Titanium tetrachloride

Titanium tetrachloride or titanium(IV) chloride is the chemical compound with the formula TiCl₄.

TiCl₄ is an important intermediate in the production of titanium metal and other titanium compounds. It is an unusual example of a liquid metal halide that is very volatile in air, where it forms spectacular opaque clouds of titanium dioxide (TiO₂) and hydrogen chloride (HCl). It is sometimes humourously referred to as "tickle".^[1]

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Properties and structure

TiCl₄ is a dense, colourless distillable liquid, although crude samples may be yellow or even red-brown. It is one of the rare transition metal chlorides that is in liquid state at room temperature, VCl₄ being another example. This distinctive property arises from the fact that TiCl₄ is molecular; that is, each TiCl₄ molecule is relatively weakly associated with its neighbours. Most metal chlorides are polymers, where the chloride atoms bridge between the metals. The attraction between the individual TiCl₄ molecules is weak, primarily van der Waals forces, and these weak interactions result in low melting and boiling points, similar to those of CCl₄.

TiCl₄ is tetrahedral, which is consistent with its description as Ti⁴⁺ surrounded by four Cl^- ligands. Ti⁴⁺ has a "closed" electronic shell, with the same number of electrons as the inert gas argon. This configuration leads to highly symmetrical structures, hence the tetrahedral shape of the molecule.

TiCl₄ is soluble in toluene and chlorocarbons, as are other non-polar species. Evidence exists that certain arenes form complexes of the type [(C₆R₆)TiCl₃]⁺. TiCl₄ reacts exothermically with donor solvents such as THF to give hexacoordinated adducts.^[2] Bulkier ligands (L) give pentacoordinated derivatives TiCl₄L.

The main problem with handling TiCl₄, aside from its tendency to release corrosive hydrogen chloride, is the formation of titanium oxides and oxychlorides that cement stoppers and syringes.

Production

TiCl₄ is produced by the Chloride process, which involves the reduction of titanium oxide ores, typically ilmenite or rutile, with carbon under flowing chlorine at 900 °C. Impurities are removed by distillation to afford pure TiCl₄.

2FeTiO₃ + 7Cl₂ + 6C → 2TiCl₄ + 2FeCl₃ + 6CO

TiCl₄ is inexpensive, thus it is typically purchased for laboratory operations.

Applications

Production of titanium metal

TiO₂ + 2Cl₂ + 2C → TiCl₄ + 2CO

Reduction of TiCl₄ using magnesium metal produces titanium metal; this is in fact the final step of the Kroll process.

2Mg + TiCl₄ → 2MgCl₂(l) + Ti

Liquid sodium has sometimes been used instead of magnesium as the reducing agent in the production of titanium metal.

4Na + TiCl₄ → 4NaCl + Ti

Production of titanium dioxide

Around 90% of the TiCl₄ production is used to make pigment; titanium(IV) oxide (TiO₂). Key is the reaction of TiCl₄ with water to form hydrochloric acid:

TiCl₄ + 2H₂O → TiO₂ + 4HCl

Or sometimes it is oxidised directly with pure oxygen:

TiCl₄ + 2O₂ → TiO₂ + 2Cl₂

Smoke-screens

In the past titanium tetrachloride has also been used to create naval smokescreens. When sprayed into the air, TiCl₄ rapidly reacts with atmospheric moisture:

TiCl₄ + 2H₂O → TiO₂ + 4HCl

The hydrogen chloride immediately absorbs more water to form tiny droplets of hydrochloric acid, which (depending on humidity) may absorb still more water, to produce large droplets that efficiently scatter light. In addition, the intensely white titanium dioxide is also an efficient light scatterer. Due to the corrosiveness of this smoke, however, TiCl₄ is no longer used.

