Novel reactivity of photoexcited iron porphyrins caged into a polyfluoro sulfonated membrane in catalytic hydrocarbon oxygenation

Article abstract of DOI:10.1039/a706237k

Heterogenization of iron porphyrins inside Nafion creates new photocatalytic systems which can be used to oxidize cyclohexene and cyclohexane with sunlight and O₂ under mild conditions (room temperature, atmospheric pressure); the polymeric matrix makes the iron porphyrin a good photocatalyst for the monooxygenation of the substrate and increases both its photocatalytic efficiency (about ten times) and its stability (turnover values > 1000).

Full text of DOI:10.1039/a706237k

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Novel reactivity of photoexcited iron porphyrins caged into a polyfluoro
sulfonated membrane in catalytic hydrocarbon oxygenation
A. Maldotti,* A. Molinari, L. Andreotti, M. Fogagnolo and R. Amadelli
Dipartimento di Chimica, Centro di Studio su Fotoreattivita` e Catalisi del CNR, Universita` degli Studi di Ferrara, Via L. Borsari
46, 44100, Ferrara, Italy
Heterogenization of iron porphyrins inside Nafion creates
new photocatalytic systems which can be used to oxidize
cyclohexene and cyclohexane with sunlight and O2 under
mild conditions (room temperature, atmospheric pressure);
the polymeric matrix makes the iron porphyrin a good
photocatalyst for the monooxygenation of the substrate and
increases both its photocatalytic efficiency (about ten times)
and its stability (turnover values > 1000).
to step (a) in Scheme 1, which is a type of reaction already
observed for many other iron porphyrin complexes in homoge-
neous solution.1a,b,8,9 In fact, photoirradiation‡ of the iron
porphyrin/Nafion composite systems in alcohol in the presence
of the spin trap phenyl-tert-butylnitrone (PBN) inside the cavity
of an EPR spectrometer yields spectra exhibiting the typical
signals of the adducts between PBN and alkoxy radicals [step
(b)].10
H
O–
As a development of an important field of research dealing with
photoexcited iron porphyrins in the catalytic oxygenation of
hydrocarbons,1 we have recently investigated photocatalytic
composite systems in which the porphyrin complexes are
confined inside cross linked polystyrene,2 bound on semicon-
ducting transition metal oxides such as TiO23 or interacting with
polyoxotungstates.4 Generally speaking, the use of heteroge-
neous media or organized systems is a suitable means to control
the reaction environment in order to promote specific processes
of interest in biomimetic catalysis by iron porphyrins.5
PBN
Ph
C
N
But
hn + O2
(b)
(c)
R
H
OR
O
(a)
RO•
FeIII(por)
O
O
•
FeIII(por) + H+
ROH +
Scheme 1
This work deals with composite systems in which iron
porphyrin complexes are caged inside Nafion,6 a commercial
polyfluorosulfonated membrane consisting of sulfonic groups
connected to a polymeric structure of polytetrafluoroethylene.
We have chosen this support because (i) it is chemically inert
also in strong oxidizing media; (ii) it is totally transparent to the
light of interest in metal porphyrin photochemistry; (iii) it is
expected to cage the monocationic porphyrin complexes inside
Since the substituents on the meso-aryl groups of [FeIII-
(tdcpp)]+ and [FeIII(tf5pp)]+ prevent the formation of a m-oxo
dimer complex,11 the very fast reaction of the iron(ii) porphyrin
with O2 is expected to restore the starting FeIII complex, so
inducing the reductive activation of O2 itself1b,12 (Scheme 1). In
this way, using dioxygen and sunlight, the iron porphyrin/
Nafion systems are able to form, in a cyclic way, reactive
intermediates which are expected to induce the oxidation of
hydrocarbons. The results obtained in the oxidation of cyclo-
hexene and cyclohexane are summarized in Table 1.
