Dioxomolybdenum(VI) and -tungsten(VI) Complexes with Multidentate Aminobisphenol Ligands as Catalysts for Olefin Epoxidation

Article abstract of DOI:10.1002/ejic.201500055

The synthesis of four molybdenum and tungsten complexes bearing tetradentate tripodal amino bisphenolate ligands with either hydroxyethylene (1a) or hydroxyglycolene (1b) substituents is reported. The molybdenum dioxo complexes [MoO₂L] (L = 2a, 2b) and tungsten complexes [WO₂L] (3a, 3b) were synthesized using [MoO₂(acac)₂] and [W(eg)₃] (eg = 1,2-ethanediolato, ethylene glycolate), respectively, as precursors. All complexes were characterized by spectroscopic means as well as by single-crystal X-ray diffraction analyses. The latter reveal, in all cases, hexacoordinate complexes in which the hydrogen atom of the hydroxy group is involved in hydrogen bonding with one of the metal oxo groups. In the case of the glycol substituent, the ether oxygen atom is coordinated to the metal whereas the hydroxy group remains uncoordinated. The complexes were tested as catalysts in the epoxidation of cyclooctene under eco-friendly conditions, using an aqueous solution of H₂O₂ as the oxidant and dimethyl carbonate (DMC) as solvent or neat conditions, as substitutes for chlorinated solvents. Molybdenum complexes 2a and 2b showed good catalytic activity using H₂O₂ without added solvent, and tungsten complexes 3a and 3b showed very high activity in the epoxidation of cyclooctene using H₂O₂ and DMC as solvents. Four new molybdenum and tungsten complexes bearing tetradentate tripodal amino bisphenolate ligands with either hydroxyethylene or hydroxyglycolene substituents were synthesized and found to catalyze olefin epoxidation reactions under eco-friendly conditions.

Full text of DOI:10.1002/ejic.201500055

FULL PAPER
DOI:10.1002/ejic.201500055
Dioxomolybdenum(VI) and -tungsten(VI) Complexes with
Multidentate Aminobisphenol Ligands as Catalysts for
Olefin Epoxidation
CLUSTER
ISSUE
Antoine Dupé,[a][‡] Md. Kamal Hossain,[b][‡] Jörg A. Schachner,[a]
Ferdinand Belaj,[a] Ari Lehtonen,*[c] Ebbe Nordlander,*[b] and
Nadia C. Mösch-Zanetti*[a]
Keywords: Molybdenum / Tungsten / Hydrogen bonds / Epoxidation / Green chemistry
The synthesis of four molybdenum and tungsten complexes
bearing tetradentate tripodal amino bisphenolate ligands
with either hydroxyethylene (1a) or hydroxyglycolene (1b)
substituents is reported. The molybdenum dioxo complexes
[MoO2L] (L = 2a, 2b) and tungsten complexes [WO2L] (3a,
3b) were synthesized using [MoO2(acac)2] and [W(eg)3] (eg
= 1,2-ethanediolato, ethylene glycolate), respectively, as pre-
cursors. All complexes were characterized by spectroscopic
means as well as by single-crystal X-ray diffraction analyses.
The latter reveal, in all cases, hexacoordinate complexes in
which the hydrogen atom of the hydroxy group is involved
in hydrogen bonding with one of the metal oxo groups. In
the case of the glycol substituent, the ether oxygen atom is
coordinated to the metal whereas the hydroxy group remains
uncoordinated. The complexes were tested as catalysts in the
epoxidation of cyclooctene under eco-friendly conditions,
using an aqueous solution of H2O2 as the oxidant and di-
methyl carbonate (DMC) as solvent or neat conditions, as
substitutes for chlorinated solvents. Molybdenum complexes
2a and 2b showed good catalytic activity using H2O2 without
added solvent, and tungsten complexes 3a and 3b showed
very high activity in the epoxidation of cyclooctene using
H2O2 and DMC as solvents.
