Dalton
Transactions
PAPER
Cite this: Dalton Trans., 2014, 43,
4451
Received 23rd September 2013,
Accepted 26th November 2013
DOI: 10.1039/c3dt52637b
www.rsc.org/dalton
Synthesis, characterization and reactivity of an
imidazolin-2-iminato aluminium dihydride†
Daniel Franz, Elisabeth Irran and Shigeyoshi Inoue*
The reaction of bis(2,6-diisopropylphenyl)imidazolin-2-imine (LH, 1) with Me
3
N·AlH
3
furnishes {μ-LAlH
2
}
2
(2).
The marked tendency of 2to release its hydride substituents is ascribed to the strong electron-donor
character of the imidazolin-2-iminato ligand. This is supported by its reactivity study and DFT calculations.
In fact, compound 2was further converted with Me
3
SiOTf, Me
2
S·BH
3
,Me
2
S·BBr
3
, and BX
3
(with X = Cl, Br,
and I) into {μ-LAl(H)OTf}
2
(3), {μ-LAl(BH
4
)
2
}
2
(4), and {μ-LAlX
2
}
2
(5, X = Br; 6, X = Cl; 7, X = I), respectively.
For all new aluminium complexes the formulation as dimers was evidenced by high resolution mass
spectrometry, as well as single-crystal X-ray diffraction analysis. A prominent structural motif of these
compounds is the square-planar four-membered Al
2
N
2
ring with two bridging bulky imidazolin-2-imino
moieties.
Introduction
Aluminium hydrides –versatile reagents with rich chemistry
For many years, chemical synthesis has been gaining immense
benefit from the distinct reactivity of the aluminium–hydrogen
bond. The strong need for highly selective transformations,
increased focus on safety considerations, and efficiency in
handling and storage are only some reasons why we find the
chemically rogue parent aluminium hydride tamed into more
suitable forms today. A variety of hydridoalanes has been tai-
lored and very often reactivity adjustment is realized by steric
congestion at the aluminium centre and by attaching strongly
electron-donating substituents to the aluminium atom. Fur-
thermore, hydridoalanes are used in a diverse range of appli-
cations. For instance, the hydroalumination of carbonyl
1,2
and
alkyne
3–7
functionalities is a common application for this
class of compounds in organic synthesis. Though the inter-
mediate aluminium species in these reactions are often
elusive, the use of aluminium hydrides such as I
8
(Fig. 1) that
bear highly sophisticated ligands grants access to all types of
isolatable and well-defined model complexes. Compound Ican
be categorized as an aluminium dihydride and is related to
the parent aluminium trihydride by replacement of one
hydride for an anionic ligand. Thus, an ancillary ligand may
be introduced, and yet, two reactive functional groups at the
metal centre are preserved for the purpose of follow-up chem-
istry. The β-diketimino group, in particular, has been a key
ligand to a rich chemistry of respective aluminium
dihydrides.
9–12
A prominent example is II (Fig. 1), which can
Fig. 1 Selected aluminium dihydride compounds. The pincer complex
I, the β-diketiminato complex II, and the trimeric cyclopentadienide III.
The guanidinate IV, the phosphinimide V, as well as the troponimide VI
(Dipp = 2,6-diisopropylphenyl).
†Electronic supplementary information (ESI) available: Procedures for the con-
version of 2with BX
3
(X = Cl, Br) and respective NMR spectra, NMR spectra of 6,
synthesis of 8, details of quantum mechanical calculations. CCDC
962234–962240 for 2–8. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3dt52637b
Institut für Chemie, Technische Universität Berlin, Straße des 17. Juni 135, Sekr. C2,
10623 Berlin, Germany. E-mail: shigeyoshi.inoue@tu-berlin.de
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be used for the activation of non-polar element–element
bonds as demonstrated by its conversion with elemental
sulphur or selenium.
9,10
In a different field of applied science
one finds chemical vapour deposition technology exploiting
the thermodynamic properties of molecular aluminium
hydrides for the purpose of creating composite
materials.
1,13–15
Aluminium hydrides are considered as fuel storage-
materials with reasonable prospects in a hydrogen-based alter-
nate energy-supply concept.
16,17
Moreover, the application of
aluminium hydrides in the synthesis of low-valent aluminium
compounds has been reported and III
18
(Fig. 1) is a very recent
example among others
19
of an intriguing nature. Recently,
scarce σ-alane complexes of transition metals (e.g. Cr, Mo, W,
Mn) implementing ligands such as IV
19
(Fig. 1) have been
described.
20,21
The particular character of the aluminium
hydride moiety in transition-metal complexes will most cer-
tainly be subject to ongoing profound study.
For advocating the diversity of ligand systems employed in
the field we would select the phosphinimide V
22
(Fig. 1) as a
rare example for the application of this particular electron
donating group in aluminium hydride chemistry. A number of
non-hydride aluminium complexes with this ligand system
have been reported, as well.
22–24
Furthermore, a troponiminato
ligand can be used to stabilize the monomeric dihydride
species in VI (Fig. 1).
25
The imidazolin-2-iminato ligand –a potent electron donor
The development of a new ligand to control the properties and
the reactivity of an aluminium hydride is one indispensable
part of this chemistry and hence an important research aim.
An imidazolin-2-imino group as an ancillary ligand can be con-
sidered a suitable bulky and electron donating group for a
novel aluminium dihydride. The imidazolin-2-iminato ligands
act as a 2σ- and either a 2π-ora4π-electron donor and, in con-
sequence, some multiple-bond character in the interaction
between the nitrogen atom and the metal centre may
result.
26–31
Fine-tuning of this ligand system with respect to its
steric and electronic properties can be conveniently accom-
plished by altering the substituents to the ring.
28,30
Pioneering
work related to main group metal complexes of the imidazo-
lin-2-iminato ligand was done by Kuhn and coworkers (VII,
Fig. 2).
32,33
Contemporary research on transition metal- and
rare earth metal complexes of this imino group is mainly
carried out by Tamm and coworkers.
28–30,34
For example, strik-
ing activity as a catalyst for propylene polymerization is
ascribed to the titanium complex VIII
30
(Fig. 2) which is rep-
resentative for the field. Most remarkably, the phosphinonitrene
IX (Fig. 2) was described by Bertrand and coworkers, thus
demonstrating the high potential of this ligand system in
main group element chemistry.
35–37
Recently, we have demon-
strated the application of the bis(2,6-diisopropylphenyl)imida-
zolin-2-imino group in the synthesis of the unprecedented
silylene X(Fig. 2).
31
Herein we report the synthesis and characterization of
the hitherto unknown bis(2,6-diisopropylphenyl)imidazolin-2-
imino aluminium dihydride {μ-LAlH
2
}
2
2and its reactivity as a
hydride transfer reagent.
