Aluminium chloride complexes with nitrobenzene: Low-temperature synthesis, structure, and stability

Article abstract of DOI:10.1023/A:1011381330763

Low-temperature IR spectroscopy was used to study intermolecular interactions in the aluminium chloride-nitrobenzene system. The 1 : 1, 2 : 1, and 1 : 2 molecular complexes of aluminium chloride with nitrobenzene were discovered. The spectral parameters w

Full text of DOI:10.1023/A:1011381330763

Russian Journal of Coordination Chemistry, Vol. 27, No. 7, 2001, pp. 469–475. Translated from Koordinatsionnaya Khimiya, Vol. 27, No. 7, 2001, pp. 504–510.
Original Russian Text Copyright © 2001 by Perlovskaya, Shilina, Smirnov.
Aluminium Chloride Complexes with Nitrobenzene:
Low-Temperature Synthesis, Structure, and Stability
O. R. Perlovskaya, M. I. Shilina, and V. V. Smirnov
Moscow State University, Vorob’evy gory, Moscow, 119899 Russia
Received April 26, 2000
Abstract—Low-temperature IR spectroscopy was used to study intermolecular interactions in the aluminium
chloride–nitrobenzene system. The 1 : 1, 2 : 1, and 1 : 2 molecular complexes of aluminium chloride with
nitrobenzene were discovered. The spectral parameters were found to correlate with the structure of the com-
plexes and their thermal stability. Above 170 K, the 1 : 2 complexes become unstable and self-ionize to give
particles with the general formula [AlCl2(RNO2)m]+AlCl–4 . The quantum-chemical calculations carried out at
the HF/6-31G level confirm the existence of three types of molecular complexes of 1 : 1, 2 : 1, and 1 : 2 com-
position. The structure of the title complexes was proposed on the basis of spectral parameters and calculation
results.
The aluminium halide complexes with nitro com- surface of a copper block cooled to 80 K. The alumin-
pounds are known to display catalytic properties in ium chloride condensation was performed on heating
some organic reactions, for example, in alkylation and of the vapors to 343–353 K. The other reagents were
isomerization of aromatic hydrocarbons [1, 2]. In order supplied at a rate controlled by fine-adjustment needle
to gain better insight into the mechanism of catalysis, valves. The condensation rate was 1014–1016 molecules
one needs data on the composition, structure, and prop- cm–2 s–1, and the film thickness did not exceed 2–5 µm.
erties of these complexes. It was shown in [3–5] that in The temperature was maintained with an accuracy of
the aluminium chloride–nitroalkane (nitromethane, ±1 K. The spectra were recorded on a Specord 75IR
1-nitropropane) systems, in addition to the known instrument in the range of 4000–400 cm–1.
equimolar associates [6–9], molecular complexes of 1 :
Ab initio quantum-chemical calculations were per-
2 or 2 : 1 composition are formed that have not been
formed using the GAMESS program package [10] at
studied previously. Paper [5] describes the low-temper-
the HF/6-31G level.
ature reactions involving these complexes. In this con-
nection, the molecular complexes of aluminium chlo-
ride with nitrobenzene are of a certain interest. The fact
that nitrobenzene lacks active hydrogen in the α-posi-
RESULTS AND DISCUSSION
tion to the nitro group prevents side reactions, which
allows one to perform unbiased comparison of com-
plexes with different compositions and structures.
The aim of this work was to study the intermolecu-
lar interactions between aluminium chloride and
nitrobenzene, to identify and examine the spectral
parameters and stability of aluminium chloride com-
plexes with nitrobenzene of different compositions.
Aluminium chloride and nitrobenzene cocondensa-
tion was carried out at molar ratios of AlCl3 : PhNO2
from 1 : 10 to 10 : 1. The spectra of nitrobenzene and
cocondensates are shown in Figs. 1 and 2.
