Molybdenum(II) diiodo-tricarbonyl complexes containing nitrogen donor ligands as catalyst precursors for the epoxidation of methyl oleate

Article abstract of DOI:10.1007/s10562-012-0890-5

The molybdenum(II) tricarbonyl complexes [Mo(CO)₃I ₂L_n] (n = 1, L = 2,2'-bipyridine, 4,4'-di-tert-butyl-2, 2'-bipyridine; n = 2, L = pyridine, 4-tert-butylpyridine) have been examined as catalyst precursors for the epoxidation of the biorenewable olefin methyl oleate with tert-butylhydroperoxide. In situ oxidative decarbonylation of the precursors gives highly active and selective molybdenum(VI) catalysts, which were identified as the one-dimensional molybdenum oxide/bipyridine polymer [MoO₃(2,2'-bipyridine)], octanuclear [Mo₈O ₂₄(4,4'-di-tert-butyl-2,2'-bipyridine)4], and the pyridinium β-octamolybdates (LH)₄[Mo₈O₂₆] for L = pyridine or 4-tertbutylpyridine.

Full text of DOI:10.1007/s10562-012-0890-5

Catal Lett (2012) 142:1218–1224
DOI 10.1007/s10562-012-0890-5
Molybdenum(II) Diiodo-Tricarbonyl Complexes Containing
Nitrogen Donor Ligands as Catalyst Precursors
for the Epoxidation of Methyl Oleate
•
Carla A. Gamelas Patrıcia Neves Ana C. Gomes Anabela A. Valente
•
•
•
´
•
•
Carlos C. Roma˜o Isabel S. Gonc¸alves Martyn Pillinger
Received: 3 July 2012 / Accepted: 5 August 2012 / Published online: 24 August 2012
Ó Springer Science+Business Media, LLC 2012
Abstract The molybdenum(II) tricarbonyl complexes
[Mo(CO)3I2Ln] (n = 1, L = 2,20-bipyridine, 4,40-di-tert-butyl-2,
20-bipyridine; n = 2, L = pyridine, 4-tert-butylpyridine)
have been examined as catalyst precursors for the epoxidation
of the biorenewable olefin methyl oleate with tert-butylhydro-
peroxide. In situ oxidative decarbonylation of the precursors
gives highly active and selective molybdenum(VI) catalysts,
which were identified as the one-dimensional molybdenum
oxide/bipyridine polymer [MoO3(2,20-bipyridine)], octanuclear
[Mo8O24(4,40-di-tert-butyl-2,20-bipyridine)4], and the pyridinium
b-octamolybdates (LH)4[Mo8O26] for L = pyridine or 4-tert-
butylpyridine.
1 Introduction
The use of molybdenum carbonyl complexes as precursors
to molybdenum(VI) catalysts for the epoxidation of olefins
has been intensively researched since 2003, when Abrantes
et al. [1] reported that highly active (g5-C5R5)MoO2Cl catalysts
(R = H, CH3, CH2Ph) could be generated in situ by reaction of
the tricarbonyl complexes (g5-C5R5)Mo(CO)3Cl with the oxi-
dant tert-butylhydroperoxide (TBHP). In the following years
various cyclopentadienyl molybdenum complexes of the
type Cp0Mo(CO)3X (Cp0 = g5-C5R5, g5-C5H4R, g5-C9H7;
X = halide, alkyl) [2–11], CpxMo(CO)3 and CpxMo(CO)2X
(Cpx = cyclopentadienyl functionalized with cycloalkyl,
N-heterocyclic carbene (NHC), oxazoline, or diphenyl phos-
phine moieties as bridging units) [12–17], Cp0Mo(CO)2
(g3-allyl) [18], and CpMo(CO)2(NHC)X [19] have been
investigated. The molybdenum(VI) species formed upon oxi-
dative decarbonylation of these precursors include Cp’MoO2X,
Cp0MoO(O2)X, [Cp0MoO2]2(l-O) and [Cp0MoO(O2)]2(l-O)
