Oligogermanes containing only electron-withdrawing substituents: Synthesis and properties

Article abstract of DOI:10.1021/acs.organomet.6b00767

A series of germanes Ar₃GeX, containing electron-withdrawing substituents [Ar = p-FC₆H₄, 1a-d, 1a (X = Cl), 1b (X = Br), 1c (X = H), 1d (X = NMe₂); p-F₃CC₆H₄, 2a-d, 2a (X = Cl), 2b (X = Br), 2c (X = H), 2d (X = NMe₂)], was synthesized and used to prepare symmetrical digermanes Ar₃Ge-GeAr₃, (p-FC₆H₄)₃GeGe(C₆H₄F-p)₃ (3), and (p-F₃CC₆H₄)₃GeGe(C₆H₄CF₃-p)₃ (4) and trigermane [(pF₃CC₆H₄)₃Ge]₂Ge(C₆F₅)2 (5) by hydrogermolysis reaction. The properties of all compounds were investigated by multinuclear NMR and for oligogermanes by UV/vis and fluorescence spectroscopy, as well as by electrochemical methods. In addition, the molecular structures of 1a, 1b, 2b, 2c, and 3-5 were studied by X-ray diffraction analysis. Compound 5 showed a significantly shifted UV/visible absorption to the red field in comparison with previously described derivatives.

Full text of DOI:10.1021/acs.organomet.6b00767

Article
pubs.acs.org/Organometallics
Oligogermanes Containing Only Electron-Withdrawing Substituents:
Synthesis and Properties
Kirill V. Zaitsev,*,† Kevin Lam,*,‡ Zhaisan Zhanabil,‡ Yerlan Suleimen,‡ Anastasia V. Kharcheva,†
Viktor A. Tafeenko,† Yuri F. Oprunenko,† Oleg Kh. Poleshchuk,§,∥ Elmira Kh. Lermontova,#
and Andrei V. Churakov#
†Department of Chemistry, Moscow State University, Leninskye Gory 1, Moscow 119991, Russia
‡Department of Chemistry, School of Science and Technology, Nazarbayev University, Astana, Kazakhstan, 010000
§National Research Tomsk Polytechnic University, Lenin Avenue, 30, Tomsk 634050, Russia
∥Tomsk State Pedagogical University, Kievskaya Street, 60, Tomsk 634061, Russia
#Russian Academy of Science, N.S. Kurnakov General and Inorganic Chemistry Institute, Leninskii pr., 31, Moscow 119991, Russia
S
* Supporting Information
ABSTRACT: A series of germanes Ar3GeX, containing electron-with-
drawing substituents [Ar = p-FC6H4, 1a−d, 1a (X = Cl), 1b (X = Br), 1c
(X = H), 1d (X = NMe2); p-F3CC6H4, 2a−d, 2a (X = Cl), 2b (X = Br), 2c
(X = H), 2d (X = NMe2)], was synthesized and used to prepare
symmetrical digermanes Ar3Ge−GeAr3, (p-FC6H4)3GeGe(C6H4F-p)3 (3),
and (p-F3CC6H4)3GeGe(C6H4CF3-p)3 (4) and trigermane [(p-
F3CC6H4)3Ge]2Ge(C6F5)2 (5) by hydrogermolysis reaction. The proper-
ties of all compounds were investigated by multinuclear NMR and for
oligogermanes by UV/vis and fluorescence spectroscopy, as well as by
electrochemical methods. In addition, the molecular structures of 1a, 1b,
2b, 2c, and 3−5 were studied by X-ray diffraction analysis. Compound 5
showed a significantly shifted UV/visible absorption to the red field in comparison with previously described derivatives.
in such species and due to the high thermodynamic stability of
decomposition products such as GeCl2. In addition, branched
catenated oligogermanes containing only electron-withdrawing
groups (such as [Cl3Ge]4Ge)11 are also very rare. However, it is
possible to assume that the presence of organic electron-
withdrawing groups should stabilize linear oligogermanes.
Surprisingly, to the best of our knowledge, only few related
oligogermanes containing organic electron-withdrawing sub-
stituents (for example, (C6F5)3Ge−Ge(C6F5)312), have been
reported in the literature, and unfortunately their physical
properties have been only superficially investigated. Therefore,
we would like to report the synthesis and investigation of the
properties of new oligogermanes containing only electron-
withdrawing substituents. It is well-known that physical
properties of derivatives of the catenated group 14 elements
heavily depend on the nature of their substituents (electronic
properties and steric hindrance),13 on the number of the
element atoms in the chain,1b and on the conformation of the
molecule.14 In the present work, we would like to disclose the
syntheses of a series of triarylgermanium derivatives and of new
oligogermanes bearing aryl groups (p-FC6H4 and p-F3CC6H4)
as well as their structural and physical studies.
INTRODUCTION
■
Nowadays significant attention is drawn to the synthesis of
novel types of compounds with the hope of finding new unique
physical properties and to establish a possible “structure−
property” relationship. Due to the σ-conjugation between the
group 14 elements E (E = Si, Ge, Sn), molecules containing
element−element bonds may be regarded as potentially
important precursors for the construction of new and unique
conductive or luminescent materials that display nonlinear
optical properties, etc.1 In general, such materials are polymers
instead of individual molecular compounds. Only recently were
molecular catenated derivatives of group 14 elements used as
building blocks for semiconductors.2 This reinforces the need
for more studies on the physical properties of the individual
molecular compounds with the hope to be able to identify the
key structural features (nature of the substituents and E,
number of the E atoms in the chain, etc.) that are influencing
the physical properties of the material.
Previous works by Satge,3 Castel,4 Draeger,5 Mochida,6
Weinert,7 Osakada,8 and Marschner9 have reported the
synthesis and the studies of oligogermane derivatives bearing
in general electron-donating groups. To the best of our
knowledge, simple germanium compounds containing only
electron-withdrawing substituents such as linear oligogermane
Cl3Ge−GeCl3 and its higher analogues are almost unknown
and highly unstable10 due to the instability of the Ge−Ge bond
Received: October 1, 2016
© XXXX American Chemical Society
A
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RESULTS AND DISCUSSION
Scheme 2. Synthesis of Germanium Hydrides and Amides
■
The focused construction of Ge−Ge bonds is a challenging
synthetic task. In the case of aryl-substituted germanes, the
hydrogermolysis reaction (i.e., reaction between a germane and
a germanium amide) developed by Weinert et al. represents
one of the few ways to access such compounds under
controlled conditions and with a good reproducibility.7a,f,g,15
This methodology leads to the desired compounds in high
yields and produces only gaseous byproducts, which make it
particularly convenient and attractive. In order to use this
methodology, the first task is to prepare a series of novel
triarylgermanium halogenides, which may then be easily
transformed into the corresponding Ar3GeH and Ar3GeNMe2
compounds.
are partially rotationally disordered, as frequently found in this
type of compounds.16
Bromobenzenes para-substituted by electron-withdrawing
groups such as p-FC6H4Br and p-F3CC6H4Br were used for
the synthesis of the desired triarylgermanes. Several synthetic
approaches could be used to prepare such compounds
including a nucleophilic substitution at the Ge center (reaction
of a Grignard or organolithium reagents with GeCl4) or an
electrophilic substitution (desymmetrization of tetraarylger-
mane by GeCl4 in the presence of a Lewis acid). It should be
noted that GeCl4 is a better starting material since it is
significantly cheaper than GeBr4.
The reaction between a Grignard reagent and GeCl4 (molar
ratio 3:1) resulted in the formation of a mixture of the
corresponding chlorides and bromides (i.e., 1a/1b and 2a/2b)
(Scheme 1) as well as other byproducts (evidently, monoaryl-,
Figure 1. Molecular structure of (p-F3CC6H4)3GeH (2c). Displace-
ment ellipsoids are shown at the 50% probability level. Minor
components of disordered CF3 groups are not shown. Hydrogen
atoms except of H(1) are omitted for clarity.
Scheme 1. Synthesis of Halogenides, 1a/1b and 2a/2b, Using
Grignard Reagents
Table 1. Selected Bond Distances (Å) and Bond Angles
(deg) for Compounds 1a, 1b, 2b, and 2c
1a
1b
2b
2c
Ar
p-FC6H4
Cl
p-F3CC6H4
Br
X
Br
H
Ge(1)−X
Ge(1)−Cav
C−Ge(1)−Cav
X−Ge(1)−Cav
2.1873(6)
1.930(2)
112.86(8)
105.81(6)
2.3126(8)
1.936(5)
2.3095(8)
1.941(4)
111.71(19)
107.11(14)
1.48
diaryl-, and tetraarylgermanes) as the minor constituents. Such
a complex mixture of compounds is typical when it comes to
the preparation of organogermanes16 and unavoidable in many
cases. To the best of our knowledge, there are only a few
reported cases in which a halogenated triarylgermane was
obtained as a sole product and in one step from the reaction
between GeCl4 and a Grignard or an organolithium reagent
(Mes3GeCl,17 (p-Tol)3GeCl,18 (C6F5)3GeCl,13 and (o-
Ph2PC6H4)3GeCl19).
