Aluminum halide-cobalt halide polynuclear complexes active in low-temperature conversion of alkanes: Formation, molecular structures, and IR spectra

Article abstract of DOI:10.1007/s11172-008-0316-0

Low-temperature FT-IR studies of products of co-condensation of aluminum chloride and cobalt chloride in hydrocarbon matrices were carried out in the temperature range 80-250 K. DFT quantum mechanical calculations of the geometry and vibrational frequenci

Full text of DOI:10.1007/s11172-008-0316-0

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Russian Chemical Bulletin, International Edition, Vol. 57, No. 11, pp. 2251—2260, November, 2008
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Aluminum halide—cobalt halide polynuclear complexes
active in lowꢀtemperature conversion of alkanes:
formation, molecular structures, and IR spectra
M. I. Shilina, R. V. Bakharev, and V. V. Smirnov
Chemistry Department, M. V. Lomonosov Moscow State University,
1 Leninskie Gory, Moscow 199991, Russian Federation.
Fax: +7 (495) 932 8836. Eꢀmail: mish@kinet.chem.msu.ru
Lowꢀtemperature FTꢀIR studies of products of coꢀcondensation of aluminum chloride and
cobalt chloride in hydrocarbon matrices were carried out in the temperature range 80—250 К.
DFT quantum mechanical calculations of the geometry and vibrational frequencies of alumiꢀ
num chloride/cobalt chloride polynuclear molecular and ionic complexes of different
compositon were carried out. Isomeric labile complexes Аl2Cl6•СoCl2 were detected under
partial matrix isolation conditions and their behavior was monitored. The interaction of the
1 : 1 and 2 : 1 aluminum halide/cobalt halide molecular complexes at 120—170 К produces
catalytically active species in lowꢀtemperature conversion of alkanes. According to the data of
in situ spectral studies and to the results of quantum chemical calculations, these species
represent ionic associates which simultaneously contain Co and Al atoms in both cationic and
anionic fragments.
Key words: IR spectroscopy, molecular complexes, ionic complexes, aluminum chloride,
cobalt chloride, low temperatures, quantum mechanical calculations, activation of alkanes.
Catalytic systems based on aluminum halides and
transition metal salts are efficient in the isomerization
of alkanes, alkylation of unsaturated and aromatic comꢀ
pounds, and cracking of heavy paraffins.1—4 Usually, the
promoting effect of transition metal compounds on
the catalytic activity of Lewis acids is associated with the
formation of complexes of stronger acidity.1,2 For instance,
the activity of catalysts based on aluminum and cobalt
halides was explained5 by the formation of highꢀspin
octahedral complexes containing an electronꢀaccepting
also significantly changes the selectivity of reactions.6,7
Incorporation of transition metal cations can cause
the appearance of not only acidic, but also other types
of active centers capable of directly interacting with
hydrocarbons and imparting the catalyst a bifunctional
character.8,9 One can assume that the promoter cations
also play an important role in the formation of active
centers in bimetallic complexes based on aluminum
halides.
The participation of transition metal cations in the
activation of alkanes is, in particular, indicated by the comꢀ
position of products of lowꢀtemperature (150—220 К)
conversion of paraffins С7—С10, which was first carried
out in our studies10,11 of aluminum halide complexes with
various promoters. It was found that, depending on the
nature of the promoter (alkyl chloride or transition metal
halide), lowꢀtemperature transformations of alkanes
follow two routes. In the presence of butyl halides, the
products of lowꢀtemperature conversion of nꢀoctane
mainly contain light hydrocarbons С4—С5 (cracking
products). Contrary to this, no cracking occurs in the
presence of bimetallic complexes. In addition to isoꢀ
alkanes, products atypical of traditional acid catalysts are
formed, namely, nꢀalkanes whose chains are one carbon
atom shorter compared to the initial reactants.10
+
species AlBr2 ; however, no direct proofs of involvement
of these complexes in the process have been obtained
so far. By and large, information on the interrelations
between the structure of complexes of transition metals
compounds with Lewis acids, their spectral characterisꢀ
tics, and catalytic activity is scarce. One should also take
into account the fact that not only the enhancement
of Lewis acidity of aluminum halide (as most authors
believe), but also direct participation of promoter cations
can play the key role in the synergistic action of two metal
salts in the interaction with organic substrate. Here, an
analogy with the action of transition metal ions introꢀ
duced into zeolites is appropriate. In particular, modifiꢀ
cation of zeolites by zinc, cadmium, cobalt, etc. cations
not only improves the conversion of hydrocarbons, but
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2209—2218, November, 2008.
1066ꢀ5285/08/5711ꢀ2251 © 2008 Springer Science+Business Media, Inc.

