New catalytic approach for nano-sized V(IV), Cr(III), Mn(II) and Fe(III)-triazole complexes: detailed spectral, electrochemical and analytical studies

Article abstract of DOI:10.1007/s11164-018-03714-y

A new series of metal ion complexes from a triazole-Schiff base was synthesized and fully characterized. The tridentate mode of coordination was the only mode of bonding in a 1:1 (M:L) molar ratio. Octahedral geometry was the only structural form proposed for all chelating compounds. This geometry was established based on UV–Vis, magnetic moments and ESR studies. The nanometer feature for all complexes was extracted from SEM images and XRD data. CV electrochemical study over Fe(III) and Mn(II) complexes (examples) displayed a i_p,c/i_p,a ratio over 1, which points to irreversible electrode couples. TGA and kinetic parameters reflect a best view about the thermal stability of all tested compounds. Molecular modeling was achieved through an advanced program, to optimize the structural forms and estimate significant parameters. All features concluded from all implemented studies orient us towards the best compounds serving in the intended catalytic application. Proceeding from this, Fe(III) and Mn(II) complexes were chosen for their heterogeneous catalytic application, as they can be used without treatment. Moreover, amounts from such complexes were ignited in open air (calcinations) to produce their corresponding oxides, Fe₂O₃ and MnO₂. The synthesized oxides were fully analyzed to establish their chemical formula, as well as extract their morphological characters, which are the main players in the catalytic field. The surface characteristic and particulate sizes show promise in a catalytic role compared to other similar purchased oxides. The original complexes and their synthesized oxides were utilized separately to synthesize biodiesel from waste oils through heterogenous catalytic process. This process was conducted through a comparative study with other referenced methods. The catalytic role of prepared oxides was clearly observed whenever their original complexes did not display catalytic features as desired. Moreover, this comparative study was prolonged versus purchased oxides (Fe₂O₃ and MnO₂) and the differentiation based on reaction yields after verification of physical features. The priorities of synthesized oxides in the catalytic process were impressive and confirm access to the desired goal.

