Hydroaluinination and hydrogallation of 1,2-bis(trimethylsilylethynyl) benzene: Formation of molecular capsules and C-C bond activation

Article abstract of DOI:10.1021/om901058a

Treatment of l,2-bis(trimethylsilylethynyl)benzene with di-tert-butylaluminum and di-tert-butylgallium hydrides afforded the simple addition products 1,2-[(Me₃Si)(R₂E)C-C(H)] ₂C₆H₄ (R = CMe₃; E = Al (1), Ga (2)), which could, not be isolated in a pure crystalline form but have been characterized unambiguously by spectroscopic methods. Addition of the Lewis base ethyldimethylamine initiated condensation reactions which gave cage compounds (3 and 4) by the release of the corresponding tri-tert-butyl element derivatives. These cages contain two aluminum or gallium atoms which are bridged by three l,2-bis(trimethylsilylethenyi)benzene spacers to form molecular capsules. The metal atoms are further coordinated by terminal amino groups. The amino ligands could not be removed from the dialuminum compound 4 without decomposition, but the ligand-free gallium, compound 5 was obtained upon heating of 3 (E = Ga) to 80 °C under vacuum. Thermolysis of the aluminum compound 1 in boiling n-hexane gave a unique reaction by the release of tri-tert-butylaluminum and the formal elimination of trimethylsilylethyne (decarbalumination). The product 6 is dimeric in the solid, state via Al-C-Al bridges and has a pentacyclic molecular structure.

Full text of DOI:10.1021/om901058a

1406 Organometallics 2010, 29, 1406–1412
DOI: 10.1021/om901058a
Hydroalumination and Hydrogallation of
1,2-Bis(trimethylsilylethynyl)benzene: Formation of Molecular Capsules
and C-C Bond Activation
Werner Uhl,*,† Alexander Hepp,† Hauke Westenberg,† Sarina Zemke,†
‡
Ernst-Ulrich Wurthwein, and Johannes Hellmann
‡
€
†
€
€
Institut fu€r Anorganische und Analytische Chemie der Universitat Munster, Corrensstrasse 30,
€
‡
D-48149 Mu€nster, Germany, and Organisch-chemisches Institut der Universitat Munster,
Corrensstrasse 40, D-48149 Mu€nster, Germany
€
Received December 10, 2009
Treatment of 1,2-bis(trimethylsilylethynyl)benzene with di-tert-butylaluminum and di-tert-butyl-
gallium hydrides afforded the simple addition products 1,2-[(Me3Si)(R2E)CdC(H)]2C6H4 (R =
CMe3; E = Al (1), Ga (2)), which could not be isolated in a pure crystalline form but have been
characterized unambiguously by spectroscopic methods. Addition of the Lewis base ethyldimethyl-
amine initiated condensation reactions which gave cage compounds (3 and 4) by the release of
the corresponding tri-tert-butyl element derivatives. These cages contain two aluminum or gallium
atoms which are bridged by three 1,2-bis(trimethylsilylethenyl)benzene spacers to form mole-
cular capsules. The metal atoms are further coordinated by terminal amino groups. The amino
ligands could not be removed from the dialuminum compound 4 without decomposition, but the
ligand-free gallium compound 5 was obtained upon heating of 3 (E = Ga) to 80 °C under vacuum.
Thermolysis of the aluminum compound 1 in boiling n-hexane gave a unique reaction by the release
of tri-tert-butylaluminum and the formal elimination of trimethylsilylethyne (decarbalumination).
The product 6 is dimeric in the solid state via Al-C-Al bridges and has a pentacyclic molecular
structure.
Introduction
proceed by condensation and release of the corresponding
trialkylelement derivatives. Alkynylaluminum or -gallium com-
pounds, for instance, gave carbaalane clusters3 or heteroada-
mantane cages4 upon treatment with dialkyl element hydrides,
while bis- or tris(tert-butylethynyl)benzenes afforded cyclo-
phane-type molecules with two or three bridging metal atoms.5
((Trimethylsilyl)ethynyl)benzenes behaved differently.6-8 Their
Hydroalumination and hydrogallation reactions1 with alkynes
are facile methods for the generation of a broad variety of un-
usual oligoaluminum or -gallium compounds.2 Many reactions
*To whom correspondence should be addressed. Fax: þ49-251-
8336660. E-mail: uhlw@uni-muenster.de.
(1) (a) Zweifel, G.; Miller, J. A. Org. React. 1984, 32, 375.
(b) Winterfeldt, E. Synthesis 1975, 10, 617. (c) Marek, J.; Normant, J.-F. Chem.
Rev. 1996, 96, 3341. (d) Eisch, J. J. In Comprehensive Organic Synthesis;
Frost, B., Flemming, I., Eds.; Pergamon: Oxford, U.K., 1991; Vol. 8, p 733.
(e) Zweifel, G. In Aspects of Mechanism and Organometallic Chemistry;
Brewster, J. H., Ed.; Plenum Press: New York, 1978; p 229. (f) Zweifel, G. In
Comprehensive Organic Chemistry; Barton, D. H. R., Ollis, W. D., Eds.;
Pergamon: Oxford, U.K., 1979; p 1013. (g) Negishi, E. Organometallics in
Organic Synthesis; Wiley: New York, 1980. (h) Saito, S. In Main Group
Metals in Organic Synthesis; Yamamoto, H., Oshima, K., Eds.; Wiley-
VCH: Weinheim, Germany, 2004; p 189. (i) Miller, J. A. In Chemistry of
Aluminum, Gallium, Indium and Thallium; Downs, A. J., Ed.; Blackie
Academic: London, 1993; p 372.
€
(4) Uhl, W.; Cuypers, L.; Neumuller, B.; Weller, F. Organometallics
2002, 21, 2365.
(5) (a) Uhl, W.; Hepp, A.; Matar, M.; Vinogradov, A. Eur. J. Inorg.
Chem. 2007, 4133. (b) Uhl, W.; Breher, F.; Haddadpour, S. Organometallics
2005, 24, 2210. (c) Uhl, W.; Haddadpour, S.; Matar, M. Organometallics
2006, 25, 159. (d) Uhl, W.; Claesener, M.; Haddadpour, S.; Jasper, B.; Hepp,
A. Dalton Trans. 2007, 417. Condensation reactions have also been
observed for some dimethylgallium compounds not obtained from hydro-
gallation: (e) Jutzi, P.; Izundu, J.; Sielemann, H.; Neumann, B.; Stammler,
H.-G. Organometallics 2009, 28, 2619. (f) Althof, A.; Eisner, D.; Jutzi, P.;
Lenze, N.; Neumann, B.; Schoeller, W. W.; Stammler, H.-G. Chem. Eur. J.
