Activation of Low-Valent, Multiply M-M Bonded Group VI Dimers toward Catalytic Olefin Metathesis via Surface Organometallic Chemistry

Article abstract of DOI:10.1021/acs.organomet.9b00787

Olefin metathesis is a broadly employed reaction with applications that range from fine chemicals to materials and petrochemicals. The design and investigation of olefin metathesis catalysts have been ongoing for over half a century, with advancements made in terms of activity, stability, and selectivity. Immobilization of organometallic complexes onto solid supports such as silica or alumina is a promising strategy for catalyst heterogenization, often resulting in increased activity and stability. Consequently, a broad range of early transition metal catalysts bearing alkyl, oxide/alkoxide, and amide ligands have been grafted onto silica and their reactivities investigated. Herein, we report a series of silica-supported tungsten and molybdenum dimers (X₃MMX₃, where M = W and Mo; X = neopentyl, tert-butoxide, and dimethyl amide) and their reactivities toward catalytic olefin metathesis. Dynamic nuclear polarization (DNP)-enhanced solid-state nuclear magnetic resonance (SSNMR), diffuse reflectance infrared Fourier transform (DRIFT), UV resonance Raman, and X-ray absorption (XAS) spectroscopies suggest that upon heterogenization the dimers bind to the surface in a monopodal fashion, with the MM triple bond remaining intact. These structural assignments were further corroborated by density functional theory (DFT) calculations. While the homogeneous dimer counterparts are inert, the supported low-valent alkyl W and Mo dimers become active for the disproportionative self-metathesis of propylene to ethylene and butenes and 4-nonene to 4-octene and 5-decene under mild conditions. The lack of activity observed for the free and supported tert-butoxide and dimethyl amide dimers likely suggests that the neopentyl groups are necessary for the formation of a putative alkylidene active species. The difference in reactivity between the free and supported dimers could be explained either by the lowering of the activation barrier of the complex through the electronic effects of the surface or by site isolation of catalytically relevant reactive intermediates.

Full text of DOI:10.1021/acs.organomet.9b00787

pubs.acs.org/Organometallics
Article
Activation of Low-Valent, Multiply M−M Bonded Group VI Dimers
toward Catalytic Olefin Metathesis via Surface Organometallic
Chemistry
́
́
Alon Chapovetsky, Ryan R. Langeslay, Gokhan Celik, Frederic A. Perras, Marek Pruski,
Magali S. Ferrandon, Evan C. Wegener, Hacksung Kim, Fulya Dogan, Jianguo Wen, Navneet Khetrapal,
Prachi Sharma, Jacob White, A. Jeremy Kropf, Alfred P. Sattelberger,* David M. Kaphan,*
and Massimiliano Delferro*
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ABSTRACT: Olefin metathesis is a broadly employed reaction
with applications that range from fine chemicals to materials and
petrochemicals. The design and investigation of olefin metathesis
catalysts have been ongoing for over half a century, with
advancements made in terms of activity, stability, and selectivity.
Immobilization of organometallic complexes onto solid supports
such as silica or alumina is a promising strategy for catalyst
heterogenization, often resulting in increased activity and stability.
Consequently, a broad range of early transition metal catalysts
bearing alkyl, oxide/alkoxide, and amide ligands have been grafted
onto silica and their reactivities investigated. Herein, we report a
series of silica-supported tungsten and molybdenum dimers
(X3MMX3, where M = W and Mo; X = neopentyl, tert-butoxide, and dimethyl amide) and their reactivities toward catalytic
olefin metathesis. Dynamic nuclear polarization (DNP)-enhanced solid-state nuclear magnetic resonance (SSNMR), diffuse
reflectance infrared Fourier transform (DRIFT), UV resonance Raman, and X-ray absorption (XAS) spectroscopies suggest that
upon heterogenization the dimers bind to the surface in a monopodal fashion, with the MM triple bond remaining intact. These
structural assignments were further corroborated by density functional theory (DFT) calculations. While the homogeneous dimer
counterparts are inert, the supported low-valent alkyl W and Mo dimers become active for the disproportionative self-metathesis of
propylene to ethylene and butenes and 4-nonene to 4-octene and 5-decene under mild conditions. The lack of activity observed for
the free and supported tert-butoxide and dimethyl amide dimers likely suggests that the neopentyl groups are necessary for the
formation of a putative alkylidene active species. The difference in reactivity between the free and supported dimers could be
explained either by the lowering of the activation barrier of the complex through the electronic effects of the surface or by site
isolation of catalytically relevant reactive intermediates.
attractive strategy for exploiting these features while benefiting
from the increases in stability, activity, and physical character-
istics afforded by the heterogeneous support.6−15 As such, there
has been a major push in recent years to investigate the
mechanisms and activities of well-studied organometallic
catalysts on silica.16−24 The catalytic performance of molecular
precursors often serves as a guideline for their selection for
heterogenization. Hence, there are a number of studies
highlighting the activity of supported early transition metal
INTRODUCTION
■
Olefin metathesis is one of the most widespread reactions
performed in the chemical industry with applications ranging
from production of fine chemicals to materials and petrochem-
ical processing.1,2 For example, propylene, globally the second
most extensively produced chemical by volume, is the
metathesis product of ethylene and 2-butene.3,4 Industrially,
propylene production is accomplished using heterogeneous
catalysts due to their robust stability, recyclability, and lower
separation costs.5 However, homogeneous catalyst systems offer
significant advantages over their heterogeneous counterparts.
