Oxidovanadium(V) Anchored to Silanol-Functionalized Polyoxotungstates: Molecular Models for Single-Site Silica-Supported Vanadium Catalysts

Article abstract of DOI:10.1021/acscatal.5b01878

The metalation of two different types of silanol-decorated polyoxotungstates, [XW₉O_34-x(^tBuSiOH)₃]^3- (X = P, x = 0; X = Sb, x = 1) and [PW₁₀O₃₆(^tBuSiOH)₂]^3-, by Cl₃VO or (ⁱPrO)₃VO has been achieved. The characterization of the resulting oxidovanadium(V) complexes [XW₉O_34-x(^tBuSiO)₃VO]^3- (X = P, 3; X = Sb, 3′) and [PW₁₀O₃₆(^tBuSiO)₂VO(ⁱPrO)]^3- (4) has been fully detailed, including X-ray analysis. These compounds are present in monomeric forms and therefore represent original molecular models for tris-grafted and bis-grafted isolated (V=O)³⁺ species dispersed onto silica. Their ability as precatalysts for the epoxidation of cyclic olefins and allylic alcohols with tert-butyl hydroperoxide (TBHP) has been studied. The results suggested that a confined tris-grafted species, such as 3 or 3′, does not act as an efficient catalyst whereas a more labile bis-grafted species, such as 4, does. To gain a better understanding, we have assessed their suitability for ligand exchange with alcohols and TBHP and performed reaction progress kinetic analysis by monitoring the epoxidation of 3-methyl-2-buten-1-ol by ¹H and ⁵¹V nuclear magnetic resonance.

Full text of DOI:10.1021/acscatal.5b01878

Research Article
pubs.acs.org/acscatalysis
Oxidovanadium(V) Anchored to Silanol-Functionalized
Polyoxotungstates: Molecular Models for Single-Site Silica-
Supported Vanadium Catalysts
́
Geoffroy Guillemot,* Edoardo Matricardi, Lise-Marie Chamoreau, Rene Thouvenot, and Anna Proust
Sorbonne Universites
Curie, 4 place Jussieu, case courrier 42, F-75252 Paris Cedex 05, France
́ ́ ́
, UPMC Univ Paris 06, CNRS, UMR 8232, Institut Parisien de Chimie Moleculaire Universite Pierre et Marie
*
S Supporting Information
ABSTRACT: The metalation of two different types of silanol-
t
3−
decorated polyoxotungstates, [XW O ( BuSiOH) ] (X = P, x
9
34−x
3
t
3−
=
0; X = Sb, x = 1) and [PW O ( BuSiOH) ] , by Cl VO or
10 36 2 3
i
(
PrO) VO has been achieved. The characterization of the
3
r e s u l t i n g o x i d o v a n a d i u m ( V ) c o m p l e x e s
t
3−
[
[
XW O ( BuSiO) VO] (X = P, 3; X = Sb, 3′) and
9
34−x
3
t i 3−
PW O ( BuSiO) VO( PrO)] (4) has been fully detailed,
10
36
2
including X-ray analysis. These compounds are present in
3+
monomeric forms and therefore represent original molecular models for tris-grafted and bis-grafted isolated (VO) species
dispersed onto silica. Their ability as precatalysts for the epoxidation of cyclic olefins and allylic alcohols with tert-butyl
hydroperoxide (TBHP) has been studied. The results suggested that a confined tris-grafted species, such as 3 or 3′, does not act
as an efficient catalyst whereas a more labile bis-grafted species, such as 4, does. To gain a better understanding, we have assessed
their suitability for ligand exchange with alcohols and TBHP and performed reaction progress kinetic analysis by monitoring the
1
51
epoxidation of 3-methyl-2-buten-1-ol by H and V nuclear magnetic resonance.
KEYWORDS: oxidovanadium, polyoxotungstates, silanol, site-isolated catalysts, tert-butyl hydroperoxide, epoxidation
INTRODUCTION
dioxo V(O) species, to square pyramidal or octahedral
2
■
species. This ongoing argument is important because the
The fundamental understanding of the mechanisms controlling
heterogeneous catalysis at a molecular level is of particular
importance in an attempt to develop efficient and selective
catalysts meeting the contemporary challenges of industrial
geometry and coordination to the surface as well as surface
distribution or aggregation of the metallic entities are key
factors that may influence the activity and selectivity of the
catalysts. This notwithstanding, true heterogeneity during
oxidation catalysis in the liquid phase is also a primary issue
because leaching of vanadium during the course of the catalytic
1
,2
processes. However, defining the chemical environment of an
active species deposited on solid surfaces is rather difficult. This
is mainly due to the effective heterogeneity and complexity of
the real surface and the difficulty in unambiguously assessing
13,14
reactions has been reported.
3
In this report, we describe the preparation of oxidovanadium-
(V) complexes of a family of silanol-decorated polyoxotung-
states that proves to be a relevant molecular model for surface
vicinal silanols of dehydroxylated silica. The structures depicted
in Figure 1, which have already been reported by some of
the structure−activity relationship. While “surface organo-
4
−7
metallic chemistry”
proved to be a relevant molecular
8
,9
approach in the rational construction of well-defined surface
species, the use of molecular models can still provide valuable
clues to improve our understanding of the intermediates and
mechanisms occurring during heterogeneous catalysis.
