Crystal structure, thermal behavior, and photochemical reactivity of a series of co-crystals of trans-1,2-bis(4-pyridyl) ethylene with dicarboxylic acids

Article abstract of DOI:10.1021/cg200681s

A series of co-crystals of trans-1,2-bis(4-pyridyl) ethylene (bpe) with aliphatic saturated or unsaturated dicarboxylic acids, of the type [(bpe)(L)] (L = dicarboxylic acid), were prepared and characterized by single crystal X-ray crystallography. In all cases chains of the type acid???bpe? ??acid were formed via strong O-H???N hydrogen bonds. The chains were further linked between them by weak C-H???O hydrogen bonds, π-π and herringbone interactions to form three-dimensional structures. Thermal analysis showed considerable elevation of the melting/decomposition point of the co-crystals with respect either to both components or to bpe, ascribed to the formation of multiple complementary C-H???O bonds as well as to π-π interactions between the two conformer molecules. When L = fumaric acid, the two components stack separately with their C=C bonds strictly parallel between them at 3.83 A from one another. However, photodimerization reaction takes place only between the olefin bonds of bpe. Photodimerization reaction between one C=C bond of the acid and that of bpe was also induced when L = trans,trans-muconic acid.

Full text of DOI:10.1021/cg200681s

Article
pubs.acs.org/crystal
Crystal Structure, Thermal Behavior, and Photochemical Reactivity of
a Series of Co-Crystals of trans-1,2-Bis(4-pyridyl) Ethylene with
Dicarboxylic Acids
Kostas Tsaggeos, Natalia Masiera, Aleksandra Niwicka, Vassiliki Dokorou, Michael G. Siskos,*
Stavroula Skoulika,* and Adonis Michaelides*
Department of Chemistry, University of Ioannina, 45110 Ioannina, Greece
*
S Supporting Information
ABSTRACT: A series of co-crystals of trans-1,2-bis(4-pyridyl) ethylene
bpe) with aliphatic saturated or unsaturated dicarboxylic acids, of the type
(bpe)(L)] (L = dicarboxylic acid), were prepared and characterized by single
(
[
crystal X-ray crystallography. In all cases chains of the type acid···bpe···acid
were formed via strong O−H···N hydrogen bonds. The chains were further
linked between them by weak C−H···O hydrogen bonds, π−π and
herringbone interactions to form three-dimensional structures. Thermal
analysis showed considerable elevation of the melting/decomposition point of the co-crystals with respect either to both
components or to bpe, ascribed to the formation of multiple complementary C−H···O bonds as well as to π−π interactions
between the two conformer molecules. When L = fumaric acid, the two components stack separately with their CC bonds
strictly parallel between them at 3.83 Å from one another. However, photodimerization reaction takes place only between the
olefin bonds of bpe. Photodimerization reaction between one CC bond of the acid and that of bpe was also induced when L =
trans,trans-muconic acid.
INTRODUCTION
Scheme 1. (a) Glutaric Acid, (b) 2,2′-Thiodiacetic Acid, (c)
,2′-Selenodiacetic Acid, (d) Fumaric Acid, (e) Glutaconic
Acid, (f) trans,trans-Muconic Acid, (g) 1,4-
■
2
The control of crystal architecture is an essential step for the
assembly of molecular crystals with desirable physical or
1
Phenylenediacrylic Acid
chemical properties. For this purpose, it is well established that
weak noncovalent directional forces are needed, such as
2
hydrogen bonding or π−π interactions. In terms of molecular
structure, this imposes the presence of appropriate functional
groups, such as carboxylic or amide groups, heteroatoms,
aromatic rings, etc. In this respect, the concept of “supra-
molecular synthons” is a useful tool for analyzing patterns of
3
interaction in organic crystals. The location of the interacting
functional groups or heteroatoms on the molecule is also a
crucial factor in crystal design as relatively small modifications
of molecular structure may bring about dramatic modifications
4
on crystal packing and/or physical or chemical properties.
