Coordinating and non-coordinating anion controlled synthesis of different nuclearity cadmium(II) complexes of 2-((E)-((pyridin-2-yl)methylimino)methyl)phenol: Crystal structure and photophysical study

Article abstract of DOI:10.1016/j.poly.2015.07.051

The tridentate Schiff base ligand 2-((E)-((pyridin-2-yl)methylimino)methyl)phenol (HL) was used to synthesize four new cadmium(II) complexes, namely [Cd₄L₆](ClO₄)₂ (1), [CdL(SCN)]_n (2), [CdL(dca)]

Full text of DOI:10.1016/j.poly.2015.07.051

Accepted Manuscript
Coordinating and non-coordinating anion controlled synthesis of different nu-
clearity cadmium(II) complexes of 2-((E)-((pyridin-2-yl)methylimino)meth-
yl)phenol: crystal structure and photophysical study
Priyanka Kundu, Ennio Zangrando, Prateeti Chakraborty
PII:
S0277-5387(15)00406-4
DOI:
http://dx.doi.org/10.1016/j.poly.2015.07.051
POLY 11437
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
6 May 2015
7 July 2015
Please cite this article as: P. Kundu, E. Zangrando, P. Chakraborty, Coordinating and non-coordinating anion
controlled synthesis of different nuclearity cadmium(II) complexes of 2-((E)-((pyridin-2-
yl)methylimino)methyl)phenol: crystal structure and photophysical study, Polyhedron (2015), doi: http://dx.doi.org/
10.1016/j.poly.2015.07.051
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Coordinating and non-coordinating anion controlled synthesis of different nuclearity
cadmium(II) complexes of 2-((E)-((pyridin-2-yl)methylimino)methyl)phenol: crystal
structure and photophysical study
Priyanka Kundu a, Ennio Zangrando b,* and Prateeti Chakraborty a,c*
a
Department of Chemistry, University of Calcutta, 92, A. P. C. Road, Kolkata -700009, India;
E-mail: prateeti_17@yahoo.co.in ; Tel: 91-33-23509937; extn.516; Fax: 91-33-23519755
bDepartment of Chemical and Pharmaceutical Sciences University of Trieste, Via L. Giorgieri 1,
34127 Trieste Italy, E-mail: ezangrando@units.it
cDepartment of Chemistry, Sarojini Naidu College for Woman, Kolkata-700028
ABSTRACT
The tridentate Schiff base ligand 2-((E)-((pyridin-2-yl)methylimino)methyl)phenol (HL) was
used to synthesize four new cadmium(II) complexes, namely [Cd4L6](ClO4)2 (1), [CdL(SCN)]n
(2), [CdL(dca)]n (3) and [CdL(N3)]n (4), with the aim of exploring the role of the coordinating
and non-coordinating anion in governing the architecture as well as the photophysical behavior
of the complexes. In addition to common physicochemical techniques, complexes 1 and 2 have
further been characterised by X-ray single crystal structural analysis. Complex 1 is a tetranuclear
cationic Cd(II) species where each metal ion has a distorted octahedral geometry. Three Cd
atoms have the N4O2 chromophoric environment, whereas the fourth and central Cd has an O6
chromophore. Complex 2 is a polynuclear species having a 2D network of (4,4) topology where
dinuclear [Cd2L3] metal entities located at the nodes are bridged by SCN− anions. The ligand and
all four complexes are highly fluorescent. Quantum yield calculations reveal that the
1

fluorescence efficiency of the complexes is higher than that of the ligand and the efficiency is
largely influenced by the nature of the co-ligand.
Keywords: Tetranuclear Cd(II); Tridentate Schiff-base; Structural diversity; Photoluminescence
1. Introduction
In recent years phenol based tridentate Schiff-base ligands have been receiving
considerable attention in coordination chemistry for their usefulness in making metal complexes
of varied nuclearity by exploiting their greater flexibility [1-9]. These complexes present not
only intriguing structural features but also potential applications in various fields, like catalysis,
pharmacology, magnetism and photoluminescence [10,11]. It is well documented that d10 metal
complexes of tridentate Schiff base ligands are very important in modern coordination chemistry
since they may adopt different structures, network topologies, electronic and optical properties.
Of the d10 metal ions Zn(II) and Cd(II), the latter is larger in size and exhibits higher flexibility in
adopting different coordination numbers and thereby is relevant for building species of varied
structural motifs [12-19]. Actually the geometry of cadmium complexes results from a balance
between the bonding energies and repulsion amongst ligands [20]. Recently we reported the co-
ligand mediated photophysical behavior and catalytic activity of a series of zinc(II) complexes of
a
potential
N,N,O-donor
tridentate
Schiff-base
ligand,
2-((E)-((pyridin-2-
yl)methylimino)methyl)phenol (HL), with Cl-, Br-, OAc- and SCN- as co-ligands. Remarkably,
these coordinating anions have been found to exhibit a significant role in governing the
geometry, catalytic activity as well as the photophysical behavior of the complexes [21]. As a
continuation of our ongoing interest towards d10 metal ion complexes of various nuclearity using
tridentate Schiff bases, we switch over here to Cd(II) species with the aim of exploring how the
2

