Activation of Molecular Hydrogen by Inter- and Intramolecular Al?N Lewis Pairs

Article abstract of DOI:10.1002/ejic.202001152

The field of frustrated Lewis pair chemistry offers many opportunities to activate molecular hydrogen, but Al?N systems have not been established yet. In this work, we describe several intermolecular classical Al?N Lewis pairs and an intramolecular ortho-

Full text of DOI:10.1002/ejic.202001152

Communications
doi.org/10.1002/ejic.202001152
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Activation of Molecular Hydrogen by Inter- and
Intramolecular AlÀ N Lewis Pairs
[a]
[a]
[a]
Alexander Bodach, Nils Nöthling, and Michael Felderhoff*
The field of frustrated Lewis pair chemistry offers many
opportunities to activate molecular hydrogen, but AlÀ N systems
have not been established yet. In this work, we describe several
intermolecular classical AlÀ N Lewis pairs and an intramolecular
ortho-ala-aminoarene for the activation of molecular hydrogen.
Their ability was investigated using the isotope exchange
reaction from HD to H and D . The herein studied intermo-
occur during a single-crystal to single-crystal transformation by
Autrey and coworkers.
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[5]
In contrast to these FLPs, the AlÀ N systems garnered far less
attention. Such studies were pursued, e.g. by Uhl and co-
[6]
workers, and rare examples for reactions with chalcogen-
[
7]
[8]
[9]
double
dehydrocoupling,
been reported.
bonds
(CO
,
2
ketones,
isocyanates ),
[10]
acryl and lactone polymerisation have
2
2
[11]
lecular Lewis pairs were based on alkylalanes and N-meth-
yldiphenylamine, while the intramolecular Lewis pair was
Noteworthy, there are very few reports on hydrogenation
(
o-TMPÀ C H )AlH ((2-(2,2,6,6-tetramethyl-piperidin-1-yl)phenyl)-
reactions using AlÀ N compounds: At first, the catalytic hydro-
6
4
2
i
i
aluminium dihydride). The activation of molecular hydrogen
was carried out in toluene under mild conditions and
monitored by H and H NMR spectroscopy. Furthermore, the
genation of imines with H
2
in the presence of HAlBu
2
(Bu=iso-
[12]
butyl) should be mentioned. The second example is a NON-
1
2
[13]
ligand stabilised Al(I) compound. Recently, the monomeric
2
7
i
i
AlÀ N interaction has been probed by Al NMR and crystallo-
Al(I) compound Al(C
6
H-2,6-(C
6
H -2,4,6-Pr
2
3
)
2
-3,5-Pr
2
) was de-
i
graphic studies. Additionally, the crystal structure of pure AlBu
scribed for hydrogen activation without the presence of a Lewis
3
[14]
was determined. These studies may attribute the pronounced
reactivities of these AlÀ N compounds to elongated AlÀ N bond
lengths.
base. Interestingly, it has been shown that aluminium metal
can be mechanochemically hydrogenated in the presence of
[
15]
one equivalent of a triethylenediamine (TEDA) or a transition
[16]
metal catalyst and piperidine.
The CÀ H activation and hydroalumination of alkynes by the
Introduction
intramolecular AlÀ N compound (o-TMPÀ C H )AlH ((2-(2,2,6,6-
6
4
2
tetramethyl-piperidin-1-yl)phenyl)-aluminium dihydride) have
[17,18]
The search for main-group-element compounds that activate
molecular hydrogen has evolved rapidly since the ground-
breaking report of a Frustrated-Lewis-Pair (FLP) by the Stephan
been previously reported.
To investigate the most crucial
test reaction for FLP chemistry, we herein report the hydrogen
1
2
activation by H and H NMR spectroscopy to follow the isotope
[1]
group. Since then, FLP chemistry has been widely explored
exchange of HD to H and D , as well as D to HD with (o-
2
2
2
[
2]
and reviewed frequently. Hitherto, the majority of combina-
tions utilise the Lewis acidity of boron and Lewis basicity of
phosphorus together with sterically demanding ligands to
inhibit adduct formation. Extension of this principle to other
Lewis acids (e.g. borenium and silylium cations as well as
alanes) or bases (e.g. amines, imines, carbenes and silylenes)
TMPÀ C H )AlH . Furthermore, we demonstrate that readily
6
4
2
i
accessible alane/amine mixtures, namely AlR (R=Me, Et, Bu)
3
with Ph NMe (N-methyldiphenylamine), as examples of inter-
2
molecular AlÀ N compounds, activate molecular hydrogen. The
pronounced reactivities of these complexes may be attributed
to the elongated AlÀ N bond lengths. Therefore, the crystal
[
3]
i
has resulted in the development of many FLP systems. Further
investigations extended the field to radical species with FLP
structures of AlMe À Ph NMe, AlEt À Ph NMe and AlBu have
3
2
3
2
3
been determined for a deeper molecular understanding.
