New pnictinogallanes [H2GaE(SiMe3)2]3 (E = P, As) - Formation, structural characterization, and thermal decomposition to afford nanocrystalline GaP and GaAs

Article abstract of DOI:10.1021/ja9731837

The new compounds [H₂GaE(SiMe₃)₂]₃ (E = P (1), As (2)), the first authenticated examples of a phosphinogallane and an arsinogallane containing the GaH₂ moiety,are prepared via efficient dehydrosilylation from the respective combinations of H₃Ga·NMe₃ and E(SiMe₃)₃ in diethyl ether or toluene. Compounds 1 and 2 are characterized by elemental analysis, NMR, IR, and mass spectrometry. Single-crystal X-ray structural studies show that the molecular structures of 1 and 2 feature a flattened six-member ring of alternating Ga and E centers. Both compounds are reasonably stable at -30°C but spontaneously decompose at ambient temperatures, 2 noticeably faster than 1, with the evolution of HSiMe₃, H₂, and E(SiMe₃)₃. The pyrolysis of 1 yields nanocrystalline GaP while the pyrolysis of solids from decayed 2 results in nanocrystalline GaAs as determined from XRD studies. Under applied pyrolysis conditions, the thermally accelerated dehydrosilylation of the precursors is accompanied by a side-evolution of CH₄ and retention of small quantities of amorphous Si/C phases.

Full text of DOI:10.1021/ja9731837

532
J. Am. Chem. Soc. 1998, 120, 532-537
New Pnictinogallanes [H2GaE(SiMe3)2]3 (E ) P, As)sFormation,
Structural Characterization, and Thermal Decomposition to Afford
Nanocrystalline GaP and GaAs
Jerzy F. Janik,†,‡ Richard L. Wells,*,‡ Victor G. Young, Jr.,§ Arnold L. Rheingold, and
Ilia A. Guzei
Contribution from the Department of Chemistry, Paul M. Gross Chemical Laboratory, Duke UniVersity,
Durham, North Carolina 27708-0346, Department of Chemistry, X-ray Crystallographic Laboratory,
UniVersity of Minnesota, Minneapolis, Minnesota 55455, and Department of Chemistry, UniVersity of
Delaware, Newark, Delaware 19716
ReceiVed September 10, 1997
Abstract: The new compounds [H2GaE(SiMe3)2]3 (E ) P (1), As (2)), the first authenticated examples of a
phosphinogallane and an arsinogallane containing the GaH2 moiety, are prepared Via efficient dehydrosilylation
from the respective combinations of H3Ga‚NMe3 and E(SiMe3)3 in diethyl ether or toluene. Compounds 1
and 2 are characterized by elemental analysis, NMR, IR, and mass spectrometry. Single-crystal X-ray structural
studies show that the molecular structures of 1 and 2 feature a flattened six-member ring of alternating Ga and
E centers. Both compounds are reasonably stable at -30 °C but spontaneously decompose at ambient
temperatures, 2 noticeably faster than 1, with the evolution of HSiMe3, H2, and E(SiMe3)3. The pyrolysis of
1 yields nanocrystalline GaP while the pyrolysis of solids from decayed 2 results in nanocrystalline GaAs as
determined from XRD studies. Under applied pyrolysis conditions, the thermally accelerated dehydrosilylation
of the precursors is accompanied by a side-evolution of CH4 and retention of small quantities of amorphous
Si/C phases.
Introduction
results in composite materials with potentially useful properties
but, on the other hand, it poses an outstanding challenge with
regard to separation of such phases, if necessary. Interestingly,
if one uses a related group 13-15 substituent isomer system,
e.g. {Al(SiMe3)3} + PCl3 (or R2PCl), the prevailing reaction
pathway under ambient conditions is not dehalosilylation as
defined above but ligand exchange with subsequent P-P
coupling side reactions.3
We have recently reported on the high yield syntheses and
structural characterizations of the novel lithium pnictidogallates,
(Et2O)2Li[µ-E(SiMe3)2]2GaH2, E ) P, As, that were obtained
from the respective combinations of LiGaH4 and E(SiMe3)3 in
Et2O Via facile dehydrosilylation.4 Under specific reaction
conditions for E ) P, we observed the formation of a minor
byproduct whose spectroscopic characterization was consistent
with the building blocks of {H2GaP(SiMe3)2}. There have also
been other precedences for the trimethylsilane elimination-
condensation reactions in related group 13-15 systems, ex-
amples of which include {BH3}/P(SiMe3)3,5 {Al(SiMe3)3}/
Trimethylhalosilane elimination, or dehalosilylation, has
proven to be a convenient route for making group 13(M)-15(E)
element bonds from the starting materials MX3 and E(SiMe3)3
and, in favorable cases, a removal of all ligands and formation
of the solid-state ME materials has been achieved.1 However,
a complete elimination of redundant Me3SiX groups is rare in
standard bulk syntheses and residual, usually amorphous Si/C
phases have been observed in the solid products in addition to
the major crystalline ME.1e,2 On one hand, this phenomenon
* Address correspondence to R. L. Wells.
† On leave from the University of Mining and Metallurgy, Krakow,
Poland.
‡ Duke University.
§ University of Minnesota.
University of Delaware.
(1) See for example: (a) Wells, R. L. Coord. Chem. ReV. 1992, 112,
273 (and references therein). (b) Aubuchon, S. R.; McPhail, A. T.; Wells,
R. L.; Giambria, J. A. Chem. Mater. 1994, 6, 82. (c) Wells, R. L.; Aubuchon,
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(d) Halaoui, L. I.; Kher, S. S.; Lube, M. S.; Aubuchon, S. R.; Hagan, C. R.
S.; Wells, R. L.; Coury, L. A. ACS Symp. Ser. 1996, 622, 178 (and references
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Schimek, G. L.; Rheingold, A. L.; Liable-Sands, L. M. Organometallics
1996, 15, 5385. (f) Wells, R. L.; Gladfelter, W. L. J. Cluster Sci. 1997, 8,
217 (and references therein). (g) Healy, M. D.; Laibinis, P. E.; Stupik, P.
