Electrochemical study of redox properties of 2,3,4,5-tetraphenyl-1-heterocyclopenta-2,4-dienes Ph4C4ER1R2 (E = Si, Ge, Sn). A new method for generation of tetraphenylgermole dianion

Article abstract of DOI:10.1023/A:1015084831035

The electrochemical behavior of the sila-, germa-, and stannacyclopentadienes (metalloles) was studied by cyclic vollammetry. The oxidation and reduction mechanisms of these heterocyclic compounds were investigated. A new method for generation of tetraphenylgermole dianion involving the electrochemical reduction of the corresponding dichlorogermole is proposed. Electrochemical oxidation of the metallole dianions was first studied taking tetraphenylgermole dianion as an example.

Full text of DOI:10.1023/A:1015084831035

Russian Chemical Bulletin, International Edition, Vol. 50, No. 11, pp. 2071—2077, November, 2001
2071
Electrochemical study of redox properties of 2,3,4,5-tetraphenyl-
1-heterocyclopenta-2,4-dienes Ph4C4ER1R2 (E = Si, Ge, Sn).
A new method for generation of tetraphenylgermole dianion
A. A. Moiseeva,a R. D. Rakhimov,a E. K. Beloglazkina,a K. P. Butin,a
K. S. Nosov,b V. Ya. Lee,b and M. P. Egorovb«
aM. V. Lomonosov Moscow State University, Chemistry Department,
119899 Moscow, Leninskie gory, Russian Federation.
Fax: (095) 939 5546. E-mail: butin@org.chem.msu.su
bN. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: (095) 135 5328. E-mail: mpe@cacr.ioc.ac.ru
The electrochemical behavior of the sila-, germa-, and stannacyclopentadienes (metalloles)
was studied by cyclic voltammetry. The oxidation and reduction mechanisms of these
heterocyclic compounds were investigated. A new method for generation of tetraphenylgermole
dianion involving the electrochemical reduction of the corresponding dichlorogermole is
proposed. Electrochemical oxidation of the metallole dianions was first studied taking
tetraphenylgermole dianion as an example.
Key words: silacyclopentadienes, germacyclopentadienes, stannacyclopentadienes,
metalloles, metallole dianions, electrochemistry.
The structure and reactivity of heteroanalogs
of cyclopentadiene, namely, 1-sila-, 1-germa-, and
1-stannacyclopentadienes (metalloles)1,2 and the aro-
maticity of their mono- and dianions3 have attracted
increasing interest. A salient feature of the electronic
structure of metalloles is low-lying LUMO. This is due
to σ*—π* conjugation and allows these compounds to
exhibit some useful properties. Recently,4,5 it was found
that metallole and, first of all, silole derivatives are
promising materials for fabrication of conducting
or semiconductor devices. For instance, homopoly-
meric poly-2,5-siloles that are structurally similar to
trans-s-cis-polyacetylenes (organic conductors) have been
the subject of intensive research (see, e.g., Ref. 6 and
references cited therein).
Up to now, studies of the redox processes with
participation of metalloles are fragmentary. In particu-
lar, information on the electrochemical properties of
metalloles is scarce and sometimes contradictory.7—11
Therefore, the aim of this work was to carry out a
systematic investigation of the redox properties of a large
group of 2,3,4,5-tetraphenylsiloles (germoles, stannoles)
in MeCN and THF solutions at a glassy-carbon (GC)
electrode and to develop a novel, electrochemical ap-
proach to the generation of metallole dianions.
tained by cyclic voltammetry (CV) technique are listed in
Table 1. Most of the compounds studied are irreversibly
oxidized in three stages. The first two of them are one-
ox
Table 1. Potentials of reduction (Epred) and oxidation (Ep
)
peaks of 2,3,4,5-tetraphenylmetalloles Ph4C4ER1R2 in MeCN
(with 0.05 M Bu4NBF4 as the supporting electrolyte, at a
glassy-carbon electrode, 200 mV s–1, vs. Ag/AgCl/KCl (sat.))