[
4]
reactivity.
(
Remarkably,
a
hydrogenation
(1-(2-(bis(penta-fluorophenyl)boryl)
phenyl)-2,2,6,6-tetramethylpiperidine) was recently found to
of
(o-TMPÀ C H )AlH was synthesised based on its correspond-
6
4
2
[
[
19]
17]
o-TMPÀ C H )B(C F )
ing lithium compound described by the Repo group
6
4
6 5 2
(Scheme 1) and AlH . In contrast, Roesky and coworkers
3
synthesised the corresponding À AlCl compound first and
2
subsequently reduced it with LiAlH .
4
[
a] A. Bodach, N. Nöthling, Dr. M. Felderhoff
Max-Planck-Institut für Kohlenforschung
Kaiser-Wilhelm-Platz 1
E-mail: felderhoff@kofo.mpg.de
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejic.202001152
©
2021 The Authors. European Journal of Inorganic Chemistry published by
Wiley-VCH GmbH. This is an open access article under the terms of the
Creative Commons Attribution Non-Commercial NoDerivs License, which
permits use and distribution in any medium, provided the original work is
properly cited, the use is non-commercial and no modifications or adap-
tations are made.
Scheme 1. Synthesis of (o-TMPÀ C
6 4 2
H )AlH and HD-exchange reaction.
Eur. J. Inorg. Chem. 2021, 1240–1243
1240
© 2021 The Authors. European Journal of Inorganic Chemistry
published by Wiley-VCH GmbH

Communications
doi.org/10.1002/ejic.202001152
The observed structure of (o-TMPÀ C H )AlH consists of a
pure HD. In an equilibrium H , HD and D exist in equimolar
2 2
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hydride-bridged dimeric form. Therein, the AlÀ N distance of
.311(2) Å is significantly longer if compared to representative
amounts. These findings indicate that the À AlH -moiety adds
2
2
significant amounts of H to the given equilibrium. To validate
2
Al-N distances in similar compounds. Some selected representa-
that the hydrogen activation involves the AlH -moiety, a
2
tive compounds and their AlÀ N bond lengths are as follows,
complementary experiment using only D2 was performed,
which resulted in the formation of HD (Supporting Information).
Since a competitive reaction may involve the activation of
[
20]
[
Me NÀ C H À AlH ] 2.118(3) Å (hydride bridged dimer),
2
10
6
2 2
[21]
[22]
Me NÀ AlMe
2.045(2) Å,
(Me N) AlH
3
2.163(2) Å,
3
3
3
2
[15]
[
TEDAÀ AlH ] 2.161(2) Å (both AlÀ N distances are equal) and
deuterated solvent, a blind experiment only with H gas under
3
n
2
[23]
TEDA-(AlMe ) 2.065(8) Å. FLPs and classic Lewis pairs can be
the same conditions was performed. This did not result in the
formation of any HD, indicating that the reaction involves
gaseous hydrogen. Based on these observations, we assume a
hypervalent Al-species, like a six-coordinate [(o-TMPÀ C H )
3
2
discriminated by atomic distances. If the bond length is in the
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[24]
order of the sum of atomic radii (d(AlÀ N)=1.97 Å) it is a Lewis
adduct. However, if the atomic distance is in the order of the
6
4
[25]
sum of crystallographic van der Waals radii (d(AlÀ N)=3.39 Å)
it indicates “Frustration”. Based on these geometrical approx-
imations, (o-TMPÀ C H )AlH should be described more convinc-
AlH D] or even a charged penta-coordinate D[(o-TMPÀ C H )
3
6
4
AlH ], to be involved in a plausible mechanism.
3
Unfortunately, due the complexity of the system (three-
phase system, diffusion, (gas) solubility and precise loadings)
we cannot provide quantitative information. In further studies,
we are planning to study the kinetics of the reaction.
6
4
2
ingly as an “active” Lewis pair than an “FLP” as reported by
[18]
Chen et al. Noteworthy, the AlÀ N distance of 2.311(2) Å in
o-TMPÀ C H )AlH is slightly longer than in BÀ P based “active”
(
6
4
2
Lewis pairs (d(BÀ P) ~2.18-2.20 Å), which exhibit classic FLP
Since the first hydrogen activation with the intramolecular
Al-N compound (o-TMPÀ C H )AlH was successful, we continued
[26]
reactivity, as described by Erker and coworkers. In contrast,
6
4
2
Repo and coworkers have reported (o-TMPÀ C H )B(C F ) as a
to use intermolecular pairs. These intermolecular pairs are
readily available by mixing alanes with amines. This offers
variable basicity of the amine and different sterically demand-
ing ligands on the aluminium. A selection of different amines
(including TEDA, lutidine, dimethylaniline) and readily available
alanes was tested without conclusive results. Similar non-
6
4
6 5 2
[19]
“
real FLP” with d(BÀ N)=3.031(4) Å.
