Homolytic versus Heterolytic Hydrogen Evolution Reaction Steered by a Steric Effect

Article abstract of DOI:10.1002/anie.202002311

Several H?H bond forming pathways have been proposed for the hydrogen evolution reaction (HER). Revealing these HER mechanisms is of fundamental importance for the rational design of catalysts and is also extremely challenging. Now, an unparalleled example of switching between homolytic and heterolytic HER mechanisms is reported. Three nickel(II) porphyrins were designed and synthesized with distinct steric effects by introducing bulky amido moieties to ortho- or para-positions of the meso-phenyl groups. These porphyrins exhibited different catalytic HER behaviors. For these Ni porphyrins, although their 1e-reduced forms are active to reduce trifluoroacetic acid, the resulting Ni hydrides (depending on the steric effects of porphyrin rings) have different pathways to make H₂. Understanding HER processes, especially controllable switching between homolytic and heterolytic H?H bond formation pathways through molecular engineering, is unprecedented in electrocatalysis.

Full text of DOI:10.1002/anie.202002311

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Accepted Article
Title: Homolytic versus Heterolytic Hydrogen Evolution Reaction
Steered via Steric Effect
Authors: Xiaojun Guo, Ni Wang, Xialiang Li, Zongyao Zhang, Jianping
Zhao, Wanjie Ren, Shuping Ding, Gelun Xu, Jianfeng Li, Ulf-
Peter Apfel, Wei Zhang, and Rui Cao
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202002311
Angew. Chem. 10.1002/ange.202002311
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10.1002/anie.202002311
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Homolytic versus Heterolytic Hydrogen Evolution Reaction
Steered via Steric Effect
Xiaojun Guo, Ni Wang, Xialiang Li, Zongyao Zhang, Jianping Zhao, Wanjie Ren, Shuping Ding, Gelun
Xu, Jianfeng Li, Ulf-Peter Apfel, Wei Zhang, and Rui Cao*
Abstract: Several H−H bond forming pathways have been proposed
for the hydrogen evolution reaction (HER). Revealing these HER
mechanisms is of fundamental importance for the rational design of
catalysts and is also extremely challenging. Herein we report an
unparalleled example of switching between homolytic and heterolytic
HER mechanisms. We designed and synthesized three nickel(II)
porphyrins with distinct steric effects by introducing bulky amido
moieties to ortho- or para-positions of the meso-phenyl groups. We
furthermore showed their different catalytic HER behaviors. For
these Ni porphyrins, although their 1e-reduced forms are active to
reduce trifluoroacetic acid, the resulted Ni hydrides − depending on
the steric effects of porphyrin rings − have different pathways to
make H2. Understanding HER processes, especially controllable
switching between homolytic and heterolytic H−H bond formation
pathways through molecular engineering, is unprecedented in
fundamentals of electrocatalysis.
state of the homolytic H−H bond formation was located based on
calculations, showing a face-to-face arrangement of the two
porphyrin macrocycles (Figure 1b). Results from calculations
and experiments using stopped-flow techniques suggest that the
formation of NiIII−H − through the protonation of NiI ion − is the
rate-limiting step, and subsequent homolytic HER between two
NiIII−H is very fast. Similar bimolecular reaction of NiIII−H to give
H2 has also been suggested to take place quickly (i.e., estimated
k > 107 M−1 s−1),[44] making the detection of NiIII−H intermediates
challenging. Because of the high bimolecular reactivity of NiIII−H,
we propose to introduce bulky substituents on both sides of the
porphyrin macrocycles to block the bimolecular mechanism.
The viability of a hydrogen-based energy society is depended on
efficient and durable production of hydrogen.[1-5] Recent efforts
have resulted in the identification of many molecular catalysts for
hydrogen evolution.[6-36] Despite these achievements, however,
understanding and further controlling reaction processes are still
highly challenging.[37,38] Metal hydrides, which can be generated
through the oxidative protonation of metal ions in their reduced
states,[39-42] are usually proposed as key intermediates to evolve
H2 through homolytic or heterolytic pathways (Figure 1a).[37,38,43]
Although these reaction paradigms have long been suggested, a
well-defined bimetallic homolytic HER has been rarely reported.
