Molybdenum(VI) dioxo complexes employing schiff base ligands with an intramolecular donor for highly selective olefin epoxidation

Article abstract of DOI:10.1021/ic301464w

Reaction of [MoO₂(η²-tBu₂pz) ₂] with Schiff base ligands HL^X (X = 1-5) gave molybdenum(VI) dioxo complexes of the type cis-[MoO₂(L ^X)₂] as yellow to light brown solids in moderate to good yields. All ligands coordinate via its phenolic O atom and the imine N atom in a bidentate manner to the metal center. The third donor atom (R₂ = OMe or NMe₂) in the side chain in complexes 1-4 is not involved in coordination and remains pendant. This was confirmed by X-ray diffraction analyses of complexes 1 and 3. Complexes 1, 3, and 5 exist as a mixture of two isomers in solution, whereas complexes 2 and 4 with sterically less demanding substituents on the aromatics only show one isomer in solution. All complexes are active catalysts in the epoxidation of various internal and terminal alkenes, and epoxides in moderate to good yields with high selectivities are obtained. In the challenging epoxidation of styrene, complexes 1 and 2 prove to be very active and selective. The selectivity seems to be influenced by the pendant donor arm, as complex 5 without additional donor in the side chain is less selective. Experiments prove that the addition of n-butyl methyl ether as intermolecular donor per se has no influence on the selectivity. The basic conditions induced by the NMe₂ groups in complexes 3 and 4 lead to lower activity.

Full text of DOI:10.1021/ic301464w

Article
pubs.acs.org/IC
Molybdenum(VI) Dioxo Complexes Employing Schiff Base Ligands
with an Intramolecular Donor for Highly Selective Olefin Epoxidation
Martina E. Judmaier, Christof Holzer, Manuel Volpe, and Nadia C. Mosch-Zanetti*
̈
Institut fur Chemie, Karl-Franzens-Universitat Graz, Stremayrgasse 16, 8010 Graz, Austria
̈
̈
S
* Supporting Information
ABSTRACT: Reaction of [MoO2(η2-tBu2pz)2] with Schiff
base ligands HLX (X = 1−5) gave molybdenum(VI) dioxo
complexes of the type cis-[MoO2(LX)2] as yellow to light
brown solids in moderate to good yields. All ligands coordinate
via its phenolic O atom and the imine N atom in a bidentate
manner to the metal center. The third donor atom (R2 = OMe
or NMe2) in the side chain in complexes 1−4 is not involved
in coordination and remains pendant. This was confirmed by
X-ray diffraction analyses of complexes 1 and 3. Complexes 1,
3, and 5 exist as a mixture of two isomers in solution, whereas
complexes 2 and 4 with sterically less demanding substituents
on the aromatics only show one isomer in solution. All
complexes are active catalysts in the epoxidation of various
internal and terminal alkenes, and epoxides in moderate to good yields with high selectivities are obtained. In the challenging
epoxidation of styrene, complexes 1 and 2 prove to be very active and selective. The selectivity seems to be influenced by the
pendant donor arm, as complex 5 without additional donor in the side chain is less selective. Experiments prove that the addition
of n-butyl methyl ether as intermolecular donor per se has no influence on the selectivity. The basic conditions induced by the
NMe2 groups in complexes 3 and 4 lead to lower activity.
worthwhile to develop further homogeneous catalysts for
selective epoxidation of styrene and other olefins.
INTRODUCTION
■
Alkene epoxidation is one of the main routes for the production
of epoxides, which are of high importance in both synthetic
organic chemistry and chemical technology.1 Among them,
styrene oxide represents one of the most interesting compounds
as it is used for the manufacture of important commercial pro-
ducts (e.g., epoxy resins, cosmetics, surface coatings, sweeteners,
perfume, drugs, etc.).2 To overcome the limitations of traditional
processes using stoichiometric amounts of peracids, research has
focused on the development of new synthetic methods. Metal
catalyzed alkene to epoxide conversion in the presence of softer
oxidants, such as H2O2, alkyl hydroperoxides, or air, has attracted
considerable interest and led to the development of highly active
catalysts.3−5 Molybdenum complexes with various types of
ligands have been tested in the epoxidation of alkenes and among
them, cis-[MoO2L2] complexes by Schiff base ligands prove to be
active catalysts.6−11 Most of these complexes show high catalytic
activity in the epoxidation of internal alkenes such as cyclooctene
and cyclohexene. The epoxidation of terminal alkenes like
styrene is more challenging because of favored ring-opening
reactions of the epoxide,12,13 leading usually to relatively low
conversions and poor selectivity. Only a very limited number of
highly selective molybdenum(VI) catalysts employing other
We have an ongoing interest in oxygen atom transfer (OAT)
chemistry mediated by high valent molybdenum com-
pounds.17−20 Only recently we started to investigate the catalytic
behavior in epoxidation reactions.13 These molybdenum dioxo
complexes contain a bidentate phenol based ligand with an adja-
cent pyrazole substituent. We found them to be highly reactive
epoxidation catalysts for various substrates but unselective for
styrene leading to several ring-opened products. The high reac-
tivity prompted us to develop molybdenum dioxo compounds
with phenol based ligands having a higher denticity to influence
the selectivity. From rhenium based epoxidation catalysis with
MTO it was shown that the addition of Lewis bases (e.g.,
pyridines and pyrazoles) modulates the Lewis acidity of the
rhenium center and thereby increases the selectivity.21 We adopt
this concept to molybdenum chemistry by introducing an
intramolecular donor in the ligand design. We focused on Schiff
base ligands as they are known to be effective ligands in oxidation
catalysis.4 Furthermore, synthetic procedures for Schiff base
ligands are widely published and are easy to modify. Here, we
report the preparation of a set of new molybdenum(VI) dioxo
complexes with bidentate phenol imine ligands equipped with a
ligands in the epoxidation of styrene were reported,14−16
a
prominent example being molybdenum(VI) dioxo half sandwich
Received: July 6, 2012
Published: August 29, 2012
complexes with cyclopentadienyl derivatives.15 Thus, it is still
© 2012 American Chemical Society
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pendant donor functionality and their highly selective catalytic
epoxidation behavior toward various olefins.
remains pendant. The formation of the complex is indicated by a
color change from light yellow to orange (for 1, 3, and 5) or
brown (for 2 and 4) of the solution. The unusual starting material
[MoO2(η2-tBu2pz)2] offers the advantage of a very easy workup
procedure, as the formed side product, bis-tert-butylpyrazole,
together with residual ligand traces can be easily separated from
the complexes via extraction with pentane or heptane. The pure
compounds are isolated as yellow to light brown solids.
Complexes 1−5 can be also prepared by more conventional
methods using either [MoO2Cl2(dme)]28 or commercially
available [MoO2(acac)2] as starting materials (Scheme 2).
Both reaction pathways would be favorable as they involve
fewer synthetic steps, but the obtained yields and purities were
significantly lower. Furthermore, we were not able to isolate
complexes 2 and 4 using [MoO2(acac)2].
RESULTS AND DISCUSSION
■
Synthesis of the Ligands. Syntheses of ligands HL1 to HL5
follow a single step procedure as described for ligand HL3 in the
literature.22 Condensation of aromatic aldehydes with the
appropriate secondary amines in methanol at room temperature
results in the formation of the Schiff base ligands as yellow
viscous oils (HL1−HL4) or solid (HL5) in quantitative yields (see
Figure 1). All ligands were characterized by common
All molybdenum complexes 1−5 are well soluble in common
organic solvents such as toluene, tetrahydrofuran (THF),
chloroform, and methanol at room temperature, but much less
so in aliphatic hydrocarbons like pentane or heptane. The
compounds are stable in the absence of moist air and can be
stored under inert conditions for several weeks (complexes 3 and
4) to months (complexes 1, 2, and 5). In general complexes 1
and 2 with a methoxy group in the pendant arm prove to be more
stable in solution than their NMe2 based counterparts 3 and 4.
