An interesting pseudo-honeycomb supramolecular arrangement obtained from the interaction between 4-aminosalicylic acid, trans-1,2-bis(4-pyridyl)ethylene and transition metal ions

Article abstract of DOI:10.1039/c2ce05771a

In this work, three new compounds containing coordination polymers named [Co(bpe)(H₂O)₄]AS₂·4H₂O (1), [Ni(bpe)(H₂O)₄]AS₂·4H₂O (2) and [Zn(bpe)(H₂O)₄

Full text of DOI:10.1039/c2ce05771a

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PAPER
An interesting pseudo-honeycomb supramolecular arrangement obtained from
the interaction between 4-aminosalicylic acid, trans-1,2-bis(4-pyridyl)ethylene
and transition metal ions†
Humberto C. Garcia, Renata Diniz and Luiz Fernando C. de Oliveira*
Received 22nd June 2011, Accepted 8th December 2011
DOI: 10.1039/c2ce05771a
In this work, three new compounds containing coordination polymers named [Co(bpe)(H
2 4
O) ]
AS $4H O (1), [Ni(bpe)(H O) ]AS $4H O (2) and [Zn(bpe)(H O) ]AS $4H O (3) (where AS is
2
2
2
2
4
2
2
2
4
2
aminosalicylate anion and bpe is trans-1,2-bis(4-pyridyl)ethylene) were synthesized and characterized
by X-ray diffraction and vibrational spectroscopy (infrared and Raman). All complexes are
isomorphous, crystallizing in a monoclinic system with space group P2 /c. The crystalline arrangement
1
shows the presence of a cavity called pseudo-honeycomb imperfect, parallel to the bc-plane formed
through hydrogen bonding between aminosalicylate anions and water molecules of crystallization. The
vibrational spectra of the compounds are very similar, in agreement with the crystal data. The Raman
ꢀ1
spectra show bands for bpe at 1642 and 1614 cm , assigned to n(C]C) and n(CC)/n(CN), respectively.
This study can be considered as the first work in the literature where the aminosalicylate anion is
associated with coordination polymers just filling the cavities of pseudo-honeycomb structures.
Introduction
construction of complicated extended arrays of molecular self-
organization and self-assemblies. This arrangement is the result
of the association of several chemical entities organized in higher
complexity and known as building blocks, giving rise to
compounds with a wide range of applications in biological, optic,
The synthesis and characterization of novel supramolecular
arrangements have become a focus of study of a large number of
scientific groups in the last few years. Work involving the
understanding of chemical bonds (such as covalent bonds) is
increasingly losing pace to that of weak intermolecular interac-
tions, such as the already known hydrogen bonding, electro-
7–9
electronic and microporous materials.
In this context, the organic ligand 4-aminosalicylic acid (HAS)
has appeared as a hopeful building block, due to the possibility of
different coordination sites, as well as its pharmacological
1–4
static, C–H/p and p-stacking interactions. The search for
a more accurate understanding of these interactions, so impor-
tant as they extend beyond the individual molecules, has devel-
oped a new branch of science which has received the suggestive
10
properties. HAS is an antibiotic used since the 1940’s in the
11
treatment of tuberculosis; on the other hand, there are only
a few studies in the literature involving this type of structure
5,6
name of supramolecular chemistry.
associated with transition metal ions, nitrogen ligands, or
12,13
The study of these weak non-covalent interactions, especially
the hydrogen bonds, plays a crucial role in fundamental bio-
logical processes, as for instance, the transfer of genetic infor-
mation and molecular recognition between receptors and
substrates. These weak interactions can be major contributors to
the binding of drugs to proteins and DNA targets and can form
the basis for developing sensors to monitor the concentration of
specific ions or molecules. In terms of coordination chemistry,
the weak supramolecular interactions are responsible for the
generating anionic building blocks,
mainly due to the insta-
bility of the aminosalicylate salt, evidenced by the darkening of
the solution.
Another organic ligand used in this study is the nitrogenous
compound named trans-1,2-bis(4-pyridyl)ethylene (bpe), which
is an excellent building block in the design and construction of
14,15
molecular complexes.
