Ge8R6: The ligands define the bonding situation within the cluster core

Article abstract of DOI:10.1039/b508379f

The disproportionation reaction of Ge(I) halides open up a way to cluster compounds with an average oxidation state of the germanium atoms inside the cluster core in between 0 and 1. Simultaneously compounds with germanium in an oxidation state greater than I are formed. During the reaction of Ge(I) bromide with one equivalent of LiR (R = 2,6-(tBuO)₂C₆H ₃) the cluster compound Ge₈R₆ and the molecular Ge(IV) compound R₃GeBr were isolated, representing the reduction and the oxidation product of the disproportionation reaction, respectively. The molecular structure of the Ge₈ cluster compound shows a highly different arrangement of the eight germanium atoms in the cluster core with respect to the only other Ge₈R₆ cluster compound where amido ligands are bound to the germanium atoms. Quantum-chemical calculations reveal that the distinct arrangement of the germanium atoms can be traced back to a different bonding situation inside the cluster frameworks, which is induced by the different ligands attached to the germanium atoms. These experimental and theoretical results show that the ligands are not only necessary for protecting the cluster core against the exterior but also have a strong influence on the bonding situation and therefore on the electronic situation inside the cluster core. Hence, the ligand influences the electronic properties and consequently the physical properties which is now seen, for the first time, in metalloid germanium cluster compounds. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b508379f

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F U L L P A P E R
Ge8R6: The ligands define the bonding situation within the
cluster core
Andreas Schnepf*a and Christian Drostb
a Institut fu¨r Anorganische Chemie der Universita¨t Karlsruhe (TH), Engesserstr., Geb.30.45,
D-76128, Karlsruhe. E-mail: schnepf@chemie.uni-karlsruhe.de; Fax: +49 (721) 608-4854;
Tel: +49 (721) 608-2951
b Institut fu¨r Anorganische Chemie der Universita¨t Leipzig, Johannisallee 29, D-04103, Leipzig.
E-mail: cdrost@rz.uni-leipzig.de; Fax: +49 (341) 9736-199; Tel: +49 (341) 9736-130
Received 14th June 2005, Accepted 29th July 2005
First published as an Advance Article on the web 31st August 2005
The disproportionation reaction of Ge(I) halides open up a way to cluster compounds with an average oxidation state
of the germanium atoms inside the cluster core in between 0 and 1. Simultaneously compounds with germanium in
an oxidation state greater than I are formed. During the reaction of Ge(I) bromide with one equivalent of LiR
(R = 2,6-(tBuO)2C6H3) the cluster compound Ge8R6 and the molecular Ge(IV) compound R3GeBr were isolated,
representing the reduction and the oxidation product of the disproportionation reaction, respectively. The molecular
structure of the Ge8 cluster compound shows a highly different arrangement of the eight germanium atoms in the
cluster core with respect to the only other Ge8R6 cluster compound where amido ligands are bound to the germanium
atoms. Quantum-chemical calculations reveal that the distinct arrangement of the germanium atoms can be traced
back to a different bonding situation inside the cluster frameworks, which is induced by the different ligands attached
to the germanium atoms. These experimental and theoretical results show that the ligands are not only necessary for
protecting the cluster core against the exterior but also have a strong influence on the bonding situation and therefore
on the electronic situation inside the cluster core. Hence, the ligand influences the electronic properties and
consequently the physical properties which is now seen, for the first time, in metalloid germanium cluster compounds.
longer than a normal Ge–Ge single bond of ca. 244 pm.9
This elongation of the homometallic bond length has been
Introduction
Molecular clusters of the general formulae GemRn with n <
attributed to multicenter bonding contributions, leading to a
m (R = organic or other groups) represent a new class of
severe distortion e.g. of the cubane type framework in the
compounds in the area of germanium cluster chemistry.1 Apart
from ligand substituted germanium atoms they also contain
additional unsubstituted or “naked” germanium atoms, only
connected to further germanium atoms. Due to the presence
cluster Ge8[N(SiMe3)2]6. We now report on the synthesis and
structural characterisation of a second example of a Ge8R6 (R =
2,6-(tBuO)2C6H3) cluster compound which—in contrast to the
former—has a nearly undistorted Ge cubane framework with
of these “naked” germanium atoms in formal oxidation state
only marginally differing homometallic distances, showing that
0, the resulting average oxidation state of germanium in the
electronic effects of the ligand can play an important role even
cluster is between 0 and 1; hence these ligand deficient clusters
for the bonding situation inside the cluster core.
may be regarded as stable intermediates between neutral, ligand
balanced clusters of formula GenRn2 and elemental germanium.
