Laser flash photolysis studies of some dimethylgermylene precursors

Article abstract of DOI:10.1021/om9000814

The laser flash photolysis of dodecamethylcyclohexagermane (1) and dimethylphenyl(trimethylsilyl)- germane (5), two known photochemical precursors to dimethylgermylene (GeMe₂), has been reinvestigated. Laser flash photolysis of 1 in hexane solu

Full text of DOI:10.1021/om9000814

Organometallics 2009, 28, 3239–3246
3239
Laser Flash Photolysis Studies of Some Dimethylgermylene
Precursors
Farahnaz Lollmahomed and William J. Leigh*
Department of Chemistry, McMaster UniVersity, 1280 Main Street West, Hamilton,
Ontario, Canada L8S 4M1
ReceiVed February 3, 2009
The laser flash photolysis of dodecamethylcyclohexagermane (1) and dimethylphenyl(trimethylsilyl)-
germane (5), two known photochemical precursors to dimethylgermylene (GeMe2), has been reinvestigated.
Laser flash photolysis of 1 in hexane solution affords markedly different results than were reported in an
earlier study of this compound in cyclohexane. The present study shows it to yield two primary transient
products, one exhibiting λmax ) 490 nm and lifetime τ e 10 ns and a second that exhibits λmax ) 470 nm
and decays on the microsecond time scale with second-order kinetics, with the concomitant growth of
absorptions centered at λmax ) 370 nm due to tetramethyldigermene (Ge2Me4). The spectrum and absolute
rate constants for quenching of the 470 nm species by acetic acid, 2,3-dimethyl-1,3-butadiene (DMB),
CCl4, and oxygen in hexane, as well as the transient behavior observed in the presence of millimolar
concentrations of THF, are all quite similar to those established recently for GeMe2 using 1-germacy-
clopent-3-ene derivatives as precursors. Two different transient products dominate the time-resolved spectra
obtained from laser flash photolysis of 5 under similar conditions, one exhibiting λmax ) 300 nm and τ
≈ 20 µs and a second exhibiting λmax ) 430 nm and τ ≈ 4 µs. The latter species was assigned to GeMe2
in earlier studies; it is re-assigned in the present work to the conjugated germene (6) derived from 1,3-
trimethylsilyl migration into the ortho-position of the phenyl ring in 5, on the basis of comparisons of
the spectrum and absolute rate constants for quenching of the species by oxygen, DMB, CCl4, acetic
acid, and acetone to those reported previously for the homologous silene derivative (11) and the
corresponding 1,1-diphenyl-substituted analogues of the germene (12) and the silene (13). Weak transient
absorptions consistent with the postpulse formation of Ge2Me4 are also detectable from 5 in deoxygenated
hexanes, but GeMe2 itself cannot be detected.
λmax ≈ 405 nm in argon at 12-18 K;15 similar spectra in
hydrocarbon matrixes were obtained by photolysis of at least
six different precursors. The direct detection of GeMe2 in
solution by laser flash photolysis methods has been much more
problematic. Several such studies were reported in the late 1980s
and early 1990s, using many of the same precursors that were
employed in the low-temperature matrix studies.8,11-13,16-18
These placed the absorption maximum of GeMe2 at various
wavelengths between 420 and 500 nm in hydrocarbon solvents,
with the spectrum, lifetime, and measured rate constants for
reaction of various substrates with the species assigned to GeMe2
varying with the precursor employed. The spectrum in the gas
phase, as established in experiments employing 193 nm laser
photolysis of two different GeMe2 precursors, is centered at λmax
≈ 480 nm,19 red-shifted quite significantly from that observed
in low-temperature matrixes.
Introduction
The chemistry and spectroscopic properties of dimethylger-
mylene (GeMe2) have been studied extensively over the past
few decades in the gas phase,1,2 in solution,3,4 and in low-
temperature matrixes,5 and much of what is known today of
germylene reactivity in solution is derived from product studies
carried out with thermal and/or photochemical precursors to this
particular germylene derivative.3,4 The species was first detected
directly by UV-vis spectrophotometry in low-temperature
matrixes,6 where its longest wavelength electronic absorption
is centered at 420-430 nm in hydrocarbons at 77 K5-14 and
* Corresponding author. E-mail: leigh@mcmaster.ca.
(1) Boganov, S. E.; Egorov, M. P.; Faustov, V. I.; Krylova, I. V.;
Nefedov, O. M.; Becerra, R.; Walsh, R. Russ. Chem. Bull. Int. Ed. 2005,
54, 483.
(2) Becerra, R.; Walsh, R. Phys. Chem. Chem. Phys. 2007, 9, 2817.
(3) Neumann, W. P. Chem. ReV. 1991, 91, 311.
(11) Wakasa, M.; Yoneda, I.; Mochida, K. J. Organomet. Chem. 1989,
366, C1.
(12) Mochida, K.; Yoneda, I.; Wakasa, M. J. Organomet. Chem. 1990,
399, 53.
(13) Mochida, K.; Kanno, N.; Kato, R.; Kotani, M.; Yamauchi, S.;
Wakasa, M.; Hayashi, H. J. Organomet. Chem. 1991, 415, 191.
(14) Mochida, K.; Tokura, S.; Murata, S. Chem. Commun. 1992, 250.
(15) Barrau, J.; Bean, D. L.; Welsh, K. M.; West, R.; Michl, J.
Organometallics 1989, 8, 2606.
(16) Bobbitt, K. L.; Maloney, V. M.; Gaspar, P. P. Organometallics
1991, 10, 2772.
(4) Neumann, W. P.; Weisbeck, M. P.; Wienken, S. Main Group Metal
Chem. 1994, 17, 151.
(5) Boganov, S. E.; Egorov, M. P.; Faustov, V. I.; Nefedov, O. M. In
The Chemistry of Organic Germanium, Tin and Lead Compounds; Rap-
poport, Z., Ed.; John Wiley and Sons: New York, 2002; Vol. 2, pp 749-
839.
(6) Sakurai, H.; Sakamoto, K.; Kira, M. Chem. Lett. 1984, 1379.
