Si-C bond cleavage in the reaction of gallium chloride with lithium bis(trimethylsilyl)amide and thermolysis of base adducts of dichloro(trimethylsilyl)amido gallium compounds

Article abstract of DOI:10.1016/S0022-328X(02)01141-5

The known cyclic dimer, (Me₃SiNSiMe₂)₂ (1), was isolated in the reaction of GaCl₃ with one equivalent of Li[N(SiMe₃)₂] at room temperature as the result of cleavage of a Si-C bond from the

Full text of DOI:10.1016/S0022-328X(02)01141-5

Journal of Organometallic Chemistry 649 (2002) 268–275
www.elsevier.com/locate/jorganchem
SiꢀC bond cleavage in the reaction of gallium chloride with
lithium bis(trimethylsilyl)amide and thermolysis of base adducts of
dichloro(trimethylsilyl)amido gallium compounds
Bing Luo, Victor G. Young Jr., Wayne L. Gladfelter *
Department of Chemistry, Uni6ersity of Minnesota, Kolthoff and Smith Halls, 207 Pleasant Street S.E., Minneapolis, MN 55455, USA
Received 20 September 2001; received in revised form 26 November 2001; accepted 26 November 2001
Abstract
The known cyclic dimer, (Me3SiNSiMe2)2 (1), was isolated in the reaction of GaCl3 with one equivalent of Li[N(SiMe3)2] at
room temperature as the result of cleavage of a SiꢀC bond from the N(SiMe3)2 anion. Although the gallium product was not
isolated from the GaCl3 reaction, pyrolysis of the new base-stabilized dichloro(silyl)amido gallium compounds,
Cl2Ga[N(SiMe3)2](quin) (2) (quin=quinuclidine), Cl2Ga[N(SiMe3)2](NMe3) (3), Cl2Ga[N(SiMe3)(tBu)](quin) (4) and
Cl2Ga[N(SiMe3)(2,6-iPrC6H3)](quin) (5), afforded MeGaCl2(quin) and MeGaCl2(NMe3) in 27–61% yields confirming a methyl
migration from Si to Ga. The SiꢀC bond cleavage and methyl migration were proposed to occur through transition states
¸¹¹¹¹¹º
containing a GaꢀNꢀSiꢀMe ring. No unusual Ga···C interaction was found in the solid-state structure of 2 that was monomeric
with the gallium atom adopting a distorted tetrahedral geometry. Improved structural data for 1 are also reported. © 2002
Elsevier Science B.V. All rights reserved.
Keywords: SiꢀC bond cleavage; Chlorogallium compounds; Amidogallium compounds; X-ray structures
generated from the reaction of GaCl3 with one equiva-
lent of Li[N(SiMe3)2](THF)n in hexane, however, details
regarding the isolation and characterization of
Cl2Ga[N(SiMe3)2](THF) are not presented.
1. Introduction
A substantial portion of Group 13 chemistry has
focused on the study of new molecular precursors to
Group 13–15 materials. Several of the precursors in-
cluding [Cl2AlN(H)SiMe3]2 [1,2], [X2GaP(SiMe3)2]2
[3,4], X3GaP(SiMe3)3 [5] (X=Cl, Br, I) and mixtures of
X3Ga (X=Cl, Br) and As(SiMe3)3 [6] produced AlN
powders, nanocrystalline GaP or GaAs by elimination
of Me3SiX. This method has not been successfully
applied to gallium nitride synthesis. The reaction of
GaCl3 with HN(SiMe3)2 was reported to eliminate 1
mol of Me3SiCl to yield [Cl2GaN(H)SiMe3]2 [7,8]. Fur-
ther, heating beyond 150° C caused partial Me3SiCl
elimination and the formation of an unidentified solid
[8]. Barry and Richeson [9] reported that
In our study, the reaction of GaCl3 with one equiva-
lent of Li[N(SiMe3)2] in benzene did not afford
Cl2Ga[N(SiMe3)2], but surprisingly a small amount of
(Me3SiNSiMe2)2 (1) and other uncharacterized prod-
ucts. To understand the reaction, a series of new base-
stabilized dichloro(trimethylsilyl)amidogallium com-
pounds, Cl2GaN[(SiMe3)R](quin) [quin=quinuclidine,
R=SiMe3 (2), tBu (4), 2,6-iPr2C6H3 (5)] and
Cl2GaN(SiMe3)2(NMe3) (3) were synthesized, charac-
terized and reacted at elevated temperatures. In this
paper we report the results of these studies.
[Li(THF)2][GaN(SiMe3)2(OSiMe3)2Cl]
(SiMe3)2](OSiMe3)2(pyridine) were synthesized from the
reactions involving Cl2Ga[N(SiMe3)2](THF) that was
and
Ga[N-
2. Experimental
2.1. Materials and general procedures
* Corresponding author. Tel.: +1-612-624-4391; fax: +1-612-626-
8659.
