Bis(pyrazolyl)methanetetracarbonyl-molybdenum(0) as precursor to a molybdenum(VI) catalyst for olefin epoxidation

Article abstract of DOI:10.1016/j.jorganchem.2012.09.019

Bis(pyrazolyl)methanetetracarbonyl-molybdenum(0), cis-[Mo(CO) ₄(BPM)] (1), was prepared from Mo(CO)₆ and the ligand bis(pyrazolyl)methane (BPM), and examined as a catalyst precursor for the epoxidation of olefins using tert-butylhydroperoxide (TBHP) as oxidant. Catalytic activities followed the sequence 1-octene < trans-2-octene < α-pinene < (R)-(+)-limonene < cis-cyclooctene, and, with the exception of α-pinene and limonene, the corresponding epoxide was always the only reaction product. Turnover frequencies for the epoxidation of cyclooctene were 580 mol mol_Mo^-1 h^-1 at 55°C and 1175 mol mol_Mo^-1 h^-1 at 75°C, which compare favourably with those found for other molybdenum carbonyl complexes used as catalyst precursors for the same reaction under similar conditions. Catalytic activities were lower in the presence of organic co-solvents, decreasing in the sequence 1,2-dichloroethane > nitromethane > ethanol > hexane > acetonitrile. It is proposed that the oxodiperoxo complex [MoO(O₂)₂(BPM)] (2) may be the active catalyst formed in situ by oxidative decarbonylation of 1, since crystals of 2 suitable for structure determination by X-ray diffraction were obtained from the reaction solution recovered after a catalytic run at 55°C with cis-cyclooctene as substrate. In support of this hypothesis, the catalytic performance of 2 for the epoxidation of cyclooctene at 55°C is very similar to that for 1.

Full text of DOI:10.1016/j.jorganchem.2012.09.019

Journal of Organometallic Chemistry 723 (2013) 56e64
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Bis(pyrazolyl)methanetetracarbonyl-molybdenum(0) as precursor to a
molybdenum(VI) catalyst for olefin epoxidation
,
Sónia Figueiredo a, Ana C. Gomes b, José A. Fernandes b, Filipe A. Almeida Paz b, André D. Lopes a
,
*
,
João P. Lourenço a, Martyn Pillinger b, Isabel S. Gonçalves b
*
a Faculty of Science and Technology, CIQA, University of the Algarve, Campus de Gambelas, 8005-139 Faro, Portugal
b Department of Chemistry, CICECO, Campus Universitário de Santiago, University of Aveiro, 3810-193 Aveiro, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Bis(pyrazolyl)methanetetracarbonyl-molybdenum(0), cis-[Mo(CO)4(BPM)] (1), was prepared from
Mo(CO)6 and the ligand bis(pyrazolyl)methane (BPM), and examined as a catalyst precursor for the
epoxidation of olefins using tert-butylhydroperoxide (TBHP) as oxidant. Catalytic activities followed the
Received 10 July 2012
Received in revised form
20 September 2012
sequence 1-octene < trans-2-octene <
a
-pinene < (R)-(þ)-limonene < cis-cyclooctene, and, with the
Accepted 26 September 2012
exception of
a
-pinene and limonene, the corresponding epoxide was always theꢀo1nly reaction product.
Turnover frequencies for the epoxidation of cyclooctene were 580 mol molMo hꢀ1 at 55 ꢁC and
Keywords:
Molybdenum
ꢀ1
1175 mol mol
hꢀ1 at 75 ꢁC, which compare favourably with those found for other molybdenum
Mo
carbonyl complexes used as catalyst precursors for the same reaction under similar conditions. Catalytic
activities were lower in the presence of organic co-solvents, decreasing in the sequence 1,2-
dichloroethane > nitromethane > ethanol > hexane > acetonitrile. It is proposed that the oxodiper-
oxo complex [MoO(O2)2(BPM)] (2) may be the active catalyst formed in situ by oxidative decarbonylation
of 1, since crystals of 2 suitable for structure determination by X-ray diffraction were obtained from the
reaction solution recovered after a catalytic run at 55 ꢁC with cis-cyclooctene as substrate. In support of
this hypothesis, the catalytic performance of 2 for the epoxidation of cyclooctene at 55 ꢁC is very similar
to that for 1.
Bis(azolyl)methane ligands
Tetracarbonyl complexes
Oxidative decarbonylation
Olefin epoxidation
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
such as tetranuclear [Mo4O12L4] for L ¼ 2-[3(5)-pyrazolyl]pyridine
[1u], octanuclear [Mo8O24L4] for L ¼ 4,40-di-tert-butyl-2,20-bipyr-
During the last decade, a wide range of molybdenum carbonyl
complexes have been examined as precursors to molybdenum(VI)
catalysts for the epoxidation of olefins [1], the cis-dehydroxylation
of olefins [2], and the oxidation of amines [3], alcohols [4] and
sulfides [5]. Catalyst generation can be carried out in situ since the
complexes undergo oxidative decarbonylation by reaction with the
oxidant, which is usually tert-butylhydroperoxide (TBHP) or
hydrogen peroxide. In our recent investigations of molybdenum
tetracarbonyl complexes of the type cis-[Mo(CO)4(L)] as catalyst
precursors for olefin epoxidation using TBHP as oxidant [1te1v],
we found that the resultant systems were capable of high epoxide
selectivities, moderate to high activities, and good stability, func-
tioning as either homogeneous or heterogeneous catalysts
depending on the type of ligand (L). A surprisingly diverse range of
molybdenum species are formed upon oxidative decarbonylation,
idine [1t] and ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate [1u], and the
one-dimensional molybdenum oxide/bipyridine polymer [MoO3L]
for L ¼ 2,20-bipyridine [1t].
