Photochemical Generation and Characterization of Di- tert-butylsilylene (SitBu2) in Solution

Article abstract of DOI:10.1021/acs.organomet.8b00902

Di-tert-butylsilylene (Si^tBu₂) has been detected directly in solution and its reactivity characterized by laser flash photolysis methods. Laser photolysis of 7,7-di-tert-butyl-7-silabicyclo[4.1.0]heptane (5) affords a transient product that exhibits _max 520 nm and decays on the microsecond time scale, concurrent with the growth of a second, much longer-lived species exhibiting _max = 290, 430 nm. The two species are assigned to di-tert-butylsilylene (Si^tBu₂) and its disilene dimer, tetra-tert-butyldisilene (^tBu₂Si-Si^tBu₂, 8), respectively. Transient absorption spectra recorded by laser photolysis of hexa-tert-butylcyclotrisilane (7) are consistent with the prompt formation of both Si^tBu₂ and 8, the former appearing as a weak shoulder absorption on the red edge of the intense absorption band due to the disilene. Absolute rate constants were determined for the reactions of Si^tBu₂ with a variety of representative silylene substrates in hexanes at 25 °C, including methanol, triethylsilane, acetic acid, acetone, molecular oxygen, and four aliphatic alkenes (1-hexene, cyclohexene, cis-cyclooctene, and 2,3-dimethyl-2-butene). Rate and(or) equilibrium constants for Lewis acid-base complexation of the silylene with diethyl ether, tetrahydrofuran, tetrahydrothiophene, and diethyl- And triethylamine were also determined, along with the UV-vis absorption spectra of the corresponding Lewis acid-base complexes. Rate constants for the reactions of dimethylsilylene (SiMe₂) and dimesitylsilylene (SiMes₂) with cis-cyclooctene, 2,3-dimethyl-2-butene, and 1-hexene are also reported, enabling a comprehensive assessment to be made of steric effects on the reactivities of simple (transient) dialkyl- And diarylsilylene derivatives in solution. The kinetic data show the reactivity of Si^tBu₂ to be intermediate between that of SiMe₂ and SiMes₂ under similar conditions, with the (2 + 1)-cycloaddition reactions with alkenes exhibiting the largest variations in rate constant as a function of substitution at Si. Absolute rate constants for the reactions of tetra-tert-butyldisilene (8) with O₂ and acetone are also reported.

Full text of DOI:10.1021/acs.organomet.8b00902

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Photochemical Generation and Characterization of Di-tert-
butylsilylene (SitBu2) in Solution
Ian R. Duffy and William J. Leigh*
Department of Chemistry & Chemical Biology, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4M1, Canada
S
* Supporting Information
ABSTRACT: Di-tert-butylsilylene (SitBu2) has been detected directly in
solution and its reactivity characterized by laser flash photolysis methods.
Laser photolysis of 7,7-di-tert-butyl-7-silabicyclo[4.1.0]heptane (5)
affords a transient product that exhibits λmax ≈ 520 nm and decays on
the microsecond time scale, concurrent with the growth of a second,
much longer-lived species exhibiting λmax = 290, 430 nm. The two species
are assigned to di-tert-butylsilylene (SitBu2) and its disilene dimer, tetra-
tert-butyldisilene (tBu2SiSitBu2, 8), respectively. Transient absorption
spectra recorded by laser photolysis of hexa-tert-butylcyclotrisilane (7) are consistent with the prompt formation of both SitBu2
and 8, the former appearing as a weak shoulder absorption on the red edge of the intense absorption band due to the disilene.
Absolute rate constants were determined for the reactions of SitBu2 with a variety of representative silylene substrates in hexanes
at 25 °C, including methanol, triethylsilane, acetic acid, acetone, molecular oxygen, and four aliphatic alkenes (1-hexene,
cyclohexene, cis-cyclooctene, and 2,3-dimethyl-2-butene). Rate and(or) equilibrium constants for Lewis acid−base
complexation of the silylene with diethyl ether, tetrahydrofuran, tetrahydrothiophene, and diethyl- and triethylamine were
also determined, along with the UV−vis absorption spectra of the corresponding Lewis acid−base complexes. Rate constants for
the reactions of dimethylsilylene (SiMe2) and dimesitylsilylene (SiMes2) with cis-cyclooctene, 2,3-dimethyl-2-butene, and 1-
hexene are also reported, enabling a comprehensive assessment to be made of steric effects on the reactivities of simple
(transient) dialkyl- and diarylsilylene derivatives in solution. The kinetic data show the reactivity of SitBu2 to be intermediate
between that of SiMe2 and SiMes2 under similar conditions, with the (2 + 1)-cycloaddition reactions with alkenes exhibiting the
largest variations in rate constant as a function of substitution at Si. Absolute rate constants for the reactions of tetra-tert-
butyldisilene (8) with O2 and acetone are also reported.
INTRODUCTION
■
Our understanding of the chemistry of silylenes, compounds
containing a divalent Si(II) center, has evolved substantially
since their debut many years ago as “short-lived intermedi-
phase8 and fluid solution9 by time-resolved UV−vis
ates”1 in various organosilicon reactions.2 Contemporary
spectrophotometry, or flash photolysis. This has allowed the
studies are mainly directed at exploring the potential utility
direct detection and characterization of the simplest carbon-
of electronically and/or kinetically stabilized isolable silylenes3
substituted silylenes in solution, including SiMe2,9a−c,10
in catalytic applications such as transition-metal-free small
SiPh2,9e−g SiMePh,9h and SiMes2 (Mes = 2,4,6-trimethylphe-
molecule activation,4 although interest remains in advancing
our understanding of basic reactions of the simpler derivatives.
nyl),9d using oligosilanes such as 2−4 as photoprecursors.
Simple hydrido, alkyl, and halo-substituted systems have been
studied in the gas phase by the groups of O. P. Strausz,11 J. M.
The majority of the known isolable silylenes contain some
Jasinski,12 and R. Walsh.8a,13
degree of electronic stabilization, imparted either via intra-
molecular or intermolecular donor coordination or direct
bonding to a heteroatom. Relatively few kinetically stable,
electronically unperturbed silylenes have yet been prepared
and studied. The notable exception to this generalization is the
well-known compound 1, which was first reported by M. Kira
and co-workers in 19995 and then studied extensively by them
over the next several years.6 The rich and diverse reactivity that
has been established for this compound has direct analogy in
the chemistry of simple transient dialkyl- and diarylsilylenes in
solution, which were studied in detail several decades ago.2a−c
Transient silylenes have been characterized using UV−vis
and infrared spectroscopy in frozen matrices,7 and in the gas
A silylene that has received particular attention in synthetic
applications is di-tert-butylsilylene (SitBu2),14 whose silver-
Received: December 13, 2018
© XXXX American Chemical Society
A
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mediated transfer from the 1,1-di-tert-butylsilirane derivative 5
to a wide variety of substrate types has been demonstrated to
be a versatile tool for the stereoselective formation of
organosilicon compounds of importance as synthetic inter-
mediates in the preparation of complex natural products.15 The
mechanism of silylene transfer in these reactions is thought to
involve silver-assisted extrusion of SitBu2 from the silirane to
form an intermediary silylsilver complex (eq 1a), preceding
formal SitBu2 transfer to the substrate.16
Et3N, respectively), and reaction with molecular oxygen (O2).
