Synthesis and characterization of a series of acylpyrazolone transition metal complexes: Crystal structures and catalytic performance in the epoxidation of cyclooctene

Article abstract of DOI:10.1016/j.ica.2020.119637

A series of transition metal complexes of 4-acetyl-3-methyl-1-phenyl-2-pyrazoline-5-one with Ni(II), Cu(II), Fe(III) and Mn (II) central metal ions have been synthesized and characterized by X-ray crystallography, UV–Vis and FTIR spectroscopy, and element

Full text of DOI:10.1016/j.ica.2020.119637

Inorganica Chimica Acta 508 (2020) 119637
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Synthesis and characterization of a series of acylpyrazolone transition metal
complexes: Crystal structures and catalytic performance in the epoxidation
of cyclooctene
a
a,⁎
b
b
Maryam Hasanzadeh Esfahani , Mahdi Behzad , Michal Dusek , Monika Kucerakova
a
Faculty of Chemistry, Semnan University, Semnan, Iran
b
Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 182 21 Praha 8, Czech Republic
A R T I C L E I N F O
A B S T R A C T
Keywords:
Pyrazolone
Epoxidation
Transition metal complexes
Cyclooctene
A series of transition metal complexes of 4-acetyl-3-methyl-1-phenyl-2-pyrazoline-5-one with Ni(II), Cu(II), Fe
(III) and Mn (II) central metal ions have been synthesized and characterized by X-ray crystallography, UV–Vis
and FTIR spectroscopy, and elemental analysis. The complexes were used as a catalyst for epoxidation of cy-
clooctene with tert-butylhydroperoxide. Various parameters, including catalyst type and amount, solvent type
and amount, oxidant/substrate ratio and time, have been optimized. The highest catalytic activity was found for
Cu(II) complex at the optimized conditions. Very high selectivity for the epoxide product (cyclooctene oxide)
was also obtained.
Crystal structure
1. Introduction
Catalytic reactivity of the complexes has been examined in the epox-
idation of cyclooctene using tert-butylhydroperoxide (TBHP) as oxidant.
Transition metal complexes with pyrazolone derivatives have
Different parameters affecting the catalytic reactivity, including cata-
lyst type and amount, solvent type and amount, oxidant/substrate ratio,
and time, was optimized. High catalytic activity was observed mainly
for the Cu(II) complex. Very high epoxide selectivity was also obtained.
shown interesting chemical and physical properties and have attracted
researcher attention [1–7]. Pyrazolones are very important due to their
wide applications in several fields in pharmacology and biology due to
their antibacterial, antimicrobial, antifungal and antioxidant, antitumor
and anti-inflammatory activities [8–16]. They have also found appli-
cations in catalysis, laser technology, chromatographic materials, and
in the petrochemical industry [17–20]. The tautomerism of such com-
pounds is also a challenging field of study due to its importance in
chemical reactivity. Most of these ligands show enol-keto tautomerism
to cause different structures and spectroscopic properties [20–22]. The
catalytic potential of several pyrazolone-based ligands has been in-
vestigated in various organic transformations. Marchetti and coworkers
have recently reviewed the applications of pyrazolone-based transition
metal complexes [2]. They have also reviewed acylpyrazolone ligands
and their transition metal complexes [3]. According to their review and
our literature survey, most of such complexes have been used in poly-
merization transformations, oxidation reactions and coupling reactions.
In the present work, we report the synthesis and characterization of
a series of Ni(II), Cu(II), Fe(III) and Mn (II) transition metal complexes
with 1-phenyl-3-methyl-4-acetyl-2-pyrazoline-5-one (PMAP). All of the
complexes were characterized by multiple techniques including FTIR,
UV–Vis, single-crystal X-ray crystallography, and elemental analysis.