One family of ligands that have led to effective molecular cata-
lysts of the type [MoO2X2(L)] and [MoO2X(L)]X (X ¼ Cl, Br) for olefin
epoxidation are poly(azol-1-yl)alkanes such as bis(pyrazolyl)
methanes, tris(pyrazolyl)methanes and tris(benzimidazolyl)
methane [6]. As part of our ongoing exploration of molybdenum
tetracarbonyl complexes, we have prepared a complex containing
the bidentate ligand bis(pyrazol-1-yl)methane, and examined its
performance for the epoxidation of olefins.
2. Experimental
2.1. General considerations
Microanalyses for CHN were performed at the University of
Aveiro. Transmission FT-IR spectra were measured on a Mattson
7000 spectrometer. Attenuated total reflectance (ATR) FT-IR spectra
* Corresponding authors. Fax: þ351 234 370084.
E-mail addresses: adlopes@ualg.pt (A.D. Lopes), igoncalves@ua.pt (I.S. Gonçalves).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.09.019

S. Figueiredo et al. / Journal of Organometallic Chemistry 723 (2013) 56e64
57
were measured on a Bruker optics Tensor 27 equipped with a Spe-
cac Golden Gate Mk II ATR accessory having a diamond top-plate
and KRS-5 focussing lenses. FT-Raman spectra were recorded on
a RFS-100 Bruker FT-Spectrometer equipped with a Nd:YAG laser
with an excitation wavelength of 1064 nm. 1H NMR spectra were
measured with a Bruker CXP 300 instrument; chemical shifts are
quoted in parts per million and referenced to tetramethylsilane.
All preparations and manipulations were carried out using
standard Schlenk techniques under nitrogen. Where appropriate,
solvents were dried by standard procedures, distilled under
nitrogen, and kept over 4 Å molecular sieves. 1H-Pyrazole (98%,
SigmaeAldrich), KOH (ꢂ99%, SigmaeAldrich), DMSO (ꢂ99%, Lab-
Scan), MgSO4 (José M. Vaz Pereira), chloroform (ꢂ99%, Sigmae
Aldrich), CH2Br2 (99%, SigmaeAldrich), Mo(CO)6 (Fluka), and
diethyl ether (99.5%, SigmaeAldrich) were purchased from
commercial sources and used as received. The ligand bis(pyrazol-1-
yl)methane (BPM) was prepared as described in the literature [7].
Satisfactory elemental analyses were obtained, and the spectro-
scopic data (1H NMR and FT-IR) were in agreement with the pub-
lished data.
982 (w), 950 (vs), 920 (m), 875 (s), 777 (w), 735 (w), 656 (w), 610
(w), 578 (m), 529 (m), 400 (w), 362 (w), 341 (w). 1H NMR (300 MHz,
25 ꢁC, DMSO-d6):
d
¼ 7.95 (d, 2H, 3-H pz), 7.49 (d, 2H, 5-H pz), 6.40
(s, 2H, CH2), 6.29 (t, 2H, 4-H pz). 13C NMR (75 MHz, 25 ꢁC, DMSO-
d6):
¼ 140.1 (3-C pz), 130.6 (5-C pz), 106.3 (4-C pz), 64.2 (CH2).
d
2.4. X-ray crystallography
Single crystals of [MoO(O2)2(BPM)] (2) were manually har-
vested from the crystallization vial, immersed in silicone grease
(Dow Corning) and mounted on a glass fibre with the help of
a Stemi 2000 stereomicroscope equipped with Carl Zeiss lenses.
Data were collected on a Bruker X8 Kappa APEX II CCD area-
detector diffractometer (Mo K
a graphite-monochromated radia-
ꢀ
tion,
l
¼ 0.71073 A) controlled by the APEX2 software package [8]
and equipped with an Oxford Cryosystems Series 700 cryostream
monitored remotely using the software interface Cryopad [9].
Images were processed using SAINTþ [10], and data were cor-
rected for absorption by the multiscan semi-empirical method
implemented in SADABS [11].
The structure was solved using the Patterson synthesis algo-
rithm implemented in SHELXS-97 [12], which allowed the imme-
diate location of the crystallographically independent Mo6þ centre
and most of the heaviest atoms. The remaining non-hydrogen
atoms were located from difference Fourier maps calculated from
successive full-matrix least-squares refinement cycles on F2 using
SHELXL-97 [12a,13]. All non-hydrogen atoms were successfully
refined using anisotropic displacement parameters. Hydrogen
atoms bound to carbon were placed at their idealized positions
using appropriate HFIX instructions in SHELXL: 23 for the eCH2e
methylene group and 43 for the aromatic CH groups of the pyr-
azolyl rings. All these atoms were included in subsequent refine-
ment cycles in riding motion approximation with isotropic thermal
displacements parameters (Uiso) fixed at 1.2 ꢃ Ueq of the parent
carbon atoms.
2.2. cis-[Mo(CO)4(BPM)] (1)
A mixture of Mo(CO)6 (2.02 g, 7.65 mmol) and the ligand BPM
(1.13 g, 7.65 mmol) was vacuum-dried for 15 min. Dry toluene
(20 mL) was then added under a continuous flow of nitrogen. The
mixture was heated at 110 ꢁC with stirring for 4 h under a contin-
uous flow of nitrogen. After cooling to room temperature, the
mixture was filtered, and the resultant greenish-yellow precipitate
was washed with diethyl ether (3 ꢃ 20 mL), and finally vacuum-
dried. Yield: 2.45 g (90%). Anal. Calcd for C11H8MoN4O4: C, 37.10;
H, 2.26; N, 15.73. Found: C, 36.73; H, 2.16; N, 15.74%. FT-IR (KBr,
cmꢀ1):
1927 (vs,
n
¼ 3150 (w), 3139 (m), 3035 (w), 2956 (w), 2017 (s,
n(CO)),
n
(CO)), 1871 (vs, (CO)), 1804 (vs, (CO)), 1515 (m), 1459
n
n
(m), 1428 (s), 1401 (s), 1330 (m), 1300 (w), 1283 (s), 1221 (m), 1096
(s), 1066 (m), 1054 (m), 980 (s), 916 (w), 892 (m), 850 (m), 762 (s),
734 (s), 730 (m), 647 (m), 607 (s), 581 (s), 560 (m), 494 (w), 463 (w),
The last difference Fourier map synthesis showed the highest
peak (0.285 e Åꢀ3) located at 0.67 A from Mo1, and the deepest hole
ꢀ
418 (m), 392 (m), 368 (s). FT-Raman (cmꢀ1):
n
¼ 3152 (w), 3138 (w),
(ꢀ0.448 e Åꢀ3) at 0.70 A from C5 close to the medium point of the
ꢀ
3036 (w), 2956 (w), 2017 (vs),1911 (w), 1871 (vs),1803 (s), 1519 (w),
1457 (w), 1427 (w), 1414 (w), 1401 (w), 1330 (w), 1280 (vs), 1240
(w), 1222 (w), 1153 (w), 1097 (w), 1066 (w), 1055 (w), 982 (m), 921
(w), 848 (w), 775 (w), 760 (w), 730 (w), 646 (w), 603 (w), 583 (w),
487 (s), 461 (m), 410 (m), 393 (w). 1H NMR (300 MHz, 25 ꢁC, DMSO-
bond with C6. Information concerning crystallographic data
collection and structure refinement details is summarized inTable 1.
