Tuning the thermodynamic onset potential of electrocatalytic O2 reduction reaction by synthetic iron-porphyrin complexes

Article abstract of DOI:10.1039/c5cc01938a

A porphyrin ligand with two β-pyrrolic electron withdrawing ester groups is synthesized and its Co complex is crystallographically characterized. The iron complex of this porphyrin ligand shows an ～200 mV positive shift in its Fe^III/II potential in organic as well as aqueous solvents and in the onset potential of ORR relative to that of an unsubstituted porphyrin.

Full text of DOI:10.1039/c5cc01938a

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ARTICLE TYPE
Tuning the Thermodynamic Onset Potential of Electrocatalytic O Reduction Reaction by Synthetic Iron-
2
Porphyrin Complexes
Sk Amanullah, Pradip Kumar Das, Subhra Samanta and Abhishek Dey*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A porphyrin ligand with two β pyrrolic electron withdrawing ester differences between heme a and heme a it is only logical to
3
explore the effect of electron withdrawing substituents on the
porphyrin ring. In a recent report, the native heme cofactor of
III/II myoglobin has been replaced with diacetyl and diformyl heme
group subsititution is synthesized and its Co complex is
crystallographically characterized. The Iron complex of this
porphyrin ligand shows an ~200 mV positive shift in its Fe
potential in organic as well as aqueous solvents and in the onset
potential of ORR relative to that of an unsubstituted porphyrin.
5
5
1
0
III/II
groups and the corresponding Fe
potential was found to
9
increase by 100 mV and 200 mV, respectively. Similarly the role
of halogenating the eight β-pyrrolic positions of cobalt porphyrins
60 and corroles have been explored and a 150-300 mV positive shift
a
Address, Department of Chemistry, Indian Association for the Cultivation
2
5-27
in formal potentials have been reported.
Thus investigation of
of Science, India, Fax: +91-33-2473 2805; Tel:+91-33-2473 4971; E-
mail: icad@iacs.res.in
1
5
the effect of introducing electron withdrawing acetyl/formyl
III/II
0
Electronic Supplementary Information (ESI) available: Synthetic details, groups on formal Fe
E
and potential of ORR by iron
characterization and Resonance Raman (rR).
porphyrins is warranted.
Oxygen reduction reaction (ORR) is important in biological
65
1
-3
respiration as well as in energy conversion.
In biology
-
2
0
cytochrome c oxidase (CcO), a heme enzyme, reduces O by 4e
4H to H O in the respiratory complex IV in higher animals.
2
2
+
4, 5
/
The active site of CcO utilizes a heme a cofactor (Fig. 1A) at the
3
oxygen binding site and a heme a (Fig. 1B) cofactor in the 70
6
adjacent electron transfer site. Apart from CcO, heme based
2
3
3
4
4
5
5
0
5
0
5
0
cofactors are found in O2 storage and transport proteins like
7
hemoglobin and myoglobin. These protein active sites use hemin
7
as their cofactor (Fig. 1C) to bind O instead of heme a in CcO.
The E for the Fe
2
3
0
III/II
process in a heme based Hb or Mb active site 7
5
0
5
Fig. 1 (A) Heme a cofactor of CcO, bonds highlighted with red
colour indicate the difference from Heme a; (B) Heme a
present in the catalytic site of bovine CcO; (C) Hemin
3
is around -100 mV whereas the same process occurs at +350 mV
8
0
in the active site of CcO. The E of CcO is > 450 mV higher than
Hb/Mb and is much closer to the thermodynamic potential of ORR
9
at pH 7; 820 mV. Hence factors contributing to this large increase
III/II
0
in the Fe
E which allows heme a to operate efficiently in
3
8
mitochondria are important to understand. One of the ostentatious
factors is the difference in the substituents on the pyrrole rings of
6
, 10
heme a and heme a3.
