Proton Relay in Iron Porphyrins for Hydrogen Evolution Reaction

Article abstract of DOI:10.1021/acs.inorgchem.1c01079

The efficiency of the hydrogen evolution reaction (HER) can be facilitated by the presence of proton-transfer groups in the vicinity of the catalyst. A systematic investigation of the nature of the proton-transfer groups present and their interplay with bulk proton sources is warranted. The HERs electrocatalyzed by a series of iron porphyrins that vary in the nature and number of pendant amine groups are investigated using proton sources whose pKa values vary from ～9 to 15 in acetonitrile. Electrochemical data indicate that a simple iron porphyrin (FeTPP) can catalyze the HER at this FeI state where the rate-determining step is the intermolecular protonation of a FeIII-H- species produced upon protonation of the iron(I) porphyrin and does not need to be reduced to its formal Fe0 state. A linear free-energy correlation of the observed rate with pKa of the acid source used suggests that the rate of the HER becomes almost independent of pKa of the external acid used in the presence of the protonated distal residues. Protonation to the FeIII-H- species during the HER changes from intermolecular in FeTPP to intramolecular in FeTPP derivatives with pendant basic groups. However, the inclusion of too many pendant groups leads to a decrease in HER activity because the higher proton binding affinity of these residues slows proton transfer for the HER. These results enrich the existing understanding of how second-sphere proton-transfer residues alter both the kinetics and thermodynamics of transition-metal-catalyzed HER.

Full text of DOI:10.1021/acs.inorgchem.1c01079

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Proton Relay in Iron Porphyrins for Hydrogen Evolution Reaction
Sarmistha Bhunia,† Atanu Rana,† Shabnam Hematian, Kenneth D. Karlin,* and Abhishek Dey*
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ABSTRACT: The efficiency of the hydrogen evolution reaction
(HER) can be facilitated by the presence of proton-transfer groups
in the vicinity of the catalyst. A systematic investigation of the
nature of the proton-transfer groups present and their interplay
with bulk proton sources is warranted. The HERs electrocatalyzed
by a series of iron porphyrins that vary in the nature and number of
pendant amine groups are investigated using proton sources whose
pKa values vary from ∼9 to 15 in acetonitrile. Electrochemical data
indicate that a simple iron porphyrin (FeTPP) can catalyze the
HER at this FeI state where the rate-determining step is the
intermolecular protonation of a FeIII-H− species produced upon
protonation of the iron(I) porphyrin and does not need to be
reduced to its formal Fe0 state. A linear free-energy correlation of the observed rate with pKa of the acid source used suggests that the
rate of the HER becomes almost independent of pKa of the external acid used in the presence of the protonated distal residues.
Protonation to the FeIII-H− species during the HER changes from intermolecular in FeTPP to intramolecular in FeTPP derivatives
with pendant basic groups. However, the inclusion of too many pendant groups leads to a decrease in HER activity because the
higher proton binding affinity of these residues slows proton transfer for the HER. These results enrich the existing understanding of
how second-sphere proton-transfer residues alter both the kinetics and thermodynamics of transition-metal-catalyzed HER.
transfer, proton-coupled electron transfer, and hydrogen-bond
formation, which are involved in the key steps in catalysis.
The reduction of protons to produce molecular hydrogen by
means of storing energy is catalyzed most efficiently by the
natural enzyme hydrogenase.34,40 To attain efficient HER and
hydrogen oxidation reaction (HOR), several groups have
attempted to incorporate proton shuttles in mononuclear
complexes using first-row transition metals (e.g., iron, nickel,
and cobalt).41−45 Such efforts are inspired by the bridging
azadithiolate ligand present in the binuclear active site of [Fe−
Fe]hydrogenase.34,46,47 Few research groups have investigated
the kinetic and thermodynamic advantage of utilizing outer-
coordination-sphere proton relays in artificial complexes
during catalysis.42,48,49 Conceivably, a proton-transfer residue
can shuttle protons, lowering the barrier, and/or act as a local
source of proton, lowering the entropic cost involved in
protonating the catalytic metal center. Thus, these phenomena
entail interplay between the pKa values of the metal center, of
these second-sphere residues, and of the bulk acid source (pH
in water). DuBois and coworkers had demonstrated that the
INTRODUCTION
■
Reactions that require both protons and electrons are
abundant in nature and are involved in both generation and
storage of energy. Of these, reactions like O2 reduction/
generation,1−6 H2 generation/oxidation7−10 and COx/SOx/
NOx reduction11−18 are of keen contemporary interest because
of the ongoing pursuit for clean energy and environment.
Efficient catalysts are required for the conversion of electricity
from renewable energy sources like solar and wind to chemical
energy for storage in the form of chemical fuels. Natural
enzymes have evolved to include sophisticated active sites
where, apart from the inner coordination sphere of the metal,
the noncovalent secondary interactions like hydrophobicity,
hydrogen bonding, and electrostatics play a major role in
defining the reactivity of these active sites.19,20 The protein
scaffold surrounding the enzyme active sites often controls the
bond formation and cleavage by means of electron-transfer,
proton-transfer, and atom-transfer reactions, imparting them
the desired rate and selectivity.19,21−23 Emulating these finer
attributes of metalloenzyme catalysis in bioinspired small
molecules has been a major area of focus in recent
records.24−27 The role of proton relays is being actively
investigated in reactions that require multiple protons and
electrons, e.g., H+ reduction,28,29 O2 reduction,30,31 CO2
reduction,32,33 H2 oxidation,34−36 and H2O oxidation.37−39
These results all stress the importance of the precise
positioning of proton-transfer groups in promoting proton
Special Issue: Advances in Small-Molecule Activation
Received: April 8, 2021
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6
Figure 1. Representative molecular structures of the iron porphyrins [FeTPP (A), FeL3 (B), FeL2 (C), and LFe (D)] studied here.
incorporation of a nitrogenous base in a diphosphine ligand
backbone, forming the complex [Ni(PNP)2]2+, readily binds
hydrogen and shows faster electrochemical hydrogen oxidation
at lower overpotential than the analogous [Ni(depp)2]2+
complex with no tethered base.43 Later, these ligands were
modified by increasing the chain length and number of bases in
[Ni(PR NR’ )2]2+ to allow better access of the nitrogenous
advance the current understanding of the roles played by these
proton shuttles.