2.5. Catalytic olefin epoxidation
d6):
d
¼ 8.18 (d, 2H, 3-H pz), 7.92 (d, 2H, 5-H pz), 6.49 (t, 2H, 4-H pz),
The liquid-phase catalytic epoxidation of cis-cyclooctene (Cy,
95%, SigmaeAldrich) was carried out with magnetic stirring
(800 rpm), under air, in closed borosilicate micro reactors (5 mL)
equipped with a valve to allow sampling. Typically, the reaction
mixtures consisted of an amount of catalyst equivalent to
43 ꢃ 10ꢀ3 mmol of molybdenum, 4.3 mmol of Cy and 6.6 mmol of
oxidant. The olefin þ oxidant mixture was pre-heated in a ther-
mostated oil bath (55 ꢁC) for 10 min, after which time the catalyst
was added. This point was considered as time zero.
tert-Butylhydroperoxide (TBHP, 5e6 M in decane, Sigmae
Aldrich) and aqueous H2O2 (30% w/w in water, SigmaeAldrich)
were used as oxidants. The reactions were performed without
adding a co-solvent or by using 2 mL of 1,2-dichloroethane (DCE),
nitromethane (MeNO2), acetonitrile (ACN), n-hexane (hex) or
ethanol (EtOH). The other substrates studied (without using co-
6.39 (s, 2H, CH2). 13C NMR (75 MHz, 25 ꢁC, DMSO-d6):
d
¼ 220.4
(CO), 146.2 (3-C pz), 133.9 (5-C pz), 107.3 (4-C pz), 62.6 (CH2).
2.3. [MoO(O2)2(BPM)] (2)
After a catalytic run at 55 ꢁC for 24 h using complex 1
(1.43 mmol) as catalyst precursor, cis-cyclooctene (143 mmol) as
substrate, and TBHP (220 mmol, 5e6 M in decane) as oxidant
(please see Section 2.5 for more details), the reaction mixture was
cooled to ambient temperature, filtered, and kept under nitrogen in
a fridge during one week, whereupon a small crop of yellow crystals
of 2 was obtained. Yield: 0.08 g (17%). Anal. Calcd for C7H8MoN4O5:
C, 25.94; H, 2.49; N, 17.29. Found: C, 25.82; H, 2.37; N, 17.37%. FT-IR
(KBr, cmꢀ1):
n
¼ 3147 (w), 3138 (w), 3118 (m), 3100 (m), 3030 (w),
2987 (w), 1516 (m),1457 (m),1430 (m),1403 (s), 1335 (w),1295 (m),
1277 (s), 1225 (m), 1153 (w), 1110 (w), 1097 (w), 1082 (m), 1065 (m),
1000 (m), 949 (vs,
(m), 776 (m), 730 (m), 655 (m), 634 (w), 603 (m), 582 (m), 535 (m),
400 (m), 366 (w). FT-Raman (cmꢀ1):
solvents) were 1-octene, trans-2-octene, (R)-(þ)-limonene and
a-
pinene. The influence of the temperature on the catalytic activity
was evaluated using Cy without co-solvents.
n(Mo]O)), 925 (w), 914 (w), 866 (s, n(OeO)), 790
The oxidation processes were monitored by gas chromatog-
raphy (GC) using a GC Chrompack CP 9001 with a 25 m OPTIMA
FFAP MachereyeNagel capillary column and a flame ionization
detector (FID), at regular intervals of 15 min during the first hour,
n
¼ 3146 (m), 3119 (w), 3100
(w), 3030 (w), 2986 (w), 1517 (w), 1465 (w), 1431 (w), 1405 (w),
1335 (w), 1275 (w), 1247 (w), 1224 (w), 1152 (w), 1110 (w), 1099 (w),

58
S. Figueiredo et al. / Journal of Organometallic Chemistry 723 (2013) 56e64
Table 1
(TBHP) as oxygen donor, and no co-solvent (other than the decane
present in the oxidant solution and n-octane used as an internal
standard for the GC measurements), at 55 ꢁC (Table 2, Fig. 1). 1,2-
Epoxy-cyclooctane (CyO) was always the only reaction product. In
the absence of catalyst, the reaction rate was negligible. As shown
in Table 3, the catalytic performance of 1, measured in terms of
turnover frequency calculated at 5 min reaction and epoxide yield
at 24 h, compares quite favourably with that reported for other
molybdenum carbonyl complexes used as catalyst precursors in the
same reaction under similar conditions.
Crystal data collection and structure refinement details for [MoO(O2)2(BPM)] (2).
