Chlorinative cleavage of germanium-germanium bonds in permethyloligogermanes initiated by photo-induced electron transfer

Article abstract of DOI:10.1016/S0022-328X(98)00709-8

Irradiation for permethyloligogermanes, Me(Me₂Ge)_nMe (n=2-5), in CCl₄-CH₃CN in the presence of 9,10-dicyanoanthracene (DCA) afforded the corresponding chlorogermanes and hexachloroethane. Fluorescence of DCA was quenched by the oligogermane with a diffusion-controlled rate. A mechanism of free-radical chlorination involving oligogermane cation radicals and a DCA anion radical is proposed for the germanium-germanium cleavage.

Full text of DOI:10.1016/S0022-328X(98)00709-8

Journal of Organometallic Chemistry 568 (1998) 121–125
Chlorinative cleavage of germanium–germanium bonds in
permethyloligogermanes initiated by photo-induced electron transfer
Kunio Mochida a,*, Hideaki Watanabe a, Shigeo Murata b, Mamoru Fujitsuka c, Osamu Ito c
a Department of Chemistry, Faculty of Science, Gakushuin. Uni6ersity, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan
b National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305-0046, Japan
c Institute for Chemical Reaction Science, Tohoku Uni6ersity, Katahira, Aoba-ku, Sendai 980-8577, Japan
Received 17 March 1998; received in revised form 18 May 1998
Abstract
Irradiation for permethyloligogermanes, Me(Me2Ge)nMe (n=2–5), in CCl4–CH3CN in the presence of 9,10-dicyanoanthracene
(DCA) afforded the corresponding chlorogermanes and hexachloroethane. Fluorescence of DCA was quenched by the oligoger-
mane with a diffusion-controlled rate. A mechanism of free-radical chlorination involving oligogermane cation radicals and a
DCA anion radical is proposed for the germanium–germanium cleavage. © 1998 Elsevier Science S.A. All rights reserved.
Keywords: Permethyloligogermanes; 9,10-Dicyanoanthracene; Photo-induced electron transfer
1. Introduction
2. Results and discussion
Organometallic compounds containing Group 14 ele-
ment–Group 14 element and Group 14 element–car-
bon bonds are excellent electron donors [1]. As
electron-rich species, they are subject to cleavage by
various organic electrophiles as well as transition metal
complexes. Since Group 14 catenates and tetraalkyl
Group 14 element compounds have rather low ioniza-
tion potentials (IPs), electron-transfer (ET) reactions
are very important in which the rate is limited by the
ability of the Group 14 element compound to transfer
an electron to an electron acceptor. While the ET
interactions between silicon–silicon | bonds and cer-
tain y acceptors have been amply investigated, there
have been few reports on the ET study of germanium–
germanium | bonds [2–7].
2.1. Chlorinati6e clea6age of permethyloligogermanes
Permethyloligogermanes, Me(Me2Ge)nMe (n=2–5),
which have low IPs (7.67–8.58 eV), are excellent elec-
tron donors [8,9]. Therefore, donor–acceptor interac-
tions with certain y acceptors are possible for the
Me(Me2Ge)nMe.
Chlorinative cleavage of the Me(Me2Ge)nMe (n=2–
5) initiated by photo-induced ET in the presence of
catalytic amounts (0.1 mol eq.) of several y electron
acceptors was examined (Eq. (1)).
hw(u\300 nm)
Me(Me2Ge)nMe+Acceptorꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢁ
0°C, 1 h
CCl /CH CN
4
3
Me(Me2Ge)mCl+Me(Me2Ge)n−mCl+(CCl3)2
(1)
We describe herein the studies on chlorinative cleav-
age of the germanium–germanium | bonds in perme-
thyloligogermanes initiated by photo-induced ET.
On irradiation for a typical permethyloligogermane,
octamethyltrigermane (Me(Me2Ge)3Me), in carbon te-
trachloride (CCl4, 0.43 ml)-acetonitrile (CH3CN, 10 ml)
in the presence of 9,10-dicyanoanthracene (DCA) (0.1
mol eq.) with a high-pressure Hg arc lamp through a
* Corresponding author.
0022-328X/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(98)00709-8

