Synthesis and structural characterization of 2,5-dihalo-3,4- dinitrothiophenes

Article abstract of DOI:10.1007/s10870-006-9160-y

Using new nitration protocols, we have been able to efficiently dinitrate 2,5-dihalothiophenes with yields of ～80-95%. The resulting products 2,5-dibromo-3,4-dinitrothiophene (1), 2,5-dichloro-3,4-dinitrothiophene (2), 2-bromo-5-chloro-3,4-dinitrothiophene (3), as well as the analogous 2-bromo-3,4-dinitrothiophene (4), all crystallize easily allowing their characterization via X-ray crystallography. Crystallization of 1 occurs in the monoclinic space group C2/c with a=14.547(3)A, b=7.3534(15)A, c=10.775(2)A, β=128.89(3)°, and Z=4. Crystallization of 2 occurs in the tetragonal space group I-42d with a=9.9398(14)A, b=9.9398(14)A, c=16.866(3)A, and Z=8. Crystallization of 3 occurs as a pseudo-merohedral twin in the triclinic space group P-1 with a=7.340(5)A, b=8.094(5)A, c=9.112(5)A, α=82.059(5)°, β=66.232(5)°, γ=63.021(5)°, and Z=2. Crystallization of 4 occurs in the triclinic space group P-1 with a=7.1787(14)A, b=7.4092(15)A, c=8.3151(17)A, α=101.67(3)°, β=96.00(3)°, γ=116.13(3)°, and Z=2. The structures of all compounds exhibit the formation of interesting solid-state assemblies due to halogen-bonding interactions between the halogen and nitro groups. Springer Science+Business Media, LLC 2007.

Full text of DOI:10.1007/s10870-006-9160-y

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Journal of Chemical Crystallography, Vol. 37, No. 6, June 2007 ( 2007)
DOI: 10.1007/s10870-006-9160-y
Synthesis and structural characterization
of 2,5-dihalo-3,4-dinitrothiophenes
Li Wen(1) and Seth C. Rasmussen(1)∗
Received August 4, 2006; accepted September 29, 2006
Published Online March 6, 2007
Using new nitration protocols, we have been able to efficiently dinitrate 2,5-
dihalothiophenes with yields of ∼80–95%. The resulting products 2,5-dibromo-
3,4-dinitrothiophene (1), 2,5-dichloro-3,4-dinitrothiophene (2), 2-bromo-5-chloro-3,4-
dinitrothiophene (3), as well as the analogous 2-bromo-3,4-dinitrothiophene (4), all crys-
tallize easily allowing their characterization via X-ray crystallography. Crystallization of
1 occurs in the monoclinic space group C2/c with a = 14.547(3) Å, b = 7.3534(15) Å,
c = 10.775(2) Å, β = 128.89(3)◦, and Z = 4. Crystallization of 2 occurs in the tetragonal
space group I-42d with a = 9.9398(14) Å, b = 9.9398(14) Å, c = 16.866(3) Å, and Z = 8.
Crystallization of 3 occurs as a pseudo-merohedral twin in the triclinic space group P-1
with a = 7.340(5) Å, b = 8.094(5) Å, c = 9.112(5) Å, α = 82.059(5)◦, β = 66.232(5)◦,
γ = 63.021(5)◦, and Z = 2. Crystallization of 4 occurs in the triclinic space group P-1 with
a = 7.1787(14) Å, b = 7.4092(15) Å, c = 8.3151(17) Å, α = 101.67(3)◦, β = 96.00(3)◦,
γ = 116.13(3)◦, and Z = 2. The structures of all compounds exhibit the formation of
interesting solid-state assemblies due to halogen-bonding interactions between the halogen
and nitro groups.
KEY WORDS: thiophene; halothiophene; nitration; halogen-bonding.
Introduction
the halides also allow further functionalization
via aryl-aryl coupling to produce more complex
copolymeric architectures.5–10 As such, the cou-
pling chemistry could be further tuned via the use
of other dihalo analogues of compound 1.
