Supramolecular assembly and Ab initio quantum chemical calculations of 2-hydroxyethylammonium salts of para-substituted benzoic acids

Article abstract of DOI:10.1021/cg301304y

Crystal structures of seven salts of 2-hydroxyethylamine with benzoic acid (1) and different substituted benzoic acids, such as p-methylbenzoic acid (2), p-methoxybenzoic acid (3), p-hydroxybenzoic acid (4), p-chlorobenzoic acid (5), p-bromobenzoic (6), and p-iodobenzoic (7) have been studied. The salts units of 1-7 serve as building blocks (BB) of the supramolecular architecture. In a crystal they are held together via proton-transferred N-H...O and normal O-H...O hydrogen bonds. The substituents on anions influence dipole-dipole interactions between anions and cations in molecular aggregates. As a result, they are organized in building blocks either via one charge-assisted (1, 2) hydrogen bond or via two (3-7) hydrogen bonds. The dispersion interaction significantly contributes to intermolecular force fields driving the organization of hydrogen bonds in BB. In all studied compounds, building blocks are consolidated into 2-D layers through the N-H...O and O-H...O hydrogen bonds. For the crystal structures of 2-7, with non-centrosymmetric space groups and the BB self-assembled by two hydrogen bonds, the macroscopic polarizations of a unit cell is practically perpendicular to the layers.

Full text of DOI:10.1021/cg301304y

Article
pubs.acs.org/crystal
Supramolecular Assembly and Ab Initio Quantum Chemical
Calculations of 2‑Hydroxyethylammonium Salts of para-Substituted
Benzoic Acids
Manuela Crisan,† Paulina Bourosh,‡ Yurii Chumakov,*,‡ Mihaela Petric,† and Gheorghe Ilia†
†Institute of Chemistry Timisoara of Romanian Academy, 24 Mihai Viteazul Boulevard, Timisoara, 300223, Romania
‡Institute of Applied Physics, Academy of Sciences of Moldova, Academiei Street 5, Chisinau, MD - 2028, Republic of Moldova
S
* Supporting Information
ABSTRACT: Crystal structures of seven salts of 2-hydrox-
yethylamine with benzoic acid (1) and different substituted
benzoic acids, such as p-methylbenzoic acid (2), p-methox-
ybenzoic acid (3), p-hydroxybenzoic acid (4), p-chlorobenzoic
acid (5), p-bromobenzoic (6), and p-iodobenzoic (7) have
been studied. The salts units of 1−7 serve as building blocks
(BB) of the supramolecular architecture. In a crystal they are
held together via proton-transferred N−H···O and normal O−
H···O hydrogen bonds. The substituents on anions influence
dipole−dipole interactions between anions and cations in
molecular aggregates. As a result, they are organized in
building blocks either via one charge-assisted (1, 2) hydrogen
bond or via two (3−7) hydrogen bonds. The dispersion interaction significantly contributes to intermolecular force fields driving
the organization of hydrogen bonds in BB. In all studied compounds, building blocks are consolidated into 2-D layers through
the N−H···O and O−H···O hydrogen bonds. For the crystal structures of 2−7, with non-centrosymmetric space groups and the
BB self-assembled by two hydrogen bonds, the macroscopic polarizations of a unit cell is practically perpendicular to the layers.
are generally stronger than those between neutral molecules,
and the ability of cations and anions to function as hydrogen-
bonding donors or acceptors can be tuned using acid−base
chemistry. Therefore, in the present study, we investigate the
molecular packing in crystalline materials using organic salts as
molecular building blocks. Aromatic carboxylic acids have
attracted our interest because of their importance in crystal
engineering and their ability to form strong and directional
hydrogen bonds.7 Many carboxylic acids are widely used in
pharmaceutical salts and co-crystals. For example, benzoic acids,
in spite of their simple structure, present complex effects as
pharmacological, plant growth regulators, antibacterial and
antifungal agents.8 It is also known that 2-hydroxyethylamine is
a good organic base and has hydrogen-bond donor sites. On
the other hand, 2-hydroxyethylamine (ethanolamine) is an
essential component of cell membranes, closely resembling that
of choline chemical behavior. Some complexes of ethanolamine
with organic and inorganic compounds appear to be promising
for optical second harmonic generation and electrooptical
properties.9 Therefore, preparation of multicomponent organic
crystals from such multifunctional molecules as aromatic
carboxylic acids and 2-hydroxyethylamine is expected to display
INTRODUCTION
■
In recent years, there has been considerable interest in the
synthesis and complex study of multicomponent organic
crystals built from acid−base complexes. Multicomponent
crystals, e.g., solvates, hydrates, co-crystals, and salts, play an
important role in the predictable assembly of supramolecular
architectures, host−guest materials, interpenetrated networks,
solid-state reactivity, topochemical reactions, pharmaceutical
co-crystals, and polymorphism.1 Supramolecular architectures
based on noncovalent interactions are fascinating and have
attracted a great deal of attention. Central in the present study
of intermolecular interactions is the hydrogen bond, which is
the most energetic and directional bond.2 Weak interactions,
such as C−H···O, C−H···N, C−H···X (X = Cl, Br, I), are of
considerable significance (in the absence of classical hydrogen
bonds) because they play an important role in the molecular
packing arrangement. Therefore, an important application is
the use of self-assembly of small molecules with hydrogen
bonds and other weaker intermolecular interactions in creation
of one-, two-, and three-dimensional networks.3 This is why
organic salts4 have been increasingly studied in crystal
engineering, other motives being their better predictability
due to robust charge-assisted hydrogen-bonded supramolecular
synthons and their greater thermal stability compared to that of
their neutral counterparts.5 Organic salts have some advantages
over neutral organic molecules:6 hydrogen bonds between ions
Received: September 7, 2012
Revised: November 21, 2012
© XXXX American Chemical Society
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amine salts were examined using UV−vis spectrophotometer at room
temperature. UV−visible spectrum was recorded in the range of 190−
800 nm with PERKIN-ELMER LAMBDA 12 UV−vis spectrometer.