15,16
us,
exhibit a number of significant peculiarities that make
them quite unique among the well-established ligand systems
that are capable of mimicking a heterogeneous oxidic
Metal oxides deposited onto a large-surface area oxide
represent an important class of heterogeneous catalysts in
which vanadium-based species have appeared as efficient
catalysts for the oxidation of hydrocarbons, alcohols, and SO2
and more recently for the oxidative dehydrogenation of light
alkanes and methanol. Depending on the nature of the
support (montmorillonite, aluminophosphates, and silicates)
and the condition of impregnation used, numerous species are
1
7−20
environment, e.g., mono- or polydentate organic silanol,
2
1−23
polyhedral oligomeric silsesquioxanes (POSS),
calix[n]-
First, they display a
24−29
30−34
arenes,
and heteropolyanions.
1
0
rigid and geometrically preorganized set of silanol moieties that
is well suited for the coordination and stabilization of high-
valent metal ions. Second, the bulky tert-butyl groups at the
1
1,12
discussed as active sites,
ranging from mononuclear to di-
or polynuclear and also to monolayers. A discussion is also
found in the literature concerning the structure of the isolated
Received: August 24, 2015
Revised: October 16, 2015
5
+
3+
V
ions, tetrahedrally coordinated (VO) being opposed to
©
XXXX American Chemical Society
7415
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Research Article
Figure 1. Representations of anions 1, 1′, and 2 used in this study, prepared as tetrabutylammonium salts (TBA).
t
43
silicon atoms create a sterical protection around the metal
coordination site. The combination of both aspects prevents
the formation of oligomers and allows the metal ion to fit in a
well-defined single site. In this context, oxidovanadium(V)
complexes of these silanol-functionalized polyoxotungstates are
derivative (n-Bu N) [(α-A-PW O )( BuSiO) VO] (TBA-3).
4 3 9 34 3
The adjunction of a base is not required; nevertheless, to avoid
the evolution of gaseous chlorhydric acid, compound TBA-3
may alternatively be prepared by reacting TBA-1 with
i
i
( PrO) VO (see eq 1; X = Cl or PrO).
3
5
+
original models for isolated V surface species. Hereafter, we
first focus on the description of their structural analogies with
silica-supported vanadium materials to validate our molecular
approach to heterogeneous systems. In addition, we have
assessed their suitability for ligand exchange with alcohols and
tert-butyl hydroperoxide (TBHP) and their potential as
catalysts in the epoxidation of alkenes.
t
3−
[PW O ( BuSiOH) ] + X VO
9
34
3
3
t
3−
→
[PW O ( BuSiO) VO] + 3HX
(1)
9
34
3
i
Clean reaction of ( PrO) VO with alcohols or silanols usually
requires anhydrous conditions to avoid hydrolysis of the
metallic precursor. Thermogravimetric profiles of TBA-1
3
samples did not show any relevant mass loss below 270 °C
t
RESULTS AND DISCUSSION
(
decomposition of BuSiOH and/or TBA), whereas thermog-
■
ravimetric profiles of TBA-2 samples are characterized by a first
mass loss (approximately 1−2%) around 200 °C. This is
attributed to the presence of water molecules (two to four
molecules per 2) so that TBA-2 should be written as TBA-2·
Synthesis. Anions 1, 1′, and 2 are based on the trilacunary
9−
9−
[
α-A-PW O ] and [α-B-SbW O ] and the dilacunary [γ-
9 34 9 33
7−
35
PW O ] Keggin-type platforms, respectively. Compared
to the closed-shell parent derivatives [PW O ] , the
10
36
3
−
1
2
40
xH O (x = 2−4). These water molecules probably stack
(multi)lacunary polyoxometalates (POMs) display nucleophilic
2
t
together forming a hydrogen bonding network on the BuSiOH
groups. It is well-established in the chemistry of silica
oxygen atoms that open the way to their functionalization using
organic electrophiles (i.e., organosilanes in this study).
Indeed, the reaction of the lacunary POMs with tert-butyl
trichlorosilane, under phase transfer conditions, affords (n-
Bu N) [(α-A-PW O )( BuSiOH) ] (TBA-1), (n-Bu N) [(α-
44
36−38
supports that water layers experiencing H-bonding with SiO2
are more strongly physisorbed and heating at ∼210 °C is
45
t
therefore needed to remove them. Indeed, after TBA-2 had
been heated at 210 °C for 3 h under vacuum (10− mbar), the
thermogravimetric profile shows no mass loss below 270 °C.
This similarity in behavior marks a certain analogy between the
silanol-decorated polyoxometalates and silica. The comparison
4
3
9
34
3
4
3
3
t
B-SbW O )( BuSiOH) ] (TBA-1′), and (n-Bu N) [(γ-
9
33
3
4
3
t
PW O )( BuSiOH) ] (TBA-2), respectively. Only minor
10
36
2
changes to the published synthesis have been made to improve
the efficiency and simplicity of the reactions. The C and C
symmetries of the native POMs engaged in the reactions are
3v
2v
1
of the H nuclear magnetic resonance (NMR) patterns of TBA-
2
and its dehydrated form is also significant (Figure S1). The
labile OH protons cannot be observed at room temperature
RT) because of fast exchange in solution on the NMR scale,
retained in anions 1, 1′, and 2. Moreover, it is worth
t
emphasizing that the structure of the BuSiOH groups at the
(
“surface” of the polyoxotungstate frameworks is directly related
1
whereas a resonance line appears at 5 ppm when H NMR is
to the symmetry of the native POMs. The rigid and 3-fold
symmetric pocket generated by the BuSiOH in anion 1 is quite
uncommon in coordination chemistry. Such a preorganized
environment is very difficult to design by organic chem-
t
performed on a dry d
3
-acetonitrile solution of the dehydrated
form of TBA-2.
