Pore size matters! Helical heterogeneous catalysts in olefin oxidation

Article abstract of DOI:10.1016/j.apcata.2015.01.040

Helical mesoporous materials of the MCM-41 type with different pore sizes were prepared, choosing as templates myristyl (C₁₄) or cetyl (C₁₆) trimethyl ammonium salts, and functionalized with Mo(II) active sites based on MoI₂(CO)₃ (1) and MoBr(η³-C₃H₅)(CO)₂ (2) fragments, respectively, using a pyridine-2-carbaldehyde ligand as anchor. The new materials were tested as the catalytic precursors in the epoxidation of cis-cyclooctene, styrene, R-(+)-limonene, trans-hex-2-en-1-ol, cis-3-hex-1-ol, and geraniol using tert-butylhydroperoxide (tbhp) as oxidant. All catalysts were moderately to highly selective toward the epoxide products. The materials with larger pores (C₁₆ template) displayed a better catalytic activity, leading in general to higher conversions and selectivities, as well as faster kinetics. For instance, geraniol is epoxidized (more than 90%) with conversions above 90%. The major achievement of these catalysts, however, is the excellent product selectivity control, which is boosted when the allyl complex 1 is used, reaching 100% of the 2S, 3R species in the epoxidation of trans-hex-2-en-1-ol. The catalysts were also found to be stable through recycling experiments and truly heterogeneous with little or no leaching.

Full text of DOI:10.1016/j.apcata.2015.01.040

Accepted Manuscript
Title: Pore size matters! Helical heterogeneous catalysts in
olefin oxidation
Author: Marta S. Saraiva Cristina I. Fernandes Teresa G.
´
Nunes Maria Jose Calhorda Carla D. Nunes
PII:
S0926-860X(15)00067-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2015.01.040
APCATA 15232
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Applied Catalysis A: General
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Revised date:
Accepted date:
15-9-2014
26-1-2015
27-1-2015
Please cite this article as: M.S. Saraiva, C.I. Fernandes, T.G. Nunes, M.J. Calhorda, C.D.
Nunes, Pore size matters! Helical heterogeneous catalysts in olefin oxidation, Applied
Catalysis A, General (2015), http://dx.doi.org/10.1016/j.apcata.2015.01.040
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Pore size matters! Helical heterogeneous catalysts in olefin oxidation
Marta S. Saraivaa, Cristina I. Fernandesa, Teresa G. Nunesb, Maria José Calhordaa*, Carla D.
Nunesa*
a Centro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, Campo Grande,
1749-016 Lisboa, Portugal
b CQE, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa,
Portugal
*Corresponding authors: Tel.: +351 217 500 196, +351 217 500 876. Fax: +351 217 500 088.
E-mail address: mjcalhorda@fc.ul.pt, cmnunes@fc.ul.pt
Highlights
 Chiral MCM porous materials were synthesized (curved channels).
 Mo(II) complexes were immobilized in the chiral materials.
 The functionalized materials oxidized selectively several chiral substrates.
 The functionalized materials were reused for several runs keeping a high activity.
Abstract
Helical mesoporous materials of the MCM-41 type with different pore sizes were prepared,
choosing as templates myristyl (C14) or cetyl (C16) trimethyl ammonium salts, and functionalized
with Mo(II) active sites based on MoI2(CO)3 (1) and MoBr(3-C3H5)(CO)2 (2) fragments,
respectively, using a pyridine-2-carbaldehyde ligand as anchor.
The new materials were tested as the catalytic precursors in the epoxidation of cis-cyclooctene,
styrene, R-(+)-limonene, trans-hex-2-en-1-ol, cis-3-hex-1-ol, and geraniol using tert-
butylhydroperoxide (tbhp) as oxidant. All catalysts were moderately to highly selective towards the
epoxide products. The materials with larger pores (C16 template) displayed a better catalytic activity,
leading in general to higher conversions and selectivities, as well as faster kinetics. For instance,
geraniol is epoxidized (more than 90 %) with conversions above 90 %. The major achievement of
these catalysts, however, is the excellent product selectivity control, which is boosted when the allyl
1
Page 1 of 24

complex 1 is used, reaching 100 % of the 2S,3R species in the epoxidation of trans-hex-2-en-1-ol.
The catalysts were also found to be stable through recycling experiments and truly heterogeneous
with little or no leaching.
Keywords
Mesoporous materials; helical materials; molybdenum; oxidation catalysis; asymmetric catalysis
1. Introduction
Organometallic complexes can efficiently and selectively catalyze many reactions and are widely
applied in industrial processes. One of their drawbacks is the difficulty of separating the products
from the reaction solution as well as recovering and recycling the catalyst. Heterogeneous catalysis,
on the other hand, offers as the main advantage the easy recovery of the catalyst from the reaction
products, making it the preferred approach in most industrial applications. As a drawback,
selectivity is usually lower than that of many homogeneous systems. Although in recent times this
gap has narrowed, designing more active and selective heterogeneous catalysts is still a priority and
a challenge. Therefore, the possibility of incorporating active complexes in materials to yield
heterogeneous catalysts has opened new routes for catalysis in recent years. 1-3 In terms of porous
inorganic supports, the family of MCM materials, in particular MCM-41, developed by Mobil in
1992, features a wide range of advantages, from the ordered structure of hexagonal channels with
diameters between 2 and 50 nm and high surface area, to the mechanical stability, and the chemical
nature of its walls with reactive SiOH groups.5,6 Recently, the search for heterogeneous catalysts
with improved enantioselectivity became an active area of research.
