Synthesis, characterization and reactivity of an imidazolin-2-iminato aluminium dihydride

Article abstract of DOI:10.1039/c3dt52637b

The reaction of bis(2,6-diisopropylphenyl)imidazolin-2-imine (LH, 1) with Me₃N·AlH₃ furnishes {μ-LAlH₂} ₂ (2). The marked tendency of 2 to 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 2 was further converted with Me₃SiOTf, Me ₂S·BH₃, Me₂S·BBr₃, and BX₃ (with X = Cl, Br, and I) into {μ-LAl(H)OTf}₂ (3), {μ-LAl(BH₄)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 Al₂N₂ ring with two bridging bulky imidazolin-2-imino moieties.

Full text of DOI:10.1039/c3dt52637b

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Synthesis, characterization and reactivity of an
imidazolin-2-iminato aluminium dihydride†
Cite this: DOI: 10.1039/c3dt52637b
Daniel Franz, Elisabeth Irran and Shigeyoshi Inoue*
The reaction of bis(2,6-diisopropylphenyl)imidazolin-2-imine (LH, 1) with Me3N·AlH3 furnishes {μ-LAlH2}2 (2).
The marked tendency of 2 to 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 2 was further converted with Me3SiOTf, Me2S·BH3, Me2S·BBr3, and BX3 (with X = Cl, Br,
and I) into {μ-LAl(H)OTf}2 (3), {μ-LAl(BH4)2}2 (4), and {μ-LAlX2}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 Al2N2 ring with two bridging bulky imidazolin-2-imino
moieties.
Received 23rd September 2013,
Accepted 26th November 2013
DOI: 10.1039/c3dt52637b
www.rsc.org/dalton
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 carbonyl1,2 and
alkyne3–7 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 I8 (Fig. 1) that
bear highly sophisticated ligands grants access to all types of
isolatable and well-defined model complexes. Compound I can
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
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).
Institut für Chemie, Technische Universität Berlin, Straße des 17. Juni 135, Sekr. C2,
10623 Berlin, Germany. E-mail: shigeyoshi.inoue@tu-berlin.de
†Electronic supplementary information (ESI) available: Procedures for the con-
version of 2 with BX3 (X = Cl, Br) and respective NMR spectra, NMR spectra of 6,
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
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
ligand to
a
rich chemistry of respective aluminium
dihydrides.9–12 A prominent example is II (Fig. 1), which can
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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 III18 (Fig. 1) is a very recent
example among others19 of an intriguing nature. Recently,
scarce σ-alane complexes of transition metals (e.g. Cr, Mo, W,
Mn) implementing ligands such as IV19 (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.
Fig. 2 Imidazolin-2-imino compounds with main group elements (VII,
IX, X) and the titanium complex VIII (Dipp = 2,6-diisopropylphenyl).
For advocating the diversity of ligand systems employed in
the field we would select the phosphinimide V22 (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
imino aluminium dihydride {μ-LAlH2}2 2 and 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 procedure22 reported by Stephan and co-
workers for the synthesis of a related phosphinimide. The stoi-
chiometric reaction of LH (1)38 with Me3N·AlH3 in toluene
affords the dimeric aluminium dihydride {μ-LAlH2}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
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- or a 4π-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 VIII30 (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
Scheme 1 Formation of the imidazolin-2-imino aluminium hydride 2
via conversion of the imine 1 and 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,
Herein we report the synthesis and characterization of
the hitherto unknown bis(2,6-diisopropylphenyl)imidazolin-2- 24 h. Dipp = 2,6-diisopropylphenyl, Tf = SO2CF3.
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aluminium dihydride II.10 The resonance pattern observed for Al(1)–N(1)–Al(1A) angles in 2 are 86.2(1)° and 93.8(1)°, respect-
the imidazolin-2-iminato moiety in the 1H and 13C{1H} NMR ively. The dihedral angle N(1A)–Al(1)–N(1)–Al(1A) of 0.0° con-
spectra of 2 resembles the one in related silicon complexes firms that these four atoms are all located in one plane. The
such as LSi(C5Me5)Br2.31 In the proton NMR spectrum (C6D6), Al(1)⋯Al(1A) distance in 2 amounts to 2.795(1) Å, which is sig-
a broad resonance at 2.60 ppm with 4 H intensity is observed nificantly longer than the metal–metal bond lengths in charac-
that can be assigned to the hydrogen atoms of the AlH2 moi- teristic donor-stabilized dialanes with four-coordinated
eties and is more shielded than those observed for amidino-, aluminium centres.19,43 The N(1)–C(1) bond length of 1.289(2)
β-diketimino-, or 1-azaallyl aluminium dihydrides (4.60 ppm– Å in 2 only differs slightly from the analogous values in silicon
4.87 ppm)39–41 and the starting material Me3N·AlH3 compounds stabilized by imidazolin-2-iminato ligands.31,38
(4.08 ppm), as well as [Me2NAlH2]3 (3.8 ppm)42 but is shifted Furthermore, it is similar to that of the guanidinate
more downfield than that of aluminium dihydride VI {μ-(Me2N)2CNAl(NMe2)2}2.44 The 5-membered imidazoline ring
(1.08 ppm, Fig. 1).25 After heating a solution of 2 in toluene to plane is tilted versus the central Al2N2 moiety by 9°. The sum
reflux temperature over a period of 12 h the NMR spectro- of the bond angles around the nitrogen atom attached to the
scopic control did not reveal any signs of decomposition aluminium centre amounts to 360°, which confirms the
which accounts for the thermal stability of this aluminium trigonal-planar geometry around the nitrogen atom of the
dihydride. Compound 2 crystallizes from hexane–toluene in imino group.
