Structural investigation of the catalytic activity of Fe(III) and Mn(III) Schiff base complexes

Article abstract of DOI:10.1016/j.poly.2021.115206

Two novel manganese (III) and iron (III) complexes of an N,N-bis(1-naphthalidimine)-o phenylene diamine ligand have been successfully synthesized and characterized by various analytical techniques including single crystal X-ray structure analysis. The lig

Full text of DOI:10.1016/j.poly.2021.115206

Polyhedron 202 (2021) 115206
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Polyhedron
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Structural investigation of the catalytic activity of Fe(III) and Mn(III)
Schiff base complexes
Sabrina Bendia a,d, Riadh Bourzami b, Jean Weiss c, Kamel Ouari a,
⇑
a Laboratoire d’Electrochimie, d’Ingénierie Moléculaire et de Catalyse Rédox (LEIMCR), Faculté de Technologie, Université Ferhat ABBAS Sétif 1, DZ-19000, Sétif, Algeria
b Emerging materials unit, Ferhat Abbas University Setif 1, 19000, Algeria
c Laboratoire de Chimie des Ligands à Architecture Contrôlée (CLAC), UMR 7177 CNRS-Université de Strasbourg, 4 rue Blaise Pascal, Strasbourg 67000, France
d Institut de la Nutrition de l’Alimentation et des Technologies Agro-Alimentaires (INATAA), Université Frères Mentouri – Constantine 1, Algeria
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel manganese (III) and iron (III) complexes of an N,N-bis(1-naphthalidimine)-o phenylene
diamine ligand have been successfully synthesized and characterized by various analytical techniques
including single crystal X-ray structure analysis. The ligand crystallizes in the monoclinic system with
space group P21/c. The detailed analyses of Hirschfeld surface and fingerprint plots provide insight into
the nature of non-covalent interactions in the ligand. Experimental data have been complemented and
interpreted in the light of Density Functional Theory calculation was performed using the B3LYP method
concerning molecular geometries, vibrational frequencies and electrochemical potentials. Cyclic voltam-
metry in dimethylformamide revealed reversible redox processes in both complexes, suggesting possible
catalytic reactivity involving electron transfer process for these complexes. The catalytic efficiency and
selectivity of manganese (III) and iron (III) complexes was tested in the oxidation of cyclohexene with
molecular oxygen. The results show that the catalytic performance depends on the nature of metal,
the most efficient catalyst in the presence of O2beingthe iron (III) complex. A plausible mechanism for
cyclohexene oxidation by complexes is proposed and discussed hereafter.
Received 13 February 2021
Accepted 6 April 2021
Available online 20 April 2021
Keywords:
Crystal structure
Iron complex
Manganese complex
DFT calculations
Electrochemical
Oxidation catalysis
Ó 2021 Elsevier Ltd. All rights reserved.
1. Introduction
of metabolic reactions [16], electrochemical sensors and
electroluminescent devices [17] or novel magnetic molecular
Schiff base ligands are well known for coordinating many
elements and stabilizing these in various oxidation states. Schiff
bases have been used in the preparation of many potential drugs
and possess a broad spectrum of biological activities [1–3] and pre-
vious reports have established that after complexation of transition
metal ions the antimicrobial activity was generally increased [4].
Amidst ligand for transition metals, pincer ligands play an impor-
tant role and their complexes have attracted tremendous interest
due to their high stability, activity and versatility [5].
The chemistry of iron and manganese complexes has been
widely investigated due to their involvement in a variety of biolog-
ical redox systems including peroxidases [6], catalases [7], super-
oxide dismutases [8], dioxygenases [9] and lipoxidases [10].
Manganese and iron chelate complexes are attractive because of
their potential use as catalysts in oxidation reactions [11]. Hence,
they are effective catalysts in alkanes [12] amines [13], alcohols
[14] and sulfides [15] oxidation and electron transfer in a range
materials [18].
Iron (III) and manganese(III) complexes, by comparison to other
transition metal ions, are usually considered as the most important
representative class of complexes exhibiting catalytic and electro-
catalytic activity in the epoxidation of alkenes [19–21]. For both
metals, the generally accepted catalytically active species for the
epoxidation reactions with different substrates and oxygen atom
donors are metal-oxo species.
Oxidation reactions are critical to numerous chemical transfor-
mations, and thus the development of oxidation reactions that rely
on renewable and environmentally friendly oxidants is a concern
for all areas of chemistry [22,23]. Indeed, molecular oxygen is an
ideal oxidant in many ways as the only byproduct of its reduction
is water. Dioxygen is non-toxic under most conditions, and its
reduction potential is more than sufficient to drive many chemical
transformations [24–27]. The coordination of O2 to a transition
metal center also facilitates oxygen transport and storage in living
organisms [28–30]. One of the first discovered dioxygen activating
heme enzymes, cytochrome P-450 [31], has provided many clues
about the catalytic cycles of dioxygen activation and oxygen
⇑
Corresponding author.
E-mail address: kamel_ouari@univ-setif.dz (K. Ouari).
https://doi.org/10.1016/j.poly.2021.115206
0277-5387/Ó 2021 Elsevier Ltd. All rights reserved.

