N-heterocyclic carbene coordinated gallanes and chlorogallanes

Article abstract of DOI:10.1016/j.jorganchem.2009.04.030

The preparation of the N-heterocyclic carbene coordinated gallium complexes [GaH₃(IXy)] (1), [GaH₃(IDipp)] (2), [GaClH₂(IMes)] (3) and [GaCl₂H(IMes)] (4), where IXy = 1,3-bis(2,6-dimethylphenyl)imidazol-2-ylidene, IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene and IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, are reported. All four complexes have been characterised by ¹H, ¹³C NMR and IR spectroscopy and, for complexes 2, 3 and 4, single crystal X-ray structure determination. These compounds represent some of the most thermally stable molecular gallium hydrides known, with 4 being the most thermally stable gallium hydride reported (dec. 274 °C). These remarkable thermal stabilities translate to significant aerobic stability such that all four compounds may be handled in dry air without significant decomposition. Compounds 2, 3 and 4 exist as distorted tetrahedra in the solid state with gallium to carbene C-donor bonds that shorten with increasing Lewis acidity of the gallium centre. Compound 2 co-crystallizes with 1 equiv. of 2,6-diisopropylphenylaniline and exhibits several weak intermolecular bonding interactions in the solid-state.

Full text of DOI:10.1016/j.jorganchem.2009.04.030

Journal of Organometallic Chemistry 694 (2009) 2934–2940
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
N-heterocyclic carbene coordinated gallanes and chlorogallanes
a
b
Marcus L. Cole a, , Samantha K. Furfari , Marc Kloth
*
a School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia
b School of Chemistry and Physics, University of Adelaide, Adelaide, South Australia 5005, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
The preparation of the N-heterocyclic carbene coordinated gallium complexes [GaH3(IXy)] (1),
[GaH3(IDipp)] (2), [GaClH2(IMes)] (3) and [GaCl2H(IMes)] (4), where IXy = 1,3-bis(2,6-dimethyl-
phenyl)imidazol-2-ylidene, IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene and IMes = 1,3-
bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, are reported. All four complexes have been characterised
by 1H, 13C NMR and IR spectroscopy and, for complexes 2, 3 and 4, single crystal X-ray structure deter-
mination. These compounds represent some of the most thermally stable molecular gallium hydrides
known, with 4 being the most thermally stable gallium hydride reported (dec. 274 °C). These remarkable
thermal stabilities translate to significant aerobic stability such that all four compounds may be handled
in dry air without significant decomposition. Compounds 2, 3 and 4 exist as distorted tetrahedra in the
solid state with gallium to carbene C-donor bonds that shorten with increasing Lewis acidity of the
gallium centre. Compound 2 co-crystallizes with 1 equiv. of 2,6-diisopropylphenylaniline and exhibits
several weak intermolecular bonding interactions in the solid-state.
Received 22 December 2008
Received in revised form 21 April 2009
Accepted 21 April 2009
Available online 3 May 2009
Keywords:
Gallium
Halide
Hydride
Structure determination
Infrared spectroscopy
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
tions that are believed to provide a low energy path to decomposi-
tion by reductive dehydrogenation [9], a mode of decomposition
The synthetic and materials utility of group 13 hydrides is well
established [1] and, despite the increased frailty of the gallium to
hydrogen bond relative to those of boron and aluminium, gallohy-
drides have found broad application [2]. We are interested in the
development of methods for the stabilisation of heavy group 13
hydride bonds [3], such as gallo- and indohydrides [4], such that
these species may be studied and their synthetic and materials
applications potential explored. Further to simply coordinating
strong Lewis bases to group 13 trihydrides, one means of accom-
plishing this goal is to substitute one or two hydrides about the
group 13 element with halides to inductively strengthen the
that becomes increasingly favourable as group 13 is descended
[9]. The significant temperature stability of the IMes supported tri-
hydride complexes depicted in Fig. 1 translates to considerable air
stability. For instance, all three compounds may be manipulated in
air for brief periods without significant decomposition.
Considering the stability of [GaH3(IMes)] (dec. 214 °C) [4d] over-
and-above simple group 15 coordinated complexes like [GaH3-
(Quin)] (dec. 100 °C) [10a] or [GaH3(PCy)3] (dec. 130 °C) [10b], it
is somewhat surprising that only three NHC coordinated gallanes
([GaH3(NHC)]) have been reported; the aforementioned
[GaH3(IMes)] [4d] [GaH3(IiPrMe)] (IiPrMe = 1,3-diisopropyl-4,5-
dimethylimidazol-2-ylidene) (dec. 180 °C) [7b] and [GaH2I(IMes)]
(dec. 177–181 °C) [11]. It is noteworthy that the decomposition
temperature of [GaH2I(IMes)] [11], which is some 40 °C below that
of its gallane congener, is far lower than that expected on the basis
of NHC coordinated haloalanes, e.g. [AlCl2H(IMes)] dec. 320 °C [3a]
vs. [AlH3(IMes)] [7a] dec. 256 °C [12]. Indeed, one would expect the
decomposition temperature of this complex to be greater than that
of [GaH3(IMes)] [4d], perhaps suggesting a different mode of
decomposition for this species [5].
remaining M–H bond(s) by halide
r-electron withdrawal [5]. This
strategy has been very successful for aluminohydrides [3a,6].