The reaction of ( PrO) VO with dehydrated TBA-2 in
i
3
3
9,40
acetonitrile leads to TBA-4 (eq 2). The release of 2 equiv of 2-
propanol can be monitored by NMR spectroscopy, and the
desired compound was obtained in good yields and recrystal-
lized from a mixture of acetonitrile and diethyl ether.
istry,
and only seldom can analogues be found in the
literature, among which the polyhedral oligomeric silsesquiox-
ane (POSS) family is probably the most relevant example.
However, compared to ligand TBA-1, the POSS ligands remain
flexible enough to easily promote the dimerization of
coordination or organometallic compounds, and therefore,
the model of site isolation at a surface (as in heterogeneous
t
3−
i
[
PW O ( BuSiOH) ] + ( PrO) VO
1
0
36
2
3
t
i
3−
i
→
[PW O ( BuSiO) VO(O Pr)] + 2 PrOH
(2)
1
0
36
2
4
1,42
systems) is lost.
The coordination site available in 1 is well-
suited for tetrahedral or pseudotetrahedral metal centers, and
Structural Characterizations. Crystals suitable for X-ray
analysis were obtained for TBA-3 and TBA-4. As expected, in
both cases, the complexes are present in monomeric forms.
Four oxygen atoms coordinate the vanadium center tetrahe-
drally, and consequently, anions 3 (or 3′) and 4 may be
3
+
indeed, the incorporation of a (VO) functionality is
straightforward. The reaction of Cl VO with TBA-1 [used as
3
such (see Experimental Section)] at room temperature in
distilled acetonitrile led to the formation of the vanadium(V)
7
416
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Research Article
considered as modeling species for tris-grafted (A) and bis-
grafted (B) species on a siloxide surface as sketched in Chart 1.
extended the stabilizing O(pπ) → V(dπ) bonding. The effect of
the angle is also corroborated by the 51V chemical shifts, as
shown by the comparison of the data collected for several
structural models [vide infra (Table 1)]. The crystal structure
also confirms that the bulky tert-butyl groups create a sterically
protected well-defined single-metal site.
Chart 1. Tris-Grafted (A) and Bis-Grafted (B) Species on a
Siloxide Surface
The crystal structure of anion 4 seems to bring up 2-fold
symmetry, although this symmetry is not crystallographically
imposed (no mirror plane). This symmetry breaking is
probably due to a slight disorder in the organic parts.
Consequently, the two V−O bond lengths, 1.73(2) and
Si
1
.74(2) Å, are similar within experimental error. The
pseudotetrahedral environment around the metal is completed
by the oxido function and the remaining i-propoxido ligand
(
Figure 3 and Figure S3). The V−O(39) bond distance of
The terminal V−O bond distances, 1.58(2) Å in anion 3 and
V
3+
1
.63(3) Å in 4, are in accordance with a (V O)
functionality and are comparable to those of related systems
12,41,46,47
reported previously.
As anion 3 possesses a crystallo-
graphically imposed C axis running through the V metal ion
3
and O(14), all the V−O bonds have the same length, 1.795(9)
Si
Å (Figure 2 and Figure S2). The geometric constraint imposed
Figure 3. Crystal structure of anion 4.
1
.67(2) Å appears to be slightly shorter than the V−O bond
si
distances, which may be the consequence of the greater σ
donor ability of the alkoxide ligand compared to that of the
siloxide ones. It should be noted again that the Si−O−V angles
in anion 4 are very obtuse, albeit smaller than in 3, 161(1)° and
1
63(1)°, which reflects an important O(pπ) → V(dπ)
stabilizing bonding [vide infra (Table 1)].
A thorough multinuclear NMR study ( H, 31P, 29Si, and
1
Figure 2. Crystal structure of anion 3.
183
W) confirms the purity of TBA-3, 3′, and 4, which exhibit
by the ligand forces the vanadium center to adopt a regular
tetrahedral geometry [O(13)−V−O(13#) = 110.3(3)°;
O(13)−V−O(14) = 108.7(3)°]. It is worth emphasizing the
primary role of the Si(1)−O(13)−V obtuse angles observed in
anion 3, 173.5(6)°. Considering the very electrophilic nature of
the expected C and C symmetries in solution. Of particular
3
2
interest, the tert-butyl group at the silanol functionality is an
ideal H NMR probe for verifying completion of the reaction. A
slight low-field shielding of the corresponding singlet is
1
observed as a consequence of the introduction of the electron
0
3+
the d -vanadium center, the more obtuse the angle, the more
deficient (VO) functionality (see Experimental Section).
Table 1. Correlation between 51V NMR and the Chemical Environments in Selected Molecular Complexes
entry
complex
mean value for the Si−O−V angle (deg)
chemical shift (ppm)
ref
1
2
3
4
5
6
(POSS)VO (monomer)
(POSS)VO (dimer)
(Ph SiO) VO
not available
143
−676
−710
−725
−730
−800
−692
41
41
47
155
3
3
[SbW O ( BuSiO) VO]3−, 3′
t
152
a
9
33
3
t
3−
[PW O ( BuSiO) VO] , 3
173.5
161.6
9
34
3
[PW O ( BuSiO) VO( PrO)]3−, 4
t
i
1
0
36
2
a
35
Estimated value from a preliminary X-ray analysis of 3′. The polyoxotungstate core in anion 1′, a B isomer, provides a slightly different
environment than in anion 1, an A isomer.
