A novel centrosymmetric dinuclear cadmium(II) Schiff base complex with unusual bridging carboxylate: Synthesis, crystal structure and luminescence properties

Article abstract of DOI:10.1016/j.molstruc.2008.10.002

A novel centrosymmetric dinuclear Cd(II) complex [CdL (CH₃COO)]₂ (1) has been synthesized by the reaction of Cd(CH₃COO)₂·2H₂O with a Schiff base ligand HL [HL = (E)-2-((pyridin-2-yl)methyleneamino)ben

Full text of DOI:10.1016/j.molstruc.2008.10.002

Journal of Molecular Structure 919 (2009) 361–365
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
A novel centrosymmetric dinuclear cadmium(II) Schiff base complex with unusual
bridging carboxylate: Synthesis, crystal structure and luminescence properties
Shyamapada Shit a, Joy Chakraborty a, Brajagopal Samanta a, Guillaume Pilet b, Samiran Mitra a,
*
a Department of Chemistry, Jadavpur University, Kolkata 700032, West Bengal, India
b Groupe de Cristallographie et Ingénierie Moléculaire, Laboratoire des Multimatériaux et Interfaces, UMR 5615, Université Claude Bernard Lyon 1, Bât. Jules Roulin,
43 Bd du 11 Novembre 1918-69622, Villeurbanne Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel centrosymmetric dinuclear Cd(II) complex [CdL (CH3COO)]2 (1) has been synthesized by the reac-
tion of Cd(CH3COO)2ꢀ2H2O with a Schiff base ligand HL [HL = (E)-2-((pyridin-2-yl)methyleneamino)ben-
zoic acid]. Title complex has been systematically characterized by elemental analysis, FT-IR, and thermal
studies. Single crystal X-ray structural analysis reveals that Cd(II) centers in 1 adopt distorted mono-
Received 12 September 2008
Accepted 2 October 2008
Available online 12 October 2008
capped octahedral geometry and held together by rare doubly
l-2, 2 carboxylato bridging which is
Keywords:
Dinuclear complex
Schiff base
Heptadentate Cd(II)
Crystal structure
believed to be first relating to anthranilic carboxylate. At room temperature, compound in the solid state
exhibits moderate photoluminescence activities indicating its potential as promising photoactive
material.
Ó 2008 Elsevier B.V. All rights reserved.
Rare l-2,2 carboxylato bridging
Photoluminescence
1. Introduction
plexes having novel structural features. The versatility of this
group as a ligand is illustrated by the variety of its coordination
The rational design and synthesis of novel coordination com-
plexes based on transition or non-transition metals and multifunc-
tional bridging ligands is of great research interest, due to their
interesting topologies and potential applications as functional
materials. The most intelligent strategy to obtain coordination
polymers is to employ appropriate bridging ligands that are capa-
ble of binding to several metal centers through direct bond forma-
tion. In this regard, Schiff base ligands have proven themselves
very effective in constructing supramolecular architectures such
as coordination polymers, double and triple helicates [1a,b] with
their additional wide applications as antibacterial, antiviral, anti-
fungal agents [2], in homogeneous or heterogeneous catalysis
[3a–e] and magnetism [4]. Schiff bases are also considered as po-
tential anticancer drugs [5a,b] and when administered as their me-
tal complexes, the anticancer activity is enhanced in comparison to
the free ligand [6a,b].
modes when acting as a bridge [7a,b]. Extensive work has been car-
ried out by using multicarboxylate ligands because of their inter-
esting structural characteristics [8a–f]. Several members of the
aminopolycarboxylates/amino acids can also be referred in this
regard.
In this context, anthranilic acid is considered as an important
and competent precursor in the designing of carboxylate incorpo-
rated Schiff base ligands because of its free primary amine func-
tionality which is useful during the condensation with an
aldehyde/ketone while its free carboxylate end is considerably
flexible from the point of view of the steric orientation in the for-
mation of less strained covalent bonds with the metal centers,
hence shows versatile bridging modes (Scheme 1a and b). Schiff
bases derived from a large number of carbonyl compounds and
amines or amino alcohols are plenty in the chemical literature,
however, the studies on their optical properties, such as fluores-
cence, are not that much ample.
