Amino?Organolithium Compounds and their Aggregation for the Synthesis of Amino?Organoaluminium Compounds

Article abstract of DOI:10.1002/ejic.202100224

Here, we present a thorough structural study of small, easily accessible amino?organolithium compounds with bridging phenyl and naphthyl moieties. Their crystal structures most likely represent their aggregation as tetramers and dimers in both hydrocarbon and ethereal solvents. These amino?organolithium compounds were further used to generate their corresponding aluminium compounds as a model system. Their crystal structures are reported too. All structures are discussed with a focus on the different steric demands and bridging moieties. Additionally, the crystal structures of neophyllithium and decomposition products of some of the compounds mentioned above are reported. This study provides additional data for the future design and synthesis of amine stabilised organolithium and -aluminium compounds.

Full text of DOI:10.1002/ejic.202100224

European Journal of Inorganic Chemistry
Accepted Article
Title: Amino-Organolithium Compounds and their Aggregation for the
Synthesis of Amino-Organoaluminium Compounds
Authors: Alexander Bodach, Jochen Ortmeyer, Bastian Herrmann, and
Michael Felderhoff
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202100224
Link to VoR: https://doi.org/10.1002/ejic.202100224
01/2020

10.1002/ejic.202100224
European Journal of Inorganic Chemistry
FULL PAPER
Amino-Organolithium Compounds and their Aggregation for the
Synthesis of Amino-Organoaluminium Compounds
Alexander Bodach, [a] Jochen Ortmeyer,[a], [b] Bastian Herrmann,[a] Michael Felderhoff*[a]
[a]
A. Bodach, Dr. J. Ortmeyer, B. Herrmann, Dr. M. Felderhoff
Department of Heterogeneous Catalysis
Max-Planck-Institut für Kohlenforschung
Kaiser-Wilhelm-Platz 1
45470, Mülheim an der Ruhr, Germany
E-mail: felderhoff@kofo.mpg.de
[b]
present adress Dr. J. Ortmeyer
Institute of Inorganic Chemistry,
RWTH Aachen University,
Landoltweg 1, 52074 Aachen, Germany
CCDC 2070399-2070412 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge, e.g.,
via www.ccdc.cam.ac.uk/data_request/cif.
Abstract: Here, we present a thorough structural study of small,
easily accessible amino-organolithium compounds with bridging
phenyl and naphthyl moieties. Their crystal structures most likely
represent their aggregation as tetramers and dimers in both
hydrocarbon and ethereal solvents. These amino-organolithium
compounds were further used to generate their corresponding
aluminium compounds as a model system. Their crystal structures are
reported too. All structures are discussed with a focus on the different
steric demands and bridging moieties. Additionally, the crystal
structures of neophyllithium and decomposition products of some of
the compounds mentioned above are reported. This study provides
additional data for the future design and synthesis of amine stabilised
organolithium and –aluminium compounds.
Currently, organolithium chemistry is evolving in several
directions: First, experimental charge density studies revealed the
nature of these unusual coordination bonds,[15] second catalytic
coupling reactions are still improved,[16] third, greener approaches
trying to avoid the usage of solvents and exploit solid
organolithium compounds in mechanochemical reactions,[17] and
fourth, they might be applied for syn-gas chemistry.[18]
Besides the crystallographic studies, which give the exact
structure, but only of (single-)crystalline compounds, NMR
spectroscopic studies reveal the aggregation in solution, which
sometimes differs, depending on, e.g., concentration and solvent.
Noteworthy for this study are several reported NMR investigations
on a series of naphthyllithium compounds[19] and several ortho-
dimethylamino-aryllithium compounds[20] in solution. Additionally,
there are approaches to utilise 7Li residual quadrupolar couplings
to identify the degree of organolithium aggregation in gels,[21]
DOSY,[22] classic 7Li,[23] and scalar Li-Li couplings[24] in NMR
spectroscopy.
Introduction
It has been more than 200 years since the discovery of lithium
metal and 100 years since the first synthesis of an organolithium
compound by Schlenk&Holtz.[1] This was followed by the report of
a convenient synthesis strategy from lithium metal and alkyl
halides by Ziegler in 1930.[2] Until now, chemists have learned to
handle, modify, and apply organolithium compounds for a wide
range of applications, even on an industrial scale.[3] One of the
most determining parameters for organolithium compounds is
their aggregation. Alkyllithium compounds are known to form
In addition to the many applications of organolithium compounds,
they can also be used to synthesise organoaluminium
compounds. These compounds garnered yet little attention aside
from the well-known Ziegler polymerization.[25] This is now
changing; several groups have recently started to explore
nitrogen-stabilised organoaluminium compounds, including Al(I)
and radical species, namely Power,[26] Braunschweig,[27]
Aldridge,[28] Stephan,[17b] and Roesky[29]. Simultaneously, there
are even reports on the activation of molecular hydrogen by
Stephan and co-workers[30] using di-iso-butylaluminium-hydride
and imines or by Aldridge and co-workers[31] using a complex
aluminium imide. Also, we recently reported on the hydrogenation
of metallic aluminium in the presence of piperidine[32] or TEDA
mostly hexamers, e.g., (iBuLi)6,[4] (iPrLi)6,[5] (nBuLi)6,[6] and
[8]
tetramers, e.g., (tBuLi)4,[6] (EtLi)4,[7] (MeLi)4
. In contrast,
aryllithium compounds form insoluble coordination polymers of
dimers, e.g., PhLi,[9] MesLi,[10] TolLi,[11] whose structures could
only be determined from X-ray powder data. Nevertheless, the
aggregation of NpLi (Np = neopentyl) and NphLi (Nph = neophyl,
2-methyl-2-phenylpropyl) in the solid-state has remained unclear.
These compounds are widely used, e.g., for the synthesis of
Schrock carbenes[12] and soluble Schlosser bases[13] because
they lack a β-H moiety and therewith do not allow β-H elimination.
