Communication: The insertion of silylene in C-H bonds; Rate constant limits and the energy barrier

Article abstract of DOI:10.1002/(SICI)1097-4601(1999)31:5<393::AID-KIN9>3.0.CO;2-2

The technique of laser flash photolysis has been used to set limits on the rate constants for the bimolecular reactions of SiH₂ with methane (CH₄) and tetramethylsilane (SiMe₄) at both ambient and elevated temperatures (ca 600 K). These limits show that the energy barriers to insertion reactions of SiH₂ in the C-H bonds of CH₄ are at least 45(±6) kJ mol^-1 and in the C-H and/or Si-C bonds of SiMe₄ are at least 23(±6) kJ mol^-1. The best thermochemical estimate of the activation energy for SiH₂+CH₄ is 59(±12) kJ mol^-1. Reasons for the greatly diminished reactivity of SiH₂ with C-H as compared with Si-H bonds are discussed.

Full text of DOI:10.1002/(SICI)1097-4601(1999)31:5<393::AID-KIN9>3.0.CO;2-2

Communication: The
Insertion of Silylene in
C9H Bonds; Rate Constant
Limits and the
Energy Barrier
ROSA BECERRA,1 ROBIN WALSH2
1Instituto de Quimica-Fisica ‘Rocasolano’, C.S.I.C., C/Serrano 119, 28006 Madrid, Spain
2Department of Chemistry, University of Reading, Whiteknights, P.O. Box 224, Reading RG6 6AD, U.K.
Received 2 September 1998; accepted 1 December 1998
ABSTRACT: The technique of laser flash photolysis has been used to set limits on the rate
constants for the bimolecular reactions of SiH2 with methane (CH4) and tetramethylsilane
(SiMe4) at both ambient and elevated temperatures (ca 600 K). These limits show that the
energy barriers to insertion reactions of SiH2 in the C9H bonds of CH4 are at least 45(Ϯ6)
kJ molϪ1 and in the C9H and/or Si9C bonds of SiMe4 are at least 23(Ϯ6) kJ molϪ1. The best
thermochemical estimate of the activation energy for SiH2 ϩ CH4 is 59(Ϯ12) kJ molϪ1. Reasons
for the greatly diminished reactivity of SiH2 with C9H as compared with Si9H bonds are
discussed. ᭧ 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 393–395, 1999
INTRODUCTION
been molecules containing Si9H bonds [3–8], and
more recently Ge9H bonds [9]. Insertion by SiH2
into these bonds is a very facile process, occurring at
close to the collision rate. By contrast, reaction with
C9H bonds is very slow. For the reaction between
SiH2 and CH4, Inoue and Suzuki [3] reported a rate
constant of (1.0 Ϯ 0.3) ϫ 10Ϫ12 cm3 moleculeϪ1 sϪ1 at
298 K, while Chu, Beach, and Jasinski [10] obtained
The simplest silylene, SiH2, is a key intermediate in
the breakdown mechanisms of silicon hydrides leading
to deposition of amorphous silicon hydride materials
(CVD). Silylenes in general are involved as reactive
intermediates in organosilicon compound breakdown
mechanisms. Interest in these species has grown, not
only because of their involvement in materials pro-
duction processes, but also because, in recent years,
they have become accessible to direct, time-resolved
monitoring by laser resonant absorption [1,2]. Since
the first study by Inoue and Suzuki [3], removal rates
a value of (2.5 Ϯ 0.5) ϫ 10Ϫ cm3 moleculeϪ sϪ ,
which, they pointed out, should be regarded as an up-
per limit since as little as 0.01% of impurity reacting
on every collision could account for this rate constant.
This reaction may be regarded as a benchmark for
C9H bond insertion, and since intramolecular
C9H insertion by silylenes is known to occur [11],
it seemed worthy of re-examination. Moreover, from
other considerations this reaction is thought to have an
14
1
1
1
constants for SiH2 (in its ground, A1, state) with a
wide variety of organic and inorganic molecules have
been measured [1,2]. Prominent amongst these have
1
activation barrier lying between 42 and 80 kJ molϪ
Correspondence to: R. Walsh
᭧ 1999 John Wiley & Sons, Inc.
CCC 0538-8066/99/050393-03
[11,12], and therefore an attempt to study it at elevated

