Three 2,5-dialkoxy-1,4-diethynyl-benzene derivatives

Article abstract of DOI:10.1107/S0108270107065523

2,5-Dieth-oxy-1,4-bis-[(trimethyl-silyl)ethyn-yl]benzene, C ₂₀H₃₀O₂Si₂, (I), constitutes one of the first structurally characterized examples of a family of compounds, viz. the 2,5-dialk-oxy-1,4-bis-[(trimethyl-silyl)ethyn-yl]benzene derivatives, used in the preparation of oligo(phenyl-ene-ethynylene)s via Pd/Cu-catalysed cross-coupling. 2,5-Dieth-oxy-1,4-diethynylbenzene, C₁₄H ₁₄O₂, (II), results from protodesilylation of (I). 1,4-Diethynyl-2,5-bis-(hept-yloxy)benzene, C₂₄H₃₄O ₂, (III), is a long alk-yloxy chain analogue of (II). The molecules of compounds (I)-(III) are located on sites with crystallographic inversion symmetry. The large substituents either in the alkynyl group or in the benzene ring have a marked effect on the packing and inter-molecular inter-actions of adjacent mol-ecules. All the compounds exhibit weak inter-molecular inter-actions that are only slightly shorter than the sum of the van der Waals radii of the inter-acting atoms. Compound (I) displays C - H...π inter-actions between the methyl-ene H atoms and the acetyl-enic C atom. Compound (II) shows π-π inter-actions between the acetyl-enic C atoms, complemented by C - H...π inter-actions between the methyl H atoms and the acetyl-enic C atoms. Unlike (I) or (II), compound (III) has weak nonclassical hydrogen-bond-type inter-actions between the acetyl-enic H atoms and the ether O atoms.

Full text of DOI:10.1107/S0108270107065523

organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
al., 2005; Meier et al., 2001). Modi®cation of OPEs is needed in
order to improve both processability and long-term stability.
One of the methods for improving their stability and solubility
ISSN 0108-2701
Three 2,5-dialkoxy-1,4-diethynyl-
benzene derivatives
JoÄao Figueira,a,b JoÄao Rodrigues,a Luca Russob and Kari
Rissanenb*
a
Â
Â
Centro de Quõmica da Madeira, LQCMM/MMRG, Departamento de Quõmica da
Universidade da Madeira, 9000-390 Funchal, Portugal, and bNanoscience Centre,
Department of Chemistry, University of Jyvaskyla, PO Box 35, FIN-40014 University
È
È
is the attachment of long linear alkoxy side chains to the main
backbone. Additionally, alkoxy side chains can lead to highly
ordered supramolecular architectures and reduce the
HOMO±LUMO gap and the band gap in the solid state
(Wackerly & Moore, 2006; Jiang et al., 2004; Zhou et al., 2004;
Zhou, Liu et al., 2003; Zhou, Zhao et al., 2003; Meier et al.,
of Jyvaskyla, Finland
È
È
Correspondence e-mail: kari.rissanen@cc.jyu.fi
Received 13 November 2007
Accepted 4 December 2007
Online 12 January 2008
È
2001; Perahia et al., 2001; Mullen & Rabe, 1998). A possible
2,5-Diethoxy-1,4-bis[(trimethylsilyl)ethynyl]benzene, C20H30-
O2Si2, (I), constitutes one of the ®rst structurally characterized
examples of a family of compounds, viz. the 2,5-dialkoxy-1,4-
bis[(trimethylsilyl)ethynyl]benzene derivatives, used in the
preparation of oligo(phenyleneethynylene)s via Pd/Cu-cata-
lysed cross-coupling. 2,5-Diethoxy-1,4-diethynylbenzene,
C14H14O2, (II), results from protodesilylation of (I). 1,4-Di-
ethynyl-2,5-bis(heptyloxy)benzene, C24H34O2, (III), is a long
alkyloxy chain analogue of (II). The molecules of compounds
(I)±(III) are located on sites with crystallographic inversion
symmetry. The large substituents either in the alkynyl group or
in the benzene ring have a marked effect on the packing and
intermolecular interactions of adjacent molecules. All the
compounds exhibit weak intermolecular interactions that are
only slightly shorter than the sum of the van der Waals radii of
the interacting atoms. Compound (I) displays CÐHÁ Á Áꢀ
interactions between the methylene H atoms and the acetyl-
enic C atom. Compound (II) shows ꢀ±ꢀ interactions between
the acetylenic C atoms, complemented by CÐHÁ Á Áꢀ inter-
actions between the methyl H atoms and the acetylenic C
atoms. Unlike (I) or (II), compound (III) has weak
nonclassical hydrogen-bond-type interactions between the
acetylenic H atoms and the ether O atoms.
way to synthesize OPEs with alkoxy side chains is by using a
two-step Sonogashira±Hagihara reaction: a palladium-cata-
lysed cross-coupling condensation between 2,5-bis(alkoxy)-
benzene halides and terminal trimethylsilylacetylenes or
2-methylbut-3-yn-2-ol as the protecting group precursors in
the presence of CuI, followed by a base-promoted deprotec-
tion of the capped acetylides (Tykwinski, 2003; Sonogashira,
2002; Takahashi et al., 1980; Zhao et al., 2007). The structures
of trimethylsilylated 1,4-diethynylbenzene (Weiss et al., 1997;
Ahmed et al., 1972) and the 2,5-bis(alkoxy) ring-substituted
derivatives 1,4-diethynyl-2,5-dimethoxybenzene and 1,4-di-
ethynyl-2,5-bis(octyloxy)benzene (Khan et al., 2003) are
known. The X-ray structures of 1,4-bis(trimethylsilylethyn-
yl)naphthalene and 9,10-bis(trimethylsilylethynyl)anthracene
were reported by Khan et al. (2004). We report here the
Comment
Among highly conjugated polymers, those composed of
alternating aryl and ethynyl units, such as the oligo(phenyl-
eneethynylene)s (OPEs), a type of monodisperse and shaped-
persistent oligomer, have been the object of increasing interest
in academic and industrial research laboratories owing to their
potential applications as molecular wires (Kushmerick et al.,
2002; Tour, 2000; Reinerth et al., 1998; Cygan et al., 1998;
Bumm et al., 1996), recti®ers (Dhirani et al., 1997), data
storage systems (Reed et al., 2001; Feringa et al., 1993),
photoluminescent and electroluminescent devices (Yama-
guchi et al., 2005), and nonlinear optical materials (Koynov et
Figure 1
An ORTEP-3 plot (Farrugia, 1997) of (I) (50% probability displacement
ellipsoids).
Acta Cryst. (2008). C64, o33±o36
DOI: 10.1107/S0108270107065523
# 2008 International Union of Crystallography o33

