Catalytic activity of molybdenum(II) complexes in homogeneous and heterogeneous conditions

Article abstract of DOI:10.1021/om501068q

The new complexes [MoBr(η³-C₃H₅)(CO)₂(L)₂] (C1) and [MX₂(CO)₃(L)₂] (M = Mo(II), X = I (C2); M = Mo(II), X = Br (C3); M = W(II), X = I (C4); M = W(II), X = Br (C5)) were synthesized by reaction of 2-amino-1,3,4-thiadiazole (L) with [MoBr(η³-C₃H₅)(CO)₂(NCCH₃)₂] (1), [MoI₂(CO)₃(CH₃CN)₂](M = Mo (2); M = W (4)), or [MoBr₂(CO)₃(CH₃CN)₂](M = Mo (3); M = W (5)) in 2:1 ratio. The five complexes were immobilized in MCM-41, yielding the materials MCM-Cn (n = 1-5), and C1 was also immobilized in silica gel (Silica-C1) and in a polyhedral oligomeric silsesquioxane (Cube-C1). Complexes and materials were fully characterized by spectroscopic techniques and elemental analysis. DFT calculations showed that several C1 isomers should coexist. The as synthesized and supported complexes were tested as catalysts on the oxidation of geraniol, cis-hex-3-en-1-ol, trans-hex-3-en-1-ol, (S)-limonene, and 1-octene. The conversions and TOF significantly depend on the complex and the nature of the substrate. The general conclusions are (i) complex C1 has the highest activity; (ii) tungsten complexes C4 and C5 are more active than the molybdenum analogues; (iii) the immobilization of the catalysts improves the performance; and (iv) silica gel and the polyhedral oligomeric silsesquioxane supports modify the selectivity, leading to products different from the one obtained with MCM for specific substrates.

Full text of DOI:10.1021/om501068q

Article
pubs.acs.org/Organometallics
Catalytic Activity of Molybdenum(II) Complexes in Homogeneous
and Heterogeneous Conditions
Maria Vasconcellos Dias,† Marta S. Saraiva,† Paula Ferreira,*,‡ and Maria Jose Calhorda*,†
́
†Departamento de Química e Bioquímica, CQB, Faculdade de Cien
Portugal
̂
cias, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa,
‡CICECO - Aveiro Institute of Materials, Department of Materials and Ceramic Engineering, University of Aveiro, 3810-193 Aveiro,
Portugal
S
* Supporting Information
ABSTRACT: The new complexes [MoBr(η3-C3H5)-
(CO)2(L)2] (C1) and [MX2(CO)3(L)2] (M = Mo(II), X =
I (C2); M = Mo(II), X = Br (C3); M = W(II), X = I (C4); M
= W(II), X = Br (C5)) were synthesized by reaction of 2-
amino-1,3,4-thiadiazole (L) with [MoBr(η3-C3H5)-
(CO)2(NCCH3)2] (1), [MoI2(CO)3(CH3CN)2](M = Mo
(2); M = W (4)), or [MoBr2(CO)3(CH3CN)2](M = Mo (3);
M = W (5)) in 2:1 ratio. The five complexes were immobilized
in MCM-41, yielding the materials MCM-Cn (n = 1−5), and
C1 was also immobilized in silica gel (Silica-C1) and in a polyhedral oligomeric silsesquioxane (Cube-C1). Complexes and
materials were fully characterized by spectroscopic techniques and elemental analysis. DFT calculations showed that several C1
isomers should coexist. The as synthesized and supported complexes were tested as catalysts on the oxidation of geraniol, cis-hex-
3-en-1-ol, trans-hex-3-en-1-ol, (S)-limonene, and 1-octene. The conversions and TOF significantly depend on the complex and
the nature of the substrate. The general conclusions are (i) complex C1 has the highest activity; (ii) tungsten complexes C4 and
C5 are more active than the molybdenum analogues; (iii) the immobilization of the catalysts improves the performance; and (iv)
silica gel and the polyhedral oligomeric silsesquioxane supports modify the selectivity, leading to products different from the one
obtained with MCM for specific substrates.
functionalization, as reported, so that the POSS can behave as a
ligand.34−38,20 Many examples of applications of these species
are known, ranging from catalysis39−51 to analytical chem-
istry52−55 and special materials.56−60
INTRODUCTION
■
The need to improve catalysts taking into account not only the
chemical process but also the environmental issues remains a
challenge. While homogeneous catalysts are usually more
controllable in terms of selectivity, heterogeneous systems allow
an easier separation of the products and recovery of the
catalyst.1,2 The mesoporous silica-based materials MCM-41,
developed by Mobil Corporation in 1992,3,4 are among the
most efficient supports, not only because they have a high
surface area inside the ordered structure of the one-dimensional
channels but also because the surface silanol groups make
functionalization with organometallic complexes, for instance,
easy. The strong covalent bonds thus formed prevent lixiviation
of the catalyst, increasing its lifetime,5−9 and lead to active
catalysts.10 While the activity is sometimes higher than in the
homogeneous parent compound,9 in others secondary
reactions take place, damaging the catalyst.7,11−14 The nature
of the surface makes these MCM-41-derived materials better
catalysts than silica,15,16 but they are more expensive. Other
forms of silica, namely, polyhedral oligomeric silsesquioxane
(POSS),17−19 have also been shown to coordinate metallic
fragments and generate multiple site catalysts.20 These species
are based on the (RSiO1.5)8 oligomer, which organizes as a cube
held by Si−O−Si bonds and carrying R groups at the
vertices.21−33 A suitable choice of these groups will allow
Epoxides are well known as very important intermediates in
the industry of fine chemicals and pharmaceuticals that can be
formed by oxidation of the corresponding olefin in the presence
of various oxygen sources. Transition metal complexes proved
to be effective catalysts for the epoxidation of alkenes, but
molybdenum and tungsten are probably the best known
transition-metal catalysts for alkene epoxidation using alkyl
hydroperoxides, because they are the metals in the industrial
ARCO-Halcon process for homogeneous olefin epoxidation
with tert-butylhydroperoxide (TBHP) as oxidant.61,62 We
reported the functionalization of a POSS with Mo(II) and its
application as epoxidation catalyst.20 Since the [MoBr(η3-
C3H5)(CO)2(L)] complex, with L = 2,2′-pyridylamine, was not
a good catalyst, neither in homogeneous conditions nor
immobilized in POSS or in MCM-41, we tried to improve
the performance of the catalyst and understand the role of the
support (POSS, silica, MCM-41). Indeed, many nitrogen donor
ligands bound to Mo(II) after substitution of NCMe in
Received: October 23, 2014
Published: April 13, 2015
© 2015 American Chemical Society
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[MoBr(η3-C3H5)(CO)2(NCCH3)2]63 have proved to be very
good precursors for olefin epoxidation, being first oxidized to
Mo(VI) active species.64 In some supports, heterogeneous
catalysts were even more effective than homogeneous catalysts.9
The ligand chosen was 2-amino-1,3,4-thiadiazole, which
offers several possibilities for coordination through the ring
atoms and bears a NH2 group, which is important for
immobilization purposes.65−67 Complexes of Co(II) and Ni(II)
with the related 2-aminothiazole were reported to inhibit
cancer cell growth, mainly owing to their complex−DNA
binding ability, suggesting other applications for the new
complexes.68
We report herein the synthesis of the [MoBr(η3-C3H5)-
(CO)2(L)2] complex with L = 2-amino-1,3,4-thiadiazole, its
characterization, and immobilization in MCM-41, POSS, and
silica, as well as the catalytic activity of the complex and the
materials in the oxidation of several alkenes. Other Mo(II)
complexes and their W(II) analogues, with the general formula
[MX2(CO)3(L)2] in which X = Br or I,69 were also synthesized
and immobilized in MCM-41, in order to extend the scope of
the study to other metal fragments.
Figure 1. FTIR spectra of [MoBr(η3-C3H5)(CO)2(NCCH3)2] (1),
the ligand 2-amino-1,3,4-thiadiazole (L), and complex [MoBr(η3-
C3H5)(CO)2(L)2] (C1).
The FTIR spectra of complexes C2−C5 are also
characterized by the strong νCO carbonyl vibrations observed,
for instance, in complex C2 at 1906, 1938, and 2008 cm−1 and
in the precursor 2 at 1921 and 2016 cm−1, while the νCN
vibration of L appears at 1597 cm−1 in C2. The different
number of carbonyl bands observed is associated with the
number of different isomers coexisting in the solid state, owing
to the possible geometries for heptacoordinate complexes.70,69
RESULTS AND DISCUSSION
■
Chemical Studies: Complexes. The allyl complex [MoBr-
(η3-C3H5)(CO)2(NCCH3)2]63 (1) reacts with the ligand 2-
amino-1,3,4-thiadiazole (L) in a 1:2 ratio to form the new
complex [MoBr(η3-C3H5)(CO)2(L)2] (C1). A similar reaction
occurs starting from the other Mo(II) and W(II) complexes,
[MoI2(CO)3(CH3CN)2] (2), [MoBr2(CO)3(CH3CN)2] (3),
[WI2(CO)3(CH3CN)2] (4), and [WBr2(CO)3(CH3C)N)2]69,7
(5), leading to the formation of the new complexes
[MX2(CO)3(L)2] (M = Mo, X = I (C2); M = Mo, X = Br
(C3); M = W, X = I (C4); M = W, X = Br (C5)), as shown in
Scheme 1.
