First gas-phase detection of dimethylstannylene and time-resolved study of some of its reactions

Article abstract of DOI:10.1021/ja012691k

Using a laser flash photolysis/laser probe technique, we report the observation of strong absorption signals in the wavelength region 450-520 nm (highest intensity at 514.5 nm) from four potential precursors of dimethylstannylene, SnMe₂, subjected to 193 nm UV pulses. From GC analyses of the gaseous products, combined with quantum chemical excited state CIS and TD calculations, we can attribute these absorptions largely to SnMe₂, with SnMe₄ as the cleanest source of the species. Kinetic studies have been carried out by time-resolved monitoring of SnMe₂. Rate constants have been measured for its reactions with 1,3-C₄H₆, MeC, CMe, MeOH, 1-C₄H₉Br, HCl, and SO₂. No evidence could be found for reaction of SnMe₂ with C₂H₄, C₃H₈, Me₃SiH, GeH₄, Me₂GeH₂, or N₂O. Limits of less than 10^-13 cm³ molecule^-1 s^-1 were set for the rate constants for these latter reactions. These measurements showed that SnMe₂ does not insert readily into C-H, Si-H, Ge-H, C-C, Si-C, or Ge-C bonds. It is also unreactive with alkenes although not with dienes or alkynes. It is selectively reactive with lone pair donor molecules. The possible mechanisms of these reactions are discussed. These results represent the first visible absorption spectrum and rate constants for any organo-stannylene in the gas phase.

Full text of DOI:10.1021/ja012691k

Published on Web 05/31/2002
First Gas-Phase Detection of Dimethylstannylene and
Time-Resolved Study of Some of Its Reactions
†
‡
‡
‡
Rosa Becerra, Sergey E. Boganov, Mikhail P. Egorov, Valery I. Faustov,
‡
‡
,§
Irina V. Krylova, Oleg M. Nefedov, and Robin Walsh*
Contribution from the Instituto de Quimica-Fisica “Rocasolano”, C.S.I.C., C/Serrano 119,
2
8006 Madrid, Spain, the N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of
Sciences, Leninsky Prospekt 47, 117913 Moscow, Russian Federation, and the Department of
Chemistry, UniVersity of Reading, Whiteknights, P.O. Box 224, Reading RG6 6AD, UK
Received December 12, 2001
Abstract: Using a laser flash photolysis/laser probe technique, we report the observation of strong absorption
signals in the wavelength region 450-520 nm (highest intensity at 514.5 nm) from four potential precursors
of dimethylstannylene, SnMe
combined with quantum chemical excited state CIS and TD calculations, we can attribute these absorptions
largely to SnMe , with SnMe as the cleanest source of the species. Kinetic studies have been carried out
by time-resolved monitoring of SnMe . Rate constants have been measured for its reactions with 1,3-
Br, HCl, and SO . No evidence could be found for reaction of SnMe with
GeH , or N
O. Limits of less than 10- cm molecule
2
, subjected to 193 nm UV pulses. From GC analyses of the gaseous products,
2
4
2
C
C
4
H
H
6
, MeCtCMe, MeOH, 1-C
4
H
9
2
2
13
3
-1 -1
2
4
, C , Me SiH, GeH , Me
H
3 8
3
4
2
2
2
s
were set for
the rate constants for these latter reactions. These measurements showed that SnMe
2
does not insert
readily into C-H, Si-H, Ge-H, C-C, Si-C, or Ge-C bonds. It is also unreactive with alkenes although
not with dienes or alkynes. It is selectively reactive with lone pair donor molecules. The possible mechanisms
of these reactions are discussed. These results represent the first visible absorption spectrum and rate
constants for any organo-stannylene in the gas phase.
Introduction
Since in recent years we have been able to generate and study
the kinetics of reactions of other members of the heavy carbene
Despite the fact that stabilized dialkylstannylenes have been
known for 25 years since the preparation of bis(bis(trimethyl-
4,5
6
family such as SiMe2 and GeMe2 by direct means using time-
resolved methods in the gas phase, we decided to turn our
attention to SnMe2. The objective of the present study was to
create SnMe2 in the gas phase, characterize its visible/UV
absorption spectrum, and study the kinetics of some of its
characteristic reactions. Although the visible/UV spectrum of
silyl)methyl)tin(II), Sn[CH(SiMe3)2]2, by Lappert’s group in
1
1
976, rather little is known about the simpler organostannylenes
such as SnMe2. Certainly the existence of Sn(CH3)2 is in little
doubt since the recording of its IR spectrum, together with that
of its deuterio analogue, Sn(CD3)2, in an argon matrix by
Neumann’s group in 1982. Neumann has indeed investigated
and documented the known chemistry of SnMe2. It appears
that although SnMe2 is fairly easy to prepare, it is a reactive
transient that does not insert into C-H or C-C bonds, and even
its addition to CdC double bonds is unknown. In the presence
of many potential reagents it simply polymerizes. Until now
such conclusions have been based on inferences from end
product analyses, i.e. lack of a specific product in reaction
mixtures where SnMe2 may plausibly be assumed to have been
generated.
1
1
2
SnMe2 is unknown, the low-lying A1 f B1 transition may
3
reasonably be expected to occur in the wavelength region 450-
6
5
20 nm, by analogy with GeMe2. This argument depends on
the similarity of energy spacings in the electronic states of SnH2
7
and GeH2. To provide a more sound basis for this, we decided
to calculate the energy of the lowest S0 f S1 electronic transition
8
,9
for SnMe2, using quantum chemical methods. The spectrum
(
4) Becerra, R.; Walsh, R. Kinetics and 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, The
Netherlands, 1995; Vol. 3, p 263.
(
5) (a) Baggott, J. E.; Blitz, M. A.; Frey, H. M.; Lightfoot, P. D.; Walsh, R.
Chem. Phys. Lett. 1987, 135, 39. (b) Baggott, J. E.; Blitz, M. A.; Frey, H.
M.; Lightfoot, P. D.; Walsh, R. J. Chem. Soc., Faraday Trans. 1988, 84,
*
To whom correspondence should be addressed. E-mail: r.walsh@
reading.ac.uk.
†
Instituto “Rocasolano”.
Zelinsky Institute.
University of Reading.
515. (c) Baggott, J. E.; Blitz, M. A.; Frey, H. M.; Walsh, R. J. Am. Chem.
‡
§
Soc. 1990, 112, 8337. (d) Baggott, J. E.; Blitz, M. A.; Frey, H. M.; Lightfoot,
P. D.; Walsh, R. Int. J. Chem. Kinet. 1992, 24, 127. (e) Al-Rubaiey, N.;
Carpenter, I. W.; Walsh, R.; Becerra, R.; Gordon, M. S. J. Phys. Chem. A
1998, 102, 8564.