Similarly, the tungsten enzymes can be divided into two
families: the aldehyde oxidoreductases and the formate de-
hydrogenases.[3] A common feature of these proteins is a
M(VI)=O moiety in the active site of the enzyme, which is
bonded to the cofactor molybdopterin.[4] In view of these
important biological functions, many examples of molybd-
enum and tungsten-containing model compounds for OAT
are known.[5,6] Molybdenum-based catalysts for oxidation
have been developed and used in important industrial pro-
cesses such as alkene epoxidation.[7]
Introduction
High-valent molybdenum and tungsten oxides are found
in a variety of metalloenzymes that take part in oxygen
atom transfer (OAT) reactions, whereby an oxygen atom is
transferred to or from a biologically relevant donor/ac-
ceptor molecule.[1] The molybdenum oxotransferase en-
zymes may be subdivided into three (structural) families
that are named after prototypical enzymes: the xanthine
oxidase, sulfite oxidase and DMSO reductase families.[2]
Tetradentate tripodal aminobisphenolate ligands have
proven to be versatile ligands able to coordinate to a variety
of metals.[8] A common type of complex with such ligands
exhibits a trianionic (O3N)3– donor atom motif around the
metal center.[9–12,13] Because such a motif creates a coordi-
nation pocket around the metal atom resembling the active
site of metalloenzymes, amino phenolate ligands have also
been employed as structural and functional models of such
metalloenzymes, for instance, the VV-dependent haloper-
oxidases.[14] By varying the structure of one arm, along with
the reaction conditions, a dianionic (O2NOR)2– coordina-
tion motif can also be achieved. The corresponding com-
plexes are useful catalysts for a broad range of reac-
tions.[15,16] In our ongoing research into the coordination
chemistry of dioxomolybdenum(VI) complexes,[17–19] we
were interested in using amino bisphenolate ligands con-
[a] Institute of Chemistry, Department of Inorganic Chemistry,
University of Graz,
Schubertstraße 1, 8010 Graz, Austria
E-mail: nadia.moesch@uni-graz.at
http://chemie.uni-graz.at/en/inorganic-chemistry/research/
moesch-zanetti-group
[b] Inorganic Chemistry Research Group, Chemical Physics,
Centre for Chemistry and Chemical Engineering, Lund
University,
Box 124, 22100 Lund, Sweden
E-mail: ebbe.nordlander@chemphys.lu.se
http://www.chemphys.lu.se/people/nordlander/
[c] Department of Chemistry, University of Turku,
20014 Turku, Finland
E-mail: ari.lehtonen@utu.fi
http://www.utu.fi/en/units/sci/units/chemistry/research/mcca/
Pages/Sub-pages%20of%20Functional%20Materials/Metal-
organic-Chemistry.aspx
[‡] These authors contributed equally to this work.
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201500055.
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taining an aliphatic alcohol as a third arm donor moiety, complexes. Only one set of resonances is visible (only two
to generate a tetradentate dianionic ligand (O2NOH)2– with aromatic and two tBu signals), consistent with formation of
the hydroxy moiety remaining intact. The presence of such symmetric O,O-trans species. The proton of the
a neutral hydroxy group in proximity to the molybdenum hydroxy group, not visible in the spectrum of the free li-
center might allow for hydrogen bonding with incoming gand, appears as a triplet at δ = 3.33 ppm in the spectrum
oxidants like tert-butylhydroperoxide (TBHP) or hydrogen of 2b, arising from coupling (J = 6.2 Hz) to the methylene
peroxide (H2O2). Such hydrogen bonds have been identified group directly bound to the hydroxy group, which appears
as important contributors to the activation of OAT reac- as a multiplet at δ = 4.10 ppm. This indicates slow or non-
tions from peroxides to olefins during catalytic epoxida- existent exchange of the alcohol proton, suggesting
tions.[20]
hydrogen interaction between this proton (hydrogen) and
The catalytic epoxidation of alkenes by dioxomolybden- the oxo group (as also observed by X-ray crystallography,
um(VI)[6,17–19,21] and dioxotungsten(VI)[22,23] complexes has see below). After several days, it was noticed that the NMR
been intensively investigated. Amino bisphenolate com- sample of 2a turned purple, indicating degradation of the
plexes have also been tested as epoxidation catalysts using complex to a possible chlorinated species. A new measure-
tert-butylhydroperoxide (TBHP) and hydrogen peroxide ment of this sample showed multiple new sets of peaks that
(H2O2) as oxidants.[9,10,24] One major issue when consider- could not be assigned.
ing catalytic reactions involves the question of how to limit
their environmental impact by establishing optimal reaction
conditions (i.e. low catalyst loading, reduction of side-prod-
uct formation and substitution of chlorinated or hazardous
solvents by non-toxic solvents or neat conditions). Within
this paper the synthesis and characterization of dioxo-
molybdenum(VI) and dioxotungsten(VI) complexes
equipped with dianionic amino bisphenolate ligands
(O2NOH)2– with a pendant hydroxy donor arm are de-
scribed. These novel complexes show high catalytic activity
Scheme 1. Amino bisphenolate ligands 1a and 1b, complexes 2a
and 2b (M = Mo) and 3a and 3b (M = W).
in the epoxidation of cyclooctene with either TBHP or
H2O2 in dimethyl carbonate (DMC)[25] and even under neat
reaction conditions, using 1 mol-% catalyst.