the monoclinic space group P21/c and the unit cell contains
DFT calculations of related aluminium dihydrides
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 2 reveals the
four-membered Al2N2 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 2 are 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
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 2 in 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 VI25 (i.e. II′, V′, and VI′; 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 2 is a strong hydride
donor reagent.
Hydride abstraction from {μ-LAlH2}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 LAlH2 moieties in the solid state
instead of the formation of dimers as evidenced by the single
crystal X-ray data of {μ-LAlH2}2 (2) and its high resolution mass
Fig. 5 Molecular structure of 3(THF)3 in the solid state. Hydrogen
spectrum (vide supra). Stimulated by a report of Stephan and 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),
coworkers about the reaction of V with trimethylsilyltriflate to
{μ-(tBu)3PNAl(H)OTf}2,22 we treated 2 with three equivalents of
Me3SiOTf (Tf = SO2CF3), which resulted in replacement of one
hydride substituent per aluminium centre by a triflate group
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)
(Scheme 1). Compound 3 {μ-LAl(H)OTf}2 was obtained in ana- 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).
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 reasonable because the diagonal relationship between these
aluminium centre in 3 is lowered in comparison to its precur- elements is one of the many verified reactivity patterns emer-
sor, a resonance pattern similar to that of 2 is observed in the ging from the periodic table.45 Recently, Harder and Spiel-
1H and 13C{1H} NMR spectra of 3 in CD3CN. Similar to 2, the mann reported the synthesis of the β-diketiminato-stabilized
molecular structure of 3 in the solid state reveals a dimer with (Dipp)nacnacAl(BH4)2 by reaction of II with DippNH2·BH3.46
a central Al2N2 core (Fig. 5). Two triflate groups are found Inspired by these findings, we set out to react 2 with four
assuming positions on the same side of the Al2N2 ring, thus equivalents of borane dimethylsulphide complex (Scheme 2).
their configuration is pseudo-cis which is reminiscent of the As the outcome of the presumed hydride transfer from the
phosphinimide congener {μ-(tBu)3PNAl(H)OTf}2.22 As marked aluminium centre to the boron atom {μ-LAl(BH4)2}2 (4) was
by the dihedral angle N(4)–Al(1)–Al(2)–N(1) = 169.6(2)° the formed as elucidated by multinuclear NMR spectroscopy,
reduced symmetry around the Al2N2 ring inflicts slight devi- single-crystal X-ray structure determination, elemental analy-
ation from its square-planar geometry that is observed for 2 sis, as well as high resolution mass spectrometric analysis.
(Fig. 3) in very high approximation. The Al–N bond lengths of
An early synthesis of the parent Al(BH4)3 goes back to
3 are found in a range from 1.849(3) Å to 1.872(3) Å and are Brown and coworkers.47 Aluminium borohydride is described
notably shorter than those in 2. This indicates that the triflate as a hazardous material, particularly because the vapour of the
substituents in 3 render the aluminium centres more electro- liquid substance may spontaneously detonate upon contact
philic, thus strengthening the interaction with the nitrogen with air and moisture. In addition, it slowly decomposes at
atoms of the adjacent ligands.
room temperature with the release of dihydrogen.47 In recent
While the replacement of one hydride at the aluminium years, Al(BH4)3 has gained prominent attention from the field
atom for OTf− proceeded readily, we were not able to isolate a of materials science16,48 and the transformation into more
related species with a ditriflated metal centre even when treat- suitable hydrogen-storage materials as ammine aluminium
ing {μ-LAlH2}2 (2) with a large excess of Me3SiOTf at an elev- borohydrides49–51 and aluminoboranes52 is explored. Given
ated temperature.
this background, it is desirable to have access to a room
As an alternative group of electrophiles that might elucidate temperature-stable and well-defined model complex such as
the hydride-donating properties of 2, we conceived boron- {μ-LAl(BH4)2}2 (4) in which the relevant aluminium centres and
centred Lewis acids. The turn from silicon to boron is borohydride moieties are unified.
