Direct detection of methylphenylgermylene and 1,2-dimethyl-1,2- diphenyldigermene - Kinetic studies of their reactivities in solution

Article abstract of DOI:10.1139/V06-107

Photolysis of 1,3,4-trimethyl-1-phenylgermacyclopent-3-ene (5) in hydrocarbon solvents containing isoprene, methanol, or acetic acid affords 2,3-dimethyl-1,3-butadiene (DMB) and the expected trapping products of methylphenylgermylene (GeMePh) in chemical yields exceeding 90%. The germylene has been detected in hexane solution by laser flash photolysis as a short-lived species (τ ～ 2 μs) exhibiting a UV-vis absorption spectrum centered at λ_max = 490 nm. It decays with second-order kinetics and a rate constant close to the diffusion-controlled limit, with the concomitant growth of a second longer-lived transient (λ_max = 420 nm) that is assigned to a mixture of (E)- and (Z)-1,2-dimethyl-1,2-diphenyldigermene (4). Absolute rate constants have been determined for the reactions of the germylene with primary and tertiary amines (n-BuNH₂ and Et ₃N, respectively), acetic acid (AcOH), a terminal alkyne and alkene, isoprene, DMB, CCl₄, and the group 14 hydrides Et₃SiH and Bu₃SnH. GeMePh is slightly more reactive than GePh₂ towards all the reagents studied in this work; both are significantly less reactive than GeMe₂ toward the same substrates. Absolute rate constants for the reactions of 4 have also been measured or assigned upper limits in every case and are compared to previously reported values for tetraphenyl- and tetramethyl-digermene with the same reagents.

Full text of DOI:10.1139/V06-107

934
Direct detection of methylphenylgermylene and
1,2-dimethyl-1,2-diphenyldigermene — Kinetic
studies of their reactivities in solution
William J. Leigh, Ileana G. Dumbrava, and Farahnaz Lollmahomed
Abstract: Photolysis of 1,3,4-trimethyl-1-phenylgermacyclopent-3-ene (5) in hydrocarbon solvents containing isoprene,
methanol, or acetic acid affords 2,3-dimethyl-1,3-butadiene (DMB) and the expected trapping products of methyl-
phenylgermylene (GeMePh) in chemical yields exceeding 90%. The germylene has been detected in hexane solution by
laser flash photolysis as a short-lived species (τ - 2 µs) exhibiting a UV–vis absorption spectrum centered at λmax
=
490 nm. It decays with second-order kinetics and a rate constant close to the diffusion-controlled limit, with the con-
comitant growth of a second longer-lived transient (λmax = 420 nm) that is assigned to a mixture of (E)- and (Z)-1,2-
dimethyl-1,2-diphenyldigermene (4). Absolute rate constants have been determined for the reactions of the germylene
with primary and tertiary amines (n-BuNH2 and Et3N, respectively), acetic acid (AcOH), a terminal alkyne and alkene,
isoprene, DMB, CCl4, and the group 14 hydrides Et3SiH and Bu3SnH. GeMePh is slightly more reactive than GePh2
towards all the reagents studied in this work; both are significantly less reactive than GeMe2 toward the same sub-
strates. Absolute rate constants for the reactions of 4 have also been measured or assigned upper limits in every case
and are compared to previously reported values for tetraphenyl- and tetramethyl-digermene with the same reagents.
Key words: germylene, digermene, kinetics, laser flash photolysis, germirane, germirene, vinylgermirane, complex,
UV–vis spectrum, insertion, addition.
Résumé : La photolyse du 1,3,4-triméthyl-1-phénylgermacyclopent-3-ène (5) dans des solvants hydrocarbonés conte-
nant de l’isoprène, du méthanol ou de l’acide acétique conduit à la formation de 2,3-diméthylbuta-1,3-diène (DMB) et
aux produits de piégeage attendus du méthylphénylgermylène (GeMePh) avec des rendements supérieurs à 90%. Fai-
sant appel à la photolyse éclair au laser et opérant dans une solution d’hexane, on a détecté la présence du germylène
sous la forme d’une espèce transitoire (τ d’environ 2µs) qui présente un spectre d’absorption UV–visible centré autour
d’un λmax de 490 nm. Sa décroissance présente une cinétique du second ordre et une constante de vitesse qui se rap-
proche de la limite d’un contrôle par la diffusion et qui est accompagnée de la croissance d’une deuxième espèce tran-
sitoire (λmax = 420 nm) qui est attribuée à un mélange (E)- et (Z)-1,2-diméthyl-1,2-diphényldigermène (4). On a déterminé
les constantes de vitesse absolues de la réaction du germylène avec des amines primaires et tertiaires (respectivement
BuNH2 et Et3N), l’acide acétique (AcOH), un alcyne terminal, un alcène terminal, l’isoprène, le DMB, le CCl4 et les
hydrures du groupes 14, Et3SiH et Bu3SnH. Le GeMePh est légèrement plus réactif que le GePh2 vis-à-vis tous les
réactifs examinés dans ce travail; les deux sont toutefois beaucoup moins réactifs que le GeMe2 par rapport aux même
réactifs. Dans tous les cas, on a aussi mesuré ou attribué des valeurs limites supérieures pour les constantes de vitesses
absolues de la réaction avec le composé (4) et on les a comparées aux valeurs rapportées antérieurement pour les réac-
tions du tétraphényl- et du tétraméthyl-digermène avec les mêmes réactifs.
Mots clés : germylène, digermène, cinétique, photolyse éclair au laser, germirane, germirène, vinylgermirane, complexe,
spectre UV–visible, insertion, addition.
[Traduit par la Rédaction] Leigh et al. 948
Introduction
and the study of their synthesis and chemistry continues to be
of considerable interest (2–6). Of particular fundamental and
practical importance is the parent molecule, GeH2, whose
chemistry has been extensively studied in the gas phase by
time-resolved kinetic methods.2 Studies of several simple
substituted derivatives by time-resolved spectroscopic meth-
ods in matrices and in solution have been reported,3 but the
solution-phase studies have been characterized by a particu-
lar lack of consistency in the spectroscopic properties and
reactivities of these species (5). We have speculated that the
problem is due to two main factors — the characteristic
weakness of the lowest energy (S0 → S1) electronic transi-
tion in divalent group 14 compounds and the pronounced
complexity of the photochemistry of the particular organo-
Germylenes are important intermediates in the thermal
and photochemical reactions of germanium compounds (1),
Received 12 March 2005. Accepted 23 May 2006. Published
on the NRC Research Press Web site at http://canjchem.nrc.ca
on 9 August 2006.
W.J. Leigh,1 I.G. Dumbrava, and F. Lollmahomed.
Department of Chemistry, McMaster University, 1280 Main
Street West, Hamilton, ON L8S 4M1, Canada.
1Corresponding author (e-mail: leigh@mcmaster.ca).
2See refs. 5 and 6 and the refs. cited therein.
3See ref. 5 for a complete bibliography.
Can. J. Chem. 84: 934–948 (2006)
doi:10.1139/V06-107
© 2006 NRC Canada

Leigh et al.
935
Scheme 1.
germanium compounds (typically oligogermanes and
oligosilagermanes) that have been employed as germylene
precursors (7). As we have recently demonstrated (8), the
combination of these factors can make transient germylenes
relatively difficult to detect, even when they are the major
products of photolysis.
Me
Ph
ν
h
[:GeMePh]
x2
Ge
+
C6H14
5
DMB
Me
Me
We recently reported the first comprehensive kinetic stud-
ies of the reactivities of diphenyl- and dimethyl-germylene
(GePh2 and GeMe2, respectively) in hydrocarbon solution,
generating and monitoring them directly by laser flash
photolysis of the 1,1-disubstituted germacyclopent-3-ene de-
rivatives 1 and 2 (9–11). Trapping experiments showed that
photolysis of these molecules proceeds very cleanly and
with remarkably high efficiency in every case, producing the
expected trapping products of the germylenes along with the
corresponding conjugated dienes in chemical yields exceed-
ing 90% and quantum yields on the order of 0.5. Absolute
rate constants were reported for dimerization of the two spe-
cies, which yields the transient digermene derivatives 3, and
for their reaction with a variety of representative substrates
such as aliphatic amines, halocarbons (RX), aliphatic al-
kynes, dienes and alkenes, oxygen, acetic acid, and the
group 14 hydrides R3MH (M = Si, Ge, Sn; R = ethyl or n-
butyl). Several of the reactions of these species were found
to be reversible, such as the [2+1]-cycloaddition reactions
with alkenes and dienes, and Lewis acid–base complexation
with tertiary amines; they are nonetheless very fast, with for-
ward rate constants on the order of 109–1010 (mol/L)–1 s–1 in
hexane at room temperature. The absolute rate constants
measured for the reactions of GeMe2 with the C—C unsatu-
rated compounds and oxygen in solution (11) were found to
agree quite well with earlier data reported by Walsh and co-
workers (6, 12) for reactions with the same or similar sub-
strates in the gas phase. The solution-phase data for the two
germylenes showed GeMe2 to be significantly more reactive
than its phenylated counterpart in every reaction that was
studied. Rate constants were also reported for the reactions
of the digermenes 3a and 3b in several cases, providing the
first indications of the effects of methyl vs. phenyl substitu-
tion on the reactivity of digermene derivatives toward vari-
ous nucleophilic and electrophilic reagents.
Ge Ge
Ph
Ph
4
(E and Z)
Scheme 2.
Me
Ge
Ph
hν
Me
Ge
Ph
+
C6H14
6
5
DMB
under the conditions of our experiments. While absolute rate
constants for these reactions could be determined in a
straightforward fashion, the weaker transient absorptions of
the dialkylgermylene made it possible only to derive esti-
mates of the equilibrium constants. The products also could
not be detected, presumably because they do not absorb at
sufficiently long wavelengths to enable detection with our
methods.
In the present paper, we report the results of a time-resolved
spectroscopic study of methylphenylgermylene (GeMePh)
and (E)- and (Z)-1,2-dimethyl-1,2-diphenyldigermene (4, as
a mixture of isomers) in hydrocarbon solution, generated by
laser flash photolysis of 1,3,4-trimethyl-1-phenylgerma-
cyclopent-3-ene (5, Scheme 1). This work represents the
first step in a more extensive study of the effects of substitu-
ents (directly attached to germanium) on germylene and
digermene reactivity, as well as an attempt to confirm and
elaborate on some of the differences in the reactivity of
GeMe2 and GePh2 towards the various substrates that have
been examined so far. Our previous results for the other two
germylenes lead to the expectation that the presence of a sin-
gle phenyl group should provide a chromophore that would
lead to stronger transient absorptions (thus enabling more
precise kinetic measurements), facilitate the detection of
transient products, and provide some moderation of reactiv-
ity (compared with GeMe2) so that meaningful structure–re-
activity relationships might eventually be derived.
R
GeR2
GeMe2
R2Ge GeR2
Ph
1
2
3
a: R = Ph
b:
a: R = H
b:
a: R = Ph
b:
R = Me
R = Me
R = Me
Results
Reasonably precise determination of equilibrium con-
stants for the (reversible) reactions with C=C bonds was
possible only with GePh2, which has a significantly higher
extinction coefficient than GeMe2 (thus affording stronger
transient absorptions) and exhibits equilibrium constants that
are in an ideal range to be measured under the conditions of
our experiments. Furthermore, the (transient) products of
these reactions could also be detected. We were less success-
ful in characterizing the reactions of GeMe2 with the same
substrates; there were clear indications of reversibility, but
the equilibrium constants are evidently significantly higher
than those for GePh2 and close to the upper limit measurable
Steady state photolysis (254 nm) of a deoxygenated hex-
ane solution of 5 (0.025 mol/L) containing isoprene
(0.1 mol/L) led to the formation of 1,3-dimethyl-1-
phenylgermacyclopent-3-ene (6) as the only detectable ger-
manium-containing product at conversions below ca. 15%
and an equal yield of 2,3-dimethyl-1,3-butadiene (DMB)
(Scheme 2). The identity of 6 was established by GC
coinjection of the crude photolysate with an authentic, inde-
pendently synthesized sample of the compound. Similarly,
photolysis of a cyclohexane-d12 solution of 5 (0.05 mol/L)
containing methanol (MeOH, 0.5 mol/L) or acetic acid
(AcOH, 0.095 mol/L), with periodic monitoring of the
© 2006 NRC Canada