Formula
C7H8MoN4O5
324.11
298(2)
Monoclinic
P21/c
11.0202(5)
7.7592(3)
12.7699(5)
Formula weight
Temperature (K)
Crystal system
Space group
ꢀ
a (A)
ꢀ
b (A)
ꢀ
c (A)
b
(ꢁ)
109.726(2)
1027.85(7)
3
ꢀ
Volume (A )
The kinetic profile with complex 1 as catalyst precursor is
typical of many complexes of the type [MoO2X2Ln] used as catalysts
in the same reaction under similar conditions in a batch reactor
(Fig. 1) [16e18]. Initially the reaction rate is high, with 50%
conversion being reached after 5 min, and then the reaction rate
drops as the substrate is consumed, with full conversion being
reached at 3 h.
Z
4
Dc (g cmꢀ3
)
2.094
1.295
m
(Mo-K
a
) (mmꢀ1
)
Crystal size (mm)
Crystal type
q
0.13 ꢃ 0.08 ꢃ 0.06
Yellow block
range (ꢁ)
3.93 to 29.13
Index ranges
Reflections collected
Independent reflections
Completeness to
ꢀ15 ꢄ h ꢄ 13, ꢀ10 ꢄ k ꢄ 10, ꢀ17 ꢄ l ꢄ 17
15 485
2766 [Rint ¼ 0.0307]
99.8%
R1 ¼ 0.0236, wR2 ¼ 0.0502
R1 ¼ 0.0302, wR2 ¼ 0.0527
m ¼ 0.0174, n ¼ 0.6862
0.285 and ꢀ0.448 e Åꢀ3
When 30% aqueous H2O2 was used as the oxidant instead of
TBHP, with 1 as catalyst precursor, no epoxidation of Cy took place.
This behaviour brings us to the studies of Mimoun and co-workers
[19], who found that the monoperoxo and diperoxo complexes of
the type [MoO(O2)Cl2L2] and [MoO(O2)2L2] showed only weak
activity as catalysts for olefin epoxidation with H2O2 or Ph3COOH.
Better results have been reported in a few selected cases [20], in
particular oxodiperoxomolybdenum/ionic liquid/H2O2 olefin
epoxidation systems [20aed]. Generally, the epoxidation activity of
oxoperoxomolybdenum complexes is much greater when using
TBHP as oxidant, which led Mimoun and co-workers to conclude
that alkyl-peroxidic rather than peroxo species are active inter-
mediates in the reaction [19]. Accordingly, experimental and
theoretical studies carried out by Kühn et al. showed that the
complexes [MoO2X2Ln] (X ¼ Cl, Br, CH3) carrying Lewis-bases
ligands undergo an equilibrium reaction with excess TBHP to give
q
¼ 29.13ꢁ
Final R indices [I > 2
s
(I)]a,b
Final R indices (all data)a,b
Weighting schemec
Largest diff. peak and hole
P
P
a
b
c
R1 ¼ jjFoj ꢀ jFcjj= jFoj:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
P
P
wR2 ¼
½wðFo2 ꢀ Fc2Þ2ꢅ= ½wðFo2Þ2ꢅ:
w ¼ 1=½s2ðFo2Þ þ ðmPÞ2 þ nPꢅ where P ¼ ðFo2 þ 2Fc2Þ=3:
then after each hour until a total of 6 h, and finally the last analysis
was performed at 24 h of reaction. The results were determined
directly from the chromatographic areas, using n-octane as internal
standard. When necessary, the reaction products were identified by
GCeMS (Agilent Technologies 5973 invert Series Mass Selective
Detector) using He as the carrier gas and a 30 m AT-WAX MS Grace
capillary column.
seven-coordinate molybdenum
h
1-alkylperoxo complexes that
selectively catalyze the epoxidation of olefins [16].
3. Results and discussion
The influence of solvent on the epoxidation of cis-cyclooctene in
the presence of 1 was investigated at 55 ꢁC (Table 2, Fig. 1).
Depending on the type of the solvent, 1,2-dichloroethane (DCE),
ethanol (EtOH), nitromethane (MeNO2), acetonitrile (ACN) or n-
hexane (hex), the catalytic activity changes considerably without
affecting the selectivity to the epoxide, which was always 100%. A
decrease in the initial rate was observed for all solvents, which
could be partially related with dilution effects and, eventually, with
changes in the reaction system that did not favour the formation of
actives species. Nevertheless, apart from hex, a full conversion of Cy
(with 100% selectivity) was obtained after 24 h for all the other
solvents, in spite of the different reaction rates observed in the first
hours.
The lack of a clear relationship between the dielectric properties
(Table 2) of the solvents and the catalytic activity may be partly
related to differences in solubility. Thus, the higher reaction rate
observed for DCE may be largely explained by the fact that complex
1 is evidently more soluble in the system containing this polar, non-
coordinating solvent. The low solubility of the catalyst in hexane,
due to the non-polar and aprotic nature of this solvent, is probably
the reason for the low epoxidation rate since it causes a decrease in
3.1. Catalyst precursor preparation
The molybdenum tetracarbonyl complex cis-[Mo(CO)4(BPM)]
(1) was obtained directly from Mo(CO)6 in excellent yield by
heating a mixture of the molybdenum hexacarbonyl and the
organic ligand in toluene at 110 ꢁC for 4 h (Scheme 1). The solid-
state FT-IR spectrum (KBr pellet) of 1 shows four active n(C^O)
normal modes in the 1800e2020 cmꢀ1 range, in agreement with
the Nujol spectrum reported previously for the same complex [14].
When the 1H NMR spectra (recorded in DMSO-d6) of the coordi-
nated and free ligand are compared, the signals for the pyrazolyl
protons are shifted downfield upon coordination, while the CH2
signal is unaffected.
3.2. Catalytic epoxidation of olefins
The tetracarbonyl complex
precursor for the liquid-phase epoxidation of olefins by using cis-
cyclooctene (Cy) as a model substrate, tert-butylhydroperoxide
1 was explored as a catalyst
Scheme 1. Preparation of the catalyst precursor 1.

S. Figueiredo et al. / Journal of Organometallic Chemistry 723 (2013) 56e64
59
Table 2
Olefin epoxidation with TBHP using complex 1 as catalyst precursor.