We undertook a systematic study on the assembly of organic
crystals through co-crystallization, as this process is a promising
route for the fabrication of compounds with improved
5
6
pharmaceutical activity novel molecular materials and reactive
7
molecular solids. In this work, we chose systems containing
unsaturated carbon−carbon bonds, potentially capable of
undergoing photochemical cycloaddition reactions. This is
achieved by bringing in close distance (up to 4.20 Å), via co-
and was exploited extensively in the crystal engineering of
1
0
organic co-crystals.
The molecular structure of the
7
8
components suggests that chains would be preferentially
crystal or salt formation, the unsaturated −CC− bonds, to
11
formed by use of the above-mentioned heterosynthon. In
satisfy Schmidt’s criterion for topochemical photocycloaddition
9
reaction. For this purpose, we undertook the assembly of co-
crystals of trans-1,2-di(4-pyridyl) ethylene (bpe), with a variety
of dicarboxylic acids (co-formers, Scheme 1). The carboxylic
acid···pyridine heterosynthon (Scheme 2) is particularly robust
Received: May 30, 2011
Revised: March 30, 2012
Published: April 10, 2012
©
2012 American Chemical Society
2187
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Article
2
through a quartz lens, a Philips (2 × 30) low pressure lamp operating
at 254 nm and a laser (20 pulses, 40 mJ) operating at 193 nm. In all
cases it was verified that the PXRD pattern of the powdered samples
was identical to the simulated ones.
Scheme 2. Cyclic R (7) Hydrogen Bond (left) and Simple
2
O−H···N Hydrogen Bond (right)
X-ray Crystallography. The X-ray diffraction intensities for
compounds 1−7 were collected at room temperature on a Bruker
P4 diffractometer, employing graphite monochromated Mo Kα
radiation. The structures were solved by direct methods and refined
on F2 using SHELX97. All non-hydrogen atoms were refined
anisotropically. All hydrogen atoms of compounds 1, 2, 5, 7, the
carboxylic hydrogen for compounds 3 and 6 and the olefin proton of
bpe for compound 6, were located from difference Fourier maps and
refined isotropically. All remaining hydrogen atoms were placed on
calculated positions and refined by using the riding model. In the case
of compound 7, one ethylene carbon of bpe appeared to be disordered
over two positions with occupancy factors 0.7 and 0.3. Because of the
existence of a center of symmetry in the middle of the double bond,
the whole bond appears to be disordered over two positions with the
above-mentioned occupancy factor. This type of shrinking of the
double bond has been observed at r.t. in many compounds having the
E-stilbene skeleton. It represents a “pedal” motion, that is, a rotation
by 180° of the double bond C1−CC−C1, around the C1···C1
direction, substantially not affecting the position of the phenyl rings to
which belong atoms C1. The occupancy factors observed in our case
the absence of flanking constituents the formation of
supramolecular sheets via C−H···O hydrogen bonds should
1
2
be promoted, as observed in many co-crystals involving
1
1,13
carboxylic acids and dipyridines.
Furthermore, it was also
shown that the supramolecular sheets are usually stacked along
the third dimension via face-to-face π−π aromatic interactions.
Of course, this does not guarantee that olefin bonds belonging
to neighboring sheets will align properly to satisfy Schmidt’s
criterion. However, based on the above-mentioned works,
especially refs 13b−d, we expected that, by using dicarboxylic
acids of various lengths, it could be possible to achieve, at least
in some cases, the proper alignment of the double bonds via a
favorable interplay of C−H···O and π−π interactions. Finally,
the use of organic diacids of different or slightly different
chemical structures gave us the opportunity to draw
correlations between thermal behavior and crystal architecture,
16
are similar to those found in the literature for r.t. studies.
14
an important issue in pharmaceutical and molecular materials.
RESULTS AND DISCUSSION
EXPERIMENTAL SECTION
■
■
All co-crystals were characterized by IR spectroscopy, X-ray
crystallography, and DSC analysis. They were also investigated
for possible photochemical reactivity.
