Electronic and steric effects on the Lewis acidities of transient silylenes and germylenes: Equilibrium constants for complexation with chalcogen and pnictogen donors

Article abstract of DOI:10.1021/om3002558

The Lewis acid-base complexation reactions of dimethyl-, diphenyl-, and dimesitylsilylene (SiMe₂, SiPh₂, and SiMes₂, respectively) and their germanium homologues (GeMe₂, GePh ₂, and GeMes₂) with diethyl ether (Et₂O), tetrahydrothiophene (THT), triethylphosphine (Et₃P), and tricyclohexylphosphine (Cy₃P) have been characterized in hydrocarbon solvents at 25 °C by laser flash photolysis methods. Together with previously reported (and some new) data for the complexation of the six transient tetrellylenes with methanol (MeOH), tetrahydrofuran (THF), and di- and triethylamine (Et₂NH and Et₃N, respectively), the results allow the first systematic assessment of the thermodynamics of Lewis acid-base complexation of simple dialkyl- and diarylsilylenes and their germanium homologues with chalcogen and pnictogen donors in solution. The equilibrium constants (K_C) for complexation of the six species with Et ₂O span a range of ca. 10⁵ M^-1, decreasing in the order SiPh₂ > SiMe₂ > GePh₂ > GeMe₂ ? SiMes₂ > GeMes₂. For each homologous MR₂ pair, K_C is consistently 10-40 times larger for the silylene than the germylene, indicating a systematic difference in binding free energy of 1.5-2.2 kcal mol^-1. Equilibrium constants have been determined for complexation of SiMes₂ and GeMes₂ with all the donors in the series except Et₃P, for which only a lower limit can be determined. Those for SiMes₂ decrease in the order Et₃P > Cy₃P > Et₂NH > THT > Et ₃N > THF > Et₂O; GeMes₂ is consistently less acidic, but its binding constants follow a similar ordering. The experimental data are supplemented with theoretical (Gaussian-4) calculations of thermochemical parameters for the complexation of SiMe₂ and GeMe₂ with 17 O, S, N, and P donors, which are shown to agree with experiment to within 1 kcal mol^-1 for the 6 systems that have also been studied experimentally. The calculations confirm that, in general, trialkylphosphines bind most strongly, followed by amines, sulfides, and then ethers and alcohols. SiMe₂ and GeMe₂ are borderline-soft Lewis acids, stronger and softer than trimethylborane in both cases.

Full text of DOI:10.1021/om3002558

Article
pubs.acs.org/Organometallics
Electronic and Steric Effects on the Lewis Acidities of Transient
Silylenes and Germylenes: Equilibrium Constants for Complexation
with Chalcogen and Pnictogen Donors
Svetlana S. Kostina, Tishaan Singh, and William J. Leigh*
Department of Chemistry and Chemical Biology, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4M1, Canada
S
* Supporting Information
ABSTRACT: The Lewis acid−base complexation reactions of
dimethyl-, diphenyl-, and dimesitylsilylene (SiMe2, SiPh2, and
SiMes2, respectively) and their germanium homologues
(GeMe2, GePh2, and GeMes2) with diethyl ether (Et2O),
tetrahydrothiophene (THT), triethylphosphine (Et3P), and
tricyclohexylphosphine (Cy3P) have been characterized in
hydrocarbon solvents at 25 °C by laser flash photolysis
methods. Together with previously reported (and some new)
data for the complexation of the six transient tetrellylenes with
methanol (MeOH), tetrahydrofuran (THF), and di- and
triethylamine (Et2NH and Et3N, respectively), the results allow the first systematic assessment of the thermodynamics of Lewis
acid−base complexation of simple dialkyl- and diarylsilylenes and their germanium homologues with chalcogen and pnictogen
donors in solution. The equilibrium constants (KC) for complexation of the six species with Et2O span a range of ca. 105 M−1,
decreasing in the order SiPh2 > SiMe2 > GePh2 > GeMe2 ≫ SiMes2 > GeMes2. For each homologous MR2 pair, KC is
consistently 10−40 times larger for the silylene than the germylene, indicating a systematic difference in binding free energy of
1.5−2.2 kcal mol−1. Equilibrium constants have been determined for complexation of SiMes2 and GeMes2 with all the donors in
the series except Et3P, for which only a lower limit can be determined. Those for SiMes2 decrease in the order Et3P > Cy3P >
Et2NH > THT > Et3N > THF > Et2O; GeMes2 is consistently less acidic, but its binding constants follow a similar ordering. The
experimental data are supplemented with theoretical (Gaussian-4) calculations of thermochemical parameters for the
complexation of SiMe2 and GeMe2 with 17 O, S, N, and P donors, which are shown to agree with experiment to within 1 kcal
mol−1 for the 6 systems that have also been studied experimentally. The calculations confirm that, in general, trialkylphosphines
bind most strongly, followed by amines, sulfides, and then ethers and alcohols. SiMe2 and GeMe2 are borderline-soft Lewis acids,
stronger and softer than trimethylborane in both cases.
intermolecular Lewis acid−base coordination is well-known to
reduce the reactivity typical of these molecules (e.g.,
dimerization)1,9 and is hence a useful tool for the preparation
of isolable derivatives.10−13 Depending on the donor, the
increase in nucleophilicity at silicon or germanium that
accompanies complexation14 can be sufficient to result in an
altogether different reactivity for the complex in comparison to
that of the free species.11b,c,13b,15,16 This has been shown to be
particularly true of phosphine and N-heterocyclic carbene
(NHC) donors, which have been shown not only to be
particularly effective stabilizers of transient silylenes17,18 and
germylenes12,13,19 but also to alter their reactivities in intriguing
and potentially useful ways.
INTRODUCTION
■
Silylenes and germylenes, the silicon and germanium analogues
of (singlet) carbenes, have been studied extensively over the
past few decades in the gas phase, in solution, and in solid
matrixes at low temperatures and have also been the subject of
numerous theoretical studies.1−5 The parent derivatives, SiH2
and GeH2, have received particular attention,4a both because of
their fundamental importance and because they are critical
intermediates in the chemical vapor deposition of solid silicon
and germanium, respectively.6 There have also been great
advances made in the synthesis and study of isolable silylene
and germylene derivatives, rendered so by some combination of
steric and electronic stabilization of the intrinsically reactive
divalent tetrel center by substituents.5,7,8
The primary driving force for the reactivity of these species is
their intrinsically high electrophilicity, which results from their
electropositive character and the vacant 3p or 4p orbital on the
tetrel center. As a result, most of their known reactions begin
with an interaction between the vacant p orbital and a
nucleophilic site in the substrate, often in the form of a discrete
Lewis acid−base complex as reaction intermediate. Intra- or
The Lewis acid−base complexation of transient silylenes20
and germylenes21 with chalcogen and pnictogen donors was
first studied experimentally by low-temperature spectroscopic
methods in the 1980s by West and Ando and their co-workers,
following an early theoretical study of the complexation and X−
Received: March 29, 2012
Published: April 27, 2012
© 2012 American Chemical Society
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H insertion reactions of SiH2 with the parent hydride donors by
Raghavachari and co-workers.22 A later computational study of
the complexation of SiH2, GeH2, and SnH2 with the chalcogen
and pnictogen hydrides predicted the Lewis acidities of the
species to decrease in the order Si > Ge > Sn and demonstrated
the more pronounced ylide character of the complexes with the
third-row donors in comparison to those with the correspond-
ing second-row homologues;14 more sophisticated theoretical
studies of the bonding in SiH2-phosphine ylides and
comparisons to those in methylenephosphorane23 and other,
related systems24 have been carried out more recently. Other
theoretical studies have ranked a variety of substituted
silylenes25 and germylenes25a according to their electro-
philicities and hardness; the latter is based on calculated
binding energies with NH3 and PH3, the relative magnitudes of
which have led to the general classification of simple silylenes
and germylenes as “hard” electrophiles.25a A more recent DFT
study by De Proft and co-workers examined the thermody-
namics of Lewis acid−base complexation of dihalogermylenes
with a variety of σ (O, S, and N) donors, in a comparative study
of the complexation of dihalogermylenes and -stannylenes with
aromatic π donors.26
Early fast kinetic studies of the complexation of SiMe2 with
various donors in solution27,28 and with ethers in the gas
phase29 established the first absolute rate constants for silylene
Lewis acid−base complexation reactions and showed them to
be essentially diffusion- or encounter-controlled in all cases,
consistent with an enthalpically barrierless process. This has
been extended by our own, more recent solution-phase studies
of the reactivities of SiMe2, GeMe2, and the corresponding
diphenyl and dimesityl derivatives,30,31 which show that
complexation with THF, alcohols, and amines is invariably
quite fast, the rates varying only modestly with variations in
structure or substitution in either the tetrellylene or the donor
or with the overall thermodynamics of the complexation
process. Equilibrium constants have been measured or
estimated in only a handful of casesfor the complexation of
the three germylene derivatives with methanol (MeOH) and
THF31c and of SiMes2 with diethylamine (Et2NH) in
hexanes30d and of SiMe2 with dimethyl ether (Me2O) and
tetrahydrofuran (THF) in the gas phase.29,32 The available
thermodynamic data are thus fairly sparse but are nevertheless
consistent with the indication from theory that silylenes are
stronger Lewis acids than germylenes of homologous structures
and that both species form stronger complexes with amines
than with O donors. The differences have never been
quantified, however. Similarly, the complexation reactions of
these species with sulfides and phosphines have not yet been
studied in a systematic way.
