A structural study of bis-(trimethylamine)alane, AlH3·2NMe3, by variable temperature X-ray crystallography and DFT calculations

Article abstract of DOI:10.1016/j.molstruc.2008.12.022

The structure of AlH₃·2NMe₃ has been investigated by single-crystal X-ray diffraction over the range of 296-173 K. Over this temperature range a phase change is observed from Cmca to Pbcm where the methyl groups convert from a statis

Full text of DOI:10.1016/j.molstruc.2008.12.022

Journal of Molecular Structure 923 (2009) 13–18
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
A structural study of bis-(trimethylamine)alane, AlH3ꢀ2NMe3, by variable
temperature X-ray crystallography and DFT calculations
*
Terry D. Humphries, Peter Sirsch, Andreas Decken, G. Sean McGrady
Department of Chemistry, University of New Brunswick, P.O. Box 4400, Fredericton, NB, Canada E3B 5A3
a r t i c l e i n f o
a b s t r a c t
Article history:
The structure of AlH3ꢀ2NMe3 has been investigated by single-crystal X-ray diffraction over the range of
296–173 K. Over this temperature range a phase change is observed from Cmca to Pbcm where the methyl
groups convert from a statistically disordered conformation to adopt a mutually eclipsed conformation at
lower temperatures. Measurement of the unit cell dimensions shows a decrease in the lengths of the a
and b axes, and an increase in that of the c axis as the temperature is lowered, with inflections apparent
between 223 and 233 K in the region of the phase change. Low-temperature DSC measurements reveal
the change from Pbcm to Cmca to occur at 218.3 K, with an enthalpy of 107.7 J molꢁ1. The molecular
structure of AlH3ꢀ2NMe3 is compared with those of related amine adducts of Group 13 hydrides, either
measured experimentally or calculated using DFT methods. 1H, 13C and 27Al NMR spectroscopy has also
been utilized to characterize AlH3ꢀ2NMe3 and its 1:1 counterpart AlH3ꢀNMe3.
Received 5 November 2008
Received in revised form 3 December 2008
Accepted 3 December 2008
Available online 16 December 2008
Keywords:
Bis-(trimethylamine)alane
Alane
X-ray crystallography
DSC
DFT calculations
NMR spectroscopy
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
with D3h symmetry. These authors also concluded that the mole-
cule undergoes partial dissociation in the gas phase, in an equilib-
The binary hydride of aluminum, AlH3 or alane, is a sterically
and electronically unsaturated moiety that reacts readily with a
range of Lewis donors, leading to 1:1 and 1:2 adducts which are
four- or five-coordinate at the Al centre, respectively [1–3]. A wide
range of such adducts has been synthesized and characterized over
the past 60 years, with donors including carbenes [4–6], phos-
phines [6–8], amines [9–12], ethers [13–15] and other O-derived
functionalities [16].
Amongst these different types of alane adducts, amine com-
plexes are the most widely studied and reported. The simplest
amine adducts; viz. AlH3ꢀNMe3 and AlH3ꢀ2NMe3, were originally
reported in 1942 by Wiberg and Stecher [9–11]. For the 1:2 adduct,
Wiberg postulated a trigonal bipyramidal (TBP) structure, a predic-
tion that was confirmed by Heitsch et al. in 1963 through a single-
crystal X-ray structure determination at room temperature [17].
Heitsch et al. concluded that the complex indeed adopts a TBP
structure, with axial amines and equatorial hydrogen ligands,
and crystallizes in an orthorhombic space group Cmca, with unit
cell parameters a = 10.10 Å, b = 8.84 Å, and c = 12.94 Å. Somewhat
surprisingly, these authors also reported a dipole moment for the
molecule in solution. The molecular structure was corroborated
in the same year by Fraser et al., who measured the gas phase IR
spectrum of AlH3ꢀ2NMe3, and interpreted it in terms of a molecule
rium between the 1:2 and 1:1 adducts and free NMe3 [18]. A
subsequent gas electron diffraction (GED) study by Mastryukov
et al. confirmed the D3h molecular structure [19]: this study made
allowance for the equilibrium described by Fraser et al., but found
no significant amounts of AlH3ꢀNMe3 or free NMe3 in the GED pat-
tern. The X-ray crystal structure of AlH3ꢀNMe3 was reported by
Raston et al., who found the compound to be dimeric in the solid
state, as postulated by Wiberg [11]. In the gas phase the molecule
is monomeric [20]. Other 1:1 alane complexes associate through
unsymmetrical bridging hydrides in the crystalline state include
the adducts with THF [16], NMe2(CH2Ph), 1-methyltetrahydropyri-
dine [21] and NMe2(CH2)3Cl [22].