Results and discussion
Introduction of the bulky imidazolin-2-imino group to the
aluminium centre
In order to introduce the bis(2,6-diisopropylphenyl)imidazo-
lin-2-iminato ligand (L
−
) to a hydridoalane moiety we found it
reasonable to adopt a procedure
22
reported by Stephan and co-
workers for the synthesis of a related phosphinimide. The stoi-
chiometric reaction of LH (1)
38
with Me
3
N·AlH
3
in toluene
affords the dimeric aluminium dihydride {μ-LAlH
2
}
2
(2)in
sufficient yield (66%) as confirmed by multinuclear NMR spec-
troscopy, high resolution mass spectrometry, single-crystal
X-ray diffraction data, and elemental analysis (Scheme 1).
In the infrared spectrum the bands at 1830 cm
−1
and
1798 cm
−1
are assigned to the Al–H bonds in 2, which is in
accordance with the IR absorptions reported for the related
Fig. 2 Imidazolin-2-imino compounds with main group elements (VII,
IX,X) and the titanium complex VIII (Dipp = 2,6-diisopropylphenyl).
Scheme 1 Formation of the imidazolin-2-imino aluminium hydride 2
via conversion of the imine 1and synthesis of its triflate derivative 3.
(i) Toluene, (1) −78 °C, 2 h, (2) rt, 24 h. (ii) Toluene, (1) 0 °C, 30 min, (2) rt,
24 h. Dipp = 2,6-diisopropylphenyl, Tf = SO
2
CF
3
.
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aluminium dihydride II.
10
The resonance pattern observed for
the imidazolin-2-iminato moiety in the
1
H and
13
C{
1
H} NMR
spectra of 2resembles the one in related silicon complexes
such as LSi(C
5
Me
5
)Br
2
.
31
In the proton NMR spectrum (C
6
D
6
),
a broad resonance at 2.60 ppm with 4 H intensity is observed
that can be assigned to the hydrogen atoms of the AlH
2
moi-
eties and is more shielded than those observed for amidino-,
β-diketimino-, or 1-azaallyl aluminium dihydrides (4.60 ppm–
4.87 ppm)
39–41
and the starting material Me
3
N·AlH
3
(4.08 ppm), as well as [Me
2
NAlH
2
]
3
(3.8 ppm)
42
but is shifted
more downfield than that of aluminium dihydride VI
(1.08 ppm, Fig. 1).
25
After heating a solution of 2in toluene to
reflux temperature over a period of 12 h the NMR spectro-
scopic control did not reveal any signs of decomposition
which accounts for the thermal stability of this aluminium
dihydride. Compound 2crystallizes from hexane–toluene in
the monoclinic space group P2
1
/cand the unit cell contains
two crystallographically independent centrosymmetric mole-
cules. In one of these molecules, the two equivalent alu-
minium atoms are disordered over two sites. Since the
structural parameters of the two molecules are almost identi-
cal, we discuss only the one for which no disordered metal
atom is observed. The single-crystal X-ray data of 2reveals the
four-membered Al
2
N
2
ring as the prominent structural motif
of the dimer (Fig. 3). The Al–N bond distances of 1.912(1) Å
and 1.915(1) Å in 2are slightly longer than those in compound
V(Al–N = 1.887(3)–1.905(3) Å).
22
The N(1)–Al(1)–N(1A) and
Al(1)–N(1)–Al(1A) angles in 2are 86.2(1)° and 93.8(1)°, respect-
ively. The dihedral angle N(1A)–Al(1)–N(1)–Al(1A) of 0.0° con-
firms that these four atoms are all located in one plane. The
Al(1)⋯Al(1A) distance in 2amounts to 2.795(1) Å, which is sig-
nificantly longer than the metal–metal bond lengths in charac-
teristic donor-stabilized dialanes with four-coordinated
aluminium centres.
19,43
The N(1)–C(1) bond length of 1.289(2)
Åin2only differs slightly from the analogous values in silicon
compounds stabilized by imidazolin-2-iminato ligands.
31,38
Furthermore, it is similar to that of the guanidinate
{μ-(Me
2
N)
2
CNAl(NMe
2
)
2
}
2
.
44
The 5-membered imidazoline ring
plane is tilted versus the central Al
2
N
2
moiety by 9°. The sum
of the bond angles around the nitrogen atom attached to the
aluminium centre amounts to 360°, which confirms the
trigonal-planar geometry around the nitrogen atom of the
imino group.
DFT calculations of related aluminium dihydrides
For obvious reasons, the reactivity of an aluminium dihydride
will largely depend on its hydride-donor strength, and one
would expect that the tendency for hydride release depends on
the electron density at the hydrogen atoms and the nature of
the Al–H bonds. Hence, we performed quantum-mechanical
calculations on a sterically reduced model compound of 2(2′,
Fig. 4) and related model compounds with a four-coordinated
Fig. 3 Molecular structure of 2in the solid state. The unit cell contains
two independent molecules and only one is shown; hydrogen atoms
(except on aluminium) and isopropyl groups have been omitted for
clarity; displacement ellipsoids are drawn at the 50% probability level.
Selected bond lengths (Å), atom⋯atom distance (Å), bond angles (°),
and dihedral angle (°): Al(1)–N(1) 1.915(1), Al(1)–N(1A) 1.912(1), Al(1)–H(1)
1.48(2), Al(1)–H(2) 1.55(2), N(1)–C(1) 1.289(2), N(2)–C(1) 1.389(2),
Al(1)⋯Al(1A) 2.795(1); H(1)–Al(1)–H(2) 118(1), N(1)–Al(1)–N(1A) 86.2(1),
Al(1)–N(1)–Al(1A) 93.8(1), Al(1)–N(1)–C(1) 131.7(1), Al(1A)–N(1)–C(1)
134.0(1); N(1A)–Al(1)–N(1)–Al(1A) 0.0. Symmetry transformation used to
generate equivalent atoms: A: −x+1,−y+2,−z+1.
Fig. 4 Calculated Wiberg bond indices of the Al–H bonds and NBO
charges at the hydride atoms for the simplified model compounds 2’,II’,
VI’, and V’.
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aluminium centre of II,
10
V,
22
and VI
25
(i.e. II′,V′,andVI′;Fig.4)
at the B3LYP/6-31G(d) level of theory (cf. the ESI†for compu-
tational details). The optimized structures of the model com-
pounds matched the experimental data. According to the NBO
charges, each H atom at the aluminium centre in 2′bears a
negative net charge (−0.396), which is more negative than
those in II′(−0.385) and VI′(−0.382), but more positive than
that in V′(−0.408). Interestingly, the calculated WBI values of
the Al–H bonds in 2′(0.786) are smaller than those in II′
(0.810), V′(0.788), and VI′(0.809) and the lower bond order
implies a higher polarization of the Al–H group. Consequently,
reactivity enhancement towards highly polarized electrophiles
may coincide. These results suggest that 2is a strong hydride
donor reagent.
