Quantitatively Analyzing Metathesis Catalyst Activity and Structural Features in Silica-Supported Tungsten Imido-Alkylidene Complexes

Article abstract of DOI:10.1021/jacs.5b03344

A broad series of fully characterized, well-defined silica-supported W metathesis catalysts with the general formula [(≡SiO)W(=NAr)(=CHCMe₂R)(X)] (Ar = 2,6-iPr₂C₆H₃ (Ar_iPr), 2,6-Cl₂C₆H₃ (Ar_Cl), 2-CF₃C₆H₄ (Ar_CF3), and C₆F₅ (Ar_F5); X = OC(CF₃)₃ (OtBu_F9), OCMe(CF₃)₂ (OtBu_F6), OtBu, OSi(OtBu)₃, 2,5-dimethylpyrrolyl (Me₂Pyr) and R = Me or Ph) was prepared by grafting bis-X substituted complexes [W(NAr)(=CHCMe₂R)(X)₂] on silica partially dehydroxylated at 700 C (SiO_2-(700)), and their activity was evaluated with the goal to obtain detailed structure-activity relationships. Quantitative influence of the ligand set on the activity (turnover frequency, TOF) in self-metathesis of cis-4-nonene was investigated using multivariate linear regression analysis tools. The TOF of these catalysts (activity) can be well predicted from simple steric and electronic parameters of the parent protonated ligands; it is described by the mutual contribution of the NBO charge of the nitrogen or the IR intensity of the symmetric N-H stretch of the ArNH₂, corresponding to the imido ligand, together with the Sterimol B₅ and pK_a of HX, representing the X ligand. This quantitative and predictive structure-activity relationship analysis of well-defined heterogeneous catalysts shows that high activity is associated with the combination of X and NAr ligands of opposite electronic character and paves the way toward rational development of metathesis catalysts.

Full text of DOI:10.1021/jacs.5b03344

Article
pubs.acs.org/JACS
Quantitatively Analyzing Metathesis Catalyst Activity and Structural
Features in Silica-Supported Tungsten Imido−Alkylidene Complexes
†
,∥
‡,∥
,‡
†,∥,⊥
‡
§
Victor Mougel, Celine B. Santiago, Pavel A. Zhizhko,
Elizabeth N. Bess, Jeno Varga,
§
,†
Georg Frater, Matthew S. Sigman,* and Christophe Coper
́
et*
†
Department of Chemistry and Applied Biosciences, ETH Zu
̈ ̈
rich, Vladimir Prelog Weg 2, 8093 Zurich, Switzerland
‡
Department of Chemistry, University of Utah, 315 South 1400 East, Salt Lake City, Utah 84112, United States
§XiMo Hungary, Zahony u. 7 H-1031 Budapest, Hungary
S Supporting Information
*
ABSTRACT: A broad series of fully characterized, well-defined
silica-supported W metathesis catalysts with the general formula
[
(
(
(≡SiO)W(NAr)(CHCMe R)(X)] (Ar = 2,6-iPr C H
Ar ), 2,6-Cl C H (Ar ), 2-CF C H (Ar ), and C F
iPr 2 6 3 Cl 3 6 4 CF3 6
2
2
6
3
5
Ar ); X = OC(CF ) (OtBu ), OCMe(CF ) (OtBuF6),
F5
3
3
F9
3 2
OtBu, OSi(OtBu) , 2,5-dimethylpyrrolyl (Me Pyr) and R = Me
3
2
or Ph) was prepared by grafting bis-X substituted complexes
[
W(NAr)(CHCMe R)(X) ] on silica partially dehydroxylated
), and their activity was evaluated with the
2‑(700)
2
2
at 700 °C (SiO
goal to obtain detailed structure−activity relationships. Quantita-
tive influence of the ligand set on the activity (turnover frequency,
TOF) in self-metathesis of cis-4-nonene was investigated using
multivariate linear regression analysis tools. The TOF of these catalysts (activity) can be well predicted from simple steric and
electronic parameters of the parent protonated ligands; it is described by the mutual contribution of the NBO charge of the
nitrogen or the IR intensity of the symmetric N−H stretch of the ArNH , corresponding to the imido ligand, together with the
2
Sterimol B and pK of HX, representing the X ligand. This quantitative and predictive structure−activity relationship analysis of
5
a
well-defined heterogeneous catalysts shows that high activity is associated with the combination of X and NAr ligands of opposite
electronic character and paves the way toward rational development of metathesis catalysts.