In this paper, electrocatalytic HER catalyzed by a series of
mononuclear iron meso-tetraphenylporphyrin (FeTPP) deriv-
atives (Figure 1) using acid sources over a range of pKa =
8.64−14.98 in acetonitrile is investigated. It has been suggested
computationally that intramolecular proton transfer results in a
reduced energy barrier.57 Using unsubstituted FeTPP as a
control, electrocatalysis by the iron porphyrins with pendant
bases is investigated. The iron porphyrins are appended with
arms bearing basic residues, which vary in both their pKa values
and numbers. The results obtained suggest that pendant basic
residues, which get protonated in the presence of bulk acid
sources, can act as local proton sources and lower the barrier of
protonation of the FeI state of the catalyst. The effect of
variation in the pKa value and number of pendant bases is
discussed in detail using a series of iron porphyrins, where both
pKa value of the pendant base and number of bases are
systematically varied.
2
2
proton shuttle to the nickel center, resulting in very high
turnover rates for electrocatalytic HER in the nickel catalyst
[Ni(PPh2NPh)2](BF4)2 (PPh2NPh = 1,3,6-triphenyl-1-aza-3,6-
diphosphacycloheptane).50−52 Shaw and coworkers established
that the arginine amino acid in a water-soluble [Ni-
(PCy2NArg2)2]8+ complex enhances the electrocatalytic activity
when these residues were protonated.36 In the above case, the
guanidium groups on the outer sphere of the molecule were
proposed to shuttle protons via the Grotthuss mechanism.36
Graham and Nocera introduced a hanging carboxylic acid/
pendant base in the second coordination sphere of iron
porphyrins, hangman porphyrins, to channel protons to the
active site to exhibit enhanced catalytic HER activity compared
to nonhangman analogues.53 The thermodynamic advantage of
reducing the overpotential for HER was also evidenced by
having a hanging carboxylic acid in a cobalt hangman
porphyrin.48 Similarly, while iron porphyrins were reported
to catalyze HER in their formal Fe0 state,8 the presence of four
hydrogen-bond donor triazole moieties (pKa in acetonitrile is
∼8)54 along with four ferrocene groups in a mononuclear iron
porphyrin FeFc4 allowed the protonation of iron(I) porphyr-
ins, reducing the overpotential of HER by 50% in the presence
of strong acids in both aqueous and organic solvents.55 Very
recently, Artero and co-workers reported HER catalyzed by a
[Co(bapbpy)Cl]+ complex containing a redox-active bipyr-
idine ligand with associated pendant proton relays.56 The
electrocatalytic behavior in response to systematic variation in
pKa of the acid source suggests that the CoI state becomes
activated for HER depending upon pKa of the acid sources
used. Conversely, systematic variations in pKa and the number
of the pendant bases along with variation of pKa of the acid
source are necessary to complement these investigations and
RESULTS AND DISCUSSION
■
Electrocatalytic Hydrogen Evolution by FeTPP. The
cyclic voltammetry (CV) of FeTPP is well documented in the
literature58−60 and exhibits three responses, corresponding to
the FeIII/II, FeII/I, and FeI/0 redox couples at −0.68, −1.5, and
−2.07 V (Figure S1), respectively (with respect to Fc+/Fc0),
with peak-to-peak separations (ΔEp) of 61, 67, and 71 mV,
respectively, at a 100 mV/s scan rate (ν) in acetonitrile. In the
presence of acids with a wide range of pKa values [e.g., tosic
acid (TsOH; pKa = 8.64), p-bromoanilinium tosylate (BA-H+;
pKa = 9.43), diisopropylanilinium tosylate (DIPA-H+; pKa =
10.62), N,N-diethylanilinium tosylate (DEA-H+; pKa = 11.42),
and collidinium tosylate (Col-H+; pKa = 14.98)] into the
acetonitrile solution of 1 mM FeTPP, an electrocatalytic HER
is observed (the tosylate salts of the conjugate acids are
generated in situ by adding TsOH to the corresponding
amine).61−63 The FeIII/II potential is not affected in the
presence of these bases because they are all noncoordinating
due to the presence of bulky groups around the potentially
B
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Figure 2. CV diagrams of 1 mM FeTPP in an acetonitrile solution with a glassy carbon working electrode using 0.1 M TBAP as the electrolyte for
the HER using four different acid sources: (A) TsOH; (B) BA-H+; (C) DIPA-H+; (D) DEA-H+; (E) Col-H+. The scan rate is 100 mV/s. A
platinum wire is used as a counter electrode. All potentials are reported with respect to the Fc+/Fc0 couple using ferrocene as an internal standard.