2006, 12, 5471.
(6) (a) Uhl, W.; Bock, H. R.; Breher, F.; Claesener, M.; Haddadpour,
S.; Jasper, B.; Hepp, A. Organometallics 2007, 26, 2363. (b) Uhl, W.;
Claesener, M. Inorg. Chem. 2008, 47, 729. (c) Uhl, W.; Claesener, M. Inorg.
Chem. 2008, 47, 4463. (d) Uhl, W.; Claesener, M.; Hepp, A. Organome-
tallics 2008, 27, 2118. (e) Uhl, W.; Claesener, M.; Hepp, A.; Jasper, B.;
(2) Uhl, W. Coord. Chem. Rev. 2008, 252, 1540.
(3) (a) Uhl, W.; Breher, F. Angew. Chem. 1999, 111, 1578; Angew.
Chem., Int. Ed. Engl. 1999, 38, 1477. (b) Uhl, W.; Breher, F.; L€utzen, A.;
Saak, W. Angew. Chem. 2000, 112, 414; Angew. Chem., Int. Ed. Engl.
2000, 39, 406. (c) Uhl, W.; Breher, F.; Grunenberg, J.; L€utzen, A.; Saak, W.
Organometallics 2000, 19, 4536. (d) Uhl, W.; Breher, F.; Mbonimana, A.;
Gauss, J.; Haase, D.; L€utzen, A.; Saak, W. Eur. J. Inorg. Chem. 2001, 3059.
(e) Stasch, A.; Ferbinteanu, M.; Prust, J.; Zheng, W.; Cimpoesu, F.; Roesky,
H. W.; Magull, J.; Schmidt, H.-G.; Noltemeyer, M. J. Am. Chem. Soc. 2002,
124, 5441. (f) Kumar, S. S.; Rong, J.; Singh, S.; Roesky, H. W.; Vidovic, D.;
Magull, J.; Neculai, D.; Chandrasekhar, V.; Baldus, M. Organometallics
2004, 23, 3496.
€
Vinogradov, A.; van W€ullen, L.; Koster, T. K.-J. Dalton Trans. 2009, 10550.
(7) (a) Uhl, W.; Bock, H. R.; Claesener, M.; Layh, M.; Tiesmeyer, I.;
€
Wurthwein, E.-U. Chem. Eur. J. 2008, 14, 11557. (b) Uhl, W.; Er, E.; Hepp,
€
A.; Kosters, J.; Layh, M.; Rohling, M.; Vinogradov, A.; W€urthwein, E.-U.;
Ghavtadze, N. Eur. J. Inorg. Chem. 2009, 3307.
(8) Uhl, W.; Claesener, M.; Haddadpour, S.; Jasper, B.; Tiesmeyer,
I.; Zemke, S. Z. Anorg. Allg. Chem. 2008, 634, 2889.
r
pubs.acs.org/Organometallics
Published on Web 02/25/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 6, 2010 1407
reactions with dialkyl or dichloro element hydrides af-
forded the relatively persistent simple addition products
with intact EX2 groups attached to one carbon atom of
the resulting CdC double bonds. However, cis/trans iso-
merization occurred, which requires intermolecular activa-
tion and is prevented by steric shielding.7 In these cases
the release of trialkylgallium and the formation of di-
vinyl- or trivinylgallium compounds has been observed
only by treatment of the monoalkyne H5C6CtCSiMe3
with dialkylgallium hydrides.8 We hoped to synthesize
novel cage compounds by the application of particu-
larly preorganized bis((trimethylsilyl)ethynes) and treated
1,2-bis((trimethylsilyl)ethynyl)benzene with di-tert-butyl-
aluminum and di-tert-butylgallium hydrides.
19.4 Hz, trans arrangement of Si and H) across the
CdC double bonds.2,6-8
Results and Discussion
Treatment of Compounds 1 and 2 with Ethyldimethylamine.
Di-tert-butylgallium hydride9 is accessible by the reaction of
tri-tert-butylgallium with stoichiometric quantities of the
gallane-amine adduct H3Ga r NMe2Et. The amine is re-
moved by distillation under vacuum. Accidentally, one
sample of the dialkylgallium hydride contained traces of
the amine by incomplete purification, which caused an
interesting and unexpected secondary reaction upon treat-
ment with the 1,2-bis(alkyne). Colorless crystals of com-
pound 3 were isolated in trace quantities. Crystal structure
determination revealed a unique structural motif with two
gallium atoms bridged by three bifunctional dialkenylben-
zene spacer ligands. Each gallium atom is further coordi-
nated by a terminal amino ligand (see below for details). An
optimized procedure comprises the reaction of the compo-
nents in a molar ratio of 3:6:2 (1,2-bis(alkyne):gallium
hydride:amine; eq 2). Hence, the quantity of the amine is
just sufficient to coordinate both gallium atoms of the cage
Reactions of 1,2-Bis((trimethylsilyl)ethynyl)benzene with
H-E(CMe3)2 (E = Al, Ga). Reactions of 1,2-bis((trimethyl-
silyl)ethynyl)benzene with di-tert-butylaluminum and
di-tert-butylgallium hydrides yielded the expected addi-
tion products 1 and 2 under different conditions (eq 1). In
accordance with our experience from previous ex-
periments2,5,7 the dialkylaluminum hydride is more re-
active and gave the quantitative consumption of the
starting compounds by stirring of the mixture at room
temperature for 16 h. Probably due to the lower polarity
of the Ga-H bond, the corresponding gallium hydride
is less reactive, and boiling n-hexane was required
for completion of the reaction. Any spontaneous secon-
dary reaction such as isomerization or condensation
(see the Introduction) was not observed even when the
mixtures were stirred at room temperature (1) or heated
(2) for several days. In particular the corresponding
tri-tert-butyl element compounds could not be detected
by NMR spectroscopy as byproducts. This behavior
corresponds to results previously obtained with diffe-
rent ((trimethylsilyl)ethynyl)benzene derivatives.2,6-8
Despite intense efforts we were not able to purify either
compound by crystallization from different solvents
(cyclopentane, toluene, pentafluorobenzene, etc.). They
were isolated as highly viscous liquids which, owing to
the NMR spectroscopic characterization, consist of
essentially one main component but contain several
unknown impurities in concentrations of up to 15%.
