Molecular symmetry and fluorine-containing supramolecular synthons as structure-differentiating agents in some bridge-flipped isomeric bis-benzylideneanilines

Article abstract of DOI:10.1039/c3ce42540a

The crystal structures of three pairs of bridge-flipped isomers are compared here in the context of whether their similarity in molecular space-filling requirements in combination with the tendency of centrosymmetric molecules to occupy crystallographic inversion centers might lead to their isomorphism. The possibility that similar fluorine-based supramolecular synthons occurring in the crystal structures of both members of a pair might promote their isomorphism is also considered. The compounds are the bis- benzylideneanilines formed by reaction of 2-fluoroaniline, 3-fluoroaniline, and 4-fluoroaniline respectively with terephthaldehyde (1-3) and by reaction of 2-fluorobenzaldehyde, 3-fluorobenzaldehyde, and 4-fluorobenzaldehyde respectively with phenylenediamine (4-6). The crystal structure of 2 is disordered, with the fluorine atom occupying two sites (95 : 5 occupation) related by rotation about the bond between the bridge nitrogen atom and the 3-fluorophenyl group. The crystal structure of 6 (HEWHAU) has been reported by previous workers. The structures of the 1:4, 2:5, and 3:6 pairs are compared to each other and to those of fluorinated simple (one-bridge) benzylideneanilines and fluorinated bis-benzylideneanilines recently described in the literature. No isomorphism is found among the bridge-flipped isomeric pairs, and none is found between positional isomers within the 1-3 or 4-6 series. The supramolecular synthon defined by a 2-F?bridge H contact is found in several of the benzylideneaniline crystal structures, but it does not compel isomorphism, nor is it specific to one type (terephthaldehyde-based or phenylenediamine-based) of isomer. Conformational variability and supramolecular synthon variety apparently serve as structure differentiators, not as isomorphism facilitators, among these bis-benzylideneanilines. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3ce42540a

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Molecular symmetry and fluorine-containing
supramolecular synthons as structure-differentiating
agents in some “bridge-flipped” isomeric
bis-benzylideneanilines†
Cite this: CrystEngComm, 2014, 16,
7226
William H. Ojala, a Barjeta Balidemaj,a Jenna A. Johnson,a Sarah N. Larsona
*
and Charles R. Ojalab
The crystal structures of three pairs of “bridge-flipped isomers” are compared here in the context of
whether their similarity in molecular space-filling requirements in combination with the tendency of cen-
trosymmetric molecules to occupy crystallographic inversion centers might lead to their isomorphism.
The possibility that similar fluorine-based supramolecular synthons occurring in the crystal structures of
both members of a pair might promote their isomorphism is also considered. The compounds are the
bis-benzylideneanilines formed by reaction of 2-fluoroaniline, 3-fluoroaniline, and 4-fluoroaniline
respectively with terephthaldehyde (1–3) and by reaction of 2-fluorobenzaldehyde, 3-fluorobenzaldehyde,
and 4-fluorobenzaldehyde respectively with phenylenediamine (4–6). The crystal structure of 2 is
disordered, with the fluorine atom occupying two sites (95 : 5 occupation) related by rotation about the
bond between the bridge nitrogen atom and the 3-fluorophenyl group. The crystal structure of 6
(HEWHAU) has been reported by previous workers. The structures of the 1:4, 2:5, and 3:6 pairs are
compared to each other and to those of fluorinated simple (one-bridge) benzylideneanilines and
fluorinated bis-benzylideneanilines recently described in the literature. No isomorphism is found among
the bridge-flipped isomeric pairs, and none is found between positional isomers within the 1–3 or
4–6 series. The supramolecular synthon defined by a 2-F⋯bridge H contact is found in several of the
benzylideneaniline crystal structures, but it does not compel isomorphism, nor is it specific to one type
(terephthaldehyde-based or phenylenediamine-based) of isomer. Conformational variability and
supramolecular synthon variety apparently serve as structure differentiators, not as isomorphism
facilitators, among these bis-benzylideneanilines.
Received 13th December 2013,
Accepted 16th February 2014
DOI: 10.1039/c3ce42540a
www.rsc.org/crystengcomm
relationship is Ar–NH–NCH–Ar′ vs. Ar–CHN–NH–Ar′ (Ar = aryl),
and in benzylideneanilines, in which the relationship is
1 Introduction
Solid-state molecular packing arrangements are a function of
not only intermolecular interactions but also molecular
space-filling requirements. To examine the influence of both
on the crystal structures of organic compounds, we are
conducting a study of the solid-state molecular packing
arrangements of pairs of molecules we have designated
“bridge-flipped isomers”,1 isomeric molecules related to each
other by the reversal of a bridge or chain of atoms connecting
two major portions of each molecule. Such isomerism is
displayed by the phenylhydrazones, in which the isomeric
Ar–CHN–Ar′ vs. Ar–NCH–Ar′ (Ar = aryl). We are interested
in identifying pairs of bridge-flipped isomers that assume
identical molecular packing arrangements and form isomor-
phous crystals. An isomorphous pair would be of interest for
their potential ability to form a continuous series of solid
solutions in which the conductivity, solubility, or color
(an especially conspicuous property of the phenylhydrazones)
might be capable of being tailored or engineered to a particu-
lar desired state. A non-isomorphous pair would be of interest
for the insights it would lend into both the intramolecular
factors (such as molecular conformation) and the inter-
molecular factors (such as intermolecular H-bonding, halogen
bonding, or π⋯π interactions) that differentiate the two crystal
structures. The benzylideneanilines and phenylhydrazones are
of particular interest to us because in these compounds,
unlike other bridge-connected compounds such as esters and
a Department of Chemistry, University of St. Thomas, St. Paul, Minnesota 55105, USA.
E-mail: whojala@stthomas.edu
b Department of Chemistry, Normandale Community College, Bloomington,
Minnesota 55431, USA
† CCDC 974614–974618. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3ce42540a
7226 | CrystEngComm, 2014, 16, 7226–7235
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amides,2 the steric differences between the reversed bridges
are relatively small and might not interfere with the formation
of isomorphous crystals.
isomorphism between bridge-flipped bis-benzylideneanilines.