The preparation of AlH3ꢀ2NMe3 is straightforward, and can be
achieved by several routes. The original Wiberg synthesis involved
the production of AlH3 in Et2O, followed by displacement of the
ether ligand by the more basic Me3N (Eqs. (1) and (2)) [9,10]. Heit-
sch et al. used a similar approach, substituting H2SO4 in place of
HCl in the first step (Eqs. (3) and 4) [17]. More recently, Ruff and
Hawthorne adopted the direct reaction between LiAlH4 and
Me3NHCl in Et2O, thereby employing the ammonium hydrochlo-
ride salt as the source of both acid and amine ligand in a single step
to form the 1:1 complex (Eq. (5)) [23]. Excess NMe3 must be con-
densed onto the amine–alane before sublimation to ensure com-
plete production of the 1:2 complex (Eq. (6)). This route was
previously employed by Schaeffer for the synthesis of BH3ꢀNMe3
using LiBH4 and NMe3ꢀHCl [24]. The procedures for preparing the
* Corresponding author.
E-mail address: smcgrady@unb.ca (G.S. McGrady).
0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2008.12.022

14
T.D. Humphries et al. / Journal of Molecular Structure 923 (2009) 13–18
1:1 and 1:2 adducts are almost identical, except that the latter
requires addition of an excess of NMe3 prior to sublimation to
guard against dissociation to the 1:1 complex.
and Yoon [30]. THF (50 mL) was added to purified LiAlH4 (1.5 g;
40 mmol), and left to stir overnight. The suspension was cooled
in an ice bath whilst conc. H2SO4 (2.0 g; 20 mmol) was added drop-
wise. A white precipitate of Li2SO4 quickly formed and H2 evolution
was apparent. The reaction mixture was stirred for 6 h and left to
stand overnight, allowing the precipitate to settle. The supernatant
solution was then filtered into a Schlenk tube. The concentration of
the resulting AlH3 solution was ca. 0.8 M. A 10 mL aliquot of this
solution was transferred into an H-tube equipped with a magnetic
stirrer flea and a coarse sintered glass frit between the two limbs.
The vessel was cooled to ꢁ196 °C and partially evacuated, and gas-
eous Me3N (9.0 g; 150 mol) was condensed on top of the frozen
solution. Upon warming, the solution melted and a white precipi-
tate was observed. The whole procedure was repeated one more
time to ensure complete reaction, then the THF solution was trans-
ferred via cannula into a separate Schlenk flask and the THF was re-
moved in vacuo, leaving a white powder on the bottom and
colourless, irregular shaped crystals that had sublimed on the side
of the flask. The remaining white powder in the H-tube was dis-
solved in toluene and washed through the frit: this solution depos-
ited cubic crystals that decomposed rapidly on exposure to air.
O
LiAlH4 þ HCl E!t2 AlH3 ꢀ Et2O þ LiCl þ H2
ð1Þ
ð2Þ
ð3Þ
ð4Þ
AlH3 ꢀ Et2O þ 2NMe3 ! AlH3 ꢀ 2NMe2 þ Et2O
THF
LiAlH4 þ H2SO4 ! AlH3 ꢀ THF þ LiSO4 þ H2
AlH3 ꢀ THF þ 2NMe3 ! AlH3 ꢀ 2NMe2 þ THF
LiAlH4 þ NHMe3Cl E!t2 AlH3 ꢀ NMe3 þ LiCl
ð5Þ
ð6Þ
O
AlH3 ꢀ NMe3 þ NMe3 ! AlH3 ꢀ 2NMe3
The 1:1 and 1:2 adducts are white, air-sensitive crystalline
materials that sublime readily at ambient temperature, and which
as described above exist in equilibrium in the gas phase [18,25,26].
Under thermolysis, they decompose readily into crystalline Al, and
Me3N and H2 gases [9]. The availability of high purity Al in this
manner has prompted a large body of research into the use of these
adducts in various types of methods of chemical vapour deposition
(CVD) to produce Al thin films [27–29].
In the solid state, AlH3ꢀ2NMe3 is known to crystallize in at least
two phases. In their earlier X-ray diffraction study, Heitsch et al.
reported the existence of a phase change at ꢁ35 °C [17]. In this pa-
per, we report an improved room temperature crystal structure for
AlH3ꢀ2NMe3, a determination of the low temperature structure for
the first time, and characterization of the phase transition between
them by low-temperature DSC. In addition, variable temperature
single-crystal X-ray diffraction has been conducted between
ꢁ100 °C and room temperature in 10 °C increments, permitting
us to monitor the temperature dependence of the crystal and
molecular structure of AlH3ꢀ2NMe3. Finally, the 1H, 13C and 27Al
NMR data for AlH3ꢀ2NMe3 are reported and compared with the cor-
responding values for the 1:1 adduct AlH3ꢀNMe3.
2.2.1.2. Method B. LiAlH4 (2.2 g; 58 mmol) and Me3NHCl (4.5 g;
47 mmol) were introduced into a Schlenk flask, which was then
cooled to ꢁ78 °C. Et2O (100 mL) was added, and the contents were
stirred and allowed to warm to room temperature. After 1 h, the
solvent was then removed in vacuo, leaving a pasty white residue.
