Two Different Structural Motifs Observed for Dimeric Dialkylaluminum and Dialkylgallium Alkynides
775 sh, 760 vs νC3C; 721 w, 685 vs, 669 w, 648 w, 629 w, 610 w (phenyl);
584 m, 565 vs, 540 vs, 442 vs, 409 s νAlC; 365 s, 345 vs, 303 w, 278 w δC3C.
small. Further stabilization results from interactions of the
terminal Al-C bonds with the σ*-orbitals of the C-C bonds
of the alkynido groups and from the back-bonding of π-
electron density into the σ*-orbitals of the terminal Al-C
bonds. Three-center bonds do not occur in the second form
(type A). The bonding is essentially determined by localized
Al-C σ-bonds formed by sp3-hybrid orbitals of aluminum
and sp-hybrid orbitals at the terminal carbon atoms of the
alkynido groups. The particular structural motif of that
type with the apparent side-on coordination of aluminum
atoms to the triple bonds is the result of interactions involv-
ing empty p-orbitals localized at the metal atoms. These
metal atom orbitals interact with the molecular orbitals for-
ming the Al-alkynido bonds and with one of the occupied
π-orbitals of the CϵC triple bond, both from the other
unit.
Synthesis of [Me2Ga-CϵC-C6H5]2 (6). Me2GaH (0.43 g;
4.27 mmol) was dissolved in 25 ml of n-hexane and treated with
0.44 g (4.27 mmol) of H-CϵC-C6H5 at room temperature. The
mixture was stirred at room temperature over night. The colorless
mixture became yellow. The solvent was removed in vacuum, and
the residue was recrystallized from toluene (20/ϩ8 °C). Yield:
0.42 g (49 %, colorless crystals). Mp. (argon, sealed capillary):
98 °C.
1H NMR (C6D6, 300 MHz): δ ϭ 7.30 (2 H, pseudo-d, ortho-H of phenyl),
6.77 (3 H, m, meta- and para-H of phenyl), 0.21 (6 H, s, Me). 13C NMR
(C6D6, 75.5 MHz): δ ϭ 133.4, 132.3, 130.4, and 128.7 (phenyl), 121.4 and
98.0 (CϵC), Ϫ2.3 (Me). IR (CsBr plates, paraffin, cmϪ1): 2100 m νCϵC;
1572 vw, 1487 m (phenyl); 1461 vs, 1377 s (paraffin); 1307 w, 1283 w δCH3;
1216 m, 1200 m, 1170 w, 1070 w, 1025 w, 1014 w, 928 w, 912 w νCC; 756 s,
730 s, 690 s, 648 s, 609 w (phenyl); 589 m, 540 m, 463 vw νGaC. Molar mass
(in benzene, cryoscopy): Found 375; calcd. 401.8.
Synthesis of [(Me3C)2Ga-CϵC-C6H5]2 (7). (i) Starting with
(Me3C)2GaH: (Me3C)2GaH (0.59 g; 3.19 mmol) was dissolved in
25 ml of n-hexane and treated with 0.35 ml (0.33 g, 3.19 mmol) H-
CϵC-C6H5 at room temperature. The mixture was stirred at room
temperature over night. The color of the mixture originally color-
less turned yellow-orange. The solvent was removed in vacuum,
and the solid residue was recrystallized from n-hexane (20/Ϫ40 °C).
Di(tert-butyl)gallium hydride exists as an equilibrium mixture of
three compounds in solution. This may be the reason why this pro-
cedure does not give sufficiently reproducible yields. (ii) Starting
Experimental Section
All procedures were carried out under dried argon. n-Hexane and
n-pentane were dried over LiAlH4, toluene over Na/benzophenone.
The usually oligomeric starting compounds (Me3C)2AlCl [18],
Me2AlH [19], (Me3C)2AlH [7], Me2GaH [8, 20], (Me3C)2GaH [7],
and (Me3C)2GaCl [21] were obtained according to literature pro-
cedures. Commercially available H-CϵC-C6H5 (Aldrich) was em-
ployed without further purification.
with (Me3C)2GaCl:
A
solution of (Me3C)2GaCl (0.49 g,
2.22 mmol) in 25 ml of n-hexane was added dropwise to a suspen-
sion of LiCϵC-C6H5 (0.24 g, 2.22 mmol) in n-hexane (the lithium
compound was obtained by the treatment of H-CϵC-C6H5 with n-
butyllithium as described before). The reaction mixture was stirred
at room temperature for 24 h and filtered. After concentration of
the filtrate, the product was isolated as colorless crystals upon cool-
ing to Ϫ30 °C. Yield: 0.36 g (57 %). Dec. p. (argon, sealed capil-
lary): 163 °C.