(
1) (a) Davidson, P. J.; Harris, D. H.; Lappert, M. F. J. Chem. Soc., Dalton
Trans. 1976, 2268. (b) Cotton, J. P.; Davidson, P. J.; Lappert, M. F. J.
Chem. Soc., Dalton Trans. 1976, 2275. (c) Cotton, J. P.; Davidson, P. J.;
Lappert, M. F.; Donaldson, J. D.; Silver, J. J. Chem. Soc., Dalton Trans.
(6) (a) Becerra, R.; Boganov, S. E.; Egorov, M. P.; Lee, V. Ya.; Nefedov, O.
M.; Walsh, R. Chem. Phys. Lett. 1996, 250, 111. (b) Becerra, R.; Egorov,
M. P.; Krylova, I. V.; Nefedov, O. M.; Walsh, R. Chem. Phys. Lett. 2002,
351, 47.
(7) (a) Olbrich, G. Chem. Phys. Lett. 1980, 73, 110. (b) Balasubramanian, K.
J. Phys. Chem. 1988, 89, 5731.
1
976, 2286.
(2) Bleckmann, P.; Maly, H.; Minkwitz, R.; Neumann, W. P.; Watta, B.;
Olbrich, G. Tetrahedron Lett. 1982, 23, 4655.
(3) Neumann, W. P. Chem. ReV. 1991, 91, 311.
10.1021/ja012691k CCC: $22.00 © 2002 American Chemical Society
J. AM. CHEM. SOC. 2002, 124, 7555-7562
9
7555

A R T I C L E S
Becerra et al.
is expected to be broad and featureless, and if it lies in this
region, this makes SnMe2 suitable for monitoring in absorption
using an argon ion laser.
On the basis of the analogy with our earlier studies4-6 of
SiMe2 and GeMe2, the rational photoprecursors for SnMe2
would be pentamethyldistannane and 1,1-dimethyl-1-stanna-
cyclopent-3-ene. Lack of availability of these compounds led
us to screen a number of other potential photoprecursors, viz.
SnMe4, Sn2Me6, Me3SnH, and PhSnMe2H. This paper describes
our initial efforts, and the first gas-phase kinetic data generated
for the species SnMe2.
this was disappointing, this compound was not crucial to the outcome
of this work and we were able to obtain useful information from it
despite the impurity. Because of the known hazards of organotin
compounds, these were prepared, handled, and vented in fume
cupboards.
All gases used in this work were degassed thoroughly prior to use.
Commercial samples of reactive substrates used in this work were
obtained as follows. Hydrocarbons (all >99%) were from Cambrian
gases. HCl (99+%), N
2
O (99.997%), MeOH (Gold label, 99+%), and
BuBr (99%) were from Aldrich. SO (99.5%) was from BDH. Me
SiH (99%) was from Fluorochem. GeH (99%) and Me GeH (98%)
n
2
3
-
15
6b
4
2
2
were prepared by us previously. Gas chromatographic analyses of
reactant and product mixtures were carried out on a Perkin-Elmer 8310
chromatograph equipped with a flame ionization detector. A 3 m
silicone oil (OV101) column operated at 60 °C (or higher) was used to
analyze most of the systems investigated, although other columns, such
as Porapak Q, were also used when necessary (for light hydrocarbons).
Retention times and peak sensitivities (GC response factors) were
calibrated with authentic samples where possible.
Experimental Section
The apparatus and equipment for these studies have been described
5
a,6
in detail previously. Only essential and brief details are therefore
included here. The target reactive transient was produced by flash
photolysis of appropriate precursor molecules (see below) using a
Coherent Compex 100 exciplex laser operating at 193 nm (ArF fill).
Transient species absorptions were monitored in real time by means
of a Coherent Innova 90-5 argon ion laser. For species characterization
all nine available lines of the probe laser were employed, but for the
kinetic studies the argon ion laser was generally only operated at 501.7
or 514.5 nm. Experiments were carried out in a spectrosil quartz cell
with demountable windows. The photolysis beam (4 cm × 1 cm cross-
section) entered the center of the cell laterally, while the probe beam
was multipassed longitudinally along the axis of the cell up to 44 times,
giving a maximum absorption path length of ca. 1.7 m. Photolysis laser
pulse energies were typically 50-70 mJ with a variation of (5%. Light
signals were measured by a dual photodiode/differential amplifier
combination, and signal decays were stored in a transient recorder
Quantum-Chemical Calculations
Quantum-chemical calculations were carried out on SiMe
2
and
GeMe as well as SnMe in order to see how the methods worked on
2
2
similar species with known visible/UV spectra. Calculations were
performed at two levels of theory. Geometry optimization and
vibrational analyses of ZMe (Z ) Si, Ge, Sn) in the ground state were
2
done using ab initio HF and DFT B3LYP16 methods. The 6-31+G(d)
basis set was used for H, C, Si, and Ge atoms. This basis set does not
exist for Sn and therefore the quasirelativistic effective core potential
(ECP) of Stevens et al.17 combined with a split valence basis set
supplemented by sets of d-functions (R ) 0.183) and diffuse
d
(Datalab DL 910) interfaced to a BBC microcomputer. This was used
sp-functions (Rsp ) 0.0231) was used instead. These basis sets are of
to average the decays of typically five photolysis laser shots (at a
repetition rate of 1 Hz or less).
Gas mixtures for photolysis were made up containing 10-30 mTorr
of the transient precursor, variable pressures of reactive substrates with
a high enough quality for these calculations. Energies of the lowest
8
vertical transitions in ZMe
2
were calculated with CIS and TD DFT
9
B3LYP methods. The calculations were carried out with GAUSSIAN
18
98 at the computer center of N. D. Zelinsky Institute of Organic
Chemistry, Russian Academy of Sciences, Moscow.
total pressures made up to 5 or 10 Torr with inert diluent (SF
6
).
Pressures were measured with capacitance manometers (MKS Baratron).
Most measurements were made at room temperature or 296 ( 2 K.
The organotin compounds used in this work were obtained or
Results
SnMe2 Precursors. Although the tin analogues to our SiMe2
and GeMe2 precursors were not available, we tested four
compounds as sources for this transient, viz. SnMe4, Sn2Me6,
Me3SnH, and PhMe2SnH. All these compounds had strong UV
absorptions at the 193 nm wavelength of photolysis. Laser
photolysis gave rise to transient absorptions from all four
compounds. Photodecomposition was accompanied in all cases
by dust formation. Interference by dust was kept to a minimum,
by keeping the exciplex laser energy low, waiting between shots
prepared as follows. Tetramethyltin, SnMe
at >99.5% purity. Hexamethyldistannane, Sn
yield) by a coupling reaction of Me SnCl with Li metal in THF solution
in an ultrasound bath similarly to the method of Mironov and
4
, was obtained from Ventron
2
Me , was made (in 70%
6
3
10
Kravchenko. It was purified by vacuum distillation to better than 95%
by GC analysis). Trimethylstannane, Me SnH, was made by the LiAlH
(
3
4
n
11
reduction of Me
3 2 2
SnCl, in Bu O solution under N . The product was
collected and purified by low-temperature distillation to >94% purity
1
13
12,13
(
by GC analysis). Its identity was confirmed by H and C NMR.
