Co-crystal and crystal: Supramolecular arrangement obtained from 4-aminosalicylic acid, bpa ligand and cobalt ion

Article abstract of DOI:10.1016/j.molstruc.2011.11.039

In this study, the synthesis, spectroscopic properties (infrared and Raman) and crystal structures of two new compounds co-crystal and crystal named HASbpa (1) and [Co(bpa)(H₂O)₄]AS₂4H₂O (2) have been reported,

Full text of DOI:10.1016/j.molstruc.2011.11.039

Journal of Molecular Structure 1010 (2012) 104–110
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Co-crystal and crystal: Supramolecular arrangement obtained
from 4-aminosalicylic acid, bpa ligand and cobalt ion
⇑
Humberto C. Garcia, Ronaldo T. Cunha, Renata Diniz, Luiz Fernando C. de Oliveira
Núcleo de Espectroscopia e Estrutura Molecular, Departamento de Química, Universidade Federal de Juiz de Fora, Juiz de Fora, MG 36036-900, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, the synthesis, spectroscopic properties (infrared and Raman) and crystal structures of two
new compounds co-crystal and crystal named HASbpa (1) and [Co(bpa)(H2O)4]AS2ꢀ4H2O (2) have been
reported, where bpa is trans-1,2-bis(4-pyridyl)ethane, HAS is 4-aminosalicylic acid and ASꢁ is aminosa-
licylate anion. The crystalline arrangement of the compound 1 exhibits a triclinic system with space
Received 3 November 2011
Received in revised form 23 November 2011
Accepted 23 November 2011
Available online 1 December 2011
ꢀ
group P1. The formation of a structure known as co-crystal, composed by building blocks in their neutral
form; being the first work of this type involving the HAS and nitrogen ligand as bpa. For compound 2, a
monoclinic system was observed with P21/c space group. The crystalline arrangement of the structure
consisted of a covalent one-dimensional cationic [Co(bpa)(H2O)4]2+ chain, which interacts by hydrogen
bonding, p-stacking and electrostatic interactions with aminosalicylate anions and water molecules that
were trapped in the crystal. These interactions form supramolecular cavities denominated as pseudo
Keywords:
Crystal and co-crystal
Raman spectroscopy
Supramolecular chemistry
Crystal structure
honeycombs. For compound 1, the infrared spectrum revealed the presence of bands at 1643 and
Molecular spectroscopy
1601 cmꢁ1 assigned to the stretching mode of CO [
m(CO)] and CC/CN groups [m(CC/CN)]. For the Raman
spectrum, these same modes appear around 1644 and 1602 cmꢁ1 related to HAS and bpa blocks, respec-
tively. For compound 2, the largest displacement of the bands compared to free ligand suggested the for-
mation of covalent bonds between bpa ligand and metallic site and loss of the proton in HAS molecule. In
the infrared spectrum we can observe the presence of bands around 1635 and 1618 cmꢁ1 attributed to
the stretching
m m(CC/CN), for the Raman spectrum these same modes appear around 1631
(COOꢁ) and
and 1619 cmꢁ1 related to ASꢁ and bpa ligand respectively.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
block used for formation of supramolecular arrangements [3,4]
and new structures known scientifically by the name of co-crystals
[5–7].
The synthesis and design of molecules capable of spontaneously
association into large and well defined systems by noncovalent
intermolecular interactions remain one of the foremost challenges
in supramolecular chemistry. Nature offers countless examples of
self-assembling processes which inspire the work of the synthetic
chemist, the creation of analogues from natural complex systems
[1]. Recent studies in the literature show that specificity and
energetic nature are key determinants of many biochemical phe-
nomena, including substrate selection by enzymes and gene regu-
lation of metabolic activity [2].
In recent years the study of supramolecular chemistry started to
have great prominence in scientific research in both physical and
biological aspects, in order to try understanding the phenomena
of self-assembly and self-organization. Feature frequently present
in chemical systems that show noncovalent interactions as hydro-
In this context, the organic ligand 4-aminosalicylic acid (HAS)
has appeared as a hopeful building block, due to its different coordi-
nation sites, as well as its pharmacological properties [8], being used
since 1940 as an antibiotic to treat tuberculosis and has also been
shown to be safe and effective in the treatment of inflammatory
bowel diseases (IBDs), namely distal ulcerative colitis and Crohn’s
disease, for over 70 years [9]. However, works involving the coordi-
nation chemistry of HAS associated with transition metal ions and
nitrogen ligands have been rarely discussed in the literature, and
have becoming a focus of great interest to our research group [10].
