Characterization of phenylene-bridged hybrid mesoporous materials incorporating arenetricarbonyl complexes (-C6H4Me(CO) 3-; Me = Cr, Mo) and their catalytic activities

Article abstract of DOI:10.1016/j.cattod.2011.10.019

The successful construction of arenetricarbonyl complexes (-C ₆H₄Me(CO)₃-; Me = Cr, Mo) was achieved through the direct modification of phenylene (-C₆H₄-) moieties of phenylene-bridged hybrid mesoporous materials (HMM-ph) by the simple chemical vapor deposition (CVD) of corresponding metal hexacarbonyls. The pore structure as well as high surface area of HMM-ph were retained even after CVD treatment. It was found that HMM-ph incorporating an arenetricarbonyl molybdenum complex (HMM-phMo(CO)₃) exhibited higher catalytic performance for the polymerization of phenylacetylene and dehydrochlorination of 2-chloro-2-methylbutane than HMM-phCr(CO)₃. Various spectroscopic investigations revealed that the strength of the chemical bond between the phenylene ligand and metal center of the arenetricarbonyl complexes affects the catalytic performance of HMM-phMe(CO)₃ (Me = Cr, Mo).

Full text of DOI:10.1016/j.cattod.2011.10.019

Catalysis Today 181 (2012) 14–19
Contents lists available at SciVerse ScienceDirect
Catalysis Today
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Characterization of phenylene-bridged hybrid mesoporous materials
incorporating arenetricarbonyl complexes (–C H Me(CO) –; Me = Cr, Mo) and
6
4
3
their catalytic activities
∗
∗
Takashi Kamegawa, Masakazu Saito, Takahiro Sakai, Masaya Matsuoka , Masakazu Anpo
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 August 2011
Received in revised form 6 October 2011
Accepted 18 October 2011
Available online 17 November 2011
6 4 3
The successful construction of arenetricarbonyl complexes (–C H Me(CO) –; Me = Cr, Mo) was achieved
through the direct modification of phenylene (–C6H4–) moieties of phenylene-bridged hybrid meso-
porous materials (HMM-ph) by the simple chemical vapor deposition (CVD) of corresponding metal
hexacarbonyls. The pore structure as well as high surface area of HMM-ph were retained even after CVD
treatment. It was found that HMM-ph incorporating an arenetricarbonyl molybdenum complex (HMM-
phMo(CO)3) exhibited higher catalytic performance for the polymerization of phenylacetylene and
dehydrochlorination of 2-chloro-2-methylbutane than HMM-phCr(CO)3. Various spectroscopic investi-
gations revealed that the strength of the chemical bond between the phenylene ligand and metal center
of the arenetricarbonyl complexes affects the catalytic performance of HMM-phMe(CO)3 (Me = Cr, Mo) .
Keywords:
Inorganic–organic hybrid
Mesoporous
Arenetricarbonyl complex
CVD
©
2011 Elsevier B.V. All rights reserved.
Polymerization of phenylacetylene
Dehydrochlorination
1
. Introduction
the direct utilization of organic moieties (–C H – = phenylene (ph))
6 4
within HMM as a “framework ligand”, i.e., the construction of
For the past several decades, siliceous mesoporous materials
arenetricarbonyl complexes (–phMe(CO) –; Me = Cr, Mo, W) by
3
have attracted considerable attention in terms of their potential
applications as an adsorbent and catalyst support as well as fillers
for chromatography [1–3]. The anchoring of organometallic com-
plexes on their surfaces as well as the incorporation of heteroatoms
such as Ti-, Cr- and Mo-atoms within their frameworks are promis-
ing ways of designing unique catalysts and photocatalysts [3–10].
On the other hand, recently, the synthesis of mesoporous materials
which have bridged organic moieties, i.e., –CH CH –, –CH CH– and
chemical vapor deposition (CVD) treatment of phenylene-bridged
hybrid mesoporous materials (HMM-ph) with corresponding metal
hexacarbonyls at elevated temperatures [21,22]. Coelho et al. have
also reported the synthesis of the same type of complexes by liquid
phase treatment of HMM-ph and their catalytic performances for
the epoxidation of olefins [23,24].