Figure 3. Left: CV overlay of 1 mM FeTPP in an acetonitrile solution with a glassy carbon working electrode using 0.1 M TBAP as the electrolyte
at a scan rate of 100 mV/s for HER using four different acid sources: (A) TsOH (pKa = 8.7; associated with protonation steps 1−3); (B) BA-H+
(pKa = 9.43; associated with protonation steps 2−4); (C) DIPA-H+ (pKa = 10.62; associated with protonation steps 2−4; (D) DEA-H+ (pKa =
11.42; associated with protonation steps 2−4); (E) Col-H+ (pKa = 14.98; associated with protonation steps 2 and 4). Platinum wire was used as a
counter electrode, and all potentials were reported versus Fc+/Fc using ferrocene as an internal standard. Right: Proposed mechanism of HER for
FeTPP with probable pathways in the presence of variable acid sources with different pKa values.
coordinating nitrogen atom (Figure S1). This is in contrast to
recent investigations using iron porphyrins, where coordinating
axial ligands were demonstrated to substantially affect the
kinetics of the HER.64
However, closer inspection of the data reveal that the HER
current overlays the reduction of FeII to FeI at −1.50 V, which
indicates H2 formation via protonation of the FeIII-H− species.
When the acid source is changed from TsOH to weaker
acids like BA-H+ (pKa = 9.43), DIPA-H+ (pKa = 10.62), DEA-
H+ (pKa = 11.42), and Col-H+ (pKa = 14.98), the catalytic
process originating from the FeI oxidation state at −1.30 V is
retained, but the irreversible electrochemical response, due to
protonation of the porphyrin ring, is not observed.
Furthermore, the catalytic current arising from protonation
of the FeI species at the FeII/I potential gets saturated; i.e., the
HER current becomes independent of the substrate [BH]+
concentration. For example, with BA-H+ and DIPA-H+, the
catalytic HER current from the FeI state becomes saturated at
5 equiv of acid. The further addition of acid into the same
solution results in another redox process at more negative
potential corresponding to HER by the protonation of FeII-H−
species (Figure 2B,C) produced by the reduction of its FeIII-
The data suggest that the electrochemical response along
with the HER current is dependent on the pKa value of the
external acid source used. For the addition of a strong acid like
TsOH (pKa = 8.64) in 1 mM FeTPP [0.1 M tetrabutylammo-
nium perchlorate (TBAP) in acetonitrile], the FeII/I redox
wave becomes irreversible and HER is observed with a peak
potential of −1.50 V. This HER current corresponds to the
process where a FeI species is protonated to form FeIII-H−,
which is then reduced to FeII-H− and becomes further
protonated to evolve H2 similar to previously reported steps
in related systems.48,65−67 There are irreversible processes
between the FeIII/II and FeII/I redox couples (Figure 2A), which
are due to protonation of the meso-carbon in the porphyrin
ligand (also known as phlorin) by a strong acid.8,68,69
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H− precursor. When the acid source is changed to DEA-H+
(pKa = 11.42), which has higher pKa value than DIPA-H+, the
catalytic activity of the FeI state is substantially reduced, as
indicated by the diminished catalytic current at the FeII/I
potential, and the catalytic current corresponding to HER
obtained by the FeII-H− species dominates (Figure 2D).
Using the Col-H+ as the acid source (pKa = 14.98), the CV
diagram of the FeII/I redox process remains unaltered and the
HER proceeds mostly through the FeII-H− species at −1.82 V
under the same condition. CV at very slow scan rates shows a
loss of reversibility of the FeII/I CV response (Figure 4) and
minimal current enhancement. Instead, the HER proceeds via
FeII-H−, implying protonation of the FeI state but not the
resulting FeIII-H− species. Thus, the electrochemical data
presented here clearly demonstrate that there are two possible
routes to the formation of H2 catalyzed by simple iron
porphyrins like FeTPP. For acid sources having pKa ≤ 11.42
(BA-H+, DIPA-H+, DEA-H+, and TsOH), the FeI state is
protonated to form a FeIII-H− species, which then accepts
another proton to generate H2 without further reduction. As
was already established, another possible route to HER after
saturation of the catalytic current or in the absence of catalysis
from the FeII/I redox state is reduction of the FeIII-H− species,
generating a FeII-H− species, which is further protonated to
eliminate H2. The latter process was investigated in detail
earlier8,64,66,67,70 and is not the focus of this investigation,
which is directed to the HER from the FeI state.
Figure 4. Scan rate dependence plot of 1 mM FeTPP in an
acetonitrile solution containing 0.1 M TBAP as the electrolyte with a
glassy carbon working electrode and platinum wire as a counter
electrode. All potentials were reported versus Fc+/Fc using ferrocene
as an internal standard using 5 equiv of collidinium (Col-H+) as the
acid source.
the FeII/I potential via the protonation of FeIII-H− produced
but rather HER is observed upon its reduction to FeII-H−.
Thus, the pKa3 value of FeIII-H− must be lower than 14.98,
and/or the rate-determining step of HER by an iron(I)
porphyrin must be the second protonation step, i.e.,
protonation of the FeIII-H− species. Alternatively, FeTPP
shows a small catalytic current from the FeI state in the
presence of DEA-H+, suggesting that the pKa3 value of FeIII-H−
is >11.42. Note that the fact that pKa of the monoanionic FeI
species is higher than that of the neutral FeIII-H− species, i.e.,
pKa2 > pKa3, is logical. Having established the basic pKa values
of the species involved in HER catalyzed by a simple FeTPP,
the effect of pendant amine groups has been investigated and is
now discussed.
The changes in the electrochemical response in FeTPP with
a change of pKa of the acid source reflect the pKa values of
different protonation events in FeTPP. Considering the
discussion further and as depicted in the mechanistic scheme
of Figure 3, there are four different protonation events under
HER conditions that manifest themselves through distinct
electrochemical responses. The CV responses allow an
estimation of the approximate pKa values for these processes.
The processes and their effect on the CV response are as
follows (note that pKa of the metal center refers to pKa of the
species before protonation or pKa of the acid source required
to protonate because protonation of the same are irreversible):
(a) Protonation of the porphyrin ring (pKa1) results in
irreversible processes between the formal FeIII/II and FeII/I
redox couples.
