Phenoxo bridged luminescent dinuclear zinc(II) and cadmium(II) complexes of 2-[[[2-(2-pyridyl)ethyl]imino]methyl]phenol: Crystal structure, photophysical and thermal studies

Article abstract of DOI:10.1016/j.poly.2012.09.017

Reactions of zinc(II) and cadmium(II) halides with an N,N,O-donor Schiff-base ligand HL (obtained by the 1:1 condensation of salicylaldehyde and 2-(2-aminoethyl)pyridine) yield six new phenoxo bridged dinuclear complexes of the general formula [Zn₂

Full text of DOI:10.1016/j.poly.2012.09.017

Polyhedron 49 (2013) 12–18
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Polyhedron
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Phenoxo bridged luminescent dinuclear zinc(II) and cadmium(II) complexes
of 2-[[[2-(2-pyridyl)ethyl]imino]methyl]phenol: Crystal structure, photophysical
and thermal studies
a,
Prateeti Chakraborty a, Averi Guha a, Sudanshu Das a, Ennio Zangrando b, , Debasis Das
⇑
⇑
a Department of Chemistry, University of Calcutta, 92, A. P. C. Road, Kolkata 700 009, India
b Dipartimento di Scienze Chimiche e Farmaceutiche, University of Trieste, Via L. Giorgieri 1, 34127 Trieste, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 July 2012
Accepted 2 September 2012
Available online 27 September 2012
Reactions of zinc(II) and cadmium(II) halides with an N,N,O-donor Schiff-base ligand HL (obtained by the
1:1 condensation of salicylaldehyde and 2-(2-aminoethyl)pyridine) yield six new phenoxo bridged dinu-
clear complexes of the general formula [Zn2L2X2] (1–3) and [Cd2L2X2] (4–6), where X = Cl, Br and I, respec-
tively. The complexes have been characterized by routine physicochemical techniques: elemental
analyses, IR, electronic spectral studies, conductivity and solid state thermal studies. Complexes 1, 2
and 6 have further been characterized by single crystal X-ray structural analyses. The ligands, as well as
Keywords:
Schiff-base
Zinc(II) complex
Cadmium(II) complex
Crystal structure
Photoluminescence
all six complexes, are highly fluorescent. For the ligand, the emission band is attributed to a p–
p⁄ transi-
tion, whereas for the complexes the emissions may be assigned to ligand-to-metal charge transfers
(LMCT). Quantum yield calculations revealed that the metal complexes exhibit more intense fluorescence
compared to the ligand, which is supposed to be due to the enhancement of rigidity of the ligand on che-
lation, which reduces the loss of energy through non-radiative channels of the intraligand emission
excited state. Thermogravimetric analyses of the complexes suggest that upon heating the thermally sta-
ble final product in the case of complexes 1–3 is ZnO, 6 gives CdO, whereas for complexes 4 and 5 the final
product remains unidentified.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
make zinc the second most abundant trace element in biology. On
the other hand, cadmium(II), though treated mainly as a toxic ele-
Study of zinc(II) and cadmium(II) complexes of Schiff-base
ligands has become a focal point of interest for researchers in re-
cent times, not only for their intriguing structural features, but also
for their potential application in various fields [1–5]. Both the ions,
of a d10 electronic configuration, do not possess ligand field stabil-
ization energy (LFSE). A balance between bonding energies and
repulsions among the ligands determines the coordination number
of zinc and cadmium in their complexes. The combined effects of
softness and lack of LFSE allow Zn(II) and Cd(II) to adopt a coordi-
nation number of four, five or six, without a marked preference for
the latter, and thus they can exhibit various geometries and struc-
tural motifs. Cd(II), being larger in size compared to Zn(II), may
show higher flexibility in adopting higher coordination numbers
[6–10]. Zinc is a good Lewis acid, particularly in complexes with
low coordination numbers, it is kinetically labile and it reduces
the pKa value of coordinated water to produce a stronger nucleo-
phile [11]. More notably, the interconversion among four-, five-
and six-coordination states is fast [12–14], and all these properties
ment to biological systems, has been recently found as the catalytic
center in a newly discovered carbonic anhydrase [15] and also in
other biological organisms. Besides their application in bio-inor-
ganic chemistry, Zn(II) and Cd(II) complexes of Schiff-bases have
interesting applications in photoluminescence studies. It has been
previously reported that zinc(II) complexes with Schiff-bases type
chelating ligands can be used as an effective emitting layer [16].