Most effective ligands reported in homogeneous asymmetric catalytic reactions share one
prominent common feature – a chiral environment in a rigid and bulky structure. This is vital to
create an asymmetric microenvironment around the catalytically active site. By the same token,
solid surfaces with confinement effects have been used with success to promote heterogeneous
asymmetric catalysis, thus preserving or improving the high product enantioselectivity of their
homogeneous analogues.7–14 Suginome and coworkers reported asymmetric reactions where the
stereocontrol was induced by chiral helical backbone polymers.15–18 Therefore, the preparation of
nanostructured materials with helical morphology opened a new field of research due to their
potential applications in chiral selective separation, as well as recognition and enantioselective
catalysis.19–23 Nonetheless, the lack of active sites is a drawback not only in mesoporous materials,
but also in the helical silica-based materials. In order to by-pass such limitations, different
2
Page 2 of 24

approaches to introduce more active sites have been developed, ranging from co-condensation or
post-synthesis procedures, to matrix doping.24,25
Although traditional oxidation catalysts were based on high oxidation state metals, such as Mo(VI),
it is known that Mo(II) complexes can act as oxidants after being oxidized in situ by tert-butyl
hydroperoxide (tbhp) to Mo(VI) species. These are catalytically active in olefin epoxidation in the
presence of tbhp as oxygen source. Moreover, compared to the widely studied Mo(VI) complexes,
the Mo(II) catalyst precursors offer usually easier handling and stability.4
Following ongoing research assessing the performance of helical mesoporous materials in
asymmetric catalysis,27 the aim of the present work consists in developing more efficient and
selective systems based on single-site active species.26 We shall address the role of the pore size
effect induced by confined space and how it can be tuned by immobilization of a variety of different
active site species. Derivatives of [MoI2(CO)3(NCCH3)2]28 and [MoBr(3-C3H5)(CO)2(NCCH3)2]29
with -diimine ligands have proven to be good catalysts in the epoxidation of olefins both in
homogeneous and heterogeneous conditions and were thus chosen for this work.
2. Results and Discussion
2.1. Synthesis and characterization of materials
The helical mesostructured silica materials were prepared according to literature, using an entropy-
driven procedure with an achiral cationic surfactant template, [CH3(CH2)14N(CH3)3Br] or
[CH3(CH2)16N(CH3)3Br], and ammonia as co-surfactant (Si14, Si16, respectively).30 The co-
surfactant induces the formation of mesoporous channels with a helical shape, leading to a
macroscopic chirality to these materials.30 They also have a hexagonal structure with silanol groups
at the surface that can be functionalized as the MCM materials.27 The immobilization of the
molybdenum complexes in the materials was made by a method reported in the literature, which
consists in the grafting of the ligand followed by reaction with the metal precursor.
The C5H4NCH=N(CH2)2CH3 was the -diimine ligand chosen to immobilize the Mo(II) complexes
in these materials, since its asymmetry might contribute to improve the enantioselectivity of the
final catalysts. In order to bind the ligand to the material, it must be functionalized with Si(OEt)3
groups, affording (C5H4NCH=N(CH2)3Si(OEt)3 (pyca). The two modified helical materials Si14-pyca
and Si16-pyca are obtained by reaction of pyca with the SiOH groups of Si14, Si16, and then
coordinate to molybdenum (Scheme 1).
Grafting of the pyca ligand was straightforward by reaction with a suspension of Si14 or Si16 in
toluene. The Mo complexes were then introduced by suspending the Si14-pyca and Si16-pyca
materials in dichloromethane and then adding the precursor complexes, [MoI2(CO)3(NCCH3)2] (1)
3
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and [MoBr(3-C3H5)(CO)2(NCCH3)2] (2), to afford the Si14-pyca-1, Si16-pyca-1, Si14-pyca-2,
and Si16-pyca-2 materials.
Scheme 1. Derivatization of Si14 and Si16 materials
CHN analyses of these materials showed 7.74 %C, 1.45 %H, and 1.85 %N, for Si14-pyca-1, 9.41
%C, 1.75 %H, and 1.92 %N, for Si14-pyca-2, 9.26 %C, 1.66 %H, and 2.03 %N for Si16-pyca-1,
and 10.11 %C, 1.82 %H, and 2.09 %N for Si16-pyca-2. Based on the N content, these results also
indicated that the loading of the pyca complex inside the pores is 0.66 mmol g-1, 0.69 mmol g-1,
0.73 mmol g-1 and 0.75 mmol g-1, respectively. In addition, the Mo contents were determined to be
4.5%, 4.1%, 5.0% and 6.4%, corresponding to 0.45 mmol g-1, 0.43 mmol g-1, 0.53 mmol g-1 and
0.68 mmol g-1 respectively, and showing that all the Mo cores are coordinated to the surface grafted
ligands.
All materials were additionally characterized by DRIFT, powder XRD, SEM and TEM, and 29Si
and 13C CP MAS-DD NMR. Sorption/desorption N2 isotherms were also carried out for textural
parameters estimation. All spectroscopic/textural characterization features discussed in the
following lines were found to be in agreement with related hybrid matrix mesoporous materials.26-30
The helical channel matrices of the as-prepared Si14 and Si16 materials were confirmed by SEM
and TEM measurements, Figure 1. The SEM images (not shown) for Si14 and Si16 materials
consist of rod-like particles. The length of the rods ranges from hundreds of nanometers to 2 μm,
whereas the rod diameter varies from 75 to 120 nm. TEM images confirm the helical channel
4
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morphology and illustrate the existence of periodic lattice fringes (Figure 1) along the rods,
indicating the presence of helical channels within the rods.22,30 The periodicity of the lattice fringes
indicates that there is a regular environment of the channels, along which reactants and products
may diffuse without many constraints. The local structure was also confirmed by powder X-ray
diffraction measurements, which agree with the published data and also make possible to confirm
that the materials Si14-pyca and Si16-pyca and Si14-pyca-1, Si16-pyca-1, Si14-pyca-2, and Si16-
pyca-2 are mesostructured.
a
b
d
c
Fig. 1. TEM (a, b, and c) and SEM (d) images of the Si14-pyca-2 materials.
