Synthesis, characterization and catalytic activity under homogeneous conditions of ethylene glycol substituted porphyrin manganese(III) complexes

Article abstract of DOI:10.1016/j.ica.2016.05.038

The catalytic efficiency and selectivity of a series of manganese(III) complexes of meso-tetrakis(pentafluorophenyl)porphyrin H₂(TPFPP) derivatives bearing one to four ethylene glycol moieties are compared in the oxidation of cis-cyclooctene an

Full text of DOI:10.1016/j.ica.2016.05.038

Accepted Manuscript
Synthesis, characterization and catalytic activity under homogeneous conditions
of ethylene glycol substituted porphyrin manganese (III) complexes
Kelly A.D.F. Castro, Fernando H.C. de Lima, Mário M.Q. Simões, M.Graça
P.M.S. Neves, Filipe A. Almeida Paz, Ricardo F. Mendes, Shirley Nakagaki,
José A.S. Cavaleiro
PII:
S0020-1693(16)30280-8
http://dx.doi.org/10.1016/j.ica.2016.05.038
ICA 17074
DOI:
Reference:
Inorganica Chimica Acta
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Revised Date:
Accepted Date:
28 February 2016
16 May 2016
17 May 2016
Please cite this article as: K.A.D. Castro, F.H.C. de Lima, M.M.Q. Simões, M.P.M. Neves, F.A. Almeida Paz, R.F.
Mendes, S. Nakagaki, J.A.S. Cavaleiro, Synthesis, characterization and catalytic activity under homogeneous
conditions of ethylene glycol substituted porphyrin manganese (III) complexes, Inorganica Chimica Acta (2016),
doi: http://dx.doi.org/10.1016/j.ica.2016.05.038
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Synthesis, characterization and catalytic activity under homogeneous
conditions of ethylene glycol substituted porphyrin manganese (III) complexes
Kelly A. D. F. Castroa,b, Fernando H. C. de Limaa, Mário M. Q. Simõesb,*, M. Graça P.
M. S. Nevesb, Filipe A. Almeida Pazc, Ricardo F. Mendesc, Shirley Nakagakia,*, José A.
S. Cavaleirob
aLaboratório de Química Bioinorgânica e Catálise, Universidade Federal do Paraná
(UFPR), CP 19081, CEP 81531-990, Curitiba, Paraná, Brasil
bDepartment of Chemistry and QOPNA, University of Aveiro, 3810-193 Aveiro,
Portugal
cDepartment of Chemistry and CICECO – Aveiro Institute of Materials, University of
Aveiro, 3810-193 Aveiro, Portugal
*Corresponding authors
e-mail address: msimoes@ua.pt (M. M. Q. Simões)
e-mail address: shirleyn@ufpr.br (S. Nakagaki)
1

ABSTRACT
The catalytic efficiency and selectivity of a series of manganese(III) complexes of
meso-tetrakis(pentafluorophenyl)porphyrin H2(TPFPP) derivatives bearing one to four
ethylene glycol moieties are compared in the oxidation of cis-cyclooctene and
cyclohexane in the presence of H2O2 at ambient temperature. The results show that
the catalysts’ performance depends on the number of ethylene glycol moieties
present and on the cocatalyst used; for both substrates, the most efficient catalyst in
the presence of H2O2 is the Mn(III) complex bearing three ethylene glycol moieties
(MnP3). Moreover, in the oxidation of cyclohexane in the presence of iodosylbenzene
(PhIO) as oxidant, all the substituted Mn(III) complexes are more efficient than the
parent non-substituted Mn(III) porphyrin complex. In addition, during the preparation
of this series of derivatives, the tetrasubstituted free-base porphyrin (P4) was further
isolated and studied in the solid state, by using single-crystal X-ray diffraction studies.
Keywords: porphyrins, ethylene glycol, manganese, oxidation, hydrogen peroxide.
2

1. Introduction
The use of synthetic macrocycles, such as porphyrins, as catalysts in oxidation
reactions has attracted a great interest in bioinorganic chemistry [1-8], especially due
to their efficiency in modelling the catalytic oxidation ability of cytochrome P-450 and
lignin peroxidases [4,9]. These two heme proteins play special roles in the
biosynthesis and degradation of important endogenous compounds, such as steroid
hormones, fatty acid derivatives or vitamins, biodegradation of the plant cell wall
constituent lignin, among others [3,10].
In the 1970s, Groves and co-workers [11] showed, for the first time, that synthetic
metalloporphyrins can mimic the catalytic activity observed in biological systems, by
using the iron(III) complex of meso-tetraphenylporphyrin (Fe(TPP)Cl) and
iodosylbenzene (PhIO) in the epoxidation of alkenes and in the hydroxylation of
alkanes. However, these early stage catalysts, the so-called first-generation porphyrin
catalysts, suffered from rapid deactivation caused by oxidative degradation of the
macrocycle [12]. Further studies resulted in the development of more stable and, in
general, more efficient catalysts, by introducing electronegative and/or bulky auxiliaries
such halogen [13-15], nitro [16] or sulfonato [4] substituents at the meso and/or β-
pyrrolic positions (the so-called second and third generation porphyrin catalysts)
[17-19]. In fact, several studies showed how the structural modification of the
macrocycle core can contribute for a better catalytic performance under homogeneous
conditions [5-8,20]. This fact is usually associated with the stereo-electronic effect
exerted by the substituents, which are able to increase the lifetime and the reactivity of
the catalytic active species in solution. Besides, the formation of dimeric species and
the oxidative self-destruction responsible for the inactivation of the catalyst are often
inhibited [17]. In this context, the use of the metallocomplexes of the
meso-tetrakis(pentafluorophenyl)porphyrin H2(TPFPP) has proven to be a surprisingly
versatile alternative presenting many possibilities of structural modification by
nucleophilic substitution of its fluorine atoms [21-24].
Manganese(III) and iron(III) porphyrins are usually considered as the most
important representative class of metalloporphyrins with catalytic activity in the
epoxidation of alkenes [5-8,25]. For both metals, the generally accepted catalytically
active species for the epoxidation reactions with different substrates and oxygen atom
donors is a metal-oxo species. Although iron(III) porphyrins can be highly active in the
epoxidation of alkenes, their efficiency can, sometimes, be limited by the formation of
3