coordinating properties of the anions affect the structural topology and the photoluminescence
behavior of the complexes. In this study the synthesized Cd(II)-perchlorate complex of HL was
used as the starting material to prepare species of various nuclearity in presence of three
−
coordinating ligands, SCN−, N3 and the dicyanoamide anion (dca) (Scheme 1) by exploiting
their versatile coordinating ability and bridging properties. The role of these pseudohalide co-
ligands to address the nuclearity as well as the photophysical property of Cd(II)-HL complexes
have been thoroughly investigated and presented in this manuscript.
N
Cd(ClO4)2
[Cd4(L)6(ClO4)2]
N
MeOH
stirring /2h
OH
NaSCN
NaN3
MeOH
Na(dca)
Reflux /2h
[Cd(L)(SCN)]n
[Cd(L)(dca)]n
[Cd(L)(N3)]n
CHO
H2N
OH
N
Scheme 1. Synthetic route to the complexes.
2. Experimental section
2.1. Starting materials
High purity salicylaldehyde and 2-aminomethylpyridine (ampy) were obtained from the
Sigma-Aldrich Company. Cadmium perchlorate, sodium thiocyanate, sodium dicyanamide and
sodium azide were purchased from Merck Chemical Company and used as received. All other
reagents and solvents used were of analytical grade and used as received.
2.2. Synthesis of the complexes
2.2.1. [Cd4L6](ClO4)2 (1)
3

A methanolic solution (5 mL) of salicylaldehyde (0.122 g, 1 mmol) was added with
constant stirring to a (10 mL) methanolic solution of 2-(aminomethyl)pyridine (0.122 g, 1 mmol)
in a dropwise manner. The resulting solution was allowed to stir for 2 h. Then a methanolic
solution of cadmium(II) perchlorate (0.629 g, 1.5 mmol) was added to it and the resulting
solution was stirred for additional 3 h. After that, the solution was filtered 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 two days. Yield 82%. Anal. Calc. For C78H66Cl2N12O14Cd4: C,
48.89; H, 3.47; N, 8.77. Found: C, 48.88; H, 3.48; N, 8.69 %. IR (cm-1): ν(C=N) 1633, ν(skeletal
vibration) 1574.
2.2.2. [CdL(SCN)]n (2)
A water-methanolic (1:1) solution of sodium thiocyanate (0.202 g, 2.5 mmol) was added
to a methanolic solution of complex 1 and the resulting solution was stirred for 2 h. After that,
the resulting solution was filtered and the filtrate was kept in a CaCl2 dessicator. Deep yellow
colored single crystals suitable for X-ray data collection were obtained from the filtrate after a
few days. Yield 86%. Anal. Calc. For C14H11N3OSCd: C, 44.05; H, 2.90; N, 11.00. Found: C,
44.01; H, 2.89; N, 11.00 %. IR (cm-1): ν(C=N) 1633, ν(skeletal vibration) 1538.
2.2.3. [CdL(dca)]n (3)
This complex was synthesized following the same procedure as for 2, just replacing
sodium thiocyanate with sodium dicyanamide (0.222 g, 2.5 mmol). A light yellow colored
crystalline solid was obtained from the filtrate after two weeks. Yield 89%. Anal. Calc. For
C15H11N5OCd: C, 46.22; H, 2.84; N, 17.98. Found: C, 46.21; H, 2.85; N, 17.88 %. IR (cm-1):
ν(C=N) 1634, ν(skeletal vibration) 1573.
2.2.4. [CdL(N3)]n (4)
4