including several of those that have been characterized
experimentally. The data are compared to various empirical
donor basicity scales in order to rank the Lewis acidities and
hard−soft behaviors of SiMe2 and GeMe2 relative to each other
and in the context of other, more conventional Lewis acids.
RESULTS
■
The transient silylenes and germylenes studied in this work
were generated and detected by laser flash photolysis of rapidly
flowed, deoxygenated solutions of compounds 1−6 in
anhydrous hexanes or cyclohexane34 and the pulses from a
KrF excimer laser (248 nm, 90−105 mJ, ca. 20 ns) for
excitation. As reported previously, the species of interest are
observed in these experiments as promptly formed transients
with UV−vis absorption bands centered at λmax 465 nm
(SiMe2),27a,34c,35 290 and 515 nm (SiPh2),34c and 290 and 580
nm (SiMes2)34c,36 for the silylenes and at λmax 470 nm
(GeMe2),34a,37 300 and 500 nm (GePh2),34a and 325 and 560
nm (GeMes2)34a,38 for the germylenes. In the absence of added
substrates they each decay on the microsecond time scale with
the concomitant formation of longer-lived absorptions in the
360−460 nm range due to the corresponding disilenes and
digermenes, formed by dimerization.34 Silylene formation is
accompanied by the formation of minor amounts of a second
transient product in the cases of 2 and 3, which gives rise to
long-lived residual absorptions (centered at 460 and 440 nm,
respectively) on which the spectra of the silylenes and
corresponding disilenes are superimposed.34c,39 These side
products are unreactive toward the substrates of interest
studied here; therefore, their presence in the photolysis
mixtures does not compromise kinetic and/or thermodynamic
measurements, and they are sufficiently long-lived that their
contributions to the absorption spectra of the resulting silylene-
donor complexes (vide infra) can be subtracted out.
Absolute rate and(or) equilibrium constants for complex-
ation of the six tetrellylenes with the four donors (Et2O, THT,
Et3P and Cy3P) were determined by monitoring the effects of
varying concentrations of added substrate on the temporal
behavior and(or) intensities of the long-wavelength transient
absorptions due to the tetrellylenes; all experiments were
carried out in hexanes at 25 °C, except those with Cy3P, which
were carried out in cyclohexane owing to its low solubility in
hexanes. The kinetic process of interest is a pseudo-first-order
approach to equilibrium, which is described by eqs 1 and 2,
In the present paper, we report the results of a fast kinetics
study of the Lewis acid−base complexation reactions of the six
homologous transient silylenes and germylenes MMe2, MPh2,
and MMes2 (M = Si, Ge) with diethyl ether (Et2O),
tetrahydrothiophene (THT), and triethyl- and tricyclohexyl-
phosphine (Et3P and Cy3P, respectively) in hexanes or
cyclohexane solution at 25 °C. The data are combined with
data reported earlier, supplemented where necessary with new
measurements for some of the systems, for the reactions of
these species with MeOH, THF, Et2NH, and Et3N under
similar conditions. The experimental data, and their implica-
tions, are extended with theoretical (Gaussian-433) calculations
of thermochemical parameters for the complexation of SiMe2
and GeMe2 with a series of 17 chalcogen and pnictogen donors,
ΔAt = ΔAres + (ΔA0 − ΔAres) exp(−kdecayt)
(1)
kdecay = k−C + kC[Q]
(2)
where kdecay is the pseudo-first-order rate coefficient for decay
of the tetrellylene, kC and k−C are the forward and reverse rate
constants for complexation with the donor (Q), ΔA0 and ΔAt
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are the transient absorbance values immediately and at time t
after the laser pulse, respectively, and ΔAres is the residual
transient absorbance due to free tetrellylene remaining at
equilibrium at the end of the initial decay. The latter undergoes
a relatively slow subsequent decay owing to dimerization. The
variation in the ratio of the initial and residual absorbances
((ΔA0)0/(ΔAres)Q) with substrate concentration is propor-
tional to the equilibrium constant (KC = kC/k−C), as shown in
eq 3.
For systems characterized by KC values between ca. 1000
M−1 and ca. 25 000 M−1, the residual absorption is nonzero at
low (substrate) concentrations and undergoes a relatively slow
decay due to the dimerization reaction; the latter process is
slowed appreciably in the presence of the substrate. In such
situations the forward rate constant can be estimated from the
rate coefficients associated with the initial fast decay (kdecay; eq
2), while the equilibrium constant can be estimated from the
variation in the residual signal level ((ΔAres)Q) with substrate
concentration according to eq 3. In practice, the determination
of kdecay values for systems in this “intermediate” range usually
required two-exponential fitting of the transient absorbance
profiles, and the corresponding (ΔAres)Q values were estimated
visually as the break point in the two-exponential decays. These
analyses are approximate but nevertheless provide values of the
forward rate and equilibrium constants with typical uncertain-
ties in the range of 10−25%.31a Figure 1b shows representative
data obtained for the complexation of SiPh2 with Et2O, as an
example of a system characterized by a KC value in this range.
Reactions characterized by equilibrium constants smaller
than ca. 1000 M−1 require relatively high concentrations of
substrate in order to produce a residual signal that is
significantly different from ΔA0, such that the approach to
equilibrium (i.e., the initial fast decay) is rendered unresolvable
from the laser pulse. Thus, addition of the substrate results only
in a reduction in the apparent initial signal intensity due to the
free tetrellylene and a concomitant decrease in its decay rate,
the latter due to retardation of the dimerization reaction. In
such situations only KC can be determined, from analysis of the
apparent initial signal intensities as a function of [Q] according
to eq 4; it should be noted that the (ΔA0)Q term in eq 4 in fact
corresponds to (ΔAres)Q in eq 3. The (ΔA0)0 values used in
(ΔA0)0/(ΔAres)Q = 1 + KC[Q]
(3)
The observed response of the tetrellylene absorptions to
added substrate adopts one of three forms, depending on the
relative magnitudes of the forward rate (kC) and equilibrium
constants (KC) for complexation, and each situation requires a
somewhat different procedure for analysis of the data and
determination of kinetic and/or thermodynamic parameters.31c
The simplest (and least informative) situation arises when KC is
in excess of ca. 25 000 M−1 or when there is a fast secondary
reaction of the complex that renders the complexation process
irreversible. When either of these situations pertains, the signal
decays to a level indistinguishable from the prepulse signal level
(i.e., ΔAres ≈ 0) at all substrate concentrations monitored and
only the forward rate constant for complexation (kC) can be
determined, from the slope of a plot of kdecay vs [Q] according
to eq 2. Figure 1a shows representative transient decays and a
plot of kdecay vs [Q] for the complexation of SiPh2 with Cy3P, as
an example of a system characterized by a KC value of this
magnitude.