Their well-defined structures allow for detailed mechanistic
studies and rapid design iteration toward performance
optimization. The incorporation of molecular catalyst motifs
onto solid supports such as silica, alumina, or zirconia is an
Special Issue: Organometallic Chemistry at Various
Length Scales
Received: November 18, 2019
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complexes bearing ancillary alkyl, oxide, and amide ligands
supported on silica.25−31 These include but are not limited to
alkylidene,27,32 imido-alkylidene,33,34 thiolate,26,35 oxo,27,36 and
hydride37,38 complexes of tungsten, molybdenum, tantalum, and
zirconium. In most of these examples, the supported metal
complex is in its highest oxidation state. To the best of our
knowledge, however, the only supported low valent tungsten
catalyst capable of effecting olefin metathesis is a supported
RESULTS AND DISCUSSION
Preparation of W2Np6 (Np = neopentyl, 1) and
■
Grafting onto SiO2−700. W2Np6 (Np = neopentyl, 1) was
prepared according to a modified literature procedure (see the
Supporting Information for details).41 A single crystal (Figure
S1), grown from toluene at −30 °C, was found suitable for single
crystal X-ray diffraction. The crystal structure of 1 revealed the
classic D3d arrangement with two tungsten atoms each
coordinated by three neopentyl ligands and joined together
via a multiple bond (Figure 1). The WW bond length is
́
organotungsten(IV) complex on silica, reported by Coperet and
co-workers.39,40 Those authors propose that reduction of the
silica supported W(VI) oxo generates a W(IV) species that can
be readily reoxidized by an olefin to generate a catalytically
relevant W(VI) alkylidene.
While the above studies paint a concise picture regarding the
reactivity of unbound and supported organometallic complexes,
there is a paucity of examples of systems bearing multimetallic
motifs. Chisholm-type dimers of the form M2X6 (where M = W
and Mo; X = alkyl, alkoxide, and amide) have structural features
that could have interesting implications for catalysis.41 The close
proximity of the two metal centers, coupled with the electron
reservoir of the metal−metal multiple bond, may allow for novel
binding and reactivity which can be advantageous for
catalysis.42,43 Studies targeting the chemical reactivity of free
and supported metal dimers toward unsaturated hydrocarbons
have been sporadic over the last few decades.44−49 For example,
upon exposure to ethylene, the W2(O-iPr)6 dimer forms a
bridging metallocyclic alkylidyne (Scheme 1).45 However,
isotopic labeling experiments indicate that no olefin metathesis
occurs under these conditions.
Scheme 1. Pathway for the Reaction of W2(O-iPr)6 with
Ethylene
Figure 1. (A) Solid-state structure of 1 viewed perpendicular to the W−
W bond. (B) Solid-state structure of 1 viewed down to the W−W bond
highlighting the 3-fold axis. Thermal ellipsoids are drawn at 50%
probability. Hydrogen atoms were omitted for clarity. Legend: tungsten
(blue), carbon (gray). (C) Chemisorption of 1 onto SiO2−700 to yield 1-
SiO2. Selected structural parameters: W(1)−W(1′) = 2.2440(5) Å,
W(1)−C(1) = 2.132(4) Å, C(1)−W(1)−W(1′) = 96.41(10)°, C(1)−
W(1)−C(1) = 118.77(4)°, C(1)−W(1)−W(1′)−C(1′) = 60.0(10)°.
W2(NMe2)6 is unreactive toward unsaturated hydrocarbons;
however, after deposition on silica it was shown to polymerize 1-
heptyne, suggesting that chemisorption on silica can impart
catalytic reactivity not observed for the molecular species.46
Postsynthetic ligand substitution of the dimethylamide ligands
with tert-butoxides was shown to produce an even more active
catalyst.47,48 The discrepancy between the reactivity of the
supported and unsupported tungsten dimers is likely due to
either the electronic and/or steric effects exerted by the silanol
ligand or the ability of the support to prevent the agglomeration
of the active species which can lead to deactivation.
In an early example of supported organometallics, W2Np6
(Np = neopentyl) was deposited onto silica and exhibited
activity for olefin metathesis.50 Unlike the unsupported parent,
the silica supported, and thermally activated (at 100 °C) W2Np6
dimer was shown to catalyze the disproportionation of propene
to ethylene and butenes. While these results were promising,
structural characterization of the system was not reported,
leaving the nature of the chemisorbed species undefined. Herein,
we synthesized and supported a series of M2X6 (M = W and Mo;
X = alkyl, alkoxide, and amide) dimers on silica and used a
combination of characterization techniques and density func-
tional theory (DFT) calculations to elucidate the binding modes
of the dimers on the surface and probe their activities toward
olefin metathesis.