In this regard, Schiff base ligands with incorporated free carbox-
ylate ends are also considered as useful because of their simulta-
neous activities as both chelator and a bridging ligand. Moreover
the free carboxylate may take part in the charge neutralization of
the target complex. Hence, the carboxylate group is one of the
most widely used bridging ligands for designing polynuclear com-
On the other hand, Cd(II) due to its d10 electronic configuration,
is particularly suited for the construction of coordination polymers
and networks. The spherical d10 configuration is associated with a
flexible coordination environment so that geometries of these
complexes can vary from tetrahedral to octahedral and severe dis-
tortions in the ideal polyhedron occur easily. Furthermore, due to
the general lability of d10 metal ion complexes, the formation of
coordination bonds is reversible which enables metal ions and li-
* Corresponding author. Tel.: +91 033 2414 6666 x2505; fax: +91 033 2414 6414.
E-mail address: smitra_2002@yahoo.com (S. Mitra).
0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2008.10.002

362
S. Shit et al. / Journal of Molecular Structure 919 (2009) 361–365
temperature indicates its potential to serve as photoactive
material.
b
a
2. Experimental
N
2.1. Materials
N
N
O
N
O
All the chemicals and solvents used for the synthesis were of
analytical grade. Pyridine-2-carboxaldehyde, Cd(CH3COO)2ꢀ2H2O,
and anthranilic acid were purchased from Aldrich Chemical Co.
Inc. and were used without any further purification. The solvents
used were of analytical grade.
O
O
2.2. Syntheses of the ligand and complex
c
2.2.1. Synthesis of (E)-2-((pyridin-2-yl)methyleneamino)benzoic acid
(HL)
The Schiff base HL was prepared following the literature proce-
dure [18]. Pyridine-2-carboxaldehyde (0.535 g, 5 mmol) was re-
fluxed with anthranilic acid (0.685 g, 5 mmol) in 50 ml methanol.
A red solution was obtained. It was subjected to TLC which re-
vealed the presence of some unreacted starting materials along
with the Schiff base product. The Schiff base product was isolated
by column chromatography in order to get it in the purified form.
The purified ligand was then evaporated under reduced pressure to
yield a gummy mass, which was dried and stored in vacuo over
CaCl2 for subsequent use. Yield: 1.061 g (87%). Anal. Calc. for
C13H10N2O2: C, 69.02; H, 4.46; N, 12.38. Found: C, 69.09; H, 4.43;
N, 12.42%.
N
N
O
O
Scheme 1. Bridging modes of anthranilic carboxylate in the deprotonated Schiff
base ligand (HL).
2.2.2. Synthesis of [CdL (CH3COO)]2 (1)
gands to rearrange during the process of polymerization to give
highly ordered network structures. Because of border-line acidity,
cadmium(II) prefers to bind both hard and soft donors which thus
widens its scope in the study of crystal engineering. Diamagnetic
cadmium(II) is chosen for its d10 configuration, permitting a wide
variety of geometries and coordination numbers. Cd(II) coordina-
tion polymers bear various topologies as the metal coordination
number can be expanded up to seven and the directional property
of the coordination bond is somewhat relaxed by the absence of
any crystal field stabilization energy in the d10 electronic configu-
ration. The terminal or blocking co-ligands, which are usually used
along with the bridging ligand to complete the metal coordination
sphere, can alter the supramolecular assembly and consequently
the type of structure formed taking the advantage of the flexibility
of the coordination sphere. Moreover, cadmium(II), though treated
mainly as a toxic element to the biological systems, similar to Pb
and Hg, has been found recently as the catalytic center in a newly
discovered carbonic anhydrase [9]. It’s complexes with nitrogen
containing ligands are also used in ligand exchange chromatogra-
phy [10]. Cadmium(II) Schiff base complexes are often found as
photochemically important too.
