Three new pseudohalide bridged dinuclear Zn(II), Cd(II) complexes of pyrimidine derived Schiff base ligands: Synthesis, crystal structures and fluorescence studies

Article abstract of DOI:10.1016/j.poly.2011.11.031

One new dinuclear Zn(II) complex, [(N₃)Zn(L₁) (μ_1,1-N₃)]₂ (1), and a dinuclear Cd(II) complex, [(N₃)Cd(L₁)(μ_1,1-N ₃)]₂ (2), of the potentially tridentate NNN-donor Schiff base ligand N-(4,6-dimethyl-pyrimidin-2-yl)-N′-(1-pyridin-2-yl-ethylidine) -hydrazine (L₁) and another dinuclear Cd(II) complex [(NCS)Cd(L ₂)(μ_1,3-NCS)]₂ (3) of a similar NNN donor Schiff base ligand, N-(4,6-dimethyl-pyrimidin-2-yl)-N′-pyridin-2- ylmethylene-hydrazine (L₂), have been synthesized and characterized by elemental analyses, IR, ¹H NMR, fluorescence spectroscopy and single crystal X-ray crystallography. The fluorescence spectral changes observed upon addition of the Zn(II) ion to a mixture of L₁ and azide showed high selectivity towards the Zn(II) ion over other metal ions. The ligands L₁ and L₂ are [1 + 1] condensation products of 2-hydrazino-4,6-dimethyl pyrimidine with 2-acetyl pyridine and pyridine-2-carbaldehyde, respectively. In the complexes 1 and 2 the two Zn(II) and Cd(II) centers are held together by μ_1,1-bridged azide ions, while in 3 the two Cd(II) centers are bridged by μ_1,3-thiocyanate ions. Complex 1 shows high chelation enhanced fluorescence compared to 2 and 3. All the metal centers have a distorted octahedral geometry.

Full text of DOI:10.1016/j.poly.2011.11.031

Polyhedron 33 (2012) 82–89
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Three new pseudohalide bridged dinuclear Zn(II), Cd(II) complexes
of pyrimidine derived Schiff base ligands: Synthesis, crystal structures
and fluorescence studies
Sangita Ray a, Saugata Konar a, Atanu Jana a, Sankar Jana a, Amarendra Patra a, Sudipta Chatterjee b,
James A. Golen c, Arnold L. Rheingold c, Sudhanshu Sekhar Mandal a, Susanta Kumar Kar a,
⇑
a Department of Chemistry, University College of Science, University of Calcutta, 92 A.P.C. Road, Kolkata 700009, India
b Department of Chemistry, Serampore College, Serampore, Hooghly 712201, India
c Department of Chemistry and Biochemistry, University of California, San Diego, CA 92093, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
One new dinuclear Zn(II) complex, [(N3)Zn(L1)(
l
1,1-N3)]2 (1), and
a
dinuclear Cd(II) complex,
Received 10 August 2011
Accepted 9 November 2011
Available online 6 December 2011
[(N3)Cd(L1)(
pyrimidin-2-yl)-N0-(1-pyridin-2-yl-ethylidine)-hydrazine (L1) and another dinuclear Cd(II) complex
[(NCS)Cd(L2)( 1,3-NCS)]2 (3) of a similar NNN donor Schiff base ligand, N-(4,6-dimethyl-pyrimidin-2-
l1,1-N3)]2 (2), of the potentially tridentate NNN-donor Schiff base ligand N-(4,6-dimethyl-
l
yl)-N0-pyridin-2-ylmethylene-hydrazine (L2), have been synthesized and characterized by elemental
analyses, IR, 1H NMR, fluorescence spectroscopy and single crystal X-ray crystallography. The fluores-
cence spectral changes observed upon addition of the Zn(II) ion to a mixture of L1 and azide showed high
selectivity towards the Zn(II) ion over other metal ions. The ligands L1 and L2 are [1 + 1] condensation
products of 2-hydrazino-4,6-dimethyl pyrimidine with 2-acetyl pyridine and pyridine-2-carbaldehyde,
Keywords:
Zinc(II) and cadmium(II) complexes
Pyrimidine derived ligands
1H NMR
X-ray crystal structures
Emission properties
respectively. In the complexes 1 and 2 the two Zn(II) and Cd(II) centers are held together by
l1,1-bridged
azide ions, while in 3 the two Cd(II) centers are bridged by 1,3-thiocyanate ions. Complex 1 shows high
l
chelation enhanced fluorescence compared to 2 and 3. All the metal centers have a distorted octahedral
geometry.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
logical significance of Zn(II) has led to the development of numerous
fluorescent chemo sensors [22]. The bioactivity of Cd is still in ques-
The complexation of group 12 metal ions with polydentate Schiff
base ligands is a well studied area of research in coordination chem-
istry [1–5]. These ligands have preparative accessibilities, structural
variety and varied denticity, forming complexes of different coordi-
nation numbers and nuclearities with interesting molecular and
crystalline architectures [6–16]. A substantial 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]. Pyrimidine
derivatives are also used in combination therapy with protease
inhibitors to fight against HIV-1, the etiological agent of acquired
immune deficiency syndrome [20,21]. Zn is an essential element
in the biosystem. It is required in genetic materials, DNA, RNA poly-
merases, the regulatory Zn finger protein [structural motif for
eukaryotic DNA-binding protein] in forming nucleic acids. The bio-
tion, but unambiguously it is reported that the specific disease itai
itai is caused by Cd poisoning [23,24].
