Reactivity of NHC Alane Adducts towards N-Heterocyclic Carbenes and Cyclic (Alkyl)(amino)carbenes: Ring Expansion, Ring Opening, and Al?H Bond Activation

Article abstract of DOI:10.1002/chem.201702166

The synthesis of mono-NHC alane adducts of the type (NHC)?AlH₃ (NHC=Me₂Im (1), Me₂Im^Me (2), iPr₂Im (3 and [D₃]-3), iPr₂Im^Me (4), Dipp₂Im (10); Im=imidazolin-2-ylidene, Dipp=2,6-diisopropylphenyl) and (NHC)?AliBu₂H (NHC=iPr₂Im (11), Dipp₂Im (12)) as well as their reactivity towards different types of carbenes is presented. Although the mono-NHC adducts remained stable at elevated temperatures, ring expansion occurred when (iPr₂Im)?AlH₃ (3) was treated with a second equivalent of the carbene iPr₂Im to give (iPr₂Im)?AlH(RER-iPr₂ImH₂) (6). In 6, {(iPr₂Im}AlH} is inserted into the NHC ring. In contrast, ring opening was observed with the sterically more demanding Dipp₂Im with the formation of (iPr₂Im)?AlH₂(ROR-Dipp₂ImH₂)H₂Al?(iPr₂Im) (9). In 9, two {(iPr₂Im)?AlH₂} moieties stabilize the ring-opened Dipp₂Im. If two hydridic sites are blocked, the adducts are stable with respect to further ring expansion or ring opening, as exemplified by the adducts (iPr₂Im)?AliBu₂H (11) and (Dipp₂Im)?AliBu₂H (12). The adducts (NHC)?AlH₃ and (iPr₂Im)?AliBu₂H reacted with cAAC^Me by insertion of the carbene carbon atom into the Al?H bond to give (NHC)?AlH₂/iBu₂(cAAC^MeH) (13–18) instead of ligand substitution, ring-expansion, or ring-opened products.

Full text of DOI:10.1002/chem.201702166

A Journal of
Accepted Article
Title: The Reactivity of NHC Alane Adducts towards N-Heterocyclic
Carbenes and Cyclic (Alkyl)(Amino) Carbenes: Ring Expansion,
Ring Opening, and Al-H Bond Activation
Authors: Udo Radius, Heidi Schneider, Andreas Hock, and Rüdiger
Bertermann
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201702166
Link to VoR: http://dx.doi.org/10.1002/chem.201702166
Supported by

10.1002/chem.201702166
Chemistry - A European Journal
FULL PAPER
The Reactivity of NHC Alane Adducts towards N-Heterocyclic
Carbenes and Cyclic (Alkyl)(Amino) Carbenes: Ring Expansion,
Ring Opening, and Al-H Bond Activation
Heidi Schneider, Andreas Hock, Rüdiger Bertermann, and Udo Radius *[a]
Dedicated to Professor Dieter Fenske on the occasion of his 75th birthday
Abstract: The synthesis of the mono NHC alane adducts of the type
R
R
Ar
N
N
N
(NHC)·AlH3 (NHC = Me2Im 1, Me2ImMe 2, iPr2Im 3 and 3-d3, iPr2ImMe
4, Dipp2Im 10) and (NHC)·AliBu2H (NHC = iPr2Im 11, Dipp2Im 12) as
well as their reactivity towards different types of carbenes is
presented. While the mono NHC adducts remain stable at elevated
temperatures, ring expansion reaction occurs when (iPr2Im)·AlH3 3 is
reacted with a second equivalent of the carbene iPr2Im to give
(iPr2Im)·AlH(RER-iPr2ImH2) 6. In 6, {(iPr2Im}AlH} is inserted into the
NHC ring. In contrast, ring opening is observed with sterically more
demanding Dipp2Im with formation of (iPr2Im)·AlH2(ROR-
Dipp2ImH2)H2Al·(iPr2Im) 9. In 9, two {(iPr2Im)·AlH2} moieties stabilize
the ring opened Dipp2Im. If two hydridic sites are blocked, the adducts
are stable with respect to further ring expansion or ring opening, as
exemplified by the adducts (iPr2Im)·AliBu2H 11 and (Dipp2Im)·AliBu2H
12. The reaction of (NHC)·AlH3 and (iPr2Im)·AliBu2H with cAACMe
yields instead of ligand substitution, ring expansion or ring opening
the products of an insertion of the carbene carbon atom into the Al–H
bond (NHC)·AlH2/iBu2(cAACMeH) 13–18.
+
RmEHX
R
N
H
H
EHnRm
X EHnRm
X
E
N
R
R
Ar
R
R
N
N
N
R
Ar
H/F
=
E
Be,
Si
B,
N
=
E
P
Si,
=
Ar
vs.
E
H, B,
Ar
O
P
Si,
C-F
C-H
Bond Activation
C, N,
E-X Bond Activation
B
O
N
crucial
or
intermediate
product
Ar
Ring
Expansion
O
H
H
B
EHnRm
O
+
R
R
N
N
Ar
N
EHnRm
Ar
=
E
Sn
P,
Reversible B-C
Bond Activation
E-E
Dehydrocoupling
Scheme 1. Carbene-mediated reactions of main-group element hydrides
Introduction
Research on N-Heterocyclic Carbenes (NHCs) and related
(occupied sp2 orbital) site which enables singlet carbenes to
undergo element element bond activation reactions beyond
simple deprotonations (Scheme 1).
[1]
molecules
demonstrated in the last decade clearly that their
application is not limited to the use as (spectator) ligands in
transition metal chemistry and catalysis or to their application as
It was demonstrated earlier, for example, that cyclic
(alkyl)(amino) carbenes (cAACs) provide a robust environment for
Si–H, B–H, and P–H bond activation, and most recently we and
others reported that even C–H and C–F[7] bond activation occurs
with formation of the E–H respectively E–E bond cleavage
products. In addition, we also showed recently that insertion of
cAACs into the B-C bond of selected arylboronate esters is
reversible and this reversibility may lead to the development of
novel carbon centered organocatalysts.[8]
We currently assume that E–H (E = H, B, C, N, Si, P) or E–E
bond activation is also the key step for any further reactivity of
NHCs and cAACs. In the last few years reactions of NHCs with
Lewis acidic hydrides such as silanes[9], boranes[10] and beryllium
hydrides[11] have been reported, which lead to ring expansion of
the NHC, while the E–H or E–E bond activation is considered to
be the crucial step of the reaction sequence.[12] We also provided
lately the first example of a ring expansion reaction of a cAAC,
using aryl catechol boronate esters. The Lewis acid/base adduct
(cAAC)·BcatAr was observed, which subsequently reacts with
[2-4]
stabilizers for subvalent main group element compounds
.
Carbenes themselves show a rich diversity in their reaction
pattern towards main group element hydrides and organyls.[5] We
and others demonstrated earlier, that in dependence on the
electronic and steric properties of the carbene and on the main
group element compound used, various reaction channels are
observed. With simple Brønsted acids like alcohols and hydrogen
halides usually the basic character of the carbene dominates and
deprotonation to the corresponding imidazolium salts occurs.[6]
However, the carbene carbon atom combines a Lewis acidic
(empty vacant pz orbital) and a Lewis basic ‘
[a]
H. Schneider, Andreas Hock, Dr. R. Bertermann, Prof. Dr. U. Radius
Institut für Anorganische Chemie
Julius-Maximilians-Universität Würzburg
Am Hubland, D-97074 Würzburg (Germany)
E-mail: u.radius@uni-wuerzburg.de
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/chem.201702166
Chemistry - A European Journal
FULL PAPER
ring expansion of the cAAC. Furthermore, we have also shown for
group 15 element hydrides that secondary and primary
phosphines may be converted with NHCs into diphosphines or
cyclooligophosphines by reductive dehydrocoupling,[9b] and
closely related dehydrocoupling was also observed for organo tin
hydrides. [13] E–H bond cleavage is likewise considered to be the
initial step for this reaction channel, leading to an instable E–H
activation product, which in turn leads to further reactions.
stabilized aluminum cation [(Dipp2Im)·AlH2][B(C6F5)3][20]. Due to
our experience on the reactivity of carbenes with phosphines,
silanes, tin hydrides and especially boranes and the existing
reports on the instability of some NHC aluminum compounds, we
decided to take a closer look on the behavior of carbenes with
different aluminum hydrides and we wish to report first results
here.
Bertrand
and
coworkers
demonstrated
earlier,
that
alkyl(amino)carbene mediated P–H bond activation of
Results and Discussion
phosphines provides the stable E–H oxidative addition products
[14]
Lewis acid-base adducts[15] with AlH3 are known for quite a
while now. There are several approaches to synthesize such
Lewis acid-base adducts: One method involves the reaction of
AlH3 (usually obtained from the reaction of LiAlH4 with H2SO4) with
a two electron donor (e.g. amines and phosphines). These
adducts are in some cases also accessible using the reaction of
LiAlH4 directly and the ligand (or the ligand salt) with salt
without any further reaction.
This example demonstrates
nicely, that the progress after E–H bond activation at a carbene
center depends strongly on the stability of the E–H oxidative
addition product and thus on the main group element hydride
employed. However, the result of the reaction between a carbene
and a main group element hydride also depends on the nature of
the carbene. We investigated recently, for example, in some detail
the reactivity of different carbenes with catechol borane HBcat.
This borane forms with backbone unsaturated NHCs (nPr2Im,
iPr2Im, iPr2ImMe and Dipp2Im) Lewis acid-base adducts, which are
stable at higher temperatures and no E–H bond cleavage or ring
expansion reaction (RER) was observed for these molecules. In
contrast, E–H bond activation occurs for the reaction of HBcat
with the cyclic alkyl(amino)carbene cAACMe, but no RER of the
cAAC occurs. If the backbone saturated NHC Dipp2H2Im is used,
adduct formation is followed by B-H activation and subsequent
ring expansion reaction (vide infra) takes place. [10f]
Donor stabilized aluminum compounds have been of interest
for several years, mainly due to the manifold application of alanes
in organic and inorganic synthesis and lately also because of the
potential of aluminum compounds as hydrogen storage systems.
Besides the well-established chemistry of amine or phosphine
stabilized mono alane adducts and adducts with oxygen based
ligands, homoleptic (amine donors) as well as heteroleptic
(amin/phosphor donors) bis alane adducts are established.[15]
NHC aluminum chemistry is developed to some extend[15q, 16] and
it is known that Lewis acid-base adducts of aluminum hydrides
are rather unstable. Besides simple decomposition of the
compounds to unidentified molecules, NHC to aNHC
rearrangement in the coordination sphere of aluminum[17] and ring
elimination. A third important method is the substitution of the
trimethylamine ligand of the adduct NMe3·AlH3.[15b-g,
15i-w, 21]
In
dependence on the basicity and the steric demand of the ligand
used, formation of the alane adducts stabilized by one or two
bases (two electron donor ligands) may occur.[15w] Known
[15q, 16a-f, 16h, 16i, 18, 22]
examples
show that NHC stabilized alanes
have been synthesized so far mainly via salt elimination from the
reaction of NHCs with lithium aluminum hydride or from base
substitution of NMe3·AlH3.[15q, 16a-e, 16i, 18, 21-22] For the presented
study we opt to synthesize the (NHC)·AlH3 adducts via the LiAlH4
route.
The adducts (Me2Im)·AlH3 1, (Me2ImMe)·AlH3 2, (iPr2Im)·AlH3
3 and (iPr2ImMe)·AlH3 4 are readily accessible from the reaction of
lithium aluminum hydride with the corresponding N-heterocyclic
carbene at 0 °C in diethyl ether (Scheme 2). In course of the
reaction, lithium hydride precipitates and can be separated by
filtration. Further workup gave the adducts 1 to 4 as colorless
crystalline solids in excellent purity and in fair to good overall yield
(1: 56 %, 2: 62 %, 3: 73 %, 4: 63 %). This method is also perfectly
suitable to synthesize the corresponding deuterated adducts, as
exemplified by the synthesis of (iPr2Im)·AlD3 3-d3. 3-d3 is obtained
starting from commercially available LiAlD4 as a colorless solid in
57 % yield.
expansion reaction of
a NHC in β-diketiminato aluminum
The adducts 1 to 4 were characterized using multinuclear
NMR and IR spectroscopy and elemental analyses. The
characteristic Al–H bond stretching vibrations of the compounds
1 – 4 were observed in the range between 1644 to 1784 cm-1 (see
SI).[15g, 15i, 15k, 15q, 15r, 15w, 21, 23] For example, two IR bands were
recorded for (iPr2Im)·AlH3 3 as a broad signal at 1719 cm-1 and a
shoulder at 1776 cm-1 in the solid state. Three sharp IR bands
were calculated for gas phase (iPr2Im)·AlH3 3 at 1762, 1767 and
1787cm-1 (scaling factor 0.9914 [24]) using DFT at the BP86/def2-
SVP level. This assignment was further proven by a comparison
of the Al-H/D IR bands of (iPr2Im)·AlH3 3 and its deuterated
isotope (iPr2Im)·AlD3 3-d3, which are significantly shifted to lower
wavenumbers at 1250 cm-1. This is in agreement with the
observation made for the amine adducts NMe3·AlH3 (the Al-H
stretch was detected as a broad band at 1792 cm-1) and
dihydrides was observed.[16i] However, the utilization of carbenes
in aluminum chemistry also facilitate the isolation of interesting
substances with new molecular structures such as
(Dipp2Im)·H2Al–AlH2·(Dipp2Im), obtained by magnesium(I)
mediated dehydrogenative coupling of (Dipp2Im)·AlH3
cAAC stabilized neutral aluminum radical[19] and the NHC
[16f, 18]
or a
R'
R'
:
1
:
2
:
3
:
4
=
=
=
=
=
=
=
R
R
R
R
Me R'
Me R' Me
iPr R'
H
+
LiAlH4
-
R
R
N
N
N
N
R
R
H
Et
rt,
0°C
2O,
Al
=
iPr R' Me
R'
R'
H
-LiH
H
H
Scheme 2. Synthesis of the compounds (Me2Im)·AlH3 1,
(Me2ImMe)·AlH3 2, (iPr2Im)·AlH3 3, and (iPr2ImMe)·AlH3 4.
This article is protected by copyright. All rights reserved.