Hydride abstraction from {μ-LAlH
2
}
2
(2)
Due to the strong electron-donor character of the iminato
ligand L
−
and its steric congestion we had reason to expect a
structural motif with discrete LAlH
2
moieties in the solid state
instead of the formation of dimers as evidenced by the single
crystal X-ray data of {μ-LAlH
2
}
2
(2) and its high resolution mass
spectrum (vide supra). Stimulated by a report of Stephan and
coworkers about the reaction of Vwith trimethylsilyltriflate to
{μ-(
t
Bu)
3
PNAl(H)OTf}
2
,
22
we treated 2with three equivalents of
Me
3
SiOTf (Tf = SO
2
CF
3
), which resulted in replacement of one
hydride substituent per aluminium centre by a triflate group
(Scheme 1). Compound 3{μ-LAl(H)OTf}
2
was obtained in ana-
lytically pure form after repeated recrystallization from THF–
hexane. It was characterized by multinuclear NMR spec-
troscopy, high resolution mass spectrometry, single-crystal
X-ray study, and elemental analysis. Though the symmetry at the
aluminium centre in 3is lowered in comparison to its precur-
sor, a resonance pattern similar to that of 2is observed in the
1
H and
13
C{
1
H} NMR spectra of 3in CD
3
CN. Similar to 2, the
molecular structure of 3in the solid state reveals a dimer with
a central Al
2
N
2
core (Fig. 5). Two triflate groups are found
assuming positions on the same side of the Al
2
N
2
ring, thus
their configuration is pseudo-cis which is reminiscent of the
phosphinimide congener {μ-(
t
Bu)
3
PNAl(H)OTf}
2
.
22
As marked
by the dihedral angle N(4)–Al(1)–Al(2)–N(1) = 169.6(2)° the
reduced symmetry around the Al
2
N
2
ring inflicts slight devi-
ation from its square-planar geometry that is observed for 2
(Fig. 3) in very high approximation. The Al–N bond lengths of
3are found in a range from 1.849(3) Å to 1.872(3) Å and are
notably shorter than those in 2. This indicates that the triflate
substituents in 3render the aluminium centres more electro-
philic, thus strengthening the interaction with the nitrogen
atoms of the adjacent ligands.
While the replacement of one hydride at the aluminium
atom for OTf
−
proceeded readily, we were not able to isolate a
related species with a ditriflated metal centre even when treat-
ing {μ-LAlH
2
}
2
(2) with a large excess of Me
3
SiOTf at an elev-
ated temperature.
As an alternative group of electrophiles that might elucidate
the hydride-donating properties of 2, we conceived boron-
centred Lewis acids. The turn from silicon to boron is
reasonable because the diagonal relationship between these
elements is one of the many verified reactivity patterns emer-
ging from the periodic table.
45
Recently, Harder and Spiel-
mann reported the synthesis of the β-diketiminato-stabilized
(Dipp)nacnacAl(BH
4
)
2
by reaction of II with DippNH
2
·BH
3
.
46
Inspired by these findings, we set out to react 2with four
equivalents of borane dimethylsulphide complex (Scheme 2).
As the outcome of the presumed hydride transfer from the
aluminium centre to the boron atom {μ-LAl(BH
4
)
2
}
2
(4)was
formed as elucidated by multinuclear NMR spectroscopy,
single-crystal X-ray structure determination, elemental analy-
sis, as well as high resolution mass spectrometric analysis.
An early synthesis of the parent Al(BH
4
)
3
goes back to
Brown and coworkers.
47
Aluminium borohydride is described
as a hazardous material, particularly because the vapour of the
liquid substance may spontaneously detonate upon contact
with air and moisture. In addition, it slowly decomposes at
room temperature with the release of dihydrogen.
47
In recent
years, Al(BH
4
)
3
has gained prominent attention from the field
of materials science
16,48
and the transformation into more
suitable hydrogen-storage materials as ammine aluminium
borohydrides
49–51
and aluminoboranes
52
is explored. Given
this background, it is desirable to have access to a room
temperature-stable and well-defined model complex such as
{μ-LAl(BH
4
)
2
}
2
(4) in which the relevant aluminium centres and
borohydride moieties are unified.
Fig. 5 Molecular structure of 3(THF)
3
in the solid state. Hydrogen
atoms (except on aluminium), isopropyl groups, and THF molecules
have been omitted for clarity; only the higher occupied site is shown for
atoms disordered over two sites; displacement ellipsoids are drawn at
the 50% probability level. Selected bond lengths (Å), atom⋯atom
distance (Å), bond angles (°), and dihedral angle (°): Al(1)–N(4) 1.849(3),
Al(2)–N(4) 1.872(3), N(1)–C(1) 1.327(5), N(2)–C(1) 1.376(5), Al(1)⋯Al(2)
2.666(2); N(1)–Al(2)–N(4) 87.7(2), Al(1)–N(1)–Al(2) 91.6(2); N(4)–Al(1)–
Al(2)–N(1) 169.6(2).
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In the infrared spectrum of 4we assign absorptions at
2492 cm
−1
and 2431 cm
−1
to be produced by the B–H bonds in
4. The symmetric and asymmetric BH stretches of Al(BH
4
)
3
have been reported at a comparable wavenumber.
53
In the
1
H
and
13
C{
1
H} NMR spectra of 4(THF-d
8
), a deviation from the
expected signal pattern for L (cf. the NMR data of 2) is found
in that the resonances of the Dipp moieties split into two
signal sets of equal intensity. The 16 hydrogen atoms of the
BH
4
groups give rise to a broad resonance at −0.33 ppm in the
1
H NMR spectrum. The chemical shift of the boron nuclei
amounts to −37 ppm (h
1/2
= 270 Hz) as observed in the
11
B{
1
H}
NMR spectrum (THF-d
8
). In the proton-coupled
11
B NMR spec-
trum, only a broad singlet at −37 ppm (h
1/2
= 380 Hz) is
observed (cf. (Dipp)nacnacAl(BH
4
)
2
:δ(
11
B) = −36.6 ppm,
quintet, J= 85.3 Hz, C
6
D
6
).
46
Obviously, the Jcoupling between
boron and hydrogen in 4is not resolved. However, the signal
broadening of 110 Hz in the proton-coupled
11
B NMR spec-
trum suggests the presence of borohydride moieties. After
storing a flame-sealed NMR tube with a sample of 4in THF-d
8
at room temperature for two weeks no signs of decomposition
were observed. However, upon heating to 60 °C overnight the
NMR spectroscopic analysis revealed a new species giving rise
to an additional imino ligand signal set in the
1
H NMR spec-
trum, as well as a second singlet in the
11
B{
1
H} NMR spectrum
at −21.5 ppm. After the sample tube had been exposed to a
temperature of 90 °C for 2 d compound 4had been consumed
entirely and the proton NMR spectrum evidenced the for-
mation of several unidentified products, as well as dihydrogen.
Crystallization of 4from toluene–THF–hexane yielded single
crystals suitable for X-ray diffraction analysis which contained
1.5 equivalents of toluene in the lattice. The X-ray study con-
firms the dimeric formulation of compound 4(Fig. 6) and the
structural parameters of the Al
2
N
2
ring and of the attached
iminato ligands resemble the geometry of the corresponding
groups in 2. The Al–N bond lengths in 4lie in a range from
1.873(2) Å to 1.889(2) Å, which marks a slight decrease in
comparison to the respective distances in 2(Al–N = 1.912(1) Å
and 1.915(1) Å; vide supra). The shorter Al–N bond lengths
coincide with marginally more acute Al–N–Al′bond angles
(92.7(1)° and 93.2(1)°) in 4as compared to 93.8(1)° in 2.
