[(≡SiO)TaV (=CH2)Cl2], the first tantalum methylidene species prepared and identified on the silica surface

Article abstract of DOI:10.1016/j.jorganchem.2013.04.050

A novel surface tantalum methylidene [(≡SiO)Ta^V (=CH ₂)Cl₂] was obtained via thermal decomposition of the well-defined surface species [(≡SiO)Ta^VCl₂Me ₂]. This first surface tantalum methylidene ever synthesized has been fully characterized and the kinetics of the a-hydrogen abstraction reaction has also been investigated in the heterogeneous system.

Full text of DOI:10.1016/j.jorganchem.2013.04.050

Journal of Organometallic Chemistry 744 (2013) 3e6
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Communication
[(^SiO)TaV(]CH2)Cl2], the first tantalum methylidene species
prepared and identified on the silica surface
*
Yin Chen, Emmanuel Callens, Edy Abou-hamad, Jean-Marie Basset
King Abdullah University of Science and Technology, KAUST Catalysis Center (KCC), 23955-6900 Thuwal, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
A novel surface tantalum methylidene [(^SiO)TaV(]CH2)Cl2] was obtained via thermal decomposition of
the well-defined surface species [(^SiO)TaVCl2Me2]. This first surface tantalum methylidene ever syn-
Article history:
Received 16 March 2013
Received in revised form
29 April 2013
thesized has been fully characterized and the kinetics of the
been investigated in the heterogeneous system.
a
-hydrogen abstraction reaction has also
Accepted 29 April 2013
Ó 2013 Elsevier B.V. All rights reserved.
Dedicated to Professor Wolfgang Herrmann,
at the occasion of his 65th birthday.
Keywords:
Surface tantalum methylidene
Surface organometallics chemistry
a
-Hydrogen abstraction
Kinetics
1. Introduction
kylidene) precursors have failed mostly because the silanol group
add into the tantalum carbon double bond rather than the expected
replacement of the alkyl group [20]. Meanwhile, in solution metal
alkyl precursors substituted with small groups are unstable and
often produce alkylidene, alkylidyne or even non identified prod-
Since the first discovery by Schrock of metal alkylidenes, these
complexes have played an important role in the development of
organometallic chemistry, olefin metathesis and generally catalysis
[1,2]. Particularly, high oxidation state tantalum alkylidenes, pre-
pared from the corresponding metal alkyl complex [3e5], have
been found to be key intermediates involved in olefin metathesis
catalysis. The late transition metal alkylidenes as ruthenium
imidazole compound discovered by Herrmann [6,7] have led to
another climax in this research field.
In heterogeneous catalysis, the study of surface supported
catalyst is essential for the improvement of catalytic performances
[8,9]. By comparison with their well-studied homogeneous coun-
terpart, surface metal alkylidenes compounds reported are much
fewer [10e16], the existence of such species often supported by
only infrared spectroscopy or chemical reactivity [17e19].
ucts due to multiple
actions [21,22].
a-hydrogen abstraction or bimolecular re-
We have always been interested in developing diversity of the
surface organometallic chemistry of tantalum alkylidene. Here, we
synthesized
a novel surface tantalum methylidene complex
[(^SiO)TaVCl2(]CH2)] on the silica surface, which derived from
thermal decomposition of the well-defined surface binary methyl
organometallic compound [(^SiO)TaVCl2Me2] that we reported
recently [23]
2. Experimental and results
Tantalum alkylidene compounds reported so far is even less
abundant. Our group has reported the first fully characterized
surface tantalum alkylidene, [(^SiO)(]CHt-Bu)(CH2t-Bu)2], but
other examples are very limited [14,16]. So far the direct grafting on
the silanols of silica with the tantalum (alkyl)(al
When [(^SiO)TaVCl2Me2] was thermally treated, methane was
observed as the main product with traces of ethane and ethylene.
From earlier work from Schrock, it is assumed that the formation of
methane results from a-hydrogen abstraction and transfer to the
remaining methyl [2]. Therefore, an immobilized tantalum meth-
ylidene complex should be observable since the bimolecular
decomposition could be avoided on the silica surface (Scheme 1).
Upon heating [(^SiO)TaVCl2Me2] 1 at 140 ꢀC for 24 h in a well-
sealed Schlenk tube, gas calibration analysis found that 1 mmol of
* Corresponding author. Tel.: þ996 28080299.
E-mail address: jeanmarie.basset@kaust.edu.sa (J.-M. Basset).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.04.050