Phenyldimethylstannane was made in a three-step synthesis using well-
known procedures. The first step was the Grignard coupling of Me
SnCl and PhMgBr to give Me SnPh . This was followed by reaction
of the latter with I in CCl to give Me
reduction of the iodide by LiAlH in ether solution. Unfortunately
the crude PhSnMe H was contaminated with a not easily separated
compound and could only be obtained in ca. 30% purity. Although
(the dust is seen to settle out from the probe beam region), and
2
-
frequent cleaning of the reaction vessel. The nature of the dust
was not investigated. As well as recording transient absorption
2
2
2
1
4
2
4
2
SnPhI. The last step was the
1
1
4
(
15) Becerra, R.; Boganov, S. E.; Egorov, M. P.; Faustov, V. I.; Nefedov, O.
2
M.; Walsh, R. J. Am. Chem. Soc. 1998, 120, 12657.
(
(
(
16) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
17) Stevens W.; Basch, H.; Krauss, J. J. Chem. Phys. 1984, 81, 6026.
18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.;
Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.;
Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98; Gaussian,
Inc.: Pittsburgh, PA, 1998.
(
8) Foresman, J. B.; Head-Gordon, M.; Pople J. A.; Frisch, M. J. J. Phys. Chem.
1
992, 96, 135.
(9) (a) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys. Lett. 1996, 256, 454. (b)
Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys. 1998, 109,
8
218.
(
10) Mironov, V. F.; Kravchenko, A. L. IzV. Akad. Nauk SSSR, Ser. Khim. 1965,
1
026.
(
11) Finholt, A. E.; Bond, A. C.; Wilzbach, K. E.; Schlesinger, H. J. J. Am.
Chem. Soc. 1947, 69, 2692.
(12) Flitcroft, N.; Kaesz, H. D. J. Am. Chem. Soc. 1963, 85, 1337.
(13) Mitchell, T. N. J. Organomet. Chem. 1973, 59, 189.
(14) Davison, A.; Rakita, P. J. Organomet. Chem. 1970, 23, 407.
7556 J. AM. CHEM. SOC.
9
VOL. 124, NO. 25, 2002

First Gas-Phase Detection of Dimethylstannylene
A R T I C L E S
spectra, we also examined briefly the precursor photochemistry
by end product analysis. This is described in a later section.
SnMe4 has a UV spectrum with λmax ) 189 nm and ꢀ ) 1.6
10 dm mol cm . End product analyses (see below) show
4
3
-1
-1
×
C2H6 to be the major photoproduct formed via a molecular
pathway. Under time-resolved conditions strong absorptions
were obtained in the 450-520 nm region. Although the
extinction coefficient of SnMe2 is not known, transient decay
traces exhibiting good signal-to-noise ratios could be obtained
with 30 mTorr of SnMe4 with averaging over 5 shots. It is
possible that as much as 20% of the transient precursor in the
irradiated volume may be decomposed per laser shot. However,
since only ca. 5% of the total vessel volume was irradiated there
is virtually no loss of precursor overall. The precursor in the
photolyzed volume is replenished by diffusion between shots.
Figure 1. Spectra obtained from zero-time transient absorptions as a
function of wavelength. Data points from different precursors are the
following: (2, SnMe4; (O) Sn2Me6; (0) Me3SnH; and (f) PhSnMe2H.
All spectra have been scaled to a common absorption at 514.5 nm. The
solid line is the best fit gas-phase spectrum for SnMe2 (see text). The dashed
line is the gas-phase GeMe2 absorption spectrum from ref 6a.
Sn2Me6 has a UV spectrum with λmax ) 220 nm and ꢀ )
4
3
-1
-1
1
.15 × 10 dm mol cm . End product analyses (see below)
show SnMe4 to be a major photoproduct, mainly formed via a
molecular pathway. Again strong absorptions were observed in
the 450-520 nm region. Under the same experimental condi-
tions (precursor concentration and total pressure) the transient
signals were about 3 times stronger than those from SnMe4.
Table 1. Absorption Wavelengths (nm) for the S0 f S1 Vertical
Transitions in ZMe2 (Z ) Si, Ge, Sn) Calculated by CIS and TD
Methods
method
SiMe
2
GeMe
2
SnMe
2
Me3SnH has a UV spectrum with λmax ) 192 nm and ꢀ )
3
3
-1
-1
CIS
TD
exptl (λmax)
437
483
450
495
459
511
>514
5
.8 × 10 dm mol cm . End product analyses (see below)
show CH4 to be a major photoproduct, mainly formed via a
molecular pathway. Again strong absorptions were observed in
the 450-520 nm region. Under the same experimental condi-
tions (precursor concentration and total pressure) the transient
signals were about 65% of those obtained from SnMe4.
458a
480b
c
a
Reference 5a. b Reference 6. c This work.
signal decays at 457.9 nm do not go to zero absorbance (even
from SnMe4), although the decay traces are very noisy because
of dust formation. This suggests the presence of either an
absorbing product or a nondecaying, absorbing reactant.
There is no previous report of this spectrum in the gas or
other phase. The evidence supporting its assignment as the S0
f S1 electronic absorption in SnMe2 includes both comparison
with the spectra of some other similar “alkyl” substituted
stannylenes and the results of our calculations of the lowest
vertical transitions in ZMe2 (Z ) Si, Ge, Sn).
Our sample (30% pure) of PhSnMe2H has a UV spectrum
4
3
-1
-1
with λmax ) 190 nm and ꢀ ) 6.3 × 10 dm mol cm . End
product analyses (see below) show benzene to be a major
photoproduct, formed mainly in a molecular pathway. Again
strong absorptions were observed in the 450-520 nm region.
Under the same experimental conditions (precursor concentra-
tion and total pressure) the transient signals were about 3× those
obtained from SnMe4 (assuming the impurity plays no role in
the photochemistry).
1
a
Lappert’s group reported the spectra of GeR2 and SnR2 [R
SnMe2 Visible Absorption Spectrum. The transient absorp-
tion spectrum was obtained by monitoring the zero-time
absorbance (obtained by fitting of exponential decay curves) at
most of the nine available wavelengths of the argon ion probe
laser for each of the precursors. In these experiments a suitable
pressure of precursor in the range 10-30 mTorr was photolyzed
in the presence of SF6 at a total pressure of 5 Torr, at photolysis
pulse energies of 65 ( 5 mJ/pulse. For each precursor the
pressure chosen was as low as possible consistent with obtaining
good signals [SnMe4, 30 mTorr; Sn2Me6, 12 mTorr; Me3SnH,
)
CH(SiMe3)2] in hexane solution. GeR2 has an absorption
maximum at 414 nm whereas SnR2 has a maximum at 495 nm.