For the synthesis of tungsten complexes 3a and 3b,
[W(eg)3] (eg, 1,2-ethanediolato)[28] was used as a precursor
and dissolved in methanol. A solution of the corresponding
ligand 1a or 1b in chloroform was added and the mixture
Results and Discussion
Ligands 1a and 1b were synthesized under neat condi- heated at 60 °C until precipitation of the product occurred.
tions in a fashion analogous to published procedures.[26] Complex 3a was isolated as a white solid albeit in poor
Yields, relative to published reports, were improved by puri- yield (34%). By using [WO2(acac)2] as the starting material,
fying 1a and 1b via column chromatography (gradient mix- 3a could be alternatively prepared in acetonitrile in signifi-
tures of EtOAc and cyclohexane as eluent) affording 1a in cantly higher yield (69%). IR spectroscopy and EI mass
75% and 1b in 67% yields. Both compounds, isolated as spectrometry confirm the formation of 3a, with characteris-
white solids, are highly soluble in common organic solvents tic resonances in IR at 948 and 918 cm–1 for the W=O
such as chlorinated solvents and hot methanol. Analytical stretching (925 and 917 cm–1 for Mo=O in 2a), and a mo-
data for 1a and 1b are consistent with previously published lecular ion peak corresponding to 711.6 g mol–1. The com-
data.[11,27]
pound is poorly soluble in common organic solvents such
For the synthesis of the molybdenum complexes, a solu- as methanol, toluene or THF, and slightly more soluble in
tion of 1 equiv. 1a or 1b, respectively, in acetonitrile was chlorinated solvents. However, as observed for 2a, upon dis-
added to a suspension of 1 equiv. of [MoO2(acac)2] in solution of the solid in CDCl3, slow degradation becomes
acetonitrile. The orange mixtures were heated at 70 °C, evident. Despite its poor solubility, 3a was partially dis-
whereupon corresponding molybdenum complexes 2a and solved in [D4]MeOH allowing acquisition of NMR data.
2b rapidly precipitated as yellow solids (Scheme 1). After The NMR spectrum obtained shows a set of resonances
filtration, 2a and 2b were isolated in good yields. The com- consistent with a symmetric species. Determination of the
plexes are soluble in common chlorinated and aromatic sol- molecular structure (vide infra) points to a six-coordinate
vents. Proton NMR analyses in deuterated chloroform con- species with a coordinated hydroxy group. Similarly, 3b was
firmed the coordination of the ligands to each metal center prepared using [W(eg)3] as the starting material and isolated
as indicated by the disappearance of the broad signals from as a white solid in 53% yield. Like 2b, NMR spectroscopic
each phenol OH proton, and by splitting of respective sing- data of 3b in deuterated chloroform revealed only one set
lets corresponding to the four CH2 protons bound to the of signals corresponding to the symmetric O,O-trans com-
aromatic groups into two doublets with large coupling con- plex and the hydrogen form the hydroxy group appeared as
stants, as is typical for Cs-symmetric amino bisphenolate a triplet at δ = 2.86 ppm (J = 5.8 Hz).
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Molecular Structures of Complexes 2a, 2b, 3a and 3b
Single crystals of 2a and 2b were obtained by slow evapo-
ration from concentrated acetonitrile solutions at room
temperature. Both complexes display a distorted octahedral
coordination environment around the Mo atom. For both
complexes, the amino bisphenolate ligand coordinates in a
dianionic, tetradentate fashion, with the hydroxy group of
the pendant arm still protonated (H3 in 2a, H6 in 2b), re-
sulting in an overall neutral dioxomolybdenum(VI) com-
plex. The coordination of the hydroxy group as a neutral
donor in 2a can be deduced from the long Mo1–O3 bond
length [2.2867(10) Å].[29] Both solid-state structures indicate
hydrogen bonding from the hydroxy hydrogen atom to the
metal oxo group. Short distances between the hydroxy oxy-
gen atom and the oxo group are observed [2.6680(19) Å in
2a and 2.7516(17) Å in 2b]. For the latter, NMR spec-
troscopy confirms this interaction. The bond lengths of the
oxo ligands O1 and O2 [1.7305(14) and 1.6952(14) Å resp.]
and of the phenolate ligands O11 and O31 [1.9403(14) Å
and 1.9337(14) Å, respectively] to the Mo center are within
the expected range for such complexes.[10,12,16,30]
The molecular structures, determined by single-crystal
X-ray diffraction analyses, are displayed in Figures 1 and 2;
selected bond lengths and angles are given in Table 1. Se-
lected details pertaining to data collection and refinement
are given in Table 4; full details are given in the Supporting
Information. For all four complexes, the positions of the H
atoms of all OH groups could be found in difference Fou-
rier maps. The O–H distances were fixed at 0.84 Å.
Table 1. Selected bond lengths [Å] and angles [°] for 2a, 2b, 3a and
3b.
