The hetero-cubane structures of the heavy alkali metal: Tert -amyloxides [MOCMe2Et]4 (M = K, Rb, Cs)

Article abstract of DOI:10.1039/c8dt01545g

A series of alkali metal (Li-Cs) alkoxides of tert-pentanol (1,1-dimethylpropan-1-ol) have been prepared by reaction of the corresponding metal with the alcohol in n-hexane or n-heptane. The compounds were purified by vacuum sublimation and crystallised in n-hexane to produce crystals suitable for single-crystal X-ray diffraction studies. The structures of the potassium, rubidium, and caesium compounds revealed tetrameric units with additional intra- and intermolecular interactions between the metal atom and alkoxide methyl groups, increasing with the size of the metal involved.

Full text of DOI:10.1039/c8dt01545g

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The Hetero-Cubane Structures of the Heavy Alkali Metal
tert-Amyloxides [MOCMe2Et]4 (M = K, Rb, Cs)
Received 00th January 20xx,
Accepted 00th January 20xx
Maximillian Kaiser and Jan Klett*
DOI: 10.1039/x0xx00000x
The series of alkali metal (Li – Cs) alkoxides of tert-pentanol (1,1-dimethylpropan-1-ol) has been prepared by reaction of
the corresponding metal with the alcohol in n-hexane or n-heptane. The compounds were purified by vacuum sublimation
and cystallised in n-hexane to produce crystals suitable for single-crystal X-ray diffraction studies. The structures of the
potassium, rubidium, and cesium compounds revealed tetrameric units with additional intra- and intermolecular
interactions between the metal atom and alkoxide methyl groups increasing with the size of the involved metal.
www.rsc.org/
This may be attributed to the poor solubilities and fierce
reactivity of such systems. Both the activation of
alkylpotassium by lithium alkoxide or the existence of mixed
Introduction
The ambiguous character of alkali metal alkoxides, the salt-like
metal-oxygen interactions in combination with an organic
lipophilic alkyl/alcohol group, makes them an important
component in many chemical reactions. These compounds are
easily accessible and in the case of the lighter alkali metals or
simple alcohols often commercially available. The comparable
low sensitivity towards air and inertness towards most organic
solvents makes them easy to handle under inert-gas
conditions. Alkali metal alkoxides have been used in a large
variety of reactions: As precursor for metal oxides,1 in the
Williamson ether synthesis,2 as moderate base3 in organic
reactions, and as additive in polymerisation reactions.4 In
1950, Morton discovered the activating role of sodium iso-
propoxide in deprotonations of ethene with pentylsodium.5
Potassium tert-butoxide in connection with n-butyllithium was
used efficiently in superbasic6 mixtures of the Lochmann-
Schlosser bases.7 In a similar reaction, alkyllithium and a
heavier alkali metal alkoxide react in a metal exchange
reaction to produce the (often insoluble) alkali metal alkyl
compound. By this method it was possible to isolate the
sodium, potassium, rubidium, and caesium compounds of
bis(trimethylsilyl)methane8 and trimethylsilylmethane.9 When
used in Lochmann-Schlosser superbasic mixtures, the alkali
metal alkyl compound is not isolated and separated from the
corresponding soluble lithium alkoxide. The formed
suspension is directly used to metalate/deprotonate organic
substrates such as benzene.10 The chemical nature of these
freshly prepared reagents could not be identified for decades.
aggregates such as [LiOtBu-KnBu] were discussed.11 The use of
the neopentyl group as alkyl component with increased
alkane-solubility allowed the isolation and characterisation of
mixed aggregates including lithium and potassium atoms, in
connection with neopentyl and tert-butoxy groups side by
side.12 Another soluble systems using 2-ethylhexyllithium and
potassium tert-amyloxide13 was used in metalation reactions,
but no characterisation was reported. The use of branched and
better soluble alkali metal alkoxides such as tert-amyloxides14
[MOtAm] in superbases leads to increased alkoxy
concentrations and often better results compared to
corresponding tert-butoxides. This is why Lochmann described
such mixtures as superbases of the second generation.15 The
combination of neopentyllithium and excess potassium
tert-amyloxide,4 organic groups with increased alkane-
solubility, leads to the formation of a soluble potassium mixed
aggregate consisting of three potassium tert-amyloxide units
and one neopentylpotassium unit.16 In contrast to more bulky
alkoxides such as 3-methyl-3-pentoxide or 3-ethyl-3-
pentoxide,14d,14e tert-amyloxide offered a good balance of
increased solubility (compared to tert-butoxide) with an
acceptable minimum of structural disorder enabling detailed
structural studies. For
a better understanding of the
interaction of tert-amyloxides with alkali metal alkyl
compounds, the knowledge about the structures is of
importance. To the best of our knowledge, only three metal
tert-amyloxides are reported, including neodymium,17 iron,18
and gallium.19 Despite their numerous uses in chemistry and to
some extent commercial availability no structures of alkali
metal tert-amyloxides were found. This is in contrast to the
corresponding tert-butoxides with numerous reported
structures. LiOtBu is found in disordered hexamers,20 but also
an octameric form is reported.21
a.Maximilian Kaiser, Dr. Jan Klett, Institut für Anorganische Chemie und Analytische
Chemie, Johannes Gutenberg-Universität Mainz, Duesbergweg 10-14, 55128
Mainz, Germany, email: klettj@uni-mainz.de
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Table 1: 1H, 13C, 7Li, and 133Cs NMR spectroscopic data [ppm] of compounds 1, 2, 3,
4, and 5 in deuterated [D6]benzene at 21°C.