10.1002/chem.201702166
Chemistry - A European Journal
FULL PAPER
solution between 105.8 and 107.8 ppm and are thus in good
agreement to the know NHC·AlH3 adducts reported so far,
Table 1. Selected NMR shifts (ppm) of the compounds (Me2Im)·AlH3 1,
(Me2ImMe)·AlH3 2, (iPr2Im)·AlH3 3, (iPr2ImMe)·AlH3 4, (Dipp2Im)·AlH3 10,
(iPr2Im)·AliBu2H 11, and (Dipp2Im)·AliBu2H 12.
(iPr2ImMe)·AlH3 (δ
107 ppm)[16a]
= =
106 ppm)[16b] and (Mes2Im)·AlH3 (δ
.
Al-H
NCN
27Al{1H} NMR
105.8 [a]
In addition to the NMR spectra in solution, we recorded the
solid state NMR spectra of selected complexes (see SI). The
resonances of the adducts 2 – 4 observed in the 13C{1H} and
27Al{1H} solid state NMR spectra are in good agreement to those
detected in solution. These results provide some evidence that
the Lewis acid-base adducts 1 to 4 adopt the same structures in
both solution and solid state (for the X-ray crystal structures see
below). The 27Al{1H} HPDec/MAS NMR spectrum of (iPr2Im)·AlH3
3 shows, for example, a signal with an isotropic shift at 105.8 ppm
with the typical line shape (MAS second-order quadrupole powder
pattern) for a tetrahedral sp3-Al atom with a quadrupolar coupling
constant of 9967 kHz and a quadrupolar asymmetry parameter of
0.07. A second isotropic shift for a conformer is detected at
105.3 ppm with a quadrupolar coupling constant of 9710 kHz and
a quadrupolar asymmetry parameter of 0.07 (see Figure 1). The
signals of both conformers are in perfect agreement with the
resonance at 106.3 ppm found in solution for (iPr2Im)·AlH3 3. The
15N VACP/MAS NMR spectrum shows one resonance at -
167.3 ppm for the nitrogen atoms of the NHC.
1H{27Al} NMR
13C{1H} NMR
(Me2Im)·AlH3 1
Me2ImMe)·AlH3 2
4.39
4.48
172.2
168.7
106.4 [a]
104.4 [b]
106.3 [a]
105.3/105.8[b]
(iPr2Im)·AlH3
3
4.53
170.3
107.8 [a]
(iPr2ImMe)·AlH3
4
4.60
3.67
4.71
3.97
170.0
178.2
173.1
180.5
103.6/103.8[b]
(Dipp2Im)·AlH3 10
(iPr2Im)·AliBu2H 11
(Dipp2Im)·AliBu2H 12
107.9 [b]
139.4 [a]
138.9 [b]
137.0 [b]
[a] 27Al{1H} NMR resonance in solution, [b] 27Al{1H} NMR resonance in the solid
state.
27Al{1H} NMR spectra in the solid state of the compounds
(Me2ImMe)·AlH3 2 and (iPr2ImMe)·AlH3 4 are likewise in perfect
accordance with the resonances observed in the solution NMR
spectra, which are compared to each other in Table 1, and show
a similar line shape as presented for compound 3.
The molecular structure of (iPr2Im)·AlH3 3 was confirmed by
X-Ray diffraction and is shown in Figure 2. The hydrogen atoms
bound to the aluminum atom of compound 3 have been located
on the electron density map and were refined isotropically. The
aluminum center is in a tetrahedral environment and coordinated
by one NHC ligand and three hydrogen atoms. The angles
spanned by the ligands are in a range between 104.1(8) and
113.5(10) °. The NHC aluminum as well as the aluminum hydride
bond lengths of 2.0375(17) and 2.0405(17) Å for Al–CNHC and
1.492(19) to 1.56(2) Å for Al–H are in good agreement to the bond
lengths of (Dipp2Im)·AlH3 (Al–NHC: 2.0556(13) Å; Al–H:
1.510(17) to 1.546(17) Å) [16d] reported earlier.
NMe3·AlD3 (the Al-D stretch was detected as a broad band at
1304 cm-1).[25]
The most important NMR shifts of the compounds 1 – 4 are
summarized in Table 1. The proton NMR spectra of the NHC·AlH3
adducts 1–4 in solution reveal sharp, significantly shifted
resonances for the NHC ligand, whereas the aluminum bound
hydrogen atoms appear as very broad peaks (range > 2.5 ppm)
due to the direct binding to the 27Al atom (l = 5/2) and the resulting
splitting in combination with the quadrupole moment (q =
14.7 fm2) of the aluminum nucleus. However, the aluminum
bound hydrogen atoms of the adducts 1 to 4 can be detected in
1
the H{27Al} NMR spectrum in the range from 4.39 to 4.60 ppm.
The proton NMR of the adduct 3 shows, for example, a doublet at
0.94 ppm for the methyl groups and a septet at 5.25 ppm for the
methine protons of the iso-propyl substituent. The singlet of the
backbone protons arises at 6.23 ppm, whereas the aluminum
1
bound hydrides were detected as a broad singlet in the H{27Al}
NMR spectrum at 4.53 ppm.
In the 13C{1H} NMR spectrum, the signals of (iPr2Im)·AlH3 3
were detected at 22.9 (iPr-CH3), 51.6 (iPr-CH) and 117.0
(NCHCHN) ppm. The carbene carbon resonances of the
(NHC)·AlH3 adducts are usually also difficult to detect due to the
quadrupole moment of the 27Al core and appear as strongly
broadened signals. The carbene carbon atom of (iPr2Im)·AlH3 3,
for example, gives rise to a signal at 170.3 ppm, significantly
shifted compared to the resonance of uncoordinated iPr2Im at
211.6 ppm. In general, the NHC-AlH3 adducts 1 – 4 with alkyl
substituted NHCs reveal carbene carbon resonances in the range
between 168.7 and 173.1 ppm. 27Al{1H} NMR resonances of the
alane adducts are typically observed in a region between 100 and
140 ppm. (tBu3P)·AlH3, for example, reveals
a signal at
Figure 1.27Al{1H} HPDec/MAS NMR spectrum of (iPr2Im)·AlH3 3.
121.8 ppm[15i], whereas the resonance of (Me3N)·AlH3 is detected
at 139.8 ppm.[15s] Compounds (Me2Im)·AlH3 1, (Me2ImMe)·AlH3 2,
(iPr2Im)·AlH3 3, and (iPr2ImMe)·AlH3 4 show broad singlets in
This article is protected by copyright. All rights reserved.

