Preparation, characterization, and reactivity of the stable indium trihydride complex [InH3{CN(Mes)C2H2N(Mes)}]

Article abstract of DOI:10.1021/om0004951

Reaction of either LiInH₄ or [InH₃(NMe₃)] with the Arduengo-type carbene :CN(Mes)C₂H₂N(Mes), Mes = C₆H₂Me₃-2,4,6, affords the indium trihydride complex [InH₃{CN(Mes)C₂H₂N(Mes)}]. This compound displays a remarkable thermal stability (dec 115 °C). In solution the products of decomposition depend on the solvent used. In toluene decomposition results in liberation of the free carbene and deposition of indium metal, and in tetrahydrofuran in the presence of indium metal hydrogen transfer occurs to give H₂CN(Mes)C₂H₂N(Mes) and indium metal, while in dichloromethane, chloride abstraction occurs from the solvent to yield [InCl₃{CN(Mes)C₂H₂N(Mes)}]. A number of related compounds have also been prepared and their structures and properties investigated. These include [GaH₃{(CN(Mes)C₂H₂N(Mes)}], which is the most stable gallium trihydride yet reported, and the indium hydride halide complex [InH₂Cl{CN(Mes)C₂H₂N(Mes)}].

Full text of DOI:10.1021/om0004951

4852
Organometallics 2000, 19, 4852-4857
P r ep a r a tion , Ch a r a cter iza tion , a n d Rea ctivity of th e
Sta ble In d iu m Tr ih yd r id e Com p lex
[In H3{CN(Mes)C2H2N(Mes)}]
Colin D. Abernethy, Marcus L. Cole, and Cameron J ones*
Department of Chemistry, University of Wales, Cardiff, P.O. Box 912,
Park Place, Cardiff, CF10 3TB, U.K.
Received J une 13, 2000
Reaction of either LiInH4 or [InH3(NMe3)] with the Arduengo-type carbene :CN(Mes)-
C2H2N(Mes), Mes ) C6H2Me3-2,4,6, affords the indium trihydride complex [InH3{CN(Mes)-
C2H2N(Mes)}]. This compound displays a remarkable thermal stability (dec 115 °C). In
solution the products of decomposition depend on the solvent used. In toluene decomposition
results in liberation of the free carbene and deposition of indium metal, and in tetrahydro-
furan in the presence of indium metal hydrogen transfer occurs to give H2CN(Mes)C2H2N-
(Mes) and indium metal, while in dichloromethane, chloride abstraction occurs from the
solvent to yield [InCl3{CN(Mes)C2H2N(Mes)}]. A number of related compounds have also
been prepared and their structures and properties investigated. These include [GaH3{CN-
(Mes)C2H2N(Mes)}], which is the most stable gallium trihydride yet reported, and the indium
hydride halide complex [InH2Cl{CN(Mes)C2H2N(Mes)}].
In tr od u ction
cyclohexyl), 2,9,10 in which the InH3 moiety is stabilized
by coordination to either a highly nucleophilic “Ar-
duengo-type” carbene or a bulky tertiary phosphine
ligand, respectively. Compound 1 is unstable in solution
above -20 °C and decomposes in the solid state above
-5 °C,7 while 2 exhibits a greater thermal stability,
decomposing slowly at 25 °C in solution and at 50 °C in
the solid state.9
We are interested in synthesizing indium trihydride
complexes of even greater thermal stability in order to
investigate the chemical reactivity of these species. The
remarkable thermal stability exhibited by the alumi-
num trihydride complex of the sterically demanding
Arduengo-type carbene 1,3-bis(2,4,6-trimethylphenyl)-
imidazol-2-ylidene, 3, viz., [AlH3{CN(Mes)C2H2N(Mes)}]
(Mes ) C6H2Me3-2,4,6), 4 (mp 246 °C, decomposition
temperature not stated),11 compared with other AlH3
complexes, which decompose at much lower tempera-
tures, e.g., [AlH3(NMe3)] (mp 78 °C, decomposition ca.
100 °C),12 suggested that the indium-containing ana-
logue [InH3{CN(Mes)C2H2N(Mes)}], 5, would also prove
to have an exceptional thermal stability. Herein, we
report the synthesis and characterization of 5 and
describe its reactivity and decomposition under certain
conditions. We also report several related complexes
incorporating 3: the indium trideuteride complex [InD3-
{CN(Mes)C2H2N(Mes)}], 5a ; the mixed indium hydride
chloride complex [InH2Cl{CN(Mes)C2H2N(Mes)}], 6; the
The chemistry of both aluminum and gallium hydride
compounds has been studied extensively.1 In particular,
Lewis base complexes of aluminum and gallium trihy-
dride have found a variety of practical applications,
which include their use as chemical vapor deposition
precursors for thin films of the group 13 metal or
semiconducting materials,2,3 as well as reagents for
hydrogenation in organic synthesis.4 Considering this,
we believe that indium trihydride complexes have the
potential to find a similar range of applications. Unfor-
tunately, however, there is a paucity of structurally
characterized compounds containing indium hydride
bonds,5 which is due both to the weakness of the In-H
bond as well as to kinetically facile decomposition
pathways, which are thought to proceed via associative
mechanisms involving In-H-In bridges.6 In order for
the field of indium hydride chemistry to be developed
and its potential applications realized, further examples
of these species need to be synthesized and their general
reactivity, stability, and decomposition pathways un-
derstood.