D.; Barron, A. R. J. Chem. Soc., Chem. Commun. 1989, 359. (h) Olshavsky,
M. A.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1990, 112,
9438. (i) Butler, L.; Redmond, G.; Fitzmaurice, D. J. Phys. Chem. 1993,
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1996, 100, 7212.
(2) (a) Paine, R. T.; Janik, J. F.; Fan, M. Polyhedron 1994, 13, 1225.
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G. E. Chem. Mater. 1995, 7, 1361. (c) Paciorek, K. J. L.; Nakahara, J. H.;
Hoferkamp, L. A.; George, C.; Flippen-Anderson, J. L.; Gilardi, R.; Schmidt,
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Kawakami, T.; Niimara, K. Ultrastructure Processing of AdVanced Materi-
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Paine, R. T. Organometallics 1989, 8, 506.
(4) Janik, J. F.; Wells, R. L.; Young, V. G., Jr.; Halfen, J. A.
Organometallics 1997, 16, 3022.
(5) Wood, G. L.; Dou, D.; Narula, C. K.; Duesler, E. N.; Paine, R. T.;
No¨th, H. Chem. Ber. 1990, 123, 1455 (and references therein).
S0002-7863(97)03183-1 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/13/1998

New Pnictinogallanes [H2GaE(SiMe3)2]3 (E ) P, As)
J. Am. Chem. Soc., Vol. 120, No. 3, 1998 533
PH3-nRn,3 and {Al(SiMe3)3}/NH36 combinations. These obser-
vations and reports turned our attention toward investigating
the {GaH3}/E(SiMe3)3 reaction systems as a potential source
for new molecular and polymeric precursors to GaE materials.
The thermal instability of the Ga-H bond7 was expected to
enhance the elimination-condensation reactivity but also could
lead to some undesirable side-decomposition pathways for these
precursors.
and a characteristic broad resonance for the GaH2 protons4 at δ
4.55 that integrated 9 to 1 as expected. Similarly, the 13C{1H}
NMR spectrum showed four regularly spaced peaks that could
be interpreted either as two closely spaced doublets or a second-
order pattern. The rather unusual appearance of the proton
doublet prompted us to run the sample at a higher field of 600
MHz with the hope to better resolve the possibly overlapped
resonances. Under this condition, the resonance pattern and
position for the SiMe3 protons remained unchanged but the
separation of the peaks in the apparent doublet decreased
slightly. If the doublet resulted from a simple coupling of one
phosphorus with one proton, the separation should have
remained the same; we thus concluded that we dealt with
complex and likely superimposed signals. On the basis of the
above, we first ruled out a planar, dimeric structure for 1 (triplet
expected for the SiMe3 protons) and, for instance, a low
symmetry trimer of the {H2GaP(SiMe3)2} species became
probable. The singlet in the 31P NMR spectrum at δ -265.8
was not of much help here but, in the related case of the trimer
[H2BP(SiMe3)2]3,5 a single phosphorus resonance was also
observed. A strong argument for the trimeric oligomerization,
at least in the evaporated state, was obtained from the mass
spectrum of 1. It clearly showed the highest m/e signal for the
trimeric parent ion and other logical fragmentation signals.
Similarly as for 1, the elemental analysis for compound 2
was consistent with the {H2GaAs(SiMe3)2} stoichiometry. On
the other hand, the compound’s spectroscopic characterization
Results and Discussion
The combinations of H3Ga‚NMe3 and E(SiMe3)3 in diethyl
ether (preferably) or toluene at ambient conditions afforded [H2-
GaE(SiMe3)2]3, E ) P (1), As (2), the first reported phosphi-
nogallane and arsinogallane containing the GaH2 moiety, as well
as their elimination-condensation polymeric products, Via very
efficient dehydrosilylation (eq 1).
H3Ga‚NMe3 + E(SiMe3)3 f
1/3[H2GaE(SiMe3)2]3 + HSiMe3 + NMe3 (1)
No reaction was detected for E ) N under comparable
conditions. This was in accord with a decreased reactivity of
the N-SiMe3 groups toward dehydrosilylation at ambient
conditions due to the strong N-Si bond and steric congestion
in N(SiMe3)3, as previously observed in the system LiGaH4/
E(SiMe3)3 (E ) N, P, As).4 Crystalline compounds 1 and 2
were obtained from the reaction mixtures in Et2O at early stages
of the reactions, and after isolation they seemed to be reasonably
stable for several days to weeks if stored at -30 °C. If any of
the compounds was left in the mother liquor it would further
react and turn to a colored, insoluble polymeric solid. The NMR
studies for samples prepared in toluene-d8 from crystalline 1
and 2 as well as from their respective yellow to orange
decomposition products showed amounts of byproducts such
as HSiMe3, H2, and E(SiMe3)3 (especially for E ) As) increasing
with time. The occurrence of HSiMe3 was consistent with
progressing facile dehydrosilylation, the target elimination-
condensation route in this study. However, the presence of the
remaining byproducts indicated other decomposition pathways
as well. The dihydrogen most likely resulted from a known
reductive decomposition of gallanes, whereas the resultant
E(SiMe3)3 were derived from a complex ligand redistribution
chemistry operating in these systems. Interestingly, the same
decomposition species were also observed by us in the related
case of the lithium pnictidogallates, (Et2O)2Li[µ-E(SiMe3)2]2-
GaH2, E ) P, As.4 It appeared that in the presently investigated
systems the decay of the molecular species was faster in toluene
than in Et2O. Also, the E ) P system decomposed slower than
the E ) As system under analogous conditions, which resulted
in a higher yield of the isolated P analogue; however, the yields
of 1 and 2 were not optimized.
