Influence Of Lithium Coordinating Additives On The Structure Of Phenyldimethylsilyllithium

Article abstract of DOI:10.1002/zaac.202100031

The synthesis and structural investigation of two dimethylphenylsilyllithium adducts with coordinating nitrogen donors quinuclidine and 1,3,5-tri-tert-butylhexahydro-1,3,5-triazine (tbtac) are presented. The structures show a comparatively long Si?Li distance and one amongst the shortest in monomeric silyllithiums reported so far, respectively. The structural investigations shed light on the influence of the donating additive for the lithium center on the structure of silyllithium compounds.

Full text of DOI:10.1002/zaac.202100031

Journal of Inorganic and General Chemistry
DOI: 10.1002/zaac.202100031 SHORT COMMUNICATION
Zeitschrift für anorganische und allgemeine Chemie
Influence Of Lithium Coordinating Additives On The
Structure Of Phenyldimethylsilyllithium
[a]
[a]
[a]
Moritz Achternbosch, Lukas Brieger, and Carsten Strohmann*
Dedicated to Prof. Christoph Janiak on the Occasion of his 60th Birthday.
The synthesis and structural investigation of two dimeth-
[
5,6]
dimethyphenylsilylllithium in the solid state and in solution.
ylphenylsilyllithium adducts with coordinating nitrogen donors
quinuclidine and 1,3,5-tri-tert-butylhexahydro-1,3,5-triazine
However, as (À )-sparteine is a comparably large ligand, steric
effects are assumed to be responsible for observations concern-
ing the structural features of these adducts such as bond length
and -angles. Another work by Daeschlein et al. showed the
crystal structure of the pmdta adduct of phenyldimeth-
ylsilyllithium and a bending of the silicon-bound phenyl
group , which is observed for a multitude of anionic silicon
species in the literature, even for heavier alkali metals. The
influence of cation complexing donors was further illustrated
(tbtac) are presented. The structures show a comparatively long
SiÀ Li distance and one amongst the shortest in monomeric
silyllithiums reported so far, respectively. The structural inves-
tigations shed light on the influence of the donating additive
for the lithium center on the structure of silyllithium com-
pounds.
[7]
[
8]
[8,9]
by Leich et al. and Schuhknecht et al. They reported on the
Introduction
hydrogenolysis of a series of alkali metal silanides Ph SiM
3
(
M=LiÀ Cs). Interestingly, only adducts with the nitrogen donor
Dimethylphenyllithiosilane and other simple organosilylanions
are known for many years and get frequently used in organic
synthesis for silylation reactions, as precursors for silyl cuprates
bases tris-[2-(dimethylamino)-ethyl]-amine (Me TREN) for potas-
6
sium and 1,4,7,10-tetramethyl-1,4,7,10-tetraaminocyclodode-
cane (Me TACD) for LiÀ Cs showed a reactivity against H ,
4
2
[1]
or silyl-Grignard reagents, to name a few. However, consider-
ing the strong structure-reactivity relationship of homologous
whereas other adducts with pmdta, crown ethers or simple thf
solvates of silyl potassium did not show any reactivity in the
[2]
[9]
alkyl lithiums , the structural investigations of simple lithiosi-
investigated reactions. According observations were reported
[3]
29
lanes are comparably scarce.
by Lerner et al. as they found a correlation between the Si-
Recent work by Coates et al. and Sheldon et al. has shown
the synthetic potential of dimethylphenyllithiosilane and other
silanides in defluoro-silylation reactions of industrially relevant
fluoroolefins and other fluorocarbons. In their investigations,
the coordination sphere around the lithium center turned out
to be a crucial factor for the reactivity of such type of
NMR shift of tBu PhSiM (M=alkali metal) and the donor abilities
2
of the used solvent, which influences the aggregation behavior
[10]
of such compounds. This clearly illustrates the importance of
choosing the correct donor base for cation complexation in
alkali metal silanides. Therefore, in this work we report the
synthesis and structural investigation of two different dimeth-
ylphenyllithiosilane adducts with nitrogen donor bases for the
lithium center, focusing on the influence on the structure by
the different donor abilities of the nitrogen additive.
[4]
compounds.
