Lecture 1c
Synthesis of Anhydrous Metal Halides
Introduction
• Anhydrous metal halides are commonly used starting
materials for many organometallic compounds
(i.e., in reactions with Grignard reagents, the formation
of metallocene, chromium(III) dithiocarbamates) or as
catalysts in organic reactions (i.e., polymerizations,
Friedel-Crafts acylations, etc.)
• Many anhydrous metal chlorides, bromides and iodides
are commercially available from Sigma-Aldrich or
Strem
• Many of them are very expensive compared to their
hydrated counterparts
• The quality often times not as high as advertised due
to prolonged storage
Purification by Distillation
• Most metal halides are solids due to the fact that they form
three-dimensional network structures. However, some of
them are liquids because they are monomeric at room
temperature:
• Trichloride: boron, nitrogen, phosphorous and arsenic
• Tetrachloride: carbon, silicon, germanium, tin, lead, titanium
and vanadium
• Pentachloride: antimony (T> -55 oC), arsenic (T> -50 oC (dec.))
• Most metal chlorides are contaminated by hydrates (due to
their Lewis acidity), oxychlorides or oxides as a result of
hydrolysis
• PCl3 (b.p.: 74.5 oC) and POCl3 (b.p.: 105.3 oC) can be separated
by fractionated distillation
Dehydration by Heating of Hydrates
• Many alkali metal chlorides can be dehydrated by heating in vacuo
LiCl*H2O
CoCl2*6 H2O


Reddish-pink
LiCl + H2O
CoCl2 + 6 H2O
Blue
dry
moist
• Cobalt(II) chloride is the indicator in Drierite®
• Hydrates of metal ions with a high formal charge are acidic
• Example: Iron(III) chloride hexahydrate (pKa([Fe(H2O)6]3+)= 2.20) cannot be
dehydrated by simple heating
• Formation of iron(III) hydroxide, iron(III) oxychlorides and hydrogen chloride
FeCl3*6 H2O = [Fe(H2O)6]Cl3

Fe(OH)3(H2O)3 + 3 HCl
• Anhydrous iron(II) halides can be obtained by the reaction of iron metal with
the corresponding acid (HX) in methanol followed by the thermal decomposition
of the solvate ([Fe(CH3OH)6]X2)
Reaction of Metal with Chlorine
• Many metals can be reacted directly with chlorine at room
temperature or elevated temperatures (synthesis reaction)
• Selenium reacts at room temperature to form Se2Cl2 (reddish-brown
liquid) and reacts further to form SeCl4 (pale yellow solid) when heated
and exposed to an excess of chlorine
• Iron and aluminum metal react with chlorine to form FeCl3 and AlCl3
• A suspension of nickel metal in 1,2-dimethoxyethane reacts with
chlorine to yield anhydrous NiCl2 and carcinogenic, chlorinated ethers
Reaction of Metal with Hydrogen Halides
• The reaction of some metals with hydrogen
chloride yields anhydrous metal chlorides
M + n HCl
MCln + n/2 H2
• MCl2: Mn(II), Fe(II), Zn, Cd, Sn(II)
• MCl3: Sc, Pr, Sm ,Gd, Dy
• The metal chlorides formed in the reaction are
not in the highest oxidation state in many cases
• A similar approach can be used for anhydrous
metal bromides
Metal Oxides and Chlorinated Hydrocarbons I
• Oxides of many metals are available as ores (i.e., Fe3O4, Fe2O3)
or are intermediates in metal processing i.e., roasting of
sulfides (Cu2S, ZnS, PbS)
• Reduction-chlorination
• Carbon and chlorine: boron, titanium, chromium
• Carbon tetrachloride: scandium, cerium, samarium
• Usage of perchloroalkenes like hexachloropropene (b.p.: 210 oC),
octachlorocyclopentene (OCP, b.p.: 285 oC), and hexachlorobutadiene
(b.p.: 210 oC)
Metal Oxides and Chlorinated Hydrocarbons II
• Boron trichloride
• Step 1: Reduction of boron oxide with carbon
powder affords a black powder, B4C
• Step 2: Chlorination of this intermediate affords
a mixture of BCl3 (b.p.: 12.5 oC) and CCl4
(b.p.: 76.6 oC) that can be separated using
fractionated distillation.