1
appeared to be simpler than that for 1. For example, the H
and 13C{1H} NMR showed only sharp singlets for the SiMe3
groups, indicating their symmetrical local environment in the
solution. The proton resonance for the GaH2 moieties at δ 4.65
was broad and featureless, and integrated as expected with the
SiMe3 proton signal. The infrared Ga-H stretch at 1834 cm-1
also supported the presence of the GaH2 moieties in the
molecule. And last, the mass spectrum showed a parent ion
and a clean fragmentation pattern for the trimer [H2GaAs-
(SiMe3)2]3. Summarizing these observations, all the character-
ization data for 1 and 2 were consistent with the trimeric
connectivities, but they also suggested some differences in the
symmetry of the two rings, especially in solution. A convincing
proof of the compounds' solid-state structures as trimers was
provided by the single-crystal X-ray diffraction studies.
The trimer [H2GaP(SiMe3)2]3 (1) appears to be the first
published example of a phosphinogallane containing the GaH2
group. It features a six-membered ring of alternating Ga and P
centers as shown in Figure 1. Curiously, the thermal parameters
for the Ga atom are about twice the magnitude of the P atom,
yet both atoms occupy structurally similar positions, i.e., both
are members of the ring. In fact, the value for Ga is similar to
that of the Si atom which is in an inherently more thermally
active side-chain position. Interestingly, unusually thermally
active group 13 atoms are also found in two related ring
systems: in [H2GaAs(SiMe3)2]3 in which P is replaced by As
The elemental analysis and spectroscopic characterization of
1 indicated a product of {H2GaP(SiMe3)2} stoichiometry. In
this regard, the infrared Ga-H stretching frequency at 1837
5
(Vide infra), and in [H2BP(SiMe3)2]3 in which Ga is replaced
by B. On the basis of crystallographic constraints for 1, the
hexagonal site symmetry imposes strict mirror-plane symmetry
on the ring atoms. However, the molecular symmetry of 1 may
be lower than the crystallographic site symmetry. Another factor
contributing to ring puckering can stem from inherent differ-
1
cm-1 was characteristic of terminal GaH2 moieties.8,9 The H
NMR spectrum obtained at 400 MHz consisted of an atypical,
significantly broadened doublet for the SiMe3 protons at δ 0.45
(6) (a) Janik, J. F.; Duesler, E. N.; Paine, R. T. Inorg. Chem. 1987, 26,
4341. (b) Janik, J. F.; Duesler, E. N.; Paine, R. T. Inorg. Chem. 1988, 27,
4335.
(7) See for example: (a) Downs, A. J., Ed. Chemistry of Aluminium,
Gallium, Indium and Thallium, Blackie-Chapman Hall: London, 1993. (b)
Jones, C.; Kousantonis, G. A.; Raston, C. L. Polyhedron 1993, 12, 1829.
(8) Pulham, C. R.; Downs, A. J.; Goode, M. J.; Rankin, D. W. H.;
Robertson, H. E. J. Am. Chem. Soc. 1991, 113, 5149 (and references therein).
(9) (a) Pulham, C. R.; Downs, A. J.; Rankin, D. W. H.; Robertson, H.
E. J. Chem. Soc., Dalton Trans. 1992, 1509. (b) Henderson, M. J.; Kennard,
C. H. L.; Raston, C. L.; Smith, G. J. Chem. Soc., Chem. Commun. 1990,
1203. (c) Baxter, P. L.; Downs, A. J.; Rankin, D. W. H.; Robertson, H. E.
J. Chem. Soc., Dalton Trans. 1985, 807. (d) Hwang, J.-W.; Hanson, S. A.;
Britton, D.; Evans, J. F.; Jensen, K. F.; Gladfelter, W. L. Chem. Mater.
1990, 2, 342.

534 J. Am. Chem. Soc., Vol. 120, No. 3, 1998
Janik et al.
Figure 1. Thermal ellipsoid diagram (35% probability ellipsoids)
showing the molecular structure of 1. All carbon-hydrogen atoms are
omitted for clarity.
Figure 2. Thermal ellipsoid diagram (35% probability ellipsoids)
showing the molecular structure of 2. All carbon-hydrogen atoms are
omitted for clarity.
ences in bonding of Ga and P atoms in the ring which result in
the Ga atoms that have greater librational freedom or, alterna-
tively, more rigid P atoms can accommodate more ring strain
compared to their neighbors. We calculated librational correc-
tions for the vibrationally averaged positions, which led to an
increase in the average Ga-P distance of only 0.006 Å.
However, even this small change indicates that the ring is
probably not planar, in accord with the analogous structures.
The ring angles in 1 illustrate some of the points discussed
above. The Ga-P-Ga angles of 126.35(10)° are significantly
larger than the neighboring P-Ga-P angles of 113.65(10)°.
Alternatively, this can be rationalized in terms of a greater ring
strain at the P centers (rigidity) than at the Ga centers (librational
freedom). A few related and structurally characterized (or
calculated) organophosphine gallane adducts H3Ga‚PR3 with
four-coordinated Ga and P atoms, and typical dative bonds,
provide the following Ga-P bond lengths: H3Ga‚P(C6H11)3,10a
2.460(2) Å; (H3Ga)2‚(PMe2CH2)2,10a 2.399(4) Å; H3Ga‚PH3
(calculated),10a 2.576 Å; H3Ga‚P(t-Bu)3,10b 2.444(6) Å; H3Ga‚
PMe3 (calculated),10b 2.550 Å. The average Ga-P length in 1
of 2.392 Å is, however, more appropriately compared with the
Ga-P distances found in the planar dimers exemplified by [Cl2-
GaP(SiMe3)2]2,11a av 2.379 Å, [Br2GaP(SiMe3)2]2,11b av 2.386
Å, [I2GaP(SiMe3)2]2,11c av 2.397 Å, [Me2GaP(SiMe3)2]2,11d
2.456(1) Å, and (Et2O)2Li[µ-P(SiMe3)2]2GaH2,4 2.4122(12) Å
or in the trimers such as planar [t-Bu2GaPH2]3,12a 2.439(3) Å,
boat [Me2GaP(i-Pr)2]3,12b av 2.442 Å, or chair [Me2GaP(Me)-
Ph]3,12c av 2.410 Å.
flattening wherein the ligands assume a close to symmetrical
arrangement above and below the core. Such a ligand config-
uration could be partly responsible for the observed equivalency
of the SiMe3 protons and carbons in the NMR spectra. The
ring flattening, with maximum displacement from planarity of
0.25 Å, is demonstrated by the close and small values of the
Ga-As-Ga-As torsion angles listed in Table 2. It also has
some impact on the ring internal angles and associated ligand
strain. The As-Ga-As average angle, 110.95°, is smaller than
the average Ga-As-Ga angle, 127.08°, indicating significant
strain at the As centers in accord with what was discussed above
for 1. This, however, seems to be compensated by the smaller
Si-As-Si angles (av 107.53°) than the H-Ga-H angles (av
127°), an apparent benefit of the small H ligand size.