As a sequel of our long standing interest in pyrimidine derived
ligands [25–28] we have prepared tridentate NNN donor Schiff
base ligands using 2-hydrazino-4,6-dimethyl pyrimidine and
pyridine containing carbonyl compounds (2-acetyl pyridine and
pyridine 2-carbaldehyde). Our interests are to observe the metal
ion coordination environments and photoluminescence using
3d/4d metal ion templates, the organic ligands L1 and L2, and suit-
able bridging units. We choose zinc and cadmium because the d10
configuration in zinc permits a wide range of symmetries and
various coordination numbers, i.e., 4, 5 or 6, relatively easily
[29–31], while cadmium shows 4, 6, 7 coordination. Moreover,
luminescent compounds are attracting much current research
interest because of their many applications, including emitting
materials for organic light emitting diodes, light harvesting mate-
rials for photo catalysis and fluorescent sensors for organic or
inorganic analyses [32]. Introduction of electron-donating groups
into the Zn(II) complexes enhances the quantum yield of the pho-
toluminescence [33]. Hence it is important to understand the
coordination chemistry and luminescence between group 12
⇑
Corresponding author. Tel.: +91 033 23508386; fax: +91 033 23519755.
E-mail address: skkar_cu@yahoo.co.in (S.K. Kar).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.11.031

S. Ray et al. / Polyhedron 33 (2012) 82–89
83
metal ions for developing new materials which can function as
fluorescent sensors [32]. In the present study (Scheme 1), using
zinc perchlorate and cadmium perchlorate as the metal precur-
out using a Perkin-Elmer 2400 II elemental analyzer. 1H NMR spec-
tra were recorded on a Bruker 300 MHz FT-NMR spectrometer
using trimethylsilane as an internal standard in d6-DMSO for com-
plexes 1, 2 and 3. Steady state absorption and fluorescence spectra
for all three complexes and the Schiff base ligands were measured
with a Hitachi UV–Vis U-3501 spectrophotometer and a Perkin-El-
mer LS50B fluorimeter, respectively, at room temperature (298 K).
All the spectral measurements were done at ꢁ10ꢀ5 to 10ꢀ6 (M)
concentration of solute in order to avoid aggregation and self-
quenching.
sors, one new dinuclear zinc(II) complex [(N3)Zn(L1)(
(1) and two new dinuclear cadmium(II) complexes,
[(N3)Cd(L1)( 1,1-N3)]2 (2) and [(NCS)Cd(L2)( 1,3-NCS)]2 (3), have
l1,1-N3)]2
l
l
been synthesized with L1 and L2. We have reported the synthetic
details, spectral characterizations, X-ray crystal structures and
fluorescence studies of 1, 2 and 3, where pseudohalides act as ter-
minal ligands as well as bridging ligands (l1,1; l1,3) in presence of
the ligands L1 and L2.
Fluorescence quantum yields of the zinc and cadmium pseudo-
halide complexes of the ligands were calculated using b-naphthol
as the reference with a known Uf (0.23) in MCH solvent, using
the following equation [34,35]
2. Experimental
R
2.1. Materials
n2f A I0ðkf Þdkf
f
Uf
¼
Uf0
R
n20f A0 If ðkf Þdkf
All chemicals were of reagent grade, purchased from commer-
cial sources and used without further purification. 2-Acetyl
pyridine, pyridine-2-carbaldehyde and methyl cyclohexane were
purchased from the Aldrich Chemical Company, USA and used
without further purification.
Caution! Although we have not encountered any problems, it
should be kept in mind that sodium azide and perchlorate com-
pounds 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.
