Specific interaction modes in the crystal structures of oligofluorinated tolanes featuring additional electron donor and acceptor groups

Article abstract of DOI:10.1016/j.jfluchem.2011.11.009

A series of partially fluorinated and specifically para-substituted tolanes (1-4) have been synthesized via palladium-catalyzed Sonogashira cross-coupling reaction. The single-crystal structures have been determined by X-ray diffraction. The molecules ado

Full text of DOI:10.1016/j.jfluchem.2011.11.009

Journal of Fluorine Chemistry 135 (2012) 231–239
Contents lists available at SciVerse ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Specific interaction modes in the crystal structures of oligofluorinated tolanes
featuring additional electron donor and acceptor groups
Mario Stein a, Ricarda Berger a, Wilhelm Seichter b, Ju¨rg Hulliger a, Edwin Weber b,
*
a Department of Chemistry and Biochemistry, University of Berne, Freiestrasse 3, CH-3012 Berne, Switzerland
b Institute of Organic Chemistry, Technische Universita¨t Bergakademie Freiberg, Leipziger Straße 29, D-09596 Freiberg, Sachsen, Germany
A R T I C L E I N F O
A B S T R A C T
Article history:
A series of partially fluorinated and specifically para-substituted tolanes (1–4) have been synthesized via
palladium-catalyzed Sonogashira cross-coupling reaction. The single-crystal structures have been
determined by X-ray diffraction. The molecules adopt a geometry being more or less disturbed from
planarity due to crystal packing effects. The packing structures are characterized by the formation of
Received 28 July 2011
Received in revised form 15 November 2011
Accepted 16 November 2011
Available online 25 November 2011
molecular stacks achieved through different modes of
p p interaction and being accomplished by other
ꢀ ꢀ ꢀ
types of weak interactions including hydrogen bonds as well as bromo and in particular fluoro involved
Keywords:
contacts.
Substituted fluorotolanes
Sonogashira cross-coupling reaction
X-ray crystal structures
Supramolecular interactions
Fluorine involved contacts
ß 2011 Elsevier B.V. All rights reserved.
1. Introduction
molecules intended for the formation of metal–ligand network
structures [17], liquid crystal behaviour [18] or crystals suitable for
Compounds showing a simple basic structure with clear
molecular geometry and having well defined substituents and
functional groups are most desirable testing systems in order to
increase the knowledge on non-covalent intermolecular interac-
tions [1,2]. Corresponding weak intermolecular contacts are a
fundamental tool in the field of crystal engineering, aiming at the
control of a crystal structure from molecular construction [3,4].
This is of paramount importance in organic materials science since
macroscopic properties of a solid such as electric and optical
behaviour [5,6] as well as structural polymorphism [7] and
catalysis [8], having widespread commercial implications, can be
selectively influenced this way. In the course of these studies,
different packing motifs in crystals including strong and weaker
non-linear optical properties like second harmonic generation
(SHG) [19], fluorination has been considered to control the
strength of neighbouring donor and acceptor groups and thus
affect their behaviour of supramolecular interaction. In this
connection, partially fluorinated tolanes [20] and hetero deriva-
tives of tolanes [21] have recently been synthesized and studied
with reference to their crystalline packing structures.
Herein we report the synthesis of
a series of similarly
fluorinated tolanes 1–4 (Scheme 1) featuring, however, selected
donor and acceptor groups in the para positions of the aryl rings
and discuss their crystal structures in the light of the consequences
coming from the fluorine substitution.
hydrogen bonding [9,10] or
p-stacking interactions [11], giving
rise to the formation of specific supramolecular synthons [1,12],
have been discussed in detail. Making a more profound under-
standing of the nature and strength of organic halogen promoted
non-covalent interactions [13] accessible, especially involving
organic bound fluorine atoms in competition with other weak
contacts [2,14], is another current challenge [15]. A motive for it is
that fluorinated organic compounds often show uncommon and
unique physical and chemical properties [16]. With relevance to
2. Results and discussion
2.1. Compounds studied
All the compounds 1–4 being substituted derivatives of tolane
were synthesized following a common protocol in the key
preparation step. This involves a Sonogashira–Hagihara cross-
coupling reaction between a halogenated arene and an aryl
substituted ethyne in the presence of a standard palladium(II)/
copper(I) catalytic system [22]. The respective aryl halides (5a, 5b)
and corresponding arylethynes (6a–6c) are specified in Scheme 2.
A complete synthetic route, exemplary shown for the preparation
* Corresponding author.
E-mail address: Edwin.Weber@chemie.tu-freiberg.de (E. Weber).
0022-1139/$ – see front matter ß 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2011.11.009

232
M. Stein et al. / Journal of Fluorine Chemistry 135 (2012) 231–239
Scheme 1. Formula structures of compounds studied.
of compound 1, is detailed in Scheme 3. Single crystals of the
compounds 1–4 were obtained by slow evaporation of solutions of
1 and 4 in acetone, 2 in chloroform and 3 in dimethylsulfoxide.
2.2. X-ray single crystal structures
The crystallographic data and structure refinement parameters
for the studied compounds (1–4) are summarized in Table 1. For
the description of the crystal structures, intermolecular contacts
within the sum of the van der Waals radii [23] have been used.
