Novel Schiff-base complexes of methyltrioxorhenium (VII) and their performances in epoxidation of cyclohexene

Article abstract of DOI:10.1016/j.jorganchem.2009.06.034

Methyltrioxorhenium (MTO) forms complexes with Schiff-bases derived from 2-pyridinecarboxaldehyde and amines. These complexes were isolated and fully characterized by NMR, IR, UV-Vis, EA, MS. Two were analyzed by X-ray crystallography, which showed that the compounds display distorted octahedral geometry in the solid state with a trans-position of Schiff-base ligand. The characterized results indicated that the more Lewis basic the ligand is, the stronger the metal-ligand interaction between the rhenium atom and the ligand. The complexes displayed high catalytic activity and selectivity when applied to the epoxidation of cyclohexene with urea hydrogen peroxide adduct (UHP) as oxidant in methanol, but poor performances with hydrogen peroxide (30%) as oxidant due to their decomposition. Experimental results revealed that the MTO Schiff-base complexes are, in general, more sensitive to water than MTO itself. Moreover, large excess of ligand is detrimental to the catalytic performance as it leads to the decomposition of the complexes.

Full text of DOI:10.1016/j.jorganchem.2009.06.034

Journal of Organometallic Chemistry 694 (2009) 3418–3424
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Novel Schiff-base complexes of methyltrioxorhenium (VII) and their performances
in epoxidation of cyclohexene
Chuan-Jiang Qiu, Yue-Cheng Zhang, Yu Gao, Ji-Quan Zhao *
School of Chemical Engineering, Hebei University of Technology, Tianjin 300130, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 May 2009
Received in revised form 24 June 2009
Accepted 24 June 2009
Available online 28 June 2009
Methyltrioxorhenium (MTO) forms complexes with Schiff-bases derived from 2-pyridinecarboxaldehyde
and amines. These complexes were isolated and fully characterized by NMR, IR, UV–Vis, EA, MS. Two
were analyzed by X-ray crystallography, which showed that the compounds display distorted octahedral
geometry in the solid state with a trans-position of Schiff-base ligand. The characterized results indicated
that the more Lewis basic the ligand is, the stronger the metal–ligand interaction between the rhenium
atom and the ligand. The complexes displayed high catalytic activity and selectivity when applied to the
epoxidation of cyclohexene with urea hydrogen peroxide adduct (UHP) as oxidant in methanol, but poor
performances with hydrogen peroxide (30%) as oxidant due to their decomposition. Experimental results
revealed that the MTO Schiff-base complexes are, in general, more sensitive to water than MTO itself.
Moreover, large excess of ligand is detrimental to the catalytic performance as it leads to the decompo-
sition of the complexes.
Keywords:
Methyltrioxorhenium
2
-Pyridinecarboxaldehyde
Schiff-base
Epoxidation
Cyclohexene
UHP
Ó 2009 Elsevier B.V. All rights reserved.
1
. Introduction
showed that MTO forms (distorted) octahedral adducts with biden-
tate Lewis bases, e.g. 2,2-bipyridine and 1,10-phenanthroline [33].
Methyltrioxorhenium (CH
lytic properties in numerous oxy functionalization reactions [1–
,10]. Among all of them MTO-catalyzed epoxidation of olefins
with hydrogen peroxide as oxidant has received particular atten-
tion since the process is environmentally friendly [4,11]. However,
due to the Lewis acidity of the rhenium center, a major limitation
of this system is the opening of the epoxide ring leading to the for-
mation of diols in the presence of water during the reaction
3
ReO
3
, MTO) has outstanding cata-
The complexes showed high catalytic activity and selectivity in the
epoxidation of olefins with H
as oxidant (Scheme 1).
2 2
O
9
On the other hand, Schiff-bases are of great importance in
homogeneous and heterogeneous reactions. They are easily syn-
thesised by the condensation of primary amines and aldehydes
or ketones and form complexes with transition metals via nitrogen
lone pair electrons. The catalytic activity of metal complexes has
been analyzed critically in various reactions such as polymeriza-
tion, oxidation, epoxidation, reduction of ketones, allylic alkylation
and Michael addition reactions [35]. The activity of these com-
plexes varied with the type of ligands, coordination sites and metal
ions.
