Singly versus Doubly Reduced Nickel Porphyrins for Proton Reduction: Experimental and Theoretical Evidence for a Homolytic Hydrogen-Evolution Reaction

Article abstract of DOI:10.1002/anie.201510001

A nickel(II) porphyrin Ni-P (P=porphyrin) bearing four meso-C₆F₅ groups to improve solubility and activity was used to explore different hydrogen-evolution-reaction (HER) mechanisms. Doubly reduced Ni-P ([Ni-P]^2-) was invo

Full text of DOI:10.1002/anie.201510001

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Communications
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International Edition: DOI: 10.1002/anie.201510001
German Edition: DOI: 10.1002/ange.201510001
Electrocatalysis
Singly versus Doubly Reduced Nickel Porphyrins for Proton
Reduction: Experimental and Theoretical Evidence for a Homolytic
Hydrogen-Evolution Reaction
Yongzhen Han, Huayi Fang, Huize Jing, Huiling Sun, Haitao Lei, Wenzhen Lai,* and Rui Cao*
Abstract: A nickel(II) porphyrin Ni-P (P = porphyrin) bear-
ing four meso-C6F5 groups to improve solubility and activity
was used to explore different hydrogen-evolution-reaction
(HER) mechanisms. Doubly reduced Ni-P ([Ni-P]2À) was
involved in H2 production from acetic acid, whereas a singly
reduced species ([Ni-P]À) initiated HER with stronger tri-
fluoroacetic acid (TFA). High activity and stability of Ni-P
were observed in catalysis, with a remarkable ic/ip value of 77
with TFA at a scan rate of 100 mVsÀ1 and 208C. Electro-
chemical, stopped-flow, and theoretical studies indicated that
a hydride species [H-Ni-P] is formed by oxidative protonation
of [Ni-P]À. Subsequent rapid bimetallic homolysis to give H2
and Ni-P is probably involved in the catalytic cycle. HER
cycling through this one-electron-reduction and homolysis
mechanism has been proposed previously but rarely validated.
The present results could thus have broad implications for the
design of new exquisite cycles for H2 generation.
proton relays, DuBois and co-workers reported that Ni
diphosphines exhibited high turnover frequencies of up to
105 sÀ1 at moderate overpotentials.[10] Nocera and co-workers
reported that hangman groups appended to metal porphyrins
could facilitate HER by mediating proton-coupled electron
transfer.[22,33,34] Recently, Cu complexes have been shown to
be very active for H2 evolution by Sun and co-workers[35] and
also by us.[36]
From these achievements, several factors have been
identified that can facilitate H2-evolution catalysis. However,
better understanding of the HER mechanism is still necessary
to provide insight for the development of efficient catalytic
systems. There are five possible reaction pathways for HER
mediated by metal complexes (Scheme 1). In many cases, H2
H
ydrogen is an ideal energy carrier to meet future energy
demands.[1–5] Its generation by proton reduction is a key step
in solar-to-chemical energy conversion.[6–8] Platinum is very
efficient in catalyzing H2 production, but the low natural
abundance and high cost of this noble metal restrict its
widespread utilization.[9,10] Progress has been made in iden-
tifying HER catalysts by the use of molecular complexes
consisting of earth-abundant metals, such as Fe,[11–15] Co,[9,16–20]
Ni,[21–25] and Mo.[26,27] For example, Co and Fe complexes of N-
based macrocyclic ligands (e.g. diglyoxime) can operate at low
overpotentials with high rates.[13,28–32] By using intramolecular
Scheme 1. Possible reaction pathways for HER mediated by metal
complexes.
evolution is triggered by the 2eÀ reduction of a catalyst to
a M(nÀ2)+ formal oxidation state, which then undergoes
[*] Y. Z. Han, H. L. Sun, H. T. Lei, Prof. Dr. W. Z. Lai, Prof. Dr. R. Cao
Department of Chemistry, Renmin University of China
Beijing 100872 (China)
oxidative protonation to form
a
hydride intermediate
n+ [10,16,18,30]
n+
À
À
H M
.