The aggregation of organolithium compounds is strongly
influenced by the electronic and steric properties of the ligand as
well as the presence of bases and solvents.[3b, 14] Here, especially
amines and ethers play a vital role.[14c]
(mechanochemical, TEDA
=
triethylenediamine)[33] and H2
activation by inter- and intramolecular Al-N Lewis pairs[34]. Among
the plenty of FLP (Frustrated Lewis Pair) literature,[35] the Al-N
systems garnered far less attention. Nevertheless, there are
noteworthy reports, e.g., by Uhl and co-workers,[36] and selected
examples of reactions with chalcogen-containing double bonds
(CO2,[37] ketones,[38] isocyanates[39]), dehydrocoupling,[40] acryl
and lactone polymerisation are among the few that have been
reported.[41]
Therefore, this study focuses on the syntheses and aggregations
of intramolecular amine-stabilised organolithium compounds,
1
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10.1002/ejic.202100224
European Journal of Inorganic Chemistry
FULL PAPER
which are common precursors for the syntheses of amine-
group-13-element (frustrated) Lewis pairs. Generally, the
organolithium compounds have been synthesised by directed
ortho-metallation or lithium-halogen-exchange reactions and
further reacted with alkylaluminium halides or aluminium hydride,
Scheme 1.
worth noting that we found a remarkable “shelf-life time” in the
order of years for compounds 1-6.
Scheme 1. General synthesis pathways of selected organolithium
compounds towards organoaluminium compounds.
A
deeper understanding of the formation principles and
geometrical changes in the aggregation of these organolithium
compounds will define further applicability for syntheses of main-
group-element compounds and serve as a model system. Here,
we present a rare example where the organolithium compounds
could be isolated donor-free and coordinated by ethers, which
most likely reflect the species present in solution. These
organolithium compounds were, for demonstration purposes,
used to synthesise intramolecular Al-N Lewis pairs and provide
alkyl derivatives of the known (o-TMP-C6H4)AlH2, which has been
used for hydrogen[34] and alkyne[42] activation.
Scheme 2. Schematic syntheses and aggregations of amine-
stabilised organolithium compounds 1-6 and neophyllithium 7.
Results and Discussion
The organolithium compounds were synthesised by lithium-
halogen-exchange reactions at low temperature or by directed
ortho-metallation with n-butyllithium based on literature
procedures,[34, 43] Scheme 2.
Remarkably, [Me2NC6H4Li]4 (1) crystallised as large cubes from
Et2O/pentane as a tetramer in a cubic crystal structure (1a) with
small voids of ~30 Å3 per unit cell, Fig. 1a. Polymorph 1a slowly
(~1 week) transformed into the respective monoclinic polymorph
(1b) without voids. Although these voids seem to be large enough
to bear H2O, its presence can be excluded due to the reactivity of
1a itself and the following phase transformation to 1b, which
contains no voids/water. The metastable cubic phase 1a seems
to undergo more likely a kinetically controlled crystallisation with
reduced packing density, and therefore the phase transformation
occurs to the more densely packed monoclinic polymorph.
The respective tetramer of [Me2NC10H6Li]2 (3) could only be
crystallised in a monoclinic form from benzene, incorporating
some benzene molecules, Fig. 1b. In contrast, the crystallisation
from ethereal solvents gave ether adducts [Me2NC10H6Li•Et2O]2
(4) and [Me2NC10H6Li•MTBE]2 (5), solely, vide infra.
Figure 1. Molecular structures of (a) [Me2NC6H4Li]4 1a (cubic
polymorph), (b) [Me2NC10H6Li]4 3 (benzene solvate, not shown),
(c) [(o-TMP-C6H4)Li]2 6 and (d) [NphLi]4 7, Ar‧‧‧Li interactions
shown as dashed lines, carbon (black), nitrogen (blue), lithium
(grey), hydrogen atoms omitted for clarity, all displacement
ellipsoids are shown with 50% probability.
A closer look at the structures of these donor-free organolithium
aggregates reveals that the tetrameric compounds 1a, 1b, and 3
each exhibit distorted heterocubanes as structure building motifs.
A comparison of the bond lengths, Table 1, reveals that the Li-N
bond length in cubic 1a is slightly shorter than in the monoclinic
form 1b while the Li-C bond length is slightly increased and the Li
‧‧‧Li distances are similar. Overall these parameters are similar
Furthermore, by increasing the steric demand from
dimethylamino- in 1 to tetramethylpiperidinyl- in [(o-TMP-C6H4)Li]2
(6), we could only crystallise a dimer in a monoclinic structure
from Et2O, surprisingly without incorporating ether, Fig. 1c. It is
2
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10.1002/ejic.202100224
European Journal of Inorganic Chemistry
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t
to the report of the co-crystal of [Me2NC6H4Li]4 (1) with BuLi, by
Stalke and co-workers[44]. In contrast, changing the phenyl (1) to
a naphthyl (3) as a bridging motif, the bond lengths stay similar
with respect to an increased deviation among approximately
equal Li-C and Li-N bond lengths, Table 1.
Table 1. Selection of characteristic Li-E interatomic distances (E = Li, C, N), including min. and max. bond lengths to α- and γ-carbon atoms, found for “equivalent”
bonds, space groups and Z’ (No of monomers in the asymmetric unit) for compounds 1a, 1b, 3, 6, and 7.
1a
1b
3
6
7
Formula
[Me2NC6H4Li]4
[Me2NC6H4Li]4
[Me2NC10H6Li •C6H6]4
[(o-TMP-C6H4)Li]2
[C10H13Li]4
̅
Space group (No.)
