Dicynamide bridged two new zig-zag 1-D Zn(II) coordination polymers of pyrimidine derived Schiff base ligands: Synthesis, crystal structures and fluorescence studies

Article abstract of DOI:10.1016/j.molstruc.2015.02.048

Two new zigzag 1-D polymeric Zn(II) coordination polymers {[Zn(L₁)(μ_1,5-dca)](H₂O)}_n (1), {[Zn(L₂)(μ_1,5-dca)](ClO₄)}_n (2) of two potentially tridentate NNO-, NNN-, don

Full text of DOI:10.1016/j.molstruc.2015.02.048

Journal of Molecular Structure 1092 (2015) 34–43
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Dicynamide bridged two new zig-zag 1-D Zn(II) coordination polymers
of pyrimidine derived Schiff base ligands: Synthesis, crystal structures
and fluorescence studies
⇑
Saugata Konar
Department of Chemistry, Jadavpur University, Kolkata 700032, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
2D polymeric dicynamide bridged
Zn(II) complexes have been
synthesized.
ꢀ
X-ray crystallographic structures of
Zn(II) complexes have been
performed.
ꢀ
ꢀ
Weak forces lead to various
supramolecular architectures.
The fluorescence study has been
performed.
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new zigzag 1-D polymeric Zn(II) coordination polymers {[Zn(L1)(l1,5-dca)](H2O)}n (1), {[Zn(L2)
(l1,5-dca)](ClO4)}n (2) of two potentially tridentate NNO-, NNN-, donor Schiff base ligands [2-(2-(4,
6-dimethylpyrimidin-2-yl)hydrazono)methyl)phenol] (L1), [1-(4,6-dimethylpyrimidin-2-yl)-2-(dipyri-
din-2ylmethylene)hydrazine] (L2) have been synthesized and characterized by elemental analyses, IR
and 1H NMR, fluorescence spectroscopy and single crystal X-ray crystallography. The dicyanamide ions
act as linkers (l1,5 mode) in the formation of these coordination polymers. Both the complexes 1 and
2 have same distorted square pyramidal geometry around the Zn(II) centres. The weak forces like
Received 25 November 2014
Received in revised form 13 February 2015
Accepted 13 February 2015
Available online 26 February 2015
Keywords:
Coordination polymers
Zinc (II)
Dicyanamide
Emission properties
Supramolecular interaction
p
ꢁ ꢁ ꢁ
p
, CAHꢁ ꢁ ꢁ
p
, anionꢁ ꢁ ꢁ
p interactions lead to various supramolecular architectures. Complex 1 shows
high chelation enhanced fluorescence compared to that of 2. The fluorescence spectral changes observed
high selectivity towards Zn(II) over other metal ions such as Mn(II), Co(II), Ni(II), Cu(II).
Ó 2015 Elsevier B.V. All rights reserved.
Introduction
different coordination numbers and nuclearities with interesting
molecular and crystalline architectures [6–16].
A substantial
The complexation of group 12 metal ions with polydentate
Schiff base ligands is a well studied area of research in coordination
chemistry [1–5]. These ligands have preparative accessibilities,
structural variety and varied denticity, forming complexes of
amount of work on pyridine and pyrazole derived Schiff base
ligands have emerged in the literature, while works on pyrimidine
derived Schiff base ligands are scarce. Pyrimidine derivatives play a
dominant role in biological systems, the ring system being present
in, for example, nucleic acids, several vitamins, co-enzymes and
antibiotics [17–19]. Zinc is an essential element in the biosystem.
It is required in genetic materials, DNA, RNA polymerases, the
⇑
Tel.: +91 9804734352.
E-mail address: saugata.konar@gmail.com
http://dx.doi.org/10.1016/j.molstruc.2015.02.048
0022-2860/Ó 2015 Elsevier B.V. All rights reserved.