This complex was prepared by adopting a similar procedure as for 2, replacing sodium
thiocyanate with sodium azide (0.162 g, 2.5 mmol). Deep yellow single crystals suitable for X-
ray data collection were obtained after a week on keeping the filtrate in a CaCl2 dessicator Yield
82%. Anal. Calc. For C13H11N3OCd: C, 46.27; H, 3.28; N, 12.45. Found: C, 46.22; H, 3.27; N,
12.40 %. IR (cm-1): ν(C=N) 1634, ν(skeletal vibration) 1574.
2.3. 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 cm-1) were recorded using a Perkin-Elmer
RXI FT-IR spectrophotometer. Fluorescence spectra of the ligand and of the complexes were
recorded in DMF solution as well as in the solid state with a Perkin-Elmer model LS 55
Luminescence spectrometer. Electronic spectra (UV-VIS) (900-250 nm) were also recorded in a
Hitachi U-3501 spectrophotometer in DMF medium.
2.4. X-ray crystal structure analysis
X-ray data collection and structure determination
Diffraction data for compounds 1 and 2 were collected on a Bruker Smart Apex
diffractometer equipped with a CCD. All the experiments were performed at room temperature
with Mo-Kα radiation (λ = 0.71073 Å). Cell refinement, indexing and scaling of the data sets
were carried out using Bruker Smart Apex and Bruker Saint packages [22]. The structures were
solved by direct methods and subsequent Fourier analyses and refined by the full-matrix least-
squares method based on F2 with all observed reflections [23]. The contribution of H atoms (at
calculated positions generated by the program SHELXL was introduced in the final cycles of
refinement. Perchlorate anions were found to be disordered, one over two positions with refined
occupancies of 0.421(13)/0.579(13) and the other by rotation about the Cl3-O30 bond (as a pivot
5

bond) with occupancies of 0.613(13)/0.387(13). All the calculations were performed using the
WinGX System, Ver 1.80.05 [24]. Crystallographic data and details of refinements are reported
in Table 1.
Table 1 Crystallographic data and details of the refinements for complexes 1 and 2.
1
C78H66Cd4Cl2N12O14
1915.92
2
C14H11CdN3OS
381.72
Empirical formula
fw
Crystal system
Monoclinic
P21/n
Monoclinic
C2/c
Space group
a, Å
22.836(3)
14.8187(18)
26.128(3)
111.967(2)
8199.9(17)
4
23.2701(17)
11.5236(9)
11.1892(8)
110.230(2)
2815.4(4)
8
b, Å
c, Å
β, deg
V, Å3
Z
Dcalcd, g cm–3
µ(Mo-Kα), mm–1
F(000)
1.552
1.801
1.156
1.697
3824
25.24
36995
0.0713
14236
6
1504
31.10
22327
0.0379
4502
θ max, deg
No. of reflections collected
Rint
No. of independent reflections

No. of obsd reflections (I > 2σ(I)) 6880
3508
No. of refined parameters
goodness-of-fit, S (F2)
R1, wR2 (I > 2σ(I))
residuals, e/Å3
891
181
0.911
1.043
0.0619, 0.1610
0.848, -1.213
0.0303, 0.0718
0.809, -0.573
3. Results and discussion
In one of our earlier studies [20] we observed that the Schiff-base formed by
condensation of 2-(2-aminoethyl)pyridine with salicylaldehyde acted as a useful ligand in
constructing highly fluorescent Zn and Cd complexes. We thus presumed that the Schiff-base
formed by condensation of salicylaldehyde with the homologous amine of 2-
(aminoethyl)pyridine would generate fluorescent complexes with d10 metal ions along with
different structures. Recently in the Zn(II) complexes of 2-((E)-((pyridin-2-
yl)methylimino)methyl)phenol we have obtained interesting structural variations and different
photoluminescence behavior, greatly influenced by the co-ligands Cl−, Br−, OAc− and SCN− [21].
For the present study we first synthesized the cadmium complex by following a template
synthesis technique employing salicylaldehyde, 2-(aminomethyl)pyridine and Cd(ClO4)2
(complex 1), then complexes 2, 3 and 4 were prepared by treating a methanolic solution of
-
complex 1 in the presence of three different pseudo halides X (where X = SCN-, dca- and N3 ).
Elemental analyses revealed that complex 1 is a tetra cadmium complex having the composition
[Cd4L6](ClO4)2, whereas the general composition of 2, 3 and 4 is Cd(L)(X). All the pseudohalide
containing complexes (2, 3 and 4) exhibit very low molar conductance (7-10 Ω-1 cm2 M-1) in
DMF medium, suggesting non-electrolytic behavior in solution, whereas 1 exhibits a very high
7

