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Hafnium(IV) chloride
Encyclopedia
Hafnium 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
the metal center in the complex occurs in nature in small amounts associated with Zirconium
minerals such as zircon, cyrtolite and baddeleyite. Zircon contains 0.05 to 2.0 percent Hafnium oxide HfO2, cyrtolite with 5.5 to 17 percent HfO2 and baddeleyite contains 1.0 to 1.8 percent HfO2.
Hafnium and zirconium compounds are extracted from ores together because Hafnium
and Zirconium
have very similar chemical and physical properties. Therefore in the synthesis of Hafnium(IV) chloride which requires HfO2, the HfO2 has to be purified from the mineral ore and this can be achieved through decomposition.
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.
Hafnium(IV) chloride is prepared by heating a mixture of HfO2 and Carbon
in a stream of Chlorine
or Sulfur monochloride under hot temperatures as shown in the equation below.
HfCl4 could also be prepared by reacting Hf(OH)4 with HCl and then boiling the reactants until crystals are formed \. Since Hf mineral ores contain Zr compounds as well, some ZrCl4 is also formed in the process of synthesizing HfCl4 therefore the product has to be purified in order to obtain pure HfCl4.
and Zr
have very similar chemical and physical properties. Both metals have the same number of d electrons and they have a small difference in atomic radii (hafnium atomic radius is 156.4 pm while that of Zr is 160 pm). This means that these two metals undergo similar reactions and form the similar coordination complexes which makes the separation of these so formed complexes difficult.
The Tetrahalides can also be separated by selectively reducing the Zirconium tetrahalide 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 is based on the difference in the reducibility between the two tetrahalides.
There have been a number of proposed processes that could be used to purify HfCl4 from ZrCl4 and some of the proposed methods include fractional distillation, fractional precicpitation, fractional crystallization and ion exchange.
contains hafnium
in the +4 oxidation state
. Solid HfCl4 is a polymer
, each of the Hf centers are bridged by chloride ligand
s as found for ZrCl4. In the gas phase, both ZrCl4 and HfCl4 are expected to adopt the monomeric tetrahedral structure seen for TiCl4.
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).
. 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.
HfCl4 can undergo the Grignard reaction for the substitution of chloride ligands. This can be achieved by reacting HfCl4 with a Grignard reagent of general formula R-Mg-X in dry diethyl ether solvent.
s, especially propylene
. Typical catalysts are derived from tetrabenzyl
hafnium.
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.
Hafnium chloride is also the precursor to highly active catalysts for the polymerization of alkenes, especially propylene. Typical catalysts are derived from tetrabenzylhafnium. It was recently reported that MgCl2 supported HfCl4 in combination with AlEt3 or Dimethyltitanocene specifically produces polypropylene.
of hafnium dioxide and hafnium silicate, used as high-k dielectric
s in manufacture of modern high-density integrated circuits.
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...
with the formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
HfCl4. This colourless solid is the precursor to most hafnium organometallic compounds. It functions as a Lewis acid
Lewis acid
]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...
and catalyst for certain alkylation
Alkylation
Alkylation is the transfer of an alkyl group from one molecule to another. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion or a carbene . Alkylating agents are widely used in chemistry because the alkyl group is probably the most common group encountered in...
and isomerism reactions. Hafnium
Hafnium
Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869. Hafnium was the penultimate stable...
the metal center in the complex occurs in nature in small amounts associated with Zirconium
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
minerals such as zircon, cyrtolite and baddeleyite. Zircon contains 0.05 to 2.0 percent Hafnium oxide HfO2, cyrtolite with 5.5 to 17 percent HfO2 and baddeleyite contains 1.0 to 1.8 percent HfO2.
Hafnium and zirconium compounds are extracted from ores together because Hafnium
Hafnium
Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869. Hafnium was the penultimate stable...
and Zirconium
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
have very similar chemical and physical properties. Therefore in the synthesis of Hafnium(IV) chloride which requires HfO2, the HfO2 has to be purified from the mineral ore and this can be achieved through decomposition.
Preparation
HfCl4 is produced by several procedures: (1) the reaction of carbon tetrachlorideCarbon tetrachloride
Carbon tetrachloride, also known by many other names is the organic compound with the formula CCl4. It was formerly widely used in fire extinguishers, as a precursor to refrigerants, and as a cleaning agent...
and hafnium oxide at above 450 °C; (2) chlorination of a mixture of HfO2 and carbon
Charcoal
Charcoal is the dark grey residue consisting of carbon, and any remaining ash, obtained by removing water and other volatile constituents from animal and vegetation substances. Charcoal is usually produced by slow pyrolysis, the heating of wood or other substances in the absence of oxygen...
above 600 °C; and (3) chlorination
Chlorination
Chlorination is the process of adding the element chlorine to water as a method of water purification to make it fit for human consumption as drinking water...
of hafnium carbide above 250 °C.