(7) Ando, W.; Tsumuraya, T.; Sekiguchi, A. Chem. Lett. 1987, 317.
(8) Tomoda, S.; Shimoda, M.; Takeuchi, Y.; Kajii, Y.; Obi, K.; Tanaka,
I.; Honda, K. Chem. Commun. 1988, 910.
(9) Ando, W.; Itoh, H.; Tsumuraya, T. Organometallics 1989, 8, 2759.
(10) Kolesnikov, S. P.; Egorov, M. P.; Dvornikov, A. S.; Kuz’min, V. A.;
Nefedov, O. M. Organomet. Chem. USSR 1989, 2, 414.
(17) Mochida, K.; Kikkawa, H.; Nakadaira, Y. J. Organomet. Chem.
1991, 412, 9.
(18) Mochida, K.; Tokura, S. Bull. Chem. Soc. Jpn. 1992, 65, 1642.
10.1021/om9000814 CCC: $40.75
2009 American Chemical Society
Publication on Web 05/06/2009

3240 Organometallics, Vol. 28, No. 11, 2009
Lollmahomed and Leigh
Most of the early solution-phase assignments were made in
conjunction with studies of the photochemistry of various group
14 catenates, such as the six- and five-membered cyclogermanes
products consistent with the formation of GeMe2 in high yields
in both cases, but they afforded markedly different results in
low-temperature matrix and solution-phase flash photolysis
experiments.13,18 For example, flash photolysis of 1 in cyclo-
hexane afforded a species decaying with second-order kinetics
and exhibiting λmax ≈ 450 nm; the decay of the species was
accompanied by the growth of absorptions assignable to
tetramethyldigermene (Ge2Me4; λmax ) 370 nm), and it was
therefore assigned to GeMe2.13 Flash photolysis of 2 under
similar conditions was reported to yield a transient exhibiting
λmax ≈ 500 nm, which was also assigned to GeMe2 based on
its kinetic behavior and the characteristic delayed formation of
the absorptions due to Ge2Me4.13,18 The spectra obtained upon
low-temperature matrix photolysis of the two compounds were
also quite different.13,18
Our own work in this area has employed the 1,1-dimethyl-
germacyclopent-3-ene derivatives 7a-c as photochemical pre-
cursors to GeMe2 (eq 2).26-29 These compounds distinguish
themselves from most of the earlier precursors by the fact that
they afford GeMe2 in essentially quantitatiVe chemical yields
and with quantum yields of ca. 50%.26,27 Laser flash photolysis
of all three compounds in hexane affords a species with a well-
defined UV-vis spectrum centered at λmax ) 470 nm26,27 (ε )
730 ( 300 dm3 mol-1 cm-1),27 which decays on the microsec-
ond time scale with second-order kinetics, coincident with the
growth of the characteristic absorptions due to Ge2Me4. The
spectrum of the first-formed species and the absolute rate
constants for its reaction with a variety of characteristic
germylene scavengers all correspond closely to the gas phase
data of Walsh and co-workers for GeMe2.1,19 The data also
correlate in rational ways with those for other transient ger-
mylenes such as GeMePh30 and GePh2,31 as well as the silicon
homologue, SiMe2.32-35 They do not agree with those assigned
to GeMe2 in any of the earlier solution-phase studies.
1
13 and 2,18 respectively, phenyl-substituted digermanes17,20 and
trigermanes11,12 (e.g., 3 and 4), and silylgermanes such as 5.16,17
In most cases, trapping experiments were consistent with the
formation of GeMe2 as a major (transient) photoproduct, and
this guided transient assignments in laser flash photolysis studies.
However, germylene formation was often accompanied by the
products of other, minor photolytic pathways, particularly with
phenylated systems. For example, the photolysis of silylgermane
5 was shown to yield trapping products consistent with the
formation of GeMe2 (55-78%16) and its coproduct (PhSiMe3),
germene 6 (20-28%16), and trace amounts of the dimethylphe-
nylgermyl and trimethylsilyl radicals (eq 1).16,17 Flash photolysis
of the compound in cyclohexane was found to afford a long-
lived transient with λmax ≈ 430 nm, which was assigned to
GeMe2 on the basis of the similarity of the spectrum to the 77
K hydrocarbon matrix spectrum, the indication from product
studies that the germylene is the major transient photoproduct,
and in one of the papers,16 an exhaustive list of rate constants
for reaction of the species with dienes and various other
substrates. We have suggested that the spectroscopic and kinetic
behavior of the species assigned to GeMe2 in these studies is
in fact more reasonably attributable to the corresponding
germene (6), which is derived from photochemical [1,3]-silyl
migration into the ortho-position of a phenyl ring in the
precursor.21 Silenes of this general structure are well known to
be formed in significant yields upon photolysis of phenylated
di- and trisilanes in solution at ambient temperatures22 and
similarly confounded early attempts to detect and study transient
phenylsilylenes by flash photolysis in solution23,24 owing to their
strong (π,π*) absorptions throughout the 400-500 nm spectral
range.25
While the assignment of GeMe2 in our laser photolysis
experiments with 7a-c seems compelling enough, it remains
to be ratified with corresponding spectral and kinetic data
obtained using a different, structurally unrelated precursor. We
note that a species with qualitatively similar spectral charac-
teristics has been detected in a recent reinvestigation36 of the
(26) Leigh, W. J.; Harrington, C. R.; Vargas-Baca, I. J. Am. Chem. Soc.
2004, 126, 16105.
(27) Leigh, W. J.; Lollmahomed, F.; Harrington, C. R. Organometallics
2006, 25, 2055.
(28) Leigh, W. J.; Lollmahomed, F.; Harrington, C. R.; McDonald, J. M.
Organometallics 2006, 25, 5424.
