Fast kinetics study of the reactions of transient silylenes with alcohols. Direct detection of silylene-alcohol complexes in solution

Article abstract of DOI:10.1021/om9009747

The kinetic behavior of dimethyl-, diphenyl-, and dimesitylsilylene in hexanes solution in the presence of methanol (MeOH), tert-butanol (t-BuOH), and the respective O-deuterated isotopomers has been studied, with the goal of elucidating a detailed mechanism for the formal O-H insertion reaction of transient silylenes with alcohols in solution. The data are in all cases consistent with a mechanism involving the intermediacy of the corresponding silylene-alcohol Lewis acid-base complexes, which have been detected directly for each of the SiMe₂-ROL and SiPh₂-ROL (L = H or D) systems that were studied. Complexation proceeds effectively irreversibly (K_eq ≥ 2 x 10⁵ M^-1) and at close to the diffusion-controlled rate in these cases. In contrast, the kinetic and spectroscopic behavior observed for SiMeS₂ in the presence of these alcohols indicates the SiMeS₂-ROL complexes are involved as steady-state intermediates, formed reversibly and 10-100 times more slowly than is the case with SiMe₂ and SiPh₂. Product formation from the silylene-alcohol complexes is shown to proceed via catalytic proton transfer by a second molecule of alcohol, the rate of which exceeds that of unimolecular intracomplex H-migration in all cases, even at submillimolar alcohol concentrations. The catalytic rate constants range from 10⁹ to 10¹⁰ M^-1 s^-1 for the SiMe₂-ROH and SiPh₂-ROH complexes, sufficiently fast that the isotope effect ranges from ca. 2.5 to close to unity for all but the SiPh₂-t-BuOL complex, where it is remarkably large (k_HH/k_DD = 10.8 ± 2.4). The value is consistent with a mechanism for catalysis involving double proton transfer within a cyclic five-membered transition state. The isotope effects on the ratio of the rate constants for catalytic proton transfer and dissociation of the SiMeS₂-MeOH and SiMeS₂-t-BuOH complexes suggest that a different mechanism for catalytic proton transfer is involved in the case of the sterically hindered diarylsilylene.

Full text of DOI:10.1021/om9009747

662 Organometallics 2010, 29, 662–670
DOI: 10.1021/om9009747
Fast Kinetics Study of the Reactions of Transient Silylenes with Alcohols.
Direct Detection of Silylene-Alcohol Complexes in Solution
William J. Leigh,* Svetlana S. Kostina, Adroha Bhattacharya, and Andrey G. Moiseev
Department of Chemistry and Chemical Biology, McMaster University, 1280 Main Street West,
Hamilton, ON, Canada L8S 4M1
Received November 8, 2009
The kinetic behavior of dimethyl-, diphenyl-, and dimesitylsilylene in hexanes solution in
the presence of methanol (MeOH), tert-butanol (t-BuOH), and the respective O-deuterated
isotopomers has been studied, with the goal of elucidating a detailed mechanism for the formal
O-H insertion reaction of transient silylenes with alcohols in solution. The data are in all cases
consistent with a mechanism involving the intermediacy of the corresponding silylene-alcohol Lewis
acid-base complexes, which have been detected directly for each of the SiMe2-ROL and
SiPh2-ROL (L = H or D) systems that were studied. Complexation proceeds effectively irreversibly
(Keq g 2 ꢀ 105 M-1) and at close to the diffusion-controlled rate in these cases. In contrast, the kinetic and
spectroscopic behavior observed for SiMes2 in the presence of these alcohols indicates the SiMes2-ROL
complexes are involved as steady-state intermediates, formed reversibly and 10-100 times more slowly
than is the case with SiMe2 and SiPh2. Product formation from the silylene-alcohol complexes is shown to
proceed via catalytic proton transfer by a second molecule of alcohol, the rate of which exceeds that of
unimolecular intracomplex H-migration in all cases, even at submillimolar alcohol concentrations. The
catalytic rate constants range from 109 to 1010 M-1 s-1 for the SiMe2-ROH and SiPh2-ROH complexes,
sufficiently fast that the isotope effect ranges from ca. 2.5 to close to unity for all but the SiPh2-t-BuOL
complex, where it is remarkably large (kHH/kDD=10.8 (2.4). The value is consistent with a mechanism for
catalysis involving double proton transfer within a cyclic five-membered transition state. The isotope
effects on the ratio of the rate constants for catalytic proton transfer and dissociation of the
SiMes2-MeOH and SiMes2-t-BuOH complexes suggest that a different mechanism for catalytic proton
transfer is involved in the case of the sterically hindered diarylsilylene.
Introduction
particularly electrophilic carbenes such as 1CH2 and the
dihalocarbenes.8,9
Insertion into the O-H bonds of simple alcohols is one of
the best-known reactions of singlet carbenes and their hea-
vier group 14 homologues, silylenes and germylenes.1-3
While most singlet carbenes react with alcohols by initial
protonation followed by nucleophilic capture of the resulting
carbenium ion,4,5 silylenes and germylenes react with these
substrates via the initial formation of the corresponding
Lewis acid-base complex followed by H-transfer from oxy-
gen to the group 14 heteroatom (eq 1).1,3,6,7 In carbene
chemistry, the analogous mechanism is thought to be pre-
ferred over that involving initial protonation only with
In the cases of simple silylenes and germylenes, the mec-
hanism of eq 1 is supported by the results of theoretical
calculations,10-19 low-temperature spectroscopic studies,20,21
*Corresponding author. E-mail: leigh@mcmaster.ca.
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pubs.acs.org/Organometallics
Published on Web 01/08/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 3, 2010 663
and kinetic studies in both the gas phase7,17,18,22-24 and solu-
tion.25-30 The intermediate Lewis acid-base complexes of
alcohols with several silylenes21,27,29 and germylenes20,28,30 have
been detected directly in low-temperature matrixes and in
solution, where their kinetic behavior has been found to be
qualitatively consistent with theoretical results for the reactions
of the parent MH2 (M = Si or Ge) and MMe2 species with
water or methanol.10-19 The calculations predict the initial
complexation step to be barrierless and of similar, moderate
exothermicity for both the silylene and germylene; the barrier
occurs in the H-migration step, which is substantially higher for
germylenes than silylenes of otherwise identical structures. In
keeping with these predictions, the complexes of SiMe2 and
SiPh2 with simple alcohols are short-lived reaction intermedia-
tes, detectable in hydrocarbon solvents only at submillimolar
alcohol concentrations,27,29 while those of GeMe2 and GePh2
are sufficiently stable to be observable in hexanes solution as
discrete species in equilibrium with the free reactants.28 The
complexes of GeMe2 and GePh2 with methanol (MeOH) are
remarkably long-lived even in neat alcohol as solvent, where
they have lifetimes in the 4-50 μs range.30
The kinetic results obtained for the GeMe2-MeOH com-
plex indicate that the barrier for unimolecular H-migration
in the complex is sufficiently high that it cannot compete with
a catalytic proton-transfer pathway involving one or more
alcohol molecules as catalyst in neutral solution. Even the
latter process is remarkably slow, as evidenced by the ca.
