Ga9(CMe3)9, an important new building block in the structural chemistry of the alkylelement(I) compounds EnRn (E = B - In)

Article abstract of DOI:10.1002/1521-3773(20010202)40:3<566::AID-ANIE566>3.0.CO;2-T

A simple preparative method has been employed for the synthesis of the novel cluster compound Ga₉ (CMe₃)₉, which contains a tricapped trigonal prism of monovalent gallium atoms. Electron transfer processes, were observed similar to those of polyboranes, leading to the reversible formation of the corresponding radical anion.

Full text of DOI:10.1002/1521-3773(20010202)40:3<566::AID-ANIE566>3.0.CO;2-T

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resin had hardened completely the remainder of the uninsulated gold
microwire was covered with a thin layer of epoxy resin so that only the
(111) face was left exposed. This procedure was successful for microcrystals
with deff ˆ 10 ± 35 mm. In the case of smaller SCUMEs, for example with a
deff of 5 mm, a slightly different procedure was used to minimize the
capacitative current.[13] In this case a 100 mm length of a gold microwire
with a diameter of 3 mm was first fixed to the larger 10 mm wire using the
gold paint. The small microcrystal was then fixed to the 3 mm wire using the
same procedure described above. Slightly different procedures were used
to fabricate SCUMEs with exposed (100) faces.
Ga9(CMe3)9, an Important New Building Block
in the Structural Chemistry of the
Alkylelement(i) Compounds EnRn
(E ˆ B ± In)**
Werner Uhl,* Lars Cuypers, Klaus Harms,
Wolfgang Kaim, Matthias Wanner, Rainer Winter,
Rainer Koch, and Wolfgang Saak
All electrochemical experiments were carried out in a water-jacketed glass
cell with the SCUME, a Pt counterelectrode, and a calomel reference
electrode containing 0.05m KCl. The cell was placed in a Faraday cage and
coaxial cables were used to connect the electrodes to the potentiostat.
Electrolyte solutions were prepared from HClO4 (Aldrich, 99.999% pure)
diluted to 0.01m in nanopure water. The solution was purged with argon for
10 min and kept under argon during the measurements. The reference
electrode was connected to the body of the cell using a Luggin capillary
containing the 0.01m HClO4 solution. Cyclic voltammograms were
Numerous organoelement cluster compounds of the ele-
ments boron, aluminum, gallium, and indium have been
published in recent literature. Most of them are difficult to
prepare and were isolated in only very poor yield. They often
contain cluster anions such as [Al77{N(SiMe3)2}20]2 or
[Ga9{Si(SiMe3)3}6] .[1] Beside these ionic derivatives a few
neutral cluster compounds such as In8Ar4 (Arˆ 2,6-dimesi-
tylphenyl) or In12{Si(CMe3)3}8[2] were synthesized in which the
number of the cluster atoms exceeds the number of the
substituents. To the best of our knowledge there is no
systematic approach to the description of their formation
and of their fascinating structural chemistry. Element(i)
compounds EnRn were isolated and structurally characterized
before only as monomers ER or as tetrahedral clusters of the
type E4R4,[3] in which R denotes very bulky substituents. A
neopentylgallium(i) compound was reported in literature,[4]
however, it was not isolated in a pure form. For many years we
have tried to control the size of EI clusters by the systematic
variation of the steric demand of their substituents. Smaller
groups should lead to the formation of larger clusters, which
are of particular interest owing to their bonding and broad
preparative application in secondary reactions. However, the
tendency to disproportionate as a consequence of insufficient
steric shielding prevented the isolation of such derivatives.
The reaction of tert-butyllithium with gallium trihalides
affords tri(tert-butyl)gallium[5] in a good yield as well as a
small quantity of elemental gallium by a redox reaction. By
systematically changing the reaction conditions we attempted
to stop the partial reduction of trivalent gallium at an
intermediate oxidation state. Finally, we obtained a green
solution from which black-green crystals of 1 were isolated in
4% yield after separation of the alkylgallium(iii) compound
[Eq. (1)]. Owing to the easy availability of compound 1 a
complete characterization was possible despite its low yield.
The crystal structure determination[6] revealed a tricapped
recorded by using
a EG&G 283 potentiostat connected to a PC
minicomputer by a GPIB IEEE-488.2 interface. The current-potential
data were acquired digitally.
Received: August 25, 2000 [Z15706]
[1] J. Heinze, Angew. Chem. 1993, 105, 1327; Angew. Chem. Int. Ed. Engl.
1993, 32, 1268.
[2] W. R. Fawcett, M. Opallo, Angew. Chem. 1994, 106, 2239; Angew.
