An oligosilsesquioxane cage functionalized with molybdenum(II) organometallic fragments

Article abstract of DOI:10.1021/om3003043

A silsesquioxane cage polymer functionalized with eight chloropropyl arms (1, T₈-PrCl) reacted with 2,2′-dipyridiylamine (DPA) to afford a new derivative with eight pendant linear chains (2, T₈-Pr-DPA). Further reaction with [Mo(η³-C₃H₅)Br(CO) ₂(NCMe)₂] afforded another derivative containing three molybdenum units (3, T₈-Pr-DPA-Mo), after substitution of the two nitrile ligands in each complex. These are the first silsesquioxane species containing DPA and the Mo(η³-C₃H₅)Br(CO) ₂ fragment. The three materials were characterized by ¹H, ¹³C, ²⁹Si, and ⁹⁵Mo NMR, FTIR, XRD, and elemental analysis, and T₈-PrCl (1) was also structurally characterized by single-crystal X-ray diffraction. It was identified as a low-temperature polymorph of this material. Elemental analysis indicated that all Cl atoms in the parent material T₈-PrCl (1) were substituted by the deprotonated DPA group in T₈-Pr-DPA (2). However, only three [Mo(η³-C₃H₅)Br(CO)₂(DPA)] units were detected in T₈-Pr-DPA-Mo (3). A comprehensive NMR study, complemented with DFT calculations, was carried out in order to detect the effect of Mo coordination on the cage silicon and on the protons and carbons of the propyl chain, but no significant effects were observed. Both ¹H and ²⁹Si chemical shifts vary upon introducing DPA but remain the same after reaction with the Mo(II) precursor. The ⁹⁵Mo NMR data reveal that the metal is not sensitive to the cage. The catalytic activity of 3 was tested as a precursor in the epoxidation of cyclooctene and styrene in the presence of TBHP. Despite the high selectivity toward the epoxides, the conversion and turnover frequencies were low, reflecting the behavior of the [Mo(η³-C₃H₅)Br(CO)₂(DPA)] complex.

Full text of DOI:10.1021/om3003043

Article
pubs.acs.org/Organometallics
An Oligosilsesquioxane Cage Functionalized with Molybdenum(II)
Organometallic Fragments
†
ima C. M. Portugal, J. M. F. Nogueira,‡,§ Paula Brandao
‡,§
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Newton L. Dias Filho, Fat
́
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, Vitor Fel
Calhorda*
‡
,§
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,‡,§
Pedro D. Vaz, Carla D. Nunes, Luis F. Veiros, Maria J. Villa de Brito, and Maria Jose
́
†
Faculdade de Engenharia de Ilha Solteira, UNESP-Univ Estadual Paulista, Departamento de Fısica e Quımica, Av. Brasil Centro, 56
́ ́
CEP 15385-000, Ilha Solteira SP, Brasil
‡
Universidade de Lisboa, Faculdade de Cien
̂
cias, Departamento de Quım
Universidade de Lisboa, Faculdade de Ciencias, Centro de Química e Bioquım
ica, Universidade de Aveiro, CICECO, 3810-193 Aveiro, Portugal
oma de Ciencias da Saude, Universidade de Aveiro, 3810-193 Aveiro, Portugal
ica Estrutural, Instituto Superior Tecnico, Universidade Tecnica de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa,
́
ica e Bioquım
́
ica, Campo Grande, 1749-016 Lisboa, Portugal
ica, Campo Grande, 1749-016 Lisboa, Portugal
§
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Departamento de Quım
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Secca
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Auton
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Centro de Quım
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Portugal
*
S Supporting Information
ABSTRACT: A silsesquioxane cage polymer functionalized with eight chloropropyl arms (1, T -PrCl) reacted with 2,2′-
8
dipyridiylamine (DPA) to afford a new derivative with eight pendant linear chains (2, T -Pr-DPA). Further reaction with
8
3
[
Mo(η -C H )Br(CO) (NCMe) ] afforded another derivative containing three molybdenum units (3, T -Pr-DPA-Mo), after
substitution of the two nitrile ligands in each complex. These are the first silsesquioxane species containing DPA and the Mo(η -
C H )Br(CO) fragment. The three materials were characterized by H, C, Si, and Mo NMR, FTIR, XRD, and elemental
3
5
2
2
8
3
1
13
29
95
3
5
2
analysis, and T -PrCl (1) was also structurally characterized by single-crystal X-ray diffraction. It was identified as a low-
8
temperature polymorph of this material. Elemental analysis indicated that all Cl atoms in the parent material T -PrCl (1) were
8
3
substituted by the deprotonated DPA group in T -Pr-DPA (2). However, only three [Mo(η -C H )Br(CO) (DPA)] units were
8
3
5
2
detected in T -Pr-DPA-Mo (3). A comprehensive NMR study, complemented with DFT calculations, was carried out in order to
8
detect the effect of Mo coordination on the cage silicon and on the protons and carbons of the propyl chain, but no significant
1
29
effects were observed. Both H and Si chemical shifts vary upon introducing DPA but remain the same after reaction with the
9
5
Mo(II) precursor. The Mo NMR data reveal that the metal is not sensitive to the cage. The catalytic activity of 3 was tested as a
precursor in the epoxidation of cyclooctene and styrene in the presence of TBHP. Despite the high selectivity toward the
3
epoxides, the conversion and turnover frequencies were low, reflecting the behavior of the [Mo(η -C H )Br(CO) (DPA)]
3
5
2
complex.
4
−6
INTRODUCTION
vertices carrying the R groups.
The size of these particles is
■
in the nanometer range, allowing them to be described as small
silica particles. The association with the R substituent gives
them a hybrid organic−inorganic character, which can be tuned
in order to obtain desirable properties. Functionalization of
these particles with catalyst precursors, for instance, leads to
new species resembling a catalyst immobilized in normal silica
Silsesquioxanes are three-dimensional oligomeric compounds
with the general formula (RSiO ) , where R can be varied
1
.5 2n
(
alkyl, hydrogen, halogen, aryl, etc.). A great number of silicon-
based polymers for a wide range of applications have been
developed in recent years, and others continue to emerge.