4 μs lifetime of the GeMe2-MeOH complex in MeOH
solution at 25 °C.30 This leads to an estimated upper limit
of k e 104 M-1 s-1 for the rate constant for catalysis by a
single solvent molecule; that for the corresponding process in
the case of the GePh2-MeOH complex is roughly an order
of magnitude smaller.30 In contrast, the SiPh2-MeOH
complex exhibits a lifetime of ca. 400 ns in hexanes solution
containing 0.15 mM MeOH.29 Under these conditions, the
complex can be observed as a weakly absorbing transient
with UV-vis spectrum centered at λmax ≈ 370 nm, which
grows in with a rise time of ca. 40 ns and then decays on a
time scale just slightly longer than that of the free silylene
under the same conditions. The species is undetectable at
higher (1-3 mM) MeOH concentrations, in experiments
on the nanosecond time scale. Similar observations were
mentioned by Levin et al. in their kinetic study of the
reactions of SiMe2 with aliphatic alcohols in cyclohexane
solution, but were evidently not pursued.27
The mechanism of the fast proton-migration/transfer
process in silylene-alcohol complexes;whether by unim-
olecular H-migration or the catalyzed pathway, or both;
remains to be established. Becerra et al. reported kinetic and
computational evidence for a mechanism, for the gas-phase
reaction of SiH2 with water, involving two molecules of the
substrate in the rate-determining step for consumption of
silylene;18 an analogous mechanism has been proposed for
singlet carbene O-H insertions on the basis of computa-
tional studies of the reaction of dichlorocarbene with water.9
It is difficult to predict what this means for the reactions of
alcohols with substituted silylenes in solution, as theory
predicts that the complexation of water with SiMe2 is less
exothermic than that with SiH2, and (unimolecular)
H-migration within the silylene-water complex proceeds
via a (slightly) lower enthalpic barrier.11 The preliminary
kinetic behavior reported earlier for the SiPh2-MeOH
complex in hexanes is consistent with either the unimolecular
or catalytic H-transfer pathway, or a combination of both,
for decay of the species in the presence of submillimolar
concentrations of MeOH.29
In this paper, we report the results of more detailed kinetic
studies of the reactions of alcohols with SiMe2, SiPh2, and
dimesitylsilylene (SiMes2) in hexanes solution, focusing on
the direct detection and characterization of their reactive
Lewis acid-base complexes with MeOH, tert-butanol
(t-BuOH), and the O-deuterated isotopomers of the two
alcohols. As in our previous study,29 the three silylenes were
generated and detected directly by laser flash photolysis of
the oligosilane derivatives 1-3, respectively.
(15) Heaven, M. W.; Metha, G. F.; Buntine, M. A. J. Phys. Chem. A
2001, 105, 1185.
(16) Heaven, M. W.; Metha, G. F.; Buntine, M. A. Aust. J. Chem.
Results and Discussion
2001, 54, 185.
(17) Alexander, U. N.; King, K. D.; Lawrance, W. D. J. Phys. Chem.
A 2002, 106, 973.
(18) Becerra, R.; Goldberg, N.; Cannady, J. P.; Almond, M. J.;
Ogden, J. S.; Walsh, R. J. Am. Chem. Soc. 2004, 126, 6816.
(19) Su, M. D. Chem. Eur. J. 2004, 10, 6073.
(20) Ando, W.; Itoh, H.; Tsumuraya, T. Organometallics 1989, 8,
2759.
(21) Gillette, G. R.; Noren, G. H.; West, R. Organometallics 1989,
8, 487.
(22) Baggott, J. E.; Blitz, M. A.; Frey, H. M.; Lightfoot, P. D.; Walsh,
R. Int. J. Chem. Kinet. 1992, 24, 127.
(23) Alexander, U. N.; King, K. D.; Lawrance, W. D. Phys. Chem.
Chem. Phys. 2003, 5, 1557.
(24) Becerra, R.; Cannady, J. P.; Walsh, R. J. Phys. Chem. A 2003,
107, 11049.
(25) Steele, K. P.; Weber, W. P. Inorg. Chem. 1981, 20, 1302.
(26) Steele, K. P.; Tzeng, D.; Weber, W. P. J. Organomet. Chem. 1982,
231, 291.
Diphenylsilylene (SiPh2). Laser flash photolysis of de-
oxygenated solutions of 1 (ca. 0.05 mM) in anhydrous
hexanes, with the pulses from a KrF excimer laser (248 nm;
ca. 20 ns; ca. 100 mJ/pulse), affords the characteristic
transient absorptions due to SiPh2 (λmax = 290 and
515 nm; τ ≈ 1.7 μs), which decay with second-order kinetics
with the concomitant growth of the longer-lived absorptions
due to the silylene dimerization product, tetraphenyldisilene
(Si2Ph4; λmax = 460 nm; τ > 50 μs).31 The absorptions due to
SiPh2 and Si2Ph4 are superimposed on the much longer-lived
ones due to a minor transient coproduct (λmax = 460 nm; τ >
50 μs), which we have assigned previously to the transient
silene 4.29 Representative transient spectra and decays are
shown in Figure 1a. It should be noted that the spectra of
Figure 1a lack the weak transient absorption centered at ca.
360 nm that was present in our earlier published spectra from
laser photolysis of 1 in hexanes;29 we believe this absorption
(27) Levin, G.; Das, P. K.; Bilgrien, C.; Lee, C. L. Organometallics
1989, 8, 1206.
(28) Leigh, W. J.; Lollmahomed, F.; Harrington, C. R.; McDonald,
J. M. Organometallics 2006, 25, 5424.
(29) Moiseev, A. G.; Leigh, W. J. Organometallics 2007, 26, 6268.
(30) Lollmahomed, F.; Huck, L. A.; Harrington, C. R.; Chitnis, S. S.;
Leigh, W. J. Organometallics 2009, 28, 1484.
(31) Moiseev, A. G.; Leigh, W. J. J. Am. Chem. Soc. 2006, 128, 14442.

