Fine tuning of the catalytic effect of a metal-free porphyrin on the homogeneous oxygen reduction

Article abstract of DOI:10.1039/c1cc11075f

The catalytic effect of tetraphenylporphyrin on the oxygen reduction with ferrocene in 1,2-dichloroethane can be finely tuned by varying the molar ratio of the acid to the catalyst present in the solution. The mechanism involves binding of molecular oxygen to the protonated free porphyrin base, in competition with ion pairing between the protonated base and the acid anion present.

Full text of DOI:10.1039/c1cc11075f

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COMMUNICATION
Fine tuning of the catalytic effect of a metal-free porphyrin on the
homogeneous oxygen reductionw
a
a
a
,a Jean-Michel Barbe,b
ˇ
´
n Trojanek, Jan Langmaier, Jakub Sebera, Stanislav Zalis
´ ´
ˇ
Antonı
Hubert H. Girault and Zdene
c
a
ˇ
k Samec*
Received 23rd February 2011, Accepted 21st March 2011
DOI: 10.1039/c1cc11075f
8
quite slow in the presence of trifluoroacetic acid (HTFA). The
The catalytic effect of tetraphenylporphyrin on the oxygen
reduction with ferrocene in 1,2-dichloroethane can be finely
tuned by varying the molar ratio of the acid to the catalyst
present in the solution. The mechanism involves binding of
molecular oxygen to the protonated free porphyrin base, in
competition with ion pairing between the protonated base and
the acid anion present.
8
homogeneous reaction was proposed to follow the overall scheme,
+
+
O + 2 Fc + 2 H - H O + 2 Fc
(1)
2
2
2
In this communication, we report a significant acceleration
of the reduction of O with Fc in DCE in the presence of
2
H TPP and HTB which, however, occurs within a rather
2
narrow range of the acid-to-H TPP molar ratio. This fine
2
Metalloporphyrins have been used to activate molecular
1
oxygen for the controlled oxygenation of hydrocarbons, as
tuning of the catalytic effect represents a new phenomenon in
2
the non-metal catalysis of the O reduction, which we have
2
well as for the reduction of O to hydrogen peroxide and/or to
studied in the present work using absorption spectroscopy,
stopped-flow kinetic measurements and the DFT calculations.
2
water in solution, at liquid/liquid interfaces, or at solid
3
4
2
electrodes. The activation process involves binding of O to
Fig. 1 (panel a) depicts the absorption spectrum of H
in the absence of HTB (black line) and a red shift of the Soret
band upon protonation of H TPP in the presence of HTB at
2
TPP
the metal center and the electron delocalization from the metal
to O , which can be considered to be a coordinated superoxide
2
5
or peroxide anion, i.e., a stronger Brønsted base. Protonation
2
ratios of 1:1 (red line) and 1 : 2.5 (blue line). Like in the
presence of tetrakis[3,5-bis(trifluoromethyl)phenyl]borate
of the coordinated O could then make it more susceptible to
2
6
reduction. The Density Functional Theory (DFT) method
À
10
TFPB ), the absorption spectrum of the base (Soret band
(
was used to investigate the role of O
7
the activation process.
2
and proton binding in
centered at 418 nm) changes into that of the diprotonated
form (Soret band centered at 438 nm) without any isosbestic
point, which indicates the intermediate formation of the
Recently, we have demonstrated that the oxygen reduction
at the polarized water/1,2-dichloroethane (DCE) interface is
monoprotonated form. Ion pairing between the mono- and
À
catalyzed by 5-(p-aminophenyl)-10,15,20-tris(pentafluoro-phenyl)-
8
diprotonated forms of H
2
TPP and the counteranion X was
porphyrin (H
2
FAP),
and 5,10,15,20-meso-tetraphenyl-
proposed to be the key factor influencing the relative values of
9
porphyrin (H TPP). The diprotonated forms of these metal-free
2
2
+
2+
porphyrins, H FAP and H TPP , were asssumed to bind
4
4
oxygen in a complex, which is reduced in the organic phase by
8
9
ferrocene (Fc), and decamethylferrocene (DMFc). The
homogeneous O reduction was found to be catalyzed by
only in the DCE solutions acidified with tetrakis-
2
2
+
4
H FAP
(
pentafluorophenyl)boric acid (HTB), while the reaction was
a
J. Heyrovsky´ Institute of Physical Chemistry of ASCR, v.v.i,
Dolejsˇkova 3, 182 23 Prague 8, Czech Republic.
