The ability of a trinuclear zinc(II) Schiff base complex to act as a photocatalyst for the degradation of methylene blue and to mimic phosphatase

Article abstract of DOI:10.1016/j.poly.2018.09.070

A new trinuclear zinc(II) complex, [Zn(μ_1,1-N₃)₂{Zn(L)(N₃)}₂]·0.5CH₃OH {HL = [2-((3-(dimethylamino)propylimino)methyl)-6-methoxyphenol]} has been synthesized and characterized. X-ray crysta

Full text of DOI:10.1016/j.poly.2018.09.070

Accepted Manuscript
The ability of a trinuclear zinc(II) Schiff base complex to act as a photocatalyst
for the degradation of methylene blue and to mimic phosphatase
Tanmoy Basak, Anik Bhattacharyya, Klaus Harms, Shouvik Chattopadhyay
PII:
S0277-5387(18)30629-6
https://doi.org/10.1016/j.poly.2018.09.070
POLY 13469
DOI:
Reference:
Polyhedron
To appear in:
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Revised Date:
Accepted Date:
4 September 2018
22 September 2018
24 September 2018
Please cite this article as: T. Basak, A. Bhattacharyya, K. Harms, S. Chattopadhyay, The ability of a trinuclear
zinc(II) Schiff base complex to act as a photocatalyst for the degradation of methylene blue and to mimic
phosphatase, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.09.070
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The ability of a trinuclear zinc(II) Schiff base complex to act as a
photocatalyst for the degradation of methylene blue and to mimic
phosphatase
Tanmoy Basaka, Anik Bhattacharyyaa, Klaus Harmsb , Shouvik Chattopadhyay*,a
aDepartment of Chemistry, Inorganic Section, Jadavpur University, Kolkata- 700032, India. Tel:
+9133-2457-2147; E-mail: shouvik.chem@gmail.com
bFachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Strasse, D-35032
Marburg, Germany
Abstract: A new trinuclear zinc(II) complex, [Zn(µ1,1-N3)2{Zn(L)(N3)}2]∙0.5CH3OH {HL= [2-
((3-(dimethylamino)propylimino)methyl)-6-methoxyphenol]} has been synthesized and
characterized. X-ray crystallographic analysis has confirmed its structure. The central zinc(II) is
hexa-coordinated and both of the terminal zinc(II) centers are penta-coordinated. The complex
has shown excellent phosphatase mimicking activity {kcat ~190 h-1 at 298 K}, evaluated by
monitoring the increase in absorbance at ~425 nm (spectrophotometrically) indicating gradual
release of p-nitrophenolate by the hydrolysis of 4-nitrophenylphosphate ester with time, when 4-
NPP (disodium 4-nitrophenylphosphatehexahydrate) has been used as substrate. The complex
has also been found to be efficient photocatalyst for the degradation of methylene blue (MB).
Keywords: Zinc(II); Schiff base; Crystal structure; Phosphatase mimicking activity;
Photocatalytic degradation.
1. Introduction
Zinc is placed in the fourth period and twelfth group in the ultra-modern periodic Table.
Its valence shell electronic configuration is 3d104s2. The only stable (accessible) oxidation state
of zinc is +2, where also the d-sub-shell is completely filled. It does not have any CFSE in any

crystal field and therefore, it may have flexible coordination environment with various
geometries, viz. tetrahedral [1], square planar [2], trigonal bipyramidal [3], square pyramidal [4]
and octahedral [5] etc. On the other hand, zinc(II) complexes have been reported to show
photoluminescence as well as electroluminescence [6]. As zinc(II) does not have any redox
property, the complexes of zinc(II) are very good candidates for mimicking hydrolytic cleavage
activities of various metalloenzymes [7-9]. Phosphatase, which catalyzes the hydrolysis of a
phosphomonoester, removing a phosphate moiety from the substrate, is one such enzyme, which
could effectively be mimicked by various zinc(II) containing complexes [10]. The catalytic
cleavage of phosphoesters is an essential reaction for living organisms, for example in processing
RNA replication and bone metabolism, and it is of ecological importance in the detoxification of
pesticides [11]. Synthetic zinc(II) complexes have been widely used for the cleavage of the P-OR
bond in phosphates (R= alkyl group). Excellent Lewis acidity, no preference for any geometry
and coordination number, capability of undergoing rapid ligand exchange (i.e. high degree of
lability), and also not having any undesired redox activity favour zinc(II) to become a good
choice for showing such activity [12]. Disodium 4-nitrophenylphosphate hexahydrate (4-NPP)
has been used as the substrate in most of the in vitro studies on phosphatase mimicking activity.
Salen type salicylaldimine Schiff bases (prepared by the condensation of several
diamines with salicylaldehyde derivatives) have been widely used to prepare various phenoxo-
bridged polynuclear complexes of zinc(II) [13-16]. N2O donor ‘half-salen’ type Schiff base
(prepared by using N-substituted diamines) have also be used by synthetic inorganic chemists to
prepare various phenoxo-bridged polynuclear complexes of zinc(II). In the present work, a
trinuclear zinc(II) Schiff base complex has been prepared using sodium azide as co-ligand. The
complex has been characterized by single crystal X-ray diffraction analysis. Phosphatase