As a consequence, switching between homolytic and heterolytic
HER pathways in a controllable manner is unprecedented.
Figure 1. (a) Proposed homolytic and heterolytic HER pathways.
(b) Calculated transition state of the bimetallic homolysis for the
H−H bond formation mediated by Ni TPFP. Bond lengths are
given in Å. Copyright 2016 Wiley-VCH.
Herein, we report the syntheses and catalytic HER features
of Ni porphyrins 1-3 (Figure 2a). Their meso-phenyl groups bear
amido moieties at ortho-positions for 1 and para-positions for 2
and 3. Consequently, comparing to 2 and 3, 1 exhibits much
larger steric resistance on both sides of the porphyrin ring. This
structural difference leads to their different electrocatalytic HER
behaviors. By studying 1e-reduced forms of 1 and 2, we show
that in both cases, singly reduced Ni porphyrins can react with
trifluoroacetic acid (TFA) to form Ni-H, but their subsequent H−H
bond forming pathways are regulated by porphyrin steric effects.
This work represents an unparalleled example of controlling
HER mechanisms by tuning steric effects of molecular catalysts.
Recently, we showed that NiIII−H, formed via the protonation
of singly reduced Ni tetrakis(pentafluorophenyl)porphyrin (TPFP),
underwent bimetallic homolysis to generate H2.[43] The transition
[a]
Dr. X. J. Guo,[+] N. Wang,[+] Dr. X. L. Li, S. P. Ding, G. L. Xu, Prof.
Dr. W. Zhang, Prof. Dr. R. Cao
Key Laboratory of Applied Surface and Colloid Chemistry, Ministry
of Education, School of Chemistry and Chemical Engineering,
Shaanxi Normal University, Xi’an 710119, China
E-mail: ruicao@ruc.edu.cn
[b]
[c]
[d]
Z. Y. Zhang
Chemistry Research Laboratory, Department of Chemistry,
University of Oxford, Mansfield Road, Oxford, OX1 3TA, UK
J. P. Zhao, W. J. Ren, Prof. Dr. J. F. Li
College of Materials Science and Optoelectronic Technology,
University of Chinese Academy of Science, Beijing 101408, China
Prof. Dr. U.-P. Apfel
Ruhr Universität Bochum, Fakultät für Chemie und Biochemie,
Anorganische Chemie I Universitätsstrasse 150, 44801 Bochum,
Germany
[e]
[+]
Prof. Dr. U.-P. Apfel
Fraunhofer UMSICHT, Osterfelder Strasse 3, 46047 Oberhausen,
Germany
Figure 2. (a) Molecular structures of Ni porphyrins 1, 2, and 3.
(b) Thermal-ellipsoid plot of the X-ray structure of 1 (50%
probability). Hydrogen atoms are omitted for clarity. (c) Space-
filling diagram of 1.
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
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The electrocatalytic HER features of 1-3 were investigated in
DMF under N2 with addition of TFA. As shown in Figure 3b, in
the presence of TFA, the 10/1− wave loses the reversibility, and a
new catalytic wave, which is well behind the 10/1− wave, appears
and increases in intensity with increasing TFA. Note that the CV
of TFA alone shows very small current under identical conditions
(Figure S12). These results suggest catalytic HER with 1, and
also suggest that although 1− is protonated with TFA, further
reduction of the resulting species under more negative potentials
is required for catalysis. Unlike 1, the first reduction wave of 2
and 3 becomes a large catalytic one upon addition of TFA
(Figure 3c and 3d, respectively). It is necessary to note that the
different electrocatalytic behavior of 1 is not caused by the
anodic shift of its reduction wave, since Ni TPFP has the first
reduction wave at −1.28 V, which becomes a large catalytic one
with addition of TFA (Figure S14).[43] This HER behavior is
similar to that of 2 and 3. Therefore, these results suggest that
the different HER behaviors of 1-3 are attributed to their different
steric effects: 1 has large steric resistance on both sides of the
Ni porphyrins 1-3 were designed and synthesized (detailed
synthetic procedures are described in Supporting Information).