Despite the large numbers of Lewis base adducts of MTO de-
scribed in the literature, only a few examples of complexes with
Schiff-bases have been reported [32,33]. Considering the outstand-
ing catalytic properties of MTO and the excellent corresponding
ability of Schiff-bases, it is very interesting to synthesize some
Schiff-bases especially the bidentate ones and their complexes
with MTO. To our knowledge, 2-pyridinecarboxaldehyde like other
aldehydes is also able to condense with amines to form Schiff-
bases. Combined with the nitrogen atom in the pyridine moiety
the generated Schiff-bases can serve as bidentate ligands which
can coordinate with metal atoms to form the corresponding com-
plexes as shown in Scheme 2. Therefore, herein we first synthesize
some Schiff-bases from 2-pyridinecarboxaldehyde and their corre-
sponding MTO complexes. Then the complexes are applied to the
[
2,3,12]. Several methods have been suggested to overcome this
problem. An efficient procedure developed to avoid this side reac-
tion requires the use of urea hydrogen peroxide adduct (UHP) as
oxidant, which enables epoxidation to be carried out in non-aque-
ous media [13–15]. A second alternative is based on the tendency
of organorhenium (VII) oxides to form Lewis acid–base adducts to
increase their coordination numbers [16–19]. Some research re-
vealed that the aromatic N-base ligands work by coordinating to
the metal center, thereby reducing the Lewis acidity of the catalyst
and accelerating the catalytic reactions additionally [17,18,20–26].
Lewis base adducts of MTO were first mentioned in 1989 by Herr-
mann et al. [27,28]. It was reported that MTO forms trigonal-bipy-
ramidal adducts with aromatic N-donor bases and certain Schiff-
bases [17,29–32]. Several bidentate Lewis base adducts of MTO
have been synthesised and fully characterized too [33,34]. Results
*
Corresponding author. Tel.: +86 22 60204279; fax: +86 22 26564733.
E-mail address: zhaojq@hebut.edu.cn (J.-Q. Zhao).
0
022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.06.034

C.-J. Qiu et al. / Journal of Organometallic Chemistry 694 (2009) 3418–3424
3419
differences can become very large (167 cmꢀ1) due to the rather
asymmetric coordination of the rhenium atom [31]. Therefore,
we conclude that complexes I–IV have an octahedral geometry,
which was confirmed by the crystal structure analysis late.
ꢀ1
Generally, C@N has a medium or strong band around 1630 cm
Scheme 1. Lewis base adducts of MTO.
for its stretching vibrations in an undisturbed situation in the FT-IR
spectra [31,39]. For the synthesised Schiff-bases (a–d) strong
ꢀ
1
bands between 1626 and 1650 cm were observed and the corre-
ꢀ
1
sponding bands were located between 1621 and 1650 cm
slightly lower than those of the Schiff-bases (shown in Table 1).
The UV–Vis absorption spectra of compounds a–d and I–IV in
methanol at room temperature are shown in Table 2. The absorp-
*
Scheme 2. Complex of transition metal with Schiff-base.
tion bands between 228 and 236 nm are derived from the p ? p
transition of imine group in the Schiff-bases, and the bands be-
*
p ? p transition in the pyr-
tween 271 and 283 nm are due to the
epoxidation of cyclohexene with UHP and 30% hydrogen peroxide
as oxidants, respectively.
idine moiety [40]. The maximum absorptions at 325 and 341 nm
*
are attributed to
p ? p transition of the phenyl moiety in the li-
gands of complexes III and IV, respectively. The above results indi-
cate that the maximum absorption is proportional to the
coordination capacity of the ligand, which is determined by the
combined results of steric and electronic effects of the group R in
the ligand as shown in Scheme 3. More detailed data are presented
in Table 2. After complexation to MTO, all maximum absorption
band exhibits a slight blue-shifted in all cases compared with that
of a free ligand. This can be attributed to the addition of Re (VII) in
the conjugated system.