S. Konar / Journal of Molecular Structure 1092 (2015) 34–43
35
regulatory Zn finger protein (structural motif for eukaryotic DNA-
binding protein) in forming nucleic acids. The biological signifi-
cance of Zn(II) has led to the development of numerous fluorescent
chemo sensors [20].
characterizations, X-ray crystal structures and fluorescence studies
of 1 and 2 have reported here.
Experimental
It is well known that the careful selection of the organic ligands
is one of the key factors for the synthesis of the coordination com-
pounds with novel structures. The synthesis and investigation of
new coordination polymers have been of great interest in the fields
of crystal engineering and coordination chemistry because of their
potential applications and intriguing architectures and topologies
[21–24]. Over the last two decades, numerous spectacular coordi-
nation polymers have been reported, such as 1D-chains and lad-
ders, 2D grids, and 3D microporous networks [25–30]. The
selection of bridging ligand is one of the key steps for these types
of syntheses.
Materials
All chemicals were of reagent grade, purchased from commer-
cial sources and used without further purification. Di (2-pyridyl)
ketone was purchased from Aldrich Chemical Company, USA and
used without further purification.
Caution! Although we have not encountered any problems, it
should be kept in mind that perchlorate compounds of metal ions
are potentially explosive especially in the presence of organic
ligands. Only a small amount of the material should be prepared
and it should be handled with care.
In the present study (Scheme 1), two new zigzag 1-D polymeric
Zn(II) coordination polymers {[Zn(L1)(
l1,5-dca)](H2O)}n (1),
{[Zn(L2)( 1,5-dca)](ClO4)}n (2) have been prepared by using pyrim-
l
Physical measurement
idine derived tridentate NNO and NNN donor Schiff base ligands
([1 + 1] condensation products of 2-hydrazino-4,6-dimethyl
pyrimidine with salicylaldehyde and di(2-pyridyl) ketone). My
interest is to observe the metal ion coordination environments
and photoluminescence using Zn(II) metal ion templates, the
organic ligands L1 and L2, and suitable bridging units. In the com-
plexes 1 and 2, the two Zn(II) centres are held together by
The infrared spectra of the complexes were recorded on a
Perkin-Elmer RX I FT-IR spectrophotometer with KBr discs (4000–
400 cmꢂ1). Elemental analyses (C, H, and N) were carried out using
a Perkin-Elmer 2400 II elemental analyzer. 1H NMR spectra were
recorded on a Bruker 300 MHz FT-NMR spectrometer using tri-
methylsilane as an internal standard in d6-DMSO for complex 1, 2
and 3. Steady state absorption and fluorescence spectra for all the
three complexes and the Schiff base ligand were measured with a
Hitachi UV/Vis U-3501 spectrophotometer and Perkin Elmer
l1,5-bridged dicyanamide ions. The fluorescence spectral changes
observed high selectivity towards Zn(II) over other metal ions such
as Mn(II), Co(II), Ni(II), Cu(II). The synthetic details, spectral
Me
N
NH2
Me
N
N
H
CHO
O
N
OH
MeOH, Reflux
N
MeOH, Reflux
Me
Me
HO
N
N
N
N
Me
N
N
H
N
Me
N
N
H
N
L2
L1
Zn(ClO4)2.6H2O, NaN(CN)2,
MeOH
Zn(ClO4)2.6H2O, NaN(CN)2,
MeOH
Complex 2
Complex 1
Scheme 1.

36
S. Konar / Journal of Molecular Structure 1092 (2015) 34–43
Table 1
LS50B fluorimeter, respectively at room temperature (298 K). All
the spectral measurements were done at ꢃ10ꢂ5 (M) concentration
of solute in order to avoid aggregation and self quenching.
Crystal data and refinement details of complexes for 1 and 2.
Compound
1
2
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
C
15H13N7OZn
C19H16ClN9O4Zn
535.25
293
0.71073
Triclinic
P-1
372.71
293
0.71073
Monoclinic
P21
Synthesis of the ligands (L1) and (L2)
The ligand L1 was synthesized and characterized using litera-
ture method [31].