Amongst Lewis base donors, bulky 1,3-bis(aryl)imidazol-2-yli-
dene N-heterocyclic carbenes (NHCs) (Fig. 1) are without peer for
the stabilisation of group 13 hydrides [1c,4d,7]. For instance, the
NHC IMes; 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, gen-
erates the most stable complexes of alane (AlH3) [7a], gallane
(GaH3) [4d] and indane (InH3) [4d] reported to date. This stability
is borne out of the donor characteristics of the NHC [8], which sat-
isfy the trihydride electronically, and the steric protection afforded
by the mesityl (2,4,6-trimethylphenyl) nitrogen substituents.
These spatially shield the MH3 unit and impede M–H–M interac-
Lewis base adducts of chlorogallanes, in-particular dichlorogall-
anes, have been studied in some depth by Schmidbaur [13], Raston
[14] and Gladfelter [15] and there are several methods for their
high yield preparation (Scheme 1). These include (i) the stoichiom-
etric reaction of gallium trichloride with organosilanes [13] and (ii)
the stoichiometric redistribution of gallium trihydrides and
* Corresponding author. Tel.: +61 2 93854678; fax: +61 2 93856141.
E-mail address: m.cole@unsw.edu.au (M.L. Cole).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.04.030

M.L. Cole et al. / Journal of Organometallic Chemistry 694 (2009) 2934–2940
2935
Fig. 1. Reported NHC complexes of group 13 trihydrides and their solid-state decomposition temperatures.
Scheme 1. High yielding methods for the preparation of mono- and dichlorogallane complexes.
trichlorides [14–16]. Herein we report the application of method
(ii) to the preparation of the monochloro and dichlorogallane
complexes [GaClH2(IMes)] (3) and [GaCl2H(IMes)] (4), which com-
plete the series [GaClnH3Àn(IMes)] (n = 0–3) in which n = 0 [4d] and
3 [17] are known, and extend the family of gallane (GaH3) NHC
complexes to [GaH3(IXy)] (1) and [GaH3(IDipp)] (2) (IXy = 1,
3-bis(2,6-dimethylphenyl)imidazol-2-ylidene, IDipp = 1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene). We also report the
spectroscopic characterisation of [GaClH2(IXy)] (3Xy) formed as a
co-product during some preparations of 1.
[7b]. These data indicate that the IDipp of 2 does not coordinate to
gallane as strongly as the less bulky NHCs of 1 and the aforemen-
tioned literature compounds (Table 1). This most likely results
from the increase in bulk at the N-substituents of IDipp vs. IXy/
IMes/IiPrMe. Curiously, the opposing trend in metal-hydride
stretching frequencies is observed in the Al–H stretching absorp-
tions of the aluminium counterparts of
1729 and 1743 cmÀ1, respectively) [7a,7c].
During some preparations of [GaH3(IXy)] (1) an additional
2 and [AlH3(IMes)]
(m
strong Ga–H absorption was observed at 1855 cmÀ1. Based on
our studies of chloroalanes supported by IMes [3a,6], this absorp-
tion is likely to be that of the monochlorogallane [GaClH2(IXy)]
(3Xy), resulting from incomplete conversion of gallium trichloride
to [LiGaH4], i.e. formation of [LiGaClH4], during in situ preparation
of this precursor by reaction of gallium trichloride with lithium hy-
dride at low temperature (cf dec. [LiGaH4] ca 0 °C). A similar
unintended preparation of [LiGaClH3], and its subsequent reactiv-
ity, has been reported by Himmel and co-workers during their
studies of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidate
(hppH) yielding the chlorogallane dimer [{GaClH(hpp)}2] [19].
The shift of Ga–H stretching absorption to higher wavenumber
for 3Xy relative to that of 1 is consistent with inductive strengthen-
ing of the gallium to hydrogen bond by halide electron withdrawal
[5]. Pure samples of 1 were acquired by extending the preparation
time used to prepare [LiGaH4], prior to reaction with the NHC IXy.
2. Results and discussion
2.1. Synthesis and spectroscopy
2.1.1. NHC complexes of gallium trihydride
The treatment of ethereal solutions of in situ prepared lithium
tetrahydridogallate with single equivalents of IXy or IDipp [18] af-
fords the NHC complexes [GaH3(IXy)] (1) and [GaH3(IDipp)] (2) by
lithium hydride substitution (Scheme 2), as evidenced by the infra-
red spectra of both crude products after removal of reaction vola-
tiles under reduced pressure. These spectra (Table 1) exhibit strong
infrared Ga–H stretching absorptions at 1789 and 1801 cmÀ1
respectively, which may be compared to the analogous absorptions
of [GaH3(IMes)] (1780 cmÀ1) [4d] and [GaH3(IiPrMe)] (1775 cmÀ1
,
)
Scheme 2. The preparations of NHC coordinated gallane and chlorogallanes reported in this article. Reaction conditions: (i) NHC = IXy (1), IDipp (2), or IMes (see (ii)), Et2O,
À40 °C to RT overnight, À LiH; (ii) toluene, 50 °C, 6 h (3) or 14 h (4).