Here n0 and n are the refractive indices of the solvents, A0 and A are
the absorbance’s, U0f and Uf are the quantum yields, and the inte-
grals denote the area of the fluorescence band for the standard
and the sample, respectively.
2.3. Syntheses of the ligands (L1 and L2)
The ligand L1 was synthesized by refluxing a methanol solution
(30 ml) of 2-hydrazino-4,6-dimethyl pyrimidine [36,37] (1.38 g,
10 mmol) with 2-acetyl pyridine (1.21 g, 10 mmol) taken in the
same solvent. The refluxing was continued for 1 h at water bath
temperature. The resulting light yellow solution was filtered and
kept at room temperature for slow evaporation. A light yellow
microcrystalline compound separated after a week. It was filtered
2.2. Physical measurements
The infrared spectra of the complexes were recorded on a
Perkin-Elmer RX
I FT-IR spectrophotometer with KBr discs
(4000–300 cmꢀ1). Elemental analyses (C, H and N) were carried
Scheme 1.

84
S. Ray et al. / Polyhedron 33 (2012) 82–89
off, washed several times with cold methanol and dried in vacuo
over fused CaCl2.
isolated by filtration and air-dried. Yield: (0.395 g, 61% with re-
spect to zinc). Anal. Calc. for C26H30N22Zn2: C, 40.06; H, 3.59; N,
L1: Yield: 2.10 g (81%); mp (°C) 140. Anal. Calc. for C13H15N5: C,
64.73; H, 6.22; N, 29.04. Found: C, 64.55; H, 6.15; N, 28.82%; IR
39.53. Found: C, 40.01; H, 3.51; N, 39.10%; IR (KBr,
m
/cmꢀ1): 2054
(vs) ( as, N3), 1579 (s) ( C@Npym), 1630 (
m
m
m
C@N); 1H NMR (d6-DMSO,
(KBr,
m
/cmꢀ1): 3270 (m) (
m
N–H), 2980 (w), 2920 (w) (
m
C–H), 1360
d/ppm): 2.41 (3H, d, 3.9 Hz, 40-CH3), 2.55 (3H, d, 3.9 Hz, 60-CH3),
2.58 (3H, s, @C–CH3), 6.95 (1H, br s, H-50), 7.65 (1H, br s, NH),
7.74 (1H, m, H-5), 8.09 (1H, br s, H-3), 8.18 (1H, br s, H-4), 8.50
(1H, br s, H-6).
(s) (dCH ), 1215 (ms) (mC@C), 1065 (m) (pym), 1640 (mC@Npym),
1635 (m3C@NH); H NMR (d6-DMSO, d/ppm): 2.46 (6H, s, 40-CH3 and
60-CH3), 2.52 (3H, s, @C–CH3), 6.81 (1H, s, H-50), 7.46 (1H, t,
7.8 Hz, H-4), 7.94 (1H, br d, 8.1 Hz, H-3), 8.69 (1H, dd, 4.0, 0.8 Hz,
H-6), 10.23 (1H, br s, NH).
1
2.4.2. Preparation of complex 2
The ligand L2 was prepared following the same procedure as
that of L1, but using pyridine-2-carbaldehyde (2.40 g, 10 mmol)
in place of 2-acetyl pyridine.
L2: Yield: 3.13 g (85%); mp (°C) 152. Anal. Calc. for C12H13N5: C,
63.43; H, 5.72; N, 30.83 Found: C, 63.22; H, 5.61; N, 30.20%; IR
Complex 2 was prepared by the same procedure as 1 using
Cd(ClO4)2ꢂ6H2O as the metal precursor. Yield: (0.548 g, 65.5%).
Anal. Calc. for C26H30N22Cd2: C, 35.64; H, 3.42; N, 35.20. Found:
C, 35.45; H, 3.39; N, 35.00%. IR (KBr,
m
/cmꢀ1): 2058 (vs) (
m
as, N3),
1575 (s) ( C@Npym), 1625 (
m
m
C@N); 1H NMR (d6-DMSO, d/ppm): 2.39
(KBr,
1570 (
m
m
/cmꢀ1): 3265 (m) (
C@NPy), 1640 (
m
N–H), 2975 (
mC–H), 1590 (mbr) (pym),
(3H, s, 40-CH3), 2.46 (3H, s, 60-CH3), 2.50 (3H, s, @C–CH3), 6.92
(1H, br s, H-50), 7.66 (2H, br s, H-5 and NH), 8.05 (1H, br s, H-3),
8.15 (1H, br s, H-4), 8.53 (1H, br s, H-6).
m
C@Npym), 1630 (
m
C@NH); 1H NMR (d6-DMSO,
d/ppm): 2.46 (6H, s, 40-CH3 and 60-CH3), 6.79 (1H, s, H-50), 7.46
(1H, dd, 7.8, 4.5 Hz, H-5), 7.96 (1H, t, 7.8 Hz, H-4), 8.13 (1H, br d,
8.1 Hz, H-3), 8.3 (1H, s, @C–H), 8.69 (1H, dd, 4.5 Hz, H-6), 11.54
(1H, br s, NH).