The
simple
thf
adduct
of
dimeth-
ylphenyllithiosilane showed a different reactivity than its
adducts with nitrogen donors such as bis(2-dimethyl-
aminoethyl)methylamine (pmdta) or N,N,N’,N’-tetrameth-
ylethane-1,2-diamine (tmeda). Daeschlein et al. reported struc-
tural
investigations
for
(À )-sparteine
adducts of Results and Discussion
In order to elucidate the influence of the coordination sphere
around the lithium center, we synthesized two dimethyl-
phenyl-lithiosilanes under application of different nitrogen
donors for the cation. Lithiosilanes 3 and 4 are easily accessible
[
a] M. Achternbosch, L. Brieger, Prof. Dr. C. Strohmann
Fakultät für Chemie und Chemische Biologie
Technische Universität Dortmund
Otto-Hahn-Str. 6, 44227 Dortmund
E-mail: carsten.strohmann@tu-dortmund.de
Supporting information for this article is available on the WWW
under https://doi.org/10.1002/zaac.202100031
by
reduction
of
commercially
available
dimeth-
[
11]
ylphenylchlorosilane 1 with lithium metal in thf and subse-
quent replacement of solvating thf molecules in 2 by the
appropriate nitrogen donor bases (Scheme 1).
©
2021 The Authors. Zeitschrift für anorganische und allgemeine
Chemie 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 adaptations are made.
In our case we chose quinuclidine and 1,3,5-tri-tert-
butylhexahydro-1,3,5-triazine (tbtac) as suitable additives, as
they are differing in their donor abilities. Lithiosilane 3
crystallizes from n-pentane at À 80°C as orange plates in space
Z. Anorg. Allg. Chem. 2021, 647, 979–983
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and 2.084(4) Å is comparable to other silyl lithiums containing
[6,7]
nitrogen donors ranging from 2.098 Å to 2.166 Å.
Analogous conditions led to lithiosilane 4, which crystallizes
from diethyl ether at À 80°C as colorless blocks in space group
P2 2 2 . Similar to 3, the asymmetric unit of 4 contains a
1
1 1
monomeric lithiosilane, in which the lithium center is coordi-
nated by the anionic silicon center and three nitrogen donors
of the tbtac. The lithium cation is located beneath the triazine
ring which leads to unfavorable angles for a coordination by
the triazine nitrogen centers. Hence, the interaction between
the donor and the lithium center is weakened. The silicon-
lithium contact is significantly shortened with 2.569(3) Å and is
to the best of our knowledge amongst the shortest reported
bond distances in monomeric triorganosilyllithium compounds
Scheme 1. Synthesis of lithiosilanes 3 and 4 via reduction of
dimethylphenylchlorosilane with lithium metal.
group P2 /n (Figure 1). The asymmetric unit of 3 in the solid
1
[13]
[14]
state contains one monomeric lithiosilane, in which the lithium
center is coordinated by two quinuclidine molecules, one thf
and the anionic silicon center. The thf donor competes with a
third quinuclidine donor for the same coordination side on the
lithium center, whereas the nitrogen is located at the position
of the oxygen. The occupation ratio is 70:30 in favour of the thf
donor. More detailed information is included in the Supporting
Material. The SiÀ Li bond length is comparably long with
to date being 2.588(4) Å and 2.598(5) Å , respectively.
The extremely short SiÀ Li distance in 4 results from the
opposite effect than the long distance in 3. Due to the
unfavourable coordination angles of the triazine ring, the
lithium cation is electronically less saturated, which leads to an
increased electrostatic interaction between silicon and lithium
(Figure 2).
In both structures a bending of the phenyl rings can be
[
5,7]
2
.710(3) Å in contrast to already known silyl lithiums ranging
observed as previously reported for other lithiosilanes.
The
[5,12]
from 2.65 Å to 2.675(5) Å.
