Influence of substituents in meso-aryl groups of iron μ-oxo porphyrins on their catalytic activity in the oxidation of cycloalkanes

Article abstract of DOI:10.1016/j.poly.2016.08.048

The aim of this work was to study the effect of substituents on the catalytic activity of iron μ-oxo porphyrins in oxidation of cycloalkanes. Electron-donating or electron-withdrawing substituents were introduced in meso-aryl groups of iron μ-oxo porphyrins. An important part of the work was the characterization of the catalysts, in particular their redox properties. Catalytic performance and selectivity were evaluated using cyclopentane, cyclohexane, and cyclooctane as model compounds. It was shown that not only electron-withdrawing but also electron-donating substituents improved the catalytic performance of iron μ-oxo complexes. Moreover, catalytic activity of iron μ-oxo porphyrins with electron-withdrawing substituents correlated with the half-wave potential E_1/2while the catalytic activity of iron μ-oxo porphyrins with electron-donating substituents increased with the decrease of reduction potential E_RED.

Full text of DOI:10.1016/j.poly.2016.08.048

Polyhedron 119 (2016) 342–349
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Influence of substituents in meso-aryl groups of iron
l-oxo porphyrins
on their catalytic activity in the oxidation of cycloalkanes
b
b
c
d
Edyta Tabor a, , Jan Połtowicz , Katarzyna Pamin , Sylwia Basa˛g , Władysław Kubiak
⇑
a J. Heyrovsky´ Institute of Physical Chemistry of the CAS, v. v. i., Dolejškova 2155/3, 182 23 Praha, Czech Republic
b Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, 30-239 Kraków, Poland
c Jagiellonian University, Department of Chemistry, Ingardena 3, 30-060 Kraków, Poland
d Faculty of Materials Science and Ceramics, AGH University of Science and Technology, Mickiewicza 30, 30-059 Kraków, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 July 2016
Accepted 27 August 2016
Available online 19 September 2016
The aim of this work was to study the effect of substituents on the catalytic activity of iron
phyrins in oxidation of cycloalkanes. Electron-donating or electron-withdrawing substituents were intro-
duced in meso-aryl groups of iron -oxo porphyrins. An important part of the work was the
characterization of the catalysts, in particular their redox properties. Catalytic performance and selectiv-
ity were evaluated using cyclopentane, cyclohexane, and cyclooctane as model compounds. It was shown
that not only electron-withdrawing but also electron-donating substituents improved the catalytic per-
l-oxo por-
l
Keywords:
Iron porphyrins
formance of iron
withdrawing substituents correlated with the half-wave potential E1/2 while the catalytic activity of iron
-oxo porphyrins with electron-donating substituents increased with the decrease of reduction potential
ERED
l-oxo complexes. Moreover, catalytic activity of iron l-oxo porphyrins with electron-
l-Oxo porphyrins
Oxidation
Cycloalkane
Molecular oxygen
l
.
Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction
structure to the requirements of different reactions [3,6–10]. How-
ever, the main drawback of practical applications of metallopor-
phyrins in oxidation processes is their deactivation under
oxidative conditions of catalytic reactions [11] by self-destruction
The process of oxidation of cycloalkanes to alcohols and ketones
is one of the fundamental reactions in organic synthesis [1–3]. It is
used to obtain valuable intermediates which are further utilized in
large-scale production of polymers [2]. Particularly, the oxidation
of cyclohexane continues to attract much attention because cyclo-
hexanone and cyclohexanol are important precursors for the pro-
duction of adipic acid and caprolactam that are used for
manufacturing of nylon-6,6 and nylon-6, respectively [2,4]. It
was shown that synthetic metalloporphyrins are effective catalysts
for the oxidation of cycloalkane to the corresponding alcohol and
ketone using molecular oxygen, which is an inexpensive, eco-
friendly and easily available oxidant [2,5,6]. Metalloporphyrins
mimic catalytic properties of cytochrome P-450: they are able to
activate molecular oxygen on metal centre and then transport
the oxygen atom to saturated hydrocarbon. The catalytic proper-
ties of metalloporphyrins depend on the type of metal center, the
three-dimensional structure of macrocycle rings, the kind and
quantity of substituents on porphyrin ring, and the type of axial
ligand [5]. Their high attractiveness in oxidation reactions is
of macrocyclic ligand or dimerization to metal
[12]. It is generally postulated that -oxo metal complexes are cat-
alytically inactive. Thus, the study of structures and catalytic activ-
ities of -oxo metalloporphyrins is currently of considerable
interest. There are only a few papers concerning the application
of -oxo metalloporphyrins as catalysts in olefin epoxidation or
hydroxylation of alkanes [3,7,8,10]. It was shown that the iron
-oxo octaethylporphyrin is inactive as catalyst for the oxidation
l-oxo complexes
l
l
l
l
of alkanes while the introduction of electron-withdrawing sub-
stituents into porphyrin rings significantly increases the activity
of this catalysts in investigated reaction [11]. Lyons et al. observed
that iron
l-oxo meso-tetraphenylporphyrin [Fe(TPP)]2O had been
also catalytically inactive in the oxidation of isobutene [9]. More-
over, the halogenation of pyrrole rings did not change its activity.