E-mail address: gladfelt@chem.umn.edu (W.L. Gladfelter).
Gallium chloride and quinuclidine were purchased
from Strem and other chemicals were obtained from
0022-328X/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(02)01141-5

B. Luo et al. / Journal of Organometallic Chemistry 649 (2002) 268–275
269
Aldrich. Gallium chloride, anhydrous trimethylamine,
quinuclidine and n-butyllithium were used as received.
Lithium bis(trimethylsilyl)amide was recrystallized from
hexanes. 2,6-Diisopropylaniline was distilled over CaH2
under reduced nitrogen pressure. N-t-Butyltrimethylsi-
lylamine, HN(SiMe3)(tBu), and chlorotrimethylsilane
were distilled under nitrogen over NaOH and P2O5,
respectively. The quinuclidine adduct of gallium chlo-
ride, GaCl3(quin), was prepared as previously described
[10]. The trimethylamine adduct of gallium chloride,
GaCl3(NMe3), was prepared as a white powder by
condensing one equivalent of NMe3 onto GaCl3.
Lithium N-t-butyltrimethylsilylamide, LiN(SiMe3)-
(tBu), was prepared as a colorless crystalline solid (64%
yield) from the reaction of HN(SiMe3)(tBu) with one
equivalent of nBuLi followed by recrystallization in
pentane. MeGaCl2 was prepared from the reaction of
Me3Ga with two equivalents of GaCl3 [11]. Diethyl
ether, pentane, hexanes, benzene and toluene were
predried over CaH2 and freshly distilled over sodium–
benzophenone under nitrogen. Benzene-d6 was distilled
over CaH2 hydride under nitrogen. All experiments
were conducted under an oxygen-free, dry-nitrogen at-
mosphere using standard Schlenk and glovebox tech-
niques.
Proton NMR spectra were obtained in benzene-d6
solutions at room temperature (r.t.) in a Varian IN-
OVA 300 spectrometer, and the residual proton (7.15
ppm) in C6D6 was used as the internal standard. In the
cases that the resonance of the residual proton was not
distinguishable from the aromatic hydrogen atoms of
the samples, the sharp singlet (0.29 ppm) from silicone
grease was used as the internal standard. The IR spec-
tra of KBr pellets were recorded in a Nicolet MAGNA-
IR 560 spectrometer. Chemical ionization mass spectra
were acquired in a Finnigan Mat 95 spectrometer using
a direct insertion probe. The samples were evaporated
at temperatures ranging from 25 to 340° C, and except
for compound 1 for which isobutane was used, the
ionization gas was methane containing 4% NH3. The
electron ionization mass spectra were collected on the
same spectrometer using a 70 eV electron flow. Melting
points were obtained in sealed, nitrogen filled capillaries
and were uncorrected. Elemental analyses were ob-
tained from Schwarzkopf Microanalytical Laboratories,
Woodside, NY or Desert Analytics, Tucson, AZ.
was much higher than the expected weight of LiCl (0.72
g). Benzene was removed from the filtrate under vac-
uum affording a yellow sticky solid. The 1H-NMR
spectrum of the solid showed more than ten singlets in
the 0.03–0.50 ppm region with the highest peaks at
0.29, 0.27 and 0.09 ppm. Sublimation of the solid at r.t.
to a cold finger at 0° C afforded colorless plates of
(Me3SiNSiMe2)2 (1) (0.12 g, 5% yield based on silicon).
1
M.p.: 42.0–43.5° C. H-NMR: l 0.09 (18H, s, SiMe3),
0.29 (12H, s, SiMe2). EI MS [assignment, % relative
intensity]: 291 [[M+H]+, 18], 275 [[M−Me]+, 100],
130 [(0.5M−Me)+, 20], 73 [Me3Si+, 14]. Other prod-
ucts were not characterized.
2.3. Synthesis of Cl2Ga[N(SiMe3)2](quin) (2)
To a stirred solution of GaCl3(quin) (1.50 g, 5.22
mmol) in 50 ml of toluene was added a solution of
Li[N(SiMe3)2] (0.874 g, 5.22 mmol) in 50 ml of toluene
at r.t. A white precipitate (LiCl) was formed immedi-
ately and the mixture was stirred for 3 h and filtered.
After the colorless filtrate was concentrated to ca. 50 ml
and stored at −20° C overnight, colorless plates were
collected (1.83 g, 85% yield). M.p.: 175° C, decom-
posed without melting. 1H-NMR: l 0.46 (18H, s,
SiMe3), 0.81 (6H, m, CH2), 1.01 (1H, m, CH), 2.88 (6H,
t, NCH2). CIMS [assignment, % relative intensity]: 413
[[M+H]+, 2.3], 397 [[M−Me]+, 25], 377 [[M−Cl]+,
36], 302 [[M−quin+H]+, 3.0], 286 [[M−quin−Me]+,
19], 266 [[M−quin−Cl]+, 52], 252 [[M−N(SiMe3)2]+,
67], 230 [{Ga+HN(SiMe3)2}+, 24], 162 [H2N(SiMe3)2+,
19], 146 [(HNSi2Me5)+, 89], 112 [(quin+H)+, 100].
