Fe-containing ionic liquids as catalysts for the dimerization of bicyclo[2.2.1]hepta-2,5-diene

Article abstract of DOI:10.1016/j.jcat.2008.05.008

Imidazolium-based Fe-containing ionic liquids (ILs), obtained from the reaction of 1-butyl-3-methylimidazolium chloride ([BMIm]Cl) with FeCl₃ or FeCl₂, were highly effective for the dimerization of bicyclo[2.2.1]hepta-2,5-diene (2,5-

Full text of DOI:10.1016/j.jcat.2008.05.008

Journal of Catalysis 258 (2008) 5–13
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Journal of Catalysis
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Fe-containing ionic liquids as catalysts for the dimerization
of bicyclo[2.2.1]hepta-2,5-diene
∗
∗
Mai Dao Nguyen, Ly Vinh Nguyen, Eun Hee Jeon, Jin Hyung Kim, Minserk Cheong, Hoon Sik Kim , Je Seung Lee
Department of Chemistry and Research Institute of Basic Sciences, Kyung Hee University, 1 Hoegi-dong, Dongdaemoon-gu, Seoul 130-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Imidazolium-based Fe-containing ionic liquids (ILs), obtained from the reaction of 1-butyl-3-methylimida-
zolium chloride ([BMIm]Cl) with FeCl3 or FeCl2, were highly effective for the dimerization of bicyclo-
Received 21 January 2008
Revised 27 March 2008
Accepted 11 May 2008
Available online 11 July 2008
[2.2.1]hepta-2,5-diene (2,5-norbornadiene, NBD) in the presence of diethylaluminum chloride (DEAC). Fe-
containing ILs produced hexacyclic endo–endo NBD dimer (Hnn) in high yield and selectivity, whereas
FeCl3 yielded large amounts of side products. The yield and selectivity of Hnn were affected by the
degree of reduction of Fe(III) compounds by DEAC to Fe(II) species as determined by X-ray photoelectron
spectroscopy. Higher yield of Hnn was obtained with the catalytic system producing larger amounts of
Fe(II) species. Theoretical calculation and experimental results support that Fe(II) is the active species for
the dimerization.
Keywords:
Ionic liquids
FeCl3
FeCl2
Diethylaluminum chloride
© 2008 Published by Elsevier Inc.
2
,5-Norbornadiene
Dimerization
Hexacyclic endo–endo isomer
1
. Introduction
active species and PR3 are believed to function as stabilizers for
the resulting active species [16]. Therefore, to obtain Hnn in high
yields and selectivities, a suitable combination of metal complex
and additives is of pivotal importance. One major problem associ-
ated with the current catalytic systems for the synthesis of Hnn is
the use of expensive Fe(acac)3 and PR3 as the key components.
Recently, there has been a considerable interest in the applica-
tion of ionic liquids (ILs) to the immobilization of volatile, precious,
and/or toxic homogeneous catalysts to improve the stability and
recovery of the catalysts [17].
In the continuous line of our work on the development of ac-
tive, selective, and cost effective catalytic systems for the dimeriza-
tion of NBD, we have found that the Fe-based ILs are highly active
and selective for the production of Hnn without use of expensive
Fe(acac)3 and PR3.
Herein, we report an active catalytic system consisting of a Fe-
containing IL and DEAC for the selective production of Hnn from
the dimerization of NBD.
Bicyclo[2.2.1]hepta-2,5-diene (2,5-norbornadiene, NBD) dimers,
ingredients of high density liquid fuels (HDLF) and lubricants, have
been prepared from the cyclodimerization of NBD in the pres-
ence of a catalyst or a catalytic system mostly based on transition
metal, such as cobalt [1–3], iron [4–7], nickel [8–11], and rhodium
[12–15]. In many cases, however, the dimerization produces a wide
range of isomeric mixtures as shown in Scheme 1, depending on
the type of catalyst employed. For instance, Rh/C catalyst favors
the formation of hexacyclic exo-endo (Hxn) as the major prod-
uct [12]. The use of three-component catalytic system consist-
ing of Co(acetylacetonate)3 (Co(acac)3), triphenylphosphine (TPP),
and diethylaluminum chloride (DEAC) or ethylaluminum dichlo-
ride (EADC) produces heptacyclic Binor-S (dodecahydro-1,2,4:5,6,8-
dimetheno-s-indecene) as the major product [1]. In contrast, the
dimerization in the presence of an iron-based three-component
catalytic system (Fe(acac)3, TPP, and DEAC or EADC) yields hexa-
2
,8 3,7 5,13 4,6
cyclic endo–endo dimer (hexacyclo[7.2.1.0 .1 .1 .0 ]tetradec-
0-ene, Hnn) as the major product [5].
2. Experimental
1
In general, the dimerization of NBD to obtain Hnn as the ma-
jor product requires the use of additives such as alkylaluminum
chloride and trialkyl or triarylphosphanes (PR3) [1,5–7,12]. Alkyl-
aluminum chlorides are known to reduce metal complexes to form
2
.1. Chemicals
NBD, DEAC, FeCl3, and FeCl2 were purchased from Aldrich
Chemical Co. and used without further purification. All of the
solvents were obtained from J.T. Baker Chemical Co. and were dis-
tilled over appropriate drying agents prior to use [18]. 1-Butyl-
3-methylimidazolium chloride ([BMIm]Cl), [BMIm]2FeCl4, and
Corresponding authors. Fax: +82 2 965 4408.