Among them, polyhedral oligomeric silsesquioxane (POSS)
cage molecules, (RSiO ) , have been attracting a great deal of
1
−3
1
.5 8
attention. The (RSiO ) oligomer consists of a cubic
arrangement of Si−O−Si bonds, with the silicon atoms in the
Received: April 13, 2012
Published: June 13, 2012
1
.5 8
©
2012 American Chemical Society
4495
dx.doi.org/10.1021/om3003043 | Organometallics 2012, 31, 4495−4503

Organometallics
Article
Scheme 1
or other support but having the features of a nanomolecule,
thereby building a bridge between homogeneous and
heterogeneous catalysts. The interest in such approaches has
been discussed in one early example addressing the formation
of [Co(CO) (H Si O )], with seven hydrogen atoms and one
coordinate to Mo to afford a stable complex, bearing at the
30
same time a N−H function able to react further (Scheme 1).
The polyhedral oligomeric silsesquioxane (POSS) cage
molecular precursors were prepared by the hydrolysis of 3-
chloropropyltriethoxysilane in methanol under acidic condi-
4
7
8
12
7
31
Co(CO) fragment in the vertices. Soon after, a POSS with
tions. This time-consuming procedure (6 weeks) with a low
4
two units was obtained and used as a precursor in the formation
of new magnetic materials, and the octakis(tetracarbonyl)
yield of 35% was recently improved. An alternative synthesis
was reported, where the condensation occurs in the presence
8
32
derivative showed catalytic activity in the hydroformylation of
of di-n-butyltin dilaurate as condensation catalyst, reducing the
time to only 4 days with an identical yield.
9
1-hexene. Other organic groups and metal-containing frag-
ments have similarly been introduced in varying proportions of
Octakis(3-chloropropyl)octasilsesquioxane (1, T -PrCl) re-
8
1
0
the vertices.
acted with DPA in a molar ratio of 1:8, in the presence of NaH
over 22 h, allowing the deprotonated form to perform the
nucleophilic substitution at the halogenated carbon atoms of 1.
The coordinating capabilities of the new ligand octakis{3-(2,2′-
These and other examples show the variety of patterns for
the distribution of R groups. While species with eight R groups
1
1−15
are more symmetric
and might be the model for a “small”
16
heterogeneous catalyst, a POSS with hydrogen atoms and
only one functional arm can be considered as a special
dipyridylamine)propyl}octasilsesquioxane (2, T -Pr-DPA) were
8
then tested by allowing a suspension in ethanol to react with an
1
7−20
3
ligand.
ethanolic solution of [Mo(η -C
H )Br(CO) (NCMe) ], under
3 5 2 2
The cages may be modified, either by aggregation to form
the same conditions as reported for the synthesis in Scheme
30
oligomers or even dendrimers or by replacement of one silicon
1. After workup, a yellow solid containing molybdenum was
obtained (3, T -Pr-DPA-Mo).
21
atom by a different one, such as vanadium. The substitution
of a Si−R vertex by an M−R group is an alternative way to
introduce transition-metal centers in the POSS cage, which
usually is carried out during the synthetic procedure, starting
from incompletely condensed silsesquioxanes. Metallocene
fragments were thus inserted, with the aim of comparing
their catalytic properties with those of the unsupported species.
8
Elemental analysis was of major importance in characterizing
the new nanomolecules 2 and 3, in order to find out the extent
of each reaction. 2 (T -Pr-DPA) was obtained in 74% yield, and
8
the analysis showed that the eight pendant arms in the parent
silsesquioxane 1 had reacted with DPA. The final species 3 (T8-
Pr-DPA-Mo) contained 8.06% of molybdenum. This value, as
well as the corresponding ones for C, H, and N, suggests that
three molybdenum units are bound to the ligand 2. The
procedure is shown in Scheme 2.
2
2,23
Several TiR-
and MCp2 (M = Zr, Hf)-containing
2
4
silsesquioxanes were synthesized, and their catalytic activity
in olefin polymerization has been studied. Zirconium alkoxides
2
5
The FTIR spectrum of T -PrCl (1) exhibits the characteristic
were also introduced for epoxidation catalysis. Several
computational approaches have been tested to deal with
8
vibrational modes of the POSS cage, namely asymmetric
−1
26−28
νSi−O−Si modes centered at 1110 cm and deformation of the
some aspects of transition-metal POSS chemistry.
−1
Si−O−Si bridges at 696 cm (Figure 1). The presence of the
In this work we explore the use of the POSS framework to
prepare a polydentate ligand, by functionalizing the cage
containing eight chloropropyl pendant arms with the bidentate
ligand 2,2′-dipyridylamine (DPA). The reaction between this
−1
propyl chain is shown by ν
modes at 2994 and 2955 cm ,
C−H
as well as β
cm . The ν
deformation modes at 1458, 1436, and 1410
stretching mode is observed at 812 cm .
C−H
−
1
−1
C−Cl
3
In the FTIR spectrum of the new material T -Pr-DPA (2)
large ligand and the metal precursor [Mo(η -C H )Br-
8
3
5
obtained after reaction with DPA, the ν
pyridine rings are observed at 1681, 1617, and 1591 cm and
modes of the
(
CO) (NCMe) ] opens the way to a large molecule, bridging
CN
2
2
−1
3
the gap between the homogeneous catalyst [Mo(η -C H )Br-
3
5
−1
the νC−N modes as two sharp bands at 1316 and 1352 cm ,
while the typical bands of the aromatic C−H groups appear at
(
CO) (DPA)] and a classic heterogeneous catalyst where the
2
complex is immobilized in a mesoporous material. The catalytic
properties of the final material as a precursor for olefin
oxidation were therefore also tested.
−
1
3
052, 3179, and 3253 cm and those of aliphatic CH at 2868
2
−1
−1
and 2943 cm , with a β
deformation mode at 1440 cm .
C−H
The bands characteristic of the POSS cage are slightly shifted
from their values in the initial compound 1 (ν
at 1027
Si−O−Si
RESULTS AND DISCUSSION
−1
−1
■
and 989 cm , Si−O−Si deformation at 760 cm ).
