[Ga51(PtBu2)14Br6] 3-: An Elementoid Gallium Cluster with Metalloid and Nonmetalloid Structural Elements

Article abstract of DOI:10.1002/anie.200352472

Metal or nonmetal? Gallium is a chameleon, both as an element and in a metalloid Ga₅₁ cluster. The cluster consists of a central Ga ₁₃, unit with the same fcc topology as the high-pressure form of Ga-IV. A second shell of Ga atoms and GaBr and GaPR₂ groups binds the central core in a corset-like molecular structure (see picture).

Full text of DOI:10.1002/anie.200352472

Angewandte
Chemie
Communications
The bonding patterns of the seven modifications of gallium show the
path of the “space molecules” GaBr(GaR) via metalloid gallium
clusters—in this case a nanoscale [Ga51(PtBu2)14Br6] cluster con-
strained by GaP units—to the metal. More information on this
fascinating chemistry is provided by H. Schnöckel et al. on the
following pages.
Angew. Chem. Int. Ed. 2004, 43, 301
DOI: 10.1002/anie.200352472
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
301

Communications
Gallium Clusters
structure, and bonding relationships in such a metalloid
gallium cluster, which, with 51 gallium atoms, is the second
largest such cluster characterized to date.
[Ga51(PtBu2)14Br6]3À: An Elementoid Gallium
Cluster with Metalloid and Nonmetalloid
Structural Elements**
A metastable solution of GaBr in toluene/THF (3:1) was
heated from À78 to À358C,[1,6] to induce the disproportiona-
tion to form metal-rich clusters and GaBr3. Subsequently, a
slight excess of LiPtBu2 suspended in toluene was added, and
the reaction mixture was heated to 608C for 2 h. After several
days in a drying oven at about 508C, compound 1 crystallizes
as black rhombuses, which contain one [Li2Br(THF)6]+ and
two [Li(THF)4]+ ions, as well as the anionic cluster [Ga51(P-
tBu2)14Br6]3À (1a). The crystal structure analysis[7] shows that
1a is a metalloid cluster with 31 “naked” and 20 ligand-
bearing Ga atoms. The number of cations indicates that the
cluster carries a threefold negative charge. Cluster 1a is
depicted in Figure 1.
Jochen Steiner, Gregor Stößer, and
Hansgeorg Schnöckel*
We have recently introduced a series of metalloid Ga clusters,
in which a core of Ga atoms is surrounded by a shell of GaR
units.[1,2] The R groups used so far (R = N(SiMe3)2, Si(SiMe3)3,
C(SiMe3)3, SitBu3) were shown to bind to their Ga atoms
exclusively through two-center, two-electron (2c2e) bonds
(Scheme 1a). In these clusters, the central cores of ligand-free
Scheme 1. Bonding principles in metalloid Gan(GaR)m clusters (see
text for details).
Ga atoms were often determined to have topologies corre-
sponding to one of the seven forms of elemental gallium,[1]
meaning that coordination numbers typical of both metals
(e.g., coordination number 12, as in Ga-III) and nonmetals
(e.g., coordination number 1, as in a-Ga, since one short and
À
six long Ga Ga distances are observed) are found. We
previously introduced the concept of metalloid or elementoid
clusters[1,3] which seems tailor-made for such Ga clusters. To
vary the influence of the outer shell, we recently introduced
PtBu2 groups.[4] Steric and electronic influences cause the
phosphorus atoms in these groups to form also bridging
bonds, which result in their increased interaction with the Gan
core (Scheme 1b).
Figure 1. Molecular structure of 1a in crystals of 1. For clarity, only the
bonds to the a-carbon atoms are shown in the PtBu2 groups. Those
atoms discussed in the text are identified by numbers.