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Russ.Chem.Bull., Int.Ed., Vol. 57, No. 11, November, 2008
Shilina et al.
A promising approach to understanding details of the
mechanism of catalytic action of Lewis acids is to carry
out lowꢀtemperature spectral studies of intermolecular
interactions under conditions of restricted molecular moꢀ
bility. This makes it possible to establish individual stages
of complexation and to follow the sequence of transforꢀ
mations of associates. Earlier,12—14 we have successfully
used this approach to detect and identify the aluminum
chloride complexes with nitroalkanes, water, and alkyl
halides.
Previously,15 a study of the lowꢀtemperature behavior
of the catalytic system АlCl3/СoCl2 at 80—240 К revealed
that the interaction of reactants can result in at least four
complexes with different composition. The order of their
formation is shown in Scheme 1. In addition to relatively
stable molecular associates 1—3 (their structures were
established using the results of spectral measurements
and quantum chemical calculations), we have detected
previously unknown labile intermediates 4. The highest
yield of products of lowꢀtemperature transformations of
alkanes was attained at an АlCl3/СoCl2 ratio of about 2 : 1
(see Ref. 10). The same conditions are also necessary for
the appearance of new absorption bands of intermediate 4
in the IR spectra of the catalyst. In this connection, we
assumed15 that these labile complexes are directly involved
in the activation of alkanes.
Experimental
Aluminum chloride (99.99%, Aldrich) was vacuum distilled
prior to sample preparation. Anhydrous cobalt (II) chloride was
obtained from crystal hydrate CoCl2•6H2O (99.9%) by evacuaꢀ
tion at 430 К for 3—4 h. Saturated hydrocarbons (HC)
(nꢀoctane, 2,2,4ꢀtrimethylpentane, and 2,4ꢀdimethylheptane)
were kept over metallic sodium for several days and then
distilled. The purity of starting reagents was monitored by IR
spectroscopy and GLC.
IR spectra of solid samples were recorded in an optical
cryostat described earlier.12 Samples were prepared by in vacuo
coꢀdeposition of reactant vapor jets on a mirror surface of a
copper plate cooled to 80 К. Aluminum chloride was conꢀ
densed by heating vapor to 343—353 К. Monomeric aluminum
chloride was obtained by additional overheating of its vapors at
up to 1000—1100 К. Cobalt chloride was condensed by heating
vapor to 820 К. The hydrocarbon feed rate was controlled
by fineꢀadjustment needle valves. The condensation rate was
1014 to 1016 molecule cm–2 s–1 and the film thickness was 4 to
15 μm. The temperature was maintained with an accuracy of
1 К. IR spectra (4000—380 cm–1) were recorded on an Infralum
FTꢀ801 spectrometer with a resolution of 0.5 to 2 cm–1. Coꢀ
condensation of aluminum chloride and cobalt chloride was
carried out using saturated hydrocarbons of different structure
(nꢀoctane, 2,2,4ꢀtrimethylpentane, and 2,4ꢀdimethylheptane)
as matrices at AlCl3/CoCl2 ratios (n) from 0.3 to 10 and
HC/AlCl3 ratios from 10 to 70.
Computational methods. Quantum mechanical calculations
of the structures and vibrational frequencies of AlCl3, Al2Cl6,
CoCl2, Co2Cl4, and their 1 : 1 and 2 : 1 molecular complexes
were carried out earlier15 using the DFT approach with the
В3LYP16 and PBE17 functionals. The results obtained using
both functionals are in reasonable agreement with each other
and with experimental data. In this work, calculations of the
structures and vibrational frequencies of the complexes were
performed with inclusion of electron correlation using the PBE
density functional17 using the “PRIRODA” program complex.18
The oneꢀelectron wave functions were expanded with an
extended TZ2p basis sets of contracted Gaussian functions of
size (311/1) for the H atom, (611111/411/11) for the C atom,
and (6111111111/611111/11) for the Al and Cl atoms. Full optiꢀ
mization of the geometry and the force field calculations were
done in the harmonic approximation.
Scheme 1
Polycrystal
The aim of the present work is to determine appropriꢀ
ate conditions for the formation of these species and to
elucidate their nature and structure. To stabilize labile
complexes, coꢀcondensation of reactant vapors and the
matrix isolation technique were used. The structures of
the complexes and their thermal behavior were studied
using a combination of in situ lowꢀtemperature IR Fouꢀ
rier spectroscopy and quantum chemical calculations. The
use of the highꢀresolution FTꢀIR spectrometer and carryꢀ
ing out experiments under better matrix isolation condiꢀ
tions (compared to a previous study15) made it possible to
revise the vibrational frequencies of the complexes 1 and
2 detected earlier, to stabilize previously unknown isoꢀ
mers of complex 3, and to determine their structures.
Results and Discussion
Aluminum chloride/cobalt chloride molecular comꢀ
plexes of the compositions 1 : 1 and 2 : 1. Earlier,15 we
have shown that vapor coꢀdeposition of aluminium
halides and cobalt halide at a small excess of hydrocarꢀ
bons (HC/AlCl3 = 3—10) at 80 К results in a number of
polynuclear complexes. If cobalt halide is taken in excess
(n = AlCl3/CoCl2 < 1), the samples mainly contain the
1 : 1 АlCl3•СoCl2 associates (1); at n > 1, the 2 : 1 comꢀ
plexes (АlCl3)2•СoCl2 (2) are mainly formed. Complexes
2 are stabilized in the form of structures with tetrahedral
or distorted octahedral environment of cobalt. The IR
spectra of the samples of nearly equimolar compositions

