Three Generations of Cobalt Porphyrins as Catalysts in the Oxidation of Cycloalkanes

Article abstract of DOI:10.1002/cssc.201802198

Three generations of cobalt porphyrins were synthesized, physicochemically characterized by FTIR and UV/Vis spectroscopy as well as cyclic voltammetry and applied as catalysts in the oxidation of cycloalkanes with atmospheric molecular oxygen under mild c

Full text of DOI:10.1002/cssc.201802198

Accepted Article
Title: Three generations of cobalt porphyrins as catalysts in oxidation of
cycloalkanes
Authors: Katarzyna Renata Pamin, Jan Połtowicz, Edyta Tabor, Sylwia
Górecka, Wojciech Kubiak, and Dorota Rutkowska-Żbik
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10.1002/cssc.201802198
ChemSusChem
FULL PAPER
Three generations of cobalt porphyrins as catalysts in oxidation
of cycloalkanes
K. Pamin*[a], E. Tabor[b], S. Górecka[c], W.W. Kubiak [d], D. Rutkowska-Zbik[a], J. Połtowicz*[a]
Abstract: Three generations of cobalt porphyrins were synthesized,
physicochemically characterized by FTIR and UV-Vis spectroscopy
as well as cyclic voltammetry and applied as catalysts in oxidation of
cycloalkanes with molecular oxygen as air under mild conditions. It
was found that all the examined catalysts were active in the tested
reaction and their catalytic activity varied with the nature and the
quantity of substituents on porphyrin ring. It has been demonstrated
that introduction of electron-withdrawing or electron-donating
substituents at the porphyrin rings increases the activity of
metallocomplexes. It was exhibit, for the first time, that the II
generation of cobalt porphyrins in cycloalkanes oxidation show
higher activity in comparison to cobalt porphyrins of III generation.
Relatively lower catalytic activity of III generation cobalt porphyrins
can be assigned to the saddle-shaped deformations of the porphyrin
macrocycle. DFT modelling of Co porphyrins and their interactions
with molecular oxygen provided motives for the observed effects. On
the basis of literature data and obtained results reaction mechanism
is proposed and discussed.
approaches[1d]. One of them relays on the variation of the
reaction conditions in the chemical process through the
introduction of a new catalysts which permits the milder reaction
condition application.
The nature has chosen metalloporphyrins as active elements of
enzymes and other biologically active compounds. Macrocyclic
complexes like metalloporphyrins have received much attention
in the field of catalytic hydrocarbon oxidation and they are good
candidates for catalytic centers. They exhibit multidentate ligand
systems that bind transition metal ions in the multiple oxidation
states. The electronic structures of porphyrin ligands can be
altered by a rational synthesis, tailoring their composition for
introduction of those elements that are necessary to achieve
defined properties.
The synthetic metalloporphyrins have been used extensively as
biomimetic catalysts for hydroxylation of hydrocarbons, oxidation
of aromatic compounds and olefins epoxidation[2a,b]
. Such
studies could help in the understanding of the complex biological
processes involved in the enzymatic reactions with cytochrome
P450[2c]
. This enzyme is responsible for hydroxylation of
nonpolar substrates[2d,e]. Therefore, the synthesis of porphyrins
and metalloporphyrins has received the a lot of attention among
scientists as highly active catalysts for hydroxylation of alkanes
and epoxidation of olefins. The currently available synthetic
methods allow one to synthesize porphyrin ligands carrying
different substituents either at meso- or β-pyrrolic positions or
both at meso- and β-pyrrolic positions (Scheme 1). Porphyrins
used for the preparation of complexes with different transition
metals can be divided into ligands of I, II and III generation. The
I generation porphyrin has no or one substituent on the phenyl
ring in porphyrin macrocycle[2d]. Lindsay synthesis[3a] permits the
development of the II generation porphyrins by the introduction
of more electron-withdrawing or/and electron-donating
substituents at the porphyrin macrocycle.