Accordingly, the Al(1)⋯Al(2) distance is shortened to 2.728(1)
Å with respect to 2.795(1) Å for the corresponding distance in
2. Concomitantly, the decrease in the Al–N distances leads to
an increase in the exocyclic imino groups’bond lengths as
exemplified by N(1)–C(1) = 1.319(2) Å in 4compared to N(1)–
C(1) = 1.289(2) Å in 2. When taking a look at how the Al–Nbond
lengths in II (Fig. 1) are affected by the transformation into
Dipp(nacnac)Al(BH
4
)
2
one finds the Al–N distance of 1.899(1) Å
in II
54
essentially unaltered upon conversion into its alu-
minium borohydride congener (Al–N = 1.898(3) Å).
46
In this
case, the bidentate β-diketiminato ligand appears to be more
rigid, allowing for less structural flexibility in its resulting alu-
minium complexes than the non-chelate-fashioned imino
group in 2and 4, respectively. The plane of the Al
2
N
2
ring in 4
intersects the planes of its adjacent imidazoline rings with di-
hedral angles of 34° and 35°, respectively. This is notably more
obscure than that in 2(9°) and presumably due to the steric
hindrance of the BH
4
moieties in comparison to the hydrides
in 2. With values between 1.07(3) Å and 1.19(2) Å the similarity
of all B–H bond lengths as a strong indicator for the formation
of BH
4
groups underlines the hydride transfer from the alu-
minium to the boron centre. Each aluminium atom can be
described as coordinated by two BH
4
−
groups in an η
2
fashion,
Scheme 2 Conversion of the aluminium dihydride 2with the dimethyl-
sulphide complexes of borane (top) or boron tribromide (bottom) yield-
ing 4and 5, respectively. (i) Toluene, (1) −78 °C, 2 h, (2) rt, 24 h.
(ii) Toluene, (1) 0 °C, 20 min, (2) 80 °C, 3 d. Dipp = 2,6-diisopropylphenyl.
Fig. 6 Molecular structure of 4(toluene)
1.5
in the solid state. Hydrogen
atoms (except on boron), isopropyl groups, and toluene molecules have
been omitted for clarity; displacement ellipsoids are drawn at the 50%
probability level. Selected bond lengths (Å), atom⋯atom distances (Å),
and bond angles (°): Al(1)–H(2A) 1.69(2), Al(1)–H(2B) 1.82(2), Al(2)–N(1)
1.873(2), Al(2)–N(4) 1.889(2), N(1)–C(1) 1.319(2), Al(1)⋯Al(2) 2.728(1),
Al(1)⋯B(1) 2.229(3), Al(1)⋯B(2) 2.220(3); N(1)–Al(1)–N(4) 87.0(1),
N(1)–Al(2)–N(4) 87.1(1), Al(1)–N(1)–Al(2) 93.2(1), Al(1)–N(4)–Al(2) 92.7(1),
Al(1)–N(1)–C(1) 134.0(1).
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which is marked by the eight Al–H distances (1.69(2)–1.82(2) Å).
Considering that the coordination number of each aluminium
atom is increased, it is a notable observation that the Al–Ndis-
tances are only slightly affected (vide supra). The smallest Al⋯B
distance in 4amounts to 2.220(3) Å and the largest is deter-
mined to be 2.229(3) Å. This is in good agreement with the
respective values in (Dipp)nacnacAl(BH
4
)
2
for which an η
2
coordination mode of the BH
4
moieties to the aluminium atom
hasalsobeendescribed.
46
Accordingly, this type of bonding is
also found for {μ-Me
3
SiOAl(BH
4
)
2
}
2
(Al⋯B = 2.156(5) Å, 2.143(4)
Å),
55
the parent Al(BH
4
)
3
(Al⋯B = 2.143(3) Å),
56
and related alu-
minium borohydrides.
57,58
Hydride–halide exchange between aluminium and boron
The hydride transfer from {μ-LAlH
2
}
2
(2)toMe
2
S·BH
3
yielding
{μ-LAl(BH
4
)
2
}
2
(4) prompted us to investigate the reactivity of 2
towards other boron-centred electrophiles. A hydride–halide
exchange reaction between a Lewis base complex of a boron
trihalide and 2might equilibrate a haloalane product due to
the comparably high stability of many hydridoborane
adducts.
59,60
One would expect the degree of hydrogenation at
the boron atom to reflect the halogenation of the aluminium
centre which would allow monitoring of the reaction progress
by proton-coupled
11
B NMR spectroscopy. We chose the boron
tribromide dimethylsulphide complex
61,62
as a suitable
bromide-transfer reagent because it can be conveniently stored
and handled as a solid under an inert atmosphere, yet, it
readily undergoes bromide–hydride exchange reactions in
solution.
62
First, we examined the conversion of 2with two
equivalents of Me
2
S·BBr
3
in C
6
D
6
by preparing an experiment
in the NMR tube. It turned out that an initial reaction took
place at room temperature as marked in the
11
B NMR spec-
trum by a doublet at −7.8 ppm (J= 160 Hz, assigned to
Me
2
S·BHBr
2
)
62
and a triplet of weaker intensity at −10.8 ppm
(J= 130 Hz, assigned to Me
2
S·BH
2
Br)
62
that superimposed with
the resonance caused by the residual starting material
(Me
2
S·BBr
3
:δ(
11
B) was observed at −10.7 ppm in C
6
D
6
). We
stored the flame-sealed NMR tube at elevated temperature
until the
1
H and
11
B NMR spectra indicated the conversion to
be complete. Accordingly, the reaction on a preparative scale
in toluene granted the analytically pure aluminium dibromide
{μ-LAlBr
2
}
2
(5) in 57% yield (Scheme 2). The formulation as a
dimeric compound is evidenced by the high resolution mass
spectrometric analysis, as well as single-crystal X-ray study
(Fig. 7).
The
1
H and
13
C{
1
H} NMR spectra (C
6
D
6
)of5differ only
insignificantly with respect to 2, taking aside the expected dis-
appearance of the broad resonance at 2.60 ppm caused by the
hydrogen atoms of the AlH
2
moieties in 2. The single-crystal
diffraction analysis of 5shows that, as in 4, 1.5 equivalents of
toluene are incorporated into the crystal lattice. The structural
parameters of the planar Al
2
N
2
ring and the adjacent imidazo-
line rings in 5are very similar to 4. The Al–N bond lengths in 5
lie in a range from 1.858(3) Å to 1.871(3) Å and are slightly
shorter in comparison to those in 4. Though rather marginal
with respect to 4the difference from the analogous bond
lengths in 2(Al–N = 1.912(1) Å, 1.915(1) Å) is now marked and
one would assume that it correlates to the stronger electron-
withdrawing nature of the bromide substituents in 5as com-
pared to the hydrides in 2. In consequence, the electrophilicity
of the aluminium centres is increased and the interaction with
the nitrogen atoms of the imino ligands is strengthened. In
accordance with the observed trend in the bond lengths com-
paring 2,4, and 5, the N–Al–N′bond angles in 5have yet again
slightly sharpened to 91.9(1)° and 92.3(1)°, respectively. The
interesting consequence of the aforementioned marginal
changes in the geometry of the Al
2
N
2
core is revealed if one
notes the Al(1)⋯Al(2) distance in 5to amount to 2.687(1) Å.