mimicking activity of the complex has been investigated spectrophotometrically using disodium
4-nitrophenylphosphate hexahydrate (4-NPP) as substrate in aqueous DMF at 298 K.
The complex has also been found effective to degrade methylene blue under visible light
irradiation. During production and usage of various textile and painting materials, many
contaminations enter to the environment [17]. Extensive research efforts have been devoted
for the last few decades to the development of heterogeneous photocatalytic technology for
eliminating those pollutants [18-23]. ZnO has a suitable band gap for photocatalytic degradation
of dyes. Literature shows many reports on methylene blue degradation using ZnO nano-particle
[24, 25]. Many zinc(II) complexes were also reported to be used in methylene blue degradation
[26, 27]. In our previous work, the ability of cadmium(II) complexes to be used as photo-
catalysts for the degradation of organic dyes has been established [28, 29]. Although the
complexes were effective to degrade methylene blue, the inherent toxicity of cadmium(II) was
a real barrier for those complexes to be used in actual practice. The application of the present
zinc(II) complex may therefore be considered as a good environment friendly alternative for the
photocatalytic degradation of methylene blue under visible light irradiation.
2. Experimental Section
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 of the complex [Zn(µ1,1-N3)2{Zn(L)(N3)}2]∙0.5CH3OH [HL = [2-((3-
(dimethylamino)propylimino)methyl)-6-methoxyphenol]

The tetradentate Schiff base ligand, HL, was prepared by refluxing N,N-dimethylpropane-
1,3-diamine (0.10 mL, 1 mmol) with 3-methoxysalicylaldehyde (152 mg, 1 mmol) in methanol
solution (10 mL) for ca. 1 h. It was not isolated but used directly for the preparation of the
complex.
A methanol solution (10 mL) of zinc(II) nitrate hexahydrate (447 mg, 1.5 mmol) was
added to the methanol solution of the ligand, HL, and the resulting solution was refluxed for ca.
30 min. A methanol–water (2:1) solution (10 mL) of sodium azide (130 mg, 2 mmol) was added
and the refluxing was continued for an additional 1 h. Single crystals, suitable for X-ray
diffraction, were obtained after few days on slow evaporation of the solution in open atmosphere.
Yield: 204 mg (~72%) based on zinc(II). Colour: Yellow; Anal. Calc. for
C26H38N16O4Zn3,0.5CH3OH (FW 850.85): C, 37.41; H, 4.59; N, 26.84; Found: C, 37.3; H, 4.5;
N, 26.9 %. IR (KBr, cm-1): 1640 (νC=N), 2086, 2059 (νN3). UV–Vis, max (nm), [εmax (L mol-1 cm-
1)] (DMF+H2O): 277 (9.21×103), 375 (3.75×103). ESI-MS m/z = 368.5721 for
[Zn(L)(H2O)(CH3OH)]+.
2.2. Physical measurement
Elemental analyses (carbon, hydrogen and nitrogen) were performed using a PerkinElmer 240C
elemental analyzer. IR spectra in KBr (4500-500 cm-1) were recorded with a PerkinElmer
Spectrum Two spectrophotometer. Electronic spectra in acetonitrile were recorded on a
Shimadzu UV-1700 UV-Vis spectrophotometer. Electrospray ionization mass spectra were
recorded using Waters QTOF Micro YA263.
2.3. X-ray crystallography
A suitable crystal of complex was used for data collection using a ‘Bruker D8 QUEST
area detector’ diffractometer equipped with Mo Kα radiation, a graded multilayer mirror