The complete set of reactions and equilibria that have been
examined are shown in Scheme 1. The data enable direct
Scheme 1
Free, uncoordinated SitBu2 has been proposed as an
intermediate in the thermolysis and photolysis of 517 (eq
1b) as well as various other substituted 1,1-di-tert-butylsilirane
derivatives such as 6a and cis- and trans-6b, from which
thermal silylene extrusion (and its reverse) have been shown to
proceed stereospecifically.18 Similarly, photolysis of hexa-tert-
butylcyclotrisilane (7) in the presence of various trapping
agents generates compounds consistent with the formation of
both SitBu2 and the marginally stable19 tetra-tert-butyldisilene
(8) as photoproducts (eq 2),20 and has been used extensively
comparisons to be made between the reactivities of SitBu2 and
the parent dialkylsilylene, SiMe2, toward a common set of
substrates, thus providing a quantitative assessment of the
effects of steric bulk on the kinetics and thermodynamics of
transient dialkylsilylene reactions in solution.
RESULTS AND DISCUSSION
■
Hexa-tert-butylcyclotrisilane (7) was prepared from di-tert-
butyldiiodosilane (tBu2SiI2)24 by a procedure adapted from
that of Weidenbruch and co-workers,20 and obtained in ≥97%
1
purity as determined by H NMR spectroscopy. 7,7-Di-tert-
butyl-7-silabicyclo[4.1.0]heptane (5) was a generous gift from
Drs. K. Woerpel and M. Prevost.
Laser photolysis of rapidly flowed, deoxygenated solutions of
5 (ca. 0.004 M) in anhydrous hexanes were carried out using a
(pulsed) KrF excimer laser for excitation (248 nm; ca. 25 ns,
40−100 mJ per pulse). Time-resolved UV−vis absorption
spectra sampled over selected time intervals after the excitation
pulse are shown in Figure 1a. The initially produced (0.16−
0.19 μs) spectrum contains a broad absorption band with an
apparent maximum centered at λmax ≈ 490 nm, which decays
over ca. 10 μs with second order kinetics and rate coefficient
by Weidenbruch and co-workers for the preparation of a wide
range of novel organosilicon compounds.21 An early laser flash
photolysis study of 7 reported the observation of a transient
product exhibiting λmax ≈ 440 nm and lifetime τ ≈ 12 ms in
degassed cyclohexane,22 which was assigned to SitBu2 on the
basis of its lifetime in spite of the spectroscopic similarities to
8.19 The silylene was subsequently characterized by UV−vis
and IR spectroscopy in low temperature matrixes after
generation by two-photon photolysis of diazidodi-tert-butylsi-
lane (9; eq 3), and found to exhibit a broad visible absorption
band over the ca. 430−590 nm spectral range (λmax ≈ 480
nm).7d It has also been the subject of theoretical studies.23
In the present paper, we report the results of laser flash
photolysis of the SitBu2 precursors 5 and 7 in solution under
ambient conditions, featuring the direct detection of SitBu2 and
characterization of its UV−vis spectrum and reaction kinetics
in a variety of well-documented silylene reactions. These
include σ-bond insertions with triethylsilane (Et3SiH) and
methanol (MeOH), formal oxa-ene addition with acetic acid
(AcOH) and acetone, (2 + 1)-cycloaddition with substituted
alkenes (1-hexene, cyclohexene, cis-cyclooctene, and 2,3-
dimethyl-2-butene (TME)), Lewis acid−base complexation
with diethyl ether (Et2O), tetrahydrofuran (THF), tetrahy-
drothiophene (THT), and di- and triethylamine (Et2NH and
2kdim/ε530 nm = (1.2
0.4) × 108 cm s−1 (eq 4).25 The
spectrum is in good qualitative agreement with that reported
previously for SitBu2 in 3-methylpentane at 77 K (λmax ≈ 480
nm).7d Accompanying the silylene decay is the growth of a
secondary product, which exhibits λmax = 290, 430 nm and is
stable over a time scale in excess of 1 ms. The spectrum and
apparent stability of this species is similarly in very good
agreement with that reported for disilene 8 in hydrocarbon
solvents (λmax = 305 nm (ε = 5200 M−1 cm−1) and 433 nm (ε
= 2800 M−1 cm−1)), produced by low-temperature photolysis
of compound 10.19 The laser photolysis results for 5 in
hexanes solution are thus consistent with the reaction scheme
of eq 5.
ΔAt = ΔA0/(1 + (2kdimΔA0/εl)t) + ΔAres
(4)
In contrast, laser photolysis experiments with deoxygenated
hexanes solutions of cyclotrisilane 7 (ca. 4 × 10−5 M) showed
evidence of two promptly formed transient photoproducts, as
illustrated in the time-resolved spectra of Figure 1b. The more
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Figure 1. Transient absorption spectra recorded by laser flash photolysis (a) of 7,7-di-tert-butyl-7-silabicyclo[4.1.0]heptane (5; 0.004 M) in
hexanes, 0.16−0.19 μs (- -) and 17.7−17.8 μs (- -) after the laser pulse; and (b) of hexa-tert-butylcyclotrisilane (7; ca. 4 × 10−5 M) in hexanes,
□
●
●
□
recorded 0.2−0.5 μs (- -) and 7.2−7.7 μs (- -) after the laser pulse. The solid dark lines in the two figures are the difference spectra (ΔAdiff
)
resulting from subtraction of appropriately scaled spectra of disilene 8 from the ∼0.2 μs spectra, in an attempt to isolate the more rapidly decaying
component of the postpulse spectrum in each case (see text). The difference spectrum in (a) is shown on the same scale as the other spectra, while
that in (b) is plotted on the scale defined by the right-hand axis. The insets show absorbance vs time profiles recorded at 530, 430, and 290 nm.