Some of the other early reports are more difficult to
rationalize. For example, photolysis of the cyclic GeMe2
oligomers 1 and 2 in cyclohexane was found to yield trapping
(19) Becerra, R.; Boganov, S. E.; Egorov, M. P.; Lee, V. Y.; Nefedov,
O. M.; Walsh, R. Chem. Phys. Lett. 1996, 250, 111.
(20) Mochida, K.; Wakasa, M.; Nakadaira, Y.; Sakaguchi, Y.; Hayashi,
H. Organometallics 1988, 7, 1869.
(21) Leigh, W. J.; Toltl, N. P.; Apodeca, P.; Castruita, M.; Pannell, K. H.
Organometallics 2000, 19, 3232.
(22) Ishikawa, M.; Kumada, M. AdV. Organomet. Chem. 1981, 19, 51.
(23) Gaspar, P. P.; Boo, B. H.; Chari, S.; Ghosh, A. K.; Holten, D.;
Kirmaier, C.; Konieczny, S. Chem. Phys. Lett. 1984, 105, 153.
(24) Gaspar, P. P.; Holten, D.; Konieczny, S.; Corey, J. Y. Acc. Chem.
Res. 1987, 20, 329.
(25) Leigh, W. J.; Moiseev, A. G.; Coulais, E.; Lollmahomed, F.; Askari,
M. S. Can. J. Chem. 2008, 86, 1105.
(29) Lollmahomed, F.; Huck, L. A.; Harrington, C. R.; Chitnis, S. S.;
Leigh, W. J. Organometallics 2009, 28, 1484.
(30) Leigh, W. J.; Dumbrava, I. G.; Lollmahomed, F. Can. J. Chem.
2006, 84, 934.
(31) Leigh, W. J.; Harrington, C. R. J. Am. Chem. Soc. 2005, 127, 5084.
(32) Levin, G.; Das, P. K.; Bilgrien, C.; Lee, C. L. Organometallics
1989, 8, 1206.
(33) Yamaji, M.; Hamanishi, K.; Takahashi, T.; Shizuka, H. J. Photo-
chem. Photobiol. A: Chem. 1994, 81, 1.
(34) Moiseev, A. G.; Leigh, W. J. Organometallics 2007, 26, 6268.
(35) Moiseev, A. G.; Leigh, W. J. Organometallics 2007, 26, 6277.
(36) Gorner, H.; Lehnig, M.; Weisbeck, M. J. Photochem. Photobiol.
A: Chem. 1996, 94, 157.

Studies of Some Dimethylgermylene Precursors
Organometallics, Vol. 28, No. 11, 2009 3241
Figure 1. Transient absorption spectra recorded by laser flash photolysis of deoxygenated solutions of dodecamethylcyclohexagermane (1)
in (a) hexanes (80-112 ns (O) and 2.0-2.1 µs (0) after the pulse) and (b) hexanes containing 15 mM THF (80-112 ns (O) and 5.38-5.41
µs (0) after the pulse). The insets show transient growth/decay profiles at 470, 370, and 310 nm, from the data sets used to construct the
spectra. The sharp spikes present at the beginning of the 470 nm decays are due to an ultrashort-lived species that decays with the laser
pulse; its spectrum (recorded at the peak of the laser pulse) is shown as the dashed lines in (a) and (b).
laser flash photolysis behavior of benzogermanorbornadiene 8,37
its absorptions superimposed on the much stronger, longer-lived
ones due to the triplet state of the coproduct of GeMe2 extrusion,
1,2,3,4-tetraphenylnaphthalene.36 Unfortunately however, no
kinetic data have been reported that might establish its identity
as the same species observed in gas-19 and solution-phase26,27
experiments with 1-germacyclopent-3-ene derivatives as GeMe2
precursors. Given the difficulties that we38 and others36 have
experienced in detecting transient germylenes from precursors
of this general structure, we decided to revisit two of the other
GeMe2 precursors that were reported in earlier studies.
et al.41 and was obtained in yields of 6-18% after purification
by column chromatography and multiple recrystallizations from
acetone. The procedure afforded the compound in g96% purity,
contaminated with e4% of cyclopentagermane 2, as determined
1
by H NMR spectroscopy and GC/MS. Unfortunately, a pure
sample of the latter compound could not be isolated from the
reaction mixtures in sufficient amounts to enable its further
study.
Laser flash photolysis of a flowed, deoxygenated solution of
1 (ca. 4.2 mM) in anhydrous hexanes, with the pulses from a
KrF excimer laser (248 nm, 25 ns, ca. 100 mJ/pulse), afforded
three distinct transient absorptions: one centered at λmax ) 490
nm that decayed with the laser pulse (τ e 10 ns), a second
centered at λmax ) 470 nm that decayed on the microsecond
time scale with second-order kinetics, and a third centered at
λmax ) 370 nm that grew in over the first ca. 2 µs after the
pulse and then decayed over ca. 100 µs, also with approximate
second-order kinetics. Figure 1a shows the time-resolved
UV-vis spectra of these species, recorded over various time
windows after the laser pulse, along with transient decay/growth
profiles at selected monitoring wavelengths to illustrate their
temporal behaviors. The spectra of the two longer lived
transients, recorded 80-112 ns and 2.0-2.1 µs after the laser
pulse, match almost precisely (in the region above ca. 320 nm)
those assigned by us previously to GeMe2 and Ge2Me4,
respectively, generated from the germacyclopenten-3-enes
7a-c.26,27 The present spectra also contain long-lived absorp-
tions in the 270-320 nm region that cannot be reliably assigned.
On the other hand, there are (at best) only qualitative similarities
between the spectra of Figure 1a and the previously reported
transient spectra from this compound;13 the latter showed
significantly more intense transient absorptions below 400 nm
than were observed in the present work, and a secondary band
centered at 450 nm, as mentioned above.
In this paper, we report the results of a reinvestigation of the
laser flash photolysis of dodecamethylcyclohexagermane (1),
carried out with the intent of refining (or otherwise explaining)
the transient spectrum that was published previously from this
compound.13 Considering the success that a number of groups
have had in laser flash photolysis studies of SiMe2 using the
silicon homologue of 1 as precursor,32-35,39,40 the lack of
agreement between the behavior reported for 113 and that of
the principle transient product in our experiments with 7a-c26-29
is particularly difficult to rationalize. We have also studied the
flash photolysis behavior of silylgermane 5, with the goal of
confirming our preliminary reassignment of the transient product
that dominates the time-resolved spectra obtained from this
compound21 and to see whether GeMe2 might in fact be
detectable in laser flash photolysis experiments, buried under
the much stronger absorptions due to the other transient
products.