936
Can. J. Chem. Vol. 84, 2006
Scheme 3.
Fig. 1. Plots of concentration vs. time showing (a) the consump-
tion of 5 and (b) the formation of trapping products from
photolysis of 5 in cyclohexane-d12 containing AcOH
(0.095 mol/L, ٗ), MeOH (0.5 mol/L, ∆), and isoprene
(0.094 mol/L, ᭺). The data from the latter have not been cor-
rected for inner filter effects.
H
Me
Ph
hν
MeGePh
Ge
+
C6H14
OR
ROH
(R = Me or Ac)
DMB
7
5
a:
b:
R = Me
R = Ac
(a)
0.045
1
photolysates by H NMR spectroscopy between 0% and ca.
25% conversion, afforded DMB and a single other product
in each case. These were tentatively identified as methoxy-
methylphenylgermane and acetoxymethylphenylgermane (7a
0.040
1
and 7b, respectively, Scheme 3) on the basis of their H
0.035
AcOH
NMR spectra. The results are consistent with formal [4+1]-
cycloreversion to yield GeMePh and DMB as the predomi-
nant photochemical reaction pathway of 5. The quantum
yield was not rigorously determined, but qualitative compar-
isons of the rate of product formation in the MeOH-trapping
experiment with those for 1a (9) and 2a (11) under the same
conditions suggest that it is similar to the value of 0.55 that
had been previously reported by us (9, 11) for the quantum
yields for formation of GePh2 and GeMe2, respectively,
from these compounds. A merry-go-round experiment was
carried out, in which 0.045 mol/L solutions of 5 in cyclo-
hexane-d12 containing MeOH (0.25 mol/L), AcOH
(0.095 mol/L), and isoprene (0.094 mol/L) were irradiated in
parallel, to assess any possible differences in reaction quan-
tum yield with trapping agent. Concentration vs. time plots,
showing the consumption of 5 and the formation of the
isoprene
MeOH
0.030
0.025
0
2
4
6
8
Time (min)
(b)
6
0.015
0.010
0.005
0.000
7a
7b
1
products (6, 7a, and 7b), measured by H NMR spectros-
copy over the 0%–20% conversion range, are shown in
Fig. 1. Correction of the isoprene concentration vs. time data
for primary inner filter effects (because of coabsorption of
the excitation light by the substrate) leads to the conclusion
that product formation using the diene as trap (at ca.
0.1 mol/L) proceeds with roughly half the efficiency as in
the experiments employing MeOH or AcOH as germylene
scavengers. This corresponds to a kQτ value of ca. 10
(mol/L)–1 for quenching of the reactive excited state of 5 by
the diene, consistent with a lifetime of 10 ns or less assum-
ing a quenching rate constant in excess of 109 (mol/L)–1 s–1.
Laser flash photolysis of continuously flowing deoxy-
genated solutions of 5 (ca. 0.0015 mol/L) in anhydrous
hexane with the pulses from a KrF excimer laser (248 nm,
-25 ns, -100 mJ) led to the formation of two transient spe-
cies, one that appeared to be formed within ca. 50 ns of the
0
2
4
6
8
Time (min)
a decay constant of k/ε510 = (1.6 0.1) × 107 cm s–1. The be-
haviour is exactly analogous to that previously reported for
1a and 1b (9) and 2a and 2b (11) under similar conditions.
We thus assign the two transients to GeMePh and its
dimerization products, (E)- and (Z)-1,2-dimethyl-1,2-
diphenyldigermene (4), respectively, (Scheme 1), which we
assume are formed as a mixture of isomers.
onset of the laser pulse (λmax = 490 nm) and another (λmax
=
415 nm) that grew to a maximum in concentration over the
first ca. 4 µs after the pulse and then decayed to baseline
over the next ca. 100 µs. Figure 2 shows transient absorption
spectra recorded 95–200 ns and 3.4–3.6 µs after excitation,
along with typical transient decay–growth profiles recorded
at 510 and 410 nm, to the red and blue, respectively, of the
absorption maxima of the two species, to isolate the tran-
sient decays as completely as possible without sacrificing
appreciably in signal intensity. The decay of the first-formed
species fits well to second-order kinetics (eq. [1]), affording
[1]
∆At = ∆A0 / (1 + 2kdim(∆A0/ελl)t)
where ∆A0 and ∆At are the transient absorbances at time = 0
and time = t after the laser pulse, ελ is the extinction coeffi-
cient at the monitoring wavelength, and l is the path length.
We next investigated the reactivities of the two species
with various germylene scavengers (see Chart 1). Addition
of AcOH, Bu3SnH, CCl4, or tert-butylacetylene (TBE) re-
sulted in effects consistent with irreversible4 scavenging of
the germylene (9–11); the (germylene) absorption at 510 nm
4 Any reaction with a (forward) rate constant of 109–1010 (mol/L)–1 s–1 and an equilibrium constant greater than ca. 25 000 (mol/L)–1 appears
to be irreversible under the conditions of our experiments, to the extent that the reacting species decay completely to baseline with pseudo-
first-order kinetics.
© 2006 NRC Canada

Leigh et al.
937
/(∆A410)max,Q (᭺) vs.
Fig. 2. Transient absorption spectra recorded 95–220 ns (᭺) and
3.4–3.6 µs (᭹) after the laser pulse from flash photolysis of a
ca. 0.0015 mol/L solution of 5 in deoxygenated, anhydrous hex-
ane. The inset shows representative transient decay and growth
profiles recorded at 410 nm and 510 nm; the solid line in the
latter is the fit of the data to second order decay kinetics
Fig. 3. Plots of kdecay (ٗ) and (∆A410 max,0
)
[Q] for scavenging of GeMePh by (a) 3,3-dimethyl-1-butyne
(TBE) and (b) tri-n-butylstannane (Bu3SnH). The solid lines are
the linear least-squares fits of the data to eqs. [2] and [3], re-
spectively. Decay and growth profiles for MePhGe and 4 were
measured at 510 and 410 nm, respectively.
0.015
(a)
15
10
5
15
10
5
(ÄA410
)max ,0 /(ÄA410)
max ,Q
0.06
kdecay/106s–1
0.010
410 nm
0.005
0.04
510 nm
0.000
0
10
20
Time (µs)
30
0.02
0.00
KSV= 5800 300 (mol/L)–1
kQ = (5.9 0.4) × 109 (mol/L)–1s–1
300
400
500
600
0
0.0
0
0.5
1.0
1.5
2.0
Wavelength (nm)
[TBE] (mmol/L)
Chart 1.
(b)
n-BuNH2
Et3N
AcOH
CCl4
(ÄA410 max ,0
)
/(ÄA410)max ,Q
10.0
7.5
5.0
2.5
0.0
10.0
7.5
5.0
2.5
0.0
kdecay/106s–1
Et3SiH
n-Bu3SnH
O2
Me3CC CH
CH2CMe3
TBE
DMP
KSV = 5000 400 (mol/L)–1
kQ= (3.7 0.4) × 109 (mol/L)–1s–1
isoprene
DMB
0.0
0.5
1.0
1.5
2.0
decayed at increased rates, completely to baseline, with
clean first-order kinetics at concentrations above those re-
quired to reduce the peak digermene signal intensities to less
than ca. 50% of their values in the absence of added reagent.
As well, both the peak intensities and growth time of the ab-
sorptions because of 4 (measured at 410 nm) were reduced
in proportion to the concentration of added scavenger. Plots
of the pseudo-first-order rate constants for germylene decay
(kdecay) vs. concentration were linear, consistent with the re-
lationship of eq. [2],
[Bu3SnH] (mmol/L)
dimerization. Typical plots are shown in Fig. 3, from the ex-
periments employing TBE and Bu3SnH as scavengers. The
absolute rate constants and KSV values for reaction of
GeMePh with the various reagents studied in this work are
collected in Table 1. Also included in the table are the
KSV/kQ ratios, calculated for each of the scavengers studied
from the KSV and kQ values. Similar effects on the
germylene and digermene absorptions to those noted above
were observed in the presence of BuNH2, even though its re-
action with GeMePh is most likely reversible (vide infra).
In several cases, the reagent also had the effect of acceler-
ating the decay of the digermene (4) signal over the concen-
tration range where it could be detected, consistent with
reaction of the scavenger with the digermene as well. The
digermene decays followed reasonably good first-order ki-
netics in the presence of the scavenger, and plots of kdecay vs.
[Q], according to eq. [2], were linear; the absolute rate con-
stants obtained are also listed in Table 1. For those scaven-
gers that did not exhibit distinctive effects on the digermene
decay kinetics, upper limits of the rate constants were esti-
[2]
kdecay = k0 + kQ[Q]
where kQ is the second-order rate constant for reaction of the
species with the added reagent (Q) and k0 is the (hypotheti-
cal) pseudo-first-order rate constant for decay in the absence
of Q. Plots of the ratios of the peak transient absorbances
due to 4, in the absence and presence of scavenger at con-
centration [Q] (∆A410,0/∆A410,Q), were also linear in every
case and were analyzed according to eq. [3],
[3]
(∆A410)max,0/(∆A410)max,Q = 1 + KSV[Q]
where KSV is a proportionality factor reflecting the effi-
ciency of the germylene scavenging reaction relative to
© 2006 NRC Canada

