Synthesis and structural characterization of a linkage isomer to a mononuclear Nickel(II) complex: Experimental and computational depiction of phosphoesterase efficiency

Article abstract of DOI:10.1016/j.molstruc.2019.127083

Synthesis and structural characterization of a linkage isomer, bis(2,2′-dipyridylamine)dithiocyanato(k-S)nickel(II), [Ni(dpa)₂(k-SCN)₂] (1) of a previously reported Ni(II) complex, [Ni(dpa)₂(k-NCS)₂] (dpa = 2,2′

Full text of DOI:10.1016/j.molstruc.2019.127083

Journal Pre-proof
Synthesis and structural characterization of a linkage isomer to a mononuclear
Nickel(II) complex: Experimental and computational depiction of phosphoesterase
efficiency
Prafullya Kumar Mudi, Rajani Kanta Mahato, Mayank Joshi, Suvendu Paul,
Angshuman Roy Choudhury, Bhaskar Biswas
PII:
S0022-2860(19)31183-4
DOI:
https://doi.org/10.1016/j.molstruc.2019.127083
MOLSTR 127083
Reference:
To appear in: Journal of Molecular Structure
Received Date: 2 May 2019
Revised Date: 11 September 2019
Accepted Date: 14 September 2019
Please cite this article as: P.K. Mudi, R.K. Mahato, M. Joshi, S. Paul, A.R. Choudhury, B. Biswas,
Synthesis and structural characterization of a linkage isomer to a mononuclear Nickel(II) complex:
Experimental and computational depiction of phosphoesterase efficiency, Journal of Molecular Structure
(2019), doi: https://doi.org/10.1016/j.molstruc.2019.127083.
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Synthesis and Structural Characterization of a Linkage Isomer to a
Mononuclear Nickel(II) Complex: Experimental and Computational
Depiction of Phosphoesterase Efficiency
a
a
b
c
Prafullya Kumar Mudi, Rajani Kanta Mahato, Mayank Joshi, Suvendu Paul,
b
a,*
Angshuman Roy Choudhury, Bhaskar Biswas
a
b
Department of Chemistry, University of North Bengal, Darjeeling 734013, India
Department of Chemical Sciences, Indian Institute of Science Education and Research, Mohali, Mohali-
1
40306, India
c
Department of Chemistry, University of Kalyani, Kalyani-741235, India

Synthesis and Structural Characterization of a Linkage Isomer to a
Mononuclear Nickel(II) Complex: Experimental and Computational
Depiction of Phosphoesterase Efficiency
a
a
b
c
Prafullya Kumar Mudi, Rajani Kanta Mahato, Mayank Joshi, Suvendu Paul,
b
a,*
Angshuman Roy Choudhury, Bhaskar Biswas
a
Department of Chemistry, University of North Bengal, Darjeeling 734013, India
b
Department of Chemical Sciences, Indian Institute of Science Education and Research,
Mohali, Mohali-140306, India
c
Department of Chemistry, University of Kalyani, Kalyani-741235, India
Synthesis and structural characterization of
a
linkage isomer, bis(2,2′-
dipyridylamine)dithiocyanato(k-S)nickel(II), [Ni(dpa) (k-SCN) ] (1) of a previously reported
2
2
Ni(II) complex, [Ni(dpa) (k-NCS) ] (dpa = 2,2′-dipyridylamine; SCN = thiocyanate) has been
2
2
reported herein. X-ray structural analysis of 1 reveals that Ni(II) complex crystallizes in
monoclinic system with P2 /c space group and exists in trans configuration. The coordination
1
geometry for 1 adopts a tetragonal structure. Supramolecular aspects of the Ni(II) complex
indicate that N atom of SCN behaves as a donor and H atom of dpa ligand acts as a receptor in
growing 1D crystalline structure along b axis in solid state. The aromatic rings in 1 prefer to
exist in face-centred stacking with interplanar distance of 3.426 Å and supports strongly to
form self-assembled 3D crystalline architecture. Hirshfeld surface analysis further
recommends this crystalline assembly in solid state. 1 exhibits promising phosphatase activity
towards disodium(4-nitrophenyl)phosphate (PNPP) in aqueous-methanolic medium. The
hydrolytic phosphatase efficiency for 1 is determined in terms of turnover number (k ) as
cat
4
-1
3
.38×10 h . Moreover, density functional theoretical computations closely replicate the
structural parameters and spectroscopic behaviour of 1.
Keywords: Bio-mimicking Chemistry, DFT Study, Linkage Isomer, Nickel(II), Phosphatase
Activity, X-ray Structure
*
Corresponding author. Tel.: +91 353 2776381.
E-mail: mr.bbiswas@rediffmail.com / icbbiswas@gmail.com