2. Experimental
All the starting materials and solvents were purchased from com-
mercial sources and were used as received. The FT-IR spectra were
obtained as KBr plate using a Shimadzu 8400S FT-IR instrument in the
−1
range of 4000–400 cm
. Melting points were obtained on a
Thermoscientific 9100 apparatus by using the simple capillary tube
method. UV–Vis spectra were obtained on a Shimadzu UV-1650 PC
spectrophotometer in DMSO solution. Elemental analysis was per-
formed on a Perkin-Elmer 2400II CHNS-O analyzer. The structures of
the products were confirmed unambiguously by single-crystal X-ray
analysis. Gas chromatography (GC) analyses were carried out with a
Capillary Bp-10 column on a GC-17A Shimadzu instrument equipped
with an FID detector working at 280 °C.
2.1. Synthesis of 1-phenyl-3-methyl-4-acetyl-2-pyrazoline-5-one (PMAP)
This ligand was synthesized following the previously reported
⁎
Corresponding author.
E-mail address: mbehzad@semnan.ac.ir (M. Behzad).
https://doi.org/10.1016/j.ica.2020.119637
Received 23 January 2020; Received in revised form 24 March 2020; Accepted 28 March 2020
Available online 30 March 2020
0020-1693/ © 2020 Elsevier B.V. All rights reserved.

M. Hasanzadeh Esfahani, et al.
Inorganica Chimica Acta 508 (2020) 119637
procedure [21]. In a typical experiment, calcium hydroxide (162 mmol,
2.6. X-ray crystallography
12 g) was added slowly to 80 mL 1,4-dioxane solution of 3-methyl-1-
3
phenyl-2-pyrazoline-5-one (86 mmol, 15 g) with gentle heating and
stirring. After 10 min, 10 mL of acetyl chloride was added dropwise
during 1–2 min. An orange paste-like product appeared, and the mix-
ture was refluxed for 1 h. The reaction mixture was then cooled on an
ice bath. 200 mL of cold hydrochloric acid (2 M) was then added to the
reaction mixture, followed by a continuous stirring until cream color
crystals appeared. The crystals were collected with a Buchner funnel
and were washed with a copious amount of water. Yield: 12.6 g. 84%.
A single crystal of (2) of size 0.06 × 0.03 × 0.02 mm and a single
crystal of 3 of size 0.06 × 0.04 × 0.04 mm3 were used for X-ray dif-
fraction study at 95 K on a SuperNova four-circles diffractometer
equipped with a CCD Atlas S2 detector, using the mirrors-collimated
Cu-Kα radiation (λ = 1.54184 Å) from a micro-focus tube, and a
Cryostream 800 + chiller.
A
larger single crystal of (1) of size
single crystal of (4) of size
3
0.15 × 0.10 × 0.07 mm3 and
a
0.18 × 0.14 × 0.11 mm were used for X-ray diffraction study at 120 K
on a Gemini four-circles diffractometer equipped with a CCD Atlas S2
detector, using the mirrors-collimated Cu-Kα radiation (λ = 0.71073 Å)
from a classical tube, and a Cryojet 5 chiller. The temperatures used for
measurement were the lowest economic values reachable on both in-
struments. The crystal structures were solved by charge flipping with
the Superflip [23] program and refined with the Jana2006 program
package [24]. The refinement was carried out against all reflections.
The structure determinations of all compounds were straightfor-
ward; no disorder was found. H atoms attached to carbon were kept in
the geometrically correct positions with a CeH distance of 0.96 Å. In
the compound of 4, the hydrogen atoms connected to oxygen atoms
were found in difference Fourier maps and refined freely. The crystal of
−
1
IR (cm ): 2933, 1612, 1593, 1544, 1496, 1458, 1436, 1363, 1330,
1087, 767.