E-mail: zdenek.samec@jh-inst.cas.cz; Fax: +420 286582307;
Tel: +420 266052011
Institut de Chimie Mole´culaire de l’Universite´ de Bourgogne,
ICMUB (UMR 5260 du CNRS), 9 avenue Alain Savary, BP 47870,
b
c
Fig. 1 (a) Absorption spectra of the air-saturated DCE solutions
À5
À5
containing: 5 Â 10 M H
2
TPP (black line), 5 Â 10 M H
2
TPP
À5
À4
21078 Dijon Cedex, France
and 5 Â 10 M HTB (red line) or 1.25 Â 10 M HTB (blue line). (b)
Laboratoire d’Electrochimie Physique et Analytique,
Ecole Polytechnique Fe´de´rale de Lausanne, CH-1015 Lausanne,
Switzerland
+
Time profile of conversion of Fc to Fc monitored at 300 nm
À1
À1
(
e = 7938 M cm ) in air-saturated DCE solutions containing
À4
À5
1
mM Fc and 1.25 Â 10 M HTB (red line), or 1 mM Fc, 1.25 Â
w Electronic supplementary information (ESI) available: Experimental
details, UV/Vis spectra of Fc and Fc , rate equations and details of
DFT calculations. See DOI: 10.1039/c1cc11075f
À4
+
10 M HTB and 5 Â 10 M H
2
TPP (blue line). Cell path length
0.1 cm (a) or 0.2 cm (b).
5
446 Chem. Commun., 2011, 47, 5446–5448
This journal is c The Royal Society of Chemistry 2011

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1 2
the equilibrium constants K and K for the two dissociation
1
steps, respectively,
0
+
À
+
À
{
(H TPP )Á(X )} # H TPP + H + X , K1
(2)
3
2
TPP )Á(XÀ)
2
+
} # {(H
TPP )Á(X )} + H + X , K
+
À
+
À
{
(H
4
2
3
2
(
3)
1
The H NMR and IR data actually suggest that the protonation
1
1
2
of H TPP is induced by ion pairing with the counteranion.
Successive formation of both the protonated forms was
followed by ion transfer voltammetry in the presence of an
+
0
Fig. 3 (a) Initial rate v of the formation of Fc from Fc (1 mM) in
À
À3
12
excess of TB (5 Â 10 M). The reported acid dissociation
the air-saturated DCE solution vs. the analytical concentration of
À
À10
M and K
constants K1a = K /[TB ] = 1.6 Â 10
=
2a
HTB (J,K) or HTFA (&) in the presence (J, &) and absence (K)
À5
1
0
À
À6
12
M, yield K = 8 Â 10
À13
2
K /[TB ] = 10
2
M
and
of 5 Â 10 M H
2
+
TPP. (b) Enhancement of the initial rate Dv
0
of the
0
1
À9
2
À
K = 5 Â 10 M , respectively, [TB ] denoting the analytical
formation of Fc vs. the analytical concentration [H
K) at the fixed ratio [HTB] /[H TPP] = 2.5 and [Fc]
and vs. the analytical concentration [Fc] of Fc in the presence of
2
TPP]
0
of H TPP
2
À3
2
0
À
10
molar concentration of TB . As expected, K is considerably
(
0
2
0
0
= 10 M,
2
0
larger than K
1
, while in the presence of small and strongly
5 Â 10À5 M H
TPP and 1.25 Â 10À4 M HTB (J).