The ultrashort-lived transient centered at ca. 490 nm, whose
spectrum is shown as the dashed line in Figure 1a, was evidently
not detected in the earlier study.13
A few quenching experiments were carried out to enable
further comparisons between the characteristics of the 470 nm
transient absorptions observed for 1 in the present work and
those reported using 7a-c26,27 as GeMe2 precursors. Addition
of 15 mM tetrahydrofuran (THF) quenched the 470 nm
Results and Discussion
1. Dodecamethylcyclohexagermane (1). Dodecamethylcy-
clohexagermane (1) was synthesized by the method of Carberry
(37) Kaletina, M. V.; Plyusnin, V. F.; Grivin, V. P.; Korolev, V. V.;
Leshina, T. V. J. Phys. Chem. A 2006, 110, 13341.
(38) Harrington, C. R.; Leigh, W. J.; Chan, B. K.; Gaspar, P. P.; Zhou,
D. Can. J. Chem. 2005, 83, 1324.
(39) Shizuka, H.; Tanaka, H.; Tonokura, K.; Murata, K.; Hiratsuka, H.;
Ohshita, J.; Ishikawa, M. Chem. Phys. Lett. 1988, 143, 225.
(40) Levin, G.; Das, P. K.; Lee, C. L. Organometallics 1988, 7, 1231.
(41) Carberry, E.; Dombek, B. D.; Cohen, S. C. J. Organomet. Chem.
1972, 36, 61.

3242 Organometallics, Vol. 28, No. 11, 2009
Lollmahomed and Leigh
its yield due to precursor quenching. Whatever the interaction
responsible for the effect, it is possible to say only that it occurs
with a bimolecular rate constant of ca. 2 × 109 M-1 s-1 or
greater. The intensity of the GeMe2 absorption at 470 nm also
appeared to be reduced modestly in O2-saturated hexanes
compared to deoxygenated solution, suggesting that the 490 nm
species might be the direct precursor to (singlet) GeMe2, but it
is again difficult to be certain of this because of the overall
weakness of the transient signals. Two reasonable possible
assignments for the 490 nm species are an excited state of 1
and the 1,6-biradical derived from Ge-Ge bond homolysis in
1 (i.e., 9; see eq 4). Better time resolution will be required for
even a tentative identification to be made of the short-lived
transient species detected in these experiments. It is worth noting
that Wakasa and co-workers recently reported evidence from
laser photolysis and product studies for the formation of a related
(1,4-) biradical from the photolysis of octa(isopropyl)cyclotet-
ragermane; the species exhibits a long-wavelength absorption
band centered at λmax ) 550 nm and lifetime τ ) 50 ( 5 ns in
cyclohexane at 25 °C.43
Figure 2. Plots of kdecay vs [Q], for quenching of the 470 nm
transient from laser photolysis of dodecamethylcyclohexagermane
(1) by 2,3-dimethyl-1,3-butadiene (DMB; O) and acetic acid
(AcOH; 0) in hexanes at 25 °C. The solid lines are the least-squares
fits of the data to eq 3.
absorption almost completely, replacing it with the longer lived
absorption centered at λmax ) 310 nm due to the GeMe2-THF
Lewis acid-base complex28 and causing a significant lengthen-
ing of the growth time of the Ge2Me4 signal; the relevant series
of transient spectra is shown in Figure 1b. The very weak signal
at 470 nm that remains detectable in the presence of THF (see
Figure 1b, inset) may be due to free GeMe2, present in
equilibrium with the complex (Keq ) 9800 ( 3800 M-1).28
These results are in excellent agreement with those reported
previously using 7b as precursor to GeMe2.28 Addition of acetic
acid (AcOH; 0.05-1 mM), 2,3-dimethyl-1,3-butadiene (DMB;
0.1-1 mM), or carbon tetrachloride (CCl4; 1-65 mM) to hexane
solutions of 1 caused enhancements in the decay rate of the
470 nm absorption and quenched the formation of the 370 nm
(Ge2Me4) absorption; the decay at 470 nm followed clean first-
order kinetics once sufficient substrate had been added to reduce
the maximum signal intensity at 370 nm to ca. 50% or less of
its value in pure hexanes. Plots of the pseudo-first-order decay
rate constant (kdecay) vs substrate concentration were linear;
analysis of the data according to eq 3 (where kQ is the second-
order rate constant for quenching by the substrate, Q, and k0 is
the (hypothetical) pseudo-first-order decay rate constant in the
absence of Q) afforded slopes of kAcOH ) (9.4 ( 0.9) × 109
M-1 s-1, kDMB ) (1.4 ( 0.1) × 1010 M-1 s-1, and kCCl4 ) (9 (
1) × 107 M-1 s-1 (see Figure 2 and Figure S1, Supporting
Information). Saturation of the solution with oxygen ([O2] )
15.7 mM42) had similar effects on the 370 and 470 nm signals
In any event, it is clear that the species that was assigned to
GeMe2 in the earlier flash photolysis study of 113 is not the
same as the one observed in the present work from the same
precursor; as indicated above, the latter corresponds closely to
the primary transient product obtained in similar experiments
with 7a-c in every respect.26,27 In addition to the difference in
the UV-vis spectra, there is also no correspondence between
the quenching rate constants reported for the (450 nm) species
detected in the earlier work by O2 (k ) 9.7 × 108 M-1 s-1),
CCl4 (k ) 4.9 × 108 M-1 s-1), and DMB (k ) 2.2 × 107 M-1
s-1 13 and those reported by us for quenching of GeMe2 by the
)
same substrates in hexane at 25 °C, in both the present and
previous studies.27 It is difficult to speculate on the true identity
of the reactive intermediate that was reported in the earlier work,
or its origins. We note that the earlier experiments were
evidently collected using static samples to which several
excitation pulses were delivered, thus allowing the buildup of
photoproducts that could very well lead to transient photoprod-
ucts of their own. In our experience, flowed sample delivery
and strictly anhydrous conditions are both essential for the
successful detection of transient germylenes in solution.