An examination of the crystallographic literature thus far gives
little reason to think so. Non-isomorphous bridge-flipped pairs
found in the Cambridge Structural Database6 (Version 5.34)
such as LICGAG and LICFUZ,7 XIGRIO8 and SANYIP/SANYIP01/
SANYIP02,9 UKUNUK and UKUPEW,10 LICGEK7 and JAYFEV,11
and FAMMAJ12 and PILSUZ13 (Fig. 1) offer little encourage-
ment that even genuinely centrosymmetric bridge-flipped
isomers can assume isomorphous crystal structures. On the
other hand, a general similarity if not actually an exact
correspondence between the crystal structures of WILSOZ
(1,4-terephthalylidene-bis(N-4′-methylaniline))14 and YAGSEG
(N,N′-bis(4-methylbenzylidene)benzene-1,4-diamine)15 (Fig. 2)
seems to us to keep the possibility open.
In addition to the role of molecular symmetry, the role of
supramolecular synthons16 in defining molecular packing
arrangements can be usefully examined in the context of
bridge-flipped isomers. The bridge might be considered a
molecular switch, allowing certain synthons to be “switched on”
or “switched off” in the solid state, depending on the bridge
orientation.10,17 We are interested in evaluating supramolecular
synthons containing fluorine in the context of bridge-flipped
isomeric molecules. The role of “organic fluorine”, the fluorine
atom covalently bonded to carbon, in solid-state molecular
packing has been studied extensively in recent years,2b,18
and the range of viewpoints concerning the ability of fluo-
rine to serve as an acceptor in hydrogen bonding and as a
participant in supramolecular synthons involving C–H⋯F,
F⋯F, and C–F⋯π contacts has inspired a substantial record
in the crystallographic literature and a range of chemical
metaphors ranging from “odd man out”19 to “the little atom
that could”.20 Recently Kaur et al.21 have examined an exten-
sive series of fluorine-substituted single-bridge benzylidene-
anilines and identified the supramolecular synthons involving
fluorine in these structures. Among their compounds are four
In even these two potentially promising families of com-
pounds, we have found isomorphism to be rare, although a
few isomorphous pairs are known. In phenylhydrazones, a
potential differentiating factor is the presence of a strong,
conventional H-bond donor, the N–H bond, in the bridge. If
the bridge-flipped isomeric phenylhydrazones bear an H-bond
acceptor capable of interacting with this N–H group, then the
different position of the N–H group within the bridge from
one isomer to the other is likely to cause significant differ-
ences between the isomers in their molecular packing. In the
examples of isomorphous bridge-flipped phenylhydrazones
we and previous workers have examined to date,1,3 an H-bond
acceptor has indeed been present, but the packing arrange-
ment has placed it roughly equidistant from the N–H and
C–H hydrogen atoms in the bridge, with the weak H-bond
donor C–H mimicking the strong H-bond donor N–H in
these particular cases. In benzylideneanilines, no similar
H-bonding difference between bridge-flipped isomers occurs;
instead, a conformational difference related to the position of
the bridge C–H can exist between the isomers. Steric inter-
action between the bridge C–H and an ortho H-atom on the
aniline ring can force the aniline ring farther out of copla-
narity with the CHN bridge atoms than the benzylidene
ring. Bridge-flipped isomeric benzylideneanilines affected by
this interaction can be expected to possess different
conformations and thus assume different molecular packing
arrangements. Nonetheless, the crystallographic literature
includes examples of benzylideneaniline structures in which
the molecules are nearly planar, which suggests to us that
the formation of isomorphous crystals of bridge-flipped
benzylideneanilines need not in all cases be prohibited by
conformational differences. Isomorphous pairs of bridge-
flipped benzylideneanilines do occur, with the isomorphism
accompanied by end-for-end disorder of the molecules in
some cases.1,4 If two opposite orientations of the molecule can
be found at
a given site in a disordered benzylidene-
aniline crystal structure, this suggests to us that the same
crystal structure could be assumed independently by two
bridge-flipped isomeric benzylideneanilines.
In isomorphous bridge-flipped benzylideneanilines, the
occurrence of the end-for-end disorder in one or both members
of the isomeric pair generally requires that the molecule(s)
be at least approximately centrosymmetric. An opportunity
for evaluating the role of molecular symmetry in the potential
isomorphism of bridge-flipped benzylideneanilines is therefore
presented by extending the range of compounds examined
from simple (one-bridge) benzylideneanilines to symmetrically
substituted bis-benzylideneanilines, which are capable of
assuming an exactly centrosymmetric conformation in the
solid state. We are interested in whether the tendency of cen-
trosymmetric molecules to occupy crystallographic centers of
symmetry in the solid state,5 specifically in combination with a
close similarity in molecular size and shape, would facilitate
Fig. 1 Some non-isomorphous pairs of centrosymmetric bridge-flipped
isomeric bis-benzylideneanilines found in the Cambridge Structural
Database.
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and deactivating fluorine-containing supramolecular synthons;
compounds they examined include the bis-benzylideneanilines
E,E-1,4-bis[2-(2,3,4,5,6-pentafluorophenyl)-2-azaethenyl]benzene
(UKUNUK, Fig. 1) and E,E-N,N′-bis(2,3,4,5,6-pentafluorobenzylidene)-
1,4-phenylenediamine (UKUPEW, Fig. 1), which also are non-
isomorphous.
In light of these previous studies, we wondered to what
extent the fluorine-containing supramolecular synthons observed
in simple benzylideneanilines would be preserved in related,
fluorine-substituted bis-benzylideneanilines, especially where
the substitution pattern of a simple fluorobenzylideneaniline
would make it essentially a half a molecule of the corre-
sponding bis-benzylideneaniline. In connection with our
interest in centrosymmetric bridge-flipped isomers, and in
spite of the absence of any published examples of isomor-
phous, fluorine-substituted benzylidenanilines of either the
single-bridge or bis-type, we also wondered if similar fluorine-
containing supramolecular synthons occurring in both mem-
bers of a bridge-flipped, centrosymmetric bis-benzylideneaniline
pair might encourage (if not compel) the formation of
isomorphous crystals. Both the recurrence of these synthons
in bis-benzylideneanilines and any tendency toward isomor-
phism they might encourage would testify to their robust-
ness and to their ultimate usefulness in crystal structure
design. Toward addressing these questions, we have determined
and describe here the crystal structures of five fluorinated
bis-benzylideneanilines (Fig. 3): E,E-1,4-bis[2-(2-fluorophenyl)-2-
azaethenyl]benzene,
azaethenyl]benzene,
1;
2;
E,E-1,4-bis[2-(3-fluorophenyl)-2-
E,E-1,4-bis[2-(4-fluorophenyl)-2-
azaethenyl]benzene, 3; E,E-N,N′-bis(2-fluorobenzylidene)-1,4-
phenylenediamine, 4; and E,E-N,N′-bis(3-fluorobenzylidene)-
1,4-phenylenediamine, 5. The crystal structure of E,E-N,N′-bis
(4-fluorobenzylidene)-1,4-phenylenediamine, 6 (HEWHAU) has
been described in a recent report by Fang and Cao.22 In these
six crystal structures we identify fluorine-containing motifs that
play a role in the molecular packing, and we compare these
motifs to those found in the fluorinated bis-
benzylideneanilines examined by Collas et al. and in the
fluorinated, single-bridge N-benzylideneanilines examined
by Kaur et al. We hope that through such comparisons we
can gain further insight into both the role of organic fluo-
rine in the solid-state packing of molecules in general and
Fig. 2 View of molecular packing in two nearly isomorphous bridge-
flipped isomeric bis-benzylideneanilines: (a) WILSOZ14 along [010] (left)
and along [100] (right), and (b) YAGSEG15 (converted from published
P21/c setting to P21/n setting to match WILSOZ) along [010] (left) and
along [001] (right). Hydrogen atoms have been omitted for clarity.