by-products resulting, for example, from allylic oxidation reactions [26]. This fact is less
limitative concerning the manganese(III) complexes of porphyrins. Additionally, some
studies by Mansuy and other groups [20,27-29] showed that Mn-porphyrins in the
presence of adequate cocatalysts are effective catalysts to activate hydrogen peroxide
for the epoxidation of alkenes [30-32]. This is considered a green and environmentally
friendly oxidant because the sole by-product is water [33].
We have recently studied the efficiency of the iron(III) and manganese(III)
complexes of meso-tetrakis(pentafluorophenyl)porphyrin H2(TPFPP) derivatives
bearing one and four ethylene glycol moieties as catalysts in the oxidation of alkenes
and alkanes with PhIO as oxidant, confirming the superior efficiency of the
manganese(III) complexes [20]. This fact, and our interest in developing oxidative
catalytic processes under environmentally benign conditions [33], prompted us to
evaluate the efficiency of the manganese(III) complexes of H2(TPFPP) derivatives
bearing one to four ethylene glycol moieties (MnP1-MnP4, Figure 1) in the oxidation
of cis-cyclooctene and cyclohexane with H2O2 as oxidant. The efficiency of the new
Mn(III)complexes with two and three aliphatic chains was also evaluated in the
presence of PhIO using the same model substrates. Moreover, the free-base
porphyrin P4 was isolated in the solid state as a crystalline material and single-crystal
X-ray diffraction studies were carried out.
2. Experimental
Chemicals were purchased from Aldrich, Sigma, or Merck, and were of
analytical grade.
2.1. Synthesis of the manganese porphyrins
The five ethylene glycol manganese(III) porphyrin derivatives MnP1-MnP4
(Figure 1) were prepared from the corresponding free-bases P1-P4 using the
adequate manganese(II) acetate salt according to the conventional methodology
reported by Kobayashi [34]. The free-base porphyrins P1, P2a, P2b, P3 (see below
for an improvement procedure of the reactions conditions) and P4 were prepared as
previously described [20]. Purple crystals of porphyrin P4 were obtained by
dissolving the porphyrin in a small amount of dimethyl sulfoxide and leaving the
resulting mixture at ambient temperature over a period of two weeks.
4

Figure 1. Schematic representation of the manganese(III) porphyrins prepared.
i) Improved synthesis of the free-base porphyrin P3: Ethylene glycol (8.6 mmol)
was dissolved in 15 mL of dry THF, and NaH (0.95 mmol) was then added. The
reaction mixture was kept under magnetic stirring and nitrogen atmosphere for 30
min. A solution of H2(TPFPP) in THF (15.4 mmol/L) was added and the reaction
mixture was heated at reflux temperature until the total consumption of the starting
porphyrin (24 h, monitored by TLC). After cooling to ambient temperature, the mixture
was diluted with dichloromethane and washed 3 times with 10 mL of water. The
solvent was evaporated under reduced pressure and the crude solid was purified by
preparative TLC using dichloromethane:methanol (9:1 v/v) as the eluent. The fourth
1
fraction was identified as the porphyrin P3 (47% yield), mp > 300 °C. H NMR: δH
ppm (CD3OD); 9.10 (broad s, 8H, H-β); 4.11 (t, J = 4.4, 6H, CH2) and 4.70 (t, J = 4.4,
6H, CH2). 19F NMR: δF ppm (CD3OD) -188.63 (dt, J = 7.3 and 21.3, 2F, F-meta), -
182.84 (dd, J = 7.3 and 21.3, 6F, F-meta), -179.13 (t, J = 21.3, 1F, F-para), -165.75
(dd, J = 7.3 and 21.3, 6F, F-ortho), -163.73 (dd, J = 7.4 and 21.3, 2F, F-ortho). UV–
VIS (CH2Cl2) λmax nm (logε): 412 (5.30), 510 (4.19), 582 (3.65) 654 (2.69). HRMS
(FAB+): m/z calcd for C50H25F17N4O6: 1101.1536 (M+H)+; found: 1101.1559.
5

