Titanium(III) chloride

Titanium(III) chloride is the inorganic compound with the formula TiCl₃. At least four distinct species have this formula; additionally hydrated derivatives are known. TiCl₃ is one of the most common halides of titanium and is an important catalyst for the manufacture of polyolefins.

Structure and bonding

In TiCl₃, each titanium atom has one ''d'' electron, rendering its derivatives paramagnetic, that is, the substance is attracted into a magnetic field. Solutions of titanium(III) chloride are violet, which arises from excitations of its ''d''-electron. The colour is not very intense since the transition is forbidden by the Laporte selection rule.

Four solid forms or polymorphs of TiCl₃ are known. All feature titanium in an octahedral coordination sphere. These forms can be distinguished by crystallography as well as by their magnetic properties, which probes exchange interactions. β-TiCl₃ crystallizes as brown needles. Its structure consists of chains of TiCl₆ octahedra that share opposite faces such that the closest Ti–Ti contact is 2.91 Å. This short distance indicates strong metal–metal interactions (see figure in upper right). The three violet "layered" forms, named for their color and their tendency to flake, are called alpha (α), gamma (γ), and delta (δ). 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 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. In contrast, the trihalides of the heavier metals hafnium and zirconium engage in metal-metal bonding. Direct Zr–Zr bonding is indicated in zirconium(III) chloride. The difference between the Zr(III) and Ti(III) materials is attributed in part to the relative radii of these metal centers.

Two hydrates of titanium(III) chloride are known, i.e. complexes containing aquo ligands. These include the pair of hydration isomers . The former is violet and the latter, with two molecules of water of crystallization, is green.

Synthesis and reactivity

TiCl₃ is produced usually by reduction of titanium(IV) chloride. Older reduction methods used hydrogen:
:2 TiCl₄ + H₂ → 2 HCl + 2 TiCl₃
More modern techniques prefer aluminum; the product is sold as a mixture with aluminium trichloride, TiCl₃·AlCl₃.

TiCl₃ can also be produced by the reaction of titanium metal and hot, concentrated hydrochloric acid; the reaction does not proceed at room temperature, as titanium is passivated against most mineral acids by a thin surface layer of titanium dioxide.
:2 Ti + 6 HCl → 3 H₂ + 2 TiCl₃

Treating TiCl₃ with tetrahydrofuran (THF) gives the light-blue colored, meridional complex, TiCl₃( THF)₃:
:TiCl₃ + 3 C₄H₈O → TiCl₃(OC₄H₈)₃
TiCl₃·AlCl₃ gives the same product.

An analogous dark green complex arises from complexation with dimethylamine. In a reaction where all ligands are exchanged, TiCl₃ is a precursor to the blue-colored complex Ti(acac)₃.

The more reduced titanium(II) chloride is prepared by the thermal disproportionation of TiCl₃ at 500 °C. The reaction is driven by the loss of volatile TiCl₄:
:2 TiCl₃ → TiCl₂ + TiCl₄

The trichloride is a Lewis acid, forming ternary hexahalide complexes with stoichiometry M₃TiCl₆. These have structures that depend on the cation (M⁺) added. Caesium chloride treated with titanium(II) chloride and hexachlorobenzene produces crystalline CsTi₂Cl₇. In these structures Ti³⁺ exhibits octahedral coordination geometry.

Applications

TiCl₃ is the main Ziegler–Natta catalyst, responsible for most industrial production of polyethylene. The catalytic activities depend strongly on the polymorph of the TiCl₃ (α vs. β vs. γ vs. δ) and the method of preparation.

Laboratory use

TiCl₃ is also a specialized reagent in organic synthesis, useful for reductive coupling reactions, often in the presence of added reducing agents such as zinc. It reduces oximes to imines. Titanium trichloride can reduce nitrate to ammonium ion thereby allowing for the sequential analysis of nitrate and ammonia. Slow deterioration occurs in air-exposed titanium trichloride, often resulting in erratic results, such as in reductive coupling reactions.

Safety

TiCl₃ and most of its complexes are typically handled under air-free conditions to prevent reactions with oxygen and moisture. Samples of TiCl₃ can be relatively air stable or pyrophoric.

References

{{Chlorides

Titanium(III) compounds
Chlorides
Titanium halides
Reducing agents