1.942(5)
109.0(2)
109.9
112.69(19)
106.01(19)
Much to our surprise, only 11 X-ray structures of
triarylgermanes have been previously reported, wherein the
majority are ortho-substituted aryl derivatives ((o-(MeSCH2)-
C6H4)3GeH,21 (o-(t-BuOCH2)C6H4)3GeH,21 (o-(Me2NCH2)-
C6H4)3GeH,22 (o-(EtO)C6H4)3GeH,23 (o-(Ph2P)-
C6H4)3GeH,19,24 (o-(Me2N)C6H4)3GeH,25 (o-(t-Bu)-
C6H4)3GeH,7e (o-MeC6H4)3GeH26) as well as also other
derivatives (Ph3GeH,27 Mes3GeH,28 (C6F5)3GeH13). No
compound containing only para-substituted aryl rings has
ever been described before. The average bond lengths for Ge−
H (1.33−2.05 Å) and Ge−C (1.93−2.06 Å) are observed.
When comparing these derivatives with 2c, it is evident that the
structural parameters are close to the smallest value of the range
(close to an unsubstituted phenyl derivative). Therefore, it
seems that the presence of an electron-withdrawing substituent
in the para-position of the aryl group does not significantly
affect the structural parameters of the molecule.
The formation of the chloride and bromide derivatives could
be explained by the exchange of halogen atoms that occurs
during the reaction. This reaction happens due to the presence
of MgHal2, which is formed in situ. Although the mixtures 1a/
1b and 2a/2b could be separated from the other byproducts
(by partial crystallization or distillation; see the Experimental
Section), it is impossible to separate the chloride from the
bromide due to the similarities in their physical properties. The
ratios for 1a/1b and 2a/2b have been determined by 19F NMR
spectroscopy and are respectively 1:4 and 6:5. Both of these
mixtures were used as obtained without further purification and
fully converted into the corresponding hydrides or amides by
7d
7d,20
using an excess of LiAlH4 or LiNMe2
in high yields
(Scheme 2).
As expected, the germanium atom in 2c has a slightly
distorted tetrahedral geometry. The aromatic rings are adopting
an unusual conformation, which differs from the typical
propeller-like structure. The two rings are almost coplanar
The molecular structure of the hydride 2c was investigated in
the solid state by X-ray analysis (Figure 1, Table 1, Table S1,
Supporting Information). In this structure, the fluorine atoms
B
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with the Ge−H bond (torsion angles H−Ge−C−C are
−13.50° and 11.62°), whereas the third ring is almost
perpendicular (torsion angles H−Ge−C−C is 88.66°).
Furthermore, in order to fully characterize 1a, 1b, 2a, and 2b
it is possible to prepare those compounds individually by
starting from the corresponding hydrides 1c and 2c. The
hydrides were subjected to thermally induced radical
halogenations and yielded the corresponding halides in high
yields (Scheme 3). Such a chlorination17a,29 and bromination30
for Ar3GeH. The 13C NMR spectra for the compounds 1a−d
showed aromatic signals as four doublets (at δ 163.8−164.6,
136.0−137.1, 129.6−131.0, 115.7−116.2 ppm) due to high
2
values of spin 13C−19F coupling constants (1J 248.7−251.0, J
20.2−20.6, 3J 7.6−8.1, 4J 3.3−3.8 Hz). For the (p-
trifluoromethyl)phenyl derivatives 2a−d, the 13C NMR
displayed aromatic signals as broad singlets (at δ 137.8−
139.4, 133.1−135.5 ppm) and quartets (in the δ range 131.1−
133.9, 125.2−125.7 ppm with 2J13C‑19F 32.2−35.9, 3J13C‑19F 3.7−
3.8 Hz), and the CF3 group was observed as a quartet (δ range
122.9−124.7 ppm, 1J13C‑19F 272.3−273.0 Hz). It is interesting to
note that the 13C chemical shifts of the aromatic ring for the
para-substituted (in relation to the Ge atom) CF3 derivatives
2a−d (ipso → ortho → para → meta) are close to the shifts of
(p-F3CC6H4)4Ge23 or of the majority of unsubstituted phenyl
derivatives.5e In contrast, 1a−d displayed chemical shifts (para
→ ortho → ipso → meta in relation to the Ge atom) that are
characteristic for electron donor para-substituted compounds.23
In the 19F NMR 1a−d were characterized by a multiplet (δ
range −108.7 to (−111.4) ppm) due to 1H−19F spin coupling,
and 2a−d showed a singlet (δ range −62.7 to (−63.4) ppm).
The molecular structures of the three halides, chloride 1a
(Figure 2) and bromides 1b and 2b (Figures 3 and 4), were
Scheme 3. Selective Synthesis of Germanium Chlorides 1a
and 2a and Bromides 1b and 2b
methodology is easy to perform and may be considered as an
alternative to the previously reported methods such as the ones
using CuCl2/CuI,7e,31 AcCl/FeCl3 (for chlorination), bro-
32
34
mine in CCl4,33 or allyl bromide/PdCl2 (for bromination).
Several attempts were made in order to obtain (p-
F3CC6H4)3GeCl (2a) by reacting the corresponding organo-
lithium reagent (formed in situ by the reaction of p-F3CC6H4Br
with an equimolar amount of n-BuLi at −30 °C) with GeCl4 (in
a molar ratio of 3:1).13 Nevertheless, all of the attempts resulted
in a mixture of mono-, di-, tetra-, and target triarylgermanes,
where (p-F3CC6H4)4Ge was the major constituent (according
to the 19F NMR spectroscopy data). The formation of a
significant amount of polymeric materials was also observed. In
this case, in contrast to when a Grignard reagent was used (see
above), we were not able to isolate the individual compounds.
At the same time, the reaction between a hypercoordinate
germanium alkoxide, such as the readily available compounds
based only on an alkyl derivative, K2[Ge{OCH(Me)CH(Me)-
O}3], with p-F3CC6H4MgBr followed by a subsequent
reduction7g,35 led, again, to an inseparable mixture of
unidentified compounds containing the target hydride (p-
F3CC6H4)3GeH only in a very low amount.
Furthermore, the attempts to perform the desymmetrization
(Kocheshkov reaction) of (p-F3CC6H4)4Ge23 by reacting it
with GeCl4 in the presence of AlCl3 failed since no reaction was
observed. Similarly, no reaction occurred when HOTf36 or
37
Figure 2. Molecular structure of (p-FC6H4)3GeCl (1a). Displacement
ellipsoids are shown at the 50% probability level.
Br2 was reacted with (p-F3CC6H4)4Ge. The absence of
electrophilic aromatic substitution in this case could be
explained by the presence of the strong electron-withdrawing
substituents, p-F3CC6H4, on the Ge atom. Thus, according to
the data obtained, it is possible to state that the synthetic
methodology using organomagnesium compounds, developed
in this work, may be regarded as a most suitable way for the
synthesis of triarylgermanes bearing electron-withdrawing
groups.
Compounds 1a−d and 2a−d were characterized by multi-
nuclear (1H, 13C, and 19F) NMR spectroscopy and elemental
analysis. For the p-fluorophenyl derivatives 1a−d, the 1H NMR
spectra display the signals of aromatic protons as two multiplets
(at δ 7.61−7.32 and 7.19−6.88 ppm) due to spin−spin
coupling with H and F atoms. In the case of the p-
(trifluoromethyl)phenyl derivatives 2a−d the proton signals
appeared as doublets (in a δ range 7.43−7.89 and 7.34−7.80
ppm, 3JH−H 7.8−8.6 Hz). The signals for Ge−H were observed
at δ 5.70 (1c) and 5.88 (2c) ppm, which are typical shift values
investigated by X-ray analysis (Table 1, Table S1, Supporting
Information) in order to determine the effect of the electronic
nature of the substituents on the germanium atom and on the
structural parameters of the germanes. Previously, only very few
examples of triarylgermanes bearing electron-withdrawing
groups were reported.
There are only seven known substituted triarylgermanium
chlorides that have been investigated by X-ray analysis
(Ph3 GeCl,3 8 (o-(MeOCH2 )C6 H4 )3 GeCl,2 9 a (o-
EtOC6H4)3GeCl,23 (o-(t-Bu)C6H4)3GeCl,7e (C6Cl5)3GeCl,33
(C6F5)3GeCl13). The germanium atom in 1a displays a
tetrahedral arrangement with its aryl rings adopting a
propeller-like conformation (torsion angles Cl−Ge−C−C are
54.24°, 60.09°, and 66.97°). Such a conformation is typical for
such derivatives. Other structural parameters, including d(Ge−
C
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unsubstituted Ph3GeBr, showing that the effect of the electronic
nature of substituents on the aryl groups is mainly affecting the
Ge bond length and the conformation of the aryl groups. At the
same time, it is evident that the corresponding chlorides and
bromides have very similar structures.
The next goal of this research was to synthesize
oligogermanes containing electron-withdrawing substituents
using known precursors as well as those obtained in this
work (1c, 1d; 2c, 2d).
