Synthesis, characterization and photocatalytic activity of a dinuclear thiocyanate bridged cadmium(II) Schiff base complex

Article abstract of DOI:10.1016/j.poly.2016.10.008

A dinuclear centrosymmetric cadmium(II) Schiff base complex, [Cd₂(L)₂(μ-1,3-SCN)₂(CH₃OH)₂], {HL?=?2-(3-(methylamino)propyliminomethyl)-4,6-dichlorophenol} has been synthesized and characterized. X-ray crystallography has confirmed its structure. The complex crystallizes in monoclinic space group C2/c with cell dimensions, a?=?26.161(1)??, b?=?8.9412(3)??, c?=?18.9687(8)??, β?=?129.202(1)°. Extended supra-molecular assemblies were generated through weak non-covalent interactions. The application of the complex as a photocatalyst for the decomposition of organic dye in visible light has been explored.

Full text of DOI:10.1016/j.poly.2016.10.008

Accepted Manuscript
Photocatalytic activity of a dinuclear thiocyanate bridged cadmium(II) Schiff
base complex for the degradation of organic dye in visible light
Sumit Roy, Klaus Harms, Shouvik Chattopadhyay
PII:
S0277-5387(16)30504-6
DOI:
http://dx.doi.org/10.1016/j.poly.2016.10.008
POLY 12260
Reference:
Polyhedron
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Revised Date:
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6 October 2016
Please cite this article as: S. Roy, K. Harms, S. Chattopadhyay, Photocatalytic activity of a dinuclear thiocyanate
bridged cadmium(II) Schiff base complex for the degradation of organic dye in visible light, Polyhedron (2016),
doi: http://dx.doi.org/10.1016/j.poly.2016.10.008
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Photocatalytic activity of a dinuclear thiocyanate bridged
cadmium(II) Schiff base complex for the degradation of organic dye
in visible light
Sumit Roya, Klaus Harmsb, Shouvik Chattopadhyaya,*
a Department of Chemistry, Inorganic Section, Jadavpur University, Kolkata – 700 032, India.
bFachbereichChemie, Philipps-Universität Marburg, Hans-Meerwein-Straße, 35032 Marburg,
Germany.
e-mail: shouvik.chem@gmail.com Tel. +91-33- 2457-2941
Abstract
A dinuclear centrosymmetric cadmium(II) Schiff base complex, [Cd2(L)2(µ-1,3-
SCN)2(CH3OH)2], {HL = 2-(3-(methylamino)propyliminomethyl)-4,6-dichlorophenol} has been
synthesized and characterized. X-ray crystallography has confirmed its structure. The complex
crystallizes in monoclinic space group C2/c with cell dimensions, a = 26.161(1) Å, b = 8.9412(3)
Å, c = 18.9687(8) Å, β = 129.202(1)°. Extended supra-molecular assemblies were generated
through weak non-covalent interactions. The application of the complex as a photocatalyst for
the decomposition of organic dye in visible light has been explored.
Keywords: Cadmium(II); Schiff base; End-to-end thiocyanate; Crystal structure;
Photoluminescence; Photocatalyst.

1. Introduction
The environmental contamination caused by organic pollutants is a great problem
nowadays. The synthesis and characterization of efficient, cheap and environment friendly
photocatalysts is, therefore, a hot topic due to their applications to purify water and air by
thoroughly decomposing organic compounds [1-4]. Methylene blue (MB) is one of the well-
known dyes widely used in several industries and has caused serious pollution. A new emerging
application of cadmium(II) complexes as photocatalyst for the degradation of organic dyes in
UV light was established in recent years [5,6]. However, there is no report of the application of
cadmium(II) Schiff base complexes in photocatalysis using visible light.
In the present work, a tridentate N2O donor salicylaldimine Schiff base has been used to
prepare a dinuclear cadmium(II) complex. The complex has been shown to have good
photocatalytic activity to decompose methylene blue in visible light. Herein, we would like to
report the synthesis characterization, self assembly and photocatalytic activity of a thiocyanate
bridged dinuclear cadmium(II) Schiff base complex.
2. Experimental
Methylene blue was purchased from Loba Chemie and used without further purification.
All other starting materials and solvents were commercially available, reagent grade, and used as
purchased from Sigma-Aldrich without further purification. Caution!!! perchlorate salts are
potentially explosive. Although no problem was encountered in the present study, only small
amounts of the materials should be prepared and they must be handled with care.