P21/c (14)
P21/n (14)
P21/n (14)
I41/a (88)
I43d (220)
Z’
1
4
4
1
4
d(Li-N) / Å
d(Li-C) / Å
2.018(3)
Min 2.030(6)
Max 2.041(6)
Min 1.984(3)
Max 2.036(3)
2.026(3)
2.243(3)
2.283(3)
2.228(3)
Min 2.213(6)
Max 2.263(6)
Min 2.184(3)
Max 2.439(3)
2.138(3)
2.452(3)
Min α 2.21(3)
Max α 2.337(3)
Min γ 2.439(3)
Max γ 2.550(3)
2.489(4)
2.622(5)
Min 2.488(8)
Max 2.678(8)
Min 2.463(4)
Max 2.629(4)
2.220(5)
Min 2.469(4)
Max 2.505(4)
d(Li‧‧‧Li) / Å
Furthermore, we have synthesised neophyllithium NphLi (7),
commonly used as a cheap replacement of neopentyllithium NpLi
by direct synthesis from neophylchloride lithium metal
analogously to the classic Schrock synthesis.[45] Both 7 and NpLi
might be used to increase the steric demand on the Al site while
offering high stability for future studies. It is worth noting that 7
and NpLi are colourless powders with a “shelf-lifetime” of several
years due to their inability to undergo β-hydride elimination.
Finally, we could elucidate the crystal structure of 7 in space
group I41/a (88) as a tetramer. The bond lengths are comparable
to those of tetrameric (m-bicyclo[2.2.1]heptan-1,1,1-triyl)-
lithium,[46] although the short Li-Cγ contacts in 7 are unique in the
CCDC database for neat, tetrameric organolithium compounds
(only H, Li, C), Table 1. In contrast, the structure of NpLi in the
solid-state remains unclear, probably due to a plastic crystalline
phase (long-range order, but no local order, e.g., molecules
rotating on their position).[47] A similar study showed hexameric
Serendipitously, we obtained a decomposition product of 1 as
hexamer [Me2NC6H4OLi]6 (8), which inserted oxygen atoms
between each pair of lithium and carbon atoms, Fig. 2b. Since we
obtained 8 from a non-specific process, we could not evaluate in
detail whether 1 cleaved Et2O or oxygen “sneaked” into the flask.
Nevertheless, the crystal structure provides insights into the
decomposition of amino-organolithium compounds. The hexamer
8 consists of two stacked Li3O3 rings, while the bridging
2-(dimethylamino)phenyl groups lead to a propeller arrangement.
The crystal structure is described disordered in a trigonal system
̅
with the rarely observed space group R3.
iBuLi[4] existing in
a plastic crystalline phase at ambient
temperature and an ordered crystalline phase at lower
temperatures.
Since a significant fraction of organolithium compounds is used in
ethereal solutions, especially for metathesis reactions, we carried
on to study the ether adducts and their aggregation. This will allow
predicting their reactivity for future synthetic protocols. The
crystallisation of 1 from Et2O led to a dimer (2) in a triclinic crystal
structure with two monomers in the asymmetric unit, Fig. 2a. The
dimer consists of a Li2C2 diamond, where the Li-coordination is
saturated in a distorted tetrahedral geometry by Et2O and amine,
respectively. This is probably the active form for the following
metathesis reaction to the respective aluminium compounds, vide
infra.
Figure 2. Molecular structures of (a) [Me2NC6H4Li•Et2O]2 2, (b)
Analogously to 1, 3 forms dimers upon crystallisation from Et2O
(4) and MTBE (5), respectively, Fig. 2c and d. Both 4 and 5 exhibit
a similar coordination geometry despite the larger linker (naphthyl
instead of phenyl), while the MTBE is disordered on two positions.
Accordingly, the bond lengths of 2, 4, and 5 are very similar, as
expected: The Li-N bonds are all ~2.1 Å long, the Li-C bonds are
in the range of 2.1-2.3 Å, the Li-O bonds are ~2.0 Å, while the Li‧
[Me2NC6H4OLi]6 8, (c) [Me2NC10H6Li•Et2O]2
4
and
(d)
[Me2NC10H6Li•MTBE]2 5, only one of the two disordered hexamers
of 8 is shown, carbon (black), nitrogen (blue), oxygen (red), lithium
(grey), hydrogen atoms omitted for clarity, all displacement
ellipsoids are shown with 50% probability.
The hexamer of 8 can be described as two stacked cyclic trimers
based on Li3O3 six-membered rings, while each amine group
additionally coordinates to a Li atom within the same ring. This
leads to a propeller shape and therewith to asymmetry. This
‧‧Li distances are ~2.4 Å, Table 2. These bond lengths are also
[49]
similar to [(Me2N)2C10H5Li•Et2O]2[48] and [(Me2N)C10H6Li•THF]2
.
3
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10.1002/ejic.202100224
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FULL PAPER
asymmetry is compensated by a disorder where both possible
orientations are similarly occupied, leading to a “pseudo-
racemate”. For visualisation, only one of those is shown in Fig. 2b.
Similar hexamers with the similar or slightly distorted core
structure of Li6O6 and a variety of coordinating amino ligands are
also reported in the CCDC database with comparable bond
lengths, e.g., ADATOP,[50] BUYSOF,[51] PIKCES[52] (CCDC
Refcodes).
Table 2. Selection of characteristic Li-E interatomic distances (E = Li, C, N, O), including min. and max. bond lengths, found for “equivalent” bonds, space groups
and Z’ (No of monomers in the asymmetric unit) for compounds 2, 4, 5, and 8.
2
4
5
8
Formula
[Me2NC6H4Li•Et2O]2
[Me2NC10H6Li•Et2O]2
[Me2NC10H6Li•MTBE]2
[ Me2NC6H4OLi ]6
̅
̅
Space group (No.)