1H
13C
Et
7Li/133Cs
Me
Et
Me
tert-C
Scheme 1. Formation of alkali metal tert-amyloxides using alkali metal and tert-
Me
CH2
1.51
1.34
1.29
1.34
1.41
Me
CH2
41.5
42.2
41.9
41.8
41.6
pentanol.
1
2
3
4
5
1.24
1.10
1.01
1.05
1.15
0.99
0.99
0.98
0.99
1.04
32.5
34.2
34.9
34.9
34.7
10.9
10.4
10.4
10.4
10.5
69.3
67.8
68.4
68.9
70.1
0.94
—
—
—
202.1
NaOtBu forms mixtures of hexamers and nonamers,20 but
recently a purely hexameric form was reported.22 KOtBu,1
RbOtBu,1 and CsOtBu23 are found as regular heterocubanes.24
Some mixed bimetallic alkali metal alkoxides were also
characterised.16,25 tert-Amyloxides are also called tert-
pentoxides, tert-amylates, 2-methyl-2-butoxides, tert-
pentylates, 1,1-dimethylpropoxides, or tert-amoxide. The
increasing use of their alkali metal compounds, even on
industrial scale, and their advantageous properties makes it
desirable to gain additional structural information.
Table 2: 1H, 13C, 7Li, and 133Cs NMR spectroscopic data [ppm] of compounds 1, 2, 3, 4,
and 5 in deuterated [D12]cyclohexane at 21°C.
1H
13C
Et
7Li/133Cs
Me
Et
Me
tert-C
Me
CH2
1.39
1.25
1.20
1.20
1.24
Me
11.1
10.5
10.3
10.3
10.2
CH2
1
2
3
4
5
1.12
1.00
0.91
0.93
0.98
0.90
0.89
0.83
0.83
0.84
32.6
34.3
34.7
34.6
33.8
41.9
42.6
42.3
42.2
41.5
69.7
68.2
68.9
69.4
70.1
1.31
—
—
—
211.2
Results and discussion
The data obtained from both series of measurements is very
similar for the corresponding tert-amyloxide compounds.
Therefore no conclusions can be made about changes of
oligomerisation depending on the different Lewis donor
properties of benzene and cyclohexane. The strongest effect in
this regard would be expected for caesium compounds, due to
Preparation of alkali metal tert-amyloxides
The alkali metal tert-amyloxides [MOtAm] were prepared
following a procedure for the preparation of KOtAm (Scheme
1).3 Pieces of excess alkali metal were stirred in n-hexane (n-
heptane in the case of sodium), and tert-pentanol (tert-
amylalcohol) was quickly added. The mixture was heated for
several days, in the meantime the metal dissolved partially
accompanied by the formation of molecular hydrogen. In the
case of potassium, rubidium, and caesium the formation of a
colourless, crystalline solid was observed which in some cases
resulted in the partial solidification of the mixture. This
precipitation is presumed to be a complex of alkali metal
alkoxide and tert-pentanol.4 In similar reactions of tert-butanol
with potassium or rubidium the solid was isolated and
characterised as polymeric chains of [KOtBu·HOtBu]∞ and
[RbOtBu·HOtBu]∞.1 As the reaction proceeded the precipitated
intermediate redissolved. After cooling the excess metal was
removed and all solvent was distilled off. The obtained
its comparable strong arene-metal interactions.27
corresponding result is found for 133Cs NMR spectra28 of
compound : 202.1 ppm in [D6]benzene, and 211.2 ppm in
A
5
[D12]cyclohexane. Compared to 133Cs chemical shifts of similar
caesium compounds measured in [D8]THF in literature (3-
methylpentoxide:29 196.9 ppm, and 3-ethyl-3-heptoxide:30
191.7 ppm),
a trend involving O-donor (THF), π-donor
(benzene), and non-donor (cyclohexane) can be observed.