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M.L. Cole et al. / Journal of Organometallic Chemistry 694 (2009) 2934–2940
Table 1
Selected solid-state physical and spectroscopic data for complexes reported in this article and relevant literature compounds
b
Compound
dec. (°C)a
IR
m
M–H (cm-1
)
Ga–H 1H NMR (ppm)c
C2H2 1H NMR (ppm)
[GaH3(IiPrMe)] [7b]
[GaH3(IXy)] (1)
180
186
166
214
263
–
274
–
177–181
1775
1789
1801
1780
1870
1855
1917
–
4.48
3.90
3.81
3.96
4.72
–
–
–
4.12
–
6.16
6.54
6.16
5.95
6.08
5.84
5.78 (in CDCl3)
5.89
[GaH3(IDipp)] (2)
[GaH3(IMes)] [4d]
[GaClH2(IMes)] (3)
[GaClH2(IXy)] (3Xy
)
[GaCl2H(IMes)] (4)
[GaCl3(IMes)][17]
[GaH2I(IMes)][11]
1863
a
Melting points and decomposition temperatures conducted using capillary sealed samples under UHP argon
All IR spectroscopy conducted as a Nujol mull with sodium chloride plates.
b
C
1H NMR spectroscopy conducted in C6D6 unless otherwise stated.
The 1H NMR spectra (C6D6) of 1 and 2, the former with mono-
chlorogallane impurity 3Xy
display the expected resonances,
baur and co-workers (cf method (i) Fig. 1) [13] is the absence of
co-products.
,
including distinct resonances for the diastereotopic isopropyl
methyls of coordinated IDipp [18] in [GaH3(IDipp)] (2) and the
GaH3 moieties (Table 1) of 1 and 2 (1 3.90 ppm, 2 3.81 ppm, no hy-
dride resonance observed for 3Xy), cf [GaH3(IMes)] 1H NMR, C6D6,
GaH3 br s 3.96 ppm [4d]. It is interesting to note that the 4,5-
C2H2 1H NMR singlet resonances of [GaH3(IXy)] (1) (6.16 ppm)
and [GaClH2(IXy)] (3Xy) (6.08 ppm) may be used to estimate the
proportions of each complex in preparations of 1, with all remain-
ing ‘‘IXy” resonances of 3Xy coincident with those of 1. The upfield
shift of the 4,5-C2H2 singlet resonance of 3Xy relative to that of 1 is
consistent with the trend observed for the series of aluminium
complexes [AlClnH3Àn(IMes)], n = 0–3 [3a,6,7a], in which the up-
field shift of the 4,5-C2H2 singlet increases with chloride content;
n = 0 6.01 ppm, n = 1 5.93 ppm, n = 2 5.86 ppm, n = 3 5.78 ppm.
Several solvent systems were used in attempts to separate 1 from
3Xy by extraction or recrystallisation, however all efforts afforded
Toluene solutions of [GaH3(IMes)] [4d] and [GaCl3(IMes)] [17]
were combined in either a 2:1 (3) or 1:2 (4) stoichiometric ratio
at room temperature, and stirred at 50 °C for 6 (3) or 14 (4) hours
(Scheme 2), followed by filtration and gradual cooling to ambient
temperature. Over this time colourless crystals of 3 and 4 depos-
ited in moderate yield (33% and 37%, respectively). These were iso-
lated by decanting of the mother liquor and characterised by IR, 1H
and 13C NMR spectroscopies (Table 1). The IR Ga–H stretch absorp-
tions of both compounds are consistent with gradual strengthen-
ing of the Ga–H bond(s) of 3 and 4 relative to [GaH3(IMes)] [4d].
For instance, there is a +90 cmÀ1 shift for monochlorogallane 3 rel-
ative to its trihydride counterpart (cf 3Xy; +66 cmÀ1 viz 1), and a
47 cmÀ1 increase in the Ga–H infrared stretching absorption of
dichlorogallane 4 relative to that of 3. These shifts are comparable
to those observed for mono-, dichloro- and bromoalanes, wherein
mean shifts of +55 and +45 cmÀ1 are observed upon substitution of
hydrides with a chloride or bromide about tertiary amine stabilised
alanes (AlH3) [6], and a +134 cmÀ1 shift is observed on dihalogen-
ation (Cl or Br) of [AlH3(IMes)] [3a,6].
samples characterising as per crude reaction mixtures for 1/3Xy
.