1
The H NMR spectrum of complex C1 is characterized by
the signals of the two protons of the ligand L (that of the ring
at 8.58 ppm and the amine proton at 7.24 ppm, both shifted
relative to their position in the spectrum of the free ligand, at
6.9 and 8.3 ppm, respectively) and those of the allyl group
(Hanti at 1.09, Hsyn at 3.37, and Hmeso at 5.77 ppm). These are
observed in the spectrum of 1 at 1.24 (Hanti), 3.45 (Hsyn), and
3.68 (Hmeso) for the most abundant isomer. The peaks
associated with the two rings and the three allyl carbon
atoms are observed in the 13C NMR spectra. In complexes C2−
C5, only the ligand L hydrogen and carbon atoms exhibit peaks
Scheme 1. Synthesis of Complexes [MoBr(η3-
C3H5)(CO)2(L)2] and [MX2(CO)3(L)2]
1
in the H and the 13C NMR spectra.
The previous spectroscopic results indicate that L
coordinates to the molybdenum and tungsten centers in all
the complexes, but only in C1 does the integration of proton
signals in the NMR spectrum allow the conclusion that two L
ligands are present. This result is confirmed in all complexes
C1−C5 by elemental analysis. None of the spectroscopic data,
however, indicate which nitrogen atom is bound to the metal.
In order to answer this question, DFT calculations (see
Computational Details) were performed. We optimized the
geometries with L bound through the two ring nitrogen atoms
(Figure 2) and considered the possibility of forming the
equatorial or the axial isomer (Scheme 2).
The new organometallic complexes were characterized by
1
elemental analysis, FTIR, and H and 13C NMR spectroscopy.
The formation of complex C1 is supported by the
appearance in the FTIR spectrum (Figure 1) of the two
νCO stretching modes, which appear at 1923 and 1948 cm−1,
shifted from their position in the precursor complex 1 (1850
and 1947 cm−1), and the deviation of the νCN stretching
mode from 1622 cm−1 in the free L to 1610 cm−1 upon
coordination. The characteristic νCN bands at 2320 and 2287
cm−1 in 1 have disappeared.
Figure 2. DFT-optimized structures of the isomers of [MoBr(η3-
C3H5)(CO)2(L)2] (C1), with relative energies (kcal mol−1).
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Scheme 2. More Stable Isomers of Complexes [MoBr(η3-
C3H5)(CO)2(L)2]
complexes 1−5, affording the functionalized materials MCM-
C1 to MCM-C5, in the presence of a base. This procedure of
immobilizing molybdenum and tungsten complexes of L is
illustrated in Scheme 3 and has been used before.11
MCM is characterized by a hexagonal unit cell, leading to
four peaks in the 2θ = 2−10° range of the X-ray powder
diffractogram, the strongest reflection being assigned to d100
and the three weaker reflections at higher angles to d110, d200
,
,
and d210, as shown in Figure 3. These peaks reflect the highly
The energy differences are small, and hydrogen bonds can be
formed. In the two equatorial isomers, E1 and E2, there is a
N−H···N hydrogen bond, and the only difference is the
nitrogen binding the metal. In the lowest energy species (E2),
the Mo−N bond is formed with the nitrogen atom adjacent to
the C−NH2 bond. The same information results from the
comparison between A1 and A2, since the same N atom makes
the Mo−N bond in the lowest energy isomer, A2. Weak N−
H···Br hydrogen bonds are formed in both A1 and A2, but in
A2 there is also a N−H···O one. It seems therefore safe to
conclude that this is the preferred coordination mode of the L
ligand to molybdenum. The preference for the isomer is not so
well defined, as E2 and A2 differ by only 1.3 kcal mol−1, and
fluxionality should be observed.
Immobilization of Complexes. The five complexes were
immobilized in MCM-41 (MCM), and the allyl derivative
[MoBr(η3-C3H5)(CO)2(L)2] (C1) was also immobilized in an
octasilsesquioxane (Cube) and in silica gel (Silica), in order to
prepare heterogeneous catalysts with different properties and
compare the role of the support on the catalytic activity of the
Mo(II) complexes in homogeneous and heterogeneous
conditions. In all cases, 3-chloropropyltriethoxysilane was
used as a spacer to react directly with the material walls using
the (RO)3Si− functionality, while the Cl side reacts with the
NH bond of the 2-amino-1,3,4-thiadiazole ligand.
Figure 3. XRD powder patterns of mesoporous materials MCM,
MCM-Pr, MCM-L, and MCM-C1.
ordered pore system.71 The lattice parameter a, calculated
assuming the hexagonal unit cell (a = 2d100/√3) is equal to
41.0 Å. Upon functionalizing MCM, all X-ray diffraction peaks
are still observed in the materials, indicating the retention of the
hexagonal structure. However, the intensity of the reflections
decreases, resulting from the immobilization of more and more
groups inside the wall after reaction with Si-OH groups with
consequent reduction of the X-ray scattering contrast between
the MCM walls and pore-filled channels.72,73
The same trend is observed after grafting the other metal
complexes to form MCM-C2, MCM-C3, MCM-C4, and
MCM-C5 (not shown). In all cases there was a clear reduction
of the peak intensities when functionalizing the pores.
We used a three-step tethering process to functionalize the
MCM and the silica gel frameworks. In this procedure, first a
silylated spacer reacts with a free silanol group of the silica-
based material. This silylation reaction can modify both the
physical and chemical properties of a given surface. After, the
chloride is replaced by the 2-amino-1,3,4-thiadiazole.
Pristine MCM obtained by a template approach8 reacted
with an excess of 3-chloropropyltriethoxysilane in dry toluene
for 24 h under reflux and with stirring. A white powder (MCM-
Pr) was filtered off and dried under vacuum at room
temperature for 4 h. Reaction of this material with the 2-
amino-1,3,4-thiadiazole led to the formation of the material
MCM-L, which was then used as a ligand toward the M(II)
The 77 K N2 adsorption/desorption isotherms of the MCM
and MCM-Pr materials (Figure 4) have the type IV shape
according to the IUPAC classification74 and are characteristic of
Scheme 3. Synthesis of Materials MCM-Pr, MCM-L, and MCM-C1 to MCM-C5
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Information, SI). The well-defined inflection on the N2
isotherm observed for the precursor material MCM (Figure
4) leads to a very narrow pore size distribution (PSD) curve,
with a maximum value of 3.60 nm. Immobilization of bulky
groups into the MCM channels is reflected in a shift of the PSD
maximum curve (dp) toward smaller pore sizes with
concomitant reduction of the intensity, indicating decreased
uniformity of the pores. The pore width decreases from 3.60
nm in the pristine MCM to 2.87 nm in MCM-C1. The same
observations can be made for all the new materials MCM-C2 to
MCM-C5.
Elemental analysis shows that an amount of metal between
3.7 and ∼5.1 wt % was introduced into the mesoporous
materials. The molybdenum(II) contents ranged from 4.0 wt %
(0.40 mmol g−1) in MCM-C1 to 3.7 wt % (0.37 mmol g−1) in
MCM-C2, to 5.1 wt % (0.51 mmol g−1) in MCM-C3. The
tungsten-containing materials revelead an amount of 4.7 wt %
(0.47 mmol g−1) in MCM-C4 and 4.0 wt % (0.40 mmol g−1) in
MCM-C5.
Figure 4. Nitrogen adsorption isotherms of materials MCM, MCM-
Pr, MCM-L, and MCM-C1 at 77 K.
The FTIR spectra of all the materials show the bands
characteristic of the asymmetric νSi−O−Si modes. They appear at
1256, 1002, and 798 cm−1 in MCM-Pr. Stretching νC−H modes
at 2800−3000 cm−1 indicate the presence of the chloropropyl-
triethoxysilane spacer, both in MCM-L and in MCM-C1, and
the allyl group in the complex. The characteristic νCO bands
reflect the coordination of the Mo(CO)2 fragment, appearing at
1944 and 1860 cm−1, slightly shifted from their position in the
spectrum of C1 as the ligand was modified. In the FTIR
spectrum of MCM-C2, vibration modes can be assigned, not
only to the silicious structure of the material (asymmetric Si−
O−Si modes at 1063 and 688 cm−1) but also to the
immobilized species: νC−H stretching modes of the propyl
chain at 2979 and 2915 cm−1; νCO stretchings at 1886, 1935,
and 2015 cm−1; νCN of the ligand at 1627 cm−1. The spectra
of materials MCM-C3 to MCM-C5 are comparable. By
comparison with the proposed structure of C1−C5, it is
considered that two immobilized L ligands in MCM-L
coordinate to one metal center of each precursor, substituting
two nitrile ligands. Indeed, there is no evidence for coordinated
acetonitrile (νCN stretching bands).
mesoporous solids. A sharp inflection is observed between the
relative pressures (p/p0) of 0.20−0.40 owing to capillary
condensation of N2 inside the primary mesoporous channels.