2a
2b
3a
3b
M1–O1
M1–O2
M1–O3
M1–O11
M1–O31
M1–N1
1.7305(14) 1.7004(9)
1.6952(14) 1.7204(8)
2.2867(10) 2.3321(9)
1.9403(14) 1.9543(9)
1.9337(14) 1.9360(9)
2.3697(16) 2.3628(10) 2.3682(15) 2.3573(19)
155.00(4) 159.43(6) 155.42(7)
1.7457(13) 1.7408(16)
1.7204(13) 1.7162(16)
2.2357(13) 2.3128(16)
1.9449(13) 1.9493(16)
1.9393(13) 1.9349(17)
O1–M1–N1 156.99(6)
Figure 1. Molecular views (50% probability level) of complexes 2a (left) and 2b (right) (solvent molecules and hydrogen atoms except for
hydroxy group are omitted for clarity). Complex 2a shows two independent molecules in the unit cell with very similar conformations;
only one molecule is shown here. The hydroxyethyl group in 2b was disordered over two orientations and refined with site occupation
factors of 0.813(4) and 0.187(4), respectively. The less occupied 2-hydroxyethoxy orientation has been omitted for clarity reasons.
Figure 2. Molecular views (50% probability level) of complexes 3a (left) and 3b (right) (solvent molecules and hydrogen atoms except for
hydroxy group are omitted for clarity).
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Single crystals of 3a and 3b were obtained by slow evapo-
ration from methanol/chloroform mixtures (1:1 v/v) at am-
bient temperature. Both complexes display a distorted octa-
hedral coordination environment around the W atom, sim-
ilar to the Mo complexes 2a and 2b. Again, for both com-
plexes the amino bisphenolate ligand coordinates in a dian-
ionic, tetradentate fashion, with the hydroxy group of the
pendant arm still protonated (H3 in 3a, H6 in 3b), resulting
in an overall neutral dioxotungsten(VI) complex. In the so-
lid-state structure of 3b, the pendant hydroxy group is ori-
entated towards the oxo ligand O1, and once again the
short distance between O6 and O1 [2.717(3) Å] confirms
formation of a hydrogen bond. The coordination of the
hydroxy group in 3a can again be deduced from the long
W1–O3 bond length, and hydrogen bonding is confirmed
by the short O3–O1 distance [2.5401(19) Å]. The bond
lengths of the oxo ligands O1 and O2 [1.7457(13) and
1.7204(13) Å resp.] and of the phenolate ligands O11 and
O31 [1.9449(13) Å and 1.9393(13) Å resp.] to the W centre
are within the expected range for similar complexes.[16,31] In
contrast, the W1–O3 distance is much longer at
2.2357(13) Å, consistent with a neutral hydroxy group.[29]
In general, the structure of 3b displays geometric param-
eters very similar to those of 2b.
Scheme 2. Substrates S1–S5 used in epoxidation experiments with
complexes 2a, 2b and 3a, 3b.
plexes 3a and 3b exerted less activity; the reaction with 3b
afforded a yield of only 70% after 24 h. For more challeng-
ing substrates S2–S5, activities as well as selectivities,
dropped significantly for all four catalysts. Mo complexes
2a and 2b showed catalytic activity for all four substrates
S2–S5. For the epoxidation of S2, high selectivities for ep-
oxide formation with moderate yields were obtained. For
styrene S3, low yields (17 and 14%, respectively) with di-
minished epoxidation selectivities (43 and 30%, respec-
tively) were observed for 2a and 2b, with benzaldehyde and
benzacetaldehyde being the main side products. For sub-
strate S4, complexes 2a and 2b showed high regioselectivity
Catalytic Epoxidation
As a benchmark experiment to assess catalytic activities, as only the endocyclic double bond underwent epoxidation.
complexes 2a, 2b, 3a and 3b were screened in the catalytic However, no selectivity for cis or trans epoxidation was ob-
epoxidation of five different olefinic substrates S1–S5 using served; both epoxides were detected by GC in a about 1:1
TBHP (5.5 m in decane) as the oxygen source (Scheme 2) ratio. One of the side products formed could be identified as
under standard experimental conditions (0.5 mL of CHCl3 a mixture of d- and l-carvone, the cyclohexanone oxidation
with 1 mol-% of catalyst and 3 equiv. of TBHP at 50 °C) as product of limonene. Finally, a catalytic profile similar to
summarized in Table 2. The five olefin substrates used were that found with S4 was obtained for α-terpinol S5, demon-
cyclooctene S1, 1-octene S2, styrene S3, limonene S4 and strating a functional group tolerance of 2a and 2b for hy-
α-terpineol S5. In the case of S4 and S5, racemic mixtures droxy groups, as expected. Tungsten complexes 3a and 3b
of the d- and l-enantiomers were used.
were less effective at catalyzing epoxidations with TBHP
Products were identified by gas chromatography. Only oxidant compared to Mo complexes 2a and 2b. This obser-
selectivity towards epoxide formation is reported and vation agrees well with previous reports.[23,32,33] No catalytic
attempts to identify enantiomers/diastereomers were not activity (Ͻ5% yield of epoxide) for 1-octene S2, styrene S3
made. As expected, the epoxidation of cyclooctene S1 with and α-terpinol S5 could be detected with 3a and 3b. Good
TBHP was achieved using all four complexes, with high yields and high selectivities were obtained only in epoxida-
selectivities. Alternatively, reactions with Mo complexes 2a tions of cyclooctene S1 (Table 2). Low yields and selectivi-
and 2b afforded high yields of epoxide (Ͼ99%) relatively ties for epoxide were observed for limonene S4. Interest-
rapidly – after 2 h and 1 h, respectively but the W com- ingly, in all catalytic experiments with W complexes 3a and
Table 2. Summary of epoxidation results u 2a, 2b and 3a, 3b with TBHP.