The latter compounds tend to decompose after a few days in
solution. We suspect that the amine group leads to more basic
conditions in comparison to the ether functionality and therefore
traces of water may have a more pronounced effect, thus
preferring the formation of polymeric molybdenum compounds.
Complexes 1−5 were characterized by NMR and IR
spectroscopy, mass spectrometry, and elemental analyses.
Crystals suitable for X-ray diffraction analyses were obtained in
the case of complexes 1 and 3.
Figure 1. Ligands HL1−HL5 employed in this study. The ligands HL3
and HL5 were previously described in the literature.22,23
spectroscopic techniques and used without further purification.
The proton of the imine group (Ar−CHN) at the aromatic ring is
indicated by a single peak in the region between 8.14 and 8.39
ppm in the 1H NMR spectra. The resonance of the
corresponding C atom (Ar-CHN) is found between 166 and
168 ppm in the 13C NMR spectra. IR measurements show a
strong absorption between 1631 cm−1 and 1634 cm−1 attributed
to the stretching vibration of the νCN group, which is in good
agreement with the literature.22,23 Mass spectrometry confirmed
the formation of the expected ligands.
1H NMR spectra of the free ligands HL1 to HL5 show a broad
resonance between 13.34 and 14.00 ppm for the aromatic OH
proton. Disappearance of this signal indicates a coordination of
the phenolic O atom to the metal center. The sharp signal of the
imine proton in the ligand (8.14 to 8.39 ppm) is shifted to higher
field and appears between 8.09 and 8.38 ppm upon complex-
ation. Mass spectrometry as well as elemental analyses confirmed
the formulation of complexes 1−5 as [MoO2(LX)2].
In principle, the design of the ligand would allow the formation
of several isomers in solution, with respect to the ligand trans to
the terminal oxygen ligands. Both ligands can coordinate via the
phenolic O atom and the imine N atom either in a symmetric way
(N,N and O,O isomer) or in an asymmetric way (N,O isomer) as
shown in Figure 2.
Ligands HL3 and HL5 were previously described in the
literature and have been used for the syntheses of several metal
complexes.22−27
Synthesis of Molybdenum(VI) Dioxo Complexes.
Molybdenum(VI) dioxo complexes [MoO2(LX)2] (X = 1−5)
are readily accessible by reaction of [MoO2(η2-tBu2pz)2]17 with 2
equiv of the ligand in dry toluene at room temperature. The
pyrazolate ligands are easily displaced by two Schiff base ligands
HLX (X = 1−5), leading to disubstituted complexes in moderate
to good yields (Scheme 1). All ligands coordinate via the
Scheme 1. Synthetic Procedure for the Preparation of Di-
substituted cis-[MoO2(LX)2] Complexes 1−5
For complexes 2 and 4 only one isomer could be observed in
solution. NMR spectra show one set of sharp resonances for just
one type of ligand, indicating a symmetric coordination of
the ligand to the metal center. Formation of the N,N isomer
(Figure 2) is probably favored over the O,O isomer because the
steric clash of the substituents on the aromatic rings is avoided.
1
On the other hand, H NMR spectra of complexes 1, 3, and 5
show together with a set of sharp resonances, a set of broad
resonances for another coordination environment.
Low temperature NMR measurements of complexes 1 and 3
resolve the broad signals and indicate the formation of a sym-
metric (major isomer) and an asymmetric isomer (minor iso-
mer) in solution in a 4:1 (complex 1) and a 2:1 (complex 3) ratio.
Figure 3 shows a comparison of the aromatic region of complex 3
at 25 °C and −35 °C. The formation of a symmetric isomer S
(N,N or O,O isomer) is indicated by the existence of one set of
phenolic O atom and the imine N atom to the metal center, the
third donor atom (R2 = OMe or NMe2) in complexes 1−4
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Scheme 2
mode of the cis-[MoO2]2+ fragment.14,29,30 The absence of a
broad band around 3250 cm−1 in the spectra of the molybdenum
complexes compared to the Schiff base ligands indicates the
coordination of the phenolic oxygen atom after deprotonation.
The characteristic stretching frequencies of the νCN band in the
free ligand (1631−1634 cm−1) are shifted to lower wave
numbers upon coordination of the imine nitrogen to the metal
center and appears at 1625−1628 cm−1.7,8,29
Figure 2. Possible symmetric and asymmetric isomers in solution.
Molecular Structure in the Solid State. Single crystals
suitable for X-ray diffraction analysis of complex 1 and 3 were
obtained from concentrated solutions in methanol at room
temperature. Complexes 1 and 3 crystallized in the monoclinic
space group Cc (1) and C2 (3) in the form of light yellow
parallelepipeds (1) or yellow tablets (3). Molecular views
of both compounds are shown in Figure 4. Selected bond lengths
and angles are given in Table 1 and crystallographic data in
Table 2.
resonances for both coordinated ligands. The broad signals
resolve into two sets of resonances of equal intensity at low
temperature (e.g., two signals for the imine groups at 8.09 and
8.36 ppm) and can thus be assigned to the asymmetric isomer AS
(N,O isomer). Molecular structures determined by X-ray
diffraction analyses of complexes 1 and 3 reveal exclusively the
symmetric N,N isomer. In several attempts we dissolved such
single crystals in benzene-d6. Their 1H NMR spectra show always
the two isomers in solution in the same ratio as in the bulk
material, pointing to a dynamic equilibrium in solution.
The X-ray structures of complexes 1 and 3 are very similar and
confirm the formation of the symmetric N,N isomer (Figure 2).
Both compounds exhibit a six-coordinate Mo atom in a distorted
octahedral geometry. The metal center is ligated by two terminal
oxygen atoms and two Schiff base ligands, each of them
coordinating via the phenolic oxygen atom and the imine N atom
to the metal center. The third donor atom in the side chain (R2 =
OMe in 1 and R2 = NMe2 in 3) is not involved in coordination
and hence the arm remains pendant. The molybdenum oxo
groups show the expected mutual cis configuration and are
located trans to the imine N atoms. All Mo−O bond lengths
[1.9556(12) Å and 1.9611(11) Å for 1 and 1.942(4) Å for 3] as
well as all MoO bond lengths [1.7013(11) Å and 1.7059(11)
Å for 1 and 1.714(4) Å for 3] are in the expected range of cis-
[MoO2]2+ complexes. The Mo−N bonds [2.3303(12) Å and
2.37454(12) Å for 1 and 2.334(5) Å for 3] are somewhat longer
because of the influence of the trans MoO ligand.7,14,29
The three compounds 1, 3, and 5 that are found in the two
isomeric forms (N,N and N,O isomer) in solution represent
derivatives with two sterically demanding tBu groups in o- and
p-positions of the phenolic ring. From a steric point of view 1, 3,
and 5 are expected to form exclusively the N,N isomer. On the
other hand, from an electronic point of view, the O,O isomer with
the more electronegative phenolic oxygen coordinated trans to
the metal oxo bond is predicted to be favored. We exclusively find
such a bonding situation in [MoO2(L)2] complexes where L
represents ß-ketiminates.19,20 For this reason, we attribute the
occurrence of two isomers in complexes 1, 3, and 5 to the higher
electron-donating capacity of the tBu groups, rendering the
phenolic oxygen more nucleophilic so, at least partially,
overruling the steric hindrance.
The IR spectra of all [MoO2(LX)2] complexes 1−5 exhibit two
strong νMoO bands in the region 900−904 cm−1 and 914−928
cm−1, characteristic for the symmetric and asymmetric stretching
1
Figure 3. H NMR spectra of complex 3 in chloroform-d at 25 °C (top) and −35 °C (bottom). The asterisk (*) denotes residual free ligand.
S corresponds to the symmetric isomer (N,N or O,O isomer), and AS corresponds to the asymmetric N,O isomer shown in Figure 2.