To guarantee the 2:1 complex, NMe3 gas was condensed into the
flask and the contents were allowed to stir for 20 min. The flask
was then warmed to 50 °C, and the contents sublimed into a sep-
arate flask cooled to ꢁ78 °C. A white crystalline powder thus ob-
tained was stored at ꢁ37 °C in a glove box.
Yield: 3.4 g (98%). 1H NMR (toluene-d8): d (ppm) = 3.42 (br, 3H,
AlH3), 2.13 (s, 18H; CH3). 27Al NMR (toluene-d8): d (ppm) = 111.49.
13C NMR (toluene-d8): d (ppm) = 47.39 (N–CH3). The unit cell was
measured by single-crystal X-ray diffraction, which confirmed
the material as AlH3ꢀ2NMe3.
2. Experimental
2.1. General considerations
2.2.2. AlH3ꢀNMe3
All manipulations were performed using standard Schlenk or dry-
box techniques in vacuo or under an atmosphere of purified nitrogen
or argon (H2O and O2 < 1 ppm). Solvents were dried and deoxygen-
ated using a Grubbs-type solvent dispensing system, degassed prior
to use, and stored under argon. All starting materials were reagent
grade from Sigma–Aldrich and were used as received, with the
exception of LiAlH4, which was purified by recrystallization from
THF. TheNMRspectra for AlH3ꢀNMe3 andAlH3ꢀ2NMe3 were recorded
intoluene-d8. 13C spectraforboth complexes andthe 1H spectrum for
AlH3ꢀNMe3 were measured using a Varian 300 NMR spectrometer.
27Al spectra for both complexes and the 1H spectrum for AlH3ꢀ2NMe3
were recorded using a Varian 400 spectrometer. 13C NMR spectra
were referencedto thetoluene-d8 singletat137.86 ppmand1Hspec-
tra were referenced to the H NMR were referenced to the toluene-d8
singlet at 7.09 ppm. 27Al NMR were referenced using Al(NO3)3 in D2O
(1.1 mol/L) as an external standard. DSC measurements were con-
ducted using a TA Instruments DSC Q200 with Perkin-Elmer Volatile
Aluminum pans. The purge gas was UHP 5.0 He at 25 mL/min. Data
were collected at 0.1 s/point. The method used included a tempera-
ture equilibration at ꢁ100 °C, isothermal for 1 min, followed by a
temperature ramp of 2 °C/min to 400 °C.
AlCl3 (3.78 g, 28 mmol) and LiAlH4 (3.69 g, 97 mmol) were
introduced into an H-tube whose two limbs were separated by a
sintered glass frit. The apparatus was cooled in an ice bath and
pentane (100 mL) was added. NMe3 (9 g; 152 mmol) was con-
densed onto the slurry, which was then allowed to warm to room
temperature and stirred overnight. The resulting solution was fil-
tered through the frit and the vessel was stored at ꢁ40 °C for
24 h, to afford a white crystalline powder. The pentane solvent
was removed in vacuo, leaving a pasty white residue. The flask
was then warmed to 50 °C, and the contents sublimed into a collec-
tion vessel cooled to ꢁ78 °C. The white powder thus obtained was
stored at ꢁ37 °C in a glove box.
Yield: 1.092 g (10.8%). 1H NMR; d (ppm) = 3.92 (br, 3H, AlH3),
1.89 (s, 9H; CH3). 27Al NMR; d (ppm) = 139.81. 13C NMR; d
(ppm) = 47.76 (CH3).
2.3. X-ray crystal structure determinations
Crystals of AlH3ꢀ2NMe3 were grown by sublimation in vacuo at
room temperature. A suitable single crystal of dimensions (mm)
0.60 ꢂ 0.50 ꢂ 0.40 was selected in a glovebox, fixed in a glass
capillary without oil and mounted on a Bruker AXS P4/SMART
1000 diffractometer. A hemisphere of data was collected using
2.2. Syntheses
graphite monochromated Mo-Ka radiation (k = 0.71073 Å) and x-
2.2.1. AlH3ꢀ2NMe3
and h-scans with a scan width of 0.3°. The unit cell parameters
were obtained by least-squares refinement of 4579 reflections
(low-temperature study at 173 K, LT) and 1780 reflections
2.2.1.1. Method A. A preliminary qualitative reaction was con-
ducted using a solution of AlH3 prepared as described by Brown

T.D. Humphries et al. / Journal of Molecular Structure 923 (2009) 13–18
15
Table 1
Crystallographic data for AlH3ꢀ2NMe3.
