Reactions of 2-chlorobutane with polynuclear bimetallic aluminum and cobalt chloride complexes: Low-temperature synthesis and structure of adducts

Article abstract of DOI:10.1007/s11172-010-0214-0

Co-condensation of aluminum and cobalt chlorides and 2-chlorobutane leads to the formation of triple complexes. Density functional method PBE was used for the quantum chemical calculations of structures and harmonic vibration frequencies. The structures of associates were established by the comparison of experimental and theoretical IR spectra. Introduction of cobalt chloride into the system 2-chlorobutane-aluminum chloride leads to the formation of a triple molecular associate, in which coordination of the organic substrate with the bimetallic complex is accomplished through the transition metal atom. The triple complex is stable in the temperature range 80-240 K and in contrast to the binary complex of butyl chloride with aluminum halide is not capable of self-ionization to form sec-butyl cation.

Full text of DOI:10.1007/s11172-010-0214-0

Russian Chemical Bulletin, International Edition, Vol. 59, No. 6, pp. 1119—1125, June, 2010
1119
Reactions of 2ꢀchlorobutane with polynuclear bimetallic
aluminum and cobalt chloride complexes:
lowꢀtemperature synthesis and structure of adducts
M. I. Shilina, V. V. Smirnov, and R. V. Bakharev
Department of Chemistry, M. V. Lomonosov Moscow State University,
1 Leninskie Gory, 119992 Moscow, Russian Federation.
Fax: +7 (495) 932 8846. Eꢀmail: mish@kinet.chem.msu.ru
Coꢀcondensation of aluminum and cobalt chlorides and 2ꢀchlorobutane leads to the formaꢀ
tion of triple complexes. Density functional method PBE was used for the quantum chemical
calculations of structures and harmonic vibration frequencies. The structures of associates were
established by the comparison of experimental and theoretical IR spectra. Introduction of
cobalt chloride into the system 2ꢀchlorobutane—aluminum chloride leads to the formation of
a triple molecular associate, in which coordination of the organic substrate with the bimetallic
complex is accomplished through the transition metal atom. The triple complex is stable in the
temperature range 80—240 K and in contrast to the binary complex of butyl chloride with
aluminum halide is not capable of selfꢀionization to form secꢀbutyl cation.
Key words: IR spectroscopy, carbocations, molecular complexes, aluminum chloride, coꢀ
balt chloride, 2ꢀchlorobutane, low temperatures, quantum chemical calculations, density funcꢀ
tional theory.
Aluminum chloride is one of the widely used acid catꢀ
alysts, its action is the most efficient in the presence of
promoting additives. Transition metal salts, which exhibit
synergism with Lewis acids in the processes of isomerizaꢀ
tion and cracking of alkanes, alkylation of unsaturated
and aromatic compounds, cracking of heavy parafins1—4
are of special interest among them. An increase in the
acidity of a Lewis acid, in particular aluminum halide, in
the presence of a transition metal is accepted as a princiꢀ
pal cause for the synergism.1,2,5 At the same time, a numꢀ
ber of experimental facts indicate a possibility of direct
interaction of transition metal cations with organic subꢀ
strates, for example, for the reactions of ions in the gas
phase6,7 or in catalytic transformations of alkanes on zeoꢀ
lites modified with transition metal ions.8,9 Product comꢀ
positions of the lowꢀtemperature conversion (170—220 К)
of parafins C7—C10, for the first time accomplished by us
on a promoted aluminum chloride, is also an evidence.10
Nontypical for the traditional acid catalysts, alkanes of
normal structure with the number of carbon atoms less by
one unit as compared to the starting number were obꢀ
tained in high yields along with alkanes of the isostructure
in the presence of transition metal halide, including cobalt
chloride.10 Here, the fraction of cracking products does
not exceed 5% as compared to 70—80% for the aluminum
chloride both individual or promoted with organic halo