In the present study, HMM-ph functionalized by arenetricar-
bonyl chromium and molybdenum complexes (HMM-phMe(CO)3;
Me = Cr, Mo) have been prepared by the CVD method and char-
acterized in detail with various spectroscopic methods. Attention
has been focused on a clear determination of the local structure
2
2
–
C H –, within their silica frameworks has been achieved by using
6 4
ꢀ
ꢀ
3
organosilanes [(R O) Si–R–Si(OR ) ] as a precursor [11–15]. The
3
direct utilization of fascinating framework organic moieties within
these inorganic–organic hybrid mesoporous materials (HMM) has
been intensively studied in various research fields. For example,
light-harvesting and light-emitting systems have been designed by
a combination of HMM and organic dyes as guest molecules [16,17].
Solid-acid and solid-base catalysts have also been prepared by the
post-synthetic direct modification of framework organic moieties
with sulfuric acid (–SO H) and amino (–NH ) groups, respectively
of –phMe(CO) – (Me = Cr, Mo) complexes by XAFS measurements.
3
Furthermore, the catalytic performance of HMM-phMe(CO)3
(Me = Cr, Mo) has been investigated by the polymerization of
phenylacetylene as well as the dehydrochlorination of 2-chloro-
2-methylbutane as model reactions in heterogeneous systems.
3
2
2. Experimental
[
18–20]. In line with such work, our group has previously reported
2.1. Materials
∗ Corresponding authors. Tel.: +81 72 254 9288; fax: +81 72 254 9910.
E-mail addresses: matsumac@chem.osakafu-u.ac.jp (M. Matsuoka),
Octadecyltrimethylammonium chloride, phenylacetylene, and
anpo@osakafu-u.ac.jp (M. Anpo).
2-chloro-2-methylbutane were purchased from Tokyo Kasei Kogyo
0
920-5861/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2011.10.019

T. Kamegawa et al. / Catalysis Today 181 (2012) 14–19
15
Co., Ltd. Chromium hexacarbonyl (Cr(CO) ), benzene tricarbonyl
2.5. Dehydrochlorination reaction of 2-chloro-2-methylbutane
6
chromium (C H Cr(CO) ), molybdenum hexacarbonyl (Mo(CO) ),
6
6
3
6
and 1,4-bis(triethoxysilyl) benzene were obtained from Aldrich.
Dry acetonitrile, dry chloroform, dry tetrahydrofuran and HCl (37%)
were obtained from Nacalai Tesque Inc. All chemicals were used
without further purification.
The dehydrochlorination reaction of 2-chloro-2-methylbutane
was carried out in a glass vessel equipped with a vacuum line con-
nector. The catalyst (HMM-phMe(CO) ; Me = Cr, Mo, 90 mg) and
3
degassed 2-chloro-2-methylbutane (0.3 ml) were charged into a
glass vessel and then heated at 358 K in an oven. At this tempera-
ture, 2-chloro-2-methylbutane and the reaction products exist in
gaseous form. After heating for 22 h, a glass vessel was cooled down
to 298 K and 1.0 ml of THF was added for the complete recovery
and diluting of the substances. Analysis of the substances was per-
formed on a gas chromatograph (Shimadzu GC-14B with a flame
2.2. Synthesis of HMM-ph and HMM-phMe(CO)3 (Me = Cr, Mo)
HMM-ph was synthesized in accordance with previous liter-
ature by Inagaki et al. using 1,4-bis(triethoxysilyl)benzene and
octadecyltrimethylammonium chloride [14]. The functionaliza-
tion of HMM-ph through the reaction of phenylene moieties
and Mo(CO)6 was carried out by simple chemical vapor deposi-
tion (CVD) treatment at 368 K [21,22]. The obtained sample was
®
ionization detector) equipped with an InertCap 1 capillary column.
3. Results and discussion
denoted as HMM-phMo(CO) , which incorporates arenetricarbonyl
3
molybdenum complexes (–phMo(CO) –) within its mesoporous
3.1. Characterization of HMM-ph and HMM-phMe(CO)3 (Me = Cr,
Mo)
3
frameworks. HMM-phCr(CO)3 was also prepared by the same pro-
cess at 398 K using Cr(CO)6 as the CVD source [21,22]. Prior to each
CVD treatment, HMM-ph was degassed at 473 K for 2 h. Benzene
tricarbonyl molybdenum (C H Mo(CO) ) as a reference complex
Fig. 1 shows the UV–vis absorption spectra of HMM-ph before
and after CVD treatment with Mo(CO)6 and chloroform solution of
the reference complexes. The peak position of C6H6Mo(CO)3 was
quite different from that of Mo(CO)6. A typical absorption peak
at around 325 nm (Fig. 1(b)) can be assigned to the metal–phenyl
intramolecular charge transfer transition of the arenetricarbonyl
metal complex [26,27]. In the case of HMM-ph, only the absorption
peak due to the ␲–␲* transition of the framework phenylene moi-
eties was observed at around 275 nm. On the other hand, the UV–vis
absorption spectrum of HMM-phMo(CO)3 shows a new intense
peak and long-tailed absorption which could not be observed for
HMM-ph at around 325 nm and above 350 nm, respectively. This
absorption spectrum of HMM-phMo(CO)3 corresponds well to the
6
6
3
was also synthesized by a method reported previously and stored
under purified argon atmosphere [25].