À
À
2
coordinated anions, such as Cl or TFA , the order of K
,10
1
and
8
K
2
is reversed.
An addition of Fc (10 M) to the air-saturated solution of
Fc proceeds at a low rate, which is practically independent of
the acid concentration (solid circles). In the presence of both
À3
À3
13
(1.39 Â 10 M) leads to the
HTB in DCE containing O
2
2
HTB and H TPP, a significant enhancement of the initial rate
+
formation of the ferrocenium cation (Fc ), which can be
identified by UV/Vis spectroscopy at 250–350 nm or
is observed, which has a sharp maximum at the HTB
À4
concentration, 1.25 Â 10 M (empty circles). In contrast,
5
00–700 nm (Fig. S1, ESIw). Fig. 1 (panel b) shows the time
+
profile of the conversion of Fc to Fc , as monitored by
when HTB is replaced by HTFA, the reaction rate decreases
8
to almost zero (empty squares), like for H FAP. The
+
2
absorption measurement at 300 nm. The formation of Fc
enhancement, Dv , of the initial rate relative to that measured
0
proceeds considerably faster in the presence of H TPP in
2
in the absence of H TPP exhibits non-linear dependences on
2
catalytic amounts (blue line) than in its absence (red line).
Fig. 2 (panel a) illustrates the gradual colour change of the
reaction mixture from green to orange. The absorption
spectrum (panel b) suggests that the diprotonated form of
2
the analytical concentrations of H TPP and of Fc, cf. the solid
and empty circles, respectively, in Fig. 3 (panel b).
These observations could be understood on the basis of the
kinetic model, which involves the O
diprotonated forms of H
TPP via the NH Á Á ÁO
bond. An increased concentration of HTB results in an
2
binding to the mono- or
H
2
TPP (Soret band centered at 438 nm) almost disappears
+
+
2
2
hydrogen
within ca. 1.5 min, while approximately 20% of the H
8,9
present remains to be bound in the monoprotonated form
+
+
2
increased number of the NH binding sites for O which,
À
10a
{
(H
3
TPP )Á(TB )} (Soret band centered at ca. 425 nm).
À
however, are simultaneously being blocked with TB . As a
The reaction then proceeds rather slowly, cf. also Fig. 1b (blue
+
line), until all the H is consumed and the free porphyrin base
result, the concentration of the O complex, and thereby the
2
O reduction rate, should pass through a maximum. Since the
2
(
Soret band centered at 418 nm) is recovered. The light pink
maximum rate is observed at the HTB to H TPP molar ratio
2
colour of its solution is overlapped by the deep orange colour
of the dissolved Fc.
that is much larger than unity, the O
assisted by the diprotonated rather than monoprotonated
TPP form. The appropriate catalytic cycle consists of the
2
reduction is likely to be
Fig. 3 (panel a) demonstrates the effects of the acid
H
2
concentration and the addition of H
+
2
TPP on the initial rate
À
2+
4 2
reversible exchange of TB bound to H TPP for O ,
v of the Fc formation, as evaluated from the initial slope of
the absorbance A vs. time plot, cf. Fig. 1 (panel b). In the
2
+
À
2+
À
{(H TPP )Á(TB ) } + O # {(H TPP )Á(TB )ÁO }
4
2
2
4
2
presence of HTB, but the absence of H
2
TPP, the oxidation of
À
+
TB , K
(4)
3
that is followed by the irreversible reduction of the bound O2,
2
+
À
+
À
{
(H TPP )Á(TB )ÁO } + Fc - {(H TPP )Á(TB )}
4
2
3
+
Fc + HO
ꢀ
+
2
(5)
2
+
À
and the regeneration of {(H
4
TPP )Á(TB )
2
} through the acid
association, eqn (3). The spontaneous reduction of the highly
ꢀ
reactive HO
2
with Fc is expected to yield H
2
O
2
as the first
The kinetic model predicts a
non-linear dependence of the reduction rate, v , on the
8
,10
stable reduction product.
