Transesterification activity by a zinc(II)-Schiff base complex with theoretical interpretation

Article abstract of DOI:10.1016/j.ica.2020.119541

The article demonstrates the synthesis, structural characterization and transesterification activity of a mononuclear zinc(II) complex, [Zn(HL)(H₂O)]·(H₂O) (1) containing a previously reported Schiff base ligand, H₃L = N,N

Full text of DOI:10.1016/j.ica.2020.119541

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Transesterification Activity by a Zinc(II)-Schiff Base Complex with Theoreti‐
cal Interpretation
Chanchal Kumar Pal, Shreya Mahato, Mayank Joshi, Suvendu Paul,
Angshuman Roy Choudhury, Bhaskar Biswas
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https://doi.org/10.1016/j.ica.2020.119541
ICA 119541
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Please cite this article as: C. Kumar Pal, S. Mahato, M. Joshi, S. Paul, A. Roy Choudhury, B. Biswas,
Transesterification Activity by a Zinc(II)-Schiff Base Complex with Theoretical Interpretation, Inorganica
Chimica Acta (2020), doi: https://doi.org/10.1016/j.ica.2020.119541
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Transesterification Activity by a Zinc(II)-Schiff Base Complex with
Theoretical Interpretation
Chanchal Kumar Pal,a Shreya Mahato,a Mayank Joshi,b Suvendu Paul,c Angshuman
Roy Choudhury, b Bhaskar Biswasa,*
aDepartment of Chemistry, University of North Bengal, Darjeeling-734013, India
bDepartment of Chemical Sciences, Indian Institute of Science Education and Research,
Mohali, Sector 81, Knowledge City, S. A. S. Nagar, Manauli PO, Mohali, Punjab 140306,
India
cDepartment of Chemistry, University of Kalyani, Kalyani 741235, West Bengal, India
Abstract
The article demonstrates the synthesis, structural characterization and transesterification
activity of a mononuclear zinc(II) complex, [Zn(HL)(H2O)].(H2O) (1) containing a previously
reported Schiff base ligand, H3L = N,N′-bis(salicylidene)-1,3-diamino-2-propanol]. X-ray
structural analysis of 1 reveals that Zn(II)-Schiff base complex crystallizes in hexagonal
crystal system with P61 space group and adopts a distorted tetrapyramidal geometry. Self-
assembled molecular units of 1 exhibit a beautiful construction of 3D crystalline architecture
through intermolecular hydrogen bond wire. Zinc(II) complex displays an enhancement of
fluorescence intensity in comparison to HL in methanol medium. Catalytic behaviour of 1
towards disodium salt of 2,4-dinitrophenylphosphate (PNPP) in aqueous-methanol medium
exhibits good transesterification activity with initial rate constant value of 1.73×10-4 min-1.
Detailed DFT calculations are also employed to cope with the geometrical parameters of 1 as
well as to explore the proposed catalytic mechanism of transesterification activity.
Keywords: DFT Study; Schiff Base; Transesterification Activity; X-ray Structure; Zinc(II)
*Corresponding author. E-mail: bhaskarbiswas@nbu.ac.in / icbbiswas@gmail.com
1. Introduction
Now-a-days, coordination compounds of 3d metal ions have created huge appeal for their
impeccable contributions and key functions in the chemistry of metallo-enzymes and biology
[1-5]. Paramount attentions have further been focused on to develop advance functional

