Photocatalytic ability of two hetero-tetranuclear complexes with CuO2Cd cores to degrade methylene blue: Influence of their structures on activity

Article abstract of DOI:10.1016/j.poly.2019.05.043

Two centrosymmetric hetero-tetranuclear copper(II)/cadmium(II) complexes, (μ_1,1-N₃)₂[(H₂O)Cu(L^R)Cd(N₃)]₂?2CH₃OH (1) and (μ_1,1-NCS)₂[CuL^R

Full text of DOI:10.1016/j.poly.2019.05.043

Accepted Manuscript
Photocatalytic ability of two hetero-tetranuclear complexes with CuO2Cd cores
to degrade methylene blue: Influence of their structures on activity
Saikat Mirdya, Tanmoy Basak, Shouvik Chattopadhyay
PII:
S0277-5387(19)30381-X
https://doi.org/10.1016/j.poly.2019.05.043
POLY 13968
DOI:
Reference:
Polyhedron
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Revised Date:
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4 February 2019
20 May 2019
21 May 2019
Please cite this article as: S. Mirdya, T. Basak, S. Chattopadhyay, Photocatalytic ability of two hetero-tetranuclear
complexes with CuO2Cd cores to degrade methylene blue: Influence of their structures on activity, Polyhedron
(2019), doi: https://doi.org/10.1016/j.poly.2019.05.043
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Photocatalytic ability of two hetero-tetranuclear complexes with
CuO2Cd cores to degrade methylene blue: Influence of their
structures on activity
Saikat Mirdya, Tanmoy Basak, Shouvik Chattopadhyay*
Department of Chemistry, Inorganic Section, Jadavpur University, Kolkata - 700032, India.
E-mail: shouvik.chem@gmail.com
Abstract: Two centrosymmetric hetero-tetranuclear copper(II)/cadmium(II) complexes, (µ1,1-
N3)2[(H2O)Cu(LR)Cd(N3)]2∙2CH3OH (1) and (µ1,1-NCS)2[CuLRCd(SCN)]2∙2CH3OH (2), have
been prepared with a compartmental reduced Schiff base ligand, H2LR=2,2'-[(2,2-dimethyl-1,3-
propanediyl)bis(iminomethylene)]bis[6-methoxy-phenol]. Both complexes have been
characterized elemental and spectral analyses. Structures of both complexes have been confirmed
by single crystal X-ray diffraction technique. Both complexes contain CuO2Cd cores. Here the
bridging mode of azide and thiocyanate is quite different. Complex 1 contains µ-1,1-azide bridges
between two cadmium(II) centres whereas in complex 2, µ-1,1-thiocyanate bridges occur between
copper(II) and cadmium(II) centres. The ability of both complexes to be used as photo-catalyst in
degrading methylene blue (MB) has been explored. The difference in photocatalytic performance
may be correlated with their structures. Complex 1 shows better catalytic activity compared to
complex 2, and can degrade almost 62% MB in 18 minutes. Comparative IR studies (before and
after the photocatalytic degradation process) confirmed the stability of both catalysts.
Keywords: Compartmental reduced Schiff base; Hetero-tetranuclear complexes; Copper(II);
Cadmium(II); Photocatalytic degradation.

