Oxidative cyclization and synthesis of benzoxazole derivatives and hydrolytic phosphatase activity studies on dinuclear diphenoxo-bridged zinc(II)complexes

Article abstract of DOI:10.1016/j.poly.2021.115048

Diphenoxo bridged dinuclear zinc complexes, [Zn₂(Phimp)₂(Cl)₂] (1) (PhimpH = (E)-2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)phenol), [Zn₂(Me-Phimp)₂(Cl)₂] (2) (Me-PhimpH = (E)-4-methyl-2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)phenol), [Zn₂(OMe-Phimp)₂(Cl)₂] (3) (OMe-PhimpH = (E)-4-methoxy-2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)phenol), were synthesized and spectroscopically characterized. The molecular structure of 2 was determined using X-ray crystallography. DFT and TD-DFT calculations were performed to optimize the molecular geometry, interpret the spectroscopic results and investigate the contribution of the ligands to the redox properties of the complexes. Phenoxyl radical complexes were generated in solution via chemical oxidation using ceric ammonium nitrate (CAN) and the redox properties were examined through cyclic voltammetric measurements. The hydrolysis of para-nitrophenylphosphate (PNPP) by all the dinuclear zinc complexes was investigated to mimic phosphatase activity. Catalytic oxidative cyclization by these complexes and synthesis of benzoxazole derivatives was investigated.

Full text of DOI:10.1016/j.poly.2021.115048

Polyhedron 199 (2021) 115048
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Oxidative cyclization and synthesis of benzoxazole derivatives and
hydrolytic phosphatase activity studies on dinuclear diphenoxo-bridged
zinc(II)complexes
Kapil Kumar, Virendra Kumar Chaudhary, U.P. Singh, Kaushik Ghosh
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, Uttarakhand, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Diphenoxo bridged dinuclear zinc complexes, [Zn2(Phimp)2(Cl)2] (1) (PhimpH = (E)-2-((2-phenyl-2-(pyr-
idin-2-yl)hydrazono)methyl)phenol), [Zn2(Me-Phimp)2(Cl)2] (2) (Me-PhimpH = (E)-4-methyl-2-((2-phe-
nyl-2-(pyridin-2-yl)hydrazono)methyl)phenol), [Zn2(OMe-Phimp)2(Cl)2] (3) (OMe-PhimpH = (E)-4-
methoxy-2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)phenol), were synthesized and spectroscopi-
cally characterized. The molecular structure of 2 was determined using X-ray crystallography. DFT and
TD-DFT calculations were performed to optimize the molecular geometry, interpret the spectroscopic
results and investigate the contribution of the ligands to the redox properties of the complexes.
Phenoxyl radical complexes were generated in solution via chemical oxidation using ceric ammonium
nitrate (CAN) and the redox properties were examined through cyclic voltammetric measurements.
The hydrolysis of para-nitrophenylphosphate (PNPP) by all the dinuclear zinc complexes was investi-
gated to mimic phosphatase activity. Catalytic oxidative cyclization by these complexes and synthesis
of benzoxazole derivatives was investigated.
Received 21 November 2020
Accepted 12 January 2021
Available online 31 January 2021
Keywords:
Dinuclear diphenoxo bridged Zn(II)
complexes
Crystal structure
Phenoxyl radical complex
Phosphatase activity
Oxidative cyclization and benzoxazole
synthesis
Ó 2021 Elsevier Ltd. All rights reserved.
1. Introduction
photophysical properties were also studied in a few reports [17–
19]. Akitshu and coworkers [20] utilized zinc complexes as fluores-
The zinc(II) ion, having filled d-orbitals (d10 electronic configu-
ration), exhibits some dissimilar properties compared to other first
row transition metals, such as redox inactivity and magnetic prop-
erties [1]. Interestingly, strong Lewis acidity and a capability for
ligand exchange are two important properties for this metal ion.
The parallel chemistry of zinc always helps to understand the
chemistry of copper as both possess a stable 2 + oxidation state
[2]. The chemistry of zinc containing dinuclear diphenoxo bridged
complexes has received considerable attention and these com-
plexes were synthesized and characterized for different objectives
[2–21]. First, dinuclear diphenoxo bridged complexes were utilized
for functional mimicking of phosphatase [3–8] nuclease [2,9] cate-
cholase [9,10] and phenoxazinone synthase [10] enzymes. Second,
a few research groups were interested in designing and synthesiz-
ing these complexes for polymerization reactions [11–15] because
they are found to be efficient catalysts for different polymerization
reactions. Third, a few groups were interested in synthesizing din-
uclear zinc complexes for some specific applications. In this regard
we would like to mention that Williams and co-workers investi-
gated aggregation induced fluorescence properties [16] whereas
cent probes for the CD19 antibody protein. Mugesh and coworkers
[21] are interested in studies on interactions with b-lactamase. As a
part of our ongoing research on structural and functional mimick-
ing of the galactose oxidase enzyme, we have been working with
dinuclear zinc complexes to understand the non-innocent proper-
ties of the phenolate ligand and generation of phenoxyl radical
complexes [2,10]. Investigation of the mechanism [22] indicated
that a copper coordinated phenoxyl radical could abstract a H atom
[23–25]. This prompted us to study the HAT (hydrogen atom trans-
fer) reaction of dinuclear diphenoxo bridged complexes and to try
and utilize this phenomenon for some organic transformations.