2.2. Synthesis of complex (1)
10 mL methanolic solution of manganese(II) acetate anhydrate
(
(
1.00 mmol, 0.17 g) was added to 15 mL methanolic solution of PMAP
2.00 mmol, 0.43 g) and the reaction mixture was refluxed for 5 h. The
resulting precipitate was collected with a Buchner funnel and dried at
room temperature. Pure crystals suitable for X-ray crystallography were
obtained by recrystallization from DMF and methanol after about
−
1
3
1
5
weeks. Yield: (0.42 g, 66%). Selected IR (cm ): 2934, 2923, 1649,
(
3) was twinned, with refined domain fractions equal to 0.8335
616, 1591, 1581, 1539, 1490, 1465, 1365, 1344, 1251, 1110, 748, and
(
9):0.1665 (9). The molecular structure plots were generated using
-
4
−1
−1
14. UV–Vis. 10 M solution in DMSO [λmax nm, (ε M cm )]: 216
36MnN : C;
Diamond 3 [25]. The crystallographic data and structure refinements
for all compounds are summarized in Table 1.
(
84200), 246 (36000). Anal. Calcd. (Found) for C30
H
6 6
O
57.00 (56.91), H; 5.70 (5.64), N; 13.30 (13.39).
2.7. General oxidation reactions
2.3. Synthesis of complex (2)
The synthesized complexes were used as a catalyst for oxidation of
This complex was prepared following a similar procedure as de-
cyclooctene by TBHP as an oxidant in different reaction conditions. In
the absence of the complexes, a minimal amount of oxidation products
was observed. The progress of the reactions was monitored by GC (Gas
chromatography). The conversion percentages (%) were calculated
scribed for (1) except Iron(II) chloride tetrahydrate (1.00 mmol, 0.20 g)
was used instead of manganese(II) acetate anhydrate. Recrystallization
from DMF and methanol afforded 0.56 g, 80%, of pure crystals of the
−1
target complex suitable for X-ray crystallography. Selected IR (cm ):
922, 1606, 1595, 1575, 1535, 1487, 1442, 1382, 1085, 759, and 694.
i f
according to the Eq. (1) in which C is the initial and C is the final
2
concentration of cyclooctene substrate. Selectivity was also calculated
according to Eq. (2) [26]. The catalytic reactions were performed in
quadruplicate and the results are the average of the obtained data.
-
4
−1
−1
UV–Vis. 10 M solution in DMSO [λmax nm, (ε M cm )]: 216
92000), 244 (31800), 350 (6600), 446 (2780). Anal. Calcd. (Found)
for C36 33FeN : C; 61.58 (61.50), H; 4.70 (4.75), N; 11.97 (12.05).
(
H
6 6
O
C
i
− C
f
Conversion% =
Selectivity% =
× 100
C
i
(1)
(2)
2
.4. Synthesis of complex (3)
C
epoxide
× 100
C
totalproducts
This complex was also prepared following a similar procedure as
described for (1) except copper(II) perchlorate hexahydrate
1.00 mmol, 0.37 g) was used instead of manganese(II) acetate anhy-
(
2
.7.1. Oxidation of cyclooctene in solvent
In a typical experiment, 10 µmol of a catalyst was dissolved in 10 mL
of freshly distilled methanol, and then, 10 mmol of cyclooctene and
0 mmol of TBHP were added. The reaction mixture was refluxed while
drate. Recrystallization from methanol yielded 0.44 g, 89%, of pure
crystals suitable for single-crystal X-ray crystallography. Selected IR
−
1
(
1
M
cm ) 3066, 1606, 1593, 1575, 1541, 1498, 1465, 1438, 1380, 1352,
2
-4
230, 1085 and 756. UV–Vis. 10 M solution in DMSO [λmax nm, (ε
being stirred, and the reaction progress was monitored at 60 min in-
tervals.
−
1
−1
cm )]: 218 (91600), 268 (17900), 340 (1000), 742 (80). Anal.
22CuN : C; 58.30 (58.22), H; 4.45 (4.38), N;
Calcd. (Found) for C24
H
4 4
O
11.34 (11.40).