10.1002/chem.201702166
Chemistry - A European Journal
FULL PAPER
N
N
N
N
N
N
H
H
H
Al
Al
H
H
N
N
H
toluene, 70°C
3
6
0.5
Dipp
N
H
Dipp
H
Dipp
Dipp
N
N
Al
N
Al
N
N
H
H
N
N
H
toluene, 70°C
H
Figure 2. Molecular structure of (iPr2Im)·AlH3 3 in the solid state (ellipsoids set
at 50% probability level). Selected bond lengths [Å] and angles [°]: Al1–C1
2.0405(17), Al1–H1 1.54(2), Al1–H2 1.56(2), Al1–H3 1.492(19), Al2–C11
2.0375(17), Al2–H4 1.53(2), Al2–H5 1.52(2), Al2–H6 1.53(2). C1-Al1-H1
108.5(7), C1-Al1-H2 104.1(8), C1-Al1-H3 105.4(8), H1-Al1-H2 112.1(11), H1-
Al1-H3 113.5(10), H2-Al1-H3 112.5(11), C11-Al2-H4 104.5(8), C11-Al2-H5
109.2(7), C11-Al2-H6 106.5(8), H4-Al2-H5 112.2(10), H4-Al2-H6 111.4(11), H5-
Al2-H6 112.5(11).
9
Scheme 3. Ring expansion and ring opening reaction of
(iPr2Im)·AlH3 3 with iPr2Im and Dipp2Im.
mixture lies in the resonance of the isopropyl methine protons,
which shifts from 5.25 ppm in 3 to 4.86 ppm and appears as a
broad resonance at higher temperatures (e.g. at 70°C). Similarly,
the 13C{1H} NMR spectrum of the solution reveals only one set of
signals and, most significantly, a distinct down field shift of the
NHC carbene carbon atom from 170.3 ppm for (iPr2Im)·AlH3 3 to
188.9 ppm, while the aluminum resonance is detected at
107.2 ppm.
Once we realized that trans-(iPr2Im)2·AlH3 5 might be an
important intermediate for this reaction, we synthesized and
isolated this compound. Starting from iPr2Im and (iPr2Im)·AlH3 3,
trans-(iPr2Im)2·AlH3 5 was isolated in 40 % yield as a colorless
solid. The resonances observed in the 1H and 13C{1H} NMR
spectra were identical to those observed in solutions investigated
for the ring opening process (vide supra). The aluminum
resonance was found at 107.2 ppm, only marginally shifted from
The NHC adducts 1 to 4 are thermally stable in solution at
least to the boiling point of toluene. Thus, a flame sealed NMR
tube of 3 in toluene-d8 was kept at 110 °C for several days without
a change of its 1H NMR spectrum. However, in course of our work
we became aware of instabilities that already occur at room
temperature. If an excess (more than one equivalent) of the NHC
iPr2Im was added to a suspension of lithium aluminum hydride, a
side product was observed in significant amounts with a rather
complicated NMR spectrum. The proton NMR spectra of these
solutions reveal aside from the symmetric resonance pattern of
the iPr2Im moiety of 3 additional resonances, indicating an
unsymmetrical iPr2Im species. In a following stoichiometric
reaction of isolated (iPr2Im)·AlH3 3 with one equivalent iPr2Im at
70 °C for 16 h, a change of the color of the solution from colorless
(color of 3) to bright orange was observed. After workup, a bright
yellow solid was isolated, highly soluble in commonly used
organic solvents like n-hexane, toluene, diethyl ether and thf and
reactive towards chlorinated solvents such as CH2Cl2 and CHCl3.
The yellow product, which was obtained in yields up to 63 %, is
highly sensitive towards air and moisture. This compound was
then identified as the ring expansion product (iPr2Im)·AlH(RER-
iPr2ImH2) 6 (RER = ring expansion reaction) using multinuclear
NMR spectroscopy, high resolution mass spectrometry and
elemental analysis. Heating of a d6-benzene solution containing
equimolar amounts of (iPr2Im)·AlH3 3 and iPr2Im to 70 °C for 9 h
(see Figure 3) demonstrated that the reaction proceeds
quantitative to the reaction product (iPr2Im)·AlH(RER-iPr2ImH2) 6.
Immediately after addition of the NHC iPr2Im to the adduct
(iPr2Im)·AlH3 3 only one set of protons of iPr2Im and (iPr2Im)·AlH3
3 is detected at room temperature, which either indicates the initial
formation of the bis NHC aluminum hydride adduct trans-
(iPr2Im)2·AlH3 5 with intact NHC moieties at the aluminum center
or a rapid exchange of the NHCs on the NMR time scale. The
major difference between the NMR spectrum of 3 and that of this
the resonance of
3 at 106.3 ppm. Trans-(iPr2Im)2·AlH3 5
rearranges even in the solid state slowly to 6, which is
recognizable by the change of the color of the solid from colorless
to yellow. (iPr2Im)·AlH(RER-iPr2ImH2) 6 was also synthesized in
solution from isolated 5. The formation of 6 can be easily spotted
in the 1H NMR spectrum (Figure 3 and Figure S43 in the SI), since
the resonance of the NHC backbone at 6.45 ppm splits into one
resonance at 6.38 ppm for the intact NHC attached to aluminum
and into two doublets at 4.85 and 5.33 ppm for the asymmetrical
NHC backbone of the ring expanded NHC. The resonances of the
methine protons at 4.86 ppm also split into one set at 5.61 ppm
(intact NHC) and into two septets at 3.12 ppm and 3.30 ppm
(methine protons of the ring expanded iPr2Im moiety). The
diastereotopic AlCH2N protons are detected as two broad
doublets at 70°C (Figure 3) at 1.86 ppm and 2.37 ppm, but lead
to sharp roofed doublets at room temperature with a geminal
coupling constant of 12.3 Hz (see Figure S37, SI). The 27Al{1H}
NMR resonance of 6 is detected as a broad singlet at 119.9 ppm,
significantly shifted from the resonance of (iPr2Im)·AlH3 3 in
solution at 106.3 ppm. The 13C{1H} NMR spectrum shows a
characteristic high field resonance for the former carbene carbon
atom AlCH2N at 29.7 ppm and two signals for the endocyclic
methine carbon atoms at 53.1 and 57.6 ppm as well as for the
methyl groups of the iso-propyl arms of the ring expanded NHC
moiety at 18.9 ppm. The intact iPr2Im moiety gives rise to a broad
This article is protected by copyright. All rights reserved.

10.1002/chem.201702166
Chemistry - A European Journal
FULL PAPER
might be explained by a bimolecular
reaction mechanism, which would
contrast our conclusions drawn from
time dependent NMR experiment.
However, 6-[H/D] may also originate
from H/D exchange between the
adducts
(iPr2Im)·AlH3
3
and
(iPr2Im)·AlD3 3-d3 before the bis NHC
adduct trans-(iPr2Im)2·AlH3 5 is even
formed. This is indeed the case, and
was verified by the independent
reaction of (iPr2Im)·AlH3
3
and
(iPr2Im)·AlH3 3-d3 in a 2:1 ratio in C6D6.
Immediately after addition of the solvent
a new set of signals was detected,
which is shifted from the resonances of
3 and 3-d3. The resonances of the
aluminum
marginally
bound
shifted
protons
at 4.52 ppm
arise
compared those of (iPr2Im)·AlH3 3 at
4.53 ppm, while the other resonances
are detected at 0.98 (iPr-CH3, 3:
Figure 3. Time-dependent 1H NMR spectrum of the reaction of (iPr2Im)·AlH3 3 with iPr2Im at 70 °C in C6D6
(blue asterisk: (iPr2Im)·AlH3 3 + iPr2Im = trans-(iPr2Im)·AlH3 5, red asterisk: (iPr2Im)·AlH(RER-iPr2ImH2) 6).
0.94 ppm, 3-d3: 0.95 ppm), 5.22 (iPr-
resonance of the carbene carbon atom at 169.8 ppm, marginally
shifted from 170.3 ppm observed for (iPr2Im)·AlH3 3. As can be
seen in Figure 3, no further intermediate was detected in solution.
The infrared spectrum of isolated 6 reveals only one characteristic
band for the Al–H stretching vibration at 1747 cm-1.
CH, 3: 5.25 ppm, 3-d3: 5.23 ppm), and 6.39 (NCHCHN, 3:
6.23 ppm, 3-d3: 6.30 ppm). The same behavior is observed for the
resonance of the deuterated compound 3-d3 in the deuterium
NMR spectra. The signal of (iPr2Im)·AlH2D 3-d1 is observed at
4.55 ppm, while the deuterium resonance of (iPr2Im)·AlD3 3-d3
arises at 4.57 ppm. In the 13C{1H} NMR spectrum the signal of the
carbene carbon atom appears slightly shifted at 169.7 ppm (3:
170.3 ppm, 3-d3: 170.3 ppm). The most significant evidence is
found in the infrared spectra of the compound, where the broad,
intensive bands of (iPr2Im)·AlH3 3 at 1719 cm-1 and (iPr2Im)·AlD3
3-d3 at 1250 cm-1 are missing. Thus, we strongly believe that the
emergence of the cross product originates from H/D exchange
already between the starting material (iPr2Im)·AlH3 3 and
A conversion vs. time diagram of the ring expansion reaction
(Figure S100) was created from the time dependent NMR study
by integration of the resonances of the starting material (integral
of the backbone of 3 minus integral of the splitted methine protons
at 3.12 and 3.30 ppm of the ring opened NHC in 6) and of the
product (integral of the splitted methine protons at 3.12 and
3.30 ppm of 6). The analysis of the kinetics (Figure S100-S103)
reveals that the monomolecular decrease of the concentration of
(iPr2Im)2·AlH3 5 and the formation of 6 follows a first order rate
law (Figure S101). In accordance with our NMR studies (formation
of a bis NHC aluminum adduct 5 after addition of iPr2Im to a
solution of (iPr2Im)·AlH3 3 in C6D6) and the kinetics presented
here we propose a monomolecular reaction mechanism (Scheme
(iPr2Im)·AlH3 3-d3. Furthermore, we assume
a
reaction
mechanism as provided in Scheme 4: [9a, 12a] In the first step the
bis NHC adduct is formed, followed from NHC insertion into one
of the Al-H bonds of (NHC)·AlH3 to give intermediate A.
Subsequent amido transfer from the NHC carbon atom to the
aluminum atom leads to intermediate B, which finally reacts with
a hydride shift from the aluminum atom to the (former) carbene
carbon atom with formation of the product 6.
4) starting from the bis NHC adduct
5 and leading to
(iPr2Im)·AlH(RER-iPr2ImH2) 6 via an intramolecular insertion of
the {(iPr2Im)·AlH} moiety into the ring of the second iPr2Im unit.
The rate constant is calculated to k = 7.32·10-5 s-1 with a half-life
of 158 min at 70 °C. In addition to the kinetics we performed
experiments with the deuterated adduct (iPr2Im)·AlD3 3-d3. The
reaction of this compound with iPr2Im afforded (iPr2Im)·AlD(RER-
iPr2ImD2) 6-d3, exclusively deuterated at the former NHC carbene
carbon atom. This demonstrates that the hydrogen atoms
transferred originate exclusively from (iPr2Im)·AlD3 3-d3 and that
the solvent is not involved in the reaction process. The reaction of
iPr2Im performed with a 1:1 ratio of (iPr2Im)·AlH3 3 and
(iPr2Im)·AlD3 3-d3 led to the formation of (iPr2Im)·AlH(RER-
iPr2ImH2) 6, (iPr2Im)·AlD(RER-iPr2ImD2) 6-d3 and the H/D cross
product (iPr2Im)·AlH/D(RER-iPr2ImHD) 6-[H/D]. The appearance
of the H/D cross product (iPr2Im)H/D(RER-iPr2ImHD) 6-[H/D]
We tried several times to obtain single crystals suitable for X-
ray diffraction of (iPr2Im)·AlH(RER-iPr2ImH2) 6 from saturated
solutions of the compound in different solvents, but failed. Usually,
the crystal quality of the material obtained was even too poor to
establish the connectivity properly. However, in one case we
isolated the hydrolyzed form of 6 as good quality crystals. The
result of the X-ray analysis of these crystals of the compound
[{(iPr2Im)·Al(RER-iPr2ImH2)}2(µ2-O)] 7 is shown in Figure 4 (left).
The central aluminum atom is coordinated in a distorted
tetrahedral environment by the carbene carbon atom and the
nitrogen atom of the ring expanded iPr2Im moiety, the intact NHC
molecule and the bridging oxygen atom. The NHC aluminum bond
This article is protected by copyright. All rights reserved.