*
E-mail addresses: khs2004@khu.ac.kr (H.S. Kim), leejs70@khu.ac.kr (J.S. Lee).
0
021-9517/$ – see front matter © 2008 Published by Elsevier Inc.
doi:10.1016/j.jcat.2008.05.008

6
M.D. Nguyen et al. / Journal of Catalysis 258 (2008) 5–13
Scheme 1. Theoretically possible dimers of NBD.
of about 1.0 × 10− Torr to minimize the surface contamination.
Each spectrum obtained was curve-deconvoluted using the XPS-
PEAK Version 4.1 software [22]. The C 1s peak at 284.6 eV was
used to correct the surface charging effect before attempting the
chemical state identification [23,24].
9
[
BMIm]FexCl3x+1 (x = 1 or 2) were prepared using the methods
described in the literature [19–21].
2
.2. Synthesis of Fe-containing ILs
The reaction of [BMIm]Cl (3.49 g, 20 mmol) with FeCl3 (3.24 g,
0 mmol) at a room temperature for 3 h produced [BMIm]FeCl4
2
2.4. Dimerization reaction
as a pale yellow liquid. Negative-ions ESI-MS spectra: m/z 197.63
−
−
[
FeCl4] , 100%; m/z 534.61 ([BMIm][FeCl4]2) , 15%.
BMIm]Fe2Cl7 was prepared in a manner analogous to that em-
Dimerization reactions of NBD were conducted in a 100 mL
3-neck round bottomed flask fitted with a reflux condenser, a ther-
mocouple well, and a dropping funnel under a nitrogen atmo-
sphere. The flask was charged with a catalyst, DEAC, toluene as a
solvent, and m-xylene as an internal standard. The flask was then
immersed in an oil bath that was maintained at a specified tem-
perature. NBD was added slowly through the dropping funnel at
the reaction temperature. After neutralization of the product mix-
ture with an aqueous solution of NaHCO3, the organic layer was
recovered and analyzed by GC, GC-Mass, and 1H NMR [25].
[
ployed for the synthesis of [BMIm]FeCl4 by reacting [BMIm]Cl with
two equivalents of FeCl3. Negative-ions ESI-MS spectra: m/z 197.63
− −
FeCl4] , 100%; m/z 358.46 [Fe2Cl7] , 39%.
[
2
.3. Instruments
The products were analyzed by gas chromatography with an
Agilent 6890 GC equipped with a flame ionized detector and a
DB wax column (30 m × 0.32 mm × 0.25 μm) and by mass
spectrometry with an Agilent 6890-5975 GC-MSD equipped with
a HP-5 capillary column (50 m × 0.2 mm × 0.5 μm). The one-
electron reduction potentials of iron-containing ILs were obtained
by means of Tafel measurement (PARSTAT 2263, Princeton Applied
Research) at an ambient temperature. A platinum working elec-
trode of 1.2 mm diameter was used with a platinum wire as the
counter electrode and a silver wire as a quasi reference electrode.
The experimental solution (0.1 M tetrabutylammonium perchlorate
in dichloromethane) was deaerated prior to measurement. The po-
larization scan was from −250 mV vs EOC (open circuit potential)
3. Results and discussion
3.1. Effects of DEAC and [BMIm]Cl
The effects of DEAC and [BMIm]Cl on the FeCl3-catalyzed
◦
dimerization of NBD were investigated at 85 C and at the mo-
lar ratio of NBD/catalyst of 50. Table 1 shows that either FeCl3 or
DEAC was inactive for the dimerization when used alone. However,
the combined use of FeCl3 and DEAC resulted in almost quantita-
tive conversion of NBD along with the formation of large quantities
of high boiling side products (entry 3). Even though the selectivity
of Hnn was as low as 13.6%, this result clearly demonstrated the
important role of DEAC in the activation of FeCl3. Interestingly, the
selectivity and yield of Hnn increased significantly when [BMIm]Cl
was added to the catalytic system consisting of FeCl3 and DEAC
(entry 6). Since the role of alkylaluminum chloride is to reduce
and/or to activate metal complexes, the activity of Fe(III)-based
catalytic system and the product composition will obviously be af-
fected by the degree of reduction of Fe(III) compound by DEAC and
−1
to +250 mV vs EOC at a rate of 10 mV s
.