Herein, we report the synthesis and characterization of three
dinuclear diphenoxo bridged zinc complexes, [Zn2(Phimp)2(Cl)2]
(1), [Zn2(Me-Phimp)2(Cl)2] (2) and [Zn2(OMe-Phimp)2(Cl)2] (3)
derived from the ligands PhimpH, Me-PhimpH and OMe-PhimpH
respectively (shown in Scheme 2). These complexes were charac-
terised by different spectroscopic methods and the molecular
structure of [Zn2(Me-Phimp)2(Cl)2] (2) was determined by X-ray
crystallography. To investigate the hydrolytic properties of these
complexes and functional mimicking of phosphatase enzyme, we
have examined the phosphatase activity. On the other hand, the
presence of a non-innocent phenolate function prompted us to
E-mail addresses: kaushik.ghosh@cy.iitr.ac.in, ghoshfcy@iitr.ac.in (K. Ghosh)
https://doi.org/10.1016/j.poly.2021.115048
0277-5387/Ó 2021 Elsevier Ltd. All rights reserved.

K. Kumar, Virendra Kumar Chaudhary, U.P. Singh et al.
Polyhedron 199 (2021) 115048
study the synthesis of benzoxazole derivatives by dehydrogenative
cyclization. Conversion of dihydroanthracene to anthracene will be
scrutinized in light of the HAT reaction.
2. Results and discussion
2.1. Synthesis and characterization of the ligands
Scheme 2. Synthetic drawing for the synthesis of complexes 1–3.
The ligands PhimpH, Me-PhimpH and OMe-PhimpH (Scheme 1)
have been synthesized by the following literature procedure [10]
and satisfactorily characterized by IR, UV–Vis and 1H NMR spectro-
scopic studies.
using the slow evaporation method. To confirm the structure of
complex 2 and the mode of coordination of the ligand (Me-
Phimp)1ꢀ, single-crystal X-ray structure determination for com-
plex 2 was carried out. The molecular structure of the diphe-
noxo-bridged dinuclear zinc complex 2 is shown in Fig. 1. The
parameters of the metal complex are given in Table S1, whilst
selected bond angles and bond distances are described in Table S2.
The crystal structure (Fig. 1, Tables S1 and S2) reveals that com-
plex 2 adopts a dinuclear structure, consisting of two pentacoordi-
nated zinc(II) ions bridged by two phenolato functional groups. To
describe the geometry around the pentacoordinated zinc(II) centre,
2.2. Synthesis and characterization of the zinc complexes
The Schiff-base ligands were synthesized by refluxing a 1:1
mixture of the corresponding substituted aldehyde and 2-(1-
phenylhydrazinyl)pyridine in methanol [10]. Generally, yellow
coloured ligands were produced in good to excellent yields. All
three complexes were synthesized by the reaction between the
Schiff-base ligands and anhydrous zinc chloride in a 1:1 M ratio,
which yielded yellowish coloured compounds (Scheme 2). We
found that the complexes are highly soluble in organic solvents,
such as acetonitrile (ACN), dichloromethane (DCM), chloroform
and N,N-dimethylformamide (DMF). The zinc complexes were
characterized by different analytical and spectroscopic techniques.
The IR spectra of the three complexes exhibited a strong band in
the range 1628–1640 cmꢀ1 (Figs. S1, S2 and S3). These bands were
we have calculated the structural index parameter
s for complex 2
[26]. The values were found to be 0.24 and 0.01 for the Zn1 and
s
Zn2 centers respectively, and these results clearly show a distorted
square pyramidal geometry around the pentacoordinated zinc cen-
tres in complex 2. For the metal centre Zn1, one imine nitrogen
atom (N6), two bridged phenolato oxygen atoms (O1 and O2)
and and one pyridine nitrogen atom (N4) constitute the equatorial
plane, whereas the axial position is occupied by a chloride ion
(Cl1). Similarly, for the other metal centre, Zn2, one imine nitrogen
atom (N3), one pyridine nitrogen atom (N1) and two bridged phe-
nolate oxygen atoms (O1 and O2) constitute the equatorial plane,
and the other chloride ion (Cl2) occupies the axial position.
Herein, we observed that in complex 2, the two chloride ions
Cl1 and Cl2 are on the same side of the Zn-Zn axis, Consequently
the ligands (Me-Phimp)ꢀ occupy the other side of the metal ions,
giving rise to an overall unique cis configuration in complex 2.
assigned to mC=N, which suggested the coordination of these ligands
to the metal ions through the imine nitrogen atoms [10].
The UV–visible spectroscopic studies were performed to evalu-
ate the electronic properties of all the synthesized zinc complexes.
The electronic spectra of the complexes (1, 2 and 3) in acetonitrile
solvent are shown in the supporting information (Fig. S4). The
absorption bands observed in the range 300–391 nm for the com-
plexes are assigned to the n-p* and p-p* intra-ligand transitions
[10]. All the zinc complexes showed a band near 300–400 nm,
which clearly indicated ligand to the metal charge transfer transi-
tions [10].