from Cp0Mo(CO)3X, and [CpMoO2(NHC)]BF4 from [CpMo
(CO)2(NHC)(NCMe)](BF4). Besides these cyclopentadienyl
complexes, several other types of molybdenum carbonyl
complexes have been studied, including the g3-allyl dicar-
bonyl complexes [Mo(g3-allyl)Cl(CO)2(bis-NHC)] [20] and
[Mo(g3-allyl)Cl(CO)2(L)] [21,22], heptacoordinate tricarbonyl
complexes [Mo(CO)3I2(L)] [23], and the tetracarbonyl com-
plexes cis-[Mo(CO)4(L)] [24–26] (L is a bidentate chelating
dinitrogen ligand such as diazabutadiene, bipyridine, pyridyli-
mine or pyrazolylpyridine). Due to the structure-directing
influence of the organodiamine ligand, oxidative decarbony-
lation of these complexes leads to a diverse range of molyb-
denum(VI) species with different catalytic performances. Some
of the species formed include tetranuclear [Mo4O12L4] for
L = 2-[3(5)-pyrazolyl]pyridine [25], octanuclear [Mo8O24L4]
for L = 4,40-di-tert-butyl-2,20-bipyridine (di-tBu-bipy) [22,
Keywords Molybdenum ꢀ Oxidative decarbonylation ꢀ
Alkenes ꢀ Epoxidation ꢀ tert-butylhydroperoxide ꢀ Methyl
oleate ꢀ Biomass conversion
C. A. Gamelas ꢀ C. C. Roma˜o
´
Instituto de Tecnologia Quımica e Biologica da Universidade
Nova de Lisboa, Av. da Republica, Estac¸a˜o Agronomica
´
´
´
Nacional, 2780-157 Oeiras, Portugal
C. A. Gamelas
Escola Superior de Tecnologia, Instituto Politecnico de Setubal,
´
2910-761 Setubal, Portugal
´
´
P. Neves ꢀ A. C. Gomes ꢀ A. A. Valente ꢀ
I. S. Gonc¸alves (&) ꢀ M. Pillinger (&)
Department of Chemistry, CICECO, University of Aveiro,
3810-193 Aveiro, Portugal
e-mail: igoncalves@ua.pt
M. Pillinger
e-mail: mpillinger@ua.pt
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Epoxidation of Methyl Oleate
1219
24] and ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate [25], and the
one-dimensional molybdenum oxide/bipyridine polymer
[MoO3L] for L = 2,20-bipyridine (bipy) [24].
University of Aveiro (by L. Carvalho). 1H, 13C{1H},
1H–1H COSY and 1H–13C HMQC NMR spectra were
recorded at room temperature on a Bruker Avance II
400 MHz spectrometer. Chemical shifts (d) are quoted in
parts per million from tetramethylsilane. Transmission
FT-IR spectra were measured on a Mattson 7000 FT-IR
spectrometer using KBr pellets. Attenuated total reflec-
tance (ATR) FT-IR spectra were measured on a Bruker
optics Tensor 27 equipped with a Specac Golden Gate Mk
II ATR accessory having a diamond top-plate and KRS-5
focusing lenses.
Despite the promising results obtained with the catalytic
systems mentioned above, there is room for further work in
the field of selective epoxidation catalysis, with one of the
most interesting aims being the development of efficient
systems for the epoxidation of biorenewable olefins [27]. Of
particular interest are plant derived olefins such as (R)-(?)-
limonene, a-pinene, and fatty acids and esters obtained from
vegetable oils [27–29]. In industry, epoxidized vegetable
oils (EVO) are currently mainly used as plasticizers and
stabilizers for polyvinyl chloride (PVC) plastics [30–33].