Anal. Calc. for C13H31Cl2GaN2Si2: C, 37.88; H, 7.58;
N, 6.80. Found: C, 37.51; H, 7.96; N, 6.73%.
2.4. Synthesis of Cl2Ga[N(SiMe3)2](NMe3) (3)
To a stirred solution of GaCl3(NMe3) (1.50 g, 6.38
mmol) in 30 ml of benzene was added a solution of
Li[N(SiMe3)2] (1.07 g, 6.38 mmol) in 20 ml of benzene
at r.t. A white precipitate (LiCl) was formed immedi-
ately after the addition of Li[N(SiMe3)2]. After the
mixture was stirred for 14 h, all the volatiles were
removed under vacuum leaving a white solid. Toluene
(10 ml) and hexanes (20 ml) were added and the
mixture was filtered. The colorless filtrate was stored at
−20° C overnight and colorless plates were collected
(1.83 g, 80% yield). M.p.: 124–130° C. 1H-NMR: l
0.39 (18H, s, SiMe3), 1.85 (9H, s, NMe3). CIMS [assign-
ment, % relative intensity]: 361 [[M+H]+, 0.3], 319
[[M−NMe3+NH4]+, 0.8], 217 [[M−N(SiMe3)2+
NH3]+, 13], 162 [(Me3Si)2NH2)+, 100], 90
[(Me3SiNH3)+, 9.2]. The sample decomposed upon
sealing in a glass vial under vacuum when prepared for
elemental analysis.
2.2. Reaction of GaCl3 with one equi6alent of
Li[N(SiMe3)2]; isolation of (Me3SiNSiMe2)2 (1)
In a typical experiment, a solution of Li[N(SiMe3)2]
(2.85 g, 17.0 mmol) in 50 ml of benzene was added
dropwise to a solution of GaCl3 (3.00 g, 17.0 mmol) in
90 ml of benzene at r.t. The mixture was stirred for 20
h and filtered to separate a white precipitate and a pale
yellow filtrate. The weight of the precipitate (1.58 g)

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B. Luo et al. / Journal of Organometallic Chemistry 649 (2002) 268–275
2.5. Synthesis of Cl2Ga[N(SiMe3)(tBu)](quin) (4)
0.95 (1H, m, CH in quinuclidine), 1.31 and 1.39 (total
12H, d, CH(CH3)2), 2.91 (6H, m, NCH2), 3.98 (2H, m,
CH(CH3)2, 7.10 (3H, s, C6H3). CIMS [assignment, %
relative intensity]: 518 [[M+NH4]+, 2.6], 501 [[M+
H]+, 4.7], 465 [[M−Cl]+, 0.8], 407 [[M−quin+
NH4]+, 7.1], 269 [{Cl2Ga(quin)(NH3)}+, 18], 250
[{H2N(SiMe3)(2,6-iPr2C6H3)}+, 58], 178 [{H3N(2,6-
iPr2C6H3)}+, 35], 112 [(quin+H)+, 100], 90
[(H3NSiMe3)+, 5.2]. Anal. Calc. for C22H39Cl2GaN2Si:
C, 52.82; H, 7.86; N, 5.60. Found: C, 52.79; H, 7.98; N,
5.53%.
To a stirred solution of GaCl3(quin) (2.00 g, 6.96
mmol) in 40 ml of benzene was added a solution of
Li[N(SiMe3)(tBu)] (1.05 g, 6.96 mmol) in 50 ml of
benzene at r.t. A white precipitate (LiCl) was formed
immediately after the addition of Li[N(SiMe3)(tBu)].
The mixture was stirred for 5 h and filtered to obtain a
pale yellow filtrate. After the filtrate was concentrated
to ca. 15 ml and stored at 0° C overnight, colorless
plates were collected (2.04 g, 74% yield). M.p.: 169–
1
171° C. H-NMR: l 0.50 (9H, s, SiMe3), 0.86 (6H, m,
t
CH2), 1.05 (1H, m, CH), 1.56 (9H, s, Bu), 2.94 (6H, m,
2.8. Synthesis of Cl2GaMe(quin) (6)
NCH2). CIMS [assignment, % relative intensity]: CIMS:
397 [[M+H]+, 1.7], 361 [[M−Cl]+, 1.9], 303 [[M−
quin+NH4]+, 1.5], 286 [[M−quin+H]+, 0.4], 267
[[M−quin−Cl+NH3]+, 1.5], 249 [[M−quin−Cl]+,
1.5], 146 [{H2N(SiMe3)(tBu)}+, 18], 112 [(quin+H)+,
100]. Anal. Calc. for C14H34Cl2GaN2Si: C, 42.45; H,
7.89; N, 7.07. Found: C, 41.40; H, 7.99; N, 6.66%.