The 1H and 13C NMR spectra of Me-PhimpH and complex 2 are
shown in Figs. S22, S23, S24 and S25 respectively. For complex 2,
the observed peaks between d 6.5 and 7.6 ppm were found to be
due to the aromatic hydrogen atoms. Multiplet signals of methy-
lene hydrogen atoms were found in the range d ꢁ 1.2–2.5 ppm.
2.3. Description of the molecular structure of complex 2
Yellow crystals of complex 2 were grown from a 1:1 solution of
acetonitrile and dichloromethane, and the crystals were obtained
Fig. 1. ORTEP diagram (40% probability level) of the complex [Zn2(Me-Phimp)2(-
Cl)2] (2). Hydrogen atoms are omitted for clarity. Selected bond lengths (Å): Zn1-
Zn2 3.1199(4), Zn1-O1 2.0217(16), Zn1-O2 2.0390(15), Zn1-N6 2.180(2), Zn1-Cl1
2.2091(8), Zn2-O1 2.0438(15), Zn2-O2 2.0208(15), Zn2-N1 2.090(2), Zn1-N4 2.0892
(18), Zn2-Cl2 2.2307(7), Zn2-N3 2.201(2).
Scheme 1. Schematic drawing of the ligands.
2

K. Kumar, Virendra Kumar Chaudhary, U.P. Singh et al.
Polyhedron 199 (2021) 115048
The axial distances Zn1-Cl1 and Zn2-Cl2 are 2.209 and 2.231 Å
respectively. The cis arrangement of the chloride ions with respect
to the Zn-Zn axis is different to the structure reported by Reedijk
et al. [26] which is consistent with the previously reported data
[2]. All the Zn-N (Npyridine as well as Nimine) distances are consistent
with the data reported by Reedijk and coworkers [26]. We have
reported that Zn-Zn distances in these types of complexes are
observed in the range 2.94–3.24 Å [10]. The Zn-Zn distance
obtained for complex 2 (3.119 Å) was found to be on the higher
side of the range. The Zn-Zn bond length in complex 2 is most sim-
ilar to the reported data of Williams and coworkers [12]. However,
the distance is higher than the values reported by Nordlander and
coworkers [7] and smaller than those reported by Mukherjee and
coworkers [4] and Sharma and coworkers [5]. For complex 2, the
Zn-O (phenolato) bond lengths are found to be in the range
2.0208 to 2.0438 Å (Table S2).
gap, corroborating the value of 3.40 eV found for complex 2. Simi-
larly, complexes 1 and 3 behave like complex 2 due to small energy
differences [10] between the HOMO and LUMO, with small shifts in
the absorption bands, and this result also correlates well with the
experimental results.
Time-dependent density functional theory (TD-DFT) was used
to evaluate the origin of the electronic properties of all the dinu-
clear zinc complexes and to explain the UV–visible spectral data.
The data obtained for complex 2 from the TD-DFT calculations
are shown in Table S5. However, the TD-DFT data of complexes 1
and 3 are given in our previous results [10]. Four main electronic
transitions were observed for complex 2. These main transitions
are (1) S0 ? S4 (H-1 ? L + 1), 375 nm, f = 0.1619, (2) S0 ? S7
(H ? L + 3), 342 nm, f = 0.0843 (3) S0 ? S9 (H-2 ? L + 1), 312 nm,
f = 0.3172 and (4) S0 ? S40 (H-3 ? L + 1), 265 nm, f = 0.1205. These
transitions can be assigned to ILCT (pp ? pp*) transitions in the
These observed distances are very similar to our previous
results [2,10], data published by Reedijk and coworkers [26] and
Das and coworkers [27]. The Zn-O-Zn bond angles of the phenoxo
bridged moiety are observed in the range 100.24–100.44°, whilst
the O-Zn-O bond angles are observed in the range 78.72–78.82°.
spectrum [10]. The probable electronic transitions of complex 2
are shown in Fig. S7.
2.5. Electrochemistry
Non-covalent interactions, hydrogen bonding networks and
p-
The redox behaviour of the zinc complexes 1–3 was investi-
gated by a cyclic voltammetric study, the data is deposited in
Table S3 and the voltammograms are displayed in Fig. S8. Com-
plexes 2 and 1 showed quasi-reversible redox waves centred at
E1\2 = 1.00 and 0.841 V vs Ag/AgCl respectively. On the other hand,
complex 3 displayed E1\2 at 0.663 and 1.13 V. These are probably
due to the successive oxidation of the phenolate function to the
phenoxyl radical [10].
stacking interactions play important roles in biology, supramolec-
ular chemistry and crystal engineering [28–30]. In complex 2,
intermolecular hydrogen bonding interactions are observed
between the aryl hydrogen atoms of the phenyl ring and the chlo-
ride ions (Fig. S5).
