[GA16(PtBu2)10]: A gallium phosphide sheathed core of four naked Ga atoms?

Article abstract of DOI:10.1002/anie.200250461

Like inverse P₄O₁₀, a nearly tetrahedral Ga₄P₁₀ shell encloses a gallium-rich core with a distorted tetrahedral Ga₄ center in the neutral metalloid cluster compound [Ga₁₆(PtBu₂)₁₀].

Full text of DOI:10.1002/anie.200250461

Angewandte
Chemie
times occur as cage molecules,[1–3] has recently revealed
several examples with unusual structures (Figure 1a–c). The
[(AlCp*)6P4] unit (Figure 1a; Cp* = C5Me5),[4] formed from
[AlCp*] and P4, and a GaP4 unit,[5] with a nearly intact P4
framework (Figure 1b) are characteristic of these structures.
Both of these compounds as well as a third species with an
AlP3 ring structure[6] are colorless or weakly colored (Fig-
ure 1c).
Cp*
a)
Cp*
b)
tBu
Al
tBu
Al
P
Cp*
Ga
tBu
GatBu3
Al
P
P
P
Al
P
Cp*
P
P
Al
P
Al
Cp*
Cp*
c)
tBu
Figure 1. Structural formulas for
a) [(AlCp*)6P4] (from [AlCp*] and
white phosphorus), b) [Ga2P4tBu6]
(from [GatBu3] and white phospho-
rus), and c) [Cp*Al(PtBu)3] (from
PtBu3 and [AlCp*]).
P
P
Cp*
Al
P
tBu
tBu
To obtain metal-rich Ga–P compounds, whose dark color
and metallic luster are suggestive of different bonding
interactions, a different synthetic route, starting from meta-
stable AlX and GaX (X = Cl, Br, I) solutions, has been
pursued, which has led to numerous metalloid cluster com-
pounds.[7] For example, we have had some success in previous
years with the synthesis of metalloid cluster compounds with
À
terminal Al N bonds. We thus obtained, among others, the
shiny cluster compound Al77R20, whose core of 57 naked
aluminum atoms is surrounded by 20 AlNR2 groups; that is it
Metalloid Gallium Cluster
À
contains 20 terminal Al N bonds.
[Ga16(PtBu2)10]: A Gallium Phosphide Sheathed
The deciding factor in these syntheses is the interplay
between disproportionation of the AlX or GaX compound to
form metal-rich species and substitution of the halides by
bulky groups, which shield the cluster core. The synthesis of
metalloid Al and Ga clusters accordingly depends on the
selective disproportionation of AlI or GaI compounds, in
which the number and type of surrounding AlR or GaR
groups determine the structure of the naked Al or Ga core.
After many successful syntheses of metalloid Al and Ga
Core of Four Naked Ga Atoms?**
Jochen Steiner, Gregor Stößer, and
Hansgeorg Schnöckel*
In accordance with the technical importance of III–V semi-
conductors, the number of publications about their potential
molecular precursors has risen explosively in the last few
years. The structural chemistry of such species, which some-
À
À
clusters with terminal Al N or Ga N bonds, we have now
obtained the first metalloid Ga cluster protected by GaPR2
groups, namely the neutral, metalloid [Ga16(PtBu2)10] mole-
cule (Compound 1, Figure 2), which, because of its Ga–P
stoichiometry, only leaves space for a central naked Ga4
tetrahedron. Cluster 1 could thus be viewed as a model
intermediate in the partial oxidation of gallium metal by
phosphorus. A (GaP)10 shell protects the Ga4 core from
complete oxidation, and the tBu groups on the phosphorus
center allow the formation of a solution-stable and shielded
cluster.
[*] Prof. Dr. H. Schnöckel, J. Steiner, Dipl.-Chem. G. Stößer
Institut für Anorganische Chemie
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 work was supported by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie.
Angew. Chem. Int. Ed. 2003, 42, 1971 – 1974
DOI: 10.1002/anie.200250461
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1971

Communications
Figure 3. Idealized structural development of 1, starting from crystal-
line gallium phosphide (zinc blende structure). a) Ga4P10 (“inverse”
P4O10); b) naked Ga4 tetrahedron at the center of the Ga4P10 unit
(somewhat reduced in size for clarity); c) insertion of six gallium
atoms into the six edges of the tetrahedron; d) insertion of two
additional gallium atoms with distortion of the Td symmetry.
Figure 2. Ball-and-stick representation of the Ga16P10 cluster frame-
work; for clarity only the P atoms of the PtBu2 groups and the central
C atoms of the tBu groups are represented. Selected bond lengths
(min./max.) [pm] and angles [8]: Ga1-Ga1 268/277, Ga1-Ga3 251/256,
Ga1-Ga4 260/267, Ga2-Ga3 251, Ga2-Ga4 261, P1-Ga4 248, P2-Ga2/
Ga3 252/242, P3-Ga2 241; Ga4-P1-Ga4 79.5, Ga2-P2-Ga3 108.