However, the halogenation of phenyl rings, which leads to the for-
mation of iron
l-oxo meso tetrakis(pentafluorophenyl)porphyrin
[Fe(TPFPP)]2O, demonstrated a positive effect on the catalytic
derived from
a possibility of tailoring the metalloporphyrin
performance in isobutene oxidation. Guo at al. investigated the cat-
alytic activity of nine iron l-oxo metalloporphyrins in cyclohexane
oxidation with PhIO as oxygen donor [3]. Their study showed an
improvement of catalytic activity in cyclohexane hydroxylation
⇑
Corresponding author. Fax: +420 286 582 307.
E-mail address: edyta.tabor@jh-inst.cas.cz (E. Tabor).
http://dx.doi.org/10.1016/j.poly.2016.08.048
0277-5387/Ó 2016 Elsevier Ltd. All rights reserved.

E. Tabor et al. / Polyhedron 119 (2016) 342–349
343
with PhIO and a better stability of iron
in comparison with the parent iron porphyrin FeTPPCl. Manganese
and iron -oxo porphyrins were applied in 2-methylbutane oxida-
tion in the presence of the same oxygen donor PhIO [8]. It was
shown that the presence of electron-withdrawing groups on por-
phyrin rings improved the catalytic selectivity of metallopor-
phyrins for the tertiary carbon–hydrogen bonds and the reaction
l
-oxo porphyrin [FeTPP]2O
for; [FeT(p-OCH3)PP]2O calculated: C-72.37%, H-4.56%, N-7.03%,
found: C-72.62%, H-4.13%, N-6.92%, for; [FeT(p-Cl)PP]2O calcu-
lated: C-64.69%, H-2.97%, N-6.88%, found: C-64.62%, H-2.83%,
N-6.84%, for: [FeT(p-CF3)PP]2O calculated: C-60.78%, H-2.55%,
N-5.91%, found: C-60.62%, H-2.65%, N-6.01%, for: [FeTPFPP]2O
calculated: C-61.70%, H-2.55%, N-5.91%, found: C-61.62%,
H-2.43%, N-5.72%).
l
rates. Iron
l-oxo porphyrin was used as catalyst in cyclohexane
oxidation with air [13]. In contrast to the monomeric iron por-
phyrin FeTPPCl, the catalytic performance of (FeTPP)2O in the oxi-
dation of cyclohexane resulted in higher yields of products and
higher turnover frequencies. In another study Guo et al. presented
2.2. Physicochemical characterization of iron porphyrins
UV–Vis spectroscopy was applied to check the progress of the
metalation of porphyrin ligands as well as the formation of iron
the use of two
l-oxo bis(tetraphenylporphinato) iron [FeTPP]2O
l
-oxo porphyrins. The position of the absorption band (the Soret
and manganese [MnTPP]2O complexes as catalysts in the oxidation
band) of the prepared iron porphyrins and their -oxo analogues
l
of ethylbenzene with air under mild conditions [7]. The authors
were compared with the literature data [3]. Measurements were
performed at room temperature using Perkin-Elmer Lambda 35
spectrophotometer with the wavelength range from 200 to
observed a higher catalytic activity of
l-oxo porphyrins in ethyl-
benzene oxidation with air and with no additive in comparison
with monomeric FeTPPCl and MnTPPCl porphyrins. The reaction
1100 nm and resolution 0.5 nm. Iron porphyrins and iron
porphyrins were dissolved in benzene.
l-oxo
mixture which consisted of iron monomeric FeTPPCl or
l-oxo
[FeTPP]2O porphyrins as catalyst, isobutylaldehyde as co-reductant
and dioxygen as the oxidant was applied in the epoxidation of
olefins [14]. The authors demonstrated a remarkable enhancement
FTIR spectroscopy was used to confirm the formation of iron
-oxo porphyrins. The FTIR spectra of iron -oxo porphyrins and
monomeric iron complexes were recorded using FTIR Nicolet 800
spectrometer in KBr pellets in the range from 4000 to 400 cmꢁ1
under atmospheric pressure.
l
l
of reactivity in the presence of
ison to FeTPPCl catalyst.
l
-oxo metalloporphyrin in compar-
Recently, we described the application of manganese
l-oxo
Cyclic voltammograms of iron l-oxo porphyrins were recorded
porphyrins with different electron-donating or electron-withdraw-
ing substituents in the oxidation of cyclooctane [10]. Our investiga-
in a three-electrode cell using a graphite paste electrode as the
working electrode, platinum coil as the auxiliary electrode and
Ag/AgCl as the reference electrode. Composite paste was prepared
by mixing synthetic graphite with Nujol. All compounds were ana-
lyzed as 0.2 M water solutions in acetate buffer of pH = 5.0 as elec-
trolyte at a scan rate of 0.05 V/s. Before the measurements the
solutions were pretreated in argon to keep air-free atmosphere
during the measurement.
tions demonstrated that manganese
l-oxo porphyrins were
catalytically active in the oxidation of cyclooctane.
