Structural elucidation, DFT calculations and catalytic activity of dioxomolybdenum(VI) complexes with N–N donor ligand: Role of halogen atom coordinated to the molybdenum centre

Article abstract of DOI:10.1016/j.poly.2021.115144

Two new isostructural mononuclear dioxomolybdenum(VI) complexes of the formula MoO₂X₂L [where, X = Cl (1), Br (2)] have been synthesized with a N–N donor 2-(3-methyl-5-phenyl pyrazol-1-yl) benzthiazole ligand (L). The reaction is carried out in open air and the Mo^VO³⁺ centre in the precursor molecule, MoOX₃L undergoes spontaneous aerial oxidation, leading to the formation of molybdenum(VI) complexes 1 and 2. The complexes are characterized by a wide range of spectroscopic techniques (IR, UV–Vis and ¹H NMR) and elemental analyses. Crystal structures of the ligand and complexes 1 and 2 have been determined by single crystal X-ray diffraction which reveal a distorted octahedral geometry around the molybdenum(VI) centre in both the complexes. The ligand and the complexes build up fascinating supramolecular assembly via several non-covalent interactions including hydrogen bonding, C–H···π and π···π interactions. Further, a detailed study of Hirshfeld surface analysis and fingerprint plots of complexes 1 and 2 are presented for understanding the intermolecular interactions involved in building self-assembled frameworks. Supportive DFT and TD-DFT calculations have also been carried out. Electrochemical properties of the complexes have been examined by cyclic voltammetry. Catalytic performance of the synthesized complexes has been evaluated for the oxidation of different olefins in the presence of hydrogen peroxide.

Full text of DOI:10.1016/j.poly.2021.115144

Polyhedron 200 (2021) 115144
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Structural elucidation, DFT calculations and catalytic activity
of dioxomolybdenum(VI) complexes with NAN donor ligand:
Role of halogen atom coordinated to the molybdenum centre
c
d
Malini Roy a, Debanjana Biswal a, Nikhil Ranjan Pramanik b, , Michael G.B. Drew , Suvendu Paul ,
⇑
Payal Kachhap e, Chanchal Haldar e, Syamal Chakrabarti a,
⇑
a Department of Chemistry, University College of Science, 92, Acharya Prafulla Chandra Road, Kolkata:700009, West Bengal, India
b Department of Chemistry, Bidhannagar College, EB-2, Sector-1, Salt Lake, Kolkata, 700064, India
c Department of Chemistry, The University of Reading, Whiteknights, Reading RG6 6AD, UK
d Department of Chemistry, University of Kalyani, Kalyani, Nadia, West Bengal- 741235, India
e Department of Applied Chemistry, Indian Institute of Technology (ISM), Dhanbad, Dhanbad-826004, Jharkhand, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new isostructural mononuclear dioxomolybdenum(VI) complexes of the formula MoO2X2L [where,
X = Cl (1), Br (2)] have been synthesized with a NAN donor 2-(3-methyl-5-phenyl pyrazol-1-yl) benzthi-
azole ligand (L). The reaction is carried out in open air and the MoVO3+ centre in the precursor molecule,
MoOX3L undergoes spontaneous aerial oxidation, leading to the formation of molybdenum(VI) com-
plexes 1 and 2. The complexes are characterized by a wide range of spectroscopic techniques (IR, UV–
Vis and 1H NMR) and elemental analyses. Crystal structures of the ligand and complexes 1 and 2 have
been determined by single crystal X-ray diffraction which reveal a distorted octahedral geometry around
the molybdenum(VI) centre in both the complexes. The ligand and the complexes build up fascinating
Received 3 January 2021
Accepted 2 March 2021
Available online 10 March 2021
Keywords:
Dioxomolybdenum(VI)
NAN donor ligand
Crystal structure
Hirshfeld surface analysis
Catalytic activities
supramolecular assembly via several non-covalent interactions including hydrogen bonding, CAHꢀꢀꢀ
p
and interactions. Further, a detailed study of Hirshfeld surface analysis and fingerprint plots of com-
pꢀꢀꢀp
plexes 1 and 2 are presented for understanding the intermolecular interactions involved in building self-
assembled frameworks. Supportive DFT and TD-DFT calculations have also been carried out.
Electrochemical properties of the complexes have been examined by cyclic voltammetry. Catalytic per-
formance of the synthesized complexes has been evaluated for the oxidation of different olefins in the
presence of hydrogen peroxide.
Ó 2021 Elsevier Ltd. All rights reserved.
1. Introduction
Benzthiazole nucleus finds its applications in biological activi-
ties viz. anticancer [14], antimicrobial [15] and fungicidal activities
Molybdenum chemistry has been mostly investigated in recent
years due to the presence of molybdenum in metalloenzymes i.e.,
nitrogenase [1], hydroxylases [2] and oxotransferases [3]. It also
plays a vital role in many of earth’s biochemical cycle [4–7].
Another important aspect of molybdenum complexes is their effi-
ciencies in the industrial catalytic processes. They show good cat-
alytic performances in some organic transformations such as olefin
metathesis [8], oxidation of alcohols [9], alkanes [10], ammoxida-
tion of propene [11], epoxidation of olefins [12] etc. since molybde-
num possess a large number of accessible oxidation states [13] as
well as coordination numbers.
[16]. Further, substituted pyrazoles also exhibit a promising role as
anticancer [17], insecticidal [18] and effective antibacterial agents
[19]. Metal complexes with specific NAN donor ligands derived
from benzthiazole and 2-pyridyl derivatives have been previously
studied [20,21].
During the previous years, there has been increasing interest in
transition metal complexes containing neutral NAN donor ligands
[22,23]. Despite the great importance of molybdenum complexes
with multidentate ligands, only a few articles have been published
in which molybdenum(VI) complexes are coordinated with neutral
bidentate NAN donor ligands [24–26].
In accordance with the versatile ligational properties of both
benzthiazole and pyrazole, attention has been focused on the syn-
⇑
Corresponding authors.
E-mail addresses: nr_pramanik@yahoo.co.in (N.R. Pramanik), schakrabar-
ti2014@gmail.com (S. Chakrabarti).
https://doi.org/10.1016/j.poly.2021.115144
0277-5387/Ó 2021 Elsevier Ltd. All rights reserved.