Nevertheless, NMR spectroscopy allowed an unambig-
uous identification of these compounds and a clear
assignment of their molecular configuration. The inte-
1
compound 3. Tri-tert-butylgallium was detected in the H
NMR spectrum of the raw product (singlet at δ 1.16).
Compound 3 is only sparingly soluble in n-hexane and
precipitated directly from the reaction mixtures. It was
isolated in a relatively low yield of 25%. A similar procedure
was applied to the synthesis of the corresponding dialumi-
num compound 4, which was isolated in 61% yield. The 1H
NMR spectra of 3 and 4 exhibit the resonances of the
ethyldimethylamino ligands and the bridging dialkenyl
groups in the expected ratios. Resonances of tert-butyl
groups are missing. The chemical shifts of the spacer ligands
are similar to those of the addition products 1 and 2. The
vinylic hydrogen atoms appear at δ 7.83 (3) and 8.15 (4). The
3JH-Si coupling constants (18.7 and 23.3 Hz, respectively)
verify the cis arrangement of hydrogen and metal atoms at
the CdC double bonds similar to that in 1 and 2. Both
compounds (3 and 4) could not be obtained by the obvious
reaction of the trihydrido element compounds H3E r N-
Me2Et10 with the bis(alkyne). The gallium trihydride is
thermally relatively unstable and decomposed by the pre-
cipitation of elemental gallium in boiling n-hexane. The
trihydridoaluminum compound gave an insoluble precipi-
tate of unknown composition. Ether as a donor instead of the
amine did not initiate these condensation reactions.
1
gration ratio of the H NMR spectra gave the expected
1:1 ratio of SiMe3 and ER2 groups. The vinylic hydrogen
atoms (δ 7.98 and 7.68, respectively) are bonded to the
R-carbon atoms of the ethenyl groups neighboring the
central benzene rings. The aluminum and gallium atoms
adopted geminal positions with the trimethylsilyl groups.
This reaction pattern follows the charge separation in the
ethynyl groups, in which the electronegativity difference
between the sp-hybridized carbon atoms and silicon
causes a partial negative charge at the β-carbon atoms
and, hence, favors the attack of the positively charged
metal atoms at this position. Hydrogen and metal atoms
are in cis positions, as unambiguously derived from
(9) Uhl, W.; Cuypers, L.; Graupner, R.; Molter, J.; Vester, A.;
€
Neumuller, B. Z. Anorg. Allg. Chem. 2001, 627, 607.
€
(10) Dorn, R.; Muller, M.; Lorberth, J.; Zimmermann, G.; Protz-
€
3
the characteristic JH-Si coupling constants (20.3 and
mann, H.; Stolz, W.; Gobel, E. O. Mater. Sci. 1993, B17, 25.

1408 Organometallics, Vol. 29, No. 6, 2010
Uhl et al.
Figure 1. Molecular structure of 3. The thermal ellipsoids are
drawn at the 40% probability level. Hydrogen atoms and methyl
˚
groups of SiMe3 are omitted. Important bond lengths (A) and an-
gles (deg): Ga(1)-C(1)=2.023(3), Ga(1)-N(1)=2.152(4), C(1)-
C(2) = 1.349(4); C(1)-Ga(1)-C(1)0 = 112.35(7), Ga(1)-C(1)-
C(2)=109.1(2), C(1)-C(2)-C(3)=129.7(3), C(2)-C(3)-C(3)0=
119.2(2). C(1)0 is generated by -x þ y þ 1, -x þ 1, z and C(3)0 by
4
2
1
-x þ /3, -x þ y þ /3, -z þ /6.
The molecular structures of compounds 3 and 4 (Figures 1
and 2) comprise two metal atoms which are bridged by
three 1,2-dialkenylbenzene groups. The phenyl groups of the
spacer ligands adopt a paddle-wheel arrangement, and the
metal atoms are coordinated by terminal ethyldimethylamino
ligands. The overall structural motif is strongly reminiscent
˚
of a spinning top. The Ga-N (2.152 A) and Al-N dis-
˚
tances (2.067 A) resemble standard values of corresponding
donor-acceptor interactions.11 All bond lengths (M-N,
CdC, etc.) are in the normal range and do not require a
detailed discussion. Only the angles M-CdC (109° on
average) and CdC-C (about 130°) deviate from the ideal
values expected for sp2- carbon atoms, which may indicate
some steric stress in the cages. The particular and unprece-
dented arrangement of the bridging ligands forms a cavity in
Figure 2. Molecular structure of 4. The thermal ellipsoids are
drawn at the 40% probability level. The hydrogen atoms and
˚
methyl groups of SiMe3 are omitted. Important bond lengths (A)
€
(11) (a) Tian, X.; Frohlich, R.; Mitzel, N. W. Z. Anorg. Allg. Chem.
2005, 631, 1442. (b) Schumann, H.; Dechert, S.; Girgsdies, F.; Heymer, B.;
Hummert, M.; Hyeon, J.-Y.; Kaufmann, J.; Schulte, S.; Wernik, S.; Wassermann,
B. C. Z. Anorg. Allg. Chem. 2006, 632, 251. (c) Uhl, W.; Vogelpohl, A. Eur.
J. Inorg. Chem. 2009, 93. (d) Tian, X.; Woski, M.; Lustig, C.; Pape, T.;
Frohlich, R.; Le Van, D.; Bergander, K.; Mitzel, N. W. Organometallics
2005, 24, 82. (e) Tian, X.; Frohlich, R.; Pape, T.; Mitzel, N. W. Organome-
tallics 2005, 24, 5294. (f) Garcia, F.; Hopkins, A. D.; Kowenicki, R. A.;
McPartlin, M.; Silvia, J. S.; Rawson, J. M.; Rogers, M. C.; Wright, D. S.
Chem. Commun. 2007, 586. (g) Uhl, W.; Abel, T.; Kosters, J.; Rogel, F. Z.