To a stirred solution of MeGaCl2 (0.500 g, 3.21
mmol) in 35 ml of Et2O was added a solution of
quinuclidine (0.357 g, 3.21 mmol) in 35 ml of Et2O at
r.t. A small amount of white precipitate was formed
after the addition of quinuclidine. Then the mixture
was stirred for 4 h and filtered. The insoluble com-
pound was not characterized. After the colorless filtrate
was concentrated to ca. 40 ml and stored at −20° C
overnight, colorless plates were collected (0.51 g, 59%
yield). M.p.: 165–167° C. 1H-NMR: l 0.06 (3H, s,
Me), 0.78 (6H, m, CH2), 1.01 (1H, m, CH), 2.57 (6H, t,
NCH2). CIMS [assignment, % relative intensity]: 379
[[M+quin+H]+, 8.1], 305 [{Cl3Ga(quin)(NH4)}+,
7.9], 285 [[M+NH4]+, 100], 269 [[M+NH3−Me]+,
13], 247 [[M+NH3−Cl]+, 18], 230 [[M−Cl]+, 4.1],
112 [(quin+H)+, 45], 69 [Ga+, 1.4]. Anal. Calc. for
C8H16Cl2GaN: C, 36.01; H, 6.04; N, 5.25. Found: C,
35.83; H, 6.34; N, 5.26%.
2.6. Synthesis of Li[N(SiMe3)(2,6-iPr2C6H3)]
To a precooled toluene solution (200 ml) of 2,6-
iPr2C6H3NH2 (8.68 g, 48.9 mmol) at −78° C was
added a hexane solution (19.6 ml) of nBuLi (48.9
mmol). After the mixture was allowed to warm to r.t.
and stirred for 2 h, a white slurry was obtained.
Me3SiCl (5.32 g, 48.9 mmol) was added to the above
slurry at r.t. The mixture was stirred for 1 h and filtered
to obtain a pale yellow filtrate. The filtrate was cooled
n
to −78° C and a hexane solution (19.6 ml) of BuLi
(48.9 mmol) was added once more. After the mixture
was allowed to warm to r.t. and stirred for 1 h, a yellow
solution was obtained. The solution was stored at
−20° C and a colorless crystalline solid was isolated.
The remaining solution was further concentrated and
stored at −20° C to afford additional product. This
process was repeated several times to yield 9.48 g (76%
2.9. Synthesis of Cl2GaMe(NMe3) (7)
NMe3 (3 ml) was condensed onto Cl2GaMe (0.300 g,
1.93 mmol) at −78° C. Then the mixture was allowed
to warm to r.t. and the excess NMe3 was evaporated
leaving a white crystalline solid (0.38 g, 91% yield).
1
yield) of Li[N(SiMe3)(2,6-iPr2C6H3)]. H-NMR: l 0.12
1
M.p.: 135–137° C. H-NMR: l −0.02 (3H, s, GaMe),
(9H, s, SiMe3), 0.89 and 1.19 (total 12H, d, CH(CH3)2),
3.49 (2H, m, CH(CH3)2), 6.87 and 6.99 (3H, m, C6H3).
1.67 (9H, s, NMe3). CIMS [assignment, % relative
intensity]: 285 [[M+NH4]+, 100], 217 [{Cl2Ga-
(NMe3)(NH3)}+, 4.9], 191 [{Cl2Me(NH3)(NH4)}+, 37],
178 [{ClGaMe(NMe3)}+, 2.1], 153 [{ClGaMe(NH3)2}+,
17], 69 [Ga+, 1.5], 60 [Me3NH+, 6.6].
2.7. Synthesis of Cl2Ga[N(SiMe3)(2,6-iPr2C6H3)](quin)
(5)
To a stirred solution of GaCl3(quin) (1.50 g, 5.22
mmol) in 40 ml of Et2O was added a solution of
Li[N(SiMe3)(2,6-iPr2C6H3)] (1.33 g, 5.22 mmol) in 40 ml
of Et2O at r.t. A white precipitate (LiCl) was formed
immediately and the mixture was stirred for 3 h and
filtered. Toluene (10 ml) was used to wash the filter
cake. After the yellow filtrate was concentrated to ca. 5
ml and stored at −20° C overnight, colorless plates
were collected (1.72 g, 66% yield). M.p.: 138–148° C.