organic compounds
structures of the three 2,5-dialkoxy-1,4-diethynylbenzene
derivatives (I)±(III).
Compound (II) does not have the bulky trimethylsilyl
groups and thus shows a different packing behaviour. Like (I),
compound (II) is symmetrical and planar with no abnormal
bond distances or angles (Fig. 3). The absence of the bulky
trimethylsilyl group allows the molecules a closer approach,
and completely different packing results, with interactions to
Compound (I) is symmetrical and has the expected planar
overall structure (Fig. 1). The bond distances and angles do
not show any abnormal values. The prevailing intermolecular
interaction (de®ned to be shorter than the sum of the van der
Waals radii of the interacting atoms) in (I) is the CÐHÁ Á Áꢀ
interaction between the methylene H atoms of the ethoxy
group and the terminal acetylenic C atom (C6ÐH6BÁ Á ÁC1),
eight separate molecules [with symmetry codes ( x, 1
1
y,
z), ( x,
1
y, z), (x,
y, 12 + z), ( x, 21 + y, 12 z),
2
(x, 12 y, 12 + z) and ( 1 + x, y, 1 + z)]. The molecules of (II)
are arranged in planes with ꢀ±ꢀ interactions between the
acetylenic C1 atoms of adjacent molecules, the C1Á Á ÁC1
ꢀ
Ê
with an interaction distance of 2.86 A and an angle of 153 , the
Ê
CÁ Á ÁC distance being 3.772 (4) A (Fig. 2; only the stronger
Ê
interactions are shown). One molecule of (I) has these inter-
actions to four separate molecules [with symmetry codes
distance being 3.224 (2) A. In addition to the ꢀ±ꢀ interactions,
in-plane CÐHÁ Á Áꢀ interaction between the acetylenic atom
1
2
1
2
(1 x, 12 + y, 12 z), (x,
y, 12 + z), (1 x, 12 + y,
y, + z)]. Another, weaker, intermolecular inter-
z)
C1 and the methyl atom H7B exist, the contact distance
Ê
1
1
and (x,
action is also present, between the H atoms of adjacent
trimethylsilyl groups [H8AÁ Á ÁH9B(1 x, 12 y, 21 + z)], with a
C1Á Á ÁH7B(x, 1 + y, z) being 2.88 A (Fig. 4). These planes
2
2
stack on top of each other with a twist angle of 36.7ꢀ between
the planes. The CÐHÁ Á Áꢀ interactions between the planes are
mediated through methylene H atoms (H6A) and the benzene
ꢀ systems (C4) of adjacent molecules, the contact distance
Ê
contact distance of 2.39 A.
1
2
1
Ê
+ z) being 2.85 A.
2
H6AÁ Á ÁC4(x,
y,
The long heptyloxy chain has an opposite effect on the
packing when compared with compound (II). Compound (III)
is symmetrical and planar, like the other two compounds, and
the bond distances and angles within the molecule are normal
(Fig. 5). Unlike (I) and (II), compound (III) exhibits, as the
only suf®ciently short intermolecular interaction, a nonclas-
sical hydrogen bond between the molecules (Fig. 6). The
contact distance between atoms ꢀH1 and O1( 1 + x, 23 y, z) is
2.54 A, with an angle of 161 and a CÁ Á2ÁO distance of
Ê
Ê
3.449 (2) A. Similar contact distances are frequently found in
nonclassical hydrogen bonds. As in (I), each molecule of (III)
interacts with four other molecules [with symmetry codes
1
2
1
2
(
x, 12 + y, z), ( 21 + x, 32 y, z), (
x, 12 + y, z)
and (12 + x, 32 y, z)]. Even though the bond lengths in all three
compounds do not show abnormal values, there is a signi®cant
variation in the bond lengths of the central benzene ring,
Figure 2
The packing (Macrae et al., 2006) of (I). For clarity, only the CÐHÁ Á Áꢀ
interactions are shown.
Figure 3
An ORTEP-3 (Farrugia, 1997) plot of (II) (50% probability displacement
ellipsoids).
Figure 4
The packing (Macrae et al., 2006) of (II), showing the in-plane
interactions.
ꢁ
o34 Figueira et al.
C20H30O2Si2, C14H14O2 and C24H34O2
Acta Cryst. (2008). C64, o33±o36