Cell constants for WILSOZ: a = 6.043(1), b = 7.842(0), c = 18.215(3) Å,
β = 92.32(1)°; cell constants for YAGSEG converted to P21/n: a = 18.69,
b = 7.156, c = 6.475 Å, β = 91.73°.
bridge-flipped isomeric pairs, none of which are isomorphous.
Collas et al.10 have studied the role of nitrogen position in the
bridges of some fluorinated aza-distyrylbenzenes in activating
Fig. 3 Bridge-flipped bis-benzylideneanilines for which the crystal
structures are compared pair-wise in this paper.
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the formation (or not) of isomorphous crystal structures by
bridge-flipped isomeric molecules in particular.
conformations and occupy two different crystallographic
inversion centers in the unit cell; the Zorky descriptor is
P21/c, Z = 4(−12). In our analyses of 1–5, only 3 assumed a
non-centrosymmetric conformation and occupied a general
position in its cell. Although molecules of 1, 2, 4, 5, and 6
are located on crystallographic inversion centers, this fact in
combination with any conformational and space-filling simi-
larity the bridge-flipped isomers possessed did not result in
the formation of isomorphous crystals.
A comparison of the bridge-flipped isomeric pairs 1 and 4,
2 and 5, and 3 and 6 with respect to their molecular confor-
mations and their fluorine-containing packing motifs follows
here. Selected torsion angles are listed in Table 2; these are
consistent with the common observation that in the solid-
state structures of benzylideneanilines the aniline ring is
twisted farther than the benzylidene ring out of coplanarity
with the bridge. Parameters related to the C–H⋯F, C–H⋯N,
and F⋯F intermolecular contacts are listed in Table 3. For
the purposes of this comparison, a contact less than the
sum of the van der Waals radii of H (1.20 Å), N (1.55 Å) or
F (1.47 Å) reported by Bondi25 and used by the Mercury26
program is here considered a “close” contact. Where the stan-
dard uncertainty in a contact parameter is not listed, the con-
tact involves a hydrogen atom placed in a calculated position
(see Experimental).
2 Results and discussion
Compounds 1–5 were prepared and crystallized by standard
methods; details of the syntheses, crystallization, and X-ray
structure determinations are given in the Experimental
section. Cell constants, structure determination details, and
refinement parameters from the single-crystal X-ray analyses
are summarized in Table 1. The structure of 2 was found
to be disordered to a small (approx. 5%) extent, with the
fluorine atom located in either of two positions related by a
180 degree rotation of the fluorophenyl group about the C–N
bond. None of the bridge-flipped isomeric pairs in the series
1–6 proved to be isomorphous, nor did any pair differing
only in the ring position of the fluorine atoms within the
terephthaldehyde group 1–3 or the phenylenediamine group
4–6, although isomorphism among other compounds differ-
ing only in the position of the fluorine substituent on an
aromatic ring is known to occur.2b,21,23 A striking example
of an apparent resistance to isomorphism is the 4-fluoro
substituted pair 3 and 6. Comparing only their cell constants
would suggest at least some structural similarity between
these two isomers, given that the two possess closely similar
unit cell axis lengths and that both crystallize with four mole-
cules per unit cell in space group P21/c, but the structure
determinations show that the packing arrangements differ
2.1 Bridge-flipped isomers E,E-1,4-bis[2-(2-fluorophenyl)-
2-azaethenyl]benzene (1) and E,E-N,N′-bis(2-fluorobenzylidene)-
1,4-phenylenediamine (4)
entirely. Molecules of
3 assume a non-centrosymmetric
conformation (see section 2.3), and they occupy general
positions in the unit cell; the Zorky24 descriptor is P21/c, Z = 4(1).
Atom numbering and molecular conformations are shown in
In contrast, molecules of
6
assume centrosymmetric
Fig. 4(a) for 1 and Fig. 4(b) for 4. Molecules of 1 are definitely
Table 1 Details of data collection and structure refinement for 1–5
Compound reference
1
2
3
4
5
Chemical formula
Formula mass
Crystal system
Space group
C20H14N2F2
320.33
Monoclinic
P21/n
C20H14N2F2
320.33
Monoclinic
P21/n
C20H14N2F2
320.33
Monoclinic
P21/c
C20H14N2F2
320.33
Monoclinic
P21/c
C20H14N2F2
320.33
Monoclinic
P21/c
a/Å
b/Å
c/Å
α/°
5.8090(7)
20.942(3)
6.6746(8)
90
9.4317(14)
3.9520(6)
20.303(3)
90
7.4680(11)
5.6986(8)
36.149(5)
90
6.9517(6)
11.5713(10)
10.0545(8)
90
17.2826(17)
6.1796(6)
7.0765(7)
90
β/°
γ/°
107.509(2)
90
94.204(2)
90
95.239(2)
90
104.6180(10)
90
92.710(1)
90
Unit cell volume/Å3
Z (formula units/cell)
Temperature/K
Radiation type
Abs. coeff., μ/mm−1
No. of refl. measured
No. of indep. refl.