Full text of DOI:10.1007/s11164-018-03714-y

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-018-03714-y
New catalytic approach for nano‑sized V(IV), Cr(III),
Mn(II) and Fe(III)‑triazole complexes: detailed spectral,
electrochemical and analytical studies
Layla Almazroia, et al. [full author details at the end of the article]
Received: 3 October 2018 / Accepted: 14 December 2018
© Springer Nature B.V. 2019
Abstract
A new series of metal ion complexes from a triazole-Schif base was synthesized
and fully characterized. The tridentate mode of coordination was the only mode of
bonding in a 1:1 (M:L) molar ratio. Octahedral geometry was the only structural
form proposed for all chelating compounds. This geometry was established based on
UV–Vis, magnetic moments and ESR studies. The nanometer feature for all com-
plexes was extracted from SEM images and XRD data. CV electrochemical study
over Fe(III) and Mn(II) complexes (examples) displayed a ip,c/ip,a ratio over 1, which
points to irreversible electrode couples. TGA and kinetic parameters refect a best
view about the thermal stability of all tested compounds. Molecular modeling was
achieved through an advanced program, to optimize the structural forms and esti-
mate signifcant parameters. All features concluded from all implemented studies
orient us towards the best compounds serving in the intended catalytic application.
Proceeding from this, Fe(III) and Mn(II) complexes were chosen for their hetero-
geneous catalytic application, as they can be used without treatment. Moreover,
amounts from such complexes were ignited in open air (calcinations) to produce
their corresponding oxides, Fe2O3 and MnO2. The synthesized oxides were fully
analyzed to establish their chemical formula, as well as extract their morphological
characters, which are the main players in the catalytic feld. The surface characteris-
tic and particulate sizes show promise in a catalytic role compared to other similar
purchased oxides. The original complexes and their synthesized oxides were utilized
separately to synthesize biodiesel from waste oils through heterogenous catalytic
process. This process was conducted through a comparative study with other refer-
enced methods. The catalytic role of prepared oxides was clearly observed whenever
their original complexes did not display catalytic features as desired. Moreover, this
comparative study was prolonged versus purchased oxides (Fe2O3 and MnO2) and
the diferentiation based on reaction yields after verifcation of physical features.
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1116
4-018-03714-y) contains supplementary material, which is available to authorized users.
Vol.:(0123456789)
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L. Almazroia et al.
The priorities of synthesized oxides in the catalytic process were impressive and
confrm access to the desired goal.
Keywords Biodiesel synthesis · Catalytic activity · Nanometer complexes ·
Electrochemical study
Introduction
Triazole-ligands with N, O and S donors are very appealing to researchers in coor-
dination chemistry. 1,2,4 Triazole and their Schif bases display outstanding phar-
macological activities as anticonvulsants [1], antimicrobials [2, 3], antitumor agents
[4], analgesics [5], and antidepressants [5]. Generally, Schif bases are exceedingly
used as versatile ligands, which are ambidextrous to coordinate with most metals and
stabilize them in diferent oxidation states [6]. On the other hand, the Fe3+ and Mn2+
-triazole complexes exhibit great catalytic activity in distinct organic reactions [7].
Such catalytic behavior was observed through shortened reaction time and low cata-
lyst tonnage. Biodiesel is an alternative energy source that has attracted the attention
of researchers in recent years. Fossil materials or the oils extracted from animals or
plants (food) are the feedstock of biodiesel production. The scope of implementing
food as a feedstock is based on the environmental conditions and soil nature. In spite
of several choices of food as raw materials, these feedstocks are mainly dependent
on the bio-fuel industry, which has an efect on the cost of food and plant resources.
Non-food feedstock is considered an alternative and renewable option in developed
countries in order to overcome obstacles to economic growth [8]. The worldwide
research has focused on biodiesel production from waste cooking oil (WCO) as a
third generation source, which has a positive efect on the efciency of such fuel.
Blending of castor oil and squandering fsh oil (50–50%) enhances the character
of biodiesel and conversion using KOH by 97% [9]. The challenges faced in the
homogenous process using NaOH or KOH solution are involved in esterifcation
of WCO. The high content of fatty acids in WCO may cause emulsion and in turn
reduced the yield of biodiesel [10]. Several materials have been used as a hetero-
geneous catalyst other than sodium hydroxide. Acetone was used as a co-solvent
for the frst time in biodiesel production with methanol and Ca2Al2O5 as a catalyst.
The yield obtained was 97% when 20% of acetone was used [11]. The catalysts have
been modifed to enhance their activity in biodiesel studies. Solid acid catalysts such
as SO−4 2/TiO2–SiO2 and SO4−2/ZrO2 have a promising efect in the bio-fuel industry
from WCO [12, 13]. The catalysts prepared with or without an organic template dis-
played a great efect in the conversion of WCO, and their activity was not afected
after many uses [14, 15]. Proceeding from all previous reports, the aim of this study
was established. First of all, we intend to synthesize a series from VO(II), Cr(III),
Fe(III), Mn(II) and Pt(IV)-triazole-Schif base complexes. Metal ions are selected
based on their broad catalytic history. Secondly, the structural and chemical for-
mulae for all new complexes, will be built based on full characterization through
experimental and theoretical tools. Finally, the comparative catalytic activity of the
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New catalytic approach for nano-sized V(IV), Cr(III), Mn(II)…
chosen complexes is the main goal in this study. Moreover, this application will be
prolonged to their corresponding oxides, which are extracted from calcinations in
open air. The catalytic behavior of such prepared oxides are compared with origi-
nal complexes, as well as the purchased oxides, toward biodiesel preparation. The
biodiesel is to be prepared from waste oil by heterogeneous catalysis versus other
referenced homogenous processes. Also, most previous research concerning hetero-
geneous catalysis depends on using an unsafe process by utilizing CaO catalyst.
Experimental
Reagents used
Trifouroacetic acid, thiocarbohydrazide, dioxane, ethanol, 3,4-dimethoxybenzalde-
hyde, acetylacetone, sulforyl chloride, diethyl ether, sodium acetate trihydrate, ani-
line, sodium nitrate, HCl are the chemicals used for preparing triazole-Schif base
derivative and were used without further purifcation. MnCl2·4H2O, CrCl3·6H2O,
FeCl3·6H2O, PtCl4 and VOSO4·XH2O were the metal salts used for complexation.
Methanol, NaOH, West Oil, Na2SO4 anhydrous and NaCl were the reagents used
for biodiesel production. All chemicals used were purchased from Merck, Sigma &
Aldrich as PDH.
Equipment
The content of elements (C, H and N) were estimated micro-analytically using a Per-
kin-Elmer 2400CHN Elemental Analyzer. The metals and conjugated anions were
determined by standard methods [16]. Also, MnO2 and Fe2O3 were analyzed using
complexometric titration after complete digestion in Conc. HNO3 [16]. FT-IR spec-
tra were extracted (KBr disc) using a FT-IR Bruker Tensor 27 spectrophotometer
within 4000–400 cm−1 range. 1HNMR spectrum was recorded in Burker 500 MHz.
The mass spectrum at 70 eV was measured by using an AEIMS 30 mass spectrom-
eter with heating rate 40 °C min−1 and mass range (50–1000). The electronic spectra
were screened (in DMSO) over 200–800 nm range using a Shimadzu UV-3600 spec-
trometer. EPR spectra of powdered complexes were recorded by Bruker EMX Spec-
trometer working in X-band (9.60 GHz) with 100 kHz modulation frequency. The
microwave power was adjusted at 1 mW and the modulation amplitude at 4 Gauss.
The low feld signal was obtained after 4 scans with tenfold increase in the receiver
gain using 2 mm quartz capillary at room temperature. The magnetic susceptibility
√
ꢀ
values were determined (at 25 °C) by μef=2.828 XM T, ϰM is the molar suscepti-
bility, which was corrected by Pascal’s constants for diamagnetism of all atoms in
the ligand using a Johnson Matthey magnetic susceptibility balance. The molar con-
ductivity values were measured in DMSO (10−3 mol L−1) at room temperature using
a JENWAY model 4070 conductance bridge. XRD patterns were recorded on an
X-ray difractometer (GNR, APD2000PRO, Italy) with a graphite monochromator
by 0.03°min−1 scanning rate using Cu/Kα1 radiation. Scanning electron microscopy
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L. Almazroia et al.
(SEM) images were obtained by using a Joel JSM-6390 instrument. Thermogravi-
metric analysis (TGA) was tested from ambient temperature up to 800 °C using a
Shimadzu TG-50 thermal analyzer under nitrogen with 10 °C min−1 heating rate.
Cyclic voltammetry measurements were executed using a potentiostat wave genera-
tor equipped with a Phillips PM 8043 X-Y recorder (in DMF solvent).
Synthesis process
Synthesis of triazole‑Schif base ligand
Equivalent moles from trifuoroacetic acid (1) and thiocarbohydrazide (2) (in water)
were refuxed for 5 h, then the mixture was cooled. The white crystals obtained,
were isolated through fltration followed by washing with cold water and recrys-
talized from water to produced pure triazole compound 3 [17]. Condensation for
aminotriazole thiol derivative 3 (2.50 mol), with 3,4-dimethoxybenzaldehyde in gla-
cial acetic acid (20 mL) under thermal heating for 5 h, produced 4-((3,4-dimeth-
oxybenzylidene)amino)-5-(trifuoromethyl)-4H-1,2,4-triazole-3-thiol (4) after fltra-
tion and re-crystallization from ethanol and dioxane mixture [18]. To intense the
number of donor sites in the ligand, compound 4 (2.50 mol) refuxed with hydra-
zonoyl chloride 5 (2.50 mol, in 30 mL dioxane) in the presence of triethylamine
(0.35 mL) for 5 h. Then the solution was poured on acidifed ice water (with HCl)
(Scheme 1 of synthesis). The solid product was fltered of and re-crystallized from
dioxane/ethanol to aford the intended ligand (6, Fig. 1) [19]. The isolated orange
1
solid product, has the following analysis data; (0.98 g, 80%), mp 174–176 °C, H
NMR (DMSO-d6) (Fig. 1S,A); 2.49 (s, 3H, COCH3), 3.76 (s, 3H, OCH3), 3.78 (s,
3H, OCH3), 6.99–8.12 (m, 8H, Ar–H), 8.16 (s, 1H,=CH), 10.47 (s, 1H, NH). MS
Scheme 1 Synthesis process of 4-[(3,4-dimethoxybenzylidene)amino]-5-(trifuoromethyl)-4H-1,2,4-tria-
zol-3-yl-2-oxo-N-phenylpropanehydrazonothioate (6)
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New catalytic approach for nano-sized V(IV), Cr(III), Mn(II)…
Fig. 1 Modeling forms of 4-((3,4-dimethoxybenzylidene)amino)-5-(trifuoromethyl)-4H-1,2,4-triazole-
3-yl-2-oxo-N-phenylpropanehydrazonothioate
m/z (%) (Fig. 1S,B); 493 (M++1, 0.3), 492 (M+, 1), 273 (6), 163 (100), 148 (32),
137(10), 120 (21), 118 (11), 102 (11), 79 (26), 77 (44), 65 (18). Anal. Calcd. for
C21H19F3N6O3S (492.48) Calcd: C, 51.22; H, 3.89; N, 17.07. Found: C, 51.39; H,
4.02; N, 17.33%.
Synthesis of metal ion complexes
The complexes were synthesized by refuxing a 1:1 molar ratio from ligand 6
(2.46 g, 5 mmol), and each metal salt (MnCl2.4H2O, CrCl3.6H2O, FeCl3.6H2O,
PtCl4, VOSO4.XH2O) in dioxane (40 mL) for 10 h. Triethylamine (1 mL) was added
to precipitate solid complexes followed by fltration (on hot), washing with methanol
and dried over CaCl2 desiccators.
Synthesis of metal oxides
Two selected complexes (Fe(III) and Mn(II)), were chosen for heterogeneous cataly-
sis testing, with the aim to prepare biodiesel fuel, comparing with their oxides iso-
lated after full calcination in open air. The calcination process of the two complexes
was performed in a porcelain crucible over a hot plate until reaching a constant
weight to assure the complete evaporation of the organic ligand, after which the
metal oxides were formed. After complete calcinations, the prepared oxides were
washed with chloroform and diethyl ether and dried carefully. The prepared oxides
are attributed to Fe2O3 (deep brown) and MnO2 (black). Their molecular formulae
were established frstly by standard analytical method, after complete digestion for
defnite weights in conc. HNO3 [16].
Biodiesel synthesis from waste cooking oils
Homogeneous catalysis
Before starting the transestrifcation, the waste oil (WO) was fltrated of to dispose
of the suspended solids and the contained moisture was dried in an oven. In a round
fask, crushed NaOH (0.5 g, 0.125 mol) was added to methanol (30 mL), then the reac-
tion mixture was stirred to dissolve the base completely. A volume of 30 mL waste oil
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L. Almazroia et al.
was measured and mixed with dissolved sodium hydroxide. The reaction mixture was
refuxed at 120 °C for 1 h with continuous stirring. The same reaction was repeated
with diferent times (2 h and 3 h). All attempts conducted to separate biodiesel by this
procedure failed, and the mixture changed to an emulsion.
Heterogeneous catalysis
Using oxides from calcinations The same process was repeated by utilizing metal
oxides extracted from calcinations for two chosen complexes (Fe(III) and Mn(II)).
The yielded oxides, Fe2O3 and MnO2 , were used instead of NaOH under refux for
