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| Names | |
|---|---|
| IUPAC names Hafnium(IV) chloride Hafnium tetrachloride | |
| Identifiers | |
3D model (JSmol) |
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| ChemSpider |
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| ECHA InfoCard | 100.033.463 |
| UNII | |
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| Properties | |
| HfCl4 | |
| Molar mass | 320.302 g/mol |
| Appearance | white crystalline solid |
| Density | 3.89 g/cm3[1] |
| Melting point | 432 °C (810 °F; 705 K) |
| decomposes[2] | |
| Vapor pressure | 1 mmHg at 190 °C |
| Structure | |
| Monoclinic,mP10[1] | |
| C2/c, No. 13 | |
a = 0.6327 nm,b = 0.7377 nm,c = 0.62 nm | |
| 4 | |
| Hazards | |
| Occupational safety and health (OHS/OSH): | |
Main hazards | irritant and corrosive |
| Flash point | Non-flammable |
| Lethal dose or concentration (LD, LC): | |
LD50 (median dose) | 2362 mg/kg (rat, oral)[3] |
| Safety data sheet (SDS) | MSDS |
| Related compounds | |
Otheranions | Hafnium tetrafluoride Hafnium(IV) bromide Hafnium(IV) iodide |
Othercations | Titanium(IV) chloride Zirconium(IV) chloride |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Hafnium(IV) chloride is theinorganic compound with theformula HfCl4. This colourless solid is theprecursor to most hafniumorganometallic compounds. It has a variety of highly specialized applications, mainly in materials science and as a catalyst.
HfCl4 can be produced by several related procedures:
Hafnium andzirconium occur together in minerals such as zircon, cyrtolite and baddeleyite. Zircon contains 0.05% to 2.0%hafnium dioxide HfO2, cyrtolite with 5.5% to 17% HfO2 and baddeleyite contains 1.0 to 1.8 percent HfO2.[9] Hafnium and zirconium compounds are extracted from ores together and converted to a mixture of the tetrachlorides.
The separation of HfCl4 and ZrCl4 is difficult because the compounds ofHf andZr have very similar chemical andphysical properties. Their atomic radii are similar: theatomic radius is 156.4 pm for hafnium, whereas that of Zr is 160 pm.[10] These two metals undergo similar reactions and form similar coordination complexes.
A number of processes have been proposed to purify HfCl4 from ZrCl4 includingfractional distillation, fractional precipitation, fractional crystallization andion exchange. The log (base 10) of the vapor pressure of solid hafnium chloride (from 476 to 681 K) is given by the equation: log10P = −5197/T + 11.712, where the pressure is measured intorrs and temperature inkelvins. (The pressure at the melting point is 23,000 torrs.)[11]
One method is based on the difference in the reducibility between the two tetrahalides.[9] The tetrahalides can in be separated by selectively reducing the zirconium compound to one or more lower halides or even zirconium. The hafnium tetrachloride remains substantially unchanged during the reduction and may be recovered readily from the zirconium subhalides. Hafnium tetrachloride is volatile and can therefore easily be separated from the involatile zirconium trihalide.
This group 4halide containshafnium in the +4oxidation state. Solid HfCl4 is apolymer with octahedral Hf centers. Of the six chloride ligands surrounding each Hf centre, two chloride ligands are terminal and four bridge to another Hf centre. In the gas phase, both ZrCl4 and HfCl4 adopt the monomeric tetrahedral structure seen for TiCl4.[12] Electronographic investigations of HfCl4 in gas phase showed that the Hf-Cl internuclear distance is 2.33 Å and the Cl...Cl internuclear distance is 3.80 Å. The ratio of intenuclear distances r(Me-Cl)/r(Cl...Cl) is 1.630 and this value agrees well with the value for the regular tetrahedron model (1.633).[10]
The compound hydrolyzes, evolvinghydrogen chloride:
Aged samples thus often are contaminated with oxychlorides, which are also colourless.
THF forms amonomeric 2:1 complex:[14]
Because this complex is soluble in organic solvents, it is a useful reagent for preparing other complexes of hafnium.
HfCl4 undergoessalt metathesis withGrignard reagents. In this way, tetrabenzylhafnium can be prepared.
Similarly, salt metathesis withsodium cyclopentadienide giveshafnocene dichloride:
With alcohols, alkoxides are formed.
These compounds adopt complicated structures.
Reduction of HfCl4 is especially difficult. In the presence ofphosphine ligands, reduction can be effected withsodium–potassium alloy:[15]
The deep green dihafnium product isdiamagnetic.X-ray crystallography shows that the complex adopts an edge-shared bioctahedral structure, very similar to the Zr analogue.
Hafnium tetrachloride is the precursor to highly active catalysts for theZiegler-Natta polymerization ofalkenes, especiallypropylene.[16] Typical catalysts are derived from tetrabenzylhafnium.
HfCl4 is an effective Lewis acid for various applications inorganic synthesis. For example,ferrocene is alkylated with allyldimethylchlorosilane more efficiently using hafnium chloride relative toaluminium trichloride. The greater size of Hf may diminish HfCl4's tendency to complex to ferrocene.[17]
HfCl4 increases the rate and control of 1,3-dipolar cycloadditions.[18] It was found to yield better results than other Lewis acids when used witharyl andaliphatic aldoximes, allowing specificexo-isomer formation.
HfCl4 was considered as a precursor forchemical vapor deposition andatomic layer deposition ofhafnium dioxide andhafnium silicate, used ashigh-κ dielectrics in manufacture of modern high-density integrated circuits.[19] However, due to its relatively lowvolatility and corrosive byproducts (namely,HCl), HfCl4 was phased out by metal-organic precursors, such as tetrakis ethylmethylamino hafnium (TEMAH).[20]
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