Thermal activation of solid-state molybdenum halide clusters with an octahedral cluster framework and their application to catalytic synthesis of 3-methylpyridine from piperidine and methanol

Article abstract of DOI:10.1246/bcsj.20150055

Solid-state molybdenum halide clusters with an octahedral metal framework MoX₂ (or [Mo₆X₈]X₂X_4/2) (X= Cl, Br, I) are applied to catalysis. When these clusters are thermally activated in a hydrogen stream above 300°C, they exhibit catalytic activity for the dehydrogenative C-methylation of piperidine with methanol, to yield 3-methylpyridine. At 400°C, the selectivity is as high as 74%. This catalytic behavior is different from that of the molecular clusters [(M₆Cl₁₂)Cl₂(H₂O)₄]?4H₂O (M= Nb, Ta) and (H₃O)₂[(M₆Cl₈)Cl₆]?6H₂O (M= Mo, W), which exhibit Bronsted acidity after thermal activation; piperidine is N-methylated to yield N-methylpiperidine selectively. Elemental analysis and thermogravimetric analysis demonstrate that the solid-state clusters partially eliminate halogen ligands during the activation. Infrared analysis of adsorbed pyridine on the activated clusters shows the presence of a Lewis acid site. This coordinatively unsaturated site of the molybdenum is catalytically active for dehydrogenative C-methylation. The formation of an η³-1-azaallyl species on the molybdenum facilitates the methylation at the 3-position of piperidine, followed by dehydrogenation to yield 3-methylpyridine.

Full text of DOI:10.1246/bcsj.20150055

Thermal Activation of Solid-State Molybdenum Halide Clusters with
an Octahedral Cluster Framework and Their Application to Catalytic
Synthesis of 3-Methylpyridine from Piperidine and Methanol
Satoshi Kamiguchi,*1,2 Ryu Kajio, Hitomi Yamada, Hidetaka Yuge,
3
4
4
5
5
3
3
Kazu Okumura, Hajime Iida, Sayoko Nagashima, and Teiji Chihara
1Advanced Catalysis Research Group, RIKEN Center for Sustainable Resource Science,
-1 Hirosawa, Wako, Saitama 351-0198
2
2Organometallic Chemistry Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198
3Graduate School of Science and Engineering, Saitama University, Shimo-Okubo, Sakura-ku, Saitama 338-8570
4School of Science, Kitasato University, Kitasato, Minami-ku, Sagamihara, Kanagawa 252-0373
5School of Advanced Engineering, Kogakuin University, Nakano-machi, Hachioji, Tokyo 192-0015
E-mail: kamigu@riken.jp
Received: February 23, 2015; Accepted: April 24, 2015; Web Released: August 15, 2015
Solid-state molybdenum halide clusters with an octahedral metal framework MoX2 (or [Mo6X8]X2X4/2) (X = Cl,
Br, I) are applied to catalysis. When these clusters are thermally activated in a hydrogen stream above 300 °C, they exhibit
catalytic activity for the dehydrogenative C-methylation of piperidine with methanol, to yield 3-methylpyridine. At
400 °C, the selectivity is as high as 74%. This catalytic behavior is different from that of the molecular clusters
[(M6Cl12)Cl2(H2O)4]¢4H2O (M = Nb, Ta) and (H3O)2[(M6Cl8)Cl6]¢6H2O (M = Mo, W), which exhibit Brønsted acidity
after thermal activation; piperidine is N-methylated to yield N-methylpiperidine selectively. Elemental analysis and
thermogravimetric analysis demonstrate that the solid-state clusters partially eliminate halogen ligands during the
activation. Infrared analysis of adsorbed pyridine on the activated clusters shows the presence of a Lewis acid site. This
coordinatively unsaturated site of the molybdenum is catalytically active for dehydrogenative C-methylation. The
3
formation of an η -1-azaallyl species on the molybdenum facilitates the methylation at the 3-position of piperidine,
followed by dehydrogenation to yield 3-methylpyridine.