Naturforsch. 2008, 63b, 117. (h) Uhl, W.; Abel, T.; Kosters, J.; Rezaeirad, B.
and angles (deg): Al(1)-C(11)=2.009(4), Al(1)-C(21)=2.017(4),
Al(1)-C(31)=2.013(4), Al(1)-N(1)=2.067(3), C(11)-C(12) =
1.348(5), C(21)-C(22) = 1.352(5), C(31)-C(32) = 1.348(5);
C(11)-Al(1)-C(21)=112.5(1), C(11)-Al(1)-C(31)=111.8(1),
C(21)-Al(1)-C(31) = 111.9(1), Al(1)-C(11)-C(12)=109.1(2),
Al(1)-C(21)-C(22)=108.5(2), Al(1)-C(31)-C(32) = 109.2(2),
C(11)-C(12)-C(34)0=129.9(3), C(21)-C(22)-C(23)=130.6(3),
C(31)-C(32)-C(33) = 129.7(3). C(34)0 is generated by -x, y,
€
€
€
€
Z. Anorg. Allg. Chem. 2009, 635, 1014. (i) Uhl, W.; Abel, T.; Hepp, A.;
Grimme, S.; Steinmetz, M. Eur. J. Inorg. Chem. 2008, 543. (j) Andrews,
P. C.; Calleja, S.; Maguire, M. Polyhedron 2006, 25, 1625.
1
-z þ /2.

Article
Organometallics, Vol. 29, No. 6, 2010 1409
the molecular center. The resulting intramolecular metal-
metal distances are similar in both compounds (Ga-Ga,
Table 1. Calculated Heats of Encapsulation (kcal/mol) Relative to
the Sum of the Free Guest and Host and Intramolecular Ga-Ga
˚
˚
˚
and Ga-Guest Distances (A)
5.86 A; Al-Al, 5.91 A). The longest distances between
carbon atoms of the cage are slightly shorter (e.g., C12-
guest
Erel
Ga Ga
3 3 3
Ga guest
3 3 3
0
0
˚
C12 or C32-C22 of compound 3 with 4.82 A). The result-
ing relatively small void in the molecular center should in
principle be suitable for the encapsulation of atoms or small
molecules (see below for further discussion).
Liþ
-70.93
-59.51
-63.16
-17.49
-83.67
-90.29
-18.90
5.949
5.384
5.871
5.989
5.793
4.077
4.769
6.016
4.975
5.114
6.040
5.454
2.974
2.809
2.935
2.994
2.896
2.039
2.385
2.318
2.487
Liþ on top
Naþ
Kþ
Agþ (ECP)
F-
Synthesis of the Solvent-Free Compound 5. The addition of
the amine initiated the condensation reactions according
to eq 2 with the formation of the novel cage compounds 3
and 4. However, the coordinative saturation of the metal
atoms may hinder the application of these compounds in
secondary reactions such as adduct formation, formation
of coordination polymers, electrochemistry, etc. Hence, it
was of particular interest to generate the solvent-free
compounds possessing tricoordinated, coordinatively un-
saturated metal atoms. The removal of the amino groups
succeeded only with the digallium compound 3 by heating
of solid samples to 80 °C under vacuum (10-3 Torr) (5; eq
2). The free amine was collected in a trap cooled with liquid
nitrogen and identified by 1H NMR spectroscopy. The
stronger Lewis acidity of aluminum compounds prevented
a similar reaction of 4. Heating under vacuum to tempera-
tures above 80 °C gave decomposition by formation of a
black solid material. The chemical shifts in the NMR
spectra of 5 are almost unchanged compared to those of
3; only the resonances of the ethyldimethylamino ligands
are missing. Despite intense efforts, we were not able to
generate single crystals of 5 for a crystal structure deter-
mination. However, spectroscopic and analytical charac-
terization, including the detection of the molecular mass in
the mass spectrum, gave clear evidence that the cage
remained intact. Further evidence came from a DOSY
NMR experiment. The ratios between the diffusion coeffi-
cients of 3 and 5 and those of the solvent (benzene) or
tetramethylsilane are almost identical, which indicates
similar Stokes radii and molecular forms.12
Cl-
Cl- (terminal) -65.04
Br-
10.42
-25.23
54.42
CN-
OCN-
N2
1.998 (Ga-C); 1.967 (Ga-N)
1.855 (Ga-N); 1.861 (O-Ga)
2.180
42.23
with the increasing size of the cation. Silver ions are the
most tightly bound (-83.7 kcal/mol). Interestingly, the
terminal coordination of Liþ to a single Ga atom from
outside of the cage is also quite exothermic in the gas phase
(-59.5 kcal/mol). A size dependence similar to that of the
alkali-metal cations was found for the halide anions, for which
we calculated exothermic encapsulation energies for fluoride
(-90.3 kcal/mol) and chloride (-18.9 kcal/mol) but an endo-
thermic value for bromide (10.4 kcal/mol). The terminal
coordination of a Cl- ion to one of the Ga atoms is calculated
to be much more favorable (-65.0 kcal/mol), thus making an
encapsulation quite unlikely. Surprisingly, the intramolecular
distances between the Ga atoms vary considerably in depen-
dence of the charge and size of the guest ion. The separation is
˚
˚
enlarged from 5.6 A in the emptyþcage to 5.9-6.0 A in the case
þ
þ
˚
of encapsulated Li , Na , and K ions but is reduced to 4.1 A
-
-
-
˚
for F and 4.8 A for Cl . Larger anions such as CN give only
a weak exothermic reaction (-25.2 kcal/mol), while the com-
plexation of OCN- (þ54.4 kcal/mol) or molecular nitrogen
(þ42.2 kcal/mol) is calculated to be strongly endothermic, which
is in accordance with the relatively small void in the center of the
molecule. NBO-charge calculations show that the negative
charge of encapsulated anions is mainly localized at the Ga
atoms (ca. 0.4-0.5 e), whereas the positive charge of cations is
partially (ca. 0.4 e) delocalized over the carbon atoms of the
cage.