Our attempts to synthesize oligogermanes bearing electron-
withdrawing groups by reacting (C6F5)3GeH with polyamide
germanes such as Me2Ge(NMe2)2 or Ph2Ge(NMe2)2 in n-
hexane or MeCN were unsuccessful even under harsh
conditions (up to 100 h at 90 °C). Furthermore, (Me2N)4Ge
showed also to be unreactive when treated with Ph3GeH or
(C6F5)3GeH in MeCN.7a Therefore, it seems that the
hydrogermolysis using monoamidogermanes remains the
easiest way to prepare oligogermanes.
Moreover, when 1c was submitted to hydrogermolysis
conditions in hexane13 with 1d or 2c with 2d, no reaction
was observed, even after 26 h of heating. This greatly differs
from the case of (C6F5)3GeH, which we previously
investigated.13 However, when hexane was replaced by
acetonitrile, the corresponding symmetric digermanes 3 and 4
were obtained in high yields (Scheme 4).
Figure 3. Molecular structure of (p-FC6H4)3GeBr (1b). Displacement
ellipsoids are shown at the 50% probability level.
Scheme 4. Synthesis of Digermanes 3 and 4
Figure 4. Molecular structure of (p-F3CC6H4)3GeBr (2b). Displace-
ment ellipsoids are shown at the 50% probability level. Minor
components of disordered CF3 groups are not shown. Hydrogen
atoms are omitted for clarity.
Thus, the result of hydrogermolysis is strongly dependent
not only on the nature of the germanes but also on the type of
solvent.
Cl) and d(Ge−C), are very similar to unsubstituted Ph3GeCl
(2.1873(6) vs 2.187(2)38a and 1.930(2) vs 1.933(5)38b Å).
To the best of our knowledge, only four triarylgermanium
bromides have been investigated by X-ray analysis, and none of
them are para-substituted. The molecular structure of 2b is
partially disordered due to the presence of the CF3 groups. In
the bromide derivatives 1b and 2b, the germanium atom has a
slightly distorted tetrahedral geometry. Introduction of
electron-withdrawing groups such as F or CF3 in para-position
of the phenyl group in aryl germanes results in a decrease of the
Ge−Br bond length in comparison with triaryl bromides that
were previously described (2.3095(8) Å for 2b and 2.3126(8) Å
for 1b vs 2.319(3) Å for Ph3GeBr39, 2.3618(5) Å for [o-
(MeOCH2(C6H4)3GeBr],29a 2.3791(6) Å for [(2,6-(t-
BuO)2C6H3)3GeBr],40 2.362(1) Å for [(o-(t-Bu)-
C6H4)3GeBr]7e). In 2b, the conformation of the aryl groups
is similar to that of 2c, where one of the aryl cycles is almost
perpendicular to the Ge−Br bond (the torsion angles Br−Ge−
C−C are 29.87°, 40.75°, and 77.20°). In sharp contrast, in 1b
the aryl rings adopt a typical propeller-like conformation (the
torsion angles Br−Ge−C−C are 51.72°, 58.91°, and 65.12°).
Other structural parameters of 1b and 2b are similar to
Unexpected results were obtained during our attempts to
prepare unsymmetrical oligogermanes. The reaction between
1d and 2c or between 1c and 2d gave only mixtures of the
corresponding symmetrical derivatives, 3 and 4. The reaction of
1d with (C6F5)3GeH led to a mixture, where the symmetric
derivative 3 was identified. Evidently, in our case, (C6F5)3GeH
may act as a proton transfer reagent.
On the contrary, when (C6F5)2GeH2 was used as the
germanium hydride source in the hydrogermolysis, the reaction
resulted in the formation of the desired product (Scheme 5).
1
Compounds 3−5 were characterized by H, 13C, and 19F
NMR, UV/visible, and fluorescence spectroscopy, elemental
analysis, and electrochemical methods. The NMR spectra of
oligogermanes 3, 4, and 5 are similar to the corresponding
Scheme 5. Synthesis of Trigermane 6
D
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germanes. The molecular structures of digermanes 3 and 4 and
trigermane 5 (Figures 5−7, Table S2, Supporting Information)
Figure 5. Molecular structure of (p-FC6H4)3Ge−Ge(C6H4F-p)3 (3).
Minor components of disordered phenyl groups are not shown.
Displacement ellipsoids are shown at the 50% probability level.
Figure 7. Molecular structure of [(p-F3CC6H4)3Ge]2Ge(C6F5)2 (5).
Minor components of disordered −CF3 groups are not shown.
Displacement ellipsoids are shown at the 30% probability level.
Selected bond lengths (Å) and bond angles (deg): Ge(1)−Ge(2)
2.4641(9), Ge(1)−C(19) 1.996(4), Ge(2)−Cav 1.958(4); Ge(2)−
Ge(1)−Ge(2A) 122.05(6), C(19)−Ge(1)−C(19A) 110.8(3),
C(19)−Ge(1)−Ge(2) 101.06(14), C(19A)−Ge(1)−Ge(2)
111.00(14), C−Ge(2)−Cav 109.9(2), C−Ge(2)−Geav 109.07(14).
Table 2. Structural Parameters of Hexaaryldigermanes
Investigated by XRD
compound
d(Ge−Ge), Å
ref
(p-Tol)3GeGePh3
(p-Tol)3GeGe(Tol-p)3
Ph3GeGePh3
2.408(1)
2.419(1)
2.437(2)
2.446(1)
2.4623(4)
2.4652(11)
7d
42
5a
5b
13
13
Ph3GeGePh3*2C6H6
(C6F5)3GeGePh3
(C6F5)3GeGe(Tol-p)3
Both germanium atoms in 3 and 4 exhibit a tetrahedral
geometry. The d(Ge−Ge) bonds in these compounds are very
close to the shortest reported ones. The introduction of
electron-withdrawing groups has a small effect and results in a
shortening of the Ge−Ge bond, especially in comparison with
the related donor−acceptor digermanes (compare Tables 2 and
3). Apparently, due to the introduction of electron-withdrawing
substituents, the electron density is moved away from the Ge
atoms to the substituents. This results in an increase of the
positive charge at the Ge center and an increase in the
covalency of the Ge−Ge bond and, therefore, as a consequence,
also in a shortening of germanium−germanium bond.
Figure 6. Molecular structure of (p-F3CC6H4)3Ge−Ge(C6H4CF3-p)3
(4). Minor components of disordered CF3 groups are not shown.
Displacement ellipsoids are shown at the 30% probability level.
in the solid state were investigated by single-crystal X-ray
diffraction analysis. Compounds 3−5, containing only electron-
withdrawing substituents at the Ge atoms, are the first examples
of oligogermanes of such type investigated by X-ray diffraction
(XRD).
According to the Cambridge Structural Database, the Ge−Ge
bond length in digermanes varies between 2.393 Å (for
Ph2(O2CCCl3)GeGe(O2CCCl3)Ph25c) and 2.713 Å (for (t-
Bu)6Ge241). Sterically hindered substituents increase this bond
length. Usually, the introduction of several electron-with-
drawing substituents decreases the bond length. At the same
time, the presence of a donor and an acceptor substituent at the
Ge center elongates the Ge−Ge bond.
Table 3. Selected Bond Distances (Å) and Bond Angles
(deg) for (p-FC6H4)3Ge−Ge(C6H4F-p)3 (3) and (p-
F3CC6H4)3Ge−Ge(C6H4CF3-p)3 (4)
3
4
Ge−Ge
Ge−Cav
C−Ge−Geav
C−Ge−Cav
2.4209(8)
1.966(8)
110.3(4)
108.6(6)
2.433(8)
1.96(4)
109.5(13)
109.4(18)
To the best of our knowledge, only five hexaaryldigermanes
have been previously investigated by XRD (Table 2).
E
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Table 4. Selected Structural Parameters for Known Trigermanes
trigermane
[Ph3Ge]2GePh2
d(Ge−Ge), Å
2.440(2)
angle Ge−Ge−Ge, deg
angle C−Ge(center)−Cav, deg
ref
121.3(1)
108.7(4)
5f
[Ph3Ge]2GeMe2
2.429(1)
120.3(1)
109.2(2)
5d
5e
43
43
44
7d
7d
13
9a
45
[ClPh2Ge]2GePh2
2.413−2.437
2.620(3)
110.4−116.7
118.6(1)
111.82(13)
110.31(15)
109.25(12)
109.36(12)
106.2(1)
[Me(t-Bu)2Ge]2Ge(t-Bu)2
[Br(t-Bu)2Ge]2Ge(t-Bu)2
[I(t-Bu)2Ge]2Ge(t-Bu)2
[(p-Tol)3Ge]2GePh2
[(p-Tol)3Ge]2Ge(p-Tol)2
[(p-Tol)3Ge]2Ge(C6F5)2
[(Me3Si)3Ge]2GeMe2
[Ph3Ge]2Ge(SiMe3)2
2.6223(1), 2.595(1)
2.660(1), 2.622(1)
2.4328(5)
113.5(1)
115.38(10)
114.80(2)
117.54(1)
124.10(3)
125.00(4)
111.949(10)
2.4404(5)
106.45(9)
108.0(2)
2.459(5)
2.4616(8)
105.35(4)
a
2.4494(3)
104.78(2)
a
Si−Ge−Si angle.