3
29
Complex [Mo(η -C H )Br(CO) (NCCH ) ] is a well-
Changes are detected upon the coordination of 2 to the
metal fragments. The ν stretching vibrations appear as very
3
5
2
3 2
known precursor for a wide range of complexes by replacement
of the nitriles with other ligands. The choice of ligands can be
tuned in order to achieve the desired properties, namely the
possibility of reacting with a given functional group using a site
different from that used for metal binding. The 2,2′-dipyridyl-
amine ligand (DPA) is a bidentate nitrogen ligand that can
CO
−1
intense absorptions at 1927 and 1844 cm , reflecting the
coordination of the Mo(CO) fragment to the POSS. These
2
−1
modes were observed at 1929 and 1840 cm in the parent
complex. The positions of ν
modes of the DPA ligand have
CN
changed to 1635 and 1583 cm− and the aromatic νC−H modes
1
4
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Scheme 2
Figure 1. FTIR spectra of T -PrCl (1), T -Pr-DPA (2), and T -Pr-
8
8
8
DPA-Mo (3).
−1
to 3282, 3235, 3195, and 3134 cm . These values are also the
3
same as in the mononuclear complex [Mo(η -C H )Br-
3
5
3
0
(
CO) (DPA)].
2
The modes assigned to the groups in the propyl chains are
−1
slightly different, as shown by the βC−H band at 1478 cm and
−1
the νC−H bands at 2998 and 2934 cm . The identity of the
silsesquioxane cage is indicated by the ν bands at 1062,
Si−O−Si
−
1
1
023, and 1011 cm and the deformation of the Si−O−Si
−1
1
bridges at 769 cm .
Figure 2. H NMR spectra of T -PrCl (1), T -Pr-DPA (2), and T -Pr-
8
8
8
1
The H NMR spectrum of T -PrCl (1) shows three signals
DPA-Mo (3).
8
(
(
Figure 2) of the propyl chain protons at 0.80 (Si−CH ), 1.88
2
−CH −) and 3.54 ppm (H C−Cl), while the corresponding
C /C , C /C , C /C , C /C , and C /C carbon atoms of
DPA.
2
2
6
6′
4
4′
5
5′
3
3′
1
1′
1
3
carbon signals are observed in the C NMR spectrum at 9.6,
2
6.6, and 47.4 ppm.
The three proton signals are shifted to 0.85 (Si−CH ), 1.83
The binding of the metal fragments leads to very small shifts
of the propyl chain proton signals (0.78, 1.84, and 2.95 ppm,
respectively, for Si−CH , −CH −, and CH −N) and those of
2
(
−CH −), and 2.92 ppm (H C−N) in T -Pr-DPA (2), the
2
2
8
2
2
2
maximum deviation being observed for the proton now next to
DPA (broad singlet at 6.87 ppm, H /H ; broad doublet at
4 4′
1
3
nitrogen. Similar effects are observed in the C NMR spectra,
with the carbon atoms CH −Si, CH −CH Si, and CH −N
appearing at 14.4, 23.0, and 63.5 ppm, respectively. The signals
at 112.2, 116.8, 138.4, 148.0, and 154.5 ppm are assigned to the
7.55−7.63 ppm, H /H , H /H ; doublet at 8.26 ppm, H /H ).
5
5′
6
6′
3
3′
Several signals are observed for the allyl ligands, indicating the
presence of several isomers, the major components appearing at
2
2
2
2
1.22, 2.85, and 3.55 ppm for Hanti, H , and Hmeso, respectively.
syn
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Organometallics
Article
The 13C NMR spectrum shows two signals for two of the
carbon atoms of the propyl chain, namely at 31.2 and 32.3 ppm
for −CH − and at 61.5 and 72.0 ppm for H C−N, significantly
the experimental 29Si NMR spectra that show equal chemical
shifts for 2 and 3.
The calculated 13C NMR chemical shifts for carbon atom
2
2
shifted from those of species 2 before binding of the metal. The
next to the nitrogen (CH −N) in 2′ and 3′ show an increase of
2
third C atom, assigned to the peak at 15.4 ppm (CH −Si), is
about 10 ppm on going from the DPA model (2′, δ 49 ppm)
2
C
further away from the DPA and the deviation is smaller. The
signals of the carbon atoms of DPA are seen at 118.1 (C /C ),
to the model with the Mo complex (3′, δ 67 ppm), in good
C
agreement with the trend verified experimentally.
6
6′
Compound 3 was also examined by 5Mo NMR spectros-
9
1
18.7 (C /C ), 137.4 (C /C ), 151.9 (C /C ), and 163.8 ppm
4
4′
5
5′
3
3′
3
(
C /C ).
copy, as were the free complex [Mo(η -C H )(CO) Br(DPA)]
1
1′
3
5
2
2
9
3
The Si NMR spectrum of T -PrCl (1) shows a signal at δ
and its precursor [Mo(η -C H )(CO) Br(NCMe) ] for
8
3 5 2 2
−67.0 ppm (Figure 3), which appears in the range observed for
comparison. Despite the low solubility of the compounds,
their 95Mo NMR spectra, displayed in Figure 4, show three to
Figure 3. 29Si NMR spectra of T -PrCl (1), T -Pr-DPA (2), and T -
8
8
8
Pr-DPA-Mo (3).
the same compound by other authors (−66.2 to −68.0 ppm)
3
m
3
and is typical of a T species (T = RSi(OSi) (OR′) ). In
m
3−m
the 29Si spectrum of T -Pr-DPA (2), a single resonance has
8
been observed at −21.9 ppm, which confirms that all the Cl
substituents have been replaced by DPA groups, leading to a
considerable change in the environment. This value is lower
than all those reported in a recent review of POSS derivatives,
9
5
3
Figure 4. Mo NMR spectra of T -Pr-DPA-Mo (3), [Mo(η -
8
3
C H )(CO) Br(DPA)], and [Mo(η -C H )(CO) Br(NCMe) ].
3 5 2 3 5 2 2
3
reflecting the nature of the DPA group. However, a single
resonance was also observed for T -Pr-DPA-Mo (3) having the
four resonances, which can be explained by the coexistence of
8
3
same chemical shift as in 2. This fact shows that coordination of
molybdenum to DPA does not influence significantly the
chemical environment of the POSS derivative. No conclusions
can therefore be drawn regarding the number of molybdenum
fragments bound to the POSS cage.
several isomers in solution, usually detected in [Mo(η -
2
9,30
C H )(CO) LLX] derivatives.