2
materials by tuning electronic and steric factors of ligands mediated through different types of
metal ions [6-16]. Among the transition metal ions, for the easy availability and cost
effectiveness, 3d metal complexes are highly dominant in nature and material science. Zinc,
one of the most significant bio-metals in nature exhibit promising opto-electronic,
luminescent, biological and catalytic properties and one of the potential candidates in bringing
novelty in the arena of advance materials [17-27].
Nature is the utmost source in scientific research which reveals invaluable impact of
coordination compounds in metal dependent bio-catalysis. Thus, considerable attentions have
been devoted in bioinorganic chemistry aiming to study on zinc complexes mediated bio-
inspired catalytic systems those are able to emphasize the active sites of metallo-
enzymes.[26,27] Zinc(II) based coordination compounds play significant role in living world
those are potentially considered as photo-active materials.[24,25] Important physico-chemical
properties including Lewis acidic behaviour, nucleophile generation, swift ligand exchange,
and stabilization of leaving group present Zn(II) ion one of the most ubiquitous candidate in
biology.[17,27] In the wide range of metallo-enzymes, phosphoesterase enzymes have great
exposures in bio-remediation of organophosphate compounds, DNA fragmentation, RNA
replication, bone metabolism, and chemotherapy [28]. Metal ions are coordinated through N
atom of histidine ligands, O-atom of aspartate, and often including a metal ion-bridging
hydroxide ion in the active site of the phosphoesterase [29].
Literature survey revealed that S. Uhlenbrock et al. [30a] produced a trinuclear zinc(II)
complex using the same Schiff base and this same Schiff base is used by R. Bagai et al. [30b]
to prepare a tetranuclear manganese complex while P.E. Kruger et al. [30c] synthesized a
dinuclear copper(II) complex by utilizing this same ligand. Eventually, we also prepared a
trinuclear Zn(II) complex, a dinuclear Fe(III) complex, a mononuclear Cu(II) complex using
this same Schiff base ligand [31]. Moreover, coordination behavior of the ligand has been
further tested towards Zn(II) ion by addition of methoxo groups to the ligand backbone of this
Schiff base [32]. In this present endeavour, we report the synthesis, spectroscopic
characterization, X-ray structure and fluorescence behaviour of a new Zn(II)-Schiff base
complex. This Zn(II) complex exhibits potential transesterification activity towards disodium
salt of 2,4-dinitrophenylphosphate (PNPP) and rate of transesterification activity was found as

3
1.73×10-4 min-1. Experimental and computational observations recommend that Zn(II)-bound
aqua molecule produces effective nucleophile in solution in the course of bio-catalysis and
proceeds through organophosphorous intermediate in solution.
2. Experimental Section
2.1. Preparation of the zinc(II)-Schiff base complex
(a) Chemicals, solvents and starting materials
High purity salisaldehyde (E-Merck, India), 1,3-diaminopropan-2-ol (Alfa-Aesar, UK),
zinc(II) chloride hexahydrate (E- Merck, India) and disodium salt of p-nitrophenylphosphate
(SRL, India) were purchased from respective concerns and used as received. All the other
reagents and solvents of analytical grade were purchased from repute commercial sources and
used without further purification.
(b) Synthesis of the Schiff base and zinc(II) complex
The synthetic procedure of the Schiff base complex was described in details elsewhere
[30,31]. Salisaldehyde (0.245 g, 2 mmol) was treated with 1,3-diaminopropan-2-ol (0.090 g,
1 mmol) in 25 mL ethanol under refluxing condition. After 8h, the reaction solution was
evaporated to yield a yellow solid compound, which was dried and stored over CaCl2 for use.
Yield, 0.270 g (90.6%). Anal cal. for C17H18N2O3 (H3L): C, 68.44; H, 6.08; N, 9.39; Found:
C, 68.48; H, 6.01; N, 9.33. 1H NMR (CDCl3) δ = 3.68 (dd, J = 12.4, 6.8 Hz, 2H), 3.84 (dd, J =
12.4, 4.0 Hz, 2H), 4.23-4.25 (m, 1H), 6.88 (t, J = 7.2 Hz, 2H), 6.96 (d, J = 8.4 Hz, 2H), 7.25
13
(dd, J = 7.6, 1.6 Hz, 2H), 7.32 (td, J = 8.8, 1.6 Hz, 2H), 8.41 (s, 2H) ppm. C NMR δ, 62.9,
70.2, 117.0, 118.5, 118.6, 131.5, 132.5, 161.1, 167.3 ppm. IR (KBr, cm-1): 1634, 1602 (vC=N),
3409 (vOH); UV-Vis (λmax, nm, 10-4 M, MeOH): ~254, 313, 404.
~10 ml methanolic solution of ZnCl2.6H2O (0.255 g, 1.0 mmol) was added to 15 ml solution
of H3L (0.300 g, 1.0 mmol) in the same solvent (15 ml). The pale yellow Schiff base solution
was turned to bright yellow and slow evaporation of the reaction mixture produced pale
yellow coloured crystalline compound after 8-10 days. Yield: 0.171 g (67.0 % based on metal

