Synthesis and structure of the silylated benzene radical anion salts [K([18]crown-6){C6H4(SiMe3)2-1,4}] and [K([18]crown-6)(THF)2][C6H2(SiMe 3)4-1,2,4,5]

Article abstract of DOI:10.1016/j.jorganchem.2010.11.040

The crystalline compound [K([18]crown-6){C₆H ₄(SiMe₃)₂-1,4}] (1) was prepared by the low-temperature reduction of the para-disilylated benzene with K/[18]crown-6 in toluene followed by recrystallisation from the

Full text of DOI:10.1016/j.jorganchem.2010.11.040

Journal of Organometallic Chemistry 696 (2011) 2161e2164
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis and structure of the silylated benzene radical anion salts [K([18]-
crown-6){C H (SiMe ) -1,4}] and [K([18]crown-6)(THF) ][C H (SiMe ) -1,2,4,5]
6
4
3 2
2
6
2
3 4
*
Peter B. Hitchcock, Michael F. Lappert , Andrey V. Protchenko
Department of Chemistry & Biochemistry, University of Sussex, Brighton, BN1 9QJ, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 July 2009
Received in revised form
6 4 3 2
The crystalline compound [K([18]crown-6){C H (SiMe ) -1,4}] (1) was prepared by the low-temperature
reduction of the para-disilylated benzene with K/[18]crown-6 in toluene followed by recrystallisation
from the same solvent. Reduction of 1,2,4,5-tetrasilylated benzene with 2(K/[18]crown-6) in toluene
12 November 2010
produced
)] [C (SiMe
(SiMe -1,2,4,5] (3). An X-ray diffraction study revealed that 1 comprised a contact ion pair with
the crown-encapsulated K cation
anion, while 3 contained a well separated cation and anion.
a
hydrocarbon-insoluble powder identified as the dianionic derivative [K([18]crown-
Accepted 17 November 2010
6
2
6
H
2
3
)
4
-1,2,4,5)] (2), which upon crystallisation from THF/Et O yielded [K([18]crown-6)(THF)
2
2
]
[
C
6
H
2
3 4
)
Keywords:
5
h
-connected to the planar ring of the substituted benzene radical
Crown ether
Silylated benzene
Potassium
Ó 2010 Published by Elsevier B.V.
Radical anion
Structure
1. Introduction
6 4 3 2
6){C H (SiMe ) -1,4}] (1) precipitated upon cooling, eqn. (1).
Compound 1 had a limited stability under ambient conditions both
in solution and as a solid and the product obtained as described
above was contaminated with some colourless microcrystalline
powder. Thus, a low-temperature procedure was developed, which
(albeit being more time-consuming) allowed the isolation of pure 1.
While less pure samples slowly decomposed even in a freezer,
recrystallised compound 1 demonstrated an improved stability
Radical anions of aromatic hydrocarbons play an important role
in chemistry as powerful reducing agents [1] or as intermediates in
arene-activated metallation reactions [2]. However, structural
studies are limited due to their instability or tendency to dimerise
in the solid state. Several structures of polycyclic or polyphenyl
radical anion salts including those of naphthalene (four structures
containing different cations) [3], anthracene [3a,4], biphenyl [5],
pyrene [6], perylene [7], and decacyclene [8] have been recorded.
However, for monocyclic hydrocarbons with no reducible func-
tional groups only toluene [9] and 1,2,4,5-tetrakis(trimethylsilyl)
benzene radical anions have been X-ray characterised [10]. Our
interest in benzenoid radical anions arose from their involvement
in the synthesis of low oxidation state organolanthanides [11].
ꢁ
remaining unchanged for months in a sealed tube at ꢀ30 C.