Co-solventa
TON (mol mol
)
TOF (mol mol hꢀ1
)
Olefin conversion (%)c
Epoxide selectivity (%)c
ꢀ1
ꢀ1
b
Olefin
T (ꢁC)
Mo
Mo
cis-Cyclooctene
20
55
None
None
None
None
5
48
38
100
40
e
63
579
379
1175
480
340
174
270
391
202
21
26/38
100/100
100/100
100/100
100/100
98/100
90/100
98/100
100/100
89/94
100/100
100/100
100/100
100/100
100/100
100/100
100/100
100/100
100/100
100/100
100/100
100/100
3/4d
(2nd batch at 55 ꢁC)
75
(2nd batch at 75 ꢁC)
None
55
55
55
55
55
55
55
55
55
MeNO2 (38)
ACN (35)
EtOH (24)
DCE (10.4)
Hex (1.9)
None
None
None
None
e
e
e
e
1-Octene
trans-2-Octene
Limonene
4
3
44
28
27/46
67/96
93/99
53/68
36
527
331
a
-Pinene
4/2e
a
b
c
Dielectric constants are given in parentheses.
Calculated from the conversion at 5 min of reaction.
Olefin conversion and epoxide selectivity at 6/24 h.
Selectivities to other products were 6/6% for carvone and 48/41% for limonene-1,2-diol.
Selectivities to other products were 13/11% for campholenic aldehyde and 9/11% for campholenic aldehyde oxide.
d
e
the number of available active species in the reaction mixture. The
much slower reactions observed in the presence of the coordi-
nating solvents ACN and ethanol (as opposed to the non-
coordinating solvents DCE and MeNO2) may be due to the partici-
pation of solvent molecules in the reaction, probably by coordina-
tion to the metal site, thereby influencing the overall kinetics (e.g.,
the formation of the active oxidizing species may become rate-
limiting) [17]. As a result, interactions between the olefin/oxidant
and the solvated and/or coordinated molybdenum species may
become unfavourable. Nitromethane, whose polarity is very similar
to that of ACN, is a non-coordinating solvent and, therefore, the
results were slightly better (98% at 6 h) than those of ACN (90% at
6 h). DCE showed a kinetic profile very similar to MeNO2.
the reaction rate increases, as observed in most cases; (b) the
stability of the catalyst decreases, resulting in deactivation. Usually,
a moderate increase in the temperature gives rise to an increase in
the reaction rate; however, high temperatures may be detrimental
due to the possibility of complex decomposition. In the present
study, when the reaction was run at room temperature (20 ꢁC, Cy as
substrate and TBHP as oxidant), the initial TOF decreased to
ꢀ1
63 mol mol
h
ꢀ1, keeping the selectivity to epoxide at 100%
Mo
(Table 2). After 6 h, a yield of about 26% was obtained, in contrast to
100% obtained at 3 h for the reaction run at 55 ꢁC, and 98% obtained
at 5 min for the reaction run at 75 ꢁC (Fig. 2). Increasing the reaction
ꢁ
temperature from 55 to 7ꢀ51 C resulted in an increase in the TOF
from 597 to 1175 mol molMo hꢀ1 (Table 2).
It is known that the catalytic activity of MoVI complexes for
olefin epoxidation is highly dependent on the reaction tempera-
ture. Raising the temperature may have two different effects: (a)
These data show that a higher reaction temperature is beneficial
in terms of initial reaction rate, which could be related with the
accelerated formation of the active species via decarbonylation of
the precursor. On the other hand, a low temperature (20 ꢁC) gives
rise to a significant decrease in the reaction rate probably due to an
ineffective formation of the active species. Despite the better
results obtained at 75 ꢁC, carrying out the reaction at 55 ꢁC may be
advantageous since a high yield of epoxide can be obtained in
a relatively short reaction time (practically 100% after 2 h), which is
very attractive regarding the reduction in energy costs of the
process.
In order to evaluate the performance of the catalytic system
with 1 for the epoxidation of other olefins, a thorough study was
carried out using the structurally different substrates 1-octene,
trans-2-octene, (R)-(þ)-limonene (present in citrus peel) and
a-
pinene (component of turpentine that can be extracted from pine
resin) (Scheme 2). Data in Table 2 summarize the results of the
catalytic activity for the different olefins tested, and Fig. 3 shows the
kinetic profiles. Both TON and TOF values follow the sequence 1-
octene < trans-2-octene <
a
-pinene < (R)-(þ)-limonene < cis-
cyclooctene. This order is in accordance with the increasing pres-
ence of alkyl substituents that cause an increase in electron density
on the double bond, thus increasing its nucleophilicity.
A comparison of the catalytic activities for the linear octenes
indicates that the catalytic system is more active in the epoxidation
of internal double bonds than of terminal ones, as would be ex-
pected based on mechanistic findings [16e18]. The respective
epoxides were the only reaction products in both reactions. trans-2-
Octene was more reactive than 1-octene, which suggests that the
catalyst promotes epoxidation of more nucleophilic olefins. This is
consistent with the fact that for (R)-(þ)-limonene the epoxidation
Fig. 1. Kinetic profiles for cis-cyclooctene epoxidation with TBHP, at 55 ꢁC, using cis-
[Mo(CO)4(BPM)] (1) as catalyst precursor and either no co-solvent (,) or in the
presence of DCE (B), MeNO2 (ꢃ), EtOH ( ), hex (þ) or ACN (>).
6

60
S. Figueiredo et al. / Journal of Organometallic Chemistry 723 (2013) 56e64
Table 3
Epoxidation of cis-cyclooctene at 55 ꢁC with TBHP (in decane) and different molybdenum carbonyl complexes as catalyst precursors.
Complexa
Mo:Cy:TBHP
Co-solvent
TOF (mol mol hꢀ1
)
Epoxide yield at 24 h (%)
Ref.
