(N-Salicylidene)aniline derived schiff base complexes of methyltrioxorhenium(VII): Ligand influence and catalytic performance

Article abstract of DOI:10.1002/asia.200800358

Methyltrioxorhenium(VII) (MTO) readily forms 1:1 adducts with several N-(salicylidene)aniline derived Schiff bases. If the aromatic rings of the N-(salicylidene)aniline ligands display non-donating or electron withdrawing substituent groups, the resulting MTO adducts show good activities in olefin epoxidations. However, steric effects seem to play a major role, leading often to instable o- and m-Schiff base-MTO adducts, while p-substituted Schiff bases usually lead to more stable adducts. In catalysis, electron-withdrawing substituents on the aniline moiety lead to better catalysts than electron donating ones. The gap between good catalysts and instable or non-existing compounds, however, is small. The general tendency, however, that good donors on the Schiff base Iigands lead to shorter Re - O(Schiff base) bridges and lower catalytic activity, while the opposite is true with acceptor ligands on the Schiff bases, seems to be quite clear.

Full text of DOI:10.1002/asia.200800358

DOI: 10.1002/asia.200800358
(
N-Salicylidene)aniline Derived Schiff Base Complexes of
Methyltrioxorhenium(VII): Ligand Influence and Catalytic Performance
AHCTUNGTRENNUNG
[
a]
[b]
[a]
[a]
Ming-Dong Zhou, Yang Yu, Alejandro Capapꢀ, Kavita R. Jain,
[b, c]
[
a]
[b]
[b]
Eberhardt Herdtweck, Xiao-Rong Li, Jun Li, Shu-Liang Zang,*
and
[
a]
Fritz E. Kꢁhn*
Abstract:
Methyltrioxorhenium
A
H
U
G
R
N
N
(VII)
base–MTO adducts, while p-substituted
Schiff bases usually lead to more stable
adducts. In catalysis, electron-with-
drawing substituents on the aniline
moiety lead to better catalysts than
electron donating ones. The gap be-
tween good catalysts and instable or
non-existing compounds, however, is
small. The general tendency, however,
that good donors on the Schiff base li-
gands lead to shorter ReÀO(Schiff
(
MTO) readily forms 1:1 adducts with
several N-(salicylidene)aniline derived
Schiff bases. If the aromatic rings of
the N-(salicylidene)aniline ligands dis-
play non-donating or electron with-
drawing substituent groups, the result-
ing MTO adducts show good activities
in olefin epoxidations. However, steric
effects seem to play a major role, lead-
ing often to instable o- and m-Schiff
base) bridges and lower catalytic activi-
ty, while the opposite is true with ac-
ceptor ligands on the Schiff bases,
seems to be quite clear.
Keywords: (N-salicylidene)aniline ·
catalysis · epoxidation · rhenium ·
Schiff bases
[
3]
Introduction
1960s however, Halcon and Arco patented a homogenous
catalytic process that allowed for a much cleaner production
of propene oxide and other oxiranes, using various alkyl hy-
droperoxides as oxidizing agents. Subsequently, in the fol-
lowing years, numerous efforts were dedicated to unravel
the mechanism of the catalytic olefin epoxidation process
From 1958 to the early 1960s, organometallic compounds
with high metal oxidation states were looked upon more as
laboratory curiosities than possible catalysts. At that time
many organic reactions were not catalysed and produced
large amounts of waste—like in the synthesis of propene ep-
[1]
[
4]
and to the search for increasingly efficient catalysts.
Although many of its congeners originally were consid-
ered to be mere curiosities, methytrioxorhenium (MTO)
soon came to be known as an important catalyst. It has
been shown that MTO is an extremely versatile catalyst or
catalyst precursor for a broad variety of organic reactions.
Among the plethora of applications, olefin epoxidation is, so
far, one of the best examined. MTO utilizes hydrogen per-
oxide as an oxidizing agent thereby producing only water as
the by-product, making such a reaction environmentally
benign unlike others of this type. MTO has the additional
advantage of being stable in an aqueous medium.
[2]
oxide—an olefin epoxidation reaction. At the end of the
[
5]
[
6]
[
a] M.-D. Zhou, A. Capapꢀ, K. R. Jain, Dr. E. Herdtweck, Prof.
Dr. F. E. Kꢁhn
Molecular Catalysis, Catalysis Research Center
Technische Universitꢂt Mꢁnchen
Lichtenbergstrasse 4,d-85747 Garching bei Mꢁnchen (Germany)
Fax : (+49)89-289-13473
E-mail: fritz.kuehn@ch.tum.de
[7]
[
8]
[
b] Y. Yu, X.-R. Li, J. Li, Prof. Dr. S.-L. Zang
Department of Chemistry
[9]
Liaoning University
Chongshan Middle Road, No. 66, 110036 Shenyang (P.R. China)
Fax : (+86)24-62202006
E-mail: slzang@lnu.edu.cn
The production of water combined with the pronounced
Lewis acidity of MTO has a side effect which is sometimes
unwanted: it furthers ring opening reactions of more sensi-
[
c] Prof. Dr. S.-L. Zang
Department of Petrochemical Engineering
Liaoning Shihua University
[10]
tive epoxides via the formation of diols. However, it was
found that Lewis base adducts of MTO, such as adducts
with nitrogen donor ligands, largely suppress the ring open-
Dandong Road, No. 1, 113001 Fushun (P.R. China)
[11]
ing reaction.