,
ꢀ
Due to paramagnetism (EPR spectra of [C
solution [12] and in frozen 2-methyltetrahydrofuran glass matrices
[
6 4 3 2
H (SiMe ) -1,4] in
2
. Results and discussion
1
13] were described earlier) the H NMR spectrum of 1 in toluene-
Following the synthesis and characterisation of the toluene
d
8
showed only two very broad partially overlapping resonances
radical anion salt [K([18]crown-6)(PhMe)] (I), which was almost
insoluble in toluene [9], we sought a more hydrocarbon-soluble
analogue. Reaction of 1,4-bis(trimethylsilyl)benzene with K metal
and a crown ether in toluene at ambient temperature gave a dark
greenebrown solution, from which black crystals of [K([18]crown-
attributed to the crown ether and the SiMe
this sample gave 1:1 mixture of
(SiMe
3
C
groups; hydrolysis of
(SiMe -1,4 and
a
6
H
4
3 2
)
C
6
H
6
3
)
2
-1,4. Similar to I, a solution of 1 in tetrahydrofuran
(^THF) was thermochromic: deep blue at ambient temperature it
ꢁ
progressively turned greenebrown upon cooling to ꢀ70 C; the
colour change may be attributed to an equilibrium between the
arene radical anion (greenebrown) and the solvated electron
(blue). Spectrophotometric studies of such an equilibrium in the
*
Corresponding author. Tel.: þ44 1273 678316; fax: þ44 1273 677196.
E-mail address: m.f.lappert@sussex.ac.uk (M.F. Lappert).
0
022-328X/$ e see front matter Ó 2010 Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2010.11.040

2162
P.B. Hitchcock et al. / Journal of Organometallic Chemistry 696 (2011) 2161e2164
,
ꢀ
case of benzene showed that [C
temperature, while e
6
H
6
]
was more stable at low
Table 1
Selected bond lengths (Å) and angles ( ) for 1
ꢁ
ꢀ
s
became the prevailing species near or above
room temperature [14].
K1eO1
K1eO2
K1eO3
K1eO4
K1eO5
K1eO6
K1/C14
K1/C15
K1/C16
K1/C17
K1/C18
K1/C13
C13eC14
C13eC18
C14eC15
C15eC16
C16eC17
C17eC18
C13eSi1
2.859(2)
2.858(3)
3.017(3)
2.845(3)
2.818(3)
3.008(2)
3.303(3)
3.124(3)
3.150(3)
3.240(4)
3.405(4)
3.514(4)
1.433(5)
1.444(5)
1.372(4)
1.435(5)
1.443(5)
1.366(5)
1.833(4)
1.836(3)
113.5(3)
113.7(3)
123.3(3)
123.1(3)
122.9(3)
123.3(3)
K2eO7
K2eO8
K2eO9
K2eO10
K2eO11
K2eO12
K2/C41
K2/C42
K2/C37
K2/C38
K2/C39
K2/C40
C37eC38
C37eC42
C38eC39
C39eC40
C40eC41
C41eC42
C37eSi3
2.802(3)
2.901(3)
2.819(3)
3.055(4)
2.901(3)
3.018(2)
3.293(4)
3.128(3)
3.165(4)
3.279(4)
3.432(4)
3.506(4)
1.443(5)
1.436(5)
1.366(5)
1.433(5)
1.428(5)
1.380(4)
1.830(3)
1.836(3)
113.3(3)
113.5(3)
123.1(3)
123.6(3)
123.3(3)
123.1(3)
Attempts to prepare a sodium analogue of 1 were unsuccessful.
No reaction was observed in toluene; in THF, a slow reaction
occurred producing a dark brown solution after two weeks (cf. Ref
[12]: no radical anion signal was observed without crown ether). A
sticky black solid was obtained upon removal of the solvent in
a vacuum. Unexpectedly, addition of hexane (in order to remove
non-crystalline material) resulted in immediate decomposition
with deposition of Na metal (as a mirror and spongy lumps); [18]
crown-6 and the starting arene were identified in the supernatant
1
solution by H NMR spectroscopy.
The molecular structure of one of the two independent mole-
cules of the crystalline compound 1 is shown as an ORTEP repre-
sentation in Fig. 1 (the second molecule has a similar structure).
