Experimental and theoretical study of aromaticity effects in the solid state architecture on squaric acid derivatives

Article abstract of DOI:10.1021/cg500264k

In this manuscript, the aromaticity of the four-membered ring in squaric acid derivatives and, particularly, how it changes when it participates in noncovalent interactions are studied combining experimental observations and theoretical analysis using the

Full text of DOI:10.1021/cg500264k

Article
pubs.acs.org/crystal
Experimental and Theoretical Study of Aromaticity Effects
in the Solid State Architecture on Squaric Acid Derivatives
Rafel Prohens,*,† Anna Portell,† Merce Font-Bardia,‡ Antonio Bauza,§ and Antonio Frontera*,§
̀
́
‡
†Unitat de Polimorfisme i Calorimetria, Centres Científics i Tecnolog
̀ ́
ics, Unitat de Difraccio de Raigs X, Centres Científics i
̀
Tecnologics, Universitat de Barcelona, Baldiri Reixac 10, 08028 Barcelona, Spain
§Departament de Química, Universitat de les Illes Balears, Crta. de Valldemossa km 7.5, 07122 Palma (Baleares), Spain
S
* Supporting Information
ABSTRACT: In this manuscript, the aromaticity of the four-membered ring in squaric acid derivatives and, particularly, how it
changes when it participates in noncovalent interactions are studied combining experimental observations and theoretical analysis
using the nucleus independent chemical shift (NICS). Moreover, the ability of the squaric acid and its derivatives to establish
hydrogen bonds, π−π stacking, and other π-interactions (anion−π, lone pair−π, and C−H/π) is related to the increase in the
aromaticity of the ring. In addition, several squaric acid derivatives (benzamh)(sqah)·H2O (1), (pipropamh2)(sqa)·2H2O (2),
(deamh)(deamsqa) (3), (phpetamh)(sqah)·(4), (amsqa)(sqa) (5), (and benzamh)(benzamsqa)·H2O (6) where benzam =
benzylamine; sqah2 = squaric acid; deamsqah = diethylamidosquaric acid; pipropam = 3,3′-(piperazine-1,4-diyl)bis(propan-
1-amine); deam = diethylamine phpetam = p-hydroxiphenylethylamine and benzamsqah = benzylamidosquaric acid) were
synthesized and characterized by single crystal X-ray diffraction analyses.
squaric acid/squaramide compounds. The aim of this
contribution is to extend the knowledge about the forces that
govern these specific π-stacked assemblies by combining X-ray
characterization and computational analysis of several squarate
and amidosquarate salts (see Scheme 1). In particular, we focus
our attention to analyze if the aromatic character of the four-
membered ring influences the solid state architecture of these
mono and dianion salts directing the type of noncovalent
interactions. Previously, a very interesting experimental and
theoretical investigation focused on alkaline squarate salts
[M2(C4O4)] (M = Li, Na, K, and Rb) was reported devoted to
correlate the structures, vibrational analysis, and aromaticity.12
In that work, the significant role of hydrogen bonding interac-
tions to water molecules was examined, and it was shown that
aromaticity and hydrogen bonds are the most important forces
driving the interactions in the solid structures of squarate ions.
In the present work, we examine the propensity of squaramide,
1. INTRODUCTION
The cyclic oxocarbon acids (CnOnH2) and their conjugated bases,
the oxocarbon dianions (CnOn)2−, have been widely inves-
tigated both theoretically and experimentally due to their highly
functionalized and interesting structures.1 In their pioneering
work, West et al.2 recognized cyclic oxocarbon dianions as
aromatic systems and suggested that they constitute a series of
aromatic compounds. The latter generalization has been
reinvestigated by others and questioned for the oxocarbons in
general,3 although there are arguments in favor of aromaticity in
the smallest members of the oxocarbon dianions series, the
deltate and squarate dianions.4 It should be mentioned that
squaramides have recently attracted attention in several fields
such as catalysis,5 host−guest chemistry,6 and transmembrane
transport.7 It is a useful supramolecular synthon to generate
interesting architectures in the solid state.8 Actually, the
utilization of squarate and hydrogen squarate salts is common
in crystal engineering9 and organic material research.10
Some of us have recently studied8 the electrostatic
compression phenomenon11 in order to explain the face-to-
face π-stacked architectures shown by a series of zwitterionic
Received: February 21, 2014
Revised: March 28, 2014
Published: April 8, 2014
© 2014 American Chemical Society
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Scheme 1. Squarate and Amidosquarate Salts 1−6 Studied in This Work
8.7 mmol) in water (25 mL). Single crystals of 4 were separated by
filtration and analyzed by single X-ray diffraction.
hydrogen squarate, and squarate salts in the solid state to
establish interactions that are typical of aromatic compounds13
(stacking interactions,14 ion−π,15 C−H/π,16 etc.). As far as our
knowledge extends, this type of analysis has not been previously
performed in the literature, especially the ability of the four-
membered ring to participate in anion−π (or lone pair−π)
and C−H/π interactions. This ability can be understood as
an experimental consequence of the aromaticity of the four-
membered ring, which is an experimentally nonmeasurable
property.
