X-Ray crystallographic analysis and differential scanning calorimetry of two polymorphic forms of 2,6-Diphenyl-1,4-dithiin

Article abstract of DOI:10.1002/hc.20035

2,6-Diphenyl-1,4-dithiin 2 exists in the two polymorphic forms. The dithiin 2L, mp 62-63°C, was assigned as the metastable form, while the dithiin 2H, mp 79-80°C, was assigned as the stable form, mainly on the basis of differential scanning calorimetry (D

Full text of DOI:10.1002/hc.20035

Heteroatom Chemistry
Volume 15, Number 6, 2004
X
-Ray Crystallographic Analysis and
Differential Scanning Calorimetry of Two
Polymorphic Forms of 2,6-Diphenyl-1,4-dithiin
Xuehua Piao, Yoshiaki Sugihara, and Juzo Nakayama
Department of Chemistry, Faculty of Science, Saitama University, Sakura-ku, Saitama-shi,
Saitama 338-8570, Japan
Received 17 May 2004
Na
2
S · 9H
2
O, with Lawesson’s reagent provides the
ABSTRACT: 2,6-Diphenyl-1,4-dithiin 2 exists in the
1
,4-dithiin 2 in good yield [1].
◦
two polymorphic forms. The dithiin 2L, mp 62–63 C,
was assigned as the metastable form, while the dithiin
◦
2
H, mp 79–80 C, was assigned as the stable form,
mainly on the basis of differential scanning calorime-
try (DSC) analysis. X-Ray crystallographic analysis
showed that the largest difference in the molecular
structure between 2L and 2H is found in the copla-
narity between the benzene ring and the double bond
part of the dithiin ring; the benzene ring and the double
bond are nearly coplanar for 2L, whereas these are far
from being coplanar for 2H. ꢀC 2004 Wiley Periodicals,
We also investigated the thermolysis of 2 and
the related 1,4-dithiins that result in the formation
of thiophenes with extrusion of a sulfur atom [3,4].
During these studies, we observed the following. At
the early stage of these studies, the 1,4-dithiin 2, syn-
Inc. Heteroatom Chem 15:424–427, 2004; Published online
in Wiley InterScience (www.interscience.wiley.com). DOI
◦
thesized by the above method, melted at 62–63 C [1].
10.1002/hc.20035
However, during the thermolysis study we obtained
◦
the dithiin 2, which had a melting point of 79–80 C
[
3]. Since then, we obtained only the high melting
INTRODUCTION
,6-Diphenyl-1,4-dithiin (2) was found to exist in two
polymorphic forms. Here we report the results of
X-ray crystallographic analysis and differential scan-
ning calorimetry (DSC) of these two polymorphic
forms.
Two decades ago, we developed a general syn-
thesis of 1,4-dithiins [1,2]. Thus, for example, sul-
furation of diphenacyl sulfide (1), easily obtainable
by treatment of phenacyl bromide or chloride with
point dithiin 2H. The low melting point dithiin 2L
never formed despite repeated syntheses or recrys-
tallizations. Although at that time we thought that
the difference of the melting point is attributed to a
polymorphism [3], we could not pursue these phe-
nomena in detail because of the nonavailability of
2L. Recently, we have reinvestigated the synthesis of
2 and obtained 2L again, thus enabling us to report
the results described below.
2
RESULTS AND DISCUSSION
Correspondence to: Juzo Nakayama; e-mail: nakaj@post.
saitama-u.ac.jp.
c 2004 Wiley Periodicals, Inc.
First of all we should mention that no differences are
found between 2L and 2H in their solution spectra
ꢀ
4
24

