Synthesis and properties of anti/syn-regioisomeric mixtures of alkyl-substituted tetracenes

Article abstract of DOI:10.3987/COM-11-12191

We prepared anti/syn-regioisomeric mixtures of alkyl-substituted tetracenes via Diels-Alder reaction between asymmetric furans and 2,6-naphthodiyne synthon. The solid-state color of the mixtures changed before and after recrystallization from Et₂O, suggesting a difference in the molecular arrangements dependent on the different alkyl substituents as well as the change in the distribution of the anti/syn regioisomers after recrystallization. Slow evaporation of the recrystallized mixtures produced single crystals suitable for X-ray analysis, which revealed that the anti regioisomer was isolated. The Japan Institute of Heterocyclic Chemistry.

Full text of DOI:10.3987/COM-11-12191

HETEROCYCLES, Vol. 83, No. 7, 2011
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HETEROCYCLES, Vol. 83, No. 7, 2011, pp. 1621 - 1629. © The Japan Institute of Heterocyclic Chemistry
Received, 26th February, 2011, Accepted, 11th April, 2011, Published online, 12th April, 2011
DOI: 10.3987/COM-11-12191
SYNTHESIS AND PROPERTIES OF anti/syn-REGIOISOMERIC
MIXTURES OF ALKYL-SUBSTITUTED TETRACENES
Chitoshi Kitamura,1* Hiroyuki Kano,1 Hideki Tsukuda,1 Takeshi Kawase,1
Takashi Kobayashi,2 and Hiroyoshi Naito2
1
Department of Materials Science and Chemistry, Graduate School of
Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2201, Japan
2
Department of Physics and Electronics, Graduate School of Engineering, Osaka
Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
e-mail: kitamura@eng.u-hyogo.ac.jp
Abstract – We prepared anti/syn-regioisomeric mixtures of alkyl-substituted
tetracenes via Diels-Alder reaction between asymmetric furans and
2,6-naphthodiyne synthon. The solid-state color of the mixtures changed before
and after recrystallization from Et2O, suggesting a difference in the molecular
arrangements dependent on the different alkyl substituents as well as the change
in the distribution of the anti/syn regioisomers after recrystallization. Slow
evaporation of the recrystallized mixtures produced single crystals suitable for
X-ray analysis, which revealed that the anti regioisomer was isolated.
INTRODUCTION
Preparation and application of organic semiconducting molecules are the subject of recent interest.
Although pentacene is an important representative molecule that shows excellent hole mobilities,1 it
suffers from instability under light and air and insolubility in common organic solvents.2 On the other
hand, tetracene has a more attractive framework because it has the advantages of air stability in its solid
state, solubility, and a low melting point though its mobility may be lower than that of pentacene. To
develop tetracene-based semiconductors,3 researchers have prepared many functionalized tetracenes.4 In
recent years, we have focused on alkyl-substituted tetracenes to obtain high solubilities, low melting
points, and good film morphologies. Thus, we have synthesized a series of tetracenes (1,4,7,10-tetraalkyl,
1,4-dialkyl, 2,3-dialkyl, and 2,3,8,9-tetraalkyl derivatives).5 We revealed that these compounds were not
only typical semiconductors but also interesting chromophores because of the unique dependences of the
solid-state color on their lengths, shapes, numbers, and substitution positions of the alkyl side chains,