This may result from the strong
bending can occur due to Pauli-repulsion between the lone pair
at the anionic silicon center and the highest occupied molecular
orbital of the phenyl ring. In order to decrease this repulsive
interaction, the arene is slightly bent and beyond that the
frontier orbital of the phenyl ring is deformed, leading to an
unsymmetrical distribution of the very same. Another possible
explanation could be a stabilizing bonding interaction between
the Cipso and the backwards lobe of the “lone pair” at silicon. In
order to increase this stabilizing interaction, the arene is bent. A
coordination of the quinuclidine as well as thf donors towards
the lithium center, therefore decreasing the electrostatic
interaction between silicon and lithium, resulting in an
elongated bond. The lithium-nitrogen distance of 2.111(4) Å
Figure 1. Molecular structure of lithiosilane 3 in the solid state. All
hydrogen atoms and disorders are omitted for clarity. The thf donor
shows an occupational disorder as it competes with a third
quinuclidine donor, whereas the nitrogen is located at the same
position as the thf oxygen. The occupancy ratio is 70:30 in favour
of the thf. The third quinuclidine is omitted for clarity. Selected
bond length [Å] and angles [°]: SiÀ Li 2.710(3), SiÀ C1 1.926(2), SiÀ C2
Figure 2. Molecular structure of lithiosilane 4 in the solid state. All
hydrogen atoms are omitted for clarity. Selected bond length [Å]
and angles [°]: SiÀ Li 2.569(3), SiÀ C1 1.9181(18), SiÀ C2 1.9197(18),
SiÀ C3 1.9171(15), N1À Li 2.135(3), N2À Li 2.133(3), N3À Li 2.162(3),
C1À SiÀ C2 101.08(11), C1À SiÀ C3 101.60(8), C2À SiÀ C3 102.64(7),
LiÀ SiÀ C3 107.32(8), N1À LiÀ N2 67.31(10), N2À LiÀ N3 66.10(9),
N3À LiÀ N1 66.39(9), SiÀ C3À C4À C5 175.56(12).
1
2
1
.921(2), SiÀ C3 1.919(2), N1À Li 2.111(4), N2À Li 2.084(4), OÀ Li
.013(4), C1À SiÀ C2 99.73(13), C1À SiÀ C3 101.90(9), C2À SiÀ C3
00.24(10), LiÀ SiÀ C3 111.47(9), SiÀ C3À C4À C5 175.46(16).
Z. Anorg. Allg. Chem. 2021, 979–983
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Journal of Inorganic and General Chemistry
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suitable indicator for this bending is the torsional angle φ
between SiÀ CÀ C À C . Both analyzed lithiosilanes 3 and 4
i
o
m
exhibit torsional angles for the phenyl group significantly
smaller than 180° (Figure 3). Interestingly, in both structures the
phenyl ring is bent to roughly the same extent. This inevitably
leads to the assumption that the choice of donor base does not
necessarily influence the bending of phenyl groups in lithiosi-
lanes. The most important crystallographic and structure refine-
ment data for lithiosilanes 3 and 4 are summarized in Table 1
(
More detailed information is given in the Supporting Informa-
tion).
NMR spectroscopy of 3 and 4 gained deeper insight into
2
9
the formed structures in solution (Figure 4). The Si-NMR shift
for 3 is À 27.9 ppm and À 26.0 ppm for 4, respectively and is in
accordance with similar systems reported in the literature so
[5]
far. Interestingly, the quinuclidine/thf adduct 3 shows a
broadened singlet at room temperature, whereas the tbtac-
coordinated lithiosilane 4 shows a quartet caused by SiÀ Li
coupling. The observed coupling pattern clearly indicates a
strong SiÀ Li interaction in 4 even in solution, thus supporting
the short SiÀ Li contact observed in the solid state of 2.569(3) Å.
2
9
Figure 4. Section of the Si-NMR spectrum of lithiosilane 3 (left)
and 4 (right) at 24°C in toluene-d .
8
The broad singlet for 3 is in well accordance with the long SiÀ Li Experimental Section
distance of 2.710(3) Å in the solid state and the presence of
strong donor bases, revealing weaker interactions between
silicon and lithium even in solution.
General Considerations
All reactions were performed in flame dried glass ware under an
inert atmosphere of argon. Solvents were dried over sodium and
distilled prior to use. Unless otherwise stated, commercially
purchased chemicals were used without further purification.
Chlorodimethylphenylsilane was distilled and stored over 4 Å
molecular sieves at 4°C. NMR spectra were recorded on a Bruker
In summary, the structural data from x-ray analyses and
NMR spectroscopy reveal a dependency between the structure
of organo-silyllithiums and the applied additive for cation
complexation. A strong donor base for complexation of the
cation leads to an elongated SiÀ Li distance, whereas a less
strong donor leads to a shortened distance with 4 representing
one of the shortest reported SiÀ Li distance in organosilyl-
lithiums so far. Further work needs to elucidate how the
coordination sphere not only influences the structure, but also
the reactivity of organosilyllithiums.
