Titanium tetrachloride

Titanium tetrachloride is the inorganic compound with the formulaTiCl₄. It is an important intermediate in the production of titanium metal and the pigment titanium dioxide.TiCl₄ is a volatile liquid. Upon contact with humid air, it forms thick clouds of titanium dioxide (TiO₂) and hydrochloric acid, a reaction that was formerly exploited for use in smoke machines. It is sometimes referred to as "tickle" or "tickle 4", as a phonetic representation of the symbols of its molecular formula (TiCl₄).

TiCl₄ is a dense, colourless liquid, although crude samples may be yellow or even red-brown. It is one of the rare transition metal halides that is a liquid at room temperature, VCl₄ being another example. This property reflects the fact that molecules ofTiCl₄ weakly self-associate. Most metal chlorides are polymers, wherein the chloride atoms bridge between the metals. Its melting point is similar to that of CCl₄.

Ti⁴⁺ has a "closed" electronic shell, with the same number of electrons as the noble gas argon. The tetrahedral structure forTiCl₄ is consistent with its description as a d⁰ metal center (Ti⁴⁺) surrounded by four identical ligands. This configuration leads to highly symmetrical structures, hence the tetrahedral shape of the molecule.TiCl₄ adopts similar structures to TiBr₄ and TiI₄; the three compounds share many similarities.TiCl₄ andTiBr₄ react to give mixed halidesTiCl_4−xBr_x, where x = 0, 1, 2, 3, 4. Magnetic resonance measurements also indicate that halide exchange is also rapid betweenTiCl₄ andVCl₄.

TiCl₄ is soluble in toluene and chlorocarbons. Certain arenes form complexes of the type[(C₆R₆)TiCl₃]⁺.TiCl₄ reacts exothermically with donor solvents such as THF to give hexacoordinated adducts. Bulkier ligands (L) give pentacoordinated adductsTiCl₄L.

TiCl₄ is produced by the chloride process, which involves the reduction of titanium oxide ores, typically ilmenite (FeTiO₃), with carbon under flowing chlorine at 900 °C. Impurities are removed by distillation.

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

The coproduction of FeCl₃ is undesirable, which has motivated the development of alternative technologies. Instead of directly using ilmenite, "rutile slag" is used. This material, an impure form ofTiO₂, is derived from ilmenite by removal of iron, either using carbon reduction or extraction with sulfuric acid. CrudeTiCl₄ contains a variety of other volatile halides, including vanadyl chloride (VOCl₃), silicon tetrachloride (SiCl₄), and tin tetrachloride (SnCl₄), which must be separated.

The world's supply of titanium metal, about 250,000 tons per year, is made fromTiCl₄. The conversion involves the reduction of the tetrachloride with magnesium metal. This procedure is known as the Kroll process:

2 Mg + TiCl₄ → 2 MgCl₂ + Ti

In the Hunter process, liquid sodium is the reducing agent instead of magnesium.

Around 90% of theTiCl₄ production is used to make the pigment titanium dioxide (TiO₂). The conversion involves hydrolysis ofTiCl₄, a process that forms hydrogen chloride:

TiCl₄ + 2 H₂O → TiO₂ + 4 HCl

In some cases,TiCl₄ is oxidised directly with oxygen:

TiCl₄ + O₂ → TiO₂ + 2 Cl₂

It has been used to produce smoke screens since it produces a heavy, white smoke that has little tendency to rise. "Tickle" was the standard means of producing on-set smoke effects for motion pictures, before being phased out in the 1980s due to concerns about hydrated HCl's effects on the respiratory system.

Titanium tetrachloride is a versatile reagent that forms diverse derivatives including those illustrated below.

A characteristic reaction ofTiCl₄ is its easy hydrolysis, signaled by the release of HCl vapors and titanium oxides and oxychlorides. Titanium tetrachloride has been used to create naval smokescreens, as the hydrochloric acid aerosol and titanium dioxide that is formed scatter light very efficiently. This smoke is corrosive, however.