L1: 1H-NMR, L1 (d6-DMSO, d): d 2.26 (6H, s, 40-CH3 and 60-CH3),
6.62 (1H, s, H-50), 6.83 (1H, m, H-5), 6.84 (1H, d, J = 7.8 Hz, H-3),
7.18 (1H, t, J = 7.5 Hz, H-4), 7.32 (1H, d, J = 7.4 Hz, H-6), 8.19 (1H,
Unit cell dimensions
a (Å)
b (Å)
c (Å)
8.1305(4)
13.8732(7)
8.7550(4)
90
112.459(2)
90
912.63(8)
2
1.356
7.646(5)
10.808(5)
14.140(5)
107.141(5)
103.244(5)
93.554(5)
1076.3(9)
2
s, H-7), 11.34 (1H, s, NH), 11.74 (1H, s, 2-OH). IR (KBr;
3247 ( NAH), 1620 ( C@C), 1566 ( C@NH), 885 (
1255 ( CAO).
m
/cmꢂ1):
mCpymAH),
a
(°)
m
m
m
b (°)
m
c
(°)
Volume (Å3)
The ligand L2 was synthesized and characterized using litera-
ture method [30].
z
Densitycal (Mg mꢂ3
)
1.652
1.314
L2: Yield: 3.13 g (85%); M.p.95 (°C) 152; Anal. Calc. for C17H16N6:
C, 69.86; H, 5.47; N, 28.76 Found: C, 69.81; H, 5.41; N, 28.30%. 1H-
NMR, L2 (d6-DMSO, d): 2.34 (6H, s, 400-CH3 and 600-CH3), 6.78 (1H, s,
500-H), 7.46 (1H, ddd, J = 8.0, 5.0, 1.5 Hz, H-5), 7.47 (1H, ddd, J = 8.0,
1.0, 1.0 Hz, H-3), 7.53 (1H, ddd, J = 8.0, 5.0, 1.5 Hz, H-50), 7.94 (1H,
ddd, J = 8.0, 1.5,1.0 Hz H-30), 7.96 (1H, ddd, J = 8.0, 8.0, 1.5 Hz, H-40),
7.97 (1H, ddd, J = 8.0, 7.5, 1.5 Hz, H-4), 8.57 (1H, ddd, J = 5.0, 1.5,
Absorption coefficient
1.361
(mmꢂ1
)
F(000)
380
544
Crystal size (mm3)
Index ranges
0.13 ꢄ 0.15 ꢄ 0.16
ꢂ9 6 h 6 10
ꢂ17 6 k 6 17
ꢂ11 6 l 6 11
281 [R(int) = .0452]
0.11 ꢄ 0.14 ꢄ 0.19
ꢂ7 6 h 6 7
ꢂ10 6 k 6 10
ꢂ12 6 l 6 14
2186 [R(int) = 0.0311]
Independent reflections
1.0 Hz, H-6), 8.84 (1H, ddd, J = 5.0, 1.5, 1.0 Hz, H-60). IR (KBr;
cmꢂ1): 3412(
NAH), 1528 ( C@N), 2920 (w) ( CAH), 1215 (ms),
C@C), 1065 (m) (pym).
m/
(Rint
)
Absorption correction
Refinement method
Multi-scan
Multi-scan
m
m
m
Full-matrix least
squares on F2
2281, 1, 223
11,977
R1 = 0.0334
wR2 = 0.0882
Full-matrix least
squares on F2
2186, 0, 309
3463
R1 = 0.2042
wR2 = 0.956
ꢂ0.57 and 0.78
(m
Data/restraints/parameters
Reflections collected
Final R indices [I > 2
Preparation of complexes
Preparation of complex [C15H15N7O2Zn] (1)
r
(I)]
Largest difference peak and ꢂ0.27 and 0.44
To a methanol solution (30 cm3) of Zn(ClO4)2, 6H2O (0.744 g,
2 mmol), a solution of the Schiff base L1 in the same solvent
(0.482 g, 2 mmol) was slowly added, followed by a solution of
sodium dicyanamide (0.356 g, 4 mmol) in a minimum volume of
aqueous methanol with constant stirring. The stirring was contin-
ued for another 2 h and filtered. The light yellow solution was kept
in a refrigerator at 10 °C which produced yellow hexagonal crystals
suitable for X-ray diffraction after 10 days. The crystals were iso-
lated by filtration and air-dried. Yield: 61%. Anal. Calc. for
hole (eÅꢂ3
)
Table 2
Selected bond distances (Å) and angles (°) in 1 and 2.