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Figure 4. Molecular structure and atom labeling scheme for complexes 1 (top) and 3 (bottom). Thermal ellipsoids have been drawn at 50% probability
level. Hydrogen atoms are omitted for clarity.
Table 1. Selected Bond Lengths (Å) and Angles (deg) of Complexes 1 and 3
Complex 1
Mo1O1
Mo1O2
1.7013(11)
1.7059(11)
Mo1O3
Mo1O4
1.9556(12)
1.9611(11)
Mo1N1
Mo1N2
2.3303(12)
2.3754(12)
O1−Mo1O2
O1−Mo1O3
O1−Mo1O4
O2−Mo1O3
O2−Mo1O4
106.15(6)
93.77(5)
97.62(5)
98.35(5)
95.98(5)
O3−Mo1O4
O1−Mo1N1
O2−Mo1N1
O3−Mo1N1
O4−Mo1N1
158.44(4)
86.58(5)
166.75(5)
84.17(5)
78.33(5)
O1−Mo1N2
O2−Mo1N2
O3−Mo1N2
O4−Mo1N2
N2−Mo1N1
164.36(5)
89.00(5)
79.90(5)
84.35(5)
78.60(4)
a
Complex 3
Mo1O1
1.714(4)
Mo1O2
1.942(4)
Mo1N1
2.334(5)
O1−Mo1O1′
O1−Mo1O2
O1−Mo1O2′
107.0(3)
94.96(19)
96.18(19)
O2−Mo1O2′
O1−Mo1N1′
O1−Mo1N1
161.2(2)
90.1(2)
O2−Mo1N1
O2−Mo1N1′
N1−Mo1N1′
80.23(16)
84.70(16)
73.1(3)
162.8(2)
a
Symmetry equivalent atoms are generated by the symmetry operator 1 − x, y, 2 − z.
Epoxidation of Alkenes. Complexes 1−5 have been
tested as catalysts in the epoxidation of several internal and
terminal alkenes using tert-butyl hydroperoxide (TBHP, 5.5 M in
decane) as oxygen source. Optimal reaction conditions regarding
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Table 2. Crystallographic Data and Structure Refinement for
Complexes 1 and 3
Table 3. Epoxidation of Cyclooctene Catalyzed by 1: Effect of
Cosolvent and Temperature
1
3
conversion (%)
a
empirical formula
formula weight
MoO6N2C36H56
708.77
MoO4N4C38H62
734.86
cosolvent
CH2Cl2
35 °C
50 °C
75 °C
80
crystal description
parallelepiped,
light yellow
tablet, yellow
CHCl3
93
C2H4Cl2
heptane
88
96
97
crystal size (mm)
0.70 × 0.31 × 0.25 0.69 × 0.15 × 0.07
monoclinic, Cc
57
crystal system, space group
unit cell dimensions
monoclinic, C2
a = 19.606(2) Å
toluene
63
a = 31.098(3) Å
b
TBHP/decane
95
b = 10.1937(8) Å b = 6.8112(7) Å
b
TBHP/H2O
<10
c = 11.9756(9) Å
α = 90°
β = 107.044(2)°
γ = 90°
3629.5(5)
4, 1.297
c = 15.1333(16) Å
α = 90°
β = 98.025(4)°
γ = 90°
2001.1(4)
2, 1.223
diethylether
25
tBuOH or MeOH
<10
a
Reaction conditions: 0.5 mol % complex 1, 1.41 mmol alkene, 2.82 mmol
(2 equiv) TBHP. Conversions were determined by GC/MS measure-
ments after 60 min. Mesitylene was used as internal standard. Com-
plete selectivity toward the epoxide was observed. Reaction was
performed without additional cosolvent (5.5 M TBHP in decane or
volume (Å3)
Z, calculated density (g cm−3
)
b
F(000)
1504.0
788.0
linear absorption coefficient μ
(mm−1
0.406
0.368
70 w% TBHP in water).
)
absorption correction
temperature
multiscan
multiscan
system (see Table 4). A further lowering to 0.1 mol % led to
lower yields with 1 or 2 equiv of TBHP. Some [MoO2]2+
100(2) K
100(2) K
wavelength (MoKα)
θ range for data collection
limiting indices
0.71073 Å
0.71073 Å
2.11 to 30.00°
−42 ≤ h ≤ 43
−14 ≤ k ≤ 14
−16 ≤ l ≤ 16
36836/9485
2.10 to 26.09°
−24 ≤ h ≤ 0
−8 ≤ k ≤ 0
−18 ≤ l ≤ 18
58693/2133
Table 4. Epoxidation of Cyclooctene Catalyzed by 1: Effect of
Catalyst and Oxidant (TBHP) Loading
a
conversion (%)
reflections collected/unique
b
b
mol % 1
Mo:substrate ratio
TBHP (1 equiv)
TBHP (2 equiv)
[R(int) = 0.0233] [R(int) = 0.0967]
1
1:100
89
87
85
74
56
31
99
98
95
84
53
33
completeness to θ max.
refinement method
0.999
0.983
0.5
1:200
full matrix least-
full matrix least-
squares on F2
squares on F2
0.25
0.1
1:400
data/restraints/parameters
9485/2/421
1.034
2133/1/211
1.085
1:1000
1:2000
1:10000
goodness-of-fit on F2
0.05
0.01
a
b
final R1, wR2 [I > 2σ(I)]
R1 = 0.0221
wR2 = 0.0576
R1 = 0.0225
wR2 = 0.0579
R1 = 0.0478
wR2 = 0.1288
R2 = 0.0496
wR2 = 0.1305
a
Reactions were carried out at 50 °C in chloroform (5 mL) using 1.41 mmol
R indices (all data)
alkene, 1.41 mmol (1 equiv), or 2.82 mmol (2 equiv) TBHP.
Conversions were determined by GC/MS measurements after 120 min.
b
largest diff. peak and hole (e Å−3
)
1.033 and −0.576 2.254 and −1.458
837295 837296
Mesitylene was used as internal standard. Complete selectivity toward
the epoxide was observed.
CCDC deposition no.
a
b
2
2
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = {∑w(Fo − Fc2)2/∑w(Fo )2}1/2
.
complexes allow lower catalyst concentrations without loss of
efficiency.7,10,31 For subsequent epoxidation reactions we chose
0.5 mol % of catalyst loading and 2 equiv of TBHP at 50 °C in
chloroform.
Scheme 3. Epoxidation of Cyclooctene
To evaluate the stability of the catalysts an experiment with
three consecutive catalytic runs was performed. Complex 1
shows full conversion in the epoxidation of cyclooctene after
60 min in the first run. After 120 min additional substrate
(1.41 mmol) and oxidant (2.82 mmol) are added for the second
run and after 24 h for the third run. The results are displayed in
Figure 5. The catalytic activity of complex 1 in the second run is
quite similar to that of the first run. After one hour reaction time
almost full conversion (93% of cyclooctene oxide) of cyclooctene
is obtained. The catalytic activity in the third run (addition of
substrate and TBHP after 24 h) is good, but the conversion rate is
lower than in the former two runs, presumably because of the
increase of tert-butanol concentration. In each run we only
observe the formation of the corresponding epoxide and no other
products caused by subsequent ring-opening reactions.
temperature, solvent, oxidant loading, and catalyst loading
were evaluated by using the substrate cyclooctene and complex
1 as catalyst (Scheme 3). Table 3 summarizes the results of tem-
perature and solvent screening. The highest conversions were
obtained at 50 °C in chloroform or without additional cosolvent.
At higher temperature (75 °C) similarly high conversions were
also reached in heptane or toluene. No conversion of epoxide is
observed in methanol, tert-butanol, or THF. Our results are in
good accordance with the literature, where higher temperatures
and chlorinated solvents7,14,30 or solvent free conditions9,10 are
preferred. For these reasons all further experiments were per-
formed in chloroform at 50 °C.