2.4. Computational studies
Density functional theory (DFT) calculations were performed to
find a correlation between the experimental and obtained results
from theoretical assumptions of all the dinuclear zinc complexes
by utilizing the B3LYP functional approach and the LANL2DZ basis
set. The Chemcraft and Gauss View 5.0 programs were used for the
formation of HOMO-LUMO orbitals (Fig. S6). The main purpose of
performing computational studies was to confirm the site of the
electronic distribution or electron loss on the ligand part of the
complex. Electron density distribution analysis was carried out
with the Gauss Sum software [31–32]. The frontier molecular orbi-
tal compositions and energy levels of complex 2 in the singlet
ground state are shown in Table S4, whilst the data for complexes
1 and 3 were reported in our previous results [10]. It is important
to mention that in all the zinc complexes, the electron density in
the HOMO orbital mainly originates on the ligand part and the con-
tribution from the metal centres in the HOMOs of complexes 1–3
was found to be negligible, which revealed the non-innocent beha-
viour of the ligands [10]. We also observed that the contribution of
the ligand without the phenolato ring was found to be about 34% in
all the three complexes. In the HOMO, the contribution of the phe-
nyl ring containing the phenolato function is around 56% and the
remaining 44% electron density is dispersed over the imine and
pyridine moieties for complex 2. In the LUMO state, the ligand
afforded a contribution of around 54% for the phenolato ring of
complex 2 and remaining 46% electron density is localized over
the imine and pyridine moieties. However, we have observed a
23% contribution for the phenolato ring in the LUMO state of com-
plexes 1 and 3 [10].
2.6. Generation of the phenoxyl radical
Phenoxyl radical species were generated by the addition of CAN
to an acetonitrile solution of the complexes. Generation of phe-
noxyl radicals from complexes 1–3 after the addition of CAN was
analyzed by a UV–Vis spectrophotometer. A representative spec-
trum for complex 2 is shown in Fig. 2 (A). The spectra for com-
plexes 1 and 3 are given in our previously reported results [2,10].
A UV–visible spectral studies were utilized for the detection of
phenoxyl radical complexes, by performing oxidation of complexes
1, 2 and 3 after addition of CAN. After the addition of an acetoni-
trile solution of CAN to an acetonitrile solution of complex 2, the
band at 334 nm disappeared. A new band at 300 nm and broad
band of low intensity at around 800 nm were observed. These
observations clearly indicate the formation of a phenoxyl radical
at 0 ºC [2,10].
Further, generation of phenoxyl radicals by the addition of CAN
to an acetonitrile solution of complex 2 was also confirmed by EPR
spectroscopic studies. After the addition of CAN to an acetonitrile
solution of complex 2, a sharp band near the value g = 1.997 was
obtained, that clearly indicates the generation of a phenoxyl radical
[2,10] in the solution (Fig. 2(B)).
3. Reactivity studies
3.1. Phosphatase activity
The results of these calculations, revealing that around 56% of
the electron density for the HOMO is localized for the phenyl ring
containing the phenolato function in 2, indicate that the phenol
function could afford radical generation. The LUMO states of all
Earlier we have described that the dinuclear zinc complexes 1–
3 have two chloride ions which are cis to each other, and both the
zinc centres are separated from each other by distances in the
range 3.118–3.178 Å. This prompted us to study the phosphatase
activity with all the zinc complexes utilizing the disodium salt of
the dinuclear zinc complexes originate from the p* orbital charac-
ter of the whole ligand, however orbitals L + 4 and L + 5 originate
from the ligand part, but with 0% character of the phenolato moiety
of the ligand. There is a small difference between the HOMO-LUMO
4-nitrophenyl phosphate hexahydrate (4-NPP) as a substrate
(Scheme 3). The hydrolytic tendency of complexes 1–3 was mea-
sured spectrophotometrically by monitoring the time evolution
3

K. Kumar, Virendra Kumar Chaudhary, U.P. Singh et al.
Polyhedron 199 (2021) 115048
Fig. 2. (A) Generation of the phenoxyl radical from 2 (10 ꢃ 10ꢀ5M) after addition of CAN (-1 equivalent, -2 equivalents and -3 equivalents) in acetonitrile solvent at 0 °C. (B)
EPR spectrum of complex 2 recorded at 298 K after addition of CAN in acetonitrile.
echol and o-aminophenol oxidase activity [10]. Herein, we have
further employed the HÅ abstraction tendency of the phenoxyl rad-
ical for the CAH bond. Activation of the imine substrate resulted in
benzoxazole derivatives via the formation of a CAO bond. Previ-
ously, we reported the synthesis of benzoxazole by nitric oxide
via a radical-mediated mechanism [34a] which clearly expresses
the role of radical species (phenoxyl) in the HAT (hydrogen atom
transfer) mechanism [23–25]. In search of effective conditions for
the non-toxic, environmentally benign synthesis of benzoxazole
derivatives, being homogeneously catalysed [34b,c] by complexes
1, 2 and 3 (Scheme 4), we selected the reaction of Schiff bases
derived from 2-amino phenol and substituted benzaldehydes
(1 mmol) as a model reaction to optimize the reaction conditions
(Table 2).
Scheme 3. A schematic depiction of the phosphate hydrolysis reaction of 4-NPP-
catalysed by the dinuclear zinc(II) complexes.
All the substrates, decorated with electron-withdrawing or
electron-donating groups on the aromatic ring, tolerate varieties
of functional groups. A full report on the effect of solvents, cata-
lysts, reaction time and catalyst loading on the synthesis of ben-
zoxazole is also provided in Table 2. A blank reaction without
catalyst was also performed to ascertain the role of the complexes
in the hydrogen atom abstraction; no product was detected.
