Hafnium(IV) chloride

Hafnium(IV) chloride is the inorganic compound with the formula HfCl₄. This colourless solid is the precursor to most hafnium organometallic compounds. It functions as a Lewis acid and catalyst for certain alkylation and isomerism reactions. Hafnium and zirconium compounds are extracted from ores together and have very similar properties. The most common contaminant in HfCl₄ is therefore ZrCl₄, and commercial samples can contain several percent zirconium.

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Preparation

HfCl₄ is produced by several procedures: (1) the reaction of carbon tetrachloride and hafnium oxide at above 450 °C;^[1] (2) chlorination of a mixture of HfO₂ and carbon above 600 °C;^[2] and (3) chlorination of hafnium carbide above 250 °C.^[3]

Structure and bonding

This Group 4 halide contains hafnium in the +4 oxidation state. Solid HfCl₄ is a polymer, each of the Hf centers are bridged by chloride ligands as found for ZrCl₄. In the gas phase, both ZrCl₄ and HfCl₄ are expected to adopt the monomeric tetrahedral structure seen for TiCl₄.^[4]

Reactivity

The compound is highly reactive toward water, evolving hydrogen chloride. Aged samples often are contaminated with the oxychloride, which is also colourless. The THF complex is monomeric and thus soluble in organic solvents, which allows this hafnium complex to react more easily.^[5]

HfCl₄ + 2 OC₄H₈ → HfCl₄(OC₄H₈)₂

Little is known about Hf(III) compounds because HfCl₄ is especially difficult to reduce, but reduction can be effected with potassium-sodium alloy^[6]:

2 HfCl₄ + 2 K + 4 P(C₂H₅)₃ → Hf₂Cl₆[P(C₂H₅)₃]₄ + 2 KCl

Deep green Hf₂Cl₆[P(C₂H₅)₃]₄ crystals form, which are diamagnetic. X-ray crystallography shows that Hf₂Cl₆[P(C₂H₅)₃]₄ has an edge-shared bioctahedral structure, very similar to the Zr analogue.

Uses

Catalyst for polymerization of propylene

Hafnium chloride is the precursor to highly active catalysts for the polymerization of alkenes, especially propylene.^[7] Typical catalysts are derived from tetrabenzylhafnium.

In organic synthesis

HfCl₄ is an effective Lewis acid for various applications in organic synthesis. For example, ferrocene is alkylated with allyldimethylchlorosilane more efficiently using hafnium chloride relative to aluminium trichloride. The greater size of Hf may diminish HfCl₄'s tendency to complex to ferrocene.^[8]

HfCl₄ increases the rate and control of 1,3-dipolar cycloadditions.^[9] It was found to yield better results than other Lewis acids when used with aryl and aliphatic aldoximes, allowing specific exo-isomer formation.

HfCl₄ is effective in catalyzing aldol condensation:^[10]

Materials science applications

HfCl₄ is the most common precursor for chemical vapor deposition of hafnium dioxide and hafnium silicate, used as high-k dielectrics in manufacture of modern high-density integrated circuits.

References

1. ^ Encyclopedia of Chemical Technology. Kirk-Othermer. Vol.11, 4th Ed. (1991)
2. ^ Hala, J. Halides, Oxyhalides and Salts of Halogen Complexes of Titanium, Zirconium, Hafnium, Vanadium, Niobium and Tantalum. Vol. 40, 176-177, (1970).
3. ^ S.V. Elinson, K.I. Petrov. Analytical Chemistry of the Elements: Zirconium and Hafnium. 11, (1969).
4. ^ Greenwood, N. N., Earnshaw, A. Chemistry of the Elements Second Ed. Butterworth-Heinemann, Boston, (1997).
5. ^ L.E. Manzer. "Tetrehydrofuran Complexes of Selected Early Transition Metals," Inorganic Synthesis. 21, 135-140, (1982).
6. ^ M. E. Riehl, S. R. Wilson, and G. S. Girolami. "Synthesis, X-ray Crystal Structure, and Phosphine-Exchange Reactions of the Hafnium(III)-Hafnium(II1) Dimer Hf₂Cl₆[P(C₂H₅)₃]₄," Inorg. Chem. 32, 218-222, (1993).
7. ^ Ron Dagani "Combinatorial Materials:Finding Catalysts Faster" Chemical and Engineering News April 7, 2003, Volume 81, Number 14, p. 10
8. ^ S. Ahn, Y. Song, B. Yoo, I. Jung. "Lewis Acid-Catalyzed Friedel-Crafts Alkylation of Ferrocene with Allylchlorosilanes," Organometallics. 19, 2777-2780, (2000).
9. ^ P. Dunn, A. Graham, R. Grigg, P. Higginson. "Tandem...cycloaddition regiochemistry by Hafnium(IV) Chloride," Chemical Communications 2035-2036, (2000).
10. ^ S. Kobayashi, T. Busujima, S. Nagayama. "A Novel Classification of Lewis Acids on the Basis of Activity and Selectivity," Chem. Eur. J. 6, 19, 3492, (2000).

Further reading

• Duraj, S. A.; Towns, R. L. R.; Baker, R. J. and Schupp, J. (1990). "Structure of cis-Tetrachlorobis(tetrahydrofuran)hafnium(IV)". Acta Crystallographica C46: 890-2.