It can not only act as a chelating
ligand, but can also serve as a terminal ligand or an uncoordi-
nated guest molecule, which may be further involved in hydrogen
bonding, C–H/p and p-stacking interactions, due to the pres-
ence of the pyridyl ring. Furthermore, magnetic properties, host–
guest and non-linear optical (NLO) materials are also observed
for this ligand, mainly when associated with transition metal
N uꢀ cleo de Espectroscopia e Estrutura Molecular, Departamento de
Qu ꢀı mica, Universidade Federal de Juiz de Fora, Campus Universit aꢀ rio
s/n, Martelos, Juiz de Fora, MG, 36036-900, Brazil. E-mail: luiz.
oliveira@ufjf.edu.br; Fax: +55 (32)21023310; Tel: +55 (32)21023310
16,17
ions.
†
Electronic supplementary information (ESI) available. CCDC
reference numbers 794353, 794354 and 794355. For ESI and
crystallographic data in CIF or other electronic format see DOI:
In the present work the synthesis, crystal structures and
vibrational spectroscopic analysis (infrared and Raman) of three
10.1039/c2ce05771a
1
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new supramolecular complexes containing aminosalicylate anion
ꢀ
samples dispersed on KBr disks and the spectral resolution was
ꢀ1
(
AS ) and nitrogen ligand trans-1,2-bis(4-pyridyl)ethylene (bpe)
acquired at 4 cm ; good signal-to-noise ratios were obtained
2+
2+
2+
as building blocks and transition metal ions (Co , Ni and Zn )
are described, together with thermal analysis. The correlation
between crystal data and vibrational spectra is also investigated,
aiming to understand the possible type of interactions which are
present in the solid state of these compounds, as well as the
influence of such interactions in the crystal packing.
from the accumulation of 128 spectral scans. Fourier-transform
Raman spectroscopy was performed using a Bruker RFS
3+
100 instrument, an Nd /YAG laser operating at 1064 nm in the
near-infrared region and a CCD detector cooled with liquid N
2
.
Good signal-to-noise ratios were obtained from 2000 scans that
were accumulated over a period of 30 min with a spectral reso-
ꢀ1
lution of 4 cm . All spectra were obtained at least twice to show
reproducibility, and there were no observed changes in band
positions or intensities. Single crystal X-ray data were collected
Experimental section
Chemicals and reagents
using an Oxford GEMINI A Ultra diffractometer with MoKa
ꢁ
(
l ¼ 0.71073 A) at room temperature (298 K) for compounds 1, 2
All chemicals used in this study were used as purchased without
and 3. Data collection, reduction and cell refinement were per-
further purification: 4-aminosalicylic acid (C
Sigma Aldrich), trans-1,2-bis(4-pyridyl)ethylene (bpe) (98.0%,
Sigma Aldrich), CoCl $6H O (98.0%, Vetec), NiCl $6H
99.9%, Sigma Aldrich) and ZnSO $7H O (99.0%, Vetec).
7 7 3
H NO , 99.0%,
formed by the CrysAlis RED, Oxford diffraction Ltd, Version
1
8
1
.171.32.38 program. The structures were solved and refined
2
2
2
2
O
19
using SHELXL-97. The empirical isotropic extinction param-
(
4
2
eter x was refined according to the method previously described
20
by Larson, and a multiscan absorption correction was
22
Synthesis
21
applied. The structures were drawn by ORTEP-3 for windows
23
ꢀ
and Mercury programs.
The aminosalicylate salt (AS ) used for vibrational measure-
ments (infrared and Raman) was prepared by the direct reaction
between 4-aminosalicylic acid and sodium carbonate solution, in
a 2 : 1 stoichiometry ratio. The resulting solution was evaporated
Results and discussions
2 4
Transition metal complexes of general formula [M(bpe)(H O) ]
(
by reduced pressure), with the formation of a white solid.
2+
2+
2+
AS
2
$4H
2
O (where M ¼ Co , Ni and Zn ) were obtained as
Elemental analysis for C H NO Na (or NaAS$2H O): calc.:
5
7
10
2
single crystals after a slow crystallization of a mixture containing
all the building blocks used in this study. In all cases, analytical
data suggest the formation of compounds with a stoichiometric
C, 38.82%; H, 4.77%; N, 6.63%; found: C, 39.03%; H, 4.95%;
N, 6.78%.
The compounds named [Co(bpe)(H
2
O)
4
]AS
O)
2
$4H
]AS
2
O (1), [Ni
$4H O (3)
2+
ꢀ
proportion of 1 : 2 : 1 (referring to M : AS : bpe), which is
slightly different from the synthesis stoichiometry (1 : 1 : 1).