They may thus exhibit unusual electronic properties and are
Experimental
of considerable current interest in terms of nanotechnological
General considerations
aspects.3 During the last two years, three different strategies
have proved their value for the preparation of these GemRn
cluster compounds with n < m.1 While the synthesis of the
cationic cluster species [Ge10(SitBu3)6I]+ with three “naked”
germanium atoms was achieved by Sekiguchi et al. via halosilane
abstraction from a cyclotrigermene precursor,4 Power and
co-workers gained the octahedral cluster Ge6R2 (R = 2,6-
Dipp2C6H3; Dipp = 2,6-iPr2C6H3)5 in a reductive coupling
reaction of the aryl germanium(II) halide Ge(Cl)R in the
presence of GeCl2·dioxane. We have recently established a
further approach towards the synthesis of cluster compounds
with unsubstituted germanium atoms, which involves the low
temperature treatment of subvalent germanium(I) bromide with
alkaline metal derivatives of appropriate ligands. We assume
that germanium rich intermediates are formed in the course
of a disproportionation reaction of Ge(I) bromide, which
are subsequently trapped by kinetic stabilization.6 Following
this route we have previously isolated the cluster compounds
Ge8[N(SiMe3)2]67 and [Ge9{Si(SiMe3)3}3]−,8 comprising two and
six “naked” germanium atoms, respectively.
All manipulations were carried out under nitrogen or in vacuo
in Schlenk-type glassware on a dual-manifold Schlenk line.
Solvents were pre-dried over sodium wire and distilled from
sodium/potassium alloy (pentane and toluene) or sodium ben-
zophenone (diethyl ether). Deuteriated benzene was degassed
prior to use. HBr and n-butyllithium in hexane (2.5 mol dm−3)
were commercial samples and used as supplied. The lithium aryl
[Li{2,6-(tBuO)2C6H3}] was prepared according to the published
procedure.10 The synthesis of Ge(I)Br and the subsequent reac-
tion was performed in a self-made apparatus.11 NMR spectra
were recorded at 298 K in C6D6 (1H, 13C) on a Bruker AVANCE
400 Spectrometer; d values are relative to Me4Si at 0 ppm; J
values are given in Hz.
Preparations of Ge(Br)[2,6-(tBuO)2C6H3]3 and
Ge8[2,6-(tBuO)2C6H3]6
Liquid germanium was treated with HBr under high vacuum
at 1550 ◦C, and the resulting gaseous products (ca. 20 mmol
GeBr) were co-condensed with a mixture of toluene–NnPr3 (5 :
1; 100 ml) at a surface at −196 ◦C. After the co-condensate was
Common to all these cluster compounds is the fact, that
some of the Ge–Ge bonds in the cluster core are considerably
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 3 2 7 7 – 3 2 8 0
3 2 7 7
©

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Table 1 Crystal data and details of structural determinations R = 2,6-
(tBuO)2C6H3
temperature resulted in a dark red solution. Work up procedures
afforded bright-yellow crystals of the germanium(IV) compound
R3GeBr 1 in about 10% yield. The molecular structure of 1
was confirmed by X-ray crystal structure analyses. A view of
compound 1 is shown in Fig. 1 together with selected bond
distances and angles.