Compounds 3 and 4 display an almost ideal gauche
oligogermane results not only in the stabilization of the
HOMO (and hence in an increase of the oxidation potential of
oligogermanes) but also in a stabilization of the LUMO.13
Therefore, the HOMO/LUMO gap, which corresponds to the
UV/visible absorption, shows some significant changes.
UV/visible absorption spectra for catenated organogerma-
nium compounds 3−5 are presented in Figure 8.
conformation46 along the Ge−Ge bond with the torsion
angle of C−Ge−Ge−Cav equal to 60.7(8)° and 60.0(19)°. All
phenyl rings are in a propeller-like conformation relative to the
Ge−Ge bond (average torsion angles are 44.03(8)° and
53.96(18)°, respectively).
The molecular structure of 5 in the solid state was also
studied by XRD analysis. Trigermanes are rare, and there are
only 11 reported compounds that have been studied by XRD
(Table 4). According to these data, in general, steric bulk of the
substituents mainly affects the structural properties of
trigermanes, while the electronic effects have a lesser impact.
The d(Ge−Ge) varied from 2.413 to 2.660 Å, and angles Ge−
Ge−Ge and C−Ge(central)−C are 110.40−125.00° and
104.78−111.82°, respectively.
In a crystal, the molecules of 5 lie on a 2-fold axis. The Ge−
Ge bond length (2.4641(9) Å) for this compound is the biggest
for the known octaaryltrigermanes (see Table 2). Thus, it
seems that the introduction of electron-withdrawing substitu-
ents at the Ge center in oligogermanes results in an elongation
of the Ge−Ge bond length.
Figure 8. UV/visible absorption spectra for compounds 3−5 (CH2Cl2,
rt).
As expected, the elongation of the germanium chain resulted
in a red shift in UV absorption (compare with 237 nm for 3,
239 nm for 4 vs 264 nm for 5). It should be noted that the
absorption of digermanes 3 and 4 bearing electron-withdrawing
groups is slightly blue-shifted in comparison with the known
digermanes (usually in the 2385f−240 nm7d range, which may
be regarded as a typical feature) due to some HOMO
stabilization. However, for trigermane 5 the absorption (264
nm) is significantly red-shifted (compare to Ph8Ge3, 238 nm;
(p-Tol)8Ge3, 253 nm; (p-Tol)3GeGePh2Ge(Tol-p)3, 251
nm).7d This could be explained by a significant LUMO
stabilization. Furthermore, the comparison between the UV
absorption of 5 with [(Me3Si)3SiGeMe2]2GeMe2 (253 nm)9a
or even with Me(Me2Ge)6Me (255 nm)6a suggests that
electron-withdrawing substituents exhibit a significant effect
on the absorption properties of oligogermanes.
The value of the Ge(2)−Ge(1)−Ge(2A) angle (122.05(6)°)
in 5 is typical of an effective σ-conjugation in the
oligogermanium chain. Unlike for 3 and 4 the substituents in
5 on the neighboring Ge atoms are in cis-conformation (the
average C−Ge(1)−Ge(2)−C torsion angle is 20.74(14)°), and
therefore, this may result in some Ge−Ge elongation of the
Ge−Ge bond. Interestingly, this conformation is atypical for
oligogermanes and apparently is caused by crystal packing. At
the same time, this conformation may be explained by the
second electronic interaction in a molecule with highly
electron-withdrawing groups. Really, according to density
functional theory (DFT) data obtained earlier,13 the HOMO
and LUMO are not only on Ge chain atoms but also on
aromatic rings. The angles around the central Ge atom, Ce−
Ge(1)−Ge and C−Ge(1)−C, are significantly different
(122.05(6)° and 110.8(3)°), and in the absence of sterically
demanding substituents such as SiMe3 groups, it is possible to
state that the electron-withdrawing substituent strongly affects
this parameter. Therefore, in addition to the transoid
conformation adopted by the catenated compounds of group
14 bearing large end groups and reported by Marschner et
al.,14,47 it is evident that the molecular geometry may be
influenced by electronic factors, i.e., by the introduction of an
electron-withdrawing substituent in the core of the molecule.
As has been described before, the introduction of an
electron-withdrawing aryl substituent into a catenated
It is interesting to note that a similar effect including
bathochromic shift has been observed earlier for molecular
oligosilanes when electron-withdrawing substituents (chlorine,
nitrophenyl groups) are introduced into their structures.48
Electrochemical investigations for compound 3 under
standard conditions in dichloromethane containing [NBu4]-
[PF6] (0.10 M) as supporting electrolyte did not show any
reduction, but a chemically irreversible anodic oxidation was
50
observed at Epa = 1.20 V Fc/Fc+ (approximately 1.69 V vs
Ag/AgCl) (see Supporting Information, Figure S44). No
rereduction wave was observed during the back scan. This
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Article
value is close to the range of 1.48−1.96 V vs Ag/AgCl of
potentials previously measured for known digermanes
(compare with 1.48 V for Ph3GeGe(Tol-p)3, 1.76 V for (p-
Tol)6Ge2, 1.96 V for Ph6Ge27d).
Table 6. Luminescence Emission Data for Known Molecular
Oligogermanes
λabs (nm),
details
a
compound
cyclo-Ge6Me12
λem (nm), details
ref
49
The electrochemistry of compound 3 was then carried out in
dichloromethane containing [NBu4][B(C6F5)4] (0.05 M) as
supporting electrolyte. The advantages of using a weakly
coordinating anion as supporting electrolyte such as the
“TFAB” anion, [B(C6F5)4]−, have been previously described.51
In the present case, we were hoping to stabilize the oxidation
product in order to be able to detect it during the back scan.
Unfortunately, even at a higher sweeping rate (1000 V/s) and
at lower temperature (−50 °C) we were not able to observe
any product during the rereduction. Again, a chemically
irreversible anodic wave was observed at Epa = 1.41 V vs Fc/
Fc+ (approximately 1.90 V vs Ag/AgCl). The formation of a
transient radical-cation 3+ would explain why the oxidation is
more difficult in [NBu4][B(C6F5)4] than in [NBu4][PF6] since
[PF6]− would better stabilize 3+ due to its higher coordination
ability. In addition to the chemical irreversibility, 3 displays an
important degree of electrochemical irreversibility. The
measured Epa − Epa/2 value for the oxidation of 3 is a
diagnostic of a slow charge transfer process with a coefficient
(β) of 0.34 in the presence of [NBu4][PF6] and of 0.30 in the
presence of [NBu4][B(C6F5)4].52
The electrochemistry of compound 4 was investigated in
several media such as dichloromethane ([NBu4][B(C6F5)4]
(0.05 M)), dichloromethane ([NBu4][PF6] (0.10 M)), and
acetonitrile ([NBu4][PF6] (0.10 M)). Unfortunately, none of
these conditions allowed us to observe any anodic process. The
presence of a strong electron-withdrawing group on the
aromatic ring in this case increased the oxidation potential
beyond the oxidation of the solvent itself. Apparently, this fact
is caused by a significant HOMO stabilization (see Table 7).
293 (250); cyclohexane,
293 K
(i-
310, CH2Cl2 370 (312), CH2Cl2
7h
45
Pr)3Ge[GePh2]4Ge(Pr-
i)3
(p-Tol)3GeGeMe3
232, CH2Cl2 286 (270), CH2Cl2; Φf
3.27%
357, 373, 393 (300), solid 45
a
Excitation wavelength λex in parentheses.
fluorescent in solution (Figure 9) as well as in the solid state
(Figure 10).
Nowadays the studies of fluorescence of molecular
oligogermanes (linear7h,45 or cyclic49) (Table 6) are very
limited in comparison with the studies conducted on polymeric
materials.53 But in general it is possible to affirm that
substituted aryl digermanes display luminescence. Therefore,
the investigation of the effect of the nature of substituents at the
Ge atom on the emission is relevant, and this would help to
identify the main structural parameters that influence the
emissive properties of oligogermanes. This would also allow
improving the optical properties of those compounds (increase
the quantum yield, shift into the red field, and narrow the
emissive line). It is possible to state that the increase of the
length of the catenated chain in molecular oligogermanes
resulted in a red shift of the emission, which is comparable to
that found in the related polymers. Furthermore, the
introduction of electron-withdrawing groups also led to the
same trend; in addition, the quantum yield in solution is also
increased (Table 5).
The fluorescence of the molecular digermanes 3−5 is
comparable to that of polygermanes (369 nm in solution and
367 nm in film for [(Me3SiOC6H4)2Ge]n54).
Table 5. Luminescence Emission Data for Compounds 3−5
a
solid state
solution
The luminescence properties of the oligogermanes were also
studied by DFT. The influence of the nature of the substituent
on the fluorescence properties was studied (Table 7).
According to these data, it is evident that the introduction of
an alkyl electron-withdrawing group results in a red shift of the
emission wavelength. At the same time, aromatic acceptor
substituents lead to a red-shifted emission in comparison to
donors or unsubstituted ones. Finally, the predicted values for 3
and 4 are in a good agreement with the experimental values
(Tables 5 and 7). Furthermore, it should be noted that the
structural parameters in oligogermanes in fundamental (S0) and
excited (S1) states differ significantly (see Supporting
Information). For instance, the Ge−Ge bond lengths (2.44−
2.48 vs 2.66−2.68 Å) are elongated, wherein the energy in S1
increased significantly.
