Journal of the Chinese Chemical Society, 2006, 53, 887-890
887
ESR Studies on a Friedel-Crafts Alkylation Reaction in [bmim]Cl-AlCl3
Ionic Liquid
Cui-Ping Zhaia,c (
Wei-Min Liua (
), Ai-Xin Songb (
) and Han-Qing Wanga* (
),
)
a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 730000, P. R. China
b
Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education,
Jinan 250100, P. R. China
c
Graduate School of the Chinese Academy of Sciences, Beijing 100039, P. R. China
The paramagnetic complexes formed in Friedel-Crafts alkylation reaction systems are invistigated by
electron spin resonance (ESR) spectroscopy, in room temperature ionic liquids system 1-butyl-3-methyllimidazolium chloride-aluminium chloride ([bmim]Cl-AlCl3). The results indicate that ESR spectra observed are due to polycyclic aromatic radical cations formed from their parent hydrocarbons. ESR spectrum of spin adduct is obtained in an ionic liquid system composed of [bmim]Cl-AlCl3. In acidic solution
the 14N hyperfine coupling constant of 4-oxo-TEMPO, 2.15 mT, is appreciably larger due to an adduct
formed with AlCl3.
Keywords: ESR; Cation radical; Ionic liquids; Friedel-Crafts alkylation.
INTRODUCTION
In recent years, Friedel-Crafts reactions in low temperature ionic liquids have prompted a significant amount
of research interest. An ionic liquids such as [bmim]ClAlCl3 and [emim]Cl-AlCl3 systems demonstrate catalytic
activity in reactions such as Friedel-Crafts acylations,1,2
alkylation reactions,3 etc. An investigation on FriedelCrafts acetylation of five simple aromatic compounds has
been performed by Adams et. al in an [emim]Cl-AlCl3 system with excellent yields and selectivities.4 They also observed that the carbocyclic aromatics, such as naphthalene,
phenanthrene and pyrene, form highly coloured compounds in acidic [bmim]Cl-AlCl3 which were probable
paramagnetic p-complexes. However, these p-complexes
were not further analysed and the type and formation mechanism of paramagnetic species were not fully understood.
For example it was not clear in these reactions whether the
p-complexes come from an intermediate or from a radical
formed by the alkylation reaction. In order to get further insight into the formation mechanism of these coloured paramagnetic complexes, it is necessary to detect and identify
the radicals produced in the ionic liquid, with the Friedel* Corresponding author. E-mail: [email protected]
Crafts alkylation method. Molten salts have been shown to
be suitable solvents for Freidel-Crafts chemistry.2 Since Clis a Lewis base and Al2Cl7- is a Lewis acid, the Lewis acidity/basicity of the ionic liquid can be manipulated by altering its composition. Mixtures of [bmim]Cl-AlCl3 are characteized as basic, acidic or neutral if the mole ratio of
AlCl3/biminCl, is less than, greater than or equal to unity.
Therefore, the main anions are AlCl4- and Cl- in basic
melts, Al2Cl7- and AlCl4- in acidic melts, and AlCl4- in neutral melts.
In the present work, we employed ESR spectroscopy
to investigate the paramagnetic complexes formed in
Friedel-Crafts reactions in a room temperature ionic liquid
system, [bmim]Cl-AlCl3, with the mole fraction of AlCl3
(XAlCl3 ) is 0.65.
RESULTS AND DISCUSSION
When the reaction mixture of C6H6 and CHCl3 (2:1,
mole ratio) was added to a [bmim]Cl-AlCl3 (XAlCl3 = 0.65)
system, the reaction took place rapidly at room temperature
and formed a deeply coloured compound. We observed the
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Zhai et al.
Table 1. The ESR spectra data of radical cations in [bmim]Cl-AlCl3 with different compositions
Mixture
Radical cations
Hyperfine coupling constants (mT)
DPA
C6H6 + CHCl3 + [bmim]Cl-AlCl3
XAlCl3 = 0.65
a(1) = 0. 263
a(2) = 0.123
a(2'3',4') = 0.044
Perylene
Perylene + [bmim]Cl-AlCl3
XAlCl3 = 0.65
ESR spectrum shown in Fig. 1.
