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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.
HfCl4 is produced by several procedures: (1) the reaction of carbon tetrachloride and hafnium oxide at above 450 °C; (2) chlorination of a mixture of HfO2 and carbon above 600 °C; and (3) chlorination of hafnium carbide above 250 °C.
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.
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.
Little is known about Hf(III) compounds because HfCl4 is especially difficult to reduce, but reduction can be effected with potassium-sodium alloy:
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.
Catalyst for polymerization of propylene
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.
HfCl4 increases the rate and control of 1,3-dipolar cycloadditions. It was found to yield better results than other Lewis acids when used with aryl and aliphatic aldoximes, allowing specific exo-isomer formation.
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.
|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.|