INTRODUCTION
bipyramidal (TBP) and square planar (SP) metallacycle
intermediates, and (iii) the occurrence of several simultaneous
deactivation pathways, such as dimerization of alkylidene
species or β-hydride elimination of metallacycles, as well as
catalyst poisoning by impurities (Scheme 1).
■
Over the last 50 years, olefin metathesis has transformed the
way in which a diverse array of compounds, ranging from
petrochemicals to advanced pharmaceuticals to polymers, can
1
be efficiently prepared. A pivotal point in the development of
these transformations came through the work of Y. Chauvin,
who demonstrated that alkylidenes and metallacyclobutanes
were key reaction intermediates in the metathesis catalytic
Despite these studies, the ligand effects and structural factors
that influence the catalytic activity still remain unclear. For
instance, it has been demonstrated that for d high-oxidation
0
2
cycle. This pioneering work led to the design of now well-
state catalysts of the general formula (X)(Y)M(E)(CHR)
0
known molecularly defined catalysts, based on either d (Mo,
[
X, Y = anionic ligands and ME = Mo/WNR or Re≡CR],
6
2,3
W and Re) or d (Ru) transition-metal alkylidene complexes.
the dissymmetry at the metal center (X ≠ Y) is key for high
activity because it facilitates coordination/decoordination of the
This mechanism has also influenced the latest development of
heterogeneous metathesis catalysts having well-defined metal
sites whose performances can now exceed their homogeneous
olefins and also destabilizes the most stable metallacyclobutane
5c,g
intermediates.
This model clearly explains the high activity
4
counterparts. Computational investigations have also provided
4d,h−k
3j,8
of both silica-supported
and MAP catalysts. Yet, it has
insights allowing for tremendous advances in the understanding
5
,6
also been revealed that having two strong electron withdrawing
anionic ligands (fluorinated phenoxy, alkoxy, and/or siloxy)
also leads to very active catalysts, indicating that other factors
of active site structure and reactivity of metathesis catalysts.
0
For the broad class of d group 6−7 homogeneous and
heterogeneous catalysts, the overall performance results from
the combination of numerous factors acting in parallel and
4j,9
are important to obtain highly active catalysts. Therefore,
4
a,b,d−f,h−k,5,7
affecting both activity and stability.
i) the barriers for olefin coordination and decoordination, (ii)
the relative stabilities and different behavior of trigonal
These include
(
Received: March 31, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b03344
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Journal of the American Chemical Society
Article
0
Scheme 1. Reaction Pathway Proposed for d Metal Alkene
Metathesis Catalysts
This can be readily quantified by parameters that measure the
pK and Sterimol B values of X and the NBO charge on N of
a
5
the parent organic molecules HX and ArNH , respectively.
2
RESULTS AND DISCUSSION
Synthesis of the Molecular Precursors. Complexes of
■
the NAr series (1a′−1e′) were prepared as previously
iPr
4
j,10
described.
Similarly, a new series of complexes bearing
12
1
1
NArCl, NArCF3, and NArF5 imido ligands were prepared by
salt metathesis of the corresponding bis(triflate) complexes
with 2 equiv of Li or K alkoxide, siloxide, or pyrrolide (eq 1).
there is a critical need to better understand and determine the
origin of activity in this general catalyst family.
Here, we have engaged in a systematic investigation of
catalyst structural properties that govern catalytic activity for a
series of supported well-defined catalysts of the general formula
We noted that the alkoxide and siloxide complexes bearing
electron-withdrawing imido ligands display lower stability than
their AriPr analogues. This feature is especially noteworthy in
the ArF5 series, preventing the synthesis of complexes 4c′ and
(
≡SiO)W(NAr)(CHCMe R)X, bearing various arylimido
2
4
d′, which were observed by NMR but could not be isolated
groups (NAr), namely 2,6-iPr C H (NAr ), 2,6-Cl C H
2
6
3
iPr
2
6
3
due to their instability (see Supporting Information for details).