2
1
. Introduction
In this modern era, investigation and exploration of new molecular properties for transition
metal based coordination compounds raised the peak of demand in the advancement of
functional materials [1-3]. Ligational motif with varying degree of coordination for different
ligands may lead to an improvement in morphologies of the coordination compounds and
physico-chemical properties as well as structural properties of the compounds may alter in
significant manner [4-6]. Nickel(II) compounds with N,N-donor ligand exhibit diverse
applications in modern science like fundamental catalysis, magnetism, bio-materials and so on
[7-10].
Among the transition metal ions, nickel(II) is very attractive towards scientific community
for its diverse functionality to serve as bio-enzymes like hydrogenase, generator of H gas, etc.
2
Among the available bio-relevant metalloenzymes, alkaline phosphatase enzyme contributes
significantly in different biological functions ranging DNA fragmentation to RNA replication
to bone metabolism as well as bio-remediation of organophosphate pesticides [11]. Active site
of Phosphatase enzyme is surrounded by N or O coordination environment using histidine and
aspartate groups [12]. Recently, it is also observed that nickel(II) complexes play key role in
phosphatase activity [13] and approved as a well-accepted alternative in pursuing catalyzing
hydrolytic reactions. Literature survey indicates that existence of Ni(II) coordination
compounds with N,N linkages are well documented [11-14]. L. Dobrzańska et al. [15] have
reported same mononuclear Ni(II) complex with dpa ligand having (k-N) mode of
coordination by NCS ligands. L. Dobrzańska et al. have synthesized this isostructural Ni(II)
complex with k-N mode of coordination by SCN ligand while we are able to produce its
linkage isomer through k-S ligational mode in coordination of thiocyanate using a different
synthetic approach. The spectroscopic properties of the previously reported Ni(II) compound
have been reported earlier [16]. Nevertheless, there are quite a few crystallographic reports on
[Ni(chelatingN,N-ligand) (NCS) ] complexes with trans- [17] or cis-octahedral geometry
2 2
[18]. In this present study, we have prepared a nickel(II) complex, bis(2,2′-dipyridylamine)
dithiocyanato(k-S) nickel(II) with tetragonal coordination geometry. Exploration on
supramolecular aspects of the Ni(II) complex indicate that N atom of SCN behaves as a donor
and H atom of dpa acts as a receptor in growing 1D and 3D crystalline structure along b axis
in solid state. The preference of face-centred stacking between aromatic rings in 1 with
interplanar distance of 3.426 Å pursues strongly to assemble 3D structure in crystalline state.

3
Moreover, this compound is quite efficient to cleave phosphoester bond hydrolytically with
-1
3
good catalytic efficacy, kcat (h )= 2.24×10 .
2
. Experimental
2
.1. Preparation of the complex
(a) Chemicals, solvents and starting materials
Pure 2,2′-dipyridylamine (Alfa-Aesar, UK), nickel(II) chloride hexahydrate (E-Merck,
India), ammonium thiocyanate (E-Merck, India), disodium salt of 4-nitrophenylphosphate
Sigma-Aldrich, USA) and all other materials were purchased from commercial sources. All
(
other chemicals and solvents were of analytical grade and were used as received without
further purification.
(b) General procedure for the synthesis of Ni(II) compound (1)
A methanolic solution of 2,2′-dipyridylamine (0.3421 g, 1.998 mM in 20 mL) was added
dropwise to an aqueous solution of hydrated nickel(II) chloride (0.2384 g, 1.002 mM in 10
mL) followed by addition of solid ammonium thiocyanate (0.1524 g, 2.002 mM). The reaction
mixture was kept on magnetic stirrer for 1h. The blue coloured solution was filtered and kept
in a capped glass tube for slowest evaporation. The beautiful blue-violet coloured single
crystals were grown in the reaction mixture and were separated out from the solution after ~50
days.
Yield: ~0.1830 g (~76.76% based on metal salt). Anal. Calc. for C H N S Ni (1): C,
22
18
8 2
-1
5
1.08; H, 3.51; N, 21.66. Found: C, 51.12; H, 3.47; N, 21.72. IR (KBr, cm ): 3216-3311 (ν
N-
H), 2159 (νSCN), 1631(νC=N).
2
.2. Physical measurements
Elemental analyses (carbon, hydrogen and nitrogen) were performed on a Perkin-Elmer
400 CHNS/O elemental analyzer. Fourier Transform Infrared (FT IR) spectra (KBr discs,
2
4
-1
000-400 cm ) were recorded using a FTIR-8400S SHIMADZU spectrophotometer in the
–
1
range 4000-400 cm . Ground state absorption spectral measurements were operated with a
1
JASCO model V-730 UV-Vis spectrophotometer. The H NMR spectra were recorded on a
Bruker DPX-500 MHz spectrometer. Electrospray ionization (ESI) mass spectrum was