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Active complexes of aluminum and cobalt chlorides
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 11, November, 2008 2253
A
(n = 1—3) exhibit the absorption bands of both types of
complexes, 1 and 2.
617
0.7
478
In this study, we have investigated the IR spectra
of aluminum halide/cobalt halide coꢀcondensates in more
detail by FTꢀIR spectroscopy in a wide range of the
HC/AlCl3 ratios (from 10 to 70) with emphasis placed on
the samples simultaneously containing complexes 1 and 2
(n = 1—3). It is these coꢀcondensates that give catalytically
active labile complexes 4 (see Scheme 1); an increase in
the degree of matrix isolation should favor stabilization of
complexes 4.
a
0.6
467
594
419
0.5
0.4
0.3
0.2
0.1
0.0
444
504
500
574
379
400
604
Figure 1 shows the IR spectra of an Аl2Cl6/СoCl2 coꢀ
condensate in nꢀoctane matrix at small excess of alumiꢀ
num chloride (n = 2.5) and HC/AlCl3 = 9. At 80 К,
not only the spectra of complexes 1 (bands at 574, 504,
and 444 cm–1) and 2 (bands at 594 and 467 cm–1),15 but
also the absorption bands of free Аl2Cl6 at 617, 478, and
419 cm–1 are observed. For the best band assignment,
these spectra were deconvoluted (Fig. 2). It was found
that as the temperature increases from 80 to 120 К, the
intensities of the aluminum chloride dimer bands deꢀ
crease while those of the bands of complexes 1 and 2 (in
particular, the bands at 574 and 594 cm–1) increase simulꢀ
taneously (Fig. 1, curve 2). These spectral changes are
due to additional complexation which accompanies enꢀ
hancement of the molecular mobility. This picture is typiꢀ
cal of lowꢀtemperature synthesis by vapor coꢀdeposition.
In the first stage, the ratio of free reactants to various
associates is mainly under kinetic control; as temperature
increases, the degree of complexation of the reactants
increases.19 In our case, the metal halides in complexes 1
and 2 become completely bound only if the temperature
raises to about 120 К. As a result, from this instant on the
spectrum exhibits only the bands of associates and alumiꢀ
num halide present in excess (Fig. 1, curve 2; Fig. 2, b).
650
600
594
550
450
ν/cm–1
A
b
0.5
0.4
0.3
0.2
0.1
0.0
444
504
574
467
617
478
419
379
400
650
600
550
500
450
400
ν/cm–1
Fig. 2. IR spectra of coꢀcondensates of AlCl3/Al2Cl6 with CoCl2
in nꢀoctane matrix (n = 2.5, HC/AlCl3 ~ 9) at 80 К (а) and
120 К (b) and their approximation by a superposition of Gaussian
curves.
It was found that the spectral picture depends not only
on the reactant ratio n = AlCl3/CoCl2, but also on the
degree of dilution of the matrix. The pattern of the changes
occurring in the samples of the same chemical composiꢀ
tion in highly diluted matrices differs from that described
above.
T(%)
The 2 : 1 complexes containing Al2Cl6 dimers. Even
if metal halides are taken in nearly equimolar amounts
(n = 1—3), at HC/AlCl3 ratios in the range from 15 to 70
the IR spectra of cocondensates recorded at 80 K exhibit
not only the bands of complexes 1 and 2, but also some
new IR bands unassignable to the spectra of the adducts
described above. The IR spectra of a cocondensate (n ≈ 2.5)
recorded in situ at different temperatures are shown in
Fig. 3 as an example. In Fig. 4 they are approximated by a
superposition of the IR spectra of individual components.
At 80 К, the IR spectra show the absorption bands of
free aluminum chloride dimer (at 617, 478, and 419 cm–1),
a superposition of two strong bands at 604 and 594 cm–1
and a weak band at 574 cm–1, and weak bands at 504, 457,
and 444 cm–1 (Fig. 3, curve 1, Fig. 4, а). The bands at 594
and 467 cm–1 can be assigned to complex 2 with distorted
octahedral environment of cobalt.15 However, the appearꢀ
5
80
4
3
2
60
1
533
40
504
20
444
419
594
600
575
463
617
478
650
550
500
450
ν/cm–1
Fig. 1. IR spectra of coꢀcondensates of AlCl3/Al2Cl6 with CoCl2
in nꢀoctane matrix (n = 2.5, HC/AlCl3 ≈ 9) at 80 (1), 120 (2),
170 (3), 180 (4), and 220 K (5).