Geometry optimization by quantum-chemical cal-
culations including dispersion correction (B97-D/def2-
TZVPP)13,14 revealed structural parameters of 5 that are
closely related to those of 3 and 4. The gallium atoms
˚
deviate from a planar environment and are 0.289 A above
the triangle formed by the three adjacent vinylic carbon
Currently it is most certainly the aesthetic appeal of these
interesting compounds that dominates. Future investiga-
tions will show whether the presented method is applicable
for the generation of larger cavities and whether the unique
bifunctionality of these compounds may be utilized in the
preparation of supramolecular structures.
atoms. The C-Ga-C angles are 117.9° on average. The
˚
Ga Ga distance in the (calculated) cage of 5 (5.60 A) is
3 3 3
˚
shortened by 0.26 A compared with the distance detected
by X-ray diffraction for the bis(amino) adduct 3. In order
to evaluate the capability of these cages to encapsulate
particular guests, DFT calculations on host-guest com-
plexes were performed (Table 1). The inclusion of alkali-
metal ions in the center of the host (D3h symmetry) is
predicted to be exothermic (-70.9 kcal/mol for Liþ, -17.5
kcal/mol for Kþ in comparison to the sum of energies of
the free host and guest). The values decrease systematically
Thermolysis of Compound 1. The amine-free aluminum
compound analogous to 5 was not accessible by heating of
the amino adduct 4 under vacuum. We hoped to synthesize
this compound through another route by the thermally
initiated elimination of tri-tert-butylaluminum from the
addition product 1. A similar reaction with the diethylgal-
lium compound Et2GaC(SiMe3)dC(H)C6H5 gave the cor-
responding trivinylgallium compound Ga[C(SiMe3)dC(H)-
C6H5]3 in an almost quantitative yield.8 We generated the
addition product 1 in n-hexane as described above and
heated the solution under reflux for 5 days (eq 3). As ex-
pected, tri-tert-butylaluminum was formed in a slow reac-
tion. Concentration of the mixture and cooling to þ4 °C gave
colorless crystals of the product 6. NMR spectroscopy
clearly revealed that a new compound has been formed
which exhibited the resonances of four different aromatic
(12) (a) Cabrita, E. J.; Berger, S. Magn. Reson. Chem. 2001, 39, S142.
(b) Wu, D.; Chen, A.; Johnson, C. S., Jr. J. Magn. Reson. A 1995, 115, 260.
(c) Cohen, Y.; Avram, L.; Frish, L. Angew. Chem. 2005, 117, 524; Angew.
Chem., Int. Ed. 2005, 44, 520.
(13) (a) Grimme, S. J. Comput. Chem. 2006, 27, 1787. (b) Weigend, F.;
Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297.
€
€
€
(14) (a) Ahlrichs, R.; Bar, M.; Har, M.; Horn, H.; Kolmel, C. Chem.
Phys. Lett. 1989, 162, 165–169. (b) Ahlrichs, R. et al. TURBOMOLE,
€
version 6.0; Universitat Karlsruhe, Karlsruhe, Germany, 2009; http://www.
turbomole.com.

1410 Organometallics, Vol. 29, No. 6, 2010
Uhl et al.
hydrogen atoms and one vinylic hydrogen atom in addition
to a tert-butyl and trimethylsilyl group in an equimolar ratio.
Figure 3. Molecular structure of 6. The thermal ellipsoids are
drawn at the 40% probability level. Hydrogen atoms and methyl
˚
groups of SiMe3 are omitted. Important bond lengths (A) andangles
(deg): Al(1)-C(1) = 1.976(1), Al(1)-C(4) = 2.120(1), Al(1)-
C(4)0 = 2.136(1), C(1)-C(2) = 1.353(2), C(3)-C(4) = 1.429(2);
Al(1)-C(4)-Al(1)0 = 76.72(4), C(4)-Al(1)-C(4)0 = 103.28(4),
Al(1)-C(1)-C(2) = 106.25(9), C(1)-C(2)-C(3) = 122.2(1),
C(2)-C(3)-C(4) = 118.4(1), C(3)-C(4)-Al(1) = 100.84(8),
C(3)-C(4)-Al(1)0 =122.81(9). Al(1)0 and C(4)0 are generated by
-x, -y þ 1, -z.
β-elimination of alkylaluminum and formation of an al-
kene.16 This part of the suggested mechanism seems to be
reasonable but relatively speculative, because we were not
able to identify the ethyne by spectroscopic methods. We did
not observe a similar reaction in our previous experiments.
The surprising C-C bond activation will stimulate further
investigations.
The constitution of 6 was clarified by crystal structure
determination (Figure 3). In accordance with the NMR
spectroscopic characterization it has only one intact vinyl
group with a terminal trimethylsilyl substituent in a mono-
meric formula unit. The aluminum atom is bonded to a
terminal tert-butyl group. It adopts a bridging position
between the β-carbon atom of the CdC double bond and a
carbanionic carbon atom of the benzene ring. The latter
atom is in an R-position to the carbon atom bearing the
intact vinyl group and may represent the position of the
missing vinyl substituent. Dimeric formula units are formed
via two 3z-2e Al-C-Al bonds in which the negatively
charged carbon atoms of the benzene rings are involved.
This situation is similar to that in triphenylaluminum or
related phenylaluminum compounds.15 The Al-C distances
Experimental Section
All procedures were carried out under purified argon. Cyclo-
pentane and n-hexane were dried over LiAlH4 and 1,2-diflu-
orobenzene over molecular sieves. tBu2AlH,9 tBu2GaH,9 and
1,2-bis(trimethylsilylethynyl)benzene17 were obtained accord-
ing to literature procedures. The assignment of the NMR
spectra is based on HMBC, HSQC, ROESY, and DEPT135
data.
˚
in the Al2C2 ring (2.120(1) and 2.136(1) A) correspond to the
values observed for those derivatives. All other bond lengths
are in the expected ranges and do not require a detailed
discussion. An overall pentacyclic molecule with an unpre-
cedented molecular structure resulted. A mechanism for the
formation of compound 6 may comprise the release of tri-
tert-butylaluminum to give a heterocyclic compound in
which both vinyl substituents are bridged by a single alumi-
num atom. We observed similar reactions with the formation
of cyclophane-type molecules in previous investigations.7 A
heterocycle similar to that postulated here which has two
CdC double bonds bridged by a chlorogallium group has
been obtained by the reaction of 1,2,4,5-tetrakis(trimethyl-
silylethynyl)benzene with HGaCl2.6b,c The unexpected clea-
vage of the C-C bond may proceed via β-elimination with
the formation of the phenylaluminum group and the release
of (trimethylsilyl)ethyne. Such a decarbalumination reaction
has been postulated in the literature in conjunction with the
Reaction of 1,2-(Me3SiCtC)2C6H4 with H-Al(CMe3)2;
Synthesis of 1. A solution of H-Al(CMe3)2 (0.241 g, 1.70 mmol)
in 10 mL of n-hexane was treated with a solution of
1,2-(Me3SiCtC)2C6H4 (0.229 g, 0.85 mmol) in 10 mL of
n-hexane. The mixture was stirred at room temperature for 16 h.