664 Organometallics, Vol. 29, No. 3, 2010
Leigh et al.
Table 1. Absolute Rate Constants (in units of 109 M-1 s-1) for
Reaction of SiMe2 and SiPh2 with Alcohols in Deoxygenated
Hexanes at 25 °C (kROL) and for Quenching of the Transient
Absorptions Due to the Corresponding Silylene-ROL
a
)
Complexes by ROL (kcat
SiMe2
SiPh2
b
c
d
e
kROL
kcat
kROL
kcat
ROL
/109 M-1 s-1 /109 M-1 s-1 /109 M-1 s-1 /109 M-1 s-1
MeOH
MeOD
t-BuOH
t-BuOD
21 ( 3
20 ( 3
14 ( 2
13 ( 1
15 ( 3
18 ( 2
14 ( 2
14 ( 1
11 ( 1
18 ( 1
17 ( 5
8.6 ( 1.2
1.4 ( 0.1
0.13 ( 0.02
8.1 ( 1.2
7.2 ( 0.9
Figure 1. Transient absorption spectra from laser flash photo-
lysis of deoxygenated solutions of 1 (ca. 0.05 mM) in hexanes:
(a) in the pure anhydrous solvent, recorded 96-128 ns (0),
1.52-1.58 μs (O), and 8.56-8.62 μs (b) after the laser pulse; and
(b) after addition of 0.3 mM t-BuOH, recorded 32-64 ns (0),
0.38-0.45 μs (O), and 4.00-4.07 μs (b) after the laser pulse. The
insets show transient decay/growth profiles recorded at moni-
toring wavelengths of 350, 460, and 530 nm. The solid line
through the 530 nm decay in (a) is the best nonlinear least-
squares fit of the data to second-order kinetics (2k/ε = (3.9 (
0.6) ꢀ 107 cm-1 s-1), while that in (b) is the best fit to first-order
kinetics (kdecay = (4.3 ( 0.3) ꢀ 106 s-1 (errors as (2σ)).
a Rate constants are the mean of two independent measurements in
each case, while errors are listed as the standard deviation of the average
value. b Monitored at 470 nm. c Monitored at 310 nm. d Monitored at
520-530 nm. e Monitored at 350 nm.
was due to the SiPh2-H2O complex, arising from inadequate
drying of the solvent and(or) sample-handling system in our
earlier study.
Figure 2. (a) Transient decay traces recorded at 350 nm, from
laser photolysis of 1 in hexanes containing 0, 0.2, and 1.0 mM
t-BuOH. (b) Transient absorption spectra from laser photolysis
of 1 in hexanes containing 0.05 M t-BuOD, recorded 90-102 ns
(0), 205-218 ns (O), and 742-755 ns (b) after the laser pulse;
the insets show transient decay traces recorded at monitoring
wavelengths of 350 and 460 nm.
within the error limits of our measurements, not unexpect-
edly considering that those for reaction of SiPh2 with the
protiated substrates are so close to the diffusion-limiting
value. The value of kMeOH obtained in the experiments with
MeOH (kMeOH = (1.8 ( 0.2) ꢀ 1010 M-1 s-1) is in reasonable
agreement with our previously reported values.29,31
Addition of small amounts of methanol or tert-butanol as
either the protiated or O-deuterated isotopomers (MeOL
and t-BuOL, respectively; L = H or D) caused the decay of
the SiPh2 absorptions to accelerate and quenched the for-
mation of Si2Ph4, both to an increasing extent with increas-
ing concentration of added alcohol. The decay of the silylene
signal obeyed clean pseudo-first-order kinetics in the pre-
sence of alcohol, and plots of the first-order decay rate
coefficients (kdecay) versus [ROL] were linear in each case.
Least-squares analysis of the data according to eq 3, where
k0 is the hypothetical pseudo-first-order decay rate coeffi-
cient in the absence of ROL and kQ is the second-order rate
constant for reaction of the monitored species (the silylene, in
The primary products of these reactions could be detected
as transients with distinct absorption bands centered at λmax
≈ 360 nm, in the wavelength range characteristic of the
complexes of SiPh2 with other O-donors such as tetrahydro-
furan (THF) and methoxytrimethylsilane.33 These absorp-
tions grew in over a time scale similar to that of the decay of
the silylene signal at a given ROL concentration, increasing
in maximum signal intensity with increasing ROL concen-
tration, and then decayed with lifetimes in the 0.2-1.5 μs
range. For example, Figure 1b shows a series of time-
resolved spectra obtained from laser photolysis of a solution
of 1 in hexanes containing 0.3 mM t-BuOH, conditions
under which free SiPh2 could still be detected (τ ≈ 230 ns).
The weak transient absorption centered at 460 nm contains
contributions from both Si2Ph4, whose formation is incom-
pletely quenched at this alcohol concentration, and the long-
lived transient coproduct (silene 4). We assign the 360 nm
transients to the corresponding SiPh2-ROL complexes and
the process(es) responsible for their decay to proton migra-
tion and(or) transfer, which transforms them to the final
reaction product, the corresponding alkoxydiphenylsilane.
this case) with the alcohol, afforded values of kQ (=kROL
)
that varied over the range (1.1-1.8) ꢀ 1010 M-1 s-1, as
summarized in Table 1.
kdecay ¼ k0 þ kQ½ROLꢁ
ð3Þ
The measured rate constants are all within a factor of 2 of
the diffusional rate constant (kdiff = 2.3 ꢀ 1010 M-1 s-1 in
hexane at 25 °C)32 and do not vary appreciably as a function
of alcohol structure. Similarly, there is little variation in the
rate constant with isotopic substitution for either alcohol
(32) Espenson, J. H. Chemical Kinetics and Reaction Mechanisms;
McGraw-Hill, Inc.: New York, 1995; pp 15-45.
(33) Moiseev, A. G.; Leigh, W. J. Organometallics 2007, 26, 6277.

Article
Organometallics, Vol. 29, No. 3, 2010 665
Figure 3. Plots of the pseudo-first-order decay rate coefficients (kdecay) of the SiPh2-ROL complexes (L = H (9) or D (0)) vs [ROL]
for (a) R = Me and (b) R = t-Bu, in hexanes at 25 °C. The solid lines are the best linear least-squares fits of the data to eq 3.