During the past two years our group has reported the
first examples of indium trihydride complexes [InH3-
{CN(Pri)C2Me2N(Pri)}], 1,7,8 and [InH3(PCy3)] (Cy )
* Corresponding author. Tel: (+44) (029) 2087 4060. Fax: (+44)
(029) 2087 4030. E-mail: jonesca6@cardiff.ac.uk.
(1) Gardiner, M. G.; Raston, C. L. Coord. Chem. Rev. 1997, 166, 1,
and references therein.
(7) Hibbs, D. E.; Hursthouse, M. B.; J ones, C.; Smithies, N. A. Chem.
Commun. 1998, 869.
(2) Simmonds, M. G.; Gladfelter, W. L. In The Chemistry of Metal
CVD; Kodas, T., Hampden-Smith, M., Eds.; VCH: Weinheim, 1994.
(3) Kidder, J . N.; Yun, H. K.; Rogers, J . W.; Pearsall, T. P. Chem.
Mater. 1998, 10, 777.
(4) Raston, C. L.; Siu A. F. H.; Tranter C. J .; Young, D. J .
Tetrahedron Lett. 1994, 35, 5915.
(8) Francis, M. D.; Hibbs, D. E.; Hursthouse, M. B.; J ones, C.;
Smithies, N. A. J . Chem. Soc., Dalton Trans. 1998, 3249.
(9) Hibbs, D. E.; J ones, C.; Smithies, N. A. Chem. Commun. 1999,
185.
(10) Cole, M. L.; Hibbs, D. E.; J ones, C.; Smithies, N. A. J . Chem.
Soc., Dalton Trans. 2000, 545.
(5) Hibbs, D. E.; Hursthouse, M. B.; J ones, C.; Smithies N. A.
Organometallics 1998, 17, 3108.
(6) Downs, A. J .; Pulham, C. R. Chem. Soc. Rev. 1994, 175.
(11) Arduengo, A. J ., III; Dias H. V. R.; Calabrese, J . C.; Davidson,
J . J . Am. Chem. Soc. 1992, 114, 9724.
(12) Ruff, J . K.; Hawthorne, M. F. J . Am. Chem. Soc. 1960, 82, 2141.
10.1021/om0004951 CCC: $19.00 © 2000 American Chemical Society
Publication on Web 10/12/2000

Stable Indium Trihydride Complex
Organometallics, Vol. 19, No. 23, 2000 4853
after which time indium metal deposited from solution. The
resulting suspension was filtered, and the solvent was removed
from the filtrate in vacuo. An H NMR spectrum (C6D6) showed
indium trichloride complex [InCl3{CN(Mes)C2H2N-
(Mes)}], 7; and the gallium trihydride complex [GaH3-
{CN(Mes)C2H2N(Mes)}], 8.
1
the residue to be :CN(Mes)C2H2N(Mes), 3.
In d iu m Meta l Ca ta lyzed Decom p osition of a Tetr a h y-
d r ofu r a n Solu tion of [In H3{CN(Mes)C2H2N(Mes)}]. In-
dium powder (0.02 g, 0.17 mmol) dried in vacuo was added to
a stable clear light yellow solution of 5 (0.25 g, 0.59 mmol) in
THF (50 mL) at 25 °C. The resulting gray suspension was
stirred for 16 h, whereupon it was filtered to yield a yellow
solution. Volatiles were removed in vacuo to yield H2CN(Mes)-
C2H2N(Mes), 9, as an off-white solid (0.17 g, 94%), mp 48 °C.
1H NMR (400 MHz, C6D6, 298 K): δ 2.29 [s, 6H, p-CH3], 2.54
[s, 12H, o-CH3], 5.12 [s, 2H, CH2], 5.66 [s, 2H, C2H2], 6.94 [s,
4H, Ar-H]. 13C NMR (100.6 MHz, C6D6, 298 K): δ 19.5 [s,
o-CH3], 21.4 [s, p-CH3], 74.5 [s, CH2], 117.6 [s, C2H2], 130.5 [s,
m-CH], 135.8 [s, p-CCH3], 137.9 [s, o-CCH3], 140.5 [s, ipso-C].
MS APCI: m/z (%) 307 [MH+, 100]. IR (Nujol) v/cm-1: 1409
(m), 1312 (m), 1286 (m), 979 (m), 871 (m), 851 (m), 784 (m).
Anal. for C21H26N2: calcd C 82.31, H 8.55, N 9.14. Found: C
81.85, H 8.61, N 8.49.