Melting points of 2-hydroxyethylammonium salts were found for finely
purifying compounds (by repeated recrystallization) using a Boetius
instrument and are uncorrected.
interesting networks and useful properties. With this back-
ground, a class of salts formed by 2-hydroxyethylamine with
aromatic carboxylic acids was prepared to investigate their
supramolecular synthons. In our previous work,6 we have
investigated the crystal structures of some organic salts
containing the same anion p-nitrobenzoic acid and different
cations, such as amino alcohols and morpholine. A structural
study revealed that those compounds form supramolecular
systems with different topologies on the basis of hydrogen
bonding arrays as a function of properties of the donor and
acceptor parts of the molecules. The objective in the present
investigation is to extend the previous work on the molecular
packing in the crystal structures of the seven 2-hydroxyethy-
lammonium salts of p-substituted benzoic acids having the same
cation. One component has a carboxyl group as an electron
donor, and the other has an amino group or a nitrogen atom as
an electron acceptor. Benzoic acids have been widely employed
in constructing supramolecular architectures mainly in
combination with organic bases, such as piperazine,10
imidazole,11 benzylamine,12 morpholinium,13 and quinoline.14
The complex formation of these compounds takes place only
via proton transfer from the carboxyl group of an organic acid
to the amino group or the nitrogen atom from the second
component. In contrast, only a few examples of crystals of
substituted benzoic acids with 2-hydroxyethylamine (ethanol-
amine) have been reported,15 including our crystals reported
earlier.6 Therefore, the searching through the Cambridge
Structural Database16,17 (version 5.33) has been performed in
order to find all salts with p-substituted benzoic acids having
the proton-transferred hydrogen bonds NH+···O− less than 2.2
Å. The total number of the found structures is 356. Among
these compounds we have observed 52, 25, 11, 4, 11, 6, 1 hits
for p-R substituted benzoic acids, where RH, OH, CH3,
OCH3, Cl, Br, I, respectively, but for the above-mentioned
substituents no compounds having the same cation have been
noticed. For of p-H, -OH, -CH3, -OCH3 substituents we found
9, 6, 3, 1 hits, respectively, where salts generate the
supramolecular synthons with different dimensions in which
the cations and anions are held together by two NH+···O1− and
XH···O2 hydrogen bonds via carboxylic group. In these
synthons, the cations are bulky and can be divided into two
groups: one group containing the fragment −OH−(CH2)n−
N+H2−, (n = 1, 2) and the other the fragment NH2−C(R)−
N+H, (RC, N), where the protonated subfragment
without NH2-group belongs to the aromatic ring.
Synthesis and Characterization of 1−7 Compounds. The title
compounds were prepared by the reaction of 2-hydroxyethylamine and
benzoic acid, p-methylbenzoic acid, p-methoxybenzoic acid, p-
hydroxybenzoic acid, p-chlorobenzoic acid, p-bromobenzoic acid and
p-iodobenzoic acid in a 1:1 molar ratio.18 Freshly distilled 2-
hydroxyethylamine was added in drops to the solution of the acid in
an appropriate solvent (diethylether for 1−4, acetone for 5−7) under
vigorous stirring at reflux for 2 h to complete the reaction. All the salts
formed were precipitated in a white crystalline state and, after cooling
at room temperature, were collected by filtration, washed with cold
diethylether, respective acetone, and dried in a vacuum for 3 h. This
reaction had a yield of about 86−93%. The salts were recrystallized to
obtain suitable crystals for the X-ray analysis by slowly cooling their
saturated ethanol solutions; the process was accompanied by a slow
evaporation of ethanol. The purity of 2-hydroxyethylamine salts was
determined via an UV spectrophotometric method, which consisted in
determining the acid concentration by measuring the UV absorbance
(in NaOH 0.1 M) at λmax of the acid.20 For the seven salts the purity
ranged between 97 and 99.5%, with an error of 0.8%.
The elemental analysis was in agreement with the expected
stoichiometry. The FT-IR spectra (KBr pellets prepared by grinding
a 5−10-mg sample with 100 mg KBr) for each of the six compounds
were consistent with salt formation.
The data of chemical/elemental analysis, IR, UV and melting point
for compounds 1−7 are provided as Supporting Information.
X-ray Measurements and Refinements. The X-ray data for 1, 3,
5, and 7 were collected at room temperature on Bruker AXS Smart
single crystal diffractometer with a CCD detector (MoKα-radiation);
for 2, 4, and 6 on Siemens P3/PC diffractometer (CuKα-radiation).