M. Roy, D. Biswal, Nikhil Ranjan Pramanik et al.
Polyhedron 200 (2021) 115144
thesis of 2-(3-methyl-5-phenyl pyrazol-1-yl) benzthiazole (L) and
its mononuclear oxomolybdenum(VI) complexes.
sor Mo(V) molecule MoOX3L in DCM solvent for 2 h in open air
(Scheme 1). The filtrate was then slowly and carefully layered with
n-hexane. Yellow colored needle shaped shiny crystals suitable for
X-ray diffraction appeared on the wall of the tube after one week.
MoO2Cl2L (1): Anal. Calcd. for C17H13N3SO2Cl2Mo (%): C, 41.63;
H, 2.65; N, 8.57; Mo, 19.59, Found: C, 41.95; H, 2.55; N, 8.51; Mo,
The construction of coordination complexes with new network
motifs is of interest for the development of new functional materi-
als and in fundamental studies of crystal engineering and
supramolecular chemistry. Hydrogen bonds are the most reliable
directional interaction in supramolecular construction. The struc-
tural complexity and solid state stability of the complexes can be
19.02; IR (KBr Pellet), cmꢁ1
:
m(C@N) 1521(s),
m
(Mo=O) 966(s), 980(vs),
m(Mo-N) 692(m); UV–Vis (CH2Cl2) [kmax /nm (
e
/dm3mol-1cmꢁ1)]:
explained by hydrogen bonding, CAHꢀꢀꢀ
p
interactions and
pꢀꢀꢀ
p
231 (11080), 286 (9289), 356(sh) (2080); 1H NMR (DMSO d6, d/
ppm): Aromatic protons 6.48–7.95 m (10H).
interactions.
Hirshfeld surface analysis facilitates a comparison of inter-
molecular interactions which are crucial for building of various
supramolecular architectures in the compounds. It is also a novel
way for polymorph analysis [27].
MoO2Br2L (2): Anal. Calcd. for C17H13N3SO2Br2Mo (%): C, 35.23;
H, 2.24; N, 7.25; Mo, 16.58, Found: C, 35.62; H, 2.20; N, 7.21; Mo,
16.02; IR (KBr Pellet), cmꢁ1
: m(C@N) 1511(s), m(Mo=O) 914(vs), 942
(vs), m(Mo-N) 695(m); UV–Vis (CH2Cl2) [kmax /nm (e
/dm3mol-
As a part of our study on molybdenum complexes, in the pre-
sent paper we have described the synthesis of mononuclear diox-
omolybdenum(VI) complexes having general formula MoO2X2L
[where, L = NAN donor 2-(3-methyl-5-phenyl pyrazol-1-yl) ben-
zthiazole ligand, X = Cl (1), Br (2)]. The investigated complexes
have been characterized by elemental analyses, spectroscopic
techniques (IR, UV–Vis, 1H NMR) and cyclic voltammetry. The crys-
tal structures of the ligand (L) and complexes 1 and 2 have - been
determined by single crystal X-ray diffraction. The crystal struc-
tures also give rise to various intriguing supramolecular architec-
tures involving interesting non-covalent interactions such as
1cmꢁ1)]: 230 (7290), 251 (5470), 303 (6444), 375 (sh) (1069); 1H
NMR (DMSO d6, d/ppm): Aromatic protons 6.46–7.93 m (10H).
2.3. Physical measurements
Elemental analyses were performed on a Perkin-Elmer 240C, H,
N analyzer. NMR spectra were recorded on a Bruker 300 L NMR
spectrometer operating at 300 MHz with TMS as internal standard.
IR spectra were recorded as KBr pellets on a Perkin-Elmer model
883 infrared spectrophotometer. Electronic spectra were recorded
using a HITACHI U-3501 UV–Vis recording spectrophotometer.
Magnetic susceptibility was measured with a PAR model 155
vibrating sample magnetometer with Hg [Co(SCN)4] as calibrant.
Electrochemical data were collected on a Sycopel model AEW2
1820F/S instrument at 298 K using a Pt working electrode, Pt aux-
iliary electrode and SCE reference electrode. Cyclic voltammo-
hydrogen bonding, CAHꢀꢀꢀ
p
interactions and
pꢀꢀꢀp interactions
which are further supported by Hirshfeld surface analysis and
associated to the finger print plots. Supportive DFT calculations
have also been carried out to compare calculated parameters with
the experimental data. The vertical electronic transitions for the
ligand and complexes in dichloromethane solvent are computed
by the TD-DFT/CPCM method which gives detailed information
concerning the absorption bands. The dioxomolybdenum(VI) com-
plexes are found to exhibit significant catalytic activities in the oxi-
dation of alkenes.
grams were recorded in DMF containing 0.1
supporting electrolyte.
M TBAP as
2.4. Crystallographic measurements
The crystallographic data for the ligand and complex 2 were
collected on a Bruker APEX-II diffractometer with CCD-area detec-
tor at 296(2) K and for complex 1 was collected on an Oxford
Diffraction X-Calibur diffractometer at 150(2) K. All data sets were
2. Materials and method
2.1. Materials
obtained with graphite-monochromated Mo-K
a (k = 0.71073 Å)
radiation. Data analysis for ligand and complex 2 were carried
out with SAINT [29] and for 1 with the CrysAlis program [30].
The crystal structures were solved by direct methods using
SHELXS-97 [31] and refined by full-matrix least-squares based on
F2 with anisotropic displacement parameter of non-hydrogen
atoms using SHELXL-2016/16 [31]. Hydrogen atoms were included
in calculated positions with thermal parameters equal to 1.2 times
that of the atom to which they were bonded. The MERCURY [32]
and DIAMOND [33] programs were used for the presentation of
the structures.
Reagent grade solvents were dried and distilled prior to use. All
other chemicals used for preparative work were of reagent grade,
available commercially and used without further purification.
2.2. Methods
2.2.1. Synthesis of the ligand
The ligand 2-(3-methyl-5-phenyl pyrazol-1-yl) benzthiazole (L)
was synthesized by the method reported previously [24,28]. White
needle shaped crystals were isolated from the filtrate which were
satisfactorily characterized by elemental analyses, various spec-
troscopy (IR, UV–Vis, 1H NMR and mass) and single crystal X-ray
diffraction.
2.5. DFT calculations
All calculations were carried out using DFT/B3LYP methodology
with the Gaussian G03 program [34]. Basis sets used were
LANL2DZ for Mo and Br, 6–31 + G* for Cl, S, N, O and 6-31G for
C, H. Geometry optimizations were carried out using the crystal
structures as starting models. Electronic transitions were obtained
using the optimized structures with dichloromethane as solvent
using the TD method.
2.2.2. Synthesis of the complexes
The dioxomolybdenum(VI) complexes of the general formula
MoO2X2L [where, L = 2-(3-methyl-5-phenyl pyrazol-1-yl) benzthi-
azole; X = Cl (1) and Br (2)] were obtained by refluxing the precur-
2.6. Hirshfeld surface analysis
It is already established that analysis of 3D Hirshfeld surfaces
[35] and 2D fingerprint plots [36] are inevitable and user-friendly
tools to redeem intermolecular interactions between the different
Scheme 1. Reaction diagram for the preparation of complexes.
2