2. Dimethylphenyl(trimethylsilyl)germane (5). Laser flash
photolysis of a flowed, deoxygenated solution of 5 (ca. 0.6 mM)
in anhydrous hexanes led to the formation of prominent transient
absorptions centered at λmax ≈ 300 nm and λmax ) 430 nm,
which decayed on the microsecond time scale with mixed-order
kinetics (see Figure 3). The behavior is in reasonably good
agreement with that reported previously for this compound in
cyclohexane at ambient temperatures.16,17 Interestingly, an
-1
and afforded an estimate of kO2 ≈ 1.4 × 108 M s-1 for the
rate constant for reaction of O2 with the 470 nm transient, from
the pseudo-first-order decay rate constant of the 470 nm
absorption (kdecay ) 2.1 × 106 s-1). The values of these four
rate constants are also in excellent agreement with those assigned
previously to GeMe2, using 7b and 7c as precursors under
similar conditions.26,27
kdecay ) k0 + kQ[Q]
(3)
The intensity of the short-lived 490 nm absorption appeared
to be reduced significantly in O2-saturated hexanes, though
because its decay coincides with that of the laser pulse, it is
not clear whether this is due to a shortening of the lifetime of
the species responsible for the absorption or to a reduction in
(42) Battino, R.; Rettich, T. R.; Tominaga, T. J. Phys. Chem. Ref. Data
1983, 12, 163.
(43) Wakasa, M.; Takamori, Y.; Takayanagi, T.; Orihara, M.; Kugita,
T. J. Organomet. Chem. 2007, 692, 2855.

Studies of Some Dimethylgermylene Precursors
Organometallics, Vol. 28, No. 11, 2009 3243
Figure 3. Transient absorption spectra from laser flash photolysis
of a solution of PhMe2GeSiMe3 (5; ∼0.6 mM) in deoxygenated
hexanes at 25 °C, recorded 0.06-0.13 µs (O) and 8.88-8.94 µs
(0) after the laser pulse. The inset shows transient decay traces
recorded at selected monitoring wavelengths between 300 and 480
nm.
Figure 4. Plots of kdecay vs [Q], for quenching of the 430 nm
transient from laser photolysis of 5 by DMB (O) and CCl4 (0) in
hexanes at 25 °C. The solid lines are the least-squares fits of the
data to eq 3.
the presence of oxygen resulted in reductions of the initial
signal intensities of both the 300 and 430 nm absorptions,
the effect on the 300 nm absorption appearing to be
considerably greater than that on the longer wavelength one.
As a result, while both species remained detectable in air-
saturated solution, only the 430 nm transient could be
detected in O2-saturated hexanes (see Figure S3, Supporting
Information). A 3-point plot of kdecay vs [O2] for the 430 nm
transient was linear, affording a slope of kO2 ) (3.8 ( 0.2)
× 108 M-1 s-1 (Figure S4, Supporting Information), roughly
5 times smaller than that reported for the same process in
the earlier studies.16,17 A bigger discrepancy exists in regards
to the effect of oxygen on the 300 nm species, for which a
quenching rate constant of kO2 ) 1.0 × 109 M-1 s-1 was
reported.17 This leads to an expected lifetime of τ ≈ 300 ns
in air-saturated hexanes, which is significantly shorter than
what we observe (see Figure S3a, Supporting Information).
The different effects of O2 on the intensities of the 300 and
430 nm absorptions could be the result of considerably faster
quenching of the 300 nm species than what was originally
reported17 or could be due to the two species originating from
different excited states of the precursor, one more efficiently
quenched by oxygen than the other. We did not pursue these
possibilities further.
absorbance-time profile recorded at 370 nm (the absorption
maximum of Ge2Me4) showed a slight hint of an initial growth
and decayed over a significantly longer time scale than the
absorptions at 300 and 430 nm, which is suggestive of the
formation of Ge2Me4 subsequent to the excitation pulse via
dimerization of GeMe2; the associated absorption band is
(barely) discernible at ca. 360 nm in the 8.9 µs spectrum of
Figure 3.
The growth component disappeared upon addition of 0.5-3.0
mM AcOH, consistent with the assignment of the species to
Ge2Me4, but decay traces recorded at 470-480 nm showed no
evidence of a minor short-lived component that could be
attributed to absorption by the germylene, at any AcOH
concentration studied. It can be concluded that the transient
absorptions due to GeMe2 are too weak relative to those of the
other transient products for the germylene to be detected in
experiments with this compound.
Transient absorption spectra recorded in hexanes containing
3.0 mM AcOH (Figure S2, Supporting Information) verified
that the two prominent absorption bands in the spectra of Figure
3 are due to two distinct species, one that is unaffected by AcOH
(λmax ) 300 nm; τ ≈ 20 µs) and one (λmax ) 430 nm) that is
quenched by the carboxylic acid. An upper limit of kAcOH < 1
× 108 M-1 s-1 was estimated for the rate constant for quenching
of the latter species by AcOH, from lifetimes measured at low
laser intensities in order to minimize second-order contributions
to the decay. Transient decay traces recorded at 300-310 nm
showed a minor fast decay component in the presence of AcOH,
suggesting that a small component of the transient absorption
in the 280-320 nm range may be due to a higher energy
absorption band associated with the 430 nm species. Mochida
and co-workers assigned the prominent component of the short-
wavelength absorption to the dimethylphenylgermyl radical,17
based on comparisons of the spectrum and the reactivity of the
species (with DMB, CCl4, and O2) to previously reported data
for this and other germyl-centered radicals.20,44,45
As was reported previously,16,17 the lifetime of the 430
nm transient was also shortened in the presence of CCl4 and
DMB. Plots of kdecay vs substrate concentration (Figure 4)
were linear, affording values of kCCl4 ) (1.3 ( 0.1) × 108
M-1 s-1 and kDMB ) (1.4 ( 0.1) × 107 M-1 s-1. The rate
constant for quenching by the diene is in good agreement
with the earlier reported values,16,17 while that for CCl4
quenching is 2-4 times smaller. The lifetime of the 430 nm
species was also shortened by added acetone, and a plot of
kdecay vs [Q] was linear with slope kacetone ) (2.1 ( 0.5) ×
106 M-1 s-1 (see Figure S8, Supporting Information). Flash
photolysis of 1 in deoxygenated THF led to quite similar
transient absorption spectra and lifetimes to those obtained
in hexanes under similar conditions (Figure S5a, Supporting
Information). In particular, the position of the 430 nm
absorption band remained unchanged in THF relative to that
in hexane, indicating little or no susceptibility of the species
responsible toward complexation with the ether solvent. This
eliminates the possibility that the 430 nm absorption is due
to a weakly bound complex of GeMe2 with the aromatic
Transient spectra recorded in air- and O2-saturated hexane
solution afforded somewhat different results from those
expected based on the earlier studies of 5.16,17In our hands,
(44) Chatgilialoglu, C.; Ingold, K. U.; Lusztyk, J.; Nazran, A. S.;
Scaiano, J. C. Organometallics 1983, 2, 1332.