2.244(1) Å, consistent with a triple bond. The WW bond
distance is the shortest reported for a homoleptic tungsten dimer
and is similar to those of other WW species.52 DFT confirmed
the presence of a triple bond with a Mayer bond order of 3.6
(Table S10). UV−vis spectroscopy revealed a prominent
absorption feature at 390 nm (Figure S2) associated with a
π→π* transition. This was supported by DFT (predicted
transition at 404 nm). The Fourier transform infrared
spectroscopy (FTIR) spectrum exhibited a set of vibrations
around 2800 cm−1 assigned to the neopentyl ν(C−H) stretching
frequencies. This assignment was supported by DFT calcu-
lations (Figure S3).
Addition of an orange solution of 1 in pentane to a suspension
of partially dehydroxylated silica (Davisil-646, pretreated under
vacuum at 700 °C, hydroxyl density53 = 1.15 OH nm−2) and
mechanical agitation over the course of 15 h resulted in the
deposition of 1 onto the silica support to produce an orange
solid (1-SiO2; 2.40 wt % W or 13.0 mmol/g via ICP) and a
colorless supernatant solution. Tracking the deposition reaction
1
by H NMR spectroscopy using 1,3,5 tri-tBu-benzene as an
internal standard revealed the disappearance of starting material
1 (0.0077 mmol) and the appearance of approximately 1 equiv
of neopentane (0.0055 mmol) with respect to 1, suggesting that
B
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1 chemisorbs to the silica via protonolysis, and that a Si−O−W
two unique methylene environments are contributing to the
signal at 100 ppm. The reasoning behind this choice of
experiment was that small chemical shift changes due to changes
in back-bonding do not influence all chemical shift tensor
elements equally; rather, the one perpendicular to the metal−
ligand bond, and the p orbital receiving the donation, is most
strongly affected.41,57−61 This component is deshielded by
increases in backbonding due to a stronger paramagnetic
shielding contribution. A SUPER method was applied to
measure the CSA.62 As can be seen in the 2D SUPER spectrum
in Figure 2B, the two sites indeed have noticeably different
CSAs, which confirms their assignments to electronically
distinct metal centers (i.e., tungsten bound to silica or not),
with the surface-bound site having the smaller CSA and the
more distant site having an isotropic chemical shift that agreed
with that of the molecular complex (Figure 2C). This difference
in CSA skews the relative intensities in the 1D projection of the
2D SUPER spectrum (Figure 2C) from which the two types of
neopentyl resonances can clearly be identified (Figure 2D). The
higher chemical shift, stronger CSA, and negative skew of the C2
carbons all suggest that this site receives stronger back-donation
from tungsten which is in agreement with its assignment to the
neopentlyl ligands coordinating to the tungsten that is not
surface bound. This was independently confirmed through DFT
calculations of the magnetic shielding tensors (Table S11).63
X-ray absorption spectroscopy (XAS) was also used to verify
that the WW bond in 1-SiO2 was still intact and to ascertain
the coordination mode of the tungsten dimer on the silica
(parameters can be found in Table 1). Upon grafting to silica,
bond is formed (Figure S4).
1
The H solid-state nuclear magnetic resonance (SSNMR)
spectrum of 1-SiO2 exhibited two peaks at 1.84 and 0.83 ppm, in
a roughly 2:9 ratio, consistent with a neopentyl environment
(Figure S5). Dynamic nuclear polarization (DNP)-enhanced
SSNMR spectroscopy was used to elucidate the structure of the
silica-supported tungsten dimer.54−56 The DNP-enhanced 13C
cross-polarization (CP) magic-angle-spinning (MAS) spectrum
of the precatalyst is shown in Figure 2A. Resonances from the
Table 1. Selected EXAFS Parameters, along with Measured
and Calculated Bond Lengths for 1 and 1-SiO2
scattering path
R/Å
σ2/× 10−3 Å2
W−C
2.163 (12)
2.259 (5)
2.117 (22)
2.285 (4)
2.3 (1.3)
1.8 (9)
21.5 (3.4)
2.7 (5)
1
WW
W−C
1-SiO2
WW
Figure 2. (A) DNP-enhanced 13C CPMAS SSNMR spectrum of 1-
SiO2. The dagger marks a benzene impurity and the asterisk marks
residual protonated 1,1,2,2-tetrachloroethane (TCE); both are from
the DNP solvent (TCE-d2). (B) The 2D separation of undistorted
powder patterns by effortless recoupling (SUPER) spectrum is shown
along with projections along the two methylene resonances to highlight
the difference in CSAs. The approximate principal components of the
chemical shift tensor for site 2 are marked with arrows. (C) Enlarged
views of the methylene region for the 13C CPMAS spectra of crystalline
1 (bottom) and 1-SiO2 (middle), as well as the 1D projection of the 2D
SUPER spectrum (top). (D) Assignment of the methylene carbon
resonances distinguished in the 2D SUPER experiment.
the oxidation state of tungsten atoms in the surface species
remained +3, as evident from the XANES data (Figure 3A).
EXAFS fit results for 1 and 1-SiO2 were consistent with a model
of a triple-bonded tungsten dimer (Figures 3B and S6−S9), with
each tungsten bound to three ligands.
