Epoxidation of olefins using a dichlorodioxomolybdenum(VI)-pyridylimine complex as catalyst

Article abstract of DOI:10.1016/j.ica.2012.01.029

The ligand N-(n-propyl)-2-pyridylmethanimine (pyim) and an immobilized analogue of this ligand (pyim-MTS) were prepared by the condensation reaction of 2-pyridinecarboxaldehyde with either n-propylamine or 3-aminopropyl groups covalently attached to a micelle-templated silica (MTS). Free and immobilized dioxomolybdenum(VI) complexes of the type MoO₂Cl₂(L ¹) (L¹ = pyim (1), pyim-MTS) were then prepared by treatment of the organic ligand or ligand-silica with the solvent adduct MoO₂Cl₂(THF)₂. MoO₂Cl ₂(pyim) (1) is a highly active catalyst for the epoxidation of olefins (cyclooctene (Cy), cyclododecene, 1-octene, trans-2-octene, R-(+)-limonene) at 55 °C using tert-butylhydroperoxide (TBHP) as the oxidant under solvent-free conditions, giving the corresponding epoxides as the only reaction products. A turnover frequency of 1855 mol mol_Mo ^-1 h^-1 was measured for the epoxidation of Cy, and the epoxide (CyO) was formed quantitatively within 4.5 h. The MTS-supported complex was less active, and exhibited temperature-dependent leaching of active species. As an alternative approach to facilitating catalyst recycling, complex 1 was investigated with the ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate as solvent. The 1/IL phase could be reused, giving ca. 95% CyO yield in each run.

Full text of DOI:10.1016/j.ica.2012.01.029

Inorganica Chimica Acta 387 (2012) 234–239
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Epoxidation of olefins using a dichlorodioxomolybdenum(VI)-pyridylimine
complex as catalyst
Salete S. Balula a,b,c, Sofia M. Bruno , Ana C. Gomes , Anabela A. Valente , Martyn Pillinger ,
a
a
a
a
a,
⇑
c
c
Isabel S. Gonçalves , Duncan J. Macquarrie , James H. Clark
a
Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal
Faculty of Science, Department of Chemistry and Biochemistry, REQUIMTE, University of Porto, 4169-007 Porto, Portugal
Clean Technology Centre, Department of Chemistry, University of York, York, YO105 DD, UK
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The ligand N-(n-propyl)-2-pyridylmethanimine (pyim) and an immobilized analogue of this ligand
Received 28 October 2011
Received in revised form 12 January 2012
Accepted 13 January 2012
Available online 28 January 2012
(
pyim-MTS) were prepared by the condensation reaction of 2-pyridinecarboxaldehyde with either n-
propylamine or 3-aminopropyl groups covalently attached to a micelle-templated silica (MTS). Free
1
1
and immobilized dioxomolybdenum(VI) complexes of the type MoO
were then prepared by treatment of the organic ligand or ligand-silica with the solvent adduct
MoO Cl (THF) . MoO Cl (pyim) (1) is a highly active catalyst for the epoxidation of olefins (cyclooctene
Cy), cyclododecene, 1-octene, trans-2-octene, R-(+)-limonene) at 55 °C using tert-butylhydroperoxide
TBHP) as the oxidant under solvent-free conditions, giving the corresponding epoxides as the only reac-
2 2
Cl (L ) (L = pyim (1), pyim-MTS)
2
2
2
2
2
Keywords:
(
(
Dioxomolybdenum(VI) complex
Pyridylimine ligand
Catalysis
ꢀ1
ꢀ1
tion products. A turnover frequency of 1855 mol molMo
h
was measured for the epoxidation of Cy,
and the epoxide (CyO) was formed quantitatively within 4.5 h. The MTS-supported complex was less
active, and exhibited temperature-dependent leaching of active species. As an alternative approach to
facilitating catalyst recycling, complex 1 was investigated with the ionic liquid (IL) 1-butyl-3-methylimi-
dazolium tetrafluoroborate as solvent. The 1/IL phase could be reused, giving ca. 95% CyO yield in each
run.
