Phosphatase-mimicking activity of a unique penta-nuclear zinc(ii) complex with a reduced Schiff base ligand: assessment of its ability to sense nitroaromatics

Article abstract of DOI:10.1039/C8NJ06549G

A unique penta-nuclear zinc(ii) complex, [(μ_1,1-N₃)₂{(N₃)Zn(L)Zn}₂Zn(L)] {H₂L= 2,2′-[(1-methyl-1,2-ethanediyl)bis(iminomethylene)]bis[6-methoxyphenol]}, has been synthesized and characteriz

Full text of DOI:10.1039/C8NJ06549G

View Article Online
View Journal
NJC
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: S. Chattopadhyay,
M. Karmakar and T. Basak, New J. Chem., 2019, DOI: 10.1039/C8NJ06549G.
This is an Accepted Manuscript, which has been through the
Volume 40 Number
1 January 2016 Pages 1–846
Royal Society of Chemistry peer review process and has been
accepted for publication.
NJC
Accepted Manuscripts are published online shortly after
New Journal of Chemistry
www.rsc.org/njc
A journal for new directions in chemistry
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
rsc.li/njc

Page 1 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
7
8
9
Phosphatase mimicking activity of a unique penta-nuclear zinc(II)
complex with a reduced Schiff base ligand: Assessment of its ability
to sense nitroaromatics
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Mainak Karmakar, Tanmoy Basak, Shouvik Chattopadhyay*
Department of Chemistry, Inorganic Section, Jadavpur University, Kolkata - 700032, India.
E-mail: shouvik.chem@gmail.com
Abstract: A unique penta-nuclear zinc(II) complex, [(µ1,1-N3)2{(N3)Zn(L)Zn}2Zn(L)] {H2L=
2,2'-[(1-Methyl-1,2-ethanediyl)bis(iminomethylene)]bis[6-methoxyphenol]}
has
been
synthesized and characterized. X-ray crystallographic analysis has confirmed its structure. The
complex has shown good phosphatase mimicking activity {kcat ~398 h-1 at 298 K}, evaluated
spectrophotometrically by monitoring the increase in absorbance at ~425 nm indicating gradual
release of p-nitrophenolate by the hydrolysis of 4-nitrophenylphosphate ester with time, when 4-
NPP (disodium 4-nitrophenylphosphate hexahydrate) has been used as substrate. The complex
behaves as a sensor for the detection of nitroaromatics in DMF solution via turn-off fluorescence
response. The solution phase sensing mechanism has been studied and a strong quenching
efficiency has been found with quenching constant (KSV) 6.497 x 104 M-1 for 3-methyl-4-
nitrobenzoic acid.
Introduction
Zinc is a very interesting metal for many reasons. It shows varieties of stereo chemistries
starting from linear (coordination number 2) to distorted trigonal dodecahedral (coordination
number 8).1 It is a d-block element (as last electron goes to d orbital), but not a transition element

New Journal of Chemistry
Page 2 of 32
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
7
8
9
(as zinc has filled d-shell both in ground state and any stable oxidation state). d10 electronic
configuration of zinc(II) affords no crystal field stabilization and therefore the stereochemistry of
a particular compound depends only on the steric necessities of the ligands and the size and
polarizing power of Zinc(II) as well.2 Thus zinc(II) usually favours four coordinate tetrahedral
complexes. However, six coordinate octahedral complexes are also reported to exist, e.g.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2+ 3
Zn(en)3 . Organozinc complexes are, on the other hand, show liner geometries, which may be
considered as extreme forms of distorted octahedral structures with two short and four long
bonds.4 Coordination numbers greater than six are rare and generally involve bidentate, O-donor
2-
ligands with a small bite angle, such as in Zn(NO3)4 .
Zinc is biologically one of the most important metals and is required to all forms of life.5
Zinc containing enzymes which have received most attention are probably carbonic anhydrase
and carboxypeptidase A. Another important zinc enzyme is phosphatase, which is a hydrolytic
enzyme and uses water to cleave a phosphoric acid monoester into a phosphate ion and an
alcohol.6 Many zinc(II) complexes are shown to behave as good catalysts in the hydrolysis of (4-
nitrophenyl)phosphate ester (phosphatase mimicking activity).6a, 7
Another importance of zinc(II) complexes lies in their strong luminescence properties,
which has been exploited by various groups to sense nitroaromatic explosives.8 Nitroaromatics
are perfect examples of electron-deficient aromatics, which are expected to quench the
fluorescence of luminescent zinc(II) complexes.9 Thus by monitoring the quenching of the
fluorescence intensity of zinc(II) complexes, nitroaromatic explosives may be detected.
In the present work, a penta-nuclear zinc(II) complex with a reduced Schiff base ligand
has been prepared and characterized. Phosphatase mimicking activity of the complex has been
investigated spectrophotometrically using disodium 4-nitrophenylphosphate hexahydrate (4-