The thermogravimetric analyses of all compounds reinforce
that the metallic complexes are hydrated. Thermogravimetric
(
bpe)(H O) ]AS $4H O (2) and [Zn(bpe)(H
2
4
2
2
2
4
2
2
were prepared by mixing 10 mL of an alcoholic solution con-
taining approximately 50 mg (0.27 mmol) of bpe and 5 mL of an
aqueous solution containing 0.27 mmol of the metal salts:
curves of the investigated compounds are shown in ESI†
ꢁ
CoCl
2 2 2 2 4 2
$6H O (65 mg), NiCl $6H O (65 mg) and ZnSO $7H O
(
Fig. S1–S3). All curves show a similar weight loss around 100 C
(
79 mg); after the addition, there was the immediate formation of
which can be attributed to the loss of eight water molecules for
a precipitate of orange, green and white colors, for the respective
salts used for synthesis. Then to this solution was added 10 mL of
a solution (water and ethanol 1/1 ratio) containing 42.0 mg
compounds 1, 2, and 3, at the calculated/experimental ratios of
2
0.88%/20.83%, 20.89%/20.66% and 20.68%/19.82%, respec-
tively. This fact suggests a weak chemical interaction between
crystallization and coordination water molecules within the
chemical environment of the obtained supramolecular struc-
tures. A second weight loss similar to all compounds can be
(
0.27 mmol) of 4-aminosalycilic acid, followed by stirring and
ꢁ
heating (50 C) until complete dissolution of the precipitate,
providing orange, green and yellow homogeneous solutions for
the Co(II), Ni(II) and Zn(II) ions. After approximately seven days
suitable single crystals were obtained, being orange for Co(II)
ꢁ
observed near 200 C, being attributed to the thermodecompo-
sition of one bpe molecule for compounds 1 (calcd/exp.: 26.42%/
[
52% yield], green for Ni(II) [63% yield] and yellow for Zn(II) [68%
2
2
7
6.77%), 2 (calcd/exp.: 26.43%/26.46%) and 3 (calcd/exp.:
yield]. Elemental analysis: [Co(bpe)(H O) ]AS $4H O (1): calc.:
2
4
2
2
6.18%/25.35%). The final residue starts to be formed above
ꢁ
C, 45.29%; H, 5.55%; N, 8.13%; found: C, 45.40%; H, 5.37%;
00 C for all compounds, and can be identified as one mol of
N, 8.16%; [Ni(bpe)(H O) ]AS $4H O (2): calc.: C, 45.30%;
2
4
2
2
metal (calcd/exp.: 8.55%/8.86%, calcd/exp.: 8.51%/8.36% and
calcd/exp.: 9.48%/9.11% for compounds 1, 2 and 3, respectively).
The structural design of the supramolecular compounds was
revealed by X-ray single crystal analysis. Tables 1–3 display the
crystallographic data, the main geometrical parameters and the
hydrogen interactions of the three new compounds containing
coordination polymers. Comparing the listed crystallographic
parameters it can be noticed that the compounds are isomor-
phous, crystallizing in a monoclinic crystal system with space
H, 5.56%; N, 8.13%; found: C, 45.73%; H, 5.50%; N, 8.20%; [Zn
(
bpe)(H O) ]AS $4H O (3): calc.: C, 44.87%; H, 5.50%; N,
2 4 2 2
8
.05%; found: C, 44.83%; H, 5.39%; N, 8.09%.