silylene in the absence of contributions from its dimerization
product. The resulting difference spectrum is included in
Figure 1b, magnified relative to the other spectra in the figure
by a factor of 5 for clarity. Indeed, the roughly symmetrical
form of the long-wavelength absorption band, which is
centered at λmax ≈ 520 nm, suggests the procedure has been
more or less successful. Essentially the same difference
spectrum is obtained from the data for 5 (see Figure 1a), by
subtraction of an appropriately scaled version of the 17.7−17.8
μs (disilene) spectrum from the prompt spectrum in order to
prominent transient exhibited strong, long-lived UV−vis
absorptions centered at λmax ≈ 290−295 and 435 nm which
exhibited lifetimes on the order of 0.1 ms and a slight growth
over the first ca. 5−10 μs after the laser pulse, consistent with it
being produced both directly (i.e., as a product of photolysis of
7) and in the first few microseconds after the pulse via a
secondary (ground state) process. The second photoproduct is
evident as a relatively weak transient absorption in the 480−
540 nm region of the spectrum, superimposed on the long-
wavelength tail of the prominent (λmax ≈ 435 nm) transient
absorption. It is also formed within the laser pulse and decays
over ca. 20 μs with second-order kinetics and decay rate
remove the contribution from a small amount of 8 that is
formed via dimerization of SitBu2 within the duration of the
laser pulse.27 The relative ΔA520 nm values obtained from laser
photolysis of the two precursors in approximately optically
matched solutions indicate that the quantum yield of SitBu2
extrusion from silirane 5 is 2−3 times higher than from 7.
The spectrum is red-shifted compared to those of both
SiMe2 (λmax = 470 nm),9a,c,f and the stable dialkylsilylene 1
(λmax = 440 nm)5 under similar conditions; the red shift
relative to the spectrum of SiMe2 has been rationalized as due
mainly to a widening of the C−Si−C bond angle by the
sterically bulky substituents,7d which destabilizes the filled n-
orbital at silicon and decreases the energy of the n → p
transition.28 Indeed, DFT calculations predict C−Si−C bond
angles of 108.2°, 97.7°, and 93.4° in SitBu2, SiMe2, and 1,
respectively (see Supporting Information); the calculated
structure of 1 is in excellent agreement with the reported X-
ray structure of the compound.5 The positions of the lowest
energy UV−vis absorption bands in the experimental spectra
of the three species are also in reasonable agreement with
theoretical predictions, made on the basis of TDB3LYP-D3/6-
31+G(d,p) calculations (see Table S1, Supporting Informa-
tion; the table also compares the calculated and experimental
spectra of 5, 7, 8, and SiMes2, which serves to benchmark the
TDDFT method that was employed).
coefficient 2k/ε530 nm = (1.9
0.5) × 108 cm s−1,26 in
reasonable agreement with that of the promptly formed
transient from laser photolysis of 5. It can thus be assigned to
SitBu2, and the more prominent, long-lived transient that is
formed as both a primary and secondary photoproduct to
disilene 8 (eq 6).
The UV−vis spectrum of SitBu2 can be extracted from the
transient spectral data for 7 (Figure 1b), by subtracting a
spectrum sampled 42.1−43.0 μs after the pulse from the 0.2−
0.5 μs spectrum. The 42.1−43.0 μs time-window corresponds
approximately to the point in the 430 nm absorbance−time
profile at which the absorbance has decayed back to its value at
the end of the laser pulse, prior to the secondary growth due to
SitBu2 dimerization (see inset, Figure 1b); thus, the procedure
should (in principle) quantitatively isolate the spectrum of the
An estimate of the molar extinction coefficient at the SitBu2
absorption maximum can be made from the reported value of
εmax for 819 and the ratio of the approximate initial absorbances
due to the silylene (ΔA0520 nm) and 8 (ΔA0430 nm) at their
respective absorption maxima in the difference and 0.2−0.5 μs
spectra of Figure 1b, respectively, assuming that the initial
concentrations of SitBu2 and 8 produced upon laser excitation
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Figure 2. Plots of kdecay vs [Q] for reaction of SitBu2 with (a) cyclohexene, (b) Et3SiH, and (c) acetone, from laser flash photolysis of solutions of 5
−5
□
○
(ca. 0.004 M; ) and 7 (ca. 4 × 10 M; ) in hexanes solution at 25 °C; the solid lines are from linear regression of the data according to eq 8.
Silylene absorbance−time profiles were recorded at a monitoring wavelength of 530 nm in all cases and were analyzed according to eq 7.
520 nm
of 7 are equal. From the approximate ΔA0 values of ΔA0
= 0.0015 0.0002 and ΔA0430 nm = 0.016 0.001 (see Figure
1b), we thus obtain a value of εmax = 260 50 M−1 cm−1 for
the long wavelength absorption band in the UV−vis spectrum
of SitBu2. The value is in the expected range for a silylene n →
3p absorption, as exemplified by SiMe2 (ε470 nm = 3400 M−1
cm−1)9c and the stable dialkylsilylene 1 (ε440 nm = 500 M−1
cm−1).5 Combined with the second order decay rate coefficient
measured with 5 (vide supra) the estimate affords a value of
with the substrate, kQ. Figure 2 shows the plots obtained in the
experiments with cyclohexene, Et3SiH, and acetone as silylene
substrates, together with the values of the rate constants
obtained with each precursor. Quenching of the silylene
absorptions from 5 and 7 by 1-hexene was also examined,
affording values of khex = (1.4 0.3) × 109 M−1 s−1 and (1.1
0.1) × 109 M−1 s−1, respectively (see Figure S9).
ΔAt = ΔAres + (ΔA0 − ΔAres)exp(−kdecayt)
(7)
2kdim = (3
1) × 1010 M−1 s−1 for the bimolecular rate
constant for dimerization of SitBu2 under these conditions. The
value is within experimental error of twice the diffusional rate
constant in hexanes at 25 °C (kdiff = 2.2 × 1010 M−1 s−1).29
The chemical yields of disilene formed via SitBu2-
dimerization in the laser photolysis experiments with 5 and 7
are only on the order of 20−30%, as estimated by comparison
of the final transient absorbance values at 430 nm to the
maximum values expected based on the initial ΔA0 values at
530 nm (from 5; Figure 1a) or 430 nm (from 7; Figure 1b).30
The inefficiency is similar to what we have typically observed
previously with other transient silylenes in solution,9f−h and is
ascribable to competing reaction of the silylene with
adventitious impurities in the solvent, most likely water.
Kinetic simulations verified that the presence of only ca. 30 μM
(≈10−4 wt %) water is sufficient to result in a disilene yield of
ca. 25%, assuming an initial silylene concentration of 10 μM
and rate constants of 1 × 1010 and 6 × 109 M−1 s−1 for
dimerization of SitBu2 and its reaction with water, respectively.