The WW dimer bond length is well-defined at 2.259(5) Å
(Table 1), in good agreement with the single-crystal X-ray
distance of 2.2440(5) Å. After tethering the molecule to the
silica support, the two tungsten atoms are inequivalent, which
complicated both the fitting and the interpretation. Although the
data range beyond k = 15 Å−1 allows for many independent
points, or floating fit parameters, the correlations and
uncertainties grow quite large for coordination number (N)
and σ2. However, the WW dimer bond length was still
measured at 2.285(4) Å, an elongation of 0.026 Å. DFT
calculations were used to corroborate the structural assignments
of 1 and 1-SiO2. W2(Np)5(OSiMe3) serving as a surrogate to 1-
SiO2 (Figure S10), the trimethylsilanol ligand (−OSiMe3), was
used as a computationally robust substitute for the silica surface.
Attempts to independently synthesize a monosubstituted
W2(Np)5(OR) model compound were unsuccessful due to
the increased reactivity of the product to further protonolysis
and disproportionation. The structures of 1 and 1-SiO2 were
optimized using the M06-L64 density functional and def2-
large number of methyl groups are observed, as well as those of
the metal bound methylene moieties, appearing as a broad
resonance from 110 to 90 ppm. This resonance has an
asymmetric line shape reminiscent of a pair of overlapping
inequivalent resonances, as would be expected for the
desymmetrized monopodal structure, as depicted in Figure
1C. The assignment was further supported by the similar 13C
NMR chemical shift observed for the methylene carbons in the
molecular precursor in the solid-state (102.9 ppm, Figure 2C).41
A
13C chemical shift anisotropy (CSA) recoupling measure-
ment was performed to enhance the resolution and confirm that
C
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Figure 4. (A) UV resonance Raman spectra of 1, 1-SiO2, and 1-SiO2
postcatalysis (30 h on stream, vida infra) (red, blue, and green,
respectively). The broadening of the WW vibration upon grafting to
silica is consistent with the polarization of the triple bond, indicative of
the monopodal binding of the dimer to the surface. (B) Overlays of the
measured (red and blue) and simulated (green and orange) Raman
spectra for 1 and 1-SiO2. The quantitative agreement between the
experimental and simulated data strengthens the assignment of the
WW triple bond. The simulated spectra were normalized with
respect to the WW stretch.
Figure 3. (A) XANES spectra of 1, 1-SiO2, WCl6, and tungsten foil.
The relative positions of the edges and white lines support a conclusion
of low-valent tungsten in the dimers. (B) Fourier transform magnitude
(solid) and real (dashed) components of the EXAFS spectra for 1 (red)
and 1-SiO2 (black). The similarities in the real components indicate
that the WW bond remains intact upon grafting to silica.
TZVPP65,66 basis set, implemented in Gaussian 09 package.67
The geometry-optimized simulated structure of 1-SiO2
exhibited an elongation of the WW bond compared to that
of 1, with an increase of 0.026 Å (from 2.298 to 2.324 Å) upon
grafting onto silica (Table 2). The calculated elongation in the
WW bond from 1 to 1-SiO2 was consistent with the
experimentally observed trend (Table 2).
UV resonance Raman spectroscopy was used to obtain
information on the nature of the WW bond (Figure 4A). The
Raman spectrum of 1 exhibited a sharp peak at 290 cm−1 which
was assigned to the WW stretching vibration. The spectrum
of 1-SiO2 exhibited a broader peak at 294 cm−1 along with a
shoulder at 278 cm−1, evidencing interfacial interaction of the
triple bond with silica, and appears to relate with the asymmetric
broad resonance of the 13C NMR experiment (see Figure 2C).
DFT calculations were employed to simulate the Raman spectra
for 1 and 1-SiO2. The calculated WW stretches at 272 and
287 cm−1 for 1-SiO2 agreed with the observed values of 278 and
294 cm−1, respectively (Figure 4B). These stretches were also
consistent with previously reported ones.68
1-SiO2 was also characterized using diffuse reflectance
infrared Fourier transform spectroscopy (DRIFTS). A DRIFTS
spectrum of 1-SiO2 showed the absorption bands in the range of
2870−2960 cm−1 assigned to the ν(C−H) stretching
frequencies of the neopentyl ligands. This result was consistent
with other literature examples.69−71 The simulated spectrum for
1-SiO2 qualitatively matched the experimental spectrum (Figure
S11). The measured UV−vis spectra of 1 and 1-SiO2 and the
simulated spectrum for 1-SiO2 all showed a strong absorption at
∼390 nm that was assigned to a π→π* transition (Figure S2).