Rint
Final R1 (I > 2σ(I))
Fnl. wR(F2) (I > 2σ(I))
Final R1 (all data)
Fnl. wR(F2) (all data)
Goodness of fit on F2
CCDC number
774.35(16)
2
173(2)
MoKα
0.098
754.7(2)
2
173(2)
MoKα
0.101
1532.0(4)
4
173(2)
MoKα
0.099
782.61(11)
2
173(2)
MoKα
0.097
754.92(13)
2
173(2)
MoKα
0.101
8930
1781
8607
1697
17 753
3550
8953
1785
4277
1692
0.0333
0.0389
0.0876
0.0591
0.0951
1.047
0.0255
0.0539
0.1382
0.0611
0.1416
1.139
0.0253
0.0411
0.1154
0.0540
0.1243
1.055
0.0387
0.0387
0.0925
0.0498
0.0994
1.074
0.0165
0.0379
0.1027
0.0453
0.1092
1.050
974614
974615
974616
974617
974618
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Table
2 Selected torsion angles (°). Corresponding angles with
different atom labels are indicated by superscripts
C6–C1–N1–C7
N1–C7–C8–C10(a)
1
2
3
43.78(19)
11.14(19)
5.2(3)
−142.0(2)
−145.21(12)
13.00(19)a
13.60(19)c
143.11(12)b
C6–C1–C7–N1
C7–N1–C8–C10(a)
4
5
6
6.2(2)
−20.1(2)
−29.79(18)
39.9(2)e
−164.02(12)
12.1(2)d
−14.2(2)f
−39.3(2)g
a
b
c
d
N1–C7–C8–C20. C16–C11–N2–C17. N2–C17–C18–C19. C3–C4–
C7–N1. e C7–N1–C8–C10. N2–C14–C15–C16. g C14–N2–C13–C11.
f
Fig. 4 Anisotropic ellipsoid (50% probability) plots of (a) compound 1
and (b) compound 4, showing atom numbering.
nonplanar with respect to the angle between the fluorophenyl
groups and the center ring, and the C–F bond and the bridge
C–H bond point in opposite directions. In contrast, molecules
of 4 are more nearly planar, and the C–F bond and the bridge
C–H bond point in the same direction. These conformational
differences between the two isomers are not trends that
hold for the remaining isomeric pairs 2:5 and 3:6, however
(see sections 2.2 and 2.3), demonstrating that the observed
conformations in 1–6 depend on intermolecular contacts as
well as on intramolecular steric requirements.
In the molecular packing arrangement of 1 (Fig. 5), C–H⋯F
contacts link the molecules into chains extending along [100].
Each of the two bridge H atoms of a given molecule is in con-
tact with a fluorine atom from a different adjacent molecule
in the chain: H7(xyz)⋯F1(−1 + x, y, z) = 2.57 Å. The robustness
of this 2-F⋯bridge-H contact is demonstrated by the existence
of a similar contact between a ring fluorine atom in the
ortho-position and the bridge hydrogen atom in the Kaur et al.
structures N-benzylidene-2-fluoroaniline, 2-fluoro-N-(3-fluoro-
benzylidene)aniline,
4-fluoro-N-(2-fluorobenzylidene)aniline,
2-fluoro-N-(4-fluorobenzylidene)aniline,
3-fluoro-N-(2-fluoro-
benzylidene)aniline, and the Collas et al. structure E,E-1,4-bis[2-
(2,3,4,5,6-pentafluorophenyl)-2-azaethenyl]benzene (UKUNUK).
Interestingly, it is absent from both polymorphs of 2-fluoro-N-
(2-fluorobenzylidene)aniline reported by Kaur et al., in which
crystallographic disorder exchanges the positions of the bridge
CH and N moieties in both structures. In these particular
structures any advantage in forming the 2-F⋯bridge-H supramolec-
ular synthon is insufficient to cause either polymorph to crys-
tallize in an ordered packing arrangement that would allow it.
In contrast to that of 1, the packing arrangement of 4
(Fig. 6) involves a fluorine–hydrogen close contact in which
Table 3 Selected intermolecular D–X⋯A contacts in 1–6 (D–X at xyz)
D
X
A
X⋯A/Å
D–X⋯A/°
Symmetry code of A
1
2
3
C7
C7
C5
C4
C4
C4
C12
C2
C9
C2
C10
C5
C4
C3
C1
C2
C1
C1
H7
H7
H5
F1
F1
F1
H12
F1
H9
F1
H10
H5
H4
F1
F1
H2
F1
F1
F2
H19
F2
F2
F1
F1
F2
F2
2.57
2.57
2.67
2.9786(13)
2.9654(13)
2.67
2.70
2.74
2.74
2.58
2.58
2.69
2.59
2.59
3.0253(16)
2.68
3.0253(16)
2.68
170
156
130
106.11(7)
104.74(7)
149
141
132
120
131
127
162
149
128
106.35(10)
133
107.80(10)
154
−1 + x, y, z
1/2 − x, −1/2 + y, 3/2 − z
−1 + x, 1/2 − y, −1/2 + z
−1 + x, 1/2 − y, −1/2 + z
−1 + x, 3/2 − y, −1/2 + z
−1 + x, 3/2 − y, −1/2 + z
1 − x, 1/2 + y, 1/2 − z
1 − x, −1/2 + y, 3/2 − z
1 − x, 1/2 + y, 3/2 − z
1 − x, −1/2 + y, 3/2 − z
1 − x, 1/2 + y, 3/2 − z
x, 3/2 − y, 1/2 + z
F2
H13
N1
H9
F1
H10
F1
N1
F1
H4
F1
F1
F1
H2
F2
F2
4
5
6
1 − x, −1/2 + y, 1/2 − z
1 − x, 1/2 + y, 1/2 − z
1 − x, −1/2 + y, 3/2 − z
1 − x, −1/2 + y, 3/2 − z
1 − x, 1/2 + y, 3/2 − z
1 − x, 1/2 + y, 3/2 − z
−x, 1/2 + y, 3/2 − z
C18
C19
C18
C18
3.0343(16)
2.76
3.0343(16)
2.76
108.90(10)
134
104.34(10)
152
−x, 1/2 + y, 3/2 − z
F2
H19
−x, −1/2 + y, 3/2 − z
−x, −1/2 + y, 3/2 − z
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the hydrogen atom is not the bridge H but is instead one
of the H atoms of the central ring: H10(xyz)⋯F1(1 − x, 1/2 + y,
3/2 − z) = 2.58 Å. The neighboring H atom on the central ring
approaches the same fluorine atom at a greater distance:
H9(xyz)⋯F1(1 − x, 1/2 + y, 3/2 − z) = 2.74 Å. The absence of the
2-F⋯bridge-H synthon and the presence of the 2-F⋯central
ring-H synthon distinguish not only the phenylenediamine
derivative 4 from the terephthaldehyde derivative 1 but also the
Collas et al. phenylenediamine derivative E,E-N,N′-bis(2,3,4,5,6-
pentafluorobenzylidene)-1,4-phenylenediamine,
UKUPEW,
from its terephthaldehyde analogue UKUNUK. In both these
cases, switching the positions of the nitrogen atoms in the
bridges from a terephthaldehyde core to a phenylenediamine
core switches off the 2-F⋯bridge-H synthon and switches on
the 2-F⋯central ring-H synthon, presumably by increasing
the acidity of the central ring H atoms, an effect noted
by previous investigators.10,21 Molecules linked by this latter
synthon in 4 are also linked by a contact between a fluorophenyl
ring H atom and the bridge N atom: H5(xyz)⋯N1(x, 3/2 − y,
1/2 + z) = 2.69 Å (Fig. 6). Both the 2-F⋯bridge-H contact and a
fluorophenyl H contact with the bridge N atom were observed
in the Kaur et al. structure N-benzylidene-2-fluoroaniline, a
benzylideneaniline that could be considered roughly
half a molecule of 1; unfortunately, a comparison with
N-(2-fluorobenzylidene)aniline, which could be considered
roughly half a molecule of 4, is not possible because in spite
of their efforts, the previous workers were unable to obtain
the compound in crystalline form.