1H-NMR: l 0.39 (9H, s, SiMe3), 0.80 (6H, m, CH2),
2.10. Pyrolysis of compound 2
Compound 2 (0.800 g, 1.94 mmol) in a Schlenk flask
(diameter, 2 cm; length, 15 cm) was heated to 210° C in
an oil bath. After heating for 1 h, the system was
connected to the vacuum source. A white crystalline
solid was sublimed to the unheated top of the flask and
a glassy, yellow solid remained on the bottom. Toluene
(15 ml) was added carefully to dissolve the residue on

B. Luo et al. / Journal of Organometallic Chemistry 649 (2002) 268–275
271
the bottom and transferred to another flask. The crys-
talline solid on the wall was dissolved in toluene (4 ml)
affording a colorless solution. After the solution was
stored at −20° C overnight, colorless plates of
MeGaCl2(quin) were collected (0.32 g, 61% yield). Its
1H-NMR and IR spectra were identical with those of
the authentic sample. The glassy yellow solid was iso-
lated after evaporation of the toluene solvent and its
1H-NMR spectrum showed a complex pattern and was
attributed to as yet unidentified products.
tion. The final cell constants were calculated from 3557
strong reflections for 1 and 5927 for 2.
The space groups P21/n for 1 and P21 for 2 were
determined based on systematic absences and intensity
statistics. Successful direct-methods solutions were ap-
plied to both structures that provided most non-hydro-
gen atoms from the E-maps. Several full-matrix,
least-squares/difference Fourier cycles were performed
which located the remainder of the non-hydrogen
atoms. All non-hydrogen atoms were refined with an-
isotropic displacement parameters. All hydrogen atoms
were placed in ideal positions and refined as riding
atoms with relative isotropic displacement parameters.
All calculations were performed using SGI INDY
R4400-SC or Pentium computers and the SHELXTL
V5.0 suite of programs. The experimental conditions
and unit cell information are summarized in Table 1.
2.11. Pyrolysis of compound 3
The procedure was similar to the pyrolysis of 2. A
sample of 3 (1.00 g, 2.78 mmol) was heated at 200° C
for 0.5 h. Crystals of MeGaCl2(NMe3) (0.33 g, 55%
yield) were obtained after the crude product was recrys-
tallized from a mixture of toluene (5 ml) and hexanes
1
(10 ml). H-NMR and IR spectra of the product were
identical with those of the authentic sample.
3. Results
2.12. Pyrolysis of compound 4
The procedure was similar to the pyrolysis of 2. A
sample of 4 (0.800 g, 2.02 mmol) was heated at 220° C
for 1 h. Crystals of MeGaCl2(quin) (0.13 g, 27% yield)
were obtained after the crude product was recrytallized
from toluene (4 ml). H-NMR and IR spectra of the
product were identical with those of the authentic
sample.
3.1. Reaction of GaCl3 with one equi6alent of
Li[N(SiMe3)2]; isolation of compound 1
The reaction of GaCl3 with one equivalent of
Li[N(SiMe3)2] in benzene afforded a white precipitate
and a yellow sticky solid after filtration and removal of
the solvent. The precipitate included LiCl but its weight
was much larger than the calculated value assuming
that one equivalent of LiCl was formed in the reaction.
The 1H-NMR spectrum of the sticky solid obtained
from the filtrate exhibited more than ten singlets in the
region of 0.03–0.50 ppm with the highest peaks at 0.29,
0.27 and 0.09 ppm. A small amount of (Me3SiNSiMe2)2
(1) (5% yield based on silicon) was isolated via sublima-
tion of the raw product. Extensive attempts to isolate
and characterize other products failed. The isolation of
1 was reproducible and its identity was unambiguously
1
2.13. Pyrolysis of compound 5
The procedure was similar to the pyrolysis of 2. A
sample of 5 (1.00 g, 2.00 mmol) was heated at 230° C
for 3 h. Crystals of MeGaCl2(quin) (0.20 g, 37% yield)
were obtained after the crude product was crystallized
1
from toluene (3 ml). H-NMR and IR spectra of the
product were identical with those of the authentic
sample.
1
confirmed by single crystal XRD, H-NMR and mass
2.14. X-ray data collection, structure solution and
spectroscopy.
refinement
Compound 1 had been previously synthesized via
several methods [12–15] and its structure was deter-
mined in 1962 [16]. The structure possessed a Si2N2 ring
with each nitrogen atom bonded to two silicon atoms
and a terminal SiMe3 group and each silicon atom to
two nitrogen atoms and two methyls (Scheme 1). The
structure obtained in our experiment was essentially the
same as that previously reported. The unit cell volume
Suitable crystals of compounds 1 and 2 were
mounted on glass fibers under N2 atmosphere. The data
collections were conducted in a Siemens SMART sys-
tem. In each experiment, an initial set of cell constants
was calculated from reflections harvested from three
sets of 20 frames. These sets of frames were oriented so
that orthogonal wedges of reciprocal space were sur-
veyed. A randomly oriented region of reciprocal space
was surveyed to the extent of 1.3 hemispheres to a
3
3
,
,
was found to be 942.26(9) A , 24 A lower than the
previously reported value. The SiꢀN bond lengths were
,
1.718(3) (terminal), 1.750(2) (bridging) and 1.752(2) A
(bridging), comparable to the previously reported val-
,
resolution of 0.84 A. Three major swaths of frames
were collected with 0.30° steps in ꢀ. The data collection
technique was generally known as a hemisphere collec-
,
ues, 1.71(4), 1.72(4) and 1.72(4) A. Selected bond
lengths and angles of 1 are given in Table 2.