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HETEROCYCLES, Vol. 83, No. 7, 2011
despite being independent of their optical properties in solution. We found that the alkyl side chains can
serve as an adjuster of the molecular arrangements to control the solid-state optical properties. Therefore,
introducing a new substitution pattern may allow the development of new optical functionalities. So far,
we have investigated single-component alkyl-substituted tetracenes in which all the alkyl groups are the
same; however, we did not consider bimolecular mixtures composed of anti 1 and syn 2 regioisomers, in
which two different types of groups were located at the 1,4,7,10-positions (Figure 1). In this study, we
report the synthesis and characterization of new alkyl-substituted tetracene mixtures, 1,7- and
1,10-dihexyltetracene (1a and 2a, respectively) and 1,7-dihexyl-4,10-diisopropyl- and
1,10-dihexyl-4,7-diisopropyltetracene (1b and 2b, respectively).
R2
R1
R1
R1
a R1 = Hex, R2 = H
b R1 = Hex, R2 = i-Pr
R1
R2
R2
R2
anti 1
syn 2
Figure 1. Chemical structures of alkyl-substituted tetracenes
RESULTS AND DISCUSSION
We prepared anti/syn mixtures of 1a/2a and 1b/2b via our recently reported procedure,5b by using
asymmetric furans (3a and 3b) and 2,6-naphthodiyne precursor 4, as shown in Scheme 1. 2-Hexylfuran
(3a) was prepared from a lithiation-nucleophilic substitution sequence. By using the same synthetic
procedure employed for 2,5-diisopropylfuran,5d we synthesized 2-hexyl-5-isopropylfuran (3b) via the
a
b
c
Hex
3b (34% in 2 steps)
Hex
Hex
OH
O
O
O
O
3a (43%)
R
Hex
O
Hex
O
Hex
O
TfO
OTf
TMS
d
O
+
+
Hex
R
O
TMS
4
Hex
R
R
R
3
a R = H,
b R = i-Pr
R
O
Hex
O
Hex
O
Hex
O
R
Hex
R
Hex
R
Hex
R
e
f
+
+
Hex
R
R
R
Hex
1
2
1a/2a (5% in 3 steps)
1b/2b (22% in 3 steps)
Scheme 1. Reagents and conditions: (a) 1. n-BuLi, TMEDA, reflux; 2. HexBr, THF, rt; (b) 1.
n-BuLi, TMEDA, reflux; 2. acetone, THF, rt; (c) H2, Pd/C, CH2Cl2, rt; (d) KF, 18-crown-6, THF,
rt; (e) H2, Pd/C, EtOH, rt; (f) conc. HCl, Ac2O, rt

HETEROCYCLES, Vol. 83, No. 7, 2011
1623
reaction of lithiated 3a and acetone followed by hydrogenolysis. Slow generation of 2,6-naphthodiyne
synthon by treating 4 with excess asymmetric furans (3a and 3b) in the presence of KF and 18-crown-6 in
THF afforded Diels–Alder cycloadducts, which were hydrogenated, dehydrated with conc. HCl and Ac2O,
and purified via column chromatography to give tetracene anti/syn-regioisomeric mixtures of 1a/2a and
1b/2b, respectively. The tetracene mixtures were soluble in common organic solvents such as hexane and
13
Et2O. We could not confirm the anti/syn ratio because there were no differences in the 1H and C NMR
signals as well as the Rf values of the anti 1 and syn 2 regioisomers.
Interestingly, after preparation, 1a/2a and 1b/2b were immediately isolated as an orange powder and a
yellow viscous solid, respectively (Figure 2). Furthermore, recrystallization from Et2O produced a visible
change in the solid-state colors, and thus 1a/2a and 1b/2b were obtained as yellow and orange powders,
respectively. Alkyl side chains do not absorb visible light and have little electronic effect on the tetracene
ring. Therefore, the differences in the solid-state colors before and after recrystallization were most likely
due to crystallochromy,6,7 i.e., a color change caused by different intermolecular interactions based on
different aggregation structures. In the step before recrystallization, we can explain the difference in color
as the different molecular arrangements of tetracene rings dependent on the different substitution patterns
of 1a/2a and 1b/2b. On the other hand, in the step after recrystallization, the result probably showed an
unbalanced distribution of anti/syn regioisomers in which the more crystalline component would
dominate the inherent molecular arrangements compared with the other component. This was supported
by the difference in the melting points before and after recrystallization: the melting points of 1a/2a and
1b/2b before recrystallization were 75–77 °C and 55–57 °C, respectively; in contrast, those of 1a/2a and
1b/2b after recrystallization were 121–122 °C and 148–150 °C, respectively.
(a)
(b)
before
after
before
after
Figure 2. Photographs of anti/syn mixtures of (a) 1a/2a and
(b) 1b/2b, showing before (left) and after (right) the
recrystallization from Et2O
We spectroscopically observed the change in the solid-state optical properties before and after
recrystallization with Et2O via absorption (Kubelka-Munk) and fluorescence spectra (Figure 3). The
spectral profiles of 1a/2a and 1b/2b before and after recrystallization showed significant differences. In
the absorption spectra, 1a/2a and 1b/2b before and after recrystallization showed structured bands. The