Olefin epoxidation
Silica supports
Ionic liquids
Ó 2012 Elsevier B.V. All rights reserved.
1
1
. Introduction
plexes of the type MoO
2
Cl
2
(L )
n
proved to be among the most effi-
cient catalysts for the liquid-phase epoxidation of olefins using
tert-butylhydroperoxide (TBHP) as the mono-oxygen source. Of
the numerous ligand types investigated, bidentate N-donor ligands
such as substituted bipyridines [18,19], diazabutadienes [20–22]
and pyrazolylpyridines [23,24] gave highly active and selective
catalysts.
Metal-catalyzed olefin epoxidation is one of the most effective
approaches for the preparation of epoxides, which are important
building blocks for the production of bulk and fine chemicals
[
1–7]. The use of oxomolybdenum(VI) complexes as catalysts for
this reaction has been extensively explored over the last 40 years,
beginning with homogeneous molybdenum(VI) catalysts in the
Halcon and Arco processes for the epoxidation of propylene
One potentially promising family of ligands that have not, to the
best of our knowledge, been studied in complexes of the type
2
+
1
[
8–14]. Complexes containing the MoO
2
core are the most com-
MoO
2
Cl
2
(L )
n
are pyridylimines. These are attractive due to their
1
mon [13], and include neutral species of the type MoO
X
2 2
(L )
X [10–12,29,30],
X(L )]Y [31] (where X = ha-
n
easy preparation (from the condensation of 2-pyridinecarboxalde-
hyde with a primary amine) and the fact that their electronic
properties are intermediate between those of the more classical li-
gands such as bipyridine and phenanthroline, and the more flexible
diazabutadienes [32]. Another advantage is that it is relatively
straightforward to prepare inorganic support materials functional-
ized with these ligands. In this way several pyridine-derived palla-
dium [33–36], platinum [37], ruthenium [37], rhodium [38],
manganese [39], cobalt [40], copper [41,42], (acetylacetona-
to)dioxomolybdenum(VI) [43] and molybdenum tri- and tetracar-
bonyl [32,44] complexes have been immobilized on silica
supports and used as catalysts for various organic transformations,
2
5
[
15–24], MoO
2 n 5 5 2
(L ) [25–28] and (g -C R )MoO
2
and cationic complexes such as [MoO
lide, alkyl, OR, OSiR
2
1
2
3
, SCN, etc., L and L represent neutral and an-
ionic ligands bearing donor atoms from groups 15 and 16, and
ꢀ
ꢀ
6
or PF ). The catalytic performance
Y = anion such as halide, BF
4
of these complexes is governed to a large extent by their solubility
and steric/electronic factors, which are all strongly influenced by
the nature of the first-sphere ligands and their substituents. Com-
⇑
Corresponding author. Tel.: +351 234 378190; fax: +351 234 370084.
E-mail address: igoncalves@ua.pt (I.S. Gonçalves).
0
020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2012.01.029

S.S. Balula et al. / Inorganica Chimica Acta 387 (2012) 234–239
235
e.g. Heck and Suzuki reactions [33–37], oxidative methyl esterifica-
tion of aldehydes [39], oxidative aromatization of Hantzsch
,4-dihydropyridines [40], atom transfer polymerization of methyl
NMR (75.5 MHz, CDCl
3
): d = 158.9 (C
6
), 152.5 (C
1
, C
).
5 3
), 140.6 (C ),
128.3, 127.8 (C , C ), 64.0 (C
2
4
7
), 22.2 (C
8
), 11.4 (C
9
1
methacrylate [41], oxidation of ethylbenzene [42] and epoxidation
of olefins [32,43,44].