derivatives. Such an influence of transition metal salts on
selectivity of the processes in the presence of Lewis acꢀ
ids was observed in the liquidꢀphase conversion of parafins
as well.1,4
One of prospective approaches to the understanding
mechanism of catalytic action of Lewis acids consists in
the study of intermolecular interactions at low temperaꢀ
tures under conditions of restricted molecular mobility. In
this case, each step of the complexation can be separated
and the sequence of the associate transformations can be
followed. This approach has been applied by us earlier for
the detection and identification of aluminum chloride
complexes with cobalt halides11,12 and alkyl halides.13 As
a result, a possibility of stabilization of a secondary butyl
cation in the reaction of 2ꢀchlorobutane and aluminum
chloride has been shown for the first time. Earlier, ion
pairs involving this cation have been registered only in the
presence of the strongest Lewis acids, for example, in the
SbF5 matrix (see Ref. 14). As it was expected, secondary
cations are less stable as compared to the tertiary and are
registered only in the limited temperature range. If tertiary
butyl cations tꢀBu+ are stable in the presence of aluminum
chloride at temperatures of 80—295 К, the secondary carꢀ
benium ions sꢀBu+ are stable only at temperatures of
130—180 K (see Ref. 13). At the same time, in the matrix
of stronger Lewis acid, SbF5, stability of ion pairs involvꢀ
ing sꢀBu+ is higher, and they are registered up to temperaꢀ
ture of 233 К.14 Due to the correlation indicated, a possiꢀ
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1097—1103, June, 2010.
1066ꢀ5285/10/5906ꢀ1119 © 2010 Springer Science+Business Media, Inc.

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Shilina et al.
bility of formation and stability of the secondary butyl
cation can be used for the comparison of the Lewis acid
strengths on a qualitative level.
1 : 1 and 2 : 1 molecular complexes. The results obtained
are in satisfactory accord between each other and with
experimental data. Earlier,13 the structure and vibration
spectra of solid 2ꢀClBu and its molecular and ionic comꢀ
plexes with aluminum chloride have been studied as well.
In the present work, calculations of structures and vibraꢀ
tion frequencies of triple complexes with allowance for the
correlation of electrons were performed by the PBE densiꢀ
ty functional (see Ref. 17) using the PRIRODA program
package (see Ref. 18). For expansion of the oneꢀelectron
wave functions, the TZ2p atomic basis sets of the grouped
Gaussian function type were used: for H atoms of the form
(311/1), for C atoms of the form (611111/411/11), for Al
and Cl atoms of the form (6111111111/611111/11), and for
Co atoms of the form (611111111111/511111111/5111).
Full optimization of geometry and calculation of harmonic
force fields were performed.
As it has been already mentioned above, the data on
the change in the aluminum chloride acidity in the reacꢀ
tion with transition metal derivatives are of principal imꢀ
portance for the understanding the promotion mechanism.
In the case if the Lewis acidity increases, the promotion
effect is well explained from the point of view of generally
accepted carbocationic mechanism; if the Lewis acidity
decreases, there is reason to suppose a direct involvement
of the transition metal ion, for example, of cobalt, into the
reaction with an organic substrate. In the present work,
the acidities of binary bimetallic polynuclear complexes of
aluminum and cobalt chlorides were evaluated based on
the product compositions of their reactions with 2ꢀchloroꢀ
butane at low temperatures. Complexes of various compoꢀ
sitions were obtained by coꢀcondensation of the halide
vapors taken in different proportions. nꢀAlkanes were used
as a matrix for stabilization of the complexes. The comꢀ
plex structures and their evolution were studied by lowꢀ
temperature IR Fourierꢀspectroscopy in situ in combinaꢀ
tion with quantum chemical calculations.