Fig. 2 Colour change (a) and absorption spectrum (b) of the reaction
À4
0
À5
analytical concentration of Fc (eqn (S5) and eqn (S6), ESIw)
and a linear dependence of v on the analytical concentration
mixture containing 5 Â 10 M H
2
TPP and 1.5 Â 10 M HTB in the
air-saturated DCE solution prior (1), and 1.5 min (2), 60 min (3) and
0
200 min (4) after the addition of 10 mM Fc.
of H TPP (eqn (S8), ESIw).
2
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 5446–5448 5447

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À
6À
À
TB { PF o Cl , indicating that small anions are likely to
block the reaction site for dioxygen.
To summarize the results, we have shown that the catalytic
effect of tetraphenylporphyrin on the reduction of molecular
oxygen can be finely tuned by the acid-to-catalyst molar ratio.
The catalytic mechanism involves the binding of O2 to the
diprotonated form of tetraphenylporphyrin, in competition
À
with the counteranion present. We conclude that TB cannot
be classified as a non-coordinating anion, on the contrary
8
,10
to the previous assumption.
2
The activation of O by
coordination to the acidic moiety of a metal-free porphyrin
1
5
or the imidazolium ring appears to be an alternative catalytic
route to the redox catalysis of the O reduction by metal
2
porphyrins. This investigation is relevant to studies of the
2
+
À
Fig. 4 DFT/M05-2x optimized structure of {(H
4
TPP )Á(TB )ÁO
2
}
non-metal sites binding O in biological enzyme-catalyzed
2
16,17
˚
system; the averaged O–H distances were calculated to be 2.338 A.
oxidations.
Mechanistic considerations are supported by
the DFT calculations.
À
Table 1 DFT stabilization energies (eV) of X or O
(H
2
in the complexes
We are grateful to Grant Agency of the Czech Republic
2
+
À
2+
À
{
4
TPP )Á(X )
2
} or {(H
4
TPP )Á(X )ÁO
2
}, respectively, with PCM
solvent correction. BSSE corrected values in vacuum are given
in parentheses
(
grant no. P208/11/0697), EPFL and European COST Action
for financial support.
À
À
6À
À
TB
Notes and references
Complex/X
Cl
PF
TPP )Á(XÀ)
2
+
1 J. T. Groves, K. Shalyaev and J. Lee, in The Porphyrin Handbook,
ed. K. M. Kadish, K. M. Smith and R. Guilard, Academic Press,
San Diego, CA, 2000, vol. 4, pp. 17–40.
{
(H
(H
4
4
2
}
0.971
0.914
0.759
(
4.222)
0.112
0.099)
(3.489)
0.101
(0.096)
(2.514)
0.118
(0.110)