2
its large anionic cavities where the SO3 groups are located;
and (iv) dioxygen concentration inside Nafion is more than ten
times higher than in organic solvent.7
The photocatalytic activity of these composite systems is
demonstrated in the oxygenation of cyclohexene and cyclohex-
ane by O2 and sunlight under mild conditions (20 °C, 760 Torr
of O2). A comparison with the photocatalytic properties of the
same iron porphyrins in homogeneous solution reveals that the
polymeric matrix strongly affects the chemioselectivity of the
oxidation process and increases both the photochemical effi-
ciency and the stability of the iron porphyrin.
Table 1 shows that irradiation of [FeIII(tdcpp)+ in EtOH–
cyclohexene (25% v/v) results in the formation of cyclohex-
2-en-1-one, cyclohex-2-en-1-ol and trans-cyclohexane-1,2-diol
monoethyl ether. Interestingly, if the same experiment is carried
out using PriOH instead of EtOH, the ether represents about
90% of the overall oxidized alkene, and allylic oxidation
products are formed only in very minor amounts. The presence
of the ether indirectly reveals the formation of cyclohexene
epoxide which, in the strong acidic environment inside Nafion,
likely undergoes a nucleophilic attack by the alcohol, leading to
the epoxide ring opening.§ Oxidation of cyclohexene is also
observed with FeIII(tf5pp)/Nafion even though the ratio of
allylic oxidation products increases significantly in PriOH. For
the latter system we obtain the maximum initial oxidation rate
The following iron(iii) porphyrins have been chosen on the
basis of their well known1 behaviour in homogeneous solution:
iron(iii) meso-tetraphenylporphyrin [FeIII(tpp)]+, iron(iii) meso-
tetrakis(2,6-dichlorophenyl)porphyrin [FeIII(tdcpp)]+, iron(iii)
meso-tetrakis(pentafluorophenyl)porphyrin
[FeIII(tf5pp)]+.
Their absorption into Nafion was carried out in CH2Cl2–alcohol
at 40 °C after swelling the membrane in EtOH or PriOH.
Typically the uptake of 0.5–0.8 mmol of complex in 1 g of resin
occurs in a few minutes.
The UV–VIS spectra of the modified resin after swelling with
EtOH or PriOH are exactly those of the starting iron porphyrin
dissolved in acidified alcohol media, which an extensive
literature8 ascribes to the monocationic species having an
alcohol molecule bound to the axial position.† We then infer
that heterogenization does not affect the nature of the iron
porphyrin axial ligand. In support of this conclusion, an EPR
spin-trapping investigation indicates that heterogenization does
not affect even the primary photochemical process consisting of
the homolytic cleavage of the FeIII axial ligand bond according
of ca. 8.5 turnovers min21
.
Photoexcitation of [FeIII(tdcpp)]+ in homogeneous alcoholic
solution gives totally different results: (i) the photooxidation is
slower than in heterogeneous conditions; (ii) no ether is
obtained; (iii) in addition to allylic oxidation products, cyclo-
hexanol and cyclohexanone are formed, as we discussed
earlier.13
The turnover values reported in Table 1 indicate that
heterogenization significantly increases (about ten times) the
stability of [FeIII(tdcpp)]3+. The Nafion matrix does not undergo
any degradation and can be utilised several times. [FeIII(tpp)]+
is much less stable inside Nafion than both [FeIII(tdcpp)]+ and
[FeIII(tf5pp)]+. This indicates that, as in homogeneous sol-
Chem. Commun., 1998
507

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Table 1 Photocatalytica oxygenation of cyclohexene and cyclohexane
explanation, the polymeric matrix may favour the above
reactions in the proximity of the metal centre and, at the same
time, inhibit the escape of radical intermediates that initiate
autooxidation processes.¶ Our previous work in the homoge-
neous phase shows that photoexcited porphyrins under aerobic
conditions can oxidise cyclohexane even in the absence of
cyclohexene.1c However, in that case, the active species are OH·
radicals which are more reactive than alkoxy radicals in
extracting hydrogen atom from hydrocarbons.16
Photocatalytic
Substrates
Productsb and related turnoverc
system
OH
O
OH
FeIII(tdcpp)/Nafion
EtOH
OEt
1700
52
380
5
520
750
OH
O
OH
FeIII(tdcpp)/Nafion
PriOH
OPri
Notes and References
* E-mail: mla@ifeuniv.unife.it
OH
OH
OH
O
O
O
OH
† Exchange of the acid membrane with NaCl causes a red shift of the Soret
band, in keeping with the deprotonation of the coordinated alcohol molecule
and the formation of a neutral porphyrin complex. In fact, in this case we
observe experimentally that the complex is released by the membrane after
a few hours in alcohol. The spectral variations described are exactly the
same as those for the starting iron porphyrin in alcohol with added
CF3SO3H or HClO4. In the acidified alcohol medium one should reasonably
have an equilibrium among more species, with the anions or the alcohol
molecules as axial ligands in the porphyrin complex.