Results and Discussion
Starting compounds. Samples obtained by coꢀcondenꢀ
sation are homogeneous in composition optically transꢀ
parent films. The use of the regime of molecular beams for
their preparation provides phase homogeneity of the sysꢀ
tem close to homogeneous media, that is confirmed by the
registration of those states of the molecules on a cooled
support, in which they were in the vapor phase at the
evaporation temperature. The regularities indicated have
been repeatedly observed earlier for various molecular sysꢀ
tems,15 including molecular systems based on aluminum
chloride as well.11—13 Structural studies of the aluminum
chloride solid films under conditions of various matrix
surroundings19,20 showed that on the condensation of aluꢀ
minum halide from the gas phase, a metastable molecular
structure of dimers Al2Cl6 is preserved in the solid films
obtained, and the content of the dimers is from 40 to 100%
depending on the extent of dilution with the second comꢀ
ponent. Transition to the crystalline state occurs only at
temperatures of 95—150 К. Vibration spectra of molecuꢀ
lar and polycrystalline forms of aluminum halides are
different.
Aluminum chloride under conditions of this experiꢀ
ment condenses from the vapor phase as a dimer Al2Cl6,
whose IR spectrum in the hydrocarbon matrix exhibits
bands at 617, 478, and 417 cm–1 related to the stretching
vibrations ν(Al—Cl) (see Refs 19—21). The IR spectrum
of the aluminum chloride monomer in the inert gas matriꢀ
ces in the region ν > 400 cm–1 contains the only band at
618 cm–1 (see Ref. 21). The IR spectrum of polycrystalꢀ
line aluminum chloride contains intensive broad bands at
605 and 503 cm–1 (see Refs 19 and 20).
Experimental
Aluminum chloride containing 99.99% of the principal subꢀ
stance (Aldrich) was distilled in vacuo before preparation of samꢀ
ples. Anhydrous cobalt chloride was obtained from the hydrate
CoCl2•6H2O (99.9%) by evacuation over 3—4 h at 430 К.
2ꢀChlorobutane (2ꢀClBu) (purity 99+%, Aldrich) was used as
purchased. Purity of starting substances was controlled by
IR spectroscopy and gasꢀliquid chromatography.
IR spectra of solid samples were recorded on an optical cryꢀ
ostat described earlier.11 The samples were prepared by coꢀ
condensation of the reagent vapors in vacuo (the pressure of
~10–2 Pa) on a copper block mirror surface cooled to 80 K. To
obtain compositionally homogeneous samples, the condensaꢀ
tion was accomplished in the regime of molecular beams.15 The
condensation rate was 1014—1016 molecules cm–2 s–1. Evaporaꢀ
tion of aluminum and cobalt chlorides was accomplished at
343—353 K and 800—850 K, respectively. The supply rate of
2ꢀchlorobutane was controlled by a needle valve of fine adjustꢀ
ment. The films were 2—10 μm thick. The sample temperatures
were varied in the range 80—250 K and kept with accuracy 1 К.
IR transmission spectra of solid samples were recorded in
the reflecting regime with the double transmission of the beam
through the sample film. IR spectra in the region 5000—380 cm–1
were obtained on a Infralum FTꢀ801 IR Fourierꢀspectrometer.
Calculation Methods
Earlier,7 we have applied methods of the density funcꢀ
tional theory with the use of the В3LYP (see Ref. 16) and
PBE (see Ref. 17) functionals for the quantum chemical
calculations of structures and vibration frequencies of
AlCl3, Al2Cl6, CoCl2, and Co2Cl4 molecules and their
No absorption bands of cobalt chloride are observed
in the region of the spectrum under study, though in
the inert gas matrices for dilutions in the proportion
~3000—1000 : 1, absorption bands of stretching vibrations

Triple complexes of sꢀBu, Al, and Co chlorides
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 6, June, 2010
1121
of monomer CoCl2 and dimer Co2Cl4 are found at 493
and 432 cm–1, respectively.22,23 Under conditions of our
experiment, associates of more complex composition are
apparently stabilized.
1224, 1069, 991, 563, 551 cm–1 for 1 or 1280, 1224, 1067,
989, 596, 570, 555 cm–1 for 2 (see Ref. 13)).