Compound 1 was first prepared by Hans
Kreis in the laboratory of Victor Meyer in
1884 through the addition of nitric acid to
2,5-dibromothiophene in sulfuric acid.11 The
following year it was also reported that 1
could be prepared by the nitration of 2,3,5-
tribromothiophene.12 These early reports
analyzed the isolated products, but did not report
the amount of product isolated and did not seem
concerned with optimization of the nitration
procedure.
2,5-Dibromo-3,4-dinitrothiophene (1) has
become an important precursor to a variety of
low band gap polymeric systems as shown in
Scheme 1.1–10 In some cases, the bromo function-
alities are used simply as blocking groups in order
to allow nitration in the β-positions.1–4 In these
cases, the bromo groups are simply eliminated
during the reduction of the nitro groups to the
corresponding amines. In other cases, however,
(1) Department of Chemistry and Molecular Biology, North Dakota
State University, Fargo, ND 58105, USA.
∗
To whom correspondence should be addressed; e-mail: seth.
rasmussen@ndsu.edu.
387
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1074-1542/07/0600-0387/0 2007 Springer Science+Business Media, LLC

388
Wen and Rasmussen
n
n
n
n

2,5-dihalo-3,4-dinitrothiophenes
389
O2N
X
NO2
X'
1) Fuming H2SO4, <20 oC
2) HNO3, <30 oC
1 X = X' = Br
2 X = X' = Cl
3 X = Br; X' = Cl
X
X'
S
S
S
O2N
Br
NO2
Br
O2N
NO2
Br
H3PO2
O2N
I
NO2
I
S
S
1
4
Scheme 2. Synthesis of various halo-3,4-dinitrothiophenes.
1
The first detailed procedure for the produc-
tion of 1 was reported by Mozingo and cowork-
ers in 1945 to give an isolated yield of 30%.13
Based on the original Kreis synthesis, this mod-
ified procedure applied the use of fuming acids
and careful control of the reaction tempera-
ture. Finally, in 2002, our group reported further
modifications consisting mainly of extending the
reaction time, resulting in yields of 45–50%.14
We have since revisited this nitration and re-
port here further modifications that give yields
of 90–95% of 1 after purification. This new
nitration procedure has been applied to a se-
ries of 2,5-dihalothiophenes (Scheme 2), and
we report here the efficient preparation of 2,5-
dibromo-3,4-dinitrothiophene (1), 2,5-dichloro-
3,4-dinitrothiophene (2) and the asymmetric
2-bromo-5-chloro-3,4-dinitrothiophene (3). The
monohalo species 2-bromo-3,4-dinitrothiophene
(4) was also prepared to further investigate the
potential halogen-bonding interactions of these
compounds. All compounds have been character-
ized via X-ray crystallography and their structural
properties included in this current report.
noted, the H NMR and 13C NMR spectra were
carried out on a 300 MHz spectrometer and
all NMR data was referenced to the chloroform
signal.
2,5-Dichlorothiophene
Chlorination of thiophene was accomplished
through the modification of a previously described
procedure.16 Thiophene (25 mL, 0.32 mole)
and perchloric acid (70%, 2.7 mL, 0.032 mole)
were combined in 200 mL of chloroform. N-
chlorosuccinimide (84 g, 0.65 mole) was added
in small portions to avoid spontaneous reflux. The
reaction mixture was heated at reflux for 30 min
and then allowed to cool to room temperature.
Water (200 mL) was added and the organic layer
separated. The organic layer was washed with an
aqueous potassium hydroxide solution (0.89 M)
and the chloroform removed by rotary evapora-
tion. The crude product was purified by vacuum
distillation to give 32.8 g (67%) of a colorless liq-
uid. bp 70–71◦C (30 mm Hg; lit.17 64.5◦C, 25 mm
1
Hg); H NMR δ 6.71 (s, 2H) consistent with lit.
value18; 13C NMR δ 127.5, 125.9.
Experimental
2,5-Dibromothiophene13 and 2,5-diiodothi-
ophene15 were prepared as previously described
in the literature. All other materials were reagent
grade and used without further purification. All
synthetic reactions described were carried out
under a nitrogen atmosphere. Unless otherwise
2-Bromo-5-chlorothiophene
2-Bromothiophene (30 mL, 0.31 mole),
perchloric acid (70%, 2.6 mL, 0.031 mole),
and N-chlorosuccinimide (62.3 g, 0.470 mole)
were combined in 450 mL of chloroform. The

390
Wen and Rasmussen
reaction mixture was heated at reflux for 1 h
and then allowed to cool to room temperature.