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Scheme 2 Conversion of the aluminium dihydride 2 with the dimethyl-
sulphide complexes of borane (top) or boron tribromide (bottom) yield-
ing 4 and 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).
In the infrared spectrum of 4 we 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(BH4)3
1
have been reported at a comparable wavenumber.53 In the H
and 13C{1H} NMR spectra of 4 (THF-d8), 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
BH4 groups give rise to a broad resonance at −0.33 ppm in the
1H NMR spectrum. The chemical shift of the boron nuclei comparison to the respective distances in 2 (Al–N = 1.912(1) Å
amounts to −37 ppm (h1/2 = 270 Hz) as observed in the 11B{1H} and 1.915(1) Å; vide supra). The shorter Al–N bond lengths
NMR spectrum (THF-d8). In the proton-coupled 11B NMR spec- coincide with marginally more acute Al–N–Al′ bond angles
trum, only a broad singlet at −37 ppm (h1/2 = 380 Hz) is (92.7(1)° and 93.2(1)°) in 4 as compared to 93.8(1)° in 2.
observed (cf. (Dipp)nacnacAl(BH4)2: δ(11B)
= −36.6 ppm, Accordingly, the Al(1)⋯Al(2) distance is shortened to 2.728(1)
quintet, J = 85.3 Hz, C6D6).46 Obviously, the J coupling between Å with respect to 2.795(1) Å for the corresponding distance in
boron and hydrogen in 4 is not resolved. However, the signal 2. Concomitantly, the decrease in the Al–N distances leads to
broadening of 110 Hz in the proton-coupled 11B NMR spec- an increase in the exocyclic imino groups’ bond lengths as
trum suggests the presence of borohydride moieties. After exemplified by N(1)–C(1) = 1.319(2) Å in 4 compared to N(1)–
storing a flame-sealed NMR tube with a sample of 4 in THF-d8 C(1) = 1.289(2) Å in 2. When taking a look at how the Al–N bond
at room temperature for two weeks no signs of decomposition lengths in II (Fig. 1) are affected by the transformation into
were observed. However, upon heating to 60 °C overnight the Dipp(nacnac)Al(BH4)2 one finds the Al–N distance of 1.899(1) Å
NMR spectroscopic analysis revealed a new species giving rise in II54 essentially unaltered upon conversion into its alu-
1
to an additional imino ligand signal set in the H NMR spec- minium borohydride congener (Al–N = 1.898(3) Å).46 In this
trum, as well as a second singlet in the 11B{1H} NMR spectrum case, the bidentate β-diketiminato ligand appears to be more
at −21.5 ppm. After the sample tube had been exposed to a rigid, allowing for less structural flexibility in its resulting alu-
temperature of 90 °C for 2 d compound 4 had been consumed minium complexes than the non-chelate-fashioned imino
entirely and the proton NMR spectrum evidenced the for- group in 2 and 4, respectively. The plane of the Al2N2 ring in 4
mation of several unidentified products, as well as dihydrogen. intersects the planes of its adjacent imidazoline rings with di-
Crystallization of 4 from toluene–THF–hexane yielded single hedral angles of 34° and 35°, respectively. This is notably more
crystals suitable for X-ray diffraction analysis which contained obscure than that in 2 (9°) and presumably due to the steric
1.5 equivalents of toluene in the lattice. The X-ray study con- hindrance of the BH4 moieties in comparison to the hydrides
firms the dimeric formulation of compound 4 (Fig. 6) and the in 2. With values between 1.07(3) Å and 1.19(2) Å the similarity
structural parameters of the Al2N2 ring and of the attached of all B–H bond lengths as a strong indicator for the formation
iminato ligands resemble the geometry of the corresponding of BH4 groups underlines the hydride transfer from the alu-
groups in 2. The Al–N bond lengths in 4 lie in a range from minium to the boron centre. Each aluminium atom can be
1.873(2) Å to 1.889(2) Å, which marks a slight decrease in described as coordinated by two BH4− groups in an η2 fashion,
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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–N dis-
tances are only slightly affected (vide supra). The smallest Al⋯B
distance in 4 amounts 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(BH4)2 for which an η2
coordination mode of the BH4 moieties to the aluminium atom
has also been described.46 Accordingly, this type of bonding is
also found for {μ-Me3SiOAl(BH4)2}2 (Al⋯B = 2.156(5) Å, 2.143(4)
Å),55 the parent Al(BH4)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-LAlH2}2 (2) to Me2S·BH3 yielding
{μ-LAl(BH4)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 2 might 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 11B NMR spectroscopy. We chose the boron
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).