Preparation of Supported Complexes. The supported
catalysts were prepared by contacting a benzene suspension of
silica partially dehydroxylated at 700 °C (SiO2‑(700), 0.26 mmol
SiOH/g) with a benzene solution of a molecular precursor
(
NAr ), 2-CF C H (NAr ), C F (NAr ), and anionic X
Cl 3 6 4 CF3 6 5 F5
ligands (X = OC(CF ) (OtBu ), OCMe(CF ) (OtBu ),
3
3
F9
3 2
F6
OtBu, OSi(OtBu) , 2,5-dimethylpyrrolyl (Me Pyr)), as pre-
3
2
sented in Scheme 2. To accomplish this, we have utilized
(
1.05 equiv with respect to the total amount of surface silanol
Scheme 2. Preparation of Well-Defined Silica-Supported
Tungsten Imido−Alkylidene Catalysts
a
groups in the silica sample) at ambient temperature for 12 h.
For the temperature sensitive NArF5 compounds 4a′−4b′,
grafting was performed in toluene, and the solution of
molecular precursor was precooled to −40 °C before addition.
Contact time in these cases was reduced to 30 min. The solid
material was decanted, washed with fresh benzene (toluene for
the low temperature grafting), and dried under high vacuum
5
(
ca. 10− mbar) for several hours.
With the use of IR spectroscopy to monitor grafting, a
−1
decrease of the isolated silanol band at 3747 cm and an
−1
appearance of broad bands at 3700−3500 cm were observed
in all cases; the latter can be associated with remaining silanols
interacting with the ligands of the grafted complex. Detection of
the corresponding alcohol, silanol, or pyrrole in the washing
solution indicates the substitution of the X ligand by a surface
siloxy group upon protonolysis with the surface silanol. The
1
amount of HX released during grafting was measured by H
NMR with ferrocene as an internal standard, indicating
elimination of 0.7−0.9 equiv of HX per initial surface silanol
group. Reduced values were observed for OSi(OtBu)
3
complexes, especially for catalysts having a low percentage of
grafting (vide infra), likely due to the liberated HOSi(OtBu)3
that was retained on the surface by strong interactions with the
remaining surface silanols. Typically, siloxide complexes give
rise to lower metal content in the sample (2.5−2.7 wt %
compared to 3.4−4.1 wt % for other complexes, Table 1), with
excess of the unreacted complex being observed in washing
solutions by NMR. We associate this behavior mainly with
significant steric bulk of this ligand, but also with its propensity
to interact strongly with surface silanols.
13
a
Designations of molecular precursors are noted with a prime (′).
multivariate linear regression analysis tools, which allow
identification of discrete molecular features that quantitatively
describe turnover frequency (TOF). The results of this
investigation are reported and point to the interplay between
the X and NAr ligands in directing the activity of the catalyst as
evaluated by TOF. In particular, high activity is associated with
the combination of X and NAr ligands of opposing electronic
character (one electron withdrawing and the other electron
donating), which is modulated by the sterics of the X ligand.
The structure of all surface complexes was further confirmed
1
with elemental analysis and solid-state NMR spectroscopy ( H,
13
19
C, and F in case of NArF5 series). The observed W/C/H/
N/F ratios (Table 1) and NMR chemical shifts (Supporting
B
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Table 1. Mass Balance and Elemental Analyses of Grafted
Complexes
times, affording reproducible results. For the detailed
procedures of preparation of cis-4-nonene and performing the
catalytic tests, see the Supporting Information.
The reaction leads to an equilibrium mixture of cis/trans 4-
octenes, 4-nonenes, and 5-decenes, corresponding to ca. 50%
conversion of nonene. Two measured outputs were typically
used to evaluate the catalytic performance: initial TOF
measured after 3 min of the reaction and time required to
reach equilibrium. No initiation period was observed with any
of the catalysts. Thus, the former measure reflects the activity
a
b
a,
c
a,
c
a,
c
a,c
cat.