Exp er im en ta l Section
Gen er a l In for m a tion . All manipulations were carried out
under anaerobic conditions using standard Schlenk glassware
and vacuum line techniques. Paraffin and ether solvents were
distilled from Na/K alloy and purged with nitrogen prior to
use. Dichloromethane was distilled from CaH2 prior to use.
1H and 13C{1H} NMR spectra were recorded with a Bruker
DPX400 and calibrated against the residual 1H or 13C reso-
nances of the solvents used. Infrared spectra were recorded
as either Nujol mulls or as solutions in toluene using a Perkin-
Elmer 1725-X Fourier transform instrument. Mass spectra
were obtained with a VG Fisons Platform II instrument under
APCI conditions. Elemental analyses were performed by the
Warwick University Microanalytical Service. Melting points
were determined in sealed glass capillaries under argon and
are not corrected.
Decom p osit ion of a Dich lor om et h a n e Solu t ion of
[In H3{CN(Mes)C2H2N(Mes)}]. A solution of [InH3{CN(Mes)-
C2H2N(Mes)}] (0.43 g, 1.02 mmol) in dichloromethane (15 mL)
at 0 °C was slowly warmed to room temperature. Upon
warming, gas evolution was observed, the clear yellow solution
was stirred for a further 7 days and filtered, and volatiles were
removed from the filtrate in vacuo to yield a white residue.
This was recrystallized from diethyl ether/dichloromethane to
give [InCl3{CN(Mes)C2H2N(Mes)}], 7, as hexagonal rods (0.36
g, 67%), dec 191 °C. 1H NMR (400 MHz, CD2Cl2, 298 K): δ
2.13 [s, 12H, o-CH3], 2.39 [s, 6H, p-CH3], 7.13 [s, 4H, Ar-H],
7.40 [s, 2H, C2H2]. 13C NMR (100.6 MHz, CD2Cl2, 298 K): δ
15.9 [s, o-CH3], 19.5 [s, p-CH3], 124.5 [s, C2H2], 128.2 [s, m-CH],
133.8 [s, o-CCH3], 133.9 [s, p-CCH3], 139.9 [s, ipso-C]. MS
APCI: m/z (%) 305 [{MH - InCl3}+, 22], 490 [{M - Cl}+, 100].
IR (Nujol) v/cm-1: 1301 (m), 1260 (m), 1230 (m), 1020 (m),
799 (m). Anal. for C21H24N2Cl3: calcd C 48.01, H 4.57, N 5.33,
Cl 20.24. Found: C 47.78, H 4.57, N 5.20.
1,3-Bis(2,4,6-trimethylphenyl)imidazol-2-ylidene,13 LiInH4,14
and [InH3(NMe3)]7 were prepared according to the published
procedures. LiInD4 was prepared by a slight modification of a
published procedure.14 InBr3 was sublimed in vacuo, and
NMe3‚HCl and quinuclidine‚HCl were dried in vacuo at 100
°C prior to use. All other reagents were used as received from
commercial sources.
P r ep a r a tion of [In H3{CN(Mes)C2H2N(Mes)}], 5. (a )
Rea ction of 3 w ith LiIn H4. A solution of 3 (1.60 g, 5.26 mmol)
in 50 mL of diethyl ether was added dropwise to a solution of
LiInH4 (0.66 g, 5.26 mmol) in 100 mL of diethyl ether at -78
°C. The resulting hazy pale yellow solution was allowed to
warm to room temperature and stirred for a further 2 h. The
diethyl ether was removed in vacuo, and toluene (80 mL) was
added. The mixture was filtered and the volume of the filtrate
reduced to 30 mL in vacuo. Pale yellow crystals of [InH3{CN-
(Mes)C2H2N(Mes)}] formed on standing for 24 h at -35 °C,
yield 1.90 g (86%). Mp: 115 °C (dec). Anal. for C21H27N2In:
calcd C, 59.73; H 6.44, N 6.64. Found: C 59.39, H 6.39, N 6.46.
MS APCI: m/z(%) 421 [{M - H}+, 33], 305 [{M - InH2}+, 100].
IR (Nujol) ν/cm-1: 1650 (s, br, In-H str); (toluene solution)
1640 cm-1 (s, br, In-H str). 1H NMR (400 MHz, C6D6, 298 K):
δ 2.29 [s, 12 H, o-CH3], 2.38 [s, 6 H, p-CH3], 5.2 [s (br), 3H,
In-H3], 6.35 [s, 2 H, NCH], 7.05 [s, 4 H, Ar-H]. 13C NMR
(100.6 MHz, C6D6, 298 K): δ 16.3 [s, o-CH3], 19.7 [s, p-CH3],
121.4 [s, NCC], 124.2 [s, m-CH], 129.8 [s, p-CCH3], 133.2 [s,
o-CCH3], 138.5 [s, ipso-c], In-C not observed.
(b) Rea ction of 3 w ith [In H3(NMe3)]. A solution of 3 (1.60
g, 5.26 mmol) in 50 mL of diethyl ether was added dropwise
to a solution of [InH3(NMe3)] (0.93 g, 5.26 mmol) in 100 mL of
diethyl ether at -78 °C. The resulting pale yellow solution was
worked up as above. Yield: 1.2 g (54%).