The solid-state decomposition points of 1 and 2, the former
without a 3Xy impurity, were recorded using capillary sealed sam-
ples under ultra high purity argon (1 dec. 186, 2 dec. 166 °C). Com-
pound 1 is more thermally robust than 2, but neither compound
exceeds the thermal stability of [GaH3(IMes)] (214 °C) [4d] (Table
1). The greater thermal stability of [GaH3(IMes)] and 1 relative to
2 may be attributed to the lessened proximity of gallane to the
methyls of these species relative to the isopropyl methyls of 2.
Thus, while it is true that the Dipp group may be used to kinetically
and sterically stabilise transient or frail chemical functions [4e] the
‘reach’ of the alkyl groups of 2 most likely accelerates decomposi-
tion, perhaps by C–H activation, at high temperature. A rationale
for the contrasting decomposition temperatures of 1 and [Ga-
H3(IMes)] [4d] is less obvious (Table 1), although it could be that
the spatial imposition of the 4-methyls of the latter serve to dis-
tance the gallane units of [GaH3(IMes)] in the solid-state, thereby
decreasing the likelihood of decomposition by Ga–H–Ga bridging
at high temperature. Both 1 and 2 may be handled in dry air (e.g.
a dessicator) without any decomposition. However, as per alumin-
ium counterparts [6], the exposure of solutions of 1 or 2 to air re-
sults in rapid decomposition to afford 2-dihydroimidazoles [4d,20].
Close comparison of the 1H NMR data of 3 and 4 with that of the
gallane [4d] and gallium trichloride [17], indicates a shift in 4,5-
C2H2 resonance for the complexes [GaClnH3Àn(IMes)] (n = 0–3) con-
sistent with that described for aluminium [3a,6,7a]. This comprises
an upfield shift with increasing chloride content (Table 1) and is
consistent with the increased Lewis acidity of the gallium centre
due to chloride electron withdrawal [5].
The solid-state decomposition temperatures of 3 and 4, ac-
quired as per 1 and 2, are significantly higher than those of the tri-
hydride [GaH3(IMes)] [4d] (dec. 214, 3 dec. 263, 4 dec. 274 °C)
making 3 and 4 the most stable molecular gallium hydrides re-
ported at this time. Like complexes 1 and 2, both compounds
may be handled in dry air, e.g. a dessicator, without decomposition.
2.2. Crystallographic studies
2.2.1. [GaH3(IDipp)]ÁNH2Dipp
Recrystallisation of reaction mixtures for 2 lead to the deposi-
tion of microcrystalline powders unsuitable for single crystal X-
ray structure determination. Small colourless blocks were isolated
on one occasion and their structure determined by X-ray methods
(Fig. 2). Table 2 contains selected bonding parameters for the com-
plexes characterised by X-ray diffraction methods herein as-well-
as relevant literature compounds. Table 3 provides a summary of
crystallographic data for the compounds reported in this article.
Compound 2 (Fig. 2) is the second structurally authenticated
NHC complex of gallium trihydride and may be compared to its
IiPrMe counterpart [7b]. During the refinement of X-ray diffraction
data for 2 it was noted that [GaH3(IDipp)] (2) had co-crystallised
2.1.2. IMes complexes of mono- and dichlorogallane
The preparation of Lewis base complexes of mono- and dichlo-
rogallane may be accomplished by the redistribution of stoichi-
ometric quantities of gallium trichloride and gallium trihydride
complexes (Fig. 1). This method was introduced by Greenwood
in 1965 [16] and utilised by the groups of Raston and Gladfelter
to prepare tricyclohexylphosphine [14] and mono/bis(quinucli-
dine) adducts of mono- and dichlorogallane [15a], respectively.
One benefit of this method relative to the silane route of Schmid-

M.L. Cole et al. / Journal of Organometallic Chemistry 694 (2009) 2934–2940
2937
Fig. 2. Molecular structure of [GaH3(IDipp)]ÁNH2Dipp (2ÁNH2Dipp): (a) 2 without NH2Dipp, 50% thermal ellipsoids, all hydrogen atoms except hydrides H(1A), H(1B) and
H(1C) omitted for clarity; (b) 2ÁNH2Dipp, 50% thermal ellipsoids, all hydrogens except those involved in intermolecular interactions omitted for clarity. Isopropyl groups
depicted as wire frames. Symmetry transformation used to generate ‘#’ atoms: ½ + x, ½ À y, ½ + z. Selected contact distances for intermolecular interactions (Å):
H(1B)ÁÁÁH(3A) 2.540(32), H(1C)ÁÁÁH(3B) 2.497(32), H(2)ÁÁÁC(30)# 2.84, H(2)ÁÁÁC(31)# 2.76, H(2)ÁÁÁC(32)# 2.75, H(3)ÁÁÁC(28)# 2.70, H(3)ÁÁÁN(3)# 2.59 (see Table 2 for further
bonding parameters).