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HETEROCYCLES, Vol. 83, No. 7, 2011
longest absorption maximum of 1a/2a before recrystallization was 498 nm, whereas that of 1a/2a after
recrystallization was 483 nm (Figure 3a). In contrast, the absorption spectrum of 1b/2b before
recrystallization was characterized by one broad band around 500 nm, whereas that of 1b/2b after
recrystallization showed the longest wavelength absorption band at 489 nm with a shoulder around 520
nm (Figure 3b). The change in the absorption edges before and after recrystallization also correlated well
with that of the solid-state colors. Thus, the absorption edges of 1a/2a before and after recrystallization
were 545 nm and 530 nm, respectively (Figure 3a), and those of 1b/2b before and after recrystallization
were 540 nm and 551 nm, respectively (Figure 3b). We also noted the differences in the fluorescence
properties before and after recrystallization. The fluorescence quantum yields of 1b/2b before and after
recrystallization (0.31 and 0.27, respectively) were larger than those of 1a/2a before and after
recrystallization (0.04 and 0.08, respectively). We also observed a great difference in the spectral shapes
between 1a/2a and 1b/2b. The spectrum of 1a/2a before recrystallization exhibited two peaks (551 and
593 nm) and a shoulder (528 nm), and that of 1a/2a after recrystallization displayed three peaks (529, 563,
and 602 nm). In contrast, the spectrum of 1b/2b after recrystallization illustrated one strong band at 560
nm, whereas that of 1b/2b before recrystallization displayed two bands at 559 and 569 nm. All the
above-mentioned results would be derived from different molecular arrangements of the tetracene
moieties.
(c)
(d)
(b)
(a)
500 600 700 800 900
Wavelength / nm
500 600 700 800 900
Wavelength / nm
400
500
600
400
500
600
Wavelength / nm
Wavelength / nm
Figure 3. Kubelka-Munk spectra of (a) 1a/2a and (b) 1b/2b in dilute KBr pellets and
fluorescence spectra of (c) 1a/2a and (d) 1b/2b in powder form, showing before
recrystallization (solid line) and after recrystallization (dotted line)
When we performed slow evaporation of the recrystallized samples from Et2O solutions, we succeeded in
obtaining some single crystals suitable for X-ray analysis. Then, we determined their crystal structures
(Figure 4).8 The molecules proved to be anti regioisomers 1a and 1b (Figure 4a). Although we attempted
recrystallization from the filtrate components under various conditions, we could not obtain any single
crystals of the syn regioisomers. In both 1a and 1b, the tetracene rings were essentially planar. Their
hexyl groups, except for the terminal methyl groups in 1a, adopted zigzag conformations, and the zigzag
planes were almost coplanar with the tetracene ring. The hexyl conformations in 1a and 1b were

HETEROCYCLES, Vol. 83, No. 7, 2011
1625
(a)
(b)
(d)
(c)
c
a
3.17 Å
b
b
c
O
7.19 Å
a
O
b
c
c
O
a
O
5.33 Å
a
8.20 Å
Figure 4. Crystal structures, showing (a) molecular structures, (b) stacking pairs, (c) layer structures,
and (d) packing diagrams of 1a (top) and 1b (bottom)
completely different from those in 1,4,7,10-tetrahexyltetracene,5b which were perpendicular to the
tetracene ring. In both 1a and 1b, there was no -overlap between the two neighboring molecules along
the stacking direction (Figure 4b). The mutual orientations were significantly different in 1a and 1b. The
interplanar distances between the tetracene planes were 3.56 Å for 1a and 3.79 Å for 1b. The difference
in the slip distance along the long molecular axis was almost 1 Å (7.19 Å for 1a and 8.20 Å for 1b), and
the difference in the slip distance along the short molecular axis was above 2 Å (3.17 Å for 1a and 5.33 Å
for 1b). In the packing structures, the molecules crystallized in layered structures while 1a and 1b appear
to be stacked in a staircase-type fashion and a brickwork-type fashion, respectively (Figure 4d). Within
the layer, one molecule of 1a is surrounded by six neighboring molecules, and one molecule of 1b is
environed by eight neighboring molecules (Figure 4c). In the crystal of 1a, the hexyl groups were
arranged in a head-to-tail regularity, and we observed alternating stripes composed of alkyl and tetracene
regions. These arrangements were slightly different from those in the crystal of 1b; the existence of
isopropyl groups broadened the intermolecular spacing between neighboring molecules. The differences
in these molecular arrangements would probably cause the differences in the solid-state optical properties.
In conclusion, we prepared anti/syn-regioisomeric mixtures of 1a/2a and 1b/2b to examine the effects of
the alkyl substitution and the mixed molecules on the solid-state color. Interestingly, the color of 1a/2a
changed from orange to yellow by recrystallization from Et2O, whereas that of 1b/2b changed from