We then tested further substrates with complexes 1−5, namely,
cyclooctene, cyclohexene, and the more challenging substrates
styrene and 4-phenyl-1-butene. All reactions are performed in
The catalyst loading can be reduced down to 0.25 mol % in the
presence of 2 equiv of TBHP without affecting efficiency of the
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Table 5. Epoxidation of Aliphatic and Aromatic Alkenes with
Complexes 1−5 and [MoO2(acac)2]
Figure 5. Activity test of complex 1 via epoxidation of cyclooctene in
three consecutive runs. After 2 and 24 h 1.41 mmol cyclooctene and
2.82 mmol TBHP were added to the initial reaction mixture. Reactions
were performed in chloroform (5 mL) at 50 °C with 0.5 mol % catalyst
loading.
a
Reaction conditions: 0.5 mol % catalyst, 1.41 mmol alkene (1 equiv),
1.41 mmol internal standard (1 equiv), 2.82 mmol (2 equiv) TBHP,
chloroform at 50 °C using 0.5 mol % catalyst, 1.41 mmol
substrate, and 2.82 mmol TBHP. In all cases control experiments
confirmed low conversion of substrate (<10%) in the absence of
catalyst.
b
c
chloroform (5 mL), 50 °C. Selectivity after 24 h. Reaction yields
were determined by GC/MS measurements; mesitylene was used as
d
e)
internal standard. Addition of Et3N (1 mol %). Due to ring-
f)
opening, benzaldehyde and diols as side products. Addition of
Generally complexes 1−5 oxidize the different olefins in fairly
high yields. While complete conversion to the corresponding
epoxide is observed for cyclohexene and cyclooctene, lower
conversions are observed for both terminal alkenes styrene and
4-phenyl-1-butene. As shown in Table 5, the pendant donor arms
as well as the substituents on the aromatic ring influence the
catalytic activity. Complexes 1 and 2 with an additional OMe
group in the side chain are more active than their NMe2 (3, 4)
and Et (5) based counterparts (e.g., in the epoxidation of
cyclooctene for 1 TOF = 359 h−1 for 3 TOF = 46 h−1 and for 5
TOF = 133 h−1). The epoxidation of styrene or 4-phenyl-1-
butene is more challenging as these substrates are less electron
rich. Herein again, the OMe based complexes 1 and 2 prove to be
more active catalysts than their NMe2 (3, 4) and Et (5) based
counterparts. Among them, both complexes 1 (R1 = tBu) and 2
(R1 = Me) with substituents in ortho and para position on the aryl
ring show excellent catalytic activities and conversions up to 80%
(1) and 76% (2) for styrene and 88% (1) and 70% (2) for
4-phenyl-1-butene are obtained after 24 h, respectively (see
Table 5). Furthermore, catalysts 1−4 are highly selective, as ring-
opening of the styrene epoxide to the corresponding diol or
aldehyde is not significant under the applied reaction conditions.
In contrast, complex 5 without a donor atom in the pending arm
shows dramatically lower conversions and poorer selectivity in
the epoxidation of styrene (39% yield after 24 h and 57%
selectivity). Such poor selectivity has been previously reported in
the literature with a similar cis-[MoO2]2+ complex coordinated to
bidentate Schiff base ligand.7 The conversions obtained with
complexes 1 and 2 in the epoxidation of styrene and 4-phenyl-1-
butene are quite high and in the same range as for some other
selective catalysts in the literature.15,32 Commercially available
[MoO2(acac)2] reacts faster in the epoxidation catalysis, as
cyclooctene or cyclohexene are converted within a few minutes
n-butyl methyl ether (1 mol %).
to the corresponding epoxide.11,33,34 Nevertheless, in the
epoxidation of styrene the formation of benzaldehyde and
further ring-opening products is observed.34
Quite similar epoxide yields for complexes 3 and 4 were
obtained by reactions performed under inert conditions (Ar).
Furthermore, the addition of molecular sieves during catalysis
has no significant effect. All catalytic reaction mixtures are
homogeneous single liquid phases.
As shown in the reaction profile of complexes 1−5 in the
epoxidation of styrene (see Figure 6), a sterically more
Figure 6. Reaction profile of complexes 1−5 in the epoxidation of
styrene.
demanding substituent on the aromatic ring has a negligible
influence on the catalytic activity, since similar reaction curves are
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obtained for complexes 1 (R1 = tBu) and 2 (R1 = Me) as well as
for complexes 3 (R1 = tBu) and 4 (R1 = Me). Nevertheless, the
reaction profile clearly demonstrates the influence of the pendant
arm in the side chain. Both complexes 1 and 2 with a OMe group
are more active than their NMe2 based counterparts 3 and 4 and
the Et based counterpart complex 5.
Table 6. Epoxidation of Aliphatic and Aromatic Alkenes with
Complexes 1, 3, and 5
The reaction mechanism in the epoxidation of alkenes is
still under debate.9,35−37 However, the more likely mechanism
includes the addition of TBHP across one terminal MoO
group, leading to the formation of Mo−OH and Mo−O−O-tBu
moieties.38 After coordination of the tBu-O-O to the metal center
its α-O atom is transferred to the alkene, producing the epoxide
under concomitant elimination of tert-butyl alcohol, yielding the
initial complex. Theoretical studies evidence the importance of H
bond formations at various steps of the catalytic cycle.35,37 Thus,
not only the oxygen atom transfer represents a barrier but also
the hydrogen atom transfers. It is noteworthy that the release of
the formed epoxide was calculated to have almost no barrier
whereas a significant energy barrier was found for the release of
the side product tert - butyl alcohol.35
The design of our complexes with pendant donor groups in the
side chain allows in principle the formation of H bonds during
the catalytic cycle, which may explain the different catalytic
activities of complexes 1−5. Both NMe2 based complexes 3 and 4
are less active, and lower epoxide conversions are observed. It
seems that the more basic NMe2 donor group is more prone
toward protonation leading to the formation of a less active
species and hence slowing down the catalytic activity. We tested
the influence of an amine (NEt3, 1 mol %) in the epoxidation of
cyclooctene using [MoO2(acac)2] (0.5 mol %) as catalyst and
TBHP as oxidants. Without amine, full conversion to the
corresponding epoxide is observed within 10 min of reaction
time, whereas the addition of the amine considerably slows down
the catalytic activity and only 10% yield of cyclooctene-oxide is
observed within 1 h of reaction time (see Table 5). An increase in
the selectivity accompanied by a decrease in the efficiency is also
observed in the rhenium based MTO system upon addition of
pyridines or pyrazoles, because of the reduced Lewis acidity of
the catalytic system.39 Such inhibiting behavior is not evident in
complexes 1, 2, and 5.
On the other hand, the higher catalytic rates of complexes 1
and 2 compared to 5 may be explained by the influence of the
OMe group on the formation of hydrogen bonds. Such bonds
may be stable enough to lower the activation barriers but not too
strong as in the case of NMe2 groups. Furthermore, the potential
coordination of the OMe group to the molybdenum may
facilitate the release of tert-butanol.
Additionally, we tested the effect of an intermolecular donor
in the epoxidation of styrene catalyzed by [MoO2(acac)2]
(0.5 mol %) in presence of n-butyl methyl ether (1 mol %).