The other ligand employed in this investigation is trans-1,2-
bis(4-pyridyl)ethane or simply bpa, which is an versatile bifunc-
tional building block used in the design and construction of
molecular complexes due to its aliphatic chain [A(CH2)2A] between
two pyridyl rings [11,12]. It cannot only act as bidentate bridging li-
gand, but can also serve as a terminal ligand or an uncoordinated
guest molecule, which may be further involved in hydrogen bonding
gen bonding, electrostatic, CAHꢀ ꢀ ꢀ
p, p-stacking and hydrophobic
interactions, which may arise mainly from the type of building
and/or
p-stacking interactions, due to the presence of aromatic ring.
⇑
Corresponding author. Tel./fax: +55 32 3229 3310.
E-mail address: luiz.oliveira@ufjf.edu.br (L.F.C. de Oliveira).
Furthermore, magnetic properties, host–guest, gas storage and
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.11.039

H.C. Garcia et al. / Journal of Molecular Structure 1010 (2012) 104–110
105
non-linear optical materials (NLOs) are also observed for this ligand,
mainly when associated to transition metal ions [13,14].
In the present work the synthesis, crystal structure and vibra-
tional spectroscopic analysis of two supramolecular compounds
containing 4-aminosalicylic acid, 1,2-bis(4-pyridyl)-ethane and co-
balt ion as building blocks are described. The correlation between
crystal data and vibrational spectra are also investigated, with the
aim of understanding the importance of solvent in the reaction and
the nature of building block composing the new supramolecular
assemblies.
by CrysAlis RED, Oxford diffraction Ltda – Version 1.171.32.38 pro-
gram [15]. The structures were solved and refined using SHELXL-
97 [16]. An empirical isotropic extinction parameter x was refined,
according to the method described by Larson [17]. A Multiscan
absorption correction was applied [18]. The structures were drawn
by ORTEP-3 for windows [19] and Mercury [20] programs. CCDC
809583 and 809582 contain the supplementary crystallographic
data for compounds 1 and 2 respectively. These data can be ob-
tained free of charge at http://www.ccdc.cam.ac.uk or from the
Cambridge Crystallographic Data Centre, 12, Union Road, Cam-
bridge CB2 IEZ, UK [Fax: (internat.) 1 44-1223/336-033; E-mail:
deposit@ccdc.cam.ac.uk].
2. Experimental section
2.1. Chemicals and reagents
3. Results and discussions
All the chemicals were used as purchased; 4-aminosalicylic acid
or HAS (C7H7NO3, 99.0%, Sigma Aldrich), trans-1,2-bis(4-pyri-
dyl)ethane or bpa (C12H12N2, 99.0%, Sigma Aldrich) and CoCl2ꢀ6H2O
(99.5%, Sigma Aldrich). All solvents were used as analytical grade.
Thermogravimetric (TG) curves of compounds HASbpa (1) and
[Co(bpa)(H2O)4]AS2ꢀ4H2O (2) are deposited as supplementary
material (Fig. S1 and S2, respectively), as well as the TG curves
for the starting materials aminosalicylic acid and bpa ligand, just
for comparison (Fig. S3 and S4, respectively).
2.2. Synthesis
The TG analysis of HASbpa (1) shows an initial mass loss at
149.4 °C, which can be attributed to the exit of one CO2 molecule
(ratio for calculated/experimental: 13.05%/13.40%) as observed in
the curve for aminosalicylic acid. A second mass loss can be ob-
served at ca. 248 °C, regarding the breaking of the aromatic rings,
and ending with the output of all the components of the sample,
with complete thermal decomposition of the analyzed organic
material and no detected residue (0% of mass). Interestingly, the
weight loss for this compound is coincident to the loss of the com-
pounds used as precursors, then suggesting a small change in their
structures, as for instance the formation of weak supramolecular
interactions. For the compound [Co(bpa)(H2O)4]AS2ꢀ4H2O (2) the
TG curve shows a first weight loss at 87.1 °C, which can be associ-
ated to the loss of eight water molecules (calcd./exp.: 20.82%/
20.36%) per mol of 2. A second weight loss can be observed at
171.5 °C, attributed to the loss related to one aminosalicylate anion
and one CO2 molecule (calcd./exp.: 28.34%/28.94%). The third loss,
observed at ca. 354.5 °C, is related to the exit of the remaining ami-
nosalicylate anion (calcd./exp.: 15.61%/15.85%), whereas the last
event ends with a mass loss starting at 427 °C, referring to the
breaking and exit of the bpa ligand (calcd./exp.: 26.62%/26.47%).
