuration with two unpaired electrons, making the compound paramagnetic. VCl3 is a Mott insulator and undergoes an antiferromagnetic transition at low temperatures.

Solid hexahydrate, [VCl2(H2O)4]Cl·2H2O, has a monoclinic crystal structure and consists of slightly distorted octahedral trans-[VCl2(H2O)4]+ 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 [VCl2(H2O)4]Cl.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 [V(H2O)6]3+.

With tetrahydrofuran, VCl3 forms the red/pink complex VCl3(THF)3. Vanadium(III) chloride reacts with acetonitrile to give the green adduct VCl3(MeCN)3. When treated with KCN, VCl3 converts to [V(CN)7]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 toV(mesityl)3.
 VCl3(THF)3 + 3 LiC6H2-2,4,6-Me3 → V(C6H2-2,4,6-Me3)3(THF) + 3 LiCl

References

Vanadium(III) compounds
Chlorides
Metal halides