Chem. Int. Ed. Engl. 1994, 33, 2131.
Â
[3] M. Hromadova, W. R. Fawcett, J. Phys. Chem. A 2000, 104, 4356.
[4] A. Hamelin, Mod. Aspects Electrochem. 1985, 16, 1.
[5] H. K. Henisch, Crystals in Gels and Liesegang Rings, Cambridge
University Press, Cambridge, 1998.
[6] P. Kratochvil, B. Sprusil, M. Heyrovsky, J. Crystal Growth 1968, 4,
360.
[7] The effective disc diameter deff is simply the diameter of the
corresponding disc electrode calculated from the actual area a of
the SCUME using the relationship a ˆ p(deff)2.
[8] C. C. Nesbitt, E. B. Milosavljevic, J. L. Hendrix, Ind. Eng. Chem. Res.
1990, 29, 1696.
[9] Gmelin Handbook of Inorganic and Organometallic Chemistry, Suppl.
Vol. B1, 8th ed., 1992, p. 187.
[10] V. Goldschmidt, Atlas der Krystallformen, Carl Winters Universitäts-
buchhandlung, Vol. IV, Heidelberg, 1918.
[11] D. R. Rolison in Ultramicroelectrodes (Eds.: M. Fleischmann, S. Pons,
D. R. Rolison, P. P. Schmidt), Datatech, Morgantown, NC, 1987,
Chap. 3.
[12] Gels form in a few days at pH 4 and very quickly at pH 7.5.[5]
[13] D. O. Wipf, A. C. Michael, R. M. Wightman, J. Electroanal. Chem.
1989, 269, 15.
GaCl3 ‡ 3LiCMe3
Ga(CMe3)3 ‡ Ga9(CMe3)9
(1)
!
LiCl
1
[*] Prof. Dr. W. Uhl, Dipl.-Chem. L. Cuypers, Dr. K. Harms
Fachbereich Chemie der Philipps Universität Marburg
Hans-Meerwein-Strasse, 35032 Marburg (Germany)
Fax : (‡49)6421-282-5653
E-mail: uhl@chemie.uni-marburg.de
Prof. Dr. W. Kaim, Dipl.-Chem. M. Wanner, Dr. R. Winter
Institut für Anorganische Chemie der Universität Stuttgart
(Germany)
Dr. R. Koch, Dipl.-Chem. W. Saak
Fachbereich Chemie der Carl von Ossietzky Universität Oldenburg
(Germany)
[**] This work was supported by Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie.
566
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trigonal prism of gallium
atoms (Figure 1). Three
ranges of Ga Ga bond
lengths were observed: short
distances of 258.8 pm (av)
were detected to the capping
Ga atoms, while longer dis-
tances of 267.0 pm were
found on the triangular faces
of the prism. Very long
Ga Ga separations were ob-
served for the edges of the
prism perpendicular to these
triangles (298.8 pm), which
indicate only weak Ga ± Ga
interactions. Thus, all galli-
um atoms within the cluster
seem to have a connectivity
of four. For comparison, the
Ga Ga single bond lengths
in tetraalkyldigallium com-
pounds Ga2R4 are about
250 pm,[7] while alkyl-substi-
tuted tetrahedral tetragal-
lanes Ga4R4 have Ga Ga
distances of about 270 pm.[8]
Compound 1 has a sur-
prisingly high thermal stabil-
ity and decomposes above
tion.[9] The reason for this may be the relatively high
symmetry of 2, which leads to energetically similar states
and the occupation of neighboring, partially degenerate
orbitals by the unpaired electron. Similar observations were
.
reported for the distorted octahedral hexaborates [B6X6]
(X ˆ halogen).[10] Also the splitting of the g components of 2
(2.173, 2.06, 1.95) is similar to that of, for example, [B6Br6]
(2.15, 2.09, 1.88).[10] Thus, these data verify a distribution of the
unpaired electron over at least large parts of the cluster
skeleton. Accordingly, the hyperfine splitting by coupling with
the isotopes 69Ga and 71Ga (each I ˆ 3/2) was not observed
owing to the overlap of the expected large number of lines.