Triethylsilane (Et3SiH), cyclohexene, and acetone have all
been shown to be efficient trapping agents for
SitBu2,7d,14c,17,18,20,31 as well as other transient silylenes,2,9b,c,g,h
so we used these substrates to compare the reactivities of the
promptly formed transients from 5 and 7 to gain further
support for a common assignment to SitBu2. As expected,
addition of each of these compounds in millimolar
concentrations to solutions of 5 and 7 resulted in shortening
of the lifetime of the 520 nm transient and a change in its
decay kinetics from second to pseudo-first order, indicative of
reaction of the species with the added substrate. With each
precursor, absorbance−time profiles were recorded at a series
of substrate concentrations and analyzed according to eq 7,
where kdecay is the pseudo-first-order decay rate coefficient at
the given substrate (“Q”) concentration. Plots of kdecay vs [Q]
according to eq 8 were linear, the slopes corresponding to the
effective second-order rate constants for reaction of the silylene
kdecay = k0 + kQ [Q]
(8)
As expected, the changes in the silylene absorptions were
accompanied by systematic reductions in the growth of the
absorptions due to 8, consistent with silylene dimerization
being quenched irreversibly in the presence of the substrate.
With both precursors, the formation of 8 via SitBu2
dimerization was eliminated completely at high substrate
concentrations, in the case of 7 leaving behind the (dominant)
portion that is formed within the laser pulse.
The results of the parallel quenching experiments with the
two precursors clearly support a common assignment of the
520 nm transient to SitBu2, and of the 430 nm species to
tBu2SiSitBu2 (8). However, for all but the reaction with
acetone (for which kQ is very close to the diffusion limit), there
appears to be a systematic dependence of the apparent rate
constant on the precursor. The values of kQ obtained with 5 as
precursor are consistently 30−60% higher than those obtained
using 7, leading to an average of k5/k7 = 1.35 0.21 for the
ratio of the rate constants from the two precursors, where the
error is quoted as the standard deviation from the mean of the
four pairs of determinations. It is unclear what causes this
discrepancy. The most likely explanation is that the rate
constants measured using 5 as precursor are artificially high
due to the gradual buildup of minor amounts of reactive
decomposition products as the experiments progress, resulting
from reaction of 5 with traces of water or oxygen in the
substrate solution as it is introduced in increasing amounts
throughout the experiment. An alternate possibility is that
SitBu2 forms a (nonreactive) complex with disilene 8 which
reduces its overall reactivity in experiments carried out using 7
as precursor, though we would not have expected such an
interaction to be favorable enough to result in an effect of this
magnitude. In any event, it seems most reasonable to attach
uncertainty limits of ca. 25% to the measured rate constants, in
recognition of these uncertainties in the data.
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Figure 3. Complexation of SitBu2 with Et2O in hexanes at 25 °C: (a) plot of (ΔA0)0/(ΔA0)Q vs [Q] from laser photolysis of 7 in hexanes
□
○
containing Et2O (see eq 9); (b) transient UV−vis spectra recorded 0.26−0.29 μs ( ) and 8.6−8.7 μs ( ) after the laser pulse, by laser photolysis
□
○
of 7 in a 0.2 M Et2O/hexanes solution; and (c) transient UV−vis spectra recorded 0.35−0.43 μs ( ) and 8.6−8.7 μs ( ) after the laser pulse, by
laser photolysis of 5 in a 0.4 M Et2O/hexanes solution. Silylene absorbance−time profiles were recorded at a monitoring wavelength of 530 nm in
all cases.
We also examined the reactivity of SitBu2 toward a variety of
other silylene substrates with known reactivities toward SiMe2
in solution (see Scheme 1), in the interest of evaluating
quantitatively the effects of the bulky tert-butyl group on
dialkylsilylene reactivity. Compound 7 (ε248 nm ≈ 1.6 × 104
M−1 cm−1) was used as the silylene precursor for most of these
experiments, as it is much more conveniently handled than the
air- and moisture-sensitive silirane (5; ε248 nm ≈ 1.7 × 102 M−1
cm−1) and has a ca. 100 fold higher molar absorptivity at the
excitation wavelength (248 nm), which means that much
smaller amounts of material are required for an experiment.
The trade-offs are that its photolysis is less efficient than that of
5, which results in weaker silylene absorptions, and the strong,
long-lived spectrum of the coproduct (disilene 8), coupled
with its own weak absorptions in the 360−370 nm range,
complicate the spectra of the transient complexes that the
silylene is expected to form with the various Lewis bases.
Addition of Et2O led to a decrease in the apparent maximum
signal intensity at 530 nm but had no effect on the decay rate
coefficient, behavior which is consistent with rapid and
reversible reaction between the transient and the substrate,
characterized by an equilibrium constant (Keq) smaller than ca.
103 M−1;32 the behavior is similar to that observed for SiMe2 in
the presence of this substrate.32a Plots of (ΔA0)0/(ΔA0)Q vs
[Q] according to eq 9, where (ΔA0)0 and (ΔA0)Q are the
apparent initial absorbances due to the silylene in the absence
and presence of the substrate, respectively, are linear, the slope
affording the equilibrium constant for Lewis acid−base
complexation of the silylene with Et2O, Keq = (15 2) M−1
(Figure 3a).
The equilibrium constant for complexation with Et2O is ca.
80 times smaller than the corresponding value for SiMe2 under
similar conditions,32a corresponding to a difference in binding
energy of Δ(ΔG°) ≈ 2.6 kcal mol−1. This quantifies the steric
effect associated with tert-butyl- for methyl-substitution at
silicon on the thermodynamics of Lewis acid−base complex-
ation with the O-donor. The result can be compared to the
difference in the binding free energies of the SiMes2- and
SiPh2-Et2O complexes of Δ(ΔG°) ≈ 5.3 kcal mol−1 in hexanes
at 25 °C.32a The equilibrium constants for complexation of
Et2O with the four transient silylenes span a range of close to 4
orders of magnitude in hexanes at 25 °C, decreasing in the
order SiPh2 ≈ 5.6SiMe2 ≈ 470SitBu2 ≈ 7900SiMes2.
Addition of submillimolar concentrations of THF to hexanes
solutions of 5 caused the silylene to decay with two-stage
kinetics, consisting of a fast initial decay to a residual
absorption that decays over an extended time scale, in a
manner consistent with a fast reversible reaction characterized
by an equilibrium constant in the range of 103 < Keq < 2.5 ×
104 M−1.32a The fast initial decay is associated with the
approach to equilibrium of the silylene-substrate reaction,
while the residual absorption (ΔAres) is associated with free
silylene remaining after equilibrium is attained and undergoing
relatively slow decay due to dimerization. Analysis of the kdecay
vs [THF] data according to eq 8 afforded the forward rate
constant for complexation of SitBu2 with THF, kTHF = (1.8
0.5) × 1010 M−1 s−1, while a value of Keq = (2.8 0.6) × 103
M−1 was obtained for the equilibrium constant from analysis of
the ΔAres data according to eq 10 (see Figure 4). Laser
photolysis of a solution of 7 in hexanes containing 5.0 mM
THF afforded a similar transient spectrum as was obtained in
the presence of Et2O (see Figure S1), and can be assigned to
the SitBu2-THF complex (λmax = 310 nm; τ ≈ 7.1 μs).
