A single iron porphyrin shows ph dependent switch between "push" and "pull" effects in electrochemical oxygen reduction

Article abstract of DOI:10.1021/acs.inorgchem.0c02408

The "push-pull"effects associated with heme enzymes manifest themselves through highly evolved distal amino acid environments and axial ligands to the heme. These conserved residues enhance their reactivities by orders of magnitude relative to small molecules that mimic the primary coordination. An instance of a mononuclear iron porphyrin with covalently attached pendent phenanthroline groups is reported which exhibit reactivity indicating a pH dependent "push"to "pull"transition in the same molecule. The pendant phenanthroline residues provide proton transfer pathways into the iron site, ensuring selective 4e-/4H+ reduction of O2 to water. The protonation of these residues at lower pH mimics the pull effect of peroxidases, and a coordination of an axial hydroxide ligand at high pH emulates the push effect of P450 monooxygenases. Both effects enhance the rate of O2 reduction by orders of magnitude over its value at neutral pH while maintaining exclusive selectivity for 4e-/4H+ oxygen reduction reaction.

Full text of DOI:10.1021/acs.inorgchem.0c02408

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A Single Iron Porphyrin Shows pH Dependent Switch between
“Push” and “Pull” Effects in Electrochemical Oxygen Reduction
Sudipta Mukherjee,† Abhijit Nayek,† Sarmistha Bhunia, Somdatta Ghosh Dey,* and Abhishek Dey*
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ABSTRACT: The “push−pull” effects associated with heme enzymes
manifest themselves through highly evolved distal amino acid
environments and axial ligands to the heme. These conserved residues
enhance their reactivities by orders of magnitude relative to small
molecules that mimic the primary coordination. An instance of a
mononuclear iron porphyrin with covalently attached pendent
phenanthroline groups is reported which exhibit reactivity indicating a
pH dependent “push” to “pull” transition in the same molecule. The
pendant phenanthroline residues provide proton transfer pathways into
the iron site, ensuring selective 4e−/4H+ reduction of O2 to water. The
protonation of these residues at lower pH mimics the pull effect of
peroxidases, and a coordination of an axial hydroxide ligand at high pH
emulates the push effect of P450 monooxygenases. Both effects enhance the rate of O2 reduction by orders of magnitude over its
value at neutral pH while maintaining exclusive selectivity for 4e−/4H+ oxygen reduction reaction.
INTRODUCTION
■
Heme-based metalloenzymes are ubiquitous in nature, and
these enzymes catalyze biologically important chemical
transformations.1−10 Heme proteins like hemoglobin (Hb)
and myoglobin (Mb) bind O2 reversibly and acts as O2 storage
and transport proteins.3,11−15 In all aerobic organisms, O2 is
reduced to H2O by cytochrome c oxidases (CcO).16−23
Cytochrome P450 utilizes O2 and catalyzes hydroxylation,
epoxidation, and oxo atom transfer of a wide variety to
substrates9,10,24−30 and is involved in catabolism and hormone
Figure 1. Active site structures of (A) peroxidase (PDB ID: 1ATJ),39
(B) Cytochrome P450 (PDB ID: 1AKD).40 Color codes: carbon→
off white, nitrogen→ blue, oxygen→ red, iron→ brown and sulfur→
yellow. Hydrogen atoms are omitted in the crystal structures for
clarity.
biosynthesis,31 while peroxidases (e.g. horseradish peroxidase,
cytochrome c peroxidase) catalyze the oxidation of organic
compounds using H2O2 as an oxidant4,32−35 and are involved
in immune response. Their diverse reactivity is attributed to
the different heme-binding axial ligand and distal resi-
dues.4,36−38 Peroxidases, for example, have highly conserved
histidine and arginine residues in their distal pocket along with
an axial histidine ligand (Figure 1A),39 while cytochrome P450
uses thiolate ligation originating from a cysteine residue and
highly conserved threonine residue in the distal pocket (Figure
1B).40 A common step within the catalytic cycle of these
oxidases and monooxygenases is the O−O bond heterolysis of
a low spin ferric hydroperoxo intermediate species (FeIII-
OOH, “compound 0”)41 to get a high valent reactive metal-oxo
intermediate called “compound I”, which is best described as
FeIVO (ferryl) species bound to a one-electron oxidized
porphyrin cation radical (formally two-electron oxidized
relative to the resting ferric state).9,10,21,24−27,29,31,32,42−49
The thiolate side chain of the cysteine residue coordinates to
the Fe atom of the heme cofactor in cytochrome P450.44 This
ends up in an anionic compound 0 species in cytochrome
P450, which is basic, and the protonation at the distal oxygen
atom of the bound hydroperoxide species occurs via a
prearranged proton transfer pathway involving the threonine
residue.50 The electron density on the thiolate (or any other
anionic trans axial ligand) is crucial for the enhancement of the
Received: August 12, 2020
https://dx.doi.org/10.1021/acs.inorgchem.0c02408
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Figure 2. (A) Cobalt(II) hangman porphyrin, (B) FeL2, (C) Fe-MARG, (D) α4-FeFC4 (Fc: Ferrocene), and (E) Fe-bisPhen (this paper).
a
Scheme 1. Synthetic Scheme of the Fe-bisPhen
a
The reagents are indicated above the arrows, and the spectroscopic data of the intermediates are included in the Supporting Information.