2.4.3. Preparation of complex 3
To a methanol solution (30 ml) of Cd(ClO4)2ꢂ6H2O (0.838 g,
2 mmol), a solution of the Schiff base L2 in the same solvent
(0.454 g, 2 mmol) was slowly added with stirring at room temper-
ature, followed by drop by drop addition of a 2 ml aqueous solution
of KSCN (0.388 g, 4 mmol). The stirring was continued for an addi-
tional 2 h and the solution was then filtered. The pale yellow solu-
tion was kept at room temperature, which produced yellow
hexagonal crystals suitable for X-ray diffraction after a week. The
crystals were isolated by filtration and air-dried. Yield: (0.556 g,
61.5%). Anal. Calc. for C28H26N14S4Cd2: C, 36.85; H, 2.85; N, 21.51.
2.4. Preparation of the complexes
2.4.1. Preparation of complex 1
To a methanol solution (30 ml) of Zn(ClO4)2ꢂ6H2O (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 so-
dium azide (0.260 g, 4 mmol) in a minimum volume of aqueous
methanol with constant stirring. The stirring was continued for
additional 2 h and filtered. The light yellow solution was kept at
room temperature, which produced yellow hexagonal crystals
suitable for X-ray diffraction after 10 days. The crystals were
Found: C, 36.67; H, 2.80; N, 21.30%. IR (KBr,
and 2098 (vs) ( as, NCS), 1578 (s) ( C@Npym), 1610 (
(d6-DMSO, d/ppm): 2.42 (3H, s, 40-CH3), 2.46 (s, 60-CH3), 6.94 (1H,
m
/cmꢀ1): 2098 (vs)
C@N); 1H NMR
m
m
m
Table 1
Experimental data for crystallographic analysis of 1, 2 and 3.
Complex
1
2
3
Empirical formula
Formula weight
T (K)
C
26H30N22Zn2
C26H30Cd2N22
875.54
100
C28H26Cd2N14S4
911.73
293
779.48
150
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
0.71073
monoclinic
C2/c
0.71073
monoclinic
C2/c
0.71073
triclinic
P1
ꢀ
16.670(4)
14.544(3)
14.556(3)
90
107.552(8)
90
3364.8(13)
4
1.539
16.927(4)
14.844(4)
14.249(4)
90
104.876(3)
90
3460.3(16)
4
1.681
1.284
9.311(5)
13.761(5)
16.253(5)
109.271(5)
92.116(5)
104.893(5)
1882.8(14)
2
1.608
1.392
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc (mg mꢀ3
)
Absorption coefficient (mmꢀ1
F(000)
)
1.485
1592
1744
904
h range (°) for data collection
Index ranges
1.9–25.6
1.9–27.9
1.3–28.1
ꢀ20 6 h 6 20
ꢀ17 6 k 6 17
ꢀ17 6 l 6 17
2.021
ꢀ21 6 h 6 22
ꢀ19 6 k 6 18
ꢀ11 6 l 6 18
1.067
ꢀ12 6 h 6 11
ꢀ18 6 k 6 18
ꢀ19 6 l 6 19
1.033
Goodness-of-fit (GOF) on F2
Completeness to h = 25.00° (%)
98.7
97.2
93.8
Independent reflections [Rint
Absorption correction
Refinement method
Data/restraints/parameters
Reflections collected
]
3162 [Rint = 0.204]
multi-scan
full-matrix least squares on F2
3162/0/229
15358
3712 [Rint = 0.030]
multi-scan
full-matrix least squares on F2
3712/0/233
12588
8597 [Rint = 0.034]
multi-scan
full-matrix least squares on F2
8597/442/0
24938
Final R indices [I > 2
r(I)]
R1 = 0.0442 wR2 = 0.2240
0.87, ꢀ1.03
0.832 and 0.793
R1 = 0.0366 wR2 = 0.0793
0.950, ꢀ0.481
0.9042 and 0.7833
R1 = 0.0439 wR2 = 0.1225
ꢀ0.89, 1.03
0.8632 and 0.8446
Largest difference peak and hole (eÅꢀ3
Maximum and minimum transmission
)

S. Ray et al. / Polyhedron 33 (2012) 82–89
85
Table 2
3. Single crystal X-ray crystallography
Selected bond distances (Å) and angles (°) in 1, 2 and 3.