Page 3 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
7
8
9
NPP) as substrate in aqueous DMF at 298 K. The complex also acts as turn-off fluorescence
chemosensor for detection of nitroaromatic compounds. Best to our knowledge, this is the first
example of any zinc(II) complex of any reduced Schiff base ligand, which has been used to
mimic phosphtase and/or to sense any nitroaromatic explosive.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Experimental
Materials
Disodium 4-nitrophenylphosphatehexahydrate was purchased from SRL Pvt. Ltd. and
used without further purification. All other chemicals were of reagent grade and used as
purchased from Sigma-Aldrich without further purification.
Caution!!! Although no problems were encountered in this work, organic ligands in the presence
of azide are potentially explosive. Only a small amount of the material should be prepared and it
should be handled with care.
Preparation
Preparation of the ligand, 2,2'-[(1-Methyl-1,2-ethanediyl)bis(iminomethylene)]bis[6-
methoxyphenol] (H2L)
A Schiff base ligand, H2L' {H2L'= N,N'-bis(3-methoxysalicylidene)propane-1,2-
diamine}, was synthesized by refluxing of 1,2-diaminopropane (3 mmol, 0.25 mL) with 3-
methoxysalicylaldehyde (6 mmol, 0.912 g) in methanol (20 mL) solution for ca. 2 h. The
solution (20 mL) was then cooled to 0°C and solid sodium borohydride (8 mmol, 0.302 g) was
added to it slowly with constant stirring. The resulting solution was acidified with glacial acetic
acid (5 mL) and then evaporated to dryness under reduced pressure in a rotary evaporator

New Journal of Chemistry
Page 4 of 32
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
7
8
9
(~60°C). The residue was dissolved in water (15 mL) and extracted with dichloromethane (15
mL). The solvent i.e. dichloromethane was evaporated under reduced pressure using a rotary
evaporator to give the reduced Schiff base ligand, H2L.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Preparation of the complex [(µ1,1-N3)2{(N3)Zn(L)Zn}2Zn(L)]
A methanol solution of zinc(II) acetate dihydrate (5 mmol, 1.095 g) was added to the
methanol solution of the ligand H2L, with stirring. The stirring was continued for additional 2 h.
An aqueous methanol (1:1) solution of sodium azide (4 mmol, 0.260 g) was then added to it and
stirred further for ca. 2 h. It was kept in open air at room temperature for 2 days. White
crystalline product was collected by filtration. X-ray quality single crystals were collected from
this crystalline product.
Yield: 1.038 g, 68% (based on zinc). Anal. Calc. for C57H71N18O12Zn5 (1527.27): C,
44.98; H, 4.37; N, 16.56 %. Found: C, 44.7; H, 4.1; N, 16.6 %. FT-IR (KBr, cm-1): 3255 (νN−H);
2945-2835(νC−H); 2064 (νN3). λmax (nm) [εmax(lit mol-1 cm-1)] (acetonitrile): 288 (1.97x104).
Physical measurements
Elemental analysis (carbon, hydrogen and nitrogen) was performed using a Perkin-Elmer
240C elemental analyzer. IR spectrum in KBr (4500-500 cm-1) was recorded with a Perkin-
Elmer Spectrum Two spectrophotometer. Electronic spectra in DMF were recorded on a JASCO
V-630 spectrophotometer. The steady-state fluorescence emission spectrum was recorded using a
Shimadzu RF-5301PC spectrofluorometer after proper background correction with individual
solvents in quartz cells with 1 cm path length. Fluorescence lifetime measurement was recorded
using Hamamatsu MCP photomultiplier (R3809) and was analyzed by using IBHDAS6 software.
Electrospray ionization mass spectra were recorded using Waters QTOF Micro YA263. The