pKa of the bound hydroperoxide, and the phenomenon is well-
known as the “push” effect.4,29,38,44,51−54 Alternatively,
imidazole functional group of histidine coordinates with
heme in HRP to produce a neutral compound 0.The histidine
and protonated arginine residue in the distal site exert a “pull”
effect,55,56 which facilitate the heterolytic O−O bond cleavage
of the FeIII-OOH (compound 0).48,57−60The H-bonding
interactions between active site ligands and the second sphere
residues play a vital role in regulating the functions of these
heme metalloenzymes. Recent investigations with synthetic
B
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Figure 3. (A) Molecular structure of Zn-bisPhen (3b). C (gray), N (blue), O (red), Zn (violet). H atoms and solvent molecules are omitted for
clarity. Two coordinated water molecules (W1 and W2) in the crystal are indicated by black circular rings. (B) The water channel (red balls) in
HRP (PDB ID: 1SCH)78 with the key arginine residue is shown, and hydrogen bonds are indicated by yellow lines. (C) Schematic representation
of the pull effect (left) in peroxidase and push effect (right) in cytochrome P450 enzymes.
iron porphyrins have revealed that the strong H-bonding
interaction to the distal oxygen atom from the protonated
distal residues polarizes the O−O bond, pulling its O−O σ*
down in energy. This allows better backbonding into the O−O
σ* from the low spin ferric center weakening the O−O bond
for further cleavage.61,62 The presence of basic residues in the
distal site and nature of axial ligation play a vital role in
regulating the pull and push mechanism, respectively.4,51
The spatial control of the hydrogen bonding and proton
delivery to the proper site is key for the generation of the high
valent compound I intermediate from compound 0. In this
spirit, the effect of H-bonding during oxygen reduction
reaction (ORR) has been explored in artificial synthetic
analogues. For example, a series of cobalt(II) hangman
porphyrins having pendant carboxylic acid residues in the
distal superstructure (Figure 2A) are found to assist the
selective reduction of O2 to H2O in acidic water medium.63
Iron porphyrins with tetratriazole residues attached to the
redox active ferrocene groups (α4-FeFc4, Figure 2D) can
selectively reduce O2 to H2O in aqueous media across a wide
pH range.64,65 The hydrogen bonding interaction favors O2
binding and reduction from the side with these hydrogen
bonding interactions.64,66−68 Recently, a series of mononuclear
iron porphyrins with distal basic residues ([FeL2, Figure 2B]69
and [Fe-MARG, Figure 2C]70), physisorbed on edge-plane
graphite (EPG) electrode, have been shown to selectively
reduce O2 to H2O at pH 7. The protonated pendant basic
residues in these iron porphyrins stabilize the FeIII-OOH
species by strong H-bonding interactions and facilitate the O−
O bond cleavage via a pull effect, promoting peroxidase activity
as well as selective 4e−/4H+ reduction in ORR.62,69−71
In this manuscript, we report a mononuclear iron porphyrin
complex containing two phenanthroline moieties above the
porphyrin plane (Figure 2E). The two distal residues in HRP is
the inspiration behind the installation of these phenanthroline
residues and its performance as an ORR electrocatalyst is
investigated. The ORR data reveal that the complex selectively
reduces O2 to water over a wide range of pH when adsorbed
on EPG electrode. The complex exhibits push effect similar to
P450 due to the axial hydroxide ligation to the metal at higher
pH. At lower pH, the protonated phenanthroline residues exert
pull effect like peroxidases. The rate of O2 reduction can be
tuned up by several orders of magnitude by taking advantage of
the pH dependent “push−pull” effect in a single mononuclear
iron porphyrin.
RESULTS
■
Synthesis and Characterization. The α,α-atropisomer of
5,10-di(ortho-aminophenyl)-15,20-diphenylporphyrin (DAPP,
Scheme 1, [2]) is utilized as starting material for the synthesis.
It has been synthesized and purified as reported previously.72
1
The H NMR of the ligand (Supporting Information, spectral
data 6[i]) indicate that it is isolated as the pure α,α-
atropisomer of 5,10-di(ortho-aminophenyl)-15,20-diphenyl-
porphyrin. The metal free ligand (bisPhen, Scheme 1,[3]) is
synthesized by condensation between DAPP (Scheme 1, [2])
and 1,10-phenanthroline-2-carbonyl chloride73 (Scheme 1,
C
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Figure 4. (A) CV (under anaerobic conditions) for Fe(III/II) couple of Fe-bisPhen catalyst at 100 mV/s scan rate at pH 7 phosphate buffer, using
100 mM KPF6 as the supporting electrolyte and using Ag/AgCl (satd. KCl) reference and Pt wire as counter electrode on edge plane graphite
surface. (B) RRDE data of Fe-bisPhen immobilized on EPG at pH 7 phosphate buffer, using 100 mM KPF6 as the supporting electrolyte, at 10
mV/s scan rate at 300 rpm rotation rate using a Pt counter electrode and Ag/AgCl (satd. KCl) as reference electrode. (C) LSV of Fe-bisPhen
physisorbed on EPG in air saturated pH 7 phosphate buffer, using 100 mM KPF6 as the supporting electrolyte at a scan rate of 100 mV/s with
multiple rotations using Ag/AgCl (satd. KCl) reference and Pt wire as counter electrode.(D) K−L plot of Fe-bisPhen catalyst (red bold line) at a
potential of −400 mV is given. and the theoretical plots for 2e− (green dashed line) and 4e− processes (black dashed line) are indicated in the
figure.