The product was then isolated and purified as
for 2,5-dichlorothiophene above to give 69.6 g
(75%) of a colorless liquid. bp 84◦C (20 mm
give 49.8 g (78%) of product; mp 100.8–101.7◦C;
13C NMR δ 140.2, 138.3, 131.2, 111.0; IR (KBr
disk) 1547, 1506, 1463, 1434, 1392, 1345, 1321,
1098, 943, 906, 811, 753, 741 cm−1.
1
Hg); H NMR δ 6.85 (d, 1H, J = 4.0 Hz), 6.68
2-Bromo-3,4-dinitrothiophene (4)
(d, 1H, J = 4.0 Hz); 13C NMR δ 130.0, 129.6,
126.9, 109.4.
Compound 3 was prepared through the modi-
fication of a previously described procedure.20
2,5-Dibromo-3,4-dinitrothiophene (3.32 g, 10
mmole) was dissolved in 50 mL of acetone that
had been cooled to 10–15◦C. Hypophosphorous
acid (50%, 9.0 mL, 50 mmole) was added to the
solution by syringe. The reaction mixture was kept
in the ice bath for an additional 5 min, and then
stirred at room temperature for 2 h, during which
the solution developed an orange-red color. The
crude product was then precipitated by addition
to water. Recrystallization from methanol gave
2.15 g (85%). mp 94.8–96.5◦C (lit.20 93–94◦C);
1H NMR δ 8.28(s, 1H); 13C NMR δ 113.1, 128.7,
140.4, 141.7; IR (KBr disk) 3110, 1546, 1507,
1450, 1412, 1334, 1140, 1064, 958, 875, 820,
788, 751 cm−1.
2,5-Dibromo-3,4-dinitrothiophene (1)
Concentrated sulfuric acid (100 mL) and
fuming sulfuric acid (30% SO3, 100 mL) were
combined in a flask equipped with a mechan-
ical stirrer and cooled with an ice bath. 2,5-
Dibromothiophene (53.8 g, 25.0 mL, 0.222 mole)
was then added maintaining a temperature below
20◦C. Nitric acid (35 mL) was then added drop-
wise, being careful to keep the temperature un-
der 30◦C. Once the addition was complete, the
mixture was allowed to react for an additional
three hours and then poured over 800 g of ice.
Upon the melting of the ice, the solid residue was
recovered by vacuum filtration and washed well
with water to produce a light yellow powder. Re-
crystallization with methanol gave 69.3 g (94%)
of analytically pure material; mp 135.8–136.7◦C;
13C NMR δ 159.7, 113.7; IR (KBr disk) 1546,
1501, 1454, 1405, 1390, 1345, 1317, 1082, 937,
900, 802, 749, 736 cm−1.
X-ray crystallography
X-ray quality crystals were grown from the
slow evaporation of methanol solutions for 1–3
and CHCl3 solutions for 4. Single crystals with
approximate dimensions of 0.2 × 0.2 × 0.2 mm
were mounted on a glass fiber and coated with
superglue. The crystals were transferred to a
Bruker SMART 1000 CCD X-ray diffractome-
ter system equipped with a Mo-target X-ray tube
(λ = 0.71073 Å) operated at 2000 watts power.
The frames were collected at ambient tempera-
ture with a scan width of 0.3◦ in ω and inte-
grated with the Bruker SAINT software package
using a narrow-frame integration algorithm. The
unit cells were determined and refined by least
squares upon the refinement of XYZ-centeroids of
reflections above 20σ(I). The data were corrected
for absorption using the SADABS program21
and refined on F2 using the WinGX software
2,5-Dichloro-3,4-dinitrothiophene (2)
Compound 2 was prepared as above from
2,5-dichlorothiophene(33.7 g, 0.222 mole)togive
44.3 g (82%) of product; mp 83.4–84.8◦C (lit.19
88–91◦C); 13C NMR δ 137.7, 128.3; IR (KBr disk)
1555, 1513, 1472, 1433, 1398, 1353, 1321, 1109,
952, 912, 820, 773, 748 cm−1.