Alcohols react withTiCl₄ to give alkoxides with the formula[Ti(OR)₄]_n (R = alkyl, n = 1, 2, 4). As indicated by their formula, these alkoxides can adopt complex structures ranging from monomers to tetramers. Such compounds are useful in materials science as well as organic synthesis. A well known derivative is titanium isopropoxide, which is a monomer. Titanium bis(acetylacetonate)dichloride results from treatment of titanium tetrachloride with excess acetylacetone:

TiCl₄ + 2 Hacac → Ti(acac)₂Cl₂ + 2 HCl

Organic amines react withTiCl₄ to give complexes containing amido (R₂N⁻-containing) and imido (RN²⁻-containing) complexes. With ammonia, titanium nitride is formed. An illustrative reaction is the synthesis of tetrakis(dimethylamido)titaniumTi(N(CH₃)₂)₄, a yellow, benzene-soluble liquid: This molecule is tetrahedral, with planar nitrogen centers.

4 LiN(CH₃)₂ + TiCl₄ → 4 LiCl + Ti(N(CH₃)₂)₄

TiCl₄ is a Lewis acid as implicated by its tendency to hydrolyze. With the ether THF,TiCl₄ reacts to give yellow crystals ofTiCl₄(THF)₂. With chloride salts,TiCl₄ reacts to form sequentially[Ti₂Cl₉]⁻,[Ti₂Cl₁₀]²⁻ (see figure above), and[TiCl₆]²⁻. The reaction of chloride ions withTiCl₄ depends on the counterion.[N(CH₂CH₂CH₂CH₃)₄]Cl andTiCl₄ gives the pentacoordinate complex[N(CH₂CH₂CH₂CH₃)₄][TiCl₅], whereas smaller[N(CH₂CH₃)₄]⁺ gives[N(CH₂CH₃)₄]₂[Ti₂Cl₁₀]. These reactions highlight the influence of electrostatics on the structures of compounds with highly ionic bonding.

Reduction ofTiCl₄ with aluminium results in one-electron reduction. The trichloride (TiCl₃) and tetrachloride have contrasting properties: the trichloride is a colored solid, being a coordination polymer, and is paramagnetic. When the reduction is conducted in THF solution, the Ti(III) product converts to the light-blue adductTiCl₃(THF)₃.

The organometallic chemistry of titanium typically starts fromTiCl₄. An important reaction involves sodium cyclopentadienyl to give titanocene dichloride,TiCl₂(C₅H₅)₂. This compound and many of its derivatives are precursors to Ziegler–Natta catalysts. Tebbe's reagent, useful in organic chemistry, is an aluminium-containing derivative of titanocene that arises from the reaction of titanocene dichloride with trimethylaluminium. It is used for the "olefination" reactions.

Arenes, such asC₆(CH₃)₆ react to give the piano-stool complexes[Ti(C₆R₆)Cl₃]⁺ (R = H,CH₃; see figure above). This reaction illustrates the high Lewis acidity of theTiCl+3 entity, which is generated by abstraction of chloride fromTiCl₄ by AlCl₃.

TiCl₄ finds occasional use in organic synthesis, capitalizing on its Lewis acidity, its oxophilicity, and the electron-transfer properties of its reduced titanium halides. It is used in the Lewis acid catalysed aldol addition Key to this application is the tendency ofTiCl₄ to activate aldehydes (RCHO) by formation of adducts such as(RCHO)TiCl₄OC(H)R.

Hazards posed by titanium tetrachloride generally arise from its reaction with water that releases hydrochloric acid, which is severely corrosive itself and whose vapors are also extremely irritating.TiCl₄ is a strong Lewis acid, which exothermically forms adducts with even weak bases such as THF and water.

• Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. San Diego, CA: Academic Press. ISBN 978-0-12-352651-9.
• Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.

• Titanium tetrachloride: Health Hazard Information
• NIST Standard Reference Database
• ChemSub Online: Titanium tetrachloride