Alkane metathesis with the tantalum methylidene [(≡SiO)Ta(=CH2)Me2]/[(≡SiO)2Ta(=CH2)Me] generated from well-defined surface organometallic complex [(≡SiO)TaVMe4]

Article abstract of DOI:10.1021/ja5113468

By grafting TaMe₅ on Aerosil₇₀₀, a stable, well-defined, silica-supported tetramethyl tantalum(V) complex, [(≡SiO)TaMe₄], is obtained on the silica surface. After thermal treatment at 150 °C, the complex is transformed into two surface tantalum methylidenes, [(≡SiO)₂Ta(=CH₂)Me] and [(≡SiO)Ta(=CH₂)Me₂], which are active in alkane metathesis and comparable to the previously reported [(≡SiO)₂TaH_x]. Here we present the first experimental study to isolate and identify a surface tantalum carbene as the intermediate in alkane metathesis. A systematic experimental study reveals a new reasonable pathway for this reaction.

Full text of DOI:10.1021/ja5113468

Communication
pubs.acs.org/JACS
Alkane Metathesis with the Tantalum Methylidene
[(SiO)Ta(CH2)Me2]/[(SiO)2Ta(CH2)Me] Generated from Well-
Defined Surface Organometallic Complex [(SiO)TaVMe4]
Yin Chen,† Edy Abou-hamad,† Ali Hamieh,† Bilel Hamzaoui,† Lyndon Emsley,‡,#
and Jean-Marie Basset*,†
†Physical Sciences and Engineering, KAUST Catalysis Center, King Abdullah University of Science and Technology (KAUST),
Thuwal 23955-6900, Saudi Arabia
‡Centre de RMN a
de Lyon, 5 rue
de la Doua, 69100 Villeurbanne, France
#Institut des Sciences et Ingen
́ ́
ierie Chimiques, Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
̀
Tres
̀
Hauts Champs, Institut de Sciences Analytiques (CNRS, ENS-Lyon, UCB Lyon 1, Universite
́
́
S
* Supporting Information
studies, isotope distribution studies, solid-state (SS) NMR,
density functional theory (DFT) calculations, determination of
the effect of substrates, etc. On the basis of those results, we
concluded that, besides C−H bond activation leading to metal−
alkyls and metal hydride olefins, intermediates should be
involved similar to those involved in olefin metathesis. We
proposed that the reaction occurred on a multifunctional, single-
site [(SiO)Ta(CH2)(H)] intermediate species and in-
volved (i) C−H bond activation on the metal hydride (Ta-H),
(ii) α-H or β-H elimination on the tantalum−alkyl formed, (iii)
olefin metathesis on the carbene moiety (TaCH2) with
metallocyclobutane as a key intermediate, and (iv) hydro-
genation of the newly formed olefins on the metal hydride (Ta-
H).8
ABSTRACT: By grafting TaMe5 on Aerosil700, a stable,
well-defined, silica-supported tetramethyl tantalum(V)
complex, [(SiO)TaMe4], is obtained on the silica
surface. After thermal treatment at 150 °C, the complex
is transformed into two surface tantalum methylidenes,
[(SiO)2Ta(CH2)Me] and [(SiO)Ta(CH2)-
Me2], which are active in alkane metathesis and
comparable to the previously reported [(SiO)2TaHx].
Here we present the first experimental study to isolate and
identify a surface tantalum carbene as the intermediate in
alkane metathesis. A systematic experimental study reveals
a new reasonable pathway for this reaction.
However, [(SiO)2TaHx]-catalyzed alkane metathesis al-
ways involves hydrogenolysis as a competing reaction,9 which is
the major initial reaction and sometimes has higher reaction rates
than alkane metathesis. The high reactivity of the proposed [(
SiO)2Ta(CH2)H] intermediate species makes it very difficult
to identify its existence and the reaction pathway. [(
SiO)Ta(CHt-Bu)(CH2t-Bu)2] and some other metal neo-
pentyl species are also able to catalyze alkane metathesis, albeit at
a lower rate.7c To determine the reaction mechanism, we focused
on isolating the methylidene intermediate and showing direct
evidence for the reaction pathway.