M. Roy, D. Biswal, Nikhil Ranjan Pramanik et al.
Polyhedron 200 (2021) 115144
asymmetric units within a crystal [37–40]. The 3D Hirshfeld sur-
faces [35] and 2D fingerprint plots [36] are generated employing
Crystal Explorer 17.5 [37]. The distance from the point on the iso-
surface to the nearest nucleus of atom outside and inside the sur-
face could be denoted by de and di respectively [35,38]. In addition,
the normalised contact distance (dnorm) could be well articulated
with de and di by the equation below:
trated in Fig. S1. A strong band assigned as m(C@N) stretching vibra-
tion near 1562 cmꢁ1 for ligand is red shifted to 1521 cmꢁ1 and
1511 cmꢁ1 in the corresponding complexes 1 and 2 respectively
indicating coordination of N atom to the metal centre [43,44].
The complexes exhibit two very strong intensity bands in the range
914–980 cmꢁ1 characteristic of symmetric and anti-symmetric
stretching vibrations of the cis-[MoO2]2+ moiety [45]. Other med-
ium intensity bands are found in the range 692–695 cmꢁ1 which
can be attributed to the m(Mo=N) stretching mode [46].
v
dW
v
dW
di ꢁ ri
de ꢁ re
dnorm
¼
þ
v
dW
v
dW
re
ri
Where, rvi dw and rvedw are the van der Waals radii of the atoms.
The magnitude of dnorm may be either positive or negative deter-
mined by the intermolecular contacts either longer or shorter than
the van der Waals separations respectively [41]. The red, white and
blue colors based on dnorm values denote the shorter contacts (ma-
jorly hydrogen bonding), contacts close to van der Waals separa-
3.3. Electronic spectra
Electronic spectra of the dioxomolybdenum(VI) complexes have
been recorded in dry dichloromethane and the spectral data are
given in the experimental section. Electronic spectra of the com-
plexes 1 and 2 are presented in Fig. S2. The complexes exhibit
strong bands in the range 286–303 nm which correspond to N
tion (majorly CHꢀꢀꢀ
p interactions) and no intermolecular
interaction respectively.
(p
from filled HOMO of the ligand, primarily N(p
empty LUMO having Mo(d ) character. Another band in the range
p
) ? Mo(dp) LMCT transitions [47] by promotion of an electron
p) in character, to the
p
2.7. Catalytic activity studies
231–251 nm may be attributed to intra-ligand charge transfer
transition [48]. The absence of d-d transition absorption bands in
the visible region confirms the presence of 4d0 electronic configu-
ration of Mo(VI).
2.7.1. Instrumentation and characterization procedures
Progress of the catalytic reaction was monitored by an Anagi-
lent 7890B gas-chromatograph fitted with a HP-5 capillary column
(30 m ꢂ 0.25 mm ꢂ 0.25
lm) and a FID detector. % conversions
3.4. 1H NMR spectra
were calculated by using calibration plot. Identity of the various
oxidation products of alkenes were identified by employing a
thermo scientific GC–MS (Model No. Trace 1300, ISQ QD) fitted
Further confirmation regarding the characterization of the com-
plexes can be obtained from 1H NMR spectral studies. 1H NMR
spectra of the complexes 1–2 were recorded in DMSO d6 solvent.
The spectral data are given in experimental section. In both the
complexes, aromatic protons appear as multiplets within the range
d 6.46–7.95 ppm.
with a TG-5MS capillary column (30 m ꢂ 0.25 mm ꢂ 0.25
lm)
and an EI+ mass detector.
3. Results and discussions
3.1. Synthesis
3.5. Electrochemistry
The NAN donor ligand, 2-(3-methyl-5-phenyl pyrazol-1-yl)
benzthiazole was prepared by the method reported previously
[28]. The ligand was satisfactorily characterized by elemental anal-
yses, IR, 1H NMR, mass spectrometry and single crystal X-ray
diffraction.
Cyclic voltammograms of the complexes 1–2 were recorded at a
Pt electrode in dry degassed DMF containing 0.1 (M) TBAP as sup-
porting electrolyte over potential range 0.00 1.50 V vs SCE. Cyclic
voltammetric results are shown in Table 1 and the voltammograms
are presented in Fig. 1 and Fig. S3. The complexes exhibit two irre-
versible one electron reduction processes in the potential range –
0.70 V to – 0.72 V and – 1.45 V to – 1.48 V respectively correspond-
ing to MoVI/MoV and MoV/MoIV couples respectively [48]. On scan
reversal, an irreversible one step two electron oxidation peak
was observed in the region + 0.75 V to + 1.09 V which is attributed
to MoIV/MoVI process [49]. It should be noted that the reduction of
dioxo Mo(VI) complexes in aprotic solvents is generally
irreversible.
Oxomolybdenum(V) complexes of the type MoOX3L (X = Cl, Br)
were initially prepared according to the literature method [28].
Dioxomolybdenum(VI) complexes 1 and 2 were synthesized by
refluxing the parent oxomolybdenum(V) complexes in dichloro-
methane solvent by aerial oxidation. Dichloromethane solution of
the complexes were layered with n-hexane and the crystals of
dioxomolybdenum(VI) complexes of the formula MoO2X2L have
been isolated after one week. The compounds are air stable in solid
state. They are readily soluble in methanol, CH2Cl2, CH3CN, DMF
and DMSO but are insoluble in water. The complexes are diamag-
netic at room temperature as is expected for d0 Mo(VI) centre [42].
All the complexes are satisfactorily characterized by spectroscopy
(IR, UV–Vis and 1H NMR spectroscopy) and cyclic voltammetry.
The ligand and the complexes have been structurally characterized
from single crystal X-ray data. Supportive DFT calculations have
also been carried out for all the compounds. Various non-covalent
interactions and their comparative dimensions in the two com-
plexes (1 and 2) have been analyzed by Hirshfeld surface analysis.
The complexes show good catalytic efficiencies towards oxidation
of alkenes.
3.6. Description of crystal structures
The molecular structure of the ligand and the complexes (1 and
2) with atom numbering schemes are presented in Fig. 2 and Figs. 4,
5. Crystallographic data, related bond angle and bond distances
Table 1
Cyclic voltammetric resultsa (V vs SCE) for the complexes 1–2 at 298 K.
Complex
E
(V)
E
(V)
pa
pc
MoVI/MoV
MoV/MoIV
MoIV/MoVI
MoO2Cl2L (1)
MoO2Br2L (2)
–0.72
–0.70
–1.48
–1.45
+1.09
+0.75
3.2. IR spectra
a
Solvent: DMF (dry, degassed); supporting electrolyte: 0.1 M TBAP; solution
strength: 10-3 M; working electrode: platinum; reference electrode: SCE; scan rate:
200 mVsꢁ1
Characteristic IR bands of the ligand and the complexes 1 and 2
are listed in the experimental section and the IR spectra are illus-
.
3