2.1. Preparation
2.1.1. Synthesis of [Cd2(L)2(µ-1,3-SCN)2(CH3OH)2]
A methanol solution of 3,5-dichlorosalicylaldehyde (1 mmol, 0.191 g) and N-methyl-1,3-
diaminopropane (1 mmol, 0.103 mL) was refluxed for 1 h to prepare a tridentate N2O donor
Schiff base, {2-(3-(methylamino)propyliminomethyl)-4,6-dichlorophenol} (HL). The Schiff base
was not isolated and was used directly for the preparation of complex. A methanol solution of
cadmium(II) perchlorate hexahydrate (1 mmol, 0.419 g) was added to the methanol solution of
Schiff base and refluxed for 1 h. A methanol solution of sodium thiocyanate (1 mmol, 0.081 g)
was then added to it and refluxed further for ca. 1 h. Single crystals, suitable for X-ray
diffraction, were obtained after few days on slow evaporation of the solution in open atmosphere.
Yield: 0.69 g (73%). Anal. Calc. for C26H34Cd2Cl4N6O4S2 (FW 925.31): C, 33.75; H, 3.70; N,
9.08 Found: C, 33.7; H, 3.6; N, 9.1 %. IR (KBr, cm-1): 1631 (υC=N), 2082 (υSCN), 3280 (υNH),
3444 (υOH), 3001-2850 (υCH). UV-Vis, λmax (nm), (εmax (L mol-1 cm-1)) (DMSO), 261 (1.2 × 104),
337 (6.4 × 103), 428 (2.2 × 103).
2.2. Physical measurements
Elemental analyses (carbon, hydrogen and nitrogen) were performed on a PerkinElmer
240C elemental analyzer. Infrared spectrum in KBr (4000-400 cm-1) was recorded using a
PerkinElmer Spectrum Two FTIR spectrophotometer. Electronic spectrum in DMSO (800-200
nm) was recorded on a PerkinElmer Lambda 35 UV-Vis spectrophotometer. Fluorescence
spectrum in DMSO was obtained on a Hitachi F-7000 Fluorescence spectrophotometer at room
temperature. The powder XRD data were collected on a Bruker D8 Advance X-ray
diffractometer using Cu Kα radiation (λ = 1.548 Å) generated at 40 kV and 40 mA. The PXRD

spectrum was recorded in a 2θ range of 5-50° using 1-D Lynxeye detector at ambient conditions.
Lifetime measurement was recorded using Hamamatsu MCP photomultiplier (R3809) and was
analyzed by using IBHDAS6 software.
2.3. X-ray crystallography
A suitable single crystal of the complex was used for data collection using a Bruker D8
QUEST area detector diffractometer equipped with graphite-monochromated Mo-Kα radiation (λ
= 0.71073 Å) at 100 K. The molecular structure was solved by direct method and refined by full-
matrix least squares on F2 using SHELXL-2014 [7]. X-ray intensity data were measured. The
frames were integrated with the Bruker SAINT Software package using a wide-frame algorithm.
Non hydrogen atoms were refined anisotropically. Hydrogen atoms attached to oxygen and
nitrogen were located by difference Fourier maps and were kept at fixed positions. Others
hydrogen atoms were placed in their geometrically idealised positions and constrained to ride on
their parent atoms. Numerical and/or multi-scan absorption corrections were applied to the data
using the program SADABS [8]. The crystallographic and refinement data of the complex are
summarized in Table 1.
2.4. Hirshfeld Analysis
Hirshfeld surfaces [9-11] and the associated two-dimensional (2D) fingerprint [12-14]
plots were calculated using Crystal Explorer, [15] with bond lengths to hydrogen atoms set to
standard values [16]. For each point on the Hirshfeld isosurface, two distances, de (the distance
from the point to the nearest nucleus external to the surface) and di (the distance to the nearest
nucleus internal to the surface), are defined. The normalized contact distance (dnorm) based on de
and di is given by