[1e]). Finally, the free ligand is metalated with iron (ferrous
bromide) or zinc acetate using standard protocols.74,75 All the
synthetic protocols for the steps depicted above (Scheme 1)
are described in details in the experimental section. These free
ligands and their Fe (Fe-bisPhen, Scheme 1, [3a]) and Zn
(Zn-bisPhen, Scheme 1, [3b]) complexes are characterized
using ESI-MS, 1H NMR, UV−visible absorption spectroscopy,
cyclic voltammetry, and single crystal X-ray diffraction
techniques (Supporting Information, Figures S1−S3 and
spectral data 6[i−vi]). Single crystals of Zn-bisPhen (dark
purple color) are obtained by diffusion of diethyl ether in a
chloroform solution of the complex. The solid-state structure
of the Zn-metalated complex is obtained using single crystal X-
ray diffraction. The complex Zn-bisPhen (Scheme 1, [3b])
N(phen)O(W1) distances are ∼2.83 and ∼2.73 Å,
indicating strong hydrogen bonding with the water molecules.
Thus, the phenanthroline residues help to establish a water
channel into the active site, which is a unique attribute of this
iron porphyrin complex and emulates the architecture of HRP
(Figure 3B). All the parameters of the crystal structure are
tabulated in the Supporting Information (Table S1).
Heterogeneous Oxygen Reduction Reaction in
Aqueous Medium. The Fe-bisPhen (Schemes 1 and 3a)
complex was physisorbed on the edge-plane graphite (EPG)
electrode, and their electrochemical response was investigated
under heterogeneous conditions at neutral pH 7 buffer. In dry
degassed pH 7 buffer, Fe-bisPhen shows electrochemically
reversible FeIII/II redox process at −222 mV vs Ag/AgCl (satd.
KCl) (Figure 4A). By integrating this redox wave, the surface
coverage of Fe-bisPhen is found to be (8.21 0.15) × 10−11
̅
crystallizes with a P1 space group (Figure 3A). The central Zn
atom is in a square pyramidal geometry with an axial water
ligand. The Zn−N distances are (2.06 0.01) Å, which are in
the range observed for other Zn(II) porphyrinates.76,77 The
metal (Zn) coordinated water molecule (W1) is directly
hydrogen bonded to one of the nitrogen atom of the pendant
phenanthroline groups, and the second water molecule (W2)
forms hydrogen bond with the other nitrogen atom of the
phenanthroline moiety and the metal(Zn) coordinated water
molecule (W1) (Figure 3A). The N(phen)O(W2) and
mol/cm2 The Fe-bisPhen complex shows electrocatalytic O2
.
reduction in air saturated pH 7 phosphate buffer solution,
which is further examined using a rotating disk electro-
chemistry (RDE) technique. Linear sweep voltammetry (LSV)
of the catalyst at different rotation rates shows a substrate
diffusion limited electrocatalytic O2 reduction current (Figure
4C) with an onset potential that overlays the reduction process
of FeIII to FeII (satd. KCl) (Figure 4A). The LSV current
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Figure 5. (A) Overlay of CV (under anaerobic conditions) for Fe(III/II) couple at 100 mV/s on edge plane graphite surface in pH 5, 6, 7, 8, 9, 10,
and 11 phosphate buffers, using 100 mM KPF6 as the supporting electrolyte and using Ag/AgCl (satd. KCl) reference and Pt wire as counter
electrode. Anaerobic CV data for additional pH values are shown in the Supporting Information (Figure S4). (B) The plot of FeIII/II potential
(green dots) with pH and its simulation (blue line) with pKa1 = 5.5, pKa2 = 8.0, and Em = −0.215 mV.
increases with increasing rotation rate of the electrode. The
kinetics of O2 reduction was determined with RDE experiment
using Koutecky−Levich (K−L) analysis79 following the
equation below:
the extent of PROS generated in situ and can be used to
measure product selectivity, e.g., the Fe-bisPhen on EPG is
found to produce (5.01 0.15) % PROS in pH 7 phosphate
buffer, suggesting a 95% selectivity for 4e−/4H+ ORR (Figure
4B), consistent with the RDE results (Figure 4C).
pH Dependence ORR. The Fe-bisPhen complex is
physisorbed on edge-plane graphite (EPG) electrode and
analyzed under aqueous conditions at various pH from 4 to 11.
In the presence of 1 atm N2, Fe-bisPhen displays electro-
chemically reversible FeIII/II redox process at all these values
(Figure 5A and Figure S4). The pH dependence shows
cathodic shift of ∼140 mV from pH 6 to 9. The pH
dependence is consistent with a FeIII−OH + e− + H+ → FeII−
OH2 proton coupled electron transfer (PCET) transition of
the Fe-bisPhen complex (Figure 5B, blue line).
In the corresponding aerated buffer solutions, the complex
shows electrocatalytic O2 reduction. LSV of the catalyst
indicates that mass transfer limited catalytic O2 reduction
current is achieved at all those negative potentials. Both RDE
and RRDE experiments (Figure 6 and Figure 7) are performed
for the determination of kinetics and selectivity of ORR at
different pH’s. Across the entire pH range evaluated, this
catalyst shows >95% selectivity for the complete four electron
reduction of oxygen to water on EPG electrode in aqueous
buffer medium (Table 1), as indicated by the slope of the I−1
vs ω−1/2. The rates obtained at these values for selective 4e−/
4H+ ORR show interesting variations. The ORR rate constant
increases from 1.85 0.02 × 105 M−1 s−1 at pH 7 to 123 11
× 105 M−1 s−1, i.e. ∼100 times higher, at pH 9 (Table 1).