(ΔA0)0/(ΔA0)Q = 1 + Keq[Q]
(9)
The absorption spectrum of the complex was obtained by
laser photolysis of a hexanes solution of 7 containing ca. 0.2 M
Et2O, and is shown in Figure 3b. The spectrum shows
absorptions due to a short-lived transient exhibiting λmax ≈ 300
nm and a lifetime of ca. 1.5 μs, whose absorptions are
superimposed on the short wavelength absorption band due to
disilene 8. A much cleaner spectrum of the species was
obtained from a solution of 5 in 0.4 M Et2O/hexanes (Figure
3c). The spectrum strongly resembles those reported for the
SiMe2-Et2O and SiMe2-THF complexes under similar con-
ditions,9b,c,32a and can be assigned with confidence to the
SitBu2-OEt2 Lewis acid−base complex.
(ΔA0)0/(ΔAres)Q = 1 + Keq[Q]
(10)
The remaining nine substrates of Scheme 1 were all found to
react efficiently with SitBu2, causing the silylene to decay with
first-order kinetics and with a rate coefficient that increased
linearly with concentration according to eq 8. The data are
shown in Figures S2−S10 (Supporting Information), while the
corresponding rate and equilibrium constants for these and the
reactions discussed above are collected in Table 1. The table
also lists the reported rate and/or equilibrium constants for the
corresponding reactions with SiMe2 and SiMes2, for compar-
E
DOI: 10.1021/acs.organomet.8b00902
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Article
corresponding complexes with SiMe2.9b,33,34 Transient UV−vis
spectra were recorded with 5 in hexanes containing 5.0 mM of
the sulfide or amine, and are shown in Figures S2b and S3b,
respectively. No new transient products were observed with
any of the other silylene substrates37 except MeOH, which will
be discussed in more detail below. Table 2 summarizes the
Table 2. UV−Vis Absorption Maxima (λmax) of the Lewis
Acid−Base Complexes of SitBu2 and SiMe2 with Chalcogen
and Pnictogen Donors in Hydrocarbon Solvents at 20−25
a
°C
λmax (nm)
substrate
SitBu2
SiMe2
b
□
○
Figure 4. Plots of kdecay ( ) and (ΔA0)0/ (ΔAres)Q ( ) vs [THF],
from laser photolysis of 5 in hexanes containing THF. The solid lines
are the least-squares fits of the data to eq 8 and 10, respectively.
none
Et2O
THF
THT
Et2NH
520
310
310
340
290
465
c
305
c
310
d
325
Table 1. Absolute Rate Constants (kQ) and Equilibrium
Constants (Keq) for the Reactions of SitBu2, SiMe2, and
SiMes2 with Various Silylene Substrates in Hydrocarbon
e
280
a
b
In hexanes at 25 °C, unless otherwise noted. Cyclohexane or
hexanes. See refs 9a−c, 10. Cyclohexane, 20 °C. Ref 9b.
c
a
Solution at 20−25 °C
d
e
Cyclohexane, 20 °C. Ref 9c. Ref 34.
b
SitBu2
SiMe2
SiMes2
UV−vis absorption maxima of each of the Lewis acid−base
complexes studied in this work, along with those reported
previously for the corresponding complexes with SiMe2.9b,f,33,34
As can be seen, the spectra of the SitBu2-donor complexes are
in most cases red-shifted slightly relative to those of the
corresponding SiMe2-donor complexes, to a similar extent (in
terms of transition energy) as is observed with the spectra of
the free silylenes.
k
Q (109 M−1 s−1
)
k
Q (109 M−1 s−1
)
kQ (109 M−1 s−1
)
substrate
[Keq/M−1
[15 2]
]
[Keq/M−1
]
[Keq/M−1
]
d
d
Et2O
THF
[1260 50]
17.3 1.5
[0.9 0.1]
[2.4 0.4]
c
e
d
18
5
c
[2800 600]
f
d
THT
13
8
2
21
16
2
7
2
d
g
[1500 100]
3.5 0.5
g
g
The absolute rate constants for complexation of SitBu2 with
the disubstituted Lewis bases are all close to the diffusional
limit and vary within a factor of 3, in the order THF > THT >
Et2NH; though small, the variation is still larger than that
observed for complexation of SiMe2 with the same series of
bases. Significantly greater discrimination between the two
silylenes is observed with the bulkier base, Et3N.
Et2NH
2
3
[6300 600]
g
e
d
Et3N
Et3SiH
0.54 0.08
0.8 0.1
9.8 0.8
3.6 0.3
[130 60]
j
0.079 0.004
c
(1.3 0.1)
h
h
MeOH
AcOH
acetone
7
2
21
16
20
3
1
1.01 0.09
e
i
4.4 0.5
18
(20 2)
−
e
The σ-bond insertion reaction of SitBu2 with Et3SiH (eq 11)
has been well documented,17,20 as has the formal oxa-ene
1
2
8.3 0.4
c
e
j
O2
0.20 0.05
0.18 0.01
(0.24 0.02)
0.99 0.05
1.1 0.1
0.47 0.04
7.8 1.0
0.032 0.004
e
j
cyclohexene
0.0028 0.0003
c
cis-cyclooctene
1-hexene
8.9 0.4
0.031 0.002
0.0102 0.0003
k
7
1
reaction with acetone, which affords silyl enol ether 11a (eq
c
(1.4 0.3)
12).31a AcOH also reacts with transient silylenes via formal
TME
0.031 0.002
9.1 0.4
0.0017 0.0001
a
In hexanes at 25 °C, unless otherwise noted. Errors are quoted as
b
twice the standard error from the least-squares analyses. Determined
c
using 7 as SitBu2 precursor unless otherwise noted. Determined using
d
e
f
g
h
5 as SitBu2 precursor. Ref 32a. Ref 9g. Ref 33. Ref 34. Ref 35.
i
j
k
Ref 36 Cyclohexane, 23 °C; ref 9d. Cyclohexane, 22 °C; ref 9b.
oxa-ene addition,9g in the present case to afford (presumably)
the corresponding acetoxysilyl hydride 11b. The mechanisms
of these reactions in the case of the parent silylene SiH2 have
been the subject of gas phase kinetics and computational
studies,8a,13,38 and are thought to proceed via reversible
complexation of the silylene with the basic site in the substrate,
with the resulting reactive complex collapsing to product via
rapid Si- or H-migration (eq 11 and 12, respectively). In these
cases the second (unimolecular) step proceeds sufficiently
rapidly that the reactive complex plays the role of a steady state
intermediate, and thus the reaction follows clean overall
ison to the values obtained for SitBu2; the rate constants for
reaction of SiMe2 and SiMes2 with cis-cyclooctene, SiMe2 with
TME, and SiMes2 with 1-hexene (see Figures S11 and S12)
were determined as part of the present study and employed
oligosilanes 2 and 4 as silylene precursors, respectively.