Chemical reactions

Organometallic and inorganic chemistry

TiCl₄ adopts similar structures to TiBr₄ and TiI₄; the three compounds share many similarities. TiCl₄ and TiBr₄ react to give mixed halides TiCl_4-xBr_x, where x = 0, 1, 2, 3, 4. Magnetic resonance measurements also indicate that halide exchange is also rapid between TiCl₄ and VCl₄.^[3]

TiCl₄ is a superb and versatile Lewis acid, as indicated by its tendency to hydrolyze, which implicates the intermediacy of TiCl₄(H₂O). With THF, TiCl₄ forms yellow crystals of TiCl₄(THF)₂. With Cl^- donors, TiCl₄ reacts to form sequentially [Ti₂Cl₉]^-, [Ti₂Cl₁₀]^2-, and [TiCl₆]^2-.^[4] Interestingly, the reaction of chloride ions with TiCl₄ depends on the counterion. NBu₄Cl reacts with TiCl₄ to give the pentacoordinate complex NBu₄TiCl₅, whereas smaller NEt₄ reacts to give (NEt₄)₂Ti₂Cl₁₀. These reactions highlight the influence of electrostatic forces on the structures of compounds with highly ionic bonding.

Much of the extensive organometallic chemistry of titanium starts from TiCl₄. Its most important reaction is with sodium cyclopentadienyl to give titanocene dichloride, TiCl₂(C₅H₅)₂. This compound is used in organic synthesis (Tebbe's reagent). Arenes, such as C₆(CH₃)₆ reacts to give [Ti(C₆(CH₃)₆)Cl₃]⁺, which is a piano-stool complex.^[5] This reaction illustrates the extraordinary Lewis acidity of the TiCl₃⁺ entity, which is derived from TiCl₄ using the even stronger chloride-abstracting agent AlCl₃.

TiCl₄ reacts with four equivalents LiNMe₂ to give Ti(NMe₂)₄, a yellow, benzene-soluble liquid.^[6] This molecule is tetrahedral, with planar nitrogen centers.^[7]

Reagent in organic synthesis

It is widely used in organic synthesis as a Lewis acid,^[8] for example in the Mukaiyama aldol reaction. Key to this application is the tendency of TiCl₄ to interact with aldehydes, RCHO, to give adducts such (RCHO)TiCl₄OC(H)R. It is also used in the McMurry reaction in conjunction with Zn, LiAlH₄, or another reducing agent in order to join two carbonyls in making a carbon-carbon double bond.

Olefin polymerisation

This compound and many of its derivatives are important precursors to Ziegler-Natta catalysts.

Reduction

Reduction of TiCl₄ yields TiCl₃. Reduction of TiCl₄ with aluminium in THF results in the light-blue THF-adduct TiCl₃(THF)₃.

Toxicity and safety considerations

Given the tendency of TiCl₄ to hydrolyze, the hazards generally arise from the effect of hydrogen chloride. TiCl₄ is a strong Lewis acid, exothermically forming adducts with even weak bases such as THF and explosively with water, again releasing HCl.

References

1. ^ [1] American Chemistry
2. ^ L. E. Manzer (1982). "Tetrahydrofuran Complexes of Selected Early Transition Metals". Inorganic Synthesis 21: 135-40.
3. ^ S. P. Webb, M. S. Gordon (1999). "Intermolecular Self-Interactions of the Titanium Tetrahalides TiX4 (X = F, Cl, Br)". J. Am. Chem. Soc. 121: 2552-2560. doi:10.1021/ja983339i.
4. ^ C. S. Creaser , J. A. Creighton (1975). "Pentachloro- and Pentabromotitanate(IV) ions". Journal of the Chemical Society, Dalton Transactions: 1402-1405. doi:10.1039/DT9750001402.
5. ^ F. Calderazzo, I. Ferri, G. Pampaloni, S. Troyanov (1996). "η⁶-Arene Derivatives of Titanium(IV), Zirconium(IV) and Hafnium(IV)". Journal of Organometallic Chemistry 518: 189-196. doi:10.1016/0022-328X(96)06194-3.
6. ^ D. C. Bradey, M. Thomas (1960). "Some Dialkylamino-derivatives of Titanium and Zirconium". Journal of the Chemical Society: 3857-3861. doi:10.1039/JR9600003857.
7. ^ M. E. Davie, T. Foerster, S. Parsons, C. Pulham, D. W. H. Rankin, B. A. Smart (2006). "The Crystal Structure of Tetrakis(dimethylamino)titanium(IV)". Polyhedron 25: 923-929. doi:10.1016/j.poly.2005.10.019.
8. ^ L.-L. Gundersen, F. Rise, K. Undheim (2004). "Titanium(IV) chloride", Encyclopedia of Reagents for Organic Synthesis, L. Paquette, J. Wiley & Sons. 

General reading

• Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
• Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.