The residue (calcd./exp.: 8.52%/8.38%) can be related to the forma-
tion of the metallic species. All these thermogravimetric data for
complex 2 strongly suggests that the presence of new covalent
bonds are the responsible for the increased stability of this struc-
ture, when compared with the temperatures involved in the ther-
modecomposition of each one of the individual components.
The crystalline nature of supramolecular compounds 1 and 2 ob-
tained from reactions involving the HAS, bpa ligand and cobalt ions
have been revealed by X-ray single crystal analysis. The crystal data
for both compounds are listed in Table 1, and some bond distances,
bond angles and hydrogen interactions are displayed in Table 2.
Compound 1 crystallizes in a triclinic system presenting space
Compound HASbpa (1) was prepared by mixing 10 mL of an
ethanol/water (4:1 v/v) solution containing approximately
100 mg (0.65 mmol) of HAS and 10 mL of an ethanolic solution
containing 120 mg (0.65 mmol) of bpa. After completing 35 days,
and the evaporation of all solvents, the formation of yellow crystals
was observed, in 85% yield. Elemental analysis: Calc.: C, 67.04%; H,
5.68%; N, 12.46%; Found: C, 67.50%; H, 5.61%; N, 12.51%.
For the coordination compound named [Co(bpa)(H2O)4]
AS2ꢀ4H2O (2), 10 mL of an ethanol/water (1:1 v/v) solution contain-
ing approximately 100 mg (0.65 mmol) of HAS was mixed with
10 mL of an ethanolic solution containing 120 mg (0.65 mmol) of
bpa. To this colorless mixture it was added by diffusion 5.0 mL of
an aqueous solution containing 155 mg (0.65 mmol) of
CoCl2ꢀ6H2O. After 15 days, suitable single orange crystals were ob-
tained, with 48% of yield. Elemental analysis: Calc.: C, 45.16%; H,
5.83%; N, 8.10%; Found: C, 45.96%; H, 5.86%; N, 8.23%. It is worth
of mention that this synthesis was performed with several other
metal ions, such as Mn2+, Ni2+, Fe2+, Zn2+ and Cu2+, however Co2+
was the only one to provide good crystals for diffraction.
2.3. Physical measurements
Thermogravimetric measurements (TG/DTA) were done using a
Shimadzu TG-60 with thermo balance. The instrument calibration
was performed using a standard sample of calcium oxalate. Sample
masses were between 4 and 6 mg, in an open platinum crucible.
The heating rate was usually 10 °C minꢁ1 from room temperature
to 1000 °C in dynamic nitrogen atmosphere, with a flow rate of
100 mL minꢁ1. Infrared spectra were obtained using a Bomem
MB-102 spectrometer fitted with a CsI beam splitter, with the sam-
ples dispersed in KBr disks and a spectral resolution of 4 cmꢁ1
.
ꢀ
Good signal-to-noise ratios were obtained from accumulation of
128 spectral scans. Fourier-Transform Raman spectroscopy was
carried out using a Bruker RFS 100 instrument, Nd3+/YAG laser
operating at 1064 nm in the near-infrared and CCD detector cooled
with liquid N2. Good signal-to-noise ratios were obtained from
2000 scans accumulated over a period of about 30 min, using
4 cmꢁ1 as spectral resolution. All spectra were obtained at least
twice to show reproducibility, and no changes in band positions
and intensities were observed. Single crystal X-ray data were col-
group P1; Fig. 1 represents the asymmetric unit of the structure,
formed exclusively by HAS and the bpa ligand, both in its neutral
form, forming a structure known as a co-crystal [21]. The absence
of water molecules of crystallization is an interesting fact in this
structure, which probably contributes to the great stability of the
crystal lattice. This fact has been observed by its thermogravimet-
ric curve, presenting a low decomposition temperature, at ca.