molar conductance (160 Ω-1 cm2 M-1), corresponding to a 1:2 electrolyte [25]. An FT-IR spectral
study reveals that all the complexes exhibit bands due to the C=N stretching of an imine group in
the range 1633-1634 cm-1 and skeletal vibrations in the range 1535-1600 cm-1, but no bands for
the free carbonyl or primary amine are observed, indicating that complete condensation has
−
occurred. The presence of ClO4 in complex 1 is attested by the broad and very strong band
centered at ~1050 cm-1 [26,27]. Complexes 2 and 4 both exhibit a sharp band at nearly 2050 and
2049 cm-1, assigned to ν(CN) and ν(N=N) stretches, respectively. Only one sharp band without
any splitting indicates that the co-ligands, SCN and N3, are likely to be coordinated only in one
binding mode [28,29]. On the other hand, the IR band centered at ~ 2200 cm-1 in complex 3 has
been assigned to the coordinated dca anion. However, from the routine physicochemical studies
it was not possible to assign the accurate structures of complexes, which demand X-ray single
crystal structural characterization. We were successful in obtaining structural characterization of
1 and 2, but failed in the case of 3 and 4 and repeated attempts to grow single crystals suitable
for X-ray diffraction remained unsuccessful.
3.1. Description of the structures
3.1.1. [Cd4L6](ClO4)2 (1)
The reaction of the tridentate Schiff base HL with Cd(ClO4)2 in methanol led to a
tetranuclear complex cation, [Cd4L6]2+ (Fig. 1), where each cadmium(II) ion presents a distorted
octahedral coordination sphere (Fig.2). The crystallographic independent unit comprises the
tetranuclear complex and two disordered perchlorate anions. Three metal ions Cd1/Cd2/Cd3 are
approximately located at the vertices of an equilateral triangle, separated by distances ranging
from 5.837(1) to 6.324(1) Å. These metals are meridionally chelated by two ligands through the
phenoxo oxygen atom and the imino and pyridine nitrogen donors in a distorted octahedral
8

coordination sphere (Fig. 2). The Cd-O, Cd-N(imino) and Cd-N(py) bond distances vary in the
range 2.230(5)-2.253(5), 2.260(8)-2.306(9) and 2.377(8)-2.455(9) Å, respectively. The bond
angles reported in Table 2 are indicative of a strong deviation from the ideal octahedral values
caused by the constraints imposed by the chelating requirements to form the five- and six-
membered rings. In addition each phenoxo oxygen atom coordinates another cadmium atom
(Cd4) located in the center of the triangle defined by the Cd1/Cd2/Cd3 ions, so that the Schiff
bases act as chelating-bridging ligands with a µ2-η2:η1:η1 coordination mode. The complex
presents a pseudo three fold symmetry with the central Cd having a distorted octahedral
coordination sphere with the CdO6 chromophore, where the Cd4-O bond lengths are in the range
2.268(5)-2.322(5) Å (Table 2). The intermetallic distances Cd1-Cd4, Cd2-Cd4 and Cd3-Cd4,
double bridged by the phenoxo oxygens, are 3.549(1), 3.5985(9) and 3.5240(9) Å, respectively,
the central Cd4 ion being slightly displaced by 0.06 Å from the plane defined by the other three
metals. The structure is the first Cd4 cluster built by Schiff base ligands, although three
coordination polymers have been reported containing tetranuclear units with Cd atoms in a
similar geometrical arrangement [30].
The complex evidences intramolecular π-π interactions, although the coordination
geometry requirements impede close facing aromatic rings. The most noteworthy interactions
realized between a pyridine and a phenyl ring are N2-C53, N4-C27, N10-C1 with centroid to
centroid distances in the range 4.15-4.43 Å. Other intramolecular ring interactions are larger than
4.78 Å.
The crystal packing (Fig. 3) evidences a void of 1001.1 Å3 within the unit cell (12.2% of
the volume), potentially available for solvent molecules. However the quality of the data and the
disorder did not allow any solvent molecule to be suitably modelled in this area.
9

Fig. 1 . Molecular structure of the tetranuclear complex cation [Cd4L6]2+in compound 1 (H atoms
not shown for the sake of clarity).
Fig. 2. Coordination sphere around the Cd1 ion in complex 1. A similar coordination geometry is
observed for Cd2 and Cd3.
10

Fig. 3. Crystal packing of compound 1 with an indication of the disordered perchlorate anions.
Table 2 Coordination bond distances (Å) and angles (°) for compound 1.
Cd(1)-O(1)
Cd(1)-O(2)
Cd(1)-N(1)
Cd(1)-N(2)
Cd(1)-N(3)
Cd(1)-N(4)
2.230(5)
2.253(5)
2.277(9)
2.409(8)
2.306(9)
2.455(10)
Cd(3)-O(5)
Cd(3)-O(6)
Cd(3)-N(9)
Cd(3)-N(10)
Cd(3)-N(11)
Cd(3)-N(12)
2.247(5)
2.246(6)
2.275(9)
2.377(8)
2.273(8)
2.400(5)
Cd(2)-O(3)
Cd(2)-O(4)
Cd(2)-N(5)
2.239(5)
2.239(6)
2.260(8)
Cd(4)-O(1)
Cd(4)-O(2)
Cd(4)-O(3)
2.300(5)
2.293(5)
2.295(5)
11