The maximum intensities of the signals due to the com-
plexes, their lifetimes (at the same ROL concentration), and
the range of alcohol concentrations over which they could
be detected all increased in the order MeOH < MeOD <
t-BuOH < t-BuOD, indicating a significant dependence of
their decay kinetics on alcohol structure and isotopic sub-
stitution. Figure 2a illustrates the observed variation in the
growth/decay profiles associated with the SiPh2-t-BuOH
complex as a function of t-BuOH concentration in a typical
experiment. The signals due to the SiPh2-t-BuOH complex
could be detected over the 0.2-5 mM concentration range in
added alcohol, above which they were too short-lived for
precise kinetic analysis to be carried out. In the low concen-
tration (0.2-1 mM) range, the signals exhibited a rapid
growth, of rise time similar to that of the decay of the free
silylene absorption at the same t-BuOH concentration. This
was followed by a two-component decay, consisting of a fast
component that accelerated with increasing t-BuOH con-
centration and a much slower component whose overall
intensity decreased with increasing concentration. The fast-
decay component of the absorption, which intensified with
increasing alcohol concentration over the 0.2-1 mM range,
is assigned to the silylene-alcohol complex. First-order rate
coefficients for decay of these absorptions were estimated by
fitting the fast component of the decays to single exponen-
tials and treating the slow component as a nondecaying
residual absorption. The SiPh2-t-BuOD complex exhibited
similar spectral characteristics at low t-BuOD concentra-
tions, but its lifetime was much less responsive to changes in
alcohol concentration. The complex remained detectable
even in the presence of 50 mM t-BuOD, where it exhibited
a lifetime τ ≈ 165 ns. The absorption due to the deuterated
complex was monitored over the 3-50 mM concentration
range in added t-BuOD, conditions under which it was
formed within the duration of the laser pulse and decayed
completely to baseline. Figure 2b shows the transient
UV-vis spectrum of the SiPh2-t-BuOD complex, obtained
by laser photolysis of 1 in hexanes containing 0.05 M
t-BuOD. Under these conditions the complex is the only
silylene-derived species detectable in solution. The spectrum
consists of an intense short-wavelength band centered below
300 nm with a long-wavelength shoulder at ca. 360 nm; the
apparent maximum at 300 nm in the spectrum of Figure 1b is
the result of spectral distortion due to bleaching of the
precursor, whose absorption onset is in the 290-300 nm
range.
Scheme 1
The signals due to the SiPh2-MeOH and SiPh2-MeOD
complexes were weaker than those of the SiPh2-t-BuOL
complexes, and their apparent lifetimes were considerably
more sensitive to MeOL concentration; they were, however,
always significantly longer than the lifetime of the free
silylene under the same conditions. First-order decay rate
coefficients were estimated over the 0.1-0.4 mM concentra-
tion range in added MeOH or MeOD, using a similar
procedure to that used for the SiPh2-t-BuOL complexes.
Figure 3 shows plots of kdecay versus ROL concentration
for the four SiPh2-ROL complexes studied in this work; the
solid lines represent the best linear least-squares fits of the
data to eq 3. The intercepts of these plots should correspond
(in principle) to the first-order rate constants for unimole-
cular proton migration in the complexes to yield the corres-
ponding alkoxysilane (i.e., k1 = k0 of eq 3), and the slopes to
the absolute rate constants for catalytic proton transfer by a
second molecule of ROL (kcat = kQ of eq 3), as shown in
Scheme 1. In practice however, the values of the intercepts
varied considerably from experiment to experiment with a
given alcohol, so we can place no significance on them
beyond noting that they are characteristically small; a rea-
sonable upper limit in the case of the SiPh2-MeOH complex
is k1 e 105 s-1. Kinetic simulations established an upper limit
of k-MeOH ≈105 s-1 for the rate constant for reversion of
the SiPh2-MeOH complex to the free reactants and thus a
lower limit of Keq g 2 ꢀ 105 M-1 for the equilibrium constant
for complexation of SiPh2 with MeOH.34 Table 1 sum-
marizes the second-order rate constants for both the primary
reaction of the free silylene with ROL (i.e., kROL) and the
(34) Kinetic simulations, employing the kMeOH and kcatMeOH values
of Table 1 showed that a value of k-MeOH = 1 ꢀ 105 s-1 or less is
required to reproduce the experimentally observed lifetimes of free SiPh2
and the SiPh2-MeOH complex at the lowest MeOH concentration
studied (0.06 mM).

666 Organometallics, Vol. 29, No. 3, 2010
Leigh et al.
Figure 4. (a) Plots of the pseudo-first-order decay rate coefficients (kdecay) vs [t-BuOL] of free SiMe2 (λmax = 465 nm; L = H (b) and D
(O)) and of the SiMe2-t-BuOL complex (λmax = 300 nm; L = H (9) and D (0)). (b) Transient absorption spectra from a deoxygenated
solution of 2 in anhydrous hexanes containing 1.16 mM t-BuOD, recorded 16-26 ns (O) and 138-150 ns (0) after the laser pulse; the
insets show transient decay traces recorded at monitoring wavelengths of 310 and 470 nm.
catalytic rate constants (kcat) obtained from analysis of the
first-order rate coefficients for decay of the complexes as a
function of ROL concentration.
Laser photolysis of ca. 0.5 mM hexanes solutions of 2
afforded the characteristic transient absorptions due to
SiMe2 (λmax = 465 nm; τ ≈ 500 ns), which decayed with
apparent first-order kinetics with the concomitant growth of
the longer-lived absorptions due to Si2Me4 (λmax = 290 and
360 nm; τ ≈ 20 μs), as reported previously.29,33,40 Addition of
0.1-1.0 mM t-BuOH resulted in reductions in the lifetime of
the silylene absorptions and quenched the formation of the
disilene, and a plot of kdeccay for the silylene versus [t-BuOH]
The data indicate that the formation of the SiPh2-MeOH
complex and its catalytic transformation to the final
(alkoxysilane) product by a second molecule of MeOH both
proceed with rate constants approaching the diffusion limit.
Interestingly, in spite of the rapidity of the catalytic process it
exhibits a primary kinetic isotope effect of kHH/kDD ≈ 2.
Catalysis is roughly an order of magnitude slower in the case
of the SiPh2-t-BuOH complex, consistent with a marked
sensitivity of the catalytic process to increased steric hin-
drance in the alcohol. The process exhibits a kinetic isotope
effect of kHH/kDD = 10.8 ( 2.4, which too seems quite
remarkable given the value of kcat ≈ 109 M-1 s-1 that
characterizes the protiated species. While proton transfers
involving sterically hindered reactants frequently exhibit
abnormally large isotope effects due to tunneling,35 espe-
cially in nonpolar solvents,36 the value is also consistent with
a mechanism involving a simultaneous double proton trans-
fer such as that shown in eq 4; the net isotope effect should be
given (in principle) by the product of the isotope effects
associated with the individual protons in flight in the transi-
tion state.37
was linear with slope kt-BuOH = (1.3 ( 0.2) ꢀ 1010 M-1 s-1
.