P r ep a r a tion of [In D3{CN(Mes)C2H2N(Mes)}], 5a . Com-
pound 5a was prepared by reacting LiInD4 (0.66 g, 5.26 mmol)
with 3 (1.60 g, 5.26 mmol) in an identical fashion to the
preparation of 5,. yield 1.61 g (75%). Mp: 114 °C (dec). MS
APCI: m/z(%) 423 [{M - D}+, 15], 306 [{M - InD2}+, 100]. IR
(Nujol) ν/cm-1: 1180 (s, br, In-D str). 1H NMR (400 MHz,
C6D6, 298 K): δ 2.27 [s, 12 H, o-CH3], 2.38 [s, 6 H, p-CH3],
6.30 [s, 2 H, NCH], 7.03 [s, 4 H, Ar-H]. 13C NMR (100.6 MHz,
C6D6, 298 K): δ 16.3 [s, o-CH3], 19.7 [s, p-CH3], 121.0 [s, NCC],
124.0 [s, m-CH], 130.1 [s, p-CCH3], 133.6 [s, o-CCH3], 137.2
[s, ipso-c]. In-C not observed.
P r ep a r a tion of [Ga H3{CN(Mes)C2H2N(Mes)}], 8. A solu-
tion of 3 (0.86 g, 2.82 mmol) in diethyl ether (20 mL) was slowly
added to a solution of LiGaH4 (2.82 mmol) in diethyl ether
(70 mL) at -78 °C. The resulting hazy light yellow solution
was warmed to room temperature and stirred for 1 h,
whereupon a colorless product precipitated from solution. This
was isolated via filtration and dried in vacuo. Extraction with
THF (30 mL) and placement at -35 °C overnight yielded air-
stable [GaH3{CN(Mes)C2H2N(Mes)}] as colorless blocks (0.94
1
g, 89%), dec 214 °C. H NMR (400 MHz, C6D6, 298 K): δ 2.15
[s, 12H, o-CH3], 2.20 [s, 6H, p-CH3], 3.96 [br s, 3H, Ga-H],
6.16 [s, 2H, C2H2], 6.86 [s, 4H, Ar-H]. 13C NMR (100.6 MHz,
C6D6, 298 K): δ 17.9 [s, o-CH3], 21.3 [s, p-CH3], 122.4 [s, C2H2],
129.6 [s, m-CH], 135.2 [s, o-CCH3], 136.1 [s, p-CCH3], 139.6
[s, ipso-C]. MS APCI: m/z (%) 305 [{MH - GaH3}+, 19], 375
[{M - H}+, 100]; IR (Nujol) v/cm-1: 1780 (s, br, Ga-H str).
(Analyses for [GaH3{CN(Mes)C2H2N(Mes)}] were found to be
consistently 2-3% high in carbon. NMR studies on the bulk
material suggest contamination with a small amount of an
unknown material, which proved difficult to remove.)
P r ep a r a tion of [In H2Cl{CN(Mes)C2H2N(Mes)}], 6. (a )
Rea ction of [In H 2Cl(NMe3)n ] w ith :CN(Mes)C2H2N(Mes).
A slurry of NMe3‚HCl (0.25 g, 2.62 mmol) in diethyl ether (15
mL) was slowly added to a solution of LiInH4 (1.41 mmol) in
diethyl ether (35 mL) at -78 °C. The resulting opaque colorless
solution was warmed to -50 °C and stirred for 2.5 h,
whereupon all the solid had reacted to yield a colorless clear
solution of [InH2Cl(NMe3)n]. Further reaction with an ethereal
(20 mL) slurry of :CN(Mes)C2H2N(Mes) (0.43 g, 1.41 mmol) at
-50 °C formed a white precipitate. The reaction mixture was
warmed to -20 °C and stirred for a further 2 h, whereupon
the precipitate was isolated by filtration. Extraction with
Th er m a l Decom p osit ion of a Tolu en e Solu t ion of
[In H3{CN(Mes)C2H2N(Mes)}]. A solution of 5 (0.35 g, 0.83
mmol) in toluene (100 mL) was heated to 100 °C for 2.5 h,
(13) Arduengo, A. J ., III; Harlow, R. L.; Kline, M. J . Am. Chem. Soc.
1991, 113, 361.
(14) Wiberg, E.; Schmidt, M. Z. Naturforch., Teil B 1957, 12, 54.

4854 Organometallics, Vol. 19, No. 23, 2000
Abernethy et al.