In the recently introduced Ga16 cluster, the Ga4 core is so
constrained by GaP units that it forms, in comparison to other
central Ga4 units in GanRm clusters, a particularly compressed
Ga4 tetrahedron.[4] Given these results, we asked ourselves if
it would be possible to synthesize larger Gan cores in
metalloid clusters with surrounding GaPR2 shells by choosing
the right conditions. In this case, we would expect centered
structures with high coordination numbers for the central
gallium atom, as in the fcc packing that was recently found in
Ga-IV at high pressures.[5] Herein we describe the synthesis,
The central gallium atom (Ga27, red, Figure 1) is sur-
rounded by twelve more Ga atoms (blue) in a slightly
À
distorted cuboctahedral environment with an average Ga Ga
distance of 284.4 pm (min./max. 276.3 pm/292.2 pm). The
À
Ga Ga distances depicted in Figure 2 show relatively little
variation; in comparison, in the Ga19R6 cluster 2 (R =
À
C(SiMe3)3), which also has a cuboctahedral environment
around the central Ga atom and an average bond length of
284.4 pm, the distances range from 272.9 to 295.5 pm.[3] The
six slightly distorted square planes of the cubooctahedron in
1a are capped with Ga5 units (orange, Figure 1), leading to
square antiprismatic units containing a plane capped by a
GaPR2 unit (Ga22, P6; assignment according to Figure 1).
Two of these six Ga5 moieties are depicted in Figure 2. Of the
four gallium atoms in the distorted square formations (Ga10,
Ga13, Ga15, Ga21), two form bonds to Br-atom-bearing Ga
[*] J. Steiner, G. Stößer, Prof. Dr. H. Schnöckel
Institut für Anorganische Chemie der Universität Karlsruhe (TH)
Engesserstrasse 15, Geb. 30.45, 76128 Karlsruhe (Germany)
Fax: (+49)721-608-4854
E-mail: Hansgeorg.Schnoeckel@chemie.uni-karlsruhe.de
[**] This research was supported by the Deutsche Forschungsgemein-
schaft and the Fonds der Chemischen Industrie.
302
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200352472
Angew. Chem. Int. Ed. 2004, 43, 302 –305

Angewandte
Chemie
Figure 4. Orientation of the 1a anions in the crystal. The distances
between the anions (in pm) show only a slight variation (min./
average/max. 20.495/21.156/21.816). One each of the tetrahedral
and octahedral holes containing the [Li(THF)4]+ or [Li2Br(THF)6]+ ions,
respectively, is highlighted.
À
Figure 2. To illustrate the variance of the Ga Ga distances, a cross-
section of 1a is highlighted (see text). The following distances [pm]
are observed: a 259.5–295.8, b 258.9–278.4, c 284.8–286.2, d 279.3–
295.3, e 275.8–278.8, f 245.5–268.1.
atoms (Ga14, Ga24), one is bound to a gallium atom with a
terminal PtBu2 group (Ga27), and the fourth binds directly to
a bridging PtBu2 group (P3). The three gallium atoms that are
directly bound to these six Ga4 squares form a distorted Ga8
cube, which is highlighted in yellow in Figure 1. Six of these
eight gallium atoms form bonds to terminal bromine atoms,
while two are bonded to terminal PtBu2 groups. Together with
the six bridging PtBu2 groups, this results in a pseudo-sixfold
axis of rotation, which is made clear by the arrangement of the
P and Br atoms in Figure 3.
The structure of 1 is unique in cluster chemistry, and
merits some discussion and substantiation. The central Ga13
unit is very symmetrical and compact. Single-point DFT
calculations[8] based on the experimentally determined struc-
3
tural data resulted in a volume of 3071 a0 , which, as expected,
lies between that of the fcc packing in the recently determined
3
Ga-IV high-pressure modification (2681 a0 ) and the volume
calculated by extrapolation of this high-pressure version to
normal pressure (3380 a0 ).[9] As a result of the coordination
3
À
number of 12, as in a typical metal, the Ga Ga distances are
larger than those in molecular compounds such as R2GaGaR2
(d(Ga Ga) = 248 pm for R = CH(SiMe3)2).[11] As can be seen
À
À
in Figure 2, the Ga Ga distances in 1a steadily decrease with
decreasing coordination number on going from the interior to
the exterior; the increasing covalent component of these
bonds also becomes clear on moving outward from the center.