This is due to the rich photophysical properties of the d10 species
as well as the easy and cost effective synthesis of their Schiff-base
complexes. Tandon and his group reported a series of complexes of
2-[[2-(2-pyridinyl)ethylimino]methyl]phenol using nitrate and
halides salts of Zn(II) and Cd(II), where only the zinc(II) nitrate
complex has been structurally characterized [17]. Our continuous
interest towards Zn(II) and Cd(II) complexes with Schiff-base li-
gands prompted us to design six phenoxo bridged dinuclear halide
complexes of these metals with the tridentate ligand 2-[[[2-(2-
pyridinyl)ethyl]imino]methyl]phenolato (L), in order to examine
the effect of halides on the structural motifs. Of these we report
herein the syntheses, characterization, photoluminescence and
thermal analysis (Scheme 1).
⇑
Corresponding authors. Tel.: +91 33 24837031; fax: +91 33 23519755.
E-mail address: dasdebasis2001@yahoo.com (D. Das).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.09.017

P. Chakraborty et al. / Polyhedron 49 (2013) 12–18
13
methanolic solution of 2-(2-aminoethyl)pyridine. The stirring was
continued for a further 15 min and the resulting solution was re-
fluxed for 2 h. Then a methanolic solution of zinc(II) chloride
(0.204 g, 1.5 mmol) was added to it, refluxing the mixture for 4 h,
followed by filtration, and the filtrate was kept in a CaCl2 desiccator.
Yellow colored single crystals suitable for X-ray data collection
were obtained from the filtrate after a week. Yield: 85%. Anal. Calc.
for C28H26O2N4Cl2Zn2 (1): C, 51.57; H, 4.33; N, 8.59. Found: C, 51.19;
2
H, 4.21; N, 8.53%. IR (cmꢀ1):
1600 and 1550.
m(C@N) 1638, m(skeletal vibration)
2.3.2. [Zn2L2Br2] (2)
This was synthesized by adopting a similar procedure as fol-
lowed for complex 1, but using zinc(II) bromide (0.337 g,
1.5 mmol) instead of zinc(II) chloride. Light yellow colored single
crystals suitable for X-ray analysis were obtained from the filtrate
after a few days. Yield: 80%. Anal. Calc. for C28H26Br2N4O2Zn2 (2): C,
45.38; H, 3.54; N, 7.56. Found: C, 45.36; H, 3.43; N, 7.41%. IR
2
2
Scheme 1. Synthetic route to the Zn and Cd complexes.
(cmꢀ1):
m(C@N) 1638, m(skeletal vibration) 1600 and 1549.
2. Experimental
2.3.3. [Zn2L2I2] (3)
2.1. Starting materials
This was synthesized following the same procedure as for 1, but
using zinc iodide (0.478 g, 1.5 mmol) in place of zinc(II) chloride. A
light yellow colored crystalline solid was obtained from the filtrate
after two weeks. Yield 85%. Anal. Calc. for C28H26I2N4O2Zn2 (3): C,
40.27; H, 3.14; N, 6.71. Found: C, 40.76; H, 3.03; N, 6.41%. IR
High purity salicylaldehyde and 2-(2-aminoethyl)pyridine
(aepy) were obtained from Merck Chemical Company. Cadmium(II)
chloride, cadmium(II) iodide, cadmium(II) bromide tetra hydrate,
zinc(II) chloride, zinc(II) bromide and zinc(II) iodide were pur-
chased from Aldrich Chemical Company and used as received.
The solvents were dried according to literature procedures. All
other chemicals were of AR grade.
(cmꢀ1):
m(C@N) 1637, m(skeletal vibration) 1600 and 1552.