2
. Results and discussions
2.1. Synthesis and spectroscopy
The reaction of primary amine with 2-pyridinecarboxaldehyde
to afford the corresponding pyridinecarboxaldimine ligand is a
well-known route to obtain bidentate ligands of nitrogen [36,37].
By changing the amine it is allowed to receive ligands as needed
and subsequently to prepare metal complexes with different phys-
ical and chemical properties. Herein, 2-pyridinecarboxaldehyde re-
acted with the primary amines smoothly to afford the Schiff-bases
as described in literature [38]. Complexes I–IV (see Scheme 3) were
synthesised by treating MTO with pyridinecarboxaldimine in
methanol at room temperature. All complexes were isolated as yel-
low crystals in yields 75–80%. In comparison with other N-coordi-
nated Lewis base adducts [33,35], compounds I–IV are slightly
more sensitive to moisture and temperature. Their color changes
from yellow to black green slowly when exposed to air, which
can be avoided by storing under a water free nitrogen atmosphere.
In the IR spectra of compounds I–IV, the symmetric Re@O
stretching vibrations appear within a narrow interval of 938–
3
Initially, the proton signals originating from the ReCH group of
non-coordinated MTO was observed at 2.67 ppm, but after com-
plexation to Schiff-bases, the methyl signal was high-field shifted
in all cases (see Table 3). It has been reported that the magnitude
of these shifting is directly related to the electron donor character
of the ligand. The higher the electron density given to the Re(VII)
center by the donor ligand, the larger the high-field shift of the
1
3
H NMR signal in the ReCH group [41]. Indeed, electron-rich pyri-
dinecarboxaldimine derivatives induced the strong up-field shift of
the methyl signal. It is noteworthy that compounds I and II show
chemical shift changes of the methyl signal around 1.5 ppm rela-
tive to non-coordinated MTO, however, the corresponding magni-
ꢀ1
9
45 cm , and the asymmetric stretching vibrations occur between
ꢀ1
9
09 and 912 cm . Compared to the vibrations of non-coordinated
Table 1
Selected IR spectroscopic data of Schiff-bases and MTO adducts.
MTO, the Re@O bands of compounds I–IV are strongly red-shifted
due to the pronounced donor capacity of the respective ligands in
the solid state. The additional electron density donated from the li-
gand to the rhenium (VII) center generally reduces the bond order
ꢀ
Compound
m
/cm 1
Imine
Pyridine
ReO
3
m
(C@N)
m
(C@N)
m
as
m
s
sꢀ as
m m
ꢀ
1
of the Re@O bonds. Besides, differences of 24–35 cm between
v
s
MTO
a
I
b
II
c
III
d
–
–
965
–
912
–
910
–
910
–
998
–
938
–
934
–
945
–
33
–
26
–
24
–
35
–
(
Re@O) and as (Re@O) are observed in the above complexes. It was
v
1650
1650
1647
1643
1623
1621
1626
1622
1587
1600
1588
1598
1588
1590
1584
1599
reported that the size of the difference between the symmetric and
asymmetric stretching is related to the geometry of the Schiff-
bases-MTO adducts. In the literatures, the differences of vibrations
of free MTO [33], monodentate and bidentate adducts of MTO
ꢀ1
ꢀ1
ꢀ1
[
32,33] are 33 cm , 60–80 cm
and 20–27 cm , respectively,
which correspond to the tetrahedral, trigonal-bipyramidal, octahe-
dral coordination geometry, respectively. In some special cases, the
IV
909
944
33
Scheme 3. Route for the synthesis of Schiff-bases (a–d) and their MTO complexes (I–IV).

3
420
C.-J. Qiu et al. / Journal of Organometallic Chemistry 694 (2009) 3418–3424
Table 2
The UV–Vis spectroscopic data of the Schiff-bases and the MTO complexes.