Fig. 5 View along [001] of molecular packing in compound 1 showing
2-F⋯bridge-H contacts as dashed lines. Hydrogen atoms not involved
in these contacts have been omitted for clarity. See Table 3 and text
for intermolecular distances and angles.
2.2 Bridge-flipped isomers E,E-1,4-bis[2-(3-fluorophenyl)-
2-azaethenyl]benzene (2) and E,E-N,N′-bis(3-fluorobenzylidene)-
1,4-phenylenediamine (5)
Atom numbering and molecular conformations are shown
in Fig. 7(a) for 2 (only the major component of the disorder
is shown) and in Fig. 7(b) for 5. As is true of 1 and 4, the
phenylenediamine derivative (5) is more nearly planar than
the terephthaldehyde derivative (2). In direct contrast to 1 and
4, in 2 the C–F bond and the bridge C–H bond point in the
same direction, while in 5 the C–F bond and the bridge C–H
bond point in opposite directions. In 2, the fluorine-containing
motif is a 3-F⋯bridge-H contact, H7(xyz)⋯F1(1/2 − x, −1/2 + y,
3/2 − z) = 2.52 Å (Fig. 8). In 5, the fluorine contact is not with
the bridge H atom but with an H atom ortho to the F atom on
the fluorophenyl ring, para-hydrogen H(4)(xyz)⋯F1(1 − x,
−1/2 + y, 1/2 − z) = 2.59 Å, a contact that links molecules of
5 into extended sheets (Fig. 9). Neither of these fluorine-
based motifs is observed in the other meta-fluoro substituted
benzylideneanilines
4-fluoro-N-(3-fluorobenzylidene)aniline,
benzylidene)aniline, 2-fluoro-N-(3-fluorobenzylidene)aniline,
3-fluoro-N-(4-fluorobenzylidene)aniline,
3-fluoro-N-(3-fluoro-
3-fluoro-N-(2-fluorobenzylidene)aniline, or N-benzylidene-3-
fluoroaniline (which could be considered roughly half a
molecule of 2) examined by Kaur et al. Where a 3-F⋯4-H
synthon appears in these structures, it is part of an R22(8)
interaction that defines dimers instead of chains. Comparison
Fig. 6 View along [100] of molecular packing in compound 4 showing
close 2-F⋯ring-H contact (upper) and close ring-H⋯bridge-N contact
(lower) as dashed lines. Hydrogen atoms not involved in these contacts
have been omitted for clarity. See Table 3 and text for intermolecular
distances and angles.
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Fig. 7 Anisotropic ellipsoid (50% probability) plots of (a) compound 2
(showing only the major component of the rotational disorder about
the C–N bond to the fluorophenyl group) and (b) compound 5,
showing atom numbering.
Fig. 9 View of molecular packing in (left) compound 5 approximately
along [001] and perpendicular to a sheet of molecules linked by 3-F⋯4-H
contacts, and in (right) UKUNUK, showing 3-F⋯4-F contacts analogous
to the 3-F⋯4-H contacts in 5. Hydrogen atoms not involved in these
contacts have been omitted for clarity. See Table 3 and text for
intermolecular distances and angles.
2.3 Bridge-flipped isomers E,E-1,4-bis[2-(4-fluorophenyl)-
2-azaethenyl]benzene (3) and E,E-N,N′-bis(4-fluorobenzylidene)-
1,4-phenylenediamine (HEWHAU) (6)
The atom numbering and molecular conformation of 3 are
shown in Fig. 10(a). For comparison, the atom numbering
and conformations of the two molecules located on two
different inversion centers in the unit cell of HEWHAU,22
compound 6, are shown in Fig. 10(b). Unlike the previous
bridge-flipped pairs, molecules of 3 and 6 are similar in the
extent to which they deviate from planarity, with angles cal-
culated using Mercury between the least-squares planes of
the center ring and the fluorophenyl rings in the two isomers
varying over a range of slightly less than three degrees (from
51.42° to 54.33°). As noted above, molecules of 3 and 6 do
differ significantly in conformation with respect to the bridges,
with molecules of 3 occupying general positions in the
unit cell and molecules of 6 occupying inversion centers. A
C–H⋯N close contact is found in 3: H(12)(xyz)⋯N1(1 − x,
1/2 + y, 1/2 − z) = 2.70 Å. Compounds 3 and 4 are the only
ones in the 1–6 series in which a close contact exists
between a C–H hydrogen atom and a bridge nitrogen atom;
the relative positions occupied by these nitrogen atoms in
the bridge, differing in terephthaldehyde derivative 3 and
phenylenediamine derivative 4, thus appear to have little
bearing on whether or not C–H contacts to these N atoms
can be made.