X-ray crystallography
All five compounds31
1, 2, 3, 4, and 5 show very good to
moderate solubility in n-hexane. Additionally, because of their
high thermal stability in this solvent these compounds could be
easily crystallised from hot n-hexane. X-ray crystallography of
crystalline solids (LiOtAm,
; CsOtAm, ) were purified in a vacuum sublimation with
pressures of about 5·10-2 mbar using temperatures between
1; NaOtAm, 2; KOtAm, 3; RbOtAm,
4
5
lithium compound 1, which crystallises in the trigonal space
group P3‾was not successful, due to problems concerning
160-180°C (except 3
: 120°C) with good to excellent yields.§
crystallographic disorder. In hexameric lithium compounds
20
32
such as (LiOtBu)6 and (LinBu)6 the six lithium atoms are
disordered over eight positions. In compound this obstacle is
NMR spectroscopy
1
joined by additional rotational disorder of the OtAm group,
resulting in a disorder between the involved methyl and ethyl
groups. Exactly the same two problems were also encountered
The good solubility of the alkali metal tert-amyloxides enables
their characterisation by NMR spectroscopy even in weakly
coordinating
[D12]cyclohexane (Table 1 and 2). The NMR spectra of
compounds and in deuterated benzene are in good
solvents
such
as
[D6]benzene
or
in sodium compound 2, preventing a satisfying refinement of
both (presumably hexameric) structures in space group C2/m.
Compared to the tert-butoxy compounds of potassium,
rubidium, and caesium, which crystallise in cubic space
groups,24 the introduction of the less symmetric OtAm group
should also result in lower crystallographic symmetry.
However, the potassium, rubidium, and caesium compounds
of OtAm are also found to consist of tetrameric units:
4
5
agreement with spectra found in literature.26 To obtain
reference data of these compounds in a solvent which is inert
towards deprotonation by highly basic systems such as
Lochmann-Schlosser superbases we also carried out NMR
spectroscopy in deuterated cyclohexane.
[MOtAm]4, M = K (3), Rb (4), and Cs (5). Potassium compound
2 | J. Name., 2012, 00, 1-3
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3
crystallised at -30°C in the highly symmetric tetragonal space alkyl group, which is responsible for the superbasic behaviour
group P4̅21c (Figure 1).
of this compound, also leads to dimerisation resulting in
[K4Np(OtAm)3]2 and therefore a substantial different solubility
in alkanes in comparison to 3.
Figure 1. Molecular structure of tetrameric [KOtAm]4, 3. Hydrogen are omitted for
clarity. Symmetry operator A: -x+1, y, -z; B: -x+1,-y+1,z; C: x, -y+1,-z. The K4O4 cube is
shaded for emphasis. Intramolecular short K···methyl contacts are shown as dashed
lines.
Figure 2. Molecular structure of tetrameric [RbOtAm]4, 4. Hydrogen and disordered
units of minor occupancy are omitted for clarity. The Rb4O4 cube is shaded for
emphasis. Intramolecular short Rb···methyl contacts are shown as dashed lines.
Table 3: Structural parameters of compounds 3, 4, and 5 in comparison to the
corresponding tert-butoxy compounds KOtBu,1 RbOtBu,1 and CsOtBu23 (distances: [Å];
angles: [°]). For 3, 4, and 5 the minimum and maximum values are given.
Table 4: Crystallographic parameters of compounds 3, polymorphic 3a, 4, and 5.
MOR
M‒O
M···Me
3.699
O‒M‒O
90.18(6)
M‒O‒M
89.82(6)
Compound
Formula
3
C5H11OK
126.24
3a
C5H11OK
126.24
4
C5H11ORb
172.61
5
C5H11OCs
220.05
KOtBu
2.6232(13)
Mr [g mol−1
]
2.6023(12)
2.6412(12)
3.498(2)
3.509(2)
90.29(4)
90.84(4)
89.16(4)
89.70(4)
3
Crystal
tetragonal
monoclinic
monoclinic
monoclinic
system
RbOtBu
2.757(4)
3.691
89.11(17)
90.89(16)
Space group
P4
̅
21c
P21/c
Ia (Cc)
P2/c
2.735(11)
2.802(10)
3.34(4)
3.71(3)
88.1(3)
91.3(3)
89.2(3)
91.9(3)
4
a [Å]
b [Å]
c [Å]
α [°]
β [°]
γ [°]
V [Å3]
Z
8.251(1)
8.251(1)
20.346(3)
90
21.876(4)
8.181(2)
16.896(3)
90
19.509(16)
8.686(6)
20.161(17)
90
19.114(3)
8.875(2)
20.401(3)
90
CsOtBu
2.924
3.865
90.92
89.08
2.879(11)
2.938(10)
3.71(2)
3.91(2)
86.0(3)
89.1(3)
90.7(3)
94.1(3)
5
90
111.92(1)
90
116.48(2)
90
117.10(1)
90
90
The four potassium atoms and the four oxygen atoms of the
OtAm groups form a regular cube with the metal atoms placed
on diagonal corners of the quadratic faces of the cube (Table
3). The three K‒O distances are found in a small range
between 2.6023(12) and 2.6416(12) Å, the angles involving the
atoms in the cubane structure are close to 90° (O‒K‒O:
90.29(4) to 90.84(4)°; K‒O‒K: 89.16(4) to 89.70(4)°).