Physical measurements
Thermogravimetric measurements (TG/DTA) were performed
using a Shimadzu TG-60 with a thermo balance; samples were
ꢁ
ꢀ1
heated at a rate of 10 C min from room temperature to
ꢁ
1
000 C in a dynamic nitrogen atmosphere, at a flow rate of
1
group P2 /c. Fig. 1 presents the crystal structure of the
ꢀ1
1
00 mL min . Infrared spectra were obtained using a Bomem
complexes, where it can be seen as an extended one-dimensional
chain, formed by the bpe ligand covalently bonded to the metal
MB-102 spectrometer fitted with a CsI beam splitter, with the
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Table 1 Crystal data for [M(bpe)(H O)
2
4
]AS
[Co(bpe)(H
38CoN
2
$4H
2
O compounds containing coordination polymers
O) ]AS $4H O (1) [Ni(bpe)(H O) ]AS
38NiN
Compound
Formula
Formula weight/g mol
Temperature/K
Crystal system
2
4
2
2
2
4
2
$4H
2
O (2)
2 4 2 2
[Zn(bpe)(H O) ]AS $4H O (3)
C
26
H
4
O
14
C
26
H
4
O
14
C
H38ZnN O
26 4 14
ꢀ
1
689.53
298(2)
689.29
298(2)
695.99
298(2)
Monoclinic
/c
Monoclinic
P2 /c
Monoclinic
P2 /c
Space group
ꢁ
P2
1
1
1
a/A
ꢁ
13.7314(2)
7.6426(2)
14.8068(3)
96.755(2)
1543.09(6)
2
0.24 ꢂ 0.32 ꢂ 0.87
2.76–32.84
1.484
13.6410(3)
7.6543(2)
14.7589(4)
96.689(2)
1530.51(6)
2
0.16 ꢂ 0.28 ꢂ 0.38
2.65–32.78
1.496
13.7232(3)
7.6423(2)
14.8063(4)
96.804(2)
1541.90(7)
2
0.29 ꢂ 0.36 ꢂ 0.93
2.76–32.83
1.499
b/A
ꢁ
c/A
b
V/A
ꢁ
3
Z
Crystal size/mm
ꢁ
q Range/
d
ꢀ
1
calc/g cm
ꢀ
1
m(Mo Ka)/mm
Transmission factors (min/max)
Reflections measured/unique
0.631
0.709
0.871
0.785/0.859
7772/3166
0.0321
2615
242
0.788/0.893
7406/3143
0.0313
2550
249
0.693/0.777
8212/3165
0.0313
2640
250
R
int
2
o
2
o
Observed reflections [F
No. of parameters refined
R[F > 2s(F )]
> 2s(F
)]
o
o
0.0396
0.1336
0.0429
0.1446
0.0328
0.1005
2
o
2
) ]
wR[F
> 2s(F
o
S
RMS peak
1.182
0.074
1.130
0.108
1.158
0.154
2 4 2 2
Table 2 Selected geometrical parameters for the [M(bpe)(H O) ]AS $4H O compounds containing coordination polymers
[
Co(bpe)(H O) ]AS $4H O (1)
2
2
4
2
[Ni(bpe)(H
O)
2 4
]AS
2
$4H
2
O (2)
2 4 2 2
[Zn(bpe)(H O) ]AS $4H O (3)
ꢁ
Bond distance/A
M–O4
M–O5
M–N2
O3–C5
O1–C7
N2–C8
N2–C12
O2–C7
C3–N1
C13–C13
Average of bond angles/
O4–M–O4
O4–M–O5
O4–M–O5
O5–M–O5
O4–M–N2
O5–M–N2
2.077(2)
2.125(2)
2.165(2)
1.354(3)
1.250(3)
1.337(3)
1.347(3)
1.278(3)
1.365(3)
1.313(6)
2.046(2)
2.079(2)
2.119(2)
1.362(3)
1.242(4)
1.332(3)
1.337(3)
1.286(3)
1.369(4)
1.340(6)
2.079(2)
2.133(2)
2.161(2)
1.355(2)
1.240(3)
1.336(3)
1.334(3)
1.297(3)
1.364(3)
1.327(5)
ꢁ
180.0
180.0
180.0
89.08(7)
90.92(7)
180.00(10)
90.25(7)
91.24(8)
89.81(8)
90.19(8)
180.0
90.06(8)
91.27(9)
89.47(7)
90.53(7)
180.00(10)
90.40(6)
91.49(7)
site, which is also coordinated to four water molecules, thus
giving rise to a polymeric repeating unit with general formula
2
+
[
2 4 n
M(bpe)(H O) ] . Two uncoordinated aminosalicylate ions act
as anions, balancing the charge of the cationic blocks. Besides the
electrostatic supramolecular interaction, the aminosalicylate
anion also interacts with the polymeric unit via hydrogen
bonding through its carboxylate group with the water molecules
involved in the coordination; completing the repeating unit are
four crystallization water molecules, which are omitted from
Fig. 1 for better visualization. The coordination sphere of
metallic ions shows a slightly distorted octahedral geometry,
where each pyridyl ring of bpe adopts an axial position, whereas
four aqua ligands are in the equatorial positions. The averages of
Fig. 1 ORTEP representation of the 1D covalently bonded chain and
ꢁ
M–O and M–N bond distances are 2.101(2) and 2.165(2) A for
ꢁ
compound 1, 2.063(2) and 2.119(2) A for compound 2 and
aminosalicylate anion. Thermal ellipsoids are drawn at the 50% proba-
bility level for the compounds [M(bpe)(H
2
O) ]AS
4
2
$4H
2
O. The water
ꢁ
2.106(2) and 2.161(2) A for compound 3, respectively. The bpe
molecules are omitted for clarity. Symmetry code (iii): ꢀx, ꢀy, ꢀz.