monochromator, and a PHOTON-100 CMOS detector at 100 K. Absorption effects were
corrected semi-empirical using multiscanned reflexions using SADABS-2016/2 [30]. The
programs used were APEX3 [31], SAINT V8.35A [32], XT V2014/1 [33], SHELXL-2018/3 [34]
and DIAMOND. Non-hydrogen atoms were refined with anisotropic thermal parameters.
Hydrogen atoms attached to oxygen atoms were located by difference Fourier maps and were
kept at fixed positions. Other hydrogen atoms were placed in their geometrically idealised
positions and constrained to ride on the atoms to which they were attached. Disorder has been
refined using constraints for both the geometry and thermal parameters. Split postions with ratio
0.667 to 0.333 have been refined for the atoms of the azide group bonded to Zn3
(N101,N102,N103)/(N201,N202,N203). The solvent MeOH molecule has been refined with two
positions of disorder with occupancy of 0.25. Due too inversion symmetry the MeOH molecule
shows a fourfold disorder in the unit cell. Only one (obviously with higher occupancy) of
disordered positions is shown in figures. Crystallographic data and refinement details of these
two complexes are given in Table 1. Important bond lengths and angles are listed in Tables 2 and
3, respectively.
2.4. Kinetic measurements of the hydrolysis of 4-NPP
Disodium 4-nitrophenylphosphate hexahydrate (4-NPP) has been used as substrate in the kinetic
study of phosphate ester bond cleavage. Aqueous DMF (97.5% DMF, v/v) has been used as
solvent to prepare solutions of substrate and catalyst (zinc(II) complex). Five sets of solutions,
having the catalyst concentration 0.05 mmol and that of substrate as 1, 1.25, 1.50, 1.75 and 2
mmol, have been prepared for the study of catalytic activity. The catalyst has been injected into
the solution of 4-NPP to start the reaction. The hydrolysis of 4-NPP at 25°C has been followed
by following the increase in absorption intensity at 425 nm in presence of catalyst due to released

4-nitrophenolate and the rate of hydrolysis has been measured by initial rate method. The
increase in absorption has been recorded for a total period of 60 min at an interval of 3 min. The
absorbance at infinity (A∞) for each set has been obtained after 12 h at 25°C.
2.5. Photocatalytic measurement
The photocatalytic experiment in aqueous solution has been carried out in usual process [35].
The catalytic degradation has been carried out separately with 100 mL of Methylene Blue (MB)
solution (10 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 4 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
In the present work, the Schiff base (HL) ligand has been synthesized by the
condensation of N,N-dimethyl-1,3-diaminopropane with 3-methoxysalicylaldehyde, following
the literature method [36]. The ligand has not been purified and used directly for the preparation
of the trinuclear zinc(II) complex. The complex has been prepared by the addition of methanol
solution of zinc(II) nitrate hexahydrate to the methanol solution of HL followed by the addition
of sodium azide. The synthetic route to the complex has been shown in Scheme 1.

Scheme 1: Synthetic route to the complex.
3.2. Description of structure
3.2.1. [Zn(µ1,1-N3)2{Zn(L)(N3)}2]∙0.5CH3OH
Single crystal X-ray diffraction analysis reveals that the complex crystallizes in triclinic
space group, P . Zn(1) and Zn(2) Zinc(II) centers are linked by one phenoxo oxygen atom of
1 1
L− and one nitrogen atom of an end-on azide forming an µ1,1-azido µ-phenoxo bridged Zn2 motif.
A third zinc(II) center is again linked with this Zn2 motif via similar bridging segment and
generates the trinuclear Zn(II)−Zn(II)−Zn(II) core. The three zinc(II) centers,
Zn(1)−Zn(2)−Zn(3), are not in linear arrangement, which is evident from the angle between three
zinc(II) centers {147.88°(1)}. The distances between the zinc(II) centers are 3.2383(4) Å {Zn(1)-
Zn(2)} and 3.2251(4) Å {Zn(2)-Zn(3)}. A perspective view of the complex has been shown in
Figure 1.
Each of the two terminal zinc(II) centers, {Zn(1)/Zn(3)}, is penta-coordinated, being
bonded to an amine nitrogen {N(1)/N(16)}, an imine nitrogen {N(5)/N(20)}, one phenoxo
oxygen {O(8)/O(23)}, of the tridentate deprotonated Schiff base (L−), one nitrogen
{N(109)/N(100)} from a terminal azide and another nitrogen {N(106)/N(103)}, from an end-on
bridging azide. The geometry of any penta-coordinated metal center may conveniently be
measured by the Addison parameter (τ) [37] [τ = (ϴ−Ф)/ 60, where ϴ and Ф are the two largest