All the resulting materials were of good quality according to the X-ray diffraction (XRD) powder
patterns (Figure 2), which confirmed the hexagonal structure of the materials, by showing 4
reflections indexed to a hexagonal cell as (100), (110), (200) and (210) in the 2-10º 2θ range.
5
Page 5 of 24

Fig. 2. Powder XRD of Si14-based parent and functionalized materials.
The d100 value of reflection (100) for Si14 and Si16 is estimated to be 36.4 Å and 40.7 Å,
respectively, corresponding to lattice constants of a = 42.0 Å and a = 47.0 Å (a = 2d100
/
).
Materials Si14-pyca, Si16-pyca, and Si14-pyca-1, Si16-pyca-1, Si14-pyca-2, and Si16-pyca-2,
obtained after stepwise functionalization with pyca and 1 or 2, still show three reflections although
with a slight deviation of the maxima toward higher 2θ values as compared to Si14 and Si16. For
Si14-pyca, the d100 value is 37.9 Å with a corresponding lattice constant a = 43.8 Å; for Si14-pyca-
1 and Si14-pyca-2 the values are, respectively, d100 = 37.6 Å and a = 43.4 Å, and d100 = 37.8 Å and
a = 43.6 Å. Similar observations were made for the set of materials Si16-pyca, Si16-pyca-1, and
Si16-pyca-2. Besides this, data from all the materials are collected in Table 1, summarizing the
evolution of their relevant textural properties until the immobilization of complexes 1 and 2. The
observed peak intensity reduction is common to all materials; it is even more significant in the
materials with the Mo cores. This is not due to a crystallinity loss, but rather to an X-ray scattering
contrast reduction between the silica walls and the pore-filling material. This has been observed for
other types of materials and is well described in the literature.31,32
Nitrogen sorption/desorption studies at 77 K were performed and have revealed that Si14 and Si16
samples exhibit a reversible type IV isotherm (Figure 2), typical of mesoporous solids (pore width
between 2 nm and 50 nm, according to IUPAC).33 The calculated textural parameters (SBET and VP)
of these materials (Table 1) agree with the literature data.34,35 The capillary
condensation/evaporation step in pristine materials appears in the 0.34–0.44 relative pressures
range, while the sharpness of this step reflects a uniform pore size distribution.
The material functionalized with the pyca ligand presents much lower N2 uptake in the isotherm,
accounting for the decreases in both SBET (33% and 31%) and VP (49% and 42%), as seen in Table
1. These findings indicate that the ligand was immobilized on the internal silica surface (Figure 3,
6
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Table 1). For the Si14-pyca-1, Si16-pyca-1, Si14-pyca-2, and Si16-pyca-2 materials, the SBET and
VP decrease relative to Si14-pyca or Si16-pyca is 71%, 48%, 77% and 55%, respectively.
This result is in agreement with the p/p0 coordinate decrease in the isotherm inflection points after
post-synthesis treatments.36 Furthermore, the maximum of the PSD curve (Figure 3) determined by
the BJH method, dBJH, for Si14 materials changes from 32.8 Å to 27.5 Å (Table 1); on the other
hand for materials Si16 the changes are between 35.4 Å and 28.5 Å, all values have shifted to lower
values after each step and the PSD curves also lose their characteristic shapes. These observations
indicate a decrease in the materials pore diameter as well as a loss of the pore size uniformity.
Textural parameters for all materials (Table 1) were also found to match values reported in the
literature for related systems.26,37,38
Table 1
Textural parameters of host and composite materials from powder XRD and N2 isotherms at 77 K,
for all prepared materials.
b
Materials
Si14
d100(Å)
36.4
37.9
37.6
37.8
40.7
42.2
42.7
41.4
A (Å)
42.0
43.8
43.6
43.4
47.0
48.7
49.3
47.8
SBET / m2.g-1 ∆SBET a/ % Vp / cm3.g-1 ∆Vp / % dBJH / (Å)
1008
678
292
520
1161
803
270
527
-
0.79
0.40
0.20
0.28
1.04
0.60
0.21
0.36
-
32.8
28.6
28.5
27.5
35.4
31.2
28.5
28.5
-33
-71
-48
-
-49
-75
-65
-
Si14-pyca
Si14-pyca-1
Si14-pyca-2
Si16
-31
-77
-55
-42
-80
-65
Si16-pyca
Si16-pyca-1
Si16-pyca-2
7
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Fig. 3. Nitrogen adsorption studies of Si14 parent and corresponding functionalized materials at
77K: isotherms (right) and pore size distribution curves (left).