Selected bond lengths and angles are given in Table 1. The molecule
consists of a crown-encapsulated potassium cation coordinated in
5
C16eSi2
C40eSi4
an approximate
h
-fashion to the planar radical anion. Due to the
substituents on the anion, the crown
C14eC13eC18
C15eC16eC17
C13eC14eC15
C14eC15eC16
C16eC17eC18
C17eC18eC13
C38eC37eC42
C39eC40eC41
C37eC38eC39
C38eC39eC40
C40eC41eC42
C41eC42eC37
presence of the bulky SiMe
3
ether is considerably distorted from its classical crown shape found
in I [9] or other salts of less sterically demanding anions. The five
KeC distances are in the range of 3.124(3)e3.405(4) Å, which is
slightly longer than in I. In the anion two CeC bonds are shorter
(
C14eC15 and C17eC18) while the other four are longer than the
corresponding bonds of the neutral arene [15]. The C13 atom is
noticeably pyramidalised (the sum of the three subtended angles is
[18]crown-6 reduction and characterised by the EPR spectroscopy
[16]). Further reduction of tetrakis(trimethylsilyl)benzene was also
ꢁ
ꢁ
3
58.2 for C13 and 359.9 for C16), which is may be attributed to
6
6
partial localisation of an unpaired electron on this atom.
2 6 2 3 4
possible: a dianionic derivative [{Li(DME)} (m-h :h -C H (SiMe ) -
It has previously been shown that trimethylsilyl substituents
stabilise aromatic radical anions [12]; however, the effect of just
two such substituents is not very pronounced. From the practical
synthesis point of view there is almost no difference in the ability of
toluene [9] or para-disilylated benzene (vide supra) to produce
crystalline radical anion salts: in each case potassium rather than
sodium was used as a reductant and a strong complexant was
required to facilitate the efficient electron transfer. With its four
1,2,4,5)] was obtained with Li in DME [17].
We
used
the
-1,2,4,5] (2) in its reaction with [{LaCp"
(SiMe -1,3) in benzene; the products contained
rather than the C (SiMe moiety, because the
latter was too bulky to bind to the LaCp
Compound 2 was insoluble in hydrocarbon solvents; unexpectedly,
recrystallisation from THF/Et O yielded the monoanionic salt [K
([18]crown-6)(THF) ][C (SiMe -1,2,4,5] (3), which was
contaminated with the enolate [K([18]crown-6)(OCHCH )] [9] as
dianionic
compound
[K([18]crown-
6)]
(Cp
2
[C
¼
6
H
h
2
(SiMe
3
)
4
2
(m-Cl)} ]
2
0
0
5
-C
5
H
3
3 2
)
coordinated C
6
H
6
6
H
2
3 4
)
0
0
2
fragment [11c].
2
SiMe
3
substituents, tetrakis(trimethylsilyl)benzene was readily
2
6
H
2
3 4
)
reduced to its radical anion with Na in 1,2-dimethoxyethane
2
(
^DME), yielding crystalline [Na(DME)
3
][C
6
H
2
(SiMe
3
)
4
-1,2,4,5] (II)
a result of THF cleavage. Apparently, 2 disproportionated into 3 and
potassium electride, Scheme 1; the equilibrium was shifted to the
products due to the instability of the latter and lower solubility of 3
under the given conditions.
[10] (earlier this radical anion was prepared by the K/dicyclohexyl
The molecular structure of the crystalline salt 3 is shown in
Fig. 2. Selected bond lengths and angles are given in Table 2. In
contrast to 1 compound 3 crystallised as a well separated ion pair,
with the K cation coordinated not only by the crown ether, but also
by two THF ligands. As a consequence the metal is more tightly
connected to the encapsulating ligand in 3 (av. KeO, 2.78 Å) than in
1
(av. KeO, 2.90 Å). The structure of the radical anion in 3 is very
similar to that in the sodium salt II [10]. It is noteworthy that as
compared to 1 the CeC bond length distribution in the C ring of 3
and II) follows a different pattern: two CeC bonds (between the
6
(
silyl-substituted C atoms, C1eC2 and C4eC5) are considerably
longer than the other four, which are nearly identical to the cor-
responding CeC bonds of the neutral arene.
The crystalline compounds [K([18]crown-6){C
1) and [K([18]crown-6)] [C (SiMe -1,2,4,5] (2) were obtained
in high yield by the reduction of C (SiMe -1,4 (at low-
temperature) and C (SiMe -1,2,4,5 (at ambient temperature),
respectively, using K/[18]crown-6 in toluene. Recrystallisation of 2
from THF/Et furnished crystals [K([18]crown-6)(THF)
-1,2,4,5] (3) contaminated with the known enolate [K
[18]crown-6)(OCHCH )]. The X-ray molecular structures of the
6 4 3 2
H (SiMe ) -1,4}]
(
2
6
H
2
3 4
)
6
H
4
3 2
)
6
H
2
3 4
)
2
O
2
]
6 2 3 4
[C H (SiMe )
(
2
Fig. 1. ORTEP representation of the molecular structure of 1 (50% ellipsoids; one of the
two independent molecules is shown).
crystalline paramagnetic compounds 1 and 3 are reported.

