H/D exchange between CH4 and CD4 catalysed by a silica supported tantalum hydride, (?SiO)2Ta-H

Article abstract of DOI:10.1039/a910239f

The silica supported tantalum hydride (?SiO)₂Ta-H 1, catalyses the H/D exchange reaction between CH₄ and CD₄ at 150 °C producing the statistical distribution of all methane isotopomers.

Full text of DOI:10.1039/a910239f

H/D exchange between CH4 and CD4 catalysed by a silica supported tantalum
hydride, (·SiO)2Ta–H
Laurent Lefort, Christophe Copéret, Mostafa Taoufik, Jean Thivolle-Cazat* and Jean-Marie Basset*
Laboratoire de Chimie OrganoMétallique de Surface UMR 9986 CNRS-CPE Lyon 43, boulevard du 11 Novembre
1918 F-69616 Villeurbanne Cedex, France. E-mail: basset@mulliken.cpe.fr
Received (in Cambridge, UK) 22nd December 1999, Accepted 1st March 2000
Published on the Web 30th March 2000
The silica supported tantalum hydride (·SiO)2Ta–H 1,
catalyses the H/D exchange reaction between CH4 and CD4
at 150 °C producing the statistical distribution of all
methane isotopomers.
catalysed by the same surface complex 1 under similar
experimental conditions has reached a TON of ca. 4 after 6 h.9
The higher activity of 1 for H/D exchange reaction is however
not surprising since this process involves C–H in place of C–C
bond cleavage for alkane metathesis.
Methane is the most thermodynamically stable alkane. Its
activation necessarily requires the cleavage of a C–H bond and,
for example, the H/D exchange reaction between CH4 and a
deuterium source (D2, D2O or a deuterated alkane or arene) is a
typical process involving C–H bond activation.1 Some of the
first examples were observed in the 1930s and 1940s using
metal surfaces.2 Numerous examples have appeared since in
heterogeneous catalysis,3 while only few homogeneous systems
are known to perform the catalytic H/D exchange with CH4.4
Surface OrganoMetallic Chemistry (SOMC) lies at the
interface between homogeneous and heterogeneous catalysis
and consists in grafting a molecular organometallic complex
onto an oxide or metal surface to generate ultimately single site
catalysts.5 This strategy has led to the synthesis and the
characterization of a silica supported tantalum hydride,
(·SiO)2Ta–H 1, a highly electrophilic complex (formally a
eight electron complex).6 It can activate the C–H bond of cyclic
alkanes, to form a tantalum cycloalkyl species, (·SiO)2Ta–
CnH2n21.7 It also catalyses both the low temperature hydro-
genolysis of alkanes8 and the new reaction of alkane met-
athesis,9 which transforms acyclic alkanes into their higher and
lower homologues. Here, we report that 1 also catalyses the H/D
exchange reaction between CH4 and CD4, leading to the
statistical formation of all possible isotopomers.
A similar H/D-exchange also occurs between deuterated
methane and hydrogen. When a 1+2 mixture of CD4/H2 (Ptot
=
200 Torr) was contacted with 1 (52 mg, 5.2 wt% Ta) at 150 °C,
the formation of d0-, d1-, d2-, d3-methanes could be detected
after 40 min by IR spectroscopy. A new stable composition of
the gas mixture, containing all the deuterated methanes, was
reached after several hours, which corresponded to the statis-
tical distribution of all H/D atoms (7.6, 15.8, 35.8, 26.6, 14.2%
of d0-, d1-, d2-, d3- and d4-methanes, respectively, cf. the
calculated theoretical composition of 6, 25, 37.5, 25, 6%). The
initial rates obtained in the exchange of CH4/CD4 and CD4/H2
do not appear to be significantly different, which is in agreement
with the cleavage of the C–H bond being the rate-determining
step for both transformations.