S. Bendia, R. Bourzami, J. Weiss et al.
Polyhedron 202 (2021) 115206
transfer reactions. Design of metal catalysts to mimic bio-oxidative
activity of cytochrome P-450 has continued to be an active area of
research [32].
31G (d. p) basis sets, and Lee-Yang-Parr correlation functional
(B3LYP) [39,40].
Hereafter, we report on the synthesis and characterization of
iron(III) and manganese(III) complexes containing N,N-bis(1-naph-
thalidimine)-o-phenylenediamine ligand as well as their applica-
tions in the catalytic oxidation of cyclohexene by molecular
oxygen. The experimental characterization can be correlated and
explained with DFT calculation as well as the origin of the electron
transfers observed by cyclic voltammetry.
2.4. Hirshfeld surface analysis
The Hirshfeld Surface (HS) and a 2D fingerprint scatter plots for
LH2were obtained using the Crystal Explorer 17.5 package [41].
The normalized contact distance (dnorm) based on both de and di
(where de is distance from a point on the surface to the nearest
nucleus outside the surface and diis distance from a point on the
surface to the nearest nucleus inside the surface) and the vdW radii
of the atom. Graphical plots of the molecular Hirshfeld surfaces
mapped with dnorm used a red-white-blue color scheme; where
red show shorter contacts, white constitute the contact around
vdW separation and blue depict longer contacts.
2. Experimental
2.1. Materials and measurements
The melting points for the ligand LH2 and its complexes of LFe
(III)Cl and LMn(III)Cl were determined with a Kofler Bench 7779
apparatus. Infrared spectra were recorded using KBr pellets on a
Shimadzu FTIR IRAffinity-1 spectrophotometer. Electronic spectra
were measured on a ShimadzuUV-1800 double-beam spectropho-
tometer using DMF as solvent. Elemental analyses were carried out
on an Elementar-Vario EL III CHNSO analyser.1HNMR spectra were
recorded on a Bruker Advance 500 MHz spectrometer usingDMSO-
d6 as solvent and tetramethylsilane (TMS) as internal standard.
Mass spectrum of the ligand LH2is obtained on a Bruker Daltonics
Flex Analysis spectrometer with MALDI-TOF procedure using
dithranol as a matrix. The molar conductivities of the compounds
were carried out on conductivitymeter MeterLab CDM-210
apparatus.
2.5. General procedure of the oxidation reaction
In a typical experiment, 10 mmol of the manganese and iron
catalysts were dissolved in 10 mL of DMF and 10 mmol of cyclo-
hexene in the presence of molecular oxygen (O2), the reaction is
carried out under bubbling with a needle introduced into the mix-
ture. The reaction mixture was refluxed while being stirred for 6 h.
A blank experiment in the absence of the complex was per-
formed and no oxidation products were observed. In gas chro-
matography, the retention times for the starting materials and
the products were determined by comparison with authentic sam-
ples. The conversion percentiles (%) are calculated by the following
equation, in which Cinitial and Cfinal are initial and final concentra-
tion of the substrate, respectively.
The electrochemical properties of the structures were investi-
gated at room temperature in DMF solutions containing tetra-n-
butyl ammonium perchlorate (TBAP), 0.1 M, as supporting elec-
trolyte. A classical three electrodes cell was used with glassy car-
bon (GC) working electrode (WE), a platinum counter electrode
and a saturated calomel electrode (SCE) reference electrode. All
potential values are given versus SCE and the ligand and complexes
solutions were 10ꢀ3 M and the scan rate used in all the voltammo-
%Conversion ¼ 100ðCinitial ꢀ CfinalÞ=Cinitial
2.6. Synthesis
2.6.1. Synthesis of the ligand LH2
The ligand was prepared by adapting literature methods
[42,43]. A mixture of 0.5 mmol(0.054 g) of 1,2-diaminobenzene
and 1 mmol (0.172 g) of 2-hydroxy-1-naphthaldehyde, in 10 mL
of methanol was refluxed with constant stirring under nitrogen
atmosphere for 3 h to yield an abundant orange precipitate that
was collected by filtration. The precipitate was washed succes-
sively with methanol (3 ꢁ 6 mL) and diethyl ether (3 x6 mL),
recrystallized from DMSO-Ethanol and dried under vacuum over-
night. Crystals suitable for X-ray analysis were obtained by slow
evaporation of DMSO. The Schiff base ligand was characterized
by elemental analysis and spectroscopic methods. Yield: 67%,
grams was set to 100 mV sꢀ1
.
2.2. X-ray crystallography
Single crystals of LH2 ligand were grown by slow evaporation of
DMSO solution at room temperature. The crystals were placed in
oil and
a
red prism single crystal of dimensions
0.40 ꢁ 0.38 ꢁ 0.28 mm3 was selected.
X-Ray diffraction data collection was carried out on a Bruker
APEX II DUO Kappa-CCD diffractometer equipped with an Oxford
mp: 221 °C.
20O2N2.0.5MeOH:C, 79.15; H, 5.13; N, 6.48%; found: C, 78.47; H,
5.14; N, 6.29%. Selected IR data (KBr pellets,
cmꢀ1): 3440
(OAH), 1616 (C@N), 1537 (C@C), 1179(CAO); UV–Vis: DMF, k
K( -
Xꢀ1 cm2 moleꢀ1) = 11.98.Analysis calculated:C28
Cryosystem liquid N2 device, using Mo-K
a
radiation
(k = 0.71073 Å). The crystal-detector distance was 38 mm. The cell
parameters were determined (APEX2 software) [33] from reflec-
tions taken from tree sets of 12 frames, each at 10 s exposure.
The structure was solved by direct methods using the program
SHELXS-97 [34]. The refinement and all further calculations were
carried out using SHELXL-97 [35]. The H-atoms were included in
calculated positions and treated as riding atoms using SHELXL
default parameters. The non-H atoms were refined anisotropically,
using weighted full-matrix least-squares on F2. A semi-empirical
absorption correction was applied using SADABS in APEX2 [36];
transmission factors: Tmin/Tmax = 2.69/28.01.
H
r
nm, [e
Mꢀ1 cmꢀ1]: 269[11300], 316[9650], 350[8130], 364[7880],
410[4580]; 1HNMR (DMSO d6, d ppm): 9.69 (s, CH@N), 7.05–8.55
(m, Ar-H); 13CNMR (DMSO d6, d ppm): 157 (C@N), 169 (CAO),
100–150 (C-Ar); MS (MALDI-TOF, dithranol): [MH]+ = 417.16
(Scheme 1).
2.6.2. Synthesis of the complexes
2.6.2.1. Synthesis of LFe(III)Cl. Title complex were obtained by the
general procedure given in the following literature methods
[44,45]. FeCl2ꢂ4H2O0.5 mmol (0.099 g) were dissolved in methanol
5 mL was added to a 5 mL methanol solution containing LH2
0.5 mmol (0.208 g). The mixture was refluxed and stirred for 2 h
under nitrogen atmosphere. A brown compound precipitated out,
washed with methanol and diethyl ether, and then recrystallized
2.3. Quantum chemical calculations
The Theoretical calculations were performed using the Gaussian
09 W program, based on Density Functional Theory (DFT) [37] with
Beck’s three parameters hybrid functional exchange [38], with 6-
2

