Insights into the thermal fragmentation of intramolecularly coordinated gallanes. A matrix-isolation FTIR study

Article abstract of DOI:10.1021/ic035496a

High-vacuum thermolyses (400-1000 °C) of the intramolecularly coordinated gallanes [Me₂N(CH₂)₃] ₂GaX [X = Cl (1), Br (2)] and (Me₂NCH₂CD ₂CH₂)₂GaBr (2D₄) have been investigated with matrix-isolation FTIR techniques. Around 850 °C, the fragmentation of these compounds was completed, and their typical IR absorptions could no longer be detected in argon matrices. Products have been identified with IR spectroscopy with the help of ab initio and DFT calculations and known literature data. In the case of precursors 1 and 2, respectively, we identified GaH₃, XGaH₂, X₂GaH, GaH, GaX, HX, HCN, CH ₄, H₂C=CH₂, H₂C=CHCH₃, H₂C=NMe, [H₂CCHCH₂]^., and H ₂C=CHCH₂NMe₂. In the case of the precursor 2D₄, we found GaH₃, GaH₂D, BrGaH₂, BrGaDH, GaH, GaD, GaBr, HBr, HCN, CH₄, H₂C=CD₂, H₂C=CDCH₃, H₂C=NMe, [H₂CCDCH ₂]^., and H₂C=CDCH₂NMe₂. Among the monomedc hydrides, BrGaH₂ and BrGaDH were isolated for the first time. These compounds have been characterized by their six normal modes, and the experimental frequencies have been compared to the results of MP2(fc)/6-311+G(2d,p) and B3LYP/6-31 1+G(2d,p) calculations. These results have been interpreted by assuming that the intramolecularly coordinated precursors 1, 2, and 2D₄ eliminate their ligands through Ga-C homolysis and β-hydrogen elimination.

Full text of DOI:10.1021/ic035496a

Inorg. Chem. 2004, 43, 3955−3964
Insights into the Thermal Fragmentation of Intramolecularly Coordinated
Gallanes. A Matrix-Isolation FTIR Study
,‡
Eliza Gemel† and Jens Mu1ller*
Department of Chemistry, UniVersity of Saskatchewan, 110 Science Place, Saskatoon,
Saskatchewan, S7N 5C9 Canada, and Lehrstuhl fu¨r Anorganische Chemie II,
Organometallics and Materials Chemistry, Ruhr-UniVersita¨t Bochum, 44780 Bochum, Germany
Received December 30, 2003
High-vacuum thermolyses (400−1000 °C) of the intramolecularly coordinated gallanes [Me2N(CH ) ] GaX [X ) Cl
2 3 2
(1), Br (2)] and (Me2NCH CD CH ) GaBr (2D ) have been investigated with matrix-isolation FTIR techniques. Around
2
2
2 2
4
850 °C, the fragmentation of these compounds was completed, and their typical IR absorptions could no longer be
detected in argon matrices. Products have been identified with IR spectroscopy with the help of ab initio and DFT
calculations and known literature data. In the case of precursors 1 and 2, respectively, we identified GaH , XGaH ,
3
2
X GaH, GaH, GaX, HX, HCN, CH , H CdCH , H CdCHCH , H CdNMe, [H CCHCH ]•, and H CdCHCH NMe2.
2
4
2
2
2
3
2
2
2
2
2
In the case of the precursor 2D , we found GaH , GaH D, BrGaH , BrGaDH, GaH, GaD, GaBr, HBr, HCN, CH ,
4
3
2
2
4
H CdCD , H CdCDCH , H CdNMe, [H CCDCH ]•, and H CdCDCH NMe2. Among the monomeric hydrides,
2
2
2
3
2
2
2
2
2
BrGaH and BrGaDH were isolated for the first time. These compounds have been characterized by their six
2
normal modes, and the experimental frequencies have been compared to the results of MP2(fc)/6-311+G(2d,p)
and B3LYP/6-311+G(2d,p) calculations. These results have been interpreted by assuming that the intramolecularly
coordinated precursors 1, 2, and 2D eliminate their ligands through Ga−C homolysis and â-hydrogen elimination.
4
Introduction
Al,10,11 Ga,12 In13,14) had been intensively used by R. A.
Fischer et al. for nitride deposition.8 To date, detailed
molecular fragmentation processes are unknown for the
majority of CVD precursors. However, this lack of knowl-
edge prevents a rational design of new precursors for
improved material properties. Recently, in a series of matrix-
isolation thermolysis experiments, we investigated the frag-
mentation of precursors of the type Me2N(CH2)3GaX2 (X )
Cl, Br, Me, N3).15-19 We have shown that the diazido gallane
The interest in group 13 nitrides AlN, GaN, InN, and Alx-
GayInzN as materials for various applications including light-
emitting devices1-7 has prompted research groups to find
single-source precursors (SSPs) for the chemical vapor
deposition (CVD) of these materials. From a chemical point
of view, azides might be ideal SSPs because the azido group
acts as an intrinsic N source by elimination of N2 as a
harmless product.8 Among the known azides of the heavier
group 13 elements, the intramolecularly coordinated precur-
(9) Wohlfart, A.; Devi, A.; Maile, E.; Fischer, R. A. Chem. Commun.
2002, 998-999.
(10) Fischer, R. A.; Miehr, A.; Sussek, H.; Pritzkow, H.; Herdtweck, E.;
Mu¨ller, J.; Ambacher, O.; Metzger, T. Chem. Commun. 1996, 2685-
2686.
(11) Mu¨ller, J.; Fischer, R. A.; Sussek, H.; Pilgram, P.; Wang, R.; Pritzkow,
H.; Herdtweck, E. Organometallics 1998, 17, 161-166.
(12) Devi, A.; Sussek, H.; Pritzkow, H.; Winter, M.; Fischer, R. A. Eur. J.
Inorg. Chem. 1999, 2127-2134.
9
sors Me2N(CH2)3Ga(N3)2 and [Me2N(CH2)3]2MN3 (M )
* To whom correspondence should be addressed. E-mail: jens.mueller@
usask.ca.
† Ruhr-Universita¨t Bochum.
‡ University of Saskatchewan.
(1) Jones, A. C.; Whitehouse, C. R.; Roberts, J. S. Chem. Vap. Deposition
1995, 1, 65-74.
(2) Akasaki, I.; Amano, H. J. Cryst. Growth 1995, 146, 455-461.
(3) Neumayer, D. A.; Ekerdt, J. G. Chem. Mater. 1996, 8, 9-25.
(4) Matsuoka, T. AdV. Mater. 1996, 8, 496-479.
(5) Ponce, F. A.; Bour, D. P. Nature 1997, 386, 351-359.
(6) Nakamura, S.; Fasol, G. The Blue Laser Diode, GaN Based Light
Emitters and Lasers; Springer: Berlin, 1997.
(7) Nakamura, S. Science 1998, 281, 951-961.
(8) Mu¨ller, J. Coord. Chem. ReV. 2002, 235, 105-119.
(13) Fischer, R. A.; Miehr, A.; Metzger, T.; Born, E.; Ambacher, O.;
Angerer, H.; Dimitrov, R. Chem. Mater. 1996, 8, 1356-1359.
(14) Parala, H.; Devi, A.; Hipler, F.; Birkner, A.; Fischer, R. A. Proc.s
Electrochem. Soc. 2001, 13, 356-364.
(15) Mu¨ller, J.; Sternkicker, H. J. Chem. Soc., Dalton Trans. 1999, 4149-
4153.
(16) Mu¨ller, J.; Sternkicker, H.; Bergmann, U.; Atakan, B. J. Phys. Chem.
A 2000, 104, 3627-3634.