A
similarly flattened ring was earlier reported for the related borane
derivative, [H2BP(SiMe3)2]3,5 in a twist chair conformation. The
Ga-As average bond length in 2, 2.4742 Å, is rather short
among the relevant four-coordinated Ga-As compounds and
can be compared with the bond distances in the related (Et2O)2-
Li[µ-As(SiMe3)2]2GaH2,4 2.4941(5) Å, [I2GaAs(SiMe3)2]2,13a av
2.471 Å, or [(Me3SiCH2)2GaAs(SiMe3)2]2,13b av 2.567 Å. Also,
the Ga-H average bond length, 1.50(3) Å, compares well with
the Ga-H distances in the closely related (Et2O)2Li[µ-E(SiMe3)2]2-
GaH2,4 where for E ) P it was 1.58(4) Å and for E ) As it
showed 1.51(5) Å. Generally, this value is in the typical range
for terminal Ga-H bond lengths as illustrated by the following
cases: Ga2H6 (gas phase),14 1.519(35) Å for the terminal
hydrogens (1.710(38) Å for the bridging hydrogens); [Me2-
NGaH2]2 (gas phase),9c 1.487(36) Å; a solid product from the
reaction between H3Ga‚NMe3 and 1,4-di-tert-butyl-1,4-diaza-
butadiene9b containing a terminal GaH2 group, 1.57(8) Å and
1.54(12) Å.
The molecular structure of [H2GaAs(SiMe3)2]3 (2) in the solid
state, the first reported arsinogallane containing the GaH2
moiety, is shown in Figure 2. It confirms a trimeric core of
alternating four-coordinated Ga and As centers. The ring has
an apparent twist boat conformation but displays a significant
(10) (a) Atwood, J. L.; Robinson, K. D.; Bennett, F. R.; Elms, F. M.;
Koutsantonis, G. A.; Raston, C. L.; Young, D. J. Inorg. Chem. 1992, 31,
2673. (b) Elms, F. M.; Gardiner, M. G.; Koutsantonis, G. A.; Raston, C.
L.; Atwood, J. L.; Robinson, K. D. J. Organomet. Chem. 1993, 449, 45.
(11) (a) Wells, R. L.; Self, M. F.; McPhail, A. T.; Aubuchon, S. R.;
Woudenberg, R. C.; Jasinski, J. P. Organometallics 1993, 12, 2832. (b)
Aubuchon, S. R.; McPhail, A. T.; Wells, R. T.; Giambria, J. A.; Bowser,
J. R. Chem. Mater. 1994, 6, 82. (c) Aubuchon, S. R.; Lube, M. S.; Wells,
R. L. Chem. Vap. Deposition 1995, 1, 1. (d) Dillingham, M. D. B.; Burns,
J. A.; Byers-Hill, J.; Gripper, K. D.; Robinson, G. H. Inorg. Chim. Acta
1994, 216, 267.
Theoretically, decomposition of the trimers, [H2GaE(SiMe3)2]3,
Via dehydrosilylation should result in the GaE binaries that,
noting the efficiency of the HSiMe3 elimination at ambient
conditions, should take place at relatively low temperatures
compared with other precursor routes, especially for E ) P. In
1a,b,e,f
this regard, the relevant X3Ga/P(SiMe3)3
(X ) halide)
dehalosilylation systems were earlier reported to form stable
(12) (a) Cowley, A. H.; Harris, P. R.; Jones, R. A.; Nunn, C. M.
Organometallics 1991, 10, 652. (b) Cowley, A. H.; Jones, R. A.; Mardones,
M. A.; Nunn, C. M. Organometallics 1991, 10, 1635. (c) Beachley, O. T.,
Jr.; Royster, T. L.; Arhar, J. R.; Rheingold, A. L. Organometallics 1993,
12, 1976.
(13) (a) Johansen, J. D.; McPhail, A. T.; Wells, R. L. AdV. Mater. Opt.
Electron. 1992, 1, 29. (b) Wells, R. L.; Pasterczyk, J. W.; McPhail, A. T.;
Johansen, J. D.; Alvanipour, A. J. Organomet. Chem. 1991, 407, 17.
(14) Pulham, C. R.; Downs, A. J.; Goode, M. J.; Rankin, D. W. H.;
Robertson, H. E. J. Am. Chem. Soc. 1991, 113, 5149.