One of the most challenging subjects in catalysis is the
development of new reactions over new catalysts. Since the
first report on molybdenum(II) chloride in 1859, many halide
[(M Cl )Cl (OH)(H O)] above 250 °C, with the evolution
6 8 3 2
of hydrogen chloride (eq 2). The resulting clusters exhibit
Brønsted acidity at the hydroxo ligand, which is the catalytic
active site (eq 3). Similarly, in the cases of the Group 5 metal
clusters, the catalytic activity is attributable to the Brønsted-
acidic hydroxo ligand on the activated clusters [(M Cl )-
1
clusters have been synthesized by combining 19 types of Group
3
­7 transition metals with four types of halogen atoms as
2
­6
ligands. Studies on these clusters have focused mainly on
syntheses and structures. The halide clusters have character-
istic features as catalysts: metal­metal bonds that are similar
to those in bulk metals, multi-center and multi-electron sys-
tems, unique intermediate oxidation states of the metal atoms,
high thermal stability, and high melting point and low vapor
pressure as molecular metal halides. However, there have been
no reports on the application of the clusters to catalysis.
We have reported on a wide variety of catalytic reactions over
molecular Group 5 metal clusters with aqua ligands, [(M X )-
6
12
8
Cl(OH)(H O) ]. In contrast, no Lewis acidity attributable to
2
3
9
,10
the metal atom is observed for these activated clusters.
ðH3OÞ2½ðM6Cl8ÞCl6ꢀ¢6H2O
!
½ðM6Cl8ÞCl4ðH2OÞ2ꢀ þ 2HCl þ 6H2O
ð1Þ
ð2Þ
ð3Þ
½
½
ðM6Cl8ÞCl4ðH2OÞ2ꢀ
!
½ðM6Cl8ÞCl3ðOHÞðH2OÞꢀ þ HCl
ðM6Cl8ÞCl3ðOHÞðH2OÞꢀ
þ
ꢁ
!
H þ ½ðM6Cl8ÞCl3ðOÞðH2OÞꢀ
6
12
X (H O) ]¢4H O (M = Nb, Ta; X = Cl, Br), and Group 6 metal
In this study, solid-state molybdenum halide clusters, MoX
2
2
2
4
2
clusters of oxonium salts, (H O) [(M Cl )Cl ]¢6H O (M = Mo,
W). The majority of the reactions are Brønsted acid catalyses.
(or [Mo X ]X X4/2) (X = Cl, Br, I), were used as catalyst pre-
6 8 2
3
2
6
8
6
2
7
cursors. These clusters have octahedral Mo metal frameworks
6
In the cases of the Group 6 metal clusters, the mechanism for
the development of the catalytically active sites have been
explained as follows. When the clusters are thermally activated
in a gas stream, they first change to aqua clusters [(M Cl )-
that are linked by bridging halide ligands to form a two-dimen-
sional array of a cluster network. No aqua ligand or oxonium
cation is included in these binary clusters. Hence, if the halogen
ligands are removed, the resulting coordinatively unsaturated
molybdenum atom can act as the catalytic active site.
2
6
8
Cl (H O) ] at 150 °C (eq 1) and then to hydroxo clusters
4
2
2
1116 | Bull. Chem. Soc. Jpn. 2015, 88, 1116–1122 | doi:10.1246/bcsj.20150055
© 2015 The Chemical Society of Japan

This paper describes the activation of the clusters and their
application to the catalytic dehydrogenative C-methylation of
piperidine with methanol to yield 3-methylpyridine. To date,
there has been only one report on this C-methylation, which is
catalyzed by mixed-metal catalysts such as Cu/Cr and Ni­Cr­
glass fixed-bed microreactor with a continuous gas flow system
1
4
was operated at atmospheric pressure. A weighed crystalline
cluster sample (20 mg) of 3 was packed in a borosilicate glass
tube (3 mm i.d.) with the aid of quartz glass wool and placed
in the center of an electric furnace. The catalyst was initially
heated from room temperature to a fixed temperature between
11
C­Na SO /SiO .
2
4
2
¹
1
2
50 and 450 °C over 15 min in a hydrogen stream (1.2 L h ),
Experimental
Materials. Solid-state molybdenum halide cluster com-
and then it was held at this temperature for 45 min. The reaction
¹
1
was initiated by feeding a mixture of piperidine (23 ¯L h ,
1
2
12
¹1
¹1
¹1
pounds [Mo Cl ]Cl Cl
(1), [Mo Br ]Br Br (2), and
4/2
0.25 mmol h ) and methanol (203 ¯L h , 5.0 mmol h ) into
the hydrogen stream using a syringe pump, without changing
the temperature. The reaction was monitored every 30 min by
sampling the reaction gas (1 mL) via a six-way valve kept
at 125 °C, followed by analysis using the on-line GLC. The
reactor effluent was frozen in an ice-water trap for determi-
nation of the material balance and subsequent analysis of the
products. Catalytic reactions using the other catalysts were
performed in the same way.