This observation is also
3
5
2
9
5
supported by published studies which indicate that Mo NMR
is sensitive to minor variations about the Mo center as a result
of subtle electronic and steric effects.
3
4,35
The range in observed
3
3
DFT calculations (see Computational Details) were
chemical shifts is −567 to −790 ppm with respect to Na MoO
2
4
performed for T -PrCl (1) and for models of the substituted
(all values are available in Table S1 in the Supporting
Information). Although no reference to 95Mo chemical shifts
of similar compounds was found in the literature, the observed
resonances lie around the central portion of the usual chemical
8
POSS with one DPA group (2′) and with one pendant Mo
complex (3′ and 3″, see Figure S1 in the Supporting
Information) aiming at a better understanding of the 29Si
NMR results, presented above. In all cases the same mean Si
chemical shift was obtained from the calculations (−74 ppm)
with a maximum deviation of 3 ppm for all Si atoms in the
various solids. The chemical shift calculated for 1 agrees well
with the experimental value (within 7 ppm), but the difference
observed in the experimental spectra of the substituted solid is
not reproduced by the calculations, most probably owing to the
simplicity of the monosubstituted models employed in the
calculations.
Despite the limitations of the DFT calculations, they can be
used to assess the influence of Mo on the Si chemical shift. In
fact, the values calculated for the Si atoms of the substituted
arms in models 2′ and 3′ are within 1.5 ppm, indicating that
both one DPA group and one Mo complex have similar
influences on the neighboring Si atom, in good agreement with
34
shift range for monomeric Mo(II) complexes. The main
3
isomer of the complex [Mo(η -C H )(CO) Br(DPA)] in
3
5
2
1
30
solution, as previously determined by H NMR spectroscopy,
corresponds to the exo conformation of the equatorial isomer
(see Scheme 1). Therefore, on the basis of the relative
intensities found on the respective 95Mo NMR spectrum, the
upfield resonance at −789 ppm can be assigned to that isomer
and the second more intense signal, at −696 ppm, to the exo/
axial isomer. The observation of Mo chemical shifts of exo
isomers at δ values lower than those of the corresponding endo
9
5
isomers is in accordance with published results involving Mo
3
35
NMR studies on Mo(η -C H )Cp(CO) derivatives. The
3
5
2
spectrum of T -Pr-DPA-Mo (3) shows signals broader than
8
those of the free complex, as expected, owing to the increase in
molecular size, but no significant differences are apparent in the
4
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Organometallics
Article
Figure 5. Molecular structure of T -PrCl (1) with silicon atoms in yellow, carbon in gray, oxygen in red, chlorine in green, and hydrogen in white.
8
The disordered chain is presented only in one position for clarity. Thermal ellipsoids are drawn at the 50% probability level.
Mo chemical shift range of those two compounds, both
(148.8(3) and 148.0(3)°) is also observed, while the Si−O−Si
angles in 1 spread over a wider range (∼146.6(1) to ∼151.8(1)
°). The substitution of −Cl by −(CH ) Cl results in an
presenting the more shielded signal at −789 ppm (also very
3
close to the corresponding signal in the precursor, [Mo(η -
2
3
C H )(CO) Br(NCMe) ], found at δ −790 ppm).
expansion of the cage that is more evident when the Si···Si
diagonal distances measured respectively along the body
(Sibd···Sibd) and the faces of the cube (Sifc···Sifc) are compared.
In the chloride analogue the Sibd···Sibd distances are 5.359(8)
and 5.331(5) Å and the Sifc···Sifc distances are 3.657(7) and
3.735(7) Å, whereas the two independent molecules of 1 have
3
5
2
2
These results show that the molybdenum chemical shift is
3
insensitive to the presence of the cage (3 compared to [Mo(η -
C H )(CO) Br(DPA)]) and also to the nature of the nitrogen
3
5
2
ligand (DPA or two acetonitriles) and is characteristic of the
metal formal oxidation state.
Interestingly, the 95Mo NMR chemical shifts calculated by
DFT for the solids substituted with one Mo complex agree very
well with the experimental trend discussed above. The Mo
chemical shift obtained for the model with an exo coordination
of the allyl ligand (3′) is 257 ppm below the corresponding shift
in the model with an endo coordination of allyl (3″) and, in
...
longer Si Si distances, in particular Sifc···Sifc distances ranging
from 4.359(1) to 4.444(1)(9) Å. The Sibd···Sibd distances vary
between 5.384(1) and 5.450(1) Å.
The three compounds were also characterized by X-ray
powder diffraction, which showed the modification of the
diffraction patterns illustrating the change of phase and the
clear loss of crystallinity (Figure S2 in the Supporting
Information).
−1
addition, 3′ is more stable than 3″ by 7 kcal mol , indicating
that the exo isomer should be the dominant one in solution.
Suitable crystals for X-ray diffraction studies were grown by
The activity of the Mo-derivatized silsesquioxane T -Pr-DPA-
8
Mo (3) as a catalyst precursor for the epoxidation of olefins was
investigated for cyclooctene (Cy8) and styrene (Sty), with tert-
butyl hydroperoxide (TBHP) as the oxygen source, at 328 K in
air with 3 mL of dichloromethane as solvent (Figure S3
diffusion of ether into a methanol solution of T -PrCl (1). In
8
the crystal structure, there are two independent molecules, both
exhibiting crystallographic inversion symmetry and one
chloropropyl chain disordered over two positions. Furthermore,
they have identical distances and angles. The molecular
structure of one of these molecules, presented in Figure 5,
shows a POSS cubic core composed of eight tetrahedral silicon
centers bridged by oxygen atoms with Si−O distances ranging
from 1.616(2) to 1.631(2) Å. The Si−C distances vary between
(
Supporting Information) and Table 1). Gas chromatography
Table 1. Conversions and Turnover Frequencies (TOF) for
Epoxidation of Cyclooctene and Styrene Promoted by T -Pr-
8
3
DPA-Mo (3) and [Mo(η -C H )Br(CO) (DPA)] in the
3
5
2
Presence of TBHP
1.836(2) and 1.858(2) Å. The crystal structure of another
polymorph of this compound was recently reported at room
a
b
cat. precursor
substrate conversion (%) TOF
3
2
temperature. This polymorph also crystallizes in the triclinic
space group P1, but the unit cell contains only one independent
T -Pr-DPA-Mo (3)
Cy8
Sty
59
36
70
43
129
80
8
̅
molecule with a centrosymmetric structure. Therefore, the
structure reported here, measured at 150 K, corresponds to a
new polymorph, which corroborates that 1 undergoes a phase
change to another triclinic structure at low temperature as
3
[
Mo(η -C H )Br(CO) (DPA)]
Cy8
Sty
63
3
5
2
116
a
b
−1 −1
Conversion at 24 h. In units of mol (mol of Mo) h .