Fig. 8 View of molecular packing in compound 2 along [010] (left)
and turned to show 3-F⋯bridge-H contacts (right). Hydrogen atoms
not involved in these contacts have been omitted for clarity. See
Table 3 and text for intermolecular distances and angles.
with N-(3-fluorobenzylidene)aniline (which could be considered
roughly half a molecule of 5) is not possible because the
compound could not be obtained in crystalline form. On the
other hand, a topological similarity between the 3-F⋯4-H
based motif in 5 and a corresponding F⋯F motif in the
Collas et al. structure UKUNUK can be identified if the role
of the 4-H atom on the fluorophenyl ring of 5 is played by
the 4-F atom in UKUNUK (Fig. 9). This kind of motif is not
found in UKUPEW, even though both 5 and UKUPEW pos-
sess the phenylenediamine core while UKUNUK possesses
the terephthaldehyde core. It is possible to form an inter-
action topologically similar to the 3-F⋯4-H motif in 5 by
exchanging the positions of the F and H atoms on the ring.
This interaction thus should be possible for para-fluorinated
derivatives and is in fact found in the Kaur et al. structure
4-fluoro-N-(4-fluorobenzylidene)aniline; however, it is not
found in the para-fluorinated compounds 3 and 6 (section 2.3).
Isomers 3 and 6, although different in overall crystal struc-
ture, possess strikingly similar intermolecular packing motifs
involving the fluorine atom if the intermolecular approaches
under consideration are allowed to extend by 0.10 Å beyond
the sum of the van der Waals radii.26 In 3 (Fig. 11(a)), both
fluorine atoms are simultaneously part of two five-membered-
ring, pincer-type interactions. The C4–F1 and C5–H5 bonds
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3/2 − y, −1/2 + z) = 2.9654(13) A; F1(xyz)⋯H13(−1 + x, 3/2 − y,
−1/2 + z) = 2.67 Å. In 6 (Fig. 11(b) and Fig. 11(c)), the two
independent molecules both engage in fluorine-containing
pincer-type interactions similar to those found in 3 but
differing in the symmetry involved (glide symmetry for 3,
screw-axial symmetry for 6). For one of these molecules, the
C1–F1 and C2–H2 bonds are directed toward F1 of a neigh-
boring molecule: F1(xyz)⋯F1(1 − x, −1/2 + y, 3/2 − z) =
3.0253(16) Å; H2(xyz)⋯F1(1 − x, −1/2 + y, 3/2 − z) = 2.68 Å,
while F1 is approached by the C1–F1 and C2–H2 bonds of
a second neighboring molecule: F1(xyz)⋯F1(1 − x, 1/2 + y,
3/2 − z) = 3.0253(16) Å; F1(xyz)⋯H2(1 − x, 1/2 + y, 3/2 − z) =
2.68 Å. For the other independent molecule, the C18–F2 and
C19–H19 bonds are directed toward F2 of a neighboring
molecule: F2(xyz)⋯F2(−x, 1/2 + y, 3/2 − z) = 3.0343(16) Å;
H19(xyz)⋯F2(−x, 1/2 + y, 3/2 − z) = 2.76 Å, while F2 is approached
by the C18–F2 and C19–H19 bonds of a second neighboring
molecule: F2(xyz)⋯F2(−x, −1/2 + y, 3/2 − z) = 3.0343(16) Å;
F2(xyz)⋯H19(−x, −1/2 + y, 3/2 − z) = 2.76 Å.
Fig. 10 Anisotropic ellipsoid (50% probability) plots of (a) compound 3
and (b) two crystallographically independent molecules of compound 6,
HEWHAU, showing atom numbering.
This dual-pincers motif is also found in the similarly bis-para-
fluorinated compound 4-fluoro-N-(4-fluorobenzylidene)aniline,
but it is not found in the other para-fluoro substituted com-
pounds 3-fluoro-N-(4-fluorobenzylideneaniline), 2-fluoro-N-(4-
fluorobenzylidene)aniline,
line, 4-fluoro-N-(2-fluorobenzylidene)aniline, N-benzylidene-4-
fluoroaniline (approximately half molecule of 3), or
4-fluoro-N-(3-fluorobenzylidene)ani-
a
N-(4-fluorobenzylidene)aniline (approximately half a molecule
of 6) examined by Kaur et al. No analogous motif is apparent
in the Collas et al. structures UKUNUK and UKUPEW.
3 Experimental
Compounds 1–5 were prepared using a standard method,
condensation of one equivalent of the “core” compound
(for 1–3, terephthaldehyde; for 4 and 5, phenylenediamine)
with two equivalents of the corresponding fluoroaniline deriv-
ative (for 1–3) or fluorobenzaldehyde derivative (for 4 and 5)
by warming the reaction mixture for 15–30 minutes in ethanol
solution. Crystals were obtained either by slow cooling of
the ethanolic reaction mixture or by recrystallization of the
crude solid from ethanol (for 1, 2, 3, and 5) or from 1 : 1
methanol : chloroform (for 4). For single-crystal X-ray studies,
compound 1 was obtained as yellow needles, mp 421–422 K;
compound 2 was obtained as yellow blocks, mp 395–396 K;
compound 3 was obtained as colorless plates, mp 423–424 K;
compound 4 was obtained as yellow plates, mp 390–391 K, and
compound 5 was obtained as yellow needles, mp 401–403 K.
Data sets were collected at 173 K using MoKα radiation
(λ = 0.71073 Å) and SMART27 (for 1) or APEXII28 (for 2–5)
software on a Siemens (for 1) or Bruker (for 2–5) CCD dif-
fractometer. Data reduction was accomplished using SAINT27
(for 1) or APEX228 (for 2–5). Absorption corrections were
applied to 1–5 using SADABS.27 Structure solution and refine-
ment were performed using SHELXTL.29 All non-hydrogen
atoms were refined anisotropically; all hydrogen atoms
were placed in calculated positions (riding model). As noted
Fig. 11 Views of dual-pincer intermolecular approaches in (a) com-
pound 3 and in (b), (c) compound 6, HEWHAU. Hydrogen atoms not
involved in these contacts have been omitted for clarity. See Table 3
and text for intermolecular distances and angles.
of a given molecule are directed toward F2 of a neighboring
glide-related molecule: F1(xyz)⋯F2(−1 + x, 1/2 − y, −1/2 + z) =
2.9786(13) Å; H5(xyz)⋯F2(−1 + x, 1/2 − y, −1/2 + z) = 2.67 Å,
while F1 is also approached by the C14–F2 and C13–H13
bonds of a second glide-related molecule: F1(xyz)⋯F2(−1 + x,
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previously, a low-intensity residual peak in the difference map
of 2 indicated disorder in the position of the fluorine atom
corresponding to a 180 degree rotation of the 3-fluorophenyl
group about the C–N bond. Modeling this disorder by
assigning two positions for the fluorine atom while applying
the restraint of equal anisotropic displacement parameters
for the two fluorine sites resulted in component percentages
after refinement of 95% and 5%. Analysis and plotting of
the structures were performed using PLATON30 and Mercury.