As is seen from Fig. 1, interaction in the system
occurs already at 80 K, which is indicated by signifi-
cant changes arising in the spectra of the cocondensates
as compared to the spectra of the reagents. The correla-
tion between the pattern of the spectrum and the
reagent ratio (Fig. 2) is similar to those previously
observed for the aluminium chloride–nitroalkane sys-
tems [3–5]. Such systems with nitromethane or 1-nitro-
propane were found to contain molecular complexes of
different compositions, namely, 1 : 1 (I), 2 : 1 (II), and
1 : 2 (III), for which the experimental data satisfacto-
rily agree with the results of quantum-chemical calcu-
EXPERIMENTAL
Before preparing the samples, aluminium chloride
(99.99%) was distilled in vacuum. Nitrobenzene and
decane were purified by standard procedures. The
reagent purity was monitored using IR spectroscopy
and gas–liquid chromatography.
IR spectra of the solid samples were recorded in the lations of the vibrational spectra [4, 11]. Table 1 assigns
optical cryostats described earlier in [3]. The samples the main vibrational frequencies of the aluminium
were prepared by vacuum cocondensation of the chloride complexes with nitrobenzene on the basis of
reagents supplied from separate outlets onto the mirror the concentrational dependences of IR spectra and their
1070-3284/01/2707-0469$25.00 © 2001 åÄIä “Nauka /Interperiodica”

470
PERLOVSKAYA et al.
1
2
1600
1400
1200
ν, cm–1
Fig. 1. IR spectra of (1) nitrobenzene and of (2) its cocondensate with aluminium chloride at AlCl : C H NO = 0.9 and T = 80 K.
3
6
5
2
comparison with the literature data. Let us consider the a band of the complicated stretching-deformation
IR spectra of the condensates in greater detail.
vibration with participation of ν(ë–N) at 1255 cm–1;
the same band (1260 cm–1) in the nitrobenzene spec-
trum has a relatively weak intensity (Fig. 1). Coordina-
tion of nitrobenzene by aluminium chloride is accom-
panied by a significant perturbation of not only the N–
O bonds but of the C–N bond as well, which is demon-
strated by the vibrational spectra. This suggestion
agrees with the data from X-ray diffraction analysis of
AlCl3 · PhNO2 [9], which exhibits a noticeable decrease
in the length of the C–N bond in complex I (1.429 Å)
as compared to that in a free molecule (1.49 Å).
Thus, the vibrational spectrum of complex I
recorded by us for cocondensates of aluminium chlo-
ride and nitrobenzene with equimolar compositions
satisfactorily agrees with the literature data, which
allowed us to go over to studies of the vibrational spec-
tra of complexes with more involved compositions that
have not been previously known.
Under the experimental conditions, aluminium
chloride condenses from the vapor phase mainly as the
Al2Cl6 dimer. In this case, the IR spectra contain bands
of stretching vibrations ν(Al–Cl) at 603, 467, and
413 cm–1. In the spectra of cocondensates with compo-
sition close to the equimolar composition (n ~ 1), in the
range of the stretching vibrations ν(Al–Cl), a new
broad band appears instead, which has a maximum at
536, a shoulder at 546 cm–1, and a band at 580 cm–1
(Fig. 2). Similar positions of the aluminium chloride
bands were observed in the spectrum of the AlCl3 ·
PhNO2 (I) complex in solution [7]. Complexation also
results in a shift of the nitrobenzene bands, with pertur-
bations spreading throughout the entire molecule
(Figs. 1, 2, Table 1). However, in this case, the shifts of
the bands are smaller (5–15 cm–1) than in the spectrum
of the complex with nitromethane (50–80 cm–1) [3]. As
compared to the frequencies of a free PhNO2, in the
The IR spectra of the samples with n > 1 differ from
spectrum of I, the νs(NO2) frequency decreases and the the spectrum considered above. Thus, in the spectrum
νas(NO2) frequency increases. Such a change in the fre- of the cocondensate with n = 3.3 (Fig. 2), in the region
quencies indicates that aluminium chloride in complex of ν(Al–Cl), in addition to the intense bands of the
I reacts with only one oxygen atom of the nitro group Al2Cl6 absorption (603, 467, and 413 cm–1), two new
[3]. When going to the AlCl3 · PhNO2 spectrum, the bands are observed, i.e., a doublet at 555, 542 cm–1 and
relative intensities of several nitrobenzene bands also a band at 588 cm–1. As for the nitrobenzene absorption
change: the intensity of the νas(NO2) band (1524 cm–1) bands, their shifts relative to the bands of a free PhNO2,
increases as compared to the intensity of the νs(NO2) in this case (n > 1), coincide in sign with those observed
band (1328 cm–1). Moreover, in the spectrum of the in the spectrum of AlCl3 · PhNO2, but exceed them in
complex, a sharp increase is observed in the intensity of absolute value (see Table 1). An increase in the alumin-
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 27 No. 7 2001

ALUMINIUM CHLORIDE COMPLEXES WITH NITROBENZENE
471
1
1
2
2
3
3
1600
1200
800
400
ν, cm–1
Fig. 2. IR spectra of the aluminium chloride and nitroben-
zene cocondensates at AlCl : C H NO equal to (1) 0.4,
3
6
5
2
(2) 1.2, and (3) 3.3 (T = 80 K).