Other potential uses of EVOs are as lubricant and anti-rust
additives [34–40], in UV-curable coatings [41–46], and as
raw materials for the synthesis of a broad variety of poly-
meric materials [47, 48]. For example, oxirane ring opening
of EVOs by reaction with alcohols can give polyols, which
are intermediates for the production of lubricants and
polyurethanes [49, 50]. EVOs are commercially produced
via the Prileshajev peracid process [29]. However, this
process has several drawbacks, such as low selectivity to
epoxidized products due to the acid-catalyzed oxirane ring
opening, the need to use excess H2O2 and long reaction
times, and corrosion problems due to the presence of a
strong acid in an oxidative environment. The use of transi-
tion metal complexes as epoxidation catalysts may eliminate
many of these problems and permit highly selective reac-
tions [29]. To this end, molybdenum-based catalytic systems
have been successfully investigated, such as [MoO2(SAP)
(EtOH)] (SAP = dianionic tridentate salicylideneimino-
phenolate chelating ligand) plus organic hydroperoxides for
the epoxidation of oleic acid [51], [MoO(O2)2(8-quino-
linol)2] plus H2O2/NaHCO3 for the epoxidation of a mixture
of methyl oleate (MO) and methyl linoleate [52],
[MoO2(acac)2] plus TBHP for the epoxidation of soybean
oil [53], and the octanuclear complex [Mo8O22(OH)4
(di-tBu-bipy)4] plus TBHP for the epoxidation of MO [54].
As part of our ongoing exploration of molybdenum
carbonyl complexes as catalyst precursors for the epoxi-
dation of bio-derived olefins, we now wish to report on the
use of the heptacoordinate molybdenum(II) tricarbonyl
complexes [Mo(CO)3I2Ln] (n = 1, L = bipy, di-tBu-bipy;
n = 2, L = pyridine, 4-tert-butylpyridine) in the catalytic
epoxidation of MO using TBHP as oxidant.
All air-sensitive operations were carried out using stan-
dard Schlenk techniques under nitrogen. Solvents were dried
by standard procedures, distilled under nitrogen and kept over
molecular sieves. Pyridine (py), 4-tert-butylpyridine (tBu-py),
2,20-bipyridine (bipy), 4,40-di-tert-butyl-2,20-bipyridine
(di-tBu-bipy), and tert-butylhydroperoxide (TBHP, 5–6 M
in decane) were obtained from Sigma-Aldrich and used as
received. [Mo(CO)3I2(NCMe)2] [55], [Mo(CO)3I2(bipy)2]
(1) and [Mo(CO)3I2(py)2] (3) [56] were prepared by fol-
lowing literature procedures, and presented satisfactory
infrared and NMR data: For 1, m(CO) = 2030vs, 1969vs,
1913vs cm-1 1H NMR (CD3COCD3) d = 9.32 (d, 2H,
;
H6,60 bipy), 8.67 (d, 2H, H3,30 bipy), 8.24 (td, 2H, H4,40
bipy), 7.70 (t, 2H, H5,50 bipy); 13C{1H} NMR
(CD3COCD3) d = 233.8 (CO), 157.6 (C2,20 and C6,60
bipy), 141.5 (C4,40 bipy), 127.9 (C5,50 bipy), 125.6 (C3,30
bipy). For 3, m(CO) = 2016s, 1998vs, 1939s, 1921vs,
1
1869sh; H NMR (DMSO-d6) d = 8.76 (br, 4H, H2 py),
8.16 (br, 2H, H4 py), 7.71 (br, 4H, H3 py); 13C{1H} NMR
(DMSO-d6) d = 147.0 (C2 py), 139.8 (C4 py), 125.1 (C3
py). [Mo(CO)3I2(di-tBu-bipy)] (2) was prepared by addition
of a freshly dried solution of di-tBu-bipy (0.25 g,
0.93 mmol) in ethanol to a solution of [Mo(CO)3I2
(NCMe)2] (0.48 g, 0.93 mmol) in ethanol. The reaction
mixture was stirred at room temperature for 90 min in the
dark. After concentration and addition of diethyl ether/n-
hexane, the resultant yellow precipitate was filtered, washed
with diethyl ether and n-hexane, and finally vacuum dried
(0.56 g, 85 %). NMR and FT-IR spectra for 2 were in
agreement with literature data [57].