T = 296(1) K
T = 173(2) K
Chemical formula
Mr
Space group
C6H21AlN2
148.23
Cmca
C6H21AlN2
148.23
Pbcm
a/Å
b/Å
c/Å
10.091(3)
8.858(3)
12.938(4)
90
8.462(2)
13.412(3)
9.640(2)
90
a
/°
b/°
90
90
c
/°
90
1156.3(6)
4
90
1094.2(5)
4
V/Å3
Z
F(000)
336
0.851
0.121
0.0501, 0.1816
1.068
336
0.900
0.128
0.0332, 0.0986
1.129
Dc/g cmꢁ3
l
/mmꢁ1
R1a(obsd), wR2b(all)
Sc
Fig. 1. Molecular structure of AlH3ꢀ2NMe3 as determined by single-crystal X-ray
diffraction at 173 K. All non-hydride hydrogen atoms are omitted for clarity and
thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å),
angles (°) and torsional angles (°): AlꢁN1 2.163(2), AlꢁN2 2.163(2), AlꢁH1 1.55(2),
AlꢁH2 1.52(2), N1ꢁC1 1.471(2), N1ꢁC2 1.470(2), N2ꢁC3 1.472(2), N2ꢁC4 1.470(2),
N1ꢁAlꢁN2 179.84(5), H1ꢁAlꢁH2 120.7(5), H1ꢁAlꢁN1 90.0(5), H2ꢁAlꢁN1 90.2(7),
AlꢁN1ꢁC1 109.82(8), AlꢁN1ꢁC2 109.0(2), C1ꢁN1ꢁC2 109.3(2), C2ꢁN1ꢁAlꢁH2
180, C4ꢁN2ꢁAlꢁH2 180, C2ꢁN1ꢁAlꢁH1 ꢁ59.3(5).
Rint (all equivalents)
0.0178
0.0264
P
P
a
R1
=
(|Fo| ꢁ |Fc|)/ |Fo|.
P
P
b
c
2
wR2 = { [w(Fo ꢁ Fc2)2]/ [w(Fo2)2]}1/2
.
P
S = [ w(Fo ꢁ Fc2)2/(no ꢁ np)]1/2
.
2
(room-temperature study at 296 K, RT) and a total number of 7161
(LT) and 3760 (RT) reflections were integrated, 1311 (LT) and 699
(RT) of which were unique [Rint = 0.0264 (LT), 0.0178 (RT)]. Raw
data were integrated with the program SAINT [31] and corrections
for absorption effects were applied using SADABS [32]. The struc-
ture was solved by a combination of direct methods (SHELXS)
[33] and iterative difference-Fourier syntheses (SHELXTL) [34].
All non-hydrogen atoms were refined using anisotropic displace-
ment parameters. For the room temperature study, hydride atoms
were refined using isotropic displacement parameters; methyl
hydrogen atoms were included in calculated positions and refined
using a riding model. All hydride atoms and methyl groups were
disordered over two positions. For the low-temperature study, all
hydrogen atoms were refined using isotropic displacement param-
eters. The number of refined parameters was 94 (LT) and 50 (RT).
The maximum residual electron density was 0.39 (LT) and 0.18
(RT) and minimum ꢁ0.11 (LT) and ꢁ0.09 eÅꢁ3 (RT). Further details
of the refinement and crystallographic data are presented in Table
1. Plots were generated using the programs ORTEP-3 [35] and PLA-
TON [36]. CCDC 705484 and 705485 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or
from the Cambridge Crystallographic Data Centre, 12, Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033).
The low-temperature (173 K) data were collected first. The crys-
tal was then allowed to warm to room temperature (296 K) where
a second data set was recorded. The variable temperature study
was then conducted with the next data set being recorded at
249 K. Data were then collected in 10 K decrements with a
20 min period allowed for equilibration. A total of 120 frames were
collected at each temperature enabling a unit cell to be deter-
mined. After the final measurement, the crystal was allowed to
warm to room temperature and the unit cell was determined a
final time to confirm that the structure had reverted to the space
group Cmca.
be true minima on the potential energy surface by calculating ana-
lytical frequencies.
3. Results and discussion
3.1. Molecular structure of AlH3ꢀ2NMe3
Fig.
1
shows the molecular structure determined for
AlH3ꢀ2NMe3 at 173 K. The molecule adopts a regular TBP array,
with three equatorial hydrides and two axial amine ligands, which
adopt a staggered conformation with respect to the central AlH3
moiety. Table 2 presents a comparison of the molecular dimen-
sions measured by us at 296 and 173 K with those reported by
Heitsch et al. at room temperature [17] and by Mastruykov in
the gas phase at 323 K [19]. In the earlier X-ray study, the hydrogen
atoms could not be located independently due to disorder. The
molecular parameters deduced from all four experiments are in
good agreement, confirming that the structures at room tempera-
ture and 173 K differs only by the increased symmetry at lower
temperature (Section 3.2).