only 1.5 h. The catalyst was separated from reaction mixture by centrifugation fol-
lowed by fltration, drying, then followed by weighing to assure the absence of weight
loss. The reaction mixture was conveyed in a separating funnel, and after 20 min,
perfect phase separation occurred. The biodiesel was the less intense phase (upper
layer), which was separated and dried over anhydrous Na2SO4 and then weighed.
Moreover, the same process was repeated using one typical oxide purchased (MnO2)
and comparing the biodiesel yields.
Using Fe(III) and Mn(II) complexes The procedure of biodiesel preparation from WO
was repeated using the original Fe(III) and Mn(II) complexes (part A) as a solid catalyst,
separately.
Electrochemical study
Cyclic voltammetry measurements were executed using a potentiostat wave generator
for dissolved complexes (in DMF solvent). The electrode consists of platinum wires of
0.5 cm diameter as the working and counter electrodes, while Ag/AgCl was used as the
−
reference electrode. Tetrabutylammonium tetrafouroborate (TBA+BF4 ) was used as a
supporting electrolyte.
Theoretical implementation
Kinetic calculations
Thermodynamic parameters have been calculated over chosen decomposition stages
(2nd and 3rd). These two stages have suitable sharpness (moderate) for extracting frac-
tion decompose at variable temperatures.
The order (n) and the energy of activation (E) were extracted. Along the scientifc
research, many researchers [20–25] have executed equations and discuss their advan-
tages in calculating thermodynamic parameters. The rate of degradation is the result of
two separate functions (temperature and conversion factor) using:
k(T)f(ꢀ),
(1)
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New catalytic approach for nano-sized V(IV), Cr(III), Mn(II)…
whereas α is a fraction decomposed at time t, k(T) is the T dependent function and
f(α) is the conversion function. The dependent function k(T) and the rate constant
are Arrhenius type:
∗
K = A e−E ∕RT
,
(2)
where R is the gas constant in (J mol−1 k−1). Substituting Eq. (2) into Eq. (1) we get
this equation:
(
)
dꢀ
dT
A
=
f(ꢀ),
(3)
−E∗∕RT
ꢁe
where φ is the linear rate of heating (dT/dt). From integration and approximation,
this equation can be obtained:
[
]
−E∗
AR
ꢁE∗
ln g(ꢀ) =
+ ln
,
(4)
RT
where g(α) is an index depending on reaction mechanism. The right hand side is
recognized as a temperature integral and has no closed form solution. So, diverse
methods have been used to estimate the temperature integral. Using Coat–Redfern
[20] and Horowitz–Metzger methods [25], the kinetic parameters for selected stages
of TGA curves of chelating compounds were computed.
DFT/B3LYP optimization
The ligand and its chelating complexes have been optimized by using Gaussian09
software [26]. DFT/B3LYP is a suitable method for optimizing the structural
forms. Log and chk fles have been visualized over the screen view of the Gauss
View program [27] and according to a numbering system. Signifcant indexes
have been deducted from output fles, as well as the optimized images. These
indexes will give good insight into the catalytic features of tested compounds.
QSAR calculation
Quantitative structure–property relationships (QSAR) parameters have been
extracted for the ligand, chelating complexes, and oxides through the HyperChem
software program (8.1). Fulfllment of the molecular mechanics force feld (MM+)
followed by semi-empirical AM1 is recognized as the optimization pre-treatment
process. The calculations have been evaluated on free parameters without fxation
until reaching an equilibrium state. Utilizing a Polake–Ribiere conjugated gradi-
ent algorithm, the internal energy for treated compounds was minimized. Char-
acteristic parameters have been deducted, including the partition coefcient (log
P), surface area and reactivity, which are known for their signifcant relation to
the compound features, especially for the feature of catalytic behavior for tested
compounds [28].
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L. Almazroia et al.
Results and discussion
Signifcant analytical and physical characters of prepared complexes and their origi-
nal ligand are shown in Table 1. A general molar ratio of 1:1 is proposed for all
complexes. The isolated complexes are stable, non-hygroscopic in nature and have
high melting points. All were insoluble in known solvents, while they are fully solu-
ble in DMSO and DMF. The molar conductivity of all complexes was tested for
10−3 M/DMSO. The Cr(III), Fe(III) and Pt(IV)-Schif base complexes display val-
ues (49.56–53.27 Ohm−1 cm2 mol−1) that agree with conducting features for mono-
anions. VO(II) and Mn(II)-Schif base complexes represent values that agree with
non-conducting features [29].
Mode of bonding, IR spectra
A comparative study of IR spectra into Schif base ligand and its corresponding
VO(II), Cr(III), Mn(II), Fe(III) and Pt(IV) complexes, refects a neutral tridentate
bonding mode. Essential IR spectral bands are shown in Table 2. Bands appearing
at 3435, 1632, 1593, 1632, 1507 and 693 cm−1 are attributed to νNH, νC=O, νC=N
(azomethine), νC=N (triazole), νN–N, δNH and νC–S transitions. The lower shifted
appearance for νC=O, νC=N(azomethine) and νC-S bands in all complex spectra
suggest tri-dentate mode of bonding towards all metal ions [30]. The complex spec-
tra, display two new bands around≈780 and≈620 cm−1, which refer to δr(H2O) and
δw(H2O) transitions, respectively. This proposal is also supported by the broadness
centered at ≈ 3420–3460 cm−1 range, which is attributed to νOH accompanied by
νNH transition. Three specifc bands appeared in the VO(II) complex spectrum, two
at 1230 and ≈ 1180 cm−1 assigned to νas(SO4) and νs(SO4) transitions, respectively,
point to bi-dentate attachment. The third one at 1120 cm−1 is assigned to ν(V=O)
vibration inside octahedral confguration [31]. Other new bands appearing in lower
wavenumber regions at≈570,≈520 and 470 cm−1 are assigned to ν(M–O), ν(M–N)
and ν(M–S) transitions. Meanwhile, the absence of ν (M–Cl) vibration band due to
its appearance out of the scanning range is applied (400–4000 cm−1).
Electronic absorption and magnetic moments
Electronic spectral data for all compounds in DMSO solvent were deduced and are
displayed in Table 3. These data are the preliminary bases for proposing the struc-
tural formula of prepared complexes (Fig. 2). The electronic transition bands in
free ligand spectrum appearing at 35,201, 27,917 and 22,568 cm−1 are attributed
to n→σ*, n→π* and π→π* transitions, respectively (concerning NS atoms).
These three transitions are attributed to donating groups, which sufer from a clear
red shift, are observed in the complex spectra. The spectrum of the [VO(SO4)
(C21H19N6O3SF3)]2(H2O) complex displays signifcant transitions at 12,188 (ν1)
2
2
and 17,672 (ν2) cm−1 are assigned to B2 →2E (ν1) and B2 →2B1(ν2) in octahedral
confguration [32]. The slightly high magnetic moment value (1.80 BM) refers to
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New catalytic approach for nano-sized V(IV), Cr(III), Mn(II)…
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L. Almazroia et al.
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New catalytic approach for nano-sized V(IV), Cr(III), Mn(II)…
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L. Almazroia et al.
Fig. 2 The optimized structures of VO(II), Cr(III), Mn(II), Fe(III) and Pt(IV) complexes (a–e, respec-
tively)
expected orbital–orbital contribution. The spectrum of the [CrCl2(C21H19N6O3SF3)
H2O]Cl·H2O complex displays three spin allowed transitions within the
18,000–30,000 cm−1 range, which coincide with octahedral arrangement. The
complex displays three bands at 15,075(ν1), 19,667(ν2) and 23,523(ν3) accord-
4
4
4
ing to B1g →4Eag, B1g →4B2g and B1g →4Ebg transitions, respectively, with D4h
or C4v symmetry [33]. The high degeneracy of triplet orbitals in octahedral geom-
etry leads to increased energy and simulated D4h symmetry. Applying known equa-
tions [34], each parameter value (429.72 cm−1) was computed. The ligand crystal
energy (10Dq=15,075) was found the in normal range. Nephelauxetic parameter
value (β=0.468) was dedcted from; β=B(complex)/Bo(free ion) relation referring to
low-energy quartet–doublet transitions. The value refects a high covalence of new σ
bonds toward chromium atoms [35]. The magnetic moment value (3.78 BM) calcu-
lated corresponds to high-spin octahedral. The spectrum of [MnCl2(C21H19N6O3SF3)
H2O]·H2O complex exhibits weak bands at 18,673; 20,680 and 25,126 cm−1.
These bands are attributed to 6A1g→4E1g, 4A1g(4G), 6A1g→4Eg(4D) and
6A1g→4T1g(4P) transitions, respectively [36]. According to d5-systems, the efec-
tive magnetic moments for high spin complexes are expected to be 6.0 BM; the
measured value was found close to normal (5.85 BM). The ligand feld parameters
were calculated (10Dq=8558.5 cm−1, B=778.04 cm−1and β=0.905) and denoting
a highly ionic feature of new Mn–L bonds inside a stable complex. The spectrum of
the [FeCl2(C21H19N6O3SF3)H2O] Cl·H2O complex displays broad bands at 19,432
and 21,831 cm−1. These bands are attributed to 6A1g →4T2g and T2g—π* transitions,
respectively. The magnetic moment measured (5.87 BM) agrees well with the high-
spin octahedral centered by Fe(III) atom. 10Dq, Racah and nephelauxetic parameters
computed were 8906.3 cm−1, 809.67 cm−1 and 0.798, which introduce nearly ionic
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New catalytic approach for nano-sized V(IV), Cr(III), Mn(II)…
Fig. 3 ESR spectrum of VO(II) and Cr(III) complexes (a and b, respectively)
Table 4 Spin–Hamiltonian parameters of VO(II)and Cr(III) complexes (A and p×10−4
)
Complex g11
g⊥
go
A11
f
A⊥ Ao
p
k
G
γ
α2
β2
(2)
(3)
1.942 1.972 1.962 168 115.59 69 102 115.12 0.846 1.990 0.276 0.623 1.031
1.976 1.984 1.981 1.437 0.465
–
–
–
–
–
–
–
–
features of new σ bonds. The low-spin Pt(IV) complexes by t2g6 eg0 confguration
1
denote to A1g ground state. Excitation yields t2g5eg1 confguration, which splits
3
into T1g+1T2g+3T2g states. The spectrum of low-spin [PtCl3(C21H19N6O3SF3)
H2O]Cl·H2O complex displays two absorption bands at 16,742 and 21,931 cm−1,
1
1
which are attributed to A1g→3T1g and A1g→1T1g allowed transitions, respec-
tively [37]. The Racah parameter, which refers to variable inter-electronic repulsion
appeared with 3d metal ion complexes, changes with Z and q values, whereas q is
the existing electron in the d-shell as dq and Z signs for efective cationic charge.
The efective metallic charges were computed through standard equation [38] and
found to be 0.633, 1.175 and 1.704 for Cr(III), Mn(II) and Fe(III) ions, respectively.
The signifcant reduction for the charge values over metal ions is suggestive of inten-
sive L→M CT.
ESR spectral study
ESR spectra of solid VO(II), Cr(III) and Mn(II) complexes (Fig. 3, 2S) were con-
ducted at 9.60 GHz (25 °C). The Hamiltonian and MO parameters (Table 4) for
spinning verify the structural formula proposed for the three complexes. The
spectrum of the VO(II) complex exhibited a relative broadness instead of eight-
lines. The g factors (parallel and perpendicular) and hyperfne (hf) A-tensors
components were calculated approximately. The parameters of g and A computed
are axially symmetric, which represent dxy as a ground orbital [39]. The values
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L. Almazroia et al.
agree with octahedral arrangement around vanadium atom [40]. The reduced G
value (1.99) to less than 4 means the high exchange interaction between nucleus
in neighboring particles [41, 42]. The MO indexes (β2 and α2) values, represent
the ionic feature of in-plane π-bonding, in comparing with σ-bonding. Aniso-
tropic parameters (A∕∕ and A⊥) were also calculated and fall in normal range [43].
7(A∕∕−A⊥)
The dipole term (P) was calculated by p =
equation. If A11 is assumed
6+3∕2(ꢀ∕ΔE1)
to be negative while A⊥ is positive, the p value will exceed greater than 270 G,
which is far from the expected value. Thus, the signs of both A11 and A⊥ were
assumed to be negative according to isotropic hf constant (Ao). McGarvey calcu-
lated p value to be + 136 G for vanadyl complexes [44] and the present value cal-
culated (115.12 G) do not deviate much. The g values are generally far from ge
(2.0023) for vanadyl complexes. This reduction is related to spin–orbit interaction
of dxy ground level, with low-lying excited states. The isotropic and anisotropic
(go and Ao) indexes were calculated from the equations Ao = (A11 + 2 A⊥) ⁄ 3 and
go = (g11 + 2 g⊥) ⁄ 3. The K value (Fermi–contact term) was estimated (0.846)
using known equation [45] by using negative anisotropic indexes. Also, ESR
spectrum of Cr(III) complex introduces parameters assert on octahedral geome-
try. Thus, by using the following expression, g = 2.0023(1 − 4λ/10Dq), the g-val-
ues were estimated. Here λ is a constant for spin–orbit coupling inside metal ion
[46, 47]. This coupling constant sufers a reduction (λ′) from free chromium(III)
ion value (λ = 90 cm−1) [48]. This is an indication for the degree of metal–ligand
covalence. The λ′(40.18) value confrms the substantial covalent character.
Nephelauxetic parameter γ was computed and refects the covalence of Cr(III)-L
bonds (γ = λ′/λ). The ESR spectrum of Mn(II) complex exhibits a broad signal
centered at gav = 1.987, according to fne structure with S = 5 spin state. This
agrees with that reported for octahedral geometry. The broadness in signals due
to the resonances between Mn(II) ions in crystal lattice leads to single line come
down.
Fig. 4 XRD patterns of Fe2O3 and MnO2
1 3