Compound
k
max/nma
*
*
*
?
Imine (
p
?
p
)
Py (p
?
p
)
Ph (p
p )
a
I
231
228
231
229
236
236
238
238
272
271
272
271
281
278
289
283
–
–
–
–
325
325
342
341
b
II
c
III
d
IV
a
Tested in methanol.
Table 3
Selected 1H NMR spectroscopic data of Schiff-base and MTO complexes I–IV.
Compound
d (ReCH
3
)
d (C@NH)
MTO
a
I
b
II
c
III
d
2.67s
–
1.36s
–
1.12s
–
1.88s
–
8.38 s
8.61 s
8.41 s
8.76 s
8.60 s
8.61 s
8.63s
8.66s
IV
1.85s
Fig. 1. PLATON drawing of complex I in the solid state. Thermal ellipsoids are at the
0% probability level.
5
1
tudes of III and IV are only about 0.8 ppm in H NMR spectra. The
results indicate that alkyl groups have stronger electron donation
capacity than aromatic ones in the synthesised bidentate ligands.
1
The H NMR signals of the imine group and pyridine moiety of
compounds I–IV are shifted to lower field, in agreement with a
changed coordination situation.
In the FAB mass spectra of compounds III and IV, only the peaks
of the Schiff-base ligands can be observed. In contrast, less infor-
mation could be obtained from FAB MS for compounds I and II.
Whereas the ESI mass spectra of compounds I–IV show the peaks
of the Schiff-base ligand and the MTO moiety separately, although
no molecular peaks of the complete molecules can be observed.
2.2. Crystal structure examinations
The crystal structures of complexes I and III in the solid state
were examined and they are shown in Figs. 1 and 2. In addition,
the selected bond lengths and angles are listed in Table 5 and
Table 6. Both complex I and III display an octahedral geometry
with a pyramidal facial arrangement of the three oxygen atoms
and a trans-position of the ligand. Two double-bonded oxygen
atoms and the ligand occupy the equatorial positions, which
can be defined as the plane of the five-membered metallacycle
Re1–N1–C5–C6–N2, while the methyl group and the remaining
oxygen atom reside in the apical sites. The same arrangement
0
is found for the reported complexes derived from 2,2 -bipyridine
Fig. 2. PLATON drawing of complex III in the solid state. Thermal ellipsoids are at
and MTO [33]. The Re=O bond distances of 1.719(4) and
the 50% probability level.
1
.724(4) Å in the case of complex I are very similar to those of
all known Re (VII) oxo complexes containing an Re@O double
bond [33,42].
imine C6–N2 distances in complex I and III are 1.280 (8) and
1.268(10) Å, respectively, which are in the range of accepted car-
bon–nitrogen double bonds [38].
The coordination of Schiff-base ligands to MTO is governed not
only by electronic but also by steric effects. The phenyl group in the
imine N-position of compound III generates even greater distortion
with the neighboring oxygen atom (O2) spin as much as 42.7°
away from the equatorial plane, comparing to the angle of 21.4°
between the plane of N2–C7–C8–C9 and equatorial position in
The Re–N bond distances are obviously governed by electronic
effect. In complex I, the N(pyridine)–Re and N(imine)–Re length
are 2.274(5) Å and 2.257(5) Å, respectively. In complex III, the cor-
responding Re–N lengths are respectively, 2.285(6) and 2.329(6) Å,
longer than those in complex III, which indicates that the larger
conjugated system of the ligand can lead to a weak N–Re bonding
interaction by reducing electron density on nitrogen atom and,
therefore, supporting the spectroscopic results given above. The

C.-J. Qiu et al. / Journal of Organometallic Chemistry 694 (2009) 3418–3424
3421
Table 4
Crystal data and details of the structure determination.