Selected bonds
Value (Å)
Selected angles
(°)
Selected crystal data for 1, 2 and 3 are given in Table 1 and se-
lected metrical parameters of the complexes are given in Table 2.
For complexes 1 and 3 data collections were made using a Bruker
SMART APEX II CCD area detector equipped with a graphite mono-
Complex 1
Zn1–N9
Zn1–N6
Zn1–N4
Zn1–N5
Zn1–N9_a
Zn1–N2
2.065(7)
2.095(6)
2.160(7)
2.181(7)
2.424(7)
2.164(6)
Zn1–N9–Zn1A
N2–Zn1–N4
N2–Zn1–N5
N2–Zn1–N6
N2–Zn1–N9
N2–Zn1–N9_a
N4–Zn1–N5
N6–Zn1–N9
103.5(3)
73.9(3)
146.8(3)
91.2(2)
114.7(2)
88.7(2)
72.9(3)
97.9(3)
chromated Mo K
a radiation (k = 0.71073 Å) source in the x scan
mode at 296(2) K. For complex 2 data collections were made using
a CCD area detector equipped with a graphite monochromated Mo
Ka radiation (k = 0.71073 Å) source in the u and x scan mode at
208(2) K. Cell parameter refinement and data reduction were car-
ried out using Bruker APEXII for complex 1 and Bruker SMART
APEXII for complexes 1 and 3. The structures of all the complexes
were solved by conventional direct methods and refined by full-
matrix least square methods using F2 data. SHELXS-97 and SHELXL-97
programs [38] were used for the solution and refinement, respec-
tively, of the structures of all the complexes.
Complex 2
Cd1–N6 _a
Cd1–N1
Cd1–N4
Cd1–N5
2.461(3)
2.319(3)
2.374(3)
2.353(3)
2.461(3)
Cd1–N6–Cd1A
N1–Cd1–N4
N1–Cd1–N5
N1–Cd1–N6
N1–Cd1–N9
N1–Cd1–N6_a
N4–Cd1–N5
N4–Cd1–N6
102.98(11)
69.18(10)
137.29(11)
89.01(10)
93.35(10)
121.22(10)
68.17(10)
82.25(9)
Cd1–N6
4. Result and discussion
Complex 3
Cd1–N11
Cd1–S2
Cd2–N12
Cd2–S1
Cd1–S3
Cd1–N1
Cd1–N4
Cd1–N5
Cd1–N6
Cd1–N2
Cd1–N5
2.337(5)
2.614(2)
2.287(5)
2.6611(19)
2.723(2)
2.374(4)
2.374(3)
2.353(3)
2.461(3)
2.348(4)
2.387(4)
N11–Cd1–S2
N12–Cd2–S1
S2–Cd1–S3
92.59(11)
90.46(12)
83.54(5)
4.1. Syntheses
S2–Cd1–N1
S2–Cd1–N2
S2–Cd1–N5
N1–Cd1–N5
N1–Cd1–N11
N1–Cd1–N11
N2–Cd1–N5
N2–Cd1–N11
N5–Cd1–N11
S3–Cd1–N1
S3–Cd1–N2
S3–Cd1–N5
S3–Cd1–N11
98.49(10)
165.74(10)
125.08(10)
136.43(13)
90.58(14)
90.58(14)
67.88(13)
94.19(14)
86.98(14)
98.64(9)
The ligands L1 and L2 were synthesized by the direct condensa-
tion reaction of 2-hydrazino-4,6-dimethyl pyrimidine with 2-acet-
yl pyridine and pyridine-2-carbaldehyde taken in methanol in a
1:1 mol proportion. Complexes 1, 2 and 3 were obtained by mixing
the ligands and the respective Zn(II) and Cd(II) salts and different
pseudohalides taken in a 1:1:2 molar ratio in methanol. X-ray
quality crystals of 1, 2 and 3 were obtained upon slow evaporation
of the suitable reaction mixtures at room temperature. Details are
given in Section 2.
91.55(10)
88.09(9)
170.43(12)
4.2. Structural description of complexes 1 and 2
A perspective view of complexes 1 and 2, with the atomic
numbering schemes, are shown in Figs. 1 and 2, respectively. Both
complexes 1 and 2 crystallize in the space group C2/c. The struc-
br s, H-50), 7.66 (1H, br s, H-5), 7.91 (1H, br s, H-3), 8.11 (1H, br s, H-
4), 8.22 (1H, br s, @C–H), 8.56 (1H, d, 4.5 Hz, H-6), 12.96 (1H, br s,
NH).
tures of 1 and 2 consist of centrosymmetric [Zn(L1)(l1,1-N3)(N3)]2
Fig. 1. Structural representation and atomic numbering scheme of 1 (H-atoms are omitted for clarity).