ii) Synthesis of the manganese porphyrins (MnP1-MnP4): The metalation of the
free-base porphyrins P1-P4 with [Mn(CH3COO)2] was performed by employing a
modification of the conventional methodology reported by Kobayashi, as described
previously by our research groups for the ethylene glycol porphyrin derivatives MnP1
and MnP4. MnP2a (C48H18F16MnN4O4): (95% yield); UV-VIS (CH3OH) λmax, nm
(logε): 457 (5.27), 497 (3.48) and 551 (3.83). HRMS-ESI(+): m/z calcd for
C48H18F18MnN4O4 1111.0421 (M+.); found: 1111.0419. MnP2b (C48H18F18MnN4O4):
(98% yield); UV-VIS (CH3OH), λmax, nm (logε): 457 (4.91), 497 (3.60) and 552 (4.04).
HRMS-ESI(+): m/z calcd for C48H18F18MnN4O4 1111.0421 (M+.); found: 1111.0425.
MnP3 (C50H23MnF17N4O6): (97% yield); UV-VIS (CH3OH) λmax, nm (logε): 457 (4.98),
497 (3.57) and 552 (3.90). HRMS-ESI(+): m/z calcd for C50H23MnF17N4O6 1153.0727
(M+.); found: 1153.0702 (Supporting information, Figures S1-S3).
2.2. Catalytic oxidation reactions
The oxidation of cis-cyclooctene (previously purified in a neutral alumina
column) or cyclohexane was carried out by using hydrogen peroxide as oxidant. The
general procedure was as follows: in a 1.5 mL reaction flask, the catalyst (about 0.5
mg of MnP) and the cocatalyst (about 7.0 mg of ammonium acetate or 10 mg of
imidazole) were suspended in acetonitrile (500 µL), followed by the substrate (0.27
mmol) and the solution of 30% (w/w) H2O2 diluted in acetonitrile (1:10). The mixture
was maintained under magnetic stirring at ambient temperature in the absence of
light. The oxidant (a solution of 30% (w/w) H2O2 diluted in acetonitrile (1:10)) was
progressively added in aliquots corresponding to a half-substrate amount at intervals
of 15 min. The catalyst/oxidant/substrate (cat/ox/sub) molar ratio was 1:1200:600 and
the reaction time was 1 h. After the reaction time, the supernatant was transferred to
a volumetric flask and the volume was completed with acetonitrile. The solutions
containing the reaction products were analysed by GC-FID and the results are
expressed in terms of conversion of the substrate, using the internal standard
methodology (octan-1-ol was employed as the internal standard). The oxidation
reactions using PhIO as oxidant were performed as described previously by us [20].
The cat/ox/sub molar ratio was 1:50:5000 and the reaction time was also 1 h.
6

2.3. Evaluation of the stability of the MnP3 and MnP4 catalysts
The stability of MnP3 and MnP4 in the oxidation reaction conditions was
monitored by UV-VIS spectroscopy by considering their characteristic absorption
Soret band at about 460 nm. In these experiments, the catalyst stability was
evaluated in the presence of solvent (acetonitrile), substrate (cis-cyclooctene) and
oxidant (H2O2). Each reaction was followed by UV-VIS at different periods of time.
2.4. Characterization techniques
1
The H and 19F NMR spectra were recorded on a Bruker AVANCE 300 at
300.13 and 282.38 MHz, respectively, using CDCl3 as solvent and TMS as the
internal reference. Mass spectrometry analyses were carried out on a Mass
spectrometer – Micromass Q-TOF-2, and HRMS were recorded on VG AutoSpec Q
and M mass spectrometers. FTIR spectra were recorded on a Biorad 3500 GX
spectrophotometer in the 400-4000 cm-1 spectral range using KBr (Aldrich, 99%)
pellets. In a typical experiment, KBr was crushed with a small amount of the solid
samples, and the spectra were collected with a resolution of 4 cm-1 and accumulation
of 32 scans. Qualitative and quantitative electronic spectra of the solution samples
were recorded on a Cary-Varian spectrophotometer, in the 200-800 nm range using a
quartz cuvette with 1 cm optical path. Electron paramagnetic resonance (EPR)
spectra of the solid materials were recorded on an EPR Bruker EMX micro X
spectrometer operating in the X band (approximately 9.5 GHz) at 77 K using liquid
N2. The catalytic oxidation reaction products were identified and quantified using gas
chromatographs Agilent 6850 and Bruker Scion 436 (FID detector) equipped with
DB-WAX type capillary columns (30 m × 0.25 mm i.d. × 0.25 µm film thickness).
Quantitative analyses were performed by using an internal standard.
2.5. Single-Crystal X-ray Diffraction Studies of P4·2(DMSO)
Single crystals of the tetrasubstituted free base porphyrin P4·2(DMSO) were
manually harvested from the crystallization vial and immersed in highly viscous
FOMBLIN Y perfluoropolyether vacuum oil (LVAC 140/13, Sigma-Aldrich) to avoid
degradation caused by the evaporation of the solvent [35]. Crystals were mounted on
Hampton Research CryoLoops with the help of a Stemi 2000 stereomicroscope
equipped with Carl Zeiss lenses. X-ray diffraction data were collected at 150(2) K on
a Bruker D8 QUEST equipped with Mo Kα sealed tube (λ = 0.71073 Å), a multilayer
7