Working on Schiff base transition metal complexes, our group
has reported a few [10–14]. However, as a part of our continuing
effort on the transition metal complexes with Schiff base ligands
incorporating free carboxylate end [15–17], we have reported
herein a novel centrosymmetric dinuclear cadmium(II) complex
[CdL(CH3COO)]2 (1) [HL = (E)-2-((pyridin-2-yl)methyleneami-
no)benzoic acid]. Systematic characterization of the complex has
been performed by elemental analysis, FT-IR, and thermal meth-
ods. Structural elucidation by the single crystal X-ray diffraction
technique reveals that Cd(II) centers in 1 adopt distorted mono-
Appropriate quantity of the solid Schiff base ligand HL (0.452 g,
2 mmol) was dissolved in dry methanol (20 ml). A solution of
Cd(CH3COO)2ꢀ2H2O (0.798 g, 3 mmol) in dry methanol (10 ml)
was added to this solution. The mixture was stirred for 2 h at room
temperature. The resulting yellow coloured solution was then fil-
tered off and the filtrate was left undisturbed. After 10 days yellow
hexagonal shaped X-ray diffraction quality single crystals sepa-
rated out on the slow evaporation of the filtrate. They were filtered,
and dried in vacuo over CaCl2. Crystals are insoluble in common po-
lar and non-polar solvents. Yield: 0.554 g (75%) with respect to the
metal substrate. Anal. Calc. for C30H24Cd2N4O8: C, 45.42; H, 3.05; N,
7.06. Found: C, 45.41; H, 3.02; N, 7.04 %.
2.3. Physical measurements
The Fourier transform infrared spectra of the ligand (HL) and 1
were recorded on a Perkin Elmer Spectrum RX I FT-IR spectropho-
tometer with a KBr disc in the range 4000–200 cmꢁ1. Elemental
analyses (C, H, and N) were carried out using a Perkin Elmer
2400 II elemental analyzer. The emission spectra for both HL and
1 were recorded at room temperature (298 K) on a Hitachi 850
fluorescence spectrophotometer. Thermogravimetric analyses
were carried out at a heating rate of 10 °C/min with a Mettler-To-
ledo star TGA/SDTA-851e thermal analyzer system in a dynamic
atmosphere of N2 (flow rate 80 ml/min) in an alumina crucible
for the range 25–630 °C.
2.4. Single crystal X-ray structure determination
A
good diffraction quality air stable single crystal of
(0.51 ꢂ 0.88 ꢂ 0.94 mm) was mounted in a Nonius Kappa CCD dif-
fractometer equipped with graphite monochromated Mo-K radi-
1
capped octahedron geometry and held together by rare l-2,2 car-
a
boxylato bridging. Such bridging mode (Scheme 1c) is rare in the
literature and is believed to be first relating to anthranilic carbox-
ylate. Solid state photoluminescence measurement of 1 at room
ation (k = 0.71073 Å). Crystal data were collected using the
software COLLECT [19a] at a temperature of 290 K. Cell refine-

S. Shit et al. / Journal of Molecular Structure 919 (2009) 361–365
363
observed at 1412 and 1545 cmꢁ1 in the complex may be assigned
Table 1
for
frequencies [
m m
s(COOꢁ) and as(COOꢁ) vibrations. The difference of these two
Crystallographic data collection and structural refinement for 1.
