Synthesis and molecular structure of an orthometalated aminogallane, [MeGaN(CH2C6H4)(CH2Ph)]2

Article abstract of DOI:10.1021/om9604381

Monitoring the thermolysis reaction of Me₃Ga with HN(CH₂Ph)₂ in toluene with multinuclear NMR spectroscopy indicates the initial formation of [Me₂GaN(CH₂Ph)₂]₂ via 1,2-elimination of CH₄ followed by additional 1,2-elimination steps that give the orthometalated gallium dimer [MeGaN(CH₂C₆H₄)(CH₂Ph)]₂ in 65% yield. This is the first reported example of orthometalation of an aromatic ring carbon by gallium. Because [MeGaN(CH₂C₆H₄)(CH₂Ph)]₂ forms prior to the complete conversion of (PhCH₂)₂NH·GaMe₃ to [Me₂GaN(CH₂Ph)₂]₂, thermolysis of a Me₃Ga/HN(CH₂Ph)₂ mixture is not a viable route to [Me₂GaN(CH₂Ph)₂]₂. Instead it was prepared by a salt elimination reaction. The single-crystal X-ray diffraction analysis establishes the dimeric nature of the orthometalated compound and indicates a structure that is consistent with the NMR chemical shift assignements obtained using both 1-D and 2-D NMR techniques.

Full text of DOI:10.1021/om9604381

Organometallics 1996, 15, 5641-5644
5641
Syn th esis a n d Molecu la r Str u ctu r e of a n Or th om eta la ted
Am in oga lla n e, [MeGa N(CH2C6H4)(CH2P h )]2
Steven J . Schauer, Charles H. Lake, Charles L. Watkins,* and
Larry K. Krannich*
Department of Chemistry, University of Alabama at Birmingham,
Birmingham, Alabama 35294
Received J une 4, 1996X
Monitoring the thermolysis reaction of Me3Ga with HN(CH2Ph)2 in toluene with multi-
nuclear NMR spectroscopy indicates the initial formation of [Me2GaN(CH2Ph)2]2 via 1,2-
elimination of CH4 followed by additional 1,2-elimination steps that give the orthometalated
gallium dimer [MeGaN(CH2C6H4)(CH2Ph)]2 in 65% yield. This is the first reported example
of orthometalation of an aromatic ring carbon by gallium. Because [MeGaN(CH2C6H4)(CH2Ph)]2
forms prior to the complete conversion of (PhCH2)2NH‚GaMe3 to [Me2GaN(CH2Ph)2]2,
thermolysis of a Me3Ga/HN(CH2Ph)2 mixture is not a viable route to [Me2GaN(CH2Ph)2]2.
Instead it was prepared by a salt elimination reaction. The single-crystal X-ray diffraction
analysis establishes the dimeric nature of the orthometalated compound and indicates a
structure that is consistent with the NMR chemical shift assignements obtained using both
1-D and 2-D NMR techniques.
In tr od u ction
investigated to determine the efficacy of this method for
the formation of dimeric aminogallanes.17 Although
peaks associated with the formation of [Me2GaN(CH2-
Ph)2]2 are initially observed in the NMR spectra of the
thermolyzed Me3Ga/HN(CH2Ph)2 reaction mixture, peaks
assignable to three intermediates that ultimately lead
The synthesis and reactivity of aminogallanes has
been of interest because of their use as precursors to
gallium nitrides.1 Traditionally, dimeric aminogallanes
have been synthesized either by alkane elimination from
the thermolysis of R3Ga2-9 with primary or secondary
amines or by salt elimination reactions from the reac-
tions of R2GaCl10-12 with LiNR′R′′. Recently, methods
to [MeGaN(CH2C6H4)(CH2Ph)]2 (1) are also observed.
Herein we report the synthesis of 1 from the thermolysis
of an equimolar mixture of Me3Ga and HN(CH2Ph)2 and
of [Me2GaN(CH2Ph)2]2 from a salt elimination reaction.
