Hafnium tetrachloride

Hafnium(IV) chloride

Names
IUPAC names

Hafnium(IV) chloride
Hafnium tetrachloride

Identifiers

CAS Number

* 13499-05-3 Y

3D model (JSmol)

* Interactive image

ChemSpider |

* 34591 Y

ECHA InfoCard | 100.033.463

PubChem CID

* 37715

UNII |

* CNZ9V5JM5H Y

CompTox Dashboard (EPA)

* DTXSID0065513

InChI

* InChI=1S/4ClH.Hf/h4*1H;/q;;;;+4/p-4 Y

Key: PDPJQWYGJJBYLF-UHFFFAOYSA-J Y

* InChI=1/4ClH.Hf/h4*1H;/q;;;;+4/p-4

Key: PDPJQWYGJJBYLF-XBHQNQODAR

SMILES

* Cl[Hf](Cl)(Cl)Cl

Properties

Chemical formula

| 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)

Solubility in water

| decomposes[2]
Vapor pressure | 1 mmHg at 190 °C
Structure

Crystal structure

| Monoclinic, mP10[1]

Space group

| C2/c, No. 13

Lattice constant

a = 0.6327 nm, b = 0.7377 nm, c = 0.62 nm

Coordination geometry

| 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

Other anions

| Hafnium tetrafluoride
Hafnium(IV) bromide
Hafnium(IV) iodide

Other cations

| Titanium(IV) chloride
Zirconium(IV) chloride

Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Y verify (what is YN ?)

Infobox references

Chemical compound

Hafnium(IV) chloride is the inorganic compound with the formula HfCl4. This colourless solid is the precursor to most hafnium organometallic compounds. It has a variety of highly specialized applications, mainly in materials science and as a catalyst.

## Preparation[edit]

HfCl4 can be produced by several related procedures:

* The reaction of carbon tetrachloride and hafnium oxide at above 450 °C;[4][5]

HfO2 \+ 2 CCl4 → HfCl4 \+ 2 COCl2

* Chlorination of a mixture of HfO2 and carbon above 600 °C using chlorine gas or sulfur monochloride:[6][7]

HfO2 \+ 2 Cl2 \+ C → HfCl4 \+ CO2

* Chlorination of hafnium carbide above 250 °C.[8]

## Separation of Zr and Hf[edit]

Hafnium and zirconium 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 of Hf and Zr have very similar chemical and physical properties. Their atomic radii are similar: the atomic 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 including fractional distillation, fractional precipitation, fractional crystallization and ion exchange. The log (base 10) of the vapor pressure of solid hafnium chloride (from 476 to 681 K) is given by the equation: log10 P = −5197/T \+ 11.712, where the pressure is measured in torrs and temperature in kelvins. (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.

## Structure and bonding[edit]

This group 4 halide contains hafnium in the +4 oxidation state. Solid HfCl4 is a polymer 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]

## Reactivity[edit]

Structure of HfCl4(thf)2.[13]

The compound hydrolyzes, evolving hydrogen chloride:

HfCl4 \+ H2O → HfOCl2 \+ 2 HCl

Aged samples thus often are contaminated with oxychlorides, which are also colourless.

THF forms a monomeric 2:1 complex:[14]

HfCl4 \+ 2 OC4H8 → HfCl4(OC4H8)2

Because this complex is soluble in organic solvents, it is a useful reagent for preparing other complexes of hafnium.

HfCl4 undergoes salt metathesis with Grignard reagents. In this way, tetrabenzylhafnium can be prepared.

4 C6H5CH2MgCl + HfCl4 → (C6H5CH2)4Hf + 4 MgCl2

Similarly, salt metathesis with sodium cyclopentadienide gives hafnocene dichloride:

2 NaC5H5 \+ HfCl4 → (C5H5)2HfCl2 \+ 2 NaCl

With alcohols, alkoxides are formed.

HfCl4 \+ 4 ROH → Hf(OR)4 \+ 4 HCl

These compounds adopt complicated structures.

### Reduction[edit]

Reduction of HfCl4 is especially difficult. In the presence of phosphine ligands, reduction can be effected with potassium-sodium alloy:[15]

2 HfCl4 \+ 2 K + 4 P(C2H5)3 → Hf2Cl6[P(C2H5)3]4 \+ 2 KCl

The deep green dihafnium product is diamagnetic. X-ray crystallography shows that the complex adopts an edge-shared bioctahedral structure, very similar to the Zr analogue.

## Uses[edit]

Hafnium tetrachloride is the precursor to highly active catalysts for the Ziegler-Natta polymerization of alkenes, especially propylene.[16] Typical catalysts are derived from tetrabenzylhafnium.