2.7.2. Solvent-free oxidations
10 µmol of the catalysts, 10 mmol of cyclooctene, and 20 mmol of
2.5. Synthesis of complex (4)
TBHP were refluxed, and the progress of the reaction was monitored
every 60 min by GC.
This complex was prepared following a similar procedure as de-
scribed for (1) except nickel(II) acetate tetrahydrate (1.00 mmol,
.25 g) was used instead of manganese(II) acetate anhydrate.
3. Results and discussions
0
Recrystallization from methanol afforded 0.44 g, 78%, of pure crystals
3.1. Synthesis of the complexes
of the target complex suitable for X-ray crystallography. Selected IR
−
1
(
1
cm ) 3124, 3110, 1620, 1596, 1583, 1504, 1490, 1467, 1458, 1365,
All of the complexes were readily prepared from the reaction of
metal salts with the PMAP ligand. Complex (2) was prepared from the
reaction of FeCl ·4H O with PMAP. Aerobic oxidation has occurred and
2 2
−4
344, 1083, 748, and 449. UV–Vis. 10 M solution in DMSO [λmax
nm, (ε M cm )]: 216 (88300), 246 (21500), 342 (7700), 406 (1 5 0),
−1
−1
6
75 (50). Anal. Calcd. (Found) for C26
H
30
N
4
NiO
6
: C; 56.40 (56.31), H;
Fe(II) ion has been oxidized to Fe(III) upon exposure to air. The same
complex has been reported previously in which the title complex was
5.42 (5.47), N; 10.12 (10.18).
2

M. Hasanzadeh Esfahani, et al.
Inorganica Chimica Acta 508 (2020) 119637
Table 1
Crystal data and final refinement details.
Crystal data
Complex (1)
36MnN
Complex (2)
66Fe
Complex (3)
22CuN
Complex (4)
Chemical formula
C
30
H
O
6 6
C
72
H
2
N
12
O
12
C
24
H
4
O
4
26 30 4 6
C H N NiO
Mr
631.6
Monoclinic
P21/n
2
1403.1
494
553.2
Monoclinic
P21/c
4
120
Crystal system
Space group
Z
Triclinic
Triclinic
−
−
P
2
95
1
P
1
95
1
Temperature (K)
120
Unit cell dimensions
a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
8.7828 (3)
9.2630 (3)
6.4811 (3)
7.1952 (3)
16.1377 (5)
8.7590 (2)
19.1441 (6)
90
110.237 (3)
90
2538.98 (14)
1.52
0.18 × 0.14 × 0.11
0.842, 1
16,810
4550
17.1305 (6)
9.9907 (3)
90
102.028 (3)
90
1470.14 (9)
4.11
0.15 × 0.10 × 0.07
0.738, 1
11,536
2633
18.2874 (6)
20.6851 (7)
71.394 (3)
85.959 (3)
83.585 (3)
3297.8 (2)
4.14
0.06 × 0.03 × 0.02
0.969, 1
20,221
11.4811 (9)
93.540 (5)
97.432 (6)
96.561 (4)
525.83 (5)
1.82
0.06 × 0.04 × 0.04
0.987, 1
3597
3
V (Å )
μ (mm−1)
Crystal size (mm )
3
T
min, Tmax
Measured reflections
Independent reflections
Observed reflections [I > 3σ(I)]
12,817
8680
0.040
3346
2409
0.028
2462
0.023
4114
0.024
R
int
R[F2 > 3σ(F2)]
wR(F2)
S
0.025
0.074
1.42
0.043
0.123
1.22
0.039
0.122
1.40
0.031
0.090
1.61
1)
No. of parameters
196
0.21, −0.24
883
0.38, −0.45
152
0.38, −0.43
340
0.18, −0.44
Δρmax, Δρmin (e Å−3)
1) Refinement program Jana2006 does not refine the weighting scheme. Therefore, S is based on weights from the experiment and usually is fairly above 1.
prepared from Fe(NO
3
)
3
·9H
2
O [27].
methyl-1-phenyl-5-pyrazolonato ligands (hereinafter named pyr-
azolonato) and two dimethylformamide ligands coordinated to the
central Mn(II) ion located in the crystallographic inversion center.