During the reaction with CH4/CD4 mixtures, the evolution of
1 could be followed by in situ IR spectroscopy. The addition at
room temperature of a mixture of d0- and d4-methanes to 1 did
not lead to a significant decrease in intensity of the n(Ta–H)
band, centered at 1825 cm21.6 However, after 45 min at 150 °C,
the n(Ta–H) band was reduced to about half its initial intensity,
and a new band at 1323 cm21 was observed, which corre-
When a 55+45 mixture of CH4/CD4 was contacted with 1 at
150 °C for 10 h, it was converted into a mixture of all the
isotopomers of methane as a statistical distribution (Fig. 1).† At
low conversion, d1- and d3-methanes were formed faster than
d2-methane, suggesting that the H/D exchange was stepwise via
the exchange of one H/D atom at a time [Fig. 1(a)]. This initial
CH4/CD4 mixture gave a mixture of methane isotopomers, of
which CD2H2 was the major product. If a 65+35 mixture of
CH4/CD4 was contacted with 1 under identical conditions, the
evolution toward a different statistical distribution of all
isotopomers was observed. The composition of the equilibrated
mixture according to GC–MS was 24, 40, 24, 12, 0% of d0-, d1-,
d2-, d3- and d4-methanes, respectively, in good agreement with
the calculated statistical distribution (18, 38, 31, 11, 1.5%),
which was no longer centered on CD2H2 since there was an
excess of H- over D-atoms in the initial mixture.
The number of turnovers for this exchange reaction can be
estimated according to eqn. (1) if considering that (a) the
formation of CDH3 and CD3H requires at least one C–H(D)
bond cleavage and formation from CH4 and CD4, respectively,
and that (b) the formation of CD2H2 necessitates at least two C–
H bond cleavages and formations from either CH4 or CD4
(1)
TON = [Q(CDH3) + Q(CD3H) + 2 Q(CD2H2)]/Q(Ta)
Q(X) represents the amount (in mol) of the compound X.
The number of turnovers as defined in eqn. (1) is underesti-
mated since it does not take into account degenerate processes
such as the exchange of a D atom with another D atom. The
calculated TON for the H/D exchange is therefore ca. 200 after
6 h of reaction. For a comparison, the metathesis of ethane
Fig. 1 CH4/CD4 exchange reaction (CH4/CD4 = 55+45; Ptot = 200 Torr;
150 °C, 36 mg of 1 (6.1 wt% Ta)). (a) Isotopomeric composition of the gas
phase vs. time. (b) Distributions of the different isotopomers at the end of
the reaction (2500 min).
DOI: 10.1039/a910239f
Chem. Commun., 2000, 663–664
This journal is © The Royal Society of Chemistry 2000
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sponded to a n(Ta–D) vibration.‡ No peaks associated with Ta–
CH3 or Ta–CD3 were detected, but these types of surface
species are usually difficult to observe by IR spectroscopy.7 In
fact, independent experiments showed that hydrolysing the
surface species formed after reacting 1 with CH4 (P = 200 Torr,
T = 150 °C, 2 days) gave methane in the gas phase, which
probably arose from the hydrolysis of a surface tantalum
methyl, (·SiO)2Ta–CH3 2. This consequently shows that 1 can
be converted into 2 under the H/D exchange experimental
conditions although the complete disappearance of both the Ta–
H and Ta–D vibration bands has never been observed.
Two mechanisms for the H/D exchange reaction between
CH4 and CD4 can be proposed. Both rely on a s-bond
metathesis C–H activation step and involve different active
species, i.e. 1 or 2 (Scheme 1).
been observed [mechanism (b), step 2]. Consequently, mecha-
nism (a) which only involves Ta–H bonds instead of Ta–C
bonds [mechanism (b)] appears to be the most consistent to
describe the H/D exchange between methanes. This observation
also agrees with previous data relative to the stability of four-
center intermediates, in which a carbon atom opposite to the
metal [Scheme 1(a)] instead of a hydrogen/deuterium atom
[Scheme 1(b)] is preferred.4e
In summary, the silica supported tantalum hydride 1 catalyses
the H/D exchange reaction in CH4/CD4 mixtures. This reaction
proceeds under mild conditions and leads to the formation of the
statistical distribution of all isotopomers of methane. Its rate is
approximately two orders of magnitude faster than that of the
alkane metathesis reaction. A mechanism in which the tantalum
hydride species is the active intermediate in the catalytic cycle
rather than the Ta–CH3 species is consistent with the experi-
mental observations.