tribromide dimethylsulphide complex61,62 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 lengths in 2 (Al–N = 1.912(1) Å, 1.915(1) Å) is now marked and
solution.62 First, we examined the conversion of 2 with two one would assume that it correlates to the stronger electron-
equivalents of Me2S·BBr3 in C6D6 by preparing an experiment withdrawing nature of the bromide substituents in 5 as com-
in the NMR tube. It turned out that an initial reaction took pared to the hydrides in 2. In consequence, the electrophilicity
place at room temperature as marked in the 11B NMR spec- of the aluminium centres is increased and the interaction with
trum by a doublet at −7.8 ppm (J = 160 Hz, assigned to the nitrogen atoms of the imino ligands is strengthened. In
Me2S·BHBr2)62 and a triplet of weaker intensity at −10.8 ppm accordance with the observed trend in the bond lengths com-
(J = 130 Hz, assigned to Me2S·BH2Br)62 that superimposed with paring 2, 4, and 5, the N–Al–N′ bond angles in 5 have yet again
the resonance caused by the residual starting material slightly sharpened to 91.9(1)° and 92.3(1)°, respectively. The
(Me2S·BBr3: δ(11B) was observed at −10.7 ppm in C6D6). We interesting consequence of the aforementioned marginal
stored the flame-sealed NMR tube at elevated temperature changes in the geometry of the Al2N2 core is revealed if one
1
until the H and 11B NMR spectra indicated the conversion to notes the Al(1)⋯Al(2) distance in 5 to amount to 2.687(1) Å.
be complete. Accordingly, the reaction on a preparative scale This value is close to the range of metal–metal bond lengths in
in toluene granted the analytically pure aluminium dibromide prominent dialanes with four-coordinated metal centres.19,43
{μ-LAlBr2}2 (5) in 57% yield (Scheme 2). The formulation as a However, we would not suggest that a bonding interaction
dimeric compound is evidenced by the high resolution mass between the Al atoms in 5 exists.
spectrometric analysis, as well as single-crystal X-ray study
(Fig. 7).
Motivated by the prospective results of the conversion of 2
with Me2S·BBr3 we decided to explore the general applicability
The 1H and 13C{1H} NMR spectra (C6D6) of 5 differ only of the hydride–halide exchange concept. The conversion of 2
insignificantly with respect to 2, taking aside the expected dis- with two equivalents of boron trichloride yielded the alu-
appearance of the broad resonance at 2.60 ppm caused by the minium dichloride {μ-LAlCl2}2 (6) (Scheme 3, cf. the ESI† for
hydrogen atoms of the AlH2 moieties in 2. The single-crystal experimental details). However, the 1H NMR spectrum of the
diffraction analysis of 5 shows that, as in 4, 1.5 equivalents of reaction mixture (CDCl3) revealed signal sets induced by yet
toluene are incorporated into the crystal lattice. The structural unassigned additional species of L, presumably intermediate
parameters of the planar Al2N2 ring and the adjacent imidazo- forms in the path of the reaction from 2 to 6. This came as a
line rings in 5 are very similar to 4. The Al–N bond lengths in 5 surprise as we would have ascribed high reactivity to the
lie in a range from 1.858(3) Å to 1.871(3) Å and are slightly uncomplexed form of BCl3. However, treating 2 with another
shorter in comparison to those in 4. Though rather marginal two equivalents of this haloborane significantly improved the
with respect to 4 the difference from the analogous bond yield of 6. Alternatively, compound 6 was synthesized by
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to 4 splitting of the resonances induced by the Dipp moieties
(Dipp = 2,6-iPr2C6H3) 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-iPr2C6H3
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 7 results 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 1H and 13C{1H} 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 5 and 6.
Scheme 3 Syntheses of the imino aluminium dihalides 5, 6, and 7
via conversion of the aluminium dihydride 2 with boron trihalides.
(i) Toluene, −78 °C → rt, 12 h. Dipp = 2,6-diisopropylphenyl.