2-Bromo-5-chloro-3,4-dinitrothiophene (3)
Compound 3 was prepared as above from 2-
bromo-5-chlorothiophene (43.8 g, 0.222 mole) to

2,5-dihalo-3,4-dinitrothiophenes
391
package.22 The structures were solved by direct
methods. All non-hydrogen atoms were refined
with anisotropic displacement coefficients using
SHELXL97.23
tration conditions result in violent, uncontrollable
reactions and little formation of the nitro prod-
ucts.24–26 The difficulty with these nitrations is
due to the presence of nitrous acid, which forms
nitrosothiophene in an autocatalytic reaction. Ni-
trosation is common in the nitration of other aro-
matics, but any resulting nitroso species are con-
verted to nitro products by the oxidative reaction
conditions. In the case of thiophene, however,
the resulting nitroso species undergo decompo-
sition,25,26 possibly via either ring opening or ox-
idation of the aromatic ring.27
While the use of acyl nitrates, nitronium
salts, or acetic anhydride-nitric acid mixtures in
glacial acetic all allow efficient production of ni-
trothiophenes by either suppressing nitrosation or
inhibiting decomposition,26 such conditions are
not reactive enough to facilitate the second nitra-
tion.25 In order to obtain the dinitro species, the
application of nitric acid-sulfuric acid mixtures is
required and often the utilization of fuming acids
is necessary to produce sufficient yields.13,14,24
Unfortunately, the yields via these stronger ni-
tration conditions are only ∼50%, likely due to
losses from competing nitrosation.
The asymmetrical thiophene 3 crystallized
as a pseudo-merohedral twin. In this case, the
triclinic lattice is simulating a higher symmetry
monoclinic lattice and the twinning can be re-
solved by applying the twin law [1 0 0, 1 − 1 0,
1 0 − 1] to give a 47.8:52.2 twin. The molecule
also possesses occupational disorder where the
molecule is flipped 180◦ so that the bromine
now occupies the space formerly occupied by
the chlorine substituent and vice versa. The disor-
der resolves into two components in the ratio of
48.3:51.7. Several restraints and constraints had to
be applied in order to reach anisotropy. A SAME
instruction was applied to correlate the 1,2 and
1,3 distances in the two disordered components.
Additionally, corresponding atoms in each dis-
order component were constrained to have the
same thermal parameters using EADP instruc-
tions. Several restraints were also applied includ-
ing SADI commands to make the N-O distances
equal and a SIMU command to restrain the thio-
phene ring carbon atoms to have similar thermal
parameters across the ring. Several bond lengths
were also restrained using DFIX commands in-
cluding those of the S to C atoms (1.714 Å), C
to C double bonds (1.340 Å), C to C single bond
(1.434 Å), and C to N bonds (1.4534 Å). The
values of these bond lengths were obtained from
the symmetrical anologues 1 and 2 also reported
here and are valid restraint values since all of
the structures were collected under identical con-
ditions. Crystal data, data collection parameters,
and refinement statistics for thiophenes 1–4 are
listed in Table 1.
Although nitrosation is discussed as a con-
sequence of nitrous acid, the term “nitrous acid”
is typically being used to refer to any N(III) con-
taining species.28,29 Such species originate from
the spontaneous decomposition of nitric acid to
produce NO2 as shown below.
1
2 HNO3 ꢀ 2 NO2 + H2O + / O2
(1)
2
This process is promoted by light exposure and
is responsible for the brownish tint that develops
in the acid over time.20 In solution, the resulting
NO2 is in facile equilibrium with N2O4 (Eq. 2),
which can in turn form either N2O3 or nitrous acid
(Eqs. 3 and 4).28–30
2 NO2 ꢀ N2O4
(2)
(3)
(4)
Results and discussion
N2O4 + H2O ꢀ HNO3 + HNO2
2 N2O4 + H2O ꢀ N2O3 + 2 HNO3
For most electrophilic substitution reactions,
thiophene exhibits similar reactivity to benzene.