Diffuse reflectance infrared spectroscopy (DRIFT) was also used to characterize the materials
(Figure 4 for the Si14 and Figure S1 for the Si16 derivatives). The DRIFT spectra of the parent
materials Si14 and Si16 are typical of a silicate with a broad band in the 3600-3000 cm-1 range due
to hydrogen bonding silanol groups. Other important features comprise the band at ca. 1634 cm-1
due to OH bending modes, and an intense broad band at 1240-950 cm-1 assigned to asymmetric
stretching vibration modes of the mesoporous framework (Si–O–Si).39 After grafting the pyca
ligand, affording the materials Si14-pyca and Si16-pyca, the DRIFT spectra show an overall
similar profile dominated by the absorptions of the host mesoporous materials. Additionally, new
bands were detected evidencing the ligand presence within the pores. The 1637 cm-1 band can be
assigned to the νC=N mode in addition to the OH bending mode of the matrix. Grafting of the pyca
ligand was also monitored by probing its νC=N mode, observed in the pyridine moiety of the free
pyca ligand 1634 cm-1. After grafting, this mode is blue shifted to 1637 cm-1, which is consistent
with the expected transformation resulting in the formation of silyl esters, as documented in the
literature.40 Upon binding the molybdenum complexes [MoI2(CO)3(CH3CN)2] (1) and [MoBr(3-
C3H5)(CO)2(CH3CN)2] (2), affording materials Si14-pyca-1, Si14-pyca-2, Si16-pyca-1, and Si16-
pyca-2, respectively, new bands characteristic of the νCO stretching modes are observed in the
DRIFT spectra. Three bands at 2040, 1975 and 1940 cm-1 and 2042, 1980 and 1944 cm-1 are
displayed in the materials containing 1, Si14-pyca-1 and Si16-pyca-1, while only two, at 1950 and
1863 cm-1 and 1952 and 1871 cm-1can be seen in the spectra of Si14-pyca-2 and Si16-pyca-2
bearing complex 2.
The shift of these bands relative to those of the precursor complexes 1 and 2 reflect the metal–
ligand binding. Additionally, the absence of the bands due to the νCN vibrational modes from the
acetonitrile (CH3CN) ligands shows that they have been replaced by the immobilized ligand.
8
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Fig. 4. Infrared Spectra of the Si14 parent and corresponding functionalized materials.
13C and 29Si solid state NMR spectroscopy studies were also performed on the materials and the
resonances observed were assigned according to the literature.28 13C CP MAS-DD NMR spectra for
Si14-pyca, Si14-pyca-1 and Si14-pyca-2 are presented in Figure 5 and in Figure S2 (Supporting
Information) for Si16-pyca, Si16-pyca-1, and Si16-pyca-2.
Fig. 5. 13C CP MAS-DD NMR spectra of Si14-pyca, Si14-pyca-1 and Si14-pyca-2 materials.
In the 13C spectra (Figure 5) three strong signals are observed in the 0 – 100 ppm range
corresponding to the aliphatic carbons closer to the SiOR group in the ligands. The signal at the
9
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lower δ is attributed to the carbon linked to Si, at 9.5 ppm for Si14-pyca, at 9.1 ppm for Si14-pyca-
1, and at 10.1 ppm for Si14-pyca-2. Several signals observed between 100 ppm to 200 ppm are
assigned to the pyridyl and imine carbons of the pyca ligand. In general the same signals are
observed in the 13C solid state NMR spectra of the materials after their reaction with the Mo core of
the precursor complexes.
Figure 6 shows the 29Si CP MAS-DD NMR spectra for pristine Si14 and the derivatized Si14-pyca,
Si14-pyca-1, and Si14-pyca-2 materials (the corresponding spectra for the Si16 derivatives can be
found in Figure S3 in Supporting Information). The parent Si14 material displays two broad
convoluted resonances in the 29Si CP MAS-DD NMR spectrum at -101.1 and -111.3 ppm, assigned
to Q3 and Q4 species of the silica framework, respectively [Qn = –Si(OSi)n(OH)4n]. A weak shoulder
is also observed at -92.1 ppm due to the Q2 species. The Q3 sites are associated with single silanols
Si-OH (including hydrogen-bonded silanols), whereas the Q2 sites correspond to geminal silanols
[Si(OH)2]. The 29Si CP MAS-DD spectrum of Si14-pyca also displays three broad signals at -92.5,
101.5, and 102.1 ppm, assigned to Q2, Q3 and Q4 organosilicon species, respectively. New signals
corresponding to the T1, T2 and T3 species of the Si(OR)3 groups of pyca ligand were also observed
at -49, -54.8 and -66.3 ppm, respectively.
The 29Si CP MAS-DD spectra in Figure 6 show that reaction of Si14-pyca with the
(organo)metallic complex [MoI2(CO)3(CH3CN)2] (1) did not significantly change the environment
of Si, as expected, indicating that the metal fragment reacted with the immobilized ligand and did
not interact with the wall surface.
Fig. 6. 29Si CP MAS-DD NMR spectra of Si14 parent and functionalized materials.
10
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All the information gathered from the different characterization techniques allows us to conclude
that the functionalization of the surface of the helically shaped pores materials with the pyca ligand
and their subsequent coordination with metal fragments was performed successfully.
2.2. Catalytic Studies
The catalytic activity of the Si14-pyca-1, Si14-pyca-2, Si16-pyca-1, and Si12-pyca-2 materials
was benchmarked in olefin epoxidation using tert-butylhydroperoxide (tbhp) as oxygen source.
Special attention was dedicated to stereoselectivity of products to test the specific helical features of
the matrix. We have therefore studied the catalytic epoxidation of cis-cyclooctene, styrene, cis-hex-
3-en-1-ol, trans-hex-2-en-1-ol, geraniol, and R-(+)-limonene (Table 2). Blank runs (without catalyst
and in the presence of oxidizing agent) gave virtually no conversion of starting material.