Compound
I
III
Empirical formula
Formula weight (g/mol)
Temperature (K)
Wave length (Å)
Crystal system
Space group
C
10
H
15
N
2
O
3
Re
13 13 2 3
C H N O Re
431.46
113(2)
Mo Ka (0.71073)
Triclinic
397.44
113(2)
Mo K
Orthorhombic
Pbca
a
(0.71073)
P1ꢀ
Crystal size (mm)
Crystal color/shape
a (Å)
b (Å)
c (Å)
0.14 ꢁ 0.18 ꢁ 0.20
0.12 ꢁ 0.18 ꢁ 0.20
Yellow/plate
7.7319(15)
7.9525(16)
10.677(2)
Yellow/needle
7.6641(15)
15.422(31)
19.275(39)
a
(°)
b (°)
(°)
90.00
90.00
90.00
2278.21(8)
80.50(3)
76.59(3)
80.03(3)
623.60(5)
c
3
V (Å )
Z
D
8
2
calc (g/cm3)
2.31733
10.661
1504.0
2.29763
9.748
408.0
Absorption coefficient (cmꢀ1)
F(0 0 0)
h Range for data collection (°)
Limiting indices
Reflections collected
2.11–25.02
2.62–25.01
ꢀ9 < h < 9, ꢀ9 < k < 9 ꢀ12 < l < 12
5535
ꢀ9 < h < 9, ꢀ15 < k < 18, ꢀ22 < l < 21
14193
Independent reflections [Rint
Refinement method
]
2007[Rint0.104]
2150[Rint0.0674]
Full-matrix least-squares technique against F2
Full-matrix least-squares technique against F2
Data/restraints/parameters
Goodness-of-fit (GOF)
2007/6/148
0.997
2150/18/173
1.091
Final R indices [I > 2
R indices (all data)
r
(I)]
R
R
1
= 0.0320, wR
= 0.0397, wR
2
= 0.0632
= 0.0654
R
R
1
= 0.0443, wR
= 0.0454, wR
2
2
= 0.1136
= 0.1146
1
2
1
Largest difference in peak/hole (e Åꢀ3)
1.40 and ꢀ1.65
3.14 and ꢀ4.50
Table 5
Selected bond angles for complex I and III.
Compound
I
Bond
Angle (°)
Bond
Angle (°)
Bond
Angle (°)
O3–Re1–O1
O3–Re1–O2
O1–Re1–O2
O3–Re1–C10
O1–Re1–C10
O2–Re1–C10
107.00(19)
105.6(2)
105.91(19)
89.9(2)
90.9(2)
152.2(2)
O3–Re1–N2
91.44(19)
157.2(2)
81.04(19)
75.5(2)
160.77(18)
87.67(18)
O2–Re1–N1
C10–Re1–N1
N2–Re1–N1
C5–N1–Re1
N2–C6–C5
81.38(18)
77.3(2)
71.67(17)
116.9(4)
119.5(6)
117.6(4)
O1–Re1–N2
O2–Re1–N2
C10–Re1–N2
O3–Re1–N1
O1–Re1–N1
C6–N2–Re1
III
O3–Re1–O2
O3–Re1–O1
O2–Re1–O1
O3–Re1–C13
O2–Re1–C13
O1–Re1–C13
106.3(2)
105.7(3)
105.4(2)
92.0(3)
92.5(3)
149.7(3)
O3–Re1–N2
O2–Re1–N2
O1–Re1–N2
C13–Re1–N2
O3–Re1–N1
O2–Re1–N1
158.0(2)
92.3(2)
79.6(2)
75.4(3)
88.6(2)
161.2(2)
O1–Re1–N1
C13–Re1–N1
N1–Re1–N2
C5–N1–Re1
N2–C6–C5
80.8(2)
75.2(2)
71.0(2)
117.3(4)
121.0(7)
115.6(5)
C6–N2–Re1
Table 6
Selected bond lengths for I and III.