122
K. Mochida et al. / Journal of Organometallic Chemistry 568 (1998) 121–125
Table 1
Chlorinative cleavage of the Ge–Ge bond of Me(Me2Ge)3Me
Acceptor
Ered (V)
Conv. (%)
Products, yield (%)a
Me3GeCl
Me5Ge2Cl
5.7
(CCl3)2
Trace
−2.29
6.0
4.6
−1.93
35.0
70.0
28.6
35.7
67.9
11.0
13.0
b
—
54.3
65.3
−0.89
70.0
69.3
26.5
a
GC and NMR yields.
b No value.
Pyrex filter at 0°C for 1 h, trimethylchlorogermane
(Me3GeCl, 65.3%) and pentamethylchlorodigermane
(Me5Ge2Cl, 69.3%) were formed together with hex-
achloroethane (Cl3CCCl3, 26.5%). The use of less than
10 mol% of DCA took longer reaction time. The reac-
tion did not occur without DCA under the same condi-
tions and it is clear that DCA should be excited first by
light of wavelength longer than 300 nm.
In mixture with other y acceptors such as naph-
thalene (Np), anthracene (An), and 1,2-dicyanonaph-
thalene (DCN) in CCl4–CH3CN, Me(Me2Ge)3Me also
afforded Me3GeCl (4.6, 28.6 and 54.3%) and Me5Ge2Cl
(5.7, 35.7, and 67.9%), respectively, together with
(CCl3)2. The radical nature of the intermediate is confi-
rmed by the formation of both chlorogermanes and
(CCl3)2 in the presence of CCl4.
and selectivity of the chloronative Ge–Ge bonds in
oligogermanes are summarized in Table 2.
From Table 2 the conversions of Me(Me2Ge)nMe
(n=2–5) roughly accord with their IPs, which decrease
as the chain length increases [9].
A clear selectivity of chlorinative cleavage between
the terminal Ge–Ge bond and the internal Ge–Ge
bond could not be observed in Me(Me2Ge)nMe (n=3–
5); this may be due to the longer Ge–Ge bond length.
2.2. Quenching experiments of DCA
In order to confirm the ET from the Me(Me2Ge)nMe
to the excited state of DCA, we then carried out the
quenching experiments of the fluorescence of DCA in
CH3CN with Me(Me2Ge)nMe. A CH3CN solution of
DCA was excited at 380 nm and the emission band
(410–560 nm, umax=434 nm) was quenched with
Me(Me2Ge)nMe to yield a good linear relationship of
the Stern–Volmer plot with a slope of kq~. Here, kq is
the fluorescence quenching rate constant, and ~ is the
fluorescence lifetime of DCA in the absence of
quencher. The value of ~ was measured to be 12 ns [11].
These results are summarized in Table 1.
From Table 1 it is evident that the conversions of
Me(Me2Ge)3Me roughly accord with electron affinity of
the acceptors [10].
Photolysis of Me(Me2Ge)nMe (n=2, 4, 5) in a mixed
solvent of CCl4–CH3CN in the presence of DCA (0.1
mol eq.) was also carried out. These results of products

K. Mochida et al. / Journal of Organometallic Chemistry 568 (1998) 121–125
123
Table 2
Chlorinative cleavage of the Ge–Ge bond of Me(Me2Ge)nMe (n=2–5) in CCl4–CH3CN
Oligogermane
IPs
Conv. (%)
Products, yield (%)a
Me3GeCl
Me5Ge2Cl
Me7Ge3Cl
Me9Ge4Cl
(CCl3)2
Me(Me2Ge)2Me
Me(Me2Ge)3Me
Me(Me2Ge)4Me
Me(Me2Ge)5Me
8.58
8.15
7.80
7.67
12.0
70.0
75.0
97.0
9.4
65.3
48.8
56.2
Trace
26.5
8.0
69.3
56.2
38.8
52.5
33.0
57.2
7.0
a GC and NMR yields.
The values of IPs and kq of Me(Me2Ge)nMe (n=2–5)
are summarized in Table 3.
2.4. Reaction mechanism
The value of kq of Me(Me2Ge)nMe increased with
decreasing of IPs as shown in Fig. 1 (r=0.971). This is
the order of increasing length of the quencher,
Me(Me2Ge)nMe (n=2–5). This indicates the quench-
ing rate is below the diffusion limit.
At first, ET from Me(Me2Ge)nMe to the excited
singlet state of DCA (DCA*) generates the geminate
radical ions composed of the DCA radical anion
(DCA−
) and the Me(Me2Ge)nMe radical cation
’
+
(Me(Me2Ge)nMe+ ). The Me(Me2Ge)nMe
’
’
under-
goes spontaneous scission to Me(Me2Ge)m+ and
Me(Me2Ge)n−m . For example, MINDO UHF calcula-
2.3. Laser flash photolysis
’
tion of the Me(Me2Ge)3Me cation radical shows that
the unpaired electron is nearly localized on the p orbital
of a Ge atom of the Me5Ge2 moiety, while the charge is
localized in the Me3Ge moiety in the most stable
configuration. This scission is also supported by MS
Laser flash photolysis (u=354 nm, pulse width 5 ns,
power 18 mJ pulse−1) of the CH3CN solution of
Me(Me2Ge)4Me with DCA was carried out to obtain
information on the reactive intermediates in the ET
reaction. Fig. 2 shows the transient absorption spectra
obtained after excitation at 100 ns and 1 ms. The broad
absorption band extending from 600 to 800 nm is the
characteristic band of the anion radical of DCA [12].
The time profile of the absorption band observed at 700
nm is included in Fig. 2. The decay of the anion radical
of DCA at 700 nm is almost constant in an acquisition
time.
’
spectra of Me(Me2Ge)3Me. Me(Me2Ge)n−m abstracts
a chlorine atom from CCl4 to afford Me(Me2Ge)n−mCl
+
’
and CCl3. On the other hand, Me(Me2Ge)m is reduced
by DCA− to afford Me(Me2Ge)m together with
DCA. Me(Me2Ge)m thus formed abstracts a chlorine
atom from CCl4 to give Me(Me2Ge)mCl and CCl3.
Finally, CCl3 either adds to DCA or dimerizes
’
’
’
’
’
(Scheme 1).
However, failure to detect radical cations of
Me(Me2Ge)4Me was partly due to the extremely short
lifetime of the radical. The umaxs of the Me(Me2Ge)4Me
cation radical generated by pulse radiolysis measure-
In contrast to the chlorinative cleavage of the sili-
con–silicon bonds in permethyloligosilanes ([2]f), that
of the germanium–germanium bonds proceeded very
smoothly and their selectivity between the terminal
Ge–Ge bond and the internal Ge–Ge bond could not
be observed.
ment was at 700 nm. This is close to the observed umax
s
of the anion radical of DCA. However, the life-lived
time of the Me(Me2Ge)4Me cation radical is estimated
to be ca. 40–50 ns [13].
Table 3
DCA fluorescence quenching with Me(Me2Ge)nMe (n=2–5) in
CH3CN
Oligogermane
IPs (eV)
kq (109 m−1 s−1
)
Me(Me2Ge)2Me
Me(Me2Ge)3Me
Me(Me2Ge)4Me
Me(Me2Ge)5Me
8.58
8.15
7.80
7.67
7.4
14.3
16.9
20.1
Fig. 1. Correlation between IP of Me(Me2Ge)nMe (n=2–5) and
fluorescence quenching rate constant of DCA–Me(Me2Ge)nMe.