Synthesis of [Me2Al-CϵC-C6H5]2 (4). A solution of dimethylalumi-
num hydride (0.6 ml; 0.53 g; 9.14 mmol) in 15 ml of n-pentane was
slowly added to phenylethyne (0.93 g, 9.12 mmol) dissolved in
15 ml of n-pentane at room temperature. Gas evolved and the solu-
tion became yellow. The mixture was further stirred for 2 h. The
solvent was removed in vacuum. The residue was recrystallized
from n-pentane (20/Ϫ15 °C) to yield colorless crystals of 4. Yield:
1.24 g (86 %). See reference [1] for characterization.
1H NMR (C6D6, 300 MHz): δ ϭ 7.55 (2 H, pseudo-d, ortho-H of phenyl),
6.87 (3 H, m, meta- and para-H of phenyl), 1.51 (6 H, s, Me). 13C NMR
(C6D6, 125.8 MHz): δ ϭ 134.2, 130.7, and 128.9 (phenyl, one resonance not
detected), 121.5 and 92.8 (CϵC), 32.2 (Me), 28.3 (GaC). IR (CsBr plates,
paraffin, cmϪ1): 2052 s νCϵC; 1592 w, 1573 w (phenyl); 1464 vs, 1377 s
(paraffin); 1360 m, 1307 vw, 1288 vw, 1280 vw δCH3; 1198 w, 1163 m,
1094 w, 1068 w, 1024 m, 1010 m, 938 w, 921 vw, 911 vw, 897 vw νCC; 814 m
νC3C; 756 s, 723 w, 689 m, 647 vw, 609 vw (phenyl); 559 m, 532 m, 514 m,
463 w, 391 m νGaC. Molar mass (in benzene, cryoscopy): Found 610; calcd.
570.2.
Synthesis of [(Me3C)2Al-CϵC-C6H5]2 (5). (i) Starting with di(tert-
butyl)aluminum chloride: n-Butyllithium (5.0 ml, 8.05 mmol, 1.6 M
solution in n-hexane) was added to a cooled (0 °C) solution of
phenylethyne (0.82 g, 8.05 mmol) in 20 ml of n-pentane. A colorless
solid (LiCϵC-C6H5) precipitated. After further stirring for 2 h at
room temperature and cooling to 0 °C, a solution of di(tert-butyl)-
aluminum chloride (1.42 g, 8.05 mmol) in 20 ml of n-pentane was
added. The mixture was warmed to room temperature and further
stirred for an additional 2 h. After filtration, all the volatile compo-
nents were removed in vacuum, and the residue was recrystallized
from n-pentane. Yield: 1.15 g (59 %). (ii) Starting with di(tert-bu-
tyl)aluminum hydride: A solution of di(tert-butyl)aluminum hydride
(1.29 g; 9.08 mmol) in 15 ml of n-pentane was slowly added to
phenylethyne (0.93 g; 9.08 mmol) dissolved in 15 ml of the same
solvent at room temperature. Gas evolution was observed, and the
solution turned yellow. The mixture was stirred for another 2 h. All
volatile components were removed in vacuum. The solid residue
was recrystallized from n-pentane. Yield: 1.71 g (78 %); yellowish
crystals. Mp (argon, sealed capillary): 185 °C.
Crystal Structure Determinations of Compounds 4 to 7
Single crystals of compounds 4, 5, and 7 were obtained on cooling
of solutions in n-hexane to ϩ8 or Ϫ15 °C. Crystals of 6 grew from
a solution in toluene at ϩ8 °C. The crystallographic data were col-
lected with a STOE IPDS diffractometer. The structures were
solved by direct methods and refined with the program SHELXL-
97 [22] by a full-matrix least-squares method based on F2. Crystal
data, data collection parameters, and structure refinement details
are given in Table 2. Four dimers, which were located on crystallo-
graphic centers of symmetry, were observed in the unit cell of 4.
¯
Compound 5 crystallizes in the triclinic space group P1 with six
1H NMR (C6D6, 300 MHz): δ ϭ 7.50 Ϫ 7.55 and 6.78 Ϫ 6.92 (5 H, m,
C6H5), 1.43 (18 H, s, CMe3). 13C NMR (C6D6, 125.8 MHz): δ ϭ 137.5,
134.4, 131.8 and 129.0 (C6H5), 119.9 and 91.5 (CϵC), 32.0 (Me), 19.1 (AlC
of CMe3). IR (CsBr plates, paraffin, cmϪ1): 2060 m νCϵC; 1595 vw, 1572 vw
(phenyl); 1464 vs, 1377 vs (paraffin); 1358 sh, 1310 w, 1283 w δCH3; 1206 w,
1175 w, 1094 vw, 1067 w, 1024 m, 1003 m, 936 m, 924 w νCC; 810 s (phenyl);
dimeric molecules in the unit cell, two of which are on crystallo-
graphic inversion centers. Some tert-butyl groups are disordered,
their methyl groups were refined on optimized split positions. The
gallium compounds 6 and 7 crystallize in the orthorhombic space
groups Pbca and Pbcn, respectively. Four dimeric formula units
Z. Anorg. Allg. Chem. 2004, 630, 1839Ϫ1845
zaac.wiley-vch.de
2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
1843
Uhl, Werner