Ligand–Metal–Ligand angles of the coordination sphere]. In the present complex, the geometry
around each terminal zinc(II) center is distorted square pyramidal with τ = 0.34 for Zn(1) and
Zn(3). Central zinc(II) center, Zn(2), has a six-coordinate pseudo-octahedral geometry being
bonded with two phenoxo oxygen atoms, O(8) and O(23), and two methoxy oxygen atoms,
O(24) and O(9), from two deprotonated Schiff base ligands and two nitrogen atoms, N(103) and
N(106), from two end-on bridging azides.
The saturated six membered chelate rings, Zn(1)−N(1)−C(2)−C(3)−C(4)−N(4) and
Zn(3)–N(16)−C(17)−C(18)−C(19)−N(20), assume half-chair conformations with puckering
parameters q = 0.6864(2) Å, θ= 9.42(2)º, φ= 338.3(1)º and q = 0.7063(2) Å, θ= 11.07(15)º, φ=
346.3(9)º respectively [38, 39].
The complex shows significant C−H∙∙∙π interactions. The hydrogen atom, H(26) attached
to the phenyl carbon atom, C(26), is involved in intermolecular C−H∙∙∙π interaction with
symmetry related (-x,-y,1-z) phenyl ring, C(7)−C(8)−C(9)−C(10)−C(11)−C(12). Two other
hydrogen atoms, H(14A) and H(29C), attached to methyl carbon atoms, C(14) and C(29)
respectively, are also involved in other intermolecular C−H∙∙∙π interactions with the symmetry
related phenyl rings, C(7)−C(8)−C(9)−C(10)−C(11)−C(12) {Symmetry transformation= -x,-y,-
z} and C(22)−C(23)−C(24)−C(25)−C(26)−C(27) {Symmetry transformation= 1-x,1-y,1-z}
respectively. These three different types of C−H∙∙∙π interactions result a 2D sheet like structure in
crystal packing (Figure 2). Details of the geometric features of C−H∙∙∙π interactions of both
complexes have been given in Table 4.

Figure 1. Perspective view of the complex with selective atom numbering scheme. Hydrogen
atoms and lattice methanol have been omitted for clarity.
Figure 2. Two dimensional supramolecular architecture of the complex via C–H∙∙∙π interactions.
Only relevant atoms have been shown.
3.3. IR and electronic spectra
In IR spectrum of the complex, a distinct band due to stretching vibrations of azomethine (C=N)
groups appear around 1640 cm-1 [40]. A sharp peak around 2086 cm-1 indicates the presence of
bridging azides. A small peak, associated with the sharp peak, is found at 2059 cm-1 corresponds
to the presence of the terminal azides [41]. The bands in the range of 2842–2942 cm−1 due to

alkyl C–H bond stretching vibrations are customarily noticed in the IR spectrum [42]. The IR
spectrum of the complex (Figure S1) has been shown in SI (Supplementary Information).
Electronic spectrum of the complex has been recorded in DMF-water medium. There are
only two bands in the UV spectrum. Band at 277 nm may be assigned as π−π* transition [43].
Band at 375 nm has been attributed to the charge transfer from n−π* centers [44]. There is no
band corresponding to d–d electronic transitions, as expected for zinc(II) complexes with d10
electronic configuration.
It is necessary to mention here that the band with absorption maximum at 375 nm spreads
over the region ~325-450 nm. In other words, the tail of the peak at 375 nm falls in the visible
region. The complex therefore absorbs visible light. This is responsible for the yellow colour of
the complex. The electronic spectrum of the complex has been shown in Figure 3.
Figure 3: UV-Vis spectrum of the complex
3.4. Powder X-ray diffraction
The experimental powder XRD patterns of bulk product of the complex is in good agreement
with simulated XRD patterns from single crystal X-ray diffraction, confirming purity of the bulk
sample. The simulated pattern was calculated from the single crystal structural data (cif file)