M. Roy, D. Biswal, Nikhil Ranjan Pramanik et al.
Polyhedron 200 (2021) 115144
CAHꢀꢀꢀ
p
interaction and two
p
ꢀꢀꢀ
p
stacking interactions (Table 3).
Intramolecular hydrogen contacts of C(36)-H(36)ꢀꢀꢀN(11) (black
dotted line) from the nine different molecules stabilize the ligand
framework by the formation of a seven-membered ring. Two mole-
cules are packed together via two CAHꢀꢀꢀ
p interactions involving C
(36F)-H(35F) and C(35F)-H(36F) with the phenyl ring of benzoyl
acetone moiety in molecule D (blue dotted lines). The crystal struc-
ture also reveals interesting
line). The interactions are further facilitated by the electron
ꢀꢀꢀ
withdrawing effect of N and S atoms in both benzthiazole and
pyrazole moieties which decreases the -electron repulsion of
pꢀꢀꢀ
p stacking interactions (red dotted
p
p
p
the rings by reducing the electron density of the corresponding
rings [50]. A packing diagram showing these contacts is presented
in Fig. 3.
3.6.2. Crystal structures of complexes 1 and 2
Both the yellow colored complexes 1 and 2 are isostructural and
crystallize with Pna21 space group. In the complexes 1 and 2, the
Mo(VI) centre adopts a distorted octahedral geometry consisting
of two cis-oxo atoms O(1), O(2), two halogen atoms X(1), X(2)
(X = Cl, Br) and two nitrogen atoms N(11) and N(24) from the
ligand (L). One oxo-oxygen atom O(2), two halogen atoms X(1), X
(2); and one nitrogen atom N(11) from benzthiazole moiety occupy
the equatorial plane. Another oxo-oxygen O(1), and nitrogen atom
N(24) from the pyrazole moiety occupy the mutually trans apical
positions [24]. Upon coordination with the ligand, a five-mem-
bered metallocycle is formed with bite angle N(11)-Mo(1)-N(24)
being 68.33(17)° and 67.8(3)° for complexes 1 and 2 respectively
[24]. The Mo(1)-O(1) and Mo(1)-O(2) bond distances are 1.684(5)
Å, 1.682(4) Å for complex 1 and 1.677(7), 1.690(6) for complex 2
respectively corresponding to Mo-Ot double bonds [51]. In both
the complexes, Mo(1)-N(11) and Mo(1)-N(24) bonds are apprecia-
bly elongated than the normal Mo-N single bond distance due to
trans influence of the strongly bound oxo-oxygen atoms O(1), O
(2). The Mo(1)-X(1) and Mo(1)-X(2) bond distances are closer to
Mo-Cl and Mo-Br single bond distances in the corresponding com-
plexes [24]. The bond angle X(1)-Mo(1)-X(2) between two mutu-
ally trans halogen atoms are found to be 158.69(7)° and 159.16
(5)° for complexes 1 and 2 respectively which largely deviate from
linearity. The bond angle O(1)-Mo(1)-N(24) being 161.8(2)° and
161.6(3)° for complexes 1 and 2 respectively, may be visualized
as the stacking effect of the complex molecules along the apical
axis. The O(1)-Mo(1)-O(2) bond angle is increased to 105.1(2)°-
105.9(4)° [52,53], while the opposite bite angle is reduced to
68.33(17)° and 67.8(3)°. In complexes 1 and 2, the molybdenum
centres are displaced by 0.297 Å and 0.304 Å respectively, from
the equatorial plane described by O(2), X(1), X(2) and N(11) in
the direction of terminal oxo-oxygen atom O(1).
Fig. 1. Cyclic voltammogram of complex 1 in DMF at 298 K.
Fig. 2. Molecular structure of ligand C. There are nine molecules in the asymmetric
unit with similar structures.
along with hydrogen bonding geometries are listed in Tables 2–6,
S1–S3
3.6.1. Crystal structure of the ligand
In complex 1, an infinite 1D chain is formed through C(22)-H
(22)ꢀꢀꢀO(1), C(25)-H(25B)ꢀꢀꢀO(1) and C(25)-H(25B)ꢀꢀꢀCl(1) hydrogen
bonding interactions (blue dotted lines) (Fig. 6, Table 6). The oxy-
gen atom O(1) acts as multiple proton acceptor. Two hydrogen
atoms from benzthiazole moiety and one hydrogen atom from
2-(3-methyl-5-phenyl pyrazol-1-yl) benzthiazole (L) behaves as
a neutral bidentate NAN donor ligand. It crystallizes in the mono-
clinic space group P21/c. The crystal structure consists of nine
molecules in the asymmetric unit with similar geometries. The
molecules are distinguished via the letters A to I inclusive but have
a common numbering scheme as shown in Fig. 2. Selected bond
distances and bond angles are given in Tables S1–S3. The N(11)-C
(19)-N(20)-C(21) angle ranges between –1.5(9) to –10.3(8) in the
nine molecules. During complexation with molybdenum, the ben-
zthiazole ring rotates about the C(19)-N(20) bond by an angle
of ~ 180° to become a NAN donor ligand similar to 2,20-bipyridine.
The orientation of the phenyl ring varies somewhat in all mole-
cules, no doubt due to packing effects, with the N(20)-C(21)-C
(31)-C(36) angle ranging from 48.1(7) to 70.5(7)o.
phenyl ring are involved in strong CAHꢀꢀꢀ
p interactions (black dot-
ted lines). The complex also exhibits intramolecular hydrogen
bonding which are indicated by red dotted lines. Repetition of
these non-bonding interactions in the complex leads to the forma-
tion of a 2D framework.
Complex
2 possesses interesting non-covalent interactions
(Fig. S4). Here also, O(1) acts as multiple proton acceptor (Table 6).
Two intermolecular hydrogen bonding interactions C(22)-H(22)ꢀꢀꢀO
(1) and C(25)-H(25A)ꢀꢀꢀO(1) (blue dotted lines) occur with pyrazole
ring hydrogen and methyl hydrogen which form a chain like archi-
tecture with neighboring molecules. The complex reveals two
Various intramolecular and intermolecular interactions play an
important role in the packing showing two hydrogen bonds, two
CAHꢀꢀꢀ
p
interactions C(32)-H(32)ꢀꢀꢀpa and C(14)-H(14)ꢀꢀꢀpb (black
4

M. Roy, D. Biswal, Nikhil Ranjan Pramanik et al.
Polyhedron 200 (2021) 115144
Fig. 3. Packing diagram of ligand structure showing the hydrogen bonding interactions (black dotted line), CAHꢀꢀꢀ
p interactions (blue dotted lines) and pꢀꢀꢀp interactions (red
dotted lines), non-bonding hydrogen atoms are omitted for clarity. (Colour online.)
Fig. 4. Molecular structure of complex 1.
Fig. 5. Molecular structure of complex 2.
dotted lines). The complex also exhibits intramolecular hydrogen
bonding which are indicated by red dotted lines.
Thus, both the complexes exhibit almost similar pattern of non-
covalent interactions except that in complex 2 bromine does not
take part in hydrogen bonding whereas chlorine participates in
hydrogen bonding in complex 1. Chlorine being more electronega-
tive than bromine is a better hydrogen bond acceptor.
greater than the experimental ones though the other features of
the structures agree well. The structure of the ligand contains nine
monomers all of which were optimized into the same geometrical
structure in which N(11)-C(19), C(19)-N(20) and N(20)-N(24) bond
lengths involving the donor sites N(11) and N(24) differ somewhat
from experimental values. In the complexes 1 and 2, Mo(1)-N(11)/
N(24) and Mo(1)-X(1)/X(2) bond distances deviate significantly in
the range of 0.06 Å to 0.10 Å and 0.05 Å to 0.12 Å respectively.
The frontier orbitals for the ligand and the complexes 1 and 2
are shown in Fig. 7 and 8, Fig. S5. The vertical electronic transitions
for the compounds are computed by TD-DFT/CPCM method in
dichloromethane solvent to obtain detailed information concern-
ing the absorption bands (Table S4). For the ligand, the main tran-
sition is from the HOMO to the LUMO at 306.59 nm with an f value
of 0.444. In the complex 1, the main transition is from the HOMO to
LUMO + 3 at 303.24 nm with an f value of 0.425. It is interesting to
3.7. DFT and TD-DFT calculations for the ligand and the complexes 1
and 2
The main objective of DFT and TD-DFT calculations on the struc-
turally characterized ligand and the complexes 1 and 2 is to inves-
tigate whether the experimental parameters for their bond
distances and the bond angles along with their electronic spectral
characteristics reveal agreement with each other. The calculated
bond parameters are compared to experimental values in Tables
S1-S3, 4-5. In general, the calculated bond lengths are slightly
5