250 °C. Other important information of this structure refers to
the deprotonation of HAS, which does not happen, and conse-
quently there is no protonation of the bpa ligand, which can be re-
lated to the excessive ethanol used in the synthesis (as a solvent)
favoring the solubilization and not the ionization, which is very
lected using a Oxford GEMINI A Ultra diffractometer with Mo K
a
(k = 0.71073 Å) at room temperature (298 K) for compound 1 and
2. Data collection, reduction and cell refinement were performed

106
H.C. Garcia et al. / Journal of Molecular Structure 1010 (2012) 104–110
Table 1
Crystal data of HASbpa (1) and [Co(bpa)(H2O)4]AS2ꢀ4H2O (2).
Compound
HASbpa (1)
[Co(bpa)(H2O)4]AS2ꢀ4H2O
(2)
Formula
C
19H19N3O3
C26H40CoN4O14
691.55
Formula weight/g molꢁ1
337.37
Crystal system
Space group
Triclinic
P1
Monoclinic
P21/c
ꢀ
a/Å
b/Å
c/Å
6.468(4)
9.151(5)
15.3978(9)
73.71(5)
83.89(5)
79.08(5)
857.5(9)
2
13.725(3)
7.685(2)
14.882(4)
90.00
95.95(2)
90.00
a
/°
b/°
c
/°
V/Å3
1561.4(7)
2
Fig. 1. ORTEP view of the HASbpa (1) crystal structure. Ellipsoids are drawn at the
50% probability level, except for hydrogen atoms which are represented by circles of
arbitrary radius. Symmetry code: (i) 2 ꢁ x, 2 ꢁ y, 2 ꢁ z.
Z
Crystal size/mm
0.27 ꢂ 0.25 ꢂ 0.10 0.72 ꢂ 0.27 ꢂ 0.15
D
calc/g cmꢁ3
1.307
1.471
l(Mo K
a
)/cmꢁ1
0.090
0.623
Transmission factors (min/
max)
Reflections measured/
unique
0.976/0.991
0.817/0.911
tions, where it can be observed that bpa ligand displays two
distinct conformations along the bc plane. The formation of a
supramolecular chain can also be observed through hydrogen
interaction between amino and carboxylic groups of HAS moieties
with the nitrogen atoms from the pyridyl group of the bpa ligands.
This interaction can be described as a graph set network, being
classified as N1 = c(38)s(6) where the symbols C and S refer to
hydrogen bonds in infinite chain and intermolecular interaction
with formation of rings, respectively, according to Bernstein et al.
[22]. Another type of hydrogen bond can also be seen through
the distance of interaction between donor and acceptor atoms,
such as the ones between O1AH1ꢀ ꢀ ꢀN2 and N1AH1NAꢀ ꢀ ꢀN3 bonds
with values of 2.556(2) and 3.013(3) Å, respectively, thus classify-
ing these interactions as moderate and weak(or long), according to
Diniz and coworkers [23]. Another important supramolecular
interaction which plays an important role to the crystal lattice sta-
6812/3497
2047
8435/3204
2642
Observed reflections
½F2o > 2
r
ðF2oÞꢃ
No. of parameters refined
246
245
R [Fo > 2
r
(Fo)]
0.0513
0.1464
1.051
0.041
0.0296
0.0890
1.057
0.062
wR [Fo2 > 2r
(Fo)2]
S
RMS peak/eꢁ Åꢁ3
Table 2
Select bond distances (Å), bond angles (°) and hydrogen interactions of HASbpa (1)
and [Co(bpa)(H2O)4]AS2ꢀ4H2O (2).
Bond distance/
Å
HASbpa
(1)
Bond distance/ [Co(bpa)(H2O)4]AS2ꢀ4H2O
Å
(2)
bility can be seen almost parallel to the c-axis, named
interaction, formed between the aromatic rings of the bpa ligand
and 4-aminosalicylic acid. Such interaction presents a centroid–
p-stacking
C10AC9
C8AC9
C8AN2
N2AC12
C10AC11
C13AC10
C13AC13
C5AN1
1.379(3)
1.373(3)
1.328(3)
1.312(3)
1.372(3)
1.504(2)
1.511(4)
1.359(2)
1.301(2)
1.243(2)
1.348(2)
Co1AO4
Co1AO5
Co1AN2
N2AC12
C10AC11
C13AC10
C13AC13
C5AN1
2.061(1)
2.121(1)
2.158(1)
1.336(2)
1.376(3)
1.505(2)
1.521(3)
1.366(2)
1.244(2)
1.290(2)
1.358(1)
centroid distance of 3.719(4) Å, whereas the CAHꢀ ꢀ ꢀ
p interactions
between hydrogen bpa ligand and the acid ring, present a distance
of 3.501(2) Å between the centroid and the carbon atom donor
[24].