[Ga16(PtBu2)10] was synthesized by the reaction of a slight
excess of a toluene suspension of LiPtBu2 with GaBr at
À788C; GaBr was prepared beforehand as a metastable
solution in toluene/THF by a cocondensation method. After
slowly warming the reaction mixture to room temperature, we
heated it to 508C for about 1 h and then removed the solvent
under vacuum. The LiBr was separated out by dissolving the
residue in toluene, which subsequently allowed us to obtain 1
from a solution in toluene as black, cubic crystals. The yield
was about 10%, which means that the dark solution contains
further species, whose isolation is the subject of ongoing
investigations.
A crystal structure analysis[9] of 1 shows it to be a neutral
Ga16 cluster compound with a distorted tetrahedral structure
(Figure 2). The tetrahedra are organized in the crystal in a
nearly body-centered cubic (certainly somewhat tetragonally
distorted) arrangement. The smallest distances between the
centers of the clusters are 1522.7 pm.
The structure of the individual Ga16P10 clusters is depicted
in Figure 3. Four Ga and ten P atoms form an adamantane-
like Ga4P10 framework, which can be described as “inverse”
P4O10 (Figure 3a) and in which the zinc blende structure of
GaP is discernible.[10]. In the center of this unit is a polyhedron
of four naked (i.e., not ligand-bearing) Ga atoms (Figure 3b).
The six edges of the Ga4P10 tetrahedron are bridged by six
more Ga atoms (Figure 3c). Figure 3d shows the result of the
insertion of two additional Ga atoms; two edges of the Ga4P10
tetrahedron are bridged by two Ga atoms, and four edges are
bridged by one Ga atom each, which disrupts the Td
symmetry.
distances to the four terminal Ga atoms are 251 and 261 pm,
respectively. The terminal Ga atoms also have the shortest
À
Ga P bonds (241 pm, coordination number 3); this bond is
only slightly longer than those in crystalline GaP (235 pm).
Thus on going from the inside to the outside of this cluster, the
À
Ga Ga interactions become increasingly covalent, as evi-
denced by the decreasing distances.[11]
The special characteristic that renders 1 unique among
metalloid Ga clusters stems from the additional bridging
capability of the PR2 groups. This allows the formation of
À
additional Ga P bonds, which constrict the cluster core like a
corset, pulling it together more tightly than is possible, for
example, with a shell of GaNR2 groups. This bridging ability
also lies behind the crucial difference between 1 and metal-
loid Ga clusters with exclusively terminal GaSiR3 groups
[12,13]
(e.g., R = tBu).
The latter surround the cluster core
À
without any additional Ga Si interactions, as many examples
have shown. To better elucidate the constriction of the central
Ga4 tetrahedron in 1,[14] we used single-point calculations
based on experimentally determined structures to determine
the volumes of the Ga4 units in three compounds with
tetrahedral Ga4 cores[15]: a) [Ga4R4] (R = Si(SiMe3)3; 2),[16]
b) 1, and c) [Ga4(GaR)6]À (R = SitBu3; 3).[17] As the average
oxidation number decreases from + 1 (2) to + 0.625 (1) to
+ 0.6 (3), the volume of the associated tetrahedra increases
from 163.5 to 172.1 to 177.0 Š3.[18] Although 1 and 3 have nearly
identical oxidation numbers, the volume for 1 is additionally
reduced by the above-mentioned constriction effect.
Further experiments on metalloid clusters with PtBu2
ligands are needed to determine whether larger cages can
form within such GaP shells. Such cages would potentially
have enough room for polyhedral Ga clusters with more than
four atoms or even Ga-centered units.
À
The Ga Ga distances in the central Ga4 tetrahedron (268/
277 pm) are longer than the distances between the central Ga
À
atoms and the bridging Ga atoms (251/256 pm). The Ga Ga
1972
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Angewandte
Chemie
HPtBu2 (d = 20.17 ppm), P2tBu4 (d = 40.19 ppm), [Ga(PtBu2)3] (d =
54.92 ppm).
The insolubility of 1 in organic solvents has thus far
prevented us from obtaining an NMR spectrum in solution.