X-ray photoelectron spectroscopy (XPS) measurements were
conducted with a PHI 5800 ESCA System of Physical Electronics
equipped with a hemispherical energy analyzer and the monochro-
matized AlKα X-ray source of 250 W. The pass energy was 93.9 eV
(
0.8 eV steps) and 58.7 eV (0.13 eV steps) for the survey and
high-resolution spectra, respectively. The collection angle for all
the measurements was 45 . Before data acquisition, the catalyst
sample was degassed for 3 h at 298 K under a reduced pressure
◦

M.D. Nguyen et al. / Journal of Catalysis 258 (2008) 5–13
7
Table 1
Activities of various catalytic systems for the dimerization of NBD
Table 2
a,b
a
Effect of molar ratio of DEAC/catalyst
Entry
Catalyst
Cb
%)
Sc
(%)
Yd
(%)
Entry
Catalyst
Molar ratio
(DEAC/catalyst)
Cb
(%)
Sc
(%)
Yd
(%)
(
1
2
3
4
5
6
DEAC
FeCl3
0.0
0.0
98.8
0.0
78.1
73.8
0.0
0.0
1
2
3
4
5
6
7
8
9
[BMIm]FeCl4
[BMIm]FeCl4
[BMIm]FeCl4
[BMIm]FeCl4
[BMIm]FeCl4
[BMIm]Fe2Cl7
[BMIm]Fe2Cl7
[BMIm]Fe2Cl7
[BMIm]Fe2Cl7
[BMIm]Fe2Cl7
[BMIm]Fe2Cl7
–
0.0
0.0
0.0
9.7
40.8
57.9
76.3
0.0
25.4
79.4
74.0
74.7
75.5
0.0
13.6
0.0
74.1
73.9
0.0
13.4
0.0
57.9
54.5
1
2
5
12.2
50.9
78.1
97.6
0.0
30.0
90.7
96.9
96.0
97.6
79.7
80.2
74.1
78.2
0.0
84.5
87.5
76.4
77.8
77.4
FeCl3–DEAC (1:5)
FeCl3–[BMIm]Cl (1:1)
[BMIm]FeCl4–DEAC (1:5)
FeCl3–[BMIm]Cl–DEAC (1:1:5)
10
–
e
1
2
5
5
a
Molar ratio of NBD/Fe(III) = 50, temperature = 85 ◦C, reaction time = 1 h.
Numbers in parentheses refer to molar ratios.
b
Conversion of NBD.
Selectivity of Hnn.
Yield of Hnn.
FeCl3 and [BMIm]Cl were added separately.
0e
1
11
c
10
d
e
a
Molar ratio of NBD/Fe(III) = 50, temperature = 85 ◦C, reaction time = 1 h.
b
c
Conversion of NBD.
Selectivity of Hnn.
Yield of Hnn.
Eight equivalents of TPP to Fe(III) was added.
d
e
−
−
.
Fig. 2. Optimized structures of (a) FeCl3, (b) [FeCl4] , and (c) [Fe2Cl7]
Fig. 1. Negative ions ESI-MS spectra of (a) [BMIm]FeCl4 and (b) [BMIm]Fe2Cl7.
pronounced. It is reported that the reaction of AlCl3 with [BMIm]Cl
produces [BMIm]AlCl4 and [BMIm]Al2Cl7 at the AlCl3/[BMIm]Cl
molar ratios of 1 and 2, respectively [26–28]. Likewise, the for-
mation of [BMIm]Fe2Cl7 from the reaction of [BMIm]Cl with two
equivalents of FeCl3 can be easily conceivable [20].
also by the nature of the reduced Fe species. FeCl3 is well known
to react with an equimolar amount of [BMIm]Cl to form an IL,
BMIm]FeCl4 [20,21]. Therefore, the higher Hnn selectivity with the
catalytic system consisting of FeCl3, [BMIm]Cl, and DEAC can be
largely attributed to the in situ formation of [BMIm]FeCl4. Follow-
ing reduction by DEAC is likely to generate an active Fe species for
the dimerization of NBD. In fact, the catalytic system composed of
[
Table 2 shows the dimerization activities of the Fe-containing
ILs, [BMIm]FeCl4 and [BMIm]Fe2Cl7. When the dimerization was
◦
carried out using [BMIm]FeCl4 at 85 C and at the molar ratio of
NBD/Fe(III) = 50 for 1 h, the conversion of NBD and the yield of
Hnn increased with increasing molar ratio of DEAC/[BMIm]FeCl4.
Interestingly, under the same experimental condition, [BMIm]-
Fe2Cl7 produced much higher yield of Hnn in comparison with
[
BMIm]FeCl4 and DEAC gave similar Hnn yield to that consisting of
FeCl3, [BMIm]Cl, and DEAC, suggesting that [BMIm]FeCl4 is formed
in situ from FeCl3 and [BMIm]Cl (entry 5).
[
BMIm]FeCl4 up to the molar ratio of DEAC/Fe(III) = 5. These
3
.2. Activities of Fe-containing ILs
results could be attributed to the structural difference between
[
BMIm]FeCl4 and [BMIm]Fe2Cl7 toward the reduction by DEAC.
To have a better insight on the activity difference between
Fe-containing imidazolium-based ILs were prepared from the
reaction of FeCl3 with [BMIm]Cl at different molar ratios of
FeCl3/[BMIm]Cl and used for the dimerization of NBD instead of
using catalytic systems composed of FeCl3 and [BMIm]Cl. As seen
in Fig. 1, ESI-MS spectra of the Fe-containing ILs show that the
[BMIm]FeCl4 and [BMIm]Fe2Cl7, the optimized structures of FeCl3,
−
−
[FeCl4] , and [Fe2Cl7] were calculated at the B3LYP (6-31+G*
for C and H, and LanL2DZ with ECP for Cl and Fe) level of the
theory using Gaussian 03 program, were compared [20]. A com-
parison of the Fe–Cl bond lengths in Fig. 2 shows that [FeCl4] has
longer Fe–Cl bond distance than FeCl3 by 0.08 Å. [Fe2Cl7] has a
−
mononuclear [FeCl4] was the major anionic Fe(III) species at the
−
molar ratio of FeCl3/[BMIm]Cl of 1. However, at the molar ratio
−
−
of 2, the formation of high nuclear Fe(III) species, [Fe2Cl7] , was

8
M.D. Nguyen et al. / Journal of Catalysis 258 (2008) 5–13
Fig. 3. Tafel plots of [BMIm]Fe2Cl7 (!) and [BMIm]FeCl4 (2) at room temperature
under N2(g). A platinum working electrode of 1.2 mm diameter was used with
a platinum wire as the counter electrode and a silver wire as a quasi reference
electrode. The experimental solution (0.1 M tetrabutylammonium perchlorate in
dichloromethane) was deaerated prior to measurement. The polarization scan was
from −250 mV vs EOC (open circuit potential) to +250 mV vs EOC at a rate of
Fig. 4. Curve-fitted XPS high-resolution Fe 2p3/2 spectra of (a) [BMIm]Fe2Cl7,
b) [BMIm]FeCl4 with DEAC, and (c) [BMIm]Fe2Cl7 with DEAC.