Table 3 allows comparison of the salient structural parameters
we have determined for AlH3ꢀ2NMe3 with those reported for the
1:1 trimethylamine adducts of boron, aluminum and gallium, as
well as their DFT-calculated counterparts. Al is unique amongst
this triad of Group 13 elements in forming
a stable 1:2
adduct. Although the Ga analogue appears theoretically stable at
the B3LYP/6-311G(d,p) level of theory [d(EꢁH) = 1.587 Å,
Table 2
Comparison of salient bond lengths (Å) and angles (°) for AlH3ꢀ2NMe3 in solid and gas
phases studied by X-ray diffraction, GED measurements and DFT calculations.
X-raya
X-ray (RT)b
X-ray (LT)b
GEDc
DFTd
AlꢁN
2.18(1)
1.48(3)
–
110(2)
–
2.174(2)
2.163(2)
2.19(2)
1.48(5)
1.53(3)
106.5(8)
–
2.248
1.474
1.616
108.7
90.0
2.4. Computational details
NꢁC
1.422(5)–1.548(6)
1.51(4)–1.56(4)
104.1(2)–114.9(2)
84(2)–95(2)
1.470(2)–1.472(2)
1.52(2), 1.55(2)
109.0(2)–110.0(2)
89.7(7)–90.2(7)
AlꢁH
DFT calculations were performed with the GAUSSIAN 03
program suite [37] using the B3LYP density functional [38–40],
along with the implemented 6-311G(d,p) basis set [41–44]. All
geometry optimizations were carried out in C3v (1:1 trimethyla-
mine adducts of B, Al and Ga) and D3h symmetry (1:2 adducts of
Al and Ga), respectively. The reported structures were found to
AlꢁNꢁC
NꢁAlꢁH
a
[17].
b
This work, study at 296 K (RT) and 173 K (LT), respectively.
[19].
This work.
c
d

16
T.D. Humphries et al. / Journal of Molecular Structure 923 (2009) 13–18
Table 3
all typical for their respective Group 13 element E, with no signif-
icant differences evident between the 1:1 and 1:2 complexes in the
cases of Al and Ga.
Comparison of selected bond lengths (Å) and angles (°) for EH3ꢀNMe3 (E = Al, Ga, B)
studied by various techniques.
EꢁH
1.211(3)
1.210
EꢁN
1.637
1.658
HꢁEꢁN
105.32(16)
105.5
The computational values presented in Table 3 are gas phase
values for the monomeric species. Although AlH3ꢀNMe3 is a dimer
in the solid state [21], it is a monomer in the gas phase [47]. There
is no experimental or computational evidence to suggest that any
of the other species in Table 3 is associated in the gas phase. Both
Ga and B have a stronger preference for 4-coordination than does
Al [3].
BH3ꢀNMe3
AlH3ꢀNMe3
Exp.a
DFTb
Exp.c
DFTb
DFTd
1.560(11)
1.602
1.609
2.063(7)
2.099
2.074
104.3(11)
99.8
100.0
GaH3ꢀNMe3
Exp.e
Exp.f
DFTb
1.511(1)
1.51(6)
1.581
2.134(4)
2.081(4)
2.196
99.3(8)
97(2)
98.5
3.2. Crystal packing of AlH3ꢀ2NMe3
a
Microwave spectroscopy [45].
This work.
Microwave spectroscopy [20].
b
c
d
e
f
Initial X-ray diffraction studies were conducted at 173 K. Analy-
sis of the diffraction pattern indicated space group Pbcm, in contrast
to the earlier room temperature study by Heitsch et al. (Cmca) [17].
However, subsequent measurements on the same crystal at room
temperature revealed space group Cmca, confirming the original re-
port and indicating a reversible phase change between these two
temperatures. A herringbone structure is evident in both phases,
indicating only a slight conformational change within the unit cell
at the phase transition (Figs. 2 and 3). The molecules are orientated
in the same direction in each case, and the unit cells differ only
slightly in their dimensions. Heitsch et al. solved their room temper-
ature structure using a disordered model corresponding to a super-
position of two orientations, 180° apart, of the trimethylamine
group, represented by a hexagon of six half-methyls. The room tem-
perature (296 K) structure presented here confirms that the NMe3 li-
gands are statistically disordered (Fig. 2). In contrast, the low
temperature structure reveals a mutually eclipsed conformation
MP2(full)/6-31G(d) calculations [46].
GED [47].
X-ray diffraction [47].
d(EꢁN) = 2.415 Å, (HꢁEꢁN) = 90.0°], no such complex has been
reported so far. As can be seen in Table 3, the EꢁN bond distance
in the 1:1 adducts increases by around 0.4 Å on progressing from
B to Al, but by less than 0.1 Å between Al and Ga. This reflects
the trend in atomic properties down Group 13, with Al and Ga each
having covalent radii of around 1.22 Å, significantly larger than B at
about 0.84 Å [48]. The experimental and calculated E–N bond dis-
tances for the 1:2 adducts of Al (Table 2) and Ga (op. cit.) are about
10% longer than the corresponding values in their 1:1 counterparts,
reflecting the increase in coordination number from 4 to 5 at the
metal centre. The experimental and theoretical E–H distances are
Fig. 2. Crystal packing of AlH3ꢀ2NMe3 at 296 K (space group Cmca). The methyl groups of the Me3N moieties are disordered and omitted for clarity, as well as the hydrogen
atoms bonded to Al. (a) View along a axis; (b) view along b axis.