Selected bonds
Value (Å)
Selected angles
(°)
Complex 1
Zn1
Zn1
Zn1
Zn1
Zn1
AO1
AN1
AN4
AN5
AN6
2.024(3)
2.068(2)
2.175(3)
2.035(4)
1.984(4)
O1
O1
O1
O1
N1
N1
N1
N4
N4
N5
Zn1
AZn1
AZn1
AZn1
AZn1
AZn1
AZn1
AZn1
AZn1
AZn1
AZn1
AN1
AN1
AN4
AN5
AN6
AN4
AN5
AN6
AN5
AN6
AN6
AN2
86.43(12)
162.22(11)
95.79(16)
90.96(16)
76.26(13)
116.89(16)
130.05(17)
95.64(15)
97.04(15)
113.01(17)
114.9(2)
C
15H15N7O2Zn: C, 48.38; H, 3.49; N, 26.34; O, 4.30. Found: C,
48.25; H, 3.45; N, 26.14; O, 4.20. IR (KBr;
/cmꢂ1): 2345
sC„N + as), 2193 (
sC@N), 1085 (ClOꢂ4 ). 1H-NMR (d6-DMSO,
m
(m
m
m
d): 2.31 (3H, s, 40-CH3), 2.38 (3H, s, 60-CH3), 6.45 (1H, t, J = 7.2 Hz,
H-5), 6.63 (1H, br d, J = 8.4 Hz, H-3), 6.72 (1H, s, H-50), 7.08 (1H,
br d, J = 8.1 Hz, H-6), 7.15 (1H, br dd, J = 8.4, 7.2 Hz, H-5), 8.20
(1H, s, H-7), 12.13 (1H, s, NH).
Preparation of complex [C19H16ClN9O4Zn] (2)
Complex 2
Complexes 2 was prepared by the same procedure as 1 using
Zn(ClO4)2ꢁ6H2O as metal precursor.
Zn01
Zn01
Zn01
Zn01
Zn01
AN1
2.180(7)
2.102(8)
2.165(7)
1.979(9)
2.014(9)
N1
N1
N1
N1
N4
N4
N4
N5
N5
N7
AZn01
AZn01
AZn01
AZn01
AZn01
AZn01
AZn01
AZn01
AZn01
AZn01
AN4
AN5
AN7
AN9
AN5
AN7
AN9
AN7
AN9
AN9
74.5(3)
AN4
AN5
AN7
AN9
147.3(3)
102.1(3)
97.1(3)
For complex 2: Yield: 65.5%. Anal. Calc. For C19H16ClN9O4Zn C,
42.61; H, 2.99; N, 23.55; O, 11.96. Found: C, 42.51; H, 2.91; N,
73.0(3)
23.45; O, 11.86. IR (KBr;
m msC„N + mas), 2188
/cmꢂ1): 2377 (
129.3(3)
120.6(3)
95.9(3)
102.0(3)
110.1(4)
(vsC@N), 1081(ClOꢂ4 ). 1H-NMR, (d6-DMSO, d): 2.37 (3H, s), 2.46
(3H, s), 6.91 (1H, s), 7.60 (4H, m), 8.01 (2H, m), 8.61 (1H, br s),
8.85 (1H, br s).