938
Can. J. Chem. Vol. 84, 2006
Table 1. Absolute rate constants (kQ) and KSV values for reactions of GeMePh and 1,2-dimethyl-1,2-diphenyl-
digermene (4) with various substrates in dry deoxygenated hexane solution at 23 °C.
GeMePha
kQ (×109 (mol/L)–1 s–1)c
PhMeGe=GeMePh (4)b
kQ (×106 (mol/L)–1 s–1)b
Reagent (Q)
KSV (mol/L)–1d
KSV/kQ (µs)
AcOH
Et3SiH
5.6 0.5
≤0.000 44
3.7 0.4
0.017 0.002g
<0.06f
4 900 300
Nde
0.88
—
140 10
—
≤390f
6 1
Bu3SnH
CCl4
O2
5 000 300
29 2
1.35
1.72
50 12
>0.85
24 3
TBE
BuNH2
5.9 0.4
11.7 1.4
5 800 300
14 200 900
0.98
1.21
24 4
5200 600
Et3N
3.9 0.4
1 120 130
0.29
—
Isoprene
DMB
DMP
8.0 09
4 2
5.1 0.7
3 800 100
175 25h
110 100
0.48
0.044
0.21
≤4e
≤12e
≤32e
aMeasured by laser flash photolysis of deoxygenated solutions of 5 (-0.0015 mol/L) in hexane. Errors are listed as 2σ from
linear least-squares analyses of the data.
bDetermined from plots of kdecay (for 4, monitored at 410 nm) vs. [Q].
cFrom plots of kdecay (for GeMePh, monitored at 510 nm) vs. [Q] according to eq. [2], over concentration ranges corresponding to
>50% quenching of the peak ∆A410 value ((∆A410
)
max,0) because of 4.
dFrom plots of [(∆A410
)max, 0/(∆A410)max,Q] vs. [Q] according to eq. [3].
eNd (not determined).
fCalculated from kdecay at the highest concentration of scavenger studied.
gThe slope of the low concentration range of (nonlinear) plots of kdecay vs. [Q].
hCorrected for primary inner filter effects.
mated from the rate constant for decay of the signal at the
highest scavenger concentration investigated, obtained by
fitting the transient decay profiles to single exponential de-
cay kinetics. These data are also listed in Table 1.
the approach to equilibrium with the scavenging product and
the residual absorption as being due to residual free
germylene present at equilibrium and undergoing slow
dimerization. This behaviour was evident over the full con-
centration range in which the germylene could be detected
in the case of DMP (0.35–2.1 mmol/L), but only over the
0.1–0.5 mmol/L concentration range in the case of isoprene,
with the decays proceeding completely to the prepulse level
(within detection limits) at higher concentrations. This is
consistent with a larger equilibrium constant for reaction
with isoprene compared with that for DMP.
The germylene decay profiles in the experiments with
DMP and isoprene were analyzed as single exponential de-
cays to a nonzero residual absorption, as in our earlier study
(10). Plots of the decay rate constants vs. scavenger concen-
tration exhibited excellent linearity (see Fig. 4b), and analy-
sis of the data according to eq. [4] afforded kQ values of
(5.1 0.7) × 109 and (8.0 0.4) × 109 (mol/L)–1 s–1 for
DMP and isoprene, respectively. The corresponding values
of KSV, obtained from analysis of the peak ∆A410 values due
to digermene 4, according to eq. [3], were 1060 100 and
3800 100 (mol/L)–1. Plots of the ratios of the initial and
residual germylene ∆A values, according to eq. [5], exhib-
ited considerable scatter, mainly because of the rather low
signal-to-noise that characterized the present series of exper-
iments. It was thus not possible to determine the equilibrium
constants with reasonable precision and reproducibility. Nev-
ertheless, qualitative comparisons of the data to those previ-
ously published for GePh2 (10) with the same substrates
suggest the Keq values to be in the range of 4000–8000
(mol/L)–1 for DMP and 12 000–15 000 (mol/L)–1 for
isoprene.
Addition of Et3N, 4,4-dimethyl-1-pentene (DMP),
isoprene, and DMB resulted in behaviour consistent with re-
versible scavenging of GeMePh, the main indication of
which is a significantly less efficient reduction in the peak
intensity of digermene absorption with increasing scavenger
concentration; formation of the digermene could still be
readily observed even at high enough substrate concentra-
tions to reduce the lifetime of the germylene to the point
where it could no longer be detected. This effect was com-
mon to all four substrates. Other anticipated effects varied
with the substrate according to the magnitudes of the for-
ward rate constant and equilibrium constant for the particu-
lar reaction. For example, addition of Et3N caused the
germylene signals to decay with clean first-order kinetics
completely to the prepulse level at all concentrations
throughout the 0.3–2 mmol/L range in added substrate, simi-
lar to the behaviour observed with the irreversible substrates;
this is consistent with an equilibrium constant significantly
in excess of ca. 25 000 (mol/L)–1 (10), the approximate up-
per limit of the value measurable under the conditions of our
experiment given a forward rate constant in the 109–
1010 (mol/L)–1 s–1 range. On the other hand, addition of
DMP and isoprene caused the germylene signal to decay
with bimodal kinetics within certain concentration ranges,
consisting of a fast, approximately first-order decay fol-
lowed by a much longer-lived residual absorption; the decay
rate of the initial component increased while the residual ab-
sorption level and its decay rate both decreased with increas-
ing substrate concentration. To illustrate this behaviour,
Fig. 4a shows a series of germylene decay traces obtained in
the presence of DMP. We associate the fast initial decay with
[4]
[5]
kdecay = k–Q + kQ[Q]
(∆A510,0/∆A510,res Q = 1 + Keq[Q]
)
© 2006 NRC Canada

Leigh et al.
939
Fig. 4. (a) Absorbance vs. time profiles for GeMePh in
deoxygenated hexane containing DMP (0, 0.35, and
1.30 mmol/L), monitored at 510 nm. The solid lines are the best
fits of the data to single exponential decays. (b) Plots of kdecay
Fig. 5. Growth and decay profiles for the signals due to
(a) GeMePh and (b) 4 in the presence of DMB (0–2.76 mmol/L),
and (c) plots of the relative peak signal intensities for the
germylene (ٗ) and digermene (᭺) absorptions vs. DMB concen-
tration according to eq. [3].
and (∆A410 max,0/(∆A410)max,Q vs. [DMP], showing the fits of the
)
data to eqs. [4] and [3], respectively.
(a)
0.008
(a)
0.010
0 mmol/L
0.35 mmol/L
1.30 mmol/L
0 mmol/L
0.006
0.004
0.002
0.000
0.50 mmol/L
0.005
0.000
1.47 mmol/L
2.76 mmol/L
0
5
10
15
0
1
2
3
Time (ìs)
Time (ìs)
(b)
0.025
0.020
(b)
(∆A410
kdecay / (106 s–1
)
max,0 / (∆A410
)
max,Q
12
10
8
12
10
8
)
0 mmol/L
0.50 mmol/L
1.47 mmol/L
2.76 mmol/L
0.015
0.010
0.005
6
6
4
2
0
4
2
0
0.000
0
20
40
60
0.0
0.5
1.0
1.5
2.0
Time (µs)
[DMP] (mmol/L)
(c)
3.0
2.5
2.0
1.5
1.0
3.0
2.5
2.0
1.5
1.0
The data obtained with DMB as scavenger were very dif-
ficult to analyze accurately. Marked reductions in the
strength of the germylene signals accompanied addition of
the diene over the 0.5–3 mmol/L concentration range, yet ef-
fected only minor reductions in the strength of the diger-
mene signals and consistent lengthening of their growth
times with increasing concentrations. Typical data, corrected
for inner filter effects because of coabsorption of the excita-
tion light by the diene, are shown in Fig. 5. An initial fast
component to the predominantly second-order germylene
decay appeared to develop over the 0–0.8 mmol/L concen-
tration range in added diene; it became distinctive enough to
allow analysis between 1.0 and 2.0 mmol/L, but was too
short-lived above 2.0 mmol/L to analyze accurately, leaving
behind a much weaker residual second-order decay. This be-
haviour is again consistent with rapid reversible reaction, but
with an equilibrium constant of less than ca. 1000 (mol/L)–1
(13). A rate constant of kQ = (4 2) × 109 (mol/L)–1 s–1 was
estimated from the decay rate constants for the fast initial
component of the bimodal decay profiles from several deter-
minations (at shorter timescales) in the presence of 1.0–
2.0 mmol/L DMB, analyzed in the same way as the data
obtained with DMP, while a value of KSV = 175 25 (mol/L)–1
0.000
0.001
0.002
0.003
[DMB] (mol/L)
was obtained from analysis of the digermene peak signal
strengths according to eq. [3].
As is evident from the raw data shown in Figs. 4 and 5,
the strength of the germylene signals decreased noticeably
with addition of reactive substrate in increasing concentra-
tions; this was generally observed with all of the substrates
studied, to an extent comparable to that shown in Fig. 4 with
DMP as scavenger. It may be the result of the fact that
© 2006 NRC Canada