The H M intermediate can either react
with a proton (known as protonolysis) to release H2 and give
E-mail: wenzhenlai@ruc.edu.cn
the starting complex Mn+ (heterolytic route, pathway A), or
ruicao@ruc.edu.cn
n+
À
react with another molecule of H M to give H2 and two
Prof. Dr. R. Cao
molecules of M(nÀ1)+ (homolytic route, pathway B). In some
School of Chemistry and Chemical Engineering
Shaanxi Normal University
Xi’an 710119 (China)
cases, oxidative protonation of the 1eÀ-reduced species
(n+1)+ [22,32,34,36]
M(nÀ1)+ occurs to form H M
,
which can react
À
with a proton to release H2 and M(n+1)+ (heterolytic route,
Dr. H. Y. Fang, H. Z. Jing
College of Chemistry and Molecular Engineering, Peking University
Beijing 100871 (China)
n+
À
pathway C) or be further reduced to form H M (path-
way D) for subsequent H2 generation through pathway A or
(n+1)+
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201510001.
À
B. Alternatively, the H M
intermediate can undergo
bimolecular homolysis to generate H2 and the starting
ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution Non-Commercial NoDerivs License, which
permits use and distribution in any medium, provided the original
work is properly cited, the use is non-commercial, and no
modifications or adaptations are made.
complex Mn+ (pathway E).
The 2eÀ reduction/protonolysis pathways (A and D) have
been well documented; however, for the 1eÀ reduction/
protonolysis pathway C, the H M(n+1)+ unit is not considered
À
to be basic enough to drive protonolysis.[34] Unlike mono-
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nuclear mechanisms, only a few examples of bimetallic
pathways B and E have been suggested to likely be involved
in HER.[28,31] For example, in a Co corrole system reported by
Dey, Gross, and co-workers, pathways A and B are suspected
to be possible on the basis of density functional theory (DFT)
calculations.[37] By simulating the electrocatalytic cyclic vol-
tammogram (CV) of a Fe diglyoxime catalyst, Winkler and
co-workers suggested a bimetallic mechanism for H2 evolu-
diamagnetism as suggested by NMR spectroscopy indicated
a formal d8 NiII electronic structure.[40]
The CV of Ni-P was measured in acetonitrile containing
0.1m nBu4NPF6. Two reversible redox couples were observed
at À1.28 and À1.82 V versus ferrocene (see Figure S1A in the
Supporting Information; all potentials reported herein are vs.
ferrocene). Their peak separations DEp were measured to be
65 mV, thus implying two one-electron events (DEp of
ferrocene: 65 mV). Their peak currents displayed a linear
correlation with the square root of the scan rate (see
Figure S2), thus indicating two diffusion-controlled electro-
chemical events. The first 1eÀ reduction is metal-centered
according to CV studies of a zinc analogue of Ni-P (see
Figure S13) and electronic absorption spectroscopic measure-
ments of 1eÀ-reduced species (see below). This assignment is
also supported by studies by SavØant[41] and Nocera[22] for the
1eÀ reduction products of similar Ni porphyrins that were
assigned as formal NiI species. The second reduction is ligand-
centered, as found in previous studies of Ni porphyrins.[22,42,43]
However, for simplicity, the formulation of [Ni-P]À and [Ni-
P]2À is used hereafter for 1eÀ- and 2eÀ-reduced species,
respectively.
tion from a H FeIII intermediate.[13] However, they also stated
À
that although simulation of the CV for a monometallic
mechanism did not give satisfactory results, this possibility
could not be ruled out. The activity comparison of a dicobal-
oxime and its monomeric analogue by Gray and co-workers
revealed no significant enhancement, which suggests that
protonolysis rather than homolysis is responsible for catal-
ysis.[38] Therefore, a well-defined bimetallic HER mechanism
has not been recognized and is of great interest and
fundamental importance.