Fig. 3 displays the repeating unit of compound 2 revealed by X-
ray single crystal analysis. The structure crystallizes in a mono-
clinic crystal system presenting P21/c space group. The molecular
one-dimensional arrangement shows the monomeric cationic frag-
ment [Co(bpa)(H2O)4]2+, displaying the cationic unit composed by
the metallic ion coordinated in a slightly distorted octahedral
geometry, to four water molecules in the equatorial positions
and two pyridyl nitrogens from the two different bpa ligands in
the axial position. The bond distances CoAO4, CoAO5 and CoAN
are 2.061(1), 2.121(1) and 2.158(3) Å, respectively. Two uncoordi-
nated aminosalicylate ions acting as anions appear neutralizing the
charge of each monomeric fragment. Furthermore, each repeating
unit has four hydration water molecules, which are also involved
in the crystal lattice stability. The bpa ligand act as bis-monoden-
tate moiety and bridge the adjacent metallic centers, adopting a
conformation Trans–Trans (or TT) with N-to-N separation distances
of 9.441(2) Å [25].
C1AO1
C1AO3
C7AO2
C1AO1
C1AO3
C7AO2
Average of bond angles/°
O3AC1AO1
O3AC1AC2
C12AN2AC8
122.01(16) O4ACo1AO5
122.06(17) O4ACo1AO5
117.23(16) O4ACo1AN2
O4ACo1AN2
88.96(6)
91.04(6)
90.48(5)
89.52 (5)
Hydrogen bonds
Dꢀ ꢀ ꢀA/Å
O1AH1ꢀ ꢀ ꢀN2
2.556(2)
O4AH4Aꢀ ꢀ ꢀO3
O5AH5Bꢀ ꢀ ꢀO6
O2AH2ꢀ ꢀ ꢀO3
O6AH6Aꢀ ꢀ ꢀO7
O7AH7Aꢀ ꢀ ꢀO1
2.729(1)
2.794(2)
2.530(1)
2.849(2)
2.801(2)
N1AH1NAꢀ ꢀ ꢀN3 3.013(3)
O2AH2ꢀ ꢀ ꢀO3
2.570(2)
DAHꢀ ꢀ ꢀA/°
O1AH1ꢀ ꢀ ꢀN2
168(2)
O4AH4Aꢀ ꢀ ꢀO3
O5AH5Bꢀ ꢀ ꢀO6
O2AH2ꢀ ꢀ ꢀO3
159(1)
167(2)
147.00
N1AH1NAꢀ ꢀ ꢀN3 172.4(18)
O2AH2ꢀ ꢀ ꢀO3
147.00
The three-dimensional arrangement verified through covalent
bonds and supramolecular interactions can be observed in Fig. 4a;
observation along a-crystallographic axis shows the covalent [Co(b-
pa)(H2O)4]2+ linear chain, extends from polymeric form only in this
direction. Interactions such as hydrogen bonds formed through
aminosalicylate anions and crystallization and coordination water
molecules can be observed in Fig. 4b, which are responsible for
supramolecular rings. These supramolecular interactions can be de-
scribed as graph set networks [22] named N1 ¼ C44ð16ÞR88ð26ÞSð6Þ
common in acids, then resulting in the formation of only neutral
building blocks. It is noteworthy the formation of the co-crystal 1
was possible only after the complete evaporation of the solvent,
which probably enabled the stacking of building blocks used in this
work. Fig. 2 shows the supramolecular arrangement of this struc-
ture in a two-dimensional level along the (011) and (101) direc-

H.C. Garcia et al. / Journal of Molecular Structure 1010 (2012) 104–110
107
Fig. 2. Supramolecular arrangement in the bc planes and ac diagonal for HASbpa (1) structure.
assigned to the stretching of the carboxyl group (
m
CO); in the case
of neutral acid this same band appears shifted to a lower wave-
number (by ca. 15 cmꢁ1), due to the formation of a resonance
structure related to the new stretching mode of the carboxylate
ꢁ
ion (mCOO ). An important spectral region for this building block
in its anionic form (ASꢁ) appears between 1400 and 1600 cmꢁ1
,
presenting some intense bands which can be assigned to the CC
stretching mode of the aromatic ring. Another very characteristic
band can be observed at 1188 cmꢁ1, referring to the
m
CO of the hy-
droxyl group. The Raman spectrum of the aminosalicylic acid
shows an intense band at 1660 cmꢁ1, assigned to the
CO of the car-
m
boxyl group, who present the same band shift, as observed in the
infrared spectrum. In the region between 1400 and 1600 cmꢁ1 it
can be seen also some intense bands which are related to the CC
stretching modes of the aromatic ring.