Unfortunately the activity of the MTO
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.200800358.
Lewis base adducts originally examined was, at least in most
Chem. Asian J. 2009, 4, 411 – 418
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
411

FULL PAPERS
cases, found to be significantly lower than that of MTO
This paper deals in particular with the synthesis and applica-
[12]
[10a]
itself.
Nevertheless, application of aromatic N-donor li-
tions of complexes 5–9.
However, our attempt to synthe-
gands in ten or twelve fold excesses together with MTO,
leads to higher activities and selectivities in epoxidation cat-
size other MTO adducts with o-, m-, and p-substituted ani-
line moieties to obtain a complete row of complexes was
not successful. The Schiff base ligands either did not react
with MTO or formed an instable product. The hypothetic
formulae of the inaccessible compounds are shown in the
Supporting Information.
[13]
alysis than the application of MTO alone. Both monoden-
tate and bidentate aromatic Lewis bases with N-donor li-
[14]
gands display this behaviour. To date, many N-base ad-
ducts of MTO have been isolated, characterized, and ap-
[15]
plied for the epoxidation of olefins as catalysts, however,
significant excess of N-base is essential for the selectivity of
those reactions.
Synthesis and Spectroscopic Characterization
It is only recently that papers have appeared dealing with
the title complexes of this work, (N-salicylidene)aniline
based Schiff base adducts of MTO and their performances
Compounds 5–9 (Scheme 1) were synthesized by treatment
of MTO with the respective Schiff bases in diethyl ether at
room temperature (Scheme 2).
The compounds 5–9 are, as complexes 1–4,
[16]
[16a]
in catalytic olefin epoxidation. It was found that some of
these complexes display high catalytic activities in cyclooc-
tene epoxidation, while others are highly unstable and de-
compose readily at room temperature within seconds. In the
case of the highly active compounds, however, no excess of
Schiff base in comparison to MTO is necessary to reach sim-
ilar selectivities as the MTO/excess N-base systems. This
work examines the influence of substituents on the aromatic
ring of the aniline moiety on the stability and catalytic per-
formance of Schiff base adducts of MTO and their perfor-
mance in the epoxidation of a broader variety of substrates.
stable at
room temperature in solution as well as in the solid state
and can be handled without decomposition. The asymmetric
Scheme 2. Synthesis of MTO compounds 5–9.
Results and Discussion
Re=O stretching vibrations in the IR spectra of compounds
5–9 are found in the region 900–960 cm on average. For
[16]
À1
In a previous work,
some Schiff base adducts of MTO
with para-substituents on the aniline moiety of the (N-salicy-
lidene)aniline ligand, so as to avoid any steric interference
of the adduct with the MTO, were described. In this work,
various ortho- and meta-substituted aniline moieties are ex-
amined and the effects of such a substitution pattern of the
Schiff base ligands on the catalytic activity of the MTO ad-
ducts are compared to the previously obtained results.
Scheme 1 represents all synthesized complexes. The non-
compounds 5–7 and 9, the ReO stretching bands show a
splitting of approximately 15 cm (Table 1). It is seen that
3
À1
in comparison to the averaged symmetric and asymmetric
stretching vibrations exhibited by non-coordinated MTO,
the respective Re=O bands of compounds 5–9 are slightly
red-shifted. This shift most likely originates from the moder-
ate charge transfer from the ligand to the Lewis acidic
MTO. Additional electron density donated from the ligand
[16a]
substituted compound 1, also described previously,
is
to the Rhenium AHCUTNGTERNNUN(G VII) center usually reduces the bond order
used as the standard. The complexes 2–4 bearing trans-sub-
stituted aniline moieties have also been described before.
of Re=O and this is observed for all the compounds 5–9.
The differences between the averaged symmetric and
asymmetric stretching vibra-
[16a]
tions as given in the last row of
Table 1 deserve some attention.
In our previous work we had
ascribed the differences be-
tween the values mainly to the
symmetry of the coordination
[16a]
around the Re atom.
How-
ever, the larger quantity of data
available now leads to a some-
what different hypothesis as an
explanation. The values ob-
tained for compounds 4 and 8
are most prominently different
from the value of the “stan-
dard” compound 1. Compounds
Scheme 1. Synthesized MTO Schiff base complexes.
412
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ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2009, 4, 411 – 418

Ligand Influence and Catalytic Performance of MTO–Schiff Base Complexes
À1
Table 1. Characteristic IR vibrations of CH
3
ReO
3
fragments (cm ) in 1–9.
supporting our original ideas about the coordination of the
Schiff base ligands to MTO.
