Microwave-assisted carbohydrohalogenation of first-row transition-metal oxides (M = V, Cr, Mn, Fe, Co, Ni, Cu) with the formation of element halides

Article abstract of DOI:10.1021/ic401914f

The anhydrous forms of first-row transition-metal chlorides and bromides ranging from vanadium to copper were synthesized in a one-step reaction using the relatively inexpensive element oxides, carbon sources, and halogen halides as starting materials. The reactions were carried out in a microwave oven to give quantitative yields within short reaction times.

Full text of DOI:10.1021/ic401914f

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Microwave-Assisted Carbohydrohalogenation of First-Row
Transition-Metal Oxides (M = V, Cr, Mn, Fe, Co, Ni, Cu) with the
Formation of Element Halides
Matthias Berger, Felix Neumeyer, and Norbert Auner*
Institut fur Anorganische Chemie, Goethe-Universitat Frankfurt, Max-von-Laue-Straße 7, 60438 Frankfurt (Main), Germany
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* Supporting Information
anhydrous iron(II) halides are also prepared from the metal.12,13
The microwave-assisted synthesis of FeCl2 under modified
conditions is also described.14 Cobalt(II) halides are produced
starting from CoCO3, and the acetyl halide11 or alternatively
CoCl2·6H2O is subsequently dehydrated with SOCl2.15 While
NiCl2 is prepared analogously, the bromide is obtained by
reacting Br2 with nickel powder in diethyl ether.16 Copper(I)
halides are available in solution from CuSO4 and SO2 with NaCl
or KBr.5
ABSTRACT: The anhydrous forms of first-row transi-
tion-metal chlorides and bromides ranging from vanadium
to copper were synthesized in a one-step reaction using the
relatively inexpensive element oxides, carbon sources, and
halogen halides as starting materials. The reactions were
carried out in a microwave oven to give quantitative yields
within short reaction times.
According to our investigations, the synthesis of first-row
transition-metal chlorides and bromides (M = V, Cr, Mn, Fe, Co,
Ni, Cu) starting from the element oxides can be simplified using a
microwave-assisted synthesis. This one-step reaction uses the
carbohydrohalogenation reaction and is an attractive alternative
to the conventional two-step process “metal oxide−metal−metal
halide” described before. For performance, carbon is mixed with
the relevant element oxide and gaseous HCl or HBr is passed
through a quartz tube while the reaction chamber is heated by
microwave absorption. The halogenation reactions occur
according to Scheme 2.
n industry as well as in academia, element halides are widely
Iused as starting materials for the synthesis of a variety of
different products. Because of the element halide functionality,
element chlorides and bromides serve as valuable precursors for
the synthesis of transition-metal complexes.1−3 Because of their
moisture-sensitive and often highly reactive polar element halide
bonds, these compounds are not available in nature and therefore
have to be synthesized from their natural ores, which are
abundant in nature as element oxides. For transformation of
oxides into their corresponding halides, different processes are
known. In principle, in a two-step synthetic route, the oxides are
reduced to the respective element in oxidation state zero with the
cheapest and most available reducing agent, carbon. The metals
are then reoxidized to a higher oxidation state with chlorine or
bromine to give the element halides (Scheme 1).4
Scheme 2. Alternative One-Step Carbohydrohalogenation for
the Synthesis of Element Halides
Scheme 1. Conventional Two-Step Synthesis of Element
Halides
Excess water from the water-containing oxides or released
from the reduction process is split under the reaction conditions
into hydrogen (H2) and carbon monoxide (CO) and thus avoids
hydrolysis of the often water-sensitive element halides
formed.17,18
When the carbohydrohalogenation experiments are per-
formed, anhydrous dark-blue CoCl2 is formed; the formation
of red-purple CoCl2·6H2O is not observed. Analogously, the
formation of anhydrous dark-green CoBr2 in contrast to red-
purple CoBr2·6H2O excludes the formation of water upon
reduction even in the cold zones of the reaction vessels (Figure
1). This convincingly demonstrates the absence of water due to
reduction by carbon to CO and H2 at temperatures above 800
°C. The formation of H2 was proven by bubbling the reaction gas
through deuterated benzene and detected by 1H NMR
spectroscopy of the resulting solution (C6D6, δ = 4.48 ppm).
On a laboratory scale, different routes for the synthesis of
element halides in the lower oxidation states are known and most
of them require long reaction times or multiple synthetic steps.5
The direct conversion of the inert oxides can only be achieved by
the highly reactive elemental halides.6 For example, in the
synthesis of water-free VCl3, vanadium metal is oxidized with
Cl2.7 The preparation of VBr3 requires vanadium metal to be
reacted with Br2 for more than 3 days at 400 °C.8 CrCl3 is
synthesized from the corresponding oxide with Cl2 at 650 °C,
with toxic phosgene being formed as a side product.9 CrBr3 is
obtained from Br2 and Cr2O3.10 Anhydrous manganese halides
are available upon reaction of the corresponding acetyl halide
with manganese acetate.11 Because of the inertness of Fe2O3,
Received: July 24, 2013
Published: October 7, 2013
© 2013 American Chemical Society
11691
dx.doi.org/10.1021/ic401914f | Inorg. Chem. 2013, 52, 11691−11693