4
salt). Anal. Calc. for C17H20N2O5Zn (1): C, 51.34; H, 5.07; N, 7.04. Found: C, 51.37; H, 5.01;
N, 7.09 %. IR (KBr, cm-1): 3471 (vOH), 1618 (vC=N), 1257 (vPhO); UV-Vis (λmax, nm, 10-4 M,
MeOH): 244, 297, 337.
2.2. Physical measurements
FTIR-8400S SHIMADZU spectrometer (Shimadzu, Kyoto, Japan) was employed to
1
record IR spectrum (KBr) of 1 in the range of 400-3600 cm–1. H NMR spectrum of ligand
was obtained in CDCl3 on a Bruker Avance 300 MHz spectrometer (Bruker, Massachusetts,
USA) at 298 K and was recorded at 299.948 MHz. Steady-state absorption spectral
measurements were performed with a JASCO model V-730 UV-Vis spectrophotometer
(Jasco, Tokyo, Japan). Elemental analyses were performed on a Perkin Elmer 2400 CHN
microanalyser (Perkin Elmer, Waltham, USA). Electrospray ionization (ESI) mass spectra of
1 and the reaction mixture of 1 in presence of pNPP were recorded on a Q-TOF MicroTM
Mass Spectrometer (Waters Corporation, Massachusetts, USA). Steady-state fluorescence
spectral measurements were performed with a Hitachi F-7000 spectrofluorimeter (Hitachi,
Tokyo, Japan). Molar conductivity measurement was performed using a LAQUA Benchtop
DS70 conductivity meter (Horiba, California, USA). The pH values of the solutions were
measured using a Labman pH meter (Wensar, Chennai, India) at 298 K.
2.3. X-ray diffraction analysis
X-ray single crystal diffraction data for 1 was collected using a Rigaku XtaLABmini
diffractometer equipped with Mercury CCD detector. The data were collected with graphite
monochromated Mo-Kα radiation (λ=0.71073 Å) at 100.0(2) K using  scans. The data were
reduced using CrysAlisPro 1.171.39.7f [33] and the space group determination was done
using Olex2. The structure was resolved by dual space method and refined by full-matrix least-
squares procedures using SHELXL-2015 software package through OLEX2 suite [34,35]. The
oxygen atom (O4) was found disordered over Axial and equatorial positions. To treat that
appropriately, the adjoining C atoms were splitted and refined as two fractions.
2.4 Hirshfeld surface analysis

5
Hirshfeld surfaces and fingerprint plots were generated using Crystal Explorer 17.5 [36]
using data of single crystal X-ray diffraction study. The function dnorm is a ratio of the
distances of any surface point to the nearest interior (di) and exterior (de) atom and the van der
Waals radii of the atoms [37,38]. The negative value of dnorm indicates the sum of di and de is
shorter than the sum of the relevant van der Waals radii, which is considered to be a closest
contact and is visualized in red colour. The white colour denotes 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 are coloured with blue. A plot of di versus de
is a fingerprint plot which identifies the presence of different types of intermolecular
interactions.
2.5. Transesterification activity of Zn(II)-Schiff base complex (1)
PNPP was employed as a standard substrate to examine transesterification activity in
aqueous-methanolic medium following a reported procedure [39,40]. An initial screening of
the hydrolytic tendency of the Zn(II)-Schiff base complex was performed until 2% formation
of 4-nitrophenolate was reached, and then its kinetic data were collected. The hydrolysis rate
of PNPP in the presence of the Zn(II) complex was measured by an initial rate method,
keeping the incremental absorbance of the electronic band fixed at 418 nm for releasing of 4-
nitrophenolate ion in aqueous-methanolic mixed solution at 298 K. For the wavelength scan,
the spectra were recorded by addition of 1×10-4 (M) Zn(II) complex solution to 1×10-3 (M)
PNPP solution for ~1 hour. Blank experiment with PNPP in absence of Zn(II) complex under
identical reaction conditions were performed and did not show any phosphatase activity.
Kinetic experiments for the phosphoester cleavage study were carried out by monitoring
the growth of the 4-nitrphenolate species at 418 nm as a function of time under the condition
of excess substrate in presence of Zn(II) complex, keeping the other parameters constant.
Kinetic experiment was performed with 1×10-4 (M) concentration of Zn(II)-Schiff base
complex and PNPP {varying the concentration from 1×10-3 to 1×10-2 (M)} in H2O-CH3OH
mixed solvent adopting UV-Vis spectrophotometer. The spectrophotometric titration was