Figure 1. (top) Transient absorbance−time profiles and (bottom) associated data plots according to eqs 2−4, all at 25 °C: (a) SiPh2−Cy3P in
cyclohexane; (b) SiPh2−Et2O in hexanes; (c) GePh2−Et2O in hexanes. The corresponding plots of the data from experiments with different
concentrations of the respective substrates are shown below each set of absorbance−time profiles.
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slightly higher Lewis basicity of Me2O compared to Et2O.41
The lower limit of KC ≥ 25 000 M−1 for complexation of SiMe2
with THF in hexanes is also consistent with the estimated gas-
phase value of KC ≈ 30 000 M−1.29,40
Transient absorption spectra were recorded for each of the
tetrellylene-donor pairs studied, generally in dilute solutions
containing the substrate at a concentration just high enough to
reduce the equilibrium concentration of the free tetrellylene to
undetectable levels. The exceptions were the spectra of the
MMes2−Et2O and MMes2−THF complexes, which were
recorded either in hexanes containing 4−5 M of the substrate
or in the neat substrate as solvent.
The spectra of the complexes were obtained by subtraction
of the spectrum remaining at the end of the monitored time
window from that immediately after the laser pulse (see Figures
S1−S28, Supporting Information) and are summarized in Table
2. Spectra recorded of the GeMes2 precursor (6) in hexanes
(ΔA0)0/(ΔA0)Q = 1 + KC[Q]
(4)
these analyses were obtained from transient decay traces
recorded in the absence of added substrate, which is
appropriate provided the laser intensity is kept the same at
each substrate concentration studied and provided the substrate
does not absorb significantly at the excitation wavelength over
the concentration range employed for the determination. It was
ensured that these requirements were met in all of the
experiments that were carried out. Figure 1c shows some of the
data obtained for the complexation of GePh2 with Et2O, as
representative of a system in this “small KC” regime. The
equilibrium constants for the SiMes2−Et2O and GeMes2−Et2O
Lewis pairs were measured using transient absorbance data
acquired with solutions containing molar concentrations of
added Et2O, keeping the solution optical densities at the laser
wavelength constant by adding the substrate as optically
matched (at 248 nm) solutions of the precursors (3 and 6,
respectively) in neat Et2O.
Similar experiments were also carried out for the SiMes2−
THF, GeMe2−Et2NH, GeMes2 − Et2NH, and GeMes2−Et3N
systems, none of which have been characterized previously. The
experiments with the GeMes2−Et3N pair were limited to
maximum substrate concentrations of 1.5 mM because the
amine absorbs (weakly) at the laser wavelength; no effect on
the germylene signal intensity could be detected over the 0−1.5
mM concentration range in Et3N, which establishes an upper
limit of KC ≤ 100 M−1 for the equilibrium constant. With this
discrete evaluation of the screening effects due to Et3N in hand,
we also re-evaluated the complexation of SiMes2 with Et3N in a
more complete fashion than in earlier work30d and determined
a value of KC = 130 60 M−1 from data recorded over the 0−
1.5 mM concentration range in added amine.
Table 2. UV−vis Absorption Maxima of the Lewis Acid−
Base Complexes of Transient Silylenes and Germylenes with
a
Chalcogen and Pnictogen Donors in Hexanes at 25 °C
λmax (nm)
donor
none
SiMe2
465
SiPh2
290,
SiMes2
GeMes2
325, 560
GePh2 GeMe2
290, 580
300,
500
470
300
310
320
280
515
300,
375
295,
b
Et2O
305
310
c,d
360
360
350
320
e
f
f
f
THF
310
310, 380
285, 380
e
c
370
(sh)
g
THT
Et2NH
325
300,
300, 400
270, 400
g
370
h
h
h
i
i
280
300
320
280, 340,
430
h
h
h
d
j
Et3N
Et3P
270
310
315
320
290
340
330
340
290
290
280
310
320
280, 310
310
Table 1 gives the absolute rate and equilibrium constants for
the 28 tetrellylene-donor systems that were studied in this
work, along with those values that have been reported
previously for complexation with THF,30a,31c Et2NH,30d,31a
Et3N,30d,31a,34a,b and THT30c under similar conditions; the
corresponding data plots are shown in Figures 1 and S1−S28 of
the Supporting Information. The value measured for the
k
Cy3P
320
340
a
b
c
This work unless otherwise noted. Cyclohexane, 20 °C.27b In the
d
e
neat substrate as solvent. Complex not detected. Reference 30a.
f
g
h
i
Reference 31c. Reference 30c. Reference 30d. Reference 31a.
Reference 34b. Cyclohexane solution.
j
k
SiMe2−Et2O pair (KC = 1260
100 M−1) is in reasonable
agreement with the value estimated from the data of Baggott et
al. for the SiMe2−Me2O system in the gas phase (KC ≈ 3600
M−1),29,40 given the uncertainties and accounting for the
containing 1.3 mM Et3N showed the germylene and Ge2Mes4
and no distinct additional absorption bands that could be
assigned definitively to the corresponding complex. Similar
features were observed in spectra recorded in neat Et2O, in
Table 1. Forward Rate (kC) and Equilibrium Constants (KC) for Complexation of Transient Silylenes and Germylenes with
a
Chalcogen and Pnictogen Donors in Hexanes at 25 °C
kC/109 M−1s−1 [KC/M−1
]
donor
Et2O
SiMe2
SiPh2
SiMes2
GeMe2
GePh2
GeMes2
b
b
b
b
b
[1260 50]
15
4
[0.9 0.1]
[110 10]
[160 10]
[0.09 0.01]
[7100 600]
c
c
b
d
d
b,d
THF
17.3 1.5
15
16
1
[2.4 0.4]
2 [1500 100]
11
2
6.3 0.6 [23 000 5000]
[1.1 0.2]
[9800 3800]
e
b
THT
21
2
3
2
7
17
12
2
3
9
1
[1000 100]
f
f
f
f
g
b
Et2NH 16
8.3 0.7
3.9 0.4
3.5 0.5 [6300 600]
7.3 0.9
2.8 0.9
8.5 0.8
3.1 0.1
[510 20]
f
b
h
h
b
Et3N
Et3P
Cy3P
9.8 0.8
16
7.9 0.8
[130 60]
8.7 0.7
14
5.6 0.5
[≤100]
1
10
1
5.2 0.6
2
5.3 0.6
i
3.5 0.2
1.5 0.1 [25 000 6000]
1.7 0.4
[3000 1000]
a
b
c
Errors are quoted as twice the standard error from linear least-squares analysis of the data according to eqs 2−4. kC indeterminable. Reference
d
e
f
g
h
i
30a. Reference 31c. Reference 30c. Reference 30d. Reference 31a. Reference 34b. Cyclohexane solution.