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Table 2. Epoxidation of Olefins (0.15 M) with TBHP (0.22 M, 1.5 equiv) in the Presence of Complexes TBA-3, 3′, or 4 in
Acetonitrile
a
entry
olefin
catalyst
temp (K)
time (h)
conversion (%)
selectivity of epoxide (%)
b
c
1
2
3
4
5
6
cis-cyclooctene
3 (1%)
343
343
343
343
343
300
18
18
18
18
6
<5
13
na
b
d
3′ (1%)
>95
b
4 (1%)
4 (1%)
4 (1%)
4 (1%)
21
>95
>95
>95
>95
b
b
b
2-propen-1-ol
60
3-methyl-2-buten-1-ol
>95
>95
6
a
b
c
Conversion based on NMR. Catalyst loading is expressed vs the quantity of olefin. The conversion is too low to ascertain the selectivity in
d
epoxide. No side products were detected by NMR.
By contrast, 31P NMR chemical shifts in anions 3 and 4 are
poorly affected, thus indicating that the oxo anion cores remain
unchanged (also correlated by the W NMR analysis of TBA-
employed. When catalysis was conducted at 343 K (70 °C), low
conversions to cyclooctene oxide were obtained (see Table 2).
The better activity observed using complex TBA-4 suggested
that the presence of an easy accessible coordination site is
significantly important. Hence, using complex TBA-4, higher
conversions were observed upon investigation of the
epoxidation of the allylic alcohols 2-propen-1-ol and 3-
methyl-2-buten-1-ol. Full conversion was reached for the
more nucleophilic olefin, and complete conversion could also
be achieved at 300 K (27 °C) in 6 h (Table 2). By using the
tris-siloxide complex (TBA-3′), a low conversion to epoxide
was also observed for 3-methyl-2-buten-1-ol (∼25% after 6 h at
300 K). Because vanadium coordination sites in anions 3 and 3′
are structurally rigid, enlargement of the coordination sphere
around the vanadium center (see below) is unlikely, so that the
observed conversions may be attributed either to a partial
decoordination from the tris-siloxide ligands or to a catalytic
activity centered on the polyoxotungstic framework itself. The
second point was discarded by control experiments revealing
that ligands TBA-1 and 2 were not active as “catalysts”. A key
point was therefore to study the reactivity of complexes TBA-3,
1
83
5
1
3
and 3′, displaying two lines in a 1:2 ratio). V NMR shows a
broad signal centered at −800 ppm for TBA-3, −730 ppm for
5
1
3
′, and −692 ppm for 4. As the V nucleus benefits from a
large shift range, its chemical shift is very sensitive to minor
changes in the local environment. Of particular interest is its
variation upon substitution of the siloxide ligands for the
i
48
alkoxide in the metallic precursor ( PrO) VO (−629 ppm).
3
Whereas a moderate high-field shielding (in the range of 40−80
4
9
ppm for three substitutions) has been reported, it is
5
1
important to note that in the siloxide series the V chemical
shifts are mainly affected by the nature of the Si−O−V angle.
The more obtuse the angle, the more high-field-shielded the
chemical shift (entries 2−5 of Table 1). It should also be
pointed out that the nature of the substituents at the silicon
atom has a weak effect on the V chemical shift (entries 3 and
), whereas in the case of alkoxide ligands RO , bulky
substituents are known to induce high-field shielding. With
regard to compound TBA-4, the less shielded chemical shift
results from the presence of a residual i-propoxide ligand (entry
5
1
−
4
48
3
′, and 4 toward an excess of TBHP or allylic alcohols.
6
).
When TBA-3 and 3′ were treated with an excess of TBHP
The sensitivity of the 51V chemical shift to the coordination
(
up to 100 equiv) in dry acetonitrile-d , no modification of the
3
5
1
1
1
31
51
sphere of the metal center makes V NMR, along with H and
NMR patterns ( H, P, and V NMR) was observed. A similar
3
1
P NMR, a useful tool for monitoring the progression of a
experiment conducted on complex TBA-4 highlighted the
i
(catalytic) reaction.
release of PrOH, which is consistent with a substitution for
Catalytic Olefin Epoxidation. Anions 3, 3′, and 4 proved
alkyl peroxide at the vanadium center and generation of the
5
+
5 5 , 5 6
to be interesting models of mononuclear V surface species.
We therefore studied the activity of these complexes as catalysts
in the epoxidation of olefins with tert-butyl hydroperoxide
supposed reactive alkyl peroxidic species
t
t
3− 51
[
PW O ( BuSiO) VO(OO Bu)] ( V NMR, δ −621):
10
36
2
VI
VI
V
IV
t
i
3−
t
(
TBHP). With W , Mo , V , and Ti catalysts, the commonly
[
PW O ( BuSiO) VO(O Pr)] + BuOOH
50,51
10 36
2
accepted mechanism is a peroxidic metal pathway
in which
t
t
3−
i
(
i) TBHP is activated by coordination to the metal through the
⇆ [PW10O36( BuSiO)2VO(OO Bu)] + PrOH
(3)
t
oxygen distal to the Bu group and (ii) olefins, as nucleophiles,
react with the metal-bound electrophilic oxygen in a back side
approach and roughly along the axis of the oxygen−oxygen
0
The ligand exchanges at the d -metal center have to proceed
through a formal five-coordinate intermediate (the oxido
function has been originally postulated as a possible basic site
for assisting the proton transfer, but alternatively, the exchange
may occur via a σ bond metathesis-type mechanism). The
bidentate bis-siloxide ligand in TBA-4 displays in the solid state
an OSi−V−OSi angle of ∼113° and therefore appears to be well-
suited to accommodate a four-coordinate (pseudotetrahedral)
or five-coordinate (trigonal bipyramidal) oxidovanadium
species (postulated transition states in Figure 4). With regard
to eq 3, it is noteworthy that the relative concentrations of both
5
2,53
bond.