Com-
pound
ER1R2
–Epred/V
Epox/V
1a
1b
1c
2a
2b
2c
2d
2e
SiH2
SiMe2
SiCl2
1.98, 2.30
2.14, 2.47
2.00, 2.33
1.73, 2.46
1.90, 2.39
1.29, 1.47, 1.81
1.35, 1.74, 2.33
1.46, 1.81, 2.16
1.39, 1.54, 2.11
1.29, 1.52, 1.96
1.34, 1.75, 2.29
1.12, 1.51, 2.04
1.14, 1.38, 1.65,
1.86, 2.33
GeH2
GeMe2
Ge(cyclo-C3H5)2 2.07, 2.39
Ge(SiMe3)2
Ge(GeEt3)2
2.40a
2.22, 2.70
2f
Ge(SnMe3)2
1.97, 2.51
1.09, 1.28, 1.62,
1.82
2g
2h
2i
3a
3b
3c
4
GeMeH
GeMeCl
GeCl2
SnMe2
SnPh2
SnBr2
CH2
1.96, 2.74
1.70, 2.52
1.31, 2.65
2.01, 2.51, 2.72 1.23, 1.49, 1.72
1.93a
1.35, 1.74, 2.05
0.72, 1.56, 2.34 1.84b
1.36, 1.63, 2.15
1.49, 1.79, 2.06
1.65, 1.84, 2.23
Results and Discussion
2.07
2.04, 2.63
1.10, 1.37, 1.86
1.40, 1.62, 2.26
5
—
The electrochemical oxidation and reduction poten-
tials of metalloles Ph4C4ER1R2 (E = Si, Ge, Sn) 1—3
and 1,2,3,4-tetraphenylcyclopentadiene, Ph4C4CH2, ob-
a At a Pt electrode.
b 1.5-Electron peak.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1980—1985, November, 2001.
1066-5285/01/5011-2071 $25.00 © 2001 Plenum Publishing Corporation

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Russ.Chem.Bull., Int.Ed., Vol. 50, No. 11, November, 2001
Moiseeva et al.
ox
–Epred/Ep (V)
electron stages (ne = 1), and the third stage corresponds
to transfer of nearly two electrons (the experimentally
found ne values vary between 1.4 and 2). The exception is
the oxidation of germoles 2e and 2f with the trialkylgermyl
and trialkylstannyl substituents at the heteroatom, re-
spectively, and compound 3c. They are oxidized in five
(2e), four (2f), and one stage (only one anodic peak
corresponding to ne = 1.5 was observed for 3c).
2.5
1
2.0
1.5
2
Metalloles 1—3 are usually reduced in two one-
electron stages; however, a third peak was observed at
high cathodic potentials for stannoles 3a and 3c. The
reduction of dihalometalloles 1c and 2i involves two
two-electron stages. Stannole 3c is oxidized in three
stages, and the first two of them also correspond to
transfer of two electrons. According to CV data in
MeCN, all the redox transitions studied are electro-
chemically and chemically irreversible.
1.0
0
2
4
σ*
red
ox
Fig. 1. Plots of –Ep
(1) and Ep
(2) for germoles
Ph4C4GeR1R2 as functions of the sum of the σ* Taft constants
of substituents R1 and R2.
R1 and R2 on the electron density distribution in
germoles.13
Since the tetraphenylbutadiene fragment is common
to all the compounds studied in this work, we investi-
gated the electrochemical behavior of (E,E)-1,2,3,4-tetra-
phenylbuta-1,3-diene (5). The shapes and potentials of
three oxidation and two reduction peaks in the CV
curves for this compound are close to those of the
corresponding peaks in the CV curves recorded under
the same conditions (MeCN, with 0.05 M Bu4NBF4 as
the supporting electrolyte) for most of tetraphenyl-
metalloles and, in particular, tetraphenylgermoles (see
Table 1). This suggests that the tetraphenylbutadiene
fragment has strong effect on the electrochemical be-
havior of metalloles 1—3. Nevertheless, the nature of
the heteroatom (E) and substituents (R1, R2) at the
E atom also affects the redox properties of metalloles.
From the data listed in Table 1 it can be seen that the
reduction of compounds Ph4C4EH2 occurs more easily
in the order of E elements: C < Si < Ge. The electron
affinity calculated in the B3LYP/6-31G(d)//AM1 ap-
proximation also increases on going from Ph4C4CH2 to
the silicon- and germanium-containing analogs.12 The
effect of substituents at the heteroatom on the reduction
potentials can be followed taking substituted germoles as
examples. Here, the reduction potentials increase in
absolute value in the order of substituents: Cl, Cl < Cl,
Me < H, H < Me, Me < SnMe3, SnMe3 < cyclo-C3H5,
cyclo-C3H5 < Et3Ge, Et3Ge < Me3Si, Me3Si. The po-
tentials of the first oxidation peaks of germoles also
depend on the nature of substituents R1, R2 and increase
in the order Me3Sn, Me3Sn < Me3Si, Me3Si < Et3Ge,
Et3Ge< Me, Me < cyclo-C3H5, cyclo-C3H5 < H,
Me < Cl, Me < H, H < Cl, Cl. A clearly seen correlation
is observed between the reduction (oxidation) potentials
of germoles 2 and the sum of the σ* Taft constants of
substituents R1 and R2 (Fig. 1)
The text below contains a more detailed consider-
ation of the reduction and oxidation of metalloles.