Synthesis of 2,2′-Selenodiacetic Acid. The synthesis was
15
carried out according to the literature. To a moderately heated
mixture composed of 1.00 g of Se, 21 mL of H O, and 21 mL of THF,
2
1
.12 g of NaBH were slowly added. The mixture was allowed to cool,
4
IR Spectroscopy. FT-IR spectra were recorded (see
Supporting Information) in order to investigate whether
some characteristic vibrations could be used for easy
identification of these compounds. Indeed, in all cases there
is an intense peak around 1605 cm− due to the stretching
vibration of the −CC− bond of the aromatic ring of bpe, and
an intense peak around 1700 cm− due to the stretching
vibration of the CO bond of the acid. Additionally, in all co-
under stirring, for about 3 h. After removal of the solid precipitate, 30
mL of water and a small quantity of petroleum ether were added to the
supernatant, followed by liquid−liquid extraction. The aqueous phase
was acidified with excess HCl and allowed to dryness. The solid
1
residue was treated with CHCl and heated until ebullition occurs. The
3
white precipitate was removed by filtration and the supernatant was
allowed to evaporate slowly, yielding small, prismatic single crystals of
the organic acid; mp 103−104 °C.
1
−1
(
bpe)(2,2′-Thiodiacetic Acid), 1. A mixture of bpe (0.0073 g, 4 ×
crystals there is a broad shoulder around 1900 cm and a
−
5
−5
−1
10
mol) and 2,2′-thiodiacetic acid (0.0060 g, 4 × 10 mol) was
broad peak around 2380 cm characteristic of the O−H···N
17
dissolved in 2 mL of solvent (THF, EtOH, MeOH). In all cases,
crystals of 1 were obtained within a few days, by slow evaporation of
the solution; mp 220−222 °C.
hydrogen bond.
X-ray Crystallography. Crystallographic and structure
refinement data for compounds 1−7 are given in Table 1. In
(
Bpe)(Glutaric Acid), 2. As compound 1, by using 0.0053 g (4 ×
−
5
all cases, chains of the type ···acid···bpe···acid··· are formed via
10
mol) of acid and EtOH or MeOH as solvents; mp 197−198 °C.
2
2
either the cyclic R (7) H-bond (compounds 4, 6, 7) or the
(
bpe)(2,2′-Selenodiacetic Acid), 3. As compound 1, by using
−5
0
.0079 g (4 × 10 mol) of acid and EtOH or MeOH as solvents; mp
strong H-bond of the type O−H···N (compounds 1−3, 5)
(Figure 1, Scheme 2). We considered as the distance cutoff for
a C−H···O interaction d(C···O) ≤ 4 Å and d(H···O) ≤ 3 Å and
1
88−190 °C.
(
Bpe)(Glutaconic Acid), 4. As compound 1, by using 0.0052 g (4
5
−
18
×
10 mol) of acid and EtOH as solvent; mp 177−179 °C.
angle cutoff (C−H···O) > 120°. The geometrical character-
(
bpe)(trans,trans-Muconic Acid), 5. As compound 1, by using
istics of these H-bonds are given in Table 2.
−5
0
.0057 g (4 × 10 mol) of acid and EtOH as solvent; mp 283−286
Crystal Structure of Compounds 1−3. Compounds 1−3
are isostructural (Table 1). The asymmetric unit contains half
of the two components. The central atom of the acid (S, Se, or
C) lies on a 2-fold axis, while the middle of the −CC− bond
of bpe lies on an inversion center. The hydrogen bonded chains
acid···bpe···acid formed (see above) are connected between
them by multiple C−H···O bonds. By means of these bonds,
each acid molecule links four different chains (Figure 2).
Additionally, neighboring molecules of bpe interact between
them by π−π aromatic interactions, involving either two
aromatic rings or one aromatic ring and one double bond
°
C.
(
bpe)(1,4-Phenylenediacrylic Acid), 6. As compound 1, by
5
−
using 0.0088 g (4 × 10 mol) of acid and DMF as solvent; mp 310−
14 °C.
Bpe)(Fumaric Acid), 7. A mixture of bpe (0.1820 g), fumaric acid
0.1162 g), DMF (4 mL), and water (6 mL) was heated for 72 h, in a
3
(
(
Teflon-lined hydrothermal reactor at 70 °C. Upon slow cooling, large
single crystals of 7 were formed; mp 280−282 °C.