The precursor material MCM thus exhibits a narrow pore size
distribution, with an average pore diameter (dp) of 3.60 nm, a
specific surface area of 1037 m2 g−1, and a pore volume of 0.97
cm3 g−1 (Table 1). All the functionalized materials, MCM-C1
Table 1. Textural Parameters of MCM, MCM-Pr, MCM-L,
MCM-C1, MCM-C2, MCM-C3, MCM-C4, and MCM-C5
Materials, from N2 Isotherms at 77 K
a
b
c
d100
(Å)
SBET
ΔSBET
Vp
ΔVp
dBJH
sample
MCM
(m2 g−1
)
(%)
(cm3 g−1
)
(%)
(nm)
35.5
33.1
32.9
32.9
32.2
33.0
30.3
30.6
1037
948
929
462
603
502
570
571
0.97
0.61
0.57
0.26
0.37
0.31
0.39
0.39
3.60
2.87
2.87
2.87
3.03
3.03
2.87
2.87
MCM-Pr
MCM-L
9
10
55
42
52
45
45
37
41
73
62
68
60
MCM-C1
MCM-C2
MCM-C3
MCM-C4
MCM-C5
Further information about the nature of the immobilized
species may be obtained from 13C and 29Si NMR spectroscopy
(Figures 5 and 6). The 13C CP MAS solid-state resonances in
the MCM-Pr material are assigned to the three carbons of the
propyl group and to the carbons of unreacted ethoxyl groups of
60
a
b
Variation of surface area relative to parent MCM-41. Variation of
c
total pore volume in relation to parent MCM-41. Median pore width
determined by the BJH method.
to MCM-C5, also display reversible type IV N2 adsorption
isotherms, indicating that the initial material structure is
maintained and some of the pores remain accessible after
binding of the metal centers. Each successive functionalization
of MCM in the three steps shown in Scheme 3 leads to a
decrease in N2 uptake, at high p/p0, and both the surface area
and pore volume decrease (Table 1), in agreement with the
reported values for this type of materials.75,76 These results
suggest that the organometallic complexes are grafted in the
internal surface of the material. The textural parameters,
calculated by the BJH method,77 are summarized in Table 1
and are comparable with others from modified mesoporous
materials previously reported.13,9
The calculated pore size distribution curves for MCM-C2
and MCM-C3 and all the materials used in its synthesis (MCM,
MCM-Pr, and MCM-L) are shown in Figure S1 (Supporting
Figure 5. 13C CP MAS NMR spectra of materials MCM-Pr, MCM-L,
and MCM-C1.
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Qn-type signals, where Qn = Si(OSi)n(OH)4−n, assigned to the
characteristic 29Si chemical environments in the MCM-41
material. Unmodified MCM displays two broad convoluted
resonances in the 29Si CP MAS NMR spectrum at δ −109.9
and −100.4 ppm, assigned to Q4 and Q3 species of the silica
framework. A weak shoulder is also observed at δ −91.2 ppm
for the Q2 species. Both Q3 and Q2 sites have free silanol groups
that are available for functionalization through esterification
reaction. After reaction with 3-chloropropyltriethoxysilane, new
organosilica environments arise, identified as Tm signals, with
the general formula Tm = R′Si(OSi)m(OR)3−m. The concom-
itant decrease of the intensity of the peaks assigned to Q2 and
Q3 is observed while, comparatively, Q4 intensity is enhanced.
T1 is mainly observed for MCM-Pr, indicating that further
condensation occurs upon functionalization.
To investigate the effect of the support on catalytic activity
(see below), complex C1 was immobilized in a POSS cage
(Cube) and in silica. Reaction between a Cube previously
functionalized with L ligands (Cube-L), containing a Si8O8 core
and eight −(CH2)3NHC2HN2S arms, and complex [Mo(η3-
C3H5)Br(CO)2(NCCH3)2] (1) in a 1:8 ratio led to a new
yellow solid (Cube-C1). Elemental analysis indicated that two
Mo(η3-C3H5)Br(CO)2(NCCH3) units were attached to each
cube and that only one coordinated nitrile was substituted
(Scheme 4).
The FTIR spectrum of Cube-C1 exhibits the characteristic
νSi−O−Si vibrational modes of the Cube cage at 1166 and 681
cm−1, the νC−H modes of the propyl chain at 2934 and 2887
cm−1, and the characteristic νCN, νC−N, and νC−H modes of the
L ligand at 1600, 1353, and 3100 and 3170 cm−1. All these
bands were observed with small shifts in the FTIR spectrum of
the Cube-C1 precursor. Two new bands of Cube-C1 observed
at 1856 and 1948 cm−1 identify the νCO stretching modes of
the organometallic fragment (1850 and 1947 cm−1 in 1), but
the νCN bands are too weak to be observed.
Figure 6. 29Si MAS NMR spectra of materials MCM, MCM-Pr,
MCM-L, and MCM-C1.
the 3-chloropropyltriethoxysilane (Figure 5). The L ligand has
two carbon atoms, but their signals are not seen in the
spectrum of MCM-L or MCM-C1. This observation may be
related with a relatively low degree of functionalization or may
be due to the presence of tertiary and quaternary carbons,
which require a long acquisition and relaxation time in the
NMR spectrometer. It should be emphasized that the signal
assigned to CH2-Cl of the MCM-Pr seems to broaden. After
reaction with the metal fragments the peaks are slightly split
and broadened. The resonances of the carbon atoms of the allyl
ligand or carbonyl groups of the metal fragment are not
evident.
29Si MAS NMR spectra of MCM, MCM-Pr, MCM-L, and
MCM-C1 are shown in Figure 6. It is possible to observe the
Scheme 4. Functionalization of Cube Containing Eight 3-Chloropropyl Arms with L, Followed by Reaction with Complex
[MoBr(η3-C3H5)(CO)2(NCCH3)2] to Form Cube-C1
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The 13C CP MAS spectrum of Cube-C1 (Figure 7, left)
shows the three signals of the propyl chain carbons at 9.8
coordination. This result is in agreement with literature, as, in a
similar functionalization, the effect of the metal was also
negligible.20 The presence of T2 species was not expected and
may be related to the presence of a small quantity of the
precursor that does not completely condensate. The possibility
that during the reaction with 2-amino-1,3,4-thiadiazole some of
silsesquioxane cages open, leading to the formation of T2
species, cannot be completely disregarded.
The reaction between complex [Mo(η3-C3H5)Br-
(CO)2(NCCH3)2] (1) and silica functionalized with the same
3-(2-amino-1,3,4-thiadiazole)propyl ligand (Silica-L,65 Scheme
5) afforded a new material (Silica-C1). Elemental analysis
shows approximately 7.63 wt % Mo (0.76 mmol g−1) of
molybdenum in Silica-C1, while the values for C, H, N, and S
suggest that one nitrile remains coordinated to the metal. The
amount of molybdenum immobilized in silica is much larger
than in MCM-41 (3.7 wt %), probably owing to the easier
access to the surface of silica than to the inside walls of the
mesoporous material.
The FTIR spectrum of Silica-C1 displays the characteristic
νCO bands at 1851 and 1947 cm−1, slightly shifted from their
position in the precursor complex 1 (1863 and 1935 cm−1), and
the νCN modes from the ligand L as a broad band at 1632
cm−1. Peaks at 2291 and 2317 cm−1 can be assigned to the
νCN bands of NCMe, indicating the presence of coordinated
ligand and suggesting the proposed coordination geometry of
the metal fragment at the silica surface as represented in
Scheme 5.
Figure 7. 13C CP MAS and 29Si MAS NMR spectra of Cube-L and
Cube-C1.
(SiCH2CH2CH2N), 22.9 (SiCH2CH2CH2N), and 41.2 ppm
(SiCH2CH2CH2N), shifted upfield compared to those in the
precursor Cube-L (signals at 10.3, 23.5, and 52.0 ppm,
respectively). Two other resonances at δ 144.3 and 158.3
ppm are observable and are assigned to the two carbons of the
thiadiazole ring. After coordination with the metal fragment 1,
another peak appears at 215.2 ppm, which may be related with
the carbon of the carbonyl groups. Signals at 34.7 and 160.45
ppm, in the Cube-C1 spectrum, are solvent peaks (dimethyl-
formamide, DMF). The assignment of the methyl protons of
coordinated acetonitrile and allyl carbons is not conclusive, as
the strong peaks from the propyl chain carbons are observed in
the same region.