Page 5 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
7
8
9
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
2 range of 5–50° using 1-D Lynxeye detector at ambient conditions.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The fluorescence quantum yield (ΦS) of the complex was calculated using tryptophan as
a reference standard in water having a quantum yield (ΦR) of 0.47, following the equation
ΦS 퐴푆 (퐴푏푠)푆 η2S
=
X
X
ηR2
Φ푅 퐴푅 (퐴푏푠)푅
Where the subscripts S and R stand for sample and reference standard respectively, Φ is the
fluorescence quantum yield, A is integrated area under fluorescence curve, Abs is absorbance,
and η is the refractive index of the solvent.
X-ray Crystallography
A suitable crystal of the complex was used for data collection using a ‘Bruker D8
QUEST area detector’ diffractometer equipped with graphite-monochromated Mo Kα radiation
( = 0.71073 Å). The molecular structure was solved by direct method and refined by full-matrix
least squares on F2 using the SHELX-14 package.10 Non-hydrogen atoms were refined with
anisotropic thermal parameters. The hydrogen atoms attached to nitrogen 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.11
The carbon atom, C(29), was disordered and therefore two sets of positions (C29 and C29′) {′=
1-x,y,1.5-z} were refined with equal occupancies (0.5 each). The details of crystallographic data
and refinements have been given in Table 1.

New Journal of Chemistry
Page 6 of 32
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
Kinetic measurements of the hydrolysis of 4-NPP
5
6
7
8
9
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.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Results and discussion
Synthesis
1,2-diaminopropane and 3-methoxysalicylaldehyde have been used to synthesize the
reduced Schiff base ligand (H2L) following the reported procedure.12 The methanol solution of
H2L on reaction with zinc(II) acetate dihydrate and sodium azide in a 3:5:4 molar ratio produced
the penta-nuclear complex. The synthetic route to the complex has been shown in Scheme 1.

Page 7 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 1: Synthetic route to the complex.
Description of structure
Single crystal X-ray diffraction analysis reveals that the complex crystallizes in
the monoclinic space group, C2/c. The structure is centrosymmetric with one zinc(II) center,
Zn(3), residing on the centre of inversion. The asymmetric unit of the complex is shown in Fig.
1. The complex is actually penta-nuclear. The perspective view of the complex with the
coordinated atoms of the reduced Schiff bases and azide ions have been shown in Fig. 2. The
complex consists of three units of reduced Schiff base ligands (deprotonated), four azides {two
terminal and two end-on bridging} and five zinc(II) centers {Zn(1), Zn(2), Zn(3), Zn(1)′ and
Zn(2)′} {Symmetry transformation ′=1-x,y,1.5-z} where zinc(II) centers are in more or less bent
arrangements with Zn(1)−Zn(2)−Zn(3) angle being 132.98 (3)°. Important bond lengths and
bond in the coordination sphere of zinc centers have been listed in Tables 2 and 3, respectively.

New Journal of Chemistry
Page 8 of 32
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fig. 1: Asymmetric unit of the complex. Hydrogen atoms have been omitted for clarity. Only
selected atoms have been numbered. Symmetry transformation ′=1-x,y,1.5-z.
The deprotonated reduced Schiff base ligand (L2-) is a compartmental ligand having inner
N2O2 and outer O4 compartments. The terminal L2- unit is containing two zinc(II) centers, Zn(1)
and Zn(2), in its inner N2O2 and outer O4 compartment, respectively. The zinc(II) center, Zn(1),
situated in the inner compartment, is penta-coordinated and being bonded to two amine nitrogen
atoms, N(1) and N(2), two phenoxo oxygen atoms, O(1) and O(2), of the deprotonated reduced
Schiff base ligand (H2L) and one terminal azide nitrogen atom, N(3). The geometry of any penta-
coordinated metal center may conveniently be measured by the Addison parameter (τ)13 [τ =
(x−y)/ 60, where x and y are the two largest Ligand–Metal–Ligand angles in the coordination
sphere]. In the present complex, the geometry around the terminal zinc(II) center, Zn(1), is
distorted square pyramidal with τ = 0.12.

Page 9 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fig. 2: Penta-nuclear moiety of the complex with selective atom-numbering scheme. Only the
coordinating atoms of the reduced Schiff base have been shown for clarity. Symmetry
transformation ′=1-x,y,1.5-z.
On the other hand, another zinc(II) center, Zn(2), is hexa-coordinated, showing distorted
octahedral geometry. The outer O4 compartment of the reduced Schiff base ligand occupies three
coordination sites of the zinc(II) center, Zn(2), by three coordinating oxygen atoms, O(1), O(2)
and O(3). Other three sites are coordinated by one methoxy oxygen atom, O(6), and one phenoxo
oxygen atom, O(5), of another reduced Schiff base ligand and one nitrogen atom, N(7), of an
end-on bridged azide. Finally, the central zinc(II) center, Zn(3), which resides on the centre of
inversion, is octahedrally coordinated by a phenoxo oxygen atom, O(5) and an amine nitrogen
atom, N(6), of the reduced Schiff base ligand and an end-on bridging azide nitrogen atom, N(7)
and also by their symmetry related counter parts, O(5)′ and N(6)′ and N(7)′. Two saturated five
membered chelate rings, Zn(1)−N(1)−C(10)–C(9)–N(2), Zn(3)−N(6)−C(28)−C(28)′−N(6)′
assume half-chair conformations (Fig. 3) with puckering parameters14 q= 0.569(10) Å, ϕ=
131.4(9)° and q= 0.493(13) Å, ϕ= 270.0(10)°, respectively.