Herein, we report that Ni-P is a highly active and stable
HER catalyst. Experimental and theoretical studies show that
a bimetallic homolysis pathway is possibly involved in the
catalytic HER cycle. H2 evolution is initiated from doubly
reduced species ([Ni-P]2À) with acetic acid or from singly
reduced species ([Ni-P]À) with the stronger acid trifluoro-
acetic acid (TFA). The intermediate [H-Ni-P] from the
oxidative protonation of [Ni-P]À is proposed to be able to
undergo homolysis to give H2 and the Ni-P starting complex
with a small activation energy barrier of 3.7 kcalmolÀ1.
We synthesized the nickel complex of 5,10,15,20-tetra-
kis(pentafluorophenyl)porphyrin, Ni-P (Figure 1). X-ray dif-
fraction studies revealed that Ni-P crystallized in the tetrag-
For comparison, we also synthesized Ni complexes of 5,15-
bis(pentafluorophenyl)-10,20-diphenylporphyrin
and
5,10,15,20-tetrakisphenylporphyrin. We found that the
replacement of meso-C6F5 by meso-C6H5 significantly
decreased the solubility and caused a cathodic shift of the
reduction waves. These results highlighted the significant
effect of pentafluorophenyl groups in regulating the redox
chemistry of Ni porphyrins. As electrocatalytic proton
reduction is closely related to the reduction of catalysts,
meso substituents with strong electron-withdrawing proper-
ties are considered to be able to cut the energy cost for
generating H2, and thus to benefit H2 evolution, despite the
fact that they might decrease the basicity of the metal center
and make the metal center less reactive to protons.[28,34] This
result may explain why the Co pophyrins without strong
electron-withdrawing groups reported by Fukuzumi and co-
workers are catalysts for O2 reduction but not as efficient for
proton reduction, as O2 reduction is initiated by CoII, whereas
CoI or Co0 are required for proton reduction. The absence of
strong electron-withdrawing substituents makes the equilib-
rium potentials for CoI or Co0 too negative to reduce
protons.[44,45]
ꢀ
onal space group I42d with the Ni atom located at the
crystallographically required S4 axis. The macrocycle of Ni-P
is saddled rather than planar, and those pentafluorophenyl
groups at trans positions are staggered, a conformation
commonly seen in metal porphyrins.[39] The four identical
À
À
À
N Ni N bond angles at 908 and Ni N bond distances at
1.9242(14) Š suggest that the Ni atom is located perfectly at
the center of the planar square defined by the four N atoms.
Ni-P was further characterized by NMR spectroscopy and
high-resolution mass spectrometry, which all confirmed the
identity and purity of the bulk sample. The neutral charge of
the molecule as observed in the X-ray crystal structure and its
Upon the addition of acetic acid (pKa = 22.3 in aceto-
nitrile),[46] the second reduction peak of Ni-P became
a catalytic wave with an onset appearing at À1.72 V (Fig-
ure 2A). The first reduction feature was not affected much,
except for the appearance of a tiny anodic wave at À1.43 V.
This phenomenon has been observed in electrocatalytic HER
with Ni and other metal porphyrins,[22,34] and was considered
to correspond to the oxidation of protonated metal species.
These results indicated that acetic acid was not strong enough
to protonate the Ni center of [Ni-P]À, and catalytic H2
evolution was initiated upon 2eÀ reduction to [Ni-P]2À.
Importantly, upon the addition of TFA (pKa = 12.7 in
acetonitrile),[46] the first reduction wave exhibited pro-
nounced catalytic activity (Figure 2B). At low TFA concen-
trations, the catalytic current ic increased linearly with the
Figure 1. Molecular structure of Ni-P (left) and thermal-ellipsoid plot
of its single-crystal X-ray structure (50% probability, right).
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tion), which gives a Faradaic efficiency of 96% and a turnover
number of 1.2 for the use of acetic acid as the proton source
with a 240 mV overpotential.