(45) Mochida, K.; Wakasa, M.; Sakaguchi, Y.; Hayashi, H. J. Am. Chem.
Soc. 1987, 109, 7942.

3244 Organometallics, Vol. 28, No. 11, 2009
Lollmahomed and Leigh
Table 1. Absolute Rate Constants for Quenching of Dimethyl- and Diphenyl-Substituted 1-Metallahexatrienes in Hydrocarbon Solvents
a In hexanes at 25 °C (this work). b In isooctane at 21 °C (ref 46). c In hexane at 23 °C (ref 21).
precursor (5),14 since such a species could not possibly
survive in THF, itself a potent complexing agent with
transient germylenes such as GeMe2.9,28 Interestingly, satura-
tion of the solution with O2 led to almost no change in the
relative signal intensities at 300 and 430 nm (Figure S5b,
Supporting Information), unlike the behavior in hexane,
where the intensity at 300 nm was reduced preferentially in
the presence of O2 (Vide supra). This is due in part to the
presence of an additional component of the 300 nm absorption
that was not present in the spectra recorded in O2-saturated
hexanes and which exhibits a lifetime of τ ≈ 5 µs. Though
we note that the lifetime of this component is similar to that
observed for the GeMe2-THF complex under these condi-
tions,29 such an assignment would be difficult to prove
because of the complexity of the absorptions in the 300 nm
region. The lifetime of the 430 nm species was shortened to
τ ≈ 160 ns in O2-saturated THF ([O2] ) 10.1 mM42),
consistent with a quenching rate constant of kO2 ≈ 6 × 108
M-1 s-1, similar to the value obtained for the species in
hexanes (Vide supra).
Table 1summarizes the absolute rate constants for reaction
of O2, AcOH, CCl4, acetone, and DMB with the 430 nm
transient from laser photolysis of silylgermane 5 in hexanes
at 25 °C. An assignment of this species to germene 6 can be
made on the basis of a comparison of these data to the
analogous values for reaction of the same substrates with
the homologous silene 11,46,47 the major product of photolysis
of disilane 10 (eq 5),22,48 coupled with a more general
knowledge of the quantitative differences in the reactivities
of homologous silenes and germenes;49,50 the requisite kinetic
data for silene 11 are included in Table 1 to facilitate the
comparison. The trends can be expected to parallel those
found in our earlier study of germene 12,21 which was
identified on the basis of similar comparisons, so we have
included the corresponding data for it and its silicon
homologue (13)21,46 in the table as well. The products of
these reactions have all been documented for the two silenes;
in the cases of the germenes the same is true only for DMB,
for which the (ene-) reactions with 616,17 and 1121 proceed
in identical fashion to those with 12 and 13.25,46
The first set of comparisons to be made relates to the effects
of phenyl-for-methyl substitution on the spectra and reactivity
of silenes and germenes of otherwise identical structures. In
silenes, phenyl-for-methyl substitution at the silenic silicon
atom results in a substantial red-shift in the lowest energy
(π,π*) absorption band and a moderate decrease in reactiv-
ity,51 as exemplified by the differences in the UV-vis spectra
of silenes 11 and 13 and the rate constants for reaction with
each of the five substrates considered in this work. Com-
parison of the corresponding parameters for the germenes
(6 and 12) shows that they follow the same general trends
as do the homologous silenes. The red-shift in the absorption
spectrum of 12 relative to that of 6 is somewhat smaller than
that exhibited by the silenes, which can be explained as being
due to poorer π-overlap between the phenyl substituents and
the GedC bond compared to the situation with the silenes.
This appears to be a general phenomenon, as the same
differences are also evident in the UV-vis absorption spectra
of homologous disilenes and digermenes, as well as silylenes
and germylenes.34
The second point of comparison focuses on the differences
in reactivity within the two homologous pairs of metallaenes,
6/11 and 12/13. The differences in the rate constants within the
two pairs of compounds mirror each other quite closely, and
are broadly similar to those observed previously with simpler
(46) Leigh, W. J.; Sluggett, G. W. Organometallics 1994, 13, 269.
(47) Leigh, W. J.; Sluggett, G. W. J. Am. Chem. Soc. 1994, 116, 10468.
(48) Steinmetz, M. G. Chem. ReV. 1995, 95, 1527.
(49) Toltl, N. P.; Leigh, W. J. J. Am. Chem. Soc. 1998, 120, 1172.
(50) Leigh, W. J.; Potter, G. D.; Huck, L. A.; Bhattacharya, A.
Organometallics 2008, 27, 5948.
(51) Morkin, T. L.; Leigh, W. J. Acc. Chem. Res. 2001, 34, 129.