TRIUMPH X-ray mirror, a PHOTON 100 CMOS detector, controlled by the APEX2
software package [36], and an Oxford Instruments Cryostrem 700+ Series low
temperature device.
Diffraction images were processed using the software package SAINT+ [37],
and data were corrected for absorption by the multiscan semi-empirical method
implemented in SADABS [38]. The structure was solved using the algorithm
implemented in SHELXT-2014 [39], which allowed the immediate location of almost
all of the heaviest atoms composing the asymmetric unit. The remaining missing and
misplaced non-hydrogen atoms were located from difference Fourier maps calculated
from successive full-matrix least-squares refinement cycles on F2 using the latest
SHELXL from the 2014 release [40,41]. All structural refinements were performed
using the graphical interface ShelXle [42].
Hydrogen atoms bound to carbon were placed at their idealized positions using
appropriate HFIX instructions in SHELXL: 43 (aromatic carbon atoms), 23 (for the –
CH2– moieties of the substituent groups) or 137 (for the terminal methyl groups of the
DMSO solvent of crystallization). These hydrogen atoms were included in
subsequent refinement cycles with isotropic thermal displacements parameters (Uiso)
fixed at 1.2 (for the two former family of hydrogen atoms) or 1.5×Ueq (solely for those
associated with the methyl groups) of the parent carbon atoms. The hydrogen atoms
associated with the terminal hydroxyl groups were placed in geometrical positions
using the HFIX 147 instruction in SHELXL in order to maximize the hydrogen bonding
interactions in which these groups are involved. The isotropic thermal displacement
parameter (Uiso) of these hydrogen atoms were fixed at 1.5×Ueq of the parent oxygen
atoms.
Crystal data for P4·2(DMSO): C56H42F16N4O10S2, M = 1299.05, monoclinic,
space group P21/c, Z = 2, a = 15.148(5) Å, b = 9.699(3) Å, c = 18.233(5) Å,
β = 97.067(4)º, V = 2658.3(13) Å3, µ(Mo-Kα) = 0.223 mm-1, Dc = 1.623 g cm-3, red
plates with crystal size of 0.19×0.16×0.03 mm3. Of a total of 34859 reflections
collected, 4843 were independent (Rint = 0.0683). Final R1 = 0.0634 [I > 2σ(I)] and
wR2 = 0.1960 (all data). Data completeness to theta = 25.24°, 99.6%.
The last difference Fourier map synthesis showed the highest peak (0.617 eÅ-3)
and the deepest hole (-0.586 eÅ-3) located at 1.67 and 0.69 Å from H26B and S1,
8

respectively. Structural drawings have been created using the software package
Crystal Impact Diamond [43].
Crystallographic data (including structure factors) for the crystal structure of
compound P4·2(DMSO) have been deposited with the Cambridge Crystallographic
Data Centre as supplementary publication No. CCDC-1438587. Copies of the data
can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge
CB2 2EZ, U.K. FAX: (+44) 1223 336033. E-mail: deposit@ccdc.cam.ac.uk.
3. Results and Discussion
3.1. Synthesis and characterization of the manganese(III) porphyrins
The synthetic strategy to obtain the manganese complexes MnP1-MnP4
bearing one to four ethylene glycol units is outlined in Figure 1 and involved the
previous preparation of the corresponding free-bases P1-P4, followed by metalation
with [Mn(AcO)2]. The free-base derivatives were obtained from the reaction of
meso-tetrakis(pentafluorophenyl)porphyrin with ethylene glycol in the presence of
NaH following the experimental procedure previously described by our research
groups [20]. Depending on the reaction conditions employed, the formation of a
derivative can be preferentially favoured. In this study we were able to slightly
improve the yield of P3 from 40% to 47% just by acting on the reaction time. The
structures of all the porphyrins P1-P4 are in accordance with the previously analytical
19
data (1H and F NMR, HRMS, UV-VIS and FTIR). Additionally, P4 was successfully
crystallized from dimethyl sulfoxide that allowed the isolation of crystals, whose
structure was unveiled from single-crystal X-ray diffraction (see below). The
metalation process of the free-base porphyrins P1, P2a, P2b, P3 and P4 with
[Mn(AcO)2] was followed by recording the electronic spectra at different reaction
times. The structure of the metalloporphyrins was confirmed by UV-VIS, FTIR
techniques and ESI-MS, with the collected analytical data of MnP1 and MnP4 being
in accordance with that already available in the literature [20].
The UV-VIS spectra of the new complexes MnP2a, MnP2b and MnP3 reveal
the characteristic Soret band at 457 nm, showing a bathochromic shift in comparison
to the Soret band of the free-base porphyrins (408 nm) and two Q-bands at 497 and
551 nm (see Supporting information, Figure S4 for MnP2a), resulting from the
alteration in the micro symmetry of the porphyrin macrocycle due to the presence of
the Mn(III) ion in the porphyrin core. Bands at 365 and 390 nm are assigned to an
9

electronic transition ligand metal charge transfer (LMCT), a1u(π),a2u(π)ꢀeg(dπ). The
weak bands at 676 and 767 nm were assigned by Boucher [44] to an electronic
transition LMCT, a1u, a2u ꢀ eg (dπ), typical of manganese porphyrins [44, 45].
The manganese(III) porphyrins exhibited the typical FTIR bands of porphyrins
(Figures not shown). The bands in the region of 3000 cm-1, ascribed to symmetric
and asymmetric NH stretching, and in the region of 1600 cm-1, referring to another
δNH vibrational mode [46] have disappeared. The bands at 1404 and 1589 cm-1 in
the complex are assigned to the acetate groups, attributed to symmetric and
asymmetric stretching of the C=O bond. The difference in the positioning of these
bands (∆ = 185 cm-1) suggests, according to Nakamoto [46], that acetate is present
as the counter-ion.
The new metalloporphyrins were also characterized by EPR in order to
identify/confirm the oxidation state of the metal present, as well as its symmetry and
possible distortions. The perpendicular microwave polarization X-band EPR spectra
of MnP2a, MnP2b and MnP3 recorded at ambient temperature and 77 K are very
similar and no signals were detected (Figures not shown). These results are in
accordance with the presence of Mn(III) in the porphyrin core [47].
3.2. Single-Crystal X-ray Diffraction Studies of P4·2(DMSO)
Porphyrin P4 was isolated in the solid state as a crystalline material co-
crystallising with two molecules of DMSO per molecular unit. Single-crystal X-ray
diffraction studies show that the molecular unit adopts in P4·2(DMSO) a
centrosymmetric geometry as depicted in Figure 2. Remarkably, contrary to that
usually observed for other porphyrinic derivatives with pendant alkylic substituent
chains, the two crystallographically independent ethylene glycol residues do not
show any structural disorder (Figure 2). We believe that this structural feature arises
mainly from the presence of the strong [dD···A = 2.747(5) Å] and somewhat directional
[<(DHA) of 145º] O–H···O intermolecular hydrogen bonding involving the two
independent terminal hydroxyl groups. This sole supramolecular interaction holds
together neighbouring individual P4 porphyrins as shown in Figure S5 (see
Supporting information).
10