394
BECERRA AND WALSH
temperatures also seemed worthwhile. We decided
also to include in our study the reaction of SiH2 ϩ
Me4Si, for which previously k Յ (2.7 Ϯ 1.5) ϫ
10Ϫ13 cm3 moleculeϪ1 sϪ1 had been reported [5], since
this molecule has the potential for Si9C bond as well
as C9H bond insertion.
and distillation, a repeat of these experiments gave a
1
rate constant of 1.7 ϫ 10Ϫ13 cm3 moleculeϪ1 sϪ . This
was the lowest value we could obtain and represents
our best upper limit for k2 for the reaction:
SiH2 ϩ SiMe4 !: product
(2)
SiH2 kinetic studies have been carried out by the
laser flash photolysis technique, the details of which
have been published previously [5,7,13]. SiH2 was
produced by photodecomposition of phenylsilane us-
ing the 193 nm line of a pulsed excimer laser. SiH2
was detected and monitored in real time by use of a
single-mode dye laser tuned to a known strong vibra-
tion-rotation transition at 17259.50 cmϪ1 in the visible
A ; X absorption band. Signal decays from 5–10
photolysis laser shots were averaged and were found
to give good first-order kinetic fits. Experiments were
carried out with gas mixtures containing a few mTorr
of phenylsilane and varying quantities of substrate gas
(CH4 up to 150 Torr, or SiMe4 up to 25 Torr). Methane
was supplied by Matheson (UHP) as 99.995% pure. It
contained no GC detectable impurities and was han-
dled carefully to avoid any contamination by noncon-
densibles (air). The tetramethylsilane (TMS) was sup-
plied by Aldrich (nmr grade) as 99.9ϩ% pure. GC
analysis indicated 99.84% purity. It was degassed be-
fore use.
At 527 K the rate constant for the purified TMS sample
was 5.89 ϫ 10Ϫ cm3 moleculeϪ sϪ . Although the
values of k2 at the two temperatures differ, they are
still of a magnitude such that they can be accounted
for by an impurity (at the 0.16% level) reacting at the
collision rate. We were not encouraged to investigate
intermediate temperatures.
13
1
1
The limiting rate constants obtained here for k1 and
k2 at ambient temperatures are less than those obtained
previously [3,5,10], supporting our conclusion that the
previous values were limits that we have improved
through the use of purer samples. Although the mag-
nitudes of the true rate constants remain almost cer-
tainly yet to be measured, nevertheless the values ob-
tained here enable us to set lower limits to their
activation energies, through use of estimated A fac-
10.5
1
1
tors of A1 ϭ 10Ϫ
cm3 moleculeϪ sϪ and A2 ϭ
10Ϫ10.0 cm3 moleculeϪ sϪ . These have been obtained
via log(A1/AϪ ) ϭ ⌬S0/R, using AϪ ϭ 1014.5 sϪ [12]
1
1
1
1
1
and ⌬S0 obtained from tabulated standard entropies for
CH4 [14], SiH2 [15], and CH3SiH3 [16]. A2 was as-
sumed to be 3 ϫ A1 on statistical grounds. Use of
these values leads to the activation energy values
shown in Table I. Clearly the benefit of the higher
temperature studies is to have provided better (higher)
limits. The uncertainties quoted allow for an uncer-
tainty of 10Ϯ0.5 in the A factors.
The reaction with CH4 was studied at 300 K and
610 K. At 300 K, decay constants ranged from
1
1
5.4 ϫ 104 sϪ in the absence of CH4 to 7.2 ϫ 104 sϪ
in the presence of 142 Torr of CH4, with values be-
tween these at intermediate pressures. Interpreted lit-
erally, this corresponds to a second-order rate constant
15
1
1
of 3.9 ϫ 10Ϫ cm3 moleculeϪ sϪ , but in reality the
variation is almost negligible, given a Ϯ10% normal
variation in decay constants. Thus this value can be
taken as an upper limit for k1 for the reaction:
It is instructive to compare these values with that
which may be obtained by consideration of the process
(Ϫ1):
SiH2 ϩ CH4 !: CH3SiH3
(1)
CH3SiH3 !: CH4 ϩ SiH2
At 610 K, decay constants were 3.8 ϫ 104 sϪ1 without
CH4 and 4.1 ϫ 104 sϪ1 in 149 Torr of CH4. This cor-
From shock tube pyrolysis studies, O’Neal and Ring
1
responds to 1.3 ϫ 10Ϫ15 cm3 moleculeϪ1 sϪ , but given
and coworkers [12] obtained (T ϭ 1150 Ϫ 1250 K):
1
1
log(kϪ /sϪ ) ϭ 14.50 Ϫ (283 Ϯ 12) kJ molϪ /RT ln10.
1
normal uncertainties probably the upper limit should
15
1
1
be taken as 4.0 ϫ 10Ϫ cm3 moleculeϪ sϪ . Inter-
mediate temperatures were not explored given the es-
sential lack of reaction at these two extremes.
Ea(1) may be obtained via: Ea(1) ϭ Ea(Ϫ1) Ϫ
Table I Summary of Derived Lower Limits for
Activation Energies
The reaction with SiMe4 was similarly studied at
296 K and 527 K. More systematic variation with sub-
strate pressures was found for this reaction, and a sec-
ond order plot at 296 K initially yielded a rate constant
Ϫ1
Ϫ1
Reaction
T/K Ea/kJ mol
T/K Ea/kJ mol
SiH2 ϩ CH4
SiH2 ϩ SiMe4 296
300
22 Ϯ 3
16 Ϯ 3
610
527
45 Ϯ 6
23 Ϯ 6
13
1
1
of 2.51 ϫ 10Ϫ cm3 moleculeϪ sϪ for this reaction.
After further drying of the TMS over molecular sieve,