(dꢆ − rꢆꢇꢈꢉ
rꢆꢇꢈꢉ
)
(dꢊ − rꢊꢇꢈꢉ
rꢊꢇꢈꢉ
)
ꢀꢁꢂꢃꢄ =ꢅ
ꢅ+ꢅ
where rivdW and revdW are the van der Waals radii of the atoms. The value of dnorm is
negative or positive depending on intermolecular contacts being shorter or longer than the van
der Waals separations. The parameter dnorm displays a surface with a red-white-blue color
scheme, where bright red spots highlight shorter contacts, white areas represent contacts around
the van der Waals separation, and blue regions are devoid of close contacts. For a given crystal
structure and set of spherical atomic electron densities, the Hirshfeld surface is unique [17] and
thus it suggests the possibility of gaining additional insight into the intermolecular interaction of
molecular crystals.
2.5. Photocatalytic Measurement
A photocatalytic experiment in aqueous solution was carried out in a typical process
[5,18]. A 36 W Hg vapour lamp was used as the visible light source and the catalytic degradation
was carried out with 100 mL of Methylene Blue solution (10 mg L-1) with the complex using as
catalyst. The mixture was stirred for 10 min in a dark environment to get a balance between
adsorption and desorption. Then, the solution was stirred constantly. A 4 mL sample was taken
from the reaction system in an interval of 5 min and the supernatant liquid obtained by
centrifugation was used for collecting the UV-Visible spectrum. The characteristic peak for
methylene blue at λ = 607 nm was employed to monitor the photocatalytic degradation.

3. Results and discussions
3.1. Synthesis
The tridentate Schiff base ligand (HL) was prepared by the condensation of N-methyl-
1,3-diaminopropane with 3,5-dichlorosalicylaldehyde [19,20]. It was not isolated and used
directly for the synthesis of the complex. The methanol solution of HL on reaction with
cadmium(II) perchlorate hexahydrate followed by the addition of sodium thiocyanate under
reflux produced the dinuclear complex, [Cd2(L)2(µ-1,3-SCN)2(CH3OH)2]. Formation of the
complex is shown in Scheme 1.
Scheme 1: Schematic representation of the preparation of the complex.
3.2. Description of structure
3.2.1. [Cd2(L)2(µ-1,3-SCN)2(CH3OH)2]
The end-to-end thiocyanate bridged centrosymmetric dinuclear complex crystallizes in
monoclinic space group, C2/c. A perspective view of the complex with selective atom-
numbering scheme is shown in Figure 1. Selected bond lengths and angles are gathered in Tables

2 and 3, respectively. The X-ray crystal structure analysis reveals that the hexa-coordinated
cadmium(II) is meridionally coordinated by one amine nitrogen atom, N(1), one imine nitrogen
atom, N(5), one phenoxo oxygen atom, O(7), of the deprotonated tridentate Schiff base ligand
(L-), one nitrogen atom, N(1'), from a coordinated thiocyanate and one oxygen atom, O(1S) from
a solvent methanol. An sulfur atom S(3')a from a symmetry related {Symmetry transformation a
= 1-x,1-y,1-z} bridging thiocyanate occupied sixth coordination site of each cadmium(II) to
complete its octahedral geometry.
The bridging thiocyanates are almost linear with the value of S-C-N angle being
178.3°(2). S-Cd-N Angles (°), Cd-S and Cd-N distances (Å) of end-to-end bridging thiocyanates
are 95.44(5)°, 2.7857(6) and 2.266(2)(Å) respectively and these values are more or less similar
with other related cadmium(II) Schiff base complexes [21-28] (Table 4). In the dinuclear moiety,
cadmium(II)···cadmium(II) distance is found to be 5.5736(5) Å. Cd(II)-O and Cd(II)-N distances
fall within the range 2.325-2.337 Å and 2.222-2.345 Å respectively, as were also observed in
similar systems [29-31]. The puckering parameters of the saturated six membered chelate ring,
Cd(1)-N(1)-C(2)-C(3)-C(4)-N(5) are q = 0.579(2) Å, θ = 20.7(2)° φ = 160.6(6)° indicating an
intermediate conformation between chair and half-chair [32].
There are significant intermolecular hydrogen bonding interactions in the solid state
structure of the complex (Table 5). The hydrogen atom, H(1S), attached to the methanol oxygen
atom, O(1S), is participated in hydrogen bonding interaction with the symmetry related phenoxo
oxygen atom, O(7)b {symmetry transformation b = 1-x,y,1/2-z}, to form a supramolecular chain,
as shown in Figure 2. It also shows intermolecular C-H···π interaction with C···centroid distance
being 3.643(2) Å. The hydrogen atom, H(4B), attached to C(4) is involved in C-H···π interaction
with the symmetry related (1-x,y,1/2-z) phenyl ring, C(7)-C(8)-C(9)-C(10)-C(11)-C(12),