Ta ble 1. Su m m a r y of Cr ysta llogr a p h ic Da ta for
Com p lexes 5, 6, a n d 7
5
6
7
formula
Mr
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
cryst syst
space group
λ, Å
C
21H27N2In
C21H26N2ClIn
456.71
C21H24N2Cl3In
525.59
16.468(2)
16.543(2)
17.220(3)
90
422.27
30.579(6)
32.414(6)
8.477(2)
90
8.783(4)
14.569(5)
16.260(5)
83.94(2)
83.60(4)
89.75(3)
2056.0(13)
triclinic
P1h
90
90
90
90
8402.3(30)
orthorhombic
Fdd2
0.71073
150(2)
16
4691.3(11)
orthorhombic
Pca21
0.71073
150(2)
0.71073
150(2)
4
T, K
Z
8
size
color
0.5 × 0.2 × 0.1 0.3 × 0.2 × 0.2 0.4 × 0.4 × 0.3
pale yellow
11.29
colorless
12.85
928
colorless
13.57
2112
µ, cm-1
F(000)
no. of reflns
collected
3456
8669
7949
7005
F igu r e 2. Molecular structure of 6. Selected bond lengths
(Å) and angles (deg): In(1)-C(1) 2.244(6), In(1)-Cl(1)
2.356(2), N(1)-C(1) 1.346(8), N(1)-C(2) 1.376(8), N(2)-C(1)
1.351(8), N(2)-C(3) 1.380(8), C(2)-C(3) 1.349(9), C(1)-In-
(1)-Cl(1) 101.71(16), N(1)-C(1)-N(2) 104.5(5), C(1)-N(2)-
C(3) 110.8(5), N(1)-C(2)-C(3) 106.1(5), N(2)-C(3)-C(2)
106.9(6).
no. of unique 3967
reflns
no. of params 223
varied
7421
463
7001
500
R {I > 2σ(I)} 0.0412
0.0538
0.1465
0.0333
0.1039
R
w {all data} 0.1133
The structures were solved by direct methods and refined on
F2 by full-matrix least-squares (SHELX97)15 using all unique
data. All non-hydrogen atoms are anisotropic with H atoms
included in calculated positions (riding model) except the
hydride ligands of 5. Empirical absorption corrections were
carried out by the DIFABS method.16 Crystal data and details
of data collections and refinement are given in Table 1. Both
compounds 5 and 6 crystallized with two crystallographically
independent molecules in the asymmetric unit. In each case,
there are no significant geometric differences between the
independent molecules, so the molecular structures of only one
are shown in Figure 1 and Figure 2.
F igu r e 1. Molecular structure of 5. Selected bond lengths
(Å) and angles (deg): In(1)-C(1) 2.253(5), N(1)-C(1) 1.353-
(6), N(1)-C(2) 1.390(6), N(2)-C(1) 1.353(6), N(2)-C(3)
1.399(6), C(2)-C(3) 1.328(8), C(1)-N(1)-C(2) 110.7(4),
N(1)-C(1)-N(2) 104.5(4), C(1)-N(2)-C(3) 110.9(4), N(1)-
C(2)-C(3) 107.5(4), N(2)-C(3)-C(2) 106.3(4).
Resu lts a n d Discu ssion
Treatment of an ethereal solution of either LiInH4 or
[InH3(NMe3)] with 1 equiv of the Arduengo-type carbene
3 at -78 °C resulted in the formation of the indium
trihydride complex [InH3{CN(Mes)C2H2N(Mes)}], 5, in
high yield after recrystallization from toluene (Scheme
1). The 1H NMR spectrum of 5 in C6D6 showed the
presence of the carbene moiety as well as a broad
resonance at δ 5.2 ppm assigned to the three hydride
ligands attached to the indium center. The chemical
shift of this resonance compares well to the indium
hydride resonances reported for [InH3{CN(Pri)C2Me2N-
(Pri)}], 1, δ 5.58 ppm,7 and [InH3(PCy3)] (Cy ) cyclo-
hexyl), 2, δ 5.61 ppm.9,10 The broadness of the indium
hydride resonance in the spectrum of 5 is due to the
high quadrupole moment of the indium center (115In
95%, I ) 9/2, 113In 5%, I ) 9/2)17 to which the hydrides
are attached. This also accounts for the absence of a
resonance for the indium-coordinated carbene center,
C(1), in its 13C NMR spectrum. The IR spectra of 5
display characteristic strong, broad absorptions centered
toluene (30 mL) at -10 °C and cooling to -35 °C overnight
afforded [InH2Cl{CN(Mes)C2H2N(Mes)}] as colorless prisms
(0.36 g, 65%), dec 119 °C. 1H NMR (400 MHz, C6D6, 298 K):
δ 2.13 [s, 12H, o-CH3], 2.17 [s, 6H, p-CH3], 6.09 [s, 2H, C2H2],
6.25 [br s, 2H, In-H], 6.84 [s, 4H, Ar-H]. 13C NMR (100.6
MHz, C6D6, 298 K): δ 17.8 [s, o-CH3], 21.1 [s, p-CH3], 122.9
[s, C2H2], 129.6 [s, m-CH], 135.0 [s, p-CCH3], 134.9 [s, o-CCH3],
140.1 [s, ipso-C]. MS APCI: m/z (%) 305 [{MH - ClInH2}+,
100], 458 [MH+, 28]. IR (Nujol) v/cm-1: 1737 (s, br, In-H str).
A reproducible microanalysis could not be obtained for 6,
presumably due to contamination of the bulk crystalline
material with small quantities of 5 or [InCl2H{CN(Mes)C2H2N-
(Mes)}].