P.B. Hitchcock et al. / Journal of Organometallic Chemistry 696 (2011) 2161e2164
2163
Scheme 1.
3.1. Preparation of [K([18]crown-6){C
6 4 3 2
H (SiMe ) -1,4}] (1)
Due to the extreme air-sensitivity and the limited thermal
stability of 1, its preparation was carried out in a vacuum-sealed
glass apparatus (thoroughly flame-dried and filled with argon prior
to the experiment) shown in Fig. 3. Potassium metal (0.120 g,
3
.07 mmol) was placed into the part A and was gently heated under
vacuum until a mirror was formed. A solution of C (SiMe -1,4
0.700 g, 3.15 mmol) and [18]crown-6 (0.880 g, 3.33 mmol) in
H
6 4
3 2
)
(
3
toluene (80 cm ) was introduced into the part B and was carefully
degassed by a freeze-pump-thaw procedure. The apparatus was
sealed off at the point C, the solution was transferred into the part A
ꢁ
and stored at ꢀ30 C for ca. 6 h. When the surface of the K mirror
became completely blocked by the product, the mixture was
quickly warmed up to ambient temperature, vigorously stirred for
several minutes and the dark greenebrown solution was carefully
ꢁ
decanted into the part B. Storing overnight at ꢀ30 C yielded black
crystals of compound 1 and the nearly colourless supernatant
solution, which was decanted back into the part A. The procedure
was repeated three times, whereafter most of the potassium had
disappeared and the product was collected in the part B. It was
Fig. 2. ORTEP representation of the molecular structure of 3 (30% ellipsoids).
. Experimental
3
washed with cold toluene (via vacuum transfer from the part A)
1
and dried in a vacuum by freezing the part A using liquid N
2
. Then
ꢁ
Standard Schlenk and vacuum line techniques were used for
the part B containing 1 (1.33 g, 70%), mp 83e87 C (decomp), was
sealed off at the point D. Crystals of 1 for the X-ray diffraction study
were prepared in a separate, small-scale experiment and were
transferred to the diffractometer directly from the toluene solution,
because dried crystals showed very weak reflections.
solvents and precursors preparation. Benzene, toluene, tetrahy-
drofuran and diethyl ether were dried using appropriate drying
agents and were stored over a sodium mirror.1,4-Bis(trimethylsilyl)
benzene [18] and 1,2,4,5-tetrakis(trimethylsilyl)benzene [10] were
prepared by literature procedures; [18]crown-6 was purchased
from Aldrich. The NMR spectra were recorded at 293 K in C
6
D
6
or
1
3.2. Isolation of [K([18]crown-6)(THF) ][C H (SiMe ) -1,2,4,5] (3)
2
6
2
3 4
C D on a Bruker DPX 300 instrument (300.1 MHz for H, 75.5 MHz
7 8
1
3
for C) and referenced internally to residual solvent resonances
1
H, 13C).