All materials catalyzed selectively the oxidation of cis-cyclooctene to the corresponding epoxide,
without formation of any by-products (Table 2, entries 1 to 4). The conversions are high, ranging
from 79 to 89 % in Si14-pyca-2, Si16-pyca-1, and Si16-pyca-2, and much lower for the Si14-pyca-
1 material (31 %). Although the Mo loading in Si14-pyca-1 material is 4.10 %, it is the lowest in all
the materials and may have a direct influence in cyclooctene epoxide yield. The kinetic profiles are
very fast for the materials reaching high conversions, but much slower for Si14-pyca-1 (Figure 7-
bottom). The complexes 1 28 and 229 exhibited conversions of 63 and 99 %, respectively, which
increased to 80 and 100 % when they were immobilized in MCM-41 (MCM-1 and MCM-2).
11
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Fig. 7. Kinetic profiles for cis-cyclooctene oxide yield (bottom) and
R-(+)-limonene conversion (top).
All materials present low conversions for the epoxidation of cis-hex-3-en-1-ol (entries 9-12) and
styrene (entries 5-8), Si14-pyca-2 reaching 75%, but all the others remaining below 56 %. For
styrene (sty), the major product detected is not the epoxide but benzaldehyde, formed by oxidative
cleavage of the former. Styrene conversion was 61 and 100 % in the presence of 1 and MCM-1. 28
Complex 2 and MCM-2 showed modest conversions of 29 and 49 %, but higher selectivities
toward the epoxide (82 and 58 %, respectively). Complex 2 and MCM-2 converted 100 and 99 %
of both cis-hex-3-en-1-ol and geraniol to the corresponding epoxides (100% for the first and with
lower selectivity for geraniol), but at higher temperatures. 29 For the epoxidation of geraniol (ger),
the large pore materials Si16-pyca-1 and Si16-pyca-1 display very high conversions and selectivity,
yielding geraniol-2,3-epoxide as the major product (Table 2, entries 18 and 20) with selectivity of
94% and 91%, respectively. The by-product is geranial, obtained in the presence of the other two
catalysts with yields of 36% (Si14-pyca-1) and 60% (Si14-pyca-2).
We extended the tests to substrates, such as trans-hex-2-en-1-ol, cis-hex-3-en-1-ol, and R-(+)-
limonene, with more functional groups, to evaluate more extensively both chemo- and
12
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stereoselectivity. As shown in Table 2, epoxidation of trans-hex-2-en-1-ol led to the formation of
hexan-1-ol-2,3-epoxide as major product in all catalytic systems tested. For R-(+)-limonene,
however, the results were different (Table 3, entries 9 to 12). Two major products, the endocyclic
6,7-epoxide and the diepoxide (2,3 and 6,7), were obtained for all catalysts in ratios close to 40/60
%, the Si14 based materials favoring the first, with low/medium conversion, and the Si16 ones
affording more diepoxide and displaying a high conversion (above 93 %). The kinetic profiles are
similar for all catalysts (Figure 7, top) during the first minutes. There is an induction period, but the
more active Si16 materials display a higher rise than the others and reach higher conversions, as
discussed.
When assessing the catalytic performance of heterogeneous systems, it is important to evaluate
catalyst stability in terms of both active center leaching and recycling. To test active site leaching,
the oxidation of trans-2-hexen-1-ol was chosen. The catalyst was removed by filtration after 2 h
reaction of a normal run, and the reaction was followed until the end of the 24 h period. The
conversion of trans-2-hexen-1-ol was 22% after 2 h, and did not change during the remaining time,
although it reached 58 % when the catalyst was not removed (entry 16). This shows clearly that the
reaction stops when the heterogeneous catalyst was filtered off the reaction slurry and that there is
little or no leaching to the homogeneous phase. It is possible to conclude that the catalysts are truly
heterogeneous in nature.
The recycling experiments were conducted using Si16-pyca-1, Si14-pyca-2, and Si16-pyca-2 as
catalyst, and trans-hex-2-en-1-ol as substrates during five successive runs (Table 2, entries 14 and
16). The catalyst was filtered, washed with dichloromethane, and dried prior to each recycling
experiment. For Si16-pyca-1, the conversion and selectivity toward the epoxide are maintained (or
slightly increased to ~97 %) during four runs both decreasing in the fifth by 22 %. The results
change a bit more when the catalyst contains the allyl complex (Si14-pyca-2, and Si16-pyca-2), but
remain close to the initial values of the first run both in conversion and in selectivity.
Table 2. Catalytic olefin epoxidation of cis-cyclooctene, styrene, cis-hex-3-en-1-ol, trans-hex-2-en-
1-ol, geraniol, and R-(+)-limonene using Si14/16-pyca-1/2 catalysts.
13
Page 13 of 24

Entry
Epoxidea
Catalyst
Conv. %b
Select. %c
100
1
2
3
4
31
83
79
89
Si14-pyca-1
Si16-pyca-1
Si14-pyca-2
Si16-pyca-2
100
O
100
100
5
6
7
8
56
75
18
29
17
11
37
28
Si14-pyca-1
Si16-pyca-1
Si14-pyca-2
Si16-pyca-2
O
9
21
50
53
33
100
100
100
100
Si14-pyca-1
Si16-pyca-1
Si14-pyca-2
Si16-pyca-2
10
11
12
13
14
15
16
65
73
Si14-pyca-1
Si16-pyca-1
Si14-pyca-2
Si16-pyca-2
OH
OH
O
97/97/98/97/75d 89/88/93/86/79d
76/65/63/51/51d 84/91/98/75/69d
58/57/54/59/56d 69/75/83/85/82d
17
18
19
20
35
94
34
91
57/36e
94
29/60e
Si14-pyca-1
Si16-pyca-1
Si14-pyca-2
Si16-pyca-2
O
OH
OH
91
21
22
23
24
57
98
73
93
51/38f
40/58f
60/35f
44/54f
Si14-pyca-1
Si16-pyca-1
Si14-pyca-2
Si16-pyca-2
O
a All reactions were carried in dichloromethane in the presence of 2 eq. of oxidant (tbhp) and 175 mg of heterogeneous at 328 K;
b
c
d
e
Calculated after 24 h; Calculated as ‘’Yield of epoxide’’/’’Conversion’’x100% calculated after 24 h; 1/2/3/4/5 run;
Selectivity for geranial; f Selectivity for diepoxide.
