Hafnium(IV) chloride

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Hafnium(IV) chloride
Hafnium(IV) chloride.jpg
General
Systematic name	Hafnium(IV) chloride Hafnium tetrachloride
Molecular formula	HfCl₄
Molar mass	320.30 g/mol
Appearance	white crystalline solid
CAS number	[13499-05-3]
Properties
phase	Solid
Solubility in water	decomposes
Melting point	432 °C (705 K)
Vapor pressure	1 mmHg at 190°C
Structure
Coordination geometry	?
Crystal structure	presumed monoclinic
Hazards
MSDS	MSDS
Main hazards	irritant and corrosive
NFPA 704	0 4 2 W
Flash point	non-flammable
Supplementary data
Structure and properties	n, ε_r, etc.
Themodynamics	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
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 HfCl₄. This Group 4 halide contains hafnium in the +4 oxidation state. HfCl₄ is precursor to many hafnium organometallic compounds. It functions as a Lewis acid and catalyst for certain alkylation and isomerism reactions.

1 Preparation
2 Composition and structure
3 Uses
4 References
5 External links

[edit] 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]

[edit] Composition and structure

HfCl₄ is non-molecular polymer wherein 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]

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.

[edit] Uses

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 Na/K + 4 P(C₂H₅)₃ → Hf₂Cl₆[P(C₂H₅)₃]₄ + 2 (Na/K)Cl

Deep green Hf₂Cl₆[P(C₂H₅)₃]₄ crystals form, which are diamagnetic. X-ray crystallography showed the structure of Hf₂Cl₆[P(C₂H₅)₃]₄ to be edge-shared bioctahedral, very similar to the Zr analogue. This hafnium dimer is used to research different bonding angles, structures, and symmetries in similar transition metal compounds.

Whereas AlCl₃ is well known as an active Lewis-acid catalyst, HfCl₄ is superior at least for the alkylation of ferrocene with allyldimethylchlorosilane. The greater size of Hf may diminish HfCl₄'s tendency to complex to ferrocene.^[7]

HfCl₄ increases the rate and control of 1,3-dipolar cycloadditions.^[8] It was found to yield better results than other Lewis acids when used with aryl and aliphatic aldoximes, allowing specific exo-isomer formation.

HfCl₄ has been proven to be an active Lewis acid, forming the aldehyde adduct selectively.^[9] The reaction is shown below:

By using HfCl₄ as a catalyst, the aldehyde product (product A) was formed in a much higher yield than product B.

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.

[edit] References

^ Encyclopedia of Chemical Technology. Kirk-Othermer. Vol.11, 4th Ed. (1991)
^ Hala, J. Halides, Oxyhalides and Salts of Halogen Complexes of Titanium, Zirconium, Hafnium, Vanadium, Niobium and Tantalum. Vol. 40, 176-177, (1970).
^ S.V. Elinson, K.I. Petrov. Analytical Chemistry of the Elements: Zirconium and Hafnium. 11, (1969).
^ Greenwood, N. N., Earnshaw, A. Chemistry of the Elements Second Ed. Butterworth-Heinemann, Boston, (1997).
^ L.E. Manzer. "Tetrehydrofuran Complexes of Selected Early Transition Metals," Inorganic Synthesis. 21, 135-140, (1982).
^ 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).
^ S. Ahn, Y. Song, B. Yoo, I. Jung. "Lewis Acid-Catalyzed Friedel-Crafts Alkylation of Ferrocene with Allylchlorosilanes," Organometallics. 19, 2777-2780, (2000).
^ P. Dunn, A. Graham, R. Grigg, P. Higginson. "Tandem...cycloaddition regiochemistry by Hafnium(IV) Chloride," Chem. Commun. 2035-2036, (2000).
^ 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).

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