Studies of Some Dimethylgermylene Precursors
Organometallics, Vol. 28, No. 11, 2009 3245
metallaene homologues such as Ph2SidCH2 and Ph2Ged
CH2.49,50,52,53 In both cases, the germene is consistently less
reactive than the homologous silene; the difference is largest
for reactions in which the polarity of the MdC bond and Lewis
acidity at the metal center play the dominant role in affecting
the rate constant, such as that with acetone and acetic acid,50,54
and smallest for reactions that proceed through radical or
diradical intermediates, such as those with CCl4 and O2.25,47 It
should be noted that the presence of the cyclohexadienyl
substituent at the metallaenic carbon in silenes of this type leads
to considerably greater reactivity toward O2 and CCl4 than is
typically observed for simpler silenes without radical-stabilizing
substituents at this position,55 and the rate constants vary much
more modestly with substituent-induced changes in SidC bond
polarity than do those for reaction with polar substrates such
as alcohols, acetic acid, and acetone.25 Thus, little difference
in reactivity between homologous germenes and silenes is
expected toward these two substrates, as is indeed observed.
The differences are somewhat larger for the ene reaction with
DMB, which proceeds with transfer of the allylic hydrogen in
the cyclohexadienyl substituent and aromatization of the ring
(see eq 6), but is still relatively small compared to the effects
on the kinetics of reactions with O-donors. With silenes, the
latter reactions are initiated by Lewis acid-base complexation
at the silenic silicon atom, and thus their rate constants are
strongly affected by substituent-induced changes in SidC bond
polarity.25 In general, reactions of this type are dramatically
slower with germenes than with the corresponding silene
homologues, owing to the substantially lower Lewis acidities
of germenes compared to silenes of otherwise identical
structures.49,50 The ca. 5000-fold smaller values of the rate
constants for quenching of 6 and 12 by acetone compared to
the corresponding values for the homologous silenes, as well
as the extremely low reactivity of 6 toward ethanol,16 are thus
also consistent with what has been reported previously for
simpler derivatives.49,50 The substantial difference in the Lewis
acidities of germene 6 and silene 11 is also manifested in their
UV-vis spectra in THF and hexane; the absorption maximum
of 11 is shifted from 425 nm in hexane to 460 nm in THF,46
while that of 6 is unaffected by the change in solvent.
acid, DMB, and CCl4, and its behavior in the presence of
millimolar concentrations of THF all agree quite closely with
those observed in similar experiments with 1,1-dimethylger-
macyclopent-3-ene derivatives as GeMe2 precursors.26-28 The
results suggest that the photolysis of 1 yields GeMe2 in
reasonably high quantum efficiency, with the only transient side-
product being an ultrashort-lived species whose UV-vis
spectrum overlaps with that of (singlet) GeMe2. The identity of
this species is not yet clear, but it is sufficiently short-lived
(τ e 10 ns) that its presence does not interfere greatly in kinetic
studies of the reactions of GeMe2 with added substrates.
Laser flash photolysis of silylgermane 5 under similar
conditions allows the detection of two main transient products,
as reported in the earlier studies of this compound in
cyclohexane.16,17 One of these was assigned previously to the
dimethylphenylgermyl radical (λmax ) 300 nm),17 while the other
(λmax ) 430 nm) was assigned to GeMe2.16,17 The latter species
is re-assigned in the present study to the transient 1,1-dimethyl-
1-germahexatriene derivative 6, formed via [1,3]-trimethylsilyl
migration into an ortho-position of the phenyl ring in the lowest
excited singlet state of 5. The germene (6) assignment for this
species is based on comparisons of its UV-vis spectrum and
reactivity with those of the homologous silene derivative
(11),46as well as with those of the analogous 1,1-diphenyl-
1-metallahexatriene homologues, germene 12 and silene 13.21
While GeMe2 is known to be formed as a major product of
photolysis of 5,16,17 the species cannot be detected at all in
laser photolysis experiments with this compound because of
the overlapping, much stronger absorptions due to the minor
transient coproduct, 6.
The various discrepancies that have been noted between the
flash photolysis behavior exhibited by 1 and 5 in the present
study and those reported in the earlier studies of these
compounds are most likely due to differences in the way the
experiments were carried out. In our experience, flowed sample
delivery is absolutely essential in experiments involving silicon-
and germanium-containing reactive intermediates (of any type),
to avoid the buildup of reaction products that are very frequently
also photoreactive and which lead to complications in experi-
ments requiring multiple laser shots for the acquisition of data.
A second complicating factor results from the high sensitivity
of transient germylenes to water and other hydroxylated
substrates, due to rapid and reversible Lewis acid-base com-
plexation. The kinetic and thermodynamic details of these
processes make (free) transient germylenessGeMe2 in particulars
uniquely difficult to detect by UV-vis spectroscopy unless
strictly anhydrous conditions are employed. These crucial
experimental details could not have been appreciated at the time
the original experiments were carried out.
Summary and Conclusions
Laser flash photolysis of dodecamethylcyclohexagermane (1)
in hexane solution allows the detection of the characteristically
weak UV-vis absorption spectrum due to GeMe2, which is
centered at λmax ) 470 nm and decays over several microseconds
with second-order kinetics, coincident with the growth of the
longer lived absorption centered at λmax ) 370 nm due to the
(transient) germylene dimer, tetramethyldigermene (Ge2Me4).
The spectrum and basic kinetic behavior, as well as the absolute
rate constants for reaction of the 470 nm species with O2, acetic
Experimental Section
1H NMR and 13C NMR spectra were recorded on a Bruker
AV200 spectrometer in CDCl3 solution and are referenced relative
to tetramethylsilane using the solvent residual proton or 13C peak
as calibrant. 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 µm; Varian Inc.). Hexanes (EMD
Omnisolv) was dried by passage through a Solv-Tek solvent
purification system and contained <10 µM water, as measured by
Karl Fischer titration. Tetrahydrofuran (Caledon reagent; HPLC
grade) was dried by refluxing over sodium metal for several days
and distilled. CCl4 was refluxed over phosphorus pentoxide and
distilled. Acetone was refluxed over anhydrous potassium carbonate
and distilled. Glacial acetic acid (Caledon) was used as received
(52) Leigh, W. J. Pure Appl. Chem. 1999, 71, 453.