using the CCDC MERCURY software. The experimental and simulated powder XRD patterns of
the complex is shown in Figure S2(SI).
3.5. Kinetic investigation through several enzyme kinetic plots
Michaelis-Menten kinetics has been used to explain the data. In this model, following equation
(Eq. (1)) is used to relate the rate of enzymatic reactions, V to the concentration of substrate, S.
푉푚푎푥 [푆]
Eq. (1)
푉 =
퐾푀 + [푆]
Here, Vmax represents the maximum rate achieved by the system, at saturating substrate
concentration. KM is the Michaelis constant and it is equal to the substrate concentration at which
the reaction rate is half of Vmax
.
Linearization of Michaelis-Menten equation produces a double reciprocal Lineweaver-Burk plot.
The Lineweaver-Burk Equation may be represented by Eq. (2).
퐾푀
1
1
1
Eq. (2)
=
+
푉
푉푚푎푥[푆] 푉푚푎푥
On the other hand, rearrangement of Michaelis-Menten equation gives Hanes equation (Eq. (3)).
In this case, the ratio of the substrate concentration (S) to the reaction rate (V) is plotted against
substrate concentration (S).
퐾푀
[푆]
푉
[푆]
Eq. (3)
=
+
푉푚푎푥 푉푚푎푥
Eadie-Hofstee equation (Eq. (4)) is sometimes used for a graphical representation of enzyme
kinetics. Here reaction rate (V) is plotted as a function of the ratio between reaction rate (V) and
substrate concentration (S):
푉
푀[푆]
Eq. (4)
푉 = 푉푚푎푥 ‒ 퐾

All these equations are used to evaluate several kinetic parameters, including turnover number
(kcat) and specificity constant (kcat /KM) for phosphatase mimicking activity of both complexes.
In enzyme kinetics, turnover number (also termed as kcat) is defined as the maximum number of
catalytic conversions of substrate molecules per unit time that a single catalytic site will execute
for a specific enzyme concentration. The specificity constant (also termed as kcat /KM ratio) is a
measure of how efficiently a catalyst converts substrates into products. This ratio is a useful
index for measuring the substrate specificity of catalyst.
An efficient catalytic activity of phosphoester bond cleavage is showing by the complex. The
time dependent spectral profile of the complex is shown in Figure 4. Under aerobic conditions
catalytic conversation of 4-nitrophenylphosphate to 4-nitrophenolate is confirmed by the gradual
increase in peak intensity at ∼425 nm (characteristic of 4-nitrophenolate absorption). A blank
experiment is also being done without catalysts under identical conditions do not show
significant growth of the band at ~425 nm. As can be observed for each complex, initially the
cleavage rate increases linearly with the increase of 4-NPP concentration but deviates gradually
from linearity and finally tends toward a saturation curve. Figure 5 represents the Michaelis-
Menten plot, Lineweaver-Burk plot, Hanes-Woolf plot and Eadie-Hofstee plot of the complex.
The kinetic parameters for the catalyzed reactions (Vmax, Km and kcat) have been determined and
summarised in Table 5.

Figure 4. Wavelength scan for the hydrolysis of 4-NPP in absence and presence of the complex
(substrate : catalyst =20:1) in 97.5% DMF recorded at 298 K at an interval of 3 mins for 1 h.
Figure 5. (a) Michaelis-Menten plot, (b) Lineweaver-Burk plot, (c) Hanes-Woolf plot and (d)
Eadie-Hofstee plot for the complex.

To elucidate active species present in the catalytic pathway during phosphoester hydrolysis, the
electron spray ionization mass spectrometry of the complexes with 4-NPP has been performed in
the positive mode. The peak at m/z=151.1455 for [(4-nitrophenolate)(Li)2]+ in the mass spectrum
of the complex with 4-NPP confirms the completion of phosphatase activity. Another peak at
m/z=575.5997 arises due to complex-substrate adduct [Zn(L1)(4-NPP)(H2O)(H)(K)]+ (theoretical
m/z=575.0424) and the peak at m/z=137.1244 has confirmed the presence of [H3PO4(Li)(H2O)]+,
which is the end product of the complete cyclic process. The full scale mass spectrum for 1:50
mixture of the complex and 4-NPP has been shown in Figure 6. The probable mechanistic
pathway of catalysis is shown in Scheme 2. Selected small range (m/z= 340−1000) of these mass
spectra have been given in Figure S3.
Figure 6. ESI-MS positive spectrum for the mixture of the complex and 4-NPP. (Full Scale).