Complex Ni-P was shown to be a highly efficient HER
catalyst in comparison with those reported previously from
the points of view of both activity and stability. Several
methods have been used previously to evaluate the perfor-
mance of various HER catalysts. As one reviewer suggested,
the direct comparison of the turnover numbers and turnover
frequencies of different catalysts is challenging (if at all
possible). Therefore, we prefer to use the ic/ip value (ip is the
peak current in the absence of an acid) to evaluate the
catalytic efficiency of different catalyst systems under similar
conditions, a ratio that has been generally reported (see
Table S2 in the Supporting Information).[13,29,47–51] For Ni-P,
the ic/ip value of 77 (see Figure S5) is remarkable and one of
the highest among well-established competent HER catalysts.
On the basis of the ic/ip value, a TOF of 1.2 ” 103 sÀ1 can be
estimated according to the generally applied method (see the
Supporting Information for details).[3,4,10,25,26] On the other
hand, the acid tolerance of Ni-P is significant: it remained
unaffected in concentrated TFA solution in the dark, whereas
cobaloxime and nickel diphosphine systems (the two well-
known HER catalyst systems) have insufficient acid tolerance
owing to the protonation of ligand-coordination sites.
Figure 2. A) CVs of 0.50 mm Ni-P in acetonitrile with 0 mm (black),
5.0 mm (red), and 10.0 mm acetic acid (blue; HOAc). B) CVs of 0.50 mm
Ni-P in acetonitrile with 0 mm (black), 5.0 mm (red), and 10.0 mm of TFA
(blue). Conditions: 0.1m Bu4NPF6, 0.07 cm2 GC working electrode,
100 mVsÀ1 scan rate, 208C.
acid concentration (see Figure S6), but reached an acid-
independent region with more than 108 mm of TFA (see
Figure S4). The overpotential determined at the half-wave
position was approximately 420 mV with 0.1m TFA. When the
acid concentration was high relative to the catalyst, ic
exhibited a linear dependence on the Ni-P concentration
(see Figure S7). These results confirmed the molecular nature
of this catalysis, and the first-order dependence of ic on both
the TFA and Ni-P concentrations indicated that the proto-
nation of [Ni-P]À could be the rate-limiting step in H2
evolution (see below). As we expected, with the Ni complex
Stopped-flow experiments were carried out to investigate
proton reduction with Ni-P. The chemical reduction of Ni-P
by sodium borohydride (NaBH4) was monitored spectroscop-
ically (Figure 3A). Upon reduction, the absorptions at 400
of
5,15-bis(pentafluorophenyl)-10,20-diphenylporphyrin,
a later onset and smaller current were observed (see Fig-
ure S10).
Constant potential elec-
trolysis (CPE) was per-
formed at À1.49 V for an
acetonitrile solution of
0.50 mm Ni-P and 0.1m
TFA in a three-compart-
ment cell. A constant cur-
rent was maintained and
a large amount of H2 gas
bubbles were observed on
the surface of glassy carbon
Figure 3. Stopped-flow experiments showing A) the generation of [Ni-P]À by NaBH4, and B,C) the reaction of
[Ni-P]À with TFA (B) and acetic acid (C).
working electrode. After
background correction, the
amount of charge passed
during electrolysis for
30 min was 0.83 C (see Figure S9), with the production of
4.2 mmol of H2 as determined by gas chromatography, thus
giving a Faradaic efficiency of 97% for H2 generation.