2.3. Preparation of the supported catalyst
In this paper, we describe the synthesis and characterization of
a dichlorodioxomolybdenum(VI) complex bearing the ligand N-(n-
The ligand-grafted MTS was prepared as follows [32–34,45,46]:
Tetraethylorthosilicate (18.8 g, 0.09 mol) was added to a solution
of dodecylamine (5.08 g, 27.5 mmol) in aqueous ethanol (46 mL
of ethanol and 53 mL of distilled water). After stirring for 18 h,
the precipitate was filtered and the template removed from the
white solid by Soxhlet extraction with ethanol. The MTS was then
dried at 100 °C. Aminopropyl-silica was prepared by the standard
method of treating MTS (5 g) with APTMS (1.79 g, 0.01 mol) in
refluxing toluene (100 mL) for 18 h. Filtration and repeated wash-
ing with ethanol and water, followed by drying at 100 °C yielded
the 3-aminopropyl-MTS (AP-MTS) as a white solid. The dried
AP-MTS (2 g) was reacted with 2-pyridinecarboxaldehyde (1.07 g,
propyl)-2-pyridylmethanimine,
a supported version of this
complex that comprises a chemically modified micelle-templated
silica, and an evaluation of the catalytic properties of both com-
pounds in olefin epoxidation systems using TBHP as the oxidant.
2
. Experimental
2.1. Materials and methods
Microanalyses for CHN were performed at the ITQB, Oeiras, Por-
0
.01 mol) in ethanol (50 mL) at room temperature for 24 h. The
tugal (by C. Almeida), and Mo was determined by ICP-AES at the
Central Laboratory for Analysis, University of Aveiro. FT-IR spectra
were obtained as KBr pellets using a FTIR Mattson-7000 infrared
spectrophotometer. A Bruker CXP 300 spectrometer was used to
resultant modified material was filtered, washed thoroughly with
toluene, diethyl ether and dichloromethane, and vacuum-dried at
1
00 °C for 2 h to give a yellow solid (pyim-MTS). Elemental analy-
ꢀ1
1
13
sis found: C, 22.6; H, 3.3; N, 7.0%. Selected FT-IR (KBr, cm ):
collect H and C NMR spectra at room temperature. Solid-state
1
3
m
9
= 1650 (m), 1590 (w), 1569 (w), 1471 (w), 1440 (w), 1076 (vs),
58 (sh), 780 (m), 696 (w), 564 (w), 461 (s). C CP MAS NMR:
C cross-polarization (CP) magic-angle-spinning (MAS) NMR spec-
1
3
tra were recorded at 125.76 MHz on a Bruker Avance 500 spec-
trometer with 3.5 ms 1H 90° pulses, 2 ms contact time, spinning
d = 161.7, 152.9, 149.2, 137.2, 124.8, 63.3, 42.5, 23.5, 10.6.
Immobilized dichlorodioxomolybdenum(VI) complexes were
2 2
prepared as follows: MoO Cl (0.56 g, 2.80 mmol) was dissolved
in THF (15 mL) and the solution stirred for 30 min at 50 °C under
nitrogen. The solution was then evaporated to dryness and the
rate of 7 kHz and 4 s recycle delays. Chemical shifts are quoted in
parts per million from tetramethylsilane.
All preparations and manipulations were carried out using stan-
dard Schlenk techniques under nitrogen. Anhydrous solvents
resultant MoO
was filtered and added to a suspension of the dried pyim-MTS (1 g)
in CH Cl (10 mL), and the mixture stirred at room temperature for
h under nitrogen. The solution was then filtered off and the solid
washed with CH Cl
2 2 2 2 2
Cl (THF) dissolved in CH Cl (10 mL). The solution
(
dichloromethane, toluene, n-hexane, THF and diethyl ether),
MoO Cl , dodecylamine, (3-aminopropyl)trimethoxysilane (APT-
2
2
2
2
MS) and 2-pyridinecarboxaldehyde were obtained from Aldrich
and used as received. Tetraethylorthosilicate was purchased from
Lancaster Chemicals. The ligand N-(n-propyl)-2-pyridylmethani-
mine (pyim) was prepared as described recently [32].