Heme a3 has one of the methyl
substituents at the C18 position of heme oxidized to an aldehyde
group and one of the vinyl group at C modified to include the
3
8
hydroxyfarnesyl group i.e. heme a has two additional electron
3
6
withdrawing group relative to heme a.
Over the last three decades several synthetic Fe-porphyrins and
their more elaborate analogues are known to exhibit ORR under
7
, 11-21
physiological conditions.
Elaborate substitutions on the meso
phenyl rings have been exploited to introduce additional 90
functionalities e.g. distal Cu site, additional redox sites, hydrogen
B
2
2,23
bonding groups etc.
On the other hand these synthetic Fig. 2 Molecular structure of the Co-PP-diester complex. C
analogues have inevitably used unsubstitutedpyrroles or
(
gray), Co (pink), N (blue), O (red). H atoms are omitted for
2
4
halogenated pyrroles for the synthesis of the porphyrin ligand.
clarity. Thermal ellipsoid plots are drawn at 30% probability
level. (CCDC number: 1051391).
These iron porphyrin complexes exhibit electrocatalytic ORR with 95
7
over potentials greater than 800 mV. The high overpotential of
A porphyrin ligand is synthesized containing two carboxylate
ester group attached to one of the pyrrole rings of the macrocycle
ORR has been a deterrent towards using these otherwise efficient
electrocatalysts for ORR. However considering the structural
28
by modifying an existing protocol. A 2,5-dimethylpyrrole ring
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bearing two ester group was synthesized via a condensation consistent with the observed difference in the organic solvent. In
reaction of a 1,4-diketo precursor (scheme 1) and methanolic NH3. air-saturated pH 7 buffer, the reversible couple disappears and is
The methyl groups are oxidized to aldehyde with ceric ammonium 50 replaced by a large irreversible reduction current representing
nitrate. Upon condensation of the 2,5-diformyl pyrrole and electrocatalytic ORR (Fig. 4). The onset potential of this
III
II
5
tripyrrane utilizing a high dilution technique under anaerobic electrocatalytic current coincides with the reduction of Fe to Fe
II
condition affords the desired porphyrin ligand. Subsequent Fe and implying that the reduced Fe species is responsible for O2
Co metallation were done using a non-coordinating weak base activation. The thermodynamic onset potential of ORR is defined
2
,4,6-collidine. The details and the characterization of the 55 where the electrocatalytic current rises. The 170 mV shift between
complexes are described in the supporting information (S4-S28).
the E1/2 of Fe-PP-diester and Fe-TPP is appropriately reflected in
the ORR onset potential (Fig. 4 red to blue). The rotating ring disc
electrochemistry (RDE) data of Fe-PP-diester (Fig. 5) indicate that
a normal substrate diffusion limited current is observed below -0.2
1
0
1
/2
60
V.The K-L plot of 1/Icat vs 1/ω indicates that the experimental
-
-
data matches the theoretical plot for a 4e process and not a 2e
process (Fig. 5, inset).
2
9
1
5
2
0
Scheme 1 The synthetic scheme of the desired complexes.
The Co-PP-diester complex crystallizes from CH Cl /Hexane
2
2
solvent with a P2(1)/n point group (Fig. 2). The complex possesses
a puckered square planar geometry. The neutral compound has Co
in its +2 valance state with Co-N average bond length 1.964Å. The
crystal of the Fe- complex could not be obtained.The rR spectra of
2
5
the complex shows the oxidation and spin state marker bands, ν
4
-
1
and ν , are at 1365 and 1550 cm , respectively, which is typical of
2
3
0
S=5/2 Fe(III) porphyrins (S30).The cyclic voltammogram of the
Fig. 4 CV of the catalyst Fe-TPP and Fe-PP-diester on the EPG
surface in pH 7 buffer, using 100 mM KPF6 as supporting
electrolyte and Pt and Ag/AgCl as counter and reference electrode
respectively, in aerobic condition. Blue, red and black lines
represent Fe-TPP, Fe-PP-diester and the background current
Fe-PP-diester shows a quasi-reversible wave with E -0.49 V with 65
1
/2
+
respect to the Fc /Fc in the presence of excess amount of imidazole
Fig. 3). The reduction potential of Fe-TPP maintaining the same
condition is -0.66 V. Therefore, the Fe-PP-diester shows 170 mV
(
III/II
0
3
5
more positive shift in its Fe E , relative to the Fe-TPP,
indicating the former is easily reduced due to the effect of electron 7
withdrawing groups.