1. Reduction
1.1. Reduction of compounds Ph4C4EH2
(E = C, Si, Ge)
Up to now, the electrochemical studies of these
compounds were reduced to 1,2,3,4-tetraphenylcyclo-
pentadiene (4)14 and cyclopentadiene.15 At transition-
metal (Fe, Pt, Ni, Co, V, Cu, Cr) electrodes, cyclo-
pentadiene is reduced at high cathodic potentials (from
–1.92 to –2.36 V in MeCN). Electrolysis of cyclo-
pentadiene is accompanied by hydrogen gas evolution.
According to coulometric studies, this reaction has a
one-electron character.15 Experiments on the reduction
of cyclopentadiene at a rotating disk-ring electrode re-
–
vealed an anodic oxidation wave of C5H5 anion at
–0.32 V. Electrolysis of cyclopentadiene in a one-com-
partment cell with a soluble Fe anode results in fer-
rocene as the main product. The mechanism proposed14
for the electrochemical reduction of cyclopentadiene 4
in DMF involves transfer of hydrogen atoms from the
•–
initial 4 to the radical anion 4 . At temperatures below
–30 °C, compound 4 is reduced in anhydrous solvents
in two one-electron quasi-reversible (chemically revers-
ible) stages to give (most likely) a dianion. However, at
higher temperatures the CV patterns become more com-
plicated due to the above-mentioned reaction:
•–
•
–
Ph4C5H2
+ Ph4C5H2
Ph4C5H3 + Ph4C5H .
Only one reduction peak of 4 in MeCN was observed
under conditions of our room-temperature experiments
(see Table 1). Two one-electron irreversible peaks were
observed for the reduction of corresponding silole 1a
and germole 2a. The mechanism of electrochemical
reduction of 1a (2a) can be described by Scheme 1.
red
–Ep
= –0.14•Σσ* + 2.07, r 2 = 0.95;
ox
Ep = –0.08•Σσ* + 1.28,
r 2 = 0.98.
The results of 2D NMR studies of germoles 2 also
point to the inductive nature of the effect of substituents
Irreversibility of the first stage of the reduction points
•–
to extremely short lifetimes of radical anions 1a
and

Electrochemical study of metalloles
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 11, November, 2001 2073
Scheme 1
I/µA
–3.00
•–
Ph
Ph
Ph
Ph
1 µA
+e
Ph
Ph
Ph
Ph
E
E
–1.60
H
H
H
H
•
–H
Ph
Ph
Ph
Ph
–2.47
–1.80
+e
•
–H
Ph
Ph
Ph
Ph
E
E
0
1
2
3
–E/V
2
H
Fig. 2. A cyclic voltammogram of the solution of dichloro-
–3
germole 2i (1•10 mol L–1) in THF, with 0.1 M Bu4NClO4
E = Si, Ge.
as the supporting electrolyte. The potential scan rate was
–1
200 mV s (20 °C).
•–
2b . Direct experimental proofs of the existence of
these radical anions were recently obtained using optical
monoalkylation product appears in the CV curve (at
–1.72 V) when dichlorogermole 2i is reduced in MeCN
in the presence of BunI. The potential of this peak
virtually coincides with the reduction potential of
chloroalkylgermole 2h (see Table 1). We failed in de-
tecting dianion 6b using analogous procedure, since it is
formed at such negative potentials at which the alkylat-
ing agent (BunI) is reduced.
•–
detection of ESR spectra.16 The lifetimes of 1a
and
•–
2b
in decane were estimated to be of the order of
several nanoseconds at 20 °C, which means that these
radical anions cannot be detected in the CV time scale.
Scheme 1 is in good agreement with the known
fact17 that the action of alkali metals on compounds 1a
and 2b in THF results in liberation of H2 and formation
of metallole dianions as the end products.
