3246
J. Am. Chem. Soc. 2000, 122, 3246-3247
Silylene Does React with Carbon Monoxide
Rosa Becerra and Robin Walsh*,‡
†
Instituto de Quimica-Fisica ‘Rocasolano’, C.S.I.C.
C/Serrano 119, 28006 Madrid, Spain
Department of Chemistry, UniVersity of Reading
Whiteknights, P.O. Box 224, Reading RG6 6AD, UK
ReceiVed January 10, 2000
2
Silylene, SiH , is known to react rapidly and efficiently with
1,2
many chemical species. Examples of its reactions include Si-H
bond insertions, CdC and CtC π-bond additions, and reactions
3
with lone pair donors. It therefore appears somewhat surprising
4
that direct, time-resolved kinetic studies by Chu et al. of the
reaction of SiH
rate constant of only 10 cm molecule
in 5 Torr He buffer gas). This corresponds to a collision efficiency
2
with CO give an upper limit for the reaction
Figure 1. Some second-order plots of the dependence of kobs on carbon
monoxide pressure at different overall pressures/Torr (SF ): O, 30; 2,
0; 0, 100; *, 200.
-
13
3
-1 -1
s
(in the gas phase
6
5
-
3
1
of less than 10 , and contrasts with the reaction of CH + CO
2
which is at least 400 times faster. The stimulus for the present
decomposition of phenylsilane using the 193 nm ArF line of a
pulsed excimer laser. SiH concentrations were monitored in real
time by means of a single-mode dye laser tuned to the known
reinvestigation of this reaction was the recent report by Maier et
5
2
al. of the IR spectrum of silaketene, H
2
SiCO (the probable
product of SiH
was supported by ab initio calculations which suggest that H
SiCO is slightly more stable than previously thought. The story
is somewhat paralleled by the situation in respect of the potential
2
+ CO), in a frozen Ar matrix at 12 K. The study
-
1
12
1
7259.50 cm ro-vibrational transition in the visible A r X
5
2
-
absorption band. Signal decays from 5 to 20 photolysis laser shots
were averaged and found to give good first-order kinetic fits.
Experiments were carried out with gas mixtures containing a few
mTorr of phenylsilane, varying quantities of CO (>99.96% pure)
6
reaction between dimethylsilylene, SiMe
2
, with CO where no gas-
7
phase reaction could be found in our laboratory while matrix
6
up to 10 Torr, and inert diluent SF to total pressures between 10
isolation studies8 strongly point to formation of Me
,9
2
SiCO.
and 500 Torr.
We reasoned that a possible cause for the failure to find reaction
in the earlier kinetic studies might have been the low pressures
employed, allied with the inefficiency of helium as a collision
partner. Thus the reaction might well initially form vibrationally
excited silaketene as shown in the scheme, but which may rapidly
redissociate via breaking of its weak bond unless collisionally
stabilized. In other words this is potentially a classic example of
an association reaction requiring a third body.
Initial studies at 10 Torr total pressure with varying CO
pressures up to 10 Torr showed a significant, although fairly small,
effect of CO on the pseudo-first-order decay constants, kobs, for
2
SiH . However, at higher total pressures the effect of added CO
was much larger. Some examples of these results are shown in
Figure 1. The linear dependence of kobs on [CO] indicates second-
order behavior, consistent with the association reaction 1. The
second-order plot gradients of Figure 1 and the rest of the data
are themselves nearly linearly proportional to total pressure over
the range 20-500 Torr. This indicates an overall third-order
reaction consistent with a third body assisted association (com-
bination) as surmised. The results give a third-order rate constant
H Si + CO a H SiCO*
9
8 H SiCO
(1)
2
2
2
M
Thus our approach was to investigate the reaction at much higher
-
31
6
-2 -1
of (6.9 ( 0.8) × 10 cm molecule s .
6
pressures and with SF , a known efficient collision partner, as
bath gas.
To gain further support for the suggested mechanism we have
carried out RRKM calculations13 based on the silaketene structure
and vibrations found by Maier et al. These calculations employed
The gas-phase kinetic studies with SiH
2
were carried out by
5
the laser flash photolysis technique, the details of which have
been published previously.1
0,11
SiH
was generated by photo-
a loose transition state consistent with our findings for other
silylene reactions of insertion11 and addition, and suggested by
the experimental results obtained here, of a fairly fast reaction.
The loose transition state vibrational assignment was obtained
by reducing the wavenumbers of the transitional modes of the
2
14
†
Instituto de Quimica-Fisica ‘Rocasolano’.
University of Reading.
‡
(
(
1) Jasinski, J. M.; Becerra, R.; Walsh, R. Chem. ReV. 1995, 95, 1203.
2) Becerra, R.; Walsh, R. Kinetics & mechanisms of silylene reactions:
A prototype for gas-phase acid/base chemistry. In Research in Chemical
Kinetics; Compton, R. G., Hancock, G., Eds.; Elsevier: Amsterdam, 1995;
Vol. 3, p 263.
silaketene (SiH wag and bend, Si-CtO bends). The bond energy
2
(
RRKM critical energy) was varied within the range 90-121 kJ
-1
-1
(
3) Gaspar, P. P.; West, R. Silylenes. In The Chemistry of Organic Silicon
Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, 1998; Vol.
, Chapter 43, p 2463.
4) Chu, J. H.; Beach, D. B.; Estes, R. D.; Jasinski, J. M. Chem. Phys.
Lett. 1988, 143, 135.
5) Maier, G.; Reisenauer, H.-P.; Egenolf, H. Organometallics 1999, 18,
mol (21.5-29.0 kcal mol ) corresponding to values found from
the various different levels of ab initio calculation. The collisional
5
2
stabilization model (effectively the efficiency of SF
6
) assumed
(
-1
an average removal of 12.0 kJ mol in a down collision consistent
(
11,14
with findings in similar reaction systems.
The key to fitting
2
155.
the results was to find the optimum parameters which cor-
responded to the third-order kinetic region (corresponding also
to the low-pressure limit for dissociation of silaketene). A grid
search gave the best fit for the loosest transition state tried, with
(
(
6) Hamilton, T. P.; Schaefer, H. F., III J. Chem. Phys. 1989, 90, 1031.
7) Baggott, J. E.; Blitz, M. A.; Frey, H. M.; Lightfoot, P. D.; Walsh, R.
Int. J. Chem. Kinet. 1992, 24, 127.
(
8) Arrington, C. A.; Petty, J. T.; Payne, S. E.; Haskins, W. C. K. J. Am.
Chem. Soc. 1988, 110, 6240.
(
9) Pearsall, M.-A.; West, R. J. Am. Chem. Soc. 1988, 110, 7229.
(
10) Baggott, J. E.; Frey, H. M.; King, K. D.; Lightfoot, P. D.; Walsh, R.;
(12) Jasinski, J. M. J. Phys. Chem. 1986, 90, 555.
Watts, I. M. J. Phys. Chem. 1988, 92, 4025.
(13) Holbrook, K. A.; Pilling, M. J.; Robertson, S. H. Unimolecular
Reactions, 2nd ed.; Wiley: Chichester, 1996.
(11) Becerra, R.; Frey, H. M.; Mason, B. P.; Walsh, R.: Gordon, M. S. J.
Chem. Soc., Faraday Trans. 1995, 91, 2723.
(14) Al-Rubaiey N.; Walsh, R. J. Phys. Chem. 1994, 98, 5303.
1
0.1021/ja000103h CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/21/2000