700
500
ν, cm–1
ium chloride content in the system (from n = 3 to n =
10) does not produce any further changes in the IR
spectra. In general, the frequency changes in the spectra
of cocondensates observed with an increase in the alu-
minium halide content in the sample are similar to the
respective changes in the spectra of the systems with
nitroalkanes. The position of bands ν(Al–Cl) in the
case of cocondensate with PhNO2 at n > 1 is close to that
of ν(Al–Cl) in the spectrum of the complex with
nitromethane (600, 562, and 542 cm–1) and 1-nitropro-
pane (598, 558, and 543 cm–1) of a 2 : 1 composition [3, 5].
Fig. 3. IR spectra of the aluminium chloride and nitroben-
zene cocondensates at T = (1) 80, (2) 180, and (3) 250 K,
AlCl : PhNO = 0.4.
3
2
confirmed by the analogous pattern of the shift of the
nitrobenzene absorption bands in the spectra of com-
plexes I and II. The quantum-chemical calculations of
the vibrational spectra of the nitromethane complexes
with aluminium chloride showed [11] that when inter-
action in the 2 : 1 complex occurs with participation of
two oxygen atoms of the nitro groups, both bands of the
One can conclude that in the AlCl3–PhNO2 system
with n > 1, complex II with a 2 : 1 composition is
formed where the aluminium chloride dimer interacts
with only one oxygen atom of the nitro group. This is stretching vibrations νas(NO2) and νs(NO2) shift toward
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 27 No. 7 2001

472
PERLOVSKAYA et al.
Table 1. Selected vibrational frequencies (cm–1) in IR spectra of nitrobenzene, Al2Cl6, and complexes I–III (solid phase,
T = 80 K)
Complexes
PhNO2, Al2Cl6
Assignment*
AlCl3 · 2PhNO2
AlCl3 · PhNO2
1524 s
Al2Cl6 · PhNO2
1526 s
1519 vs
1510 vs
1475 m
1343 s
1524 s
1519 s
1467 m
1338 s
1334 s
1256 m
νas(NO2)
1458 m
1328 m
1455 m
1326 m
ν(Ph)
νs(NO2)
1260 w
603 s
1255 vs
1243 vs
Component of (δ(CH) + ν(CN) + …)
ν8[Al2Cl6]
534 s
580 m
546 sh
536 s
588 vs
555 s
500 w. sh
ν(Al–Cl)
542 m. sh
440 w
467 s
ν16[Al2Cl6]
ν13[Al2Cl6]
413 m
* Assignment made according to [7, 12].
low frequencies. A satisfactory agreement with the
experimental data was obtained in the calculations of
the spectra of complexes which exhibit interaction of
aluminium halide with one oxygen atom and which
retain one of the bridging bonds of the Al2Cl6 dimer. In
this case, the calculation suggests similar changes in
the spectra of complexes I and II.