Introduction
The oxyfuntionalization of cycloalkanes into more valuable
products such as alcohols and ketones is one of the most
important and one of the fundamental reactions in organic
synthesis[1a]. Particularly, the oxidation of cyclohexane attracts
constantly much attention because cyclohexanol and
cyclohexanone are important intermediates for the production of
adipic acid or caprolactam. Both of them are used in the
chemical industry for production of polyamids like Nylon-6,6 or
Nylon-6, respectively. In the industry, the system currently in use
operates with cobalt naphthenate as catalyst, molecular oxygen
as air, reaction temperature in the range 155-160oC and air
pressure of 1 MPa. The conversion around 4% and selectivity of
about 85% accomplished in this process means that a large
stream of unreacted cyclohexane is constantly circulating in the
installation for cyclohexane oxidation[1b,c]. Therefore, this makes
the oxidation of cyclohexane the least efficient of all major
industrial chemical processes.
Type of substituents:
meso-positions
o-position on phenyl ring
p-position on phenyl ring
m-position on phenyl ring
The XXI century is called “the century of new materials” because
only the synthesis of new materials can allow mankind to
maintain the current development. It is also the century which
brings a new demands for the society, such as the idea of the
sustainable development which can be reached using different
pyrrolic position
β-position
[a]
K. Pamin, J. Połtowicz, D. Rutkowska-Zbik
Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish
Academy of Sciences,
Scheme 1. Co porphyrin with substituents on meso- and β-pyrrolic positions.
Niezapominajek 8, 30-239 Krakow, Poland
E-mail: ncpamin@cyf-kr.edu; ncpoltow@cyf-kr.edu.pl; nczbik@cyf-
kr.edu.pl;
Metalloporphyrins of the II generation have been extensively
studied as catalysts with different oxygen donors[3b,c]. The
demand for the synthesis of porphyrin ligands with the
substitution on both meso- and β-pyrrolic positions resulted in III
generation porphyrins on the way of the evolution of the
synthetic procedures involving the perhalogenation of the I and II
generation porphyrins. Perhalogenation of porphyrin ligand was
the method of choice for the formation of the increased electron
deficiency of the metalloporphyrin. The enhanced catalytic
[b]
[c]
E. Tabor
J. Heyrovský Institute of Physical Chemistry of the CAS
Dolejškova 2155/3, 182 23 Praha, Czech Republic
edyta.tabor@jh-inst.cas.cz
S. Górecka
Departament of Chemistry
Jagiellonian University
Gronostajowa 2, 30-387 Krakow, Poland
gorecka.syl@gmail.com
[d]
W.W. Kubiak
Faculty of Materials Science and Ceramics
AGH University of Science and Technology
Mickiewicza 30, 30-059 Krakow, Poland
kubiak@agh.edu.pl
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10.1002/cssc.201802198
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activity observed for the II and III generation of porphyrins is
related to the protection of macrocyclic ligand from the oxo-
dimer formation and oxidative self-destruction during catalysis.
The synthetic iron and manganese porphyrins are among the
most studied metallocomplexes as catalysts in different
oxidation processes[4a-f,2a,2f-h]. On the contrary, there are only a
few examples in the literature dealing with the cobalt porphyrins
UVVis Absorption Spectra
UVVis spectra of metalloporphyrins are fingerprints of their
structures. Generally, metalloporphyrins possess characteristic
UVVis spectra with 2 distinct regions: Soret band at about 380 –
500 nm and Q bands at 500 – 750 nm. First region includes the
transition from the ground state S0 to the second excited state
S2 and the resulting band is called the Soret or B band. The
second region covers weak transitions from the ground state S0
to the first excited state S1 provoking the formation of rather
weaker intensity Q bands. Their relative intensity is dependent
on the kind and the position of substituents on the macrocyclic
ring. UVVis spectra of Co porphyrins were measured as 10-5
solutions in dichloromethane and the results are gathered in
Table 1.
catalyzed oxidation of cycloalkanes[4a-d,2a,2i]
.
Recently, we have described the application of both manganese
and iron porphyries as well as their µ-oxo analogs with different
electron-donating or electron-withdrawing substituents in the
oxidation of cyclooctane to its oxygenates[5a,b]. Our investigations
demonstrated that these metalloporphyrins were catalytically
active in the oxidation process and their activity depend on the
structure of porphyrin macrocycle. It was also found that the
yield of products in hydroxylation reaction shows an almost
linear relationship with the number of the halogens on the
porphyrin macrocycle.