As with Et2O and THF, laser photolysis of 5 or 7 in the
presence of THT and Et2NH led to the formation of new
transient absorptions at shorter wavelengths, which could also
be assigned to the corresponding Lewis acid−base complexes
with SitBu2 based on comparisons to the spectra of the
F
DOI: 10.1021/acs.organomet.8b00902
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Article
second order kinetics.8,9g This is also observed in the present
cases with SitBu2.
With water and alcohols, however, catalysis by a second
molecule of substrate provides a lower energy pathway for H-
transfer than unimolecular H-migration (see eq 13), which
SitBu2 can be seen to decrease over a 40-fold range with
increasing alkyl substitution on the alkene, in the order 1-
hexene ≈ cis-cyclooctene > cyclohexene > TME. Interestingly,
there is a 5−6 fold reduction in the rate constant for reaction
of SitBu2 with cyclohexene compared to that for cis-
cyclooctene, which parallels the relative thermal stabilities of
the silirane products from the two cycloalkenes (5 and 12,
respectively; Figure 5).14c The lower thermodynamic stability
applies both in solution35 and the gas phase.39 The resulting
kinetic behavior in solution is thus more complicated, and
varies depending on the stability of the intermediate complex
relative to the reactants. With SiMe2 and SiPh2 the reaction
with alcohols exhibits the characteristics of two distinct,
essentially irreversible second order reactions, each first order
in ROH concentration, and with the complex building up to
detectable amounts at intermediate alcohol concentrations, its
lifetime then decreasing as the ROH concentration is increased
further. On the other hand, no intermediate complex can be
detected in the case of SiMes2, and the decay of the silylene
exhibits a mixed first- and second-order dependence on ROH
concentration, consistent with the complex playing the role of
a steady state intermediate in the case of the more highly
hindered (and much less Lewis acidic) silylene.35
Figure 5. Calculated structures and energies of the most stable
conformers of the (2 + 1)-cycloadducts of SitBu2 with cyclohexene
(5) and cis-cyclooctene (12), calculated relative to the isolated
reactants at the B3LYP-D3/6-31+G(d,p) level of theory. The methyl-
hydrogens have been omitted for clarity.
The reaction of SitBu2 with MeOH leads to kinetic behavior
more like that of SiMe2 and SiPh2 than SiMes2, in that the
silylene decay rate coefficient follows a first-order dependence
on MeOH concentration, affording a linear plot of kdecay vs
[MeOH] with slope kMeOH = (7 2) × 109 M−1 s−1 (Figure
S5). The rate constant is intermediate between those for the
initial complexation step in the reactions of MeOH with SiMe2
and SiMes2 (kSiMe2 ≈ 3kSitBu2 ≈ 24kSiMes2),35 and can thus be
associated with the formation of the SitBu2-MeOH complex.
Indeed, a weak, relatively short-lived transient absorption could
be detected at 310 nm upon laser photolysis of solutions of 7
in hexanes containing 0.1−1.2 mM MeOH, superimposed on
the longer-lived short wavelength band due to disilene 8. The
species decayed with first order kinetics, and a plot of kdecay vs
[MeOH] was linear with slope kcat = (1.5 0.5) × 109 M−1 s−1
(Figure S5). The value is roughly an order of magnitude
smaller than that for MeOH-catalyzed H-transfer in the SiMe2-
of the cyclohexene-adduct (5) relative to 12 is also predicted
theoretically (see Figure 5 and Supporting Information), and is
due presumably to unfavorable steric interactions between the
endo-tert-butyl substituent and the flagpole H atoms in the C6-
ring of the compound,16b coupled with the increased ring
strain that results from the enforced boat-conformation of the
C6-ring. The ca. 30-fold lower rate constant for reaction of
SitBu2 with TME compared to those with cis-cyclooctene or 1-
hexene can be ascribed to steric destabilization of the transition
state for the reaction, in which the initial bonding interaction is
between the nonbonded p-orbital on silicon (perpendicular to
the 3-atom bonding plane of the silylene) and the π-MO of the
alkene, and the silylene is oriented with its substituents
approximately bisecting the alkene CC bond.42 The rate
constants for reaction of the series of alkenes with SiMes2 are
systematically 20−100× smaller than those for SitBu2, but the
spread in reactivity throughout the series is similar.
MeOH complex, kcat = (1.5
0.3) × 1010 M−1 s−1,35
presumably the result of steric destabilization of the transition
state for the proton-shuttling process (eq 13).
In the experiments employing 7 as the SitBu2 precursor, we
also monitored the effects of the various substrates on the
decay kinetics of the photolysis coproduct, tetra-tert-
butyldisilene (8), up to a maximum substrate concentration
of at least 5 mM in most cases. Most had no discernible effect
on the lifetime of 8, which under the conditions of our laser
photolysis experiments was typically on the order of 0.1 ms.
The exceptions were molecular oxygen and acetone, which
were both found to cause discrete acceleration of the decay
rate coefficient of the disilene over the concentration ranges
that were studied. The most reactive substrate is oxygen, for
which a value of kO2 = (2.3 0.1) × 107 M−1 s−1 was estimated
from the slope of a three-point plot of kdecay vs [O2],
constructed from rate coefficients obtained in argon-purged,
air-saturated, and oxygen-saturated hexanes at 25 °C (Figure
S13). The rate constant is in line with those reported
(2 + 1)-Cycloaddition reactions of SitBu2 with simple
alkenes have also been well documented,14c,21f,40 as they have
as well for other transient silylenes in solution and in the gas
phase.41 In the present work we report bimolecular rate
constants for reaction of SitBu2 with 1-hexene, 2,3-dimethyl-2-
butene (TME), cyclohexene, and cis-cyclooctene as represen-
tative cyclic and acyclic alkenes with varying degrees of alkyl
substitution about the CC bond. To enable broader
comparisons, we have also determined rate constants for
reaction of the same series of alkenes with SiMe2 and SiMes2
(see Table 1).