1. Introduction
Different groups of coordination chemists were synthesizing varieties of homo and hetero
polynuclear complexes of transition and non-transition metals with salen type di-Schiff base
ligands (prepared by the condensation of several diamines with salicylaldehyde) for long [1-5].
In this method, the ability of phenoxo oxygen atoms of salicylaldimine Schiff bases was utilized
to bridge different metal centres [6-11]. Our group was also engaged in the synthesis and
characterization of such complexes [12-14]. Several ‘half salen’ type mono-condensed Schiff
bases have also been used to prepare such complexes [15-18]. Ability of many of these
complexes in mimicking several enzymes, e.g. phosphatase, phenoxazinone synthase, catechol
oxidase etc was investigated [19-25]. Opto-electronic properties of few such complexes were
explored [26, 27]. Few complexes were found effective to degrade methylene blue under visible
light irradiation [28-30]. The denticity of these ‘salen’ type Schiff bases may be increased by
using 3-alkoxysalicylaldehyde instead of salicylaldehyde [31-33]. In this method, ‘salen’ type
hexadentate N2O4 donor compartmental Schiff bases were produced affording a facile synthetic
route to multimetallic complexes with greater ease. N,N′-bis(3-methoxysalicylidene)-2,2-
dimethylpropane-1,3-diamine is a well known compartmental Schiff base, containing inner N2O2
and outer O4 compartments. We have prepared many heterometallic complexes with this ligand
[34, 35]. Many other groups were also used this ligand to prepare hetero-metallic complexes [36-
38]. Usually, transition metals are placed in the inner N2O2 compartment as their radii matches
well with the radius of this compartment. On the other hand, non-transition metals of larger size
usually fit well in the outer O4 compartment [39-43].
In this work, we have reduced the Schiff base ligand with borohydride to get a more
flexible reduced Schiff base ligand which was then used to prepare tetranuclear heterometallic

copper(II)/cadmium(II) complexes, (µ1,1-N3)2[(H2O)Cu(LR)Cd(N3)]2∙2CH3OH (1) and (µ1,1-
NCS)2[CuLRCd(SCN)]2∙2CH3OH, H2LR=2,2'-[(2,2-dimethyl-1,3-
where
propanediyl)bis(iminomethylene)]bis[6-methoxy-phenol]. Both complexes were characterized
with single crystal X-ray diffraction analysis. Both complexes could be used as photo-catalyst to
degrade methylene blue under visible light irradiation, although complex 1 was found to be
better catalyst than complex 2. Methylene blue is an organic dye pollutant released by textile
industries and it is necessary to destroy methylene blue from industrial waste to minimize
environmental pollution [44-46]. A probable mechanistic pathway for the photocatalytic
degradation of the organic dye by these complexes has been proposed. The difference in their
photocatalytic performance has also been correlated with their structures.
2. Experimental
All starting materials and solvents were commercially available, reagent grade, and used
as purchased from Sigma-Aldrich without further purification.
Caution!!! Although no problem was encountered in this work, azide complexes are
potentially explosive. Only a small amount of the materials should be prepared and they must be
handled with care.
2.1. Preparation
2.1.1. Preparation of the ligand, 2,2'-[(2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)]bis[6-
methoxy-Phenol] (H2LR)
A hexadentate Schiff base ligand, H2L [N,N'-Bis(3-hydroxysalicylidene)-2,2-dimethyl-
1,3-propanediamine], was synthesized by refluxing of 2,2-dimethyl-1,3-propanediamine (0.2

