Infrared spectra of indium hydrides in solid hydrogen and of solid iadane

Article abstract of DOI:10.1002/anie.200353216

One little indium: Reactions of laser-ablated indium atoms with pure hydrogen give sharp IR absorptions corresponding to InH intermediate species. Irradiation at 193 nm maximizes further reaction to give the indane monomer InH₃. Annealing provides evidence for In₂H₆ and ultimately InH₃ → 1/2 In₂ H₆ → 1/n (InH₃)_n the sharp absorption bands are replaced with a broad IR band centered at 1460 cm^-1 assigned to solid indane (InH₃)_n [see Equation].

Full text of DOI:10.1002/anie.200353216

Communications
Matrix Isolation
Indium (99.99%) was melted into a steel cup and the soild
indium sample was rotated to minimize local heating of the
target. Aseries of infrared spectra were recorded after matrix
sample preparation to form indium-hydride intermediate
species in a solid-hydrogen matrix at 3.5 K, and after various
subsequent treatments (Figure 1). Analogous experiments
with D2 give the spectra shown in Figure 2.
Infrared Spectra of Indium Hydrides in Solid
Hydrogen and of Solid Indane**
Lester Andrews* and Xuefeng Wang
ꢀ
The inherent weakness of the In H bond might be useful for
the design of precursors for chemical vapor deposition (CVD)
to make indium-containing semiconductor devices. Afew
interesting indium hydride complexes have been prepared
[1–3]
from InH3NMe3 and LiInH4
but the primary indium
hydride, indane (InH3), has endured a controversial existence.
Claims for the early preparation of solid indane by Wiberg
et al.[4] could not be reproduced.[5] Mass spectroscopic inves-
tigation[6] of the comparative stabilities of AlH3, GaH3, and
InH3 also casts doubt on the stability of (InH3)n. Reviews
continue to state that indane has not yet materialized[7] and
theoretical calculations suggest that solid indane lacks room-
temperature stability.[8] However, bridging In H In bonds
ꢀ ꢀ
formed on indium-rich InP semiconductor surfaces are stable
at room temperature.[9]
There is spectroscopic evidence for the intermediate InH,
InH2, and InH3 molecules.[10–14] Recently Mitzel concluded
that the next question remaining to be answered concerns the
possible isolation of InH3.[15] In contrast, thalium hydride
(TlH) has only been investigated in the gas phase,[10] and there
is no experimental evidence to date for TlH2 and TlH3.
We recently reported the preparation of dialane using the
reaction of laser-ablated aluminum with pure H2 during
condensation at 3.5 K.[16] The reaction first formed AlH, UV
photolysis and annealing in solid hydrogen then gave AlH3
and Al2H6. After evaporation of the H2 matrix host, a solid
alane film remained on the CsI window until removed by
cleaning at room temperature. This film exhibited broad IR
absorptions at 1720 and 720 cmꢀ1, which are almost identical
to the 1760 and 680 cmꢀ1 bands reported for solid (AlH3)n
Figure 1. Infrared spectra in the 1900–1000 cmꢀ1 region for laser-
ablated indium codeposited with normal isotopic hydrogen at 3.5 K.
a) Pure H2 and In deposited, b) after 193 nm irradiation for 3 min,
c) after 193 nm irradiation for 58 min (total), d) after annealing to
6.2 K, e) after annealing to 8 K and removing the H2 matrix host,
f) spectrum recorded at 70–100 K, and g) spectrum recorded at
180–200 K.
samples containing only bridging Al H Al linkages.[16–19] We
ꢀ ꢀ
report herein the first successful preparation of solid (InH3)n
using this new cryogenic method, and its spectroscopic
characterization as being isostructural to solid (AlH3)n. This
work refutes the Wiberg claim[4] for the synthesis of (InH3)n.
The laser-ablation experiment in conjunction with pure
hydrogen as a matrix at 3.5 K has been described else-
where.[16,17,20] Clearly the ablation laser energy must be
limited or the heat load from material and radiation directed
to the cold window will prevent the deposition of solid
hydrogen.