2a
2b
3a
3b
Yield (Sel.)[a] t[b]
Yield (Sel.)[a] t[b]
Yield (Sel.)[a] t[b]
Yield (Sel.)[a] t[b]
[%]
[h]
[%]
[h]
[%]
[h]
[%]
[h]
S1
Ͼ99 (100)
40 (100)
17 (43)
61 (63)
48 (50)
2
Ͼ99 (100)
25 (100)
14 (30)
64 (70)
46 (58)
1
85 (100)
Ͻ5 (n.d.)
Ͻ5 (n.d.)
10 (14)
24
24
24
24
24
70 (100)
Ͻ5 (n.d.)
Ͻ5 (n.d.)
20 (34)
24
24
24
5
S2
24
24
4
24
24
6
S3
S4[c]
S5[c]
24
24
Ͻ5 (n.d.)
Ͻ5 (n.d.)
24
[a] Maximum yield of epoxide, selectivity (Sel.) to epoxide given in brackets; n.d.: not determined. [b] Time to reach maximum yield of
epoxide, maximum experiment time was 24 h. [c] Sum of both possible diastereoisomers.
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Table 3. Epoxidation of cyclooctene using H2O2.[a]
2a
2b
3a
3b
Yield (Sel.)[b]
[%]
T
[h]
Yield (Sel.)[b]
[%]
t
[h]
Yield (Sel.)[b]
[%]
t
[h]
Yield (Sel.)[b]
[%]
t
[h]
CHCl3
CHCl3
DMC
DMC
Neat
39
Ͼ99
16
34
58
4
20
4
20
4
20
59
Ͼ99
13
45
2
4
20
4
20
4
20
40
Ͼ99
80
Ͼ99
15
40
4
20
4
20
4
20
36
Ͼ99
71
Ͼ99
0
0
4
20
4
20
4
20
Neat
Ͼ99
95
[a] Cyclooctene (0.45 mmol), 3 equiv. H2O2 (50% in water), cat. (1 mol-%), solv. [0.45 m] or neat, 50 °C. Monitored by GC with mesitylene
as internal standard. [b] Selectivity for cyclooctene oxide was 100%.
3b we detected oxidation products of decane (solvent of oxi- bearing a short arm with a hydroxy group exhibits good
dant TBHP, e.g. 4-decanone, 5–10%), which could point to activity (59% yield after 4 h). To investigate the role of the
a possible catalytic activity in alkane oxidation for 3a and hydroxy group during the catalytic reaction, a molybdenum
3b.
complex similar to 2a but bearing a methoxy group instead
By designing tripodal bisphenolate complexes bearing a of a hydroxy group was synthesized following a previously
hydroxy group in one arm, we sought to achieve efficient reported procedure.[16] This complex was tested as a catalyst
catalytic epoxidation using eco-friendly conditions. To do for cyclooctene epoxidation under the conditions described
so, we tested 2a, 2b and 3a, 3b as catalysts for epoxidation in Table 3. When using DMC as solvent, the yield of epox-
of cyclooctene using aqueous hydrogen peroxide as oxidant ide reached 20% after 4 h and 55% after 24 h; the complex
instead of TBHP solutions in decane. The advantage of with the methoxy group showing better activity than 2a.
using H2O2 is that only water is present as a byproduct at Under neat reaction conditions, the yield reached 44% after
the end of the reaction, instead of tert-butanol and decane. 4 h and 97% after 24 h, the methoxy catalyst exhibited sim-
More importantly, dimethyl carbonate, a non-toxic and ilar activity to that of 2a. These results indicate that the
non-hazardous solvent,[25] was tested as a solvent substitute presence of the hydroxy group in the arm of the catalysts
for chloroform. Catalytic experiments using H2O2 in neat does not significantly affect the catalytic reaction nor does
conditions were also performed (Scheme 3). Results are it increase the activity of the complexes. Despite this obser-
summarized in Table 3.
vation, the complexes 2a, 2b and 3a, 3b are interesting cata-
lysts relative to previously reported systems using eco-
friendly conditions. Morlot et al.,[21m] reported the synthesis
of molybdenum complexes that could catalyze cyclooctene
epoxidation under neat reaction conditions with very good
activity (88% yield and 89% selectivity after 5.5 h for epox-
ide using 1 mol-% [Mo] catalyst), but using TBHP as oxi-
dant. Vrdoljak et al.[21p] used similar conditions (TBHP/
water, no added solvent) for cyclooctene epoxidation cata-
lyzed by molybdenum and tungsten complexes and faced
selectivity issues (53% selectivity for the epoxide in the best
case). Jimtaisong et al.,[23] used molybdenum complexes
with H2O2 in acetonitrile or ethanol but only observed
moderate yields (35% after 24 h in the best case). The best
catalytic systems using H2O2 as oxidant are the cyclo-
pentadienyloxido-molybdenum and -tungsten complexes re-
ported by Dinoi et al.,[34] exhibiting high activity at low
catalyst loading but requiring MeCN as added solvent. In
the present study, complexes 2a and 2b are active Mo cata-
lysts using H2O2 without added solvents, and 3a and 3b are
very active catalysts using H2O2 and a more eco-friendly
solvent than MeCN.