32
suggested earlier. Indeed, the cell parameters, a = 10.2067(6)
Å, b = 15.2037(9) Å, and c = 15.5094(9) Å, differ significantly
from those determined in the high-temperature polymorph (a
coupled to mass spectrometry (GC-MS) analysis was
performed to monitor the catalytic epoxidation. The same
3
=
10.020(1) Å, b = 10.133(1) Å, and c = 12.855(1) Å). The
study was performed using the complex [Mo(η -C H )Br-
3
5
30
distances and angles within the POSS structure are identical in
both polymorphs.
The X-ray single-crystal structure determination of the
(CO) (DPA)], in order to compare the mononuclear species
2
under the same conditions, with the nanomolecular species 3.
Both the material T -Pr-DPA-Mo (3) and the complex
8
3
related Cl Si O revealed a structure with 3
̅
crystallographic
[Mo(η -C H )Br(CO) (DPA)] catalyzed selectively the oxida-
8
8
12
3
5
2
symmetry and three independent Si−O distances of 1.595(4),
tion of the two substrates to the corresponding epoxides,
without formation of diols. The results for the complex were
36
1
.605(4), and 1.610(4) Å, which are slightly shorter than in
30
T -PrCl (1). A trend toward slightly narrower O−Si−O angles
8
reported previously and follow the described activity of the
4
499
dx.doi.org/10.1021/om3003043 | Organometallics 2012, 31, 4495−4503

Organometallics
Article
3
family of [Mo(η -allyl)X(CO) (N−N)] complexes with X =
in the same nanomolecule, the conversion and TOF for both
cycloctene and styrene remaining relatively small in 3.
2
Cl, Br, allyl = C H , C H O, and N−N = bidentate nitrogen
3
5
5
5
3
7
ligand, although the DPA ligand does not lead to a very
efficient catalyst. This complex immobilized in two different
types of clays (PILC and PCH) also exhibited a similar (low)
activity. The total conversions are very modest compared to
others obtained when the catalytic precursors were analogous
Mo(II) systems bearing different nitrogen ligands, used either
EXPERIMENTAL SECTION
■
General Procedures. All reagents were obtained from Aldrich and
used as received. Commercial grade solvents were dried and
deoxygenated by standard procedures (Et O, THF, and toluene over
2
Na/benzophenone ketyl; CH Cl2 over CaH ), distilled under
37
2
2
under homogeneous conditions or after being immobilized in
nitrogen, and kept over 4 Å molecular sieves. The complexes
3
8
3
3
29a
3
MCM-41.
For instance, even the [Mo(η -C H )Br-
[Mo(η -C H )Br(CO) (NCCH ) ]
and [Mo(η -C H )Br-
3
5
3
5
2
3
2
3 5
3
0
(
(
CO) (NCCH ) ] precursor exhibited a TOF of 222 mol
(CO) (DPA)] were prepared according to literature procedures.
2
3 2
−1
2
−1
37
mol of Mo)
h
and 100% conversion at 24 h (Cy8).
FTIR spectra were obtained as KBr pellets and diffuse reflectance
−
1
measurements (DRIFT) using 2 cm resolution on a Nicolet 6700
The results given in Table 1 (and Figure S3) show that both
systems achieve similar catalytic conversions in the oxidation of
−1
instrument in the 400−4000 cm range. Powder XRD measurements
were taken on a Philips PW1710 instrument using Cu Kα radiation
cyclooctene and styrene; however, the initial activity of 129 mol
1
13
mol of Mo)−1
−1
filtered by graphite. H and C solution NMR spectra were obtained
with a Bruker Avance 400 spectrometer. Chemical shifts are quoted in
ppm from TMS.
(
h
(TOF) of T -Pr-DPA-Mo (3) in the
8
3
epoxidation of Cy8 exceeds that of [Mo(η -C H )Br-
3
5
−1
−1
(
CO) (DPA)] (63 mol (mol of Mo) h ). The TOF for
29
2
Si NMR spectra in CDCl were recorded on a Bruker Avance 400
3
oxidation of styrene is higher for the complex than for 3 (116 vs
spectrometer at 79.49 MHz with a 10 mm broad-band probe. Silicon
resonances were referenced to internal standard TMS. A pulse
sequence with inverse gated decoupling (waltz-16 decoupling applied
during acquisition time only) was used, since the magnetogyric ratio
for 29Si is negative and NOE might suppress the signal. The Si
spectrum of T -PrCl (1) in CDCl was obtained using the following
−1
−1
8
0 mol (mol of Mo) h ). This kind of inversion in the TOF
of Cy8 (higher) and Sty (lower) after immobilization has been
observed before in the catalytic epoxidation of these substrates
with related homogeneous and MCM41-supported catalysts
bearing other diimines. A possible explanation may arise from
the fact that epoxidation of Cy8 and Sty requires different
29
8
3
parameters: 31.8 kHz (400 ppm) sweep width, 1.0 s acquisition time,
2
9
3
9
27 μs (90°) pulse width, 5 s relaxation delay, 64 k data points. The Si
acid−base properties of the catalyst.
spectra of compounds T -Pr-DPA (2) and T -Pr-DPA-Mo (3), less
8
8
The coordination of a Mo(II) complex to the POSS cage to
soluble than 1, were obtained using a DEPT sequence optimized for
coupling with two protons (DEPT 45) and a coupling constant of 7
Hz9.
form the organometallic species T -Pr-DPA-Mo (3) did not
8
improve the properties of the catalyst precursors. On the other
hand, the selectivity toward the epoxide, as confirmed by mass
spectrometry, has remained as high as in the other systems
investigated.