Therefore, the turnover number was determined to be 4.2
with an applied overpotential of 540 mV, which is comparable
to reported competent HER catalysts.[10,47] The linear rela-
tionship between the charge and time suggested the stability
of Ni-P during catalysis. After CPE for 30 min, no new species
formed, as evidenced by thin-layer chromatography and UV/
Vis and NMR spectroscopy, which proved that neither
a demetalated porphyrin nor other porphyrin products were
generated during CPE (see Figure S8). Furthermore, CPE
performed at À1.82 V with a solution of 0.50 mm Ni-P and
10 mm acetic acid in acetonitrile generated a total of 0.24 C
charge and 1.2 mmol H2 in 90 min (after background correc-
and 520 nm decreased, whereas the absorptions at 342, 435,
and 550 nm increased. The characteristic isosbestic points at
381, 413, and 534 nm indicated a clean transformation from
Ni-P to its reduced state. These changes as well as the final
electronic absorption spectrum closely resemble those
observed for many other 1eÀ-reduced Ni porphyrins,[41–43,52]
thus suggesting the formation of singly reduced species, [Ni-
P]À (see the Supporting Information for experimental
details). Upon 1eÀ reduction, the Soret band at 400 nm split
into two bands at 402 and 435 nm (but the overall intensity of
the Soret band did not significantly decrease), and no
absorption peaks developed in the 600–720 nm range (see
Figure S12). These results are consistent with metal-centered
reduction to give a formal NiI species, as established for metal
porphyrins.[22,42,43,53,54]
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Complete recovery of the initial UV/Vis spectrum upon
the addition of TFA to a freshly prepared [Ni-P]À solution
revealed catalyst regeneration (Figure 3B). This study further
confirmed the molecular nature of Ni-P catalysis. On the basis
of this result, we propose that [Ni-P]À first undergoes
oxidative protonation with TFA to yield an [H-Ni-P] inter-
mediate. Because the bimetallic homolysis of [H-Ni-P] yields
H2 and two equivalents of Ni-P, a pathway that is fully
consistent with the experimental results and is found to
proceed with a large thermodynamic driving force and a very
small activation energy barrier (see below), we suggest that
such a bimetallic homolytic route is possibly involved in the
catalytic cycle. Direct protonolysis of [H-Ni-P] for H2
evolution could be ruled out in this system, since this process
would give H2 and [Ni-P]+, a species not observed in our
stopped-flow experiments. This phenomenon is consistent
took 27 ms to reach completion. This result is in accordance
with CV studies, in which catalytic activity could be initiated
upon the formation of [Ni-P]À with TFA as the proton source,
but catalytic H2 evolution with acetic acid as the proton
source was observed upon the formation of [Ni-P]2À under
electrochemical conditions. On the basis of these results and
those from electrochemical studies, we can conclude that
oxidative protonation of [Ni-P]À is the rate-limiting step for
H2 evolution, and is followed by a fast bimetallic homolysis
reaction. Therefore, the changes in the UV/Vis spectrum only
showed the kinetic behavior of the protonation step in this
two-step process, and the detection of the [H-Ni-P] inter-
mediate is highly challenging, if it is even possible, under our
experimental conditions.
To gain more insight into HER with Ni-P, we studied
possible catalytic mechanisms by DFT calculations. The
results are summarized in Figure 4. It was found that Ni-P
has a closed-shell singlet ground state, and that the triplet
state is 4.3 kcalmolÀ1 higher in energy. The first 1eÀ-reduced
species [Ni-P]À has two close-lying doublet states: [NiI-P]À
(n+1)+
À
with the fact that H M
drive protonolysis. For example, hangman Co porphyrins with
is typically not basic enough to
three meso-C6F5 groups have been shown to electrocatalyze
III
HER with the involvement of H Co species.[34] However,
À
III
À
H Co needs to be
À
further reduced to H
CoII (pathway D in
Scheme 1) for the pro-
tonolysis to yield H2.
Thermodynamic anal-
ysis of HER indicated
III
À
that H Ni (defined
as [H-Ni-P] herein)
should have a smaller
driving force (by
0.41 V) to react with
III
À
a proton than H Co
does (see Scheme S1
and the relevant dis-
cussion in the Sup-
porting Information).