Corresponding hydrogen bond type interactions (C–Hꢀ ꢀ ꢀO, C–
Hꢀ ꢀ ꢀN, C–Hꢀ ꢀ ꢀF, N–Hꢀ ꢀ ꢀN) as well as C–Fꢀ ꢀ ꢀ , N–Oꢀ ꢀ ꢀO, N–Oꢀ ꢀ ꢀN,
p
Scheme 3. Synthetic route exemplary demonstrated for compound 1.
Brꢀ ꢀ ꢀO and Fꢀ ꢀ ꢀF contacts are presented in Table 2.
2.2.1. Nitropentafluoro-substituted tolane 1
of their longitudinal axes. In this arrangement, the ethynyl moiety
˚
Crystallization of 1 from acetone yielded yellow plates of the
monoclinic space group P21/c with one molecule in the asymmetric
unit of the cell. The molecule (Fig. 1(a)) deviates from planarity and
adopts a dihedral angle of nearly 9.98 between the mean planes of
the aromatic rings. The nitro substituent is slightly tilted (ca. 2.58)
with respect to the aromatic ring to which it is attached.
In the crystal structure, the molecules are stacked along the b-
axis in a head-to-head fashion. Within a given stack, consecutive
of each molecule is sandwiched in a distance of 3.50 A between the
perfluorophenyl and nitrophenyl rings of two neighbouring
molecules which suggests the presence of p(CBB C)ꢀ ꢀ ꢀp(arene)
interactions [20,21] (Fig. 2). These findings are consistent with a
recently arisen theoretical interaction model based on molecular
electrostatic potentials [24] which provides a new view for simple
explanation of the packing behaviour of molecules in crystalline
perfluoroarene–arene complexes [25,26]. In this model, the
geometries of arene stacking are induced by local substituent
effects with the proximal vertex of the other ring rather than being
˚
molecules are strictly parallel but displaced by 3.64 A in direction
caused by traditional
[27,28] that was already questioned before [29]. The electron-
withdrawing nature of the ring substituents in relieves
electrostatic repulsion between the aryl -clouds which in the
p-polarization or quadrupole interaction
1
p
present case produces a longitudinal displacement of neighbouring
molecules. The molecular planes of adjacent stacks are oriented
approximately orthogonal thus forming an overall herringbone
pattern. There appears to be no significant non-covalent bonding
based on fluorine interactions [15]. The shortest C–Hꢀ ꢀ ꢀF contact
˚
[C(2)–H(2)ꢀ ꢀ ꢀF(1) 2.68 A, 1428] is beyond the sum of the van der
˚
Waals radii (2.67), whereas the Fꢀ ꢀ ꢀF distance (2.94 A) represents
the van der Waals limit. This suggests that fluorine involved
interactions hardly contribute to crystal stabilization of 1.
However, an unusual feature of the crystal structure is the short
distance between the nitro substituents of two adjacent molecules
˚
in the structure, showing Oꢀ ꢀ ꢀO separations of 2.82 and 2.91 A,
which is less than twice the van der Waals radius of oxygen
˚
(3.04 A). The structure excerpt displayed in Fig. 3 shows the
contact mode between the nitro substituents. Due to the
orthogonal orientation of the interacting molecules I and II, their
nitro substituents approach each other in a perpendicular fashion
with N–Oꢀ ꢀ ꢀO and N–Oꢀ ꢀ ꢀN angles (symmetry: ꢁx, 0.5 + y, 0.5 ꢁ z)
being 136.1 and 113.48, respectively. The N–Oꢀ ꢀ ꢀO angle between
the symmetry related molecules I and III which adopt a coplanar
arrangement (symmetry: ꢁx, 1 ꢁ y, 1 + z) is 131.38. In this context,
it should be noted that similar close Oꢀ ꢀ ꢀO contacts have also been
observed in the crystal structures of a picric acid complex [30] and
of 2,4,6-trinitrotoluene [31].
Scheme 2. Formula structures of fluoroarene and ethynylarene intermediates.

M. Stein et al. / Journal of Fluorine Chemistry 135 (2012) 231–239
233
Table 1
Crystallographic and structure refinement data of the compounds studied.