The primary exception to this, however, is the
nitration of thiophene, where most standard ni-
As shown in Eqs. 5–7, nitrosonium (NO+)
can be produced by either the heterolytic

392
Wen and Rasmussen
Table 1. Crystal Data, Data Collection Parameters, and Refinement Statistics for Halo-3,4-Dinitrothiophenes 1–4
1
2
3
4
CCDC deposit no.
Formula
Formula Weight
Temperature (K)
Crystal System
Space Group
a (Å)
614780
C4Br2N2O4S
331.94
293(2)
monoclinic
C2/c
14.547(3)
7.3534(15)
10.775(2)
90.00
128.89(3)
90.00
897.1(5)
4
2.451
614778
C4Cl2N2O4S
243.02
293(2)
tetragonal
I-42d
9.9398(14)
9.9398(14)
16.866(3)
90.00
90.00
90.00
616834
C4BrClN2O4S
287.48
293(2)
triclinic
P-1
7.340(5)
8.094(5)
9.112(5)
82.059(5)
66.232(5)
63.021(5)
440.9(5)
614779
C4HBrN2O4S
253.04
293(2)
triclinic
P-1
7.1787(14)
7.4092(15)
8.3151(17)
101.67(3)
96.00(3)
116.13(3)
379.31(19)
2
b (Å)
c (Å)
α (◦)
β (◦)
γ (◦)
V (Å3)
1666.4(4)
8
1.937
Z
2
dcalc (g cm−3
)
2.166
5.183
2.215
5.667
µ (mm−1
Absorption correction
)
9.250
1.010
SADABS (multi-scan)
SADABS (multi-scan)
0.9058 and 0.8236
6169
SADABS (multi-scan)
0.600000 and 0.415237
7878
SADABS (multi-scan)
0.3213 and 0.3213
3328
Max. and min. transmission 0.3982 and 0.1779
Reflections collected
Unique reflections
7693
910
920
1563
1328
[Rint = 0.0984]
[Rint = 0.0569]
[Rint = 0.0376]
[Rint = 0.0385]
Observed reflections
557
636
1037
1002
Final R indices [I > 2σ(I)] R1 = 0.0391
R1 = 0.0322
wR2 = 0.0476
R1 = 0.0728
wR2 = 0.0565
1.045
R1 = 0.0286
wR2 = 0.0632
R1 = 0.0625
wR2 = 0.0753
0.961
R1 = 0.0457
wR2 = 0.1179
R1 = 0.0645
wR2 = 0.1291
1.126
wR2 = 0.0794
R indices (all data)a
R1 = 0.0899
wR2 = 0.1009
1.049
Goodness-of-fit on F2
aR1 = ꢀ(||Fo| − |Fc||)/ꢀ|Fo|,wR2 = [ꢀ(w(Fo2 − Fc2)2)/ꢀ(Fo2)2]1/2, Goodness-of-fit on F2 = [ꢀ(w(Fo2 − Fc2)2)/(n − p)]1/2, where n is
the number of reflections and p is the number of parameters refined.
dissociation of the nitrogen oxides or the protona-
tion of the nitrous acid. The resulting nitrosonium
cation then acts as the nitrosating agent.
It has been found in both our research and
that of Peter Ba¨uerle,31 that the order and method
of acid addition can play a large role in the ni-
tration of dihalothiophenes. Thus, the slow ad-
dition of HNO3 to fuming H2SO4 solutions of
the dihalothiophene results in yields nearly dou-
ble those achieved using the former methods of
adding the dihalothiophene to a mixture of the
fuming acids.13,14 It is our belief that the increased
yields result from maintaining a large excess of the
fuming H2SO4 in comparison to the added HNO3.
This large excess then affects several equilibria in
a manner favorable to nitration and, more impor-
tantly, acts to inhibit nitrosation.
N2O3 ꢀ NO+ + NO−2
N2O4 ꢀ NO+ + NO−3
(5)
(6)
HA + HNO2 ꢀ NO+ + H2O + A− (7)
Nitration from nitric acid solutions is accom-
plished by protonation of HNO3 (Eq. 8), followed
by the loss of water to form the nitronium ion
(Eq. 9).28 As the nitronium ion acts as the nitrat-
ing agent, nitration is dependent on the amount of
nitronium produced.