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Table 3. Electronic Energies, Standard Enthalpies, and Free Energies (298.15 K, in kcal mol−1) of Complexation of SiMe2 and
a
b
GeMe2 with Chalcogen and Pnictogen Donors and M−X Distances (Å) in the Corresponding Complexes, Calculated at the
Gaussian-4 Level of Theory Relative to the Isolated Reactants
SiMe2
GeMe2
donor
H2O
rSi−X
ΔEel
ΔH298 K
ΔG298 K
rGe−X
ΔEel
ΔH298 K
ΔG298 K
d
d
2.156
2.081
2.121
2.092
2.055
2.529
2.472
2.477
2.465
2.073
2.058
2.067
2.090
2.142
2.400
2.350
2.351
−11.1
−15.1
−15.6
−15.6
−18.2
−9.3
−19.7
−19.6
−20.8
−22.1
−26.9
−29.7
−27.7
−28.6
−14.0
−25.2
−28.5
−9.0
−13.0
−13.5
−13.4
−16.0
−7.3
−17.7
−17.7
−18.9
−19.5
−24.2
−27.0
−24.8
−25.7
−11.6
−23.0
−26.2
+1.3
−1.8
−2.5
−1.4
−4.8
2.309
−10.0
−13.3
−14.8
−13.8
−17.1
−9.2
−18.5
−19.2
−19.6
−19.2
−23.5
−26.4
−24.8
−26.3
−12.6
−22.8
−26.2
−8.1
−11.5
−12.8
−12.0
−15.1
−7.5
−16.7
−17.3
−17.8
−16.9
−21.2
−24.0
−22.3
−23.9
−10.5
−20.8
−24.2
+1.5
−1.8
−2.1
−0.9
−3.6
+1.8
c
d
d
d
d
d
d
MeOH
Me2O
Et2O
THF
H2S
2.251
2.297
d
2.251
2.236
d
+2.9
2.728
Me2S
Et2S
−6.0
−5.9
−7.3
−8.0
−12.1
−14.1
−11.5
−13.2
−0.4
2.589
2.616
2.588
2.222
2.198
2.207
2.256
2.269
2.533
2.474
2.468
−6.1
−5.5
−7.0
−6.6
−10.2
−12.1
−9.2
−12.4
−0.3
−9.7
−13.0
THT
c
NH3
c
c
MeNH2
Me2NH
Et2NH
c
Me3N
PH3
Me2PH
Me3P
−11.2
−14.3
a
b
c
For the chalcogen complexes, unless otherwise noted the anti conformer is the lowest in energy. B3LYP/6-31G(2df,p) distances. Data for the
SiMe2-derived complexes are from ref 30d. Gauche conformer.
d
Figure 2. Calculated (B3LYP/6-31G(2df,p)) structures of the complexes of SiMe2 with (from left to right) Me2O, Me2S, Me3N, and Me3P, along
with the Si−X bond distances (in Å) and the C−X−C (top), X−Si−C (middle), and C−Si−C (bottom) bond angles (in degrees).
addition to a rising absorption at the blue edge of the spectral
window that may be assignable to the GeMes2−Et2O complex.
Computational Studies. The complexation of SiMe2 and
GeMe2 with a selection of representative second- and third-row
chalcogen and pnictogen donors was studied computationally
at the Gaussian-4 (G4)33 level of theory. The various donors
that were studied are summarized in Table 3, along with the
calculated (B3LYP/6-31G(2df,p)) M−X bond distances in the
complexes (rM−X) and the G4 electronic energies and standard
enthalpies and free energies (ΔEel, ΔH°, and ΔG°,
respectively) of the complexes relative to the free reactants;
the results for the complexes of SiMe2 with MeOH, NH3, and
the methylamines were available from an earlier study.30d Each
of the structures was confirmed to be an energy minimum by
inspection of the Hessian matrix, which contained no negative
eigenvalues in any case. Two minimum-energy conformers
were located for the Me2O and Me2S complexes, one
(designated gauche) in which one of the chalcogen substituents
approximately bisects the C−M−C bond angle in the
tetrellylene moiety, and one (designated anti) in which the
tetrellylene lone pair approximately bisects the C−X−C bond
in the donor moiety. These are shown in Figure 2 for the
complexes of SiMe2 with Me2O and Me2S; the optimized
structures of the complexes with Me3N and Me3P are also
shown along with selected geometric data. The structures
suggest that the consistently shorter M−X bond distances in
the gauche conformers is due to a larger amount of s character
in the bonding orbital at the chalcogen atom compared to that
in the anti conformers. This was verified by Natural Bond
Orbital (NBO) calculations (vide infra), which showed the O
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Table 4. Zero-Point Energy Corrected Electronic Energies, Calculated Dative vs Covalent M−X Bond Length Differences
a
b
(ΔrM−X), Complexation-Induced Changes in Natural Charge at the Heteroatoms (ΔZM, ΔZX), and Mayer−Wiberg M−X
b
Bond Indices (PM−X) for the Complexes of SiMe2 and GeMe2 with Representative Chalcogen and Pnictogen Donors
SiMe2
ΔZSi
GeMe2
ΔZGe
c
c
donor
ΔE0
ΔrSi−X/Å (%)
ΔZX
PSi−X
ΔE0
ΔrGe−X/Å (%)
ΔZX
PGe−X
MeOH(g)
Me2O(g)
Me2O(a)
Me2S(g)
Me2S(a)
Me2NH
Me3N
−12.6
−13.4
−13.5
−17.0
−17.5
−26.3
−25.1
−22.9
−26.1
0.43 (26)
0.46 (28)
0.51 (31)
0.25 (12)
0.30 (14)
0.33 (19)
0.40 (23)
0.07 (3)
−0.08
−0.08
−0.09
−0.27
−0.27
−0.14
−0.14
−0.41
−0.42
−0.03
−0.05
−0.04
+0.14
+0.14
−0.06
−0.07
+0.20
+0.21
0.198
0.220
0.172
0.478
0.431
0.284
0.252
0.709
0.714
−11.4
−12.5
−12.9
−16.1
−16.9
−23.6
−23.6
−20.8
−24.3
0.47 (26)
0.49 (27)
0.51 (29)
0.33 (15)
0.34 (15)
0.37 (20)
0.42 (23)
0.15 (6)
−0.06
−0.05
−0.06
−0.22
−0.23
−0.12
−0.11
−0.37
−0.38
−0.03
−0.04
−0.03
+0.11
+0.11
−0.05
−0.05
+0.17
+0.19
0.156
0.147
0.142
0.382
0.369
0.250
0.221
0.614
0.631
Me2PH
Me3P
0.07 (3)
0.13 (6)
a
b
ΔrM−X = rM−X(complex) − rM−X(Me2RM−XMen), B3LYP/6-31G(2df,p) distances, given in Å; percentages are given in parentheses. From NBO
analysis of the HF/6-31G(d) densities; the calculated charges at M in free SiMe2 and GeMe2 are ZSi = 1.087 and ZGe = 1.038, respectively, while
those in the donors are ZO = −0.789 (MeOH) and −0.634 (Me2O), ZS = 0.214 (Me2S), ZN = −0.727 (Me2NH) and −0.558 (Me3N), and ZP =
c
0.656 (Me2PH) and 0.920 (Me3P). Relative to the isolated donor and acceptor, in kcal mol−1.
atoms in the MMe2−Me2O complexes to be approximately sp2
hybridized in the gauche conformers, in which the bonding
geometry at O is planar, and approximately sp3 hybridized in
the anti conformers. Several conformers were found for the
diethyl-substituted systems, which were differentiated by the
anti or gauche arrangement with respect to the MMe2
substituents and also by the conformations of the ethyl groups.
In any event, the differences in energy between the various
conformers were found to be small, amounting to less than 1.2
kcal mol−1 in every case. Table 3 contains the calculated data
for only the lowest energy conformers of the various structures
that were located. The calculated binding energies of the
complexes of both SiMe2 and GeMe2 with the parent (hydride)
donors increase in the order H2S < H2O < PH3 < NH3, in good
general agreement with the results of earlier calculations for
made of the differences in Lewis acidity of silylenes and
germylenes of homologous structures and of its response to
changes in substitution at the tetrel center, as they apply to
complexation with a common Lewis base. As can be seen from
Table 1, the KC values for complexation with Et2O span a range
of roughly 5 orders of magnitude between that of the weakest
Lewis acid in the series (GeMes2) to the strongest (SiPh2),
corresponding to a range in binding free energies of −5.2 to
+1.7 kcal mol−1, where the standard state is a hexanes solution
containing 1 M concentrations of each of the reactants at 25
°C. For both sets of compounds the trend in Lewis acidities
follows the order MPh2 > MMe2 ≫ MMes2, reflecting the σ-
and π-electron-withdrawing effects of the phenyl substituent
relative to methyl, which leads to enhanced acidity, and the
(much larger) destabilizing effect of steric hindrance on the
complexation equilibria in the cases of the diaryltetrellylenes.