Similar results were obtained with MeOH (kMeOH = (2.1 (
0.3) ꢀ 1010 M-1 s-1) and with the O-deuterated alcohols as
substrates. As with SiPh2, the rate constants for silylene
quenching by the deuterated alcohols were the same as those
obtained with the protiated analogues in both cases. The
values of kMeOH and kt-BuOH determined in these experiments
(see Table 1) are in good agreement with the previously
reported values in hexanes or cyclohexane solution.27,29,38,39
Inspection of transient growth/decay profiles at 300-310
nm, in the range characteristic of SiMe2-O-donor com-
plexes,21,27,33,40 revealed the presence of new transient ab-
sorptions that behaved similarly to those due to the
SiPh2-ROH complexes, increasing in both maximum in-
tensity and decay rate with increasing alcohol concentration
until they became too short-lived to be detected. These
absorptions, which we assign to the corresponding SiMe2-
ROL complexes, were superimposed on a longer-lived com-
ponent (λmax = 290 nm) whose contribution to the overall
decay diminished as the alcohol concentration was in-
creased; this is due most likely to Si2Me4 and(or) a higher
oligomerization product, whose yield decreases with increas-
ing efficiency of silylene trapping by the added alcohol. The
absorptions due to the complexes exhibited apparent life-
times longer than those of the free silylene under the same
conditions in every case; the SiMe2-MeOL complexes were
detectable over the 0.08-0.5 mM alcohol concentration
range, while the corresponding t-BuOL complexes were
detectable over the 0.1-2 mM concentration range. They
were too short-lived at higher alcohol concentrations to be
accurately resolved from the negative-going signal due to
sample fluorescence.
The intercepts of the plots of Figure 3b afford upper limits
of k1 e 1 ꢀ 106 s-1 and k1 e 5 ꢀ 105 s-1 for the rate constants
for unimolecular H(D) migration in the SiPh2-t-BuOH and
SiPh2-t-BuOD complexes, respectively.
Dimethylsilylene (SiMe2). The kinetics of the reactions of
SiMe2 with MeOL and t-BuOL were studied using dodecamethyl-
cyclohexasilane (2) as the silylene precursor.27,29,33,38-40
(35) Bell, R. P. The Tunnel Effect in Chemistry; Chapman and Hall:
London, 1980.
(36) Jarczewski, A.; Hubbard, C. D. J. Mol. Struct. 2003, 649, 287.
(37) Gerritzen, D.; Limbach, H. H. J. Am. Chem. Soc. 1984, 106, 869.
(38) Levin, G.; Das, P. K.; Lee, C. L. Organometallics 1988, 7, 1231.
(39) Shizuka, H.; Tanaka, H.; Tonokura, K.; Murata, K.; Hiratsuka,
H.; Ohshita, J.; Ishikawa, M. Chem. Phys. Lett. 1988, 143, 225.
(40) Yamaji, M.; Hamanishi, K.; Takahashi, T.; Shizuka, H.
J. Photochem. Photobiol. A: Chem. 1994, 81, 1.
Figure 4a shows the plots of kdecay versus [t-BuOL] that
resulted from the experiments with that alcohol, while
Figure 4b shows transient absorption spectra and represen-
tative absorption-time profiles recorded with a solution of 2

Article
Organometallics, Vol. 29, No. 3, 2010 667
in hexanes containing ca. 1.2 mM of the deuterated substrate
at 25 °C. The lifetime of SiMe2 under the conditions of
Figure 4b was ca. 70 ns, while that of the SiMe2-t-BuOD
complex (λmax = 300 nm) was ca. 130 ns; the formation of
Si2Me4 is almost completely quenched at this concentration,
as evidenced by the weak shoulder on the long-wavelength
Scheme 2
edge of the absorption band due to the complex (λmax
≈
300 nm). The analogous plots of kdecay versus [MeOL] from
the experiments with MeOH and MeOD as substrate are
shown in Figure S2 of the Supporting Information. The
various rate constants determined in these experiments are
listed in Table 1 along with the corresponding values for
SiPh2.
to the (probably greater) contributions from other pathways
involving alcohol oligomers. Our experiments were carried out
with isotopically pure substrates at relatively low alcohol con-
centrations where the monomeric form of the alcohol is essen-
tially the only species present. The results indicate that under
these conditions the isotope effects on both the complexation
and catalytic proton-transfer steps are approximately unity
within experimental error for both MeOH and t-BuOH.
Dimesitylsilylene (SiMes2). Our preliminary kinetic study
of the reaction of SiMes2 with MeOH in hexanes afforded a
linear plot of kdecay versus [MeOH] (kQ = (8.2 ( 0.3) ꢀ
108 M-1 s-1),29 consistent with a reaction mechanism leading
to overall second-order kinetics (first-order in SiMes2; first-
order in MeOH) over the alcohol concentration studied
(1-8 mM). However, the plot exhibited a negative intercept,
which suggests a higher order dependence on alcohol con-
centration that functions only at low (<2 mM) concentra-
tions. The simplest change to the mechanism of Scheme 1
that could result in this behavior is the introduction of
reversibility in the first step of the reaction. The resulting
working mechanism, with the additional provision that the
rate constant for unimolecular H-transfer within the com-
plex is vanishingly small (i.e., k1 , kcat[ROL] at all ROL
concentrations studied), is shown in Scheme 2. The predicted
dependence of the pseudo-first-order rate coefficient for
silylene decay, assuming the steady-state approximation
holds for the intermediate complex (vide infra), is given in
eq 5. It should be noted that this expression predicts a linear
dependence of kdecay on [ROL] (with slope equal to the
complexation rate constant, kROL) in the high concentration
range, where kcat[ROL] . k-ROL, and a gradual change to a
squared dependence on [ROL] as [ROL] f 0, where catalytic
proton transfer involving the complex takes over as the rate-
controlling step for silylene decay.
Again, the lifetimes of both free SiMe2 and the SiMe2-
MeOH complex responded in parallel fashion to variations
in MeOH concentration, with the lifetime of the complex
being roughly twice longer than that of the free silylene at the
same alcohol concentration throughout the range studied.