4
recorded using a Q-tof-micro quadruple mass spectrometer. Molar conductivity measurement
was done using a Horiba LAQUA Benchtop DS 70 conductivity meter.
2
.3. X-ray diffraction study
Single crystal X-ray diffraction data of the nickel(II) complex were collected using a
Rigaku XtaLABmini (Fixed 2Theta and Distance) diffractometer equipped with Mercury375R
2×2 bin mode) CCD detector. The data were collected with graphite monochromated Mo-Kα
radiation (λ=0.71073 Å) at 100 K using ω scans. The data were reduced using CrysAlisPro
.171.38.46 [19] and the space group determination was done using Olex2. The structure was
(
1
resolved by dual space method using SHELXT-2015 [20a] and refined by full-matrix least-
squares procedures using the SHELXL-2015 [20b] software package through OLEX2 suite
[20c].
2
.4. Hirshfeld surface calculations of 1
Hirshfeld surfaces and fingerprint plots were generated using Crystal Explorer 17.5
[21] using data of single crystal X-ray diffraction study. The function dnorm was a ratio of the
distances of any surface point to the nearest interior (d ) and exterior (d ) atom and the van der
i
e
Waals radii of the atoms [22,23]. The negative value of dnorm indicated that the sum of d and
i
de was shorter than the sum of the relevant van der Waals radii, which was considered to be a
closest contact and was visualized in red colour. The white colour denoted intermolecular
distances close to van der Waals contacts with dnorm equal to zero whereas contacts longer
than the sum of van der Waals radii with positive dnorm values were coloured with blue. A plot
of d versus d was a fingerprint plot that identified the presence of different types of
i
e
intermolecular interactions.
.5. Phosphatase activity by the nickel(II) complex
2
Disodium salt of (4-nitrophenyl)phosphate (PNPP) was used as a convenient substrate to
explore the nature of cleavage of PNPP in aqueous-methanolic solution following a previously
reported procedure [24]. An initial screening of the hydrolytic tendency of Ni(II) complex was
performed until 2% formation of 4-nitrophenolate (2 hours) was reached, and then its kinetic
data were collected. The cleavage rate of PNPP in the presence of the nickel complex was
determined through initial rate method. The incremental optical band in absorption spectrum

5
was fixed at 414 nm owing to release of p-nitrophenolate ion in aqueous-methanolic solution
-4
at 25 °C. The electronic spectra were recorded by adding a solution of 1×10 M Ni(II)
-3
complex with 1×10 M PNPP during the course of wavelength scan for 2 hours. Blank
experiment with PNPP in absence of Ni(II) complex under identical reaction conditions didn’t
exhibit any phosphatase activity.
Kinetic experiments of the phosphoester cleavage of PNPP in presence of Ni(II) complex
were performed by monitoring the growth of the p-nitrophenolate band at 414 nm as a
function of time under the condition of excess substrate (PNPP). Kinetic experiment was
-4
performed with 1 (at a constant concentration of 1×10 M) and PNPP (varying the
-3
-2
concentration from 1×10 M to 1×10 M) in aqueous-methanolic solution adopting UV-Vis
spectrophotometer. The incremental effect of absorbance in the electronic spectrum was
recorded for a period of 2 hours at 8 min intervals. All measurements were performed in
triplicate.
2
.6. Computational methodology
Avoiding symmetrical restrictions all quantum mechanical computations were performed
using Gaussian 09W programme suite [25]. For all atoms including the metal ion, B3LYP
theoretical model and widely used 6-311G basis set [26] were compiled with Density
Functional Theory (DFT) for ground state calculations. Initially, the preliminary structures
were optimized in vacuum and then in acetonitrile (ε=35.688) employing well-established
Integral Equation Formalism Polarizable Continuum Model (IEFPCM) [27]. Thereafter, the
stability of all the complexes in vacuum and acetonitrile were confirmed separately through
stability and IR frequency calculations. The images of molecular structure and frontier
molecular orbitals (FMOs) were extracted from checkpoint files using GaussView 5.0.8 [25].
For more details of computational methodology readers could follow somewhere else [28].
3
. Results and discussion
3
.1. Synthesis and formulation
2 2
The [Ni(dpa) (k-SCN) ] complex has been prepared (Scheme 1) in the form of single
crystals by addition of hydrated nickel(II) chloride with the chelator (dpa) in aqueous
methanolic medium in a sealed glass tube. The structural formulation has been determined
through different analytical techniques. The geometry of the Ni(II) complex (1) is confirmed