(
−1
10 mV s
.
bridging and six terminal Cl atoms. The bond distance between Fe
and the bridging Cl is 2.44 Å, which is longer by 0.15 Å than that in
−
[
FeCl4] . From the structural analysis of Fe(III) compounds and the
experimental results on the NBD dimerization, it is likely that the
role of [BMIm]Cl is to facilitate the reduction of FeCl3 by length-
−
−
ening Fe–Cl bond through the formation of [FeCl4] and [Fe2Cl7] ,
and to stabilize the resulting active species.
The reduction of Fe-containing ILs by DEAC was supported by
the Tafel measurement (see Fig. 3). The one-electron reduction po-
−
−
tentials of [FeCl4] and [Fe2Cl7] were observed at −0.192 and
0
.001 V, respectively. This is a strong indication that Fe(III) is
reduced to Fe(II) and the reduction of [Fe2Cl7] is easier than
FeCl4] . In this context, the higher catalytic performance of
BMIm]Fe2Cl7 can be ascribed to the easier reduction by DEAC aris-
−
−
[
[
ing from the presence of a loosely bound bridging chloride ligand
−
in [Fe2Cl7] , thereby resulting in the generation of larger amounts
of active Fe species.
The investigation of the effect of DEAC/Fe(III) molar ratio
on the NBD dimerization strongly implies that the reduction of
[
BMIm]Fe2Cl7 by DEAC is easier than that of [BMIm]FeCl4. As listed
2 7
Fig. 5. Curve-fitted XPS high-resolution Cl 2p spectrum of (a) [BMIm]Fe Cl ,
(b) [BMIm]FeCl4 with DEAC, and (c) [BMIm]Fe2Cl7 with DEAC.
in Table 2, the NBD conversions and Hnn yields increased with
increasing molar ratio of DEAC/Fe(III) for both [BMIm]FeCl4 and
[BMIm]Fe2Cl7, but the amount of DEAC required to achieve sim-
at 711.2, 709.8, and 706.7 eV, respectively [22,23,29]. As shown
ilar yield of Hnn was much less for [BMIm]Fe2Cl7 than that for
in Fig. 4, [BMIm]Fe Cl7 exhibited two distinct peaks of Fe 2p3/2
2
[
BMIm]FeCl4.
at 713.6 and 711.8 eV corresponding to Fe(III) with the area ratio
The easier reduction of [BMIm]Fe2Cl7 by DEAC was further con-
of 1:1.7, implying that two different coordination environments ex-
◦
firmed from the analysis of gas evolution during the reduction
process. The amount of n-butane and ethyl chloride measured
by GC and GC-Mass was much larger for [BMIm]Fe2Cl7 than for
ist for Fe(III). When [BMIm]Fe Cl7 was treated with DEAC at 85 C,
2
the high-resolution spectrum of Fe 2p3/2 showed three component
peaks at 710.5, 709.1, and 706.0 eV associated with Fe(III), Fe(II),
and Fe(0), respectively. The area ratio of Fe(III)/Fe(II)/Fe(0) was
calculated as 1/3.2/2.4, indicating that about 85% of Fe(III) was re-
duced to Fe(II) and Fe(0). In contrast, the reduction of [BMIm]FeCl4
by DEAC showed two component peaks at 711.1 and 709.3 eV cor-
responding to Fe(III) and Fe(II) with the area ratio of Fe(III)/Fe(II) =
1/1.3, clearly demonstrating that [BMIm]Fe2Cl7 can be more easily
reduced by DEAC than [BMIm]FeCl4.
Further information on the chemical structure of the active
species was obtained from a close analysis of the Cl 2p spec-
trum. As seen in Fig. 5, the chloride ions in [BMIm]Fe2Cl7 were in
two different energy states, evidenced by the two peaks at 200.7
[
BMIm]FeCl4.
3
.3. Mechanistic investigation
To elucidate the nature of active species and to have a bet-
ter understanding the role of [BMIm]Cl and DEAC, the high-
resolution XPS analysis of the catalytic systems was conducted.