2.2 Preparation of [Mo(CO)3I2(tBu-py)2] (4)
tBu-py (0.16 mL, 1.06 mmol) was added to a solution of
[Mo(CO)3I2(NCMe)2] (0.27 g, 0.53 mmol) in dichloro-
methane, and the reaction mixture was stirred at room
temperature for 20 min in the dark. After removal of the
solvent under reduced pressure, the resultant light brown
solid was washed with diethyl ether and finally vacuum
dried (0.30 g, 81 %). Anal. Calcd. for C21H26I2MoN2O3:
C, 35.82; H, 3.72; N, 3.98. Found: C, 35.70; H, 3.76; N,
3.94 %. FT-IR (KBr, cm-1): m(CO) 2013vs, 1944vs,
2 Experimental
2.1 Materials and Methods
Microanalyses for CHN were performed at the ITQB (by
C. Almeida) and ICP-OES were performed at the
123

1220
C.A.Gamelas et al.
1
1
1911vs. H NMR (DMSO-d6): d = 8.73 (br, 4H, H2 py),
7.87 (br, 4H, H3 py), 1.33 (s, 18H, CMe3). 13C{1H} NMR
(DMSO-d6): d = 213.9 (CO), 204.3 (CO), 167.6 (C4 py),
143.7 (C2 py), 124.2 (C3 py), 35.7 (CMe3), 29.7 (CMe3).
709s, 675s, 554w, 523w. H NMR (DMSO-d6): d = 8.66
(br, 8H, H2 py), 7.97 (t, 4H, H4 py), 7.54 (dd, 8H, H3 py).
Data for 4* Anal. Calcd. for C36H56Mo8N4O26: C,
25.02; H, 3.27; N, 3.24. Found: C, 25.50; H, 3.46; N,
3.00 %. FT-IR (KBr, cm-1): m = 3263w, 3083w, 2964m,
2928m, 2358w, 2339w, 1635m, 1597w, 1500w, 1463w,
1375w, 1263w, 1211w, 1102w, 1023w, 949s, 914m, 837m,
2.3 Catalytic Epoxidation of MO
1
The reactions of MO (99 %, Sigma-Aldrich) with TBHP were
carriedout at75 °C in batch borosilicatemicroreactors (5 mL
capacity), under magnetic stirring (1,000 rpm). The initial
molar ratios of molybdenum:olefin:oxidant were 1:100:152
(18 lmol Mo), and 1 mL of 1,2-dichloroethane (DCE, 99 %,
Sigma-Aldrich) or 0.3 mL of the ionic liquid (IL) 1-butyl-
3-methylimidazolium tetrafluoroborate ([bmim]BF4, Merck)
were used as cosolvent. The acquired cosolvents and oxidant
solution were dried prior to the catalytic tests: DCE was
treatedwithCaH2 atroomtemperatureovernightwithstirring,
799s, 708m, 659m, 559w, 525w. H NMR (DMSO-d6):
d = 8.68 (br, 8H, H2 py), 7.74 (d, 8H, H3 py), 1.38 (s,
36H, CMe3).
3 Results and Discussion
3.1 Preparation of Catalyst Precursors
The complexes [Mo(CO)3I2Ln] (n = 1, L = bipy (1), di-tBu-
bipy (2); n = 2, L = pyridine (3), 4-tert-butylpyridine (4))
(Chart 1) were readily prepared in very good yields by
replacement of the acetonitrile ligands in [Mo(CO)3I2
(NCMe)2] by the nitrogen donor ligands. Since the bis-aceto-
nitrile complex can be synthesized in excellent yield from
Mo(CO)6 by reaction with MeCN to give [Mo(CO)3(NCMe)3]
followed by reaction with I2, complexes 1–4 are potentially
interesting as catalyst precursors due to their easy preparation
from cheap starting materials [58]. To the best of our knowl-
edge, the 4-tert-butylpyridine (tBu-py) complex 4 has not been
previously reported. The FT-IR spectrum of 4 measured in KBr
pellets shows three very strong m(CO) bands at 2013, 1944 and
1911 cm-1 typical of complexes of the type [Mo(CO)3I2Ln].