The X-ray structure and complete 1H and 13C NMR
chemical shift assignments for this unique orthometal-
lated gallium compound are reported.
have been described using the reactions of Me2Ga-
13,14
C5H5
with primary or secondary amines and of
R3Ga15 (R ) Me, Et) with Me3SnNR′R′′ as routes to
aminogallanes.
As an extension of our work on the thermolysis of
Me3Al with a homologous series of secondary amines,16
the thermolysis of Me3Ga with these amines was
Resu lts a n d Discu ssion
The Me3Ga/NH(CH2Ph)2 reaction system was studied
in order to monitor the ease of 1,2-CH4 elimination and
of possible formation of the dimeric aminogallane
[Me2GaN(CH2Ph)2]2. This was part of a systematic
study of the influence of secondary amine steric bulk
on the viability of using thermolysis to prepare a series
of aminogallanes. Monitoring the reaction as a function
X Abstract published in Advance ACS Abstracts, November 15, 1996.
(1) Neumayer, D. A.; Ekerdt, J . G. Chem. Mater. 1996, 8, 9.
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103.
1
of temperature and time by H and 13C NMR spectros-
copy revealed the initial formation of (PhCH2)2NH‚-
GaMe3 at room temperature. After heating of the
mixture for 1 day at 120 °C, the 1H NMR spectrum
indicated the partial conversion to [Me2GaN(CH2Ph)2]2
as a result of 1,2-elimination of CH4. After continued
heating of the mixture over a period of 5 days, spectral
data showed further conversion to the dimer and
appearance of peaks in the CH2 and Me3Ga spectral
regions assignable to Me2GaN(CH2Ph)2Ga(Me2)N(CH2-
Ph)2‚GaMe3. When the temperature was increased to
135 °C, additional peaks appeared and were assigned
(13) Beachley, O. T., J r.; Royster, T. L., J r.; Arhar, J . R.; Rheingold,
A. L. Organometallics 1993, 12, 1976.
(14) Beachley, O. T., J r.; Rosenblum, D. B.; Churchill, M. R.; Lake,
C. H.; Toomey, L. M. Organometallics 1996, 15, 3653.
(15) Nutt, W. R.; Murray, K. J .; Gulick, J . M.; Odom, J . D.; Ding,
Y.; Lebioda, L. Organometallics 1996, 15, 1728.
(16) Thomas, C. J .; Krannich, L. K.; Watkins, C. L. Polyhedron 1993,
12, 389.
to the partially orthometallated species MeGa(C6H4-
(17) Unpublished results.
S0276-7333(96)00438-4 CCC: $12.00 © 1996 American Chemical Society

5642 Organometallics, Vol. 15, No. 26, 1996
Schauer et al.
Sch em e 1
CH2)N(CH2Ph)Ga(Me)2N(CH2Ph)2‚GaMe3, 2. Contin-
ued heating at 135 °C results in further conversion of
[Me2GaN(CH2Ph)2]2 to 2 and the transformation of 2
to MeGa(C6H4CH2)N(CH2Ph)Ga(Me)N(CH2C6H4)(CH2-
Ph)‚HN(CH2Ph)2, 3. Increasing the temperature to 140
°C and heating for 8 days resulted in an increase in the
intensity of peaks associated with 2 and 3, a downfield
shift in the chemical shift of the original adduct as the
Me3Ga is in exchange with 2, and the appearance with
peaks associated with 1. Upon cooling of the mixture
to room temperature, crystals of 1 formed. Thus, the
NMR data support a stepwise elimination of 2 equiv of
CH4 from [Me2GaN(CH2Ph)2]2 which results in the
orthometalation of one benzyl moiety on each nitrogen
Figu r e 1. Molecular structure and atom-numbering scheme
for [MeGaN(CH2C6H4)(CH2Ph)]2.
(Scheme 1). [MeGaN(CH2C6H4)(CH2Ph)]2 (1) is ob-
tained in 65% yield after recrystallization. These
results also suggest that the thermolysis is not a viable
route to [Me2GaN(CH2Ph)2]2 because peaks assignable
to 2 and 3 were observed in the NMR spectra before
the (PhCH2)2NH‚GaMe3 adduct was converted to
[Me2GaN(CH2Ph)2]2. Instead, it was synthesized in 78%
yield by the reaction of Me2GaCl with LiN(CH2Ph)2.