M. Roy, D. Biswal, Nikhil Ranjan Pramanik et al.
Polyhedron 200 (2021) 115144
Table 2
Crystal data and details of refinement for ligand and complexes 1–2.
Ligand (L)
Complex 1
Complex 2
Chemical formula
Formula weight (M)
Crystal system
Space group
a (Å)
C
17H13N3S
C17H13SCl2N3O2Mo
490.20
orthorhombic
P n a 21
18.4618(13)
14.1674(16)
7.1798(9)
90
1877.9(3)
4
C17H13SBr2N3O2Mo
579.12
orthorhombic
P n a 21
18.598(4)
14.619(3)
7.2402(16)
90
1968.5(7)
4
291.36
monoclinic
P21/c
30.254(6)
15.648(3)
31.389(4)
120.333(13)
12,826 (4)
36
b (Å)
c (Å)
b (o)
V (Å3)
Z
Crystal size
Temperature (K)
0.2 ꢂ 0.18 ꢂ 0.16
0.24 ꢂ 0.05 ꢂ 0.05
0.24 ꢂ 0.05 ꢂ 0.05
293(2)
150(2)
296(2)
DC (g cmꢁ3
)
1.358
0.223
1.734
1.111
1.954
4.848
l
(Mo K
a
) (mmꢁ1
)
F (000)
5472
0.919
976
0.959
1120
1.037
Goodness of fit on F2
No. of independent
reflections
24,648
3479
3101
No. of reflections with
I > 2r
(I)
7830
2309
2896
R1, wR2 [I > 2
r(1)]
R1 = 0.0607
wR2 = 0.1219
R1 = 0.0465
wR2 = 0.0735
R1 = 0.0339
wR2 = 0.0825
Max, min residual
electron density e/Å3
0.180, ꢁ0.274
0.603, ꢁ0.517
1.802, ꢁ0.404
P
P
P
P
R =
ΙΙF0Ι – ΙFcΙΙ /
ΙF0Ι; wR(F2) = [ w(ΙF0Ι2 – ΙFcΙ2)2
/
w ΙF0Ι4]1/2
Table 3
Geometry of hydrogen-bonding interactions in the crystal structure of the ligand.
D-H
A
H∙∙∙A (Å)
D∙∙∙A (Å)
D-H∙∙∙A (ꢀ)
Symmetry code
C(36A)-H(36A)
C(36B)-H(36B)
C(36C)-H(36C)
C(36D)-H(36D)
C(36E)-H(36E)
C(36F)-H(36F)
C(36G)-H(36G)
C(36H)-H(36H)
C(36I)-H(36I)
C(36F)-H(36F)
C(35F)-H(35F)
N(11A)
N(11B)
N(11C)
N(11D)
N(11E)
N(11F)
N(11G)
N(11H)
N(11I)
pa
2.80
3.16
2.79
2.82
3.25
2.76
2.88
3.29
2.76
3.74
3.18
4.10
3.84
3.074(8)
3.296(8)
3.029(8)
3.132(8)
3.330(8)
3.059(8)
3.153(8)
3.356(8)
3.079(8)
99
90
97
101
87
100
98
86
x,y,z
x,y,z
x,y,z
x,y,z
x,y,z
x,y,z
x,y,z
x,y,z
101
x,y,z
x,3/2-y,-1/2 + z
x,3/2-y,-1/2 + z
x,3/2-y,1/2 + z
x,3/2-y,1/2 + z
pa
pb
pd
–
–
pc
pe
a
b
c
Centroid of C(36D),C(31D),C(32D),C(33D),C(34D),C(35D)
Centroid of C(19D),N(11D),C(12D),C(17D),S(18D)
Centroid of C(12F),C(13F),C(14F),C(15F),C(16F),C(17F)
Centroid of N(24D),C(23D),C(22D),C(21D),N(20D)
Centroid of C(19F),N(11F),C(12F),C(17F),S(18F)
d
e
Table 4
Bond distances (Å) in complexes 1 and 2 with calculated dimensions from DFT/B3LYP optimization.
Bond Distance (Å)
Complex 1 (X = Cl)
Obs.
Complex 2 (X = Br)
Obs.
Calc.
Calc.
Mo(1)-O(1)
Mo(1)-O(2)
Mo(1)-N(24)
Mo(1)-N(11)
Mo(1)-X(1)
Mo(1)-X(2)
N(11)-C(19)
1.684(5)
1.682(4)
2.329(5)
2.342(5)
2.3595(19)
2.359(2)
1.300(7)
1.708
1.708
2.427
2.426
2.410
2.410
1.306
1.677(7)
1.690(6)
2.332(7)
2.349(7)
2.5050(15)
2.5074(14)
1.297(11)
1.708
1.705
2.392
2.414
2.627
2.627
1.307
6

M. Roy, D. Biswal, Nikhil Ranjan Pramanik et al.
Polyhedron 200 (2021) 115144
Table 5
Bond angles (deg) in complexes 1 and 2 with calculated dimensions from
DFT/B3LYP optimization.
Bond Angle (°)
Complex 1 (X = Cl)
Complex 2 (X = Br)
Obs.
Calc.
Obs.
Calc.
O(1)-Mo(1)-O(2)
O(1)-Mo(1)-N(24)
O(2)-Mo(1)-N(24)
O(1)-Mo(1)-N(11)
O(2)-Mo(1)-N(11)
N(24)-Mo(1)-N(11)
O(1)-Mo(1)-X(2)
O(2)-Mo(1)-X(2)
N(24)-Mo(1)-X(2)
N(11)-Mo(1)-X(2)
O(1)-Mo(1)-X(1)
O(2)-Mo(1)-X(1)
N(24)-Mo(1)-X(1)
N(11)-Mo(1)-X(1)
X(2)-Mo(1)-X(1)
105.1(2)
161.8(2)
93.0(2)
106.0
161.4
92.6
94.3
159.8
67.1
97.5
97.9
79.5
78.8
97.5
98.1
79.4
79.2
154.1
105.9(4)
161.6(3)
92.3(3)
93.9(3)
160.1(3)
67.8(3)
95.0(3)
95.0(3)
81.03(19)
81.47(18)
96.0(3)
106.6
161.6
91.8
94.2
159.2
67.4
97.1
97.5
80.1
79.5
97.1
97.6
80.1
79.5
155.4
93.5(2)
161.4(2)
68.33(17)
95.72(19)
95.82(17)
80.86(14)
81.90(13)
96.43(18)
97.81(18)
82.13(14)
79.95(13)
158.69(7)
98.9(3)
82.97(19)
80.16(18)
159.16(5)
Table 6
Geometry of hydrogen-bonding interactions in the crystal structures of the complexes 1 and 2.
D-H
A
H∙∙∙A (Å)
D∙∙∙A (Å)
D-H∙∙∙A (ꢀ)
Symmetry code
Complex 1
C(25)-H(25B)
C(22)-H(22)
C(13)-H(13)
C(25)-H(25A)
C(25)-H(25C)
C(36)-H(36)
C(25)-H(25B)
C(32)-H(32)
C(14)-H(14)
C(15)-H(15)
O(1)
O(1)
O(1)
O(2)
O(2)
S(18)
Cl(1)
pa
2.756
2.977
2.436
2.947
2.750
3.166
3.037
2.698
2.993
3.609
3.296(9)
3.394(8)
3.152(9)
3.163(9)
3.163(9)
3.125(8)
3.988(8)
116
109
134
94
107
79
1/2-x, y-1/2, z-1/2
1/2-x, y-1/2, z-1/2
x, y, z
x, y, z
x, y, z
x, y, z
171
1/2-x, y-1/2, z-1/2
1-x, 1-y, z + 1/2
1-x, 2-y, z + 1/2
1-x, 2-y, z + 1/2
pb
pb
Complex 2
C(22)-H(22)
C(25)-H(25A)
C(25)-H(25C)
C(25)-H(25B)
C(13)-H(13)
C(36)-H(36)
C(32)-H(32)
C(14)-H(14)
O(1)
O(1)
O(2)
O(2)
O(1)
S(18)
pa
2.889
2.598
2.795
2.842
2.447
3.19
3.40(1)
3.30(1)
3.13(1)
3.13(1)
3.17(1)
3.14(1)
116
130
102
98
135
78
1/2-x, y-1/2, z-1/2
1/2-x, y-1/2, z-1/2
x, y, z
x, y, z
x, y, z
x, y, z
1-x, 1-y, z + 1/2
1-x, 1-y, z + 1/2
2.719
2.955
pb
a
Centroid of C(34),C(35),C(36),C(31),C(32),C(33)
Centroid of C(15),C(16),C(17),C(12),C(13),C(14)
b
note that of the five lowest LUMOs, all are concentrated on the
MoO2Cl2 moiety with the exception of the LUMO + 3. The frontier
orbitals for 2 are very similar to those for 1.
tribute more profoundly in complex 1 than in complex 2. There-
fore, it is evident that not only hydrogen bonding but also van
der Waals forces and CAHꢀꢀꢀ
p interactions, play an essential role
in stabilization of both the crystals.
3.8. Hirshfeld surface analysis
3.9. Catalytic oxidation of alkenes
From the 3D Hirshfeld surfaces and 2D fingerprint plots of both
complex 1 and complex 2 (Fig. 9), it is clear that HꢀꢀꢀH van der
Typical oxidation of alkenes produces a number of active prod-
ucts ranging from epoxide, diols to ketones and acids [54–58]. As
an oxidant hydrogen peroxide is the most attractive choice, since
it is green, cheap and easy to handle [59,60]. Catalytic potentials
of Mo complexes 1 and 2 have been assessed against the homoge-
neous oxidation of a number of alkenes namely, cyclohexene, cis-
cyclooctene and styrene in presence of 30% aqueous hydrogen per-
oxide. To achieve the maximum substrate conversion (%) of alke-
nes, oxidation of cyclohexene catalyzed by catalyst complex 1
was considered as a model reaction. To attain the optimized reac-
tion conditions, a number of reaction parameters such as amounts
of catalyst, oxidant and solvent, nature of the oxidant and temper-
ature were studied. In common practice, the oxidation of cyclohex-
ene was performed by reacting cyclohexene (5.0 mmol, 0.414 g)
with 30% aqueous hydrogen peroxide (15.0 mmol, 1.70 g) in pres-
Waals interactions, CAHꢀꢀꢀ
p
interactions of aromatic rings, OꢀꢀꢀH
hydrogen bonding interactions and possibly ClꢀꢀꢀH hydrogen bond-
ing interactions (BrꢀꢀꢀH for complex 2) are the major intermolecular
forces operating among the asymmetric units within the crystal
structures of the two complexes. Though not significant, SꢀꢀꢀH
and NꢀꢀꢀH interactions also contribute to some extent to the stabil-
ity of the crystals. Along with fingerprint plots, the contribution of
individual interactions is well tabulated in Table S5 for both the
complexes. Moreover, the pie charts (Fig. 9, right) clearly reveal
the percentage of contribution of different intermolecular interac-
tions in the two complexes. Complexes 1 and 2 can be easily distin-
guished if we focus on their fingerprint plots. The OꢀꢀꢀH hydrogen
bonding interaction in complex 2 is more prominent in comparison
to that in complex 1. On the contrary, CAHꢀꢀꢀp interactions con-
7