Upon temperature increase in the range of 80–
290 K, the IR spectrum of complex I does not change
and this complex remains stable up to room tempera-
ture. In the systems containing excess aluminium chlo-
ride, temperature growth is accompanied by transfor-
mation of the molecular Al2Cl6 to the crystal form [16]
and its further evaporation. As a result, at room temper-
ature, the spectra of the systems with n > 1 contain only
bands of complex II.
The IR spectra of samples with n < 1 do not coincide
with the spectra described above (Fig. 2 and Table 1).
The region of the stretching vibrations ν(Al–Cl) con-
tains bands with maxima at 534 and 500 cm–1. The
position and the relative intensity of the bands from
nitrobenzene also change, the values of shifts νas(NO2)
and νs(NO2) being minimum in this case (see Table 1).
Under similar conditions (at n < 1), aluminium chloride
forms the 1 : 2 complex with nitroalkanes via interac-
As the temperature is increased, the spectra of the
systems containing complex III with n < 1 undergo
substantial changes. At a temperature of ~150 K, the
band at 534 cm–1 disappears and a new band appears
instead with the maximum at 495 cm–1 (Fig. 3), which
is assigned to a degenerate vibration ν(Al–Cl) in the
AlCl–4 anion [12]. The band at 495 cm–1 was observed
tion of two nitrobenzene molecules with the aluminium in the spectrum of a solution of the AlCl3 · CH3NO2
chloride monomer [3, 4]. In this case, vibrations ν(Al–
Cl) appear at 523, 505 and 528, 506 cm–1 in the spectra
of complexes with nitromethane and 1-nitropropane,
respectively. Comparison of the data in Table 1 with the
spectra of complexes with nitroalkanes shows that the
AlCl3 · 2PhNO2 complex (III) is formed in the AlCl3–
PhNO2 system at n < 1. Complex III is likely to have a
trigonal-bipyramidal structure similar to that of the
AlCl3 complexes with ethers [13, 14] and nitroalkanes
[3–5]. The formation of the 1 : 2 aluminium chloride
complex with nitrobenzene was previously reported in
[15]; however, spectral data on such complexes are not
available, which is, probably, due to their insufficient
stability. The thermal stability studies confirmed the
suggestion on the lower stabilities of these complexes
as compared to those of I and II.
complex in nitromethane [8] during the formation of
the ionic particles [AlCl2(CH3NO2)m]+AlCl4– , (m = 2–
4). Simultaneously, the nitrobenzene band positions
also change. It is most probable that in cocondensates
of aluminium chloride with nitrobenzene at n <1, the
temperature growth is followed by the self-ionization
of complex III to give ionic structures
[AlCl2(C6H5NO2)m]+AlCl4– (IV).
Quantum-Chemical Studies
In order to confirm the conclusions made on the
basis of the spectral data, ab initio quantum-chemical
calculations were performed with complete geometry
optimization at the level HF/6-31G. A number of local
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ALUMINIUM CHLORIDE COMPLEXES WITH NITROBENZENE
473
Table 2. Main geometric parameters of MeNO2, PhNO2, AlCl3, and of their complexes with different composition according
to the data of quantum-chemical calculation at the HF/6-31G level
The Al–O
r(Al–O), r(N–OAl),
NOAlangle,
deg
Compound
PhNO2
r(N–O), Å r(C–N), Å r(Al–Cl), Å q(N), e q(AlCl3), e bond
Å
Å
order
1.23
1.22
1.45
1.48
+0.22
+0.33
MeNO2
AlCl3
2.16
AlCl3 · MeNO2
AlCl3 · PhNO2
Al2Cl6 · MeNO2
1.91
1.89
1.85
1.27
1.27
1.27
1.18
1.19
1.18
1.49
1.43
1.49
2.20
+0.38
+0.27
+0.4
–0.14
–0.14
–0.14
0.197
0.201
0.194
138.7
138.3
148.7
2.20
2.17
2.32(bridge)
2.44(bridge)
2.19
Al2Cl6 · PhNO2
AlCl3 · 2MeNO2
AlCl3 · 2PhNO2
1.86
1.27
1.19
1.42
1.49
+0.27
–0.16
–0.20
–0.25
0.219
141.7
2.29(bridge)
2.51(bridge)
2.24
2.01
1.25
1.25
1.19
+0.4
0.163
0.163
145.2
2.01*
1.19*
1.49* 2.24
2.28
+0.4*
171.3*
2.01
1.26
1.20
1.43
2.23
+0.26
0.190
131.7
2.00*
1.27*
1.20*
1.43* 2.26
2.27
+0.27*
0.175*
178.1*
* Parameters for the second ligand molecule in the 1 : 2 complexes.