Evaporation of the solvent gave a reddish highly viscous liquid,
which could not be purified by crystallization from different
noncoordinating solvents. Its purity was sufficient for a spectro-
scopic characterization. 1H NMR (C6D6, 400 MHz): δ 7.98
3
(2 H, s, JH-Si = 20.3 Hz, vinylic H), 7.33 (2 H, m, 3,6-H of
aromatic ring), 7.08 (2 H, m, 4,5-H of aromatic ring), 1.20 (36 H,
s, AlCMe3), 0.12 (18 H, s, SiMe3). 13C NMR (C6D6, 100 MHz):
δ 159.5 (CdC-Al), 153.9 (CdC-Al), 141.3 (1,2-C of aromatic
ring), 128.7 (3,6-C of aromatic ring), 127.6 (4,5-C of aromatic
ring), 29.9 (Al-CMe3), 19.2 (Al-C), 1.6 (SiMe3). 29Si NMR
(C6D6, 79.5 MHz): δ -13.4.
(15) (a) Barber, M.; Liptak, D.; Oliver, J. P. Organometallics 1982, 1,
1307. (b) Malone, J. F.; McDonald, W. S. J. Chem. Soc., Dalton Trans.
1972, 2649. (c) Brauer, D. J.; Kr€uger, C. Z. Naturforsch. 1979, 34b, 1293.
(16) Eisch, J. J.; Burlinson, N. E.; Boleslawski, M. J. Organomet.
Chem. 1976, 111, 137.
(17) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N.
Synthesis 1980, 627.

Article
Organometallics, Vol. 29, No. 6, 2010 1411
Table 2. Crystal Data, Data Collection Parameters, and Structure Refinement Details for Compounds 3, 4, and 6
3 C6H14
3
4 4C6H4F2
3
6
formula
cryst syst
space group
Z
C
62H94Ga2N2Si6
rhombohedral
C80H110Al2F8N2Si6
monoclinic
C2/c
4
153(2)
1.154
29.474(6)
15.050(3)
19.347(4)
90
98.46(3)
90
8489(3)
0.177
C30H46Al2Si2
triclinic
P1
1
153(2)
1.094
9.1789(7)
9.3838(7)
10.810(1)
95.770(1)
99.954(1)
118.907(1)
784.3(1)
0.185
R3c
6
153(2)
1.154
14.9203(3)
14.9203(3)
52.633(3)
90
90
120
10 147.1(6)
0.938
temp, K
Dcalcd, g/cm3
˚
a, A
˚
b, A
˚
c, A
R, deg
β, deg
γ, deg
V, 10-30 m3
μ, mm-1
cryst dimens, mm
radiation
θ range, deg
index ranges
0.05 ꢀ 0.03 ꢀ 0.03
0.40 ꢀ 0.23 ꢀ 0.03
Mo KR; graphite monochromator
1.80-29.33
-40 e h e þ40
-20 e k e þ20
0 e l e þ26
11 518 (Rint = 0.0604)
412
0.13 ꢀ 0.12 ꢀ 0.03
1.76-25.04
1.96-31.96
-17 e h e þ17
-17 e k e þ17
-62 e l e þ62
2007 (Rint = 0.0778)
119
-13 e h e þ13
-13 e k e þ13
-15 e l e þ15
4937 (Rint = 0.0213)
160
no. of unique rflns
no. of params
R1 (rflns I > 2σ(I))
wR2 (all data)
0.0389 (1606)
0.1125
þ1.063/-1.012
0.0789 (5205)
0.2175
þ1.084/-0.766
0.0402 (3862)
0.1110
þ0.391/-0.222
max/min residual electron density, 1030 e/m3
Reaction of 1,2-(Me3SiCtC)2C6H4 with H-Ga(CMe3)2:
Synthesis of 2. A solution of 1,2-(Me3SiCtC)2C6H4 (0.290 g,
1.07 mmol) in 10 mL of n-hexane was added to a solution of
H-Ga(CMe3)2 (0.397 g, 2.15 mmol) in 50 mL of n-hexane at
room temperature. The mixture was heated under reflux for
16 h. Cooling to room temperature and evaporation of the
solvent gave a highly viscous liquid, which could not be purified
by crystallization from different noncoordinating solvents. Its
m, 824 s, 793 vw, 766 m, 748 s, F(CH3Si); 719 w (paraffin); 681 m
νas(SiC); 644 m νs(SiC); 602 vw, 557 m, 484 w, 463 vw, 422 m
ν(GaN), ν(GaC), δ(CC). MS (EI, 20 eV, 160 °C; m/z (%)): 954
(1), 956 (2), 957 (1), 958 (1) [Mþ - 2NMe2Et], 682 (3), 684 (5),
685 (6), 686 (3) [Mþ - 2NMe2Et - C6H4{C(H)dC(SiMe3)}2],
341 (100), 343 (74) [Ga{(Me3Si)CdC(H)}2C6H4].
Reaction of 1,2-(Me3SiCtC)2C6H4 with H-Al(CMe3)2 and
NMe2Et: Synthesis of 4. A solution of 1,2-(Me3SiCtC)2C6H4
(0.311 g, 1.15 mmol) in 20 mL of n-hexane was added to a
solution of H-Al(CMe3)2 (0.327 g, 2.30 mmol) in 50 mL of
n-hexane. The mixture was treated with 0.083 mL of NMe2Et
(0.056 g, 0.77 mmol) at room temperature and stirred for 24 h.
After about 2 h the product started to precipitate as a colorless
solid. The suspension was concentrated. The solvent was re-
moved by decantation, and the solid was washed twice with
1
purity was sufficient for a spectroscopic characterization. H
NMR (C6D6, 400 MHz): δ 7.68 (2 H, s, 3JH-Si = 19.4 Hz, vinylic
H), 7.36 (2 H, m, 3,6-H of aromatic ring), 7.09 (2 H, m, 4,5-H of
aromatic ring), 1.27 (36 H, s, GaCMe3), 0.09 (18 H, s, SiMe3).