Table 2
Selected bond lengths (Å) and angles (°) for complexes reported in this article and relevant literature compounds.
Compound
Ga–C (Å)
Ga–H (Å)
H–Ga–H/Cl–Ga–Cl (°)
C–Ga–H (°)
C–Ga–Cl/I (°)
N–C–N (°)
[GaH3(IiPrMe)] [7b]
[GaH3(IDipp)] (2)
[GaClH2(IMes)]a (3)
[GaCl2H(IMes)]a (4)
[GaCl3(IMes)] [17]
[GaH2I(IMes)] [11]
2.071(5)
2.0545(14)
2.030(3)
2.005(6)
1.954(4)
2.022(4)
1.58(5), 1.62(3)
1.46(2), 1.47(2), 1.54(2)
1.85(3), 1.51(4)
2.00(5)
114(2), 111(2)
115.8(11), 112.7(12),111.6(12)
117.6(19)
110.99(10)
107.86(4)
112(2), 102(2)
104.8(9), 104.5(8), 106.3(8)
107.2(10), 107.0(16)
112.3(13)
–
–
105.6(4)
104.15(12)
104.2(3)
105.6(5)
102.9(3)
104.4(4)
105.07(10)
106.24(18), 108.3(18)
113.26(8)
–
–
1.52(4),1.59(5)
120(2)
111.4(18), 109.5(19)
105.23(12)
a
Two unique molecules are present in the asymmetric unit. Only bonding parameters for the lowest numbered unit reported here.
Table 3
its subsequent inclusion in the small number of crystals isolated,
presumably results from concentration of toluene samples of 2.
Thus, although this impurity was not evident in the bulk sample,
subsequent IR, 1H NMR and C, H, N microanalytical data collected
on these crystals confirms that NH2Dipp is included. The IR spec-
trum of a crystal of 2ÁNH2Dipp exhibits a broad stretch at
3368 cmÀ1 (N–H) in addition to the characteristic Ga–H single
stretch of 2 at 1801 cmÀ1, and a 1H NMR spectrum (C6D6) of a crys-
tal from the same batch exhibits broad resonances at 3.81 and
3.28 ppm, in a 3:2 integration ratio. These correspond to the
GaH3 and the NH2 hydrogens, respectively. This co-crystallisation
clearly indicates that 2 does not react with NH2Dipp in toluene
solution.
Summary of crystallographic data for compounds characterised by single crystal X-
ray structure determination.
2ÁNH2Dipp
C39H58GaN3
638.60
P21/n
12.8389(4)
18.1264(6)
15.9583(5)
93.761(2)
3705.9(2)
4
3
4a
Mol. formula
Mol. weight
Space group
a (Å)
b (Å)
c (Å)
C42H52Cl2Ga2N4
823.22
Pca21
16.142(3)
14.413(3)
17.646(4)
90
4105.4(14)
4
1.332
C42H50Cl4Ga2N4
892.10
Pca21
17.1580(6)
16.4134(6)
15.6186(5)
90
4398.5(3)
4
1.347
b (°)
V (Å3)
Z
Dcalc, g cmÀ3
1.145
0.771
l
(mmÀ1
)
1.476
1.501
Reflections collected
Unique reflections
Parameters varied
R(int)
74 716
10 747
420
0.0726
0.0382
0.0972
49 982
10 994
479
0.0870
0.0467
0.0976
46 495
11 889
489
0.0864
0.0702
0.2183
The N–C–N angle of 2 (Fig. 2a) is 104.15(12)°, which is typical
for coordinated NHCs ([AlH3(IMes)]: 104.2(2)°) [7a]. The Ga(1)–
C(1) bond length of 2.0545(14) Å is considerably longer than that
of [GaCl3(IMes)] (1.954(4) Å) [17] and longer than that of [GaCl3
(IDipp)] (2.016(2) Å) [17] indicating the increased Lewis acidity
of gallium trichloride relative to gallane, and the increased steric
bulk of IDipp vs. IMes. The gallium–hydrogen contacts of 1.46(2),
1.47(2) and 1.54(2) Å compare well with those of [GaH3(IiPrMe)]
(1.58(5) and 1.62(3) Å) [7b] and are consistent with gallium hy-
dride bonds of group 15 Lewis base adducts of gallane [10]. In
terms of intermolecular contacts, the 4,5-C2H2 of the IDipp of 2
R1
wR2 (all data)
a
Isostructural with aluminium counterpart [6].
with residual 2,6-diisopropylaniline (NH2Dipp). This was carried
forward as a minor impurity in samples of IDippÁHCl used to pre-
pare IDipp [18]. The high boiling point of NH2Dipp (257 °C), and
(H(2) and H(3)) act as C–H acceptors to arene
p-donation from

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M.L. Cole et al. / Journal of Organometallic Chemistry 694 (2009) 2934–2940
the lattice NH2Dipp (C(28)–C(33)) (Fig. 2b). Furthermore, two of
the Ga–H bonds (H(1B) and H(1C)) interact with the hydrogens
of the NH2 functionality of the lattice aniline (Fig. 2b). These weak
intermolecular forces hold the NH2Dipp in a defined position in the
crystal lattice (see graphical abstract illustration) but are not evi-
dent in solution (cf 1H NMR and IR data for 2 vs. 2ÁNH2Dipp). All
of the intermolecular interactions described are within accepted
bonding parameters for atom–atom or C–HÁÁÁ
p-arene contacts of
this type (see Fig. 2b caption) [21].