P1 (2)
P21/c (14)
P21/c (14)
R3 (148)
Z’
2
1
1
1
d(Li-N) / Å
2.103(2)
2.138(2)
2.113(3)
2.164(2)
2.043(3)
2.065(3)
Min 2.161(2)
Max 2.238(2)
d(Li-C) / Å
2.219(3)
2.189(3)
2.224(2)
2.261(2)
Min 2.667(4)
Max 2.772(3)
d(Li‧‧‧Li) / Å
2.448(4)
2.468(3)
2.344(5)
2.429(3)
2.597(3)
2.260(3)
1.973(2)
d(Li-O) / Å
1.930(3)
2.052(2)
Min 1.917(2)
Max 1.985(2)
Since we know that different solvents do not only change the
solubility of organolithium compounds but also change their
aggregation (vide supra), we chose to use these organolithium
compounds deliberately in Et2O to increase their reactivity
through smaller reactive dimers instead of tetramers (1 and 3) in
hydrocarbon solvents. To investigate this reactivity, we chose a
hitherto rarely studied and currently reviving field of simple main-
group element compounds. Accordingly, Al-N compounds, where
an increased Al-N distance could possibly lead to frustration, were
selected as target molecules. Similar Al-N compounds recently
garnered particular interest for their potential to activate molecular
hydrogen as Lewis pairs with typical FLP reactivity.[34]
However, up to now we did not obtain the desired scope of single
crystals. Especially, 16-18 resisting our crystallisation attempts.
Nevertheless, we could crystallise the ether adduct
(o-TMP-C6H4)AlEt2•Et2O (17) from Et2O and the decomposition
product of 18 namely [(o-TMP-C6H4)Al(Np)(ONp)]2 (19) from a
pentane solution that was left open to air.
Subsequently, the above-mentioned organolithium compounds
(mostly etherates) were reacted with dialkylaluminiumchlorides
R2AlCl or freshly prepared AlH3, Scheme 3. First, Me2NC6H4AlMe2
(9) was synthesised and crystallised from hexane/Et2O.
Surprisingly, from an Et2O-solution of the crude product of 9within
several weeks, the side-product (Me2NC6H4)2AlCH3 (10) could be
crystallised as minor species from Et2O. This may point to a
disproportionation reaction, where also AlMe3 should have been
formed and probably left to the gas phase. Analogously to 9, the
respective naphthalene-bridged compounds 12 and 13 have been
synthesised.[53] [Me2NC10H6AlH2][54] (11) was synthesised by a
different method using AlH3 and its crystal structure confirmed
with a higher qualty dataset (100 K instead of ambient
measurement, Tab. 3), and additionally Me2NC10H6AliBu2 (14)
was prepared similarly to 9, Scheme 3. To investigate the
influence of the steric demand of the amino group, the compounds
(o-TMP-C6H4)AlR2 (R = H, Me, Et, Np) (15-18) were synthesised.
The synthesis of 15 has been reported previously,[34] while the
others were synthesised analogously to above-mentioned
compounds, see experimental section.
Scheme 3. Syntheses of amine-stabilised organoaluminium compounds 9-18
from R-Li compounds.
Remarkably, the crystal structure of 9 is described in the rarely
̅
observed space group R3 (148) with an Al-N bond length of
2.103(2) Å, Fig. 3a, Table 3. All bond lengths are comparable to
the ones reported by Schumann et al.[53] for the naphthylene-
bridged derivative 12 and its respective Et-derivative 13 and
within the expected range (d(Al-N) = 2.103(2) Å, d(Al-C) =
1.965(2) - 1.996(2) Å,), Table 3.
In contrast, the Al-N bond length in 10, described in space group
Iba2 (45), is slightly elongated to 2.201(2) Å in comparison to 9 as
expected for a penta-coordinated Al species, Fig. 3b, Table 3. The
Noteworthy for the synthesis of 18 is that Np2AlCl was
synthesised from a modified solvothermal procedure,[55] and also
mechanochemically using a common shaker mill and LiCl as
milling medium.
4
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10.1002/ejic.202100224
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bond lengths in the crystal structure of 10 are similar to those of
the chloro-derivative reported by Kannan et al.[38b] and the
naphthylene bridged Et-derivative reported by Schumann et
al.[53a]
Increasing the steric bulk to TMP, we could only crystallise the
Et2O-adduct 17, Fig. 3c, which has a drastically increased Al-N
distance of 3.294(2) Å). On the one hand, this Al-N distance is in
the range of FLPs reported, in the order of the sum of
crystallographic van der Waals radii[56] (d(Al-N) = 3.39 Å). On the
other hand, 17 is not representative of an FLP because an Et2O
molecule fills the free coordination site of the Al atom.
Notably, ~30% of one Al-Et group is described as a Cl atom,
which may hint at an undesired side reaction. In this side reaction,
both Et and Cl of the Et2AlCl could react with 6 at approximately
similar rates to give a mixed product with predominantly Et groups
but also Cl atoms. This would be unexpected. However, Et and Cl
both consist of seventeen electrons. Hence, our fitted Cl atom
might be disordered Et group, but we could not obtain a suitable
disordered model and the centre of the electron density should be
more diffuse for an Et group.
The partly-oxidised product 19 forms inversion-symmetric
oxygen-bridged dimers in a monoclinic crystal structure, Fig. 3d.
The bond lengths are all as expected, based on the sum of atomic
radii.[57] Interestingly, the Al-N distance increases to 3.504(2) Å for
this alcoholate compared to the ether adduct 17.
Figure 3. Molecular structures of (a) Me2NC6H4AlMe2 9, (b) (Me2NC6H4)2AlCH3
10, (c) (o-TMP-C6H4)AlEt2•Et2O 17 (disorder/Cl atom not shown for clarity), and
(d) [(o-TMP-C6H4)Al(Np)(ONp)]2 19, carbon (black), nitrogen (blue), oxygen
(red), aluminium (green), hydrogen atoms omitted for clarity, all displacement
ellipsoids are shown with 50% probability.
Table 3. Selection of characteristic Al-E interatomic distances (E = C, N, O, Al), including min. and max. bond lengths, found for “equivalent” bonds, space groups
and Z’ (No of molecules/monomers in the asymmetric unit) for compounds 9, 10, 17, and 19. For comparison, the respective entries for 12 and 13 are reported
here as well.[53]
9
10
11
12[53a]
13[53b]
17
19
Formula
C10H16AlN
C17H23AlN2
C12 H14 Al N
C14H18AlN
C16H22AlN
C22.50 H40.65 Al Cl0.30
O
N
[C25H44Al NO]2
Space group (No.)