Single crystal X-ray crystallography
Bruker SMART APEX II CCD area detector area detector equipped
with graphite monochromated Mo K radiation (k = 0.71073Å)
source in and scan mode at 293(2) K. Cell parameters refine-
ment and data reduction were carried out using Bruker SMART
[32] and Bruker SAINT softwares for all the complexes. The struc-
tures were solved by direct and refined by full-matrix least-
squares based on F2 using the SHELXS-97 and SHELXL-97 programs
a
Selected crystal data for 1 and 2 are given in Table 1 and
selected metrical parameters of the complexes are given in
Table 2. For complexes 1 data collections were made using
Bruker SMART APEX II CCD area detector equipped with graphite
u
x
monochromated Mo K
a radiation (k = 0.71073 Å) source in x scan
mode at 150(2) K. For complex 2, data collection was made using

S. Konar / Journal of Molecular Structure 1092 (2015) 34–43
37
Fig. 1. Molecular structure with atom numbering scheme of 1 (Asymmetric unit).
Fig. 2. Molecular structure with atom numbering scheme of 2 (Asymmetric unit).

38
S. Konar / Journal of Molecular Structure 1092 (2015) 34–43
Fig. 3. Polymeric 1D zigzag chain running along a-axis in 1 (H2O molecules are omitted for clarity).
Fig. 4. Polymeric 1D zigzag chain along c-axis in 2 (perchlorate ions are omitted for clarity).
[33]. For 1 and 2 non hydrogen atoms were refined anisotropically
till the convergence is attained. Better quality crystals for 2 could
not be prepared after several trials.
by dicyanamide ligands. The complexes 1 and 2 crystallize in space
group P21 and P-1. The unit cell of 1 comprises of one molecule and
that of 2 contains two molecules. The geometry around the Zn(II)
centres may be described as distorted square pyramidal (s = 0.44
for 1 and 0.32 for 2) [34]. The structure of 1 consists of the neutral
mononuclear unit, [Zn(L1)(dca)](H2O) in which the basal plane for
1 is formed by N4 from pyrimidine N atom, N1 from azomethine N
atom, O1 from hydroxyl group, N5 from bridging dicyanamide and
that of 2 consists of neutral mononuclear unit, [Zn(L2)(dca)2](ClO4)
where the basal plane is formed by N1 from pyrimidine N atom, N4
from azomethine N atom, N5 from pyridine N atom, N9 from dicya-
namide bridge. In both the complexes the axial positions are occu-
pied by N atom from bridging dicyanamide (N7 for 1, N9 for 2). The
ZnAN axial bond distances of 1.983 Å (for 1) and 1.979 Å (for 2) are
different from that of the ZnAN distances in the basal planes of 1
and 2. The angle subtended at the Zn centre by the N atom of
two bridging 1,5 dicyanamide anions is 113.01° and 110.07° for 1
and 2 and the distance between two Zn centres is 8.130 Å and
7.646 Å for 1 and 2. The NCN bent angle of the dicyanamide ligand
is 128.30° and 132.22° for 1 and 2. The Polymeric 1D zigzag chain
of the coordination polymers of 1 and 2 has been shown in Figs. 3
and 4 respectively. It has been observed that the aromatic benzene
ring, Cg3 (Cg3 = C1AC2AC3AC4AC5AC6) undergoes strong face-
Results and discussion
Synthesis
The ligand L1 and L2 were synthesized by the direct con-
densation reaction of 2-hydrazino-4,6-dimethyl pyrimidine with
salicylaldehyde and di(2-pyridyl) ketone respectively, taken in
methanol in 1:1 M proportion. The complexation behaviour of L1
and L2 towards Zn(II) metal salt were investigated. Complexes 1
and 2 were obtained by mixing the ligands and respective Zn(II)
salts and sodium dicyanamide taken in 1:1:2 M ratio in methanol.
X-ray quality crystals of 1 and 2 were obtained upon slow evap-
oration of the suitable reaction mixtures at room temperature.