mL, 2 mmol) with 3-methoxysalicylaldehyde (~610 mg, 4 mmol) in methanol solution (10 mL)
for ca. 2 h. The Schiff base was not purified but used directly for the preparation of the reduced
Schiff base ligand, H2LR. The methanol solution (10 mL) was then cooled to 0°C and solid
sodium borohydride (8 mmol, 310 mg) was added to it slowly with constant stirring. Then the
resulting solution was acidified with glacial acetic acid (2 mL) and stirred for 10 minutes. The
solution was evaporated to dryness under reduced pressure in a rotary evaporator (~60°C). The
residue was dissolved in water (15 mL) and extracted with dichloromethane (15 mL). The
organic phase was dried over anhydrous sodium acetate and the solvent i.e. dichloromethane was
evaporated under reduced pressure using a rotary evaporator to give the reduced Schiff base
ligand, H2LR.
2.1.2. Preparation of the complex (µ-1,1-N3)2[(H2O)Cu(LR)Cd(N3)]2∙2CH3OH (1)
A methanol solution (5 mL) of copper(II) acetate monohydrate (~400 mg, 2 mmol) was
added to the methanol solution (10 mL) of the reduced Schiff base ligand H2LR, with constant
stirring. A methanol (10 mL) solution of cadmium(II) acetate dihydrate (~540 mg, 2 mmol) was
then added to it and the stirring was continued for about 2h. A methanol–water (2:1) solution (10
mL) of sodium azide (260 mg, 4 mmol) was added and the resulting mixture was refluxed for ca.
2h. Crystalline product was obtained after few days on very slow evaporation. Single crystals,
suitable for X-ray diffraction, were isolated from the crystalline product.
Yield: 914 mg (~67%) based on copper(II). Anal. Calc. for C44H68Cd2Cu2N16O12 (FW:
1365.06): C, 39.88; H, 4.46; N, 17.72. Found: C, 39.7; H, 4.3; N, 17.9 %, FT-IR (KBr, cm-1):
3412 (νO−H), 3246-3166 (νN−H), 2972-2835 (νC−H), 2060 (νµ-1,1-N3), 2035 (νN3). UV–Vis, λmax
(nm), [εmax (L mol−1 cm−1)] (DMF): 242 (1.5 × 104), 280 (7.9 × 103), 336 (8.9 × 102), 414 (1.0 ×
103), 591 (2.2 × 102).

2.1.3. Preparation of the complex (µ-1,1-NCS)2[CuLRCd(SCN)]2∙2CH3OH (2)
Complex 2 was prepared in a similar method to that of complex 1, except that sodium
thiocyanate (~330 mg, 4 mmol) was added instead of sodium azide. Crystalline product was
obtained after few days on very slow evaporation. Single crystals, suitable for X-ray diffraction,
were isolated from the crystalline product.
Yield: 905 mg (~65%) based on copper(II). Anal. Calc. for C48H64Cd2Cu2N8O10S4 (FW:
1393.23): C, 41.57; H, 4.25; N, 8.43. Found: C, 41.3; H, 4.1; N, 8.6%, FT-IR (KBr, cm-1): 3220-
3145 (νN−H), 2972-2835 (νC−H), 2081 (νµ-1,1-NCS), 2017 (νSCN). UV–Vis, λmax (nm), [εmax (L mol−1
cm−1)] (DMF): 240 (1.8 × 104), 280 (1.0 × 104), 336 (1.1 × 103), 410 (1.3 × 103), 595 (2.7 × 102).
2.2. Physical measurement
Elemental analysis (carbon, hydrogen and nitrogen) was performed using a Perkin-Elmer
240C elemental analyzer. IR spectra in KBr (4500-500 cm-1) were recorded with a Perkin-Elmer
Spectrum Two spectrophotometer. Electronic spectra were recorded on a JASCO J-630
spectrophotometer.
2.3. X-ray crystallography
Suitable crystals of both complexes were used for data collection using a ‘Bruker D8
QUEST area detector’ diffractometer equipped with graphite-monochromated Mo Kα radiation
( = 0.71073 Å). The molecular structures were solved by direct method and refined by full-
matrix least squares on F2 using the SHELXL-18/1 package [47]. Non-hydrogen atoms were
refined with anisotropic thermal parameters. The hydrogen atoms attached to nitrogen and
oxygen atoms were located by difference Fourier maps and were kept at fixed positions. All