1
2
2
(4 ꢁ 20 mL), and finally vacuum-dried to give
the supported material designated as Mo-pyim-MTS with a molyb-
denum loading of 2.5 wt.%. Elemental analysis found: C, 20.9; H,
ꢀ1
3
.1; N, 5.6%. Selected FT-IR (KBr, cm ):
m = 1649 (m), 1599 (w),
2 2
2.2. MoO Cl (pyim) (1)
1
590 (w), 1570 (w), 1469 (w), 1439 (w), 1076 (vs), 940 (sh), 780
1
3
(
m), 695 (w), 566 (w), 459 (s). C CP MAS NMR: d = 162.0, 159.0,
48.8, 140.0, 127.6, 124.4, 64.2, 42.5, 21.6, 10.2.
2
MoO Cl
2
(0.25 g, 1.25 mmol) was dissolved in THF (20 mL) and
1
the solution stirred for 30 min at 50 °C under nitrogen. The ligand
pyim (0.19 g, 1.25 mmol) was then added and the reaction mixture
stirred at room temperature for 1 h. The solution was evaporated
to dryness, and the product washed with hexane (4 ꢁ 15 mL) and
dried under reduced pressure for 3 h to give a pale purple solid
2.4. Catalysis
The liquid-phase catalytic epoxidation of olefins (cis-cyclooc-
tene (Cy), 1-octene, trans-2-octene, cyclododecene, R-(+)-limo-
nene) was carried out at 55 °C, under air (atmospheric pressure)
in a closed borosilicate 10 mL reaction vessel equipped with a mag-
netic stirrer and immersed in a thermostated oil bath. In a typical
experiment, the molybdenum:olefin mole ratio was 0.003 and the
olefin/oxidant mole ratio was ca. 0.67 (TBHP, 5.5 M in decane),
(
0.34 g, 78%). Anal. Calc. for C
9 2 2 2
H12Cl O MoN (347.05): C, 31.14;
H, 3.48; N, 8.07. Found: C, 31.31; H, 3.65; N, 7.77%. Selected FT-
ꢀ
1
IR (KBr, cm ):
m = 1651 (m), 1644 (m), 1599 (vs, C@N), 1567 (w),
1
478 (m), 1307 (s), 940 (vs, Mo@Osym), 903 (vs, Mo@Oasym), 778
1
(
s), 645 (m), 377 (m), 347 (vs, MoꢀCl). H NMR (300.1 MHz,
CDCl
.94 (d, 1H, H
), 1.12 (t, 3H, H
9
3
): d = 9.54 (d, 1H, H
), 7.85 (m, 1H, H
) (see Scheme 1 for atom numbering).
1 6
), 8.43 (s, 1H, H ), 8.24 (dt, 1H, H
3
),
without additional solvent or using 150
lL of the ionic liquid (IL)
7
H
4
2
), 4.34 (t, 2H, H
7
), 2.28 (m, 2H,
1
-butyl-3-methylimidazolium tetrafluoroborate. The reaction mix-
1
3
8
C
ture involving complex 1 as catalyst without additional solvent
was a solution (homogeneous), and when the IL was used as sol-
vent, a biphasic liquid–liquid system was obtained where the cat-
alyst was dissolved in the IL phase (system denoted as 1/IL). Prior
to use, the TBHP solution was dried using activated molecular
sieves (3 Å), and the IL was dried under vacuum for 2 h at 100 °C.