Cd(2)-N(6)
Cd(2)-N(7)
Cd(2)-N(8)
2.419(8)
2.261(9)
2.395(9)
Cd(4)-O(4)
Cd(4)-O(5)
Cd(4)-O(6)
2.322(5)
2.296(6)
2.268(5)
O(1)-Cd(1)-O(2)
O(1)-Cd(1)-N(1)
O(1)-Cd(1)-N(2)
O(1)-Cd(1)-N(3)
O(1)-Cd(1)-N(4)
O(2)-Cd(1)-N(1)
O(2)-Cd(1)-N(2)
O(2)-Cd(1)-N(3)
O(2)-Cd(1)-N(4)
N(1)-Cd(1)-N(2)
N(1)-Cd(1)-N(3)
N(1)-Cd(1)-N(4)
N(2)-Cd(1)-N(3)
N(2)-Cd(1)-N(4)
N(3)-Cd(1)-N(4)
78.17(18)
80.0(3)
O(5)-Cd(3)-O(6)
O(5)-Cd(3)-N(9)
O(5)-Cd(3)-N(10)
O(5)-Cd(3)-N(11)
O(5)-Cd(3)-N(12)
O(6)-Cd(3)-N(9)
O(6)-Cd(3)-N(10)
O(6)-Cd(3)-N(11)
O(6)-Cd(3)-N(12)
N(9)-Cd(3)-N(10)
N(9)-Cd(3)-N(11)
N(9)-Cd(3)-N(12)
N(10)-Cd(3)-N(11)
N(10)-Cd(3)-N(12)
N(11)-Cd(3)-N(12)
78.24(19)
79.4(3)
140.5(3)
131.5(2)
99.2(3)
144.3(3)
125.5(2)
91.8(2)
133.7(3)
103.4(2)
78.2(2)
122.3(3)
100.2(3)
80.4(3)
136.1(3)
70.6(3)
136.6(2)
71.3(3)
143.3(3)
87.4(3)
151.4(3)
96.5(3)
86.3(3)
88.6(3)
105.2(3)
71.2(3)
111.1(2)
71.4(3)
O(3)-Cd(2)-O(4)
O(3)-Cd(2)-N(5)
76.76(19)
82.5(3)
O(1)-Cd(4)-O(2)
O(1)-Cd(4)-O(3)
75.96(19)
100.87(19)
12

O(3)-Cd(2)-N(6)
O(3)-Cd(2)-N(7)
O(3)-Cd(2)-N(8)
O(4)-Cd(2)-N(5)
O(4)-Cd(2)-N(6)
O(4)-Cd(2)-N(7)
O(4)-Cd(2)-N(8)
N(5)-Cd(2)-N(6)
N(5)-Cd(2)-N(7)
N(5)-Cd(2)-N(8)
N(6)-Cd(2)-N(7)
N(6)-Cd(2)-N(8)
N(7)-Cd(2)-N(8)
Cd(1)-O(1)
142.3(3)
121.1(2)
90.2(3)
O(1)-Cd(4)-O(4)
O(1)-Cd(4)-O(5)
O(1)-Cd(4)-O(6)
O(2)-Cd(4)-O(3)
O(2)-Cd(4)-O(4)
O(2)-Cd(4)-O(5)
O(2)-Cd(4)-O(6)
O(3)-Cd(4)-O(4)
O(3)-Cd(4)-O(5)
O(3)-Cd(4)-O(6)
O(4)-Cd(4)-O(5)
O(4)-Cd(4)-O(6)
O(5)-Cd(4)-O(6)
Cd(3)-O(5)
174.1(2)
85.82(19)
97.25(18)
82.93(18)
106.00(18)
91.17(18)
166.69(19)
74.1(2)
124.8(2)
96.0(3)
81.3(3)
137.5(3)
71.4(3)
150.2(3)
92.5(3)
169.67(18)
109.81(19)
99.6(2)
93.5(3)
117.0(3)
71.2(4)
81.89(19)
76.79(19)
2.248(5)
2.246(6)
2.274(9)
2.377(8)
2.274(8)
2.401(5)
2.230(5)
2.252(5)
2.279(9)
2.408(8)
2.307(9)
2.456(10)
Cd(1)-O(2)
Cd(3)-O(6)
Cd(1)-N(1)
Cd(3)-N(9)
Cd(1)-N(2)
Cd(3)-N(10)
Cd(1)-N(3)
Cd(3)-N(11)
Cd(1)-N(4)
Cd(3)-N(12)
Cd(2)-O(3)
Cd(2)-O(4)
2.240(5)
2.240(6)
Cd(4)-O(1)
Cd(4)-O(2)
2.300(5)
2.292(5)
13