R3GeBr 1
Ge8R6 2·Et2O
Formula
Mr
C42H63O6BrGe
816.42
150
C88H136O13Ge8
1982.69
150
T/K
Crystal system
Space group
Triclinic
Triclinic
¯
¯
P1 (no. 2)
P1 (no. 2)
˚
a/A
10.042(2)
12.137(2)
17.962(4)
90.49(3)
90.97(3)
107.26(3)
2090.0(7)
2
1.732
1.297
22843
8495
13.856(3)
13.981(3)
15.252(3)
64.18(3)
73.81(3)
61.41(3)
2326.4(8)
1
2.603
1.415
25179
10222
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
l/mm−1
Dc/g cm−3
Reflns measured
Reflns obsd (I > 2rI)
Rint
4.59
1.051
5.95
0.954
GOF
R1 (I > 2r)
wR2 (all data)
0.0359
0.1056
0.0382
0.0852
◦
Fig. 1 Molecular structure (ball and stick model; hydrogen atoms are
omitted for clarity) of R3GeBr (1): Selected bond lengths (pm) and
angles (◦): Ge1–Br1 237.91(6), Ge1–C1 195.8(2), Ge1–C10 196.6(2),
Ge1–C20 197.0(2); C1–Ge1–C10 119.18(9), C1–Ge1–C20 113.83(10),
C10–Ge1–C20 113.65(9).
allowed to warm to −78 C an emulsion consisting of a dark-
red oil in a pale yellow solution was obtained. This emulsion
was subsequently treated with Li[2,6-(tBuO)2C6H3] (4.373 g,
21.4 mmol) dissolved in toluene (100 ml) at −78 ◦C. The reaction
mixture was then slowly allowed to warm to room temperature
to yield a dark red solution. Removing of the solvent in vacuo
yielded a dark red oil, which upon extraction with pentane gave a
likewise coloured solution. Concentration of the pentane extract
afforded bright yellow crystals of Ge(Br)[2,6-(tBuO)2C6H3]3 1
(484 mg, 0.6 mmol, 10%). 1H NMR (250 MHz, C6D6): d 1.16 (s,
9 H, CH3), 1.49 (s, 9 H, CH3), 6.53 (d, J = 8.2 Hz, 1 H, CH),
6.65 (d, J = 8.2 Hz, 1 H, CH), 7.07 (t, J = 8.2 Hz, 1 H, CH)
ppm.
The most striking structural feature of compound 1 are the
bond angles subtended at the central Ge atom by the 2,6-di-tert-
butoxyphenyl groups, being greater than the ideal tetrahedral
angle of 109.5◦. While the Cipso–Ge1–Cipso angles range from
113.6 to 119.2◦, the Cipso–Ge1–Br1 angles are diminished to
an average value of 102.4◦. Hence, compound 1 comprises a
largely distorted tetrahedral arrangement, which is ascribed to
a severe steric repulsion of the spatial encumbered 2,6-di-tert-
butoxyphenyl groups. This is also reflected in a slight increase
in the Cipso–Ge1 (av. 196.5 pm) and a more pronounced increase
in the Ge1–Br1 bond lengths with 237.9 pm, when compared
with corresponding values for the sterically less encumbered
compound Ph3GeBr12 with an average value of 193.5 pm for the
Cipso–Ge bond lengths for two independent molecules and an
average value of 232 pm for the Ge–Br distances. The distances
between the oxygen and germanium atoms for 1 are shorter
than the sum of their van der Waals radii (362 pm), with values
ranging from 288.4 to 321.2 pm, respectively. However, their
close proximity appears inevitable and a strong interaction thus
remains doubtful. This is in contrast to the related compound
The overlaying pentane extract was filtered and evaporated
in vacuo leading to a dark red oil which was taken up in ether.
Concentration of the dark red ether solution yielded a small
amount of orange–red crystals of Ge8[2,6-(tBuO)2C6H3]6 2.
X-Ray crystallography
Table 1 contains the crystal data and details of the X-ray
structural determination for Ge(Br)[2,6-(tBuO)2C6H3]3 1 and
Ge8[2,6-(tBuO)2C6H3]6 2. The data were collected at 150 K on
a Bruker IPDS II diffractometer employing monochromated
˚
Mo-Ka radiation (0.71073 A) from a sealed tube and equipped
with an Oxford Cryosystems cryostat. A numeric absorption
correction was applied using the optically determined shape of
the crystals. The structure was solved by direct methods and
refined by full-matrix least-square techniques. The non hydrogen
atoms were refined anisotropically and the hydrogen atoms were
calculated using a riding model. In 2·Et2O the ether molecule
is disordered over two positions. The disordered atoms were
refined isotropically using a split model with an occupancy of
0.5 : 0.5. The cluster compound Ge8[2,6-(tBuO)2C6H3]6 2 lies
about an inversion centre. Therefore only half of the molecule is
present in the asymmetric unit.