producing a supramolecular chain as illustrated in Figure 3. There is no other significant
supramolecular interaction present in the complex.
3.3. IR, electronic and fluorescence spectra
In the IR spectrum of the complex, a band corresponding to the azomethine (C=N)
stretching vibration appears at 1611 cm-1 [29-31]. Sharp band corresponding to thiocyanate
stretching vibration appears at 2082 cm-1 [33,34]. Broad bands in the range of 3001-2850 cm-1
are assigned as alkyl C-H bond stretching vibrations [35,36]. IR spectrum of the complex
exhibits a sharp peak at 3280 cm-1, which may attributed to the N-H stretching vibration [37,38].
Broad band due to O-H stretching vibration is customarily noticed around 3444 cm-1 in IR
spectrum of the complex [39-41].
Electronic spectrum of the complex exhibits three bands of which the band at 261 nm
may be attributed to ligand based π-π* transition. On the other hand, bands at 337 and 428 nm
may be assigned as n-π* transitions [34,42,43]. The complex in DMSO, on exciting at 337 nm,
exhibits luminescence at 395 nm. This may be assigned as intra-ligand (π-π*) fluorescence [35].
The lifetime of the excited state is 18.20 ns at room temperature (Table 6) and relative
fluorescence quantum yield [42,43] for the complex is found to be 0.1082.
3.4. Photocatalytic activity
The distinct performance on the photocatalytic degradation of methylene blue (MB), an
organic dye, was discovered in the complex in an aqueous solution under visible light irradiation.
As shown in Figure 5, the residual concentration of the dye in an aqueous solution gradually
decreased as a function of the increasing reaction time, indicating that the complex can degrade
the organic dye. With increase in complex concentration, the rate of degradation was also

increased (Figure 5). Again, as shown in Figure 6, it is shown that the intensity of the
characteristic absorption peak of MB decreased with the increase of irradiation time in the
degradation process.
The complex is highly efficient for the degradation of MB and the ratio of the
degradation reaches about 64% using only 1 mg of the complex (Figure 5, curve III). Use of 2
and 3 mg of the complex, the ratio of the degradation reaches about 77 and 81%, respectively
(Figure 5, curves IV,V). The ratio of the degradation reaches about 85% using 4 mg of the
complex (Figure 5, curve VI). The reaction depends on 1st Order Kinetics and the details data on
average rate constants for this degradation are given in Table 7. To prove the high photocatalytic
efficiency of the complex to MB, comparative experiments without catalyst under visible light
irradiation were performed, which showed only very little decomposition. We had also studied
the photocatalytic activity of cadmium(II) perchlorate hexahydrate to MB under similar
conditions (Figure 5, curve II), which showed that the rate of degradation was still very slow
(only 13% using 4 mg of bare cadmium(II).
3.5. Powder X-ray diffraction
The experimental powder XRD patterns of the bulk product is in fine agreement with the
simulated XRD patterns obtained from single crystal X-ray diffraction data (cif), confirming the
purity of the bulk sample (Figure 7).
3.6. Hirshfeld surface analysis
The Hirshfeld surfaces of complex, mapped over dnorm, shape index and curvedness, is
illustrated in Figure 8. The intermolecular interactions appear as distinct spikes in the 2D