In order to have approximate isothermal reaction conditions when
charging the reactor, the olefin and the catalyst were pre-heated to
the desired reaction temperature prior to the addition of TBHP,
which was also pre-heated to 55 °C (this instant was taken as zero
time). The course of the reaction was monitored using a Varian
3
5
800 GC equipped with a capillary column (Chrompack, CP-SIL
CB, 50 m ꢁ 0.32 mm ꢁ 0.5 m) and a flame ionization detector,
as carrier gas. The products were identified by GC–MS
l
Scheme 1. Synthesis of complex 1 showing the atom numbering for the NMR
assignments.
using H
2

2
36
S.S. Balula et al. / Inorganica Chimica Acta 387 (2012) 234–239
(
(
Trace GC 2000 Series (Thermo Quest CE Instruments) – DSQ II
Thermo Scientific), equipped with a fused silica capillary DB-5
type column (30 m ꢁ 0.25 mm; 0.25
lm film thickness)), using
He as carrier gas.
3
. Results and discussion
3.1. Synthesis and characterization
The complex MoO
method of replacing the THF molecules in the solvent adduct
MoO Cl (THF) by the pyridylimine ligand (Scheme 1). Elemental
2 2
Cl (pyim) (1) was prepared by the standard
2
2
2
analysis, FT-IR and NMR data were fully consistent with the suc-
cessful substitution of the THF ligands. In particular, the FT-IR
ꢀ1
spectrum contains a pair of bands at 940 and 903 cm , which
are assigned to the symmetric and asymmetric stretching vibra-
2
+
tions of the cis-[MoO
2
]
group, respectively. The corresponding
frequencies for the THF adduct lie at higher values (ca. 960 and
ꢀ
1
9
20 cm ), consistent with the stronger coordination of the pyr-
ꢀ1
idylimine ligand in 1. In the range of 1550–1650 cm , the free li-
gand exhibits one strong band at 1650 cm
ꢀ1
assigned to the
Fig. 1. 13C CP MAS NMR spectra and selected assignments for the modified
materials (a) AP-MTS, (b) pyim-MTS and (c) Mo-pyim-MTS.
m
(C@N) vibration of the imine group, and two medium intensity
ꢀ1
bands at 1568 and 1587 cm assigned to pyridyl ring stretching
vibrations. After complexation with the MoO Cl fragment, the
(imine) mode shifts to 1599 cm , and the two (pyridyl ring)
2
2
ꢀ1
m
m
represented in Scheme 2) and/or bipodally anchored (i.e., species
ꢀ1
bands are replaced by one weak band at 1567 cm . A strong band
at 347 cm for 1 is assigned as a Mo–Cl stretching vibration, in
agreement with that normally observed for complexes of the type
of the type RSi(OSi)
may estimate that about 40% of the NH
2
(OH)). From the C/N molar ratio of 3.8 we
groups in AP-MTS were
ꢀ1
2
converted to pyridylimine groups. Furthermore, the total loading
1
MoO
Cl
2 2
(L ) bearing bidentate Lewis-base ligands [15,24]. In the
of anchored n-propylsilyl moieties can be estimated as ca.
1
ꢀ1
H NMR spectrum of 1, the pyridine and imine resonances appear
as expected. The protons (H and H ) attached to the carbon atoms
3.6 mmol g . FT-IR spectroscopy provided useful information on
2
the reaction of the NH groups with 2-pyridinecarboxaldehyde.
1
6
adjacent to the pyridyl and imine nitrogens show deshielding from
the corresponding resonances for the free ligand (especially evi-
dent for H
reflects a reduction in the electron density at nitrogen due to coor-
dination with the Mo center.
Thus, in addition to the characteristic vibrations of the silica sup-
ꢀ
1
port observed between 400 and 1400 cm , the spectrum of
ꢀ1
1
: d = 8.54 ppm for pyim versus 9.54 ppm for 1), which
pyim-MTS contains two weak bands at 1569 and 1590 cm , as-
signed to pyridyl ring stretching vibrations. A slightly more intense
VI
ꢀ1
band at 1650 cm is attributed to overlapping C@N stretching
An immobilized analogue of complex 1 was prepared by carry-
ing out sequential reactions to functionalize a micelle templated
silica (Scheme 2). The presence of pyridylimine functions in the
modified material pyim-MTS was confirmed by 1 C CP MAS NMR
spectroscopy (signals for the imine carbon (C@N) at 161.7 ppm
(from the imine group) and water (adsorbed from the atmosphere)
bending modes.