Below pH 6, the rate again increases up to ∼40 times to ∼60 ×
105 M−1 s−1 (Table 1) by lowering the pH down to 4. Such U-
shaped rate profile across this pH range (Figure 8), which is
absent in iron porphyrins with triazole-based hydrogen
bonding distal pocket, indicates two different pH dependent
mechanisms at play and responsible for enhancing the rate of
ORR.65
−1
i−1 = iK(E)−1 + iL
(1)
where iK is the kinetic current expressed as
iK(E) = nFA[O2]kcatΓ
catalyst
(2)
(3)
and iL is the levich current and expressed as
iL = 0.62nFA[O2](DO2)2/3 1/2v−1/6
ω
where n is the number of electrons transferred to the substrate,
F is the Faraday constant, A is the macroscopic area of the disc
(0.096 cm2), [O2] is the concentration of O2 in an air saturated
buffer (0.21 mM) at 25 °C, kcat is the second-order rate
constant of catalytic O2 reduction, Γcatalyst is the catalyst surface
coverage in mol/cm2, DO2 is the diffusion coefficient of O2
(1.96 × 10−5 cm2 s−1) at 25 °C, ω is the angular velocity of the
disc, and v is the kinematic viscosity of the solution (0.009 cm2
s−1) at 25 °C.80
Plot of the inverse of the catalytic current (I−1) at multiple
rotation rates across several potentials in the mass transfer
region with the inverse square root of the angular rotation rate
(ω−1/2) is linear. The second-order rate constant of electro-
catalytic O2 reduction can be calculated from K−L plot using
eq 2. The kcat values obtained using the intercept on the EPG
electrode surface for the Fe-bisPhen complex is (1.85 0.02)
× 105 (Figure 4D, red line). The slope obtained from the
experimental data is almost identical to that predicted for a 4e−
process (Figure 4D, black line) and very different from that
predicted for a 2e− process (Figure 4D, green line). The
selectivity for O2 reduction is better quantified specifically by
monitoring the partially reduced oxygen species (PROS)
formation via rotating ring disc electrochemistry
(RRDE).79,81,82 In an RRDE experiment, any PROS, like
−
DISCUSSIONS
O2 and H2O2, produced at the working electrode reaches at
■
the Pt ring encircling the working electrode due to the
hydrodynamic force created by rotation of the electrode. The
Pt ring is held at a fixed potential (0.7 V at pH 7) such that it
oxidizes the PROS back to O2 and generates an oxidative
current. The ratio of ring to disk current is a direct measure of
During the past several years, investigation of heterogeneous
electrochemical ORR catalyzed by iron porphyrin using in situ
surface enhanced resonance Raman spectroscopy coupled to
rotating disc electrochemistry has been used to investigate the
mechanism of electrochemical ORR catalyzed by different
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Figure 6. LSV data of Fe-bisPhen catalyst physisorbed on EPG surface at a scan rate of 100 mV/s with multiple rotations in pH 5, 6, 8, 9, 10, and
11 phosphate buffers using 100 mM KPF6 as the supporting electrolyte and Ag/AgCl (satd. KCl) reference and Pt wire as counter electrode.
(inset) K−L plot of the Fe-bisPhen catalyst (red bold line). The theoretical plots for 2e− (green dashed line) and 4e− (black dashed line) processes
are indicated in the inset. The extended K−L plot of the Fe-bisPhen catalyst is also provided in the Supporting Information (Figure S6). The RDE
data for additional values are shown in Supporting Information (Figure S5).
synthetic heme and heme/Cu systems (Table 2).83−87 Under
heterogeneous aqueous conditions, irrespective of nature of
axial ligands (H2O in the absence of a covalently attached axial
ligand) and distal metal, this six coordinate (6C) low spin (LS)
FeIII-OOH species gets accumulated during catalysis, indicat-
ing that the rate of O−O bond cleavage of the above species is
the rate-determining step for ORR catalyzed by iron
porphyrins (Table 2).86,87 The selectivity of ORR is
determined from the site of protonation of an intermediate
6C LS FeIII-OOH species formed during oxygen reduction.88 If
protonation occurs at the proximal oxygen, then H2O2 is
released (2e−/2H+ reduction product) (Scheme 2, pathway 2).
On the other hand, protonation at the distal oxygen facilitates
O−O bond cleavage, and H2O is produced (4e−/4H+ O2
reduction product) (Scheme 2, pathway 1).56,89 These results
all suggest that the 6C LS FeIII-OOH species, which have been
observed in almost all cases investigated, is important in
controlling not only the selectivity but also the overall rate of
reaction under these reaction conditions (Scheme
2).69,83,88,90−92 Introduction of H-bonding residues in the
distal pockets of mononuclear iron porphyrin could stabilize
this FeIII-OOH species and direct selective protonation of the
distal oxygen of a FeIII-OOH species favoring facile and
selective 4e−/4H+ oxygen reduction.69 These conclusions were
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Figure 7. RRDE data of Fe-bisPhen catalyst deposited on the EPG surface at a scan rate of 10 mV/s with 300 rpm rotation in pH 5, 6, 8, 9, 10, and
11 phosphate buffers using 100 mM KPF6 as the supporting electrolyte and Ag/AgCl (satd. KCl) reference and Pt wire as counter electrode. The
RRDE data for additional values are shown in Supporting Information (Figure S7).