The Fe 2p3/2 and Cl 2p peaks were deconvoluted using XPS-
PEAK Version 4.1 software using a Gaussian–Lorentzian peak pro-
file function and a Shirley background [22]. It is reported that the
characteristic Fe 2p3/2 peaks for Fe(III), Fe(II), and Fe(0) appeared

M.D. Nguyen et al. / Journal of Catalysis 258 (2008) 5–13
9
Table 3
species. Since the catalytic system consisting of FeCl2, [BMIm]Cl,
and DEAC is active and metallic Fe(0) is inactive for the dimeriza-
tion, the complete reduction of FeCl2 by DEAC to inactive metallic
Fe(0) is highly unlikely. In this regard, the major role of DEAC in
the FeCl2-based catalytic system is believed to be the activation
a
Activities of catalytic systems based on Fe(0) and FeCl2 for the dimerization of NBD
Entry
Catalyst
Cb
%)
Sc
(%)
Yd
(%)
(
e
1
Fe(0)
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
58.2
80.1
88.9
91.2
85.3
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
79.4
73.8
78.7
77.5
83.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
46.2
59.1
70.0
70.7
70.8
e
2
Fe(0)–[BMIm]Cl (1:1)
Fe(0)–TPP (1:5)
of in situ formed [BMIm] FeCl4 rather than the reduction of Fe(II)
2
e
3
+
to Fe(0). As an active Fe(II) species, [Fe(II)Cl] was considered as
e
4
Fe(0)–[BMIm]Cl–DEAC (1:1:5)
Fe(0)–[BMIm]Cl–TPP–DEAC (1:1:3:5)
FeCl2
FeCl2–[BMIm]Cl (1:1)
FeCl2–DEAC (1:5)
FeCl2–[BMIm]Cl–DEAC (1:0.5:2.5)
FeCl2–[BMIm]Cl–DEAC (1:1:2.5)
FeCl2–[BMIm]Cl–DEAC (1:2:5)
FeCl2–[BMIm]Cl–DEAC (1:3:5)
Fe(acac)3–TPP–DEAC (1:4:10)
e
the most plausible active species on the basis of theoretical cal-
5
+
culation. Moreover, [Fe(II)Cl] can easily accommodate two NBD
6
7
8
9
+
molecules to form [Fe(II)Cl(NBD) ] , an intermediate to H (see
2
nn
Fig. 6c).
−
As an active Fe(0) species, Fe(0) with a Cl ligand, [Fe(0)Cl]
−
1
0
could be regarded as a plausible active Fe(0) species because
there is no neutral ligand like TPP to stabilize Fe(0) species in
our catalytic system. Even though any decisive evidence for the
1
1
1
2
13
−
a
Molar ratio of NBD/Fe = 50, temperature = 85 ◦C, reaction time = 1 h. Num-
generation [Fe(0)Cl] has not been observed, there is a possibil-
−
bers in parentheses refer to molar ratios.
b
ity that [Fe(0)Cl] could function as an active species because it
Conversion of NBD.
Selectivity of Hnn.
Yield of Hnn.
Fe(0): Fe powder, Fe nanoparticles, or Fe(CO)5.
can bind two NBD molecules to form a stable 18-electron species.
c
−
However, the possibility of [Fe(0)Cl] to act as an active species
d
e
was excluded based on the theoretical calculation. Nevertheless,
the involvement of other type of Fe(0) species such as [Fe(NBD)]
and [Fe(NBD)2] cannot be ruled out even though the stabilization
of Fe(0) by NBD would be much weaker than that by TPP [30].
It is noteworthy that the activity of an Fe-containing ionic liq-
uids is not significantly affected by the presence of TPP (Table 2,
entry 10), demonstrating that Fe-containing ionic liquid does not
generate Fe(0) species like [Fe(TPP)] or [Fe(TPP)2], which are pro-
posed as active species for the dimerization of NBD in the presence
of a catalytic system consisting of Fe(acac)3, TPP, and DEAC [30].
and 199.2 eV. The presence of two peaks with the area ratio of
1:1.8 indicates that the charge is asymmetrically distributed on
−
two Fe atoms of Fe2Cl as can be expected from the geome-
try of Fe2Cl7 shown in Fig. 2. The Cl 2p spectra of [BMIm]FeCl4
and [BMIm]Fe2Cl7 after being reduced by DEAC also exhibited
two peaks at 199.8 and 198.6 eV for [BMIm]FeCl4 and 200.7 and
7
−
199.0 eV for [BMIm]Fe2Cl7 with the area ratios of 1:1.3 and 1:5.7,
respectively. The binding energy of Cl will be smaller for Fe(II)
than that for Fe(III), and therefore the higher Cl content in the
lower binding energy state for [BMIm]Fe2Cl7 species implies that
◦
For comparison, NBD dimerization was also conducted at 85 C
with the catalytic system consisting of Fe(acac)3, TPP, and DEAC.
The NBD conversion and Hnn yield were little lower than those ob-
tained from either FeCl2–[BMIm]Cl–DEAC (1:2:5) or [BMIm]Fe2Cl7–
DEAC (1:5), suggesting that Fe-containing ionic liquid-based cat-
alytic systems are somewhat superior to that based on Fe(acac)3
[
[
BMIm]Fe2Cl7 is more susceptible to the reduction by DEAC than
BMIm]FeCl4.
As shown in Fig. 4, the catalytic system producing higher
yields of Hnn showed higher contents of Fe(II) and Fe(0) species.