New Pnictinogallanes [H2GaE(SiMe3)2]3 (E ) P, As)
J. Am. Chem. Soc., Vol. 120, No. 3, 1998 535
N(SiMe3)3 was obtained from Aldrich and used as received. 1H, 13C-
{1H}, and 31P NMR spectra were recorded on the Varian Unity 400
and Unity 600 spectrometers at 25 °C from toluene-d8 solutions and
referenced Vs SiMe4 by generally accepted methods. Mass spectra were
collected on a JEOL JMS-SX 102A spectrometer operating in the EI
mode at 20 eV; the assignment of the ion fragments was supported by
comparison with theoretical ion distributions. IR spectra of solids and
gaseous pyrolysis products were acquired by use of KBr pellets and a
gas cell, respectively, on a BOMEM Michelson MB-100 FT-IR
spectrometer. A calibrated manifold was used for volume estimations
of reaction gases. TGA/DTA analyses were acquired under an UHP
nitrogen flow on a TA Instruments SDT 2960 simultaneous TGA/DTA
apparatus. Elemental analyses were provided by E + R Microanalytical
Laboratory, Corona, NY. Melting points (uncorrected) were determined
with a Thomas-Hoover Uni-melt apparatus for samples flame sealed
in glass capillaries. Single-crystal X-ray diffraction study for 1 was
performed at the University of Delaware, Department of Chemistry
and Biochemistry, Newark, DE, on a Siemens P4 diffractometer, and
study for 2 was carried out at the University of Minnesota, X-ray
Crystallographic Laboratory, Minneapolis, MN, on a Siemens SMART
Platform CCD system with in both cases Mo KR radiation (λ ) 0.71073
Å) at 203(2) K for 1 and at 173(2) K for 2. All calculations were
carried out with the SHELXTL V5.0 or V5.03 suite of programs;18 the
structures were solved by direct methods. XRD data were collected
by using mineral oil coated samples on a Phillips XRD 3000
diffractometer utilizing Cu KR radiation; the average particle sizes of
GaP and GaAs pyrolysis products were calculated by using the Scherrer
equation applied to the (111) diffraction lines of the cubic GaP and
GaAs.
adducts at room temperature and only prolonged sonication or
reflux in hexane caused a removal of one Me3SiX equivalent
and the formation of [X2GaP(SiMe3)2]2. The pyrolyses of these
adducts at 450 °C under vacuum resulted in the formation of
mostly nanocrystalline GaP but also in retention of some Si/C
phases from thermal cracking of residual SiMe3 groups.1e On
1a,f,h,i
the other hand, the X3Ga/As(SiMe3)3
dehalosilylation
system was found for X ) Cl to eliminate at room temperature
approximately 2 equiv of Me3SiCl, but for X ) Br, I only 1
equiv of Me3SiX was observed. The pyrolyses of these
precursors yielded GaAs solids apparently containing none or
very little Si/C residues.
In the present study, the pyrolyses of the precursors from
the {GaH3}/P(SiMe3)3 system, compound 1 and the related
colored polymeric solid, under applied conditions afforded
nanocrystalline GaP with the average crystallite size of 2.3 (450
°C, vacuum) or 29.4 nm (600 °C, vacuum), and 5.1 nm (475
°C, vacuum), respectively. However, a mismatch between the
theoretical and observed TGA weight losses, the presence of
CH4 in the pyrolysis off-gases, and the elemental analysis of
the pyrolysates all clearly indicated an incomplete HSiMe3
elimination and the resulting cracking of the SiMe3 groups that
eventually led to the retention of Si/C amorphous phases in the
final product. Additionally, the detection of the P(SiMe3)3
byproduct pointed to other decomposition side reactions as well.
Interestingly, the analyzed silicon and carbon contents were
similar to those observed for some of the pyrolysis products
from the X3Ga/P(SiMe3)3 dehalosilylation systems.1e Regarding
the {GaH3}/As(SiMe3)3 system, the pyrolysis of pure compound
2 was not possible due to its fast decomposition at ambient
conditions. However, the reflux in xylenes (bp 137-144 °C)
of the colored polymeric solid obtained in this system resulted
in a product with an XRD spectrum that indicated the onset of
GaAs crystallinity. The pyrolysis of the polymeric solid at 450
°C under vacuum yielded nanocrystalline GaAs with the average
crystallite size of 3.4 nm as determined by XRD. Similarly
with the analogous phosphorus system, the analytical evidence
supported the retention of the Si/C species in the final product.
Additionally, there appeared to be a significant excess of
gallium, i.e., Ga/As was found at 1.5/1.0. This feature was a
likely consequence of the low barrier decomposition of the
precursors along the E(SiMe3)3 and H2 elimination pathways
which were much more acute for E ) As than for E ) P. These
pathways would have been expected to lead to some gallium
rich phase due to arsenic depletion (decay of GaH2 moieties
associated with H2 evolution plus the formation and removal
of the soluble As(SiMe3)3 byproduct). It should be stressed,
however, that the major crystalline product by XRD was the
nanosized GaAs and both the minor gallium rich and Si/C
byproducts seemed to be separate. In summary, the facile
HSiMe3 elimination-condensation chemistry for these precursors
was found to coexist and compete with other decomposition
pathways associated with the inherent fragility of the Ga-H
bonds.
Reaction System H3Ga‚NMe3/P(SiMe3)3. (1) Synthesis of [H2GaP-
(SiMe3)2]3 (1) in Et2O. A sample of freshly prepared H3Ga‚NMe3 (0.40
g, 3.0 mmol) was dissolved in 30 mL of Et2O. To this was added a
solution of P(SiMe3)3 (0.75 g, 3.0 mmol) in 15 mL of Et2O at room
temperature. After being stirred overnight, the solution was partially
evacuated to 5-10 mL and stored at -30 °C, which resulted in the
formation of small colorless crystals. The crystals were separated from
the mother liquor and evacuated for 10 min (0.48 g). After the mother
liquor was further concentrated to less than 5 mL, another batch of
crystals was obtained at -30 °C (0.10 g). Total yield, 0.58 g or 78%
based on eq 1 (vide infra). Storage of solid 1 at room temperature for
several hours to days resulted in a gradual color change to yellow.
X-ray quality crystals were first obtained at -30 °C from both the
ethereal and toluene solutions of 1; however, serious twinning problems
prevented a satisfactory crystallographic refinement. Crystals of 1 of
better quality were eventually obtained from a related reaction system
in which 1 appeared to be a major byproduct.19 Melting behavior:
150-160 °C, change of color to brown-black; no melting to 300 °C.