The ESR proton hyperfine constants obtained from
this spectrum are given in Table 1. The hyperfine pattern
was proved to be virtually identical to that of 9,10-diphenylanthracene in concentrated H2SO4,6 but all lines are
broader (see Fig. 1a), and this spectrum can, therefore, be
assigned to 9,10-diphenyl-anthracene radical cation (
)
with ge = 2.0026. The formation of
may be explained
in Scheme I.7
Gases evolved from the reaction mixture were found
to be H2 and HCl. It is assumed that a certain amount of HCl
remains in ionic liquids. In view of the fact that protons are
considered to be necessary for Friedel-Crafts-type reactions,8,9 we assume that the presence of H+ in ionic liquids
is beneficial for the alkylation of aromatic compounds with
alkyl halide.3
The acidic chloroaluminate (XAlCl3 > X[bmim]Cl) ionic
liquid contains primarily the anions AlCl4- and Al2Cl7-.10 It
suggests that they may be the catalysts for Friedel-Crafts
Fig. 1. (a) ESR spectrum of 9,10-diphenyl anthracene
radical cation obtained from C6H6 and CHCl3 in
[bmim]Cl-AlCl 3 (X AlCl 3 = 0.65) at room temperature. (b) ESR spectrum of 9,10-diphenyl
anthracene in 98% H2SO4 at room temperature.
a(1) = 0.302
a(2) = 0.401
a(3) = 0.042
Scheme I The formation mechanism of 9,10-diphenylanthracene radical cation (
)
alkylation of benzene with various alkyl chlorides. These
active species can catalyze the alkylation of C6H6 to yield a
binary substituted product which proceeded via an intermolecular Scholl11 condensation reaction to yield DPA; the
latter was converted into the corresponding radical cations
, under the influence of a molten chloroaluminate system, whose structure has been confirmed by our ESR study.
As we know, ionic liquids show acidity when a Lewis
acid (e.g., AlCl3) is used in excess; the acid strength of an
ionic liquid can be controlled by changing the ratio of
AlCl 3 and [bmim]Cl. In acidic chloroaluminate (XAlCl3 >
ESR on a Alkylation Reaction in Ionic Liquid
X[bmim]Cl) the melts contain primarily the anions AlCl4- and
Al2Cl7-:
2AlCl4- ƒ Al2Cl7-+ClIn order to study the influence of acid strength on the
formation of radical cations, we estimated the behaviour of
[bmim]Cl-AlCl3 with different mole fractions. The results
show that the aromatic alkylation reaction cannot occur
when XAlCl3 £ 0.55 and no ESR spectra were observed.
However, the addition of the mixtures of C6H6 and CHCl3
to [bmim]Cl-AlCl3 results in the appearance of an ESR signal, when XAlCl3 is increasing from 0.55 to 0.65, with an increase of Lewis acid strength.
We also observed that when perylene, a polycyclic
hydrocarbon, was added to the [bmim]Cl-AlCl3 (XAlCl3 =
0.65) system, a deeply blue compound was produced.
Fig. 2 exhibits the ESR spectrum observed in this system and the hyperfine coupling constants are summarized
in Table 1. This hyperfine pattern was proved to be virtually identical to that observed for perylene in concentrated
H2SO4.12
This spectrum could, therefore, be assigned to the
perylene radical cation with ge = 2.0032. Based on this observation, we suggest that perylene radical cations are produced by one electron transfer from their parent compound
to Al2Cl7- or AlCl4-, which probably act as electron acceptors. Our initial results indicated that the ESR spectra observed are due to the polycyclic aromatic radical cations
formed from their parent hydrocarbons in room temperature an ionic liquid [bmim]Cl-AlCl3 system. The radical
cations generated are quite persistent and accumulate to
form deeply coloured mixtures.13
In this work, we have also employed a spin probe
method to detect the sorts of chloroaluminate[III] species in
Fig. 2. ESR spectrum of perylene radical cation obtained in [bmim]Cl-AlCl 3 (X AlCl 3 = 0.65) at
room temperature.