6
carried out for a total period of 70 mins with 06 minutes interval. All measurements were
performed in triplicate and arithmetic average was reported.
2.6. Computational details
Avoiding all types of symmetrical constraints, all quantum chemical computations were
executed with Gaussian 09W program suite [41] and GaussView 5.0 was exercised to extract
graphical figures as well as to view Gaussian input and output files [41]. For ground and
excited singlet state computational calculations, density functional theory (DFT) and Time
Dependent Density Functional Theory (TD-DFT) were respectively considered. For
computational calculations, B3LYP theoretical model and 6-311G basis set [42] were
employed for both the metal and non-metallic elements. Initially, the Zn(II)-complex was
optimized in vacuum and global minima was confirmed through IR frequency calculation and
stability check-up. Then, optimized structures in vacuum were again optimized in methanol
(MeOH, ε=32.613) following Integral Equation Formalism Polarisable Continuum Model
(IEFPCM) [43] to include solvent effect and corroborate with the experimental results. For
further details of computational specifications, readers could follow somewhere else [44].
3. Results and discussion
3.1. Synthesis and formulation of the Zn(II)-Schiff base complex (1)
The Zn(II)-Schiff base complex was prepared in single crystals by mixing of Zn(II)
chloride with an (N,O)-donor Schiff base ligand in 1:1 mole ratio in an aqueous methanolic
medium (Scheme1). Variation of Zn(II) salts in this reaction might lead to different Zn(II)-
Schiff base complexes. It was already observed and reported that Zn(II) acetate salt in reaction
with this Schiff base produced an acetate-bridged trizinc(II)-Schiff base complex [31]. The
structural composition of 1 was determined by different spectroscopic and analytical
techniques like IR, UV-Vis, 1H NMR, ESI-Ms, molar conductivity including single crystal X-
ray diffraction study.

7
OH
OH
N
N
N
N
CH
ZnCl2
CH
Zn
HC
HC
O
O
HO
OH
OH2
CH3OH-H2O
Scheme 1. Synthetic route of the Zn(II)-Schiff complex (1).
3.2. Description of crystal structure and optimized molecular structure
The coordination geometry of this Zn(II)-Schiff base complex is determined by X-ray
structural analysis which reveals that Zn(II) complex crystallizes in hexagonal crystal system
with P61 space group. An ellipsoidal plot of the crystal structure of 1 is shown as Figure 1.
The structural refinement parameters are tabulated in Table 1. Selected bond angles and bond
distances of 1 are listed in Table 2. The Zn(II) centre exists in a distorted tetrapyramidal
geometry in crystalline phase as Zn(II) centre is out of plane which created by O1O2N1N2
atoms of ligand backbone. The average Zn-N bond distance values are longer than average
Zn-O bond distance values (Zn1-Nav = 2.116 Å; Zn1-Oav = 2.021Å) in the pyramidal base.
The apical bond distance is found shortest (1.983 Å) among all the metal-ligand donor
coordination in the Zn(II) complex. Literature survey suggests that this ligand can exhibit a
diversified mode of coordination based on the nature of metal ions. In general, this Schiff base
may be considered as a pentadentate chelating ligand although during the formation of metal
chelates, H3L acts as a tetradentate chelator towards Cu(II) and Zn(II) ions, and exhibits its
pentadentate behaviour towards Fe(III) ions.[31] Isolation of single crystals for
monocopper(II) complex, diiron(III) complex and a trinuclearzinc(II) complex employing this
ligand strongly account on this fact.[31] Moreover, H3L is utilized to develop d-block metallo-
clusters in crystallize phase with greater magnetic importance [30]. In real sense, H3L is an
example of flexidentate ligand in coordination chemistry. To examine the nature of weak
forces in self-assembled crystalline architecture for this Zn(II)-Schiff base complex, it is
observed that phenolate oxygens (O1,O2) and coordinated O(O3) in aqua molecule act as
donors while H atoms attached to aromatic-C (C10A) and coordinated aqua molecule (H3A,

8
H3B) behave as accepter in the construction of 3D crystalline architecture through strong to
moderate H-bonding interactions [Fig. S1; O3-H3A…O1, 2.21 Å, O3-H3B…O1, 1.93Å, C10-
H10A…O2, 2.57 Å].
Previously, we reported the preparation and crystal structure of an isostructural Zn(II)-
Schiff base complex, [Zn(HL1)(H2O)] (where H3L1 = N,N′-bis(3-methoxysalicylidene)-1,3-
diamino-2-propanol) using o-vanilin instead of salicyldehyde [20]. Interestingly, adding of
methoxy substituent to the ligand skeletal didn’t effect a much and displays almost identical
mode of coordination towards Zn(II) ion. A comparison of structural aspects between X-ray
structures of [Zn(HL)(H2O)] and [Zn(HL1)(H2O)] (Table S1) clearly present the tetradentate
chelating mode of coordination for both the Schiff base ligands [20]. From crystallographic
perspectives crystal system and space group are entirely different. Solvents of crystallization
is found for [Zn(HL)(H2O)] structure (Table S1).
Structural correlation between the DFT optimized structure and X-ray structure
arecopmpared in Table Table 2 employing DFT study. Also, the optimized molecular
structure of Zn(II)-Schiff base complex in vacuum (Fig. 2) resembles well with crystal
structure. Computationally calculated bond length and bond angle values are in well
agreement with the crystallographic bond distance and bond angle values (Table 2). The mode
of coordination for ligand to metal centre exhibits close similarities between the coordination
geometry of theoretical and XRD structure. A superimposition of the XRD structure with
optimized structure is also performed using PyMOL 1.3 software and presented in Fig. S2.
This atom to atom superimposition between the XRD and optimized structure of 1 validates
the computational results more profoundly.
3.3 Hirshfeld surface analysis
The hirshfeld surface of Zn(II) complex is shown in Figure S3 and display surface is
plotted with dnorm. The hirshfeld surface is unique for given crystal structure which proposes
the opportunity of locating additional understanding to the crystal. The quantitative measures