R¯3 (148)
1
Iba2 (45)
1
P21/c, (14)
1
P212121 (19)
1
P21/c (14)
4
P21/n (14)
1
P21/n (14)
1
Z’
d(Al-N) / Å
2.103(2)
2.201(2)
2.237(2)
2.124(2)
2.069(2)
Min 2.056(3)
Max 2.070(3)
3.293(2)
3.504(2)
d(Al-Al) / Å
d(Al-C) / Å
2.7561(8)
1.979(2)
2.002(2)
2.003(2)
2.881(2)
-
1.965(2)
1.970(2)
1.996(2)
1.995(2)
1.998(2)
2.000(2)
1.971(3)
1.972(3)
1.984(2)
Min 1.961(3)
Max 1.981(3)
1.964(2)
1.989(2)
1.999(2)
1.964(2)
d(Al-O) / Å
1.853(2)
1.883(2)
Knowing that the above-mentioned Al-N compounds may be
eligible for H2 activation, based on our previous study,[34] the HD-
activation was tested in the same manner. However, this did not
lead to conclusive results. The reasons for this inactivity can be
found in the crystal structures: The etherate 17 cannot act like an
FLP with a coordinating Et2O molecule, and 16 (as well as 9-14)
might not be sterically hindered enough. This may point out the
need for synthesising compounds with sterically more demanding
ligands, an AlH2-moiety, or a suitable amine of lower basicity.
Conclusion
Here, a thorough structural study of aggregated amine-stabilised
organolithium compounds and their respective dimeric ether
adducts, which are most likely the soluble reacting species, is
demonstrated. The discussed donor-free dimethylamino-phenyl
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SHELXL.[62] All non-hydrogen atoms were refined with anisotropic
displacement parameters, while the hydrogen atoms were refined
isotropically.
or –naphthyl lithium compounds form tetramers while their ether
adducts are dimers. In contrast, the TMP-phenyl lithium
compound forms only dimers due to the increased steric bulk.
Furthermore, these organolithium compounds were used to
synthesise the respective aluminium compounds. Based on our
previous study,[34] we tested their ability to activate molecular
hydrogen by HD isotope exchange reactions, which did not show
any conclusive reactivity. Therefore, their crystal structures were
analysed to investigate the reasons for their inactivity towards
H2/HD. This points strongly towards the need for the future
development of compounds with steric more demanding ligands.
Additionally, the structure of NphLi (7) as a common alternative to
NpLi and robust bulky ligand transferring agent as well as several
decomposition products of the mentioned organolithium
and -aluminium compounds are reported for a broader view on
the described compounds. All reported organolithium compounds
have a remarkable shelf-life time in the order of years, which is
especially surprising for the ether adducts. This study presents
valuable structural information for the future design and synthesis
of these kinds of molecules.
Syntheses
NpLi (neopentyllithium)[45] and Np3Al (tris(neopentyl)aluminium) have
been synthesised and characterised as described in literature[17b]. 12 ((8-
(dimethylamino)naphthalenyl)-1-dimethylaluminium)
(dimethylamino)naphthalenyl)-1-diethylaluminium)
and
were
13
prepared
((8-
according to literature procedure.[53b] (o-TMP-C6H4)I ((2-(2,2,6,6-
tetramethyl-piperidine-1-yl)phenyl)-iodide) and 6 ((2-(2,2,6,6-tetramethyl-
piperidine-1-yl)phenyl)-lithium) have been synthesised according to
literature procedures[43b]. 6 was recrystallised from Et2O and the obtained
single crystals were harvested for diffraction experiments. 15 has been
described in our previous communication[34] as well as by Roesky and co-
workers.[42a]
Np2AlCl (bis(neopentyl)aluminium chloride) was synthesised based on a
modified literature procedure.[55] A mixture of AlCl3 (97mg, 0.73mmol, 1
eq.) and Np3Al (350 mg, 1.5 mmol, 2 eq.) was heated in toluene to 80 °C
for 3 h, then cooled to ambient and filtered. The residue was washed twice
with 10 mL toluene. The filtrate was dried in vacuo and the product therein
was purified by recrystallisation from pentane solution and sublimation in
vacuo at 90 °C for several hours to yield 356 mg (1.7 mmol, 80%) of the
colourless product. 1H NMR (300 MHz, C6D6) δ/ppm = 1.14 (s, 9H), 0.69
(s, 2H). 13C (75 MHz, C6D6) δ/ppm = 34.7, 31.5; 27Al NMR could not be
observed.
Experimental Section
General procedures: All manipulations have been performed under inert
argon atmosphere using standard Schlenk and glovebox techniques. All
solvents were purified and dried by distillation from Na/benzophenone and
stored over molecular sieves (3–4 Å, dried in vacuo at 250 °C for several
hours). All chemicals, including organolithium and -aluminium compounds,
were used as received from common suppliers or prepared vide infra. For
a representative set of the described compounds, mass spectrometry and
elemental analysis were attempted, but none of them was successful due
to the high reactivity of these compounds, especially towards traces of
water and air.
Np2AlCl – mechanochemical approach: AlCl3 (0.125 g, 0.93 mmol, 1 eq.),
NpLi (0.45 g, 1.87 mmol, 2 eq.), 1.5 g LiCl as milling medium, and four
balls (ø = 10 mm, steel) were charged into a 20 mL stainless steel milling
vial and shook three times for (10 min + 5 min break) each in a Retsch
MM200 shaker mill at 50% power. The product was extracted with pentane
and dried in vacuo to give a colourless solid. Yield: 0.51 g (2.49 mmol,
90%).