Scheme 2. Plausible mechanism for the hydrolysis of 4-NPP catalyzed by zinc(II) complex.
3.6. Photocatalytic activity
To reveal the catalytic oxidation properties of zinc(II), the catalytic degradation of dyes are
chosen as a model reaction. In this experiment, methylene blue (MB) is chosen as typical organic
pollutant target. The degradation efficiencies are defined as C/C0, where C and C0 represent the
remnant and initial concentration of organic dyes, respectively. As shown in Figure 6, the
residual concentration of the dye in an aqueous solution gradually decreased as a function of the
increasing reaction time, indicating that the complex degrade organic dyes. The complex is
highly efficient for the degradation of MB and the ratio of the degradation reaches around 90%
using only 5 mg of the complex.
To prove the high photocatalytic efficiency of the complex to MB, comparative experiments
without catalyst under visible light irradiation are have been performed, which showed only very
little decomposition. The photocatalytic activity of zinc (II) nitrate hexahydrate have also been
studied to MB under similar conditions (mentioned in the Figure. 7), which showed that the rate
of degradation was still very slow only 10% using 5 mg of bare zinc(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. The degradation profile is shown in Figure S4 (SI).
Figure 7. Degradation efficiency of MB in presence of zinc(II). Experimental conditions:[MB]:
10 mg/L; Amount of Catalyst: 5mg.
A probable mechanistic pathway for the photocatalytic degradation of organic dye is shown in
Scheme 3 [45]. The first step of the process is excitation of electron (e−) from the valence band
(VB) to the conduction band (CB) which creates equal amounts of positives vacancies (holes, h)
in VB. Here it is known that the holes act as powerful oxidants and electrons as powerful
reductants. The second step is the adsorption of oxygen (O2) or the hydroxyl (OH−) adsorbed on
the surfaces of complex could interact with the electrons (e−) on the CB or the hole (h) on the
VB, respectively, which would probably lead to the formation of the hydroxyl radicals (ȮH).
The final step is the effective degradation of methylene blue (MB) by the ȮH radicals.
In order to verify whether the complex is capable of maintaining its structural integrity during
photocatalytic decomposition of MB, PXRD experiments for the complex were performed
during the course of photocatalytic reactions (Figure S3). The PXRD experiments indicated that
the patterns are nearly identical when compared with the pure complex. Hence the PXRD

experiments imply that the complex maintains its structural integrity even after getting involved
in the photocatalytic reactions (Figure 8). It is interesting to mention here that the complex also
exhibits reasonable chemical stability as established by its unaltered PXRD results thereby
suggesting that the crystallinity of the complex is retained even after the exposure to aqueous
solutions (Figure 8).
Scheme 3. Probable mechanistic pathway for the degradation of organic dye (MB).
Figure 8. The powder XRD patterns of complex before and after the photocatalytic degradation.

It is very interesting to add here that both the absorption maxima of the complex fall in the UV
range but the complex is still acting as a photo-catalyst under visible light irradiation. This
apparently seems to be paradoxical. However, a close inspection of its absorption spectrum
indicates that the band with λmax at 375 nm has a long tail in the visible region (Figure S2). This
is responsible for its absorption in the visible region. As was already mentioned, this tail of the
band at 375 nm is the origin of the yellow colour of the complex. The complex absorbs visible
light (400-450 nm, as explained above) to excite electrons from VB to CB. Consequently, holes
are supposed to be produced on zinc(II) and thereby the zinc(II) centre is oxidized to some
extent. It then oxidizes methylene blue and itself is reduced back to zinc(II) state. A comparative
study has also been done to compare the degradation efficiency of the complex with recently
reported zinc(II) and cadmium(II) complexes (Table 6).
4. Concluding remarks
The whole work may be concluded in two statements. Firstly, the synthesis, characterization and
photocatalytic
activity
of
a
new
trinuclear
zinc(II)
complex,
[Zn(µ1,1-
N3)2{Zn(L)(N3)}2]∙0.5CH3OH, provides an efficient method of synthesizing polynuclear zinc(II)
complexes with efficient catalytic ability for the photo-degradation of organic pollutant under
visible light irradiation indicating potential application of the complex both in environmental and
industrial chemistry. Secondly, the complex has the ability for biomimetic hydrolytic cleavage of
P−O bonds and therefore may be used as a phosphatise mimic.
Appendix A. Supplementary data
IR, UV-VIS spectrum, selected small range ESI-MS spectrum and degradation profile of
methylene blue of the complex are given in the supporting information part. CCDC 1848198
contains the supplementary crystallographic data for the complex.