13
Ge(Br)[2-(MeOCH2)C6H4]3 having germanium clearly in a
trigonal bipyramid geometry with the three Cipso atoms in the
equatorial andthe bromineandone oxygen atom (Ge–O 285 pm)
occupying the apical positions, while the residual two oxygen
atoms are located, on average, 465 pm away from the Ge centre.
The Ge atom in 1 appears to be to some extent positively
charged by virtue of the Ge1–Br1 bond elongation. Compound
1 may thus be regarded as being intermediate between an
uncharged derivative and a separate ion pair [R3Ge+]Br−.
1
The H NMR spectrum of 1 in benzene-d6 at room temper-
ature reveals two signals for the tert-butoxy groups, which is
in accordance with restricted rotation of the Cipso–Ge bonds in
solution.
CCDC reference numbers 255632 (1) and 255631 (2).
See http://dx.doi.org/10.1039/b508379f for crystallographic
data in CIF or other electronic format.
The successful isolation of compound 1, in which the ger-
manium atom exhibits an oxidation state of +IV is indicative
of a disproportionation reaction as the synthesis started from
Ge(I) bromide with no oxidizing reagents present in the reaction
solution. Hence, compounds containing germanium in an
average oxidation state between 0 and 1 were expected to be
also present in the reaction mixture. Indeed, further work up
Results and discussion
Treatment of dissolved germanium(I) bromide—which is pre-
pared in a co-condensation reaction and trapped at low
temperature11—with the lithium derivative [Li(l-R)]3 (R =
2,6-(tBuO)2C6H3)10 at −78 ◦C and slow warming to room
3 2 7 8
D a l t o n T r a n s . , 2 0 0 5 , 3 2 7 7 – 3 2 8 0

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procedures afforded the germanium cluster compound Ge8R6
(R = 2,6-(tBuO)2C6H3) 2, which was isolated as orange–red
crystals and characterized by X-ray crystal structure analysis.
The molecular structure of 2 (Fig. 2) consists of a Ge8-cubane
framework in which six of the eight germanium atoms are ligand-
substituted. The remaining two “naked” germanium atoms are
unsubstituted with respect to the ligand R and only connected to
further germanium atoms. Therefore the average oxidation state
of the germanium atoms inside the cluster amounts to 0.75.
Interestingly, the cubane geometry in 2 is nearly undistorted;
the Ge–Ge distances between the ligand-substituted germanium
atoms are only 2 pm shorter with 249.3 pm than the Ge–Ge
distances between the “naked” and the substituted germanium
atoms with 251.7 pm. Furthermore, the Ge–Ge–Ge angles
within the cluster core differ only marginally (88.8–92.5◦) from
the ideal value of 90◦.
A comparison of the Ge–Ge distances in 2 with those found
for the fully substituted Ge8-cluster compounds Ge8R8 (R = 2,6-
Et2C6H3, 249.0 pm;14 R = CMe2CHMe2, 251.6 pm15) reveals an
almost identical Ge–Ge bonding situation. The elimination of
two ligands therefore appears to lead only to very minor changes
in the arrangement of the eight germanium atoms of the cluster
core. This is in sharp contrast with the situation found for the
cluster compound Ge8[N(SiMe3)2]6 3, where a heavily distorted
cubane like structure was observed (Ge–Ge(R) 250 pm; Ge(R)–
Ge(R) 267 pm; R = N(SiMe3)2).7 A summary of the important
structural features of the only two cluster compounds with the
general formula Ge8R6 known to date is given in Table 2.
To understand the nature of these differences, we performed
quantum-chemical calculations on the model compounds Ge8R6
(R = NH2 3ꢀ, C6H5 2ꢀ).16 The calculated Ge–Ge distances
(Table 3) are hereby in good agreement with those established
by X-ray crystal structure analysis, excluding steric reasons
for the different arrangements of the germanium atoms in
both cluster frameworks. The different arrangement of the
eight Ge atoms in 2 and 3 appears to be the outcome of a
different bonding situation inside both cluster frameworks. The
results of the quantum-chemical calculations revealed a more
delocalised bonding electron situation for 3ꢀ than for 2ꢀ. This
difference becomes particularly apparent if the shared electron
numbers (SENs) of an Ahlrichs–Heinzmann population analysis
are considered.17 For instance, the SEN of the three-centered
bonding component in the three-membered (GeR)3 ring are
more than twice as large for 3ꢀ (0.13) as for the corresponding
ring in 2ꢀ (0.06, Table 3).