An X-ray crystallographic analysis of 1 was per-
formed, and the molecular structure is given in Figures
1 and 2. Details of the data collection are listed in Table
1, and selected interatomic distances and angles are
listed in Table 2. Each dimeric molecule 1 crystallized
with a center of symmetry, indicating that the four-
membered Ga2N2 core must possess a planar conforma-
tion. This fact coupled with the internal angles Ga(1)-
N(2)-Ga(1A) ) 89.3(1)° and N(2)-Ga(1)-N(2A) )
90.7(1)° reveals that the four-membered Ga2N2 core
possesses a slightly distorted square geometry. The
bond distances associated with this core are Ga(1)-
N(2A) ) Ga(1A)-N(2) ) 2.018(3) Å and Ga(1)-N(2) )
Ga(1A)-N(2A) ) 2.045(3) Å (this pair is also associated
with the five-membered orthometalated rings) and are
within the range of other dimeric aminogallanes (2.01-
2.08 Å).6,8,9,11,12,15 The Ga-N bond pair associated with
the orthometalated rings is longer than that of the other
set of Ga-N bond distances. This feature may not
necessarily be attributed to the presence of the ortho-
F igu r e 2. Molecular structure of [MeGaN(CH2C6H4)(CH2-
Ph)]2 showing only the ligands on one side of the Ga2N2
plane. Calculated hydrogen atoms for the methylenes are
included.
metalated ring since other symmetric aminogallane
dimers show similar variations.
The geometry around the gallium atoms is best
described as a distorted tetrahedron, with the largest
distortions being the interligand angles C(1)-Ga(1)-
C(15) ) 130.5(2)° and N(2)-Ga(1)-C(15) ) 88.4(1)°.
These distortions can be attributed to the orthometa-
lation of the benzyl group. The geometry around the
bridging nitrogen atom can also be described as a
distorted tetrahedron, with angles varying from 89.3(1)
to 117.4(2)°. This range in variation is similar to that
observed in other aminogallane dimers.6,8,9,11,12,15 The
five-membered orthometalated ring is slightly puckered
along the Ga(1)-C(9) axis with an angle of 13.5°

An Orthometalated Aminogallane
Organometallics, Vol. 15, No. 26, 1996 5643
Ta ble 1. Cr ysta llogr a p h ic Da ta for
[MeGa N(CH2C6H4)(CH2P h )]2
empirical formula
fw
C30H32Ga2N2
560.0
cryst system
space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/c
9.0502(15)
19.5400(29)
7.5642(6)
99.548(10)
1319.1(7)
2
1.410
2.063
576
â (deg)
V (Å)
Z
Dcalcd (g cm-3
)
abs coeff (mm-1
F(000)
)
2θ range (deg)
scan type
5.0-45.0
ω-2θ
index ranges
reflcns collcd
indepdt reflcns
obsd reflcns
(h,-k,+l
1776
1625 (Rint ) 1.70%)
1253 (|Fo| > 6.0σ(|Fo|))
0.4713/0.5739
0.49
-0.32
155
10.5:1
min/max transm
largest diff peak e A-3
largest diff hole, e A-3
no. of params refined
data-to-parm ratio
final R indices (6σ data)
R indices (all data)
goodness-of-fit
F igu r e 3. 1H COSY NMR spectrum of the CH2 region of
the 140 °C reaction mixture. The eight marked resonances
(3) are assigned to 3. Overlapping resonances occur at 4.32,
4.28, and 3.77 ppm. The doublet at 3.55 ppm is assigned
to the exchanged averaged adduct, (PhCH2)2NH‚GaMe3.
R ) 3.29, Rw ) 3.87
R ) 4.21, Rw ) 5.61
0.83
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for [MeGa N(CH2C6H4)(CH2P h )]2
Ga(1)-N(2)
N(2)-Ga(1A)
Ga(1)-C(1)
N(2)-C(2)
C(2)-C(3)
2.045 (3)
2.018 (3)
1.947 (5)
1.468 (5)
1.519 (5)
1.503 (6)
Ga(1)-N(2A)
Ga(1)-Ga(1A)
Ga(1)-C(15)
N(2)-C(9)
2.018 (3)
2.854 (1)
1.971 (4)
1.487 (5)
1.382 (5)
1.398 (5)
2,3
J
spectroscopies for the protonated and quaternary
CH
carbons, respectively.
