Mono- and digallane complexes of a tridentate amido-diamine ligand

Article abstract of DOI:10.1039/b503705k

Bis(2-dimethylaminoethyl)amido gallane, H₂GaN(CH ₂CH₂-NMe₂)₂, that melts at 27 °C and remains stable upon heating at 55 °C for two days, was synthesized either from the reaction of the quinuclidine adduct of monochlorogallane with the lithium salt of the corresponding amine, or from the reaction of trimethylamine gallane and the amine; the latter affords an unusual co-product with both GaH₂ and GaH₃ bonded to the same amido nitrogen. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b503705k

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Mono- and digallane complexes of a tridentate amido-diamine ligand{
Bing Luo, Benjamin E. Kucera and Wayne L. Gladfelter*
Received (in Berkeley, CA, USA) 11th March 2005, Accepted 16th May 2005
First published as an Advance Article on the web 9th June 2005
DOI: 10.1039/b503705k
Bis(2-dimethylaminoethyl)amido gallane, H2GaN(CH2CH2-
NMe2)2, that melts at 27 uC and remains stable upon heating
at 55 uC for two days, was synthesized either from the reaction
of the quinuclidine adduct of monochlorogallane with the
lithium salt of the corresponding amine, or from the reaction of
trimethylamine gallane and the amine; the latter affords an
unusual co-product with both GaH2 and GaH3 bonded to the
same amido nitrogen.
Compound 1 melted at 27 uC. Its vapor pressures, obtained
from multiple distillation experiments, were 0.20, 0.37, 0.69 and
0.91 Torr at 40, 48, 57 and 61 uC, respectively. While heating 1
under vacuum at 80 uC resulted in evaporation without
decomposition, it started to decompose under N2 at 100 uC
forming HN(CH2CH2NMe2)2 and gray particles that were
presumably elemental gallium. Under 1 atm of N2, 1 survived
for 48 h at 55 uC in a glass vial, or 24 h at 50 uC when in contact
with stainless steel, and then gradually decomposed as evidenced
by the formation of gray particles. Although 1 is air sensitive, it is
not pyrophoric. These results suggest that 1 may have practical
utility as a gallium precursor in vapor deposition processes (CVD,
MOCVD, etc.).
For the chemical vapor deposition of gallium-containing films,
there is interest in developing nonpyrophoric, alternative pre-
cursors to trimethyl or triethyl gallium. It is also desirable for these
precursors to minimize carbon contamination. Gallium hydride
complexes that do not contain direct gallium–carbon bonds are
attractive candidates.1 Gallane itself, a hydrogen-bridged dimer
[GaH3]2, decomposes above 230 uC.2 Room-temperature isolable
gallium hydride derivatives are generally stabilized with bulky
ligands or they exist as oligomers as exemplified by [2,6-
(Me2NCH2)2C6H3]GaH2,3 [H2GaNMe2]3,4 H2GaN(SiMe3)2-
(quin) (where quin 5 quinuclidine),5 and [tBu(H)GaNEt2]2.6 The
increase in molecular mass renders them less volatile. Although
several hydrides with smaller ligands including [H2GaNH2]3,7
[H2GaNHNMe2]2,8 H3Ga(quin)9 and [H2GaN3]n10 have been used
to prepare GaN and GaAs nanocrystals or films at low tempera-
tures, hydride-based precursors with the merits of long-shelf life
and high volatility have yet to be discovered. To address these
problems, we used a tridentate nitrogen ligand to prepare gallium
hydride complexes, and herein report the synthesis and characteri-
zation of thermally robust, volatile bis(2-dimethylaminoethyl)-
amido gallane, H2GaN(CH2CH2NMe2)2 (1), which is a liquid
above 27 uC. Along with the synthesis of 1, an unusual complex in
which the amido nitrogen bridges between a gallium dihydride and
a trihydride was isolated and structurally characterized.