6
8
1
2
4/2
6
8
2
3
[
Mo I ]I I
(3) were synthesized according to published
6
8
2 4/2
procedures, and crystals of these clusters were crushed for use
as catalysts. The organic substrates were commercial products
(
>99%), and were employed as received. Silica­alumina (JRC-
SAL-2, Catalysts & Chemicals Industries Co., Ltd.) and H-
beta (JRC-Z-HB25(1), Sud-Chemie Catalysts Japan, Inc.) were
obtained from Catalysis Society of Japan as reference catalysts.
Characterization. Scanning electron microscopy (SEM)
was carried out using a HITACHI S-4200 microscope (accel-
eration voltage of 5 kV). Powder X-ray diffraction (XRD) data
were recorded using a Rigaku RINT-2000 X-ray diffractometer
Conversion and selectivity are defined as follows: con-
version = products/(products + recovered piperidine) © 100
(mol %), and selectivity = product/(total amount of prod-
ucts) © 100 (mol %), based on piperidine.
¹
1
employing Cu Kα radiation and a scan rate of 2.5° min .
Raman spectra of the cluster samples (20 mg) in a glass reaction
tube were recorded in situ using a Kaiser Optical Systems
HoloLab 5000 spectrometer and an Nd:YAG laser operating at
Results and Discussion
Catalytic Reactions. Pulverized crystals of 3 were placed
in a glass reaction tube and activated at 400 °C in a hydrogen
stream for 1 h. The reaction was initiated by introduction of a
mixture of piperidine and methanol, in a mole ratio of 1:20,
into the hydrogen stream at 400 °C. The reaction profile is
plotted in Figure 1. The catalytic activity decreased with time.
As the SEM images of 3 show (Figure 2), there was practically
no change in the particle size during the reaction: the primary
particle size after both the activation and the subsequent 3-h
reaction was ca. 100 nm. The deactivation was probably due to
coke deposition on the catalyst. Coke formed from piperidine
is reported to deactivate in a gas­solid-phase reaction under
­
= 532 nm, with a 7.6 mm focusing lens. The data counts were
accumulated 30 times in 1-s intervals. Attenuated total reflec-
tion Fourier transform infrared (ATR­FTIR) spectra (4000­650
¹
1
cm ) were recorded using a SensIR Technologies Travel-IR
with a ZnSe beam splitter. Thermogravimetric (TG) analyses
were performed on a Seiko TG/DTA 6200 instrument. Cluster
samples (ca. 30 mg) were loaded into a platinum crucible and
¹1
heated from room temperature at a rate of 10 °C min in a
¹
1
hydrogen flow (15 L h ). Brunauer­Emmett­Teller (BET) sur-
face area was measured by physical adsorption of dinitrogen
at 77 K using a Bel Japan BELSORP-max. The temperature-
programmed desorption (TPD) of ammonia was measured using
a Bel Japan TPD-1-AT(NH3) equipped with a mass spectrom-
eter (MS) for continuous analysis of the desorbed species.
Cluster samples (ca. 30 mg) were placed in a quartz tube and
purged with helium, followed by activation at 400 °C for 1 h in a
hydrogen stream. Ammonia vapor (100 mmHg) was introduced
to the samples at 100 °C, and excess ammonia was removed by
evacuation. The samples were then heated from 100 to 500 °C at
1
5
hydrogen. Catalyst deactivation by coke formation is inevi-
1
6
table in gas­solid-phase reactions at higher temperatures.
2
2
1
1
5
0
5
0
70
60
50
¹
1
40
a heating rate of 10 °C min in a helium stream. The amount of
desorbed ammonia was measured with the MS detector (m/z =
3
2
0
0
1
6). Elemental analyses were carried out by the Materials
Characterization Team at RIKEN. Catalytic reaction products
were analyzed using a gas-liquid chromatograph (GLC) (GL
Sciences 353B gas chromatograph fitted with a flame ionization
detector with a DB-1 capillary column) and a GC/MS (Hewlett-
Packard 5890 gas chromatograph coupled with a Jeol Automass
System II, with a DB-1 capillary column). They were identified
by comparison with authentic samples. Catalytic activities were
determined using an on-line GLC (Shimadzu GC14B gas
chromatograph fitted with a flame ionization detector with an
Amipack packed column).