Acknowledgements
T.B. thanks the CSIR, India, for awarding a Junior Research Fellowship [Sanction No.
09/096(0861)/2016-EMR-I]. S.C. gratefully acknowledges UGC-CAS II programme,
Department of Chemistry, Jadavpur University, for financial support under the head
[Chemicals/Consumables/Glassware].
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Table: 1 Crystal data and refinement details of the complex.
Formula
C26H38N16O4Zn3,0.5CH3OH
Formula Weight
850.85
100
Temperature (K)
Crystal system
Triclinic
P
1 1
Space group
a(Å)
10.4169(5)
11.6055(5)
16.2790(7)
96.784(2)
103.558(2)
106.475(2)
2
b(Å)
c(Å)
α
β
γ
Z
dcalc (g cm-3)
μ (mm-1)
F(000)
1.571
2.043
874

Table: 2. Selected bond
lengths (Å) of the complex.
Total Reflections
43440
8266
Unique Reflections
Table: 3. Selected bond
angles (°) of the complex.
Observed data[I >2 σ (I)]
7250
No. of parameters
R(int)
514
0.030
R1, wR2 (all data)
0.0304, 0.0645
R1, wR2 [I > 2 σ (I)]
0.0245, 0.0619
2.0368(14)
2.0976(17)
2.0732(17)
2.1719(16)
1.9966(17)
2.0049(13)
2.3005(14)
2.0608(18)
2.0005(12)
2.2816(13)
2.0537(16)
2.0384(12)
2.0927(17)
2.0919(16)
2.003(13)
Zn(1)–O(8)
Zn(1)–N(1)
Zn(1)–N(5)
Zn(1)- N(106)
Zn(1)–N(109)
Zn(2)–O(8)
Zn(2)–O(9)
Zn(2)–N(106)
Zn(2)–O(23)
Zn(2)–O(24)
Zn(2)–N(103)
Zn(3)–O(23)
Zn(3)–N(16)
Zn(3)–N(20)
Zn(3)–N(100)
Zn(3)–N(103)
2.1816(16)

O(8)–Zn(1)–N(1)
115.47(6)
86.49(6)
75.18(6)
140.83(6)
90.83(7)
101.48(7)
103.52(7)
160.99(7)
96.92(7)
94.25(7)
73.51(5)
78.38(6)
151.87(6)
155.59(5)
88.05(5)
116.98(6)
88.63(5)
84.63(5)
91.13(6)
73.54(5)
79.03(6)
117.80(6)
152.31(6)
93.46(6)
102.66(7)
111.37(6)
85.84(5)
140.6(4)
160.70(6)
Table 4: Geometric features (distances in
Å and angles in °) of the C-H···π
O(8)–Zn(1)–N(5)
O(8)–Zn(1)–N(106)
O(8)–Zn(1)–N(109)
N(1)–Zn(1)–N(5)
interactions for the complex.
N(1)–Zn(1)–N(106)
N(1)–Zn(1)–N(109)
N(5)–Zn(1)–N(106)
N(5)–Zn(1)–N(109)
N(106)–Zn(1)–N(109)
O(8)–Zn(2)–O(9)
O(8)–Zn(2)–N(106)
O(9)–Zn(2)–N(106)
O(8)–Zn(2)–O(23)
O(8)–Zn(2)–O(24)
O(8)–Zn(2)–N(103)
O(9)–Zn(2)–O(23)
O(9)–Zn(2)–O(24)
O(9)–Zn(2)–N(103)
O(23)–Zn(2)–O(24)
O(23)–Zn(2)–N(103)
O(23)–Zn(2)–N(106)
O(24)–Zn(2)–N(103)
O(24)–Zn(2)–N(106)
N(103)–Zn(2)–N(106)
O(23)–Zn(3)–N(16)
O(23)–Zn(3)–N(20)
O(23)–Zn(3)–N(100)
N(20)-Zn(3)-N(103)