In the infrared spectrum of free trans-1,2-bis(4-pyridyl)ethane
ligand, an intense band at 1597 cmꢁ1 can be observed, tentatively
assigned to the coupled stretching modes of the pyridyl rings
Fig. 3. ORTEP view of the [Co(bpa)(H2O)4]AS2ꢀ4H2O (2) crystal structure. Ellipsoids
are drawn at the 50% probability level, except for hydrogen atoms which are
represented by circles of arbitrary radius. The water molecules are omitted for
clarity. Symmetry code: (i) 2 ꢁ x,1 ꢁ y, ꢁz; (ii) 1 ꢁ x,1 ꢁ y, ꢁz.
Analysis of topological parameters of
p
-stacking [24] shows that
involving CC and CN bonds (mCC CN), and the bands at 1414 and
/
m
the rings of the aminosalicylic anion and bpa ligand provide a cen-
troid–centroid interaction less than 3.80 Å, more precisely
3.798(2) Å. Another important supramolecular interaction can be
noted between the hydrogen atom of the aminosalicylate anion
and the aromatic ring of the another aminosalicylate anion; such
830 cmꢁ1 are attributed to (mring
+ mCH) and dCH, respectively. In
the Raman spectrum the most intense and characteristic bands
are the ones at 1598 and 995 cmꢁ1, assigned to the (mCC
mring modes, respectively.
/mCN) and
It is straightforward to mention that the coordination to metal
ions, such as Co(II), Ni(II) and Zn(II), lead to a shift for higher wave-
numbers for the main observed bands for the nitrogenous ligands,
more specifically bpa, bpe and bpy according to previous investiga-
tion [31,32]. This fact can be used as a fingerprint for the coordina-
tion mode between metal ions and all of these nitrogenous ligands.
In the infrared spectrum of compound 1 it can be observed an
intense multiplet in the 3500–3100 cmꢁ1 region; the bands at
interaction gives rise to a NAHꢀ ꢀ ꢀ
p interaction, showing a distance
value for the nitrogen atom from the anion and the centroid of other
anion equal to 3.568(2) Å. An interesting fact observed by Garcia
et al. [26], involving systems containing aminosalicylate anion,
nitrogen ligand trans-1,2-bis(4-pyridyl)ethylene (bpe) and metallic
ions (cobalt, nickel and zinc), have demonstrated the same coordi-
nation mode between the bpe ligand and the metallic ion, with the
formation of a polymer chain involved by a cavity, named as pseudo
honeycomb. The supramolecular compound synthesized in that
investigation also presented the same functional group (P21/c)
and network parameter values very close to the obtained for com-
plex 2, then suggesting that changing the bpe ligand by bpa ligand
shows no influence in the metal coordination mode, and especially
in the crystalline arrangement of the supramolecular structure.
The vibrational spectra of all ligands and complexes investi-
gated in this work are displayed in Figs. 5 and 6 (infrared and Ra-
man spectra, respectively). All spectra are very similar and in
agreement with the crystal data. The main vibrational modes are
summarized in Table 3, as well as the tentative assignment based
on similar chemical systems [27–30].
3467 and 3393 cmꢁ1 are assigned to the mNH (symmetric and
2
asymmetric modes) of the HAS, whereas the ones at 3304 and
3196 cmꢁ1 are referred to the
mCH of the bpa ligand. All these bands
appear intense due to the absence of water molecules in the solid
structure. As noted in the same region for compound 2, there is the
presence of a broad band, assigned to the stretching modes of OH
and NH2 groups. Other intense band for
1 is observed at
1643 cmꢁ1, assigned to the mCO mode of the carboxyl group. It
can be noticed that this same vibrational mode undergoes a small
shift when compared to the pure HAS spectrum, thus suggesting
the maintenance of the proton in the building block and a conse-
quent formation of a hydrogen bond with bpa, which contributes
to the weakening of the hydroxyl bond. On the other hand, for
compound 2 the mCOO mode appears at 1635 cmꢁ1, caused by
ꢁ
In the infrared spectrum of the aminosalicylic acid, two bands of
medium intensity can be observed at 3389 and 3497 cmꢁ1, respec-
tively assigned to the stretching modes (symmetrical and asym-
metrical) of NH2 group, and these modes appear frequently
omitted in compounds which present water molecules in their
composition. Other strong band observed at 1649 cmꢁ1 can be
the exit of the proton present in the HAS building block and also
to the formation of resonance structures of the carbonyl group.