minima were discovered on the potential energy sur-
Complex AlCl3 · 2PhNO2. The coordination poly-
face of the system; such minima correspond to several hedron of Al in the AlCl3 · 2PhNO2 complex is a
slightly distorted trigonal bipyramid; the coordination
number of aluminium is equal to 5 (Table 2, Fig. 4c).
The complex has a loose structure which allows hin-
dered rotation of nitrobenzene about the O–Al–O axis.
This is indicated by the slight (within the limits of
1.5 kcal/mol) changes in the energy observed upon
changes in the angle of rotation of the PhN–O fragment
about this axis. The order of two Al–O bonds in this
complex is somewhat different (0.175 and 0.19). The
charges on the nitrogen atoms are +0.26 and +0.27e.
The charge transfer to the aluminium-containing frag-
ment is −0.25e; such a value is typical of sufficiently
stable complexes. However, the order of the Al–O
bonds is, nevertheless, lower, while its length (~2.0 Å)
is higher, than in the case of complexes I and II.
complexes.
Complex AlCl3 · PhNO2. According to calcula-
tions, PhNO2 is coordinated in the complex via the oxy-
gen atom (Fig. 4a). The main geometric parameters are
given in Table 2. The relatively short Al···O distance
(1.89 Å) obtained by calculation corresponds to a
strong intermolecular interaction. The charge transfer
(CT) from nitrobenzene to AlCl3 is 0.14e, while the
bond order for Al···O is 0.201 and the charge on the
nitrogen is +0.27e.
Complex Al2Cl6 · PhNO2. The main parameters are
listed in Table 2. One of the two bridging bonds of the
Al2Cl6 dimer in complex II remains unchanged
(Fig. 4b); the structure of the –O–AlCl2–Cl–AlCl3 frag-
The same basis was used to perform quantum-chemical
calculation of analogous complexes with nitromethane.
The results obtained are presented in Table 2.
Calculations of the energy of formation of the
nitromethane and nitrobenzene complexes with different
ment is similar to that of the Al2Cl–7 anion. The charge
transfer from nitrobenzene to the Al-containing frag-
ment is 0.16e.The Al–O bond is relatively short (1.86
Å), and its order is relatively high for the molecular
complexes (0.219) which correspond to a stable com- compositions (with account for the energy of the alumin-
plex.
ium chloride dimerization) gave the following results.
Complex
∆E, kcal/mol
AlCl3 · PhNO2 Al2Cl6 · PhNO2 AlCl3 · 2PhNO2 AlCl3 · MeNO2 Al2Cl6 · MeNO2 AlCl3 · 2MeNO2
–29.7
–35.4
–39.1
–26.8
–38.3
–40.6
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 27 No. 7 2001

474
PERLOVSKAYA et al.
(a)
(b)
Cl
H
H
C
H
C
2.20
1.07
H
C
1.39
Al
1.89
C
C
O
2.20
1.39
1.19
O
1.27
1.07
H
Cl
1.43
C
1.42
Cl
H
C
N
C
N
Cl
Cl
1.19
C
2.17
1.27
O
1.86
C
C
C
O
Al
H
2.29
H
H
Cl
H
Cl
2.19
2.51
2.19
Cl Cl
H
(c)
C
H
C
H
C
1.39
1.07
C
H
H
C
H
C
1.43
H
1.39
C
O
C
1.20
1.27
Cl
1.43
Cl
N
1.07
1.20
2.23
N
1.26
2.26
C
O
C
H
2.00
2.01
O
Al
O
C
2.27
C
Cl
H
H
Fig. 4. Geometry of (a) AlCl · C H NO , (b) Al Cl · C H NO , and (c) AlCl · 2C H NO calculated at the HF/6-31G level (with
3
6
5
2
2
6
6
5
2
3
6
5
2
bond lengths in Å).