When the substrates are allylic (or homoallylic)
alcohols, the high chemoselectivity observed in the epoxide
products has been explained by a mechanism that proceeds by
the coordination of both reacting species, TBHP and allylic
5
4
alcohol. Such a mechanism requires that the precatalysts are
substitution labile, and therefore, we could anticipate complex
TBA-4 to be much more reactive than 3 and 3′.
We first studied the oxidation of cyclohexene and cis-
cyclooctene (0.15 M) with TBHP (0.22 M) in the presence of
a catalytic amount of complexes TBA-3, 3′, and 4 (1 mol % vs
olefin) in acetonitrile. At room temperature and even after a
long reaction time (18−24 h), no significant conversions to
epoxides were observed for any of the olefins and complexes
51
species can be assessed by V NMR and confirmed that TBHP
has a 10-fold lower affinity for vanadium than alcohols do (K
3
57
∼ 0.08). These experiments also pointed out that the V−O
Si
bonds in anions 3 and 4 are stable toward solvolysis in the
7
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during a period of ∼30 min (Figure 5 and Figure S5). The
conversion into epoxide was found to be concomitant with the
Figure 5. Plot of epoxide formation vs time in the oxidation of 3-
methyl-2-buten-1-ol (0.25 M) by TBHP (0.27 M) in CD CN at 300
3
K, catalyzed by 1 mol % TBA-3′ (◇) or TBA-4 (○).
increase in the magnitude of the signal centered at −639 ppm
51
(
V NMR) mentioned above. This strongly suggests that a
Figure 4. Proposed chemical scheme for the reaction of 4 and
analogues with TBHP and postulated transition states.
confined tetrahedral oxidovanadium species such as that in
complex TBA-3′ is not active toward the epoxidation of
58
olefins and that the observed catalytic activity is most
probably due to an equilibrium displacement to a partially
decoordinated species in the presence of excess alcohols and
TBHP. The release or leaching of vanadium in the liquid phase
during oxidation reactions is well-known in heterogeneous
systems. True heterogeneity has been associated with lower
catalytic activity as a result of the vanadium being more strongly
presence of hydroperoxides, as opposed to other systems
4
1
(Figure 4).
Similarly, we studied by 51V NMR the addition of an excess
of 3-methyl-2-buten-1-ol to solutions of the oxidovanadium
species in dry acetonitrile-d . Complexes TBA-3 and 3′ are
3
almost not affected; albeit, the formation of a new vanadium
species in a very low yield was revealed by the observation of a
minor line at −654 and −639 ppm (Figure S4). By contrast, the
addition of 3-methyl-2-buten-1-ol to a solution of TBA-4
59
bonded to the support. Similarly, in our systems, it should be
pointed out that after a long period of catalysis the formation of
t
51
48
( BuO)
VO was clearly evidenced by V NMR (−672 ppm),
3
5
1
resulted in the formation of two distinct lines in the V NMR
spectrum corresponding to the chemical equilibrium between
as a result of the leaching of a detectable amount of vanadium
in the presence of an increasing amount of tert-butyl alcohol
(produced inevitably as a byproduct).
t
TBA-4 at −692 ppm and [PW O ( BuSiO) VO(OCH CH
1
0
36
2
2
3−
C(CH ) )] at −678 ppm (K = 0.83). The presence of an
excess of olefin (100 equiv as under the catalysis conditions)
Kinetic Analysis. Monitoring of the evolution of complex
3
2
4
TBA-4 by 51V NMR clearly indicated two main resting states
t
shifts the equilibrium toward the new species [NMR ratio of
for the catalyst. Over the first 60 min, [PW10
O36( BuSiO)
VO-
2
1
31
3−
1
:99 (see also Figure S4)]. Remarkably, H and P NMR did
(OCH
in the remainder of the reaction, [PW10O36( BuSiO) VO-
2
2
CHC(CH
3 2
) )] is the dominating species, whereas
t
not reveal the presence of freed ligand TBA-2, suggesting that
t
3−
V−O bonds in anion 4 are stable toward alcoholysis.
(O Bu)] appeared as the new dominating species (see Figure
S6; a progressive enrichment in the latter species is mainly due
to the increasing amount of BuOH, a stronger σ donor ligand).
Si
t
i
3−
[
PW O ( BuSiO) VO(O Pr)]
t
1
0
36
2
This observation suggests that the reaction of
+
(CH ) CCHCH OH
3
2
2
t
3−
[
PW O ( BuSiO) VO(OCH CHC(CH ) )]
with
1
0
36
2
2
3 2
t
3−
⇆
[PW O ( BuSiO) VO(OCH CHC(CH ) )]
+
To gain a better understanding of the evolution of the
catalysis over time, we then decided to monitor by NMR the
epoxidation of 3-methyl-2-buten-1-ol by TBHP using TBA-3′
and 4 as (pre)catalysts. Typically, to a 2.5 mM solution of
TBHP is the rate-determining step, at least in the first stage
of the catalysis, and that the reaction rate should depend on
both concentrations. An analysis of the initial rates at different
concentrations of the reagents or catalyst indicated a kinetics
1
0
36
2
2
3 2
i
PrOH
(4)
close to first order toward [TBHP] and half-order toward the
0
total vanadium concentration, [TBA-4] , in the range of studies
0
(Table 3, Figure 6, and Figure S7).
complex TBA-3′ (or 4) in acetonitrile-d were successively
Continuous monitoring of reaction progress also allows us to
draw a reliable picture of the kinetics over time and most
interestingly versus substrate concentration. Figure 7 displays
a graphical rate equation of the rate versus TBHP
concentration. The straight-line relationship indicates that the
epoxidation reaction exhibits a kinetics close to first-order
toward [TBHP] in the range of 0.274 to 0.12 M (from 0 to 180
min) and therefore a pseudo-zero-order in [olefin] (this result
3
added 3-methyl-2-buten-1-ol (0.25 M) and TBHP (0.27 M).