The crystallographic data and the experimental details are summarized
in Table 1. Structure solution and refinement were carried out using
the SHELX-97 programs.21 Non-hydrogen atoms were refined
anisotropically. The empirical absorption correction from the
calculated and observed structure factors has been calculated for
compounds 5−7.22 Hydrogen atoms were placed in calculated
positions with their isotropic displacement parameters riding on
those of parent atoms; the H-atoms of NH3 groups of 2-
hydroxyethylamine were found from differential Fourier maps. The
geometric parameters for hydrogen bonds are given in Table 3.
Selected bond lengths and valence angles for 1−7 are listed in
Supporting Information.
Geometric parameters have been calculated and the figures made
with the use of the PLATON software.23 Hydrogen atoms that are not
involved in hydrogen bonding were omitted from the representation of
crystal packings. Crystallographic data for compounds 1−7 were
deposited in the Cambridge Crystallographic Data Center (CCDC
853483, 887607−887609 and 853486−853488, respectively).
In the present paper, we report the synthesis, crystallographic
studies, and ab initio quantum calculations of 2-hydroxyethyl-
amine with benzoic acid (1) and different substituted benzoic
acids, such as p-methylbenzoic acid (2), p-methoxybenzoic acid
(3), p-hydroxybenzoic acid (4), p-chlorobenzoic acid (5), p-
bromobenzoic (6), and p-iodobenzoic (7). Our previous
studies reported biological activity and low-toxicity of these
compounds.18−20 Single-crystal X-ray diffraction experiments
have been carried out for all of the investigated salts in order to
analyze their supramolecular architectures.
RESULTS AND DISCUSSION
■
The 1:1 stoichiometric reaction of p-substituted benzoic acids
(1−7) with 2-hydroxyethylamine leads to the formation of
stable solid salts in accordance with the appropriate “rule of
three”. A distinction between formation of a salt and/or co-
crystal and a proper understanding of the process are very
important concerning chemical/pharmaceutical development
aspects. The pKa difference (pKa of base − pKa of acid) is a tool
for predicting salt or co-crystal formation. A larger ΔpKa
(greater than 3) will result in salt formation, and a smaller
ΔpKa (less than 0) will lead to co-crystal formation. But the
ΔpKa value ranging between 0 and 3 was unable to give a clear-
cut distinction between co-crystals and salts.25 The differences
in pKa values between the base and acid in all studied
EXPERIMENTAL SECTION
■
General. All the reagents were purchased from Fluka AG (Buchs
SG) and used without any further purification. 2-Hydroxyethylamine
was freshly distilled before any use. The FT-IR spectra were recorded
between 4000−400 cm−1 on a JASCO − FT/IR-4200 spectrometer,
using the KBr pellet technique at a scanning speed of 16 mm/s with a
resolution of 4.0 cm−1. The optical properties of the 2-hydroxyethyl-
C
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Table 2. Bond Lengths (Å) in Carboxylate Groups and Absolute Values of Their Differences
d (Å)
1
2
3A
3B
4
5
6
7
d1(C9−O2)
d2(C9−O3)
|d1-d2|
1.244(1)
1.269(1)
0.025
1.248(4)
1.269(4)
0.021
1.239(3)
1.271(3)
0.032
1.268(3)
1.249(3)
0.019
1.262(3)
1.257(3)
0.005
1.247(2)
1.271(2)
0.024
1.245(4)
1.269(4)
0.024
1.23(1)
1.25(1)
0.02
Figure 1. ORTEP drawing for 1−7 with the atomic labeling and charge-assisted hydrogen bonds represented by dashed lines. Thermal ellipsoids are
shown with the 50% probability level.
compounds were greater than 3, which determines the extent of
proton transfer (Table 3). The studied compounds are water-
soluble substances and were obtained in high yields (86−93%)
via an easy one-step synthesis. They have well-defined melting
points, generally lower for the salt forms than for the respective
benzoic acids; this shows that the neutrality achieved by salt
D
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a
Table 3. Tabulated ΔpKa and Hydrogen-Bonding Geometry (Å, °) for Compounds 1−7
numbers of HB ΔpKa
D−H···A
d(D···H), (Å) d(H···A), (Å) d(D···A), (Å) ∠(DHA), deg symmetry transformation for H-acceptor
1
1
2
3
4
5.31
5.13
5.03
N(1)−H(1)···O(2)
N(1)−H(2)···O(3)
N(1)−H(3)···O(3)
O(1)−H(1)···O(3)