5
10
0
0
0
1
2
3
4
5
6
7
t / h
Figure 1. A typical reaction profile of piperidine with
methanol catalyzed by [Mo6I8]I2I4/2 (3) in a hydrogen
stream at 400 °C. Legend: conversion ( ); selectivity for 3-
methylpyridine ( ); selectivity for N-methylpiperidine ( );
and selectivity for pyridine ( ).
Catalytic Measurements. The general procedure for the
catalytic reaction is described for 3. A conventional vertical
Bull. Chem. Soc. Jpn. 2015, 88, 1116–1122 | doi:10.1246/bcsj.20150055
© 2015 The Chemical Society of Japan | 1117

CH
3
+
CH3OH
+
H2O
+
3H2
N
N
H
Scheme 1.
a
b
+
CH3OH
+
H2O
N
N
H
CH
3
Scheme 2.
200 nm
200 nm
Hence, the selectivity was dependent not on the activation
temperature but on the reaction temperature. According to Le
Chatelier’s principle, a reaction that increases the number of
molecules is thermodynamically favorable at higher temper-
atures under constant pressure. The temperature-dependent
selectivity is consistent with this principle: the number of mole-
cules remains unchanged in the N-methylation but increases in
the dehydrogenative C-methylation.
Figure 2. SEM images of [Mo6I8]I2I4/2 (3): (a) after 1-h
activation with hydrogen at 400 °C and (b) after subsequent
3-h reaction with piperidine and methanol in a hydrogen
stream at 400 °C.
2
2
1
1
5
0
5
0
70
6
5
4
3
2
1
0
0
0
0
0
0
Table 1 shows the catalytic activity of some halide clusters
and related compounds treated at 400 °C. All the solid-state
molybdenum clusters 1­3 exhibited catalytic activity (Entries
1­3), in spite of their small BET surface area (1.2, 1.2, and
2
¹1
3
.6 m g for 1, 2, and 3, respectively). The main reaction was
the dehydrogenative C-methylation, which yielded 3-methyl-
pyridine with 59­74% selectivity. In contrast, the molecular
clusters exhibited different selectivities. The niobium, tantalum,
and tungsten clusters selectively catalyzed the N-methylation
to yield N-methylpiperidine (Entries 4, 6, and 7). The molecular
molybdenum chloride cluster yielded 3-methylpyridine as the
main product; however, its selectivity (54%) was much lower
than that of the solid-state molybdenum chloride 1 (74%)
5
0
0
250
300
350
400
450
Reaction temperature / °C
Figure 3. Effect of temperature on activity and selectivity
in the reaction of piperidine with methanol over [Mo6I8]-
I2I4/2 (3) in a hydrogen stream at 3 h after the start of the
reaction. Legend: conversion ( ); selectivity for 3-methyl-
pyridine ( ); selectivity for N-methylpiperidine ( ); and
selectivity for pyridine ( ).
(
Entry 5). As we previously reported, these molecular clusters
supported on silica gel predominantly catalyzed the N-methyl-
ation of aliphatic amines, including piperidine. Table 1 also
shows the catalytic activity of conventional Brønsted acids such
as silica­alumina and H-beta, which exclusively catalyzed the
N-methylation (Entries 8 and 9). Thus, the catalytic behavior of
17
1
­3 differed from that of the Brønsted acids.
Catalytic Active Sites. Figure 4 illustrates the XRD
3
-Methylpyridine was obtained with a nearly constant selec-
tivity of 64% after a period of 3 h (Scheme 1). The turnover
frequency (TOF) per cluster during the reaction period of 3­4 h
was 3.78 h , assuming that all the cluster cores were active.
The material balance was 94% at 3­4 h after the start of the
reaction.
The effect of the reaction temperature on activity and
selectivity over 3 is shown in Figure 3, where the activation
and the subsequent reaction were performed without changing
the temperature. The catalytic activity developed above 300 °C,
and it increased with increasing temperature. The main reaction
changed, depending on the temperature: N-methylation afford-
ed N-methylpiperidine below 350 °C (Scheme 2), and dehy-
drogenative C-methylation afforded 3-methylpyridine above
patterns of the solid-state clusters 1­3. All of these clusters
exhibited no appreciable changes in the XRD patterns after the
activation and the subsequent 3-h reaction of piperidine with
methanol at 400 °C; no formation of Mo metal was observed.