Stokes
λ
shift
Stokes
Φ
f c
em b
compound
(nm)
(nm)
λem (nm) shift (nm) (%)
(p-FC6H4)3Ge−
378
141
350/364
sh
113/127
3.57
Ge(C6H4F-p)3 (3)
(p-F3CC6H4)3Ge−
Ge(C6H4CF3-p)3
(4)
437
476
198
335/345
sh
96/106
11.77
(p-F3CC6H4)3Ge−
212
376
112
12.36
Ge(C6F5)2-
Ge(C6H4CF3-p)3
(5)
a
Spectra were recorded in CH2Cl2, excitation wavelength λex 290 nm.
b
c
Excitation wavelength λex 320 nm. Quantum yield.
Electrochemical investigations for 5 were performed in
conditions found for 3. As expected, in this case there are two
oxidation waves, Epa = 1.27 and 1.63 V vs Fc/Fc+
(approximately 1.76 and 2.09 V vs Ag/AgCl) (see Supporting
Information, Figure S45). These data are higher than the ones
obtained earlier for trigermanes7d (1.696 and 2.052 V for
Ph8Ge3, 1.498 and 1.860 V for (p-Tol)3GeGePh2Ge(Tol-p)3,
1.542 and 1.865 V for (p-Tol)8Ge3 vs Ag/AgCl), which shows
the stabilization of the HOMO in 5 due to the presence of
electron-withdrawing groups.
CONCLUSIONS
■
A series of electron-deficient triarylgermanes (1a−d, 2a−d),
containing p-fluorophenyl and (p-trifluoromethyl)phenyl
groups, have been synthesized, wherein organomagnesium
reagents gave suitable results. These derivatives were used in
hydrogermolysis reactions in order to prepare di- (3, 4) and
trigermanes (5). Introduction of electron-withdrawing sub-
stituents into the oligogermane core resulted in a red shift in
UV absorption in comparison to the known related derivatives.
Furthermore, oligogermanes bearing electron-withdrawing
Finally, the fluorescence of the compounds 3−5 was studied
by fluorescence spectroscopy (Table 5). All compounds are
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Table 7. Luminescence Emission Data for Substituted Oligogermanes According to DFT Calculations
compound
Me3GeGeMe3
fluorescence, λ, nm
oscillator strength
transition
HOMO, eV
350
373
391
342
338
335
305
306
389
350
343
402
357
349
0.47
0.59
0.36
0.17
0.16
0.27
0.23
0.22
0.23
0.01
0.14
0.39
0.30
0.30
LUMO−HOMO
LUMO−HOMO
LUMO−HOMO
LUMO+1−HOMO
LUMO+2−HOMO
LUMO−HOMO
LUMO+1−HOMO
LUMO+2−HOMO
LUMO−HOMO
LUMO+1−HOMO
LUMO+2−HOMO
LUMO−HOMO
LUMO+1−HOMO
LUMO−HOMO
−6.37
−9.23
−5.62
(F3C)3GeGe(CF3)3
Ph3GeGePh3
(p-Tol)3GeGe(Tol-p)3
(p-FC6H4)3GeGe(C6H4F-p)3 (3)
(p-F3CC6H4)3GeGe(C6H4CF3-p)3 (4)
−5.74
−6.68
last cases, the absence of detectable amounts of organic impurities was
established by high-field multinuclear NMR spectroscopy.
1H NMR (400.130 MHz), 13C NMR (100.613 MHz), and 19F
(376.498 MHz) spectra were recorded with a Bruker 400 or Agilent
400MR spectrometer at 298 K. Chemical shifts are given in ppm
relative to internal Me4Si (1H and 13C NMR spectra) or external
CFCl3 (19F spectra). Mass spectra (EI-MS, 70 eV) were recorded on a
Finnigan MAT INCOS 50 quadropole mass spectrometer with direct
insertion; all assignments were made with reference to the most
abundant isotopes. Elemental analyses were carried out by the
Microanalytical Laboratory of the Chemistry Department of Moscow
State University using a HeraeusVarioElementar instrument. MALDI
mass spectra were recorded on an Autoflex II Bruker spectrometer
(fwhm resolution 18 000) equipped with a nitrogen laser operating at
a wavelength of 337 nm and with a time-of-flight mass analyzer
operating in reflection mode. The accelerating voltage was 20 kV, and
anthracene (Acros, 99%) was used as the matrix. UV/visible spectra
were obtained using an Evolution 300 Thermo Scientific two-ray
spectrophotometer with a 0.10 cm long cuvette. Fluorescence spectra
at room temperature were recorded with a Hitachi F-7000
spectrofluorimeter. The fluorescence quantum yields were measured
with respect to rhodamine.
Figure 9. Florescence emission spectra for 3−5 (λex 290 nm) in
solution in CH2Cl2.
Known compounds (C6F5)2GeH2, (C6F5)3GeH,13 Me4Ge,45 and
Ph4Ge36 were obtained by employing published procedures.
Dimethyldichlorogermane, Me2GeCl2. The improved proce-
dure with detailed conditions was used.55 At 0 °C under strong stirring
acetyl chloride (51.00 mL, 56.50 g, 0.72 mmol) was added dropwise to
the mixture of Me4Ge (26.00 g, 200.00 mmol) and anhydrous AlCl3
(112.00 g, 840.00 mmol). The reaction mixture was slowly warmed to
room temperature and stirred for 5 h, then heated at 100 °C for 1 h.
Volatile materials were distilled twice using an effective condenser,
giving dimethyldichlorogermane as a colorless liquid, bp 119−123 °C
Figure 10. Fluorescence emission spectra for 3−5 (λex 320 nm) in the
solid state.
groups displayed luminescence properties. The presence of
acceptor groups results in a red-shifted emission. The found
results would stimulate investigations in design and further
investigation of materials based on group 14 elements.
1
(12 mmHg). Yield: 18.40 g (53%). H NMR (δ, ppm, CDCl3): 1.20
(s, 6H, CH3). 13C NMR (δ, ppm, CDCl3): 10.77 (CH3).
EXPERIMENTAL SECTION
■
Diphenyldichlorogermane, Ph2GeCl2. GeCl4 (2.20 g, 10.20
mmol) was added to the mixture of Ph4Ge (3.93 g, 10.30 mmol) and
AlCl3 (0.24 g, 1.80 mmol). The procedure of freezing in liquid
nitrogen, evacuation, and slow warming to room temperature was
repeated three times, and then the solution obtained was heated at 125
°C (oil bath temperature) for 4 h. After cooling to room temperature
the reaction mixture was treated with concentrated HCl and extracted
with benzene (3 × 30 mL). Combined organic phases were dried over
MgSO4; then all volatile materials were removed under reduced
pressure, and the residue was fractionized using an oil bath for heating
(150−180 °C). Diphenyldichlorogermane (2.23 g, 73%) was isolated
as a colorless oil, bp 115−120 °C (0.17 mmHg), bp 120−124 °C (0.01
mmHg).56 1H NMR (δ, ppm, CDCl3): 7.77−7.79 (m, 4H, aromatic
protons), 7.51−7.58 (m, 6H, aromatic protons). 13C NMR (δ, ppm,
CDCl3): 129.01, 131.80, 132.67, 134.29 (aromatic carbons). NMR
spectra correspond to known data.5c
General Remarks. All manipulations were performed under a dry,
oxygen-free argon atmosphere using standard Schlenk techniques.
GeCl4 (Aldrich), p-FC6F4Br (Aldrich), and p-F3CC6F4Br (Aldrich)
were distilled prior to use. n-BuLi (Aldrich) and LiAlH4 (Aldrich) were
used as supplied from commercial sources. Solvents were dried using
the usual procedures. Tetrahydrofuran, diethyl ether, and triethylamine
were stored under solid KOH and then distilled over sodium/
benzophenone. Toluene and n-hexane were refluxed and distilled over
sodium. Dichloromethane, CCl4, and acetonitrile were distilled under
CaH2. C6D6 was distilled over sodium under argon. CDCl3 was
distilled over CaH2 under argon. The purity of the new compounds
has been established based on elemental analysis or MALDI mass
spectrometry. Me2GeCl2 and Ph2GeCl2 are known compounds; for
Me2Ge(NMe2)2 and Ph2Ge(NMe2)2 elemental analysis was not
performed due to their extreme sensitivity; for compounds 1d and
2d elemental analysis data are also sensitive to moisture traces. In the
H
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Synthesis of Functionalized Triarylgermanes. The Mixture
of (p-FC6H4)3GeCl/(p-FC6H4)3GeBr, 1a/1b. Synthesis of p-
FC6H4MgBr. A solution of p-FC6H4Br (33.00 mL, 300.00 mmol) in
THF (100 mL) was added dropwise to a suspension of Mg (7.05 g,
290.00 mmol) in THF (40 mL). Then the reaction mixture was
refluxed for 2 h, cooled to room temperature, and filtered. The
solution of p-FC6H4MgBr in THF obtained was used without further
purification.