This value is close to the range of metal–metal bond lengths in
prominent dialanes with four-coordinated metal centres.
19,43
However, we would not suggest that a bonding interaction
between the Al atoms in 5exists.
Motivated by the prospective results of the conversion of 2
with Me
2
S·BBr
3
we decided to explore the general applicability
of the hydride–halide exchange concept. The conversion of 2
with two equivalents of boron trichloride yielded the alu-
minium dichloride {μ-LAlCl
2
}
2
(6) (Scheme 3, cf. the ESI†for
experimental details). However, the
1
H NMR spectrum of the
reaction mixture (CDCl
3
) revealed signal sets induced by yet
unassigned additional species of L, presumably intermediate
forms in the path of the reaction from 2to 6. This came as a
surprise as we would have ascribed high reactivity to the
uncomplexed form of BCl
3
. However, treating 2with another
two equivalents of this haloborane significantly improved the
yield of 6. Alternatively, compound 6was synthesized by
Fig. 7 Molecular structure of 5(toluene)
1.5
in the solid state. Hydrogen
atoms, isopropyl groups, and toluene molecules have been omitted for
clarity; displacement ellipsoids are drawn at the 50% probability level.
Selected bond lengths (Å), atom⋯atom distance (Å), and bond
angles (°): Br(2)–Al(1) 2.282(1), Br(4)–Al(2) 2.298(1), Al(1)–N(4) 1.871(3),
Al(2)–N(1) 1.858(3), N(1)–C(1) 1.318(4), Al(1)⋯Al(2) 2.687(1); Br(1)–Al(1)–Br(2)
110.29(4), N(1)–Al(1)–N(4) 87.7(1), N(1)–Al(2)–N(4) 88.1(1), Al(1)–N(1)–Al(2)
92.3(1), Al(1)–N(4)–Al(2) 91.9(1), Al(1)–N(1)–C(1) 134.0(2).
Paper Dalton Transactions
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reaction of bis(2,6-diisopropylphenyl)imidazolin-2-imino lithium
34
{LLi}
2
(8) with aluminium trichloride (cf. the ESI for details on
our synthesis of 8and its molecular structure in the solid
state, Fig. S3†). The formation of 6was evidenced by high
resolution mass spectrometry, NMR spectroscopy, as well as
X-ray crystallographic analysis. The essential structural charac-
teristics of 6strongly resemble the heavier halide congener 5
(cf. the ESI for the molecular structure of 6in the solid state,
Fig. S1†). Furthermore, parallels exist to the bulky phosphini-
mide {μ-(
t
Bu)
3
PNAlCl
2
}
2
reported by Stephan and coworkers.
23
Interestingly, 2,6-dimethylimidazolin-2-imino aluminium
dichloride VII, a less sterically congested derivative of 6, forms
a six-membered Al
3
N
3
ring in the solid state in which most Al-
centred bond angles nearly fit the ideal value of a tetrahedron
(109.5°).
32
Thus, the square-planar geometry of the Al
2
N
2
core
in 6must result from the bulkiness of L in that it imposes
smaller Al–N–Al′angles than the iminato ligand in VII.
On the other hand, when we reacted 2with two equivalents
of BBr
3
, quantitative conversion to 5was observed (NMR spec-
troscopic control, Scheme 3). Obviously, the reactivity of the
heavier haloborane towards 2is higher which is in line with
the general trend in the Lewis acidity of boron trihalides that
increases with rising atom number.
63,64
It has to be pointed
out that no elevated temperature is required for the reaction
with boron tribromide to proceed which contrasts the conver-
sion of 2with Me
2
S·BBr
3
and, yet again, underlines the corre-
lation between formation of the imino-substituted aluminium
halide and the Lewis acidity of the haloborane employed.
To conclude our study on the hydride–halide exchange, we
reacted 2with two equivalents of boron triiodide (Scheme 3).
It turned out that the reaction readily proceeded to give the
aluminium diiodide {μ-LAlI
2
}
2
(7) which we isolated in nearly
quantitative yield (92%). In accordance with our previous
results, the high Lewis acidity of the boron triiodide promoted
the reaction.
63,64
Similar to its lower halide congeners 5and 6,
the single crystal X-ray data (Fig. S2, cf. the ESI†) and the high
resolution mass spectrum of 7support its dimeric formu-
lation. From a mixture of THF–hexane, compound 7crystal-
lizes with three equivalents of THF incorporated into the
lattice. The
1
H NMR spectrum (C
6
D
6
)of7shows the expected
singlet for the four protons of the imidazoline ring of the
imino ligands. Notably, in contrast to 2,5, and 6and similar
to 4splitting of the resonances induced by the Dipp moieties
(Dipp = 2,6-
i
Pr
2
C
6
H
3
) into two signal sets of equal intensity
occurs. In consequence, two doublets are observed for the
hydrogen atoms in positions 3 and 5 of the 2,6-
i
Pr
2
C
6
H
3
groups (7.27 ppm, 7.20 ppm, J= 8 Hz each) and two septets
(3.61 ppm, 3.44 ppm, J= 7 Hz each), as well as two doublets of
doublets result. Very likely, the splitting in 7results from hin-
dered rotation of the Dipp moieties due to the comparably
large atom radius of iodine. As for 4, the characteristic devi-
ation of the
1
H and
13
C{
1
H} NMR spectra of 7(vide supra)is
not reflected in its molecular structure in the solid state and,
accordingly, strong similarities exist to the lower halide con-
geners 5and 6.
Precedence for the transformation of Al–H groups into Al–X
functionalities (with X = Cl, Br or I) using BX
3
or Lewis base
adducts thereof is scarcely found in the literature. To the best
of our knowledge, the only related examples refer to the halo-
genation of Al–H moieties in carbaalane clusters employing
the higher haloboranes.
65,66
Obviously, the direct conversion
of an aluminium hydride complex with X
2
offers an alternative
methodology for halogenation at the metal centre. For this
procedure we found few reports in the literature merely report-
ing on respective iodinations.