10.1021/ic035496a CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/02/2004
Inorganic Chemistry, Vol. 43, No. 13, 2004 3955

Gemel and Mu1ller
Chart 1
[Me2NCH2CD2CH2]2GaBr (2D4). In accordance with the method
described for 2, compound 2D4 was synthesized. GaBr3 (1.16 g,
3.75 mmol) and LiCH2CD2CH2NMe2 (0.75 g, 8.06 mmol) gave
analytically pure 2D4 (0.86 g, 2.67 mmol, 71%). 1H NMR (400.13
MHz, C6D6): δ 0.60 (s, br, 4H, GaCH2), 1.74 (s, br, 2H, NCHH),
1.96 (s, 12H, NCH3), 2.46 (s, br, 2H, NCHH). 13C{1H } (100.62
MHz, C6D6), δ 9.60 (GaCH2), 45.50 (NCH3), 60.50 (NCH2). MS
(70 eV): m/z (%) ) 326 (6) [M+], 245 (5) [M+ - Br], 238 (12)
[M+ - H2CCD2CH2N], 88 (20) [H2CCD2CH2N+], 58 (100) [H2-
CN(CH3)2+]. Anal. Calcd for C10H20D4N2GaBr (325.94): C, 36.85;
N, 8.59. Found: C, 36.98; N, 8.81.
Matrix Isolation. The matrix apparatus consists of a vacuum
line (Leybold Turbovac 151; Leybold Trivac D4B) and a Displex
CSW 202 cryogenic closed-cycle system (ADP Cryogenics Inc.)
fitted with CsI windows. Compounds were kept in a small heatable
metal container connected to the matrix apparatus. In a typical
experiment, the starting compound was connected to high vacuum
at constant temperatures (57-70 °C for 1; 75-85 °C for 2 and
2D4) while a flow of argon was conducted over the sample (Linde
6.0; 1.25 sccm). This gaseous mixture was passed through a
thermolysis oven and deposited onto the CsI window at 15 K for
45-90 min. The oven was fitted with an Al2O3 tube with two
parallel inner canals (outer diameter 4 mm; inner diameter 1 mm;
the last 15 mm heated with tungsten wire); one of the inner canals
was equipped with a thermocouple (Thermocoax: NiCrSi/NiSi),
and the second one was used for the argon/precursor mixture. The
hot end of the pyrolysis tube was 25 mm away from the cooled
CsI window to ensure that maximum amounts of volatile fragments
emerging from the oven were trapped in matrices. To ensure that
the results are reproducible, we performed four extensive series of
thermolysis experiments; first series with 1, second with 2, third
with 2D4, followed by a last series with precursor 2 again.
After a deposition onto a CsI window at 15 K, the respective
matrix was cooled to 10-11 K for IR measurements. IR spectra of
the matrices were recorded on a Bruker EQUINOX 55 in the range
4000-200 cm-1 with a resolution of 0.5 and 1.0 cm-1 (KBr beam
splitter for 4000-400 cm-1 and Mylar beam splitter for the far-IR
region from 700 to 200 cm-1).
Measured IR frequencies of matrix-isolated species are
indicated as follows: O×O, BrGaH2 (see Table 1); O×b, BrGaDH
(see Table 2); O×O, ClGaH2 (1978.3, 1965.4 cm-1); ×O×, Br2-
GaH (1991.7, 596.7, 445.3 cm-1); OOO, GaH3 (1923.0, 758.5,
717.3 cm-1); ObO, GaDH2 (1923.3, 1921.1, 1385.6, 756.8, 660.5,
626.1 cm-1); O, GaH (1514.0 cm-1); b, GaD, 1090.6 cm-1; a,
H2CdNCH3 (1659.1, 1469.5, 1440.3, 1220.6, 1026.3, 478.4 cm-1);
b, H2CdCH2 (1440.3, 947.4 cm-1); b′, CH2dCD2 (945.8, 749.9
cm-1); c, CH4 (1305.7 cm-1); d, [H2CCHCH2]• (1388.7, 983.5,
801.1 cm-1); d′, [H2CCDCH2]• (803.0, 786.6 cm-1); e, H2Cd
CHCH3 (998.0, 908.6, 578.3 cm-1; tentative assignment, see text);
e′, H2CdCDCH3 (908.6 cm -1; tentative assignment, see text); f,
H2CdCHCH2NMe2 (1417.4, 1350.1, 1266.8, 1181.4, 1173.2,
1096.4, 1043.7, 1038.4, 1000.5, 963.2, 922.6, 920.2, 918.0, 854.9,
850.0 cm-1); f ′, H2CdCDCH2NMe2 (1349.7, 1181.4, 1096.2,
1043.7, 1038.4, 1033.4, 922.7, 919.8 cm-1); g, HCN (3306.0, 720.8
cm-1); X, Me2N(CH2)3GaBrH (max at 1902.2 cm-1); XD, Me2-
NCH2CD2CH2GaBrH (max at 1902.2 cm-1); Y, Me2N(CH2)3GaClH
(max at 1904.1 cm-1).
does not fragment via â-hydrogen elimination and the
detected thermolysis products could be explained by assum-
ing a homolysis of the Ga-C bond in the initial stages of
the process.18 Interestingly, we identified monomeric GaN3
as a thermolysis product which might be a key intermediate
of the nitride deposition with Me2N(CH2)3Ga(N3)2.17
In the present paper we report on first insights into the
fragmentation of organometallic gallanes that are equipped
with two dimethylaminopropyl ligands. This might be a first
step toward a better understanding of the pyrolysis behavior
of azide SSPs of the general formula [Me2N(CH2)3]2MN3
(M ) Al, Ga, In) (Chart 1).
Experimental Section
General Remarks. All manipulations were performed under an
inert atmosphere of purified argon using Schlenk techniques. The
solvents were dried by standard procedures, distilled, and stored
under argon and molecular sieves (4 Å). NMR spectra were obtained
on a Bruker Advance DRX 400 (ambient temp; 400.13 and 100.62
MHz for 1H and 13C, respectively) calibrated against residual protons
of the deuterated solvents. 1H and 13C chemical shifts are reported
relative to TMS. Elemental analyses (C, H, N) were measured on
a Carlo Erba EA 1110 CHNS-O. MS data were obtained on a
Varian MAT CH5 (70 eV). The reagents Li(CH2)3N(CH3)2,20
LiCH2CD2CH2NMe2,19 and [Me2N(CH2)3]2GaCl (1)21 were prepared
according to the literature procedures and analyzed by standard
methods. Compound 1 (75-80 °C) and GaBr3 (80-90 °C) were
sublimed in a vacuum (10-2-10-3 mbar) prior to use.
[Me2N(CH2)3]2GaBr (2). To a solution of GaBr3 (1.60 g, 5.17
mmol) in Et2O (30 mL) and toluene (30 mL) Li(CH2)3NMe2 (1.01
g, 10.85 mmol) was added at -78 °C (acetone/dry ice bath).
Subsequently, the cooling bath was removed and the reaction
mixture stirred for 12 h. All at ambient temperature, volatile
components of the colorless clear solution were removed in a
vacuum, toluene (20 mL) was added to the remaining solid, and
the solution was separated by filtration from the colorless precipi-
tate.Toluene was removed in a vacuum at ambient temperature to
leave 1.80 g of a crude material behind. After two sublimations,
1
analytically pure 2 was obtained (1.42 g, 4.41 mmol, 85%). H
NMR (400.13 MHz, C6D6): δ 0.62 (s, br, 4H, GaCH2), 1.55 (s,
br, 2H, GaCH2CHH), 1.75 (s, br, 4H, GaCH2CHH and NCHH),
1.96 (s, 12H, NCH3), 2.47 (s, br, 2H, NCHH). 13C{1H} (100.62
MHz, C6D6): δ 9.92 (GaCH2), 22.36 (GaCH2CH2), 45.46 (NCH3),
60.66 (NCH2). MS (70 eV): m/z (%) ) 322 (6) [M+], 241 (10)
[M+ - Br], 236 (22) [M+ - (CH2)3N], 86 (26) (C5H12N+), 58 (100)
[H2CN(CH3)2+]. Anal. Calcd for C10H24N2GaBr (321.94): H, 7.51;
C, 37.31; N, 8.70. Found: H, 7.20; C, 36.91; N, 8.63.