Anal. Found (calcd) for C18H60Ga3P3Si6: C, 28.72 (28.93); H, 7.85
(8.09). 1H NMR (400 MHz): δ 0.45 (54H, SiMe3; high-neck doublet,
apparent JP-H coupling of 4.6 Hz), 4.55 (6H, GaH2, br). 1H NMR (600
MHz): δ 0.45 (54H, SiMe3; high-neck doublet, apparent JP-H coupling
of 3.6 Hz), 4.54 (6H, GaH2). 13C{1H} NMR (400 MHz) (intensity):
δ 1.982 (80), 1.955 (100), 1.929 (100), 1.902 (80). 13C NMR (600
MHz) (intensity): δ 1.998 (85), 1.979 (100), 1.965 (80), 1.946 (90).
31P{1H} NMR (400 MHz): δ -265.8. MS: m/e (intensity)(ion): peak
clusters around 746 (3)([H2GaP(SiMe3)2]3 or M), 731 (1)(M - Me),
673 (1)(M - SiMe3), 568 (25)(M - P(SiMe3)2), 496 (100)([H2GaP-
(SiMe3)2]2 - 2H or M* - 2H), 483 (2)(M* - Me), 425 (13)(M* -
SiMe3), 409 (2)(M* - SiMe3 - Me - H), 318 (12)(M* - P(SiMe3)2
- 3H), 247 (40)(H2GaP(SiMe3)2 - H or M** - H; possible contribution
from P(SiMe3)3 at m/e 250), 235 (2)(P(SiMe3)3 - Me), 178 (28)(HP-
(SiMe3)2), 163 (11)(HP(SiMe3)2 - Me), 147 (15)(P(SiMe3)2 - 2Me),
Experimental Section
73 (26)(SiMe3), 59 (2)(HSiMe2). IR: ν(Ga-H) 1837 cm-1
.
General Techniques. All experiments were performed with stan-
dard vacuum/Schlenk techniques. Solvents were distilled from sodium
benzophenone ketyl or Na/K alloy prior to use. H3Ga‚NMe3,15
(18) SHELXTL-plus V5.03 (1) or V5.0 (2), Siemens Industrial Automa-
tion, Inc., Madison, WI.
17
P(SiMe3)3,16 and As(SiMe3)3 were prepared by literature methods.
(19) The reactions of (Et2O)2Li[µ-P(SiMe3)2]GaH2 with some compounds
of the type RnMX3-n (M ) group 13 element; R ) H, alkyl, aryl; X )
halogen; n ) 0, 1, 2), performed in our laboratory, have yielded [H2GaP-
(SiMe3)2]3 as a major byproduct. X-ray quality crystals of the trimer were
obtained from the reaction between (Et2O)2Li[µ-P(SiMe3)2]GaH2 and Me2-
BBr in toluene (Wells, R. L.; Jouette, J. R.; Janik, J. F. To be submitted for
publication).
(15) Shirk, A. E.; Shriver, D. F. Inorg. Synth. 1977, 17, 42.
(16) Becker, G.; Ho¨lderich, W. Chem. Ber. 1975, 108, 2484.
(17) (a) Becker, G.; Gutenkunst, G.; Wessely, H. J. Z. Anorg. Allg. Chem.
1980, 462, 113. (b) Wells, R. L.; Self, M. S.; Johansen, J. D.; Laske, J. A.;
Aubuchon, S. R.; Jones, L. J. Inorg. Synth. 1997, 31, 150.

536 J. Am. Chem. Soc., Vol. 120, No. 3, 1998
Janik et al.
Table 1. Crystallographic Data and Measurements for 1 and 2
possible H2), approximately 0.8 mmol (0.01 g); condensables (HSiMe3),
1.8 mmol or 0.13 g. Weight of volatiles, 0.14 g. An XRD pattern for
the black product matched JCPDS file 12-191 for cubic GaP; average
particle size, 5.1 nm.
1
2
molecular formula
formula weight
crystal system
space group
a, Å
b, Å
c, Å
C18H60Ga3P3Si6
747.27
hexagonal
P63/m
11.8477(2)
11.8477(2)
16.7845(4)
90
2040.36(7)
2
C18H60Ga3As3Si6
879.12
monoclinic
P21/n
11.9593(2)
16.7702(3)
20.3940(4)
90.3640(10)
4090.14(13)
4
Reaction System H3Ga‚NMe3/As(SiMe3)3. (1) Synthesis of
[H2GaAs(SiMe3)2]3 (2) in Et2O. A sample of freshly prepared H3-
Ga‚NMe3 (0.40 g, 3.0 mmol) was dissolved in 20 mL of Et2O. To
this, a solution of As(SiMe3)3 (0.88 g, 3.0 mmol) in 20 mL of Et2O
was added at room temperature. After being stirred overnight, the
yellow solution was concentrated to 5-10 mL and stored at -30 °C.
After 1 to 2 days, the solution darkened and some colorless crystals
appeared, 0.21 g (24% based on eq 1 (vide infra)). X-ray quality
crystals were obtained at -30 °C from the ethereal solution of 2 after
separating freshly formed crystals from the mother liquor. If the crystals
were left in the liquor at -30 °C or higher temperatures, they would
soon disappear, the solution acquiring an orange color, and a yellow-
orange solid precipitating. Also, increased concentrations of the
reactants accelerated the formation of the insoluble product. For
crystals, melting behavior: 130-150 °C, change of color to black; no
melting to 300 °C. Anal. Found (calcd) for C18H60Ga3As3Si6: C, 24.37
(24.59); H, 6.68 (6.88). 1H NMR: δ 0.49 (54H, SiMe3), 4.65 (6H,
GaH2, br). 13C{1H} NMR: δ 2.15. MS. m/e (intensity)(ion): peak
clusters around 877 (3)([H2GaAs(SiMe3)2]3 - H or M - H), 805 (3)-
(M - SiMe3), 791 (2)(M - SiMe3 - Me + H), 731 (1)(M - 2SiMe3
- H), 657 (83)(M - 3SiMe3 - 2H or M - As(SiMe3)2), 586 (100)([H2-
GaAs(SiMe3)2]2 or M*), 512 (100)(M* - SiMe3 - H), 497 (11)(M* -
SiMe3 - Me - H), 363 (10)(M* - As(SiMe3)2 - 2H or M* - 3SiMe3
- 4H), 291 (100)(H2GaAs(SiMe3)2 - H or M** - H), 222 (93)(HAs-
(SiMe3)2), 206 (56)(As(SiMe3)2 - Me), 191 (11)(As(SiMe3)2 - 2Me),
134 (31)(HAsSiMe2), 73 (100)(SiMe3), 69 (4)(Ga), 59 (32)(HSiMe2).