Coordination of the tBu-py ligand is indicated by the shift of the
pyridyl ring stretching vibration from 1,597 cm-1 for the free
ligandto1,619 cm-1 for 4. The1HNMRspectrum of4 (25 °C,
DMSO-d6) shows the expected two broad signals at d 8.73 and
7.87 for the pyridine protons, and a singlet at d 1.33 for the
methyl groups.
˚
followed by distillation and storage over activated 4 A
molecular sieves; [bmim]BF4 was treated at 100 °C under
˚
vacuum for 2 h; TBHP was dried using activated 3 A
molecular sieves. The reactors were charged with MO and
cosolvent and immersed in a thermostated oil bath (at 75 °C)
for 10 min, followed by addition of TBHP (which was pre-
heated at the reaction temperature). It is considered that the
catalytic reaction began at the instant that TBHP was added to
the reactor. The course of the reactions was monitored by GC
using a Varian V3800 equipped with a BR-5 capillary column
(30 m 9 0.25 mm 9 0.25 lm). Methyl decanoate was used
as internal standard. The reaction products were identified by
GC–MS (Trace GC 2000 Series Thermo Quest CE Instru-
ments GC; Thermo Scientific DSQ II) using He as carrier gas.
2.4 Isolation of Oxidized Metal Compounds 1*–4*
Reactions were carried out as described in Sect. 2.3 (with
DCE as cosolvent), but on a larger scale and without MO.
After 24 h the reaction mixtures were concentrated, diethyl
ether added, and the resultant solids isolated by filtration,
washed with CH2Cl2, n-hexane and diethyl ether, and
finally vacuum-dried. FT-IR data for 1* (obtained from 1)
and 2* (obtained from 2) were in agreement with published
data for the molybdenum oxide/bipyridine polymer [MoO3
(bipy)] and the octanuclear complex [Mo8O24(di-tBu-bipy)4],
respectively [24]. Compounds 3* (obtained from 3) and 4*
(obtained from 4) were identified as the pyridinium b-oc-
tamolybdates (C5H5NH)4[Mo8O26] and [(CH3)3CC5H4
NH]4[Mo8O26], respectively, on the basis of elemental
analyses, FT-IR spectra, and NMR data.
3.2 Catalytic Epoxidation of MO
The reaction of MO with TBHP at 75 °C using complexes
1–4 as catalyst precursors and a Mo:MO:TBHP molar ratio
of 1:100:152 gave methyl 9,10-epoxystearate (MES) as the
main product in up to 71 % yield at ca. 80 % conversion,
24 h (Scheme 1). No MES was formed in the absence
of catalyst precursor. By-products included methyl-9,
10-dihydroxyoctadecanoate, which may be formed via
hydrolysis of MES with adventitious water present in the
reaction mixture (since the substrate was not pre-dried and
the catalytic reactions were carried out under atmospheric
air), and other non-identified compounds. The dependence
of MES yield on MO conversion was comparable for
1/2 (58–62 % yield at 65–72 % conversion) and 3/4
(16 % yield at ca. 25 % conversion), Fig. 1. However,
Data for 3* Anal. Calcd. for C20H24Mo8N4O26: C,
15.97; H, 1.61; N, 3.72. Found: C, 16.20; H, 1.36; N, 3.59 %.