940
Can. J. Chem. Vol. 84, 2006
Fig. 6. Transient absorption spectra recorded at selected time in-
tervals after the laser pulse for a solution of 5 (0.0015 mol/L) in
deoxygenated hexane containing DMB (2.76 mmol/L). The inset
shows a decay trace recorded at a monitoring wavelength of
280 nm.
germylene formation appears to lag slightly behind the laser
pulse; this is evident from the [DMP] = 0 trace of Fig. 4a,
which suggests that the signal grows with a rise time of ca.
50 ns, significantly longer than the ca. 25 ns width of the la-
ser pulse. This behaviour suggests that germylene formation
is not a direct excited (singlet) state process, but rather en-
sues from a secondary intermediate. If this is true, then a re-
duction in germylene signal strength with increasing
scavenger concentration would be expected, as reaction of
the germylene with the scavenger becomes increasingly
competitive with its formation. The effect on signal strength
was much more pronounced in the experiments with
isoprene and DMB as scavengers, suggesting an additional
mechanism for quenching of the signals beyond that exhib-
ited by the other scavengers. Plots of the relative peak tran-
sient absorbances at 510 nm vs. concentration (cf. eq. [3])
afforded Stern–Volmer constants of 1000 150 (mol/L)–1
and 650 40 (mol/L)–1 for isoprene and DMB (see Fig. 5c),
respectively.
0.08
0.06
0.06
0.04
0.02
0.00
0.04
0
5
10
time (µs)
15
1.25 – 1.38 µs
4.05 – 4.48 µs
9.38 – 9.98 µs
0.02
-0.00
300
400
500
600
On the other hand, significant increases in the strength of
the transient absorptions at 260–300 nm occurred with in-
creasing DMB concentration; these absorptions, which are
centered at ca. 285 nm and are superimposed on those formed
in the absence of the scavenger (vide supra), decayed over a
similar timescale as the weak germylene absorption at
510 nm at low diene concentrations. Figure 6 shows a series
of transient absorption spectra recorded at various time inter-
vals after the laser pulse for the solution containing
2.76 mmol/L DMB. Higher diene concentrations could not
be investigated in this case because of coabsorption at the
excitation wavelength by the diene.
Wavelength (nm)
Fig. 7. Transient absorption spectra recorded by flash photolysis
of a solution of 5 (ca. 0.0015 mol/L) in deoxygenated, anhy-
drous hexane containing (a) Et3N (2.0 mmol/L) and (b) isoprene
(20 mmol/L). The insets show growth and decay profiles re-
corded at 350 nm and 285 nm, respectively.
0.02
(a)
0.03
0.01
New long-lived transient absorptions in the 270–350 nm
range were also observed in the presence of the amines and
the other C—C unsaturated compounds. The absorption
maximum of the species that was formed in the presence of
Et3N (λmax = 330 nm, Fig. 7a) was red-shifted sufficiently
from the absorptions due to the free germylene and its oligo-
merization products to enable a growth in the signal to be
detected at the upper end of the amine concentration range
studied; the growth rate constant was found to be in reason-
able agreement with that for decay of the germylene at the
same amine concentration, consistent with the new species
being formed by reaction of GeMePh with the amine. For
example, a trace recorded at 350 nm with a solution of 5
containing 1.5 mmol/L Et3N exhibited a growth rate con-
stant of (10.5 1.8) × 106 s–1, which agrees reasonably well
with the decay rate constant of kdecay = (6.5 0.5) × 106 s–1,
measured for the germylene signal at 510 nm at the same
amine concentration. We assign the species formed in the re-
action to the Lewis acid–base complex 8a. The analogous
product (8b) is formed in the presence of BuNH2 and exhib-
its λmax = 310 nm; however, the expected growth of its ab-
sorption could not be resolved because of overlap with other
transient absorptions that are present in the short wavelength
region at scavenger concentrations at which the germylene
signal could still be detected. The same was true of the prod-
ucts formed in the presence of isoprene, DMP, and TBE,
which absorb in the 270–285 nm range.
0.02
0.00
0
1
2
3
Time (µs)
0.01
0.00
165–218 ns
2.88–3.56 ìs
300
400
500
600
Wavelength (nm)
(b)
0.03
0.02
0.01
0.06
0.04
0.02
0.00
0
1000
2000
3000
Time (µs)
0–25 ìs
3.43–3.46 ìs
0.00
300
400
500
600
Wavelength (nm)
Transient decay profiles and spectra of the new products
formed in the presence of isoprene, DMP, and TBE were re-
© 2006 NRC Canada

Leigh et al.
941
Table 2. UV absorption maxima and lifetimes of transient prod-
corded at relatively high (3.3–200 mmol/L) substrate con-
centrations to minimize contributions to the lifetimes from
germylene dimerization or from overlapping transient ab-
sorptions because of GeMePh or its oligomerization prod-
ucts. It was possible to employ reasonably high substrate
concentrations in the experiments with DMP, and the life-
time of the species formed from the alkene was found to be
approximately independent of concentration over the 0.1–
0.35 mol/L range. The species formed from isoprene absorbs
with λmax = 285 nm and decays with clean pseudo-first-order
kinetics in the presence of 0.02 mmol/L diene (τ - 670 µs).
The spectra and lifetimes of these species are similar to
those observed previously from GePh2 with the same sub-
strates (10) and we tentatively assign them to the three-
membered germanocycles 9a–11. The species absorbing
with λmax - 285 nm in the transient spectra of Fig. 6 is simi-
larly assigned to vinylgermirane 9b. Table 2 contains a sum-
mary of the absorption maxima and lifetimes of the primary
transient products formed from reaction of GeMePh with
these compounds, while Fig. 7 shows examples of the spec-
tra and decays obtained in the presence of Et3N and isoprene.
ucts from the reaction of GeMePh with amines and C—C unsat-
urated compounds in hexane at 23 °C.
Tentative
assignment
λmax
(nm)
τ (ms)
(conc. in mmol/L)
Scavenger
BuNH2
Et3N
8b
8a
310
330
0.015 (0.3)
0.014 (1.5)
Isoprene
DMB
DMP
9a
9b
10
11
285
285
275
270
0.670 (20)
Nda
2.6 (350)
>2000 (3.3)
TBE
aNd (not determined).
studies of 1a and 2a, which were carried out under the same
conditions of lamp intensity and concentration, indicate sim-
ilar quantum yields for germylene extrusion (Φ - 0.5) from
the three compounds.
The UV–vis spectrum of GeMePh in a 3-methylpentane
(3-MP) matrix at 77 K was reported in the late 1980s by two
different groups; Ando et al. (15) generated the species by
photolysis of the 1,4-germanonaphthalene derivative 12 and
reported a UV spectrum exhibiting λmax = 440 nm, while
Mochida and co-workers (16) employed the photolysis of
trigermane 13 and reported λmax = 456 nm in 3-MP at 77 K
and λmax = 440 nm in cyclohexane solution at room tempera-
ture. The latter group also reported rate constants for reac-
tion of the species in cyclohexane solution with DMB (k =
2.2 × 106 (mol/L)–1 s–1), Et3SiH (k = 4.1 × 106 (mol/L)–1 s–1),
and CCl4 (k = 6.5 × 107 (mol/L)–1 s–1) (16). A transient ex-
hibiting a similar UV absorption spectrum but much differ-
ent reactivity toward DMB and CCl4, from flash photolysis
of poly(methylphenylgermylene) (14), was later assigned to
GeMePh by the same group (17). The results reported in the
present work using 5 as precursor differ significantly from
those of the earlier studies, but are fully consistent with the
spectroscopic and kinetic data reported by us recently for
GePh2 and GeMe2 under similar conditions (9–11).
+
MeGePh
NRR'R"
GeMePh
_
R
9
8
a. R = H
b
a. R = R' = R" = Et
b
. R = Me
. R = n-Bu; R' = R" = H
MeGePh
MeGePh
t-Bu
t-BuCH2
10
11
Saturation of a hexane solution of 5 with oxygen caused a
ca. 40% drop in the peak absorbance due to digermene 4
compared with its value in deoxygenated solution, but had no
discernible effect on the decay characteristics of the
germylene signal. An upper limit of kO < 6 × 107 (mol/L)–1 s–1
Me
Ph
Ge
2
for quenching of GeMePh was thus obtained from the
pseudo-first-order rate constant for decay of the germylene
in O2-saturated hexane (kdecay - 1 × 106 s–1), using a value of
0.015 mol/L for the concentration of O2 (14) and with the
assumption that the decay of the germylene is dominated by
its reaction with oxygen under these conditions. Modest re-
ductions in the lifetime of the digermene were observed with
Ph
Ph
Me
(MePhGe)n
Ph Ge(GeMe3)2
Ph Ph
14
(n ~ 28)
13
12
increasing O2 concentration, affording a value of kO
(2.4
=
2
0.3) × 107 (mol/L)–1 s–1 from analysis of the
For example, the long wavelength absorption maximum of
the initially formed transient from laser photolysis of 5
digermene decays according to eq. [2].
(λmax = 490 nm) lies between those of GePh2 (λmax
=
=
Discussion
500 nm, ε500 = 1850 dm3 mol–1 cm–1 (9)) and GeMe2 (λmax
470–480 nm (9, 11, 12), ε475 = 730 dm3 mol–1 cm–1 (11)).
The spectrum also shows a somewhat stronger absorption
band centered at λmax - 290 nm due to the S0 → S2 transi-
tion, which is shifted ca. 10 nm to the blue of the corre-
sponding band in the spectrum of GePh2 (9). We note that
the long wavelength absorption maximum of GeMePh is the
same as that reported for the silicon homologue (SiMePh) in
As we found previously for the related compounds 1 and
2 (9), steady state photolysis of 5 in the presence of MeOH
(0.5 mol/L), AcOH (0.095 mol/L), or isoprene (0.1 mol/L)
affords the expected trapping products of GeMePh in high
yields, along with 2,3-dimethyl-1,3-butadiene (DMB). Qual-
itative comparisons of the rate of product formation in the
MeOH-trapping experiment to those obtained in our earlier
© 2006 NRC Canada