other hydrogen atoms were placed in their geometrically idealized positions and constrained to
ride on their parent atoms. Multi-scan empirical absorption corrections were applied to the data
using the program SADABS [48]. The details of crystallographic data and refinements have been
given in Table 1. Important bond lengths and angles have been listed in Tables 2 and 3,
respectively.
2.4. Photocatalytic measurement
The photocatalytic experiment in aqueous solution has been carried out in usual process
[49]. The catalytic degradation has been carried out separately with 100 mL of Methylene Blue
(MB) solution (20 mg L-1) with the complex using as catalyst. The mixture has been stirred for
10 min in a dark environment to get a balance between adsorption and desorption. The solution
has been then stirred constantly. A 3 mL sample has been taken from the reaction system in an
interval of 3 min and the supernatant liquid obtained by centrifugation has been used for
collecting the UV–Vis spectrum. The characteristic peak for methylene blue (~600 nm) has been
employed to monitor the photocatalytic degradation.
3. Results and discussion
3.1. Synthesis
2,2-dimethyl-1,3-diaminopropane was refluxed with 3-methoxysalicylaldehyde in a 1:2
ratio to form a N2O4 donor compartmental Schiff base ligand, H2L, following a literature method
and NaBH4 was used as a reducing agent to prepare reduced Schiff base ligand (H2LR) [50, 51].
This reduced Schiff base (H2LR) on reaction with copper(II) acetate monohydrate followed by the
addition of cadmium(II) acetate dihydrate and sodium azide in methanol gave rise to complex 1.
The complex 2 was synthesized in similar method to that of complex 1, except that sodium

thiocyanate is used instead of sodium azide. Synthetic route to the formation of both complexes 1
and 2 have been shown in Scheme 1.
Scheme 1: Synthetic route to complexes 1 and 2
3.2. Description of structures
3.2.1. (µ1,1-N3)2[(H2O)Cu(LR)Cd(N3)]2∙2CH3OH (1)
Structure determination reveals that complex 1 crystallizes in the monoclinic space group
P21/c. The complex is a centrosymmetric zigzag tetramer and is build up of two dinuclear units,
[(H2O)Cu(LR)Cd(N3)], connected through two end-on azide bridges. A perspective view of the
complex with selective atom numbering scheme has been shown in Fig. 1. In the asymmetric

unit, the copper(II) centre, Cu(1), and cadmium(II) centre, Cd(1), respectively occupy the inner
N2O2 and outer O4 sites of a potential compartmental reduced Schiff base H2LR, with a
Cu(1)∙∙∙Cd(1) distance of 3.378(6) Å.
Copper(II) centre is penta-coordinated being bonded to two amine nitrogen atoms, N(1)
and N(2), and two phenoxo oxygen atoms, O(1) and O(2), of the deprotonated reduced Schiff
base ligand, (LR)2- and one oxygen atom, O(5), from a coordinated water molecule. The
geometry of any penta-coordinated metal centre may conveniently be measured by the Addison
parameter (τ) [52]. The geometry of copper(II) centre is slightly distorted square pyramidal with
τ = 0.06 (τ = 0 indicates perfect square pyramid). On the other hand, cadmium(II) centre, Cd(1),
is hepta-coordinated and its geometry is distorted pentagonal bipyramidal. Cd(1) is coordinated
by two phenoxo oxygen atoms, O(1) and O(2), two methoxy oxygen atoms, O(3) and O(4), and
one nitrogen atom, N(3), of an end-on bridging azide, which constitute the pentagonal plane. The
sixth site is occupied by a nitrogen atom, N(6), from a terminal azide. The seventh coordination
site is occupied by another nitrogen atom, N(3), from an end-on bridging azide, which bridges
Cd(1) and its symmetry {′= 1-x,1-y,1-z} related counterpart, Cd(1′). The saturated six-membered
chelate ring [Cu(1)–N(1)–C(9)–C(10)–C(13)–N(2)] has envelope conformation with puckering
parameters, q = 0.523(6) Å; θ = 23.0(5)°; ϕ = 177.6(14)° [53, 54].