Precedence for the transformation of Al–H groups into Al–X
reaction of bis(2,6-diisopropylphenyl)imidazolin-2-imino lithium34 functionalities (with X = Cl, Br or I) using BX3 or Lewis base
{LLi}2 (8) with aluminium trichloride (cf. the ESI for details on adducts thereof is scarcely found in the literature. To the best
our synthesis of 8 and its molecular structure in the solid of our knowledge, the only related examples refer to the halo-
state, Fig. S3†). The formation of 6 was evidenced by high genation of Al–H moieties in carbaalane clusters employing
resolution mass spectrometry, NMR spectroscopy, as well as the higher haloboranes.65,66 Obviously, the direct conversion
X-ray crystallographic analysis. The essential structural charac- of an aluminium hydride complex with X2 offers an alternative
teristics of 6 strongly resemble the heavier halide congener 5 methodology for halogenation at the metal centre. For this
(cf. the ESI for the molecular structure of 6 in the solid state, procedure we found few reports in the literature merely report-
Fig. S1†). Furthermore, parallels exist to the bulky phosphini- ing on respective iodinations.19,67–69 These comprise only one
mide {μ-(tBu)3PNAlCl2}2 reported by Stephan and coworkers.23 example68 in which an aluminium hydride bearing a mono-
Interestingly,
2,6-dimethylimidazolin-2-imino
aluminium dentate N-donor ligand is subject to conversion with I2 and,
dichloride VII, a less sterically congested derivative of 6, forms thus, is strongly related to our report of a non-chelate-
a six-membered Al3N3 ring in the solid state in which most Al- fashioned iminato system. Notably, a mixture of products is
centred bond angles nearly fit the ideal value of a tetrahedron obtained in this report and we interpret this in terms of an
(109.5°).32 Thus, the square-planar geometry of the Al2N2 core endorsement of our halogenation approach with BX3 as an
in 6 must result from the bulkiness of L in that it imposes alternative option to X2. Furthermore, it should be pointed out
smaller Al–N–Al′ angles than the iminato ligand in VII.
that our conversions of imidazolin-2-iminato lithium {LLi}2 (8)
On the other hand, when we reacted 2 with two equivalents with AlBr3 or AlI3 did not result in the formation of 5 and 7,
of BBr3, quantitative conversion to 5 was observed (NMR spec- respectively. Hence, our investigation of the hydride–halide
troscopic control, Scheme 3). Obviously, the reactivity of the exchange between aluminium and boron contributes to
heavier haloborane towards 2 is higher which is in line with modern inorganic chemistry in that it may provide access to
the general trend in the Lewis acidity of boron trihalides that aluminium halide complexes where standard halogenation
increases with rising atom number.63,64 It has to be pointed and salt metathesis protocols fail.
out that no elevated temperature is required for the reaction
with boron tribromide to proceed which contrasts the conver-
sion of 2 with Me2S·BBr3 and, yet again, underlines the corre-
lation between formation of the imino-substituted aluminium
halide and the Lewis acidity of the haloborane employed.
Conclusions
To conclude our study on the hydride–halide exchange, we We have described the synthesis of the dimeric bis(2,6-di-
reacted 2 with two equivalents of boron triiodide (Scheme 3). isopropylphenyl)imidazolin-2-imino aluminium dihydride
It turned out that the reaction readily proceeded to give the {μ-LAlH2}2 (2). The strong electron-donating properties of the
aluminium diiodide {μ-LAlI2}2 (7) which we isolated in nearly iminato ligand result in the ability of 2 to act as a potent
quantitative yield (92%). In accordance with our previous hydride-transfer reagent. This was demonstrated by the syn-
results, the high Lewis acidity of the boron triiodide promoted thesis of {μ-LAl(H)OTf}2 (3), {μ-LAl(BH4)2}2 (4), {μ-LAlBr2}2 (5),
the reaction.63,64 Similar to its lower halide congeners 5 and 6, {μ-LAlCl2}2 (6), and {μ-LAlI2}2 (7) via conversion of 2 with
the single crystal X-ray data (Fig. S2, cf. the ESI†) and the high several Lewis acids (i.e. Me3SiOTf, Me2S·BH3, Me2S·BBr3, BCl3,
resolution mass spectrum of 7 support its dimeric formu- BBr3, and BI3). All new imidazolin-2-iminato aluminium com-
lation. From a mixture of THF–hexane, compound 7 crystal- pounds reported in this paper form dimers with a prominent
lizes with three equivalents of THF incorporated into the square-planar Al2N2 structural motif and the essential charac-
1
lattice. The H NMR spectrum (C6D6) of 7 shows the expected teristics of the molecular structure in the solid state stay
singlet for the four protons of the imidazoline ring of the largely untouched when replacing the hydride substituents for
imino ligands. Notably, in contrast to 2, 5, and 6 and similar halides or tetrahydridoborates.
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Importantly, aluminium dihydride 2 possesses potential for 3.63 g, 66% (Found: C, 75.11; H, 8.82; N, 9.67%. Calc. for
applications such as hydroalumination and activation of un- C54H76Al2N6 [863.18]: C, 75.14; H, 8.87; N, 9.74%). Decomp.:
polarized element–element bonds (e.g. S–S, Se–Se bonds), as well >260 °C.
as in the field of hydrogen-storage materials. We are currently
1
IR: ˜ν/cm−1 = 1830, 1798 (AlH). H NMR (500.1 MHz, C6D6):
exploring the reactivity of 2 in this regard. In addition, we will δ = 7.29 (t, J = 8, 4 H, DippH-4), 7.11 (d, J = 8, 8 H, DippH-3,5),
target cationic, as well as low-valent aluminium compounds by 5.85 (s, 4 H, NCH), 3.02 (sept, J = 7, 8 H, CH(CH3)2), 2.60 (br,
applying halide abstraction- and reductive dehalogenation pro- 4 H, AlH2), 1.45 (d, J = 7, 24 H, CH(CH3)2), 1.12 (d, J = 7, 24 H,
tocols, respectively, using 5, 6, and 7.