Compound
I
Bond
Length (Å)
Bond
Length (Å)
Bond
Length (Å)
Re1–O1
Re1–O2
Re1–O3
1.724(4)
1.758(4)
1.719(4)
Re1–C10
Re1–N1
Re1–N2
2.219(6)
2.274(5)
2.257(5)
N1–C5
C5–C6
C6–N2
1.357(7)
1.461(9)
1.280(8)
III
Re1–O1
Re1–O2
Re1–O3
1.758(5)
1.713(5)
1.713(5)
Re1–C13
Re1–N1
Re1–N2
2.227(8)
2.285(6)
2.329(6)
N1–C5
C5–C6
C6–N2
1.357(0)
1.453(10)
1.268(10)
complex I. Moreover, in the process of synthesizing Schiff-bases-
MTO complexes, when a 6-methyl or iso-propyl are present in
the pyridine moiety, no product was received, which indicates that
the alkyl in C-6 of pyridine can lead to an excessively bulky ligand
which cannot coordinate to MTO. Similar behavior was reported
for the unsuccessful synthesis of a 2,9-dimethyl-4,7-diphenyl-
2.3. Application in epoxidation catalysis
Compounds I–IV were examined as catalysts for the epoxida-
tion of cyclohexene with UHP and 30% hydrogen peroxide as oxi-
dants, respectively.
A catalyst: oxidant: substrate ratio of
1:200:100 was used in all experiments. The results are shown in
Table 7. Blank reactions showed that no significant amount of
epoxide was formed in the absence of catalyst.
1
,10-phenanthroline MTO adduct, only free ligand and MTO were
obtained in this case [33].

3
422
C.-J. Qiu et al. / Journal of Organometallic Chemistry 694 (2009) 3418–3424
Table 7
Epoxidation of cyclohexene using calalyst I–IV.
Entry
Catalyst
Ligand (mmol)a
Oxidant
UHP
H O
2 2
UHP
Time (h)
Conversion (%)b
Selectivity (%)c
1
2
3
4
5
6
7
8
9
MTO
MTO
I
I
I
I
II
II
II
–
–
–
–
2.5
3.0
8.0
1.5
0.5
0.5
8.0
3.0
0.5
0.5
2.0
3.0
2.0
2.0
3.0
3.0
2.0
2.0
>99
99
>99
25
2.5
1.2
>99
23
6.4
6.0
>99
>99
30
>99
30
>99
99
39
60
>99
99
44
60
>99
30
47.5
30
>99
2
87.5
77.5
H
2
H
2
H
2
O
O
O
2
2
2
a (0.01)
ab(0.02)
–
UHP
–
H
2
H
2
H
2
O
O
O
2
2
2
b (0.01)
b (0.02)
–
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
II
III
III
III
III
IV
IV
IV
IV
UHP
–
H
2
H
2
H
2
O
O
O
2
2
2
c (0.01)
c (0.02)
–
16.2
>99
>99
15
UHP
–
H
2
H
2
H
2
O
O
O
2
2
2
d (0.01)
d (0.02)
9.5
a
Reaction conditions: 2.5 mmol of cyclohexene; 5.0 mmol of oxidant; 0.025 mmol of catalyst; 3.5 ml of methanol; 25 °C; figures in bracket are the amount of ligand
mmol).
(
b
Calculated by GC.
c
Calculated by GC.
When UHP was used as oxidant for the epoxidation of cyclohex-
that n-propylamine and iso-propylamine were detected by GC in
the reaction mixtures catalyzed by I and II, respectively.
ene, no significant amount of diol was observed. All catalytic reac-
tions showed high reaction rate and completed within 3 h.
Complexes III and IV showed similar catalytic activity for the epox-
idation of cyclooctene, and the epoxide yield reached up to100%
after 4 h. The catalytic activities of complexes I and II, both of
which are prepared from the ligands derived from alkylamines,
are relatively low, and the reaction time must be prolonged to
When Schiff-base c or d was used as an additive in the case with
III or IV as catalyst, the selectivity for epoxide increased, however,
the catalytic activity decreased obviously and no further increase
of conversion was observed after 2.0 h. In addition, the color of
the reaction mixture changed from yellow to colorless in the first
1 h as noted in the cases of I and II. Thus it can be concluded that
this type complex of MTO is more sensitive to water than MTO it-
self [31], and furthermore, an excess of Schiff-base ligand can
accelerate the decomposition of the complex. These can explain
why more Schiff-base addition led to catalytic systems with very
low activity of compounds I–IV. This result is in contrast to the
behavior of bipyridine-MTO adducts, which can significantly accel-
erate the catalytic reactions when applied in large excess [16–19].