The electronic effect associated with phenyl for methyl
42
SiH2,14,22 GeH2,14 and GeMe2 at lower levels of theory. The
trends in the calculated M−X bond distances as a function of M
and X for the Me2X (X = O, S) and Me2XH (X = N, P)
complexes are also similar to those found in the earlier
calculations for the parent hydrides,14,22 with the bond lengths
increasing in the order N < O < P < S for both the silylene and
germylene complexes and being systematically longer in the
GeMe2 complexes by roughly 10% for the O and N donors and
ca. 5% for the S and P donors.
substitution appears to be somewhat larger for the silylenes
GePh2
(KCSiPh2/KCSiMe2 = 5.6 0.6) than for the germylenes (KC
/
GeMe2
KC
= 1.5
0.3), which we attribute tentatively to
preferential enhancement of the Lewis acidity of SiPh2
compared to GePh2, owing to the somewhat more effective π
overlap that is possible between the phenyl substituents and the
3p orbital on Si, compared to that involving the 4p orbital on
Table 4 gives, for selected MMe2−donor Lewis pairs in the
GeMe2
Ge. We note in passing that the KCGePh2/KC
ratio for
series, the differences between the M−X bond distances (rM−X
)
complexation with Et2O is somewhat smaller than those for
complexation with MeOH, t-BuOH, and THF,31c which
together indicate that GePh2 is the stronger Lewis acid by an
average factor of 3 1. In any event, the difference disappears
in the complexes and those in the corresponding Me2RM−
XMen (R = H, Me) compounds, which were optimized at the
same (B3LYP/6-31G(2df,p)) level of theory as employed in
the G4 compound method. The table also gives the differences
between the natural (NPA) charges at M and X in the
complexes and those in the free reactants and the Mayer−
Wiberg M−X bond indices (PM−X), obtained from natural bond
orbital (NBO) analysis of the HF/6-31G(d) densities that were
generated as part of the G4 calculations.
SiMe2
GeMes2
in the MMes2 derivatives (i.e., KCSiMes2/KC
≈ KC
/
KCGeMe2), in which π overlap is reduced by steric-induced
twisting of the aryl rings out of the (near) coplanar
arrangement that is possible in the phenylated derivatives.
Mesityl for phenyl substitution gives rise to reductions in KC of
103−104, indicating differences in binding free energies of 5−6
More complete details of the computational results for
representative complexes in the series are given in the
Supporting Information.
kcal mol−1 between the corresponding MMes2 and MPh2
GeMes2
complexes with Et2O. The KCGePh2/KC
ratio varies with
steric factors in the Lewis base, from values of ca. 220−250 for
MeOH and t-BuOH,31c through ca. 2000 for Et2O, to ca. 20
000 for THF.31c Similar trends can be expected to hold for the
silylenes, though the actual numbers for MeOH and THF
cannot be measured for one reason or another (vide infra).
DISCUSSION
■
The successful determination of equilibrium constants for
complexation of the six transient silylenes and germylenes with
Et2O allows the first quantitative experimental assessment to be
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The results for each of the three silylene−germylene pairs
show the silylene to be the consistently stronger Lewis acid by
roughly an order of magnitude in KC, corresponding to
differences in binding free energies of 1.4−2.2 kcal mol−1. This
is in good agreement with theoretical predictions for other
trialkylphosphines (even the relatively hindered derivative,
Cy3P) are significantly stronger donors than the secondary and
tertiary amines toward both SiMes2 and GeMes2 in solution at
ambient temperatures. The same is true of the sulfide (THT)
compared to its second-row homologue, THF.
The data for all nine of the experimentally characterized
MR2−donor Lewis pairs are shown in Figure 3, in the form of a
silylene− and germylene−O-donor systems.14,25a,43 The differ-
Ge
ence is somewhat larger for the MPh2−Et2O pairs (KCSi/KC
=
44 7) than for the MMe2−Et2O (KCSi/KC = 12 2) and
Ge
Ge
MMes2−Et2O (KCSi/KC = 10
2)) systems, presumably
reflecting the more effective π overlap that is possible in SiPh2
in comparison to the germanium homologue, as discussed
above.
Equilibrium constant measurements are not possible for
complexation of the three silylenes with MeOH, owing to the
rapidity with which the complexes react to yield the net O−H
insertion products via (catalytic) H transfer.30b This is not the
case for the corresponding germylenes, whose complexes with
MeOH are sufficiently stable that they can be detected even in
the neat alcohol as solvent.44 On the basis of the differences
observed for the Et2O complexations and the experimental KC
values for complexation of MeOH with the germylenes,44 it is
possible to derive estimates of KC ≈ 104, 1.5 × 105, and 150
M−1 for complexation of MeOH with SiMe2, SiPh2, and SiMes2,
respectively, in hexanes at 25 °C. Estimates of KC ≈ 105 and 106
M−1 can be similarly derived for the equilibrium constants for
complexation of THF with SiMe2 and SiPh2, respectively.
These are both larger than the upper measurable limit that is
accessible with our experimental method, as we concluded in
our earlier reported studies of these species in solution.30a
Given the uncertainties, the predicted value of KC for SiMe2−
THF in hexanes is in reasonable agreement with the value (of
KC ≈ 30 000 M−1) estimated from the gas-phase data of
Baggott et al. (vide supra).29,40 This, along with the reasonably
close agreement between the estimated KC value for complex-
ation of SiMe2 with Me2O in the gas phase and that measured
in the present work for complexation with Et2O in hexanes,
shows that solvation effects on the complexation equilibria are
minimal, at least for the more weakly bound Lewis pairs in the
series.
We knew from previous work that the equilibrium constants
for complexation of MMe2 and MPh2 with the ami-
nes30d,31a,d,34b are also larger than can be measured by our
experimental method; thus, it was not particularly surprising to
find that the same is also true for complexation with THT30c
and the trialkylphosphines. On the other hand, the increased
steric interactions afforded by mesityl for phenyl substitution
results in measurable KC values for all the SiMes2− and
GeMes2−donor pairs except those with Et3P, which are again
larger than the measurable limit. Remarkably, the variation in
the equilibrium constants with Lewis base for SiMes2 coincides
nearly perfectly with that observed for GeMes2, the KC values
increasing in the order Et2O < THF < Et3N < THT < Et2NH <
Cy3P < Et3P for SiMes2 and in the same order but with THT
and Et2NH reversed for GeMes2. As we found for the
complexations with Et2O, the silylene displays a significantly
higher Lewis acidity than the germylene toward all the donors
in the series; the difference in KC is roughly a factor of 10 for
Et2O, Et2NH, and Cy3P but reduces to a factor of 1.5−2 for
THF and THT. The reasons for the different behavior of the
last two Lewis bases are not clear but may be due to entropic
effects, owing to the rigidity of their cyclic structures. In
contradiction with the general classification of silylenes and
germylenes as hard electrophiles,25a the results indicate that
Figure 3. Plot of experimental free energies of complexation of
GeMes2 vs those of SiMes2 with the same donor in hexanes at 25 °C
●
( ); the dashed line is the least-squares fit of the six data points. The
points for the MPh2−Et2O, MMe2−Et2O, and MMe2−THF systems
○
are also included in the plot ( ) (reference state: 1 M solution, 25
°C).
plot of the binding free energies of the germylene complexes
versus the corresponding values for the homologous silylene
complexes, where the reference state is a 1 M solution in
hexanes at 25 °C. The plot accentuates the parallels between
the Lewis acid−base complexation behavior of the silylene and
germylene systems; least-squares analysis of the six data points
for the MMes2−donor Lewis pairs affords a slope of 1.0 0.1,
indicating that the difference in Lewis acidity between SiMes2
and GeMes2 is more or less independent of the donor and the
strength of the Lewis acid−base interaction, over a 6−7 kcal
mol−1 range in binding energy. The intercept corresponds to
the average difference in the binding energies of the complexes
of SiMes2 and GeMes2 with a given donor, which is 0.8 0.5
kcal mol−1 in favor of (i.e., ΔG is more negative for) the
silylene.