These porphyrins have meso-phenyl groups with similar amido
substituents at ortho-positions for 1 and para-positions for 2 and
3. For 1, bulky pivalamido substituents provide sufficient steric
resistance to hinder free rotation of the phenyl groups, leading to
different spatial isomers (Scheme S2). The αβαβ isomer is more
favorable thermodynamically and can be enriched.
Complex 1 is structurally characterized (Figure 2b), showing
the desired αβαβ configuration with the four pivalamido groups
having alternating upward and downward orientations. The Ni
atom is located at the porphyrin center, and the NiII oxidation
state is suggested by four Ni−N bonds of 1.913(2)-1.928(2) Å
and by charge balance calculation. Both sides of the porphyrin
macrocycle are sterically protected (Figure 2c). The purity and
identity of 1 are further confirmed by NMR (Figure S2) and high-
resolution mass spectrometry (Figure S3). Complexes 2 and 3
were also structurally characterized (Figure S10), showing small
steric resistance at the direction perpendicular to porphyrin rings. porphyrin ring, while 2 and 3 have small steric resistance at the
The cyclic voltammogram (CV) of 1 in dimethylformamide
(DMF, 0.1 M Bu4NPF6) under N2 shows two reversible 1e-redox
waves at −1.54 and −2.12 V vs ferrocene (Figure 3a and S11, all
potentials reported in this work are referenced to ferrocene). The
peak currents display linear correlation with square root of scan
rates (Figure S11), indicating diffusion-controlled processes. For
simplicity, we assign these two redox waves to be 10/1− and 11−/2−
respectively. The CVs of 2 and 3 display reversible 1e-reduction
wave at −1.63 and −1.64 V, respectively (Figure 3a). Both 1 and
2 have pivalamido units substituted at the meso-phenyl groups.
However, X-ray structure analysis shows that the porphyrin ring
of 1 is substantially saddled due to the steric hinderance of bulky
ortho-substituents, while the porphyrin ring of 2 is almost planar
because the pivalamido units are para-substituted. This severe
distortion from the planar geometry is known to affect electronic
structure of metal porphyrins,[45] leading to 90 mV anodic shift of
the first reduction wave of 1 as compared to 2 in electrochemical
measurements.
direction perpendicular to the porphyrin ring.
The molecular nature and catalytic stability were confirmed.
For 1 and 2, their catalytic currents show first-order dependence
on the concentrations of TFA (Figure S15) and catalyst (Figure
S16). During bulk electrolysis with TFA, the currents maintained
constant, the accumulated charges increased linearly with time,
and their UV-vis spectra after electrolysis showed almost no
changes as compared to those before electrolysis (Figure S17
for 1 and Figure S18 for 2). All these results confirm that both 1
and 2 are stable by functioning as HER catalysts.
In order to understand HER mechanisms, we generated 1−
and 2− and studied their reaction with TFA. As shown in Figure
4a, upon 1e reduction of 1 with one equivalent of KC8, the Soret
band at 417 nm and the Q band at 529 nm decrease in intensity,
and meanwhile new peaks at 448, 543 and 589 nm appear. The
changes are accompanied with isosbestic points at 350, 405,
435, 517 and 538 nm (Figure 4b), suggesting clean conversion
from 1 to 1−. During the reduction, the Soret band does not show
a redshift, and the absorption in the 550-700 nm range does not
increase, suggesting a metal-centered reduction to give NiI.[43,46]
In electron paramagnetic resonance (EPR) analysis, 1 with a NiII
state is EPR-silent, while 1− shows typical NiI signals under the
liquid helium temperature (Figure 4d),[46] confirming a metal-
centered reduction.
,
Addition of excess TFA to 1− at −20 °C leads to a new UV-
vis spectrum (Figure 4a), and during this process, well-defined
isosbestic points at 392, 431 and 495 nm are observed (Figure
4c). No broad and intense absorption peaks in the 600-800 nm
range, which are characteristic for phlorins,[47-49] are observed in
UV-vis measurement. This result argues against the protonation
at the porphyrin ring. In addition, the EPR of the resulted species
shows significantly large g values, which are indicative of a NiIII
state (Figure 4d).[50-52] It is worth noting that the tiny signal with g
= 1.99 is due to residual 1−. Based on these results, we can
propose that the oxidative protonation of NiI with TFA gives
NiIII−H,[53] which has insufficient basicity for heterolytic
protonolysis with TFA.[43,54] This NiIII−H of 1 is also unlikely to
undergo bimetallic homolysis due to the large steric resistance
on both sides of the porphyrin ring. Thus, further reduction of
Figure 3. (a) CVs of Ni porphyrins 1, 2, and 3 in DMF. CVs of 1
(b), 2 (c), and 3 (d) in DMF with addition of TFA. Conditions: 0.5
mM catalyst, glassy carbon working electrode, 100 mV s−1 scan
rate, 23 °C.