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.[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 with aryl and aliphatic aldoximes, allowing specific exo-isomer formation.

### Microelectronics applications[edit]

HfCl4 was considered as a precursor for chemical vapor deposition and atomic layer deposition of hafnium dioxide and hafnium silicate, used as high-k dielectrics in manufacture of modern high-density integrated circuits.[19] However, due to its relatively low volatility and corrosive byproducts (namely, HCl), HfCl4 was phased out by metal-organic precursors, such as tetrakis ethylmethylamino hafnium (TEMAH).[20]

## References[edit]

1. ^ a b Niewa R., Jacobs H. (1995) Z. Kristallogr. 210: 687
2. ^ Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). Boca Raton, FL: CRC Press. p. 4.66. ISBN 1-4398-5511-0.
3. ^ "Hafnium compounds (as Hf)". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
4. ^ Kirk-Othmer Encyclopedia of Chemical Technology. Vol. 11 (4th ed.). 1991.
5. ^ Hummers, W. S.; Tyree, Jr., S. Y.; Yolles, S. (1953). Zirconium and Hafnium Tetrachlorides. Inorganic Syntheses. Vol. 4\. p. 121\. doi:10.1002/9780470132357.ch41. ISBN 9780470132357.
6. ^ Hopkins, B. S. (1939). "13 Hafnium". Chapters in the chemistry of less familiar elements. Stipes Publishing. p. 7.
7. ^ Hála, Jiri (1989). Halides, oxyhalides and salts of halogen complexes of titanium, zirconium, hafnium, vanadium, niobium and tantalum. Vol. 40 (1st ed.). Oxford: Pergamon. pp. 176–177. ISBN 978-0080362397.
8. ^ Elinson, S. V. and Petrov, K. I. (1969) Analytical Chemistry of the Elements: Zirconium and Hafnium. 11.
9. ^ a b Newnham, Ivan Edgar "Purification of Hafnium Tetrachloride". U.S. patent 2,961,293 November 22, 1960.
10. ^ a b Spiridonov, V. P.; Akishin, P. A.; Tsirel'Nikov, V. I. (1962). "Electronographic investigation of the structure of zirconium and hafnium tetrachloride molecules in the gas phase". Journal of Structural Chemistry. 3 (3): 311. doi:10.1007/BF01151485. S2CID 94835858.
11. ^ Palko, A. A.; Ryon, A. D.; Kuhn, D. W. (1958). "The Vapor Pressures of Zirconium Tetrachloride and Hafnium Tetrachloride". The Journal of Physical Chemistry. 62 (3): 319. doi:10.1021/j150561a017. hdl:2027/mdp.39015086446302.
12. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. pp. 964–966. ISBN 978-0-08-037941-8.
13. ^ Duraj, S. A.; Towns; Baker; Schupp, J. (1990). "Structure of cis-Tetrachlorobis(tetrahydrofuran)hafnium(IV)". Acta Crystallographica. C46 (5): 890–2. doi:10.1107/S010827018901382X.
14. ^ Manzer, L. E. (1982). "Tetrahydrofuran Complexes of Selected Early Transition Metals". Inorg. Synth. 21: 135–140. doi:10.1002/9780470132524.ch31. ISBN 978-0-470-13252-4.
15. ^ Riehl, M. E.; Wilson, S. R.; Girolami, G. S. (1993). "Synthesis, X-ray Crystal Structure, and Phosphine-Exchange Reactions of the Hafnium(III)-Hafnium(III) Dimer Hf2Cl6[P(C2H5)3]4". Inorg. Chem. 32 (2): 218–222. doi:10.1021/ic00054a017.
16. ^ Ron Dagani (2003-04-07). "Combinatorial Materials: Finding Catalysts Faster". Chemical and Engineering News. p. 10.
17. ^ Ahn, S.; Song, Y. S.; Yoo, B. R.; Jung, I. N. (2000). "Lewis Acid-Catalyzed Friedel−Crafts Alkylation of Ferrocene with Allylchlorosilanes". Organometallics. 19 (14): 2777. doi:10.1021/om0000865.
18. ^ Graham, A. B.; Grigg, R.; Dunn, P. J.; Higginson, P. (2000). "Tandem 1,3-azaprotiocyclotransfer–cycloaddition reactions between aldoximes and divinyl ketone. Remarkable rate enhancement and control of cycloaddition regiochemistry by hafnium(iv) chloride". Chemical Communications (20): 2035–2036. doi:10.1039/b005389i.
19. ^ Choi, J. H.; Mao, Y.; Chang, J. P. (2011). "Development of hafnium based high-k materials—A review". Materials Science and Engineering: R: Reports. 72 (6): 97. doi:10.1016/j.mser.2010.12.001.
20. ^ Robertson, John (2006). "High dielectric constant gate oxides for metal oxide Si transistors". Reports on Progress in Physics. 69 (2): 327–396. Bibcode:2006RPPh...69..327R. doi:10.1088/0034-4885/69/2/R02. S2CID 122044323.