The protonation of FeI to form FeIII-H− occurs in the
presence of all acids (pKa = 9.43−14.98) used here, and
subsequent protonation of FeIII-H− (11.42 < pKa3 < 14.98) to
produce H2 results in the catalytic HER current at the FeII/I
potential. A feature of the catalytic behavior that had eluded
past investigations that used strong acids is saturation of the
HER current originating from the FeI species. Such a behavior
is suggestive of a fast precatalytic equilibrium binding step
between the FeIII-H− species and substrate, proton source,
involved. This is supported by a plot of icat/ip versus
equivalents of acid used, which shows that saturation is
achieved at a much lower concentration for acids having a
lower pKa (Figure 5A), i.e., better hydrogen-bond donors. This
situation is characteristic of Michaelis−Menten-type enzyme
kinetics, where a pre-equilibrium enzyme substrate binding
step precedes the rate-determining step and saturation of the
rate is observed at higher substrate concentration. Because the
TPP ligand framework does not offer a distinct binding site for
these anilinium ions from the acid source, it is likely that the
FeIII-H− species associates with the anilinium ion of the
corresponding conjugate acid in solution via intermolecular
dihydrogen bonding interaction before catalysis occurs. Such
interactions are weakened in the presence of electrolytes.
Accordingly, saturation behavior of the HER current is lost
when the concentration of the electrolyte is increased to 1 M
(Figures 5B and S2).
(b) Protonation of FeI (pKa2) to form FeIII-H− results in an
irreversible nature of the FeII/I redox couple.
(c) Protonation of FeIII-H− (pKa3) results in a HER current
at the FeII/I redox potential (−1.48 V).
(d) Protonation of the FeII-H− species (pKa4) results in a
HER current at a more cathodic potential, that beyond the
FeII/I redox process (−1.86 V).
Protonation of the porphyrin ring can occur in the presence
of a strong acid like TsOH (pKa = 8.64) and does not occur in
BA-H+ (pKa = 9.43), indicating that pKa of the porphyrin ring
lies below 9.43 (pKa1) and above 8.64. Protonation of the FeII-
H− species (pKa4), resulting in the catalytic HER current, can
be achieved even with Col-H+, specifying that the FeIII-H−
species is formed and reduced to generate FeII-H− species; the
pKa value of the FeII-H− species (>14.98) is greater than those
of all of the acid sources investigated here. The FeII/I redox
process is visible in the presence of Col-H+ and becomes
irreversible only at very slow scan rates (Figure 4), suggesting
that the protonation of FeI is slow (does not occur at scan rates
higher than 100 mV/s) and pKa2 is greater than 14.98. Despite
protonation of FeI at slow scan rates, HER is not observed at
Effect of a Distal Base on the Electrochemical HER.
The porphyrins are grouped in two classes: (a) varying the pKa
value of the distal base; (b) varying the number of basic
residues having the same pKa.
a. Effect of the pKa Value of the Distal Base. The
approximate pKa value of the acidic forms of pyridine in FeL2
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Figure 5. icat/ip plot for the catalytic wave at the FeII/I redox potential with the addition of equivalents of four different acids for FeTPP in the
presence of (A) 100 mM TBAP and (B) 1 M TBAP.
6
Figure 6. CV overlay of 1 mM FeL2, FeL3, and LFe in an acetonitrile solution for HER with a glassy carbon working electrode and a platinum
wire counter electrode using 0.1 M TBAP as the electrolyte at a scan rate of 100 mV/s with respect to a Fc+/Fc0 couple using four different acid
sources: (A) TsOH (pKa = 8.64) associated with protonation steps 1−4 for all; (B) BA-H+ (pKa = 9.43) associated with protonation steps 2−4 for
all; (C) DIPA-H+ (pKa = 10.62) associated with protonation steps 2−4 for all; (D) DEA-H+ (pKa = 11.42) associated with protonation steps 2−4
(Figure 3, right) for FeL2 and FeL3 and processes 2 and 4 for 6LFe; (E) Col-H+ (pKa = 14.98) associated with mainly processes 2 and 4 for all of
the complexes except for FeL3, where the protonation step 3 is involved.
is 13.11, and that of the primary amine in FeL3 is 16.9 in
acetonitrile.62,63 In acetonitrile, the HER by FeL2 and FeL3 is
probed in the presence of 5 equiv of [BH+]-OTs, where the
pKa value of the conjugate acid is varied from 9.43 to 14.98.
Protonation of the ring (i.e., step 1 in Figure 3, right), which
has pKa < 9.43, is not observed in any of these cases for the
acid sources having pKa ≥ 9.43. The FeIII/II CV for FeL2
moves anodically by 60 mV once the pKa value of the acid is
lowered below the pKa value of the pendant pyridine (13.11),
suggesting protonation of the pendant pyridine by the acid
source in the solvent accompanying the reduction. Similarly,
the FeIII/II CV for FeL3 is shifted anodically by 60 mV even
when Col-H+ is used as the acid source because the pKa value
of the pendant amine in FeL3 (16.9) is higher than that of Col-
H+ (14.98). The protonation of FeII-H− (pKa > 14.98)
resulting in a HER current at potentials below that of the FeII/I
redox process is observed for all of these acids. The
protonation of FeI (pKa > 14.98), as is evident by a slight
anodic shift and the irreversible nature of the FeII/I redox wave,
is observed for all of the acids used here (Figure 6). In these
cases, the HER current gradually increases in intensity and
shifts anodically as the pKa value of the acid source is lowered
(Figure 6). The electrocatalytic hydrogen evolution by FeIII-H−
protonation remarkably slows in FeTPP when using a weaker
acid source like DEA-H+ (pKa = 11.42), but significant
catalytic current enhancement occurs in these two catalysts
bearing pendant bases. The HER from Col-H+ results in a
small catalytic current in FeL2 with a pendant pyridine but is
clearly observed with the pendant alkyl amine in FeL3. Unlike
FeTPP, the HER by FeI is observed with Col-H+ (pKa = 14.98)
even under high scan rates in FeL2 and FeL3 (Figure S3),
suggesting that pKa of FeIII-H− produced in these systems is
E
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Figure 7. CV diagrams of 1 mM FeL2 in an acetonitrile solution with a glassy carbon working electrode using 0.1 M TBAP as the electrolyte for
HER using four different acid sources: (A) BA-H+; (B) DIPA-H+; (C) DEA-H+; (D) Col-H+. The scan rate is 100 mV/s. Platinum wire is used as
the counter electrode. All potentials are reported with respect to a Fc+/Fc0 couple using ferrocene as an internal standard.