Physical Measurements. DSC analysis was performed on a
Schimadzu 60 apparatus under N stream (30 mL/min), at a scan rate
2
of 10 °C/min. Melting points have been determined as the onset
temperatures of endotherms. FT-IR spectra were recorded on a
Perkin-Elmer spectrum GX FT-IR spectrophotometer, using the KBr
(
Figure 2). The latter type of interaction is rarely mentioned in
1
13
the literature, although it seems to play an important role in
technique. H NMR and C NMR were recorded on a Bruker AMX-
50 MHz spectrometer. Irradiation experiments were carried out with
an OSRAM 400 W high pressure Hg lamp, by focusing its light
19
2
biological systems. Neighboring double bonds of bpe are
nearly parallel (their planes form a dihedral angle in the range
2
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Figure 1. Chains of the type acid···bpe···acid formed via a cyclic
hydrogen bond (compound 6) (bottom) and a simple O−H···N
hydrogen bond (compound 5) (top). Color codes: N (green), O
(red), C (gray), S (yellow).
Table 2. Geometrical Characteristics of the Cyclic Hydrogen
Bonds for Compounds 1−7
D−H···A
D···A (Å)
H···A (Å)
D−H···A (°)
1
2
3
4
O2−H8···N
O2−H8···N
O2−H10···N
O3−H3···N3
C4−H4···O4
O2−H2···N12
C13−H13···O
O2−H7···N
O2−H2···N
C5−H5···O1
O2−H2···N
C5−H5···O1
2.708(5)
2.572(3)
2.572(10)
2.547(6)
3.304(7)
2.606(6)
3.376(7)
2.610(4)
2.657(6)
3.274(6)
2.617(5)
3.229(5)
1.54
1.57
1.65
1.73
2.67
1.80
2.74
1.65
1.67
2.69
1.80
2.54
168
168
148
171
126
169
126
158
175
121
176
131
5
6
7
Figure 2. View of the structure of compound 1. Each chain interacts
with four neighboring ones, located on two layers above and below the
chain, via C−H···O hydrogen bonds (top), inside a layer the molecules
of bpe interact by aromatic interactions (bottom).
5
.9−9.9° for compounds 1−3) and the distance between their
centers is in the range 4.62−4.80 A, too far for topochemical [2
2] cycloaddition reaction. Indeed, no photochemical reaction
+
was observed upon 1 h of irradiation.
Crystal Structure of Compound 4. In an attempt to bring
the olefin bonds more closely, we used glutaconic acid (scheme
1
e), which is a partly planar molecule. Furthermore, we hoped
that the asymmetry of this molecule could probably lead to a
polar structure of potential interest to nonlinear optics.
20
2
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Indeed, this co-crystal crystallizes in the polar space group Pc
Crystal Structure of Compound 5. In order to further
improve the relative alignment of the olefin bonds, we chose as
co-former the trans,trans-muconic acid which has a planar
carbon backbone (Scheme 1f) and, in the present crystal
structure, adopts a strictly planar conformation. Both
components are centrosymmetric. The chains acid···bpe···acid
form planar layers through C−H···O bonds.
(
Table 1). Moreover, in this case, the absence of inversion
center in the acid brings about the formation of zigzag chains
with a zigzag angle of about 109°. Parallel chains are arranged
along b to form planar layers (Figure 3). In this case, the olefin
As the two components are of approximately equal length,
they are arranged, inside a layer, in an alternant way (Figure
13b
4
). Neighboring layers interact further through C−H···π, C−
Figure 3. View of the structure of compound 4. The zigzag
acid···bpe···acid chains form layers bound mainly via aromatic
interactions (top). Chains belonging to adjacent layers interact
between them by C−H···O hydrogen bonds (bottom).