Cd(2)-N(5)
Cd(2)-N(6)
Cd(2)-N(7)
Cd(2)-N(8)
2.260(8)
2.419(8)
2.261(9)
2.396(9)
Cd(4)-O(3)
Cd(4)-O(4)
Cd(4)-O(5)
Cd(4)-O(6)
2.295(5)
2.323(5)
2.295(6)
2.268(5)
O(1)-Cd(1)-O(2)
O(1)-Cd(1)-N(1)
O(1)-Cd(1)-N(2)
O(1)-Cd(1)-N(3)
O(1)-Cd(1)-N(4)
O(2)-Cd(1)-N(1)
O(2)-Cd(1)-N(2)
O(2)-Cd(1)-N(3)
O(2)-Cd(1)-N(4)
N(1)-Cd(1)-N(2)
N(1)-Cd(1)-N(3)
N(1)-Cd(1)-N(4)
N(2)-Cd(1)-N(3)
N(2)-Cd(1)-N(4)
N(3)-Cd(1)-N(4)
78.17(18)
80.0(3)
O(5)-Cd(3)-O(6)
O(5)-Cd(3)-N(9)
O(5)-Cd(3)-N(10)
O(5)-Cd(3)-N(11)
O(5)-Cd(3)-N(12)
O(6)-Cd(3)-N(9)
O(6)-Cd(3)-N(10)
O(6)-Cd(3)-N(11)
O(6)-Cd(3)-N(12)
N(9)-Cd(3)-N(10)
N(9)-Cd(3)-N(11)
N(9)-Cd(3)-N(12)
N(10)-Cd(3)-N(11)
N(10)-Cd(3)-N(12)
N(11)-Cd(3)-N(12)
78.24(19)
79.5(3)
140.5(3)
131.5(2)
99.1(3)
144.2(3)
125.5(2)
91.8(2)
133.6(3)
103.4(2)
78.2(2)
122.3(3)
100.2(3)
80.3(3)
136.2(3)
70.6(3)
136.5(2)
71.2(3)
143.4(3)
87.4(3)
151.4(3)
96.4(3)
86.3(3)
88.7(3)
105.3(3)
71.3(3)
111.2(2)
71.5(3)
O(3)-Cd(2)-O(4)
76.77(19)
O(1)-Cd(4)-O(2)
75.96(19)
14

O(3)-Cd(2)-N(5)
O(3)-Cd(2)-N(6)
O(3)-Cd(2)-N(7)
O(3)-Cd(2)-N(8)
O(4)-Cd(2)-N(5)
O(4)-Cd(2)-N(6)
O(4)-Cd(2)-N(7)
O(4)-Cd(2)-N(8)
N(5)-Cd(2)-N(6)
N(5)-Cd(2)-N(7)
N(5)-Cd(2)-N(8)
N(6)-Cd(2)-N(7)
N(6)-Cd(2)-N(8)
N(7)-Cd(2)-N(8)
82.6(3)
142.3(3)
121.1(2)
90.2(3)
124.8(2)
95.9(3)
81.2(3)
137.5(3)
71.4(3)
150.2(3)
92.5(3)
93.4(3)
117.0(3)
71.2(4)
O(1)-Cd(4)-O(3)
O(1)-Cd(4)-O(4)
O(1)-Cd(4)-O(5)
O(1)-Cd(4)-O(6)
O(2)-Cd(4)-O(3)
O(2)-Cd(4)-O(4)
O(2)-Cd(4)-O(5)
O(2)-Cd(4)-O(6)
O(3)-Cd(4)-O(4)
O(3)-Cd(4)-O(5)
O(3)-Cd(4)-O(6)
O(4)-Cd(4)-O(5)
O(4)-Cd(4)-O(6)
O(5)-Cd(4)-O(6)
100.86(19)
174.1(2)
85.80(19)
97.26(19)
82.96(18)
106.02(19)
91.11(19)
166.7(2)
74.1(2)
169.67(18)
109.79(19)
99.6(2)
81.89(19)
76.8(2)
3.1.2. [Cd(L)(SCN)]n (2)
The X-ray structural analysis of compound 2 evidenced the formation of a 2D network of
the composition [CdL(SCN)]n. Basically the structure is composed of dinuclear entities with the
metals separated by 3.496(1) Å and located about a crystallographic two-fold axes passing
normal to the Cd2O2 moiety, as depicted in Fig. 4. The crystallographic independent cadmium
ion is chelated by the Schiff base ligand through the phenoxo oxygen atom (Cd-O = 2.2505(16)
Å), the imino (Cd-N = 2.3031(18) Å) and pyridine nitrogen donor (Cd-N = 2.353(2) Å). A
selection of bond distances and angles is reported in Table 3. The distorted octahedral geometry
15