Figure 8. CV diagrams of 1 mM FeL3 in an acetonitrile solution with a glassy carbon working electrode using 0.1 M TBAP as the electrolyte for
HER using four different acid sources: (A) BA-H+; (B) DIPA-H+; (C) DEA-H+; (D) Col-H+. The scan rate is 100 mV/s. Platinum wire is used as
the counter electrode. All potentials are reported with respect to a Fc+/Fc0 couple using ferrocene as an internal standard.
higher than or similar to (due to hydrogen bonding with the
pendant group) those of the pendant bases (pyridine in FeL2
and amine for FeL3) in these porphyrins. Similarly, pKa
enhancement by hydrogen bonding was observed in the
same system for an axial −OOH ligand.31 Thus, the
protonation of FeIII-H−, which is completely inhibited in
FeTPP, is enabled in the presence of pendant bases from Col-
H+, allowing HER in FeL2 and FeL3. Note that a blank glassy
carbon electrode does not show HER at these potentials with
these acid sources in the absence of a catalyst under similar
experimental conditions (Figure S7), but rather a HER current
is observed at more negative potentials, as previously
reported.71
Both FeL2 (Figure 7A−D) and FeL3 (Figure 8A−D) show
a clear saturation of the HER current catalyzed by the FeI state
using the different acid sources. Controlled potential
electrolysis experiments at −1.2 V (vs Ag/AgCl reference
electrode) confirm the release of H2 gas during a headspace gas
F
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analysis in gas chromatography (GC; Figure S4). Although the
HER current saturates in FeL2 and FeL3 with increasing
concentration of [BH+] like FeTPP, the saturation and activity
is not lost for FeL2 in the presence of 1 M electrolyte (Figure
S5). Thus, saturation of the HER current from the FeI state in
these neutral iron(I) porphyrins having protonated bases does
not involve a precatalytic bonding/association equilibrium of
FeIII-H− with the external proton source [BH]+ but rather
reflects an intramolecular proton transfer from the protonated
pendant base to FeIII-H−. Protonation of the pendant base in
the presence of these conjugate acids is evident from the shift
in the FeIII/II CV depending on the pKa value of the acid source
and equivalents of the external acid source needed to protonate
the pendant base and to reach saturation of the catalytic
current at the FeII/I potential; unsurprisingly, increase with its
pKa (Figure S6). For example, 3 equiv of BA-H+ (pKa = 9.43)
is enough to protonate the pendant pyridine in FeL2 (pKa =
13.11), while 5 equiv of DEA-H+ (pKa = 11.42) is required to
achieve the same (Figure S6).
The results presented above clearly demonstrate the
advantage of having a pendant base in iron porphyrins during
HER, where these bases allow the protonation and/or proton
translocation to its FeIII-H− center, resulting in higher HER
currents at the FeII/I potential than FeTPP for the same acid
source. Given that the pendant base is protonated by external
acid sources having pKa values of less than that of the pendant
base and considering the fact that the rate-determining step is
H+ transfer to the FeIII-H− species, these results may imply that
the pendant bases facilitate the electrcatalytic HER via
enhancement of the protonation rate of the FeIII-H− species
produced, and the fact that this HER current becomes
independent of the external conjugate acid concentration,
further indicates that this protonation step is likely intra-
molecular. Further insight into this is obtained from the
catalytic rates presented in a later section.
the three pyridines in 6LFe form a “proton sponge” with a very
high first pKa. For these acid sources, the catalytic current of
HER increases because of protonation of the FeIII-H− species;
i.e., the FeI state can catalyze HER similar to that in FeL2 and
FeL3. The magnitude of the HER current from the FeIII-H−
6
species is smaller in LFe relative to FeL2. In fact, while FeL2
shows a catalytic current at FeII/I redox potential from DEA-H+
(pKa = 11.42), 6LFe having three pyridines does not show such
a current enhancement from the FeI state and the CV diagram
of the FeII/I redox wave becomes irreversible owing to the
protonation and formation of a FeIII-H− species (Figure S8).
Thus, 6LFe, which features a greater number of pendant bases
relative to FeL2, shows a weaker catalytic current for HER.
This is because the three pyridines together can act as proton
sponge, causing the intramolecular protonation of FeIII-H− to
be slow (see DFT Calculations).