Table 1. UVVis spectra of Co porphyrins.
In this paper we continue our study on the catalytic activity of
metalloporphyrins in the oxidation of cycloalkanes to
cycloketone and cycloalcohol. We have applied as catalysts
three generations of cobalt porphyrins with various electron-
donating or electron-withdrawing substituents to modify their
physicochemical properties and catalytic activity. Their
structures are presented in Figure1. We exhibit here, for the first
time, that the II generation Co porphyrins are catalytically more
active than those of III generation in the oxidation of
cycloalkanes with molecular oxygen.
Catalyst
Soret band
[nm]
Q bands
[nm]
TPP
CoTPP
CoTTP
CoT(p-Cl)PP
CoTMP
CoTDCPP
CoTPFPP
CoTDCPβN6P
CoTDCPβCl8P
CoTPFPβBr8P
417
410
412
411
412
410
404
446
436
442
514 538 585 620
528
529
529
529
531 560
526 554
546
556 585
558 588
Z1
Z
X
Z1
X
Y
Y
Z1
Y
Z
In Table 1, the positions of Soret band for TPP ligand and series
of Co porphyrins as well as one to four characteristic bands in
the visible region are listed. Insertion of cobalt atom and different
substituents into the porphyrin ring influences both the position
of Soret band and the intensity and number of Q bands. The
largest variations in Soret band locations are observed for
substituents in β-pyrollic positions (Scheme 1). Moreover,
introduction of cobalt results, for porphyrins of I and II generation,
in blue shift of Soret band. However, for III generation Co
porphyrins with substituents in β-pyrollic and meso-positions
Soret band is red-shifted. For example, the introduction of
electron-withdrawing groups like NO2 or Cl into CoTDCPP
induces the change of Soret band position from 410 nm to 446
nm for NO2 or 436 nm for Cl substituents. Similarly, in the case
of CoTPFPP, the incorporation of Br groups renders the Soret
band red shift from 404 nm for CoTPFPP to 442 nm for
CoTPFPβBr8P.
X
N
X
Z1
Y
N
Co
Z1
Y
X
N
N
X
Y
Z1
Y
Z
Y
X
Z1
X
Z
Z1
Co porphyrin
X
Z
Z1
Y
NoS[a]
CoTPP
CoTTP
H
H
H
Cl
CH3
F
Cl
Cl
F
H
H
H
H
H
H
F
H
H
F
H
H
H
H
H
H
NO2
Cl
Br
0
4
4
CH3
Cl
H
CH3
F
CoT(p-Cl)PP
CoTDCPP
CoTMP
8
Cyclic Voltammetry Measurements
Redox properties of Co porphyrins were studied using cyclic
voltammetry, which is reliable method to asses oxidation skills of
12
20
14
16
28
CoTPFPP
CoTDCPβN6P
CoTDCPβCl8P
CoTPFPβBr8P
H
H
F
studied catalysts. Table
2 presents the results of the
voltammetric measurements for the series of Co porphyrins. All
the studied Co porphyrins undergoes one-electron oxidation
processes. Oxidation of unsubstituted CoTPP porphyrin, treated
here as reference sample, occurs at 280 mV. However, the
introduction of substituents on the porphyrin ring modifies
oxidation potential. For example, the addition of electron-
withdrawing groups either to meso- or to β-pyrollic positions on
the porphyrin macrocycle induces easier reduction and more
difficult oxidation. By contrast, in the presence of electron-
donating groups on Co porphyrin reduction is harder while
oxidation proceeds more readily.
[a] Number of substituents.
Figure 1. Co porphyrins: I generation (blue color), II generation (orange color),
III generation (green color)
Results and Discussion
Syntheses and Characterizations
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10.1002/cssc.201802198
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previous studies, which were performed in the presence of
manganese[5a] or iron[5b] porphyrins bearing electron-donating or
electron-withdrawing groups, also confirm catalytic performance
of Co porphyrins in the oxidation of cyclooctane by molecular
oxygen.