2.3. Characterization of HMM-ph and HMM-phMe(CO)3 (Me = Cr,
Mo)
The X-ray diffraction (XRD) patterns were collected by a Shi-
madzu XRD-6100 using Cu K␣ radiation (ꢀ = 1.5406 A˚ ). The UV–vis
spectra were obtained using a Shimadzu UV-2200A spectropho-
tometer. The FT-IR spectra were recorded with a resolution of
−
1
4
cm in transmission mode using a JASCO FT-IR 660 Plus appara-
superimposed absorption of HMM-ph and C H Mo(CO) , show-
6
6
3
tus. The Cr K-edge X-ray absorption fine structure (XAFS) spectra
were obtained at the BL-12C facility of the Photon Factory at the
High-Energy Acceleration Research Organization (KEK) in Tsukuba,
ing the formation of arenetricarbonyl molybdenum complexes
–phMo(CO) –) through the reaction of phenylene moieties of
(
3
HMM-ph and Mo(CO)6 under CVD conditions (Eq. (1)):
(1)
It should be noted that HMM-phCr(CO)3 also exhibits a typical
UV–vis absorption peak at around 320 nm and long-tailed absorp-
tion above 350 nm (data not shown) due to the arenetricarbonyl
Japan. The Mo K-edge XAFS spectra were also recorded at the
BL-01B1 facility of SPring-8 at the Japan Synchrotron Radiation
Research Institute (JASRI). All spectra were recorded in fluores-
cence mode with a Si(1 1 1) two-crystal monochromator at 298 K.
The extended X-ray absorption fine structure (EXAFS) data were
examined using an analysis program (Rigaku REX2000). In order
to obtain the radial structure function, Fourier transformations of
chromium complexes (–phCr(CO) –) [21,22].
3
The formation of –phMe(CO) – (Me = Cr, Mo) complexes within
3
the framework of HMM-ph was also confirmed by FT-IR measure-
ments. As shown in Fig. 2, HMM-phCr(CO)3 and HMM-phMo(CO)3
−1
exhibit typical FT-IR peaks at 1985–1980 and 1940–1840 cm
,
3
−1
k -weighted EXAFS oscillations in the range of 3–12 A˚ were per-
which can be assigned to the a1 and e vibrational mode of
the areneMe(CO)3 (Me = Cr, Mo) complexes, respectively [28–30].
These results clearly indicate the successful construction of
formed.
2.4. Polymerization reaction of phenylacetylene
–
phMe(CO) – (Me = Cr, Mo) complexes which are coordinated by
3
a phenylene moiety of HMM-ph as a “framework ligand” and con-
firm the thermal stability of these complexes at temperatures as
high as 473 K. The position of the a1 vibrational mode of HMM-
The polymerization reaction of phenylacetylene (PA) was car-
ried out in a two-necked flask (100 ml) equipped with a reflux
condenser and gas adapter connected to an oil bubbler. A mixture of
−1
phMo(CO)3 (1984 cm ) was observed to shift slightly to higher
the catalyst (HMM-phMe(CO) ; Me = Cr, Mo, 300 mg) and degassed
−1
3
wavenumber regions than that of HMM-phCr(CO)3 (1981 cm ).
pure PA (2.0 ml) was heated at 353 K with magnetic stirring for
4 h. After the polymerization reaction, the reaction mixture was
It has been reported that the wavenumber of the a1 vibrational
mode of the arenetricarbonyl chromium complexes is closely
related to the strength of the metal–arene bond, i.e., the higher
the wavenumber of the a1 vibrational mode, the weaker the
strength of the metal–arene bond of the complex [30,31]. The high
wavenumber of the a1 vibrational mode of HMM-phMo(CO)3 indi-
2
added to THF and thoroughly stirred for a washing of the catalyst.