The crystal structures of 1­3 were retained throughout the activ-
ation and the reaction. The Raman spectra of 1­3 are shown in
¹1
¹1
Figure 5. The chloride 1 exhibited a peak at 199 cm , assign-
1
8
able to the breathing motion of the Mo octahedron (T ), and
6
2g
this peak was retained after the activation and the subsequent 3-
h reaction. Similarly, a peak attributable to the Mo octahedron
6
1
9
20
(A ) for the bromide 2 and iodide 3 was retained after the
1
g
activation and the reaction. These observations demonstrate that
the Mo6 cluster frameworks of 1­3 remained intact throughout
the catalysis.
4
00 °C (Scheme 1). When 3 activated at 400 °C was subjected
to the reaction at 300 °C, the selectivity was similar to that
when the treatments were both performed at 300 °C: the selec-
tivity for N-methylpiperidine was 72% at 7% conversion.
When 1­3 were heated progressively in a hydrogen stream,
evolution of hydrogen halide was detected above 300 °C,
at which temperature the catalytic activity of 3 developed
1118 | Bull. Chem. Soc. Jpn. 2015, 88, 1116–1122 | doi:10.1246/bcsj.20150055
© 2015 The Chemical Society of Japan

Table 1. Methylation of Piperidine over Various Halide Clusters and Related Compoundsa)
Selectivity/%d)
Entry
Catalyst
Conversion/%b)
TOF/h¹1 c)
3
-Methylpyridine
N-Methylpiperidine
Otherse)
1
2
3
4
5
6
7
8
9
[Mo6Cl8]Cl2Cl4/2 (1)f)
[Mo6Br8]Br2Br4/2 (2)
[Mo6I8]I2I4/2 (3)
[(Nb6Cl12)Cl2(H2O)4]¢4H2O
(H3O)2[(Mo6Cl8)Cl6]¢6H2O
[(Ta6Cl12)Cl2(H2O)4]¢4H2O
(H3O)2[(W6Cl8)Cl6]¢6H2O
SiO2­Al2O3 (low alumina)
H-beta
17.7
21.2
14.4
9.7
9.2
44.8
9.9
99.8
98.8
6.3
1.11
4.07
3.78
1.45
1.40
9.67
2.16
74.2
58.7
64.0
21.5
53.7
0.8
15.1
0.0
0.0
23.3
30.5
31.2
77.2
27.5
98.5
78.7
98.8
92.0
36.1
2.5
10.8
4.8
1.3
18.8
0.7
6.2
1.2
8.0
24.9
1
0
Mo metal
0.98g)
39.0
11
None
0.0
a) Catalyst 20 mg. At 400 °C and 3 h after the start of the reaction. b) Conversion = products/(products + piperidine) © 100 (mol %)
based on piperidine. c) Turnover frequency per cluster core at 3­4 h after the start of the reaction based on piperidine. d) Selectivity =
product/(total amount of products) © 100 (mol %) based on piperidine. e) 3,4,5.6-Tetrahydropyridine, pyridine, etc. f) Catalyst 40 mg.
g) Per metal atom.
Mo metal
[Mo
6
Cl
8
]Cl
2
Cl4/2 (1)
T2g, Mo6
[
6 8 2
Mo Cl ]Cl Cl4/2 (1)
After reaction
After activation
Untreated
After reaction
After activation
Untreated
[
Mo
6
Br
8
]Br
2
Br4/2 (2)
4/2 (3)
40
[
6 8 2
Mo Br ]Br Br4/2 (2)
A1g, Mo6
After reaction
After activation
Untreated
After reaction
After activation
[
Mo
6 8 2
I ]I I
Untreated
After reaction
After activation
Untreated
[Mo
6 8 2
I ]I I4/2 (3)
A1g, Mo6
0
10
20
30
50
60
After reaction
After activation
Untreated
θ
2 /°
Figure 4. XRD patterns of [Mo6Cl8]Cl2Cl4/2 (1), [Mo6Br8]-
Br2Br4/2 (2), and [Mo6I8]I2I4/2 (3) before activation, after
1
-h activation with hydrogen at 400 °C, and after sub-
350
300
250
200
150
Raman shift / cm-
1
sequent 3-h reaction with piperidine and methanol in a
hydrogen stream at 400 °C. The XRD pattern of molybde-
num metal is also shown.