respectively. (Inset) CV of the catalyst in N atmosphere at 50
2
0
mV/s.
7
8
8
5
0
5
Fig. 5 LSV of the catalyst deposited on EPG surface at multiple
rotations in pH 7 buffer, using 100 mM KPF as the supporting
6
Fig. 3 CV of the Fe-TPP and Fe-PP-diester complexes in MeOH
solvent (containing excess amount of imidazole) having 100 mM
TBAP as the supporting electrolyte, glassy carbon as the working
electrolyte. (Inset) K-L plot of the catalyst (red line). The dashed
4
0
-
-
lines indicate the theoretical plots for 4e (blue) and 2e (green)
transfers respectively.
+
electrode and Fc /Fc as the internal standard.
90
The O reduction for this catalyst is investigated over a wide
2
In the absence of O at pH 7, the redox potential of the Fe-PP-
2
III/II
range of pH (3-10). The linear sweep voltammetry (LSV) shows a
gradual shift of the E (ORR) towards positive values as the pH is
lowered where E (ORR) represents the potential at which the
electrocatalytic current of the oxygen reduction reaction (ORR)
achieves half maximum (Fig. 6A). A plot of E (ORR) with pH
diester physiadsorbed on EPG surface shows a Fe
process at -
4
5
0.15 V vs Ag/AgCl i.e. 0.05 V vs NHE (Fig. 4, inset, red). The CV
of Fe-TPP physiadsorbed on EPG surfaces in degassed pH 7 buffer
III/II
shows a Fe
process at -0.32 V vs Ag/AgCl (Fig. 4,inset, blue), 9
5
2
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show a ~ 52±2 mV/pH unit change between pH 4-10 (Fig. 6B) Yamaguchi, K. Shinzawa-Itoh, R. Nakashima, R. Yaono, S.
indicating the involvement of a single proton coupled electron
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Yoshikawa, Science, 1995, 269, 1069-1074.
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. J. P. Collman, R. Boulatov, C. J. Sunderland, L. Fu, Chem. Rev.
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8. P. L. Dutton, D. F. Wilson, C.-P. Lee, Biochemistry, 1970, 9,
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1.S. Chatterjee, K. Sengupta, S. Samanta, P. K. Das and A. Dey,
Inorg. Chem., 2013, 52, 9897-9907.
2.S. Samanta, K. Sengupta, K. Mittra, S. Bandyopadhyay A. Dey,
1
Fig. 6 A) LSV of the catalyst deposited on EPG surface at different
Chem. Commun , 2012, 48, 7631-7633.
pH. B) Catalytic half peak potential (E) vs. pH fitted with a line 7
13. S. Samanta, P. K. Das, S. Chatterjee, K. Sengupta, B. Mondal, A.
Dey, Inorg. Chem., 2013, 52, 12963-12971.
having a slope of 52 mV/pH.