We have also studied by means of density functional theory
(DFT) calculations the aromatic character of squaric acid
and its derivatives. Moreover, we have studied changes in the
aromatic character of the ring upon complexation with
hydrogen bond donors, acceptors, and both simultaneously.
An increase in the aromatic character of the four-membered
ring upon complexation can be related to its ability to
concurrently form other types of noncovalent interactions in
the solid state involving the π-system of the ring. We have
used the Cambridge Structural Database17 and the new crystal
structures reported herein to analyze this topic. Finally, among
the known criteria for aromaticity, we have used Schleyer’s18
nucleus independent chemical shift (NICS) because it does
not require the use of complicated equations and fictitious
reference systems.1a
Synthesis of 5. Compound 5 was obtained from a 1:1 mixture of
squaric acid (0.10g, 0.88 mmol) and 3,4-bis(2-dimethylamino-
ethylamino)-cyclobut-3-ene-1,2-dione (0.23 g, 0.88 mmol) in water
(10 mL). The suspension was stirred for 4 h, and it was cooled to r.t.
The needle shaped crystals of 5 were filtered and analyzed by single
X-ray diffraction.
Synthesis of 6. Compound 6 was prepared from a 1:2 mixture of
squaric acid (0.21 g, 17.6 mmol) and benzilamine (1.89 g, 17.6 mmol)
in water (6 mL). The suspension was refluxed for 3 h, and the solution
was slowly cooled to r.t. Some crystals appeared after 3 days at r.t. and
one prismatic crystal was analyzed by single X-ray diffraction.
2.3. X-ray Crystallographic Analysis. Single crystal X-ray
diffraction intensity data of the title complexes were collected using
a MAR345 diffractometer with an image plate detector, equipped with
graphite monochromated Mo Kα radiation (λ = 0.71073 Å). Lorentz-
polarization and empirical absorption correction SADABS19 were
applied. The structure of the title compounds were solved by direct
method and refined by the full-matrix least-squares technique on F2
with anisotropic thermal parameters to describe the thermal motions
of all non-hydrogen atoms using the programs SHELXS97 and
SHELXL97,20 respectively. All calculations were carried out using
PLATON21 and WinGX system Ver-1.64.22 All hydrogen atoms were
located from difference Fourier map and refined isotropically. A
summary of crystal data and relevant refinement parameters is given in
Table 1.
2.4. Theoretical Methods. The energies of all compounds and
complexes included in this study were optimized at the M06-2X/6-
311+G* level of theory within the program Gaussian-09.23 NICS
values were obtained at the same level of theory by calculating the
absolute NMR shielding using the GIAO method24 at the ring centers,
at 0.5 and 1.0 Å above them in order to reduce the local shielding of
nearby σ-bonds which complicates the analysis for small rings. The
maximum diatropic ring current effect for benzene is in the center of
the ring but is offset by the paratropic contribution of the C−H and
C−C σ-bonds. These paratropic effects decrease more rapidly than the
diatropic ones away from the center. Therefore, NICS values above the
ring minimize the paratropic effect and give a more reliable indication
of aromaticity. As it is mentioned below, the NICS value computed at
0.5 Å above the ring centroid is used for the discussion of the results.
We have selected this distance because the protonated squaric acid
derivatives have their maximum value of NICS at 0.5 Å, and then it
decreases rapidly at higher distances (see Table 2). In contrast, for the
rest of compounds (neutral and anionic) the variation of the value of
NICS from 0.5 to 1 Å is very small. Therefore, the utilization of
NICS(0.5) values is a good compromise to deal simultaneously with
protonated, neutral, and anionic squaric acid derivatives.
2. EXPERIMENTAL AND THEORETICAL METHODS
2.1. Materials and Measurements. All chemicals used were
reagent grade and used as received from Sigma-Aldrich.