124
K. Mochida et al. / Journal of Organometallic Chemistry 568 (1998) 121–125
age of the Ge–Ge bond of Me(Me2Ge)3Me is de-
scribed. A degassed sealed Pyrex tube containing a
mixed solvent of CH3CN (10 ml)–CCl4 (4.5 mmol, 0.43
ml) of Me(Me2Ge)3Me (0.09 mmol) and DCA (0.009
mmol) was irradiated with a high-pressure Hg arc lamp
(450 W) for 1 h at 0°C. NMR, GC, and GC-MS
analysis of the resulting mixture showed Me3GeCl
(0.059 mmol, 65.3%), Me5Ge2Cl (0.062 mmol, 69.3%),
and (CCl3)2 (0.024 mmol, 26.5%) with internal standard
methods.
3.3. Quenching experiment of DCA* with
Me(Me2Ge)nMe (n=2–5)
Fluorescence spectra were recorded with a Hitachi
850 fluorescence spectrometer, and spectral corrections
for emission were made. The emission and excitation
band-pass was fixed to 5 nm. Fluorescence quenching
was measured by measuring the fluorescence intensity
ratio Io/I at 25°C, where Io and I are fluorescence
intensities in the absence and in the presence of
quencher, respectively. Since the fluorescence spectrum
did not change upon quenching, Io/I was replaced by
the intensity ratio at the wavelength of maximum inten-
sity. The samples were not deoxygenated.
Fig. 2. Transient absorption spectra obtained by 354 nm laser flash
photolysis of DCA–Me(Me2Ge)nMe in CH3CN. The insert shows
the absorption-time profile at 700 nm.
3. Experimental details
1H spectra were recorded with a Varian Unity-Inova
400 MHz NMR. The GC-MS spectra were recorded
using a JEOL JMS-DX 303 mass spectrometer. The
UV-visible spectra were recorded on a Shimadzu UV
2200 spectrometer. The fluorescence spectra were
recorded on a Hitachi 850 fluorescence spectrometer.
Gas chromatography was done on a Shimadzu GC 8A
with 1 m 20% SE30 and 30% Apiezon L columns.
3.4. Time-resol6ed optical absorption
The sample was placed in a quartz cell with an
optical length of 5 mm and degassed by four freeze–
pump–thaw
cycles.
The
concentrations
of
3.1. Materials
Me(Me2Ge)4Me were ca. 10−3 M. The solution was
excited by a Nd:YAG laser (Quanta-Ray GCR-130, 6
ns, fwhm) used for the probe beam, which was detected
with a Si-PIN photodiode (Hamamatsu, C5331-SPL)
after passing through the quartz cell and a monochro-
mator (Ritsu, MC-10N). The output from the Si-PIN
photodiode was recorded with a digitizing oscilloscope
(HP 54510B, 300 MHz). The details of the laser photol-
ysis apparatus have been published elsewhere ([7]b).
Permethyloligogermanes, Me(Me2Ge)nMe (n=2–5),
Me3GeCl, Me5Ge2Cl, Me7Ge3Cl, and Me9Ge4Cl were
prepared as described ([7]a). 9,10-DCA, Np, An, 1,2-
DCN, CCl4, CH3CN, and (CCl3)2 are commercially
available.
3.2. Photochemical reactions of
permethyloligogermanes, Me(Me2Ge)nMe (n=2–5) in
the presence of DCA in CCl4–CH3CN
Acknowledgements
As a representative example, the chlorinative cleav-
The authors thank Mitsubishi Material Co., Ltd., for
the gift of germanium tetrachloride.
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