The solid-state 13C CP MAS NMR spectra of both Silica-L
and Silica-C1 show (Figure 8, left) the signals from the
aliphatic carbon chain at 9.4 ppm (SiCH2CH2CH2N), 25.9
ppm (SiCH2CH2CH2N), and 35.4 and 45.4 ppm
(SiCH2CH2CH2N), as well as a signal from the SiOCH2CH3
group at 46.2 ppm, resulting from incomplete hydrolysis of the
precursor. The signals at 143.4 and 155.3 ppm can be assigned
to the thiadiazole ring carbons. All values are consistent with
previously reported data for this type of silicon bond.24 No
signal from the coordinated nitrile carbon atoms or the allyl
carbons can be assigned, owing to the presence of several peaks
29Si MAS NMR spectra (Figure 7, right) present only Tm 29Si
chemical environments, from which T3 and T2 species can be
identified. They are observed at −68.5 and −60.5 ppm,
respectively,19,78 and do not change significantly upon metal
Scheme 5. Synthesis of Materials Silica-Pr, Silica-L, and Silica-C1
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Figure 8. 13C CP MAS NMR (left) and 29Si CPMAS NMR spectra
(right) of Silica-C1 and Silica-L.
in the respective region. The carbonyl carbon cannot be easily
identified on the 13C CP MAS NMR spectrum of Silica-C1.
The 29Si CP MAS NMR spectrum (Figure 8, right) of the
precursor silica gel material displays signals of Q4 species at
−109.2 ppm, Q3 at −99.3 ppm, and a shoulder at about −89.7
ppm from Q2. After reaction with the spacer and the ligand, the
functionalized Silica-L shows an enhancement of the Q4
intensity as a result of the quantitative reduction of Q3 and
Q2. Reaction with the Mo(II) fragment has a negligible effect of
the chemical shifts of Qn species.
Figure 9. Conversions calculated after 24 h reaction for complexes C1,
C2, C3, C4, and C5 (top) and for materials MCM-C1, MCM-C2,
MCM-C3, MCM-C4, MCM-C5, Silica-C1, and Cube-C1 (bottom).
ger, cis-3, and trans-3. When it is immobilized, conversions
become closer to 100% for all substrates, except trans-3 and 1-
oct in MCM-C1 (86.1%, 4%), S-lim and 1-oct in Cube-C1
(53.7%, 66%), and 1-oct in Silica-C1 (2.5%). For instance, 1-
oct has been found very difficult to oxidize by several similar
catalysts.11 The immobilization in all the tested supports
increases the activity of complex C1 in general.
T3 and T2 species are present in the spectra of Silica-L and
Silica-C1 and result from the reaction of the silylated spacer.
Upon metal coordination, the intensity of the T2 species
becomes less important relative to T3, probably due to further
condensation of the germinal silicon species under the reaction
conditions.
The immobilization of complex 1 in the three supports and
2015 cm−1 occurs in different ways. Experimental evidence
indicates that in MCM two acetonitriles of [MoBr(η3-
C3H5)(CO)2(CH3CN)2] (1) are replaced by two thiadiazole
rings hanging from the inner walls, while only one seems to be
substituted both in the Cube cage, where the arms are directed
away from one another (Cube-C1), and in silica (Silica-C1),
where the surface can be considered locally flat. The amount of
molybdenum introduced in the support is about twice as large
in silica as in MCM-41, probably owing to the absence of
constraints.
Catalytic Studies. All the new complexes C1−C5 and
materials MCM-C1, MCM-C2, MCM-C3, MCM-C4, MCM-
C5, Cube-C1, and Silica-C1 were tested as precursors in the
catalytic oxidation of alkenes: geraniol (ger), cis-hex-3-en-1-ol
(cis-3), trans-hex-3-en-1ol (trans-3), (S)-limonene (S-lim), and
1-octene (1-oct). The reaction was carried out in dichloro-
methane, with tert-butyl hydroperoxide (TBHP, in decane) as
oxygen donor, at 328 K (see details in the Experimental
Section). No reaction took place in the absence of a catalyst.
The total conversions, calculated after 24 h reaction, the
turnover frequencies (TOF), and the selectivity of each catalyst
are presented in Figures 9 and 10 and in Tables 2−4.
The conversions of all substrates with all the catalysts are
shown in Figure 9. The first observation is that complex
[MoBr(η3-C3H5)(CO)2(L)2] (C1) converts all the substrates,
except 1-oct, with significant yields, reaching almost 100% for
The two molybdenum complexes [MoI2(CO)3(L)2] (C2)
and [MoBr2(CO)3(L)2] (C3) achieve very low (or no)
conversions of all the substrates. The immobilization of C2
in MCM barely improves the performance, while MCM-C3
reaches conversions of 50−60% for ger, trans-3, and S-lim.
Substitution of Mo by W has dramatic consequences in the
capability of the complexes C4 and C5 to oxidize all the
substrates, except 1-oct, with yields close to 100% for trans-3,
for instance, in both cases. Enhanced activity of W complexes
relative to the Mo ones has been described, for instance, in the
epoxidation of cis-cyclooctene catalyzed by N-heterocyclic
carbene analogues of C1.79 The material MCM-C4 displays
much higher conversions than C4 for all the substrates, but the
immobilization of C5 leads to almost complete loss of activity.
In general, all heterogeneous catalysts achieve higher
conversions of all the substrates than their homogeneous
counterparts, although there is a relationship: the best
homogeneous catalysts give rise to the best heterogeneous
ones.
The highest TOF (474 mol molcatalyst−1 h−1), calculated after
10 min reaction, is observed in the oxidation of S-lim in the
presence of Silica-C1 (Table 4). Homogeneous catalysts C4
and C5 reach high TOFs of 450 and 331 mol molW h−1 for
−1
trans-3 and ger, respectively (Tables 2, 3). The lowest TOFs
are obtained for the oxidation of 1-oct (Table 3), associated
with conversions under 30% after 24 h reaction. Materials
containing the allyl complex C1 as well as the two tungsten
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Figure 10. Kinetic profiles of the oxidation of each of the five substrates in the presence of complex [MoBr(η3-C3H5)(CO)2(L)2] (C1) and materials
MCM/Silica/Cube-C1.
Table 2. Conversions (%) and Turnover Frequencies (TOF/mol molMo−1 h−1) in the Oxidation of cis-3 and trans-3 Catalyzed
by Complexes C1 to C5, Materials MCM-C1 to MCM-C5, Silica-C1, and Cube-C1
a
catalyst
C1
substrate
TOF*
conv
98.9
catalyst
substrate
TOF
conv
99.3
cis-3
52
26
0
MCM-C1
cis-3
17
38
trans-3
cis-3
96.2
trans-3
cis-3
86.1
b
C2
0
MCM-C2
MCM-C3
MCM-C4
MCM-C5
Silica-C1
19
10.5
trans-3
cis-3
0
0
trans-3
cis-3
0
2
0
C3
0
0
18.8 (8 h)
56.3
trans-3
cis-3
0
0
trans-3
cis-3
5
C4
8.5
450
6
97.9
191
266
6
100
trans-3
cis-3
98.9 (4 h)
89.1
trans-3
cis-3
95.1 (8 h)
6.1
C5
trans-3
cis-3
330
2
99.6
trans-3
cis-3
17
60
137
10.2
Cube-C1
99.4
97.5
trans-3
402
99.2 (4 h)
trans-3
96.3
a
b
Calculated after 10 min. After 1 h.
complexes C4 and C5 exhibit the highest TOFs and
conversions for the majority of the substrates (Tables 2−4).
The kinetic profile of the oxidation of the five substrates in
the presence of complex C1 or the material obtained by its
immobilization (MCM-C1, Silica-C1, Cube-C1) is shown in
Figure 10.
containing Mo(VI) is different in each case, similar catalytic
species are probably formed.64,81
The immobilization of C1 in MCM-C1 has only a slight
effect on the TOF and conversions of all substrates. On the
other hand, Cube-C1 can oxidize 1-oct with almost 60%
conversion (only 38.6% with C1).
The kinetic profile curves of oxidation in Figure 10 for
MCM-C1, Silica-C1, and Cube-C1, showing the conversions
as a function of time, reveal that the maximum rate is achieved
very fast, except for the oxidation of 1-oct over Silica-C1, where
an induction period of more than 8 h is needed. A similar
induction period takes place when the catalyst is C4 or C5.
Although the required period to form the active species
The immobilization of the inactive complexes C2 and C3
does not make C2 more active (conversion and TOF), but
confers some activity to C3. Complexes C4 and C5 exhibit high
TOFs for oxidation of trans-3, but C5 loses its activity upon
immobilization in MCM. Faster kinetics also occurs in the
oxidation of S-lim with the heterogeneous catalyst MCM-C4
(but not MCM-C5).