942
Can. J. Chem. Vol. 84, 2006
Scheme 4.
a hydrocarbon matrix at 77 K (λmax = 490 nm (18)). Such
similarities are also evident in the UV–vis spectra of GeMe2
and SiMe2 (λmax = 453–465 nm (18, 19)), as well as those of
GePh2 and SiPh2 (λmax = 495 nm (18)). It thus seems clear
that the transient absorptions assigned to GeMePh in the ear-
lier time-resolved studies in solution, using 13 and 14 as
precursors, must be due to other more strongly absorbing
transient products; as we have shown previously for related
systems (8), the likely candidates are conjugated germene
derivatives and (or) germanium-centered radicals. For exam-
ple, the conjugated germene derivative (15) that might be
anticipated from photolysis of 13 can be predicted to exhibit
an absorption maximum in the 430–450 nm range and ex-
hibit moderate reactivity toward CCl4 and DMB, based on
analogies taken from related systems (7, 8, 20).
+
H
NR'R"R'''
R2Ge GeR2
O
_
R2GeOAc
R2Ge
AcOH
x2
R2Ge
(?)
NR'R"R'''
O2
H
R'3MH
R'
R'X
R2Ge MR3
[:GeR2]
[R2GeX]
(M = Si, Sn)
R'C CH
R'
R' GeR2
GeR2
GeR2
Me
R'
R'
Ge
GeMe3
H
GeMe3
GeR2
R'
15
Similarly, the absorption maximum of the second-formed
species (λmax = 415 nm) falls between those of Ge2Ph4 (3a,
λmax = 440 nm (9)) and Ge2Me4 (3b, λmax = 370 nm (9, 11,
21, 22)) and can thus be assigned with confidence to (E)-
and (or) (Z)-1,2-dimethyl-1,2-diphenyldigermene (4,
Scheme 1), formed most likely as a mixture of isomers. The
spectra of (E)- and (Z)-4 are also quite similar to those re-
ported by Sekiguchi et al. (23) for the silicon homologues,
(E)- and (Z)-1,2-dimethyl-1,2-diphenyldisilene [(E)- and (Z)-
16, λmax = 417 and 423 nm, respectively] in cyclohexane
solution. An absorption maximum of λmax = 420 nm was
reported earlier for 16 in a 3-MP glass at 77 K, formed (pre-
sumably as a mixture of isomers) by dimerization of SiMePh
(18).
scavenging of the germylene is reversible or not. A reason-
ably reliable indicator of reversibility in the data is the
KSV/kQ ratio, which (within certain limitations (11)) provides
a normalized measure of the efficiency with which a given
scavenger suppresses germylene dimerization as a result of
its competing reaction with the digermene precursor (i.e., the
germylene). The KSV/kQ ratio tends toward significantly
smaller values (indicating lower scavenging efficiencies) for
those substrates that react with the germylene reversibly
compared with those for irreversible scavengers. Because
KSV is measured from the relative peak intensities of the sig-
nal due to digermene 4, its magnitude is also affected by the
reactivity of the digermene with the scavenger. This leads to
artificially high values of the efficiency parameter, which
become particularly significant when the reactivity of the
digermene toward the scavenger is within a factor of ca. 5 of
that of the germylene; the smaller the differential reactivity,
the greater the distortion in the magnitude of KSV, and
KSV/kQ, from their true values. Thus, the KSV/kQ ratios for
those reagents for which this differential reactivity is rela-
tively small (e.g., BuNH2, CCl4, and O2) can be expected to
overestimate the efficiency (i.e., the rate relative to that of
dimerization) of the scavenging reaction. For this reason, we
have not attached an absolute value to the rate constant for
scavenging of GeMePh by O2 based on the KSV value and
report only the upper limit of the rate constant as defined by
the lifetime of the germylene in O2-saturated solution; simi-
larly, the value estimated previously for the reaction of
Me
Ph
Me
Me
Si Si
Si Si
-16
Ph
Ph
Ph
Me
-16
E
Z
Absolute rate constants have been determined in this work
for the reaction of GeMePh with a selection of representa-
tive germylene scavengers in hexane solution. The reactions
studied include O-H insertion with AcOH, Lewis acid–base
complexation with primary and tertiary amines, M-H inser-
tions with trialkylhydridosilanes and -stannanes, reaction
with terminal alkenes, alkynes, and conjugated dienes, chlo-
rine atom abstraction from CCl4, reaction with oxygen, and
dimerization, as summarized in Scheme 4. While product
studies specific to GeMePh have not been carried out in
most instances, the course of these reactions are well estab-
lished for GeMe2 and other reactive germylenes (1, 10, 11,
24, 25) and all of them have direct analogies in silylene
chemistry (26).
GePh2 with O2 (kO - 3 × 107 (mol/L)–1 s–1) (10) should also
2
be considered an upper limit. In any event, it is clear from
the data of Table 1 that those scavengers that react at least
ca. three times faster with GeMePh than with 4 fall into two
groups, those with KSV/kQ ≥ 0.9 (AcOH, Bu3SnH, O2, CCl4,
and TBE) and those with KSV/kQ < 0.5 (Et3N, isoprene,
DMB, and DMP). The first group is comprised of those re-
agents that react with GeMePh irreversibly, while the second
As we found in our earlier studies of GePh2 and GeMe2
(10, 11), two distinct types of kinetic behaviour are observed
in the signals due to GeMePh and 4, depending on whether
© 2006 NRC Canada

Leigh et al.
943
consists of the reversible scavengers. The KSV/kQ value for
BuNH2 would seem to suggest that it belongs in the group
of irreversible scavengers, but as it reacts with digermene 4
only twice slower than it does with GeMePh, the KSV value
(and hence KSV/kQ) is likely to be significantly overesti-
mated. This reaction is most likely reversible, like that with
Et3N (vide infra).
The absolute rate constants for the various reactions of
GeMePh that have been studied in the present work are in
most cases the same, within experimental error, as the corre-
sponding values for GePh2 (9, 10) and are consistently a fac-
tor of two to eight times smaller than those for reaction of
GeMe2 with the same substrates (11). The smallest varia-
tions throughout the series are exhibited by the rate con-
stants for dimerization and Lewis acid–base complexation
with the primary amine, which proceed within a factor of ca.
2 of the diffusional rate constant in hexane at 25 °C in all
three cases (11). An estimate of kdim - 2 × 1010 (mol/L)–1 s–1
for the rate constant for dimerization of GeMePh can
be obtained from the measured second-order decay
coefficient (k/ε510 = (1.6 0.1) × 107 cm s–1), assuming an
extinction coefficient intermediate between those of GePh2
strengths of the GeMePh absorptions in laser flash
photolysis experiments with these substrates. Apparent
Stern–Volmer constants on the order of 1000 (mol/L)–1 have
been determined from analysis of the peak germylene
absorbances as a function of diene concentration over the 0–
3 mmol/L concentration range, which is consistent with dif-
fusion-controlled quenching (k - 2 × 1010 (mol/L)–1 s–1) of
an intermediate possessing a lifetime of τ - 50 ns. However,
product studies indicate that product formation remains effi-
cient in the presence of isoprene in concentrations as high as
ca. 0.1 mol/L; compound 6, the expected product of trapping
of GeMePh by the diene, is formed in only modestly (ca.
2×) lower quantum yield than the corresponding products of
trapping of the germylene by MeOH or AcOH. This indi-
cates that whatever phenomenon is primarily responsible for
the loss in signal strength in the flash photolysis experi-
ments, it does not lead to a reduction in the yield of
germylene-derived products; this effectively rules out
energy-transfer quenching of an excited state of 5 as the pro-
cess responsible for the effect. In this regard, it is notewor-
thy that in the absence of added scavengers the germylene
signals appear to be characterized by a ca. 50 ns growth,
which is discrete from the temporal characteristics of the la-
ser pulse. This could be indicative of the involvement of an
additional intermediate such as 1,5-biradical 17, formed via
Ge—C bond cleavage in the reactive excited state of 5,
which undergoes further fragmentation to yield GeMePh and
DMB. Such a species would also be expected to undergo
ring closure to yield 5 and 9b competitively with fragmenta-
tion. The same temporal feature was also evident in our ear-
lier studies of GePh2 (generated from 1b) (9) and GeMe2
(generated from 2a) (11), although the effects of added
diene on the signal strengths were much less pronounced
than in the present case. It is reasonable to expect that such a
species might be reactive toward dienes, though again, the
product studies demand that it must do so reversibly.
Neumann and co-workers (27, 28) isolated products corre-
sponding to the addition of two molecules of diene to
GeMe2 in their early studies of the reactions of GeMe2 with
aliphatic dienes; we have found no evidence for the forma-
tion of such products in the present instance, although the
highest diene concentration we have employed in product
studies is only 0.1 mol/L. Further work will be required to
address the possible involvement of such intermediates in
the photochemistry of 1-germacyclopent-3-ene derivatives
and the reaction of the corresponding germylenes with
dienes in a more conclusive way.
(εmax ~ 1850 dm3 mol–1 cm–1) (9) and GeMe2 (εmax
~
730 dm3 mol–1 cm–1) (11). The results suggest that phenyl
for methyl substitution has relatively small effects on the re-
activity of simple germylenes toward this diverse set of re-
agents, but confirm the earlier conclusion that phenyl
substitution has a distinct moderating effect on germylene
reactivity (11). The present results indicate that most of the
effect on the rate constant arises with substitution of the first
alkyl substituent in GeMe2 and replacing the second has
much smaller consequences.
As might be expected, the equilibrium constants for the
reversible reactions of GeMePh also appear to lie between
those for GeMe2 and GePh2, although we have not been able
to measure them with reasonable precision owing to the low
signal-to-noise ratios that characterize the present experi-
ments. Nevertheless, the equilibrium constants appear to be
more sensitive to germylene substitution than the absolute
rate constants for the forward component of the reaction. For
example, the rate constants for reaction of the three
germylenes with the alkene DMP vary by less than a factor
of 2, while the equilibrium constants vary between Keq
-
2500 (mol/L)–1 for GePh2 (10) and Keq - 20 000 (mol/L)–1
for GeMe2 (11); these should be compared with the qualita-
tive estimate of 4000–8000 (mol/L)–1 for reaction of
GeMePh with the same substrate. The estimated equilibrium
constant for reaction of GeMePh with isoprene (Keq
-
15 000 (mol/L)–1) also falls within the range defined by
GePh2 and GeMe2. The total range in Keq exhibited by the
reactions of DMP with the three germylenes corresponds to
a range of ca. 1.3 kcal/mol (1 cal = 4.184 J) in standard free
energy.
Ge(Me)Ph
·
CH2
·
17
The two dienes have remarkably different effects on the
germylene and digermene growth–decay profiles, which can
be attributed primarily to a substantial difference in the equi-
librium constants for the reactions of these substrates cou-
pled with similar absolute rate constants for formation of the
primary products. Interestingly, they also appear to quench
the formation of the free germylene, to the extent that they
cause significant reductions in the initial (peak) signal
Addition of the dienes leads to the formation of new
strongly absorbing transients exhibiting λmax = 285 nm in
both cases, which are assigned to the corresponding
vinylgermirane derivatives 9a and 9b. The absorption spec-
trum and lifetime of the species formed from reaction of
GeMePh with isoprene (9a, τ - 670 µs, Fig. 7b) are quite
close to those of the analogous product from reaction of
© 2006 NRC Canada