(17) Mu¨ller, J.; Bendix, S. Chem. Commun. 2001, 911-912.
(18) Mu¨ller, J.; Wittig, B.; Sternkicker, H.; Bendix, S. J. Phys. IV 2001,
11, 17-22.
(19) Mu¨ller, J.; Wittig, B.; Bendix, S. J. Phys. Chem. A 2001, 105, 2112-
2116.
(20) Thiele, K.-H.; Langguth, E.; Mu¨ller, G. E. Z. Anorg. Allg. Chem. 1980,
462, 152-158.
(21) Schumann, H.; Seuss, T. D.; Just, O.; Weimann, R.; Hemling, H.;
Go¨rlitz, F. H. J. Organomet. Chem. 1994, 479, 171-186.
Theoretical Calculations. All calculations have been carried out
using the GAUSSIAN 98 program package.22 All geometry
optimizations have been performed without geometrical constraints.
As a hybrid HF-DFT method, the three-parameter exchange
functional of Becke23 with the correlation functional by Lee, Yang,
and Parr24 has been used (B3LYP). All MP2 calculations have
3956 Inorganic Chemistry, Vol. 43, No. 13, 2004

Thermal Fragmentation of Gallanes
Table 1. Observed IR Frequencies and Calculated Harmonic Frequencies of ClGaH2 and BrGaH2
MP2(fc)/6-311+G(2d,p)
B3LYP/6-311+G(2d,p)
freqa [cm-1 int [km mol-1
]
exptl
obsd/calcd
freq ratio
obsd/calcd
freq ratio
freqa [cm-1
]
freqa [cm-1
]
int [km mol-1
]
]
ClGaH2
75.8
212.4
79.0
213.7
47.2
134.2
A1
ν1
ν2
ν3
ν4
ν5
ν6
νs(GaH2)
δ(GaH2)
ν(GaCl)
νas(GaH2)
δ(HGaCl)
γ(H)
1964.9
731.4
407.2
1978.2
510.0
619.9
2060.2
751.1
421.5
2065.4
499.4
624.4
0.9537
0.9738
0.9661
0.9578
1.0212
0.9928
2026.4
725.7
399.8
2043.2
495.2
620.0
59.6
155.9
69.4
192.9
35.9
0.9697
1.0079
1.0185
0.9682
1.0299
0.9998
B1
B2
87.0
BrGaH2
93.7
252.2
40.8
205.8
32.9
116.7
A1
ν1
ν2
ν3
ν4
ν5
ν6
νs(GaH2)
δ(GaH2)
ν(GaBr)b
νas(GaH2)
δ(HGaBr)
γ(H)
1958.3
725.5
297.7
1973.2
494.8
609.6
2056.7
750.7
302.6
2063.3
489.0
616.0
0.9521
0.9664
0.9838
0.9563
1.0119
0.9896
2021.2
722.5
291.6
2039.7
479.6
604.7
72.6
201.0
37.4
186.3
24.3
0.9689
1.0042
1.0209
0.9674
1.0317
1.0081
B1
B2
75.4
a Listed are wavenumbers concerning the main isotopomer. b ν(GaBr) shows an isotopic pattern (see Figure 2).
Table 2. Observed IR Frequencies and Calculated Harmonic Frequencies of BrGaDH
MP2(fc)/6-311+G(2d,p)
B3LYP/6-311+G(2d,p)
exptl
obsd/calcd
freq ratio
obsd/calcd
freq ratio
freq [cm-1
]
freq [cm-1
]
int [km mol-1
]
freq [cm-1
]
int [km mol-1
]
BrGaDH
153.6
191.9
39.1
76.1
28.2
A′
ν1
ν2
ν3
ν4
ν5
ν6
νs(GaH)
δ(GaHD)
ν(GaBr)
ν(GaD)
δ(HGaBr)
γ(HD)
1966.7
654.1
297.6
1415.1
393.9
534.7
2059.8
673.7
302.1
1467.4
391.4
537.8
0.9548
0.9709
0.9851
0.9644
1.0064
0.9942
2030.3
649.5
291.2
1446.3
382.8
528.0
133.6
152.1
36.0
64.8
21.7
58.2
0.9687
1.0071
1.0220
0.9784
1.0290
1.0127
A”
89.9
been performed with the frozen core (fc) approximation. Analysis
of the vibrational frequencies at all applied levels of theory
confirmed that respective molecules correspond to local minima at
a potential energy surface. The absolute energies in Hartrees and
zero point vibrational energies, ZPVE, in kJ/mol (values in paren-
theses) are as follows: ClGaH2, -2386.336575 (37.74) [B3LYP/
6-311+G(2d,p)], -2383.924573 (38.41) [MP2(fc)/6-311+G(2d,p)];
BrGaH2,-4500.253451(36.84)[B3LYP/6-311+G(2d,p)],-4496.781171
(37.55) [MP2(fc)/6-311+G(2d,p)]; Me2N(CH2)3GaHCl, isomer
A/isomer B (Figure 6), -2632.150653 (500.96)/-2632.149958
(501.15) [HF/6-31G(d)], -2636.321693 (469.33)/-2636.320868
(469.36) [B3LYP/6-31G(d)], -2638.346768 (463.93), -2638.346042
(463.62) [B3LYP/6-311G(d,p)]; Me2N(CH2)3GaHBr, isomer A/iso-
mer B (Figure 6), -4742.572371 (500.35)/-4742.571812 (500.66)
[HF/6-31G(d)], -4747.842113 (469.08)/-4747.841438 (468.91)
[B3LYP/6-31G(d)], -4752.268753 (463.02), -4752.268071 (462.97)
[B3LYP/6-311G(d,p)].
The B3LYP/6-311+G(2d,p) harmonic frequencies [cm-1] for
XGaH• are as follows (intensities [km/mol] in parentheses): ClGaH•
1675.5 (129.9), 509.9 (35.2), 373.4 (64.4); BrGaH• 1676.0 (135.8),
497.1 (31.6), 269.2 (35.3).
The isotopic pattern of Figure 2 was calculated on the B3LYP/
6-311+G(2d,p) level with the assumption of a Gauss line shape
profile. The calculated unscaled harmonic frequencies (cm-1) and
intensities (km/mol; in parentheses) for ν3 for the four main
isotopomers with natural abundances in percent are the following:
30.5% H269Ga79Br 291.57 (37.4), 29.9% H269Ga81Br 289.87 (36.9),
20.0% H271Ga79Br 289.41 (36.9), 19.6% H271Ga81Br 287.69 (36.4).
Results and Discussion
Thermolysis of [Me2N(CH2)3]2GaX [X ) Cl (1) and X
) Br (2)]. We carried out a series of high-vacuum ther-
molyses in the temperature range 400-1000 °C of com-
pounds 1 and 2, respectively. Around 850 °C, the fragmen-
tations of the starting compounds were completed, and their
typical IR absorptions could no longer be detected in argon
matrices.
The B3LYP/6-311+G(2d,p) harmonic frequencies [cm-1] for
BrGaD2 are as follows (intensities [km/mol] in parentheses): 1455.2
(98.6), 1432.8 (40.0), 514.6 (107.5), 437.8 (41.0), 346.6 (13.5),
290.9 (35.8).