Table 2. Cyclic voltammograms of Co porphyrin catalysts
Co porphyrin
Potential
EOX, [mV]
Number of
Co porphyrin
generation
substituents
CoTMP
CoTTP
CoTPP
CoT(p-Cl)PP
CoTDCPP
CoTPFPP
CoTDCPβN6P
CoTDCPβCl8P
CoTPFPβBr8P
271
278
280
282
292
295
290
290
308
4
12
-
4
8
20
14
16
28
I
II
I
I
II
Catalytic results
Three members of homologous series, namely cyclopentane,
cyclohexane or cyclooctane were applied as substrates and
studied in oxidation catalyzed by cobalt porphyrin complex
CoTPP (Scheme 2).
II
III
III
III
O
OH
+
cyclopentane
cyclopentanone cyclopentanol
O
OH
+
CoTPP
O
The presence of electron-withdrawing groups reduces the
electron density on the metal atom. On the other hand, electron-
donating substituents act in quite the opposite way, rendering
the increase of electron density in the vicinity of metal atom.
2
cyclohexane
cyclooctane
cyclohexanone cyclohexanol
O
OH
+
cyclooctanone
cyclooctanol
90
Scheme 2. Cycloalkanes oxidation.
CoTDCPP
80
CoTPFPP
The main products of cycloalkane oxidation with molecular
oxygen are cycloketone and cycloalcohol (Scheme 2). Blank test
of liquid phase oxidation of cyclooalkane carried out in the
absence of a catalyst showed lack of oxygenates in the reaction
conditions. For cyclopentane oxidation, in the presence of
CoTPP, after 6 h of reaction only 3.8% yield to ketone and 1.2%
yield to alcohol were obtained (Figure 3).
CoTMP
70
CoTDCPCl8P
CoTDCPN6P
CoTPFPBr8P
60
50
40
30
CoT(p-Cl)PP
30
% cycloketone
% cycloalcohol
CoTTP
25
type of substituents:
electron-donating
20
15
10
5
electron-withdrawing
CoTPP
270 275 280 285 290 295 300 305 310
Eox, mV
Figure 2. Catalytic activity of cobalt porphyrins with different substituents as a
function of their oxidation potentials. The yield is based on molecular oxygen
in the reaction vessel.
0
Introduction of electron-donating substituents on the porphyrin
macrocycle (CoTTP and CoTMP) renders the decrease of their
oxidation potentials. On the contrary, oxidation potential of
cobalt porphyrins grows upon the addition of electron-
withdrawing groups and generally is higher than the one
C H
5 10
C H
6 12
C H
8 16
Figure 3. Oxidation of C5, C6 and C8 cycloalkanes by unsubstituted CoTPP.
established for CoTPP. Figure
2 depicts the relationship
between the yield to oxygenates and the oxidation potential of
the Co porphyrins. The presence of both, the electron-donating
and the electron-withdrawing substituents on porphyrin ring
improves the catalytic activity in comparison with unsubstituted
CoTPP. In particular, for electron-donating groups on porphyrin
ring, with the decrease of oxidation potential Eox the yield to
oxygenates grows. In the presence of electron-withdrawing
substituents the increase of the Eox is accompanied by the rise
of the yield to oxygenates. These results are in line with findings
reported by Guo[6] for iron µ-oxo porphyrins in the cyclohexane
hydroxylation with iodosobenzene as oxygen donor. Our
The oxidation of cyclohexane resulted in 15.4% yield to
cyclohexanone and 2.9% yield to cyclohexanol. In the similar
reaction
conditions,
cyclooctane
was
converted
to
cyclooctanone and cyclooctanol with yields of 29.5% and 6.3%,
respectively.
Taking into account the above results the following order of
catalytic activity in the studied reaction was established:
cyclooctane
>
cyclohexane
>
cyclopentane, and hence
cyclooctane was finally selected for further investigations.
It is well-known that the selective catalytic oxidation of C-H
bonds results in the formation of hydroperoxides[7]. However,
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iodometric titration of reaction mixture, analysed after 6 h of the
reaction, enables the evaluation of the amount of cyclooctane
hydroperoxide as 0.06%, which is too low to have any impact on
the results of the further GC analysis. Hydroperoxide once
formed upon the action of the catalyst in the course of the
reaction transforms to alcohol and ketone. In our reaction
conditions cycloalkyl hydroperoxide in the presence of cobalt
porphyrin catalysts almost completely decomposes.
catalysts pertaining to the group of II generation Co porphyrins.