19,67–69
These comprise only one
example
68
in which an aluminium hydride bearing a mono-
dentate N-donor ligand is subject to conversion with I
2
and,
thus, is strongly related to our report of a non-chelate-
fashioned iminato system. Notably, a mixture of products is
obtained in this report and we interpret this in terms of an
endorsement of our halogenation approach with BX
3
as an
alternative option to X
2
. Furthermore, it should be pointed out
that our conversions of imidazolin-2-iminato lithium {LLi}
2
(8)
with AlBr
3
or AlI
3
did not result in the formation of 5and 7,
respectively. Hence, our investigation of the hydride–halide
exchange between aluminium and boron contributes to
modern inorganic chemistry in that it may provide access to
aluminium halide complexes where standard halogenation
and salt metathesis protocols fail.
Conclusions
We have described the synthesis of the dimeric bis(2,6-di-
isopropylphenyl)imidazolin-2-imino aluminium dihydride
{μ-LAlH
2
}
2
(2). The strong electron-donating properties of the
iminato ligand result in the ability of 2to act as a potent
hydride-transfer reagent. This was demonstrated by the syn-
thesis of {μ-LAl(H)OTf}
2
(3), {μ-LAl(BH
4
)
2
}
2
(4), {μ-LAlBr
2
}
2
(5),
{μ-LAlCl
2
}
2
(6), and {μ-LAlI
2
}
2
(7)via conversion of 2with
several Lewis acids (i.e. Me
3
SiOTf, Me
2
S·BH
3
,Me
2
S·BBr
3
, BCl
3
,
BBr
3
, and BI
3
). All new imidazolin-2-iminato aluminium com-
pounds reported in this paper form dimers with a prominent
square-planar Al
2
N
2
structural motif and the essential charac-
teristics of the molecular structure in the solid state stay
largely untouched when replacing the hydride substituents for
halides or tetrahydridoborates.
Scheme 3 Syntheses of the imino aluminium dihalides 5,6, and 7
via conversion of the aluminium dihydride 2with boron trihalides.
(i) Toluene, −78 °C →rt, 12 h. Dipp = 2,6-diisopropylphenyl.
Dalton Transactions Paper
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Importantly, aluminium dihydride 2possesses potential for
applications such as hydroalumination and activation of un-
polarized element–element bonds (e.g. S–S, Se–Se bonds), as well
as in the field of hydrogen-storage materials. We are currently
exploring the reactivity of 2in this regard. In addition, we will
target cationic, as well as low-valent aluminium compounds by
applying halide abstraction- and reductive dehalogenation pro-
tocols, respectively, using 5,6, and 7.
Experimental
General considerations
All experiments and manipulations were carried out under dry
oxygen-free nitrogen using standard Schlenk techniques or in
an MBraun drybox containing an atmosphere of purified nitro-
gen. Glass junctions were coated with the PTFE-based grease
Merkel Triboflon III. Solvents were dried by standard methods
and freshly distilled prior to use. NMR spectra were recorded
on Bruker Avance II 200 or Avance III 500 spectrometers.
Chemical shift values are referenced to (residual) solvent
signals (
1
H and
13
C{
1
H} NMR),
70
to external Et
2
O·BF
3
(
11
B and
11
B{
1
H} NMR) or to external CFCl
3
(
19
F NMR). Jcoupling
values are reported in Hz. Abbreviations: s = singlet, d =
doublet, t = triplet, sept = septet, br = broad, n.o. = not
observed, n.r. = not resolved, Dipp = 2,6-diisopropylphenyl, L =
bis(2,6-diisopropylphenyl)imidazolin-2-imino. High resolution
mass spectra were recorded on a Thermo Fisher Scientific LTQ
Orbitrap XL using an APCI ion source and providing the
analyte dissolved in toluene, C
6
D
6
, or THF. Elemental analyses
were performed by the microanalytical laboratory of the Insti-
tut für Chemie, Technische Universität Berlin. Reagents pur-
chased from commercial sources were used as received if not
stated otherwise. Boron tribromide was stored over mercury.
Boron triiodide (95% grade) was supplied by Sigma-Aldrich
Corporation. Borane dimethylsulphide complex (5–6% excess
dimethylsulphide) was purchased from abcr GmbH & Co. KG.
LH,
38
Me
3
N·AlH
3
,
71
and Me
2
S·BBr
361
were synthesized accord-
ing to the reported procedures. Cf. the ESI†for the synthesis of
{LLi}
2
(8).
Synthetic procedures
Synthesis of {μ-L(AlH
2
)}
2
(2). A solid mixture of LH
(1, 5.12 g, 12.7 mmol) and Me
3
N·AlH
3
(1.13 g, 12.7 mmol) was
cooled to −78 °C. Under stirring, toluene (40 mL) was added
via a syringe and the reaction was allowed to proceed for 2 h
with the cooling applied followed by a period of 24 h at room
temperature (caution: gas evolution!). From the resulting solu-
tion the volatiles were evaporated under reduced pressure and
the residue was recrystallized from a hot mixture of hexane–
toluene (60 mL/10 mL). After crystal formation had been com-
pleted at room temperature the supernatant was decanted and
stored at low temperature (4 °C and −30 °C consecutively) for
3 d yielding a second crop. By this method, single crystals suit-
able for X-ray diffraction analysis could be obtained. The
crystal fractions were combined and dried in vacuo. Yield:
3.63 g, 66% (Found: C, 75.11; H, 8.82; N, 9.67%. Calc. for
C
54
H
76
Al
2
N
6
[863.18]: C, 75.14; H, 8.87; N, 9.74%). Decomp.:
>260 °C.
IR: ν
˜
/cm
−1
= 1830, 1798 (AlH).
1
H NMR (500.1 MHz, C
6
D
6
):
δ= 7.29 (t, J= 8, 4 H, DippH-4), 7.11 (d, J= 8, 8 H, DippH-3,5),
5.85 (s, 4 H, NCH), 3.02 (sept, J= 7, 8 H, CH(CH
3
)
2
), 2.60 (br,
4H,AlH
2
), 1.45 (d, J= 7, 24 H, CH(CH
3
)
2
), 1.12 (d, J=7,24H,
CH(CH
3
)
2
);
13
C{
1
H} NMR (125.8 MHz, C
6
D
6
): δ= 149.9 (NCN),
148.8 (DippC-1), 133.0 (DippC-2,6), 130.3 (DippC-4), 124.2
(DippC-3,5), 115.7 (NCH), 29.1 (CH(CH
3
)
2
), 25.5 (CH(CH
3
)
2
),
22.6 (CH(CH
3
)
2
).
HRMS: m/zfound (calc.): 895.5728 [M + H + 2O]
+
(100%),
(895.5733); 861.5676 [M −H]
+
(33), (861.5679); 877.5627
[M + O]
+
(27), (877.5628).