944
Can. J. Chem. Vol. 84, 2006
Scheme 5.
membered ring compounds derived from reaction of the
germylene with two molecules of substrate, most likely via
the intermediacy of the corresponding three-membered ger-
manocycle (1, 27). We thus suspect that the relatively slow
decay of these species is due to these secondary reactions,
perhaps involving biradical intermediates formed by (single)
Ge—C bond cleavage processes. In the case of 10 this pro-
cess occurs with a rate constant of k - 500 s–1 in the pres-
ence of 0.1–0.35 mol/L alkene, roughly half that observed
for the analogous species formed from reaction of GePh2
with DMP under similar conditions (11). The process occurs
roughly 103 times more slowly than cycloreversion to regen-
erate GeMePh and the alkene, based on an estimate derived
from the estimated Keq value and measured rate constant for
the (forward) reaction of GeMePh with the alkene. The
somewhat shorter lifetime observed for vinylgermirane 9a is
consistent with an additional contribution to its decay due to
unimolecular isomerization, yielding 6.
The reactions of GeMePh with BuNH2 and Et3N also re-
sult in the formation of detectable transient products, which
can be identified as the Lewis acid–base complexes 8a and
8b, respectively. The equilibrium constants for formation of
these complexes are in considerable excess of ca. 25 000
(mol/L)–1, the approximate upper limit of Keq for which re-
sidual germylene absorptions can be detected under the pres-
ent experimental conditions. This was also found for the
corresponding amine complexes with GePh2 and GeMe2 (10,
11), so it is not possible to comment on the relative stabili-
ties of the amine complexes of this series of germylenes.
Again, however, the rate constants for their formation are
very similar. The only hint of a difference between the three
germylenes in their reaction with amines is in the absorption
spectra of the complexes, which are in both cases shifted
significantly to the blue of those formed from GePh2 (10),
and significantly red-shifted compared with the complexes
from GeMe2 (11). Ando et al. (15) suggested that the posi-
tions of the absorption maxima of the complexes of amines,
sulfides, phosphines, and ethers with a given germylene re-
flects their relative stabilities, in that they correlate broadly
with Lewis base strength, shifting to the blue as the strength
of the complex increases. This suggests that with a given
amine, GeMePh forms the somewhat stronger Lewis acid–
base complex and hence possesses somewhat higher Lewis
acid strength than GePh2. The opposite is true in regards to
GeMe2. For a given amine, the absorption maxima of the
three germylene complexes vary to a much greater extent
than those of the free germylenes. It is interesting to note
that since the complex with BuNH2 can be detected as a dis-
crete, relatively long-lived intermediate, the formal H-
migration process required to convert it to the putative N-H
insertion product must be relatively slow. We previously re-
ported that the reaction of GePh2 with primary and second-
ary amines affords mainly polymeric material, and the
germylamines derived from formal N-H insertion are formed
in very low yields (10).
GeMePh
MeGePh
MeGePh
R
τ
1 /
+
R
R
6. R = H
5
9
. R = Me
a:
R = H
b: R = Me
GePh2 with the same diene (λmax = 285 nm, τ - 500 µs) (10).
It should be noted that the lifetime of the isoprene-derived
product has been determined in the presence of sufficiently
high diene concentrations to suppress digermene formation
more or less completely, and where the decay of the species
follows clean first-order kinetics. We thus tentatively associ-
ate the decay rate constant with that for further
(unimolecular) rearrangement to yield the stable final
product of the reaction (1, 27, 29), the corresponding
germacyclopent-3-ene derivative 6 (Scheme 5). Unfortu-
nately, the greater absorptivity of DMB at the laser excita-
tion wavelength precluded experiments at sufficiently high
diene concentrations to determine a lifetime for 9b under
comparable conditions. It is interesting to note that the rela-
tive equilibrium constants for formation of 9a and 9b from
GeMePh indicate that 9b is ca. 1.5 kcal/mol less thermody-
namically stable than 9a, with respect to cycloreversion back
to the free germylene and the diene.
The intermediates formed in the reactions of GeMePh
with the alkene (DMP) and alkyne (TBE) are also relatively
long-lived and have been detected as discrete species with
absorption maxima in the 270–285 nm range and lifetimes
of ca. 2.6 ms or greater (see Table 2). The corresponding in-
termediates formed in the reactions of these substrates with
GePh2 exhibit similar absorption maxima (λmax = 275 nm in
both cases), but somewhat shorter lifetimes (τ ~ 1.2 and
600 ms, respectively) (10); the analogous products could not
be detected from GeMe2 under similar conditions, presum-
ably because they absorb too far to the blue to be detectable
under the conditions of our experiments (11). We similarly
assign these products to the three-membered germanocycles
10 and 11.
The significantly longer lifetimes of 9a–11 compared to
those of the corresponding adducts from GePh2 indicate that
the greater degree of stabilization of the GeMePh adducts
(as measured by the equilibrium constants for their forma-
tion) is also reflected in the rate constants for their further
reaction. The processes responsible for the decay of the
germirane (10) and germirene (11) at high substrate concen-
trations are unfortunately not clear; our earlier study of the
reaction of DMP with GePh2 indicated that it led to the for-
mation of germanium-containing polymers, while the reac-
tion with TBE produced a primary product that underwent
further thermal or photochemical reaction under the condi-
tions employed for its generation (10). Indeed, very few sta-
ble examples of such compounds are known (30–36), unlike
the situation with the corresponding silicon analogues,
which enjoy much greater thermodynamic stability (36–38).
As mentioned previously, the reactions of GeMe2 with
alkenes, dienes, and alkynes are known to yield five-
The effects of methyl vs. phenyl substitution on the reac-
tivity of digermenes 3a, 3b, and 4 are somewhat greater and
more varied than is the case with the corresponding germyl-
enes. We have been successful in determining absolute rate
constants for the reactions of 4 with four of the substrates
studied (BuNH2, AcOH, CCl4, and O2) and have determined
© 2006 NRC Canada

Leigh et al.
945
upper limits for the rate constants in the other cases. As
mentioned earlier, we make the assumption that dimerization
of GeMePh proceeds nonstereospecifically to yield a mix-
ture of the (E)- and (Z)-isomers of the digermene, which
seems reasonable enough considering that the reaction is
fully diffusion-controlled. In most cases, the rate constants
again fall between those of Ge2Me4 (3b) and Ge2Ph4 (3a),
and since the mechanistic implications of these results have
been discussed in some detail (11), they will not be elabo-
rated on further here. The exception is the value for reaction
with BuNH2, which indicates somewhat greater reactivity
for 4 compared with the tetraphenyl derivative. This is per-
haps suggestive of a small steric effect on the rate of nucleo-
philic attack at the Ge=Ge bond in Ge2Ph4 (3a), which is
thought to be the rate-determining step of the reaction (11).5
It is also interesting to note that the rate constant for reaction
with O2 is ca. 30× lower than that reported (39) for the
homologous disilenes, (E)- and (Z)-16, providing the first
quantitative indication of the differences in reactivity of
digermenes and disilenes of otherwise identical substitution.
The reaction of disilenes and digermenes with oxygen has
been reasonably well-studied and is known to yield the cor-
responding 1,2-dimetallaoxetane in both cases (40–42), pre-
sumably via the intermediacy of a triplet 1,4-(metalla-oxa)-
biradical intermediate (43). Comparison of the rate constants
for reaction of oxygen with the series of digermenes Ge2Me4
produced by photoextrusion of GeMePh from germa-
cyclopent-3-ene derivative 5, which affords GeMePh in
>90% chemical yield and high quantum efficiency, based on
the results of steady-state trapping experiments with metha-
nol, acetic acid, and isoprene. The germylene is detectable
as a weakly absorbing transient species with λmax = 490 nm
by laser flash photolysis of 5 in hexane solution, where it
decays by diffusion-controlled dimerization to yield 4 (λmax
=
415 nm). The trends in the experimental UV–vis absorption
spectra of GeMe2, GeMePh, and GePh2 mirror those in the
spectra of the corresponding silylenes and similar correla-
tions are found in the spectra of the corresponding
digermenes and disilenes.
Absolute rate constants for reaction of GeMePh with sev-
eral representative germylene scavengers have been mea-
sured, including primary and tertiary amines, acetic acid,
triethylsilane, tri-n-butylstannane, O2, CCl4, two aliphatic
dienes, and a terminal alkene and alkyne. With every reagent
studied, GeMePh reacts with a similar absolute rate constant
to that previously reported for GePh2 under similar condi-
tions and both germylenes are significantly less reactive than
GeMe2 toward the same substrates. Equilibrium constants
falling between those determined earlier for GeMe2 and
GePh2 have been estimated for the reversible reactions of
GeMePh with amines, the alkene, and the dienes; the pri-
mary products of these reactions have also been detected as
long-lived transient intermediates exhibiting similar UV–vis
absorption spectra to the analogous products from GePh2.
More detailed mechanistic studies of the photochemistry
of arylgermacyclopent-3-ene derivatives and the reactions of
transient germylenes and digermenes, including those with
oxygenated substrates, are currently in progress in our labo-
ratory.
(3b, kO = 5.0 × 107 (mol/L)–1 s–1) (11), Ge2Me2Ph2 (4, kO
=
2
2
2.4 × 107(mol/L)–1 s–1), and Ge2Ph4 (3a, kO = 4.8 × 106
2
(mol/L)–1 s–1) (10) indicates that phenyl for methyl substitu-
tion results in a consistent decrease in reactivity, spanning a
factor of about 10 throughout the series. To our knowledge,
there is little information available on the sensitivity of the
rate constant to substituent for the reaction of O2 with
disilenes.
Similar trends in reactivity are evident in the rate con-
stants for the reactions of 3a, 4, and 3b with CCl4 and
AcOH (10, 11); the former is thought to proceed via initial
chlorine atom abstraction to yield a (singlet) radical pair
(44–46), while the latter most likely proceeds via 1,2-
addition (of H-OAc) across the Ge=Ge bond, as is known
for the reaction of alcohols with digermenes (9, 21) and
disilenes (39, 47, 48). Interestingly, the opposite trend in re-
activity is observed with the strong nucleophile, BuNH2, for
which the rate constants for the reactions with both 4 and
Ge2Ph4 (3a) are ca. two orders of magnitude greater than
that reported for Ge2Me4 (3b) (11). The difference in the re-
activities of 3a and 3b toward this substrate is consistent
with frontier molecular orbital control of the reaction, as the
rate constants correlate with the relative LUMO energies of
the two compounds (11).
Experimental
1H and 13C NMR spectra were recorded on Bruker AV
200 or AV 600 spectrometers in deuteriochloroform or cyc-
lohexane-d12 solution and were referenced to the residual
solvent proton and 13C signals, respectively. Gas chromato-
graphic analyses were carried out using a Hewlett-Packard
5890 gas chromatograph equipped with a conventional
heated injector, a flame ionization detector, a Hewlett-
Packard 3396A integrator, and a SPB-5 capillary column
(25 m, 0.25 µm, Supelco, Inc.). Gas chromatographic – mass
spectrometric (GC–MS) separations employed a Hewlett-
Packard 5890 Series II gas chromatograph equipped with a
HP-5971A mass selective detector and a SPB-5 capillary
column (25 m, 0.25 µm, Supelco, Inc.). High-resolution
electron impact mass spectra and exact masses were deter-
mined on a Micromass TofSpec 2E mass spectrometer. Exact
masses were determined for the molecular ion (M+), em-
ploying a mass of 12.000 000 for carbon-12. Column chro-
matography employed a 30 mm × 800 mm column using
silica gel (120 g, acid-washed, 230–400 mesh ASTM, parti-
cle size 0.040–0.063 mm, Silicycle) and pentane as the
eluant. Steady state photolyses employed a Rayonet® photo-
chemical reactor (Southern New England Ultraviolet Co.,
Summary and conclusions
The direct detection and characterization of the reactivi-
ties of methylphenylgermylene (GeMePh) and (E)- and (Z)-
1,2-dimethyl-1,2-diphenyldigermene (4), formed as a mix-
ture by dimerization of the germylene, have been accom-
plished for the first time. The two species have been
5 W.J. Leigh and L.A. Huck. To be published.
© 2006 NRC Canada