S. Bendia, R. Bourzami, J. Weiss et al.
Polyhedron 202 (2021) 115206
MeOH
O
HC
N
N
CH
Reflux, 3h
2
+
- H2O
N2 atmospher
OH
HO
OH
H2N
NH2
MeOH
Normal
atmospher
Reflux, 2 h
HC
N
N
CH
FeCl2.4H2O
MnCl2
M
Cl
O
O
M= Fe, Mn
Scheme 1. Synthesis of the ligand (LH2) and the corresponding complexes.
from DMSO/methanol. Yield: 78%; mp > 300 °C.
K
(
Xꢀ1 cm2-
nitrogen to the metal center [48]. In addition, the stretching vibra-
tion of CAO,
C-O/naphtholate, appears at 1179 cmꢀ1 in the ligand
moleꢀ1) = 11.67. Analysis calculated for C28H18O2N2FeꢂH2O: C,
m
68.87; H, 4.13; N, 5.74. Found: C, 68.98; H, 4.30; N, 5.59%; IR
form, whereas in the complexes LFe(III)Cl and LMn(III)Cl, it is
shifted to higher energy by 20 cmꢀ1 suggesting that the metallic
center causes an increase of the electronic density in the vicinity
of the oxygen. This behavior can be interpreted as a result of coor-
dination of both deprotonated oxygen of naphtolic group to the
metal center giving N2O2 tetra-coordinated complexes [49]. The
DFT calculations strongly corroborate the experimental results
and our hypotheses (Table 1).
(KBr pellets,
r
cmꢀ1): 3452 (OAH), 1598 (C@N), 1535(C@C),
1187 (CAO); UV–Vis: DMF, k nm, [e
Mꢀ1 cmꢀ1]: 266[3750], 341
[2770], 409[1790], 476[1240].
2.6.2.2. Synthesis of LMn(III)Cl. To a 5 mL methanolic solution of
MnCl20.5 mmol (0.063 g,), a methanolic solution of the ligand
LH2 0.5 mmol (0.208 g) was added dropwise. The mixture was
refluxed and stirred for 2 h under nitrogen atmosphere and then
filtered. The precipitate, washed with methanol and diethyl ether,
was re-crystallized from DMF-methanol. Yield: 70%; mp > 300 °C.
3.3. Electronic absorption spectra
K
(
Xꢀ1 cm2 moleꢀ1
)
=
26.41.Analysis calculated:C28H18N2O2-
Mnꢂ0.5H2O: C, 70.30; H, 4.00; N, 5.86. Found: C, 69.79; H, 4.26;
N, 5.44%; IR (KBr pellets,
cmꢀ1): 3439 (OAH), 1607 (C@N),
In the spectrum of the ligand (Fig. 1), the band located at
460 nm is assigned to the n ?
(CH@N) group [50]. In the spectra of the metal complexes, these
n ? * transitions, are shifted to lower frequencies indicating that
p* transition from the azomethine
r
1536 (C@C), 1181 (CAO); UV–Vis: DMF, k nm, [e
Mꢀ1 cmꢀ1]: 270
p
[5380], 340[4350], 412[2850], 472[2030].
the imine nitrogen atoms are involved in coordination to the metal
ion. The bands at higher energies 260 and 310 nm are associated
with the aryl
p ? p* transitions. The spectra of the complexes
3. Results and discussion
show tow intense bands in the high-energy, at 345 and 410 nm,
attributed to L ? M charge transfer bands (MLCT) [51].
3.1. Elemental analysis and molar conductance
3.4. NMR spectra
The analytical data are in good agreements with the proposed
structures of the Schiff base ligand and their complexes, corrobo-
rating the occurrence of a half-molecule of methanol in the LH2
ligand. However, the complexes show a ligand–metal of stoichio-
metric ratio of 1:1 affording mononuclear compounds with the
presence of a molecule of water and a half one for respectively
the LFe(III)Cl and LMn(III)Cl complexes.
The observed molar conductance of the ligand and the com-
plexes in DMF (10ꢀ3 M) solution are in the range11.67_26.41 Xꢀ1
cm2 molꢀ1, the molar conductance values are non-electrolytic nat-
ure for the complexes, the Schiff base ligand is also regarded as a
non-electrolyte [46].
The 1H NMR spectrum of the ligand (Fig. 2 (a)) exhibits a mul-
tiplet in the region 6.60–8.80 ppm assignable to aromatic protons
(CH-Ar) of the benzene and naphthalene ring. The sharp singlet at
9.69ppmis due to azomethine proton (CH@N). The phenolic proton
is not observed because of the exchange with the water present in
the DMSO d6.
The 13C NMR spectrum of the ligand is in agreement with the
different types of magnetically nonequivalent carbons (Fig. 2(b)).
The resonance around 157 ppm can be assigned to the azomethine
carbon (–C@N–). The signal observed at 169 ppm corresponds to
the naphtholate (–CAO–). The carbon atoms of the benzene and
naphthalene ring occurring over a broad range 110–140 ppm.
3.2. Infrared spectra
The IR spectra of LH2 and their complexes were analyzed in the
region 4000–400 cmꢀ1. The spectra of ligand and complexes dis-
play IR absorption bands at 1616 and 1594 cmꢀ1, respectively,
which are attributed to C@N transitions of the azomethine moiety
[47]. The shift band of the LFe(III)Cl and LMn(III)Cl complexes
towards lower wave number indicates coordination of the imine
3.5. Mass spectrometry
The mass spectra of the ligand were recorded in MALDI TOF ion-
ization mode (Fig. 3). The protonated molecular ion peak [MH]+ of
the ligand appears at m/z = 417.16, which is in good agreement
with the proposed formula.
3