‡ Irradiation was carried out at 20 ± 1 °C with a Hanau Q 400 mercury lamp
(15 mW cm22), under oxygen at 760 Torr. Selection of wavelength between
330 and 400 nm was performed by use of a glass cut-off filter.
§ This statement is confirmed by the observation that the epoxide in acidic
solution undergoes a nucleophilic attack with the formation of the same
trans-cyclohexane-1,2-diol monoalkyl ether during the photocatalytic
oxidation.
FeIII(tf5pp)/Nafion
EtOH
OEt
280
200
30
310
550
OH
FeIII(tf5pp)/Nafion
PriOH
OPri
1800
1100
22
OH
FeIII(tpp)/Nafion
PriOH
OPri
31
OH
O
O
OH
FeIII(tdcpp)/EtOH
¶ The effect of the matrix is not just that of providing an acid environment.
In fact, photooxidation of the cycloalkenes by [FeIII(tdcpp)]+ dissolved in
EtOH or PriOH acidified with trifluoromethanesulfonic acid leads to the
formation of a mixture of various oxygenation products which, on the other
hand, does not include the ethers.
(homogeneous solution)
44
79
100
32
FeIII(tdcpp)/Nafion
PriOH
< 10–5 mol dm–3
1 (a) K. S. Suslick and R. A. Watson, New. J. Chem., 1992, 16, 633; (b)
A. Maldotti, C. Bartocci, G. Varani, A. Molinari, P. Battioni and
D. Mansuy, Inorg. Chem., 1996, 35, 1126; (c) L. Weber, R. Hommel,
J. Behling, G. Haufe and H. Hennig, J. Am. Chem. Soc., 1994, 116,
2400.
FeIII(tdcpp)/Nafion
PriOH
OH
O
OH
OPri
OH
11
4
80
22
2 E. Polo, R. Amadelli, V. Carassiti and A. Maldotti, Inorg. Chim. Acta,
1992, 192, 1.
3 R. Amadelli, M. Bregola, E. Polo, V. Carassiti and A. Maldotti, J. Chem.
Soc., Chem. Commun., 1992, 1355.
4 A. Maldotti, A. Molinari, R. Argazzi, R. Amadelli, P. Battioni and
D. Mansuy, J. Mol. Catal., 1996, 114, 141.
a
b
Six hours photoirradiation, see footnote ‡. Reaction products were
determined by gas chromatography and gas mass analyses. Reported values
are ±10%. Cyclohexane-1,2-diol monoethers have been separated on an
SiO2 column and characterised by proton NMR spectroscopy. Mol of
c
product formed per mol of consumed iron porphyrin.