ꢀ1
b
Mo
cis-[Mo(CO)4(BPM)] (1)
cis-[Mo(CO)4(di-tBu-bipy)]
cis-[Mo(CO)4(pzpy)]
1:100:150c
1:100:150d
1:100:150d
1:100:150c
1:100:150d
1:100:200d
1:100:150d
1:100:200d
1:100:150d
1:100:150d
1:100:200d
1:100:200d
1:100:200d
None
DCE
579 (5)
9 (10)
100
82
92
100
95
100
100
100
100
100
27
This work
[1t]
[1u]
[1v]
[1m]
[15]
[1o]
[1d]
[1j]
None
None
None
CHCl3
None
None
None
None
None
CHCl3
None
67 (10)
372 (5)
297 (10)
900 (5)
500 (10)
820 (5)
313 (5)
310 (10)
40 (15)
960 (5)
1400 (2)
cis-[Mo(CO)4(pyim)]
[Mo(h
3-allyl)Cl(CO)2(DAB)]
CpMo(CO)3Cl
CpMo(CO)3Cl
CpMo(CO)3Me
CpMo(CO)3Me
CpMo(CO)2(
h
3-allyl)
[1o]
[1p]
[1r]
CpMo(CO)2(NHC)Cl
[CpoxMo(CO)2(MeCN)]BF4
CpPPh2Mo(CO)2Cl
100
100
[1s]
a
Di-tBu-bipy ¼ 4,40-di-tert-butyl-2,20-bipyridine; pzpy ¼ ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate; pyim ¼ N-(n-propyl)-2-pyridylmethanimine; DAB ¼ 1,4-(2,6-dimethyl)
phenyl-2,3-dimethyldiazabutadiene; NHC ¼ 1,3-dibenzylimidazol-2-ylidene; (CpOx and CpPPh2) pendant oxazoline or diphenylphosphine groups are coordinated through the
N or P-atoms to the molybdenum centre.
b
The reaction time used to calculate the TOF is given in parentheses.
c
Order of addition of reagents ¼ [(Cy þ TBHP) þ Mo complex].
d
Order of addition of reagents ¼ [(Mo complex þ Cy) þ TBHP].
is mainly promoted at the double bond of the ring (C1eC2) instead
of the terminal double bond (C8eC9). However, -pinene,
selective to the corresponding epoxide, under the applied reaction
conditions, originating wide spectrum of products, which
a
a
a substituted and cyclo-olefin, has a significantly lower reactivity
because it is a bicyclic molecule with a proximal dimethyl bridge
that may cause steric hindrance when it approaches the active
species.
No correlation could be established between the polarity of the
substrate molecules and the catalytic activity (this could be
important since in the absence of solvent the volume of substrate in
the reaction medium is considerable): for example, the dielectric
included carvone and limonene-1,2-diol from (R)-(þ)-limonene,
and campholenic aldehyde and campholenic aldehyde oxide from
a
-pinene (Scheme 2). After 24 h, 99% conversion of (R)-(þ)-limo-
nene was achieved, while the epoxide and diol yields were only 4
and 41%, respectively. For both the (R)-(þ)-limonene and -pinene
a
reactions, small amounts of other unidentified products were
detected. The multiplicity of reaction products obtained from these
monoterpenes results in low concentrations of the final products,
thereby increasing the cost of recovery of these compounds.
The catalytic stability of the active species formed from 1 was
investigated by carrying out two 24 h runs at 55 ꢁC, using a sub-
strate:oxidant molar ratio of 1:1.6. After the first run, the reaction
vessel was recharged with substrate and oxidant in identical
amounts to those used in the first run. Initially, the reaction was
somewhat slower in the second run (Table 2), but after 3 h the
kinetic curves converged to give the same conversion of 100% at 6 h
(Fig. 2). As mentioned above, for a reaction temperature of 75 ꢁC,
complete conversion of Cy was achieved in the first run after 5 min.
To assess the stability of the catalyst at this higher temperature,
a second run was carried out as described above. The same effect
was observed, i.e., the reaction was somewhat slower in the second
run (Table 2), but after 3 h the kinetic curves converged to give
similar conversion in the range of 95e100% up to 6 h (Fig. 2). For
both reaction temperatures, since the catalyst was not separated
from the reaction products of the first run, the accumulation of tert-
BuOH, a possible coordinating solvent, could cause a lower initial
reaction rate [16,17]. Additionally, a more dilute reaction mixture
and a decrease in the reaction temperature that occurred when the
new amounts of substrate and oxidant were added may also
contribute to the lower initial rate. Nevertheless, the same level of
conversion is achieved at longer times, which attests to the high
stability of the catalyst.
constants of (R)-(þ)-limonene and
a-pinene are similar (2e3, at
20 ꢁC), but the respective conversions were not. The reactions of
cis-cyclooctene, 1-octene and trans-2-octene gave the corre-
sponding epoxide as the only product throughout 24 h reaction. In
contrast, the reactions of (R)-(þ)-limonene and
a-pinene were not
With the aim of obtaining a better understanding of the
molybdenum species formed upon use of complex 1 as a catalyst
precursor for the epoxidation of olefins, a separate reaction was
performed with the same precursor:Cy:TBHP ratios (1:100:152) but
involving larger quantities (please see Section 2.3 for details). After
24 h of reaction at 55 ꢁC, the reaction mixture was cooled to
ambient temperature, filtered, and kept in a fridge during one
week, whereupon a small crop of yellow crystals suitable for X-ray
diffraction was obtained. As will be described below, the XRD
Fig. 2. Kinetic profiles for cis-cyclooctene epoxidation with TBHP using cis-
[Mo(CO)4(BPM)] (1) as catalyst precursor and no co-solvent, at 20 ꢁC (C run 1), 55 ꢁ
(, run 1, ꢃ run 2), and 75 ꢁC (þ run 1, B run 2).
