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Na6[Ge10{Fe(CO)4}8]18THF A Centaur Polyhedron of Germanium Atoms.

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Cluster Compounds
DOI: 10.1002/anie.200600928
Na6[Ge10{Fe(CO)4}8]·18 THF: A Centaur
Polyhedron of Germanium Atoms**
Andreas Schnepf* and Christian Schenk
Metalloid cluster compounds of germanium of the general
formula [GenRm] (n > m)[1] contain ligand-bound germanium
atoms as well as “naked” germanium atoms that form
exclusively GeGe bonds.[2] Various synthetic strategies are
known for incorporating naked germanium atoms into metalloid germanium cluster compounds: the reductive elimination
of a leaving group XY can provide naked germanium atoms if
the X- and Y-bound germanium atoms are further connected
only to other germanium atoms in the precursor. This
synthetic route was proposed by Sekiguchi et al. for the
preparation of the cationic cluster compound [Ge10(SitBu3)6I]+ (1).[3] Another synthetic strategy uses the reductive coupling reaction of a substituted germanium(II) halide
RGeCl with an adequate reducing agent such as C8K in the
presence of a germanium(II) halide (e.g. GeCl2·dioxane).
Thus, Power and co-workers synthesized the metalloid cluster
compounds [Ge6Ar2] (2, Ar = C6H3-2,6-Dipp2 ; Dipp = C6H32,6-iPr2)[4] and [Ge5R4] (3, R = CH(SiMe3)2).[5]
We established a third alternative for the preparation of
metalloid cluster compounds by using the disproportionation
reaction of a molecular germanium(I) halide.[6] Following this
synthetic route we were able to synthesize the metalloid
{(OtBu)2C6H3}6] (5), and [Ge9{Si(SiMe3)3}3] (6).[9]
By using the ternary solvent mixture NnBu3/THF/CH3CN
in the cocondensation reaction[10] of the high-temperature
molecule GeBr, we are able to isolate a solution of GeBr from
the cocondensation apparatus for the first time. Herein, we
report on a first reaction of this GeBr solution with CollmanGs
reagent Na2[Fe(CO)4] and formation of the new cluster
compound Na6[Ge10{Fe(CO)4}8]·18 THF (7).
The dark red solution of GeBr was treated with CollmanGs
reagent at 40 8C. Slow warming of the reaction mixture to
room temperature led to a nearly black reaction solution.
Work up of the reaction mixture yielded a dark red diethyl
ether extract, from which red crystals of the compound
Na2[Ge{Fe(CO)4}3]·4 dioxane (8) precipitated[11] upon addi-
[*] Dr. A. Schnepf, C. Schenk
Institut f!r Anorganische Chemie
Universit't Karlsruhe (TH)
Engesserstrasse, Geb. 30.45, 76128 Karlsruhe (Germany)
Fax: (+ 49) 721-608-4854
[**] We gratefully acknowledge the financial support of the Deutsche
Forschungsgesellschaft. We also thank Prof. Schn>ckel for helpful
Supporting information for this article is available on the WWW
under or from the author.
Angew. Chem. Int. Ed. 2006, 45, 5373 –5376
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
tion of dioxane. Further work up of the extract yielded
another reaction product in the form of black crystals.
Recrystallization from THF led to black crystals of the cluster
compound 7 which were suitable for X-ray crystal structure
analysis. The molecular structure of 7 is shown in Figure 1.
form a direct germanium–germanium bond. Therefore, as in
the above-mentioned metalloid germanium cluster compounds, 7 consists of ligand-bound germanium atoms as well
as naked germanium atoms.
Since the arrangement of the 10 germanium atoms in the
cluster core can also be described as a fusion of two different
polyhedra (cube and icosahedron), this arrangement can be
called a “centaur polyhedron”.[12] In Figure 1 the cubic part
(left) is highlighted by a dark-colored polyhedron and the
icosahedral part (right) by a light-colored polyhedron. The
centaur polyhedron is well-known in solid-state chemistry
representing a textbook example for coordination number 10.
However, in Group 14 cluster chemistry it is a new structural
The formation of 7 can formally be described, starting
from GeBr, by Equation (1).
½Ge10 Br10 þ 8 Na2 ½FeðCOÞ4 ! Na6 ½Ge10 fFeðCOÞ4 g8 þ 10 NaBr ð1Þ
Figure 1. Molecular structure of 7; THF molecules are omitted for
clarity. The central Ge10 unit is highlighted by polyhedron illustration.
The {Fe(CO)4} units are light-colored, and the six sodium cations are
Cluster compound 7 consists of a Ge10 framework in
which eight of the ten germanium atoms are each bound to
the iron atom of a {Fe(CO)4} ligand (GeFe: 243 pm). The
{Fe(CO)4} ligands are further bound to six sodium cations,
which are arranged in the form of a distorted octahedron with
NaNa distances between 9 and 11 H (Figure 1). Each
sodium cation is additionally bound to three THF molecules.