2.3.4. [Cd2L2Cl2] (4)
This was synthesized by adopting a similar procedure as fol-
lowed for complex 1, but using cadmium(II) chloride (0.301 g,
1.5 mmol). A deep yellow colored crystalline solid was obtained
2.2. Methods and instrumentation
Elemental analyses (C, H and N) were performed using a
Perkin-Elmer 240C elemental analyzer. Infrared spectra in KBr
pellets (400–4500 cmꢀ1) were recorded using a Perkin-Elmer RXI
FT-IR spectrophotometer. Electronic spectra (UV–Vis) in dimethyl-
formamide (900–250 nm) were recorded in a Hitachi U-3501
spectrophotometer. Thermal analysis (DT-TGA) was carried out
using a Mettler Toledo (TGA/SDTA 851) thermal analyzer in a dy-
namic atmosphere of dinitrogen (flow rate: 30 cc/minꢀ1). All the
samples were heated in a platinum crucible at a rate of 10 degree
per minute with inert alumina as a reference from 30 to 700 °C.
Fluorescence spectra of the ligand and the complexes were re-
corded in DMF solution with a Perkin-Elmer model LS 55 Lumines-
cence spectrometer.
Table 1
Crystallographic data and details of structure refinement for complexes 1, 2 and 6.
1
2
6
Empirical
formula
Formula weight 652.17
Crystal system
Space group
a (Å)
C
28H26Cl2N4O2Zn2 C28H26Br2N4O2Zn2 C28H26Cd2I2N4O2
741.09
monoclinic
P21/c
9.0425(6)
9.3057(6)
16.8869(12)
90.0
103.433(3)
90.0
929.13
triclinic
P1
monoclinic
P21/c
ꢀ
8.9712(10)
9.1489(10)
16.5733(18)
90.0
103.060(1)
90.0
1325.1(3)
2
1.635
9.0703(3)
10.7354(4)
16.8230(6)
90.448(1)
104.762(1)
106.571(1)
1512.51(9)
2
b (Å)
c (Å)
a
(°)
b (°)
(°)
c
Volume (Å3)
Z
1382.10(16)
2
1.781
2.3. Synthesis of the complexes
Dcalc (Mg mꢀ3
)
2.040
All the six complexes were synthesized adopting two different
methods. In the former procedure the ligand was isolated by reflux-
ing a 1:1 mixture of salicylaldehyde and 2-(2-aminoethyl)pyridine
for 30 min in absolute ethanol, as reported earlier [17]. Then the
corresponding metal complexes were prepared following the same
procedure reported therein. The alternative method adopted was
the template synthetic technique where ZnX2 and CdX2 metal salts
were allowed to react with the Schiff-base ligand formed in situ via
the condensation of salicylaldehyde and 2-(2-aminoethyl)pyridine,
maintaining the same molar ratio. The complexes formed using the
two different synthetic methodologies were the same. The prepara-
tion, composition and other physicochemical characteristics of all
six complexes using template technique are given below.
l
(Mo K
a
)
)
2.046
4.660
3.478
(mm–1
F(000)
hmax (°)
Reflections
collected
Unique
reflections
Rint
Observed
664
26.37
9811
736
25.71
13503
880
24.84
19509
2645
2632
5204
0.0253
2334
0.0863
1901
0.0716
4164
I > 2r(I)
Parameters
Goodness of fit
224
1.054
172
1.041
343
1.041
(F2)
R1 (I > 2
wR2 (I > 2
(e/Å3)
r
(I))a
(I))b
0.0262
0.0651
0.390, ꢀ0.230
0.0391
0.0873
0.486, ꢀ0.590
0.0345
0.0840
1.081, ꢀ0.951
r
D
q
2.3.1. [Zn2L2Cl2] (1)
a
A
methanolic solution (5 ml) of salicylaldehyde (0.122 g,
R1
wR2 = [
=
R
||F0| ꢀ |Fc||/
R|F0|.
w(F0 ꢀ Fc2)2/
R .
w(F02)2]1/2
b
2
R
1 mmol) was added dropwise with constant stirring to a 10 ml

14
P. Chakraborty et al. / Polyhedron 49 (2013) 12–18
Table 2
2.4. X-ray data collection and crystal structure determinations
Selected bond distances (Å) and angles (°) for compounds 1–2.