CoTMP complex with twelve CH3 groups demonstrates slightly
higher yield to cyclooctanone (58.5%), however the yield to
cycloalcohol is comparable with the results observed for III
generation of Co porphyrins. Although CoTMP complex
possesses exclusively electron-donating substituents its high
catalytic performance in the studied reaction is not surprising.
Similar metalloporphyrins, MnTMP and FeTMP, have been
widely applied and they demonstrated superior catalytic activity
in oxidation processes[4a,8b-c]. For CoTPFPP with 20 fluorine
substituents the yield to cyclooctanone slightly diminishes
(56.3%) but on the other hand this catalyst is the most active in
the formation of cyclooctanol. CoTDCPP porphyrin having 8 Cl
groups in phenyl rings is the most active among all the studied
catalysts in the oxidation of cyclooctane yielding 68.8% of
cyclooctanone and 14.2% of cyclooctanol. Summing up, it is
worth notice that II generation Co porphyrins are more
catalytically active than those of III generation. Entirely similar
phenomenon was described by Meunier et al.[8d] who showed
that Mn porphyrins of II generation (MnTDCPP) are more
catalytically active than those of III generation (MnTMβBr8P) in
the cyclohexane hydroxylation by KHSO5. Moreover, it was also
demonstrated that III generation MnTDCPβCl8P is less active
than II generation MnTDCPP in the olefins epoxidation with
hypochlorite[8e]. We postulate that the motive for the reverse
order of catalytic activity in cyclooctane oxidation is spatial
deformation of porphyrin ring structure from planar to saddle-
shape.
In this paper we report the effect of the Co porphyrin ring
structure of the series of three generations cobalt porphyrins on
their catalytic activity in the oxidation of cyclooctane with
molecular oxygen. The main products of the studied reaction are
cyclooctanone and cyclooctanol. The formation of secondary
oxidized products such as 1,5-cyclooctanedione and 1,4-
cyclooctandione was not confirmed by GC analysis. As catalysts
the unsubstituted cobalt porphyrins or Co porphyrins with one
substituent at the phenyl ring (I generation), those bearing more
substituents at the phenyl rings (II generation) and cobalt
porphyrins with groups both at the phenyl and pyrrole rings (III
generation) were applied. The results summarized in Figure 4
clearly show that all the examined macrocyclic catalysts were
active in the studied reaction and their catalytic activity varied
with the nature and the quantity of substituents at the porphyrin
ring.
cyclooctanone
cyclooctanonol
70
60
50
40
30
20
10
0
GEN I
GEN III
GEN II
Basing on the literature[9] and our investigations the following
mechanism of oxidation with Co porphyrins as catalysts was
proposed. According to the literature data[9a,10] we propose the
radical chain reaction mechanism.
-
O2
+
+
Co(II)Por
Co(III)Por-OO
H
(1)
(2)
.
.
-
Co(III)Por-OO
Co(III)Por- O2 H
OO
.
.
+
O2
(3)
(4)
P
8
P
T
T
P
6
P
8
.
PP
PP
PP
MP
T
OO
OOH
)PP
N
H
l
T
Cl
C
Br


o
o
PF
T
o

D
T
-C
P
+
P
+
C
C
C
P
C
o
(p
T
C
o
D
T
C
D
T
C
PF
o
T
o
C
o
o
C
.
.
OO
C
C
OO
OH
+
O
+
Figure 4. Cyclooctane oxidation in the presence of Co porphyrins. The yields
are based on molecular oxygen in the reaction vessel.
O2
.
+
(5)
(6)
As one can see catalysts can be divided into three groups,
depending on the porphyrin generation. In the case of I
generation metalloporphyrins, the lowest catalytic activity in
cyclooctane oxidation is observed for unsubstituted CoTPP.
CoTTP complex with four electron-donating groups (CH3)
demonstrates significant increase of yield to cyclooctanone
(41.5%). And the last complex in this group of catalysts, CoT(p-
Cl)PP, belonging to the I generation metalloporphyrins with four
electron-withdrawing groups (Cl) shows further rise of
cyclooctanone yield. At the same time yield to cyclooctanol
.
.
.