New Journal of Chemistry
Page 10 of 32
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fig. 3: Half chair conformation of two saturated five membered chelate rings.
It is to be noted here that the conformation of the reduced Schiff base moiety, attached to
the central zinc(II) centre, Zn(3), is interesting, as N2O2 donor atoms of the ligand are not in
same plane. Rather both phenoxo oxygen atoms are displaced from the planes of two nitrogen
atom one in upward and another in downward direction, thereby enabling both oxygen atoms to
coordinate two [(N3)Zn(L)Zn] units, facilitating the formation of the penta-nuclear complex, (µ-
1,1-N3)2{(N3)Zn(L)Zn}2Zn(L).
The solid state structure of the complex has been stabilized through various non-covalent
interactions, such as hydrogen bonding and C−H∙∙∙π interactions. The hydrogen atom, H(1),
attached to the amine nitrogen atom, N(1), has been participated in intermolecular hydrogen
bonding interaction with the symmetry related azide nitrogen atom, N(3)#, {symmetry
#
transformation: = 1.5-x,0.5-y,1-z}. The hydrogen bonding interaction forms a one-dimensional
array (Fig. 4) and the distance between the donor (N1) and acceptor (N3#) is 3.026(11) Å with
N(1)–H(1)···N(3)#= 157(6)°. On the other hand, the hydrogen atom, H(19A), attached with the
carbon atom, C(19), has been involved in intramolecular C−H∙∙∙π interaction with a phenyl ring,
C(21)−C(22)−C(23)−C(24)−C(25)−C(26). The measured C−H∙∙∙π interaction distance is
3.475(13) Å for C(19)−H(19A)∙∙∙Cg(14) {Cg(14) = center of gravity of the phenyl ring,

Page 11 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
C(21)−C(22)−C(23)−C(24)−C(25)−C(26)}. The C−H∙∙∙π interaction of the complex has been
shown in Fig. 5.
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fig. 4: Intermolecular hydrogen bonding interactions in the complex. Only the relevant hydrogen
atoms have been shown for clarity.
Fig. 5: Intramolecular C−H∙∙∙π interaction. Only the relevant hydrogen atoms have been shown
for clarity.

New Journal of Chemistry
Page 12 of 32
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
IR, UV-Vis and Fluorescence and mass spectra
5
6
7
8
9
A strong band at 2071 cm-1 in IR spectrum of the complex indicates the presence of
azide.15 Bands around 3255 cm-1 in the IR spectrum of the complex is observed due to amine
N−H bond stretching.16 Broad bands in the range of 2945-2835 cm-1 due to alkyl C−H stretching
vibrations are routinely noticed in IR spectrum of the complex.17 IR spectrum of the complex
(Fig. S3) has been shown in ESI (Electronic Supplementary Information). Electronic spectrum of
the complex consists of one band at 288 nm, which may be assigned as intra-ligand charge
transfer transition.18 The complex exhibits fluorescence in DMF solution upon excitation at
288 nm. The emission occurs at 330 nm. This is assigned as intra-ligand charge transfer π→π*
fluorescence.12a,19 The UV-Vis and fluorescence spectra of the complex are shown in Fig. 6.
Fluorescence lifetime of the complex was measured in DMF solution at room temperature. The
Stoke shift between absorption (288 nm) and emission (330 nm) spectrum is 4419 cm-1. Average
lifetime of the complex is about 55.47 ns (at λex = 288 nm). Details of the luminescence data and
decay profile of the complex are listed in Tables S1 and Fig. S4 (ESI), respectively.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The mass spectral analysis of the complex reveals that the complex breaks into the
mononuclear unit in DMF. The mass spectrum of the complex shows a base peak at m/z
431.1450 (calc. 431.0925) which confirms the formation of [ZnL(Na)]+. Another peak at m/z
347.2397 (calc. 347.1971) arises due to the free ligand [H2L(H)]+. Other peaks at m/z 159.0639
and 233.1577 are due to the fragments of the ligand. The mass spectrum of the complex has been
shown in Fig. 7.

