Titanium tetrabromide

Titanium tetrabromide is the chemical compound with the formula TiBr₄. It is the most volatile transition metal bromide. The properties of TiBr₄ are an average of TiCl₄ and TiI₄. Some key properties of these four-coordinated Ti(IV) species are their high Lewis acidity and their high solubility in nonpolar organic solvents. TiBr₄ is diamagnetic, reflecting the d⁰ configuration of the metal centre.^[1]

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Preparation and structure

This four-coordinated complex adopts a tetrahedral geometry. It can be prepared via several methods: (i) from the elements, (ii) via the reaction of TiO₂ with carbon and bromine (see Kroll process), and (iii) by treatment of TiCl₄ with HBr.

Reactions

TiBr₄ forms adducts such as TiBr₄(THF)₂ and [TiBr₅]^-.^[2] With bulky donor ligands, such as 2-methylpyridine (2-Mepy), five-coordinated adducts form. TiBr₄(2-MePy) is trigonal bipyramidal with the pyridine in the equatorial plane.^[3]

TiBr₄ has been used as a Lewis-acid catalyst in organic synthesis.^[4]

TiBr₄ reacts rapidly with TiCl₄ to give a statistical mixture of the tetrahalides, TiBr_4-xCl_x (x = 0-4). The mechanism of this exchange is a matter of some dispute; a reasonable pathway involves the intermediacy of dimers.^[5]

Safety

TiBr₄ hydrolyzes rapidly, potentially dangerously, to release hydrogen bromide.

References

1. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
2. ^ Colin S. Creaser and J. Alan Creighton (1975). "Pentachloro- and pentabromo-titanate(IV) ions". J. Chem. Soc., Dalton Trans.: 1402 - 1405. doi:10.1039/DT9750001402.
3. ^ Hensen, K.; Lemke, A.; Bolte, M. (2000). "Tetrabromo(2-methylpyridine-N)-titanate(IV)". Acta Crystallographica C56: e565 - e566. doi:10.1107/S0108270100015407.
4. ^ B. Patterson, S. Marumoto and S. D. Rychnovsky (2003). "Titanium(IV)-Promoted Mukaiyama Aldol-Prins Cyclizations". Org. Lett. 5 (17): 3163-3166. doi:10.1021/ol035303n.
5. ^ S. P. Webb and 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.