M. Roy, D. Biswal, Nikhil Ranjan Pramanik et al.
Polyhedron 200 (2021) 115144
Fig. 6. Packing diagram of complex 1 along c-axis showing the hydrogen bonding interactions (blue dotted lines) and CAHꢀꢀꢀ
p
interactions (black dotted lines). Intramolecular
hydrogen bonds are shown separately (red dotted lines). Non-bonding hydrogen atoms are omitted for clarity. (Colour online.)
Fig. 7. Frontier orbitals in ligand, (a) LUMO + 4, (b) LUMO + 3, (c) LUMO + 2, (d) LUMO + 1, (e) LUMO, (f) HOMO, (g) HOMO-1, (h) HOMO-2, (i) HOMO-3, (j) HOMO-4.
ence of 1.06 ꢂ 10-3 mmol of catalyst at 80 °C temperature in 5.0 mL
of methanol for 8 h. A single quadrupole GC–MS was used to con-
firm the identity of the oxidation products of various alkenes.
i.e., 0.45 ꢂ 10-3, 0.61 ꢂ 10-3, 0.81 ꢂ 10-3, 1.06 ꢂ 10-3, 1.22 ꢂ 10-3
,
1.42 ꢂ 10-3 mmol were used while a fixed amount of cyclohexene
(5.0 mmol, 0.414 g) was reacted with aqueous H2O2 (15.0 mmol,
1.70 g) in 5.0 mL of methanol at 80 °C for 8 h.
Fig. 10 (a) clearly indicates that a maximum of 90.2% conversion
was achieved by 1.06 ꢂ 10-3 mmol of catalyst after 8.0 h. On the
other hand, 0.45 ꢂ 10-3, 0.61 ꢂ 10-3, 0.81 ꢂ 10-3 1.22 ꢂ 10-3 and
1.42 ꢂ 10-3 mmol of catalyst exhibit 11.2%, 68.1%, 83.6%, 49.7%,
and 15% conversions respectively after 8 h at 80 °C. Reduction in
substrate conversion (%) with increasing amount of catalyst may
be due to the deactivation of active catalyst through desorption/-
chemisorption of reactant molecules on two separate catalyst cen-
tres in solution [65] which lower the chance of mutual effective
3.9.1. Catalytic oxidation of cyclohexene
Oxidation of cyclohexene catalyzed by catalyst complex 1 in
presence of hydrogen peroxide produces 2-cyclohexene-1-ol
(Cynol), cyclohexene epoxide (Cyox), cyclohexane 1,2-di-ol
(Cydiol) and 2-hydroxycyclohexaone (Cynon) (Scheme 2). These
are the typical products and have been reported in literature by
others previously [61–64].
The impact of amount of catalyst on cyclohexene oxidation is
shown in Fig. 10 (a and b). Five different amounts of catalysts
8

M. Roy, D. Biswal, Nikhil Ranjan Pramanik et al.
Polyhedron 200 (2021) 115144
Fig. 8. Frontier orbitals in complex 1, (a) LUMO + 4, (b) LUMO + 3, (c) LUMO + 2, (d) LUMO + 1, (e) LUMO, (f) HOMO, (g) HOMO-1, (h) HOMO-2, (i) HOMO-3, (j) HOMO-4.
Fig. 9. Hirshfeld surface plot mapped with dnorm (left); All in-all out fingerprint plot (middle); Pie chart for contribution of different interactions in percentage (right).
Scheme 2. Catalytic oxidation products of cyclohexene.
collisions. Therefore, 1.06 ꢂ 10-3 mmol of catalyst amount was
considered as optimum. In search of the best substrate to oxidant
ratio for the homogeneous oxidation of cyclohexene, three differ-
ent substrate to oxidant ratios i.e., 1:2, 1:3 and 1:4 were studied,
using a fixed amount of cyclohexene (5.0 mmol, 0.414 g) and cat-
alyst precursor (1.06 ꢂ 10-3 mmol) in 5.0 mL of methanol at 80 °C
9

