Titanium tetrachloride

Titanium tetrachloride
IUPAC name Titanium tetrachloride
Systematic name Tetrachlorotitanium
Other names Titanium(IV) chloride
Identifiers
CAS number	7550-45-0 Y
PubChem	24193
ChemSpider	22615 Y
EC number	231-441-9
UN number	1838
MeSH	Titanium+tetrachloride
RTECS number	XR1925000
Jmol-3D images	Image 1
SMILES Cl[Ti](Cl)(Cl)Cl
InChI InChI=1S/4ClH.Ti/h4*1H;/q;;;;+4/p-4 Y Key: XJDNKRIXUMDJCW-UHFFFAOYSA-J Y InChI=1/4ClH.Ti/h4*1H;/q;;;;+4/p-4/rCl4Ti/c1-5(2,3)4 Key: XJDNKRIXUMDJCW-FOGBWSKZAG
Properties
Molecular formula	TiCl4
Molar mass	189.679 g mol-1
Exact mass	187.823357881 g mol-1
Appearance	Colourless liquid
Density	1.726 g cm-3
Melting point	-25 °C, 248.6 K, -12 °F
Boiling point	135 °C, 408 K, 275 °F
Solubility in water	Reacts
Vapor pressure	1.3 kPa (at 20 °C)
Viscosity	827 μPa s
Structure
Coordination geometry	Tetragonal
Molecular shape	Tetrahedral
Dipole moment	0 D
Thermochemistry
Std enthalpy of formation ΔfHo298	-804.16 kJ mol
Standard molar entropy So298	221.93 J K–1 mol–1
Hazards
MSDS	MSDS
EU Index	022-001-00-5
EU classification	C
R-phrases	R14, R34
S-phrases	(S1/2), S7/8, S26, S36/37/39, S45
NFPA 704	0 3 2 W
Related compounds
Related titanium chlorides	Titanium(II) chloride Titanium(III) chloride
Related compounds	Hafnium(IV) chloride Titanium(IV) bromide Titanium(IV) fluoride Titanium(IV) iodide Zirconium(IV) chloride
Y tetrachloride (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Titanium tetrachloride is the inorganic compound with the formula TiCl₄. It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. TiCl₄ is an unusual example of a metal halide that is highly volatile. Upon contact with humid air, it forms spectacular opaque clouds of titanium dioxide (TiO₂) and hydrogen chloride (HCl).

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 halides that is a liquid at room temperature, VCl₄ being another example. This property reflects the fact that TiCl₄ is molecular; that is, each TiCl₄ molecule is relatively weakly associated with its neighbours. Most metal chlorides are polymers, wherein 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.

Ti⁴⁺ has a "closed" electronic shell, with the same number of electrons as the inert gas argon. The tetrahedral structure for TiCl₄ 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 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.^[1]

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 adducts TiCl₄L.

Production

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 of TiO₂, is derived from ilmenite by removal of iron, either using carbon reduction or extraction with sulfuric acid. Crude TiCl₄ contains a variety of other volatile halides, including vanadyl chloride (VOCl₃), silicon tetrachloride (SiCl₄), and tin tetrachloride (SnCl₄), which must be separated.

Applications

Production of titanium metal

The world's supply of titanium metal, about 4M tons per year, is made from TiCl₄. The conversion takes place by the reduction of the chloride with magnesium metal, and yields titanium metal and magnesium chloride. This procedure is the final step of the Kroll process:

2 Mg + TiCl₄ → 2 MgCl₂ + Ti

Liquid sodium has also been used instead of magnesium as the reducing agent.

Production of titanium dioxide

Around 90% of the TiCl₄ production is used to make the pigment titanium dioxide (TiO₂). The conversion involves hydrolysis of TiCl₄, a process that forms hydrogen chloride:^[3]

TiCl₄ + 2 H₂O → TiO₂ + 4 HCl

In some cases, TiCl₄ is oxidised directly with oxygen:

TiCl₄ + O₂ → TiO₂ + 2 Cl₂

Chemical reactions

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

Hydrolysis and related reactions

The most noteworthy reaction of TiCl₄ is its easy hydrolysis, signaled by the release of corrosive hydrogen chloride and the formation of titanium oxides and oxychlorides, as described above for the production of TiO₂. In the past titanium tetrachloride has also been used to create naval smokescreens. 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 highly refractive titanium dioxide is also an efficient light scatterer. Because of the corrosiveness of its smoke, however, TiCl₄ is no longer used.

Alcohols react with TiCl₄ to give the corresponding 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.