2.2. Synthesis. Synthesis of 1. Compound 1 was prepared from
a 1:1 mixture of squaric acid (2.01 g, 17.6 mmol) and benzilamine
(1.89 g, 17.6 mmol) in water (25 mL). The suspension was refluxed
for 3 h, and the solution was slowly cooled to r.t. The resulting crystals
were filtered, washed with acetone (2 × 5 mL), and dried under a
vacuum overnight. The yield was 97%. Single crystals of 1 were
analyzed by single X-ray diffraction.
Synthesis of 2. Compound 2 was prepared from a 1:1 mixture of
squaric acid (0.50 g, 4.4 mmol) and 1,4-bis(3-aminopropyl)piperazine
(0.44 g, 2.2 mmol) in water (10 mL). The yield was 80%. Single
crystals of 2 were analyzed by single X-ray diffraction.
Synthesis of 3. Compound 3 was prepared from a 1:1 mixture of
squaric acid (0.20 g, 1.7 mmol) and diethylamine (1.28 g, 17.5 mmol)
in water (10 mL). The suspension was stirred for 4 h, and the solid
was recrystallized in water at 70 °C by slowly cooling to 5−8 °C.
Single crystals of 3 were filtered and analyzed by single X-ray
diffraction.
3. RESULTS AND DISCUSSION
3.1. Preliminary Theoretical Study. We have first studied
the effect of both protonation and deprotonation of squaric
Synthesis of 4. Compound 4 was prepared analogously to 1 from a
1:1 mixture of squaric acid (1.09 g, 9.6 mmol) and tyramine (0.94 g,
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contrast the squarate dianion (13) and monoanions 14 and 15
are moderately aromatic. In fact compound 14 has exactly the
half NICS value compared to benzene, thus indicating a very
modest aromatic character. The squarate dianion exhibits a
complete equalization of the C−C bonds due to equal popula-
tion of the four resonance forms shown at the bottom of
Scheme 2. The aromatic resonance form has a strong charge
separation and consequently is likely not important. These
anionic compounds are clear examples where structural criteria
for aromaticity based either on bond length (e.g., Julg26
aromaticity index) or on bond order considerations (e.g., Bird27
and Jug28 indexes) are not adequate since they predict high
aromatic character for squarate dianion. It should be also
emphasized that the aromatic resonance form of squarate
dianion can be stabilized electrostatically by the countercation
and/or hydrogen bonding interactions and, consequently,
increasing the relative importance of this resonance form.
We have also computed a series of dimers and trimers based
on squaric acid and squaramide in order to investigate the
influence of the hydrogen bonding on the aromatic character.
They are represented in Scheme 3, and the NICS(0.5) values
are summarized in Table 3. From the inspection of the results,
some interesting issues arise. First, in the squaric acid dimer
(16), the aromatic character of ring A decreases (H-bond
donor) and, conversely, it increases in ring B (H-bond
acceptor), which is in agreement with the expectation taking
into account that the di-O-protonated squaric acid is as
aromatic as benzene and that the squarate dianion is only
moderately aromatic. Second, for squaramide both rings
increase their aromatic character in the dimer (17) compared
to the monomer. A likely explanation is that ring A upon com-
plexation reduces the delocalization of the lone pair of electrons
on nitrogen into the ring, and, consequently, the aromaticity of
the ring increases. This result agrees with the experimental
hydrogen bond acceptor/donor dual character of squaramide
Table 2. Computed NICS (ppm) at the M06-2X/6-311+G*
Level of Theory for Compounds 7−15
compound
NICS(0)
NICS(0.5)
NICS(1)
7
−1.9
−2.3
−0.3
−7.2
−8.1
−6.8
0.8
−5.9
−6.4
−5.7
−10.0
−10.0
−10.2
−5.6
−4.8
−5.4
−9.6
−5.0
−5.4
−5.4
−6.6
−6.2
−6.9
−6.0
−5.3
−5.1
−10.4
8
9
10
11
12
13
14
1.4
15
−0.7
−7.4
benzene
acid derivatives (compounds 7−9, see Scheme 2) on their
aromatic character, and the results are gathered in Table 2. For
comparison purposes and to make the discussion tractable,
we have included the NICS value of benzene at the same level.