Figure 2. Schematic representation of the centrosymmetric porphyrinic molecular unit
present in compound P4·2(DMSO). Non-hydrogen atoms are represented as thermal
ellipsoids drawn at the 50% probability level. Hydrogen atoms are represented as small
spheres with arbitrary radii. Please note: the asymmetric unit of compound P4·2(DMSO) is
composed of half of the porphyrin molecule and one DMSO solvent molecule of
crystallization.
The close packing in the crystal structure of P4·2(DMSO) is mainly driven by
the need to closely pack individual porphyrin molecules while fulfilling the
aforementioned, structure directing O–H···O intermolecular hydrogen bonds. It is,
however, important to emphasize that a number of weak C–H···O and C–H···F also
exist in the crystal structure globally contributing to structural robustness. The close
packing of individual P4 molecules leads to the presence of structural voids that are
filled by the DMSO solvent molecules as depicted in Figure 3.
11

Figure 3. Schematic representation of the crystal packing of compound P4·2(DMSO) viewed
in perspective along the [010] direction of the unit cell. Porphyrin molecules are represented
in ball-and-stick mode while the DMSO solvent molecules of crystallization in space filling
mode.
3.3. Investigation of the catalytic activity
We have recently reported that the porphyrin complexes MnP1 and MnP4 are
able, in the presence of iodosylbenzene and under homogeneous conditions, to very
efficiently epoxidize cis-cyclooctene (88% vs 96%) and also to hydroxylate
cyclohexane (52% vs 64%) [20]. In the present study we intend to verify if that
efficiency is maintained in the Mn complexes of H2(TPFPP) bearing two and three
ethylene glycol moieties (MnP2a, MnP2b and MnP3). Additionally, it is important to
verify if the distribution of the aliphatic chains in the disubstituted complexes affects
(or not) the catalytic efficiency. As a preliminary experiment, in order to evaluate if the
new complexes are catalytic active and stable under the reaction conditions,
iodosylbenzene was used as oxidant and cis-cyclooctene and cyclohexane were
studied as model compounds.
Figure 4 summarizes the catalytic results obtained in the epoxidation of cis-
cyclooctene in the presence of MnP2a, MnP2b and MnP3 together with the previous
12

results already reported for MnP1 and MnP4 [20]. Figure 4 also compiles the catalytic
results obtained in the hydroxylation of cyclohexane with this series of
manganese(III) porphyrins. The catalytic tests confirmed the efficiency of the glycol
substituted manganese(III) porphyrins in the oxidation of cis-cyclooctene. The
epoxide yields decreased in the following order: MnP3 > MnP4 > MnP2b > MnP1 >
MnP2a. The epoxide yield obtained for MnP2a, MnP2b and MnP1 was very similar,
only MnP2a yield was slightly lower than the other two. These results suggest that
the substitution of two of the electronegative p-fluorine atoms by ethylene glycol
substituents did not significantly affect the stability of the porphyrin core. The better
result observed for MnP3 (98%) and MnP4 (96%) can be justified by the stability of
the catalytic species found for this two complexes, as expected for this family of
second generation porphyrins [20].
Cyclooctene oxide
Cyclohexanol
Cyclohexanone
98
96
100
90
80
70
60
50
40
30
20
10
0
89
88
85
64
59
52
45
35
10
MnP1
MnP2a
MnP2b
MnP3
MnP4
Figure 4. Cyclooctene and cyclohexane oxidation by PhIO catalysed by MnP1-MnP4 under
homogeneous conditions. Yields were calculated based on the amount of PhIO added.
Results represent reactions performed in duplicate or triplicate.
Cyclohexane is also a very useful model substrate to study catalytic systems
involving MnP/PhIO system. Besides the catalytic efficiency, some information about
selectivity can also be obtained by using this substrate, because the alcohol and the
ketone, the main products frequently observed, can be formed. If compared to cis-
cyclooctene, cyclohexane is obviously harder to oxidize. In fact, manganese(III)
porphyrins are known to be efficient and selective catalysts for cyclohexane oxidation
under homogeneous conditions with high selectivity to the alcohol. The proposed
13

mechanism for the oxidation of cyclohexane involves the abstraction of a hydrogen
atom from the substrate by a metal-oxo catalytic species, which generates an alkyl
radical and a hydroxo-metalloporphyrin in the solvent cage (Scheme 1). Oxygen
rebound preferably occurs within the cage, forming the alcohol [48-50].
Figure 4 shows that all the MnP compounds studied in this manuscript were
selective to the alcohol, its yields decreasing in the following order: MnP4 > MnP3 >
MnP1 > MnP2b > MnP2a. The tetra substitution of the p-fluorine atoms improved the
efficiency of MnP4 if compared with the other complexes, affording alcohol with
higher selectivity (64%). For MnP3 the selectivity for the alcohol is close to that
observed for MnP4 (59% vs 64%, respectively) suggesting, in a resembling way
when using cis-cyclooctene as substrate, similar stability of the catalytic species.
In general, the here described modified manganese(III) porphyrins afforded
higher product yields than its parent Mn(TPFPP)Cl (34% alcohol and 6% ketone)
under similar conditions [20]. The inclusion of the –OR groups at the p-positions of
the meso-aryl groups of the macrocycle increases the catalytic efficiency (MnP4 >
MnP3 > MnP1 > MnP2b), except for MnP2a that exhibits lower selectivity (78%) for
cyclohexanol relatively to Mn(TPFPP)Cl (85% selectivity). Such behaviour could also
be attributed to the stability of the catalytic active species.
Scheme 1. Proposed mechanism for cyclohexane hydroxylation catalysed by Mn(III)
porphyrins (MnP).
For MnP2a the formation of cyclohexanone was also observed (35%
cyclohexanol and 10% cyclohexanone) because the produced alcohol probably
14

