Polymeric membranes: The role this support plays in the reactivity of the different generations of metalloporphyrins

Article abstract of DOI:10.1016/j.molcata.2004.11.014

In this work, we investigated the catalytic activity of representatives of the three generations of ironporphyrins, namely [5,10,15,20- tetraphenylporphyrin]iron(III) chloride (Fe(TPP)Cl), [5,10,15,20-tetrakis(2,6- dichlorophenyl)porphyrin]iron(III) chloride (Fe(TDCPP)Cl), and [5-mono(pentafluorophenyl)10,15,20-tris(2,6-dichlorophenyl)2,3,7,8,12,13,17, 18-octachloroporphyrin] iron(III) (Fe(PCl₈)Cl), occluded in a polymeric film based on poly(dimethylsiloxane) (PDMS), in the oxidation of cyclohexane by iodosylbenzene. The catalytic results show the influence of the polymeric support on the reactivity of the three generations of metalloporphyrins, its importance in concentrating the substrate close to the catalytic site, and the protection it renders the catalyst against auto-oxidative destruction.

Full text of DOI:10.1016/j.molcata.2004.11.014

Journal of Molecular Catalysis A: Chemical 229 (2005) 137–143
Polymeric membranes: the role this support plays in the reactivity
of the different generations of metalloporphyrins
a
a
b
b
´
´
´
Maria C.A.F. Gotardo , Andre A. Guedes , Marco A. Schiavon , Nadia M. Jose ,
b
a,∗
´
I. Valeria P. Yoshida , Marilda D. Assis
a
Laborato´rio de Bioinorgaˆnica, Departamento de Qu´ımica, FFCLRP, Universidade de Sa˜o Paulo,
Av. Bandeirantes 3900, 14040-901 Ribeira˜o Preto, SP, Brazil
b
Instituto de Qu´ımica, UNICAMP, CP 6154, 13083-970 Campinas, SP, Brazil
Received 23 August 2004; received in revised form 19 November 2004; accepted 20 November 2004
Available online 7 January 2005
Abstract
In this work, we investigated the catalytic activity of representatives of the three generations of ironporphyrins, namely [5,10,15,20-
tetraphenylporphyrin]iron(III) chloride (Fe(TPP)Cl), [5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin]iron(III) chloride (Fe(TDCPP)Cl),
and [5-mono(pentafluorophenyl)10,15,20-tris(2,6-dichlorophenyl)2,3,7,8,12,13,17,18-octachloroporphyrin] iron(III) (Fe(PCl8)Cl), occluded
in a polymeric film based on poly(dimethylsiloxane) (PDMS), in the oxidation of cyclohexane by iodosylbenzene. The catalytic results show
the influence of the polymeric support on the reactivity of the three generations of metalloporphyrins, its importance in concentrating the
substrate close to the catalytic site, and the protection it renders the catalyst against auto-oxidative destruction.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Metalloporphyrins; Poly(dimethylsiloxane); Membrane; Catalysis; Oxidation
1. Introduction
␤-pyrrole rings of the porphyrin macrocycle [11,12]. Such
substituents confer steric hindrance to the catalyst, avoiding
its auto-oxidative destruction [13], and activate the catalytic
species, generally a metallo-oxo porphyrin ␲-cation radical
[MeIVO(P•+)], making it more electrophilic and reactive to-
wards the substrate [14,15]. Metalloporphyrin immobiliza-
tiononsolidsupportsoffersnumerousadvantages, suchasthe
prevention of catalyst self-destruction, increased regioselec-
tivity in some reactions, and easy catalyst recovery and re-use
[1–4]. Another advantage of supported metalloporphyrins is
related to the possibility of their mimicking the protein cavity
that is present in natural enzymes. It is well known that the
steric effects imposed by the active site environment are re-
sponsible for the selectivity encountered within the biological
systems [16,17].
The application of new materials in biomimetical chem-
istry has been extensively studied in order to prepare catalysts
thataremorestableandselectiveundertheemployedreaction
conditions. Another feature that is pursued in these catalysts
is their easy recovery from the reaction media and the pos-
sibility of having them recycled. One example of such new
materials is metalloporphyrins immobilized on, or entrapped
in zeolites, mesoporous materials, and clays [1–4].