Since (o-TMPÀ C H )AlH may describe the lower limit for
6
4
2
FLP-type reactivity for the Al-N system, it raises the questions
can it activate molecular hydrogen? Therefore, we chose the
common isotope exchange reaction of HD to H2 and D2 in
toluene-d to address this question.
conclusive results were obtained when only trisalkylalanes or
8
i
A slurry of (o-TMPÀ C H )AlH in toluene-d was charged with
HAlBu were used.
6
4
2
2
1
HD gas, and the isotope exchange reaction was followed via H
However, successful HD-exchange experiments with
i
NMR, Figure 1, Supporting Information. Initially, the HD triplet
N-methyldiphenylamine (Ph NMe) and AlR (R=Me, Et, Bu)
2
3
1
occurs at 4.46 ppm ( J=43 Hz), while the H singlet at 4.50 ppm
were conducted and monitored by NMR spectroscopy,
Scheme 2.
2
is increasing over time (~1 h) and gentle heating (additional
i
1
~
90 min, 60°C). This indicates the activation of HD, allowing
The reaction of AlBu À Ph NMe with HD was followed via H
3
2
2
the formation of H and D (D was not measured due to low
and H NMR spectroscopy. We observed the formation of H
2
2
2
2
sensitivity). In another, longer experiment, the HD-exchange
and D2 within an hour at ambient temperature, Figure 2,
1
reaction resulted in H amounts above the expected yields for
Supporting Information. Quantification via H NMR spectro-
2
[27]
scopy (corrected for the presence of para-hydrogen, but not
considering diffusion processes) resulted in a specific rate of
À 1 À 1
À 1 À 1
~
0.033 molmolcat
h
(~91 μmolgcat h ). This specific rate is
of the same order of magnitude as the hydrogenation reactions
[2b]
with common FLP catalysts.
Similar HD-exchange experi-
ments with AlR À Ph NMe (R=Me, Et) show that they convert
3
2
HD just at the detection level and therewith significantly slower
i
than AlBu À Ph NMe, Supporting Information. We could not
3
2
1
detect any change of the employed AlÀ N compounds using H
NMR spectroscopy, which points to a catalytic function.
Since the HD-exchange reactions proceed slower with less
bulky alkylalanes, we expect a correlation to steric reasons, such
1
Figure 1. H NMR spectra of the HD-exchange reaction with (o-TMPÀ C
6
H
4
)
Scheme 2. Hydrogen activation by HD isotope exchange using alane-amine
mixtures.
AlH
2
in Tol-d
8
, for the full-spectrum see Supporting Information.
Eur. J. Inorg. Chem. 2021, 1240–1243
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1241
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Communications
doi.org/10.1002/ejic.202001152
i
AlBu it is preferably on the monomeric side under ambient
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conditions. These equilibria are strongly dependent on temper-
[
29]
ature and concentration.
i
A crystal of AlBu suitable for single X-ray diffraction could
3
be grown by using in situ crystallisation in a sealed glass
capillary, Figure 4, Supporting Information. The obtained struc-
ture consists of an iso-butylene-bridged dimer. The AlÀ C bond
lengths are 1.972(3) Å and 1.983(4) Å for the terminal alkyl
groups, while the bridging alkyl groups exhibit AlÀ C bond
i
lengths of 2.128(4) Å and 2.209(2) Å. It is noteworthy that AlBu
3
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[
30]
exhibits slightly longer AlÀ C bonds than dimeric AlMe .
3
2
7
Furthermore, Al NMR spectroscopic studies showed a
similar trend: In Al spectra, the chemical shift depends on the
coordination number of the aluminium atoms, while the signal
2
7
width increases strongly with lowered symmetry and bulkier
1
2
Figure 2. H and H NMR spectra of the HD-exchange reaction with
[31]
ligands up to unobservable broadness.
A simple 1D “zg”
i
AlBu
the H
3
À Ph
2
NMe in Tol-d after 60 min at ambient temperature, referenced to
8
1
pulse sequence (π/2-acquisition) experiment is hard to phase
for broad peaks leading to a wavy background. Additionally, an
artificial peak from the probe also overlaps to a certain extent.
Therefore, we used an aring sequence (π/6 – τ – π/6 – τ – π/6 –
acquisition), enabling an easier phasing, flat background and a
reduced artificial signal (Supporting Information).