86
S. Ray et al. / Polyhedron 33 (2012) 82–89
Fig. 2. Structural representation and atomic numbering scheme of 2 (H-atoms are
omitted for clarity).
and [Cd(L1)(l1,1-N3)(N3)]2 dimers, formed by the union of two
ZnL1(N3)2 and CdL1(N3)2 fragments, respectively. The unit cells
of 1 and 2 comprise of six molecules. Each zinc(II) and cad-
mium(II) cation shows a distorted octahedral geometry. In 1,
the equatorial plane is formed by N2 from pyrimidine, N4 from
azomethine, N5 from pyridine and the N9 atom from the bridging
azide group. In the case of 2, N1 from pyrimidine, N4 from azo-
methine, N5 from pyridine and the N9 atom from a bridging azide
form the equatorial plane. In both complexes the axial positions
are occupied by a nitrogen atom from the bridging azide (N9A
for 1, N6A for 2) and another nitrogen atom from a dangling azide
(N6 for 1 and N9 for 2). The Zn and Cd atoms are shifted by a dis-
tance of 0.246 and 0.269 Å downward from the equatorial plane
towards the dangling azide unit, respectively. The Zn–N1, 1-azide
bond distances of 2.059 and 2.420 Å are a little different from that
of the Cd–N1, 1-azide bond distances of 2.461 and 2.270 Å,
respectively. The N1, 1-azide–Zn–N1, 1-azide bond angle of
76.2° is smaller than the N1, 1-azide–Cd–N1, 1-azide bond angel
of 77.02°. The existence of an inversion center causes the Zn1–
N9–Zn1A–N9A and Cd1–N6–Cd1A–N6A units to form a parallelo-
gram in each case (the sides and angles are a = 2.42 Å, b = 2.065 Å,
76.49° and 103.51° for 1, a = 102.98 Å, b = 77.02 Å, 2.461° and
2.270° for 2) In this dimeric unit, the Znꢂ ꢂ ꢂZn separation distance
is 3.532 Å, which is smaller than the Cdꢂ ꢂ ꢂCd separation distance
of 3.704 Å. The bridging Zn1–N9–Zn1A angle of 103.51° is slightly
larger than the bridging Cd1–N6–Cd1A angle of 102.98°. The crys-
tal structure of this compounds can be described as a 1D layer
due to intermolecular hydrogen bonding (Fig. 4). Both the com-
plexes are stabilized by a network of intermolecular hydrogen
bonding, H3A of N3 and N6 of the dangling azide for 1 and
H1N of N3 and N9 of the dangling azide for 2 (Fig. 5). The details
of the hydrogen bonding parameters are given in Table 3.
Fig. 3. Structural representation and atomic numbering scheme of 3 (H-atoms are
omitted for clarity).
thiocyanate form the equatorial plane for Cd1, whilst N10 pyrimi-
dine, N7 azomethine, N6 pyridine and S1 from another bridging
thiocyanate form the equatorial plane for Cd2. The axial positions
are occupied by N11 from a bridging thiocyanate and S3 from a
dangling thiocyanate for Cd1, and those for Cd2, by N12 from a
bridging thiocyanate and S4 from a dangling thiocyanate. Both
the Cd atoms are shifted slightly towards the bridging thiocyanate,
0.042 Å for Cd1 and 0.025 Å for Cd 2. The Cd1–N11 and Cd1–S2
bond distances, 2.337 and 2.614 Å, are somewhat different to the
Cd2–N12 and Cd2–S1 bond distances, 2.287 and 2.6611 Å, respec-
tively and the N11–Cd1–S2 bond angle of 92.59° is larger than the
N12–Cd2–S1 bond angle of 90.46°. In this dimeric unit, the Cdꢂ ꢂ ꢂCd
separation is 5.791 Å.