Synthesis of {μ-LAl(H)OTf}
2
(3). Under ice-cooling,
Me
3
SiOTf (3.07 g, 2.50 mL, 13.8 mmol) was gradually added
via a syringe to a stirred solution of 2(3.63 g, 4.2 mmol) in
toluene (20 mL). After 30 min, the ice-bath was removed and
the reaction continued at room temperature for 24 h. The vola-
tiles were removed under reduced pressure, the residue
washed with hexane (30 mL) and subjected to recrystallization
from hexane–THF (70 mL/70 mL). The solid material that
formed at −30 °C was isolated and dried in vacuo yielding
1.96 g of the product with sufficient purity for further conver-
sions (NMR spectroscopic control). In order to obtain analyti-
cally pure 3the crude product was dissolved in hexane–THF
(15 mL/35 mL) and a slight amount of insoluble material was
removed from the resulting solution by filtration through a
frit. The crystallization commenced after storage at 4 °C for 1 d
and was allowed to complete at −30 °C over a period of
3 d. A solid fraction produced by this method provided X-ray
quality single crystals. The crystalline material was separated
from the mother liquor and dried in vacuo. Yield: 1.05 g,
22% (Found: C, 58.62; H, 6.52; N, 6.96; S, 5.23%. Calc. for
C
56
H
74
Al
2
F
6
N
6
O
6
S
2
[1159.31]: C, 58.02; H, 6.43; N, 7.25;
S, 5.53%). Decomp.: 235–245 °C.
1
H NMR (200.1 MHz, CD
3
CN): δ= 7.46 (t, J=8,4H,
DippH-4), 7.24 (d, J= 8, 8 H, DippH-3,5), 6.74 (s, 4 H, NCH),
2.59 (n.r., 8 H, CH(CH
3
)
2
), 1.21 (n.r., 24 H, CH(CH
3
)
2
), 1.05 (d,
J= 7, 24 H, CH(CH
3
)
2
), n.o. (AlH);
13
C{
1
H} NMR (50.3 MHz,
CD
3
CN): δ= 152.5 (NCN), 148.6 (DippC-1), 132.6 (DippC-2,6),
132.5 (DippC-4), 126.0 (DippC-3,5), 119.4 (NCH), 29.7 (CH(CH
3
)
2
),
25.7 (CH(CH
3
)
2
), 22.8 (br, CH(CH
3
)
2
), n.o. (CF
3
);
19
F NMR
(188.3 MHz, CD
3
CN): δ=−78.3, −79.3 (CF
3
).
HRMS: m/zfound (calc.): 1009.5107 [M −C−3F −3O −S]
+
(100%), (1009.5126); 1157.4551 [M −H]
+
(8), (1157.4568);
1159.4712 [M + H]
+
(5), (1159.4724).
Synthesis of {μ-L(Al(BH
4
)
2
)}
2
(4). In a Schlenk tube
equipped with a PTFE-coated magnetic stirrer bar and a
rubber septum 2(0.366 g, 0.42 mmol) was dissolved in toluene
(5 mL) and the resulting mixture was cooled to −78 °C. Under
stirring, a freshly prepared solution (0.5 M) of borane
dimethylsulphide complex in toluene (3.5 mL, 1.8 mmol) was
added dropwise via a syringe over a period of 5 min. The reac-
tion mixture was slowly warmed to room temperature over-
night. A colourless suspension formed that was stirred for an
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additional 6 h, after which the precipitation was completed by
gradual addition of hexane (9 mL). The septum was replaced
by a glass stopper, and the solid was allowed to sediment over-
night. Via a syringe, the supernatant was withdrawn (16 mL)
and the residual solvent was evaporated under reduced
pressure. The remaining colourless powder was dried in vacuo
(3 h, 9 × 10
−2
mbar), and stored under nitrogen for 2 d before
a sample was subjected to elemental analysis. Yield: 0.27 g,
67% (Found: C, 71.43; H, 9.40; N, 8.94%. Calc. for
C
54
H
88
Al
2
B
4
N
6
[918.52] × 0.5 C
7
H
8
[92.14]: C, 71.60; H, 9.61;
N, 8.71%). Decomp.: >260 °C.
IR: ν
˜
/cm
−1
= 2492, 2431 (BH).
1
H NMR (500.1 MHz,
THF-d
8
): δ= 7.33 (t, J= 8, 4 H, DippH-4), 7.20 (d, J=8,4H,
DippH-3,5), 7.16 (d, J= 8, 4 H, DippH-3,5), 6.58 (s, 4 H, NCH),
3.18 (sept, J= 7, 4 H, CH(CH
3
)
2
), 2.97 (sept, J=7,4H,
CH(CH
3
)
2
), 1.29 (d, J= 7, 12 H, CH(CH
3
)
2
), 1.23 (d, J=7,12H,
CH(CH
3
)
2
), 1.11 (d, J= 7, 12 H, CH(CH
3
)
2
), 0.89 (d, J=7,12H,
CH(CH
3
)
2
), −0.34 (br, h
1/2
= 170 Hz, 16 H, BH);
13
C{
1
H} NMR
(125.8 MHz, THF-d
8
): δ= 151.5 (NCN), 148.2 (ArC), 147.6 (ArC),
135.0 (ArC), 131.4 (ArC), 125.8 (ArC), 124.8 (ArC), 119.9 (NCH),
29.4 (CH(CH
3
)
2
), 28.8 (CH(CH
3
)
2
), 27.2 (CH(CH
3
)
2
), 24.4
(CH(CH
3
)
2
), 23.7 (CH(CH
3
)
2
), 23.4 (CH(CH
3
)
2
);
11
B NMR
(64.2 MHz, THF-d
8
): δ=−37 (n.r., h
1/2
= 380 Hz);
11
B{
1
H} NMR
(64.2 MHz, THF-d
8
): δ=−37 (h
1/2
= 270 Hz).
HRMS: m/zfound (calc.): 875.6002 [M −10 H −3B]
+
(11%),
(875.6012).
Synthesis of {μ-LAlBr
2
}
2
(5). A Schlenk tube equipped with a
PTFE-coated magnetic stirrer bar and a glass stopper was
charged with 2(0.223 g, 0.26 mmol) and Me
2
S·BBr
3
(0.175 g,
0.56 mmol) in the dry box. Under Schlenk conditions, the
solid mixture was cooled to 0 °C and, with stirring, toluene
(4.5 mL) was gradually added via a syringe. Stirring was contin-
ued at 0 °C for 20 min followed by replacement of the cooling
for an oil-bath which was heated to 80 °C. After a short induc-
tion period, the Schlenk vessel was isolated from the inert-gas
system by closing the tap and the reaction proceeded at this
temperature for 3 d. Using a frit, the warm reaction mixture
was filtered in order to separate a slight amount of insoluble
material. From the filtrate, crystals formed upon standing at
room temperature overnight. In order to complete the crystalli-
zation process it was stored at 4 °C and at −30 °C for a period
of 24 h each time. By this method, crystals suitable for X-ray
diffraction analysis were obtained. After decantation of the
cold supernatant the solid residue was dried in vacuo for 3 h
and stored for 1 d under nitrogen before it was subjected to
elemental analysis. Yield: 0.19 g, 57% (Found: C, 58.05;
H, 6.63; N, 6.62%. Calc. for C
54
H
72
Al
2
Br
4
N
6
[1178.77] × C
7
H
8
[92.14]: C, 57.65; H, 6.34; N, 6.61%).