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HETEROCYCLES, Vol. 83, No. 7, 2011
yellow to orange. The difference in the solid-state color was probably due to the different intermolecular
interactions based on the different aggregation structures as well as the unbalanced distribution of the
anti/syn regioisomers after recrystallization from Et2O. X-ray analysis of single crystals prepared by slow
evaporation from Et2O revealed that anti 1 was isolated as single crystals.
EXPERIMENTAL
All reagents were commercially available and used without further purification. 2,6-Naphthodiyne
precursor 4 was prepared according to the literature method.5b All reactions were performed under
nitrogen atmosphere. Column chromatography was performed on a Wako C-300 silica-gel column
(45–75 m). Melting points were measured on a Yanaco melting point apparatus. 1H and 13C spectra were
measured with a Bruker-Biospin DRX500 FT spectrometer. EI-MS spectra were measured on a Shimadzu
GCMS-QP5050A. Elemental analyses were carried out on a Yanaco MT-5 CHN corder. Absorption and
fluorescence spectra in solution were recorded with a HITACHI U3500 spectrophotometer and a
HITACHI F2500 spectrophotometer (ex = 365 nm), respectively. Kubelka-Munk spectra were measured
using a HITACHI U3010 spectrophotometer equipped with a 60 integrating sphere attachment.
Fluorescence spectra in the solid state were measured on a Hamamatsu Photonics PMA11 calibrated
optical multichannel analyzer with a solid-state blue laser (ex = 377 nm). Absolute quantum yields were
determined with a Labsphere IS-040-SF integrating sphere.
Preparation of 2-hexylfuran (3a)
To an ice-cooled mixture of furan (2.0 mL, 27.6 mmol) and TMEDA (10 mL), 1.6 M n-BuLi in hexane
(20 mL, 32.0 mmol) was added slowly. The mixture was refluxed for 1 h to accomplish mono-lithiation.
The mixture, which changed to a brown suspension, was allowed to cool to rt and was then cooled with
an ice bath. To the ice-cooled mixture, a solution of 1-bromohexane (4.8 mL, 34.4 mmol) in THF (15
mL) was added dropwise. The mixture was stirred at rt for 16 h. After quenching with water, the crude
product was extracted with Et2O, washed with brine, and dried over Na2SO4. After evaporation of the
solvents, the residue was subjected to column chromatography (hexane) on silica gel to remove polar
components. Distillation under reduced pressure (22 mmHg, > 90 °C) provided 3a a colorless oil (1.82 g,
1
43%). H NMR (500 MHz, CDCl3)  0.88 (t, J = 6.8 Hz, 3H), 1.26–1.37 (m, 6H), 1.59–1.65 (m, 2H),
2.60 (t, J = 7.7 Hz, 2H), 5.95 (d, J = 3.0 Hz, 1H), 6.26 (dd, J = 1.8, 3.0 Hz, 1H), 7.27 (d, J = 1.8 Hz, 1H);
13C NMR (126 MHz, CDCl3)  13.96, 22.59, 27.97, 28.04, 28.90, 31.62, 104.46, 109.90, 140.50, 156.45.
Preparation of 2-hexyl-5-isopropylfuran (3b)
To an ice-cooled mixture of furan 3a (3.00 g, 19.7 mmol) and TMEDA (6.3 mL), 1.6 M n-BuLi in hexane
(18 mL, 28.8 mmol) was added slowly. The mixture was refluxed for 1 h. The mixture was allowed to