The system aluminum chloride—cobalt chloride—2ꢀchloꢀ
robutane. Similar picture is also characteristic of the
IR spectrum of the threeꢀcomponent coꢀcondensate of
aluminum and cobalt halides and 2ꢀClBu at the ratio of
reagents ~1 : 1 : 1. The IR spectra of the organic halide
in the free state and in the threeꢀcomponent system
2ꢀClBu•CoAlCl5 are shown in Fig. 1. It is seen that when
the triple complex is formed, absorption bands of the startꢀ
ing 2ꢀClBu disappear and new bands appear, for example,
at 1292, 1282, 1229, 1069, 990, 564, 503, 470 cm–1. Vibraꢀ
tion frequencies of the threeꢀcomponent system are given
in Table 1. A comparison of the IR spectra of complex
2ꢀClBu•CoAlCl5, binary complex 1, and the starting reꢀ
agents shows that the vibration frequencies of 2ꢀchlorobuꢀ
tane in the triple system are close to those observed in the
spectrum of 2ꢀClBu•AlCl3, the deviations, as a rule, do
not exceed 5 cm–1. Note that the IR spectra of complexes
1 and 2 are close enough as well.13
2ꢀChlorobutane condenses from the gas phase as three
conformers SCH´, SHH´, and SH´H´, that is reflected in the
IR spectra by splitting a number of bands to three compoꢀ
nents. The splitting mentioned is clearly seen in the region
700—600 cm–1, which corresponds to the stretching viꢀ
brations of the C—Cl bond. The proportion of the band
intensities for different conformers is similar to their disꢀ
tribution in the spectra of 2ꢀchlorobutane, liquid or isolatꢀ
ed in the argon matrix,24,25 with predominance of the most
stable SH´H´ configuration. The ratio of conformers does
not virtually depend on temperature. The presence of varꢀ
ious conformational states of 2ꢀchlorobutane in the conꢀ
densates obtained, in contrast to the only conformer obꢀ
served in the crystalline state,24 can be considered as an
additional evidence that condensates obtained by us are
close to the homogeneous systems.
Molecular complexes. The system aluminum chloride—
2ꢀchlorobutane. The coꢀcondensation of aluminum chloꢀ
ride with 2ꢀchlorobutane leads to the emergence of new
absorption bands in the IR spectra, which are absent in
the spectra of starting reagents. Earlier,13 study of the specꢀ
tra of binary coꢀcondensates has been performed in details
in a wide range of the reagent proportions and temperaꢀ
tures (80—250 К), as well as using partial isolation in the
alkane matrices. It was established that in the binary coꢀ
condensates of aluminum chloride with 2ꢀClBu, molecuꢀ
lar and ionic complexes are formed; conditions for their
formation and interconversions are shown in Scheme 1.
However, in the lowꢀfrequency region of the spectrum
(700—400 cm–1) where metal—chlorine stretching vibraꢀ
tions are found (Fig. 2), considerable differences in the
binary (Fig. 2, 1 and 2) and triple (see Fig. 2, 3) systems
are present already at 80 K. In this case, no bands at 563
and 551 cm–1 typical of complex 1 (see Fig. 2, 1) are
observed, with broad intensive bands at 566, 502, 470, and
446 cm–1 emerging instead (Fig. 2, 3). The frequencies
recorded for the triple systems do not agree with the specꢀ
tra of binary complexes AlCl3•CoCl2 either (Fig. 2, 2),
which were found earlier11,12 during study of coꢀcondenꢀ
sates of aluminum and cobalt chlorides for wide ranges of
the reagent ratios (from 0.3 to 10) and temperatures
(80—250 К). Among associates of various composition and
structure revealed in works,11,12 complexes AlCl3•CoCl2
and (AlCl3)2•CoCl2 have spectral characteristics the closꢀ
est to those observed for the triple coꢀcondensates. The
superposition of spectra of these complexes exhibiting at
574, 504, 444 cm–1 and at 594, 467 cm–1, respectively, is
shown in Fig. 2, 2. From Fig. 2, it is seen that a possible
Scheme 1
T (%)
1
According to Ref. 13, the reaction of Al2Cl6 with
2ꢀClBu at 80 K mainly leads to molecular complexes
2ꢀClBu•AlCl3 (1). When aluminum chloride is in excess
amount, the 1 : 2 complexes 2ꢀClBu•Al2Cl6 (2) are also
formed along with complexes 1. Transformation of comꢀ
plexes 1 and 2 to ionic associates is observed at temperaꢀ
tures above 120 K. The IR spectra of 2ꢀClBu and its comꢀ
plex 1 are given in Table 1. As it is seen from the data in
Table 1, the formation of molecular associates is accomꢀ
panied by the extinction of a number of absorption bands
of the starting reagents (for example, 1298, 1237, 1076,
993, 617, 603 cm–1) and emergence of new bands (1282,
2
80
40
1400
1200
1000
800 ν/cm–1
Fig. 1. IR spectra of 2ꢀchlorobutane (1) and coꢀcondensates of
2ꢀchlorobutane with aluminum chloride and cobalt chloride of
the form 2ꢀClBu•CoAlCl5 (2) at 80 К.