High-angle annular dark field scanning transmission electron
microcopy (HAADF-STEM) was used to ascertain the
morphology of the surface species. 1-SiO2 was heated to 100
°C under dynamic vacuum for 5 h to remove any adventitious
organics that could interfere with the measurement. Raman and
DRIFTS data suggested that the surface species remained intact
post heat treatment (Figures S12 and S13). A STEM image of 1-
Table 2. WW Bond Lengths for 1 and 1-SiO2 as Obtained
a
from Various Techniques
complex (measurement)
W−W bond length/Å
1 (XRD)
1 (XAS)
1 (DFT)
2.244(1)
2.259(5)
2.298
1-SiO2 (XAS)
1-SiO2 (DFT)
ΔW−W (XAS)
ΔW−W (DFT)
2.285(4)
2.324
0.026(6)
0.026
a
The agreement between the experimental results (XAS) and the
simulated data (DFT) suggests that the WW bond lengths increase
upon heterogenization onto a silica support.
D
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SiO2 (Figure 5A and S14) shows a collection of regions of high
electron density (shown in yellow) corresponding to the
Figure 5. (A) STEM-HAADF image of 1-SiO2. The brightly colored
yellow regions correspond to individual tungsten atoms. Circled in
white are two pairs of tungsten dimers oriented perpendicular with
respect to the electron beam. (B, C) Histograms of tungsten−tungsten
separation extracted from the HAADF-STEM micrograph. The
measured distances between the metal centers, D1 and D2, were 2.71
and 2.91 Å, respectively.
tungsten atoms.72−75 Pairs of WW oriented perpendicular
to the electron beam are highlighted in white circles. The center-
to-center distances between the tungsten atoms were measured
at 2.71 and 2.91 Å (Figure 5B−C). These results suggest that the
dimers are not agglomerated on the silica.
Catalytic Olefin Metathesis with 1-SiO2. The activity of
1-SiO2 in the disproportionative self-metathesis of propylene
was evaluated in an air-free plug flow reactor (Scheme 2). In a
Figure 6. Reaction profiles for 1-SiO2 during a 30 h plug flow run (top)
and the product distribution of 1-SiO2 throughout the 30 h period
(bottom). Conditions: room temperature, 20 cc/min, 2% ethylene in
argon. The parent complex, 1, did not show any propensity toward
metathesis.
Scheme 2. Reaction Scheme and Conditions for the
Disproportionation of Propylene to Ethylene and 2-Butene
and alkyl environments in the 13C SSNMR spectrum (Figure
S16) all suggest that the majority of the dimers on the surface
remained intact after the 30 h plug flow experiment. Taken
together, these observations suggest that the supported tungsten
dimers are not quantitatively converted to an active alkylidene
species. However, a pathway in which a trace amount of 1-SiO2
is transformed into a highly active species cannot be ruled out.
Thermogravimetric analysis (TGA) of 1-SiO2 was employed in
an attempt to observe the mass loss associated with α-
elimination to produce neopentane (Figure S37). A mass loss
at ∼100 °C could plausibly be assigned to the α-elimination.
However, inspection of 1-SiO2 thermolyzed at 100 °C by
DRIFTS and Raman spectroscopy did not identify any
resonances that could be assigned to alkylidene species (Figures
S12 and S13), and 13C SSNMR of the post catalysis material,
which had not deactivated significantly, also did not reveal an
alkylidene resonance (Figure S16).
The activity of 1-SiO2 toward solution-phase disproportio-
nation of cis-4-nonene in toluene (0.26 M) at 80 °C was also
assessed (Scheme 3 and Figure 7). Sampling over the course of
the first 25 min showed a low conversion rate indicative of an
induction period consistent with observations in the flow reactor
(3.5% conversion at 22 min). Following the induction period,
the activity of the material ramped up, giving rise to a rapid
increase in activity over the next 60 min after which point
equilibrium was reached (∼50% conversion at the ca. 80 min
mark). The reaction mixture was then filtered to remove 1-SiO2,
and the metathesis activity of the supernatant solution was
typical experiment, 100 mg of material was charged into a glass
reactor tube (OD = 0.25 in., ID = 0.15 in.). Propylene (2.1% in
argon) was fed into the reactor at a flow rate of 20 mL/min at
room temperature (∼22.0 °C) over the course of 30 h. The gas
outlet was connected to a gas chromatograph (GC-FID) which
was used to identify and quantify the product profile of the
mixture. These conditions were chosen in order to avoid
operation at equilibrium conversion (∼34%) and study the
stability of the supported dimers.51
As seen in Figure 6, 1-SiO2 is a competent propylene
metathesis catalyst. At the start of the plug flow run, 1-SiO2
exhibited a 5 h induction period as its conversion gradually
increased from 6.6 to 19.8%. Similar profiles have been observed
for other monometallic systems and are suggestive of the
formation of a tungsten alkylidene moiety.69 Following the
induction period, 1-SiO2 achieved a maximum performance of
20.0% conversion followed by a slow deactivation (16.0%
conversion at 30 h). The product distribution of the reaction
remained relatively constant with a trans-2-butene to cis-2-
butene ratio of 2.3, consistent with the thermodynamic ratio.
Postcatalysis 1-SiO2 (30 h on stream) was characterized using
DRIFTS, Raman, and 13C SSNMR spectroscopies. The
persistence of the WW in the Raman spectrum (Figure 4A),
the ν(C−H) stretches in the DRIFTS spectrum (Figure S15),
E
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Scheme 3. Reaction Scheme and Conditions for the
Disproportionation of 4-Nonene to 4-Octene and 5-Decene
a
a
Both cis and trans isomers are present in the reaction mixture.