Quantum-chemical calculations of the methyl compound
Ga9Me9 (1a) verified the structural parameters of 1 with three
characteristic ranges of Ga Ga distances and the elongation
of the cluster parallel to its threefold rotation axis.[11] For
comparison, we also calculated the radical anion 2a which was
reversibly generated by an electrochemical experiment and
detected by ESR spectroscopy.[11] Remarkably, the nonagal-
lium cluster showed behavior similar to the corresponding
boron compounds [B9X9]n (X ˆ H, halogen; n ˆ 0,1,2).[12] The
addition of one electron leads to a strengthening of the
Ga Ga interaction parallel to the long edges of the trigonal
prism and to a compression of the cluster (Ga Ga: from 298.2
in 1a to 281.6 pm in 2a). The distances to the capping Ga
atoms remained almost unchanged (256.3 (1a) and 254.4 pm
(2a)), while the edges of the triangles of the prism became
longer (267.3 (1a) and 275.1 pm (2a)). Thus, an almost
undistorted equilateral trigonal prism is present in the radical
anion.
Figure 1. Molecular structure of 1;
the tert-butyl groups are omitted for
clarity. Only one of the two inde-
pendent molecules is drawn, the
average values are, however, calcu-
lated from both independent mole-
cules. Important bond lengths [pm]:
Ga Ga distances to the capping Ga
atoms (Ga4, Ga5, Ga6 (Ga7, Ga13,
Ga15)): 256.9(1) ± 261.9(1) (av
258.8); Ga Ga distances at the
edges of the triangle of the prism
between the atoms Ga1, Ga2, Ga3
(Ga8, Ga9, Ga13 and Ga10, Ga11,
Ga14): 264.06(9) ± 268.96(9) (av
267.02); Ga1-Ga1' 300.7(1), Ga2-
Ga2' 304.6(1), Ga3-Ga3' 291.2(1)
[291.39(9) ± 306.6(1) in the second
molecule]; Ga-C 202.5.
2288C with the formation of elemental gallium. Powdered
samples of 1 remained unchanged in air over a period of about
30 min. The UV/Vis spectrum showed two absorptions in the
visible region at 480 and 600 nm. In comparison to gallium(iii)
compounds, the proton resonances of the tert-butyl groups in
the 1H NMR spectrum are only slightly shifted to a lower field
by about 0.3 ppm (d ˆ 1.48). However, an extreme low-field
shift (d < 30 in compounds possessing trivalent gallium atoms)
was observed for the resonances of carbon atoms attached to
gallium (d ˆ 100.4). Tetrahedral E4R4 clusters bearing tris-
(trialkylsilyl)methyl substituents showed a less dramatic shift
of d ˆ 60 ± 64.[8] An absorption at 513 cm 1 in the IR spectrum
was assigned to the Ga ± C stretching vibration. A 71Ga NMR
spectrum could not be recorded owing to the quadrupole
moment of gallium and the low local symmetry at each
gallium atom.
According to cyclic voltammetric investigations the uptake
of an electron is electrochemically reversible; it leads in o-
difluorobenzene or dichloromethane to the corresponding
radical anion 2, accompanied by a lightening of the color of
the solution.[9] The radical anion 2 was stable at room
temperature and was further reduced at an about 1 V more
negative potential in an irreversible two-electron reduction
step. Cooling to 788C did not lead to an improvement of the
reversibility behavior. The oxidation was also completely
irreversible in a multielectron process in which the current
intensity corresponded approximately to the loss of one
electron per gallium atom.
For the first time, the influence of the steric demand of the
substituents on the size of a third main group cluster was
verified by the synthesis of compound 1. This will stimulate
further experiments toward the synthesis of other alkylele-
ment(i) compounds bearing smaller alkyl groups and having
novel structures. Furthermore, the synthesis of 1 opens the
access to a chemistry similar to that of polyborates. As
impressively shown with the related compounds E4R4,[3] we
expect a broad preparative application of 1, in particular in
those cases in which the high steric shielding of the derivatives
previously employed prevented secondary reactions.
Experimental Section
n-Pentane and n-hexane were dried over LiAlH4. Freshly sublimed GaCl3
(7.33 g, 0.042 mol) in n-hexane (90 mL) was treated with tert-butyllithium
(78 mL of a 1.6m solution in n-hexane) over a period of 1 h. The mixture
was heated under reflux for 16 h, cooled to room temperature, and filtered.
The volatile components were removed in vacuum at room temperature.
At the same temperature and at a pressure of 10 3 Torr tri(tert-butyl)gal-
lane was distilled in a trap cooled with liquid nitrogen (yield: 6.83 g, 68%).
The green residue was dissolved in pentane (25 mL) and filtered. The
filtrate was evaporated, the residue was thoroughly evacuated and
recrystallized from n-pentane or diisopropyl ether (yield: 0.19 g, 4%).