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Table 3. Conversions (%), Turnover Frequencies (TOF/mol molMo−1 h−1), and Selectivity in the Oxidation of Geraniol and 1-
Octene Catalyzed by the Complexes C1 to C5, Materials MCM-C1 to MCM-C5, Silica-C1, and Cube-C1
catalyst
C1
substrate
TOF
conv
geranial
8.1
Z-2,3
Z-6,7
1-octanal
epox
ger
137
4
94.4 (4 h)
38.6
87.0
4.9
1-oct
ger
4.8
67.7
56.3
53.9
22.0
2.1
95.2
32.3
43.7
46.1
78.0
97.9
27.6
43.4
78.7
16.8
99.8
77.2
C2
32
2
25.7
56.4
43.8
0
43.6
56.2
93.4
50
0
1-oct
ger
4.5
C3
32
7
12.8
0
1-oct
ger
2.9
C4
59.4
3
68.0
6.6
50
1-oct
ger
13.2
C5
331
2
96.2
0
1-oct
ger
4.9
MCM-C1
MCM-C2
MCM-C3
MCM-C4
MCM-C5
Cube-C1
Silica-C1
89
4
97.9
0
51.0
61.5
25.5
49.6
85.2
100
49.0
9.2
48.2
50.4
14.8
0
1-oct
ger
61.6 (4 h)
27.9
46
3
29.3
26.3
0
1-oct
ger
2.5
72.4
56.6
21.3
83.2
0.8
16
3
51.9
1-oct
ger
17.8
306
25
55
2
97.3 (1.5 h)
19.8
1-oct
ger
14.2
0
1-oct
ger
10.4
117
66
308
3
97.6 (6 h)
57.4
0
1-oct
ger
95.7
0
100
0
1-oct
2.5
22.8
The remaining factor that might prove determining in the
choice of a support is the selectivity. The five substrates and the
oxidation products, which were observed, are sketched in
Scheme 6.
Table 4. Conversions (%), Turnover Frequencies (*TOF/
−1
mol molMo h−1), and Selectivity in the Oxidation of (S)-
Limonene Catalyzed by Complexes C1 to C5, Materials
MCM-C1 to MCM-C5, Silica-C1, and Cube-C1
The two substrates cis-hex-3-en-1-ol (cis-3) and trans-hex-3-
en-1-ol (trans-3) are selectively oxidized to their epoxide by all
the catalysts, except the inactive C2 and C3, in the presence of
TBHP (Table 4).
Z-
E-
Z-Lim- E-Lim-
catalyst
C1
TOF
conv
Limox Limox
OH
OH
diepox
37.9 54.5
40.2
0
48.6
0
5.6
0
5.6
0
0
0
0
0
0
0
C2
C3
C4
C5
0
0
0
Two products were detected in the catalytic oxidation of 1-
octene, namely, 1-octanal and 1,2-epoxyoctane (epox, Table 3).
Complex C1 exhibits a high selectivity for the epoxide, 95.2%.
Upon immobilization in MCM-C1, the selectivity increases to
97.9% in epoxide. The other two materials, Silica-C1 and
Cube-C1, also produce more epoxide than octanal. In
particular, Cube-C1 has 99.8% selectivity toward epox.
Associated with the 57.4% conversion of the substrate, it is
the best catalyst to epoxidize 1-oct. The most selective catalyst
for converting 1-oct into octanal (83.2%) is MCM-C5, but the
conversion is only 10.4%.
Both geraniol and (S)-limonene can be oxidized to a larger
number of products (Scheme 6, Tables 3 and 4), as there are
two CC bonds in both substrates (yielding several epoxides)
and one OH functionality in geraniol (aldehyde). Stereo-
isomers of some of the possible products can be formed, and
some of them can be detected in standard GC-MS conditions
(see the Experimental Section).
Three products were observed in the oxidation of geraniol
(Table 3, Scheme 6): the aldehyde (geranial) and the two
epoxides Z-2,3-oxyrane (Z-2,3) and Z-6,7-oxyrane (Z-6,7).
Cube-C1 and Silica-C1 afford only one epoxide, Z-2,3 (100%),
which is also obtained in yields between 85% and 93% with C4,
C1, and MCM-5. Silica-C1 also displays the highest
conversion. This system might be promising as a catalyst,
since experimental conditions may be optimized with
0
0
0
0
0
43
27
10
54.1
100
100
39.1
51.0
56.8
50.7
42.9
43.0
4.5
2.7
0.1
5.7
3.4
0.1
MCM-
C1
(4 h)
MCM-
C2
2
8
17.3
58.5
79.4
3.1
31.4
19.1
16.2
17.3
33.3
9.2
48.4
62.2
76.0
46.8
44.2
8.9
8.8
9.4
11.4
9.4
0
0
MCM-
C3
MCM-
C4
164
3
3.4
4.4
0
MCM-
C5
16.1
10.5
0.1
19.8
12.0
0.2
0
Cube-
C1
14
474
53.7
(8 h)
0
Silica-
C1
100
(0.5 h)
81.6
The silica gel support leads to materials that perform better
than MCM-41 or Cube derivatives, in terms of conversions and
TOFs. This may be due not only to a higher percentage of
molybdenum loaded in silica gel but also to the absence of
structural constraints to access the active species. In MCM-41,
the species are supported inside the pores, and in silica gel the
active species are located at the surface. Except for oxidizing 1-
oct, silica gel seems thus a very convenient (also cheap) support
material.
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Scheme 6. Substrates and Oxidation Products Detected
significant results.11 Several catalysts, namely, C4, C5, MCM-1,
MCM-C4, and MCM-5, reveal a chemoselectivity for
epoxidation but without discrimination between the two
types of CC bonds, so that a 50:50 ratio of Z-2,3 and Z-
6,7, close to statistic, is formed (except for C4 and MCM-5).
C1 leads to the three products, with a larger amount of Z-2,3.
In MCM-1 the two epoxides are formed in 51:49 ratios, and
only Z-2,3 is obtained with Cube-C1 and Silica-C1. The allylic
system (C1) shows well the effect of the support. The activity
of the other complexes is not influenced in a clear way by the
metal (Mo or W) or the halide (Br or I), and grafting them in
MCM does not improve much the selectivity of the
homogeneous catalysts. Geranial is the one major product
when using the molybdenum catalysts C2 or C3 (56.4% and
43.6%).
promising catalysts, with ∼100% conversion for the latter,
despite the low TOF (slow initiation).
In the S-lim oxidation, complex C1 is the most active and
selective, as a homogeneous catalyst, supported in MCM-C1
(99.8% limox) and supported in silica. The material Silica-C1
becomes highly selective toward diepoxidation. The oxidation
reaction outcome can be tuned by the support.
Complexes C2 and C3 containing molybdenum are inactive,
reaching a conversion of ∼60% when immobilized (MCM-C3).
The tungsten complexes are active, as well as their
heterogeneous counterparts, with C5 reaching 100% con-
version.
SUMMARY AND CONCLUSIONS
■
The new complexes [MoBr(η3-C3H5)(CO)2(L)2] (C1) and
the Mo(II) and W(II) [MX2(CO)3(L)2] (M = Mo, X = I (C2);
M = Mo, X = Br (C3); M = W, X = I (C4); M = W, X = Br
(C5)) were synthesized and immobilized in MCM-41. Analysis
and spectroscopic data suggest that the metal binds two L
ligands supported in this material. Complex C1 was also
immobilized in two other supports, namely, silica gel and a
polyhedral oligomeric silsesquioxane (Cube). In both, the
metal is coordinated to only one L ligand of the surface, and
one acetonitrile remains.
The oxidation of S-lim is in general slow (small TOFs), with
the exception of the silica-supported catalyst Silica-C1 with a
−1
TOF of 474 mol molMo h−1. This reaction can lead to a
variety of products, and five were observed (Scheme 6, Table
4): stereoisomers of 1,2-epoxy-p-meth-8-ene (Z-limox and E-
limox), stereoisomers of 2-methyl-5-(1-methylvinyl)-
cyclohexan-1-ol (Z-lim-OH and E-lim-OH), and 1,2:8,9-
diepoxy-p-menthane (diepoxide).
The diepox formed only with the active Silica-C1 catalyst,
with 81.6% selectivity, no trace of this product being detected
in any other system. The stereoisomers Z-lim-OH and E-lim-
OH were formed in small amounts or were trace products of
the reactions catalyzed by all systems, except C2 and C3
(none). These two complexes were completely inactive and
unable to oxidize the S-lim substrate.