Fig. 3. Crystal packing of AlH3ꢀ2NMe3 at 173 K (space group Pbcm). Here the methyl groups of the Me3N moieties exclusively adopt a staggered arrangement with respect to
the central AlH3 moiety. All hydrogen atoms are omitted for clarity. (a) View along a axis; (b) view along c axis.

T.D. Humphries et al. / Journal of Molecular Structure 923 (2009) 13–18
17
Fig. 4. Unit cell axis dimensions and cell volume as a function of temperature between +23 and ꢁ100 °C. Axes are labeled with respect to the structure at 296 K.
for the NMe3 groups (Fig. 3), such that the methyl groups adopt a
staggered arrangement with respect to the central AlH3 moiety
(see Fig. 1 for detail). The eclipsed conformation is preferred at low
temperatures on account of its increased molecular symmetry and
its correspondingly lower conformational entropy. As temperature
increases, the molecules adopt a statically disordered conformation
because of the higher entropy of this arrangement.
The molecular structure of AlH3ꢀ2NMe3 determined from these
experiments is discussed in Section 3.1. Closest intermolecular
contacts in the crystal at 173 K were between AlꢁHꢀꢀꢀHꢁC with a
distance of 2.58(3) Å and CꢁHꢀꢀꢀHꢁC, at 2.60(2) Å. These distances
are significantly longer than the sum of the van der Waals radii and
give no indication for significant intermolecular interactions [49]
or unconventional hydrogen bonding [50] between the molecules
of AlH3ꢀ2NMe3 in the solid state.
between ꢁ50 and ꢁ60 °C. This inflection corresponds to the change
in space group from Cmca to Pbcm: as this change involves intra-
molecular adjustments of the orientation of the NMe3 ligands with
respect to each other, it has a barely discernible effect on the
supramolecular structure and unit cell dimensions. Heitsch et al.
[17] estimated the phase change to occur around ꢁ35 °C; however,
our study places it around 20 °C lower than this value. The facile
thermal cycling of the single crystal used in this study is likely
assisted by the small changes in unit cell dimensions at the phase
transition: a more discontinuous structural change would likely
cause the crystal to fracture or deteriorate. Between ꢁ30 and
ꢁ60 °C a significant amount of disorder is evident in the structure,
which complicated the analysis of the data in terms of an ortho-
rhombic unit cell. The disorder is due to the simultaneous presence
of both high temperature and low temperature molecular confor-
mations of AlH3ꢀ2NMe3 within the crystal lattice over this temper-
ature regime.
3.3. Variable temperature X-ray diffraction study of AlH3ꢀ2NMe3
Upon lowering the temperature, the a and b axes of the unit cell
were observed to contract while the c axis expanded; these two
effects largely compensated for each other resulting in only a mar-
ginal decrease in the unit cell volume. The temperature depen-
dence of these axes and the corresponding cell volume is
depicted in Fig. 4, where an inflection can be seen for a and c
3.4. Low-temperature DSC study of AlH3ꢀ2NMe3
Inordertodeterminemore preciselythetemperatureofthephase
transition and to measure its enthalpy, low-temperature DSC exper-
iments were carried out. At ꢁ54.82 °C, a small endothermic peak was
observed in the trace, with an enthalpy of 107.7 J molꢁ1 (Fig. 5). This
corresponds to the transition from the Pbcm to Cmca structure, and
the temperature measured for this transition corresponds well with
the variable temperature X-ray data collected.
3.5. NMR spectroscopic study of AlH3ꢀ2NMe3 and AlH3ꢀNMe3
Surprisingly, no NMR spectroscopic characterization of
AlH3ꢀ2NMe3 appears to have been published; nor have 13C and
27Al NMR data been reported for AlH3ꢀNMe3. Table 4 compares
the 1H, 13C and 27Al chemical shifts measured by us for AlH3ꢀ2NMe3
Table 4
1H, 13C and 27Al chemical shifts measured for AlH3ꢀNMe3 and AlH3ꢀ2NMe3.
1H
13C
27Al
Al–H
C–H
a
AlH3ꢀNMe3
AlH3ꢀNMe3
3.92
3.70
3.42
1.89
1.87
2.13
47.8
–
47.4
139.8
–
111.4
b
a
AlH3ꢀ2NMe3
All chemical shifts reported in ppm.
Fig. 5. Low-temperature DSC trace for a sample of AlH3ꢀ2NMe3. The endothermic
a
peak at ꢁ54.82 °C (
to Cmca.
D
H = 107.7 J molꢁ1) corresponds to the phase change from Pbcm
This study (toluene-d8).
b
[52] (Benzene d6).