Figure 4. View of the structure of compound 5. The chains form
planar layers via multiple C−H···O hydrogen bonds (top). Adjacent
layers interact between them mainly by aromatic interactions (bottom)
The arrow indicates the pair of double bonds undergoing [2 + 2]
cycloaddition photoreaction.
bonds of bpe are parallel between them with a center-to-center
distance equal to the b axis (4.52 Å) and an offset angle δ
21
H···O, and π−π interactions (Figure 4). The latter type of
interaction involves aromatic ring−aromatic ring, aromatic ring-
double bond and double bond−double bond interactions. The
distance between the olefin bond of bpe and one olefin bond of
the acid is 4.01 Å, with an angle of 33° between their planes.
Photochemical [2 + 2] cycloaddition reaction between these
bonds was observed upon 1 h of irradiation (see below).
Crystal Structure of Compound 6. In an attempt to
obtain a better overlap of π-orbitals, we used 1,4-phenyl-
enedicarboxylic acid (Scheme 1g) as co-former. Both
components are centrosymmetric. As in the previous case,
planar layers are formed via C−H···O bonds (Figure 5).
However, in this case, most of these bonds are formed between
the molecules of the acid. The chains are further linked by face-
to-face aromatic interactions between the bpe molecules and
herringbone, instead of the more favorable for olefin alignment,
face-to-face interactions, between the aromatic rings of the acid
and of bpe (Figure 5). As a consequence, no favorable contacts
between olefin bonds are observed, confirmed by the absence
of any photochemical reactivity upon 1 h of irradiation.
equal to 37.8° (where δ is the angle formed between the center-
to-center vector of a pair of double bonds and the
perpendicular from the center of one bond to the plane of
the other and obtains the value 0° for perfect overlap), while
the olefin bonds of the acid are also parallel between them with
the same center-to-center distance and δ equal to 33.5°. The
layers are linked between them by multiple C−H···O bonds,
leading, thus, to a 3D structure (Figure 3). The geometry of the
olefin bonds is more favorable for [2 + 2] cycloaddition
reaction, with respect to compounds 1−3.
Nevertheless, we observed no reaction upon 1 h of
irradiation. The 4.52 Å distance is above the limit generally
quoted for topochemical reactions (4.20 Å). However,
9
Schmidt had pointed out that “the limit of 4.2 Å is set by
the absence of experimental data in the range 4.2−4.7 Å, and is
not to be construed necessarily as an upper limit of
dimerizability. At 4.7 Å and above the reaction does not take
place.”
Actually, there are some examples of compounds with olefin
separation in the range 4.2−4.7 Å, which are photochemically
Crystal Structure of Compound 7. In this compound, the
coformer is fumaric acid (Scheme 1d). Both components are
centrosymmetric. The packing of the acid···bpe···acid chains is
similar to that of compounds 1−3 (Figure 6). The main
2
2
active. We estimate that, in this case, the zigzag arrangement
of the chains does not allow the olefin bonds to get closer for
photoreaction to occur.
2
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and propose some interpretations based on the structural
results. The corresponding diagrams are shown in Figure 7. In
all cases melting of compounds 1−7 was accompanied by
decomposition. The existence of a double melting/decom-
position peak in the case of 3 prompted us to carry out
complementary TGA experiments, which showed the existence
of two partially overlapped decomposition events which proved
impossible to separate completely (Figure S8, Supporting
Information). The differences between the melting/decom-
position temperatures of the co-crystals and of the correspond-
ing co-formers are summarized in Table 3. In the case of the
isostructural co-crystals 1−3, there is significant elevation of the
melting/decomposition temperature with respect to both
components (Figure 7a−c). In this group we observe that the
higher the melting point of the co-former acid the higher the
melting point of the corresponding co-crystal, in agreement
24
with the results found by others. This is also in line with the
packing coefficients of these compounds. What is the structural
origin of these observations?
Figure 5. View of the structure of compound 6. The chains form layers
via multiple C−H···O hydrogen bonds (top). Chains belonging to
adjacent layers interact between them mainly by aromatic interactions
A plausible answer is that highly organized solid state
(
bottom).