Comparisons with Theory. Since experimental limitations
prevent the measurement of equilibrium constants for
complexation of the less hindered tetrellylenes with the S, N,
and P donors, we turned to theoretical calculations in order to
expand the comparison of the binding thermodynamics of
SiMe2−donor and GeMe2−donor complexes to cover a broader
range in donor strength, using the Gaussian-4 (G4) compound
method of Curtiss and co-workers.33 The G4 method has been
shown to reproduce experimental thermochemical data for a
wide variety of organic and main-group systems with an average
chemical accuracy of ca. 1 kcal mol−1,33 and indeed the
calculated binding free energies of the SiMe2−Et2O, SiMe2−
Me2O,29,40 SiMe2−THF,29,40 GeMe2−Et2O, GeMe2−
MeOH,31c and GeMe2−THF complexes (see Table 3) match
the experimentally determined values (after conversion to the
gas-phase reference state; see Table 5) to within this limit of
uncertainty in all cases. It should be noted that the calculations
predict the same ordering of stability of the SiMe2− and
GeMe2−O-donor complexes as a function of donor as was
found experimentally, and they successfully reproduce the small
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Table 5. Experimental Gibbs Free Energies (Reference State, Gas Phase, 1 atm, 25 °C) for Lewis Acid−Base Complexation of
Silylenes and Germylenes with Chalcogen and Pnictogen Donors in Hexanes at 25 °C (in kcal mol−1)
ΔG
SiMe2
SiPh2
SiMes2
GeMe2
GePh2
GeMes2
MeOH
Et2O
a
a
a
−2.1 0.1
−0.9 0.1
−3.5 0.3
a, c
−2.9 0.2
−1.1 0.1
−4.0 0.1
a, c
+0.3 0.3
+3.3 0.4
+1.8 0.1
−2.2 0.1
−1.8 0.1
−2.3 0.1
−4.2 0.1
a, c
−3.4 0.1
a, c
+2.0 0.1
+1.4 0.1
−2.4 0.1
−3.3 0.1
−1.0 0.4
a, b
b
THF
THT
Et2NH
Et3N
a, c
a, c
a, c
a, c
a, c
ad
,
a, c
a, c
a, c
a, c
≥−0.8
a, c
Et3P
a, c
a, c
a, c
a, c
i
Cy3P
a, c
b
a, c
−4.1 0.1
a, c
a, c
−2.8 0.2
a
c
d
KC indeterminable. Gas phase; see ref 39. Upper limit is −6.0 kcal mol−1. Lower limit.
differences in the experimental binding energies of the SiMe2
and GeMe2 complexes with a common O donor. The
calculations also predict nearly equal binding free energies for
the SiMe2 and GeMe2 complexes with each of the three sulfides
(Me2S, Et2S, and THT; see Table 3), as was found
experimentally for the MMes2−THT complexes. The calcu-
lated values of ΔG° for the MMe2−THT complexes (ΔG ≈ −7
kcal mol−1; 1 atm gas phase at 25 °C as reference state)
correspond to KC values on the order of 5 × 106 M−1, which is
consistent with the indication from experiment that they are
greater than the measurable upper limit of our experimental
method. Again, solvation effects appear to have a minimal effect
on the complexation equilibria in comparison to the gas phase,
for the weaker Lewis bases in the series at least.
Figure 4. Plot of calculated (G4) binding free energies of GeMe2−
donor complexes vs those of the corresponding SiMe2−donor
The calculated binding energies of the complexes of both
SiMe2 and GeMe2 with the parent chalcogen and pnictogen
●
complexes ( ). The experimental data points for the MMe2−O-
△
○
donor ( ) and MMes2−donor ( ) Lewis pairs (see Figure 3) are also
included in the figure (standard state gas phase, 1 atm, and 25 °C).
hydrides increase in the order H2S < H2O < PH3 < NH3, in
14,22
agreement with previous theoretical results for SiH2
and
GeH2.14 The ordering differs from that found by Su and Chu
for the complexes with GeMe2 (H2S < PH3 < H2O < NH3) at
the B3LYP/6-311G(d) and MP2/6-311G(d) levels of theory,42
which appear to predict anomalously high values of the binding
energy for the GeMe2−H2O complex. Alkyl substitution in the
donor leads to marked increases in basicity, more so for the
third-row donors than the second,45 and together with steric
effects (which are greater for the second-row donors than the
third) produces a complete change in the order of stabilities of
the corresponding complexes in comparison to those found for
the parent chalcogen and pnictogen hydrides.
The overall trends are illustrated in Figure 4, a plot of the
calculated binding free energies of the 17 GeMe2−donor
complexes versus those of the corresponding SiMe2 complexes.
The experimental data from Figure 3 are also included in the
plot (after conversion to the gas-phase reference state; see
Table 5), in order to illustrate the level of agreement between
theory and experiment for the MMe2−Et2O and MMe2−THF
Lewis pairs and the relationship between the theoretical
estimates for the MMe2−donor systems and the experimentally
determined ΔG values for the sterically encumbered MMes2−
donor systems. As can be seen in the figure, theory predicts a
similar overall trend in the binding energies of the MMe2−
donor systems as a function of the donor as is observed
experimentally for the MMes2 systems, with the trialkylphos-
phine (Me3P) forming the strongest complexes and the acyclic
ethers and MeOH the weakest, stronger only than those with
H2S, H2O, and PH3. The calculations suggest that the
difference in Lewis acidities of SiMe2 and GeMe2 increases
very modestly as the Lewis base strength of the donor increases
over a 20 kcal mol−1 range in binding free energy, as revealed
by the nonunit slope of the plot (0.87 0.03).
As individual data sets, the calculated binding energies of the
SiMe2 and GeMe2 complexes correlate with the (experimental)
proton affinity (PA) values of the donors,45,46 albeit fairly
roughly. This is illustrated in Figure 5, again with the available
experimental data for the corresponding MMe2−donor and
MMes2−donor systems included for comparison; the data
points for the MMes2−Et3P complexes are shown in the plots
as upward-pointing arrows, to signify the fact that only lower
limits of the binding energies could be established exper-
imentally for these two systems. Both correlations improve
quite significantly (R2 ≥ 0.947) if the data points for Et2O and
Et2NH are excluded from the analyses. This is justifiable,
considering that these are the only systems for which the
favored conformation of the donor moiety in the complexes is
different from the all-trans conformation favored by the free
donor; torsional strain effects thus decrease the stabilities of
these MMe2−donor complexes by a small (2−3 kcal mol−1)
increment which is not accounted for by the PA values of the
two donors. Similar-quality correlations are also observed with
the experimental donor gas basicities,45 not surprisingly, given
that the proton affinities and gas basicities of the series of Lewis
bases studied in this work are strongly correlated45 (so too are
the calculated ΔH and ΔG values for the MMe2−donor
complexes; see Figure S29, Supporting Information); we have
employed proton affinities to illustrate the correlation with
Brønsted basicity for the simple reason that it is the only
experimental measure of gas-phase basicity that is available for
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●
△
Figure 5. Plots of calculated (G4; ) and experimental (hexanes; ) standard free energies of complexation of chalcogen and pnictogen donors
with (a) SiMe2 and (b) GeMe2 vs the (experimental) proton affinities of the donors. The experimental values for the SiMes2− and GeMes2−donor
□
complexes ( ; hexanes) are also shown in the plots. The reference state is the gas phase at 1 atm and 25 °C in all cases.
Figure 6. Plots of calculated (G4) gas-phase binding enthalpies of (a) SiMe2 and (b) GeMe2 with chalcogen and pnictogen donors, vs the two-
parameter functions defined by least-squares fitting of the data to eq 5.
Cy3P.46 Not unexpectedly given the increased steric inter-
actions, the MMes2−donor systems show only a trend of
increasing binding free energy with increasing donor PA, but no
significant correlation. The calculations predict nearly equal
binding energies for the complexes of SiMe2 with Me2NH,
Me3N, and Me3P and the same as well for the corresponding
GeMe2 complexes. This is due at least in part to a much greater
sensitivity of the binding energies of the tertiary amine
complexes to steric destabilization; the fact that the MMes2−
Et3N complexes are at least 5 kcal mol−1 less stable than the
MMes2−Et3P complexes shows that the preferential destabili-
zation of the tertiary amine complexes increases even further as
steric bulk is built into the tetrellylene.