In this contribution we continue our study on the catalytic
activity of l-oxo metalloporphyrins in the oxidation of cycloalka-
nes with molecular oxygen. Various electron-donating or
electron-withdrawing substituents were introduced on the iron
l
-oxo macrocyclic rings to modify their electrochemical properties
Cyclic voltammetry (CV) measurements of monomeric iron por-
phyrins were performed using standard three-electrode system.
Platinum electrodes were used as the auxiliary electrode (thin
metal plates) and working electrode (spherical shape and area
10 mm2), Ag/AgCl/saturated KCl was the reference electrode. Com-
plexes were analyzed in 0.1 M solution of tetrabutylammonium
perchlorate (TBAClO4) and dichloromethane (two-times distilled).
The potential was varied in the range from ꢁ1.5 to +1.5 V at the
scan rate of 0.20 V/s.
and catalytic activity.
2. Experimental
2.1. Preparation of iron porphyrins and their l-oxo complexes
All the studied iron porphyrins contained the electron-donating
or electron-withdrawing substituents introduced in the meso-aryl
positions of porphyrin macrocycle. To simplify the descriptions of
iron porphyrins the phrase ‘‘substituents in porphyrin ring” will
be used implying that substituents are in meso-aryl positions in
porphyrin rings. The porphyrin ligands with electron-donating
substituents (H2TTP, H2T(p-OCH3)PP), electron-withdrawing sub-
stituents (H2T(p-Cl)PP, H2T(p-CF3)PP, H2TPFPP) and unsubstituted
H2TPP were prepared by Lindsey method, where pyrrole was con-
densated with an appropriate aldehyde [15]. Metalation of por-
phyrin ligands (H2TTP, H2T(p-OCH3)PP, H2T(p-Cl)PP, H2T(p-CF3)
PP, H2TPFPP, H2TPP) was carried out using FeCl2ꢀ4H2O in DMF
[16]. All the monomeric iron porphyrins contained a Cl anion as
axial ligand. The monomeric iron porphyrins were purified by suc-
cessive chromatography on a silica gel column [3].
2.3. Catalytic experiments
Oxidation of cycloalkanes by molecular oxygen was carried out
in the stainless steel batch reactor at 393 K and under the air pres-
sure of 10 atm. Cycloalkane to oxygen molar ratio was set at 6.5.
The amount of iron
l-oxo or monomeric iron porphyrin giving
the final concentration 3.3 ꢂ 10ꢁ5 M of the catalyst in the reaction
mixture was dissolved in 1 ml of benzene and added to 60 ml of
cycloalkane. Reactor filled with substrate and the catalyst was
pre-treated under the argon flow to remove air and to provide an
inert atmosphere. Then the rector was heated to 393 K and air
was introduced. After 6 h of reaction time the oxidation was
stopped by immersing the hot reactor in a cold water bath. Yields
of products were calculated based on the oxygen quantity in the
batch reactor for all catalytic tests. Reaction products were ana-
lyzed using Agilent Technologies 6890N chromatograph equipped
with Innovax chromatography column (30 m). The yield values
were verified by an addition of internal standard, chlorobenzene,
at the end of the reaction. Cycloalcohol and cycloketone were the
only oxygen-containing products, together with traces of cycloalk-
ane hydroperoxide. The blank experiment confirmed that the
cycloalkane was not oxidized by O2 in the absence of catalyst.
Iron l-oxo porphyrins were obtained by repeated pass of mono-
meric iron porphyrin (FeTPPCl, FeTTPCl, FeT(p-Cl)PPCl, FeT(p-CF3)
PPCl, or FeT(p-OCH3)PPCl) dissolved in benzene with 5% of metha-
nol through alumina column [3]. The solvents were evaporated
under vacuum and the final product was washed with distilled
water. Fig. 1 illustrates the structure of prepared catalysts (elemen-
tal analysis for: [FeTPP]2O calculated: C-78.11%, H-4.17%, N-8.28%,
found: C-78.02%, H-4.13%, N-8.12%, for: [FeTTP]2O calculated:
C-78.69%, H-4.95%, N-7.65%, found: C-78.12%, H-4.83%, N-7.25%,

344
E. Tabor et al. / Polyhedron 119 (2016) 342–349
Fig. 1. Structure of monomeric iron porphyrins (a) and their l-oxo analogues (b).