It has previously assigned the convention that if a molecule has
less than half of the magnitude of NICS value in comparison to
that of benzene (NICS(0.5) = −9.6 ppm), it is not aromatic.25
In Table 2, we have included the NICS values at several
distances, and it can be clearly observed that the NICS
computed at the ring center is clearly affected by paratropic
effects in the four-membered rings. In addition, the com-
putation of NICS at 1 Å is not adequate for the four-membered
rings since the ring current diatropic effect vanishes. Therefore,
the NICS(0.5) values will be used for the discussion of
aromaticity in the squaric acid and its derivatives. The NICS
values computed for the neutral compounds 7−9 indicate that
they are moderately aromatic, in agreement with previous
results.1 The di-O-protonated compounds 10−12 are aromatic
(approximately equal to benzene) because the 2e-aromatic
resonance form is predominant (see right of Scheme 2). In
Scheme 2. Compounds 7−15 Studied in This Work and Resonance Forms of a Generic Di-O-protonated Compound and
Squarate Dianion 13
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Scheme 3. Dimers 16−18 and Trimers 19−21 Studied in This Work
monoanion 14, which also increases its aromatic character in
the self-assembled dimer 18. Third, the central ring C of the
squaric acid trimer (19), which simultaneously participates
in hydrogen bonding interactions as donor and acceptor, very
modestly increases its aromatic character compared to 10.
Remarkably, the central ring C of the squaramide trimer (20)
significantly increases its aromaticity. Therefore, the simulta-
neous participation of the squaramide in H-bonding
interactions as donor and acceptor contributes to increase the
aromaticity of the four-membered ring, confirming previous
results observed for dimer 17. Finally, the central ring in trimer
21 significantly increases its aromatic character with respect to
the naked dianion (13); therefore, the squarate dianion is also
expected to form π-interactions in the solid state.
Table 3. Computed NICS(0.5) (ppm) at the M06-2X/
6-311+G* Level of Theory for Compounds 16−19
a
compound
A
B
C
16
17
18
19
20
21
−5.1 (0.8)
−7.2 (−0.8)
−5.2 (−0.4)
−5.3 (0.6)
−7.4 (−1.0)
−4.6 (1.3)
−6.6 (−0.7)
−7.2 (−0.8)
−5.2 (−0.4)
6.8 (−0.9)
−7.6 (−1.2)
−4.6 (1.3)
−6.1 (−0.2)
−7.7 (−1.3)
−7.1 (−1.5)
a
The NICS variation with respect to the monomers 7−15 is indicated
in parentheses.
derivatives, acting as binding units in several receptors for both
anions and cations. We have also examined the dimer of the
Figure 1. CSD search results for squaramide (top) and squaric acid (bottom), and partial views of some X-ray structures retrieved from the search.
The CSD codes are indicated. Distances in Å.
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Figure 2. CSD search results for mono squarate anion and partial views of some X-ray structures retrieved from the search. The CSD codes are
indicated. Distances in Å.
We have examined the Cambridge Structural Database, to
learn the solid state features of squaric acid. In particular we
focused our study on analyzing the noncovalent interactions
involving the four-membered ring and comparing them to the
typical aromatic interactions (π−π, ion−π, C−H/π). This type
of analysis that relates the aromaticity to the ability to form
anion−π, lone pair−π, and C−H/π interactions is unprece-
dented and can be understood as an experimental confirmation
of the aromaticity of the four-membered ring. We have per-
formed several searches that have been restricted to only
organic compounds; therefore, organometallic compounds have
not been considered. The results and some examples are shown
in Figures 1−3, and we have divided the searches into neutral,
anionic, and dianionic systems. For squaric acid, we have found
15 X-ray structures in the CSD, and in only two of them the
π-system of the four-membered ring is a mere spectator.
In the rest of structures, it participates in typical aromatic
π-interactions, including π−π stacking (8 hits), lp(anion)−π (3
hits), and C−H/π (1 hit) interactions. Some selected examples
for each type of binding are shown in Figure 1 (bottom). For
the squaramide search, a similar result is found since in only 5
of a total of 31 structures the four-membered ring does not
participate in π-interactions. The rest of structures present
Figure 3. CSD search results for squarate dianion and partial views of
some X-ray structures retrieved from the search. The CSD codes are
indicated. Distances in Å.
interesting assemblies in the solid state dominated by a
combination of H-bonds and π-interactions (π−π, C−H/π, and
lp(anion)−π). This result agrees well with the theoretical study
(vide supra) that showed a significant increase in the aromatic
character of the squaramide ring when it participates in
H-bonding interactions as donor and acceptor, which is the
case of the WECCAK structure (Figure 1).
noted the importance of this result, since a strong electrostatic
repulsion between the dianionic squarate moieties is expected.