Figure 8. (A) Solid-state structure of 2 and (B) schematic of 2-SiO2.
Thermal ellipsoids are drawn at 50% probability. Hydrogen atoms were
omitted for clarity. Legend: molybdenum (teal), carbon (gray).
Selected structural parameters: Mo(1)−Mo(1′) = 2.1654(7) Å,
Mo(1)−C(1) = 2.148(2) Å, C(1)−Mo(1)−Mo(1′) = 96.33(7)°,
C(1)−Mo(1)−C(1) = 118.80(3)°, C(1)−Mo(1)−Mo(1′)−C(1′) =
60.0(10)°.
adopts a monopodal structure, like that seen for 1-SiO2. 2-SiO2
was also characterized by DRIFTS spectroscopy as exhibiting
absorption bands in the range of 2870−2960 cm−1, which were
assigned to the ν(C−H) stretches of the neopentyl ligands
(Figure S23). The DRIFTS of 1-SiO2 and 2-SiO2 were
qualitatively similar suggesting that the materials are structurally
analogous (Figure 11). A simulated DRIFTS spectrum for 2-
SiO2 matches the peak assignments (Figure S24). The UV−vis
spectra of 2 and 2-SiO2 were also qualitatively similar (Figure
S18). Like the tungsten system, unsupported complex 2 is
unreactive toward olefins, while 2-SiO2 is active in the catalysis
of metathesis reactions. Air-free propylene disproportionation
plug flow experiments on 2-SiO2 were carried out similarly to
those of 1-SiO2. Material 2-SiO2 showed a different activity
profile than that of 1-SiO2 (Figure 9) with a maximum
Figure 7. Plot of conversion over time for the disproportionation of
nonene catalyzed by 1-SiO2 in toluene at 80 °C to give octene and
decene.
tested using fresh nonene. Sampling of the reaction mixture over
the course of 10 h showed no change in the product distribution
(Figure S17), confirming that the active species did not leach
from 1-SiO2 into the solution.76 While the formation of
catalytically active tungsten alkylidene species during the
reaction cannot be ruled out, free neopentane, which would
be diagnostic of such a process, was not detected in the reaction
mixture.
Evaluation of Molybdenum Homologue. Mononuclear
molybdenum complexes, both unsupported and supported, have
also been extensively studied with regard to their olefin
metathesis capabilities.35,77−79 In fact, well-defined molybde-
num alkylidene complexes supported on silica are often more
active for olefin metathesis than their tungsten homologues. The
molybdenum analogue, Mo2Np6 (2), was prepared according to
a modified literature procedure (see the Supporting Information
for details) and grafted onto silica in a similar fashion to afford
material 2-SiO2 (Figure 8).
The UV−vis and FTIR spectra of 2 were qualitatively similar
to those of 1 (Figures S18 and S19). 1H NMR loading
experiments under reaction conditions similar to those of 1-SiO2
showed a decrease in the concentration of the Mo2Np6 dimer
and the appearance of approximately 1 equiv of free neopentane,
suggesting that 2 is also chemisorbed onto the silica via
protonolysis and Si−O−Mo bond formation (Figure 8B and
Figure S20) to yield a monopodal structure. ICP analysis of 2-
SiO2 confirmed a loading of 1.23% by mass or 12.8 mmol/g. The
molar loading of molybdenum for 2-SiO2 is comparable to the
molar loading of tungsten in 1-SiO2. A 1H SSNMR spectrum of
2-SiO2 revealed two peaks at 1.84 and 0.94 ppm, in a 2:9 ratio
similar to the spectrum of 1-SiO2 (Figure S21). Similarly, the
DNP-enhanced 13C CPMAS NMR spectrum of 2-SiO2 (Figure
S22) featured resonances from both the tert-butyl carbons as
well as the Mo-bound methylenes. In this case, the peak from the
methylenes did not show the resolution of the two distinct
environments, as was seen for 1-SiO2, but the peak was
broadened toward lower frequency, suggesting that the complex
Figure 9. Reaction profiles for 1-SiO2 (green), 2-SiO2 (yellow), and 3-
SiO2 (blue). Conditions: room temperature, 20 cc/min, 2% ethylene in
argon. Note that the y-axis is logarithmic.
conversion of 20.3% at the onset of the plug flow experiment.
Soon thereafter, 2-SiO2 experienced a drop in performance in
the first 5 h (conversion rate of 1.0%) which did not recover for
the rest of the run. The trans-/cis-butene ratio, while still
favoring the thermodynamically more stable trans alkene,
dropped from 3 to 1.4, as the plug flow run commenced (Figure
S24).
F
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Effect of the Ancillary Ligand. The metathesis activities of
1-SiO2 and 2-SiO2 were hypothesized to proceed through a
pathway that involves a neopentylidene generated by α-
elimination. To test this hypothesis, tert-butoxide
(W2(O-tBu)6 and Mo2(O-tBu)6, 3 and 4, respectively) and
dimethylamide (W2(NMe2)6 and Mo2(NMe2)6, 5 and 6,
respectively) ligated dimers, were grafted onto silica and
investigated by procedures analogous to 1 and 2 (Figure 10).