272
B. Luo et al. / Journal of Organometallic Chemistry 649 (2002) 268–275
Table 1
Crystal data and structure refinement parameters for compounds 1 and 2
Empirical formula
Formula weight
Temperature (°C)
Color
C10H30N2Si4 (1)
290.72
C13H31Cl2Ga2Si2 (2)
412.20
−100
Colorless
Block
0.30×0.25×0.25
P21/n
Colorless
Plate
0.90×0.60×0.40
P21
Habit
Size (mm3)
Space group
Unit cell dimensions
,
a (A)
6.6553(4)
13.1345(7)
11.1129(6)
104.076(1)
942.26(9)
6.7764(2)
,
b (A)
13.3672(4)
11.3334(3)
105.6180(10)
988.69(5)
,
c (A)
i (°
V (A )
3
,
Z
2
,
u (A)
0.71073
v (cm−1
)
3.00
17.77
Total reflections
4718
7070
Independent reflections
Parameters
1648 [Rint=0.0773]
74
0.0583, 0.1522
3393 [Rint=0.0273]
181
0.0230, 0.0519
R1 [I\2|(I)], wR2 (all data)a
a R1=ꢀꢁꢁFoꢁ−ꢁFcꢁꢁꢀꢁFoꢁ and wR2={ꢀ[w(F2o−Fc2)2]/ꢀ[w(Fo2)2]}1/2, where w=1/[|2(Fo2)+(aP)2+(bP)], P=(Fo2+2F2c)/3 and a, b are constants.
3.2. Synthesis and characterization of compounds 2–5
and the structure of 2
the three-coordinate N(2) was 357.0(5)° and the two Si
atoms were slightly non-planar with Ga(1) and N(2) in
a manner such that the two SiMe3 groups bent away
from the quinuclidine ligand. The Ga(1)ꢀN(2)ꢀSi(1)
and Ga(1)ꢀN(2)ꢀSi(2) bond angles were 116.64(16) and
121.43(18)°, respectively. The distortion from a perfect
planar geometry on N(2) and the different SiꢀNꢀGa
bond angles were attributed to the ligand steric interac-
tions as indicated by the short distances of C(3)···C(9)
Compounds 2–5 were isolated as colorless crystalline
solids in good yields (74–85%) from the reactions of
GaCl3(quin) or GaCl3(NMe3) with the corresponding
lithium amide compounds (Scheme 2), and were charac-
terized by elemental analysis and spectroscopic meth-
ods. The structure of 2 was determined by single crystal
XRD. The new reactant, Li[N(SiMe3)(2,6-iPr2C6H3)],
was obtained as a colorless solid in 76% yield from the
reactions described in Section 2.
,
,
(3.680 A), C(5)···C(13) (3.530 A) and C(10)···C(12)
,
(3.264 A). This crowdedness was also reflected in the
shorter van der Waal’s contacts of Ga(1)···C(13) (3.534
The CꢀHꢀN analysis results for compounds 2, 4 and
5 were satisfactory. In our repeated efforts to prepare a
sample of 3 for elemental analysis the crystals of 3
turned into a glassy solid within several hours in a glass
vial that had been sealed under vacuum. Repeated
analytical data from these samples were inconsistent
with each other suggesting decomposition had oc-
curred. The molecular ions (+1) were found with the
intensities ranging from 0.5 to 4.7% of the base peaks in
the chemical ionization mass spectra of all the four
Scheme 1.
Table 2
1
compounds. The H-NMR spectra of 2–5 were consis-
,
Selected bond lengths (A) and bond angles (°) for (Me3SiNSiMe2)2
(1)
tent with their formulas. It was noteworthy that the two
doublets at 1.31 and 1.39 ppm in the spectrum of 5
were due to the magnetic inequivalency of the two
methyls in each ꢀCHMe2 group.
Selected bond lengths and angles for 2 are given in
Table 3. In the structure of 2 (Fig. 1), the gallium atom
adopted a distorted tetrahedral geometry with the bond
angles N(2)ꢀGa(1)ꢀN(1) [113.56(7)°], N(2)ꢀGa(1)ꢀCl(1)
[115.13(11)°] and N(2)ꢀGa(1)ꢀCl(2) [118.46(10)°] being
significantly larger than other angles on gallium
[100.57(9)–104.67(2)°]. The sum of the angles around
Bond lengths
Si(1)ꢀN(1)
Si(1)ꢀN(1A)
Si(2)ꢀN(1)
1.752(2)
1.750(2)
1.718(3)
Si(1)ꢀC(1)
Si(1)ꢀC(2)
1.864(3)
1.865(3)
Bond angles
N(1)ꢀSi(1)ꢀN(1A)
N(1)ꢀSi(1)ꢀC(1)
N(1)ꢀSi(1)ꢀC(2)
N(1A)ꢀSi(1)ꢀC(1)
N(1A)ꢀSi(1)ꢀC(2)
89.45(12) C(1)ꢀSi(1)ꢀC(2)
115.23(13) Si(1)ꢀN(1)ꢀSi(1A)
114.02(13) Si(1)ꢀN(1)ꢀSi(2)
113.76(13) Si(2)ꢀN(1)ꢀSi(1A) 133.88(13)
115.13(14)
108.5(2)
90.55(12)
134.53(13)

B. Luo et al. / Journal of Organometallic Chemistry 649 (2002) 268–275
273
3.3. Isolation of compounds 6 and 7 from thermolysis
of compounds 2–5
When compounds 2–5 were heated to 200–230° C
for a period of 0.5–3 h, methyl abstraction yielded 6 or
7 in 27–61% yields based on gallium (Scheme 3). These
products were purified by sublimation and recrystalliza-
tion from toluene or a mixture of toluene and hexanes.