The CH2 portion of the 1H NMR spectrum of the
reaction mixture, after being heated to 140 °C, shows
16 resonances for 2 and 8 resonances for 3. Analysis of
C(3)-C(4)
C(9)-C(10)
C(10)-C(15)
1
N(2)-Ga(1)-C(1)
C(1)-Ga(1)-C(15)
115.0(2) N(2)-Ga(1)-C(15)
130.5(2) N(2)-Ga(1)-N(2A)
88.4(1)
90.7(1)
an H COSY NMR spectrum of the mixture (Figure 3)
for this region leads to an assignment of four non-
equivalent CH2 groups for 2 [4.43 and 3.89 ppm, ∆δ )
0.54 ppm; 4.30 and 3.79 ppm, ∆δ ) 0.51 ppm; 4.30 and
4.02 ppm, ∆δ ) 0.28 ppm; 3.92 and 3.75 ppm, ∆δ ) 0.17
ppm]. The 1H NMR spectrum of the Me-Ga region also
supports the proposed structure of 2 in that there are
three resonances of 1:1:1 intensity assignable to 2. The
C(1)-Ga(1)-N(2A) 115.1(2) C(15)-Ga(1)-N(2A) 107.0(1)
Ga(1)-N(2)-C(2)
C(2)-N(2)-C(9)
115.4(2) Ga(1)-N(2)-C(9)
110.8(3) Ga(1)-N(2)-Ga(1A)
107.2(2)
89.3(1)
117.4(2)
113.1(3)
109.4(3)
118.1(4)
C(2)-N(2)-Ga(1A) 115.1(2) C(9)-N(2)-Ga(1A)
N(2)-C(2)-C(3)
C(9)-C(10)-C(15)
116.2(3) N(2)-C(9)-C(10)
120.1(4) Ga(1)-C(15)-C(10)
Ga(1)-C(15)-C(14) 132.4(3) C(10)-C(15)-C(14)
1
eight resonances for 3 in the CH2 region of the H NMR
between the planes defined by Ga(1)-C(9)-C(10)-C(15)
and Ga(1)-N(2)-C(9). This five-membered ring system
intercepts the Ga2N2 core at approximately 70.2°.
spectrum are assigned to two nonequivalent CH2 groups
[4.45 and 3.70 ppm, ∆δ ) 0.75 ppm; 4.09 and 3.43 ppm,
∆δ ) 0.66 ppm]. The observed spectral pattern is very
similar to that for 1, with the ∆δ values being the same
and the eight resonances in 3 being displaced upfield,
about 0.13 ppm. The Me-Ga region shows only one
resonance each for 1 (0.18 ppm) and 3 (0.054 ppm).
Compound 1 represents, to our knowledge, the first
reported example of orthometalation of an aromatic ring
carbon by gallium. Similar orthometalation reactions
have been reported for organoaluminum compounds,18-20
as well as for aminoalanes.21-23 Of related interest are
the cyclometalation reactions that involve the metala-
tion of alkyl groups at the ortho-position on an aromatic
ring. These have been reported for several ami-
noalanes24,25 and aminogallanes.8
1
The H NMR spectrum of 1 consists of a single sharp
resonances for the Me-Ga groups but two sets of AB
quartets for the CH2 groups indicating that the meth-
ylene protons are chemically nonequivalent. The aro-
matic region indicated two distinct subspectra, one for
the metalated benzyl groups and one of the normal
benzyl groups. The 1H NMR chemical shift assignments
were made using 1-D homonuclear decoupling, 2-D
COSY, and 2-D NOESY experiments. The individual
aromatic and methylene group subspectra were deter-
mined from the homonuclear decoupling and 2-D COSY
spectra on the basis of 3J HH coupling. The final chemical
shift assignments were determined by combined use of
the 2-D NOESY spectra and the calculated hydrogen
positions from the X-ray structure (Figure 2). The 13C
NMR spectrum of 1 consisted of one resonance for the
Me-Ga groups, two resonances for the methylene
carbons, and ten resonances for the aromatic carbons
as expected from the symmetry of the structure. The
13C NMR chemical shift assignments were carried out
(18) Schmidbaur, H.; Wolfsberger, W. J . Organomet. Chem. 1969,
16, 188.