Mixtures of two major products (endocyclic epoxide and diepoxide) were obtained in the oxidation
of R-(+)-limonene with all the catalysts, and for geraniol (2,3-epoxide and geranial) with the small
14
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pore materials Si14-pyca-1 and Si14-pyca-2. Only for the large pore materials was the geraniol
oxidation selective toward the epoxide (>90 %) with high conversion. Oxidation of styrene always
led to benzaldehyde as major product after 24 h, though at shorter reaction times (8 h) epoxide is
generally the major product.
The evaluation of the enantioselectivity induced by the chiral nature of the heterogeneous catalysts
was the aim of a set of tests. The oxidation of cis-hex-3-en-1-ol, trans-hex-2-en-1-ol and R-(+)-
limonene was performed in conditions allowing the determination of the enantiomeric ratios. The
results (Table 3) show that the epoxide from cis-hex-3-en-1-ol is not obtained with high optical
selectivity. The two diastereomers 3R,4S and 3S,4R are formed in almost 50:50 amounts, except in
the presence of Si14-pyca-2, containing the allylic bromide complex 2, where the latter is slightly
preferred (66 %). Both cis and trans diastereomers of R-(+)-limonene 6,7-epoxides are also
obtained independently of the catalyst, in a 1:2 ratio, with a preference for the trans with three of
them and 1:1 with Si14-pyca-1. This result differs from what was obtained in the oxidation of S-(-)-
limonene by complex 2 and MCM-2 (100 % conversions), where the main product was the
diepoxide in 96 and 88 %. Although the % of the cis and trans epoxides was too small, they were in
2:1 and 1:1 amounts.
The situation is completely different when it comes to epoxidation of the trans-hex-2-en-1-ol
substrate. The diastereomeric ratio is much higher in general, with a preference for the 2S,3R
diastereomer relative to the 2R,3R, but both Si14-pyca-2 and Si16-pyca-2 lead to 100 % of the
2S,3R. These two catalysts contain the complex [MoBr(3-C3H5)(CO)2(pyca)] (2) immobilized in
the two different materials (Table 3).
Table 3
Product selectivity
15
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%
Catalyst
(3R,4S)a,b
(3S,4R)a,b
49
45
34
52
55
66
54
Si14-pyca-1
Si16-pyca-1
Si14-pyca-2
Si16-pyca-2
45
O
OH
(2S,3R)a,c
(2R,3R)a,c
69
90
31
10
0
Si14-pyca-1
Si16-pyca-1
Si14-pyca-2
Si16-pyca-2
100
100
0
O
O
cis-limoxb
trans-limoxb
56
27
34
37
44
73
66
63
Si14-pyca-1
Si16-pyca-1
Si14-pyca-2
Si16-pyca-2
a
Determined by chiral GC; b Values refers to enantiomeric ratio;
c Values refers to diastereomeric ratio.
These heterogeneous catalyst show reduced performance compared to other previous reported ones,
also based on helical mesoporous materials with Mo(II) complexes, but with a 2,2’-bipyridine
derived ligand.27 On the other hand, both complexes 1 and 2 bearing the pyca ligand and
immobilized in MCM-41 (MCM-1 and MCM-2) have proved to be active catalysts and they
display some chiral discrimination.28,29 It binds to the material wall by its long arm, while the ligand
in other materials had a bidentate and bifunctionalized ligand that might be bound to the surface
more rigidly. These topological differences may control the activity of the catalysts and explain the
different behavior. Pore diameter does not significantly influence the selectivity, although it affects
the kinetics, as discussed above. For a given catalyst the large pore system consistently performs
16
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better than the narrow pore one, independently of the substrate (Figure 8). The allylic complexes 2
are in general more selective than complexes 1, despite the nature of the material.
Fig. 8. Substrate (cis-cyclooctene, cis-hex-3-en-1-ol, trans-hex-2-en-1-ol, and R-(+)-limonene)
conversion kinetics vs pore size profile. Please notice the different ranges in the vertical scales.
3. Experimental Section
3.1. General
All reagents were obtained from Aldrich and used as received. Commercial grade solvents dried
and deoxygenated by standard procedures (Et2O, tetrahydrofuran and toluene over Na /
benzophenone and CH2Cl2 over CaH2), distilled under nitrogen, and kept over 4 Å molecular
sieves. The pyca ligand28 was prepared according to literature methods. The helical mesostructured
silica materials synthesis followed a reported procedure with achiral cationic surfactant templates,
using ammonia as co-surfactant and [CH3(CH2)14N(CH3)3Br] to prepare Si14 materials, or
[CH3(CH2)16N(CH3)3Br] to prepare Si16.30
FTIR spectra were obtained as Diffuse Reflectance (DRIFT) measurements on a Nicolet 6700 in the
400-4000 cm-1 using 2 cm-1 resolution. Powder XRD measurements were taken on a Philip
17
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Analytical PW 3050/60 X’Pert PRO (theta/2 theta) equipped with X’Celerator detector and with
automatic data acquisition (X’Pert Data Collector (v2.0b) software), using a monochromatized Cu-
Kα radiation as incident beam, using 40 kV and 30 mA.