Rate Determination Using the Foot-of-the-Wave
Analysis (FOWA). The catalytic rates of HER from the FeI
state can be determined either by using the FOWA of the
́
kinetic region developed by Costentin and Saveant at which
the HER current depends on the substrate concentration
(acid)74 or by analyzing icat/ip at acid concentrations where the
scan-rate-independent HER current plateaus.75−77 In our cases,
the electrochemical processes deviate from the ideal S-shaped
behavior, and the catalytic rates are analyzed under purely
kinetic conditions at the foot of the wave in the absence of any
side phenomena. Here, we have evaluated the reaction rates for
the heterolytic reaction pathway (ECCE mechanism) from the
slope of the linear fit of i/ip versus [1 + exp(F/RT)(E −
E
cat/2)]−1 (Figures S9−S13) using eq 1 by which the FOWA
expression can be generally represented.78,79
i/ip = 2.24n[(RT/Fν)kCA0]1/2/[1 + exp(F/RT)
(E − Ecat/2)]
(1)
b. Effect of the Number of Bases on the HER. The number
of pendant bases was varied from one pyridine to three
where n = number of electrons transferred from the electrode,
T is the temperature, R is the gas constant, ip = CV peak
current of one electron-transfer process, i = catalytic peak
current, k is the second-order rate constant of the rate-
6
pyridines for complexes FeL2 and LFe, respectively (Figure
1). The CV data for the FeL2, FeL3, and 6LFe complexes with
1 mM catalyst concentration were recorded in an acetonitrile
solution using the same set of acids (pKa = 8.64 − 14.98). The
0
determining step, F is the Faraday constant, CA is the acid
0
concentration, kCA = kobs is the first-order rate, ν is the scan
CV of these catalysts without any acid showed the FeIII/II
,
rate, and E − Ecat/2 is the difference between the applied
potential (E) and the potential of the half-peak catalytic
current (Ecat/2) in the presence of a substrate.
FeII/I, and FeI/0 redox processes (peak separation of 75 10
mV) having potentials similar to FeTPP, indicating that the
incorporation of these bases in the porphyrin ligand does not
alter the electronic structure of the iron center. Note that the
0
The first-order rates (kCA ) of HER for all of these
complexes are determined at 5 equiv of acid concentration
with respect to the catalyst concentration (Table 1), where the
6
CV response of the LFe complex is inherently complicated
because of the counteranion dissociation and coordination of
one of the free pyridines to the FeIII and FeII states, likely that
previously established.72,73 The large potential shifts and
multiple catalytic waves of FeIII/II and FeII/I redox couples in
the presence of BA-H+ and DIPA-H+ can be attributed to the
protonation of pyridine (Figure S8). Because there are three
pyridines, there is the possibility of multiple protonations in
6LFe. This is the case for BA-H+ and DIPA-H+ (Figure 6, green
and red lines) where the shifts in the FeIII/II and FeII/I
processes are more than that observed for DEA-H+ and Col-
H+ (Figures 6, blue and yellow lines, and S8). Furthermore, for
DEA-H+ and Col-H+, the shifts become saturated upon using 1
equiv of acid, suggesting that in these two cases only one
proton binds and the binding of this proton is more favorable
than in FeL2, which has a single pendant pyridine but requires
more than 1 equiv of acid that has pKa > 10, suggesting that
0
Table 1. First-Order Reaction Rate kCA (s−1)
FeL2
(13.11)
FeL3
(16.9)
6LFe
acid (pKa)
FeTPP
(13.11)
BA-H+ (9.42)
4.9 0.5
2.7 0.3
1.2 0.3
ND
10.4 0.6
9.4 0.5
4.9 0.5
0.8 0.2
5.9 0.7
7.6 0.4
3.9 0.3
1.1 0.4
1.3 0.4
1.8 0.3
0.5 0.4
ND
DIPA-H+ (10.62)
DEA-H+ (11.42)
Col-H+ (14.98)
pendant bases are protonated (see above). The rate of HER for
FeTPP drops from 4.9 0.5 to 1.2 0.3 s−1 when the pKa
value of the acid source is increased from 9.43 in BA-H+ to
11.42 in DEA-H+. This is due to stronger N−H bonds in acids
having higher pKa, which would result in higher activation
energies for the rate-determining protonation step by the acid
G
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Figure 9. Plot of log(rate) versus pKa of the external acid source for FeTPP (green), FeL2 (violet), and FeL3 (red) in the presence of 100 mM
TBAP (A) and the same for FeTPP (green) and FeL2 (brown) with 100 mM TBAP and 1 M TBAP, respectively (B).
6
Figure 10. DFT-optimized structures of (A) FeIIIL2-H−, (B) FeIIIL3-H−, and (C) LFeIII-H− with protonated distal bases.
[BH+]-OTs−, leading to H2 release. The rates of HER are, in
general, estimated to be higher for iron porphyrins with
pendant bases like FeL2 and FeL3 relative to FeTPP for any
acid source including Col-H+, where the rate for FeTPP is not
measurable. Overall, these pendant bases help to accelerate the
catalytic current for HER compared to that for FeTPP. For
structurally analogous complexes FeL2 and FeL3 (the distance
between the proton bearing a base and iron is the same), the
rate is always higher in FeL2, which has a lower pKa (13.11)
for the pendant group than does FeL3 as long as the pKa value
of the acid source is less than that of the pendant base; i.e., the
pendant base is protonated. This is due to the stronger N−H
bond strength of the protonated amine with higher pKa in
FeL3 (pKa = 16.9), which makes it a weaker proton donor
relative to the protonated pyridine in FeL2 (pKa = 13.11).
When Col-H+ is used as the acid source, the rate of FeL3
(higher pKa than Col-H+) is greater than that of FeL2 (lower
pKa than Col-H+) because the pendant base is protonated in
the former and only partially (with ΔpKa ∼ 1.6 of pendant
bases Py-H+ and Col-H+) in the latter. The CV experiments
suggest that the rate-determining step for HER is the
protonation of FeIII-H− species. Normally, FOWA yields the
rate of the chemical step after the redox event, leading to the
formation of an active catalyst (EC), i.e., FeI + H+ → FeIII-H−.