(19) Eisch, J . J .; Kaska, W. C. J . Am. Chem. Soc. 1962, 84, 1501.
(20) Eisch, J . J .; Kaska, W. C. J . Am. Chem. Soc. 1966, 88, 2976.
(21) Waggoner, K. M.; Olmstead, M. M.; Power, P. P. Polyhedron
1990, 9, 257.
(22) Lee, B.; Pennington, W. T.; Robinson, G. H. Inorg. Chim. Acta
1991, 190, 173.
using 2-D 13C{1H} heteronuclear correlated J CH and
(23) Hoberg, H. Liebigs Ann. Chem. 1971, 746, 86.
1

5644 Organometallics, Vol. 15, No. 26, 1996
Schauer et al.
Syn th esis of [Me2Ga N(CH2P h )2]2. A solution of Me2GaCl
(2.00 g, 14.8 mmol) in hexane (20 mL) was slowly added to a
50 mL hexane solution of LiN(CH2Ph)2 (14.8 mmol, previously
prepared from the reaction of H(NCH2Ph)2 and LiBun) at -40
°C. LiCl precipitated from the solution upon warming the
mixture to room temperature. After the mixture was stirred
overnight at room temperature, the solution was decanted and
the remaining solids were washed with hexane. The hexane
was then removed in vacuo, leaving a white solid, which was
purified by recrystallization from toluene at -15 °C. A 78%
yield was obtained. Mp: 135-137 °C. 1H NMR (δ): 0.21 (s,
12 H, GaCH3); 4.15 (s, 8 H, NCH2); 6.94 (m, 8H, H-2), 7.01
(m, 4H, H-4), 7.03 (m, 8H, H-3) (NCH2C6H5). 13C NMR (δ):
-5.91 (GaCH3), 54.35 (NCH2), 127.03, C-4; 128.06, C-3; 129.14,
C-2; 137.64, C-1 (NCH2C6H5). IR (cm-1): 3086 (m), 3060 (m),
3028 (s), 2958 (s), 2912 (s), 2861 (m), 1306 (w), 1209 (m), 1153
(w), 1086 (w), 1064 (m), 1032 (w), 1018 (w), 964 (w), 952 (w),
907 (m), 882 (m), 738 (vs), 697 (s), 665 (w), 813 (m), 571 (m),
530 (w), 512 (w), 492 (w), 474 (m). Cryoscopic molecular
weight: calcd, Mr 592.11; obsd Mr (molality), 563 (0.095). Anal.
Calcd (found) for C32H40Ga2N2: C, 64.91 (63.74); H, 6.81 (6.77);
N, 4.73 (4.78).
Exp er im en ta l Section
Gen er a l P r oced u r es. Standard inert-atmosphere tech-
niques were used for the synthesis and manipulation of all
compounds using a high-vacuum line in conjunction with an
inert-atmosphere drybox (Vacuum Atmospheres HE-43 DriLab).
Toluene was distilled under an atmosphere of nitrogen over
calcium hydride. HN(CH2Ph)2 (Aldrich) was dried over KOH
and distilled under vacuum prior to use. Me3Ga was obtained
as a gift from Morton Advanced Materials, Danvers, MA.
Benzene-d6 (Isotec) was stored over molecular sieves.
1H and 13C NMR spectral data were collected on a Bruker
DRX 400 NMR spectrometer using a 0.2 M C6D6 solution of
the compound and were referenced with respect to the solvent.