6
by single crystal X-ray diffraction study. We have employed several attempts to produce
similar linkage isomers employing sodium cyanate but failed to produce corresponding
compound using our reaction methodology. Our synthetic approach is highly useful for the
production of thiocyanate based linkage isomers.
N
C
H
N
S
Ni
S
MeOH+H O
2
N
N
N
N
NiCl2
+
HN
+ NH SCN
NH Cl
4
4
NH +
N
N
.
C
N
Scheme 1. Preparative procedure of Ni(II) complex (1).
3
.2. Depiction of X-ray structure and supramolecular aspects of long range crystalline
architectures
X-ray diffraction analysis of the nickel(II) complex reveals that nickel(II) compound
crystallizes in a monoclinic system with P2 /c space group. The ellipsoidal plot of the
1
mononuclear nickel(II) complex is displayed in Figure 1. The nickel(II) ion exists in
tetragonal coordination geometry where the square plane consists of two units of dpa ligands.
Two thiocyanate ions coordinate at axial position to the Ni(II) centre in this centrosymmetric
structure and satisfies the cationic charges of Ni(II) ion. Bond distances and bond angles
measurement in the primary zone of coordination indicates that Ni(II) ion adopts a tetragonal
coordination geometry in crystalline phase. Two independent bond angles, ∠ N1-Ni1-N3 and
∠
N3-Ni1-N1* in square plane are found to contain 86.20°(7) and 93.80°(7) respectively
whereas only one symmetry adopted bond angle, ∠ S1-Ni1-S1* is found as 180°. Two
independent Ni–N bond lengths are also observed of about 2.0240(17) and 2.0085(2) Å
although one Ni1-S1 bond length is found as 2.8880(6) Å. The single crystal X-ray diffraction
refinement parameters for 1 are shown in Table 1. Important bond lengths and angles for 1 are
also summarized in Table 2.
Interestingly, we have collected Ni(II) complex in trans-form as blue-violet coloured crystals
from the solution and more surprisingly S-end of the thiocyanate ions are used to coordinate
Ni(II) ion. Probably, effect of creating strong hydrogen bonds by N atom of SCN ligand with
the H atoms of dpa facilitates the coordination by S-end of SCN to Ni(II) ion along axial side.

7
Investigation on the nature of growth of supramolecular architectures in crystalline phase
for this Ni(II) complex, it is revealed that H-atoms attached to dpa and N-atom of the
thiocyanate act significant role in constructing a 1D H-bonded network along b axis. N2-
H2…N4 and C4-H4…N4 are the main responsible force in the construction of 3D crystalline
architecture in solid state (Fig. 2, Table S1). Furthermore, face-centred stacking between
aromatic rings has provided additional stability in forming self assembled 3D crystalline
architecture in solid state. The Ni(II) complex has been found to have face-centred stacking
between the aromatic rings. The pyridine rings of one dpa ligand are stacked with another
pyridine ring of other dpa unit with centroid-centroid distance of 3.426 Å and interplanar
distance of 3.476Å.
Comparison of structural aspects between the Ni(II) compound prepared by L. Dobrzańska et
al. [15] and its linkage isomer, [Ni(dpa)
linkage isomers has been found to exist in trans configuration. The isomers are crystallized in
monoclinic system with identical space group, P2 /c. Both the compounds have and displayed
2 2
(k-SCN) ] (1), it has been observed that both the
1
close relationship between bond distances and bond angles around Ni(II) ion. However, some
important structural parameters like cell volume, cell dimension, resolution factor, H bonding
interactive distances, and face-centred stacking between aromatics for our synthesized Ni(II)
complex differ largely from its linkage isomer reported by L. Dobrzańska.
3
.3. Hirshfeld analysis of nickel(II) complex
Hirshfield surface area of Ni(II) complex has been determined to reveal the nature of H
bonding and face-centred stacking between aromatics in forming supramolecular structure for
3
this Ni(II) complex. The nickel(II) complex has been found to have 528.41Å surface volume
2
and 428.47Å surface area (Fig. S1). Red area shows close contact present in the molecules
with other adjacent molecules in crystalline phase. This molecular interaction has presented
the N-H…N strong hydrogen bonding with nitrogen atom of SCN molecule as displayed in
Fig 3. Percentage share of each element in close interaction (Table S2) and fingerprint plots
(
Figure 3) display the same type of interaction by the surface area. Fingerprint plot of Hin
-
Allout and All and N has showed of intense interaction between N-H of molecule and N of
in
out
SCN which have exhibited by the sharp point peaks in the 2D fingerprint plot.

