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The Polyhedral Gallium Subhalide [Ga24Br22]10THF The First Step on the Path to a New Modification of Gallium.

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Cluster Compounds
The Polyhedral Gallium Subhalide
[Ga24Br22]� THF: The First Step on the Path to a
New Modification of Gallium?**
Taike Duan, Elke Baum, Ralf Burgert, and
Hansgeorg Schnckel*
To date, [Ga8I8]�PEt3 is the only structurally characterized
gallium(i) subhalide.[1] In this compound the gallium atoms
form an eight-membered planar ring with two center?two
electron (2c?2e) bonds. Although we have recently synthesized a series of metalloid gallium clusters from GaX-solution
(X = Cl, Br, I),[2, 3] no further examples of Gallium subhalides
have been reported.[4] However, we have shown that the
polyhedral aluminum subhalide, [Al22X20]� THF (1; X = Cl,
Br) can be prepared from metastable AlCl or AlBr solution.[5, 6] The topology of the 22 aluminium atoms in 1 is
unexpected: 10 aluminium atoms are directly bonded by
2c?2e bonds to a central Al12 icosahedron such that two
aluminum atoms at the apexes of the Al12 icosahedron are
?naked?. An equivalent framework has not even been
reported for boron, although there are conjoined B12 icosahedra in a-boron. Based on this discovery and in collaboration with other research groups, we found that the
formation of 1, as an intermediate from the disproportionation of AlX to aluminum metal and aluminum trihalide, could
be plausibly interpreted as a step in the formation of new
modification of aluminum with an analogous structure to aboron.[6] This conclusion has been further supported by
theoretical studies by H;ussermann et al.[7] Furthermore,
they found that for gallium a hypothetical modification with
an a-boron type structure is energetically only 5 kJ mol 1
higher than a-gallium (for aluminum it would be
22 kJ mol 1). With an expansion of the volume by about
20 %, the gallium atoms of a-gallium would rearrange and
approach an a-boron type framework.[7] Motivated by these
studies, we directed our investigations towards preparing the
first polyhedral gallium subhalide, particularly a gallium
subhalide with the same structure as the [Al22X20]� THF
clusters. The existence of such a cluster with an icosahedral
Ga12 core was further suggested by the recently reported
partially substituted Ga22 gallium subhalide cluster
[Ga22Br10R10].[8] Herein we report the novel compound
[Ga24Br22]� THF (2), which may be the precursor of this
kind of Ga22-core compound.
[*] M.Sc. T. Duan, Dr. E. Baum, Dipl.-Chem. R. Burgert,
Prof. Dr. H. Schn,ckel
Institut f/r Anorganische Chemie
Universit3t Karlsruhe (TH)
Engesserstrasse 15, Geb. 30.45, 76128 Karlsruhe (Germany)
Fax: (+ 49) 721-608-4854
[**] This work was supported by the Deutsche Forschungsgemeinschaft
and the Fond der Chemischen Industrie.
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
A dark-brown to black solution of GaBr in a mixture of
THF and toluene (1:20), made by cocondensation of the
gaseous components (GaBr, THF, toluene),[2, 3] is slowly
warmed from 78 8C to room temperature over several
days. After the separation of precipitated metallic gallium and
the stepwise concentration of the solution under vacuum, 2
crystallizes in the form of small orange square plates, the
crystallization continues for several weeks. The crystal growth
is interrupted every few days by the removal by filtration of
precipitated gallium. Most of the crystals are very small
(< 0.1 mm), mechanically and chemically highly labile, and
even a reaction is observed in the anhydrous perfluoropolyether oil at room temperature during the preparation of the
crystals for X-ray structural analysis. The synthesis of 2 is
repeatable although yields are low (2 % to 16 %), and a
suitable crystal for X-ray structural analysis can seldom be
selected. The X-ray structural analysis[9] indicates 2 is an
inversion-symmetric compound with a central, slightly distorted Ga12 icosahedron to which the other 12 Ga atoms are
joined with 2c?2e bonds (Figure 1). The Ga Ga bond lengths
Figure 1. Molecular structure of [Ga24Br22]� THF (2), C and H atoms
of THF are omitted for clarity.
in the central Ga12 icosahedron range from 255 pm to 267 pm.