0.89
0.89
0.89
0.82
1.81
2.15
2.01
1.96
2
2.693(1)
2.955(1)
2.884(1)
2.783(1)
170
150
166
176
x, y, z
−x, −y, −z
−x + 1/2, y − 1/2, z
x − 1/2, −y − 1/2, −z
5
6
7
8
N(1)−H(1)···O(2)
N(1)−H(2)···O(3)
N(1)−H(3)···O(2)
O(1)−H(1)···O(2)
0.89
0.89
0.89
0.82
1.98
1.90
2.12
2.01
3
2.865(3)
2.757(3)
2.946(5)
2.829(3)
173
163
153
176
x, y, z
−x, −y, z
x, −y, z + 1/2
−x + 1, −y, z
9
N(1A)−H(1)···O(3A)
N(1A)−H(2)···O(2B)
N(1A)−H(3)···O(2A)
O(1A)−H(1)···O(3A)
N(1B)−H(1)···O(3B)
N(1B)−H(2)···O(3A)
N(1B)−H(3)···O(2B)
O(1B)−H(1)···O(3B)
C(10A)−H(1)···O(1B)
C(10B)−H(1)···O(1A)
0.89
0.89
0.89
0.82
0.89
0.89
0.89
0.82
0.96
0.96
1.96
1.97
1.89
1.94
1.90
2.09
1.89
1.90
2.45
2.41
4
2.778(3)
2.850(3)
2.714(3)
2.691(3)
2.779(3)
2.937(3)
2.770(3)
2.711(3)
3.355(3)
3.341(3)
153
169
154
153
169
159
168
169
157
163
x − 1, y, z
−x + 1, y + 1/2, −z + 1/2
x, y, z
10
11
12
13
14
15
16
17
18
x, y, z
x − 1, y, z
x, y, z
x, y, z
x, y, z
−x + 5/2, −y + 1, z + 1/2
−x + 5/2, −y + 1, z + 1/2
19
20
21
22
23
24
25
4.96
N(1)−H(1)···O(2)
N(1)−H(2)···O(2)
N(1)−H(3)···O(3)
O(1)−H(1)···O(3)
O(4)−H(1)···O(1)
C(1)−H(1)···O(4)
C(8)−H(1)···O(2)
0.89
0.89
0.89
0.82
0.82
0.97
0.93
1.91
1.86
2.17
1.89
1.88
2.56
2.48
5
2.788(2)
2.733(2)
2.878(2)
2.690(2)
2.694(2)
3.485(3)
3.241(3)
167
167
136
165
173
160
140
x, y, z
x, y + 1, z
x, −y + 2, z − 1/2
x, y, z
x + 1/2, −y + 1/2, z + 1/2
x − 1/2, y + 1/2, z − 1
x, −y + 1, z + 1/2
26
27
28
29
30
5.52
5.5
N(1)−H(1)···O(1)
N(1)−H(2)···O(2)
N(1)−H(3)···O(3)
O(1)−H(1)···O(3)
C(1)−H(1)···O(3)
0.89
0.89
0.89
0.82
0.97
2.01
1.82
1.91
1.89
2.58
6
2.894(2)
2.701(2)
2.793(2)
2.652(2)
3.467(3)
173
172
169
155
153
x + 1/2, −y + 1/2, z
x, y, z
x + 1, y, z
x, y, z
x + 1/2, −y − 1/2, z
31
32
33
34
35
N(1)−H(2)···O(1)
N(1)−H(1)···O(2)
N(1)−H(3)···O(3)
O(1)−H(1)···O(3)
C(1)−H(1)···O(3)
0.89
0.89
0.89
0.82
0.97
2.01
1.80
1.89
1.84
2.54
7
2.896(4)
2.686(4)
2.776(4)
2.625(4)
3.426(5)
175
177
171
161
152
x + 1/2, −y + 3/2, z
x, y, z
x + 1, y, z
x, y, z
x + 1/2, −y + 1/2, z
36
37
38
39
40
5.5
N(1)−H(1)···O(1)
N(1)−H(3)···O(2)
N(1)−H(2)···O(3)
O(1)−H(1)···O(3)
C(1)−H(1)···O(3)
0.89
0.89
0.89
0.81
0.97
2.10
1.87
1.99
1.84
2.53
2.91(1)
2.66(1)
2.76(1)
2.59(1)
3.41(1)
152
146
143
154
150
x + 1/2, −y + 3/2, z
x, y, z
x + 1, y, z
x, y, z
x + 1/2, −y + 1/2, z
a
D and A are hydrogen bond donor and acceptor atoms. ΔpKa = pKa (base) − pKa (acid) calculating using the pKa data from SRC PhysProp
Database.24 All pKa values have been determined in aqueous solutions.
forming of zwitterions dramatically decreases the salt melting
points.26 The melting-point range for all synthesized salts did
not exceed ca. 0.5 °C, which is indicative of their high purity.
Elemental analysis indicated 1:1 stoichiometry for all
compounds.
Furthermore, the salts were characterized via UV−vis and
FT-IR spectroscopy to obtain more information on the
chemical structures and the functional groups of those
compounds. UV data obtained in NaOH 0.1 M showed that
λmax values are identical for salts and acids; this confirms the
existence of the same anion in both compounds.
The UV−Vis spectral studies indicate the cut off wavelength
lying between 225.6 and 249 nm for 1−3; 5−7 and 280.3 nm
for 4, corresponding to the energy gap of 5.5−5 eV for 1−3;
5−7 and 4.4 eV for 4, which are typical values of insulating
materials. The forbidden energy gap was estimated from the
.
values of λmax using the relation Eg = 1.243 × 103/λmax 27 There
are no absorption bands in the entire visible region; hence these
E
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a
Table 4. Computed Dipole Moments (μ, Debye) and Isotropic Dipole Polarizabilities (P, Bohr3) for Anions
1
2
3A
3B
4
5
6
μx
−10.7060
−0.1008
−0.0269
10.6782
80.83
−13.4545
−0.1289
0.0375
−15.0769
0.3747
−15.4475
−0.7077
−0.0854
15.4104
94.89
−12.4760
1.2149
−12.1969
−0.1186
0.0427
−15.0908
−0.1190
−0.0164
15.1225
100.98
μy
μz
−0.1285
15.0273
94.23
−0.2504
11.0607
101.22
μtotal
P
13.4147
91.11
12.1882
93.49
a
The scheme of coordinate system for HF calculations is given in Supporting Information. This system of coordinates where the x axis is
perpendicular to the O2···O3 line is common for all studied compounds.
compounds are potential candidates for nonlinear optical
applications.