2
signal at 4.50 ppm and D
2
at 4.50 ppm for comparison. The J HÀ D
coupling of 43 Hz is shown. # indicates trace impurities, for the full-spectrum
see Supporting Information.
as an elongated AlÀ N distance. However, this hypothesis could
not be conclusively investigated, since we were unable to grow
For AlEt we observed a four-coordinated Al species in the
3
,
i
27
suitable crystals of the liquid AlBu À Ph NMe. On the other
dimer [AlEt ] ( Al δ=157 ppm) which is only slightly shifted
3 2
3
2
2
7
i
hand, single crystals of AlMe À Ph NMe could be grown, and the
for Ph NMeÀ AlEt ( Al δ=164 ppm). In contrast, AlBu showed
3
2
2
3
3
27
liquid AlEt À Ph NMe could be crystallised in situ on a single-
two signals in the Al NMR spectrum for three- and four-
coordinated species identified as a monomer AlBu (major
species, Al δ=283 ppm) and a dimer [AlBu ] (minor species,
Al δ=155 ppm). In the respective NMR spectrum of
3
2
i
crystal diffractometer (Supporting Information). This method
has already been used successfully several times, including the
3
2
7
i
3
2
[28]
27
determination of an absolute structure.
i
i
Both alane-amine mixtures form classic Lewis adducts upon
crystallisation, Figure 3, Supporting Information. In a direct
comparison of Ph NMeÀ AlMe and Ph NMeÀ AlEt , the AlÀ N
AlBu À Ph NMe, the signal of the monomer AlBu is almost
3
2
3
27
unchanged ( Al δ=268 ppm) and the four-coordinated species
i
27
seems to be the dimer [AlBu ] ( Al δ=157 ppm) and not an
2
3
2
3
3 2
bond length increases from 2.159(2) Å to 2.184(2) Å. This can be
attributed to the different steric demands. We, therefore, expect
amine adduct. However, the uncertainty of the position of a
broad peak has to be taken into account.
i
that the Ph NMeÀ AlBu has at least the same AlÀ N bond length
Based on our results we expect that the AlÀ N distance in
2
3
i
or rather an even longer AlÀ N distance, which enables FLP-type
reactivity.
AlBu À Ph NMe is elongated compared to thus of the AlEt and
3
2
3
AlMe derivatives. This could explain the classic FLP reactivity
3
A closer look at the nature of alkylalanes reveals that all of
them exist in a monomer-dimer equilibrium. For AlEt3 this
equilibrium is almost exclusively on the dimer side while for
towards hydrogen as an active Lewis pair similar to (o-
TMPÀ C H )AlH .
6
4
2
In this work, we have demonstrated that inter- and intra-
molecular Al-N compounds with elongated bonds act as
Figure 3. Molecular structures of Ph
right) at 100 K, Al (green), C (black), N (blue), hydrogen atoms omitted for
clarity, see Supporting Information.
2
NMeÀ AlMe
3
(left) and Ph
2
NMeÀ AlEt
3
i
(
Figure 4. Molecular structure of dimeric AlBu
3
at 100 K, Al (green), C (black),
hydrogen atoms omitted for clarity, see Supporting Information.
Eur. J. Inorg. Chem. 2021, 1240–1243
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Communications
doi.org/10.1002/ejic.202001152
“
active” Lewis Pairs. Isotope exchange reaction studies using HD
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For (o-TMPÀ C H )AlH an increased H /HD ratio beyond the
6
4
2
2
expectation for pure HD and the exchange of D to HD was
2
found. Based on these results, we conclude a reaction pathway
presumably includes a hypervalent Al species. We further
continue to investigate these systems and their intermediates
with an emphasis on their activity and structure. Especially the
elongation of AlÀ N distances will be focussed. These studies
may allow for catalytic hydrogenations if suitable substrates
and conditions tolerant towards the strong Lewis acidic
aluminium compounds are used.
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1
1
1
1
1
1
1
1
1
1
2
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2
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2
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2
2
3
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3
3
4
4
4
4
4
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5
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Deposition Numbers 2031283 (for Ph NMeÀ AlEt ), 2031284
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3
i
1559.
(
for AlBu ), and 2031285 (for Ph NMeÀ AlMe ) contain the
3
2
3
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Access Structures service www.ccdc.cam.ac.uk/structures.
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Acknowledgements
The authors thank Markus Kochius for measuring the NMR spectra
27
of pressurised samples, Dr. Markus Leutzsch for developing the Al
NMR aring sequence, Bastian Herrmann for helping to synthesise
the described compounds as well as Dr. Lauren E. Longobardi, Dr.
Richard Goddard and Prof. Dr. Ferdi Schüth for helpful discussions.
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Keywords: Aluminum · Main group elements · Hydrogen ·
Crystal growth · Structure elucidation
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Manuscript received: December 22, 2020
Revised manuscript received: January 29, 2021
Accepted manuscript online: February 1, 2021
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