Metalloporphyrin systems have been studied as models
for cytochrome P-450 for over 20 years [5–10]. The search
for improved models has led to the design and synthesis of
the first, second, and third generations of metalloporphyrins,
which bear increasing substitution of hydrogen atoms for
bulky, electron-withdrawing groups, on the meso-phenyl and
Polymeric membranes have been considered innovative
materials for metalloporphyrin immobilization. Such support
presents many advantages concerning its affinity for reagents,
which is the main property of polymers. The hydrophobic
∗
Corresponding author. Tel.: +55 16 602 3799; fax: +55 16 633 8151.
E-mailaddress: mddassis@usp.br (M.D. Assis).
1381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2004.11.014

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M.C.A.F. Gotardo et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 137–143
Fig. 1. Ironporphyrins of 1st, 2nd and 3rd generation.
membrane acts as a barrier, isolating and controlling the ac-
cess of both the substrate and the oxidant to the active site,
thus avoiding the presence of excess polar substances in its
vicinity, and rendering a hydrophobic environment for sub-
strate binding. Therefore, polymeric membrane-immobilized
metalloporphyrins can act as shape-selective catalysts where
the polymer isolates the catalytic complex, avoiding its inac-
tivation by aggregation or self-oxidative destruction [18].
Poly(dimethylsiloxane) (PDMS), a dense hydrophobic
elastomer, is one of the most important polymers in the silicon
chemistry because of its good thermal and chemical stabili-
ties. It has been successfully applied to the immobilization of
oxygenation catalysts, such as phthalocyanines in zeolite-Y,
and epoxidation catalysts, such as Mn-salen [19–21].
In this work, we compare the catalytic activity of
representatives of the three generations of ironpor-
phyrins, namely [5,10,15,20-tetraphenylporphyrin]iron(III)
chloride (Fe(TPP)Cl), [5,10,15,20-tetrakis(2,6-dichloro-
phenyl)porphyrin]iron(III) chloride (Fe(TDCPP)Cl), and
[5-mono(pentafluorophenyl)10,15,20-tris(2,6-dichlorophe-
nyl)2,3,7,8,12,13,17,18-octachloroporphyrin] iron(III) (Fe
(PCl8)Cl) (Fig. 1) occluded in a polymeric film based
on poly(dimethylsiloxane) (PDMS), in the oxidation of
cyclohexane by iodosylbenzene. These films belong to a
class of hybrid organic–inorganic materials that present,
apart from an hydrophobic component (PDMS, in this case),
the polar clusters as cross-linking centers of PDMS chains.
These clusters display a great number of functional Si OH
groups to promote condensation reaction with the Si OH of
the chain-end of PDMS. The design of such hybrid network
is an alternative for the production of multi-functional
materials with polar and apolar nano-domains which can
lead to a vast number of applications, since the combination
of the intrinsic properties of each compound results in
solubility, viscosity, and thermal behavior that are distinct
from those observed with the traditional polymeric systems.
Our aim herein is to investigate the role of this new support
in the reactivity of the different generations of encapsulated
metalloporphyrins.
2. Experimental
2.1. Materials
All compounds used in this study were commercially
available from Aldrich or Sigma and were of analytical grade
purity unless otherwise stated. Dichloromethane was distilled
˚
andstoredon4 Amolecularsieves. Acetonitrileandmethanol
˚
were stored on 3 A molecular sieves. Iodosylbenzene (PhIO)
was prepared by the hydrolysis of iodosylbenzene diacetate
following the method of Sharefkin and Saltzman [22], and
its purity was 96%, as determined by iodometric titration.
Fe(TDCPP)Cl and Fe(TPP)Cl were purchased from Mid-
Century and used as received. Pentaerythritholtriacrylate
(PETA) and isopropanol were purchased from Aldrich.
2-Aminoethyl-3-aminopropyltrimethoxysilane (AS) and
poly(dimethylsiloxane) (PDMS), both with molecular
weight of ∼2200 g mol−1, were supplied by Dow Corning.
The di-n-butyltindilauryl complex, 5 wt.% in hexane (Sn
catalyst), and tetraethoxysilane (TEOS) were supplied by
Gelest.