vetted by investigating the kinetics of O−O bond heterolysis in
organic solvents as well where a covalently attached pendent
distal amine/guanidine residues (Figure 2B, C) could enhance
the rate of heterolytic O−O bond cleavage by several orders of
magnitude relative to a control sample without pendant
groups.62 The pendant phenanthroline used here results in a
water channel which is poised to translocate the proton to the
bound axial -OOH ligand of a putative FeIII-OOH intermediate
from bulk water. This is evident from the hydrogen bonded
water network observed crystallographically in the Zn-bisPhen-
OH2 complex (Figure 3A) between the pendant phenanthro-
line and the bound axial ligand.
The rate of 4e−/4H+ electrochemical ORR by Fe-bisPhen at
pH 7 is (1.85 0.02) × 105 M−1 s−1, which matches those
reported for mononuclear iron porphyrins without push or pull
effects exerted by anionic trans axial ligands and pendant
proton transfer residues, respectively.64,65 Experimentally, the
ORR rate increases by 2 orders of magnitude as the pH of the
buffer solution is increased from 7 to 11 (Figure 8, blue
points). At a higher pH the trans axial H2O ligand of a putative
solvent bound H2OFeIII-OOH species is likely to be
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imidazole and (1.75 0.09) × 106 M−1 s−1 with Cl− as axial
ligands (Table 3). Imidazole and Cl−, both being better axial
ligands to a FeIII-OOH species relative to H2O, affect an order
of magnitude faster ORR relative to H2O. Note that the
determination of rates using K−L analysis can only be
performed for complexes that are sufficiently stable, which
requires them to be selective for 4e−/4H+ ORR across a wide
range of pH values. Thus, simpler porphyrins like FeTPP94
cannot be included in this comparison. However, the
tetraferrocene functionalized FeTPP (α4-FeFc4, Figure 2D) is
selective for 4e−/4H+ ORR in this pH range, and its rates have
been reported earlier.64 A similar rise in ORR rate is also
observed (Figure 8, purple dots) in α4-FeFc4, where the pKa
for the change is different due to the fact that the hydrogen
bonding group in α4-FeFc4 is a triazole instead of a
phenantroline in Fe-bisPhen. This observation adds credence
to the proposal that a trans OH− ligand, produced at higher
values, is responsible for acceleration of ORR rates by a factor
of 100.
Table 1. Electrochemical ORR Data of Fe-bisPhen at
Different pH Values
pH
kcat (M−1s−1) × 10−5
PROS (%)
4
4.67
5
6
7
8
9
10
11
(59.8 0.7)
(65.9 1.2)
(27.4 0.4)
(6.09 0.09)
(1.85 0.02)
(29.0 0.6)
(123 11.0)
(121 10.0)
(169 15.0)
(2.52 0.3)
(0.39 0.4)
(2.09 0.25)
(3.86 0.2)
(5.01 0.15)
(3.03 0.1)
(1.43 0.2)
(0.03 0.1)
(0.03 0.05)
When the pH of the solution is lowered from 7 to 4, the
ORR rate increases from (1.85 0.02) × 105 to ∼60 × 105
M−1 s−1, i.e., a 40-fold increase (Table 1, Figure 8). A plot of
the ORR rate with pH (Figure 8) indicates a pKa of (5.6 0.1)
for this enhancement of rate at low pH, which matches very
well the pKa of phenanthroline (∼5) in water.95 The α4-FeFc4
complex which bears distal triazole residues having pKa < 2
does not show any rate enhancement (Figure 8, purple) at this
pH. Thus, protonation of the pendant phenanthroline residues
enhance the rate of ORR by 40-fold. A phenanthroline
strapped porphyrin had also proposed rate enhancement of
ORR due to phenantroline protonation in organic solvents.68
Similar enhancement of the ORR rate was observed for iron
porphyrins with pendant pyridine/amines which, on proto-
nation at neutral pH values, were found to be efficient in
accelerating the rate of ORR by orders of magnitude.62,69,70
In summary, installation of two phenanthroline groups in a
mononuclear iron porphyrin enables it to reduce O2 selectively
to H2O over a wide range of pH values. At low values, the
phenanthrolines are protonated and behave like the protonated
histidine of HRP, exerting a pull effect (Scheme 3) and
resulting in a 40-fold increase in ORR rate. At high pH values,
the trans axial water gets deprotonated to a hydroxide, which
exerts a push effect (Scheme 3) similar to cysteine in cytP450,
resulting in 100-fold increase in ORR rate. The pyridyl groups
(a) directly hydrogen bond to axial ligands on the metal center
which (b) results in a water channel. Both features allow direct
delivery of protons from bulk solvent to the iron center,
imparting selectivity for 4e−/4H+ ORR while the pH
dependent push−pull effect tunes the rates favorably over 2
orders of magnitude.
Figure 8. Plot of log of second order rate constant or ORR vs pH.
The red line indicates the fit to the experimental data (blue
diamonds) using two pKa values of 5.6 0.1 and 7.9 0.1.