Again, a selectivity of only 65% concerning styrene epoxide is
observed, thereby proving that a donor per se does not increase
the selectivity as observed in complexes 1−4, but it requires an
intramolecular donor (see Table 5).
a
Reaction conditions: 0.5 mol % catalyst, 1.41 mmol alkene (1 equiv),
1.41 mmol internal standard (1 equiv), 2.82 mmol (2 equiv) TBHP,
chloroform (5 mL), 50 °C. Selectivity after 24 h. Reaction yields
were determined H NMR spectroscopy; dichloroethane was used as
internal standard. Reaction yields were determined by HPLC
chromatography; mesitylene was used as internal standard. Reaction
yields were determined by GC/MS measurements; mesitylene was
used as internal standard. Due to ring-opening, benzaldehyde and
b
c
1
d
e
f
g
acetophenone as side products. Dipentene dioxide (limonene
dioxide) as side product.
yields is in good accordance with the literature.7,8,30 Significantly
higher conversions are obtained in the epoxidation of cis-stilbene
with respect to the epoxidation of trans-stilbene but in both cases
with high stereoselectivities for complex 1. Similar results have
been observed in the literature, in some cases with higher epoxide
conversions.8,30 After 24 h, the formation of side products
because of ring-opening in the epoxidation of trans-stilbene is
apparent for complex 5. α-Terpineol and limonene are
interesting substrates, as both are naturally occurring. After 24 h
we observe full conversion of α-terpineol to the corresponding
epoxide. The remaining hydroxyl group is not affected, as already
shown by others in the literature.7 The epoxidation of limonene
is more challenging, as the substrate includes two quite different
alkene moieties, one internal and one terminal. For complexes
1 and 5, complete conversion is obtained, but selectivities are
low as the formation of the double epoxide, dipentene
dioxide (limonene dioxide), occurs after 4 h. Complex 3 is
more selective, as only the formation of 1,2 epoxy limonene is
observed. Nevertheless, this is most probably caused by the low
epoxidation rate of complex 3. Some catalysts in the literature are
more selective, as only the formation of 1,2 epoxy limonene is
observed.14 Negligible conversions with all catalysts are observed
using ally phenyl ether as substrate. To sum up, complex 1 with a
OMe substituent in the pendant arm is more reactive than its
NMe2 based counterpart complex 3. Complex 5 is as reactive as
complex 1.
With complexes 1, 3, and 5 epoxidation of a variety of different
other aliphatic and aromatic alkenes was explored. Results are
summarized in Table 6. All alkenes are oxidized in moderate to
high yields, with complexes 1 and 5 to be generally more reactive
than 3.
Both aliphatic alkenes, 1-octene and 2-propenol, are
epoxidized in high yields and good selectivities. The hydroxyl
group of 2-propenol is accepted under these reaction conditions
as we do not observe further products. The range of the observed
After one catalytic run catalyst 1 could be isolated as a solid
material after evaporation of the solvent and subsequent addition
of heptane. Both, IR and MS (EI) measurements of this material
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All 1H and 13C NMR spectra were recorded on a Bruker Avance III
indicate that complex 1 remains unchanged. Furthermore we
were concerned with possible decomposition reactions based on
the more reactive N atom in complexes 3 and 4. Therefore, we
investigated the possible oxidation of the NMe2 moiety by
addition of TBHP to solutions of the ligand as well as the
complex 3. However, no formation of N oxide could be observed
as NMR spectra remained unchanged. Attempts to isolate any
intermediate molybdenum(VI) tert-butyl hydroperoxide com-
plex have not yet been successful. The synthesis of molybdenum
peroxo complexes by treatment of 1 with H2O2 or reaction of
HL1 with [MoO(O2)2·DMF]40 have likewise been unsuccessful.
Furthermore, the use of H2O2 as terminal oxidant did not yield
significant amounts of the corresponding epoxide.
Spectrometer (300 MHz for H and 75 MHz for 13C NMR). The H
NMR spectroscopic data are reported as s = singlet, d = doublet, t =
triplet, m = multiplet or unresolved, br = broad signal, coupling con-
stants are reported in hertz (Hz), and chemical shifts are given in parts
per million (ppm) relative to the solvent residual peak. All deuterated
solvents were purchased from Deutero GmbH and dried over molecular
sieves. IR spectra were measured as solid samples on a Bruker Alpha B
Diamond FTIR spectrometer. Mass spectra have been measured on an
Agilent 5973 MSD-Direct Probe using the EI ionization technique. GC-
MS measurements were performed on an Agilent 7890A with an Agilent
19091J-433 column coupled to a mass spectrometer type Agilent
5975C. Elemental analyses were carried out using a Heraeus Vario
Elementar automatic analyzer at the Institute of Inorganic Chemistry at
the University of Technology in Graz.
1
1
Lastly, it is noteworthy that the formation of heterogeneous
catalysts with our [MoO2(LX)2] complexes should be possible.
The noncoordinated donor group in the side chain seems to be a
good linking moiety for their immobilization onto inorganic
solids or organic polymers. Such polymer supported Schiff base
complexes of different metal ions are well-known in the literature
and serve as highly active catalysts in various types of reactions
(e.g., polymerizations, oxidations).5,24,26,27,41
Syntheses of the Ligands. Ligand HL1. 2-Methoxyethylamine
(1.05 g, 14 mmol) was added to a solution of 3,5-di-tert-butyl-2-hydroxy-
benzaldehyde (3.28 g, 14 mmol) in 50 mL of MeOH. The mixture was
refluxed overnight. After cooling to room temperature, the solution was
dried with Na2SO4. The solvent was removed under vacuum to afford a
yellow viscous oil, which was used without further purification. Yield:
3.80 g (93%).
1H NMR (300 MHz, chloroform-d, 298 K): δ 1.30 (s, 9H, Ar−
C(CH3)3), 1.44 (s, 9H, Ar−C(CH3)3), 3.36 (s, 3H, OCH3), 3. 66 (t,
3
3JH−H = 5.4 Hz, 2H, N(CH2)2OCH3), 3.74 (t, JH−H = 5.4 Hz, 2H,
N(CH2)2OCH3), 7.09 (d, 4JH−H = 2.4 Hz, 1H, Ar-H), 7.37 (d, 4JH−H
=
CONCLUSIONS
■
2.4 Hz, 1H, Ar-H), 8.37 (s, 1H, Ar−CHN), 13.70 (s, 1H, Ar−OH). 13C
NMR (75 MHz, chloroform-d, 298 K): δ 29.64 (Ar−C(CH3)3), 31.71
(Ar−C(CH3)3), 34.33 (Ar-C(CH3)3), 35.22 (Ar-C(CH3)3), 59.16
(N(CH2)2OCH3), 59.23 (N(CH2)2OCH3), 72.16 (N(CH2)2OCH3),
118.06 (Ar), 126.18 (Ar-H), 127.12 (Ar-H), 136.83 (Ar), 140.18 (Ar),
158.29 (Ar−OH), 167.63 (Ar-CHN). IR (ATR, cm−1): 1632 (s, CN),
1467 (m), 1458 (m), 1440 (s), 1390 (m), 1361 (m), 1273 (m), 1239
(m), 1121 (s), 971 (w), 923 (w), 904 (w), 877 (w), 839 (m), 827 (m).
MS (EI) (70 eV) m/z (%): 291.2 (36.5) [M]+, 276.3 (100.0) [M-CH3]+,
260.2 (3.8) [M-OCH3]+, 57.1 (8.7) [C4H9]+.
We have been able to prepare a series of new [MoO2(LX)2]
complexes with Schiff base ligands using the uncommon η2
coordinated [MoO2(η2-tBu2pz)2] complex as starting material.
All complexes are readily accessible in moderate to good yields.
With the more sterically demanding tBu substituent on the aryl
ring (complexes 1, 3, and 5) a mixture of two isomers in solution
is obtained, whereas only one isomer in solution is detected for
both methyl substituted complexes 2 and 4. Low temperature
NMR measurements of complexes 1 and 3 clearly indicate the
formation of one symmetric (major isomer) and one asymmetric
isomer (minor isomer). X-ray diffraction analyses show the
ligands to be, in all cases, symmetrically coordinated to the metal
center. Complexes 1−5 have been tested as catalysts in the
epoxidation of various alkenes using TBHP as oxidants. Among
them, complexes 1 and 2 prove to be highly selective in the
epoxidation of styrene, and epoxide conversions up to 80% are
obtained after 24 h. They belong to a rare group found in the
literature, which undergo selective epoxide formation of this
challenging substrate in high yields. Complex 5 with no side
chain functionality is significantly less selective in the epoxidation
of styrene. Both NMe2 based complexes 3 and 4 are less active.