W, wt %
HX
C/W
H/W
N/W
F/W
2
2
2
2
2
3
3
3
3
4
4
4
a
b
c
d
e
a
b
c
e
3.87
4.14
2.81
3.90
4.15
3.62
3.90
2.27
3.54
3.46
3.08
4.13
0.7
0.7
0.6
0.8
0.9
0.8
0.8
0.1
0.9
0.9
0.7
0.9
20(20)
21(20)
27(28)
20(20)
22(22)
23(21)
22(21)
36(29)
22(23)
23(24)
20(20)
14(15)
19(18)
49(42)
27(24)
23(23)
23(16)
23(19)
63(43)
23(24)
20(22)
17(15)
21(20)
1(1)
1(1)
2(1)
1(1)
3(2)
1(1)
1(1)
1(1)
2(2)
1(1)
1(1)
2(2)
9(9)
6(6)
−
−
−
d
14(12)
9(9)
(
(
TOF3 min), and the latter is also related to catalyst stability
time to thermodynamic conversion).
2(3)
3(3)
Interested in investigating the effect of particular ligand
combinations on catalytic activity, we focus our discussion on
TOF3 min (Table 2). The detailed catalytic data is found in the
d
a
13(14)
11(11)
5(5)
d
b
e
22(22)
a
b
According to elemental analysis. Amount of HX released per surface
Table 2. Catalytic Activity of Grafted Complexes in Self-
Metathesis of cis-4-Nonene
c
silanol group determined by NMR. Theoretical value given in
parentheses. Grafting performed at low temperature.
a
d
NAriPr
NArCl
NArCF3
NArF5
OtBu
5
9
45
55
50
51
49
−
67
59
44
33
−
81
−
Information) correspond to those expected in accordance with
Scheme 2.
Me Pyr
2
OSi(OtBu)3
OtBuF6
41
A typical side-reaction that is reported to accompany grafting
of alkylidene complexes onto silica surface is protonation of the
alkylidene ligand by the surface silanol group leading to
metathesis-inactive species (≡SiO)W(NAr)(CH CMe R)X .
75
38
OtBuF9
115
25
a
0.8 M toluene solution, batch reactor, 30 °C, catalyst loading 0.1 mol
W (calc. on the basis of elemental analysis); activity is expressed as
initial TOF after 3 min of reaction (TOF3 min, min ).
2
2
2
%
Presence of these species can be assessed by the reduced
amount of HX released during grafting and increased C, H, F,
etc. content in elemental analysis data, but these criteria may be
−
1
misleading in the case of OSi(OtBu) and other ligands that can
3
Supporting Information. The data presented in Table 2
indicates a bifurcated trend when the entire series of various
imido and X ligands is assessed. Within the NAriPr series,
increases in catalytic activity are associated with the enhanced
remain on the surface (see above). Unfortunately, these species
are also difficult to observe by solid-state NMR without using
labeled molecular precursors. We have previously observed
significant amounts of protonation for W(NAr )(CHtBu)-
iPr
4j
electron-withdrawing character of the X ligand. However, the
X complexes with the most electron-donating X ligands in the
2
4j
inverse correlation is observed for NArF5 where decreasing
electron-withdrawing ability improves catalytic activity. With
NArCl and NArCF3 spanning the transition between these two
extreme scenarios, two main trends combining effects of both X
and imido ligands on catalytic activity are observed.
set, namely OtBu and OtBu , as well as for aryloxide complex
F3
4h
W(O)(CHCMe Ph)(OHMT) . As was shown for OtBu
2
2
complex 1d′, the amount of protonated species could be
4j
reduced by carrying out grafting at low temperature. This
approach was successfully applied here to prepare complex 2d,
but preliminary study of grafting of 3d′ and 4d′ showed almost
no tBuOH release and the materials obtained displayed
negligible catalytic activity toward olefin metathesis. We
associate this behavior with loss of the alkylidene ligand via
protonation. Thus, the ability of these complexes to undergo
protonation seems to correlate with their instability in solution
(1) The activity gradually decreases throughout the series
with X = OtBuF9 and increases for the series with X =
Me Pyr when changing from NAriPr to more electron-
2
withdrawing imido groups.
(2) While moving across the NAriPr series, the activity
correlates with the electron-withdrawing character of the
X ligand, but the opposite trend is observed within the
NArCF3 and NArF5 series. Catalysts bearing the NArCl
imido group seem to be an intermediate case; they do
not display any pronounced maximum activity within the
series.
(
see Supporting Information).
Catalytic Activity. All of the grafted catalysts were tested in
self-metathesis of cis-4-nonene according to the previously
4j
established procedure (Scheme 3). It should be noted that the
performances of olefin metathesis catalysts are strongly affected
by the purity of the substrate. All of the data presented was
obtained by strictly applying the same conditions of substrate
purification and catalyst manipulation and using a single batch
of purified cis-4-nonene. The tests were repeated multiple
Overall, these data indicate a cooperative influence of NAr
and X ligands on catalytic activity.