Compound
1
2
3
4
Empirical formula
Formula weight
Crystal system
Space group
C14H4F5NO2
313.18
Monoclinic
P21/c
C154H4F5N
293.19
C15H6F4N2
290.22
Triclinic
P21/n
C14H4F4Br1NO2
374.09
Monoclinic
P21/c
Triclinic
P-1
˚
a (A)
20.003(4)
5.0291(6)
12.107(2)
90.0
13.2995(10)
9.623(1)
19.7667(15)
90.0
7.8158(17)
5.8014(8)
27.052(6)
90.0
8.5057(13)
8.7859(12)
9.1781(13)
91.380(12)
102.526(12)
92.975(11)
668.21(17)
2
˚
b (A)
˚
c (A)
a
b
g
(8)
(8)
(8)
90.391(15)
90.0
106.393(6)
90.0
97.204(18)
90.0
3
˚
V (A )
Z
1217.9(3)
4
2426.9(4)
8
1216.9(4)
4
F(0 0 0)
Dc (mg mꢁ3
624
1168
584
364
)
1.708
1.605
1.584
1.859
m
(mmꢁ1
)
0.166
0.149
0.139
3.129
Data collection
Temperature (K)
223(2)
223(2)
173(2)
223(2)
9734
No. of collected reflections
9184
34137
7669
Within the
u
-limit (8)
2.0–25.7
1.6–29.3
1.5–25.7
ꢁ9/9, ꢁ7/6, ꢁ32/32
2293
2.3–29.3
Index ranges ꢂh, ꢂk, ꢂl
No. of unique reflections
Rint
Refinement calculations: full-matrix least-squares on all F2 values
Weighting expression wa
ꢁ24/24, ꢁ6/6, ꢁ14/14
2289
ꢁ18/18, ꢁ13/13, ꢁ23/27
ꢁ11/11, ꢁ12/10, ꢁ12/12
6566
3610
0.0629
0.0600
0.0659
0.0601
[
s2(Fo2) + (0.0367P)2
[
s2(Fo2) + (0.0551P)2
[
s2(Fo2) + (0.0287P)2
[
s2(Fo2) + (0.0940P)2
+(0.0000P)ꢁ1
199
+ (0.0000P)ꢁ1
200
]
+ (0.0000P)ꢁ1
380
]
+ (0.0000P)ꢁ1
215
]
]
No. of refined parameters
No. of F values used [I > 2
Final R-indices
s(I)]
1290
3251
1182
2036
R(=
S
|
D
F|/
S
|Fo|)
0.0385
0.0390
0.0382
0.0592
wR on F2
0.0827
0.1093
0.0744
0.1644
S (=goodness of fit on Fꢁ23)
0.873
0.862
0.787
0.961
˚
Final Drmax
/
Drmin (e A
)
0.15/ꢁ0.13
0.23/ꢁ0.16
0.19/ꢁ0.22
0.91/ꢁ0.99
a
P = (Fo2 + 2Fc2)/3.
Table 2
Selected hydrogen bond and other non-covalent interactions in the compounds 1–4.
˚
Atoms involved
Symmetry
Distances (A)
Angles (8)
D–H
Dꢀ ꢀ ꢀA
Yꢀ ꢀ ꢀA
Hꢀ ꢀ ꢀA
Xꢀ ꢀ ꢀA
D-Hꢀ ꢀ ꢀA
X–Y
X-Yꢀ ꢀ ꢀA
(X = C,N; Y = O,F, Br)
(A = N, O, F, p)
1
C(6)–H(6)ꢀ ꢀ ꢀO(1)
C(5)–H(5)ꢀ ꢀ ꢀO(2)
C(2)–H(2)ꢀ ꢀ ꢀF(1)
N(1)–O(1)ꢀ ꢀ ꢀO(2)
N(1)–O(1)ꢀ ꢀ ꢀN(1)
N(1)–O(2)ꢀ ꢀ ꢀO(2)
C(14)–F(5)ꢀ ꢀ ꢀF(2)
C(11)–F(2)ꢀ ꢀ ꢀF(5)
2
x, ꢁ0.5ꢁy, 0.5 + z
ꢁx, ꢁ1 ꢁ y, 1 ꢁ z
x, 1.5 ꢁ y, ꢁ0.5 + z
ꢁx, ꢁ0.5 + y, 0.5 ꢁ z
ꢁx, ꢁ0.5 + y, 0.5 ꢁ z
ꢁx, ꢁ1 ꢁ y, 1 ꢁ z
x, 1.5 ꢁ y, ꢁ0.5 + z
x, 1.5 ꢁ y, 0.5 + z
0.94
0.94
0.94
1.22
1.22
1.22
1.34
1.34
3.354(2)
3.504(2)
3.470(2)
3.887(2)
3.573(2)
3.740(2)
4.140(2)
3.168(2)
2.72
2.67
2.68
2.91
2.91
2.83
2.94
2.94
125
149
142
136.1
113.4
131.3
147.9
87.0
C(5A)–H(5A)ꢀ ꢀ ꢀN(1)
C(3)–H(3)ꢀ ꢀ ꢀN(1A)
C(5)–H(5)ꢀ ꢀ ꢀF(5A)
C(13)–F(4)ꢀ ꢀ ꢀcentroid(A0)a
C(11A)–F(2A) ꢀcentroid(A0)a
3
x, 0.5 ꢁ y, ꢁ0.5 + z
x, 0.5 ꢁ y, 0.5 + z
ꢁ1 + x, y, z
0.94
0.94
0.94
1.34
1.33
3.591(2)
3.533(2)
3.258(2)
4.345(2)
4.351(2)
2.66
2.60
2.66
3.04
3.05
170
171
122
1 ꢁ x, 2 ꢁ y, 0.5 ꢁ z
1 ꢁ x, 0.5 + y, 0.5 ꢁ z
163.7
167.4
C(6)–H(6)ꢀ ꢀ ꢀF(1)
C(3)–H(3)ꢀ ꢀ ꢀF(3)
N(1)–H(1B)ꢀ ꢀ ꢀN(1)
N(1)–H(1A)ꢀ ꢀ ꢀN(2)
C(3)–H(3)ꢀ ꢀ ꢀN(2)
C(10)–F(1)ꢀ ꢀ ꢀF(3)
C(13)–F(3)ꢀ ꢀ ꢀF(1)
4
1 ꢁ x, 1 ꢁ y, ꢁz
0.93
1.00
0.91
0.91
1.00
1.35
1.34
3.568(2)
3.357(2)
3.452(2)
3.265(2)
3.468(2)
3.023(2)
3.014(2)
2.74
2.61
2.64
2.43
2.64
2.88
2.88
149
132
149
153
140
82.8
82.5
ꢁx, ꢁ1 ꢁ y, ꢁz
0.5 ꢁ x, 0.5 + y, 0.5 ꢁ z
ꢁ0.5 + x, ꢁ0.5 ꢁ y, 0.5 + z
ꢁ0.5 + x, ꢁ0.5 ꢁ y, 0.5 + z
x, 1 + y, z
x, ꢁ1 + y, z
C(3)–H(3)ꢀ ꢀ ꢀO(1)