Since the dianion 6b is less stable in MeCN than in
THF, we studied the reduction of dichlorogermole 2i in
THF. The CV data for compound 2i in THF (Fig. 2)
show that two anodic peaks with different heights appear
in the CV curve when the potential is back scanned.
Taken together, the peaks correspond to a two-electron
oxidation of the product that formed in the second
reduction stage. Assuming the germole dianion 6b to be
this product, the anodic peaks at –2.47 and –1.80 V
must correspond to successive oxidation of 6b.
To confirm this assumption, we first studied the
electrochemical oxidation of the metallole dianion tak-
ing 6b, obtained by the action of four equivalents of
metallic lithium in THF on dichlorogermole 2i, as an
example. The CV curve (Fig. 3) for the oxidation of 6b
1.2. Reduction of dihalometalloles Ph4C4EHal2
(E = Si, Ge, Sn; Hal = Cl, Br).
Electrochemical generation
of 2,3,4,5-tetraphenylgermole dianion
Dichlorosilole 1c and dichlorogermole 2i are re-
duced in two two-electron irreversible stages (Scheme 2).
This corresponds to successive elimination of two Cl
anions and the formation of mono- and dianions of the
–
corresponding metalloles (the highly labile radical anion
•–
2i
was detected only by OD ESR spectroscopy.16)
Scheme 2
Ph
Ph
Ph
Ph
I/µA
+2e
–Cl
Ph
Ph
Ph
Ph
E
E
–2.53
1 µA
Cl
Cl
Cl
–1.98
Ph
Ph
+2e
–2.53
–Cl
Ph
Ph
E
2
–1.67
6a,b
0
1
2
–E/V
E = Si (a), Ge (b).
–3
Fig. 3. Cyclic voltammograms of a 10
M Ph4C4GeLi2
Intermediate formation of a monoanion is confirmed
by the fact that an additional reduction peak of the
solution in THF, with 0.1 M Bu4NClO4 as the supporting
electrolyte. The potential scan rate was 200 mV s (20 °C).
–1

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Moiseeva et al.
also exhibits two oxidation peaks at –2.53 and –1.67 V,
whose potentials are close to those of the anodic peaks
in the CV curve of dichloride 2i (–2.47 and –1.80 V,
respectively). Slight differences between these curves
seem to be due to the nature of the counterion (Li+ or
Bu4N+). The most plausible oxidation mechanism of
dianion 6b is presented in Scheme 3.
reduction of dihalogermoles and dihalosiloles with three
equivalents of metallic potassium in THF.19—22
The formation of tetraphenylgermole dianion in the
electrochemical reduction of its dihalo derivative is a
new method for generation of metallole dianions.
In contrast to 1c and 2i, reduction of dibromostannole
3c proceeds in three stages. The first two two-electron
stages probably involve cleavage of the Sn—Br bonds.
However, this can hardly result in the stannole dianion
as the end product of the reduction of 3c. Stannole
dianions have not been reported as yet. In addition, the
results of a study of the reduction of 1,1-dialkylstannoles
with methyllithium showed that these compounds are
prone to elimination of the organotin fragment to give a
dilithium salt of butadiene22:
Scheme 3
•–
Ph
Ph
Ph
Ph
–e
Ph
Ph
Ph
Ph
Ge
2
Ge
••
7
6b
H
H
–e
H
H
H
Li
MeLi
+ M e2SnR2
SnR2
Li
Ph
Ph
H
Ph
Ph
H
Ph
1/2
Ph
Ph
Ph
H
Ge Ge
The third peak (at –2.34 V) in the CV curve of 3c
can originate from the reduction of the ring opening
product and it is observed at more negative potentials
than that of the cathodic peak of tetraphenylbutadiene 5
(–2.04 V). Indeed, the reduction potential of, e.g.,
(1,2,3,4-tetraphenylbuta-1,3-dienyl)dimethyltin chloride
is –2.25 V.10 Cleavage of the carbon—tin bond can
occur already at the early reaction stages. This is indi-
cated by the results of our PM3 calculations of the
geometry of tetraphenylstannole radical anion. For com-
parison, we performed analogous calculations for com-
pounds Ph4C4EH2 (E = C, Si Ge, Sn) and their radical
anions (Table 2).