10.1002/chem.201702166
Chemistry - A European Journal
FULL PAPER
aluminum
hydride
and
2
iPr
N
iPr
N
iPr
N
iPr
N
iPr
N
iPr
iPr
iPr
N
equivalents of iPr2Im in Et2O
(Figure S104 in the SI). In the
cationic part [(iPr2Im)4·AlH2]+ of
these ionic compounds, the
aluminum cation is surrounded by
four NHC ligands and two trans-
configured hydride ligands, which
span an almost ideal octahedral
coordination polyhedron around
the metal ion. In the anionic part of
8, one {AlH3} unit is coordinated in
a Lewis acid-base interaction by
the NHC in 2-position, while
another {AlH3} moiety is bound to
the 4-position at the backbone of
the same NHC, i.e. one iPr2Im
ligand coordinates in its “normal”
and “abnormal” mode and thus
bridges two {AlH3} units. The bond
distances between the aluminum
N
N
H
H
H
N
N
iPr
iPr
Al
Al
N
Al
Al
N
iPr
N
iPr
N
iPr
N
iPr
N
iPr
i
ii
iPr
H
H
H
H
H
H
H
H
H
A
3
5
bis NHC alane adduct
hydrogen migration
iPr
N
iPr
iPr
iPr
H
N
iPr
N
H
N
H
N
N
H
iPr
H
H
H
H
H
N
N
Al
N
Al
N
Al
N
Al
H
N
N
N
iPr
iPr
iPr
N
iPr
N
iPr
iPr
iv
iii
H
H
iPr
iPr
iPr
iPr
6
B
ring stabilisation
bond cleavage
C-N
iPr
by migration
hydride
a second
of
+
insertion
{( 2Im)AlH2}
Scheme 4. Proposed mechanism for the ring expansion reaction of (iPr2Im)·AlH3
(iPr2Im)·AlH(RER-iPr2ImH2) 6 via the bis-NHC adduct trans-(iPr2Im)·AlH3 5.
3 with iPr2Im to give
distance corresponds with 2.0709(18) Å to the one
observed(iPr2Im)·AlH3 3 in with 2.0405(17) Å. The Al–CRER-NHC
bond length of 1.999(2) Å and the Al–NRER-NHC of 1.8514(17) Å do
not deviate from the literature known data of aluminum carbon
(1.99 Å [26]) and aluminum nitrogen (1.88 Å [16i]) single bonds.
We also tried several times to obtain single crystals suitable
for X-ray diffraction of trans-(iPr2Im)2·AlH3 5 from saturated
solutions of the compound in different solvents, but weren’t
successful likewise. Instead we characterized the compound
atom and the NHCs in the cation of approximately 2.179 Å are
larger compared to (iPr2Im)·AlH3 3 (Al–CNHC: 2.039 Å), as
expected for higher coordination numbers, whereas the Al-C
distances of the anionic part of 8 (2.048(3) Å and 2.006(3) Å) are
in perfect agreement to those observed for 3. Aluminum hydride
cations are relatively scarce and Stephan et al. presented only
recently the closely related compounds [(Dipp2Im)·AlH2][B(C6F5)4]
and [(Cy2Im)2AlH][B(C6F6)4], which are the first examples of NHC
stabilized aluminum cations.[16h] To our knowledge 8 is the first
example of a NHC stabilized cationic aluminum center in the
coordination number six. However, if crystals of 8 were dissolved
in solvents like benzene, toluene, diethyl ether or THF, we
isolated the adduct (iPr2Im)2·AlH3 5 as the main component,
usually contaminated with traces of 6. NMR investigations in C6D6
[(iPr2Im)4·AlH2][(H3AlaiPr2Im)·AlH3]
8
(Figure
4,
right)
crystallographically, which was obtained from the reaction of
(iPr2Im)·AlH3 3 with iPr2Im in Et2O repeatetly. Similarly, we
repeatedly crystallized [(iPr2Im)4·AlH2] [(iPr2Im)2Li(AlH4)2] with
one molecule (iPr2Im)·AlH3 3 in the unit cell starting from lithium
Figure 4. Molecular structure of the hydrolyses product of [{(iPr2Im)·Al(RER-iPr2ImH2)}2(µ2-O)] 7 (left) and of [(iPr2Im)4·AlH2][(H3AlaiPr2Im)·AlH3] 8 (right) in the solid
state (ellipsoids set at 50% probability level). Selected bond lengths [Å] and angles [°]: 7: Al–C1 2.0709(18), Al–C11 1.999(2), Al–N4 1.8514(17), Al–O1 1.6996(6).
C1-Al1-C11 108.01(8), C1-Al1-O1 108.38(6), C11-Al1-O1 114.48(6), N4-Al1-O1 118.69(6), C1-Al1-N4 104.10(7), C11-Al1-N4 102.29(8). 8: Al1–C1 2.181(3), Al1–
C11 2.189(3), Al1–C21 2.170(3), Al1-C31 2.176(3), Al1–H101 1.62(2), Al1–H100 1.60(3); C1-Al1-C11 91.43(11), C1-Al1-C21 179.23(12), C1-Al1-C31 89.94(11),
C11-Al1-C21 88.88(11), C11-Al1-C31 177.97(12), C21-Al1-C31 89.77(11), C1-Al1-H100 88.7(9), C1-Al1-H101 90.4(9), C11-Al1-H100 91.8(9), C11-Al1-H101 89.4(8),
C21-Al1-H100 90.6(9), C21-Al1-H101 90.3(9), C31-Al1-H100 89.7(9), C31-Al1-H101 89.1(8), H100-Al1-H101 178.5(13). Anion: Al2–C41 2.048(3), Al2–H104 1.52(3),
Al2–H103 1.55(3), Al2–H102 1.55(3), Al3–C42 2.006(3), Al3–H107 1.58(3), Al3–H106 1.51(3), Al3–H105 1.58(3); C41-Al2-H102 106.2(12), C41-Al2-H103 106.8(10),
C41-Al2-H104 107.2(11), H102-Al2-H103 111.1(16) H102-Al2-H104 115.1(17), H103-Al2-H104 110.0(15).
This article is protected by copyright. All rights reserved.

10.1002/chem.201702166
Chemistry - A European Journal
FULL PAPER
revealed the formation of 3, 5, 6, and some hydrogenated NHC
iPr2ImH2 from these crystals in solution. Thus, complex 5 seems
to be in a rather complicated equilibrium with other species in
solution, from which ionic 8 seems to crystallize preferentially.
The reaction of (iPr2Im)·AlH3 3 with the sterically more
demanding NHC Dipp2Im follows a different pathway (Scheme 3).
Instead of a ring expansion reaction the ring opening of the
Dipp2Im carbene was observed at 70 °C. The isolated product
(iPr2Im)·AlH2(ROR-Dipp2ImH2)H2Al·(iPr2Im) 9 was identified via
solution and solid state NMR spectroscopy, high resolution mass
spectrometry and X-ray diffraction. Compound 9 is, similar to 6,
highly sensitive towards air and moisture. The molecular structure
of 9 confirms the ring opening of the sterically more demanding
NHC Dipp2Im with two molecules (iPr2Im)·AlH3 in combination
with formation of one Al–C and one Al–N bond and migration of
two hydrogen atoms (one hydrogen atom of each aluminum atom)
to the former carbene carbon atom of Dipp2Im (Figure 5). Both
aluminum atoms adopt a tetrahedral structure, in which Al1 is
coordinated by one iPr2Im, two hydrogen atoms and the former
carbene carbon atom C1, whereas the second aluminum atom
Al2 is bound to the nitrogen atom of the ring opened Dipp2Im, to
an iPr2Im ligand and two hydrogen atoms. The Al–NHC bond
lengths of 2.0518(4) and 2.066(4) Å are similar to those found in
shifted at 5.03 ppm (iPr-CH) and 6.10 ppm (backbone) compared
to the signal set of {(iPr2Im)·Al2H2-NDipp} at 5.29 ppm (iPr-CH)
and 6.15 ppm (backbone). The aluminum bound hydrogen atoms
were observed as broad singlets at 4.22 and 4.72 ppm. In the
13C{1H} NMR spectra the resonance of the former carbene carbon
atom of Dipp2Im is detected at 42.3 ppm, broadened by the
quadrupole moment of the aluminum atom and significantly high
field shifted compared to free Dipp2Im at 220.5 ppm. The signals
of the carbene carbon atoms of iPr2Im are observed at 168.6 ppm
for {(iPr2Im)·Al2H2-NDipp} and 171.1 ppm for {(iPr2Im)·Al1H2–
CH2}. In the IR spectrum of 9 the Al-H stretches are recorded at
1687, 1731 and 1787 cm-1. The aluminum resonances of
(iPr2Im)·AlH2(ROR-Dipp2ImH2)H2Al·(iPr2Im)
9
were
not
detectable in the solution 27Al{1H} NMR spectra, but were
observed in the 27Al{1H} HPDec/MAS NMR at 108.8 ppm
(CQ(quad) = 8821±10 kHz, η(quad) = 0.53) and at 119.5 ppm
(CQ(quad) = 10649±10 kHz, η(quad) = 0.43).
Different reactivity of group 13 element hydrides towards
bulky NHCs have been reported so far. While the catechol borane
adduct (Dipp2Im)·BcatH is stable even at higher temperatures, the
adduct (Dipp2H2Im)·BcatH undergoes at room temperature ring
expansion reaction to afford (RER-Dipp2H2Im(H2))B-cat-
Bcat·(Dipp2H2Im). Bertrand et al. observed for the reaction of
pinacol borane with the backbone saturated NHC Dipp2H2Im the
formation of the dimeric ring opened compound pinB(ROR-
Dipp2H2Im)2Bpin,[14] similarly as we reported the formation of a
ring opening product for the reaction with Dipp2H2Im. The
saturated NHC Dipp2H2Im reacts in a 2:2 ratio to yield a NHC ring
expansion product at room temperature via C–N bond cleavage
and further migration of the hydrides from two HBcat molecules to
the former carbene-carbon atom. When equimolar amounts of
(iPr2Im)·AlH3
3 (Al–NHC 2.0375(17) & 2.0405(17) Å). The
unsaturated backbone of Dipp2Im remains in the ring opening
product intact, as can be seen from the C2–C3 bond length of
1.337(5) Å, which is characteristic for a C–C double bond.
In the proton NMR spectrum of 9, the characteristic resonance
for the AlCH2N protons is detected as a singlet at 2.85 ppm,
whereas the signals of the backbone of the ring opened Dipp2Im
are detected as two roofed doublets at 5.70 and 6.05 ppm. The
resonances of each iPr2Im moiety are well separated from each
other, those of the {(iPr2Im)·Al1H2–CH2} unit are slightly high field
(iPr2Im)·AlH3
3 and Dipp2Im were used for the reaction,
(iPr2Im)·AlH2(ROR-Dipp2ImH2)H2Al·(iPr2 Im ) 9 and free Dipp2Im
were isolated in a ratio of 1:1, and no further
ring expansion or ring opening or exchange
of the Lewis base NHC at aluminum occurs.
We assume that the ring opening of
Dipp2Im is due to the steric demand of the
NHC. In contrast to this, the reaction of
(iPr2Im)·AlH3 3 with Mes2Im gives even at
higher temperatures no ring expansion or
ring opening reaction. When other Lewis-
bases like amines (NEt3) or phosphines
(PPh3) were reacted with (iPr2Im)·AlH3 3 at
70 °C for 2 days, no reaction was observed,
neither
ligand
exchange
or
ring
expansion/opening of the NHC.
Since we observed ring expansion or
ring opening, respectively, for the reactions
of (iPr2Im)·AlH3 3 with the small, alkyl
substituted NHC iPr2Im and the sterically
more demanding aryl substituted Dipp2Im,
we became interested in (i) the stability of
AlH3 base adducts of sterically more
demanding NHCs and (ii) the stability of
NHC aluminum hydride base adducts, if we
Figure 5. Molecular structure of (iPr2Im)·AlH2(ROR-Dipp2ImH2)H2Al·(iPr2Im) 9 in the solid state
(ellipsoids set at 50% probability level). Selected bond lengths [Å] and angles [°]: Al1–C1 1.983(4),
Al1–C11 2.051(4), Al1–H1 1.41(4), Al1–H2 1.59(4), C1–N1 1.449(4), N1–C2 1.394(4), C2–C3
1.337(5), N2–C3 1.415(4), N2–Al2 1.839(3), Al2–C21 2.066(4), Al2–H3 1.48(4), Al2 H4 1.60(4). C1-
Al1-C11 112.39(17), C1-Al1-H1 120.9(15), C1-Al1-H2 101.5(13), C11-Al1-H1 104.0(15), C11-Al1-H2
101.4(14), C11-Al1-H2 101.4(14), H1-Al1-H2 115(2), C21-Al2-N2 114.97(14), C21-Al2-H3 105.7(14),
C21-Al2-H4 100.4(14), N2-Al2-H3 111.2(14), N2-Al2-H4 109.7(14), H3-Al2-H4 114.6(19).
This article is protected by copyright. All rights reserved.