C(5)–H(5)ꢀ ꢀ ꢀO(2)
C(3)–H(3)ꢀ ꢀ ꢀF(1)
C(5)–H(5)ꢀ ꢀ ꢀF(3)
C(12)–Br(1)ꢀ ꢀ ꢀO(1)
C(12)–Br(1)ꢀ ꢀ ꢀO(2)
C(14)–F(4)ꢀ ꢀ ꢀF(4)
ꢁx, 1 ꢁ y, 1 ꢁ z
1 ꢁ x, 1 ꢁ y, 2 ꢁ z
ꢁ1 + x, y, z
0.94
0.94
0.94
0.94
1.86
1.86
1.40
3.424(2)
3.469(2)
3.102(2)
3.472(2)
4.988(2)
4.963(2)
3.017(2)
2.63
2.70
2.63
2.68
3.19
3.19
2.91
142
140
111
x, 1 + y, 1 + z
143
1 ꢁ x, ꢁ1 + y, ꢁ1 + z
1 ꢁ x, ꢁ1 + y, ꢁ1 + z
1 ꢁ x, ꢁy, ꢁz
161.6
157.7
80.3
a
Means the center of the triple bond [C(7)–C(8)].

234
M. Stein et al. / Journal of Fluorine Chemistry 135 (2012) 231–239
Fig. 1. ORTEP plots of the molecular structures of compounds 1–4 including atom labelling. Thermal ellipsoids are drawn at 50% probability level.
2.2.2. Cyanopentafluoro-substituted tolane 2
The basic supramolecular units of the crystal structure are given
˚
Crystallization of 2 from chloroform yielded colourless crystals
of the monoclinic space group P21/c with two crystallographically
independent molecules within the asymmetric part of the unit cell.
A perspective view of the molecular structure is depicted in
Fig. 1(b). The molecules are approximately planar with a dihedral
angle of 3.08 between the aromatic moieties.
by C–Hꢀ ꢀ ꢀN bonded molecular dimers [d(Hꢀ ꢀ ꢀN) 2.60, 2.66 A],
which are assembled to stacking-like structure domains (Fig. 4)
extending parallel to the crystallographic 1 0 1 plane. Within this
arrangement, the stacking order of molecules can be described as
AABBAA. . ., which means that every fourth molecule along the
stacking axis is congruent. Also in the crystal of 2 the lack of arene
Fig. 2. Packing diagram of 1 viewed down the crystallographic c-axis. Oxygen atoms are displayed as dotted, nitrogen atoms as hatched and fluorine atoms as dark grey circles.
Broken double lines represent (arene) interactions.
(ethynyl)ꢀ ꢀ ꢀ
p
p

M. Stein et al. / Journal of Fluorine Chemistry 135 (2012) 231–239
235
Fig. 3. Packing excerpt of 1 showing the mode of interactions between the nitro substituents of the molecules. Oxygen atoms are displayed as dotted, nitrogen atoms as
hatched and fluorine atoms as dark grey circles. Broken lines represent Oꢀ ꢀ ꢀO and Oꢀ ꢀ ꢀN contacts.
stacking is attributable to the electron-withdrawing character of
the arene substituents which depletes the -electron density of
both aromatic rings. In this structure, the hydrogen bonded ring
motifs of the dimers are located between the perfluorophenyl and
benzonitrile part of neighbouring molecules. In a similar fashion as
in the aforementioned crystal structure, molecular planes of
adjacent stacks are nearly perpendicular. Interstack association is
p
˚
restricted to weak C–Hꢀ ꢀ ꢀF contacts [C(5)–H(5)ꢀ ꢀ ꢀF(5A) 2.66 A,
1228] and C–Fꢀ ꢀ ꢀ(CBB C) interactions involving the fluorine atoms
F(4) and F(2A) and the central ethynyl unit of the molecule 2. With
reference to the latter interactions, both the Fꢀ ꢀ ꢀcentroid(CBB C)
˚
distances of 3.04 (3.05) A, being less than the sum of van der Waals
˚
radii (3.17 A), and the well defined geometry of this contacts (nC–
Fꢀ ꢀ ꢀcentroid 163.7, 167.48), demonstrate evidence of fluorine-
centered bonding based on the
s-hole approach [32].