5. Characterization of the ligand and the complex species
5.1. 1H NMR spectra
The structures of the ligands were secured from their 300 MHz
1H NMR spectra in d6-DMSO. The proton chemical shift assign-
ments of the ligand L2, received support from the observed cou-
pling patterns and the coupling constants involved therein. The
4.3. Structural description of complex 3
A perspective view of complex 3, with the atomic numbering
scheme, is shown in Fig. 3. Complex 3 crystallizes in the space
group P1. The structure of 3 consists of a dimer formed by two
signal at d 8.69 (1H, d, J = 4.5 Hz) in L2 is typical of an a-proton res-
onance in a pyridine system with a low J0 value and thus was as-
signed to H-6, which is coupled to H-5 (d 7.46, 1H, dd, J = 7.8 and
4.5 Hz). The latter signal is coupled to a signal at d 7.96 (1H, t,
J = 7.8 Hz, H-4) which is in turn coupled to a resonance at d 8.13
(1H, br d, J = 8.1 Hz) for H-3. A sharp singlet at d 8.33 (1H) is in con-
sonance with the presence of an azomethine proton, whilst that at
d 6.79 (1H, s) is in accord with the H-5 proton of the 4,6-dialkylpyr-
imidine unit. The appearance of two aromatic methyl’s at the same
position (d 2.46, 6H, s) is in line with the presence of a symmetric
ꢀ
fragments, where CdL2NCS units are bridged by two
l1,3-NCS an-
ions in an equatorial axial bridging mode. Each cadmium center
is attached to a dangling thiocyanate (Fig. 3). The unit cell com-
prises of two molecules. Here the ligand L2 acts as a neutral triden-
tate NNN donor. Each Cd atom has a distorted octahedral geometry
with a N4S2 chromophore. Three nitrogen atoms, N5 pyrimidine,
N2 from azomethine, N1 from pyridine, and S2 from the bridging

S. Ray et al. / Polyhedron 33 (2012) 82–89
87
Fig. 4. Hydrogen bonding interactions in complex 1.
Fig. 5. Hydrogen bonding interactions in complex 2.
coordination of L1 and L2, as appropriate, to Cd(II) and Zn(II). It
was observed that H-3, H-6 and 40-CH3 experienced upfield shifts
to the extent of 0.18–0.22, 0.13–0.18 and 0.04–0.07 ppm, respec-
tively, while H-4, H-5 and H-50 exhibited downfield shifts of about
0.15–0.24, 0.20–0.28 and 0.11–0.15 ppm, respectively.
Table 3
Details of hydrogen bond distances (Å) and angles (°) for 1 and 2.
D–Hꢂ ꢂ ꢂA
Complex 1
N3–H3Aꢂ ꢂ ꢂN6
Complex 2
d(D–H)
0.8600
0.82(5)
d(Hꢂ ꢂ ꢂA)
2.2200
2.19(5)
d(Dꢂ ꢂ ꢂA)
2.897(9)
2.936(5)
<(DHA)
136.00
152(5)
5.2. Infrared spectroscopy
N3–H1 Nꢂ ꢂ ꢂN9
Symmetry transformations used to generate equivalent atoms:
For 1: 3/2 ꢀ x, 1/2 ꢀ y, 2 ꢀ z.
The infrared spectra of the complexes are consistent with the
structural data given in this paper. The coordination mode of an
For 2: 3/2 ꢀ x, 1/2 ꢀ y, 1 ꢀ z.
azide is usually detected by an intense IR band due to mas(N3) which
occurs above 2000 cmꢀ1. In general, it appears above 2055 cmꢀ1
for a l1,1 bridging azide. The strong sharp bands at 2054 and
2058 cmꢀ1 in the IR spectra of 1 and 2 are attributed to the pres-
ence of a l1,1 bridging azide [39–42]. Complex 3 shows a strong
pyrimidine unit. There is also a low field signal at d 11.54 (1H, br s)
for the arylhydrazone NH proton. The spectrum of the ligand L1 is
quite similar to that of L2 but it lacks the signal for the azomethine
proton, the place being taken by a vinyl methyl signal at 2.52 (3H,
s) and corroborating the assigned structure of L1.
band at 2125 cmꢀ1 due to
mas(NCS) of a l1,3-NCS anion [43,44].
The strong mC@N azomethine bands occurring at 1630, 1625 and
1610 cmꢀ1 for 1, 2 and 3, respectively, are shifted considerably to-
wards lower frequencies compared to that of the free Schiff base
ligands (1635 cmꢀ1 for L1 and 1630 cmꢀ1 for L2), indicating coordi-
nation of the imino nitrogen atom [46].
In the complex (3) of L2 there are six sets of signals for protons
on sp2 hybridized carbons, as in the parent ligand L2, but the aro-
matic methyl groups are now non-equivalent because of the coor-
dination with one pyrimidine nitrogen, and they consequently
appear at different positions (d 2.42 and 2.46, 3H each, br s). Sim-
ilar observations were also made in the cases of the Cd and Zn
complexes 2 and 1, formed from L1 and N3.