Figure 2. Infrared spectra in the 1400–700 cmꢀ1 region for laser-
ablated indium codeposited with deuterium at 3.5 K. a) Pure D2 and In
deposited, b) after 193 nm irradiation for 20 min, c) after 193 nm irra-
diation for 40 min (total), d) after annealing to 9.0 K, e) spectrum
recorded at 30–60 K after removal of the D2 matrix host, f) spectrum
recorded at 110–130 K, and g) spectrum recorded at 200–220 K.
[*] Prof. Dr. L. Andrews, Dr. X. Wang
Department of Chemistry
University of Virginia
Charlottesville, VA 22904-4319 (USA)
Fax: (+1)434-924-3710
E-mail: lsa@virginia.edu
Infrared spectra after 193 nm irradiation, Figure 1b and
Figure 2b, show the pronounced growth of new strong bands
at 1393.4 and 997.7 cmꢀ1,respectively, which are between the
[**] This work was supported by the National Science Foundation
(CHE00-78836).
gas phase (1424.8 and 1023.5 cmꢀ1
)
and solid argon
[10–12]
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DOI: 10.1002/anie.200353216
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Angewandte
Chemie
(1387.4 and 995.9 cmꢀ1)[13] fundamentals for InH and InD,
respectively. The weaker 1628.9/1563.3 cmꢀ1 and 1175.4/
1126.3 cmꢀ1 band pairs are assigned to InH2 and InD2,
respectively, as these bands are 13–15 cmꢀ1 higher the
corresponding bands documented in argon matrix.[13] Con-
tinued irradiation at 193 nm, Figure 1c and Figure 2c, max-
imizes the intensity of new absorptions at 1760.9 and
1266.2 cmꢀ1 (Table 1) which are assigned to InH3 and InD3
in solid H2 and solid D2, respectively, as they appear at slightly
higher wavenumbers than the 1754.5 and 1261.2 cmꢀ1 argon-
further annealing to 7.0 K (not shown). Similar behavior was
found for deuterium: Weak new bands were observed at 1297,
1291, 943, and 767 cmꢀ1 on annealing to 9–12 K.
The spectrum after removing the H2 matrix host shows
that the sharp InH3 product absorptions are replaced by a
broad (300 cmꢀ1 full-width at half-maximum) band centered
at 1460 ꢁ 20 cmꢀ1 (Figure 1e). Similar behavior is observed
for In and D2: the strong InD3 band at 1266.2 cmꢀ1 is replaced
by a broad (200 cmꢀ1 full-width at half-maximum) band
centered at 1060 ꢁ 20 cmꢀ1 (Figure 2e). Spectra were
recorded for both samples continuously as they warmed to
room temperature. The broad 1460 cmꢀ1 band in H2 experi-
ments is observed in spectra recorded at 30–60 K and up to
160 K, is decreased in the 160–180 K range, and is absent
above 180 K (Figure 1g). The broad 1060 cmꢀ1 band in
spectra with D2 at 30–60 K (Figure 2e) remains in the spectra
recorded up to 180–200 K and is absent above 200 K
(Figure 2g). Indium metal is deposited on the window at
room temperature.
matrix bands[13]
.
Table 1: Infrared absorptions [cmꢀ1] observed for indium and thallium
hydrides in solid hydrogen or deuterium at 3.5 K.
In, H2
In, D2
Tl, H2
Tl, D2
Hydride
1760.9
1628.9
1563.3
1407.7
1393.4
979.6
1266.2
1175.4
1126.3
1011.6
997.7
1748.4
1520.0
1390.2
1254.6
1098.8
1007.5
MH3
MH2
MH2
MH site
MH
Electronically excited indium hydride is formed very
efficiently in these experiments by activation with 193 nm
excitation[21] of In in an endothermic (46 kcalmolꢀ1)[10]
Equation (1). This electronically excited InH* reacts straight-
away with the hydrogen matrix cage to form InH3 [Eq. (2)].