Inspired by our recent syntheses of various surface methyl-
metal compounds, e.g., [(SiO)WMe5],10 [(SiO)Ta-
Me2Cl2],11 and [(SiO)Ta(CH2)Cl2],12 here we report the
isolation of an alkane metathesis intermediate, the surface
tantalum methylidene [(SiO)xTa(CH2)Me3−x] (x = 1, 2),
which is the simplest analogue of the presumed active species
[(SiO)Ta(CH2)(H)]. This compound was prepared by
thermal treatment of the well-defined surface tantalum species
[(SiO)TaMe4] (Scheme 1). Both [(SiO)TaMe4] 2 and
tantalum methylidene methyl compounds 3a and 3b can catalyze
the alkane metathesis reaction with a reactivity comparable to
that of [(SiO)2TaHx]. By performing a systematic study of the
he catalytic transformation of alkanes remains one of the
T
most important challenges in chemistry, considering the
inertness of the C−H and C−C bonds but also the application
potential,1 for converting short alkanes into longer chain
analogues usually is a value-adding process. Our work in this
area began with the discovery of the highly electrophilic metal
hydride [(SiO)3Zr-H] supported on silica, which is able to
activate the C−H and C−C bonds of alkanes at very moderate
temperature and transform polyethylene into diesel-range
gasoline.2 Later, in 1997, we reported the discovery of the highly
electrophilic tantalum hydride supported on a silica surface, [(
SiO)2TaHx],3 which can catalytically transform alkanes into their
lower and higher homologues (eq 1, T > 25 °C). By analogy to
2CnH2n+2 ⇌ Cn−iH2(n−i)+2 + Cn+iH2(n+i)+2
n ≥ 2, i = 1, 2, ..., (n − 1)
(1)
olefin metathesis, and different from “tandem metathesis of
paraffins”,1b,f,4 this reaction was dubbed “alkane metathesis”.3a,5
An alumina-supported tungsten hydride, W(H)3/Al2O3, was
found to have better activity in subsequent studies.6
A clear understanding of the reaction mechanism is essential
for catalyst design and improvement of the catalyst’s reactivity
and selectivity. We previously performed a series of mechanistic
studies with [(SiO)2TaHx] as the catalyst,5,7 including kinetic
Received: November 4, 2014
Published: December 26, 2014
© 2014 American Chemical Society
588
DOI: 10.1021/ja5113468
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Journal of the American Chemical Society
Communication
per mole of tantalum, respectively. This compound is quite stable
at room temperature, and no decomposition is detected after
storage for more than 1 week; i.e., there is no change in the SS-
NMR spectrum.
Scheme 1. Production of [(SiO)TaMe4] and [(
SiO)Ta(CH2)Me2]/[(SiO)2Ta(CH2)Me]
After [(SiO)TaMe4] was heated at 150 °C for 4 h, methane
was the only product in the gas phase (1.1 0.2 equiv of methane
per mole of tantalum). IR shows that the bands at 2932 cm−1 for
ν(C−H) and 1397 cm−1 for δ(C−H) both decreased after the
treatment (SI, Figure S3). The 1H MAS NMR spectrum (Figure
2A and SI, Figure S5) shows that the peak at 0.9 ppm disappears
reaction mechanism, we found that the Ta−Me bond can directly
activate C−H as an initiation step.
[(SiO)TaMe4] can be easily and efficiently prepared by
grafting TaMe5 (synthesized in situ, see SI, Figure S1) onto the
13
surface of Aerosil700
.
After washing three times with pentane
and evacuation at room temperature (10−5 mbar) for 3 h, grayish
powder product 2 was obtained (Scheme 1). After chem-
isorption of TaMe5, the IR peak of the silanols at 3747 cm−1
disappeared, and new bands appeared at 2932 cm−1 for ν(C−H)
and 1397 cm−1 for δ(C−H) (SI, Figure S2). The H MAS SS-
1
NMR spectrum of 2 displays a main signal at 0.9 ppm (Figure 1A
Figure 2. (A) 1H MAS NMR, (B,C) 2D contour plots of the aliphatic
region of the DQ and TQ quantum proton SS-NMR correlation spectra,
(D) 13C CP-MAS NMR, and (E) 2D contour plot of the aliphatic region
of 1H−13C HETCOR spectrum of [(SiO)Ta(CH2)Me2]/[(
SiO)2Ta(CH2)Me].