1 [(2-(dimethylamino)phenyl)-1-lithium]4. 1 was synthesised based on a
modified literature procedure.[43b] 2-Bromo-dimethylaniline (5 mL, 3.75 g,
34.7 mmol, 1 eq.) was dissolved in 25 mL pentane, the solution cooled to
Caution: All organoaluminium and –lithium compounds, especially as neat
liquids, as well as aluminium hydrides used for syntheses, are severely air-
/moisture-sensitive and pyrophoric. Aluminium halides are air-/moisture-
n
0°C and BuLi (14 mL, 2.5 M in hexanes, 35 mmol, 1.04 eq.) was added
sensitive and decompose to volatile acids. Guidelines can be found in the
within 15 min. The reaction mixture was stirred for 3 h at 0 °C, then filtered
and the residue washed with 6x4 mL pentane and dried in vacuo to yield
the colourless product (3.75 g, 29.5 mmol, 85%). Crystallisation by a gas-
phase diffusion process (an Et2O solution in a vial surrounded by pentane)
yielded large blocks of the cubic polymorph 1a which transformed over
58]
literature for organolithium compounds,[3a,
whereas the handling of
organoaluminium compounds is similar with respect to their higher
sensitivity and the possibility to use them as neat liquids in many cases.
NMR spectra were collected at 298 K on Bruker Avance III 300nano or
AV400 spectrometers in 5 mm diameter NMR tubes. 1H and 13C chemical
shifts (δ) are reported in ppm relative to non-deuterated solvent signals
described by Fulmer et al.,[59] coupling constants are given in Hz common
abbreviations are used. For 27Al NMR spectra, an aring sequence of three
pulses was used as described previously.[34]
1
several days to the monoclinic polymorph 1b. H NMR (300 MHz, C6D6)
δ/ppm =8.29 (m, 1H) 7.26 (m, 2H), 7.03 (m, 1H), 2.08 (s, br, 6H). 7Li NMR
(117 MHz, C6D6) δ/ppm = 3.63. 13C NMR (75 MHz, C6D6) δ/ppm = 166.3,
140.3, 126.2, 119.0, 46.7.
2 [(2-(dimethylamino)phenyl)-1-lithium-etherate]2. 2 was obtained upon
crystallisation from an Et2O solution of 1 at -18 °C. 2 decomposes in C6D6
solution to Et2O and 1, indicated by its NMR spectrum.
Single-crystal structure analysis Suitable single crystals were selected
under inert oil and mounted on a MiTeGen loop, and transferred into the
cooled nitrogen stream (100 K, Oxford Cryostream 700, Oxford
Cryosystems) on the goniometer. The three used instruments were Bruker
AXS Mach3 instruments with a) Bruker AXS rotating Anode FR591 (45 kV,
30 mA, Cu-Kα1 λ = 1.5406 Å) with Incoatec Montel 200 Optics and an
APEX II CCD detector; b) Bruker AXS rotating Anode FR591 (50 kV, 40
mA, Mo-Kα1 λ = 0.7093 Å) with Incoatec Montel 200 Optics and a Kappa
CCD detector; c) Incoatec, Micro Focus IµS 1.0 (50 kV, 1 mA, Mo-Kα1 λ =
0.7093 Å) with Incoatec Helios MX Optics and an APEX II CCD detector.
The obtained diffraction data were evaluated with SuperGUI (collect and
supergui, Bruker AXS BV, 1997-2009) and EVALCCD[60] or the APEX3
software package and a multi-scan absorption correction was applied
using XREP (Version 2014/2, Bruker-AXS, 2014) or SADABS.[61] All
Structures were solved with SHELXS or SHELXT and refined with
3
[(8-(dimethylamino)naphthalenyl)-1-lithium]4. To
a
solution of
dimethylaminonaphthalene (3.0 mL, 3.1 g, 18 mmol, 1 eq.) in 8 mL MTBE
nBuLi (7.5 mL, 2.5 M in hexanes, 19 mmol, 1 eq.) was added within 10 min
and the reaction mixture was stirred overnight, then filtered and the
residue washed with 8x10 mL pentane. The filtrate was dried in vacuo to
obtain the colourless product (2.61 g, 14.7 mmol, 81%). Single crystals
1
were grown from a benzene solution. H NMR (300 MHz, C6D6) δ/ppm =
8.27 (dd, J = 6.0, 1.4 Hz, 1H), 7.70 (ddd, J = 8.2, 4.5, 1.3 Hz, 2H), 7.39 –
7.29 (m, 1H), 7.13 – 7.03 (m, 2H), 2.20 (s, 3H) 1.66 (s, 3H). 7Li NMR (117
MHz, C6D6) δ/ppm = 4.88. 13C NMR (75 MHz, C6D6) δ/ppm = 156.3, 143.5,
136.1, 135.8, 127.2, 125.3, 125.0, 116.5, 51.0, 55.6.
6
This article is protected by copyright. All rights reserved.

10.1002/ejic.202100224
European Journal of Inorganic Chemistry
FULL PAPER
4
[(8-(dimethylamino)naphthalenyl)-1-lithium-etherate]2.
4
was
14 (8-(dimethylamino)naphthalenyl)-1-di-iso-butylaluminium. Di-iso-butyl-
aluminium hydride (8 mL, 25% in hexanes, 11.3 mmol, 1 eq.) was slowly
added to a suspension of 4 (2.80 g, 11.1 mmol, 1 eq.) in 30 mL Et2O at -
78 °C. After stirring for 1h, the reaction mixture was slowly warmed to
ambient and stirred overnight, filtered and the filtrate dried in vacuo. The
product was obtained after distillation under reduced pressure (145 °C /
0.01 mbar) as colourless oil (1.82 g, 5.9 mmol, 53%). 1H NMR (300 MHz,
C6D6) δ/ppm = 8.11 (dd, J = 6.4, 1.2 Hz, 1H), 7.68 – 7.47 (m, 3H), 7.09 –
7.17(m, 1H) 6.71 (dd, J = 7.5, 1.0 Hz, 1H), 2.25 (s, 6H), 2.08 (hept, J =
6.8 Hz, 2H), 1.20 (d, J = 6.5 Hz, 12H), 0.48 – 0.18 (m, 4H). 13C NMR (75
MHz, C6D6) δ/ppm = 151.4, 137.5, 135.9, 133.8, 127.9, 126.0, 124.6, 114.2,
49.0, 28.9, 28.8, 22.8; 27Al NMR (78.21 MHz, C6D6) δ/ppm = 175 (FWHM
~5.7 kHz).