However, if the redox event is followed by two consecutive
chemical steps (ECC), FOWA should reflect the rate of the
second step if that is lower than the rate of the first chemical
step, which is the case here (i.e., FeIII-H− + H+ → FeIII + H2).
The relationship between ΔG (Gibbs free energy of the
reaction) and ΔG⧧ (Gibbs free energy of activation) describing
the transition state (TS) can be approximated as
ΔG⧧ = αΔG + C
(2)
where α is the extent of conversion from the reactant to the
product and C is the constant.
The rate constant (k) and equilibrium constant (K) are
related to ΔG⧧ and ΔG, respectively.
k = kBT/h exp(−ΔG⧧/RT)
(3)
K = exp(−ΔG/RT)
(4)
When eqs 3 and 4 are combined in eq 2 and its logarithmic
form is taken, the expression becomes
(5)
ln k = α ln K + C′
Again, pKa = −ln K.
ln k = −αpKa + C′
(6)
For the set of iron porphyrin complexes used here, FeTPP
(Figure 9A, green), FeL2 (Figure 9A, violet), and FeL3 (Figure
9A, red), the log of the rate constant varies linearly with pKa of
the acid sources. The slopes obtained from the linear free
H
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energy plots are different for these complexes using the same
conjugate acid sources having different pKa values. The slopes
of the lines are 0.35, 0.20, and 0.11 for FeTPP, FeL2, and
FeL3, respectively. The slope of the plot of log(k) versus pKa is
indicative of the nature of TS involved in the protonation of a
FeIII-H− species according to the previously established
assumption where a smaller slope indicates a reactant-like
early TS;80 i.e., the extent of proton transfer is less to the FeIII-
H− center. The magnitude of the slope is lowest for FeL3,
where pKa of the pendant base is the highest (intramolecular
proton transfer irrespective of the external acid sources used)
and FeTPP, without pendant groups (intermolecular proton
transfer), exhibits the highest slope (green trace, Figure 9A,B).
Although, qualitatively, the generalization of the pKa depend-
ence of log(rate) seems reasonable, neither FeL2 nor FeL3
shows rates that are independent of pKa of the external acids to
suggest purely intramolecular control of the rate-determining
FeIII-H− protonation step. To limit any association of the
external acid with FeL2 and FeL3 akin to FeTPP, the HER
rates were obtained for FeL2 (slope 0.20 in Figure 9A) using 1
M TBAP (Figure S13 and Table S1). The resulting log(k)
versus pKa plot for FeL2 approaches linearity (slope 0.01;
brown trace, Figure 9B), indicating mostly intramolecular
proton transfer to FeIII-H− in the rate-determining step; i.e.,
the rate of HER becomes almost independent of pKa of the
external proton source. Density functional theory (DFT)
calculations are used to understand proton transfer to FeIII-H−
from the protonated pendant group to obtain insight into the
difference in the intramolecular H−H bond formation step
determining step of HER is intramolecular proton transfer
from the protonated pendant nitrogen base to a FeIII-H−
species. Having pendant residues covalently attached to the
porphyrin facilitates this rate-determining protonation step by
providing a local source of proton mimicking the desired roles
played by such residues in naturally occurring enzyme active
sites. The rate of proton transfer is, however, lower for pendant
bases with high pKa because of stronger N−H bonds, resulting
in more reactant like TS. Importantly, increasing the number
of bases results in a lowering of the catalytic rate due to a
competing proton sponge effect.
EXPERIMENTAL DETAILS
■
Materials. All reagents were of the highest grade commercially
available and were used without further purification unless mentioned.
The reagents for the synthesis and other chemicals like ferrocene, p-
toluenesulfonic acid (TsOH), bromoaniline, N,N-diethylaniline, 2,6-
diisopropylaniline, collidine, and tetrabutylammonium perchlorate
(TBAP, [Bu4N][ClO4)]) were purchased from Sigma-Aldrich. The
nitrogenous bases were freshly distilled and used after purification
whenever it was required, and acetonitrile was dried using the Pure
Process Technology solvent system. All of the bases and solvent were
degassed using a freeze−pump−thaw technique and stored in an
inert-atmosphere glovebox.
Electrochemical Measurement. All electrochemical experiments
were performed using a CH Instruments model CHI710D
bipotentiostat electrochemical analyzer. A platinum wire electrode
was used as a counter electrode. The measurements were made
against a silver wire reference electrode using ferrocene as an internal
standard. Anaerobic experiments were performed inside a glovebox
under a nitrogen atmosphere. The glassy carbon electrode was used as
a working electrode, which was freshly polished to remove all
contamination before every use. The polished electrode was then
rinsed with water and the solvent used for electrochemical
measurements. The electrodes used for homogeneous electro-
chemistry were purchased from CH Instruments.
6
between FeL2, FeL3, and LFe.
DFT Calculations. Geometry-optimized DFT calculations
are used to investigate the interaction between the protonated
distal base and FeIII-H−. Optimized structures show that the
FeIII-H− species in both FeL2 and FeL3 show strong hydrogen-
bonding interaction between the protonated distal base and
hydride ligand (Figure 10A−B) with H---H distances of 1.39
and 1.30 Å, respectively. This interaction is also referred to as a
dihydrogen bonding.55,81 The energy difference between the
hydrogen-bonded structure and the structure where the
hydrogen-bonding BH+ group is rotated away is 8−17 kcal/
mol in the gas phase (Figure S14). A similar energy for
dihydrogen bonding was estimated for the active site of the
mononuclear active site of mononuclear Fe-hydrogenase.81
The optimized structure of the hydrogen-bonded FeIII-H− in a
Synthesis. The catalysts (FeL2 and FeL3) were synthesized in our
laboratory following an earlier reported procedure.31 6LFe was
synthesized in Prof. Karlin’s laboratory following a previous literature
report.82 These catalysts are pictorially represented in Figure 1.