is completed by a phenoxo oxygen atom from a symmetry related ligand (Cd-O(1)’ = 2.3135(17)
Å) and by two SCN− ions, one bound via a nitrogen atom, the other via a sulfur atom, with Cd-N
and Cd-S bond lengths of 2.272(2) and 2.7631(7) Å, respectively. The SCN− bridges the metals
with Cd-S-CN and Cd-N-CS angles of 104.58(9) and 153.5(2)°, respectively. The coordination
bond distances agree with those found in the discrete dinuclear complex [Cd2(L')2(NCS)2] [31]
and
in
the
1D
polymer
[Cd2(L')2(NCS)2]n
where
L'
=
2-[(2-
dimethylaminoethylimino)methyl]phenol [32,33].
Thus the Schiff base HL behaves in a chelating-bridging mode as for Cd1/Cd2/Cd3 of the
tetranuclear derivative 1, but the thiocyanate anions displace one of the N,N,O-ligands with a
rearrangement of the coordination donors about the Cd atom, as described. The crystal packing
analysis shows a two-dimensional network parallel to the bc plane, having a 4,4 topology (Figs.
5 and 6), where SCN− anions act as bridging ligands. Assuming the midpoints of the dinuclear
entities as nodes of this net, the edges of the rhomboidal grid structure are of 8.031 Å. Adjacent
2D nets are marginally interdigitated to provide weak π-π interactions, the pyridine ring centroids
being spaced at 4.630(3) Å, but slipped by 2.717 Å (Fig. 7).
16

Fig. 4. The dinuclear entity in compound 2 with the labelling scheme of the metal coordination
sphere.
Fig. 5. View down axis a of the 2D network of (4, 4) topology in compound 2.
17

Fig. 6. A simplified picture (Schiff bases removed for clarity) of the 2D network in compound 2.
Fig. 7. View of the crystal packing down axis b showing three 2D nets (in different colors)
marginally inter digitated in compound 2.
18

Table 3 Coordination bond distances (Å) and angles (°) for compound 2.
Cd-O(1)
Cd-N(3)
Cd-N(1)
2.2505(16)
2.272(2)
Cd-O(1')
Cd-N(2)
Cd-S(1'')
2.3135(17)
2.353(2)
2.3031(18)
2.7631(7)
O(1)-Cd-N(3)
O(1)-Cd-N(1)
N(3)-Cd-N(1)
O(1)-Cd-O(1')
N(3)-Cd-O(1')
N(1)-Cd-O(1')
O(1)-Cd-N(2)
N(3)-Cd-N(2)
N(1)-Cd-N(2)
106.31(7)
78.63(6)
174.81(7)
80.03(6)
89.01(7)
93.48(6)
149.61(8)
103.57(8)
71.70(7)
O(1')-Cd-N(2)
O(1)-Cd-S(1'')
N(3)-Cd-S(1'')
N(1)-Cd-S(1'')
O(1')-Cd-S(1'')
N(2)-Cd-S(1'')
C(14'')-S(1'')-Cd
C(14)-N(3)-Cd
95.12(7)
99.70(5)
86.31(6)
91.31(5)
175.04(4)
87.59(7)
104.58(9)
153.5(2)
Symmetry codes (') -x+1/2,-y+1/2,-z+1; ('') x,-y+1,z-1/2
As a matter of fact, the structural analysis of complex 1 shows the possible intramolecular π-π
interactions are more significant than the weaker interactions originating between the
interdigitated 2D layers in 2.
3.2. Electronic properties
The electronic spectra of the ligand and all the four Cd complexes were recorded in DMF
medium (Fig. 8). The ligand exhibits band maxima at 240, 268 and 316 nm, the former two
19