The THF-rinsed catalyst was then separated by centrifugation. The
product was precipitated in methanol, washed with methanol and
then purified by reprecipitation from THF. Only a methanol insolu-
ble polymer was recovered. The polymer yield was determined by
gravimetry.
cates that the metal–arene bond of –phMo(CO) – is weaker than
3
that of –phCr(CO) –, showing good agreement with previously
3

1
6
T. Kamegawa et al. / Catalysis Today 181 (2012) 14–19
Fig. 1. (a and b) Transmission and (c and d) diffuse reflectance UV–vis spectra of
a) Mo(CO)6 in chloroform, (b) C6H6Mo(CO)3 in chloroform, (c) HMM-ph, and (d)
Fig. 3. XRD patterns of (a) HMM-ph and (b) HMM-phMo(CO)3.
(
HMM-phMo(CO)3.
above 523 K under vacuum. During this CO desorption process, the
reported trends showing that the metal–arene bond is weaker for
areneMo(CO)3 than areneCr(CO)3 [32].
Moreover, it was also found that these FT-IR peaks completely
disappeared after evacuation at 573 K for 1 h by the decomposition
sample color of HMM-phMe(CO) gradually changed from pale yel-
3
low to gray due to the formation of corresponding metal clusters.
The mesoporous structures of HMM-ph and HMM-phMo(CO)3
were also investigated by XRD measurements. As shown in Fig. 3,
the diffraction peaks due to the two-dimensional hexagonal meso-
of (–phMe(CO) –; Me = Cr, Mo). CO desorption analysis from HMM-
3
phMe(CO) also revealed that the CO desorbed from the complexes
3
porous structure and periodicity of the SiO1.5–C H –SiO
units
6
4
1.5
within the wall of the mesoporous framework were observed
◦
◦
in the 2ꢁ region from 1.5 to 50 [14]. A lattice constant (a0
=
√
2
d100
/
3) of the two-dimensional hexagonal structure was deter-
mined to be 51.5 A˚ from the position of an intense XRD peak in
the region of 2ꢁ < 5 . The shape and intensity of these diffraction
◦
peaks were found to be almost the same for both samples, showing
that CVD treatment with Mo(CO)6 scarcely affected the structure
of HMM-ph. The incorporation of –phCr(CO) – complexes within
3
HMM-ph was also achieved without destruction of HMM-ph (data
not shown), as reported in previous literature [21,22]. The surface
areas of HMM-ph, HMM-phCr(CO) and HMM-phMo(CO)3 were
3
2
2
2
determined to be 806 m /g, 771 m /g and 765 m /g, respectively,
supporting the above experimental results. Moreover, the amount
of –phMe(CO) – (Me = Cr, Mo) complexes were determined to be
3
3
.9 wt.% as Cr metal and 1.0 wt.% as Mo metal by ICP analysis for
HMM-phCr(CO)3 and HMM-phMo(CO) , respectively.
3
3
.2. XAFS investigations on the local structures of –phMe(CO) –
3
(Me = Cr, Mo) complexes
The local structure of –phCr(CO) – complexes within HMM-
3
phCr(CO)3 was investigated by XAFS measurements. The X-ray
absorption near-edge structure (XANES) spectra are sensitive to
the local structure and electronic state of X-ray absorbing atoms
(Cr). As shown in Fig. 4, the shape of the XANES spectrum of HMM-
phCr(CO)3 differs from that of Cr(CO) , the CVD source of the Cr
6
metal. The peak position of HMM-phCr(CO)3 also corresponded
well to that of C H Cr(CO) . As shown in Fig. 4(a), Fourier transform
Fig. 2. FT-IR spectra of samples after evacuation at (a and b) 473 K and (a’ and b’)
6
6
3
5
73 K for 1 h (Samples: (a and a’) HMM-phCr(CO)3 and (b and b’) HMM-phMo(CO)3).
of the EXAFS spectrum of HMM-phCr(CO)3 exhibited two peaks due

T. Kamegawa et al. / Catalysis Today 181 (2012) 14–19
17
Fig. 5. (A–D) XANES and (a–d) Fourier transform of the EXAFS spectra of (A, a)
HMM-phMo(CO)3, (B, b) C6H6Mo(CO)3, (C, c) Mo(CO)6, and (D, d) Mo foil.