10.1002/chem.201702166
Chemistry - A European Journal
FULL PAPER
volatiles were removed in vacuo, both adducts were isolated by
crystallization from n-hexane at -60 °C as colorless solids in yields
of 56 % (11) and 57 % (12). Important NMR data of both
compounds are summarized in Table 1. These data fit well into
the general picture as described previously. However, the
aluminum resonance of (iPr2Im)·AliBu2H 11 in the solution
27Al{1H} NMR spectrum at 139.4 ppm is significantly shifted
compared to the AlH3 adducts (cf. 3: 106.3), and are in perfect
agreement with the resonance detected in the solid state 27Al{1H}
HPDec/MAS NMR spectrum at 138.9 ppm. Compound 12,
however, shows similar to 10 no detectable resonance in the
27Al{1H} NMR spectrum, while in the solid state 27Al{1H}
HPDec/MAS NMR spectrum of the compound a signal with an
isotropic shift at 137.0 ppm was observed. The quadrupolar
N
N
Dipp
Dipp
10
+
LiAlH4
Al
Al
Et
rt, -LiH
2O
H
H
H
-
0°C
R
R
N
N
+
0.5
N
N
R
R
iBu H)
:
:
{(Al
}
2
2
=
R iPr
11
=
12 R Dipp
Et
2O
iBu
iBu
H
- rt
0°C
Scheme 5. Synthesis of the compounds (Dipp2Im)·AlH3 10, (iPr2Im)·AliBu2H 11
and (Dipp2Im)·AliBu2H 12.
coupling constant was determined to 15860 kHz with
a
quadrupolar asymmetry parameter of 0.29. A. The Al-H stretching
vibrations of 11 and 12 were detected at 1681 cm-1 (11) and at
1702 cm-1 (12) in the IR spectrum of the complexes.
block two of the hydridic positions. Therefore, the adducts
(Dipp2Im)·AlH3 10, (iPr2Im)·AliBu2H 11 and (Dipp2Im)·AliBu2H 12
were synthesized (Scheme 5). (Dipp2Im)·AlH3 10 was prepared
from the reaction of the NHC with lithium aluminum hydride, as
described earlier by Jones et al.[16d] (Dipp2Im)·AlH3 was isolated
in yields of 65 % as a colorless solid and was identified using
multinuclear NMR spectroscopy, elemental analysis and IR
spectroscopy. The Al–H signals of (Dipp2Im)·AlH3 10 were
recorded at 3.67 ppm, slightly shifted compared to those of
(iPr2Im)·AlH3 3 at 4.53 ppm. The 13C{1H} NCN resonance of
Dipp2Im in 10 was observed at 178.2 ppm, at lower field
compared to 1 – 4, which is in accordance with the comparatively
deep field shifted NCN resonance of uncoordinated Dipp2Im
(220.5 ppm). (Dipp2Im)·AlH3 10 reveals with our settings no
detectable resonance in the 27Al{1H} solution NMR spectrum,
while in the solid-state 27Al{1H} HPDec/MAS NMR spectrum of the
compound a signal with an isotropic shift at 107.9 ppm was
detected. This resonance reveals the typical line shape (MAS
second-order quadrupolar powder pattern) for a tetrahedral sp3-
Al atom with a quadrupolar coupling constant of 11943 kHz and a
quadrupolar asymmetry parameter of 0.1. The 15N VACP/MAS
NMR spectrum shows one resonance at -180.0 ppm for the two
nitrogen atoms of the symmetrical (Dipp2Im)·AlH3 conformer. In
the IR spectrum of adduct 10 the characteristic Al-H stretching
vibrations were detected at 1725, 1741 and 1777 cm-1.
The addition of iPr2Im to a solution of (iPr2Im)·AliBu2H 9 leads
to either the formation of a compound (iPr2Im)2·AliBu2H or to a
rapid exchange of the NHC ligands on an NMR time scale, similar
as observed for the formation of (iPr2Im)2·AlH3 5. Only one set of
protons is detected for the NHC in these solutions, However, in
contrast to the ring expansion reaction observed for (iPr2Im)·AlH3
3 / iPr2Im to give (iPr2Im)·AlH(RER-iPr2ImH2) 6, no further ring
expansion was detected for (iPr2Im)·AliBu2H / iPr2Im up to a
temperature of 110 °C (for several days). Similar observations
were made for the reaction of (Dipp2Im)·AliBu2H 12 with one
equivalent of iPr2Im. Thus, blocking of two hydridic sites leads to
adducts that seem to remain stable with respect to ring expansion.
Another class of stable carbenes, cyclic (alkyl)(amino)
carbenes (cAACs) have attracted an enormously growing
attention lately. These cAACs arise from the formal replacement
of one of the electronegative amino substituents of NHCs by a
stronger σ-donor alkyl group [27] Compared to NHCs, cAACs have
a smaller HOMO-LUMO gap and are thus stronger electrophiles
and nucleophiles. [28] These features enable cAACs to activate
R'
R'
In contrast to the observations made for the alkyl substituted
NHC adduct (iPr2Im)·AlH3 3, the bulkier (Dipp2Im)·AlH3 10 shows
no reaction towards Dipp2Im. However, immediately after addition
of Dipp2Im to solutions of (Dipp2Im)·AlH3 10 only one set of
protons for the two NHCs were detected and the methine protons
N
thf
rt
N
N
N
+
R
R
H
Al
R
Al
H
N
H
H
H
H
N
R'
R
1
of the Dipp substituents broadened significantly in the H-NMR
R'
spectrum, which accounts either for an rapid coordination
equlilibrium or the formation of a compound (Dipp2Im)2·AlH3.
However, despite many efforts, the resonance of the carbene
carbon atom was not detected in the 13C{1H} NMR spectra.
Heating of these solutions up to 110°C for several days in toluene-
d8 did not lead to any further reaction.
:
:
:
:
:
1
=
=
=
=
=
=
=
=
:
:
:
:
:
R
R
R
R
R
Me R'
Me R' Me
iPr R'
H
=
=
=
=
=
=
=
=
13
14
15
16
17
R
Me R'
Me R' Me
iPr R'
H
2
3
4
R
R
R
R
H
H
=
iPr R' Me
=
iPr R' Me
=
Dipp R'
H
10
=
H
Dipp R'
For a closer investigation of the reactivity if two hydridic sites
of the alane are blocked, the NHC adducts (iPr2Im)·AliBu2H 11
and (Dipp2Im)·AliBu2H 12 were synthesized from the reaction of
di-iso-butyl aluminum hydride with the corresponding N-
heterocyclic carbene in diethyl ether at 0 °C (Scheme 5). After all
Scheme 6. Synthesis of the Al–H bond activation products
(NHC)·AlH2(cAACMeH) (NHC = Me2Im 13, Me2ImMe 14, iPr2Im 15, iPr2ImMe 16,
Dipp2Im 17).
This article is protected by copyright. All rights reserved.