The two pyrazolonato ligands take the enol tautomeric form and
they are deprotonated on the OH group on the pyrazole ring. The
geometry around the central metal ion is pseudo-octahedral with the
two OO' type pyrazolonato ligands in the basal plane of the octahedron
in trans form, and two DMF molecules coordinated in apical positions
through their oxygen atoms. It could also be seen that the two pyr-
azolonato ligands are in anti configuration with respect to the enolato
3
3
.2. Description of the crystal structures
.2.1. Description of the crystal structure of (1)
Molecular structure of the complex (1) is shown in Fig. 1 with the
common atom numbering scheme. Crystallography data and final
structural refinement parameters are summarized in Table 1. Selected
bond lengths and angles are also collected in Tables 2 and 3, respec-
tively. Complex (1) is neutral containing two monoanionic 4-acetyl-3-
i
oxygen of the pyrazoline ring (i.e. O3 and O3 as shown in Fig. 1).
According to Table 1, the two Mn-O1 and Mn-O3 bond distances are
almost equal (2.1539 (9) and 2.1579 (8) Å). Besides, the two C1-O1 and
C4-O3 bond distances (1.2556 (15) and 1.2642 (15) Å) are also almost
equal, which implies that the two carbonyl groups are similar. The
other two Mn-O bond distances are longer than the basal Mn-O bond
distances (Mn-O2: 2.2107 (9) Å) which implies a z-out Jahn-Teller
distortion and is usual for Mn(II) octahedral complexes [28]. The in-
version center at the position of Mn(II) causes the exact location of the
central metal ion on the basal plane, and 180° of trans O-Mn-O bond
angles. All of the bond distances and bond angles are in the same range
as those of the previously reported similar complexes [29,30].
3.2.2. Description of the crystal structure of (2)
Fig. 2 shows one molecule of the complex (2) with the same atom
numbering scheme as used for the complex (1). The asymmetric unit of
the complex (2) consists of two crystallographically independent, but
chemically identical and geometrically very similar molecules. The
complex is neutral with three monoanionic pyrazolonato ligands co-
ordinated to the central Fe(III) metal ion. Like in the complex (1), the
ligands are deprotonated from the OH group of the pyrazole ring. The
coordination geometry of the metal center is distorted octahedral, and
ligands are coordinated in a facial coordination geometry with respect
to the enolato oxygen of the pyrazoline ring (i.e. O2, O4 and O6 in
Fig. 2). The average Fe-O bond length from the pyrazolonato oxygen
atoms, i.e. Fe1-O2, Fe1-O4 and Fe1-O6 (1.977 Å) is slightly shorter than
the other Fe-O average bond lengths (2.034 Å). This is in contrary to
those of complex (1) in which the average bond lengths from different
Fig. 1. Molecular structure of the complex (1) with an atom numbering scheme
common for all reported complexes. Thermal ellipsoids are drawn at 50%
probability level. Hydrogen atoms are shown as spheres of arbitrary radii.
Symmetry code (i) −x, 1 − y, −z.
3

M. Hasanzadeh Esfahani, et al.
Inorganica Chimica Acta 508 (2020) 119637
Table 2
Selected bond lengths (Å) around the central metal ions.