946
Can. J. Chem. Vol. 84, 2006
Branford, Connecticut) equipped with a merry-go-round ap-
paratus and six RPR-2537 lamps (254 nm).
the compound was at least 99% according to GC analysis.
The compound exhibited the following spectroscopic data:
1H NMR (CDCl3, 200 MHz) (52) δ: 0.56 (s, 3H), 1.74 (m,
10 H), 7.31–7.51 (m, 5H). 13C NMR (CDCl3) δ: –4.5, 19.4,
26.4, 128.0, 128.5, 130.9, 133.3, 143.9. MS m/z: 248 (65),
246 (45), 244 (35), 233 (38), 231 (28), 229 (20), 166 (70),
164 (43), 151 (100), 149 (85), 147 (80). Exact mass calcd.
for C13H1874Ge: 248.0620; found: 248.0617.
2,3-Dimethyl-1,3-butadiene, isoprene, phenylmagnesium
bromide (3.0 mol/L solution in diethyl ether), methyl-
magnesium bromide (3.0 mol/L solution in diethyl ether),
and magnesium were used as received from Sigma-Aldrich.
Hexanes (Caledon Reagent) was dried by refluxing for sev-
eral days under nitrogen with sodium–potassium amalgam
followed by distillation, or by passage through activated alu-
mina under nitrogen using a Solv-Tek solvent purification
system (Solv-Tek, Inc., Berryville, Virginia). Diethyl ether
(Caledon Reagent) and tetrahydrofuran (Caledon Reagent)
were dried by the latter method. Each of the scavengers in-
vestigated in this work were obtained from commercial
sources in the highest purity available. The amines were
refluxed over solid KOH for 12 h and distilled, while tri-
ethylsilane (Et3SiH) and tri-n-butylstannane (Bu3SnH) were
stirred at room temperature for 18 h over lithium aluminum
hydride and distilled at atmospheric pressure (Et3SiH) or un-
der mild vacuum (Bu3SnH). CCl4 was refluxed over phos-
phorus pentoxide and distilled. 4,4-Dimethyl-1-pentene
(DMP) was dried by passage through a silica gel column,
while isoprene, DMB, and 3,3-dimethyl-1-butyne (TBE)
were distilled at atmospheric pressure. 1,1-Dichloro-3,4-
dimethylgermacyclopent-3-ene and 1,1-dichloro-3-methyl-
germacyclopent-3-ene were prepared as previously described
(9, 49, 50). Deuterated solvents were used as received from
Cambridge Isotopes Laboratories.
1,3,4-Trimethyl-1-phenyl-1-germacyclopent-3-ene (5) was
synthesized by a procedure similar to that of Mazerolles and
Manuel (51). In a flame-dried apparatus consisting of a
250 mL two-necked round-bottomed flask, reflux condenser,
nitrogen inlet, addition funnel, and magnetic stirrer was pre-
pared a solution of 1,1-dichloro-3,4-dimethyl-1-germacyclo-
pent-3-ene (3.7 g, 0.016 mol) in anhydrous diethyl ether
(80 mL) under nitrogen. The solution was cooled to 0 °C
using an ice bath, and then a stirred solution of phenyl-
magnesium bromide (5.7 mL of a 3.0 mol/L solution in di-
ethyl ether, 0.0171 mol) in dry diethyl ether (20 mL) was
added dropwise over 1 h. The cooling bath was removed af-
ter 2 h and the white suspension was allowed to stir at room
temperature for another 20 h. A solution of methyl-
magnesium bromide (6.3 mL of a 3.0 mol/L solution in di-
ethyl ether, 0.019 mol) in anhydrous diethyl ether (20 mL)
was then added dropwise over 1 h, and the reaction mixture
was then stirred under reflux for a further 3 h. After cooling,
the resulting yellow suspension was hydrolyzed with satu-
rated aqueous ammonium chloride (40 mL) over 20 min,
transferred to a separatory funnel, and the aqueous and or-
ganic fractions were separated. The aqueous fraction was ex-
tracted with diethyl ether (3 × 60 mL), and then the
combined ether fractions were washed with distilled water
(2 × 30 mL), 5% aqueous sodium bicarbonate (2 × 30 mL),
dried with anhydrous magnesium sulfate (30 min), and fil-
tered. The solvent was removed on a rotary evaporator to
yield a yellow-green liquid (4.4 g). Distillation under vac-
uum (bp = 103–105 °C at 0.3 mm Hg, 1 mm Hg =
133.322 Pa) afforded 5 (3.78 g, 0.015 mol, 86%) as a color-
less liquid. Further purification by column chromatography,
using pentane as eluant, was performed until the purity of
1,3-Dimethyl-1-phenyl-1-germacyclopent-3-ene (6) was
prepared in similar fashion. A solution of 1,1-dichloro-3-
methyl-1-germacyclopent-3-ene (2.0 g, 0.009 mol) in anhy-
drous diethyl ether (40 mL) was introduced into a flame-
dried apparatus consisting of a two-necked round-bottomed
flask (100 mL), reflux condenser, nitrogen inlet, addition
funnel, and magnetic stirrer. After cooling to 0 °C, a solu-
tion of phenylmagnesium bromide (3.3 mL of a 3.0 mol/L
solution in diethyl ether, 0.010 mol) in dry diethyl ether
(20 mL) was added dropwise over 1.5 h with stirring. The
cooling bath was removed after 2 h and the suspension was
stirred at room temperature for another 15 h. A solution of
methylmagnesium bromide (4.1 mL of a 3.0 mol/L solution
in diethyl ether, 0.012 mol) in anhydrous diethyl ether
(20 mL) was then added dropwise over 1.5 h, and the result-
ing mixture was stirred under reflux for a further 4 h. After
cooling, the resulting yellow suspension was hydrolyzed
with saturated aqueous ammonium chloride (40 mL) over
20 min and transferred to a separatory funnel, where the
aqueous and organic fractions were separated. The aqueous
fraction was extracted with diethyl ether (3 × 60 mL) and
then the combined ether fractions were washed with distilled
water (2 × 30 mL), 5% aqueous sodium bicarbonate (2 ×
30 mL), dried with anhydrous magnesium sulfate (30 min),
and filtered. The solvent was removed on a rotary evaporator
to yield a yellow oil (2.3 g), which was purified by silica gel
column chromatography with pentane as the eluting solvent.
This afforded a colorless oil (1,9 g, 0.008 mol, 89%) in
>99% purity, according to GC analysis, which was identified
1
as 6 on the basis of the following spectroscopic data. H
NMR (CDCl3, 200 MHz) δ: 0.58 (s, 3H), 1.23–1.80 (m,
7 H), 5.64 (s, 1H), 7.24–7.50 (m, 5H). 13C NMR (CDCl3) δ:
–4.2, 14.2, 22.7, 31.7, 125.4, 128.1, 128.6, 130.9, 133.2,
140.3. MS m/z: 234 (35), 232 (25), 230 (18), 220 (67), 219
(36), 217 (15), 214 (10), 166 (40), 151 (100), 149 (85), 147
(50), 122 (20). Exact mass calcd. for C12H1674Ge: 234.0464;
found: 234.0446.
Steady state photochemical trapping experiments were
carried out in quartz NMR tubes using 0.045 mol/L solu-
tions of
5
in cyclohexane-d12 containing methanol
(0.5 mol/L), acetic acid (0.095 mol/L), or isoprene
(0.094 mol/L), monitoring the course of the reactions over
1
the 0%–40% conversion range by H NMR spectroscopy.
The tubes were sealed with rubber septa and deoxygenated
with a stream of dry argon for ca. 20 min prior to photolysis.
The photolyses afforded equal yields of 2,3-dimethyl-1,3-
butadiene (DMB; δ: 1.96, 4.88, 4.97) and a single additional
major (>90%) product in each case. The identity of the
1
isoprene adduct (6) was established by H NMR spectros-
copy, GC–MS, and GC coinjection of the crude photolysate
with the authentic sample described above. The MeOH and
AcOH adducts 7a and 7b, respectively, were identified on
© 2006 NRC Canada