Fig. 4. (A–D) XANES and (a–d) Fourier transform of the EXAFS spectra of (A, a)
HMM-phCr(CO)3, (B, b) C6H6Cr(CO)3, (C, c) Cr(CO)6, and (D, d) Cr metal.
in homogeneous systems [32,37,38]. Here, the same reaction was
carried out on HMM-phMo(CO)3 aiming at their applications for
heterogeneous catalysts. During the polymerization reaction of PA,
the powder color of HMM-phMo(CO)3 changed from pale yellow
to dark orange. Fig. 6 shows the UV–vis absorption spectra of PA
and the products obtained by reprecipitation from THF after the
reactions. The reaction product shows a maximum band at around
to the presence of neighboring carbon atoms (Cr–Cph, Cr–CCO) at
around 1.8 A˚ and the oxygen atoms of CO (Cr–(C)OCO) at around
2
.5 A˚ (without phase-shift correction). Curve fitting analysis of the
Fourier transform of EXAFS provided detailed insight into the bond
distance (R) and coordination number (CN) of the –phCr(CO) –
3
complexes. These values were determined for the Cr–CCO bond
(
R = 1.86 A˚ , CN = 3.0), Cr–Cph bond (R = 2.24 A˚ , CN = 6.0) and Cr–OCO
2
20–280 nm with a shoulder band above 300 nm, which were
bond (R = 2.99 A˚ , CN = 3.0), showing good accordance with the X-ray
crystallographic data of C H Cr(CO) [31,33,34].
assigned to the ␲–␲* transition of phenyl group and the ␲–␲* inter-
band transition of the polymer (polyphenylacetylene) main chain,
respectively [39]. The formation of polyphenylacetylene (PPA) was
also confirmed by FT-IR investigations (Fig. 7(inset)). PA shows
6
6
3
The local structure of the –phMo(CO) – complexes was also con-
3
firmed by XAFS measurements. Fig. 5 shows the XANES spectra of
HMM-phMo(CO)3 and some reference complexes of known struc-
tures. The shape of the XANES spectrum of HMM-phMo(CO)3 was
quite similar to that of C H Mo(CO) as the reference complex of
6
6
3
the arenetricarbonyl molybdenum structure. Fourier transform of
the EXAFS spectra for HMM-phMo(CO)3 and C H Mo(CO) also
6
6
3
exhibited two peaks due to the presence of neighboring car-
bon atoms (Mo–Cph, Mo–CCO) at around 1.9 A˚ and oxygen atoms
of CO (Mo–(C)OCO) at around 2.6 A˚ (without phase-shift correc-
tion). Curve fitting analysis of these peaks revealed the following
parameters for the Mo–CCO bond (R = 1.97 A˚ , CN = 3.0), Mo–Cph
bond (R = 2.38 A˚ , CN = 6.0) and Mo–OCO bond (R = 3.11 A˚ , CN = 3.0),
showing good coincidence with those of C H Mo(CO) determined
6
6
3
by X-ray crystallographic characterizations [34–36]. These results
indicated that the –phMe(CO) – (Me = Cr, Mo) complexes were suc-
3
cessfully constructed within HMM-ph without the formation of Cr
and Mo metal clusters.
3.3. Polymerization of phenylacetylene on HMM-phMe(CO)3
(Me = Cr, Mo)
It has been reported that arenetricarbonyl complexes exhibit
Fig. 6. UV–vis absorption spectra of (a) chloroform solution of PA and (b) chloroform
catalytic activities for the polymerization of phenylacetylene (PA)
solution of the obtained product by polymerization of PA on HMM-phMo(CO)3.

1
8
T. Kamegawa et al. / Catalysis Today 181 (2012) 14–19
Fig. 7. Yields of polyphenylacetylene (PPA) in the polymerization reaction of PA
on each catalyst. Inset shows the FT-IR spectra of (a) PA and (b) PPA produced on
HMM-phMo(CO)3.
typical FT-IR peaks at around 3300 and 2100 cm−1 due to
the stretching vibration of the C–H (alkyne) and C C bonds,
respectively, and these peaks gradually decreased during the poly-
merization reaction accompanied by increases in the typical FT-IR
peaks of the polymerized products. The recovered reaction product
shows FT-IR peaks due to the stretching vibration of the conjugated
Fig. 8. XRD patterns of (a) HMM-phMo(CO)3 after polymerization of PA and (b)
HMM-phMo(CO)3 washed in THF after measurement of (a).