9
like volume (507.83 Å3), area (451.69 Å2), globularity (0.681) and asphericity (0.368) also
computed using hirshfeld surface. In this molecule hydrogen bonding between
oxygen and hydrogen atoms which is seen in red spots on the hirshfeld surface (Fig. S3).
Percentage share of each element in close interaction (Table S2) and fingerprint plots (Fig. 3)
are showing the area of intense interaction with other molecule. In the fingerprint plot also, the
direction of interaction is displayed towards O of bridging molecule from oxygen of water
molecule as seen in red region of hirshfeld surface.
3.4. Experimental observations on solution properties of Zn(II) complex and calculated
frontier orbitals
The zinc(II) compound is soluble in common polar solvents like methanol, acetonitrile
and water. The electronic spectrum of Schiff base in methanolic solution consists of high
intensity electronic transitions at 254 and 313 nm which are assignable to * and n*
transitions correspond to –C=N- chromophore of Schiff base ligand (Fig. S4).[20,31] The
optical spectrum for the Zn(II)-Schiff base complex in methanol medium exhibits electronic
transitions at 244, 297 and 337 nm (Fig. S4). Electronic transitions correspond to 244 and 297
nm are assignable to π→π*/n→π* transitions while the electronic band at 337 nm represents
the intra-ligand charge transfer in the Zn(II)-Schiff complex. The electronic bands for zinc(II)
complex remains almost unaffected after keeping the solution for 3 days in MeOH at RT.
This observations account on the structural rigidity for 1.
TD-DFT is employed to cope the electronic properties of Zn(II) complex in excited states.
To gain the insights of the nature of optical spectrum of Zn(II) complex, TD-DFT studies are
performed. The low energetic electronic transitions in HOMOLUMO/ HOMO-
1LUMO+1 and HOMO-2LUMO+2 (Fig. 4) portray a deep view of the level of chemical
reactivity for this Zn(II) complex in MeOH. The electronic cloud of HOMO, HOMO-1 and
HOMO-2 are majorly located on the ligand moiety and on contrary, the electronic cloud of
LUMO, LUMO+1 and LUMO+2 are redistributed throughout the ligand moiety avoiding the
Zn(II) portion. Also, there occurs a reorientation of electron cloud within the ligand moiety on
transition from filled to unfilled FMOs. This indicates the possibility of intra ligand charge

10
transfer (ILCT) phenomenon [45]. Therefore, from computational calculation, it may be
supported the possibility of ILCT. Also, the HOMO-LUMO, HOMO-1 to LUMO+1 transition
energies 3.92 eV (316 nm), 4.19 eV (296 nm) are very close to absorption spectra of the
Zn(II) complex. Thus, it may be recommended that electronic transitions among these FMOs
are majorly contributed for UV-Vis spectrum of the Zn(II) complex.
Molar conductivity of the Zn(II) complex is recorded at 298 K. A solution of 1.240×10–3 (M )
concentration produces molar conductance as 6.1 µS/m. Molar conductivity data for this
Zn(II) complex recommends about the non-electrolytic nature and solution phase stability for
the Zn(II)-Schiff base complex.
The luminescence spectrum of the Zn(II) compound (λex, 244, 297 nm) in methanol (Fig.
S5) portrays the emission behaviour of the Zn(II)-Schiff base complex. The emission
wavelength is found to be at ~ 357 nm (Fig. S5). It is observed that the fluorescence intensity
enhances by more than 2 fold in comparison to the fluorescence intensity of the Schiff base.
Incorporation of Zn(II) ion (3d10 system) in Schiff base may bring structural rigidity in
solution phase and this phenomenon accounts in favour of enhancement of fluorescence
intensity over the ligand [10b,18,20].
3.6. Transesterification study of Zn(II)-Schiff base complex
A 10-4 M aq. methanolic solution of zinc(II)-Schiff base complex is added to the solution
of 10-3 (M) PNPP and a new electronic band with incremental absorbance at 418 nm is started
to develop (Fig. 5). This new electronic band at max, 418 nm may be attributed to the growth
of 4-nitrophenolate ion in solution in this course of transesterification activity [6b,8b,22,39].
The observed spectral changes are displayed in Fig. 5.
The kinetic parameters related to transesterification study are determined following initial
slope method. The initial rate constant, V(min-1) is found as 1.73×10-4 (Error = 8.37×10-5 )
which is determined from the plot of log[A/(A - At)] versus time (Fig. 6) in this catalytic
transesterification study [6b,8b,22,39]. The nature of plot seems to be a first order kinetics and
maintains a good linearity with R2 = 0.978. Control experiments are also performed to
examine the non-catalytic role of Schiff base, and the experiments under similar reaction