Leigh et al.
947
1
6. S.E. Boganov, M.P. Egorov, V.I. Faustov, I.V. Krylova, O.M.
Nefedov, R. Becerra, and R. Walsh. Russ. Chem. Bull. 54, 483
(2005).
the basis of comparisons of their H NMR spectra to those
of closely related compounds (9, 11, 53). Neither compound
survived attempted GC separation without significant de-
1
7. W.J. Leigh, N.P. Toltl, P. Apodeca, M. Castruita, and K.H.
composition. Methoxymethylphenylgermane (7a): H NMR
Pannell. Organometallics, 19, 3232 (2000).
8. C.R. Harrington, W.J. Leigh, B.K. Chan, P.P. Gaspar, and D.
Zhou. Can. J. Chem. 83, 1324 (2005).
9. W.J. Leigh, C.R. Harrington, and I. Vargas-Baca. J. Am.
Chem. Soc. 126, 16105 (2004); Erratum, 128, 1394 (2006).
10. W.J. Leigh and C.R. Harrington. J. Am. Chem. Soc. 127, 5084
(2005).
(C6D12, 600 MHz) δ: 0.59 (d, 3H, J = 2.3 Hz), 3.45 (s, 3H),
5.68 (q, 1H, J = 2.3 Hz), 7.28 (m, 3H), 7.48 (m, 2H).
Acetoxymethylphenylgermane (7b): 1H NMR (C6D12,
600 MHz) δ: 0.79 (d, 3H, J = 2.3 Hz), 1.88 (s, 3H), 6.01 (q,
1H, J = 2.3 Hz), 7.28 (m, 3H), 7.58 (d, 2H).
Laser flash photolysis experiments employed the pulses
from a Lambda Physik Compex 120 excimer laser, filled
with F2–Kr–Ne (248 nm, -25 ns, 100 5 mJ) mixtures and
a Luzchem Research mLFP-111 laser flash photolysis sys-
tem, modified as described previously (9). Solutions were
prepared at concentrations such that the absorbance at the
excitation wavelength (248 nm) was between ca. 0.7 and 0.9
and were flowed continuously through a thermostatted 7 ×
7 mm Suprasil flow cell connected to a calibrated 100 mL
reservoir, fitted with a glass frit to allow bubbling of nitro-
gen or argon gas through the solution for at least 30 min
prior to and throughout the duration of each experiment. The
glassware, sample cells, and transfer lines used for these ex-
periments were stored in a 85 °C vacuum oven when not in
use, and the oven was vented with dry nitrogen just prior to
assembling the sample-handling system at the beginning of
an experiment. Solution temperatures were measured with a
Teflon-coated copper–constantan thermocouple inserted di-
rectly into the flow cell. Transient decay and growth rate
constants were calculated by nonlinear least-squares analysis
of the absorbance–time profiles using the Prism 3.0 software
package (GraphPad Software, Inc., San Diego, California)
and the appropriate user-defined fitting equations, after im-
porting the raw data from the Luzchem mLFP software. Re-
agents were added directly to the reservoir by microliter
syringe as aliquots of standard solutions. Rate constants
were calculated by linear least-squares analysis of decay rate
concentration data (five to seven points) that spanned as
large a range in transient decay rate as possible. Errors are
quoted as twice the standard deviation obtained from the
least-squares analyses.
11. W.J. Leigh, F. Lollmahomed, and C.R. Harrington. Organo-
metallics, 25, 2055 (2006).
12. R. Becerra, S.E. Boganov, M.P. Egorov, V.Ya. Lee, O.M.
Nefedov, and R. Walsh. Chem. Phys. Lett. 250, 111 (1996).
13. R. Becerra, C.R. Harrington, P.P. Gaspar, W.J. Leigh, I.
Vargas-Baca, R. Walsh, and D. Zhou. J. Am. Chem. Soc. 127,
17469 (2005).
14. R. Battino, T.R. Rettich, and T. Tominaga. J. Phys. Chem. Ref.
Data, 12, 163 (1983).
15. W. Ando, H. Itoh, and T. Tsumuraya. Organometallics, 8, 2759
(1989).
16. M. Wakasa, I. Yoneda, and K. Mochida. J. Organomet. Chem.
366, C1 (1989).
17. K. Mochida, K. Kimijima, and H. Chiba. Organometallics, 13,
404 (1994).
18. M.J. Michalczyk, M.J. Fink, D.J. De Young, C.W. Carlson,
K.M. Welsh, R. West, and J. Michl. Silicon Germanium Tin
Lead Compds. 9, 75 (1986).
19. G. Levin, P.K. Das, C. Bilgrien, and C.L. Lee. Organo-
metallics, 8, 1206 (1989).
20. W.J. Leigh and G.W. Sluggett. Organometallics, 13, 269 (1994).
21. K. Mochida, T. Kayamori, M. Wakasa, H. Hayashi, and M.P.
Egorov. Organometallics, 19, 3379 (2000).
22. M.B. Taraban, O.S. Volkova, V.F. Plyusnin, Y.V. Ivanov, T.V.
Leshina, M.P. Egorov, O.M. Nefedov, T. Kayamori, and K.
Mochida. J. Organomet. Chem. 601, 324 (2000).
23. A. Sekiguchi, I. Maruki, and H. Sakurai. J. Am. Chem. Soc.
115, 11460 (1993).
24. B. Klein, W.P. Neumann, M.P. Weisbeck, and S. Wienken. J.
Organomet. Chem. 446, 149 (1993).
25. W.P. Neumann, M.P. Weisbeck, and S. Wienken. Main Group
Met. Chem. 17, 151 (1994).
26. P.P. Gaspar and R. West. In The chemistry of organic silicon
compounds. Vol. 2. Edited by Z. Rappoport and Y. Apeloig.
John Wiley and Sons, New York. 1998. p. 2463.
27. W.P. Neumann, E. Michels, and J. Kocher. Tetrahedron Lett.
28, 3783 (1987).
28. M. Schriewer and W.P. Neumann. J. Am. Chem. Soc. 105, 897
(1983).
29. E.C.L. Ma, K. Kobayashi, M.W. Barzilai, and P.P. Gaspar. J.
Acknowledgement
We thank the Natural Sciences and Engineering Research
Council of Canada (NSERC) for financial support, the
McMaster Regional Centre for Mass Spectrometry (Hamil-
ton, Ontario) for mass spectrometric analyses, and Teck-
Cominco Metals Ltd. (Vancouver, British Columbia) for a
generous gift of germanium tetrachloride.
Organomet. Chem. 224, C13 (1982).
30. W. Ando, H. Ohgaki, and Y. Kabe. Angew. Chem. Int. Ed.
Engl. 33, 659 (1994).
31. H. Ohgaki, Y. Kabe, and W. Ando. Organometallics, 14, 2139
(1995).
References
32. M.P. Egorov, S.P. Kolesnikov, Y.T. Struchkov, M.Y. Antipin,
S.V. Sereda, and O.M. Nefedov. J. Organomet. Chem. 290,
C27 (1985).
33. A. Krebs and J. Berndt. Tetrahedron Lett. 24, 4083 (1983).
34. F. Meiners, W. Saak, and M. Weidenbruch. Z. Anorg. Allg.
Chem. 628, 2821 (2002).
1. W.P. Neumann. Chem. Rev. 91, 311 (1991).
2. J. Barrau and G. Rima. Coord. Chem. Rev. 180, 593 (1998).
3. M. Weidenbruch. Eur. J. Inorg. Chem. 1999, 373 (1999).
4. N. Tokitoh and R. Okazaki. Coord. Chem. Rev. 210, 251 (2000).
5. S.E. Boganov, M.P. Egorov, V.I. Faustov, and O.M. Nefedov.
In The chemistry of organic germanium, tin, and lead com-
pounds. Vol. 2. Edited by Z. Rappoport. John Wiley and Sons,
New York. 2002. p. 749.
35. N. Tokitoh, K. Kishikawa, T. Matsumoto, and R. Okazaki.
Chem. Lett. 827 (1995).
© 2006 NRC Canada

948
Can. J. Chem. Vol. 84, 2006
45. M.-D. Su. Inorg. Chem. 43, 4846 (2004).
46. M.-D. Su. J. Phys. Chem. A, 108, 823 (2004).
47. H. Sakurai. In The chemistry of organic silicon compounds.
Edited by Z. Rappoport and Y. Apeloig. John Wiley and Sons,
New York. 1998. p. 827.
36. M. Driess and H. Grutzmacher. Angew. Chem. Int. Ed. Engl.
35, 828 (1996).
37. J.A. Boatz, M.S. Gordon, and L.R. Sita. J. Phys. Chem. 94,
5488 (1990).
38. D.A. Horner, R.S. Grev, and H.F. Schaefer, III. J. Am. Chem.
Soc. 114, 2093 (1992).
48. T. Veszpremi, M. Takahashi, B. Hajgato, and M. Kira. J. Am.
39. T.L. Morkin, T.R. Owens, and W.J. Leigh. In The chemistry of
organic silicon compounds. Vol. 3. Edited by Z. Rappoport and
Y. Apeloig. John Wiley and Sons, New York. 2001. p. 949.
40. S. Masamune, S.A. Batcheller, J. Park, W.M. Davis, O.
Yamashita, Y. Ohta, and Y. Kabe. J. Am. Chem. Soc. 111,
1888 (1989).
Chem. Soc. 123, 6629 (2001).
49. S.P. Kolesnikov, I.S. Rogozhin, and O.M. Nefedov. Izv. Akad.
Nauk SSSR, Ser. Khim. 2297 (1974).
50. O.M. Nefedov, S.P. Kolesnikov, and A.I. Ioffe. Izv. Akad.
Nauk SSSR, Ser. Khim. 602 (1976).
51. P. Mazerolles and G. Manuel. Bull. Soc. Chim. Fr. 56, 327
(1965).
41. M.S. Samuel, M.C. Jennings, and K.M. Baines. J. Organomet.
Chem. 636, 130 (2001).
42. R. Okazaki and R. West. Adv. Organomet. Chem. 39, 231
(1996).
52. K.L. Bobbitt, V.M. Maloney, and P.P. Gaspar. Organometallics,
10, 2772 (1991).
53. M. Massol, J. Satge, P. Riviere, and J. Barrau. J. Organomet.
43. K. McKillop, R. West, T. Clark, and H. Hofmann. Z.
Naturforsch. B: Chem. Sci. 49, 1737 (1994).
44. M.S. Samuel, M.C. Jennings, and K.M. Baines. Organo-
metallics, 20, 590 (2004).
Chem. 22, 599 (1970).
© 2006 NRC Canada