2 2 2
Treatment of pyim-MTS with a solution of MoO Cl (THF) in
3
CH
2
Cl gave the final supported material Mo-pyim-MTS with a
2
ꢀ1
molybdenum content of 2.5 wt.% (0.26 mmol g ). Despite the
low uptake of complex, which corresponds to roughly 25% of the
loading of pyridylimine groups in Mo-pyim-MTS (based on a nitro-
gen content of 5.6 wt.% and assuming no change in the relative
and the adjacent methylene carbon (CH
Fig. 1). A peak at 42.5 ppm indicates the presence of unreacted
NH groups (CH NH ). This explains why the C/N molar ratio deter-
2
N@C) at 63.3 ppm;
2
2
2
mined from the elemental analysis results (3.8) was lower than the
theoretical value of 4.5 for the surface-bound N-(n-propyl)-2-pyr-
idylmethanimine groups. The lack of a peak due to methoxy groups
in the C CP MAS NMR spectrum of pyim-MTS suggests that the
n-propylsilyl groups are tripodally anchored to the surface (as
2
proportion of pyim and unreacted NH groups), changes in the
FT-IR spectrum of Mo-pyim-MTS when compared with pyim-
MTS were indicative of partial complexation of the pyim groups
with molybdenum. Thus, while bands remained at 1570, 1590
1
3
ꢀ
1
and 1650 cm
for the uncoordinated ligand, a new band at
Scheme 2. Preparation of the ligand-grafted MTS and immobilized analogue of complex 1.

S.S. Balula et al. / Inorganica Chimica Acta 387 (2012) 234–239
237
ꢀ1
appeared at 1599 cm and can be attributed to the
m(imine) mode
tend to be more reactive, which is consistent with the mechanistic
of coordinated pyim groups. This parallels the behavior observed
for complex 1. Similarly, the 13C CP MAS NMR spectra of pyim-
MTS and Mo-pyim-MTS are quite similar except that the signal
assigned to the imine carbon of pyim-MTS at 162 ppm decreases
in intensity for Mo-pyim-MTS and a new peak appears at
proposals reported in the literature for TBHP-based epoxidations of
1
olefins with complexes of the type MoO
2
X (L )
2 n
. It is generally ac-
cepted that the reaction mechanisms involve the coordination of
the oxidant to the metal center (which acts as a Lewis acid increas-
ing the oxidizing power of the peroxo group of TBHP), and the
epoxidation of the substrate by nucleophilic attack on an electro-
philic oxygen atom of the active oxidizing species [15,22,48,50].
Accordingly, the more electron-rich olefins are expected to be
more reactive. On the other hand, steric effects cannot be ruled
out: the relatively bulky cyclic olefin cyclododecene is less reactive
than Cy (Table 2).
1
59.0 ppm (Fig. 1). A comparable shift from 161.3 to 158.9 ppm
was observed upon complexation of pyim to give 1.
3.2. Catalytic olefin epoxidation
When complex 1 was used as a catalyst for the epoxidation of
cis-cyclooctene (Cy) with TBHP at 55 °C, cyclooctene oxide (CyO)
was formed as the only product in 93% yield at 10 min reaction,
In face of the fairly high catalytic activity of complex 1 (as a
homogeneous catalyst), we set out to investigate ways to immobi-
lize the complex and therefore facilitate catalyst separation from
the reaction products and reuse. Two approaches were tested: (a)
Anchoring of the complex on a chemically modified micelle-tem-
plated silica (Mo-pyim-MTS), and (b) complex 1 dissolved in an io-
nic liquid (IL), specifically 1-butyl-3-methylimidazolium
tetrafluoroborate (denoted 1/IL).