Decomposition (under argon, sealed capillary): 2288C. 1H NMR
(500 MHz, C6D6): d ˆ 1.48; 13C NMR (125.8 MHz, C6D6): d ˆ 100.4
(GaC), 32.2 (CH3). IR (KBr pellet): nÄ [cm 1] ˆ 1465vs, 1387w, 1358vs,
1308m, 1242vs (d(CH3)); 1188w, 1158vs, 1005m (ns(C3C)); 984m, 937m
(CH3 rocking); 805vs (nas(C3C)); 513m (n(GaC)); UV/Vis (n-hexane): lmax
[nm] (lge): 245 (4.6), 480 (3.5), 600 (2.9).
The ESR spectroscopic characterization of the radical 2 was
only possible at very low temperature owing to fast relaxa-
Received: October 12, 2000 [Z15944]
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567

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[1] A. Ecker, E. Weckert, H. Schnöckel, Nature 1997, 387, 379; W. Köstler,
G. Linti, Angew. Chem. 1997, 109, 2758; Angew. Chem. Int. Ed. Engl.
1997, 36, 2644; A. Schnepf, G. Stöûer, R. Köppe, H. Schnöckel, Angew.
Chem. 2000, 112, 1709; Angew. Chem. Int. Ed. 2000, 39, 1637; A.
Schnepf, G. Stöûer, H. Schnöckel, Z. Anorg. Allg. Chem. 2000, 626,
1676.
[2] B. E. Eichler, N. J. Hardman, P. P. Power, Angew. Chem. 2000, 112,
391; Angew. Chem. Int. Ed. 2000, 39, 383; N. Wiberg, T. Blank, H.
Nöth, W. Ponikwar, Angew. Chem. 1999, 111, 887; Angew. Chem. Int.
Ed. 1999, 38, 839; G. Linti, A. Rodig, Chem. Commun. 2000, 127.
[3] S. T. Haubrich, P. P. Power, J. Am. Chem. Soc. 1998, 120, 2202. Review
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18, 239.
Stereoselective Multiple Functionalization of
Pyrylium Salts by Domino Reactions with
2-Silyloxybuta-1,3-dienes**
Uwe Beifuss,* Kim Goldenstein, Frank Döring,
Christopher Lehmann, and Mathias Noltemeyer
The development of multicomponent domino reactions is
particularly attractive as these allow the stereoselective
conversion of several simple substrates into complex target
molecules, such as annulated or bridged polycycles, in a single
step.[1] Pyrylium salts are easily accessible, reactive hetero-
arenes that react preferably with nucleophiles. Apart from
transformations that proceed with conservation of the pyran
ring, we are also familiar with reactions in which the primary
adducts are stabilized by ring opening or subsequent ring
transformation.[2] In contrast, only a few stereoselective
reactions that start from pyrylium salts are known to result
in complex ring systems. One exception is the 1,3-dipolar
cycloadditions of 3-oxidopyrylium salts, which have initially
been developed by Sammes et al. and later by Wender,
[4] O. T. Beachley, Jr., M. J. Noble, R. D. Allendoerfer, J. Organomet.
Chem. 1999, 582, 32.
[5] R. A. Kovar, H. Derr, D. Brandau, J. O. Callaway, Inorg. Chem. 1975,
14, 2809.
[6] Crystal structure of 1: Crystals from toluene at 308C; C36H81Ga9 ´
0.33C7H8, orthorhombic, space group Pnnm; a ˆ 1900.73(5), b ˆ
2181.55(4), c ˆ 3751.45(7) pm, Vˆ 15555.5(6) Š3, Z ˆ 12, 1calcd
ˆ
1.498 gcm 3, crystal dimensions: 0.50 Â 0.45 Â 0.36 mm, diffractome-
ter Stoe IPDS, MoKa radiation, 193 K, measurement range: 3.9 <
2V< 52.08, 335 exposures, Df ˆ 0.68, 15480 independent reflections,
11165 reflections F > 4s(F), m ˆ 4.62 mm 1, numerical absorption
correction; programs SHELXTL PLUS REL 4.1 and SHELXL-97,
746 parameters, R1 ˆ 0.045 and wR2 (all data) ˆ 0.127, max./min.
Ä
Mascarenas, and Magnus and co-workers to provide an
3
residual electron density: 2.77/ 3.02 Â 1030 em
. Compound 1
attractive route to various ring systems and natural products.[3]
One concept that may be applied to yield products of a
higher complexity in diastereomerically pure form from
simple components is the multiple functionalization of
positively charged heteroarenes. Here, A first reacts regiose-
lectively with a nucleophile at C-2. The resulting enol ether B
can then react with an electrophile at C-3 forming the Michael
acceptor C. Subsequent transformation with a nucleophile at
C-6 and an electrophile at C-5 yield the trisubstituted D and
the tetrasubstituted heterocycle E, respectively (Scheme 1).