synthesised as described in the literature[43a] and crystallised from an
Et2O/hexane/pentane mixture. 1H NMR (300 MHz, C6D6) δ/ppm = 7.70 (d,
J =8.0 Hz, 1H) 7.44 (d, J = 8.2Hz, 1H), 7.35 (ddd, J = 8.4, 6.8, 1.4 Hz, 1H),
7.29 (m, 2H), 6.88 (d, J = 7.4 Hz, 1H), 3.26 (t, J = 7.0 Hz, 4H), 2.60 (s, 6H),
1.12 (t, J = 7.0 Hz, 6H). 7Li NMR (117 MHz, C6D6) δ/ppm = 1.75. 13C NMR
(75 MHz, C6D6) δ/ppm = 128.7, 126.1, 125.4, 124.7, 123.4, 114.4, 65.9,
60.8, 45.1, 15.6.
5 [(8-(dimethylamino)naphthalenyl)-1-lithium-(methyl-tert-butyl-)etherate]2.
Single-crystals of 5 were obtained upon recrystallisation of donor-free 3 in
MTBE/pentane. 5 decomposes in C6D6 to 3 and MTBE.
7 Neophyllithium (2-methyl-2-phenylpropyl lithium). Neophylchloride (10
mL, 10.47 g, 62.1 mmol, 1 eq.) was added to a suspension of lithium
granules (1% Na, 1.72 g, 248 mmol, 4 eq.) in 80 mL hexane. The reaction
mixture was refluxed for 4 d. The resulting suspension was filtered over 1
cm of medium-coarse SiO2. The residue was washed with pentane and
toluene, and the filtrate was dried in vacuo. Recrystallisation of the crude
product in 200 mL of pentane/hexane yielded 4.0 g of the desired product.
Further concentration of the mother liquor resulted in additional 0.80 g. The
product was obtained as colourless powder 4.80 g (34 mmol, 55%). Single
crystals were grown from a toluene/pentane solution. 1H NMR (300 MHz,
C6D6) δ/ppm = 7.44 – 7.39 (m, 2H), 7.28 – 7.21 (m, 2H), 7.08 – 7.01 (m,
1H), 1.31 (s, 6H), -1.04 (s, 2H). 7Li NMR (117 MHz, C6D6) δ/ppm = 0.76.
13C NMR (75 MHz, C6D6) δ/ppm = 153.2, 129.9, 126.3, 124.9, 39.8, 35.8,
31.5.
16 (2-(2,2,6,6-tetramethyl-piperidine-1-yl)phenyl)-dimethyl aluminium. A
solution/slurry of 6 (0.50 g, 2.24 mmol, 1 eq.) in 15 mL toluene was cooled
to -78°C and Me2AlCl (2.24 mL, 1 M in hexane, 2.24 mmol, 1 eq.) was
added slowly. The reaction mixture was stirred cooled for 10 min then
warmed naturally to ambient and stirred for additional 4 h after reaching
ambient. The resulting suspension was filtered and the residue washed
twice with 5 mL toluene. The filtrate was dried in vacuo to yield the
yellowish solid 0.425 g (1.56 mmol, 69%). 1H NMR (300 MHz, C6D6) δ/ppm
= 7.69 (dt, J = 6.9, 1.3 Hz, 1H), 7.23 – 7.17 (m, 1H), 7.09 – 7.02 (m, 2H),
1.58 (m, 4H), 1.38 (s, 6H), 1.03 – 0.94 (m, 2H), 0.90 (s, 6H), 0.03 (s, 6H).
Partial 13C NMR (75 MHz, C6D6) δ/ppm = 155.8, 135.9, 130.6, 127.3, 127.0,
122.7, 59.6, 35.6, 33.3, 29.63, 16.78. Al-CH3 could not be observed due
to broadening. 27Al NMR (78 MHz, C6D6) δ/ppm = 175.6 (FWHM could not
be determined due to overlap with the artificial signal of the probe).
8 [(lithium 2-(dimethylamino)phenolate)]6. 8 was obtained serendipitously
by a non-specific process upon attempting to crystallise 4 in Et2O in a vial
surrounded by pentane by a gas-diffusion process.
17
(2-(2,2,6,6-tetramethyl-piperidine-1-yl)phenyl)-diethyl
aluminium-
etherate. A solution/slurry of 6 (0.370 g, 1.66 mmol, 1 eq.) in 15 mL Et2O
was cooled to -78°C and Et2AlCl (1.66 mL, 1 M in hexane, 1.66 mmol, 1
eq.) was added slowly. The reaction mixture was stirred cooled for 10 min
then warmed naturally to ambient and stirred overnight. The resulting
suspension was filtered, the residue washed twice with 5 mL Et2O and the
filtrate dried in vacuo to yield yellowish oil and crystals. The colourless
crystals were harvested as the etherate 17 of the desired product 0.165 g
(0.44 mmol, 26%). 1H NMR (300 MHz, C6D6) δ/ppm = 7.56 (ddd, J = 6.9,
1.9, 0.7 Hz, 1H), 7.40 – 7.23 (m, 1H), 7.20 – 7.05 (m, 1H), 7.02 – 6.84 (m,
1H), 3.28 (q, J = 7.0 Hz, 4H, Et2O), 1.48 (m, 6H), 1.37 (t, J = 8.1 Hz, 6H),
9 (2-(dimethylamino)phenyl)dimethylaluminium. Me2AlCl (6.3 mL, 1 M in
hexane, 6.3 mmol, 1 eq.) was added slowly to a solution of 1 (0.80 g,
6.3 mmol, 1 eq.) in 20 mL Et2O at -78 °C within 10 min. The reaction
mixture was warmed to -40 °C and stirred for 3 h. The suspension was
filtered and the residue washed twice with 7 mL Et2O. The filtrate was dried
carefully in vacuo to grow single crystals of 9 (0.632g, 3.57 mmol, 57%).