Solid state NMR measurements were performed at room temperature on a Tecmag/Bruker 300 wide
bore spectrometer operating at 59.60 and 75.47 MHz for the observation of 29Si and 13C resonances,
respectively. The standard magic angle spinning (MAS) cross-polarization – dipolar decoupling RF
pulse sequence (CP-DD) was carried out at ca. 4 kHz spinning rate. For the acquisition of 29Si
spectra, were selected 5 ms contact time, 6 s recycling delay and a number of scans always higher
than 3000; the Hartmann-Hahn condition was optimized using tetrakis-trimethylsilyl-silane and
tetramethylsilane (tms) was the external reference to set the chemical shift scale (δ = 0). 13C spectra
were recorded with 2 ms contact time, 4s recycling delay and a number of scans higher than 900.
The Hartmann-Hahn condition was optimized using glycine, also the external reference to set the
chemical shift scale (13CO at 176.1 ppm).
The N2 sorption measurements were obtained in an automatic apparatus (ASAP 2010;
Micromeritics). 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 (PSD) were calculated by the BJH method using the modified Kelvin equation with
correction for the statistical film thickness on the pore walls 41,42. The statistical film thickness was
calculated using Harkins-Jura equation in the p/p0 range from 0.1 to 0.95. Microanalyses for CHN
and Mo quantification were performed at CACTI, University of Vigo. CHN analyses were
performed on a Fisons EA 1108; Mo quantification was performed on a Perkin Elmer Optima
4300DV using In as internal standard.
SEM images were obtained on a FEG-SEM (Field Emission Gun Scanning Electron Microscope)
from JEOL, model JSM-7001F. TEM images were obtained on a Hitachi microscope, model H8100
with a LaB6 filament using an acceleration tension of 200 kV.
3.2. Synthesis of Si14-pyca and Si16-pyca
A solution of the pyca ligand (0.70 g, 1.1 mmol) in 10 mL of toluene was added to a suspension of
1 g of the materials Si14 or Si16 in 10 mL of toluene. The mixture was heated at 373 K during 9
hours. The resulting solid was filtered, washed with 4x20 mL of dichloromethane and dried under
vacuum.
Si14-pyca
Elemental analysis (%): found C 10.33, H 1.61, N 2.44.
18
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DRIFT (KBr, ν/cm-1): 3412 (s), 3266 (w), 2977 (m), 2928 (m), 1637 (s), 1618 (s), 1475 (m), 1444
(m), 1385 (m), 1236 (s), 1086 (s), 963 (s), 799 (m).
13C CP/MAS (DD) NMR (δ ppm): 9.5 (C10), 16.6 (OCH2CH3), 23.7 (C9), 50.9 (C8), 58.0
(OCH2CH3), 100-200 (Cpyridine,imine).
29Si CP MAS-DD NMR (δ ppm): -49.0 (T1), -54.8 (T2), -66.3 (T3), -92.5 (Q2), -101.5 (Q3), -112.1
(Q4).
Si16-pyca
Elemental analysis (%): found C 11.85, H 1.73, N 2.72.
DRIFT (KBr, ν/cm-1): 3480 (s), 3415 (s), 2981 (w), 2827 (w), 1637 (s), 1618 (s), 1572 (s), 1561 (s),
1413 (s), 1385 (s), 1236 (s), 1082 (s), 966 (w), 791 (m).
13C CP MAS-DD NMR (δ ppm): 9.5 (C10), 16.6 (OCH2CH3), 23.7 (C9), 50.9 (C8), 58.6
(OCH2CH3), 100-200 (Cpyridine,imine).
29Si CP/MAS (DD) NMR (δ ppm): -57.5 (T2), -101.3 (Q3), -109.5 (Q4).
3.3. Synthesis of Si14-pyca-1 and Si16-pyca-1
A solution of [MoI2(CO)3(CH3CN)2] (0.335 g, 0.65 mmol) in 5 mL of dichloromethane was added
to a suspension of 0.9 g of Si14-pyca or Si16-pyca in 15 mL of dichloromethane. The mixture was
stirred under inert atmosphere at room temperature overnight. The resulting solid was filtered,
washed with 2x20 mL of dichloromethane and dried several hours under vacuum.
Si14-pyca-1
Elemental analysis (%): found C 7.74, H 1.45, N 1.85, Mo 4.50.
DRIFT (KBr, ν/cm-1): 3409 (s), 3259 (w), 3235 (w), 2979 (w), 2931 (w), 2040 (s), 1975 (s), 1940
(s), 1637 (s), 1618 (s), 1448 (w), 1408 (w), 1385 (m), 1235 (s), 1088 (s), 958 (m), 802 (m), 473 (s).
13C CP MAS-DD NMR (δ ppm): 9.1 (C10), 16.9 (OCH2CH3), 20.8 (C9), 42.5 (C8), 58.6
(OCH2CH3), 100-200 (Cpyridine,imine).
29Si CP MAS-DD NMR (δ ppm): -48.3 (T1), -56.5 (T2), -92.9 (Q2), -101.1 (Q3), -109.7 (Q4).
Si16-pyca-1
Elemental analysis (%): found C 9.26, H 1.66, N 2.03, Mo 5.00.
DRIFT (KBr, ν/cm-1): 3415 (s), 2980 (w), 2931 (w), 2853 (w), 2042 (s), 1980 (s), 1944 (s), 1618
(s), 1466 (w), 1447 (m), 1385 (m), 1237 (s), 1087 (s), 962 (m), 802 (s), 459 (s).