Page 13 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fig. 6: (a) UV-Vis and (b) fluorescence spectra (upon excitation at 288 nm, conc. 10−5 M) of the
complex in DMF.
Fig. 7: Mass spectrum of the complex.

New Journal of Chemistry
Page 14 of 32
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
Powder X-ray diffraction
5
6
7
8
9
The experimental powder XRD patterns of bulk product of the complex is in good
agreement with simulated XRD pattern from single crystal X-ray diffraction, confirming purity
of the bulk sample (Fig. S5 (ESI)). The simulated pattern was calculated from the single crystal
structural data (cif file) using the CCDC Mercury software.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Nitro Sensing
The fluorescence study has been performed to investigate the potential application of the
complex as a sensor for nitroaromatics. The fluorescence spectrum of the complex has been
recorded in DMF solution in the presence of various organic analytes such as benzene, toluene,
benzaldehyde, phenol, aniline, p-cresol, and aromatic nitro compounds. Significant quenching of
fluorescence intensity has only been observed upon addition of nitroaromatics, such as
nitrobenzene, 1,3-dinitrobenzene, nitrotoluene, 3-nitrobenzoic acid, 3-methyl-4-nitro benzoic
acid (3M4NBA) (Fig. 8). This observation revealed that the complex could be used as a potential
sensor for selective detection of nitroaromatics. The reason for the significantly high quenching
of fluorescence intensity by nitroaromatics compared to other aromatics is most likely due to
their more electron deficient nature (Fig. 9). The maximum quenching of fluorescence intensity
has been observed upon addition of 3-methyl-4-nitrobenzoic acid in comparison to other
nitroaromatics.

Page 15 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fig. 8: Fluorescence spectra of the complex in methanol (excitation wavelength 288 nm and
conc. 10−5 M) upon the addition of various organic analytes (5.0 equiv).
Fig. 9: Relative changes in fluorescent intensity of the complex in the presence of different
organic analytes (5.0 equiv).

New Journal of Chemistry
Page 16 of 32
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
7
8
9
In order to check the quenching capability, fluorescence titration of the complex (10-5 M)
has been performed by gradual addition (10 µL) of nitroaromatic compounds (10-3 M) in DMF
solution. The decrease in the fluorescence intensity has been shown in Fig. 10. The quenching
efficiency of nitroaromatics has been quantitatively determined by the Stern-Volmer equation
(1).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
퐼0
Eq. (1)
= 1 + 퐾푠푣 [푄]
퐼
Where, KSV is the quenching constant (M-1), [Q] is the molar concentration of the
quencher, I0 and I are luminescence intensities before and after adding the quencher,
respectively. At low concentrations, the Stern-Volmer plot of 3-methyl-4-nitrobenzoic acid is
linear where as at higher concentrations, consequently deviate from linearity and turn upward
(Fig. 11), which may be due to self-absorption.20 The Stern-Volmer quenching constant (Ksv) is
calculated from the slope of the linearly fitted curve (inset Fig. 11) is 6.497 × 104 M-1 (R2=
0.9987) which is moderately higher for 3-methyl-4-nitrobenzoic acid. The limit of detection
(LOD) of 3-methyl-4-nitrobenzoic acid is 3.8 × 10−5 M (0.38 ppm) calculated for the complex.
The detection limit of nitroaromatics towards complex is calculated using the equation, LOD =
3σ/k, where σ is the standard deviation in blank measurements.21 The slope of the plot of
fluorescence intensity versus concentration of the complex gives k.

Page 17 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fig. 10: Fluorescence spectra of the complex in DMF (excitation wavelength 288 nm and conc.
10−5 M) upon increasing concentration of 3-methyl-4-nitrobenzoic acid.
Fig. 11: Stern−Volmer plot of the complex.
In order to determine the binding mechanism of nitroaromatics with the complex in
solution 1H NMR titration has been performed with varying amounts of 3-methyl-4-nitrobenzoic

New Journal of Chemistry
Page 18 of 32
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
7
8
9
acid (0 and 5 equiv) in DMSO-d6 (Fig. 12). Similar titration has also been done with 4-
nitrobenzoic acid (Fig. S6 in ESI). We observe that aromatic protons are gradually shifted to the
downfield region with increasing concentration of nitroaromatics. This observation can be
explained if it is assumed that nitroaromatics withdraw electron density from the complex and
shift protons toward the deshielded region. Except this shifting in peak positions, everything else
remains practically constant in each step of this NMR titration. This confirms that any ligands of
the complex are not substituted during the interaction of the complex with the nitroaromatics.
Mass spectrum of the complex with 3-methyl-4-nitrobenzoic acid (Fig. S7 in ESI) shows a base
peak at m/z 793.2122 (calc. 793.1675) and thereby supports the formation of a mononuclear
complex with 3-methyl-4-nitrobenzoic acid [ZnL(3M4NBA)2(Na)]+.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fig. 12: 1H NMR titration of the complex with varying amounts of 3-methyl-4-nitrobenzoic acid
in DMSO-d6.