J. Chin. Chem. Soc., Vol. 53, No. 4, 2006
889
the acidic ionic liquids. 2,2,6,6-Tetramethyl-4-oxopiperidine-1-oxyl (4-oxo-TEMPO) has been widely used as a
probe molecule for determination of active species. When
a chloroform solution of 4-oxo-TEMPO is added to
[bmim]Cl-AlCl3 (XAlCl3 = 0.65), an ESR spectrum (Fig. 3)
with the magnetic parameters of ge = 2.0058, aN = 2.15 mT
and aAl = 0.98 mT have been obtained. This spectrum can
be assigned to the spin adduct 1, which is derived from the
4-oxo-TEMPO (nitroxyl radical) and AlCl4- according to
the following ligand exchange (shown in Scheme II), i.e.,
the ligand exchange reactions between 4-oxo-TEMPO and
AlCl4- led to the formation of spin adduct 1. The 14N hyperfine constants of 4-oxo-TEMPO, 2.15 mT, is appreciably larger due to the adduct formation with AlCl3.
Scheme II Formation of the spin adduct 1
Nitroxide complexes with AlCl3 in ionic liquids are
stable at room temperature over three days. The ESR spectrum of spin adduct 1 exhibits 18-line spectra and each of
the three 14N (I = 1) lines is split into six by the interaction
with a single 27Al (I = 5/2) nucleus (Fig. 3). The presence of
hyperfine splitting of 27Al is indicative of 4-oxo-TEMPO
coordinated to Chloroaluminate [III] species in [bmim]ClAlCl3.
Hoffman and Eames14 first reported the existence of
this adduct in CCl4 saturated with AlCl3. They measured
the hyperfine coupling constants due to N and Al nuclei and
found aN = 2.05 mT and aAl = 0.88 mT in the acidic melt.
4-oxo-TEMPO with the unpaired electron is a strong
electron donor and a single electron transfer process can
Fig. 3. ESR spectrum of spin adduct 1.
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J. Chin. Chem. Soc., Vol. 53, No. 4, 2006
readily occur in the presence of electron acceptors. AlCl4- is
a strong electron acceptor and serves as very effective spin
trap for 4-oxo-TEMPO. Thus the stable radical adducts
formed and a characteristic ESR spectrum was observed.
CONCLUSIONS
1. The polycyclic aromatic hydrocarbons were obtained through Friedel-Crafts alkylation reactions of monocyclic aromatics with CHCl3 undergoing intermolecular
Scholl11 condensation reaction and were converted into the
corresponding radical cations,
, in acidic chloroaluminate ionic liquids [bmim]Cl-AlCl3 (XAlCl3 = 0.65).
2. The paramagnetic compounds (radical cations) are
produced by one electron transfer from polycyclic aromatic
hydrocarbons.
3. The ESR spectrum of 4-oxo-TEMPO was obtained
in [bmim]Cl-AlCl3. The 14N hyperfine coupling constants
of 4-oxo-TEMPO and the 27 Al hyperfine coupling constants of AlCl4- are 2.15 mT and 0.98 mT, respectively.
EXPERIMENTAL
Chemicals
Perylene, 2,2,6,6-tetramethyl-4-oxopiperidine-1oxyl (4-oxo-TEMPO) and 9,10-diphenyl anthracene were
purchased from Aldrich Chemical and used without further
purification. The purity of anhydrous aluminum chloride is
99.9%.
Sample preparation
Ionic liquids used in this study were synthesized according to the previously published methods5 and all ionic
liquids were cleaned and dried under vacuum at 70 °C for
72 h to remove organic solvents and water.
Acidic ionic liquids [bmim]Cl-AlCl3 were prepared
by the slow addition of the desired amount of AlCl3 to the
imidazolium salt. The reaction was left stirring overnight at
0 °C for 24 h, in order to allow a perfect homogenisation of
the resulting [bmim]Cl-AlCl3 .3
During a Friedel-Crafts reaction the evolution of
Zhai et al.
gases removed the dissolved oxygen in the solution that allowed the resolved ESR spectra to be observed. The sample
was placed in the resonance cavity and its ESR spectrum
was recorded.
ESR spectra
The 4 mm O. D. Pyrex tubes were used in the ESR
studies and the samples were prepared directly in the tubes.
The samples were not degassed in the vacuum line. The
ESR spectra were obtained on a Varian E-115 spectrometer
operating at microwave frequency of 9.5 GHz at room temperature. The magnetic field was calibrated with a Varian
E-500 NMR Gaussmeter, and the microwave frequency
was measured with a frequency counter model H/P 5341 A.
Received September 28, 2005.
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