MTO
1
2
3
4
5
6
7
8
9
Assignment
Usually molecules of the type PhCH=NPh will display a
medium strength IR band at about 1650 cm , arising from
1
1
9
9
368 1376 1375 1375 1380 1369
–
1375 1385 1363 CH
def
3
asym
À1
205 1216 1210 1215
–
1247 1245 1252
–
1217 CH
def
3
sym
the C=N stretching mode, if undisturbed. For the ligand
complexes 5–9, these bands are found between 1601–
98
65
1005 1007 1012 1035 999 1000 1001 1011 1005 ReO
str
3
sym
À1
À1
1
619 cm (Table 2). This lowering of 40–50 cm is in a simi-
928 935 950 925
19 911 921 914
930 931 930
913 918 918
926 ReO
913 str
3
asym
lar order of magnitude as the reduction observed in com-
9
18
[16a]
9
pounds 1–4
and also strengthens the assumption of the
5
9
67
76
576 576 573 525
950 951 961 958
575 576 553 558 558 ReC str
947 950 950 965 948 ReO str.
averaged
existence of an intramolecular hydrogen bond between the
phenolic hydrogen and nitrogen atoms. After complexation,
the proton is attached to the nitrogen atom, the C=N vibra-
33
81.5 84
76.5 120.5 77.5 76
76.5 93
83.5
A
H
U
G
R
N
U
G
s
Àn
a
)
3
tions being observed at higher wave numbers ranging from
À1
1
633–1650 cm .
1
In the H NMR spectra of compounds 1–9 (Table 3), it is
seen that the proton signals are only very slightly shifted to
high field as compared to the value of MTO. It is observed
that the new complexes with ortho- and meta- substituted
derivatives show very similar properties with respect to the
4
and 8 have donor ligands in the para-position of the ani-
line moiety. The weaker donors in compound 2 and 9 lead
to a considerably less pronounced enlargement of the value,
while the electron acceptor Cl leads to a slight reduction of
the difference between the symmetric and asymmetric
stretching vibrations. The ortho- and meta- substitutions lead
only to smaller effects than those observed for the para-sub-
stituted compounds (comparable to the order of magnitude
observed for compound 3). It can be concluded that their
electronic effects on the attached MTO moiety therefore is
on average less pronounced than that of the substituents in
the para position.
1
Table 3. Selected H NMR data (ppm) of MTO and compounds 1–9 in
3
CDCl or DMSO.
d
MTOÀCH3
d
N (O)ÀH
Compound
CDCl
3
DMSO
CDCl
3
DMSO
MTO
2.67
2.62
2.63
2.62
2.61
2.63
2.60
2.61
2.60
–
1.91
–
–
–
–
–
–
1.91
1.90
1.90
–
–
–
–
–
–
–
–
1
2
3
4
5
6
7
8
9
13.25
13.49
13.02
13.45
13.07
13.22
13.49
13.52
–
[16a]
As has already been pointed out for complexes 1–4,
the absence of OH stretching bands of the Schiff bases in
the region around 3400 cm indicates the presence of a
strong intramolecular hydrogen bond with the nitrogen
atom of the imine group to form a six-membered ring. The
À1
13.37
13.30
13.41
broad band with a specific fine structure in the range 2900–
À1
2
400 cm is most likely from the phenolic OH stretching
1
feature characteristic of a strong hydrogen bond. There are
other low-frequency bands, which are characteristic for a
phenolic OH group in the spectra of the pure Schiff base li-
gands. These bands, together with the OH stretching vibra-
tions, are also conspicuously absent in the spectra of the
newly synthesized compounds 5–9 (see Table 2), thereby
H NMR spectra as the para-substituted derivatives. The
values in the coordinating solvent DMSO are virtually iden-
tical with that of MTO. DMSO, being a coordinating solvent
might be able to replace Schiff base ligands comparatively
easily, particularly when used in large excess, as is the case
here. These observations support the notion of a very weak
MTO–Schiff base interaction.
These results further indicate
À1
Table 2. Selected IR (KBr) data (cm ) for compounds 5–9. The respective pure ligand vibrations are given
that Schiff bases are most likely
even more weakly bonded to
MTO than several of the N-
donor adducts examined before.
The N (O) bonded hydrogen
atom (proton) is in all cases ob-
served between 13.02 and
13.52 ppm. The shift differences
between the different com-
pounds however, are too small
to allow for a profound discus-
sion of the influence of the
varying Schiff base substituents.
for sake of comparison.