The reaction of Cy with TBHP in the presence of Mo-pyim-MTS
at 55 °C gave CyO as the only product in 85% yield at 24 h reaction
(Fig. 2). Hence, the catalytic reaction was much slower than that
observed for 1 (using the same Mo:Cy:TBHP mole ratio of
0.3:100:150). The reaction rate for Mo-pyim-MTS levels-off
significantly with reaction time, which may be partly due to (i)
‘‘non-productive decomposition’’ of TBHP, and/or (ii) catalyst deac-
tivation. Hypothesis (i) was ruled out since iodometric titrations
indicated negligible TBHP decomposition in the presence of Mo-
pyim-MTS (without Cy) during 24 h. The catalyst stability was
investigated at reaction temperatures in the range of 35–65 °C
(Fig. 3). The CyO yield at 24 h decreases with decreasing reaction
temperature; nevertheless, at nearly room temperature the CyO
yield is relatively high (ca. 50% CyO at 24 h/35 °C). Leaching tests
were performed by interrupting each reaction after 1 h and filter-
ing the mixture (while still at the reaction temperature) through
which corresponds to
a
turnover frequency (TOF) of
ꢀ1
ꢀ1
1
855 mol molMo
h
(Fig. 2). This performance is much superior
to that usually encountered for dichlorodioxomolybdenum(VI)
complexes bearing bidentate nitrogen-donor ligands, used as cata-
lysts in the same reaction under similar conditions. Only com-
plexes bearing substituted bipyridine ligands have recently been
reported to exhibit comparable initial reaction rates [17–19].
Although caution must be applied when comparing the TOF of 1
with those reported in the literature (in the present study, a
molybdenum:olefin mole ratio of 0.003 was used, which is lower
than the usual ratio of 0.01, and in most published studies it is
not specified whether approximate isothermal reaction conditions
were used when charging the reactor), Table 1 lists such values for
0
complexes bearing 2,2 -bipyridine and substituted derivatives,
diazabutadienes and pyrazolylpyridines.
The catalytic performance of complex 1 was further investi-
gated in the reaction of more demanding olefins (1-octene,
trans-2-octene, cyclododecene, R-(+)-limonene) with TBHP, at
5
5 °C (Table 2). For all the tested substrates only the respective
epoxides were formed. A comparison of the catalytic results for
the linear olefins 1-octene and trans-2-octene indicates that the
latter is more reactive than the former (conversion of trans-2-oc-
tene at 24 h is nearly double that observed for 1-octene). These re-
sults may be due to electronic effects in favor of the epoxidation of
the more substituted (electron-rich) double bond. This hypothesis
is supported by the fact that for the diene R-(+)-limonene the epox-
idation occurs mainly at the endocyclic (more substituted) double
bond rather than the exocyclic (terminal) one (at 99% total epox-
ides yield, the mole ratio of 1,2-epoxy-p-menth-8-ene/1,2:8,9-
diepoxy-p-menthane was 3). Hence, the more substituted olefins
a 0.2 lm PTFE GVS membrane, and stirring the resultant reaction
solution for a further 23 h at the appropriate temperature. The
increment in conversion between 1 and 24 h reaction is denoted
as
DFiltCat; for each reaction temperature, DFiltCat is compared
with the increment in conversion observed in the presence of
Mo-pyim-MTS (without filtration) for the same time interval (de-
noted
bution exists;
is completely homogeneous;
D
Cat). If
D
FiltCat/
D
Cat > 0, a homogeneous catalytic contri-
Cat = 1 suggests that the catalytic reaction
FiltCat/ Cat = 0 suggests that the
D
FiltCat/
D
D
D
catalytic reaction is heterogeneous in nature. In the temperature
range of 55–65 °C the catalytic reaction was completely homoge-
neous (
than 55 °C, the ratio
D
FiltCat/
D
Cat ffi 1), Fig. 3. For reaction temperatures lower
D
FiltCat/ Cat tended to decrease with
D
decreasing temperature, suggesting that the homogeneous cata-
lytic contribution decreased, albeit remaining significant (at
3
5 °C, DFiltCat/DCat = 0.5). Possibly, there may be some supported
metal species that are weakly adsorbed and others that are more
strongly adsorbed, leading to leaching phenomena even at low
reaction temperatures. The active species may be of different types,
i.e. supported or dissolved; at this stage, the structure(s) of the ac-
tive metal species is unknown.