New catalytic approach for nano-sized V(IV), Cr(III), Mn(II)…
XRD and SEM studies
XRD patterns were recorded for all prepared compounds and are displayed in Fig. 4,
3S (2θ from 10 to 90°). The purity of tested compounds were established in com-
paring with reactant patterns by known methods [49]. Essential parameters were
computed (Table 5) over patterns for nano-crystalline compounds (ligand, VO(II),
Fe(III), Mn(II) complexes, Fe2O3 and MnO2). However, Cr(III) and Pt(IV) com-
plexes do not exhibit well-split peaks refecting their amorphous character (Table 6).
Crystallite sizes, d-spacing, crystal strain (ε) and dislocation density (δ) are the
parameters computed using known equations [50]. The particulate sizes calculated
were found excellently in the nanometer range. The reduced values of dislocation
density and strain refect regularity of crystals. SEM is an additive tool used for good
insight about surface topography and crystallinity for solid compounds. Two images
from Fig. 5, 4S exhibit particles in a rock shape (ligand and its VO(II) complex).
However, the other images introduce particles in spherical shape, which are mainly
found in nanometer sizes. The purity of tested compounds were concluded from the
morphology diference from that of free ligand [51–53]. Fe(III) and Mn(II) com-
plexes were chosen to be the potential catalyst in preparing biodiesel. This selection
is based on the feature of particles as, their nano-sizes and spherical shapes. Such
feature is a preferable character for catalytic performance. Moreover, their exrtacted
oxides have displayed particles in a certain nanometer range and preferable mor-
phology in comparing with their original complexes.
Thermal study
TGA analysis is an essential method used to verify the chemical formulae for com-
pounds under consideration. All investigated compounds display low thermal stabil-
ity, due to starting decomposition at low temperature <30 °C. Most of the tested
complexes decompose through three stages except for the free ligand and its VO(II)
complex, which exceed four stages. The successive degradation stages represent
the distortion of most coordination sphere until ≈ 700 °C. The residual parts were
found in high percentage yield for all compounds, which refects the strength of
most bonds around the central atom. A small diference between calculated and
Table 5 XRD spectral data of nano-crystalline compounds
Complex
Size (nm)
2θ
I/Io (%)
d (Å)
β (FWHM)
ε
δ(Å−2
)
1
2
4
5
20.78
18.21
23.66
18.80
12.24
14.96
25.44
15.40
12.06
31.56
33.10
29.21
266
120
92
128
110
72
3.498
5.749
7.333
2.833
2.704
3.055
0.7143
0.8024
0.6154
0.8004
0.3571
0.3356
0.2025
0.0794
0.3726
0.1810
3.2169
2.796
0.2316
0.3016
0.1786
0.2829
0.6675
0.4468
Fe2O3
MnO2
1 3

