Titanium(III) chloride

Titanium(III) chloride is the chemical compound with the formula TiCl₃. This deceptively simple name describes several distinct species as well as a hydrated salt. TiCl₃ is one of the most common halides of titanium and is an important catalyst for the manufacture of polyolefins.

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Electronic properties

In TiCl₃, each Ti atom has one d electron, rendering most of its forms paramagnetic, i.e. the substance is attracted into a magnetic field. The paramagnetism of TiCl₃'s contrasts with the diamagnetism (the property of being repelled from a magnetic field) of the trihalides of hafnium and zirconium. These heavier metals engage in metal-metal bonding, but the Ti(III) ion does not typically.

Solutions of titanium(III) chloride have violet coloration which arises from excitations of its d-electron. The colour is not very intense since the transition is forbidden.

Structure

Solid TiCl₃ can be obtained in one of four forms or polymorphs. These forms can be distinguished by crystallography as well as studies on their magnetic properties which probes exchange interactions.^[1]

Beta form

β-TiCl₃ crystallizes as brown needle. The structure of this material features chains of TiCl₆ octahedra that share opposite faces such that the closest Ti-Ti is 2.91 Å. This short distance allows strong metal-metal bonding.

"Violet layered" forms

The three violet layered" forms, named for their color and its tendency to flake, are called alpha, gamma, and delta. In α-TiCl₃, the chloride anions are hexagonal close-packed. In γ-TiCl₃, the chlorides anions are cubic close-packed. Finally, disorder in shift successions, causes an intermediate between alpha and gamma structures, called the delta (δ) form. The TiCl₆ share edges in each form, with 3.60 Å being the shortest distance between the titanium cations. This large distance between titanium cations precludes direct metal-metal bonding.

Ternary alkali halides

Caesium chloride treated with titanium(II) chloride and hexachlorobenzene produces crystalline CsTi₂Cl₇. The product contains 1:2 molar mixtures of CsCl and TiCl₃. CsCl₃ and Cl₄ stack alternatively in an ABAC sequence with Ti³⁺ in one quarter of the octahedral holes.^[2]

Synthesis and selected handling properties

Titanium(IV) chloride can be reduced to TiCl₃; this conversion is generally accomplished electrochemically. It is sold as a mixture with aluminium trichloride. This mixture can be separated to afford TiCl₃(THF)₃.^[3]

Ti³⁺ is prepared in situ as part of the method of analyzing titanium-containing samples such as ores. Thus, acidic solutions of Ti⁴⁺ are reduced using Jones reductor with amalgamated zinc.^[4] The resulting Ti³⁺ containing product is analyzed by redox titration.

TiCl₃ and most of its complexes are handled under an inert atmosphere to prevent reactions with oxygen. Slow deterioration occurs in air-exposed titanium trichloride, often results in erratic results, e.g. in reductive coupling reactions^[5].

Applications in Ziegler-Natta catalyst

TiCl₃ is a useful Ziegler-Natta catalyst, although the catalytic activities vary with the method of preparation.^[6]

Other reactions of Ti(III) chloride

TiCl₃ forms a variety of coordination complexes, most of which are octahedral. The light-blue crystalline adduct TiCl₃(THF)₃ forms when TiCl₃ is refluxed with tetrahydrofuran.^[7]

TiCl₃ + 3C₄H₈O → TiCl₃(OC₄H₈)₃

A dark green charge-neutral complex arises from complexation with dimethylamine:

TiCl₃ + 3Me₂NH → TiCl₃(NHMe₂)₃ + CH₃Cl

TiCl₃ reacts with acetylacetone to form a tris acetylacetonate complex.

TiCl₃ + 3NH₄(acac) → Ti(acac)₃ + 3NH₄Cl

The product is used as cross-linking agent for cellulose films used in polyethylene catalysts. Ti(acac)₃ air-oxidizes to give orange TiO(acac)₂, which is ineffective in cross-linking.^[8]

Above 200 °C, TiCl₃ undergoes ammonolysis.⁶

The ternary halides, such as A₃TiCl₆, have structures that depend on the cation (A) added.^[9]

References

1. ^ Starr, C.; Bitter, F.; Kaufman, A.R.Lippard, S. "Halides & Halide Complexes" in (1968) Progress in Inorganic Chemistry, Cotton (Ed.) Volume 9, (John Wiley & Sons, Inc.) pp. 6
2. ^ Holm, R. H. "Stabilized Low Oxidation States" in (1971) Progress in Inorganic Chemistry, Lippard, S. J. (Ed.) Volume 14, (John Wiley & Sons, Inc.) pp. 17
3. ^ Jongen, L. and Meyer, G. (2004). "Caesium heptaiododititanate(III), CsTi₂I₇". Zeitschrift für Anorganische und Allgemeine Chemie 630: 211-212. DOI
4. ^ Kisova, L.; Sotkova, S.; Konemdova, I. (1994). "Electrode Kinetics of the Ti(IV)/Ti(III) System in Water and in Water Dimethylformamide and Water Dimethyl Sulfoxide Mixed Solvents". Collection of Czechoslovak Chemical Communications 59: 1279-1286. DOI
5. ^ Fleming, M. P; McMurry, J. E. (1981). "Reductive Coupling of Carbonys to Alkenes: Adamantylideneadamantane". Organic Syntheses 60: 113. PDF
6. ^ Ueno, H.; Imanishi, K.; Ueki, S.; Kohara, T. (2000). "Kinetics Study of Propene Polymerization with Porous Titanium Trichloride". Chemical Society of Japan 7: 495. Abstract
7. ^ Jones, N. A.; Liddle, S. T.; Wilson, C.; Arnold, P. L. (2007). "Titanium(III) Alkoxy-N-heterocyclic Carbenes and a Safe, Low-Cost Route to TiCl₃(THF)₃". Organometallics, 26: 755 - 757. DOI: 10.1021/om060486d..
8. ^ Manzer, L. E., "Tetrahydrofuran Complexes of Selected Early Transition Metals", in (1982) Inorganic Syntheses, Flacker, J. P. (Ed.) Volume 21, pp. 137
9. ^ Cor, M.; Lewis, J.; Nyholm, R. S., S. "Titanium" in (1966) Progress in Inorganic Chemistry, Lippard, S. J. (Ed.) Volume 7, (John Wiley & Sons, Inc.) pp. 391
10. ^ Fowles, G. W. A. "Reacting Halides with Liquid Ammonia" in (1965) Progress in Inorganic Chemistry, Lippard, S. J. (Ed.) Volume 6, (John Wiley & Sons, Inc.) pp. 2
11. ^ Hinz, D.; Gloger, T. and Meyer, G. (2000). "Ternary halides of the type A₃MX₆. Part 9. Crystal structures of Na₃TiCl₆ and K₃TiCl₆". Zeitschrift für Anorganische und Allgemeine Chemie 626: 822-824. Abstract

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