We started the reaction by taking 0.2 mol% of the catalyst (com-
plex 1, 2 or 3) at 60 °C in acetonitrile solvent. These reactions,
catalysed by complexes 1, 2 and 3, yielded 48, 54 and 55% of ben-
zoxazole after 12 h. It was observed that complex 1 exhibited com-
paratively lower catalytic activity than 2 and 3. A change of the
solvent from acetonitrile to DMF-H2O (1: 9) at 60 °C, gave rise to
only 7, 11 and 13% yields of product formation for catalysts 1, 2
and 3 respectively. To shorten the time and enhance the product
yield, the reaction was performed in the presence of additives.
The addition of TEMPO enhanced the catalytic activity, which
was clearly observed from the product yields [23]. TEMPO can
act as an oxidizing agent as well as a radical scavenger. Hence,
the enhancement of the product yield in the presence of TEMPO
could be because TEMPO might generate more phenoxyl radical
species in solution by a one-electron oxidation of the phenolate
group of ligand, which in turn abstract Hꢂ from the substrate mole-
cules. Notably, only 18% yield of benzoxazole was observed in the
presence of 3 mol% of TEMPO in the absence of catalysts. However,
3 mol% of TEMPO in the presence of 0.2 mol% of catalysts 2 and 3
afforded 79–94% product yield at 60 °C after 12 h. Similarly, com-
plex 1 produced 68% yield of benzoxazole. Different solvents were
also tested, but no appreciable changes in the product yield were
observed. It was observed that the reaction at room temperature
was very slow and it took a longer time to afford benzoxazole with
of 4-nitrophenolate band at 423 nm through a wavelength scan in
the DMF/H2O solvent system [33a]. The changes in the spectral fea-
tures of 1–3 upon the addition of PNPP are shown in Fig. 3 as rep-
resentative scans.
The kinetic investigation of the zinc complexes 1–3 were car-
ried out by the initial slope method, monitoring the growth of
the p-nitrophenolate band at 423 nm as a function of time over
2 h. The initial rate constant for a particular complex and substrate
mixture was determined from the log [A
(Fig. S9) for all the zinc complexes.
a/(A
a
ꢀ At)] vs time plot
In all cases, a first-order kinetic dependence was observed at
low concentrations of 3,5-DTBC, whereas higher concentrations
resulted in saturation kinetics.
The observed rates versus concentration of substrate data were
analyzed and then based on the Michaelis Menten approach of
enzymatic kinetics (Fig. S10), the kinetic parameters Vmax, KM, Kcat
for of the dinuclear zinc complexes were investigated from a Line-
weaver-Burk (double reciprocal) plot of 1/V versus 1/[S] (Fig. S11).
All the kinetic parameters are given in Table 1.
The catalytic activity order of the complexes, 2 > 3 > 1, demon-
strates a direct correspondence to the Zn. . .Zn separation (3.170 Å
for 1, 3.119 Å for 2 and 3.155 Å for 3), showing the shorter Zn. . .Zn
separation is better more qualified to accommodate the phosphate
group [33a]. Also, the catalytic activity of complex 2 is better than
some reports available in the literature [5].
3.2. Catalytic activity of the complexes 1, 2 and 3 for the synthesis of
benzoxazole derivatives
The Hꢂ abstraction tendency of the phenoxyl radical was uti-
lized in our previous results for benzyl alcohol oxidation, and cat-
4

K. Kumar, Virendra Kumar Chaudhary, U.P. Singh et al.
Polyhedron 199 (2021) 115048
Fig. 3. (A) Wavelength scan for the hydrolysis of 4-NPP in the absence and presence of complex 2 (substrate:catalyst = 20:1) in solution (97.5% DMF and 2.5% H2O) recorded
at 25 °C after intervals of 5 min. [4-NPP] = 1 ꢃ 10ꢀ3 M; [complex 2] = 0.05 ꢃ 10ꢀ3 M, (B) for complex 1, (C) for complex 3. The arrow shows the change in absorbance with the
reaction time.
Table 1
Table 2
Kinetic parameters for the hydrolytic cleavage of PNPP catalyzed by 1–3 in aqueous
DMF at 25 °C.
Optimization of the reaction conditions for the synthesis of benzoxazole.
Entry
Catalyst
Additive
Solvent
Yield (%)
Complex
Vmax (M sꢀ1
)
Km (M)
kcat (hꢀ1
)
1
2
3
4
5
6
7
8
9
10
1
2
3
1
2
3
1
2
3
–
__
__
__
__
__
__
TEMPO
TEMPO
TEMPO
TEMPO
DMF + H2O
DMF + H2O
DMF + H2O
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
7
1
2
3
4.9 ꢃ 10ꢀ4
7.8 ꢃ 10ꢀ3
9.59 ꢃ 10ꢀ3
9.6 ꢃ 10ꢀ3
1.78 ꢃ 104
4.1 ꢃ 104
2.77 ꢃ 104
11
13
48
54
55
68
79
94
18
11.64 ꢃ 10ꢀ4
7.7 ꢃ 10ꢀ4
of phenol did not show any difference in the product yield. Elec-
tron-withdrawing substituents, such as -Cl and -NO2 groups on
the aryl ring, yielded products in excellent yields as compared to
electron-donating groups like-Me and -OMe substituted on the
aryl ring.