13C NMR (C6D6, 100 MHz): δ 163.9 (CdC-Ga), 150.1
(CdC-Ga), 140.9 (1,2-C of aromatic ring), 129.1 (3,6-C of
aromatic ring), 127.3 (4,5-C of aromatic ring), 30.3 (Ga-CMe3),
29.4 (Ga-C), 1.7 (SiMe3). 29Si NMR (C6D6, 79.5 MHz): δ
-12.5.
small portions of n-hexane. Yield of 4 0.5(n-hexane): 0.246 g
3
(61%). Mp (argon, sealed capillary): 240 °C dec. Anal. Calcd for
Reaction of 1,2-(Me3SiCtC)2C6H4 with H-Ga(CMe3)2 and
NMe2Et: Synthesis of 3. A solution of 1,2-(Me3SiCtC)2C6H4
(0.312 g, 1.16 mmol) and H-Ga(CMe3)2 (0.426 g, 2.31 mmol) in
70 mL of n-hexane was treated with 0.083 mL of NMe2Et (0.056 g,
0.77 mmol) at room temperature. The yellow solution was
heated under reflux for 16 h. The color of the mixture changed to
brown, and a colorless solid precipitated. After the mixture was
cooled to room temperature, the solvent was decanted, and the
solid was washed several times with small portions of n-hexane.
C56H94Al2N2Si6 0.5C6H14 (1017.9 þ 43.1): C, 66.8; H, 9.6; N,
3
2.6. Found: C, 67.1; H, 9.3; N, 2.3. 1H NMR (C6D6, 400 MHz): δ
8.15 (6 H, s, 3JH-Si = 23.3 Hz, vinylic H), 7.27 (6 H, m, 3,6-H of
aromatic ring), 7.16 (6 H, m, 4,5-H of aromatic ring), 3.20 (4 H,
q, 3JH-H = 7.1 Hz, NCH2), 2.35 and 2.36 (each 6 H, s, NCH3),
1.23 and 0.89 (n-hexane), 0.65 (6 H, t, 3JH-H = 7.1 Hz, NCH2-
CH3), 0.19 (54 H, s, SiMe3). 13C NMR (C6D6, 100 MHz): δ 164.2
(CdC-Ga), 155.6 (CdC-Ga), 144.2 (1,2-C of aromatic ring),
128.1 (3,6-C of aromatic ring), 126.6 (4,5-C of aromatic ring), 52.8
(NCH2), 43.8 (NCH3), 32.0, 23.1, and 14.3 (n-hexane), 14.3
(NCH2CH3), 4.1 (SiMe3). 29Si NMR (C6D6, 79.5 MHz): δ
-10.4. IR (CsI plates, paraffin, cm-1): 1913 vw, 1692 w, 1643 m,
1551 s, 1524 s, phenyl, ν(CdC); 1449 vs, 1375 s (paraffin); 1305 vw,
1254 m, 1242 s, δ(CH3); 1198 vw, 1171 w, 1084 s, 1022 w, 1003 w,
ν(CN), ν(CC); 957 vw, 918 s, 880 m, 864 m, 823 m, 770 m, 748 s,
F(CH3Si); 721 w (paraffin); 679 m, νas(SiC); 644 m, 625 w, νs(SiC);
573 w, 557 vw, 532 vw, 490 s, 446 vw, 430 s, ν(AlN), ν(AlC),
δ(CC). MS (EI, 20 eV, 160 °C; m/z (%)): 870 (1.5), 871 (1.2), 872
(1) [Mþ - 2NMe2Et], 855 (1), 856 (1), 857 (0.7) [Mþ - 2NMe2Et
- Me], 797 (2), 798 (2), 799 (1) [Mþ - 2NMe2Et - SiMe3].
Synthesis of the Amine-Free Compound 5. The solid amino
adduct 3 (0.303 g, 0.28 mmol) was heated under vacuum (10-3
Torr) to 80 °C for 16 h. The amine was removed quantitatively.
It was collected in a trap cooled by liquid nitrogen. The amine-
free compound 5 remained as a colorless solid in a quantitative
yield and in very high purity. Yield: 0.250 g (95%). Mp (argon,
Yield of 3 0.5(n-hexane): 0.110 g (25%); 3 can be crystallized
3
from cyclopentane. Mp (argon, sealed capillary): 155 °C dec.
Anal. Calcd for C56H94Ga2N2Si6 0.5C6H14 (1103.3 þ 43.1): C,
3
1
61.8; H, 8.9; N, 2.4. Found: C, 61.9; H, 8.8; N, 2.4. H NMR
(C6D6, 400 MHz): δ 7.83 (6 H, s, 3JH-Si = 18.7 Hz, vinylic H),
7.34 (6 H, m, 3,6-H of aromatic ring), 7.12 (6 H, m, 4,5-H of
3
aromatic ring), 2.76 (4 H, q, JH-H = 7.2 Hz, NCH2), 2.24
(12 H, s, NCH3), 1.23 and 0.89 (n-hexane), 0.79 (6 H, t, 3JH-H
=
7.2 Hz, NCH2CH3), 0.18 (54 H, s, SiMe3). 13C NMR (C6D6, 100
MHz): δ 159.9 (CdC-Ga), 156.6 (CdC-Ga), 142.5 (1,2-C of
aromatic ring), 128.0 (3,6-C of aromatic ring), 126.9 (4,5-C of
aromatic ring), 53.9 (NCH2), 45.0 (NCH3), 32.0, 23.1, and 14.4
(n-hexane), 8.3 (NCH2CH3), 3.4 (SiMe3). 29Si NMR (C6D6, 79.5
MHz): δ -10.1. IR (CsI plates, paraffin, cm-1): 1653 w, 1558 w,
1537 s, 1529 s, phenyl, ν(CdC); 1454 vs, 1377 vs (paraffin); 1308
vw, 1254 m, 1242 s, δ(CH3); 1202 vw, 1175 w, 1096 w, 1086 m,
1026 m, 1011 m, ν(CN), ν(CC); 945 vw, 916 s, 899 w, 878 m, 864

1412 Organometallics, Vol. 29, No. 6, 2010
Uhl et al.