1
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ligand presents a bis-monodentate coordination mode between
ꢁ
supramolecular cavity formed by the hydrogen interaction can
4
8
two adjacent metallic centers (Co/Co ¼ 13.731(2) A, Ni/Ni ¼
be represented as a net graph: N ¼ C (16)R (26)S(6), where the
1
4
8
ꢁ
ꢁ
1
3.641(2) A, Zn/Zn ¼ 13.723(2) A) in a one-dimensional
symbols C, R and S describe an infinite chain, ring set and
intramolecular interaction respectively, whereas the subscript
numbers indicate the quantities of donor atoms and the super-
infinite chain. It can also be observed that the two pyridyl rings of
the bpe ligand are coplanar, since both carbon atoms of the
aliphatic chain present p bonds.
25,26
script numbers indicate the acceptor atoms in each set.
Fig. 2(a) shows an interesting supramolecular arrangement
Another two types of supramolecular hydrogen bonding inter-
actions can be observed: one between two parallel polymer
chains and two crystallization water molecules, and other
between the aminosalicylate anion, the polymeric chains and the
crystallization water molecules, generating net graphs classified
parallel to the a-crystallographic axis involving one-dimensional
2+
polymer chain [M(bpe)(H
2
O)
4
]
n
common to the three synthe-
sized complexes. Another interaction occurs only through
hydrogen bonding between aminosalicylate anions and water
molecules of crystallization with the formation of a 2D cavity
which encapsulates the one-dimensional polymer chain
throughout the full length. The one-dimensional polymer chain
interacts with the supramolecular cavity through hydrogen
interactions between the coordination water molecules and the
crystallization water molecules, as well as the carboxylic and
hydroxyl groups from the aminosalicylic anion.
2
3
3
as N2 ¼ R (34) and N ¼ C (16)R (27), respectively. These
4
3
4
4
supramolecular interactions can be seen in Fig. S4 and S5, ESI†.
Fig. 3 displays the two-dimensional supramolecular arrange-
ment formed parallel to the bc-plane through holes which are
connected to each other by hydrogen bonds between amino-
salicylate anions and crystallization water molecules. Each one of
these connectors gives rise to three independent cavities in the
structure, resembling imperfect hexagons; the interaction
between them creates a new macrostructure that resembles
a honeycomb. However, such supramolecular structure only
exists through intermolecular forces, more precisely by hydrogen
interactions; due to this fact, the system can be classified as an
Fig. 2(b) shows that the cavity parallel to the bc-plane consists
of three aminosalicylate anions and five water molecules, which
interact via hydrogen bonds through the carboxyl and hydroxyl
groups. These interactions are classified as moderate and weak
ꢁ
because the interaction distances 2.797(3) A (O1/O7), 2.830
ꢁ
ꢁ
ꢁ
(
3) A (O6/O7), 2.835(3) A (O2/O6) and 2.934(3) A (O7/O3)
for complexes 1, 2 and 3 (see Table 3) between donor and
ꢁ
receptor atoms present in the structure are 2.5 A < x < 2.8 A or
ꢁ
ꢁ
24
x > 2.8 A. The geometrical parameters of these hydrogen
bonds are listed in Table 3. Another classification for this
Fig. 2 Two different views of the 1D covalent linear chain [M(bpe)
2+
(H
2
O)
4
]
n
: (a) along the a-crystallographic axis and (b) in the bc-plane
Fig. 3 Supramolecular interactions of the cavities, with formation of the
involved in the cavity.
pseudo-honeycomb structure.