Figure 5. Raman spectra of [Mo6Cl8]Cl2Cl4/2 (1),
Mo6Br8]Br2Br4/2 (2), and [Mo6I8]I2I4/2 (3) before activa-
[
tion, after 1-h activation with hydrogen at 400 °C, and after
subsequent 3-h reaction with piperidine and methanol in a
hydrogen stream at 400 °C.
(
1
Figure 3). Elemental analysis revealed that the Mo content of
, 2, and 3 increased by 2.1, 0.2, and 1.6%, respectively, after
the activation at 400 °C. These observations also revealed the
partial loss of the halogen ligands, as they are binary com-
pounds. Figure 6 illustrates the TG profiles of 1­3 treated in a
hydrogen stream. All the clusters exhibited a slight weight loss
up to 470 °C, followed by a significant one-step weight loss
above 470 °C. The total weight losses of 42, 61, and 71% for
Figure 7 shows the FT-IR spectra of 1­3 after the activation
in a hydrogen stream at 400 °C and the subsequent adsorption of
pyridine. None of these clusters exhibited peaks at 1634, 1536,
¹1
and 1486 cm assignable to pyridinium ion, which is indica-
tive of the absence of a Brønsted acid site. In contrast, a peak
¹
1
1
, 2, and 3, respectively, correspond to the almost complete
loss of the halogen ligands. The formation of Mo metal above
50 °C for 1­3 was confirmed by XRD analysis.
at around 1445 cm assignable to coordinatively bonded pyri-
2
1
dine was observed in these spectra, which indicates the devel-
opment of coordinatively unsaturated sites on the molybdenum
5
Bull. Chem. Soc. Jpn. 2015, 88, 1116–1122 | doi:10.1246/bcsj.20150055
© 2015 The Chemical Society of Japan | 1119

atom of 1­3. The order of the peak intensity (1 > 2 > 3) was the
practically no 3-methylpyridine was formed (Scheme 4b).
These observations show that the dehydrogenative C-methyl-
ation did not proceed via 3-methylpiperidine. When piperidine
alone was allowed to react under the same reaction conditions,
a partial dehydrogenation product 3,4,5,6-tetrahydropyridine
was obtained almost exclusively, and almost no pyridine was
formed (Scheme 4c). The reaction of pyridine with methanol
was attempted, but 3-methylpyridine was not obtained at all
(Scheme 4d). These observations show that the dehydrogen-
ative C-methylation did not proceed via pyridine, although
pyridine has been proposed as an intermediate for the dehydro-
same as that of the acid amount detected using the NH -TPD:
3
¹1
0
.80, 0.37, and 0.04 mol kg for 1, 2, and 3, respectively.
Taking into account all the characterization results described
above, the mechanism for the development of an active site
on 1­3 can be illustrated as shown in Scheme 3. When these
clusters were heated in a hydrogen stream at around 400 °C, the
halogen ligand was partially removed as hydrogen halide, with
the retention of both the crystal structure and the octahedral
Mo metal framework. The resulting coordinatively unsaturated
6
sites on the molybdenum atom would be catalytically active for
the dehydrogenative C-methylation.
11
genative C-methylation over the mixed-metal catalysts. It has
been reported that some secondary aldimines are deprotonated
at the 2-position to yield 1-azaallyl anions, which are sub-
sequently alkylated at the 2-position with alkyl cation formed
Reaction Mechanism. Piperidine and related compounds
were allowed to react over 3, and the results are summarized
in Scheme 4. The reaction of piperidine with methanol did not
yield 3-methylpiperidine (Scheme 4a). When 3-methylpiperi-
dine was allowed to react under the same reaction conditions,
[
6 8 2
Mo Cl ]Cl Cl4/2 (1)
After activation
After activation
1
1
1
00
0
.4%
.5%
.4%
+
pyridine
8
6
4
2
0
0
0
0
12Cl
2%
6
Mo
[
6 8 2
Mo Cl ]Cl Cl4/2 (1)
1443
00
0
[
6 8 2
Mo Br ]Br Br4/2 (2)
8
6
4
2
0
0
0
0
0
After activation
1
2Br
After activation
+ pyridine
2%
[
Mo
6 8 2
Br ]Br Br4/2 (2)
6Mo
1445
00
[Mo
6
I
8
]I
2
I4/2 (3)
0
After activation
8
6
4
2
0
0
0
0
1
2I
After activation
+ pyridine
2%
1447
[Mo
6
I
8
]I
2
I4/2 (3)
6
Mo
1
600
1500
1400
ν
/ cm-
1
0
200
400
600
800
T / °C
Figure 7. FT-IR spectra of pyridine adsorbed on [Mo6Cl8]-
Cl2Cl4/2 (1), [Mo6Br8]Br2Br4/2 (2), and [Mo6I8]I2I4/2
(3) after 1-h activation with hydrogen.