The free base H2(PCl8) was prepared by chlo-
rination of zinc 5-(pentafluorophenyl)-10,15,20-tris(2,6-
dichlorophenyl)porphyrin with N-chlorosuccinimide fol-
lowed by demetallation with trifluoroacetic acid, as described
previously [3,23]. Iron porphyrin, Fe(PCl8)Cl, was obtained
by metallation of the free ligand with iron(II) chloride di-
hydrate in acetonitrile. UV–vis (CH2Cl2), Fe(PCl8)Cl: λ,
nm (ε, ×102 L mol−1 cm−1): 394 (854); 440 (933); m/z
[FAB+]: 1239.566 [100% − relative abundance] correspond-
ing to FeC44H9N4F535Cl1137Cl3.
2.2. Preparation of the polymeric membranes
The synthetic procedure followed for the obtention of
polymeric membranes (PM), and of polymeric membranes
containing encapsulated ironporphyrin started from the first-
generation dendrimeric units prepared in situ from PETA
(2.6 wt.%) and AS (7.7 wt.%), by the addition-type Michael

M.C.A.F. Gotardo et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 137–143
139
reaction. These dendrimersact ascross-linkable centersin the
preparation of the PDMS network, making it a self-supported
film. In these films, the amount of PDMS was 79.7 wt.%,
and TEOS (10 wt.%) was present as co-cross-link. The mem-
branes were obtained using isopropanol (10 mL) as solvent,
by mixing the reagents for 30 min at 40 ◦C, in the pres-
ence of Sn catalyst [24]. Afterwards, 10 mL dichloromethane
was added, followed by the addition of a volume of cat-
alyst solution in dichloromethane that was enough to pro-
vide 0.02 wt.% iron porphyrin in relation to the total mass
of the polymeric membrane. The mixtures were cast on
Teflon® Petri dishes for 10 days. The samples were then
dried at 50 ◦C, in a vacuum oven, for 48 h, resulting in
the Fe(TPP)Cl-PM, Fe(TDCPP)Cl-PM and Fe(PCl8)Cl-PM
membranes with 0.02 wt.% catalyst.
GC analysis was performed on a Varian 3400 CX
chromatograph with a hydrogen flame ionization detec-
tor using a DB-wax (1 ␮m thickness) megabore column
(30 m × 0.538 mm). Nitrogen was used as the carrier gas. The
results were recorded and processed on a Varian workstation.
The morphology of the membranes was analyzed by scan-
ning electron microscopy (SEM) using a JEOL-JSM T-300
microscope, operating at 20 kV. The observed surface was
obtained by coating the cryogenic fracture with a thin gold
layer.
3. Results and discussion
3.1. Characterization of the occluded catalyst
2.3. Sorption measurements
The polymeric membrane used was a PDMS-based mem-
brane, cross-linked by polar first-generation dendrimers.
TEOS units also acted as co-cross-linkable centers. Indeed,
the resulting polymeric membrane displayed a well-defined
glass transition at approximately −120◦C, due to relaxation
of the PDMS chains [24,25]. Since the ironporphyrins were
soluble in the medium where the membranes were pre-
pared, the occluded catalysts were homogeneously dispersed
into the PDMS chains, resulting in completely homoge-
nous and transparent membranes. Fe(TPP)Cl, Fe(TDCPP)Cl,
and Fe(PCl8)Cl iron(III)porphyrins (Fig. 1) were success-
fully encapsulated in the membranes, as shown by the
UV–vis spectra of the materials, which display Soret bands
at 414, 424, and 488 nm for Fe(TPP)Cl-PM, Fe(TDCPP)Cl-
PM, and Fe(PCl8)Cl-PM, respectively (Fig. 2). The ab-
sorption of Fe(TDCPP)Cl-PM and Fe(PCl8)Cl-PM are red-
shifted, if compared to the parent iron(III)porphyrins in
dichloromethane solution (416 and 440 nm, respectively).