Compound
Imine group
(C=N)
Phenolic OH group and coupled ring vibrations
n
A
H
U
G
R
N
U
G
b [OH]
n(CX)
n(CX)
N(CX)
g(OH)
C
C
C
C
C
C
C
C
C
C
13
H
13
H
14
H
14
H
14
H
14
H
15
H
15
H
13
H
13
H
10Cl NO
10Cl NO·CH ReO (5)
3 3
1615 s
1633 s
1601 s
1650 s
1617 vs
1643 s
1617 vs
1634 s
1619 s
1645 s
1367 m, sh
1281 s
1056 m, sh
818 m
755 vs
13NO
13NO
2
1368 m
1287 s
1280 s
1258 s
1241vs
1045 s
834 s
835 s
838 s
841 m
752 s
754 vs
754 s
741 s
2
·CH
3
3
3
3
ReO
ReO
ReO
ReO
3
3
3
3
(6)
(7)
(8)
(9)
13N O
13N O·CH
1365 m
1035 m
1107 m, sh
1047 s
15NO
15NO
11NO
11NO
2
2
2
2
1367 m
·CH
·CH
1391 m, sh
Notation of vibrational modes: u
ACHTUNGERTNNUNG( C=N), C=N stretching; b(OH), hydrogen bonded OH in-plane deformation;
u
(CX), substituent sensitive aromatic ring stretchings; g(OH) phenolic out-of-plane vibrations.
Chem. Asian J. 2009, 4, 411 – 418
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
413

S.-L. Zang, F. E. Kꢁhn et al.
FULL PAPERS
These results help to realize that the border between sta-
bility, instability, and non-existence of these Schiff base ad-
ducts is seemingly quite narrow. The reasons whether or not
a complex can be isolated or prepared seems to depend on
both steric and electronic factors, and steric hindrance
seems to be a good reason to account for the non-existence
of some of the complexes.
X-ray Crystal Structures of Compounds 5–9
The solid-state structures of the examined compounds are
shown in Figure 1–5, and selected bond distances and bond
angles given in Tables 4–6.
It is observed that both trans (compounds 5, 7–9, Fig-
ures 1, 3–5) and cis (compound 6, Figure 2) structures, with
Figure 3. ORTEP style plot of compound 7 in the solid state. Thermal el-
lipsoids are drawn at the 50% probability level. The molecule is located
on a mirror plane.
Figure 1. ORTEP style plot of compound 5 in the solid state. Thermal el-
lipsoids are drawn at the 50% probability level. The molecule is located
on a mirror plane.
Figure 4. ORTEP style plot of compound 8 in the solid state. Thermal el-
lipsoids are drawn at the 50% probability level.
Figure 2. ORTEP style plot of compound 6 in the solid state. Thermal el-
Figure 5. ORTEP style plot of compound 9 in the solid state. Thermal el-
lipsoids are drawn at the 50% probability level.
lipsoids are drawn at the 50% probability level.
414
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ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2009, 4, 411 – 418

Ligand Influence and Catalytic Performance of MTO–Schiff Base Complexes
Table 4. Selected bond lengths (ꢄ) of compounds 5–9.
distances for all compounds are around 1.713(3) ꢄ on aver-
distances in the compounds are between
2.166(3) (compound 6) and 2.271(4) ꢄ (compound 9), which
age. The ReÀO
5
6
7
8
9
bridge
ReÀOterminal
1.708(4)
1.714(3)
1.721(2)
1.724(2)
2.166(3)
2.116(3)
1.320(4)
1.301(4)
1.419(4)
1.693(6)
1.697(5)
1.697(5)
2.309(5)
2.089(10)
1.315(9)
1.301(9)
1.438(10)
1.697(6)
1.701(6)
1.703(6)
2.234(6)
2.073(9)
1.324(9)
1.295(9)
1.416(9)
1.690(6)
1.714(5)
1.719(6)
2.271(4)
2.098(8)
1.311(7)
1.305(7)
1.418(8)
1
1
.717(3)
.717(3)
are in agreement with the IR results and consistent with
[16]
what was found in our previous work. It should be noted
ReÀObridge
2.286(3)
2.116(5)
1.316(5)
1.310(6)
1.416(6)
that the cis-configured adducts 4 and 6 display the shortest
ReÀCH
3
ReÀO
bond distances. They are 2.153(5) ꢄ and
CÀObridge
bridge
methyleneÀN
arylÀN
2.166(3) ꢄ, while all other compounds exhibit ReÀO
C
C
bridge
bond distances above 2.23 ꢄ. The cis configuration obvious-
ly allows a closer proximity of the Schiff base ligand to the
Re center, as has previously been noted.
[16b]
The ReÀO
bridge
Table 5. Comparison of ReÀCH
MTO and compounds 1–9.
and ReÀObridge bond lengths (ꢄ) for
3
increases in the order 4<6<8<2<1<9<5<3<7. The
ReÀC bond distances seem to (inversely) correlate very
ReÀCH
ReÀObridge
3
roughly with this pattern, but the bond distance differences
are even less pronounced and are largely within the error
margins (see Table 5), which makes a well-founded discus-
sion of the ReÀC distances impossible. However, it is note-
[
10]
MTO
2.063(2)
2.119(7)
2.116(3)
2.073(9)
2.084(8)
2.112(12)
2.098(8)
2.116(5)
2.095(11)
2.089(10)
4
6
8
2
1
9
5
3
7
2.153(5)
2.166(3)
2.234(6)
2.243(4)
2.269(7)
2.271(4)
2.286(3)
2.286(5)
2.309(5)
worthy that only Schiff base ligands substituted with good
electron donor groups tend to form cis adducts with
[16]
MTO. Within the group of the trans adducts the general
tendency towards longer ReÀO bonds with decreasing
donor ability of the Schiff base ligand is clearly seen. How-
ever, the particularly long ReÀO bond in 7 might arise from
a steric effect.