5 E. I. Stiefel, in Bioinorganic Catalysis, ed. J. Reedijk, Marcel Dekker,
New York, Basel, Hong Kong, 1993, pp. 21–27; L. Barloy, P. Battioni
and D. Mansuy, J. Chem. Soc., Chem. Commun., 1990, 1365;
J. T. Groves and S. B. Ungashe, J. Am. Chem. Soc., 1990, 112, 7796.
6 Nafion is a Du Pont Nemours registered trademark.
7 Z. Ogumi, T. Kuroe and Z. Takehara, J. Electrochem. Soc., 1985, 132,
2601.
8 C. Bizet, P. Morliere, D. Brault, O. Delgado, M. Bazin and R. Santus,
Photochem. Photobiol., 1981, 34, 315; A. Maldotti, C. Bartocci,
R. Amadelli and V. Carassiti, J. Chem. Soc., Dalton Trans., 1989,
1197.
9 C. Bartocci, A. Maldotti, G. Varani, P. Battioni, V. Carassiti and
D. Mansuy, Inorg. Chem., 1991, 30, 1255.
10 A. Ledwith, P. J. Russel and L. M. Sutcliffe, Proc. R. Soc. London, Ser.
A, 1973, 332, 151; A. Maldotti, C. Bartocci, R. Amadelli and
V. Carassiti, Inorg. Chim. Acta, 1983, 74, 275.
11 R. Cheng, L. Latos-Grazynsky and A. L. Balch, Inorg. Chem., 1982, 21,
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12 A. Maldotti, C. Bartocci, C. Chiorboli, A. Ferri and V. Carassiti,
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13 A. Maldotti, C. Bartocci, R. Amadelli, G. Varani, E. Polo and
V. Carassiti, in Chemistry and Properties of Biomolecular Systems, ed.
E. Rizzarelli and T. Theophanides, Kluwer Academic, Dordrecht,
Boston, London, 1991.
14 D. N. Hendrickson, M. G. Kinnard and K. S. Suslick, J. Am. Chem. Soc.,
1987, 109, 1243.
utions,1c,9 halogen substituents on the meso aryl groups play a
fundamental role in restoring the starting iron porphyrin during
the photocatalytic cycle, avoiding both a too fast oxidative
degradation of the porphyrin ring and the formation of m-oxo-
dimers.
Upon irradiation of FeIII(tdcpp)/Nafion in PriOH containing
cyclohexane (25% v/v) the concentration of oxidation products
was < 1025 mol dm23, on the same timescale as for
cyclohexene. On the other hand, the alkane undergoes hydrox-
ylation when cyclohexene is present as a co-substrate (cyclo-
hexane 12.5% v/v, cyclohexene 12.5% v/v). This suggests that
cyclohexene plays a dominant role in the formation of active
monooxygenating species during the photocatalytic process.
Cyclohexene is expected to capture efficiently alkoxy
radicals originating from the photochemistry of iron porphyrins
through an allylic hydrogen abstraction process [step (c) in
Scheme 1].14 This hypothesis is confirmed by experiments in
which the intensity of the EPR signal of the adduct between
PBN and alkoxy radicals is followed as a function of irradiation
time. We observed that the signal intensity is unchanged in the
presence of cyclohexane, while it is reduced by about 75% if
cyclohexene is present. Apparently, the latter is able to compete
efficiently with PBN in the reaction with the radical inter-
mediates giving relatively stable cyclohexenyl radicals. Reac-
tion of O2 with these radicals in the presence of the iron
porphyrin can give efficient monooxygenating species1b as in
the catalytic cycle of cytochrome P450.15 As a tentative
15 D. Mansuy, Pure Appl. Chem., 1990, 62, 741; B. Meunier, Chem. Rev.,
1992, 92, 1411; M. Sono, M. P. Roach, E. D. Coulter and J. H. Dawson,
Chem. Rev., 1996, 96, 2841.
16 S. W. Benson, J. Chem. Educ., 1965, 42, 503.
Received in Basel, Switzerland, 26th August 1997; 7/06237K
508
Chem. Commun., 1998