Scheme 3. Epoxidation of cyclooctene with complexes 2a, 2b and
3a, 3b under eco-friendly conditions.
In contrast to the observations with TBHP, tungsten
complexes 3a and 3b showed higher catalytic epoxidation
activities in reactions using H2O2; conversion of cyclo-
octene to cyclooctene oxide was found to be quantitative in
chloroform. Interestingly, the tungsten complexes showed
excellent activity when epoxidations were performed in di-
methyl carbonate instead of CHCl3. Reactions were found
to be two times faster, reaching 80% yield of epoxide using
3a and 71% using 3b after 4 h. Because of their low solubili-
ties in H2O2 solution and cyclooctene, 3a and 3b showed
limited activity when reactions were performed in the ab-
sence of solvent. Reduced activity could be expected using
molybdenum complexes 2a and 2b with H2O2 as oxidant
when compared to tungsten complexes, as already observed
in the literature.[23,33,34] For this reason, examples of Mo/
H2O2 catalytic systems are scarce.[33,35] However, quantita-
tive conversion of cyclooctene could be achieved with Mo
complexes 2a and 2b, using CHCl3 as solvent or, more im-
Conclusions
Four new molybdenum and tungsten complexes, 2a, 2b
portantly, under neat conditions. In the latter case, data and 3a, 3b, with tetradentate tripodal aminobisphenolate
shows an induction period for complex 2b, but complex 2a ligands bearing a hydroxy group in one arm were synthe-
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the residual solvent peak. Electron impact Mass spectra were re-
corded with an Agilent Technologies 5975C inert XL MSD instru-
ment using the direct insertion technique. Results are denoted as
cationic mass peaks, unit is the mass/charge ratio. Gas chromatog-
raphy mass spectroscopy measurements (GC–MS) have been per-
formed with a gas chromatograph type Agilent 7890 A (column
type Agilent 19091J–433), coupled to an Agilent 5975 C mass spec-
trometer. Samples for infrared spectroscopy were measured using
a Bruker Optics ALPHA FT-IR Spectrometer. IR bands are re-
ported with wave number [cm–1] and intensities (br = broad, vs. =
sized and fully characterized. The complexes were tested
as catalysts for epoxidation of cyclooctene in eco-friendly
conditions, using aqueous solutions of H2O2 as oxidant and
dimethyl carbonate (DMC) as solvent or neat conditions as
substitutes for chlorinated solvents. Molybdenum com-
plexes 2a and 2b converted cyclooctene to its epoxide in
more than 95% yield using H2O2 without added solvent,
and tungsten complexes 3a and 3b showed very high ac-
tivity in cyclooctene epoxidation using H2O2 and DMC as
solvents (80% and 71% yield, respectively, after 24 h). very strong, s = strong, m = medium, w = weak). A Heidolph
Parallel Synthesizer 1 was used for all epoxidation experiments.
Elemental analyses were carried out at the Microanalytical Labora-
tory of the University of Vienna using a EuroVector EA3000 in-
strument.
These results highlight the potential of aminobisphenolate
molybdenum and tungsten-based complexes as epoxidation
catalysts amenable to eco-friendly conditions. Since com-
plexes 2a and 2b were also able to convert different sub-
strates (styrene, 1-octene, terpenes) using TBHP in CHCl3,
future applications of eco-friendly conditions to these more
challenging epoxidation reactions may be readily envi-
sioned.
X-ray Structure Determination: X-ray data collection was per-
formed with a Bruker AXS SMART APEX 2 CCD diffractometer
by using graphite-monochromated Mo-Kα radiation (0.71073 Å)
from a fine-focus sealed tube at 100 K. SHELXS-97[37] was used as
structure solution and structure refinement program (Table 4).
Full-matrix least-squares on F2 was employed as refinement
method. Further details on the solution of the structures can be
found in the supporting information. Crystallographic data (ex-
cluding structure factors) for the compounds reported in this article
were deposited with the Cambridge Crystallographic Data Center
as supplementary publication numbers.