The 1 : 1 nitromethane complex is slightly less sta- the other hand, in the AlCl3 · PhNO2 complex, other
ble, which agrees with the calculated lengths and bond fragments of the ligand molecule experience greater
orders for Al–O: 1.91, 1.89 Å and 0.197, 0.201 for perturbation, for instance, the C–N bond is much
AlCl3 · CH3NO2 and AlCl3 · PhNO2, respectively. The shorter.
results of the quantum-chemical calculations are con-
Similar regularities were also revealed by calcula-
firmed by X-ray diffraction data. For instance, experi-
tions for two other types of complexes (see Table 2),
mental Al–O distances for complexes with MeNO2 and
although the complexes with nitrobenzene (2 : 1 and 1 :
PhNO2 are 1.867 and 1.856 Å, respectively [9]; i.e., the
2) turned out to be less stable than the respective com-
discrepancy between these and our data is only ~2%.
plexes with nitromethane.
Coordination leads to elongation of the N–O bond,
It should be noted that in all complexes with
this elongation for the complex with MeNO2 (1 : 1)
nitrobenzene, the positive charge is delocalized on the
being greater (0.04 Å) than for the complex with
aromatic ring, for which reason it is substantially lower
PhNO2 (0.03 Å). Hence, the N–O(Al) bond in the com-
on the nitrogen atom (0.27Â) than on the corresponding
plex with MeNO2 is disturbed to a greater extent than
complexes with nitromethane (0.4Â).
in the more stable complex with PhNO2. This conclu-
sion complies with the experimental data: shifts of the
Thus, aluminium chloride reacts with nitrobenzene
bands due to the stretching and deformation vibrations to give, in addition to the known equimolar complexes,
of the nitro group in the IR spectrum of AlCl3 · PhNO2 the 1 : 2 and 2 : 1 adducts that have not been known pre-
are not essential, whereas for the complex with viously. The intermolecular interactions in all these
nitromethane, they amount to 50 to 80 cm–1 [3–5]. On complexes is significantly affected by the aromatic sub-
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ALUMINIUM CHLORIDE COMPLEXES WITH NITROBENZENE
475
8. Dalibart, M., Derouault, J., and Granger, P., Inorg.
Chem., 1982, vol. 21, no. 6, p. 2241.
9. Lanfranchi, M., Pellinghelli, M.A., Predieri, G., et al.,
J. Chem. Soc., Dalton Trans., 1993, no. 9, p. 1463.
10. Schmidt, M.W., Baldridge, K.K., Boatz, J.A., et al.,
QCPE Bull., 1990, p. 52.
stituent. As a result, the stability of the aluminium chlo-
ride complexes with nitrobenzene, according to calcu-
lations, is comparable and, in some cases, is even
higher than the stability of complexes with
nitromethane.
11. Shilina, M.I., Kuramshina, G.M., Bondarenko, G.N.,
and Leonova, I.I., Tezisy dokladov XVIII Vserossiiskogo
Simpoziuma molodykh uchenykh po khimicheskoi kine-
tike (Abstracts of the Papers, XVIII All-Russia Sympo-
sium of Young Scientists on Chemical Kinetics),
Klyaz’ma, 2000, p. 43.
ACKNOWLEDGMENTS
This work was supported by the Russian Foundation
for Basic Research, project no. 98-03-33145.
12. Nakomoto, K., Infrared and Raman Spectra of Inorganic
and Coordination Compounds, Chichester: Wiley, 1986.
Translated under the title IK spektry i spektry KR neor-
ganicheskikh i koordinatsionnykh soedinenii, Moscow:
Mir, 1991, p. 146.
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