In conclusion, 4 MTO Schiff-base complexes were prepared and
characterized thoroughly. The complexes displayed high catalytic
activity and selectivity in the epoxidation of cyclohexene with urea
hydrogen peroxide adduct (UHP) as oxidant in methanol, but poor
performances in the case of hydrogen peroxide (30%) as oxidant
due to the decomposition of them. The results showed that the
complexes are more sensitive to water than MTO itself. Moreover,
large excess of ligand is detrimental to the catalytic performance as
it leads to the decomposition of the complexes.
8
.0 h for getting a 100% epoxide yield. These results match well
with the spectroscopic data of the complexes presented above
and show that the low electron donating ligand (complex III and
IV) leads to a relatively weak Re–N bond, but to an active catalyst.
Stronger electron donating ligand leads to a stronger Re–N bond
(
complex I and II), but a lower reactive catalyst. This can be ex-
plained by the fact that the ligands can exert their influence on
the Lewis acidity of the Re atom and the adjacent terminal ligands.
If the Lewis acidity of the metal is reduced, the formation of cata-
lytically active mono and bisperoxo complexes is hampered. Addi-
tionally, a decrease in the electron deficiency of the peroxo-oxygen
atoms makes the catalyst less prone to be nucleophiliclly attacked
by an olefin [32,43]. In addition, the solvent remained yellow dur-
ing the whole reaction time in all catalytic reactions. This is similar
to the observations made with several aromatic N-ligated MTO
complexes which are usually quite stable under oxidative condi-
tions [31,32].
In the cases where complexes I and II were used as catalyst, use
3. Experimental
of 30% H
2 2
O as oxidant, without an excess of the ligand, resulted in
only a 20% conversion, and after 0.5 h no further increase in con-
version was observed. When a mixture of a complex and its corre-
sponding ligand was applied to the epoxidation reaction, in
contrast with the case of UHP as oxidant, the reaction mixture be-
came colorless after 30 min, and a poor reaction conversion was
noted. However, in the cases of complex III and IV, the catalytic
reaction achieved 100% conversion after 3 h. Different from the
case of UHP as catalyst, significant diol was observed, the selectiv-
ity for the epoxide is only 30% and 2% with III and IV as catalyst,
respectively. The much lower catalytic activities of I and II than
those of III and IV may be derived from the much easer hydrolysis
of the pyridineimine moiety of the former two complexes than the
later ones. It is obvious that the hydrolysis happened for I and II
but not for III and IV is due to the greater steric hindrance present
in the latter complexes. This deduction is confirmed by the fact
3.1. Reagent and methods
Methyltrioxorhenium (MTO) was purchased from Alfa Aesar. 2-
Pyridinecarboxaldehydes were prepared according to literature
methods. All other reagents were obtained from commercial
sources. Methanol was dried by standard procedures, distilled un-
der nitrogen and kept over 4 Å molecule sieves. Other reagents
were used as received. All preparations and manipulations were
carried out under an oxygen and water free nitrogen atmosphere
using the standard Schlenk techniques.
3.2. Physical measurements
1H NMR spectra were obtained in DMSO or CDCl
Bruker AC 400 spectrometer. IR spectra in KBr pellets were
using a
3

C.-J. Qiu et al. / Journal of Organometallic Chemistry 694 (2009) 3418–3424
3423
recorded on
a
Bruker Vector-22 spectrophotometer. Melting
3.4.4. Complex IV
Yellow solid, Yield: 81%, M.p.100 °C (decomposition); UV–Vis:
(see Table 1); H NMR (DMSO): 1.85(s, ReCH , 1H), 3.80(s, OCH ,
3 3
points were determined on a Perkin XT-4 microscopic analyzer.