Metallacyclobutane Intermediates. Previously, we were
able to spectroscopically characterize surface metallacyclobu-
tane intermediates obtained by exposure of grafted tungsten
1
3
Scheme 3. cis-4-Nonene Self-Metathesis
imido−alkylidene complexes 1a−1e to bis( C-labeled)-
4e,j
ethylene. Within the NAr series, we found that the ratio
iPr
between TBP and SP metallacycles (that could be estimated on
13
the basis of C solid-state NMR spectroscopy) could serve as a
measure of σ-donating ability of X ligand, which correlated with
the catalytic activity of grafted complexes. Herein, we
investigated whether this trend is also observed when changing
C
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Article
the imido ligand. Several grafted complexes of NAr and
Cl
1
3
NArCF3 series were exposed to bis( C-labeled)ethylene under
the previously reported conditions. TBP/SP ratios of surface
metallacyclobutanes were determined from solid-state NMR.
The results summarized in Table 3 demonstrate that the imido
Table 3. TBP/SP Ratio of Metallacyclobutanes Obtained by
1
3
Exposure of Grafted Complexes to C-Labeled Ethylene
NAriPr
NArCl
NArCF3
OtBuF9
OtBuF6
OtBu
88/12
67/33
0/100
86/14
67/33
0/100
92/8
61/39
−
ligand in these cases has no influence on the TBP/SP ratio,
which was primarily dependent on the nature of the X ligand.
Accordingly, with this broader series of catalysts, no direct
correlation between TBP/SP ratio and catalytic activity of
grafted complex is observed (Tables 2 and 3).
Parameterization. The reversal of TOF trends observed
for X = Me Pyr as compared to the weaker σ-donating ligand X
2
=
OtBuF9 (Table 2), along with increasingly electrophilic imido
groups, suggests a cooperative effect between the X and NAr
moieties in tuning the catalyst activity. To further understand
how this cooperative effect impacts the activity (TOF),
potentially influencing the design of new tungsten imido−
14
alkylidene metathesis catalysts, we analyzed electronic and
1
5
steric parameters that were hypothesized to be significant for
describing molecular changes to X and NAr. From a panel of
the molecular descriptors including IR frequencies and
intensities, NBO charges,
Sterimol values derived from the parent organic molecules,
1
4
16,17
pK , proton affinity, and
a
1
8
Figure 1. (A) Parameters hypothesized to be significant in
quantitatively describing the TOF. (B) Multivariate linear regression
model to predict TOF.
HX and ArNH (Figure 1A), multivariate linear regression
2
analysis was employed to relate these normalized parameters to
the TOF of the tungsten imido−alkylidene catalysts. With the
use of a training set (red squares, Figure 1B), regression
analysis afforded a model composed of two interaction terms:
(
NBON,ArNH2 = −0.811). While catalyst 4e, which incorporates
the least acidic X ligand, Me Pyr (pK = 17.97 of the amine),
2
a,HX
results in its highest TOF when combined with the most
electron-poor aryl imido of the series, NArF5 (NBON,ArNH2
the NBO charge of the ArNH2 nitrogen (NBON,ArNH2
)
combined with a steric term, Sterimol B of HX (B5,HX), and
=
5
the same NBO charge combined with the pK of HX (pKa,HX).
−0.779). Two of the highest TOFs were observed using X and
NAr ligand combinations with opposite NBON,ArNH2 and pKa,HX
values. This is consistent with the inclusion of the interaction
term in the model (NBON,ArNH2)(pKa,HX), which supports the
empirical observation of an interplay between the electronic
contribution of these ligands that tunes the catalyst and affects
the TOF. Furthermore, we also observed that the NBON,ArNH2
measurement correlates well with the intensity of the
symmetric N−H stretch, IN−H(s),ArNH2, a descriptor for changes
in dipole moment as well as conjugative effects (see Supporting
Information, Figurse S44 and S45).
a
Further substantiation of this regression model was provided
2
via external validation (blue squares, Figure 1B, R = 0.95, slope
=
1.01, intercept = 1.2). As depicted in Figure 1, the model
displays accurate and predictive capabilities for estimating the
TOF in olefin metathesis reactions involving silica-supported
tungsten imido−alkylidene catalysts.