FT-IR (KBr, cm-1): m = 3107w, 3069w, 2963w, 2927w,
2360w, 2341w, 1633m, 1609sh, 1532w, 1486m, 1475m,
1261m, 1204w, 1097m, 1027m, 946s, 912s, 839m, 802m,
123

Epoxidation of Methyl Oleate
1221
Chart 1 Catalyst precursors used in this work
considerable differences in reaction rate (based on con-
versions at 6, 24 h) were observed for 1–4, following the
order 2 (65, 78 %) [ 1 (47, 72 %) [[ 4 (23, 37 %) [ 3
(16, 25 %) (Fig. 1). A comparison of 1/2 and 3/4 suggests
that the introduction of the tert-butyl substituents in the
aromatic ligands enhances the reaction rate. A similar
effect has been reported in the literature for molybdenum
tetracarbonyl complexes of the type cis-[Mo(CO)4(L)],
where L is bipy or di-tBu-bipy, tested as catalyst precursors
in the epoxidation of cis-cyclooctene with TBHP; the
complex with L = di-tBu-bipy was more soluble and led to
a faster catalytic reaction [24]. Complex 2 is apparently
more soluble than the other three complexes, which may
account for faster in situ formation of the active species,
enhancing the overall reaction rate of MO.
Scheme 1 Identified products in the reaction of MO with TBHP at
75 8C using complexes 1–4 as catalyst precursors
overlapping Mo–O–Mo stretching vibrations), 904 and
940 cm-1 [m(Mo = O)] for 2*. For 3* and 4*, the spectra are
consistent with the presence of the pyridinium b-octamo-
lybdates (C5H5NH)4[Mo8O26] (3*) and ((CH3)3CC5H4NH)4
[Mo8O26] (4*), respectively. The former compound was
previously obtained by the reaction of MoO3ꢀ2H2O with
pyridine [61]. In accordance with that reported for pyridini-
um isopolymolybdates [61, 62], 3* exhibits bands at 1633,
1609, 1532 and 1486 cm-1, which indicate that the pyridine
ring is protonated at the N atom. In the region of the Mo–O
vibrations (500–950 cm-1), the bands exhibited by 3* (946,
912, 839, 802, 709, 675, 554 and 523 cm-1) and 4* (949,
914, 837, 799, 708, 659, 559 and 525 cm-1) match those
reported for other b-[Mo8O26]4- clusters [63, 64].
The reaction of MO with TBHP in the presence of
1*–4* (using the same molar ratios of Mo:MO:TBHP), at
75 °C, was much faster than that observed for the corre-
sponding precursor complexes 1–4 (Fig. 1). These results
may be partly due to the fact that the primary oxidative
decarbonylation step is not required for the (pre-oxidized)
complexes 1*–4*, whereas it is necessary for 1–4. The
dependence of MES yield on MO conversion was similar
for 1*–4* and at least 99 % MES yield could be reached
within 24 h in all cases. A comparison of the MES selec-
tivities at ca. 20–35 % conversion shows that the epoxide
selectivity is poorer for the precursor complexes: 91 % (1),
93 % (2), 62 % (3), 71 % (4), [99 % (1*–4*). A likely
explanation is that the in situ conversion of the precursor
complexes is accompanied by oxidation of liberated iodide
with TBHP to give molecular iodine and even (hypo)iodite
ions, which may play a catalytic role favoring side reac-
tions or epoxide ring opening [65, 66]. In parallel to that
observed for the precursor complexes 1 and 2, the reaction
The complexes bearing bidentate ligands (1 and 2) lead to
enhanced reaction rates in comparison to those possessing the
corresponding monodentate ligands (3 and 4, respectively),
Fig. 1. These results may be at least partly related with the
different types (and properties) of the active species formed
and/or their rates of formation. The transformation of the
precursor complexes was evidenced by color changes in the
reaction mixtures after addition of TBHP. In the few reports
dealing with complexes of the type [Mo(CO)3X2Ln]
(X = Br, I; n = 1 or 2 for L = bidentate or monodentate
ligand, respectively) used as catalyst precursors for liquid-
phase epoxidation of olefins with TBHP [23, 59, 60], the
oxidized metal compounds were not identified. In the present
work, the oxidized metal compounds could be isolated by
scaling up the catalytic experiments. The FT-IR spectra of all
four compounds (1*, 2*, 3* and 4* from 1, 2, 3 and 4) show
the absence of m(CO) bands (Fig. 2). For 1* and 2*, the
spectra match those reported previously for the compounds
[MoO3(bipy)] and [Mo8O24(di-tBu-bipy)4], respectively [22,
24]. In addition to bands due to bipy or di-tBu-bipy, the
spectra exhibit several bands in the range of 600–950 cm-1
due to Mo–O vibrations. The characteristic bands are found at
622, 882 and 914 cm-1 for 1*, and at 700–850 (several
123

1222
C.A.Gamelas et al.