Fig. 1. Perspective view of complex 1 with selective atom numbering. Hydrogen atoms and the
lattice methanol molecule have been omitted for clarity.
3.2.2. (µ1,1-NCS)2[CuLRCd(SCN)]2∙2CH3OH (2)
Complex 2 crystallizes in triclinic space group, P . It is basically a centrosymmetric
1
cyclic tetramer with [−Cd−(µ1,1-NCS)CuO2Cd(µ1,1-NCS)−Cu−] core, in which two hetrero-
dinuclear [CuLRCd(SCN)] units are linked through two end-on thiocyanate bridges. A
perspective view with selective atom numbering scheme has been shown in Fig. 2. Cu(1)∙∙∙Cd(1′)
{symmetry transformation ′= 1-x,1-y,1-z} distance in the tetrameric unit is 3.617(9) Å.
Copper(II) centre, Cu(1), is square pyramidal being coordinated equatorially by two
amine nitrogen atoms, N(1) and N(2), and two phenoxo oxygen atoms, O(1) and O(2), of the
deprotonated reduced Schiff base, (LR)2-. The fifth position is occupied by a nitrogen atom, N(3′),
of the bridging thiocyanate. The square pyramidal geometry of copper(II) centre is confirmed by
τ, here τ = 0.01 [52]. On the other hand, the coordination geometry of cadmium(II) centre, Cd(1),

is distorted octahedral, where two phenoxo oxygen atoms, O(1) and O(2), and two methoxy
oxygen atoms, O(3) and O(4), of the reduced Schiff base constitute the equatorial plane. One
axial site is occupied by a sulfur atom, S(1), from terminal thiocyanate and another axial site is
occupied by nitrogen atom, N(3), from thiocyanate which is act as a bridge between the
cadmium(II) centre, Cd(1) and the symmetry related copper(II) centre, Cu(1′). The saturated six-
membered chelate ring [Cu(1)–N(1)–C(9)–C(10)–C(13)–N(2)] has envelope conformation with
the puckering parameters, q = 0.524(6) Å; θ = 148.2(5)°; ϕ = 2.6(11)° [53, 54].
Fig. 2. Perspective view of complex 2 with selective atom numbering. Hydrogen atoms and the
lattice methanol molecule have been omitted for clarity.
3.3. Comparison of structures of 1 and 2
It is interesting to note here that in complex 1, cadmium(II) centre of one asymmetric unit
is connected with a cadmium(II) centre of a symmetry related counterpart via end-on azide
bridges. This makes the copper(II) centres free to be used in any catalytic reaction. On the other

hand, cadmium(II) centre of one asymmetric unit in complex 2 is connected with a copper(II)
centre of a symmetry related dinuclear unit via end-on thiocyanate bridges. Thus both copper(II)
and cadmium(II) are confined in a cyclic tetrameric unit, making copper(II) less available to take
part any catalytic reaction.
3.4. Supramolecular interactions
Solid state structures of both complexes are stabilized through significant non-covalent
interactions such as hydrogen bonding and C−H∙∙∙π. Complex 1 shows five significant hydrogen
bonding interactions. The hydrogen atom, H(1), attached to amine nitrogen atom, N(1), involve
in hydrogen bonding interaction with the symmetry related (x,1.5-y,-0.5+z) oxygen atom, O(6j),
of the lattice methanol molecule. Another hydrogen atom, H(5C), attached to oxygen atom, O(5),
of coordinated water molecule participate in hydrogen bonding interaction with the symmetry
related (1-x,1/2+y,3/2-z) azide nitrogen atom N(8i). These two intermolecular hydrogen bonding
interactions form a two dimensional layer structure. On the other hand, the oxygen atom, O(6),
of the lattice methanol molecule forms hydrogen bond with the hydrogen atom, H(5D), attached
with the oxygen atom, O(5), of the coordinated water molecule. Again, hydrogen atom, H(6), of
the same lattice methanol molecule, forms an additional hydrogen bond with azide nitrogen
atom, N(6). Another hydrogen atom, H(2), attached to amine nitrogen atom N(2), participates in
hydrogen bond formation with the azide nitrogen atom, N(5). Two dimensional layer structure of
the complex has been shown in Fig. 3 and selected small part of the two dimensional structure of
hydrogen bonding has been shown in Fig. 4. Details of the hydrogen bonding interactions have
been given in Table 4.