OOH
O
Co(II)Por
Co(III)Por
OH
+
+ Co(III)Por
.
OOH
OO
+
+
H
(7)
+
Co(II)Por
.
O
OH
+
H
.
(8)
(9)
+
remains almost stable for all three
I
generation
OH
O
metalloporphyrins. III generation Co porphyrins, namely
CoTDCPβCl8P, CoTPFPβBr8P and CoTDCPβN6P, exhibit
continued growth of both, the yield to cyclooctanone and to
cyclooctanol. However, relatively moderate catalytic activity of
these considerably substituted Co porphyrins is rather surprising
since it has been repeatedly demonstrated that introduction of
electron-withdrawing or electron-donating substituents at the
Co(III)Por
1/2 O
H
+
H O
2
+
2
Scheme 3. Cycloalkanes oxidation in the presence of Co porphyrins – the
mechanism
porphyrin rings increases the activity of metallocomplexes[8a]
And finally, the highest catalytic activity can be observed for the
.
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Scheme 3 presents set of reactions where the main reaction
products are cycloctanone, cyclooctanol and traces of
cyclohydroperoxide. Reaction starts with the activation of
molecular oxygen by Co porphyrin which leads to the formation
of superoxide (1) species. In the next step the latter abstracts
hydrogen from cycloalkane forming the cycloalkyl radical R• (2)
of macrocyclic ring (Figures 5A and 5B), generation II
complexes are only slightly ruffled (Figure 5C), while those of III
generation exhibit saddle deformation (Figure 5D). Their distinct
geometry is also reflected by the average Co-N bond lengths: in
the four-coordinated complexes they are equal ca. 1.98 Å for
generation I and II complexes, but are diminished to ca. 1.93 Å
in the case of generation III catalysts due to their saddle
deformation. Upon molecular oxygen binding to the cobalt ion
and promoting the initiation of radical chain reaction[9c,11a,b]
Chain propagation step begins with the reaction of cycloalkyl
.
radical R• and molecular oxygen to cycloperoxyl radical ROO• (3). the average Co-N distances (N-CoO2) are slightly elongated in
Cycloperoxyl radical ROO• may further react with the next
molecule of cycloalkane producing cycloalkyl hydroperoxide
ROOH and cycloalkyl radical R• (4). In the fifth step cycloketone
and cycloalcohol appear as a result of the recombination of
cycloperoxyl radicals ROO• establishing at the same time chain
termination step. Cycloalkyl hydroperoxide ROOH in the
presence Co porphyrin decomposes both, in the homolytic[9a,11c]
or heterolytic[9a,11d] manner. In the homolytic decomposition
cycloalkoxyl radical RO• and hydroxyl radical OH• are produced
(6). On the other hand heterolytic decomposition results in the
formation of cycloperoxyl radical ROO• (7). Additional source of
cycloalcohol is the reaction of cycloalkoxyl radical RO•, produced
in (6), with cycloalkane molecule (8). Cycloalcohol arising in the
previous steps may also be converted to cycloketone in the
presence of the catalyst (9). Perhaps step (9) is the main reason
for the high yield of cycloketone and only limited formation of
cycloalcohol observed in our catalytic system. In contrast to our
catalytic studies on cycloalkanes it is worth to recall that for
oxidation of linear and branched alkanes in the presence of
cytochrome P-450 the main reaction product is always
the most of the studied systems except for CoTPP-O2 and
CoT(p-Cl)PP-O2. This indicates that the molecular oxygen
molecule pulls out Co ion from the macrocycle plane in most of
the cases.
cycloalcohol[11e]
.
DFT calculations
DFT modelling was employed to elucidate the dependency
between catalytic properties of the Co porphyrins observed in
the cycloalkanes oxidation with molecular oxygen and the spatial
geometry of the studied structures. Table 3 gathers the results of
the DFT calculations for the series of Co porphyrins. Doublet
ground state of all the studied cobalt porphyrins with or without
bound molecular oxygen molecule is found in agreement with
RN-Co = 1.981Å
B
+ O2
RN-CoO2 = 1.964Å
earlier theoretical studies of similar complexes[12a,b]
molecular oxygen is always bound in an end-on mode.
.