Similar results were obtained in experiments using MeOD as
substrate. The intercepts of the plots of kdecay versus [ROH]
for the SiMe2-MeOH and SiMe2-t-BuOH complexes were
both indistinguishable from zero, establishing an upper limit
-1
of k1 e105
s
for the rate constant for unimolecular
H-migration within the complex in both cases. Thus, cata-
lysis by a second molecule of alcohol provides the only
detectable proton-transfer pathway for product formation
from the intermediate complexes, as was found to be the case
for the corresponding SiPh2-ROH complexes. In contrast
to the SiPh2-ROH complexes, however, the catalytic pro-
cess exhibits an isotope effect of approximately unity for
both the SiMe2-MeOH and SiMe2-t-BuOH complexes.
It is unclear to what extent we should expect the present
kinetic data to correlate with the relative rate data published
in the early studies by Steele and Weber of the reactions of
SiMe2 with aliphatic alcohols in cyclohexane solution.25,41
The early data, which were derived from competitive trap-
ping experiments, afford a value of kMeOH/kt-BuOH ≈ 2.1 for
the relative rates of product formation from reaction of
SiMe2 with the two alcohols in cyclohexane at ambient
temperatures, which agrees well with the ratio of the catalytic
rate constants (kcatMeOH/kcatt-BuOH = 1.9 ( 0.6) determined
in the present work. However, our data indicate kinetic
isotope effects of approximately unity for both steps in the
reactions of SiMe2 with MeOH and t-BuOH, which is at
odds with the values of kH/kD ≈ 2 established by Steele and
Weber, for the reactions of EtOH(D) and t-BuOH(D) with
SiMe2, on the basis of relative product yields.25 The dis-
agreement may be due to the vastly different experimental
conditions used in the early experiments compared to the
present ones. The competition experiments were all carried
out in cyclohexane containing molar concentrations of the
alcohols,25 where the substrates exist largely in (hydrogen-
bonded) oligomeric form rather than the monomeric form
that dominates alcohol speciation at the submillimolar con-
centrations employed in our kinetic experiments.42,43 Further-
more, the early isotope effect experiments were carried out
using mixtures of the protiated and deuterated alcohols, and
hence the measured isotope effect contains contributions from a
minimum of four isotopomeric reaction pathways, in addition
2
kdecay ¼ k0 þ kROLkcat½ROLꢁ =ðk-ROL þ kcat½ROLꢁÞ
ð5Þ
Thus, information on the magnitudes of kcat and k-ROL
should be accessible (in principle) from analysis of the
concentration dependence of the pseudo-first-order rate
constants for silylene decay in the low concentration range
where the nonlinear behavior prevails. Such an analysis is
made potentially difficult by the fact that as [ROL] f 0,
dimerization takes over as the main reaction channel avai-
lable to the silylene, which should result in decay profiles that
follow mixed pseudo-first- and second-order kinetics and are
hence more difficult to analyze quantitatively. In principle, it
should be possible to reduce the contribution of the second-
order component due to silylene dimerization by working at
low excitation laser intensities, but we have little flexibility in
this regard because the silylene absorptions are relatively
weak. Nevertheless, the presence of a primary isotope effect
(41) Steele, K. P.; Weber, W. P. J. Am. Chem. Soc. 1980, 102, 6095.
(42) Landeck, H.; Wolff, H.; Goetz, R. J. Phys. Chem. 1977, 81, 718.
(43) Griller, D.; Liu, M. T. H.; Scaiano, J. C. J. Am. Chem. Soc. 1982,
104, 5549.

668 Organometallics, Vol. 29, No. 3, 2010
Leigh et al.
Table 2. Kinetic Data for the Reaction of SiMes2 with Alcohols in
Deoxygenated Hexanes at 25 °Ca
ROL
kROL/109 M-1 s-1
(kcat/k-ROL)/M-1
MeOH
MeOD
t-BuOH
t-BuOD
1.01 ( 0.09
1.26 ( 0.25
0.136 ( 0.005
0.136 ( 0.009
550 ( 150
220 ( 100
36 ( 7
64 ( 5
a From analysis of kdecay vs [ROL] data according to eq 6; listed as the
average ( standard deviation of three independent determinations in
each case. The silylene decay was monitored at 580 nm.
substrates with adventitious water in the solvent might be a
problem (below ca. 0.1 mM), yet below the ranges where
oligomerization effects begin to seriously complicate alcohol
speciation (above ca. 0.01 M for MeOH; above ca. 0.08 M for
t-BuOH).42,43,45
As expected, the plots reveal a nonlinear dependence of
kdecay on [ROL] in the low alcohol concentration ranges,
which changes to a linear dependence at intermediate alcohol
concentrations and above. The kinetic behavior at MeOH
concentrations higher than 1.0 mM is in excellent agreement
with our earlier study;29 linear least-squares analysis of the
present data, excluding those data points below 1.5 mM
MeOH, affords a slope of (8.6 ( 0.3) ꢀ 108 M-1 s-1. As
expected, the linear portion of the plot for MeOD is dis-
placed to higher concentrations compared to that for
MeOH; this is consistent with a normal kinetic isotope effect
on the catalytic rate constant (kcat), although the effect is
apparently quite small. Interestingly, the linear portion of
the plot for t-BuOD is displaced to lower concentrations
compared to that for t-BuOH, consistent with an inverse
kinetic isotope effect on kcat for the bulkier alcohol.
Attempts to fit the data to eq 5 failed to yield unambiguous
values for k-ROL and kcat, so the expression was recast in the
analytically more tractable form of eq 6. The results of fitting
the data to the latter equation are shown as the solid lines in
the plots of Figures 5 and S4, while the kROL and kcat/k-ROL
values afforded by the analyses are listed in Table 2. Each of
the values listed is the average of three independent deter-
minations.
Figure 5. Plots of the pseudo-first-order rate constants for the
decay of SiMes2 vs [MeOL] (L = H, (b) or D (O)) in hexanes at
25 °C. The solid lines are the nonlinear least-squares fits of the
data to eq 6.
on the catalytic proton-transfer process should be at least
qualitatively evident in the resulting kdecay versus [ROL]
plots, as an extension of the concentration range over which
the curvature is displayed for the deuterated alcohol com-
pared to that of the protiated isotopomer. The result should
be a displacement of the linear portion of the plot to higher
concentrations for the deuterated alcohol, with the extent of
the displacement depending on the magnitude of the isotope
effect on kcat
.