Complex (1)
Complex (2)
Complex (3)
Complex (4)
Mn1-O2
Mn1-O2i
Mn1-O2
Mn1-O2i
Mn1-O3
Mn1-O3i
2.1539 (9)
2.1539 (9)
2.2107 (9)
2.2107 (9)
2.1579 (8)
2.1579 (8)
Fe2-O7
Fe2-O9
Fe2-O8
Fe2-O10
Fe2-O11
Fe2-O12
Fe1-O6
Fe1-O5
Fe1-O4
Fe1-O2
Fe1-O1
Fe1-O3
2.008 (2)
2.045 (2)
1.987 (2)
1.982 (2)
2.035 (2)
1.977 (2)
1.965 (2)
2.017 (2)
1.979 (2)
1.986 (2)
2.035 (2)
2.049 (2)
Cu1-O1
Cu1-O1i
Cu1-O2
Cu1-O2i
1.9319 (19)
1.9319 (19)
1.9171 (17)
1.9171 (17)
Ni1-O2
Ni1-O1
Ni1-O4
Ni1-O5
Ni1-O3
1.9966 (12)
2.0386 (9)
2.0107 (9)
2.0842 (12)
2.0314 (12)
oxygen atoms of the chelating ligands were almost equal. On the other
hand, the average CeO bond lengths on the pyrazolone rings (1.278 Å)
are almost similar to the average CeO bond lengths of the acetyl groups
(
1.268 Å) which indicates that the negative charge is delocalized on the
two oxygen atoms. The trans and cis O-Fe-O bond angles substantially
deviate from the ideal 180° and 90°, respectively, which is another
evidence for the distortion from the ideal octahedron. All of the bond
lengths and bond angles are similar to those of the previously reported
similar complexes [27,31–32]. The crystal structure of the same com-
plex was previously reported in [27] as a monoclinic structure with
space group Cc. The Herein reported complex is a triclinic isostructural
−
compound with
P
space group. The two structures were compared
1
using CrystalCMP software [33] which showed that the two complexes
have totally different packing and hence, the different structures are the
result of this difference (see Fig. S1).
3.2.3. Description of the crystal structure of (3)
The synthesis of the title complex was reported previously, but
without crystal structure [34]. Fig. 3 shows the molecular structure of
the complex (3) with a common atom numbering scheme. This complex
is also neutral in which two pyrazolonato ligands are coordinated to the
central Cu(II) ion in the anti configuration with respect to the enolato
i
oxygen of the pyrazoline ring (i.e. O2 and O2 , see Fig. 3). The geometry
around the central metal ion is square planar, and the central metal ion
located at en inversion center lies almost exactly on the plane formed
by the four coordinating oxygen atoms. The two different Cu–O bond
lengths are similar, and the two different CeO bond lengths, i.e. C1-O1
Fig. 2. Molecular structure of complex (2) with an atom numbering scheme
common for all reported complexes. Thermal ellipsoids are drawn at 50%
probability level. Hydrogen atoms are shown as spheres of arbitrary radii.
(
1.267 (3) Å) and C4-O2 (1.280 (3) Å) are also similar which is an
3
.2.4. Description of the crystal structure of (4)
Fig. 4 shows the molecular structure of complex (4) with a common
indication of the delocalization of the negative charge on the two dif-
ferent oxygen atoms. All of the bond lengths and bond angles are si-
milar to those of the previously reported similar complexes [35,36].
atom numbering scheme. As could be seen from Fig. 4, the molecule of
Table 3
Selected bond angles around the central metal ion.