Single crystals of 1–2 and 6 were mounted on a glass fiber and
coated with epoxy resin. Data collections were carried out at room
temperature on a Bruker Smart Apex diffractometer equipped with
1, X = Cl
2, X = Br
Zn(1)–O(1)
Zn(1)–O(10)
Zn(1)–N(1)
Zn(1)–N(2)
Zn(1)–X(1)
1.9982(15)
2.1304(13)
2.1149(17)
2.1175(17)
2.2449(7)
2.001(3)
2.131(3)
2.112(4)
2.124(4)
2.3873(7)
a CCD and Mo Ka radiation (k = 0.71073 Å). Cell refinement, index-
ing and scaling of the data sets were done using the Bruker Smart
Apex and Bruker Saint packages [18]. All the structures were
solved by direct methods and subsequent Fourier analyses [18],
and refined by the full-matrix least-squares method based on F2
with all observed reflections [19]. Hydrogen atoms were placed
at geometrically calculated positions and constrained to ride on
the atom to which they are attached. All the calculations were per-
formed using the WinGX System, Ver 1.80.05 [20]. Crystal data and
details of refinements are given in Table 1, and a selection of bond
lengths and angles are presented in Tables 2 and 3.
O(1)–Zn(1)–O(10)
O(1)–Zn(1)–N(1)
O(1)–Zn(1)–N(2)
O(1)–Zn(1)–X(1)
N(1)–Zn(1)–N(2)
N(1)–Zn(1)–O(10)
N(1)–Zn(1)–X(1)
N(2)–Zn(1)–O(10)
N(2)–Zn(1)–X(1)
O(10)–Zn(1)–X(1)
78.40(6)
88.88(6)
118.54(7)
117.04(5)
83.76(7)
161.57(7)
101.19(5)
90.61(6)
124.27(5)
96.49(5)
78.67(12)
89.36(13)
119.70(14)
115.32(9)
83.62(14)
162.23(14)
100.83(11)
90.91(13)
124.85(10)
96.18(9)
Zn(1)–O(1)–Zn(1)0
101.60(6)
101.33(12)
3. Results and discussion
Primed atoms at ꢀx + 1, ꢀy + 2, ꢀz + 2 (for 1) and ꢀx + 1, ꢀy + 1, ꢀz + 1 (2).
3.1. Synthesis and characterization
In one of our earlier studies [21] we observed that 2-(2-amino-
ethyl)pyridine may act as useful ancillary fragments in constructing
unprecedented highly fluorescent Cd-halide polymeric species.
Thus the Schiff-base formed by condensation of salicylaldehyde
with such amines would generate structurally interesting fluores-
cent complexes with d10 metal ions. For the present study we have
prepared six complexes by following the template synthetic tech-
nique, where ZnX2 and CdX2 (X = Cl, Br, I) are treated with the
N,N,O-chelating ligand L formed in situ via condensation of salicyl-
aldehyde and 2-(2-aminoethyl)pyridine. The molecular composi-
tion of the complexes may generally be formulated as [M2L2X2]
on the basis of elemental analyses. The complexes exhibit very
Table 3
Selected bond distances (Å) and angles (°) for compound 6.
Cd(1)–O(1)
Cd(1)–O(10)
Cd(1)–N(2)
Cd(1)–N(1)
Cd(1)–I(1)
2.206(3)
2.299(3)
2.313(5)
2.320(4)
2.7053(5)
Cd(2)–O(2)
Cd(2)–O(200)
Cd(2)–N(3)
Cd(2)–N(4)
Cd(2)–I(2)
2.234(3)
2.295(3)
2.300(5)
2.378(5)
2.7383(6)
O(1)–Cd(1)–O(10)
O(1)–Cd(1)–N(1)
O(1)–Cd(1)–N(2)
O(1)–Cd(1)–I(1)
N(1)–Cd(1)–N(2)
N(1)–Cd(1)–O(10)
N(1)–Cd(1)–I(1)
N(2)–Cd(1)–O(10)
N(2)–Cd(1)–I(1)
O(10)–Cd(1)–I(1)
77.33(14)
82.32(15)
111.26(16)
115.43(11)
78.64(17)
151.05(15)
107.18(12)
89.74(15)
133.31(12)
100.11(10)
O(2)–Cd(2)–O(200)
O(2)–Cd(2)–N(3)
O(2)–Cd(2)–N(4)
O(2)–Cd(2)–I(2)
N(3)–Cd(2)–N(4)
N(3)–Cd(2)–O(200)
N(3)–Cd(2)–I(2)
N(4)–Cd(2)–O(200)
N(4)–Cd(2)–I(2)
O(200)–Cd(2)–I(2)
76.68(13)
82.47(15)
114.50(15)
128.27(10)
78.71(16)
151.19(16)
104.36(13)
92.01(14)
117.14(11)
104.14(10)
low molar conductance (7–10
X
ꢀ1 cm2 Mꢀ1) in DMF medium sug-
gesting that they are non-electrolytes in solution. A FT-IR spectral
study reveals that all the complexes exhibit bands due to a C@N
stretch in the range 1635–1640 cmꢀ1 and skeletal vibrations in
the range 1535–1600 cmꢀ1, but no bands for the free carbonyl or
primary amine are observed, indicating that a complete condensa-
tion has occurred. As verified by routine physicochemical studies,
the same complexes have also been obtained following a reported
procedure [17].