Significant changes are normally observed when these iron
porphyrins are immobilized on solid supports [26], and they
can be attributed to the distortion of the porphyrin ring pro-
voked by the support. Distortions in the porphyrin ring pla-
narity lead to a destabilization of the HOMO, but not of the
LUMO orbital, resulting in a decrease in the HOMO–LUMO
energy gap, which is responsible for the red-shift of the Soret
band [27,3]. No shift was observed in the Soret band of oc-
cluded Fe(TPP)Cl (412 nm). This is because this less robust
ironporphyrin can fit better in the membrane cavity without
any distortion of the porphyrin ring.
Thesorptionofcyclohexane, theoxidationproduct(cyclo-
hexanol) and the solvents (methanol and dichloromethane) in
the polymeric membrane were investigated by dipping 50 mg
of the membrane into the appropriate liquid. After 24 h of im-
mersion, the swollen membrane was weighted again after the
removal of the excess liquid. The supernatant was analyzed
by UV–vis spectrum in order to check if the metalloporphyrin
had leached from the membrane during the sorption experi-
ments.
2.4. Oxidation reactions
The encapsulated iron(III)porphyrin (FeP) (7.0 ×
10−9 mol, 40 mg membrane containing FeP 0.02 wt.%)
or iron(III)porphyrin in homogeneous solution (7.0 ×
10−9 mol) was stirred with cyclohexane and iodosyl-
benzene in dichloromethane:methanol (1:1). The cata-
lyst:substrate:oxidant molar ratio was 1:2000:4000, in
the case of condition 1; and for condition 2, the cata-
lyst:oxidant:substrate molar ratio was 1:4000:990.000. After
24 h, the membrane was removed, the internal standard
was added, and the products were analyzed by GC. All
the reactions were carried out at room temperature with
magnetic stirring. After the reactions, the membranes were
immersed in dichloromethane for 24 h in order to extract
the oxidized products that could remain inside the polymer.
The extracts were analyzed by GC. Control reactions were
carried out in the same conditions, using the membrane
containing no iron(III)porphyrin.
The cryogenic fractures observed on the surface of the
dense membranes both in the presence and in the absence
of the occluded complexes showed similar and homoge-
neouspatterns, withoutanymorphologicaldetailinthehigher
magnification micrography, as can be seen in Fig. 3A for
Fe(TDCPP)Cl-PM. In the lower magnification micrography
(Fig. 3B), a rough surface was observed, due to the cryogenic
fracture typical of PDMS membranes.
2.5. Instrumentation
UV–vis spectra were obtained on a Hewlett-Packard 8452,
diode array spectrometer. The spectra were recorded in
10 mm path length quartz cells (Hellma). The spectra of the
occluded catalysts were registered by maintaining the mem-
brane directly in the optical way, using the membrane con-
taining no iron porphyrin as reference.
The affinity of reagents and products for the membrane
phase can be easily determined by measuring the sorption of
the compounds in the membrane. The more a polymer swells

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M.C.A.F. Gotardo et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 137–143
file of the encapsulated catalysts, as there is competition be-
tween this solvent and the substrate for the catalytic center.
Methanol and cyclohexanol do not swell the membranes sig-
nificantly.
No leaching of the catalysts was observed in any of the
supernatants, proving that the catalysts are maintained inside
the membranes.
3.2. Catalytic studies
The catalytic activity of the three generations of polymeric
membrane-occluded metalloporphyrins was evaluated by us-
ing cyclohexane as substrate, and iodosylbenzene as oxidant.
Takingintoaccounttheaffinityofthesupportforthereagents,
two reaction conditions were employed. In condition 1, a cat-
alyst:substrate:oxidant molar ratio of 1:2000:4000 was used,
which corresponds to 1.5 × 10−5 mol substrate in 1.5 mL sol-
vent. Such condition employed highly diluted substrate in the
reaction medium, since our aim was to evaluate the ability of
the support in concentrating apolar reagents in its interior.
Moreover, as the oxidant is polar and could be poorly sorbed
into the membrane, it was used in excess in order to favor its
sorption by the dragging effect.
A
catalyst:oxidant:substrate
molar
ratio
of
1:4000:990.000 was employed in condition 2, corre-
sponding to a 1:1 cyclohexane:dichloromethane/methanol
volume ratio. Such condition aimed at favoring the sorption
of cyclohexane with respect to the solvent dichloromethane,
since the latter displays higher affinity for the polymeric
support (Fig. 4).