Further research concerning the mechanism of such epoxidation
reactions is in progress.
Ligand HL2. The synthesis of the ligand followed the procedure
described above. 2-Methoxyethylamine (1.26 g, 16.8 mmol) was added
to a solution of 3,5-dimethyl-2-hydroxybenzaldehyde42 (2.10 g, 14 mmol)
in 50 mL of methanol. After workup, the ligand was obtained as a light
orange viscous oil, which was used without further purification. Yield:
2.76 g (95%).
1H NMR (300 MHz, chloroform-d, 298 K): δ 2.24 (s, 6H, 2x
3
Ar-CH3), 3.34 (s, 3H, N(CH2)2OCH3), 3.63 (t, JH−H = 5.5 Hz, 2H,
3
N(CH2)2OCH3), 3.71 (t, JH−H = 5.4 Hz, 2H, N(CH2)2OCH3), 6.86
(s, 1H, Ar-H), 6.98 (s, 1H, Ar-H), 8.25 (s, 1H, Ar−CHN), 13.35 (s, 1H,
Ar−OH). 13C NMR (75 MHz, chloroform-d, 298 K): δ 15.49 (Ar-CH3),
20.35 (Ar-CH3), 58.94 (N(CH2)2OCH3), 59.13 (N(CH2)2OCH3),
72.00 (N(CH2)2OCH3), 117.75 (Ar), 125.57 (Ar), 126.95 (Ar), 129.05
(Ar-H), 134.28 (Ar-H), 157.26 (Ar−OH), 166.54 (Ar-CHN). IR (ATR,
cm−1): 1632 (s, CN), 1605 (m), 1474 (m), 1438 (m), 1266 (s), 1120
(s), 1033 (m), 969 (w), 956 (w), 857 (m), 837 (m). MS (EI) (70 eV)
m/z (%): 207.2 (91.2) [M]+, 192.1 (2.4) [M-CH3]+, 176.1 (16.7)
[M-OCH3]+, 162.1 (83.8) [M-CH2OCH3]+, 149.1 (25.5) [(M-
(CH2)2OCH3)+H]+, 135.1 (100.0) [(M-N(CH2)2OCH3)+H]+.
Ligand HL4. The ligand HL4 was prepared in analogous manner to
the ligand HL1. N,N-Dimethylethylene-diamine (1.48 g, 16.8 mmol)
was added to a solution of 3,5-dimethyl-2-hydroxybenz-aldehyde42
(2.10 g, 14 mmol) in methanol (50 mL). The product was obtained as a
dark brown viscous oil, which was used without further purification.
Yield: 2.96 g (96%).
EXPERIMENTAL SECTION
■
General Remarks. All reactions involving air-sensitive compounds
were carried out under an atmosphere of dry argon using standard
Schlenkline or glovebox techniques. [MoO2(acac)2] and tert-butyl
hydroperoxide (TBHP, 5.5 M in decane, over molecular sieves 4 Å)
were purchased from Sigma Aldrich and used as received. All other
chemicals were obtained from different suppliers and used without
further purification. Solvents were purified via a Pure-Solv MD-4-EN
solvent purification system from Innovative Technology, Inc. Methanol
was refluxed over activated magnesium for at least 24 h and then distilled
prior to use. Chloroform was extracted three times with water and then
stirred for 24 h over CaCl2. After filtration, the solvent was refluxed over
P2O5 for 2 h and then distilled. 3,5-Dimethyl-2-hydroxybenzaldehyde,42
the ligands (HL3 and HL5)22,23 as well as the metal precursors
[MoO2(η2-tBu2pz)2]17 and [MoO2Cl2(dme)]28 were prepared accord-
ing to literature procedures.
1H NMR (300 MHz, chloroform-d, 298 K): δ 2.16 (s, 3H, Ar−CH3),
2.17 (s, 3H, Ar−CH3), 2.20 (s, 6H, N(CH3)2), 2.51 (t, 3JH−H = 6.9 Hz,
3
2H, N(CH2)2N(CH3)2), 3.57 (t, JH−H = 6.9 Hz, 2H, N(CH2)2N-
(CH3)2), 6.75 (s, 1H, Ar-H), 6.89 (s, 1H, Ar-H), 8.14 (s, 1H, Ar−CHN),
13.34 (s, 1H, Ar−OH). 13C NMR (75 MHz, chloroform-d, 298 K): δ
15.30 (Ar-CH3), 20.15 (Ar-CH3), 45.56 (N(CH2)2N(CH3)2), 57.55
(N(CH2)2N(CH3)2), 59.83 (N(CH2)2N(CH3)2), 117.55 (Ar), 125.27
(Ar), 126.56 (Ar), 128.75 (Ar-H), 133.97 (Ar-H), 157.08 (Ar−OH),
165.52 (Ar-CHN). IR (ATR, cm−1): 1631 (s, CN), 1605 (m), 1460 (m),
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1438 (m), 1266 (s), 1040 (m), 1020 (m), 958 (w), 935 (w), 905 (w),
855 (m). MS (EI) (70 eV) m/z (%): 220.1 (11.7) [M]+, 176.1 (2.5)
[M-N(CH3)2]+, 162.1 (2.6) [M-CH2N(CH3)2]+, 135.1 (6.8) [(M-
N(CH2)2N(CH3)2)+H]+, 58.1 (100.0) [CH2N(CH3)2]+.
1H NMR (300 MHz, chloroform-d, 238 K, A (major isomer) and B
(minor isomer) A:B = 2:1) δ 1.24 (s, 18H, 2x C(CH3)3, B), 1.28 (s, 9H,
C(CH3)3, A), 1.40 (s, 9H, C(CH3)3, B), 1.42 (s, 9H, C(CH3)3, A), 1.98
(s, 6H, N(CH2)2N(CH3)2, B), 2.09 (s, 6H, N(CH2)2N(CH3)2, A), 2.19
(m, 3H, N(CH2)2N(CH3)2, B), 2.29 (s, 6H, N(CH2)2N(CH3)2, B),
2.40 (m, 1H, N(CH2)2N(CH3)2, A), 2.61 (m, 1H, N(CH2)2N(CH3)2,
A), 2.96 (m, 2H, N(CH2)2N(CH3)2, B), 3.52 (m, 3H, N(CH2)2
N(CH3)2, 2A+B), 4.00 (m, 1H, N(CH2)2N(CH3)2, B), 4.25 (m, 1H,
N(CH2)2N(CH3)2, B), 7.08 (s, 1H, Ar-H, B), 7.15 (d, 4JH−H = 3.0 Hz,
2H, Ar-H, A+B), 7.46 (s, 1H, Ar-H, B), 7.49 (d, 4JH−H = 3.0 Hz, 2H, Ar-
H, A+B), 8.09 (s, 1H, Ar−CHN, B), 8.28 (s, 1H, Ar−CHN, A), 8.36 (s,
1H, Ar−CHN, B). 13C NMR (75 MHz, chloroform-d, 298 K, major
isomer A) δ 30.34 (C(CH3)3), 31.66 (C(CH3)3), 34.51 (C(CH3)3),
35.34 (C(CH3)3), 45.72 (N(CH2)2N(CH3)2), 56.95 (N(CH2)2N-
(CH3)2), 59.95 (N(CH2)2(CH3)2), 121.69 (Ar), 128.45 (Ar-H),
129.81 (Ar-H), 138.80 (Ar), 142.45 (Ar), 160.49 (Ar-O), 168.46 (Ar-
CHN). IR (ATR, cm−1): 1625 (CN), 1561 (m), 1439 (m), 1413 (m),
1392 (m), 1360 (m), 1266 (m), 1246 (m), 1202 (m); 1174 (m), 1039
(m), 993 (w), 914 (s, MoO), 902 (s, MoO), 843 (s), 752 (s), 615
(m), 551 (s), 486 (m), 434 (m). MS (EI) (70 eV) m/z (%): 736.6 (0.1)
[M]+, 706.6 (5.4) [(M-NCH3)-H]+, 691.5 (2.3) [(M-N(CH3)2)-H]+,
433.2 (31.6) [M-C19H31N2O]+, 417.1 (1.6) [M-C19H31N2O2]+, 303.2
(7.2) [C19H31N2O]+, 58.1 (100.0) [CH2N(CH3)2]+. Anal. Calcd. for
MoO4N4C38H62: C, 62.11; H, 8.50; N, 7.62. Found: C, 62.34; H, 8.30;
N, 7.95%.