The presence of bpa ligand in the structure of both compounds
can be inferred by the vibrational bands observed at 1601 and
1618 cmꢁ1, regarding to the mCC
/mCN modes of the pyridyl ring for

108
H.C. Garcia et al. / Journal of Molecular Structure 1010 (2012) 104–110
2þ
Fig. 4. (a) Views of the 1D covalent linear chain ½CoðbpaÞðH2OÞ4ꢃn along a-crystallographic axis and (b) ring formation by hydrogen bonds along the bc-plane involving the
covalent chain.
Fig. 5. Infrared spectra of HASbpa (1) and [Co(bpa)(H2O)4]AS2ꢀ4H2O (2); for
Fig. 6. Raman spectra of HASbpa (1) and [Co(bpa)(H2O)4]AS2ꢀ4H2O (2); for
comparison purposes the spectra of the precursors (HAS and bpa) are also
comparison purposes the spectra of the precursors (HAS and bpa) are also
displayed.
displayed.
compounds 1 and 2, respectively. When comparing this mode for
both co-crystal and crystal and the free ligand (occurring at
1597 cmꢁ1), the observed band shift for higher wavenumbers can
be explained by the coordination to the metal ion in complex 2,
thus suggesting a strengthening of this chemical bond and conse-
quently for the vibrational mode. For compound 1 the small band
shift can be attributed to the weak supramolecular interactions,
such as hydrogen bonds, observed through its crystalline structure.
The same interpretation made above for the Raman spectra of
compounds 1 and 2 can be applied to the infrared spectra for both
compounds. Compound 1 displays an intense band at 1644 cmꢁ1
which can be assigned to the CO mode; the small observed wave-
,
m
number shift when compared to HAS can be explained based on
the existence of hydrogen bonding between carboxylic acid group
and the bpa ligand, clearly seen in the crystalline arrangement
showed in Fig. 1. For compound 2, the same band appears shifted
to a lower wavenumber, occurring at 1631 cmꢁ1 and assigned to
ꢁ
the mCOO vibrational mode, thus suggesting a further bond weak-
ening due to the formation of the corresponding ASꢁ anion. Other

H.C. Garcia et al. / Journal of Molecular Structure 1010 (2012) 104–110
109
Table 3
Raman (R) and infrared (IR) wavenumbers (in cmꢁ1) and tentative assignment of the most important bands observed for HASbpa (1) and [Co(bpa)(H2O)4]AS2ꢀ4H2O (2).
HAS
IR
bpa
IR
HASbpa (1)
[Co(bpa)(H2O)4]AS2ꢀ4H2O (2)
Tentative assignment
R
R
IR
R
IR
R
529 m
520 m
549 w
590 m
di.p CO(h)
do.p. CH + dwag CH2
di.p C@O
mCC
547 vs
601 w
753 s
782 vs
589 w
754 m
802 m
751 m
770 w
795 m
dwag. NH2
830 vs
991 m
829 s
833 m
966 w
848 w
968 w
1022 s
1197 w
1230 w
1300 s
do.p CH
mring
mring
di.p CH
mCAO(h)
di.p OH(h)
mCC(c)
964 w
995 vs
1216 s
1002 s
1024 w
1194 m
1227 m
1313 s
1352 m
1389 vs
1458 vs
1580 vs
1618 vs
1231 m
1296 vs
1226 m
1338 vs
1221 m
1298 s
1339 s
1416 s
1454 m
1232 m
1342 vs
1414 s
1390 m
1458 m
mring + do.p CH
1450 vs
mCC
mCC
mCC
1592 m
1597 vs
1598 s
1601 s
1620 s
1643 vs
2930 w
3043 w
3196 s
3304 s
3393 m
3467 s
1602 m
1620 s
1644 s
2927 m
3065 vs
1619 s
/mCN
1626 s
1649 vs
1633 m
1660 s
mC@O
mC@O
mCH2
mCH
mCH
mCH
1635 vs
2929 sh
3073 sh
3225 m
3342 s
1631 s
2920 s
2924 s
3054 vs
3229 w
3338 w
3389 s
3497 m
3387 w
3454 m
3494 sh
mNH
2 sy
mNH
2 as
Abbreviations: vs very strong; s, strong; m, medium; w, weak; i.p., in-plane; o.p., out-of-plane; wag., waging; sh., shoulder; (c), carbonyl group; (h), hydroxyl group.
bands observed for compound 1, related to the bpa ligand, are ob-
served at 1602 and 1002 cmꢁ1, assigned to the mCC
mCN and mring
modes, respectively. For complex 2 the same bands appear shifted,
being observed at 1619 and 1021 cmꢁ1, suggesting coordination
between the metal ion and nitrogen ligand bpa, which is evidenced
by X-ray diffraction data.