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but is combined with experimental disadvantages: chlorine is an
inherent toxic and corrosive gas that forms phosgene and
tetrachloromethane under the required reaction conditions. In
the case of bromine as the bromination agent, separation and
transfer of the products is critical because unreacted bromine will
stick on the products formed and has to be removed in a separate
purification step. In contrast, the use of the less corrosive and at
room temperature gaseous hydrogen halides HX (X = Cl, Br)
simplifies the experimental procedure and is thus strongly
recommended over the use of other mostly liquid or solid
element halides that are technically used for halogenation
reactions (e.g., PCl5, SbCl3).1 Furthermore, no reaction of HCl
with CO to give phosgene and tetrachloromethane is known in
the literature.
After completion of carbohydrohalogenation, separation of
the products from impurities such as remaining excess carbon or
oxide residues is very simple. Because of the high temperature
gradient within the quartz tube, the products sublime into the
colder zones and are then mechanically collected as pure
compounds. Alternatively, for quantitative isolation, the product
is washed off with water to yield the hydrated form of the halides.
In the case of CuBr, isolation with water has to be replaced by
aqueous HBr as the solvent.
All products were formed in the crystalline state and identified
and characterized by a combination of powder X-ray diffraction
(PXRD) and energy-dispersive X-ray (EDX) analyses (Table 1).
Figure 1. Anhydrous products (a) CoCl2 and (b) CoBr2. For
comparison, the hydrolyzed products CoCl2·6H2O (c) and CoBr2·
6H2O (d) are shown.
Table 1. Products of Carbohalogenation
a
element oxide halogenating agent product (PXRD) ratio M:X (EDX)
b
V2O5
HCl
HBr
HCl
HBr
HCl
HBr
HCl
HBr
HCl
HBr
HCl
HBr
HCl
HBr
VCl3
1:2.4 0.2
1:2.1 0.3
1:3.2 0.3
1:3.1 0.3
1:2.0 0.1
1:3.0 0.2
1:2.0 0.1
1:2.9 0.3
1:2.0 0.1
1:2.2 0.1
1:1.1 0.2
1:2.0 0.1
1:2.0 0.1
1:2.2 0.1
For the reduction of element oxides, different kinds of carbon
sources can be applied. Because of its very attractive microwave
absorption coefficient, the use of graphite gives the best results.19
For the use of different carbon sources with a lower microwave
absorption coefficient, graphite might be admixed to the oxide/
carbon mixture to reach reaction temperatures of about 1000−
1400 °C. For achievement of the quantitative yields of halides
with respect to their metal oxides, an excess of carbon is required.
In case carbon is depleted, the reduction stops and excess
element oxide remains. This complicates a separation of the pure
halogenated products. Because of the high reaction temperatures
required and the transparency against microwave irradiation20,21
needed for halogenation, the reaction vessel consists of quartz. A
side reaction giving SiCl4 from SiO2 is not observed. Carbon does
not react with the smooth inner surface of the quartz tube; for
tetrachlorosilane formation the SiO2 sources have to be carefully
mixed with the reducing agent.14
c
b
VOBr2
CrCl3
CrBr3
MnCl2
MnBr2
FeCl2
FeBr2
CoCl2
CoBr2
Cr2O3
MnO2
Fe2O3
Co3O4
CuO
b
b
c
c
CuCl/CuCl2
CuBr/CuBr2
NiCl2
b
Ni2O3
NiBr2
a
b
Value of four measurements. The product was partly oxidized prior
c
to EDX measurements. The product was partly oxidized prior to
PXRD measurements.
Instead of microwave heating, carbohydrohalogenation can
also be performed in conventional ovens but then require
significantly longer reaction times. For comparison, for equal
molar amounts of metal oxides, the microwave-assisted reaction
needs about 15 min for complete consumption of the material,
while in conventional ovens at 1000 °C, most of the starting
material is still left after the same time period. Besides, other
disadvantages of conventional heating are the higher energy costs
and the longer heating and cooling times.22 When microwave
irradiation of the reaction mixtures is performed, a colored
plasma or white sparks and high energetic arcs are visible in most
reactions. In these reaction zones, the temperatures are obviously
much higher, leading to high or quantitative yields of the
products.
Representative samples showed no carbon impurities, as
confirmed by CHN combustion analysis. Generally, the
anhydrous forms of the salts are isolated. The products formed
show the metal atoms in the lower oxidation states: V3+, Cr3+,
Mn2+, Fe2+, Co2+, Cu+, and Ni+. For the oxidation-sensitive
compounds of Fe2+ and Cu+, PXRD analysis proves the lower
oxidation state, but prior to the EDX analyses, the salts were
readily oxidized under air to form the respective Fe3+ and Cu2+
species. For verification of a unified product formation, the
residues of the reaction mixtures were checked by EDX and only
excess carbon and traces of the halides could be identified.
In summary, we have described the syntheses of anhydrous
element chlorides and bromides of the first-row transition metals
(V, Cr, Mn, Fe, Co, Ni, and Cu). Compared to conventional
methods, the microwave-assisted syntheses provide shorter
For performance of the halogenation reactions described, the
use of elemental chlorine or bromine gas is, in principle, possible
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Communication
reaction times, simple product isolation, and higher yields of the
desired products. Carbon, graphite, or carbon-containing
materials were applied as reducing agents; hydrogen halides
were used for halogenations. The use of the latter is
recommended because of lower costs, lower toxicity, and lower
corrosivity compared to element halides. In all experiments, the
water-free metal salts were isolated in lower oxidation states and
characterized by PXRD and EDX spectroscopy.
ASSOCIATED CONTENT
* Supporting Information
Experimental section and photographs. This material is available
free of charge via the Internet at http://pubs.acs.org.
■
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AUTHOR INFORMATION
Corresponding Author
*E-mail: auner@chemie.uni-frankfurt.de. Fax: (+49)069-798-
29188.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We acknowledge support from Thalia Vavaleskou (EDX
analysis) and Edith Alig (PXRD analysis).
■
REFERENCES
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