Hafnium(IV) chloride

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.

Preparation
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.

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.

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.

Uses
The THF complex is monomeric and thus soluble in organic solvents, which allows this hafnium complex to react more easily.
 * 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:
 * 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.

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.

HfCl4 has been proven to be an active Lewis acid, forming the aldehyde adduct selectively. 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.