Table 2. Fe−O and O−O Vibrations Identified in Situ Using
SERRS-RDE during ORR Catalyzed by Different
Systems83−87
Fe O
O O
system
(cm−1
)
(cm−1
)
FeTPP
PhO trans axial ligand
Fe/Cu Mb* (CcO biosynthetic models)
Fe/Cu
synthetic model 6L-Fe(Im)Cu (Im-
Imidazole)
634
587
ND
ND
545
830
830
801
819
847
6L-Fe(OH2)Cu
deprotonated to a −OH trans axial ligand. The pKa of this
change is estimated to be 7.9 0.1 from the data (Figure 8,
red line). Similar 100-fold enhancement in ORR rate was also
observed for thiolate bound iron porphyrins.93 Due to its
greater push effect, a hydroxide ligand can increase the pKa of
FeIII-OOH species and making the O−O bond cleavage faster
to release water akin to a thiolate axial ligand. To validate such
a possibility, an imidazole ligand (neutral; stronger σ donor
than H2O) and dissolved Cl− axial ligand (π anionic ligand;
weaker than OH−) are used to investigate the ORR at pH 7
(Figure S8 and S9). The ORR data show that, while the
selectivity of ORR remains the same for both these axial
ligands, the rate of ORR increased from (1.85 0.02) × 105
EXPERIMENTAL DETAILS
■
Materials and Methods. All reagents used are of the highest
grade commercially available. 1,10-Phenanthroline, hydrogen peroxide
solution 30% (w/w) in H2O, benzoyl chloride, anhydrous ferrous
bromide (FeBr2), and 2,4,6-collidine were purchased from Sigma-
Aldrich. All solvents, zinc acetate [Zn (OAc)2·2H2O], thionyl
chloride, triethylamine, and Na2SO4 were purchased from Merck
and used without further purification. Unless otherwise mentioned,
solvents were distilled, dried, and deoxygenated pursuant to
established protocols for use in the glovebox. Preparation and
handling of air-sensitive materials were carried out under an inert
atmosphere in the glovebox. Unless otherwise mentioned, all reactions
were performed at room temperature, and column chromatography
M−1 s−1 with H2O to (1.71
0.07) × 106 M−1 s−1 with
H
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Scheme 2. Schematic Representation of the Factors Determining the 2e−/2H+ vs 4e−/4H+ Selectivity in O2 Reduction by Iron
a
Porphyrin Complexes
a
The shaded oval ring represents the porphyrin ligand; red and blue color denote the proximal and distal oxygen atoms of a bound hydroperoxide
ligand, respectively.
Table 3. Rate of ORR at pH 7 (kcat, M−1s−1)
Fe-bisPhen
Fe-bisPhen Cl−
Fe-bisPhen Imz
α4-FeFc4
(1.85 0.02) × 105
(1.75 0.09) × 106
(1.71 0.07) × 106
(1.20 0.20) × 105
Scheme 3. Schematic Representation of O−O Bond Activation by the Push Effect and the Pull Effect Using Fe-bisPhen
a
Catalyst
a
The shaded oval ring represents the porphyrin ligand. Two phenyl rings are omitted in the left and right structures for clarity.
was performed on silica gel (60−120 mesh) with neutral alumina.
Elemental analyses were performed on a PerkinElmer 2400 series II
CHN analyzer. Electrospray ionization (ESI) mass spectra were
recorded with a Waters QTOF Micro YA263 instrument. The
absorption spectra are measured in the SHIMADZU spectrograph
(UV-2100). NMR tubes were purchased from Wilmad-Lab Glass.
Room temperature 1H NMR spectra were collected on a Bruker DPX-
500 spectrometer. X-ray single crystal data were collected at 120 and
298 K using radiation on a Bruker SMART APEX diffractometer
equipped with CCD area detector using graphite monochromatic Mo
Kα (λ = 0.71073 Å) radiation. The APEX II software package was
used for data collection, data reduction, and structure solution
refinement. The hydrogen atoms were refined isotropically, and their
locations were determined from a difference Fourier map. Structure
solving was done using the direct method and refined in a routine
manner.
AgCl (saturated KCl) aqueous reference electrode with scan rates
varying from 50 to 500 mV/s.
Electrode Modification. Physiabsorption on EPG. A 50 μL
portion of catalyst from a 1 mM solution of the respective catalysts
in chloroform (CHCl3) was deposited on a freshly cleaned EPG
electrode mounted on a RDE setup. After evaporation of the solvent,
the surface was rinsed with CHCl3, sonicated in ethanol, and
thoroughly dried with N2 gas. Finally, the modified electrodes were
washed with milli-Q water before using them for electrochemical
experiments.
Coverage Calculation. The coverage of a catalyst on an electrode
is estimated by the integrated area under the corresponding
oxidation/reduction currents of the respective species obtained
from their voltammogram under N2. The experiments were repeated
three times and average values with standard deviation has been
reported. The value can be obtained from the equation: Γ = Q/nFA
[where Q = integrated area under the reduction/oxidation current in
coulomb, n = no of electron involved in the process (n = 1), F =
96 500 C, and A = microscopic area of the disk (0.096 cm2)]
Rotating Disc Voltammetry. The RDE measurements were
performed on a CHI 700E bipotentiostat with a Pine Instruments
modulated speed rotor fitted with an E6 series Change-disc tip. The
Electrochemical Measurements (Heterogeneous). Cyclic
Voltammetry. The cyclic voltammograms (CV) are recorded on a
CH Instrument bipotentiostat model 700E. A glassy carbon electrode
(2 mm diameter) was used as a working electrode. A Pt wire was used
as a counter electrode. The measurements were made against an Ag/
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Scheme 4. Synthetic Scheme of the Fe-bisPhen Imz
graphite surface was cleaned by polishing it uniformly on a Silicon
carbide grinding paper. The complex was physisorbed on the disc as
described above. The RDE experiments were carried out by
measuring LSV at 50 mV/s scan rate at different rotation rates
using Ag/AgCl (saturated KCl) reference and Pt counter electrodes.