19
Similarly, Kira et al. reported the spectra of 1-E-2,2,5,5-tetra-
19b
(
trimethylsilyl)cyclopentyl-idenes-1 (E ) Ge,19aSn ) in hexane
solution. Their absorption maxima are located at 450 and 484
nm, respectively.These are the only published “alkyl” substituted
stannylene spectra, although there are plenty of reports of spectra
2
0
of aryl and other stabilized stannylenes. Given that the gas-
phase spectrum of GeMe2 is red shifted relative to solution,6a
this should place the gas-phase absorption maximum for SnMe2
somewhat to longer wavelengths than 514.5 nm, consistent with
our findings.
2
0 mTorr; PhSnMe2H, 10 mTorr]. Because the quantum yields
are not known, the absorptions were all scaled to a common
value (100) at 514.5 nm. The results are shown in Figure 1,
which also shows the GeMe2 spectrum, obtained by us
previously,6a for reference. The absorption signals from all
precursors show, for the most part, a common trend of
decreasing with decreasing wavelength. The solid line has been
drawn through the data obtained with SnMe4 as precursor,
because it is the cleanest source (see later). The stronger
absorptions at shorter wavelengths from Sn2Me6 and PhSnMe2H
The results of our CIS and TD B3LYP calculations on the
lowest S0 f S1 transition are shown in Table 1. Both methods
give good agreement with experiment for SiMe2 and GeMe2.
(
19) (a) Kira, M.; Ishida, S.; Iwamoto, T.; Ichinoche, M.; Kabuto, C.; Ignatovich,
l.; Sakurai, H. Chem. Lett., 1999, 263. (b) Kira, M.; Yanchibara, R.; Hirano,
R.; Kabuto, C.; Sakurai, H. J. Am. Chem. Soc., 1991, 113, 7785.
20) Boganov, S. E.; Egorov, M. P.; Faustov, V. I.; Nefedov, O. M. Spectroscopic
studies and quantum-chemical calculations of short-lived germylenes,
stannylenes and plumbylenes. In The chemistry of organic germanium, tin
and lead compounds; Rappoport, Z., Ed.; Wiley, Chichester, UK, 2002, in
press.
(
(in particular) suggest the presence of other absorbing inter-
mediates. Another feature, not shown in the figure, is that the
J. AM. CHEM. SOC.
9
VOL. 124, NO. 25, 2002 7557

A R T I C L E S
Becerra et al.
Of the three ZMe2 species only SiMe2 has had its UV spectrum
studied before at a high theoretical level. The best previously
calculated value21 of the energy for the S0 f S1 transition in
SiMe2, obtained at the CISD-Q/TZP level, corresponds to a
wavelength of 456 nm and compares favorably with our results.
The calculations give a singlet ground state for all three ZMe2
2
0
species in agreement with their known multiplicities. The
SnMe2 species has C2V symmetry in the ground state although
SiMe2 and GeMe2 have C2 symmetry due to slight rotations of
the methyl groups. However, the energy differences between
C2 and C2V species (methyl hydrogens in the opposed position)
for the SiMe2 and GeMe2 species are very small (less than 0.8
-
1
kJ mol ). For SnMe2 the S0 f S1 electronic transition can be
1
1
denoted A1 f B1. The calculations place the S0 f S1 transition
in SnMe2 at 459 (CIS) and 511 nm (TD). Both values are
reasonably close to the highest absorption obtained here of 514.5
nm, although the true band maximum may be at slightly longer
wavelength. Of the two methods used TD DFT gives better
agreement with experiment. Our calculations nicely reproduce
the small shifts to longer wavelength experimentally observed
for the ZMe2 series. It is also interesting to note that the data
show that the energy of the S0 f S1 transition in all ZMe2
species is amazingly insensitive to the nature of the central atom,
Z.
In summary, both the wavelength location of this spectrum
and the similarities between that from SnMe4 and the other
precursors provide strong evidence that this is indeed a part of
the visible spectrum of SnMe2. This is further supported by the
photochemistry reported in a later section.
Figure 2. Second-order plots of the dependence of kobs on reactive substrate
pressures (monitoring wavelengths shown in parentheses): MeCtCMe (O,
501.7 nm); MeOH (4, 501.7 nm; 2, 514.5 nm); and HCl (f, 514.5 nm).
Table 2. Gas-Phase Rate Constants for SnMe2 at 296 ( 2 K
k/cm molecule s-1
3
-1
substrate
-
11
1
,3-C4H6
(5.97 ( 0.17) × 10
(7.84 ( 0.12) × 10
(2.60 ( 0.10) × 10
(8.08 ( 0.35) × 10
(2.88 ( 0.28) × 10
(3.35 ( 0.12) × 10
-12
CH3CtCCH3
CH3OH
HCl
-
-
-
12
13
12
1-C4H9Br
-11
SO2
-
14
C3H8
Me3SiH
GeH4
Me2GeH2
C2H4
N2O
e3.1 × 10
e6.2 × 10
e3.1 × 10
e9.3 × 10
e1.0 × 10
e9.3 × 10
-14
-
-
-
14
14
14
Gas-Phase Rate Constants for Reactions of SnMe2. For
kinetic studies, SnMe4 was used as the SnMe2 precursor. This
was because the other precursors all showed some evidence of
producing other transients additional to SnMe2 (see the next
section). The transient from SnMe4 was monitored at both 501.7
and 514.5 nm, in the strongest absorption region. Decay traces
-14
set upper limits for the rate constants. Apart from these, one
other substrate, viz. O2, was also studied, but a linear second-
order plot was not obtained. If a second-order reaction does
take place, it has a rate constant somewhere in the range 0.4-
(2 to 5 shot averages) were found to fit exponential forms
-
11
3
-1 -1
consistent with first-order kinetics. Linear least-squares fitting
up to 75% or greater for each decay trace was used to obtain
the first-order decay constant, kobs, for reactive substrates. For
unreactive substrates decay traces were somewhat noisier, and
fitting was only possible up to 60% in some cases. Rate
constants were found to be independent of excimer laser energy
1.7 × 10
cm molecule s . No attempt was made at this
stage to investigate the overall pressure dependence of any of
these reactions.
The Search for End Products (Reaction Photochemistry).
As a preliminary to these studies a brief investigation of the
photolysis products of each precursor (at low conversions, ca.