1H NMR (400 MHz, C6D6) δ/ppm = 7.74 – 7.70 (m, 1H), 7.27 (ddd, J = 6.8,
4.6, 1.6 Hz, 2H), 6.93 – 6.89 (m, 1H), 2.54 (s, 6H), -0.26 (d, J = 22.1 Hz,
6H). 13C NMR (101 MHz, C6D6) δ/ppm = 160.8, 137.9, 128.9, 127.4, 117.4,
116.3, 47.8, -9.9 (br). 27Al NMR (78.21 MHz, C6D6) δ/ppm = 177 (FWHM
could not be determined).
1.07 (s, 12H), 0.78 (t, J = 7.0 Hz, 6H, Et2O), 0.47 (q, J = 8.1 Hz, 4H). 13
C
NMR (75 MHz, C6D6) δ/ppm = 155.2, 137.4, 130.7, 129.7, 127.1, 125.7,
66.1, 57.0, 39.4, 32.6, 30.0, 29.1, 18.1, 14.2, 10.3, 4.8. 27Al NMR (78 MHz,
C6D6) δ/ppm = 179.3 (FWHM could not be determined due to overlap with
the artificial signal of the probe).
10 Bis(2-(dimethylamino)phenyl)(methyl)aluminium. 10 was only obtained
as minor species upon a single recrystallisation experiment of crude 9 in
Et2O which may result from a disproportionation to 10 and AlMe3.
18 (2-(2,2,6,6-tetramethyl-piperidine-1-yl)phenyl)-di-neopentyl aluminium
etherate. Np2AlCl (367 mg, 1.79 mmol, 1 eq.) was added as toluene
solution to a slurry of 6 (0.40 g, 1.78 mmol, 1 eq.) in toluene and the
reaction mixture stirred for 4 h at ambient temperature. The resulting
suspension was filtered and the residue washed twice with toluene and the
filtrate was dried in vacuo. The crude product was extracted subsequently
with Et2O and then pentane and the filtrate dried in vacuo to yield a
yellowish compound of the Et2O adduct. Yield 367 mg (0.80 mmol, 45%)
1H NMR (300 MHz, C6D6) δ/ppm = 7.76 – 7.67 (m, 1H), 7.24 – 7.18 (m,
1H), 7.11 – 6.98 (m, 2H), 3.28 (q, J = 7.0 Hz, 4H, Et2O), 1.75 – 1.43 (m,
6H), 1.36 (s, 6H), 1.34 (s, 18H), 1.10 (t, J = 7.0 Hz, 6H, Et2O), 1.00 (s, 6H),
0.91 (s, 4H). Partial 13C NMR (75 MHz, C6D6) δ/ppm = 155.9, 136.4, 130.3,
127.4, 127.0, 124.2, 65.8, 58.7, 36.7, 35.6, 33.2, 33.0, 31.2, 17.0, 15.1. Al-
CH2 could not be observed due to broadening. 27Al NMR could not be
observed.
11 (8-(dimethylamino)naphthalenyl)-1-aluminium hydride has been
reported by Hair et al.[54] However, here we report a different synthesis
procedure. AlCl3 (0.62 g, 4.7 mmol, 0.25 eq.) in 15 mL Et2O was added to
a solution of LiAlH4 (0.54 g, 14 mmol, 0.75 eq.)) in 45 mL Et2O at 0 °C and
the resulting suspension stirred for 20 min. The suspension was filtered
onto a suspension of 4 (3.00 g, 11.9 mmol, 1 eq.) in 60 mL Et2O within 5
min. The resulting suspension was stirred at ambient temperature
overnight, then filtered, and the residue was washed twice with 10 mL Et2O.
The residue was collected as pure colourless product (0.39 g, 16%). The
filtrate was dried in vacuo and the obtained residue extracted with toluene,
filtered off, and washed twice with 10 mL toluene. This second filtrate was
dried in vacuo, and the crude product sublimated at up to 170 °C/0.02
mbar to obtain additional product (1.66 g, 70%). Total yield 2.05 g (10.3
mmol, 86%). 1H NMR (300 MHz, C6D6) δ/ppm = 8.09 (dd, J = 6.3, 0.8 Hz,
1H), 7.62 (ddd, J = 20.6, 8.3, 1.1 Hz, 2H), 7.44 (dd, J = 8.2, 6.4 Hz, 1H),
7.15 (m, 1H), 6.79 (dd, J = 7.4, 1.1 Hz, 1H), 4.81(s, br, 2H, Al-H), 2.46(s,
br, 6H). 13C NMR (75 MHz, C6D6) δ/ppm = 136.8, 133.9, 128.2, 127.7,
127.3, 125.1, 115.5, 49.5. 27Al NMR could not be observed.
19 (2-(2,2,6,6-tetramethyl-piperidine-1-yl)phenyl)-neopentyl-neopentoxy
aluminium. Single crystals of 19 were harvested from a sample of 18 in
pentane after leaving it open to air.
Acknowledgements
7
This article is protected by copyright. All rights reserved.

10.1002/ejic.202100224
European Journal of Inorganic Chemistry
FULL PAPER
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Entry for the Table of Contents
A structural study of amino-organolithium compounds is presented with an emphasis on their aggregation as tetramers and dimers.
These compounds were further used to generate their corresponding aluminium compounds whose crystal structures as well as
those of neophyllithium and decomposition products are reported too. This study provides additional data for the future design and
synthesis of amino-organolithium and –aluminium compounds.
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