1311
940
709.9
909.7
652.9
M2H2
(MH3)n
1460.0[a]
1060.0[a]
–
–
[b]
[b]
[a] Broad bands appeared after warming above 7(10) K to remove H2(D2)
and disappeared above 200–210 K. [b] Not observed for Tl.
193 nm
*
In þ H2
InH þ H
ð1Þ
ð2Þ
ƒƒƒ!
Annealing the H2 sample to 6.2 K increases the InH2
absorptions and sharpens the InH band with little effect on
the InH3 absorption (Figure 3). Further annealing to higher
temperatures was carried out to search for the possible
*
InH þ H2 ! InH3
The In2H2 (In2D2) product is detected at 979.6
(709.9) cmꢀ1, which is slightly higher than the recently
reported argon-matrix counterparts,[22] as expected. Anneal-
ing to 6.5 K (Figure 3c) triples the intensity of the 979.6 cmꢀ1
In2H2 band and sharpens the InH absorption. Further
annealing to 7.2 K increases In2H2 at the expense of InH
and decreases InH2 and InH3, while the above seven bands
formation of In2H6: Aneon “overcoat” enabled the solid H
2
to be annealed above 7 K. The InH3 monomer decreases and
the weak absorptions at 1820, 1803, 1297, and 1059 cmꢀ1
increase on annealing in the 7–8 K range. These absorptions,
labeled DI for diindane, increase together on annealing to
7.2 K (Figure 3d) and are joined by others in the lower region
at 718, 535 and 526 cmꢀ1 (not shown) These absorptions are
destroyed by 193 nm radiation, and regenerated in part by
ꢀ ꢀ
increase in intensity. Higher polymers of InH with In H In
linkages should absorb near In2H2 (or to lower wavenumber).
Hence, the broad 1460 and 1060 cmꢀ1 bands can be assigned
to absorptions of solid (InH3)n and (InD3)n (Figure 1 and 2).
The positions of the broad bands in the In/H2 and In/D2
spectra define a H/D ratio 1460/1060 = 1.377 appropriate for
ꢀ
an In H stretching vibration (InH1,2,3 intermediates exhibit
H/D ratios ranging from 1.386 to 1.394). Solid alane exhibits a
broad band (AlH3)n/(InH3)n ratio (1720/1460 = 1.18), which is
comparable to the Al/In frequency ratios in solid hydrogen
for the MH (1.16), MH2 (1.12, 1.15), and MH3 (1.07)
molecules (M = metal), and for the calculated parallel ring-
stretching mode (b3u) frequency ratio for M2H6 molecules
(1483/1280 = 1.16). This result demonstrates that the broad
ꢀ ꢀ
bands in (AlH3)n and (InH3)n are due to fundamental Al H
ꢀ ꢀ
Al and In H In stretching vibrational modes, respectively,
and implies that network solid (AlH3)n and (InH3)n have
similar structures although a different coordination environ-
ment of In cannot be excluded on this basis.
Solid gallane has an oligomer structure with both terminal
Figure 3. Infrared spectra in the 1840–1740 and 1340–940 cmꢀ1
regions for laser-ablated indium co-deposited with hydrogen at 3.5 K.
a) Pure H2 and In deposited, b) after 193 nm irradiation for 15 min,
c) after annealing to 6.2 K, and d) after annealing to 7.2 K.
and bridging Ga H bonds.[23] The chemistry of Al, Ga, and In
ꢀ
is influenced by a d-orbital contraction for Ga, which makes
Ga atoms smaller and more electronegative. In contrast Al
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Communications
and In are more electropositive, and better Lewis acids, and
stabilize the sixfold coordination network[18] with M H M
bridge bonds in solid (AlH3)n and in solid (InH3)n.