The importance of the results shown here can be seen through
the principles of self-assembly and self-organization, covered un-
der the focus of supramolecular chemistry. The solvent mixture
used in the synthesis of compound 1, water/ethanol (1:4 v/v), has
shown that the hydrogen atom present in the HAS structure is
more difficult to dissociate, making the building blocks used in this
investigation (bpa and HAS) remaining in their neutral form. The
crystallization of co-crystal (compound 1) is stabilized only
line arrangement formed by a cationic polymeric chain named
[Co(bpa)(H2O)4]2+
containing supramolecular interactions be-
/
,
tween the build blocks ASꢁ, bpa and water molecules (from crys-
tallization and coordination) in the structure, responsible for the
stability of the system. The synthesis of compound 2 suggests that
the entire system adopts a preferred coordination when compared
to the formation of co-crystals or molecular structures formed only
by the isolated HAS and bpa building blocks. In other words, this
investigation addresses the importance of knowing previously
the physical chemistry properties of the solvent (or mixture of sol-
vents) and the building blocks used for the preparation of different
supramolecular complexes. The vibrational spectra (infrared and
Raman) have been very important tools to characterize and very
conclusive to confirm structures. For compound 1, the infrared
spectrum has revealed the presence of bands of high intensity at
1643 and 1601 cmꢁ1, assigned to the mCO and mCC/CN modes,
whereas in the Raman spectrum these same modes appear at
1644 and 1602 cmꢁ1 related to HAS and bpa blocks, respectively.
For compound 2, the largest wavenumber shifts of the bands, when
compared to the free ligands, have suggested the formation of
covalent bonds between bpa ligand and the metallic ion, as well
as the loss of the HAS proton. In the infrared spectrum it can be ob-
serve the presence of bands at 1635 and 1618 cmꢁ1, assigned to
through interactions such as hydrogen bonds and p-stacking. From
our knowledge, this type of crystallization through neutral blocks
can be considered the first work in literature involving aminosali-
cylic acid associated with nitrogen ligands. Another interesting fact
is related to the synthesis of compound 2, where the change in the
solvent mixture water/ethanol (1:1 v/v), associated to the insertion
of a metal ion in the reaction, lead to the formation of the ASꢁ an-
ion and the bpa coordination.
ꢁ
the mCOO and mCC/CN modes. In the Raman spectrum these same
4. Conclusions
modes appear at 1631 and 1619 cmꢁ1, related to ASꢁ and bpa,
respectively.
In summary, in this study we have reported the synthesis and
results of the spectroscopic (infrared and Raman) and structural
data for two novel supramolecular compounds named HASbpa
(1) and [Co(bpa)(H2O)4]AS2ꢀ4H2O (2). For compound 1 the crystal-
line arrangement shows a structure known as co-crystal, which is
formed only by building blocks in their neutral form through
Acknowledgements
The authors thank CNPq, CAPES, FAPEMIG (PRONEX 04370/10,
CEX-APQ-00617) and FINEP (PROINFRA 1124/06) for financial sup-
port and also LabCri (Departamento de Física – UFMG) for the
X-ray facilities.
supramolecular interaction such as hydrogen bonding,
p-stacking
and CAHꢀ ꢀ ꢀ
p
. The formation of this co-crystal can be attributed
to the mixture of solvents used in the synthesis, which directly
influenced the dielectric constant of the system, forcing the perma-
nence of the proton covalently bonded to the HAS block. For com-
pound 2 the change in solvent mixture associated with insertion of
a metal ion in the reaction showed the formation of a new crystal-
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molstruc.2011.11.039.

110
H.C. Garcia et al. / Journal of Molecular Structure 1010 (2012) 104–110
[16] G.M. Sheldrick, SHELXL-97 – A Program for Crystal Structure Refinement,
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