Following research on the selective mono- and bisfunctional-
crystallizes with 1.5 independent molecules per asymmetric unit, one
molecule is located on a crystallographic mirror plane. The toluene
molecules are strongly disordered, their hydrogen atoms were not
considered. Crystallographic data (excluding structure factors) for the
structure reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publica-
tion no. CCDC-150591 (1). Copies of the data can be obtained free of
charge on application to CCDC, 12 Union Road, Cambridge
CB21EZ, UK (fax: (‡44)1223-336-033; e-mail: deposit@ccdc.cam.
ac.uk).
[7] W. Uhl, M. Layh, T. Hildenbrand, J. Organomet. Chem. 1989, 364, 289;
X. He, R. A. Bartlett, M. M. Olmstead, K. Ruhlandt-Senge, B. E.
Sturgeon, P. P. Power, Angew. Chem. 1993, 105, 761; Angew. Chem.
Int. Ed. Engl. 1993, 32, 717.
[8] W. Uhl, W. Hiller, M. Layh, W. Schwarz, Angew. Chem. 1992, 104,
1378; Angew. Chem. Int. Ed. Engl. 1992, 31, 1364; W. Uhl, A.
Jantschak, J. Organomet. Chem. 1998, 555, 263.
[9] Cyclic voltammogram of 1 in CH2Cl2/0.1m Bu4NPF6 or 1,2-difluoro-
benzene/0.1m Bu4NPF6 at 298 K: Reversible reduction at E(1/2) ˆ
‡
1.74 V (vs. [Fe(C5H5)2] /[Fe(C5H5)2]0); irreversible two-electron
reduction at 2.70 V; irreversible multielectron oxidation at 0.40 V
(100 mVs 1). ESR spectrum of 2, generated electrochemically in
CH2Cl2/0.1m Bu4NPF6 at 298 K, measured in a glassy frozen solution
at 3.4 K: g1 ˆ 2.173, g2 ˆ 2.06, g3 ˆ 1.95. Owing to very fast relaxation
no signal was observed above 110 K.
[10] [B6X6] (X ˆ halogen): V. Lorenzen, W. Preetz, F. Baumann, W. Kaim,
Inorg. Chem. 1998, 37, 4011.
[11] Structural parameters of the model compounds 1a and 2a were
optimized at the HF/6-31G* level with the program package
Gaussian 98. This level has previously been shown to be reliable for
gallium compounds. Ga9Me9 (1a): Ranges of Ga Ga distances:
255.1 ± 257.6 pm to the capping Ga atoms (av 256.3 pm), 266.6 ±
268.4 pm for the edges of the triangles of the prism (av 267.3 pm),
294.7 ± 303.1 pm for the edges of the prism parallel to the threefold
rotation axis (av 298.2 pm); Ga C 197.7 pm (av). [Ga9Me9] 2a:
Ranges of Ga Ga distances: 253.8 ± 255.1 pm to the capping Ga atoms
(av 254.4 pm), 274.7 ± 275.5 pm for the edges of the triangles of the
prism (av 275.1 pm), 280.8 ± 282.7 pm for the edges of the prism
parallel to the threefold rotation axis (av 281.6 pm); Ga C 200.3 pm.
[12] L. M. McKee, Z.-X. Wang, P. von R. Schleyer, J. Am. Chem. Soc. 2000,
122, 4781; H. Binder, R. Kellner, K. Vaas, M. Hein, F. Baumann, M.
Wanner, R. Winter, W. Kaim, W. Hönle, Y. Grin, U. Wedig, M.
Schultheiss, R. K. Kremer, H. G. von Schnering, O. Groeger, G.
Engelhardt, Z. Anorg. Allg. Chem. 1999, 625, 1059.
Scheme 1. Multiple functionalization of positively charged heteroarenes.
[*] Priv.-Doz. Dr. U. Beifuss, Dr. K. Goldenstein, Dipl.-Chem. F. Döring
Institut für Organische Chemie der Georg-August-Universität Göt-
tingen
Tammannstrasse 2, 37077 Göttingen (Germany)
Fax : (‡49)551-39-9660
E-mail: ubeifus@gwdg.de
Dipl.-Chem. C. Lehmann, Dr. M. Noltemeyer
Institut für Anorganische Chemie der Georg-August-Universität
Göttingen
Tammannstrasse 4, 37077 Göttingen (Germany)
[**] This work was supported by the Fonds der Chemischen Industrie.
Supporting information for this article is available on the WWW under
http://www.angewandte.com or from the author.
568
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Angew. Chem. Int. Ed. 2001, 40, No. 3