1
H NMR (500.1 MHz, C
6
D
6
): δ= 7.31 (t, J= 8, 4 H, DippH-4),
7.22 (d, J= 8, 8 H, DippH-3,5), 5.84 (s, 4 H, NCH), 3.31 (sept,
J= 7, 8 H, CH(CH
3
)
2
), 1.57 (d, J= 7, 24 H, CH(CH
3
)
2
), 0.99 (d,
J= 7, 24 H, CH(CH
3
)
2
);
13
C{
1
H} NMR (125.8 MHz, C
6
D
6
):
δ= 151.5 (NCN), 147.6 (DippC-1), 134.3 (DippC-2,6), 131.3
(DippC-4), 125.3 (DippC-3,5), 119.1 (NCH), 28.9 (CH(CH
3
)
2
),
24.3 (CH(CH
3
)
2
), 20.8 (CH(CH
3
)
2
).
HRMS: m/zfound (calc.): 1178.2132 [M]
+
(100%), (1178.2142).
Synthesis of {μ-LAlCl
2
}
2
(6). A stirring suspension of freshly
sublimed AlCl
3
(0.38 g, 2.8 mmol) in toluene (8 mL) was
cooled to 0 °C and treated dropwise with a solution of {LLi}
2
(8, 1.11 g, 1.4 mmol) in toluene (19 mL) over a period of
10 min. The resulting reaction mixture was stirred at 0 °C for
30 min, then the cooling was removed and the reaction contin-
ued for 12 h at room temperature. After filtration via a canula
(glass fiber-coated inlet) the solvent was evaporated from the
pale yellow solution under reduced pressure, the residue was
washed with hexane (two portions of 15 mL, removed 13 mL of
supernatant each time), and the crude product was dried
in vacuo. Yield: 0.74 g, 54% (cf. the ESI†for depiction of the
NMR spectra of the crude product in CDCl
3
). Single crystals
suitable for X-ray diffraction analysis were obtained after
dilution of the filtered reaction mixture with hexane (10 mL)
and storage at 4 °C for 1 d.
1
H NMR (500.1 MHz, C
6
D
6
): δ= 7.30 (t, J= 8, 4 H, DippH-4),
7.22 (d, J= 8, 8 H, DippH-3,5), 5.87 (s, 4 H, NCH), 3.17 (sept,
J= 7, 8 H, CH(CH
3
)
2
), 1.54 (d, J= 7, 24 H, CH(CH
3
)
2
), 1.00 (d,
J= 7, 24 H, CH(CH
3
)
2
);
13
C{
1
H} NMR (125.8 MHz, C
6
D
6
):
δ= 151.8 (NCN), 147.6 (DippC-1), 133.5 (DippC-2,6), 131.1
(DippC-4), 125.0 (DippC-3,5), 118.3 (NCH), 28.9 (CH(CH
3
)
2
),
26.0 (CH(CH
3
)
2
), 23.8 (CH(CH
3
)
2
).
HRMS: m/zfound (calc.): 1001.4233 [M + H]
+
(28%),
(1001.4252); 981.4605 [M −Cl+2H+O]
+
(26), (981.4620).
Synthesis of {μ-LAlI
2
}
2
(7). In the drybox, a solid mixture of
2(0.511 g, 0.59 mmol) and boron triiodide (0.488 g,
1.25 mmol) was prepared in a Schlenk vessel equipped with a
PTFE-coated magnetic stirrer bar and a rubber septum.
Under Schlenk conditions, it was cooled to −78 °C and, with
stirring, toluene (8 mL) was gradually added via a syringe
along the inner glass wall of the cold reaction vessel. The
resulting reaction mixture was slowly allowed to warm to
room temperature overnight. After 18 h the solvent was evap-
orated under reduced pressure and the colourless solid
residue was dried in vacuo. It was redissolved in a mixture of
hexane–THF (6 mL/12 mL) and, using a frit, a slight amount
of insoluble material was removed by filtration. The filtrate
was stored at 4 °C for 24 h. Separation of the crystalline
product had commenced and was completed at −30 °C
for a period of 2 d. The fraction, thus obtained, contained
single crystals suitable for X-ray diffraction analysis. The
cold supernatant was decanted and the solid was dried
in vacuo. Yield: 0.74 g, 92% (Found: C, 47.81; H, 5.63;
N, 5.87%. Calc. for C
54
H
72
Al
2
I
4
N
6
[1366.77]: C, 47.45; H, 5.31;
N, 6.15%).
1
H NMR (500.1 MHz, C
6
D
6
): δ= 7.32 (t, J= 8, 4 H, DippH-4),
7.27 (d, J= 8, 4 H, DippH-3,5), 7.20 (d, J= 8, 4 H, DippH-3,5),
5.76 (s, 4 H, NCH), 3.61 (sept, J= 7, 4 H, CH(CH
3
)
2
), 3.44 (sept,
J= 7, 4 H, CH(CH
3
)
2
), 1.63 (d, J= 7, 12 H, CH(CH
3
)
2
), 1.60 (d,
J= 7, 12 H, CH(CH
3
)
2
), 1.03 (d, J= 7, 12 H, CH(CH
3
)
2
), 0.93 (d,
J= 7, 12 H, CH(CH
3
)
2
);
13
C{
1
H} NMR (125.8 MHz, C
6
D
6
):
δ= 151.2 (NCN), 147.6 (ArC), 147.4 (ArC), 136.0 (ArC), 131.6
(ArC), 126.2 (ArC), 125.5 (ArC), 120.4 (NCH), 28.9 (CH(CH
3
)
2
),
28.8 (CH(CH
3
)
2
), 26.5 (CH(CH
3
)
2
), 25.9 (CH(CH
3
)
2
), 24.4
(n.r., 2 × CH(CH
3
)
2
).
Dalton Transactions Paper
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HRMS: m/zfound (calc.): 1239.2567 [M −I]
+
(100%),
(1239.2583); 1257.2661 [M −I+2H+O]
+
(75), (1257.2689);
1367.1680 [M + H]
+
(6), (1367.1706).
Single crystal structure determination
Data for the single-crystal structure determination of 2,3,4,
and 7were collected on an Agilent SuperNova diffractometer
equipped with a CCD area Atlas detector and a mirror mono-
chromator utilizing CuK
α
radiation (λ= 1.5418 Å). Data for the
single-crystal structure determination of 5,6, and 8were
collected on an Oxford-Diffraction Xcalibur diffractometer
equipped with a CCD area detector Sapphire S and a graphite
monochromator utilizing MoK
α
radiation (λ= 0.71073 Å).
The crystal structures were solved by direct methods and
refined on F
2
using full-matrix least squares with
SHELXL-97.
72
The positions of the H atoms of the carbon
atoms were calculated and considered isotropically according
to a riding model. The positions of the H atoms attached to
the aluminium or the boron atoms at the compounds 2,3, and
4were found in the electron-density map and were refined
without restraints of the interatomic distances.
Acknowledgements
We are exceptionally grateful to the Alexander von Humboldt
foundation (Sofja Kovalevskaja Program) for financial support.
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