Ph
Ph
Ge
••
Ph
Ph
8
9
–e
Oligomers
The first stage of the oxidation results in the germylene
radical anion 7, which then either is oxidized to germylene
8 or, similarly to other germylene radical anions, under-
goes a dimerization18 into dianion 9. The oxidation of 9
leads to oligomeric products. The existence of dianions
of the type 9 was confirmed by their synthesis by the
Table 2. Selected geometric parameters of molecules Ph4C4EH2 (E = C, Si Ge, Sn) and their radical anions obtained
from PM3 calculations and X-ray diffraction study
E
Method
Bond length/Å
Bond angle /deg
E—C(2)
C(2)—C(3)
C(3)—C(4)
C(2)—E—C(5) E—C(2)—C(3) C(2)—C(3)—C(4)
Ph4C4EH2
1.472
(1.478)
1.484
(1.511)
1.477
(1.507)
1.483
C
PM3
1.506
(1.496)
1.849
1.363
(1.366)
1.352
(1.358)
1.346
(1.356)
1.344
(1.352)
104
(104)
92
(92.6)
90.9
(90.9)
83
109
(109)
108.1
(107.5)
106.5
(107.4)
108
109
(109)
115.8
(116)
118
X-ray analysis
PM3
X-ray analysisa (1.865)
PM3
X-ray analysisb (1.921)
PM3 2.143
X-ray analysisc (2.126)
Si
Ge
Sn
1.922
(117.5)
120
(1.511)
Radical anions Ph4C4EH2
(84.6)
•–
C
Si
Ge
Sn
PM3
PM3
PM3
PM3
PM3
1.506
1.843
1.919
2.279
2.201
1.412
1.417
1.421
1.418
1.457
105
95
93
107
105
104.5
109
110
117.5
119
121
120
1.399
1.392
1.363
1.370
78
112
a For 1,1-dimethyl-2,3,4,5-tetraphenylsilole.23
b For 1,1-diethynyl-2,3,4,5-tetraphenylgermole.24
c For hexaphenylstannole.25

Electrochemical study of metalloles
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 11, November, 2001 2075
By and large, the calculated geometry of molecules
Ph4C4EH2 is in reasonable agreement with the experi-
mental data. Since no structural data for the metallole
radical anions are available, it was useful to compare the
geometric parameters of radical anions Ph4C4EH2 cal-
culated in this work with the results of recent DFT
calculations of the same radical anions Ph4C4EH2 (E = C,
Si, Ge).12 Both computational approaches showed that
This assumption (i.e., the formation of a dianion of the
type 10) is confirmed by the isolation of 1-phenyl-1,3,4-
trimethyl-1-silacyclopent-3-ene after hydrolysis of the
product of the reduction of 1-phenyl-1,3,4-trimethylsilole
with alkali metal.1
However, we cannot also rule out the possibility for
fragmentation of the metallole radical anion to occur,
which can be followed by the opening of the hetero-
cycle. This process seems to be most plausible for
stannoles (see above).
•–
the five-membered ring in radical anions Ph4C4EH2
remains planar and that the endocyclic bonds C(2)—C(3)
(C(4)—C(5)) are lengthened while the C(3)—C(4) bonds
are shortened as compared to the corresponding bond
lengths in the neutral molecules. The calculated struc-
ture of tetraphenylstannole radical anion also remains
planar; however, the configuration of the tin atom changes
from tetrahedral, as is the case of other metallole radical
anions, to a distorted trigonal-pyramidal. The C(2)—Sn
(C(5)—Sn) bonds were found to be longer than in the
corresponding neutral molecule (see Table 2), one of
them being appreciably longer (by 0.078 Å) than the
other. This is not observed for other metalloles (E = Si,
Ge) and cyclopentadiene, where the endocyclic E—C
bond lengths are the same as or slightly smaller than in
the corresponding neutral molecules. This suggests that
the tetraphenylstannole radical anion exhibits a pro-
nounced trend to cleavage of the C—Sn bonds.