Structural description
Description of structure of complexes 1 and 2
The Molecular structure of complexes 1 and 2 with atom num-
bering schemes are shown in Figs. 1 and 2 respectively. The struc-
tures of 1 and 2 consist of well isolated chains of Zn atoms bridged
to-face
pꢁ ꢁ ꢁp stacks (Fig. 5 and Table 3) with a pyrimidine ring,

S. Konar / Journal of Molecular Structure 1092 (2015) 34–43
39
Fig. 5. 2D supramolecular chain structure of complex 1 generated through the
pꢁ ꢁ ꢁp along c axis (H2O molecules are omitted for clarity).
Table 3
Geometric features (distances in Å and angles in degrees) of the
pꢁ ꢁ ꢁp interactions obtained for 1.
Complex
Cg(Ring I)ꢁ ꢁ ꢁCg(Ring J)
Cg(3)ꢁ ꢁ ꢁCg(4)
Cgꢁ ꢁ ꢁCg
3.831(4)
Cg(I)ꢁ ꢁ ꢁPerp
3.746
Cg(J)ꢁ ꢁ ꢁPerp
3.820
a
b
c
Symmetry
Complex 1
15.22
4.23
12.03
ꢂ1 + X, Y, Z
a
= Dihedral angle between ring I and ring J (°); b = Cg(I) ? Cg(J) or Cg(I) ? Me vector and normal to plane I (°); c = Cg(I) ? Cg(J) vector and normal to plane J (°);
CgACg = distance between ring Centroids (Å); CgIꢁ ꢁ ꢁPerp = perpendicular distance of Cg(I) on ring J (Å); CgJꢁ ꢁ ꢁPerp = Perpendicular distance of Cg(J) on ring I (Å);
Cg(4) = centre of gravity of ring [C1AC2AC3AC4AC5AC6]; Cg(3) = centre of gravity of ring [N3AC8AN4AC12AC11AC10] for complex 1.
Cg4 (Cg4 = N3AC8AN4AC12AC11AC10) leads to the formation of
a 2D unit in 1. In addition, each molecule of 1 is assembled by
Characterization of the ligands and the complex species
1H NMR spectra
CAHꢁ ꢁ ꢁ
p interactions (Fig. 6) involving the one C(methyl)AH
group, C(9)AH(9B) donor group of pyrimidine ring and another
aromatic benzene ring Cg(4) (Cg4 = C1AC2AC3AC4AC5AC6) of
symmetry ꢂX, 1/2 + Y, 1ꢂZ (Table 4). In 2, each molecule of 2 is
The structure of the ligands was well secured from their high
resolution 1H NMR spectra in DMSO-d6. The proton chemical shift
assignments of the ligand L1 given in the experimental section
received support from observed coupling patterns and coupling
constants involved therein. A part of the spectrum was typical of
the presence of unsymmetrically substituted 1, 2-disubstituted
benzene moiety. The upfield signal at d 6.84 (1H, d, J = 7.8 Hz) in
L1 was typical of an aromatic proton (H-3) ortho to a phenolic
hydroxyl. The same signal was coupled to H-4 resonance (d 7.18,
1H, t, J = 7.5 Hz) and the latter was again coupling to a signal at d
6.83 (1H, m, H-3) which was in turn coupled to a resonance at d
assembled by CAHꢁ ꢁ ꢁ
p interactions (Fig. 7) involving the one
C(methyl)AH group, C(4)AH(4A) donor group of pyrimidine
ring and another pyridine ring Cg(4) (Cg4 = N5AC8AC9A
C10AC11AC12) of symmetry 1 ꢂ X, ꢂY, 1 ꢂ Z (Table 4). The non
coordinating atoms O(3) of
orientated towards the
Cg5 (Cg5 = N6AC13AC14AC15AC16AC17) in such an angle
a dangling perchlorate anion is
p-face (Fig. 8) of the pyridine ring
that it leads to three significant anionꢁ ꢁ ꢁ
p interactions in 2
(Table 5).