Figure 1 shows a representative IR spectrum of the
intermediates of a thermolysis of 2 at 850 °C trapped in argon
at 15 K (Figure 1, spectrum A). The most informative area
of this rich spectrum is the gallium hydrogen stretching
region at the high-frequency end of spectrum A. In this range,
four sharp and intense bands at 1991.7, 1973.2, 1958.3, and
1923.0 cm-1 are present. The smallest one at 1991.7 cm-1
is certainly caused by Br2GaH, which exhibits two other IR
bands at 596.7 cm-1 [δ(BrGaH)] and at 445.5 cm-1 [γ(H)]
in the depicted spectrum A (indication ×O×).15 The IR band
at 1923.0 cm-1 can be assigned to the degenerated Ga-H
stretching modes of monomeric GaH3. Moreover, our
measured IR bands at 758.5 cm-1 [δas(GaH2)] and 717.3
(22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
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A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,
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Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
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L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.;
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Inorganic Chemistry, Vol. 43, No. 13, 2004 3957

Gemel and Mu1ller
Figure 1. Spectrum A shows the matrix-isolated products of a thermolysis
of 2 at 850 °C trapped in an excess of argon at 15 K (see Experimental
Section for details): O×O, BrGaH2; ×O×, Br2GaH; OOO, GaH3; O, GaH;
a, H2CdNCH3; b, H2CdCH2; c, CH4; d, [H2CCHCH2]•; e, H2CdCHCH3;
f, H2CdCHCH2NMe2; g, HCN. Spectrum B is the calculated IR spectrum
of BrGaH2 (O×O) at the B3LYP/6-311+G(2d,p) level (Table 1).
Figure 2. Observed (spectrum A) and calculated (spectrum B) isotopic
pattern of ν3 of BrGaH2 (scaling factor of 1.021; see Experimental Section,
last paragraph for details).
ness of the assignment for the hitherto unknown bromogal-
lane BrGaH2. Additional support comes from a distinct
isotopic pattern for the Ga-Br stretch ν3 which is caused
by two gallium and two bromine isotopes. Only this mode
is expected to give a detectable natural isotopic pattern within
our spectral resolution. Figure 2 compares the measured IR
band (spectrum A) with the predicted isotopic pattern. Even
though the intensity of this IR band is quite low, the exact
match of the detected and measured isotopic pattern shown
in Figure 2 proves the assignment to be correct.
cm-1 [γ(H)] match with the published data of this well-
known molecule (published values are 1923.2, 758.7, and
717.4 cm-1; Figure 1 indication OOO).25 We assigned two
remaining IR bands at 1973.2 and 1958.3 cm-1 to the
antisymmetric and symmetric Ga-H stretching modes of
monomeric BrGaH2, respectively. In the case of the chloride
compound 1, the Ga-H stretching modes absorb at 1978.2
and 1964.9 cm-1. Even without any calculations, there is no
doubt that monomeric ClGaH2 was formed in the pyrolysis
of the gallane 1, because ClGaH2 was photochemically
synthesized in argon matrices and had been characterized
by IR spectroscopy, ab initio calculations, and normal
coordinate analysis.26 Within the experimental error, our
measured frequencies as well as intensities of ClGaH2 match
with the published data.
Table 1 shows the experimental and calculated IR data of
ClGaH2 and BrGaH2. For the assignment of harmonic
frequencies we have chosen the MP2 and B3LYP method
with a 6-311+G(2d,p) basis set. The C2V symmetrical
gallanes ClGaH2 and BrGaH2 are expected to show six
normal modes belonging to the irreducible representations
A1(3), B1(2), and B2(1). We assigned all six normal modes
to measured IR bands for ClGaH2 and for the hitherto
unknown BrGaH2. A calculated IR spectrum of BrGaH2 is
depicted in Figure 1 to illustrate the assignment and the fit
between theory and experiment. This fit is also demonstrated
in Table 1 by observed/calculated frequency ratios which
were found in the ranges 0.9521-1.0212 (ClGaH2) and
0.9522-1.0119 (BrGaH2) at the MP2 level and in the ranges
0.9682-1.0299 (ClGaH2) and 0.9674-1.0317 (BrGaH2) at
the B3LYP level of theory (Table 1).
One of the most intense IR bands in Figure 1 appears at
1514.0 cm-1 (indication O). This absorption is present in
all measured thermolysis matrix spectra of 1 and 2, respec-
tively. Its relative intensity with respect to IR bands of other
molecules varies at different thermolysis temperatures, and
we could not identify other IR bands that showed the same
behavior. Furthermore, the small half-band-width of this IR
band hints to a small molecule. The reported stretching fre-
quency of monomeric GaH that was matrix isolated in Ar
(1514.1 cm-1,27 1513.8 cm-1 28) matches with our value, and
consequently, we assign the IR band at 1514.0 cm-1 to GaH.
In addition to gallium hydrides discussed so far, we have
identified organic and inorganic species resulting from the
decomposition of 1 and 2, respectively. These assignments
are based on known literature data including our experience
with fragmentations of compounds equipped with the di-
methylaminopropyl ligand. As it is indicated in Figure 1,
we identified H2CdNCH3,29,30 CH4,31 H2CdCH2,32 H2Cd
CHCH3,33 [H2CCHCH2]•,34 HCN,35,36 and Me2NCH2CHd
CH2.16 Furthermore, GaX (X ) Cl,37 Br 15) and HX (X )
Cl, Br)38-40 are among the thermolysis products, but their
typical IR bands are out of the range of spectrum A.
A comment is needed on the identification of propene,
H2CdCHCH3 (see Figure 1, indication e). We found three
IR bands at 998.0, 908.6, and 578.3 cm-1 that match well
As expected, similar frequency ratios are observed for
similar normal modes of ClGaH2 in comparison with BrGaH2
on both levels of theory. This strongly supports the correct-
(25) Pullumbi, P.; Bouteiller, Y.; Manceron, L.; Mijoule, C. Chem. Phys.
(27) Xiao, Z. L.; Hauge, R. H.; Margrave, J. L. Inorg. Chem. 1993, 32,
1994, 185, 25-37.
642-646.
(26) Ko¨ppe, R.; Schno¨ckel, H. J. Chem. Soc., Dalton Trans. 1992, 3393-
(28) Pullumbi, P.; Mijoule, C.; Manceron, L.; Bouteiller, Y. Chem. Phys.
1994, 185, 13-24.
3395.
3958 Inorganic Chemistry, Vol. 43, No. 13, 2004

Thermal Fragmentation of Gallanes
with published values of 997.8, 909.9, and 578.6 cm-1 33
for this species. However, the IR band at 998.0 cm-1 is in-
distinguishable from an IR absorption of allyldimethylamine,
and the IR band at 578.3 cm-1 is very weak (Figure 1). The
most intense and, hence, the most indicative IR band is the
one at 908.6 cm-1 that would correspond to a CH2 wagging
mode of propene. According to recent calculations41 the next
most intense modes for propene are CH stretches, but the
CH stretching region of the thermolysis experiments with 2
is highly populated by IR bands and an assignment would
be uncertain. That means that the identification of propene
is based on a tentative assignment of only two IR bands.
Thermolysis of (Me2NCH2CD2CH2)2GaBr (2D4). For a
deeper insight into the fragmentation processes, we per-
formed thermolysis studies with the deuterated gallane
(Me2NCH2CD2CH2)2GaBr (2D4). By using the selectively
deuterated donor ligand Me2NCH2CD2CH2, one can experi-
mentally determine if â-hydrogen elimination occurs. In a
former study, we were able to show that the dimethylgallane
Me2NCH2CD2CH2GaMe2 fragments mainly via a â-hydrogen
elimination path resulting in monomeric DGaMe2.19 As in
the case of the nondeuterated compound 2, we performed a
series of high-vacuum thermolyses with compound 2D4 in
the temperature range 400-1000 °C.
Figure 3. Experimental IR spectra of products of the thermolysis 2D4
(spectrum A) and 2 (spectrum B) at 850 °C, respectively, trapped in an
excess of argon at 15 K. Spectrum A: O×O, BrGaH2; O×b, BrGaDH;
ObO, GaDH2. Spectrum B: ×O×, Br2GaH; O×O, BrGaH2; OOO, GaH3.
This interaction is negligible for BrGaDH caused by a large
difference between atomic masses of H and D. Consequently,
BrGaDH is expected to show an “undisturbed” Ga-H
stretching motion. We have assigned all six fundamental
vibrations of BrGaDH to measured IR bands for this Cs
symmetrical molecule (5 × A′ and 1 × A′′). Figure 4 shows
an IR spectrum of matrix-isolated species from the ther-
molysis of 2D4 with O×b indicating the assignment for
BrGaDH. All experimental and calculated vibrational data
of BrGaDH are compiled in Table 2.