Laser photolysis of deoxygenated hexanes solutions of the
SiMes2 precursor (3) afforded the characteristic absorptions
due to free SiMes2 (λmax = 290 and 580 nm), which decayed
with second-order kinetics with the concomitant growth of
the long-lived absorption due to the dimerization product,
tetramesityldisilene (Si2Mes4; λmax = 420 nm; τ0 > 20 s), as
reported previously.29,44 Addition of as little as 0.2 mM
MeOH to the solutions caused a noticeable shortening of
the silylene decay (monitored at 580 nm) and suppression
of the characteristic growth of the disilene absorption at
420 nm. As anticipated, the silylene absorptions decayed
with mixed-order kinetics in the presence of the alcohol over
the 0.2 to ca. 1.3 mM concentration range, above which the
decay profiles fit well to first-order kinetics. First-order
decay rate coefficients over the lower range in [MeOH] were
estimated by fitting the data to two first-order exponentials
(i.e., treating the second-order component, which dominates
the earliest portion of the decay profile, as a first-order
decay) and taking the slower of the two decay coefficients
as that due to reaction with the alcohol; those at higher
concentrations were obtained from fits to simple first-order
kinetics. Decay rate coefficients were measured at a total of
12-15 different alcohol concentrations, up to that required
for the silylene lifetime to be reduced to 150 ns or less. A
similar procedure was followed using MeOD, t-BuOH, and
t-BuOD as substrates. Figure 5 shows the resulting plots of
kdecay versus [ROL] for MeOH and MeOD; the correspond-
ing ones for t-BuOL (Figure S4, Supporting Information)
show similar curvature, except they span a ca. 10-fold higher
range in alcohol concentration. It should be noted that the
alcohol concentration ranges studied in these experiments
are well above the range where exchange of the deuterated
2
kdecay ¼ kROLðkcat=k-ROLÞ½ROLꢁ =
ð1 þ ðkcat=k-ROLÞ½ROLꢁÞ
ð6Þ
The use of the steady-state approximation in the deriva-
tion of eqs 5 and 6 can be justified by the fact that transient
spectra recorded in the presence of MeOH (3.9 mM; Figure
S4a, Supporting Information) or t-BuOH (0.05 M; Figure
S4c, Supporting Information), where the lifetime of the
silylene is reduced to ca. 10% of its value in the absence of
added alcohol, showed only short-lived absorptions due to
free SiMes2 and no evidence of new transient absorptions in
the 300-420 nm range that might be assigned to the corres-
ponding SiMes2-ROH complexes;21 lifetimes measured in
this spectral range were nearly identical to those measured at
570-580 nm, as was also the case in the absence of added
alcohol. Analogous results were obtained in experiments
carried out at both lower and higher concentrations of
MeOH. We measured transient spectra from a hexanes
solution of 3 containing 0.6 M THF (Figure S4b, Supporting
Information) in order to determine how the expected spectra
of SiMes2-ROH complexes should appear in fluid solution,
(44) Conlin, R. T.; Netto-Ferreira, J. C.; Zhang, S.; Scaiano, J. C.
Organometallics 1990, 9, 1332.
(45) Valero, J.; Gracia, M.; Gutierrez Losa, C. J. Chim. Phys.
Phys.-Chim. Biol. 1980, 77, 65.

Article
Organometallics, Vol. 29, No. 3, 2010 669
assuming that the appearance of the spectrum should be
roughly independent of structural differences in the O-donor,
as has been shown previously for SiMe2 and SiPh2.33 The
spectrum of the SiMes2-THF complex in hexanes at 25 °C
of the corresponding silylene-alcohol Lewis acid-base com-
plex, which proceeds to the corresponding alkoxysilane via
catalyzed proton transfer from oxygen to silicon, with a second
molecule of the alcohol acting as catalyst. Complex formation is
effectively irreversible with sterically unhindered silylenes such
as SiMe2 and SiPh2 and proceeds at or close to the diffusion-
controlled rate in hexanes solution. As a result, the correspond-
ing silylene-alcohol complexes can be detected directly at low
alcohol concentrations as short-lived reaction intermediates
with their own distinctive UV/vis spectra. A lower limit of
Keq g 2 ꢀ 105 M-1 has been estimated for the equilibrium
constant for formation of the SiPh2-MeOH complex, corres-
ponding to a free energy difference of ΔG ≈ -7.2 kcal mol-1
for the complex relative to the free reactants in hexanes solution
at 25 °C.
The involvement of catalysis in the mechanism for the
transformation of the complexes to product is revealed by a
dependence of the lifetimes of the complexes on alcohol
concentration and isotopic substitution; this process is also
extremely rapid, proceeding with a rate constant similar to
that of the initial complexation step. The large deuterium
kinetic isotope effect exhibited by t-BuOH catalyzed proton
transfer in the SiPh2-t-BuOH complex, for which a value of
kHH/kDD = 10.8 ( 2.4 has been determined in hexanes at
25 °C, is consistent with a mechanism involving concerted,
double proton transfer within a cyclic five-membered transi-
tion state. An upper limit of k1 e 105 s-1 has been established
for the rate constants for unimolecular H-migration in the
complexes of SiMe2 and SiPh2 with MeOH, consistent with a
free energy of activation of ΔG‡ ≈ 11 kcal mol-1 or greater at
25 °C. Thus, of the two unimolecular decay channels avail-
able to the silylene-alcohol complex, H-migration to pro-
duce the alkoxysilane product must surmount a significantly
higher free energy barrier than reversion to the free reactants,
in agreement with earlier theoretical studies of the reactions
of SiH2 with water and alcohols.11,15,18
The reactions of MeOH and t-BuOH with sterically
protected diarylsilylenes such as dimesitylsilylene exhibit
much different kinetic behavior compared to that of the
parent diarylsilylene, SiPh2. The difference arises because of
a substantially lower equilibrium constant for silylene-
alcohol complexation in the case of the sterically hindered
silylene compared to those exhibited by the less hindered
derivatives, which leads to kinetics consistent with reversible
complexation in the first step of the reaction followed by
alcohol-catalyzed proton transfer in the second step. As a
result, the corresponding silylene-alcohol complexes are
involved as steady-state intermediates and cannot be de-
tected in fluid solution. The rates of alcohol-catalyzed pro-
ton transfer relative to those for dissociation of the
complexes exhibit small deuterium isotope effects, which
vary between normal and inverse depending on the alcohol.