located porphyrin ligands in
to lower wavelength. Thus, UV–Vis spectroscopy confirms the for-
mation of iron -oxo porphyrins.
l-oxo adducts shifted the Soret band
3. Results
l
3.1. UV–Vis spectroscopy
Fig. 2 shows the UV–Vis spectra of TTP, FeTTPCl and [FeTTP]2O
dissolved in benzene. The differences in the UV–Vis spectrum of
porphyrin ligand TTP (Soret band 419 nm, Fig. 2a) and product of
metalation FeTTPCl are visible. The most remarkable change was
the absence of some of the Q bands and a shift of the Soret band
UV-Vis spectroscopy was employed to monitor the particular
stages of the catalyst preparation. UV-Vis spectrum of porphyrin
ligand (Fig. 2) contains the Soret band (390–420 nm) and four Q
absorption bands (450–700 nm) [12].
as a result of the interaction between
p-electrons of porphyrin
The positions of Soret bands of synthesized iron
l-oxo
ligand with the d-electrons of iron ions. The position of the Soret
band for [FeTTP]2O 409.0 nm, Fig. 2c) is shifted to lower wave-
lengths in comparison with its monomeric analogue FeTTPCl
(421.0 nm, Fig. 2b) due to the interaction of two closely located
porphyrins and iron porphyrins with different types of sub-
stituents are gathered in Table 1. The interaction of two closely
porphyrin ligands in
l-oxo adduct. Similar tendency is observed
for all the synthesized iron l-oxo porphyrins (Table 1).
0.1
3.2. FTIR spectroscopy
c
FTIR spectroscopy was applied to investigate the formation of
iron -oxo porphyrins. Table 1 summarizes the positions of the
bands characteristic for the -oxo bridge in the investigated iron
-oxo porphyrins. Fig. 3 compares FTIR spectra of monomeric
l
l
l
FeTTPCl and
l-oxo [FeTTP]2O porphyrin. FTIR spectrum of
[FeTTP]2O exhibits two maxima at 893 and 878 cmꢁ1 (Fig. 3b) indi-
cating the formation of the oxygen bridge between two iron atoms
b
a
in
l-oxo complex [3,7,8,13]. FTIR spectra of studied iron l-oxo
porphyrin [FeTTP]2O demonstrate the bands in the region 910–
880 cmꢁ1, which are characteristic for asymmetric stretching
vibrations of
l-oxo bridge (Fe–O–Fe) [3]. These emission bands
300
400
500
600
700
are not observed for monomeric iron porphyrins (Fig. 3a). The
changes in the position of the band assigned to Fe–O–Fe vibration
are the result of the presence of various types of substituents on
the porphyrin macrocycle.
Wavelength / nm
Fig. 2. UV–Vis spectra of (a) TTP, (b) FeTTPCl and (c) [FeTTP]2O dissolved in
benzene.

E. Tabor et al. / Polyhedron 119 (2016) 342–349
345
Table 1
Results of UV-Vis, FTIR, and cyclic voltammetry measurements for iron
l
-oxo porphyrins and their parent analogues.
a
b
a
c
Iron
l-oxo porphyrin
kSoret
nm
ERED
V
IR vibration
cmꢁ1
Iron porphyrin
kSoret
nm
E1/2
V
[FeTPP]2O
[FeTTP]2O
[FeT(p-OCH3)PP]2O
[FeT(p-Cl)PP]2O
[FeT(p-CF3)PP]2O
[FeTPFPP]2O
408.0
409.0
422.0
410.0
414.0
409.5
ꢁ0.60
ꢁ1.38
ꢁ1.07
ꢁ0.29
+0.60
+0.83
864; 892
878; 893
872; 895
876; 894
881; 889
890; 887
FeTPPCl
FeTTPCl
FeT(p-OCH3)PPCl
FeT(p-Cl)PPCl
FeT(p-CF3)PPCl
FeTPFPPCl
420.1
421.0
423.5
419.5
417.0
419.5
ꢁ0.29
ꢁ0.35
ꢁ0.34
ꢁ0.27
ꢁ0.24
+0.08
a
b
c
Measurements in benzene.
Reduction potential.
Half-wave potential.
a
b
a
840 860 880 900 920 940 960
-1
Wavenumber / cm
b
b
a
-400
-200
0
200
400
Potential / mV
400
800
1200
1600
2000
Fig. 4. Cyclic voltammogram of [FeTPP]2O after one scan (a) and after 20 scans (b)
in acetate buffer (pH = 5.0).
Wavenumber / cm-1
Fig. 3. Comparison of FTIR spectra of monomeric iron FeTTPCl (a) and iron
[FeTTP]2O porphyrin (b).
l-oxo
results in the increased value of E1/2 of monomeric iron porphyrin.