The Ga5 units on the square surfaces of the cuboctahedron
form square Ga8 antiprisms, which, by virtue of their bond
lengths (Figure 2), differ significantly from those in the
similarly structured Ga8R8 cluster (R = C13H9) 3.[12] The
2À
À
average Ga Ga distance within the Ga4 plane in 1a is thus
Figure 3. Space-filling model of the Pand Br atoms directly bound to
the Ga51 cluster, depicted in the same orientation of 1a as in Figure 2.
280.4 pm, versus 251.9 pm in 3, whereas the average distance
of 266.2 pm between the Ga atoms that lie between the Ga4
planes in 1a is somewhat smaller than the corresponding
distance of 270.7 pm in 3. The topology of these square-
antiprismatic Ga8 units on the one hand, and the almost
perfect cuboctahedral arrangement around the central Ga
atom in 1a on the other hand resemble the different bonding
pattern in modifications of elemental gallium: that is, both
nonmetalloid (Ga8 unit) and metalloid (Ga core) bonding
types are present in 1a.
The arrangement of the [Ga51(PtBu2)14Br6]3À cluster
anions 1a in the crystals of 1 is shown in Figure 4. The
roughly 2 nm clusters are in a slightly distorted cubic close-
packed arrangement, in which all of the tetrahedral holes are
filled with [Li(THF)4]+ ions and all of the octahedral holes
with [Li2Br(THF)6]+ ions. This is similar to the Li3Br structure
typical of C60M3 fullerides, but the diameter of the cluster
anion 1a is about three times as large as that of the fulleride
To back up our topological interpretation of the bonding
relationships with a population analysis and calculated NMR
3À
ion C60
.
Angew. Chem. Int. Ed. 2004, 43, 302 –305
www.angewandte.org
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
303

Communications
chemical shifts, DFT calculations[13] were carried out on
[1] A. Schnepf, H. Schnöckel, Angew. Chem. 2002, 114, 6000 – 6021;
Angew. Chem. Int. Ed. 2002, 41, 3533 – 3554.
[2] K. Weiß, H. Schnöckel, Z. Anorg. Allg. Chem. 2003, 629, 1175 –
1183.
[3] A. Schnepf, G. Stösser, H. Schnöckel, J. Am. Chem. Soc. 2000,
122, 9178 – 9181.
[4] J. Steiner, G. Stösser, H. Schnöckel, Angew. Chem. 2003, 115,
2016 – 2019; Angew. Chem. Int. Ed. 2003, 42, 1971 – 1974.
[5] a) Z. Li, J. S. Tse, Phys. Rev. B 2000, 62, 9900 – 9902; b) L. Bosio,
J. Chem. Phys. 1978, 68, 1221; c) O. Schulte, W. B. Holzapfel,
Phys. Rev. B 1997, 55, 8122; d) K. Takemura, K. Kobayashi, M.
Arai, Phys. Rev. B 1998, 58, 2482; e) H. G. von Schnering, R.
Nesper, Acta Chem. Scand. 1991, 45, 870; f) U. Häussermann, S.
Simak, I. Abrikosov, S. Lidin, Chem. Eur. J. 1997, 3, 904; g) X. G.
Gong, G. Chiarotti, M. Parinello, E. Tosatti, Phys. Rev. B 1991,
43, 14277.
model compounds in which the PtBu2 group was replaced by
either PH2 or P(CH3)2 groups (1b and 1c). These calculations
showed that the relationship between the very different
distances in 1a and in the model compounds (1b and 1c) is
very well reproduced. However it is evident that the ligands
have a strong influence over the absolute values of these
distances. This was also true of the calculated 71Ga NMR and
31P NMR chemical shifts and the partial charges calculated
from the population analyses. Because reliable calculations
are not yet possible with the PtBu2 group, we will refrain from
further discussion at this point. However, given the current
data, it is possible to conclude that, unlike in the single-shelled
C60,[14] B12H122À [15] or Pb94À [16] clusters, the 71Ga and 31P NMR
,
chemical shifts for our clusters cannot be simply interpreted
through their NICS values and their spherical aromaticity. On
the contrary, in these multishelled metalloid clusters we see
that the 71Ga signal for the central Ga atom, for example, is
shifted significantly (e.g. by 250 ppm for 1c) downfield
relative to the signals for the Ga atoms of the inner
cuboctahedron, as in Ga metal. To clarify this, we intend to
carry out further calculations as well as solid-state NMR
experiments, as 1a is not soluble in common organic
solvents.[17] Such quantum-chemical and experimental inves-
tigations should make it possible to draw further conclusions
from the topology (distances and coordination numbers)
about the metalloid bonding relationships in the center as well
as the more covalent bonding in the second shell and
periphery of the Ga framework.