General Synthetic Procedure for Molybdenum(VI) Dioxo
Complexes. The respective ligand (0.62 mmol, 2 equiv) was dissolved
in 2 mL of dry toluene and slowly added to a solution of 150 mg of
[MoO2(η2-tBu2pz)2] (0.31 mmol, 1 equiv) in 10 mL of toluene. The
formation of the complex was immediately indicated by a change of
color. To ensure complete formation of the complex, the solution was
stirred overnight at room temperature. The mixture was then filtered
through a pad of Celite, and the solvent was removed in vacuo. The
residue was washed twice with 5 mL of pentane or heptane, affording the
pure compounds as yellow to light brown solids.
[MoO2(L1)2] (1). The synthesis of complex 1 followed the general
procedure described above. A solution of the ligand HL1 (0.18 g, 0.63
mmol) was slowly added to a solution of [MoO2(η2-tBu2pz)2] (0.15 g,
0.31 mmol) in toluene. After purification, compound 1 was obtained as a
yellow solid. Yield: 0.17 g (77%).
1H NMR (300 MHz, chloroform-d, 238 K, A (major isomer), B
(minor isomer) A:B = 4:1) δ 1.25 (s, 9H, C(CH3)3, B), 1.27 (s, 9H,
C(CH3)3, B), 1.29 (s, 18H, C(CH3)3, A+B), 1.41 (s, 18H, 2x C(CH3)3,
A+B), 3.14 (s, 3H, N(CH2)2OCH3, B), 3.26 (s, 3H, N(CH2)2OCH3, A),
3.34 (s, 3H, N(CH2)2OCH3, B), 3.46 (m, N(CH2)2OCH3, overlapping
signals A+B), 3.67 (m, NCH2CH2OCH3, overlapping signals A+B), 3.83
(m, 2H, NCH2CH2OCH3, B), 4.15 (m, 1H, N(CH2)2OCH3, B), 4.29
(m, 1H, N(CH2)2OCH3, B), 7.10 (d, 4JH−H = 2.2 Hz, 1H, Ar-H, B), 7.21
(d, 4JH−H = 2.5 Hz, 2H, Ar-H, A+B), 7.48 (d, 4JH−H = 2.2 Hz, 2H, Ar-H,
B), 7.50 (d, 4JH−H = 2.5 Hz, 1H, Ar-H, A), 8.10 (s, 1H, Ar−CHN, B),
8.30 (s, 1H, Ar−CHN, A), 8.38 (s, 1H, Ar−CHN, B). 13C NMR
(75 MHz, chloroform-d, 298 K, major isomer A) δ 30.24 (C(CH3)3),
31.63 (C(CH3)3), 34.50 (C(CH3)3), 35.36 (C(CH3)3), 59.11 (N-
(CH2)2OCH3), 59.17 (N(CH2)2OCH3), 71.51 (N(CH2)2OCH3),
121.54 (Ar), 128.63 (Ar-H), 129.93 (Ar-H), 138.84 (Ar), 142.53
(Ar), 160.33 (Ar-O), 168.99 (Ar-CHN). IR (ATR, cm−1): 1628 (s, C
N), 1440 (m), 1414 (w), 1390 (w) 1268 (m), 1252 (s), 1236 (m), 1178
(m), 1178 (m), 1124 (m), 1072 (m), 928 (m, MoO), 904 (s, MoO),
841 (s), 752 (m), 617 (m), 594 (w), 547 (br, s), 482 (m), 429 (m). MS
(EI) (70 eV) m/z (%): 710.5 (2.3) [M]+, 694.5 (0.7) [M-O]+, 678.5
(0.4) [M-2O]+, 625.4 (6.5) [(M-CHN(CH2)2OCH3)+H]+, 420.1
(100.0) [M-C18H28NO2]+, 404.1 (1.7) [M-C18H28NO3]+, 291.1 (7.0)
[(C18H28NO2)+H]+, 57.1 (12.5) [C4H9]+. Anal. Calcd. for
MoO6N2C36H56·0,37 CH2Cl2: C, 59.00, H, 7.72; N, 3.78. Found: C,
59.24; H, 7.50; N, 3.89%.
[MoO2(L4)2] (4). The synthesis of complex 4 followed the general
procedure described above. A solution of the ligand HL4 (0.14 g, 0.63
mmol) in toluene was slowly added to a solution of [MoO2(η2-tBu2pz)2]
(0.15 g, 0.31 mmol) in toluene. After purification, compound 4 was
obtained as a light brown solid. Yield: 0.11 g (63%).
1H NMR (300 MHz, chloroform-d, 298 K) δ 2.11 (s, 6H, N(CH3)2),
2.26 (s, 3H, Ar−CH3), 2.27 (s, 3H, Ar−CH3), 2.58 (m, 2H,
N(CH2)2N(CH3)2), 3.45 (m, 2H, N(CH2)2N(CH3)2), 6.98 (s, 1H,
Ar-H), 7.14 (s, 1H, Ar-H), 8.18 (s, 1H, Ar−CHN). 13C NMR (75 MHz,
chloroform-d, 298 K) δ 16.63 (Ar-CH3), 20.52 (Ar-CH3), 45.76
(N(CH3)2), 59.01 (N(CH2)2N(CH3)2), 60.21 (N(CH2)2N(CH3)2),
120.71 (Ar), 127.99 (Ar), 129.46 (Ar), 130.96 (Ar), 131.50 (Ar-H),
136.88 (Ar-H), 159.12 (Ar-O), 167.29 (Ar-CHN). IR (ATR, cm−1):
1625 (s, CN), 1571 (w), 1454 (w), 1259 (s), 1227 (m), 1171 (w),
1028 (w), 926 (s, MoO), 900 (s, br, MoO), 867 (m), 846 (s), 823
(s), 781 (w), 749 (m), 611 (m), 549 (m), 514 (m), 417 (m). MS (EI)
(70 eV) m/z (%): 568.3 (0.1) [M]+, 523.2 (2.9) [(M-N(CH3)2)-H]+,
482.2 (7.6) [M-N(CH2)2N(CH3)2]+, 349.1 (35.5) [M-(C13H19N2O)]+,
219.1 (9.3) [C13H19N2O]+, 72.1 (15.3) [(CH2)2N(CH3)2]+, 58.2
(100.0) [CH2N(CH3)2]+. Anal. Calcd. for MoO4N4C38H62: C, 55.12, H,
6.76; N, 9.89. Found: C, 54.93; H, 6.51; N, 10.28%.
[MoO2(L2)2] (2). The synthesis of complex 2 followed the general
procedure described above. A solution of the ligand HL2 (0.13 g, 0.63
mmol) in toluene was slowly added to a solution of [MoO2(η2-tBu2pz)2]
(0.15 g, 0.31 mmol) in toluene. After purification, compound 2 was
obtained as a light brown solid. Yield: 0.066 g (82%).