ꢀ ꢀ
We also find evidence for In2H6 molecules: This dimer is
produced during the later stages of the annealing at 6–8 K
before the solid film is formed, which suggests a small barrier
to dimerization. Although the amount of In2H2 also increases
on annealing by dimerization reaction (2InH!In2H2), the
further addition of 2H2 to form In2H6 probably requires
substantial activation energy. The indane dimerization reac-
tion (2InH3!In2H6) is calculated to be exothermic by only
1 kcalmolꢀ1.[8] The new 1820, 1803, 1297, 1059, 718, 535, and
526 cmꢀ1 absorptions are appropriate for dibridged In2H6
based on comparisons with recent MP2 calculations[8]
(Table 2) and the spectrum of Al2H6 in solid hydrogen.[16,17]
The two terminal In-H2 modes at 1820 and 1803 cmꢀ1 are
higher than for InH3, as calculated and observed, and as found
for the aluminum analogs. The two In-H-In bridging modes at
1297 and 1059 cmꢀ1 are 35 cmꢀ1 above and 30 cmꢀ1 below the
Figure 4. Infrared spectra in the 1800–1260 cmꢀ1 region for laser-
ablated thallium codeposited with hydrogen at 3.5 K. a) Pure H2 and Tl
deposited, b) after 193 nm irradiation for 5 min, c) after l>240 nm
irradiation for 10 min, and d) after annealing to 8 K.
low yield of TlH3.[8,25] Finally, this result casts
doubt on the claimed synthesis of
(TlH3)n.[26,27]
Table 2: Calculated and observed infrared active vibrational modes for dibridged In2H6 (D2h).
Symmetry
Calcd[a]
[cmꢀ1
Intensity[a]
Observed[b]
[cmꢀ1
Intensity[b]
Observed[c]
[cmꢀ1
]
H/D ratio
]
]
b3u
1841
1262
607
1089
588
1837
753
202
(139)
(1372)
(834)
(420)
(259)
(522)
(261)
(10)
1820
1297
535
1059
526
(0.004)
(0.011)
(0.034)
(0.005)
(0.008)
(0.002)
(0.005)
1297
943
–
767
–
1.403
1.375
–
1.381
–
Received: October 31, 2003 [Z53216]
Keywords: hydrides · indium ·
b2u
b1u
.
IR spectroscopy · matrix isolation · thallium
1803
718
1291
–
1.397
–
[a] Ref. [8]: intensities (kmmolꢀ1). [b] This work, observed in hydrogen matrix on annealing to 6–8 K:
Intensities (integrated absorbance). [c] This work, observed for In2D6 in deuterium matrix on warming to
9–12 K.
[1] D. E. Hibbs, M. B. Hursthouse, C. Jones,
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Al2H6.
[4] E. Wiberg, O. Dittmann, M. Z. Schmidt, Z.
Similar experiments with laser-ablated thallium and
hydrogen (deuterium) give weak bands at 1311 (940) cmꢀ1,
which increased on ultraviolet irradiation and are slightly
lower than gas-phase TlH (TlD) fundamentals[10] of 1345.3
(963.7) cmꢀ1 and are due to diatomic thallium hydride
molecules in solid H2 (D2). Weak absorptions are observed
at 1520.0 (1098.8) and at 1390.2 (1007.5) cmꢀ1 for the TlH2
(TlD2) dihydrides and at 1748.4 (1254.6) cmꢀ1 for the TlH3
(TlD3) trihydrides (see Table1). Our TlH3 frequency is in
excellent agreement with the MP2 prediction.[8] Thallium
hydride spectra are illustrated in Figure 4. In addition we
observe weak absorptions for the Tl2H2 (Tl2D2) dimers at
909.7 (652.9) cmꢀ1, in spite of the fact that Tl2H2 was not
predicted to be stable.[8] Our TlH3 band absorbance is much
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expected to be much less than that observed for GaH3 and
AlH3.[16,17,24,25] Evaporation of the H2 (D2) matrix gave no
evidence of broad absorption bands in the 1500–700 cmꢀ1
region that might be due to solid thallium hydride. Our failure
to observe (TlH3)n at low temperature is due to the inherent
instability of the TlIII hydride, which is also manifested in a
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