4
Y. Chen et al. / Journal of Organometallic Chemistry 744 (2013) 3e6
TaVCl2Me2] (Fig. 1a), it is more easily detected after heat decom-
position, with the same intensity. The small peak at 1.2 ppm cor-
responds to traces of the starting material on the silica surface.
Proton double (DQ)- and triple (TQ)-quantum correlation
spectra under 22 kHz MAS provide autocorrelation peaks observed
on the diagonal of the 2D DQ spectrum for all the protons (Fig. 1c)
and only an autocorrelation peak is observed in the TQ spectrum for
the peak at 1.2 ppm (1.2 ppm in F2 and 3.7 ppm in F1) (Fig. 1d). The
fact that the signal at 7.5 ppm exhibits autocorrelation in the double
quantum autocorrelation but does not exhibit an autocorrelation in
the triple quantum is a proof that it is a CH2 species. These results
confirm that the signal at 7.49 ppm corresponds to a (]CH2)
group [24].
Scheme 1. The synthesis of [(^SiO)TaV(]CH2)Cl2] 2.
For a higher resolution of the 13C CP MAS NMR spectroscopy, 13
C
[(^SiO)TaVCl2Me2] 1 produced 0.93 mmol of methane. Another
0.90 mmol of methane was produced upon hydrolysis of the surface
complex. Infrared spectroscopy shows this complex exhibits two
peaks for the CeH stretching vibrations (2961, 2858 cmꢁ1).
Elemental analysis shows a ratio of Ta:Cl:C ¼ 1 ꢂ 0.1:1.89 ꢂ
0.1:0.97 ꢂ 0.1 (expected: 1:2:1) in agreement with the expected
monopodal methylidene species.
labeled [(^SiO)TaVCl2Me2] 1* (97%) was synthesized. After heating
the sample to 140 K, the 13C CP/MAS NMR spectrum of 2* displays
formation of a novel peaks at 230 ppm (Fig. 2b) for an sp2 carbon.
Other 13C NMR signals belong to the remained trace amount of
[(^SiO)TaVCl2Me2] on the silica surface.
2D 1He13C HETCOR NMR analysis (Fig. 2c) with short contact
time (0.2 ms) shows a clear correlation between the methylidene
protons at 7.5 ppm and the carbon at 230 ppm, confirming the
existence of tantalum methylidene species on the surface.
2.1. NMR study
1H- MAS NMR spectrum of species 2 displays 4 peaks at 0.0, 1.2,
1.9 and 7.5 ppm (Fig. 1b). The weak signal at 0.0 ppm is probably
due to methane or trace amount of ^SiMe (vide infra for further
comments, Fig. S3). One broad peak at 7.5 ppm is due to the
methylidene group of [(^SiO)TaV(]CH2)Cl2]. The peak at 1.9 ppm
corresponds to unreacted silanols on the silica surface, which ap-
pears as a shoulder peak of the methyl group NMR signal of [(^SiO)
2.2. Kinetics study of the conversion of [(^SiO)TaVCl2Me2] to
[(^SiO)TaV(]CH2)Cl2]
In order to determine the kinetics of formation of this methyl-
idene species, the reaction was firstly performed in a fixed bed
reactor with a PID control unit, 1 was heated under argon flow
Fig. 1. (a) 1H MAS NMR of [(^SiO)TaVMe2Cl2], (b) 1H MAS NMR of [(^SiO)TaV(]CH2)Cl2] (c), (d) two dimensional contour plots of the aliphatic region of the double (DQ) and triple
(TQ) quantum proton solid-state NMR correlation spectra of [(^SiO)TaV(]CH2)Cl2] (more details see Supplementary material).