There is a strong chemoselectivity toward the epoxidation of
the inner ring CC bond to give the stereomeric pair Z-limox
and E-limox. Similar amounts of the Z and the E isomers are
formed in the presence of C1 (40.2%, 48.6%), C4 (39.1%,
50.7%), C5 (51.0%, 42.9%), MCM-C1 (56.8%, 43.0%), MCM-
C2 (31.4%, 48.4%), and Cube-C1 (33.3%, 44.2%). A larger
amount of E-limox is obtained with MCM-C3 (62.2%) or
MCM-C4 (76.0%). If the issue is not the discrimination
between Z- and E-limox, C1 and MCM-C1 can be considered
All the complexes and the functionalized materials were
tested in catalytic oxidation reactions of a series of substrates
with TBHP. In general, the allyl complex C1 is more active
than the C2−C5 family. The molybdenum derivatives C2 and
C3 are inactive in most conditions. Although their performance
improves after immobilization in MCM, they remain poor
catalyst precursors. The grafting of C1, C4, and C5 on MCM
modifies the results of the catalytic reactions. 1-Octene can be
oxidized with almost 60% conversion by C1 supported on the
polyhedral oligomeric silsesquioxane (Cube-C1), significantly
better than 38.6% with C1. cis-Hex-3-en-1-ol (cis-3) and trans-
hex-3-en-1-ol (trans-3) are selectively oxidized to their epoxide
by all the active catalysts. C4 and Cube-C1 have the highest
TOFs (>400 mol molMo−1 h−1). The oxidation of geraniol leads
to three products. MCM-1 leads to a racemic mixture of the
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University of Aveiro. High-resolution mass spectra were performed at
2,3-epoxides with 97% conversion, while in the presence of
both Cube-C1 and Silica-C1 only Z-2,3 is detected (100%).
C1 and MCM-C1 can be considered promising catalysts in
the oxidation of S-lim if the racemic limox is desired, the latter
achieving almost 100% conversion.
́ ́
Laboratorio Central de Analises, University of Aveiro.
1H and 13C solution NMR spectra were obtained with a Bruker
Avance 400 spectrometer. 29Si and 13C solid-state NMR spectra were
performed at University of Aveiro, operating at 79.49 and 100.62
MHz, respectively, on a Bruker Avance 400P spectrometer (9.4 T).
29Si MAS NMR spectra were recorded with 40° pulses, spinning rates
of 5.0−5.5 kHz, and 60 s recycle delays. 29Si CP MAS NMR spectra
were recorded with 5.5 μs 1H 90° pulse, 2 ms contact time, a spinning
rate of 8 kHz, and 4 s recycle delays. Chemical shifts are quoted in
ppm from TMS. 13C spectra (solid state) were recorded at 125.76
MHz on a Bruker Avance 500 spectrometer.
In conclusion, this evaluation of several homogeneous and
heterogeneous catalysts containing 2-amino-1,3,4-thiadiazole
coordinated to molybdenum or tungsten indicates that the
allylic species display higher conversions and TOFs than the
others. The polyhedral oligomeric silsesquioxane requires a
lengthy synthesis but offers the best way to oxidize 1-octene.
The support can significantly modify the selectivity of geraniol
oxidation, silica gel (Silica-C1) being the best support to obtain
Z-2,3 (much cheaper than the oligomeric silsesquioxane), while
MCM-C1 yields the racemic 2,3-epoxide with high conversions.
The different performance between the heterogeneous catalysts
may be related, at least partly, to the metal coordination sphere
and also with the different environment of the metal fragment
in the support. It must be considered that in MCM-41 the
metal fragment is probably heterogeneously distributed along
the material channel with a significant steric confinement and
diffusion constraints of reactants and catalytic products. In the
cases of Cube-C1 and Silica-C1, the metal centers should be
more easily available since no significant steric restraints are
expected. We must emphasize that the metal content between
heterogeneous catalysts is very different, being much higher in
Cube and Silica supports than in MCM-41. This may have an
influence on the final catalytic activity.
Powder XRD measurements were taken on a Philips Analytical PW
3050/60 X’Pert PRO (θ/2θ) equipped with an X’Celerator detector
and with automatic data acquisition (X’Pert Data Collector (v2.0b)
software), using monochromatic Cu Kα radiation as the incident
beam, operating at 40 kV−30 mA. XRD diffraction patterns were
obtained by continuous scanning in a 2θ range of 2° to 10° with a 2θ
step size of 0.017° and a scan step time of 99.695 s.
The N2 sorption measurements were obtained in an automatic
apparatus (ASAP 2010; Micrometrics). BET specific surface areas
(SBET, p/p0 from 0.03 to 0.13) and specific total pore volume, Vp, were
estimated from N2 adsorption isotherms measured at 77 K. The pore
size distributions were calculated by the BJH method using the
modified Kelvin equation, with correction for the statistical film
thickness on the pore walls.81,82 The statistical film thickness was
calculated using the Harkins−Jura equation in the p/p0 range from 0.1
to 0.95.83,84 Prior to the measurements, samples were degassed, and
physisorbed water was removed by heating at 723 K for MCM and
413 K for the derivatized materials (to minimize the destruction of the
functionalities) under vacuum for 2 h.
[MoBr(η3-C3H5)(CO)2(C2H3N3S)2] (C1). A solution of [MoBr(η3-
C3H5)(CO)2(CH3CN)2] (0.36 g, 1 mmol) in CH2Cl2 (15 mL) was
added to a heated solution of the L ligand C2H3N3S (0.22 g, 2.2
mmol) in benzene (30 mL). The resulting mixture was refluxed for 10
h under a nitrogen atmosphere. The solvent was concentrated, and the
mixture was left in the cold overnight to precipitate. Hexane was added
to the remaining solution to obtain more precipitate. The solid was
washed with hexane and dried under vacuum. Yield: 0.42 g (88.2%).
Anal. Calcd for MoBrC9H11N6O2S2·0.5C2H3N3S (%): C, 22.84; N,
19.98; H, 2.40; S, 16.25. Found: C, 23.05; N, 21.69; H, 2.91; S, 14.61.
1H NMR (400.13 MHz, (CD3)2SO): δ 1.09 (d, 2H, Hanti), 3.37 (m,
The different reactivity pattern may be associated, at least
partly, with the metal coordination sphere.
EXPERIMENTAL SECTION
■
General Considerations. All reagents were obtained from Aldrich
and used as received. The work involving sensitive compounds was
carried out using standard Schlenk techniques. Commercial-grade
solvents were dried and deoxygenated by standard procedures (THF,
toluene, and benzene over Na/benzophenone ketyl; CH2Cl2 over
CaH2), distilled under nitrogen, and kept over 4 Å molecular sieves.
The organometallic complexes [MoBr(η3-C3H5)(CH3CN)2(CO)2]
(1)63 and [MX2(CH3CN)2(CO)3] (M = Mo, X = I 2; M = Mo, X
= Br 3; M = W, X = I 4; M = W, X = Br 5)7,69 were prepared as
described before.
2H, Hsyn), 5.77 (m, 1H, Hmeso), 7.24 (s, 2H, H1), 8.58 (s, 1H, H2). 13
C
NMR (100.62 MHz, (CD3)2SO): δ 61.6 (Callyl), 75.2 (Callyl), 168.7
(C3), 143.3 (C2). IR (KBr, νcm−1): 3322 (s), 3092 (s), 2786 (s),
2749 (m), 1948 (vs), 1923 (vs), 1610 (s), 1520 (s), 1379 (w), 1339
(m), 1219 (m), 1106 (w), 1022 (w), 930 (w), 892 (w), 807 (m), 784
(w), 638 (w), 574 (w), 485 (m). HR-MS (EI+): observed m/z 397.00
([M − Br]+), 369.00 ([M − Br − CO]+), 296.00 ([M − Br −
(C2H3N3S)]+); calcd m/z for MoC9H11N6O2S2 ([M − Br]+) 396.94,
calcd m/z for MoC8H11N6OS2 ([M − Br − CO]+) 368.95, calcd m/z
for MoC7H8N3O2S ([M − Br − (C2H3N3S)]+) 295.94.
[MX2(CO)3(C2H3N3S)2] (M = Mo or W and X = I or Br) (C2−C5).
The ligand L (2-amino-1,3,4-thiadiazole, C2H3N3S (0.22 g, 2.2
mmol)) was completely dissolved in benzene with heating and
stirring, and a solution of [MX2(CO)3(CH3CN)2] (1 mmol) in
CH2Cl2 (15 mL) was then added. After 12 h of reflux under N2,
hexane was added, and a precipitate formed. The product was filtered,
washed three times with hexane, and dried under vacuum.