The structure of 1,14 as shown in Fig. 1, was monomeric with a
crystallographically imposed two-fold axis passing through the
Ga(1)–N(2) bond. The gallium adopted a distorted trigonal
bipyramidal geometry with the two NMe2 groups occupying the
axial positions. The hydrides were located from the electron
density map. In the literature, similar structures were found for
[2,6-(Me2NCH2)2C6H3]GaH2 and H2GaCl(quin)2.13 It is note-
3
worthy that [2,6-(Me2NCH2)2C6H3]GaH2 also exhibited a very
high thermal stability; its vapor survived up to 350 uC.3 We
speculate that the high thermal stability of these chelating five-
coordinate gallium compounds arises from the blocking of the fifth
gallium coordination site that otherwise might be used in a
decomposition reaction.
A second route to 1 was explored and is shown in Scheme 1. In
12
a typical run, an Et2O solution (20 mL) of HN(CH2CH2NMe2)2
Compound 111 was initially synthesized from the reaction
12
shown in eqn. 1. A solution of LiN(CH2CH2NMe2)2 (1.21 g,
7.33 mmol) in 40 mL of Et2O was added to a solution of
H2GaCl(quin)13 (1.60 g, 7.33 mmol) in 40 mL of Et2O at room
temperature. The mixture was stirred for 14 h and filtered. The
solvent and volatile quinuclidine were removed from the filtrate
under vacuum affording a pale-yellow liquid. Fractional distilla-
tion under reduced pressure afforded 1 (1.26 g, 75% yield) as a
colorless, mobile liquid, which slowly solidified at ambient
temperature to form a colorless, crystalline solid.
H2GaCl(quin) + LiN(CH2CH2NMe2)2 A
H2GaN(CH2CH2NMe2)2 + quin + LiCl
Fig. 1 Structure of 1 showing 50% thermal ellipsoids. The hydrogen
(1)
atoms except for those on Ga are omitted for clarity. Selected bond
˚
distances (A) and angles (u): Ga(1)–N(2) 1.874(2), Ga(1)–N(1) 2.2826(16),
Ga(1)–H(1) 1.47(2); N(1)–Ga(1)–N(2) 79.04(4), N(2)–Ga(1)–H(1) 121.0(8),
{ Dedicated to the memory of Ian P. Rothwell
*gladfelt@chem.umn.edu
N(1A)–Ga(1)–H(1) 97.2(8), N(1)–Ga(1)–H(1) 94.0(8).
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Scheme 1
(9.05 g, 56.8 mmol) was added to a dry ice–2-propanol cooled
Et2O solution (200 mL) of approximately 1 equivalent of freshly
prepared H3Ga(NMe3).15 The reaction mixture was refluxed for
two days followed by evaporation of Et2O to give a colorless
Fig. 2 Structure of 2 showing 50% thermal ellipsoids. The hydrogen
atoms except for those on Ga are omitted for clarity. Selected bond
˚
distances (A) and angles (u): Ga(1)–N(1) 2.3140(18), Ga(1)–N(2)
1.9922(17), Ga(1)–N(3) 2.2730(17), Ga(2)–N(2) 2.0582(18), Ga(1)–H(1D)
1.42(3), Ga(1)–H(1E), 1.43(2); N(1)–Ga(1)–N(2) 82.15(7), N(1)–Ga(1)–
N(3) 162.93(7), N(2)–Ga(1)–N(3) 82.68(7), N(2)–Ga(1)–H(1D) 114.5(11),
N(3)–Ga(1)–H(1D) 88.9(11), N(1)–Ga(1)–H(1D) 90.3(11), N(2)–Ga(1)–
H(1E) 119.9(10), N(3)–Ga(1)–H(1E) 96.6(10), N(1)–Ga(1)–H(1E)
97.7(10), H(1D)–Ga(1)–H(1E) 125.6(15), Ga(1)–N(2)–Ga(2), 113.67(9).
mixture of 1,
a
new compound (2) and unreacted
HN(CH2CH2NMe2)2 as measured by NMR. Pentane (70 mL)
was added to the mixture, which was then filtered. Upon cooling, 2
was isolated from the filtrate as a colorless crystalline solid (1.85 g,
22% yield based on gallium).