13C CP MAS-DD NMR (δ ppm): 10.1 (C10), 17.2 (OCH2CH3), 30.2 (C9), 51.8 (C8), 59.6
(OCH2CH3), 100-200 (Cpyridine,imine).
19
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29Si CP MAS-DD NMR (δ ppm):-48.3 (T1), -56.5 (T2), -92.9 (Q2), -101.1 (Q3), -109.7 (Q4).
3.4. Synthesis of Si14-pyca-2 and Si16-pyca-2
A solution of [MoBr(ƞ3-C3H5)(CO)2(CH3CN)2] (1.0 mmol, 0.355 g) in 5 mL of dichloromethane
was added to a suspension of 0.9 g of Si14-pyca or Si16-pyca in 15 mL of dichloromethane. The
mixture was stirred in inert atmosphere at room temperature overnight. The resulting solid was
filtered, washed with 2x20 mL of dichloromethane and dried under vacuum.
Si14-pyca-2
Elemental analysis (%): found C 9.41, H 1.75, N 1.92, Mo 4.10.
DRIFT (KBr, ν/cm-1): 3415 (s), 3245 (w), 2980 (w), 2924 (w), 2853 (w), 1950 (s), 1863 (s), 1639
(s), 1618 (s), 1474 (m), 1447 (m), 1385 (m), 1235 (s), 1085 (s), 964 (m), 797 (s), 592 (w), 452 (s).
13C CP MAS-DD NMR (δ ppm): 9.1 (C10), 17.2 (OCH2CH3), 42.8 (C9), 54.1 (C8), 59.6
(OCH2CH3), 100-200 (Cpyridine,imine).
29Si CP MAS-DD NMR (δ ppm): -57.3 (T2), -101.5 (Q3), -109.7 (Q4).
Si16-pyca-2
Elemental analysis (%): found C 10.11, H 1.82, N 2.09, Mo 6.40.
DRIFT (KBr, ν/cm-1): 3409 (s), 3260 (w), 2978 (w), 2927 (w), 1952 (s), 1871 (s), 1618 (s), 1468
(w), 1445 (w), 1385 (m), 1238 (s), 1090 (s), 967 (m), 806 (s), 452 (s).
13C CP MAS-DD NMR (δ ppm): 8.8 (C10), 17.2 (OCH2CH3), 41.8 (C9), 52.8 (C8), 59.3
(OCH2CH3), 100-200 (Cpyridine,imine).
29Si CP MAS-DD NMR (δ ppm):-48.7 (T1), -56.1 (T2), -101.5 (Q3), -108.5 (Q4).
3.5. Catalytic studies
The materials were tested as catalysts in the epoxidation of olefins and allylic alcohols, such as, cis-
cyclooctene, styrene, 1-octene, trans-hex-2-en-1-ol, cis-3-hex-1-ol, geraniol and R-(+)-limonene,
using t-butylhydroperoxide (tbhp) as oxidant (Aldrich, 5.5 M in decane). The catalytic oxidation
tests were carried out at 328 K under air in a vessel equipped with a magnetic stirrer and a
condenser. In a typical experiment the vessel was loaded with olefin or alcohol (100%), internal
standard (dibutyl ether, DBE), catalyst (175 mg), oxidant (200%) and 3 mL of dichloromethane as
solvent. The final volume of the reaction was ca. 6 mL. Addition of the oxidant determined the
initial time of the reaction. Conversion, product yields and stereochemistry were monitored by
sampling periodically and analyzing them using a Shimadzu QP2100-Plus GC/MS system and a
capillary column (Teknokroma TRB-5MS/TRB-1MS or Restek Rt-βDEXsm) operating in the linear
20
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velocity mode. Leaching and recycling experiments, in general, were carried out as described above
for trans-hex-2-en-1-ol epoxidation using material Si16-pyca-2 and Si16-pyca-1 as catalyst;
conversion and product yields were monitored as described above. For the leaching experiments
after 2 h reaction, the catalyst was filtered off, and the reactions continued under the same
conditions.
In the case of the recycling experiments, after each cycle (24 h), the catalyst was filtered, washed
with dichloromethane, and dried prior to reuse in a new catalytic cycle.
4. Conclusions
In this work we assessed the capacity of helical mesoporous materials, with different pore diameter,
and containing two different kinds of immobilized Mo(II) complexes, to induce chirality in catalytic
reactions. The materials with both pore diameters were studied in parallel to test symmetric and
asymmetric substrates, to evaluate stereoselectivity enhancement.
The helical nature of the materials was confirmed by TEM experiments. As main conclusions, the
performance of the materials synthesized in the presence of the longer C16 chain surfactants (larger
pore) was better, leading in general to higher conversions and selectivities (Table 2). In addition,
they also showed faster kinetics, probably owing to the easier diffusion fluxes of reactants and
products inside the pores. On the other hand, the use of a metal complex containing an asymmetric
ligand in the active site is found to enhance product stereoselectivity independently of the host
material. The not very high activity of these catalysts was compensated by their robustness, as
attested in the recycling experiments by the comparable conversions and selectivities along
four/five runs, and their heterogeneous nature, probed in experiments in which leaching was not
observed to any meaningful extent.
Acknowledgments
Authors thank FCT, POCI and FEDER (Projects PTDC/QUI/71576/2006, PEst-
OE/QUI/UI0612/2014, RECI/QEQ-QIN/0189/2012 and PEst-OE/QUI/UI0100/2014). M.S.S. and
CIF thank FCT for research grants (SFRH/BPD/88082/2012) and (SFRH/BD/81029/2011),
respectively.
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*Graphical Abstract (for review)
Page 24 of 24