S. Bendia, R. Bourzami, J. Weiss et al.
Polyhedron 202 (2021) 115206
Table 1
Selected experimental and theoretical infrared frequencies (cmꢀ1) and their assignments for LH2, LMnCl and LFeCl.
LH2
Exp
LFeCl
Exp
LMnCl
Exp
Assignments
Calculated
Unscaled
Calculated
Unscaled
Calculated
Unscaled
Scaled
scaled
scaled
1616
1179
1537
3440
–
1630
1213
1526
3116
–
1581
1176
1490
3336
–
1598
1187
1535
–
493
552
1553
1208
1534
–
485
551
1506
1151
1487
–
470
534
1607
1181
1536
–
556
476
1677
1230
1590
–
577
445
1626
1145
1542
–
560
431
C@N
CAO
C@C
–OH
MꢀN
MꢀO
–
–
–
and 41.83°, but the molecule of LMn(III)Cl (Fig. 4c) is less distorted.
Fig. 4d shows that the angles between the plans are 14.92°, 27.76°
and 42.60° in the case of the LMn(III)Cl complex, a strikingly differ-
ent situation than in the LFe(III)Cl complex (Fig. 3e) in which pla-
narity with an out of the plane positioning of the Fe is observed
(Fig. 4f).
Some characteristic bond distances and angles are listed in
Table 3.
€The LH2 unit cell is shown in the Fig. 5. The crystal is mono-
clinic with P21/c space group and the cell parameters are
a = 13.353(4) Å, b = 7.561(2) Å, c = 24.350(8)Å and b = 90.959
(10)°. The filling rate of the LH2 unit cell is about four molecules
(Z = 4), the unit cell is characterized by (x y z) identity, an inversion
center at the point (000), a glide plane perpendicular to direction
[010] with glide component [0 0 ½] and finally 2-fold screw axis
with the direction [010] at the points (0 y¼) with screw compo-
nent [0 ½ 0].
Fig. 1. Electronic spectra of the ligand and the corresponding complexes in DMF.
3.7. Hirshfeld surface analysis
The Hirshfeld Surface (HS) demonstrates the intermolecular
interactions in the crystalline structures. The Fig. 6 shows the HS
of the ligand LH2mapped over the ranges: dnorm[ꢀ0.1792, 1.2866]
Å, shape index [ꢀ1.0, 1.0] Å and curvedness [ꢀ4.0, 0.4] Å.
The red spots correspond to the closer Oꢂ ꢂ ꢂH/Hꢂ ꢂ ꢂO contacts due
to the NAHꢂ ꢂ ꢂO hydrogen bonds. The white areas mark the places
where the distance between the neighbouring atoms is close to the
sum of the Van derWaals radii of the considered atoms, they indi-
cate Hꢂ ꢂ ꢂH interactions. The blue areas illustrate the domain where
the neighbouring atoms are farthest apart to interact with each
other (Fig. 6 (a)).
3.6. Structural data
The ligand LH2has been characterized by X-ray diffraction. Its
solid state structure matches the optimized theoretical structure
obtained by DFT calculation, whereas the molecular structures of
LMn(III)Cl and LFe(III)Cl were only obtained by DFT calculations.
Fig. 4 summarizes the geometry of the ligand and the calculated
geometries of the complexes. Table 2 summarizes the crystal data,
the data collection and the structure refinement parameters and
Fig. 5shows the unit cell of the ligand LH2.
The XRD analysis proves that the asymmetric unit of LH2 ligand
(Fig. 4a) is very far from a planar geometry; the two naphthalene
moieties and the single aryl belong to three different plans, as
shown in Fig. 4b. The angles between the plans are 28.57°, 39.02°
Fig. 7a illustrates the 2D fingerprint of the totality contacts con-
tributing to the HS of the LH2 ligand. The graph shown in Fig. 6b
represent the Hꢂ ꢂ ꢂO/Oꢂ ꢂ ꢂH contacts between the hydrogen atoms
located inside the HS and the oxygen atoms located at the external
10,0
9,5
9,0
8,5
8,0
7,5
7,0
180
170
160
150
140
130
120
110
100
ppm
(a)
ppm(b)
Fig. 2. NMR spectra of LH2ligand in DMSO d6: (a) 1HNMR, (b) 13CNMR.
4