10-25%) was undertaken with 25 to 100 photolysis laser shots
of ca. 40-50 mJ/pulse. On the basis of GC analysis, the major
product of photodecomposition of SnMe4 (0.12 Torr in 100 Torr
of N2) was C2H6 (89%). Small amounts of CH4 (1.5%), Me3-
SnH (6%), and Sn2Me6 (3.5%) were also formed. Experiments
with added oxygen had little effect on the C2H6 yield but reduced
the Me3SnH and Sn2Me6 yields to almost zero. The lack of effect
on C2H6 shows that it cannot be produced via CH3 radical
recombination. These results strongly suggest the major primary
process for SnMe2 to be
(within a 40-80 mJ/pulse range) and also the number of laser
shots (up to 20 shots). At a given substrate pressure the
reproducibility of kobs values was within (10% of the mean,
although often better than this. Substrates were chosen to cover
a selection of potential reaction types. No reactions were found
with C3H8 (10 Torr), Me3SiH (10 Torr), GeH4 (10 Torr), Me2-
GeH2 (10 Torr), C2H4 (30 Torr), and N2O (10 Torr). Reactions
were, however, found with 1,3-C4H6 (butadiene), CH3CtCCH3,
MeOH, HCl, 1-C4H9Br, and SO2. In these studies, the partial
pressure of reactive substrate was varied in a systematic way
to test the dependence of kobs. These experiments yielded
reasonable linear plots as expected for second-order kinetics.
Examples of such plots are shown in Figure 2 for the reactions
with butyne-2, methanol, and HCl. The second-order rate
constants found from the slopes of these plots are collected in
Table 2. The uncertainties shown are single standard deviations.
For the unreactive substrates, the lack of reaction was used to
SnMe + hν (193 nm) f SnMe + C H
4
2
2
6
with a minor contribution from
SnMe + hν (193 nm) f SnMe + Me
4
3
with Sn2Me6 coming from SnMe3 recombination and Me3SnH
probably from SnMe3 disproportionation.
(21) Grev, R. S.; Schaefer, H. F., III J. Am. Chem. Soc. 1986, 108, 5804.
7
558 J. AM. CHEM. SOC. VOL. 124, NO. 25, 2002
9

First Gas-Phase Detection of Dimethylstannylene
A R T I C L E S
The photodecomposition of Sn2Me6 (0.3 Torr in 100 Torr of
N2) yielded C2H6 and SnMe4 in almost equal amounts with ca.
PhSnMe H + hν (193 nm) f PhSnMe + CH4
2
5% of Me3SnH. Experiments with added oxygen approximately
PhSnMe H + hν (193 nm) f PhSnH + C H
2
2
6
halved the yield of SnMe4 while having little effect on the C2H6
yield. This suggests that a substantial proportion of the SnMe4
comes via Me + SnMe3 combination. Clearly the SnMe3
radicals must be in excess or the C2H6 yield would also have
been significantly affected. These results suggest that g50%
of the primary process for Sn2Me6 comes via
PhSnMe H + hν (193 nm) f SnMe + C H
2
2
6
6
These analytical studies strongly indicate the formation of
SnMe as a primary product of photodissociation of all four
precursors, broadly consistent with spectral evidence. They also
2
suggest the formation of other transient stannylenes from Sn2-
Me6 (Me3SnSnMe), Me3SnH (MeSnH), and PhSnMe2H (Ph-
SnH, PhSnMe). The spectra found from each precursor show
some differences, suggestive that there may be species other
than SnMe2 present. This evidence led us to believe that the
photodecomposition of SnMe4 was likely to be the cleanest
source of SnMe2.
The main objective of the analytical studies was to character-
ize the precursor photochemistry. Additionally a brief search
by GC of the products in the reaction systems of SnMe2 with
Sn Me + hν (193 nm) f Sn Me + C H
6
2
6
2
4
2
The nature of the Sn2Me4 product is unknown but the ease of
,1-elimination of C2H6 from SnMe4 suggests that it is more
likely to be Me3SnSnMe than Me2SnSnMe2. Interestingly,
published theoretical calculations indicate that H3SnSnH is
slightly more stable than H2SnSnH2.22 The next most significant
process (ca. 25%) is
1
Sn Me + hν (193 nm) f SnMe + SnMe
4
2
6
2
1,3-C4H6 and butyne-2 indicated the clear presence of a product
with a further 15-20% contribution from
Sn Me + hν (193 nm) f 2SnMe
3
in the former case and a possible product in the latter. However,
the small size of the gas sample and the lack of any synthetic
samples of potential products left us unable to characterize these
new GC peaks. Since this was not the main objective of this
work, product studies were not further pursued here.
2
6
Methyl radicals must also be formed. They could arise either
via a further minor primary process or via secondary decom-
position of vibrationally excited SnMe3.
Discussion
The photodecomposition of Me3SnH (0.29 Torr in 100 Torr
of N2) gave rise to CH4 (45%) and C2H6 (50%) as the major
products with ca. 1.5% of Sn2Me6 as a minor product and some
Although photochemical decomposition of organostannanes
has often been used as a source of tin-centered free radicals in
solution, there seem to have been no previous gas-phase
23
3
.5% of unidentified peaks. Experiments with added oxygen
photochemical studies of the stannanes used in this work. In
reduced slightly the absolute yields of both CH4 and C2H6 and
largely eliminated the minor products. The relative yields of
C2H6 and CH4 were now 45% and 55%, respectively. This
suggests a part of the C2H6 is coming via Me radical recom-
bination. These results show that there are two major primary
processes, viz.
24
low-temperature matrices it has been reported that SnMe4
24a
24b
photolysis gives rise to Me2SndCH2 and Me3SnH photolysis
produces SnMe2, identified by M o¨ ssbauer spectroscopy (also
24b
Me2SnH2 forms MeSnH ). The measurements reported here
are strongly indicative of the formation of SnMe2 in the gas
phase and the kinetic studies represent the first experimental
determination of gas-phase rate constants for this species (and
indeed any stannylene). The results show that SnMe2 does not
insert into C-H, Si-H, or Ge-H bonds (nor, indeed, into C-C,
Si-C, and Ge-C bonds) at room temperature on the time scale
of these experiments. It also does not readily add to simple Cd
C double bonds (insofar as C2H4 is representative) although it
appears to react with the CtC triple bond (but see later). It
reacts rapidly with 1,3-butadiene. It does not react with N2O,
one potential n-type lone pair donor, although it does react with
MeOH, another lone pair donor. Reactions with halides are
Me SnH + hν (193 nm) f SnMe + CH
4
3
2
Me SnH + hν (193 nm) f MeSnH + C H
6
3
2
with the first of these slightly predominating. Minor contribu-
tions probably come from
Me SnH + hν (193 nm) f Me SnH + Me
3
2
Me SnH + hν (193 nm) f SnMe + H
3
3
observed. These findings are, in general terms, consistent with
It is also possible that the Me radicals are formed via secondary
decomposition of vibrationally excited stannylenes.
The photodecomposition of PhSnMe2H (60 mTorr in 100 Torr
of N2) produced CH4, C2H6 ,and C6H6 (benzene) as well as small
amounts of other products. Because of the presence of the
impurity it was impossible to provide a quantitative description.