likely to be originating from π→π* transitions and the latter one due to n→π* transitions. All
four complexes display three strong bands at ~235, 270 and 378 nm. The bands are likely to
originate from a ligand to metal charge transfer due to the d10 electronic configuration of Cd(II)
[34]. The band at lower wavelength (~235 nm) corresponds to an intraligand charge transfer,
whereas the other two bands are originate from ligand to metal charge transfers.
Fig. 8. UV-Vis spectra of complexes 1-4 and the ligand as 10-4 M solutions in DMF.
3.3. Photoluminescence properties
The fluorescence properties of the ligand as well as all the four complexes were
examined in DMF medium on performing photo-excitation at a λmax(ex) of 370 nm. Fig. 9 depicts
the emission spectra of the Cd(II) complexes along with that of the ligand in the inset with an
excitation λ
of 370 nm. The highest emission band for the ligand is observed at 492 nm,
max(ex)
whereas the emission band maxima for all the Cd(II) complexes are shifted to lower wavelengths
20

(Table 4), consistent with previous observations reported by us in similar systems [21]. Just like
the previous case, the observed blue shift may be rationalized by considering that the excited
states originating from the Cd(II) complexes are basically ligand-centered in nature due to the
inability of the d10 electronic configuration center to participate in low-energy charge transfers
for metal-centered transitions. Quantum yield calculations (Table 4) show that all the metal
complexes are much more fluorescent in comparison to the ligand, a feature likely due to the
greater rigidity of the ligand system attained upon complexation. From the fluorescence spectra
as well as quantum yield calculations it is also evident that the co-ligands play a significant role
in controlling the photoluminescence properties of the complexes. The influence of co-ligands on
−
−
the fluorescence efficiency follows the trend: ClO4 > SCN− > N3 > dca, which is most probably
originating from co-ligand mediated structural variations leading to different extents of the π-π
interactions. With a greater the extent of the π-π interaction, a higher fluorescence efficiency is
expected [35,36]. X-ray single crystal structural analyses clearly point out stronger π-π
interactions in complex 1 in comparison to complex 2, justifying the observed higher
fluorescence efficiency of the former complex.
21

Fig. 9. Fluorescence spectra of complexes 1-4 in DMF medium. (λexc = 370 nm). Probe
concentrations are 10-5 M.
Table 4 Photophysical parameters of the metal complexes in DMF on excitation at λ= 370 nm.
Complex
Quantum yield
0.683
Emission (λmax, nm)
[Cd4L6](ClO4)2 (1)
[Cd(L)(SCN)]n (2)
[Cd(L)(dca)]n (3)
[Cd(L)(N3)]n (4)
Ligand HL
457
458
458
457
492
0.465
0.218
0.325
0.222
4. Conclusion
The flexibility
of
the
tridentate
Schiff-base
ligand
2-((E)-((pyridin-2-
yl)methylimino)methyl)phenol (HL) has been exploited to synthesize Cd(II) complexes of
different nuclearity. The non-coordinating perchlorate ions induce the formation of an
22

unprecedented tetranuclear Cd(II) complex (1), as evidenced from X-ray single crystal structural
characterization. The use of complex 1 as a starting material in the presence of different
pseudohalides led to the in situ synthesis of three new complexes, 2 (SCN), 3 (dca) and 4 (N3).
The X-ray single crystal structural characterization of 2 reveals that it is a 2D coordination
polymer. All four complexes show fluorescence efficiency to a much greater degree than the
ligand itself, mainly due to the conformational rigidity imposed after complexation. The extent
of fluorescence of the complexes is observed to be co-ligand dependent and follows the order
−
−
−
ClO4 > SCN− > N3 > dca. The higher efficacy of the ClO4 containing complex (1) than that of
the thiocyanato species (2) is most likely related to the stronger π-π interactions observed in 1
than in 2, as detected from the X-ray structural analysis. Although the correct structure of
complexes 3 and 4 could not be elucidated, the physicochemical studies along with
photophysical properties suggest a structure similar to 2, with a slightly different extent of π-π
interactions conditioned by the different length provided by the linkers.
Appendix A. Supplementary data
FT-IR spectra of the complexes. CCDC 1061606 and 1061607 contain the supplementary
crystallographic data for the paper. 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.]
23

Acknowledgements
The authors wish to thank Professor Debasis Das, Department of Chemistry, University of
Calcutta for his constant support. P.K. is thankful to Shailabala Biswas Research Foundation,
University of Calcutta, Kolkata, India for financial support.
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26

Graphical abstract
27

Graphical abstract (synopsis)
Four Cd(II) complexes, having different nuclearity, have been synthesized using three
coordinating and one non-coordinating anion, and two of the complexes have been characterized
by single crystal X-ray crystallography. Their fluorescence behavior was investigated in solution
which reveals that π-π interactions in the complexes are accountable for controlling the
fluorescence efficiency of the synthesized complexes.
28