S. Bendia, R. Bourzami, J. Weiss et al.
Polyhedron 202 (2021) 115206
and reciprocal. It is characterized by two symmetrical points situ-
ated at the top and left and at the bottom right in the region
1.07 Å < (de + di) < 2.57 Å, these Oꢂ ꢂ ꢂH contacts represent 13.3%
of all the intermolecular contacts.
The graph shown in Fig. 7c illustrates the 2D fingerprint associ-
ated with the hydrogen atoms (rvdW = 1.20 Å). It is characterized by
extremity that points to the origin diagonally and corresponding to
diꢃdeꢃ 1.2 Å, which reveals the presence of closer Hꢂ ꢂ ꢂH contacts
within the LH2 crystal. They have the largest contribution to the
total HS (45.9%). The decomposition of the 2D fingerprint also
shows other contacts: Cꢂ ꢂ ꢂH (24.9%, Fig. 7d), CAN (2.07%), CAO
(0.4%), CAS (0.2%), HAS (0.9%) and HAN (0.3%).
3.8. Experimental hydrogen bonds
The hydroxyl groups are involved in intramolecular OAHꢂ ꢂ ꢂN
hydrogen bonds influencing the molecular conformations. The
amino hydrogen atom forms the center of intramolecular hydrogen
bonds, which includes the NAHꢂ ꢂ ꢂO hydrogen bond of 2.5499(16)
Å within the keto amino part. The proton transfer from the hydro-
Fig. 3. Molecular cluster of the masse spectrum of LH2 ligand, MALDI-TOF in
dithranol.
Fig. 4. Molecular structures (a) asymmetric unit of the ligandLH2provided by hydrogen bonds (b) optimized structure of LMn(III)Cl (c) optimized structure of LFe(III)Cl.
5

S. Bendia, R. Bourzami, J. Weiss et al.
Polyhedron 202 (2021) 115206
Table 2
and Cꢂ ꢂ ꢂH contacts. In addition, the DMSO molecules play a crucial
role in the chain as shown in Fig. 8b. The contact between LH2 and
DMSO molecules is maintained via Oꢂ ꢂ ꢂH and Cꢂ ꢂ ꢂH links, along the
c axis the contact Cꢂ ꢂ ꢂC makes the molecules parallel with a dis-
tance 7.562 Å, (Fig. 8 (c)).
Data collection and structure refinement parameters.
Compound
LH2.DMSO
30H26N2O3S
Molecular formula
Molecular weight (g/mol)
C
494.59
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Monoclinic
P21/c
5. Quantum calculations results
13.353 (4)
7.561 (2)
24.350 (8)
90.959 (10)
2458.43(13)
4
5.1. Molecular electrostatic potential (MEP)
b (°)
V (Å3)
Z
The total electron density surfaces mapped with electrostatic
potential for LH2, LFe(III)Cl and LMn(III)Cl are shown in Fig. 9, the
surface of LH2 exhibits a positive potential in the most geometric
zones, and the most negative potential indicated by a red color is
principally centered on the oxygen atoms, those sites can host
nucleophilic ions as transition metal ions, a weak negative poten-
tial around the cycle indicated by a yellow color can be observed.
Furthermore, the surfaces of LMn(III)Cl and LFe(III)Clare very
comparable. The Fe(III) and Mn(III) are coordinated to the oxygen
and nitrogen atoms, where as the apical chlorine bears a strong
negative charge indicated by a red color, the comparison with
the ligand surface, the potentials generated by the cycle are more
positive for the both complex molecules. The non-homogeneity
of the potential distributions for the three molecules results in dif-
ferent values of the dipolar moment 2.644, 5.471 and 5.796 Debye
for LH2, LMn(III)Cl and LFe(III)Cl respectively.
Dcalc (g.cmꢀ3
)
1.336
Crystal size (mm3)
Crystal description
Crystal color
0.40 ꢁ 0.38 ꢁ 0.28
Prism
Red
173(2)
k = 0.71073 Å
0.168
1040
17304/5917
0.0173
ꢀ16 ? 17; ꢀ9 ? 9; –32 ? 32
1.53 ? 28.01
4939
Temperature (K)
Radiation Mo-K
a1
Absorption coefficient (mmꢀ1
F(000)
)
Reflections collected/unique
Rint
Range/indices (h, k, l)
Tetalimit
No. of observed data
I > 2Sigma(I)
No. of variables
No. of restraints
328
0
1.032
0.434, 0.1090
0.0543, 0.1196
2
Goodness of fit on F
R1, wR2 [IP2Sigma(I)]a
R1, wR2 (all data)a
5.2. Molecular orbitals (MOs)
The molecular orbitals and their relative energies give an
important idea on electronic properties and chemical reactions
[53]. The Fig. 10 shows the calculated orbitals for LH2, LMnCl,
LFeCl-a channel and LFeCl-b channel. It can be seen that all elec-
trons of various molecular orbitals are almost delocalized on the
whole molecular structure except for hydrogen atoms.
The gap energy is defined as the energy difference between the
HOMO and LUMO. The calculated energies are 3.17, 2.965, 2.861
and 3.094 eV, for LH2, LMnCl, LFeCl-a channel and LFeCl-b channel
respectively, these high values indicate that the molecules are
stable. Other characteristic physicochemical parameters are sum-
marized in Table 5.
Fig. 5. Unit cell of the ligand LH2.
6. Electrochemical study
xyl oxygen atom of the parent aromatic aldehyde to the imino
nitrogen atom requests a small amount of energy, this proton
transfer causes a remarkable conformation changes within the
The electrochemical measurements have been carried out, for
the ligand LH2 and the complexes LFe(III)Cl and LMn(III)Cl, in
DMF solution, for their high solubility, under nitrogen saturated
atmosphere in the potential range 1600 to ꢀ2200 mV/SCE at scan
rate 100 mV/s. The Fc+/Fc, redox couple of ferrocene, was used as
an internal standard.
molecule, especially in the p-electron distribution [52].
Table 4 summarizes the intramolecular hydrogen bonds with
corresponding donors and acceptors angles and bond lengths.
The planarity within the naphthaldimine part of the molecule is
preserved since the pseudo aromatic six membered chilate rings
formed by intramolecular hydrogen bonds (H1OꢀO1ꢀC17ꢀC8ꢀC7ꢀ-
N1 and O2ꢀC28ꢀC19ꢀC18ꢀN2ꢀH2N) are almost coplanar with cor-
responding naphthalene rings (C17 to C8 and C28 to C18). The
twisting of the naphthaldimine moiety toward the phenyl ring is
described by torsion angles C7ꢀN1ꢀC1ꢀC6 and C18ꢀN2ꢀC6ꢀC1
being 148.23 (1) and 160.2 (1)° respectively.
The cyclic voltammogram of the ligand shows irreversible ano-
dic and cathodic peaks (Fig. 11), appearing at Epa1 = ꢀ1.6, Epa2 = 0.9,
E
pa3 = 1.2 V/SCE and at Epc1 = ꢀ1.5, Epc2 = ꢀ1.7 V/SCE respectively.
These irreversible waves are attributed to the oxidation of the
bonds of the naphtholate moiety and the benzene ring.
p
The voltammetric study of LFe(III)Cl complex, in the range 100
reversible redox couple at
to ꢀ500 mV/SCE, shows
a
E1/2 = ꢀ320 mV/SCE, assignable to the FeIII/FeII couple [54].
Fig. 12a shows multiple scans that resulted in superposable cyclic
voltammograms of the redox couple. Minimal variation is observed
for the potential Ep when the scanning speed increases, which is
characteristic of a reversible system. A linear relationship is
observed between the cathodic and the anodic peak currents and
the square root of the scanning speed (v1/2) in the range 10 to
4. 3D network
The structure of the 3D network of LH2 ligand is enriched by
weak contacts maintaining the stability and the structural cohe-
sion. Fig. 8 shows the 3D network in the views (a) along the b axis
(b) according to the a, b plan (c) along the c axis. Along the b axis,
LH2 molecules form chains in which they are attached by Hꢂ ꢂ ꢂH
100 mV/s (Fig. 12(c)). The value
D
Ep = (Epa ꢀ Epc) is 80 mV/SCE
6