Symmetry
C-H···Cg(Ring)
H···Cg (Å) C-H···Cg (°)
C···Cg (Å)
3.568(2)
3.713(2)
3.629(2)
C(14)-H(14A)···Cg(9)a
C(26)-H(26)···Cg(9)b
C(29)-H(29c)···Cg(10)c
2.63
2.80
2.67
161
162
165
transformati
a
ons: = -x,-
y,1-z; b= -
x,-y,-z; c= 1-
x,1-y,1-z;For the complex:
Cg(9) = Center of gravity of the ring [C(7)-C(8)-C(9)-C(10)-C(11)-C(12)] and Cg(10) = Center
of gravity of the ring [C(22)-C(23)-C(24)-C(25)-C(26)-C(27)].
Table 5. Kinetic parameters of both complexes for phosphatase mimicking activity using various
enzyme kinetic plots at 25 °C in DMF-water medium.
Enzyme kinetic plots
Vmax (M S-1)
KM (M)
kcat (h-1)
kcat/KM (h-1 M-1)
Michaelis-Menten Plot 1.5894 x 10-4
Lineweaver-Burk plot 1.5894 x 10-4
1.9321 x 10-3
1.9321 x 10-3
1.9366 x 10-3
1.9300 x 10-3
190.728
190.728
190.900
190.704
9.8715 x 104
9.8715 x 104
9.8574 x 104
9.8810 x 104
Hanes-Woolf plot
Eadie-Hofstee plot
1.5915 x 10-4
1.5892 x 10-4
Table 6. Kinetic studies for the degradation of methylene blue by some previously reported
compounds.
Degradation time
(min)
Degradation
percentage
Compounds
Refs.
FeOOH–ZnAl-LDH
180
40
95%
95%
[23]
[25]
Y-doped ZnO (at 12 mol%)
[ZnLa]
180
120
97%
[26]
[27]
[Zn(Lb)(DCTP)]n
96.9%

[Zn (Lc) (DCTP)]n
120
40
81.4%
99%
[27]
[28]
[{CdLd(µ-1,1-N3)}2Cd(µ-1,1-
N3)2∙1.76CH3OH]n
[Zn3(Le)2(µ-O2CCH3)2]
[Mn(Lf)(AIA)]
60
80%
21%
[33]
[35]
180
[Cu4I4Cu(Lg)2]n
[Fe(Lh)]
300
75
90%
[46]
96.5%
82.8%
84.7%
[47]
[Co(Li)(tbta)]∙H2O}n
[Co(Li)(nip)]n
135
135
[48]
[48]
This
[Zn(µ1,1-N3)2{Zn(L)(N3)}2]
18
90%
Complex
La=
2,2'-Dihydroxybenzylideneaniline;
Lb=
1,3-bis(5,6-dimethylbenzimidazol-1-
Ld=
2-(3-
ylmethyl)benzene;
Lc=
1,4-bis(benzimidazol-1-ylmethyl)benzene];
Le=
(dimethylamino)propyliminomethyl)-6-ethoxyphenol;
N,N'-bis(salicyaldehydene)-1,3-
diaminopropan-2-ol; Lf= 2,5-bis(3-pyridyl)-3,4-diaza-2,4-hexadiene (3-bpdh); Lg= (1S)-1-(5-
tetrazolyl)ethylamine); Lh= 5-(2-(N-benzoyl-5-carboxyl)-1-amino)-10,15,20-triphenyl porphyrin;
Li= 1,5-bis(benzimidazolyl)pentane.
LDH= layered double hydroxide; H2DCTP = 2,5-dichloroterephthalic acid; AIA= 5-
aminoisophthalicacid; H2tbta = tetrabromoterephthalic acid; H2nip = 5-nitroisophthalicacid.
Graphical Abstract (Synopsis)
The ability of a trinuclear zinc(II) Schiff base complex to act as a
photocatalyst for the degradation of methylene blue and to mimic
phosphatase
Tanmoy Basak, Anik Bhattacharyya, Klaus Harms, Shouvik Chattopadhyay

A trinuclear zinc(II) Schiff base complex has been synthesized and characterized by
single crystal X-ray crystallography. 2D sheet like structure of C−H∙∙∙π interactions of the
complex has been explored. The complex exhibits excellent phosphatase mimicking activity. The
complex also catalyzes the degradation of methylene blue under visible light irradiation.
Graphical Abstract (Pictogram)
The ability of a trinuclear zinc(II) Schiff base complex to act as a
photocatalyst for the degradation of methylene blue and to mimic
phosphatase
Tanmoy Basak, Anik Bhattacharyya, Klaus Harms, Shouvik Chattopadhyay*