CV. Homogeneous CV experiments were carried out in an
acetonitrile solution of a 1 mM/0.5 mM catalyst with 100 mM
TBAP (supporting electrolyte) in an electrochemical cell. The scan
rate was 100 mV/s in most cases unless mentioned otherwise. An
internal standard ferrocence was dissolved in the electrolytic solution,
and the potentials are reported with respect to a Fc+/Fc0 couple. A
stock solution of ferrocene was prepared, which was added to the
solution of the complex before electrochemical measurements. Then
two stock solutions of equal strength (250 mM/500 mM) were
prepared for the base (B) used and the acid (TsOH). In order to
perform the catalysis with different acids, 10 equiv (with respect to
the catalyst) of the base to the solution and then increasing
equivalents of acid was gradually added up to 10 equiv so that in situ
generated conjugate acid [BH+]OTs− behaves as the required acid
source. The direct reduction of anilinium is sufficiently negative on
glassy carbon such that it does not interfere in the analyzed region.
Bulk Electrolysis and Hydrogen Detection by GC. Hydrogen
evolution was confirmed by controlled potential electrolysis at −1.2 V
versus Ag/AgCl in acetonitrile at room temperature. Headspace gas
analysis was performed by GC fitted with a thermal conductivity
detector using helium as the carrier gas in a sealed electrochemical cell
containing a 0.5 mM solution of FeL2, 50 equiv of DEAH+ as a
substrate, a glassy carbon working electrode with a large surface area
(1 cm2), a platinum counter electrode, and a Ag/AgCl/1 M KCl
reference electrode. Here, we performed electrolysis at a fixed
potential for a certain period under the conditions mentioned above
and obtained a substantial amount of H2, which was confirmed by
GC.
6
singly protonated LFe molecule reveals that the dihydrogen-
bonding distance is 3.35 Å (Figure 10C) relative to 1.39 Å
(Figure 10A) in FeL2. In fact, the proton appears to be nicely
lodged in the proton sponge created by the three pyridines.
Logically, this dihydrogen-bonding interaction is weak and is
unlikely to contribute to proton transfer to the FeIII-H− species
6
for the catalytic HER. Consistently, LFe with three pyridines
is much slower toward the catalytic activity from the FeI state
than that of FeL2 with only one pyridine (Table 1).
SUMMARY
■
The pKa values of the different possible protonation events in
HER by iron porphyrins are estimated using proton sources
with varying pKa values. Protonation of the FeIII-H− species by
an external proton source is likely to be the rate-determining
step of the HER catalyzed by FeTPP. The role of pendant
bases in HER are investigated using a series of synthetic iron
porphyrins where both the pKa and number of bases are
systematically varied. The results show that the rate-
I
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DFT Calculations. DFT calculations of the complexes were
carried out using the BP86 functional with unrestricted formal-
ism.83−86 For these complexes, a split basis set (6-311g* on the iron
atom and 6-31g* on carbon, hydrogen, oxygen, and nitrogen atoms)
was used. The frequencies were calculated on the optimized
structures. An energy minimum is confirmed by performing frequency
calculations on the fully optimized structure using the same basis set
as that used for optimization to ensure no imaginary mode is present.
The final energy calculations were performed using the 6-311+g*
basis set on all atoms in the PCM model using acetonitrile as a solvent
and a convergence criterion of 10−10 hartree.87,88 The energies
reported are the total free energies computed using zero-point
energies and entropy (298 K).
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■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c01079.
Additional experimental data, electrochemical data, and
optimized coordinates (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Abhishek Dey − School of Chemical Science, Indian
Association for the Cultivation of Science, Kolkata 700032,
India; orcid.org/0000-0002-9166-3349;
Email: abbeyde@gmail.com
Kenneth D. Karlin − Department of Chemistry, John Hopkins
University, Baltimore, Maryland 21218, United States;
orcid.org/0000-0002-5675-7040; Email: kkarlin1@
jhu.edu
Authors
Sarmistha Bhunia − School of Chemical Science, Indian
Association for the Cultivation of Science, Kolkata 700032,
India
Atanu Rana − School of Chemical Science, Indian Association
for the Cultivation of Science, Kolkata 700032, India;
orcid.org/0000-0002-2397-5869
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Selective Photocatalytic CO2 Reduction in Water through Anchoring
of a Molecular Ni Catalyst on CdS Nanocrystals. J. Am. Chem. Soc.
2017, 139, 7217−7223.
Shabnam Hematian − Department of Chemistry and
Biochemistry, University of North Carolina at Greensboro,
Greensboro, North Carolina 27402, United States;
orcid.org/0000-0002-0788-7615
(16) Chen, L.; Guo, Z.; Wei, X.-G.; Gallenkamp, C.; Bonin, J.;
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c01079
̀
Anxolabéhere-Mallart, E.; Lau, K.-C.; Lau, T.-C.; Robert, M.
Molecular Catalysis of the Electrochemical and Photochemical
Reduction of CO2 with Earth-Abundant Metal Complexes. Selective
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Author Contributions
†S.B. and A.R. contributed equally to this work.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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This research is funded by DST-SERB Grants EMR/2016/
008063 and DST/TMD/HFC/2K18/90. S.B. and A.R.
acknowledge the Integrated Ph.D. program of IACS. S.H.
and K.D.K. acknowledge the National Institutes of Health
(USA) for financial support.
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