1
51
60
Each step was followed by H and V NMR at 300 K. These
experiments brought to light the fact that the complexes display
completely different kinetic profiles. When complex TBA-4 was
used as a precatalyst, the formation of epoxide started readily
−
1
(
r = 1.51 mM min ), whereas when complex TBA-3′ was
0
used, the formation of epoxide was not significantly observed
7
419
DOI: 10.1021/acscatal.5b01878
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ACS Catalysis
Research Article
Table 3. Kinetic Data for the Vanadium-Catalyzed
Epoxidation of 3-Methyl-2-buten-1-ol with TBHP in
Acetonitrile at 300 K ([olefin] ≈ [TBHP] )
0
0
3
a
a
r × 103
b
entry
[4] × 10
0
[TBHP]0
0
1
2
3
4
5
6
1.44
2.5
0.274
0.143
0.274
0.505
0.710
0.274
1.10
0.83
1.51
3.29
4.92
2.51
8.35
a
b
Concentrations in moles per liter. Rates in molar per minute.
Figure 7. Graphical rate equation (Table 3, entry 3): rate vs tert-butyl
hydroperoxide (TBHP) concentration. The dashed-line relationship
corresponds to first-order kinetics in the concentration of substrate.
CONCLUSIONS
Silanol-decorated [XW O ( BuSiOH) ] (X = P, x = 0; X =
Sb, x = 1) and [PW O ( BuSiOH) ] polyoxotungstates
■
t
3−
9
34−x
3
t
3−
1
0
36
2
have been used as ligands in an elegant approach to model
surface silanols. The preparation and structural characterization
o f o x i d o v a n a d i u m ( V ) c o m p l e x e s ( n -
t
Bu N) [XW O ( BuSiO) VO] (for TBA-3, X = P and x =
4
3
9
34−x
3
0
; for TBA-3′, X = Sb and x = 1) and (n-
t
i
Bu N) [PW O ( BuSiO) VO( PrO)] (TBA-4) have been
4
3
10 36
2
fully detailed, including X-ray analysis. These complexes
provide interesting models for tris- and bis-grafted single-site
silica-supported vanadium catalysts, respectively. The kinetic
study of the epoxidation reaction of alkenes and allylic alcohols
by tert-butyl hydroperoxide clearly confirmed that an isolated
3+
tetrahedral (VO) complex is not an active species. Instead,
a partial decoordination appears to be a prerequisite for
allowing the oxidation to take place. This point has been
inferred from 51V NMR spectroscopy and from the much
higher catalytic efficiency displayed by complex TBA-4, a
homogeneous analogue to bipodal vanadium oxido alkoxide
surface species. The originality of our models stems from the
ability of lacunary polyoxotungstates to generate a rigid and
geometrically preorganized set of silanol moieties. The bulky
tert-butyl groups at the silicon atoms create a sterical protection
that ensures the oxidovanadium will fit in a well-defined single
site, a fundamental condition for developing a relevant
molecular approach to oxo surfaces. Major applications of
polyoxotungstates in catalysis rely on their thermal and
oxidative robustness that makes them efficient and recyclable
catalysts with a low environmental impact, and we are therefore
confident that the further preparation, structures, and catalytic
properties of metallic complexes based on ligands 1 and 2 will
also provide helpful information concerning the formation and
analysis of surface species.
Figure 6. Plot of the variation of [epoxide] vs time (top) in the
oxidation of 3-methyl-2-buten-1-ol with TBHP in CD CN at 300 K
3
−3
catalyzed by TBA-4 (2.5 × 10 M) [[olefin] ≈ [TBHP] = 0.143
0
0
(
□), 0.274 (◇), 0.505 (○), and 0.710 M (△)]. Plot of ln(r ) vs
0
2
ln([TBHP] ) (bottom) [slope = 1.12; f(0) = −4.96; R = 0.995].
0
describes a saturation kinetics in [olefin] at this first stage of the
epoxidation reaction). Then the curvature in the graphical rate
equation suggests a more complex relationship between the
61
rate and the substrate concentration at longer times. This may
be attributed to the change in the nature of the predominant
EXPERIMENTAL SECTION
■
t
t
3−
resting state that becomes [PW O ( BuSiO) VO(O Bu)]
General Procedures. The lacunary polyoxotungstates,
1
0
36
2
6
4
64
(
see above): the replacement of the alkoxide by the olefin may
K [α-A-PW O ],
Na [α-B-SbW O ],
and Cs [γ-
9
9
34
9
9
33
7
6
5,66
therefore impose a kinetic dependence toward [olefin] (this
PW O ],
and their silanol derivatives, (n-Bu N) [(α-A-
4 3
67
10
36
t
byproduct inhibition is known and is described well in this type
PW O )( BuSiOH) ] (TBA-1), (n-Bu N) [(α-B-SbW O )-
9 34 3 4 3 9 33
68
6
2
t
of reaction). This deviation in the kinetics at a lower
concentration of TBHP (thus at a higher concentration of
( BuSiOH) ] (TBA-1′), and (n-Bu N) [(γ-PW O )-
3
4
3
10 36
t
16
( BuSiOH) ] (TBA-2), respectively, were prepared as
2
t
BuOH) may also be consistent with the presence of an
reported in the literature. All manipulations were conducted
under an inert atmosphere using standard Schlenk techniques.