Page 19 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
Kinetic investigation through several enzyme kinetic plots
5
6
7
8
9
Michaelis-Menten kinetics has been used to explain the data. In this model, following
equation (Eq. (2)) is used to relate the rate of enzymatic reactions, V to the concentration of
substrate, S.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
푉푚푎푥 [푆]
Eq. (2)
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. (3).
퐾푀
1
1
1
Eq. (3)
=
+
푉
푉푚푎푥[푆]
푉푚푎푥
On the other hand, rearrangement of Michaelis-Menten equation gives Hanes equation (Eq. (4)).
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. (4)
=
+
푉푚푎푥
푉푚푎푥
Eadie-Hofstee equation (Eq. (5)) 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
Eq. (5)
푉 = 푉푚푎푥 ― 퐾푀
[푆]

New Journal of Chemistry
Page 20 of 32
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
7
8
9
All these equations are used to evaluate several kinetic parameters, including turnover
number (kcat) and specificity constant (kcat/KM) for phosphatase mimicking activity of the
complex. 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. The higher the specificity
constant, the more the enzyme prefers that substrate.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
An efficient catalytic activity of phosphoester bond cleavage is showing by the complex.
The time dependent spectral profile of the complex is shown in Fig. 13. 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. Fig. 14 represents the Michaelis-
Menten plot, Lineweaver-Burk plot, Hanes-Woolf plot and Eadie-Hofstee plot of the complex. A
measurably high value of catalytic constant (kcat≈ 398 h-1) has been found for this complex. The
kinetic parameters for the catalyzed reactions (Vmax, Km and kcat) have been determined and
summarised in Table 4.

Page 21 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fig. 13: Wavelength scan (for 1h) 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.

New Journal of Chemistry
Page 22 of 32
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fig. 14: (a) Michaelis-Menten plot, (b) Lineweaver-Burk plot, (c) Hanes-Woolf plot and (d)
Eadie-Hofstee plot for the complex.
To reveal active species present in the catalytic process of phosphoester hydrolysis, the
positive mode electron spray ionization mass spectrometry of the complexes with 4-NPP has
been performed. The peak at m/z = 431.1273 (theoretically at m/z = 431.09) corresponding to
[ZnL(Na)]+ confirms the separation of one mononuclear unit of the complex from the penta-
nuclear molecule. The other peak at m/z = 841.2535 (theoretically at m/z = 841.19) is due to
[(ZnL)2Na]+ and this peak confirms the dinuclear moiety of the complex with two ZnL unit. The
peak at m/z=919.1649 (theoretically at m/z = 919.19) has confirmed the presence of
[(ZnL)2(H3PO4)(H)]+ which ensures the attachment of phosphate group with the dinuclear moiety

Page 23 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
7
8
9
of the complex. Finally, the peak at m/z = 137.0735 has confirmed the presence of
[H3PO4(Li)(CH3OH)]+ (theoretically at m/z = 137.02), which is the end product of the complete
cyclic process. The high value of the catalytic constant indicates the rapid conversion of
substrate (4-NPP) into product (4-nitrophenol). The rapid conversion of substrate into product
may be responsible for not having any peak corresponding to the complex-substrate adduct in the
mass spectrum. The probable mechanistic pathway of catalysis is shown in Scheme 2. Mass
spectrum for the mixture of the complex and 4-NPP has been given in Fig. 15.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 2: Plausible mechanism for the hydrolysis of 4-NPP catalyzed by zinc(II) complex.