â, deg
V, Å3
Z
radiation (wavelength, Å) Mo KR (0.71073)
Mo KR (0.71073)
µ, mm-1
2.268
4.554
temp, K
203(2)
1.216
173(2)
1.428
D
calcd, g/cm3
crystal dimens, mm
max/Tmin
Θ range for data
collection, deg
0.4 × 0.4 × 0.4
1.00/0.50
1.98-28.28
0.38 × 0.35 × 0.30
1.0000/0.7527
1.57-25.02
T
no. of reflcns collected
7582
20340
no. of independent reflcns 1681 (Rint ) 0.0371) 7149 (Rint ) 0.0334)
data/restraints/parameters 1673/0/53
7149/15/295
0.0503; 0.0866
0.0801; 0.0943
1.096
R1 (I>2σ(I));awR2b
R1 (all data);awR2b
goodness of fitc
0.0893; 0.1989
0.1102; 0.2254
1.127
final max/min ∆F, e/Å-3 0.879/-0.926
0.903/-0.763
2
2
a Rint ) ∑|Fo2 - Fo |/∑|Fo |; R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 )
[∑[w(Fo - Fc2)2]/∑[w(Fo )2]]1/2 where w ) 1/σ2(Fo ) + (aP)2 + bP.
2
2
2
c GooF ) S ) [∑[w(Fo - Fc2)2]/(n - p)1/2
.
2
IR: ν(Ga-H) 1834 cm-1
.
(2) Reaction of H3Ga‚NMe3 with P(SiMe3)3 in Toluene. The
reaction in toluene was performed similarly as above. After 2 days at
room temperature, the mixture turned yellow and, after an additional 2
days at -30 °C, orange. At the end of the fourth day, the volatiles
were removed and the resulting orange solid was evacuated for 3 h.
(2) Reaction of H3Ga‚NMe3 with As(SiMe3)3 in Toluene. The
reaction in toluene was performed similarly as above. Coloration of
the solution with concurrent formation of colorless crystals were
observed at -30 °C 1 day past addition, as in the Et2O reaction system.
Upon concentrating the mixture to approximately 10 mL, a yellow-
orange insoluble solid formed and no crystals were observed anymore.
After one week of storage at -30 °C, the volatiles were removed
affording an orange solid. EA: Ga, 29.29; As, 36.79; C, 19.39; H,
4.84; Ga/As ) 0.9/1.0; H/C ) 3.0/1.0; C/As ) 3.3/1.0. 1H and 13C-
{1H} NMR spectra for a toluene-d8 slurry of the solid showed peaks
for As(SiMe3)3, HSiMe3, and H2 and a weak, very broad feature at δ
0.7. IR: ν(Ga-H) 1830 cm-1 (br).
(3) Thermal Decomposition Studies. Pyrolysis of the Toluene-
Insoluble Solid. A TGA curve for the orange solid isolated one week
after addition (as above) showed two stages for weight changes: 30-
250 °C, 13% and 250-500 °C, 9%; total weight loss, 22%. For
pyrolysis, 0.32 g of the orange solid was placed in a sublimer that was
attached to a -196 °C cold trap. The system was briefly evacuated
and closed under static vacuum. The sublimer was heated at 200 °C
for 2 h and at 475 °C for 3 h, and volatiles were collected. Some
orange-brown solid deposited on the colder parts. The final product
was a black solid, 0.20 g, or weight loss of 0.12 g. EA: Ga, 48.23;
As, 34.26; Si, 3.03; C, 0.23; H, 0.12; Ga/As ) 1.5/1.0; As/Si ) 4.2/
1.0. Noncondensables (CH4 and possible H2), approximately 0.5 mmol
(0.008 g); condensables (HSiMe3), 1.1 mmol (0.081 g). Weight of
volatiles, 0.09 g. An XRD pattern for the black product matched
JCPDS file 14-450 for cubic GaAs; average particle size, 3.4 nm. In
another experiment, some of the orange solid was refluxed in xylenes
(bp 137-144 °C) for 40 h. Soon after the beginning of reflux, the
solid turned dark brown-black. Upon completion, the dark solid was
dried under vacuum. An IR spectrum for the solid contained some
weak vibrations for CH3 groups but no Ga-H stretches. An XRD
spectrum showed two broad halos at 2Θ 28° and 49° that were
consistent with the onset of GaAs crystallinity.
1
The solid could not be redissolved in fresh toluene afterward. A H
NMR study of the toluene-d8 slurry of the orange solid showed abundant
P(SiMe3)3, some HSiMe3, H2,8 and a very broad peak feature at δ 0.6;
no discernible Ga-H resonances were observed between δ 4 and 6.
IR: ν(Ga-H) 1842 cm-1 (br).