Lewis acid−base (AB) pair into individual contributions from
electrostatic (E) and covalent (C) bonding components.48
Figure 6 shows the least-squares fits of the calculated binding
enthalpies to eq 5, incorporating all the systems for which
donor EB and CB constants have been assigned except the
MMe2−Me3N pairs (vide infra). The analyses afford values of
CA = 4.4 0.2 and EA = 4.5 0.4 (kcal mol−1)1/2 for SiMe2 (R2
= 0.974) and CA = 4.1 0.3 and EA = 3.6 0.6 (kcal mol−1)1/2
for GeMe2 (R2 = 0.926), where the errors are quoted as the
standard errors from the two-parameter least-squares fits. We
note that the bases in the series span a range of ca. 750 in EB/
CB ratio, which helps to maximize the accuracy of the EA and CA
values determined from such analyses.48
Plots of the calculated binding enthalpies of the MMe2−
donor complexes against BF3 affinity values (CH2Cl2 solution,
298 K)41 for the six donors for which parameters are available
are suggestive of two-family correlations. Those for the
complexes with Et2O, Me2O, THF, and Me3N correlate
strongly with BF3 affinities (R2 ≥ 0.990), while the −ΔH
values for the THT and Me3P complexes are 8−9 kcal mol−1
larger than the correlations for the second-row donors would
predict. The Lewis acid properties of SiMe2 and GeMe2 are
clearly remarkably similar to one another, but they do not
exhibit behavior typical of hard Lewis acids such as BF3.
Significantly better correlations are observed between the
calculated binding enthalpies of the SiMe2−donor and GeMe2−
donor complexes and the donor EB and CB constants of Drago
and co-workers.47 The analysis employs eq 5, an empirical
relationship that separates the binding enthalpy of a given
−ΔH = EAEB + CACB
(5)
The results of the EC analysis suggest that SiMe2 and GeMe2
are of borderline hardness, significantly softer and more acidic
than (for example) BMe3 (EA = 2.90; CA = 3.60)47 but
qualitatively similar to it in terms of the tendency to engage in
covalent vs electrostatic binding with Lewis bases. The roughly
equal CA and EA values implies, in essence, that the nature of
the Lewis base alone (as defined by its EB/CB ratio) defines the
nature of the bonding interaction in a given complex, whether
predominantly electrostatic (e.g., water and alcohols; EB/CB >
2.5), predominantly covalent (e.g., sulfides and phosphines; EB/
CB < 0.1), or a mixture of both (e.g., ethers and amines; EB/CB
≈ 1 and 0.2−1, respectively). The only outliers in the
correlations are the MMe2−Me3N Lewis pairs, for which the
calculated binding enthalpies are in both cases ca. 3 kcal mol−1
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smaller than the correlations predict they should be. The
discrepancy can be attributed to a steric effect on the binding
enthalpy, which is not accounted for in the E and C
parameters.48 The effect is absent in the complexes with the
primary and secondary amines, whose favored conformations
have an N−H bond bisecting the two methyl groups in the
tetrellylene moiety. On the other hand, the fact that the binding
enthalpies of the MMe2−Me3P complexes obey the correlations
defined by the other MMe2-donor pairs indicates that steric
factors are much less important than in the amine complexes,
and that there is no detectable stabilization from π back-
bonding (which the E and C parameters also do not account
for). The latter conclusion is consistent with a recent
theoretical evaluation of the bonding in the parent SiH2−PH3
complex.23
The variations in the calculated structures, charge distribu-
tions, and M−X bond orders of the complexes tell a similar
story, more or less. The data for selected complexes in the
series are shown in Table 4, the structural information in the
form of the percent differences between the M−X bond lengths
in the complexes and in the corresponding covalent
compounds Me2RM−XMen, which decrease in the order O >
N > S > P, and the charge distributions in the form of the
differences between the NPA charges at M and X in the
complexes and those in the free donors and acceptors, which
increase along with the M−-X bond orders in the same order as
the bond length differences decrease. In partial contrast to the
conclusions derived from the EC analysis, the NPA charge
analysis indicates a clear difference between the bonding
characteristics of the complexes with the second- and third-row
donors, the data for the former being consistent with
predominantly electrostatic binding and the latter with
predominantly covalent binding, especially for the phosphine
complexes. The role of steric effects in the binding to amines is
clearly evident in the data for the complexes with Me2NH and
Me3N, where the latter complexes possess longer bonds and
smaller bond orders compared to the complexes with the
former. In contrast, the bond distances, charge polarizations,
and bond orders of the Me2PH and Me3P complexes are
identical; steric effects are absent (more or less) because the
bonds to phosphorus are longer than those to nitrogen.
Differences in steric factors may also be partly responsible for
the inversion in the relative stabilities of the ether and sulfide
complexes relative to those of the complexes with the parent
hydrides, H2O and H2S.
Electronic Spectra of Silylene- and Germylene-Donor
Complexes in Solution. For each of the three silylene−
germylene pairs, the UV−vis spectra of the corresponding Et2O
complexes are quite similar to one another, with the absorption
maximum of the germylene−Et2O complex appearing at
slightly shorter wavelength than that of the corresponding
silylene−Et2O complex. For the diaryltetrellylenes the relative
positions of λmax for the complexes are the same as those for the
uncomplexed (free) species (i.e., with the GeAr2−Et2O
complexes absorbing at shorter wavelengths than the
corresponding SiAr2−Et2O complexes), while the opposite is
true of the dimethyl systems. In any event, it is clear that the
magnitude of the spectral shift that occurs upon complexation
does not correlate with the relative magnitudes of the binding
energies; the spectral shift is generally somewhat greater for the
germylene than the silylene, regardless of the fact that the
germylene−ether complex is the more weakly bound of the
two. The same trend holds for the tetrellylene−THF
complexes, for which the equilibrium constants and underlying
binding energies are larger owing to the higher Lewis basicity of
THF compared to Et2O.41 For most of the donors that have
been studied, the trends in the UV−vis absorption character-
istics of the various complexes are somewhat different in
solution than in low-temperature matrixes.20b,21b Nevertheless,
as they do under low-temperature matrix conditions, in fluid
solution silylene− and germylene−chalcogen complexes
consistently absorb at significantly longer wavelengths than
the corresponding pnictogen complexes, perhaps reflecting the
generally weaker binding that characterizes the former
complexes compared to the latter ones. However, there is no
consistent variation in λmax with the donor atoms within the
same group, the direction and magnitude of the spectral shift
appearing to vary from (tetrellylene) system to system.
Kinetics of Lewis Acid−Base Complexation. The kinetic
data show complexation to be invariably fast, proceeding in all
cases with rate constants within a factor of 20 of the diffusion
limit in hexanes solution (kdiff ≈ 2.3 × 1010 M−1 s−1 at 25 °C),
with only modest variations with substituents in either the
tetrellylene or the donor, or with the overall reaction
thermochemistry. This is consistent with complexation being
an essentially barrierless process enthalpically. Both SiPh2 and
GePh2 react roughly half as fast as their methyl homologues
with the pnictogen donors, despite the fact that complexation
with the phenyl-substituted systems is thermodynamically more
favorable; similar differences in rate are observed for the
reactions of GePh2 with the chalcogen donors compared to
GeMe2, but not for SiPh2. The small rate retardation may be a
reflection of the greater steric demand of the phenyl substituent
compared to methyl, since SiMes2 reacts 2−4 times slower than
SiPh2 with each of the S, N, and P donors for which rate
constant measurements are possible. There is a general trend of
marginally faster complexation rates with the heavier donor
element within a group, all else being equal; for example,
GeMe2 and GePh2 react slightly faster with THT than with
THF, and both the germylenes and silylenes react roughly twice
as fast with Et3P than with Et3N. This is undoubtedly a
reflection of the longer equilibrium bond distances in the
complexes with third-row donors compared to those with
second-row donors (vide supra and refs 14, 22, 25a, and 42)
and thus a lower sensitivity in the transition state to steric
factors associated with the donor substituents. Interestingly,
there is essentially no dependence of the rate constant on the
tetrel element except for the complexations with THF, for
which the silylenes (SiMe2 and SiPh2) are faster by a factor of
about 2. This is also the case for complexation of the MMe2 and
MPh2 systems with alcohols;30b,31c thus, the trend appears to be
general for O-donor systems.