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this hydride under more negative potential is required for
catalysis (Figure 3b). Further reduction of metal hydrides to
make more active ones for heterolytic HER has been proposed
previously.[37,55-58] Note that 1− shows no changes under N2 and
can be oxidized with O2 to regenerate 1 (Figure S19). This result
suggests that 1− has sufficient stability for these studies.
EPR of 22− shows no obvious signals (Figure S21), supporting a
metal-based reduction. Addition of excess TFA to 22− also leads
to the complete regeneration of 2 (Figure S21). This behavior
indicates the formation of NiII−H, which can undergo heterolytic
protonolysis with TFA to release H2 and the NiII porphyrin.
Moreover, 1− can be reduced by one more electron with KC8
to give 12−. This process displays isosbestic points at 392, 426,
494 and 530 nm in UV-vis (Figure S20). EPR of 12− displays no
obvious signals (Figure S20), suggesting metal-based reduction
to give Ni0. Addition of excess TFA to 12− fully recovers 1 (Figure
S20). This result suggests that the oxidative protonation of 12−
produces NiII−H and confirms that NiII−H is basic enough to
undergo heterolytic protonolysis with TFA to make H2.
Figure 5. (a) UV-vis spectra of 2, 2−, and 2− with added TFA in
DMF. (b) UV-vis change upon 1e-reduction of 2. (c) UV-vis
change of 2− upon adding excess TFA as monitored by stopped-
flow technique at −20 °C. (d) Liquid-helium temperature EPR
spectra of 2, 2−, and 2− with addition of excess TFA.
To further confirm the homolytic HER from NiIII−H of 2, we
studied the hydride transfer reaction between NaHB(OAc)3 and
NiIII. The hydride transfer ability of NaHB(OAc)3 was confirmed
by its reaction with GaIII TPFP, giving the known GaIII−H with the
same electronic absorption spectrum as we reported previously
(Figure S22).[26] Notably, no reduction of GaIII porphyrin and no
H2 evolution were observed during this process. Next, the CV of
2 shows a quasi-reversible 1e oxidation wave at 0.51 V (Figure
S23b). Consequently, we can electro-oxidize 2 to make 2+. As
shown in Figure S24a, upon 1e oxidation, the Soret band at 420
nm and the Q band at 532 nm decrease in intensity, while the
absorption at 464 nm increases. Because absorption in the 550-
700 nm range does not increase, this result suggests a metal-
centered oxidation. Significantly, subsequent reaction of 2+ with
one equivalent of NaHB(OAc)3 fully recovers 2 (Figure S24b and
S24c), and during this process, a total of 0.40 equivalent of H2
was evolved (Figure S24d). This gives a yield 79% for homolytic
HER. Note that the reaction of 2 with NaHB(OAc)3 gives no H2
and no change in UV-vis spectroscopy (Figure S25), confirming
that NaHB(OAc)3 is not able to reduce 2. This observation
excludes the possibility that 2+ is first reduced by NaHB(OAc)3 to
generate 2− and protons, which then undergo HER. Electro-
oxidation of 1 to 1+ displayed similar changes in UV-vis
spectroscopy (Figure S26a). However, unlike 2+, the reaction of
1+ and NaHB(OAc)3 gave a UV-vis spectrum, which was not
comparable to that of 1 but was almost identical to that obtained
by adding TFA to 1− (Figure S26b and S26c). Therefore, we can
assign the resulting species as the NiIII−H of 1. Importantly, no
H2 was evolved during this reaction (Figure S26d). These results
suggest that (1) NiIII−H is actually involved in the HER for both 1
and 2 and (2) NiIII−H of 2 can undergo homolytic HER.