The cations in 1−7 adopt the +Syn-Clinal and −Syn-Clinal
conformations.23 The N1C1C2O1 torsion angles in 1, 3A, 5,
and in 2, 3B, 4 are equal to 59.8°, 79.2°, 55.1° and −62.0,
−58.2°, −68.8°, respectively. For each of compounds 1 and 2,
there are two possibilities to form building blocks using the
following charge-assisted hydrogen bonds with N···O separa-
The synthesized salts are IR active species, and their
absorption bands occur in different regions. The technique
used in the present work brings about plenty of additional
information on hydrogen-bonded complexes. Salt formation
could be identified by the presence of bands arising from the
asymmetric and symmetric vibrations of the COO− group
occurring at 1650−1540 cm−1 and 1450−1360 cm−1 and the
absence of bands at 1710−1680 cm−1 and 1320−1210 cm−1,
characteristic for CO in a carboxylic group.28 A supple-
mentary proof of the salt formation was the appearance of the
C−O stretch belonging to primary alcohols (1100−1000 cm−1,
−CH2OH from ethanolamine), and an additional peak to
medium absorption between 2220 and 1820 cm−1 characteristic
of primary amine salts.
Crystal Structure Analysis. The salts units of 1−7 serve as
building blocks of the supramolecular architecture, and in
crystal they are self-assembled via ionic N−H···O and normal
O−H···O hydrogen bonds (HB) (Figure 1, Table 3).
The identical space groups, in close similarity with chemical
structures and lattice parameters of 5−7, suggest some degree
of isostructurality between them. The unit cell similarity index
is P = [{(a + b + c)/(a0 + b0 + c0)} − 1], where a, b, c and a0,
b0, c0 are the orthogonalized lattice parameters of the related
crystals.29 For similar unit cells P = 0, in our case the values of
P(56), P(57), P(67) are equal to 0.003, 0.02, and 0.017,
respectively. The structures of 5−7 are obviously isostructural,
which is evident by the high value of the maximal volumetric
isostructurality index,29 which is described as follows: Imax = [(2
min{V1,V2})/(V1+V2)] × 100%, where V1, V2 are the volumes
of the compared fragments. In our case V1, V2 are the molecular
volumes of building blocks in asymmetric unit cells. For
isostructural pairs (5−6), (5−7), and (6−7), Imax are close to
100% and equal to 99.8%, 97.4%, 97.6%, respectively.
Therefore, for crystal structures of 5−7 we will further describe
only compound 5.
tions: 2.693 (1) (1A), 2.884(1) (1B), 2.865 (3) (2A) and
31
́
2.757(3) (2B) Å (Table 3). For this reason, the total HF
energies have been calculated for these systems in order to
choose the optimal building blocks: −627.2256 (1A),
−627.2196 (1B), −666.2585 (2A), −666.22324 (2B) a.u.
Thus, the 1A and 2A BB were selected to study the
supramolecular architecture of compounds 1−7. The cation
was the same for all studied compounds; therefore, the analysis
of both dipole moments (DM) and isotropic polarizabilities31
of anions was carried out so as to understand why in 1 and 2
anions and cations are held together by one proton-transferred
hydrogen bond while in 3−7 by two hygrogen bonds (Table
4).
The increase of the dipole moment in 2, when hydrogen in 1
is replaced by the methyl group, can be referred to the
inductive effect, which leads to the development of charges. But
the value of DM in 2 is greater than in 4 and 5, which may be
due to the substituent effect. The hydroxyl group is an electron-
withdrawing one inductively but it is an electron-donating
group through resonance. Halogen substituents are a little
unusual and they are both inductive electron withdrawing
(electronegativity) and resonance donating (lone pair dona-
tion). For example, the charges of Cl and Br atoms in 5 and 6
are equal to 0.109 and −0.010, respectively. However, the
dipole polarizabilities in 3−6 are greater than in 1, 2, which
leads to increasing the instantaneous dipoles formation in 3−6
because the polarizability refers to the ability of the electron
distribution in a molecule to change temporarily. The
fluctuating dipole corresponds to the permanent dipole;
therefore, the polar molecules interact both through their
permanent dipoles and through the correlated, instantaneous
fluctuations in those dipoles. The polarizability affects
dispersion forces which play a significant role in the formation
of two N−H···O and O−H···O hydrogen bonds between the
cations and anions in 3−7. The two above-mentioned
hydrogen bonds are also observed when an H atom in para-
position is substituted by an electron withdrawing NO2- group.6
Thus in the studied molecular aggregates the substituent effect
influences both the dipole−dipole interactions and the
dispersion interactions between anions and cations, which
leads to intermolecular force fields driving the organization of
one charge-assisted (1, 2) hydrogen bond or two (3−7)
hydrogen bonds in building blocks. The complexation energies
of single molecular building blocks ΔE1 show a better
complexation efficiency for the proton-transfer complexes of
3A, 3B and 4 than for 1 and 2 (Table 5), while the values of
In building blocks of 1 and 2 the cations and anions are held
together by one charge-assisted hydrogen bond, while in 3−7
by two (N−H···O and O−H···O) hydrogen bonds. The
proton-transferred N−H···O hydrogen bond plays the main
role in formation of these pairs and it is present in all
investigated salts. The building blocks of 3−7 generate the
30
2
supramolecular synthons R2 (9). The anions in 1−7 form
practically planar systems because the dihedral angles between
the least-squares plane of the phenyl ring
A
(C3C4C5C6C7C8) and the least-squares plane of the COO−
groups range from 1.8° to 5.7°. In the asymmetric unit cell of 3,
there are two independent building blocks (3A and 3B) where
carbon atoms of methyl groups lie practically in the plane of
phenyl rings. The torsion angles of C5−C6−O4−C10 range
from 1.8° to 3.0°.