Figure 11. DRIFTS spectra for (1−6)-SiO2 normalized to the intensity
of the silica peak at 3750 cm−1
dimethylamine ligand (Figure 11).47 Finally, the H SSNMR
1
and DRIFTS spectra of 6-SiO2 were qualitatively similar to
those of 5-SiO2, suggesting that 6-SiO2 was the Mo homologue
of 5-SiO2 (Figures 11 and S32). These data suggest that 3−6
were successfully chemisorbed onto the silica surface and were
coordinated to the metal centers in a monopodal fashion.
Air-free propylene disproportionation plug flow experiments
with materials (3−6)-SiO2 were run in a fashion similar to those
on (1, 2)-SiO2. Out of the four materials, only 3-SiO2 catalyzed
the metathesis of propene (Figures 10 and S33), albeit with
much lower activity compared to the alkyl congeners. With an
initial conversion of 0.03%, the activity of 3-SiO2 gradually
increased over the course of 30 hours at which point it reached a
conversion of 0.28% (Figure S33). The trans/cis ratio for the
formed products was close to unity for the duration of the
experiment, likely reflecting the kinetic product selectivity at low
conversion (Figures S33 and S34). Upon removal of 3-SiO2
from the plug flow reactor, a distinct color change from red to
black was observed, suggestive of the formation of a new species
upon exposure to the reactant stream; no other materials in this
report demonstrated such a change. The activity, or lack thereof,
observed for (3−6)-SiO2 supports the hypothesis that the
metathesis reaction proceeds through a metal−neopentylidene
intermediate, formed via H-α-elimination from a neopentyl
group.
Figure 10. Structures of (3−6)-SiO2.
The grafting of 3−6 proceeded in accordance to the
techniques employed for 1 and 2. 1H NMR loading experiments
under similar reaction conditions to those of 1-SiO2 showed the
disappearance of 3−6 over the course of 1 h, but neither tert-
butanol nor dimethylamine was observed (Figures S25, S27,
S29, and S31). This can be explained by the physisorption of the
protonated ligand onto the silica, as previously observed.72 The
results of the ICP analysis of the grafted materials (3−6)-SiO2
can be found in Table 3. The metal loading was roughly 13
mmol/g for all the systems.
Table 3. ICP Data for Materials (1−6)-SiO2
material
metal
wt %
mmol/g
1-SiO2
2-SiO2
3-SiO2
4-SiO2
5-SiO2
6-SiO2
W
Mo
W
Mo
W
Mo
2.40
1.23
2.78
1.30
2.33
1.29
13.0
12.8
15.1
13.6
12.6
13.5
CONCLUSION
■
In this work, we investigated the reactivity of a family of silica
supported tungsten and molybdenum dimers toward olefin
metathesis. Using a combination of spectroscopic techniques, it
was shown that for all six compounds, chemisorption occurs by a
protonolysis reaction to release 1 equiv of ligand per metal
complex, resulting in a monopodal surface organometallic
species. In contrast with its molecular parent, 1-SiO2 is a
competent metathesis catalyst in both plug flow (propylene) and
batch (nonene) reactor modes, making it the first example of a
well-defined system featuring a supported W(III) precatalyst.
The reactivity observed for alkyl complexes (1−2)-SiO2
compared to those observed for alkoxide and amide complexes
(3−6)-SiO2, coupled with the induction period for 1-SiO2, are
consistent with a mechanism which proceeds through the
formation of a neopentylidine generated by α-elimination,
although other mechanisms of alkylidene formation are
plausible. The discrepancy between the reactivity of 1 and 1-
SiO2 could be explained by the lowering of the activation barrier
1
The H SSNMR spectra of 3-SiO2 and 4-SiO2 exhibited a
broad singlet in the 1.00−2.00 ppm range consistent with an
asymmetric binding mode to the surface (Figures S26 and S28).
The DRIFTS spectra for 3-SiO2 and 4-SiO2 had motifs similar
to those of 1-SiO2 and 2-SiO2 to which they are structurally
analogous (Figure 11). 5-SiO2 was previously prepared and
characterized and thus was used as a reference.46 The H
1
SSNMR spectrum of 5-SiO2 exhibited a broad multiplet in the
0−5 ppm range (Figure S30). This was consistent with a
rotationally locked dimer on the surface and agreed with the
previously reported spectrum for 5-SiO2.47 DRIFTS character-
ization of 5-SiO2 revealed a series of peaks in the range of 2760−
3000 cm−1 attributed to the ν(C−H) stretches of the
G
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of 1 through the electronic effects of the silanol ligand or by site
isolation of reactive intermediates on the surface.80 More
detailed experimental and computational investigations into the
mechanism by which 1-SiO2 catalyzes the metathesis of
propylene are underway.