Fig. 1 Kinetic profiles for the
reaction of MO with TBHP, at
75 °C, in the presence of 1–4
(a) or 1*–4* (c), and the
dependence of MES yield on
MO conversion for 1–4 (b) or
1*–4* (d), using DCE as
cosolvent: (open diamond)-1/
1*, (open square)-2/2*, (open
triangle)-3/3*, (open circle)-4/
4*. Results for 1* using the IL
[bmim]BF4 as cosolvent are
also shown (dashed lines are
visual guides): run 1 (asterisk),
run 2 (times)
reaction of MO with TBHP (39/82 % MES yield at
41/85 % conversion, reached at 6/24 h reaction, 1,2-
dichloroethane as cosolvent, 55 °C) [54].
Compounds 3* and 4* are very active and gave similar
results, which is consistent with the identification of both
compounds as pyridinium b-octamolybdates. The presence
of the tert-butyl substituent on the pyridinium counter-ion in
4* does not significantly influence the reaction of MO:
97/99 % MES yield at 2/6 h reaction (Fig. 1). M. Jia and
co-workers [67] recently reported that a b-octamolydate-
based polyoxometalate modified by imidazole moieties and
containing imidazolium counter-ions was an active catalyst
for the epoxidation of olefins, and the catalytic reactions
were heterogeneous in nature (e.g., 82 % conversion of cis-
cyclooctene after 6 h reaction at 35 °C, initial molar ratio
Mo:olefin:TBHP = 0.048:1:1, chloroform as solvent).
Although compounds 3* and 4* did not dissolve completely
in the reaction medium, the catalytic reactions took place at
least to a certain extent in homogeneous phase. This was
assessed in separate assays in which the solid phase was
removed by filtration after 5 min reaction (at the reaction
temperature of 75 °C), and the reaction taking place in the
solution phase was monitored for a further 115 min. The
increment in MO conversion in this time interval (Dfilt) was
compared with that observed for the corresponding catalytic
test without filtration (Dcat): values of Dfilt/Dcat were 0.3
for 3* and 0.7 for 4* (Dfilt/Dcat [ 0 indicates the existence
of a homogeneous catalytic contribution). The Dfilt/Dcat
Fig. 2 FT-IR spectra of 1* and 1*-IL (ATR), and 2*–4* (KBr disc)
of MO was faster for 2* than for 1*. Some of us recently
reported that the octanuclear complex [Mo8O22(OH)4
(di-tBu-bipy)4] is an active and selective catalyst in the
123

Epoxidation of Methyl Oleate
1223
values may be less than unity due to the interruption of the
dissolution of the metal species upon filtration of the solid at
the initial stage of the reaction. For 1* and 2*, Dfilt/
Dcat = 1 (for 1*, filtration was performed at 1 h, further
monitoring the reaction until 24 h; for 2*, filtration was
performed at 5 min, further monitoring the reaction until
2 h), in agreement with the literature data [24].
bidentate nitrogen donor ligands are convenient precursors
to oxomolybdenum(VI) catalysts for the selective epoxi-
dation of MO using TBHP as oxidant. Reaction with the
oxidant TBHP results in oxidative decarbonylation and loss
of the iodo ligands, giving the one-dimensional molybde-
num oxide/bipyridine polymer [MoO3L] for L = 2,20-
bipyridine, octanuclear [Mo8O24L4] for L = 4,40-di-tert-
butyl-2,20-bipyridine, and the pyridinium b-octamolybdates
(LH)4[Mo8O26] for L = pyridine or 4-tert-butylpyridine.