F
dx.doi.org/10.1021/cg301304y | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
a
Table 5. Computed Total SCF Energies (E, au) and Energy Differences for 1−6
1
2
3A
3B
4
5
6
E1
−627.2256
−417.6843
−209.3798
−0.1615
−0.1911
−0.0355
−666.2585
−456.7011
−209.3768
−0.1806
−0.20241
−0.0436
−741.0897
−531.5491
−209.3563
−0.1843
−741.1035
−531.5483
−209.3644
−0.1908
−702.0901
−492.5430
−209.3628
−0.1843
−0.193
−0.0175
−1086.1522
−876.6099
−209.3671
−0.1752
−3196.5822
−2987.0414
−209.3627
−0.1781
Eanion
Ecation
ΔE1
ΔE2
ΔE3
−0.2015
−0.028
−0.1836
−0.0168
−0.1904
−0.0245
a
E1 denotes the SCF energy of single molecular building block.
Table 6. Shortest Intermolecular Contacts (d) between Pairs of Building Blocks and Computed Total SCF Energies (E, au) and
Energy Differences for 1−6
1A
2.884
1B
2.783
1C
2.955
2A
2.757
2B
2.946
2C
2.829
3A
2.778
3B
2.779
d (Å)
Etot
−1254.4804
−627.2402
−0.1761
−0.0292
3AB
−1254.4819
−627.241
−0.1769
−0.0307
3BA
−1254.4979
−627.249
−0.1849
−0.0467
4A
−1332.5762
−666.2881
−0.2102
−0.0592
4B
−1332.5548
−666.2774
−0.1995
−0.0378
5A
−1332.5509
−666.2755
−0.1976
−0.0339
5B
−1482.2116
−741.1058
−0.2004
−0.0322
6A
−1482.2347
−741.1174
−0.2047
−0.0277
6B
E2
ΔE2
ΔE3
d (Å)
Etot
2.850
−1482.236
−741.1178
−0.2087
2.937
−1482.2024
−741.1012
−0.1921
2.878
−1404.2094
−702.1047
−0.1989
2.694
−1404.1859
−702.0921
−0.1872
2.793
−2172.3339
−1086.167
−0.1899
2.894
−2172.3084
−1086.1542
−0.1967
2.776
−6393.2016
−3196.6008
−0.1967
2.896
−6393.1762
−3196.5881
−0.184
E2
ΔE2
ΔE3
−0.0428
−0.0092
−0.0292
−0.0057
−0.0295
−0.0372
−0.0372
−0.0118
Figure 2. (a) Fragment of the layer showing synthons formation and main hydrogen bonds in Table 2. (b) Crystal packing of 1 representing the 2-D
layers parallel to (001) plane. (c) Fragment of molecular packing in the crystal of 1.
ΔE1 for 5 and 6 are comparable with that in 2. In order to
estimate how the environment (i.e., the neighboring building
blocks in the studied crystal structures) influenced the
complexation energies ΔE1, the calculations of the following
quantities were performed (Table 6): (a) the total SCF
energies (Etot) for nearest pairs of building blocks in 1−6; (b)
the total energies (E2) of a single building block belonging to
the given pair of nearest building blocks. The shortest
intermolecular contacts for non-hydrogen atoms (d) between
each pair of building blocks in the investigated compounds, as
well as the energies for complexation of neighboring building
blocks (ΔE3) in the pairs and for the given building blocks
(ΔE2) are presented in Table 6. The averaged values of ΔE2
and ΔE3 are summarized in Table 5. The presence of
neighboring building blocks in the studied crystal structures
has led to a higher complexation efficiency for the salts units of
1−6 which are building blocks of the supramolecular
architecture (Table 5). From the point of view of crystal
growth and design, it is important to analyze the energies for
complexation of building blocks in 1−6. The complexation
energies ΔE3 (Tables 5 and 6) show better complexation
efficiency for 1 and 2 in comparison with that in complexes 3−
G
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Crystal Growth & Design
Article
Figure 3. (a) The chains along the a direction where the synthons are related by N(1)···O3 (6) hydrogen bonds. (b) View of layer formation where
chains are joined by N(1)···O2 (7) hydrogen bonds. (c) Fragment of molecular packing in the crystal of 2.
Figure 4. (a) Chains along the b direction in 3 where building blocks are linked by N1A···O2B (10) and N1B···O3A (14) hydrogen bonds. (b) View
of layer formation where translation related along the a axis chains are joined by N1A···O3A (9) and N1B···O3B (13) HB. (c) The crystal packing of
3 showing interactions between layers via C(10A)···O(1B) (17) and C(10B)···O(1A) (18) hydrogen bonds.
6 because in the BB of 1 and 2 the cations and anions are held
together by one charge-assisted hydrogen bond N−H···O while
in 3−6 by two hydrogen bonds. It means that in 1 and 2 the
O−H···O hydrogen bond is involved in linking with the
neighboring building blocks, whereas in 3−6 the given
hydrogen bond participates in formation of building blocks.