8
3
.4. Theoretical modelling of the structure and frontier molecular orbitals
Computational modeling has been performed to correlate the structural information
between X-ray structure and optimized Ni(II) structure. The optimized molecular structure of
in vacuum (Fig. 4) exhibits close similarity with the X-ray structure. Subsequently, the bond
lengths and bond angles have been observed in well agreement with the experimental values
Table 2). The optimized structure in vacuum has also defined the same degree of
1
(
coordination motifs for dipyridyl amine and thiocyanate ions and has attested the formation of
Ni(II) structure in trans conformation. We have also superimposed the XRD structure with
computationally optimized structure using PyMOL 1.3 software and presented in Fig. S2. The
experimental and theoretical geometry parameters for this Ni(II) linkage isomer are already
summarized in Table 2. Both the structures well superimposed on each-other. There is little
discrepancy in orientation of SCN groups of computationally optimized structures. Probably
this is due to ignorance of hydrogen bonding in computational calculations as observed in
experimental XRD structure between two asymmetric units. The green and brown structure
presents the experimental XRD and computationally optimized structure respectively.
Calculation of energies of the frontier orbitals of the nickel(II) linkage isomer in MeCN
exhibit that the energy gap between HOMO→LUMO and HOMO-1/2→LUMO+1/2 (Fig. 5)
portray a clear picture about the electronic activity of 1 in solution. It is well established that
low energy gap between HOMO and LUMO provides higher chemical activity. It is found that
electronic cloud of HOMO, HOMO–1 and HOMO–2 orbitals locate majorly on Ni(II) ion;
however electronic cloud of LUMO, LUMO+1 and LUMO+2 orbitals dominate extensively
over ligand moieties. Such large and prominent shift of electronic cloud indicates the
possibility of LMCT behaviour. Also, HOMO-LUMO, (HOMO–1)-(LUMO+1) and (HOMO–
2
)-(LUMO+2) transition energies 2.36 eV (525 nm), 2.62 eV(473 nm) and 2.85 eV (435 nm)
respectively have been well justified the experimental absorption spectrum of 1 in acetonitrile
medium.
3
.5. Phosphatase activity of [Ni(dpa) (k-SCN) ] and its mechanistic implications
2 2
The cleavage efficiency of the Ni(II) complex has been examined in aqueous-methanolic
medium by spectrophotometry. Disodium (4-nitrophenyl)phosphate hexahydrate (PNPP) is
considered as a standard substrate. Evolution of 4-nitrophenolate in aqueous-methanolic
medium is monitored by scanning wavelength from 200 to 900 nm at 25 °C. Furthermore, the