C

S. Figueiredo et al. / Journal of Organometallic Chemistry 723 (2013) 56e64
61
Scheme 2. Substrates and their respective epoxidation products using 1 as catalyst precursor and TBHP as oxidant, at 55 ꢁC, without co-solvent.
analyses identified the structure as the oxodiperoxo complex
[MoO(O2)2(BPM)] (2). Complex 2 could also be obtained in 47%
yield by carrying out the reaction of 1 with TBHP in the absence of
bidentate N,N-ligand such as a substituted pyrazolylpyridine
[21a,22]. A catalytic run for the epoxidation of cis-cyclooctene
carried out with 2 under the same conditions used for the catalyst
precursor 1 gave a kinetic profile similar to that obtained with 1
(Fig. 4). Hence, there is a high probability that the active catalyst
formed in situ from 1 is the oxodiperoxo complex [MoO(O2)2(BPM)]
Cy. The complex was characterized by FT-IR, FT-Raman, 1H and 13
C
NMR spectroscopy. In the solid-state, the FT-IR spectrum of 2 shows
bands at 949 cmꢀ1 (Mo]O)), 866 cmꢀ1
(OeO)), 656, 582 and
535 cmꢀ1
(Mo(O2)2)), which are all characteristic of [MoO(O2)2Ln]
(n
(n
ꢀ1
h
ꢀ1) is
Mo
(n
(2). The initial catalytic activity for 2 (TOF z 550 mol mol
complexes bearing either two monodentate ligands or one biden-
tate ligand (L) [21]. The corresponding bands appear at 950, 875,
slightly higher than that reported for [MoO2Cl2(2,2-di(1-pyꢀra1zolyl)
propane)] under similar conditions (TOF ¼ 460 mol molMo hꢀ1
)
656, 578 and 529 cmꢀ1 in the Raman spectrum. No
n
(C^O) bands
[6a]; after 4 h reaction the epoxide yield with 2 was 99.2% vs. 62%
were observed. The 1H and 13C NMR spectra of 2 in DMSO-d6 show
pyrazole (dH ¼ 7.95, 7.49, 6.29 ppm; dC ¼ 140.1, 130.6, 106.3 ppm)
and CH2 (dH ¼ 6.40 ppm; dC ¼ 64.2 ppm) signals. When compared
to the spectra of the free ligand in the same solvent [14], the signals
for 2 are unshifted, which may mean that the ligand BPM is weakly
coordinated to the metal centre and/or is displaced by the deuter-
ated solvent molecules.
As mentioned above, seven-coordinate molybdenum peroxo
complexes of the type [MoO(O2)2Ln] are well known catalysts for
the epoxidation of olefins with TBHP, especially when L is a neutral
for the dichlorodioxomolybdenum(VI) complex.
3.3. Crystal structure description of [MoO(O2)2(BPM)] (2)
Compound 2 crystallized in the monoclinic space group P21/c,
with the asymmetric unit being composed of a whole complex
molecule ultimately formulated as [MoO(O2)2(BPM)]. The metal
centre is heptacoordinated to a terminal oxido, two h2-O,O0-per-
oxido and one k2-N,N0-bispyrazolylmethane ligands. If each of the
peroxido ligands is considered as a single coordination site, the

62
S. Figueiredo et al. / Journal of Organometallic Chemistry 723 (2013) 56e64
Fig. 5. Asymmetric unit of [MoO(O2)2(BPM)] (2) showing a whole molecular unit. Non-
hydrogen atoms are represented as thermal ellipsoids drawn at the 50% probability
level, while hydrogen atoms are represented as small spheres with arbitrary radii. The
atomic labelling is provided for all non-hydrogen atoms. For selected bond lengths and
angle ranges referring to the Mo6þ coordination sphere see Table 4.
Fig. 3. Oxidation of cis-cyclooctene (,), (R)-(þ)-limonene (ꢃ), -pinene (þ), trans-2-
a
octene (C) and 1-octene (
6) with TBHP using cis-[Mo(CO)4(BPM)] (1) as catalyst
precursor, at 55 ꢁC, without co-solvent.
ꢀ
coordination environment around the metal centre can be envis-
aged as a highly distorted trigonal bipyramid (Fig. 5). The poly-
hedron distortion can be related with either the internal angles or
the coordination distances. In this geometry the apical positions are
occupied by the oxido ligand O1 and by one of the nitrogen atoms
and 2.3666(16) A], with the longest being attributed to the apical
nitrogen atom (see Table 4 for additional geometry details). This
structural feature has been observed in a series of compounds
having the general formula [MoO(O2)2(L)2] (where L represents
a pyrazole or pyridine group) and similar coordination geometry, as
revealed by a systematic search of the Cambridge Structural Data-
base (CSD, version 5.33, November 2011 with three updates e May
2012) [23]. For this family of compounds, which comprises 11
of the neutral ligand (N3 in this case). The MoeO distances are
ꢀ
1.6854(15)
A
for the Mo]O bond and in the 1.9160(15)e
ꢀ
1.9597(15) A range for the single MoeO bonds. The trans influ-
ence of the former double bond is remarkable in this molecule, with
the two MoeN bonds being statistically very distinct [2.1913(16)
ꢀ
members, the MoeN distances range from ca. 2.16 to ca. 2.25 A for
the equatorial nitrogen atom and from ca. 2.28 to ca. 2.43 A for the
ꢀ
apical one [20b,c,22b,24], which agree well with the results for 2.
While the internal polyhedral angles in 2 involving the apical and
the equatorial atoms range from 77.83(6) to 104.25(8)ꢁ, those
subtended by the medium points of the peroxido ligands and the
equatorial nitrogen atoms range from ca. 109 to ca. 111ꢁ (see Table 4
Table 4
6þ
ꢀ
Selected bond lengths (in A) and angles (in degrees) for the Mo coordination
environment present in [MoO(O2)2(BPM)] (2).