Hydrogen and carbon atoms are labeled according to Figure
1. 2-D H-1H COSY, 1H-1H NOESY, 13C{1H} HETCOR, and
1
long-range HETCOR NMR spectra were obtained using stan-
dard Bruker programs. The NOESY spectra were obtained
as a function of mixing time to eliminate any transferred NOE
effects. FTIR data were collected using a Bruker Vector 22
FT spectrometer using samples prepared as Kel-F (halocarbon)
and Nujol mulls on KBr plates. Elemental analysis was
performed by E+R Microanalytical laboratory, Inc., Corona,
NY.
X-r a y Da ta Collection a n d Str u ctu r e Refin em en t. A
crystal of 1 (0.30 × 0.25 × 0.25 mm) was sealed into a thin-
walled capillary under anerobic conditions. Data were col-
lected at 296 K on an Enraf-Nonius CAD4 diffractometer using
graphite-monochromated Mo KR radiation. Details of the data
collection are listed in Table 1. The structure was solved by
a Patterson Synthesis. All crystallographic calculations were
accomplished with the Siemens SHELXTL-PC program pack-
age.26 All positional and anisotropic thermal parameters were
refined for all non-hydrogen atoms. Hydrogen atoms were
placed in calculated positions with the appropriate geometry.
Hydrogen atoms were placed in calculated positions with the
appropriate geometry. The Ueq of each hydrogen atom was
set equal to that of the carbon atom to which it was bound.
Syn th esis of [MeGa N(CH2C6H4)(CH2P h )]2. A 2.004 g
(17.5 mmol) amount of Me3Ga, 3.444 g (17.5 mmol) of HN(CH2-
Ph)2, and 30 mL of toluene were mixed at room temperature.
The mixture was heated to reflux, and the progress of the
reaction was monitored on aliquots over a 5 day period using
1H and 13C NMR spectroscopy. 1 was isolated from the
reaction solution in 65% yield by recrystallization at -15 °C.
Mp: 193-195 °C. 1H NMR: δ 0.18 (s, 3 H, H1), 3.56 (d, 1 H,
J ) 15.2 Hz, H2c), 3.83 (d, 1 H, J ) 16.2 Hz, H9a), 4.22 (d, 1
H, J ) 15.2 Hz, H2d), 4.58 (d, 1 H, J ) 16.2 Hz, H9b), 7.07 (d,
1 H, H11), 7.16 (t, 1 H, H13), 7.25 (m, 1 H, H6), 7.26 (m, 1 H,
H12), 7.32 (m, 2 H, H4 and H8), 7.35 (m, 2 H, H5 and H7),
7.66 (d, 1 H, H14). 13C NMR: δ -10.8 (C1), 58.2 (C2), 59.6
(C9), 124.2 (C11), 126.6 (C13), 127.5 (C6), 127.8 (C4 and C8),
128.6 (C12), 128.8 (C5 and C7), 135.5 (C14), 140.6 (C3), 143.9
(C15), 149.2 (C10). IR (cm-1): 3040 (m), 2919 (vs), 2840 (s),
1485 (w), 1443 (m), 1428 (m), 1352 (m), 1299 (w), 1251 (w),
1231 (w), 1195 (m), 1150 (w), 1089 (s), 1069 (s), 1045 (m), 1020
(m), 956 (w), 931 (m), 886 (w), 822 (w), 726 (s), 682 (s), 660
(m). Anal. Calcd (found) for C30H32Ga2N2: C, 64.34 (64.68);
H, 5.76 (5.92); N, 5.00 (4.98).
Su p p or tin g In for m a tion Ava ila ble: Tables listing po-
sitional and thermal parameters and complete bond distances
and angles and 1H and 13C 1-D and 2-D NMR spectra (11
pages). Ordering information is given on any current mast-
head page.
OM9604381
(24) Hitchcock, P. B.; J asim, H. A.; Lappert, M. F.; Williams, H. D.
Polyhedron 1990, 9, 245.
(25) Dillingham, M. D. B.; Hill, J . B.; Lee, B.; Schauer, S. J .;
Pennington, W. T.; Robinson, G. H. J . Coord. Chem. 1993, 28, 337.
(26) Siemens SHELXTL-PC Manual, Release 4.1; Siemens Analyti-
cal Instruments: Madison, WI, 1990.