Gallium Hydrides and Deuterides. The results of a
thermolysis of 2D4 at 850 °C are illustrated in Figures 3 and
4. Figure 3 depicts the typical IR range for Ga-H stretches
and compares the results of 2D4 with those of the nondeu-
terated precursor 2.
From a first glance at the IR spectra of Figure 3, it is
obvious that the most intense IR bands of the thermolysis
of 2D4 are due to the formation of BrGaH2 (Figure 3A,
indication O×O). However, a new IR band at 1966.7 cm-1
appears right in the middle of the two IR bands of BrGaH2
(Figure 3A, indication O×b). The frequency and the fact
that it is only a single IR band in the typical Ga-H stretching
region can be interpreted as being caused by the monodeu-
terated species BrGaDH. The differences between two
frequencies of the asymmetric and symmetric Ga-H modes
of BrGaH2 are due to the interaction of the two H atoms.
As expected, similar vibrational modes of the two isoto-
pomers BrGaH2 and BrGaDH lead to similar obsd/calcd
frequency ratios (Tables 1 and 2). One particular interesting
mode is the Ga-D stretching mode which gives an experi-
mental isotopic shift of 1.390 if we compare the ν(GaH) with
the ν(GaD) of BrGaDH. The calculated value of 1.404 on
both levels of theory is slightly higher than the measured
one. A similar difference between predicted and measured
isotopic shifts was determined for the two isotopomers Me2-
GaH and Me2GaD: isotopic shifts of 1.387 (exptl) and of
1.404 [MP2(fc)/6-311+G(2d,p) level].15
The identification of BrGaH2 and BrGaDH in the ther-
molysis experiments with the selectively deuterated com-
pound 2D4 raises the question if BrGaD2 is among the
thermolysis products. On the basis of the predicted vibra-
tional spectrum of BrGaD2, we could not find experimental
hints for this species (see Experimental Section, last para-
graph, for details). Therefore, we assume that if the com-
pound is among the thermolysis products, the amount is
negligible low in comparison with BrGaDH.
Additional differences between the thermolysis experi-
ments with 2D4 and 2 can be seen in Figure 3. Fragmentation
of 2 results in one intense IR band at 1923.0 cm-1 (GaH3)
whereas 2D4 results in a “doublet” at 1923.5 and 1921.2
cm-1. These facts point to gallane with a reduced symmetry,
and they hint to GaDH2 (C2V symmetry). Fortunately, the
complete set of possible deuterides of the parent gallane
GaDH2, GaD2H, and GaD3 are known in argon matrices.25
The published vibrational frequencies and intensities of
(29) Hinze, J.; Curl, R. F., Jr. J. Am. Chem. Soc. 1964, 86, 5068-5070.
(30) Stolkin, I.; Ha, T.-K.; Gu¨nthard, H. H. Chem. Phys. 1977, 21, 327-
347.
(31) Huang, J. W.; Graham, W. R. M. J. Chem. Phys. 1990, 93, 1583-
1596.
(32) Andrews, L.; Johnson, G. L.; Kelsall, B. J. J. Chem. Phys. 1982, 76,
5767-5773.
(33) Andrews, L.; Johnson, G. L.; Kelsall, B. J. J. Am. Chem. Soc. 1982,
104, 6180-6186.
(34) Nandi, S.; Arnold, P. A.; Carpenter, B. K.; Nimlos, M. R.; Dayton,
D. C.; Ellison, G. B. J. Phys. Chem. A 2001, 105, 7514-7524.
(35) Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1963, 39, 712-715.
(36) Lanzisera, D. V.; Andrews, L. J. Phys. Chem. A 1997, 101, 824-
830.
(37) Samsonova, E. D.; Osin, S. B.; Shevel’kov, V. F. Russ. J. Inorg. Chem.
1988, 33, 1598.
(38) Bowers, M. T.; Flygare, W. H. J. Chem. Phys. 1966, 44, 1389-1404.
(39) Barnes, A. J.; Hallam, H. E.; Scrimshaw, G. F. Trans. Faraday Soc.
1969, 65, 3150-3158.
(40) our measured values are 2888.2, 2785.7, and 2701.6 cm-1 for (HCl)x;
2569.0, 2452.1, and 2431.9 cm-1 for (HBr)x.
(41) Radziszewski, J. G.; Downing, J. W.; Gudipati, M. S.; Balaji, V.;
Thulstrup, E. W.; Michl, J. J. Am. Chem. Soc. 1996, 118, 10275-
10284.
Inorganic Chemistry, Vol. 43, No. 13, 2004 3959

Gemel and Mu1ller
Figure 4. Products of a thermolysis of 2D4 at 850 °C trapped in an excess of argon at 15 K (see Experimental Section for details): O×O, BrGaH2; O,
GaH; O×b, BrGaDH; b, GaD; ObO, GaDH2; a, H2CdNCH3; b′, CH2dCD2; c, CH4; d′, [H2CCDCH2]•; e′, H2CdCDCH3; f ′, H2CdCDCH2NMe2; g,
HCN.
GaDH2 (1924.0, 1921.3, 1385.4, 756.9, 660.6, and 626.3
cm-1) match very well with our assignments for GaDH2
(1923.5, 1921.2, 1385.6, 756.8, 660.6, and 626.1 cm-1
(Figures 3 and 4; indication ObO). Very weak IR bands at
758.5 and 717.4 cm-1 indicate the presence of small amounts
of GaH3. However, the absorption of ν(GaH) of GaH3 is
obscured by the IR band at 1923.5 cm-1 assigned to GaDH2.
As in the case of the thermolysis of compound 2, the
fragmentation of the deuterated precursor 2D4 results in the
formation of GaH (Figure 4, indication O). Furthermore, for
all thermolysis experiments an intense IR band at 1090.6
cm-1 appears in the IR spectra. We assign the intense band
at 1090.6 cm-1 to GaD. Our detected frequency matches very
well with the published value for GaD (1090.3 cm-1 27).
Interestingly, the IR band of GaD is more intense than the
one of GaH (Figure 4; indications b and O). The relative
amounts of GaD/GaH would be ca. 2.8:1.0 (Figure 4; peak
heights have been taken as a measure of intensity), if one
takes the calculated transition moments of 426 km/mol for
GaD (1113.5 cm-1) and 839 km/mol for GaH (1562.9 cm-1)
into account (B3LYP/6-311+G(2d,p) level22). However, the
GaD/GaH ratio depends on the applied thermolysis temper-
ature: with increasing temperature the relative amount of
GaD strongly decreases.42 So far, this reproducible effect is
not understood.
of 2D4 can be traced back to deuterated organic molecules.
The two most intense new IR absorptions were found at
945.8 and 749.9 cm-1 and match well with the published
values of H2CdCD2 (945 and 749 cm-1).32 A thorough study
of the matrix-isolated allyl radicals [H2CCHCH2]•, [H2-
CCDCH2]•, and [D2CCDCD2]• with polarized IR absorption
spectroscopy have been published recently.34 This paper also
provides an overview of the previous investigations of allyl
radicals. The published value of the most intense IR band
of [H2CCHCH2]• is 801 cm-1 [ν(CH2)wag] followed by the
second intense mode at 983 cm-1 [ν(CH)oop].34 As mentioned
before, these data are in excellent agreement with the IR
bands at 801.1 and 983.6 cm-1 that we assigned to [H2-
CCHCH2]• in the thermolysis experiments of precursor 2;
these IR bands are absent in the case of the precursor 2D4.