Synthesis of the Mixture of (p-FC6H4)3GeCl/(p-FC6H4)3GeBr, 1a/
1b. At 0 °C the solution of p-FC6H4MgBr in THF obtained as stated
above was added dropwise to a solution of GeCl4 (11.00 mL, 97.00
mmol) in THF (120 mL). The reaction mixture was refluxed for 19 h.
Then THF (180 mL) was distilled off, hexane (200 mL) was added,
the mixture was filtered, and the solid was washed with hexane (2 × 50
mL). All volatile materials were removed from the mother liquor, and
the residue was recrystallized twice from n-hexane. The mixture of the
compounds 1a/1b (1:4, according to 19F NMR) was obtained (9.16 g,
22% based on GeCl4) as a white glass. This mixture was used for
synthesis of 1c without further purification.
Tris(p-fluorophenyl)germanium Dimethylamide, (p-
FC6H4)3GeNMe2 (1d). A solution of the mixture of (p-
FC6H4)3GeCl/(p-FC6H4)3GeBr (1a/1b) (5.36 g, 12.53 mmol) in
toluene (40 mL) was added to a suspension of LiNMe2 (0.80 g, 15.68
mmol) in toluene (40 mL). The reaction mixture was stirred at room
temperature for 3 d and then filtered. All volatile materials were
removed under reduced pressure. Compound 2d (4.33 g, 86%) was
isolated as a colorless, low-melting substance. 1H NMR (δ, ppm,
C6D6): 7.37−7.32, 6.91−6.86 (2m, each 6H, aromatic protons), 2.59
(s, 6H, NMe2). 13C NMR (δ, ppm, C6D6): 164.44 (d, 1J13C‑19F = 248.7
3
4
Hz), 137.07 (d, J13C‑19F = 7.6 Hz), 130.96 (d, J13C‑19F = 3.8 Hz),
115.90 (d, 2J13C‑19F = 20.2 Hz) (aromatic carbons), 41.32 (NMe2). 19
F
NMR (δ, ppm, C6D6): −110.58 to (−110.66) (m, 3F, 3 p-C6H4F).
Anal. Calcd for C20H18F3GeN (401.9988): C, 59.75; H, 4.51; N, 3.48.
Found: C, 57.78; H, 4.73; N, 3.06.
The Mixture of (p-F3CC6H4)3GeCl/(p-F3CC6H4)3GeBr, 2a/2b.
Synthesis of p-F3CC6H4MgBr. A solution of p-F3CC6H4Br (42 mL,
67.52 g, 300.00 mmol) in THF (40 mL) was added dropwise to a
suspension of magnesium (7.30 g, 300.00 mmol) in THF (100 mL).
After the addition reaction the mixture was refluxed for 2 h. The black
solution of the Grignard reagent was filtered and used further without
purification.
Tris(p-fluorophenyl)germanium Chloride, (p-FC6H4)3GeCl
(1a). Catalytic quantities of 2,2′-azobisisobutyronitrile (AIBN)
(0.0457 g, 0.28 mmol, 5 mol %) were added to the solution of
tris(p-fluorophenyl)germane, 1c (2.00 g, 5.57 mmol), in CCl4 (40
mL). The procedure of freezing in liquid nitrogen, evacuation, and
slow warming was repeated three times, and then the solution
obtained was heated at 90 °C for 3 h. After cooling to room
temperature all volatile materials were removed under reduced
pressure. The residue was recrystallized from n-hexane. Compound
Synthesis of the Mixture of (p-F3CC6H4)3GeCl/(p-F3CC6H4)3GeBr
(2a/2b). At 0 °C the solution of the Grignard reagent obtained as
stated above was added dropwise to a solution of GeCl4 (11.18 mL, 98
mmol) in THF (120 mL) during 2 h. The reaction mixture was
warmed to room temperature and refluxed for 12 h. Then THF (120
mL) was distilled off, and hexane (150 mL) was added. The mixture
obtained was filtered. The volatile materials were removed under
reduced pressure, and the residue was fractionalized, giving two
fractions. The first fraction, 5.81 g (20% based on GeCl4), bp 110−150
°C (0.08 mmHg), is a mixture of 2a/2b (6:5). The second fraction,
2.09 g (8% based on GeCl4), bp 155−170 °C (0.08 mmHg), is 2b,
which solidifies on standing at room temperature. Compound 2b may
be recrystallized from hexane, giving an analytically pure substance.
1
1a (2.06 g, 94%) was isolated as a white powder. H NMR (δ, ppm,
CDCl3): 7.61−7.56, 7.19−7.14 (2m, each 6H, aromatic protons). 13C
1
NMR (δ, ppm, CDCl3): 164.59 (d, J13C‑19F = 251.0 Hz), 136.00 (d,
3J13C‑19F = 8.1 Hz), 129.73 (d, 4J13C‑19F = 3.7 Hz), 116.15 (d, 2J13C‑19F
=
20.5 Hz) (aromatic carbons). 19F NMR (δ, ppm, CDCl3): −108.79 to
(−108.87) (m, 3F, 3 p-C6H4F). Anal. Calcd for C18H12ClF3Ge
(383.3761): C, 54.96; H, 3.07. Found: C, 55.18; H, 2.88. Crystals
suitable for X-ray analysis were obtained after recrystallization from n-
hexane.
1
Compound 2a, H NMR (δ, ppm, CDCl3): 7.89, 7.80 (2d, each 6H,
3JH−H = 7.8 Hz, aromatic protons). 13C NMR (δ, ppm, CDCl3):
2
138.74 (ipso-Ge), 133.86 (q, J13C‑19F = 32.8 Hz), 133.13 (ortho-Ge),
Tris(p-fluorophenyl)germanium Bromide, (p-FC6H4)3GeBr
(1b). Catalytic quantities of benzoyl peroxide (0.0678 g, 0.28 mmol,
5 mol %) were added to the solution of tris(p-fluorophenyl)germane,
1c (2.00 g, 5.57 mmol), and N-bromosuccinimide (1.09 g, 5.13 mmol)
in toluene (40 mL). The solution obtained was refluxed for 3 h. After
cooling to room temperature all volatile materials were removed under
reduced pressure. The residue was recrystallized from n-hexane.
Compound 1b (2.15 g, 88%) was isolated as a white powder. 1H NMR
(δ, ppm, CDCl3): 7.61−7.56, 7.18−7.13 (2m, each 6H, aromatic
125.76 (q, 3J13C‑19F = 3.8 Hz) (aromatic carbons), 123.47 (q, 1J13C‑19F
=
272.8 Hz, CF3). 19F NMR (δ, ppm, CDCl3): −63.40 (s, 9F, 3CF3).
1
Compound 2b, H NMR (δ, ppm, CDCl3): 7.76, 7.73 (2d, each 6H,
3JH−H = 8.6 Hz, aromatic protons). 13C NMR (δ, ppm, CDCl3):
2
137.80 (ipso-Ge), 134.49 (ortho-Ge), 133.11 (q, J13C‑19F = 32.9 Hz),
125.60 (q, 3J13C‑19F = 3.7 Hz) (aromatic carbons), 123.70 (q, 1J13C‑19F
=
272.6 Hz, CF3). 19F NMR (δ, ppm, CDCl3): −63.31 (s, 9F, 3CF3).
Tris(p-(trifluoromethyl)phenyl)chlorogermane, (p-
F3CC6H4)3GeCl (2a). A catalytic quantity of AIBN (0.0226 g, 0.14
mmol, 5 mol %) was added to the solution of (p-F3CC6H4)3GeH (2c)
(1.40 g, 2.75 mmol) in CCl4 (40 mL). The reaction mixture was
frozen in liquid nitrogen, evacuated under high vacuum, slowly
warmed to room temperature, and heated at 95 °C for 2 h. Then
volatile materials were removed under reduced pressure, and the
residue was recrystallized from n-hexane. Compound 2a (1.34 g, 90%)
was isolated as a colorless oil, which solidifies on standing. Anal. Calcd
for C21H12ClF9Ge (543.3986): C, 46.42; H, 2.23. Found: C, 46.24; H,
2.25.
1
protons). 13C NMR (δ, ppm, CDCl3): 164.52 (d, J13C‑19F = 251.0
3
4
Hz), 136.11 (d, J13C‑19F = 8.1 Hz), 129.64 (d, J13C‑19F = 3.3 Hz),
116.09 (d, 2J13C‑19F = 20.5 Hz) (aromatic carbons). 19F NMR (δ, ppm,
CDCl3): −108.90 to (−108.97) (m, 3F, 3 p-C6H4F). Anal. Calcd for
C18H12BrF3Ge (437.8271): C, 49.38; H, 2.76. Found: C, 49.02; H,
2.82. Crystals suitable for X-ray analysis were obtained after
recrystallization from n-hexane.
Tris(p-fluorophenyl)germane, (p-FC6H4)3GeH (1c). At 0 °C
LiAlH4 (0.06 g, 1.58 mmol) was added to a solution of the mixture of
1a/1b (0.65 g, 1.52 mmol) in ether (40 mL). The reaction mixture
was stirred for 6 h; then 2 M aqueous H2SO4 (40 mL) was added
dropwise, the organic phase was separated, and the aqueous layer was
extracted with ether (3 × 20 mL). Combined organic phases were
dried over anhydrous Na2SO4; then the solvent was removed under
reduced pressure, and the residue was recrystallized from n-hexane.