[MoO2(L5)2] (5). The synthesis of complex 5 followed the general
procedure described above. A solution of the ligand HL5 (0.18 g, 0.63
mmol) in toluene was slowly added to a solution of [MoO2(η2-tBu2pz)2]
(0.15 g, 0.31 mmol) in toluene. After purification, compound 5 was
obtained as a yellow solid. Yield: 0.12 g (54%).
1H NMR (300 MHz, chloroform-d, 298 K) δ 2.24 (s, 3H, Ar-CH3),
2.28 (s, 3H, Ar-CH3), 3.26 (s, 3H, OCH3), 3.53 (m, 3H,
N(CH2)2OCH3), 3.72 (m, 1H, N(CH2)2OCH3), 7.01 (d, 1H, Ar-H),
7.16 (d, 1H, Ar-H), 8.20 (s, 1H, Ar−CHN). 13C NMR (75 MHz,
chloroform-d, 298 K) δ 16.48 (CH3), 20.45 (CH3), 59.18 (N-
(CH2)2OCH3), 61.00 (N(CH2)2OCH3), 71.59 (N(CH2)2OCH3),
120.53 (Ar), 127.99 (Ar-H), 129.57 (Ar-H), 131.70 (Ar), 136.90
(Ar), 158.81 (Ar-O), 167.79 (Ar-CHN). IR (ATR, cm−1): 1626 (s,
CN), 1571 (m), 1473 (m), 1267 (m), 1229 (m), 1110 (s), 988 (w),
959 (w), 921 (s, MoO), 901 (s, MoO), 839 (s), 829 (s), 749 (m),
612 (m), 550 (s), 516 (s), 470 (m), 421 (s). MS (EI) (70 eV) m/z (%):
542.2 (4.3) [M]+, 510.1 (0.4) [M-2O]+, 457.1 (3.4) [(M-CHN-
(CH2)2OCH3)+H]+, 336.1 (100.0) [M-C12H16NO2]+, 207.1 (7.8)
[(C12H16NO2)+H]+, 59.1 (7.9) [(CH2)2OCH3]+. Anal. Calcd. for
MoO6N2C24H32·0.4 CH2Cl2: Found: C, 51.00; H, 5.75; N, 4.87. Found:
C, 51.05; H, 5.46; N, 4.97%.
1H NMR (300 MHz, chloroform-d, 298 K, major isomer A) δ 0.75 (t,
3JH−H = 7.3 Hz, 3H, N(CH2)3CH3), 1.13 (m, 2H, N(CH2)3CH3), 1.31
(s, 9H, C(CH3)3), 1.44 (s, 9H, C(CH3)3), 1.52 (m, 1H, N(CH2)3CH3),
1.76 (m, 1H, N(CH2)3CH3), 3.46 (m, 2H, N(CH2)3CH3), 7.16 (s,
4JH−H = 2.4 Hz, 1H, Ar-H), 7.53 (s, 4JH−H = 2.4 Hz, 1H, Ar-H), 8.26 (s,
1H, Ar−CHN). 13C NMR (75 MHz, chloroform-d, 298 K) δ 13.8
(N(CH2)3CH3), 20.54 (N(CH2)3CH3), 30.21 (C(CH3)3), 31.65
(C(CH3)3), 33.58 (N(CH2)3CH3), 34.51 (C(CH3)3), 35.41
(C(CH3)3), 60.06 (N(CH2)3CH3), 121. 59 (Ar), 128.18 (Ar-H),
129.69 (Ar-H), 138.93 (Ar), 142.35 (Ar), 160.43 (Ar-O), 167.02 (Ar-
CHN). IR (ATR, cm−1): 1628 (s, CN), 1563 (w), 1439 (m), 1391
(w), 1360 (w), 1270 (m), 1249 (s), 1202 (w), 1180 (w), 917 (m, Mo
O), 904 (s, MoO), 844 (s), 772 (w), 754 (m), 551 (m), 432 (m). MS
(EI) (70 eV) m/z (%): 706.6 (9.9) [M]+, 690.6 (5.9) [M-O]+, 623.5
(100.0) [M-(CHN(CH2)3CH3)+H]+, 57.2 (61.4) [C4H9]+. Anal.
Calcd. for MoO4N2C38H60: C, 64.75, H, 8.58; N, 3.97. Found: C,
65.03; H, 8.50; N, 3.97%.
[MoO2(L5)2] (3). The synthesis of complex 3 followed the general
procedure described above. A solution of the ligand HL3 (0.19 g, 0.63
mmol) was slowly added to a solution of [MoO2(η2-tBu2pz)2] (0.15 g,
0.31 mmol) in toluene. After purification, compound 3 was obtained as a
yellow solid. Yield: 0.16 g (68%).
Epoxidation. In a typical epoxidation reaction, catalyst (7.05 × 10−3
mmol, 0.5 mol %), the corresponding alkene (1.41 mmol, 1 equiv), and
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internal standard (1.41 mmol, 1 equiv) were combined in dry chloro-
form (5 mL). After stirring the mixture for 5 min, the epoxide reaction
was started with the addition of TBHP (0.5 mL of a 5.5 M solution in
decane, 2.82 mmol, 2 equiv). The reactions were monitored quantita-
tively by GC/MS (cyclooctene, cyclohexene, styrene, 4-phenyl-1-
butene, α-terpineol, R-(+)-limonene), HPLC (cis-stilbene, trans-
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1
stilbene), or H NMR (1-octene and 2-propenol) analyses. At fixed
́
E. Z. Naturforsch. 2004, 59b, 1223−1228. (d) Sobczak, J. M.; Ziolkowski,
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acetate (for GC/MS) or acetonitrile (for HPLC). Mesitylene was used
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1
as internal standard for GC/MS and HPLC measurements. H NMR
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spectra were measured in chloroform-d using dichloroethane as internal
standard.
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X-ray Structure Determination. For X-ray structure analyses the
crystals were mounted onto the tip of glass fibers, and data collection
was performed at 100 K using graphite monochromated MoKα radiation
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(λ = 0.71073 Å) with a BRUKER-AXS SMART APEX II diffractometer
̌
̌ ́ ́
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2
reduced to Fo and corrected for absorption and polarization using
SAINT43 and SADABS44 programs, respectively. The structures were
solved by direct methods (SHELXS-9745 or SIR9246) and refined by
full-matrix least-squares techniques against F2 (SHELXL-9747). If not
otherwise stated, all non-hydrogen atoms were refined with anisotropic
displacement parameters without any constraints. All hydrogen atoms
were located in calculated positions to correspond to standard bond
lengths and angles. All diagrams were drawn with 50% probability
thermal ellipsoids, and all H atoms were omitted for clarity. For complex
1 the C15 atom was split and modeled as two-site disorder with 0.6/0.4
occupancy. The crystals for complex 3 were all heavily twinned; data set
was collected from a crystal which seemed to be the best from the
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the refinement as neither ISOR nor DELU constraints could prevent it
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−3
́
from going NPD. The high residual electron density (+ 4.4 e Å ) is
located close to the Mo1 atom. Crystallographic data (excluding
structure factors) for the structures of compounds 1 and 3 reported in
this paper have been deposited with the Cambridge Crystallographic
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ASSOCIATED CONTENT
Baumgartner, J.; Belaj, F.; Jancik, V.; Herbst-Irmer, R.; Mosch-Zanetti,
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́
(21) (a) Coperet, C.; Adolfsson, H.; Sharpless, K. B. Chem. Commun.
̈
■
S
* Supporting Information
Crystallographic data in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: nadia.moesch@uni-graz.at.
Author Contributions
The manuscript was written through contributions of all authors.
All authors have given approval to the final version of the
manuscript.
(24) Cameron, P. A.; Gibson, V. C.; Redshaw, C.; Segal, J. A.; Bruce, M.
D.; White, A. J. P.; Williams, D. J. Chem. Commun. 1999, 1883−1884.
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Notes
The authors declare no competing financial interest.
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