S. Bendia, R. Bourzami, J. Weiss et al.
Polyhedron 202 (2021) 115206
Table 3
Selected experimental and theoretical bond lengths (Å) and angles values (°).
LH2
LMn(III)Cl
LFe(III)Cl
Selected bonds
X-ray (Å)
Calc. (Å)
Selected bonds
Calc. (Å)
Selected bonds
Calc. (Å)
C1–N1
C6–N2
C7–N1
C17–O1
C18–N2
C28–O2
1.410(17)
1.409(17)
1.298(17)
1.327(18)
1.311(17)
1.290(18)
1.41
1.41
1.298
1.328
1.311
1.291
Mn-O1
Mn-O2
Mn-N1
Mn-N2
Mn-Cl
O1-C17
O2-C28
N1-C7
N2-C18
N1-C1
N2-C6
1.858
1.826
1.898
2.042
2.208
1.291
1.32
1.333
1.301
1.416
1.408
Fe-O1
Fe-O2
Fe-N1
Fe-N2
Fe-Cl
O1-C17
O2-C28
N1-C7
N2-C18
N1-C1
N2-C6
1.873
1.873
1.922
1.922
2.284
1.32
1.32
1.329
1.329
1.426
1.426
Selected angles
C2-C1-N1
C6-C1-N1
C5-C6-N2
C1-C6-N2
X-ray (°)
Calc. (°)
122.66
117.52
122.26
117.69
121.31
122.53
117.18
122.15
122.8
Selected angles
O2-Mn-O1
O1-Mn-N1
O1-Mn-N2
O2-Mn-N1
N1-Mn-N2
O2-Mn-N2
N1-Mn-Cl
N2-Mn-Cl
O1-Mn-Cl
Calc. (°)
94.2
94.15
172.22
132.89
80.16
85.95
107.58
87.19
99.66
116.47
Selected angles
O2-Fe-O1
O1-Fe-N1
O2-Fe-N1
O1-Fe-N2
N1-Fe-N2
O2-Fe-N2
N1-Fe-Cl
Calc. (°)
87.78
91.35
163.73
163.75
84.93
91.35
94.74
94.73
101.35
101.36
122.70(12)
120.00(13)
122.26(13)
117.69(12)
121.31(13)
122.53(13)
117.18(13)
122.15(13)
122.81(13)
118.91(14)
120.95(12)
124.23(12)
N1-C7-C8
O1-C17-C8
O1-C17-C16
N2-C18-C19
O2-C28-C19
O2-C28-C27
C7-N1-C1
N2-Fe-Cl
O1-Fe-Cl
O2-Fe-Cl
118.91
120.95
124.23
O2-Mn-Cl
C18-N2-C6
Fig. 6. Hirshfeld surfaces mapped with (a) dnorm, (b) shape index and (c) curvedness of LH2.
and the intensity ratio ipa/ipc is 0.83, these values reveal a reversi-
ble redox process, a typical behavior for an electron transfer pro-
cess controlled by diffusion.
both complexes LFe(III)Cl and LMn(III)Cl can be calculated using
the oxidation and reduction waves respectively following the
empirical relations EHOMO = ꢀ[Eoxydation-E1/2(ferrocene) + 4.8] eV and
ELUMO = ꢀ[Ereduction-E1/2(ferrocene) + 4.8] eV.
The cyclic voltammetric studies of LMn(III)Cl complex exhibit a
reversible redox couple at E1/2 = ꢀ115 mV/SCE corresponding to
MnIII/MnII [55]. These results are consistent with a reversible
one-electron reduction of the LMn(III)Cl complex (Fig. 12b). The
variation of peak current parameters as a function of scan rate is
plotted in Fig. 12c.This is slightly smaller than the corresponding
value for the one-electron reduction in LMn(III)Cl complex.
The electrochemical responses of the two LFe(III)Cl and LMn(III)
Cl complexes are almost identical which indicate that the elec-
troactive species, in DMF, for the iron and the manganese com-
plexes have probably the same redox character. It is mentioned
that a similar process in Fe(III)/Fe(II) has been observed in the iron
(III) mononuclear compounds derived from the related Schiff base
ligand [56].
Furthermore, the DFT calculation can predict the reduction and
oxidation potentials, since the first reduction and oxidation occurs
at the HOMO and LUMO and the other oxidation reduction occurs
at the other molecular orbitals, generally, a correlation between
cyclic voltammetry waves and frontier orbitals energies can be
established. The HOMO and LUMO energies of the ligand LH2 and
For the ligand, all oxidation and reduction peaks correspond to
the removing or addition of electrons from or to the conjugated
p
system, the potentials are summarized in Table 6, which illustrate
that the waves affect the first and the second levels of FMOs. The
first oxidation/reduction affects mostly the orbitals around the
naphthalene moieties and the second affects the orbitals around
the benzene rings in Fig. 10.The comparison of experimental and
theoretical values of oxidation and reduction potentials is illus-
trated in Table 6. Moreover, the experimental and theoretical gap
are 2.4, 2.9 and 3.17, 3.94 respectively.
In the LFe(III)Cl complex, the redox couple observed at
E1/2 = ꢀ0.32 V corresponds to the addition of an electron in
a-
LUMO, ELUMO = ꢀ[E-0.32-E1/2(ferrocene) + 4.8] = ꢀ3.6 eV, indicating
that the first oxido-reduction event takes place at the metallic cen-
ter Fe(III) in Fig. 10.
Also, the manganese complex has almost a comparable calcu-
lated molecular orbital energies as the ligand, in addition, the the-
oretical and experimental gaps of the ligand LH2 and the
manganese complex are nearly equal (Table 5), according to the
7