40
S. Konar / Journal of Molecular Structure 1092 (2015) 34–43
Fig. 6. 2D supramolecular chain structure of complex 1 generated through the CAHꢁ ꢁ ꢁ
p along c axis.
Table 4
Geometric features (distances in Å and angles in degrees) of the CAHꢁ ꢁ ꢁp interactions obtained for 1 and 2.
Complexes
CAHꢁ ꢁ ꢁCg(Ring)
Hꢁ ꢁ ꢁCg (Å)
CAHꢁ ꢁ ꢁCg (°)
Cꢁ ꢁ ꢁCg (Å)
Symmetry
Complex 1
Complex 2
C9AH9Bꢁ ꢁ ꢁCg(4)
C4AH4Aꢁ ꢁ ꢁCg(4)
2.82
2.88
128
126
3.492(9)
3.528(11)
ꢂX, 1/2 + Y, 1 ꢂ Z
1 ꢂ X, ꢂY, 1 ꢂ Z
For complex 1, Cg(4) = centre of gravity of ring [C1AC2AC3AC4AC5AC6] and for complex 2, Cg(4) = centre of gravity of ring [N5AC8AC9AC10AC11AC12].
7.32 (1H, br d, J = 7.4 Hz) for H-6. A sharp singlet at d 8.19 (1H) was
in consonance with the presence of the ACH@NA proton while
that at d 6.62 (1H, s) was commensurate with the presence of
H-5 proton in 4,6-dialkylpyrimidine unit. The appearance of two
aromatic methyls at the same position (d 2.26, 6H, s) was in line
with the presence of a locally symmetric 2-pyrimidyl unit. There
were also a pair of low field signals at d 11.34 (1H, br s) and
11.74 (1H, br s) for arylhydrazone NH and chelated phenolic OH
protons. The 500 MHz 1H spectrum of ligand L2 in DMSO-d6 dis-
played singlet signals at d 2.26 (6H) and 6.80 (1H) which were
quite similar to that of L1 in respect of the 4, 6-dimethyl-2-pyrim-
idyl unit present therein. The signals at d 8.58 (1H, ddd, J = 5.0, 1.5,
1.0 Hz) and 8.84 (1H, ddd, J = 5.0, 1.5, 1.0 Hz) with low Jo value in L2
appearing at different positions (d 2.31 and 2.38, 3H each, br s).
It is noteworthy, that solution phase 1H NMR spectral studies of
the complexes 2 adduced for their similarity in the nature of
coordination of L2 to Zn (II). It was observed that H-3, H-5, H-6
and 400-CH3 experienced downfield shift upon coordination of
metal with pyridine nitrogen.
Infrared spectra
The infrared spectra of the complexes are consistent with the
structural data given in this paper. Complexes 1 and 2 exhibit
strong absorption bands in the region 2345–2140 cmꢂ1 that corre-
spond to mC„N modes for dca groups, attributed to mas
+ ms(C„N)
were typical of
a-proton resonance in pyridine system and thus
combination modes, mas(C„N) and s(C„N). The shift towards higher
m
assigned to H-6 and H-60, respectively. In the complex 1 of L1 with
Zn(II) salt and N(CN)ꢂ2 there were six sets of signals for protons on
sp2 hybridised carbon as in parent ligand L1 but upfield shift of aro-
matic protons H-3, H-5 and H-6 were noticed in the complex.
Again, the aromatic methyls are now nonequivalent because of
coordination of pyrimidine nitrogen with Zn and consequently
wave numbers for the peaks of dca in 1 and 2 when compared to
those of free dicyanamide is consistent with the coordination of
dca in the complexes. These three bands appear at 2344, 2239,
2193 cmꢂ1 for complex 1, at 2377, 2237, 2188 cmꢂ1 for complex
2 [35]. The strong mC@N azomethine bands occurring at 1608,
1597 for 1 and 2 respectively are shifted considerably towards

S. Konar / Journal of Molecular Structure 1092 (2015) 34–43
41
Fig. 7. CAHꢁ ꢁ ꢁp interaction along b-axis in 2 (perchlorate ions are omitted for clarity).
lower frequencies compared to that of the free Schiff base ligands
(1635 cmꢂ1), indicating coordination of the imino nitrogen atom
[36].
the Zn+2 ion which show high selectivity compared to other metal
ions (Fig. 10).