A liquid mixture of 1,
HN(CH2CH2NMe2)2 and a residue of 2 was obtained after
removal of pentane from the mother liquor. Distillation at reduced
pressures afforded 6.55 g of 1 (50% yield based on gallium).
Crystalline 2 was stable at low temperatures, but gradually
decomposed over a period of a few days at room temperature. For
a freshly prepared sample, it decomposed at 69 uC. The IR
spectrum (KBr pellet) exhibited nGa–H absorptions at 1797, 1871
and 1888 cm21. The structure of 2 (Fig. 2)16 was unprecedented in
the sense that the bonding of N(2) was covalent to the GaH2 and
dative to the GaH3. The corresponding N(2)–Ga(1) and N(2)–
as oligomeric rods and forms GaN when reacted at higher
temperatures.18 We are applying the –N(CH2CH2NMe2)2 ligand
to a number of other metals and using 1 to deposit III–V
semiconductor films. These results will be reported in subsequent
publications.
This research was supported by a grant from the National
Science Foundation (CHE-03159540).
Bing Luo, Benjamin E. Kucera and Wayne L. Gladfelter*
Department of Chemistry, University of Minnesota, 207 Pleasant St.
SE, Minneapolis, MN 55455. E-mail: gladfelt@chem.umn.edu;
Fax: +1 612-626-0859; Tel: +1 612-624-4391
˚
Ga(2) bond lengths were 1.9922(17) and 2.0582(18) A, respectively.
This is significantly different from that observed in cyclic
(R2GaNR92)n and (R2GaNHR9)n (where R, R9 5 H, alkyl or
aryl groups, and n 5 2 or 3) compounds, where the bridging
Ga–N bond lengths were essentially equal.17 At room temperature
the 13C NMR spectrum of 2 in toluene-d8 consisted of resonances
at 47.7 (NMe2), 55.3 (CH2) and 58.0 ppm (CH2). The inability to
resolve distinct methyl resonances was also reflected in the 1H
NMR spectrum. The spectrum, however, displayed broad singlets
at 4.51 and 4.78 ppm attributable to the GaH3 and GaH2 groups.
At 283 uC the proton methyl resonances were resolved consistent
with the solid state structure.
Notes and references
1 S. Aldridge and A. J. Downs, Chem. Rev., 2001, 101, 3305.
2 C. R. Pulham, A. J. Downs, M. J. Goode, D. W. H. Rankin and
H. E. Robertson, J. Am. Chem. Soc., 1991, 113, 5149.
3 A. H. Cowley, F. P. Gabba¨ı, D. A. Atwood, C. J. Carrano,
L. M. Mokry and M. R. Bond, J. Am. Chem. Soc., 1994, 116, 1559.
4 C. Jones, F. C. Lee, G. A. Koutsantonis, M. G. Gardiner and
C. L. Raston, J. Chem. Soc., Dalton Trans., 1996, 829.
5 B. Luo, V. G. Young, Jr. and W. L. Gladfelter, Inorg. Chem., 2000, 39,
1705.
6 L. Grocholl, S. A. Cullison, J. Wang, D. C. Swenson and E. G. Gillan,
Inorg. Chem., 2002, 41, 2920.
7 J.-W. Hwang, J. P. Campbell, J. Kozubowski, S. A. Hanson, J. F. Evans
and W. L. Gladfelter, Chem. Mater., 1995, 7, 517.
8 B. Luo and W. L. Gladfelter, Chem. Commun., 2000, 825; B. Luo,
S.-Y. Lee and J. M. White, Chem. Mater., 2004, 16, 629.