The amount of reduced Fe species at the same molar ratio of
DEAC/Fe(III) was larger for [BMIm]Fe2Cl7 than for [BMIm]FeCl4. It
is not clear at the moment which reduced species, Fe(II) or Fe(0),
is responsible for the formation of Hnn, but Fe(II) species seems to
be more in charge of the formation of Hnn. This is somewhat sup-
ported by the separate experiments with FeCl2 and Fe(0). As seen
in Table 3, the dimerization did not proceed at all when Fe powder,
Fe nanoparticles, or Fe(CO)5 was used as the catalyst, irrespective
of the presence of [BMIm]Cl and/or DEAC. The reaction was also
performed with Fe(0) in the presence of TPP because Fe(0) species
such as [Fe(TPP)] and [Fe(TPP)2] were proposed as active species
for the NBD dimerization in the presence of a catalytic system
composed of Fe(acac)3, TPP, and DEAC [30]. However, the presence
of TPP did not affect the dimerization with Fe(0), clearly show-
ing that active Fe(0) species is not generated from Fe powder, Fe
nanoparticles, or Fe(CO)5.
(
see Tables 2 and 3).
From the experimental results, the formation of active species
from [BMIm]Fe2Cl7, [BMIm]FeCl4, and [BMIm]2FeCl4 by the inter-
action with DEAC are postulated and depicted in Scheme 2. The
reduction of [BMIm]Fe2Cl7 would proceed through ethyl trans-
fer from DEAC and subsequent reductive elimination of n-butane
∗
(or ethyl chloride) to generate reduced ([BMIm]Fe2Cl5) and
EtAlCl2. Theoretical calculation shows that dimeric structure of
∗
(
(
[
[BMIm]Fe2Cl5) is hard to exist and thus it is assumed that
[BMIm]Fe2Cl5) rapidly transforms into an active Fe(II) species,
(BMIm) (Fe(II)Cl) ][Fe(II)Cl4]
∗
+
+
2−
as soon as it forms. Since the
dimerization reaction proceeds only in the presence of [BMIm]Cl,
the involvement of [BMIm] moiety in the active species can be
rationalized to a certain extent. Alternatively, ([BMIm]Fe2Cl5) can
be further reduced to metallic Fe(0) by interacting with DEAC. In
fact, metallic Fe(0) precipitates was observed during the dimeriza-
tion with [BMIm]Fe2Cl7.
+
∗
On the contrary, FeCl2 exhibited high NBD conversion and Hnn
yield, but only in the co-presence of both [BMIm]Cl and DEAC.
These results indicate that Fe(II) is an active species and both
Similarly to [BMIm]Fe2Cl7, [BMIm]FeCl4 can be reduced by
∗
DEAC to give ([BMIm]FeCl3) , which can be transformed into
+
−
[
(
Fe(II)Cl] [(BMIm)Cl2] or further reduced to Fe(0) (see Eqs. (1)–
5)).
[
BMIm]Cl and DEAC are playing pivotal roles in the activation of
FeCl2. The yield of Hnn increased with increasing molar ratio of
BMIm]Cl/FeCl2 up to 2 and then remained almost unchanged on
2
[BMIm]FeCl4 + 2DEAC
[
∗
→
2([BMIm]FeCl3) + 2EtAlCl2 + n-C4H10,
(1)
(2)
further increase of the molar ratio, implying that [BMIm]2FeCl4 is
the precursor to an active species. The behavior of [BMIm]Cl–FeCl2
system is in contrast with that of [BMIm]Cl–FeCl3, with which the
yield of Hnn decreased with increasing content of [BMIm]Cl, again
demonstrating the importance of Fe(II) species. Like in the case of
FeCl3, the Fe–Cl bond of FeCl2 becomes lengthened by the interac-
tion with [BMIm]Cl through the formation of [BMIm]2FeCl4 [31,32],
thereby facilitating the interaction with DEAC to generate active
∗
+
−
2
2([BMIm]FeCl ) → 2[Fe(II)Cl] [(BMIm)Cl ] ,
3
∗
(
[BMIm]FeCl3) + DEAC
→
Fe(0) + [BMIm]Cl + EtAlCl2 + C2H5Cl,
(3)
∗
2
([BMIm]FeCl3) + 2DEAC
→
Fe(0) + [BMIm]Cl + 2EtAlCl2 + n-C4H10,
(4)
(5)
[BMIm]Cl + EtAlCl2 → [BMIm]EtAlCl3.

10
M.D. Nguyen et al. / Journal of Catalysis 258 (2008) 5–13
+
+
+
+
Fig. 6. Optimized structures of: (a) NBD, 1; (b) Hnn, 2; (c) [Fe(II)Cl(NBD)2] , 3a; (d) [Fe(III)Cl2(NBD)2] , 3b; (e) [Fe(II)Cl(Hnn)] , 4a; (f) [Fe(III)Cl2(Hnn)] , 4b.
−
+
Unlike the cases of [BMIm]Fe2Cl7 and [BMIm]FeCl4, the activa-
7.9 kcal mol 1, suggesting that coordination of NBD to [Fe(II)Cl]
+
tion of [BMIm]2FeCl4, formed from the interaction of FeCl2 with
two equivalents of [BMIm]Cl, seems to proceed in a different way.
The evolution of n-butane or ethyl chloride was not observed at
is more feasible than that to [Fe(III)Cl2] (see supplementary ma-
terial).