The
membrane-occluded
iron(III)porphyrins
Fe(TDCPP)Cl and Fe(TPP)Cl presented excellent effi-
ciency for cyclohexane oxidation, in the condition of highly
diluted substrate (condition 1), as shown in Table 1, leading
to 42 and 37 turnovers of oxidized products, respectively
(Table 1, entries 4 and 5). These ironporphyrins were not
efficient catalysts for cyclohexane oxidation in homogeneous
solution, in the same experimental condition (Table 1, entries
1 and 2). This is because the low substrate concentration and
the low substrate/oxidant molar ratio favor competitive re-
actions, such as solvent oxidation, PhIO disproportionation,
among others, in homogeneous systems. These results show
the great ability of the polymeric support in concentrating
the apolar substrate in the vicinity of the catalyst active
site, even at diluted concentrations, and considering that
one of the reaction solvents (dichloromethane) presents
higher affinity for the membrane than the substrate itself and
undergoes preferable sorption (Fig. 4).
Fig. 2. UV–vis spectra of the (A) Fe(TPP)Cl-PM, (B) Fe(TDCPP)Cl-PM,
and (C) Fe(PCl8)Cl-PM. The membrane without ironporphyrin was used as
reference.
in a determined solvent, the higher its affinity for this com-
pound. The ability of the membrane to adsorb the solvent and
substrates is essential to establish the reaction conditions in
thecatalyticstudies. Thesorptionvaluesafter24 hofswelling
for the compounds of interest in our study are shown in Fig. 4.
Dichloromethane, which is the solvent used in this study,
is more readily adsorbed by these hydrophobic materials than
cyclohexane. These results are reflected in the catalytic pro-
The only cyclohexane oxidation product obtained
when the reaction was catalyzed by Fe(TPP)Cl-PM or
Fe(TDCPP)Cl-PM was cyclohexanol. The high selectivity
for the alcohol indicates that the oxidation mechanisms prob-
ably involve the same catalytic species as the one observed
in homogeneous solution, the high valent metal-oxo por-
phyrin ␲-cation radical, (•+P)FeIV O, which is formed from
the reaction between the oxidant and the iron(III)porphyrin

M.C.A.F. Gotardo et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 137–143
141
Fig. 3. Cryogenic surface fracture micrographies of the Fe(TDCPP)Cl occluded-polymeric membrane at different magnification: (A) ×3500; (B) ×75.
In the case of Fe(PCl8)Cl-PM, a third-generation metal-
loporphyrin, no product was formed in condition 1 (Table 1,
entry 6). Although the polymeric membrane is capable of
concentrating the substrate in the surroundings of the cat-
alytic site, the eight additional chloro substituents in the
␤-pyrrole positions of this ironporphyrin ring offer great
steric hindrance to the approach of cyclohexane. Moreover,
as shown by its UV–vis spectrum, this bulky ironporphyrin
presents a distorted ring when it is present between the poly-
mer chains, a factor that may disfavor the interaction be-
tween the catalyst and the oxidant, or the catalyst and the
substrate.
The catalytic results obtained with the Fe(TPP)Cl-PM sys-
tem show the benefits of designing new materials for catalyst
immobilization. In diluted condition (condition 1), this first-
Fig. 4. Room-temperature sorption measurements for dichloromethane,
methanol, cyclohexanol, and cyclohexane in FeP-PM.
generation ironporphyrin led to the same catalytic efficiency
as the second-generation ironporphyrin Fe(TDCPP)Cl-PM,
which is known to be a very efficient catalyst in homogeneous
solution, according to the literature [28–31]. In solution, total
destruction of Fe(TPP)Cl was observed in the reaction condi-
tions employed in this work, as seen from solution bleaching.
The Fe(TPP)Cl-PM system, however, kept the same coloring
after the oxidation reactions, indicating that the porphyrin
[14,15]. After being produced, cyclohexanol is quickly ex-
pelled from the hydrophobic membrane, as can be inferred
from its low sorption value (Fig. 4), avoiding its further ox-
idation to the corresponding ketone. This was confirmed by
the absence of the alcohol in the dichloromethane extracts
after the reaction.