25
structures may account for high melting points. Indeed, the
numerous weak C−H···O bonds and π−π interactions, not
15,26
present in the crystal structures of the pure compounds,
stabilize mutually the two components, accounting for the
elevation of the melting point. The remaining co-crystals 4−7
are not isostructural and therefore possible interpretation of
thermal behavior is more tentative, as many other parameters
such as symmetry, entropy, robustness, size, and shape of
individual components also play a role in thermal stability. We
presume, however, that the existence of numerous weak
interactions may account for the elevation of the melting
point of the co-crystals with respect to that of bpe.
Photochemical Reactivity of Compounds 5 and 7. A
powdered sample of 5 was irradiated for 1 h, with a 400 W high
1
pressure lamp (focused radiation). The H NMR spectrum
(
DMSO-d solvent) (Figure 8) shows that about 50% of the
6
starting material is converted to the corresponding cyclobutane
derivative. The [2 + 2] photodimerization takes place between
the double bond of bpe and one of the double bonds of
trans,trans-muconic acid (see description of the structure of 5),
leaving the second double bond as substituent to the
cyclobutane ring. The photodimer shows peaks at 8.47 ppm
and at 7.24 ppm attributed to the aromatic protons of pyridine
rings, a doublet of doublets at 6.63 ppm (J = 15.5 and 9.25 Hz,
respectively) which is attributed to the vinyl proton of the
carbon connected to the cyclobutane ring while the other vinyl
proton appears as a doublet at 5.85 ppm (d, J = 15.5 Hz). The
cyclobutane protons showed peaks at 4.64, 4.48, and 3.80 ppm
in the form of triplets (Figure 8). The fourth proton of the ring
is covered by the strong solvent absorption. The remaining
Figure 6. View of the structure of compound 7. The arrangement of
the chains is similar to that of compounds 1−3 although the two
components, in this case, are stacked more efficiently than in 1−3. The
arrow indicates the pair of double bonds undergoing [2 + 2]
cycloaddition photoreaction.
difference between them is that, in this case, the two
components are stacked more efficiently than in compounds
1
−3, through stronger π−π interactions (Figure 6). The olefin
3
5
bonds of each component are strictly parallel between them
peaks at 8.60 and 7.61 ppm (dd, J = 4.8 Hz and J = 1.6 Hz) as
well as at 7.54 ppm are due to the unreacted bpe and those
appearing at 7.37 and 6.33 ppm to the unreacted trans,trans-
muconic acid. Further investigation of the product obtained is
in progress.
with a center-to-center distance equal to the length of the a axis
(
3.829 Å) and an offset angle δ equal to 22.7°. It is worth
noting that the short length of the molecule of fumaric acid
allows formation of C−H···O bonds involving also the olefin
protons of bpe. This geometric arrangement fulfils the criteria
for photochemical [2 + 2] cycloaddition reaction for both
components (see below). As far as we know, there are only two
previous examples in which both components of the co-crystal
are potentially capable of undergoing photochemical [2 + 2]
A powdered sample of 7 was irradiated with a 400 W high
pressure lamp (focused radiation), and the progress of the
1
13
photoreaction was investigated by H NMR and C NMR
spectroscopy. The photodimer 1,2,3,4-tetra(4-pyridyl)-
cyclobutane (Scheme 3) was exclusively formed, as evidenced
23
1
27
cycloaddition reaction.
in the H NMR spectra (Figure 9), by the gradual
disappearance of the olefinic singlet at 7.54 ppm, as well as
the doublets of aromatic protons at 8.60 and 7.61 ppm (dd, J =
Thermal Analysis. Differential scanning calorimetry (DSC)
was used to determine the thermal behavior of co-crystals 1−7
2
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Crystal Growth & Design
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Figure 7. DSC diagrams of the co-crystals (green) 1 (a), 2 (b), 3 (c), 4 (d), 5 (e), 6 (f), 7 (g) plotted together with those of bpe (black) and of the
co-former acid (red). Temperature is in °C.
Table 3. Melting Point and Packing Coefficient Data for
Compounds 1−7
Δm.p.
co-
crystal-
bpe)
(°C)
Δm.p.
