Titanium(III) chloride

Titanium(III) chloride

β-TiCl3 viewed along the chains TiCl3 solution
Names
Other names titanium trichloride titanous chloride
Identifiers
CAS Number	7705-07-9 Y
ChemSpider	56398 Y
EC Number	231-728-9
Jmol interactive 3D	Image
PubChem	62646
RTECS number	XR1924000
InChI InChI=1S/3ClH.Ti/h3*1H;/q;;;+3/p-3 Y Key: YONPGGFAJWQGJC-UHFFFAOYSA-K Y InChI=1/3ClH.Ti/h3*1H;/q;;;+3/p-3 Key: YONPGGFAJWQGJC-DFZHHIFOAS
SMILES Cl[Ti](Cl)Cl
Properties
Chemical formula	TiCl3
Molar mass	154.225 g/mol
Appearance	red-violet crystals hygroscopic
Density	2.64 g/cm3
Melting point	425 °C (797 °F; 698 K) (decomposes)
Boiling point	960 °C (1,760 °F; 1,230 K)
Solubility in water	very soluble
Solubility	soluble in acetone, acetonitrile, certain amines; insoluble in ether and hydrocarbons
Refractive index (nD)	1.4856
Hazards
Main hazards	Corrosive
Safety data sheet	External MSDS
Related compounds
Other anions	Titanium(III) fluoride Titanium(III) bromide Titanium(III) iodide
Other cations	Scandium(III) chloride Chromium(III) chloride Vanadium(III) chloride
Related compounds	Titanium(IV) chloride Titanium(II) chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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 Ti atom has one d electron, rendering its derivatives paramagnetic, i.e. the substance is attracted into a magnetic field. The trihalides of hafnium and zirconium: in these heavier metals engage in metal-metal bonding. 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, 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. In contrast, 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.^[1]

Synthesis and reactivity

TiCl₃ is produced usually by reduction of titanium(IV) chloride. Older reduction methods used hydrogen:^[2]

2 TiCl₄ + H₂ → 2 HCl + 2 TiCl₃

It is conveniently reduced with aluminium and sold as a mixture with aluminium trichloride, TiCl₃·AlCl₃. This mixture can be separated to afford TiCl₃(THF)₃.^[3] The complex adopts a meridional structure.^[4]

Its hydrate can be synthesised by dissolving titanium in aqueous hydrochloric acid.

2 Ti + 6 HCl + 3 H₂O → 2 TiCl₃(H₂O)₃ + 3 H₂

TiCl₃ forms a variety of coordination complexes, most of which are octahedral. The light-blue crystalline adduct TiCl₃(THF)₃ forms when TiCl₃ is treated with tetrahydrofuran.^[5]

TiCl₃ + 3 C₄H₈O → TiCl₃(OC₄H₈)₃

An analogous dark green complex arises from complexation with dimethylamine. In a reaction where all ligands are exchanged, TiCl₃ is a precursor to the tris acetylacetonate complex.

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:^[6]

2 TiCl₃ → TiCl₂ + TiCl₄

The ternary halides, such as A₃TiCl₆, have structures that depend on the cation (A⁺) added.^[7] Caesium chloride treated with titanium(II) chloride and hexachlorobenzene produces crystalline CsTi₂Cl₇. In these structures Ti³⁺ exhibits octahedral coordination geometry.^[8]

Applications

TiCl₃ is the main Ziegler-Natta catalyst, responsible for most industrial production of polypropylene. The catalytic activities depend strongly on the polymorph and the method of preparation.^[9]

Laboratory use

TiCl₃ is also a 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.^[10] Titanium trichloride can reduce nitrate to ammonium ion thereby allowing for the sequential analysis of nitrate and ammonia.^[11] Slow deterioration occurs in air-exposed titanium trichloride, often resulting in erratic results, e.g. in reductive coupling reactions.^[12]

Safety

TiCl₃ and most of its complexes are typically handled under an air-free conditions to prevent reactions with oxygen and moisture. Depending on the method for its preparation, samples of TiCl₃ can be relatively air stable or pyrophoric.^[2][13]

References