In 1, R2 (12) antidromic intermolecular rings around a
4
symmetry center are formed by building blocks via N1···O2 (1)
and N1···O3 (2) hydrogen bonds. These synthons are further
related by a glide planes perpendicular to [1 0 0] with the glide
component [0 0.5 0] and 2-fold screw axes with direction [100]
and screw component [0.5 0 0] due to N1···O3 (3) and
H
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Crystal Growth & Design
Article
Figure 5. (a) View of the H-bonded chain running along the b direction due to translation. Building blocks form chains via N1···O2 (20) hydrogen
bonds. (b) These chains are joined by glide planes through the N1···O3 (21) and C8···O2 (25) hydrogen bonds and assemble into 2-D layers. (c)
Fragment of molecular packing in 4 showing interactions between layers O4···O1 (23) and C1···O4 (24) hydrogen bonds.
Figure 6. (a) Fragment of the chain in 5. BB form the chains due to translation along the a direction via N1···O3 (28) hydrogen bonds. (b) These
chains are joined by glide planes through N1···O1 (26) and C1···O3 (30) hydrogen bonds forming the layers. (c) Fragment of molecular packing in
the crystal of 5.
O1···O3 (4) HB and assemble into 2-D layers parallel to the
angle between the HCg vector and the normal to the aromatic
ring).23 For the C1−H···Cg(A) (−1/2 + x, y, 1/2 − z)
interaction, the distance between the H atom and A centroid is
2.94 Å, while the angle γ is 14.41°. The C1−H bonds and
phenyl rings lie in different layers. The X−H···Cg interaction is
observed within the layers for C5−H bonds, where H···Cg(A)
(1 + x, y, z) distances and γ values are equal to 2.89 Å and
13.69°, respectively.
(001) plane (Figure 2a,b, Table 3). According to the criterion
proposed by others:23 the distance of Cg(J1)···Cg(J2) < 6.0 Å,
́
β < 60.0°, where β is the angle between the Cg(J1)Cg(J2)
vector and the normal to the J1 ring, there are the π−π stacking
interactions between the phenyl rings Cg(A) and Cg(A) (1/2 +
x, y, 1/2 − z), laying in different layers of 1 (Figure 2c). The
Cg(J1)···Cg(J2) distance between the centroids of these
moieties is 4.81 Å and β = 9.5°. In 1 there is a X−H···Cg
(π-ring) interaction (H···Cg < 3.0 Å, γ < 30.0°, where γ is the
In 2, the R2 (14) antidromic intermolecular rings around the
2
2-fold rotation axes are formed by building blocks via N1···O2
I
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Crystal Growth & Design
Article
(5) and O1···O2 (8) hydrogen bonds. These synthons are
further related by a translation due to N(1)···O3 (6) HB and
assemble into chains developed along the a direction in the unit
cell. The chains are further consolidated into 2-D layers parallel
to the (010) plane through the N(1)···O2 (7) hydrogen bonds
via the glide planes which are perpendicular to a axis with glide
component [0 0 0.5] (Figure 3a,b, Table 3). There are the π−π
stacking interactions between the phenyl rings Cg(A) and
Cg(A) (−x, y, −1/2 + z), laying in different layers of 2 (Figure
3c).
electron delocalization.6 Formation of single or multiple
hydrogen bonds at one oxygen atom should cause the
associated C−O bond to lengthen. Thus the differences in
C−O bond lengths in carboxylate groups vary between 0.005
and 0.032 Å in 1−7 that is more than three times their esd’s
(Table 2), with the exception of compounds 4 and 7.
As expected from the non-centrosymmetric space groups for
crystal structures of 2−7, they may exhibit second-harmonic
generation that is related to nonlinear optical applications. For
the studied compounds, the calculations of macroscopic
polarizations (P) of unit cell using the SIESTA method32
show a sizable value of P mainly directed along the direction of
the c axis for 3, 5−7 and the a axis for 4 (Table 7).
The Cg(J1)···Cg(J2) distance between the centroids of these
́
moieties is 5.03 Å and β = 12.0°.
In the crystal structure of 3, the building blocks form the
helix-like chains along b direction due to 2-fold screw axis via
N1A···O2B (10) and N1B···O3A (14) hydrogen bond (Figure
4a, Table 3). Translations related along a axis chains are further
consolidated into 2-D layers parallel to (001) plane through
N1A···O3A (9) and N1B···O3B (13) hydrogen bonds (Figure
4b). The layers in 3 are linked by C(10A)···O(1B) (17) and
C(10B)···O(1A) (18) hydrogen bonds (Figure 4c).
Table 7. Macroscopic Polarization Per Unit Cell (P, Debye)
along Lattice Vectors
CN
Pa
Pb
Pc
2
3
4
5
6
7
14772.898521
10047.748369
18638.082667
4517.534205
4572.795134
4517.534205
140319.877225
24175.314416
3371.104062
5535.750654
5603.466959
5508.859092
12895.569495
38077.850088
8656.148086
15703.515070
13704.447941
17177.652181
In 3 there are both π−π stacking interactions between the
phenyl rings Cg(A(C3AC4AC5AC6AC7AC8A)) and Cg-
(B(C3BC4BC5BC6BC7BC8B)) and X−H···Cg(A,B) interac-
tions within the layers. The Cg(A)···Cg(B) (1 − x, −1/2 + y,
1/2 − z) and Cg(B)···Cg(A) (−x, 1/2 + y, 1/2 − z)) distances
between the centroids of these moieties are equal to 4.922 and
4.875 Å, and the values of β angles are equal to 14.71 and
14.92°. The H···Cg(A) (1 − x, 1/2 + x, 1/2 − z) and
H···Cg(B) (−x, 1/2 + x, 1/2 − z) separations and γ angles for
distances C5B−H and C5A−H are equal to 2.90, 2.92 Å and
7.8, 12.9°, respectively.