By-products from the thermal decomposition of 2–5
1
were glassy, yellow or brown solids whose H-NMR
spectra exhibited unresolved, complicated patterns. At-
tempts to isolate and characterize these materials were
unsuccessful. Compounds 6 and 7 were also prepared
independently from the reactions of MeGaCl2 with
quinuclidine and NMe3, respectively.
Scheme 2.
Table 3
,
Selected bond lengths (A) and angles (°) for Cl2Ga[N(SiMe3)2](quin)
(2)
Bond lengths
4. Discussion
Ga(1)ꢀN(1)
Ga(1)ꢀN(2)
Ga(1)ꢀCl(1)
2.0293(18) Ga(1)ꢀCl(2)
1.8658(17) N(2)ꢀSi(1)
2.1952(8)
1.757(3)
1.747(3)
4.1. Proposed mechanisms for the SiꢀC bond clea6age
and methyl migration from Si to Ga
2.2011(8)
N(2)ꢀSi(2)
Bond angles
N(1)ꢀGa(1)ꢀN(2)
N(1)ꢀGa(1)ꢀCl(1)
N(1)ꢀGa(1)ꢀCl(2)
N(2)ꢀGa(1)ꢀCl(1)
N(2)ꢀGa(1)ꢀCl(2)
113.56(7)
100.57(9)
102.19(9)
115.13(11) Si(1)ꢀN(2)ꢀSi(2)
118.46(10)
Cl(1)ꢀGa(1)ꢀCl(2)
Ga(1)ꢀN(2)ꢀSi(1)
Ga(1)ꢀN(2)ꢀSi(2)
104.67(2)
116.64(16)
121.43(18)
118.95(10)
As previously mentioned, [9] [Li(THF)2][GaN-
(SiMe3)2(OSiMe3)2Cl] was synthesized from the reac-
tion of GaCl3 with one equivalent of Li[N(SiMe3)2]-
(THF)n followed by addition of two equivalents of
LiOSiMe3 in THF. Ga[N(SiMe3)2](OSiMe3)2(pyridine)
was obtained if hexane was used as the solvent in the
second step and pyridine was added [9]. These results
indicated that Cl2Ga[N(SiMe3)2](THF) formed in the
first step was stable for at least a period of time
allowing its reaction with LiOSiMe3. With the use of
benzene in this work, Cl2Ga[N(SiMe3)2], if formed, was
unstable at room temperature. The isolation of 1, which
possessed SiMe2 moieties, clearly indicated the cleavage
of a SiꢀC bond presumably from a gallium-coordinated
N(SiMe3)2 ligand. The loss of
a b-methyl from
N(SiMe3)2 could lead to a reactive intermediate,
‘Me3SiNꢁSiMe2’, dimerization of which would afford
compound 1. ‘Me3SiNꢁSiMe2’ had been previously pro-
posed to be the intermediate in several reactions leading
to 1 [15,16]. Because of the complexity with the pres-
ence of Cl, Li and Ga in the reaction system
Fig. 1. Molecular structure and atom labeling scheme for
Cl2Ga[N(SiMe3)2](quin) (2). Atoms are shown at the 50% probability
level. All hydrogen atoms are omitted for clarity.
,
,
A) and Ga(1)···C(8) (3.752 A), whereas the regular van
der Waals radii of Ga and a methyl were 1.9 [17] and
,
2.0 A [18], respectively. The dative Ga(1)ꢀN(1) bond
,
length [2.0293(18) A] was longer than that in
,
Cl2GaH(quin) [2.017(3) A] [10] due to the larger steric
repulsion in 2, but shorter than the corresponding
,
GaꢀN bond in H2Ga[N(SiMe3)2](quin) [2.108(2) A] [19]
reflecting the higher Lewis acidity on gallium for the
Cl2Ga[N(SiMe3)2] species. The Ga(1)ꢀN(2) bond length
,
[1.8658(17) A] was typical for the terminal GaꢀN bonds
,
(1.82–1.94 A) [20].
Scheme 3.