D
m
=
m
s(COOꢁ) ꢁ
m
as(COOꢁ)], 133 cmꢁ1 indicates the
Chemical formula
Formula weight
Crystal system
Space group
a (Å)
C30H24Cd2N4O8
793.34
Monoclinic
P21/a (no.14)
bridging coordination modes of the carboxylate group of the Schiff
base [20,21]. Sharp band appearing at 375 cmꢁ1 corresponds to
m(CdAO) is also observed for 1 [14]. Observed strong sharp bands
9.4019(2)
in the regions 1605–1597, 1485–1460, 1445–1420, 1055–1040
and 1015–1005 cmꢁ1 for 1 may be correlated to the coordinated
pyridine ring [20].
b (Å)
c (Å)
17.6839(5)
10.0512(3)
90
a
(°)
b (°)
116.4890(10)
c
Z
(°)
90
2
3.2. Crystal structure of [CdL (CH3COO)]2 (1)
T (K)
290
kMo
(Å)
0.71073
1.762
1.479
784
2.3–28.8
3984
3854
2768
2768; 199
R1 = 0.0399, wR2 = 0.0492
R1 = 0.0552, wR2 = 0.0610
0.0399
0.46
K
a
Single crystal X-ray structure analysis shows that the structure
of 1 consists of a discrete dimeric unit. An ORTEP view of the asym-
metric unit of the complex has been shown in the Fig. 1. Structure
determination reveals the centrosymmetric nature of the dimer in
which the coordination environments of metal atoms are alto-
gether satisfied by deprotonated Schiff base ligands and acetate
ions. The Cd(II) ions in the complex are hepta coordinated. Five
coordinating positions of each Cd(II) center is satisfied by five do-
nor atoms from two Schiff base ligands and the other two coordi-
nating positions are satisfied by the chelating acetate ion. The
coordination geometry of cadmium atoms [Cd1 and its symmetry
related counterpart] can be best described as a distorted mono-
capped octahedron. A pyridine nitrogen N6 and an imine nitrogen
N7 along with five oxygen donors (O4 and O5, coming from chelat-
ing acetate whereas O2 and O3, from a chelating anthranilic car-
boxylate of the same Schiff base ligand, remaining oxygen atom
O2* is a symmetry related counterpart of O2 coming from another
Schiff base ligand) constitute the N2O5 donor set around the Cd(II)
metal center [Cd1AN6, 2.366(4); Cd1AN7, 2.366(3); Cd1AO2,
2.412(3); Cd1AO2*, 2.379(3); Cd1AO3, 2.426(3); Cd1AO4,
2.299(3) and Cd1AO5, 2.391(4) Å (translation of symmetry codes
to equivalent positions: * = 1 ꢁ x, ꢁy, ꢁz)]. The carboxylate group
of acetate ligand acts as a bidentate chelating group while the free
anthralinic carboxylate end of the Schiff base acts as bridging as
Dc (g cmꢁ3
)
l
(mmꢁ1
F(000)
)
h Range (°)
Total data
Unique data
Observed data [I > 2
Nref ; Npar
r
(I)]
Final R Indices[I > 2
R indices (all data)
Rint
r(I)]
4
qmax (e Åꢁ3
qmin (e Åꢁ3
)
)
4
ꢁ1.44
Goodness-of-fit on F2
1.0587
ments were carried out using DENZO-SCALEPACK [19a]. No signif-
icant intensity variation was observed for each of them. A total of
3984 reflections (3854 unique reflections, Rint = 0.065) were col-
lected applying the boundary condition I > 3r(I). No absorption
correction was applied to the data sets of 1. Data reduction was
performed using DENZO-SCALEPACK for 1 [19a]. According to the
observed systematic extinctions, the structure of 1 was solved by
direct methods using the SIR97 [19b] program combined with Fou-
rier difference syntheses and refined against F using reflections
maintaining the boundary condition I > 3r(I) (CRYSTALS program)
well as chelating groups (totally tridentate) in a l-2,2 mode, where
[19c]. The molecular graphics and crystallographic illustrations of
the complex were prepared using the CAMERON [19d], ORTEP
[19e] programs. For all non-hydrogen atoms, the anisotropic dis-
placement parameters have been refined successfully. Hydrogen
atoms of the aromatic rings and the imino groups were placed geo-
metrically and refined as a riding model taken from a Fourier dif-
ference map and refined with isotropic thermal parameters.