The highest E1/2 value is observed for FeTPFPPCl having 20 fluorine
atoms in the porphyrin ring.
3.3. Cyclic voltammetry
3.4. Catalytic results
The redox properties of synthesized iron porphyrins were
studied using cyclic voltammetry. Cyclic voltammograms of iron
porphyrins indicate one-electron transfer process attributed to
Three cycloalkanes (cyclopentane, cyclohexane, and cyclooc-
tane), were applied as substrates and studied in the oxidation cat-
the Fe(III)/Fe(II) redox couple. The cathodic (Epc) and anodic (Epa
)
alyzed by iron l-oxo porphyrin [FeTTP]2O (Scheme 1).
current peak potential were used to calculate the value of the
half-wave potential E1/2, where E1/2 = (Epc + Epa)/2.
The main products of cycloalkane oxidation with molecular
oxygen are cycloketone and cycloalcohol. No oxidation occurs in
the absence of the catalyst. Only 4.1% yield to ketone and 1.1% yield
to alcohol were obtained for cyclopentane, in the presence of
[FeTTP]2O after 6 h (Fig. 5). Under analogous conditions, cyclohex-
ane was converted to ketone and alcohol with yields of 17.4% and
3.4%, respectively. The oxidation of cyclooctane catalyzed by
[FeTTP]2O resulted in 40.3% yield to ketone and 8.0% yield to alco-
hol. Iodometric titration of reaction mixture after 6 h permits the
evaluation of the amount of cyclooctane hydroperoxide as 0.06%,
which is too low to have an impact on the results of the subsequent
GC analysis [17].
The following order of catalytic activity has been established:
cyclooctane > cyclohexane > cyclopentane. Therefore, the most
reactive one among investigated cycloalkanes, cyclooctane, was
selected for further investigations.
The results of oxidation of cyclooctane with air over iron l-oxo
porphyrins and their monomeric analogues containing either elec-
Cyclic voltammograms of iron
l-oxo porphyrin [FeTPP]2O are
shown in Figs. 4 and S1 (Supplementary data). They indicate the
irreversible decomposition of [FeTPP]2O complex (Fig. 4A) to
monomeric iron porphyrin (Fig. 4B). The similar cyclic voltammo-
grams were obtained for all the iron
l-oxo porphyrins. Because of
the irreversible decomposition of all the
l-oxo porphyrins during
the electrochemical measurement, it is impossible to establish
their half-wave potential E1/2. Instead, a reduction potential ERED
of iron l-oxo porphyrins was measured.
In Supplementary data Figs. S2 and S3 show the cyclic voltam-
mograms of FeTTPCl (with electron-donating substituents) and
FeTPFPPCl (with electron-withdrawing substituents). The calcu-
lated value of E1/2 for FeTTPCl was -0.35 V and for FeTPFPPCl
+0.08 V, confirming that the value of E1/2 is influenced by the type
of substituents on porphyrin macrocycle. FeTTPCl exhibits lower
E1/2 value than FeTPFPPCl. Table 1 presents values of E1/2 for all
studied monomeric iron porphyrins. The increase of electron-with-
drawing character of the substituents on porphyrin macrocycle
tron-donating or electron-withdrawing substituents are summa-
rized in Table 2. Both, the iron
l-oxo and monomeric porphyrins

346
E. Tabor et al. / Polyhedron 119 (2016) 342–349
Scheme 1. Oxidation of cycloalkanes.
procedure, and reused three times with only a slight decrease in
catalytic activity as it is shown in Table 2 for the [FeTPP]2O
catalyst.
ketone
alcohol
40
30
20
10
0
Fig. 6 presents the correlation between the catalytic activity of
iron
as well as between iron monomeric porphyrins and the half-wave
potential E1/2 (Fig. 6B). The catalytic activity of iron -oxo por-
l-oxo porphyrins and the reduction potential ERED (Fig. 6A)
l
phyrins with electron-donating substituents increases with the
decrease of the ERED value. Conversely, the catalytic activity of iron
l
-oxo porphyrins with electron-withdrawing substituents
increases with the increase of the value of ERED. Among iron
-oxo porphyrins with electron-donating substituents, [FeTTP]2O
l
with the lowest reduction potential of ꢁ1.38 V provided the high-
est yield of oxygenates (48.3%). The order of catalytic activity in the
oxidation of cyclooctane over iron
l-oxo porphyrins with either
electron-donating or electron-withdrawing substituents was simi-
lar to monomeric iron porphyrins. The presence of electron-donat-
ing or electron-withdrawing substituents in iron
l-oxo and
monomeric porphyrins enhanced their catalytic activity in com-
parison with the unsubstituted porphyrins. This conclusion is sup-
ported by the fact that both [FeTPFPP]2O having 40 fluorine atoms
and FeTPFPPCl with 20 fluorine atoms were the most active in the
oxidation of cyclooctane possessing the highest value of ERED
(+0.83 V) or E1/2 (+0.08 V), respectively.