L. Almazroia et al.
Table 6 Plausible thermogravimetic analysis for all investigated complexes (1–6)
Molecular formula of complex
Temp. range (°C) Mass (%)
Expelled molecule
Calc. Found
(1) [C21H19N6O3SF3]
25.12–101.32
101.33–180.42
180.42–350.54
350.55–534.71
26.54–189.92
189.92–398.43
398.43–610.52
610.55–730.32
Residue
12.60 12.62
32.73 32.71
20.73 20.74
33.93 33.93
2(OCH3)
C9H9N2O
C7H4N
C3N3SF3
2H2O
SO4 +C9H10O2
C6H6N2
(2) [VO(SO4)(C21H19N6O3SF3)]2(H2O)
5.21
5.22
35.61 35.88
15.35 15.29
15.76 15.75
28.07 27.86
25.25 25.46
17.64 17.84
17.34 17.27
39.77 39.43
10.92 10.80
N2C2F3
VO2 +C4H3N2S
2H2O+1.5Cl2 +OCH3
C7H9N2
(3) [CrCl2(C21H19N6O3SF3)H2O]Cl·H2O 26.75–241.46
241.46–401.44
401.44–613.48
Residue
28.66–160.99
161.00–291.55
291.63–431.46
431.71–701.60
Residue
C8H7O
[Cr(C5N4OSF3)]
2H2O+0.5Cl2
0.5Cl2 +OCH3
C6H5
(4) [MnCl2(C21H19N6O3SF3)H2O]·H2O
9.84
8.49
9.74
8.77
18.21 18.78
47.46 48.09
10.35 10.51
32.01 32.09
15.36 15.32
42.28 42.08
20.55 20.53
26.26 26.27
15.95 16.02
37.23 37.18
C8H7O
[Mn(C6H4N6OSF3)]
2H2O+0.5Cl2
Cl2 +C9H10O2
C6H6N2
[Fe(C6H3N4OSF3)]
2H2O+2Cl2
C15H15O2
(5) [FeCl2(C21H19N6O3SF3)H2O]Cl·H2O 27.83–260.43
260.43–458.41
458.2–705.61
Residue
(6) [PtCl3(C21H19N6O3SF3)H2O]Cl·H2O 32.4–220.67
220.67–450.76
450.77–760.84
Residue
C2HN4F3
[Pt(C4H3N2OS)]
Fig. 5 Scanning electron micrographs of Fe2O3 and MnO2
1 3