Fig. 2 Molecular structure of 2 (tBu-groups are omitted for clarity;
thermal ellipsoids with 50% possibility). Selected bond lengths (pm)
and angles (◦): Ge1–Ge2a 249.13(10), Ge1–Ge4 251.59(11), Ge2–Ge4
252.92(11), Ge4–Ge4a 448.30(11), Ge1–C1 197.8(3); Ge2a–Ge1–Ge4
92.57(3), Ge1–Ge4–Ge2 88.70(4), Ge2a–Ge1–C1 123.45(10).
Table 2 Comparison of the different structures of the cluster com-
pounds Ge8[2,6-(tBuO)2C6H3]6 2 and Ge8[N(SiMe3)2]6 3
The ligand attached to the germanium center clearly modifies
the bonding situation inside the cluster core, leading to more
or less localized bonding electrons inside the cluster core. To
establish the attributing factor of the ligand we investigated the
influence of the lone pair at the ligand atom directly bound to the
germanium center by quantum chemical calculations, employing
the model compounds Ge8R6 (R = CH3, SiH3, C6H5, NH2, PH2,
OH; Table 3). The results of the calculations demonstrate, that a
ligand possessing a free electron pair on the hetero atom directly
bound to the germanium atom (R = NH2, PH2, OH) leads to a
higher degree of delocalisation of bonding electrons within the
cluster core. In contrast, ligands R with no free electron pair at
the hetero atom bound to germanium (R = CH3, SiH3, C6H5)
result in more localised bonding electrons in the cluster core
(Table 3). Hence, in the case of the Ge8R6 cluster compounds
the ligand attached to the cluster core is not only of importance
for shielding the cluster core against the exterior but also has a
strong influence on the bonding situation inside the cluster core,
2,6-(tBuO)2C6H3
N(SiMe3)2
I
252.9
249.4
249.5
251.6
250.2
267.2
266.1
249.9
II
III
IV
a
b
c
88.7
90.5
89.9
90.8
81.3
101.4
75.3
d
101.8
Table 3 Results of quantum chemical calculations for the model cluster compounds Ge8R6 (distances in pm; SEN cf . ref. 17)
Ligand (R)
CH3
SiH3
C6H5
NH2
PH2
OH
d(Ge–GeR)
d(GeR–GeR)
259.7
251.0
256.6
252.6
259.0
251.8
254.8
266.3
257.8
265.5
256.7
266.6
SEN (Ge–GeR)
SEN (GeR–GeR–GeR)
0.93
0.06
1.04
0.05
0.97
0.06
1.10
0.13
1.03
0.16
1.14
0.14
D a l t o n T r a n s . , 2 0 0 5 , 3 2 7 7 – 3 2 8 0
3 2 7 9

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which is established for the first time in the results presented
here.
3 M. S. El-Shall, S. Li, D. Graiver and U. Pernisz, in Nanotechnology—
Molecularly Designed Materials, ed. G. M. Chow and K. E. Gon-
salves, ACS Symp. Ser. 622, Washington, DC, 1996.
4 A. Sekiguchi, Y. Ishida, Y. Kabe and M. Ichinohe, J. Am. Chem. Soc.,
2002, 124, 8776.
5 A. F. Richards, H. Hope and P. P. Power, Angew. Chem., 2003, 115,
4205; A. F. Richards, H. Hope and P. P. Power, Angew. Chem., Int.
Ed., 2003, 42, 4071.
6 A. Schnepf, Phosphorus, Sulfur, Silicon, Relat. Elem., 2004, 179, 695.
7 A. Schnepf and R. Ko¨ppe, Angew. Chem., 2003, 115, 940; A. Schnepf
and R. Ko¨ppe, Angew. Chem., Int. Ed., 2003, 42, 911.
8 A. Schnepf, Angew. Chem., 2003, 115, 2728; A. Schnepf, Angew.
Chem., Int. Ed., 2003, 42, 2624.