2.2.2. IMes complexes of mono- and dichlorogallane
Chlorogallane complexes 3 and 4 were also characterised by sin-
gle crystal x-ray diffraction and their molecular structures are de-
picted in Figs. 3 and 4, respectively. In both cases the hydride
ligands were located from difference maps and refined isotropically.
Selected bond lengths (Å) and angles (°) are provided in Table 2 with
relevant bond distances and angles for salient literature com-
pounds. Table 3 contains a summary of crystal measurement and
refinement data for the molecular structures reported in this article.
The four-coordinate IMes complexes 3 and 4 crystallise with
two molecular units in the asymmetric unit. The two distinct units
exhibit comparable bonding parameters. Accordingly, only one
molecule is discussed here. The ‘‘Ga(IMes)” subunits of 3 and 4
are similar to those of their iodogallane counterpart [GaH2I(IMes)]
[11] prepared by the reaction of [GaH3(IMes)] with monovalent
gallium iodide, from Baker and Jones, and the molecular structure
of [GaCl3(IMes)] [17]. Unfortunately, the solid-state structure of
[GaH3(IMes)] is not available for comparison.
Fig. 4. Molecular structure of lowest numbered unique molecule of [GaCl2H(IMes)]
(3) (50% thermal ellipsoids POV-RAY illustration). All hydrogen atoms excepting
H(1A) omitted for clarity. See Table 2 for selected bonding parameters.
The Ga–C bonding contacts and N–C–N angles of 3 (2.030(3) Å
and 104.2(3)°) and 4 (2.005(6) Å, 105.6(5)°) compare well with
those of [GaH2I(IMes)] (2.022(4) Å and 104.4(4))°) [11] but differ
markedly to those of [GaCl3(IMes)] (1.954(4) Å and 102.9(3)°)
[17] wherein the higher Lewis acidity of gallium trichloride de-
creases the Ga–C bond length.
The Ga–H contacts of 3 and 4 are of length 1.85(3), 1.51(4) (3)
and 2.00(5) Å (4) (Table 2). For 3, the shorter contact lies coplanar
with the ‘‘Ga-NHC” plane. Overall these contacts are significantly
longer than those observed in 2ÁNH2Dipp, but this increase in bond
length is consistent with similar increases observed for the chlo-
roalane counterparts of 3 and 4 relative to [AlH3(IMes)] [6,7a].
However, it is interesting to note that, despite the questionable
reliability of M–H bond determination by x-ray methods, the
extension of these bonds is contrary to expectation based on trends
lower than that of 4. This trend is also observed for quinuclidine
adducts of mono- and dichlorogallane [15a] and the aluminium
congeners of 3 and 4 [6].
3. Conclusion
In summary, we have extended the family of gallohydride NHC
species with the addition of four fully characterised complexes that
exhibit spectroscopic and structural trends consistent with the rel-
ative Lewis acidities of the gallohydrides and the sterics of the
NHCs used. The structures of the compounds studied by X-ray
methods exhibit intermolecular bonding interactions owing to
inclusion of co-crystallised aniline (2), or trends consistent with
Bent’s rule (3 and 4) [22]. The co-crystallisation of NH2Dipp with
2 suggests either that the pKb of 2 may be too high for deprotona-
tion of this aniline, or there is a steric impediment to reaction.
From thermal decomposition data, it is clear that IMes is a superior
ligand for the stabilisation of gallane relative to IDipp and IXy, and
that chloride substitution of hydrides leads to increased gallohy-
dride stability culminating in 4, which is the most stable molecular
gallium hydride reported. The stabilities of the compounds re-
ported herein make them ideal candidates for ‘bottleable’ hydro-
gallation reagents in organic synthesis.
in
m
Ga–H IR data [6].
The relative pyramidalisation of the gallium centres of 3, 4, [Ga-
Cl3(IMes)] [17]. and [GaH2I(IMes)] [11] (Table 2) is consistent with
trends in gallium ‘hybridisation’ expected according to Bent’s rule
[22]. For instance, the Cl–Ga–Cl angle of 4 is greater than that of
[GaCl3(IMes)] (Table 2), and the H–Ga–H angle of 3 approaches
the ideal angle for sp2-hybridisation with a Cl–Ga–C angle that is
The preparation and reactivity of Lewis base adducts of group
13 halohydrides is ongoing in our laboratory and will form the ba-
sis of forthcoming publications.