11
conditions are found non-responsive towards transesterification activity of PNPP. A literature
survey on the rate of catalytic transesterification study of zinc(II) complexes is made to
compare the level of activity of this zinc(II) with reported compounds (Table 3) [46-52].
Previous scientific investigations made by renowned scientists and their group provide
some source of lights in proposing the mechanistic pathway for the course of
transesterification study. In general, Lewis acid activation, leaving group activation and metal-
nucleophile attack to substrate remain the catalytic route of transesterification activity for the
enzymes (Scheme 2) [6b,8b,22,39, 46-54].
Scheme 2. Modes of activation for metal catalysed transesterification activity towards
phosphoester substrates
ESI-Ms of this Zn(II) complex in presence of PNPP is recorded and the rise of important
peaks with isotopic distribution pattern is noted. The spectrum (Fig. S6) displays an important
peak at m/z 596.6761 (cald. 596.8680) which is attributed to the existence of substrate-
enzyme adduct in solution phase. Another important peak at m/z 459.8944 in the mass
spectrum is indicative of the presence of organophosphorous intermediate species and peak at
161.1078 m/z is evident for the development of by phenolate species (calcd. m/z 161.09) in
solution. The presence of such peaks in ESI-Ms provides sufficient light about the
nucleophilic attack by aqua molecules to P atom of PNPP which actually facilitates hydrolytic
cleavage of PNPP. Zn(II) ions always behave as a good Lewis acid and this property
facilitates the generation of Zn(II)-OH nucleophile. In this regard, pKa value of the
coordinated aqua molecule is calculated which strongly suggests the nucleophilic behavior
towards PNPP. This phenomenon represents the formation of organophosphorus intermediate

12
by nucleophilic attack to PNPP, and consequently, 4-nitrophenolate ion evolutes in solution.
Finally, it decomposes to the active form of Zn(II)-Schiff base complex.
Based on information obtained from literatures and spectroscopic as well as analytical
observations, the proposed mechanistic cycles may be presented as follows. (Scheme 3)
[6b,8b,22,39,46-54].
O
OH
Zn
OH
Zn
OH
Zn
O
O
P
OR
N
N
O
N
N
N
N
CH
CH
HC
CH
HC
HC
O
O
O
O
OH
O
O
OH
OH2
O
R =
NO2
O
P
OR
OH
H2PO
4
OH
OH
OH
N
N
N
N
N
N
CH
CH
Zn
HC
CH
Zn
O
HC
Zn
O
HC
O
O
O
O
O
O
O
O
O
O
O
OH
P
HO
O
P
HO
O H
P
OR
O
OR
NO2
Scheme 3. Plausible catalytic cycle for phosphoester cleavage study by Zn(II)-Schiff base
Recently, different scientific groups focused on exploration of mechanistic aspect of the
phosphoestarese activity of dinuclear Zn(II) complexes theoretically employing density
functional theory [54-57]. However, an effort is made to explore the plausible catalytic
pathways of transesterification study by this mononuclear zinc(II)-Schiff base complex based
on systematic DFT study.
To rationalize the catalytic pathway of Zn(II) complex DFT study is carried out to perform
detailed computational calculations. All the intermediates (I-V, Scheme 4) of the catalytic
process are optimized in methanol and the Cartesian coordinate values of the intermediates are
presented in Tables S3-S7 (ESI) respectively. Detailed exploration of literature of catalysis
mechanisms alongwith spectroscopic as well as analytical observations enforced us to propose
the above mentioned mechanistic pathway (Scheme 2) [6b,8b,22,39,46-53]. Hence, from

13
optimized geometries, calculations are made to determine the binding energies of the
intermediates aiming to establish their thermodynamic criteria of formation within the cyclic
pathway. Initially, the Zn(II) complex (I) loses a proton and converted into the active form of
Zn(II)-Schiff base complex (II). Then, the active form (II) initiates nucleophilic attack to
PNPP and generates the intermediate III which converted into IV. The Intermediate (IV)
releases 4-nitrophenolate ion in solution and converted in (V). Finally, the intermediate (V)
regenerates the active form of the catalyst (II) to initiate the cycle again and again.
Scheme 3. DFT optimized structures of the intermediates of plausible mechanistic cycle of
catalytic activity of the Zn(II) complex along with interaction energy values.