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Shilina et al.
Table 1. Experimental (80 К) and calculated by the DFT/PBE/3z method wave numbers (cm–1) and intensitiesa (km mol–1) in the IR
spectra of 2ꢀchlorobutane and its molecular complexes with aluminum chloride and aluminum and cobalt chlorides
2ꢀClBu
2ꢀClBu•AlCl3 (1)
2ꢀClBu•CoAlCl5
Experiment Calculation
Assignmentb
Experiment
Calculation
Experiment
Calculation
3A
3B
2972 s
2938 s
3062 (16)
3045 (30)
3039 (31)
3035 (26)
3014 (2)
2999 (5)
2970 (23)
2965 (15)
2971 (13)
1465 (7)
1454 (7)
1450 (6)
1444 (13)
1435 (1)
1359 (11)
1352 (6)
1330 (1)
1303 (3)
1251 (1)
1214 (19)
1143 (13)
1125 (1)
2975 s
2939 s
3075 (3)
3056 (16)
3050 (21)
3044 (5)
2989 (6)
3028 (1)
2979 (12)
2971 (11)
2944 (11)
1455 (15)
1450 (13)
1446 (7)
1432 (8)
1416 (2)
1373 (6)
1361 (14)
1348 (1)
1277 (14)
1266 (6)
1215 (9)
1143 (15)
1108 (3)
2978 s
2939 s
3069 (6)
3055 (20)
3048 (17)
3044 (9)
3000 (6)
3030 (2)
2977 (16)
2972 (8)
2949 (9)
1457 (11)
1452(13)
1442 (2)
1434 (11)
1413(4)
1368 (8)
1361(12)
1351 (3)
1279 (1)
1267 (20)
1208 (16)
1144 (17)
1106 (2)
3075 (3)
3057 (18)
3050 (14)
3046 (2)
2993 (5)
3028 (0)
2978 (11)
2973 (5)
2944 (8)
1455 (13)
1450(14)
1444 (2)
1430 (12)
1408 (4)
1371 (8)
1363 (14)
1354 (1)
1277 (10)
1262 (14)
1202 (16)
1143 (23)
1108 (2)
ν(Me)
ν(Me)
2929 s
2917 m
2878 m
2926 m
2917 w
2878 m
2925 s
2914 w
2879m
ν(СН2)
ν(СН)
ν(Me)
2846 w
1461 s
2847 w
1460 s
2849 m
1458 s
ν(СН2)
δ(Me)
1455 s
1443 s
1432 m.sh
1378 s
1362 w
1321 vw
1298 s
1287 s
1237 s
1158 s
1122 m
1076 s
1063 m
1025m
993 s
1455 s
1440 s
1452 s
1442 s
δ(CH2)
δ(Me)
1381 s
1383 s
1323 vw
1282 s
1287 s
1224 s
1160 s
1122 m
1321 w
1282 s
1292 m
1229 s
1160 s
1121 m
1069 m
1057 m
1021 w
0990 s
0950 m
0839 m
0781 s
0—c
χ(CH)
w(CH2)
sc(CH2), χ (CH)
i
χ (CH)
i
ν(CC)
ν(CC)
ν(CC), r(Me)
1045 (12)
1002 (6)
0958 (5)
0935 (9)
0831 (9)
0767 (4)
0652 (29)
0408 (1)
1069 s
1024 m
0991 s
0950 s
0839 m
0778 s
0—c
0—c
0563 s.br
0551 s.br
1054 (8)
1015 (6)
1050 (10)
1014 (8)
0974 (17)
0934 (8)
0826 (10)
0757 (20)
0511 (31)
0444 (1)
0574 (144)
0489 (210)
1048 (7)
1013 (9)
ν(CC)
r(Me)
r(Me)
ν(CC)
r(CH2)
ν(CCl)
γ(CCC)
ν(AlCl)
ν(AlCl), ν(CCl)
0972 (16)
0937 (11)
0825 (6)
0765 (18)
0512 (22)
0443 (2)