The X-ray crystal structures
reveal that the proton is bound
to the C=N group and not to
Table 6. Selected bond angles (8) of compounds 5–9.
5
6
7
8
9
O
O
O
O
terminalÀReÀOterminal
119.04(10)
103.77(13)
103.49(13)
119.34(12)
89.15(13)
116.87(12)
116.63(12)
80.04(11)
85.51(10)
166.23(11)
77.48(12)
117.7(4)
118.3(4)
119.1(4)
119.9(4)
94.6(5)
95.6(4)
96.0(4)
79.5(3)
86.8(3)
87.6(3)
175.4(3)
118.1(3)
119.2(3)
120.4(3)
94.5(3)
94.5(3)
96.2(4)
81.3(2)
86.3(2)
87.2(2)
the oxygen atom.
1
1
19.04(10)
19.63(14)
119.6(2)
119.6(2)
95.4(2)
95.4(2)
96.6(4)
82.0(3)
85.3(2)
85.3(2)
178.6(3)
terminalÀReÀCH
94.9(2)
3
Applications as Epoxidation
Catalysts
9
9
5.08(12)
5.08(12)
terminalÀReÀObridge
83.81(15)
Compounds 5–9 were examined
as catalysts for the epoxidation
177.5(3) of cyclooctene, 1-octene, and
styrene with hydrogen peroxide
and compared to the results ob-
8
8
5.54(10)
5.54(10)
bridgeÀReÀCH
178.75(17)
3
respect to the orientation of the methyl groups and the
Schiff base ligand, can be seen. This is largely attributed to
packing effects in the crystals. It is noteworthy, however,
tained for the previously described complexes 1–4. Details
of the catalytic reaction are given in the Experimental Sec-
tion. Blank reactions showed that there is no significant for-
mation of epoxide in absence of the catalyst. A catalyst/oxi-
dant/substrate ratio of 1:200:100 was used in all cases. All
catalytic reactions followed first-order kinetics in which the
reaction conversion increases steadily for the first two hours
and then slows down.
Complexes 3 and 5, which have electron withdrawing
Schiff base ligands, show the highest activity (turnover fre-
quency, determined after 5 min reaction time) and conver-
sions of cyclooctene of above 70% after 2 h and of 90%
after 4 h are reached. The Schiff base with the Cl-substituent
in the p-position leads to a slightly less active complex (com-
pound 3) than the ortho derivative (compound 5). Com-
pounds 2 and 7, having donor substituents at the Schiff base,
display an opposite effect. The reasonably good activity
that the ÀOCH substituted Schiff bases form the only cis-
3
oriented MTO complexes, all others display trans configura-
tion of the Re-bound CH group to the Re-bound Schiff
3
base oxygen atom. However, we have recently described a
related compound, displaying both cis- and trans-arrange-
[16b]
ments in the solid state.
This is a strong indication that
packing forces are the main reasons for a certain configura-
tion rather than any ligand or substituent influences.
Table 4 gives selected bond distances for the compounds
5
–9. The ReÀC bond distances in ꢄ for the compounds 5–9
are 2.116(5), 2.116(3), 2.089(10), 2.073(9), and 2.098(8) ꢄ,
respectively, which are on average slightly longer than in
free MTO (2.063(2) ꢄ). This effect is already known from
Lewis base adducts of MTO. It contributes to the higher la-
bility of the ReÀCH bond and accordingly, adducts of
(350 mol/
ed by compound 2 compares to a TOF of approximately
200 mol/(molꢅh) and a conversion of approximately 65%
ACHTUNGTRENNGNU( molꢅh)) and conversion (ca. 70 after 4 h) exhibit-
3
MTO are generally somewhat less stable than the non-coor-
dinated organometallic compound alone. The Re=O bond
ACHTUNGTRENNUNG
Chem. Asian J. 2009, 4, 411 – 418
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
415

S.-L. Zang, F. E. Kꢁhn et al.
FULL PAPERS
after 4 h in the case of compound 7, in which the methyl
group is now at the ortho position. However, it is seen that
the compounds 4 (ca. 50% after 4 h), 6 (ca. 45% after 4 h)
and 8 (50% after 4 h) do not show a very high conversion
nor a high activity in the cases of 6 and 8 (TOF=150 (6)
While cyclooctene is epoxidized fast and selectively, the
epoxidation of 1-octene is slower, but still highly selective
(no diols are formed). With the more sensitive substrate sty-
rene, however, epoxides are the predominant products only
during the first 2 h of the reaction (epoxide yields are be-
tween 30 and 40% after 2 h, diols are formed usually in
smaller amounts). However, after 4 h and particularly after
24 h reaction time, this picture has changed drastically and
diols are either predominant or the only products present
(see also the Supporting Information).
and 200 (8) mol/ ACHTUNGTRENNUNG( molꢅh)). It is interesting to note that these
compounds have electron donating ÀOR ligands on the
Schiff base ligand. Compound 9, containing a p-substituted
Schiff base ligand displays a moderately good conversion of
ca. 70% after 4 h reaction time and a medium activity of ca.