New Journal of Chemistry
Page 24 of 32
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fig. 15: ESI-MS positive spectrum for the mixture of the complex and 4-NPP.
Concluding remarks
The synthesis and characterization of a new penta-nuclear zinc(II) complex with reduced
Schiff base ligand is described in this paper. X-ray crystal structure determination confirmed its
structure. The complex has the ability for biomimetic hydrolytic cleavage of P−O bonds and
therefore may be used as a phosphatase mimic. The complex has also been used for
nitroaromatics sensing using fluorescence quenching activity.
Acknowledgements
M.K. thanks the UGC, 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,

Page 25 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
7
8
9
Jadavpur University, for financial support under the head [Chemicals/Consumables/Glassware].
We are thankful to Dr. Saurabh Das, Associate Professor, Department of Chemistry, Jadavpur
University, for providing the facility of UV-Vis spectrophotometer.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Appendix A. Electronic Supplementary Information
IR, UV-VIS spectrum, fluorescence decay profile, powdered X-ray diffraction analysis of
the complex and NMR spectra of complex and 4-nitrobenzoic acid adduct are given in the
electronic supplementary information part. CCDC 1870759 contains the supplementary
crystallographic data for the complex.
References
1 (a) M. Yamakawa, R. Noyori, J. Am. Chem. Soc., 1995, 117, 6327-6335; (b) G. Parkin, Chem.
Rev., 2004, 104, 699-767; (c) A. Bhattacharyya, S. Sen, K. Harms, S. Chattopadhyay,
Polyhedron, 2015, 88, 156–163; (d) A. Majumder, G. M. Rosair, A. Mallick, N. Chattopadhyay,
S. Mitra, Polyhedron, 2006, 25 1753–1762; (e) H. Sakiyama, R. Mochizuki, A. Sugawara, M.
Sakamoto, Y. Nishida, M. Yamasaki, J. Chem. Soc., Dalton Trans., 1999, 997–1000; (f) M.
Mirzaei, A. Hassanpoor, H. Alizadeh, M. Gohari, A. J. Blake; J. Mol. Struc., 2018, 1171, 626-
630.
2 (a) A. L. Johnson, N. Hollingsworth, G. Kociok-Köhn, K. C. Molloy, Inorg. Chem., 2008, 47,
12040-12048; (b) D. J. D. Wilson, C. M. Beavers, A. F. Richards, Eur. J. Inorg. Chem., 2012,
1130–1138.
3 (a) G. J. Palenik, D. W. Wester, Inorg. Chem., 1978, 17, 864-870; (b) Y.-Y. Liu, J.-F. Ma, J.
Yang, Z.-M. Su, Inorg. Chem., 2007, 46, 3027-3037; (c) I. S. Ahuja, R. Singh, Inorg. Nucl.
Chem. Letters, 1973, 9, 289-294; (d) K. Krishnan, R. A. Plane, Inorg. Chem., 1966, 5, 852-857.

New Journal of Chemistry
Page 26 of 32
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
7
8
9
4 (a) E. Rijnberg, J. Boersma, J. T. B. H. Jastrzebski, M. T. Lakin, A. L. Spek, G. V. Koten,
Organometallics, 1997, 16, 3158-3164; (b) C. A. Wheaton, P. G. Hayes, Dalton Trans., 2010,
39, 3861–3869.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
5 (a) S. Mandal, Y. Sikdar, D. K. Maiti, G. P. Maiti, S. K. Mandal, J. K. Biswas, S.Goswami,
RSC Adv., 2015, 5, 72659–72669; (b) C. Patra, A. K. Bhanja, A. Mahapatra, S. Mishra, K. D.
Saha, C. Sinha, RSC Adv., 2016, 6, 76505–76513; (c) K. E. Dalle, F. Meyer, Eur. J. Inorg.
Chem., 2015, 3391–3405; (d) S. A. Bellingham, B. Guo, A. F. Hill, Biol. Cell., 2015, 107, 389–
418.
6 (a) R. Sanyal, A. Guha, T, Ghosh, T. K. Mondal, E. Zangrando, D. Das, Inorg. Chem., 2014,
53, 85-96; (b) S. Bosch, P. Comba, L. R. Gahan, G. Schenk, Inorg. Chem., 2014, 53, 9036-9051.
7 R. Sanyal, X. Zhang, P. Kundu, T. Chattopadhyay, C. Zhao, F. A. Mautner, D. Das, Inorg.
Chem., 2015, 54, 2315-2324.
8 (a) A. K. Gupta, K. Tomar, P, K. Bharadwaj, New J. Chem., 2017, 41, 14505-14505; (b) S.
Roy, I. Mondal, K. Harms, S. Chattopadhyay, Polyhedron, 2019, 159, 265–274; (c) W.-C.
Song1, X.-Z. Cui1, Z.-Y. Li, E.-C. Yang, X.-J. Zhao, Sci. Rep., 2016, 6, 34870.
9 (a) S. Dasgupta, E. Zangrando, I. Majumder, ChemistrySelect, 2017, 2, 7073–7081; (b) M. E.
Germain, M.l J. Knapp, J. Am. Chem. Soc., 2008, 130, 5422-5423.
10 G.M. Sheldrick, Acta Crystallogr., Sect., 2008, A 64, 112-122.
11 G. M. Sheldrick, SADABS, V2014/5, Software for Empirical Absorption Correction,
University of Göttingen, Institute fur Anorganische Chemieder Universitat, Gottingen, Germany,
1999–2003.