X-Ray Crystallographic Analysis and DSC of Two Polymorphic Forms of 2,6-Diphenyl-1,4-dithiin 425
FIGURE 1 IR spectra of 2L and 2H for KBr disk.
1
13
such as H NMR, C NMR, and UV–Vis spectra, but
many differences are found in the IR spectra when
they were taken for KBr disk (Fig. 1), thus indicating
that these two are polymorphic forms and not iso-
mers. In addition, once we had 2L in hand, we were
able to convert 2H to 2L by using a piece of the crys-
tals of 2L as the seed of crystallization. Conversion
of 2L to 2H was also possible in a similar way.
FIGURE 3 Molecular structure and relevant bond length,
bond angle, and dihedral angle data of 2H.
in the coplanarity between the benzene ring and the
double bond part of the dithiin ring. For 2L, the ben-
zene ring and the double bond are nearly coplanar,
◦
◦
with small torsion angles of 11.8(2) and −5.4(2) for
Molecular structures of 2L and 2H, which were
viewed from two different directions, are shown in
Figs. 2 and 3, respectively, along with the relevant
bond lengths and angles. Reportedly, the 1,4-dithiin
ring, including that of the parent compound [5], ex-
ists in a boat conformation in the crystalline state [6].
The present analyses revealed that both 2L and 2H
also adopt boat conformations with flap and boat an-
gles summarized in Table 1. The largest difference in
the molecular structure between 2L and 2H is found
C
4
C
3
C
7
C
8
and C 14, respectively. On
5
C
6
C
13
C
the other hand, the benzene ring and the double bond
in 2H are far from being coplanar, with torsion an-
◦
◦
gles of 39.7(3) and −26.7(3) for C
14, respectively.
The molecular structure 2A in Fig. 4 shows the
4
C
3
C
7
C and
8
C
5
C
6
C
13
C
optimized (energy-minimized) conformation of the
dithiin obtained by DFT MO calculations (B3LYP/6-
3
11+G(2d,p) level). The calculations were also car-
ried out by input of the geometric parameters ob-
tained by X-ray analysis of 2L and 2H. The use of
the geometric parameters of 2L and that of 2H both
led to the same structure 2B (Fig. 5). These show that
the calculated molecular structures well agree with
that of 2H and not of 2L (see also Table 1). Thus,
although it is not adequate to extrapolate the results
of calculations on a single molecule to the molecule
FIGURE 2 Molecular structure and relevant bond length,
bond angle, and dihedral angle data of 2L.
FIGURE 4 Calculated optimized structure 2A of 2,6-
diphenyl-1,4-dithiin (B3LYP/6-311+G(2d,p) level).

4
26 Piao, Sugihara, and Nakayama
TABLE 1 Flap Angle and Boat Angle Data of 2L, 2H, 2A, and 2B
Plane
2L
2H
2A
2B
◦
a
Flap angle ( )
S1 C3 C4/S1 C6 C5
S2 C5 C6/S2 C4 C3
C3 S1 C6/C3 C4 C6
C3 S1 C6/C3 C6 C5
C4 S2 C5/C3 C4 C5
C4 S2 C5/C4 C5 C6
127.9(3)
133.9(3)
142.4(3)
142.9(3)
149.8(3)
149.3(3)
130.8(3)
136.8(3)
145.1(3)
145.1(3)
151.7(3)
151.8(3)
132.8
150.2
141.0
156.8
132.8
149.2
141.1
157.1
◦
b
Boat angle ( )
a
The flap angle is defined as the angle between two S C C planes.
The boat angle is defined as the angle between a C S C plane and the plane of the olefinic carbon atoms.
b
in the crystals, it would be safe to say that the dithiin
molecules in 2H are adopting the energy-minimized
conformation.
EXPERIMENTAL
Preparation of the Low Melting Point
,6-Diphenyl-1,4-dithiin (2L)
2
Figures 6 and 7 show the molecular packing di-
agrams of 2L and 2H, respectively. Both 2L and 2H
are yellow monoclinic needles that belong to the
space group of P21/c. The crystals of the former have
A mixture of 1.15 g (4.3 mmol) of diphenacyl sul-
fide and 2.70 g (9.1 mmol) of Lawesson’s reagent
in 20 mL of toluene was heated at reflux for
1.5 h. The resulting organge mixture was cooled
to room temperature and the insoluble materi-
als were removed by filtration. The filtrate was
−3
a slightly larger density (1.362 g cm ) than those of
−3
the latter (1.360 g cm ). This will originate from fact
that the conformation of the dithiin molecule in 2L is
more compact than that of 2H because the benzene
ring and the double bond are coplanar for 2L.
washed with water, dried over MgSO , and evapo-
4
rated. The residue was chromatographed on a col-
Both experimental findings and calculations de-
scribed above imply that 2H is the stable form, and
thus 2L is the metastable one. In order to obtain
more rigid experimental evidence for this, 2L and 2H
were subjected to DSC analysis. Figures 8 and 9 show
umn of silica gel with CH Cl /hexane (1:10) as the
2
2
eluent to give 1.05 g (91%) of 2,6-diphenyl-1,4-
dithiin as an orange solid. Crystallization of the
solid from MeOH gave 0.86 g (74%) of single crys-
tals of the low melting point dithiin 2L (mp 62–
◦
DSC curves of 2L and 2H, respectively. 2L melted
63 C), which are suitable for X-ray crystallographic
◦
at 63.4 C with a ꢀH
m
[heat of fusion (melting)] of
analysis.
−
1
−1
2
7
0.9 kJ mol (77.7 mJ mg ), while 2H melted at
7.5 C with a ꢀH
The single crystals of the high melting point
dithiin 2H, synthesized by us previously, were suit-
able enough for X-ray crystallographic analysis.
◦
−1
m
of 24.6 kJ/mol (91.5 mJ mg ).
Therefore, ꢀH
m
of 2H is greater than that of 2L by
−
1
3.7 kJ mol , thus providing clear evidence that 2H
1
is the stable form.
Spectroscopic Data of 2L and 2H. H NMR (400
In conclusion, we uncovered that 2 exists in two
polymorphic forms, the metastable form 2L and the
stable form 2H.
MHz, CDCl ) δ 6.55 (s, 2H), 7.31–7.39 (m, 6H), 7.63–
3
1
3
7.65 (m, 4H); C NMR (100.6 MHz, CDCl ) δ 118.3,
3
127.0, 128.6, (accidental overlap of two peaks), 137.1,
1
3
1
39.2; C NMR (100.6 MHz, C
6
D
6
) δ 118.9, 127.2,
1
28.6, 128.7, 137.4, 138.8; UV/Vis (hexane) λmax (ε)
FIGURE 5 Calculated optimized structure 2B of 2,6-
diphenyl-1,4-dithiin obtained on the basis of the geometric
parameters of the X-ray analyses (B3LYP/6-31G(d) level).
FIGURE 6 Molecular packing diagram of 2L.