Table 1
Conversion and selectivity in the dehydrochlorination of 2-chloro-2-methylbutane
on HMM-ph and HMM-phMe(CO)3 (Me = Cr, Mo).
−1
double bond at around 1600 cm and typical C–H stretching vibra-
−1
Catalyst
Conversion % Selectivity/%
2-Methyl-1-butene 2-Methyl-2-butene
tion of the olefinic proton at 3020 cm
[38], which were quite
different from those observed for the original PA. These results
clearly indicate that the polymerization of PA proceeds on HMM-
phMo(CO)3 in heterogeneous systems to form PPA. The average
molecular weight of the recovered PPA was ca. 8400, which was
almost the same as that of PPA obtained in homogeneous systems.
Fig. 8 shows the XRD patterns of the recovered catalyst before
and after washing with THF. After the polymerization reaction
HMM-ph
0
–
–
HMM-phCr(CO)3
HMM-phMo(CO)3 21.8
13.3
8.5
8.3
91.5
91.7
6
the ␩ -arene moieties was key for the polymerization reaction.
Considering that the metal–arene bond of –phCr(CO) – is stronger
◦
3
of PA, the peak intensities (2ꢁ < 10 ) due to the two-dimensional
than that of –phMo(CO) – [32], it can be concluded that the strong
3
hexagonal mesoporous structure became weak, although the peaks
phenylene–Cr metal bonding of –phCr(CO) – prevents the efficient
◦ ◦
10 < 2ꢁ < 50 ) due to the periodicity of the SiO –C H –SiO
1.5 6 4 1.5
3
(
coordination of PA to the Cr metal center, hence decreasing the
reaction rate as compared to that on HMM-phMo(CO)3.
units scarcely changed as compared to those of the original HMM-
◦
phMo(CO)3 (Fig. 3(b)). Moreover, the peak intensities (2ꢁ < 10 )
of HMM-phMo(CO) , which was washed in THF after measure-
ment (Fig. 8(a)), almost completely recovered its original intensities
3
3.4. Dehydrochlorination reaction of 2-chloro-2-methylbutane
(
Fig. 8(b)). These results suggest that the polymerization reaction
The dehydrochlorination of 2-chloro-2-methylbutane was
carried out to investigate the catalytic performance of HMM-
phMe(CO)3 (Me = Cr, Mo) in the gas phase (Eq. (2)):
of PA proceeds catalytically and PPA as a product was mainly
formed within the mesopores of HMM-phMo(CO)3 without the
(2)
destruction of the mesoporous frameworks. Moreover, compar-
ative studies revealed that HMM-phMo(CO)3 exhibited a higher
As shown in Table 1, the dehydrochlorination products, i.e.,
2-methyl-1-butene and 2-methyl-2-butene were formed in this
reaction system accompanied by the formation of HCl. The turnover
numbers defined as the ratio of the total amount of products to
yield for the formation of PPA than HMM-phCr(CO) . As shown in
3
Fig. 7, under the same reaction conditions, the yield of PPA was
1
8% on HMM-phMo(CO)3 and 2.1% on HMM-phCr(CO) , respec-
the amount of –phMe(CO) – (Me = Cr, Mo) complexes were 4.8
3
3
tively. Two kinds of reaction processes for the polymerization of
PA have thus far been reported [32,37]. One is the metathesis reac-
tion after the formation of an alkyl vinylidene complex. The other
proceeds by way of 2 + 2 cycloaddition propagations and a ring
opening reaction that follows. In both reaction processes, the coor-
dination of PA on the metal center through the ring slipping of
(Cr) and 56.6 (Mo), respectively, indicating that the –phMe(CO) –
complexes catalyzed dehydrochlorination even in the gas phase.
In this reaction, HMM-phMo(CO)3 exhibited quite higher cat-
3
alytic performance than HMM-phCr(CO) . It has been reported
3
that arenetricarbonyl chromium and molybdenum complexes
act as catalysts for the dehydrochlorination reactions of organic

T. Kamegawa et al. / Catalysis Today 181 (2012) 14–19
19
compounds [40,41]. In these reactions, the tridentate arene ligand
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[
[
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bidentate ligand, creating coordinatively unsaturated sites which
play an important role as the catalytically active sites [40,41].
The observed high catalytic performance of HMM-phMo(CO)3 for
the dehydrochlorination of 2-chloro-2-methylbutane can thus be
ascribed to the relatively weak Mo–arene bond strength which
facilitates the partial dissociation of the arene ligand from the Mo
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