Compound 1c (0.46 g, 84%) was isolated as a white powder. 1H NMR
(δ, ppm, CDCl3): 7.48−7.43, 7.12−7.07 (2m, each 6H, aromatic
protons), 5.70 (s, 1H, GeH). 13C NMR (δ, ppm, CDCl3): 163.87 (d,
1J13C‑19F = 248.7 Hz), 136.75 (d, 3J13C‑19F = 7.6 Hz), 130.22 (d, 4J13C‑19F
Tris(p-(trifluoromethyl)phenyl)bromogermane, (p-
F3CC6H4)3GeBr (2b). N-Bromosuccimide (0.54 g, 3.03 mmol) was
added to a solution of (p-F3CC6H4)3GeH (2c) (1.40 g, 2.75 mmol)
and a catalytic quantity of benzoyl peroxide (0.0339 g, 0.14 mmol, 5
mol %) in toluene (20 mL). The reaction mixture was refluxed for 2 h.
Then volatile materials were removed under reduced pressure, and the
residue was extracted with n-hexane and recrystallized from n-hexane.
Compound 2b (1.47 g, 92%) was isolated as a white powder. Anal.
Calcd for C21H12BrF9Ge (587.8496): C, 42.91; H, 2.06. Found: C,
42.87; H, 1.98. Crystals suitable for X-ray analysis were obtained after
recrystallization from n-hexane.
2
= 3.8 Hz), 115.75 (d, J13C‑19F = 20.6 Hz) (aromatic carbons). 19F
NMR (δ, ppm, CDCl3): −111.25 to (−111.33) (m, 3F, 3 p-C6H4F).
Anal. Calcd for C21H13F9Ge (508.9535): C, 49.56; H, 2.57. Found: C,
49.58; H, 2.48.
Tris(p-(trifluoromethyl)phenyl)germane, (p-F3CC6H4)3GeH
(2c). At 0 °C LiAlH4 (0.05 g, 1.20 mmol) was added to a solution
of the mixture of 2a/2b (0.50 g, 0.89 mmol) in ether (40 mL). The
I
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reaction mixture was stirred for 6 h; then 2 M H2SO4 (40 mL) was
added dropwise, the organic phase was separated, and the aqueous
layer was extracted with ether (3 × 20 mL). Combined organic phases
were dried over anhydrous Na2SO4. Then the solvent was removed
under reduced pressure, and the residue was recrystallized from n-
hexane. Compound 2c (0.34 g, 76%) was isolated as a white powder.
3), 169 ([(p-FC6H4)Ge + H]+, 10). UV/vis (CH2Cl2), λmax nm (ε,
M
C, 60.40; H, 3.38. Found: C, 58.42; H, 3.08. MALDI, m/z (rel %): 716
([M]+, 100). Crystals suitable for X-ray analysis were obtained after
recrystallization from chloroform.
−1 cm−1): 237 (4.1 × 104). Anal. Calcd for C36H24F6Ge2 (715.8462):
Hexa(p-(trifluoromethyl)phenyl)digermane, (p-F3CC6H4)3Ge−Ge-
(C6H4CF3-p)3 (4): white powder, yield 0.37 g (46%). 1H NMR (δ, ppm,
CDCl3): 7.59, 7.34 (2d, 3JH−H = 7.7 Hz, each 12H, aromatic protons).
13C NMR (δ, ppm, CDCl3): 139.56 (ipso-Ge), 135.44 (ortho-Ge),
3
1H NMR (δ, ppm, CDCl3): 7.67, 7.64 (2d, each 6H, JH−H = 8.3 Hz,
aromatic protons), 5.88 (s, 1H, GeH). 13C NMR (δ, ppm, CDCl3):
2
138.65 (ipso-Ge), 135.36 (ortho-Ge), 131.98 (q, J13C‑19F = 32.5 Hz),
132.21 (q, 2J13C‑19F = 32.6 Hz), 125.63 (q, 3J13C‑19F = 3.7 Hz) (aromatic
125.25 (q, 3J13C‑19F = 3.7 Hz) (aromatic carbons), 122.96 (q, 1J13C‑19F
=
1
carbons), 123.80 (q, J13C‑19F = 272.4 Hz, CF3). 19F NMR (δ, ppm,
273.0 Hz, CF3). 19F NMR (δ, ppm, CDCl3): −63.17 (s, 9F, 3CF3).
Anal. Calcd for C21H13F9Ge (508.9535): C, 49.56; H, 2.57. Found: C,
49.58; H, 2.48. Crystals suitable for X-ray analysis were obtained after
recrystallization from n-hexane.
CDCl3): −63.21 (s, 18F, 6 p-C6H4CF3). MS EI, m/z (rel %): 1016
([M]+, 3), 509 ([(p-F3CC6H4)3Ge + H]+, 100), 490 ([(p-
F3CC6H4)3Ge + H − F]+, 38), 219 ([(p-F3CC6H4)Ge + H]+, 15).
UV/vis (CH2Cl2), λmax nm (ε, M−1 cm−1): 239 (2.3 × 104). Anal.
Calcd for C42H24F18Ge2 (1015.8912): C, 49.66; H, 2.38. Found: C,
49.12; H, 2.13. MALDI, m/z (rel %): 1016 ([M]+, 100). Crystals
suitable for X-ray analysis were obtained after recrystallization from
CH2Cl2/n-hexane.
Tris(p-(trifluoromethyl)phenyl)germanium Dimethylamide,
(p-F3CC6H4)3GeNMe2 (2d). A solution of (p-F3CC6H4)3GeCl (2a)
(2.86 g, 5.26 mmol) in toluene (15 mL) was added to a suspension of
LiNMe2 (0.32 g, 6.30 mmol) in toluene (40 mL). The reaction
mixture was stirred at room temperature for 4 d and then filtered. All
volatile materials were removed under reduced pressure. Compound
2d (2.56 g, 88%) was isolated as a colorless oil that solidifies on
2,2-Bis(pentafluorophenyl)-1,1,1,3,3,3-hexakis(p-(trifluoro-
methyl)phenyl)trigermane, (p-F3CC6H4)3Ge−Ge(C6F5)2-Ge(C6H4CF3-
p)3 (5). (C6F5)2GeH2 (0.16 g, 0.40 mmol) was added to a solution
of (p-F3CC6H4)3GeNMe2 (0.44 g, 0.80 mmol) in MeCN (30 mL).
The procedure of freezing in liquid nitrogen, evacuation, and warming
to room temperature was repeated three times. The mixture obtained
was heated at 100 °C for 46 h. Then all volatile materials were
removed under reduced pressure, and the residue was extracted with
CH2Cl2, filtered, and recrystallized from CH2Cl2/n-hexane. Com-
pound 6 was isolated as orange crystals, yield 0.32 g (57%). 1H NMR
(δ, ppm, CDCl3): 7.54, 7.30 (2d, 3JH−H = 7.6 Hz, each 12H, aromatic
protons). 13C NMR (δ, ppm, CDCl3): 138.27 (ipso-Ge), 135.10
1
standing at room temperature. H NMR (δ, ppm, C6D6): 7.43, 7.34
(2d, 3JH−H = 8.1 Hz, each 6H, aromatic protons), 2.52 (s, 6H, NMe2).
13C NMR (δ, ppm, C6D6): 139.38 (ipso-Ge), 135.43 (ortho-Ge),
2
3
132.20 (q, J13H‑19F = 32.2 Hz), 125.34 (q, J13H‑19F = 3.7 Hz)
1
(aromatic carbons), 124.72 (q, J13H‑19F = 272.3 Hz, CF3), 41.19
(NMe2). 19F NMR (δ, ppm, C6D6): −62.76 (s, 9F, 3CF3). Anal. Calcd
for C23H18F9GeN (551.0213): C, 50.04; H, 3.29; N, 2.54. Found: C,
49.24; H, 3.03; N, 2.38.
Bis(dimethylamido)dimethylgermane, Me2Ge(NMe2)2.
Me2GeCl2 (6.03 g, 35.00 mmol) was added dropwise to a suspension
of LiNMe2 (3.90 g, 76.40 mmol) in ether (40 mL). The reaction
mixture was stirred at room temperature for 20 h and then was
refluxed for 5 h. The mixture was filtered, and the residue was carefully
fractionized. Me2Ge(NMe2)2 (5.17 g, 78%) was isolated as a colorless
liquid, bp 142−143 °C. 1H NMR (δ, ppm, C6D6): 2.54 (s, 12H,
NMe2), 0.18 (s, 6H, Me2Ge). 13C NMR (δ, ppm, C6D6): 40.41
(NMe2), −5.24 (Me2Ge). Elemental analysis data are unsatisfactory
due to high sensitivity of the compounds to the traces of moisture.