1
13
Avance III HD spectrometer at 24°C. H and C chemical shifts (δ)
1
were referenced to the solvent residual peak [C D H: δ( H)=
6
5
1
7
.16 ppm, C D H: δ( H)=2.08 ppm ] and the solvent peak [C D :
7 7 6 6
13 13 29
δ( C)=128.4 ppm, C D : δ( C)=20.4 ppm], respectively. Si NMR
7
8
2
9
were calibrated using tetramethylsilane [δ( Si)=0.0 ppm] as exter-
nal standard and the spectra were recorded using the INEPT pulse
sequence. Spin-spin coupling constants (J) are reported in Hertz
1
(
Hz). Coupling patterns in the H NMR spectra are abbreviated as
following: s (singlet), br s (broadened singlet), d (doublet), t (triplet),
q (quartet), m (multiplet), dq (doublet of quartets), dspt (doublet of
septets).
X-RAY structure determination
Single crystal X-ray data were collected on a Bruker D8 Venture
four-circle diffractometer from Bruker AXS GmbH, equipped with a
Photon II CPAD detector. The measurements were carried out at
1
00 K using MoKα radiation (λ=0.71703 Å). Radiation source: IμS Mo
microfocus source by Incoatex GmbH, equipped with HELIOS mirror
optics and a single-hole collimator from Bruker AXS GmbH. Air and
moisture sensitive crystals were mounted using the X-TEMP
2
[15]
system in combination with a SMZ1270 stereo microscope from
Nikon Metrology GmbH. The crystals were mounted on Micro-
Mounts, MicroLoops or MicroGrippers from MiTeGen in perfluor-
opolyalkylethers. APEX3 in connection with SAINT (integration) and
SADABS (adsorption correction) from Bruker AXS GmbH were used
for the data collection. The molecular structures were solved with
[
16]
[17]
SHELXT
and refined using SHELXL.
The final processing of
2
[18]
structures was carried out with OLEX . All non-hydrogen atoms
were refined anisotropically and all carbon-bound hydrogen atoms
Figure 3. Side-view of the bent phenyl group in lithiosilane 4 in the
solid state (left) and torsional angles of 3 and 4 (right).
Z. Anorg. Allg. Chem. 2021, 979–983
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published by Wiley-VCH GmbH.

Journal of Inorganic and General Chemistry
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Table 1. Most important crystal and structure refinement data for 3 and 4.
3
4
Empirical formula
Formula weight
C26.9H46.5LiN O OSi
448.39
C H LiN Si
397.64
2.3
0.7
23 44
3
Crystal system
Space group
monoclinic
P2 /n
1
orthorhombic
P2 2 2
1 1 1
a/Å
b/Å
c/Å
α/°
β/°
γ/°
9.9152(5)
17.1270(9)
15.7470(9)
90
97.581(2)
90
2650.7(2)
4
1.124
10.129(3)
15.619(3)
16.021(5)
90
90
90
2534.5(11)
4
1.042
3
Volume/Å
Z
À 3
1calc/gcm
μ/mm
À 1
0.108
0.105
F(000)
986.0
4.596 to 52.792
0.0697
880.0
2
R
Θ range/°
3.642 to 75.726
0.0597
1.041
int
2
Goodness-of-fit on F
1.038
R [I�2σ (I)]
R1 =0.0592,
R1 =0.0515,
wR =0.1527
wR =0.1475
2
2
wR (all data)
R1 =0.0727,
R1 =0.0562,
wR =0.1646
wR =0.1516
2
2
À 3
Δ1 (min, max)/eÅ
0.44/À 0.40
1.18/À 0.17
were placed in geometrically calculated positions according to a
riding model.