fingerprint plot (Figure 9). Details of Hirshfeld surfaces analysis are given in Electronic
Supplementary Information (ESI).
4. Conclusion
In conclusion, a dinuclear thiocyanate bridged cadmium(II) Schiff base complex has been
synthesized and characterized by elemental, spectral and single crystal X-ray diffraction analysis.
The complex exhibits efficient photocatalytic ability in the decomposition of methylene blue
under visible light. This is the first example of any X-ray characterized cadmium(II) Schiff base
complex with photocatalytic application. These results provide an opening into a promising new
field of efficient photocatalytic degradation of organic pollutant. We are now working hard to get
better yield of the reaction and to synthesize other photocatalytic cadmium(II) complexes.
Acknowledgement
This work was supported by the DST, India, under FAST Track Scheme (Order
No.SR/FT/CS-118/2010, dated 15/02/2012).
Appendix A. Supplementary data
Crystallographic data for the analysis have been deposited with the Cambridge
Crystallographic data Centre, CCDC No 1062962. The data can be obtained free of charge from
CCDC via www.ccdc.cam.ac.uk/data_request/cif.

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Table 1: Crystal data and refinement details of the complex
Formula
C26H34Cd2Cl4N6O4S2
Formula Weight
Temperature (K)
Crystal system
Space group
a(Å)
925.31
100
Monoclinic
C2/c
26.161(1)
8.9412(3)
18.969(1)
(90)
b(Å)
c(Å)
α(°)
129.202(1)
β(°)
(90)
γ(°)
Z
4
dcalc (g cm-3)
µ (mm-1)
1.788
1.711
F(000)
1840
Total Reflections
Unique Reflections
Observed data [I >2 σ (I)]
No. of parameters
R(int)
17214
3956
3539
209
0.033
R1, wR2 (all data)
R1, wR2 [I > 2 σ (I)]
0.0251,0.0441
0.01950.0425

Table 2: Selected bond lengths (Å) of complex.
Cd(1)–O(1S)
2.345(1)
2.222(2)
2.337(3)
2.325(2)
2.266(2)
2.786(1)
Cd(1)–O(7)
Cd(1)–N(1)
Cd(1)–N(5)
Cd(1)–N(1')
Cd(1)–S(3')a
Symmetry transformation a =1-x,1-y,1-z.
Table 3: Selected bond angles (°) of complex.
O(1S)–Cd(1)–O(7)
O(1S)–Cd(1)–N(1)
97.78(6)
88.30(7)
89.59(5)
166.82(5)
78.83(6)
89.58(7)
87.48(5)
173.39(6)
94.11(7)
87.95(4)
97.85(8)
85.42(4)
171.92(8)
95.44(4)
88.28(4)
O(1S)–Cd(1)–N(5)
O(7)–Cd(1)–N(1)
O(7)–Cd(1)–N(5)
N(1)–Cd(1)–N(5)
O(1S)–Cd(1)–N(1')
S(3')a–Cd(1)–O(1S)
O(7)–Cd(1)–N(1')
S(3')a–Cd(1)–O(7)
N(1)–Cd(1)–N(1')
S(3')a–Cd(1)–N(1)
N(1')–Cd(1)–N(5)
S(3')a–Cd(1)–N(1')
S(3')a–Cd(1)–N(5)
Symmetry transformation a =1-x,1-y,1-z.