Figure 6. TG profiles of [Mo6Cl8]Cl2Cl4/2 (1), [Mo6Br8]-
Br2Br4/2 (2), and [Mo6I8]I2I4/2 (3) in a hydrogen stream.
X
Mo
H
2
HX
Scheme 3.
1120 | Bull. Chem. Soc. Jpn. 2015, 88, 1116–1122 | doi:10.1246/bcsj.20150055
© 2015 The Chemical Society of Japan

X
(
(
(
a)
b)
c)
CH
CH
3
Mo
X
1
4.4% conversion
0.0% selectivity
+
CH3OH
N
H
N
H
H
2
CH
3
HX
CH
3
3
1
3
4.4% conversion
0.0% selectivity
N
N
H
N
H
N
CH
3
Mo
N
N
H
Mo
Mo
9.6% conversion
8.4% selectivity
9
H
2
H
2
N
H
N
N
1
.2% selectivity
CH
3
N
N
Mo
Mo
(
d)
CH
3
2
.2% conversion
0.0% selectivity
+
CH3OH
CH
3
H
N
N
N
N
H
2
Mo
H
Mo
H
Scheme 4.
+
3+
CH
from an alkyl halide.22 Similarly, the transfer of hydrogen at the
-position of the 3,4,5,6-tetrahydropyridine to the molybdenum
H
2
H O
3
3
CH
3
OH
yields an η -1-azaallyl group, which could be methylated at the
-position with methyl cation formed from methanol; there are
+
H
3
3
23
some molybdenum complexes with an η -1-azaallyl ligand,
which is the nitrogen analogue of η -oxoallyl ligand.
Scheme 5.
3
24
Taking these reaction results into consideration, a plausible
mechanism of the dehydrogenative C-methylation over the
molybdenum clusters is illustrated in Scheme 5. When the coor-
dinatively unsaturated cluster is formed by the activation under
hydrogen, the average formal oxidation number of the molyb-
denum decreases from +2 to +1.67, assuming that two halogen
Conclusion
When the solid-state molybdenum halide clusters [Mo X ]-
X X (X = Cl (1), Br (2), I (3)) were thermally activated in
6
8
2
4/2
a hydrogen stream above 300 °C, the halogen ligands were
partially removed as hydrogen halide, leading to the formation
of the coordinatively unsaturated Lewis acidic molybdenum
atom. The activated clusters catalyzed the dehydrogenative
C-methylation of piperidine with methanol to yield 3-methyl-
pyridine. The selective methylation at the 3-position is attrib-
utable to the intermediate formation of the η -1-azaallyl group
coordinating to the molybdenum atom. There are no reports for
the dehydrogenative C-methylation via the η -1-azaallyl group.
ligands are removed from the Mo core in the activation. The
6
catalytic cycle is initiated by coordination of piperidine to
the coordinatively unsaturated molybdenum atom, succeeded
by dehydrogenation to yield 3,4,5,6-tetrahydropyridine which
coordinates to the molybdenum atom through a π-bond. Clusters
of electron-poor early transition metals are favorably coordi-
nated by π-donor ligands, which can occupy either a terminal, a
μ2-briding, or a μ3-face-capping site.25 The hydrogen of the
tetrahydropyridine at the 3-position is eliminated to form an
3
3
This selectivity was different from the selectivities of the
molecular clusters [(M Cl )Cl (H O) ]¢4H O (M = Nb, Ta),
6
12
2
2
4
2
3
η -1-azaallyl group and a face-capping hydride. The formal
and (H O) [(M Cl )Cl ]¢6H O (M = Mo, W) which exhibit
3 2 6 8 6 2
oxidation number of the molybdenum reverts to +2. Then, the
Brønsted acidity.