[6] C. Dohmeier, D. Loos, H. Schnöckel, Angew. Chem. 1996, 108,
141 – 161; Angew. Chem. Int. Ed. Engl. 1996, 35, 129 – 149.
[7] Crystal structure data for 1: Mr = 7185.02. Crystal dimensions
0.6 ” 0.1 ” 0.05 mm, monoclinic, space group C2/c, b =
110.26(3)8,
a = 34.7025(12) Š,
b = 21.8163(5) Š,
c =
37.5622(14) Š, V= 26677.3(15) Š˚, Z = 4, 1calcd = 1.789 gcm˚,
mMo = 6.222 mmÀ1, qmax. = 25.05(, 112878 measured reflections,
23411 independent reflections (R(int.) = 0.1459). Absorption
correction: numeric (min./max.-transmission 0.6469/0.9396),
R1 = 0.0560, wR2 = 0.1295. STOE-IPDS-diffractometer (MoKa
radiation, l = 0.71071 Š), 150 K. The structure was solved by
direct methods and refined against F2 for all observed reflec-
tions. Programs used: SHELXS and SHELXTL (G. M. Shel-
drick, Universität Göttingen). CCDC-215886 contains the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/conts/
retrieving.html (or from the Cambridge Crystallographic Data
Centre, 12, Union Road, Cambridge CB21EZ, UK; fax: (+
44)1223-336-033; or deposit@ccdc.cam.ac.uk).
4À
Given the recently attained results from the Ga84R20
cluster (4)[18] (metallic conductivity,[19] superconductivity[20]),
we are also planning similar investigations for 1. In addition to
the threefold negative charge of 1a, a partial lower charge
(for example 1À), which could be balanced by a dilution in the
cationic lattice of the Li3Bi lattice structure of the Ga51 cluster
(see Figure 4), is also conceivable for 1. Since according to
latest results, the cluster 4 could be present as both a three-
and fourfold negatively charged species in the same crystal,[21]
such cluster doping could provide an approach for the
clarification of the so far not understood conductivity
phenomena.
[8] The molecular volume was calculated with the GAUSSIAN 98
program package on the SCF level with a 3-21G basis set. We
carried out single-point calculations based on the experimentally
determined geometry of 1, in which the ICPM solvation model
was used to place a shell of uniform electron density (4 ”
10À3 eŠÀ3) around the Ga13 unit. a) IPCM: J. B. Foresman,
T. A. Keith, K. B. Wiberg, J. Snoonian, M. J. Frisch, J. Phys.
Chem. 1996, 100, 16098; b) Gaussian98 (RevisionA.7), M. J.
Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, R. E.
Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D.
Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V.
Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C.
Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q.
Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K. Raghava-
chari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, B. B. Stefanov, G.
Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L.
Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A.
Nanayakkara, C. Gonzalez, M. Challacombe, P. M. W. Gill, B. G.
Johnson, W. Chen, M. W. Wong, J. L. Andres, M. Head-Gordon,
E. S. Replogle, J. A. Pople, Gaussian, Inc., Pittsburgh, PA, 2001.
Experimental Section
LiPtBu2 (1 g, 6.57 mmol) was suspended in toluene (50 mL) and
cooled to 08C in an ice bath. In a second flask, a 0.32m GaBr solution
(17 mL, 5.44 mmol) in toluene/THF (3:1) was warmed from À78 to
À358C over several hours. The ligand was added to this latter solution
through a stainless steel canula, which caused the reaction mixture to
warm from À35 to À258C. The reaction mixture was warmed to room
temperature over 10 h and then held at 608C for 2 h. This resulted in a
black solution, from which 1 (250 mg, 0.035 mmol) crystallized over
several days at 508C in the form of black rhombuses.