In the second approach for improving the recyclability of 1, the
1/IL catalytic system was tested in the reaction of Cy with TBHP at
5
5 °C. The reaction system was biphasic liquid–liquid consisting of
a lower yellow IL phase containing the catalyst and TBHP, and an
upper colorless organic phase containing the olefin. CyO was
formed as the only product in 96% yield at 24 h. The reaction
was slower for 1/IL than for 1 (using the same Mo:Cy:TBHP mole
ratio of 0.3:100:150), which may be partly due to mass transfer
limitations in the former case (Fig. 2). After 24 h CyO was extracted
Fig. 2. Kinetic profiles of the epoxidation of cis-cyclooctene with TBHP at 55 °C in
the presence of 1 (4), Mo-pyim-MTS (s) and 1/IL (ꢁ).

2
38
S.S. Balula et al. / Inorganica Chimica Acta 387 (2012) 234–239
Table 1
Catalytic epoxidation of cis-cyclooctene with TBHP in the presence of complexes of the type MoO
1
a
2
Cl
2
(L ) possessing bidentate nitrogen-donor ligands .
0
Moꢀ1 ꢀ1
)b
h
Nitrogen donor ligand
R
–
R
–
TOF (mol mol
Ref.
1855 (10 min)
This work
N
N
H
H
H
H
H
CH
H
CH
CH
COOEt
COOMe
H
65 (10 min)
25 (10 min)
248 (10 min)
1135 (5 min)
1950 (3 min)
1916 (3 min)
[47]
[48]
[48]
[17]
[18]
[19]
R
R
c
3
N
N
3
, (CH
2 5
) CH
3
3
R'
R'
–
–
–
–
–
–
850 (3 min)
179 (15 min)d
85 (20 min)e
[20]
[22]
[49]
N
N
N
N
N
N
Ph
Ph
–
–
–
H
CH
(CH
190 (10 min)
328 (10 min)
347 (10 min)
[24]
[23]
[23]
R'
N
N
N
2
CO
) CH
2 3 3
2
Et
a
Reaction conditions: Mo:Cy:TBHP mole ratio = 0.3:100:150 (this work) or ꢂ1:100:150 (literature data), without a co-solvent (unless otherwise
specified), 55 °C. Epoxide selectivity was 100% unless otherwise stated.
b
Values in brackets are the reaction times at which the turnover frequency was calculated.
c
d
e
ꢀ1 ꢀ1
A much higher TOF of 923 mol molMo
Decane was used as co-solvent.
h
(calculated at 5 min reaction) has since been reported for the same complex [17].
1,2-Cyclooctanediol was also formed.
Table 2
a
Catalytic epoxidation of olefins with TBHP, in the presence of complex 1 at 55 °C .
Olefin
Reaction time
Conversion (%)
93/100
Epoxide selectivity (%)
100/100
1
0 min/4.5 h
cyclooctene
2
4 h
93
100
cyclododecene
2
2
4
4 h
4 h
.5 h
43
82
99
100
100
100b
1
-octene
trans-2-octene
R-(+)-limonene
a
Reaction conditions: Mo:Cy:TBHP mole ratio = 0.3:100:150.
b
Total epoxides selectivity. The mole ratio of 1,2-epoxy-p-menth-8-ene/1,2:8,9-diepoxy-p-menthane was 3.

S.S. Balula et al. / Inorganica Chimica Acta 387 (2012) 234–239
239
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FCT), the Programa Operacional Ciência e Inovação (POCI) 2010,
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