The quantities of samples obtained to date have been too
small for solid-state NMR measurements because although
the synthesis is reproducible, the yields are very low. We have
thus initially calculated[19] the chemical shifts d (in ppm) for
the 69Ga and 31P nuclei (numbering according to Figure 2):
Ga1 + 414, Ga2 + 393, Ga3 + 527, Ga4 À54; P1 + 125, P2
À2, P3 À8. These results demonstrate that the electronic
relationships in 1 are far more complex than the previously
described topology would lead us to expect. Thus only four of
the sixteen Ga atoms (Ga4) show a slight upfield shift
(À54 ppm). This group of 2 ” 2 Ga atoms is unusual because
the distance between every two Ga atoms is only 2.99 Š,
despite the PR2 bridges. This results in a Ga4-P1-Ga4 angle of
only 79.58. It is noteworthy that the 31P chemical shift of the
bridging PR2 groups (+ 125 ppm) differs substantially from
that of the other PR2 ligands. All other Ga atoms are shifted
significantly downfield in the direction of metallic Ga (e.g.,
a-Ga ca. + 4500 ppm);[21] that is, even further downfield
than the chemical shift observed for crystalline GaP (69Ga:
+ 307 ppm).[22] There are also no striking parallels between
the 31P chemical shifts of 1 and solid GaP (31P: + 146 ppm).[23]
Despite their partially similar structures, the polar environ-
ment of four Ga atoms surrounded by four P atoms in solid
GaP clearly leads to a significantly different local electronic
Received: October 31, 2002
Revised: December 27, 2002 [Z50461]
Keywords: cluster compounds · gallium · metal–metal
.
interactions · phosphides · structure elucidation
[1] a) R. L. Wells, A. P. Purdy, A. T. McPhail, C. G. Pitt, J. Organo-
met. Chem. 1986, 308, 281; b) A. M. Arif, B. L. Benac, A. H.
Cowley, R. L. Geerts, R. A. Jones, K. B. Kidd, J. M. Power, S. T.
Schwab, J. Chem. Soc. Chem. Commun. 1986, 1543; c) O. T.
Beachley, Jr., J. P. Kopasz, H. Zhang, W. E. Hunter, J. T.
Atwood, J. Organomet. Chem. 1987, 325, 69; d) C. G. Pitt,
K. T. Higa, A. T. McPhail, R. L. Wells, Inorg. Chem. 1986, 25,
2483; e) K. M. Waggoner, P. P. Power, J. Am. Chem. Soc. 1991,
113, 3385.
[2] a) K. M. Waggoner, H. Hope, P. P. Power, Angew. Chem. 1988,
100, 1765; Angew. Chem. Int. Ed. Engl. 1988, 27, 1699; b) R. A.
Bartlett, P. P. Power, J. Am. Chem. Soc. 1990, 112, 3660.
[3] R. L. Wells, A. P. Purdy, A. T. McPhail, C. G. Pitt, J. Chem. Soc.
Chem. Commun. 1986, 487.
[4] C. Dohmeier, H. Schnöckel, C. Robl, U. Schneider, R. Ahlrichs,
Angew. Chem. 1994, 106, 225 – 227; Angew. Chem. Int. Ed. Engl.
1994, 33, 199.
[5] M. B. Power, A. R. Barron, Angew. Chem. 1991, 103, 1403–
1404; Angew. Chem. Int. Ed. Engl. 1991, 30, 1353 – 1354.
[6] C. Üffing, C. von Hänisch, H. Schnöckel, Z. Anorg. Allg. Chem.
2000, 626, 1557 – 1560.
[7] A. Schnepf, H. Schnöckel, Angew. Chem. 2002, 114, 6000 – 6021;
Angew. Chem. Int. Ed. 2002, 41, 3533 – 3554.
À
structure than in 1 (e.g. for P2), with its largely nonpolar P C
bonds.
The above results indicate that despite its expected
complex electronic structure, the unique structure of 1,
which consists of a naked Ga4 core surrounded by a shell of
GaP in which the terminal P atoms are saturated with tBu
groups, can be regarded as an intermediate in the oxidation of
a Gan particle by phosphorus.
Like many other highly symmetric species, the single
molecules of 1 have a nearly body-centered cubic arrange-
ment within the crystal. If the fragmentation of 1 can be
induced thermally in such a way as to remove the tBu groups
without decomposing the cluster core, we believe that one of
nanochemistry's dreams will have been realized: a specific
arrangement of metal atom clusters surrounded by a semi-
conducting protective shell in a topologically defined manner.
We are currently conducting experiments in this direction.
[8] A. Ecker, E. Weckert, H. Schnöckel, Nature 1997, 387, 379.
[9] Crystal structure data for [Ga16(PtBu2)10]: Mr = 2567.46. Crystal
ꢀ
dimensions 0.3” 0.2 ” 0.2 mm, tetragonal, space group P42(1)c,
a = 17.792(3), c = 17.157(3) Š, V= 5431.3(2) Š3, Z = 2, 1calcd
=
1.560 gcmÀ3, mMo = 4.078 mmÀ1, qmax = 20.818, 45703measured
reflections, 2854 independent reflections (R(int.) = 0.2759),
absorption correction: numeric (min./max. transmission 0.6845/
0.7362), R1 = 0.0637, wR2 = 0.0788. Stoe IPDS diffractometer
(MoKa radiation, (l = 0.71073Š), 150 K). The structure was
solved by direct methods and was refined against F2 for all
observed reflections. Programs used: Shelxs und Shelxtl (G. M.