4. Experimental
Diethyl ether was dried over sodium and freshly distilled from
sodium diphenylketyl before freeze–thaw degassing prior to use.
Toluene was dried over sodium and freshly distilled from potas-
sium before freeze–thaw degassing prior to use. Lithium tetrahydr-
idogallate was prepared and used in situ by reaction of gallium
trichloride with excess lithium hydride (ca 30 equiv.) at À30 °C
in diethyl ether. [GaH3(IMes)] [4d], [GaCl3(IMes)] [17] IXy and
Fig. 3. Molecular structure of lowest numbered unique molecule of [GaClH2(IMes)]
(3) (50% thermal ellipsoids POV-RAY illustration). All hydrogen atoms excepting
hydrides H(1A) and H(2A) omitted for clarity. See Table 2 for selected bonding
parameters.

M.L. Cole et al. / Journal of Organometallic Chemistry 694 (2009) 2934–2940
2939
IDipp [18] were prepared by literature procedures. All manipula-
tions were performed using conventional Schlenk or glovebox
techniques under an atmosphere of ultra high purity argon in
flame-dried glassware. Infrared spectra were recorded as Nujol
mulls using sodium chloride plates on a Nicolet Nexus FTIR spec-
trophotometer. 1H and 13C{1H} NMR spectra were recorded using
Bruker spectrometers (see below for MHz) with chemical shifts ref-
erenced to the residual 1H resonances of the deutero-benzene sol-
vent (d 7.16 and 128.39 ppm, respectively). Melting points were
determined in sealed glass capillaries under argon and are uncor-
rected. All microanalyses were conducted by the Campbell Micro-
analytical Laboratory, Chemistry Department, University of Otago,
P.O. Box 56, Dunedin, New Zealand. Single crystal X-ray data col-
lections were undertaken at Monash University or UNSW on Bru-
ker Apex II (2ÁNH2Dipp), Enraf-Nonius Kappa (3) or Bruker X8
Apex (4) instruments. Further details are listed in a separate sec-
tion below.
at 50 °C and slow cooling to ambient temperature yielded 3 as col-
ourless blocks (0.18 g, 33%), m.p. 263 °C (dec.). Anal. Calc. for
C21H26ClGaN2: C, 61.28; H, 6.37; N, 6.81; Found: C, 60.30; H,
6.39; N, 6.58%; 1H NMR (300 MHz, C6D6, 300 K): d 2.03 [s, 12H,
o-CH3], 2.05 [s, 6H, p-CH3], 4.72 [br s, 2H, Ga–H], 5.95 [s, 2H,
C2H2], 6.71 [s, 4H, m-H]; 13C NMR (75.5 MHz, C6D6, 300 K): d 18.0
[s, o-CH3], 21.4 [s, p-CH3], 123.0 [s, C2H2], 129.9 [s, m-CH], 135.1
[s, p-CCH3], 135.4 [s, o-CCH3], 140.2 [s, ipso-C], 172.5 [s, NCN]; IR
(Nujol) v/cmÀ1: 1870 (sharp s, Ga–H str).
4.4. [GaCl2H(IMes)] (4)
[GaCl3(IMes)] (0.1 g, 0.26 mmol) in toluene (10 cm3) was added
dropwise to a solution of [GaH3(IMes)] (0.25 g, 0.52 mmol) in tolu-
ene (10 cm3) at ambient temperature. The resulting light yellow
solution was warmed to 50 °C and stirred for 14 h. Filtration at
50 °C and slow cooling to ambient temperature yielded 4 as thin
plates (0.26 g, 37%), m.p. 274 °C (dec.). Anal. Calc. for
C21H25Cl2GaN2: C, 56.54; H, 5.65; N, 6.28; Found: C, 56.39; H,
5.75; N, 6.27%; 1H NMR (300 MHz, C6D6, 300 K): d 2.02 [s, 12H,
o-CH3], 2.04 [s, 6H, p-CH3], 5.84 [s, 2H, C2H2], 6.69 [s, 4H, m-H];
13C NMR (75.5 MHz, C6D6, 300 K): d 17.6 [s, o-CH3], 20.9 [s, p-
CH3], 123.2 [s, C2H2], 129.5 [s, m-CH], 133.0 [s, p-CCH3], 135.0 [s,
o-CCH3], 140.3 [s, ipso-C]; IR (Nujol) v/cmÀ1: 1917 (br s, Ga–H str).