5Mo NMR spectra were recorded on a Bruker Avance 400
spectrometer with a 10 mm broad-band probe at 26.05 MHz; a version
of the antiacoustic ringing pulse sequence (from the Bruker library)
was used in order to reduce baseline distortions. Saturated solutions of
the samples in DMSO-d were measured at 303 K and the chemical
CONCLUSIONS
All chloropropyl side chains in the POSS cage compound 1
T -PrCl) reacted with DPA, affording a new species (2, T -Pr-
6
■
shifts referenced to 2 M Na MoO in D O (pH 11) as the external
2
4
2
standard. The pulse parameters were as follows: 57.5 kHz sweep
width, 31 μs (90°) pulse width, 0.14 s acquisition time, 0.01 to 1 s
relaxation delay, 16 k data points. Zero filling to 32 k and an
exponential window function was applied with a line broadening of
(
8
8
DPA) with eight DPA arms. Further reaction with the Mo(II)
3
complex [Mo(η -C H )Br(CO) (NCMe) ] afforded the new
3
5
2
2
1
50 Hz before Fourier transform to obtain a reasonable signal to noise
derivative 3 (T -Pr-DPA-Mo), which most likely contains three
8
1
(S/N) ratio. Topspin3 software from Bruker was used for data
processing.
Microanalyses (C, N, H) were performed at the University of Vigo,
and atomic absorption for determination of molybdenum was
performed on a Unicam 929AA spectrometer at the Departamento
molybdenum atoms per cage. NMR spectroscopy of H and
Si showed that the chemical shifts of the protons in the propyl
2
9
chain changed when DPA was introduced but were insensitive
to the binding to molybdenum, the same happening to the
signals of the Si in the cage, a fact which was confirmed by DFT
calculations. A change in the C chemical shifts (propyl chain)
was observed upon introduction of the metal, but the low
quality of the spectrum could not be improved upon because of
low solubility. These are, to our knowledge, the first POSS
cages functionalized with eight DPA, to build a large ligand, and
de Quım
̂
ica e Bioquımica, Faculdade de Ciencias da Universidade de
́ ́
1
3
Lisboa.
Synthesis of Octakis(3-chloropropyl)octasilsesquioxane (1,
T -PrCl). 3-Chloropropyltriethoxysilane (225 mL) was added to a
8
stirred mixture of methanol (4 L) and concentrated hydrochloric acid
(35 mL) under a slow continuous nitrogen purge. The reaction
3
mixture was stirred for 6 weeks. The resultant solution was then
filtered and dried to give a white solid in 35% yield.
with DPA and three [Mo(η -C H )Br(CO) (DPA)] molecules
3
5
2
to build a large species behaving as a small model of a
heterogeneous catalyst. The precursor 1 was structurally
characterized by single-crystal X-ray diffraction and shown to
be the low-temperature polymorph of a recently reported
Anal. Calcd for Si O C H Cl (1036.9): C, 27.80; H, 4.67.
8
12 24 48
8
−1
Found: C, 27.94; H, 4.93. IR (KBr; ν, cm ): 2994 (m), 2955 (m),
193 (vs), 1111 (s). H NMR (400.13 MHz, CDCl , 298 K; δ, ppm):
0.80 (t, 2H, H ), 1.88 (t, 2H, H ), 3.54 (t, 2H, H ). C{ H} NMR
1
1
3
1
3
1
1
2
3
3
2
compound, differing in the packing and the size of the unit
(100.61 MHz, CDCl
3
, 298 K; δ, ppm): 9.64 (C
1
), 26.55 (C ), 47.35
2
29
1
(
C3). Si{ H} NMR (79.49 MHz, CDCl , 298 K; δ, ppm): −66.97.
cell.
3
3
Octakis[3-(2,2′-dipyridylamine)propyl]octasilsesquioxane
Since the family of [Mo(η -C H )Br(CO) (N) ] complexes
has been shown to promote efficiently the catalytic epoxidation
of olefins in the presence of TBHP, the activities of 3 (T -Pr-
DPA-Mo) and the related DPA complex (already known)
were compared. [Mo(η -C H )Br(CO) (DPA)] is not among
the most active of the possible precursors, but there is no visible
3
5
2
2
(
2, T -Pr-DPA). A solution of 2,2′-dipyridylamine (1.310 g, 7.7 mmol)
8
in anhydrous dimethylformamide (10 mL) was added via an addition
funnel, over a period of 30 min, to a stirred suspension of sodium
hydride (60% dispersion in mineral oil; 0.308 g, 7.7 mmol) in
anhydrous dimethylformamide (10 mL), at 0 °C under nitrogen. The
8
30
3
3
5
2
mixture was stirred continuously for 2 h at that temperature. T -PrCl
8
cooperative effect induced by the presence of several Mo atoms
(1; 1.0 g, 0.96 mmol) and 10 mL of anhydrous dimethylformamide
4
500
dx.doi.org/10.1021/om3003043 | Organometallics 2012, 31, 4495−4503

Organometallics
Article
were then added to the reaction mixture. The temperature was raised
to room temperature, and stirring was continued for 22 h to afford a
pale yellow solution. Distilled water was then added, and the desired
product was extracted with chloroform. The resulting solution was
dried over anhydrous sodium sulfate, filtered, and evaporated to
The molecular diagram presented was drawn with the Mercury 2.3
4
1
software available from CSD.
Catalytic Studies. T -Pr-DPA-Mo (3) and [Mo(η -C H )Br(CO)
3
8
3
5
2(DPA)] were tested in the epoxidation of cis -cyclooctene and
styrene, using as oxidant tert- butyl hydroperoxide (TBHP). The
catalytic oxidation of the two olefins was carried out at 328 K under a
normal atmosphere in a reaction vessel equipped with a magnetic
stirrer and a condenser. The vessel was loaded with olefin (100%),
internal standard (DBE), catalyst (1%), oxidant (200%), and 3 mL of
solvent. The course of the reaction was monitored by GC-MS analysis.