* v
* t
* e

Hafnium compounds

Hf(II)|

* HfB2

Hf(III)|

* HfI3

Hf(IV)|

* HfC
* HfBr4
* HfCl4
* HfF4
* HfI4
* Hf(C5H7O2)4
* Hf(OSO2CF3)4
* HfO2
* Hf(NO3)4
* HfSiO4
* HfS2
* La2Hf2O7
* Ta4HfC5
* CHf2N

* v
* t
* e

Salts and covalent derivatives of the chloride ion

| HCl |  |  |  | He

|
|
|
|

LiCl | BeCl2 | B4Cl4
B12Cl12
BCl3
B2Cl4
+BO3 | C2Cl2
C2Cl4
C2Cl6
CCl4

+CO3 | NCl3
ClN3

+NO3 | ClxOy
Cl2O
Cl2O2
ClO
ClO2
Cl2O4
Cl2O6
Cl2O7
ClO4
+O | ClF
ClF3
ClF5 | Ne
NaCl | MgCl2 | AlCl
AlCl3 | Si5Cl12
Si2Cl6
SiCl4 | P2Cl4
PCl3
PCl5
+P | S2Cl2
SCl2
SCl4
+SO4 | Cl2 | Ar
KCl | CaCl
CaCl2 |  | ScCl3 | TiCl2
TiCl3
TiCl4 | VCl2
VCl3
VCl4
VCl5 | CrCl2
CrCl3
CrCl4 | MnCl2
MnCl3 | FeCl2
FeCl3 | CoCl2
CoCl3 | NiCl2 | CuCl
CuCl2 | ZnCl2 | GaCl
GaCl3 | GeCl2
GeCl4 | AsCl3
AsCl5
+As | Se2Cl2
SeCl2
SeCl4 | BrCl | Kr
RbCl | SrCl2 |  | YCl3 | ZrCl3
ZrCl4 | NbCl3
NbCl4
NbCl5 | MoCl2
MoCl3
MoCl4
MoCl5
MoCl6 | TcCl3
TcCl4 | RuCl2
RuCl3
RuCl4 | RhCl3 | PdCl2 | AgCl | CdCl2 | InCl
InCl2
InCl3 | SnCl2
SnCl4 | SbCl3
SbCl5 | Te3Cl2
TeCl2
TeCl4 | ICl
ICl3 | XeCl
XeCl2
XeCl4
CsCl | BaCl2 | *  | LuCl3 | HfCl4 | TaCl3
TaCl4
TaCl5 | WCl2
WCl3
WCl4
WCl5
WCl6 | ReCl3
ReCl4
ReCl5
ReCl6 | OsCl2
OsCl3
OsCl4
OsCl5 | IrCl2
IrCl3
IrCl4 | PtCl2
PtCl4 | AuCl
(Au[AuCl4])2
AuCl3 | Hg2Cl2
HgCl2 | TlCl
TlCl3 | PbCl2
PbCl4 | BiCl3 | PoCl2
PoCl4 | AtCl | Rn
FrCl | RaCl2 | **  | LrCl3 | RfCl4 | DbCl5 | SgO2Cl2 | BhO3Cl | Hs  | Mt  | Ds  | Rg  | Cn  | Nh  | Fl  | Mc  | Lv  | Ts  | Og

*  | LaCl3 | CeCl3 | PrCl3 | NdCl2
NdCl3 | PmCl3 | SmCl2
SmCl3 | EuCl2
EuCl3 | GdCl3 | TbCl3 | DyCl2
DyCl3 | HoCl3 | ErCl3 | TmCl2
TmCl3 | YbCl2
YbCl3
**  | AcCl3 | ThCl3
ThCl4 | PaCl4
PaCl5 | UCl3
UCl4
UCl5
UCl6 | NpCl3 | PuCl3 | AmCl2
AmCl3 | CmCl3 | BkCl3 | CfCl3 | EsCl2
EsCl3 | FmCl2 | MdCl2 | NoCl2
*[CID]: Compound ID
*[EPA]: U.S. Environmental Protection Agency
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