(co-
crystal-
acid)
(°C)
(
m.p.
cocrystal
(°C)
m.p.
bpe
m.p.
acid
packing
coefficient
(%)
compound
(°C) (°C)
1
2
3
4
5
6
7
220
155
155
155
155
155
155
135
101
103
134
293
387
233
65
85
72.0
70.9
71.4
68.2
71.1
70.9
71.5
197
42
96
a
188, 199
177
33, 44
22
85, 96
43
283
128
−10
310, 380
280
155, 225 −77, −7
125 47
155
15
1
a
Figure 8. H NMR spectrum of compound 5 after 1 h of irradiation.
Reported value 109 °C.
4
.8 Hz and J = 1.6 Hz) and the appearance of a singlet at 4.66
(CO) and 133.97 ppm in 13C NMR remained unchanged,
confirming that only bpe is photodimerized. This is easily
explained by the fact that the molecular extinction coefficient of
bpe is much higher than that of fumaric acid. As a consequence,
upon irradiation of the material, bpe starts first to dimerize
while the fumaric components bound to the reactive pair of
bpes move in opposite directions and the distance between the
centers of the olefin bonds reaches about 6 Å (Scheme 3).
Indeed, further irradiation for about 1 h did not modify the
reaction products. In the case of co-crystals of 2,2′-bpe with
ppm (cyclobutane ring) and the shifting of aromatic protons at
.33 and 7.22 ppm (dd, J = 4.8 Hz and J = 1.6 Hz), respectively.
These peaks are consistent with the rctt stereochemistry of the
8
13
cyclobutane ring (Scheme 3). Furthermore, in the C NMR
7
f
spectra (Figure S10) the chemical shifts of 150.17, 143.38,
30.60, and 121.26 ppm, assigned to trans-1,2-bis(4-pyridyl)-
1
ethylene, decrease while new chemical shifts attributed to the
cyclobutane product appear at 149.23, 148.15, 123.30, and
4
4.38 (C−H of cyclobutane ring) ppm. The chemical shifts of
1
23
fumaric acid at 6.63 ppm (singlet) in H NMR and 167.97
fumaric or mesaconic acid, the two co-formers are stacked in
2
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Scheme 3. 1,2,3,4-Tetra(4-pyridyl)cyclobutane Is Formed by
Dimerization of bpe, upon Irradiation of 7
The main influence of the dimerization in the IR spectrum of 7
a
(Figure S7, Supporting Information) is that the double broad
−1
peak at about 2362 cm , due to the stretching vibration of the
17
O−H···N hydrogen bond, becomes more intense and single.
This is probably because, upon dimerization, the cyclic
hydrogen bond is broken and a simple O−H···N hydrogen
bond is formed.
CONCLUSION
■
In conclusion, a series of co-crystals of bpe with saturated or
olefinic dicarboxylic acids of various lengths as co-formers were
prepared as single crystals either by slow solvent evaporation or
a
The molecules of fumaric acid (blue) move away from one another
during photoreaction.
hydrothermally. As expected, chains were formed via either
2
robust cyclic R (7) or strong O−H···N hydrogen bonds,
2
such a way that concomitant [2 + 2] cycloaddition could occur.
However, the photodimer from 2,2′-bpe was the main product
and this was ascribed to the major absorption capability of 2,2′-
bpe (at 302 nm) with respect to the acids. Moreover, in this
case the photoreaction between the acid molecules was possible
because, in our opinion they do not move as apart as in our
case. Irradiation of 7 at 193 nm by using a laser (≈15 min, 40
mJ) and at 254 nm with a low pressure lamp, resulted again in
dimerization of bpe only. The maximum yield of the
photodimerization reaction was about 80%. This value may
be related to the shrinking of the double bond of bpe present in
this structure (see Experimental Section). The reaction
proceeds smoothly upon irradiation of single crystals with
ordinary lamp but is accompanied by severe damaging of the
crystal, prohibiting structure determination by X-ray diffraction.