The apparently different bonding situation of clusters 2 and
3 should also have an influence on the physical properties. The
most obvious difference is the color of the compounds. While
crystalline 2 is orange–red, crystalline 3 is almost black.
Further physical and theoretical studies are necessary in order
to evaluate the differences of both compounds and to gain deeper
insight of ligand influence towards the bonding situation in the
cluster units.
Concluding remarks
9 A. F. Wells, Structural Inorganic Chemistry, Oxford, 5th edn, 1984,
p. 1279.
10 S. Harder, J. Boersma, L. Brandsma, J. A. Kanters, A. J. M.
Duisenberg and J. H. van Lenthe, Organometallics, 1991, 10, 1623.
The compounds R3GeBr
1
and Ge8R6
2
(R
=
2,6-
(tBuO)2C6H3) have been prepared from Ge(I) bromide and
Li[2,6-(tBuO)2C6H3], representing the oxidation and the reduc-
tion product of the disproportionation reaction of the Ge(I)
halide Ge(I)Br. Comparing the bonding situation in 2 with
another Ge8R6 cluster compound (R = N(SiMe3)2) reveals that
the ligand R is not only of importance for a protection of the
germanium framework but also has a strong influence on the
bonding situation inside the cluster core. In the case of the
cluster compound Ge8R6 the bonding situation inside the cluster
core is strongly affected by the ligands, leading to different
physical properties of the isolated clusters. This aspect of ligand
influence might be of general importance for other metalloid
cluster compounds what has to be verified in the future.
11 A. Schnepf and R. K
o¨ppe, Z. Anorg. Allg. Chem., 2002, 2914.
12 H. Preut and F. Huber, Acta Crystallogr., Sect. B: Struct. Crystallogr.
Cryst. Chem., 1979, 35, 83.
13 Y. Sugiyama, T. Matsumoto, H. Yamamoto, M. Nishikawa, M.
Kinoshita, T. Takei, W. Mori and Y. Takeuchi, Tetrahedron, 2003,
59, 8689.
14 A. Sekiguchi, T. Yatabe, H. Kamatani, C. Kabuto and H. Sakurai,
J. Am. Chem. Soc., 1992, 114, 6260.
15 M. Unno, K. Higuchi, K. Furuya, H. Shioyama, S. Kyushin, M.
Goto and H. Matsumoto, Bull. Chem. Soc. Jpn., 2000, 73, 2093.
16 Quantum-chemical calculations were carried out with the RI-DFT
version of the Turbomole program package, by employing the
Becke–Perdew 86-functional and an SVP basis set. The electronic
structure was analyzed with the Ahlrichs–Heinzmann population
analysis based on occupation numbers. Turbomole: O. Treutler and
R. Ahlrichs, J. Chem. Phys., 1995, 102, 346; J. P. Perdew, Phys.
Rev. B, 1986, 33, 8822; BP-86-Funktional: A. D. Becke, Phys. Rev.
Acknowledgements
¨
A, 1988, 38, 3098; K. Eichkorn, O. Treutler, H. Ohm, M. Ha¨ser
We gratefully acknowledge the financial support by the Deutsche
Forschungsgemeinschaft. We also thank Prof. Schno¨ckel for
helpful discussions.
and R. Ahlrichs, Chem. Phys. Lett., 1995, 240, 283; RI-DFT: A.
Scha¨fer, H. Horn and R. Ahlrichs, J. Chem. Phys., 1992, 97, 2571;
SVP: E. R. Davidson, J. Chem. Phys., 1967, 46, 3320; Ahlrichs–
Heinzmann population analysis: K. R. Roby, Mol. Phys., 1974, 27,
81; R. Heinzmann and R. Ahlrichs, Theor. Chim. Acta, 1976, 42, 33;
C. Erhardt and R. Ahlrichs, Theor. Chim. Acta, 1985, 68, 231.
17 The SENs for bonds are a reliable measure of the covalent bonding
strength. For example the SEN for the Ge–Ge single bond in the
model compound R3Ge–GeR3 (R = NH2) is 1.04.
References
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2 A. Sekiguchi and H. Sakurai, Adv. Organomet. Chem., 1995, 37, 1.
3 2 8 0
D a l t o n T r a n s . , 2 0 0 5 , 3 2 7 7 – 3 2 8 0