0972 (14)
0929 (13)
0826 (11)
0753 (22)
0502 (80)
0445 (2)
959 s
844 s
791 s
607 vs
0—c
618d
0568 (147)
0545 (131)
0565 vs
0503 s
0470 s
0444 s
0420 w
0387 w
0569 (178)
0491 (10)
0362 (33)
0389 (68)
0380 (67)
0410 (103)
0382 (80)
ν(AlCl), ν(СоCl)
a Given in parentheses.
b
ν are stretching, δ, γ — bending, χ and χ — outꢀofꢀplane and inꢀplane deformation, respectively, r — rocking, sc — scissoring,
i
w — wagging deformation vibrations.
c Overlap with the ν(Al—Cl) vibrations.
d For monomer AlCl3 in the region above 400 cm–1 in the Ar matrix.21
formation of the associates mentioned during coꢀcondenꢀ
sation of AlCl3, CoCl2 and 2ꢀClBu should not lead to the
spectrum shown in Fig. 2, 3. First of all, this is related to
the most intensive band at 566 cm–1 belonging to the
stretching vibrations ν(Al—Cl), whereas similar vibrations
in complexes AlCl3•CoCl2 are found at 574—594 cm–1 of
the IR spectra. Additional evidence for the formation of
a new associate in the triple system is its thermal stability.
It turned out that the elevation of temperature from 80 to
240 K produces no considerable changes in the IR specꢀ
trum of the triple coꢀcondensate in contrast to the spectra
of various binary coꢀcondensates studied earlier,11—13 in
which the complex transformations causing a sharp change
in the spectral picture occur already at 130—170 K.
To sum up, spectrum of the triple coꢀcondensate of
2ꢀClBu, aluminum chloride, and cobalt chloride on the

Triple complexes of sꢀBu, Al, and Co chlorides
T (%)
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 6, June, 2010
1123
1.512
1.518
1.529
2.268
1
2.343
2.346
Al
2.363
Co
1.907
2
3
2.262
472
596
560
444
503
417
420
3A
617
548
479
598
1.510
1.529
447
1.936
506
2.538
475
1.515
567
550
2.161
2.342
Al
Co
650
600
500
450
400
ν/cm–1
2.161
Fig. 2. Lowꢀfrequency IR spectra of binary coꢀcondensates of
aluminum chloride with 2ꢀchlorobutane (1) and cobalt chloride (2)
and triple coꢀcondensate 2ꢀClBu•CoAlCl5 (3) at 80 К.
2.154
3B
2.540
Fig. 3. Models of triple complexes of 2ꢀchlorobutane, aluminum
whole cannot be described by superposition of spectra of
the inꢀpair interactions. Therefore, a new associate is
formed, by all appearances, a triple molecular complex.