The addition of the oxidant determines the initial time of the reaction.
Samples were taken every 15 min during the first 1 h and then after 2,
4, 6, 8, and 24 h of reaction. They were diluted with dichloromethane
and chilled in an ice bath. For the destruction of tert-butyl
hydroperoxide, a catalytic amount of manganese dioxide was added.
The resulting slurry was filtered and the resulting extract injected into
the GC-MS system. The conversion of each olefin, measured by the
formation of the corresponding epoxide, was quantified through GC-
MS using convenient calibration plots recorded prior to the reaction
dryness under vacuum to give the modified silsesquioxane T -Pr-DPA
8
in 74% yield (1.5 g).
Anal. Calcd for Si O C104H112N24 (2114.8): C, 59.06; N, 15.90; H,
8
12
−1
5
(
.34. Found: C, 59.18; N, 15.56; H, 5.90. IR (KBr; ν, cm ): 3255
m), 3179 (m), 3025 (s), 2846 (m), 1681 (w), 1617 (s), 1591 (s),
440 (vs), 1352 (s), 1316 (m), 1055 (vs), 1027 (vs), 989 (vs), 760 (s).
1
1
H NMR (400.13 MHz, CDCl , 298 K; δ, ppm): 0.85 (t, 16H, H ),
3
9
1
3
.23 (s, 16H, H ), 2.92 (s, 16H, H ), 6.80 (t, 16H, H /H ), 7.56 (m,
8 7 4 4′
13
1
2H, H /H and H /H ), 8.25 (d, 16H, H /H ). C{ H} NMR
5
5′
6
6′
3
3′
(
100.61 MHz, CDCl , 298 K; δ, ppm): 14.4 (C ), 23.0 (C ), 63.5
3
9
8
(
C ), 112.2 (C /C ), 116.8 (C /C ), 138.4 (C /C ), 148.0 (C /C ),
7 6 6′ 4 4′ 5 5′ 3 3′
29
1
1
54.5 (C /C ). Si{ H} NMR (79.49 MHz, CDCl , 298 K; δ, ppm):
1
1′
3
−
21.87.
Octakis[3-(2,2-dipyridylamine)propyl]octasilsesquioxane-
Mo(allyl)Br(CO) ] (3, T -Pr-DPA-Mo). Octakis[3-(2,2-
[
course. Blank experiments using TBHP/MnO and olefin were carried
2
8
2
dipyridylamine)propyl]octasilsesquioxane, T -Pr-DPA (2; 1.0 g, 0.47
out, and no measurable activity was found. No reaction took place
without a metal-containing catalyst.
8
3
mmol), was added to a stirred solution of [Mo(η -C H )Br-
3
5
(
CO) (NCCH ) ] (1.34 g, 3.8 mmol) in anhydrous ethanol (25
GC-MS analysis was performed by using an Agilent 6890 Series gas
chromatograph, equipped with an Agilent 7683 automatic liquid
sampler and coupled to an Agilent 5973N mass selective detector
(Agilent Technologies, Little Falls, DE). The GC analyses were
performed on a TRB-5MS (30 m × 0.25 mm i.d. × 0.25 μm film
thickness) capillary column (5% diphenyl, 95% dimethylpolysiloxane;
Teknokroma, Barcelona, Spain), and helium was used as carrier gas
maintained in the constant pressure mode (7.36 psi). The oven
temperature was programmed from 60 °C (held for 3 min) to 200 °C
(held for 2 min) at 10 °C/min. The inlet was set at 250 °C operating
with a split ratio of 25:1, and 1 μL of each sample was injected. The
transfer line, ion source, and quadrupole analyzer temperatures were
maintained at 280, 230, and 150 °C, respectively, and a solvent delay
of 3 min was selected. The mass selective detector operated in the full-
scan mode acquisition, and electron ionization mass spectra in the
range 35−550 Da were recorded at 70 eV electronic energy with an
ionization current of 34.6 μA. Data analysis and instrument control
were performed by the MSD ChemStation software (G1701CA,
version C.00.00; Agilent Technologies). For the determination of the
conversion percentages, the ratio of the abundance areas of olefins and
DBE was calculated.
2
3 2
mL) under nitrogen, and the mixture was stirred overnight at room
temperature. The solvent was removed by filtration and the solid was
washed three times with anhydrous dichloromethane and dried, giving
1
.3 g of T -Pr-DPA-Mo (98% yield).
8
Anal. Calcd for Si O C119H127N Br Mo (2934.3; 3 Mo units): C,
8
18
24
3
3
4
4
8.72; N, 11.46; H, 4.36; Mo, 9.81. Found: C, 47.34; N, 10.68; H,
.62; Mo, 8.06. IR (KBr; ν, cm ): 3282 (s), 3235 (m), 3195 (s), 3134
−1
(
s), 2998 (m), 2934 (w), 1927 (vs), 1843 (vs), 1635 (s), 1594 (m),
1
583 (s), 1478 (vs), 1369 (m), 1062 (s), 1023 (s), 1011 (vs), 769
1
(
vs). H NMR (400.13 MHz, CDCl , 298 K; δ, ppm): 0.78 (s bd, 16H,
3
H ), 1.22 (s, bd, allyl), 1.84 (s bd, 16H, H ), 2.85 (s, bd, allyl), 2.95 (s
9
8
bd, 16H, H ), 3.55 (s, bd, allyl), 6.87 (s bd, 16H, H /H ), 7.55−7.63
7
4
4′
13
1
(
d bd, 32H, H /H and H /H ), 8.26 (d bd, 16H, H /H ). C{ H}
5 5′ 6 6′ 3 3′
NMR (100.61 MHz, CDCl , 298 K; δ, ppm): 15.4 (C ), 31.2/32.3
3
9
(
(
C ), 50.2 (allyl), 61.5/72.0 (C ), 75.1 (allyl), 118.1 (C /C ), 118.7
8 7 6 6′
29
1
C /C ), 137.4 (C /C ), 151.9 (C /C ), 163.8 (C /C ). Si{ H}
4
4′
5
5′
3
3′
1
1′
9
5
NMR (79.49 MHz, CDCl , 298 K; δ, ppm): −21.91. Mo NMR
(
−
3
26.05 MHz, DMSO-d , 303 K; δ, ppm): −615, −678 (endo isomers);
6
709 (exo/axial); −789 (exo/equatorial).