underwent catalytic overoxidation to ketone or, at the stage of oxygen rebound
mechanism, there may be a leak of the alkyl radical causing it to react with other
species in solution, ultimately resulting in several other products [50]. The two MnP2
complexes gave a total yield of 45%, depending on the position of the ethylene glycol
substituent. The opposite or trans-isomer MnP2b was, however, more selective for
the alcohol than the adjacent or cis-isomer MnP2a. This interesting behaviour may
suggest that it is possible to modulate the selectivity just by modifying the macrocycle
structure.
Considering the stability and the efficiency of this series of catalysts in the
epoxidation of cis-cyclooctene with PhIO, the following studies were focused on its
catalytic behaviour when hydrogen peroxide was employed as oxidant. The catalytic
activity using hydrogen peroxide for the compounds reported in this manuscript is
unprecedented. It is well-known that when hydrogen peroxide is used as oxidant in
the presence of Mn porphyrins the presence of a cocatalyst is required [13,51]. In our
study, we have decided to perform the oxidation reactions in the presence of two
well-known cocatalysts, ammonium acetate and imidazole. The function of the
cocatalyst is to promote the formation of the active catalytic species; it acts as an
acid-base catalyst to facilitate the axial heterolytic cleavage of the O-O bond and also
to transfer one oxygen atom to the metal, in order to generate the metal-oxo active
species [12]. Figure 5 summarizes the results obtained in the oxidation of cis-
cyclooctene in the presence of the different catalysts prepared in this work. In these
experiments all the reactions were performed under the same cat/ox/sub molar ratio
conditions (1:1200:600) and the oxidant was progressively added at intervals of 15
min. Reactions were stopped after 1 h (the same reaction time when using PhIO as
oxidant).
The gradual addition of the oxidant is important to minimize the possible
catalase-like activity of the catalyst that promotes hydrogen peroxide dismutation
and, in this way, to maximize the formation of the desired oxidation product, the
epoxide. In reactions catalysed by Mn(III) porphyrins in the presence of an excess of
hydrogen peroxide, the active catalytic species can also catalyse the conversion of
hydrogen peroxide to molecular oxygen and water, ultimately leading to low yields
[4,13,15,20]. As mentioned above, the Mn(III) porphyrins are able to promote
hydrocarbon oxidation with hydrogen peroxide mainly in the presence of a cocatalyst,
in order to promote the heterolytic cleavage of the O-O bond of hydrogen peroxide.
15

The Mn(III) porphyrins frequently used are robust catalysts (from the second and
third generation of porphyrins) and the cocatalysts can be a nitrogen base (e.g.,
imidazole or pyridine), acetic acid, benzoic acid or ammonium acetate. It is well
established that the presence of the cocatalyst favours the formation of a Mn(V)-oxo
porphyrin species that is considered as the active oxidative species.
60
52.0
Mn(TPFPP)Cl
50
MnP1
37.7
MnP2a
40
32.8
MnP2b
30
MnP3
20.5
MnP4
20
13.6
10.6
8.2
10
5.0
3.1
2.7
0.5
0.5
0.4
0.4
0.4
0.2
0.1 0.1
H2O2
0
H2O2 + Imidazole
H2O2 + Ammonium
acetate
Figure 5. Cyclooctene oxidation by H2O2 aqueous solution catalysed by MnP1-MnP4 and
Mn(TPFPP)Cl under homogeneous conditions. Yields were calculated based on the amount
of substrate. Results represent reactions performed in duplicate or triplicate.
In Figure 5, the very low conversions obtained in the absence of a cocatalyst
(0.1% - 0.5%) corroborate its importance when hydrogen peroxide is used as
oxidant. The cocatalyst is involved in several reaction steps, mainly in the removal of
the proton of hydrogen peroxide in order to facilitate the heterolytic cleavage of the
O-O bond of the intermediate; it also acts as axial ligand to Mn, thus favouring the
formation of a leaving group (Scheme 2).
However, for all the ethylene glycol substituted catalysts (MnP1-MnP4), low
conversions were also observed when imidazole was used as cocatalyst. The best
result was obtained for the MnP3 complex, showing 5.0% of conversion towards the
epoxide. A different situation was observed using ammonium acetate as cocatalyst.
All the catalysts were able to promote the epoxidation process, though with different
overall efficiency. The most efficient catalyst was MnP3 (52% yield) followed by
Mn(TPFPP)Cl (37.7% yield) and MnP4 (32.8% yield). The remaining catalysts MnP1,
16

