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Hafnium(IV) chloride



Hafnium(IV) chloride
IUPAC name Hafnium(IV) chloride
Hafnium tetrachloride
Identifiers
CAS number 13499-05-3
Properties
Molecular formula HfCl4
Molar mass 320.30 g/mol
Appearance white crystalline solid
Melting point

432 °C (705 K)

Solubility in water decomposes
Vapor pressure 1 mmHg at 190°C
Structure
Crystal structure presumed monoclinic
Hazards
MSDS MSDS
Main hazards irritant and corrosive
NFPA 704
0
4
2
 
Flash point non-flammable
Related Compounds
Other anions Hafnium(IV) fluoride
Hafnium(IV) bromide
Hafnium(IV) iodide
Other cations Titanium(IV) chloride
Zirconium(IV) chloride
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox disclaimer and references

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.

Additional recommended knowledge

Contents

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.
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Hafnium(IV)_chloride". A list of authors is available in Wikipedia.
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