10.1002/chem.201702166
Chemistry - A European Journal
FULL PAPER
small molecules such as CO, H2, P4 and other enthalpically
strong single bonds, such as B-H, B-C, C-H, and C-F. [7-8, 29]
Surprisingly, the reaction of lithium aluminum hydride with
cAACMe affords in our hands just the starting material and the
adduct (cAACMe)·AlH3 was not formed. An alternative procedure
for the synthesis of cAAC alane adducts would be ligand
substitution starting from (NHC)·AlH3 with cAACMe. However, in
this case insertion of the cAAC into the Al–H bond is observed, i.
e. the reaction of (NHC)·AlH3 with cAACMe leads to Al–H bond
activation to afford compounds of the type (NHC)·AlH2(cAACMeH)
(Scheme 6). These compounds represent the products of an
oxidative addition of (NHC)·AlH3 to cAACMe, which seems to be
independent on the steric demand of the NHC bound to the
aluminum atom. The reaction of cAACMe with (NHC)·AlH3 (NHC =
Me2Im 1, Me2ImMe 2, iPr2Im 3, iPr2ImMe 4), coordinated with small
alkyl substituted NHCs, leads already at room temperature to
(NHC)·AlH2(cAACMeH) (NHC = Me2Im 13, Me2ImMe 14, iPr2Im 15,
iPr2ImMe 16) in form of colorless solids in good yield after workup
(13: 58 %, 14: 66 %, 15: 78 %, 16: 66 %). NMR spectroscopic
studies on the reaction of (iPr2Im)·AlH3 3 with an equimolar
amount of cAACMe revealed that product formation proceeds
already at temperatures as low as -78 °C quantitatively to give
(iPr2Im)·AlH2(cAACMeH) 15. The formation of a likely bis carbene
adduct (iPr2Im)(cAAC)·AlH3 was not observed at these
temperatures. For the reaction of cAACMe with the adduct of the
sterically more demanding NHC Dipp2Im, (Dipp2Im)·AlH3 10, Al–
H bond activation also occurs at room temperature and the
product (Dipp2Im)·AlH2(cAACMeH) 17 was isolated as a colorless
solid in 49 % yield. The compound (iPr2Im)·AlH2(cAACMeH) 15 is
stable up to 105 °C for several days. Similarly to the E–H bond
activation product (cAACMeH)Bcat [10f] no ring expansion reaction
or ring opening reaction was observed up to this temperature.
Furthermore, (iPr2Im)·AlH2(cAACMeH) 15 does not react with an
additional equivalent of cAACMe, i.e. the two remaining Al–H
bonds are stable with respect to further attack.
broadened by the interaction with the 27Al nucleus. The resonance
of the NHC carbene carbon atom lies in the range observed for
the (NHC)·AlH3 adducts, e. g. at 171.8 ppm for 15 compared to
170.3 ppm in 3. In general, the aluminum resonances of the
compounds (NHC)·AlH2(cAACMeH) are difficult to detect in the
27Al{1H}
NMR
spectrum
to
our
experience.
For
(Me2Im)·AlH2(cAACMeH) 13, (Me2ImMe)·AlH2(cAACMeH) 14 and
(iPr2Im)·AlH2(cAACMeH) 15 the resonances appear as very broad
signals at 116.5 ppm (13), 117.3 ppm (14) and 115.0 ppm (15).
For the compounds (iPr2ImMe)·AlH2(cAACMeH) 16 and
(Dipp2Im)·AlH2(cAACMeH) 17 we could not detect signals in the
solution 27Al{1H} NMR spectra. The chemical shifts of the
resonances of 13 in the solid-state 13C{1H} and 27Al{1H}
HPDec/MAS NMR spectra match very good with those recorded
in solution, which shows that solid-state and solution structure
agree. For example, the resonance of 13 arises with an isotopic
shift at 117.5 ppm in the 27Al{1H} HPDec/MAS NMR spectrum, a
quadrupolar coupling constant of 11380 kHz and a quadrupolar
asymmetry parameter of 0.48. In the 15N VACP/MAS the
resonance of the NHC ligand is observed at -192.8 ppm, whereas
the nitrogen atom of the former cAAC substituent emerges at -
316.5 ppm. All the compounds (NHC)·AlH2(cAACMeH) 13 – 17
show Al–H stretching vibrations in the infrared spectrum in the
characteristic region between 1688 and 1804 cm-1 (see Table 2
and SI).
The molecular structure of 14, 16 and 17 were determined by X-
ray
diffraction
(see
Figure
6
and
Table
3).
(Me2ImMe)·AlH2(cAACMeH) 14 crystallizes in the triclinic space
group P1, (iPr2ImMe)·AlH2(cAACMeH) 16 in the monoclinic space
�
group P21/n and (Dipp2Im)·AlH2(cAACMeH) 17 in the orthorhombic
space group P212121. These molecules adopt a tetrahedral
structure at the aluminum atom, spanned by the NHC (Me2ImMe
for 14, iPr2ImMe for 16 and Dipp2Im for 17), the cyclic
(alkyl)(amino)carbene rest and the two remaining hydrogen
Compounds 13 – 17 were characterized by multinuclear NMR
spectroscopy, elemental analyses, infrared spectroscopy and
high resolution mass spectrometry. Important data are
summarized in Table 2. The 1H and 13C{1H} NMR spectra of 13 –
17 reveal resonances for each set of hydrogen atoms and carbon
atom of the cAAC separately due to the chirality of the former
carbene carbon atom. For example, the methyl groups of the
cAAC backbone of (iPr2Im)·AlH2(cAACMeH) 15 were detected as
four separate singlets at 1.14, 1.31, 1.47 and 1.48 ppm and the
diastereotopic CH2 protons as roofed doublets of an AB spin
system at 1.95 and 2.04 ppm. The resonances of the cAAC Dipp
iso-propyl methyl protons emerge as doublets at 1.18 and 1.38
ppm, and the methine protons as two septets at 3.73 and
4.50 ppm. The resonances of the NHC ligand methine and
backbone protons remain magnetic equivalent, which accounts
for free rotation around the Al-CNHC axis. The aluminum bound
protons, which are diastereotopic in 13 – 17, were detected as
two broad singlets (e.g. at 3.90 and 4.33 ppm for 15) in the
1H{27Al} NMR spectra of the compounds. In the 13C{1H} NMR
spectra the resonance of the former cAACMe carbene carbon
atom is shifted significantly from 313.5 ppm in the cAAC itself to
a signal in the range between 65.8 ppm and 72.2 ppm, which is
Table 2. Selected NMR shifts (ppm) of the compounds
(Me2Im)·AlH2(cAACMeH) 13, (Me2ImMe)·AlH2(cAACMeH) 14, (iPr2Im)·AlH2
(cAACMeH) 15, (iPr2ImMe)·AlH2(cAACMeH) 16, (Dipp2Im)·AlH2(cAACMeH) 17,
(iPr2Im)·AliBu2(cAACMeH) 18.
AlCH
1H{27Al}
NMR
AlH2
1H{27Al}
NMR
CAACAlCH
13C{1H}
NMR
NHCNCN
13C{1H}
NMR
Al-H
stretching
vibration
1688,
1796
13
14
3.35
3.39
4.00, 4.25
4.07, 4.36
66.6
66.7
173.6
170.9
1749,
1797,
1687
1739,
1773
15
16
3.39
3.44
3.90, 4.33
3.89, 4.37
67.3
67.3
171.8
171.9
1721,
1772
1779,
1804
17
18
3.02
3.12
3.02, 3.47
–
65.8
72.2
179.8
174.4
–
This article is protected by copyright. All rights reserved.

10.1002/chem.201702166
Chemistry - A European Journal
FULL PAPER
Figure 6. Molecular structure of (Me2ImMe)·AlH2(cAACMeH) 14, (iPr2ImMe)·AlH2(cAACMeH) 16 and (Dipp2Im)·AlH2(cAACMeH) 17 in the solid state (ellipsoids set at
50% probability level). Selected bond lengths [Å] and angles [°]: 14 (left): Al–C1 2.064(2), Al–C11 2.039(2), Al–C31 2.020(8) Al–H1 1.52(2), Al–H2 1.52(2). C1-Al-
C11 104.89(9), C1-Al-C31 114.5(2), C1-Al-H1 104.3(8), C1-Al-H2 104.0(8), C11-Al-H1 120.7(8), C11-Al-H2 107.0(8), C31-Al-H1 93.0(9), C31-Al-H2 125.1(9), H1-
Al-H2 114.2(12). 16 (middle): Al–C1 2.0868(19), Al–C21 2.0309(19), Al–H1 1.58(2), Al–H2 1.53(2). C1-Al-C21 107.74(8), C1-Al-H1 102.0(7), C1-Al-H2 103.8(8),
C21-Al-H1 112.6(7), C21-Al-H2 114.7(8), H1-Al-H2 114.5(11). 17 (right): Al–C1 2.1011(19), Al–C31 2.0226(19), Al–H1 1.50(2), Al–H2 1.52(3). C1-Al-C31 111.44(7),
C1-Al-H1 100.1(9), C1-Al-H2 102.2(9), C31-Al-H1 115.1(9), C31-Al-H2 114.1(9), H1-Al-H2 112.2(13).
Table 3. Selected bond lengths of the compounds (Me2ImMe)·AlH2
(cAACMeH) 14, (iPr2ImMe)·AlH2(cAACMeH) 16, and (Dipp2Im)·AlH2(cAACMeH)
17..
atoms. The Al–CNHC bond length of 14 (2.064(2) Å) and 16
(2.0868(19) Å) differ only marginally from aluminum carbene
carbon atom bond distance observed in (iPr2Im)·AlH3
3
14
16
17
(2.0405(17) Å). The Al–CcAAC bond lengths of 2.039(2) Å (14),
2.0309(19) Å (16) and 2.0226(19) Å (17) fall in the same range,
but are slightly longer compared to other typical NHC stabilized
aluminum alkyl compounds (1.9 to 2.0 Å)[17, 26, 30]
We also tried to react the compounds (iPr2Im)·AliBu2H 11 and
(Dipp2Im)·AliBu2H 12 with cAACMe (Scheme 7). The compound
with the sterically more demanding carbene (Dipp2Im)·AliBu2H 12
proved to be unreactive, while the reaction of (iPr2Im)·AliBu2H 11
with cAAC led to (iPr2Im)·AlH2(cAACMeH) 18, which was isolated
as a colorless solid in 41 %.
Al–CNHC
Al–CCAAC
Al–H1
2.064(2)
2.039(2)
1.52(2)
1.52(2)
2.0868(19)
2.0309(19)
1.58(2)
2.1011(19)
2.0226(19)
1.50(2)
Al–H2
1.53(2)
1.52(3)
Compound 18 was characterized using multinuclear NMR
spectroscopy, elemental analysis, and infrared spectroscopy.
NMR and IR spectroscopy of this compound is in accordance with
the other Al-H oxidative addition adducts (see Table 2 and SI). 1H
and 13C{1H} NMR spectra reflect as described above the
asymmetry of the compound, which is also reflected in the proton
resonances of the iso-butyl substituent. The iBu-CH3 groups give
N
=
R
iPr
N
N
N
+
R
R
H
rt
thf,
Al
Al
=
iBu
iBu
iBu
iBu
H
N
Figure 7. Molecular structure of (iPr2Im)·AliBu2(cAACMeH) 18 in the solid state
(ellipsoids set at 50% probability level). Selected bond lengths [Å] and angles
[°]: Al–C1 2.1419(18), Al–C10 2.0423(17), Al–C30 2.0056(18), Al–C34
2.0131(18). C1-Al-C10 106.48(7), C1-Al-C30 105.27(7), C1-Al-C34 113.74(7),
C10-Al-C30 107.59(7), C10-Al-C34 109.09(7), C30-Al-C34 114.22(8).
N
18
:
:
11
R
iPr
=
12 R Dipp
no
12
reaction with
Scheme 7. Synthesis of the Al–H bond activation products
(iPr2Im)·AliBu2(cAACMeH) 18.
This article is protected by copyright. All rights reserved.