250 mol/ ACHTUNGTRENNUNG( molꢅh). An overview of the catalytic performance
of the compounds synthesized is given in Figure 6 and the
turnover frequencies of the compounds for cyclooctene ep-
oxidation are shown in Table 7.
Conclusions
While the good result obtained for compound 3 might be
ascribed to the better electron accepting capability of the
Cl-ligand at the para-position, in compound 5, a combina-
Several Schiff bases readily form stable complexes with
MTO and have distorted trigonal-bipyramidal structures.
Others, however, lead to unstable complexes or prevent,
seemingly dependent on their steric demand, a reaction with
MTO. The electronic differences leading to stable complexes
and/or good catalysts seem to be quite small since they are
not clearly reflected from the gathered data. However, it
seems that Schiff bases with para substituents at the aniline
moiety lead in general to more stable complexes with MTO
than their more sterically hindered o- and m- derivatives.
Furthermore, the ReÀO(Schiff base ligand) bond distance
was found to roughly correspond to the donor ability of the
ligand. Stronger donors seem to have the tendency to crys-
tallize as cis-derivatives with MTO (i.e., the ReÀCH group
3
in the cis-position to the bridging oxygen) since this configu-
Figure 6. Conversions of the substrate cyclooctene to cyclooctene epoxide
after 2 h (hatched bars), 4 h (gray bars) and after 24 h (black bars) in the
presence of the compounds 1–9 as catalysts and H O as oxidant (cata-
2 2
ration allows a shorter ReÀO bond distance. If a reaction of
a Schiff base and MTO takes place, the phenolic proton of
the ligand is transferred to a ligand imine group upon coor-
lyst/substrate/oxidant=1:100:200; reaction temperature: 208C).
dination to Re, which means that the Lewis acidic Rhenium-
À
À1
Table 7. TOF (h ) for cyclooctene epoxidation after 5 min for com-
A( VII) is coordinated to O and forms a comparatively short
H
U
G
R
N
U
G
pounds 1–9.
ReÀO bond. In solution, the molecules seem to be flexible
with respect to their structures. Nevertheless, different
Schiff base ligands have a pronounced influence on the cata-
lytic performance of the complexes. Thus, whilst particularly
Compound
TOF
Compound
TOF
1
2
3
4
300
350
375
325
5
6
7
8
9
400
150
200
200
250
ÀOCH groups (good donors) on the aniline moiety of the
3
Schiff bases lead to reduced catalytic activities of the stable
adducts, other Schiff bases, namely those with electron with-
drawing Cl-ligands (acceptors), lead to active and selective
epoxidation catalysts. An excess of ligand, however, always
leads to rapid decomposition of the catalyst. Given the
ready availability and room temperature stability of several
of the title complexes, together with the good catalytic activ-
ity and high selectivity of some of them, they appear to be
good alternatives to the, on average, more temperature sen-
sitive MTO N-donor complexes as epoxidation catalysts.
The Schiff base adducts of the latter can also be prepared
and applied in situ. In contrast to N-donor adducts, no pro-
nounced ligand excess is necessary to achieve high yields
and selectivities in olefin epoxidation catalysis for substrates
like cyclooctene and 1-octene. However, sensitive products
such as styrene epoxide are quantitatively transformed to
diols after prolonged reaction times.
tion of steric and electronic effects may contribute to the
comparatively good results. In the case of compound 2 the
donor ability of the methyl ligand reduces the activity some-
what, when compared to compound 3. For compounds 6 and
7
, displaying a combination of donor abilities and steric hin-
drance, the activities are lower than in the case of the un-
substituted, standard compound 1. In general, with the sole
exception of compound 5, steric hindrance seems to be neg-
ative for the catalytic activity of any of the examined com-
plexes, in accord to what would be naively expected. The
particularly good electron donating effects of the ÀOR
groups also cause a reduced activity (compounds 4 and 8) in
comparison to compound 1, having no additional substitu-
ents residing on the Schiff base ligand.