However, it was verified that none of these products was
substantially suppressed in the presence of added oxygen. This
suggests that all of the following primary processes are occurring
to some extent:
what is already known from solution and end product studies.3
The observed lack of reactivity with many of the substrates
previously found reactive toward silylenes and germylenes is
not too surprising. As far as bond insertion is concerned, SiMe2
5c
6b
and GeMe2 insert fairly slowly into Si-H and Ge-H bonds,
respectively. From studies with SiH2 and GeH2 (which are more
reactive than SiMe2 and GeMe2, respectively) it seems that the
(23) Gordon, C. M.; Long, C. The photochemistry of organometallic compounds
of germanium, tin and lead. In The chemistry of organic germanium, tin
and lead compounds; Patai, S., Ed.; Wiley: Chichester, UK, 1995; Vol. 1,
Chapter 14, p 723.
(24) (a) Obayashi, C.; Sato, H.; Tominaga, T. J. Radioanal. Nucl. Chem. 1992,
164, 365. (b) Yamada, Y.; Kumagawa, T.; Yamada, Y. T.; Tominaga, T.
J. Radioanal. Nucl. Chem. 1995, 201, 417
(
22) (a) M a´ rquez, A.; Gonz a´ lez, G. G.; Fern a´ ndez Sanz, J. Chem. Phys. 1989,
38, 99. (b) Trinquier, G. J. Am. Chem. Soc. 1990, 112, 2130. (c) Trinquier,
G. J. Am. Chem. Soc. 1991, 113, 144.
1
J. AM. CHEM. SOC.
9
VOL. 124, NO. 25, 2002 7559

A R T I C L E S
Becerra et al.
Table 3. Comparison of Addition Rate Constants of the Dimethyl
Table 4. Some Bond Dissociation Energies (kJ mol-1)
Heavy Carbenes to Unsaturates at Room Temperature
radical, R
substrate
C2H4
,3-C4H6
SiMe
2
GeMe
2
SnMe
2
a
Me
3
Snb
bond
C
2
H
5
-
-
-
11 a,b
11 a
-11 c
-11 c
-11 e
-14 d
-11 d
-12 d
2.2 × 10
7.5 × 10
1.7 × 10
2.0 × 10
1.2 × 10
3.4 × 10
<1.0 × 10
6.0 × 10
7.8 × 10
R-Hc
R-Clc
423 ( 2
351 ( 2
296 ( 2
236 ( 2
395 ( 2
370 ( 2
322 ( 17
425 ( 17
381 ( 17
320 ( 17
488 ( 17
295 ( 17
1
d
10 a
CH3CtCCH3
c
R-Br
c
R-I
a
Reference 5b. b Reference 5e. c Reference 6a. d This work. e Reference
R-OHc
R-CH3c
3
0.
heavy methylenes get less reactive with increasing size, i.e.
GeH2 has lower rate constants than SiH2.25 Moreover, reactivity
is affected by the strength of the substrate bond for insertion.
Thus GeH2 insertion into Si-H bonds25 is considerably slower
than SiH2 insertion into Ge-H bonds.26 Thus from these
considerations SnMe2 is unlikely to insert easily into any M-H
bond stronger than Sn-H, and given the sluggishness of GeMe2
insertion into Ge-H,6b even this is likely to be very slow.
The lack of reaction of SnMe2 with simple CdC bonds is
almost certainly connected with the weakness of the interaction,
or to put it another way, the strain involved in forming the
stannirane ring. Studies of GeH2 with C2H427 and C3H6 show
that germirane rings are highly strained (more so than siliranes),
and therefore stanniranes can be expected to be even more
strained. There are no published theoretical calculations on
stanniranes. There are, however, calculations by Boatz, Gordon,
and Sita29 on stannirene, which show that this ring, although
a
o
-1
from ref 38. b ∆Hfo for Me3SnX
c o
∆
Hf (C2H5) ) 121 kJ mol
o
-1
compounds, including ∆H (Me Sn) ) 130 kJ mol from ref 39. ∆H
f
for other compounds from ref 40. Estimated using ∆Hvap ) 40 kJ mol
f
3
d
-1
.
2
to trap SnMe2 itself with dienes. Theoretical calculations at
the MNDO level suggest a synchronous 1,4 cycloaddition
mechanism and indeed Neumann’s group found evidence for
this by reacting Lappert’s stannylene with a stereolabeled
diallenyl compound. We would, however, reserve judgment
on this until more definitive experiments have been done using
SnMe2 itself with simpler dienes (without steric encumbrance).
3
3
3
4
The evidence for SiMe2 and GeMe2 cycloadditions to dienes
favors 1,2 addition followed by ring expansion.3
28
5,36
The
observation of a high rate constant in this work is of itself not
definitive as to the mechanism. Table 3 compares the gas-phase
rate constants of addition reactions of the dimethyl heavy
carbenes, under the conditions of measurement. The pattern for
butadiene seems erratic, although since the pressure dependences
of these reactions have not been studied it is premature to draw
any hard conclusions. It seems to us not impossible that SnMe2
could initially form a vinylstannirane followed by rapid ring
expansion to the stannacyclopentene.
-1
highly strained, is weakly bound by some 52 kJ mol relative
to SnH2 + C2H2 (calculation at the MP2/3-21G(d)//RHF/3-21G-
(d) level). This weak binding suggests that the potential
tetramethylstannirene product of the reaction
SnMe + MeCtCMe f c-Me SnC Me
2
The reactions of SnMe2 with HCl and 1-BuBr are consistent
2
2
2
1b
with the known affinity of Lappert’s stannylene with halides.
3
7
should redissociate very readily. This makes it questionable
whether such a product could be stabilized at room temperature.
The measured rate constant for this reaction is surprisingly high
The solution studies by Neumann et al. of SnMe2 and SnBu2
could not establish conclusively that the observed bond insertion
products were necessarily those of a free stannylene. There is
also the issue of whether this insertion reaction proceeds directly
or via halogen abstraction, illustrated, for the current case, as
follows:
5b
30
(
although not as high as those of SiMe2 and GeMe2 with
MeCtCMe; Table 3) and the decay traces show no obvious
signs of reversibility. However, the reaction may be less efficient
at low pressures, where stabilization of the product should be
less effective. We plan to investigate the pressure dependence
of this reaction. If there is another pathway for this reaction it
is not obvious what it might be. It is worth pointing out,
however, that a stannirene has previously been prepared, albeit
from a strained cycloalkyne and a highly stabilized stannylene.