M. Roy, D. Biswal, Nikhil Ranjan Pramanik et al.
Polyhedron 200 (2021) 115144
Fig. 10. (a) Effect of amount of catalyst, (b) Impact of cyclohexene to oxidant ratio, (c) Influence of solvent amount, (d) Effect of nature of oxidant and (e) Influence of
temperature on the homogeneous oxidation of cyclohexene catalyzed by catalyst 1. For detailed reaction conditions refer main text.
and the reaction was monitored for 8 h. While increasing the sub-
strate to oxidant ratio from 1:2 to 1:3, the % conversion increases
from 41.9% to 90.2%. Further increasing the ratio to 1:4 does not
improve the substrate conversion (94.8%) significantly but does
achieve comparable conversion (91.4%) with a very shorter time
period (only 4 h). With 1:4 ratio, comparable substrate conversion
(%) to that of 1:3 ratio can be attained by utilizing only half the
time. Hence, a substrate to oxidant ratio of 1:4 and a time period
of 4 h were chosen to optimize the other reaction parameters.
To optimize the volume of solvent, three different volumes of
solvent, 5 mL, 10 mL and 15 mL, were considered while reacting
cyclohexene (5.0 mmoL, 0.414 g) with 30% H2O2 (20.0 mmol,
2.26 g) in presence of catalyst complex 1 (1.06 ꢂ 10-3 mmol) at
80 °C for 4 h. The effect of volume of solvent on % conversion has
been shown in Fig. 10 (c). On increasing the volume of solvent from
5 mL to 10 mL, the % conversion decreases significantly from 91.4%
to only 15.7%. On further increasing the volume of solvent from
10 mL to 15 mL, the oxidation reaction proceeds with difficulty
and only 6.6% conversion is observed after 4 h. This is because
the additional volume of solvent reduces the chance of effective
collision between the reactant molecules. Hence, 5 mL of solvent
was chosen as best for the optimized reaction conditions.
Similarly, the influence of nature of oxidant was also studied
and presented in Fig. 10 (d). Three different oxidants such as,
hydrogen peroxide (H2O2), tert-butylhydroperoxide (TBHP) and
di-tert-butylperoxide (DTBP) were employed while reacting fixed
amount of cyclohexene (5.0 mmol, 0.414 g) and catalyst precursor
complex 1 (1.06 ꢂ 10-3 mmol) with 30% H2O2 (20.0 mmol, 2.26 g)
in 5 mL of methanol at 80 °C for 4 h. Fig. 10 (d) clearly reveals that
TBHP and DTBP show poor substrate conversion (%) in comparison
to H2O2. With H2O2, 91.4% conversion was achieved whereas 10.7%
and 8.6% conversions were achieved with DTBP and TBHP respec-
10

M. Roy, D. Biswal, Nikhil Ranjan Pramanik et al.
Polyhedron 200 (2021) 115144
tively. Hence, in comparison to TBHP and DTBP, green and cheaper
H2O2 was considered as efficient oxidant and used to optimize the
other parameters.
Finally the impact of temperature was checked and illustrated
in Fig. 10 (e). By applying the above optimized reaction conditions
i.e., 5.0 mmol cyclohexene (0.414 g), 1.06 ꢂ 10-3 mmol catalyst
precursor complex 1, 20.0 mmol 30% H2O2 (2.26 g), 5 mL of metha-
nol and 4 h homogeneous oxidation of cyclohexene was performed
at three different temperatures, 70 °C, 80 °C and 90 °C. By increas-
ing the temperature from 70 °C to 80 °C, conversion increases from
18.4% to 91.4% but further increasing the temperature results in no
significant improvement and therefore 80 °C was considered as the
optimum temperature.
Scheme 3. Liquid phase oxidation products of cis-cyclooctene catalyzed by catalyst
complexes 1 and 2.
2, 91.9% and 81.6% conversions were achieved respectively within
12 h under the optimized reaction conditions with an enhanced
TOF value. A blank reaction under optimized reaction conditions
produces only 5.1% conversion.
Likewise, homogeneous catalytic oxidation of styrene in the
presence of hydrogen peroxide, produces phenyl acetaldehyde
(Pacd), benzaldehyde (Bzd), benzoic acid (Bza) and 2-hydroxy-1-
phenylethanone (hap). These are common oxidation products of
liquid phase oxidation of styrene and frequently reported in a
number of literatures [72–74] (Scheme 4). Under the optimized
reaction conditions, catalyst complex 1 shows 19.5% conversion
with a TOF value of 2.299 ꢂ 102hꢁ1 and catalyst complex 2 exhibits
5.6% conversion with a TOF value of 0.786 ꢂ 101hꢁ1 (Table 8).
Interestingly, under the optimized reaction conditions both the
catalysts produce only two oxidation products benzoic acid (Bza)
and phenyl acetaldehyde (Pacd).
With 77.2% selectivity, catalyst complex 1 generates Bza as a
major product and with 22.8% selectivity Pacd appears only as a
minor product. Catalyst complex 2 exhibits similar trends of %
selectivity for Bza and Pacd. By extending the reaction time while
keeping the original optimization reaction conditions, significant
improvement of substrate conversion of oxidation of styrene by
catalyst complex 1 and 2 was observed. However, two new oxida-
tion products benzoic acid (Bza) and 2-hydroxy-1-phenylethanone
(hap) were obtained. Detailed selectivity (%) of the individual oxi-
dation products of styrene under optimized reaction conditions are
shown in Table 8. With 12 h reaction time, catalyst complex 1
shows 86.8% conversion with a TOF value of 3.419 ꢂ 102hꢁ1
whereas catalyst complex 2 leads to 58.2% conversion with a TOF
value of 2.724 ꢂ 102hꢁ1. A blank reaction under the same reaction
conditions produced only 0.4% conversion.
All the experimental details, substrate conversion and TOF val-
ues of cyclohexene oxidation catalyzed by catalyst complex 1 at
different reaction conditions are listed in Table 7 and it can be seen
that Sl. no. 8 represents the fully optimized reaction conditions.
Under the optimized reaction conditions, catalyst complex 1 shows
91.4% conversion with a TOF value of 1.077 ꢂ 103hꢁ1 whereas cat-
alyst complex 2 secures 51.7% conversion with a TOF value of
7.261 ꢂ 102hꢁ1. With 36.4% and 38.7% selectivity, cynon becomes
the major oxidation product of cyclohexene catalyzed by both the
catalyst complexes. Catalyst complex 1 shows the following order
of % selectivity of the oxidation products: Cynon (36.4%) > Cydiol
(29.8%) > Cynol (18.4%) > Cyox (15.3%) while the order: Cynon
(38.7%) > Cydiol (27.5%) > Cyox (17.8%) > Cynol (15.9%) is observed
for catalyst complex 2 (Table 7). This indicates that complex 1 is an
efficient catalyst in terms of % conversion and TOF values com-
pared to complex 2 towards the catalytic oxidation of cyclohexene.
This result can be explained in terms of electro negativity of the
halogen atoms. Chlorine being more electronegative than bromine
accelerates oxidation of cyclohexene, thereby the activity increases
in the order Cl > Br [66]. A blank reaction under the same opti-
mized reaction conditions produces only 7.3% conversion.
Optimized reaction conditions described above were extended
toward the liquid phase homogeneous oxidation of cis-cyclooctene
and styrene. Catalytic oxidation of cis-cyclooctene by catalyst com-
plex 1 in presence of hydrogen peroxide produces cyclooctene
oxide as a solo product [65,67–71] with 22.8% conversion and a
TOF value of 2.688 ꢂ 102hꢁ1. On the other hand, catalyst complex
2 shows 24.1% conversion with TOF value of 3.384 ꢂ 102hꢁ1. Both
the catalysts produce cyclooctene oxide with 100% selectivity
(Scheme 3).
A comparative plot presenting % conversion vs time for the oxi-
dation of cyclohexene, cis-cyclooctene and styrene by catalysts
complex 1 and 2 is shown in Fig. 11. It is evident from Fig. 11 that
under the optimized reaction conditions, catalyst complex 1 and 2
show higher catalytic activity towards cyclohexene. Cis-cyclooc-
tene behaves moderately whereas styrene shows least reactivity
which is in agreement with the fact that the terminal double bonds
Although the % conversion observed for the catalyst complex 1
and 2 is not ideal, significant enhancement has been observed by
increasing the time scale while keeping the original optimized
reaction conditions. Thus, in presence of catalyst complex 1 and
Table 7
Conversion and TOF values observed under various reaction conditions in the homogeneous oxidation of cyclohexene catalyzed by complexes 1 and 2.
Sl. no.
Cat.Comp.
Amount(mmol)
Oxidant(g, mmol)
Temp.(°C)
MeOH(mL)
Conv. (%)
Time (h)
TOF (hꢁ1
)
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
0.45 ꢂ 10-3
0.61 ꢂ 10-3
0.81 ꢂ 10-3
1.06 ꢂ 10-3
1.22 ꢂ 10-3
1.42 ꢂ 10-3
1.06 ꢂ 10-3
1.06 ꢂ 10-3
1.06 ꢂ 10-3
1.06 ꢂ 10-3
1.06 ꢂ 10-3
1.06 ꢂ 10-3
1.06 ꢂ 10-3
1.06 ꢂ 10-3
1.06 ꢂ 10-3
0.89 ꢂ 10-3
H2O2 (1.70, 15)
H2O2 (1.70, 15)
H2O2 (1.70, 15)
H2O2 (1.70, 15)
H2O2 (1.70, 15)
H2O2 (1.70, 15)
H2O2 (1.13, 10)
H2O2 (2.26, 20)
H2O2 (2.26, 20)
H2O2 (2.26, 20)
H2O2 (2.26, 20)
TBHP (2.574, 20)
DTBP (2.984, 20)
H2O2 (2.26, 20)
H2O2 (2.26, 20)
H2O2 (2.26, 20)
80
80
80
80
80
80
80
80
80
80
80
80
80
70
90
80
5
5
5
5
5
5
5
5
11.2
68.1
83.6
90.2
49.7
15.0
41.9
94.8
91.4
15.7
6.6
8
8
8
8
8
8
8
8
4
4
4
4
4
4
4
4
1.55 ꢂ 102
6.976 ꢂ 102
6.45 ꢂ 102
5.318 ꢂ 102
2.545 ꢂ 102
0.66 ꢂ 102
2.471 ꢂ 102
5.589 ꢂ 102
1.077 ꢂ 103
1.851 ꢂ 102
0.778 ꢂ 102
1.014 ꢂ 102
1.26 ꢂ 102
2.167 ꢂ 102
1.1 ꢂ 103
9
5
10
11
12
13
14
15
16
10
15
5
5
5
8.6
10.7
18.4
93.3
51.7
5
5
7.261 ꢂ 102
11