Silica gel and octasilsesquioxane functionalized with L (2-amino-
1,3,4-thiadiazole) were obtained from N. L. Dias Filho. The cage
polymer octakis(3-chloropropyl)octasilsesquioxane (Cube) was pre-
pared by stirring 3-chloropropyltriethoxysilane in methanol for 6
weeks under acidic conditions. Reaction with the ligand L, in a molar
ratio of 1:12, under reflux in anhydrous dimethylformamide, for 12 h,
led to octakis[3-(2-amino-1,3,4-thiadiazole)propyl]octasilsesquioxane
(Cube-L).65,20 Silica gel functionalized with 3-chloropropyl was
obtained by reaction of silica gel, previously activated (heated for 4
h at 150 °C under 10−3 Torr), with 3-chloropropyltriethoxysilane in
dry xylene. After drying the resulting white powder, Silica-Cl, under
vacuum and degassing (10−3 Torr) for 6 h at room temperature, it was
reacted with 2-amino-1,3,4-thiadiazole in dimethylformamide for 40 h
at 110 °C.80
MCM-41 and functionalized materials were prepared according to a
methodology previously described, using [(C14H29)N(CH3)3]Br as
template agent.8 Prior to the grafting experiment, physisorbed water
was removed from MCM by heating at 540 °C under vacuum (10−2
Pa) for 6 h.
FTIR spectra were obtained as KBr pellets, and diffuse reflectance
(DRIFT) measurements (materials) using cm−1 resolution were
obtained on a Nicolet 6700 in the 400−4000 cm−1 range.
Microanalyses (C, N, S, H) and ICP (Mo and W) were performed
[MoI2(CO)3(C2H3N3S)2] (C2). Yield: 0.51 g (80.5%). Anal. Calcd
for MoI2C7H6N6O3S2 (%): C, 13.22; N, 13.21; H, 0.95. Found: C,
1
13.50; N, 12.94; H, 1.04. H NMR (400.13 MHz, (CD3)2SO): δ 8.72
(s, H1). 13C NMR (100.62 MHz, (CD3)2SO): δ 169.1 (C3), 144.6
(C2). IR (KBr ν cm−1): 3340 (m) 3263 (s), 3135 (m), 3070 (w), 2008
(s), 1932 (vs), 1906 (s), 1756 (s), 1602 (s), 1597 (vs), 1346 (s), 1226
(m), 1012 (s), 923 (m), 892 (m), 732 (m), 583 (w).
[MoBr2(CO)3(C2H3N3S)2] (C3). Yield: 0.43 g (80.0%). Anal. Calcd
for MoBr2C7H6N6O3S2 (%): C, 15.51; N, 15.50; H, 1.12; S, 11.83.
1
Found: C, 15.67; N, 14.28; H, 1.46; S, 8.13. H NMR (400.13 MHz,
at CACTI, University of Vigo, and at Laborator
́
io Central de Anal
́
ises,
(CD3)2SO): δ 8.73 (s, H1). 13C NMR(100.62 MHz, (CD3)2SO): δ
1475
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Article
169.4 (C3), 144.4 (C2). IR (KBr ν cm−1): 3407 (s), 3257 (m), 3141
(m), 3084 (w), 1955 (s), 1911 (vs), 1853 (s), 1791 (vs), 1601 (vs),
1507 (vs), 1350 (m), 1228 (m), 1020 (m), 927 (m), 801 (m), 685
(w).
[WI2(CO)3(C2H3N3S)2] (C4). Yield: 0.71 g (98.0%). Anal. Calcd for
WI2C7H6N6O3S2 (%): C, 11.61; N, 11.61; H, 0.84; S, 8.86. Found: C,
11.36; N, 11.44; H, 1.05; S 9.12. 1H NMR (400.13 MHz, (CD3)2SO):
δ 8.64 (s, H1). 13C NMR (100.62 MHz, (CD3)2SO): δ 169.0
(C3),143.8 (C2). IR (KBr ν cm−1): 3480 (m), 3382 (vs), 3264 (vs),
3173 (s), 3092 (vs), 2005 (s), 2000 (vs), 1913 (vs), 1600 (vs), 1516
(s), 1332 (w), 1244 (w), 1008 (m), 924 (m), 886 (m), 822 (m), 593
(m).
CH2N), 59.2 (SiOCH2CH3). 29Si MAS NMR (δ ppm): −58.9 (T2),
−67.4 (T3), −101.4 (Q3), −109.8 (Q4). IR (KBr ν cm−1): 3386, 2955,
2850 (s, νN−H), 2000, 1960, 1872 (s, νCO), 1627 (s,νCN).
Octakis[3-(2-amino-1,3,4-thiadiazole)propyl[MoBr-
(CO)2(CH3CN)]octasilsesquioxane (Cube-C1). A solution of
[MoBr(η3-C3H5)(CO)2(CH3CN)2] (2.27 g, 6.4 mmol) in dry toluene
(10 mL) was added to a suspension of Cube-L (1 g, 0.64 mmol)
(10:1) in dry toluene (20 mL). The reaction mixture was refluxed
under N2 overnight. The resulting material was then filtered, washed
with CH2Cl2, and dried under vacuum for 3 h. Yield: 0.83 g (59.2%).
Anal. Calcd for Mo2Br2C54H80N26O16Si8S8 (%): C, 29.72; N, 16.69; H,
3.69, S, 11.75; Mo, 8.79. Found: C, 28.68; N, 14.81; H, 3.81; S, 8.31;
Mo, 8.21. 13C CPMAS NMR: δ 9.8 (SiCH2), 22.9 (CH2CH2CH2),
41.2 (CH2N), 144.4 and 158.3 (C from the thiadiazole ring). 29Si MAS
NMR (δ ppm): −60.5 (T2), −68.5 (T3). IR (KBr ν cm−1): 3170 (w),
3100 (w), 2934 (m), 2887 (m; νN−H), 1942, 1856, (s; νCO), 1794,
1663 (s; νCN), 1600 (m), 1520 (m), 1459 (m), 1421 (m), 1353 (m),
1251 (m), 1166 (s), 1034 (s), 918 (m), 681 (m).
[WBr2(CO)3(C2H3N3S)2] (C5). Yield: 0.55 g (87.4%). Anal. Calcd
for WBr2C7H6N6O3S2 (%): C, 13.35; N, 13.34; H, 0.96. Found: C,
1
13.55; N, 13.57; H, 1.18. H NMR (400.13 MHz, (CD3)2SO): δ 8.66
(s, H1). 13C NMR (100.62 MHz, (CD3)2SO): δ 168.8 (C3),143.9
(C2). IR (KBr ν cm−1): 3242 (s), 3195 (s), 3095 (s), 2006 (m), 1952
(vs), 1855 (vs), 1599 (vs), 1533 (vs), 1335 (m), 1205 (m), 1079 (m),
887 (s), 841 (m), 705 (m), 591 (m).
Silica-C1. A solution of [MoBr(η3-C3H5)(CO)2(CH3CN)2] (0.5 g,
1.4 mmol) in dry toluene (10 mL) was added to a suspension of 1 g of
the material Silica-L in dry benzene (20 mL), and the mixture was
refluxed under N2 overnight. The resulting solid was filtered, washed
with CH2Cl2, and dried under vacuum for 3 h. Anal. Found (%): C,
6.22; N, 1.97; H, 1.01; S, 0.94; Mo, 7.63. 13C CPMAS NMR: δ 9.4
(SiCH2), 25.9 (CH2CH2CH2), 35.4, 45.4 (CH2N), 46.2
(SiOCH2CH3). 29Si MAS NMR (δ ppm): −60.4 (T2), −69.7 (T3),
−89.7 (Q2), −99.3 (Q3), −109.2 ppm (Q3). IR (KBr ν cm−1): 3011
(w), 2940 (w), 2887 (w; νN−H), 2291, 2317 (s; νCN), 1955, 1853 (s;
νCO), 1632, 1539 (m; νCN), 1457 (m), 1412 (m), 1280 (m), 1043
(m), 926 (m), 808 (m), 683 (w).
Catalytic Studies. The new complexes C1−C5 and materials
MCM-C1, MCM-C2, MCM-C3, MCM-C4, MCM-C5, Cube-C1, and
Silica-C1 were tested as precursors in the catalytic oxidation of
alkenes: geraniol (ger), cis-hex-3-en-1-ol (cis-3), trans-hex-3-en-1ol
(trans-3), (S)-limonene (S-lim), and 1-octene (1-oct), using TBHP as
oxidant. The catalytic oxidation tests were carried out at 328 K under
air in a reaction vessel (25 mL) equipped with a magnetic stirrer and a
condenser. In a typical experiment the vessel was loaded with the
substrate (100%), internal standard (dibutyl ether), catalyst (1%),
oxidant (200%), and 2 mL of solvent. The reaction mixture was
refluxed for 24 h, the addition of the oxygen donor determining the
initial time. Samples of 100 μL were obtained 10 and 30 min after the
reaction started and after 1 h, 1 h 30 min, 2 h, 4 h, 6 h, 8 h, and 24 h.
Each sample was diluted in 1 mL of dichloromethane. In order to
destroy the hydrogen peroxide and stop the reaction, each sample was
treated with a catalytic amount of manganese oxide (Mn2O7). The
filtrate was then injected in a GCMS column, and the course of the
reactions was monitored by quantitative GC analysis. Blank experi-
ments were made using only substrate and TBHP. A conversion of
<0.05% occurs in the absence of the catalysts. This autoxidation
process was already observed and reported for some substrates.85,86 All
the products identified for each substrate are shown in Scheme 6.