Table 4: S-Cd-N Angle(°), Cd-S and Cd-N distances of bis-EE(NCS)2 bridging moieties of
some selected cadmium(II) Schiff base complexes
Complex
CSD Ref
code
S-Cd-N
Cd-S (Å)
Cd-N
Centro-
Ref.
Angle(°)
95.44(5)
symmetric
-
2.7857(6) 2.266(2) Yes
2.7873(9) 2.228(2) Yes
This work
[Cd2(L)2(µ-1,3-SCN)2(CH3OH)2]
[Cd2(L1)2(µ-1,3-SCN)2(CH3OH)2]
[Cd(NCS)(µ-1,3-SCN)(L2)]2
[Cd2(L3)2(µ-1,3-SCN)2(CH3OH)2]
[Cd2(L4)2(NCS)2(µ-1,3-NCS)2]
[Cd2(L4)2(NCS)2(µ-1,3-NCS)2]
[Cd2(L5)2(NCS)2(µ-1,3-NCS)2]
[Cd2(L6)2(NCS)2(µ-1,3-NCS)2]
[Cd2(L7)(µ-1,3-NCS)2(NCS)2]n
UNAMIH 91.04(7)
FIBKUX 92.4(1)
SASXAM 91.84(6)
IJOLOJ 90.77(5)
IJOLOJ01 91.14(5)
OPUJUF 91.1(2)
21
22
23
24
25
26
27
28
2.616(8)
2.315(8) Yes
2.7527(8) 2.244(2) Yes
2.6679(6) 2.292(2) Yes
2.616(8)
2.849(2)
2.828(1)
2.616(1)
2.315(8) Yes
2.204(6) Yes
2.272(3) Yes
2.387(5) Yes
CESTOK 85.89(9)
XIWMOG 93.8(1)
HL = 2-(3-(methylamino)propyliminomethyl)-4,6-dichlorophenol
HL1 = 2-(2-(dimethylamino)ethyliminomethyl)-4-chlorophenol
L2 = N,N-diethyl-Nʹ-(1-pyridin-2-yl-ethylidene)-ethane-1,2-diamine
HL3 = 4-nitro-2-(2-(dimethylamino)ethyliminomethyl)phenol
L4 = (2-morpholino-N-(1-(2-pyridin-2-yl)ethylidene)ethanamine
L5 = Nʹ-(pyridine-2-yl)benzylidine)ethane-1,2-diamine
L6 = N,N-dimethyl- Nʹ-(1-pyridylmethylidene)-propane-1,3-diamine
L7 = N-(1-pyridin-2-ylformylidene)-N-[2-(4-{2-[(1-pyridin-2ylformylidene)
amino]ethyl}piperazin-1-yl)ethyl]-amine

Table 5: Hydrogen bond distances (Å) and angles (º) of complex.
D–H···A
D–H
0.77
H···A
D···A
∠D–H···A
O(1S)–H(1S)···O(7)b
1.84
2.600(2)
175(4)
D, donor; H, hydrogen; A, acceptor. Symmetry transformation b = 1-x,y,1/2-z.
Table 6: The details data of the photoluminescence and time-resolved photoluminescence decay
of complex.
λex(nm)
λem(nm)
A1(%)
40.25
τ1(ns)
A2(%)
59.75
τ2(ns)
τav(ns)
χ2
337
394
2.95
19.74
18.20
1.1416
Table 7: The details data of the rate constants for the degradation of organic dye.
Amount of complex (mg) taken in 10
Average rate constant
mg L-1 aqueous solution of MB
1
7.01 x 10-4
8.87 x 10-4
8.90 x 10-4
9.29 x 10-4
2
3
4

Figure 1: Perspective view of complex with selective atom-numbering scheme. Hydrogen atoms
are omitted for clarity.

Figure 2: Supramolecular chain in complex, generated through hydrogen bonding interactions.
Only relevant atoms are shown. Symmetry transformation b = 1-x,y,1/2-z.

Figure 3: Supramolecular chain in complex, created throughC-H···π interactions. Only relevant
atoms are shown.
Figure 4: Lifetime decay profile of complex.

Figure 5: Changes in concentration of MB as a function of reaction time in the presence of
different amount of complex (curves III-VI), without complex (curve I) and with Cadmium-salt
(curve II) under visible light irradiation.
Figure 6: UV-VIS absorption spectra of MB solution with the complex (sample taken = 4 mg),
used as a catalyst.

Figure 7: Experimental and simulated powder XRD patterns of complex
Figure 8: Hirshfeld surfaces mapped over dnorm (a), shape index (b) and curvedness (c) of the
complex.

Figure 9: Fingerprint plots of the complex: full (a) and resolved into O···H/H···O (b) and
Cl···H/H···Cl (c) contacts showing the percentages of contacts contributed to the total Hirshfeld
surface area of the complex.

Graphical Abstract (synopsis)
Photocatalytic activity of a dinuclear thiocyanate bridged
cadmium(II) Schiff base complex for the degradation of organic dye
in visible light
Sumit Roy, Klaus Harms, Shouvik Chattopadhyay
One new photo-luminescent dinuclear cadmium(II) complex with salicylaldimine Schiff
base ligand and thiocyanate as co-ligand has been synthesized and characterized. The structure
has been confirmed by single crystal X-ray analysis. Under visible light irradiation, the complex
can acts as a highly efficient photocatalyst for the decomposition of organic dye.