Acknowledgements
Support of this work by the National Science Foundation
through grant DMR05-20415 administered by the Materials
Research Facilities Network (MRFN) is gratefully acknowl-
edged. Acknowledgement for their support is also made to the
donors of the American Chemical Society Petroleum Research
Fund and to the University of St. Thomas Department of
Chemistry. The authors thank Dr. Victor G. Young, Jr.
(Director) of the X-ray Crystallographic Laboratory of the
University of Minnesota Department of Chemistry for his
assistance and Dr. Doyle Britton (Department of Chemistry) and
Dr. William B. Gleason (Department of Laboratory Medicine
and Pathology) of the University of Minnesota for helpful
discussions.
4 Conclusion
Our examination of 1–6 has not provided any new examples
of isomorphous bridge-flipped isomers. Instead, it has dem-
onstrated that structural features such as molecular con-
formation and intermolecular contacts that must be closely
similar in order for bridge-flipped isomeric pairs to assume
the same molecular packing arrangement actually display wide
variability in these bis-benzylideneanilines. Their conforma-
tional flexibility ensures that even when a centrosymmetric
conformation is possible for these molecules, no guarantee
exists that both isomers will assume it. The occurrence of
fluorine-based supramolecular synthons in the molecular
packing is undoubtedly significant with respect to how the
molecules of individual bis-benzylideneanilines self-assemble,
but the influence of these synthons apparently does not
extend to compelling isomorphism between bridge-flipped
isomeric bis-benzylideneanilines. Although we have found
one particular fluorine-containing supramolecular synthon,
the 2-F⋯bridge-H contact, to be sufficiently robust to occur
in a variety of benzylideneanilines of both the single-bridge
and bis-type, it is not sufficiently robust to occur in both of
the 2-fluorinated bridge-flipped bis-benzylideneanilines 1 and
4 and contribute to their isomorphism. At the same time,
the fact that its occurrence is not specific to only one bridge
orientation (2-fluoroaniline vs. 2-fluorobenzylidene) – i.e. to
one position of the “synthon switch” – may limit its useful-
ness to some extent as a reliable supramolecular synthon in
future crystal engineering applications. In the one instance
in the 1–6 series, namely 3 and 6, in which a pair of bridge-
flipped isomeric benzylideneanilines do engage in similar
fluorine-implicated intermolecular interactions, these interactions
are insufficient to cause the two isomers to assume the same
packing arrangement. The occurrence of particular fluorine-
containing supramolecular synthons in the crystal structures
of those single-bridge benzylideneanilines that can be con-
sidered “half-molecules” of analogous bis-benzylideneanilines
has proved in our examples not to be a reliable predictor of
which fluorine-containing supramolecular synthons will occur
in the related bis-benzylideneaniline. Ultimately, it appears
that flexibility in assuming a variety of molecular conforma-
tions and capability of engaging in a variety of fluorine-
containing supramolecular synthons, both factors that might
facilitate isomorphism, instead have been used by the bis-
benzylideneanilines in our study to avoid it.
Notes and references
1 W. H. Ojala, J. M. Smieja, J. M. Spude, T. M. Arola,
M. K. Kuspa, N. Herrera and C. R. Ojala, Acta Crystallogr.,
Sect. B: Struct. Sci., 2007, 63, 485–496.
2 Isomorphism between bridge-flipped amides has been
reported; see for examples (a) P. Mocilak and J. F. Gallagher,
CrystEngComm, 2011, 13, 5354–5366; (b) D. Chopra and
T. N. Guru Row, CrystEngComm, 2008, 10, 54–67.
3 G. Ferguson, C. Glidewell, J. N. Low, J. M. S. Skakle and
J. L. Wardell, Acta Crystallogr., Sect. C: Cryst. Struct. Commun.,
2005, 61, o613–o616.
4 Two such pairs are (a) the 4-CH3-4′-Cl benzylideneanilines:
T. R. Welberry, B. D. Butler and A. P. Heerdegen, Acta
Chim. Hung., 1993, 130, 327–345; I. Bar and J. Bernstein,
Acta Crystallogr., Sect. B: Struct. Sci., 1983, 39, 266–272;
K. J. Haller, A. D. Rae, A. P. Heerdegen, D. C. R. Hockless
and T. R. Welberry, Acta Crystallogr., Sect. B: Struct. Sci.,
1995, 51, 187–197; J. Harada, M. Harakawa and K. Ogawa,
Acta Crystallogr., Sect. B: Struct. Sci., 2004, 60, 589–597; (b)
the 4-CH3–4′-NO2 benzylideneanilines: H. B. Bürgi,
J. D. Dunitz and C. Züst, Acta Crystallogr., Sect. B: Struct.
Crystallogr. Cryst. Chem., 1968, 24, 463–464; H. B. Bürgi and
J. D. Dunitz, Helv. Chim. Acta, 1970, 53, 1747–1764;
M. B. Hursthouse and A. Karaulov, 2003, private communication
to CCDC; J. Harada, M. Harakawa and K. Ogawa,
Acta Crystallogr., Sect. B: Struct. Sci., 2004, 60, 578–588;
O. S. Filipenko, V. I. Ponomarev and L. O. Atovmyan,
Dokl. Akad. Nauk SSSR, 1976, 229, 1113–1116; J. M. Cole,
J. A. K. Howard and G. J. McIntyre, Acta Crystallogr.,
Sect. B: Struct. Sci., 2001, 57, 410–414; V. I. Ponomarev,
O. S. Filipenko, L. O. Atovmyan, S. S. Grazhulene,
S. A. Lempert and V. D. Shigorin, Kristallografiya, 1977, 22,
394–396; O. S. Filipenko, V. D. Shigorin, V. I. Ponomarev,
L. O. Atovmyan, Z. Sh. Safina and B. L. Tarnopol'skii,
Kristallografiya, 1977, 22, 534–541.