2.2.3. Aminocyanotetrafluoro-substituted tolane 3
Crystal growing of 3 from a solution in DMSO yielded yellow
plates which show the monoclinic space group P21/n with the
asymmetric unit cell containing one molecule. A perspective view
of the molecular structure is presented in Fig. 1(c). From the
structural viewpoint, donor–acceptor functionalized diaryl acet-
ylenes should be characterized by a redistribution of charge from
donor to acceptor across the conjugated
p-electron system, giving
rise to ground-state charge-transfer activity [33]. As a matter of
fact, a detailed analysis of the molecular structure of 3 reveals only
a small partial quinoid character of the tetrafluorobenzonitrile
part, which is obvious from bond distances within the aromatic
˚
ring ranging from 1.360(3) to 1.396(4) A. In contrast, the
aminophenyl donor group is far from being planar, but the plane
of the amino substituent is inclined at an angle of nearly 308 with
regard to the phenyl ring plane. This can be seen as an indication
for the difference in conjugation of the two aromatic entities, while
the bridging ethynyl unit possesses a distinctive single bond–triple
bond alternation pattern. Nevertheless, our findings are consistent
with the structures of a previously studied series of donor–
acceptor (amino–nitro) para-substituted diarylalkynes [34].
In the crystal structure of 3 (Fig. 5), the molecules are stacked in
direction of the crystallographic a-axis with an antiparallel
arrangement of consecutive molecules. The centroidꢀ ꢀ ꢀcentroid
distances between the aromatic rings of interacting molecules are
Fig. 4. Packing diagram of 2. Nitrogen atoms are displayed as hatched and fluorine
atoms as dark grey circles. Broken double lines represent C–Hꢀ ꢀ ꢀN hydrogen bonds.
˚
3.78 (symmetry: ꢁx, ꢁy, ꢁz) and 4.11 A (symmetry: 1 ꢁ x, ꢁy, ꢁz).

236
M. Stein et al. / Journal of Fluorine Chemistry 135 (2012) 231–239
Fig. 5. Packing diagram of 3 viewed down the crystallographic a-axis. Nitrogen atoms are displayed as hatched and fluorine atoms as dark grey circles. Broken lines represent
hydrogen bonds and Fꢀ ꢀ ꢀF contacts.
Due to the above mentioned theoretical interaction model [24], the
donor–acceptor character of the substituents in implies
the effective overlapping area between aromatic units and thus
diminishes the strength of stacking forces. The well-balanced ratio
between strong molecular donors and acceptors induces a tight
network of hydrogen bonds. The molecular stacks are interlinked
3
electrostatic potential values of opposite signs for the aromatic
molecular building blocks which should lead to a electrostatic
attraction between dissimilar arene rings of consecutive mole-
cules. However, strong directional interactions induce a molecular
offset within the stacking arrangement which significantly reduces
˚
via N–Hꢀ ꢀ ꢀN(amine) [N(1)–H(1B)ꢀ ꢀ ꢀN(1) 2.64 A, 1408] and N–
˚
Hꢀ ꢀ ꢀN(nitrile) hydrogen bonds [N(1)–H(1A)ꢀ ꢀ ꢀN(2) 2.43 A, 1538] as
˚
well as weak Fꢀ ꢀ ꢀF contacts [15] [F(1)ꢀ ꢀ ꢀF(3) 2.88 A]. Moreover, the
Fig. 6. (a) Packing diagram of 4 viewed down the crystallographic b-axis. Oxygen atoms are displayed as dotted, nitrogen atoms as hatched, bromine atoms as cross-hatched
and fluorine atoms as dark grey circles. Broken lines represent hydrogen bonds and Fꢀ ꢀ ꢀF contacts. (b) Mode of arene stacking.

M. Stein et al. / Journal of Fluorine Chemistry 135 (2012) 231–239
237
nitrile nitrogen is involved in weak C–Hꢀ ꢀ ꢀN interactions [9] [C(3)–
fashion, the bromine atom in 4 acts as a halogen bonding donor
site, leading to a crystal structure which is constructed of
molecular C–Brꢀ ꢀ ꢀO bonded head-to-tail chains. They are further
assembled to 2D stacking-like domains with an opposite running
direction of neighbouring strands. The offset between molecules
induces a stacking order of aromatic residues which is different
from 3 and can be described as phenylF–(phenyl)2–phenylF. In all
cases, weak C–Hꢀ ꢀ ꢀF and C–Fꢀ ꢀ ꢀF contacts are likely to be present,
the distances of which are near the van der Waals limit. Hence, it is
questionable whether they are relevant for crystal structure
stabilization of the compounds.
˚
H(3)ꢀ ꢀ ꢀN(2) 2.64 A, 1408].