The hydrogen atom, H(22A), attached to the carbon atom, C(22), of the lattice methanol
molecule, is involved in a C−H∙∙∙π interaction with the symmetry related (x,1.5-y,0.5+z) phenyl
ring [C(2)−C(3)−C(4)−C(5)−C(6)−C(7)]. Two dimensional layer structure of C−H∙∙∙π interaction
has been shown in Fig. 5 and selected small part of C−H∙∙∙π interaction with specific atom
numbering has been shown in Fig. 6. Details of the geometric feature of the C−H∙∙∙π interaction
have been given in Table 5.
Fig. 3. Hydrogen bonded layer structure of complex 1. Only the relevant hydrogen atoms have
been shown.

Fig. 4. Selected small part of hydrogen bonded structure of complex 1. Only the relevant
hydrogen atoms have been shown. Symmetry transformation i= x,1.5-y,0.5+z, j= 1-x,0.5+y,1.5-z
Fig. 5. Two dimensional supramolecular C–H∙∙∙π interactions in complex 1. Only the relevant
hydrogen atoms have been shown for clarity. Symmetry transformation: #= x,1.5-y,0.5+z.

Fig. 6. Selected small part of C–H∙∙∙π interactions of complex 1. Only the relevant hydrogen
atoms have been shown. Symmetry transformation #= x,1.5-y,0.5+z.
Three types of supramolecular interactions have been observed in complex 2 (H-bonding,
C−H∙∙∙π and π∙∙∙π interactions). The hydrogen atom, H(2N), attached to amine nitrogen atom,
N(2), of the reduced Schiff base is engaged in intramolecular hydrogen bonding interaction with
the same symmetry related oxygen atom, O(5), of lattice methanol molecule, as shown in Fig. 7.
Details of the hydrogen bonding interaction have been gathered in Table 4.
The hydrogen atom, H(24B), attached to the carbon atom, C(24), of lattice methanol
molecule
is
involved
in
the
C−H∙∙∙π
interaction
with
a
phenyl
ring
[C(15)−C(16)−C(17)−C(18)−C(19)−C(20)]. The C−H∙∙∙π interaction of the complex has been
shown in Fig. 8. Detail of the C−H∙∙∙π interaction has been given in Table 5. Complex 2 shows a
significant π∙∙∙π stacking interaction (Fig. 9) between an aromatic ring [C(2)–C(3)–C(4)–C(5)–
C(6)–C(7)] with a symmetry-related (1−x,1−y,1−z) aromatic ring [C(15)–C(16)–C(17)–C(18)–
C(19)–C(20)]. Details of the π∙∙∙π interaction have been given in Table 6.

Fig. 7. Hydrogen bonding structure of complex 2 with lattice methanol molecule. Only the
relevant hydrogen atoms have been shown.
Fig. 8. C−H∙∙∙π interaction of complex 2 with lattice methanol molecule. Only the relevant
hydrogen atoms have been shown.

Fig. 9. Intramolecular π∙∙∙π stacking interactions in complex 2. Hydrogen atoms and lattice
methanol molecules have been omitted for clarity.
3.5. Hirshfeld surfaces analysis
The Hirshfeld surfaces of both complexes mapped over dnorm (range -0.1 Å to 1.5 Å),
shape index and curvedness have been shown in Fig. 10. Red spots on these surfaces denote the
dominant interactions. Analysis of Hirshfeld surfaces indicates that there are interactions mainly
between nitrogen and hydrogen atoms in complex 1, whereas, there are significant interactions
between sulphur and hydrogen atoms in complex 2. Other visible spots in Hirshfeld surfaces
correspond to C···H and H···H contacts. The intermolecular interactions appear as distinct
spikes in the 2D fingerprint plot (Fig. S1).