Instead, two absorptions at 803.0 and 786.6 cm-1 appear in
this region (Figure 4, indication d′). We assign the IR band
at 803.0 cm-1 to ν(CH2)wag and the lower frequency mode
to the ν(CD)oop of [H2CCDCH2]•. These assignments are
based on the published data for this monodeuterated spe-
cies: the two most intense IR bands of this compound were
detected at 802 and 787 cm-1 with relative intensities of 1.00:
0.71.34
In a former thermolysis matrix-isolation study, we have
shown that Me2NCH2CD2CH2GaMe2 fragments to give
monomeric DGaMe2 and H2CdCDCH2NMe2. The mono-
deuterated allyldimethylamine is a relatively weak IR
absorber, but because Me2NCH2CD2CH2GaMe2 mainly frag-
ments via â-hydrogen elimination, we could clearly identify
the most intense IR bands of this species, i.e., two bands at
2819.8 and 2779.6 cm-1, a “triplet” at 1043.7, 1038.4, and
1033.4 cm-1, and a “doublet” at 922.7 and 919.8 cm-1. In
thermolysis experiments with 2D4, the CH stretching region
and the area around 1040 cm-1 are very populated with IR
bands, and an assignment of IR bands to H2CdCDCH2NMe2
is possible but uncertain. However, a “doublet” at 922.8 and
919.7 cm-1 gives a clear hint to the presence of the
monodeuterated amine (Figure 4, indication f ′).
Carbon-Containing Species and Further Products. The
identification of products resulting from fragmentation of the
chelating ligands gives important mechanistic insights into
the thermolysis process. A comparison between the spectra
resulting from compound 2 with those from 2D4 directly
shows similarities and dissimilarities. Fragmentation of 2D4
resulted in CH4, HCN, and H2CdNCH3, but H2CdCH2, [H2-
CCHCH2]•, and Me2NCH2CHdCH2 could not be detected.
On the other hand, new IR bands in the thermolysis spectra
(42) In a continuous series of experiments, the following IR values were
measured in absorbance units: 0.099 (GaD):0.078 (GaH) at 850 °C,
0.044 (GaD):0.068 (GaH) at 900 °C, and 0.024 (GaD):0.045 (GaH)
at 1000 °C.
3960 Inorganic Chemistry, Vol. 43, No. 13, 2004

Thermal Fragmentation of Gallanes
As mentioned before, we tentatively assigned three IR
bands at 998.0, 908.6, and 578.3 cm-1 in the thermolysis
experiments of 2 to H2CdCHCH3 (Figure 1, indication e).
In the thermolysis experiments with 2D4, a weak IR band
was again detected at 908.6 cm-1. The value and the shape
of this IR band matches exactly with the one that we assigned
in experiments with 2 to H2CdCHCH3. From a chemical
point of view, the monodeuterated propene H2CdCDCH3
is expected as a pyrolysis product of 2D4. To the best of our
knowledge, an IR spectrum of matrix-isolated H2CdCDCH3
is not available in the literature. However, in the course of
an investigation of transition moment directions, the IR
spectra of propene and some of its isotopomers had been
determined in stretched polyethylene at 12 K.41 Both
isotopomers, H2CdCHCH3 and H2CdCDCH3, exhibit their
most intense IR band at 908 cm-1. This is in accordance
with CCSD/6-311G(d,p) calculations that were discussed in
the same publication.41 Against this background, it is possible
that the IR band at 908.6 cm-1 in the spectrum of Figure 4
is due to the presence of H2CdCDCH3, but this only a
tentative assignment.
Figure 5. Formation of intermediates at thermolysis temperatures of
750 °C (thermolysis products trapped in an excess of argon at 15 K;
y-axis in arbitrary units; see Experimental Section for details). Spectrum
A shows matrix-isolated products of a thermolysis of 1: O×O, ClGaH2;
OOO, GaH3; Y, Me2N(CH2)3GaClH. Spectrum B shows matrix-isolated
products of a thermolysis of 2: O×O, BrGaH2; OOO, GaH3; X, Me2N-
(CH2)3GaBrH. Spectrum C shows matrix-isolated products of a thermolysis
of 2D4: O×O, BrGaH2; O×b, BrGaDH; ObO, GaDH2; XD, Me2NCH2-
CD2CH2GaBrH.
In addition to the thermolysis products discussed so far,
we could identify (HBr)x (2569.0, 2452.1, 2431.9 cm-1)38,39
and GaBr (299.6, 297.7, 295.7 cm-1)15 by comparison with
their known IR frequencies. On the basis of published data
of (DBr)x, we could not find any IR absorptions for this
species.38 Keeping in mind that the IR transition moment is
reduced by ca. 50% from HX to DX, it might be the case
that (DBr)x is present but its IR absorptions are below our
detection limit.
Lower Thermolysis Temperature Range. We have
discussed results of thermolyses at 850 °C of the precursors
1, 2, and 2D4, respectively. Under these conditions, a
complete fragmentation occurs. However, around 600 °C new
IR bands in the thermolysis spectra already indicate a
beginning fragmentation.
Figure 5 shows the Ga-H stretching region of three
representative spectra obtained from thermolysis experiments
of precursors 1, 2, and 2D4, respectively. First, for all
different precursors a broad but well-structured IR band
around 1900 cm-1 is present. The intensity of this structured
IR band increases from 600 to ca. 750 °C, it decreases at
higher temperatures, and it is not detectable any longer at
850 °C. The compounds that are causing these IR resonances
are obvious intermediates in the fragmentation process. If
one compares the chloride 1 with the bromide 2 (Figure
5A,B), the structure of the IR bands around 1900 wavenum-
bers are comparable; only the frequencies are slightly
different. For 1 the most intense IR band appears at 1904.1
cm-1 whereas for 2 it appears at 1902.2 cm-1. On the other
hand, the shape and the position of the intermediate IR band
are the same for precursors 2 and 2D4 (Figure 5B,C). The
frequency of the structured IR band suggests that intermedi-
ates with Ga-H bonds are causing these resonances.
Interestingly, the fragmentation of 2D4 does not result in
broad and structured IR bands that are shifted to the typical
Ga-D stretching region. The broadness of the IR bands
indicate that the corresponding molecules are relatively large
in comparison with hydrides such as XGaH2 (X ) Cl, Br).
Furthermore, the intermediate IR bands appear at lower
frequencies compared with those of the Ga-H stretches of
X2GaH and XGaH2 (X ) Cl, Br), a fact that can be
interpreted as being due to a higher coordination number of
the gallium atom for these intermediates.
All these experimental results can be explained if we
assume an intermediate gallium hydride that is equipped with
only one chelating ligand; i.e., Me2N(CH2)3GaXH in the case
of 1 (X ) Cl) and 2 (X ) Br) and Me2NCH2CD2CH2GaBrH
in the case of 2D4. On the basis of this hypothesis, we
performed HF and B3LYP calculations using 6-31G(d) and
6-311G(d,p) basis sets for the proposed intermediates (Figure
6). We could optimize only two geometries that differ from
each other in the position of the envelope tip of the five-
membered ring with respect to the substituents H and X
(Figure 6). This is illustrated in Figure 6, where two different
perspectives are shown for the bromide Me2N(CH2)3GaBrH.
Isomer A exhibits C3 as the envelope tip, whereas isomer B
exhibits C2 as the envelope tip. The view along the Ga-N
donor bond reveals that for A the Ga-H bond is staggered
with respect to the NMe2 moiety; for B the GaClH moiety
shows an eclipsed arrangement with respect to NMe2. In
principle, a compound with saturated five-membered ring
in an envelope conformation with one chiral center could
result in 10 enantiomeric pairs (C1 point group symmetries).
Several attempts have been undertaken to find more than
the two isomers depicted in Figure 6, but all optimizations
resulted either in A or B. For example, if the positions of
the H and X atoms in isomer A were exchanged to get a
starting geometry, the optimization resulted in one enanti-
omer of isomer B; a similar procedure starting with B
resulted in isomer A. As expected, the calculation shows that
the most intense IR bands are caused by Ga-H stretching
modes. Table 3 compiles the Ga-H harmonic frequencies,
the intensities, and the relative energies of the two isomers
at three levels of theory.