Sheldrick, Universität Göttingen). CCDC-196943contains the
supplementary crystallographic data for this paper. These data
can be obtained online free of charge at www.ccdc.cam.ac.uk/
conts/retrieving.html (or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB21EZ; Fax:
(+ 44)1223-336-033; or deposit@ccdc.cam.ac.uk).
[10] Quantum-chemical investigations of discrete molecular Ga16P12
species show that structures with tetrahedral geometry are in
fact preferred. V. Tozzini, F. Buda, A. Fasolino, Phys. Rev. Lett.
2000, 85, 4554 – 4557.
[11] This point is emphasized by the fact that according to quantum-
chemical calculations, the average atomic volume of each of the
four central Ga atoms is 43Š 3, while that for all 16 of the atoms
is only 34 Š3.
[12] A. Donchev, A. Schnepf, G. Stößer, E. Baum, H. Schnöckel, T.
Blank, N. Wiberg, Chem. Eur. J. 2001, 7, 3348.
[13] A. Rodig, G. Linti, Angew. Chem. 2000, 112, 3076; Angew.
Chem. Int. Ed. 2000, 39, 2952.
[14] Analysis of the molecular orbitals of 1 shows that the four MOs
of the central Ga4 unit (analogous to 2) are in fact significantly
separated from the neighboring MOs.
[15] The molecular volumes were calculated with the Gaussian 98
program package at the SCF level with a 3-21G* basis set. We
Experimental Section
LiPtBu2 (235 mg, 1.5 mmol) was suspended in toluene (25 mL) and
cooled to À788C. A 0.26m solution of GaBr (5 mL, 1.3mmol) in
toluene/THF (3:1) was added dropwise with a stainless steel cannula.
This mixture was stirred and allowed to warm to room temperature
over 6 h, resulting in a dark black solution. The solution was then
heated to 508C for 1 h under slightly reduced pressure. A pale solid
precipitated (LiBr). The solvent was removed under vacuum, to leave
a dark oily residue, which was redissolved in toluene (20 mL). The
toluene extract was subsequently separated from the precipitated
LiBr with a filter cannula and stored at + 78C for the crystallization.
After several weeks we obtained small black crystals of 1 (21 mg,
0.008 mmol). Those species remaining in the solution that could be
identified by 31P{1H} NMR spectroscopy (101 MHz, C6D6) are:
Angew. Chem. Int. Ed. 2003, 42, 1971 – 1974
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1973

Communications
carried out single-point calculations based on the experimentally
determined geometries of 1, 2, and 3, in which the “ICPM
solvation model” was used to place a shell of uniform electron
density (4 ” 10À3 eŠÀ3) around the Ga4 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, A. G. Baboul,
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, 1998.
[16] G. Linti, J. Organomet. Chem. 1996, 520, 107; G. Linti, A. Rodig,
Chem. Commun. 2000, 127; N. Wiberg, K. Ameluxen, H.-W.
Lerner, H. Nöth, W. Ponikwar, H. Schwenk, J. Organomet.
Chem. 1999, 574, 246.
[17] M. Kehrwald, W. Köstler, A. Rodig, G. Linti, T. Blank, N.
Wiberg, Organometallics 2001, 20, 860.
[18] For better comparison, 3 was considered as a neutral species,
analogous to 1 and 2.
[19] The 69Ga NMR shifts were first based on the calculated shift in
GaCp, and were subsequently set in reference to the usual
standard ([Ga(H2O)6]3+). The calculations were carried out in
the S4 point group with the PMSHIFT module of the Turbomole
program package with DFT (BP86-Functional, SVP basis).
a) Turbomole: O. Treutler, R. Ahlrichs, J. Chem. Phys. 1995, 102,
346; b) BP86-Functional: 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. ꢀhm, 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 with SCF: M. Häser, R. Ahlrichs, H. P. Baron, P. Weiss, H.
Horn, Theor. Chim. Acta 1992, 83, 455.
À
[20] In crystalline GaP the Ga Ga distance is 3.85 Š. A similar value
is found for the distance between Ga2 and Ga3in 1 (4.01 Š).
[21] O. N. Bakharev, N. Zelders, H. B. Brom, A. Schnepf, H.
Schnöckel, L. J. de Jong, Eur. Phys. J., submitted .
[22] O. H. Han, H. K. C. Timken, E. Oldfield, J. Chem. Phys. 1988,
89, 6046.
[23] L. J. Humphries, R. E. J. Sears, J. Phys. Chem. Solids 1975, 36,
1149.
1974
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Angew. Chem. Int. Ed. 2003, 42, 1971 – 1974