9
structural stability of the Ni(II) linkage isomer has been examined through time dependent
measurements of molar conductance and electronic spectra in solution phase with 30 mins
interval. Initially, the optical spectrum for this Ni(II) liknage isomer in methanolic solution
exhibits sharp electronic transitions at 249 and 308 nm which are assignable to the π-π* / n-π*
electronic transitions of ligand origin (Fig. S3) [2d]. After 30 mins, the positions of existing
electronic bands are shifted promptly to lower energy region (260 & 328 nm; Red shift) along
with development of a broad optical band at 623 nm. The positional variation of the optical
bands strongly recommend about the changes in ligand coordination to Ni(II) centre or most
probably the displacement of thiocyanate ion from Ni(II) centre in methanol medium [29].
Further measurement with another 30 min interval i.e., after 1 h, the position of the electronic
bands remains very close to position of the bands appeared in measurements after 30 mins.
-4
The electrical molar conductivity measurements of 1 in 10 M dry methanolic solution show a
2
-1
non-electrolytic behaviour with molar conductance value of 07 Scm mol . After 30 mins, the
2
-1
solution of the Ni(II) isomer exhibits molar conductance value 93 Scm mol which is a
definite signature for the 1:1 electrolytic behavior in solution phase. With progress of time
2
-1
(
with 30 mins interval till 2h), molar conductivity value is found as 99 Scm mol which
consolidates the dissociation of one thiocyanate ion and represents a 1:1 type electrolyte
during a period of 2h (Fig. S4). The changes in electronic bands and molar conductance value
with the progress of time are indicative of structural instability of this nickel complex in
solution phase.
Characteristics changes in the spectral bands upon addition of Ni(II) complex to PNPP
have been observed and is shown in Fig. 6. The spectra have been recorded for 2 hours at 8
min intervals. Upon addition of Ni(II) complex to the solution of PNPP, a new optical band at
4
14 nm with incremental absorbance (Fig. 6) has been detected during the spectrophotmetric
titration. A close inspection on absorbance vs time plot at 414 nm has revealed that the
changes of absorbance difference remain almost parallel to time and maintain a first order
kinetics. The incremental absorption band at 414 nm is assignable to the evolution of 4-
-4
nitrophenolate in solution. Phosphatase activity has also been measured employing 10 M aq.
methanolic solution of hydrated nickel(II) chloride and dipyridylamine ligand (dpa)
-4
independently under similar experimental conditions. However, independent addition of 10
-3
M solution of NiCl .6H O and dpa to 10 M PNPP didn’t produce any significant effect in
2
2
optical region which strongly recommends about non-functional activity of NiCl
dpa in solution (Fig. S5).
2 2
.6H O and

10
The details of kinetics for phosphatase activity have been carried out following the initial
-1
-4
slope method. The calculated first order rate constant, V(min ) is found as 1.61×10 (Error =
-6
2
.81×10 ) (Fig. S6). The kinetic parameters (V , K , k ) for Ni(II) complex catalyzed
max M cat
reactions have been calculated using Michaelis-Menten approach of enzymatic kinetics (Table
). Important kinetic parameters for phosphoesterase activity by the nickel(II) complex have
been determined through plotting of log[A /(A - A )] values against time. Moreover, a
3
α
α
t
comparison has also been made to examine the catalytic efficacy of this linkage isomer with
other reported nickel(II) complexes in Table 3 and it is found that this Ni(II) compound
exhibits good phosphoester cleavage efficiency compared to reported Ni(II) complexes
[24,30,31].
To explicate the involvement of active species in studying phosphoester cleavage activity
in aqueous-methanol medium, ESI mass spectral analysis of PNPP in presence of Ni(II)
complex has been recorded in positive mode. In situ ESI-MS spectral measurements of the
reaction mixture remain helpful in predicting the mechanistic pathway of the phosphoesterase
activity in aqueous-methanol. The reaction mixture between Ni(II) complex and PNPP of 1:10
ratio in aqueous-methanol has exhibited the rising of new characteristics peaks with isotropic
distribution pattern. In the solution of mixture between Ni(II) complex and PNPP,
characteristics peaks in ESI-Ms have been observed at m/z 161.90, 478.07 and 637.25 which
+
+
are assignable to [Na-nitrophenolate] (Theoretical m/z 161.09), [Ni(dpa) (k-SCN)(H O)]H
2
2
2 2
(Theoretical m/z 478.2) and [Ni(dpa) (PNPP)(H O)] (Theoretical m/z 637.19) adducts in
solution (Fig. S7). Initially, it is believed that the Ni(II) complex undergoes hydrolysis to form
an aquatic Ni(II) complex which in presence of PNPP has made an adduct between substrate
and catalyst. In the later stage, the coordinated aqua molecules have made a nucleophilic
attack that probably formed a P-O-Ni cycle and subsequently rearranged to release
nitrophenolate and phosphoric acid through cleavage of phosphor-ester linkage (Scheme 2).
Most of the proposed adducts and chemical species have been identified through ESI-Ms in
aqueous methanolic medium. The experimental findings have been further attested by the
energy and stability of the optimized reaction intermediates in solution phase.

11
Scheme 2. Proposed catalytic cycles for the phosphatise activity of Ni(II) linkage isomer
With our own curiosity, we have optimized few adducts those have been probably
operative in the course of catalytic cycle and calculated the stabilization energy values for
those intermediates. The proposed reactive intermediates and adducts including their
stabilization energies are given in Scheme 3. The stabilization energy (∆E) of the possible
intermediates has been calculated by considering the difference between potential energy of
the Ni(II) complex (Ecomplex) and summation of the potential energies of the individual ions
2
+
-
(
Ni , SCN )/molecules (water).
∆E = EComplex – EComponents
Where, ∆E = Stabilization energy or binding energy of the metal-ligand complex; EComplex
=
Potential energy of metal-ligand complex; EComponent = Summation of potential energies of the
II
–
individual ions (Ni , SCN etc.)/molecules (water).