The rate constants for reaction of SiMe2 with the four
alkenes are all within a factor of ca. three of the diffusional rate
constant in hexanes at 25 °C (kdiff = 2.2 × 1010 M−1 s−1) and
consequently show no discernible trend with alkene structure,
a trend which is similar to that established by Walsh and co-
workers for the reactions of SiMe2 with a more extensive list of
aliphatic alkenes in the gas phase.41e In contrast, those for
previously for tetramethyldisilene (13; kO2 = (2.9
0.4) ×
108 M−1 s−1)9g and tetrakis(trimethylsilyl)disilene (14, kO2
=
(1.1 0.1) × 106 M−1 s−1).43 Considering that 8 is known to
have a lifetime of several hours in the absence of air and
G
DOI: 10.1021/acs.organomet.8b00902
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
moisture,19 the ca. 0.1 ms lifetime that is observed for 8 in our
experiments with 7 in argon-purged hexanes is consistent with
a residual O2 concentration of ca. 4 × 10−4 M. As expected, the
lifetime of 8 could be lengthened to >1 ms when generated by
laser photolysis of 7 in freeze−pump−thaw degassed hexanes
solution. Acetone was found to react with 8 with an estimated
rate constant of kacetone = (4 2) × 105 M−1 s−1.
substituents. The differences in reactivity between the three
silylenes span a particularly large range for the (2 + 1)-
cycloaddition reactions, again with SitBu2 exhibiting reactivity
intermediate between those of SiMe2 and SiMes2.
EXPERIMENTAL SECTION
■
1H and 13C NMR spectra were recorded at 600.13 MHz (1H) and
150.90 MHz (13C{1H}), respectively, on a Bruker AV600
spectrometer in chloroform-d or benzene-d6 solution, and were
referenced to the solvent residual proton and 13C signals, respectively.
29Si spectra were also recorded on a Bruker AV600 spectrometer using
1
the 2D H−29Si HMBC pulse sequence and were referenced to an
external solution of tetramethylsilane. High-resolution electron impact
mass spectra and exact masses were determined on a Waters
Micromass GCT Premier mass spectrometer using electron impact
ionization (70 eV). Infrared spectra were recorded using a Nicolet
6700 FTIR spectrometer. Melting points were measured using a
Mettler FP82 Hot Stage mounted on an Olympus BH-2 microscope
and controlled by a Mettler FP80 Central Processor, and are
uncorrected. Static UV−visible absorption spectra were recorded
using a Cary 50 UV−vis spectrophotometer. Column chromatog-
raphy and silica microcolumns were prepared using SiliaFlash P60
40−63 μm (230−400 mesh) silica gel (Silicycle). Alumina packed
microcolumns were prepared using neutral (60−325 mesh)
Brockmann Activity I alumina (Fisher Scientific).
SUMMARY AND CONCLUSIONS
■
Laser flash photolysis of 7,7-di-tert-butyl-7-silabicyclo[4.1.0]-
heptane (5) and hexa-tert-butylcyclotrisilane (7) affords SitBu2,
which has been detected for the first time in solution under
ambient conditions. The silylene exhibits a broad absorption
band centered at λmax ≈ 520 nm and decays with the
concurrent growth of tetra-tert-butyldisilene (8), the spectral
characteristics of which (λmax = 290, 430−435 nm) are in good
agreement with the previously reported UV−vis spectrum.19
The silylene is the sole initial transient photoproduct from
photolysis of 5, and decays with a second-order rate coefficient
of 2k/ε530 nm = (1.2
0.4) × 108 cm s−1 to form 8. The
All commercially available materials were used as received from the
suppliers or further purified prior to use as described below; solvents
were all reagent grade or better. Lithium wire (99.9%, Sigma-Aldrich)
was cut and washed with dry hexanes. Di-tert-butylsilane (tBu2SiH2;
95%, Gelest) and iodine (I2; 99.8%, VWR) were used as received
from the suppliers. Naphthalene (99%, Sigma-Aldrich) was purified
by sublimation. Hexanes (Caledon) and diethyl ether (Et2O;
Caledon) were dried by passage through activated alumina under
nitrogen using a Solv-Tek solvent purification system (Solv-Tek Inc.).
Pentane (Caledon) and 35−60 petroleum ether (Caledon) were used
as received. Tetrahydrofuran (THF; Caledon) for flash photolysis
experiments was dried by stirring over lithium aluminum hydride
(LiAlH4) overnight and distilling immediately prior to use.
Alternatively, THF for synthesis was distilled under nitrogen from
sodium/benzophenone. Tetrahydrothiophene (THT; 99%, Sigma-
Aldrich) was refluxed over anhydrous sodium sulfate and distilled
immediately prior to use.44 Diethylamine (Et2NH; 99.5%, Sigma-
Aldrich), and triethylamine (Et3N; ≥99%, Sigma-Aldrich) were
stirred over solid sodium hydroxide under nitrogen and distilled
immediately prior to use. Triethylsilane (Et3SiH; 98%, Gelest) was
stirred over LiAlH4 overnight and distilled. Glacial acetic acid (AcOH;
Caledon) was used as received from the suppliers. Methanol (MeOH;
Caledon) was distilled from sodium methoxide. Acetone (Caledon)
was distilled from calcium sulfate.44 Cyclohexene (≥99%, Sigma-
Aldrich) was stirred over maleic anhydride overnight before being
distilled and passed through a silica microcolumn.44 cis-Cyclooctene
(95%, Sigma-Aldrich) was distilled from NaOH pellets and then
passed through an alumina microcolumn.44 1-Hexene (99%, Sigma-
Aldrich) and 2,3-dimethyl-2-butene (TME; ≥99%, Sigma-Aldrich)
were stirred over calcium hydride or lithium aluminum hydride and
distilled immediately prior to use. Sealed ampules of benzene-d6
(Cambridge Isotope) were used as received from the suppliers, while
chloroform-d was dried by distillation from CaH2.
formation of SitBu2 as a primary photoproduct from cyclo-
trisilane 7 is less obvious in laser experiments with this
precursor because the accompanying formation of 8, with its
much stronger absorption at 430−435 nm, obscures the
relatively weak silylene absorptions above 500 nm. Never-
theless, the significantly shorter lifetime of SitBu2 compared to
8 allowed for its routine detection at 520−530 nm, on the red
edge of the disilene absorption band; the species was again
found to decay with second order kinetics and rate coefficient
of 2k/ε530 nm = (1.9
0.5) × 108 cm s−1, in reasonable
agreement with the value obtained using 5 as precursor.
Combined with the estimate of εmax ≈ 250 M−1 cm−1 for the
extinction coefficient at 520−530 nm, the kinetic data indicate
the dimerization of SitBu2 is fully diffusion-controlled.