Table 3 Selected geometrical parameters of [M(bpe)(H O)
2
4
]AS
2
$4H
2
O, taking into account the hydrogen interactions
[
Co(bpe)(H
2
O) ]AS
4
2
$4H
2
O (1)
[Ni(bpe)(H
2
O)
4
]AS
2
$4H
2
O (2)
2 4 2 2
[Zn(bpe)(H O) ]AS $4H O (3)
ꢁ
D/A/A
N1/O7
O3/O2
O4/O1
O4/O2
O5/O6
O6/O7
O6/O2
O7/O3
O7/O1
D–H/A/
3.021(3)
2.534(3)
2.732(3)
2.726(3)
2.792(3)
2.830(3)
2.835(3)
2.934(3)
2.797(3)
3.024(4)
2.523(3)
2.731(3)
2.747(3)
2.776(3)
2.844(4)
2.836(3)
2.925(3)
2.790(3)
3.017(3)
2.522(2)
2.726(2)
2.738(2)
2.780(3)
2.839(3)
2.822(3)
2.945(3)
2.792(3)
ꢁ
N1–HN1A/O7
O3–H3/O2
O4–H4A/O1
O4–H4B/O2
O5–H5A/O6
173(3)
147.00
158.00
164(3)
158.00
162(5)
147.00
175(3)
177(3)
172(3)
159(3)
147.00
172(2)
172(3)
168(2)
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imperfect pseudo-honeycomb. Inside each cavity, which
comprises the observed pseudo-honeycomb parallel to the crys-
tallographic a axis, the presence of the one-dimensional polymer
2+
2 4 n
chain generated by its monomeric fragment [M(bpe)(H O) ]
can be seen, covalently expanded in only one dimension. The
presence of hydrogen bonds in the pseudo-honeycomb structure
generates a 2D system parallel to the bc-plane; this arrangement
interacts with other arrangements (or neighbors) via hydrogen
bonding parallel to the a-axis, thus causing the structure to
present a 3D supramolecular arrangement. Besides the hydrogen
bonds, which contribute significantly to the stability of the
crystal lattice, other supramolecular interactions can also be
observed between the rings of the pyridyl moiety and the ami-
nosalicylate anion, as for instance the p-stacking interactions,
ꢁ
showing centroid–centroid distances of 3.699(2) A, 3.701(2) A
ꢁ
ꢁ
and 3.701(2) A and horizontal offsets of 1.435(3), 1.429(2), and
1
.434(3) for compounds 1, 2 and 3, respectively. The N–H/p
interactions (similar to C–H/p) can also be seen between the
hydrogen of the amino group of one aminosalicylate and the ring
of another aminosalicylate anion, with N/p distances of
Fig. 4 Infrared spectra of [M(bpe)(H
ꢀ
2
O) ]AS
4
2
$4H
2
O (where M ¼ Co,
Ni and Zn, AS is aminosalicylic anion and bpe is 1,2-bis(4-pyridyl)-
ꢀ
ethylene); for comparison purposes the spectra of the ligands (AS and
ꢁ ꢁ ꢁ
3
.516(2) A, 3.497(2) A and 3.512(2) A for compounds 1, 2 and 3,
bpe) are also displayed.
27,28
respectively.
It is important to mention that the literature has
shown only a few examples of pseudo-honeycombs; for instance,
2
9
Kulynych and Shimizu have applied the term ‘‘pseudo’’ for the
formation of irregular hexagons, with formation of covalently
linked cavities originated from the interaction between 5-sulfoi-
assigned to the CC stretching modes of the phenyl ring. Another
ꢀ
1
very characteristic band can be observed at 1188 cm , referring
to the n(C–O) of the hydroxyl group. The Raman spectrum of the
ꢀ1
30
sophthalic acid and copper ion, whereas Perruchas et al. have
used the same term for the description of the intermolecular
forces through cationic blocks responsible for the creation of the
arrangements which resemble a honeycomb. In our study, the
interesting feature is the cavity formed by the mixture between
anionic and neutral molecular blocks. However, the major
difference rests on the guest moiety, which in our case is the
polymeric chain formed by metal ions and bpe ligand; from our
knowledge, this can be considered as the first work in the liter-
ature reporting this type of structure for aminosalicylate anions.
The vibrational spectra of all ligands and complexes investi-
gated in this work are displayed in Fig. 4 and 5 (infrared and
Raman spectra, respectively). All spectra are very similar and in
agreement with the crystal data, which indicates the same crys-
talline arrangement for all compounds obtained. The main
vibrational modes are summarized in Table 4, as well as the
aminosalicylate anion shows a strong band at 1625 cm , also
tentatively assigned to the n(C]O) of the carboxyl group, and in
ꢀ1
the region 1400–1600 cm some intense bands can be seen which
are related to the (CC) stretching modes of the phenyl ring, in the
same way as observed in the infrared spectrum.
In the infrared spectrum for the free bpe ligand, a very intense
ꢀ1
band at 1597 cm can be assigned to the coupled n(CC)/n(CN)
stretching modes of the pyridyl rings, whereas two other impor-
ꢀ1
tant and intense bands appear at around 1416 and 989 cm are
31–34
tentative assignment based on similar chemical systems.