increasing amount of released oxidovanadium species [VO-
63
(
OR) ] that will influence the kinetics of the overall reaction.
tert-Butyl hydroperoxide (TBHP, 5.5 M in decane), Cl VO, and
3
3
7
420
DOI: 10.1021/acscatal.5b01878
ACS Catal. 2015, 5, 7415−7423

ACS Catalysis
Research Article
(iPrO) VO were purchased from Aldrich or Strem and used as
Synthesis of TBA [PW O ( BuSiO) VO( PrO)] (TBA-4).
t
i
3
3
10 36
2
1
31
1
t
received. H NMR and P{ H} NMR spectra were recorded at
room temperature in 5 mm outside diameter tubes on a Bruker
AvanceII 300 spectrometer equipped with a QNP probehead or
on a Bruker AvanceIII 600 spectrometer equipped with a
Dehydrated TBA [PW O ( BuSiOH) ] (2) (402 mg, 0.12
3 10 36 2
mmol) was placed in a Schlenk tube. Under an argon
atmosphere, freshly distilled acetonitrile (4 mL) was then
added followed by 30 μL (0.13 mmol) of ( PrO) VO at room
temperature and the solution was stirred overnight. The
resulting greenish solution was then layered by 11 mL of freshly
distilled diethyl ether. After 4 days, the formation of crystals
was observed, the solution was removed, and the crystalline
i
3
5
1
183
BBFO probehead. V (157.9 MHz) NMR and W (25 MHz)
NMR spectra were recorded on a Bruker AvanceIII 600
spectrometer equipped with a low-frequency BBO probehead
1
at room temperature in 10 mm outside diameter tubes. For H,
chemical shifts are referenced with respect to tetramethylsilane
by using the solvent signals as a secondary standard. For
P{ H}, chemical shifts were measured by the substitution
solid was washed with diethyl ether and dried under vacuum:
1
153 mg (37% yield); H NMR (CD CN, 300.13 MHz, 300 K)
3
3
1
1
3
δ 5.68 (sept, J = 6 Hz; 1H, (CH ) CHO-V), 3.17 (m, 24 H,
3
2
3
method and are given with respect to 85% H PO . With regard
NCH
2
TBA), 1.66 (m, 24 H, CH
2
TBA), 1.53 (d, J = 6 Hz;
3
4
183
to W NMR, the chemical shifts are given with respect to a 2
M Na WO aqueous solution and were determined by the
6H, (CH ) CHO-V), 1.43 (m, 24 H, CH TBA), 1.15 (s, 18 H,
3 2 2
t
31
CH BuSi), 1.02 (t, 36 H, CH TBA); P NMR (CD CN,
2
4
3
3
3
51
substitution method using a saturated D O solution of
121.5 MHz, 300 K) δ −15.05 (s); V NMR (CD CN, 105.2
2
3
tungstosilicic acid H SiW O as a secondary standard (δ
MHz, 300 K) δ −692 (ω = 60 Hz). Elemental analysis calcd
4
12 40
1/2
−
103.8). Elemental analyses were performed by the Service de
(%) for C H N O PSi VW (3501.2): C, 20.24; H, 3.83;
59
133
3
40
2
10
microanalyses of the ICSN-CNRS (Gif-sur-Yvette, France).
N, 1.20. Found: C, 20.13; H, 3.80; N, 1.14.
t
Dehydration of TBA [PW O ( BuSiOH) ] (TBA-2).
X-ray Crystallography. A single crystal of each compound
was selected, mounted onto a cryoloop, and transferred in a
cold nitrogen gas stream. Intensity data were collected with a
Bruker Kappa-APEXII diffractometer with graphite-monochro-
mated Mo Kα radiation (λ = 0.71073 Å). Data collections were
performed with the APEX2 suite (Bruker). Unit cell parameter
refinement, integration, and data reduction were performed
with SAINT (Bruker). SADABS (Bruker) was used for scaling
and multiscan absorption corrections.
3
10 36
2
t
Samples of TBA [PW O ( BuSiOH) ] were dehydrated by
3
10 36
2
−3
being heated at 210 °C for 3 h under dynamic vacuum (101
mbar) and stored under an inert atmosphere before use: H
NMR (CD CN, 300.13 MHz, 300 K) δ 5 (s, 2H, SiOH), 3.2
3
(
m, 24 H, NCH TBA), 1.6 (m, 24 H, CH TBA), 1.4 (m, 24
2
2
t
H, CH TBA), 1.1 (s, 18 H, CH BuSi), 1.0 (t, 36 H, CH
TBA); P NMR (CD CN, 121.5 MHz, 300 K) δ −14.9.
2
3
3
31
3
t
Synthesis of TBA [PW O ( BuSiO) VO] (TBA-3). One
3
9
34
3
t
69
gram (0.306 mmol) of TBA [PW O ( BuSiOH) ] (1) was
In the WinGX suite of programs, the structures were
3
9
34
3
70
71
placed in a Schlenk tube, and under an argon atmosphere,
freshly distilled acetonitrile (20 mL) was added. The solution
determined with either Sir92 or SUPERFLIP and refined by
72
full-matrix least-squares methods using SHELXL-14. Non-
hydrogen atoms were essentially refined anisotropically. In
TBA-3, disordered carbon atoms of tert-butyl moieties were
refined isotropically. In TBA-4, ADPs of TBA cations were
restrained to be similar to or to follow a rigid bond condition.