Conversions after 24 h reaction, TOFs (turnover frequencies after 10
min of reaction), and selectivities for the different products observed
were calculated.
MCM-Pr. A suspension of 1 g of MCM-41 in 30 mL of toluene was
treated with an excess of 3-chloropropyltriethoxysilane (2.0 mL) and
allowed to reflux for 24 h. The product was filtered, washed with 4 ×
20 mL of dichloromethane, and dried under vacuum. 13C CPMAS
NMR (δ ppm): 7.7, 9.1 (SiCH2), 16.0 (SiOCH2CH3), 25.2
(CH2CH2CH2), 46.2 (CH2Cl), and 59.3 (SiOCH2CH3). 29Si MAS
NMR (δ ppm): −48.3 (T1), −57.5 (T2), −68.1 (T3), −102.0 (Q2),
−109.0 (Q4). IR (KBr, ν cm−1): 2979, 2927, 2895 (vs, νN−H), 1873
(s), 1704 (m), 1627 (s).
MCM-L. A solution of 2-amino-1,3,4-thiadiazole (L) (0.121 g, 1.2
mmol) in dry toluene (15 mL) was added to a suspension of 1 g of the
material MCM-Pr in toluene (20 mL). The mixture was refluxed
under N2 for 12 h. The resulting solid was filtered, washed with
MeOH, and dried under vacuum for 3 h. 13C CPMAS NMR (δ ppm):
7.6, 9.1 (SiCH2), 16.1 (SiOCH2CH3), 24.7 (CH2CH2CH2), ∼46.0
(CH2Cl and CH2NH), and 59.3 (SiOCH2CH3). 29Si MAS NMR (δ
ppm): −59.1 (T2), −65.7 (T3), −101.5 (Q3), −109.9 (Q4). IR (KBr, ν
cm−1): 2960, 2870 (s, νN−H), 1880 (m), 1640 (s,νCN), 1490 (m).
MCM-C1−MCM-C5. A solution of each complex C1−C5 (1
mmol) in dry toluene (10 mL) was added to a suspension of 1 g of the
material MCM-L in dry toluene (20 mL). The mixture was refluxed
under N2 overnight. The resulting solid was then filtered, washed with
CH2Cl2, and dried under vacuum for 3 h.
MCM-[MoBr(η3-C3H5)(CO)2(C2H3N3S)2] (MCM-C1). Anal. Found
(%): C, 7.12; N, 1.16; H, 1.68; Mo, 4.02. 13C CPMAS NMR (δ ppm):
7.6, 8.8 (SiCH2), 15.4 (SiOCH2CH3), 25.4 (CH2CH2CH2), 46.8
(CH2Cl and CH2N), 57.9 (SiOCH2CH3). 29Si MAS NMR (δ ppm):
−58.9 (T2), −67.6 (T3), −102.9 (Q3), −110.3 (Q4). IR (KBr, ν cm−1):
3254, 3162, 3102 (s, νN−H), 1944, 1860 (m, νCO), 1608 (s,νCN).
MCM-[MoI2(CO)3(C2H3N3S)2] (MCM-C2). Anal. Found (%): C,
6.75; N, 1.38; H, 1.78; Mo, 3.7. IR (KBr ν cm−1): 3360, 2979 (s,
νN−H), 2015, 1935, 1886 (m, νCO), 1627 (s,νCN). 13C CPMAS
NMR (δ ppm): 8.8 (SiCH2), 11.9 (SiOCH2CH3), 25.8
(CH2CH2CH2), 45.6 (CH2Cl and CH2N), 58.5 (SiOCH2CH3). 29Si
MAS NMR (δ ppm): −58.8 (T2), −67.9 (T3), −103.9 (Q3), −109.9
(Q4).
MCM-[MoBr2(CO)3(C2H3N3S)2] (MCM-C3). Anal. Found (%): C,
6.97; N, 1.51; H, 1.52; Mo, 5.05. 13C CPMAS NMR (δ ppm): 9.0
(SiCH2), 16.2 (SiOCH2CH3), 25.9 (CH2CH2CH2), 45.5 (CH2Cl and
CH2N), 58.0 (SiOCH2CH3). 29Si MAS NMR (δ ppm): −58.5 (T2),
−66.6 (T3), −102.8 (Q3), −109.9 (Q4). IR (KBr ν cm−1): 3311, 2963
(s, νN−H), 2003, 1981, 1893 (s, νCO), 1617 (s, νCN).
Epoxidation of Geraniol, cis-Hex-3-en-1-ol, trans-Hex-3-en-
1-ol, (S)-limonene, and 1-Octene. Substrate (800 mg: 5.2 mmol of
geraniol, 7.99 mmol of cis-hex-3-en-1-ol, and 7.99 mmol of trans-hex-2-
en-1-ol), 5.87 mmol of S(−)-limonene, 7.1 mmol of 1-octene, 800 mg
of dibutyl ether (internal standard), 1 mol % of catalyst, TBHP (5.5 M
in n-decane; 1.89 mL for geraniol, 2.91 mL for cis-hex-3-en-1-ol and
trans-hex-3-en-1-ol, 2.14 mL for (S)-limonene, and 2.58 mL for 1-
octene), and 4 mL of dichloromethane (CH2Cl2).
MCM-[WI2(CO)3(C2H3N3S)2] (MCM-C4). Anal. Found (%): C,
7.36; N, 2.03; H, 1.61; W, 4.7. 13C CPMAS NMR (δ ppm): 8.9
(SiCH2), 14.9 (SiOCH2CH3), 25.7 (CH2CH2CH2), 44.8 (CH2Cl and
CH2N), 58.5 (SiOCH2CH3). 29Si MAS NMR (δ ppm): −59.7 (T2),
−68.4 (T3), −103.2 (Q3), −110.4 (Q4). IR (KBr ν cm−1): 3366, 2957,
2848 (s, νN−H), 1994, 1903, 1870 (s, νCO), 1626 (s,νCN).
MCM-[WBr2(CO)3(C2H3N3S)2] (MCM-C5). Anal. Found (%): C,
6.99; N, 1.43; H, 1.59; W, 3.67. 13C CPMAS NMR (δ ppm): 8.3
(SiCH2), 15.0 (SiOCH2CH3), 25.7 (CH2CH2CH2), 44.8 (CH2Cl and
Computational Studies. DFT87 calculations were performed
using Gaussian 0388 and the PBE1PBE functional. That functional uses
a hybrid generalized gradient approximation (GGA), including a 25%
mixture of Hartree−Fock89 exchange with DFT exchange−correlation,
given by the Perdew, Burke, and Ernzerhof functional (PBE).90 All
geometry optimizations were performed without symmetry constraints
using a LanL2DZ basis set91 augmented with an f-polarization
1476
DOI: 10.1021/om501068q
Organometallics 2015, 34, 1465−1478

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Article
function92 for Mo and a standard 6-31G(d,p)93 for the remaining
elements. The calculations were performed for the real molecule of
complex C1.
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ASSOCIATED CONTENT
* Supporting Information
■
(15) Tangestaninejad, S.; Moghadam, M.; Mirkhani, V.;
Mohammadpoor-Baltork, I.; Ghani, K. Inorg. Chem. Commun. 2008,
11, 270−274.
S
A table (S1) with pore size distribution curves of materials and
a text .xyz file of the Cartesian coordinates of all calculated
structures. This material is available free of charge via the
Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
(18) Quadrelli, E. A.; Basset, J.-M. Coord. Chem. Rev. 2010, 254,
707−728.
■
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ACKNOWLEDGMENTS
■
́
Brandao, P.; Felix, V.; Vaz, P. D.; Nunes, C. D.; Veiros, L. F.; Brito, M.
̃
J. V. D.; Calhorda, M. J. Organometallics 2012, 31, 4495−4503.
This work was financed by FEDER funds through Programa
Operacional Factores de Competitividade−COMPETE and by
National Funds from FCT (Fundacao para a Ciencia e
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Tecnologia) by the projects PTDC/QUI/71576/2006, UID/
MULTI/00612/2013, and FCOMP-01-0124-FEDER-015644
(PTDC/QUI-QUI/113678/2009). We thank CICECO-Aveiro
Institute of Materials (Ref. FCT UID /CTM /50011/2013),
financed by national funds through the FCT/MEC and when
applicable co-financed by FEDER under the PT2020 Partner-
ship Agreement. P.F. thanks FCT for Programa Investigador
FCT. M.V.D. and M.S.S. thank FCT for research grants
(SFRH/BD/37690/2007 and SFRH/BD/48640/2008). A. P.
Carvalho and J. Pires are thanked for their kind help with the
adsorptions measurements, and C. D. Nunes is thanked for
preliminary tests. We thank N. L. Dias Filho for a loan of the
ligand-functionalized silsesquioxide.
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