5.3. Emission behavior
It is noteworthy that solution phase 1H NMR spectral studies of
the complexes 1–3 indicated their similarity in the nature of
The emission spectra of the Schiff base ligands (L1 and L2) and
their Zn(II) and Cd(II) complexes (1, 2 and 3) were recorded at

88
S. Ray et al. / Polyhedron 33 (2012) 82–89
Table 4
L1
150
100
50
Emission data and calculated quantum yields for complexes 1–3.
Complex1
Complex2
L2
Compound Abs (kmax
)
Emission (kem
(nm)
)
Quantum yield
(nm)
(U
) ꢃ 103
Complex3
1
2
3
344
338
340
505
505
470
6.263
0.404
0.812
6. Conclusion
0
The pyrimidine derived NNN donor Schiff base ligands L1 and L2
form two dinuclear Zn(II) and Cd(II) complexes with L1, and one
Cd(II) complex with L2, where the Zn and Cd centers are held by
l1,1 azide ions in 1 and 2, and end-on bridged thiocyanate ions
450
500
550
600
Wavelength(nm)
Fig. 6. Fluorescence emission properties of the free ligands, L1 and L2, and
complexes 1, 2 and 3.
(l1,3) bridging the Cd(II) centers in 3. In all three complexes each me-
tal center has a distorted octahedral geometry. The azide bridged
complexes are isostructural. The ligands are fluorescent silent, but
the Zn complex shows a strong chelation induced enhanced fluores-
cence compared to its Cd analogue. The fluorescence silent behavior
of the ligands may probably be due to the presence of several non-
bonding electron pairs on the nitrogen donors. These electrons are
involved in coordinate bond formation with metal ions during com-
plexation. The ligand n–p⁄ transitions are not allowed and the flex-
ible bonds of the ligands cause the non-radiative channel to be
active. During the metal binding process, non-radiative channels
and flexible bonds are inactive due to strong binding. The Zn(II)
ion, being harder than the Cd(II) ion, forms strong bonds with nitro-
gen donors. This strong binding of the ligand with Zn(II) may be the
reason for its chelation enhanced fluorescence.
Acknowledgements
Fig. 7. Fluorescence emission (10ꢀ4 M) in the presence of Ni2+, Co2+, Mn2+, Cu2+
,
Cd2+ and Zn2+ (from bottom to top in CH3OH).
Sangita Ray acknowledges the University Grants Commission
(UGC), New Delhi, India [Sanc. No. F. 5-1/2007(BSR)/5-16/
2007(BSR), dated 12.02.2006] for a J.R.F. Financial support [Sanc.
No. SR/S1/IC-27/2008, dated 13.03.2009] from the Department of
Science & Technology (DST), New Delhi, India, is also gratefully
acknowledged. X-ray crystallography was performed at the DST
funded National Single Crystal Diffractometer facility at the
Department of Chemistry, University of Calcutta.
room temperature (398 K) in methanol solvent on excitation of
the corresponding absorption maxima of the ligand, and all the
spectra are shown in Fig. 6. In the cases of both ligands, L1 and
L2 have no such emission intensities but their metal complexes
show enhanced fluorescence with emission maxima at ꢁ505 nm
for 1 and 2 and at 470 nm for complex 3. The quenching of the
emission intensity of the ligands probably is due to the occur-
rence of photoinduced electron transfer processes in the presence
of a lone pairs of electrons of the donor atoms within the ligands
[45]. The enhancement of the emission intensities of metal com-
plexes may be due to the metal ligand chelation [46] or the in-
crease in conformational rigidity of the ligands upon
coordination [47]. The quenching of the emission intensity of a
Schiff base ligand in the presence of transition metal ions is a
common fact, but enhancement of fluorescence through complex-
ation is, however, of much interest as it opens up the opportunity
for photochemical applications of these complexes [48]. In the
present study, the fluorescence spectra changes observed upon
addition of various metal ions (Fe3+, Co2+, Ni2+, Cu2+, Hg2+ and
Zn2+) indicate that a 1:1 mixture of L1 and azide in methanol
show a high selectivity for Zn2+ (Fig. 7) compared to the other
metal ions. Since the ligands have a too low emission intensity
to calculate the quantum yield, we have only calculated quantum
yields for the metal complexes 1, 2 and 3 using b-naphthol in
MCH Uf (0.23) as a secondary standard, and the results are shown
in Table 4. From the quantum yield data in Table 4 it is clear that
the quantum yields of the complexes increase with enhancement
of the fluorescence intensity.
Appendix A. Supplementary data
CCDC 817977, 817978 and 817979 contain the supplementary
crystallographic data for 1, 2 and 3. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.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.
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