Interestingly, the OtAm group does not show any rotational
disorder. Several intramolecular and intermolecular
interactions of the potassium with alkoxy methyl groups can
be observed, the shortest of these interactions are in the
1385.1(2)
8
2805.2(10)
16
3058(4)
16
3080.6(9)
16
ρ
calcd [g cm−3
]
1.211
1.196
1.500
1.898
measured
reflections
7105
1664
29399
6662
12649
5571
37696
7562
independent
reflections
R(int)
0.0365
0.0222
(0.228)
0.0568
(0.0571)
0.0903
0.0719
(0.1172)
0.1439
(0.1588)
0.0742
0.0406
(0.1737)
0.0818
(0.1045)
0.0828
0.0695
(0.0965)
0.2090
(0.2364)
R1
(all data)
wR2
(all data)
range of 3.498(2) and 3.509(2) Å. When the crystallisation of
3
CCDC
1837111
1837112
1837113
1837114
Number31
was attempted at room temperature a polymorphic form was
obtained in the monoclinic space group P21/c (3a). Disorder of
one of the four OtAm groups and an overall lower quality of
the collected data makes it reasonable to restrict the
discussion to the tetragonal form. The structure of the
homometallic potassium neopentyl/tert-amyloxide mixed
In compound
4 (Figure 2), which crystallises in the monoclinic
space group Ia (equivalent to space group Cc) three of the four
OtAm groups are disordered over two positions. Despite the
lower symmetry, the Rb4O4 cubane structure is fairly regular
with O‒Rb‒O and Rb‒O‒Rb angles close to 90° (Table 3). In
aggregate16 K4Np(OtAm)3 can be derived from tetrameric
3 by
replacement of one OtAm group by a neopentyl group. This
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comparison to
3,
more short intramolecular and This work was supported by Internal University Research
intermolecular interactions of rubidium with methyl groups of Funding of the University of Mainz. We thank Prof. K.
the OtAm units can be observed (Rb···Me: 3.34(4) to 3.71(3) Klinkhammer for his generous support of this work and helpful
Å).
discussions. We also thank Dr. D. Schollmeyer and R. Jung-
Pothmann for X-ray crystallographic measurements, and
especially Dr. M. Mondeshki for NMR measurements.
Notes and references
§ See supporting information for experimental details,
crystallographic and NMR spectroscopic data
1
2
3
4
5
M. H. Chisholm, S. R. Drake, A. A. Naiini and W. E. Streib,
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Figure 3. Molecular structure of tetrameric [CsOtAm]4, 5. Hydrogen and disordered
units of minor occupancy are omitted for clarity, only one of the two
crystallographically independent units is shown. Symmetry operator A: -x+1, y, -z+0.5.
The Cs4O4 cube is shaded for emphasis. Intramolecular short Cs···methyl contacts are
shown as dashed lines.
6
7
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Compound
P2/c, is found in two crystallographic independent units. One
of four OtAm groups shows disorder. also forms reasonably
regular Cs4O4 cubane structures, despite the less symmetric
structural environment. Compared to and , it shows
5, which crystallises in the monoclinic space group
5
3
4
considerably more intra- and intermolecular Caesium···methyl
contacts (Cs···Me: 3.71(2) to 3.91(2) Å). These additional
intermolecular interactions, caused by the increased size of
caesium, may be the reason for the considerably lower
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The potassium, rubidium, and caesium compounds of
tert-amylalcohol were identified as tetramers in the solid state
by single crystal X-ray crystallography. This is in accordance
with the corresponding tert-butoxy compounds, which were
identified as tetramers in highly symmetric crystalline
environment. However, the tert-amyloxide group can
introduce disorder and asymmetries into the crystalline
arrangement. This property and the increased lipophilicity of
the organic group in such ‘amphiphile’ alkali metal alkoxy
compounds may contribute to the increased solubility
compared to the corresponding tert-butoxy compounds.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
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Hetero-cubane structures were revealed for the tert-amyloxide compounds of potassium, rubidium,
and caesium.

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39x19mm (300 x 300 DPI)