Conclusion
Emission properties
The pyrimidine derived NNO and NNN donor Schiff base ligands
The emission spectra of the Schiff base ligands L1, L2 and their
Zn(II) complexes (1 and 2) were recorded at room temperature
(298 K) in methanol solvent by exciting at the absorption maxima
of the ligands (Fig. 9). In both the cases, L1 and L2 have low fluores-
cence intensity with emission maxima at ꢃ433 nm and ꢃ420 nm,
respectively, whereas, the metal complexes show red shifted
enhanced fluorescence emission with maxima at ꢃ456 nm for
complex 1 and 526 nm for complex 2 (Table 3). The low emission
intensity of the ligand may be due to occurrence of photo induced
electron transfer processes in the presence of lone pairs of elec-
trons of the donor atoms within it [37]. The enhancement of emis-
sion intensity of Zn(II) and Cd(II) complexes is probably due to the
metal ligand chelation [38] or increase in conformational rigidity of
the ligands upon complexation [39]. In the present study, the addi-
tion of different metal ions (Ni+2, Co+2, Mn+2, Cu+2, Cd+, 2 Zn+2) to a
1:1 mixture of L1 and dicyanamide in methanol indicates that it is
L1 and L2 along with l1,5 bridging dicyanamide ions form 1D poly-
meric zigzag chains of Zn(II) complexes 1 (with L1), 2 (with L2). In
all the complexes, two zinc complexes have square pyramidal
environment around the metal centres. The ligands are fluores-
cence silent but the Zn(II) complex of L1 i.e. complex 1 shows high-
est chelation enhanced flourescene compared to the Zn(II)
complexes of L2 i.e. complex 2. It seems interesting to conduct a
systematic study on a designed series of transition metal com-
plexes with the intention to understand the way in which different
supramolecular interactions cooperate and direct the supra-
molecular assembly of the molecular building blocks with varia-
tion in the ligand moieties. Fluorescence silent behaviour of the
ligand may probably be due to the presence of non bonding elec-
tron pairs on its nitrogen and oxygen donors (for L1) and nitrogen
donors (for L2). These electrons are involved in coordinate bond
formation with metal ions during complexation. During metal

42
S. Konar / Journal of Molecular Structure 1092 (2015) 34–43
Fig. 8. Anion-pi interaction in complex 2 along b axis.
Table 5
Geometric features (distances in Å and angles in degrees) of the anionꢁ ꢁ ꢁp interactions obtained for 2.
Complex
YAXꢁ ꢁ ꢁCg(J)
Cl1AO3ꢁ ꢁ ꢁCg(5)a
Xꢁ ꢁ ꢁCg(J)
3.400(14)
Yꢁ ꢁ ꢁCg(J)
4.148(6)
\YAXꢁ ꢁ ꢁCg(J)
112.4(7)
XAPerp
Complex 2
3.329
For complex 2, Cg(5) = centre of gravity of ring [N6AC13AC14AC15AC16AC17].
a
Symmetry element: 1 ꢂ X, ꢂY, 1 ꢂ Z.
Fig. 9. Fluorescence emission property of free ligand (L1), (L2) and complexess 1 and
2.
Fig. 10. Fluorescence emission (10ꢂ4 M) in the presence of L1, Ni2+
Mn2+,Cu2+,Cd2+ and complex 1 (from bottom to top in CH3OH).
, ,
Co2+

S. Konar / Journal of Molecular Structure 1092 (2015) 34–43
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43
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L1 using its N, O donors with comparatively harder Zn(II) may be
the reason for chelation enhanced fluorescence of 1 compared to
2 of L2 having only N donor atoms.
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