SUMMARY AND CONCLUSIONS
■
The scattered handful of thermodynamic data for Lewis acid−
base complexation of simple dialkyl- and diarylsilylenes and
dialkyl- and diarylgermylenes in solution that existed at the
onset of this work has been expanded quite substantially,
through the determination of equilibrium constants for
complexation of SiMes2 and GeMes2 with two ethers, a sulfide,
a secondary and tertiary amine, and two trialkylphosphines and
of those for complexation of the homologous MMe2 and MPh2
pairs with Et2O, all in hydrocarbon solvents at 25 °C. The
results reveal a systematically higher Lewis acidity for the
silylene compared to the germylene homologue by roughly 1
pK unit, which is more or less independent of substitution in
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THF. 1,1,3,3-Tetramethyl-2,2-diphenyl-1,2,3-trisilacyclohexane (2)34c
and 2,2-dimesityl-1,1,1,3,3,3-hexamethyltrisilane (3)36 were synthe-
sized as reported previously and were purified by column
chromatography on silica gel with hexanes as eluent, followed by
recrystallization from methanol (2) or hexanes (3). The germacyclo-
pent-3-enes 4−6 were synthesized and purified according to previously
reported procedures.31d,34a,b All six compounds were judged to be
>97% pure by GC/MS analysis and 1H NMR spectroscopy.
Tetrahydrofuran (THF, Caledon Reagent) was refluxed over CaH2
for 2 h, distilled into a flask containing sodium, refluxed over sodium
for 5 days, and finally distilled immediately prior to use. Cyclohexane
(Caledon Reagent) was refluxed over sodium for several days and
distilled immediately prior to use. Tetrahydrothiophene (THT, 99%
Sigma-Aldrich) was purified by distillation from anhydrous sodium
sulfate.50 Diethylamine (Et2NH, 99.5% Sigma-Aldrich) and triethyl-
amine (Et3N, ≥99% Sigma-Aldrich) were refluxed over solid NaOH
for several hours and then distilled under nitrogen. Triethylphosphine
(PEt3, 99% Sigma-Aldrich) was distilled under argon prior to use,
while tricyclohexylphosphine (PCy3, ≥ 94% Sigma-Aldrich) was
recrystallized from hexane; all manipulations of the phosphines were
carried out in a glovebox under a dry argon atmosphere, and purity
was checked by 31P NMR spectroscopy. Hexanes (EMD OmniSolv)
and Et2O (Caledon Reagent) were dried by passage through activated
alumina under nitrogen using a Solv-Tek solvent purification system
(Solv-Tek, Inc.).
the tetrellylene or the identity of the donor over a range of ca.
105 M−1 in the binding constant, or ca. 7 kcal mol−1 in binding
free energy. For both series of tetrellylenes, the Lewis acidity
decreases in the order MPh2 > MMe2 ≫ MMes2, reflecting the
σ- and π-electron-withdrawing effects of the phenyl substituent
relative to methyl, which leads to enhanced Lewis acidity, and
the (much larger) destabilizing effect of steric hindrance on the
complexation equilibria. The data for the MMes2 systems show
that third-row chalcogen and pnictogen donors complex
significantly more strongly than their second-row homologues
with both silylenes and germylenes, O donors binding the
weakest, trialkylphosphines the strongest, and sulfides and
amines jostling for the middle positions according to steric
factors, to which complexation with amines is particularly
sensitive. They thus exhibit behavior that is characteristic of
“borderline soft” Lewis acids. Complexation is accompanied by
characteristic blue shifts of the UV−vis absorption spectra of
the tetrellylenes, but while they are generally larger for
pnictogen than chalcogen donors, there is no systematic
variation with the binding energy of the complex. The forward
rate constants for complexation are typically close to diffusion
controlled and exhibit only slight systematic variations with
structure in either the tetrellylene or the donor or with the
overall thermodynamics of the process.
Laser flash photolysis experiments were carried out using a Lambda
Physik Compex 120 excimer laser filled with F2/Kr/Ne (248 nm, 20
ns, 98−110 mJ/pulse) and a Luzchem Research mLFP-111 laser flash
photolysis system, modified as described previously.34a The solutions
were prepared in deoxygenated anhydrous hexanes such that the
absorbance at 248 nm was between 0.4 and 0.7. The solutions were
flowed rapidly through a 7 × 7 mm Suprasil flow cell connected to
calibrated 100 or 250 mL reservoirs, which contain a glass frit to allow
bubbling of argon gas through the solution for 40 min prior to and
throughout the experiment. 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. The glassware, sample cell, and transfer
lines were dried in a vacuum oven (65−85 °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.
The Et3P and Cy3P stock solutions were prepared in a glovebox in
volumetric flasks, transferred to a 25 mL Schlenk bomb, and attached
to a Schlenk line under a dry Ar atmosphere. The stopcock on the
bomb was replaced with a rubber septum, and the solution was then
withdrawn using a 1 or 2.5 mL gastight syringe, which was inserted
through the septum on the reservoir containing the precursor solution
to allow the addition of the desired amounts of the substrate to the
reservoir solution. The syringe was left attached to the reservoir until
the end of the experiment to avoid contact with air and moisture.
Transient absorbance−time profiles were recorded by signal
averaging of data obtained from 10−40 individual laser shots. Decay
rate constants were calculated by nonlinear least-squares analysis of the
transient absorbance−time profiles using the Prism 5.0 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 intensity as
possible. Errors are quoted in all cases as twice the standard error
obtained from the least-squares analyses.
These conclusions are corroborated and extended by
computational chemistry, using the chemically accurate
Gaussian-4 compound method to calculate binding enthalpies
and free energies of the complexes of SiMe2 and GeMe2 with
17 representative chalcogen and pnictogen donors in the gas
phase, several from the same group of donors that has been
studied experimentally in solution. For six of the MMe2−O-
donor systems, the theoretical binding free energies match the
experimental gas- or solution-phase values to within 1 kcal
mol−1, giving some confidence in the accuracy of the
computational method as applied to the other donors in the
series. The calculated data reveal a difference in Lewis acidity
between SiMe2 and GeMe2 similar to that obtained
experimentally, though they suggest that the difference
increases (modestly) with increasing donor basicity over a ca.
20 kcal mol−1 range in binding energy. The calculations indicate
that both SiMe2 and GeMe2 coordinate with Me3P and Me3N
to afford complexes of nearly equal binding energies, in contrast
to the experimental data for the dimesityl systems, which
indicate stronger binding to Et3P by at least 5 kcal mol−1
relative to Et3N. The differences reflect the much greater
sensitivity of the binding energy to steric effects in the case of
the amines relative to trialkylphosphines. The calculated
binding enthalpies correlate remarkably well with the empirical
two-parameter donor basicity scale of Drago and co-workers.
The correlations verify the conclusion that SiMe2 and GeMe2
are borderline soft Lewis acids, stronger and softer than
unhindered trialkylboranes such as BMe3.
Further studies of the Lewis acid−base chemistry of heavy
group 14 carbene analogues, and its effects on their reactivity,
are in progress.
EXPERIMENTAL SECTION
■
1H NMR and 31P NMR spectra were recorded on a Bruker AV600
spectrometer in CDCl3 solution. GC/MS analyses were carried out on
a Varian Saturn 2200 GC/MS/MS system equipped with a VF-5 ms
capillary column (30 m × 0.25 mm; 0.25 mm; Varian, Inc.).
ASSOCIATED CONTENT
■
S
* Supporting Information
Tables and figures giving additional kinetic data determined in
laser flash photolysis experiments, details of the computational
studies, and calculated Cartesian coordinates and energies for
Dodecamethylcyclohexasilane (1) was synthesized using literature
procedures49 followed by several recrystallizations from 7/1 ethanol/
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selected complexes. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: leigh@mcmaster.ca.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the Natural Sciences and Engineering
Research Council of Canada for financial support, Teck
Cominco Ltd. for a generous gift of germanium tetrachloride,
B. Cowie (McMaster University) for technical assistance, and
R. Walsh (University of Reading) and P. Ayers (McMaster
University) for helpful discussions. 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.
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