Figure 4. (a) UV-vis spectra of 1, 1−, and 1− with added TFA in
DMF. (b) UV-vis change upon 1e-reduction of 1. (c) UV-vis
change of 1− upon adding excess TFA as monitored by stopped-
flow technique at −20 °C. (d) Liquid-helium temperature EPR
spectra of 1, 1−, and 1− with addition of excess TFA.
Complex 2 can also be singly and doubly reduced. Upon 1e-
reduction, the Soret band at 420 nm decreases, while absorption
at 472 and 606 nm increase, displaying isosbestic points at 411
and 439 nm (Figure 5a and 5b). Unlike 1−, intermediate 2− has
considerably enhanced absorption in the 550-650 nm range,
suggesting that it is better described as a NiI species with some
NiII porphyrin p-anion radical character. This is supported by
EPR analysis of 2−, showing increased porphyrin p-anion radical
character (Figure 5d).[46] Addition of TFA to 2− leads to complete
recovery of the UV-vis spectrum of 2 with isosbestic points at
379, 441 and 488 nm (Figure 5a and 5c). No intermediate can
be identified in stopped-flow studies even at −20 °C (the lowest
temperature of the instrument). The resulted solution is EPR-
silent (Figure 5d), which is consistent with the formation of 2. On
the basis of these results, we propose that the oxidative
protonation of 2− to make NiIII−H is the rate-limiting step and
subsequent bimolecular homolysis of NiIII−H occurs immediately
to make H2 and 2. This is consistent with electrocatalytic studies,
showing that the reduction wave of 20/1− becomes a large
catalytic wave with TFA and catalytic currents increases linearly
with 2 (Figure S16b).
The reduction of 2− by one more equivalent of KC8 displays
isosbestic points at 399 and 435 nm in UV-vis (Figure S21).
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Moreover, we measured the CV of 1 with addition of one
equivalent of TFA. A small but significant reduction peak, which
is behind and separated from the 10/1− reduction wave, appears
at Ep,c = –1.77 V (Figure S27). This reduction event is very close
to the onset of catalytic HER wave with 1. Thus, we attributed
this reduction wave to the 1e-reduction of NiIII−H and proposed
that this reduction made the hydride more active for heterolytic
protonolysis with TFA. Unlike 1, with one equivalent of TFA, the
CV of 2 shows a catalytic wave at the 20/1− reduction potential
and a new reduction wave at Ep,c = –1.93 V, which is well behind
and separated from the 20/1− wave (Figure S28). Based on these
results, we can attribute this new reduction wave at Ep,c = –1.93
V to the reduction of NiIII−H of 2.
2, which is also supportive of the involvement of bimolecular
homolytic mechanism in the HER with 2.
Scheme 1. Proposed catalytic HER mechanisms with 1 and 2,
showing the switching between homolytic (red) and heterolytic
(blue) pathways regulated by the steric effects of Ni porphyrins.
Taking these results together, we can make two conclusions.
First, the protonation occurs at the Ni site of 1− and 2−. Dissimilar
electrocatalytic behaviors of 1 and 2 strongly argue against the
protonation at meso-C sites, as proposed by theoretical studies
on hangman porphyrins.[59] The reason is that pivalamido groups
are substituted at the ortho- and para-positions for 1 and 2,
respectively, and thus their meso-C sites are expected to have
similar electronic and steric properties. This is not consistent
with their dissimilar electrocatalytic HER behaviors. In addition,
the hydride species formed by 1− with TFA does not show broad
and intense absorption peaks in the 600-800 nm range in UV-vis
spectrum, which are characteristic for phlorins, and shows EPR
signals for NiIII. These results support the protonation at the Ni
site. More importantly, the hydride transfer from NaHB(OAc)3 to
1+ gives UV-vis spectrum of NiIII−H, and the reaction between
NaHB(OAc)3 and 2+ fully recovers 2 and generates H2. All these
reaction features of 1 and 2 are consistent with the involvement
of Ni-based hydride intermediates in the HER.