4. Conclusion
Scheme 4. Synthesis of benzoxazole from 2-(benzyledeneamino)phenol [cata-
lyst = zinc complexes 1, 2 and 3].
Following are the major findings and conclusions of our present
study:
1. Dinuclear zinc complexes 1–3 have been synthesized by utiliz-
ing tridentate ligands and were characterized. The molecular
structure of the representative complex 2 was determined by
X-ray crystallography.
55% and 43% yields using complexes 2 and 3 respectively after
24 h.
On increasing the catalyst loading at room temperature, the
product yield was increased to some extent.
2. DFT calculations clearly indicate the non-innocent property of
the phenolate function and phenoxyl radical complexes were
generated in solution. Complex 3 provided the most stable phe-
noxyl radical complex, whereas 1 gave rise to the least stable
one.
Under the optimized reaction conditions, different types of ben-
zoxazole derivatives were synthesized (Table 3). Herein, we used
two different derivatives of o-aminophenol, one with a substituent
at the para-position to phenol and the other one is un-substituted
o-aminophenol. Substitution of a methyl group at the para-position
5

K. Kumar, Virendra Kumar Chaudhary, U.P. Singh et al.
Polyhedron 199 (2021) 115048
Table 3
Substrate scope for benzoxazole synthesis.
Conditions: imine substrates (1 mmol), catalyst (0.2 mol%), TEMPO (3 mol%), solvent (acetonitrile 5 mL),
time = 12 h.
3. Functional mimicking of phosphatase enzyme was investigated
5.3. X–ray crystallography
by studying the hydrolysis of disodium 4-nitrophenyl phos-
phate (4-NPP).
A yellow coloured crystal of zinc complex 2 was used for the
determination of the solid-state structure. The X-ray diffraction
data of the metal complex was obtained at 273 K on a Bruker
Kappa Apex-II CCD diffractometer, with the use of graphite
4. These complexes were exploited as homogeneous catalysts for
the synthesis of benzoxazole derivatives via oxidative cycliza-
tion. The hydrogen atom abstraction ability of the phenoxyl
radical complexes were utilized to achieve the synthesis of
benoxazole derivatives from Schiff bases of benzaldehyde and
o-aminophenol. HÅ abstraction and CAH bond activation of the
imine carbon atom resulted in CAO bond formation.
monochromatic Mo-Ka radiation (k = 0.71073 Å). The structure
solution, refinement and data outputs were carried out with the
help of the SHELXTL program [36]. To refine the hydrogen and
non-hydrogen atoms, isotropic and anisotropic thermal parame-
ters were used respectively. The image was created with the help
of the MERCURY 3.7 program. The crystal data and other relevant
information are given in the ESI (Table S1).
The application of these complexes for other catalytic activities
are under progress.
5. Experimental
5.4. Computational details
5.1. Materials
Theoretical calculations were carried out for all the ligands,
namely PhimpH, Me-PhimpH and OMe-PhimpH, as well as their
binuclear zinc complexes 1, 2, and 3. DFT and TD-DFT calculations
[37–39] were carried out on the optimized geometrical structure
(Fig. S22) by utilizing the B3LYP functional approach and the
LANL2DZ basis set [40]. The TDDFT technique yielded progressively
the exact electronic excitation energies. The HOMO-LUMO were
prepared by utilizing the Gauss View 5.0 software [41]. All the
DFT calculations were performed with the assistance of the Gaus-
sian 09 W software [42]. Gauss Sum 2.1 was utilized to determine
the percentage contributions from groups or atoms in the molecu-
lar orbitals.
All the chemicals were purchased from commercial sources and
used as received. Using standard procedures, the solvents were dis-
tilled prior to use. All solvents used for spectroscopic studies were
HPLC grade. Analytical grade reagents, sodium hydride (Acros), sal-
icylaldehyde (SRL), 2-hydroxy-5-methoxylbenzaldehyde and 2-
hydroxy-5-methylbenzaldehyde were prepared according to the
literature [35]. Disodium (4-nitrophenyl)phosphatehexahydrate
(4-NPP), (Avra Synthesis) was used as purchased. The analytical
grade reagents anhydrous ZnCl2 (Merck limited Mumbai), cerric
ammonium nitrate and benzaldehyde (Himedia), and TEMPO
(Sigma Aldrich) were used as purchased.