States; Center for Catalysis and Surface Science and Department
of Chemistry, Northwestern University, Evanston, Illinois 60208,
United States; orcid.org/0000-0002-5599-065X
Fulya Dogan − Chemical Sciences and Engineering Division,
Argonne National Laboratory, Lemont, Illinois 60439, United
States; orcid.org/0000-0001-7997-266X
Jianguo Wen − Center for Nanoscale Materials, Argonne National
Laboratory, Lemont, Illinois 60439, United States; orcid.org/
0000-0002-3755-0044
Navneet Khetrapal − Department of Chemistry, Chemical
Theory Center, and Supercomputing Institute, University of
Minnesota, Minneapolis, Minnesota 55455, United States
Prachi Sharma − Department of Chemistry, Chemical Theory
Center, and Supercomputing Institute, University of Minnesota,
Minneapolis, Minnesota 55455, United States; orcid.org/
0000-0002-1819-542X
Jacob White − Department of Chemistry, Chemical Theory
Center, and Supercomputing Institute, University of Minnesota,
Minneapolis, Minnesota 55455, United States
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.9b00787.
■
sı
General discussion of synthetic and experimental
procedures, instrumentation, characterization of materi-
als, and rate laws and their derivations (PDF)
XYZ coordinate files for computational assessment of
ligand displacement from vanadium to support mecha-
nistic discussion (ZIP)
Accession Codes
CCDC 1968533−1968534 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033.
A. Jeremy Kropf − Chemical Sciences and Engineering Division,
Argonne National Laboratory, Lemont, Illinois 60439, United
States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.9b00787
AUTHOR INFORMATION
Corresponding Authors
Alfred P. Sattelberger − Chemical Sciences and Engineering
Division, Argonne National Laboratory, Lemont, Illinois 60439,
United States; Email: asattelberger@anl.gov
Funding
■
This work was supported by the U.S. Department of Energy
(DOE), Office of Basic Energy Sciences, Division of Chemical
Sciences, Geosciences, and Biosciences, Catalysis Science
Program under contract nos. DE-AC-02-06CH11357 (Argonne
National Laboratory) and DE-AC02-07CH11358 (Ames
Laboratory). Use of the Advanced Photon Source was supported
by the U.S. DOE, Office of Science, and Office of the Basic
Energy Sciences, under contract no. DE-AC-02-06CH11357.
MRCAT operations were supported by the DOE and the
MRCAT member institutions. Use of the TEM at the Center for
Nanoscale Materials at Argonne National Laboratory is
supported by the U.S. Department of Energy, Office of Science,
Office of Basic Energy Sciences, under contract no. DE-AC-02-
06CH11357. The computational work was supported by the
Inorganometallic Catalyst Design Center, an EFRC funded by
the DOE, Office of Basic Energy Sciences (DE-SC0012702).
P.S. acknowledges a doctoral dissertation fellowship from the
University of Minnesota. H.K (UV Raman work) was supported
by the US Department of Energy, Office of Basic Energy
Sciences, through grant no. DE-FG02-03ER15457 to the
Institute for Catalysis for Energy Processes (ICEP) at
Northwestern University. F.K.D. acknowledges support from
the Vehicle Technologies Office at the U.S. Department of
Energy Efficiency and Renewable Energy
David M. Kaphan − Chemical Sciences and Engineering Division,
Argonne National Laboratory, Lemont, Illinois 60439, United
States; orcid.org/0000-0001-5293-7784;
Email: kaphand@anl.gov
Massimiliano Delferro − Chemical Sciences and Engineering
Division, Argonne National Laboratory, Lemont, Illinois 60439,
United States; orcid.org/0000-0002-4443-165X;
Email: delferro@anl.gov
Authors
Alon Chapovetsky − Chemical Sciences and Engineering Division,
Argonne National Laboratory, Lemont, Illinois 60439, United
States
Ryan R. Langeslay − Chemical Sciences and Engineering Division,
Argonne National Laboratory, Lemont, Illinois 60439, United
States; orcid.org/0000-0003-2915-9309
Gokhan Celik − Chemical Sciences and Engineering Division,
Argonne National Laboratory, Lemont, Illinois 60439, United
States; orcid.org/0000-0001-8070-5219
́
́
Frederic A. Perras − U.S. DOE Ames Laboratory, Ames, Iowa
50011, United States; orcid.org/0000-0002-2662-5119
Marek Pruski − U.S. DOE Ames Laboratory, Ames, Iowa 50011,
United States; Department of Chemistry, Iowa State University,
Ames, Iowa 50011, United States; orcid.org/0000-0001-
7800-5336
Notes
The authors declare no competing financial interest.
Magali S. Ferrandon − Chemical Sciences and Engineering
Division, Argonne National Laboratory, Lemont, Illinois 60439,
United States
Evan C. Wegener − Chemical Sciences and Engineering Division,
Argonne National Laboratory, Lemont, Illinois 60439, United
States
ACKNOWLEDGMENTS
■
We thank Laura Gagliardi (University of Minnesota) for useful
discussions and Charlotte Stern (Northwestern University) for
crystallographic support. We also thank Tom Gilbert (Northern
Illinois University) for helpful discussions on the M2Np6
syntheses.
Hacksung Kim − Chemical Sciences and Engineering Division,
Argonne National Laboratory, Lemont, Illinois 60439, United
H
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Organometallics XXXX, XXX, XXX−XXX