3
-position carbon is methylated by methyl cation formed from
methanol. The formation of methyl cation intermediate from
This work was supported by a Grant-in-Aid (No. 25410079)
for Scientific Research (C) from the Ministry of Education,
Culture, Sports, Science and Technology.
methanol has been proposed in various methylations,2
6,27
some
of which are catalyzed by molecular metal halide clusters, as we
9
have previously reported: aromatic methylation of toluene, N-
18
methylation of aliphatic and aromatic amines, 2-methylation
of pyridine, and O- and C-methylation of phenol.28 Then, three
hydrogens of the ring on the molybdenum atom are eliminated
to yield 3-methylpyridine.
References
1
2
W. Blomstrand, J. Prakt. Chem. 1859, 77, 88.
S. C. Lee, R. H. Holm, Angew. Chem., Int. Ed. Engl. 1990,
29, 840.
Bull. Chem. Soc. Jpn. 2015, 88, 1116–1122 | doi:10.1246/bcsj.20150055
© 2015 The Chemical Society of Japan | 1121

3
4
5
6
7
J. D. Corbett, Pure Appl. Chem. 1990, 62, 103.
17 S. Kamiguchi, I. Takahashi, S. Nagashima, A. Nakamura,
T. Chihara, J. Cluster Sci. 2007, 18, 935.
18 D. Bublitz, W. Preetz, Z. Anorg. Allg. Chem. 1996, 622,
1107.
19 W. Preetz, D. Bublitz, H. G. Von Schnering, J.
Saßmannshausen, Z. Anorg. Allg. Chem. 1994, 620, 234.
20 P. Brückner, W. Preetz, M. Pünjer, Z. Anorg. Allg. Chem.
1997, 623, 8.
21 S. J. Hilsenbeck, V. G. Young, Jr., R. E. McCarley, Inorg.
Chem. 1994, 33, 1822.
22 P. Sulmon, N. De Kimpe, N. Schamp, Synthesis 1989, 8.
23 P. J. Daff, A. Monge, P. Palma, M. L. Poveda, C. Ruiz, P.
Valerga, E. Carmona, Organometallics 1997, 16, 2263.
24 Y. Takagi, S. Teratani, K. Tanaka, J. Catal. 1972, 27, 79.
25 P. Atkins, T. Overton, J. Rourke, M. Weller, F. Armstrong,
Shriver & Atkins Inorganic Chemistry, 5th ed., Oxford University
Press, New York, 2010, p. 470.
26 J.-H. Kim, T. Kunieda, M. Niwa, J. Catal. 1998, 173, 433.
27 H. Vinek, M. Derewinski, G. Mirth, J. A. Lercher, Appl.
Catal. 1991, 68, 277.
J. D. Corbett, Pure Appl. Chem. 1992, 64, 1395.
J. D. Corbett, NATO ASI Ser., Ser. C 1992, 382, 27.
N. Prokopuk, D. F. Shriver, Adv. Inorg. Chem. 1998, 46, 1.
S. Nagashima, S. Kamiguchi, T. Chihara, Metals 2014,
4
, 235.
8
, 84.
9
S. Kamiguchi, S. Nagashima, T. Chihara, Metals 2014,
S. Kamiguchi, S. Nishida, H. Kurokawa, H. Miura, T.
4
Chihara, J. Mol. Catal. A: Chem. 2005, 226, 1.
S. Kamiguchi, T. Mori, M. Watanabe, A. Suzuki, M.
1
0
Kodomari, M. Nomura, Y. Iwasawa, T. Chihara, J. Mol. Catal. A:
Chem. 2006, 253, 176.
1
1
A. B. Tereshko, V. A. Tarasevich, N. G. Kozlov, Russ. J.
Org. Chem. 1995, 31, 258.
12
13
14
P. Nannelli, B. P. Block, Inorg. Synth. 1970, 12, 170.
J. C. Sheldon, J. Chem. Soc. 1962, 410.
S. Kamiguchi, M. Noda, Y. Miyagishi, S. Nishida, M.
Kodomari, T. Chihara, J. Mol. Catal. A: Chem. 2003, 195, 159.
G. C. Hadjiloizou, J. B. Butt, J. S. Dranoff, J. Catal. 1992,
35, 27.
G. F. Froment, Catal. Rev.: Sci. Eng. 2008, 50, 1.
1
5
1
28 S. Nagashima, S. Kamiguchi, S. Ohguchi, T. Chihara,
J. Cluster Sci. 2011, 22, 647.
1
6
1122 | Bull. Chem. Soc. Jpn. 2015, 88, 1116–1122 | doi:10.1246/bcsj.20150055
© 2015 The Chemical Society of Japan