Furthermore, thermo-
dynamic comparison
of pathways C and E
revealed that HER
through the bimetallic
homolysis route has
Figure 4. Energy diagram for HER catalyzed by Ni-P. Free-energy values are given in kcalmolÀ1 and electrochemical
potentials in volts.
a larger driving force (by 0.52 V) as compared to mononu-
clear protonolysis. Note that NaBH4 will be immediately
consumed upon mixing with TFA. Less than 6.60 ” 10À2 mmol
of H2 will be generated in this process in a 20 mL optical cell
equipped with the stopped-flow instrument. Generated H2
will have a negligible influence on the measurement, as about
6.85 ” 10À2 mmol of H2 can be dissolved in 20 mL of acetoni-
trile.[55]
As shown in Figure 3B, the reaction was completed in less
than 1 ms, which is the shortest data-collection timescale with
our stopped-flow instrument. To get more information, we
replaced TFA with acetic acid to slow down the reaction. The
addition of a large excess of acetic acid to the solution of [Ni-
P]À caused similar changes in the UV/Vis spectrum (Fig-
ure 3C), thus implying the regeneration of the starting
catalyst. As we expected, the reaction was much slower and
and [NiII-PCÀ]À, generated from the reduction of Ni and
porphyrin ring, respectively. Our DFT calculations show that
the energetic ordering of these two states is functional-
sensitive (see Table S3). Given the challenges of correctly
predicting the energetic ordering in transitional-metal com-
plexes with DFT, the doublet [NiI-P]À was taken as the ground
state, in agreement with the experimental results. The 2eÀ-
reduced species was found to be a NiI porphyrin radical
dianion with a triplet ground state. The calculated first and
second reduction potentials of Ni-P are À1.19 and À1.77 V, in
good agreement with the experimental values (À1.28 and
À1.82 V). More significantly, the calculated separation
between the first and second reduction of Ni-P was 0.58 V,
which agrees very well with the value of 0.54 V from
experimental CV measurements.
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Oxidative protonation of [Ni-P]À to generate [H-Ni-P]
information to enable energy input for the formation of H2 to
was calculated to be endothermic by 23.2 or 36.3 kcalmolÀ1
when TFA or acetic acid was used as the proton source,
respectively. Although this protonation process is uphill in
energy, combined stopped-flow and UV/Vis experiments
indicated that proton attack on [Ni-P]À can occur at a high
acid concentration. To explore a possible bimolecular mech-
anism for H2 evolution, the homolytic reaction pathway of [H-
Ni-P] was investigated. It was found that this reaction
proceeds via a transition state (TS) with a very small
activation energy barrier of 3.7 kcalmolÀ1 and a large exo-
thermicity of 93.6 kcalmolÀ1 (pathway I in Figure 4). On the
other hand, further 1eÀ reduction of [H-Ni-P] gave [H-Ni-P]À
at a calculated potential of À1.17 V, which is close to the
potential of the first reduction of Ni-P (À1.19 V). This result
suggested the possibility that [H-Ni-P] could be further
reduced by 1eÀ at the electrode during electrocatalysis.
Subsequent protonolysis of [H-Ni-P]À to release H2 and the
Ni-P starting catalyst was calculated to be exothermic by
40.6 kcalmolÀ1 with TFA as the proton source (pathway II in
Figure 4). Under conditions of chemical reduction (by
NaBH4), the reaction pathway via [H-Ni-P]À is not likely.
On the basis of stopped-flow and DFT studies, we can
conclude that a bimolecular homolysis mechanism for H2
evolution is likely in this system, and the calculated small
activation energy barrier of 3.7 kcalmolÀ1 is supportive of
a fast bimetallic reaction, as established from our stopped-
flow measurements.
be decreased. Therefore, this study will have broad implica-
tions for the design of new exquisite cycles for H2 generation.
Acknowledgements
This research was supported by grants from the “Thousand
Young Talents” program in China, the National Natural
Science Foundation of China (21101170 and 21573139), the
Fundamental Research Funds for the Central Universities,
and the Research Funds of Renmin University of China.
Keywords: bimetallic reactions · electrocatalysis · homolysis ·
hydrogen evolution · nickel porphyrins
How to cite: Angew. Chem. Int. Ed. 2016, 55, 5457–5462
Angew. Chem. 2016, 128, 5547–5552
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