(b) Rea ction of Qu in u clid in e‚HCl w ith [In H3{CN(Mes)-
C2H2N(Mes)}]. Solid quinuclidine‚HCl (0.09 g, 0.61 mmol) was
added to a stirred solution of [InH3{CN(Mes)C2H2N(Mes)}]
(0.25 g, 0.59 mmol) in diethyl ether (70 mL) at -20 °C. The
resulting white slurry was stirred for 1 h, whereupon the solid
had reacted to yield a light yellow solution. This was filtered,
and the filtrate was concentrated in vacuo at -10 °C and
placed at -35 °C overnight to yield [InH2Cl{CN(Mes)C2H2N-
(Mes)}] as colorless prisms (0.13 g, 50%), which analyzed as
above.
(15) Sheldrick, G. M. SHELX97; University of Go¨ttingen, 1997.
(16) Walker, N. P. C.; Stuart, D. Acta Crystallogr. Sect. A 1983, 39,
X-r a y Cr ysta l Str u ctu r e Deter m in a tion s. Crystals of 5,
6, and 7 that were suitable for single-crystal X-ray diffraction
were mounted in silicone oil. All crystallographic measure-
ments were made using an Enraf-Nonius CAD4 diffractometer.
158.
(17) Downs, A. J . In Chemistry of Aluminium, Gallium and Thal-
lium; Downs, A. J ., Ed.; Blackie Academic: Glasgow, 1993; Chapter
1.

Stable Indium Trihydride Complex
Organometallics, Vol. 19, No. 23, 2000 4855
Sch em e 1
at 1650 cm-1 (Nujol) or 1640 cm-1 (toluene solution),
both of which were assigned to the stretching modes of
its In-H bonds. The observed frequencies are close to
ylidenes (ca. 102°) and imadazolium cations (ca. 108°).11
The molecule is monomeric and does not display any
intermolecular interactions in the solid state. It is
noteworthy that compound 5a is isostructural and
isomorphous to 5.
those described for 1, 1640 cm-1 7,8 and 2, 1661 cm-1 9
.
,
It is believed that, as with most Lewis base adducts of
AlH3 or GaH3,1 single In-H stretching absorptions were
obserevd due to effective overlap of the expected stretch-
ing bands.
The most remarkable feature of complex 5 is its
surprisingly high thermal stability. In the solid state it
is stable under an inert atmosphere up to 115 °C.
Interestingly, the stability of 5 in solution is dependent
on the solvent. Toluene solutions of 5 are stable at room
temperature for periods in excess of three months.
However, on heating in toluene at 100 °C decomposition
occurs after 2.5 h via loss of the carbene ligand and
deposition of indium metal. Similarly, Lewis base loss
is the primary decomposition process for complexes of
AlH3 and GaH318 and the indium trihydride complex 2.10
It was observed that when a toluene solution of 5 was
heated at 100 °C, no decomposition of the complex
occurred until traces of indium metal appeared in
suspension. As soon as this occurred, the decomposition
process went to completion within 2 minutes. This
suggests that the decomposition was catalyzed by in-
dium metal. To test this hypothesis, indium metal
powder was added to a toluene solution of 5 at room
temperature. After 12 h an NMR spectrum of the soluble
material showed that 5 had decomposed and that only
the free carbene, 3, was present. In contrast, when
For purpose of comparison the indium trideuteride
complex [InD3{CN(Mes)C2H2N(Mes)}], 5a , was pre-
1
pared by a method analogous to that of 5. Its H NMR
and 13C NMR spectra are almost identical with those
of 5 with the exception that no hydride resonance is seen
1
in the H NMR spectrum of 5a . As expected, there was
a significant shift to lower frequency (1180 cm-1) for the
In-D stretch of 5a in its infrared spectrum compared
with the In-H stretch of 5. The magnitude of this shift
is consistent with the difference in the reduced masses
of the two atom pairs (i.e., InD and InH).
An X-ray crystal structure analysis of 5 was carried
out (Figure 1). Unfortunately, none of the hydride
ligands could be located from the difference maps, but
the In-C(1) distance, 2.253(5) Å, compares well to that
reported, 2.260(6) Å, for the closely related compound,
1. In addition, the bond lengths and angles within the
carbene heterocycle are similar to those reported for the
aluminum-containing analogue, 4.11 As expected, the
N-C(carbene)-N angle of 104.5° is typical for Ar-
duengo-type carbenes coordinated to metal centers and
lies between the normal value for free imidazol-2-
(18) J ones, C.; Koutsantonis, G. A.; Raston, C. L. Polyhedron 1993,
12, 1829.