New catalytic approach for nano-sized V(IV), Cr(III), Mn(II)…
−
Fig. 6 Cyclic voltammogram of Fe(III) and Mn(II) complexes (a and b, respectively) in DM-TBA+ BF4
at 100 mV/s versus Ag/AgCl electrode
Table 7 Cyclic voltammetric data of two metal ion complexes (example) in DMF-TBA+ BF4− a
Complex
First electrode couple
Second electrode couple
Ep,a
Ep,c
ΔEp
E1/2
Ep,a
Ep,c
ΔEp
E1/2
MnCl2(C21H −0.3019
19N6O3SF3)
−0.9974
0.6955
−0.6496
0.7511
0.4489
0.3022
0.5995
H2O]·H2O
[FeCl2(C2
1H19N6O
3SF3)H2O]
Cl·H2O
−0.4289
−0.9993
0.5704
−0.7141
0.7494
0.4709
0.2785
0.3747
aScan rate=100 mV s −1; E1/2 =(Ep,c +Ep,a)/2
found decomposed masses, may be attributed to the small overlap between follow-
ing stages.
Electrochemical study
Electrochemical cyclic voltammogram (Fig. 6) was conducted in DMF-TBA+
−
BF4 (0.1 mol L−1 TBATFB) for [MnCl2(C21H19N6O3SF3)H2O]·H2O and
[FeCl2(C21H19N6O3SF3)H2O]Cl·H2O complexes. The applied potential range from
−1.5 to 1.5 V, was sufcient for cycle execution by two well-determined electrode
couples. The voltammetric data are displayed in Table 7. The Fe(III) voltammogram
exhibits a safe assigned reduction wave for irreversible FeIII/FeII couple. The E1/2
and ΔEp values are −0.7141 and 0.5704 V for FeIII L+e−1 ↔FeIIL. The second
electrode couple for FeII/FeIII displays E1/2 and ΔEp values at 0.5995 and 0.3022 V
[54]. The ratio of ip,c/ip,a >1 at a scanning voltage range (1–100 mV) represents
the irreversible nature of the electrode couple [55]. The relation between cathodic
peak current (ip,c) for FeII/FeIII electrode couple, with the square route of sweep rate
(υ1/2), suggests a difusion-controlled electrochemical process (Fig. 7). Depend-
ing on voltammetric response attributed to FeII/FeIII on a sweep rate, the analyte
1 3