The reaction of SnMe2 with butadiene again comes as no
surprise. Lappert’s stannylene could be trapped with 2,3-
dimethylbuta-1,3-diene1b and Neumann’s group have shown that
BuBr + SnMe f Bu..Br..SnMe f BuSnMe Br
2
2
2
BuBr + SnMe f Bu + BrSnMe f BuSnMe Br
2
2
2
31
This question is open to further investigation by product analysis,
which we plan to undertake in a more detailed study. However,
it is possible to comment on the likelihood of abstraction from
the thermochemical viewpoint, since the overall abstraction
reaction involves breaking a primary C-Br bond and making
a trialkyltin-Br bond. The relevant bond dissociation energies
for this and related cases are shown in Table 4. The data are
Lappert’s stannylene adds to a number of other dienes to give
a set of 1-stannacyclopent-3-enes,3 although they were unable
2
(
25) (a) Becerra, R.; Walsh, R. Phys. Chem. Chem. Phys. 1999, 1, 5301. (b)
Becerra, R.; Boganov, S. E.; Egorov, M. P.; Faustov, V. I.; Nefedov, O.
M.; Walsh, R. Phys. Chem. Chem. Phys. 2001, 3, 184.
(32) Hillner, K. Dr rer. nat. Thesis, University of Dortmund, 1986 (cited in ref
3).
(26) Becerra, R.; Boganov, S. E.; Walsh, R. J. Chem. Soc., Faraday Trans.
(33) Dewar, M. J. S.; Friedheim, J. E.; Grady, G. L. Organometallics 1985, 4,
1784.
1
998, 94, 3569.
(
27) (a) Becerra, R.; Boganov, S. E.; Egorov, M. P.; Nefedov, O. M.; Walsh,
R. Paper presented at the 6th International Conference on the Chemistry
of Carbenes and Related Intermediates, St. Petersburg, Russia, May 28-
(34) Marx, R.; Neumann, W. P.; Hillner, K. Tetrahedron Lett. 1984, 25, 625.
(35) (a) Lei, D.; Hwang, R. J.; Gaspar, P. P. J. Organomet. Chem. 1984, 271,
1. (b) Lei, D.; Gaspar, P. P. J. Chem. Soc., Chem Commun. 1985, 1149.
(c) Lei, D.; Gaspar, P. P. Res. Chem. Intermed. 1989, 12, 103. (d) Bobbitt,
K. L.; Gaspar, P. P. J. Organomet. Chem. 1995, 499, 17.
(36) Bobbitt, K. L.; Maloney, V. M.; Gaspar, P. P. Organometallics 1991, 10,
2772.
(37) (a) Schr o¨ er, U.; Neumann, W. P. Angew. Chem., Int. Ed. Engl. 1975, 14,
246. (b) Neumann, W. P.; Schwartz, A. Angew. Chem., Int. Ed. Engl. 1975,
14, 812. (c) Watta, B.; Neumann, W. P.; Sauer, J. Organometallics 1985,
4, 1954.
3
0, 1998. (b) Becerra, R.; Boganov, S. E.; Egorov, M. P.; Nefedov, O. M.;
Walsh, R. Paper presented at the 15th International symposium on Gas
Kinetics, Bilbao, Spain, Sept 6-10, 1998.
(
(
(
28) Becerra, R.; Walsh, R. J. Organomet. Chem. 2001, 636, 50.
29) Boatz, J. A.; Gordon, M. S.; Sita, L. R. J. Phys. Chem. 1990, 94, 5488.
30) Becerra, R.; Egorov, M. P.; Krylova, I. V.; Nefedov, O. M.; Walsh, R.
Unpublished results.
(31) Sita, L. R.; Bickerstaff, R. D. J. Am. Chem. Soc. 1988, 110, 5208.
7560 J. AM. CHEM. SOC.
9
VOL. 124, NO. 25, 2002

First Gas-Phase Detection of Dimethylstannylene
A R T I C L E S
taken from the best sources we are aware of.38-40 It can be
clearly seen that the Sn-hal bonds are considerable stronger
that their C-hal counterparts. The situation is slightly more
complicated, however, because the overall energy gain is
reduced by the contribution of the divalent state stabilization
pathways (unpublished calculations and experiments in our
4
7
labs suggest that the analogous reaction of SiH2 + CO2 f
H2SiO + CO is anything but direct). It is premature, without
further experiments, to discuss these reactions in detail. We
consider here only their thermochemical feasibility. The key
4
1
energy (DSSE) of SnMe2. The value of this quantity is not
quantity will be the stability of Me SnO about which little seems
2
-
1 42
48-50
known with certainty, but is estimated as 100 ( 20 kJ mol .
to be known. Provided the SndO double bond
is as strong
This makes the abstraction reactions at best marginal, although
or stronger than D(OSdO) and likewise D(NNdO), then these
reactions are feasible. The values for D(OSdO) and D(NNd
33
the MNDO calculations favored it in the case of MeI + SnMe2.
It should be pointed out, however, that the evidence from the
solution studies of Lappert’s stannylene43 with alkyl halides
favors the abstraction mechanism.
51
O) are easily calculated from well-established thermochemistry
-1
at 552 and 167 kJ mol . Since the Sn-O single bond in Me -
3
-
1
Sn-OH has a strength of ca. 488 kJ mol (see Table 4) this
The reaction of SnMe2 with n-type lone pair donors such as
CH3OH comes as no surprise, given the facility with which
seems to imply that reaction of Me Sn with SO (by this route)
2
2
52,53
should at best be marginal,
whereas with N2O it is a
silylenes5
d,44
and germylenes
45,46
react with this type of mol-
reasonable possibility. Because this argument appears contrary
to our findings, clearly there are other considerations involved
here. It should be added that there are no studies of SiMe2 or
GeMe2 with these molecules although SiH2 is known to react
ecule. Thus we should expect the following mechanism for this
reaction:
5
4
55
with N2O and recent studies have shown that both SiH2 and
5
6
GeH2 react with SO2.
Since SnMe2 is rather unreactive toward several of the
potential substrate molecules investigated here, it is legitimate
to enquire what its fate is in these systems. The decays under
these conditions are usually noisy and not very well character-
ized. When fitted in the usual way to exponentials, kobs values
of (2-5) × 10 s are obtained. The question that arises is
whether these traces might be mistaken for second order, i.e.
hyperbolic, decays, corresponding to the reaction
An interesting question here is whether the initial formation
of the donor-acceptor (zwitterion) complex from SnMe2 +
5d
MeOH is reversible. The evidence from SiMe2 studies is that,
at room temperature, the reaction of SiMe2 + MeOH is not
reversible although SiMe2 + Me2O is. The analogous reactions
4
-1
45
of GeMe2 do not show reversibility at room temperature. The
reactions of SiH2 + MeOH4 and Me2O
4c
44a,b
are both reversible
46
at higher temperatures and unpublished investigations suggest
similar results for GeH2 + MeOH and GeH2 + Me2O. There is
no evidence of reversibility in our present room temperature
study but we may expect that at higher temperatures this may
well occur, since there is no reason to believe that the Sn‚‚‚O
bond in the intermediate complex should be any stronger than
the Si‚‚‚O bond in the analogous complex.