(53) Toltl, N. P.; Stradiotto, M. J.; Morkin, T. L.; Leigh, W. J.
Organometallics 1999, 18, 5643.
(54) Mosey, N. J.; Baines, K. M.; Woo, T. K. J. Am. Chem. Soc. 2002,
124, 13306.
(55) Morkin, T. L.; Owens, T. R.; Leigh, W. J. In The Chemistry of
Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; John Wiley
and Sons: New York, 2001; Vol. 3, pp 949-1026.

3246 Organometallics, Vol. 28, No. 11, 2009
Lollmahomed and Leigh
from Sigma-Aldrich. 2,3-Dimethyl-1,3-butadiene (DMB; Aldrich)
was purified by passage of the neat liquid through a silica gel
microcolumn. Phenylpentamethyldisilane was prepared by the
published method.56
wavelength (248 nm) was between ca. 0.7 and 0.9. The solutions
were pumped continuously through a thermostated 7 × 7 mm
Suprasil flow cell connected to a calibrated 100 mL reservoir using
a Masterflex 77390 peristaltic pump fitted with Teflon tubing (Cole-
Parmer Instrument Co.), at an appropriate flow rate to ensure
reproducible signal strengths and decay kinetics from one laser shot
to the next (typically 3-5 mL/min). The calibrated reservoir was
fitted with a glass frit to allow bubbling of argon, air, or oxygen
gas through the solution for at least 30 min prior to and then
throughout the duration of each experiment. The glassware, sample
cells, and transfer lines used for these experiments 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. Reagents were added
directly to the reservoir by microliter syringe as aliquots of standard
solutions. Solution temperatures were measured with a Teflon-
coated copper/constantan thermocouple inserted in the thermostated
sample compartment in close proximity to the sample flow cell
and are considered accurate to within 2 °C.
Dodecamethylcyclohexagermane (1) was synthesized from dichlo-
rodimethylgermane (7.58 g, 0.046 mol; Gelest) and lithium powder
(2.45 g, 0.11 mol, 30% dispersion in mineral oil) in anhydrous THF,
according to the procedure of Carberry et al.41 Silica gel column
chromatography using hexanes as eluant, followed by several
recrystallizations from acetone, afforded a mixture (0.26 g, 6%,
mp 206-210 °C; 1H NMR, δ 0.34 (s)) containing g96% of
dodecamethylcyclohexagermane (1) and e4% of decamethylcy-
clopentagermane (2), as determined by GC/MS. The mass spectra
of the two compounds agreed well with the reported spectra.41
A
second preparation using chopped lithium wire instead of powder
afforded the compound in 18% yield after purification as described
above. Vacuum sublimation of a portion of the sample afforded
material exhibiting mp 195-204 °C, which GC/MS analysis
revealed consisted of g99% 1. The two preparations afforded almost
identical results in subsequent laser flash photolysis experiments.
Dimethylphenyl(trimethylsilyl)germane (5) was synthesized from
chlorodimethylphenylgermane (1.5 g, 0.0069 mol; Gelest), chlo-
rotrimethylsilane (3.02 g, 0.028 mol; Aldrich), and lithium wire
(0.19 g, 0.028 mol) in anhydrous THF, according to the procedure
of Bobbitt et al.16 Silica gel column chromatography using hexanes
Transient decay rate constants were calculated by nonlinear least-
squares analysis of the 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. Quenching rate constants were
calculated by linear least-squares analysis of decay rate-concentration
data that incorporated at least 7 points and spanned as large a range
in transient decay rate as possible. Errors are quoted as twice the
standard error from the least-squares analysis, and thus reflect the
precision associated with the individual experiment.
1
as eluant afforded the product as a colorless oil (0.6 g, 34%; H
NMR, δ 0.18 (s, 9H), 0.44 (s, 6H), 7.31-7.35 (m, 3H), 7.43-7.46
(m, 2H); 13C NMR, δ -4.55 (GeMe2), -1.54 (SiMe3), 127.58
(para), 127.83 (ortho or meta), 133.55 (ortho or meta), 142.36 (ipso);
MS, m/z (I) 254 (14; M+), 252 (12), 250 (6), 239 (52; (M - Me)+),
237 (39), 235 (23), 181 (19; (M - SiMe3)+), 179 (14), 177 (11),
136 (16), 135 (100), 75 (13), 73 (34), 45 (22), 44 (7), 43 (19). The
spectra agree well with those reported by Bobbitt et al.,16 except
for minor differences in the 13C NMR spectra.
Laser flash photolysis experiments employed the unfocused
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 system, modi-
fied as described previously.26 The system employs signal averaging
(of 5-30 individual experiments, carried out a repetition rate of
ca. 0.3 Hz) for the acquisition of transient absorbance-time profiles
at selected monitoring wavelengths; spectra over various time
windows after the laser pulse are obtained from a series of such
profiles, recorded at predefined monitoring wavelengths throughout
the desired range in an automated sequence. Sample solutions were
prepared at concentrations such that the absorbance at the excitation
Plots of the maximum transient signal intensities at 450, 470,
and 490 nm from laser photolysis of 1 (in hexanes) vs laser pulse
energy, where the latter was varied using neutral density filters,
were linear in each case, verifying that both the 470 and 490 nm
transients are derived from single-photon processes. The upper limit
of 10 ns for the lifetime of the 490 nm species was derived from
nonlinear least-squares analysis of transient decays recorded with
a sampling frequency of 0.64 ns/pt and a bandwidth of 300 MHz.
Acknowledgment. We thank P. P. Gaspar for helpful
comments, the Natural Sciences and Engineering Research
Council of Canada for financial support, and Teck-Cominco
Metals, Inc. for a generous gift of germanium tetrachloride.
Supporting Information Available: Time-resolved UV-vis
absorption spectra and details of kinetic analyses. This material is
available free of charge via the Internet at http://pubs.acs.org.
(56) Gilman, H.; Peterson, D. J.; Wittenberg, D. Chem. Ind. 1958, 1479.
OM9000814