Quantum chemical calculations for the vibration spectra
of possible triple associates performed confirmed this
suggestion.
Quantum chemical calculations performed using the
PBE/3z density functional predict formation of two isoꢀ
meric 1 : 1 : 1 complexes 2ꢀClBu•AlCl3•CoCl2 (3) (Fig. 3).
In the isomeric complex 3A, coordination of the 2ꢀClBu
molecule is accomplished through the cobalt atom, in the
complex 3B, through the aluminum atom. The experiꢀ
mental and calculated vibration spectra of the triple coꢀ
condensate of aluminum and cobalt chlorides and 2ꢀClBu
are given in Table 1.
chloride, and cobalt chloride calculated by the PBE/3z method.
lowꢀfrequency region agrees better with those calculated
for complex 3A. More substantiated conclusion can be
drawn from the comparison of energies of formation of
the complexes from starting reagents.
Figure 4 shows the diagram of the energy changes durꢀ
ing complexation. It is seen that the gain in energy for the
formation of complex 3A (–46.5 kcal mol–1) from the
starting halides is much higher than in the case of complex
ΔE/kcal mol–1
2ꢀClBu, AlCl3, CoCl2
0
As it is seen from the data in Table 1, the experimental
spectrum of 2ꢀClBu observed in the threeꢀcomponent
complexes is in satisfactory accord with the calculated
vibration frequencies of complexes 3A and 3B. In this
case, the spectra of complexes 3A and 3B are close to each
other in the highꢀfrequency region and differ little from
the calculated spectrum of the binary associate 1. On the
contrary, in the lowꢀfrequency region, in which the metꢀ
al—chlorine vibrations are found, differences are obvious.
The experimental spectrum of the triple system cannot be
already described by the spectrum of complex 1, rather it
satisfactorily agrees with the spectra of complexes 3A and
3B. Close positions of the vibration frequencies does not
allow us to make an unambiguous conclusion on the site
of coordination of 2ꢀClBu in the triple system. Relative
intensities of the experimental bands of the complex in the
–10
–20
–30
–40
–12.84
–15.53
2ꢀClBu•CoCl2
2ꢀClBu•AlCl3
2ꢀClBu•AlCl3•CoCl2
(3B)
–28.4
–25.85
CoCl2•AlCl3
2ꢀClBu•CoCl2•AlCl3
(3A)
–46.5
Reaction coordinate
Fig. 4. Energy diagram for the formation of complexes of
2ꢀchlorobutane with metal halides according to the data calcuꢀ
lated by the PBE/3z method.

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Shilina et al.
3B (–25.9 kcal mol–1). In addition, the latter value is less
than the energy of formation of the double complex
CoCl2•AlCl3 by 2.5 kcal mol–1. Therefore, according to
the calculated data the formation of complex 3B, in which
coordination of 2ꢀchlorobutane occurs at the aluminum
atom, is energetically unfavorable.
Scheme 2
2ꢀClBu + AlCl3
2ꢀClBu•AlCl3
sꢀBu+•[AlnCl3n+1
]
–
To sum up, the reaction of 2ꢀClBu with aluminum and
cobalt halides leads to the triple molecular complex
2ꢀClBu•CoCl2•AlCl3 of the structure 3A, in which coorꢀ
dination of 2ꢀchlorobutane is accomplished through the
cobalt atom. Let us now consider whether ionization
of such associates with the formation of carbocations
is possible, as it happens with the binary complexes
2ꢀClBu•AlCl3.
2ꢀClBu + AlCl3 + CoCl2
2ꢀClBu•CoAlCl5
sꢀBu+
Molecular complexes (for example, BuCl•AlCl3)
formed by coꢀcondensation (80 К) of 2ꢀchlorobutane with
aluminum chloride undergo transformation at ~120 K to
the ionic structures containing secꢀbutyl cation and polyꢀ
chloroaluminate anion. The condensation of 2ꢀClBu with
aluminum and cobalt halides at 80 K leads to the triple
molecular complex 2ꢀClBu•CoAlCl5 stable in the temꢀ
perature range 80—240 K and undergoing no ionization to
form secꢀbutyl cation.