Hafnium(IV) chloride is prepared by heating a mixture of HfO2 and Carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
in a stream of Chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
or Sulfur monochloride under hot temperatures as shown in the equation below.
-
- HfO2 + 4Cl + C → HfCl4 + CO2
HfCl4 could also be prepared by reacting Hf(OH)4 with HCl and then boiling the reactants until crystals are formed \. Since Hf mineral ores contain Zr compounds as well, some ZrCl4 is also formed in the process of synthesizing HfCl4 therefore the product has to be purified in order to obtain pure HfCl4.
Purification
The purification of HfCl4 tends to be a difficult endeavor because HfHafnium
Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869. Hafnium was the penultimate stable...
and Zr
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
have very similar chemical and physical properties. Both metals have the same number of d electrons and they have a small difference in atomic radii (hafnium atomic radius is 156.4 pm while that of Zr is 160 pm). This means that these two metals undergo similar reactions and form the similar coordination complexes which makes the separation of these so formed complexes difficult.
The Tetrahalides can also be separated by selectively reducing the Zirconium tetrahalide 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 is based on the difference in the reducibility between the two tetrahalides.
There have been a number of proposed processes that could be used to purify HfCl4 from ZrCl4 and some of the proposed methods include fractional distillation, fractional precicpitation, fractional crystallization and ion exchange.
Structure and bonding
This Group 4 halideHalide
A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...
contains hafnium
Hafnium
Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869. Hafnium was the penultimate stable...
in the +4 oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
. Solid HfCl4 is a polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
, each of the Hf centers are bridged by chloride ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s as found for ZrCl4. In the gas phase, both ZrCl4 and HfCl4 are expected to adopt the monomeric tetrahedral structure seen for TiCl4.
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).
Reactivity
The compound is highly reactive toward water, evolving hydrogen chlorideHydrogen chloride
The compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry...
. Aged samples often are contaminated with the oxychloride, which is also colourless. The THF
ThF
Follicular B helper T cells , are antigen-experienced CD4+ T cells found in the B cell follicles of secondary lymphoid organs such as lymph nodes, spleens and Peyer's patches, and are identified by their constitutive expression of the B cell follicle homing receptor CXCR5...
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 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
X-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
shows that Hf2Cl6[P(C2H5)3]4 has an edge-shared bioctahedral structure, very similar to the Zr analogue.
HfCl4 can undergo the Grignard reaction for the substitution of chloride ligands. This can be achieved by reacting HfCl4 with a Grignard reagent of general formula R-Mg-X in dry diethyl ether solvent.
-
- RMg-X + HfCl4 → HfX4
Uses
Hafnium chloride is the precursor to highly active catalysts for the polymerization of alkeneAlkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
s, especially propylene
Propylene
Propene, also known as propylene or methylethylene, is an unsaturated organic compound having the chemical formula C3H6. It has one double bond, and is the second simplest member of the alkene class of hydrocarbons, and it is also second in natural abundance.-Properties:At room temperature and...
. Typical catalysts are derived from tetrabenzyl
Benzyl
In organic chemistry, benzyl is the term used to describe the substituent or molecular fragment possessing the structure C6H5CH2-. Benzyl features a benzene ring attached to a CH2 group.-Nomenclature:...
hafnium.
HfCl4 is an effective Lewis acid for various applications in organic synthesis
Organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
. For example, ferrocene
Ferrocene
Ferrocene is an organometallic compound with the formula Fe2. It is the prototypical metallocene, a type of organometallic chemical compound consisting of two cyclopentadienyl rings bound on opposite sides of a central metal atom. Such organometallic compounds are also known as sandwich compounds...
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
Aryl
In the context of organic molecules, aryl refers to any functional group or substituent derived from an aromatic ring, be it phenyl, naphthyl, thienyl, indolyl, etc....
and aliphatic aldoximes, allowing specific exo-isomer formation.
Hafnium chloride is also the precursor to highly active catalysts for the polymerization of alkenes, especially propylene. Typical catalysts are derived from tetrabenzylhafnium. It was recently reported that MgCl2 supported HfCl4 in combination with AlEt3 or Dimethyltitanocene specifically produces polypropylene.
Materials science applications
HfCl4 is the most common precursor for chemical vapor depositionChemical vapor deposition
Chemical vapor deposition is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer is exposed to one or more volatile precursors, which react and/or...
of hafnium dioxide and hafnium silicate, used as high-k dielectric
High-k Dielectric
The term high-κ dielectric refers to a material with a high dielectric constant κ used in semiconductor manufacturing processes which replaces the silicon dioxide gate dielectric...
s in manufacture of modern high-density integrated circuits.