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J.K. Vohs et al. / Polyhedron 21 (2002) 531–534
2. Experimental
ment converged at R1=0.050, wR2=0.11 for 10 369
collected reflections.
2.1. General comments
3. Results and discussion
Standard Schlenk technique was employed in con-
junction with an inert atmosphere dry-box (MBraun
LabMaster 130). Diethyl ether was distilled over
sodium/potassium benzophenone under an atmosphere
of nitrogen prior to use. Bromopentamethylbenzene
was purchased from Lancaster Synthesis, Inc. (Wind-
ham, NH) while gallium(III) chloride was obtained
from Strem Chemical Co. (Newburyport, MA). All
reagents were used without further purification. Pen-
tamethylphenylmagnesium bromide was prepared by
reaction of bromopentamethylbenzene with magnesium
in diethyl ether. Elemental analyses were performed by
E & R Microanalytical Laboratories (Parsipanny, NJ).
Single crystal X-ray diffraction data were collected on a
Bruker SMART™ CCD instrument.
The first organogallium compound, triethylgallium,
was reported by Dennis and Patnode [4] almost 70
years ago. Unlike its aluminum analog, triethylgallium
is a trigonal planar-based monomer as opposed to a
tetrahedral-based dimer containing electron deficient
AlꢀCEtꢀAl bridges. Decades later the molecular struc-
ture of triphenylgallium, Ph3Ga, [5] would confirm the
gallium center residing in a virtually idealized trigonal
planar environment and a mean GaꢀC bond distance of
,
1.957 A. Again, the trigonal planar-based monomeric
nature of triphenylgallium is easily contrasted with the
dimeric structure of di-m-phenyl-bis(diphenylalu-
minum), Ph6Al2 (i.e. the triphenylaluminum dimer),
with tetrahedral aluminum centers and electron defi-
cient three center-two electron AlꢀCPhꢀAl bridges [6,7].
However, careful examination of the structure of
triphenylgallium reveals subtle factors at work con-
tributing to this observed orientation. In particular, one
of the phenyl rings of Ph3Ga is co-planar with the
GaC3 basal plane while the remaining two phenyl rings
were observed with dihedral angles of 13.2 and 31.6°.
Moreover, while the immediate coordination about the
gallium center in Ph3Ga is certainly trigonal planar, an
examination of the unit cell packing reveals weak inter-
2.2. Synthesis of (Me5C6)3Ga
A reaction vessel was charged with diethyl ether and
GaCl3 (0.880 g; 5.00 mmol). Pentamethylphenylmagne-
sium bromide (3.86 g; 15.0 mmol) was added to this
solution at −78 °C. This reaction mixture was allowed
to stir overnight, slowly warming to room temperature
(r.t.). The colorless solution was filtered leaving from a
white precipitate (MgBrCl). Solvent reduction of the
filtrate, coupled with subsequent cooling, afforded col-
orless, needle crystals of (Me5C6)3Ga (0.910 g, 31.0%
yield): melting point (m.p.): 163 °C. Calc. (found) for
,
molecular interactions (3.42 A) between a gallium atom
of one Ph3Ga molecule and the phenyl p-electrons of a
neighboring Ph3Ga molecule. Thus, when taken in a
larger context, the coordination of the gallium center in
triphenylgallium approaches five-coordinate trigonal
bipyramidal.
Almost two decades would pass before the synthesis
and molecular structure of trimesitylgallium, Mes3Ga
(Mes=1,3,5-Me3C6H2),[8] would be reported. The ster-
ically demanding Mes3Ga molecule resides about a
threefold crystallographic axis with approximate D3
1
C33H45Ga: C, 77.50 (77.42); H, 8.90 (8.67)%. H NMR
(400 MHz, 298 K, THF-d8): l=1.82 (s, 18H, m-CH3),
1.90 (s, 9H, p-CH3), 2.01 (s, 18H, o-CH3); 13C NMR
(400 MHz, 298 K, CDCl8): l=16.3, 23.7, 25.4, 68.9,
131.8, 135.0, 137.6.
2.3. Structural solution and refinement for (Me5C6)3Ga
A
colorless needle crystal of (Me5C6)3Ga was
,
symmetry. While the GaꢀC bond distance of 1.968 A
and the trigonal planar coordination about the gallium
center in Mes3Ga compares with the values obtained
mounted in a thin-walled glass capillary under an inert
atmosphere of nitrogen. X-ray intensity data were mea-
sured at r.t. on a Bruker SMART™ CCD-based X-ray
diffractometer system with graphite-monochromated
,
for Ph3Ga (GaꢀC: 1.957 A), the dihedral angle of the
mesityl rings relative to the GaC3 plane is noteworthy.
Each of the aromatic rings of the mesityl ligands reside
in a propeller arrangement about the gallium atom at
an angle of 55.9°. The gallium center is sufficiently
protected to prevent diethyl ether coordination. Indeed,
the orientation of the three mesityl ligands of trimesityl-
gallium preclude the weak secondary interactions
present in triphenylgallium and the associated sec-
ondary trigonal bipyramidal coordination of the metal
center. The molecular structure of trimesitylaluminum
,
Mo Ka radiation (u=0.71073 A). The structure was
solved by direct methods using the SHELXTL-6.1 [3]
system of computer programs. The non-hydrogen
atoms were refined anisotropically while hydrogen
atoms were placed in idealized positions with their
coordinates and thermal parameters riding on the at-
tached carbon atoms. Crystallographic Data: triclinic,
(
,
space group P1 (No. 2), a=9.003(9) A, b=10.713(1)
,
,
A, c=16.066(2) A, h=107.63 (3)°, i=97.63 (3)°, k=
90.08(3)°, for Z=2. Full-matrix least-squares F2 refine-
Vohs, Jason K.