14
Most incredibly, the binding energies of all the intermediates are found to be negative that
allowed the thermodynamic criteria of formation of the mentioned intermediates. Therefore, it
can be stated that the above mentioned mechanism (Scheme 3) must be the plausible
mechanism of the transesterification activity of the Zn(II) complex.
4. Conclusion
In summary, we synthesize a Zn(II)-Schiff base complex with a flexidentate chelator and
determined its square pyramidal coordination geometry with single crystal X-ray analysis
including different spectroscopic and analytical techniques. DFT computations recommend
about intra-ligand charge transfer properties of 1 in solution phase. The complex shows good
photo-luminescence property in methanolic solution. This Zn(II)-Schiff base complex displays
good transestrification reaction of PNPP in aq. methanolic medium with initial rate constant,
V(min-1) = 1.73×10-4. The computational calculations interpret the experimental observations
very well. Synthesis of bio-inspired Zn(II)-Schiff base complexes as catalysts aren’t highly
abundant in literatures and investigation of bio-mimicking property of this Zn(II) complex
may provide valuable information about biological process. Good luminescent properties of
this Zn(II)-Schiff base complex may bring advancement in designing opto-electronic devices.
Moreover, this mononuclear Zn(II) complex can be employed as a metallo-ligand to develop
multidimensional structural aspects in Zn(II) based coordination compounds.
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 1900186
for the Zn(II) complex.
Acknowledgement
BB thanks Science and Engineering Research Board (SERB) for financial support under the
Teacher Associateship for Research Excellence (TARE) (No-TAR/2018/000473 dtd 01-3-

15
2019). ARC thanks the X-ray facility of the Department of Chemical Sciences, IISER Mohali
for single crystal X-ray diffraction data collection.
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20
Table 1. Crystallographic data and structure refinement parameters for Zn(II)-Schiff base
complex.
Parameters
Compound 1
Empirical formula
Formula weight
T (K)
C17H18N2O5Zn
379.70
100
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å))
0.71073
Hexagonal
P61
26.492(3)
26.492(3)
5.1701(8)
90.0
b (Å)
c (Å)
α (°)
β (°)
90.0
γ (°)
120.0
V (Å3)
3142.4(7)
6
Z
ρ (gm cm–3)
Absorption coefficient (mm–1)
F(000)
1.204
1.191
1176
Crystal size (mm3)
Index ranges (h, k, l)
Reflections collected
0.1×0.1×0.11
-38,28; -33,40; -7, 7
23830
Independent reflections
R(int)
14147
0.077
Final R indices [I>2sigma(I)]
Largest diff. peak and hole
R1 = 0.0695, wR2 = 0.1681
–3
1.49 and -0.46 e.Å

21
Table 2. Selected bond distances (Å) and bond angles (°) for Zn(II) complex.
Bond length (Å)
XRD
DFT
2.012
1.956
2.182
XRD
DFT
2.060
2.060
Zn1-O1
Zn1-O2
Zn1-O3
2.025(5)
2.017(5)
1.983(5)
Zn1-N2
Zn1-N1
2.142(7)
2.093(9)
Bond angles (˚)
XRD
DFT
96.1
XRD
DFT
94.3
O2-Zn1-O3
O1-Zn1-O2
O1-Zn1-N1
O2-Zn1-N1
O1-Zn1-O3
106.2(2)
81.80(2)
88.1(3)
O3-Zn1-N2
O3-Zn1-N1
O1-Zn1-N2
O1-Zn1-N1
N1-Zn1-N2
100.2(3)
106.5(3)
159.0(3)
88.1(3)
91.2(3)
110.3
88.1
136.1
156.1
88.1
147.0(3)
100.2 (2)
127.7
72.27
88.6