Maximum, minimum peaks (eÅꢁ3) in the final difference Fourier
synthesis were found as 0.46, and ꢁ1.44, respectively. All the rele-
vant crystallographic data and structure refinement parameters for
1 are summarised in Table 1.
two oxygen atoms of the carboxylate group of two bridging anthra-
nilic carboxylate chelate and also bridge the two adjacent Cd(II)
centers, thus yielding a four-membered {CdAOACAO}2 metallacy-
cle [22] with a CdꢀꢀꢀꢀCd separation of 3.902(3) Å. Thus two adjacent
cadmium centers are connected to each other by a doubly oxo-
bridging mode of this carboxylate through the atoms O2 and
3. Results and discussion
3.1. Fourier transform infrared spectra
The solid state Fourier transform infrared spectra of HL and 1
were recorded on a Perkin Elmer Spectrum RX I FT-IR spectropho-
tometer in the range 4000–200 cmꢁ1. The samples were studied as
powder dispersed in KBr pellets. HL displayed a sharp strong peak
at 1642 cmꢁ1 corresponds to imine (CH@N) group. HL also showed
two bands at 1605 and 1413 cmꢁ1 assignable to as(COOꢁ) and
m
m
s(COOꢁ), respectively, for free carboxylate. The ligand (HL) shows
strong sharp bands at 1586, 1473 and 1430 cmꢁ1 correspond to the
pyridine skeleton [20].
However, the azomethine stretching frequency is lowered by
30 cmꢁ1 upon complexation, indicating the coordination of imine
(CH@N) nitrogen, which is further supported by bands in the range
Fig. 1. ORTEP view of the dimeric unit of 1, with atom labels. Displacement
ellipsoids are shown at 50% probability level. The hydrogen atoms are omitted for
clarity (translation of symmetry codes to equivalent positions: * = 1 ꢁ x, ꢁy, ꢁz).
of 455 cmꢁ1 corresponding to
m(Cd–N) observed for 1. Bands

364
S. Shit et al. / Journal of Molecular Structure 919 (2009) 361–365
Table 2
Selected bond lengths (Å) and bond angles (°) for complex 1.
Bond lengths
(Å)
Bond angles
(°)
Cd1AO2
Cd1AO4
Cd1AO5
Cd1AN6
Cd1AN7
Cd1AO2*
Cd1AO3*
2.412(3)
2.299(3)
2.391(4)
2.366(4)
2.366(3)
2.379(3)
2.246(3)
O2ACd1AO4
O2ACd1AO5
O2ACd1AN6
O2ACd1AN7
O2ACd1AO2*
O2ACd1AO3*
O4ACd1AO5
O4ACd1AN6
O4ACd1AN7
O2*ACd1AO4
O3*ACd1AO4
O5ACd1AN6
O5ACd1AN7
O2*ACd1AO5
O3*ACd1AO5
N6ACd1AN7
O2*ACd1AN6
O3*ACd1AN6
O2*ACd1AN7
O3*ACd1AN7
O2*ACd1AO3*
Cd1AO2AC10
Cd1AO2ACd1*
Cd1*AO2AC10
Cd1*AO3AC10
Cd1AO4AC11
Cd1AO5AC11
Cd1AN6AC14
Cd1AN6AC15
Cd1AN7AC9
Cd1AN7AC19
146.56(11)
92.45(9)
108.33(11)
67.55(10)
70.90(9)
104.66(10)
55.26(10)
102.50(13)
113.02(11)
97.77(11)
92.06(11)
137.40(12)
85.55(12)
89.55(11)
129.13(13)
69.87(12)
131.97(10)
81.90(13)
137.87(10)
145.29(13)
54.10(11)
119.2(2)
109.10(11)
93.3(2)
91.3(2)
93.9(2)
89.9(3)
125.7(3)
116.6(3)
122.4(3)
117.0(3)
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
Fig. 2. Luminescence spectra of HL and complex 1 (kexc = 435 nm for HL and 380 nm
for 1).
energy through radiationless decay of the intraligand emission ex-
cited state [27,28].