Bis(dimethylamido)diphenylgermane, Ph2Ge(NMe2)2. At 0
°C a solution of Ph2GeCl2 (7.19 g, 24.15 mmol) in ether (20 mL) was
added dropwise to a suspension of LiNMe2 (2.71 g, 53.00 mmol) in
ether (40 mL). The reaction mixture was stirred at room temperature
overnight and then was refluxed for 5 h. All volatile materials were
removed under reduced pressure, toluene (40 mL) was added, and the
suspension obtained was filtered. Then the solvent was removed under
reduced pressure. Ph2Ge(NMe2)2 (6.67 g, 88%) was isolated as a
colorless liquid. 1H NMR (δ, ppm, C6D6): 7.71−7.65 (m, 4H,
aromatic protons), 7.24−7.16 (m, 6H, aromatic protons), 2.71 (s,
12H, NMe2). 13C NMR (δ, ppm, C6D6): 135.21, 135.01, 129.72,
128.56 (aromatic carbons), 40.87 (NMe2). Elemental analysis data are
unsatisfactory due to high sensitivity of the compounds to the traces of
moisture.
2
(ortho-Ge), 132.56 (q, J13C‑19F = 32.9 Hz), 125.40 (br s) (aromatic
carbons), 123.59 (q, 1J13C‑19F = 273.0 Hz, CF3). Carbons of C6F5 were
not found due to low intensity and the high value of nuclear coupling.
19F NMR (δ, ppm, CDCl3): −63.31 (s, 18F, 6 p-C6H4CF3), −123.21
to (−123.26) (m, 4F, C6F5), −148.26 to (−148.34) (m, 2F, p-C6F5),
−158.76 to (−158.92) (m, 4F, C6F5). MS EI, m/z (rel %): 509 ([(p-
F3CC6H4)3Ge + H]+, 100), 218 ([(p-F3CC6H4)Ge]+, 12), 167
([C6F5]+, 9). UV/vis (CH2Cl2), λmax nm (ε, M−1 cm−1): 264 (2.4 ×
104). Anal. Calcd for C54H24F28Ge3 (1422.6436): C, 45.59; H, 1.70.
Found: C, 45.08; H, 1.82. MALDI, m/z (rel %): 1423 ([M]+, 100).
Crystals suitable for X-ray analysis were obtained after recrystallization
from CH2Cl2/n-octane.
X-ray Crystallography. Experimental intensities were measured
on a Bruker SMART APEX II (for 1a, 2b, 2c, and 3) and on a
STADIVARI Pilatus (for 1b, 4, and 5) diffractometer using ω-scan
mode. Absorption correction based on measurements of equivalent
reflections was applied. The structures were solved by direct methods
and refined by full matrix least-squares based on F2 with anisotropic
thermal parameters for all non-hydrogen atoms. All aromatic hydrogen
atoms were placed in calculated positions. In 2c, the germanium H
atom was found from difference Fourier synthesis. All H atoms were
refined using a riding model. In the structures 2b, 2c, 4, and 5 all −CF3
groups are rotationally disordered over two or three positions. They
were refined with restrained C−F and F···F distances (SADI). In 3, all
phenyl groups are rotationally disordered over two positions. Minor
components of these disordered substituents were refined with
constrained C···C bond lengths (AFIX 66). Details of X-ray studies
are given in Tables S1 and S2 (Supporting Information). The crystals
of 2c were pseudomerohedrally twinned (β = 90.063(1)°) with twin
law 1 0 0 0 −1 0 0 0 −1 and domain ratio 0.635(4)/0.365(4).
Crystal data are deposited in the Crystallographic Data Centre as
supplementary publications under the CCDC numbers 1505504−
1505509. This information may be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Synthesis of Oligogermanes with Electron-Withdrawing
Groups. General Procedure for Oligogermane Synthesis. Corre-
sponding triarylgermane (0.80 mmol) was added to the solution of
triarylgermanium dimethylamide (0.80 mmol) in MeCN (20 mL).
The procedure of freezing in liquid nitrogen, evacuation, and warming
to room temperature was repeated three times. The mixture obtained
was heated at 100 °C for 86 h; then all volatile materials were removed
under reduced pressure, and the residue was extracted with CH2Cl2,
filtered, and recrystallized from CH2Cl2/n-hexane.
Hexa(p-fluorophenyl)digermane, (p-FC6H4)3Ge−Ge(C6H4F-p)3 (3):
1
white powder, yield 0.32 g (56%). H NMR (δ, ppm, CDCl3): 7.18−
7.13, 7.02−6.96 (2m, each 12H, aromatic protons). 13C NMR (δ,
1
3
ppm, CDCl3): 163.81 (d, J13C‑19F = 249.5 Hz), 136.93 (d, J13C‑19F
=
7.6 Hz), 131.49 (d, 4J13C‑19F = 3.8 Hz), 115.95 (d, 2J13C‑19F = 19.8 Hz)
(aromatic carbons). 19F NMR (δ, ppm, CDCl3): −111.16 to
(−111.25) (m, 6F, 6 p-C6H4F). MS EI, m/z (rel %): 716 ([M]+,
12), 359 ([(p-FC6H4)3Ge + H]+, 100), 264 ([(p-FC6H4)2Ge + H]+,
Electrochemistry. Electrochemical measurements were carried out
using an Autolab 302N potentiostat interfaced through Nova 2.0
software to a personal computer. Electrochemical measurements were
performed in a glovebox under oxygen levels of less than 5 ppm using
J
DOI: 10.1021/acs.organomet.6b00767
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solvent that had been purified by passing through an alumina-based
purification system. Diamond-polished glassy carbon electrodes of 3
mm diameter were employed for cyclic voltammetry (CV) scans. CV
data were evaluated using standard diagnostic criteria for diffusion
control and for chemical and electrochemical reversibility. The
experimental reference electrode was a silver wire coated with
anodically deposited silver chloride and separated from the working
solution by a fine glass frit. The electrochemical potentials in this paper
are referenced to the ferrocene/ferrocenium couple, as recommended
elsewhere.57 The ferrocene potential was obtained by its addition to
the analyte solution58 at an appropriate time in the experiment.
[NBu4][B(C6F5)4] was prepared as previously described.59
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ASSOCIATED CONTENT
* Supporting Information
■
S
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2006, 25 (13), 3211−3219. (b) Amadoruge, M. L.; Gardinier, J. R.;
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2011, 696 (18), 2993−2999. (f) Samanamu, C. R.; Amadoruge, M. L.;
Schrick, A. C.; Chen, C.; Golen, J. A.; Rheingold, A. L.; Materer, N. F.;
Weinert, C. S. Organometallics 2012, 31 (11), 4374−4385. (g) Schrick,
E. K.; Forget, T. J.; Roewe, K. D.; Schrick, A. C.; Moore, C. E.; Golen,
J. A.; Rheingold, A. L.; Materer, N. F.; Weinert, C. S. Organometallics
2013, 32 (7), 2245−2256. (h) Roewe, K. D.; Rheingold, A. L.;
Weinert, C. S. Chem. Commun. 2013, 49 (75), 8380−8382.
(8) (a) Tanabe, M.; Hanzawa, M.; Ishikawa, N.; Osakada, K.
Organometallics 2009, 28 (20), 6014−6019. (b) Tanabe, M.; Osakada,
K. Organometallics 2010, 29 (21), 4702−4710. (c) Tanabe, M.;
Hanzawa, M.; Osakada, K. Phosphorus, Sulfur Silicon Relat. Elem. 2011,
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Organometallics 2012, 31 (21), 7386−7393.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00767.
NMR spectra for the compounds synthesized in this
work (Figures S1−S41); tables of crystallographic data
(Tables S1, S2) for compounds 1a,b, 2b,c, 3, 4, and 5;
electrochemistry CV curves (Figures S42−S44); compu-
tational details; calculated structures (Figures S45−S56)
(PDF)
Crystallographic data for compounds 1a,b, 2b,c, 3, 4, and
5 (CIF)
Calculated structures (XYZ)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: zaitsev@org.chem.msu.ru.
*E-mail: kevin.lam@nu.edu.kz.
■
ORCID
Kirill V. Zaitsev: 0000-0003-3106-8692
Notes
The authors declare no competing financial interest.
(9) (a) Hlina, J.; Zitz, R.; Wagner, H.; Stella, F.; Baumgartner, J.;
Marschner, C. Inorg. Chim. Acta 2014, 422, 120−133. (b) Marschner,
C.; Hlina, J. Catenated Compounds − Group 14 (Ge, Sn, Pb). In
Comprehensive Inorganic Chemistry II (2nd ed.); Reedijk, J.;
Poeppelmeier, K., Eds.; Elsevier: Amsterdam, 2013; pp 83−117.
(c) Hlina, J.; Baumgartner, J.; Marschner, C. Organometallics 2010, 29
(21), 5289−5295.
ACKNOWLEDGMENTS
■
This work was financially supported by the Russian President
Grant for Young Russian Scientists (MK-1790.2014.3, K.V.Z.),
in part by M.V. Lomonosov Moscow State University Program
of Development (K.V.Z.), Nazarbayev University (ORAU grant
for Medicinal Electrochemistry, K.L.), and the Ministry of
Education and Science of Kazakhstan (K.L.). We are grateful to
Dr. S.V. Gruener (MSU) for his valuable advice and assistance
in obtaining the initial germanes.
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DOI: 10.1021/acs.organomet.6b00767
Organometallics XXXX, XXX, XXX−XXX