2.2.4. Bromonitrotetrafluoro-substituted tolane 4
Crystallization of 4 from acetone yielded colourless plates of the
space group P-1 (Z = 2). A perspective view of the molecule is
shown in Fig. 1(d). The twist angle between the aromatic rings is
about 15.08, whereas the nitrophenyl moiety adopts a nearly
˚
planar conformation. The C–Br bond distance of 1.863(4) A, which
is less than the average value of bromophenyl derivatives (ca.
˚
1.90 A), indicates a partial double bond character, which may be
ascribed to polarization effects of the fluorine atoms. As for 3, the
central ethynyl fragment shows alternating single bond–triple
bond character.
Another point worth mentioning is that the donor/acceptor
character of 3 and 4 may give rise to a potential ground-state
charge-transfer activity, which should be represented by a partial
‘‘quinoid’’ resonance structure of the molecules [33]. Surprisingly,
the analysis of bond lengths around the central ethynyl fragment
reveals that neither the fluorination nor the presence of para-
substituents show any significant influence on the C(sp2)–C(sp)
and C(sp)–C(sp) bond distances since they were found in
conformity with plain tolane [36]. This means that the ethynyl
unit in 3 and 4 does not take part in perceptible charge-transfer
[34].
In summary, it is illustrated that the tolanes 1–4, although
featuring rather similar conformational geometry of the basic
molecular framework (only the compound 4 possesses moderate
distortion of the aryl rings from coplanarity), show different
packing structures determined both by the functional groups and
The crystal structure of 4 (Fig. 6) is composed of linear
supramolecular strands with the molecules being connected in a
head-to-tail fashion via C–Brꢀ ꢀ ꢀO contacts. Geometric parameters
˚
[d(Brꢀ ꢀ ꢀO) 3.19, 3.19 A; nC–Brꢀ ꢀ ꢀO 161.6, 157.78] show these
contacts as being typical of halogen bonds [35]. These chains are
stacked along the crystallographic b-axis. Within the stacking
structure, each phenyl ring is located between an equivalent ring
and an unequivalent fluorinated ring with centroidꢀ ꢀ ꢀcentroid
˚
distances of 3.67 (phenylꢀ ꢀ ꢀphenyl) and 3.85 A (phenylꢀ ꢀ ꢀꢀ ꢀ ꢀ
fluorophenyl).Interstack association is accomplished by numerous
weak interactions comprising C–Hꢀ ꢀ ꢀO hydrogen bonds [9]
˚
˚
[d(Hꢀ ꢀ ꢀO) 2.63, 2.70 A] and C–Hꢀ ꢀ ꢀF [d(Hꢀ ꢀ ꢀF) 2.63, 2.68 A] as well
˚
as Fꢀ ꢀ ꢀF contacts [d(Fꢀ ꢀ ꢀF) 2.91 A].
the fluorine substitution but with an obvious preference to
p-
3. Conclusions
stacking interactions involving the fluorinated and non-fluorinated
phenyl as well as the ethynyl moiety. This is a behaviour largely
corresponding with previous studies [20,34], thus making a
potential guiding principle for the purpose of crystal engineering
such as the alignment of molecules required for a particular
materials property.
A comparative inspection of the crystal structures of the
partially fluorinated and differently para-substituted tolanes 1–4
carried out in terms of a recent theoretical model based on
molecular electrostatic potentials (ESPs) [24] reveals some
interesting features as specified in the following.
Analogous to the solid phase structures of tolane [36] and its
functionalized derivatives [3,5–7], the compounds 1–4 adopt a
geometry which is more or less disturbed from planarity due to
crystal packing effects. The tendency to form columnar packing
structures is characteristic for this kind of compounds and
resembles those found in a variety of 1:1 complexes of an aromatic
hydrocarbon and a perfluoroarene component [25,26]. However,
ideal alternate stacking is only realized in co-crystals with weak
interstack association. In such systems, local substituent effects
have been discussed as a driving force for the stacking formation.
In the molecular structures 1–4, the presence of para-
substituents at the phenyl part and the fluorinated ring in 3 and
4 affects the electrostatic potentials of the respective arene unit.
The electron withdrawing character of the substituents in 1
4. Experimental
4.1. General
Melting points were determined using a microscope heating
stage PHMK Rapido (VEB Wa¨getechnik) and are uncorrected. IR
spectra were measured on FT-IR 510 Nicolet as KBr pellets. 1H NMR
and 13C NMR spectra were obtained using a Bruker Avance 300
Spectrometer at 300 MHz (1H) or 75 MHz (13C); measurements
were carried out at 25 8C with TMS as an internal standard. 19F
NMR spectra were recorded on Bruker DNRX 400 at 376 MHz with
trichlorofluoromethane as external standard. Coupling constants
are given in Hz and resonance multiplicities are described as s
(singlet), d (doublet), t (triplet), m (multiplet). Assignment of the
signals base on increment calculation [39]. Mass spectra (GC/MS)
were determined with a Perkin Elmer SCIEX instrument. TLC was
performed on aluminium plates coated with SiO260F254 (Merck).
For column chromatography Merck silica gel 60 (0.063–0.1 mm)
was used. Solvent for Sonogashira–Hagihara coupling reactions
was deoxygenated prior to use by ultrasound (20 min) while
bubbling argon through the solution.