MnP2a, and MnP2b showed epoxide yields ranging from 8.2% to 13.6%. The parent
non-substituted complex Mn(TPFPP)Cl showed to be more efficient than the ethylene
glycol substituted ones in the presence of imidazole (20.5% epoxide yield).
The low conversion obtained with imidazole is probably linked to the different
stages in which the cocatalyst participates. Johnstone and co-workers [52] studied
the effect of a number of cocatalysts, including imidazole, by employing Mn(III)
porphyrins as catalysts and hydrogen peroxide as oxidant. The authors concluded
that one of the problems was centred on the amount of oxidant used, the
destruction/oxidation of imidazole. Furthermore, when acetonitrile is used as solvent,
there is a strong possibility of destruction of imidazole, forming peroxymidic acid. For
the reactions performed using PhIO as oxidant and an excess of substrate, low
conversions were registered if compared to those with hydrogen peroxide. A larger
excess of oxidant is, however, used in the latter conditions. The large excess of
oxidant may, thus, be responsible for the destruction of the catalyst in homogeneous
catalysis, eventually resulting in lower overall efficiency.
L
ROH
H2O2
MnIII
RH
L
L
MnIII
MnV
O
O
H
OH
L
L
MnIII
H2O
O
OH
H
L
Scheme 2. Different roles of the cocatalyst in oxidation reactions using H2O2 and Mn(III)
porphyrins as catalysts.
17

Besides the porphyrin structure resistance that frequently justifies best catalytic
results [15,17,20,53-57], the choice of the solvent is also important. The low catalytic
activity observed for MnP1, MnP2a and MnP2b if compared with MnP3 and MnP4
can be partly explained by the partial solubility of the catalyst in acetonitrile under the
conditions studied, since the inclusion of the ethylene glycol moieties increases the
overall polarity of the catalyst. In addition, Mn(III) porphyrins may have pronounced
catalase-like activity, thereby reducing epoxidation yields, and the chemical structure
of the Mn(III) porphyrins could also affect this catalase activity. Knowing that the
electronegative groups generally increase the lifetime of the catalytically active metal-
oxo species, the absence of these groups should conversely reduce the catalytic
activity, explaining the lower activity observed for the MnP1 and MnP2 complexes, if
compared with the parent Mn(TPFPP)Cl under the same reaction conditions.
However, the epoxide yield decrease observed for the mono and the disubstituted
complexes was not registered for the trisubstituted one, and less drastic than for the
tetrasubstituted Mn(III) complex.
Recently, we have reported that the presence of bulkier substituents on the
porphyrin macrocycle can affect the ring symmetry and, consequently, influence the
formation and stabilization of the catalytic active oxo-species [20,33]. For this
ethylene glycol substituted manganese porphyrin series, the MnP3 catalytic activity
was better than MnP4 (52% vs 32.8% of epoxide yield, respectively), suggesting that
the MnP3 catalytic species is more stable than the manganese tetrasubstituted MnP4
under this experimental conditions. The amount of hydrogen peroxide employed may
be responsible for the destruction of the catalyst in homogeneous catalysis, which
may explain the lower efficiency of MnP4 relatively to MnP3.
In order to explain the results obtained for MnP3 and MnP4, the stability of
these catalysts in homogeneous catalysis was further investigated. Figure 6
illustrates the MnP3 and MnP4 degradation during the oxidation reaction by
monitoring the decrease of the manganese(III) porphyrin characteristic band at 457
nm in the presence of cis-cyclooctene and hydrogen peroxide. Throughout the
reaction, the destruction of both MnP3 and MnP4 can be observed, since a decrease
in the Soret band is registered, but in the first 5 minutes the characteristic Soret band
has the greatest overall reduction for both complexes. Figure 6 shows the
percentage of the remaining compound over time. These results justify the dissimilar
catalytic activity observed for each of the complexes under the reaction conditions
18

investigated. The smallest degradation of MnP3 was expected since it presents the
best catalytic performance in the oxidation of cis-cyclooctene (Figure 5).
Figure 7 summarizes the results obtained in the oxidation of cyclohexane in the
presence of the different catalysts with and without a cocatalyst. The data reveal high
selectivity for cyclohexanol (ol) for most of the catalysts, in line with that reported in
the literature [52].
100
MnP3
MnP4
90
80
70
60
50
40
30
20
10
0
0
5
10 15 20 25 30 35 40 45 50 55 60
Time (min
)
Figure 6. UV-VIS spectrophotometric study of the MnP3 or MnP4 solutions in the
presence of peroxide hydrogen and cis-cyclooctene at different reaction times and
1:1200:600 cat/ox/sub molar ratio. The Soret band intensity was monitored at 457
nm.
When hydrogen peroxide was used as the oxidant, a similar behaviour to that
registered for the oxidation of cis-cyclooctene is observed. In the absence of a
cocatalyst no product formation was observed. For this substrate neither MnP3 nor
MnP4 were able to efficiently produce cyclohexanol using imidazole as cocatalyst.
The MnP1-MnP4 derivatives studied in this manuscript showed catalytic activity
in the oxidation of cyclohexane in the presence of ammonium acetate only. As
observed with PhIO, the inclusion of ethylene glycol groups increases the catalytic
activity in the following order: MnP3 > MnP4 > MnP2b > MnP2a > MnP1 (16.1%,
15.6%, 11.7%, 7.4% and 1.3%, respectively). MnP3 and MnP4 show a better
catalytic performance than the parent Mn(TPFPP)Cl (16.1% and 15.6% vs 12%)
suggesting that both catalysts are more stable than Mn(TPFPP)Cl. In addition, higher
selectivity for the alcohol is observed for MnP3. These results showed that the
19