The
ΔE=-49.3 kJ
Table 3. Selected results of the DFT calculations.
[b]
[d]
[e]
Catalyst
N-Co[a]
Co-O2
q(O2)[c]
EO2
N-CoO2
[Ǻ]
[Ǻ]
[kJ/mol]
[Ǻ]
C
CoTMP
CoTTP
CoTPP
CoT(p-Cl)PP
CoTDCPP
CoTPFPP
CoTDCPβN6P
CoTDCPβCl8P
CoTPFPβBr8P
1.980
1.982
1.982
1.981
1.982
1.981
1.934
1.935
1.926
1.867
1.870
1.870
1.872
1.878
1.884
1.917
1.897
1.903
-0.22
-0.19
-0.16
-0.15
-0.11
-0.15
-0.08
-0.10
-0.14
-47.1
-47.0
-50.3
-49.3
-40.4
-38.5
-29.3
-44.0
-33.1
1.991
1.993
1.967
1.964
1.992
1.992
1.954
1.956
1.947
+ O2
ΔE=-40.4 kJ
RN-CoO2 = 1.992Å
RN-Co = 1.982Å
[a] average N-Co bond length. [b] atom bond length between Co and O2. [c]
total ESP charge on O2. [d] binding energy of O2. [e] average N-Co bond
length after binding O2.
Examination of the structures geometry for all the studied
systems reveals the differences among the three generations of
Co porphyrins (see Supplementary Information for Co porphyrin
structures). Co porphyrins of I generation show planar structure
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All the synthesized cobalt porphyrin catalysts are catalytically
active in the oxidation of cycloalkanes with molecular oxygen.
Their catalytic performance depends on the nature and number
of substituents introduced into meso-aryl and β-pyrrolic positions
of porphyrin macrocycle. 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 reaction products. It is noteworthy that II generation
of cobalt porphyrins is more active than III generation. Relatively
lower catalytic activity of III generation cobalt porphyrins can be
assigned to the saddle-shaped deformations of the porphyrin
macrocycle. DFT modelling of Co porphyrins and their
interactions with molecular oxygen provided motives for the
observed effects. The yield to oxygenates grows for Co
porphyrins substituted with electron-donating groups on
porphyrin ring with the decrease of oxidation potential Eox. In the
presence of electron-withdrawing substituents the increase of
the Eox is accompanied by the rise of the yield to oxygenates.
Basing on the literature data and our investigations the oxidation
mechanism of cycloalkanes by cobalt porphyrins is proposed.
D
RN-CoO2 = 1.956Å
+ O2
ΔE=-44.0 kJ
RN-Co = 1.935Å
Figure 5. DFT calculations for unsubstituted CoTPP (A), CoT(p-Cl)PP with 4Cl
substituents (B), CoTDCPP with 8 Cl substituents (C) and CoTDCPβCl8P with
16 Cl substituents (D) and upon binding O2.
The distortions from the planarity found for the III generation Co
porphyrins seems to be large enough to constitute steric
hindrances in the process of O2 binding to cobalt ion. This can
be seen confronting the Co-O2 bond lengths which are longer for
the III generation in comparison to the I and II generation. Co-O2
bond lengths of Co porphyrins with electron-donating
substituents are shortened in comparison with CoTPP catalyst
treated here as a reference. However, in the presence of
electron-withdrawing groups Co-O2 bond lengths become longer,
comparing with CoTPP.
Experimental Section
General
All the reagents used in this work were purchased and used directly
without further purification. The I generation porphyrins TPP, TTP and
T(p-Cl)PP were synthesized by condensation of pyrrole and appropriate
aldehyde according to the Rothemund and Menotti procedure[2d,e]. Two
steps Lindsay procedure[3a] involving acid-catalyzed condensation of
pyrrole with the appropriate aldehyde into porphyrinogen, followed by its
oxidation with quinone to porphyrin ligand was applied to obtain II
generation porphyrin ligands TDCPP and TPFPP. Porphyrin ligand TMP
was acquired using modified Lindsay method[13a]. The preparation of III
generation porphyrin TDCPβCl8P was accomplished by the chlorination
In general, this effect is reflected by the molecular oxygen
binding energies. Figure 6 exhibits a linear correlation between
Co-O2 bond length and the molecular oxygen binding energies
EO2, similar as shown in[12a]
.