The following compounds were prepared according to litera-
ture procedures: 4-bromo-2,3,5,6-tetrafluorobenzonitrile (5a)
(from pentafluorobenzonitrile with lithium bromide) [40], 4-
ethynylnitrobenzene (6a) [41] and 4-ethynylaniline (6c) [42]
(from the respective p-substituted iodobenzenes and trimethylsi-
lylacetylene followed by deprotection). 4-Iodonitrobenzene, 4-
iodobenzonitrile, 4-iodoaniline as well as pentafluoroiodobenzene
and bis(triphenylphosphane)palladium(II) chloride were pur-
chased from commercial sources (Aldrich, Alfa, Aesar, Lancaster).
induces repulsion between the
p-clouds of dissimilar aromatic
units. This property results in a considerable longitudinal offset of
parallel molecules giving rise to p(CBB C)ꢀ ꢀ ꢀp(arene) interactions. In
principle, the crystal structure of the corresponding nitrile 2 shows
a similar packing behaviour, but due to the lateral offset of
neighbouring molecules, the stacking structure lacks
pꢀ ꢀ ꢀp
interactions. Instead, C–Hꢀ ꢀ ꢀN bonds which link the molecules
to dimers, as well as intermolecular contacts of the C–Fꢀ ꢀ ꢀp(CBB C)
type, taking geometric parameters of the
s
-hole concept into
account [32] stabilize the crystal packing. The complementary
character of the para-substituents of compound 3 induces a
stacking arrangement with an antiparallel orientation of consecu-
tive molecules, which is typical for the crystal structures of
phenyl–fluorophenyl ethynes [18,20,37] and corresponding stil-
benes [38]. Due to the donor/acceptor ability of 3, the molecular
stacks are interlinked by a tight network of non-covalent bonds
including N–Hꢀ ꢀ ꢀN and C–Hꢀ ꢀ ꢀN type hydrogen bonds. In a similar

238
M. Stein et al. / Journal of Fluorine Chemistry 135 (2012) 231–239
4.2. Syntheses of compounds 1–3
cooling down to room temperature, the mixture was poured into
water (10 mL). The precipitate which formed was collected,
washed with water and dissolved in chloroform. The organic
layer was washed with water and the aqueous phase extracted
with chloroform. The combined organic phases were dried
(Na2SO4) and evaporated under reduced pressure. Column
chromatography (SiO2, eluent: hexane–ethyl acetate, 3:1, v/v)
led to the isolation of a small amount (ꢄ1% yield) of the product as
The respective halogenated oligofluorobenzene (2.0 mmol) and
the corresponding terminal ethynyl compound (2.2 mmol) were
dissolved in degassed triethylamine (20 ml). To this solution, the
catalyst, being composed of bis(triphenylphosphane)palladium(II)
chloride (22 mg, 0.03 mmol) and copper(I) iodide (5.6 mg,
0.03 mmol), was added and the mixture was stirred at 50 8C
under argon for 16 h. The mixture was filtered through Celite, the
filtration residue washed with diethyl ether and the combined
organic layers evaporated. Column chromatography (SiO2, eluent :
hexane–ethyl acetate) yielded the pure compounds. Specific
details for each compound are given below.
yellow solid. IR (KBr): v
max (cmꢁ1) 3068 (CHAr), 2229 (CBB N), 1604,
1523, 1499 (C55CAr), 1112 (C–F), 964, 843, 831 (CHAr, 1,4-disubst.).
3
1H NMR (CDCl3): dH 7.75 (d, JHH = 8.9 Hz, 2H, Ar–H), 7.67 (s, 2H,
Ar–H); 13C NMR (CDCl3): C 78.8 (CBB CarF), 99.8 (t, 3JCF = 18 Hz, ArF),
d
102.1 (CBB CarH), 103.4 (C–Br), 123.9 (ArH), 128.2 (ArH), 132.9 (ArH),
145.0 (d, 1JCF = 245 Hz, ArF), 147.0 (d, 1JCF = 254 Hz, ArF), 148.1 (C–
3
4.2.1. 4-[(Pentafluoropheny)ethyny]nitrobenzene (1)
NO2); 19F NMR (CDCl3): dF ꢁ134.4 (d, JFF = 19 Hz), ꢁ150.8 (d,
Pentafluoroiodobenzene (5a) (0.59 g, 2.0 mm) and 4-(ethynyl)-
nitrobenzene (6a) (0.33 g, 2.2 mmol) were used; ratio of the
elution solvents hexane:ethyl acetate 10:1 (v/v). Yield 0.36 g (57%)
yellow solid; mp: 162–164 8C, lit. [43] mp: 137–138 8C. IR (KBr):
vmax (cmꢁ1) 3104, 3085 (CHAr), 2223 (CBB C), 1595, 1522, 1501
(C55CAr), 1540, 1347 (NO2), 1106 (C–F), 992, 967, 855 (CHAr, 1,4-
3JFF = 18 Hz). MS (GC/MS) m/z: 374 [M]+.