Fig. 10. Hirshfeld surfaces of both complexes 1 and 2 mapped over dnorm (left), shape index
(middle), curvedness (right).
3.6. IR and electronic spectra
In the IR spectrum of complex 1, a sharp peak at 2060 cm-1 and an associated small peak
at 2036 cm-1 indicate the presence of bridging and terminal azides [55-57]. In IR spectrum of
complex 2, there are two successive strong peaks observed at 2081 and 2017 cm−1, indicating the
presence of terminal (S-bonded) and end-on bridged thiocyanates (N-bonded), respectively [58].
A moderately strong band due to stretching vibration of N−H bond appears around 3213-3166
cm-1 for both complexes [59]. A broad peak around 3412 cm-1 indicates the presence of
coordinating water molecule in complex 1 [60]. Bands near the range of 2972–2835 cm−1 are due
to alkyl C–H bond stretching vibrations, which are customarily noticed in the IR spectra of both
complexes [61]. The IR spectra of both complexes have been shown in Fig. S2 (Electronic
Supplementary Information, ESI).

Electronic spectrum of each complex in DMF displays one absorption band in the visible
2
2
region around 590 nm which may be considered as T2g(D) ← Eg(D) transition for copper(II)
[35,60]. Bands around 410 nm (407 nm for complex 1 and 414 nm for complex 2) may be
attributed to LMCT transition from the N donor centres of Schiff base to copper(II) [62, 63].
Another bands around 335 nm (for both 1 and 2) may be attributed to LMCT transition from the
O donor centres of Schiff base to cadmium(II) [64]. The high intensity band occur at 242 (for 1)
and 240 (for 2) nm is attributed to π→π* transition of the ligand. Another intense band observe
in the region of 275-285 is correspond to n→π* transition for both complexes [62, 65]. The UV-
vis spectra of both complexes have been shown in Fig. 11 and Fig. S3 (ESI), respectively.
Fig. 11. UV-vis spectra of complex 1. Inset shows the visible range spectrum.

3.7. Photocatalytic activity
The ability of both complexes (1 and 2) to be used as photo-catalyst for the degradation
of organic dye, viz. methylene blue (MB), has been investigated under visible light irradiation.
The degradation efficiencies are defined as C/C0, where C and C0 represent the residual and
initial concentration of organic dyes, respectively. The degradation efficiencies (C/C0) is
calculated by the following equation:-
Degradation efficiencies (%)= (C0−C)/C0 × 100.
As shown in Fig. 12, the residual concentration of the dye in an aqueous solution
gradually decreased as a function of the increasing reaction time, indicating that both complexes
degrade organic dyes. Both complexes are efficient for the degradation of MB and the ratio of
the degradation reaches around 62% (for complex 1) and 30% (for complex 2) using 10 mg of
complexes.
To prove the photocatalytic efficiency of both complexes to MB, comparative
experiments without catalyst under visible light irradiation have been performed, which showed
only very little decomposition. The photocatalytic activity of cadmium(II) acetate and copper(II)
acetate have also been studied to MB under similar conditions (mentioned in the Fig. 13), which
showed that the rate of degradation was still very slow using 10 mg of bare cadmium(II) as well
as bare copper(II). It is revealed that the intensity of the characteristic absorption peak of MB
decreased with the increase of irradiation time in the degradation process. Kinetic plots of both
complexes have been shown in Fig. 13.

Fig. 12: UV–Vis absorption spectra of MB solution with both complexes (sample taken = 10
mg), used as a catalyst.
Fig.13. Degradation efficiency of MB in presence of complex 1 and complex 2. Experimental
condition [MB]= 20 mg/L.
In order to verify whether both complexes are capable of maintaining its structural
integrity during photocatalytic decomposition of MB, IR experiments for both complexes were
performed during the course of photocatalytic reactions. The IR experiments indicated that the