In rotating ring disc electrochemistry (RRDE) partially reduced
oxygen species (PROS) produced on the working electrode is
detected on a Pt ring electrode, which is held at a potential (0.7 V)
1
4. K. Sengupta, S. Chatterjee, S. Samanta A. Dey, Proc. Natl. Acad.
Sci., 2013, 110, 8431-8436.
where it can oxidize the PROS produced from O2 and this 15. E. Kim, E. E. Chufán, K. Kamaraj, K. D. Karlin, Chem. Rev.
oxidation current can be used for the quantitative determination of 80 2004, 104, 1077-1134.
the amount of PROS generated during O reduction. Fe-PP-diester 16.C. T. Carver, B. D. Matson, J. M. Mayer, J. Am. Chem. Soc., 2012,
produces about 1±0.5% PROS at pH 7 when physiadsorbed on
EPG (S31). Note that it is unlikely that such high selectivity for
ORR can be obtained by an iron porphyrin complex having only
one iron. It is likely that deposition of multilayers of this complex 85
on the graphite surface allows facile bimetallic catalysis as is
known to be the case for porphyrin and corroles lacking distal
2
3
4, 5444-5447.
1
7. J. P. Collman, N. K. Devaraj, R. A. Decréau, Y. Yang, Y.-L.
Yan, W. Ebina, T. A. Eberspacher and C. E. D. Chidsey, Science,
2007, 315, 1565-1568.
1
8. Z. Halime, H. Kotani, Y. Li, S. Fukuzumi, K. D. Karlin, Proc
3
0-32
Natl. Acad. Sci., 108, 13990-13994.
super-structures which inhibit dimer formation.
1
9.E. E. Chufãn, S. C. Puiu, K. D. Karlin, Acc. Chem. Res., 2007, 40,
In summary, inclusion of two electron withdrawing ester groups
in the β-pyrrolic position of tetraphenylporphyrin, similar to the
5
63-572.
electron withdrawing formyl groups present in the β-pyrrolic 90 20. B. Sun, Z. Ou, D. Meng, Y. Fang, Y. Song, W. Zhu, P. V.Solntsev,
position of heme a , induces a ~200 mV positive shift in the formal V. N. Nemykin, K. M. Kadish, Inorg. Chem., 53, 8600-8609.
3
III/II
Fe
potential in both organic and aqueous medium. The onset 21. K. Mittra, S. Chatterjee, S. Samanta and A. Dey, Inorg. Chem., 52,
potential of ORR is also found to shift by ~200 mV relative to an
unsubstituted porphyrin. These data indicate that 1) the two ester
1
2
4317-14325.
2. J. P. Collman, R. A. Decreau, Chem. Commun., 2008, 5065-5076.
III/II
0
substituents on heme a can shift the formal Fe E by ~200 mV 95
relative to heme and 2) substituting the β-pyrrolic positions with
additional electron withdrawing ester/formyl groups may be
explored to reduce the significant overpotential involved in
electrocatalytic ORR by, otherwise efficient (stable and fast), iron
23. S. Samanta, K. Mittra, K. Sengupta, S. Chatterjee and A. Dey,
Inorg. Chem., 2013, 52, 1443-1453.
2
4. M. W. Grinstaff, M. G. Hill, E. R. Birnbaum, W. P. Schaefer, J. A.
Labinger, H. B. Gray, Inorg. Chem., 1995, 34, 4896-4902.
5.a)A. Mahammed, B. Mondal, A. Rana, A. Dey, Z. Gross
2
porphyrin electrocatalysts.
100 Chem. Commun., 2014, 50, 2725,b)A Schechter, M tanevsky, A.
Acknowledgement
Mahammed, Z Gross, Inorg.chem., 2011,51, 22-24 c)E. Steene, A.
The work is funded by DST (SB/S1/IC-25-2013). S.A. and P.K.D. Dey, A. Ghosh, J. Am. Chem. Soc., 2003, 125,6300-16309.
acknowledge CSIR-JRF and SRF fellowship respectively. S.S 26. J. P. Collman, M. Kaplun and R. A. Decreau, Dalton Trans., 2006,
acknowledges the Integrated Ph.D. Program of IACS.
5
54-559.
1
1
1
05 27. a) K. M. Kadish, L. Frãmond, Z. Ou, J. Shao, C. Shi, F. C. Anson,
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