Metal Oxides and Chlorinated Hydrocarbons III
• Reflux with OCP
Metal oxide
Product (time)
Nb2O5
NbCl5 (5 min)
Fe2O3
FeCl3 (15 min)
TiO2
TiCl4 (30 h)
V2O5
VOCl3 (15 min), VCl3 (6-8 h)
WO3
WOCl4 (2 h)
MoO2
MoOCl3 (15 min)
• Most reactions employing CCl4 as reagent are carried out in an
autoclave under elevated temperatures.
• Careful: The bomb builds up a significant pressure due to the
formation of carbon monoxide (CO) and phosgene (COCl2),
which are both highly toxic. The bomb has to be vented in a
well-ventilated hood.
Dehydration of Metal Chloride Hydrates using
Thionyl chloride, Trimethylsilyl chloride or Ketals I
• In these reactions, the water is chemically consumed in
the reaction with thionyl chloride or a ketal
• Thionyl chloride: produces sulfur dioxide, hydrogen chloride as
byproducts (both noxious!)
• Examples: NiCl2, CoCl2, CrCl3
• Problems:
• Thionyl chloride very difficult to obtain because it is a restricted
substance
• Thionyl chloride is contaminated by S2Cl2, SO2Cl2, SCl2
• Dimethoxyketal: methanol and acetone
Dehydration of Metal Chloride Hydrates using
Thionyl chloride, Trimethylsilyl chloride or Ketals II
•
The reaction of metal chloride hydrates with trimethylsilyl
chloride leads to the formation of hydrogen chloride (gas)
and hexamethyldisiloxane (b.p.: 100 oC), which is removed
by distillation
• MCln*x H2O + 2x (CH3)3SiCl
MCln + x ((CH3)3Si)2O + 2x HCl
• The anhydrous chlorides are obtained in the absence of a solvent
(M=Ba, Zn, Cu(II), Fe(III))
• If the reaction is carried out in THF, the reaction will afford
THF adducts (i.e., CrCl3(THF)3, ZnCl2(THF)2)
•
Tin(II) chloride (SnCl2 * 2 H2O) can be dehydrated by the
reaction with acetic acid anhydride
•
SnCl2*2 H2O + 2 (CH3CO)2O
SnCl2 + 4 CH3COOH
Special Methods
• Reaction of Metal Oxides with ammonium halides
• M2O3 + 6 NH4X
• M=Ln; X=Cl, Br, I
2 MX3 + 6 NH3 + 3 H2O
• Reaction of Metal acetates with acyl halides
• M(OAc)n*mH2O + (n+m) AcX
MXn + n Ac2O + m AcOH + m HX
• M=Mn-Cu (n=2), Cr, Nd, Sm (n=3), X=Cl, Br, I
• Reaction of anhydrous Metal acetates with hydrogen
halides
• M(OAc)n + n HX
MXn + n AcOH
• M=Cr, X=Cl, Br; M=Mo, X=Cl
Lower Chlorides of Tungsten I
• The direct chlorination of a metal at elevated
temperatures usually leads to the highest oxidation
state possible with chloride as ligand
• Many lower oxidation state chlorides are not
available by direct chlorination due to the high
oxidation potential of chlorine itself
• They are be obtained by reduction using different
reducing agents like hydrogen, alkenes, carbonyls,
metals, etc. or by co-proportion reactions with low
oxidation states
Lower Chlorides of Tungsten II
• Tungsten(VI) chloride is obtained by direct chlorination
of tungsten metal
• Tungsten(V) chloride is obtained from the reduction of WCl6
with hydrogen under mild conditions or by the reaction with
aluminum metal at 475 oC or with C2Cl4
• Tungsten(IV) chloride can be obtained from
• the reaction of tungsten metal with WCl5 in a bomb reaction
(syn-proportion)
• the reaction of the WCl6 with tin, antimony (97 %) or bismuth
(83 %, contaminated by Bi) (reduction)
3 WCl6 + 2 M