4. Discussion
Fig. 5. Oxidation of various cycloalkanes by [FeTTP]2O complex. The yields to
ketone or alcohol were calculated based on the oxygen quantity in batch reactor of
the beginning of the reaction.
4.1. Influence of substituents on iron
on their catalytic activity
l-oxo and monomeric porphyrins
Results of catalytic study reveal the influence of substituents on
porphyrin ring on their catalytic performance. Unsubstituted
l
are active in the oxidation of cyclooctane by molecular oxygen to
cyclooctanone and cyclooctanol. Regardless of the type of sub-
stituents, the reaction proceeds with a high selectivity to cyclooc-
tanone and low selectivity to cyclooctanol. The highest yield of
-oxo [FeTPP]2O complex (H⁄) showed the lowest catalytic activity
among iron l–oxo porphyrins (Fig. 6A) and demonstrated the low-
est catalytic activity among all the studied catalysts. The introduc-
tion of either electron-donating or electron-withdrawing
substituents into porphyrin rings increased the yields to cyclooc-
tanol and cyclooctanone. The presence of electron-donating
(–CH3 and –OCH3) or electron-withdrawing (–Cl, –CF3, and –F) sub-
products for iron
l-oxo porphyrins was observed for [FeTPFPP]2O
with 20 electron-withdrawing fluorine atoms (54.5%). [FeTPFPP]2O
is twice as active in this reaction as unsubstituted [FeTPP]2O por-
phyrin (23.9%). Unsubstituted iron
l-oxo porphyrin [FeTPP]2O
demonstrates the lowest catalytic activity in comparison with
the catalysts containing either electron-donating or electron-with-
drawing substituents. The same tendency can be observed for iron
porphyrins, where FeTPPCl exhibits the lowest catalytic activity.
Moreover, the catalyst can be easily recovered from reaction mix-
ture, cooled down to room temperature via filtration and drying
stituents in iron
tial ERED (Table 1).
The results of catalytic activity for series of iron
electron-donating and electron-withdrawing substituents have
the form of V-shaped curves (Fig. 6A), where the minimum of cat-
l-oxo porphyrins affected their reduction poten-
l-oxo with
alytic activity occurs for unsubstituted iron
l-oxo porphyrin

E. Tabor et al. / Polyhedron 119 (2016) 342–349
347
Table 2
Oxidation of cyclooctane over
l
-oxo and monomeric iron porphyrins with either electron-donantig or electron-withdrawing substituents.
Catalyst
Abbreviated Symbols
Yield toa,b
Yield toa,b
C-one/C-ol
TOFc
hꢁ1
C-one
%
C-ol
%
C-one + C-ol
FeTTPCl
[FeTTP]2O
FeTPPCl
CH3
CH⁄3
H
50.5
40.3
33.5
20.5
19.8
18.5
42.6
23.9
47.7
29.5
40.9
25.0
71.0
46.0
8.0
8.0
6.3
3.4
3.4
3.1
7.4
4.0
8.0
4.6
7.4
4.0
10.8
8.5
58.5
48.3
39.8
23.9
23.2
21.6
50.0
27.9
55.7
34.1
48.3
29.0
81.8
54.5
6.3
5.0
5.3
6.0
5.8
6.0
5.8
6.0
6.0
6.4
5.5
6.3
6.6
5.4
21940
18110
14930
9170
8700
8100
18750
10460
20890
12790
18110
10880
30680
20440
[FeTPP]2O
H⁄
[FeTPP]2Od
H⁄
[FeTPP]2Oe
H⁄
FeT(p-OCH3)PPCl
[FeT(p-OCH3)PP]2O
FeT(p-CF3)PPCl
[FeT(p-CF3)PP]2O
FeT(p-Cl)PPCl
[FeT(p-Cl)PP]2O
FeTPFPPCl
OCH3
OCH⁄3
CF3
CF⁄3
Cl
Cl⁄
5F
[FeTPFPP]2O
5F⁄
a
Reaction conditions: 120 °C, 10 atm, C8H16:O2 = 6.5, reaction time 6 h.
b
c
Calculated on the base of oxygen quantity in batch reactor.
mol productꢀmol catalystꢁ1ꢀhꢁ1
.
d
e
Second run.
Third run.
Fig. 6. Yield of cyclooctane oxidation products as a function of reduction potential ERED for iron
l–oxo porphyrins (A) and as a function of half-wave potential E1/2 for
monomeric iron porphyrins (B).