and five new peaks appear at 6.4, 3.1, 1.2, 0.4, and −0.1 ppm. The
1
latter four signals autocorrelate in 2D DQ and TQ H−1H
homonuclear dipolar correlation spectra and thus can be
assigned to Ta-Me groups (Figure 2B,C). The peak at 6.4 ppm
can be assigned to TaCH2, as an autocorrelation is limited to
the DQ. The signal at −0.1 ppm is attributed to Si−Me groups
resulting from the surface methyl transfer, which is also
supported by the autocorrelation in DQ and TQ (Figure
2B,C). The 13C CP-MAS NMR spectrum (13C, 97% enriched)
gives parallel results (Figure 2D): the peak at 72 ppm ascribed to
[(SiO)TaMe4] disappears after the thermal treatment, and
new peaks are observed at 216, 61, 57, 49, and −7 ppm. The 2D
1H−13C HETCOR NMR spectrum with a short contact time of
0.2 ms shows that the CH3 protons (3.1, 1.2, and 0.4 ppm)
correlate with carbon atoms (61, 57, and 49 ppm, respectively),
and the proton signal at 6.4 ppm has strong correlation with the
carbon signal at 216 ppm (Figure 2E). These data support the
assignment of the methylidene moiety, TaCH2. The carbon
peak at −7 ppm correlates with proton peak at −0.1 ppm
attributed to Si−Me group, suggesting the formation of
bipodal species 3b.11 By analogy with our recent NMR and DFT
Figure 1. (A) 1H MAS NMR, (B,C) 2D contour plots of the aliphatic
region of the DQ and TQ proton SS-NMR correlation spectra, (D) 13C
CP-MAS NMR, and (E) 2D contour plot of the aliphatic region of the
1H−13C HETCOR spectrum of [(SiO)TaMe4].
and SI, Figure S4) and a weak-intensity peak at 1.90 ppm,
assigned to the small amount of unreacted silanols. The 13C-
enriched compound 2* (97% enriched) was synthesized with the
same protocol starting from 13CH3I for better carbon NMR
sensitivity. The 13C CP-MAS NMR spectrum of 2* gives only
one signal at 72 ppm. 1H−13C HETCOR NMR recorded with a
contact time of 0.2 s shows that the proton signal at 0.9 ppm is
correlated with the carbon signal at 72 ppm (Figure 1E). The
1H−1H double- (DQ) and triple-quantum (TQ) correlation
spectra under 22 kHz MAS (Figure 1B,C) support that the signal
belongs to a methyl group,4b as autocorrelation peaks are
observed on the diagonal of the two-dimensional (2D) DQ and
TQ spectra (0.9 ppm in F2, and 1.8 and 2.7 ppm in F1,
respectively). Elemental analysis gives 5.30% Ta, 1.41% C, and
0.33% H, with <0.1% Cl and a ratio of Ta/C/H = 1/4.0 0.3/
11.5 1.0 (theoretical ratio 1/4.0/12.0). Hydrolysis and hydro-
genolysis of 2 produced 4.0 0.3 and 3.8 0.3 mol of methane
study on [(SiO)W(CH)Me2],10 as well as the H−1H
1
correlation observed between the methyl groups and the
methylidene moiety in DQ and TQ experiments (Figure
1
2B,C), the methyl groups at 13C 49 and 57 ppm and H 0.4
and 1.2 ppm can be assigned to the monopodal species 3a, while
the methyl groups at 13C 61 ppm and 1H 3.1 ppm are assigned to
the bipodal species 3b. Elemental analysis gives 5.41% Ta, 1.04%
C, and 0.25% H, with a ratio of Ta/C/H = 1/2.9 0.3/8.2 1.0.
Because the methylidene species 3 is much more thermally
stable than precursor 2, hydrolysis of 3 produced 2.6 0.2 mol of
589
DOI: 10.1021/ja5113468
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Journal of the American Chemical Society
Communication
Table 1. Alkane Metathesis with [(SiO)Ta(CH2)Me2]/[(SiO)2Ta(CH2)Me] As Prepared
product selectivity (%)
a
b
alkane
ethane
TON
C1
C2
C3
C4
C5
C6
C7
C8
2.5
c
18 (2.8)
49 (7.8)
65 (10.4)
119(18.0)
50.6
11.6
5.3
−
46.5
−
41.9
1.3(0.2)
propane
butane
45.4
15.1
2.7
32.8(6.5)
−(2.6)
21.8(0.5)
5.5(1.7)
29.3(3.4)
−(1.7)
0.9(0.2)
3.8(0.9)
42.5(0.8)
1.2
pentane
1.0
17.2
9.0(1.1)
a
b
After 120 h of reaction in a batch reactor (Ta/alkane ratio ≈ 630, 150 °C, 0.95 atm). TON is expressed in moles of alkane transformed per mole
of Ta. The values in parentheses are conversions. The percentage of the isomer partition.
c
methane per mole of tantalum. All these characterization data
support the generation of the tantalum methylidene species on
the silica surface as a mixture of [(SiO)Ta(CH2)Me2] and
[(SiO)2Ta(CH2)Me][Si−Me]. Several attempts to
avoid the formation of the bipodal species failed, as the methyl
transfer to silicon is an easy process when the Ta-methyl is close
to a highly reactive Si-O-Si.