5 (a) A. I. Kitaigorodskii, in Organic Chemical Crystallography,
Consultants Bureau, New York, 1961, pp. 172 and 429; (b)
A. I. Kitaigorodskii, in Molecular Crystals and Molecules,
Academic Press, New York, 1973, p. 35; (c) C. P. Brock and
7234 | CrystEngComm, 2014, 16, 7226–7235
This journal is © The Royal Society of Chemistry 2014

View Article Online
CrystEngComm
J. D. Dunitz, Chem. Mater., 1994, 6, 1118–1127; (d) F. H. Allen
Paper
18 (a) R. Berger, G. Resnati, P. Metrangolo, E. Weber and
J. Hulliger, Chem. Soc. Rev., 2011, 40, 3496–3508; (b)
A. R. Choudhury and T. N. Guru Row, CrystEngComm,
2006, 8, 265–274; (c) I. Y. Bagryanskaya, Y. V. Gatilov,
A. M. Maksimov, V. E. Platonov and A. V. Zibarev, J. Fluorine
Chem., 2005, 126, 1281–1287; (d) A. R. Choudhury and
T. N. Guru Row, Cryst. Growth Des., 2004, 4, 47–52; (e)
V. R. Vangala, A. Nangia and V. M. Lynch, Chem. Commun.,
2002, 1304–1305; (f) M. L. Renak, G. P. Bartholomew,
S. Wang, P. J. Ricatto, R. J. Lachicotte and G. C. Bazan, J. Am.
Chem. Soc., 1999, 121, 7787–7799; (g) J. D. Dunitz and R. Taylor,
Chem.–Eur. J., 1997, 3, 89–98; (h) J. A. K. Howard, V. J. Hoy,
D. O'Hagen and G. T. Smith, Tetrahedron, 1996, 52, 12613–12622;
(i) L. Shimoni and J. P. Glusker, Struct. Chem., 1994, 5, 383–397.
19 J. D. Dunitz, ChemBioChem, 2004, 5, 614–621.
and W. D. S. Motherwell, Acta Crystallogr., Sect. B: Struct. Sci.,
2002, 58, 407–422; (e) J. W. Yao, J. C. Cole, E. Pidcock,
F. H. Allen, J. A. K. Howard and W. D. S. Motherwell,
Acta Crystallogr., Sect. B: Struct. Sci., 2002, 58, 640–646; (f)
D. Britton and W. B. Gleason, Acta Crystallogr., Sect. C:
Cryst. Struct. Commun., 2003, 59, o439–o442.
6 F. H. Allen, Acta Crystallogr., Sect. B: Struct. Sci., 2002, 58,
380–388.
7 W. H. Ojala, T. M. Arola, N. Herrera, B. Balidemaj and
C. R. Ojala, Acta Crystallogr., Sect. C: Cryst. Struct. Commun.,
2007, 63, o207–o211.
8 S. Chakraborty, R. H. Laye, R. L. Paul, R.G. Gonnade,
V. G. Puranik, M. D. Ward and G. K. Lahiri, J. Chem. Soc.,
Dalton Trans., 2002, 1172–1179.
9 SANYIP: N. Hoshino, T. Inabe, T. Mitani and Y. Maruyama,
Bull. Chem. Soc. Jpn., 1988, 61, 4207–4214; SANYIP01: T. Inabe,
N. Hoshino, T. Mitani and Y. Maruyama, Bull. Chem. Soc. Jpn.,
1989, 62, 2245–2251; SANYIP02: S.-S. Zhang, S.-J. Ye, T. Zhang
and X.-M. Li, Chem. Res. Chin. Univ., 2004, 20, 562–564.
10 A. Collas, R. De Borger, T. Amanova and F. Blockhuys,
CrystEngComm, 2011, 13, 702–710.
11 H. M. Cho, J. S. Moore and S. R. Wilson, Acta Crystallogr.,
Sect. E: Struct. Rep. Online, 2005, 61, o3773–o3774.
12 D. A. J. Harding, E. G. Hope, K. Singh and G. A. Solan,
Polyhedron, 2012, 33, 360–366.
20 K. Reichenbächer, H. I. Süss and J. Hulliger, Chem. Soc. Rev.,
2005, 34, 22–30.
21 G. Kaur, P. Panini, D. Chopra and A. R. Choudhury,
Cryst. Growth Des., 2012, 12, 5096–5110.
22 Z. Fang and C. Cao, J. Mol. Struct., 2013, 1036, 447–451.
23 K. Donnelly, J. F. Gallagher and A. J. Lough, Acta Crystallogr.,
Sect. C: Cryst. Struct. Commun., 2008, 64, o335–o340.
24 (a) P. M. Zorky, J. Mol. Struct., 1996, 374, 9–28; (b) P. M. Zorkii,
V. K. Bel'skii, S. G. Lazareva and M. A. Porai-Koshits,
J. Struct. Chem., 1967, 8, 267–270.
25 A. Bondi, J. Phys. Chem., 1964, 68, 441–451.
13 M. Odabasoglu, O. Büyükgüngör, K. Sunil and B. Narayana,
Acta Crystallogr., Sect. E: Struct. Rep. Online, 2007, 63, o4145–o4146.
14 W. Thyen and P. Zugenmaier, Cryst. Res. Technol., 1994, 29,
449–456.
15 H. Li, Y. Xiang and H. Han, Acta Crystallogr., Sect. E: Struct.
Rep. Online, 2011, 67, o2953.
16 K. Merz and V. Vasylyeva, CrystEngComm, 2010, 12, 3989–4002.
17 W. H. Ojala, K. M. Lystad, T. L. Deal, J. E. Engebretson,
J. M. Spude, B. Balidemaj and C. R. Ojala, Cryst. Growth
Des., 2009, 9, 964–970.
26 C. F. Macrae, P. R. Edgington, P. McCabe, E. Pidcock,
G. P. Shields, R. Taylor, M. Towler and J. van de Streek,
J. Appl. Crystallogr., 2006, 39, 453–457.
27 Bruker, SMART, SAINT, and SADABS, 2004, Bruker AXS, Inc.,
Madison, Wisconsin, USA.
28 Bruker, APEX2, 2005, Bruker AXS, Inc., Madison, Wisconsin, USA.
29 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr.,
2008, 64, 112–122.
30 A. L. Spek, Acta Crystallogr., Sect. D: Biol. Crystallogr., 2009,
65, 148–155.
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