12
Scheme 3. Optimized structures of the proposed reactive intermediates during catalytic
phosphoester cleavage of PNPP with Ni(II) complex with corresponding stabilization energy
values.
Theoretical calculations reveal that the 2nd intermediate has highest binding energy (–900.04
–
1
kJ Mol ) (1st row, Scheme 3) among the three intermediates and strongly recommends in
nd
favour of better stability of the 2 intermediate. Consequently, it is also justified that
conversion from dithiocyanato-Ni(II) to thiocyanato-aqua-Ni(II) complex during
solubilisation of the nickel(II) complex in aq. methanolic medium is in accordance with
computational calculations and depicted in Scheme 2. On the other hand, upon addition of
PNPP to aq. methanolic solution of Ni(II) complex, theoretical calculations have preferred the
–
1
lowest interaction energy (1403.06 kJ Mol ) of the hydrogen bonded PNPP complex (2nd
row, Scheme 3) and clearly justified the possibility of formation of hydrogen bonded
intermediate during the course of phosphatase activity as proposed in Scheme 2.
On the basis of ESI mass spectrum for the reaction mixture and theoretical calculations, it may
be considered that the primary stage of Ni(II) complex in aq. methanolic solution provides
simple substitution at Ni(II) centre which has attested the most possible existence of second
structure in solution phase of as most stable structure (1st row images, Scheme 3). While
enhanced stabilization energy of the second intermediate in second row structures (1st row
images, Scheme 3) has clearly suggested it as the most probable substrate-catalyst
intermediate for the course of phosphoestarase cleavage activity by this Ni(II) complex. The

13
m/z values of the possible intermediates and adducts have been obtained through ESI-MS that
has well resembled with theoretical values calculated for those adducts (Scheme 3).
4
. Conclusion
Herein, we have synthesized a linkage isomer of nickel(II) complex, bis(2,2′-
dipyridylamine)dithiocyanato(k-S)nickel(II),
[Ni(dpa) (k-SCN) ](1)
(dpa
=
2,2′-
2
2
dipyridylamine) and structurally characterized employing different analytical techniques. X-
ray structural analysis reveals that nickel(II) ion in 1 adopts tetragonal coordination geometry
and exists in a centrosymmetric structure. Nitrogen atoms of SCN ion and hydrogen atoms of
dpa play an important role in constructing 1D and 3D supramolecular architecture along b axis
in crystalline phase. Face-centred stacking between the pyridine rings of different dipyridyl
amine ligands having interplanar distance of 3.426Å lead to a self assembled 3D crystalline
architecture. The hydrolytic phosphatase activity for this mononuclear nickel complex has
4
-1
been determined in terms of high turnover number (k ) as 3.38×10 h . Computational
cat
outcomes employing density functional theory has been used to correlate structural parameters
and spectroscopic behaviour. More importantly, the coordination geometry for most of the
linkage isomers has been determined through spectroscopic analysis although we have
successfully prepared the blue-violet coloured single crystals for this nickel(II) linkage
isomer. Furthermore, the mode of coordination for thiocyanate in complexation with metal
ions is observed through nitrogen (k-NCS) and from this perception, this nickel(II) linkage
isomer is a limited addition to the molecular library.
5
. Supplementary data
Crystallographic data are available free of charge from The Director, CCDC, 12 Union
Road, Cambridge, CB2 1EZ, UK (fax: +44-1223-336033; E-mail: deposit@ccdc.cam.ac.uk or
www: http://www.ccdc.cam.ac.uk) upon request, quoting deposition number CCDC 1900185
for the Ni(II) compound.
Acknowledgement
The work is supported financially by the Science and Engineering Research Board
(SERB), New Delhi, India under TARE scheme (NO. TAR/2018/000473). The single crystal
X-ray facility of Department of Chemical Sciences, IISER Mohali is acknowledged for the