Some C−C distances were also restrained to be similar in the
latter compound. Hydrogen atoms were placed at calculated
positions.
was cooled to 0 °C, and 32 μL (0.337 mmol) of Cl VO was
3
added. The resulting reddish solution was allowed to warm to
room temperature to give a colorless solution. After being
stirred at room temperature overnight, the solution was layered
with diethyl ether to afford crystallization of the compound.
The crystals were filtered off, washed with diethyl ether, and
1
dried under vacuum: 730 mg (72% yield); H NMR (CD CN,
3
3
2
00.13 MHz, 300 K) δ 3.16 (m, 24 H, NCH TBA), 1.66 (m,
CCDC 1414350 and 1414351 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Catalytic Olefin Epoxidation. To a screw-capped Schlenk
tube (equipped with a rotaflo-type valve) were added under
argon the catalyst (50 μmol), acetonitrile (3 mL), the olefin
(0.5 mmol, 0.15 M), and tert-butyl hydroperoxide (0.75 mmol,
0.22 M). The mixture was stirred while being heated at the
desired temperature (300 or 343 K) for 18 h. Dilution by
diethyl ether gave a turbid suspension from which the solid may
be taken off (by filtration or centrifugation). Conversions were
assigned by NMR analysis.
2
2
4 H, NCH CH TBA), 1.43 (sext, J = 7.2 Hz; 24 H,
2
2
H,H
t
2
CH CH TBA), 1.17 (s, 27 H, CH BuSi), 1.0 (t, J = 7.2
Hz; 36 H, CH CH TBA); { H} P NMR (CD CN, 121.5
MHz, 300 K) δ −15.2; { H} C NMR (CD CN, 75.5 MHz,
3
2
3
3
H,H
1
31
2
3
3
1
13
3
00 K) δ 66.3 (C(CH ) ), 59.4 (NCH ), 27.1 (C(CH ) ), 24.4
3 3 2 3 3
1
83
(
(
(
NCH CH ), 20.4 (CH CH ), 14.0 (CH CH ); W NMR
CH CN/CD CN, 12.5 MHz, 300 K) δ −80.7 (3W), −157.9
6W); Si NMR (CD CN, 59.6 MHz, 300 K) δ −55 (ω
2 2 2 3 2 3
3
3
29
=
3
1/2
5
1
8
0 Hz, broad signal due to the coupling with quadripolar V);
5
1
V NMR (CD CN, 78.9 MHz, 300 K) δ − 800 (ω = 240
3
1/2
Hz). Elemental analysis calcd (%) for C H N O PSi VW
6
0
135
3
38
3
9
(3327.4): C, 21.66; H, 4.09; N, 1.26. Found: C, 21.71; H, 3.99;
N, 1.21.
Monitoring by NMR. A screw-capped 5 mm outside
diameter NMR tube under argon was charged with complex
t
TBA [SbW O ( BuSiO) VO] (TBA-3′). The title com-
3
9
33
3
pound was prepared following the same procedure as for the
TBA-4 (4.8 mg, 1.37 μmol) and dry acetonitrile-d (0.5 mL). 3-
3
1
synthesis of TBA-3: H NMR (CD CN, 300.13 MHz, 300 K) δ
Methyl-2-buten-1-ol (17 μL, 166 μmol) and TBHP (5.5 M in
decane, 30 μL, 165 μmol) were successively added to the
3
3
1
.12 (m, 24 H, NCH TBA), 1.63 (m, 24 H, NCH CH TBA),
2 2 2
2
1
31
51
.38 (sext, J = 7.2 Hz; 24 H, CH CH TBA), 1.25 (s, 27 H,
mixture. Each step was monitored at 300 K by H, P, and V
NMR on a Bruker AvanceIII 600 spectrometer equipped with a
BBFO probehead. The acquiring parameters (relaxation delays)
have been optimized to ensure an accurate integration
measurement. The NMR data set (integral measurements)
was fitted to a polynomial function to generate a full profile.
H,H
2
3
t
2
29
CH BuSi), 0.98 (t, J = 7.2 Hz; 36 H, CH CH TBA); Si
3
H,H
2
3
NMR (CD CN, 59.6 MHz, 300 K) δ −51 (ω = 100 Hz,
broad signal due to the coupling with quadripolar V);
3
1/2
5
1
183
W
51
(
CD CN, 12.5 MHz, 300 K) δ −117 (3W), −119.2 (6W); V
3
NMR (CD CN, 78.9 MHz, 300 K) δ −730 (ω = 200 Hz).
3
1/2
7
421
DOI: 10.1021/acscatal.5b01878
ACS Catal. 2015, 5, 7415−7423

ACS Catalysis
Research Article
The functions were then differentiated to obtain rate versus
time data (see Figure S8).
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.5b01878.
■
*
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thermal treatment, evolution of the V NMR of
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methyl-2-buten-1-ol, close-up of the plot of epoxide
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AUTHOR INFORMATION
Corresponding Author
Fax: +33 1 44 27 38 41. Telephone: +33 1 44 27 35 22. E-
■
*
(32) Carabineiro, H.; Villanneau, R.; Carrier, X.; Herson, P.; Lemos,
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(
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