CH(CH3)2); 13C{1H} NMR (125.8 MHz, C6D6): 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(CH3)2), 25.5 (CH(CH3)2),
22.6 (CH(CH3)2).
Experimental
General considerations
HRMS: m/z found (calc.): 895.5728 [M + H + 2O]+ (100%),
(895.5733); 861.5676 [M − H]+ (33), (861.5679); 877.5627
[M + O]+ (27), (877.5628).
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 (1H and 13C{1H} NMR),70 to external Et2O·BF3 (11B and
11B{1H} NMR) or to external CFCl3 (19F NMR). J coupling
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, C6D6, 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.
Synthesis of {μ-LAl(H)OTf}2 (3). Under ice-cooling,
Me3SiOTf (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 3 the 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
C56H74Al2F6N6O6S2 [1159.31]: C, 58.02; H, 6.43; N, 7.25;
S, 5.53%). Decomp.: 235–245 °C.
1H NMR (200.1 MHz, CD3CN): δ = 7.46 (t, J = 8, 4 H,
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(CH3)2), 1.21 (n.r., 24 H, CH(CH3)2), 1.05 (d,
J = 7, 24 H, CH(CH3)2), n.o. (AlH); 13C{1H} NMR (50.3 MHz,
CD3CN): 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(CH3)2),
LH,38 Me3N·AlH3,71 and Me2S·BBr3 were synthesized accord-
61
ing to the reported procedures. Cf. the ESI† for the synthesis of
{LLi}2 (8).
Synthetic procedures
Synthesis of {μ-L(AlH2)}2 (2). A solid mixture of LH 25.7 (CH(CH3)2), 22.8 (br, CH(CH3)2), n.o. (CF3); 19F NMR
(1, 5.12 g, 12.7 mmol) and Me3N·AlH3 (1.13 g, 12.7 mmol) was (188.3 MHz, CD3CN): δ = −78.3, −79.3 (CF3).
cooled to −78 °C. Under stirring, toluene (40 mL) was added
HRMS: m/z found (calc.): 1009.5107 [M − C − 3F − 3O − S]+
via a syringe and the reaction was allowed to proceed for 2 h (100%), (1009.5126); 1157.4551 [M − H]+ (8), (1157.4568);
with the cooling applied followed by a period of 24 h at room 1159.4712 [M + H]+ (5), (1159.4724).
temperature (caution: gas evolution!). From the resulting solu-
Synthesis of {μ-L(Al(BH4)2)}2 (4). In a Schlenk tube
tion the volatiles were evaporated under reduced pressure and equipped with a PTFE-coated magnetic stirrer bar and a
the residue was recrystallized from a hot mixture of hexane– rubber septum 2 (0.366 g, 0.42 mmol) was dissolved in toluene
toluene (60 mL/10 mL). After crystal formation had been com- (5 mL) and the resulting mixture was cooled to −78 °C. Under
pleted at room temperature the supernatant was decanted and stirring,
a freshly prepared solution (0.5 M) of borane
stored at low temperature (4 °C and −30 °C consecutively) for dimethylsulphide complex in toluene (3.5 mL, 1.8 mmol) was
3 d yielding a second crop. By this method, single crystals suit- added dropwise via a syringe over a period of 5 min. The reac-
able for X-ray diffraction analysis could be obtained. The tion mixture was slowly warmed to room temperature over-
crystal fractions were combined and dried in vacuo. Yield: night. A colourless suspension formed that was stirred for an
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additional 6 h, after which the precipitation was completed by
Synthesis of {μ-LAlCl2}2 (6). A stirring suspension of freshly
gradual addition of hexane (9 mL). The septum was replaced sublimed AlCl3 (0.38 g, 2.8 mmol) in toluene (8 mL) was
by a glass stopper, and the solid was allowed to sediment over- cooled to 0 °C and treated dropwise with a solution of {LLi}2
night. Via a syringe, the supernatant was withdrawn (16 mL) (8, 1.11 g, 1.4 mmol) in toluene (19 mL) over a period of
and the residual solvent was evaporated under reduced 10 min. The resulting reaction mixture was stirred at 0 °C for
pressure. The remaining colourless powder was dried in vacuo 30 min, then the cooling was removed and the reaction contin-
(3 h, 9 × 10−2 mbar), and stored under nitrogen for 2 d before ued for 12 h at room temperature. After filtration via a canula
a sample was subjected to elemental analysis. Yield: 0.27 g, (glass fiber-coated inlet) the solvent was evaporated from the
67% (Found: C, 71.43; H, 9.40; N, 8.94%. Calc. for pale yellow solution under reduced pressure, the residue was
C54H88Al2B4N6 [918.52] × 0.5 C7H8 [92.14]: C, 71.60; H, 9.61; washed with hexane (two portions of 15 mL, removed 13 mL of
N, 8.71%). Decomp.: >260 °C.