2
SnMe f Sn Me
2 2 4
From the experimental conditions (percent absorption, path
3
3
-1
length) and an assumed extinction coefficient of 10 dm mol
-1
57
cm (cf. that for SiMe2 ), a maximum concentration of SnMe2
1
3
-3
of 5.8 × 10 molecule cm ( )1.8 mTorr) can be estimated.
Regarding the potential reactions of SnMe2 with SO2 and
N2O, we may consider the likelihood of the O-atom transfer
steps
If the recombination reaction occurred at the collision rate (ca.
-
10
3
-1 -1
3
× 10
a half-life of 5.8 × 10 s, corresponding to a kobs value of 1.2
10 s . Thus at the maximum rate this reaction is just too
cm molecule s ) then this would correspond to
-
5
4
-1
×
SnMe + SO f Me SnO + SO
2
2
2
slow to occur under the conditions of this study. Given the
uncertainty in the extinction coefficient it is possible that it may
contribute to the SnMe2 decay. In his review Neumann states
SnMe + N O f Me SnO + N
2
2
2
2
3
Clearly from our results only the first of these is a possibility,
and even then the reaction may be indirect or have other
that “simple stannylenes like SnMe2, ... polymerise very rapidly,
(
47) Becerra, R.; Cannady, J. P.; Walsh, R. J. Phys. Chem. A. 2002, 106, 4922.
49
(
38) Berkowitz, J.; Ellison, G. B.; Gutman, D. J. Phys. Chem. 1994, 98, 2744.
39) Martinho Sim oˆ es, J. A.; Liebman, J. F.; Slayden, S. W. Thermochemistry
of organometallic compounds of germanium, tin and lead. In The chemistry
of organic germanium, tin and lead compounds; Patai, S., Ed.; Wiley:
Chichester, UK, 1995; Vol. 1, Chapter 5, p 245.
(48) Theoretical calculations suggest that species such as dimethylstannanone,
with double bonds between oxygen and tin, are not likely to exist. Despite
(
5
0
this a stannanone has been implicated in stannaoxetane decomposition.
(49) (a) Kapp, J.; Remko, M.; Schleyer, P. v. R. J. Am. Chem. Soc. 1996, 118,
5745. (b) Kapp, J.; Remko, M.; Schleyer, P. v. R. Inorg. Chem. 1997, 36,
4241.
(50) (a) Rodi, A. K.; Anselme, G.; Ranaivonjatovo, H.; Escudie, J. Chem
Heterocycl. Compd. (Engl. Transl.) 1999, 35, 965. (b) Rodi, A. K.; Anselme,
G.; Ranaivonjatovo, H.; Escudie, J. Khim. Geterosikl. Soedin. 1999, 8, 1098.
(51) Benson, S. W. Thermochemistry, 2nd ed.; Wiley-Interscience: New York,
1976.
(52) There are strong indications that in the ethylene analogues of higher
members of Group 14, the double bond dissociation energy is less than
that of the single bond, although whether this extends to the ketone
analogues is not known.
(53) Jacobsen, H.; Ziegler, T. J. Am. Chem. Soc. 1994, 116, 3667.
(54) Becerra, R.; Frey, H. M.; Mason, B. P.; Walsh, R. Chem. Phys. Lett. 1991,
185, 415.
(55) Becerra, R.; Goldberg, N.; Walsh, R. Unpublished results.
(56) Becerra, R.; Walsh, R. Unpublished results.
(57) (a) Drahnak, T. J.; Michl, J.; West, R. J. Am. Chem. Soc. 1979, 101, 5427.
(b) Raabe, G.; Vancik, H.; West, R.; Michl, J. J. Am. Chem. Soc. 1986,
108, 671.
(
40) Cox, J. D.; Pilcher, G. Thermochemistry of Organic and Organometallic
Compounds; Academic Press: London, UK, 1970.
(41) Becerra, R.; Walsh, R. Thermochemistry. In The Chemistry of Organosilicon
Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, UK,
1
998; Vol. 2, Chapter 4, p 153.
(
42) Walsh, R. Unpublished calculation.
5
3
(
43) (a) Gynane, M. J. S.; Lappert, M. F.; Miles, S. J.; Power, P. P. J. Chem.
Soc., Chem. Commun. 1976, 256. (b) Gynane, M. J. S.; Lappert, M. F.;
Miles, S. J.; Carty, A. J.; Taylor, N. J. J. Chem. Soc., Dalton Trans. 1977,
2
009.
(
44) (a) Becerra, R.; Carpenter, I. W.; Gutsche, G. W.; King, K. D.; Lawrance,
W. D.; Staker, W. S.; Walsh, R. Chem. Phys. Lett. 2001, 333, 83. (b)
Alexander, U. N.; King, K. D.; Lawrance, W. D. Phys. Chem. Chem. Phys.
2
001, 3, 3085. (c) Alexander, U. N.; King, K. D.; Lawrance, W. D. J.
Phys. Chem A 2002, 106, 973.
(
45) Becerra, R.; Boganov, S. E.; Egorov, M. P.; Nefedov, O. M.; Walsh, R.
Unpublished results.
(46) Alexander, U. N. Ph.D. thesis, Flinders University of South Australia, 2000.
J. AM. CHEM. SOC.
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A R T I C L E S
Becerra et al.
8
-1 -1
k ) 10 M s ...”. However, to be rapid in the gas phase the
much lower than this, since otherwise large amounts of the likely
product, Sn Me , would have been seen in the GC product
3
rate constant has to be more than 10 faster still. We suspect
2
6
that in the absence of reactive substrates or in the presence of
unreactive ones the slow decays are caused by a mixture of
self-reaction and reaction with other intermediates produced
simultaneously in the photodecomposition of SnMe4 (e.g. Me
radicals). Although slow these decays are still too fast to be
attributable to diffusion or reaction with dust. The decay
constants do not appear to be dependent on SnMe4 precursor
analyses.
We plan to extend these studies and hope to throw further
light on the kinetic behavior of SnMe2 in future work.
Acknowledgment. We thank the following: INTAS-RFBR
(project IR-97-1658) and NATO (project PST.CLG.975368).
-
11
R.B. also thanks the DGICYT (Spain) for support under projects
PB98-0537-C02-01 and BQU2000-1163-C02-01. S.E.B., M.P.E.,
I.V.K., V.I.F., and O.M.N. thank RFBR (projects 00-15-97387,
pressure, which allows us to set an upper limit of 3 × 10
3
-1 -1
cm molecule for the rate constant of the reaction
s
0
1-03-32630 and 00-03-33001). R.W. also thanks EPSRC
SnMe + SnMe f products
2
4
(project C7410).
This upper limit is still quite high and not very restricting.
However, we suspect that the rate constant for this reaction is
JA012691K
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VOL. 124, NO. 25, 2002