Page 27 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
7
8
9
12 (a) A. Hazari, L. K. Das, R. M. Kadam, A. Bauzá, A. Frontera, A. Ghosh, Dalton Trans.,
2015, 44, 3862-3876; (b) K. Sundaravel, E. Suresh, K. Saminathan, M. Palaniandavar, Dalton
Trans., 2011, 40, 8092-8107.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
13 A. W. Addison, T. N. Rao, J. Reedijk, J. van Rijn, G. C. Verschoor, J. Chem. Soc., Dalton
Trans. 1984, 1349-1356.
14 (a) D. Cremer, J. A. Pople, J. Am. Chem. Soc., 1975, 97, 1354–1358; (b) D. Cremer, Acta
Crystallogr., Sect. B: Struct. Sci., 1984, 40, 498–500.
15 T. Basak, K. Ghosh, C. J. Gómez-García, S. Chattopadhyay, Polyhedron, 2018, 146, 42–54.
16 (a) T. Basak, K. Ghosh, S. Chattopadhyay, Polyhedron, 2018, 146, 81-92; (b) S.
Chattopadhyay, M. S. Ray, S. Chaudhuri, G. Mukhopadhyay, G. Bocelli, A. Cantoni, A. Ghosh,
Inorg. Chim. Acta, 2006, 359, 1367-1375 (c) G. Socrates, Infrared and Raman Characteristic
Group Frequencies. Tables and Charts, 2001, John Wiley & Sons, Chichester, vol. 3, p. 108–113.
17 S. Roy, T. Basak, S. Khan, M. G. B. Drew, A. Bauza, A. Frontera, S. Chattopadhyay,
ChemistrySelect, 2017, 2 , 9336– 9343.
18 T. Basak, A. Bhattacharyya, M. Das, K. Harms, A. Bauza´, A. Frontera, S. Chattopadhyay,
ChemistrySelect, 2017, 2, 6286– 6295.
19 K. Visvaganesan, R. Mayilmurugan, E. Suresh, M. Palaniandavar, Inorg. Chem., 2007, 46,
10294-10306.
20 Z.-J. Wang, L. Qin, J.-X. Chen, H.-G. Zheng, Inorg. Chem., 2016, 55, 10999−11005.
21 A. Kumar, A. Kumar and D. S. Pandey, Dalton Trans., 2016, 45, 8475–8484.

New Journal of Chemistry
Page 28 of 32
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
Table 1 Crystal data and refinement details of the complex.
7
8
9
Formula
C57H71N18O12Zn5
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Formula Weight
1527.27
Temperature (K)
Crystal System
Space group
a (Å)
273
Monoclinic
C2/c
27.683(3)
18.147(2)
13.1612(14)
90.297(4)
2
b (Å)
c (Å)
β (˚)
Z
dcal (g cm-3
1.534
)
μ(mm-1)
F(000)
1.860
3140
Total reflection
Unique Reflections
68560
6932
Observe data[I>2σ(I)]
7935
R(int)
0.097
R1, wR2 (all data)
0.1054, 0.2235
0.0707, 0.2009
R1, wR2 [I>2(I)]

Page 29 of 32
New Journal of Chemistry
View Article Online
DOI: 10.1039/C8NJ06549G
1
2
3
4
5
6
Table 2 Selected bond lengths (Å) of the complex.
7
8
9
2.013(4)
2.049(5)
2.115(7)
2.126(8)
1.957(8)
2.043(4)
1.991(4)
2.381(4)
1.971(4)
2.213(4)
2.089(5)
2.063(4)
2.189(8)
2.176(5)
Zn(1)-O(1)
Zn(1)-O(2)
Zn(1)-N(1)
Zn(1)-N(2)
Zn(1)-N(3)
Zn(2)-O(1)
Zn(2)-O(2)
Zn(2)-O(3)
Zn(2)-O(5)
Zn(2)-O(6)
Zn(2)-N(7)
Zn(3)-O(5)
Zn(3)-N(6)
Zn(3)-N(7)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60