416
www.chemasianj.org
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2009, 4, 411 – 418

Ligand Influence and Catalytic Performance of MTO–Schiff Base Complexes
Experimental Section
at the window of a rotating anode (nonius fr591) and graphite mono-
chromated MoKa radiation (l=0.71073 ꢄ). Data collections were per-
formed at 173 K (oxford cryosystems). Reflections were integrated, cor-
rected for Lorentz, polarization, absorption effects, and arising from the
scaling procedure for latent decay. The structures were solved by a com-
bination of direct methods and difference Fourier syntheses. All non-hy-
drogen atoms were refined with anisotropic displacement parameters. All
hydrogen atoms were calculated in ideal positions (5: riding model, dCÀ
Methods and Instrumentation
All preparations and manipulations were initially performed using stan-
dard Schlenk techniques in an Argon atmosphere. However, it was found
that the syntheses can also be performed under (dry) air without prob-
lems. Solvents were dried by standard procedures (n-hexane and Et
2
O
over Na/benzophenone; CH Cl over CaH ), distilled under argon and
2
2
2
H
=0.95 and 0.98 ꢄ 6: riding model, dNÀH =0.88 ꢄ and dCÀH =0.95 and
used immediately (as in the case of THF) or kept over 4 ꢄ molecular
sieves. Elemental analyses were performed with a Flash EA 1112 series
elemental analyser. H NMR were measured in CDCl
0
.98 ꢄ). Isotropic displacement parameters were calculated from the
parent carbon/nitrogen atom (Uiso(H) =1.2/1.5Ueq(C)). Full-matrix least-
1
3
with a mercury-
2
2 2
squares refinements were carried out by minimizing w
A
H
U
G
R
N
U
G
o
ÀF
c
) with the
VX 300 spectrometer and a 400 MHz Bruker Avance DPX-400 spectrom-
eter. IR spectra were recorded on a Perkin–Elmer FT-IR spectrometer
using KBr pellets as IR matrix. CI-MS spectra (isobutene as CI gas)
were obtained using a Finnigan MAT 90 mass spectrometer. Catalytic
runs were monitored by GC methods on a Hewlett–Packard instrument
HP 5890 Series II equipped with a FID, a Supelco column Alphadex 120
and a Hewlett–Packard integration unit HP 3396 Series II. The Schiff
shelxL-97 weighting scheme. The final residual electron density maps
show no remarkable features. Specials 5: Small extinction effects were
corrected with the SHELXL-97 procedure [e=0.0105(5)]. The hydrogen
atom located at the nitrogen atom was found in the final difference Four-
ier maps and was allowed to refine freely (dNÀH =0.80(7) ꢄ).
CCDC 702104 (5), CCDC 702105 (6) contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre at www.ccdc.cam.a-
[
17]
base ligands were prepared as described previously.
[
18a–g]
c.uk/data_request/cif.
Synthesis
X-ray Crystal Determination of Compounds 7–9
Compounds 5–9 were prepared as follows:
MTO (0.2 g, 0.8 mmol) was dissolved in diethyl ether (5 mL) and an
equally concentrated solution of ligand (0.8 mmol) in diethyl ether
The diffraction data were obtained with a Bruker Smart 1000 CCD dif-
fractometer operating at 50 kV and 30 mA using MoKa radiation (l=
0.71073 ꢄ). Data collection was performed at 293 K with a diffraction
measurement method and reduction was performed using the SMART
and SAINT software with frames of 0.38 oscillation in the range 1.5<q<
26.28. An empirical absorption correction was applied using the
SADABS program. The structures were solved by direct methods and all
non-hydrogen atoms were subjected to anisotropic refinement by full-
(
3
5 mL) was added to the stirred solution at room temperature. After 20–
0 min the yellow solution was concentrated in an oil pump vacuum to
ca. 3 mL and the orange or red precipitate was obtained by filtration,
washed with n-hexane (10 mL) and dried under reduced pressure.
1
Compound 5: (colour: red) Yield: 85%; H NMR (400 MHz, CDCl
3
,
RT): d=13.07 (s, 1H; NH), 8.70 (s, 1H; CH=N), 7.51–7.34 (m, 6H; Ph),
.11–6.94 (m, 2H; Ph), 2.63 ppm (s, 3H; MTOÀCH
); IR (KBr): see
Tables 1 and 2; MS (70 eV, CI): m/z (%): 232.00 (100)
2
7
3
matrix least squares on F using the SHELXTL package. All hydrogen
atoms were generated geometrically (C
assigned appropriate isotropic thermal parameters, and included in struc-
ÀH bond lengths fixed at 0.96 ꢄ),
+
+
[
C
13
H
10ClNO+H ] , 336.0 (13.53), 463.0 (3.19); elemental analysis: calcd
2
(
%) for C14
H
13ClNO
4
Re (480.92): C 34.96, H 2.72, N 2.91; found: C 35.07,
ture factor calculations in the final stage of F refinement. CCDC 648768
H 2.77, N 2.95.
(7), CCDC 648766 (8), CCDC 648767 (9) contain the supplementary crys-
tallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre at
1
Compound 6: (colour: orange) Yield: 86%; H NMR (300 MHz, CDCl
RT): d=13.25 (s, 1H; NH), 8.60 (s, 1H; CH=N), 7.40–7.25 (m, 3H; Ph),
3
,
[
18h–k]
www.ccdc.cam.ac.uk/data_request/cif.