S. Bendia, R. Bourzami, J. Weiss et al.
Polyhedron 202 (2021) 115206
Fig. 7. (a) Fingerprint plots Full contact, (b) resolved into Oꢂ ꢂ ꢂH, (c) resolved into Hꢂ ꢂ ꢂH and (d) resolved Cꢂ ꢂ ꢂH for the ligand LH2.
Table 4
Hydrogen-bond geometry (Å, °)of LH2.
D—Hꢂ ꢂ ꢂA
D—H
Hꢂ ꢂ ꢂA
Dꢂ ꢂ ꢂA
D—Hꢂ ꢂ ꢂA
N2— H2Nꢂ ꢂ ꢂ O2
O1—H1ꢂ ꢂ ꢂ N1
0.88
0.84
1.84
1.79
2.5499(16)
2.5384(16)
135.7
147.6
Fig. 8. 3D network in the views (a) along the b axis (b) according to the b, a plan (c) along the c axis.
8

S. Bendia, R. Bourzami, J. Weiss et al.
Polyhedron 202 (2021) 115206
Fig. 9. Total electron density map with electrostatic potential of LH2, LMnCl and LFeCl.
Fig. 10. Frontiers Molecular Orbitals plots.
presentations HOMO, LUMO, HOMO-1 and LUMO + 1 of complex
LMn(III)Cl, the orbitals of the metal and of the ligand are over-
lapped, Fig. 10. Thus DFT calculation corroborates the experimental
electrochemical data.
7. Chemical reactivity
The reactivity of our compounds could be discussed mostly on
the basis of its electronic orbitals and their energies [57] which
clearly control the various steps of the catalytic cycle. The high val-
9