3 WCl4 + 2 MCl3 (M=Sb (130 oC), Bi (290 oC))
• the reaction of tungsten(VI) chloride with tungsten hexacarbonyl in
boiling chlorobenzene (syn-proportion)
• 2 WCl6 + W(CO)6
3 WCl4 + 6 CO ↑
Lower Chlorides of Tungsten III
• Tungsten(II) chloride is obtained by reduction of various
higher chlorides with hydrogen at temperatures between
450-500 oC (Note: at higher temperatures it disproportionates
into WCl4 and tungsten metal)
• Tungsten(III) chloride is obtained from the dichloride, which
is reacted with chlorine gas at 100 oC
Metal Carbonyls as Reactant
• Metal carbonyl compounds can be regarded as metal atoms in an
inert matrix (CO)
• Formation of CrCl3, MoBr4, MoI3, WBr5, WI3 from the metal
hexacarbonyl and the halogen
• Formation of Mo(OAc)2 (exhibits a Mo-Mo quadruple bond) from
the molybdenum hexacarbonyl and acetic acid
Mo(CO)6 + 2 Br2
2 Mo(CO) 6 + 4 CH3COOH
MoBr4 + 6 CO
Mo2(CH3COO)4 + 12 CO + 2 H2
• The reaction of tungsten hexacarbonyl with phosphorus
pentachloride in acetonitrile affords WCl4(CH3CN)2
• The reaction of molybdenum hexacarbonyl with iodine
in tetrahydrofuran affords MoI3(OC4H8)3 while the
solvent-free reaction in a Pyrex tube yields MoI3
Lewis Base Adducts I
• Lower chlorides of tungsten like WCl4L2 (L=THF, DME) or
molybdenum MoCl4L2 (L=Et2O, DME) can be obtained by
the reduction of WCl6 or MoCl5 using cyclopentene (C5H8)
or allyltrimethylsilane ((C3H5)Si(CH3)3)
• The reaction of MoCl5 with acetonitrile at room temperature
affords MoCl4(CH3CN)2, which can be reduced with tin metal
to form MoCl3(CH3CN)3
• The reaction of the NbCl5 or NbBr5 with varying amounts of
tributyltin hydride in the presence of ethers like THF or
1,2-dimethoxyethane leads to the formation of ether adducts of
niobium chlorides or niobium bromides in lower oxidation states
and hydrogen gas.
• NbX5 + Bu3SnH + 2 THF
• NbX5 + 2 Bu3SnH + DME
NbX4(THF)2 + Bu3SnX + 0.5 H2
NbX3(DME) + 2 Bu3SnX + H2
Lewis Base Adducts II
• Lewis base adducts are also preferred if the anhydrous metal
halides are either too unreactive due to their network
structures or too reactive because of their Lewis acidity
• Strong Lewis acids (i.e., MCl4 (M=Ti, Zr, Hf, Sn) have to be
dissolved in an inert solvent (i.e., dichloromethane, toluene)
when reacted with strong Lewis base (i.e., THF, diethyl ether,
acetonitrile)
• The resulting adducts MCl4(THF)2 can display both
cis-configuration (M=Ti, Zr, Hf, Re, Gd) and trans-configuration
(M=Ti, V, Mo(III), W(III), Sn, Te, many Ln)
• CrCl3(THF)3 is formed in the reflux of anhydrous chromium
chloride in THF in the presence of traces of zinc metal as
catalyst (which reduced Cr(III) to Cr(II) intermittently)
• Many MCl3(THF)3 adducts (M=Sc, Ti, V, Cr, Mo, W, Rh, Lu,
Bi, In) display mer-configuration
Colors of Metal Salts
• Hydrated and anhydrous metal salts are often
very different in color
Compound
VCl3
CrCl3
MnCl2
FeCl2
FeCl3
CoCl2
NiCl2
CuCl2
ZnCl2
Color of Hydrate
Green-gray (6)
Green (6)
Pink (4)
Light blue (4)
Brown (6)
Pink-red (6)
Green (6)
Blue-green (2)
White (2)
Color of anhydride
Purple-peach
Pink
Pale pink
White/pale brown
Black
Blue
Yellow-brown
Yellow
White