Rotating Ring Disk Electrochemistry: Partially Reduced Oxygen
Species Detection and Calculation. The platinum ring was polished
using alumina powder (grit sizes: 1 μ, 0.3 μ, and 0.05 μ) and
electrochemically cleaned. The EPG disc with the catalyst is inserted
in the RRDE assembly which is then mounted on the rotor and
immersed into a cylindrical glass cell equipped with Ag/AgCl
reference (saturated KCl) and Pt counter electrodes. In this
technique, the potential of the disk is swept from positive to negative,
and when O2 is reduced, any H2O2, i.e., a 2e− reduction product of O2
produced in the working disk electrode is radially diffused to the
encircling Pt ring, which is held at a constant potential of 0.7 V and
oxidizes the H2O2 back to O2. The ratio of the 2e−/2H+ current
(corrected for collection efficiency) at the ring and the catalytic
current at the disk is expressed as PROS and it provides an in situ
measure of the 2e−/2H+ reduction side reaction. The collection
efficiency (CE) of the RRDE setup is measured in a 2 mM
K3Fe(CN)6 and 0.1 M KNO3 solution at 10 mV/s scan rate and 300
rpm rotation speed. A 17 1% CE is generally recorded during these
experiments. The potential at which the ring is held during the
collection experiments at pH 7 for detecting H2O2 is 0.7 V. The
PROS are generally determined at potentials where the Pt ring current
is at the maximum.
silica gel (100−200 mesh size) and characterized by 1H NMR
spectroscopy, where the peak integration indicated only a single
1
atropisomer in the product. H NMR (400 MHz, CDCl3, 25 °C): δ,
ppm = 8.96−8.93 (m, 8H), 8.27−8.25 (m, 3H), 7.91 (t, 2H), 7.82−
7.78 (m, 5H), 7.59 (t, 3H), 7.21−7.18 (m, 3H), 7.02 (d, 2H), 3.50 (s,
4H), −2.60 (s, 2H).
Bisphenanthroline Substituted α, α-5,10-Di(ortho-aminophen-
yl)-15,20-diphenylporphyrin (bisPhen). At first 1,10-phenanthroline-
2-carboxylic acid (673 mg,3 mmol) was refluxed with 30 mL of SOCl2
solvent for 1 h under inert atmosphere. Then, SOCl2 was evaporated,
and solid light-yellow mass was obtained. The dry solid DAPP
(644.78 mg,1 mmol) was added to that yellow mass under inert
condition. The system was put under vacuum for 15 min to make it
free from oxygen and moisture. After that, 35 mL of dry DCM was
added followed by 10 mL of dry THF. The resulting mixture was
neutralized with dry triethylamine (1.08 mL, 7.8 mmol) under ice
cold conditions and stirred for 4 days. Then, the final mixture was
evaporated in a rotary evaporator and worked up with a dichloro-
methane−water mixture and was dried over anhydrous Na2SO4 and
evaporated to obtain solid product. The compound was further
purified by column chromatography using neutral alumina with
DCM-MeOH (5%) mixture as the eluent, and purple colored
compound was isolated (bisPhen, 3, Scheme 1).Yield: (898.61 mg,
85%); 1H NMR (400 MHz, CDCl3, 25 °C): δ, ppm = 10.55 (s, 2H),
9.18 (s, 2H), 8.90 (d, 2H), 8.66 (d, 2H), 8.59 (d, 2H), 8.52 (s, 2H),
8.21 (d, 3H), 8.04 (d, 5H), 7.94 (t, 3H), 7.67−7.57 (m, 13H), 6.79
(d, 2H), 6.57 (t, 4H), 5.30 (s, 3H), 4.96 (s,2H), 3.65 (d,1H), −2.70
(s, 2H). UV−vis (CH2Cl2): λmax = 423 nm (Soret), 516 nm, 551 nm,
591 nm, 650 nm (Q bands). ESI-MS (positive ion mode in ACN):
m/z (%) =1057.3721(100) [bisPhen]H+
Electrochemical Measurements (Homogeneous). Cyclic
voltammetry measurement of homogeneous, nonaqueous solution
containing 1 mM Fe-bisPhen was collected on a CH Instruments
(CHI) model 710D potentiostat using a three-electrode config-
uration. Glassy carbon (3 mm, CHI), platinum wire, and silver wire
electrodes were used as the working, auxiliary, and reference
electrodes, respectively. Ferrocene was used as internal standard.
[Bu4N][ClO4] (100 mM) was used as supporting electrolyte.