Catalytic cycle for the formation of Hnn from two molecules of
NBD by [Fe(II)Cl] was theoretically investigated. The optimized
◦
+
temperatures lower than 60 C upon interaction with DEAC, sug-
gesting that the reduction of [BMIm]2FeCl4 is much more diffi-
cult compared with those of [BMIm]Fe2Cl7 and [BMIm]FeCl4. Even
at 85 C, only a small amount of n-butane was produced. From this
structures of reaction intermediates and transition states were de-
picted in Fig. 7, which shows the order of stepwise formation of
three σ bonds by breaking three π bonds (see supplementary ma-
terial for further information on the structures). Reaction profile
◦
result, it is likely that the activation proceeds through the abstrac-
−
∗
+
tion of Cl from [BMIm]2FeCl4 by DEAC to form ([BMIm]FeCl3)
of the dimerization reaction of NBD by [Fe(II)Cl] shown in Fig. 8
and [BMIm]Et2AlCl2. Even though the reduction of [BMIm]2FeCl4
by DEAC is hard to take place, there still is a possibility that small
portion of [BMIm]2FeCl4 is reduced to Fe(0) along with the forma-
tion of n-butane and [BMIm]EtAlCl3 (Scheme 2).
indicates that the rate determining step is the formation of the
first σ bond connecting two NBD molecules with the activation
−
1
energy of 35.8 kcal mol . As a whole, the reaction is exergor-
−1
nic by 13.1 kcal mol . For comparison, the potential energies for
the formation of intermediates from the [Fe(III)Cl2] -assisted NBD
+
To support our postulate that cationic Fe(II) species is an active
+
species in the dimerization of NBD, the interactions of [Fe(II)Cl]
dimerization were also calculated (see supplementary material).
+
and [Fe(III)Cl2] with NBD were theoretically investigated at the
B3LYP (6-31+G* for C and H, and LanL2DZ with ECP for Cl and
Fe) level of the theory using Gaussian 03 program [20]. Com-
pounds 1, 2, 3a, and 4a in Fig. 6 are the optimized structures of
NBD, Hnn, [Fe(II)Cl(NBD)2] , and [Fe(II)Cl(Hnn)] , respectively. For
comparison, optimized structures of [Fe(III)Cl2(NBD)2] (3b) and
3.4. Effect of molar ratio of NBD/Fe(III)
The effect of NBD/Fe(III) molar ratio on the dimerization of NBD
+
+
◦
was also investigated. The reactions were conducted at 85 C and
+
at the DEAC/Fe(III) molar ratio of 5 in the presence of a catalytic
system consisting of DEAC and [BMIm]FeCl4 or [BMIm]Fe2Cl7. As
shown in Fig. 9, the conversion of NBD decreased with increas-
+
[
Fe(III)Cl2(Hnn)] (4b) were also obtained. Calculation of free en-
ergy of formation shows that 3a is more stable than 3b by about

M.D. Nguyen et al. / Journal of Catalysis 258 (2008) 5–13
11
+
Scheme 2. Plausible mechanisms for the active species, [Fe(II)Cl]
.
+
Fig. 7. The optimized structures of reaction intermediates and transition states of dimerization of NBD using [Fe(II)Cl] as an active species (TS = transition state; A and B =
reaction intermediates).
ing molar ratio of NBD/Fe(III), but the selectivity of Hnn increased
slowly with the increase of the molar ratio up to 100.
of NBD increased with the temperature rose when the dimeriza-
tion reactions were conducted with the molar ratio of NBD/Fe(III)
at 100. The yield of Hnn, however, increased with the increase of
◦
3
.5. Effect of temperature
temperature only up to 85 C, and then decreased on further in-
crease of temperature, most likely due to the acceleration of NBD
polymerization and to the decomposition of the active species.
The dimerization reactions of NBD were carried out at vari-
ous temperatures using a catalytic system consisting of DEAC and
BMIm]FeCl4 or [BMIm]Fe2Cl7. As shown in Table 4, the conversion
◦
Larger amounts of metallic particles were observed at 100 C than
◦
[
at 85 C.

12
M.D. Nguyen et al. / Journal of Catalysis 258 (2008) 5–13
+
Fig. 8. Reaction profile of the dimerization reaction of NBD by [Fe(II)Cl]
.
◦
Fig. 10. Effect of reaction time on the dimerization reaction of NBD at 85 C:
1) conversion of NBD with [BMIm]Fe2Cl7; (2) conversion of NBD with [BMIm]-
FeCl4; (!) TON of Hnn with [BMIm]Fe2Cl7; (") TON of Hnn with [BMIm]FeCl4; (TON
mol of Hnn/mol of Fe(III), molar ratios of NBD/Fe(III) and DEAC/Fe(III) were set
(
=
at 100 and 5, respectively).
Table 5
a
Catalyst recycling study with [BMIm]Fe2Cl7
Cycle
No.
Conversion
of NBD (%)
Selectivity
of Hnn (%)
Yield of
Hnn (%)
1
2
3
4
94.1
88.3
66
86.4
85.9
87.7
86.2
81.3
75.8
57.9
42.0
48.7
a
Molar ratio of NBD/Fe(III) = 100/1, DEAC/catalyst = 5, temperature = 85 ◦C,
reaction time = 1 h.
both catalysts. It seems that the system reaches a maximum of
conversion in less than an hour.