Obviously, the counterions also play a prominent role in the
final structure. Relevant examples are included in Figure 3,
and in all cases the squarate forms a three-component assembly
resembling complex 20 studied above. In this complex, ring C
(squarate dianion) increased its aromatic character significantly.
Therefore, the ability of this ring, even in its formal dianionic
form, to participate in π−π interactions is related to its aromatic
character.
3.2. Description of Squaric Acid Derivatives 1−6. We
have synthesized and characterized six new squaric salts. The
structural analysis of five out of the six new compounds reveals
well-defined crystal architectures through π−π stacking in
combination with charge-assisted hydrogen bonding interac-
tions as expected. The packing motif is similar to the ones
presented by our previously published8 zwitterionic squaramido
compounds and consists of a variety of electrostatically com-
pressed assemblies. In an effort to correlate the solid state
architecture of compounds 1−6 and the aromaticity of the four-
membered ring, we have performed further theoretical
The results for the monoanionic search are presented in
Figure 2. A significant number of structures have been found
(78), and in most of them the squarate ring participates in π−π
stacking interactions. As an example, the ODEZOO structure
is shown in Figure 2 that forms in the solid state the self-
assembled dimer that we have previously studied theoretically.
As aforementioned, the aromatic character of this dimer (18)
increases with respect to the monomer (14) and likely increases
the ability to establish short π−π interactions as observed in
ODEZOO structure (3.23 Å). The number of structures that
form C−H/π and lp(anion)−π interactions is lower (7 and 10,
respectively), and two selected examples are shown in Figure 2.
It should be emphasized that only in four structures the four-
membered ring does not establish any type of π-interaction.
Finally, the results for the dianionic search are gathered in
Figure 3. In this case we have found 51 structures, and in only
12 of those the ring does not participate in π-interactions. In 26
structures the ring participates in π−π interactions. It should be
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Figure 4. Solid state structures of compounds 1−6 with indication of the NICS(0.5) values.
Figure 5. Crystal structure of the dianions 2 (a) and 5 (b) showing centroid−centroid distances (water molecules not shown).
calculations. We have specially focused our attention on the
interplay between H-bonding interactions in the molecular
plane (σ-interactions) and the aromaticity of the ring and,
consequently, π-interactions. As described above, both dianions
(2 and 5) and monoanions 1, 4, and 6 participate in π−π
stacking interactions, and, concurrently, several hydrogen
bonds are also present. Obviously the final solid state
architecture is dominated by an intricate combination of
interactions, and we have analyzed some of them. In Figure 4
we represent the six salts with indication of the NICS(0.5)
values. Interestingly all compounds exhibit higher aromatic
character than the isolated mono- and dianions 13−15 as a
consequence of the interaction with the counterion. Moreover,
the squaramide ring in compound 5 is also more aromatic
than compound 8 (−6.4 ppm, see Scheme 2 and Table 2) as a
consequence of the π−π stacking interaction with squaric acid
dianion.
The crystal structure of the dianion 2 shows a distorted
electrostatically compressed assembly of disquarates in a zigzag
fashion with a centroid−centroid distance of 4.05 Å. Squarate
dianions are bridged with others from equivalent assemblies via
strong H-bonds with water molecules (see Figure 5). On the
other hand, 5 forms discrete assemblies of two squaramides and
two disquarate rings establishing two different π−π stacking
interactions between them, with a centroid−centroid distance
3.95 Å for the squaramide-squarate and 3.63 Å for squaramide−
squaramide interactions. Strong charge-assisted NH···O inter-
actions between both kind of rings are also established.
Squarate dianions are bridged with disquaramide molecules via
strong H-bonds with water molecules.
In compound 1, the squarate monoanions form infinite
chains by means of O···H−O interactions as previously
reported29 that are interconnected through parallel displaced
π−π interactions with a centroid−centroid distance of 3.71 Å.
CH−π interactions between aromatic hydrogens and mono-
squarate ring are also observed with a centroid−aromatic
hydrogen distance of 3.39 Å (see Figure 6). In this case water
molecules establish strong H-bonds simultaneously with both
monoanions and benzilammonium cations.
Similarly, in compound 4 infinite chains are also formed (see
Figure 7); however the donor−acceptor hydrogen bonding
groups are in a less frequent30 relative position 1,3 than the 1,2
position in compound 1 (see Figure 6b). In this compound the
stacked four-membered rings display a centroid−centroid
distance of 3.33 Å, and strong H-bonds are formed between
phenol and squarate groups.