Synthesis. 1,10-Phenanthroline-2-carboxylic Acid. 1,10-Phenan-
throline-2-carboxylic acid (1e, Scheme 1) was prepared following the
previous reported procedure starting from 1,10-phenanthroline (1a,
Scheme 1)73 after characterization of the product obtained in every
Bisphenanthroline Substituted α,α-1,2-Diaminotetraphenyl Iron
Porphyrin (Fe-bisPhen). The bisPhen ligand (50 mg, 0.0473 mmol)
was dissolved in dry degassed THF, and about 30 μL of collidine was
added to it and stirred for 5 min. Then, FeBr2 (40.8 mg, 0.189 mmol)
was added to the solution, and it was stirred overnight. After the
completion of the reaction, the solvent was removed, and the reaction
mixture was worked up with a dichloromethane−water mixture after
treatment with dil. HCl (4 M, 30 mL) to remove excess FeBr2. The
organic layer was dried with Na2SO4 and evaporated through a rotary
evaporator. The reddish-brown colored solid was isolated (Fe-
bisPhen, 3a; Scheme 1). Yield: (47.23 mg, 90%); Elemental analysis
calcd (%) for [Fe-bisPhen] (C70H42FeN10O2Br): C 75.68, H 3.81, N
12.61; Found: C 71.31, H 3.71, N 11.71; 1H NMR (500 MHz,
CDCl3, 25 °C): δ, ppm = 77.43, 77.31 (pyrollic protons), 11−14
(meta hydrogens of phenyl rings of the porphyrin); UV−vis
(CH2Cl2): λmax = 416 nm (Soret), 511 nm, 585 nm, 685 nm (Q
bands); ESI-MS (positive ion mode in ACN): m/z (%) = 1110.2841
(100) [FeIII-bisPhen]+
1
step with ESI-MS and H NMR spectroscopy.
α,α-5,10-Di(ortho-aminophenyl)-15,20-diphenylporphyrin
(DAPP). The α,α-atropisomer of 5,10-di(ortho-aminophenyl)-15,20-
diphenylporphyrin (2, Scheme 1) was synthesized and separated
accordingly following a previously reported procedure.72 The α,α-
isomers that have their 2-aminophenyl groups above the same face of
the porphyrin macrocycle can be separated in a column. They are
more polar than the α,β-isomers which have their 2-aminophenyl
groups above the two opposite faces of the porphyrin. Again, the 5,10-
isomers which have their 2-aminophenyl groups on two adjacent
meso-aryl substituents are slightly more polar than the 5,15-isomers.
So, the pure α,α-atropisomer of 5,10-di(ortho-aminophenyl)-15,20-
diphenylporphyrin is isolated by using column chromatography using
Bisphenanthroline Substituted α,α-1,2-Diaminotetraphenyl Zinc
Porphyrin (Zn-bisPhen). To a solution of the bisPhen ligand (100
mg, 0.095 mmol) in 25 mL of THF was added Zn(OAc)2·2H2O (58
mg, 0.316 mmol), and the reaction was stirred for 6 h. The solvent
was evaporated and extracted with DCM. After work up, the organic
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layer was collected and dried over anhydrous Na2SO4. Finally, purple
colored solid compound (Zn-bisPhen; 3b, Scheme 1) was isolated.
Yield: (95.39 mg, 90%). 1H NMR (400 MHz, CDCl3, 25 °C): δ, ppm
= 9.75 (s, 2H), 9.06 (s, 2H), 8.91 (d, 2H), 8.80 (s,2H), 8.75 (d, 2H),
8.54 (d, 2H), 8.27 (d, 2H), 8.08 (s, 2H), 7.98 (d, 3H), 7.89 (t, 2H),
7.76 (d, 2H), 7.68−7.61(m, 7H), 7.53(d, 4H), 6.96(d, 2H), 6.84(d,
2H), 6.71 (d, 2H), 6.03 (q,2H), 5.41 (d, 2H).UV−vis (CH2Cl2): λmax
= 428 nm (Soret), 558 and 598 nm (Q bands).
Imidazole Bound Fe-bisPhen (Fe-bisPhen Imz). For preparation
of 1 mL of a 2 mM stock solution, we added 2.22 mg of Fe-bisPhen in
1 mL of DCM in a glass tube. Separately, we had 3.40 mg of imidazole
in 250 μL of DCM. Then, the two solutions were mixed in the glass
tube, and the resulting solution was precipitated to isolate solid
product by adding excess nonpolar solvent such as hexane to the
mixture and drying under vacuum. The imidazole bound Fe-bisPhen
(3c, Scheme 4) was characterized by absorption spectroscopy. Yield:
(2.14 mg, 91%). UV−vis (CH2Cl2): λmax = 423 nm (Soret), 556 nm
(Q-band).
ACKNOWLEDGMENTS
■
This research was funded by Department of Science and
Technology grant (EMR/008063). S.M. and A.N. acknowl-
edge SRF from CSIR, India and DST-INSPIRE, India,
respectively. S.B. acknowledges the IACS Int-PhD program.
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ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02408.
UV−visible spectroscopy data, homogeneous cyclic
voltammetry data, heterogeneous electrochemical data,
crystallographic data, and spectral data (PDF)
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AUTHOR INFORMATION
■
Corresponding Authors
Somdatta Ghosh Dey − School of Chemical Sciences, Indian
Association for the Cultivation of Science, Kolkata 700032,
West Bengal, India; orcid.org/0000-0002-6142-2202;
Email: icsgd@iacs.res.in
Abhishek Dey − School of Chemical Sciences, Indian Association
for the Cultivation of Science, Kolkata 700032, West Bengal,
India; orcid.org/0000-0002-9166-3349; Email: icad@
iacs.res.in
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Abhijit Nayek − School of Chemical Sciences, Indian Association
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Sarmistha Bhunia − School of Chemical Sciences, Indian
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Notes
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