Inorganic Chemistry, Vol. 43, No. 13, 2004 3961

Gemel and Mu1ller
eliminate the signals of a remaining starting compound
electronically. For alanes and gallanes of the type Me2N(CH2)3-
MX2, this procedure helped to identify intermediates, but
for any of the three starting gallanes described in this paper
an electronic elimination was not successful. It seems that
the IR spectra of the nonfragmented precursors are dependent
on oven temperatures. That means that a subtraction of a
remaining starting compound from a particular spectrum of
a thermolysis is not applicable, because an IR spectrum of
this precursor without any thermolysis products at these
specific thermolysis conditions is unknown. This can be
rationalized if we assume that one donor group is lifted off
from the gallium atom to give a tetracoordinated species (eq
1). This equilibrium will shift to the right side with increasing
temperatures. In addition to the proposed equilibrium, one
needs to take into account that the starting precursors with
the pentacoordinated gallium atom can exist in different
isomers that are distinguished by the position of the envelope
tip of the five-membered rings. Furthermore, one expects
many isomers for the tetracoordinated species of eq 1 caused
by different conformations for the five-membered ring and
the open propyl arm.
Figure 6. Calculated equilibrium geometries of proposed intermediates
Me2N(CH2)3GaXH. Depicted are the bromides at the B3LYP/6-311G(d,p)
level of theory. Each isomer is shown from two different perspectives
(Keller, E. SCHAKAL 99, A Computer Program for the Graphical
Representation of Molecular and Crystallographic Models; University of
Freiburg: Freiburg, Germany, 2001).
Conclusions
We have identified products of thermolysis reactions of
the intramolecularly coordinated precursors 1, 2, and 2D4.
How are these products formed? Matrix-isolation technique
allows the reaction products to be identified, but not the
complete decomposition mechanisms, which must therefore
remain speculative. A matrix-isolation pyrolysis investigation
aims at getting snapshots of gaseous reaction products. One
intends to identify intermediates that are fragments of an
investigated precursor. However, a major difficulty is due
to the fact that it is impossible to suppress further reactions
of intermediates. These “secondary” reactions can occur in
the gas phase and in the evolving matrix. All IR spectra of
thermolysis experiments show many IR absorptions, and we
have no doubt that the matrices contain other products than
those identified so far. All intense IR bands have been
assigned to reaction products, and we assume that these are
the main products. Furthermore, comparison between the
nondeuterated precursor 2 and its deuterated counterpart 2D4
gives important insights into the decomposition of the organic
ligands.
In principle, one expects two major fragmentation path-
ways for the loss of the chelating ligand: homolysis of a
Ga-C bond and â-hydrogen elimination. If â-hydrogen
elimination were occurring exclusively, then allyldimethy-
lamine and XGaH2 would be the only products. In particular,
2D4 would have resulted in BrGaD2, which is obviously not
the case. Therefore, we conclude that for a single precursor
molecule, both ligands are not detached via â-hydrogen
Both isomers have a similar absolute energy with A being
slightly preferred over B at all applied levels of theory (Table
3). If one assigns calculated harmonic Ga-H stretching
frequencies for isomer A and B to the measured IR bands at
1904.1 and 1902.2 for 1 and 2, respectively, reasonable obsd/
calcd frequency ratios are obtained; e.g., compound 1 results
in ratios of 0.9422 [HF/6-31G(d)], 0.9614 [B3LYP/6-31G-
(d)], and 0.9670 [B3LYP/6-311G(d,p)] for isomer A. At all
applied levels of theory, the predicted frequencies of the
chlorides are always a few wavenumbers higher than the ones
of the bromides. These differences are 4.7 cm-1 (isomer A)
and 2.7 cm-1 (isomer B) at the HF level and 5.3/3.1 cm-1
(isomer A) and 12.0/10.4 cm-1 (isomer B) at the two B3LYP
levels. These differences are not in perfect agreement with
the experimental determined difference of 2 cm-1 but do not
contradict the experiment either (Figure 6). The unequivocal
identification of a complex molecule like the proposed
intermediate Me2N(CH2)3GaXH by just one type of vibra-
tional mode is impossible. Therefore, we tried to find other
IR bands that belong to the set of resonances around 1900
cm-1. In the temperature range 600-800 °C, the fragmenta-
tion of the precursor is incomplete, and between 600 and
700 °C the most intense IR bands are caused by undecom-
posed precursor 1, 2, and 2D4, respectively. This makes the
assignment of further IR bands to the proposed intermediate
very difficult. In cases such as this, often it is possible to
3962 Inorganic Chemistry, Vol. 43, No. 13, 2004

Thermal Fragmentation of Gallanes
Table 3. Calculated Harmonic Ga-H Stretching Frequencies [cm-1], Intensities (in Parentheses in km/mol), and Relative Energies (in Brackets in
kJ/mol)a for Isomers A and B of Me2N(CH2)3GaXHb
X ) Cl
X ) Br
HF/6-31G(d)
B3LYP/6-31G(d)
B3LYP/6-311G(d,p)
HF/6-31G(d)
B3LYP/6-31G(d)
B3LYP/6-311G(d,p)
isomer Ab
isomer Bb
2020.9
(242.6)
[0.00]
2022.7
(247.7)
[1.82]
1980.5
(203.0)
[0.00]
1984.0
(206.3)
[2.17]
1969.1
(215.4)
[0.00]
1967.7
(217.6)
[1.91]
2016.2
(246.2)
[0.00]
2020.0
(251.2)
[1.48]
1975.2
(205.1)
[0.00]
1972.0
(208.4)
[1.77]
1966.0
(213.3)
[0.00]
1957.3
(215.4)
[1.79]
a Without inclusion of ZPVE. b See Figure 6.
Scheme 1. Proposed Fragmentation Paths for Precursor 2D4 after the Loss of the First Chelating Ligand
eliminations. On the other hand, this means that homolysis
of Ga-C bonds occurs. The indications that this is the case
are as follows. For the nondeuterated precursors, we detected
H2CdCH2, H2CdNMe, CH4, and HCN; the deuterated
precursor 2D4 resulted in the same species except for ethylene
for which D2CdCH2 instead of H2CdCH2 was formed. The
experimental fact that D2CdCH2 has been found as the only
isotopomer of ethylene implies that this fragment originates
from the chelating ligand CH2CD2CH2NMe2. These findings
are in accordance with the proposed eq 2, where a dimeth-
ylaminopropyl radical eliminates ethylene to give a new
tensity of the IR bands at 803.0 and 786.6 cm-1 strongly
increased. At temperatures of 900 and 1000 °C allyldim-
ethylamine was no longer detectable but [H2CCDCH2]• still
resulted in intense signals. We interpreted this behavior as
an indication that allyldimethylamine is produced by the
thermolysis process and is further decomposed to give allyl
radicals and Me2N• (eq 3). The latter radical presumably is
a source for H•, HCN, Me•, and H2CdNCH3. This interpreta-
tion is supported by the known method to produce allyl
radicals by gas-phase pyrolysis of allyl halides.34
•
radical Me2NCH2 . This radical might eliminate Me• to give
the detected species H2CdNMe, and further decompositions
could result in HCN.
The occurrence of allyldimethylamine suggests that â-hy-
drogen elimination is involved in the thermolysis process.