As a result, the reported complex may be regarded as an excel-
lent candidate of violet–blue light emitting materials, since the
condensed material is highly thermally stable and insoluble in
common polar and non-polar solvents [24,26]. Enhancement of
fluorescence through complexation is, however, of much interest
as it opens up the opportunity for photochemical applications of
these complexes. Factors like a simple binding of ligand to the
d10 metal ions [29], an increased rigidity in structure of the com-
plexes [30], a restriction in the photoinduced electron transfer
(PET) [31], etc. may be assigned for the increase in the
photoluminescence.
Translation of symmetry codes to equivalent positions: * = 1 ꢁ x, ꢁy, ꢁz.
O2*. Such bridging modes exhibited by the single anthranilic car-
boxylate is almost rare in the chemical literature of the related sys-
tems and believed to be first of its own kind. The relevant bond
distances and angles are summarized in Table 2.
4. Conclusion
In this paper we have reported a novel centrosymmetric dinu-
clear cadmium(II) complex with a Schiff base ligand incorporating
free carboxylate end. Systematic characterizations of the complex
by elemental analysis, FT-IR, and thermal methods have also been
discussed. X-ray structural analysis reveals that each Cd(II) center
in 1 adopts distorted monocapped octahedral geometry and held
3.3. Thermogravimetric analysis
Thermogravimetric analysis of the complex reveals that it is
thermally stable up to 160 °C after which it undergoes decomposi-
tion in two steps. A total mass loss of 14.50% corresponds to the re-
lease of two chelating acetate ions per formula unit observed
between 161 and 230 °C in the TGA curve. Another mass loss takes
place between 231 and 605 °C corresponds to the release of two
Schiff base ligands indicated by a total mass loss of 64%.
together by doubly
l-2,2 bridging anthranilic carboxylates which
is rare in literature and believed to be first relating to anthranilic
carboxylate. Solid state photoluminescence measurement at room
temperature indicates its potential to serve as photoactive
material.
3.4. Photoluminescence properties
5. Supplementary data
The emission spectra of HL and 1 in their solid state were re-
corded at room temperature. The luminescence spectra of both
HL and 1 are depicted in Fig. 2. It is noteworthy that HL shows a
broad blue-fluorescent emission around 520 nm upon excitation
at 435 nm [Fig. 2: solid brown line1] while complex 1 exhibits a very
strong blue-shifted fluorescent emission around 465 nm upon exci-
tation at 380 nm [Fig. 2: solid blue line]. Therefore, the emission of
1 may not be assigned to intraligand fluorescent emission but be
attributed to ligand-to-metal charge transfer (LMCT) [23–26]. In
comparison HL molecule, 1 displays intense luminescence, undoubt-
edly owing to the chelating of the ligand to the metal center. This en-
hances the ‘‘rigidity’’ of the ligand and thus reduces the loss of
X-ray crystallographic data in the ‘CIF’ format corresponding to
the complex reported in this paper has been deposited with the
Cambridge Crystallographic Data Center and supplementary crys-
tallographic data for this paper can be obtained free of charge on
request at http://www.ccdc.cam.ac.uk/data_request/cif [or from
the Cambridge Crystallographic Data Centre (CCDC), 12 Union
Road, Cambridge CB2 1EZ, UK; fax: +44(0)1223-336033; email: de-
posit@ccdc.cam.ac.uk], quoting the CCDC number 636956 for 1.
Acknowledgements
Joy Chakraborty is grateful to the University Grants Commis-
sion, New Delhi, Government of India for the award of a Senior Re-
search Fellowship to him. The financial assistance from AICTE, New
Delhi, Government of India is also gratefully acknowledged.
1
For interpretation of the references to color in this figure, the reader is referred to
the web version of this article.

S. Shit et al. / Journal of Molecular Structure 919 (2009) 361–365
365
[b] M.K. Saha, D.K. Dey, B. Samanta, A.J. Edwards, W. Clegg, S. Mitra, Dalton
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