Cd(1)–O(1)–Cd(10)
102.67(14)
Cd(2)–O(2)–Cd(200)
103.32(13)
Symmetry codes : (0) ꢀx + 1, ꢀy, ꢀz; (00) ꢀx, ꢀy + 1, ꢀz + 1.
from the filtrate after a few weeks. Yield: 80%. Anal. Calc. for
3.2. Description of the crystal structures
C
28H26Cl2N4O2Cd2 (4): C, 45.01; H, 3.51; N, 7.51. Found: C, 44.96;
H, 3.48; N, 7.41%. IR (cmꢀ1):
1598 and 1543.
m(C@N) 1638, m(skeletal vibration)
The X-ray structural analyses of complexes 1, 2 and 6 revealed
the formation of discrete dinuclear complexes of the formulae
[Zn2L2X2], where X = Cl, Br, and [Cd2L2I2]. All the complexes, having
a rhomboid M2O2 core, are closely comparable and are located
about a crystallographic inversion center. Each metal is chelated
by the pyridine and imino N donors and by the phenolato oxygen,
that in turn acts as a bridge towards the other metal ion. The met-
als complete a highly distorted trigonal bipyramidal coordination
with a halogen atom.
Complexes 1 and 2 present a similar structure. An ORTEP view
[22] of 1 is displayed in Fig. 1 and that of 2 is shown in a supple-
mentary file. The bond lengths and angles for 1 and 2 (Table 2)
indicate comparable coordination values, with the clear exception
of the Zn-Cl and Zn-Br distances of 2.2449(7) and 2.3873(7) Å,
respectively. The oxygen bridge forms an asymmetric linkage to
the zinc atoms with distinct Zn–O(1) (2.00 Å) and Zn–O(10)
(2.13 Å) bond distances (mean value for the two complexes), while
the Zn–N bond lengths fall in the range 2.1149(17)–2.124(4) Å. The
different halide does not affect the intermetallic distances, which
are similar to within their esd’s, of 3.2006(5) and 3.1969(9) Å, in
2.3.5. [Cd2L2Br2] (5)
This was synthesized adopting the same previous procedure
for 4, but using cadmium(II) bromide tetrahydrate (0.516 g,
1.5 mmol), instead of cadmium(II) chloride. A yellow solid was ob-
tained from the filtrate after three days. Yield 82%. Anal. Calc. for
C
28H26Br2N4O2Cd2 (5): C, 40.27; H, 3.14; N, 6.71. Found: C, 40.06;
H, 3.23; N, 6.41%. IR (cmꢀ1):
1596 and 1542.
m(C@N) 1636, m(skeletal vibration)
2.3.6. [Cd2L2I2] (6)
This was synthesized adopting the same procedure as for 4, but
using cadmium (II) iodide (0.549 g; 1.5 mmol), instead of cad-
mium(II) chloride. Deep yellow colored single crystals suitable
for X-ray data collection were obtained from the filtrate after one
day. Yield 90%. Anal. Calc. for C28H26N4O2I2Cd2 (6): C, 36.19; H,
2.82; N 6.03. Found: C, 36.13; H, 2.68; N, 5.93%. IR (cmꢀ1):
m(C@N) 1638, m(skeletal vibration) 1598 and 1547.

P. Chakraborty et al. / Polyhedron 49 (2013) 12–18
15
Fig. 1. Ortep view (35% thermal ellipsoid probability) of complex 1 with atom labels of the independent unit. Primed atoms at ꢀx + 1, ꢀy + 2, ꢀz + 2. The same atom label
scheme is applied to 2 by substituting the chlorine Cl1 with bromine Br1.
