A Ga4N8 cage structure formed by reaction of trimethylgallium with phenylhydrazine

Article abstract of DOI:10.1039/a800266e

Methane elimination during thermolysis of GaMe₃ with PhHNNH₂ yields sequentially dimeric [Me₂Ga{μ-N(H)N(H)Ph}]₂, tetrameric [MeGa(μ-N(H)NPh}]₄ and, ultimately, GaN; the X-ray structure of the tetramer shows a novel Ga₄N₈ cage.

Full text of DOI:10.1039/a800266e

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A Ga4N8 cage structure formed by reaction of trimethylgallium with
phenylhydrazine
David W. Peters,a,b Maurice P. Power,a Edith D. Bourretb and John Arnold*a,b
†
a Department of Chemistry, University of California, Berkeley, California 94720-1460, USA
b Materials Sciences Division, Lawrence Berkeley National Laboratory, California 94720, USA
Methane elimination during thermolysis of GaMe3 with
PhHNNH2 yields sequentially dimeric [Me2Ga{m-
N(H)N(H)Ph}]2, tetrameric [MeGa{m-N(H)NPh}]4 and, ulti-
mately, GaN; the X-ray structure of the tetramer shows a
novel Ga4N8 cage.
structure, shown in Fig.1, consists of a tetrameric unit that
resides on the intersection of three mutually perpendicular
mirror planes. The geometry about gallium is distorted
tetrahedral, with angles ranging from 121.3(1) [N(1)–Ga(1)–
C(1)] to 94.4(1) [N(2*)–Ga(1)–N(2*)]. The bond length of Ga
to the four-coordinate N(2*) [1.993 (3) Å] is longer than that to
the three-coordinate N(1) [1.914(2) Å], although the mean Ga–
N distance of 1.966(3) Å falls within the reported range for
gallium amido complexes.1–5 The N–N bond length of 1.489(3)
Å is longer than in the only other crystallographically
characterized Ga hydrazide complex [Et2Ga{m-N(H)NPh2}]2,
[1.457(8) and 1.446(8) Å]11 and is somewhat longer than
predicted for a N–N single bond (1.454 Å), a result that may be
attributed to the bridging nature of the hydrazine ligand18 in the
cage species. Owing to the high quality of the data obtained, the
hydrazine hydrogen was located in the Fourier difference map
and refined isotropically. Fig. 2 shows a view of the unit cell
which highlights the rather large open core of the structure (ca.
4.5 Å) and the fact that there are no unusually short
intermolecular contacts. We are unaware of previous reports of
such a cage structure for Ga, although we note that a related
boron compound [ButBN(H)N(H)]4 was characterized some
time ago.19
A number of research groups have investigated the chemistry of
molecules containing Ga–N bonds,1–5 with most of the recent
emphasis directed at using these compounds as single-source
precursors to the wide bandgap semiconductor GaN.6–13 Here
we describe some new results on the use of hydrazines as a
source of nitrogen including the formation of two intermediates
formed on elimination of 1, then 2 equiv. of methane during the
thermolysis reaction between GaMe3 and PhHNNH2. Both
complexes have been isolated and fully characterized and the
latter is shown to have an unusual structure featuring a cage with
a Ga4N8 core;14 this species eliminates a final equivalent of
methane to form GaN at higher temperatures.
As shown in Scheme 1, 1 equiv. of methane is eliminated in
the reaction of GaMe3 and PhHNNH2 at room temperature in
toluene. We saw no evidence for the presumed monomeric
intermediate ‘Me2Ga–N(H)N(H)Ph’, instead the colorless di-
meric product was isolated in 88% yield.15
Thermolysis of the cage complex at 700 °C under a hydrogen
atmosphere leads to the formation of hexagonal GaN (identified
Me
Ga
PhHN
H
H
toluene, 20 °C
–2MeH
Me
2GaMe3
+
2H2N–NHPh
N
N
Ga
Me
NHPh
Me
C(6*)
C(5*)
Scheme 1
C(4*)
C(5*)
The solid-state structure of the dimer is very similar to that of
the closely related species [Et2Ga{m-N(H)NPh2}]2, prepared by
the metathesis reaction between Et2GaCl and LiN(H)NPh2,11
with both compounds crystallizing as anti conformers.16
Upon further heating, the dimer undergoes a second methane
elimination reaction as shown in Scheme 2.
C(6*)
C(7*)
C(7*)
C(4*)
C(3*)
C(2*)
N(2*)
C(3*)
C(2*)
N(2*)
H
Me
Me
C(1*)
N
Me
Ga
Ga
N
Ga
H
N
N(1*)
H
Me
Ga(1*)
toluene, 111 °C
–2MeH
PhHN
H
H
Ga(1*)
N
C(1*)
N
N
N
N
Ga
Me
N
NHPh
N(1*)
Ga
Ga
N
Me
Ga(1*)
Me
N(1)
H
Me
N(1*)
N(2*)
C(1*)
Ga(1)
N(2)
Scheme 2
As monitored by 1H NMR spectroscopy, the reaction
proceeds in a quantitative fashion and microcrystalline product,
which is much less soluble than the starting material, precip-
C(2*)
C(3*)
C(1)
C(2)
C(7*)
itates from the reaction solvent in 84% yield.15 The H NMR
1
C(3)
spectrum shows a simple pattern consistent with only single
methyl, N–H and phenyl environments15 and we saw no
evidence of fluxional behavior between 280 and 110 °C. Since
these data were insufficient to unambiguously determine the
structure of the molecule, an X-ray study was carried out.
Crystals of [MeGaN(H)NPh]4 were grown by thermolysis of
[Me2GaN(H)N(H)Ph]2 in toluene at 105 °C.17 The solid state
C(6*)
C(7)
C(6)
C(4*)
C(4)
C(5)
C(5*)
Fig. 1 X-Ray crystal structure of [MeGaN(H)NPh]4
Chem. Commun., 1998
753

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Fig. 2 Unit cell of [MeGaN(H)NPh]4
16 Details will be given in a full paper.
17 Crystallographic data [MeGaN(H)NP]4, C28H36Ga4N8, Mw = 763.53,
tetragonal, space group I41/a (no. 88), a = 14.0460(2), c = 15.581(2)
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We thank the donors of the Petroleum Research Fund
(administered by the American Chemical Society) for partial
support of this work.
Å, U
= = 4, Dc =
2970.81(6) Å3, Z 1.71 g cm23, graphite
monochromated Mo-Ka radiation (l = 0.710 69 Å), m(Mo-Ka) = 36.3
cm21. A total of 6973 reflections (1433 unique, Rint = 2.73%) were
collected on a colourless crystal of dimensions 0.24 3 0.29 3 0.31 mm
at 2111 °C on a Siemens SMART diffractometer/CCD area detector; an
empirical absorption correction was applied. The structure was solved
by direct methods and refined by full matrix least squares procedures.
All non-hydrogen atoms were refined isotropically. The hydrogen atom
on N(1) was located from the Fourier difference map and refined
Notes and References
isotropically. Final residuals at convergence were R = 0.0227 and Rw
=
0.0281 for 1046 data with F2 > 3s(F2) and GOF = 1.23. CCDC
182/782.
†E-mail: arnold@violet.berkeley.edu
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Received in Bloomington, IN, USA, 9th January 1998; 8/00266E
754
Chem. Commun., 1998