4.4. X-ray structure determination
Crystals of 1–4 suitable for structure analysis were obtained by
slow evaporation of solutions of the respective compounds in
acetone (1, 4), chloroform (2), and dimethylsulfoxide (3). The
intensity data were collected on a Stoe Mark II-Image Plate
˚
diffractometer with MoK radiation (l = 0.71073 A). Reflections
a
disubst.). 1H NMR (CDCl3): H 7.74 (d, 3JHH = 8.9 Hz, 2H, Ar–H), 8.27
d
(d, 3JHH = 10.23 Hz, 2H, Ar–H); 13C NMR (CDCl3):
d
C 77.8 (CBB CArF),
2
98.9 (CBB CArH), 99.3 (t, JCF = 18 Hz), ArF, 123.9 (ArH), 128.3 (ArH),
132.9 (ArH), 137.9 (d, 1JCF = ꢁ256 Hz, ArF), 142.3 (d, 1JCF = ꢁ257 Hz),
were corrected for background, Lorentz and polarization effects.
Preliminary structure models were derived by application of direct
methods [44] and were refined by full-matrix least squares
calculation based on F2 for all reflections [45]. With the exception
of the hydrogens in 3, all other hydrogen atoms were included in
the models in calculated positions and were refined as constrained
to bonding atoms.
All crystal data and experimental parameters are summarized
in Table 1. Crystallographic data (excluding structure factors) for
the structures in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publi-
cation nos. CCDC 818168–818171. Copies of the data can be
obtained, free of charg on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: +44 1223 336033 or deposit
@ccde.cam.ac.uk).
1
147.4 (d, JCF = 258 Hz), ArF, 148.1 (C–NO2); 19F NMR (CDCL3);
3
3
d
F = ꢁ134.1 (d, JFF = 23 Hz), ꢁ150.6 (t, JFF = 21 Hz), ꢁ161.1 (t,
3JFF = 21 Hz). MS (GC/MS) m/z: 313 [M]+.
4.2.2. 4-[(Pentafluorophenyl)ethynyl]benzonitrile (2)
Pentafluoroiodobenzene (5a) (0.59 g, 2.0 mmol) and 4-(ethy-
nyl)nitrobenzene (0.25 g, 2.2 mmol) were used; ratio of the elution
solvents hexane:ethyl acetate 3:1 (v/v). Yield 0.35 g (76%)
colourless solid; mp: 163 8C. IR (KBr): vmax (cmꢁ1) 3068 (CHAr),
2229 (CBB N), 1604,1523, 1499 (C55CAr), 1112 (C–F), 964, 843, 831
(CHAr, 1,4-disubst.). 1H NMR (COCl3):
dH 7.67 (s, 2H, Ar–H), 7.67 (s,
2H, Ar–H); 13C NMR (CDCl3): dC 99.3 (CBB CArF), 99.32 (CBB CArH),
2
99.6 (t, JCF = 18 Hz, ArF), 11.2 (C–CN), 118.2 (CBB N), 1256.3 (ArH),
1
132.3 (ArH), 132.5 (ArH), 137.8 (d, JCF = ꢁ255 Hz, ArF), 142.2 (d,
1JCF = 257 Hz, ArF), 147.4 (d, 1JCF = ꢁ246 Hz, ArF); 19F NMR (CDCl3)
3
3
dF ꢁ135.3 (d, JFF = 21 Hz), ꢁ150.8 (t, JFF = 21 Hz), ꢁ161.1 (t,
Acknowledgement
3JFF = 21 Hz). MS (GC/MS) m/z: 293 [M]+.
We thank Dr. A. Neels from the Crystallographic Service of the
University of Neuchaˆtel for solving the crystal structures.
4.2.3. 4-[(4-Aminophenyl)ethynyl]-2,3,5,6-tetrafluorobenzonitrile
(3)
4-Bromo-2,3,5,6-tetrafluorobenzonitrile (0.51 g, 2.0 mmol) and
4-ethynylaniline (0.26 g, 2.2 mmol) were used. The reaction
mixture was filtered and the residue was washed with acetone
(2 ꢃ 10 mL), instead of diethyl ether; ratio of the elution solvents
hexane:ethyl acetate 3:1 (v/v). Yield 0.11 g (19%) yellow solid; mp:
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jfluchem.2011.11.009.
140 8C (dec.). IR (KBr):
1649, 1622, 1596, 1518, 1488 (C55CAr), 1178 (C–F), 979, 835 (CHAr
v
max (cmꢁ1) 3451, 3360 (NH2), 2213 (CBB N),
,
References
1,4-disubst.). 1H NMR (DMSO-d6): dH 6.01 (s, 2H, NH2), 6.60 (d,
3JHH = 6.5 Hz, 2H, Ar–H), 7.29 (d, 3JHH = 6.5 Hz, 2H, Ar–H); 13C NMR
(DMSO-d6): dC 72.4 (C–CBB N), 91.5 (ArH), 104.8 (CBB N), 108.3
(CBB CArF), 109.9 (CBB CArH), 110.9 (ArF), 113.6 (ArH), 133.7 (ArH),
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NH2); 19F NMR (DMSO-d6): dF ꢁ134.8 (m), ꢁ136.8 (m). MS (GC/MS)
m/z: 290 [M]+.
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