18
T.D. Humphries et al. / Journal of Molecular Structure 923 (2009) 13–18
[5] A.J. Arduengo, H.V.R. Dias, J.C. Calabrese, F. Davidson, J. Am. Chem. Soc. 114
(1992) 9724.
[6] J.L. Atwood, K.W. Butz, M.G. Gardiner, C. Jones, G.A. Koutsantonis, C.L. Raston,
K.D. Robinson, Inorg. Chem. 32 (1993) 3482.
[7] F.M. Elms, M.G. Gardiner, G.A. Koutsantonis, C.L. Raston, J.L. Atwood, K.D.
Robinson, J. Organomet. Chem. 449 (1993) 45.
[8] F.R. Bennett, F.M. Elms, M.G. Gardiner, G.A. Koutsantonis, C.L. Raston, N.K.
Roberts, Organometallics 11 (1992) 1457.
[9] O. Stecher, E. Wiberg, Ber. Dtsch. Chem. Ges. 75 (1942) 2003.
[10] E. Wiberg, H. Graf, M. Schmidt, R. Uson, Z. Naturforsch. B 7 (1952) 578.
[11] E. Wiberg, H. Graf, R. Uson, Z. Anorg. Allg. Chem. 272 (1953) 221.
[12] C.J. Harlan, S.G. Bott, A.R. Barron, J. Chem. Crystallogr. 28 (1998) 649.
[13] F.M. Brower, N.E. Matzek, P.F. Reigler, H.W. Rinn, C.B. Roberts, D.L. Schmidt, J.A.
Snover, K. Terada, J. Am. Chem. Soc. 98 (1976) 2450.
and AlH3ꢀNMe3. As would be expected, the corresponding features
are quite similar. The hydride chemical shift for both the 1:1 and
2:1 complexes appears as a very broad singlet, on account of effi-
cient relaxation by the quadrupolar 27Al nucleus. This effect is
more pronounced in the 2:1 complex; with its low symmetry
TBP geometry at the Al nucleus the FWHM of 486 Hz is signifi-
cantly larger than the corresponding value of 63 Hz for the 1:1
counterpart, which displays a pseudo tetrahedral environment at
Al. In a similar vein, the 27Al resonances of the 2:1 and 1:1 adducts
have FWHM values of 1164 and 724 Hz, respectively. The addi-
tional amine ligand in the 2:1 complex causes the hydride 1H
and 27Al resonances to appear at lower frequencies than in the
1:1 species, presumably on account of the increased electron den-
sity at the Al centre. These NMR spectral features are in general
accord with those reported for other amine–alane complexes, such
as AlH3ꢀNMe2Et reported by Frigo et al. [51].
[14] G.K. Lund, J.M. Hanks, H.E. Johnston, Production of
2007.
a-alane, 2007066839,
[15] G.K. Lund, J.M. Hanks, H.E. Johnston, Method for the production of
2005222445, 2005.
a-alane,
[16] I.B. Gorrell, P.B. Hitchcock, J.D. Smith, Chem. Commun. (1993) 189.
[17] C.W. Heitsch, R.W. Parry, C.E. Nordman, Inorg. Chem. 2 (1963) 508.
[18] G.W. Fraser, B.P. Straughan, N.N. Greenwood, J. Chem. Soc. (1963) 3742.
[19] V.S. Mastryukov, A.V. Golubinskii, L.V. Vilkov, J. Struct. Chem. 20 (1979)
788.
4. Conclusions
[20] H.E. Warner, Y. Wang, C. Ward, C.W. Gillies, L. Interrante, J. Phys. Chem. 98
(1994) 12215.
[21] F.M. Elms, C. Jones, C.L. Raston, K.D. Robinson, J. Am. Chem. Soc. 113 (1991)
8183.
[22] P.C. Andrews, M.G. Gardiner, C.L. Raston, V.A. Tolhurst, Inorg. Chim. Acta 259
(1997) 249.
[23] J.K. Ruff, M.F. Hawthorne, J. Am. Chem. Soc. 82 (1960) 2141.
[24] G.W. Schaeffer, E.R. Anderson, J. Am. Chem. Soc. 71 (1949) 2143.
[25] C.W. Heitsch, Nature 195 (1962) 995.
[26] C.M.B. Marsh, H.F. Schaefer, J. Phys. Chem. 99 (1995) 195.
[27] D.B. Beach, S.E. Blum, F.K. Legoues, J. Vac. Sci. Technol. A 7 (1989) 3117.
[28] C. Popov, B. Ivanov, V. Shanov, J. Appl. Phys. 75 (1994) 3687.
[29] T.H. Baum, C.E. Larson, High Temp. Sci. 27 (1989) 237.
[30] H.C. Brown, N.M. Yoon, J. Am. Chem. Soc. 88 (1966) 1464.
[31] SAINT 7.23A, Bruker AXS Inc., Madison, WI, USA, 2006.
[32] G.M. Sheldrick, SADABS, Bruker AXS Inc., Madison, WI, USA, 2004.