1 and 2, respectively. The four membered Zn2O2 moiety is bent
with respect to the phenolato, forming a dihedral angle of ca. 14°
where L0 is a phenol based compartmental ligand having two coor-
dinating pyridine arms [26].
between the two species. The index
s
, which indicates the degree
The crystal packing of complexes 1 and 2 shows the presence
of non-conventional C–H. . .X hydrogen-bonds with a C5. . .X1
distance of 3.523(3) and 3.619(5) Å, respectively, forming a 2D net-
work developed parallel to the bc plane, as depicted for 1 in Fig. 2.
of distortion for the penta-coordination, is calculated to be 0.62
for both complexes, and the geometry is best described as distorted
trigonal bipyramidal where N(1)–Zn(1)–O(10) of ca. 162° is the ax-
ial direction. A zero value for
s
suggests a perfect square pyramidal,
A few C–H. . .
p-ring interactions are observed in the crystal pack-
and a value of one stands for a perfect trigonal bipyramidal geom-
etry [23]. The Zn–O and Zn–N distances are closely comparable
with those reported in the isostructural azido [Zn2L2(N3)2] [24]
and nitro derivative [Zn2L2(NO3)2] [17], and also in our dinuclear
macrocycle complexes comprising a central Zn2O2 fragment [25].
Likewise, the Zn–Cl, Zn-O and Zn–N bond lengths are in agreement
with the geometrical data measured in the complex [Zn2L0Cl3],
ing, but there are no significant p–p
stacking interactions.
An ORTEP view [22] of 6 is depicted in Fig. 3. It shows two half
independent units arranged about a center of symmetry, similarly
to those described for the Zn complexes. The Cd–O bond distances,
in the range 2.206(3)–2.299(3) Å, differ for the asymmetric bridg-
ing oxygen by 0.1 and 0.06 Å for the two independent complexes.
The values of the Cd–N bond lengths fall in the range from
Fig. 2. Crystal packing of 1 viewed down axis a showing the C–H. . .Cl hydrogen bonds.

16
P. Chakraborty et al. / Polyhedron 49 (2013) 12–18
Fig. 3. Ortep view (35% thermal ellipsoid probability) of one of the two complexes of 6 with atom labeling scheme of the independent unit. Primed atoms at ꢀx + 1, ꢀy, ꢀz.
2.300(5) to 2.378(5) Å. It is worth noting that the bond values are
longer for Cd2 in comparison to those of Cd1 by ca. 0.04 Å (Table 3).
250
This is clearly evident by comparing the Cd–O, Cd–N(imino) and
Cd–I distances (2.206(3) vs 2.234(3), 2.320(4) vs 2.378(5) and
4
2.7053(5) vs 2.7383(6) Å, respectively). Among the coordination
bond angles, differences are observed for those involving the io-
dine, with O–Cd–I angles of 115.43(11) and 128.27(10)°, and
N(py)–Cd–I of 133.31(12) and 117.14(11)° in the two complexes.
These discrepancies are reflected in the trigonal index, which is
0.30 for Cd1 and 0.38 for Cd2, thus it is lower than in 1 and 2. In
the two independent complexes the bridging metals are
3.5179(7) and 3.5529(8) Å far apart, and the Cd2O2 moiety forms
a dihedral angle of ca. 17° with the phenolato ring in both com-
200
150
100
50
5
6
plexes. Some C–H. . .p ring interactions are detected in the crystal
packing with H-centroid of the aromatic ring distances in the range
2.85–3.00 Å.
Ligand
0
350
400
450
500
550
600
Wavelength (nm)
Fig. 5. Fluorescence spectra of ligand, cadmium(II) complexes in DMF (kexc = 316 -
nm). Probe concentrations are 10ꢀ5 M.
Table 4
Photophysical parameters of ligand and metal complexes in DMF with excitation at
k = 316 nm.
Complex M2L2X2
Emission (kmax in nm)
Quantum yield
1
2
3
4
5
6
445
448
448
452
445
453
362
0.56
0.44
0.36
0.72
0.60
0.34
0.32
Fig. 4. Fluorescence spectra of ligand, zinc(II) complexes in DMF (kexc = 316 nm).
Ligand HL
Probe concentrations are 10ꢀ5 M.

P. Chakraborty et al. / Polyhedron 49 (2013) 12–18
17
Fig. 6a. Thermogram of complex 1.