Crystallography. Crystals of 1 with suitable quality for a single-
crystal X-ray determination were grown from diffusion of ether into a
methanol solution of 1. Crystal data: C H Cl O Si P , M
Epoxidation of cis-Cyclooctene: cis-cyclooctene (800 mg, 7.3
mmol), 800 mg of dibutyl ether (internal standard), 1 mol % of
catalyst, 2.65 mL of TBHP (5.5 M in n-decane), and 3 mL of CH Cl .
=
r
24
48
8
12
8
2
1
1
9
0
036.94, triclinic, space group P1
̅
, Z = 2, a = 10.2067(6) Å, b =
2
2
5.2037(9) Å, c = 15.5094(9) Å, α = 99.157(3)°, β = 103.552(3)°, γ =
Epoxidation of Styrene: styrene (800 mg, 7.7 mmol), 800 mg of
dibutyl ether (internal standard), 1 mol % of catalyst, 2.65 mL of
TBHP (5.5 M in n-decane), and 3 mL of CH Cl .
3
−3
9.353(3)°, V = 2259.3(2) Å , ρ(calcd) = 1.524 Mg m , μ(Mo Kα) =
−1
.761 mm , 120 818 reflections collected, 20 206 unique reflections
Rint= 0.0315). The final R values were R1 = 0.0581 and wR2 = 0.1581
for 14 010 reflections with I > 2σ(I) and R1 = 0.0924 and wR2 =
2
2
(
Computational Details. All calculations were performed using the
42
Gaussian 03 software package and the B3LYP hybrid functional.
4
3
0
.1887 for all hkl data.
The X-ray data were collected on a CCD Bruker APEX II
That functional includes a mixture of Hartree−Fock exchange with
33
DFT exchange correlation, given by Becke’s three-parameter
44
diffractometer at 150(2) K using graphite-monochromated Mo Kα
radiation (λ = 0.710 73 Å). The crystal was positioned at 35 mm from
the CCD, and the spots were measured using a counting time of 10 s.
Data reduction including a multiscan absorption correction was carried
out using the SAINT-NT software from Bruker AXS. The structure
was solved using SHELXS-97 and refined using full-matrix least
functional with the Lee, Yang, and Parr correlation functional,
45,46
which includes both local and nonlocal terms.
All geometry
optimizations were performed without symmetry constraints using the
4
7
following basis set (b1): a LanL2DZ basis set augmented with an f
4
8
49
polarization function for Mo and a standard 6-31G(d,p) set for the
remaining elements. NMR shielding tensors were calculated using the
40
50
squares in SHELXL-97. The disordered chloropropyl chains were
refined with the chlorine and the two subsequent carbon atoms
occupying two alternative positions with their occupancies set to 0.5
for the first molecule, whereas for the second molecule refined
occupancies of x and 1 − x, with x equal to 0.29(1), were used.
Anisotropic thermal parameters were used for all non- hydrogen
atoms. The C−H hydrogen atoms were included at calculated
positions and refined with isotropic parameters equivalent to 1.2 times
those of the atom to which they were attached. The residual electronic
density was in the range +2.79 to −1.87 Å, with the highest positive
peak 0.75 Å within the coordination sphere of a disordered chloride.
gauge-independent atomic orbital method (GIAO) with the same
functional, an improved basis set (b2), and geometries optimized at
51
the B3LYP/b1 level. Basis b2 consisted of a standard 3-21G set with
4
7
52
an added f polarization function for Mo and a 6-311+G(2d,p) set
for the remaining elements. The calculations were performed for the
real molecule of silsesquioxane 1 and for models of the substituted
solids with only one substitution each, owing to computational
limitations. The model 2′ corresponds to a solid with only one DPA
substituent, and models 3′ and 3″ correspond to the POSS substituted
with one Mo complex with the allyl ligand in the exo and in the endo
conformations, respectively (see Figure S1, Supporting Information).
4
501
dx.doi.org/10.1021/om3003043 | Organometallics 2012, 31, 4495−4503

Organometallics
Article
(
21) (a) Wada, K.; Mitsudo, T.-A. Catal. Surveys Asia 2006, 9, 229−
ASSOCIATED CONTENT
■
2
4
41. (b) Bent, M.; Gun’Ko, Y. J. Organomet. Chem. 2005, 690, 463−
*
S
Supporting Information
Mo NMR data (Table S1), a representation of the models
68.
9
5
(22) Crocker, M.; Herold, R. H. M.; Orpen, A. G.; Overgaag, M. T.
used in the DFT calculations (Figure S1), X-ray powder
diffraction data (Figure S2), catalytic activity (Figure S3),
atomic coordinates for all optimized species, and a CIF file
giving crystal data for 1. This material is available free of charge
via the Internet at http://pubs.acs.org.
A. J. Chem. Soc., Dalton Trans. 1999, 3791−3804.
23) Monticelli, O.; Cavallo, D.; Bocchini, S.; Frache, A.; Carniato,
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(
(
(
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AUTHOR INFORMATION
́
(26) Solans-Monfort, X.; Filhol, J.-S.; Coperet, C.; Eisenstein, O. New
■
J. Chem. 2006, 30, 842−850.
Notes
(27) Hossain, D.; Pittman, C. U., Jr.; Hagelberg, F.; Saebo, S. J. Phys.
The authors declare no competing financial interest.
Chem. C 2008, 112, 16070−16077.
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ACKNOWLEDGMENTS
■
2
(
F.C.M.P. thanks the FCT for a research grant (SFRH/BD/
4598/2005). L.F.V. (PTDC/QUI-QUI/099389/2008), L.F.V.
and M.J.V.d.B. (PEst-OE/QUI/UI0100/2011), and M.J.C.,
C.D.N., P.D.V., and J.M.F.N. (PEst-OE/QUI/UI0612/2011)
thank the FCT for funding. N.L.D.F. thanks FAPESP, UNESP
and CAPES for scholarships.
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