organic compounds
Compound (II) was obtained by adding K2CO3 (1.61 g, 11.6 mmol)
to (I) (1.91 g, 5.3 mmol) in a purged Schlenk tube. Degassed CH2Cl2
(7 ml) and MeOH (3 ml) were then added to the solids, forming a
yellow solution, and stirring was begun. After 4 h at room tempera-
ture, the now cloudy yellow mixture was ®ltered and evaporated
under vacuum, yielding a light-yellow solid. Colorless crystals
suitable for X-ray diffraction were obtained as previously described
for compound (I) (yield 73.8%, m.p 347±348 K). 1H NMR (500 MHz,
CDCl3): ꢁ 6.89 (2H, s, Ar), 4.07 (4H, q, CH2, JHH = 14 Hz), 3.40 (2H, s,
HÐCC), 1.43 (6H, t, CH3, JHH = 14 Hz); 13C NMR (126 MHz,
CDCl3): ꢁ 153.83, 117.92, 113.36, 82.42, 79.79, 65.20, 14.75; IR (KBr):
3288 (s), 2984 (s), 2932 (s), 2883 (s), 2107 (s), 1932 (s), 1496 (m), 1218
(m), 1040 (s), 674 (m). MS (ESI): 158 [M+ ± (C2H5)2 + H2], 214 (M+)
m/z.
Figure 5
An ORTEP-3 (Farrugia, 1997) plot of (III) (50% probability displace-
ment ellipsoids).
Compound (III) was prepared by re¯uxing 4,40-[2,5-bis(heptyl-
oxy)-1,4-phenylene]bis(2-methylbut-3-yn-2-ol) in toluene with an
excess of NaOH for 4 h. After this, the still hot solution was ®ltered
and concentrated under vacuum. Good quality single crystals were
obtained by allowing the yellow solution to cool to room temperature
(yield 99%, m.p. 346±347 K). 1H NMR (250 MHz, CDCl3): ꢁ 6.95 (2H,
s, Ar), 3.97 (4H, t, CH2, JHH = 27 Hz), 3.32 (2H, s, HÐCC), 1.76 (4H,
Q, CH2, JHH = 27 Hz), 1.469±1.257 [16H, (CH2)4], 0.89 (3H, d, CH3,
JHH = 9); 13C NMR (65 MHz, CDCl3): ꢁ 153.86, 117.64, 113.15, 82.23,
79.65, 69.54, 31.62, 28.99, 28.85, 25.72, 22.44, 13.93; IR (KBr): 3265 (s),
2927 (s), 2932 (s), 2866 (s), 2105 (w), 1497 (s), 1275 (m). MS (ESI):
158 [M+ ± (C2H5)2 + H2], 354 (M+) m/z.
Compound (I)
Crystal data
3
Ê
C20H30O2Si2
Mr = 358.62
V = 1162.9 (5) A
Z = 2
Figure 6
Monoclinic, P21=c
Mo Kꢃ radiation
ꢄ = 0.16 mm
T = 173 (2) K
0.25 Â 0.2 Â 0.15 mm
1
Ê
a = 10.132 (1) A
The packing (Macrae et al., 2006) of (III), showing the CÐHÁ Á ÁO
Ê
b = 9.887 (2) A
interactions.
Ê
c = 12.169 (4) A
ꢂ = 107.46 (2)ꢀ
which is typical for this type of symmetrical tetrasubstituted
benzenes. The bond length between the unsubstituted C4
atom and the alkoxy-substituted C5 atom is on average 0.034
Data collection
Nonius KappaCCD diffractometer
9367 measured re¯ections
2032 independent re¯ections
1568 re¯ections with I > 2ꢅ(I)
Rint = 0.039
Ê
[for (I)], 0.015 [for (II)] and 0.010 A [for (III)] shorter than the
other bond lengths in the benzene ring.
Re®nement
R[F2 > 2ꢅ(F2)] = 0.048
wR(F2) = 0.175
S = 0.76
113 parameters
H-atom parameters constrained
Experimental