supernatant each time), and the crude product was dried
IR: ˜ν/cm−1 = 2492, 2431 (BH). 1H NMR (500.1 MHz, in vacuo. Yield: 0.74 g, 54% (cf. the ESI† for depiction of the
THF-d8): δ = 7.33 (t, J = 8, 4 H, DippH-4), 7.20 (d, J = 8, 4 H, NMR spectra of the crude product in CDCl3). Single crystals
DippH-3,5), 7.16 (d, J = 8, 4 H, DippH-3,5), 6.58 (s, 4 H, NCH), suitable for X-ray diffraction analysis were obtained after
3.18 (sept, J = 7, 4 H, CH(CH3)2), 2.97 (sept, J = 7, 4 H, dilution of the filtered reaction mixture with hexane (10 mL)
CH(CH3)2), 1.29 (d, J = 7, 12 H, CH(CH3)2), 1.23 (d, J = 7, 12 H, and storage at 4 °C for 1 d.
CH(CH3)2), 1.11 (d, J = 7, 12 H, CH(CH3)2), 0.89 (d, J = 7, 12 H,
1H NMR (500.1 MHz, C6D6): δ = 7.30 (t, J = 8, 4 H, DippH-4),
CH(CH3)2), −0.34 (br, h1/2 = 170 Hz, 16 H, BH); 13C{1H} NMR 7.22 (d, J = 8, 8 H, DippH-3,5), 5.87 (s, 4 H, NCH), 3.17 (sept,
(125.8 MHz, THF-d8): δ = 151.5 (NCN), 148.2 (ArC), 147.6 (ArC), J = 7, 8 H, CH(CH3)2), 1.54 (d, J = 7, 24 H, CH(CH3)2), 1.00 (d,
135.0 (ArC), 131.4 (ArC), 125.8 (ArC), 124.8 (ArC), 119.9 (NCH), J = 7, 24 H, CH(CH3)2); 13C{1H} NMR (125.8 MHz, C6D6):
29.4 (CH(CH3)2), 28.8 (CH(CH3)2), 27.2 (CH(CH3)2), 24.4 δ = 151.8 (NCN), 147.6 (DippC-1), 133.5 (DippC-2,6), 131.1
(CH(CH3)2), 23.7 (CH(CH3)2), 23.4 (CH(CH3)2); 11B NMR (DippC-4), 125.0 (DippC-3,5), 118.3 (NCH), 28.9 (CH(CH3)2),
(64.2 MHz, THF-d8): δ = −37 (n.r., h1/2 = 380 Hz); 11B{1H} NMR 26.0 (CH(CH3)2), 23.8 (CH(CH3)2).
(64.2 MHz, THF-d8): δ = −37 (h1/2 = 270 Hz).
HRMS: m/z found (calc.): 875.6002 [M − 10 H − 3B]+ (11%), (1001.4252); 981.4605 [M − Cl + 2 H + O]+ (26), (981.4620).
(875.6012).
HRMS: m/z found (calc.): 1001.4233 [M + H]+ (28%),
Synthesis of {μ-LAlI2}2 (7). In the drybox, a solid mixture of
Synthesis of {μ-LAlBr2}2 (5). A Schlenk tube equipped with a 2 (0.511 g, 0.59 mmol) and boron triiodide (0.488 g,
PTFE-coated magnetic stirrer bar and a glass stopper was 1.25 mmol) was prepared in a Schlenk vessel equipped with a
charged with 2 (0.223 g, 0.26 mmol) and Me2S·BBr3 (0.175 g, PTFE-coated magnetic stirrer bar and a rubber septum.