Preparation of lithiosilane 4: To a suspension of lithium metal
(62.0 mg, 8.93 mmol, 7.64 eq.) in thf (4 ml) chlorodimethyl-phenyl-
silane (0.20 g, 1.17 mmol, 1 eq.) was added and the solution was
stirred for 4 h at 0°C. After complete reduction, the thf was
Syntheses
removed in vacuo and the residue was dissolved in Et O. The
2
supernatant was decanted and transferred into a fresh Schlenk
tube. After addition of tbtac (0.33 g, 1.29 mmol, 1.1 eq.) the solution
was stored at À 80°C for crystallization. Lithiosilane 4 was obtained
as colorless crystals. The supernatant was decanted and the crystals
were washed three times with cold (À 78°C) n-pentane and
subsequently dried in vacuo to yield 4 (0.27 g, 0.68 mmol, 58%).
Synthesis of tbtac: The synthesis of tbtac was carried out according
to a modified instruction of Khoma et al. To an aqueous solution
[
19]
of formaldehyde (30 wt% in H O, 6.72 ml, 82.9 mmol, 1 eq.) was
2
added t-butylamine (6.06 g, 82.9 mmol, 1 eq.) at 0°C. The reaction
was kept stirring for 24 hours at 0°C. Subsequently the aqueous
solution was extracted three times with Et O, the combinded
2
The crystals were dissolved in dry toluene-d and submitted for
8
organic phases were dried over NaSO and concentrated in vacuo.
1
4
NMR measurements. H NMR (600 MHz, toluene-d ) δ=0.80 (s, 6H,
À 1
8
Kugelrohr distillation (115°C, 2·10 mbar) gave tbtac as a colorless
SiCH ), 1.12 [s, 27H, NC(CH ) ], 3.73 (s, 6H, NCH N), 7.11–7.20 (m, 2H,
1
3
3 3
2
liquid (5 g, 20 mmol, 24%). H-NMR (400 MHz, C D ) δ=1.14 [s,
6
6
CH ), 7.30–7.33 (t, 1H, J=7.43 Hz), 7.41–7.43 (m, 1H, CH ), 7.87 (d,
1
13
ar
ar
2
7H; NC(CH ) ], 3.77 [s, 6H, NCH N], ppm. { H} C-NMR (101 MHz,
1
13
3
3
2
J=6.69 Hz, 1H, CH ) ppm, { H} C NMR (151 MHz, toluene-d ) δ=
ar
8
C D ) δ=28.2 [s, 9C, NC(CH ) ], 53.5 [s, 3C; NC(CH ) ], 64.6 (s, 3C,
6
6
3 3
3 3
7
.34 (2C, SiCH ) 27.8 [9C, NC(CH ) ], 53.1 [3C, NC(CH ) ], 64.15 (3C,
3 3 3 3 3
NCH N) ppm.
2
NCH N), 123.6 (1C, Cpara), 126.9 (2C, Cmeta), 134.1 (2C, Cortho), 162.8
2
1
29
Preparation of lithiosilane 3: To a suspension of lithium metal
56.0 mg, 8.07 mmol, 7 eq.) in thf (4 ml) was added chlorodimeth-
(1C, Cipso) ppm, { H} Si NMR (119 MHz, toluene-d
8
) δ=À 26.0 (q, 1Si;
(
Si) ppm.
ylphenylsilane (0.21 g, 1.23 mmol, 1 eq.). The reaction was stirred
for 4 h at 0°C before the thf was removed in vacuo and the residue
was dissolved in n-pentane. The supernatant was decanted and
transferred into a fresh Schlenk tube. Subsequently, quinuclidine
Acknowledgements
(
0.78 g, 7.02 mmol, 6 eq.) was added at room temperature and the
resulting solution was stored at À 80°C for crystallization. Lithiosi-
lane 3 was obtained as orange plates. The supernatant was
decanted and the crystals were washed three times with cold
Open access funding enabled and organized by Projekt DEAL.
Keywords: lithiosilane · silanides · lithium · silicon · reductions
(
À 78°C) n-pentane and subsequently dried in vacuo. The dried
crystals were dissolved in dry toluene-d and submitted for NMR
8
measurements. Due to solubility issues of 3 in toluene, the NMR
spectra were recorded at room temperature. However, as the
concentration of 3 in solution is still low at room temperature, the
signals from H and C NMR are not assigned at this point. For more
detailed information and full spectra see the SI. { H} Si NMR
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983 © 2021 The Authors. Zeitschrift für anorganische und allgemeine Chemie
published by Wiley-VCH GmbH.