L. Almazroia et al.
Fig. 7 The dependence of peak currents (ip,c and ip,a, frst electrode couple) and υ1/2 for Fe(III) and
Mn(II) complexes (a and b, respectively)
depolarizer concentration and the decrease in ip,c/υ1/2, are the same for ECE (elec-
trochemical reaction coupled). This may introduce a mechanism type for irreversible
frst-order reaction, as intermediated between two successive charge transfers (for
one electron). The voltammogram of the Mn(II) complex exhibits the same feature
of two well-resolved electrode couples attributed to its irreversible nature. The frst
electrode couple for MnII/MnI, has E1/2 and ΔEp values of −0.6496 and 0.6955 V
for MnII L+e−1 ↔MnIL. The second electrode couple for MnI/MnII displays E1/2
and ΔEp values at 0.5995 and 0.3022 V. Also, the relation between ip,c for FeII/FeIII
electrode couple and υ1/2 suggests a difusion-controlled electrochemical process
(Fig. 7). Moreover, this relation introduces irreversible frst-order reaction mecha-
nism [56].
Theoretical implementation
Kinetics
The kinetic parameters were calculated (Table 8) from plotted relations (Fig. 5S) for
Coats-Redfern (CR) [20] and Horowitz–Metzger (HM) equations. This was carried
out over two successive degradation stages for all compounds under examination
(2nd and 3rd). The chosen stages found suitable features of sharpness and broad-
ness, which leads to production of exact fractions decomposing at each defned
temperature. The two comparative methods were used to determine accuracy [25].
The order of reaction (n), frequency factor (A), activation energy (E), enthalpy
(∆H*), entropy (∆S*) and Gibbs free energy change (∆G*) were estimated. The
infuence of metal atom on the decomposition behavior was clearly noted from
this study. Gibbs free energy (∆G*) and activation enthalpy (∆H*) were estimated
from ∆G*=∆H*−T∆S* and ∆H*=E*−RT relations. Essential features are listed
as follows: (1) E, is the activation energy, which rises from one stage to another,
refecting the rigidity of M–L bonds. (2) ∆S*, is the entropy, which has negative
values. This refects the presence of well-ordered fragments, which rise along the
1 3

New catalytic approach for nano-sized V(IV), Cr(III), Mn(II)…
1 3

L. Almazroia et al.
1 3

New catalytic approach for nano-sized V(IV), Cr(III), Mn(II)…
degradation stages. (3) ∆H*, is the enthalpy of activation (positive sign), which
introduces an endothermic process. (4) ∆G*, is the Gibbs free energy, which dis-
plays a non-spontaneous decomposition process, as well as the hardness of residual
fragments than the original molecule. ∆G* values were generally overridden from
one step to another along the decomposition process [57].
DFT/B3LYP optimization
The DFT method was suitable for optimizing all compounds (Fig. 2) under con-
sideration using Gaussian09 software. Using chk fles, which were visualized over
the program screen [58], HOMO and LUMO MO images (Fig. 8a, b), as well as
Fig. 8 a The frontier molecular orbital (HOMO) of the ligand and its complexes. b The frontier molecu-
lar orbitals (LUMO) of the ligand and its complexes
1 3

L. Almazroia et al.
Fig. 8 (continued)
energies were taken. HOMO images display the focus of the frontier level over the
coordinating atoms, which appears excellently in the ligand image and is verifed
through complexes. This feature verifes the intensive presence of charge over theses
atoms, which leads to their priority in coordination towards the central atoms.
Extracted parameters
Essential parameters were computed for all tested compounds using known rela-
tions [59] based on frontier energy gaps (EL − EH, ΔE) (Table 9). Global elec-
trophilicity index (ω), global of hardness (η), electronegativity (χ), global of
softness (S), electronic chemical potential (μ) and absolute softness (σ) are the
parameters of concern. The ability of charge transfer of L → M, which looked
at the efective feature in the coordination process, was examined through the
1 3

New catalytic approach for nano-sized V(IV), Cr(III), Mn(II)…
1 3

L. Almazroia et al.
electrophilicity index (ω). With the reduced electrophilic nature of the ligand,
the high CT character towards metal atoms appeared. This parameters introduce
stability accompanied by acquiring more electrons from the environment, which
appeared clearly with Fe(III) and Mn(II) complexes. σ and η indexes refer to the
constancy and molecular reactivity, which introduce the priority of Fe(III) and
Mn(II) complexes. The global hardness and softness indexes (η and S) are two
opposing features examining the degree of fexibility of tested compounds. The
hardness observed for Fe(III) and Mn(II) complexes refects their high stability.
The high softness recorded for the free ligand compared to its complexes intro-
duces its high fexibility in the coordination process. The reduced ΔE values in
the complexes supports the stability of coordinating compounds compared to the
free ligand, especially for Fe(III) and Mn(II) complexes. These qualities have
shown an advantage for the two selected complexes, Fe(III) and Mn(II), in cata-
lytic application.
Log fles data
Log fles summarize signifcant parameters such as charges, oscillator strength,
excitation energy and dipole moments. The computed data are displayed in
Table 10. The charges over OSN donors in the free state (in the ligand), were
found sufcient, to be promising coordinators, while, for other nitrogen atoms
in triazole ring, found to a little extent for exclusion from coordination. When-
ever coordinating donors (OSN) in some complexes represent charge rise, this
may refer to the π-acceptor character of their groups. Oscillator strength is a
good indicator for the radiation-emission or absorption possibility in electronic
transitions. The value is close to 1, which means a strong transition, while the
computed values were far from unity. This refects the lower electronic transi-
tion efciency especially for electronically intensive metal complexes [60]. The
high excitation energy recorded for the tested complexes display their stabilized
ground states. The dipole moment values, were found exceeds than free ligand
except for VO(II), Fe(III) and Mn(II) complexes. This may refer to the infuence
of the metal atoms in the CT process.
Table 10 Apparent charges, dipole moment (D), oscillator strength (ʄ) and excitation energies (E)
Compound O25
S11
N10
N14*
N15*
M
D
ʄ
E(nm)
(Debye)
Ligand
−0.372630 0.556936 −0.309553 −0.224968 −0.290408
–
8.0016 0.0051
366.54
VO(II)+L −0.390113 0.628464 −0.308168 −0.261140 −0.295510 0.885506
6.4558 0.0003 5606.64
Cr(III)+L −0.397998 0.624383 −0.309660 −0.248537 −0.287132 0.637898 13.4519 0.0007 1476.45
Mn(II)+L −0.401505 0.626214 −0.307596 −0.320140 −0.248528 0.328804
Fe(III)+L −0.359414 0.559964 −0.156013 −0.272944 −0.272650 0.246939
5.8923 0.0002 5843.58
5.4324 0.0001 11176.1
Pt(IV)+L
−0.323028 0.382803 −0.023786 −0.261167 −0.300565 0.134467 10.7276 0.0002 12071.1
*Non-bonded atoms
1 3