The protonation step shown in equation 8
can occur in neat solutions of HNO3, but then
two equivalents are required to produce each
HA + HNO3 ꢀ H2NO+3 + A−
H2NO+3 → NO+2 + H2O
(8)
(9)

2,5-dihalo-3,4-dinitrothiophenes
393
equivalent of NO+2 . The use of a large excess of
the fuming H2SO4 accomplishes the protonation
step, while disulfuric acid present in the fuming
acid can also absorb the water produced during
the formation of the nitronium ion (Eq. 10).
halothiophene. The monohalo analogue 2-bromo-
3,4-dinitrothiophene (4) was also prepared by the
debromination of 1 in order to further understand
the structure-function relationships in these halo-
dinitrothiophenes.
As thiophenes 1–3 contain no hydrogens, 13C
NMR is a primary method for initial characteri-
zation. This is complicated, however, as the β-
carbons bound to the nitro functionalities of thio-
phenes 1–4 have very slow relaxation times and
their signals are typically not seen under stan-
dard NMR parameters. Thus, to obtain quality
13C spectra, delay times of ∼3 s are necessary
in order to acquire sufficient signal from the β-
carbons. The spectra collected under these condi-
tions exhibit no other unusual characteristics and
give expected chemical shifts.
Thiophenes 1–3 crystallize easily from
methanol to give X-ray quality crystals. Crystals
of the monobromo species 4 could not be obtained
using the same conditions, but suitable crystals
could be obtained from chloroform, thus allowing
the characterization of all compounds via X-ray
crystallography. While our group has previously
reported the structure of thiophene 1,14 the data
has been recollected to ensure that thiophenes 1–3
were all characterized under identical conditions
and to provide in this report a more refined struc-
ture of thiophene 1. The data presented here for
1 is nearly identical to the previously reported
structure, but does give a slightly smaller unit
cell volume and some minor deviations in bond
lengths and angles. Thermal ellipsoid plots of 1–4
are shown in Fig. 1 and selected bond lengths and
angles are given in Table 2.
H2S2O7 + H2O ꢀ 2H2SO4
(10)
The removal of water reduces contributions
from Eqs. 3 and 4, which lead to NO+ produc-
tion and nitrosation. While the removal of wa-
ter will not affect the contribution of NO+ from
Eq. 6, any remaining NO+ produced can be fur-
ther reduced through the formation of nitrosyl-
sulfuric acid (NO+HSO−4 ) as shown in equations
11–13.29,32
HSO4− + NO+ ꢀ NO+HSO4−
(11)
H2SO4 + HNO2 ꢀ NO+HSO4− + H2O (12)
2 H2SO4 + N2O4 ꢀ NO+HSO4− + HSO4−
+ NO+2 + H2O
(13)
In dilute H2SO4 solutions, NO+HSO4− can
react with water to reform nitrous acid. In concen-
trated solutions (>72%), however, NO+HSO4− is
stable and accumulates in the acidic solution.32
While nitrosylsulfuric acid can still act as a nitro-
sation agent via dissociation, the stability of this
species in the fuming H2SO4 effectively limits
its reactivity with the thiophene reagents. Thus,
maintaining a high effective concentration of the
fuming H2SO4 both inhibits NO+ production and
converts the remaining NO+ to the less reactive
nitrosylsulfuric acid, effectively inhibiting nitro-
sation and resulting in significantly improved ni-
tration yields.
All thiophenes retain their planar conforma-
tions as expected for such aromatic rings. As can
be seen from Table 2, however, the double bonds
of thiophenes 1–3 are slightly shorter than those
of the unfunctionalized thiophene parent,33 which
can be attributed to the donor-acceptor nature of
the substituents.14 Comparing the analogous bond
lengths for thiophene 4, it can be seen that the dou-
ble bond containing the bromo substituent agrees
well with the bonds of 1–3. The other double bond,
however, lacks the electron-rich bromide and
Using these new nitration protocols, we have
been able to efficiently dinitrate 2,5-dibromo-,
2,5-dichloro-, and 2-bromo-5-chloro-thiophene
with yields of ∼80–95% for the corresponding
dihalodinitrothiophenes 1–3. These new methods
have the added benefit of no longer requiring the
use of fuming nitric acid. Attempts to nitrate 2,5-
diiodothiophene under these conditions, however,
were unsuccessful as the strong acids resulted in
immediate polymerization of the more reactive di-

394
Wen and Rasmussen
Fig. 1. Ellipsoid Plots of halo-3,4-dinitrothiophenes 1–4 at the 50% probability level.