The NBO charge on the ArNH nitrogen, which is obtained
2
through natural population analysis according to Seybold and
co-workers, was identified to be one of the most correlative to
pK ’s of anilines out of several empirical and quantum chemical
a
19
methods to assign partial charges. Since the NBON,ArNH2
From the obtained regression model, the other interaction
term (NBON,ArNH2)(B5,HX) indicates that the size of the X
ligand is also influencing the TOF of the tungsten imido−
accounts for the electron density on the ArNH nitrogen
2
through identification of which orbitals were involved in
2
0
bonding, it represents the donating ability of the imido
nitrogen toward the tungsten metal center, which then further
alkylidene catalyst. For example, the ligands X = OSi(OtBu)
3
and X = Me
toward a more electrophilic NAr. However, this increase with
Pyr resulted in the same increasing TOF trend
2
21
delocalizes to the alkylidene carbon. The dipole being
established between the tungsten and the alkylidene carbon is
stabilized by the electron-withdrawing nature of the X ligand,
which can be quantified by the pKa,HX. Of the catalysts
evaluated, the highest TOF was observed for the tungsten
imido−alkylidene catalyst 1a, composed of the more acidic X
the ligand X = OSi(OtBu) does not result from an increased
3
pKa,HX but can be attributed to its large size in terms of Sterimol
B .
5
CONCLUSION
Following the preparation of a wide range of silica-supported
tungsten imido−alkylidene metathesis catalysts [(≡SiO)W(
■
ligand, OtBu (pK
= 4.91 of the alcohol), in combination
F9
a,HX
with the NAr with the most electron-rich substituent, NAriPr
D
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NAr)(CHCMe R)X] bearing various NAr and X supporting
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2
■
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the series was observed for the catalysts bearing NAr and X
ligands with opposing electronic properties. This cooperative
effect was quantitatively exhibited through multivariate linear
regression analysis, which demonstrated that the NBO charge
(
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parameters of the parent organic molecule HX and ArNH , i.e.,
2
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a
5
intensity of the symmetric N−H stretch of ArNH . Further
2
4
studies to improve the TOF of these tungsten alkylidene
catalyst will be focused on using more electron-rich imido
groups as well as expanding the scope on sterically differ-
entiated X ligands. While previous computational studies
describe how the steric and electronic nature of the ligands
could affect the activity of imido−alkylidene catalysts, this study
highlights the synergy and provides a quantitative relationship
between the electronic and steric influences of X and NAr and
the activity of supported metathesis catalysts, thus paving the
way to develop improved catalysts in a more rational way.
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ASSOCIATED CONTENT
■
*
S
Supporting Information
́
Synthesis and characterization of molecular precursors and
grafted complexes, IR and solid-state NMR spectra, catalytic
data, and computational details. The Supporting Information is
available free of charge on the ACS Publications website at
DOI: 10.1021/jacs.5b03344.
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AUTHOR INFORMATION
■
(
(
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Corresponding Authors
ccoperet@inorg.chem.ethz.ch
sigman@chem.utah.edu
́
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*
*
́
et, C.; Eisenstein, O.
Author Contributions
∥
́
These authors contributed equally.
1
́
Notes
The authors declare no competing financial interest.
́
⊥
On leave from A. N. Nesmeyanov Institute of Organoelement
Solans-Monfort, X.; Clot, E.; Eisenstein, O.; Lesage, A.; Emsley, L. J.
Am. Chem. Soc. 2008, 130, 5886−5900. (e) Leduc, A. M.; Salameh, A.;
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ACKNOWLEDGMENTS
■
This article is dedicated to the memory of Yves Chauvin. We
thank Mr. Maxence Valla for assistance with the solid-state
NMR measurements. We acknowledge Dr. Csaba Hegedus and
Dr. Levente Ondi for help and discussion with the synthesis of
some of the molecular precursors. The Center for High
Performance Computing at the University of Utah is gratefully
acknowledged. V.M. is supported by a ETH fellowship
cofunded ETH Zurich-Marie Curie action for people, FEL-
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8 12-2). We also thank the NSF for partial support of this
work (CHE-1361296).
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