Experimental Section
General: Unless otherwise specified, all experiments were per-
formed under atmospheric conditions with standard laboratory
equipment. Commercially available chemicals and solvents were
used as received and no further purification or drying operations
were performed. Syntheses of ligands 1a and 1b[11,26,27] and the
precursor complexes [W(eg)3][28] and [WO2(acac)2][36] have been
published previously. Flash purifications were carried out on a
Biotage Isolera ISO-4SV automatic flash purification system with
UV detection at 254 and 280 nm. A gradient program was used
with EtOAc and cyclohexane mixtures (4–18% of the ester) as elu-
ent. The 1H and 13C NMR spectra were recorded with a Bruker
Optics Instrument 300 MHz or a Varian Inova 500 MHz spectrom-
eter. Peaks are denoted as singlet (s), doublet (d), doublet of dou-
blets (dd), triplet (t) and multiplet (m), Ar denotes aromatic pro-
tons. Chemical shifts are reported in ppm and are referenced using
CCDC-1043407 (for 2a), 1043408 (for 2b), 1043409 (for 3a),
1043410 (for 3b) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Epoxidation of Olefins: In a typical experiment, catalyst (2–3 mg,
1 mol-%) were dissolved in 1 mL of the respective solvent, mixed
with substrate (1 equiv., [0.45 m]), mesitylene (10 μL as an internal
standard) and heated to the respective reaction temperature. Oxi-
dants (3 equiv.) were then added. Aliquots for GC–MS (20 μL)
were withdrawn at given time intervals, quenched with MnO2 and
Table 4. Selected crystallographic data and structure refinement data for 2a, 2b, 3a and 3b.
2a
2b
3a
3b
Empirical formula
Crystal description
Crystal size
Crystal system, space group
Unit cell dimensions
2(C32H49MoNO5)·C2H3N C34H53MoNO6
C32H49NO5W·CH4O
needle, yellow
C34H53NO6W
plate, yellow
needle, yellow
needle, yellow
0.27ϫ0.12ϫ0.06 mm
triclinic, P1
0.28ϫ0.10ϫ0.09 mm
monoclinic, P21/n
a = 10.5571(3) Å
b = 11.6945(3) Å
c = 27.1485(8) Å
0.28ϫ0.12ϫ0.05 mm
monoclinic, P21/n
a = 10.6420(3) Å
b = 11.3195(3) Å
c = 27.6060(8) Å
0.20ϫ0.20ϫ0.04 mm
monoclinic, P21/n
a = 10.5340(4) Å
b = 11.6582(4) Å
c = 27.2429(11) Å
¯
a = 10.8267(3) Å
b = 15.4947(4) Å
c = 20.8325(6) Å
α = 109.4953(13)°
β = 91.5192(12)°
γ = 100.5836(12)°
2
β = 96.8380(10)°
β = 93.6295(10)°
β = 97.029(2)°
Z
4
2
4
Reflections collected/unique
Significant unique reflections
R(int), R(sigma)
Completeness to θ = 30.0°
Refinement method
Data/parameters/restraints
Goodness-of-fit on F2
Final R indices
38263/18764
24944/9687
8690 with IϾ2σ(I)
0.0212, 0.0243
99.7%
22397/9617
8731 with IϾ2σ(I)
0.0209, 0.0277
99.5%
28896/9682
8258 with IϾ2σ(I)
0.0318, 0.0353
99.9%
14385 with IϾ2σ(I)
0.0326, 0.0518
99.7%
full-matrix least squares on F2
18764/817/5
1.009
R1 = 0.0350,
wR2 = 0.0805
R1 = 0.0559,
wR2 = 0.0889
9687/421/2
1.039
R1 = 0.0246,
wR2 = 0.0620
R1 = 0.0291,
wR2 = 0.0647
9617/410/2
1.095
R1 = 0.0213,
wR2 = 0.0429
R1 = 0.0252,
wR2 = 0.0438
9682/412/0
1.035
R1 = 0.0234,
wR2 = 0.0528
R1 = 0.0320,
wR2 = 0.0558
[IϾ2σ(I)]
R indices (all data)
Eur. J. Inorg. Chem. 2015, 3572–3579
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FULL PAPER
diluted with HPLC grade EtOAc. The reaction products were ana-
lysed by GC–MS (Agilent Technologies 7890 GC System), and the
epoxide produced from each reaction mixture was quantified vs.
mesitylene as the internal standard.
0.275 mmol) in methanol (5 mL). The orange mixture was stirred
and heated at 70 °C. After 2 h stirring, the white precipitate was
filtered off and washed with cold methanol affording 3a in 53%
1
yield (0.11 g). H NMR (300 MHz, CDCl3, 298 K): δ = 7.37 (d, J
= 2.4 Hz, 2 H), 6.97 (d, J = 2.4 Hz, 2 H), 4.48 (d, J = 13.8 Hz, 2
H), 4.40 (dd, J1 = 4.5, J2 = 2.6 Hz, 2 H), 4.06 (m, 2 H), 4.00 (d, J
= 13.8 Hz, 2 H), 3.96 (t, J = 5.8 Hz, 2 H), 3.07 (t, J = 5.8 Hz, 2
H), 2.86 (t, J = 5.8 Hz, 1 H), 1.46 (s, 18 H), 1.29 (s, 18 H) ppm.