The building blocks of 4 form the chains due to translation
along the b direction via N1···O2 (20) HB. These chains are
further related by a glide plane perpendicular to [0 1 0] with
the glide component [0 0 0.5] through the N1···O3 (21) and
C8...O2 (25) HB and assemble into 2-D layers parallel to (100)
plane (Figure 5a,b, Table 3). The layers in 4 are linked by
O4···O1 (23) and C1···O4 (24) hydrogen bonds (Figure 5c)
due to the glide plane perpendicular to [0 1 0] with the glide
component [0.5 0 0.5] and the centering vector [0.5 0.5 0].
In the crystal structure of 5 the building blocks form the
chains along the a direction due to translation via N1···O3 (28)
hydrogen bond (Figure 6a, Table 3). The chains are further
consolidated into 2-D layers through N1···O1 (26) and
C1···O3 (30) hydrogen bonds. These layers propagate parallel
to the (001) plane where the chains are related by glide planes
perpendicular to [0 1 0] with the glide component [0 0 0.5]
(Figure 6b).
Within the layers in 5 there are short phenyl rings
Cg(A)···Cg(A) (−1/2 + x, 1/2 − y, z) as well as X−H···Cg(A)
(−1/2 + x, 1/2 − y, z) interactions. The distance between the
centroids of cycles is equal to 4.738 Å and β = 14.5°. The
H···Cg(A) separation and γ angles for distance C5−H are 2.81
Å and 8.2°, respectively. There are van der Waals interactions
between the layers with short Cl···O1(2 − x, 1 − y, 1/2 + z)
contacts which are equal to 3.263(2) Å (Figure 6c).
There is an effect of hydrogen bonding on the geometry of
carboxylate groups. While all carboxylate groups in 1−7
participate as hydrogen-bonding acceptors, the C−O bond
lengths vary significantly with the number and type of
hydrogen-bonding donors linked to the oxygen atom. In the
absence of hydrogen bonding and other electronic perturba-
tions, the C−O bond lengths should be equal because of
For all these structures, macroscopic polarizations are
practically perpendicular to layers formed by building blocks
via hydrogen bonds. For 2, where the building blocks are self-
assembled by one hydrogen bond, the orientation of P is not
related to crystal packing.
CONCLUSIONS
■
2-Hydroxyethylammonium salts of para-substituted benzoic
acids were synthesized and characterized by X-ray diffraction
analysis. Their structures display a number of certain common
structural features. In the crystal packing, the building blocks of
1−7 are self-assembled via proton-transferred N−H···O and
normal O−H···O hydrogen bonds. In the building blocks of 1
and 2, the cations and anions are held together by one charge-
assisted hydrogen bond N−H···O while in 3−7 by two
hydrogen bonds. In the studied compounds, the substituent
effect influences dipole−dipole interactions as well as the
dispersion interaction between anions and cations, which leads
to intermolecular force fields driving the organization of one
charge-assisted (1, 2) hydrogen bond or two (3−7) hydrogen
bonds in building blocks. The complexation energies ΔE1 of
single molecular building blocks show a better complexation
efficiency for the proton-transfer complexes of 3A, 3B and 4
than for 1 and 2, while the values of ΔE1 for 5 and 6 are
comparable with that in 2. However, the environment of
building blocks in the studied crystal structures has influenced
the complexation energies ΔE1. The presence of neighboring
building blocks in crystal structures has led to a higher
complexation efficiency for the salts units of 1−7 which are
building blocks of the supramolecular architecture. The crystal
growth of the studied compounds depends on the energies for
complexation of neighboring building blocks (ΔE3). The values
ΔE3 show a better complexation efficiency for 1 and 2 in
comparison with complexes 3−6, because in the buiding blocks
of 1 and 2 the cations and anions are held together by one
charge-assisted hydrogen bond N−H···O while in 3−6 by two
hydrogen bonds. It means that in 1 and 2 the O−H···O
hydrogen bond is involved in linking the neighboring building
J
dx.doi.org/10.1021/cg301304y | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
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ASSOCIATED CONTENT
* Supporting Information
The X-ray crystallographic information files (CIF) are available
for compounds 1−7. Data on chemical analyses (elemental
analysis, IR, UV and melting point), selected bond lengths and
angles and computational details are provided free of charge via
the Internet at http://pubs.acs.org/.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: xray52@mail.ru; telephone: +37322738154; fax
+37322738149.
■
(12) Parshad, H.; Frydenvang, K.; Liljefors, T.; Sorensen, H. O;
Larsen, C. Int. J. Pharm. 2002, 237, 193−207.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was financially supported by Project 2.2 of the
Institute of Chemistry Timisoara of Romanian Academy. The
authors thank Dr. Gabriele Bocelli for X-ray data collection.
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