274
B. Luo et al. / Journal of Organometallic Chemistry 649 (2002) 268–275
due to their smaller size which promotes the stability of
the dimeric structures. The reactions of GaCl3 with two
and three equivalents of LiN(SiMe3)2 in ether afforded
stable ClGa[N(SiMe3)2]2 [23] and Ga[N(SiMe3)2]3,
[23,24], respectively. Both are monomeric with trigonal
planar gallium and nitrogen atoms [23]. Apparently, the
reduced Lewis acidities in these complexes compared
with that in the suggested dichlorogallium intermediate
(Scheme 4) is insufficient to induce methyl migration.
Monomeric Cl2BN(SiMe3)2 was prepared from the
reaction of BCl3(OEt2) with LiN(SiMe3)2 [25] or the
reaction of BCl3(NEt3) with HN(SiMe3)2 [26]. Me3SiCl
elimination took place from Cl2BN(SiMe3)2 at higher
temperatures affording trimeric (ClBNSiMe3)3 [25]. The
stability of Cl2BN(SiMe3)2 was largely attributed to the
reduced Lewis acidity on B because of the pp–pp
interaction between N and B. However, compound 1
and R2N(Me)BN(SiMe3)2, where R=Et and iPr,
formed when R2N(Cl)BN(SiMe3)2 reacted with one
equivalent of Na[N(SiMe3)2] in refluxing xylene [25].
Finally, the reactions of AlCl3 and InCl3 with one
equivalent of Li[N(SiMe3)2] afforded ClAl[N(SiMe3)2]2
(30% yield) [27] and Me(Cl)In[N(SiMe3)2] (no exact
yield but a statement that the yield was lower than 65%
was given), [28] respectively.
Scheme 4.
and the reactive nature of the SiꢁN compound, [21]
dimerization would not be expected to be the only
reaction, perhaps explaining the low yield of 1 and
formation of other uncharacterized products.
Strong bases, quinuclidine and NMe3, stabilized the
species Cl2Ga[N(SiMe3)2], implying that the coordina-
tive unsaturation of gallium was responsible for the
SiꢀC bond cleavage in the reaction of GaCl3 with
Li[N(SiMe3)2]. The isolation of good yields of 6 (61%)
and 7 (55%) from the thermolysis of 2 and 3 indicated
that at higher temperatures a b-methyl migrates from a
SiMe3 group to Ga. The pyrolysis of 4 and 5 indicated
that this type of methyl migration was general.
Because of the electronegativity difference between Si
and C, the methyls in the SiMe3 groups were partially
negatively charged. The delocalization of the lone pair
electrons of the N to Si through the pp–dp interaction
in N(SiMe3)2 enhanced the negative charges on C
atoms. Although an intermolecular process could not
be ruled out, the SiꢀC bond cleavage in the reaction of
GaCl3 and Li[N(SiMe3)2] possibly occurred via an inter-
mediate with bridging Me and N between Ga and Si as
shown in Scheme 4. The higher reaction temperatures
needed to promote the b-methyl elimination in 2–5
5. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC nos. 170545 and 170546 for com-
pounds 1 and 2, respectively. Copies of this information
may be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge CB12 1EZ, UK
(Fax: +44-1223-336033; e-mail: deposit@ccdc.cam.
ac.uk or www: http://www.ccdc.cam.ac.uk).
were consistent with
a required quinuclidine or
trimethylamine dissociation to form an open coordina-
tion site on gallium. Compared with the quinuclidine
adducts (2, 4 and 5), the decomposition of compound 3
took place at a lower temperature, consistent with the
lower basicity, and thus, the lower stabilizing power, of
NMe3 over quinuclidine. This observation supports the
above mechanism, however, more quantitative studies
would be required to clarify the nature of the b-methyl
elimination step. Finally, a methyl migration from sili-
con to gallium was previously found in the reaction of
GaCl3 with SiMe4 for which an intermediate with
bridging Cl and Me between Si and Ga was suggested
by Schmidbaur and Findeiss [22].
6. Note added in proof
A report has appeared describing the formation of
[MeGaCl2]2 from the reaction of GaCl3 with N(SiMe3)3.
See C.J. Carmalt, J.D. Mileham, A.J.P. White, D.J.
Williams, J.W. Steed, Inorg. Chem. 40 (2001) 6035
4.2. Comparisons with related compounds and reactions
in Group 13 chemistry
Acknowledgements
The authors gratefully acknowledge support from the
National Science Foundation.
The amido-bridging ligands in [Cl2GaN(H)SiMe3]2
and [Cl2GaN(Me)SiMe3]2 were stable at room tempera-
ture, and partial elimination of Me3SiCl rather than a
methyl migration occurred for [Cl2GaN(H)SiMe3]2 at
elevated temperatures [8]. The enhanced stability of
NHSiMe3 and N(Me)SiMe3 containing complexes com-
pared to those bearing a N(SiMe3)2 ligand is probably
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