[FeTPP]2O (H⁄). The introduction of electron-donating substituents
into the porphyrin macrocycle increased the electron density on
the metal center which resulted in the lower value of the reduction
as co-reductant and molecular oxygen as oxidant was observed
[14]. Improvement of catalytic activity in oxidation of ethyloben-
zene with
l-oxo [FeTPP]2O and [MnTPP]2O porphyrins was also
potential ERED of iron
is the most active catalyst among iron
tron-donating substituents. The presence of electron-withdrawing
substituents (–Cl, –CF3, and –F) on iron -oxo porphyrins reduces
l
-oxo porphyrin [FeTTP]2O (CH⁄3). The latter
described [7]. Also our previous findings showed that the presence
of electron-donating and electron-withdrawing groups in man-
ganese porphyrins improve their catalytic performance in the
oxidation of cyclooctane by molecular oxygen [10].
l-oxo porphyrins with elec-
l
the electron density on the metal atoms and increases the value
The similar catalytic behavior of monomeric iron porphyrins in
cyclooctane oxidation was observed (Fig. 6B). FeTPFPPCl (5F) (with
20 atoms of fluorine) induces the highest value of half-wave poten-
tial (+0.08 V) and the highest catalytic activity in the oxidation of
cyclooctane with the highest yield of oxygenates (81.8%). Unsubsti-
tuted FeTPPCl (H) porphyrin demonstrates the lowest catalytic
activity for monomeric iron porphyrins in the studied reaction sys-
tem. The positive influence of the electron-withdrawing sub-
stituents on monomeric metalloporphyrins on their catalytic
properties in the oxidation processes is well known [6,9,10,18–
23] (See Fig. 7).
of ERED. The [FeTPFPP]2O porphyrin (5F⁄) has the highest ERED
potential (+0.83 V) among studied iron
l-oxo porphyrins with
either the electron-withdrawing or the electron-donating sub-
stituents. Guo reported that in the cyclohexane hydroxylation with
iodozobenzene PhIO as oxygen donor catalyzed by iron l-oxo por-
phyrins the presence of the electron-donating and electron-with-
drawing substituents improves their catalytic activity [3].
Unexpectedly, significant enhancement of catalytic activity of
l-oxo [FeTPP]2O porphyrin in comparison with monomeric
FeTPPCl porphyrin in the olefin epoxidation with isobutyraldehyde

348
E. Tabor et al. / Polyhedron 119 (2016) 342–349
Increase of ERED of iron µ-oxo porphyrins or E1/2 of monomeric iron porphyrins
electron-donating substituents
electron-
decrease of electron-donating properties
decrease of catalytic activity
increase of electron-
increase of catalytic activity
H
Fig. 7. Influence of substituents on iron porphyrin rings on the value of the reduction ERED and the half-wave E1/2 potentials, on the catalytic activity.
4.2. Mechanism of the oxidation of cycloalkanes over iron
porphyrins
l
-oxo
oxidation but in argon (oxygen-free) atmosphere was performed
(Fig. 8a). Spectroscopic investigations revealed that the [FeTPP]2O
porphyrin (green solution) in the oxygen-free atmosphere reacts
with cycloalkane resulting in the abstraction of hydrogen from
Many studies have been devoted to the oxidation of hydrocar-
bons with O2 and metalloporphyrins as catalysts although the
mechanism of this reaction is still a subject of discussion. Two
main routes of oxidation of hydrocarbons with metalloporphyrins
were proposed [6,9,12]. The first one suggests direct oxygen activa-
tion on metalloporphyrin with the formation of oxo species. The
second mechanism assumes a hydrocarbon activation on metallo-
porphyrin molecule [12,24]. It was shown that the catalytic oxida-
tion of cycloalkanes in liquid phase with molecular oxygen in the
presence of metalloporphyrins proceeds through radical-chain
mechanism [6,9,25–27].
the hydrocarbon molecule and the formation of
l-hydroxo diiron
porphyrin Fe(III)Por–O(H)–Fe(III)Por (red solution) [28]. On the
other hand, the positions of Soret band of the [FeTPP]2O porphyrin
(Fig. 8c) before and after the reaction of cyclooctane with molecu-
lar oxygen are identical (Fig. 8b), which means that catalyst struc-
ture remains intact. Moreover, the [FeTPP]2O porphyrin was
recycled three times in cyclooctane oxidation with only a small
decrease of catalytic activity (Table 2).
However, it is well known [29] that the
l-oxo iron porphyrin
easily forms monomeric iron porphyrins during the oxidation reac-
tion. Our electrochemical investigations also confirmed this obser-
vation (Fig. 4) indicating the irreversible decomposition of
[FeTPP]2O complex (Fig. 4A), possibly to monomeric iron porphyrin
FeTPPOH.