5.2. Methods
5.5. EPR spectroscopy
Elemental analyses were carried out microanalytically with an
Elemenlar Vario EL III. Infrared spectra were recorded on a Thermo
Nicolet Nexus FTIR spectrophotometer and were obtained in KBr
pellets using 16 scans (in cm─1). Electronic absorption spectra of
all the ligands and metal complexes were obtained with an Evolu-
tion 600, Thermo Scientific UV–Vis spectrophotometer. 1H NMR
spectra were taken in deuterated solvents on a Bruker AVANCE,
500.13 MHz spectrometer. EPR spectra were taken on a Bruker
EMX EPR spectrometer. The cyclic voltammetric study was per-
formed on a CH-600 electro analyzer in dichloromethane solution
with 0.1 M tetrabutylammonium perchlorate (TBAP) as the sup-
porting electrolyte. The working electrode, reference electrode
and auxiliary electrode were a glassy carbon electrode, Ag/AgCl
electrode and Pt wire respectively. The concentration of the com-
pound was ꢄ10ꢀ3 M. The ferrocene/ferrocenium couple appeared
X-band EPR spectra were obtained on a Bruker EMX EPR spec-
trometer with the following spectrometer settings: microwave fre-
quency, 9.44 GHz; microwave power, 1 mW; modulation
frequency, 100 kHz; modulation amplitude, 5G.
6. Synthesis and characterization of the ligands and metal
complexes
6.1. Syntheses of the ligands
The ligands PhimpH, Me-PhimpH and OMe-PhimpH were syn-
thesized by the previously reported experimental method [33b].
The substituted salicylaldehydes were synthesized by the formyla-
tion of the corresponding phenols using a previously reported pro-
cedure [35]. The NMR spectra of the ligand Me-PhimpH are shown
in the supporting file (Figs. S23-S24).
at E1/2/V(
DEp/mV) = +0.46(143) vs. Ag/AgCl (scan rate 0.1 V/s) in
dichloromethane under the same experimental conditions.
6

K. Kumar, Virendra Kumar Chaudhary, U.P. Singh et al.
Polyhedron 199 (2021) 115048
6.2. Syntheses of the metal complexes
nitrophenolate at 424 nm [43]. After three days the final A values
1
for each set were obtained.
6.2.1. Synthesis of [Zn2(Phimp)2(Cl)2] (1)
To a solution of PhimpH (0.145 g, 0.5 mmol) in acetonitrile
(10 mL), sodium hydride (0.012 g, 0.5 mmol) was added consecu-
tively in acetonitrile, and the resulting solution was stirred for
40 min. A solution of anhydrous ZnCl2 (0.085 g, 0.5 mmol) in ace-
tonitrile was then added to the reaction mixture with constant stir-
ring. After 8 h stirring, a yellow-colored complex precipitated out.
The solvent was evaporated and the solid was separated by filtra-
tion. A yellow-colored compound resulted after redissolving the
solid in methanol, then layering this solution with hexane. This
compound was washed with diethyl ether and dried under air.
Yield: 76%. Anal. Calcd for C36H28N6Cl2O2Zn2: C, 55.55; H, 3.63;
N, 10.80. Found: C, 55.38; H, 3.72; N, 10.64. IR (KBr pellets,
6.4. General procedure for benzoxazole synthesis
The Schiff base substrate (1 mmol) was dissolved in 5 mL ace-
tonitrile. To this solution, 0.1 mol % of catalyst was added, then
3 mol% of TEMPO was added at room temperature. The reaction
mixture was stirred at 60 °C for 12 h. The solvent was then evapo-
rated, followed by addition of ethyl acetate. The reaction mixture
was filtered and the filtrate was purified by column chromatogra-
phy using silica gel (mesh size 60–120) and hexane as the eluent.
The product yield was calculated based on the yield obtained after
column chromatography. The NMR spectra of the compounds 3a–
3e, formed after catalysis, are shown in Figs. S12–S21.
cmꢀ1):
m(C@N), 1637, 1571, 1483, 1440, 1346, 1306, 1218, 1146,
1056, 958, 879, 811, 772, 697, 564. UV–vis (CH3CN kmax, nm (e,
in Mꢀ1 cmꢀ1)): 335 (50939), 307 (35606), 235 (28290).
6.5. NMR of the benzoxazole derivatives
6.2.2. Synthesis of [Zn2(Me-Phimp)2(Cl)2] (2)
2-Phenylbenzoxazole (3a): White solid. 1H NMR (500 MHz,
CDCl3) d, ppm: 8.27 (m, 2H), 7.79–7.77 (m, 1H), 7.60–7.58 (m,
1H), 7.54–7.53 (m, 3H), 7.36 (m, 2H). 13C NMR (126 MHz, CDCl3)
d, ppm: 163.0, 150.7, 142.1, 131.5, 128.9, 127.6, 127.2, 125.1,
124.5, 120.0, 110.6.
This compound was prepared by following the same procedure
as described above for 1, except using Me-PhimpH instead of
PhimpH. The NMR spectra of complex 2 are shown in the support-
ing file (Fig. S25-S26). Yield: 72%. Anal. Calcd for C38H32N6Cl2O2-
Zn2: C, 56.60; H, 4.00; N, 10.42. Found: C, 56.47; H, 4.23; N,
6-Methyl-2-phenylbenzoxazole (3b): White solid. 1H NMR
(400 MHz, CDCl3) d, ppm: 8.39 (q, J = 8.9 Hz, 4H), 7.60 (s, 1H),
7.50 (d, J = 8.3 Hz, 1H), 7.24 (d, J = 8.5 Hz, 1H), 2.51 (s, 3H). 13C
NMR (126 MHz, CDCl3) d, ppm: 157.1, 152.2, 142.3, 135.7, 133.3,
129.6, 128.8, 128.6, 120.0, 115.8, 114.9, 21.6.