It is worth noting that the same photochemical reaction takes
place without irradiation, just by exposing the sample to the
laboratory sunlight for about 3 months (conversion ≈ 60%).
further linked between them by weak noncovalent interactions
to afford 3D extended structures. As a consequence, the melting
point of all co-crystals was either higher than its pure individual
components or between them. It is worth noting that
symmetric acids (axis 2 for 1−3, center of symmetry for 5−
7) generated centrosymmetric structures, whereas the asym-
metric glutaconic acid (co-crystal 4) induced formation of a
polar structure. Moreover, acids with planar conformation (co-
crystals 5−7) bring the olefin bonds of molecules belonging to
neighboring chains at closer distance (3.83 Å to 4.60 Å) than
those that do not possess such a planarity. Photochemical [2 +
2] cycloaddition reaction was induced only in 5 and 7 which
conform to Schimdt’s topochemical postulate. However, in the
case of co-crystal 7, in which the two components stack
separately with their olephin bonds strictly parallel between
them at 3.83 Å from one another, only bpe was photo-
dimerized, presumably because of its high absorption
coefficient.
16
1
Figure 9. H NMR spectra of compound 7 at different irradiation times.
2
193
dx.doi.org/10.1021/cg200681s | Cryst. Growth Des. 2012, 12, 2187−2194

Crystal Growth & Design
Article
Tocher, D. A.; Vickers, M.; Karamertzanis, P. G.; Price, S. L. Cryst.
Growth Des. 2009, 9, 2881.
ASSOCIATED CONTENT
Supporting Information
■
*
S
(
11) Pedireddi, V. R.; Chatterjee, S.; Ranganathan, A.; Rao, C. N. R.
Tetrahedron 1998, 54, 9457.
12) Sharma, C. V. K.; Panneerselvam, K.; Pilati, T.; Desiraju, G. R. J.
Eleven figures containing the IR spectra of compounds 1−7,
the IR spectrum of 7 at different irradiation times, the 13
NMR spectrum of compound 7, the TG diagram of compound
and one diagram containing the progress of the cycloaddition
C
(
Chem. Soc., Perkin Trans. 1993, 2209.
3
(13) (a) Shan, N.; Bond, A. D.; Jones, W. Cryst. Eng. 2002, 5, 9.
(b) Zhang, J.; Wu, L.; Fan, Y. J. Mol. Struct. 2003, 660, 119. (c) Zhang,
J.; Ye, L.; Wu, L.-X. Acta Crystallogr. 2005, C61, 038. (d) Ebenezer, S.;
Muthiah, P. T. J. Mol. Struct. 2011, 990, 281.
reaction of compound 7. This material is available free of
charge via the Internet at http://pubs.acs.org.
(
14) (a) Schultheiss, N.; Newman, A. Cryst. Growth Des. 2009, 9,
AUTHOR INFORMATION
Corresponding Author
E-mail: vskoul@uoi.gr (S.S.); amihail@uoi.gr (A.M.);
msiskos@cc.uoi.gr (M.G.S.).
■
*
2
6
950. (b) Cincic, D.; Friscic, T.; Jones, W. Chem. Mater. 2008, 20,
623.
(15) Doudin, K. I.; Berge, R. K.; Borve, K. J.; Songstad, J.; Tornrous,
K. W. J. Mol. Struct. 2000, 554, 149.
16) Galli, S.; Mercandelli, P.; Sironi, A. J. Am. Chem. Soc. 1999, 121,
767.
17) Aakeroy, C. B.; Salmon, D. J.; Smith, M. M.; Desper, J. Cryst.
(
3
(
Notes
The authors declare no competing financial interest.
Growth Des. 2006, 6, 1033.
(18) Desiraju, G. R..; Steiner, T. The Weak Hydrogen Bond; Oxford
University Press: New York, 1999.
ACKNOWLEDGMENTS
We thank Dr. A. Ikiades for carrying out the laser irradiations.
■
(19) Tsumoto, K.; Yokota, A.; Tanaka, Y.; Vi, M.; Tsumuraya, T.;
Fujii, I.; Kumagai, I.; Nagumo, Y.; Oguri, H.; Inoue, M.; Hirama, M. J.
Biol. Chem. 2008, 283, 12259.
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