The bonds from the Ga12 icosahedron to terminal 12 Ga
atoms (Ga2 Ga1) are shorter at about 240.0 0.4 pm, this is
expected since they are also bonded to Br atoms and have a
higher oxidation state leading to shorter covalent radius.
Compared to the central Ga12 icosahedron, the external Ga12
icosahedron is strongly distorted owing to the varied substitution of the outer gallium atoms: Two opposing apical
gallium atoms (Ga1/Ga1?) are bonded to three Br atoms, one
exclusively, and the other two shared with neighboring
gallium atoms. All the other external gallium atoms (such as
Ga5/Ga5?) bond with two bromine atoms and one oxygen atom
from the THF molecule. The Ga O bond length is on average
199 2 pm, which is in good agreement with DFT calcula-
DOI: 10.1002/anie.200453786
Angew. Chem. Int. Ed. 2004, 43, 3190 ?3192
tions.[10] The arrangement of [Ga24Br22]� THF clusters in the
crystal corresponds to a distorted body-centered topology
similar to the a-tungsten structure.
The Ga Ga bond lengths in the central Ga12 icosahedron
(Ga2-Ga3, Ga3-Ga4)of 2 (261 6 pm) are in the range seen in
other compounds containing a Ga12 icosahedron: [Ga12(fluorenyl)10]2 (3; 264 6 pm),[11] [Ga22Br2{N(SiMe3)2}10Br10]2 (4;
259 2 pm),[8] and [Ga22Br{N(SiMe3)2}10Br10]3 (5; 270 12 pm).[8]
The topology of the Ga24 framework of 2 is the same of
the Ga22 clusters 4 and 5, which implies that the subhalide
cluster 2 may react further with the GaBr3 present in the
solution to give Ga2Br4 and the hypothetic [Ga22Br20]� THF
cluster analogous to 1 and may thus be the precursor of the
target Ga22-subhalide cluster as well as 4 and 5. Comparison
of 1 and 2 shows that the 22 Al atoms or 24 Ga atoms are
surrounded by a cage formed by 32 non-metal atoms (1: 20 Br
atoms and 12 O atoms from THF, 2: 22 Br atoms and 10 O
atoms from THF). Single-point self-consistent field (SCF)
calculations based on the experimentally determined geometries were performed to calculate the volume[12] of the
shells of the same electron density formed by the 32 nonmetal atoms in 1 and 2. They indicate that, although 2 has
more metal atoms, the volume occupied by the 24 Ga atoms is
almost 3 % smaller than that of the 22 Al atoms in 1. The
result is consistent with the difference in the atomic radius of
gallium and aluminum calculated from the [R2M-MR2]
compounds (R = CH(SiMe3)2 ; M = Ga 254.1(1) pm and M =
Al: 266.0(1) pm),[13] which shows the gallium atom radius is
4.6 % smaller. Although 1 and 2 have the same sum of Br and
O atoms, they arrange in quite different ways: In 1, 20 Br
atoms adopt the geometry of a highly distorted pentagonal
dodecahedron with the 12 O atoms of THF molecule in the
centers of the pentagonal faces; in 2 the 22 Br atoms and 10 O
atoms form three twisted eight-membered rings and two
twisted four-membered rings.
To test the hypothesis that 2, just like 1, is an intermediate
in the decomposition of the monohalide to an a-boron type
modification of gallium or aluminum, respectively, quantum
chemical calculations were carried out[10] to compare the
energy of 2 to GaBr and GaBr3/Ga. In combination with
thermodynamic data from the literature[10, 14] we get an energy
level diagram that describes the disproportionation of GaBr
and THF to a-gallium and GaBr3�THF (Figure 2). The
diagram shows that, GaBr disproportionates firstly to compound 2, then to an a-boron type modification of gallium, and
finally to the thermodynamically stable a-gallium and GaBr3.