-30
of ZnTDCPP with Cl2 followed by the in situ removal of Zn with HCl[13b]
.
Ligand TPFPβBr8P was prepared by halogenation of NiTPFPP with Br2
and the in situ removal of Ni with HCl[13b]. Porphyrin ligand TDCPβN6P
was synthesized using Mansuy-Battioni method[13c] consisting in the
reaction of porphyrin ligand TDCPP with fuming acid HNO3. All the
synthesized porphyrin ligands were metalated by condensation of
-35
-40
-45
-50
porphyrin with cobalt acetate according to the Adler procedure[13d]
.
UV-Vis measurements were carried out on a Perkin Elmer Lambda 35
double beam spectrophotometer, using quartz cells of 1 cm optical path.
Cyclic voltammograms were recorded in a three-electrode cell using Co
porphyrin modified working electrode, platinum coil as auxiliary electrode
and Ag/AgCl as reference electrode. Co porphyrin solid sample was
homogenized with graphite powder and Nujol to form thick paste which
was then packed into the electrode well. Cobalt porphyrins were
analyzed in acetic buffer of pH=5 as electrolyte, with the scan rate of 25
mV/s. Prior to the measurements the solution was de-aerated with argon
to keep air-free atmosphere over the solution during the measurement.
1,86 1,87 1,88 1,89 1,90 1,91 1,92
Co-O2, [Å]
Figure 6. Linear correlation (R2 = 0.78) between Co-O2 bond length and
dioxygen binding energies EO2
.
Catalytic reaction
The catalytic oxidation of cyclooctane (cyclopentane or cyclohexane)
which simultaneously played the role of the reaction medium was
performed in 1L stainless steel batch reactor at the optimum temperature
of 120oC, under the air pressure of 10 atm and with the molar ratio of
cycloalkane to oxygen set at 6.5. In the typical experiment, cobalt
porphyrin having the concentration of 3.3  10-4 M was introduced into the
60 mL of substrate and the reaction was started when the required
reaction conditions were attained. After 6 h of reaction time, the oxidation
was stopped by immersing the hot reactor in a cold water bath and the
reaction mixture was analyzed by means of Agilent Technologies 6890 N
gas chromatograph equipped with a FID detector and Innowax (30 m)
capillary column. The percentage yield of oxygenates (calculated as the
amount of desired product obtained divided by the theoretical yield
predicted by the reaction stoichiometry) was determined in the presence
Finally, the negative charge accumulated on molecular oxygen
ligand (q(O2)) indicates that it is bound in the O2 form, thus
-
implying its nucleophilic character. The negative charge is
located mostly on the outer oxygen atom, exposing it towards
the incoming organic substrate. The O2 ligand is more
nucleophilic in the systems with electron donating-substituents
(CoTTP-O2, CoTMP-O2). The presence of electron-withdrawing
substituents leads to the smaller negative charge on O2 as
compared to the reference CoTPP-O2 complex. Charges on O2
confirm donor or acceptor character of substituents increasing or
reducing their values referring to the unsubstituted CoTPP.
Conclusions
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10.1002/cssc.201802198
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of chlorobenzene as internal standard. The conversion and yield of
products were calculated on the basis of oxygen quantity in batch
reactor.
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This research was supported in part by PL-Grid Infrastructure.
Keywords: cobalt porphyrin • cycloalkanes oxidation • DTF
calculations • molecular oxygen • sustainability
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Reversed catalytic activity: Three
generations of cobalt porphyrins with
K. Pamin*, E. Tabor, S. Górecka, W.W.
Kubiak , D. Rutkowska-Zbik, J.
Połtowicz*
electron-withdrawing
or
electron-
donating substituents were applied as
catalysts in oxidation of cycloalkanes
with molecular oxygen as air under
mild conditions. It was shown, for the
first time, that the II generation of
cobalt porphyrins in cycloalkanes
oxidation show higher activity in
comparison to cobalt porphyrins of III
generation.
Page No. – Page No.
I and II generation
III generation
Three generations of cobalt
porphyrins as catalysts in oxidation
of cycloalkanes
O
O
Co porphyrin
air,120oC,10 atm
+
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