patterns are nearly identical when compared with the pure complex. Hence the IR experiments
imply that both complexes maintain their structural integrity even after getting involved in the
photocatalytic reactions. IR spectra of both complexes 1 and 2 (before and after degradation)
have been shown in Fig. 14 and S4, respectively.
Fig.14: IR spectra of complex 1 before (blue) and after (red) photodegradation process.
The first step of the process is excitation of electrons from the valence band to the
conduction band which creates equal amounts of positives vacancies or holes in valence band. It
is well-established that holes act as powerful oxidants and electrons as powerful reductants. The
second step is adsorption of oxygen or water on the surfaces of catalyst. Electrons in the
conduction band is scavenged by oxygen molecule to produce O2•− anion radical, which provides
peroxide radical (•OOH) and then decomposed to active hydroxyl radical (•OH). On the other
hand, the holes in the valence band interact with H2O or −OH to produce highly active hydroxyl

radical (•OH). Finally, the effective degradation of methylene blue is occurred by these generated
hydroxyl radicals (•OH).
3.8. Structure-activity relationship
The photocatalytic degradation study has been carried out with visible light irradiation.
So the degradation process must be initiated with visible light absorption. Absorption spectra of
both complexes contain peaks around 590 nm in the visible region, originating from copper(II)-
based 2T2g (D) ← 2Eg (D) transition. Thus copper(II) centres may be considered as the initiator of
the degradation process. As already mentioned, copper(II) centre in complex 1 is relatively more
free to take part in any reaction for its open zigzag structure. High efficiency of complex 1 to
degrade the dye (MB) is therefore justified from its structure. On the other hand, copper(II) is
rigidly imprisoned in a cyclic tetranuclear skeleton in complex 2 and is less free to take part in
reaction. This explains the less catalytic efficiency of complex 2 compared to complex 1.
A comparative study has also been done to compare the degradation efficiency of these
complexes with all reported complexes, which has been characterized structurally. The
comparative study has been shown in Table 7. This indicates our complexes are good enough to
be explored in industrial chemistry, although better complexes are also known in literature.
However, it should be noted that many of the previously reported catalysts have been prepared
using Schiff bases as ligands and therefore their application in aqueous medium is limited (Schiff
bases are susceptible to hydrolysis). Our complexes are containing reduced Schiff bases as
ligands and may, therefore, be used in aqueous medium without any problem related to
destruction of the ligands via hydrolysis.

4. Concluding remarks
Synthesis and X-ray characterization of two new hetero-tetranuclear copper(II)–
cadmium(II) complexes with same reduced Schiff base ligand have been described in this paper.
In both complexes 1 and 2, copper(II) and cadmium(II) resides on N2O2 and O4 donor sites,
respectively. The bridging mode in complex 2 is quite different than that of complex 1. In
complex 1, two cadmium(II) centres are bridged by end-on bridging azide whereas in complex 2,
the copper(II) and cadmium(II) is bridged by end-on bridging thiocyanate instead of two
cadmium(II) centres. Both complexes show significant supramolecular interactions in their solid
state structures. Photocatalytic degradation study of both complexes gives a reliable result as it is
the first report of the degradation study for heterometallic complexes with reduced Schiff base
ligands. Both complexes have catalytic ability for the degradation of organic pollutant, MB,
under visible light irradiation. Complex 1 shows a good catalytic degradation property than
complex 2. The structural variation upon the degradation process has also been rationalized.
Finally, efficient catalytic ability of both complexes for the photo-degradation of organic
pollutant under visible light irradiation indicates the potential application of the complex in
industrial chemistry.
Acknowledgements
S.M. thanks the CSIR, India, for awarding a Junior Research Fellowship. T.B. thanks the
CSIR, India, for awarding a Senior Research Fellowship [Sanction No. 09/096(0861)/2016-
EMR-I]. S.C. gratefully acknowledges UGC-CAS II program, Department of Chemistry,
Jadavpur University, for financial support under the head [Chemicals/Consumables/Glassware].
Appendix A. Electronic Supplementary Information

UV-VIS spectrum of complex 2 and Hirshfeld surface analysis and IR spectrum of both
complexes 1 and 2 are given in the electronic supplementary information part. CCDC
1891988−1891989 contains the supplementary crystallographic data for both complexes 1 and 2,
respectively.
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