When 13C-labeled [(SiO)TaMe*4] reacts with a stoichio-
metric amount of C5H12 (150 °C, 250 mbar), 13CH4 is produced
without any 13C-labeled metathesis product, indicating that the
labeled methyl or methylidene group does not take part in the
metathesis step of this reaction (SI, Figures S17−S20).
Generally, only alkane products are found in the gas phase
after the alkane metathesis reaction (SI, Figure S9). However,
after reaction with ethane for 4 h, upon hydrolysis, the surface
organometallic species gives not only alkanes but also alkene
products (as propene and four different butenes; SI, Figure
S10).7f By varying the contact time, we already observed that
alkenes are produced in the reaction as intermediates,7e probably
by β-H elimination from the Ta-alkyls. SS-NMR results also
support that after reaction of [(SiO)TaMe4] with 13CH3-CH3
for a few hours, the 13C MAS NMR (SI, Figure S7) shows two
peaks at 105 and 118 ppm corresponding to alkenes (they
disappear from the NMR spectrum after hydrolysis with H2O).
If, instead, the surface species is hydrolyzed with D2O, partially
deuterated alkane products are identified by GC-MS at M+1, M
+2, and M+3 (singly, doubly, and triply deuterated alkanes); the
peaks are found with high intensity (SI, Figures S21−S24).
Meanwhile, the ethane produced in this treatment is found as
C2H4D2 with a main peak at M+2, indicating the ethyl group
exists on the surface mainly as a TaCHCH3 moiety.
These methyl methylidene speciesthe simplest homologues
of the proposed active species, [(SiO2)Ta(CH2)H]also
are important intermediates themselves. Alkane metathesis with
[(SiO)Ta(CH2)Me2]/[(SiO)2Ta(CH2)Me] as cata-
lyst was performed with our previously reported standard
conditions in a batch reactor. When propane (630 equiv) was
brought into contact with 3 at 150 °C, after 120 h, the propane
was converted into methane (11.6%), ethane (45.4%), butanes
(32.8%), and pentanes (5.5%) with a conversion of 7.8%, which
gives a turnover number (TON) of 49. The results for ethane,
butane, and pentane are listed in Table 1. These results are
comparable with the catalytic results obtained using [(
SiO)2TaHx].3a We also found that 2 (the precursor) has
productivity and selectivity similar to those of 3 in alkane
metathesis, which indicates that the alkane metathesis catalyzed
by 2 and 3 might go through the same reaction pathway.
Further insight into the mechanism was achieved by labeling
experiments. The reaction of [(SiO)TaMe*4] (98% labeled)
and 13CH3-CH3 (50% 13C-labeled) was monitored by SS-NMR.
Within the first hour, 2 was converted into methylidene: strong
peaks corresponding to 3 were observed, while the peak at 72
ppm for 2 disappeared (SI, Figure S6). After 6 h, the intensity of
the peak for the methylidene group at 216 ppm sharply
decreased, and another carbene species, presumably a Ta-
ethylidene, appeared at 220 ppm (SI, Figure S7). Starting from
compound 3*, after reacting with 13CH3-CH3 for 5 h, a similar
13C SS-NMR spectrum was obtained (SI, Figure S8). These
results support that 2 and 3 catalyze the alkane metathesis via the
same pathway.
Next we monitored the metathesis reaction by in situ IR (SI,
Figures S11 and S12). A small peak at 2270 cm−1, generally
assigned to Si-H (SI, Figure S13), is observed after reaction of
[(SiO)TaMe4] with ethane or pentane for 6 h. The presence
of this Si-H indicates the formation of the Ta-H intermediate
(likely monopodal) in the reaction, which would react with the
surface Si-O-Si bridge as a secondary reaction to produce
the Si-H moiety. The formation of the Ta-H also is supported
by formation of H2 in the reaction as a byproduct (SI, Scheme
S1). Furthermore, the addition of 0.5% H2 to the alkane has little
effect on the reaction (SI, Table S2).