10.1002/chem.201702166
Chemistry - A European Journal
FULL PAPER
products remain stable at higher temperatures and no cAAC ring
expansion occurs. The reaction of (iPr2Im)·AliBu2H 11 with
cAACMe leads also to Al–H bond activation, whereas the
compound of the sterically more demanding Dipp2Im ligand,
(Dipp2Im)·AliBu2H 12, proofed to be unreactive with respect to Al–
H bond activation. In total, three major decomposition pathways
of carbene alane adducts have been identified here, i.e. carbene
ring expansion and ring opening as well as carbene insertion into
the Al–H bond. These reactions require an excess of the carbene
(i.e. NHC:Al > 1:1), 1:1 Lewis acid-base adducts NHC·AlH3 are
stable. Furthermore, our results also show that ring expansion
and ring opening may be easily suppressed just by blocking two
of the Al-H sites, as demonstrated here for (NHC)·AliBu2H, which
is unreactive with respect of NHC ring destruction. However, Al–
H bond activation with cAACMe still happens for (iPr2Im)·AliBu2H
11. In this case it is helpful to additionally increase the steric
demand of the NHC employed, i.e. using the more bulky
(Dipp2Im)·AliBu2H 12, which also suppresses the insertion of
cAACMe into the Al–H bond. Further investigations to substantiate
these concepts are currently underway.
rise to 4 doublets at 0.80, 1.06, 1.28 and 1.43 ppm, whereas
the methine protons give rise to two multiplets at 2.02 and
2.36 ppm. The iBu-CH2 protons were detected as multiplets at -
0.45, -0.17, 0.53 and 1.08 ppm. Similar splitting of the iBu-CH3
carbon resonances occurs in the 13C{1H} NMR spectrum. The
molecular structure of (iPr2Im)·AlH2(cAACMeH) 18 (Figure 7)
confirms the connectivity of the compound, which adopts a slightly
distorted tetrahedron at aluminum. The Al–CNHC bond of
2.1419(18) Å is slightly longer then the bond lengths found in 14,
16 and 17, whereas the Al–CcAAC and Al–iBu distances
(2.0423(17), 2.0056(18) and 2.0131(18) Å) are similar to those
observed before.
Conclusions
NHC alane adducts (NHC)·AlH3 (NHC = Me2Im 1, Me2ImMe 2,
iPr2Im 3 and 3-d3, iPr2ImMe 4, Dipp2Im 10) are readily accessible
from the reaction of the NHC with lithium aluminum
hydride/deuteride with lithium hydride/deuteride elimination. The
secondary
alane
adducts
(iPr2Im)·AliBu2H
11
and
(Dipp2Im)·AliBu2H 12 have been synthesized from the reaction of
the NHC carbenes with di-iso-butylaluminum hydride. These
adducts are stable at 110 °C for several days in solvents like
toluene. However, if an excess of the NHC is present, NHC ring
expansion or NHC ring cleavage occurs, depending on the NHC
used. The reaction of (iPr2Im)·AlH3 3 with iPr2Im was investigated
in some detail. Upon addition of the NHC, a bis(carbene) alane
Acknowledgements
This work was supported by the Julius-Maximilians-Universität
Würzburg and the Deutsche Forschungsgemeinschaft (DFG RA
720/13-1). Christoph Mahler is acknowledged for the
measurement of the high resolution mass spectra.
adduct (iPr2Im)2·AlH3
5
forms, which decomposes to
Keywords: Aluminum • N-heterocyclic Carbene • cAAC • E–H
(iPr2Im)·AlH(RER-iPr2ImH2) 6. NMR studies reveal that this is a
process of first order kinetics, presumably a reaction along the
sequence (i) insertion of an NHC into one of the Al-H bonds of
(NHC)·AlH3, (ii) subsequent intramolecular amido transfer from
the NHC carbon atom to the Lewis acidic aluminum, and (iii) a
hydride shift from the aluminum atom to the (former) carbene
carbon atom with formation of the final product 6. The reaction of
the deuterated adduct (iPr2Im)·AlD3 3-d3 with iPr2Im led
exclusively to (iPr2Im)·AlD(RER-iPr2ImD2) 6-d3, which supports
the idea of an intramolecular concerted reaction pathway.
However, the cross experiment using a 1:1 mixture of 3 and 3-d3
and iPr2Im as starting material led to some deuterium scrambling
in the ring expanded product 6, but we could also demonstrate
that the deuterium scrambling already takes place in the starting
materials (i.e. in a mixture of 3 and 3-d3). The reaction of
(iPr2Im)·AlH3 3 with the sterically more demanding N-heterocyclic
carbene Dipp2Im follows a different pathway and the ring opening
of Dipp2Im was observed at 70 °C to give (iPr2Im)·AlH2(ROR-
Dipp2ImH2)H2Al·(iPr2Im) 9. The NHC adducts of the
(iPr2Im)·AliBu2H 11 and (Dipp2Im)·AliBu2H 12 were stable with
respect to NHC ring expansion or NHC ring opening.
Bond Activation • Ring Expansion Reaction
[1]
[2]
a) M. Asay, C. Jones, M. Driess, Chem. Rev. 2011, 111, 354-396; b) F.
E. Hahn, M. C. Jahnke, Angew. Chem. Int. Ed. 2008, 47, 3122-3172; c)
M. Melaimi, M. Soleilhavoup, G. Bertrand, Angew. Chem. Int. Ed. 2010,
49, 8810-8849; d) O. Schuster, L. Yang, H. G. Raubenheimer, M.
Albrecht, Chem. Rev. 2009, 109, 3445-3478; e) J. Vignolle, X. Cattoën,
D. Bourissou, Chem. Rev. 2009, 109, 3333-3384.
a) A. C. Filippou, O. Chernov, G. Schnakenburg, Angew. Chem. 2009,
121, 5797-5800, Angew. Chem. Int. Ed. 2009, 48, 5687-5690; b) R. S.
Ghadwal, H. W. Roesky, S. Merkel, J. Henn, D. Stalke, Angew. Chem.
2009, 121, 5793-5796, Angew. Chem. Int. Ed. 2009, 48, 5683-5686; c)
Y. Xiong, S. Yao, M. Driess, J. Am. Chem. Soc. 2009, 131, 7562-7563;
d) R. Kinjo, B. Donnadieu, M. A. Celik, G. Frenking, G. Bertrand, Science
2011, 333, 610-613; e) K. C. Mondal, H. W. Roesky, M. C. Schwarzer,
G. Frenking, B. Niepötter, H. Wolf, R. Herbst-Irmer, D. Stalke, Angew.
Chem. 2013, 125, 3036-3040, Angew. Chem. Int. Ed. 2013, 52, 2963-
2967; f) K. C. Mondal, H. W. Roesky, M. C. Schwarzer, G. Frenking, I.
Tkach, H. Wolf, D. Kratzert, R. Herbst-Irmer, B. Niepötter, D. Stalke,
Angew. Chem. 2013, 125, 1845-1850, Angew. Chem. Int. Ed. 2013, 52,
1801-1805; g) Y. Xiong, S. Yao, S. Inoue, J. D. Epping, M. Driess,
Angew. Chem. 2013, 125, 7287-7291, Angew. Chem. Int. Ed. 2013, 52,
7147-7150; h) M. Soleilhavoup, G. Bertrand, Acc. Chem. Res. 2015, 48,
256-266; i) D. Lutters, C. Severin, M. Schmidtmann, T. Müller, J. Am.
Chem. Soc. 2016, 138, 6061-6067; j) Y. Xiong, S. Yao, G. Tan, A. Inoue,
M. Driess, J. Am Chem. Soc. 2013, 135, 5004-5007.
Compared to NHCs, cAACs possess a smaller HOMO-LUMO
gap and are thus stronger electrophiles and nucleophiles. The
reaction of (NHC)·AlH3 with cAACMe leads to Al–H bond cleavage
and insertion of the cAAC into the Al–H bond to afford compounds
of the type (NHC)·AlH2(cAACMeH) (NHC = Me2Im 13, Me2ImMe 14,
iPr2Im 15, iPr2ImMe 16, Dipp2Im 17). These Al–H oxidative addition
[3]
a) Y. Wang, Y. Xie, P. Wei, R. B. King, H. F. Schaefer, P. von R.
Schleyer, G. H. Robinson, Science 2008, 321, 1069-1071; b) Y. Wang,
G. H. Robinson, Chem. Commun. 2009, 5201-5213; c) H. Braunschweig,
R. D. Dewhurst, K. Hammond, J. Mies, K. Radacki, A. Vargas, Science
This article is protected by copyright. All rights reserved.