Second, bimolecular homolysis of NiIII−H is involved in the
HER with 2. After oxidative protonation of 2− with TFA, there are
two possible pathways for the resulted NiIII−H species to evolve
H2: it can undergo homolytic HER or it requires further reduction
by residual 2− to become active for heterolytic protonolysis with
TFA. However, the reduction potential of NiIII−H is more negative
than the 20/1− redox potential by 300 mV. Thus, 2− is not able to
reduce NiIII−H. Consequently, further reduction of NiIII−H by 2− to
become active for HER is not likely. In addition, the reaction of
2+ with one equivalent of NaHB(OAc)3 can fully recovers 2 and
generate H2 with a yield of 79%. This is a strong support of the
homolytic HER from NiIII−H as no protons exist in this system.
Based on results from electrochemical and chemical studies,
we demonstrate that 1− and 2− show different mechanisms to
evolve H2 when treated with TFA (Scheme 1). Because of bulky
pivalamido groups located on both sides of the porphyrin ring of
1, the bimolecular homolytic mechanism of NiIII−H of 1 is blocked.
As a result, NiIII−H of 1 requires further 1e-reduction to become
more active for the heterolytic protonolysis with TFA. In sharp
contrast, because of the presence of small steric resistance at
the direction perpendicular to the porphyrin ring of 2, the NiIII−H
generated from 2− and TFA can undergo bimetallic homolysis to
make H2. Moreover, unlike singly reduced forms, doubly reduced
1 and 2 are able to reduce TFA to yield NiII−H species, and
these NiII−H have similar reaction features to undergo heterolytic
protonolysis with TFA to generate H2. This further underlines the
dissimilar reaction features of NiIII−H forms obtained from 1 and
In conclusion, we herein report that Ni porphyrins, which
have distinct steric effects by introducing bulky amido moieties
to ortho- or para-positions of meso-phenyl groups, display
different HER behaviors. Although 1e-reduced Ni porphyrins are
able to react with TFA, the resulting hydride intermediate −
depending on the steric resistance along the perpendicular
direction of porphyrin rings − will either undergo direct bimetallic
homolysis to evolve H2 or require further reduction for heterolytic
protonolysis. Such switching between homolytic and heterolytic
HER pathways via the regulation of molecular steric effects is
unprecedented. Our results also demonstrate that for reduced Ni
porphyrins, the oxidative protonation occurs at Ni sites rather
than meso-C sites. This work improves the understanding of the
HER mechanisms and provides valuable insights into catalyst
design with more consideration from steric effects.
Acknowledgements
We are grateful for support from the “Thousand Talents Program”
of China, Fok Ying-Tong Education Foundation for Outstanding
Young Teachers in University, National Natural Science
Foundation of China (Grants 21101170, 21573139, and
21773146), Fundamental Research Funds for the Central
Universities, and Research Funds of Shaanxi Normal University.
U.-P. A. is grateful for the financial support by the Deutsche
Forschungsgemeinschaft (Emmy Noether grant AP242/2-1,
under Germany's Excellence Strategy – EXC 2033 – 390677874
– RESOLV) and the Fraunhofer Internal Programs under Grant
No. Attract 097-602175.
Keywords: steric effect • hydrogen evolution • homolytic •
heterolytic • nickel porphyrin
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10.1002/anie.202002311
Angewandte Chemie International Edition
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Homolytic vs heterolytic HER: Ni
porphyrins with distinct steric effects
by introducing amido units to ortho- or
para-positions of meso-phenyl groups
display different catalytic HER
Xiaojun Guo, Ni Wang, Xialiang Li,
Zongyao Zhang, Jianping Zhao, Wanjie
Ren, Shuping Ding, Gelun Xu, Jianfeng
Li, Ulf-Peter Apfel, Wei Zhang, and Rui
Cao*
behaviors. Depending on steric
resistance on both sides of porphyrin
((Insert TOC Graphic here))
Page No. – Page No.
rings, hydride intermediates, formed
through the oxidative protonation of
1e-reduced Ni porphyrins, either
undergo bimetallic homolysis or is
required to be further reduced for
heterolytic protonolysis to evolve H2.
Homolytic versus Heterolytic
Hydrogen Evolution Reaction Steered
via Steric Effect
This article is protected by copyright. All rights reserved.