(3) Thermal Decomposition Studies. (a) Pyrolysis of 1. The
weight changes by TGA started at 120 °C. Two severely overlapping
weight loss stages between 120 and 230 °C were observed with a total
weight loss of 55.5%. No additional weight loss occurred up to 500
°C. Calculated weight change for: [H2GaP(SiMe3)2]3 ) 3GaP +
6HSiMe3, 59.6%. A sample of 1 [0.23 g (0.31 mmol)] was pyrolyzed
in a sublimator under vacuum at 200 °C for 2 h and at 450 °C for 3 h
with the collection of volatiles. Small amounts of a viscous liquid
were observed to condense on colder parts of the apparatus (confirmed
to be P(SiMe3)3 by NMR) and the main product was a black solid,
0.12 g (theoretical 0.10 g). EA: Ga, 61.26; P, 27.16; Si, 5.59; C, 1.00;
H, 0.48; Ga/P ) 1.0/1.0; P/Si ) 4.4/1.0; C/Si ) 0.4/1.0. Noncon-
densables (CH4 and H2), approximately 0.65 mmol (0.01 g); condens-
ables (HSiMe3), 0.95 mmol (0.07 g). Weight of volatiles, approximately
0.08 g. An XRD pattern for the black product matched JCPDS file
12-191 for cubic GaP; average particle size, 2.3 nm. Another sample
of 1 was pyrolyzed under vacuum at 400 °C for 2 h and at 600 °C for
0.5 h, and an XRD pattern for the black product matched JCPDS file
12-191 for cubic GaP; average particle size, 29.4 nm.
(b) Pyrolysis of the Toluene-Insoluble Solid. Continuous weight
changes by TGA for the orange product from the reaction in toluene
commenced at 30 °C with most losses occurring in the 100-200 °C
range, 20%, and smaller losses to 400 °C, 3%; total weight change to
500 °C, 23%. In another experiment, 0.38 g of the orange solid was
placed in a sublimer that was attached to a -196 °C cold trap. The
system was briefly evacuated and then closed under static vacuum.
The sublimer was heated at 200 °C for 2 h and at 475 °C for 3 h with
the collection of volatiles. Some brown solid deposited on the colder
parts. The final product was a shiny black solid, 0.22 g or weight loss
of 0.16 g. EA: Ga, 64.76; P, 28.69; Si, 2.64; C, 3.53; H, 0.79; Ga/P
) 1.0/1.0; P/Si ) 9.9/1.0; C/Si ) 3.1/1.0. Noncondensables (CH4 and
Structural Analyses of 1 and 2. Suitable colorless crystals of 1
and 2 were sealed in glass capillaries. Specimens of 2 were stored at
-30 oC prior to determinations and a locally designed low-temperature
transfer device was used to keep the sample cold (<-40 °C) during
its removal from the capillary and subsequent transfer to the goniometer.
For 1, systematic absences in the data and diffraction symmetry

New Pnictinogallanes [H2GaE(SiMe3)2]3 (E ) P, As)
J. Am. Chem. Soc., Vol. 120, No. 3, 1998 537
Table 2. Selected Bond Distances (Å) and Angles (deg) for 1 and 2 with Estimated Standard Deviations in Parentheses
1
2
Bond Lengths
As(1)-Ga(1)
P(1)-Ga(1)
2.387(2)
2.397(2)
2.4752(8)
2.4695(8)
2.4717(9)
2.4733(8)
2.4831(8)
2.4721(8)
1.50(3)
1.49(3)
2.355
2.349(2)/2.361(2)
1.857
1.847(7)/1.868(7)
P(1)-Ga(1B)
As(1)-Ga(3)
As(2)-Ga(1)
As(2)-Ga(2)
As(3)-Ga(2)
As(3)-Ga(3)
Ga(1)-H(1)
Ga(1)-H(2)
As-Si(av)
Ga(1)-H(4)
P-Si
1.50 (assumed)
2.267(3)
As-Si; min/max
Si-C (av)
Si-C; min/max
Si-C (av)
Si-C; min/max
1.827
1.810(12)/1.847(9)
Bond Angles
Ga(1B)-P(1)-Ga(1)
P(1A)-Ga(1)-P(1)
H(4)-Ga(1)-H(4A)
Si(1B)-P(1)-Si(1)
126.35(10)
113.65(10)
107.7 (assumed)
107.9(2)
Ga(3)-As(1)-Ga(1)
Ga(3)-As(3)-Ga(2)
Ga(1)-As(2)-Ga(2)
As(2)-Ga(1)-As(1)
As(2)-Ga(2)-As(3)
As(1)-Ga(3)-As(3)
H(2)-Ga(1)-H(1)
H(4)-Ga(2)-H(3)
H(6)-Ga(3)-H(5)
Si(2)-As(1)-Si(1)
Si(4)-As(2)-Si(3)
Si(6)-As(3)-Si(5)
H(2)-Ga(1)-As(2)
H(2)-Ga(1)-As(1)
H(1)-Ga(1)-As(2)
H(1)-Ga(1)-As(1)
127.99(3)
126.57(3)
126.69(3)
111.76(3)
110.64(3)
110.49(3)
132(4)
129(4)
121(4)
108.74(7)
105.95(8)
107.91(7)
97(3)
H(4)-Ga(1)-P(1)
108.8
95(3)
117(3)
102(3)
Torsion Angles
Ga(2)-As(2)-Ga(1)-As(1)
Ga(1)-As(2)-Ga(2)-As(3)
Ga(1)-As(1)-Ga(3)-As(3)
-2.19(6)
-16.38(6)
-4.86(6)
indicated any of the hexagonal space groups P63, P63/m, and P6322.
E-statistics strongly favored the centrosymmetric alternative, P63/m,
which proved to be correct on the basis of the results of refinement.
For 2, systematic absences and intensity statistics determined the space
group P21/n. The structures were solved by direct methods. All non-
hydrogen atoms were refined with anisotropic displacement parameters.
For 1, all hydrogen atoms were placed in ideal positions and refined
as riding atoms with relative isotropic displacement parameters. In
the case of 2, the C hydrogens were treated as above, but the Ga
hydrogens were all located from the difference map and were refined
isotropically; the Ga-H bond lengths were constrained to be similar
to each other by using the SHELXL constraint SADI. Details of the
data collection for 1 and 2 are summarized in Table 1, and Table 2
contains selected bond distances and angles.
Acknowledgment. R.L.W. thanks the Office of Naval
Research for its financial support.
Supporting Information Available: Crystal packing dia-
grams for 1 and 2; tables of bond distances, bond and torsion
angles, anisotropic temperature factor parameters, and atomic
fractional coordinates for 1 and 2 (12 pages). See any current
masthead page for ordering and Internet access instructions.
JA9731837