4.1. [GaH3(IXy)] (1) (and [GaClH2(IXy)] (3Xy))
A cooled (À40 °C) solution of IXy (0.60 g, 2.17 mmol) in diethyl
ether (50 cm3) was added to a stirred slurry of LiGaH4 (2.32 mmol)
also in diethyl ether (70 cm3). The resulting grey/brown reaction
mixture was stirred below 0 °C for 2 h and gradually warmed to
room temperature overnight. The brown solution was filtered
and the solvent removed in vacuo to afford 1 as an off-white pow-
der that was recrystallised from the minimum volume of toluene
(430 mg, 72%), m.p. 186 °C (dec.). Samples of 1 repeatedly gave
microanalyses low in carbon and nitrogen. This presumably results
from incomplete combustion. Example analysis: Anal. Calc. for
C19H23GaN2: C, 65.36; H, 6.64; N, 8.02; Found: C, 55.68; H, 7.37,
N, 5.21%; 1H NMR (400.13 MHz, C6D6, 300 K); d 2.12 [s, 12H,
CH3], 3.90 [br s, 3H, Ga–H], 6.16 [s, 2H, C2H2], 6.72 [s, 6H, C6H3];
13C NMR (100.62 MHz, C6D6, 300 K); d 17.2 (CH3), 121.6 (C2H2),
121.8 (p-C6H3), 129.0 (o-C6H3), 134.6 (m-C6H3), 135.0 (ipso-C6H3),
138.9 (NCN); IR (Nujol) v/cmÀ1: 1789 (br s, Ga–H str). Additional
4.5. X-ray structure determination
Crystalline samples of 2ÁNH2Dipp, 3 and 4 were mounted on
glass fibres in silicone oil at À150(2) °C (123(2) K). A summary of
crystallographic data can be found in Table 3 and salient bond
lengths and angles and listed in Table 2. Hydrogen atoms were re-
fined in calculated positions (riding model) with the exception of
hydride ligands and the aniline NH2 hydrogens of 2ÁNH2Dipp,
which were located and refined isotropically in all cases. Data were
collected using graphite monochromated Mo K
a X-ray radiation
(k = 0.71073 Å) on a Bruker Apex II (2ÁNH2Dipp), Enraf-Nonius Kap-
pa (3), or Bruker X8 Apex (4) CCD diffractometer, and data were
corrected for absorption by empirical methods using SADABS. Struc-
tural solution and refinement was carried out using the SHELX suite
of programs [23]. Flack tests for compounds 3 and 4 are supportive
of the correct structure and space group refinement. For compound
4, a peak of electron density >1.5 e ÅÀ3 is placed 0.01 Å from H(1A).
This hydride is associated with an unusually long Ga to H contact.
data for 3Xy
6.72 [s, 6H, C6H3]; IR (Nujol)
:
1H NMR; d 2.12 [s, 12H, CH3], 6.08 [s, 2H, NCH],
/cmÀ1; 1855 (br, s, Ga–H str).
m
4.2. [GaH3(IDipp)] (2) and [GaH3(IDipp)]ÁNH2Dipp (2ÁNH2Dipp)
A cooled (À40 °C) solution of IDipp (1.12 g, 2.88 mmol) in
diethyl ether (20 cm3) was added to a stirred slurry of LiGaH4
(2.84 mmol) also in diethyl ether (100 cm3). The grey/brown reac-
tion mixture stirred below 0 °C for 2 h and gradually warmed to
room temperature overnight. Filtration and removal of volatiles
in vacuo afforded crude 2 as a beige powder (993 mg, 76%), m.p.
166 °C (dec.). During one preparation, recrystallisation from tolu-
ene afforded a small number of colourless blocks that characterised
as 2ÁNH2Dipp, m.p. 136 °C. Anal. Calc. for C39H58GaN3 (2ÁNH2Dipp):
C, 73.23; H, 9.30; N, 6.57; Found: C, 72.71; H, 9.28, N, 6.45%; 1H
NMR (NH2Dipp free sample) (300.13 MHz, C6D6, 300 K); d 1.15
[d, J = 6.3 Hz, 12H, CH(CH3)2], 1.53 [d, J = 6.3 Hz, 12H, CH(CH3)2],
2.76 [sept, J = 6.3 Hz, 4H, CH(CH3)2], 3.81 [br s, 3H, Ga–H], 6.54 [s,
2H, C2H2], 7.19 [d, J = 6.9 Hz, 4H, m-C6H3], 7.36 [t, J = 6.9 Hz, 2H,
p-C6H3]; 13C NMR (100.62 MHz, C6D6); d 23.0 (CH(CH3)2), 24.5
(CH(CH3)2), 28.6 (CH(CH3)2), 118.5 (C2H2), 123.2 (p-C6H3), 123.8
(o-C6H3), 130.2 (m-C6H3), 134.8 (ipso-C6H3), 145.2 (NCN); IR (Nujol)
Acknowledgements
The authors would like to thank the Australian Research Council
(DP0558562) for financial support of this research, and thank Dr.
Craig M. Forsyth (Monash University) for the collection of single
crystal X-ray diffraction data for compounds 3 and 4.
Appendix A. Supplementary material
CCDC 714187, 714188 and 714189 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via http://www.ccdc.cam.ac.uk/data_request/cif. Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.jorganchem.2009.04.030.
m
/cmÀ1; 1801 (br, s, Ga–H str).
4.3. [GaClH2(IMes)] (3)
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yellow solution was warmed to 50 °C and stirred for 6 h. Filtration
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