10.1002/chem.201702166
Chemistry - A European Journal
FULL PAPER
2012, 336, 1420-1422; d) C. Jones, A. Sidiropoulos, N. Holzmann, G.
Frenking, A. Stasch, Chem. Commun. 2012, 48, 9855-9857.
C. Jones, C. L. Raston, Chem. Commun. 1992, 1349-1351; g) F. R.
Bennett, F. M. Elms, M. G. Gardiner, G. A. Koutsantonis, C. L. Raston,
N. K. Roberts, Organometallics 1992, 11, 1457-1459; h) J. S. Cha, H. C.
Brown, J. Org. Chem. 1993, 58, 3974-3979; i) F. M. Elms, M. G.
Gardiner, G. A. Koutsantonis, C. L. Raston, J. L. Atwood, K. D. Robinson,
J. Organomet. Chem. 1993, 449, 45-52; j) I. B. Gorrell, P. B. Hitchcock,
J. D. Smith, Chem. Commun. 1993, 189-190; k) J. L. Atwood, G. A.
Koutsantonis, F.-C. Lee, C. L. Raston, Chem. Commun. 1994, 91-92; l)
C. L. Raston, J. Organomet. Chem. 1994, 475, 15-24; m) M. M.
Andrianarison, M. C. Ellerby, I. B. Gorrell, P. B. Hitchcock, J. D. Smith,
D. R. Stanley, Dalton Trans. 1996, 211-217; n) C. Jones, F. C. Lee, G.
A. Koutsantonis, M. G. Gardiner, C. L. Raston, Dalton Trans. 1996, 829-
833; o) P. C. Andrews, C. L. Raston, B. W. Skelton, A. H. White, Chem.
Commun. 1997, 245-246; p) M. G. Gardiner, C. L. Raston, Coord. Chem.
Rev. 1997, 166, 1-34; q) S. G. Alexander, M. L. Cole, M. Hilder, J. C.
Morris, J. B. Patrick, Dalton Trans. 2008, 6361-6363; r) S. G. Alexander,
M. L. Cole, C. M. Forsyth, Chem. Eur. J. 2009, 15, 9201-9214; s) T. D.
Humphries, P. Sirsch, A. Decken, G. Sean McGrady, J. Mol. Struct.
2009, 923, 13-18; t) D. Lacina, J. Reilly, J. Johnson, J. Wegrzyn, J.
Graetz, J. Alloys Compd. 2011, 509, S654-S657; u) B. M. Wong, D.
Lacina, I. M. B. Nielsen, J. Graetz, M. D. Allendorf, J. Phys. Chem 2011,
115, 7778-7786; v) M. Bodensteiner, A. Y. Timoshkin, E. V. Peresypkina,
U. Vogel, M. Scheer, Chem. Eur. J. 2013, 19, 957-963; w) T. D.
Humphries, K. T. Munroe, A. Decken, G. S. McGrady, Dalton Trans.
2013, 42, 6953-6964; x) J. M. Davidson, T. Wartik, J. Am. Chem. Soc.
1960, 82, 5506-5506.
[4]
a) K. C. Mondal, P. P. Samuel, H. W. Roesky, R. R. Aysin, L. A. Leites,
S. Neudeck, J. Lübben, B. Dittrich, N. Holzmann, M. Hermann, G.
Frenking, J. Am. Chem. Soc. 2014, 136, 8919-8922; b) K. C. Mondal, S.
Roy, B. Dittrich, D. M. Andrada, G. Frenking, H. W. Roesky, Angew.
Chem. 2016, 128, 3210-3213, Angew. Chem. Int. Ed. 2016, 128, 3158-
3161; c) M. Arrowsmith, H. Braunschweig, T. E. Stennett, Angew. Chem.
2017, 129, 100-120, Angew. Chem Int. Ed. 2017, 129, 100-120; d) G.
Bertrand, M. Sioleilhavoup, M. Melaimi, R. Jazzar, Angew. Chem. Int.
Ed. 2017, DOI: 10.1002/anie.201702148.
[5]
[6]
S. Würtemberger-Pietsch, U. Radius, T. B. Marder, Dalton Trans. 2016,
45, 5880-5895.
a) V. Jancik, L. W. Pineda, J. Pinkas, H. W. Roesky, D. Neculai, A. M.
Neculai, R. Herbst-Irmer, Angew. Chem. 2004, 116, 2194-2197, Angew.
Chem. Int. Ed. 2004, 43, 2142-2145; b) L. W. Pineda, V. Jancik, H. W.
Roesky, D. Neculai, A. M. Neculai, Angew. Chem. 2004, 116, 1443-1445,
Angew. Chem. Int. Ed. 2004, 43, 1419-1421; c) A. Jana, I. Objartel, H.
W. Roesky, D. Stalke, Inorg. Chem. 2009, 48, 798-800; d) A. C. Filippou,
O. Chernov, B. Blom, K. W. Stumpf, G. Schnakenburg, Chem. Eur. J.
2010, 16, 2866-2872; e) S. Inoue, C. Eisenhut, J. Am. Chem. Soc. 2013,
135, 18315-18318, f) S. Ahmad, T. Szilvási, S. Inoue, Chem. Commun.
2014, 50, 12619-12622.
[7]
[8]
a) U. S. D. Paul, U. Radius, Chem. Eur. J. 2017, 23, 3993-4009; b) Z. R.
Turner, Chem. Eur. J. 2016, 22, 11461-11468.
A. F. Eichhorn, S. Fuchs, M. Flock, T. B. Marder, U. Radius, Angew.
Chem. 2017, DOI: 10.1002/ange.201701679, Angew. Chem. Int. Ed.
2017, DOI: 10.1002/anie.201701679
[16] a) A. J. Arduengo, H. V. R. Dias, J. C. Calabrese, F. Davidson, J. Am.
Chem. Soc. 1992, 114, 9724-9725; b) M. D. Francis, D. E. Hibbs, M. B.
Hursthouse, C. Jones, N. A. Smithies, Dalton Trans. 1998, 3249-3254;
c) R. J. Baker, M. L. Cole, C. Jones, M. F. Mahon, Dalton Trans. 2002,
1992-1996; d) R. J. Baker, A. J. Davies, C. Jones, M. Kloth, J.
Organomet. Chem. 2002, 656, 203-210; e) S. G. Alexander, M. L. Cole,
M. Hilder, J. C. Morris, J. B. Patrick, Dalton Trans. 2008, 6361-6363; f)
S. J. Bonyhady, D. Collis, G. Frenking, N. Holzmann, C. Jones, A.
Stasch, Nature Chemistry 2010, 2, 865-869; g) S. J. Bonyhady, N.
Holzmann, G. Frenking, A. Stasch, C. Jones, Angew. Chem. Int. Ed.
2016, 55, 1-6; h) S. J. Urwin, D. M. Rogers, G. S. Nichol, M. J. Cowley,
Dalton Trans. 2016, 45, 13695-13699; i) M. D. Anker, A. L. Colebatch, K.
J. Iversen, D. J. D. Wilson, J. L. Dutton, L. García, M. S. Hill, D. J. Liptrot,
M. F. Mahon, Organometallics 2017.
[9]
a) D. Schmidt, J. H. J. Berthel, S. Pietsch, U. Radius, Angew. Chem.
2012, 124, 9011-9015; b) H. Schneider, D. Schmidt, U. Radius, Chem.
Commun. 2015, 51, 10138-10141.
[10] a) S. M. I. Al-Rafia, R. McDonald, M. J. Ferguson, E. Rivard, Chem. Eur.
J. 2012, 18, 13810-13820; b) D. Franz, S. Inoue, Chem. Asian J. 2014,
9, 2083-2087; c) T. Wang, D. W. Stephan, Chem. Eur. J. 2014, 20, 3036-
3039; d) S. Pietsch, U. S. D. Paul, I. A. Cade, M. J. Ingleson, U. Radius,
T. B. Marder, Chem. Eur. J. 2015, 21, 9018-9021; e) M. Eck, S.
Würtemberger-Pietsch, A. Eichhorn, J. H. J. Berthel, R. Bertermann, U.
S. D. Paul, H. Schneider, A. Friedrich, C. Kleeberg, U. Radius, T. B.
Marder, Dalton Trans. 2017, 46, 3661-3680; f) S. Würtemberger-Pietsch,
H. Schneider, T. B. Marder, U. Radius, Chem. Eur. J. 2016, 22, 13032-
13036.
[17] G. Schnee, O. Nieto Faza, D. Specklin, B. Jacques, L. Karmazin, R.
Welter, C. Silva López, S. Dagorne, Chem. Eur. J. 2015, 21, 17959-
17972.
[11] a) M. Arrowsmith, M. S. Hill, G. Kociok-Köhn, D. J. MacDougall, M. F.
Mahon, Angew. Chem. Int. Ed. 2012, 51, 2098-2100; b) M. Arrowsmith,
M. S. Hill, G. Kociok-Köhn, Organometallics 2015, 34, 653-662.
[12] a) K. J. Iversen, D. J. D. Wilson, J. L. Dutton, Dalton Trans. 2013, 42,
11035-11038; b) K. J. Iversen, D. J. D. Wilson, J. L. Dutton,
Organometallics 2013, 32, 6209-6217; c) M. R. Momeni, E. Rivard, A.
Brown, Organometallics 2013, 32, 6201-6208; d) R. Fang, L. Yang, Q.
Wang, Organometallics 2014, 33, 53-60; e) K. J. Iversen, D. J. D. Wilson,
J. L. Dutton, Dalton Trans. 2014, 43, 12820-12823; f) M.-D. Su, Inorg.
Chem. 2014, 53, 5080-5087; g) P. Hemberger, A. Bodi, J. H. J. Berthel,
U. Radius, Chem. Eur. J. 2015, 21, 1434-1438; h) K. J. Iversen, D. J. D.
Wilson, J. L. Dutton, Dalton Trans. 2015, 44, 3318-3325.
[18] S. J. Bonyhady, N. Holzmann, G. Frenking, A. Stasch, C. Jones, Angew.
Chem. 2016, DOI: 10.1002/ange.201610601, Angew. Chem. Int. Ed.
2016, DOI: 10.1002/anie.201610601.
[19] B. Li, S. Kundu, A. C. Stueckl, H. Zhu, H. Keil, R. Herbst-Irmer, D. Stalke,
B. Schwederski, W. Kaim, D. M. Andrada, G. Frenking, H. W. Roesky,
Angew. Chem. Int. Ed. 2017, 56, 397-400.
[20] L. L. Cao, E. Daley, T. C. Johnstone, D. W. Stephan, Chem. Commun.
2016, 52, 5305-5307.
[21] R. J. Baker, A. J. Davies, C. Jones, M. Kloth, J. Organomet. Chem. 2002,
656, 203-210.
[13] a) C. P. Sindlinger, L. Wesemann, Chem. Sci. 2014, 5, 2739-2746; b) H.
Schneider, M. J. Krahfuß, U. Radius, Z. Anorg. Allg. Chem. 2016, 642,
1282-1286.
[22] a) S. G. Alexander, M. L. Cole, S. K. Furfari, M. Kloth, Dalton Trans.
2009, 2909-2911; b) C. Fliedel, G. Schnee, T. Aviles, S. Dagorne, Coord.
Chem. Rev. 2014, 275, 63-86; c) W. W. Schoeller, G. D. Frey, Inorg.
Chem. 2016, 55, 10947-10954.
[14] G. D. Frey, J. D. Masuda, B. Donnadieu, G. Bertrand, Angew. Chem.
2010, 122, 9634-9637, Angew. Chem. Int. Ed. 2010, 49, 9444-9447.
[15] a) E. C. Ashby, J. Am. Chem. Soc. 1964, 86, 1882-1883; b) E. M. Marlett,
W. S. Park, J. Org. Chem. 1990, 55, 2968-2969; c) J. L. Atwood, F. R.
Bennett, F. M. Elms, C. Jones, C. L. Raston, K. D. Robinson, J. Am.
Chem. Soc. 1991, 113, 8183-8185; d) J. L. Atwood, K. D. Robinson, C.
Jones, C. L. Raston, Chem. Commun. 1991, 1697-1699; e) J. L. Atwood,
F. R. Bennett, C. Jones, G. A. Koutsantonis, C. L. Raston, K. D.
Robinson, Chem. Commun. 1992, 541-543; f) J. L. Atwood, S. G. Bott,
[23] J. Weidlein, U. Müller, K. Dehnicke, Schwingungsfrequenzen I,
Hauptgruppenelemente, Thieme, Stuttgart, Germany, 1981.
[24] M. Boronat, P. Viruela, A. Corma, J. Phys. Chem. 1996, 100, 16514-
16521.
[25] G. W. Fraser, N. N. Greenwood, B. P. Straughan, J. Chem. Soc. 1963,
3742-3749.
[26] M. Wu. M, A. M. Gill, L. Yunpeng, L. Falivene, L. Yongxin, R. Ganguly,
L. Cavallo, F. Garcia, Dalton Trans. 2015, 44, 15166-15174.
This article is protected by copyright. All rights reserved.

10.1002/chem.201702166
Chemistry - A European Journal
FULL PAPER
[27] V. Lavallo, Y. Canac, C. Präsang, B. Donnadieu, G. Bertrand, Angew.
Chem. 2005, 117, 5851-5855, Angew. Chem. Int. Ed. 2005, 44, 5705-
5709.
11005-11014; d) U. S. D. Paul, U. Radius, Organometallics 2017, 36,
1398-1407.
[29] S. Styra, M. Melaimi, C. E. Moore, A. L. Rheingold, T. Augenstein, F.
Breher, G. Bertrand, Chem. Eur. J. 2015, 21, 8441-8446.
[30] a) X.-W. Li, J. Su, G. H. Robinson, Chem. Commun. 1996, 2683-2684;
b) A.-L. Schmitt, G. Schnee, R. Welter, S. Dagorne, Chem. Commun.
2010, 46, 2480-2482.
[28] a) G. D. Frey, V. Lavallo, B. Donnadieu, W. W. Schoeller, G. Bertrand,
Science 2007, 316, 439-441; b) O. Back, M. Henry-Ellinger, C. D. Martin,
D. Martin, G. Bertrand, Angew. Chem. 2013, 125, 3011-3015, Angew.
Chem. Int. Ed. 2013, 52, 2939-2943; c) U. S. D. Paul, C. Sieck, M.
Haehnel, K. Hammond, T. B. Marder, U. Radius, Chem. Eur. J. 2016, 22,
This article is protected by copyright. All rights reserved.

10.1002/chem.201702166
Chemistry - A European Journal
FULL PAPER
Entry for the Table of Contents (Please choose one layout)
Layout 1:
FULL PAPER
(NHC)·AlH3 and
Heidi Schneider, Andreas Hock, Rüdiger
Bertermann, Udo Radius*
R'
R'
cAAC
rt
(NHC)·AliBu2H react with N-
heterocyclic carbenes and
cAACs with NHC ring
expansion, NHC ring opening,
or cAAC mediated Al–H bond
cleavage.
Dipp
N
iPr2Im
N
N
R
R
∆
T
H
Page No. – Page No.
Al
R
Al
H
The Reactivity of NHC Alane Adducts
towards N-Heterocyclic Carbenes and
Cyclic (Alkyl)(Amino) Carbenes: Ring
Expansion, Ring Opening, and Al-H
Bond Activation
N
H
H
H
N
H
N
N
R'
R
H
>
stable 110°C
H
H
E-H Bond
Activation
Al
R'
iPr N
N iPr
Dipp
H
Dipp
H
N
Ring
Expansion
Reaction
Al
N
Dipp2Im
N
N
H
H
∆
Al
T
N
N
H
H
Ring
Reaction
Opening
This article is protected by copyright. All rights reserved.