Presented results confirmed the ability of iron
to oxidize cycloalkanes. Our previous work showed that man-
ganese -oxo porphyrins are active in the oxidation of cyclooctane
l-oxo porphyrins
l
with molecular oxygen, but the yields of products were lower in
comparison with monomeric manganese porphyrins [10]. To
understand the reaction mechanism, the catalytic experiment with
In the presence of iron
l-oxo porphyrins, the oxidation of
cycloalkanes in liquid phase with molecular oxygen proceeds
through radical-chain mechanism. We propose the following
mechanism of cycloalkane oxidation with molecular oxygen. The
propagation step (Eq. (1)) begins with the abstraction of hydrogen
iron
l
-oxo porphyrin in reaction conditions similar to cyclooctane
atom from cycloalkane (RH) by iron
l
-oxo complex followed by
the formation of cycloalkyl radical RÅ and
l
-hydroxo diiron com-
4
409.5
plex [28,30]. The latter easily decomposes resulting in iron
hydroxo complex Fe(III)Por-OH and iron porphyrin Fe(II)Por (Eq.
(2)). Due to the abstraction of hydrogen from cycloalkane by
hydroxo iron complex Fe(III)Por-OH the additional source of both
cycloalkyl radical RÅ and iron porphyrin Fe(II)Por appears (Eq.
(3)). Cycloperoxyl radical, formed in the reaction of cycloalkyl rad-
ical with molecular oxygen (Eq. (4)), can react further with
cycloalkane resulting in the formation of cycloalkyl hydroperoxide
(Eq. (5)). Cycloalkyl hydroperoxide can be decomposed in a homo-
lytic (Eq. (6)) or heterolytic (Eq. (7)) manner. Bimolecular dispro-
portionation process (Eq. (8)) of cycloperoxyl radicals ROOÅ is a
source of cycloketone and cycloalcohol constituting the major
chain-termination step. The reaction of Fe(II)Por with molecular
3
408
408
2
oxygen leads to the regeneration of iron
(Eqs. 9–12).
l-oxo porphyrin
a
FeðIIIÞPor—O—FeðIIIÞPor þ RH ! FeðIIIÞPor—OðHÞ—FeðIIIÞPor þ RÅ
ð1Þ
1
FeðIIIÞPor—OðHÞ—FeðIIIÞPor ! FeðIIIÞPor—OH þ FeðIIÞPor
FeðIIIÞPor—OH þ RH ! FeðIIÞPor þ H—OH þ RÅ
RÅ þ O2 ! ROOÅ
ROOÅ þ RH ! ROOH þ RÅ
ROOH þ FeðIIÞPor ! ROÅ þ OHÅ
ð2Þ
ð3Þ
ð4Þ
ð5Þ
ð6Þ
ð7Þ
b
c
0
300
400
500
600
700
800
Wavelenght / nm
Fig. 8. UV–Vis spectra of (a) [FeTPP]2O after the reaction in oxygen-free atmo-
sphere, (b) [FeTPP]2O after cyclooctane oxidation with molecular oxygen and (c)
initial [FeTPP]2O, dissolved in benzene.
ROOH þ FeðIIIÞPor ! ROOÅ þ Hþ

E. Tabor et al. / Polyhedron 119 (2016) 342–349
349
ROOÅ þ ROOÅ ! ROH þ RCO þ O2
ð8Þ
ð9Þ
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FeðIIÞPor þ O2 ! FeðIIÞPor ꢀ ꢀ ꢀ O2
FeðIIÞPor ꢀ ꢀ ꢀ O2 þ FeðIIÞPor ! FeðIIIÞPor—O—O—FeðIIIÞPor
FeðIIIÞPor—O—O—FeðIIIÞPor ! 2FeðIVÞPor@O
FeðIVÞPor ¼ O þ FeðIIÞPor ! FeðIIIÞPor—O—FeðIIIÞPor
ð10Þ
ð11Þ
ð12Þ
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All synthesized iron l-oxo complexes are catalytically active in
the oxidation of cycloalkanes by molecular oxygen. Their catalytic
performance depends on the type and number of substituents
introduced in meso-aryl positions of porphyrin macrocycles. It
was shown that not only electron-withdrawing but also electron-
donating substituents on porphyrin rings significantly improve
the catalytic activity towards the formation of oxygenates. The cat-
alytic activity of iron
substituents correlated with the half-wave potential E1/2 while the
catalytic activity of iron -oxo porphyrins with electron-donating
l-oxo porphyrins with electron-withdrawing
l
substituents increased with the decrease of reduction potential
ERED. Moreover, the it was confirmed that the [FeTPP]2O catalyst
can be reused with only a small decrease of catalytic activity.
Based on the literature data and our investigations it was possi-
ble to propose the oxidation mechanism of cycloalkanes by iron
l
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Appendix A. Supplementary data
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Metalloporphyrins with Molecular Oxygen,, in: A. George (Ed.), InTech, 2011.
[30] S.K. Ghosh, S. Bhowmik, D. Sil, S.P. Rath, Chem. Eur. J. 19 (2013) 17846.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2016.08.048.
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