For clarity, one of the two independent disordered components of 3 are shown.
exhibits a longer bond length in good agreement
with the parent thiophene. The halide identity (Br
vs. Cl) seems to have little effect on the length of
the double bond.
interesting supramolecular assemblies. As shown
in Fig. 2, the halogen-bonded assemblies of the
dibrominated 1 result in co-parallel 2D planar
sheets. Within these 2D arrays, each molecule of
–
The donor-acceptor nature of the substituents
in thiophenes 1–4 also allows interesting inter-
molecular interactions via halogen bonding.34,35
Here the arylhalide plays the role of halogen
bond donor and the oxygens of the nitro groups
act as conventional Lewis bases. In addition
to acting as halogen-bond acceptors, the NO2
groups within each thiophene also act as strong
electron-withdrawing groups and pull electron
density away from the halogens, thus activating
them towards the resulting halogen-bonding
interaction.36 As shown in Table 3, all such
halogen-oxygen interactions in 1–4 are 0.162–
1 exhibits a Br O contact of 2.965 Å with four of
its neighbors. This distance is approximately 88%
that of the sum of the van der Waals radii37 and the
◦
–
O · · · Br C angles are fairly linear at 172.04 . The
result of these interactions is that all molecules of
1 in each sheet have the same orientation and all
dipole moments are co-aligned. Alternating 2D
sheets have opposing orientations, thus balancing
the dipole moments of the neighboring sheets.
In contrast, the dichloro analogue 2 exhibits
a significantly different assembly as shown in
Fig. 3. In this case, each molecule of 2 exhibits
–
double Cl O contacts with each neighbor result-
0.405 Å shorter than the sum of the van der Walls
ing in ribbon-like assemblies in which alternating
◦
radii37 and give O · · · X C angles of ∼163–172 .
molecules have opposite orientations. The Cl O
–
–
As two donors and two acceptors are present
on each of the thiophenes 1–3, this results in some
contacts here are 3.108 Å, approximately 95%
of the sum of the van der Waals radii,37 with

2,5-dihalo-3,4-dinitrothiophenes
395
Table 2. Selected Bond Distances (Å) and Angles (deg) in Halo-3,4-Dinitrothiophenes 1–4
Parameter
thiophenea
1
2
3
4
–
S C(1)
1.714
1.370
—
—
—
—
1.423
—
—
—
1.713(6)
1.339(8)
1.858(5)
1.455(7)
1.228(6)
1.193(6)
1.446(9)
—
1.711(3)
1.340(4)
1.707(3)
1.452(3)
1.232(3)
1.213(3)
1.422(5)
—
1.715(8)
1.352(8)
1.69(3)
1.463(8)
1.230(6)
1.226(7)
—
1.439(8)
1.353(8)
1.723(8)
1.93(2)
1.703(5)
1.341(8)
1.862(5)
1.448(6)
1.221(6)
1.213(7)
—
1.424(7)
1.373(7)
1.680(6)
(0.9300)
–
–
–
–
–
–
–
–
C(1) C(2)
C(1) X(1)
C(2) N(1)
N(1) O(1)
N(1) O(2)
C(2) C(2 )
C(2) C(3)
C(3) C(4)
b
ꢁ
—
—
—
—
—
—
–
–
S(1) C(4)
C(4) X(2)
b
—
–
–
S C(1) C(2)
111.47
—
—
111.9(4)
125.4(5)
116.0(6)
125.8(6)
91.7(4)
112.2(3)
—
—
—
—
112.30(18)
124.3(2)
117.4(3)
125.5(3)
90.86(19)
112.26(14)
—
111.6(11)
125(2)
116.3(13)
128.0(13)
—
111.9(4)
123.3(5)
117.9(4)
124.2(5)
—
–
–
N(1) C(2) C(1)
O(1) N(1) C(2)
O(1) N(1) O(2)
– –
C(1) S C(1 )
–
–
–
–
—
ꢁ
92.17
112.45
—
—
—
ꢁ
–
–
C(1) C(2) C(2 )
—
—
–
–
C(1) C(2) C(3)
– –
112.6(17)
91.8(7)
111.5(12)
112.0(17)
112.0(4)
92.7(3)
111.0(4)
112.4(5)
C(1) S C(4)
—
—
—
–
–
S C(4) C(3)
–
–
C(2) C(3) C(4)
—
aDetermined from gas phase microwave spectroscopy, Katritzky, A. R.; Pozharskii, A. F. Handbook of
Heterocyclic Chemistry, 2nd ed.; Pergamon: New York, 2000; p. 61.