Epoxide selectivity in the first run appears to be reduced
due to the presence of oxidized iodine species in the
reaction mixtures. This disadvantage is offset by the fact
that the precursor complexes are easy to prepare from
cheap starting materials, and in the second catalytic run
using the recovered catalysts the epoxide selectivity was
always at least 99 %. For the catalyst [MoO3(bipy)], the
use of an IL as cosolvent facilitates the separation of the
reaction products. The IL phase including the Mo(VI)
catalyst can be readily recycled by washing with hexane
and drying, and the Mo(VI) catalyst can be reused without
significant drop in catalytic performance.
ICP-OES of the liquid phases separated from the reac-
tion mixtures after 6 h (MO conversion [82 %) by filtra-
tion through a 0.2 lm PTFE membrane filter indicated that
the amount of dissolved molybdenum was 1, 30, 26 and
31 % of the initial molar amount of molybdenum for 1*,
2*, 3* and 4*, respectively. Considering that for the four
compounds the catalytic reaction is homogeneous in nature
and assuming that all molybdenum centers of the dissolved
compounds possess intrinsic catalytic activity, the turnover
number at 6 h is highest for 1* (mol mol-M1o): 8,180
(1*) [ 380 (3*) % 326 (2*) % 316 (4*).
Since 1* acts as a homogeneous catalyst the development
of procedures for its separation from the reaction mixture and
reuse is desirable. A possible approach consists of dissolving
the catalyst in an IL, which poorly dissolves the substrate/
products. The readily available and relatively cheap IL
[bmim]BF4 seems attractive and has been successfully
employed in molybdenum-based catalytic systems for the
epoxidation of a mixture of MO and methyl linoleate;
[MoO(O2)2(8-quinolinol)2], H2O2 and NaHCO3 [52]. In the
present work, the upper liquid phase consisted of MO which
was hardly detected in the lower IL phase (ascertained by GC
analysis). This IL-based catalytic system led to 86 % MES
yield at 89 % conversion, 24 h, 75 °C (Fig. 1). After the first
run, the reaction products were easily separated from the
reaction mixture by adding n-hexane (immiscible with the IL;
diethyl ether or ethyl acetate were similarly effective) to the
reactor, followed by centrifugation and removal of the upper
liquid phase. The complete removal of reactant/products
from the IL mixture was checked by GC analysis. Prior to
reuse, the IL mixture was treated at 40 °C under vacuum for
1 h. A second batch run was initiated by adding MO and
TBHP to the IL mixture in the same amounts to those used in
run 1. The kinetic behavior observed for run 2 is comparable
to that of run 1, and the dependence of MES yield on MO
conversion is roughly coincident for the two runs (Fig. 1).
The ATR FT-IR spectrum of the recovered catalyst (washed
with n-hexane and dried at room temperature overnight to
give 1*-IL) was similar to that of 1* (Fig. 2), suggesting that
the catalyst is fairly stable and recyclable in the IL medium.
ˆ
Acknowledgments We are grateful to the Fundac¸a˜o para a Ciencia
e a Tecnologia (FCT), QREN, FEDER, COMPETE, the European
Union and the Associate Laboratory CICECO (Pest-C/CTM/LA0011/
2011) for continued support and funding. The FCT is acknowledged
for a post-doctoral grant to PN (SFRH/BPD/73540/2010). The NMR
spectrometers are part of The National NMR Network (REDE/1517/
RMN/2005), supported by POCI 2010 and the FCT.
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