The metathesis reaction also was performed under dynamic
conditions in a fixed-bed reactor coupled with an in situ GC
detector. Contacting 2 (0.043 mmol Ta) with ethane (1 bar, 3.0
mL/min, ∼3.1 mol of ethane/(mol of Ta·min)) at 150 °C in a
dynamic reactor gave a mixture of hydrogen, methane, propane,
isobutane, and butane as the products with time (SI, Figure
S14a). Trace amounts of ethylene, propylene, and butenes were
also detected. Methane was produced in a large quantity initially
and reached a maximum at 50 min, and then it decreased quickly
to a level similar to that of propane, which may be due to the
generation of methylidene species and σ-bond metathesis with
the C−H group of the alkanes. The production of propane and
hydrogen also reached a maximun with time and then decreased
slowly. In total, approximately 0.592 mmol of methane, 0.441
mmol of propane (0.151 mmol less than methane, 3.2 equiv
relative to Ta), and 0.038 mmol of hydrogen (0.8 equiv relative
Isotopic distribution in the alkane metathesis reaction was
then followed by GC-MS. When 2 and 3 were reacted with a
stoichiometric amount of C5D12 (for details see SI) at 150 °C,
after a few minutes, CH3D was observed as the main product in
the gas phase, with trace amount of metathesis products (SI,
Figures S15 and S16). This indicates that the Ta−CH3 bond can
directly activate the C−D bond of C5D12 and produce
monodeuterated methane, likely by σ-bond metathesis. The
alkane metathesis mechanism we proposed previously involves
an initial elementary step of C−H bond activation with surface
Ta-H,7b,e leading to a Ta-alkyl. Here we demonstrate that the C−
H activation could be achieved with a Ta-alkyl group via σ-bond
metathesis. When the reaction with C5D12 was performed under
the same experimental conditions as the reaction with C5H12
(Table 1), the rate decreased by a factor of 2.4, suggesting a
possible kinetic isotope effect.14
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Journal of the American Chemical Society
Communication
to Ta) were produced. The TON reached 22, and the catalyst
slowly lost its activity after 1200 min (SI, Figure S14b).
On the basis of the experimental results from this and our
previous studies,6,7 the following mechanism is proposed
(Scheme 2): 3 as the active species reacts with alkane to produce
AUTHOR INFORMATION
Corresponding Author
jeanmarie.basset@kaust.edu.sa
Notes
■
The authors declare no competing financial interest.
Scheme 2. Proposed Mechanism for Ethane Metathesis with
the Precursor [(SiO)TaMe4]
ACKNOWLEDGMENTS
We thank KAUST for the generous financial support. This paper
is dedicated to Yves Chauvin.
■
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■
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methane and Ta-alkyl species 4, followed by α-H abstraction,
producing ethylidene species 5, which is supported by the
observed new 13C NMR peak at 220 ppm (SI, Figure S7) and
production of C2H4D2 in the hydrolysis treatment. Further σ-
bond metathesis with ethane removes the methyl group (original
methylidene moiety) to produce 6; intermediate species 7 is
produced with a following β-H abstraction. The metathesis
product is produced following the metallocyclobutane mecha-
nism described by Chauvin15 via intermediates 8 and 9.
Intramolecular insertion of the formed higher olefin into the
tantalum hydride, followed by σ-bond metathesis between
ethane and propyl-tantalum, gives the metathesis product.
Alternatively, propylene can dissociate to produce 11, leaving a
possibility of σ-bond metathesis between ethane and the
tantalum hydride. The Ta-H intermediate species 7−9 can lead
to surface Si-H by reaction with the siloxane bridge, or to H2
by reaction with alkane via σ-bond metathesis. The catalyst
possibly is deactivated because of these secondary reactions, and
these byproducts lead to different deuterated alkanes when they
meet D2O.
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In summary, we present the first experimental study to isolate
the tantalum methylidene intermediate species 3 for alkane
metathesis, prepared easily from the well-defined surface species
[(SiO)TaMe4]. Both 2 and 3 have reactivity and selectivity in
alkane metathesis comparable to those previously reported for
[(SiO)2TaHx], and catalyze this reaction via the same
pathway. The reaction intermediates for alkane metathesis are
identified for the first time by SS-NMR and other character-
ization methods. We have developed a new catalyst for alkane
metathesis which can be prepared easily, opening a new way for
catalyst design.
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ASSOCIATED CONTENT
* Supporting Information
Experiment details, and IR, NMR, GC, MS, and other
characterization data. This material is available free of charge
via the Internet at http://pubs.acs.org.
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S
(15) Her
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isson, J.-L.; Chauvin, Y. Makromol. Chem. 1971, 141, 161.
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DOI: 10.1021/ja5113468
J. Am. Chem. Soc. 2015, 137, 588−591