10.32. IR (KBr pellets, cmꢀ1):
m
(C@N), 1633, 1562, 1480, 1423,
1331, 1315, 1290, 1021, 851, 752, 700, 554. UV–vis (CH3CN kmax,
nm (
, in Mꢀ1 cmꢀ1)): 373 (29444), 332 (36666), 301 (38888),
e
246 (48666).
6-Methoxy-2-phenylbenzoxazole (3c): White solid. 1H NMR
(500 MHz, CDCl3) d, ppm: 7.97 (m, 3H), 7.57 (t, J = 7.4 Hz, 2H),
7.46 (t, J = 7.7 Hz, 3H), 4.34 (s, 3H). 13C NMR (126 MHz, CDCl3) d,
ppm: 153.35, 146.79, 139.66, 134.82, 132.65, 131.02, 130.80,
130.22, 122.45, 115.83, 109.30, 20.00.
6.2.3. Synthesis of [Zn2(OMe-Phimp)2(Cl)2] (3)
This compound was prepared by following the same procedure
as described above for 1, except using OMe-PhimpH instead of
PhimpH. Yield: 71%. Anal. Calcd for C38H32N6Cl2O4Zn2: C, 54.44;
H, 3.85; N, 10.02. Found: C, 54.10; H, 3.63; N, 10.35. IR (KBr pellets,
6-Chloro-2-phenylbenzoxazole (3d): White solid. 1H NMR
(500 MHz, CDCl3) d, ppm: 7.95–7.98(m, 3H), 7.58(t, J = 7.6 Hz,
2H), 7.47 (t, J = 7.6 Hz, 1H). 13C NMR (125 MHz, CDCl3) d, ppm:
162.02,150.72, 141.08, 137.73, 129.22, 128.81, 125.30, 124.61,
120.05, 110.57.
cmꢀ1):
(CH3CN kmax, nm (
m
(C@N), 1628, 1564, 1483, 1414, 1356, 1022, 652. UV–vis
e
, in Mꢀ1 cmꢀ1)): 391 (16451), 333 (22548),
303 (22225), 246 (28290).
6.3. Phosphatase activity
6-Nitro-2-phenylbenzoxazole (3e): White solid. 1H NMR
(500 MHz, CDCl3) d, ppm: 8.44 (d, J = 8.7 Hz, 2H), 8.39 (d,
J = 8.7 Hz, 2H), 7.83 (d, J = 7.2 Hz, 1H), 7.64 (d, J = 7.8 Hz, 1H),
7.46–7.40 (m, 2H). 13C NMR (126 MHz, CDCl3) d, ppm: 160.6,
151.0, 141.9, 132.8, 128.4, 126.3, 125.2, 124.2, 120.7, 115.0, 110.9.
The dinuclear metal complexes show efficient phosphatase
activity. Para-nitrophenylphosphate (PNPP) was utilized as the
model substrate to explore the phosphatase activity. All experi-
ments were carried out in 97.5% DMF/H2O medium [43,44]. Solu-
tions of the substrate 4-NPP and the zinc complexes were freshly
prepared, and the total volume maintained was 3 mL. An initial
screening of the hydrolytic tendency of all the metal complexes
was performed until 2% formation of 4-nitrophenolate was reached
and then the kinetic data were collected. The rate of hydrolysis of
PNPP in the presence of zinc complexes 1–3 was measured spec-
trophotometrically by monitoring the UV absorption of the p-
nitrophenolate anion at 423 nm as a function of time at 25 °C
[44]. The investigation contained five sets, having a complex con-
centration of 0.05 mmol and substrate concentrations of 0.5 (10
equiv.), 0.7 (14 equiv.), 1.0 (20 equiv), 1.2 (24 equiv.) and 1.5 (30
equiv.) mmol. The stock solutions of 2 mM of the complexes and
the substrate were set up in a DMF/H2O mixture. The reactions
CRediT authorship contribution statement
Kapil Kumar: Conceptualization, Methodology. Virendra
Kumar Chaudhary: . U.P. Singh: . Kaushik Ghosh: Supervision,
Conceptualization, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
were performed by injecting 50 lL of the complex and various vol-
umes of the substrate (0.5, 0.74, 1.1, 1.34 and 1.7 mL) in a cuvette
and making the final volume up to 3 mL with solvent. After com-
plete mixing, the spectra were recorded at 25 °C, over 2 h at regular
time intervals of 3 min, for all complexes. All the experiments were
carried out at least twice, and average values were taken. The reac-
tions were corrected for the extent of ionization of 4-nitropheno-
late at 25 °C, utilizing the molar extinction coefficient for 4-
Acknowledgements
K.G. is thankful to CSIR, New Delhi, India for financial assistance
(01(2942)/18/EMR-II dated 01-05-2018). K.K. is thankful to UGC
and V.K.C. is thankful to CSIR, India, for financial assistance. We
are thankful for the Central Instrumental Facility, IIT Roorkee.
7

K. Kumar, Virendra Kumar Chaudhary, U.P. Singh et al.
Polyhedron 199 (2021) 115048
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