M. Roy, D. Biswal, Nikhil Ranjan Pramanik et al.
Polyhedron 200 (2021) 115144
Scheme 4. Homogeneous oxidation products of styrene catalyzed by complexes 1 and 2.
Table 8
Results of homogeneous oxidation of cis-cyclooctene and styrene in presence of catalyst complex 1 and 2. The % conversion and TOF values attained after 4 h and 12 h are
presented.
Sl. no.
Cat. Comp.
Substrate
Conv. (%)
% Selectivity
Time (h)
TOF (hꢁ1
)
Cyclooctene Oxide
1
2
3
4
1
2
1
2
Cis-Cyclooctene
Cis-Cyclooctene
Cis-Cyclooctene
Cis-Cyclooctene
22.8
24.1
91.9
80.6
100
100
100
100
4
4
12
12
2.688 ꢂ 102
3.384 ꢂ 102
3.612 ꢂ 102
3.773 ꢂ 102
% Selectivity
(Pacd)
22.8
41.1
11.8
54.5
(Bzd)
(hap)
-
–
7.1
10.8
(Bza)
-
–
46.7
23.8
5
6
7
8
1
2
1
2
Styrene
Styrene
Styrene
Styrene
19.5
5.6
86.8
58.2
77.2
58.8
34.4
10.8
4
4
12
12
2.299 ꢂ 102
0.786 ꢂ 102
3.419 ꢂ 102
2.724 ꢂ 102
and complexes reveal that the experimental and theoretical struc-
tural parameters are in good agreement with each other. TD-DFT/
CPCM calculations have also been carried out to identify the molec-
ular orbitals and experimentally determined transitions. Strong
hydrogen bonding, CAHꢀꢀꢀ
p
and
p p interactions are present in
ꢀꢀꢀ
both the ligand and the complexes which raise the stability and
integrity of their supramolecular assemblies. Hirshfeld surface
analysis of the complexes uncover the van der Waals interactions,
CAHꢀꢀꢀ
p interactions as well as hydrogen bonding interactions
which play significant role in stabilization of both the complexes.
The complexes exhibit promising catalytic activities in the oxida-
tion of alkenes. Complex 1 shows higher catalytic performance
than complex 2 towards oxidation of cyclohexene due to presence
of more electronegative chlorine atom.
CRediT authorship contribution statement
Fig. 11. % Conversion vs time plot for the oxidation of cyclohexene, cis-cyclooctene
and styrene catalyzed by complexes
conditions for 4 h.
1 and 2 under the optimized reaction
Malini Roy: Data curation, Investigation, Validation, Writing -
original draft, Writing - review & editing. Debanjana Biswal: Data
curation, Investigation. Nikhil Ranjan Pramanik: Conceptualiza-
tion, Methodology, Supervision. Michael G.B. Drew: Software, For-
mal analysis. Suvendu Paul: Software, Formal analysis. Payal
Kachhap: Investigation, Validation. Chanchal Haldar: Methodol-
ogy. Syamal Chakrabarti: Conceptualization, Methodology,
Supervision.
are less reactive than the non-conjugated or non-terminal double
bonds. Moreover, double bonds conjugated with aryl rings are less
reactive than the others [75]. Comparable reactivities of catalyst
complex 1 and 2 towards cis-cyclooctene and styrene can be
attained by extending the optimized reaction time up to 12 h.
Declaration of Competing Interest
4. Conclusions
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.
Two mononuclear dioxomolybdenum(VI) complexes of the for-
mula MoO2X2L [where, X = Cl (1), Br (2)] have been synthesized
with the NAN donor ligand 2-(3-methyl-5-phenyl pyrazol-1-yl)
benzthiazole (L). The ligand and the complexes 1 and 2 are struc-
turally characterized by single crystal X-ray diffraction. The elec-
trochemical behavior of the complexes has been analyzed by
cyclic voltammetry. The crystal structures of the isostructural com-
plexes reveal distorted octahedral geometry around the Mo center
in which [MoO2]2+ moiety coordinates with NAN donor ligand and
two halogen atoms in such a fashion that the halogens are mutu-
ally trans to each other. Relevant DFT calculations of the ligand
Acknowledgements
DST-FIST is acknowledged for providing single crystal XRD, at
Department of Chemistry, University of Calcutta for crystal data
collection of the ligand. We thank the University of Reading and
the EPSRC for funding the diffractometer used to collect data for
complex 1. Single crystal X-ray data for complex 2 was collected
12

M. Roy, D. Biswal, Nikhil Ranjan Pramanik et al.
Polyhedron 200 (2021) 115144
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at the DBT-funded CEIB program (Project No. BT/01/CEIB/11/V/13)
awarded to Department of Organic Chemistry, IACS, Kolkata. We
are thankful to Ms. Poulami Chakraborty for crystal data collection
of complex 2. The authors also thank Ms. Oiendrilla Sarkar for her
sustained interest in this work.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.poly.2021.115144.
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13