7
.03–6.82 (m, 5H; Ph), 3.84 (3H, s, OCH
3
), 2.60 ppm(3H, s, MTOÀCH
3
);
IR
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(KBr): see Tables 1 and 2; MS (70 eV, CI): m/z (%): 228.1 (100)
Catalytic Reactions
+
+
+ +
[
C
14
H
13NO
2
+H ] , 251.0 (48.15) [CH
3
3
ReO +H ] ; elemental analysis:
calcd (%) for C15
C 37.99, H 3.24, N 2.88.
H
16NO
5
Re (476.50): C 37.82, H 3.36, N 2.94; found:
Method A: cis cyclooctene (800 mg, 7.3 mmol), 1.00 g mesitylene (inter-
nal standard), (30% aqueous solution; 1.62 mL, 14.6 mmol)
2 2
H O
1
(0.64 mL, 6.24 mmol,) and 1 mol% (73 mmol) of the catalyst (5–9) were
mixed.
Compound 7: (colour: red) Yield: 78%; H NMR (400 MHz, CDCl
3
,
RT): d=13.49 (s, 1H; NH), 8.61 (s, 1H; CH=N), 7.45–7.28 (m, 3H; Ph),
7
2
.23–7.10 (m, 4H; Ph), 6.97–6.94 m, 1H; Ph), 2.61 (s, 3H; MTOÀCH
3
),
Method B: 1-octene (343.2 mg, 3.12 mmol), 429 mg mesitylene (internal
standard), H O (30% aqueous solution) (0.64 mL, 6.24 mmol,) and
2 2
1 mol% (31.3 mmol) of catalyst (5–9) were mixed.
.40 ppm (s, 3H; CH ); IR (KBr): see Tables 1 and 2; MS (70 eV, CI):
3
+
+
+
m/z (%): 212.10 (100) [C14
mental analysis: calcd (%) for C15
N 3.04; found: C 39.17, H 3.57, N 3.09.
H
13NO+H ] , 250.1 (1.52) [CH
3
ReO
3
] ; ele-
H
16
O
4
NRe (461.06): C 39.12, H 3.50,
Method C: Styrene (250 mg, 2.39 mmol), 100 mg mesitylene (internal
standard), H O (30% aqueous solution) (0.53 mL, 4.78 mmol,) and
2
2
1
Compound 8: (colour: orange) Yield: 80%; H NMR (300 MHz, CDCl
3
,
1 mol% (24 mmol) of catalyst (5–9) were mixed.
RT): d=13.43 (s, 1H; NH), 8.60 (s, 1H; CH=N), 7.38–7.26 (m, 4H; Ph),
Olefins, mesitylene (internal standard) and compounds 5–9 as catalysts
were added to the reaction vessel under standard conditions. The reac-
tion began with the addition of H O . The course of the reaction was
7
.03–6.90 (m, 4H; Ph), 4.08–4.03 (q, 2H; CH
2
), 2.60 (s, 3H; MTOÀCH
3
),
1
.45–1.42 (t, 3H; CH ); IR (KBr): see Tables 1 and 2; MS (70 eV, CI):
3
2
2
+
+
m/z (%): 242.1 (100) [C15
mental analysis: calcd (%) for C16
N 2.86; found: C 39.19, H 3.74, N 2.94.
H
15NO
2
+H ] , 298.1 (25.54), 483.0 (11.78); ele-
monitored by quantitative GC analysis (cyclooctene and styrene) and
GC-MS analysis (1-octene). Samples were taken in regular time intervals,
diluted with CH Cl , and treated with a catalytic amount of MgSO and
H
18
O
5
NRe (490.52): C 39.18, H 3.70,
2
2
4
1
Compound 9: (colour: red) Yield: 82%; H NMR (400 MHz, DMSO,
RT): d=13.41 (s, 1H; NH), 9.66 (s, 1H; OH), 8.90 (s, 1H; CH=N), 7.60–
2
MnO to remove water and to destroy the excess of peroxide. The result-
ing slurry was filtered and the filtrate injected into a GC column. The
conversion of cyclooctene, 1-octene, styrene and the formation of the cor-
7
1
.57 (m, 1H; Ph), 7.38–7.30 (m, 3H; Ph), 6.97–6.82 (m, 4H; Ph),
.90 ppm (s, 3H, MTO-CH ); IR (KBr): see Tables 1 and 2; MS (70 eV,
2
responding oxides were calculated from calibration curves (r =0.999) re-
3
+
CI): m/z (%): 212.1 (68.32) [C13
H
11NO
2
ÀH] , 251.0 (60.02)
+H ] ; elemental analysis: calcd (%) for NRe
462.47): C 36.36, H 3.05, N 3.03; found: C 36.41, H 3.09, N 3.05.
corded prior to the reaction course.
+
+
[
(
CH
3
ReO
3
14 14 5
C H O
X-ray Crystal Determination of Compounds 5 and 6
General: Preliminary examination and data collection were carried out
on an area detecting system (5: stoe ipds 2t; 6: nonius k-CCD device)
Chem. Asian J. 2009, 4, 411 – 418
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
417

S.-L. Zang, F. E. Kꢁhn et al.
FULL PAPERS
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Received: September 16, 2008
Revised: November 11, 2008
Published online: December 15, 2008
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