9 J. S. Foord, T. J. Whitaker, D. O’Hare and A. C. Jones, J. Cryst.
Growth, 1994, 136, 127; J. L. Atwood, S. G. Bott, F. M. Elms, C. Jones
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10 J. McMurran, D. Dai, K. Balasubramanian, C. Steffek, J. Kouvetakis
and J. L. Hubbard, Inorg. Chem., 1998, 37, 6638.
11 Compound 1. 1H NMR (300 MHz, C6D6, 25 uC): d 2.08 (12H, s,
NMe2), 2.36 (4H, t, NCH2CH2NMe2), 3.10 (4H, t, NCH2CH2NMe2),
4.90 (2H, br s, GaH2); 13C NMR (75 MHz, C6D6, 25 uC): d 45.71 (s,
It was intriguing that 2 was isolated nearly quantitatively from
the reaction of 1 with one equivalent of H3Ga(NMe3) in Et2O at
temperatures up to room temperature. This result demonstrated
the strong Lewis basicity of the amido ligand in 1. This strong
basicity was further confirmed by the inability to displace
H2GaN(CH2CH2NMe2)2 from 2 with quinuclidine: no reaction
occurred after 2 and quinuclidine were stirred in a 1 : 1 molar ratio
in Et2O at room temperature for 4 h.
The stability of 1 does not preclude reactions with Group 15
reagents. At 150 uC in an autoclave, 1 reacted under 12 atm of
NH3 in a period of 40 min to give mainly [HGaNH]n which exists
3464 | Chem. Commun., 2005, 3463–3465
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NMe2), 50.56 (s, CH2), 61.04 (s, CH2). MS (CI):
[H2GaN(CH2CH2NMe2)2 + H]+ (230 amu, 31.3% of the total ion
abundance). IR (NaCl window): nGaH, 1781 cm21. Anal. calc. for
C8H22GaN3: C, 41.78; H, 9.64; N, 18.27%. Found: C, 41.22; H, 8.69; N,
18.06%.
15 N. N. Greenwood, A. Storr and M. G. H. Wallbridge, Inorg. Chem.,
1963, 2, 1036; D. F. Shriver and R. W. Parry, Inorg. Chem., 1963, 2,
1039.
16 Crystal data of 2. C8H25Ga2N3, M 5 302.75, orthorhombic,
3
˚
˚
a 5 6.1939(14), b 5 12.977(3), c 5 17.601(4) A, U 5 1414.8(6) A ,
T 5 173 K, space group P212121 (no. 19), Z 5 4, m(Mo-Ka) 5
3.779 mm21, 17128 reflections measured, 3255 unique (Rint 5 0.0376)
which were used in all calculations. The final wR(F2) was 0.0531
(all data). CCDC 266261. See http://www.rsc.org/suppdata/cc/b5/
b503705k/ for crystallographic data in CIF or other electronic
format.
12 H. Luitjes, M. Schakel and G. W. Klumpp, Synth. Commun., 1994, 24,
2257.
13 B. Luo, V. G. Young, Jr. and W. L. Gladfelter, Chem. Commun., 1999,
123.
14 Crystal data of 1. C8H22GaN3, M 5 230.01, orthorhombic,
3
˚
˚
a 5 11.010(2), b 5 11.706(2), c 5 9.4523(19) A, U 5 1218.2(4) A ,
T 5 173 K, space group Pbcn (no. 60), Z 5 4, m(Mo-Ka) 5
2.221 mm21, 13559 reflections measured, 1412 unique (Rint 5 0.0362)
which were used in all calculations. The final wR(F2) was 0.0771 (all
data). CCDC 266262. See http://www.rsc.org/suppdata/cc/b5/b503705k/
for crystallographic data in CIF or other electronic format.
17 For a review, see: C. J. Carmalt, Coord. Chem. Rev., 2001, 223,
217.
18 B. L. Kormos, J. A. Jegier, P. C. Ewbank, U. Pernisz, V. G. Young, Jr.,
C. J. Cramer and W. L. Gladfelter, J. Am. Chem. Soc., 2005, 127,
1493.
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