22
Table 3. Kinetic parameters for transesterification activity by Zn(II)-Schiff base complex in
aqueous methanol medium.
kcat (s-1)
Reference
Complex
Substr Medium
ate
[Zn(HL)(H2O)].(H2O)*
[Zn2L]
NPP
NPP
methanol-water
7.55×10−2
Present
46
acetonitrile-water,
476−203
[Zn2(L2)(μ-OAc)2(MeCN)2][PF6]
[MPGN-ZnII]
HPNP
BNPP
BNPP
methanol-water
water-methanol
water
3.44× 10−4
3.60× 10−5
47
48
[Zn2(LH−2)]2+
2.26 × 10−6 49
4.60 × 10−6 50
5.74 × 10−7 51
[Zn2(L2H)(AcO)(H2O)](PF6)]0.2H2O BNPP
methanol-water
water
[Zn(bpy)Cl2]
[Zn(HL2)X2]
BNPP
NPP
DMF-H2O
2-10
52
*Vmax (M s-1) = 7.5510-6 (Std. Error 2.2110-7); Km (M) = 2.2710-3 (Std. Error 5.3810-4)
Where NPP =Nitro phenyl phosphate; BNPP = Bis(4-nitrophenyl)phosphate.; HPNP = 2-
Hydroxypropyl(4-nitrophenyl)-phosphate

23
Fig 1. ORTEP diagram of Zn(II)-Schiff base complex (1) with 30% ellipsoid probability.
Fig 2. Optimized structure of Zn(II) complex in vacuum obtained using B3LYP/6-311G
theoretical model.

24
All in – All out
Zn in – All out
O in – All out
N in – All out
H in – All out
C in – All out
All in – Zn out
All in – O out
All in – N out

25
All in – H out
All in – C out
Fig 3. Fingerprint plot of Zn(II)-Schiff base complex
Fig. 4. Images of frontier molecular orbital of Zn(II) complex in IEFPCM/MeOH solvent
system.

26
Fig 5. Increase of 4-nitrophenolate band at 418 nm after addition of zinc(II)-Schiff base
complex to 100 equivalents of PNPP in aq. methanolic solution. (The spectra were recorded
after every 06 min). Inset: Absorbance vs Time plot at defined wavelength.
Fig 6. Initial rate determination for phosphoester cleavage activity by Zn(II)-Schiff base
complex towards PNPP using plot of time vs log[A/(A-At)] of the same solution.

27
Transesterification Activity by a Zinc(II)-Schiff Base Complex with
Theoretical Interpretation
Chanchal Kumar Pal,a Shreya Mahato,a Mayank Joshi,b Suvendu Paul,c Angshuman
Roy Choudhury, b Bhaskar Biswasa,*
aDepartment of Chemistry, University of North Bengal, Darjeeling-734013, India
bDepartment of Chemical Sciences, Indian Institute of Science Education and Research,
Mohali, Sector 81, Knowledge City, S. A. S. Nagar, Manauli PO, Mohali, Punjab 140306,
India
cDepartment of Chemistry, University of Kalyani, Kalyani 741235, West Bengal, India
The present study accounts on the synthesis, structural characterization, self-assembled
crystalline architecture, and transesterification study of a mononuclear zinc(II) complex,
[Zn(HL)(H2O)].(H2O) (1) containing a previously reported Schiff base ligand, H3L = N,N′-
bis(salicylidene)-1,3-diamino-2-propanol].

28
Highlights
 Synthesis and characterization of a tetrapyramidal zinc(II)-Schiff base complex
 Zn(II) complex crystallizes in hexagonal crystal system with P61 space group.
 This Zn(II)-Schiff base complex displays good transesterification activity with initial
rate constant, V(min-1) = 1.73×10-4
 Computational studies agree well with the experimental results in this course of study.
Chanchal Kumar Pal: Conceptualization, Curation, Formal analysis,
Methodology, Investigation; Shreya Mahato: Formal analysis, Investigation,
Draft manuscript preparation, Mayank Joshi: Visualization; Suvendu Paul:
Software; Angshuman Roy Choudhury: Validation; Bhaskar Biswas:
Writing-Reviewing and Editing, Supervision.

29
Conflict of Interest and Authorship Conformation Form
Please check the following as appropriate:
o
All authors have participated in (a) conception and design, or analysis and
interpretation of the data; (b) drafting the article or revising it critically for
important intellectual content; and (c) approval of the final version.
o
o
o
This manuscript has not been submitted to, nor is under review at, another journal
or other publishing venue.
The authors have no affiliation with any organization with a direct or indirect
financial interest in the subject matter discussed in the manuscript
The following authors have affiliations with organizations with direct or indirect
financial interest in the subject matter discussed in the manuscript:
Author’s name
Affiliation
Chanchal Kumar Pal
University of North Bengal, Darjeeling-734013