2
+
À
{
TPP )Á(X )ÁO
2
}
2
(a) S. Fukuzumi, S. Mochizuki and T. Tanaka, Inorg. Chem., 1989,
8, 2459; (b) S. Fukuzumi, K. Okamoto, C. P. Gros and
R. Guilard, J. Am. Chem. Soc., 2004, 126, 10441.
(
2
The binding and activation of
À
O
2
in the complex
3
(a) I. Hatay, B. Su, F. Li, M. Mendez, T. Khoury, C. P. Gros,
J. M. Barbe, M. Ersoz, Z. Samec and H. H. Girault, J. Am. Chem.
Soc., 2009, 131, 13453; (b) B. Su, I. Hatay, A. Trojanek, Z. Samec,
T. Khoury, C. P. Gros, J. M. Barbe, A. Daina, P.-A. Carrupt and
H. H. Girault, J. Am. Chem. Soc., 2010, 132, 2655; (c) R.
Partovi-Nia, B. Su, F. Li, C. P. Gros, J. M. Barbe, Z. Samec and
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2
+
{
(H
4
TPP )Á(X )ÁO
2
}, and the effects of ion pairing with the
À
À
À
À
counteranion X = Cl , PF or TB , as well as the solvent
6
À
effect on the stabilization energy of O and of X were treated
2
by the quantum chemical DFT method (ESIw). The optimized
2
+
À
structure of {(H TPP )Á(TB )ÁO } is depicted in Fig. 4;
4
2
À
À
6
. The
4 (a) E. Song, C. Shi and F. C. Anson, Langmuir, 1998, 14, 4315;
b) C. Shi and F. C. Anson, Inorg. Chem., 1998, 37, 1037.
analogous structures were found for Cl and PF
stabilization energies were calculated at the M05-2X/
-311++G** level for the M05-2X/6-31G* optimized
geometries, following the procedure that has recently been
(
5
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6
1
4
6 J. P. Collman, P. S. Wagenknecht and J. E. Hutchison, Angew.
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used to evaluate weak interactions in the Lewis pairs. Table 1
demonstrates that the stabilization energies of O (i.e., the
2
2
+
À
work required to extract O
2
from {(H
4
TPP )Á(X )ÁO
2
}) are
somewhat lower than those calculated for the end-on O
2
7
adduct with Co porphyrin (0.28 eV), which is known to
2
catalyze the oxygen reduction to H O and/or H O. It is
2
2
2
+
2
À
worth noticing that the binding of O in {(H TPP )Á(TB )Á
2
4
9
´
A. Trojanek, J. Langmaier, B. Su, H. H. Girault and Z. Samec,
Electrochem. Commun., 2009, 11, 1940.
2
O } polarizes the O–O bond; the Mulliken charge at the O
atom attached to H is À0.091, the remote O atom carries the
1
0 (a) G. De Luca, A. Romeo, L. M. Scolaro, G. Ricciardi and
A. Rosa, Inorg. Chem., 2007, 46, 5979; (b) R. Karaman and
T. C. Bruice, Inorg. Chem., 1992, 31, 2455.
charge of +0.251. The electron delocalization should facilitate
the activation of O
5
porphyrin. As expected, the stabilization energy of X (i.e.,
2
, similar to the complex with a metal
À
¨
11 Y. Zhang, M. X. Li, M. Y. Lu, R. H. Yang, F. Liu and K. A. Li,
J. Phys. Chem. A, 2005, 109, 7442.
À
2+
À
the work required to extract X from {(H
4
TPP )Á(X )
2
})
1
2 B. Su, F. Li, R. Partovi-Nia, C. Gros, J.-M. Barbe, Z. Samec and
H. H. Girault, Chem. Commun., 2008, 5037.
decreases considerably with increasing size of the counter-
anion. The data in Table 1 also show that the inclusion of the
solvent effect substantially diminishes the stabilization
energies of the counteranions, while the stabilization
1
1
3 P. Luhring and A. Schumpe, J. Chem. Eng., 1989, 34, 250.
4 G. Er o+ s, H. Mehdi, I. Pa
pai, T. A. Rokob, P. Kiraly, G. Tarkanyi
´ ´ ´ ´
and T. Soos, Angew. Chem., Int. Ed., 2010, 49, 6559.
´
15 D. S. Choi, D. H. Kim, U. S. Shin, R. R. Deshmukh, S. Lee and
C. E. Song, Chem. Commun., 2007, 3467.
16 P. F. Widboom, E. N. Fielding, Y. Liu and S. D. Bruner, Nature,
energy for O2 varies only slightly. The difference between
À
the stabilization energies of X and O
2
points to a much
2007, 447, 342.
7 Y. Goto and J. P. Klinman, Biochemistry, 2002, 41, 13637.
stronger bond of the counteranion, which follows the order,
1
5
448 Chem. Commun., 2011, 47, 5446–5448
This journal is c The Royal Society of Chemistry 2011