0.56 mmol) in the dry box. Under Schlenk conditions, the Under Schlenk conditions, it was cooled to −78 °C and, with
solid mixture was cooled to 0 °C and, with stirring, toluene stirring, toluene (8 mL) was gradually added via a syringe
(4.5 mL) was gradually added via a syringe. Stirring was contin- along the inner glass wall of the cold reaction vessel. The
ued at 0 °C for 20 min followed by replacement of the cooling resulting reaction mixture was slowly allowed to warm to
for an oil-bath which was heated to 80 °C. After a short induc- room temperature overnight. After 18 h the solvent was evap-
tion period, the Schlenk vessel was isolated from the inert-gas orated under reduced pressure and the colourless solid
system by closing the tap and the reaction proceeded at this residue was dried in vacuo. It was redissolved in a mixture of
temperature for 3 d. Using a frit, the warm reaction mixture hexane–THF (6 mL/12 mL) and, using a frit, a slight amount
was filtered in order to separate a slight amount of insoluble of insoluble material was removed by filtration. The filtrate
material. From the filtrate, crystals formed upon standing at was stored at 4 °C for 24 h. Separation of the crystalline
room temperature overnight. In order to complete the crystalli- product had commenced and was completed at −30 °C
zation process it was stored at 4 °C and at −30 °C for a period for a period of 2 d. The fraction, thus obtained, contained
of 24 h each time. By this method, crystals suitable for X-ray single crystals suitable for X-ray diffraction analysis. The
diffraction analysis were obtained. After decantation of the cold supernatant was decanted and the solid was dried
cold supernatant the solid residue was dried in vacuo for 3 h in vacuo. Yield: 0.74 g, 92% (Found: C, 47.81; H, 5.63;
and stored for 1 d under nitrogen before it was subjected to N, 5.87%. Calc. for C54H72Al2I4N6 [1366.77]: C, 47.45; H, 5.31;
elemental analysis. Yield: 0.19 g, 57% (Found: C, 58.05; N, 6.15%).
H, 6.63; N, 6.62%. Calc. for C54H72Al2Br4N6 [1178.77] × C7H8
[92.14]: C, 57.65; H, 6.34; N, 6.61%).
1H NMR (500.1 MHz, C6D6): 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),
1H NMR (500.1 MHz, C6D6): δ = 7.31 (t, J = 8, 4 H, DippH-4), 5.76 (s, 4 H, NCH), 3.61 (sept, J = 7, 4 H, CH(CH3)2), 3.44 (sept,
7.22 (d, J = 8, 8 H, DippH-3,5), 5.84 (s, 4 H, NCH), 3.31 (sept, J = 7, 4 H, CH(CH3)2), 1.63 (d, J = 7, 12 H, CH(CH3)2), 1.60 (d,
J = 7, 8 H, CH(CH3)2), 1.57 (d, J = 7, 24 H, CH(CH3)2), 0.99 (d, J = 7, 12 H, CH(CH3)2), 1.03 (d, J = 7, 12 H, CH(CH3)2), 0.93 (d,
J = 7, 24 H, CH(CH3)2); 13C{1H} NMR (125.8 MHz, C6D6): J = 7, 12 H, CH(CH3)2); 13C{1H} NMR (125.8 MHz, C6D6):
δ = 151.5 (NCN), 147.6 (DippC-1), 134.3 (DippC-2,6), 131.3 δ = 151.2 (NCN), 147.6 (ArC), 147.4 (ArC), 136.0 (ArC), 131.6
(DippC-4), 125.3 (DippC-3,5), 119.1 (NCH), 28.9 (CH(CH3)2), (ArC), 126.2 (ArC), 125.5 (ArC), 120.4 (NCH), 28.9 (CH(CH3)2),
24.3 (CH(CH3)2), 20.8 (CH(CH3)2).
28.8 (CH(CH3)2), 26.5 (CH(CH3)2), 25.9 (CH(CH3)2), 24.4
HRMS: m/z found (calc.): 1178.2132 [M]+ (100%), (1178.2142). (n.r., 2 × CH(CH3)2).
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HRMS: m/z found (calc.): 1239.2567 [M − I]+ (100%), 13 M. D. Francis, D. E. Hibbs, M. B. Hursthouse, C. Jones
(1239.2583); 1257.2661 [M − I + 2 H + O]+ (75), (1257.2689);
and N. A. Smithies, J. Chem. Soc., Dalton Trans., 1998,
3249.
1367.1680 [M + H]+ (6), (1367.1706).
14 R. J. Baker, M. L. Cole, C. Jones and M. F. Mahon, J. Chem.
Soc., Dalton Trans., 2002, 1992.
Single crystal structure determination
15 S. Singh and H. W. Roesky, J. Fluorine Chem., 2007, 128,
369.
16 S.-i. Orimo, Y. Nakamori, J. R. Eliseo, A. Züttel and
C. M. Jensen, Chem. Rev., 2007, 107, 4111.
Data for the single-crystal structure determination of 2, 3, 4,
and 7 were collected on an Agilent SuperNova diffractometer
equipped with a CCD area Atlas detector and a mirror mono-
chromator utilizing CuKα radiation (λ = 1.5418 Å). Data for the
single-crystal structure determination of 5, 6, and 8 were
collected on an Oxford-Diffraction Xcalibur diffractometer
equipped with a CCD area detector Sapphire S and a graphite
monochromator utilizing MoKα radiation (λ = 0.71073 Å).
The crystal structures were solved by direct methods and
refined on F2 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
4 were found in the electron-density map and were refined
without restraints of the interatomic distances.
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