14
data collection. This work is dedicated to Dr. Indranil Kar, Principal, Surendranath College,
Kolkata 700009 who remains a constant source of inspiration to me.
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17
Table 1 Crystallographic refinement parameters of [Ni(dpa) (k-SCN) ] (1)
2
2
Crystal parameters
Empirical formula
Formula weight
1
22 18 8 2
C H N S Ni
517.27
100 K
Temperature
Wavelength
0.71073 Å
Crystal system
Monoclinic
Space group
Unit cell dimensions
P2 /c
1
o
a = 7.5588(3)Å, α= 90 ; b = 17.4779(5)Å , ß=
02.785(4) ; c = 8.3349(3)Å, ᵞ = 90°
o
1
Volume
3
1
073.84(7) Å
2
Z
Density (calculated)
3
1
.60 g/cm
-1
.128 mm
Absorption coefficient
Reflections collected
1
7831
Independent reflections
R(int)
1887
0.031
1.08
2
Goodness-of-fit on F
R indices (all data)
R1 = 0.0339, wR2 = 0.0919
-3
Largest diff. peak and hole
0
.88 and -0.58 e. Å
Table 2 Selected bond lengths (Å) and bond angles (°) for [Ni(dpa) (k-SCN) ] (1) obtained
2
2
from the XRD structure and from DFT study
Bond lengths (Å)
Experiment
DFT
Experiment
2.0085(18)
DFT
Ni1-S1
2.8880(6)
3.187
1.9225
Ni1-N3
1.9239
Ni1-N1
2.0240(17)
Bond angles (º)
Experiment
92.16(6)
DFT
Experiment
86.20(7)
DFT
S1-Ni1-N3
S1-Ni1-N1
92.70
91.79
174.53
89.75
91.34
N1-Ni1-N3
86.45
177.40
93.05
176.84
95.69(5)
N3-Ni1-N3* 180.00
N1-Ni1-N3* 93.80(7)
N1-Ni1-N1* 180.00
S1-Ni1-S1* 180.00
S1-Ni1-N3* 87.84(6)
S1-Ni1-N1* 84.31(5)
*
Symmetric atom occupies to trans position and applicable to centrosymmetric system

18
Table 3 Kinetic parameters for hydrolytic cleavage of PNPP catalyzed by 1 in aq. MeOH
-1
-1
Complex
Vmax(M s )
K (M)
m
k ( h )
Ref
cat
-
4
4
-3
4
1
*
9.39×10
4.09×10
1.19 ×10
3.38 ×10
Present
[24]
-
-4
4
[
[
[
[
Ni L (NCS)(OAc)(H O) (MeOH) ]
2 2 2 0.5 0.5
2.81×10
2.94×10
-
4
-3
3
Ni(bpy) (OH )(NO )](NO )
2
2
3
3
2.34×10
1.02×10
8.42 ×10
[2d]
II
.
-
5
-3
2
Ni (µ-OHCH ) (L) (CH OH) ] 2CH OH
4
3 2
2
3
2
3
1.65×10
5.78 ×10
5
.92 ×10
[30]
1
Ni (L )(H O) (NCS) )]
2
2
2
2
3
2
.91 ×10
[31]
-
1
-5
-4
*
Std. Error for Vmax (MS ) = 2.46×10 ; Std. Error for K
m
(M)= 1.10×10
Fig. 1. An ellipsoidal plot of nickel(II) complex with 30% ellipsoid probability

19
…
Fig. 2. Formation of 1D (Top) and 3D (Below) crystalline architectures through H N
interactions (pink dotted) in 1

20
All in – All out
N in – All out
All in – Ni out
Ni in – All out
H in – All out
All in – S out
S in – All out
C in – All out
All in – N out

21
All in – H out
All in – C out
Fig. 3. Fingerprint plot of Ni(II) complex.
Fig. 4. Optimized structure of 1 in vacuum using DFT/B3LYP/6-311G theoretical model.

22
Fig. 5. Frontier molecular orbitals of 1 along with the corresponding transition energy values
calculated using TD-DFT/B3LYP/6-311G theoretical model and IEFPCM/acetonitrile solvent
system.
Fig. 6. Enhancement of absorbance for gradual production of phenolate species at 414 nm
upon addition of 1 to 100 equivalents of PNPP in aqueous medium. The spectra were recorded
after every 8 min at 298 K.

Highlights
•
Synthesis and X-ray structure of
a
linkage isomer, bis(2,2′-
dipyridylamine)dithiocyanato(k-S)nickel(II), [Ni(dpa) (k-SCN) ] of a previously
2
2
reported nickel(II) complex, [Ni(dpa) (k-NCS) ]
2
2
4
–1
•
•
The nickel(II) complex efficiently cleave PNPP with TON, 3.38×10 h .
The theoretical findings employing DFT have been well corroborated with the
experimental results.