Complex (1)
Complex (2)
Complex (3)
Complex (4)
O1-Mn1-O1i
O1-Mn1-O3
O1-Mn1-O3i
180.0 (5)
85.05 (3)
94.95 (3)
94.95 (3)
85.05 (3)
180.0 (5)
O6-Fe1-O5
O6-Fe1-O4
O6-Fe1-O2
O6-Fe1-O1
O6-Fe1-O3
O5-Fe1-O4
O5-Fe1-O2
O5-Fe1-O1
O5-Fe1-O3
O4-Fe1-O2
O4-Fe1-O1
O4-Fe1-O3
O2-Fe1-O1
O2-Fe1-O3
O1-Fe1-O3
90.15 (9)
88.29 (9)
97.43 (9)
92.15 (9)
173.97 (10)
98.64 (9)
170.92 (10)
86.46 (9)
85.36 (9)
86.64 (9)
174.88 (9)
88.40 (9)
88.24 (9)
87.41 (9)
91.60 (9)
O1-Cu1-O1a
O1-Cu1-O2
180.0 (5)
85.75 (8)
94.25 (8)
94.25 (8)
180.0 (5)
85.75 (8)
O2-Ni1-O1
O2-Ni1-O4
O2-Ni1-O5
O2-Ni1-O3
O1-Ni1-O4
O1-Ni1-O5
O1-Ni1-O3
O4-Ni1-O5
O4-Ni1-O3
O5-Ni1-O3
91.07 (4)
88.36 (4)
89.29 (5)
179.31 (4)
179.10 (4)
88.78 (4)
89.52 (4)
91.91 (4)
91.06 (4)
90.35 (5)
a
O1-Cu1-O2
O2-Cu1-O1
i
a
O3-Mn1-O1
i
i
a
O1 -Mn1-O3
O2-Cu1-O2
O3-Mn1-O3i
O1 -Cu1-O2
a
a
4

M. Hasanzadeh Esfahani, et al.
Inorganica Chimica Acta 508 (2020) 119637
range of the previously reported similar structures [35,36].
3.3. Description of the spectroscopic data
The IR spectra of the complexes could be described based on the
results of the crystal structures. In the IR spectra of the complexes, the
−
1
presence of a very intense band at around 1600 cm
could be attrib-
uted to the stretching vibrations of the carbonyl groups of the acyl-
pyrazolone ligands. As followed from the crystal structures of the
complexes, the two CeO groups were almost identical due to the de-
localization of the negative charge between the two carbonyl groups,
and hence, the CeO bond lengths were between the single and double
bonds. The IR spectroscopy also confirms such delocalization. The IR
spectra of double bond carbonyl groups are usually observed above
−1
1650 cm
and delocalization of negative charge shifts stretching vi-
Fig. 3. Molecular structure of complex (3) with an atom numbering scheme
common for all reported complexes. Thermal ellipsoids are drawn at 50%
probability level. Hydrogen atoms are shown as spheres of arbitrary radii.
Symmetry code (i) −x, 1 − y, 1 − z.
bration to lower wavenumbers. The observation of the ν(CeO) at
−1
around 1600 cm
is clear evidence of such delocalization. In the IR
spectrum of the complex (1), the signal due to the ν(CeO) of the N,N-
−1
dimethylformamide ligands were observed at 1649 cm
and con-
firmed the coordination of these ligands. The M−O stretching vibra-
−1
tions were observed at low wavenumbers at around 700 cm . The
aromatic and aliphatic ν(CeH) were also observed at around
−1
3
000 cm . Supplemental Figs. S2–S6 show the IR spectra of the PMAP
ligand and complexes (1–4), respectively.
The UV–Vis spectra of the complexes could also be described based
on the crystal structural data. In the UV–Vis spectra of the complexes,
the very intense bands at around 215 and 250 nm were assigned to the
π → π* transitions. Similar signals were observed for the acylpyr-
azolone ligand at approximately similar region [21]. The main differ-
ence between the UV–Vis spectra of the ligand and the complexes were
due to the LMCT which were observed at around 340 nm. The d-d
transitions for the complexes (1) and (2) were not observed, even at
higher concentrations, most probably due to their very weak absor-
bances. This behavior is usual for high spin d5 complexes. The d-d
transitions for complexes (3) and (4) were observed above 600 nm only
at higher concentrations. These data are comparable to previously re-
ported similar complexes [28–37]. Besides, the UV–vis spectra of the
complexes did not change by time, which means that the complexes
were stable in solutions.
Fig. 4. Molecular structure of complex (4) with an atom numbering scheme
common for all reported complexes. Thermal ellipsoids are drawn at 50%
probability level. Hydrogen atoms are shown as spheres of arbitrary radii.