3
Ê
Áꢆmax = 0.21 e A
3
Compound (I) was obtained by Sonogashira cross-coupling (Weder
&
Ê
0.52 e A
2032 re¯ections
Áꢆmin
=
Wrighton, 1996) of 1,4-diethoxy-2,5-diiodobenzene (1.50 g,
3.59 mmol) with trimethylsilylacetylene (TMSA; 1.5 ml, 10.54 mmol)
in dried tetrahydrofuran (20 ml) and N-ethyldiisopropylamine
(25 ml). The reaction was catalysed by PdCl2(PPh3)2 (0.293 g,
0.42 mmol) and CuI (0.06 g, 0.32 mmol). After 16 h of stirring at
318 K under a nitrogen atmosphere, the dark-brown mixture was
®ltered and evaporated under vacuum. Puri®cation by column
chromatography (silica gel S60, CH2Cl2/cyclohexane 1:3) gave a dark-
yellow solid that was further puri®ed by sublimation. The resulting
solid was dissolved in CH2Cl2, forming a light-yellow solution which
was evaporated slowly in air at room temperature, affording colorless
crystals in 35% yield (m.p. 305±309 K). 1H NMR (500 MHz, CDCl3):
ꢁ 6.89 (2H, s, Ar), 4.02 (4H, q, CH2, JHH = 14 Hz), 1.40 (4H, t, Me,
JHH = 14 Hz), 0.29 (18H, s, Me); 13C NMR (126 MHz, CDCl3): ꢁ
153.97, 117.75, 101.06, 100.24, 65.30, 14.79, 0.058; IR (KBr): 2969 (s),
2151 (s), 1499 (s), 1394 (s), 1213 (sh), 1043 (s), 840 (br), 760 (s).
Compound (II)
Crystal data
3
Ê
C14H14O2
Mr = 214.25
V = 604.7 (3) A
Z = 2
Monoclinic, P21=c
Mo Kꢃ radiation
1
Ê
Ê
a = 9.748 (2) A
ꢄ = 0.08 mm
T = 173 (2) K
b = 8.890 (2) A
Ê
c = 7.566 (2) A
0.3 Â 0.2 Â 0.2 mm
ꢂ = 112.74 (3)ꢀ
Data collection
Nonius KappaCCD diffractometer
4943 measured re¯ections
1045 independent re¯ections
910 re¯ections with I > 2ꢅ(I)
Rint = 0.024
ꢁ
Acta Cryst. (2008). C64, o33±o36
Figueira et al.
C20H30O2Si2, C14H14O2 and C24H34O2 o35

organic compounds
Re®nement
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wR(F2) = 0.095
S = 1.07
74 parameters
H-atom parameters constrained
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3
Ê
Áꢆmax = 0.14 e A
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Ê
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Áꢆmin
=
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Ê
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a = 6.7246 (2) A
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Ê
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Ê
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1458 re¯ections with I > 2ꢅ(I)
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3
Ê
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Ê
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Áꢆmin
=
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described at the back of the journal.
ꢁ
o36 Figueira et al.
C20H30O2Si2, C14H14O2 and C24H34O2
Acta Cryst. (2008). C64, o33±o36