13C NMR (75 MHz, CDCl3, 298 K): δ = 157.00, 143.55, 138.35,
124.68, 123.44, 122.01 (Ar-C), 68.99 (CH2), 62.51 (CH2), 61.43
(CH2), 54.36 (CH2), 35.20 [C(CH3)], 34.46 [C(CH3)], 31.79 (CH3),
[MoO2(1a)] (2a): Ligand 1a (0.305 g, 0.61 mmol) was dissolved in
hot acetonitrile (5 mL) and added to a solution of [MoO2(acac)2]
(0.2 g, 0.61 mmol) in acetonitrile (5 mL). The orange mixture was
stirred and heated at 70 °C to dissolve all solid material. After
20 min. stirring, the yellow precipitate was filtered and washed with
cold acetonitrile to afford 2a in 66% yield (0.25 g). 1H NMR
(300 MHz, CDCl3, 298 K): δ = 7.29 (d, J = 2.4 Hz, 2 H), 6.98 (d,
J = 2.4 Hz, 2 H), 4.47 (d, J = 13.7 Hz, 2 H), 3.91 (d, J = 13.7 Hz,
2 H), 3.76 (t, J = 5.3 Hz, 2 H), 2.96 (t, J = 5.6 Hz, 2 H), 1.40 (s,
18 H), 1.29 (s, 18 H) ppm. 13C NMR (75 MHz, CDCl3, 298 K): δ =
158.73, 142.99, 137.23, 124.10, 123.58, 122.25 (Ar-C), 62.68 (CH2),
58.49 (CH2), 55.47 (CH2), 35.17 [C(CH3)3], 34.48 [C(CH3)3], 31.81
30.33 (CH ) ppm. IR (ATM): ν = 2949, 1473, 1439, 1359, 1242,
˜
3
1226, 1203, 1167, 1040, 954, 944 (w, W=O), 899 (s, W=O), 880, 848,
832, 757, 746, 602, 559, 496 cm–1. C34H53NO6W (755.65): calcd. C
54.04, H 7.07, N 1.85; found C 53.91, H 6.93, N 1.96.
Supporting Information (see Footnote on the first page of this arti-
cle) Further X-ray crystallographic details for each structure, the
crystal structure data and structure refinement data, as well as ste-
reoscopic ORTEP plots of compounds 2a, 2b, 3a and 3b.
(CH ), 30.28 (CH ) ppm. IR (ATR): ν = 2958, 1471, 1439, 1262,
˜
3
3
1171, 928 (w, Mo=O), 917 (w, Mo=O), 874, 855, 757, 561,
473 cm–1. C32H49MoNO5·0.5CH3CN·0.45H2O (652.3): calcd. C
60.76, H 7.94, N 3.22; found C 60.51, H 7.66, N 2.97.
[MoO2(1b)] (2b): Ligand 1b (0.332 g, 0.61 mmol) was dissolved in
hot acetonitrile (5 mL) and added to a solution of [MoO2(acac)2]
(0.2 g, 0.61 mmol) in acetonitrile (5 mL). The orange mixture was
stirred and heated at 70 °C to dissolve all solid material. After
20 min. stirring, the yellow precipitate was filtered and washed with
cold acetonitrile to afford 2b in 74% yield (0.30 g). 1H NMR
(300 MHz, CDCl3): δ = 7.31 (d, J = 2.4 Hz, 2 H), 6.99 (d, J =
2.4 Hz, 2 H), 4.46 (dd, J1 = 4.6, J2 = 2.8 Hz, 2 H), 4.24 (d, J =
13.8 Hz, 2 H), 4.10 (m, 2 H), 4.05 (t, J = 5.8 Hz, 2 H), 3.99 (d, J
= 13.9 Hz, 2 H), 3.33 (t, J = 6.2 Hz, 1 H), 2.99 (t, J = 5.8 Hz, 2
H), 1.44 (s, 18 H), 1.29 (s, 18 H) ppm. 13C NMR (75 MHz, CDCl3):
δ = 159.21, 143.32, 137.11, 124.29, 123.65, 122.26 (Ar-C), 77.37
(CH2), 67.20 (CH2), 61.04 (CH2), 60.71 (CH2), 53.78 (CH2), 35.22
[C(CH3)3], 34.48 [C(CH3)3], 31.77 (CH3), 30.34 (CH3) ppm. IR
Acknowledgments
Financial support by the Austrian Science Fund (FWF) (grant
number P26264-N19) is gratefully acknowledged. This work was
carried out within the framework of EU COST Action CM1003,
Biological oxidation reactions – mechanisms and design of new
catalysts. M. K. H. thanks the European Commission for an Eras-
mus Mundus predoctoral fellowship.
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Received: January 20, 2015
Published Online: April 27, 2015
Eur. J. Inorg. Chem. 2015, 3572–3579
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