In the infrared spectrum of the aminosalicylate anion can be
ꢀ1
observed a broad band around 3200–3500 cm region, assigned
to the stretching modes of H O and NH groups. These same
2
2
bands are also observed for the three complexes, suggesting the
hydration of all synthesized compounds, previously confirmed
by the data from elemental analysis and X-ray diffraction. The
ꢀ1
strong band observed at 1637 cm can be assigned to the n(C]
O) of the carboxyl group; this mode appears to be shifted by ca.
ꢀ1
1
5 cm to lower wavenumbers when compared to the acidic
species, due to the loss of the acidic hydrogen atom and
a consequent increase of the electronic delocalization over the
carboxyl group in the anionic species formed. In a vibrational
study involving infrared, Raman and SERS, Panicker and
Fig. 5 Raman spectra of [M(bpe)(H O)
2
4
]AS
2
$4H
2
O (where M ¼ Co, Ni
32
coworkers have assigned the main modes of aminosalicylic acid
ꢀ
and Zn, AS is aminosalicylic anion and bpe is 1,2-bis(4-pyridyl)
ꢀ
and its corresponding salt, especially in the spectral region
ethylene); for comparison purposes the spectra of the ligands (AS and
ꢀ1
between 1400 and 1600 cm , which presents some intense bands
bpe) are also displayed.
1
816 | CrystEngComm, 2012, 14, 1812–1818
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ꢀ
1
2 4
Table 4 Raman (R) and infrared (IR) wavenumbers (in cm ) and tentative assignment of the most important bands observed in the [M(bpe)(H O) ]
a
AS
2
$4H
2
O spectra
[Co(bpe)(H
AS
2
O)
O (1)
4
]
[Ni(bpe)(H
AS
2
O)
O (2)
4
]
[Zn(bpe)(H
AS
2
O)
4
]
NaAs
IR
bpe
IR
2
$4H
2
2
$4H
2
2 2
$4H O (3)
R
R
IR
R
IR
R
IR
R
Tentative assignment
4
59 w
440 w
532 m
465 w
464 w
465 w
d(CC)
d(C–O) + d(C]O)
d(CH) + d(C–C)
d(CC)
Ring breathing
d(CC)
d(NH)
d(CC)
d(C–N) + n(C–C)
d(CH)
d(CH)
5
56 vs
557 m
623 m
559 m
625 m
557 m
623 m
6
7
8
29 m
06 m
10 m
6
70 w
701 w
797 m
829 m
704 w
795 m
829 m
702 w
795 m
829 m
777 m
750 w
751 w
751 w
2
8
29 vs
97 vs
8
8
31 m
62 m
834 m
858 m
880 w
879 w
881 w
8
79 w
d(C]C)
9
73 m
973 m
968 m
968 m
968 m
d(NH)
2
Ring breathing
d(CH) + d(C–C)
n(C–O)
n(C–C)
n(C–O)
d(CH)
n(CC)
9
996 m
1018 m
1151 m
1194 w
1020 m
1022 m
1151 m
1194 w
1023 w
1020 m
1151 m
1194 w
1021 m
1
1
168 w
188 m
h
1
1
1
197 s
1201 m
1243 w
1339 w
1201 s
1243 w
1339 w
1201 s
1245 w
1340 w
1
1
1
1
1
228 m
304 s
388 s
448 s
508 m
1232 s
1318 s
1392 s
1450 m
1221 w
1221 w
1223 w
h
237 m
342 w
1315 s
1352 m
1390 s
1315 s
1352 m
1391 s
1313 s
1352 m
1391 vs
d(CH) + n(CC)
n(C–O)
n(CC)
n(CC)
n(CN) + n(CC)
n(CC)
n(CN) + n(CC)
n(CC)/n(CN)
n(C]O)
c
1
418 vs
1458 vs
1458 vs
1458 s
1
492 w
1518 m
1578 vs
1612 s
1514 m
1578 vs
1612 s
1514 m
1580 vs
1612 vs
1556 s
1597 vs
1548 w
1596 vs
1555 w
1614 vs
1554 w
1614 vs
1556
1614 vs
1637 s
1625 m
1641 vs
3054 w
1643 s
1642 vs
3066 w
1643 s
1641 vs
3065 w
1643 s
1643 vs
3064 w
n(C]C)
n(CH)
n(CH)
n(OH)
v
3
036 w
3062 m
3068 sh
3340 br
3456 m
3074 sh
3335 br
3456 m
3072 sh
3350 br
3456 m
3
3
334 m
402 m
2
n(NH )
a
Abbreviations: vs, very strong; s, strong; m, medium; w, weak; v, vinyl; sh, shoulder; br, broad. The subscript c designates a motion of the carboxyl
group, the subscript h a motion of a hydroxyl group.