Absolute rate or equilibrium constants have been measured
for reaction of SitBu2 with 14 substrates in hexanes solution at
25 °C, detailing the reactivity of the silylene toward Lewis
acid−base complexation with ethers, sulfides and amines, σ-
bond insertion with MeOH and Et3SiH, oxa-ene addition with
acetone and AcOH, (2 + 1)-cycloaddition reactions with a
series of aliphatic alkenes, and reaction with molecular oxygen.
The Lewis acid−base reactions proceed at close to the
diffusion-controlled rate in most cases, to form the
corresponding Lewis acid−base complexes, which have also
been detected. Their UV−vis spectra are blue-shifted relative
to that of the free silylene (as expected), and red-shifted by 5−
15 nm relative to the spectra of the corresponding complexes
formed with SiMe2. The equilibrium constant for complexation
with Et2O is ca. 2 orders of magnitude smaller than that for
complexation of the ether with SiMe2; since the rate constants
for complexation of the two silylenes with THF are nearly
equal, it can be concluded that the difference in equilibrium
constants mainly reflects the difference in thermodynamic
stability of the SiMe2- and SitBu2-Et2O complexes with respect
to dissociation. The rate constants measured for the other 12
reactions are generally smaller than the corresponding ones for
SiMe2, but consistently larger than those of dimesitylsilylene
(SiMes2) under similar conditions, as one would expect from
consideration of the differences in steric bulk of the three
Di-tert-butyldiiodosilane (tBu2SiI2) and hexa-tert-butylcyclotrisilane
(7) were prepared according to the procedures of Weidenbruch and
co-workers.20,24 Compound 7 was purified by column chromatog-
raphy (silica gel, hexanes), followed by recrystallization from pentane,
1
to a purity of ≥97% as determined by H NMR spectroscopy (mp
175−188 °C (dec); lit:20 182 °C (dec)). Spectroscopic data for the
compound are in good agreement with the reported spectra,20 except
for the chemical shift of the quaternary carbon signal in the 13C
spectrum, which was reported at 31.0 ppm in C6D6; we observed the
resonance at 27.2 ppm. Complete spectral data for 7 are as follows:
1H NMR (C6D6) δ 1.42 (s, 54 H); 13C{1H} NMR (C6D6) δ 27.2
H
DOI: 10.1021/acs.organomet.8b00902
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(C(CH3)3), 35.1 (C(CH3)3); 29Si NMR (C6D6) δ 3.6; IR, cm−1
(relative intensity) 2951 (w), 2853 (s), 1483 (m), 1473 (w), 1361
(m), 1165 (m), 937 (w), 810 (s), 481 (w), 438 (m); UV−vis
(hexanes) λmax, nm 208, 237, 353(sh), 388; EI-MS, m/z (relative
intensity) 426.4 (18, M+), 376.2 (9), 314.2 (9, (Si3t-Bu4 + 2H)+),
285.2 (9, (Si2t-Bu4 + H)+), 228.2 (18, (Si2t-Bu3 + H)+), 185.1 (12),
169.1 (39, (Si2t-Bu2 − H)+), 127.1 (52), 91.1 (51), 73.0 (100), 57.1
AUTHOR INFORMATION
Corresponding Author
*E-mail: leigh@mcmaster.ca.
ORCID
Ian R. Duffy: 0000-0001-6805-4943
William J. Leigh: 0000-0002-4905-8436
Notes
■
+
(35, C4H9 ); HRMS (C24H54Si3) calc. 426.3533, found 426.3529.
The samples of 7,7-di-tert-butyl-7-silabicyclo[4.1.0]heptane (5)
employed in this study were provided in sealed ampules by Prof. K.
Woerpel and Dr. M. Prevost. Dodecamethylcyclohexasilane (2) and
2,2-dimesityl-1,1,1,3,3,3-hexamethyltrisilane (4) were synthesized and
purified according to the previously reported procedures.45
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Natural Sciences and Engineering Research
Council of Canada for financial support for this work through
Discovery Grant #38211-2012, D. Emslie, K. Kolpin, and J.
Price for technical support, and K. Woerpel and M. Prevost for
their generosity and advice. Part of this work was made
possible by the facilities of the Shared Hierarchical Academic
Research Computing Network (SHARCNET: www.sharcnet.
ca) and Compute/Calcul Canada.
Laser flash photolysis experiments were carried out using a Lambda
Physik Compex 120 excimer laser filled with F2/Kr/Ne (248 nm, 20
ns, 40−100 mJ) and a Luzchem Research mLFP-111 laser flash
photolysis system, modified as described previously.46 Solutions of the
silylene precursors (5 or 7) were prepared in deoxygenated anhydrous
hexanes such that the absorbance of the resulting solution at 248 nm
in a 7 × 7 mm quartz cell was in the range 0.4−0.7. The solutions
were flowed through a 7 × 7 mm Suprasil flow cell from calibrated
100 or 250 mL reservoirs, which contain a glass frit to allow bubbling
of argon gas through the solution for at least 30 min prior to, and then
throughout, the experiment. Samples of 5 were dispensed in a
glovebox and were transferred rapidly in the required amounts to the
(solvent-filled, argon-purged) reservoirs using a microliter syringe to
minimize contact with air and moisture. The flow cell was connected
to a Masterflex 77390 peristaltic pump fitted with Teflon tubing
(Cole-Parmer Instrument Co.) which pulls the solution through the
cell at a constant rate of 2−3 mL/min. Solution reservoirs were flame-
dried under an atmosphere of nitrogen or argon, while the sample cell
and transfer lines were dried in a vacuum oven (55−75 °C) before
use. Solution temperatures were measured with a Teflon-coated
copper/constantan thermocouple inserted into the thermostated
sample compartment in close proximity to the sample cell. Substrates
were added directly to the reservoir by microliter syringe as aliquots of
standard solutions. Standard solutions were prepared in 10 or 25 mL
volumetric flasks using known quantities (determined by mass and/or
volume, in the latter case using literature densities at 25 °C) of
substrate. Air and O2-saturated solutions were prepared by bubbling
the appropriate gas through the solution for at least 20 min prior to
and during measurement.
REFERENCES
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Transient absorbance−time profiles were recorded by signal
averaging of data obtained from 10−50 individual laser shots.
Decay rate coefficients were calculated by nonlinear least-squares
analysis of the transient absorbance−time profiles using the Prism
7.0d software package (GraphPad Software, Inc.) and the appropriate
user-defined fitting equations, after importing the raw data from the
Luzchem mLFP software. Rate and equilibrium constants were
calculated by linear least-squares analysis of transient absorbance data
that spanned as large a range in transient decay rate or initial signal
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00902.
Additional kinetic data determined in laser flash
photolysis experiments; details of the computational
studies and tables of calculated energies and UV-vis
spectra for computed structures (PDF)
Cartesian coordinates for computed structures (XYZ)
I
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value is
530nm
J
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