A possibility of formation of ionic complexes in the sysꢀ
tem 2ꢀClBu•CoCl2•AlCl3. Formation of secꢀbutyl cation
in the binary systems is accomplished, as we have already
mentioned, by selfꢀionization of molecular complexes 1
or 2 at temperatures above 120 K (see Scheme 1). Figure 5, 1
shows the IR spectrum of the corresponding ionic comꢀ
plex sꢀBu+[Al3Cl10]–, which has been studied by us in
details earlier.13 Very close spectrum has been obtained
for sꢀBu+ in the SbF5 matrix as well.14 The spectrum of
the triple coꢀcondensate 2ꢀClBu•CoCl2•AlCl3 (ratio of
reagents ~1 : 1 : 1) recorded after heating to 150 К is also
shown in Fig. 5. It is seen that the spectrum of 2ꢀchloroꢀ
butane in the triple coꢀcondensate contains no bands charꢀ
acteristic of sꢀBu+: there are absent, for example, bands at
1269, 1207, 1051, and 836 cm–1. No considerable changes
are observed either in the lowꢀfrequency region upon temꢀ
perature elevation from 80 to 150 K (cf. Fig. 2 and 5).
The IR spectrum of the triple complex 3 remains unꢀ
changed upon further heating up to 240 К. The indicated
facts gave an evidence that only molecular complexes staꢀ
ble in a wide range of temperatures are formed in the triple
system containing 2ꢀchlorobutane and aluminum and coꢀ
balt halides.
In conclusion, a suggestion made earlier in the literaꢀ
ture that the addition of transition metal compounds to
aluminum chloride increases electronꢀwithdrawing propꢀ
erties of the Lewis acid1,2,5 found no confirmation, at least,
in this case. Introduction of cobalt chloride to the system
2ꢀchlorobutane—aluminum chloride leads to the formaꢀ
tion of stable triple complex, capable of no selfꢀionization
with the formation of secꢀbutyl cation. Moreover, in
the triple complex the transition metal ion, rather than
aluminum ion, is the center, which is involved into
the interaction with the organic substrate. Apparently,
the main part should belong to this ion in the reactions
with alkanes as well. This conclusion is in complete
agreement with the results of quantum chemical calcuꢀ
lation by a density functional method for the complexes
of aluminum and cobalt halides (components of the caꢀ
talytic system) with propane.26 In the triple complexꢀ
es, the interaction with propane is accomplished through
the transition metal atom, rather than through the
stronger Lewis center, the aluminum atom. In this case,
a strong polarization of the CH bonds takes place, the
direction of which is opposite to that occurring upon
the action of typical Lewis acids, including aluminum haꢀ
lide. According to the calculation, the reaction of alkane
with aluminum chloride decreases negative charge on
the carbon atom. At the same time, the reaction of an
alkane with molecular or ionic bimetallic complexes leads
to the opposite effect: electron density on the carbon
atom increases, that promotes formation of metalꢀalkyl
compounds, in which organic radical has a carbanionic
character.
Comparison of two systems (Scheme 2) shows that the
processes of complexation in them considerably differ.
T (%)
1
589
770
1269
1282
1208
1227
546
502
2
781
470
ν/cm–1
Fig. 5. IR spectra of the ionic complex sꢀBu+[Al3Cl10 – (1) and
565
1400
1200
1000
800
600
]
This work was financially supported by the Rusꢀ
sian Foundation for Basic Research (Project No. 08ꢀ03ꢀ
00171).
the triple system of the form 2ꢀClBu•CoAlCl5 (2) at 150 К.
Vertical lines represent frequencies of the ionic complex absent
in the triple coꢀcondensate.

Triple complexes of sꢀBu, Al, and Co chlorides
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 6, June, 2010
1125
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Received March 27, 2009;
in revised form February 17, 2010