b1: X(1) = Br, 2: X(1) = Cl, 3: X(1) = Cl, X(2) = Br, 4: X(1) = Br, X(2) = H.
◦
–
corresponding O · · · Cl C angles of 162.92 . The
smaller interpenetration of the van der Waals
volumes and reduced linearity of the interaction
are indicative of a weaker interaction for the
chloride donor, which is consistent with known
halogen-bonding trends.34 Each ribbon has an
exactly perpendicular edge-to-face orientation
with each of its neighboring ribbons, with four
ribbons per unit cell.
Unfortunately, the mixed bromochloro
analogue 3 exhibits positional disorder within its
crystalline lattice, thus making it difficult to com-
–
–
pare the Br O vs. Cl O contacts in this mixed sys-
tem. However, the halogen-bonding assemblies
of 3 are the same as those of the dibromo parent
1, with average halogen−O contacts of ∼3.0 Å.
This average value also falls midway between
the halogen-bonding contacts observed in the
dibromo analogue 1 and the dichloro analogue 2.
Removing one of the halogen donors in the
monobromo analogue 4 gives assemblies exhibit-
ing properties of both the previously discussed
assemblies of 1 and 2. As shown in Fig. 4,
thiophene 4 adopts ribbon-like structures simi-
lar to those of thiophene 2. Here, however, the
ribbons are comprised of head-to-head dimeric
Table 3. Selected Distances (Å) and Angles (deg) for Halogen
Bonding Interactions in 1–4
Parameter
1
2
3a
4
Br · · · O(1)
Cl · · · O(1)
2.965
—
172.04
—
—
3.108
—
∼2.94
∼3.07
∼170
∼168
3.117
—
165.50
—
–
C(1) Br · · · O(1)
–
species via two complementary Br O contacts.
–
C(1) Cl · · · O(1)
162.92
–
The Br O contacts here are 3.117 Å, approx-
aValues are only estimated due to positional disorder in structure.
imately 92% of the sum of the van der Waals

396
Wen and Rasmussen
Fig. 2. 2D Halogen-bonded sheets of 1.
radii,37 with corresponding O · · · Br C angles of
165.5◦. Thus, the halogen-oxygen interactions
here are a little stronger than those displayed
by thiophene 2, but significantly weaker than
those displayed by the dibromo species 1. These
halogen-bonded dimeric units are then extended
into the ribbon-like assemblies via weak comple-
mentary hydrogen-oxygen interactions, in a man-
phene 2, however, the ribbons of thiophene 4 form
parallel stacks more similar to the stacked 2D
sheets of thiophene 1.
–
Conclusion
Using new nitration protocols, we have been
able to efficiently dinitrate 2,5-dihalothiophenes
with yields of ∼80–95%. In addition to these
increased yields, these new methods have
–
ner analogous to the previously described Br O
interactions. Unlike the ribbons formed by thio-
Fig. 3. Perpendicular halogen-bonded ribbons of 2.

2,5-dihalo-3,4-dinitrothiophenes
397
Fig. 4. Face (A) and side views (B) of halogen-bonded ribbons of 4.
the added benefit of no longer requiring the
use of fuming nitric acid, greatly reducing
the cost and difficulty of these nitrations. The
resulting 2,5-dihalo-3,4-dinitrothiophenes 1–3
produced, as well as the analogous 2-bromo-3,4-
dinitrothiophene 4, all crystallize easily allowing
their characterization via X-ray crystallography.
The structures of all compounds exhibit the
formation of interesting solid-state assemblies
due to halogen-bonding interactions between the
halogen and nitro functionalities.
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