Table 1
Results obtained for the cyclohexane oxidation reactions by PhIO catalyzed by FeP in homogeneous systems or by FeP-PM
Entry
Catalyst
C-ol (Ton)
C-one (Ton)
Total Ton
C-ol/C-one ratio
1
2
3
4
5
Fe(TPP)Cl (homogeneous solution)a
Fe(TDCPP)Cl (homogeneous solution)a
Fe(PCl8)Cl (homogeneous solution)a
Fe(TPP)Cl-PMa
nd
nd
nd
37
42
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
37
42
–
–
–
–
–
–
–
Fe(TDCPP)Cl-PMa
6
Fe(PCl8)Cl-PMa
7
8
9
10
11
12
Fe(TPP)Cl (homogeneous solution)b
Fe(TDCPP)Cl (homogeneous solution)b
Fe(PCl8)Cl (homogeneous solution)b
Fe(TPP)Cl-PMb
395
570
1152
313
605
443
316
445
530
195
375
242
711
1015
1682
508
980
685
1.3
1.3
2
1.6
1.6
1.8
Fe(TDCPP)Cl-PMb
Fe(PCl8)Cl-PMb
a
Condition 1 – 1500 ␮L DCM: MeOH (1:1); – 2 ␮L cyclohexane (1.5 × 10−5 mol).
b
Condition 2 – 750 ␮L DCM: MeOH (1:1); – 750 ␮L cyclohexane (6.9 × 10−3 mol); nd: not detected; C-ol: cyclohexanol; C-one: cyclohexanone; Ton: mol
product/mol FeP (turnover number). Blank reactions (only PM was used): nd.

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M.C.A.F. Gotardo et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 137–143
structure remained unchanged throughout the reaction. The
polymer is, therefore, efficiently acting to avoid oxidative
self-destruction, one of the problems encountered when first-
generation metalloporphyrins are used in oxidation reactions
in homogeneous solution [7].
The results obtained for cyclohexane oxidation carried
out in excess substrate (condition 2) were different from
those obtained in condition 1. In homogeneous solution,
the presence of excess substrate disfavors the occurrence of
the competitive reactions, leading to high turnover numbers
(Table 1, entries 7–9). The observed catalytic efficiency was
Fe(PCl8)Cl > Fe(TDCPP)Cl > Fe(TPP)Cl, which is expected
since an increase in the number of electron-withdrawing sub-
stituents in the periphery of the porphyrin ring improves cat-
alyst performance by increasing the electrophilicity of the
active species [32–35].
In the case of immobilized ironporphyrins, the small
piece of the membrane that is used in the reactions has lim-
ited substrate and/or oxidant sorption capacity, which seems
to be the limiting reaction factor at higher substrate con-
centrations. This fact explains the lower turnover numbers
obtained for the heterogeneous systems in this condition
(685, 980, and 508 for Fe(PCl8)Cl-PM, Fe(TDCPP)Cl-PM,
and Fe(TPP)Cl-PM, respectively, Table 1, entries 10–12),
if compared to those obtained in homogeneous solution. In
this condition, Fe(TDCPP)Cl-PM was more efficient than
Fe(PCl8)Cl-PM because of the greater steric hindrance of the
latter catalyst within the membrane, as discussed previously.
Fe(TDCPP)Cl-PM was also more efficient than Fe(TPP)Cl-
PM, different from what occurs in condition 1. In this case,
the excess substrate shows these complexes have different
intrinsic reactivity.
It is important to note that the selectivity for cyclohexanol
is higher with the occluded catalyst than with the less robust
porphyrins in homogeneous solution (Table 1, entries 10 and
11 versus 7 and 8), showing that the hydrophobic membrane
does play a role in selecting the substrate by its polarity,
which is a role similar to that of the protein matrix in P-450
enzymes.
metalloporphyrin and higher than that of the third-generation
catalyst.
The catalytic results obtained with these systems show that
polymeric membranes of different polarity and environment
can be designed, aiming at the selective oxidation of other
substrates.
Acknowledgements
We thank CAPES, CNPq and FAPESP for financial sup-
port.
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