The cluster core consists of a distorted cubic arrangement of
eight {GeFe(CO)4} units. Two of the six rectangular faces are
each capped by a naked germanium atom (Ge9 and Ge10,
Figure 2). Furthermore, the two capping germanium atoms
Figure 2. Ge/Fe core of 7. Selected bond lengths [pm] and angles [8]:
Ge6-Fe6 242.21(18), Ge8-Fe8 243.45(16), Ge6-Ge2 249.77(13), Ge6Ge8 252.63(13), Ge2-Ge1 252.16(14), Ge1-Ge10: 276.47(14); Ge10Ge9 269.56(14), Ge9-Ge8 274.53(16); Ge8-Ge6-Ge2 91.44(5), Ge8-Ge9Ge10 100.72(4), Ge9-Ge10-Ge1 99.36(5).
Considering this hypothetical reaction scheme,[13] the
average oxidation state of the germanium atoms of 7 can be
assigned as + 1. Since the oxidation state of the naked
germanium atoms should be assigned as zero, the remaining
germanium atoms are formally assigned an average oxidation
state of + 1.25. Consequently, 7 can be described as an
internal disproportionation product of GeBr.[14]
Within the Ge10 polyhedron the GeGe bond lengths vary
between 250 and 276 pm, whereby the germanium atoms with
the higher coordination number form the longer GeGe
bonds. Thus, the average GeGe bond length for germanium
atoms with coordination number four is 252 pm, and the
average GeGe bond length of the naked germaniums atoms
with coordination number five is 267 pm. This finding
corresponds to the same trend observed for Zintl ions, in
which the germanium atoms with the higher coordination
number also form the longer GeGe bonds. Furthermore, the
GeGe bond lengths in 7 are in the same range as in Zintl ions
(Ge52 : 247–270 pm,[15] Ge93 : 257–286 pm,[16] Ge94 : 253–
296 pm,[17] Ge186 : 249–284 pm[18]) although the average
oxidation state of the germanium atoms is positive in 7 and
negative in the Zintl ions.
In contrast, the average GeGe bond lengths in 7 are on
average 10 pm larger than those in [Ge6{Cr(CO)5}6]2 (9),[19]
the only other cluster compound in which transition-metal
fragments are attached as ligands to the cluster core.[20] These
differences may be attributed to the presence of the naked
germanium atoms in 7. Moreover, the difference in formal
oxidation state of the germanium atoms in these two
compounds is relevant. As described above, the average
oxidation state of 7 is + 1. Assuming analogous germanium–
ligand interactions in 7 and 9, which is quite realistic as nearly
the same GeM bond lengths in 7 (243 pm) and 9 (241 pm)
are found, a more positive average oxidation state (+ 1.67) is
obtained for cluster compound 9. Therefore, with respect to
the average oxidation state of the germanium atoms, the
cluster compound 7 can be classified between the Zintl ions
and the octahedral cluster compound 9. This classification is
further supported by the average GeGe bond lengths of
these compounds.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 5373 –5376
As the Zintl ions[21] and the cluster compound 9 can be
described by WadeGs rules,[22] the question arises as to whether
7 also fulfills WadeGs rules. Assuming that the two electrons of
the GeFe bond are supplied by the germanium atom, thus
leading to the ideal number of 18 valence electrons for each
iron atom, and if the naked germanium atoms further bear a
lone pair of electrons, every germanium atom in 7 provides
two electrons for cluster bonding. Together with the six
electrons of the negative charge of the cluster fragment
[Ge10{Fe(CO)4}8]6, this interpretation leads to 26 electrons
for cluster bonding (2 n + 6; n = 10). For cage compounds with
2n + 6 bonding electrons an arachno structure is expected
according to WadeGs rules, and the structure of 7 can indeed
be described as a distorted arachno structure. Adding two
more germanium atoms would lead to an icosahedral closo
The formal description of 7 by WadeGs rules points to
delocalized bonding electrons within the cluster core. This
assumption is supported by preliminary quantum-chemical
calculations on the model compound [Ge10{Fe(CO)4}8]6, for
which a structure analogous to 7 was calculated.[23] By using
an Ahlrichs–Heinzmann population analysis, SENs (shared
electron numbers)[24] of three-center bonding contributions
between 0.04 and 0.28 were calculated. In the triangles of the
icosahedral part of the centaur polyhedron, an average SEN
for the three-center bonding components of 0.28 (e.g. Ge1Ge7-Ge10: 0.284; Ge3-Ge9-Ge10: 0.283) is found. In contrast, the average SEN for the three-center bonding components in the triangles of the cubic part decreases to 0.07
(e.g. Ge3-Ge6-Ge7: 0.09; Ge8-Ge6-Ge7: 0.06). Thus, the
bonding situation of the cluster core changes from “localized”
in the cubic part to “delocalized” in the icosahedral part, thus
representing a completely new bonding situation in metalloid
cluster compounds.
These first results show the synthetic potential of this new
metastable solution of GeBr. Furthermore, the average
oxidation states of 7 (+ 1) and 8 (+ 4) lead to the suggestion
that species that have an average oxidation state between one
and zero should be present in the reaction solution, and such
species may thus contain more “naked” germanium atoms.