Fig. 6b. Thermogram of complex 6.
In all the present complexes, and also in similar Zn derivatives
[17,24], the centrosymmetric symmetry induces the two pyridine
rings to be located in trans positions in order to avoid steric clashes
amongst the aromatic hydrogens of the two basic structural units.
230, 270, 316 and 370 nm (Figs. 7 and 8, provided in the SI file).
These may be assigned to ligand-to-metal charge transfers for their
d10 electronic configuration. The bands at lower wavelengths cor-
respond to intraligand charge transfers, whereas the other bands
other originate from ligand to metal charge transfers. Considering
the two bands at low energy, one may be due to Xꢀ ? M and the
other due to PhOꢀ ? M charge transfers [17].
3.3. Electronic spectra
Electronic spectra of the ligand as well as all of the six com-
plexes were recorded in DMF medium. The ligand exhibits band
3.4. Fluorescence spectral study
maxima at 239 and 316 nm, which may be attributed to
p–
p⁄
and n–p⁄ transitions, respectively. Both the zinc(II) and cad-
Fluorescence spectra of the ligand as well as all the six com-
plexes were examined in DMF medium at room temperature.
mium(II) complexes displayed four strong absorption bands near

18
P. Chakraborty et al. / Polyhedron 49 (2013) 12–18
Table 5
nol (HL) yields dinuclear complexes of the general formulation
[M2L2X2], with the phenoxo oxygen acting as a bridging donor to
the metal ions. The ligand is fluorescent, but the fluorescence inten-
sity increases upon complexation, most probably due to the increas-
ing rigidity of the molecule. Although the halide ligands have no
influence on the structural motif, their electronic properties have a
considerably effect on the fluorescence behavior, and the fluores-
cence intensity decreases with increasing the atomic weight of the
halide. Solid state thermal analyses suggest that the zinc–chloro
and cadmium–iodo complexes show the highest thermal stability
in the series.
Theoretical and experimental weight loss (from thermograms) of the complexes.
Complex End product
(assumed)
Final
temperature
(°C)
Theoretical
weight loss (%) weight loss (%)
Experimental
1
2
3
6
ZnO
ZnO
ZnO
CdO
620
650
620
650
87.534
89.001
90.358
86.209
79.282
87.753
83.018
82.621
Photo-excitations have been performed at kmax(ex) of 316 and
370 nm. However, it is interesting to note that no appreciable
change in emission band maxima or in quantum yields for the
complexes, as well as for the ligand, was observed on changing
the wavelength. Figs. 4 and 5 depict the emission spectra of the
Zn(II) and Cd(II) complexes along with that of the ligand with an
excitation kmax(ex) of 316 nm. Emission spectra obtained on excita-
tion at 370 nm are provided in the supplementary information. The
highest emission band for the ligand is observed at 362 nm,
whereas the emission band maxima for all the Zn(II) and Cd(II)
complexes are shifted to higher wavelengths. These red shifts sug-
gest successful complexation between the metal salts and the li-
gand in all cases. Quantum yield calculations (Table 4) show that
in comparison to the ligand, all the metal complexes are more fluo-
rescent, and this is supposedly due to the greater rigidity of the li-
gand system attained upon complexation. From the figures, as well
as the quantum yield calculations, it is also clear that the halide
ions have a significant role in the photoluminescence property of
the complexes. The fluorescence shows the following trend for
both the Zn(II) and Cd(II) complexes: chloro > bromo > iodo; thus
the highest efficiency is seen for the chloro derivative. This can
be explained by considering heavy atom perturbation for which
energy dissipation through non-radiative channels increases, and
consequently the fluorescence intensity/quantum yield decreases.
Acknowledgements
The authors wish to thank CSIR, New Delhi [01(2464)/11/EMR-
II dt 16-05-11 to D.D.] for financial support and the University of
Calcutta for providing the facility of a single crystal X-ray diffrac-
tometer from the DST FIST program.
Appendix A. Supplementary data
CCDC 874689, 874690 and 874691 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; or
e-mail: deposit@ccdc.cam.ac.uk. Supplementary data associated
with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.poly.2012.09.017.
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4. Conclusion
The reaction of zinc or cadmium halide (MX2) with the tridentate
N,N,O Schiff-base ligand 2-[[[2-(2-pyridyl)ethyl]imino]methyl]phe-