sealed capillary): 181 °C dec. Anal. Calcd for C48H72Ga2Si6
(957.1): C, 60.2; H, 7.6. Anal. Found: C, 59.5; H, 7.5. 1H NMR
(C6D6, 400 MHz): δ 7.83 (6 H, s, 3JH-Si = 17.7 Hz, vinylic H),
7.37 (6 H, m, 3,6-H of aromatic ring), 7.05 (6 H, m, 4,5-H of
aromatic ring), 0.20 (54 H, s, SiMe3). 13C NMR (C6D6, 100
MHz): δ 160.3 (CdC-Ga), 155.7 (CdC-Ga), 140.1 (1,2-C of
aromatic ring), 127.4 (4,5-C of aromatic ring), 127.3 (3,6-C of
aromatic ring), 2.4 (SiMe3). 29Si NMR (C6D6, 79.5 MHz): δ
-10.0. IR (CsI plates, paraffin, cm-1): 1921 vw, 1595 vw, 1568
vw, 1537 s, phenyl, ν(CdC); 1458 vs, 1375 vs (paraffin); 1304 vw,
1273 w, 1259 s, 1244 vs, δ(CH3); 1177 m, 1153 w, 1088 m, 1038 w,
1024 w, 1008 w, ν(CC); 974 vw, 948 vw, 916 s, 897 s, 872 vs, 839
vs, 795 sh, 741 vs, F(CH3Si); 721 s (paraffin); 683 m, νas(SiC); 644
m, νs(SiC); 604 vw, 561 w, 529 w, 476 w, 413 s, ν(GaC), δ(CC).
MS (EI, 20 eV, 160 °C; m/z (%)): 956 (3), 957 (5), 958 (4), 959 (3)
[Mþ þ H], 681 (3), 683 (8), 684 (5), 685 (8) [Mþ - C6H4{C-
(H)dC(SiMe3)}2 - H], 341 (100), 343 (82) [GaC6H4{C(H)d
C(SiMe3)}2þ].
(C1 of the aromatic ring attached to Al), 29.2 (CMe3), 17.7 (Al-
CMe3),1.0 (SiMe3). 29Si NMR (C6D6, 79.5 MHz): δ -6.6. IR
(CsI plates, paraffin, cm-1): 1923 w, 1869 w, 1593 w, 1537 m,
1514 w, phenyl, ν(CdC); 1456 vs, 1375 vs (paraffin); 1307 w,
1246 vs, δ(CH3); 1171 m, 1159 m, 1107 w, 1088 w, 1038 m, 1001
m, ν(CC); 934 m, 918 m, 839 s, 810 sh, 752 s, F(CH3Si); 721 s
(paraffin); 687 w, νas(SiC); 645 w, 629 w, νs(SiC); 592 m, 529 w,
455 m, 428 m, ν(AlC), δ(CC). MS (EI, 20 eV, 100 °C; m/z (%)):
459 (37) [Mþ - CMe3], 258 (38) [1/2Mþ], 201 (100) [1/2Mþ
-
CMe3].
Crystal Structure Determinations of Compounds 3, 4, and 6.
Single crystals were obtained by crystallization from a dilute
solution in n-hexane (3, þ20/-45 °C; 6, þ20/þ4 °C) or from 1,2-
difluorobenzene (þ20/-25 °C; 4). The crystallographic data
were collected with Bruker APEX (3 and 6) and Stoe IPDS
diffractometers (4). The structures were solved by direct meth-
ods and refined with the program SHELXL-9718 by a full-
matrix least-squares method based on F2. Crystal data, data
collection parameters, and structure refinement details are given
in Table 2. The Ga and N atoms of compound 3 are located on
crystallographic 3-fold rotation axes. Hence, the ethyldimethyl-
amino ligands are statistically disordered; the methyl and
ethyl groups were refined on split positions. The crystals enclose
one disordered n-hexane molecule per formula unit of 3. Com-
pound 4 crystallized as very thin platelets. The molecules are
located on crystallographic 2-fold rotation axes. The crystals
enclose four molecules of 1,2-difluorobenzene per formula unit
of 4. The dimeric molecules of 6 are located on crystallographic
centers of symmetry. Further details of the crystal structure
determinations are available from the Cambridge Crystallo-
graphic Data Center on quoting the depository number CCDC-
Thermolysis of Compound 1: Synthesis of 6. A solution of 1,2-
bis((trimethylsilyl)ethynyl)benzene (0.194 g, 0.72 mmol) in
10 mL of n-hexane was added to a solution of di-tert-butylalu-
minum hydride (0.204 g, 1.44 mmol) in 10 mL of n-hexane. As
shown by 1H NMR spectroscopy, compound 1 was formed as
an intermediate. The mixture was heated under reflux for 5 days.
Its color changed to red. The solution was concentrated at room
temperature and cooled to þ4 °C to isolate a colorless solid of 6.
Yield: 0.105 g (56%). Mp (argon, sealed capillary): 134 °C dec.
Anal. Calcd for C30H46Al2Si2 (516.8): C, 69.7; H, 9.0; Al, 10.4.
Found: C, 69.7; H, 9.0; Al, 10.4. 1H NMR (C6D6, 400 MHz): δ
3
8.31 (2 H, d, JH-H = 7.0 Hz, H-C6 of the aromatic ring,
Al-C-CH), 8.11 (2 H, s, 3JH-Si = 9.6 Hz, vinylic H), 7.17 (2 H, m,
H-C4 of the aromatic ring), 7.11 (2 H, m, H-C3 of the
aromatic ring), 7.10 (2 H, m, H-C5 of the aromatic ring),
0.69 (18 H, s, CMe3), 0.40 (18 H, s, SiMe3). 13C NMR (C6D6,
100 MHz): δ 167.3 (CdC-Al), 164.2 (C2 of the aromatic ring
attached to the vinylic carbon atom), 160.0 (CdC-Al), 148.5
(C6 of the aromatic ring), 136.4 (C4 of the aromatic ring), 127.5
(C3 of the aromatic ring), 127.2 (C5 of the aromatic ring), 124.5
756900 (3 C6H14), CCDC-756901 (4 4C6H4F2), and CCDC-
756902 (6).
3
3
Acknowledgment. We are grateful to the Deutsche
Forschungsgemeinschaft for generous financial support.
Supporting Information Available: CIF files giving the crystal
data for compounds 3 C6H14, 4 4C6H4F2, and 6 and tables
3
3
(18) SHELXTL-Plus, REL. 4.1; Siemens Analytical X-Ray Instru-
ments, Inc., Madison, WI, 1990. Sheldrick, G. M. SHELXL-97, Program
giving optimized geometries of compound 5 and the complexes
given in Table 1. This material is available free of charge via the
Internet at http://pubs.acs.org.
€
€
€
for the Refinement of Structures; Universitat Gottingen, Gottingen, Ger-
many, 1997.