3
3
[9] The volume of the Ga13 unit in 2 is 3119 a0 , a value of 2955 a0
was calculated for a naked Ga13 molecule[10] (see reference [8]).
[10] K. Weiß, R. Köppe, H. Schnöckel, Int. J. Mass Spectrom. 2002,
214, 383 – 395.
Received: July 25, 2003 [Z52472]
Published Online: December 9, 2003
[11] W. Uhl, M. Layh, T. Hildebrand, J. Organomet. Chem. 1989, 364,
289.
[12] A. Schnepf, G. Stösser, H. Schnöckel, Z. Anorg. Allg. Chem.
2000, 626, 1676 – 1680.
[13] The 69Ga NMR shifts were first based on the calculated shift for
GaCp, and were subsequently set in reference to the usual
Keywords: cluster compounds · gallium · structure elucidation
.
304
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2004, 43, 302 –305

Angewandte
Chemie
standard ([Ga(H2O)6]3+). The calculations were carried out in
the Ci point group with the PMSHIFT module of the TURBO-
MOLE program package with DFT (BP 86 functional, SVP
basis). TURBOMOLE: a) O. Treutler, R. Ahlrichs, J. Chem.
Phys. 1995, 102, 346; b) Funcional BP 86: A. D. Becke, Phys.
Rev. A 1998, 38, 3098; J. P. Perdew, Phys. Rev. B 1996, 33, 8822;
c) RIDFT: K. Eichkorn, O. Treutler, H. Öhm, M. Häser, R.
Ahlrichs, Chem. Phys. Lett. 1995, 242, 652; K. Eichkorn, F.
Weigend, O. Treutler, R. Ahlrichs, Theor. Chem. Acc. 1997, 97,
119; d) GIAO with DFT: G. Schreckenbach, T. Ziegler, J. Phys.
Chem. 1995, 99, 606; GIAO mit SCF: M. Häser, R. Ahlrichs,
H. P. Baron, P. Weiss, H. Horn, Theor. Chim. Acta 1992, 83, 455.
[14] Z. Chen, H. Jiao, A. Hirsch, P. von R. Schleyer, Angew. Chem.
2002, 114, 4485 – 4488; Angew. Chem. Int. Ed. 2002, 41, 4309 –
4312.
[15] P. von R. Schleyer, K. Najafian, Inorg. Chem. 1998, 37, 3454 –
3470.
[16] a) A. Hirsch, Z. Chen, H. Jiao, Angew. Chem. 2001, 113, 2916 –
2920; Angew. Chem. Int. Ed. 2001, 40, 2834 – 2838; b) V.
Queneau, S. C. Sevov, Inorg. Chem. 1998, 37, 1358 – 1360; c) E.
Todorov, S. C. Sevov, Inorg. Chem. 1998, 37, 3889 – 3891; d) J.
Campbell, D. A. Dixon, H. P. A. Mercier, G. J. Schrobilgen,
Inorg. Chem. 1995, 34, 5798 – 5809.
[17] NMR experiments with 2 have shown that it has thus far not
been possible to reconcile calculated 71Ga NMR chemical shifts
with those obtained experimentally. Clearly relativistic effects,
among others, play a nontrivial role in the Ga core.
[18] A. Schnepf, H. Schnöckel, Angew. Chem. 2001, 113, 733 – 737;
Angew. Chem. Int. Ed. 2001, 40, 712 – 715.
[19] O. N. Bakharev, N. Zelders, H. B. Brom, A. Schnepf, H.
Schnöckel, L. J. de Jong, Eur. Phys. J. 2003, D24, 101 – 104.
[20] J. Hagel, M. T. Kelemen, G. Fischer, B. Pilawa, J. Wosnitza, E.
Dormann, H. v. Löhneysen, A. Schnepf, H. Schnöckel, U. Neisel,
J. Beck, J. Low Temp. Phys. 2002, 129, 133 – 142.
[21] A. Schnepf, E. Herrling, B. Pilawa, E. Weckert, A. Meents, B.
Jee, H. Schnöckel, Inorg. Chem., in press.
Angew. Chem. Int. Ed. 2004, 43, 302 –305
www.angewandte.org
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
305