ꢀ
1
attributed to the n(CC) and n ring respectively. The Raman
band at 1612 cm , assigned to the n(CC)/n(CN) stretching of the
ꢀ1
ꢀ1
spectrum displays two intense bands at 1641 and 1596 cm ,
which can be assigned to the n(C]C) stretching referring to the
carbon atoms of the aliphatic chain between the pyridyl rings and
n(CC)/n(CN) of the same ring respectively. Another less intense
band which shows the same characteristic as before is located at
pyridyl ring; this same band appears to be shifted by ca. 15 cm
ꢀ1
compared to the free ligand which occurs at 1597 cm , as
previously assigned as a mode involving pyridine ligands in the
35
coordination environment. The presence of the aminosalicylate
anion in the formation of the supramolecular structure is
ꢀ1
ꢀ1
9
96 cm and attributed to ring breathing. For the bpe ligand the
observed through a band of medium intensity at 1194 cm ,
spectral regions (in both infrared and Raman) around 1590 and
referring to the n(C–O) of the hydroxyl group. Other vibrational
ꢀ1
1
650 cm , where CC and CN stretching modes are expected, are
modes characteristic of this structure are seen in the 1350–
ꢀ1
very important in the confirmation of coordination or any other
disturbance caused to the structure, found through the simple
displacement of the vibrational modes.
1550 cm region, and assigned to the stretching of the CC modes
of the phenyl ring. The spectroscopic analysis indicates the
disappearance of the vibrational mode referring to the carbonyl
ꢀ1
For the compounds 1, 2 and 3 the infrared spectrum exhibits
stretch at 1637 cm , which may be understood by a symmetry
change in the structure, thus favoring the intensity decrease of
the CO stretching in the infrared technique. The Raman spectra
of complexes 1, 2, and 3 showed a very similar profile, with the
high intensity bands related to the bpe ligand. For instance, two
vibrational modes can be mentioned occurring at 1614 and
ꢀ1
an intense band around 1643 cm that can be attributed to the
carbon–carbon stretching mode in the aliphatic chain present
between the two pyridyl rings of the bpe ligand. The interesting
feature in the appearance of this band is the ratification of the
bpe ligand coordination, since this mode has no activity through
the infrared technique when the ligand is in the free form.
Another vibrational mode which confirms the presence of this
ligand in the formation of the supramolecular complex is the
ꢀ1
1021 cm , assigned to the n(CC)/n(CN) and ring breathing
modes, respectively. As discussed before, both vibrational modes
appear to be shifted to high wavenumber, thus suggesting the
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coordination of the nitrogen atom of bpe to the metal site.
Another two modes of high intensity, but which present a small
wavenumber shift after coordination to the metal site, appear at
the X-ray facilities. Part of this work is a collaboration of
members of the Rede Mineira de Qu ꢀı mica supported by
FAPEMIG.
ꢀ
1
1
642 and 1201 cm , assigned to the n(C]C) and n(C–C) modes,
present in the aliphatic chain between the pyridyl rings. Another
intriguing fact about the Raman spectra is the decrease or
absence of the main vibrational modes concerning the charac-
terization and/or confirmation of the aminosalicylate anion in
the supramolecular structure. Vibrations such as the aromatic
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2
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Conclusions
In this paper three new supramolecular complexes have been
1
reported named [Co(bpe)(H O) ]AS $4H O (1), [Ni(bpe)(H O) ]
4
2
4
2
2
2
AS $4H O (2) and [Zn(bpe)(H O) ]AS $4H O (3). All
2
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u€ y u€ kg u€ ng o€ r and
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1
/c) were observed. All structures show one
1
2+
polymeric [M(bpe)(H
2
O)
4
]
n
chain covalently linked parallel to
1
the a-crystallographic axis. Each one of the polymer chains
appears to be involved in a two-dimensional supramolecular
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Acknowledgements
3
The authors thank CNPq, CAPES, FAPEMIG (PRONEX
3
3
04370/10) and FINEP (PROINFRA 1124/06) for financial
support and also LabCri (Departamento de F ꢀı sica—UFMG) for
1
818 | CrystEngComm, 2012, 14, 1812–1818
This journal is ª The Royal Society of Chemistry 2012