ESI and FAB mass spectrometry was performed on a VG ZAB-
HS mass spectrometer. Elemental analyses were performed on
an Elementar Vario E1. Reaction products were analyzed on a
Shandong Lunan Ruihong gas chromatograph, SP-6800A,
equipped with an FID detector.
1
1H), 7.02(d, J = 6.8 Hz, PhH, 2H), 7.41(d, J = 6.8 Hz, PhH, 2H),
7.51–7.54(m, pyH,1H), 7.96(t, J = 7.6 Hz, pyH, 1H), 8.15(d,
J = 7.6 Hz, pyH, 1H), 8.66(s, HC@N, 1H), 8.71 (d, J = 8.0 Hz, pyH,
1H); IR: 3086w, 2972w, 2930w, 2838w, 1622 m, 1599s, 1511vs,
ꢀ
1
1
(
2
3
296s, 1262s, 1025s, 944s, 909vs, 851vs, 831s, 541w cm ; MS
+ꢀ
+
FAB) m/z: 213.0(100)[M MTO], MS (ESI) m/z: 213.5(M ꢀMTO),
3.3. X-ray diffraction studies
+
51.1[M ꢀL]; Anal. Calc. for C14
15 2 4
H N O Re (461.49): C, 36.44; H,
.28 N, 6.07. Found: C, 36.55; H, 3.32; N, 6.08%.
Diffraction data for complex I and III were collected with a Bru-
ker AXS APEX CCD diffractometer equipped with a rotation anode
at 113(2) K using graphite-monochromated Mo K radiation
k = 0.71073 Å). Data were collected over the full sphere and were
corrected for absorption. Structure solutions were found by the
Patterson method. Structure refinement was carried out by full-
matrix least-squares on F2 using SHELXL-97 with first isotropic and
later anisotropic displacement parameters for all non-hydrogen
atoms [44]. A summary of the most important crystallographic
data is given in Table 4.
3
.5. Catalytic experiments
a
(
The catalytic reactions were carried out under continuous stir-
ring in a glass flask immersed in a water bath with temperature
control. In a typical experiment, 0.025 mmol of the catalyst,
3
luted hydrogen peroxide (solution, 30 wt% in water) were mixed
in the flask under agitation until the reaction temperature was
reached. At this time, 2.5 mmol of the substrate was added (time
zero). Small aliquots were taken at selected reactions times. The
products were analyzed by gas chromatography in a capillary col-
umn using a FID detector.
.5 ml of methanol, and a given amount (5 mmol) of UHP or di-
3.4. Synthesis of Schiff-base-MTO complexes I–IV
Schiff-base ligand was prepared according to the previously de-
scribed method [34]. In a typical procedure, 0.5 mmol of MTO was
added to a stirred solution of 0.5 mmol of corresponding Schiff-
base in 5 ml of methanol at room temperatures. A few seconds la-
ter, a yellow precipitate was formed. The yellow precipitate was
collected by filtration, washed with 1 ꢁ 3 ml of methanol, and
dried under reduced pressure.
Acknowledgement
This work was supported by NSFC of China (Grant No.
2
0776035).
Appendix A. Supplementary data
3
.4.1. Complex I
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2009.06.034.
Yellow solid, Yield: 75%, M. p. 94 °C (decomposition); UV–Vis:
1
(
1
see Table 1); H NMR (CDCl
3
) 1.04(t, J = 8.0 Hz, CH
1H), 1.98–2.07(m, CH CH CH
CH , 3H), 7.58(t, J = 6.4 Hz, PyH, 1H), 7.95(d,
J = 7.6 Hz, PyH, 1H), 8.07(t, J = 7.6 Hz, PyH, 1H), 8.61(s, HC = N,
2
CH
2 3
CH , 3H),
.36(s, ReCH
3
,
2
2
3
, 2H), 3.99(t,
J = 7.2 Hz, CH
2
CH
2
3
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