Experimental Section
A solution of GeBr (14 mL, 0.29 m in CH3CN/THF/nBu3N 2:2:1;
4.06 mmol) at 30 8C was added to Na2[Fe(CO)4]·1.5 C4H8O2 (1.2 g,
3.46 mmol) at 78 8C. The stirred reaction mixture was slowly
warmed to room temperature and resulted in a black solution.
Removal of the solvent in vacuum yielded a black residue. Extraction
with diethyl ether gave a dark red solution from which, after the
addition of dioxane and storage at + 78, orange-red crystals of 8
(100 mg, 0.1 mmol, 10 %) precipitated. After separation of the
crystals and further concentration of the extract, black crystals were
formed (ca. 10 mg). Recrystallization of these crystals from THF
yielded black crystals of 7.
Crystal structure data for 8: Mr = 974.66 g mol1, crystal dimensions 0.3 N 0.2 N 0.1 mm, triclinic, space group P1̄, a = 10.614(2), b =
11.087(2), c = 19.205(4) H, a = 103.51(3), b = 92.17(3), g = 114.38(3)8,
V = 1978.4(7) H3, Z = 2, 1calcd = 1.636 g cm3, mMo = 1.928 mm1,
2 qmax = 51.928, 14 098 measured, 7203 independent reflections
(R(int.) = 0.0343), absorption correction: numerical (min./max. transAngew. Chem. Int. Ed. 2006, 45, 5373 –5376
mission 0.6125/0.9032), R1 = 0.0298, wR2 = 0.0615. Stoe IPDS II
diffractometer (MoKa radiation (l = 0.71073 H), 120 K).
Crystal structure data for 7: Mr = 3504.83 g mol1, crystal dimensions 0.4 N 0.35 N 0.15 mm, monoclinic, space group P2(1)/n, a =
18.557(4), b = 27.664(6), c = 26.884(5) H, b = 92.11(3)8, V =
13 790(10) H3, Z = 4, 1calcd = 1.688 g cm3, mMo = 3.055 mm1, 2 qmax =
50.248, 80 226 measured, 23 999 independent reflections (R(int.) =
0.0948), absorption correction: numerical (min./max. transmission
0.3549/0.7065), R1 = 0.0735, wR2 = 0.1623. Stoe IPDS II diffractometer (MoKa radiation, (l = 0.71073 H), 100 K).
The structures were solved by direct methods and refined against
F2 for all observed reflections. Programs used: SHELXS and
SHELXL.[25] CCDC-298741 (7) and -298740 (8) contain the supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via
Received: March 9, 2006
Revised: April 5, 2006
Published online: July 17, 2006
Keywords: centaur polyhedron · cluster compounds ·
density functional calculations · germanium
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[2] According to the original definition from SchnRckel and coworkers, metalloid cluster compounds of germanium contain
more germanium–germanium contacts than germanium–ligand
contacts and contain germanium atoms (termed “naked”
germanium atoms) that participate exclusively in germanium–
germanium interactions. As a result of this definition metalloid
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[11] Compound 8 is formed from Na2[Fe(CO)4] and a germanium
halide by metathesis. A suitable germanium halide is GeBr4, the
oxidation product of the disproportionation reaction:
GeBr4 + 3 Na2[Fe(CO)4]!Na2[Ge{Fe(CO)4}3] + 4 NaBr. Therefore, the central germanium atom in 8 can be assigned a
formal oxidation state of + 4, and thus every iron atom has the
ideal value of 18 valence electrons.
[12] Similar to the description of the centaur in Greek mythology
(half human, half horse), a polyhedron built through the fusion
of two different polyhedrons (here: half cube, half icosahedron)
can be called a centaur polyhedron. C. Rocaniere, J. P. Laval, P.
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[13] Neither [Ge10Br10] nor any [(GeBr)n] species have been synthesized so far. However, in analogy to the characterized EX species
of Group 13 (E = Ga, Al; X = halogen)[14] the synthetic method
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
introduced by us might give access to such germanium subhalides.
A similar situation is known in Group 13 chemistry, in which the
cluster compound [Al22Br20]·10 THF can also be seen as an
internal disproportionation product of AlBr. However, 7 is not a
binary compound since the halogen atoms are substituted by the
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compounds do not belong to the compounds described here. B.
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Quantum-chemical calculations were carried out with the RIDFT version of the TURBOMOLE program package by
employing the Becke–Perdew 86 functional. The basis sets
were of SVP quality. The electronic structure was analyzed
with the Ahlrichs–Heinzmann population analysis as based on
occupation numbers. TURBOMOLE: O. Treutler, R. Ahlrichs,
J. Chem. Phys. 1995, 102, 346; BP 86 functional: J. P. Perdew,
Phys. Rev. B 1986, 33, 8822; A. D. Becke, Phys. Rev. A 1988, 38,
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Shared electron numbers (SENs) for bonds are a reliable
measure of the strength of covalent bonding. For example, the
SEN for the GeGe single bond in the model compound R3Ge
GeR3 (R = NH2) is 1.04.
G. M. Sheldrick, SHELXTL, Version 5.1, Bruker AXS, 1998.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 5373 –5376
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