4856 Organometallics, Vol. 19, No. 23, 2000
Abernethy et al.
indium metal powder was added to a stable solution of
5 in tetrahydrofuran under identical conditions, decom-
position occurred overnight and the only tetrahydrofu-
ran-soluble decomposition product was found to be
H2CN(Mes)C2H2N(Mes), 9. This suggests that in tet-
rahydrofuran hydrogen transfer occurs from the indium
center to the carbenic carbon of the ligand. This
hypothesis was confirmed by decomposing a THF solu-
tion of 5a in a similar way to afford D2CN(Mes)C2H2N-
(Mes). Complex 5 is also unstable at room temperature
when dissolved in dichloromethane. Chloride abstrac-
tion from the solvent takes place at 25 °C to form [InCl3-
{CN(Mes)C2H2N(Mes)}], 7, in moderate (67%) yield.
A photolytic decomposition of 5 was also attempted
in order to gain further insight into the different
decomposition pathways displayed by 5. After 12 h of
ultraviolet irradiation of a toluene solution of 5, indium
coordinated Arduengo-type carbene. Complex 8 is stable
in air and is the most thermally stable (dec 214 °C)
GaH3 complex yet reported. Its thermal stability can be
compared with that of 10, which decomposes at 180 °C.8
Unfortunately, single crystals of suitable quality for
X-ray diffraction could not be obtained.
Despite the ease of formation of bis-carbene complexes
of indium trihalides, no evidence of bis-carbene com-
plexes of indium trihydrides has been observed.7 It has
been suggested that this is due to the lower Lewis
acidity of InH3 with respect to InX3 (X ) Cl, Br).8 Bis-
phosphine complexes of indium trihydride have, how-
ever, recently been reported.10 To attempt the formation
of a bis-carbene indium trihydride complex, the Ar-
duengo-type carbene :CN(Me)C2Me2N(Me), 11, was
reacted with an equimolar amount of 5 in toluene at
room temperature. After 4 h indium metal was depos-
ited and a 1H NMR spectrum showed that the only
products remaining in solution were 3 and 11. This
result suggests that instead of a five-coordinate complex
forming, carbene exchange took place at the indium
center to form [InH3{CN(Me)C2Me2N(Me)}], which then
decomposes under the experimental conditions em-
ployed. The thermal instability of this less sterically
protected InH3 complex has been previously reported.21
1
metal was deposited and a H NMR spectrum revealed
that a complex mixture of products had been formed.
No further workup was attempted.
The remarkable thermal stability of 5, both in solution
and the solid state, may be explained by a combination
of the large steric bulk and high nucleophilicity of 3.
With respect to the latter, Nolan et al. have demon-
strated that 3 binds to metals with a greater M-L bond
enthalpy than tricyclohexylphosphine and other similar
tertiary phosphines.19 Therefore, the greater stability
of 5 with respect to 2 is not surprising. Compound 5
also has a significantly enhanced thermal stability
relative to that of the closely related indane complex 1
(dec ca. -5 °C). This cannot be due to differences in the
nucleophilicities of the carbene ligands, as these are
similar. Instead, the lower stability of 1 is due to it
decomposing via a different pathway, which is believed
to involve metalation of the isopropyl carbene substit-
uents by the InH3 moiety with a concomitant loss of H2.
Compound 5 does not decompose via a related metala-
tion of the ortho methyl groups of its mesityl substitu-
ents, perhaps for steric reasons, but instead its primary
thermal decomposition process is ligand loss or dihy-
droimidazole formation, depending on the conditions
employed. Interestingly, metalation of the methyl sub-
stituents of 3 has recently been observed in a Rh(I)
complex, though in this case the process was presum-
ably more facile due to it being an oxidative insertion
reaction.20
To extend the synthetic utility of stable indium
hydride complexes, we wished to synthesize the mixed
indium hydride halide complexes [InH(3-x)Clx{CN-
(Mes)C2H2N(Mes)}] (x ) 1 or 2), which would enable us
to carry out further chemistry at the indium centers via
metathesis reactions. The reaction of [InH2Cl(NMe3)n]
with an equimolar amount of 3 yielded [InH2Cl{CN-
(Mes)C2H2N(Mes)}], 6, in 65% yield. It was also pro-
duced in 50% yield by the reaction of quinuclidine‚HCl
with 5. In this instance the quinuclidine‚HCl acted as
a source of Cl-, and 6 was produced with the elimination
of hydrogen. Attempts were made to prepare a pure
sample of [InHCl2{CN(Mes)C2H2N(Mes)}] by similar
procedures, but all met with failure.