Mo1eO1
Mo1eO2
Mo1eO3
Mo1eO4
Mo1eO5
Mo1eN1
Mo1eN3
O2eO3
1.6854(15)
1.9597(15)
1.9219(16)
1.9468(16)
1.9160(15)
2.1913(16)
2.3666(16)
1.466(2)
O4eO5
1.472(2)
O1eMo1eO5
O1eMo1eO3
O5eMo1eO3
O1eMo1eO4
O5eMo1eO4
O3eMo1eO4
O1eMo1eO2
O5eMo1eO2
O3eMo1eO2
O4eMo1eO2
104.25(8)
103.68(8)
89.35(7)
100.34(8)
44.80(7)
132.42(7)
101.34(7)
131.38(7)
44.36(6)
O1eMo1eN1
O5eMo1eN1
O4eMo1eN1
O2eMo1eN1
O5eMo1eN3
O1eMo1eN3
O3eMo1eN3
O4eMo1eN3
O2eMo1eN3
N1eMo1eN3
90.93(7)
131.55(7)
87.63(7)
88.05(6)
83.00(7)
168.72(7)
84.81(6)
78.55(6)
79.40(6)
77.83(6)
Fig. 4. Epoxidation of cis-cyclooctene with TBHP in the presence of cis-
[Mo(CO)4(BPM)] (1) (,) or [MoO(O2)2(BPM)] (2) (þ), at 55 ꢁC, without co-solvent.
157.95(7)

S. Figueiredo et al. / Journal of Organometallic Chemistry 723 (2013) 56e64
63
Table 5
4. Conclusions
ꢀ
Weak interactions (distances in A and angles in degrees) present in the crystal
structure of [MoO(O2)2(BPM)] (2).a
The present work has shown that a molybdenum tetracarbonyl
complex bearing a bis(pyrazolyl)methane ligand is a precursor to
a very active and stable catalyst for olefin epoxidation, with the
catalytic activity comparing favourably with that previously found
for analogous complexes bearing either bipyridine, pyrazolylpyr-
idine or pyridylimine derivatives as ligands. Whereas the reactions
of cis-cyclooctene, 1-octene and trans-2-octene gave the corre-
sponding epoxide as the only product, the reactions of (R)-
p
/
pb
Intercentroid distance
3.7338(12)
3.3414(14)
CgA/CgAi
CgB/CgBii
DeH/Cg
d(D/Cg)
3.414(3)
<(DHCg)
144
C2eH2/CgBi
DeH/A
d(D/A)
3.486(3)
3.359(3)
3.046(3)
3.212(3)
3.476(3)
<(DHA)
175
141
123
129
C1eH1/O4iii
C4eH4A/O1iv
C4eH4B/O2c
C5eH5/O2v
C7eH7/O3iv
(þ)-limonene and
a-pinene were not selective to the correspond-
ing epoxide, under the applied reaction conditions, originating
a wide spectrum of products. Taking into account that the oxodi-
peroxo complex [MoO(O2)2(BPM)] (2) was isolated in a small
amount from the catalytic reaction mixture obtained after epoxi-
dation of cis-cyclooctene, and that the catalytic performance of 2
for cis-cyclooctene epoxidation matches that observed for 1, it
seems reasonable to conclude that fast oxidative decarbonylation of
cis-[Mo(CO)4(BPM)] (1) may occur in situ to give 2 as the active
catalyst. In the future, spectroscopic investigations of the reaction
of the precursor with the oxidant may help to confirm the nature of
the molybdenum species formed.
169
a
Symmetry operations used to generate equivalent atoms: (i) 2 ꢀ x, 1 ꢀ y, 2 ꢀ z;
(ii) 1 ꢀ x, 1 ꢀ y, 2 ꢀ z; (iii) 2 ꢀ x, ꢀy, 2 ꢀ z; (iv) x, ½ ꢀ y, ½ þ z; (v) x, 1 þ y, z.
b
CgA ¼ centroid of the ring {N1, N2, C1eC3}, CgB ¼ centroid of the ring {N3, N4,
C5eC7}.
c
Intramolecular interaction.
for detailed geometrical details). One remarkable aspect of the
coordination geometry around the Mo6þ metal centre concerns the
fact that the atoms of the coordination sphere are distributed in
two almost perpendicular medium planes [angle between of
89.68(10)ꢁ]: one comprising the peroxido ligands [largest deviation
of 0.014(2) A for both O3 and O5] and the other comprising the
atoms Mo1, N1, N3 and O1 [largest deviation of 0.007(2) A for O1].
Atom Mo1, which is located in the second plane, is displaced only
0.371(1) A from the first plane.
In the absence of strong supramolecular interactions, the crystal
packing in 2 is mainly governed by the need to fill the available
space and by a handful of weak interactions such as CeH/O, Ce
H/
details and Fig. 6 for a schematic representation). Each of the
aromatic rings interacts through stacking with symmetry-
Acknowledgements
ꢀ
ꢀ
We are grateful to the Fundação para a Ciência e a Tecnologia
(FCT), QREN, FEDER, COMPETE, the European Union and the Asso-
ciate Laboratory CICECO (Pest-C/CTM/LA0011/2011) for continued
support and funding. The FCT is acknowledged for financial support
towards the purchase of the single-crystal diffractometer, for a PhD
grant to SF (SFRH/BD/45116/2008), and for a post-doctoral grant to
JAF (SFRH/BPD/63736/2009).
ꢀ
p and pep stacking interactions (see Table 5 for geometrical
pep
Appendix A. Supplementary material
related aromatic rings, with the distances between centroids
ꢀ
being smaller than 3.7338(12) A. This leads to the formation of
CCDC-890671 contains the supplementary crystallographic data
(including structure factors) for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary
data associated with this article can be found, in the online version,
at http://dx.doi.org/10.1016/j.jorganchem.2012.09.019.
a supramolecular chain parallel to the a-axis of the unit cell. A Ce
interaction [C/Cg distance of 3.414(3) A] also participates
in the constitution of this chain. Supramolecular chains close pack
ꢀ
H/p
with the assistance of weak CeH/O interactions [C/O distance
ꢀ
smaller than 3.486(3) A].
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