X-Ray Crystallographic Analysis and DSC of Two Polymorphic Forms of 2,6-Diphenyl-1,4-dithiin 427
FIGURE 9 DSC curve of 2H.
FIGURE 7 Molecular packing diagram of 2H.
◦
−1
of 2 C min . The average values of three scans were
given. Each scan was carried out by using 3–4 mg of
samples.
2
2
45.5 (29400), 309 nm (7700); MS (EI, 70 eV) m/z
68 (M ); IR, see Fig. 1.
+
ACKNOWLEDGMENTS
X-Ray Crystallographic Data
We thank Professors H. Nakahara, A. Ishii, and H.
Saito for discussion, Professor A. Sakamoto for cal-
culations, and Dr. M. Villeneuve for determination of
the DSC curves.
2
L:
C
16
H
12
S
2
,
F
w
= 268.40, monoclinic, a =
˚
1
9
3.503(1), b = 5.570(1), c = 21.130(2) A, β =
◦
˚
3
4.98(1) , V = 1308.8(2) A , space group P2
1
/c,
3
Z = 4, D = 1.362 g/cm , yellow needles, µ (Mo-Kꢀ) =
−
1
0
(
.384 mm , R
all data), GOF = 0.957.
, F
1
= 0.0548 [I > 2ꢁ(I)], wR = 0.1444
2
REFERENCES
2
H: C16
H
12
S
2
w
= 268.40, monoclinic, a =
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˚
1
1
1.624(1), b = 6.085(1), c = 19.220(2) A, β =
2
2, 1527.
◦
˚
3
05.440(5) , V = 1310.41(13) A , space group P2
1
/c,
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2] For reviews on 1,4-dithiins, see (a) Kobayashi, K.;
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3
Z = 4, D = 1.360 g/cm , yellow needles, µ (Mo-Kꢀ) =
−
1
0
.383 mm , R
1
= 0.0439 [I > 2σ(I)], wR = 0.1059
2
(all data), GOF = 0.942.
[
[
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◦
2
6, 1793.
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[
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FIGURE 8 DSC curve of 2L.