Organic amines react with TiCl₄ to give complexes containing amido (R₂N^--containing) and imido (RN^2--containing) complexes. With ammonia, titanium nitride is formed. An illustrative reaction is the synthesis of tetrakis(dimethylamido)titanium Ti(NMe₂)₄, a yellow, benzene-soluble liquid:^[4] This molecule is tetrahedral, with planar nitrogen centers.^[5]

4 LiNMe₂ + TiCl₄ → 4 LiCl + Ti(NMe₂)₄

Complexes with simple ligands

TiCl₄ is a Lewis acid as implicated by its tendency to hydrolyze. With the ether THF, TiCl₄ reacts to give yellow crystals of TiCl₄(THF)₂. With chloride salts, TiCl₄ reacts to form sequentially [Ti₂Cl₉]⁻, [Ti₂Cl₁₀]²⁻ (see figure above), and [TiCl₆]²⁻.^[6] Interestingly, the reaction of chloride ions with TiCl₄ depends on the counterion. NBu₄Cl and TiCl₄ gives the pentacoordinate complex NBu₄TiCl₅, whereas smaller NEt₄⁺ gives (NEt₄)₂Ti₂Cl₁₀. These reactions highlight the influence of electrostatic forces on the structures of compounds with highly ionic bonding.

Redox

Reduction of TiCl₄ with aluminium results in one-electron reduction. The trichloride (TiCl₃) and tetrachloride have contrasting properties: the trichloride is a 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 adduct TiCl₃(thf)₃.

Organometallic chemistry

Main article: Organotitanium compound

The organometallic chemistry of titanium typically starts from TiCl₄. 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 as C₆(CH₃)₆ react to give the piano-stool complexes [Ti(C₆R₆)Cl₃]⁺ (R = H, CH₃; see figure above).^[7] This reaction illustrates the high Lewis acidity of the TiCl₃⁺ entity, which is generated by abstraction of chloride from TiCl₄ by AlCl₃.

Reagent in organic synthesis

TiCl₄ finds limited but diverse use in organic synthesis, capitalizing on its Lewis acidity and its oxophilicity.^[8] Illustrative is the Mukaiyama aldol reaction. Key to this application is the tendency of TiCl₄ to activate aldehydes (RCHO) by formation of adducts such (RCHO)TiCl₄OC(H)R. It is also used in the McMurry reaction in conjunction with zinc, LiAlH₄. These reducing agents generate Ti(III) derivatives that couple ketones, leading to alkenes.

Toxicity and safety considerations

Hazards posed by titanium tetrachloride generally arise from the release of hydrogen chloride (HCl). TiCl₄ is a strong Lewis acid, exothermically forming adducts with even weak bases such as THF and explosively with water, releasing HCl.

References

1. ^ S. P. Webb, M. S. Gordon (1999). "Intermolecular Self-Interactions of the Titanium Tetrahalides TiX4 (X = F, Cl, Br)". J. Am. Chem. Soc. 121 (11): 2552–2560. doi:10.1021/ja983339i. 
2. ^ L. E. Manzer; Deaton, Joe; Sharp, Paul; Schrock, R. R. (1982). "Tetrahydrofuran Complexes of Selected Early Transition Metals". Inorganic Synthesis. Inorganic Syntheses 21: 135–40. doi:10.1002/9780470132524.ch31. ISBN 9780470132524. 
3. ^ Hans G. Völz et al. “Pigments, Inorganic” in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2006. doi:10.1002/14356007.a20_243.pub2
4. ^ D. C. Bradey, M. Thomas (1960). "Some Dialkylamino-derivatives of Titanium and Zirconium". Journal of the Chemical Society: 3857–3861. doi:10.1039/JR9600003857. 
5. ^ 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 (4): 923–929. doi:10.1016/j.poly.2005.10.019. 
6. ^ C. S. Creaser , J. A. Creighton (1975). "Pentachloro- and Pentabromotitanate(IV) ions". Journal of the Chemical Society, Dalton Transactions (14): 1402–1405. doi:10.1039/DT9750001402. 
7. ^ 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. 
8. ^ L.-L. Gundersen, F. Rise, K. Undheim (2004). "Titanium(IV) chloride". Encyclopedia of Reagents for Organic Synthesis (L. Paquette ed.). New York: J. Wiley & Sons. 

General reading

• Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
• Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Oxford: Butterworth-Heinemann. ISBN 0080379419. 

External links

• Titanium tetrachloride: Health Hazard Information
• ESIS: European chemical Substances Information System
• NIST Standard Reference Database
• ChemSub Online: Titanium tetrachloride