HETEROCYCLES, Vol. 83, No. 7, 2011
1627
cool to rt and was then cooled with an ice bath. To the ice-cooled mixture, a solution of acetone (5.0 mL,
68.0 mmol) in THF (7.5 mL) was added dropwise. Then the mixture was stirred at rt for 16 h. After
quenching with water, the crude alcohol was extracted with AcOEt, washed with brine, and dried over
Na2SO4. After evaporation of the solvents, the residue was roughly purified by column chromatography
(CHCl3) on silica gel to give a brown oil (1.88 g). Because of its difficult purification, the product
containing some impurities was used in the next reaction. A solution of the brown oil (1.88 g) in CH2Cl2
(50 mL) was hydrogenated over 10% Pd/C (100 mg) under atmospheric pressure at rt for 6 h. The catalyst
was removed by filtration and the filtrate was concentrated under reduced pressure. Column
chromatography (hexane) on silica gel afforded 3b as a yellow oil (1.29 g, 34% from 3a). 1H NMR (500
MHz, CDCl3)  0.88 (t, J = 7.0 Hz, 3H), 1.22 (d, J = 6.9 Hz, 6H), 1.28–1.39 (m, 6H), 1.59–1.63 (m, 2H),
13
2.56 (t, J = 7.6 Hz, 2H), 2.85–2.90 (m, 1H), 5.81–5.84 (m, 2H); C NMR (126 MHz, CDCl3)  14.07,
21.20, 22.59, 27.79 (2C), 28.08, 28.92, 31.61, 102.74, 104.57, 154.65, 159.91.
Preparation of an anti/syn mixture of 1,7- and 1,10-dihexyltetracene (1a/2a)
KF (1.12 g, 19.3 mmol) was added to a solution of 2,6-naphthodiyne precursor 4 (2.60 g, 4.57 mmol),
furan 3a (1.54 g, 10.1 mmol), and 18-crown-6 (4.80 g, 18.2 mmol) in THF (80 mL). The mixture was
stirred at room temperature for 18 h. Water was added, and the resulting mixture was extracted with
CHCl3. The combined organic layer was washed with brine and dried over Na2SO4. After removal of the
solvents, the residue was roughly purified by column chromatography (CHCl3/hexane = 1/2) to afford a
bis(furan) adduct (162 mg) as a yellow oil. Because of its difficult purification, the adduct was then used
in the next reaction. A solution of the adduct (162 mg) in EtOH (30 mL) was hydrogenated over 10%
Pd/C (33 mg) under atmospheric pressure at rt for 4 h. The catalyst was removed by filtration, and the
filtrate was concentrated under reduced pressure. To the residue was added a cold solution of conc. HCl
(1 mL) and Ac2O (5 mL) at 0 °C, and the mixture was stirred at rt for 40 min. Water was added, and the
resulting mixture was extracted with CHCl3. The combined organic layer was washed with brine and
dried over Na2SO4. After removal of the solvents, purification by column chromatography (hexane) on
silica gel afforded tetracene 1a/2a as an orange solid (94 mg, 5% from 4). mp 75–77 °C. Recrystallization
of the orange solid from Et2O produced a yellow solid (27 mg, 1.5% from 4). mp 121–122 °C. 1H NMR
(500 MHz, CDCl3)  0.92 (t, J = 7.1 Hz, 6H), 1.36–1.40 (m, 8H), 1.49–1.55 (m, 4H), 1.88–1.91 (m, 4H),
3.22 (t, J = 7.8 Hz, 4H), 7.21 (d, J = 6.5 Hz, 2H), 7.32 (dd, J = 6.5, 8.5 Hz, 2H), 7.88 (d, J = 8.5 Hz, 2H),
13
8.71 (s, 2H), 8.81 (s, 2H); C NMR (126 MHz, CDCl3)  14.16, 22.73, 29.65, 30.23, 31.83, 33.37,
122.34, 124.24, 124.85, 126.86, 127.39, 129.76, 130.58, 132.06, 138.74; MS m/z 396 (M+, 100); UV-vis
(hexane) max (log ) = 420 (3.56), 446 (3.71), 476 nm (3.71); Fluorescence (hexane) em = 486, 514, 551
nm ( = 0.18). Anal. Calcd for C30H36: C, 90.85; H, 9.15. Found: C, 90.70; H, 9.19.