3
.4. Catalytic studies
The catalytic oxidation of cyclooctene with the four new catalysts
gave three products, as shown in Scheme 1. Various reaction condi-
tions, including catalyst type, amount of catalyst, solvent type, solvent
amount, reaction time, and oxidant to substrate ratio, were optimized.
Fig. 5 summarizes the results for the optimization of different para-
meter. Table 4 also shows the results of the epoxidation of cyclooctene
under optimized conditions for all of the complexes. The optimized
reaction time was found to be 24 h and complex (3) with Cu central
metal ion was found to be the most efficient catalyst. Then, the amount
of the catalyst was optimized, using 10 mmol of cyclooctene, 20 mmol
of TBHP, reflux in 10 mL of methanol for 24 h and the optimized
amount was found to be 15 µmol. The catalytic reaction was also per-
formed in 10 mL of different solvents i.e. ethanol, methanol, acetoni-
trile and chloroform at reflux, in which the complexes were sufficiently
soluble. Other reaction conditions were kept constant. The solvent-free
complex (4) is neutral, and the central Ni(II) metal ion is coordinated to
two anionic pyrazolonato ligands and two neutral methanol ligands. In
contrary to complexes (1) and (3), the two pyrazolonato ligands in the
complex (4) are in syn configuration with respect to the enolato oxygen
of the pyrazoline ring (i.e. O2 and O4 as shown in Fig. 4). The geometry
around the central metal ion is pseudo-octahedral in which the two
pyrazolonato ligands occupy the basal plane, and the two methanol
ligands occupy the apical positions. As could also be seen from Table 1,
the four basal Ni-O bond lengths are almost equal. Besides, the average
CeO bond length on the pyrazolone rings (2.003 Å) is almost equal to
the average CeO bond length of the acetyl groups (2.034 Å) which
again means that the negative charge is delocalized on the two different
oxygen atoms. The other bond lengths and bond angles are in the same
Scheme 1. The products of catalytic epoxida-
tion of cyclooctene with the studied catalysts.
5

M. Hasanzadeh Esfahani, et al.
Inorganica Chimica Acta 508 (2020) 119637
Fig. 5. Results of the optimization of different parameters affecting the epoxidation of cyclooctene by complex (3). The data are the average of four different
experiments.
Table 4
the other reaction conditions as mentioned above.
The results of the catalytic epoxidation of cyclooctene with different catalysts.
Based on data in Table 4, the Cu(II) complex with square planar
geometry showed the highest catalytic activity while the Fe(III) com-
plex with tris-chelate octahedral structure showed the lowest activity.
The lower catalytic activity of Fe(III) complex could be due to the lack
of coordination sites available for interaction with the reactants since
the highest observed coordination number (CN) for Fe(III) is six. On the
other hand and by considering the crystal structures, the Cu(II) complex
with square planar geometry and CN = 4 has two available coordina-
tion sites for such interactions; hence, it is rational to conclude that the
metal ion-reactants interactions are essential. Complexes (1) and (4)
with CN = 6 also showed low catalytic activity which confirms the
above statement. It follows the previously reported metal ion-oxidant
mechanism [38–40]. These findings further confirm that the central
metal ion is an important factor in catalytic reactions such as olefin
Complex
Conversion percent
Epoxide selectivity%
Other products%
(1)
(2)
(3)
(4)
57.8
37.0
84.7
39.1
79.4
81.4
82.8
87.6
20.6
18.6
17.2
12.4
Conditions: 15 μmol of each catalyst, 10 mmol cyclooctene, 20 mmol TBHP,
reflux, 24 h in methanol. The data are the average of four different experiments.
reaction was also done. Methanol was found as the best solvent in
which the conversion reached about 85% while with other solvents it
was below 50%. The oxidant to substrate ratio was then optimized and
2:1 oxidant:substrate ratio was found as the optimized ratio, keeping
6

M. Hasanzadeh Esfahani, et al.
Inorganica Chimica Acta 508 (2020) 119637
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