6 2 3 4
A solution of C H (SiMe ) -1,2,4,5 (0.734 g, 2.00 mmol) and [18]
(
3
crown-6 (1.145 g, 4.33 mmol) in toluene (100 cm ) was introduced
into a Schlenk flask containing a K mirror (0.156 g, 4.00 mmol) at
ambient temperature. The mixture was stirred vigorously for 2 d
(the flask was tilted periodically from one side to the other to
ensure that the stirring bar moved all over the flask, clearing the
potassium surface from the precipitate). When all the potassium
had disappeared, the dark brown precipitate was filtered off,
washed with toluene and dried in a vacuum yielding 2 (1.850 g,
Table 2
Selected bond lengths (Å) and angles ( ) for 3
ꢁ
KeO1
2.747(3)
2.825(3)
2.768(3)
2.734(5)
1.845(4)
1.854(4)
1.401(5)
1.405(5)
1.462(4)
115.5(3)
127.5(4)
116.1(3)
129.0(3)
128.3(3)
115.1(3)
128.1(3)
KeO2
KeO4
KeO6
KeO8
Si2eC2
Si4eC5
C1eC2
C3eC4
2.788(3)
2.796(3)
2.831(3)
2.692(4)
1.850(4)
1.851(4)
1.474(5)
1.405(5)
1.404(5)
116.4(3)
116.3(3)
128.3(4)
115.5(3)
115.0(3)
128.4(3)
115.4(3)
KeO3
KeO5
KeO7
Si1eC1
3
95%). A sample of 2 (0.200 g, 0.205 mmol) in hexane (10 cm ) was
Si3eC4
C1eC6
treated with dry air (until the dark colour disappeared) and then
3
with water (10 cm ). Evaporation of the hexane layer produced
C2eC3
C4eC5
6 2 3 4
C H (SiMe ) -1,2,4,5 (0.072 g, 0.196 mmol) and titration of the
water layer with 0.1 M HCl revealed the presence of 0.40 mmol of
KOH, thus confirming the 1:2 stoichiometry. The remaining
C5eC6
C6eC1eC2
C2eC3eC4
C4eC5eC6
Si1eC1eC2
Si2eC2eC1
Si3eC4eC3
Si4eC5eC4
C1eC2eC3
C3eC4eC5
C5eC6eC1
Si1eC1eC6
Si2eC2eC3
Si3eC4eC5
Si4eC5eC6
1
Care should be taken when freezing large volumes of toluene as frozen toluene
spontaneously undergoes glassy-crystalline phase transition with a violent crack,
which may result in breaking a glass ampoule.

2164
P.B. Hitchcock et al. / Journal of Organometallic Chemistry 696 (2011) 2161e2164
at 173(2) K. The crystals were coated in oil and then directly
mounted on the diffractometer under a stream of cold nitrogen gas.
2
Each structure was refined on all F using SHELXL-97 [19].
Absorption correction was applied for 3 using an empirical method
(psi-scans), while no correction was applied for 1. The illustration
of Figs. 1 and 2 used ORTEP-3 for Windows. Further details are in
Table 3.
Appendix A. Supplementary material
CCDC 734703 and 734704; contain the supplementary crystal-
lographic data for the structural analysis of the compounds 1 and 3.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
Fig. 3. Glass apparatus for the synthesis and crystallisation of air-sensitive and ther-
mally unstable compounds.
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C
3
C
[
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a (Å)
24
H
6
46KO Si
2
38 8 4
H78KO Si
6526.
525.89
Monoclinic
P2 /c (No. 14)
18.8913(6)
14.9526(7)
21.9469(7)
104.775(2)
5994.4(4)
8
1.17
0.29
24662
10473 (0.069)
824.5
Monoclinic
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114.72(2)
9859(4)
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1.10
0.25
6941
6847 (0.021)
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ꢁ
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( )
3
V (Å )
[
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int
(R )
[
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s
(I)
6131
4796
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6847/0/460
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Final R indices [I > 2
R indices (all data)
s
(I)]
R
R
1
¼ 0.064, wR
2
2
¼ 0.123 R
1
1
¼ 0.056, wR
2
2
¼ 0.134
¼ 0.153
1
¼ 0.128, wR
¼ 0.147 R
¼ 0.089, wR
(
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(
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2
O and
[
[
ꢁ
stored at ꢀ10 C; after several days a mixture of dark brown blocks
and needles of 3 and a colourless microcrystalline powder was
obtained.
[14] R.A. Marasas, T. Iyoda, J.R. Miller, J. Phys. Chem. A 107 (2003) 2033.
[
[
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3
.3. Crystal data and refinement details for 1 and 3
[
[
18] M. Laguerre, J. Dunogues, R. Calas, N. Duffaut, J. Organomet. Chem. 112 (1976)
49.
Diffraction data were collected on a Enraf-Nonius Kappa-CCD
19] G.M. Sheldrick, SHELXL-97, Program for Crystal Structure Refinement.
diffractometer using monochromated Mo-K
a
radiation,
l
0.71073 Å
University of Göttingen, 1997.