In compound 3 we observe two different C−H/π
interactions (centroid−alkyl hydrogen distances of 3.32 and
3.40 Å, respectively) and a strong H-bond between the anionic
oxygen atom and the protonated amine; however, no π−π
stacking interactions are established (see Figure 8).
Finally, in compound 6, stacked dimers of the monoanion
with a centroid−centroid distance of 3.44 Å are formed.
Interestingly, the usual self-complementary squaramide/
squaramide interaction through H-bonding is formed in a
non-coplanar fashion (see Figure 9).
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Figure 6. Crystal structure of 1 showing centroid−centroid distance (a) and H-bonding chains (b) (water molecules not shown).
Figure 9. Crystal structure of 6 showing centroid−centroid distances
(water molecules not shown).
Figure 7. Crystal structure of 4 showing centroid-centroid distance (a)
and H-bonding chains (b).
chains formed by the squarate monoanions in compounds 1
and 4. It can be observed that the central squarate presents a
NICS value of −5.3 ppm in the model of 1 and −5.7 ppm in
the model of 4. The NICS value (see Table 1) for compound
14 (squarate monoanion) is −4.8 ppm, indicating that the
formation of the infinite chain increases the aromaticity of
the ring and the ability to establish π−π interactions, as
corroborated by the solid state architecture of both compounds.
We have also analyzed the NICS value in a model of com-
pounds 3 and 6, where the amidosquarate anion forms a strong
H-bond with the ammonium counterion and concurrently
establishes a C−H/π interaction. The NICS value slightly
increases from −5.4 to −5.6 ppm upon formation of the
hydrogen bond in agreement with previous results. In order to
validate the models used for 3 and 6, we have also performed
calculations using the solid state geometries of these salts and
evaluated the NICS(0.5) values in the absence and presence of
the counterion (see Figure 10, right). The results using the
crystallographic coordinates also indicate a significant increase
in the aromatic character of the ring if the countercation is
included in the calculation, thus confirming the strong influence
of the ammonium cations on the aromaticity of the ring in the
X-ray structures.
Figure 8. Crystal structure of 3 showing centroid−alkyl hydrogen
distances.
We have evaluated theoretically the trimers shown in
Figure 10 that are used as models for the H-bonded infinite
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Figure 10. M06-2X/6-311+G* optimized structures of several theoretical models and X-ray structures of compounds 3 and 6. The NICS values
(ppm) of some four-membered rings are indicated.
́
(d) von Rague Schleyer, P.; Najafian, K.; Kiran, B.; Jiao, H. J. Org.
4. CONCLUSIONS
Chem. 2000, 65, 426−431 and references cited therein.
In this manuscript, the aromatic character of the four-
membered ring in squaric acid derivatives and, principally,
how it changes when it participates in noncovalent interactions
have been studied theoretically using the nucleus independent
chemical shift (NICS). Moreover, the aromaticity in these rings
has been also studied using an unprecedented criterion, which
is the analysis of the noncovalent interactions involving the ring
in the solid state that has been performed using the Cambridge
Structural Database. Apart from confirming the previously
observed ability of the four-membered ring to participate in
π-stacking interactions, it should be emphasized the significant
number of structures where the ring participated in ion−π and
C−H/π interactions that has not been previously analyzed in
detail. Moreover, several squaric acid derivatives have been
synthesized and characterized by single crystal X-ray diffraction
analyses to further analyze this topic and confirm the ability of
the squaric acid derivatives to establish hydrogen bonds and
π−π stacking interactions in the solid state.
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ASSOCIATED CONTENT
■
S
* Supporting Information
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Experimental characterization and X-ray crystallographic
information files (CIF) of the synthesized squaric acid
derivatives are available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
*(R.P.) E-mail: rafel@ccit.ub.edu.
*(A.F.) E-mail: toni.frontera@uib.es.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(10) (a) Qin, C.; Numata, Y.; Zhang, S.; Yang, X.; Islam, A.; Zhang,
K.; Chen, H.; Han, L. Adv. Funct. Mater. 2014, DOI: 10.1002/
adfm.201303769. (b) Dega-Szafran, Z.; Dutkiewicz, G.; Kosturkiewicz,
■
A.B. and A.F. thank DGICYT of Spain (CTQ2011-27512/BQU
and CONSOLIDER INGENIO CSD2010-00065, FEDER
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́ ́
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Govern Balear (Project 23/2011, FEDER funds) for funding.
Z.; Katrusiak, A.; Szafran, M. J. Mol. Struct. 2012, 1018, 28−34.
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