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HETEROCYCLES, Vol. 83, No. 7, 2011
Preparation of an anti/syn mixture of 1,7-dihexyl-4,10-diisopropyl- and 1,10-dihexyl-4,7-
diisopropyltetracene (1b/2b)
The title compound was prepared following the same procedure as described above for 1a/2a except that
reagents KF (927 mg, 16.0 mmol), 2,6-naphthodiyne precursor 4 (2.20 g, 3.80 mmol), furan 5b (1.70 g,
9.10 mmol), 18-crown-6 (3.71 g, 13.9 mmol), THF (100 mL), 10% Pd/C (200 mg), and EtOH (50 mL)
were used. The title compound was obtained as a yellow viscous solid (400 mg, 22% from 4). mp
55–57 °C. Recrystallization of the yellow solid from Et2O produced an orange solid (350 mg, 19% from
1
4). mp 148–150 °C. H NMR (500 MHz, CDCl3)  0.93 (t, J = 6.9 Hz, 6H), 1.32–1.59 (m, 24H),
1.88–1.93 (m, 4H), 3.21 (t, J = 7.8 Hz, 4H), 3.94–4.00 (m, 2H), 7.23–7.26 (m, 4H), 8.86 (s, 2H), 8.94 (s,
13
2H); C NMR (126 MHz, CDCl3)  14.20, 22.76, 23.63, 28.68, 29.73, 30.18, 31.88, 33.35, 120.16,
122.71, 123.40, 123.98, 128.93, 130.44, 131.09, 136.57, 142.53; MS m/z 480 (M+, 100); UV-vis (hexane)

max (log ) = 424 (3.69), 451 (3.94), 481 nm (3.93); Fluorescence (hexane) em = 494, 522, 558 nm ( =
0.11). Anal. Calcd for C36H48: C, 89.94; H, 10.06. Found: C, 89.88; H, 10.24.
X-Ray crystallographic analysis
Crystal data for 1a: C30H36, Fw = 396.59, T = 223 K, triclinic, space group P1, a = 7.648(4), b =
8.428(4), c = 10.406(5) Å,  = 67.430(17),  = 74.293(19),  = 79.68(2)°, V = 594.1(5) Å3, Z = 1, Dcalcd
=
1.108 g cm−3, 4735 reflections measured, 2680 independent, 146 parameters refined, GOF = 1.091, R1 =
0.087 [I > 2(I)], wR2 = 0.303 (all data). Crystal data for 1b: C36H48, Fw = 480.74, T = 223 K, monoclinic,
space group C2, a = 19.570(3), b = 7.5746(11), c = 9.8068(14) Å,  = 99.493(3)°, V = 1433.8(3) Å3, Z =
2, Dcalcd = 1.114 g cm−3, 7041 reflections measured, 3222 independent, 175 parameters refined, GOF =
0.876, R1 = 0.087 [I > 2(I)], wR2 = 0.369 (all data). Single crystals were grown from Et2O solutions by
slow evaporation. X-Ray diffraction data were collected on a Rigaku Mercury diffractometer for 1a and a
Rigaku RAXIS RAPID diffractometer for 1b with a graphite−monochromated Mo-K ( = 0.7107 Å)
radiation. The structures were solved by direct methods using SIR2004.9 All non-hydrogen atoms were
refined anisotropically by full-matrix least-squares on F2 using SHELXL97.10 All the H atoms were
geometrically positioned and refined using a riding model. All calculations were performed using the
WinGX program package.11 Crystallographic data for the structures in this paper have been deposited
with the Cambridge Crystallographic Data Centre as CCDC 805752 (1a) and 805751 (1b).
ACKNOWLEDGEMENTS
This study was supported by the Grant-in-Aid from JSPS and by the University of Hyogo. We thank the
Instrument Center of the Institute for Molecular Science for the X-ray structural analysis and Chisso
Petrochemical Corporation for the gift of reagents.

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7
There were no remarkable differences in the optical properties in solution between 1a/2a and 1b/2b
as well as before and after recrystallization.
8. We observed that each molecule had two disordered sets of alkyl groups. For clarity, minor sites are
omitted in Figure 4.
9. M. C. Burla, R. Caliandro, M. Camalli, B. Carrozzini, G. L. Cascarano, L. De Caro, C. Giacovazzo,
G. Polidori, and R. Spagna, J. Appl. Cryst., 2005, 38, 381.
10. G. M. Sheldrick, Acta Crystallogr., 2008, A64, 112.
11. L. J. Farrugia, J. Appl. Cryst., 1999, 32, 837.