The postulated formation of the a-boron type modification of
gallium also seems to be plausible under the experimental
conditions: at low temperature (from 78 8C to ca. 0 8C) and
under vacuum GaBr oligomers of large volume are formed as
intermediates. By volume reduction of the primary species
cluster 2 is formed by association and disproportionation of
the oligomers in the solution, then the a-boron type gallium
and finally the thermodynamically stable a-gallium is formed.
Together with the results of the different metalloid
gallium clusters, especially the recently published Ga22
cluster,[8] the results presented herein clearly illustrate that
the connectivity diversity in the seven gallium modificaAngew. Chem. Int. Ed. 2004, 43, 3190 ?3192
Figure 2. Energy level diagram for the disproportionation of molecular
GaBr to a-gallium and GaBr3 via the intermediate [Ga24Br22]� THF (2)
and the hypothetical a-boron type gallium modification, based on DFT
calculation and literative values.[10, 14]
tions,[3, 8, 15] is also reflected in the of structural variety of the
metalloid gallium clusters and the connectivities of their
gallium frameworks. From the structure of 2 and the energy
level diagram it seems plausible that a new modification of
elemental gallium with an a-boron type structure may be
experimentally realizable.
Experimental Section
GaBr (48 mmol) was cocondensed with toluene (100 mL) and THF
(5 mL) according to the literature method.[5] A portion (25 ml) of the
ca. 0.41m black or dark-brown GaBr solution was warmed from
78 8C to room temperature under vacuum over several days. The
elemental gallium precipitate was separated and the dark filtrate was
concentrated in stages and kept at room temperature under vacuum.
Repeated concentration and filtration steps generated crystals of 2
from the filtrate in several weeks in the form of yellow orange square
plates. The yields of 2, owing to its extreme sensitivity to experimental
conditions, range between 2 % and 16 %. Cluster 2 is mechanically
and chemically highly labile, it decomposes even in solution as well as
during the mounting in anhydrous perfluoropolyether oil for X-ray
diffraction at room temperature. Mounting a crystal in cooled
anhydrous perfluoropolyether oil makes X-ray diffraction at low
temperature possible and gives suitable data for the structural
Received: January 19, 2004 [Z53786]
Keywords: cluster compounds � gallium � structural analysis �
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[9] Crystal structure analysis of 2: Mr = 4144.28 g mol 1, Crystal
dimensions:0.5 R 0.2 R 0.1 mm, monoclinic, space group P2(1)/n,
b = 91.318(8)8,
a = 14.9402(11),
b = 21.52433(12),
16.4147(18) S, V = 5277.2(8) S3, Z = 2, 1calcd = 2.608 g cm 3,
Mm = 14.372 mm 1, qmax = 27.11, 19 826 reflections measured, of
which 9477 were independent, (R(int) = 0.0645). Numerical
absorption correction:(min/max Transmission 0.4008/0.6454),
R1 = 0.0799, WR2 = 0.2134, GoF on F2 = 1.025, completeness of
2V = 81.4 %, QrestMax/QrestMin = 2.776/ 2.040. STOE-IPDS diffractometer (MoKa radiation, l = 0.71071), 150 K. The structure
was solved by direct methods and refined against F2 for all
observed reflections.Software used: ShelXS and ShelXtl (G. M.
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conts/retrieving.html (or from the Cambridge Crystallographic
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(+ 44) 1223-336-033; or
[10] The quantum chemical calculation were carried out with the
modules in the program package TURBOMOLE with DFT
(BP 86 functional, SVP-Basis). If possible, the energy of the
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which shows only few percent deviation. The literative value of
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transition of solid gallium. The energy of gallium with a-boron
type structure was taken from the work of H;ussermann et al.[7] ,
in which the a-boron analogue gallium modification is about
5 kJ mol 1 higher than the a-gallium modification. TURBOMOLE: a) O. Treutler, R. Ahlrichs, J. Chem. Phys. 1995, 102,
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[12] The molecular volumes were calculated with Gaussian 98 of
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Ga24Br22O10 and Al22Br20O12 units using the IPCM (isodensity
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Angew. Chem. Int. Ed. 2004, 43, 3190 ?3192
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polyhedra, gallium, subhalide, step, first, modification, ga24br22, 10thf, new, path
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