selectivity can be modulated by the position and number of –OR groups: while MnP3
showed the best performance concerning selectivity for the alcohol, MnP2a was
instead more selective for the ketone.
17.6
11.2% ol +
16.1% ol
4.4% one
Mn(TPFPP)Cl
15.6
MnP1
9.7% ol
+ 2.3% one
13.6
11.6
9.6
6.7% ol +
5.0% one
12% ol
MnP2a
MnP2b
MnP3
3.0 % ol +
4.4% one
MnP4
7.6
5.6
3.6
1.3%
ol
1.6
-0.4
H2O2
H2O2 + Imidazole
H2O2 + Ammonium acetate
Figure 7. Cyclohexane oxidation by H2O2 catalysed by MP1-MnP4 and Mn(TPFPP)Cl in
homogeneous conditions. Conversions were calculated based on the amount of substrate.
Results represent reactions performed in duplicate or triplicate. Cyclohexanol (ol) and
cyclohexanone (one). Under these conditions the conversion values were in the range of
0.5%
4. Conclusions
The effects of the inclusion of one to four –OR moieties at the meso-aryl p-
positions of H2(TPFPP) in the catalytic efficiency of their manganese(III) complexes
was studied for the oxidation of cis-cyclooctene and cyclohexane using PhIO and
hydrogen peroxide. It was found that the inclusion of an appropriate number of those
bulky groups into the parent H2(TPFPP), in association with the remaining
electronegative fluorine atoms, resulted in efficient and selective homogeneous
catalysts for alkenes epoxidation and alkanes hydroxylation with both oxidants. For
both substrates, the most efficient catalyst in the presence of H2O2 is the Mn(III)
complex bearing three ethylene glycol moieties (MnP3). It was possible to modulate
catalyst selectivity and efficiency by choosing the number of ethylene glycol moieties.
20

Acknowledgments
The authors are grateful to Conselho Nacional de Desenvolvimento Científico e
Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nível
Superior (CAPES) and Universidade Federal do Paraná (UFPR) for financial support.
The authors also thank Fundação para a Ciência e a Tecnologia (FCT, Portugal),
QREN, FEDER and COMPETE founds for the QOPNA (PEst-C/QUI/UI0062/2013;
FCOMP-01-0124-FEDER-037296), and Universidade de Aveiro for financial support.
Part of this work was developed within the scope of the project CICECO-Aveiro
Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID /CTM
/50011/2013), financed by national funds through the FCT/MEC and when
appropriate co-financed by FEDER under the PT2020 Partnership Agreement.
Authors further wish to thank CICECO for specific funding towards the purchase of
the single-crystal X-ray diffractometer. Kelly A. D. F. Castro also thanks CNPq for the
granted post-doctoral scholarship (Process: 151512/2013-2 and 201107/2014-7).
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24

SUPPORTING INFORMATION
Synthesis, characterization and catalytic activity under homogeneous
conditions of ethylene glycol substituted porphyrin manganese (III) complexes
Kelly A. D. F. Castroa,b, Fernando H. C. de Limaa, Mário M. Q. Simõesb, M. Graça P. M.
S. Nevesb, Filipe A. Almeida Pazc, Ricardo F. Mendesc, Shirley Nakagakia, José A. S.
Cavaleirob
aLaboratório de Química Bioinorgânica e Catálise; Universidade Federal do Paraná
(UFPR), CP 19081, CEP 81531-990, Curitiba, Paraná, Brazil.
bDepartment of Chemistry and QOPNA, University of Aveiro, 3810-193 Aveiro,
Portugal.
cDepartment of Chemistry, CICECO – Aveiro Institute of Materials, University of Aveiro,
3810-193 Aveiro, Portugal
Contents
Figure S1. HRMS-ESI(+): m/z spectrum of MnP2a………………………………...............2
Figure S2. HRMS-ESI(+): m/z spectrum of MnP2b………………………...………….……2
Figure S3. HRMS-ESI(+): m/z spectrum of MnP3……………………………...……...…....3
Figure S4. UV-VIS spectrum of MnP2a in methanol………………………………....…..…4
Figure S5. Schematic representation of the sole O–H···O hydrogen bonding interactions
(via the pendant ethylene glycol moieties of the porphyrin molecules) present in the
crystal structure of compound P4·2(DMSO)……………………………………….….……..5
1

1111.04189
C48 H18 O4 N4 F18 Mn
0.29313 ppm
100
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
1113.04697
1109.04763
1110.05071
1114.05006
1114
1105.70973
1106
1107.05530
1108
1117.11614
1116 1118
1101.75749 1103.80516
1102 1104
1120.08109
1120
0
1110
1112
1122
1124
m/z
Figure S1. HRMS-ESI(+): m/z spectrum of MnP2a.
1111.04255
C48 H18 O4 N4 F18 Mn
0.88964 ppm
100
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
1113.04773
15
10
5
1109.04867
1110.05163
1108.03900
1107.03408
1106 1108
1114.05080
1114
1117.11635
1116 1118
1101.83171
1102
1105.03746
1104
0
1110
1112
1120
1122
1124
m/z
Figure S2. HRMS-ESI(+): m/z spectrum of MnP2b.
2

Figure S3. HRMS-ESI(+): m/z spectrum of MnP3.
3

457
551
497
365
390
767
676
480 510 540 570 600 630 660 690 720 750 780
350
400
450
500
550
600
650
700
750
800
Wavelength (nm)
Figure S4. UV-VIS spectrum of MnP2a in methanol.
4

Figure S5. Schematic representation of the sole O–H···O hydrogen bonding interactions (via
the pendant ethylene glycol moieties of the porphyrin molecules) present in the crystal
structure of compound P4·2(DMSO). Hydrogen bond geometrical details: O4–H4···O3i with
dD···A = 2.747(5) Å and <(DHA) of 145º. Symmetry transformation used to generate
equivalent atoms: (i) 1+x, 0.5+y, 1.5-z. Color scheme as in Figure 1. Hydrogen bonding
interactions depicted as dashed green lines.
5