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Russ.Chem.Bull., Int.Ed., Vol. 57, No. 11, November, 2008
Shilina et al.
T(%)
Table 1. Total energies (Е) and formation energies (ΔЕ) of complexes
of aluminum chloride dimer with cobalt chloride Аl2Cl6·CoCl2
(3) obtained from density functional PBE/3z calculations
70
60
50
40
30
20
10
0
4
Compound
–E/au
–ΔE/kcal mol–1
3а
3b
3c
5548.089089
5548.067332
5548.053605
26.5
12.8
4.2
3
2
533
1
574
444
pronounced when comparing the superpositions of the
spectra of individual components (Fig. 2, а and Fig. 4, а).
Unlike the spectrum in Fig. 2, а of the sample with a low
degree of dilution (HC/AlCl3 < 10), which is satisfactoꢀ
rily described by the sum of the spectra of complexes 1
and 2, the experimental spectrum in Fig. 4, a can reasonꢀ
ably be described only using additional absorption bands.
The new adduct we have detected manifests itself
in the spectra mainly at high dilution of the matrix
(HC/AlCl3 = 15—50). Under these conditions, the role
of kinetic factors in the interaction between reactants
becomes much more significant,19 i.e., stabilization of
the less thermodynamically favorable complexes becomes
more probable. Taking into account the fact that,
depending on the temperature of aluminum chloride
vapor (see Experimental), this compound in the course
of coꢀcondensation is partially or completely present as a
dimer, it is logical to assume stabilization of adducts
containing aluminium chloride dimers with unbroken
bridging bond Al—Cl—Al in the initial stage. Our quanꢀ
tum mechanical calculations predict the possibility of forꢀ
mation of three relatively stable structures (3a—с) of these
complexes from Al2Cl6 dimer. The calculated total enerꢀ
gies and stabilization energies of these systems are listed
in Table 1 and the structures are shown in Fig. 5.
Spectral band assignment and the structure of comꢀ
plexes. According to calculations, the attachment of
cobalt chloride normal to the plane of the Al—Cl—Al
bridging bonds leads to cleavage of one bridging bond
with the formation of complex 3а (see Fig. 5), the most
stable out of three possible structures. The stabilization
energy of 3a calculated relative to the sum of the energies
of noninteracting aluminum chloride dimer and cobalt
chloride monomer is 26.5 kcal mol–1, being somewhat
lower than the similarly calculated stabilization energy of
complexes 2 (35.1 and 30.8 kcal mol–1).15 The lengths
of the bridging bonds Со—Сl are in the range from 2.35 to
2.4 Å, which is close to the corresponding bond lengths in
complexes 2.
594
504
500
463
478
575
419
617
650
600
550
450
ν/cm–1
Fig. 3. IR spectra of Al2Cl6/CoCl2 coꢀcondensates in nꢀoctane
matrix (n ≈ 2.5, HC/AlCl3 ~ 30) at 80 (1), 118 (2), 148 (3), and
220 K (4).
ance of other bands provided the lack of intense absorption
at 574 cm–1 characteristic of complex 1 (see Ref. 15) indiꢀ
cates the formation of yet another adduct characterized
by the absorption bands at 604, 504, 457, and 445 cm–1
.
The bands of the previously unknown complex are most
A
617
a
478
0.8
419
0.6
467
457
0.4
604
600
594
574
444
504
500
400
0.2
0
379
650
550
450
400 ν/cm–1
A
b
0.4
0.3
0.2
0.1
0
594
604
444
504
467
478
574
419
400
617
379
When a cobalt chloride molecule is bonded to aluꢀ
minium chloride dimer in another fashion, with retention
of both bridging bonds in the latter molecule, calculations
predict the formation of energetically less favorable comꢀ
plexes 3b and 3c (see Fig. 5). The structure of 3c has
already been considered earlier;15 here the cobalt atom is
in nonequivalent environment of chlorine atoms and the
650
600
550
500
450
400 ν/cm–1
Fig. 4. IR spectra of Al2Cl6/CoCl2 coꢀcondensates in nꢀoctane
matrix (n ≈ 2.5, HC/AlCl3 ~ 30) at 80 К (а) and 130 К (b) and
their approximation by a superposition of Gaussian curves.