THE INSERTION OF SILYLENE IN C9H BONDS
395
(⌬H0 Ϫ RT). ⌬H0 is estimated from ⌬Hf 0 for values
rier less of a hindrance. The best examples of this are
the 1,3 C9H insertions of alkylsilylenes such as
ethylsilylene [1,2,11,22] (to form silirane rings).
for CH4 [14], SiH2 [2,17] and CH3SiH3 [16,17]. This
1
gives Ea(1) ϭ 59 Ϯ 12 kJ molϪ where the uncer-
tainty largely comes from the experimental value for
Ea(Ϫ1) but also from the uncertain temperature cor-
rection (from 1200 to 298 K). Nevertheless, this value
is consistent with the experimental result and close
enough to it to encourage belief that an experimental
measurement might be possible in the future. Until that
We thank the Direccion General de Investigacion Cientifica
y Technica (DGICYT), Spain for support to RB under proj-
ect PB94-0218-CO2-01.
1
1
is achieved, 59 Ϯ 12 kJ molϪ (14 Ϯ 3 kcal molϪ )
represents the best current estimate for Ea(1), based,
as it is, on current, recently revised, thermochemistry
BIBLIOGRAPHY
1
[17]. Previous estimates of Ea(1)/kJ molϪ were 42
[11] and 80 [12] from experiment (and thermochem-
istry) and 115 [18] and 87 [19] from ab initio theoret-
ical calculations. Unfortunately, there is no informa-
tion to estimate Ea(2). We can merely note that
insertion of SiH2 into either the C9H or Si9C bonds
of SiMe4 requires a substantial barrier.
1. Becerra, R; Walsh, R. In Research in Chemical Kinetics;
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ence between the facile insertion by SiH2 into Si9H
bonds and the difficult, activated insertion into C9H
bonds is related to the differing bond polarities. In the
former case, polarization is Siϩ 9HϪ and the elecro-
philic silylene has no difficulty approaching the neg-
atively charged H atom initially. By contrast, the po-
larization in the latter case is CϪ 9Hϩ and the silylene
access to the desired electron pair is hindered by the
positively charged H atom. However, undoubtedly
other factors are involved since the analogous inser-
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1
tion reaction of methylene (CH2, A1 state) into C9
H bonds is not hindered. In fact the insertion reactions
of CH2 into C9H, Si9H, and Ge9H are all bar-
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1
rierless processes [20]. One explanation for the differ-
ence between C9H insertion reactions of SiH2 and
CH2 could be the presence of the low-lying triplet state
of CH2 (the triplet state of SiH2 is higher in energy)
which could participate in configurational mixing to
lower the barrier for 1CH2 ϩ CH4 as discussed by Sax
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1
and Olbrich [21] in comparing the analogous SiH2
and 1CH2 insertion reactions with H2. It has been noted
[20] that insertion efficiencies of 1CH2 with the Group
14 hydrides (CH4, SiH4, and GeH4 ) correlate with
changes in bond strengths. In a crude sense this is also
true for SiH2, but bond dissociation energy values of-
fer no model to predict the magnitude of the activation
energy for SiH2 ϩ CH4.
Finally, it is worth noting that while the magnitude
of the barrier makes silylene insertion into a C9H
bond a process unlikely to be observed as an inter-
molecular, that is, bimolecular, process, it is known to
occur as an intramolecular, that is, unimolecular, pro-
cess. This is at least in part because the insertion site
is near the silylene center, making the activation bar-
16. Doncaster, A. M.; Walsh, R. J Chem Soc Faraday Trans
2 1986, 82, 707.
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Apeloig, Y. Eds.; Wiley: New York, 1998; Vol. II, pp
153–180.
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metallics 1991, 10, 3513.