Complex 6 was characterized by 1H and 13C NMR and
IR spectroscopy together with an X-ray crystal structure
analysis. A broad resonance due to the two hydride
ligands attached to the indium center occurred at δ 6.25
1
ppm in the H NMR spectrum of 6. The downfield shift
of this resonance compared with that of the hydride
ligands of 5 is probably due to the influence of the
electronegative chloride ligand also bound to the indium
center. The presence of the chloride ligand also serves
to strengthen the In-H bonds in the complex by a
negative inductive effect, as evidenced by their IR
To gain further insights into the stabilities of [MH3-
{CN(Mes)C2H2N(Mes)}] compounds, we decided to syn-
thesize the gallium-containing analogue [GaH3{CN-
(Mes)C2H2N(Mes)}], 8. To this end, the free carbene, 3,
was reacted with an equimolar amount of LiGaH4 in
diethyl ether. The product 8 was formed in 89% yield
stretching modes. These are centered at 1737 cm-1
,
1
and characterized by H and 13C NMR together with
which is at a considerably higher frequency than for the
corresponding In-H stretching modes for 5. It is
noteworthy that uncoordinated InH2Cl has recently
been prepared and isolated in a solid argon matrix, and
its properties have been studies.22
1
APCI MS and IR spectroscopy. The H NMR spectrum
(C6D6) showed a broad resonance at 3.96 ppm, which
was assigned to the three hydride ligands of the GaH3
unit and can be compared with the hydride resonance
(4.48 ppm) seen for [GaH3{CN(Pri)C2Me2N(Pri)}],10. A
resonance (139.6 ppm) assigned to the carbenic carbon
of 8 was also observed in the 13C NMR spectrum, which
is in the region expected for a carbenic carbon of a metal-
The X-ray crystal structure of 6 (Figure 2) revealed
that this compound, like 5, is monomeric in the solid
state without any intermolecular contacts. This is also
consistent with the structures of 7 (Figure 3) and the
(19) Huang, J .; Schanz, H.-J .; Stevens, E. D.; Nolan, S. P. Organo-
metallics 1999, 18, 2370.
(20) Huang, J .; Stevens, E. D.; Nolan, S. P. Organometallics 2000,
19, 1194.
(21) Smithies, N. A. Ph.D. Thesis, Cardiff University, 1999.
(22) Himmel, H.-J .; Downs, A. J .; Greene, T. M. J . Am. Chem. Soc.
2000, 122, 922.

Stable Indium Trihydride Complex
Organometallics, Vol. 19, No. 23, 2000 4857
Unsurprisingly, the In-Cl distance in 6, 2.356(2) Å, is
very close to the average In-Cl distance observed in 7,
2.352 Å. In 7 the geometry at In(1) is close to tetrahedral
(av C(1)-In(1)-Cl angle - 110.7°), while in 6 it appears
to be distorted, as the C(1)-In(1)-Cl(1) angle is 101.71-
(16)°.
Con clu sion
The indium hydride complex [InH3{CN(Mes)C2H2N-
(Mes)}], 5, has been synthesized in high yield and
possesses exceptional thermal stability. The decomposi-
tion of this compound has been studied under a variety
of conditions and occurs via different pathways depend-
ing on the conditions employed. The related complexes
[GaH3{CN(Mes)C2H2N(Mes)}], 8; [InH2Cl{CN(Mes)-
C2H2N(Mes)}], 6; and [InCl3{CN(Mes)C2H2N(Mes)}], 7,
have also been prepared and their structures and
properties compared to those of 5. We are currently
investigating the reactivity of 5 and related compounds
toward a variety of organic24 and inorganic25 substrates.
F igu r e 3. Molecular structure of 7. Selected bond lengths
(Å) and angles (deg): In(1)-C(1) 2.200(7), In(1)-Cl(1)
2.355(2), In(1)-Cl(2) 2.349(2), In(1)-Cl(3) 2.353(2), N(1)-
C(1) 1.336(8), N(1)-C(2) 1.366(9), N(2)-C(1) 1.334(7),
N(2)-C(3) 1.372(8), C(2)-C(3) 1.329(10), C(1)-In(1)-Cl-
(1) 108.7(2), C(1)-In(1)-Cl(2) 112.09(15), C(1)-In(1)-Cl-
(3) 111.45(15), Cl(1)-In(1)-Cl(2) 107.35(7), Cl(1)-In(1)-
Cl(3) 111.45(15), Cl(2)-In(1)-Cl(3) 107.63(7), N(1)-C(1)-
N(2) 106.7(6), C(1)-N(2)-C(3) 109.7(5), N(1)-C(2)-C(3)
108.1(6), N(2)-C(3)-C(2) 106.5(6).
related indium bromide complex [InBr3{CN(Pri)C2Me2N-
(Pri)}].23 The In-C(1) distances in 6, 2.244(6) Å, and 7,
2.200(7) Å, compare well to those observed in 5, 2.253-
(5) Å, and [InBr3{CN(Pri)C2Me2N(Pri)}], 2.199(5) Å.
Ack n ow led gm en t. We gratefully acknowledge the
financial support of the Engineering and Physical
Sciences Research Council (Postdoctoral Fellowship for
C.D.A. and Studentship for M.L.C.).
(23) Black, S. J .; Hibbs, D. E.; Hursthouse, M. B.; J ones, C.; Malik,
K. M. A.; Smithies, N. A. J . Chem. Soc., Dalton Trans. 1997, 4313.
(24) Abernethy, C. D.; Cole, M. L.; Davies, A. J .; J ones, C. Tetra-
hedron Lett., in press.
(25) Abernethy, C. D.; Cole, M. L.; J ones, C. Manuscript in prepara-
tion.
Su p p or t in g In for m a t ion Ava ila b le: Full crystallo-
graphic data for compounds 5, 6, and 7. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM0004951