This suggests that a different mechanism for catalysis may
operate in the case of the sterically hindered silylene, such
as a stepwise process involving sequential protonation/
deprotonation or deprotonation/protonation. Interestingly,
of the three silylenes studied in the present work, SiMes2
exhibits kinetic characteristics that are qualitatively most simi-
lar to those reported by Walsh and co-workers for the gas-
phase reaction of the parent silylene (SiH2) with water.18 The
similarity is due to the fact that in both systems dissociation of
the intermediate silylene-ROH complex back to the free
reactants is competitive with the catalytic proton-transfer
process that transforms it to the final product.
was found to consist of a strong band centered at λmax
=
300 nm coupled with a shoulder centered at ca. 400 nm. This
appears to be different from the reported UV/vis spectrum of a
photolyzed sample of 3 in a 95:5 3-methylpentane-THF
matrix at 77K (λmax = 328 nm), which was assigned to
the SiMes2-THF complex.21 It is, however, quite similar to
the spectrum of the SiPh2-t-BuOD complex (Figure 2) under
similar conditions, the main difference being a 30-40 nm shift
of the spectrum to longer wavelengths. Comparison of the
spectrum of the SiMes2-THF complex to that of free SiMes2
suggests that the putative SiMes2-ROH complexes, if they
were present in appreciable concentrations, should be detect-
able selectively in the 380-390 nm monitoring wavelength
range. None of the spectra recorded for solutions of 3 contain-
ing MeOH or t-BuOH exhibited discrete transient absorptions
within this spectral window, suggesting that the concentrations
of the complexes do not grow to detectable levels. The mechan-
istic situation contrasts that for SiMe2 and SiPh2, where the
rates of product formation from the complex far exceed those
of its dissociation to the free reactants even at very low alcohol
concentrations, and as a result the complexes can be detected
within limited concentration ranges.
The data of Table 2 indicate that the steric bulk afforded
by the mesityl substituents in SiMes2 has a pronounced effect
on the rate constant for the initial complexation step in the
reaction with MeOH, which is more than an order of
magnitude smaller for SiMes2 than for SiPh2. Increasing
the steric bulk in the alcohol further reduces the complexa-
tion rate constant, as evidenced by the ca. 7-fold lower value
of kt-BuOH compared to kMeOH. As we found for the more
reactive silylenes, the complexation rate constants for both
alcohols exhibit isotope effects indistinguishable from unity,
as expected considering that they should be secondary effects
and hence quite small. Surprisingly, the isotope effects on the
kcat/k-ROH ratios are also quite small and appear to vary
between a small normal effect for MeOH and an inverse
effect for t-BuOH. Since we can reasonably expect at most a
small (potentially inverse) secondary isotope effect on
k-ROL, we conclude that the isotope effects on kcat must be
quite small for both alcohols.
The data for the protiated complexes indicate that product
formation is ca. 10 times more efficient for the MeOH
complex compared to the bulkier t-BuOH complex, which
is consistent with faster catalysis and(or) slower reversion to
free reactants in the former compared to the latter. The very
modest kinetic isotope effect on the kcat/k-MeOH ratio, which
transforms to an inverse isotope effect in the case of t-BuOH,
argues for a different mechanism for the catalytic proton-
transfer process in the complexes with the bulkier silylene,
compared to the concerted double proton-transfer process
that may be operable for the SiPh2-ROH complexes. A
reasonable possibility is a sequential deprotonation/proton-
ation process, which would be expected to be less sterically
demanding than the concerted double proton transfer pro-
posed for the SiPh2-ROL complexes.
Summary and Conclusions
The well-known O-H insertion reaction of dialkyl- and
diarylsilylenes with alcohols proceeds via the initial formation

670 Organometallics, Vol. 29, No. 3, 2010
Leigh et al.
Further mechanistic studies of the reactions of transient
silylenes and their germanium homologues are in progress.
flow cell from a calibrated 250 mL reservoir, fitted with a glass
frit to allow bubbling of argon gas through the solution for at
least 30 min prior to and then throughout the duration of each
experiment, using a Masterflex 77390 peristaltic pump fitted
with Teflon tubing (Cole-Parmer Instrument Co.). The sample
cell and transfer lines were dried before use in a vacuum oven at
65-85 °C, while the reservoir was flame-dried and allowed to cool
under an argon atmosphere. Reagents were added directly to the
reservoir by microliter syringe as aliquots of standard solutions.
Transient decay rate constants were calculated by nonlinear
least-squares analysis of the absorbance-time profiles using the
Prism 4.0 or 5.0 software packages (GraphPad Software, Inc.) and
the appropriate user-defined fitting equations, after importing
the raw data from the Luzchem mLFP software. Rate constants
were calculated by linear least-squares analysis of decay rate-
concentration data (8-15 points). Errors in absolute second-order
rate constants are quoted as twice the standard error obtained
from the least-squares analyses or as the standard deviation of
2-3 replicate determinations.
Experimental Section
1,1,3,3-Tetramethyl-2,2-diphenyl-1,2,3-trisilacyclohexane (1),29
dodeccamethylcyclohexasilane (2),27 and 2,2-dimesityl-1,1,1,3,3,3-
hexamethyltrisilane (3)46 were prepared and purified by the re-
ported methods and exhibited spectral and analytical data similar
to those published previously. Hexanes (EMD OmniSolv) was
dried by passage through activated alumina under nitrogen using a
Solv-Tek solvent purification system (Solv-Tek, Inc.). Methanol
and tert-butanol (anhydrous), methanol-Od (99 atom % D), and
tert-butanol-Od (99 atom % D) were used as received from Sigma-
Aldrich Chemical Co. Tetrahydrofuran (Caledon Reagent) was
refluxed for several days over sodium metal and distilled.
Laser flash photolysis experiments employed the pulses from
a Lambda Physik Compex 120 excimer laser filled with F2/Kr/
Ne mixtures (248 nm; ca. 20 ns; 90-120mJ/pulse) and a Luzchem
Research mLFP-111 laser flash photolysis system, modified as
described previously.47 Solutions of 1-3 in anhydrous hexanes
were prepared at concentrations (0.06-0.5 mM)29 such that
the absorbance at the excitation wavelength (248 nm) was ca.
0.7 and were flowed (ca. 3 mL/min) through a 7 ꢀ 7 mm Suprasil
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada for financial
support of this work.
Supporting Information Available: Plots of kdecay vs [ROL]
and transient absorption spectra from laser photolysis of 3 in
hexanes containing MeOH, t-BuOH, and THF. This material is
available free of charge via the Internet at http://pubs.acs.org.
(46) Tan, R. P.; Gillette, G. R.; Yokelson, H. B.; West, R. Inorg.
Synth. 1992, 29, 19.
(47) Leigh, W. J.; Harrington, C. R.; Vargas-Baca, I. J. Am. Chem.
Soc. 2004, 126, 16105.