Fig. 9. Effect of the molar ratio of NBD/Fe(III) on the dimerization reaction of
NBD: (1) conversion of NBD with [BMIm]Fe2Cl7; (2) conversion of NBD with
3.7. Catalyst recycling
[BMIm]FeCl4; (!) TON of Hnn with [BMIm]Fe2Cl7; (") TON of Hnn with [BMIm]-
FeCl4 (TON = mol of Hnn/mol of Fe(III), reaction condition: molar ratio of DEAC/
To test the catalyst stability and recyclability, the dimeriza-
tion reaction of NBD was performed with [BMIm]Fe2Cl7 for 1 h
◦
Fe(III) = 5, temperature = 85 C, reaction time = 1 h).
◦
at 85 C at the ratios of NBD/Fe(III) = 100/1, DEAC/catalyst = 5,
Table 4
respectively. After the reaction, most of volatile compounds were
removed under a reduced pressure and the remaining catalyst mix-
ture was reused for further reactions with fresh charges of NBD
and toluene. As seen in Table 5, both NBD conversion and Hnn yield
were found to decrease continuously with the recycles. This can be
largely attributed to the reduction of Fe(II) to inactive Fe(0) and to
the accumulation of high boiling side products.
a
Effect of temperature on the dimerization reaction of NBD
Entry
Catalyst
Temperature
Cb
(%)
Sc
(%)
Yd
(%)
◦
(
C)
1
2
3
4
5
6
7
8
9
[BMIm]FeCl4
[BMIm]FeCl4
[BMIm]FeCl4
[BMIm]FeCl4
[BMIm]FeCl4
[BMIm]Fe2Cl7
[BMIm]Fe2Cl7
[BMIm]Fe2Cl7
[BMIm]Fe2Cl7
[BMIm]Fe2Cl7
25
40
60
85
100
25
40
60
85
100
1.7
57
1
3.7
31.6
40.4
36.2
2.3
5.3
40.1
48.5
63.1
3.4
69.5
78.8
83.2
57.3
68.9
74.6
82
4
. Conclusion
8.2
6.1
85.2
95.4
97.4
69.9
84.3
76.7
Fe-containing IL catalysts, [BMIm]FeCl4, [BMIm]Fe2Cl7, and
88.4
78.7
10
[BMIm]2FeCl4 were highly effective for the selective dimerization
of NBD to produce Hnn in high yield. By employing these catalysts,
the use of expensive phosphanes was avoided and the required
amount of reducing agent (DEAC) was greatly reduced. From the
mechanistic and theoretical studies on the active species, experi-
mental results on the dimerization, and Tafel measurements, it is
a
Molar ratio of NBD/Fe(III) = 100/1, DEAC/catalyst = 5, temperature = 85 ◦C,
reaction time = 1 h.
b
Conversion of NBD.
Selectivity of Hnn.
Yield of Hnn.
c
d
+
postulated that the active species, [Fe(II)Cl] , is in charge of the
3
.6. Effect of reaction time
Hnn formation. The roles of [BMIm]Cl are believed to promote the
generation of the active species through the formation of easily
reducible [BMIm]FexCl3x+1 (x = 1 or 2) or [BMIm]2FeCl4, and to
stabilize the resulting active species.
Further study is in progress to improve the catalyst stability and
recyclability using polymer-supported catalysts for the practical ap-
plication.
Effect of reaction time was also examined using [BMIm]FeCl4
◦
and [BMIm]Fe2Cl7 at 85 C. The molar ratios of NBD/Fe(III) and
DEAC/Fe(III) were set at 100 and 5, respectively. As shown in
Fig. 10, the conversion of NBD increased rapidly with the reaction
time, but the rate of increase was much slower after 30 min for

M.D. Nguyen et al. / Journal of Catalysis 258 (2008) 5–13
13
Acknowledgment
[13] H. Brunner, F. Prester, Tetrahedron: Asymm. 1 (1990) 589.
[14] S. Tanaka, C. Dubs, A. Inagaki, M. Akita, Organometallics 24 (2005) 163.
[
15] S.K. Brayshaw, J.C. Green, G. Kociok-Kohn, E.L. Sceats, A.S. Weller, Angew. Chem.
Int. Ed. 45 (2006) 452.
We acknowledge financial support by a grant (CF2-101-1) from
Carbon Dioxide Reduction & Sequestration Research Center, one of
the 21st Century Frontier Programs funded by the Ministry of Sci-
ence and Technology of Korean government.
[16] T. Ooi, J. Morikawa, D. Uraguchi, K. Maruoka, Tetrahedron Lett. 40 (1999) 2993.
[17] P. Wasserscheid, J. Ind. Eng. Chem. 13 (2007) 325.
[
18] W.L.F. Armarego, C.L.L. Chai, in: Purification of Laboratory Chemicals, fifth ed.,
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[19] V. Farmer, T. Welton, Green Chem. 4 (2002) 97.
Supplementary material
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298.
21] Y. Yoshida, G. Saito, J. Mater. Chem. 16 (2006) 1254.
2
[
The online version of this article contains additional supple-
mentary material.
[22] Y.J. Kim, C.R. Park, Inorg. Chem. 41 (2002) 6211.
[
23] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, in: J. Chanstain, R.C. King
Jr. (Eds.), Handbook of X-Ray Photoelectron Spectroscopy—A Reference Book
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