New catalytic approach for nano-sized V(IV), Cr(III), Mn(II)…
QSAR study
By implementing HyperChem 8.1 software, the optimization of compounds under
investigation was achieved using MM+ followed by semi-empirical method.
QSAR parameters executed ofered very signifcant features about the compounds
(Table 11). Surface area, reactivity and polarizability were the most signifcant
parameters among all. These parameters give the best view about the reactivity atti-
tude for tested compounds. The high surface area of the compound gives a promis-
ing potential catalytic activity [61]. Also, the reactivity and polarizability act in the
same way, which verifes the feature expected from the surface area value. From
this view, distinguished hetero-catalysis was expected from Fe(II) and Mn(II) com-
plexes. Also, the extracted oxides for the same complexes from calcinations were
also tested to aggregate a good view about their expected activity. Also, a promising
feature was appeared but in a little extent in comparing with the original complexes.
Heterogeneous catalytic application
The data displayed in this study such as XRD and SEM ofer distinguished charac-
teristics for Fe(III) and Mn(II) complexes, as well as their calcination residual oxides
(Fe2O3 and MnO2). The characteristics such as distinguished nanometer sizes and the
regular morphological shapes, gave promising catalytic expectation based on their
wide surface area. This feature is considered the most efective feature in catalytic role.
But, the complexes failed to play the desired role in catalytic behavior, while their own
oxides succeed in a promising way. The unsuccessful behavior of the two complexes
may discussed as follows: (1) their TGA curves display lower thermal stability. This
may lead to the probability of their decomposition during their catalytic role, as well as
their noticeable solubility during the catalytic process (which is observed by the impact
of their deep colors on reaction mixture). (2) The catalytic reactions were conducted
at 150 °C, which is sufcient for starting decomposition for the two complexes. The
applicable temperature is highly suitable for thermally stable oxides, while the success-
ful catalytic role was recorded for their corresponding oxides (MnO2 and Fe2O3, from
Table 11 Essential QSAR computation parameters over the ligand, its complexes and specifc oxides
Function
HL1
VO(II)-L Cr(III)-L Mn(II)-L Fe(III)-L Pt(II)-L Fe2O3 MnO2
Surface area
(approx) (Å2)
691.00 865.04
865.79
847.09
888.42
869.64
881.62
868.03
848.35 378.05 289.68
863.77 261.02 196.97
Surface area (grid)
761.82 872.70
(Å2)
Volume (Å3)
1280.13 1521.34 1518.69 1556.95 1552.26 1543.72 335.61 240.05
−15.17 −34.68 −28.83 −20.13 −20.15 −12.64 −10.45 −7.62
Hydration energy
(Kcal mol–1)
Log P
2.54
46.36 142.86
492.48 46.98
1.73
1.56
141.62
48.27
0.99
145.52
50.07
1.32
144.72
49.67
2.93
148.45
51.46
3.77 −0.07
Reactivity (Å3)
Polarizability (Å3)
8.24
6.59
2.90
2.71
1 3

L. Almazroia et al.
Scheme 2 The trans-estrifcation of the waste oil to biodiesel
Table 12 Essential physical characters for prepared biodiesel
Catalyst
Yield % of methyl ester
Density
Viscosity
Cloud point
NaOH
0% (emulsion)
78%
72%
–
–
–
3
3
6
Nano-sized Fe2O3
Nano-sized MnO2
Purchased MnO2
ASTM-6751
0.8643
0.8820
0.8703
0.86–0.90
3.752
3.771
3.781
1.9–6.0
52%
−3–12
Biodiesel (reference)
calcinations). This successful behavior was expected based on the following features:
their particulate nano-sizes, as well as their surface morphology. Also, it was confrmed
by structural optimization data and distinguished QSAR parameters (surface area and
reactivity). The synthesis of biodiesel was accomplished upon using waste sunfower
oil (WO). This oil comprises mainly oleic acid and other fatty acids. The trans-esterif-
cation process (Scheme 2), which leads to conversion reaction by using each suggested
catalyst (two complexes, two synthesized oxides and purchased MnO2), was moni-
tored [62]. This process was compared with another referenced process, using NaOH
as a homogeneous catalyst, but this process failed (Fig. 6S) at the applied conditions
of preparation (methanol & oil ratio, weight of catalyst and temperature applied) [63].
However, all oxides accomplish the synthesis of biodiesel from waste oil perfectly.
But, the prepared oxides from complex calcinations succeeded surprisingly compared
to similar purchased complexes (Fig. 6S). The conclusion was based on comparative
product yields. Here, we considered some physical features for synthesized biodiesel as
the only evidences for its preparation [63–65]. The percentage yields and other physi-
cal features, exhibited in Table 12, agree perfectly with that reported previously in the
literature [63–65].
Conclusion
The coordination mode for octahedral complexes isolated was suggested based
on variable techniques. XRD and SEM studies exhibit nanometer characteristic
sizes for all discussed compounds. Fe(III) and Mn(II) complexes, as well as their
1 3

New catalytic approach for nano-sized V(IV), Cr(III), Mn(II)…
calcination residues (Fe2O3 and MnO2), were chosen for heterogeneous catalysis,
for biodiesel synthesis, from waste oils. Also, their original complexes were tested
for biodiesel synthesis, but no desired results were given. This catalytic process was
conducted from a comparative view, versus others published by using homogeneity.
The distinguished performance of synthesized oxides appeared for purchased MnO2.
Moreover, the unsuccessful trial to synthesize biodiesel using NaOH (referenced
process) as a homogeneous catalyst, has been conducted at the applicable conditions
with our catalysts. Finally, the preparation of biodiesel from waste oils was executed
using new calcinations of residual oxides. Thus, the main aim in this research and
was achieved.
Acknowledgements This work was supported by grants from King Abdulaziz City for Science and Tech-
nology in Riyadh, Riyadh, Saudi Arabia under Project Grants No. 37-170.
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Afliations
Layla Almazroia1 · Reem K. Shah1 · Nashwa M. El‑Metwaly1,2
Thoraya A. Farghaly1,3
·
*
Nashwa M. El-Metwaly
n_elmetwaly00@yahoo.com
1 3

New catalytic approach for nano-sized V(IV), Cr(III), Mn(II)…
1
Department of Chemistry, Faculty of Applied Science, Umm Al-Qura University, Makkah,
Saudi Arabia
2
Chemistry Department, Faculty of Science, Mansoura University, Mansoura, Egypt
3
Chemistry Department, Faculty of Science, Cairo University, Cairo, Egypt
1 3