This interpretation is based on our experience with alanes
and gallanes of the type Me2N(CH2)3MX2 (M ) Al and X
) Cl, Br;43,44 M ) Ga and X ) Cl,15 Br,15 N3,17 Me16,19
)
This raises the question: is â-hydrogen elimination in-
volved at all in the decomposition process? In the temp-
erature range 700-850 °C, we have identified H2CdCDCH2-
NMe2 among other reaction products. Additional support for
the formation of allyldimethylamine comes from the detec-
tion of allyl radicals. Compound 2 resulted in [H2CCHCH2]•
whereas 2D4 selectively gave [H2CCDCH2]•. First of all, this
suggests that this C3 fragment originates from the propyl
group of the chelating ligand. Interestingly, at thermolysis
temperatures of 700 °C the signals of H2CdCDCH2NMe2
are clearly visible, but the two strongest IR absorptions of
[H2CCDCH2]• at 803.0 and 786.6 cm-1 are just distinguish-
able from the background. This situation changes with
increasing temperature. From 700 to 750 to 800 °C we
measured a small increase in intensities for both products,
but from 800 to 850 °C the intensity of the 922.8/919.7
“doublet” (H2CdCDCH2NMe2) decreased whereas the in-
where we have shown that only the dimethylgallane frag-
ments via a â-hydrogen elimination route. Only in this
particular case were we able to identify allyldimethylamine
among the reaction products. In all other cases, the chelating
ligand was eliminated through radical processes; e.g., the
selectively deuterated species Me2NCH2CD2CH2MBr2 frag-
mented to give monomeric HMBr2 and not DMBr2.44
The precursor compounds 1, 2, and 2D4 eliminate their
chelating ligands one after the other. This is suggested by
the detection of intermediate gallium hydrides Me2N(CH2)3-
GaXH and Me2NCH2CD2CH2GaBrH, respectively (Figures
5 and 6). IR bands of respective deuterides could not be
detected. These results can be rationalized if one assumes
that the first donor ligand is eliminated by a homolysis of a
(43) Mu¨ller, J.; Wittig, B. Eur. J. Inorg. Chem. 1998, 1807-1810.
(44) Mu¨ller, J.; Wittig, B. Proc.sElectrochem. Soc. 2001, 13, 124-128.
Inorganic Chemistry, Vol. 43, No. 13, 2004 3963

Gemel and Mu1ller
Ga-C bond (depicted for precursor 2D4 in Scheme 1). The
resulting gallium radical [Me2NCH2CD2CH2GaBr]• might
further react in three different ways (Scheme 1).
identified for the first time in 1994 from co-deposition
reactions of Ga and H atoms.25 It is quite possible that GaH3,
which was observed in the thermolysis of precursor 1 and
2, results from reactions of Ga and H atoms. The formation
of Ga atoms in a CVD process using the diazido precursor
Me2N(CH2)3Ga(N3)2 were monitored with REMPI mass
spectrometry.46 Therefore, we assume that a fragmentation
of compounds 1, 2, and 2D4 also produces Ga atoms.
Interestingly, precursor 2 results in GaH3 and GaH whereas
2D4 results in GaH3, GaDH2, GaH, and GaD. Among the
gallium(III) species, GaDH2 is the main compound. Among
the gallium(I) species, GaD is the prominent compound with
the GaD/GaH ratio being strongly dependent on applied
thermolysis temperatures. To date, we cannot explain these
experimental facts. However, if the hydrides and deuterides
exclusively result from reactions of H, Ga, and D atoms in
the evolving matrices, then one would expect a strong
dominance of gallium hydrides, because H atoms diffuse
faster than D atoms and the concentration of H atoms is
expected to be much higher than the one of D atoms (H to
D ratio in 2D4 is 5 to 1).
In summary, we have shown that the intramolecularly
coordinated gallium chlorides and bromides, chemically
rather simple compounds, exhibit a complex fragmentation
behavior. Involved in the loss of the organic ligands are
Ga-C homolysis as well as â-hydrogen elimination. Among
the thermolysis products, we have identified the monomeric
gallanes BrGaH2 and BrGaDH for the first time. The
precursors investigated within this study serve as model
compounds for the azides of the type [Me2N(CH2)3]2MN3
(M ) Al,10,47 Ga,12 In13,47-49), which had been used for the
group 13 nitride deposition (MOCVD). Recently, we have
started to investigate the fragmentation process of these azido
species with the intent of a deeper understanding of the
MOCVD process. We hope to report on the results shortly.
First, route A shows a decomposition to GaBr and another
equivalent of a dimethylaminopropyl radical discussed
before. Second, the gallium radical could trap an H atom
(route B) to give the hydride Me2NCH2CD2CH2GaBrH. This
interpretation is consistent with our results obtained with
precursors of the type Me2N(CH2)3MX2, which resulted in
monomeric hydrides even though â-hydrogen elimination
was not involved.15,18,43,44 These results clearly show a
principle difficulty within matrix-isolation pyrolysis inves-
tigations. Instead of an intermediate radical, a hydrogen
saturated “secondary” product was identified in matrix. Third,
route C illustrates the possibility that first â-hydrogen
elimination occurs to give allyldimethylamine and GaBrD•
followed by a saturation reaction with H atoms. Via route B
and C, the presence of BrGaDH and H2CdCDCH2NMe2 can
be explained. However, precursor 2 would result in BrGaH2
via route B and C, but this dihydride is the main product in
the experiments with 2D4, too. Therefore, route B and C
cannot be the main path to BrGaH2, and the question remains
how this dihydride is formed. The known compound ClGaH2
was synthesized in Ar matrices by co-condensation reactions
of GaCl with H2.26 Gallium(I) chloride was introduced into
the vacuum system by passing Cl2 over liquid Ga at 1170
K. Finally, irradiation with an Hg high-pressure lamp gave
ClGaH2.26 Recently, the mechanism of the reaction of H2
with MCl (M ) Ga, In) was investigated with theoretical
methods.45 This work confirmed that the molecules MCl and
H2 do not react spontaneously. However, a spontaneous
formation of monomeric gallium hydrides had been obtained
in matrix-isolation experiments from thermally produced
metal atoms co-condensed with H atoms.25 On the basis of
these results, one can speculate that the dihydrides XGaH2
(X ) Cl, Br) are formed from GaX and H atoms, with the
latter being produced from the radical fragmentation path-
ways of the chelating ligand (Scheme 1; eqs 2 and 3). A
formation of XGaH2 from GaX and H atoms would proceed
through intermediates XGaH•, and consequently, we tried
to identify these unknown radicals. According to B3LYP
calculations, the Ga-H stretching modes are expected at
1675.5 cm-1 and at 1676.0 cm-1 with intensities of 130 and
136 km/mol for ClGaH• and BrGaH•, respectively (see
Experimental Section for details). However, we could not
find IR bands in the expected region that could be assigned
to XGaH•. Of course, this does not mean that these radicals
are not trapped in matrices; they might be present in small
amounts. Additionally, a formation of BrGaDH from GaBr
with H and D atoms is also feasible.
Acknowledgment. This work was supported by the Fonds
der Chemischen Industrie, by the Deutsche Forschungsge-
meinschaft, and by a Discovery Grant of the Natural Sciences
and Engineering Research Council of Canada.
Supporting Information Available: Calculated bond lengths
and angles for H2GaX (X ) Cl, Br) at B3LYP/6-311+G(2d,p) and
MP2(fc)/6-311+G(2d,p), and Cartesian coordinates for all calcu-
lated structures. This material is available free of charge via the
Internet at http://pubs.acs.org.
IC035496A
(46) Scha¨fer, J.; Wolfrum, J.; Fischer, R. A.; Sussek, H. Chem. Phys. Lett.
1999, 300, 152-156.
(47) Fischer, R. A.; Miehr, A.; Ambacher, O.; Metzger, T.; Born, E. J.
Cryst. Growth 1997, 170, 139-143.
It seems likely that the presence of hydrogen atoms is the
cause for the formation of GaH3. This species had been
(48) Fischer, R. A.; Sussek, H.; Miehr, A.; Pritzkow, H.; Herdtweck, E. J.
Organomet. Chem. 1997, 548, 73-82.
(49) Devi, A.; Parala, H.; Rogge, W.; Wohlfart, A.; Birkner, A.; Fischer,
R. A. J. Phys. IV 2001, 11, 577-584.
(45) Himmel, H.-J. J. Chem. Soc., Dalton Trans. 2002, 2678-2682.
3964 Inorganic Chemistry, Vol. 43, No. 13, 2004