S. Bendia, R. Bourzami, J. Weiss et al.
Polyhedron 202 (2021) 115206
Table 5
Calculated HOMO and LUMO energies and some related physicochemical parameters.
Compound
LH2
LMnCl
LFeCl-
a
LFeCl-b
Electronic band energies
EHOMO (eV)
ꢀ5.200
ꢀ5.675
ꢀ6.092
ꢀ2.030
ꢀ1.737
ꢀ0.658
3.17
ꢀ5.265
ꢀ5.693
ꢀ5.856
ꢀ2.300
ꢀ2.174
ꢀ1.906
2.965
ꢀ5.952
ꢀ5.962
ꢀ6.469
ꢀ3.091
ꢀ2.648
ꢀ2.295
2.861
ꢀ5.756
ꢀ6.169
ꢀ6.442
ꢀ2.662
ꢀ2.507
ꢀ2.252
3.094
E
E
HOMO-1(eV)
HOMO-2(eV)
ELUMO (eV)
E
E
LUMO+1(eV)
LUMO+2(eV)
Energy gap (D) (eV)
Electrochemical parameters
Dipolar moment (Debye)
Ionization potential (I)
Electron affinity (A)
2.644
5.20
2.03
1.585
0.631
3.615
3.491
ꢀ0.069
5.471
5.265
2.30
1.452
0.688
3.782
4.925
ꢀ0.066
5.796
5.952
3.091
1.430
0.699
4.521
7.146
ꢀ0.055
5.796
5.756
2.662
1.547
0.646
4.209
5.725
ꢀ0.059
Global hardness (
g
)
Global softness (
r
)
Electronegativity (v)
Global electrophilicity (
x)
Chemical potential (
l)
I = ꢀEHOMO, A = ꢀELUMO
,
v
= ꢀ[1/2(ELUMO + EHOMO)],
g
= 1/2(ELUMO- EHOMO), x = g. r = 1/g, l = ꢀ1/2(A + I).
v2/2
Also, the catalytic rates, for the investigated LM(III)Cl com-
plexes, are linked to the redox potential of the M+3/M+2couple dur-
ing the catalytic cycle. According to the mechanistic hypothesis,
the Lewis acidity of the metal (III) center in the metal (II)-oxo inter-
mediate significantly affects the redox potential of the complexes
and, therefore, the degree of the catalytic reactivity [58].
Hence, a relationship between the catalytic properties of the
complexes and the redox potentials can be established. Since the
electronic effects control the reactivity in the catalytic cycle, cyclic
voltammetry is a useful tool to investigate the mechanism of the
catalysis and for the understanding of the structure/reactivity rela-
tionship in our LM(III)Cl complexes. When the half-wave potential
values, E1/2, approaching the zero value, the activity of the com-
plexes is appreciable in the aerobic oxidation [59]. Herein, The
E1/2 values of the complexes LMn(III)Cl and LFe(III)Cl are respec-
tively ꢀ115 and ꢀ320 mV/SCE, therefore, this would imply that a
window of E1/2 values exists and these values initially favor the fix-
ation of molecular oxygen by the complexes prior to the start of the
effective catalysis.
Fig. 11. Cyclic voltammogram of LH2in 0.1 M TBAP/DMF at 100 mV/s.
ues of the respective HOMO energies (EHOMO
)
ꢀ5.265 and
ꢀ5.952 eV for LMn(III)Cl and LFe(III)Cl complexes reflect a low
donor character whereas the low values of the respective LUMO
energies (ELUMO) ꢀ2.300 and ꢀ3.091 eV reflect an acceptor charac-
ter in chemical transformations. The relatively small gap 2.965 and
2.861 eV implies low kinetic stability and high chemical reactivity
[56] and the negative HOMO and LUMO energies indicate that the
complexes are globally predisposed to act as reducing agents. Fur-
8. Catalytic reactions
thermore, the high values of dipolar moment (l) 5.796 and 5.471 D
The catalysts LFe(III)Cl and LMn(III)Cl complexes oxidize the
cyclohexene molecule in presence of molecular oxygen and give
the cyclohexene oxides as 2-cyclohexen-1-one,2-cyclohexen-1-ol
and epoxy-cyclohexane (Table 7 and Fig. 13). Both the catalysts
of the LMn(III)Cl and LFe(III)Cl complexes is an indicator of a high
polarity of the covalent bonds correlating with the molecular dis-
tribution of electrons [57].
Fig. 12. Cyclic voltammograms in DMF/0.1 M TBAP of (10ꢀ3 M) (a): LFe(III)Cl, (b):LMn(III)Cl. (c): Plot of cathodic currents vs. the square root of sweep rate (v1/2).
10

S. Bendia, R. Bourzami, J. Weiss et al.
Polyhedron 202 (2021) 115206
Table 6
Experimental and calculated potential.
Compound
LH2
Epa1
Epa2
Epa3
Epc1
Epc2
Experimental
Theoretical
ꢀ1.60
0.90
0.90
1.20
1.37
ꢀ1.50
ꢀ2.27
ꢀ1.77
ꢀ2.56
/
Table 7
Catalytic oxidation of cyclohexene with O2 by LFe(III)Cl and LMn(III)Cl complexes.
Catalyseurs
Time (h)
Conversion (%)
Products (%)
Epoxy-cyclohexane
2-Cyclohexen-1-ol
2-Cyclohexen-1-one
LFe(III)Cl
LMn(III)Cl
Oxidant free
6
6
6
96
97
–
20
23
–
16
17
–
64
60
–
are selective to cyclohexen-1-one over the two other oxidation
products and this result is revealed by gas chromatography.
From the GC data, it is observed that the LFe(III)Cl and LMn(III)
Cl complexes exhibited the highest yield for cyclohexen-1-one, 64
and 60%, respectively. The iron and manganese complexes are
included in the family of ‘‘best” catalysts for the oxidation of olefins
[55]. Thus, screening for oxidation catalysis using molecular oxy-
gen as the terminal oxidant identified the iron and the manganese
complex as the oxidation catalysts is worthy of continued
development.
The electron-rich cyclohexene has a greater reactivity than the
terminal olefins. This reflects the electrophilic nature of the oxygen
of the iron-oxo intermediate to the cyclohexene. The proposed
mechanism of cyclohexene oxidation catalyzed by the LFe(III)Cl
and LMn(III)Cl complexes consists of 3 steps and involves the bind-
ing of molecular oxygen and substrate to the iron and manganese
Fig. 13. Oxidation of cyclohexene using LFe(III)Cl and LMn(III)Cl catalysts.
N
N
O
OH
MIII
O2
+
+
O
O
O
Cl
Cl
Cl
N
N
N
N
MV
M V
O
O
O
O
O
O
O
O2
Cl
N
N
MV
O
O
-O
O
M = Fe, Mn
Fig. 14. Formation of different intermediate species during the cyclohexene oxidation.
11

S. Bendia, R. Bourzami, J. Weiss et al.
Polyhedron 202 (2021) 115206
-6.3
Acknowledgement
The Authors gratefully acknowledge the financial support from
The Directorate General for Scientific Research and Technological
Development (DGRSDT) in Algeria. They also acknowledge the help
to access the NMR Facility and Microanalysis Service at Chemistry
Institute of the University of Strasbourg (UNISTRA), France.
Cyclohexene
-6.4
-6.5
Epoxy-cyclohexane
2-Cyclohexene-1-one
2-Cyclohexene-1-ol
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.poly.2021.115206.
-8.4
-8.5
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