We are grateful to CNRS and ESCPE Lyon for financial
support. We would also like to thank Dr O. Maury, Professors
I. P. Rothwell (Purdue University) and R. A. Andersen (UC
Berkeley) for fruitful discussions.
Notes and references
† A statistical distribution corresponds to a random distribution of all
hydrogen and deuterium atoms among the five possible isotopomers of
methane and can be calculated by the coefficients of the polynom: (d0
+
d4)4, where d0 and d4 represent the initial amount of non-deuterated and
perdeuterated methane, respectively. The consumption of CH4 and CD4 as
well as the formation of the different isotopomers of methane (d1, d2, d3)
were monitored by IR spectroscopy. CH4, CDH3, CD2H2, CD3H and CD4
show characteristic IR bands at 1347 cm21 (CH4), 1156 cm21, (CDH3),
1090 cm21 (CD2H2), 1034 cm21 (CD3H) and 994 cm21 (CD4). The
isotopomeric composition of the gas phase was also measured by GC–MS
at the end of the reaction. The determination of the isotopomeric distribution
was evaluated by minimizing the sum of the square difference of the
respective peaks of calculated theoretical and experimental spectra. The
calculated theoretical spectra were generated by incrementing the selectiv-
ity of various relative amounts of d0-, d1-, d2-, d3- and d4-methanes, which
fragmentation patterns were taken from Compilation of mass spectral data,
ed. A. Cornu and R. Massot, Heyden & Son Limited, London, 1966. The
mass spectrum of a mixture of variously labelled methanes was considered
according to a peak distribution in the range m/z 13–20.
‡ Changes in the intensity of the Ta–D vibration band are not easily
observed since this band appears at the border of the spectral window of
silica and is partially truncated.
1 R. H. Crabtree, Chem. Rev., 1995, 95, 987.
2 K. Morikawa, W. S. Benedict and H. S. Taylor, J. Am. Chem. Soc., 1936,
58, 1445; G. Parravano, E. F. Hammel and H. S. Taylor, J. Am. Chem.
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Scheme 1
3 C. Kemball, Adv. Catal., 1959, 11, 223; L. Guczi and Z. Karpinski,
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A two-step experiment, in which 1 is first contacted with CD4
(step 1) and then with CH4 (step 2) under the same conditions,
should provide a different isotopomeric distribution depending
on the reaction mechanism (Scheme 2).
4 (a) A. E. Shilov, Activation and Functionalization of Alkanes, ed. C. L.
Hill, Wiley Interscience, New York, 1989; (b) W. D. Jones and J. A.
Maguire, Organometallics, 1986, 5, 590; (c) P. L. Watson, J. Chem. Soc.,
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Bercaw, J. Am. Chem. Soc., 1987, 109, 203.
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Lécuyer, F. Quignard and A. Choplin, New. J. Chem., 1994, 18, 115.
6 V. Vidal, A. Théolier, J. Thivolle-Cazat, J.-M. Basset and J. Corker,
J. Am. Chem. Soc., 1996, 118, 4595.
Scheme 2
7 V. Vidal, A. Théolier, J. Thivolle-Cazat and J.-M. Basset, J. Chem. Soc.,
Chem. Commun., 1995, 991.
8 M. Chabanas, V. Vidal, C. Copéret, J. Thivolle-Cazat and J.-M. Basset,
Angew. Chem., Int. Ed., 2000, in press.
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276, 99; V. Vidal, A. The´olier, J. Thivolle-Cazat and J.-M. Basset,
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Chem., Int. Ed., 1999, 38, 1952.
Therefore, the reaction of CD4 (30 Torr) with 1 at 150 °C for
3 h (step 1) led to the formation of CD3H as the sole methane
isotopomer in the gas phase. After removing the gas phase, the
addition of 20 Torr of CH4 (step 2) followed by a 3 h reaction
at 150 °C gave CDH3 as the only deuterated product present in
the gas phase. This shows that despite the probable formation of
Ta–CD3 species in step 1 (vide supra), it does not participate
efficiently in the H/D exchange process since CD3H has not
664
Chem. Commun., 2000, 663–664