2. Oxidation
Based on the results available at the moment, it is
impossible to draw definite conclusions about the oxida-
tion mechanism of metalloles. We can only say that
substituents at the heteroatom E affect the oxidation
potential by inductive mechanism (see Fig. 1). Most
likely, the first electron is detached from the π-system of
tetraphenylbutadiene. This is confirmed by close values
of the first oxidation potentials of some metalloles con-
taining no substituents with strong inductive effect at the
E atom and the first oxidation potential of tetraphenyl-
butadiene 5 (see Table 1). Directions of further frag-
mentation of the radical cation obtained will apparently
depend on the nature of substituents R1 and R2. For
instance, the most probable oxidation scheme of the
compounds Ph4C4EH2 (E = C, Si, Ge) can look as
follows:
1.3. Reduction of other metalloles
The mechanism of electrochemical reduction of
1,1-Alk2- and 1,1-(R3E)2-substituted metalloles is still
to be clarified. The most probable route involves further
reduction of the metallole radical anion to dianion 10
followed by hydrolysis of the latter (Scheme 4).
Ph
Ph
–e
•+
Ph4C4EH2
–H+
Ph
Ph
E
H
H
Scheme 4
Ph
Ph
H
Ph
Ph
•–
Ph
Ph
Ph
Ph
+e
Ph
Ph
Ph
Ph
1/2
Ph
Ph
Ph
E
E
E
E
–e
–H+
H
Ph
Ph
Ph
Ph
1/2
Ph
E
E
Alk
Alk
Alk
Alk
+e
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
E
Ph
Ph
Ph
•
E
–e
Ph
Ph
Ph
Ph
E
1/2
Ph
E
Alk
Alk
Alk
Alk
10
Ph
Ph
+2 H+
The oxidation of metalloles 2d—f (fragmentation of
their radical cations is accompanied by elimination of
Alk3E+, where E = Si, Ge, Sn) can also proceed by
analogous scheme. Finally, the oxidative decomposition
of metalloles can be accompanied by cleavage of the
endocyclic C(sp2)—E bond, as was found in studies of
Ph
Ph
Ph
Ph
E
Alk
Alk

2076
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 11, November, 2001
Moiseeva et al.
was extracted with ether, and dried over CaCl2. Ether was
removed in vacuo, and the residue was recrystallized from
hexane. The yield of 1,2,3,4-tetraphenylbuta-1,3-diene was
2.1 g (70%), m.p. 184 °C (cf. 185—186 °C for cis,cis-isomer40).
1H NMR (CDCl3, δ): 6.30 (s, 2 H, C—H); 6.70—7.50 (m,
20 H, Ph). Mass spectrum (m/z): 358 [M]+.
the reactions of metalloles with halogens, oxygen, or
peracids (see review1 and references cited therein).
Experimental
Electrochemical studies were carried out using a PI-50-1.1
potentiostat. The working electrode was a glassy-carbon rod
(d = 1.8 mm) or a platinum wire (d = 3.5 mm), the supporting
electrolyte was a 0.05 M Bu4NBF4 solution in MeCN or a
0.1 M Bu4NClO4 solution in THF, and the reference electrode
was Ag/AgCl/KCl(sat.). All measurements were performed in
argon atmosphere.
This work was carried out with the financial support
of the Russian Federation Ministry of Industry, Science,
and Technologies in the framework of the State Subpro-
gram "Fundamental Problems of Modern Chemistry"
(Project No. 9.3.03) and of the INTAS (Grant No.
97-30344).
Quantum-chemical PM3 calculations with full optimiza-
tion of the molecular and ion geometry were carried out using
the "Hyperchem" program package (Hypercube Inc., Gainesville
–1
(FL), USA). A gradient magnitude of less than 10 cal mol–1 Å
References
was chosen as convergence criterion.
Acetonitrile was purified by successive distillation over
CaH2 (10 g L–1), H2SO4 (5 mL L–1), KNO3 (1 g L–1), and
P2O5 (5 g L–1). Tetrahydrofuran was distilled over sodium
benzophenone ketyl in argon atmosphere and then immedi-
ately transferred into the electrochemical cell.
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1
crystals. The yield was 2.53 g (45%), m.p. 138 °C. H NMR
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spectrum (m/z): 478 [M]+, 356 [M – GeMeCl]+.
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(2g). To a suspension of LiAlH4 (0.15 g, 4 mmol) in 20 mL of
anhydrous ether, a suspension of 1-methyl-1-chloro-2,3,4,5-
tetraphenyl-1-germacyclopenta-2,4-diene (1.5 g, 3.13 mmol)
in 200 mL of anhydrous ether was added portionwise (small
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205—207 °C. H NMR (CDCl3, δ): 0.80 (d, 3 H, CH3); 5.50
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(E,E)-1,2,3,4-Tetraphenylbuta-1,3-diene (5). To a solu-
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Received May 7, 2001;
in revised form June 21, 2001