Y. Chen et al. / Journal of Organometallic Chemistry 744 (2013) 3e6
5
-12
-13
-14
-15
-16
0.0022
0.0024
0.0026
0.0028
1/T (K -1
)
Fig. 4. Eyring plot of the reaction (ln(k/T) vs 1/T).
with the former results, we can take the methane generation rate as
the reaction rate, online GC monitoring found the methane gen-
eration rate decreased exponentially with the time. The equation
fits with a first-order kinetics reaction, in agreement with studies of
Schrock [25] and Xue [26], which reported also a first order kinetics
for the conversion of Ta(CH2XMe3)5 into Ta(]CHXMe3)(CH2XMe3)3
(X ¼ C, Si).
To obtain the thermodynamics parameters, thermal decompo-
sition of [(^SiO)TaVCl2Me2] was monitored under a static condition
(experimental details see Supplementary material). In a well-sealed
Schlenk tube with known volume, [(^SiO)TaVCl2Me2] was
decomposed at specified temperature, the quantity of the methane
produced was quantified.
The thermal decomposition at six different temperatures (be-
tween 353.1 and 453.1 K) has been determined (see Supplementary
material). Eyring plot of ln(k/T) vs 1/T gave an activation enthalpy of
Fig. 2. (a) 13C CP MAS spectrum of [(^SiO)TaVMe2Cl2], (b) 13C CP MAS spectrum (c)
and contour plot of the aliphatic region of a 1He13C HETCOR spectrum recorded with a
contact times of 0.2 ms spectrum of [(^SiO)TaV(]CH2)Cl2] (experimental details are
given in Supplementary material).
Hꢂ
¼
9.3
ꢂ
0.2 kcal/mol and an activation entropy of
(atmosphere pressure, 17.1 mL/min) at 140 ꢀC (more information
see Supplementary material), methane was observed with traces
amounts (less than 0.1%) of ethane and ethylene (Fig. 3). Methane
accounts for more than 99.8% of the gaseous products; combined
D
D
Sꢂ ¼ ꢁ26.0 ꢂ 1.5 (eu) for the conversion of 1 to 2 (Fig. 4).
Arrhenius plot gave the
DH
¼
10.1
ꢂ
0.2 (kcal/mol) and
D
S ¼ ꢁ39.0 ꢂ 2 (eu) (see Fig. S7). These values close to the exper-
imental values Schrock observed for the -hydrogen abstraction of
TaCp(CH2CMe3)Cl2 in chloroform (
H ¼ 10.7 ꢂ 0.5 (kcal/mol),
S ¼ ꢁ36.0 ꢂ 2 (eu)) [25,26], also comparable to the DFT calculated
value for -H abstraction of TaMe5 (
a
D
D
a
DH ¼ 7.2 kcal/mol) [27]. Further
application and catalysis research works are still in progress.
3. Conclusions
By employing surface organometallic chemistry, we have been
able to synthesize a novel tantalum methylidene species on the
silica surface, [(^SiO)TaV(]CH2)Cl2], and monitored its formation
by
a-H abstraction. The thermodynamic values supported
a
monomolecular
a-hydrogen abstraction mechanism and highlights
how surface organometallic species is an effective tool to study a
transformation which would not be otherwise observable in a ho-
mogeneous system.
Acknowledgements
Fig. 3. The thermal decomposition of [(^SiO)TaVCl2Me2] at 140 ꢀC in a fixed bed
reactor (black line, experimental data; red line, exponential fitting with first order law)
(For interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.).
We thanks KAUST for the financial support and YC gratefully
acknowledges generous donation from SABIC.

6
Y. Chen et al. / Journal of Organometallic Chemistry 744 (2013) 3e6
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Appendix A. Supplementary material
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(1995) 4288.
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G.J. Sunley, J. Am. Chem. Soc. 126 (2004) 13391.
Supplementary material associated with this article can
be found in the online version, at http://dx.doi.org/10.1016/j.
jorganchem.2013.04.050.
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