[33] G.M. Sheldrick, SHELXS, University of Göttingen, Germany, 2001.
[34] G.M. Sheldrick, SHELXTL 6.14, Bruker AXS Inc., Madison, WI, USA, 2000.
[35] L.J. Farrugia, J. Appl. Crystallogr. 30 (1997) 565.
This study has improved on and extended the structural deter-
mination of AlH3ꢀ2NMe3 published by Heitsch et al. in 1963. We
have confirmed and improved the Cmca room temperature struc-
ture in this earlier study. At ambient temperature, the orientation
of the methyl groups in the Me3N–Al–NMe3 moiety is statistically
disordered. As the temperature of the crystal is decreased, there is
a phase change to Pbcm at around ꢁ55 °C, which we have charac-
terized independently by low-temperature DSC experiments. In
the low temperature Pbcm phase, the orientation of the amine
methyl groups in the Me3N–Al–NMe3 moiety is exclusively mutu-
ally eclipsed, which allows each NMe3 group to adopt a staggered
arrangement with the planar central AlH3 unit. The NMR spectral
characteristics of AlH3ꢀ2NMe3 and its 1:1 counterpart are reported
and are in accord with those of related amine–alane complexes.
[36] A.L. Spek, J. Appl. Crystallogr. 36 (2003) 7.
[37] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
J.A. Montgomery, T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J.
Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson,
H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.
Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian,
J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O.
Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K.
Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S.
Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K.
Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J.
Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L.
Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M.
Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A.
Pople, Gaussian 03, Revision D.02, Wallingford, CT, 2004.
Acknowledgements
The authors would like to thank Keelie Munroe for assistance
with the DFT calculations presented in Tables 2 and 3, Michael
Johnson for the DSC measurements reported in Fig. 5 and Dr. Larry
Calhoun for NMR spectroscopy assistance. We are grateful to
NSERC and CFI for support of this work, and we thank the Atlantic
Computational Excellence Network (ACEnet) for providing access
to their computing facilities.
[38] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
[39] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.
[40] B. Miehlich, A. Savin, H. Stoll, H. Preuss, Chem. Phys. Lett. 157 (1989) 200.
[41] A.D. McLean, G.S. Chandler, J. Chem. Phys. 72 (1980) 5639.
[42] R. Krishnan, J.S. Binkley, R. Seeger, J.A. Pople, J. Chem. Phys. 72 (1980) 650.
[43] R.C. Binning Jr., L.A. Curtiss, J. Comput. Chem. 11 (1990) 1206.
[44] L.A. Curtiss, M.P. McGrath, J.P. Blaudeau, N.E. Davis, R.C. Binning Jr., L. Radorn, J.
Chem. Phys. 103 (1995) 6104.
[45] J.R. Durig, Y.S. Li, J.D. Odom, J. Mol. Struct. 16 (1973) 443.
[46] I. Abderrahim Boutalib, N.-G. Abdellah Jarid, F. Toms, J. Phys. Chem. A 105
(2001) 6526.
Appendix A. Supplementary data
Cartesian coordinates and geometrical parameters at the B3LYP/
6-311G(d,p) level of theory for BH3ꢀNMe3, AlH3ꢀNMe3, AlH3ꢀ2NMe3,
GaH3ꢀNMe3 and GaH3ꢀ2NMe3. Supplementary data associated with
this article can be found, in the online version, at doi:10.1016/
j.molstruc.2008.12.022.
[47] P.T. Brain, H.E. Brown, A.J. Downs, T.M. Greene, E. Johnsen, S. Parsons, D.W.H.
Rankin, B.A. Smart, C.Y. Tang, J. Chem. Soc. Dalton Trans. (1998) 3685.
[48] B. Cordero, V. Gomez, A.E. Platero-Prats, M. Reves, J. Echeverria, E. Cremades, F.
Barragan, S. Alvarez, J. Chem. Soc. Dalton Trans. (2008) 2832.
[49] A. Bondi, J. Phys. Chem. 68 (1964) 441.
[50] T. Steiner, G.R. Desiraju, Chem. Commun. (1998) 891.
[51] D.M. Frigo, G.J.M. Vaneijden, P.J. Reuvers, C.J. Smit, Chem. Mater. 6 (1994) 190.
[52] D.G. Hendrick, C.W. Heitsch, J. Phys. Chem. 71 (1967) 2683.
References
[1] S. Cucinella, A. Mazzel, W. Marconi, Inorg. Chim. Acta 4 (1970) 51.
[2] C. Jones, G.A. Koutsantonis, C.L. Raston, Polyhedron 12 (1993) 1829.
[3] M.G. Gardiner, C.L. Raston, Coord. Chem. Rev. 166 (1997) 1.
[4] M.D. Francis, D.E. Hibbs, M.B. Hursthouse, C. Jones, N.A. Smithies, J. Chem. Soc.
Dalton Trans. (1998) 3249.